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A detailed collection of definitions of additives used in coatings formulation.
You can purchase PCI's 2011 Additives Handbook on CD for $29.95 plus shipping. The CD will also include a directory of additive suppliers and distributors. Contact Andrea Kropp at kroppa@pcimag.com for details.
Additives belong to a broad and diffuse category of key components in a coating formulation. They comprise a small percentage in that formulation, usually less than 5%, but their impact is significant. Additive function is almost always very specific in nature. Some additives are multi-purpose; for example, they may be important to the manufacturing process as well as to the coating’s performance. In recent years, multi-purpose additives have been developed, thus allowing the use of fewer additives in many formulations. Occasionally the use of one additive will require the use of another to counter some undesirable effect of the first.
Some additives are proprietary products with highly specific functions that work well in some systems but cannot be used in others. In addition, because of the proprietary nature of many additives, their chemical composition is not disclosed. This can make general recommendations difficult. In addition, this lack of structural knowledge means that additive substitutions cannot be made on the basis of fundamental structural chemistry.
The focus on green technology, lower cost and safer products has led to the introduction of newer additives and chemistries. The industry demands that green additives perform the same or better than their traditional counterparts and that they combine performance, sustainability and efficiency along with lower cost. With a large number of additives available for a particular problem, formulators can find themselves in trouble if the wrong additive is initially selected or added to alleviate or correct a problem. Correct additive selection is important to success, and such selection is made through vendor assistance or years of experience.
Please note that there are a number of new nano-sized additives on the market today that are difficult to categorize. Their functions are varied and tend to overlap our traditional categories. For this reason we have included a number of these types under the Nanotechnology section.
The following is a brief description of various coating additives along with some generic examples. The majority of additive types are represented.
Abrasion is a phenomenon caused by the mechanical action of rubbing, scraping or erosion. It has two forms, marring or wearing. Mar abrasion is the permanent deformation of a surface, but the deformation does not break the surface. Wear abrasion is removal of a portion of the surface by some kind of mechanical action: wind erosion, sliding back and forth of an object, wear of tires on traffic paint, and so on. The surface removal is gradual and progressive in nature. Abrasion resistance is a combination of basic factors such as elasticity, hardness, strength (both cohesive, tensile and shear strength), toughness, and, especially in the case of wear resistance, thickness. In addition, abrasion resistance is intimately related to scratching and slip. Thus, compounds that enhance these properties will improve abrasion resistance.
The nature of the polymeric resin and the pigments affect abrasion resistance. In the case of the pigments, it should be noted that extender pigments are noted for their ability to contribute to a variety of mechanical properties. Examples of compounds that have been used to enhance abrasion resistance include: silica glass spheres, specialty glass spheres such as UVT™ Sunspheres, and similar compounds that improve hardness. Certain silicones and other oils will decrease surface friction, making it easier for objects to slide over the surface and thus reduce wear abrasion. Increasing crosslink density by use of higher functionality oligomers and/or larger amounts of crosslinking agents has been used to improve abrasion resistance.
Waxes have also been used to improve slip and thereby abrasion. Hard waxes resist abrasion better than soft materials. Both PE and PTFE waxes function by the ball bearing mechanism, while the softer microcrystalline waxes work via the layer (bloom) mechanism.
The use of nano-sized materials in coating formulations can significantly improve scratch resistance. These improvements can be used in clear topcoats, ink over-print varnishes and pigmented finishes. The commercial availability of nanoparticles allows formulators to obtain new properties that were unachievable in the past, not only in scratch resistance but many other physical performance attributes.
For nanoparticles to be of use in transparent coatings, it is critical that aggregates present in the powder be dispersible to their primary particle size in the coating formulation to avoid rapid settling and excessive light scattering. In addition, it is critical that the dispersed primary particles avoid re-aggregation during the coating curing process.
Thousands of scratch-resistant coating applications are present in our everyday lives. Examples of these applications include coatings for wood floors, safety glasses, electronic displays, automotive finishes and polycarbonate panels. Improving the mar, scratch and/or abrasion resistance in these transparent coating applications is a major challenge, particularly with regard to not affecting the other performance attributes of the coating.
Inorganic Fillers
Incorporation of inorganic fillers into coatings to improve mechanical properties is well known. Drawbacks associated with this approach can include loss of transparency, reduced coating flexibility, loss of impact resistance, increase in coating viscosity and appearance of defects. To overcome these defects, a filler material should impart improved scratch resistance without causing the aforementioned drawbacks. Nanomaterials have the potential to overcome many of these drawbacks because of their inherent small size and particle morphology.
Maintaining transparency in a coating containing inorganic filler particles is a challenge. Four properties dictate the degree of transparency in a composite material: film thickness, filler concentration, filler particle size, and the difference in refractive index between the bulk coating and the filler particle.
Silica particles, colloidal or fumed, and clays are among the most widely studied inorganic fillers for improving the scratch/abrasion resistance of transparent coatings. These fillers are attractive from the standpoint that they do not adversely impact the transparency of coatings due to the fact that the refractive indices of these particles (fumed silica = 1.46; bentonite clay = 1.54) closely match those of most resin-based coatings. The drawback to silica-based fillers is that high concentrations of the particles are generally required to show a significant improvement in the scratch/abrasion resistance of a coating, and these high loadings can lead to various other formulation problems associated with viscosity, thixotropy and film formation.
Alumina
The use of alumina particles in transparent coatings is much more limited even though alumina is significantly harder than silica-based materials and, as a scratch- and abrasion-resistant filler, higher performance at lower loadings is often observed. For alumina particle sizes greater than 100 nm, the high refractive index (1.72) results in significant light scattering and a hazy appearance in most clear coatings. Currently, only high-refractive-index coatings, such as the melamine-formaldehyde resins used in laminate production, can use submicron alumina for scratch resistance and maintain transparency.
To use alumina as a scratch-resistant filler in transparent coatings, the particle size must be sufficiently small to overcome its refractive index mismatch. A Physical Vapor Synthesis (PVS) process has been developed that allows production of nonporous crystalline metal oxides having primary particle sizes less than 100 nm at economically viable rates with essentially no byproducts or waste streams.
Two grades of aluminum oxide can be produced using the PVS process: NanoTek™ and NanoDur™ alumina. Both grades feature a mixture of γ- and δ-crystal phases and are spherical in shape, but the grades differ in terms of primary particle size. NanoTek alumina has a surface area of 35 m2/g corresponding to a mean particle size of 48 nm, whereas NanoDur alumina has a surface area of 45 m2/g with a mean particle size of 37 nm.
There is a proprietary particle dispersion stabilization process that involves specific surface treatments designed to yield nanoparticles that are compatible with a variety of different coating formulations. For example, stable dispersions of metal oxide nanoparticles can be prepared in solvents such as water, alcohols, polar and nonpolar hydrocarbons, plasticizers, and even directly in acrylate monomers with the appropriate surface-treatment process. These surface treatments allow solids levels of up to 60 wt% to be dispersed, and yet maintain a sufficiently low viscosity for ease of blending.
The use of highly concentrated, non-aggregated nanoparticle dispersions allows incorporation of the nanoparticles into a coating formulation without substantial dilution of the formulation with the dispersion liquid. This feature is particularly important in 100%-solids coating formulations wherein the nanoparticle is dispersed in one of the reactive monomers.
Within a given coating class, formulations that result in harder/stiffer coatings tend to show greater improvement with alumina incorporation than formulations that lead to softer/more elastomeric coatings. In addition, transparent coating formulations that exhibit crosslinking upon curing, such as UV-curable, 2K polyurethane, and melamine-based coatings, show greater improvement in their scratch resistance upon alumina nanoparticle incorporation compared to transparent coatings that do not crosslink but rather coalesce, such as emulsion-based coatings.
SNC
SNC is an abbreviation for silica nanocomposites that are composed of colloidal silica particles with an organic surface modification. These particles, which improve the scratch and abrasion resistance of a variety of coatings including radiation-curable formulations, are produced by a unique process that results in monodispersed, non-agglomerating spheres with a diameter of about 20 nm. The flexible manufacturing process is also capable of producing a broad range of cationic (epoxide) and free-radical (acrylate) radiation-curable oligomeric composite materials. These products are stable, transparent and have low viscosity, even at a silica loading of 60%.
Nanoscale materials for coatings also include complex silicon oxides and aluminum silicates. Nanoparticles of these materials have been incorporated into automotive coating formulations that have good sag resistance. The cured coatings have excellent chip and scratch resistance, outstanding appearance, superior sandability, and resistance to water spotting and acid etching. Some properties, such as scratch resistance, are maintained after accelerated weathering.
Sol-gel
It is also possible to improve the scratch- and wear-resistance properties of a coating as well as its photostability/weatherability by the addition of nanoparticles prepared by sol-gel processing. This method has the advantage in that it starts from existing, well-developed formulations to which a sol containing nanoparticles is added. After curing, the modified systems give transparent coatings with high wear and scratch resistance.
Very often, hybrid (organic-inorganic) materials are produced by sol-gel. The most common way to produce nanocomposites is to form, in-situ, an inorganic phase by hydrolysis and condensation of alkoxides or alkoxysilanes. A further curing results in covalent bonding between the organic and inorganic phase.
These products increase the epoxy-amine reaction rate and subsequently reduce the possibility of undesired blushing or blooming reactions. Controlled use of the amount and type of accelerator ensures minimal impact on the cured binder performance. Although there are numerous products capable of accelerating epoxy-amine reactions, the most commonly used are: tertiary amines (e.g., DMP-30 = 2,4,6-tris-[dimethylaminomethyl]-phenol), phenol derivatives (e.g., nonylphenol), alcohols (e.g., benzyl alcohol) or acids (e.g., salicylic acid). Be aware that adding accelerator will significantly reduce the pot-life of the binder system.
Iron phosphate conversion coatings for general industrial application use accelerators, such as molybdate or vanadate, to provide active sites for iron phosphate deposition. Alkyd resin technology frequently uses metal driers to promote air oxidation and resin crosslinking. The activity of cobalt or manganese drier metals can be enhanced by the addition of chelating agents, also known as drier accelerators. The chelating agents function by stabilizing the valence state of the metal so that the oxygen up-take rate is maximized. One such chelating agent is 1,10-phenanthroline.
Acid catalysts are used to accelerate chemical reactions. Strong acids such as p-toluene sulfonic acid (PTSA) are frequently used. Also used are catalysts based on dodecylbenzene sulfonic acid (DDBSA) and hexafluorophosphoric acid. In using strong acids as catalysts, acid strength does not necessarily influence the cure rate but it does affect some film properties. The most widely used of the strong acids is PTSA. Weaker acids, such as butylphosphoric, those based on aromatic phosphates and various carboxylic acids, are also used in some coatings systems. Blocked acid catalysts are also used for many crosslinking reactions.
Adhesion promoters improve a coating’s ability to withstand mechanical separation from a substrate. That is, they improve adhesive strength. Quite often these compounds contain two different functional ends, one of which will interact with the substrate and the other that will interact with the coating binder. Examples of the various coupling agents are the silanes, which are trihydrolyzable; the titanates, which can be mono-, di-, and tetrahydrolyzable; and the chromiums, which are complex in nature.
For metal surfaces that are to be coated, this is particularly important because metals, as a class, are unstable. The pure metal is always oxidizing to the metal oxide on the surface of the metal substrate. Exposure to moisture, oxygen and salts accelerates the process. Almost all coatings contain microvoids through which oxygen, small molecules like water, and ionic materials can diffuse. If the coating can remain bonded to the metal, then the damage done by these diffuse agents will be nonexistent. In other words, corrosion can be prevented. It is, therefore, very important to do all that is possible to maximize adhesion.
For some materials this involves a mechanical roughening of the substrate surface to increase the surface area for physical absorption. Chemical pretreatments such as zinc/iron phosphate and various other materials have also been used because tightly bound phosphated surfaces will retard access to the metal and, therefore, impede corrosion.
Typically, organofunctional silanes have been used in coatings as adhesion promoters because they provide a polar functional group to contribute to increased bonding to a mineral substrate. They also are hydrolyzable and provide wetting ability and surface activity. The silanes are moisture sensitive and will hydrolyze over time to silanols. This is not a problem in solventborne coatings systems but can cause problems for waterborne systems. The silanes react with both the polymer and the substrate to form covalent bonds across the interface. Silane adhesion promoters are used in urethane, epoxy, acrylic and latex systems.
Receptive inorganic surfaces are those that have hydroxyl groups attached to elements such as Si, Al, Ti and Fe. Nonreceptive surfaces, such as boron, and alkaline earth oxides, do not form stable covalent bonds with silanols. A number of different commercial silane coupling agents are used in coatings. Levels that range from 0.05-1.0% are generally effective.
Methacrylic phosphate monomers that improve adhesion to metal, concrete, glass and other inorganic substrates and that can be used in both water- and solventborne formulations are available. Some methacrylic phosphate monomers improve metal adhesion and also significantly improve corrosion resistance. There are also acrylic phosphate functional monomers that improve adhesion to various metal substrates. The acrylic reactive group provides a higher reaction rate in UV- and EB-curable applications.
Other adhesion promoters that are in the marketplace are titanates (such as isopropyl tris-[N-ethylaminoethylamino] titanate), zircoaluminates, zirconates, aryl/alkyl phosphate esters and proprietary metal organic compounds. The titanates and zirconates suffer from moisture sensitivity as well, so caution is necessary when using them with waterborne systems. Neo-alkoxy products are claimed to not have this problem. Alkyl/aryl phosphate esters, zircoaluminates and the metal organic promoters are stable in waterborne coatings. They are quite different in chemical nature and therefore the formulator needs to evaluate them separately.
Epoxy/methoxy functional additives are effective in promoting adhesion of a variety of coating systems to glass, aluminum and steel. Methacrylate/methoxy functional additives improve adhesion of free radical cured resins, such as polyacrylates, to inorganic substrates. Epoxy functional silanes improve adhesion and water resistance of a variety of coating systems to inorganic substrates. Amine/methoxy functional additives improve adhesion and water resistance of coatings and adhesives when bonded to glass or metal substrates.
Radiation-curable Coatings
Radiation-curable coatings applied over a variety of metal surfaces can pose an adhesive challenge. There are proprietary phosphate ester monomers that are best used on an additive basis to provide enhanced adhesive properties. These acid-functional monomers (AFMs) range in functionality from mono to tri, with differing acid level content. The AFM additives promote adhesion to a variety of metal substrates including aluminum, cold-rolled steel and tin-plated steel as well as promoting adhesion to wood and plastic substrates. AFMs should not be used with amines, as instability may result.
Powder Coatings
The same precautions regarding clean substrates and pretreatments that apply to liquid coatings are advised for powder coatings. Adhesion promoters such as the silanes and titanates may also be used to enhance adhesion. Silanes designed for use in powder coatings have an organo functionality that has an affinity for the powder resin system. The organo-silane must orient itself at the coating-substrate interface. The choice of organo-silane is usually governed by the resin system, and experimental screening is advised to determine which promoter provides the most improvement. Adhesion promoter types commonly used in powder include mercapto-silanes, amino-silanes, carboxyl/hydroxyl-silanes, and carboxyl-silanes.
Plastic Substrates
Due to high chemical stability, low price, excellent balance of physical properties, possible recycling, etc., the amount of polypropylene (PP) and thermoplastic olefin (TPO) consumed by automotive parts, household electrical appliances and molded general goods businesses is increasing. However, PP and TPO are materials with low surface energy that make painting and adhesion problematic, hence chlorinated polyolefin (CPO) has found wide use as an adhesion promoter. Solventborne CPOs have traditionally been used. Excellent adhesion between TPO substrates and CPO can be obtained as the result of good wetting and higher dispersion interaction, which are affected by the properties of the CPO’s chlorine content, crystallinity, melting temperature, molecular weight and its polydispersity.
There are several factors that can affect the performance of a CPO-based adhesion promoter. Application parameters play a significant role in designing a system that will provide optimum adhesion performance. Of particular importance is the temperature at which a coating applied to a PP or TPO part is cured or baked. In addition, substrate and CPO composition can influence overall adhesion performance.
Coating bake temperature is the temperature at which the coating applied to the TPO part is cured. Coating bake temperature can have an effect on the interaction between a CPO-based adhesion promoter and the surface of TPO, which can affect performance. For best results, coating adhesion is enhanced when the coated TPO parts are baked at temperatures over 100
Chemical agents used to destroy algae. Algae are chlorophyll-containing plants whose green color is often masked by a brown or red pigment. Representative compounds include gluteraldehyde, methylenebis(thiocyanate), quaternary ammonium compounds and zinc oxide.
Blocking is a key performance parameter for architectural application and, for industrial and OEM applications, block resistance is important in the manufacture of roll stock that will be unrolled at a later date. It is also important for reducing the need for storage space for freshly painted parts. ASTM D 4949 may be used to measure blocking performance. Factors affecting blocking include the coating surface free energy, topography of the coating, the hardness and the Tg of the polymer.
One approach to improve the block resistance of a coating is to introduce a surface-active agent that will bloom to the top – the air interface – of a film as it dries/cures.
Carbinol-functional silicone polyether copolymers impart mar resistance and anti-blocking properties in addition to leveling and wetting. Methacrylate functional silicone polyether copolymers provide consistent and long-lasting slip, mar resistance and anti-blocking to UV-cured coatings.
Fluorochemical additives can be mixed into coating formulations, often as post-adds, and migrate to the air interface where they can provide effective protection where it is most needed, without affecting recoat adhesion. Common applications include latex semi- and high-gloss architectural paints used on doors and window trim, and applications where painted parts are stacked for shipping.
Waxes decrease blocking so that unwanted transfer or adhesion to a contacted surface is prevented. This can be very important for materials that are coated, dried and stacked for storage and shipping. Waxes can be used in any type of coating that could benefit from mar resistance and/or a slip aid. Both water- and solventborne metal coatings benefit from added lubricity and abrasion resistance. HDPE, paraffin and Carnauba waxes are typically used to counteract blocking. Anti-blocking agents are also very useful for any type of items that are coated, dried and immediately stacked or rolled up for storage or shipment.
It should be noted that anti-blocking additives, since they tend to gather near the top surface of a film can change the appearance of a film – the gloss level. The type of paint is also a variable.
Related phenomena caused by surface energy gradients are picture framing, Bénard cell formation, orange peel, edge-pull and picture framing. In general, such defects are caused by the fact that the coating will flow from a low-surface-energy area to a high (relatively)-surface-energy area. A variety of surfactants will alleviate or remove this problem, but the fluorochemical surfactants are particularly effective in many cases.
ANTI-CRAWLING AGENT
A chemical additive that prevents the wet coating film from receding from small contaminated or otherwise altered areas of the substrate, leaving the area with only a very thin coating and often having the appearance of being uncoated. Changing solvent systems or the addition of a low-energy surfactant (silicones or polyether-modified silicones) may solve the problem.
ANTI-FLOODING AGENT
A chemical added to a multi-pigmented coating that prevents the separation and concentration of one pigment at the surface of the film upon drying or in a dispersion.
The terms flooding and floating are often used interchangeably within the industry but they are distinct events. Both events occur in the liquid coating phase. Some have referred to flooding as being a horizontal separation of the pigments as opposed to floating, which is a vertical separation. Some also refer to floating as being a problem on the surface of the film as opposed to flooding, which occurs beneath the surface of the film.
The terms are not standardized and both are the result of an uneven distribution of the pigment that appears in the film as it is drying. Some also use the term floating to describe a mottled effect and the term flooding to describe a surface color that is uniform but darker or lighter in color than it should be.
ANTI-FOAMING AGENT
See Defoamer, Foam Control
An anti-foam additive is used in coatings manufacture to prevent the formation of foam, or it is added to a mixture to destabilize foam and act as a bubble breaker, breaking the foam that has already formed.
The terms ‘defoamer’ and ‘antifoaming’ agents are often used interchangeably. In fact, they are not quite the same. A defoamer is a surface-active agent that stops the foam and breaks the bubble once it has been formed. It is a bubble breaker. An antifoaming agent prevents the formation of foam so it never forms. The term ‘foam-control agent’ is a more appropriate term to use and they function by a variety of mechanisms to prevent or rupture foam.
Quite often defoamers are proprietary blends of various ingredients such as mineral oils, organic solids and surface-active compounds; fatty oils, surfactants and silica derivatives; alcohols, silica derivatives and surface-active compounds; esters, mineral oils and silica derivatives; and the like. Such blends are compounded for special problems such as:
• quick foam “knock-down” coupled with long-lasting foam prevention in latex manufacture and latex coatings;
• controlling foam in stripping operations;
• foam control in food applications where FDA product acceptability is needed;
• foam control in paper coating and dyestuff applications;
• foam elimination in effluent systems;
• prevent air entrainment and/or facilitate air release from coatings during the filtration and filling processes;
• prevention of foam formation during roller application of architectural coatings;
• aid in the breaking of bubbles formed during roller application of latex coatings; and
• prevention of air entrainment in flash tanks and stripping columns.
With such product and specific end-use complexity, it is desirable to contact a supplier for product suggestions, point of application and amounts needed to efficiently eliminate difficulties.
These additives protect underwater marine hulls, or other coated structures, from harmful effects of marine life in coastal areas. Antifouling paint is used to protect the bottoms of ships against living organisms that attach themselves to the hull. These organisms cause reduced slippage (the ship advances more slowly and consumes more fuel) and can increase the weight of the ship, thereby affecting safety. Although they keep algae and barnacles from attaching to ship bottoms, tin-based antifouling agents have an impact on other marine organisms.
For years in the marine industry, the use of tributyltin (TBT) with cuprous oxide was an effective antifouling system that deterred algae, barnacles and other organisms from adhering to the ship’s hull. However, TBT is being phased out of use because of safety concerns for a variety of marine life and its long half-life that causes it to accumulate in the environment. There are a number of co-biocides used with cuprous oxide that are replacing the TBT-based systems. These include zinc and copper pyrithione and isothiazolinone (4,5-dichloro-2-n-octyl-4-isothiazolin-3-one)-based products. All of these co-biocide antifoulants have been used successfully in Japan for many years.
Cuprous oxide is a fungicide that deters attachment of marine organisms such as barnacles and mussels and thus is an active ingredient in antifouling paint. It is typically used with other co-biocides to enhance paint performance and improve the coating’s resistance against algae and slime attachment. The use of an algicidal co-biocide minimizes the attachment of algae and grasses to the coating surface.
There are barnacle inhibitors based on natural bioproducts – small molecules with menthol-like configurations. In particular, a menthol derivative called menthol propylene glycol carbonate is a GRAS (generally recognized as safe) food ingredient used as a cooling agent in gum and cosmetics. This chemical has repellent characteristics against barnacles as well as other insects.
The International Maritime Organization (IMO), in late 2001, approved a ban on the application of antifouling marine paints containing organotins such as TBT, beginning on January 1, 2003. The IMO agreement also required removal or sealing of all TBT-based coatings from vessels by Jan. 1, 2008. The actions affect all vessels engaged in international voyages. For more information, see the following website: www.antifoulingpaint.com.
Tin-free products have been on the market for more than 10 years, and marine paint companies offer the same performance guarantee as given with the tin-containing coatings. While most ships dry-dock every three years or less, some owners want a dry-dock interval of five years. Several of the new self-polishing systems can now meet that five-year standard ensuring most, if not all, marine markets can be serviced using a tin-free product. Registration requirements in the United States and Europe mean the new tin-free products have already undergone significant testing that show they are environmentally preferable to TBT.
With the approval at IMO of a ban on antifouling systems using organotins acting as biocides, a significant amount of work is being done on new antifouling systems. The goal is to find a renewable, natural material that is safe to humans and not harmful to the environment.
ECONEA (1H-Pyrrole-3-carbonitrile, 4-bromo-2-(4-chlorophenyl)-5-trifluoromethyl) is a metal-free antifouling agent that protects vessels against aquatic growth without a potential negative impact on the marine environment compared to traditional, copper-based antifoulants. It is being applied to U.S. Coast Guard aluminum hull crafts, eliminating the risk of galvanic corrosion that can occur when copper-based paints are applied to aluminum. Research has shown that ECONEA provides effective protection against fouling at substantially lower concentrations in the paint formula than traditional copper-based antifouling agents. Unlike copper compounds, ECONEA degrades rapidly once released at the surface of the paint – thereby not accumulating in the marine environment.
Currently marine coating research is highly focused on foul release coatings – i.e., those that are biocide free and do not release additives into the environment. These coatings function by using polymers that prevent living organisms from attaching themselves to the surface of the coating.
ANTI-FREEZING AGENT
A chemical that will prevent freezing or damage resulting from the freezing and thawing of a coating’s composition. Typical examples are ethylene and propylene glycol.
ANTI-GELLING AGENT
A chemical that prevents the progressive, irreversible increase in consistency of a pigment-vehicle composition usually caused by an interaction between the vehicle and pigment, reaction between resin and solvent, or polymerization of the vehicle. Some anti-gel agents for air-dry and stoving systems are based on a ketoxime and a phosphorous ester salt. They delay/prevent thickening, which can occur as a result of oxidation or condensation of the binder. They also reduce the reaction of the pigments with the vehicle and prolong the shelf life of a coating.
ANTI-LIVERING AGENT
A chemical that prevents the progressive, irreversible increase in consistency of a pigment-vehicle composition, usually caused by an interaction between the vehicle and pigment, or polymerization of the vehicle. Two typical anti-livering agents are 2-amino-2-methyl-1-propanol and N,N-dimethylethylamine. Livering is the slow, irreversible change that increases the consistency of a formulated (pigment-vehicle combination) coating. It usually results from a strong interaction between the vehicle and the pigment or other solid, dispersed material. However, it can be caused by a slow polymerization of the vehicle and entrainment of the pigment or filler. Livering often can be seen as a thick skin covering the surface of previously opened and stored cans of paint.
ANTIMICROBIAL AGENT
See Biocides/Fungicides
The term ‘antimicrobials’ is generally defined as substances, or mixtures of substances, used to destroy or suppress the growth of harmful microorganisms, whether they be bacteria, fungi or virus, in, or on, a substrate or article where it is not desired. Historically, the term ‘antibiotics’ is used in reference to controlling bacterial infections specifically in humans. For substances that control or inhibit yeast and fungi, the term used is ‘anti-fungal,’ whether on animate or inanimate substrates. Similarly, for inhibiting viruses, we refer to anti-viral substances.
Besides these various terms for antimicrobial concepts, in many industries terms like bactericides, fungicides, algaecides, virucides, preservatives and biocides are commonly used. For example, in the coatings industry, the term ‘biocide’ is historically employed to indicate preserving a wet formulation from microbial spoilage which, in most other industries like personal care or household markets, would be defined as ‘preservatives’.
In addition to these various terms to indicate chemicals that inhibit or destroy microorganisms, we are also exposed to layman’s terminology of ‘mildewcide’ or ‘moldicide’ to refer to chemicals controlling unsightly biological defacement on surfaces that are ‘black’. Even though it is implied primarily for fungal growth, it can also be caused by a consortium of other microorganisms including lower forms of algae, moss, amoeba, protozoa, etc. For non-microbiologists, these terms may be confusing, with each term meaning different things depending on the implied uses and claims by different suppliers. All of these chemicals can be referred to as antimicrobials irrespective of their applications, target organisms and mode of activity under a given condition.
Government regulations on treated articles, including coatings with antimicrobials, and guidelines on claims come under the Federal Insecticide Fungicide and Rodenticide Act [FIFRA], EPA and FDA. Under FIFRA, an antimicrobial product that claims to control microorganisms such as bacteria and fungi requires registration. According to the EPA, an article or a substance that is treated with, or containing, a registered pesticide is defined as a treated article and is limited to protecting the article itself from microbial spoilage or contamination [for example paints supplemented with fungicides or bactericides to protect the paint in storage or after application, or preservative treatments used in wood to protect wood against insects or fungal infestation].
The EPA in 2000 issued a Pesticide Regulation Notice 2000-1 to clarify the Agency’s policy with respect to the scope of the treated article exemption. It is a guidance document that focuses on the various types of antimicrobial claims that the EPA considers acceptable or unacceptable.
Antimicrobials used in public or non-public health claim products have to go through a battery of tests using acceptable microbiological methods to show efficacy in the product in use. There are many test methods in microbiology described to demonstrate the antimicrobial nature of a substance and when it is incorporated in an article. There are various screening stages adopted that may include primary, secondary and in-use final testing. A typical primary-screening protocol involves testing for the minimum inhibitory concentration [MIC] of the chemical to be incorporated in an article. Normally the MIC is determined in an in-vitro system like growth media against a set of bacteria, fungi, virus and/or algae depending on the target microorganism. After getting selected in the primary screen, the substance enters the secondary screening process in an in-vivo system, meaning the application matrix in which it is expected to be incorporated. Because not all selected antimicrobial substances are expected to be universally acceptable in varied systems, they next go through the rigor of compatibility, stability and efficacy testing evaluations. Some examples of this rigor in coatings applications include the chemical stability in the formulation, pH compatibility, heat stability, color acceptance, shelf storage longevity, exterior weather sustainability, etc., in addition to testing for its bio-availability to function as an antimicrobial throughout the process.
With so many scenarios for defining an antimicrobial, the challenge becomes how to test and what test methods to use for demonstrating and claiming broadly the ‘antimicrobial’ property of a treated article such as a coating. There are several ASTM methods available such as: ASTM D 5589, ASTM D 5590, ASTM D 3273 and ASTM D 2574. Also used is the Japanese Industrial Standard JIS Z 2801-2000: Antimicrobial Products-Test for Antimicrobial Activity and Efficacy. This method was originally developed to test the antibacterial activity of silver ions impregnated in rigid hydrophobic polymers. The method was developed by a consortium of workers comprised of manufacturers of silver-based antimicrobial agents, government-based research organizations and universities, and under organizations such as the Society of Industrial Technology for Antimicrobial Articles (SIAA).
This method is a quantitative measurement method that tests survival of low-dose bacterial inoculum deposited between the tested antimicrobial surface and a thin plastic film that keeps the inocula wet and nourished in a nutrient-rich environment throughout the 24 h incubation at 35 ºC. This differs from other traditional methods of testing antimicrobial resistance in coatings surfaces where the surface is not kept deliberately wet or moist with a nutrient medium. Inoculum survival and growth in the other methods depends only on the moisture from either media or humidity created in the incubated unit, so only microorganisms that can survive some level of desiccation on the surfaces in 24 h can be recovered and can be compared to the non-treated surface. In this respect, the JIS Z 2801 method is so severe it is not necessarily a realistic surface contamination model for dry walls and other coated vertical surfaces.
In spite of this limitation, JIS Z 2801 has emerged as one of the industry standards for perhaps the ‘worst case scenario’ of a surface that retains wetness and permits microbial survival. Following the method as written for hydrophobic coatings surfaces, yields results that may provide useful information, but for hydrophilic and porous substrates and surfaces, a deviation in the inoculum delivery and validation for each different substrate would be required to get useful data. There is no one universal test protocol or one pesticide product that can demonstrate all the antimicrobial properties on all surfaces.
The cause of degradation is almost always by oxygen or UV radiation attack (see UV Absorbers and Light Stabilizers). Oxygen attack can be by oxygen or ozone and it may occur under ambient or elevated temperature conditions. At elevated temperatures antioxidants decrease thermal oxidation and formation of peroxide radicals that, in turn, can cause formation of colored chromophores and/or other deleterious changes in coating properties.
Effective antioxidants include the p-phenylenediamine derivatives such as the N,N´-diaryl, the N,N´-alkyl-aryl, and the N,N´-dialkyl compounds, but these compounds have a tendency to discolor or scorch during cure. Also effective and widely used are the hindered phenolic compounds, such as 2,6-di-t-butyl-4-methylphenol (BHT), octyl and certain higher alkyl phenols, phosphites (trisnonylphenol phosphite, triphenyl phosphite, tris(2,4-di-tertbutylphenyl phosphite, bis(2,4-dicumylphenyl) pentaerythritol phosphite), and synergists that are mixtures of antioxidants that act in a synergistic manner. Antioxidants are usually used in a concentration range of 0.1-0.5%, and they are consumed in the stabilization process.
For powder coatings, the purpose of the antioxidant is to minimize thermal degradation of the polymer in the coating. Thermal degradation causes yellowing and a reduction in mechanical and chemical properties. Usually hindered phenols are used as primary antioxidants. Organo-phosphites are sometimes used as secondary antioxidants. An organo-phosphite acts synergistically with a hindered phenol to check the thermal degradation of the polymers. These compounds act as peroxide decomposers and can be incorporated in a 2 or 3 to 1 ratio (hindered phenol to organo-phosphite). Hindered phenol-based antioxidants are usually used at a level of 0.2 – 0.8% of the binder. Higher levels will cause yellowing. It should be noted that antioxidants do not reduce degradation from UV light exposure.
Various classes of antioxidants have different thermal stabilization mechanisms. The classic high-molecular-weight hindered-phenolic antioxidant is effective as an oxygen-centered radical scavenger, but can suffer from “pinking” in the presence of combustion gases like NOx, which can be found in gas oven exhaust. The phosphite antioxidants act as decomposers of hydroperoxides and provide protection during high temperature processing and/or curing cycles. The new lactone antioxidants function as a carbon or oxygen centered radical scavengers and inhibit auto-oxidation. The development of a high-performance “phenol free” antioxidant blend exploits synergistic effects between phosphite and lactone properties while illuminating the possibility of pinking. There is a synergistic effect when using different antioxidants blended together and the combination can meet heat and processing stability requirements.
ANTI-RUST AGENTS
See Corrosion Inhibitors/Flash Rust Inhibitors
Anti-rust agents are compounds that, when added to water, alleviate or lessen rust formation. The term also refers to a variety of chemicals used to prevent corrosion on the surface of iron or other ferrous metals resulting in the formation of products consisting largely of hydrous ferric oxides. The same or different agents may be used to prevent flash rusting, early rusting or to provide long-term corrosion resistance. Anti-rust agents are important in preventing rusting in cans or other metal containers of waterborne paints.
This is a common term used to cover a class of compounds used to increase the viscosity of paints, and control or prevent sagging of a coating film during application and curing. The potential for sagging tends to increase as wet film thickness increases and/or as drying is prolonged. Most of the anti-sag agents impart a thixotropic rheology to the paint.
Treated clays are the main type of thixotropic agent used in alkyd systems. These are Bentonite clays that have been treated so that they build a gel structure by hydrogen bonding in the paint. They are either added into the mill base where the dispersion shear breaks the agglomerates, or added as a pre-gel. These clays are of two types depending on whether or not they need a polar activator. The activators are usually alcohols. The choice of a clay is dependent on the system.
In many latex systems, structure is formulated into the coating and settling is not as much of a problem as it can be in solvent systems. For some aqueous systems the choice of colloid is very important. A variety of thickeners are available for waterborne systems. Assistance should be sought from the supplier of these various types of materials.
An anti-settling agent is a suspension or rheological additive whose function is to prevent or retard pigment settling, and to maintain uniform consistency of the coating during storage and application.
ANTI-SILKING AGENT
See Anti-Flooding
Additive used to prevent a particular type of float that results in parallel hairline striations of different colors running throughout the film of a pigmented coating.
ANTI-SKID AGENTS
See Anti-Slip Agent
Slip-resistant or anti-skid additives are compounds that function directly opposite of slip additives or lubricants. They make it more difficult to move one surface past another surface. Compounds such as the colloidal silicas are used for this purpose. In the marine coating segment, antislip or antiskid coatings are those that reduce the sliding and slipping of humans and cargo on decks. Silica sand is widely used as a point-of-application stir-in additive in floor, deck and stairway coatings.
Skin formation may occur in a dipping tank that is open to the atmosphere. Here the skin may begin forming as surface gel particles that are formed by oxidation. As time passes, the gels coalesce into a continuous film as further oxidation takes place. Additives such as the oximes, particularly 2-butanone oxime, phenolics, solvents, and retention aids are used. Oximes such as butyraldoxime, 2-butanone ketoxime (methylethylketoxime or MEKO, which is the most widely used compound), and cyclohexanone oxime are compounds that complex with polymerization catalysts and prevent polymerization. Phenolics such as hydroquinone, 2,6-di-t-butyl-4-methoxyphenol (BHT), o-alkylphenol, and similar compounds act as antioxidants that retard auto-oxidation reactions. These are used at low levels of about 0.05% to as much as 0.2%. Solvents dissolve the components that cause the skinning. Currently undisclosed composition products are on the market from various suppliers. Retention aids prevent applied paint films from drying too rapidly. Waterborne paints usually contain retention aids that slow down the evaporation of water.
ANTI-SLIP AGENT
See Anti-Skid Agent
Any material added to a coating that will reduce or eliminate the hazard of slipping on the surface of the dried film. Surface roughness or an increase in coefficient of friction accomplishes this. A coating surface on floors, curbs, streets, porches, decks and so forth may be slippery particularly if damp and, therefore, there is often a need to add an anti-slip agent to the coating to enhance the surface roughness. Polyolefins with high COFs (coefficient of friction) are widely used in both commercial and consumer formulas for floor finishes.
This is not to be confused with the slip or lubricity that many coatings are designed to have particularly in manufacturing, packaging and transporting coated goods.
ANTISTATIC AGENTS
Anti-stats are materials that, when added to the coating or applied to the film, make it less conductive, or less attractive to dust, lint or other airborne particulates.
Conventional antistatic agents used to increase the conductivity of polymeric materials so as to permit dissipation of electrostatic charges can be separated into four general categories.
1. Hydroscopic surfactants such as tertiary fatty amines and their quaternary ammonium salts, monoacyl glycerides, monoalkyl and dialkyl phosphates, alkane sulfonates and sulfonamides work by blooming to the surface and attracting a conductive film of atmospheric moisture. These antistatic surfactants are humidity-dependent and work on the chemical principal of limited polymer solubility, blooming to the polymeric surface to provide sites for water absorption from the atmosphere. Examples are: glycerol monostearate, stearyl phosphate, dodecylbenzene and sulfonamide.
2. Conductive pigments, metal powders and other additives, which dissipate the electronic charge proportionate to their loading in the polymer. Carbon black, graphite fiber, metal powders, barium titanate powders, potassium titanate whiskers, metal-doped silicas, TiO2 and fibers provide a low-resistance pathway to dissipate the electrostatic charge and provide permanent antistatic protection.
3. Metallocenes that provide a low-energy transfer of electrons between adjacent aromatic layers. The primary example is bis(methyl)cyclopentadienyl cobalt.
4. A new class of antistatic agents based on combined neoalkoxy titanates and/or zirconates and subsequent tri-neoalkoxy zirconates that can be added in minor amounts during compounding.
Antistatic agents are in hydrophobic coatings such as the silicones to improve the coating’s resistance to dirt pick-up. These additives are usually cationic in nature, but in certain instances they are nonionic hydrophilic compounds.
Anti-static additives enhance the electrical conductivity of electrostatic spray paints, improve gloss, reduce the Faraday effect in powder coatings, and retard dust attraction on the finished product. (The Faraday cage effect is observed in the powder coating of parts that have recesses, inside corners, channels or protrusions on the surface of the substrate. The Faraday cage is the area of the part where the external electric field created by the gun does not penetrate.)
These static dissipative materials can be blended internally with powder coatings to enhance the electrostatic spray characteristics of the coating and minimize dust attraction. This increases the transfer efficiency, which improves penetration into corners and recesses. The anti-stats seem to have no adverse affect on the physical characteristics of the final powder coating such as: impact resistance, pencil hardness, gloss, gel time, color, adhesion, cure time, salt spray and condensing humidity. These agents are able to control gassing and minimize pinholing in the coating.
For powder coatings both charge control and antistatic agents are used. Charge-control agents improve the transfer efficiency and the ability of the powder to penetrate the Faraday cage areas. The function of the antistat agents is to improve the ability of the coating to conduct extraneous electrical charges to ground. These additives are used to decrease the surface resistivity of the powder and the applied powder coating.
Antistatic additives reduce the powder resistivity so that the powder particles charge more efficiently. This in turn improves the overall efficiency of the coating process. Several types of materials are used to reduce resistivity. Quaternary ammonium salts (cationic) or alkyl sulfonates (anionic) based on fatty acid derivatives are often used.
Some of the cationic antistatic agents are catalysts for epoxy-containing powders and have a tendency to cause yellowing when baking. Barium titanate is also used to promote powder-charging characteristics.
ASSOCIATIVE THICKENERS
See Thickeners
Associative thickeners are polymeric compounds that provide consistent and reliable control of coating rheology during manufacture and use. They obtain their efficiency presumably by association between thickener molecules or thickener and latex particles.
BACTERICIDES
See Biocides/Fungicides
Bactericides are additives that will kill bacteria (single-celled aerobic or anaerobic organisms) that can cause a variety of problems in liquid coatings and coating films. Examples of suitable compounds include: hexahydro-1,3,5-tris (2-hydroxyethyl-s-triazine), sodium pyrithione, isothiazolinone-based chemicals, 1-(3-chloroallyl)-3,5,7-tri-aza-1-azonia-adamantane chloride, formaldehyde-releasing compounds, and biguanides (polyhexamethylene biguanide [PHMB]).
BARRIER COATING ADDITIVE
See Seal Coating
Because coatings are largely organic in nature, they provide a source of food for microorganisms. Microorganisms are found everywhere and they work around the clock trying to cause viscosity loss, putrefaction, gas formation, emulsion breakdown, and other undesirable physical and chemical changes. These attacking species can cause discoloration, marring, loss of adhesion and finally coating failure.
Microorganisms can contaminate paint in different ways during the manufacturing process. Unsanitary conditions may exist, such as for the raw materials (including thickeners, extenders, pigments, emulsions, surfactants and defoamers), water (process water and recycled water), containers and equipment (tanks, pipes, hoses, etc.).
The ability of some microorganisms to attach to surfaces and form adherent biofilms is also important. Biofilms are functional consortia of microbial cells entrapped within an extensive matrix of extracellular polymer (glycocalyx) produced by them. Biofilms can be formed in water systems, processing tanks and other areas. Biofilms may be sources of contamination of the product, and may cause corrosion, scaling, the reduction of heat transfer efficiency and other problems in addition to the spoilage. Microbial biofilms are usually resistant to biocide treatments or disinfectants. Depending on the growth conditions (nutrients, minerals, gas composition, temperature, pH, water activity, etc.), microorganisms can reproduce very rapidly in the paint.
Coatings need to be protected from microbe attack, and there are a number of microorganisms that the formulator needs to be conscious of when formulating paints. Sometimes the terminology of available agents is confusing: biocide, mildewcide, fungicide, algaecide (also spelled algicide) and so forth. Product literature and the suppliers will certainly assist in this regard. Many of these additives are multi-purpose and can curtail the growth of a number of organisms.
Biocides (or microbiocides) are substances that will kill organisms and thus are used to protect coatings from biological attack caused by algae, fungi and other organisms that propagate in moist environments – particularly in warm climates. The “-cide” nomenclature refers to compounds that kill, in this case, microorganisms. A biostat prevents or interferes with the growth of the organism but does not kill it. The additives are often further defined as follows to describe the particular types or organisms that are killed or affected.
Algaecide/Algicide - Chemical agent used to destroy algae.
Bactericide - Compound used at low levels to kill bacteria.
Bacteriostat - Substance that prevents or slows the growth of bacteria.
Biocide - A chemical agent capable of killing organisms responsible for microbiological degradation.
Efficacy - The effect of the microbiocide on the target organism or group of organisms; can be measured as percent killed versus a control containing no biocide. Efficacy can be expressed as MIC, or minimum inhibitory concentration, against a specific organism.
Fungicide - Chemical agent that destroys, retards or prevents the growth of fungi and spores.
Fungistat - Compound that inhibits the growth of a fungus, or prevents the germination of its spores.
Mildewcide - Chemical agent that destroys, retards or prevents the growth of mildew.
Spectrum - Refers to the effect a microbiocide may have on more than one organism such that a broad-spectrum biocide will be affective against more than one group of target organisms.
For coatings we are concerned with bacteria (aerobic and anaerobic), fungi (multicellular [molds], unicellular [yeasts]) and algae (green and blue-green). The addition of an in-can preservative will protect coatings in the wet state during storage and transport. But after a coating has been applied and dried, it becomes susceptible to colonization by fungi and/or algae.
Biocidal agents are available to work both “in-the-can or batch” and also in the dried film. For this reason, many manufacturers include a biocide (anti-microbial) agent in the formulation of the paint so that it can kill both bacteria and yeasts that can be present. If not corrected before they start, microorganisms can lead to the production of gases – this can occur in the container and result in can lids popping and cans distending, offensive odors emanating and loss of film and application properties. Bacterial enzymes and certain fungi attack organic thickeners, and this can lead to viscosity changes in the liquid coating. The pH of the paint can be affected and the paint can undergo discoloration.
Microbial contaminants can be introduced with water (process water, wash water), with raw materials (latex, fillers, pigments, etc.) and by poor plant hygiene. Bacteria are the most common spoilage organisms, but fungi and yeasts are sometimes responsible for product deterioration. Spoilage of waterborne products, which may go unnoticed until the product reaches the consumer, can result in significant economic loss. Good plant hygiene and manufacturing practices, when combined with the use of an optimized biocide, will minimize the risk of microbial spoilage.
Enzymes are organic catalysts, which means that they are not consumed in any reactions. They are produced by living cells and are protein in chemical nature. Bacteria and their enzymes can degrade the organic components of paint – the polymer and its organic additives. One enzyme molecule can change hundreds of organic molecular structures and degrade them. The most obvious immediate result is a loss of viscosity. This renders the product unstable and unusable. This is more of a problem for architectural coatings, which tend to be warehoused, shipped and then stored on shelves for longer periods than typical industrial coatings, which are usually consumed rapidly.
Manufacturers and formulators need to be conscious of the fact that the dried paint film is subject to microbe attack from mold, mildew and algae – particularly in certain climates where temperature and humidity encourage microbe growth. For dried coating films, algae and fungi can cause discoloration, dirt entrapment, cracking, blistering and loss of adhesion. A loss of adhesion is commonly associated with fungi growth as well as corrosion on certain substrates due to the moisture produced by fungi. Dependent on the climate, many exterior surfaces and roofs may be subject to algae growth, and not all fungicides are necessarily effective against algae. Certain areas of the world have already recognized this as a serious problem and one of concern for the preservation of exterior buildings.
The type of microorganism that can attack the coating depends on many factors including the presence of nutrients, the moisture content and the composition of both the substrate and the coating itself. Moisture is affected by the amount of rainfall, dew, humidity, temperature and time of year. Local environment conditions such as surfaces that are sheltered from wind and shaded areas also have an impact on microbial growth. Nutrient sources include constituents of the coating, partially biodegradable material from other microorganisms or simply dirt. The substrate may affect the pH of the surface and make it suitable for microbe growth. Fungi favor acidic conditions such as those provide by wood and some species of wood are more susceptible to fungi attack than others. Algae favor alkaline conditions such as those provided by masonry.
For use in architectural coatings, it is important that the fungicidal material have a low solubility in water so that it is not readily leached out of the paint film. It should also not cause any weathering effect such as fading, chalking or discoloration. Some antimicrobial agents can cause fading in architectural coatings; therefore, it is always wise to expose the formulation to Weather-Ometer testing.
There are thousands of kinds of fungi and algae throughout the world. However, only a relatively few disfigure and deteriorate exterior paint films. In general, research on painted panels and structures from around the world indicates that two types of fungi are the dominant causative agents of disfigurement and degradation of modern exterior paint films. These fungi were identified as Alternaria sp. and Aureobasidium (Pullularia) pullulans. Aureobasidium pullulans is the fungus predominantly responsible for the development of mildew in exterior paints. The Pseudomonas species attacks paints, joint compounds, roof coatings, exterior insulation and finishing systems and clear finishes in the can.
There are many effective biocides available for use. It is important to understand the operation of these agents and the differences in their activity. Some may be effective against certain bacteria in one concentration and effective against fungi in another concentration. Some biocides may be biocidal in certain concentrations and in other concentrations exhibit biostatic behavior. It is very important for the formulator to work with the supplier of these agents to understand their use and mode of action. Blends of biocides may often be used to enhance coating performance, as one biocide alone cannot always provide the desired results under demanding and varying climate conditions.
Some of the typical chemistries of these agents include: formaldehyde donors; ortho-phenylphenol (OPPs); isothiazolinone derivatives (such as 2-n-octyl-4-isothiazolin-3-one [OIT]); guanides and biguanides (such as PHMB or polyhexamethylene biguanide); carbamates (such as 3-iodo-2-propynylbutyl carbamate [IPBC]) and dithiocarbamates; copper or sodium or zinc pyrithione; benzimidazoles; n-haloalkylthio compounds; 1-(3-chloroallyl)-3,5,7-tri-aza-1-azonia-adamantane chloride; tetrachloroisophthalonitriles; cis[1-(3-chloroallyl)-3,5,7-tri-aza-1-azonia-adamantane] chloride and 2,2-dibromo-3-nitrilopropionamide (DBNPA); and quaternary ammonium compounds. These are but a few examples of the many agents available to the formulator.
Some biocides on the market today are two-for-one and eliminate the need for separate in-can preservatives and mildewcides. DCOIT – 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one is an example of one such biocide, which controls bacteria that cause coatings to degrade in the can and prevent mildew growth after the films dry. This particular biocide controls a wide range of microorganisms including fungi, algae and bacteria.
The microbiological activity of 2,2-dibromo-3-nitrilopropionamide (DBNPA) was documented as a seed fungicide in 1947 and later as an antimicrobial agent. DBNPA, when formulated as a 20% solution in water and polyethylene glycol, is completely miscible with water and readily disperses upon introduction into a water-based system. The DBNPA molecule begins functioning as an antimicrobial agent immediately upon introduction into a system; the rate of this activity is not affected by pH, and antimicrobial control is usually achieved before complete degradation occurs. The combination of instantaneous antimicrobial activity and rapid chemical breakdown makes this a cost-effective and environmentally friendly biocide.
It is used as a quick-kill biocide and short-term preservative in water-containing systems that require microbe control; it is ideal for the treatment of wastewater generated during the manufacture of paint. The collection and reuse of all wash water used to rinse paint mixing vats has been emphasized as crucial to achieving environmentally responsible production. This wash water contains a high concentration of paint solids and is usually heavily contaminated with microorganisms; it must be decontaminated prior to its re-introduction into the paint production process. DBNPA is ideal for this type of application.
Mildewcide (fungicide) and algaecide testing has been very confusing for paint companies. A paint formulation’s resistance to attack by fungi and algae is the most difficult performance characteristic to determine; testing requires a specialized laboratory with trained personnel to work accurately with fungi and algae.
The use of a single active ingredient may be sufficient to protect a coating against in-can spoilage or dry film defacement, but in many cases it may be advantageous to use a blend of the actives. For example, the combination of certain active ingredients can result in synergy whereby lesser amounts of each active are needed to bring about the same inhibitory effect as the use of either active alone. Thus blends of actives may allow manufacturers to protect a product at reduced levels, providing not only a potential cost benefit but also a product that is more environmentally friendly.
Even more important is for formulators to recognize the fact that even a minor change in a formulation may have a major effect on the biocide in that formulation. It is crucial with every change in formulation that the coating be tested for biocide efficacy. Some of the common factors that will decrease biocide efficiency are: pH, temperature of addition to the batch, nonionic surfactants, solubility, the presence of other formulation additives that deactivate the biocide, UV radiation and so forth. The only way to determine the efficacy of a biocide in a coating is through testing.
In testing various biocides in coatings there can be significant differences in performance of paint systems by exposure location. Since there can be such a wide variation in product performance by location, it is very dangerous to rely on data from one exposure site to assess how a national paint product might perform. To truly assess the potential commercial performance of paint systems, they should be tested at a variety of locations across the country. Laboratory tests alone are not sufficient to assess how well a particular film preservative will perform in the field.
For evaluating mildew resistance of interior coatings there is some uncertainty about which test method to follow for a realistic assessment of the coating. Testing coatings in interior environments presents different challenges than for exterior coatings. There are several test methods and each claim to be the best for predicting in-service performance. The most popular are Mil Spec 810F, ASTM D 5590 and ASTM D 3273. ASTM C 1338 and ASTM G 21 as well as a test method from the Forest Products Laboratory.
Caution is urged in testing because evaluating coatings on non-porous substrates like vinyl charts that resist wetting by moisture makes it more difficult for fungi to proliferate on the coating surface. Therefore using these charts may over estimate the coating performance.
Most interior paints are used on typical gypsum wall board with paper facing both sides. They absorb more moisture and retain it under humid conditions and thus are frequently susceptible to mold growth. So to evaluate interior paints, ASTM D 5590 with paper as a substrate or a variation of ASTM D 3273 with fungal inoculum deposited directly on the surface of a paper-based wall board may provide more realistic results.
Antimicrobial Polymer Emulsions
The traditional biocidal agents often are lost over time due to leaching or degradation of the film. Recently, driven by the European Biocides Directive, many of these traditional biocides are coming under greater scrutiny regarding their potential affect on the environment. There is also concern regarding the ability of microorganisms to easily adapt to the additives used and build resistance over time. The result is a growing need for alternative solutions that provide sustainable antimicrobial protection while minimizing many of the potential issues related to the use of conventional additives.
There is new, recently developed antimicrobial technology that offers unique ways to mitigate the effects of microbes on products. Unlike conventional active ingredients, the technology relies on antimicrobial polymers that either have inherent antimicrobial characteristics, or incorporate a conventional antimicrobial additive encapsulated or embedded into a polymer.
Since the active ingredient, or part of the molecule that is primarily responsible for the antimicrobial action, is attached to a polymer or uniformly embedded into the polymer at a nanoscopic level, these materials provide a more sustained and effective antimicrobial action over time.
In addition to the antimicrobial action, these same materials can provide polymer-related attributes such as binding, adhesion, barrier and bonding properties that conventional antimicrobial additives cannot. This multifunctional aspect may be of value where an ingredient is required to perform more than one function and thereby help in delivering a simpler and cost-effective solution. The intention for these new materials is thus not a direct replacement of the active ingredients used today but to provide additional benefits that cannot be provided in terms of durability, uniformity, consistency and greater functionality.
Recent data shows that this polymeric route to antimicrobial functionality may be well suited for applications in textiles, nonwovens, medical products, hygiene and personal care products, building materials and a variety of formulated products including coatings and adhesives. The requirements of the specific application determine which of the two approaches provides the best combination of antimicrobial and polymer properties.
Inherent Antimicrobial Polymer Approach
The inherent approach to antimicrobial functionality relies on the fact that the active site is part of the polymer and is tightly bound to the polymer backbone. The polymers are waterborne, easy to handle and environmentally favorable. Also, unlike conventional antimicrobial additives, these polymers are high-molecular weight materials and, therefore, less likely to be of concern regarding potential toxicology. These polymers can be designed to provide additional attributes such as static control, permeability, barrier and strengthening properties, and can be tailored to suit a given application need.
These materials would also be of interest in areas where a conventional AI (active ingredient) may not be desirable. Such applications could include hygiene products, medical devices and products or personal care products where the additional multifunctional aspects of this polymeric approach may be of greater value.
Unlike conventional antimicrobial additives that function by disrupting a biochemical pathway, these polymers function by breaching the integrity of the cell wall of the microbe. It is therefore believed that they are less likely to contribute to development of resistance in the targeted microbes. A variety of active sites and polymer backbones can be designed to suit a given application. Since they are waterborne they can easily be deposited on surfaces by well-known processes such as coating, spraying, saturation or wet end deposition.
Active Ingredient Carrier Polymer Approach
The active ingredient (AI) approach involves the incorporation of active ingredients into a polymer whether it is inherently antimicrobial, as in the case above, or if the polymer is inert. The AI is typically incorporated during the polymerization process in such a way that it is uniformly distributed into the polymer at a nanoscopic level.
This embedding of the AI into the polymer matrix provides complete additive coverage of the surface in a more uniform and consistent manner thus increasing the longevity of the antimicrobial effect. In this way it is possible to get increased efficacy and sustainability from an AI while providing the benefits of a polymer in terms of ease of handling and durability.
It is also possible to use this approach to deliver a concentrated dose of the AI into a given formulation or application, coatings for example, where the AI is finely distributed into a polymeric carrier used only to deliver the AI. This is possible only because the method of incorporation allows high levels of the AI to be suspended into a polymer at a nanoscopic level without affecting the clarity of the polymer film.
This approach also makes it possible to enhance the inherently antimicrobial polymer with AI by creating a “tunable” antimicrobial polymer where the activity can be selectively controlled using either the additive or the polymer as the situation demands.
Polymers containing the AI are waterborne and will have the performance properties associated with waterborne technology. Waterborne polymers are useful in applications where an additive is desirable, and would allow formulators to design sustainable solutions without many of the handling and durability issues related to the conventional additive approach. The choice of polymers and AI that can be used to provide a solution is varied and can be tailored to meet specific needs.
These new antimicrobial technologies that include inherently antimicrobial polymers as well as polymers that have encapsulated active ingredients provide a comprehensive approach to providing antimicrobial benefits across a wide variety of applications where sustainable antimicrobial functionality is needed, combined with the stated benefits of a polymeric offering. These technologies present real value to customers interested in providing the multifunctional benefits derived from a polymer that has inherent or embedded antimicrobial characteristics.
Polymeric Design Approach
A new “antimicrobial paint” developed at MIT can kill influenza viruses that land on surfaces coated with it, potentially offering a new weapon in the battle against a disease that kills nearly 40,000 Americans per year. If applied to doorknobs or other surfaces where germs tend to accumulate, the new substance could help fight the spread of the flu. The coating’s polymers poke holes in the membranes that surround influenza viruses.
The new substance can kill influenza viruses before they infect new hosts. The “antimicrobial paint,” which can be sprayed or brushed onto surfaces, consists of spiky polymers that poke holes in the membranes that surround influenza viruses. Influenza viruses exposed to the polymer coating were essentially wiped out. The researchers observed a more than 10,000-fold drop in the number of viruses on surfaces coated with the substance.
The polymers are also effective against many types of bacteria, including human pathogens Escherichia coli and Staphylococcus aureus, deadly strains of which are often resistant to antibiotics. For example, S. aureus causes serious problems in hospitals, where it can spread among patients and health care workers.
The new coating acts in a very different way from the many antibacterial products – such as soaps, sponges, cutting boards, pillows, mattresses and even toys – that are now on the market. Those products kill bacteria but not viruses and depend on a timed release of antibiotics, heavy metal ions or other biocides. Once all of the biocide has been released, the antimicrobial activity disappears.
With this type of polymeric architecture it is highly unlikely that bacteria will develop resistance because it would be difficult for bacteria to evolve a way to stop the polymer spikes from tearing holes in their membranes. Repeated testing thus far suggests that the microbes are not becoming resistant and the polymer coating is 99% effective.
The MIT researchers are working with industrial and military partners such as Boeing and the Natick Army Research Center to develop the coatings for practical use. Once the polymer coating is applied to a surface, it should last about as long as a regular coat of paint.
Silver Ions
Silver-based antimicrobials are compounds that contain silver whose beneficial properties make it useful as an antimicrobial agent. Such silver-containing compounds are stable in the presence of light and ultraviolet radiation, are thermally stable, offer broad spectral activity against target organisms, and usually have a low order of toxicity, which can be determined from manufacturers’ literature.
The effectiveness of silver products is based on the slow and continuous leaching of superfine silver ions that interact with the metabolism of microorganisms. Silver ions can inhibit enzyme activity, especially those containing sulfur. In doing so, they have a major influence on the energy metabolism of these microorganisms. Products containing silver demonstrate a broad level of antimicrobial effectiveness, however, significantly less activity is observed for attack of fungus when compared to bacteria. Examples of these antimicrobials are products where silver is embedded into base materials, such as special zeolites or glass. Furthermore, combinations of Ag and Zn used in zeolites may lead to synergistic effects.
AgION™
A silver-bearing zeolite antimicrobial compound, known as AgION, has been incorporated into polymer-based coatings to control the growth of harmful bacteria, mold and mildew. These coatings may be applied to stainless or carbon steel products for use in appliance, food processing and HVAC applications. The active ingredients in the coating are the silver ions that are held within an alumino-silicate zeolite carrier particle. The silver ions exchange with other ions (counter ions) that may be present in nutrients or moisture and in this manner are transported to areas that support microbial growth. The antimicrobial properties of the silver ion have been recognized for a long time.
Silver Plus Benzisothiazoline
The combination of silver or silver with benzisothiazoline gives a new and very effective preservative system. By using this combination, the amount of sensitizing benzisothiazoline can be significantly reduced and, if an excess of benzisothiazoline is used, the well-known photosensitivity of silver compounds is reduced. This combination is a highly effective and safe new preservative system for coatings.
SmartSilver™
Coatings enhanced with SmartSilver antimicrobial silver nanotechnology are effectively protected against a wide range of molds, fungi and bacteria, making it a highly efficient industrial antimicrobial. It can be incorporated into aqueous and polar organic solventborne coatings, powder coatings and injection-molded plastics. SmartSilver additives do not impact the mechanical or flame-resistant properties of coatings, and are stable against UV light and high temperatures. The dispersible powders are highly concentrated, silver-containing antimicrobial additives made with proprietary stabilizers, and deliverable as powders or pre-dissolved dispersions. Typical use levels range from 0.1-1.0% based on application and process conditions. The dispersed additives release silver ions when in contact with moisture, inhibiting the growth of microbes.
Ca(OH)2
Ca(OH)2-based surface coatings, now available in a variety of pigmented colors, act to prevent infections – in particular influenza, sinus infections, pneumonia, allergic rhinitis, asthma and some indications are it is beneficial for anthrax as well. These one-application, antimicrobial-antibiotic surface coatings are effective against all classes of microbes including bacteria, viruses, fungus and algae. The coating combines calcium hydroxide as its active biocide with a special Bi-Neutralizing Agent (BNA), a biopharmaceutical whose mode of action keeps the coating continuously working year after year.
Calcium hydroxide inhibits the growth of common microorganisms on the coating’s surface through its high alkalinity (at levels normally incompatible with the life of microorganisms). Normally, hydrated lime is highly susceptible to atmospheric attack – carbon dioxide in the air quickly converts calcium hydroxide to calcium carbonate, reducing its alkalinity and rendering it ineffective. With BNA, calcium hydroxide is safely stabilized by this patented technology into a semi-permeable calcium hydroxide-encapsulated matrix system. This specially engineered matrix system protects hydrated lime from atmospheric degradation, preserving its antimicrobial-biocidal potency long after the coating is applied.
What is unique is its benign nature to humans and its lethal nature to microbes. The active ingredient is coming from a naturally occurring mineral and has been used for centuries as a safe and effective way to kill pathogens.
The end-use product is waterborne, fast drying, virtually odorless and remarkably contains no VOCs. It offers a widespread solution to the problems caused by the presence of the common microorganisms. Using an antimicrobial agent registered by the U.S. EPA, this multi-patented technology works by creating a surface coating that resists the growth of microbes on its surface for over six years.
BLOCK-RESISTANT ADDITIVE
See Anti-Blocking Agent
Chemical agent that prevents the undesirable sticking together or adhesion of painted surfaces under normal or specified conditions of pressure, temperature and humidity.
BODYING AGENT
See Thickeners
BRIGHTENERS (OPTICAL)
The brighteners are usually fluorescent dyes or pigments that absorb UV radiation and re-emit it as violet-blue light that gives yellowish-white coatings a brighter, whiter appearance. They are used to increase the luminance factor and to remove the yellow undertone of white or off-white materials. Small amounts of blue dyes are also used to achieve the same result.
The optical brighteners or fluorescent whitening agents (FWA) are colorless to weakly colored organic compounds. In solution or applied to a substrate they impart bluish-white effects, have good light fastness, and excellent heat resistance and chemical stability.
BURNISH-RESISTANT ADDITIVE
Agent that improves the resistance of a coating to increases in gloss or sheen due to rubbing or polishing.
Lightweight spherical additives called hollow glass microspheres can be used for coatings formulations. These hollow glass microspheres offer scrub and burnish properties, in addition to viscosity control, thermal insulation and sound-dampening characteristics, and improved performance properties. Spherical particles composed of styrene and acrylic polymers are also used for improving mar resistance as well as ceramic microspheres.
High-molecular-weight micronized polyethylene or micronized polypropylene are used in architectural formulations for increased burnish resistance. These waxes effectively replace silicas in both waterborne and solvent systems without settling.
Polishing or burnishing a satin or flat clear coating occurs when silica is used in the formula to flatten the finish look. The problem with silica is that it orientates to the surface of the coating, and when the coating is rubbed, washed or marred, the top layer of silica is quickly worn away. With the use of nano particles, once again a synergetic effect is created between the silica and nano, giving a uniform distribution to the silica. This gives the coating scratch resistance and no more polishing (glossing up).
Many of the crosslinking reactions used to form durable films are accelerated by the use of catalysts. For example, melamine-crosslinked systems, polyurethanes and epoxies make use of catalysts. Some systems use acids as catalysts: phosphoric, carboxylic or sulfonic acids [such as para-toluene sulfonic acid (PTSA) and dodecyl benzene sulfonic acid (DDBSA)] can be used. Others make use of typical Lewis acid metal catalysts or Lewis tertiary amine base catalysts.
Acid catalysts (and blocked acid catalysts) are used to accelerate the reaction between the crosslinking resin and the primary resin. Good crosslinking is desirable so that the final cured film will show improved properties. By increasing the molecular weight of the crosslinked product, improvements are gained in chemical, humidity and detergent resistance, corrosion resistance, film flexibility and film hardness.
Typical acid catalysts used in coatings are: dinonylnaphthalene disulfonic acid (DNNDSA), dinonylnaphthalene sulfonic acid (DNNSA), dodecylbenzene sulfonic acid (DDBSA), para-toluene sulfonic acid (PTSA), and alkyl acid phosphate (AAP). The relative catalyst strength is: PTSA>DNNDSA>DDBSA>DNNSA>phosphates. In general, the sulfonic and blocked sulfonic acids are strong acids, whereas the carboxylic and phosphates are considered weak acids.
The formulator has to balance the properties of the catalyst with that of the crosslinking agent, the cure temperature and time, the pH of the system, and the desired final properties of the coating. This is not trivial and the raw-material suppliers have guidelines that can be followed.
A new class of blocked sulfonic acid catalysts, derived from aromatic sulfonic acids, has been developed that promotes the crosslinking of hydroxyl-functional polymers with amino-formaldehyde crosslinking agents such as hexamethoxymethyl melamine, especially in coil coatings. These catalysts are particularly effective in coil primer formulations containing calcium ion exchange anti-corrosive pigments. In addition, the unique deblocking profile of these catalysts provides the so-called snap cure at the desired peak metal temperature and within the specified time.
In coil primers, this new class of blocked sulfonic acids provide outstanding cure, viscosity stability upon oven aging, and corrosion/salt spray resistance. In addition to resistance to basic pigment deactivation, these catalysts reduce solvent popping defects and provide excellent adhesion/intercoat adhesion while allowing extended storage of formulated coatings. Products from this class of catalysts also are effective in topcoats where their cure response allows release of volatiles before cure, thereby preventing popping, while providing storage stability.
Dibutyltin dilaurate (DBTDL) and dibutyltin diacetate (DBTDA) are well-established catalysts for the isocyanate-hydroxyl reaction in the formation of urethane coatings. DBTDL is efficient but, as with any catalyst, problems such as reactivity and hydrolysis of ester groups may occur. Diazabicyclo[2.2.2]octane (DABCO) is a commonly used tertiary amine catalyst. The tertiary amines are effective for use with aromatic isocyanates.
There are, however, some unique non-tin catalysts based on bismuth, aluminum and zirconium that are useful for these same reactions. The non-tin catalysts are environmentally more acceptable and offer advantages such as: faster cure rate, improved pot life, improved catalysis in cationic electrocoating and reduced hydrolysis of polyester resins. Catalyst deactivation can occur because of water, resins with high acid numbers, anions and pigments that are carrying water into the formulation.
Oxidation Catalysts
Oxidation reactions are of great interest in fine chemistry, at both the laboratory and industrial scale. There are numerous applications using the conversion of primary alcohols into aldehydes.
Two types of reactions could fit in with this scheme. First of all, there are stoichiometric reactions involving the use of strong oxidizing agents such as: the complex chromium (VI) oxide/pyridine (Collins or Sarret reagent); pyridinium chlorochromate (Corey reagent); oxalyl chloride/DMSO (Swern reagent); dimethylsulphide/N-chlorosuccinimide (Corey-Kim reagent); Dess-Martin periodinane; SO3/pyridine; KMnO4; MnO2; RuO4, etc. Secondly, there are catalytic dehydrogenation reactions with catalysts such as copper chromite, Raney nickel, palladium acetate, etc. All these reactions do not fit in perfectly with the responsible care approach, which is now a priority in all chemical reactions. In effect, they present a number of drawbacks such as a high amount of metal waste, poor selectivity, safety issues in some cases, or harsh conditions.
A new family of green catalysts has appeared in the last few years. TEMPO, or 2,2,6,6-tetramethyl-1-piperidinyloxy radical, is the most representative member of this family, and its efficiency in oxidation reactions is well documented. The oxidation of alcohols into aldehydes, ketones and carboxylic acids uses a catalytic amount of the nitroxyl radical and a stoichiometric amount of an oxidant such as sodium hypochlorite, m-chloroperbenzoic acid, sodium bromite, sodium chlorite, trichloroisocyanuric acid, bis(acetoxy)iodobenzene, n-chlorosuccinimide, or oxygen in combination with CuCl or RuCl2(PPh3)3. The nitroxyl radical is converted into an active species, which is the corresponding oxoammonium ion, and is then able to oxidize various substrates.
Among these substrates, alcohols are converted into aldehydes, ketones or acids; diols into lactones; sulfides into sulfoxides; benzylic ethers into esters; or α-hydroxy-lactame into anhydrides. Nevertheless, the TEMPO or hydroxyl-TEMPO structures present several drawbacks such as poor thermal stability, strong volatility with a tendency to sublimation, high solubility in water with ensuing difficulties in treating the aqueous wastes, non-negligible toxicity, and a complex synthesis route involving several reaction steps.
A new green nitroxyl catalyst called Oxynitrox S100 is on the market and was designed for oxidation reactions. High activities and selectivities are achieved for different types of alcohols and its use can be extended to polyols or carbohydrates. It is classified as a green catalyst as it does not contain any metal. Additionally, it can efficiently replace classic metal catalysts such as copper chromite, chromium derivatives, catalysts based on ruthenium, molybdenum, silver, cerium, etc. It belongs to the family of nitroxyl radicals. It features an oligomeric structure that contains several TEMPO moieties.
Its high molecular weight (between 2000 and 3000 g/mol) makes it particularly suitable for possible recycling without losing any oxidation efficiency. It is generally used in homogeneous conditions, and the high molecular weight allows easy recovery of the end products by simple distillation.
The conditions that are generally followed for the use of Oxynitrox S100 correspond to a biphasic medium. The general procedure uses sodium hypochlorite as oxidant, Oxynitrox S100 as catalyst, dichloromethane, ethyl acetate or toluene as solvent, and sodium bromide as co-oxidant. This co-catalyst leads to the in situ formation of NaOBr, which is a more efficient oxidant than NaOCl. The degree of oxidation of the final product can be controlled by the amount of sodium hypochlorite: when using 1 to 1.3 NaOCl equivalents, or when using 2 NaOCl equivalents, the primary alcohol is respectively converted into the corresponding aldehyde or acid.
Powder Coatings
Caution is advised when using a catalyst in a powder coating formulation. The point in using the catalyst is to increase cure speed during the bake. Excessive catalysis can cause a pre-reaction to occur in the extruder, which in turn causes the formation of ‘gel bits’ or localized crosslinking. These gel particles cause defects in the finished film. Significant crosslinking in the extruder can also damage the equipment.
Catalysts are used at low levels based on the binder – usually around 0.1 – 2.0%. Many of the polyester manufacturers offer resins that contain catalysts and they provide information regarding the bake temperature. Catalysts are also provided as masterbatches to improve distribution into the powder mixture. Typical examples of catalysts for powder coatings are: Lewis base, ammonium salts, dibutyl tin dilaurate, dibutyl tin oxide, stannous octoates, sulfonic acid/amine, peroxides, methyl tolyl sulfonamide, dimethyl stearyl amine, onium, and cyclic amidine. Again the catalyst is specific to the binder type.
A non-yellowing catalyst for uretdione crosslinked powder coatings has been developed that promotes the reaction of polyols and uretdione crosslinkers in powder coatings. This catalyst, K-KAT XK-602 uretdione, is designed to improve on the two most common issues associated with using uretdione technology – high temperature curing and yellowing in the presence of common catalysts.
Cure temperatures can be lowered by 30
COALESCENTS (COALESCING AGENT)
Coalescing aids have a high boiling point which, when added to a coating, contributes to film formation by way of temporary plasticization (softening) of the vehicle. These aids facilitate the transition from liquid to solid state during the latex drying or film formation process. Coalescing agents have been traditionally used to obtain good film formation. Levels of coalescent affect drying time and ultimate film properties. Important properties of a coalescing agent include: hydrolytic stability, water solubility, evaporation rate, freezing point, odor, color and safety and regulatory concerns.
Architectural latex paints are made from a variety of different polymers that are selected based on performance requirements and cost. The monomers used in these polymers determine the glass transition temperature (Tg), which characterizes the hardness of the final polymer at a given temperature. The Tg and polymer type influence the amount and type of solvent required to coalesce the polymer. Substrate, application, dry time, compatibility, VOC regulations and efficiency all play a role in determining the type of solvent or combination of solvents to be used.
A conventional coalescent temporarily lowers the Tg, providing mobility to the polymer chains. The softened polymer can then flow and fuse with other polymer chains in the system, creating a protective, decorative film. To be effective, the coalescent has to remain in the film after the water has evaporated to ensure that a homogeneous film develops. A conventional coalescent will evaporate out of the film after a period of time and the film will regain its initial Tg and hardness. Various coalescents can be used individually or in combination to help formulators optimize performance in architectural coatings, while meeting VOC regulations.
Typical coalescing aids are compounds such as aromatic hydrocarbons, esters, ester alcohols, glycols, glycol ethers and glycol ether esters. As would be expected, the nature of the polymer – its solubility and affinity for various compounds – will affect the particular coalescing agent chosen. EB (ethylene glycol butyl ether) seems to have been the preferred coalescent agent in many industrial coatings for years. This is not by accident. While it is a fast-evaporating solvent, it appears to also have more of a swelling effect on emulsion particles that some of the other fast-evaporating aids. It is very efficient in lowering the MFFT of emulsions.
Other typical examples of coalescing solvents are: ethylene glycol monobutyl ether (butyl CELLOSOLVE™), propylene glycol n-butyl ether, diethylene glycol monobutyl (butyl CARBITOL™), ethylene glycol monohexyl ether, dipropylene glycol methyl ether, dipropylene glycol t-butyl ether, dipropylene glycol n-butyl ether, ester alcohol or 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate [(TMB)Texanol™], 2-ethylhexyl benzoate. The various trade names can cause confusion at times. Butyl CARBITOL is a low-volatility coalescent and should be used to ensure proper film formation when air drying or using long flash-off times. Low-volatility coalescents are necessary when air drying in high-humidity conditions.
Water-insoluble coalescents, such as hexyl Cellosolve, butyl Propasol™, etc., are hydrophobic solvents and can cause seeding if not premixed with water miscible solvents.
Specific high-purity esters of fatty acids and their blends can also be used to provide low-odor, VOC-free, renewable coalescents to enhance the performance of latexes used in low-VOC paints.
Coalescents based on renewable sources combine various long-chain acids with either a short-chain alcohol like methanol (the most economical option) or higher-chain alcohols and glycols or even glycol esters. Glycerol as an alcohol is not suitable for this use, as glycerol esters can have a negative impact on the adhesion of the paint film. Very short-chained acids like C8 are also not suitable as they contain VOCs. Longer-chain esters with a higher molecular weight form softer films and act as plasticizers. New “green” coalescents that are high in efficiency, low in toxicity and improve early hardness development are a step in the right direction for making better paint, especially outdoor paint.
A high-purity version of propylene glycol mono-oleate based on renewable oleic acid was introduced to meet stringent VOC requirements in the global consumer coatings market. A second-generation propylene glycol monoester with C-18 fatty acid mixtures is available that has better color and offers an improved value. A third product, based on renewable technology, is a high-purity version of linear short chain fatty esters, which is VOC-free based on the European definition but is over 90% VOC by Federal EPA method 24. This product has been found to be over 30% more efficient than trimethyl pentanediol monoisobutyrate ester (TMB) in many popular types of latexes and offers improved hardness development and dirt pick-up resistance. These renewable-based coalescents are naturally derived, low-odor agents and can be used in all types of decorative paints and result in improved performance and application properties, while helping to achieve compliance with new VOC regulations.
New developments in polymer technology have introduced latex emulsions for solvent-free architectural coatings. Architectural paints using this new technology do not require the use of coalescing agents, plasticizers or cosolvents to achieve good film-formation properties. Test results have shown that formulated solvent-free paints maintain the properties of low-temperature appearance, durability, film formation and open time normally expected from quality conventional latex paints. Also, a significant advantage is the low-odor characteristics of these paints, both during and after application. Formulating approaches to solvent-free architectural coatings are now focused on the selection of raw materials that would not contribute any solvents or VOCs to the system. These ingredients include pigment dispersants and surfactants, rheology modifiers, preservatives and substrate wetting agents.
There has also been development of new coalescing agents that are partially or completely free of VOCs. The efficacy of these materials in all varieties of formulations with different polymers has not been fully investigated.
Reduction of VOC using VOC-exempt solvents is not always simple, and formulating with them can be a challenge. These solvents usually bring higher cost and some have very low flash points, which leads to increased shipping cost. Careful formulation evaluations need to be conducted.
COLLOID STABILIZERS
These additives stabilize the very finely divided particles that have a size of about 0.01-1 micron (µm). Compounds such as animal glue, casein, cellulose ethers, guar gum, gum Arabic, poly(vinyl alcohol) and similar compounds are used as colloid stabilizers.
Corrosion inhibitors are compounds that improve a coating’s ability to protect aluminum, brass, copper and steel. The term refers to a variety of materials used to prevent the oxidation of metals, including surface treatments, undercoats, and additives or elements alloyed to the surface of the metal. Corrosion poses a major potential problem for metal surfaces that are typically protected through the use of zinc-rich coatings, the use of anti-corrosive pigments and the application of a barrier coat.
Flash rust inhibitors are often used to prevent in-can corrosion during the storage of waterborne coatings. Inhibitors also may prevent the corrosion of ferrous metals during the drying time of waterborne coatings. Sodium nitrite typically has been used in the past. Other types of materials include the following: organic zinc complexes, salts of dodecylnaphthalenesulfonic acid, ammonium benzoate, 2-aminomethoxypropanol and amine neutralized thiosuccinic acid.
Long-term corrosion inhibitors have low water solubility and are often used in combination with anti-corrosion pigments although they may be used alone. Some examples of these materials include: metal salts of aminocarboxylates, salts of dodecylnaththalenesulfonic acids, zinc salts of cyanuric acid, zirconium or amine complexes of toluylpropionic acid, and tridecylamine salts of thiosuccinic acid.
There are also organic, heavy-metal-free corrosion inhibitors to help industry meet regulatory trends toward restricting the use of lead, zinc and chromate. These corrosion inhibitors can improve wetting and adhesion over traditional corrosion inhibitors, and can be used to formulate high-gloss coatings, as well as corrosion-resistant clear coatings.
In terms of corrosion inhibitors, some of the most effective and widely used anticorrosive pigments such as red lead (PbO4), lead silica-chromate (4(PbCrO4· PbO) + 3(SiO2· 4PbO)), zinc chromate (ZnCrO4), zinc tetraoxychromate (ZnCrO4 · 4 Zn(OH)2) and strontium chromate (SrCrO4), have been and continue to be under heavy scrutiny due to the hazards posed to humans and the environment. Lead compounds are deemed toxic, zinc and strontium chromate are classified as carcinogenic and most recently, according to the EU Directive 004/73/CE, zinc phosphate has been determined to be a danger to the aquatic media.
In general, the latest global trend is to design coatings that comply with the environmental regulations that now exist. These “Eco-Friendly” or “Green” coating systems contain only non-toxic, non-reportable raw materials to ensure no hazard to humans and the environment. The industry has found it very difficult to obtain the same level of performance with the “Eco-Friendly” systems as compared to the non-compliant systems.
Coatings that meet the eco-friendly definition are high- or 100%-solids systems, powder coatings, UV or EB curing coatings, low/zero VOC, no heavy metal content, zinc-free or systems that contain no reportable compounds or ingredients in order to meet green label compliant status. Therefore, eco-friendly corrosion inhibitors should not contain heavy metals or non-reportable compounds, and be zinc-free in order to meet the green label compliant standard.
Ever since the use of chromates was restricted, the industry has been forced to use a variety of different non-toxic corrosion inhibitors specifically designed for a given substrate or resin type in an attempt to match the efficiency and versatility that chrome-based inhibitors offered. But now coatings formulators are demanding today’s non-toxic inhibitors offer as much universal application in a wide range of binders and protective coatings as their toxic counterparts.
Zinc phosphate (Zn3(PO4)2 · X H2O) was the first and most widely used non-toxic inhibitor for replacing lead and chrome-based inhibitors. Historically, standard zinc phosphate has demonstrated acceptable performance in real outdoor exposure, but less efficiency compared to chromates in marine environments and in accelerated weathering tests such as salt spray and cyclic corrosion (i.e., prohesion). However its user-friendly, low cost, universal application, and good package stability in a variety of general-purpose industrial and protective coating applications, made zinc phosphate the most popular choice early on for replacing chrome and lead-based inhibitors
Today, in order to meet the eco-friendly labelling demands, zinc phosphate and modified zinc-containing inhibitors can no longer be used. This has caused yet another dilemma for the inhibitor suppliers as the current offering of non-zinc inhibitors on the market have generally shown inferior anti-corrosion performance in accelerated corrosion tests, especially on steel substrates, as compared to most zinc-based inhibitors. Also, current zinc-free inhibitors are very limited in their application scope, performing well in some coating systems and poorly in others.
Today in the marketplace there are available mixed metal calcium-strontium phosphate complexes deposited on silicate cores that provide good overall performance. Also an organo-modified version of this same type of complex produces additional advantages in the areas of film formation, adhesion promotion and substrate wetting.
The basic formula for these products is: [Sr·P2O5·SiO2·XH2O], which represents a mixed metal phosphate complex on a silicate core. These complexes provide excellent direct anodic inhibition from the combination of the calcium and strontium cations but also provide good cathodic inhibition due to the basicity/alkalinity of the silica core. Its basic nature reduces the amount of oxygen needed to passivate the formation of rust.
The organic surface treatment shows improved mechanical properties in terms of better wetting in organic systems without decreasing its performance in waterborne systems, reduced pigment/binder interface, which makes the flow of water and electrolytes through the organic coating difficult and at the same time protects the pigment making it more inert when reactive resins or those with high acid values are used.
Most of the organosilanes have one organic substituent and three hydrolyzable substituents. For surface treatment applications, the alkoxy groups of the trialkoxysilanes are hydrolyzed to form silanol-containing species. The final result of reacting an organosilane with a substrate ranges from altering the wetting or adhesion characteristics of the substrate, utilizing the substrate to catalyze chemical transformations at the heterogeneous interface, ordering the interfacial region, and modifying its partition characteristics. It includes the ability to effect a covalent bond between organic and inorganic materials.
Titanates, zirconates and aluminates are used as coupling agents. Titanates act as organometallic chemical bridges between two dissimilar phases such as an inorganic pigment and a polymer binder – or as an adhesion promoter for a coating on a metal substrate. They provide an alternate to silane coupling agents and couple to non-silane reactive substrates such as CaCO3, carbon black and phthalo blue.
Titanates have six functions.
1. Coupling to form atomic monolayers on inorganics rendering them hydrophobic and organophilic, thus allowing complete dispersion and deagglomeration of pigments and extenders with minimum shear and work energy. They promote adhesion, significantly lower system viscosity and shift the critical pigment volume concentration (CPVC).
2. Catalysis to lower bake times and temperatures; to induce increased strain strength to the polymer binder to increase mandrel flexibility and reverse impact strength; to compatibilize dissimilar organics; and to synthesize polyesters.
3. Heteroatom to phosphatize and prevent corrosion or intumesce and add flame retardance.
4. Alkyl/aryl functionality to create polarity for adhesion and compatibilization.
5. Thermoset functionality to densify the degree of crosslink and hardness.
6. Molecular structuring to create stable organometallics.
Designer waxes are also excellent coupling agents and since they are produced by metallocene catalysis it is possible to tailor their properties to individual applications. They are important additives in wood-plastic composites (WPCs). WPCs are a new type of natural fiber composite with up to 90% wood fiber or wood flour content. The plastic matrix is usually polypropylene or polyethylene, although polyvinylchloride is also used. Since wood and plastic differ, especially in their polarity, suitable coupling agents such as polyolefin grafted with maleic anhydride must be added. They significantly improve the mechanical parameters of tensile stress and bending load in WPCs. The polar functional groups of the coupling agent react with the OH groups of the wood and form a genuine chemical bond. The non-polar areas of the coupling agent have strong affinity to the non-polar polyolefin chains.
Solvents that cause two immiscible liquids to homogeneously mix are also referred to as coupling agents.
CRAZE-RESISTANCE ADDITIVE
A chemical that will prevent the formation of fine random cracks or fissures on or under a surface of plaster, cement, mortar, concrete, ceramic coating, or paint film caused by shrinkage.
Epoxy-functional silane crosslinking agents for waterborne coatings are also available that will crosslink under heat or at room temperature on alkaline substrates if used in combination with recommended catalysts. These are designed primarily for carboxyl- or amino-functional acrylic latexes or polyurethane dispersions. The mechanism of crosslinking involves the epoxysilane’s dual chemical functionality. The epoxy portion of the molecule is reactive with the matrix resin and the alkoxysilane portion crosslinks after hydrolysis by condensation forming siloxane bonds. The alkoxysilane also can react with surfaces to improve wet adhesion of the coating, or with fillers to improve pigment binding.
Crosslinking causes changes in physical and chemical properties. That is, it causes changes in hardness, tensile strength, modulus, elongation, solubility, swelling and other properties. For example, crosslinking renders a soluble or thermoplastic polymer into a thermoset polymer that is three-dimensional in nature and insoluble. However, the crosslinked polymer system may be swellable to different degrees in liquids that may have been solvents for the uncrosslinked polymer. The greater the degree of crosslinking or the number of bonds formed between chains, the lower will be the degree of swelling in any particular solvent.
UV-Powder Coatings
The latest development in powder coatings is the combination of powder coating technology and UV technology. This technology is making strong inroads especially for temperature-sensitive substrates. The most suitable approach for a coating formulation is the use of a major binder and a crosslinker. The crosslinker may control the network density for the coating, while the binder determines properties of the coating such as discoloration, out door stability, mechanical properties, etc. Furthermore this approach will lead to a more homogenous concept in powder coatings applications as a category bringing similitude to thermosetting coatings where crosslinkers such as TGIC and β–hydroxyl amides are used. A crosslinker should present properties quite specific for the application intended: molecular weight, high functionality, and physical properties compatible to the application.
The existing UV-curing powder coating systems generally build on two binders. There is an option to the existing systems that consists of using unsaturated amorphous or amorphous-crystalline polyesters comprising maleic/fumaric moieties as reactive double bonds in combination with crystalline crosslinkers containing unsaturated reactive groups.
PSG di-acrylate (CAS number: 85286-82-4) is rated as having high UV-DSC reactivity in powder coatings formulations. Coatings using a crystalline acrylic ester may be easily formulated using as a starting point any unsaturated polyester of maleic/fumaric type. Addition of a small quantity of PSG-diacrylate may improve the performance in commercial systems of lower reactivity. This approach appears to be a reliable option in UV powder coating formulations and makes possible the use of typical unsaturated polyesters as binders in powder coatings.
CURING AGENT
See Hardener
Additive that promotes the curing of a film. Anything that promotes, enhances or ensures mechanical or optical property development of a coating (usually liquid) by means of chemical and/or physical change such as through evaporation, polymerization or other means. The curing agent can be anything such as thermal energy, radiation energy, a catalyst, various substances or other means to enhance such property development. Some curing agents are termed “hardeners.” Crosslinking agents are also curing agents.
Recent advances in new amine-functional curing agents have improved performance of waterborne two-package epoxy coating systems. These new amine adducts offer performance advancements along with the ability to meet near-zero VOC levels. Those that are designed for cementitious applications in industrial and do-it-yourself markets demonstrate water-reducible epoxy coatings with superior handling, application properties, improved resistance to chemicals and staining and visible end of pot life.
In waterborne systems, foam occurs as both macro- or microfoam and is manifested as pinholes and air bubbles in the dried film. Macrofoam is readily seen as large bubbles often held together in a honeycomb-like structural formation. Microfoam is often recognized as fine spherical shaped bubbles.
For solventborne systems, microfoam is often a problem and pinholes form from small air bubbles, which create channels as they move through the drying film. During the drying process the viscosity of the coating increases and the channels remain open and fixed in the dried film. Pinholes severely reduce protection properties of a coating, and they may allow salts and moisture to penetrate the coating and deleteriously affect the substrate.
Microfoam is caused by air entrapment during manufacture and/or application, during the mixing of two-component systems, or from byproducts formed from chemical reactions. Deaerators work by coalescing small bubbles into larger bubbles that can rise to the surface considerably faster than microbubbles.
DEFLOCCULANT
See Dispersant or Dispersing Agent
A material that prevents pigments in suspension from forming floccs.
Additive used to reduce or eliminate foam in a coating or coating constituent. The terms ‘defoamer’ and ‘antifoaming’ agent are often used interchangeably. In fact, they are not quite the same. A defoamer is a surface-active agent that stops the foam and breaks the bubble once it has been formed. It is a bubble breaker. An antifoaming agent prevents the formation of foam so it never forms.
The term “foam-control agent” is a more appropriate term to use. In an aqueous formulation, it is almost impossible (at acceptable use levels) to totally eliminate all foam. The correct foam control agent will help to prevent foam formation, but more importantly, it will allow the dried film to be free of foam and any resultant film defects that might result from an air void in a film.
There is a difference between macrofoam and microfoam. Macrofoam is located mostly on the coating surface and is surrounded by a duplex film with two liquid/air interfaces (double layer), whereas microfoam occurs inside of a coating film (air entrapment) and is characterized by a single liquid/air interface. These two types of foam also differentiate defoamers from deaerators. Defoamers are mostly effective against macrofoam, whereas deaerators suppress microfoam. In practice, the terms are frequently confused and used interchangeably. Many of the commercial products are optimized to prevent macro- as well as microfoam.
Both kinds of foam impair the surface optics of the coating and cause surface irregularities, as well as reduce gloss and transparency. Microfoam also adversely affects the coating’s protective properties because the effective film thickness is reduced and pinholes can form from the micro bubbles.
The function of defoamers is based on disturbance of the double layer of the macrofoam lamella. Substances with very low surface tension are used as they are not wetted by the foam bubble. Foam-stabilizing substances move away from the defoamer droplet, which finally causes collapse of the bubble. Surfactants are often used with defoamers to improve the spreading of the defoamer droplet on the bubble surface.
Foam may be introduced at various stages of manufacture and use of the coating. The raw materials used to make a coating, such as surfactants, dispersants, etc., enable foam to form. Entrapped air, or foam, is introduced into the manufacture of most paints as part of the process. Manufacturing care must be taken to avoid entrapping air during production by choosing the correct stirring equipment and stirring conditions. Letting the product stand for as long as possible is also helpful in preventing air entrapment.
High levels of foam may occur during the milling stage, and defoamers are often used as a component in a grinding paste. Due to the activity of some surfactants at the air/water interface, foam is often created and stabilized in both the pre-mixing and milling chambers of dispersing equipment. Foam slows down the process of dispersion and adversely affects the moisture resistance of the coating. Using silicone anti-foam agents may present an additional potential for surface defects.
Foaming occurs during application to some degree, depending on the method of application. For example, curtain coating carries entrapped air continuously around in the system. Entrapped air also occurs with airless spray systems. Airless spray does not use compressed air. Paint is pumped at increased fluid pressures through a small opening at the tip of the spray gun to achieve atomization. When the pressurized paint enters the low-pressure region in front of the gun, the sudden drop in pressure causes the paint to become an aerosol. Airless spraying has several distinct advantages over conventional air-spray methods. It is more efficient than the air spray because airless spray is less turbulent and, therefore, less paint is lost in bounce back. The droplets that are formed are usually larger than conventional spray guns and produce a heavier paint coat in a single pass. The system is also more portable, production rates are nearly double and transfer efficiency is usually greater. Other advantages include the ability to use high-viscosity coatings and to have good penetration in recessed areas of work pieces. One major disadvantage of airless spray is that pinhole formation from air entrapment is possible. Air-assisted airless spray is similar to airless application except that a small amount of atomizing air is used to further improve coating atomization. Spray application in relatively low humidity conditions or in high-temperature conditions can increase the tendency for foam entrapment.
Latex paints are stabilized with surfactants that easily generate foam under agitation. Elimination of this foam is essential for the manufacturing process, for storage and for good application properties. Foam reduction can also be somewhat controlled by optimizing the settings on a spray gun and by adjusting the viscosity/solids level in the formulation.
Foam is a dispersion of a relatively large volume of gas in a small volume of liquid. Gases are soluble in liquid media to different extents and are influenced by temperature. As the paint film starts to dry, the dissolved gases try to escape in the form of bubbles. A bubble, as a sphere, requires the least amount of surface energy. Large bubbles rise faster than small ones and collect on the surface. They are often covered by a surface film of surfactant or other additive in the coatings system. On the surface, the bubbles pack side by side as densely as possible. In some systems, densely packed microfoam can form on the surface and remain there even after the film has cured. This is possible for high-build systems like some plastisols.
In the process of bubble escape, tiny pores may be formed in the film. In lower solids films that dry quickly, the viscosity of the coating is increasing quickly with drying. As this is happening, the smaller micro bubbles are still rising to the surface but quite slowly; in the process these bubbles can form small channels. If the rising bubble penetrates the surface, lack of flow allows for the formation of pinholes – a surface defect. Sometimes these microbubbles cannot penetrate the surface, but they will push a very thin, viscous layer of coating to the front surface. This layer will remain on the surface after drying or curing and becomes a spherical blister.
Good defoamers not only need to be good bubble breakers, but they need to be able to keep the action sustained and maintain good defoaming over time in both oven and room temperature aging studies. Good defoamers need to be insoluble in the foaming system. If the product is too soluble it will only increase foaming. Defoamers need to have excellent dispersibility throughout the systems. Spreadability is the ability of the product to spread evenly and uniformly on the surface, coating the bubble particles and eliminating them. They work by lowering the surface tension around the bubble and cause them to coalesce to larger bubbles and eventually to break.
Most foam-control agents for aqueous systems consist of carrier, actives (hydrophobic materials) and other additives that enhance spreading, compatibility, product stability, etc.
Some examples of carriers include mineral oils, vegetable oils, glycols, glycol ethers, alcohols, silicone oils and water. Three types of actives are most common: hydrophobic silica, hydrophobic silicone and organic materials that are hydrophobic/lipophilic. Many foam-control agents are blends of the above actives. The other additives vary from surfactants, co-solvents, thickeners, etc.
The actual foam-control agent that functions the best is based on its spreading rate, compatibility, persistency and cost/performance. Many of the above factors oppose each other in a formula. For example, the most compatible is usually the least persistent; the least compatible often spreads the best; etc.
Many defoamers are colloidal suspensions of particles that act as seeds to allow bubbles to collect and burst. These types of additives can be simple alcohols, oils or complex silicone oils on fine particle silicas. Defoamers should spread instantaneously on the bubble surfaces and into the underlying layers, and cause immediate rupture of the bubble.
Silicone defoamers are usually based on a polysiloxane-type structure. For example, there are acrylate-functional polydimethylsiloxanes, polyether-modified polydimethylsiloxane, etc. To select the proper antifoam, the formulator needs to be aware of the nature of the foaming agent, the foaming tendency, the solubility and concentration, pH, temperature and viscosity of the system. Each of these factors has a direct influence on the antifoaming agent of choice. Some examples of other compounds that function as defoamers are waxes, fatty acids, aluminum stearate; amyl, capryl, decyl, nonyl, and octyl alcohols; caster, corn, mineral, pine, silicone, and turkey red oil; palmitic and stearic acid diethylene glycol monolaurate, sulfonic acid salts, tributyl citrate, and tributyl phosphate.
Some foam-control agents have an effect on gloss; but not all do, so the formulator must carefully evaluate this effect. Color acceptance of emulsion paints is also important, and some defoamers can have a negative influence on color development that needs to be evaluated.
Foam-control agents are very formula/system dependent and the best way of selecting them is to test them. This is usually done quickly in the lab by adding various defoamers to a known volume of paint sample, shaking for a set time, and then measuring the height of the foam that is generated. The method actually works quite well.
Foam-control agents should be used with some degree of caution to minimize cratering. It is important for wood sealers and topcoats that the defoamers used do not cause surface defects or create haziness in the final finish. Incorrect use of antifoaming agents can cause craters, fisheyes, floating, flooding, crawling, etc.
New, liquid, mineral-oil-free defoamers based on renewable raw materials have been introduced into the market. These new defoamers are ideally suited for use in the manufacture of synthetic latex, waterborne matte to satin finish architectural coatings, plasters, and aqueous adhesives. They display remarkable performance characteristics retaining defoaming capabilities even after prolonged paint storage. Some of these new materials may be used as an alternative to replace tributylphosphate (TBP).
Another approach has been to replace mineral oil with natural oils like soybean, rape seed, sunflower and rice bran oils.
There are no universal defoamers, although certain types function better with certain emulsions, certain plant processing requirements, etc. Most suppliers are willing to provide technical support to make the selection process easier.
DEGASSING AGENTS
Specific chemicals that allow for the release of volatiles in a molten powder film. Volatiles can be interstitial air, blocking agents, and low-molecular-weight polymeric fractions.
Benzoin is typically the choice for degassing powder coatings. In the absence of benzoin, the air bubbles start to shrink very slowly as a result of a diffusion-controlled process. Quite remarkably, in the presence of benzoin, the bubble shrinkage process is accelerated to such an extent that most air bubbles disappear before any significant increase in the viscosity occurs due to the curing of the coating. This suggests that benzoin functions by accelerating the rate of bubble shrinkage. Yellowing side effects of benzoin on powder coatings may be associated with the formation of benzyl, the common oxidation product of benzoin.
DENATURANT
An unpleasant or toxic substance that is deliberately added to make a product unfit for human consumption.
DETACKIFICATION AGENTS
Paints used in automotive finishing operations are a tacky material and tend to adhere to the surfaces of spray booths, particularly in the sump and drain areas. To maintain the design intent of the paint spray booth the paint overspray must be constantly removed from the sump to prevent clogging of the sump drain and recirculating system. In order to assist in the removal of the oversprayed paint from the air and to provide efficient operation of the down-draft, water-washed paint spray booths utilize paint detackifying chemical agents. The detackification products are commonly introduced into the water that is recirculated in the paint spray booth system.
Paint spray booths are typically 100 – 300 feet in length and usually contain many robotic and manual spray zones. The temperature and humidity are rigorously controlled in these systems. As vehicles are painted in these booths, a certain amount of paint does not contact the article being painted and forms a fine mist of paint in the air space surrounding the article. This paint must be removed from the air. To accomplish this, the contaminated air is pulled through the paint spray booth by exhaust fans. A curtain of circulating water is maintained across the path of the air in such a way that the air must pass through the water curtain to reach the exhaust fans. As the air passes through the water curtain, the paint mist is “scrubbed” from the air and carried to a sump basin (sludge pit) usually located below the paint spray booth. In this area, the paint particles are separated from the water so that the water may be recycled and the paint particles disposed of as paint sludge.
The paint detackifiers, or “denaturants” commonly added to these systems are either melamine-formaldehyde-based or based upon acrylic acid chemistry. The innovative nature of the chitosan detackifier (BC4200NP technology) lies in the fact that it is derived from chitin, the waste product of food production, namely shell fish harvesting. The solid chitin derived from these operations is treated with sodium hydroxide at an elevated temperature to produce chitosan, also called poly(glucosamine), that represents a deacetylated chitin. The chitosan produced in this way yields a glucosamine polysaccharide structurally similar to cellulose. The degree of deacetylation can be controlled by temperature and reaction time. The chitosan produced, also in a solid state, is not readily soluble in water, but can be rendered more water soluble by the addition of various acids such as acetic, sulfuric, hydrochloric, citric, sulfamic and mixtures thereof.
On a pound for pound basis, the BC4200NP technology is either cost neutral or less expensive than the current paint detackifiers in the market place. Further, field studies indicate that the BC4200NP technology may reduce the overall cost of such programs by lowering detackifier usage and by decreasing the use of ancillary chemicals, i.e., liquid caustic and biocides.
The melamine-formaldehyde detackifiers that BC4200NP is replacing are derived from non-renewable natural gas supplies and contain residual amounts of free formaldehyde as a necessary consequence of the resin production operation. The acrylic acid-based detackifiers are derived from non-renewable crude oil feed stocks and their price is therefore subject to the global oil market. Further, since the chitosan-based is less acidic than the traditional products, less sodium hydroxide is necessary in an operating system to control pH resulting in less overall chemical usage. Additional field studies have also indicated that less detackifier is necessary to treat a given amount of paint and this also makes the BC4200NP more attractive from an application cost perspective.
The environmental advantages of this technology are summarized as follows: no residual free formaldehyde: raw material is not derived from natural gas and/or crude oil and therefore does not utilize non-renewable resources; it is obtained from the waste products of food production (crab, lobster and shrimp shells); low acidity level; chitosan, the main component, has anti-microbial properties.
DESICCANTS
Desiccants are materials that have a high affinity for water absorption and are used as drying agents. The most commonly used desiccants are silica gel or calcium oxide.
DETERGENT
A detergent is a surfactant that is used to clean soiled surfaces. They may be anionic, nonionic or cationic.
DILUENT
See Reactive Diluent
A dispersant is an additive that increases the stability of a suspension of powders (pigments) in a liquid medium. The pigment-dispersing step is the most difficult and time/energy consuming part of the paint manufacturing process. This is because of the difference in surface tension between the liquids (polymers and solvents) and the solids (pigments and extenders).
Pigments and extenders are often received as agglomerates and then are subjected to a grinding process that incorporates the pigment into the vehicle during paint manufacture. During the grinding the agglomerates are dissociated into a dispersion of particles. In the process, dispersants (surfactants) are used that mainly prevent reassociation of the pigment and extender particles. The dispersants are adsorbed onto the particles and hinder close approach of particles by charge repulsion effects (ionic dispersants) or by steric effects (nonionic dispersants). Wetting and dispersing agents are used to stabilize pigment dispersions. They often use steric hindrance to avoid flocculation of the pigment particles.
Since a dispersant is used to help the suspension of fine particles of a solid in a liquid phase, it must be able to totally wet the solid particle and also be able to interact with the dispersing medium. Dispersants act by being absorbed on the surface of the pigment particles with the other end of the molecule exposed and usually charged. Since like charges repel, the individually coated pigment particles repel each other and stay uniformly dispersed. Because the individual pigments are all so chemically different – some are attracted to water and some are not – the dispersants have to be chemically different. When changing pigmentation in a formula, the dispersant needs to be checked quite carefully to see if it is stable in the formulation. Good dispersion provides the end user with better hiding and color stability.
Pigment dispersants are necessary to improve the wetting of the pigment and enhance its suspension. The choice of dispersant depends on the nature of the pigments and if the coating is a waterborne or solventborne system. Dispersants are available for organic and inorganic pigment systems and for both solvent- and waterborne systems. The proper choice of dispersant can be useful for obtaining higher gloss, hiding power and lower viscosity.
Pigment dispersion is one of the most important, critical and complex steps in paint manufacture. Usually pigments are dispersed in a mill base. A mill base is a concentrated mixture of pigments, a small amount of the binder and solvent and a wetting and dispersing additive.
In the wetting stage, the liquid in the mill displaces air from the pigment surfaces. Shear force, applied by a mill, is necessary to break down pigment agglomerates. As pigment particles become adequately dispersed, dispersants that are now on the surface of the pigment particles prevent reagglomeration of the pigment.
Because of their hydrophobic nature, organic pigments are difficult to disperse in aqueous media. Surface-active compounds that are amphoteric in nature are usually effective for both organic and inorganic pigments that are particularly hydrophobic.
In aqueous systems, dispersants must disperse the pigment and form an affiliation between the pigment and the dispersing medium that is water. There are several types of dispersants commonly used in latex paints, for example, phosphates (such as potassium tripolyphosphate) and several sodium phosphates (sodium hexametaphosphate, sodium polyphosphate, sodium phosphate (tribasic). The potassium salt is preferred because it is less soluble. Ionic types such as the sodium salts of water-soluble polymers are also very common.
There are ionic water-loving (hydrophilic) and oil-loving (lipophilic) types available on the market. The hydrophobic part of the molecule is attracted to the pigment particle, and surrounds the particle with an ionic layer. Pigment separation by mutual repulsion takes place. These cannot be relied on for in-can stability and need to be used then with nonionic surfactants.
Ionic and anionic surfactants are also essential for good dispersion, and contribute to the latex paint system by helping to optimize hiding power, flow, and leveling. The amount of dispersant used is very small and is directly related to the total surface area of the pigment used. Again, too much or too little of these agents can affect in-can stability and other properties of the paint.
There are new nonionic deflocculation dispersing and wetting additives for waterborne coating systems that have been designed specifically to provide superior performance in wetting, dispersing and deflocculation of pigment particles and to comply with environmental regulations. These are zero-VOC and alkyl phenol-free to allow reformulation of existing platforms to comply with new requirements.
Pigments are usually easier to disperse in solventborne resin systems; however, higher solids systems will sometimes show pigment flocculation problems. This can be the result of the lower-molecular-weight polymers or oligomers having less of a buffering effect on the pigments and thereby permitting closer pigment-to-pigment contact. Amphoteric dispersants have been developed that are effective for preventing flocculation of carbon black and other organic and inorganic pigments in higher-solids systems. These dispersants have also been able to keep viscosity from increasing in some coatings where the polymer’s solubility is causing rheology problems.
The most widely used wetting agents in the past were primarily alcohol and alkyl phenol ethoxylates used alone or blended with anionic surfactants. Alkyl phenol alkoxylates (APAOs) include octylphenol (OPEs) and nonylphenol (NPEs) ethoxylates. These were preferred as wetting agents because they were cost effective and provided a number of structural, compositional and performance attributes. APEOs have become less acceptable due to the effects of degradation products on aquatic life forms and their potential effect of fertility of organisms. Consequently many manufacturers and raw material suppliers are gradually phasing out APAO compounds despite the absence of any legislative guideline prohibiting their use. However, by exploiting narrow--range ethoxylation and newer alcohol feedstocks, the properties of non-APEO surfactants are able to match those of the APEOs.
Some typical dispersing agents for organic and inorganic pigments are salts of monosulfonic acid such as: calcium or zinc sulfonate. Acetylenic diols are also sometimes added. Another class of dispersing agents is known as the aromatic ethoxylates. This type of molecule contains a relatively large anchor group that is a polycyclic aromatic structure. No amine or acidic groups are present, so the compound is pH neutral. Aromatic ethoxylates allow high pigment loading and do not tend to foam. Water resistance and gloss retention are both good.
Another widely used class of dispersants is based on soya lecithin. These have been used for many years as pigment wetting agents, grinding aids and dispersion stabilizers in oleoresinous formulas. This class of agent is also available in water-soluble or water-dispersible forms. These are used in much the same applications in latex formulations. The water-compatible versions are also very useful in formulating universal colorants and as agents in both water and solvent formulas to improve universal colorant compatibility and color development.
Polymeric dispersants (hyperdispersants) are a new class of dispersants that have significantly higher molecular weight than conventional dispersants. These dispersants provide improved pigment stability and increase tinctorial properties through their ability to wet and stabilize pigments. The result is cost improvement through faster dispersion, which results in increased throughput, lower pigment loading requirements due to enhanced color strength, and good rheological properties as well as low foaming characteristics. Performance is enhanced due to resulting high gloss, good flow and leveling (surface properties), and improved transparency.
Polymeric dispersants contain polymeric chains for stearic stability in solution and they must be able to be strongly absorbed onto the particle surface. Many functional polymers and/or copolymers are capable of functioning in this fashion. Examples are polyurethane or polyacrylate with tertiary amine functionality, or polyester.
High-molecular-weight dispersants are linear or branched chain polyurethane or polyacrylates that have molecular weights between 5,000 and 30,000 g/mol. These dispersants have pendant anchoring groups, which adsorb onto the surface of the organic pigment particle by hydrogen bonding, dipole-dipole interactions or Van der Waals forces. The remainder of the polymeric dispersant is large enough to cause steric stabilization. The level of dispersant is important because performance depends on the optimum amount of saturation by the dispersant of the pigment surface. Polyurethanes function well for viscosity depression in the mill base, which can lead to higher pigment loading. The polyacrylates are more compatible in nonpolar and highly polar systems.
Controlled polymerization techniques such as GTP (group transfer polymerization) and controlled radical polymerization (CRP) techniques such as ATRP (atom transfer radical polymerization), NMP (nitroxyl-mediated polymerization), and RAFT (reversible addition-fragmentation chain transfer) provide opportunities to customize polymers in a way that is not possible with conventional random copolymerization methods. Each of these techniques has its own characteristics that define to a certain extent the properties of the resulting polymeric additives. These techniques are applied for the synthesis of custom-made structured polymers. Compared to free radical polymerization (FRP) they all allow much better control over polymer structure and molecular weight distribution and they all have their specific characteristics.
Due to the fact that the chain growth mechanisms are different in GTP and CRP, it is clear that the resulting polymers will have different properties and these differences also can show up in their performance as wetting and dispersing additives. Controlled polymerization techniques are not only a tool to synthesize block copolymers, they also make other polymer architectures accessible. Gradient copolymers are one of the polymer architectures that possess a continuous transition of the properties of the respective monomers along the whole polymer chain; these polymers have very unique properties for pigment wetting and stabilization.
In random copolymers the pigment anchoring areas and binder-compatible areas are randomly distributed and generally too small for a strong stabilization effect. In block copolymers (as well as in graft copolymers) the two different areas are large enough for good stabilization of the particles in a pigment dispersion. But the strict separation of the properties in the polymeric structure can cause other drawbacks. Such polymeric structures may have a strong tendency toward foam stabilization. Micelle formation, mainly due to the incompatibility of the anchoring groups with the liquid medium, is also observed and pigment wetting with the additive can become difficult.
The gradient copolymer performs better because there is a gradually increasing concentration of anchor groups along the polymer chain. This obviously helps to increase compatibility, reduces micelle formation but still allows excellent steric stabilization of the pigment particles. These structural differences have a strong impact on the performance of the polymeric additives as wetting and dispersing additives.
Polymeric wetting and dispersing additives are useful for all types of pigments, whereas low-molecular-weight structures with only one or few anchor groups are mainly used for inorganic pigments.
Silicone and silane pigment-treatment additives can cause pigments to disperse evenly throughout coatings formulations. The materials can also help reduce pigment floating from application through cure, producing consistent color. They are effective in low concentrations, minimizing recoatability concerns.
In an ink formulation the pigment must be dispersed and stabilized to achieve good color strength, gloss, transparency and transfer efficiency. Stable, concentrated dispersions with small particles and narrow particle size distribution potentially can lead to higher gloss and color strength per unit mass of pigment. The push towards waterborne formulations and increasing pigment loading underlines the need for good dispersion and stabilization properties.
The use of polyether amines (PEAs) as dispersants for carbon black has been investigated. PEAs contain primary amino groups attached to the terminus of a polyether backbone. The polyether backbone is based either on propylene oxide (PO), ethylene oxide (EO), or mixed EO/PO. Adsorption, contact angle, and color density measurements indicate that a polyether amine having a high ethylene oxide content and high molecular weight provides the best performance. Surface tension and interfacial tension values decrease with decreasing ethylene oxide. Molecular weight has little impact on surface tension and interfacial tension. Low HLB dispersants (below 5) exhibit poor dispersion stability.
Pigment settling is not always prevented by using wetting and dispersing additives, but there is a noted distinction between ‘hard’ and ‘soft’ settling. Soft settling may result from weak flocculation of properly wetted pigments and stirring can easily redisperse the pigment. Hard settling occurs due to improper adsorption of dispersant. The result is reagglomeration of the pigment that can only be corrected with high-shear mixing.
Driers are not the same as curing agents, which chemically react with functional groups in the polymer. Driers are catalytic in nature and do not chemically react with the polymeric material. Driers promote or accelerate the drying, curing or hardening of oxidizable coatings vehicles. The most common driers that have been used in paints are metallic salts of monocarboxylic acids, usually C8-C10 branched acids, such as naphthenic acid, and neodecanoic acid as an example. The choice of acid does not seem to have an effect on the drying. The choice of acid primarily affects solubility, stability and efficacy. The acid makes the metal soluble in the resin system. Insoluble salts, such as acetates or chlorides, do not function as driers. Drier levels are always expressed as percent metal on resin solids. Use levels are generally in the ranges of 0.01-0.6%. The metal in all cases is the cation and has the capability of more than one oxidation state.
The drying of oil-based paints occurs through a process that is characterized by oxygen absorption, followed by the formation of peroxide, and subsequent peroxide decomposition. The presence of driers in the paint accelerates the oxygen absorption and the resultant drying of the film.
Traditional alkyds absorb oxygen into their double bonds and break those bonds to form free radicals that can undergo polymerization to give a dry film. Driers accelerate this process, improving the rate of absorption and utilization of the oxygen. Alkyds would dry to a soft film without driers over several days. Driers provide tack-free times of a few hours and hard dry overnight. A metal carboxylate drier catalyzes or promotes the crosslinking of resin polymers or drying oils.
Driers are classified as:
1. Oxidation (catalytic, top driers or surface driers). Examples are: cobalt, manganese, vanadium, cerium and iron.
2. Polymerization (crosslinking drier). Examples are: zirconium, lanthanum, neodymium, aluminum, bismuth, strontium, and barium.
3. Auxiliary (promoters) driers or catalysts. These are typically: calcium, potassium, lithium, and zinc. The first three increase the rate of top dry and zinc usually inhibits the top dry.
The oxidative driers promote the absorption of oxygen by the film as well as catalyzing the formation and decomposition of peroxides. These driers promote the surface dry of a coating. The top driers catalyze the decomposition of the peroxides formed by the reaction of the oxygen in the air with the resin or drying oil. This leads to the formation of direct polymer-to-polymer crosslinks (top drying) and also the formation of hydroxyl groups and carbonyl groups on the resin polymer and the drying oil. The hydroxyl groups are then available for through drying or crosslinking by the through driers, which form oxygen-metal-oxygen bridges or crosslinks between polymers.
The proper balance of driers in an oxidation-curing system is essential to stability, rate of cure and development of film properties.
Cobalt is the most active drier and a strong oxidizer. It top dries the film very rapidly. Care must be taken because excess cobalt causes wrinkling and color changes in light-colored paints. Excess cobalt has also caused gelation in some varnishes.
Manganese is also an active drier and is a potent oxidizer as well. It promotes polymerization to a greater degree than cobalt. It is often used alone in certain baking finishes. In air-dry systems it is used along with auxiliary driers. Manganese has a dark color, which can limit its use.
Iron seems to promote rapid drying by polymerization and hence is used widely in baking finishes where the dark color is permissible. For air-dry finishes, it is useful in eliminating film tack of some paints.
Rare Earth driers perform better than zirconium under marginal conditions such as high humidity or low temperatures.
Lithium improves the efficiency of other driers and is the preferred esterification catalyst in alkyd manufacture.
Zinc is an auxiliary drier and, in conjunction with cobalt, produces a harder film. It retards the surface drying in order to prevent wrinkling and allows freer access to oxygen, thus permitting hardening through the entire film. Zinc naphthenate is a good wetting agent and often improves gloss.
Calcium is used as an auxiliary drier, usually in conjunction with zirconium. It frequently performs better than any other auxiliary drier in baking finishes. Calcium driers are often added to the grind portion of the paint as auxiliary dispersants.
Zirconiums are used mostly with calcium as a replacement for lead. These show improved gloss, color, and gloss and color retention compared to lead, but do not perform as well as lead under adverse conditions such as low temperature and high humidity.
Aluminum offers outstanding polymerization and yellowing resistance without viscosity instability. Neodymium is a replacement for calcium and zirconium in VOC-compliant resins. It is cost effective and has proven performance. Neodymium and lanthanum are recommended for low-temperature/high-humidity applications. Vanadium is excellent for heavy film build (4+ mils) but has shown some discoloration in white coatings.
Waterborne coatings present a different sort of problem when using driers. The presence of surface-active agents, in addition to ammonia and amines and various resins, make proper selection important – particularly so that seeding does not occur. For waterborne systems some potential problems are as follows: incompatibility between the resin and the drier; potential for seeding; potential for resin discoloration. There are many good water-reducible driers including the rare earths: lanthanum, cerium and neodymium. Vanadium octoate is also available in a water-emulsifiable form.
Driers in high-solids systems also present the following possible problems: resin viscosity build, resin yellowing and slow dry times.
The best sources for starting-point drier combinations are the resin manufacturers who have already found combinations that develop desirable properties.
The change from natural acids like naphthenic acids to synthetic acids like octoates or neodecanoates provided more uniform products to the industry.
DRIER STABILIZERS
Compounds that prevent drier absorption or dissipation resulting in the loss of catalytic power of driers.
DYES (FOR USE IN STAINS)
Dyes are soluble colorants, which do not scatter light, but which absorb certain wavelengths and transmit others. Dyes are generally soluble in a solvent, or they may exist in such a finely dispersed state that they do not scatter light and behave as if they were in solution. Pigments are basically insoluble in the medium they are in.
Dyes are frequently used in the printing and coatings industry where a high level of transparency is required. Dyes have also found their way into automotive finishes because of advanced polymer technology – again because of their transparency and color properties. In general, dyes fall into one of three categories: metal-complex dyes, basic dyes and fat-soluble dyes.
For wood applications, dyes may offer deeper penetration of the wood surfaces and less grain hiding. However, they also fade more quickly than pigmented stains and require more effort to prepare the wood. Water-based dyes tend to raise the grain on many woods because the water penetrates the wood and raises the tiny fibers. Wood should be wetted first, then sanded down, before applying water-based dyes. Nongrain-raising (NGR) dyes need to be used in a nongrain-raising solvent. They dry faster than water-based counterparts, so application must be faster to avoid lap marks.
Liquid, high-concentration anionic dyes with good light fastness properties can be diluted in water or water/alcohol/glycol mixtures and optimized for interior use in water-based and water-containing wood stains.
Neozapon® dyes are metal-complex dyes, available in powder and, in some cases, high-concentration liquid form (Neozapon® L). They are highly soluble in polar solvents, but almost completely insoluble in water. Due to their limited weather fastness, these dyes are recommended for interior use.
ELECTROCONDUCTIVE ADDITIVES
Compounds that alter the conductivity or resistivity of a system. Silicon carbide is used in the manufacture of coatings to formulate conductive coatings. It is used in paints that conduct electrical charges away from large motors. Sizes are F400, F800. Silicon carbide is also used for wear-resistant coatings, such as for tanks, tubes, etc. It is usually mixed with an epoxy or resin matrix. It is also put on cutting tools with a proprietary method of application.
EMULSIFIER
See Surfactants
Emulsifiers fall into the category of surface-active materials, or surfactants. In general they are used to produce stable mixtures of two partially immiscible liquids. Emulsifiers allow us to make water-in-oil (W/O) emulsions and oil-in-water (O/W) emulsions. They promote the ease of mixing or dispersibility by lowering the surface tension of the liquid, much as a wetting agent lowers the surface tension of the liquid for application to a solid substrate.
ENZYME-BASED ADDITIVES
Enzyme-based additives can be mixed with coatings to create novel biologically and chemically active coatings, including those that self-decontaminate and detoxify organophosphorus compounds such as nerve gases and pesticides. A new understanding of enzyme biochemical capabilities is leading to the development of innovative, biocatalytic “smart coatings”.
Many organophosphorus (OP) compounds are potent cholinesterase inhibitors, accounting for their widespread use as insecticides and chemical warfare agents. Common organophosphorus agents include the chemical warfare agents tabun (GA), soman (GD), sarin (GB), cyclosarin, VX and its isomeric analog Russian VX (R-VX). Historically, most approaches to chemical agent decontamination are post-exposure, focusing on the treatment of surfaces after exposure has occurred and been subsequently detected.
Novel enzyme additives are being commercialized that not only remain stable in paint but, remarkably, remain active for extended periods of time. One self-decontaminating coating additive, OPDTOXTM, is biocatalytic in nature, and represents a paradigm shift for chemical agent decontamination. OPDTOX can serve either as a stand-alone decontamination method or as a complementary approach to existing decontamination techniques and products. When incorporated into a coating, the additive creates a reactive surface that will initiate the process of decontamination immediately upon exposure to organophosphorus pesticides and neurotoxins.
Applied in advance of exposure, painted surfaces containing OPDTOX can continue to degrade organophosphorus compounds after repeated exposures and remain active following washing. The scientific basis of the product lies in the ability of bacterially derived enzymes to efficiently degrade organophosphorus compounds. The results are novel coatings that self-decontaminate following exposure to organophosphorus compounds, which includes many important environmental and security targets such as the nerve agents VX, GD, GB, thickened nerve agents, and pesticides such as malathion [o,o-dimethyl-S-(1,2-dicarbethoxyethyl)dithiophosphate], parathion [o,o-diethyl-o-(4-nitrophenyl) phosphorothioate] and coumaphos [o-(3-chloro-4-methyl-2-oxo-2H-1-benzopyran-7-yl)o,o-diethyl phosphorothioate].
The enzyme of choice for OP decontamination is organophosphorus hydrolase. Of the six major groups of enzymes approximately 80% of industrial enzymes are hydrolases. Hydrolases are enzymes that catalyze the hydrolytic cleavage of C-O, C-N, and C-C bonds. A class of hydrolase known as organophosphorus hydrolase (OPH) can also cleave the P-O, P-F, and P-S bonds of the organophosphorus compounds.
Of the currently stockpiled nerve agents, VX is the most toxic, as well as the most persistent in the environment after release. In addition to the initial inhalation route of exposure common to such agents, persistent agents such as VX and thickened soman pose threats through dermal absorption. Enzymatic hydrolysis of OPs occurs when the compound is cleaved at the phosphoryl center’s chemical bond resulting in predictable byproducts that are acidic in nature but benign from a neurotoxicity perspective. By comparison, chemical hydrolysis can be much less specific and in the case of VX may produce byproducts that are extremely toxic. Although a number of enzymes have been identified that are capable of detoxifying OP compounds, OPH has the broadest substrate specificity. The substrate range of OPH includes numerous insecticides (paraoxon, parathion, coumaphos) and the neurotoxic chemical warfare agents and their analogs. Catalytic specificities for this family of compounds have been shown to range from rates that are diffusion limited (i.e., paraoxon, P-O bond, kcat=104 sec-1) to rates that are six to eight orders of magnitude lower (i.e., acephate, P-N bond). The enzyme is composed of two identical monomers associated to form a remarkably stable dimeric enzyme with a thermal Tm of approximately 75
Gloss is one of the key properties of a coating. The observed gloss of a film is dependent upon the angle and intensity of incident and reflected light from a surface. It is a function of the smoothness of the coating’s surface. A very smooth surface will reflect most of the light rays, so that the angle of incidence is the same as the angle of reflectance. As a surface becomes less smooth, incident light rays are reflected at various angles, and the surface is perceived as being less shiny and of lower gloss. In order for light to be reflected in many directions instead of at the same angle, it is necessary to introduce some degree of micro-roughness to the surface of the film. The reduction in gloss to give satin, semi-gloss, flat or matte finishes can be controlled by adding pigments and extenders – or materials called flatting agents.
Any tiny irregularities in a coating film will cause light to scatter. Flatting agents reduce gloss by imparting micro-roughness to the paint surface as it dries and cures. The rough surface diffuses light rather than reflecting it. The result to the eye is a matte surface. These agents include particles with dimensions that range from 1 to 50 microns in diameter. Thermoplastic flatting agents are being increasingly used for this application, and interestingly, the smaller particles reduce gloss to a greater extent than do the larger particles.
Gloss is also affected by conditions under which a film forms, such as the coating application method, drying temperature, humidity, rate of solvent evaporation and solvent composition. These factors can cause an increase or decrease in gloss, depending on the particular coating formulation. Factors such as high temperature or slow solvent evaporation rate, which allow the polymeric resin to relax and find the most thermodynamically favorable position, tend to give the smoothest surfaces and highest gloss. The method of applying a coating (spray, brush and roller) has an influence on the gloss as well.
The components in a formulation can also affect gloss. As film formation proceeds, volatile or nonvolatile leveling aids are sometimes required to assist relaxation of the polymeric film former to minimize the surface area of the coating and give a smoother surface. Other materials such as slip and anti-blocking aids can be used; these are meant to migrate to the surface of the film to perform their intended function. These materials change the characteristics of the surface and hence the gloss.
Flatting agents are available in different chemical compositions and particle sizes. Common inorganic materials include silicas for clear coatings, and extender pigments, such as clays, talcs and carbonates, for pigmented systems. Hydrophilic materials, which include some silicas, must be stored under conditions that prevent moisture pick-up by hydration. The inadvertent addition of moisture to solventborne systems can be detrimental, while the additional water of hydration in waterborne systems might have to be compensated for, to retain the desired solids level.
Silicas
Silicas have typically been used as the traditional flatting agents in the industry. Silicas with large particle size generally improve flatting efficiency; surface smoothness and clarity of the film will decline. Small silicas improve surface smoothness but are therefore less efficient at flatting.
The performance of silica flatting agents is also influenced by surface treatments such as organic wax treatment and inorganic surface treatment. Surface treatment is used to prevent settling and also to improve mechanical properties and often adhesion.
Silicas can be used in solvent and aqueous systems, and in virtually all types of resin systems: acrylics, vinyls, polyesters, nitrocellulosics, urethanes and alkyds. The reduction of gloss can have a significant corresponding effect on increasing viscosity that makes the job of formulation sometimes difficult. Silicas are available as: precipitated silica, fumed silica, diatomaceous silica and silica gels.
The synthetic silica flatting agents are used to produce low-gloss finishes in organic solventborne systems, waterborne and high-solids coatings. Typically, waterborne finishes cannot withstand high shear that can destroy the emulsion and cause foaming. So the flatting agent must be easy to wet out and incorporate into the coating under low-shear conditions.
Precipitated and Fumed
Precipitated and fumed silicas are not resistant to overgrinding. In some cases, the overgrinding may occur under relatively low shear such as obtained in mixing while tinting a batch of paint. Silica gels are resistant. Fumed silica has a pronounced effect on thickening; silica gels do not. In fact, their impact on viscosity is minimal.
Diatomaceous Earth
Diatomaceous earth (silica, SiO2) has been used extensively in the coatings arena as a very efficient flatting agent, achieving appearance characteristics from a dead flat to a silky low luster in a range of coatings. It imparts increased toughness and durability, added ‘tooth’ for adhesion, and improved sanding properties. Because these microscopic particles are irregularly shaped, they diffuse light and are used to impart degrees of flatness to coating films.
Diatomite consists of the siliceous skeletal remains of single-cell aquatic plants known as diatoms. The diatomite structure is porous with microscopic voids that serve to control vapor permeability and for the reduction of blistering and peeling and faster dry time. The use of this type of material results in uniform gloss and sheen reduction.
Silica Gels
Modern silica gel technology enables the production of highly pure, porous products. A silica gel is an amorphous form of silica composed of nearly 100% silicon dioxide (SiO2) produced synthetically in a liquid process. Silica gels belong to a class of synthetic silica materials known as hydrated silicas, which have an average water content of 6% to 8% by weight. Silica gels are produced from the acid treatment of an aqueous sodium silicate solution.
Mixing sulphuric acid and sodium silicate under controlled conditions produces the base gel from which all types of silica gels are made. This glass-like, solid gel material is broken down into granules, then washed and dried to produce the highly porous material needed for matting agents. Physical parameters such as porosity, pore size and surface area can be manipulated to produce a range of different silica gel types. The physical properties of silica gel differ from other specialty silicas. The internal structure of silica gel is composed of a large network of interconnected microscopic pores that attract and hold water, hydrocarbons and other chemicals by the mechanism of physical adsorption and capillary condensation. This huge pore volume and extensive surface area gives the silica gel many of its unique properties.
The rigidity of the silica gel particle is higher than that for precipitated silica and thus better resists the shear forces arising during the manufacture and application of the paint. Silica gels can influence rheological properties, although other agents are typically preferred for rheological control purposes.
The efficiency of the matting agent depends on the type of silica, particle size distribution and porosity of the particle. A high porosity is a key feature of a modern matting agent as it enables the formulator to limit the addition rate of a matting agent in a formulation. Unfavorable viscosity effects as well as limitations in coating surface properties can be avoided. A new milestone in micronized silica gel technology was reached by achieving a pore volume of more than 2 ml/g for a micronized silica. Today, matting agents for industrial applications are available in a variety of particle sizes.
The application segments for silica gel matting agents include coil coatings, industrial wood coatings, general industrial and automotive coatings as well as printing inks, leather/textile coatings and decorative coatings. Coil, wood and furniture coatings are the applications with the largest volumes, but radiation-cured coatings and other coating systems as well as printing inks also employ silica gels. The solventborne paint sector has been more important for silica gel than waterborne coatings. However, the pressure to reduce VOC emissions and formulate solvent-free paints has led to the development of grades of silica gel, which are suitable for formulation in powder, ultraviolet, and waterborne coatings.
Surface-treated silica gel flatting agents for high-performance OEM and DIY wood coatings and other clear finishes are available. They contain an organic additive that aids in dispersion and suspension, and imparts mar- and burnishing-resistance to the finished surface.
Silica gels are more resistant to overgrind compared to precipitated silica or fumed silica. This resistance provides gloss stability for demanding applications such as coil coatings. Synthetic silica gels contain internal voids – thereby giving pore volume or void volume. As pore volume increases, the specific volume of the particles increases while maintaining a constant weight. The result is more particles per unit weight, which yields higher flatting efficiency. The increase in pore volume must be balanced against application viscosity.
Selection and Applications
It is not easy to select the best flatting agent. It depends on the end use of the coating, as well as dry film properties required: clarity, smoothness, mar resistance and weatherability. Also the choice of flatting agent can make a significant difference in application characteristics such as viscosity, re-dispersion and gloss stability. Optimum flatting efficiency results when a flatting agent is selected that has optimum pore volume – that is, more particles per unit weight.
Particle size is also important, as it will affect flatting and surface clarity and smoothness. In general, the larger the average size of the flatting agent, the rougher the surface and the lower the ratio of sheen:gloss. For certain markets this becomes very important and formulators need to determine which agents to use depending on this consideration.
Average particle size can affect the flatting efficiency and loading level at constant film thickness. Silicas are available in a variety of particle size ranges. In general, the larger sizes generally give higher flatting efficiency and are also less sensitive to variations in film thickness. Larger sizes also give rougher surfaces.
In some cases, the flatting agents yield a smoother surface than the full gloss coating does, which indicates that thin film coatings can retain the features of the substrates. This can be important in paper coatings and wood sanding sealers, where it is desirable to obtain a smoother surface by filling voids in the substrate.
Silicas are used in architectural coatings and varnishes, coil coating and general industrial finishes, industrial wood finishes, topcoats and primers for corrosion resistance, and textured and suede effect finishes. Some coatings segments need very high-quality synthetic silicas – coil coatings in particular. Coil coatings are also very dependent on gloss stability.
Clear coatings for fine furniture and other wood products are flatted to obtain a satin finish. The satin finish is a desirable aesthetic feature, obtained by lowering the 60
Surface tension holds a liquid together and causes it to take the smallest possible volume. A drop of liquid on a solid surface will cover a larger or smaller area depending on both the surface tension of the liquid and the surface tension of the substrate. For example, a drop of water, which has high surface tension, will bead up as small as possible on a clean and waxed automobile. The same drop of water will tend to completely spread out and ‘wet’ another portion of the automobile that is not so clean or waxed. A liquid will ‘wet’ a substrate when the substrate has an equal or higher surface tension than the liquid itself. Sufficient wet film thickness is also important for a smooth surface.
The wetting of the substrate and leveling of the liquid film depend on the surface tension of the coating. Both processes have opposing requirements in terms of surface tension. If the surface tension is too high, poor wetting occurs, along with the formation of possible defects such as craters. If the surface tension is too low, poor leveling occurs and can cause orange peel.
Because of the recent advances in technology within the industry in areas of powder and waterborne systems, good flow is more important than ever. Some of the resins that are being introduced exhibit poor wetting and flow characteristics. This is such an important area because good flow is necessary to eliminate possible surface defects such as craters, fish eyes, orange peel and pinholes.
Many surface defects develop during the application of the coating. Application methods can influence leveling. For instance, some brushes, rollers and direct roller coaters can produce uneven surface films that require leveling for a smooth finish. Other methods that produce a smooth surface – spraying, curtain coating, reverse roller coating – need to maintain that smoothness during the baking and curing process.
Leveling agents function by influencing the viscosity throughout the film. Solvents with good solvating power and a gradual evaporation rate influence the “open” time necessary for leveling. Leveling agents can also be pigment dispersants that function by preventing the flocculation of pigments.
Flow modifiers can differ greatly in their chemical structure and in their ability to affect surface tension and promote leveling within any given coating system. As such, they need to be evaluated in a formula at several different usage levels to determine the optimum level needed in any given formulation. Even more importantly, a given flow agent’s effectiveness will vary from system to system. The natures of the resin, other additives and application technique will all affect the flow agent. For this reason, great care must be taken when selection is made and laboratory work must be conducted to check on effectiveness and surface defects.
Some flow modifiers are designed as ‘general purpose’, while some are very specific. Some are more effective at controlling craters while others are better at imparting good leveling. This is why proper selection is so important.
Flow agents are available for powder systems, solvent and waterborne applications. The most commonly used flow agents are: silicones, surfactants, fluorinated alkyl esters, solvents and polyacrylates.
Silicones form an important group of flow control agents. These consist of:
1. Polydimethylsiloxanes (silicone oils). - Increasing chain length gives materials with higher viscosities. Higher molecular weight means reduced solubility in coating systems and less compatibility. Lower-molecular-weight siloxanes (dimethyl units <60 for example) are used for surface flow control and to mask floating of pigments. Higher-molecular-weight polydimethylsiloxanes are effective as defoamers. Those products with molecular weight greater than 1400 are incompatible and are responsible for causing craters. They can be used to produce hammertone finishes.
2. Methylphenylsiloxanes.
3. Organically modified polysiloxanes. - Organo-modified siloxanes have distinctly different properties than their original polydimethylsiloxane counterparts, with respect to silicone crater formation, and other silicone-caused surface defects. The compounds are derived from low-molecular-weight polydimethylsiloxanes, and rather than having only methyl functionality, various organic chains replace certain methyl groups to increase the compound’s compatibility with coatings and inks. The organic portion of the molecule can be polyether, polyester or a long alkyl chain.
The most important modification is the polyether chemistry derived from ethylene oxide and/or propylene oxide. Hydrophilic character, i.e., water compatibility, increases as a function of the ethylene oxide content so that it is even possible to synthesize water-soluble silicone-based additives. Factors influencing the properties of modified siloxanes are silicone content, and the type and location of organic groups on the molecule.
Polyether-modified siloxanes simultaneously influence the following effects in coatings. Evaporation of solvent from an applied film causes differences in temperature, surface tension, solvent concentration and density within the film. Solvent-rich material rises in the center of these cells, while material having a lower solvent concentration moves downward from the edges of the cells. As a result, the surface tension in the center of the cell is lower than at the edges. Material flow occurs from the lower surface tension to the higher surface tension areas, forming valleys in the center of the cells and ridges at the edges. Bénard cells can be particularly problematic in systems containing mixed pigments. In clear coatings containing a matting agent, the larger pigment particles are forced out of the zones of higher flow-rate to the center of the cell. Bénard cell effects can be suppressed with modified siloxane-based flow additives. The addition of modified siloxane compounds to a formulation suppresses the surface tension of an uncured film to a uniformly low level, which is altered during solvent evaporation. Therefore, no differences in surface tension occur on the coating surface. Uniform drying and uniform flow are achieved, due to the elimination of surface tension variations. Another benefit, the requirement of good flow, is maintained because low surface tension facilitates complete substrate wetting.
Silicone and acrylate additives are typically used in combination in standard coating formulations. Acrylates improve the flow and leveling while the silicone contributes enhanced substrate wetting and prevents cratering. A silicone macromer-modified polyacrylate is available that incorporates both acrylate and silicone characteristics. In high-polarity coatings, the additive brings about a massive reduction in surface tension, therefore providing good substrate wetting with no significant reduction of the dry surface energy. The silicone part provides good anti-crater properties without increasing surface slip. The acrylate backbone provides excellent leveling.
Silicones are multipurpose additives because, in addition to control of flow, they also assist as slip agents, prevent floating, and in general, are very effective at low levels. One of the most valuable characteristics of silicone flow additives is the prevention of cratering together with the reduction of sagging.
Silicones concentrate at the paint surface and, because they are surface agents, in addition to reducing the surface tension they influence the slip resistance of the coating surface. [For more information, see Slip Agents]
The chemical structure of silicone additives (molecular weight and functional groups on side chains) makes them more or less compatible with the resin solution. Very incompatible silicones will cause surface defects (craters). Compatible silicones will reduce surface tension and slip. Those whose structures cause them to be somewhere between compatible and incompatible will function as defoamers.
Fluorocarbons are the most highly surface-active group of coating additives. Fluorosurfactants aid in wetting and flow because they decrease surface and interfacial tension. Some low-energy substrates, such as polyethylene or metal surfaces contaminated with oil, are very difficult to wet. Fluorochemical agents function quite effectively and do not usually affect other properties such as water sensitivity. Fluorosurfactants are leveling aids because they minimize surface tension gradients between the resin and solvent that resists leveling. They are generally used at very low levels (0.05-0.10%) and excesses can cause severe foaming.
High-solids systems, powder-coating systems and lower-VOC coatings are particularly in need of flow modifiers because the traditional wetting role of the solvents has been greatly reduced or, in the case of powder coatings, totally eliminated. The reduced solvent content of higher-solids coatings causes an increase in surface tension that results in poor substrate wetting, crawling, cratering and sometimes surface defects on the coating. Waterborne coatings in particular are prone to flow defects, particularly when applied over oily substrates or contaminated surfaces. The solventborne systems were quite forgiving of minor amounts of oil or other contaminations. Waterborne coatings are not at all forgiving over substrates such as metal, plastic, glass and concrete. Raw-material suppliers are continuously introducing new products to overcome these problems.
Powder Coatings
For powder coatings, the quality of the final film depends on the film-forming process. This process includes the coalescence of individual powder particles, the wetting of the substrate by the melted but not cured powder coating and the flow of the irregular film into a uniform film. Both viscosity (the resistance to wetting and leveling) and surface tension (the driving force) play a role in the process of film formation.
If the surface tension is too high, poor wetting occurs and craters may form. If the surface tension is too low the leveling is also affected adversely and orange peel may result. Polyacrylates and polysiloxanes are two types of leveling additives in use.
Flow modifiers for powder coatings are designed to address the surface imperfections, such as orange peel and craters.
Acrylic powder coatings can be contaminants for other nonacrylic powder coatings. The result is usually some type of cratering formation. Interestingly, acrylic flow agents are also used in powder coatings as leveling aids. These materials are melt-mixed into the powder composition. It is their controlled incompatibility that allows them to equalize the surface tension of the air-to-solid interface. If the acrylic flow agent was dry-blended into the powder coating, craters would surely be a result. The melt-mixing allows this additive to do its job.
The majority of flow modifiers for powder coatings are alkyl acrylates. These modifiers work by enhancing flow and leveling during the bake cycle. There are surface irregularities that must be overcome during the application and cure of powder coatings. In addition, the particle size and distribution can affect flow. Just as in liquid coatings, incomplete leveling of the surface will result in severe orange peeling. Orange peel for powder coatings is frequently caused by either uneven application, or a nonuniform particle size distribution of the powder particles. Sometimes nonuniform pigment distribution can also cause orange peel. Polyesters, in particular, seem to be susceptible to this defect.
Cratering and trapped air or other volatiles that result in pinholing are also critical. For example, if a powder resin cures too rapidly, it can trap gas and pinholes will result.
Some of the acrylic flow modifiers behave as a plasticizer and lower melt viscosity, and increase the leveling time to produce a smooth surface. Acrylic flow modifiers have been used effectively to provide high-quality surface coatings prepared from a variety of vehicles: epoxy, melamine, urethane, silicone rubber, acrylic, alkyd, phenolic, EVA copolymers, polyester and cellulosic. Certain melamine-formaldehyde resin crosslinkers, such as methylated/butylated coetherified melamine-formaldehyde resins, also provide good flow and leveling and help to prevent pinholes, craters and picture framing in coatings. Levels of use of flow modifiers are different than in liquid coatings, and systems need to be well evaluated by the formulator.
For powder coating, polyacrylates (both homopolymers and copolymers) are the most widely used flow-control agents. These are moderate- to high-viscosity liquids and are usually supplied as a powdered master batch dispersed on silica particles.
Additives based on polyacrylate technology, encapsulated by a solid (silica free) shell, are available as dry powders for powder coatings. These additives promote leveling while maintaining maximum gloss levels.
Silicones are also used and are typically more effective in reducing surface tension. Polyether and polyester-modified polysiloxanes are preferred to poly(dimethyl) siloxane because these are very active and are prone to cause defects such as pinholing, craters, haze, etc. The modified polysiloxanes are provided in a powdered form on silica particles.
The fluorocarbons are very effective in powder coatings but are not often used because they are expensive. They are, however, very effective, particularly on contaminated or oily metal substrates.
You can purchase PCI's 2011 Additives Handbook on CD for $29.95 plus shipping. The CD will also include a directory of additive suppliers and distributors. Contact Andrea Kropp at kroppa@pcimag.com for details.
Additives belong to a broad and diffuse category of key components in a coating formulation. They comprise a small percentage in that formulation, usually less than 5%, but their impact is significant. Additive function is almost always very specific in nature. Some additives are multi-purpose; for example, they may be important to the manufacturing process as well as to the coating’s performance. In recent years, multi-purpose additives have been developed, thus allowing the use of fewer additives in many formulations. Occasionally the use of one additive will require the use of another to counter some undesirable effect of the first.
Some additives are proprietary products with highly specific functions that work well in some systems but cannot be used in others. In addition, because of the proprietary nature of many additives, their chemical composition is not disclosed. This can make general recommendations difficult. In addition, this lack of structural knowledge means that additive substitutions cannot be made on the basis of fundamental structural chemistry.
The focus on green technology, lower cost and safer products has led to the introduction of newer additives and chemistries. The industry demands that green additives perform the same or better than their traditional counterparts and that they combine performance, sustainability and efficiency along with lower cost. With a large number of additives available for a particular problem, formulators can find themselves in trouble if the wrong additive is initially selected or added to alleviate or correct a problem. Correct additive selection is important to success, and such selection is made through vendor assistance or years of experience.
Please note that there are a number of new nano-sized additives on the market today that are difficult to categorize. Their functions are varied and tend to overlap our traditional categories. For this reason we have included a number of these types under the Nanotechnology section.
The following is a brief description of various coating additives along with some generic examples. The majority of additive types are represented.
ABRASION-RESISTANCE IMPROVERS
See Slip Aid, NanotechnologyAbrasion is a phenomenon caused by the mechanical action of rubbing, scraping or erosion. It has two forms, marring or wearing. Mar abrasion is the permanent deformation of a surface, but the deformation does not break the surface. Wear abrasion is removal of a portion of the surface by some kind of mechanical action: wind erosion, sliding back and forth of an object, wear of tires on traffic paint, and so on. The surface removal is gradual and progressive in nature. Abrasion resistance is a combination of basic factors such as elasticity, hardness, strength (both cohesive, tensile and shear strength), toughness, and, especially in the case of wear resistance, thickness. In addition, abrasion resistance is intimately related to scratching and slip. Thus, compounds that enhance these properties will improve abrasion resistance.
The nature of the polymeric resin and the pigments affect abrasion resistance. In the case of the pigments, it should be noted that extender pigments are noted for their ability to contribute to a variety of mechanical properties. Examples of compounds that have been used to enhance abrasion resistance include: silica glass spheres, specialty glass spheres such as UVT™ Sunspheres, and similar compounds that improve hardness. Certain silicones and other oils will decrease surface friction, making it easier for objects to slide over the surface and thus reduce wear abrasion. Increasing crosslink density by use of higher functionality oligomers and/or larger amounts of crosslinking agents has been used to improve abrasion resistance.
Waxes have also been used to improve slip and thereby abrasion. Hard waxes resist abrasion better than soft materials. Both PE and PTFE waxes function by the ball bearing mechanism, while the softer microcrystalline waxes work via the layer (bloom) mechanism.
The use of nano-sized materials in coating formulations can significantly improve scratch resistance. These improvements can be used in clear topcoats, ink over-print varnishes and pigmented finishes. The commercial availability of nanoparticles allows formulators to obtain new properties that were unachievable in the past, not only in scratch resistance but many other physical performance attributes.
For nanoparticles to be of use in transparent coatings, it is critical that aggregates present in the powder be dispersible to their primary particle size in the coating formulation to avoid rapid settling and excessive light scattering. In addition, it is critical that the dispersed primary particles avoid re-aggregation during the coating curing process.
Thousands of scratch-resistant coating applications are present in our everyday lives. Examples of these applications include coatings for wood floors, safety glasses, electronic displays, automotive finishes and polycarbonate panels. Improving the mar, scratch and/or abrasion resistance in these transparent coating applications is a major challenge, particularly with regard to not affecting the other performance attributes of the coating.
Inorganic Fillers
Incorporation of inorganic fillers into coatings to improve mechanical properties is well known. Drawbacks associated with this approach can include loss of transparency, reduced coating flexibility, loss of impact resistance, increase in coating viscosity and appearance of defects. To overcome these defects, a filler material should impart improved scratch resistance without causing the aforementioned drawbacks. Nanomaterials have the potential to overcome many of these drawbacks because of their inherent small size and particle morphology.
Maintaining transparency in a coating containing inorganic filler particles is a challenge. Four properties dictate the degree of transparency in a composite material: film thickness, filler concentration, filler particle size, and the difference in refractive index between the bulk coating and the filler particle.
Silica particles, colloidal or fumed, and clays are among the most widely studied inorganic fillers for improving the scratch/abrasion resistance of transparent coatings. These fillers are attractive from the standpoint that they do not adversely impact the transparency of coatings due to the fact that the refractive indices of these particles (fumed silica = 1.46; bentonite clay = 1.54) closely match those of most resin-based coatings. The drawback to silica-based fillers is that high concentrations of the particles are generally required to show a significant improvement in the scratch/abrasion resistance of a coating, and these high loadings can lead to various other formulation problems associated with viscosity, thixotropy and film formation.
Alumina
The use of alumina particles in transparent coatings is much more limited even though alumina is significantly harder than silica-based materials and, as a scratch- and abrasion-resistant filler, higher performance at lower loadings is often observed. For alumina particle sizes greater than 100 nm, the high refractive index (1.72) results in significant light scattering and a hazy appearance in most clear coatings. Currently, only high-refractive-index coatings, such as the melamine-formaldehyde resins used in laminate production, can use submicron alumina for scratch resistance and maintain transparency.
To use alumina as a scratch-resistant filler in transparent coatings, the particle size must be sufficiently small to overcome its refractive index mismatch. A Physical Vapor Synthesis (PVS) process has been developed that allows production of nonporous crystalline metal oxides having primary particle sizes less than 100 nm at economically viable rates with essentially no byproducts or waste streams.
Two grades of aluminum oxide can be produced using the PVS process: NanoTek™ and NanoDur™ alumina. Both grades feature a mixture of γ- and δ-crystal phases and are spherical in shape, but the grades differ in terms of primary particle size. NanoTek alumina has a surface area of 35 m2/g corresponding to a mean particle size of 48 nm, whereas NanoDur alumina has a surface area of 45 m2/g with a mean particle size of 37 nm.
There is a proprietary particle dispersion stabilization process that involves specific surface treatments designed to yield nanoparticles that are compatible with a variety of different coating formulations. For example, stable dispersions of metal oxide nanoparticles can be prepared in solvents such as water, alcohols, polar and nonpolar hydrocarbons, plasticizers, and even directly in acrylate monomers with the appropriate surface-treatment process. These surface treatments allow solids levels of up to 60 wt% to be dispersed, and yet maintain a sufficiently low viscosity for ease of blending.
The use of highly concentrated, non-aggregated nanoparticle dispersions allows incorporation of the nanoparticles into a coating formulation without substantial dilution of the formulation with the dispersion liquid. This feature is particularly important in 100%-solids coating formulations wherein the nanoparticle is dispersed in one of the reactive monomers.
Within a given coating class, formulations that result in harder/stiffer coatings tend to show greater improvement with alumina incorporation than formulations that lead to softer/more elastomeric coatings. In addition, transparent coating formulations that exhibit crosslinking upon curing, such as UV-curable, 2K polyurethane, and melamine-based coatings, show greater improvement in their scratch resistance upon alumina nanoparticle incorporation compared to transparent coatings that do not crosslink but rather coalesce, such as emulsion-based coatings.
SNC
SNC is an abbreviation for silica nanocomposites that are composed of colloidal silica particles with an organic surface modification. These particles, which improve the scratch and abrasion resistance of a variety of coatings including radiation-curable formulations, are produced by a unique process that results in monodispersed, non-agglomerating spheres with a diameter of about 20 nm. The flexible manufacturing process is also capable of producing a broad range of cationic (epoxide) and free-radical (acrylate) radiation-curable oligomeric composite materials. These products are stable, transparent and have low viscosity, even at a silica loading of 60%.
Nanoscale materials for coatings also include complex silicon oxides and aluminum silicates. Nanoparticles of these materials have been incorporated into automotive coating formulations that have good sag resistance. The cured coatings have excellent chip and scratch resistance, outstanding appearance, superior sandability, and resistance to water spotting and acid etching. Some properties, such as scratch resistance, are maintained after accelerated weathering.
Sol-gel
It is also possible to improve the scratch- and wear-resistance properties of a coating as well as its photostability/weatherability by the addition of nanoparticles prepared by sol-gel processing. This method has the advantage in that it starts from existing, well-developed formulations to which a sol containing nanoparticles is added. After curing, the modified systems give transparent coatings with high wear and scratch resistance.
Very often, hybrid (organic-inorganic) materials are produced by sol-gel. The most common way to produce nanocomposites is to form, in-situ, an inorganic phase by hydrolysis and condensation of alkoxides or alkoxysilanes. A further curing results in covalent bonding between the organic and inorganic phase.
ABSORBENTS
Absorption is a process wherein a material is taken up and held, or retained, by another material. The material taken up is called the “absorbate” and the material that retains the material from the absorption process is called the “absorbent.” Thus, absorbents are materials that are able to take up another material with the formation of a homogeneous mixture. For example, cotton fibers will take up moisture, charcoal will take up a gas, baking soda will take up odors, silica gels will take up moisture; certain pigments, clays or extenders will take up oils and others will take up moisture; and so on. This should be contrasted with adsorption, which is a surface phenomenon and wherein adsorbed molecules can have markedly different properties than those of absorbed molecules. Compounds such as zeolites or molecular sieves are adsorbents that take up compounds by the adsorption process (See Moisture Scavenger).ACCELERATORS
See HardenersThese products increase the epoxy-amine reaction rate and subsequently reduce the possibility of undesired blushing or blooming reactions. Controlled use of the amount and type of accelerator ensures minimal impact on the cured binder performance. Although there are numerous products capable of accelerating epoxy-amine reactions, the most commonly used are: tertiary amines (e.g., DMP-30 = 2,4,6-tris-[dimethylaminomethyl]-phenol), phenol derivatives (e.g., nonylphenol), alcohols (e.g., benzyl alcohol) or acids (e.g., salicylic acid). Be aware that adding accelerator will significantly reduce the pot-life of the binder system.
Iron phosphate conversion coatings for general industrial application use accelerators, such as molybdate or vanadate, to provide active sites for iron phosphate deposition. Alkyd resin technology frequently uses metal driers to promote air oxidation and resin crosslinking. The activity of cobalt or manganese drier metals can be enhanced by the addition of chelating agents, also known as drier accelerators. The chelating agents function by stabilizing the valence state of the metal so that the oxygen up-take rate is maximized. One such chelating agent is 1,10-phenanthroline.
ACID CATALYSTS
See CatalystsAcid catalysts are used to accelerate chemical reactions. Strong acids such as p-toluene sulfonic acid (PTSA) are frequently used. Also used are catalysts based on dodecylbenzene sulfonic acid (DDBSA) and hexafluorophosphoric acid. In using strong acids as catalysts, acid strength does not necessarily influence the cure rate but it does affect some film properties. The most widely used of the strong acids is PTSA. Weaker acids, such as butylphosphoric, those based on aromatic phosphates and various carboxylic acids, are also used in some coatings systems. Blocked acid catalysts are also used for many crosslinking reactions.
ACID SCAVENGERS
Acid scavengers remove the small amounts of acid that are formed during the lifetime of a coating or ink. For example, when vinyl copolymers are aged small amounts of hydrochloric acid are formed as they age. An acid scavenger reacts with the acid, which removes it from the system so it cannot cause harm to the coating, substrate or abutting objects. Compounds such as cycloaliphatic epoxides and soybean oil epoxide, which readily react with strong acids, are examples of acid scavengers.ADHESION PROMOTERS
See Coupling AgentsAdhesion promoters improve a coating’s ability to withstand mechanical separation from a substrate. That is, they improve adhesive strength. Quite often these compounds contain two different functional ends, one of which will interact with the substrate and the other that will interact with the coating binder. Examples of the various coupling agents are the silanes, which are trihydrolyzable; the titanates, which can be mono-, di-, and tetrahydrolyzable; and the chromiums, which are complex in nature.
For metal surfaces that are to be coated, this is particularly important because metals, as a class, are unstable. The pure metal is always oxidizing to the metal oxide on the surface of the metal substrate. Exposure to moisture, oxygen and salts accelerates the process. Almost all coatings contain microvoids through which oxygen, small molecules like water, and ionic materials can diffuse. If the coating can remain bonded to the metal, then the damage done by these diffuse agents will be nonexistent. In other words, corrosion can be prevented. It is, therefore, very important to do all that is possible to maximize adhesion.
For some materials this involves a mechanical roughening of the substrate surface to increase the surface area for physical absorption. Chemical pretreatments such as zinc/iron phosphate and various other materials have also been used because tightly bound phosphated surfaces will retard access to the metal and, therefore, impede corrosion.
Typically, organofunctional silanes have been used in coatings as adhesion promoters because they provide a polar functional group to contribute to increased bonding to a mineral substrate. They also are hydrolyzable and provide wetting ability and surface activity. The silanes are moisture sensitive and will hydrolyze over time to silanols. This is not a problem in solventborne coatings systems but can cause problems for waterborne systems. The silanes react with both the polymer and the substrate to form covalent bonds across the interface. Silane adhesion promoters are used in urethane, epoxy, acrylic and latex systems.
Receptive inorganic surfaces are those that have hydroxyl groups attached to elements such as Si, Al, Ti and Fe. Nonreceptive surfaces, such as boron, and alkaline earth oxides, do not form stable covalent bonds with silanols. A number of different commercial silane coupling agents are used in coatings. Levels that range from 0.05-1.0% are generally effective.
Methacrylic phosphate monomers that improve adhesion to metal, concrete, glass and other inorganic substrates and that can be used in both water- and solventborne formulations are available. Some methacrylic phosphate monomers improve metal adhesion and also significantly improve corrosion resistance. There are also acrylic phosphate functional monomers that improve adhesion to various metal substrates. The acrylic reactive group provides a higher reaction rate in UV- and EB-curable applications.
Other adhesion promoters that are in the marketplace are titanates (such as isopropyl tris-[N-ethylaminoethylamino] titanate), zircoaluminates, zirconates, aryl/alkyl phosphate esters and proprietary metal organic compounds. The titanates and zirconates suffer from moisture sensitivity as well, so caution is necessary when using them with waterborne systems. Neo-alkoxy products are claimed to not have this problem. Alkyl/aryl phosphate esters, zircoaluminates and the metal organic promoters are stable in waterborne coatings. They are quite different in chemical nature and therefore the formulator needs to evaluate them separately.
Epoxy/methoxy functional additives are effective in promoting adhesion of a variety of coating systems to glass, aluminum and steel. Methacrylate/methoxy functional additives improve adhesion of free radical cured resins, such as polyacrylates, to inorganic substrates. Epoxy functional silanes improve adhesion and water resistance of a variety of coating systems to inorganic substrates. Amine/methoxy functional additives improve adhesion and water resistance of coatings and adhesives when bonded to glass or metal substrates.
Radiation-curable Coatings
Radiation-curable coatings applied over a variety of metal surfaces can pose an adhesive challenge. There are proprietary phosphate ester monomers that are best used on an additive basis to provide enhanced adhesive properties. These acid-functional monomers (AFMs) range in functionality from mono to tri, with differing acid level content. The AFM additives promote adhesion to a variety of metal substrates including aluminum, cold-rolled steel and tin-plated steel as well as promoting adhesion to wood and plastic substrates. AFMs should not be used with amines, as instability may result.
Powder Coatings
The same precautions regarding clean substrates and pretreatments that apply to liquid coatings are advised for powder coatings. Adhesion promoters such as the silanes and titanates may also be used to enhance adhesion. Silanes designed for use in powder coatings have an organo functionality that has an affinity for the powder resin system. The organo-silane must orient itself at the coating-substrate interface. The choice of organo-silane is usually governed by the resin system, and experimental screening is advised to determine which promoter provides the most improvement. Adhesion promoter types commonly used in powder include mercapto-silanes, amino-silanes, carboxyl/hydroxyl-silanes, and carboxyl-silanes.
Plastic Substrates
Due to high chemical stability, low price, excellent balance of physical properties, possible recycling, etc., the amount of polypropylene (PP) and thermoplastic olefin (TPO) consumed by automotive parts, household electrical appliances and molded general goods businesses is increasing. However, PP and TPO are materials with low surface energy that make painting and adhesion problematic, hence chlorinated polyolefin (CPO) has found wide use as an adhesion promoter. Solventborne CPOs have traditionally been used. Excellent adhesion between TPO substrates and CPO can be obtained as the result of good wetting and higher dispersion interaction, which are affected by the properties of the CPO’s chlorine content, crystallinity, melting temperature, molecular weight and its polydispersity.
There are several factors that can affect the performance of a CPO-based adhesion promoter. Application parameters play a significant role in designing a system that will provide optimum adhesion performance. Of particular importance is the temperature at which a coating applied to a PP or TPO part is cured or baked. In addition, substrate and CPO composition can influence overall adhesion performance.
Coating bake temperature is the temperature at which the coating applied to the TPO part is cured. Coating bake temperature can have an effect on the interaction between a CPO-based adhesion promoter and the surface of TPO, which can affect performance. For best results, coating adhesion is enhanced when the coated TPO parts are baked at temperatures over 100
ALGAECIDES
See Biocides/FungicidesChemical agents used to destroy algae. Algae are chlorophyll-containing plants whose green color is often masked by a brown or red pigment. Representative compounds include gluteraldehyde, methylenebis(thiocyanate), quaternary ammonium compounds and zinc oxide.
ANTI-BLOCKING AGENT
Additive used to prevent the undesirable sticking together or adhesion of painted surfaces under moderate pressure, or specified conditions of pressure, temperature and humidity; or during storage, manufacture, or use. Blocking is a measure of the coating’s ability to resist adhesion to itself (on another freshly coated surface) or adhesion to other substrates, for example, weather-stripping, doors, hardware etc. A well-known example of blocking is when a freshly painted window frame is too rapidly closed. Sometimes it can be very difficult to open the window again.Blocking is a key performance parameter for architectural application and, for industrial and OEM applications, block resistance is important in the manufacture of roll stock that will be unrolled at a later date. It is also important for reducing the need for storage space for freshly painted parts. ASTM D 4949 may be used to measure blocking performance. Factors affecting blocking include the coating surface free energy, topography of the coating, the hardness and the Tg of the polymer.
One approach to improve the block resistance of a coating is to introduce a surface-active agent that will bloom to the top – the air interface – of a film as it dries/cures.
Carbinol-functional silicone polyether copolymers impart mar resistance and anti-blocking properties in addition to leveling and wetting. Methacrylate functional silicone polyether copolymers provide consistent and long-lasting slip, mar resistance and anti-blocking to UV-cured coatings.
Fluorochemical additives can be mixed into coating formulations, often as post-adds, and migrate to the air interface where they can provide effective protection where it is most needed, without affecting recoat adhesion. Common applications include latex semi- and high-gloss architectural paints used on doors and window trim, and applications where painted parts are stacked for shipping.
Waxes decrease blocking so that unwanted transfer or adhesion to a contacted surface is prevented. This can be very important for materials that are coated, dried and stacked for storage and shipping. Waxes can be used in any type of coating that could benefit from mar resistance and/or a slip aid. Both water- and solventborne metal coatings benefit from added lubricity and abrasion resistance. HDPE, paraffin and Carnauba waxes are typically used to counteract blocking. Anti-blocking agents are also very useful for any type of items that are coated, dried and immediately stacked or rolled up for storage or shipment.
It should be noted that anti-blocking additives, since they tend to gather near the top surface of a film can change the appearance of a film – the gloss level. The type of paint is also a variable.
ANTI-CRATERING AGENT
A chemical additive used to prevent the formation of small, bowl-shaped depressions in a coating film. Very small areas of contamination that have a low surface tension will produce a surface energy gradient that pulls the coating away from the contaminant to form a bowl-shaped depression surrounded by a raised rim. As a result, the coating film is left with a roughly rounded depression – either a crater if the central contaminant is not visible or a “fish eye” if the central area is visible. As the coating pulls away from the contaminant and starts to lose solvent it becomes higher in viscosity and is not able to flow out, with the result being the thicker rim of the crater. Craters are typically 1-3 mm in diameter. Craters are not always caused by a contaminant within the coating or on the substrate. Frequently airborne particles will come in contact with the wet surface of a newly applied coating to cause cratering. Certain coatings tend to be more prone to these surface defects than others. Low-molecular-weight resins that do not dry quickly are prone to this. Coatings with defoamers and wetting agents may also cause surface defects.Related phenomena caused by surface energy gradients are picture framing, Bénard cell formation, orange peel, edge-pull and picture framing. In general, such defects are caused by the fact that the coating will flow from a low-surface-energy area to a high (relatively)-surface-energy area. A variety of surfactants will alleviate or remove this problem, but the fluorochemical surfactants are particularly effective in many cases.
ANTI-CRAWLING AGENT
A chemical additive that prevents the wet coating film from receding from small contaminated or otherwise altered areas of the substrate, leaving the area with only a very thin coating and often having the appearance of being uncoated. Changing solvent systems or the addition of a low-energy surfactant (silicones or polyether-modified silicones) may solve the problem.
ANTI-FLOAT AGENT
A chemical added to single- or multi-color pigmented coatings that prevents the separating or floating apart of one or more pigments from the other. This prevents a streaked or mottled effect in the final film. The effect may happen even if a single pigment is used if the particle sizes of the pigment are varied over a broad range. Silking is a type of float confined to parallel striations in the film; dipping and flow coating are more prone to the effect of silking.ANTI-FLOODING AGENT
A chemical added to a multi-pigmented coating that prevents the separation and concentration of one pigment at the surface of the film upon drying or in a dispersion.
The terms flooding and floating are often used interchangeably within the industry but they are distinct events. Both events occur in the liquid coating phase. Some have referred to flooding as being a horizontal separation of the pigments as opposed to floating, which is a vertical separation. Some also refer to floating as being a problem on the surface of the film as opposed to flooding, which occurs beneath the surface of the film.
The terms are not standardized and both are the result of an uneven distribution of the pigment that appears in the film as it is drying. Some also use the term floating to describe a mottled effect and the term flooding to describe a surface color that is uniform but darker or lighter in color than it should be.
ANTI-FOAMING AGENT
See Defoamer, Foam Control
An anti-foam additive is used in coatings manufacture to prevent the formation of foam, or it is added to a mixture to destabilize foam and act as a bubble breaker, breaking the foam that has already formed.
The terms ‘defoamer’ and ‘antifoaming’ agents are often used interchangeably. In fact, they are not quite the same. A defoamer is a surface-active agent that stops the foam and breaks the bubble once it has been formed. It is a bubble breaker. An antifoaming agent prevents the formation of foam so it never forms. The term ‘foam-control agent’ is a more appropriate term to use and they function by a variety of mechanisms to prevent or rupture foam.
Quite often defoamers are proprietary blends of various ingredients such as mineral oils, organic solids and surface-active compounds; fatty oils, surfactants and silica derivatives; alcohols, silica derivatives and surface-active compounds; esters, mineral oils and silica derivatives; and the like. Such blends are compounded for special problems such as:
• quick foam “knock-down” coupled with long-lasting foam prevention in latex manufacture and latex coatings;
• controlling foam in stripping operations;
• foam control in food applications where FDA product acceptability is needed;
• foam control in paper coating and dyestuff applications;
• foam elimination in effluent systems;
• prevent air entrainment and/or facilitate air release from coatings during the filtration and filling processes;
• prevention of foam formation during roller application of architectural coatings;
• aid in the breaking of bubbles formed during roller application of latex coatings; and
• prevention of air entrainment in flash tanks and stripping columns.
With such product and specific end-use complexity, it is desirable to contact a supplier for product suggestions, point of application and amounts needed to efficiently eliminate difficulties.
ANTIFOULING AGENTS
See Biocides/FungicidesThese additives protect underwater marine hulls, or other coated structures, from harmful effects of marine life in coastal areas. Antifouling paint is used to protect the bottoms of ships against living organisms that attach themselves to the hull. These organisms cause reduced slippage (the ship advances more slowly and consumes more fuel) and can increase the weight of the ship, thereby affecting safety. Although they keep algae and barnacles from attaching to ship bottoms, tin-based antifouling agents have an impact on other marine organisms.
For years in the marine industry, the use of tributyltin (TBT) with cuprous oxide was an effective antifouling system that deterred algae, barnacles and other organisms from adhering to the ship’s hull. However, TBT is being phased out of use because of safety concerns for a variety of marine life and its long half-life that causes it to accumulate in the environment. There are a number of co-biocides used with cuprous oxide that are replacing the TBT-based systems. These include zinc and copper pyrithione and isothiazolinone (4,5-dichloro-2-n-octyl-4-isothiazolin-3-one)-based products. All of these co-biocide antifoulants have been used successfully in Japan for many years.
Cuprous oxide is a fungicide that deters attachment of marine organisms such as barnacles and mussels and thus is an active ingredient in antifouling paint. It is typically used with other co-biocides to enhance paint performance and improve the coating’s resistance against algae and slime attachment. The use of an algicidal co-biocide minimizes the attachment of algae and grasses to the coating surface.
There are barnacle inhibitors based on natural bioproducts – small molecules with menthol-like configurations. In particular, a menthol derivative called menthol propylene glycol carbonate is a GRAS (generally recognized as safe) food ingredient used as a cooling agent in gum and cosmetics. This chemical has repellent characteristics against barnacles as well as other insects.
The International Maritime Organization (IMO), in late 2001, approved a ban on the application of antifouling marine paints containing organotins such as TBT, beginning on January 1, 2003. The IMO agreement also required removal or sealing of all TBT-based coatings from vessels by Jan. 1, 2008. The actions affect all vessels engaged in international voyages. For more information, see the following website: www.antifoulingpaint.com.
Tin-free products have been on the market for more than 10 years, and marine paint companies offer the same performance guarantee as given with the tin-containing coatings. While most ships dry-dock every three years or less, some owners want a dry-dock interval of five years. Several of the new self-polishing systems can now meet that five-year standard ensuring most, if not all, marine markets can be serviced using a tin-free product. Registration requirements in the United States and Europe mean the new tin-free products have already undergone significant testing that show they are environmentally preferable to TBT.
With the approval at IMO of a ban on antifouling systems using organotins acting as biocides, a significant amount of work is being done on new antifouling systems. The goal is to find a renewable, natural material that is safe to humans and not harmful to the environment.
ECONEA (1H-Pyrrole-3-carbonitrile, 4-bromo-2-(4-chlorophenyl)-5-trifluoromethyl) is a metal-free antifouling agent that protects vessels against aquatic growth without a potential negative impact on the marine environment compared to traditional, copper-based antifoulants. It is being applied to U.S. Coast Guard aluminum hull crafts, eliminating the risk of galvanic corrosion that can occur when copper-based paints are applied to aluminum. Research has shown that ECONEA provides effective protection against fouling at substantially lower concentrations in the paint formula than traditional copper-based antifouling agents. Unlike copper compounds, ECONEA degrades rapidly once released at the surface of the paint – thereby not accumulating in the marine environment.
Currently marine coating research is highly focused on foul release coatings – i.e., those that are biocide free and do not release additives into the environment. These coatings function by using polymers that prevent living organisms from attaching themselves to the surface of the coating.
ANTI-FREEZING AGENT
A chemical that will prevent freezing or damage resulting from the freezing and thawing of a coating’s composition. Typical examples are ethylene and propylene glycol.
ANTI-GELLING AGENT
A chemical that prevents the progressive, irreversible increase in consistency of a pigment-vehicle composition usually caused by an interaction between the vehicle and pigment, reaction between resin and solvent, or polymerization of the vehicle. Some anti-gel agents for air-dry and stoving systems are based on a ketoxime and a phosphorous ester salt. They delay/prevent thickening, which can occur as a result of oxidation or condensation of the binder. They also reduce the reaction of the pigments with the vehicle and prolong the shelf life of a coating.
ANTI-LIVERING AGENT
A chemical that prevents the progressive, irreversible increase in consistency of a pigment-vehicle composition, usually caused by an interaction between the vehicle and pigment, or polymerization of the vehicle. Two typical anti-livering agents are 2-amino-2-methyl-1-propanol and N,N-dimethylethylamine. Livering is the slow, irreversible change that increases the consistency of a formulated (pigment-vehicle combination) coating. It usually results from a strong interaction between the vehicle and the pigment or other solid, dispersed material. However, it can be caused by a slow polymerization of the vehicle and entrainment of the pigment or filler. Livering often can be seen as a thick skin covering the surface of previously opened and stored cans of paint.
ANTI-MARRING AGENT
A chemical or composition that will enhance the ability of a coating to resist damage caused by light abrasion, impact or pressure. Silicone polyether copolymers impart mar resistance, improve leveling and reduce cratering, pinholing, orange peel and cissing. Carbinol functional silicone polyether copolymers impart mar resistance and anti-blocking properties in addition to leveling and wetting. Methacrylate functional silicone polyether copolymers provide consistent and long-lasting slip, mar resistance and anti-blocking to UV-cured coatings. Waxes are frequently used to improve mar resistance. (See Slip-Aid and Abrasion-Resistance for more discussion.)ANTIMICROBIAL AGENT
See Biocides/Fungicides
The term ‘antimicrobials’ is generally defined as substances, or mixtures of substances, used to destroy or suppress the growth of harmful microorganisms, whether they be bacteria, fungi or virus, in, or on, a substrate or article where it is not desired. Historically, the term ‘antibiotics’ is used in reference to controlling bacterial infections specifically in humans. For substances that control or inhibit yeast and fungi, the term used is ‘anti-fungal,’ whether on animate or inanimate substrates. Similarly, for inhibiting viruses, we refer to anti-viral substances.
Besides these various terms for antimicrobial concepts, in many industries terms like bactericides, fungicides, algaecides, virucides, preservatives and biocides are commonly used. For example, in the coatings industry, the term ‘biocide’ is historically employed to indicate preserving a wet formulation from microbial spoilage which, in most other industries like personal care or household markets, would be defined as ‘preservatives’.
In addition to these various terms to indicate chemicals that inhibit or destroy microorganisms, we are also exposed to layman’s terminology of ‘mildewcide’ or ‘moldicide’ to refer to chemicals controlling unsightly biological defacement on surfaces that are ‘black’. Even though it is implied primarily for fungal growth, it can also be caused by a consortium of other microorganisms including lower forms of algae, moss, amoeba, protozoa, etc. For non-microbiologists, these terms may be confusing, with each term meaning different things depending on the implied uses and claims by different suppliers. All of these chemicals can be referred to as antimicrobials irrespective of their applications, target organisms and mode of activity under a given condition.
Government regulations on treated articles, including coatings with antimicrobials, and guidelines on claims come under the Federal Insecticide Fungicide and Rodenticide Act [FIFRA], EPA and FDA. Under FIFRA, an antimicrobial product that claims to control microorganisms such as bacteria and fungi requires registration. According to the EPA, an article or a substance that is treated with, or containing, a registered pesticide is defined as a treated article and is limited to protecting the article itself from microbial spoilage or contamination [for example paints supplemented with fungicides or bactericides to protect the paint in storage or after application, or preservative treatments used in wood to protect wood against insects or fungal infestation].
The EPA in 2000 issued a Pesticide Regulation Notice 2000-1 to clarify the Agency’s policy with respect to the scope of the treated article exemption. It is a guidance document that focuses on the various types of antimicrobial claims that the EPA considers acceptable or unacceptable.
Antimicrobials used in public or non-public health claim products have to go through a battery of tests using acceptable microbiological methods to show efficacy in the product in use. There are many test methods in microbiology described to demonstrate the antimicrobial nature of a substance and when it is incorporated in an article. There are various screening stages adopted that may include primary, secondary and in-use final testing. A typical primary-screening protocol involves testing for the minimum inhibitory concentration [MIC] of the chemical to be incorporated in an article. Normally the MIC is determined in an in-vitro system like growth media against a set of bacteria, fungi, virus and/or algae depending on the target microorganism. After getting selected in the primary screen, the substance enters the secondary screening process in an in-vivo system, meaning the application matrix in which it is expected to be incorporated. Because not all selected antimicrobial substances are expected to be universally acceptable in varied systems, they next go through the rigor of compatibility, stability and efficacy testing evaluations. Some examples of this rigor in coatings applications include the chemical stability in the formulation, pH compatibility, heat stability, color acceptance, shelf storage longevity, exterior weather sustainability, etc., in addition to testing for its bio-availability to function as an antimicrobial throughout the process.
With so many scenarios for defining an antimicrobial, the challenge becomes how to test and what test methods to use for demonstrating and claiming broadly the ‘antimicrobial’ property of a treated article such as a coating. There are several ASTM methods available such as: ASTM D 5589, ASTM D 5590, ASTM D 3273 and ASTM D 2574. Also used is the Japanese Industrial Standard JIS Z 2801-2000: Antimicrobial Products-Test for Antimicrobial Activity and Efficacy. This method was originally developed to test the antibacterial activity of silver ions impregnated in rigid hydrophobic polymers. The method was developed by a consortium of workers comprised of manufacturers of silver-based antimicrobial agents, government-based research organizations and universities, and under organizations such as the Society of Industrial Technology for Antimicrobial Articles (SIAA).
This method is a quantitative measurement method that tests survival of low-dose bacterial inoculum deposited between the tested antimicrobial surface and a thin plastic film that keeps the inocula wet and nourished in a nutrient-rich environment throughout the 24 h incubation at 35 ºC. This differs from other traditional methods of testing antimicrobial resistance in coatings surfaces where the surface is not kept deliberately wet or moist with a nutrient medium. Inoculum survival and growth in the other methods depends only on the moisture from either media or humidity created in the incubated unit, so only microorganisms that can survive some level of desiccation on the surfaces in 24 h can be recovered and can be compared to the non-treated surface. In this respect, the JIS Z 2801 method is so severe it is not necessarily a realistic surface contamination model for dry walls and other coated vertical surfaces.
In spite of this limitation, JIS Z 2801 has emerged as one of the industry standards for perhaps the ‘worst case scenario’ of a surface that retains wetness and permits microbial survival. Following the method as written for hydrophobic coatings surfaces, yields results that may provide useful information, but for hydrophilic and porous substrates and surfaces, a deviation in the inoculum delivery and validation for each different substrate would be required to get useful data. There is no one universal test protocol or one pesticide product that can demonstrate all the antimicrobial properties on all surfaces.
ANTIOXIDANT
A chemical compound that prevents oxygen from reacting with other compounds that are susceptible to oxidation. Antioxidants are additives that are expected to prolong the lifetime of a coating and thus assist in maintaining its original high-performance characteristics as long as possible. Components of a coating, adhesive, ink or sealant are subjected to opportunities for degradation during component manufacture, storage, and transportation as well as during application and final use.The cause of degradation is almost always by oxygen or UV radiation attack (see UV Absorbers and Light Stabilizers). Oxygen attack can be by oxygen or ozone and it may occur under ambient or elevated temperature conditions. At elevated temperatures antioxidants decrease thermal oxidation and formation of peroxide radicals that, in turn, can cause formation of colored chromophores and/or other deleterious changes in coating properties.
Effective antioxidants include the p-phenylenediamine derivatives such as the N,N´-diaryl, the N,N´-alkyl-aryl, and the N,N´-dialkyl compounds, but these compounds have a tendency to discolor or scorch during cure. Also effective and widely used are the hindered phenolic compounds, such as 2,6-di-t-butyl-4-methylphenol (BHT), octyl and certain higher alkyl phenols, phosphites (trisnonylphenol phosphite, triphenyl phosphite, tris(2,4-di-tertbutylphenyl phosphite, bis(2,4-dicumylphenyl) pentaerythritol phosphite), and synergists that are mixtures of antioxidants that act in a synergistic manner. Antioxidants are usually used in a concentration range of 0.1-0.5%, and they are consumed in the stabilization process.
For powder coatings, the purpose of the antioxidant is to minimize thermal degradation of the polymer in the coating. Thermal degradation causes yellowing and a reduction in mechanical and chemical properties. Usually hindered phenols are used as primary antioxidants. Organo-phosphites are sometimes used as secondary antioxidants. An organo-phosphite acts synergistically with a hindered phenol to check the thermal degradation of the polymers. These compounds act as peroxide decomposers and can be incorporated in a 2 or 3 to 1 ratio (hindered phenol to organo-phosphite). Hindered phenol-based antioxidants are usually used at a level of 0.2 – 0.8% of the binder. Higher levels will cause yellowing. It should be noted that antioxidants do not reduce degradation from UV light exposure.
Various classes of antioxidants have different thermal stabilization mechanisms. The classic high-molecular-weight hindered-phenolic antioxidant is effective as an oxygen-centered radical scavenger, but can suffer from “pinking” in the presence of combustion gases like NOx, which can be found in gas oven exhaust. The phosphite antioxidants act as decomposers of hydroperoxides and provide protection during high temperature processing and/or curing cycles. The new lactone antioxidants function as a carbon or oxygen centered radical scavengers and inhibit auto-oxidation. The development of a high-performance “phenol free” antioxidant blend exploits synergistic effects between phosphite and lactone properties while illuminating the possibility of pinking. There is a synergistic effect when using different antioxidants blended together and the combination can meet heat and processing stability requirements.
ANTI-RUST AGENTS
See Corrosion Inhibitors/Flash Rust Inhibitors
Anti-rust agents are compounds that, when added to water, alleviate or lessen rust formation. The term also refers to a variety of chemicals used to prevent corrosion on the surface of iron or other ferrous metals resulting in the formation of products consisting largely of hydrous ferric oxides. The same or different agents may be used to prevent flash rusting, early rusting or to provide long-term corrosion resistance. Anti-rust agents are important in preventing rusting in cans or other metal containers of waterborne paints.
ANTI-SAG AGENTS
See Thickeners, Rheology ModifiersThis is a common term used to cover a class of compounds used to increase the viscosity of paints, and control or prevent sagging of a coating film during application and curing. The potential for sagging tends to increase as wet film thickness increases and/or as drying is prolonged. Most of the anti-sag agents impart a thixotropic rheology to the paint.
Treated clays are the main type of thixotropic agent used in alkyd systems. These are Bentonite clays that have been treated so that they build a gel structure by hydrogen bonding in the paint. They are either added into the mill base where the dispersion shear breaks the agglomerates, or added as a pre-gel. These clays are of two types depending on whether or not they need a polar activator. The activators are usually alcohols. The choice of a clay is dependent on the system.
In many latex systems, structure is formulated into the coating and settling is not as much of a problem as it can be in solvent systems. For some aqueous systems the choice of colloid is very important. A variety of thickeners are available for waterborne systems. Assistance should be sought from the supplier of these various types of materials.
ANTI-SETTLING AGENT
See Rheology Modifiers, ThickenersAn anti-settling agent is a suspension or rheological additive whose function is to prevent or retard pigment settling, and to maintain uniform consistency of the coating during storage and application.
ANTI-SILKING AGENT
See Anti-Flooding
Additive used to prevent a particular type of float that results in parallel hairline striations of different colors running throughout the film of a pigmented coating.
ANTI-SKID AGENTS
See Anti-Slip Agent
Slip-resistant or anti-skid additives are compounds that function directly opposite of slip additives or lubricants. They make it more difficult to move one surface past another surface. Compounds such as the colloidal silicas are used for this purpose. In the marine coating segment, antislip or antiskid coatings are those that reduce the sliding and slipping of humans and cargo on decks. Silica sand is widely used as a point-of-application stir-in additive in floor, deck and stairway coatings.
ANTI-SKINNING AGENTS
Anti-skinning agents prevent the formation of an insoluble film or skin formation on the surface of a liquid coating during storage or during application. Coatings that cure by an oxidative mechanism are most susceptible to skinning. Replacing air with an inert atmosphere such as nitrogen in the headspace of storage containers is an effective way to prevent skinning during storage.Skin formation may occur in a dipping tank that is open to the atmosphere. Here the skin may begin forming as surface gel particles that are formed by oxidation. As time passes, the gels coalesce into a continuous film as further oxidation takes place. Additives such as the oximes, particularly 2-butanone oxime, phenolics, solvents, and retention aids are used. Oximes such as butyraldoxime, 2-butanone ketoxime (methylethylketoxime or MEKO, which is the most widely used compound), and cyclohexanone oxime are compounds that complex with polymerization catalysts and prevent polymerization. Phenolics such as hydroquinone, 2,6-di-t-butyl-4-methoxyphenol (BHT), o-alkylphenol, and similar compounds act as antioxidants that retard auto-oxidation reactions. These are used at low levels of about 0.05% to as much as 0.2%. Solvents dissolve the components that cause the skinning. Currently undisclosed composition products are on the market from various suppliers. Retention aids prevent applied paint films from drying too rapidly. Waterborne paints usually contain retention aids that slow down the evaporation of water.
ANTI-SLIP AGENT
See Anti-Skid Agent
Any material added to a coating that will reduce or eliminate the hazard of slipping on the surface of the dried film. Surface roughness or an increase in coefficient of friction accomplishes this. A coating surface on floors, curbs, streets, porches, decks and so forth may be slippery particularly if damp and, therefore, there is often a need to add an anti-slip agent to the coating to enhance the surface roughness. Polyolefins with high COFs (coefficient of friction) are widely used in both commercial and consumer formulas for floor finishes.
This is not to be confused with the slip or lubricity that many coatings are designed to have particularly in manufacturing, packaging and transporting coated goods.
ANTISTATIC AGENTS
Anti-stats are materials that, when added to the coating or applied to the film, make it less conductive, or less attractive to dust, lint or other airborne particulates.
Conventional antistatic agents used to increase the conductivity of polymeric materials so as to permit dissipation of electrostatic charges can be separated into four general categories.
1. Hydroscopic surfactants such as tertiary fatty amines and their quaternary ammonium salts, monoacyl glycerides, monoalkyl and dialkyl phosphates, alkane sulfonates and sulfonamides work by blooming to the surface and attracting a conductive film of atmospheric moisture. These antistatic surfactants are humidity-dependent and work on the chemical principal of limited polymer solubility, blooming to the polymeric surface to provide sites for water absorption from the atmosphere. Examples are: glycerol monostearate, stearyl phosphate, dodecylbenzene and sulfonamide.
2. Conductive pigments, metal powders and other additives, which dissipate the electronic charge proportionate to their loading in the polymer. Carbon black, graphite fiber, metal powders, barium titanate powders, potassium titanate whiskers, metal-doped silicas, TiO2 and fibers provide a low-resistance pathway to dissipate the electrostatic charge and provide permanent antistatic protection.
3. Metallocenes that provide a low-energy transfer of electrons between adjacent aromatic layers. The primary example is bis(methyl)cyclopentadienyl cobalt.
4. A new class of antistatic agents based on combined neoalkoxy titanates and/or zirconates and subsequent tri-neoalkoxy zirconates that can be added in minor amounts during compounding.
Antistatic agents are in hydrophobic coatings such as the silicones to improve the coating’s resistance to dirt pick-up. These additives are usually cationic in nature, but in certain instances they are nonionic hydrophilic compounds.
Anti-static additives enhance the electrical conductivity of electrostatic spray paints, improve gloss, reduce the Faraday effect in powder coatings, and retard dust attraction on the finished product. (The Faraday cage effect is observed in the powder coating of parts that have recesses, inside corners, channels or protrusions on the surface of the substrate. The Faraday cage is the area of the part where the external electric field created by the gun does not penetrate.)
These static dissipative materials can be blended internally with powder coatings to enhance the electrostatic spray characteristics of the coating and minimize dust attraction. This increases the transfer efficiency, which improves penetration into corners and recesses. The anti-stats seem to have no adverse affect on the physical characteristics of the final powder coating such as: impact resistance, pencil hardness, gloss, gel time, color, adhesion, cure time, salt spray and condensing humidity. These agents are able to control gassing and minimize pinholing in the coating.
For powder coatings both charge control and antistatic agents are used. Charge-control agents improve the transfer efficiency and the ability of the powder to penetrate the Faraday cage areas. The function of the antistat agents is to improve the ability of the coating to conduct extraneous electrical charges to ground. These additives are used to decrease the surface resistivity of the powder and the applied powder coating.
Antistatic additives reduce the powder resistivity so that the powder particles charge more efficiently. This in turn improves the overall efficiency of the coating process. Several types of materials are used to reduce resistivity. Quaternary ammonium salts (cationic) or alkyl sulfonates (anionic) based on fatty acid derivatives are often used.
Some of the cationic antistatic agents are catalysts for epoxy-containing powders and have a tendency to cause yellowing when baking. Barium titanate is also used to promote powder-charging characteristics.
ASSOCIATIVE THICKENERS
See Thickeners
Associative thickeners are polymeric compounds that provide consistent and reliable control of coating rheology during manufacture and use. They obtain their efficiency presumably by association between thickener molecules or thickener and latex particles.
BACTERICIDES
See Biocides/Fungicides
Bactericides are additives that will kill bacteria (single-celled aerobic or anaerobic organisms) that can cause a variety of problems in liquid coatings and coating films. Examples of suitable compounds include: hexahydro-1,3,5-tris (2-hydroxyethyl-s-triazine), sodium pyrithione, isothiazolinone-based chemicals, 1-(3-chloroallyl)-3,5,7-tri-aza-1-azonia-adamantane chloride, formaldehyde-releasing compounds, and biguanides (polyhexamethylene biguanide [PHMB]).
BARRIER COATING ADDITIVE
See Seal Coating
BIOCIDES/FUNGICIDES/ANTIMICROBIALS
See EnzymeBecause coatings are largely organic in nature, they provide a source of food for microorganisms. Microorganisms are found everywhere and they work around the clock trying to cause viscosity loss, putrefaction, gas formation, emulsion breakdown, and other undesirable physical and chemical changes. These attacking species can cause discoloration, marring, loss of adhesion and finally coating failure.
Microorganisms can contaminate paint in different ways during the manufacturing process. Unsanitary conditions may exist, such as for the raw materials (including thickeners, extenders, pigments, emulsions, surfactants and defoamers), water (process water and recycled water), containers and equipment (tanks, pipes, hoses, etc.).
The ability of some microorganisms to attach to surfaces and form adherent biofilms is also important. Biofilms are functional consortia of microbial cells entrapped within an extensive matrix of extracellular polymer (glycocalyx) produced by them. Biofilms can be formed in water systems, processing tanks and other areas. Biofilms may be sources of contamination of the product, and may cause corrosion, scaling, the reduction of heat transfer efficiency and other problems in addition to the spoilage. Microbial biofilms are usually resistant to biocide treatments or disinfectants. Depending on the growth conditions (nutrients, minerals, gas composition, temperature, pH, water activity, etc.), microorganisms can reproduce very rapidly in the paint.
Coatings need to be protected from microbe attack, and there are a number of microorganisms that the formulator needs to be conscious of when formulating paints. Sometimes the terminology of available agents is confusing: biocide, mildewcide, fungicide, algaecide (also spelled algicide) and so forth. Product literature and the suppliers will certainly assist in this regard. Many of these additives are multi-purpose and can curtail the growth of a number of organisms.
Biocides (or microbiocides) are substances that will kill organisms and thus are used to protect coatings from biological attack caused by algae, fungi and other organisms that propagate in moist environments – particularly in warm climates. The “-cide” nomenclature refers to compounds that kill, in this case, microorganisms. A biostat prevents or interferes with the growth of the organism but does not kill it. The additives are often further defined as follows to describe the particular types or organisms that are killed or affected.
Algaecide/Algicide - Chemical agent used to destroy algae.
Bactericide - Compound used at low levels to kill bacteria.
Bacteriostat - Substance that prevents or slows the growth of bacteria.
Biocide - A chemical agent capable of killing organisms responsible for microbiological degradation.
Efficacy - The effect of the microbiocide on the target organism or group of organisms; can be measured as percent killed versus a control containing no biocide. Efficacy can be expressed as MIC, or minimum inhibitory concentration, against a specific organism.
Fungicide - Chemical agent that destroys, retards or prevents the growth of fungi and spores.
Fungistat - Compound that inhibits the growth of a fungus, or prevents the germination of its spores.
Mildewcide - Chemical agent that destroys, retards or prevents the growth of mildew.
Spectrum - Refers to the effect a microbiocide may have on more than one organism such that a broad-spectrum biocide will be affective against more than one group of target organisms.
For coatings we are concerned with bacteria (aerobic and anaerobic), fungi (multicellular [molds], unicellular [yeasts]) and algae (green and blue-green). The addition of an in-can preservative will protect coatings in the wet state during storage and transport. But after a coating has been applied and dried, it becomes susceptible to colonization by fungi and/or algae.
Biocidal agents are available to work both “in-the-can or batch” and also in the dried film. For this reason, many manufacturers include a biocide (anti-microbial) agent in the formulation of the paint so that it can kill both bacteria and yeasts that can be present. If not corrected before they start, microorganisms can lead to the production of gases – this can occur in the container and result in can lids popping and cans distending, offensive odors emanating and loss of film and application properties. Bacterial enzymes and certain fungi attack organic thickeners, and this can lead to viscosity changes in the liquid coating. The pH of the paint can be affected and the paint can undergo discoloration.
Microbial contaminants can be introduced with water (process water, wash water), with raw materials (latex, fillers, pigments, etc.) and by poor plant hygiene. Bacteria are the most common spoilage organisms, but fungi and yeasts are sometimes responsible for product deterioration. Spoilage of waterborne products, which may go unnoticed until the product reaches the consumer, can result in significant economic loss. Good plant hygiene and manufacturing practices, when combined with the use of an optimized biocide, will minimize the risk of microbial spoilage.
Enzymes are organic catalysts, which means that they are not consumed in any reactions. They are produced by living cells and are protein in chemical nature. Bacteria and their enzymes can degrade the organic components of paint – the polymer and its organic additives. One enzyme molecule can change hundreds of organic molecular structures and degrade them. The most obvious immediate result is a loss of viscosity. This renders the product unstable and unusable. This is more of a problem for architectural coatings, which tend to be warehoused, shipped and then stored on shelves for longer periods than typical industrial coatings, which are usually consumed rapidly.
Manufacturers and formulators need to be conscious of the fact that the dried paint film is subject to microbe attack from mold, mildew and algae – particularly in certain climates where temperature and humidity encourage microbe growth. For dried coating films, algae and fungi can cause discoloration, dirt entrapment, cracking, blistering and loss of adhesion. A loss of adhesion is commonly associated with fungi growth as well as corrosion on certain substrates due to the moisture produced by fungi. Dependent on the climate, many exterior surfaces and roofs may be subject to algae growth, and not all fungicides are necessarily effective against algae. Certain areas of the world have already recognized this as a serious problem and one of concern for the preservation of exterior buildings.
The type of microorganism that can attack the coating depends on many factors including the presence of nutrients, the moisture content and the composition of both the substrate and the coating itself. Moisture is affected by the amount of rainfall, dew, humidity, temperature and time of year. Local environment conditions such as surfaces that are sheltered from wind and shaded areas also have an impact on microbial growth. Nutrient sources include constituents of the coating, partially biodegradable material from other microorganisms or simply dirt. The substrate may affect the pH of the surface and make it suitable for microbe growth. Fungi favor acidic conditions such as those provide by wood and some species of wood are more susceptible to fungi attack than others. Algae favor alkaline conditions such as those provided by masonry.
For use in architectural coatings, it is important that the fungicidal material have a low solubility in water so that it is not readily leached out of the paint film. It should also not cause any weathering effect such as fading, chalking or discoloration. Some antimicrobial agents can cause fading in architectural coatings; therefore, it is always wise to expose the formulation to Weather-Ometer testing.
There are thousands of kinds of fungi and algae throughout the world. However, only a relatively few disfigure and deteriorate exterior paint films. In general, research on painted panels and structures from around the world indicates that two types of fungi are the dominant causative agents of disfigurement and degradation of modern exterior paint films. These fungi were identified as Alternaria sp. and Aureobasidium (Pullularia) pullulans. Aureobasidium pullulans is the fungus predominantly responsible for the development of mildew in exterior paints. The Pseudomonas species attacks paints, joint compounds, roof coatings, exterior insulation and finishing systems and clear finishes in the can.
There are many effective biocides available for use. It is important to understand the operation of these agents and the differences in their activity. Some may be effective against certain bacteria in one concentration and effective against fungi in another concentration. Some biocides may be biocidal in certain concentrations and in other concentrations exhibit biostatic behavior. It is very important for the formulator to work with the supplier of these agents to understand their use and mode of action. Blends of biocides may often be used to enhance coating performance, as one biocide alone cannot always provide the desired results under demanding and varying climate conditions.
Some of the typical chemistries of these agents include: formaldehyde donors; ortho-phenylphenol (OPPs); isothiazolinone derivatives (such as 2-n-octyl-4-isothiazolin-3-one [OIT]); guanides and biguanides (such as PHMB or polyhexamethylene biguanide); carbamates (such as 3-iodo-2-propynylbutyl carbamate [IPBC]) and dithiocarbamates; copper or sodium or zinc pyrithione; benzimidazoles; n-haloalkylthio compounds; 1-(3-chloroallyl)-3,5,7-tri-aza-1-azonia-adamantane chloride; tetrachloroisophthalonitriles; cis[1-(3-chloroallyl)-3,5,7-tri-aza-1-azonia-adamantane] chloride and 2,2-dibromo-3-nitrilopropionamide (DBNPA); and quaternary ammonium compounds. These are but a few examples of the many agents available to the formulator.
Some biocides on the market today are two-for-one and eliminate the need for separate in-can preservatives and mildewcides. DCOIT – 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one is an example of one such biocide, which controls bacteria that cause coatings to degrade in the can and prevent mildew growth after the films dry. This particular biocide controls a wide range of microorganisms including fungi, algae and bacteria.
The microbiological activity of 2,2-dibromo-3-nitrilopropionamide (DBNPA) was documented as a seed fungicide in 1947 and later as an antimicrobial agent. DBNPA, when formulated as a 20% solution in water and polyethylene glycol, is completely miscible with water and readily disperses upon introduction into a water-based system. The DBNPA molecule begins functioning as an antimicrobial agent immediately upon introduction into a system; the rate of this activity is not affected by pH, and antimicrobial control is usually achieved before complete degradation occurs. The combination of instantaneous antimicrobial activity and rapid chemical breakdown makes this a cost-effective and environmentally friendly biocide.
It is used as a quick-kill biocide and short-term preservative in water-containing systems that require microbe control; it is ideal for the treatment of wastewater generated during the manufacture of paint. The collection and reuse of all wash water used to rinse paint mixing vats has been emphasized as crucial to achieving environmentally responsible production. This wash water contains a high concentration of paint solids and is usually heavily contaminated with microorganisms; it must be decontaminated prior to its re-introduction into the paint production process. DBNPA is ideal for this type of application.
Mildewcide (fungicide) and algaecide testing has been very confusing for paint companies. A paint formulation’s resistance to attack by fungi and algae is the most difficult performance characteristic to determine; testing requires a specialized laboratory with trained personnel to work accurately with fungi and algae.
The use of a single active ingredient may be sufficient to protect a coating against in-can spoilage or dry film defacement, but in many cases it may be advantageous to use a blend of the actives. For example, the combination of certain active ingredients can result in synergy whereby lesser amounts of each active are needed to bring about the same inhibitory effect as the use of either active alone. Thus blends of actives may allow manufacturers to protect a product at reduced levels, providing not only a potential cost benefit but also a product that is more environmentally friendly.
Even more important is for formulators to recognize the fact that even a minor change in a formulation may have a major effect on the biocide in that formulation. It is crucial with every change in formulation that the coating be tested for biocide efficacy. Some of the common factors that will decrease biocide efficiency are: pH, temperature of addition to the batch, nonionic surfactants, solubility, the presence of other formulation additives that deactivate the biocide, UV radiation and so forth. The only way to determine the efficacy of a biocide in a coating is through testing.
In testing various biocides in coatings there can be significant differences in performance of paint systems by exposure location. Since there can be such a wide variation in product performance by location, it is very dangerous to rely on data from one exposure site to assess how a national paint product might perform. To truly assess the potential commercial performance of paint systems, they should be tested at a variety of locations across the country. Laboratory tests alone are not sufficient to assess how well a particular film preservative will perform in the field.
For evaluating mildew resistance of interior coatings there is some uncertainty about which test method to follow for a realistic assessment of the coating. Testing coatings in interior environments presents different challenges than for exterior coatings. There are several test methods and each claim to be the best for predicting in-service performance. The most popular are Mil Spec 810F, ASTM D 5590 and ASTM D 3273. ASTM C 1338 and ASTM G 21 as well as a test method from the Forest Products Laboratory.
Caution is urged in testing because evaluating coatings on non-porous substrates like vinyl charts that resist wetting by moisture makes it more difficult for fungi to proliferate on the coating surface. Therefore using these charts may over estimate the coating performance.
Most interior paints are used on typical gypsum wall board with paper facing both sides. They absorb more moisture and retain it under humid conditions and thus are frequently susceptible to mold growth. So to evaluate interior paints, ASTM D 5590 with paper as a substrate or a variation of ASTM D 3273 with fungal inoculum deposited directly on the surface of a paper-based wall board may provide more realistic results.
Antimicrobial Polymer Emulsions
The traditional biocidal agents often are lost over time due to leaching or degradation of the film. Recently, driven by the European Biocides Directive, many of these traditional biocides are coming under greater scrutiny regarding their potential affect on the environment. There is also concern regarding the ability of microorganisms to easily adapt to the additives used and build resistance over time. The result is a growing need for alternative solutions that provide sustainable antimicrobial protection while minimizing many of the potential issues related to the use of conventional additives.
There is new, recently developed antimicrobial technology that offers unique ways to mitigate the effects of microbes on products. Unlike conventional active ingredients, the technology relies on antimicrobial polymers that either have inherent antimicrobial characteristics, or incorporate a conventional antimicrobial additive encapsulated or embedded into a polymer.
Since the active ingredient, or part of the molecule that is primarily responsible for the antimicrobial action, is attached to a polymer or uniformly embedded into the polymer at a nanoscopic level, these materials provide a more sustained and effective antimicrobial action over time.
In addition to the antimicrobial action, these same materials can provide polymer-related attributes such as binding, adhesion, barrier and bonding properties that conventional antimicrobial additives cannot. This multifunctional aspect may be of value where an ingredient is required to perform more than one function and thereby help in delivering a simpler and cost-effective solution. The intention for these new materials is thus not a direct replacement of the active ingredients used today but to provide additional benefits that cannot be provided in terms of durability, uniformity, consistency and greater functionality.
Recent data shows that this polymeric route to antimicrobial functionality may be well suited for applications in textiles, nonwovens, medical products, hygiene and personal care products, building materials and a variety of formulated products including coatings and adhesives. The requirements of the specific application determine which of the two approaches provides the best combination of antimicrobial and polymer properties.
Inherent Antimicrobial Polymer Approach
The inherent approach to antimicrobial functionality relies on the fact that the active site is part of the polymer and is tightly bound to the polymer backbone. The polymers are waterborne, easy to handle and environmentally favorable. Also, unlike conventional antimicrobial additives, these polymers are high-molecular weight materials and, therefore, less likely to be of concern regarding potential toxicology. These polymers can be designed to provide additional attributes such as static control, permeability, barrier and strengthening properties, and can be tailored to suit a given application need.
These materials would also be of interest in areas where a conventional AI (active ingredient) may not be desirable. Such applications could include hygiene products, medical devices and products or personal care products where the additional multifunctional aspects of this polymeric approach may be of greater value.
Unlike conventional antimicrobial additives that function by disrupting a biochemical pathway, these polymers function by breaching the integrity of the cell wall of the microbe. It is therefore believed that they are less likely to contribute to development of resistance in the targeted microbes. A variety of active sites and polymer backbones can be designed to suit a given application. Since they are waterborne they can easily be deposited on surfaces by well-known processes such as coating, spraying, saturation or wet end deposition.
Active Ingredient Carrier Polymer Approach
The active ingredient (AI) approach involves the incorporation of active ingredients into a polymer whether it is inherently antimicrobial, as in the case above, or if the polymer is inert. The AI is typically incorporated during the polymerization process in such a way that it is uniformly distributed into the polymer at a nanoscopic level.
This embedding of the AI into the polymer matrix provides complete additive coverage of the surface in a more uniform and consistent manner thus increasing the longevity of the antimicrobial effect. In this way it is possible to get increased efficacy and sustainability from an AI while providing the benefits of a polymer in terms of ease of handling and durability.
It is also possible to use this approach to deliver a concentrated dose of the AI into a given formulation or application, coatings for example, where the AI is finely distributed into a polymeric carrier used only to deliver the AI. This is possible only because the method of incorporation allows high levels of the AI to be suspended into a polymer at a nanoscopic level without affecting the clarity of the polymer film.
This approach also makes it possible to enhance the inherently antimicrobial polymer with AI by creating a “tunable” antimicrobial polymer where the activity can be selectively controlled using either the additive or the polymer as the situation demands.
Polymers containing the AI are waterborne and will have the performance properties associated with waterborne technology. Waterborne polymers are useful in applications where an additive is desirable, and would allow formulators to design sustainable solutions without many of the handling and durability issues related to the conventional additive approach. The choice of polymers and AI that can be used to provide a solution is varied and can be tailored to meet specific needs.
These new antimicrobial technologies that include inherently antimicrobial polymers as well as polymers that have encapsulated active ingredients provide a comprehensive approach to providing antimicrobial benefits across a wide variety of applications where sustainable antimicrobial functionality is needed, combined with the stated benefits of a polymeric offering. These technologies present real value to customers interested in providing the multifunctional benefits derived from a polymer that has inherent or embedded antimicrobial characteristics.
Polymeric Design Approach
A new “antimicrobial paint” developed at MIT can kill influenza viruses that land on surfaces coated with it, potentially offering a new weapon in the battle against a disease that kills nearly 40,000 Americans per year. If applied to doorknobs or other surfaces where germs tend to accumulate, the new substance could help fight the spread of the flu. The coating’s polymers poke holes in the membranes that surround influenza viruses.
The new substance can kill influenza viruses before they infect new hosts. The “antimicrobial paint,” which can be sprayed or brushed onto surfaces, consists of spiky polymers that poke holes in the membranes that surround influenza viruses. Influenza viruses exposed to the polymer coating were essentially wiped out. The researchers observed a more than 10,000-fold drop in the number of viruses on surfaces coated with the substance.
The polymers are also effective against many types of bacteria, including human pathogens Escherichia coli and Staphylococcus aureus, deadly strains of which are often resistant to antibiotics. For example, S. aureus causes serious problems in hospitals, where it can spread among patients and health care workers.
The new coating acts in a very different way from the many antibacterial products – such as soaps, sponges, cutting boards, pillows, mattresses and even toys – that are now on the market. Those products kill bacteria but not viruses and depend on a timed release of antibiotics, heavy metal ions or other biocides. Once all of the biocide has been released, the antimicrobial activity disappears.
With this type of polymeric architecture it is highly unlikely that bacteria will develop resistance because it would be difficult for bacteria to evolve a way to stop the polymer spikes from tearing holes in their membranes. Repeated testing thus far suggests that the microbes are not becoming resistant and the polymer coating is 99% effective.
The MIT researchers are working with industrial and military partners such as Boeing and the Natick Army Research Center to develop the coatings for practical use. Once the polymer coating is applied to a surface, it should last about as long as a regular coat of paint.
Silver Ions
Silver-based antimicrobials are compounds that contain silver whose beneficial properties make it useful as an antimicrobial agent. Such silver-containing compounds are stable in the presence of light and ultraviolet radiation, are thermally stable, offer broad spectral activity against target organisms, and usually have a low order of toxicity, which can be determined from manufacturers’ literature.
The effectiveness of silver products is based on the slow and continuous leaching of superfine silver ions that interact with the metabolism of microorganisms. Silver ions can inhibit enzyme activity, especially those containing sulfur. In doing so, they have a major influence on the energy metabolism of these microorganisms. Products containing silver demonstrate a broad level of antimicrobial effectiveness, however, significantly less activity is observed for attack of fungus when compared to bacteria. Examples of these antimicrobials are products where silver is embedded into base materials, such as special zeolites or glass. Furthermore, combinations of Ag and Zn used in zeolites may lead to synergistic effects.
AgION™
A silver-bearing zeolite antimicrobial compound, known as AgION, has been incorporated into polymer-based coatings to control the growth of harmful bacteria, mold and mildew. These coatings may be applied to stainless or carbon steel products for use in appliance, food processing and HVAC applications. The active ingredients in the coating are the silver ions that are held within an alumino-silicate zeolite carrier particle. The silver ions exchange with other ions (counter ions) that may be present in nutrients or moisture and in this manner are transported to areas that support microbial growth. The antimicrobial properties of the silver ion have been recognized for a long time.
Silver Plus Benzisothiazoline
The combination of silver or silver with benzisothiazoline gives a new and very effective preservative system. By using this combination, the amount of sensitizing benzisothiazoline can be significantly reduced and, if an excess of benzisothiazoline is used, the well-known photosensitivity of silver compounds is reduced. This combination is a highly effective and safe new preservative system for coatings.
SmartSilver™
Coatings enhanced with SmartSilver antimicrobial silver nanotechnology are effectively protected against a wide range of molds, fungi and bacteria, making it a highly efficient industrial antimicrobial. It can be incorporated into aqueous and polar organic solventborne coatings, powder coatings and injection-molded plastics. SmartSilver additives do not impact the mechanical or flame-resistant properties of coatings, and are stable against UV light and high temperatures. The dispersible powders are highly concentrated, silver-containing antimicrobial additives made with proprietary stabilizers, and deliverable as powders or pre-dissolved dispersions. Typical use levels range from 0.1-1.0% based on application and process conditions. The dispersed additives release silver ions when in contact with moisture, inhibiting the growth of microbes.
Ca(OH)2
Ca(OH)2-based surface coatings, now available in a variety of pigmented colors, act to prevent infections – in particular influenza, sinus infections, pneumonia, allergic rhinitis, asthma and some indications are it is beneficial for anthrax as well. These one-application, antimicrobial-antibiotic surface coatings are effective against all classes of microbes including bacteria, viruses, fungus and algae. The coating combines calcium hydroxide as its active biocide with a special Bi-Neutralizing Agent (BNA), a biopharmaceutical whose mode of action keeps the coating continuously working year after year.
Calcium hydroxide inhibits the growth of common microorganisms on the coating’s surface through its high alkalinity (at levels normally incompatible with the life of microorganisms). Normally, hydrated lime is highly susceptible to atmospheric attack – carbon dioxide in the air quickly converts calcium hydroxide to calcium carbonate, reducing its alkalinity and rendering it ineffective. With BNA, calcium hydroxide is safely stabilized by this patented technology into a semi-permeable calcium hydroxide-encapsulated matrix system. This specially engineered matrix system protects hydrated lime from atmospheric degradation, preserving its antimicrobial-biocidal potency long after the coating is applied.
What is unique is its benign nature to humans and its lethal nature to microbes. The active ingredient is coming from a naturally occurring mineral and has been used for centuries as a safe and effective way to kill pathogens.
The end-use product is waterborne, fast drying, virtually odorless and remarkably contains no VOCs. It offers a widespread solution to the problems caused by the presence of the common microorganisms. Using an antimicrobial agent registered by the U.S. EPA, this multi-patented technology works by creating a surface coating that resists the growth of microbes on its surface for over six years.
BLOCK-RESISTANT ADDITIVE
See Anti-Blocking Agent
Chemical agent that prevents the undesirable sticking together or adhesion of painted surfaces under normal or specified conditions of pressure, temperature and humidity.
BODYING AGENT
See Thickeners
BRIGHTENERS (OPTICAL)
The brighteners are usually fluorescent dyes or pigments that absorb UV radiation and re-emit it as violet-blue light that gives yellowish-white coatings a brighter, whiter appearance. They are used to increase the luminance factor and to remove the yellow undertone of white or off-white materials. Small amounts of blue dyes are also used to achieve the same result.
The optical brighteners or fluorescent whitening agents (FWA) are colorless to weakly colored organic compounds. In solution or applied to a substrate they impart bluish-white effects, have good light fastness, and excellent heat resistance and chemical stability.
BURNISH-RESISTANT ADDITIVE
Agent that improves the resistance of a coating to increases in gloss or sheen due to rubbing or polishing.
Lightweight spherical additives called hollow glass microspheres can be used for coatings formulations. These hollow glass microspheres offer scrub and burnish properties, in addition to viscosity control, thermal insulation and sound-dampening characteristics, and improved performance properties. Spherical particles composed of styrene and acrylic polymers are also used for improving mar resistance as well as ceramic microspheres.
High-molecular-weight micronized polyethylene or micronized polypropylene are used in architectural formulations for increased burnish resistance. These waxes effectively replace silicas in both waterborne and solvent systems without settling.
Polishing or burnishing a satin or flat clear coating occurs when silica is used in the formula to flatten the finish look. The problem with silica is that it orientates to the surface of the coating, and when the coating is rubbed, washed or marred, the top layer of silica is quickly worn away. With the use of nano particles, once again a synergetic effect is created between the silica and nano, giving a uniform distribution to the silica. This gives the coating scratch resistance and no more polishing (glossing up).
CATALYSTS
Catalysts are additives that will increase the rate of a chemical reaction but are not consumed or changed in the reaction process. Catalysts have widely varying compositions that depend on the nature of the reaction being catalyzed.Many of the crosslinking reactions used to form durable films are accelerated by the use of catalysts. For example, melamine-crosslinked systems, polyurethanes and epoxies make use of catalysts. Some systems use acids as catalysts: phosphoric, carboxylic or sulfonic acids [such as para-toluene sulfonic acid (PTSA) and dodecyl benzene sulfonic acid (DDBSA)] can be used. Others make use of typical Lewis acid metal catalysts or Lewis tertiary amine base catalysts.
Acid catalysts (and blocked acid catalysts) are used to accelerate the reaction between the crosslinking resin and the primary resin. Good crosslinking is desirable so that the final cured film will show improved properties. By increasing the molecular weight of the crosslinked product, improvements are gained in chemical, humidity and detergent resistance, corrosion resistance, film flexibility and film hardness.
Typical acid catalysts used in coatings are: dinonylnaphthalene disulfonic acid (DNNDSA), dinonylnaphthalene sulfonic acid (DNNSA), dodecylbenzene sulfonic acid (DDBSA), para-toluene sulfonic acid (PTSA), and alkyl acid phosphate (AAP). The relative catalyst strength is: PTSA>DNNDSA>DDBSA>DNNSA>phosphates. In general, the sulfonic and blocked sulfonic acids are strong acids, whereas the carboxylic and phosphates are considered weak acids.
The formulator has to balance the properties of the catalyst with that of the crosslinking agent, the cure temperature and time, the pH of the system, and the desired final properties of the coating. This is not trivial and the raw-material suppliers have guidelines that can be followed.
A new class of blocked sulfonic acid catalysts, derived from aromatic sulfonic acids, has been developed that promotes the crosslinking of hydroxyl-functional polymers with amino-formaldehyde crosslinking agents such as hexamethoxymethyl melamine, especially in coil coatings. These catalysts are particularly effective in coil primer formulations containing calcium ion exchange anti-corrosive pigments. In addition, the unique deblocking profile of these catalysts provides the so-called snap cure at the desired peak metal temperature and within the specified time.
In coil primers, this new class of blocked sulfonic acids provide outstanding cure, viscosity stability upon oven aging, and corrosion/salt spray resistance. In addition to resistance to basic pigment deactivation, these catalysts reduce solvent popping defects and provide excellent adhesion/intercoat adhesion while allowing extended storage of formulated coatings. Products from this class of catalysts also are effective in topcoats where their cure response allows release of volatiles before cure, thereby preventing popping, while providing storage stability.
Dibutyltin dilaurate (DBTDL) and dibutyltin diacetate (DBTDA) are well-established catalysts for the isocyanate-hydroxyl reaction in the formation of urethane coatings. DBTDL is efficient but, as with any catalyst, problems such as reactivity and hydrolysis of ester groups may occur. Diazabicyclo[2.2.2]octane (DABCO) is a commonly used tertiary amine catalyst. The tertiary amines are effective for use with aromatic isocyanates.
There are, however, some unique non-tin catalysts based on bismuth, aluminum and zirconium that are useful for these same reactions. The non-tin catalysts are environmentally more acceptable and offer advantages such as: faster cure rate, improved pot life, improved catalysis in cationic electrocoating and reduced hydrolysis of polyester resins. Catalyst deactivation can occur because of water, resins with high acid numbers, anions and pigments that are carrying water into the formulation.
Oxidation Catalysts
Oxidation reactions are of great interest in fine chemistry, at both the laboratory and industrial scale. There are numerous applications using the conversion of primary alcohols into aldehydes.
Two types of reactions could fit in with this scheme. First of all, there are stoichiometric reactions involving the use of strong oxidizing agents such as: the complex chromium (VI) oxide/pyridine (Collins or Sarret reagent); pyridinium chlorochromate (Corey reagent); oxalyl chloride/DMSO (Swern reagent); dimethylsulphide/N-chlorosuccinimide (Corey-Kim reagent); Dess-Martin periodinane; SO3/pyridine; KMnO4; MnO2; RuO4, etc. Secondly, there are catalytic dehydrogenation reactions with catalysts such as copper chromite, Raney nickel, palladium acetate, etc. All these reactions do not fit in perfectly with the responsible care approach, which is now a priority in all chemical reactions. In effect, they present a number of drawbacks such as a high amount of metal waste, poor selectivity, safety issues in some cases, or harsh conditions.
A new family of green catalysts has appeared in the last few years. TEMPO, or 2,2,6,6-tetramethyl-1-piperidinyloxy radical, is the most representative member of this family, and its efficiency in oxidation reactions is well documented. The oxidation of alcohols into aldehydes, ketones and carboxylic acids uses a catalytic amount of the nitroxyl radical and a stoichiometric amount of an oxidant such as sodium hypochlorite, m-chloroperbenzoic acid, sodium bromite, sodium chlorite, trichloroisocyanuric acid, bis(acetoxy)iodobenzene, n-chlorosuccinimide, or oxygen in combination with CuCl or RuCl2(PPh3)3. The nitroxyl radical is converted into an active species, which is the corresponding oxoammonium ion, and is then able to oxidize various substrates.
Among these substrates, alcohols are converted into aldehydes, ketones or acids; diols into lactones; sulfides into sulfoxides; benzylic ethers into esters; or α-hydroxy-lactame into anhydrides. Nevertheless, the TEMPO or hydroxyl-TEMPO structures present several drawbacks such as poor thermal stability, strong volatility with a tendency to sublimation, high solubility in water with ensuing difficulties in treating the aqueous wastes, non-negligible toxicity, and a complex synthesis route involving several reaction steps.
A new green nitroxyl catalyst called Oxynitrox S100 is on the market and was designed for oxidation reactions. High activities and selectivities are achieved for different types of alcohols and its use can be extended to polyols or carbohydrates. It is classified as a green catalyst as it does not contain any metal. Additionally, it can efficiently replace classic metal catalysts such as copper chromite, chromium derivatives, catalysts based on ruthenium, molybdenum, silver, cerium, etc. It belongs to the family of nitroxyl radicals. It features an oligomeric structure that contains several TEMPO moieties.
Its high molecular weight (between 2000 and 3000 g/mol) makes it particularly suitable for possible recycling without losing any oxidation efficiency. It is generally used in homogeneous conditions, and the high molecular weight allows easy recovery of the end products by simple distillation.
The conditions that are generally followed for the use of Oxynitrox S100 correspond to a biphasic medium. The general procedure uses sodium hypochlorite as oxidant, Oxynitrox S100 as catalyst, dichloromethane, ethyl acetate or toluene as solvent, and sodium bromide as co-oxidant. This co-catalyst leads to the in situ formation of NaOBr, which is a more efficient oxidant than NaOCl. The degree of oxidation of the final product can be controlled by the amount of sodium hypochlorite: when using 1 to 1.3 NaOCl equivalents, or when using 2 NaOCl equivalents, the primary alcohol is respectively converted into the corresponding aldehyde or acid.
Powder Coatings
Caution is advised when using a catalyst in a powder coating formulation. The point in using the catalyst is to increase cure speed during the bake. Excessive catalysis can cause a pre-reaction to occur in the extruder, which in turn causes the formation of ‘gel bits’ or localized crosslinking. These gel particles cause defects in the finished film. Significant crosslinking in the extruder can also damage the equipment.
Catalysts are used at low levels based on the binder – usually around 0.1 – 2.0%. Many of the polyester manufacturers offer resins that contain catalysts and they provide information regarding the bake temperature. Catalysts are also provided as masterbatches to improve distribution into the powder mixture. Typical examples of catalysts for powder coatings are: Lewis base, ammonium salts, dibutyl tin dilaurate, dibutyl tin oxide, stannous octoates, sulfonic acid/amine, peroxides, methyl tolyl sulfonamide, dimethyl stearyl amine, onium, and cyclic amidine. Again the catalyst is specific to the binder type.
A non-yellowing catalyst for uretdione crosslinked powder coatings has been developed that promotes the reaction of polyols and uretdione crosslinkers in powder coatings. This catalyst, K-KAT XK-602 uretdione, is designed to improve on the two most common issues associated with using uretdione technology – high temperature curing and yellowing in the presence of common catalysts.
Cure temperatures can be lowered by 30
COAGULANTS
Coagulants are additives that are useful for wastewater and water clarification. Quite often coagulants are inorganic compounds that are used at low levels of about 1-20 ppm to yield low turbidity influent and effluent waters and wastewater. In the case of wastewater, up to about 100 ppm are used. Some medium-molecular-weight, highly-charged cationic polymers are used as coagulants. These polymers have also been used to de-emulsify oily wastewater, and to aid drainage and fiber retention in paper forming operations.COALESCENTS (COALESCING AGENT)
Coalescing aids have a high boiling point which, when added to a coating, contributes to film formation by way of temporary plasticization (softening) of the vehicle. These aids facilitate the transition from liquid to solid state during the latex drying or film formation process. Coalescing agents have been traditionally used to obtain good film formation. Levels of coalescent affect drying time and ultimate film properties. Important properties of a coalescing agent include: hydrolytic stability, water solubility, evaporation rate, freezing point, odor, color and safety and regulatory concerns.
Architectural latex paints are made from a variety of different polymers that are selected based on performance requirements and cost. The monomers used in these polymers determine the glass transition temperature (Tg), which characterizes the hardness of the final polymer at a given temperature. The Tg and polymer type influence the amount and type of solvent required to coalesce the polymer. Substrate, application, dry time, compatibility, VOC regulations and efficiency all play a role in determining the type of solvent or combination of solvents to be used.
A conventional coalescent temporarily lowers the Tg, providing mobility to the polymer chains. The softened polymer can then flow and fuse with other polymer chains in the system, creating a protective, decorative film. To be effective, the coalescent has to remain in the film after the water has evaporated to ensure that a homogeneous film develops. A conventional coalescent will evaporate out of the film after a period of time and the film will regain its initial Tg and hardness. Various coalescents can be used individually or in combination to help formulators optimize performance in architectural coatings, while meeting VOC regulations.
Typical coalescing aids are compounds such as aromatic hydrocarbons, esters, ester alcohols, glycols, glycol ethers and glycol ether esters. As would be expected, the nature of the polymer – its solubility and affinity for various compounds – will affect the particular coalescing agent chosen. EB (ethylene glycol butyl ether) seems to have been the preferred coalescent agent in many industrial coatings for years. This is not by accident. While it is a fast-evaporating solvent, it appears to also have more of a swelling effect on emulsion particles that some of the other fast-evaporating aids. It is very efficient in lowering the MFFT of emulsions.
Other typical examples of coalescing solvents are: ethylene glycol monobutyl ether (butyl CELLOSOLVE™), propylene glycol n-butyl ether, diethylene glycol monobutyl (butyl CARBITOL™), ethylene glycol monohexyl ether, dipropylene glycol methyl ether, dipropylene glycol t-butyl ether, dipropylene glycol n-butyl ether, ester alcohol or 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate [(TMB)Texanol™], 2-ethylhexyl benzoate. The various trade names can cause confusion at times. Butyl CARBITOL is a low-volatility coalescent and should be used to ensure proper film formation when air drying or using long flash-off times. Low-volatility coalescents are necessary when air drying in high-humidity conditions.
Water-insoluble coalescents, such as hexyl Cellosolve, butyl Propasol™, etc., are hydrophobic solvents and can cause seeding if not premixed with water miscible solvents.
Specific high-purity esters of fatty acids and their blends can also be used to provide low-odor, VOC-free, renewable coalescents to enhance the performance of latexes used in low-VOC paints.
Coalescents based on renewable sources combine various long-chain acids with either a short-chain alcohol like methanol (the most economical option) or higher-chain alcohols and glycols or even glycol esters. Glycerol as an alcohol is not suitable for this use, as glycerol esters can have a negative impact on the adhesion of the paint film. Very short-chained acids like C8 are also not suitable as they contain VOCs. Longer-chain esters with a higher molecular weight form softer films and act as plasticizers. New “green” coalescents that are high in efficiency, low in toxicity and improve early hardness development are a step in the right direction for making better paint, especially outdoor paint.
A high-purity version of propylene glycol mono-oleate based on renewable oleic acid was introduced to meet stringent VOC requirements in the global consumer coatings market. A second-generation propylene glycol monoester with C-18 fatty acid mixtures is available that has better color and offers an improved value. A third product, based on renewable technology, is a high-purity version of linear short chain fatty esters, which is VOC-free based on the European definition but is over 90% VOC by Federal EPA method 24. This product has been found to be over 30% more efficient than trimethyl pentanediol monoisobutyrate ester (TMB) in many popular types of latexes and offers improved hardness development and dirt pick-up resistance. These renewable-based coalescents are naturally derived, low-odor agents and can be used in all types of decorative paints and result in improved performance and application properties, while helping to achieve compliance with new VOC regulations.
New developments in polymer technology have introduced latex emulsions for solvent-free architectural coatings. Architectural paints using this new technology do not require the use of coalescing agents, plasticizers or cosolvents to achieve good film-formation properties. Test results have shown that formulated solvent-free paints maintain the properties of low-temperature appearance, durability, film formation and open time normally expected from quality conventional latex paints. Also, a significant advantage is the low-odor characteristics of these paints, both during and after application. Formulating approaches to solvent-free architectural coatings are now focused on the selection of raw materials that would not contribute any solvents or VOCs to the system. These ingredients include pigment dispersants and surfactants, rheology modifiers, preservatives and substrate wetting agents.
There has also been development of new coalescing agents that are partially or completely free of VOCs. The efficacy of these materials in all varieties of formulations with different polymers has not been fully investigated.
Reduction of VOC using VOC-exempt solvents is not always simple, and formulating with them can be a challenge. These solvents usually bring higher cost and some have very low flash points, which leads to increased shipping cost. Careful formulation evaluations need to be conducted.
COLLOID STABILIZERS
These additives stabilize the very finely divided particles that have a size of about 0.01-1 micron (µm). Compounds such as animal glue, casein, cellulose ethers, guar gum, gum Arabic, poly(vinyl alcohol) and similar compounds are used as colloid stabilizers.
CORROSION INHIBITORS
See Anti-RustCorrosion inhibitors are compounds that improve a coating’s ability to protect aluminum, brass, copper and steel. The term refers to a variety of materials used to prevent the oxidation of metals, including surface treatments, undercoats, and additives or elements alloyed to the surface of the metal. Corrosion poses a major potential problem for metal surfaces that are typically protected through the use of zinc-rich coatings, the use of anti-corrosive pigments and the application of a barrier coat.
Flash rust inhibitors are often used to prevent in-can corrosion during the storage of waterborne coatings. Inhibitors also may prevent the corrosion of ferrous metals during the drying time of waterborne coatings. Sodium nitrite typically has been used in the past. Other types of materials include the following: organic zinc complexes, salts of dodecylnaphthalenesulfonic acid, ammonium benzoate, 2-aminomethoxypropanol and amine neutralized thiosuccinic acid.
Long-term corrosion inhibitors have low water solubility and are often used in combination with anti-corrosion pigments although they may be used alone. Some examples of these materials include: metal salts of aminocarboxylates, salts of dodecylnaththalenesulfonic acids, zinc salts of cyanuric acid, zirconium or amine complexes of toluylpropionic acid, and tridecylamine salts of thiosuccinic acid.
There are also organic, heavy-metal-free corrosion inhibitors to help industry meet regulatory trends toward restricting the use of lead, zinc and chromate. These corrosion inhibitors can improve wetting and adhesion over traditional corrosion inhibitors, and can be used to formulate high-gloss coatings, as well as corrosion-resistant clear coatings.
CORROSION-INHIBITIVE PIGMENT
A pigment that, when made into or added to a paint, has the property of minimizing corrosion of the substrate to which the paint is applied.In terms of corrosion inhibitors, some of the most effective and widely used anticorrosive pigments such as red lead (PbO4), lead silica-chromate (4(PbCrO4· PbO) + 3(SiO2· 4PbO)), zinc chromate (ZnCrO4), zinc tetraoxychromate (ZnCrO4 · 4 Zn(OH)2) and strontium chromate (SrCrO4), have been and continue to be under heavy scrutiny due to the hazards posed to humans and the environment. Lead compounds are deemed toxic, zinc and strontium chromate are classified as carcinogenic and most recently, according to the EU Directive 004/73/CE, zinc phosphate has been determined to be a danger to the aquatic media.
In general, the latest global trend is to design coatings that comply with the environmental regulations that now exist. These “Eco-Friendly” or “Green” coating systems contain only non-toxic, non-reportable raw materials to ensure no hazard to humans and the environment. The industry has found it very difficult to obtain the same level of performance with the “Eco-Friendly” systems as compared to the non-compliant systems.
Coatings that meet the eco-friendly definition are high- or 100%-solids systems, powder coatings, UV or EB curing coatings, low/zero VOC, no heavy metal content, zinc-free or systems that contain no reportable compounds or ingredients in order to meet green label compliant status. Therefore, eco-friendly corrosion inhibitors should not contain heavy metals or non-reportable compounds, and be zinc-free in order to meet the green label compliant standard.
Ever since the use of chromates was restricted, the industry has been forced to use a variety of different non-toxic corrosion inhibitors specifically designed for a given substrate or resin type in an attempt to match the efficiency and versatility that chrome-based inhibitors offered. But now coatings formulators are demanding today’s non-toxic inhibitors offer as much universal application in a wide range of binders and protective coatings as their toxic counterparts.
Zinc phosphate (Zn3(PO4)2 · X H2O) was the first and most widely used non-toxic inhibitor for replacing lead and chrome-based inhibitors. Historically, standard zinc phosphate has demonstrated acceptable performance in real outdoor exposure, but less efficiency compared to chromates in marine environments and in accelerated weathering tests such as salt spray and cyclic corrosion (i.e., prohesion). However its user-friendly, low cost, universal application, and good package stability in a variety of general-purpose industrial and protective coating applications, made zinc phosphate the most popular choice early on for replacing chrome and lead-based inhibitors
Today, in order to meet the eco-friendly labelling demands, zinc phosphate and modified zinc-containing inhibitors can no longer be used. This has caused yet another dilemma for the inhibitor suppliers as the current offering of non-zinc inhibitors on the market have generally shown inferior anti-corrosion performance in accelerated corrosion tests, especially on steel substrates, as compared to most zinc-based inhibitors. Also, current zinc-free inhibitors are very limited in their application scope, performing well in some coating systems and poorly in others.
Today in the marketplace there are available mixed metal calcium-strontium phosphate complexes deposited on silicate cores that provide good overall performance. Also an organo-modified version of this same type of complex produces additional advantages in the areas of film formation, adhesion promotion and substrate wetting.
The basic formula for these products is: [Sr·P2O5·SiO2·XH2O], which represents a mixed metal phosphate complex on a silicate core. These complexes provide excellent direct anodic inhibition from the combination of the calcium and strontium cations but also provide good cathodic inhibition due to the basicity/alkalinity of the silica core. Its basic nature reduces the amount of oxygen needed to passivate the formation of rust.
The organic surface treatment shows improved mechanical properties in terms of better wetting in organic systems without decreasing its performance in waterborne systems, reduced pigment/binder interface, which makes the flow of water and electrolytes through the organic coating difficult and at the same time protects the pigment making it more inert when reactive resins or those with high acid values are used.
COUPLING AGENT
Coupling agents are compounds that promote adhesion between dissimilar compounds. These compounds are often used for the surface modification of fillers, wherein they attach themselves to the filler by means of a hydrolysis reaction and then leave a functional group available for reaction with the coating, ink or adhesive. The main general classes of coupling agents are comprised of the silanes and titanates.Most of the organosilanes have one organic substituent and three hydrolyzable substituents. For surface treatment applications, the alkoxy groups of the trialkoxysilanes are hydrolyzed to form silanol-containing species. The final result of reacting an organosilane with a substrate ranges from altering the wetting or adhesion characteristics of the substrate, utilizing the substrate to catalyze chemical transformations at the heterogeneous interface, ordering the interfacial region, and modifying its partition characteristics. It includes the ability to effect a covalent bond between organic and inorganic materials.
Titanates, zirconates and aluminates are used as coupling agents. Titanates act as organometallic chemical bridges between two dissimilar phases such as an inorganic pigment and a polymer binder – or as an adhesion promoter for a coating on a metal substrate. They provide an alternate to silane coupling agents and couple to non-silane reactive substrates such as CaCO3, carbon black and phthalo blue.
Titanates have six functions.
1. Coupling to form atomic monolayers on inorganics rendering them hydrophobic and organophilic, thus allowing complete dispersion and deagglomeration of pigments and extenders with minimum shear and work energy. They promote adhesion, significantly lower system viscosity and shift the critical pigment volume concentration (CPVC).
2. Catalysis to lower bake times and temperatures; to induce increased strain strength to the polymer binder to increase mandrel flexibility and reverse impact strength; to compatibilize dissimilar organics; and to synthesize polyesters.
3. Heteroatom to phosphatize and prevent corrosion or intumesce and add flame retardance.
4. Alkyl/aryl functionality to create polarity for adhesion and compatibilization.
5. Thermoset functionality to densify the degree of crosslink and hardness.
6. Molecular structuring to create stable organometallics.
Designer waxes are also excellent coupling agents and since they are produced by metallocene catalysis it is possible to tailor their properties to individual applications. They are important additives in wood-plastic composites (WPCs). WPCs are a new type of natural fiber composite with up to 90% wood fiber or wood flour content. The plastic matrix is usually polypropylene or polyethylene, although polyvinylchloride is also used. Since wood and plastic differ, especially in their polarity, suitable coupling agents such as polyolefin grafted with maleic anhydride must be added. They significantly improve the mechanical parameters of tensile stress and bending load in WPCs. The polar functional groups of the coupling agent react with the OH groups of the wood and form a genuine chemical bond. The non-polar areas of the coupling agent have strong affinity to the non-polar polyolefin chains.
Solvents that cause two immiscible liquids to homogeneously mix are also referred to as coupling agents.
CRAZE-RESISTANCE ADDITIVE
A chemical that will prevent the formation of fine random cracks or fissures on or under a surface of plaster, cement, mortar, concrete, ceramic coating, or paint film caused by shrinkage.
CROSSLINKING AGENT
Crosslinking agents are compounds that convert a thermoplastic into a thermoset. They are multifunctional chemical compounds that react with functionality on the macromolecular chains and thereby form a thermoset or three-dimensional polymeric material. The crosslinking agents are of di- or higher functionality, and they become an integral part of the final thermoset material except for materials lost in a condensation crosslinking process, for example. These agents can range from low-molecular-weight to polymeric materials. Examples of crosslinking agents are melamines, such as hexamethoxymelamine, guanamines and other aminoplasts, isocyanates, epoxides, amines, and so forth.Epoxy-functional silane crosslinking agents for waterborne coatings are also available that will crosslink under heat or at room temperature on alkaline substrates if used in combination with recommended catalysts. These are designed primarily for carboxyl- or amino-functional acrylic latexes or polyurethane dispersions. The mechanism of crosslinking involves the epoxysilane’s dual chemical functionality. The epoxy portion of the molecule is reactive with the matrix resin and the alkoxysilane portion crosslinks after hydrolysis by condensation forming siloxane bonds. The alkoxysilane also can react with surfaces to improve wet adhesion of the coating, or with fillers to improve pigment binding.
Crosslinking causes changes in physical and chemical properties. That is, it causes changes in hardness, tensile strength, modulus, elongation, solubility, swelling and other properties. For example, crosslinking renders a soluble or thermoplastic polymer into a thermoset polymer that is three-dimensional in nature and insoluble. However, the crosslinked polymer system may be swellable to different degrees in liquids that may have been solvents for the uncrosslinked polymer. The greater the degree of crosslinking or the number of bonds formed between chains, the lower will be the degree of swelling in any particular solvent.
UV-Powder Coatings
The latest development in powder coatings is the combination of powder coating technology and UV technology. This technology is making strong inroads especially for temperature-sensitive substrates. The most suitable approach for a coating formulation is the use of a major binder and a crosslinker. The crosslinker may control the network density for the coating, while the binder determines properties of the coating such as discoloration, out door stability, mechanical properties, etc. Furthermore this approach will lead to a more homogenous concept in powder coatings applications as a category bringing similitude to thermosetting coatings where crosslinkers such as TGIC and β–hydroxyl amides are used. A crosslinker should present properties quite specific for the application intended: molecular weight, high functionality, and physical properties compatible to the application.
The existing UV-curing powder coating systems generally build on two binders. There is an option to the existing systems that consists of using unsaturated amorphous or amorphous-crystalline polyesters comprising maleic/fumaric moieties as reactive double bonds in combination with crystalline crosslinkers containing unsaturated reactive groups.
PSG di-acrylate (CAS number: 85286-82-4) is rated as having high UV-DSC reactivity in powder coatings formulations. Coatings using a crystalline acrylic ester may be easily formulated using as a starting point any unsaturated polyester of maleic/fumaric type. Addition of a small quantity of PSG-diacrylate may improve the performance in commercial systems of lower reactivity. This approach appears to be a reliable option in UV powder coating formulations and makes possible the use of typical unsaturated polyesters as binders in powder coatings.
CURING AGENT
See Hardener
Additive that promotes the curing of a film. Anything that promotes, enhances or ensures mechanical or optical property development of a coating (usually liquid) by means of chemical and/or physical change such as through evaporation, polymerization or other means. The curing agent can be anything such as thermal energy, radiation energy, a catalyst, various substances or other means to enhance such property development. Some curing agents are termed “hardeners.” Crosslinking agents are also curing agents.
Recent advances in new amine-functional curing agents have improved performance of waterborne two-package epoxy coating systems. These new amine adducts offer performance advancements along with the ability to meet near-zero VOC levels. Those that are designed for cementitious applications in industrial and do-it-yourself markets demonstrate water-reducible epoxy coatings with superior handling, application properties, improved resistance to chemicals and staining and visible end of pot life.
DEAERATORS
Deaerators prevent microfoam and pinhole formation in a coating. They are very important in high-viscosity and high-solid systems, and absolutely vital for airless applications. Silicones (polydimethyl siloxanes), mineral oil, organic polymers (polyethers, polyacrylates), modified polysiloxanes (such as polyether-modified polysiloxane with hydrophobic solids) and fluorinated silicones are examples of materials that will function as deaerators.In waterborne systems, foam occurs as both macro- or microfoam and is manifested as pinholes and air bubbles in the dried film. Macrofoam is readily seen as large bubbles often held together in a honeycomb-like structural formation. Microfoam is often recognized as fine spherical shaped bubbles.
For solventborne systems, microfoam is often a problem and pinholes form from small air bubbles, which create channels as they move through the drying film. During the drying process the viscosity of the coating increases and the channels remain open and fixed in the dried film. Pinholes severely reduce protection properties of a coating, and they may allow salts and moisture to penetrate the coating and deleteriously affect the substrate.
Microfoam is caused by air entrapment during manufacture and/or application, during the mixing of two-component systems, or from byproducts formed from chemical reactions. Deaerators work by coalescing small bubbles into larger bubbles that can rise to the surface considerably faster than microbubbles.
DEFLOCCULANT
See Dispersant or Dispersing Agent
A material that prevents pigments in suspension from forming floccs.
DEFOAMER
See Deaerators for discussion on microfoam.Additive used to reduce or eliminate foam in a coating or coating constituent. The terms ‘defoamer’ and ‘antifoaming’ agent are often used interchangeably. In fact, they are not quite the same. A defoamer is a surface-active agent that stops the foam and breaks the bubble once it has been formed. It is a bubble breaker. An antifoaming agent prevents the formation of foam so it never forms.
The term “foam-control agent” is a more appropriate term to use. In an aqueous formulation, it is almost impossible (at acceptable use levels) to totally eliminate all foam. The correct foam control agent will help to prevent foam formation, but more importantly, it will allow the dried film to be free of foam and any resultant film defects that might result from an air void in a film.
There is a difference between macrofoam and microfoam. Macrofoam is located mostly on the coating surface and is surrounded by a duplex film with two liquid/air interfaces (double layer), whereas microfoam occurs inside of a coating film (air entrapment) and is characterized by a single liquid/air interface. These two types of foam also differentiate defoamers from deaerators. Defoamers are mostly effective against macrofoam, whereas deaerators suppress microfoam. In practice, the terms are frequently confused and used interchangeably. Many of the commercial products are optimized to prevent macro- as well as microfoam.
Both kinds of foam impair the surface optics of the coating and cause surface irregularities, as well as reduce gloss and transparency. Microfoam also adversely affects the coating’s protective properties because the effective film thickness is reduced and pinholes can form from the micro bubbles.
The function of defoamers is based on disturbance of the double layer of the macrofoam lamella. Substances with very low surface tension are used as they are not wetted by the foam bubble. Foam-stabilizing substances move away from the defoamer droplet, which finally causes collapse of the bubble. Surfactants are often used with defoamers to improve the spreading of the defoamer droplet on the bubble surface.
Foam may be introduced at various stages of manufacture and use of the coating. The raw materials used to make a coating, such as surfactants, dispersants, etc., enable foam to form. Entrapped air, or foam, is introduced into the manufacture of most paints as part of the process. Manufacturing care must be taken to avoid entrapping air during production by choosing the correct stirring equipment and stirring conditions. Letting the product stand for as long as possible is also helpful in preventing air entrapment.
High levels of foam may occur during the milling stage, and defoamers are often used as a component in a grinding paste. Due to the activity of some surfactants at the air/water interface, foam is often created and stabilized in both the pre-mixing and milling chambers of dispersing equipment. Foam slows down the process of dispersion and adversely affects the moisture resistance of the coating. Using silicone anti-foam agents may present an additional potential for surface defects.
Foaming occurs during application to some degree, depending on the method of application. For example, curtain coating carries entrapped air continuously around in the system. Entrapped air also occurs with airless spray systems. Airless spray does not use compressed air. Paint is pumped at increased fluid pressures through a small opening at the tip of the spray gun to achieve atomization. When the pressurized paint enters the low-pressure region in front of the gun, the sudden drop in pressure causes the paint to become an aerosol. Airless spraying has several distinct advantages over conventional air-spray methods. It is more efficient than the air spray because airless spray is less turbulent and, therefore, less paint is lost in bounce back. The droplets that are formed are usually larger than conventional spray guns and produce a heavier paint coat in a single pass. The system is also more portable, production rates are nearly double and transfer efficiency is usually greater. Other advantages include the ability to use high-viscosity coatings and to have good penetration in recessed areas of work pieces. One major disadvantage of airless spray is that pinhole formation from air entrapment is possible. Air-assisted airless spray is similar to airless application except that a small amount of atomizing air is used to further improve coating atomization. Spray application in relatively low humidity conditions or in high-temperature conditions can increase the tendency for foam entrapment.
Latex paints are stabilized with surfactants that easily generate foam under agitation. Elimination of this foam is essential for the manufacturing process, for storage and for good application properties. Foam reduction can also be somewhat controlled by optimizing the settings on a spray gun and by adjusting the viscosity/solids level in the formulation.
Foam is a dispersion of a relatively large volume of gas in a small volume of liquid. Gases are soluble in liquid media to different extents and are influenced by temperature. As the paint film starts to dry, the dissolved gases try to escape in the form of bubbles. A bubble, as a sphere, requires the least amount of surface energy. Large bubbles rise faster than small ones and collect on the surface. They are often covered by a surface film of surfactant or other additive in the coatings system. On the surface, the bubbles pack side by side as densely as possible. In some systems, densely packed microfoam can form on the surface and remain there even after the film has cured. This is possible for high-build systems like some plastisols.
In the process of bubble escape, tiny pores may be formed in the film. In lower solids films that dry quickly, the viscosity of the coating is increasing quickly with drying. As this is happening, the smaller micro bubbles are still rising to the surface but quite slowly; in the process these bubbles can form small channels. If the rising bubble penetrates the surface, lack of flow allows for the formation of pinholes – a surface defect. Sometimes these microbubbles cannot penetrate the surface, but they will push a very thin, viscous layer of coating to the front surface. This layer will remain on the surface after drying or curing and becomes a spherical blister.
Good defoamers not only need to be good bubble breakers, but they need to be able to keep the action sustained and maintain good defoaming over time in both oven and room temperature aging studies. Good defoamers need to be insoluble in the foaming system. If the product is too soluble it will only increase foaming. Defoamers need to have excellent dispersibility throughout the systems. Spreadability is the ability of the product to spread evenly and uniformly on the surface, coating the bubble particles and eliminating them. They work by lowering the surface tension around the bubble and cause them to coalesce to larger bubbles and eventually to break.
Most foam-control agents for aqueous systems consist of carrier, actives (hydrophobic materials) and other additives that enhance spreading, compatibility, product stability, etc.
Some examples of carriers include mineral oils, vegetable oils, glycols, glycol ethers, alcohols, silicone oils and water. Three types of actives are most common: hydrophobic silica, hydrophobic silicone and organic materials that are hydrophobic/lipophilic. Many foam-control agents are blends of the above actives. The other additives vary from surfactants, co-solvents, thickeners, etc.
The actual foam-control agent that functions the best is based on its spreading rate, compatibility, persistency and cost/performance. Many of the above factors oppose each other in a formula. For example, the most compatible is usually the least persistent; the least compatible often spreads the best; etc.
Many defoamers are colloidal suspensions of particles that act as seeds to allow bubbles to collect and burst. These types of additives can be simple alcohols, oils or complex silicone oils on fine particle silicas. Defoamers should spread instantaneously on the bubble surfaces and into the underlying layers, and cause immediate rupture of the bubble.
Silicone defoamers are usually based on a polysiloxane-type structure. For example, there are acrylate-functional polydimethylsiloxanes, polyether-modified polydimethylsiloxane, etc. To select the proper antifoam, the formulator needs to be aware of the nature of the foaming agent, the foaming tendency, the solubility and concentration, pH, temperature and viscosity of the system. Each of these factors has a direct influence on the antifoaming agent of choice. Some examples of other compounds that function as defoamers are waxes, fatty acids, aluminum stearate; amyl, capryl, decyl, nonyl, and octyl alcohols; caster, corn, mineral, pine, silicone, and turkey red oil; palmitic and stearic acid diethylene glycol monolaurate, sulfonic acid salts, tributyl citrate, and tributyl phosphate.
Some foam-control agents have an effect on gloss; but not all do, so the formulator must carefully evaluate this effect. Color acceptance of emulsion paints is also important, and some defoamers can have a negative influence on color development that needs to be evaluated.
Foam-control agents are very formula/system dependent and the best way of selecting them is to test them. This is usually done quickly in the lab by adding various defoamers to a known volume of paint sample, shaking for a set time, and then measuring the height of the foam that is generated. The method actually works quite well.
Foam-control agents should be used with some degree of caution to minimize cratering. It is important for wood sealers and topcoats that the defoamers used do not cause surface defects or create haziness in the final finish. Incorrect use of antifoaming agents can cause craters, fisheyes, floating, flooding, crawling, etc.
New, liquid, mineral-oil-free defoamers based on renewable raw materials have been introduced into the market. These new defoamers are ideally suited for use in the manufacture of synthetic latex, waterborne matte to satin finish architectural coatings, plasters, and aqueous adhesives. They display remarkable performance characteristics retaining defoaming capabilities even after prolonged paint storage. Some of these new materials may be used as an alternative to replace tributylphosphate (TBP).
Another approach has been to replace mineral oil with natural oils like soybean, rape seed, sunflower and rice bran oils.
There are no universal defoamers, although certain types function better with certain emulsions, certain plant processing requirements, etc. Most suppliers are willing to provide technical support to make the selection process easier.
DEGASSING AGENTS
Specific chemicals that allow for the release of volatiles in a molten powder film. Volatiles can be interstitial air, blocking agents, and low-molecular-weight polymeric fractions.
Benzoin is typically the choice for degassing powder coatings. In the absence of benzoin, the air bubbles start to shrink very slowly as a result of a diffusion-controlled process. Quite remarkably, in the presence of benzoin, the bubble shrinkage process is accelerated to such an extent that most air bubbles disappear before any significant increase in the viscosity occurs due to the curing of the coating. This suggests that benzoin functions by accelerating the rate of bubble shrinkage. Yellowing side effects of benzoin on powder coatings may be associated with the formation of benzyl, the common oxidation product of benzoin.
DENATURANT
An unpleasant or toxic substance that is deliberately added to make a product unfit for human consumption.
DETACKIFICATION AGENTS
Paints used in automotive finishing operations are a tacky material and tend to adhere to the surfaces of spray booths, particularly in the sump and drain areas. To maintain the design intent of the paint spray booth the paint overspray must be constantly removed from the sump to prevent clogging of the sump drain and recirculating system. In order to assist in the removal of the oversprayed paint from the air and to provide efficient operation of the down-draft, water-washed paint spray booths utilize paint detackifying chemical agents. The detackification products are commonly introduced into the water that is recirculated in the paint spray booth system.
Paint spray booths are typically 100 – 300 feet in length and usually contain many robotic and manual spray zones. The temperature and humidity are rigorously controlled in these systems. As vehicles are painted in these booths, a certain amount of paint does not contact the article being painted and forms a fine mist of paint in the air space surrounding the article. This paint must be removed from the air. To accomplish this, the contaminated air is pulled through the paint spray booth by exhaust fans. A curtain of circulating water is maintained across the path of the air in such a way that the air must pass through the water curtain to reach the exhaust fans. As the air passes through the water curtain, the paint mist is “scrubbed” from the air and carried to a sump basin (sludge pit) usually located below the paint spray booth. In this area, the paint particles are separated from the water so that the water may be recycled and the paint particles disposed of as paint sludge.
The paint detackifiers, or “denaturants” commonly added to these systems are either melamine-formaldehyde-based or based upon acrylic acid chemistry. The innovative nature of the chitosan detackifier (BC4200NP technology) lies in the fact that it is derived from chitin, the waste product of food production, namely shell fish harvesting. The solid chitin derived from these operations is treated with sodium hydroxide at an elevated temperature to produce chitosan, also called poly(glucosamine), that represents a deacetylated chitin. The chitosan produced in this way yields a glucosamine polysaccharide structurally similar to cellulose. The degree of deacetylation can be controlled by temperature and reaction time. The chitosan produced, also in a solid state, is not readily soluble in water, but can be rendered more water soluble by the addition of various acids such as acetic, sulfuric, hydrochloric, citric, sulfamic and mixtures thereof.
On a pound for pound basis, the BC4200NP technology is either cost neutral or less expensive than the current paint detackifiers in the market place. Further, field studies indicate that the BC4200NP technology may reduce the overall cost of such programs by lowering detackifier usage and by decreasing the use of ancillary chemicals, i.e., liquid caustic and biocides.
The melamine-formaldehyde detackifiers that BC4200NP is replacing are derived from non-renewable natural gas supplies and contain residual amounts of free formaldehyde as a necessary consequence of the resin production operation. The acrylic acid-based detackifiers are derived from non-renewable crude oil feed stocks and their price is therefore subject to the global oil market. Further, since the chitosan-based is less acidic than the traditional products, less sodium hydroxide is necessary in an operating system to control pH resulting in less overall chemical usage. Additional field studies have also indicated that less detackifier is necessary to treat a given amount of paint and this also makes the BC4200NP more attractive from an application cost perspective.
The environmental advantages of this technology are summarized as follows: no residual free formaldehyde: raw material is not derived from natural gas and/or crude oil and therefore does not utilize non-renewable resources; it is obtained from the waste products of food production (crab, lobster and shrimp shells); low acidity level; chitosan, the main component, has anti-microbial properties.
DESICCANTS
Desiccants are materials that have a high affinity for water absorption and are used as drying agents. The most commonly used desiccants are silica gel or calcium oxide.
DETERGENT
A detergent is a surfactant that is used to clean soiled surfaces. They may be anionic, nonionic or cationic.
DILUENT
See Reactive Diluent
DISPERSANT/(DISPERSING AGENT)
See SurfactantsA dispersant is an additive that increases the stability of a suspension of powders (pigments) in a liquid medium. The pigment-dispersing step is the most difficult and time/energy consuming part of the paint manufacturing process. This is because of the difference in surface tension between the liquids (polymers and solvents) and the solids (pigments and extenders).
Pigments and extenders are often received as agglomerates and then are subjected to a grinding process that incorporates the pigment into the vehicle during paint manufacture. During the grinding the agglomerates are dissociated into a dispersion of particles. In the process, dispersants (surfactants) are used that mainly prevent reassociation of the pigment and extender particles. The dispersants are adsorbed onto the particles and hinder close approach of particles by charge repulsion effects (ionic dispersants) or by steric effects (nonionic dispersants). Wetting and dispersing agents are used to stabilize pigment dispersions. They often use steric hindrance to avoid flocculation of the pigment particles.
Since a dispersant is used to help the suspension of fine particles of a solid in a liquid phase, it must be able to totally wet the solid particle and also be able to interact with the dispersing medium. Dispersants act by being absorbed on the surface of the pigment particles with the other end of the molecule exposed and usually charged. Since like charges repel, the individually coated pigment particles repel each other and stay uniformly dispersed. Because the individual pigments are all so chemically different – some are attracted to water and some are not – the dispersants have to be chemically different. When changing pigmentation in a formula, the dispersant needs to be checked quite carefully to see if it is stable in the formulation. Good dispersion provides the end user with better hiding and color stability.
Pigment dispersants are necessary to improve the wetting of the pigment and enhance its suspension. The choice of dispersant depends on the nature of the pigments and if the coating is a waterborne or solventborne system. Dispersants are available for organic and inorganic pigment systems and for both solvent- and waterborne systems. The proper choice of dispersant can be useful for obtaining higher gloss, hiding power and lower viscosity.
Pigment dispersion is one of the most important, critical and complex steps in paint manufacture. Usually pigments are dispersed in a mill base. A mill base is a concentrated mixture of pigments, a small amount of the binder and solvent and a wetting and dispersing additive.
In the wetting stage, the liquid in the mill displaces air from the pigment surfaces. Shear force, applied by a mill, is necessary to break down pigment agglomerates. As pigment particles become adequately dispersed, dispersants that are now on the surface of the pigment particles prevent reagglomeration of the pigment.
Because of their hydrophobic nature, organic pigments are difficult to disperse in aqueous media. Surface-active compounds that are amphoteric in nature are usually effective for both organic and inorganic pigments that are particularly hydrophobic.
In aqueous systems, dispersants must disperse the pigment and form an affiliation between the pigment and the dispersing medium that is water. There are several types of dispersants commonly used in latex paints, for example, phosphates (such as potassium tripolyphosphate) and several sodium phosphates (sodium hexametaphosphate, sodium polyphosphate, sodium phosphate (tribasic). The potassium salt is preferred because it is less soluble. Ionic types such as the sodium salts of water-soluble polymers are also very common.
There are ionic water-loving (hydrophilic) and oil-loving (lipophilic) types available on the market. The hydrophobic part of the molecule is attracted to the pigment particle, and surrounds the particle with an ionic layer. Pigment separation by mutual repulsion takes place. These cannot be relied on for in-can stability and need to be used then with nonionic surfactants.
Ionic and anionic surfactants are also essential for good dispersion, and contribute to the latex paint system by helping to optimize hiding power, flow, and leveling. The amount of dispersant used is very small and is directly related to the total surface area of the pigment used. Again, too much or too little of these agents can affect in-can stability and other properties of the paint.
There are new nonionic deflocculation dispersing and wetting additives for waterborne coating systems that have been designed specifically to provide superior performance in wetting, dispersing and deflocculation of pigment particles and to comply with environmental regulations. These are zero-VOC and alkyl phenol-free to allow reformulation of existing platforms to comply with new requirements.
Pigments are usually easier to disperse in solventborne resin systems; however, higher solids systems will sometimes show pigment flocculation problems. This can be the result of the lower-molecular-weight polymers or oligomers having less of a buffering effect on the pigments and thereby permitting closer pigment-to-pigment contact. Amphoteric dispersants have been developed that are effective for preventing flocculation of carbon black and other organic and inorganic pigments in higher-solids systems. These dispersants have also been able to keep viscosity from increasing in some coatings where the polymer’s solubility is causing rheology problems.
The most widely used wetting agents in the past were primarily alcohol and alkyl phenol ethoxylates used alone or blended with anionic surfactants. Alkyl phenol alkoxylates (APAOs) include octylphenol (OPEs) and nonylphenol (NPEs) ethoxylates. These were preferred as wetting agents because they were cost effective and provided a number of structural, compositional and performance attributes. APEOs have become less acceptable due to the effects of degradation products on aquatic life forms and their potential effect of fertility of organisms. Consequently many manufacturers and raw material suppliers are gradually phasing out APAO compounds despite the absence of any legislative guideline prohibiting their use. However, by exploiting narrow--range ethoxylation and newer alcohol feedstocks, the properties of non-APEO surfactants are able to match those of the APEOs.
Some typical dispersing agents for organic and inorganic pigments are salts of monosulfonic acid such as: calcium or zinc sulfonate. Acetylenic diols are also sometimes added. Another class of dispersing agents is known as the aromatic ethoxylates. This type of molecule contains a relatively large anchor group that is a polycyclic aromatic structure. No amine or acidic groups are present, so the compound is pH neutral. Aromatic ethoxylates allow high pigment loading and do not tend to foam. Water resistance and gloss retention are both good.
Another widely used class of dispersants is based on soya lecithin. These have been used for many years as pigment wetting agents, grinding aids and dispersion stabilizers in oleoresinous formulas. This class of agent is also available in water-soluble or water-dispersible forms. These are used in much the same applications in latex formulations. The water-compatible versions are also very useful in formulating universal colorants and as agents in both water and solvent formulas to improve universal colorant compatibility and color development.
Polymeric dispersants (hyperdispersants) are a new class of dispersants that have significantly higher molecular weight than conventional dispersants. These dispersants provide improved pigment stability and increase tinctorial properties through their ability to wet and stabilize pigments. The result is cost improvement through faster dispersion, which results in increased throughput, lower pigment loading requirements due to enhanced color strength, and good rheological properties as well as low foaming characteristics. Performance is enhanced due to resulting high gloss, good flow and leveling (surface properties), and improved transparency.
Polymeric dispersants contain polymeric chains for stearic stability in solution and they must be able to be strongly absorbed onto the particle surface. Many functional polymers and/or copolymers are capable of functioning in this fashion. Examples are polyurethane or polyacrylate with tertiary amine functionality, or polyester.
High-molecular-weight dispersants are linear or branched chain polyurethane or polyacrylates that have molecular weights between 5,000 and 30,000 g/mol. These dispersants have pendant anchoring groups, which adsorb onto the surface of the organic pigment particle by hydrogen bonding, dipole-dipole interactions or Van der Waals forces. The remainder of the polymeric dispersant is large enough to cause steric stabilization. The level of dispersant is important because performance depends on the optimum amount of saturation by the dispersant of the pigment surface. Polyurethanes function well for viscosity depression in the mill base, which can lead to higher pigment loading. The polyacrylates are more compatible in nonpolar and highly polar systems.
Controlled polymerization techniques such as GTP (group transfer polymerization) and controlled radical polymerization (CRP) techniques such as ATRP (atom transfer radical polymerization), NMP (nitroxyl-mediated polymerization), and RAFT (reversible addition-fragmentation chain transfer) provide opportunities to customize polymers in a way that is not possible with conventional random copolymerization methods. Each of these techniques has its own characteristics that define to a certain extent the properties of the resulting polymeric additives. These techniques are applied for the synthesis of custom-made structured polymers. Compared to free radical polymerization (FRP) they all allow much better control over polymer structure and molecular weight distribution and they all have their specific characteristics.
Due to the fact that the chain growth mechanisms are different in GTP and CRP, it is clear that the resulting polymers will have different properties and these differences also can show up in their performance as wetting and dispersing additives. Controlled polymerization techniques are not only a tool to synthesize block copolymers, they also make other polymer architectures accessible. Gradient copolymers are one of the polymer architectures that possess a continuous transition of the properties of the respective monomers along the whole polymer chain; these polymers have very unique properties for pigment wetting and stabilization.
In random copolymers the pigment anchoring areas and binder-compatible areas are randomly distributed and generally too small for a strong stabilization effect. In block copolymers (as well as in graft copolymers) the two different areas are large enough for good stabilization of the particles in a pigment dispersion. But the strict separation of the properties in the polymeric structure can cause other drawbacks. Such polymeric structures may have a strong tendency toward foam stabilization. Micelle formation, mainly due to the incompatibility of the anchoring groups with the liquid medium, is also observed and pigment wetting with the additive can become difficult.
The gradient copolymer performs better because there is a gradually increasing concentration of anchor groups along the polymer chain. This obviously helps to increase compatibility, reduces micelle formation but still allows excellent steric stabilization of the pigment particles. These structural differences have a strong impact on the performance of the polymeric additives as wetting and dispersing additives.
Polymeric wetting and dispersing additives are useful for all types of pigments, whereas low-molecular-weight structures with only one or few anchor groups are mainly used for inorganic pigments.
Silicone and silane pigment-treatment additives can cause pigments to disperse evenly throughout coatings formulations. The materials can also help reduce pigment floating from application through cure, producing consistent color. They are effective in low concentrations, minimizing recoatability concerns.
In an ink formulation the pigment must be dispersed and stabilized to achieve good color strength, gloss, transparency and transfer efficiency. Stable, concentrated dispersions with small particles and narrow particle size distribution potentially can lead to higher gloss and color strength per unit mass of pigment. The push towards waterborne formulations and increasing pigment loading underlines the need for good dispersion and stabilization properties.
The use of polyether amines (PEAs) as dispersants for carbon black has been investigated. PEAs contain primary amino groups attached to the terminus of a polyether backbone. The polyether backbone is based either on propylene oxide (PO), ethylene oxide (EO), or mixed EO/PO. Adsorption, contact angle, and color density measurements indicate that a polyether amine having a high ethylene oxide content and high molecular weight provides the best performance. Surface tension and interfacial tension values decrease with decreasing ethylene oxide. Molecular weight has little impact on surface tension and interfacial tension. Low HLB dispersants (below 5) exhibit poor dispersion stability.
Pigment settling is not always prevented by using wetting and dispersing additives, but there is a noted distinction between ‘hard’ and ‘soft’ settling. Soft settling may result from weak flocculation of properly wetted pigments and stirring can easily redisperse the pigment. Hard settling occurs due to improper adsorption of dispersant. The result is reagglomeration of the pigment that can only be corrected with high-shear mixing.
DRIERS
A drier is a compound that catalyzes or accelerates the drying (curing) of oil, paint, printing ink or varnish, or the crosslinking of polymers or drying oils. Driers are usually metallic – metal carboxylates.Driers are not the same as curing agents, which chemically react with functional groups in the polymer. Driers are catalytic in nature and do not chemically react with the polymeric material. Driers promote or accelerate the drying, curing or hardening of oxidizable coatings vehicles. The most common driers that have been used in paints are metallic salts of monocarboxylic acids, usually C8-C10 branched acids, such as naphthenic acid, and neodecanoic acid as an example. The choice of acid does not seem to have an effect on the drying. The choice of acid primarily affects solubility, stability and efficacy. The acid makes the metal soluble in the resin system. Insoluble salts, such as acetates or chlorides, do not function as driers. Drier levels are always expressed as percent metal on resin solids. Use levels are generally in the ranges of 0.01-0.6%. The metal in all cases is the cation and has the capability of more than one oxidation state.
The drying of oil-based paints occurs through a process that is characterized by oxygen absorption, followed by the formation of peroxide, and subsequent peroxide decomposition. The presence of driers in the paint accelerates the oxygen absorption and the resultant drying of the film.
Traditional alkyds absorb oxygen into their double bonds and break those bonds to form free radicals that can undergo polymerization to give a dry film. Driers accelerate this process, improving the rate of absorption and utilization of the oxygen. Alkyds would dry to a soft film without driers over several days. Driers provide tack-free times of a few hours and hard dry overnight. A metal carboxylate drier catalyzes or promotes the crosslinking of resin polymers or drying oils.
Driers are classified as:
1. Oxidation (catalytic, top driers or surface driers). Examples are: cobalt, manganese, vanadium, cerium and iron.
2. Polymerization (crosslinking drier). Examples are: zirconium, lanthanum, neodymium, aluminum, bismuth, strontium, and barium.
3. Auxiliary (promoters) driers or catalysts. These are typically: calcium, potassium, lithium, and zinc. The first three increase the rate of top dry and zinc usually inhibits the top dry.
The oxidative driers promote the absorption of oxygen by the film as well as catalyzing the formation and decomposition of peroxides. These driers promote the surface dry of a coating. The top driers catalyze the decomposition of the peroxides formed by the reaction of the oxygen in the air with the resin or drying oil. This leads to the formation of direct polymer-to-polymer crosslinks (top drying) and also the formation of hydroxyl groups and carbonyl groups on the resin polymer and the drying oil. The hydroxyl groups are then available for through drying or crosslinking by the through driers, which form oxygen-metal-oxygen bridges or crosslinks between polymers.
The proper balance of driers in an oxidation-curing system is essential to stability, rate of cure and development of film properties.
Cobalt is the most active drier and a strong oxidizer. It top dries the film very rapidly. Care must be taken because excess cobalt causes wrinkling and color changes in light-colored paints. Excess cobalt has also caused gelation in some varnishes.
Manganese is also an active drier and is a potent oxidizer as well. It promotes polymerization to a greater degree than cobalt. It is often used alone in certain baking finishes. In air-dry systems it is used along with auxiliary driers. Manganese has a dark color, which can limit its use.
Iron seems to promote rapid drying by polymerization and hence is used widely in baking finishes where the dark color is permissible. For air-dry finishes, it is useful in eliminating film tack of some paints.
Rare Earth driers perform better than zirconium under marginal conditions such as high humidity or low temperatures.
Lithium improves the efficiency of other driers and is the preferred esterification catalyst in alkyd manufacture.
Zinc is an auxiliary drier and, in conjunction with cobalt, produces a harder film. It retards the surface drying in order to prevent wrinkling and allows freer access to oxygen, thus permitting hardening through the entire film. Zinc naphthenate is a good wetting agent and often improves gloss.
Calcium is used as an auxiliary drier, usually in conjunction with zirconium. It frequently performs better than any other auxiliary drier in baking finishes. Calcium driers are often added to the grind portion of the paint as auxiliary dispersants.
Zirconiums are used mostly with calcium as a replacement for lead. These show improved gloss, color, and gloss and color retention compared to lead, but do not perform as well as lead under adverse conditions such as low temperature and high humidity.
Aluminum offers outstanding polymerization and yellowing resistance without viscosity instability. Neodymium is a replacement for calcium and zirconium in VOC-compliant resins. It is cost effective and has proven performance. Neodymium and lanthanum are recommended for low-temperature/high-humidity applications. Vanadium is excellent for heavy film build (4+ mils) but has shown some discoloration in white coatings.
Waterborne coatings present a different sort of problem when using driers. The presence of surface-active agents, in addition to ammonia and amines and various resins, make proper selection important – particularly so that seeding does not occur. For waterborne systems some potential problems are as follows: incompatibility between the resin and the drier; potential for seeding; potential for resin discoloration. There are many good water-reducible driers including the rare earths: lanthanum, cerium and neodymium. Vanadium octoate is also available in a water-emulsifiable form.
Driers in high-solids systems also present the following possible problems: resin viscosity build, resin yellowing and slow dry times.
The best sources for starting-point drier combinations are the resin manufacturers who have already found combinations that develop desirable properties.
The change from natural acids like naphthenic acids to synthetic acids like octoates or neodecanoates provided more uniform products to the industry.
DRIER STABILIZERS
Compounds that prevent drier absorption or dissipation resulting in the loss of catalytic power of driers.
DYES (FOR USE IN STAINS)
Dyes are soluble colorants, which do not scatter light, but which absorb certain wavelengths and transmit others. Dyes are generally soluble in a solvent, or they may exist in such a finely dispersed state that they do not scatter light and behave as if they were in solution. Pigments are basically insoluble in the medium they are in.
Dyes are frequently used in the printing and coatings industry where a high level of transparency is required. Dyes have also found their way into automotive finishes because of advanced polymer technology – again because of their transparency and color properties. In general, dyes fall into one of three categories: metal-complex dyes, basic dyes and fat-soluble dyes.
For wood applications, dyes may offer deeper penetration of the wood surfaces and less grain hiding. However, they also fade more quickly than pigmented stains and require more effort to prepare the wood. Water-based dyes tend to raise the grain on many woods because the water penetrates the wood and raises the tiny fibers. Wood should be wetted first, then sanded down, before applying water-based dyes. Nongrain-raising (NGR) dyes need to be used in a nongrain-raising solvent. They dry faster than water-based counterparts, so application must be faster to avoid lap marks.
Liquid, high-concentration anionic dyes with good light fastness properties can be diluted in water or water/alcohol/glycol mixtures and optimized for interior use in water-based and water-containing wood stains.
Neozapon® dyes are metal-complex dyes, available in powder and, in some cases, high-concentration liquid form (Neozapon® L). They are highly soluble in polar solvents, but almost completely insoluble in water. Due to their limited weather fastness, these dyes are recommended for interior use.
ELECTROCONDUCTIVE ADDITIVES
Compounds that alter the conductivity or resistivity of a system. Silicon carbide is used in the manufacture of coatings to formulate conductive coatings. It is used in paints that conduct electrical charges away from large motors. Sizes are F400, F800. Silicon carbide is also used for wear-resistant coatings, such as for tanks, tubes, etc. It is usually mixed with an epoxy or resin matrix. It is also put on cutting tools with a proprietary method of application.
EMULSIFIER
See Surfactants
Emulsifiers fall into the category of surface-active materials, or surfactants. In general they are used to produce stable mixtures of two partially immiscible liquids. Emulsifiers allow us to make water-in-oil (W/O) emulsions and oil-in-water (O/W) emulsions. They promote the ease of mixing or dispersibility by lowering the surface tension of the liquid, much as a wetting agent lowers the surface tension of the liquid for application to a solid substrate.
ENZYME-BASED ADDITIVES
Enzyme-based additives can be mixed with coatings to create novel biologically and chemically active coatings, including those that self-decontaminate and detoxify organophosphorus compounds such as nerve gases and pesticides. A new understanding of enzyme biochemical capabilities is leading to the development of innovative, biocatalytic “smart coatings”.
Many organophosphorus (OP) compounds are potent cholinesterase inhibitors, accounting for their widespread use as insecticides and chemical warfare agents. Common organophosphorus agents include the chemical warfare agents tabun (GA), soman (GD), sarin (GB), cyclosarin, VX and its isomeric analog Russian VX (R-VX). Historically, most approaches to chemical agent decontamination are post-exposure, focusing on the treatment of surfaces after exposure has occurred and been subsequently detected.
Novel enzyme additives are being commercialized that not only remain stable in paint but, remarkably, remain active for extended periods of time. One self-decontaminating coating additive, OPDTOXTM, is biocatalytic in nature, and represents a paradigm shift for chemical agent decontamination. OPDTOX can serve either as a stand-alone decontamination method or as a complementary approach to existing decontamination techniques and products. When incorporated into a coating, the additive creates a reactive surface that will initiate the process of decontamination immediately upon exposure to organophosphorus pesticides and neurotoxins.
Applied in advance of exposure, painted surfaces containing OPDTOX can continue to degrade organophosphorus compounds after repeated exposures and remain active following washing. The scientific basis of the product lies in the ability of bacterially derived enzymes to efficiently degrade organophosphorus compounds. The results are novel coatings that self-decontaminate following exposure to organophosphorus compounds, which includes many important environmental and security targets such as the nerve agents VX, GD, GB, thickened nerve agents, and pesticides such as malathion [o,o-dimethyl-S-(1,2-dicarbethoxyethyl)dithiophosphate], parathion [o,o-diethyl-o-(4-nitrophenyl) phosphorothioate] and coumaphos [o-(3-chloro-4-methyl-2-oxo-2H-1-benzopyran-7-yl)o,o-diethyl phosphorothioate].
The enzyme of choice for OP decontamination is organophosphorus hydrolase. Of the six major groups of enzymes approximately 80% of industrial enzymes are hydrolases. Hydrolases are enzymes that catalyze the hydrolytic cleavage of C-O, C-N, and C-C bonds. A class of hydrolase known as organophosphorus hydrolase (OPH) can also cleave the P-O, P-F, and P-S bonds of the organophosphorus compounds.
Of the currently stockpiled nerve agents, VX is the most toxic, as well as the most persistent in the environment after release. In addition to the initial inhalation route of exposure common to such agents, persistent agents such as VX and thickened soman pose threats through dermal absorption. Enzymatic hydrolysis of OPs occurs when the compound is cleaved at the phosphoryl center’s chemical bond resulting in predictable byproducts that are acidic in nature but benign from a neurotoxicity perspective. By comparison, chemical hydrolysis can be much less specific and in the case of VX may produce byproducts that are extremely toxic. Although a number of enzymes have been identified that are capable of detoxifying OP compounds, OPH has the broadest substrate specificity. The substrate range of OPH includes numerous insecticides (paraoxon, parathion, coumaphos) and the neurotoxic chemical warfare agents and their analogs. Catalytic specificities for this family of compounds have been shown to range from rates that are diffusion limited (i.e., paraoxon, P-O bond, kcat=104 sec-1) to rates that are six to eight orders of magnitude lower (i.e., acephate, P-N bond). The enzyme is composed of two identical monomers associated to form a remarkably stable dimeric enzyme with a thermal Tm of approximately 75
FLATTING (MATTING) AGENTS
A wide variety of natural and synthetic materials that are added to coatings to primarily affect gloss are called flatting agents. When light is reflected from a smooth surface, the surface appears to be glossy. When light is scattered as it hits the surface, instead of being reflected, a matte appearance will be created.Gloss is one of the key properties of a coating. The observed gloss of a film is dependent upon the angle and intensity of incident and reflected light from a surface. It is a function of the smoothness of the coating’s surface. A very smooth surface will reflect most of the light rays, so that the angle of incidence is the same as the angle of reflectance. As a surface becomes less smooth, incident light rays are reflected at various angles, and the surface is perceived as being less shiny and of lower gloss. In order for light to be reflected in many directions instead of at the same angle, it is necessary to introduce some degree of micro-roughness to the surface of the film. The reduction in gloss to give satin, semi-gloss, flat or matte finishes can be controlled by adding pigments and extenders – or materials called flatting agents.
Any tiny irregularities in a coating film will cause light to scatter. Flatting agents reduce gloss by imparting micro-roughness to the paint surface as it dries and cures. The rough surface diffuses light rather than reflecting it. The result to the eye is a matte surface. These agents include particles with dimensions that range from 1 to 50 microns in diameter. Thermoplastic flatting agents are being increasingly used for this application, and interestingly, the smaller particles reduce gloss to a greater extent than do the larger particles.
Gloss is also affected by conditions under which a film forms, such as the coating application method, drying temperature, humidity, rate of solvent evaporation and solvent composition. These factors can cause an increase or decrease in gloss, depending on the particular coating formulation. Factors such as high temperature or slow solvent evaporation rate, which allow the polymeric resin to relax and find the most thermodynamically favorable position, tend to give the smoothest surfaces and highest gloss. The method of applying a coating (spray, brush and roller) has an influence on the gloss as well.
The components in a formulation can also affect gloss. As film formation proceeds, volatile or nonvolatile leveling aids are sometimes required to assist relaxation of the polymeric film former to minimize the surface area of the coating and give a smoother surface. Other materials such as slip and anti-blocking aids can be used; these are meant to migrate to the surface of the film to perform their intended function. These materials change the characteristics of the surface and hence the gloss.
Flatting agents are available in different chemical compositions and particle sizes. Common inorganic materials include silicas for clear coatings, and extender pigments, such as clays, talcs and carbonates, for pigmented systems. Hydrophilic materials, which include some silicas, must be stored under conditions that prevent moisture pick-up by hydration. The inadvertent addition of moisture to solventborne systems can be detrimental, while the additional water of hydration in waterborne systems might have to be compensated for, to retain the desired solids level.
Silicas
Silicas have typically been used as the traditional flatting agents in the industry. Silicas with large particle size generally improve flatting efficiency; surface smoothness and clarity of the film will decline. Small silicas improve surface smoothness but are therefore less efficient at flatting.
The performance of silica flatting agents is also influenced by surface treatments such as organic wax treatment and inorganic surface treatment. Surface treatment is used to prevent settling and also to improve mechanical properties and often adhesion.
Silicas can be used in solvent and aqueous systems, and in virtually all types of resin systems: acrylics, vinyls, polyesters, nitrocellulosics, urethanes and alkyds. The reduction of gloss can have a significant corresponding effect on increasing viscosity that makes the job of formulation sometimes difficult. Silicas are available as: precipitated silica, fumed silica, diatomaceous silica and silica gels.
The synthetic silica flatting agents are used to produce low-gloss finishes in organic solventborne systems, waterborne and high-solids coatings. Typically, waterborne finishes cannot withstand high shear that can destroy the emulsion and cause foaming. So the flatting agent must be easy to wet out and incorporate into the coating under low-shear conditions.
Precipitated and Fumed
Precipitated and fumed silicas are not resistant to overgrinding. In some cases, the overgrinding may occur under relatively low shear such as obtained in mixing while tinting a batch of paint. Silica gels are resistant. Fumed silica has a pronounced effect on thickening; silica gels do not. In fact, their impact on viscosity is minimal.
Diatomaceous Earth
Diatomaceous earth (silica, SiO2) has been used extensively in the coatings arena as a very efficient flatting agent, achieving appearance characteristics from a dead flat to a silky low luster in a range of coatings. It imparts increased toughness and durability, added ‘tooth’ for adhesion, and improved sanding properties. Because these microscopic particles are irregularly shaped, they diffuse light and are used to impart degrees of flatness to coating films.
Diatomite consists of the siliceous skeletal remains of single-cell aquatic plants known as diatoms. The diatomite structure is porous with microscopic voids that serve to control vapor permeability and for the reduction of blistering and peeling and faster dry time. The use of this type of material results in uniform gloss and sheen reduction.
Silica Gels
Modern silica gel technology enables the production of highly pure, porous products. A silica gel is an amorphous form of silica composed of nearly 100% silicon dioxide (SiO2) produced synthetically in a liquid process. Silica gels belong to a class of synthetic silica materials known as hydrated silicas, which have an average water content of 6% to 8% by weight. Silica gels are produced from the acid treatment of an aqueous sodium silicate solution.
Mixing sulphuric acid and sodium silicate under controlled conditions produces the base gel from which all types of silica gels are made. This glass-like, solid gel material is broken down into granules, then washed and dried to produce the highly porous material needed for matting agents. Physical parameters such as porosity, pore size and surface area can be manipulated to produce a range of different silica gel types. The physical properties of silica gel differ from other specialty silicas. The internal structure of silica gel is composed of a large network of interconnected microscopic pores that attract and hold water, hydrocarbons and other chemicals by the mechanism of physical adsorption and capillary condensation. This huge pore volume and extensive surface area gives the silica gel many of its unique properties.
The rigidity of the silica gel particle is higher than that for precipitated silica and thus better resists the shear forces arising during the manufacture and application of the paint. Silica gels can influence rheological properties, although other agents are typically preferred for rheological control purposes.
The efficiency of the matting agent depends on the type of silica, particle size distribution and porosity of the particle. A high porosity is a key feature of a modern matting agent as it enables the formulator to limit the addition rate of a matting agent in a formulation. Unfavorable viscosity effects as well as limitations in coating surface properties can be avoided. A new milestone in micronized silica gel technology was reached by achieving a pore volume of more than 2 ml/g for a micronized silica. Today, matting agents for industrial applications are available in a variety of particle sizes.
The application segments for silica gel matting agents include coil coatings, industrial wood coatings, general industrial and automotive coatings as well as printing inks, leather/textile coatings and decorative coatings. Coil, wood and furniture coatings are the applications with the largest volumes, but radiation-cured coatings and other coating systems as well as printing inks also employ silica gels. The solventborne paint sector has been more important for silica gel than waterborne coatings. However, the pressure to reduce VOC emissions and formulate solvent-free paints has led to the development of grades of silica gel, which are suitable for formulation in powder, ultraviolet, and waterborne coatings.
Surface-treated silica gel flatting agents for high-performance OEM and DIY wood coatings and other clear finishes are available. They contain an organic additive that aids in dispersion and suspension, and imparts mar- and burnishing-resistance to the finished surface.
Silica gels are more resistant to overgrind compared to precipitated silica or fumed silica. This resistance provides gloss stability for demanding applications such as coil coatings. Synthetic silica gels contain internal voids – thereby giving pore volume or void volume. As pore volume increases, the specific volume of the particles increases while maintaining a constant weight. The result is more particles per unit weight, which yields higher flatting efficiency. The increase in pore volume must be balanced against application viscosity.
Selection and Applications
It is not easy to select the best flatting agent. It depends on the end use of the coating, as well as dry film properties required: clarity, smoothness, mar resistance and weatherability. Also the choice of flatting agent can make a significant difference in application characteristics such as viscosity, re-dispersion and gloss stability. Optimum flatting efficiency results when a flatting agent is selected that has optimum pore volume – that is, more particles per unit weight.
Particle size is also important, as it will affect flatting and surface clarity and smoothness. In general, the larger the average size of the flatting agent, the rougher the surface and the lower the ratio of sheen:gloss. For certain markets this becomes very important and formulators need to determine which agents to use depending on this consideration.
Average particle size can affect the flatting efficiency and loading level at constant film thickness. Silicas are available in a variety of particle size ranges. In general, the larger sizes generally give higher flatting efficiency and are also less sensitive to variations in film thickness. Larger sizes also give rougher surfaces.
In some cases, the flatting agents yield a smoother surface than the full gloss coating does, which indicates that thin film coatings can retain the features of the substrates. This can be important in paper coatings and wood sanding sealers, where it is desirable to obtain a smoother surface by filling voids in the substrate.
Silicas are used in architectural coatings and varnishes, coil coating and general industrial finishes, industrial wood finishes, topcoats and primers for corrosion resistance, and textured and suede effect finishes. Some coatings segments need very high-quality synthetic silicas – coil coatings in particular. Coil coatings are also very dependent on gloss stability.
Clear coatings for fine furniture and other wood products are flatted to obtain a satin finish. The satin finish is a desirable aesthetic feature, obtained by lowering the 60
FLOCCULANTS
Flocculants are chemical compounds that cause controlled flocculation in a coatings system to give the formulator the ability to optimize color development and adjust pigment mobility. As a paint film dries, flow currents transport solvent to the surface. Smaller pigment particles may be transported with the solvent and result in a higher concentration of smaller sized pigments at the surface. This is called flooding and can be controlled through the use of flocculating additives.FLOW AND LEVELING AGENTS/FLOW MODIFIERS
Flow and leveling agents are chemical compounds that increase a coating’s mobility after application, thus enabling the process of leveling. They reduce the surface tension of the wet coating and, more importantly, maintain a uniform surface tension over the entire surface area. Flow is the resistance to movement by a liquid material. Leveling is a measure of the ability of a coating to flow out after application so as to obliterate any surface irregularities such as brush marks, orange peel, or craters.Surface tension holds a liquid together and causes it to take the smallest possible volume. A drop of liquid on a solid surface will cover a larger or smaller area depending on both the surface tension of the liquid and the surface tension of the substrate. For example, a drop of water, which has high surface tension, will bead up as small as possible on a clean and waxed automobile. The same drop of water will tend to completely spread out and ‘wet’ another portion of the automobile that is not so clean or waxed. A liquid will ‘wet’ a substrate when the substrate has an equal or higher surface tension than the liquid itself. Sufficient wet film thickness is also important for a smooth surface.
The wetting of the substrate and leveling of the liquid film depend on the surface tension of the coating. Both processes have opposing requirements in terms of surface tension. If the surface tension is too high, poor wetting occurs, along with the formation of possible defects such as craters. If the surface tension is too low, poor leveling occurs and can cause orange peel.
Because of the recent advances in technology within the industry in areas of powder and waterborne systems, good flow is more important than ever. Some of the resins that are being introduced exhibit poor wetting and flow characteristics. This is such an important area because good flow is necessary to eliminate possible surface defects such as craters, fish eyes, orange peel and pinholes.
Many surface defects develop during the application of the coating. Application methods can influence leveling. For instance, some brushes, rollers and direct roller coaters can produce uneven surface films that require leveling for a smooth finish. Other methods that produce a smooth surface – spraying, curtain coating, reverse roller coating – need to maintain that smoothness during the baking and curing process.
Leveling agents function by influencing the viscosity throughout the film. Solvents with good solvating power and a gradual evaporation rate influence the “open” time necessary for leveling. Leveling agents can also be pigment dispersants that function by preventing the flocculation of pigments.
Flow modifiers can differ greatly in their chemical structure and in their ability to affect surface tension and promote leveling within any given coating system. As such, they need to be evaluated in a formula at several different usage levels to determine the optimum level needed in any given formulation. Even more importantly, a given flow agent’s effectiveness will vary from system to system. The natures of the resin, other additives and application technique will all affect the flow agent. For this reason, great care must be taken when selection is made and laboratory work must be conducted to check on effectiveness and surface defects.
Some flow modifiers are designed as ‘general purpose’, while some are very specific. Some are more effective at controlling craters while others are better at imparting good leveling. This is why proper selection is so important.
Flow agents are available for powder systems, solvent and waterborne applications. The most commonly used flow agents are: silicones, surfactants, fluorinated alkyl esters, solvents and polyacrylates.
Silicones form an important group of flow control agents. These consist of:
1. Polydimethylsiloxanes (silicone oils). - Increasing chain length gives materials with higher viscosities. Higher molecular weight means reduced solubility in coating systems and less compatibility. Lower-molecular-weight siloxanes (dimethyl units <60 for example) are used for surface flow control and to mask floating of pigments. Higher-molecular-weight polydimethylsiloxanes are effective as defoamers. Those products with molecular weight greater than 1400 are incompatible and are responsible for causing craters. They can be used to produce hammertone finishes.
2. Methylphenylsiloxanes.
3. Organically modified polysiloxanes. - Organo-modified siloxanes have distinctly different properties than their original polydimethylsiloxane counterparts, with respect to silicone crater formation, and other silicone-caused surface defects. The compounds are derived from low-molecular-weight polydimethylsiloxanes, and rather than having only methyl functionality, various organic chains replace certain methyl groups to increase the compound’s compatibility with coatings and inks. The organic portion of the molecule can be polyether, polyester or a long alkyl chain.
The most important modification is the polyether chemistry derived from ethylene oxide and/or propylene oxide. Hydrophilic character, i.e., water compatibility, increases as a function of the ethylene oxide content so that it is even possible to synthesize water-soluble silicone-based additives. Factors influencing the properties of modified siloxanes are silicone content, and the type and location of organic groups on the molecule.
Polyether-modified siloxanes simultaneously influence the following effects in coatings. Evaporation of solvent from an applied film causes differences in temperature, surface tension, solvent concentration and density within the film. Solvent-rich material rises in the center of these cells, while material having a lower solvent concentration moves downward from the edges of the cells. As a result, the surface tension in the center of the cell is lower than at the edges. Material flow occurs from the lower surface tension to the higher surface tension areas, forming valleys in the center of the cells and ridges at the edges. Bénard cells can be particularly problematic in systems containing mixed pigments. In clear coatings containing a matting agent, the larger pigment particles are forced out of the zones of higher flow-rate to the center of the cell. Bénard cell effects can be suppressed with modified siloxane-based flow additives. The addition of modified siloxane compounds to a formulation suppresses the surface tension of an uncured film to a uniformly low level, which is altered during solvent evaporation. Therefore, no differences in surface tension occur on the coating surface. Uniform drying and uniform flow are achieved, due to the elimination of surface tension variations. Another benefit, the requirement of good flow, is maintained because low surface tension facilitates complete substrate wetting.
Silicone and acrylate additives are typically used in combination in standard coating formulations. Acrylates improve the flow and leveling while the silicone contributes enhanced substrate wetting and prevents cratering. A silicone macromer-modified polyacrylate is available that incorporates both acrylate and silicone characteristics. In high-polarity coatings, the additive brings about a massive reduction in surface tension, therefore providing good substrate wetting with no significant reduction of the dry surface energy. The silicone part provides good anti-crater properties without increasing surface slip. The acrylate backbone provides excellent leveling.
Silicones are multipurpose additives because, in addition to control of flow, they also assist as slip agents, prevent floating, and in general, are very effective at low levels. One of the most valuable characteristics of silicone flow additives is the prevention of cratering together with the reduction of sagging.
Silicones concentrate at the paint surface and, because they are surface agents, in addition to reducing the surface tension they influence the slip resistance of the coating surface. [For more information, see Slip Agents]
The chemical structure of silicone additives (molecular weight and functional groups on side chains) makes them more or less compatible with the resin solution. Very incompatible silicones will cause surface defects (craters). Compatible silicones will reduce surface tension and slip. Those whose structures cause them to be somewhere between compatible and incompatible will function as defoamers.
Fluorocarbons are the most highly surface-active group of coating additives. Fluorosurfactants aid in wetting and flow because they decrease surface and interfacial tension. Some low-energy substrates, such as polyethylene or metal surfaces contaminated with oil, are very difficult to wet. Fluorochemical agents function quite effectively and do not usually affect other properties such as water sensitivity. Fluorosurfactants are leveling aids because they minimize surface tension gradients between the resin and solvent that resists leveling. They are generally used at very low levels (0.05-0.10%) and excesses can cause severe foaming.
High-solids systems, powder-coating systems and lower-VOC coatings are particularly in need of flow modifiers because the traditional wetting role of the solvents has been greatly reduced or, in the case of powder coatings, totally eliminated. The reduced solvent content of higher-solids coatings causes an increase in surface tension that results in poor substrate wetting, crawling, cratering and sometimes surface defects on the coating. Waterborne coatings in particular are prone to flow defects, particularly when applied over oily substrates or contaminated surfaces. The solventborne systems were quite forgiving of minor amounts of oil or other contaminations. Waterborne coatings are not at all forgiving over substrates such as metal, plastic, glass and concrete. Raw-material suppliers are continuously introducing new products to overcome these problems.
Powder Coatings
For powder coatings, the quality of the final film depends on the film-forming process. This process includes the coalescence of individual powder particles, the wetting of the substrate by the melted but not cured powder coating and the flow of the irregular film into a uniform film. Both viscosity (the resistance to wetting and leveling) and surface tension (the driving force) play a role in the process of film formation.
If the surface tension is too high, poor wetting occurs and craters may form. If the surface tension is too low the leveling is also affected adversely and orange peel may result. Polyacrylates and polysiloxanes are two types of leveling additives in use.
Flow modifiers for powder coatings are designed to address the surface imperfections, such as orange peel and craters.
Acrylic powder coatings can be contaminants for other nonacrylic powder coatings. The result is usually some type of cratering formation. Interestingly, acrylic flow agents are also used in powder coatings as leveling aids. These materials are melt-mixed into the powder composition. It is their controlled incompatibility that allows them to equalize the surface tension of the air-to-solid interface. If the acrylic flow agent was dry-blended into the powder coating, craters would surely be a result. The melt-mixing allows this additive to do its job.
The majority of flow modifiers for powder coatings are alkyl acrylates. These modifiers work by enhancing flow and leveling during the bake cycle. There are surface irregularities that must be overcome during the application and cure of powder coatings. In addition, the particle size and distribution can affect flow. Just as in liquid coatings, incomplete leveling of the surface will result in severe orange peeling. Orange peel for powder coatings is frequently caused by either uneven application, or a nonuniform particle size distribution of the powder particles. Sometimes nonuniform pigment distribution can also cause orange peel. Polyesters, in particular, seem to be susceptible to this defect.
Cratering and trapped air or other volatiles that result in pinholing are also critical. For example, if a powder resin cures too rapidly, it can trap gas and pinholes will result.
Some of the acrylic flow modifiers behave as a plasticizer and lower melt viscosity, and increase the leveling time to produce a smooth surface. Acrylic flow modifiers have been used effectively to provide high-quality surface coatings prepared from a variety of vehicles: epoxy, melamine, urethane, silicone rubber, acrylic, alkyd, phenolic, EVA copolymers, polyester and cellulosic. Certain melamine-formaldehyde resin crosslinkers, such as methylated/butylated coetherified melamine-formaldehyde resins, also provide good flow and leveling and help to prevent pinholes, craters and picture framing in coatings. Levels of use of flow modifiers are different than in liquid coatings, and systems need to be well evaluated by the formulator.
For powder coating, polyacrylates (both homopolymers and copolymers) are the most widely used flow-control agents. These are moderate- to high-viscosity liquids and are usually supplied as a powdered master batch dispersed on silica particles.
Additives based on polyacrylate technology, encapsulated by a solid (silica free) shell, are available as dry powders for powder coatings. These additives promote leveling while maintaining maximum gloss levels.
Silicones are also used and are typically more effective in reducing surface tension. Polyether and polyester-modified polysiloxanes are preferred to poly(dimethyl) siloxane because these are very active and are prone to cause defects such as pinholing, craters, haze, etc. The modified polysiloxanes are provided in a powdered form on silica particles.
The fluorocarbons are very effective in powder coatings but are not often used because they are expensive. They are, however, very effective, particularly on contaminated or oily metal substrates.
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