Pigmentation — Its Importance
Proper pigmentation drives not only the aesthetics of the film, such as gloss, opacity and color, but many of the film’s mechanical and performance properties, as well as the extent of changes wrought by the environment upon such properties. Adequate pigmentation is often the primary buttress against aging and radiation (UV and heat) induced degradation of the binder. In spite of all of these things, pigments and the all important volumetric relationship between the pigment package and the selected binder generally remain aspects of our technology that do not receive the same adequacy of treatment among those responsible for the transfer and reiteration of the technology from one generation to the next, as do coating binders.
Extender Pigments — The Forgotten Component
If pigments are considered technologically, if not economically, the less crucial constituents of the paint film, then surely the true orphans of our discipline are the extender pigments. In spite of their widespread use in almost all coatings and their elevation from “fillers” to “extenders,” these pigments, in many eyes, remain the “pig ears” among the silks and satins of the modern formulation. Much less attention is directed toward information on these materials, and insofar as extenders are concerned, the modern course and compendium of learning more often than not relegates these materials to less time than they do a discussion of any single binder type, or perhaps no more than a few short paragraphs.This was not always the case. Payne1 and Nylen and Sunderland2 directed more space to their treatment of extenders than they did to their treatment of white pigments. It is, however, sadly apparent that those formulators who have followed these greats have been sorely remiss in not providing would-be apprentices to our trade with updated expansions of this portion of our technology.
It is not that such treatment is not pertinent to this modern industry — far from it. In today’s technology, where toxicology and compliance are major concerns, there is much to be salvaged from the judicious manipulation of the extender package.
In this article, we can do no more than broach the subject. We will, however, attempt an approach more in keeping with today’s predominantly engineering-based schemes in treating extender technology. The more common types of extenders in use today by the coatings industry are listed in Table 1.
Engineering Aspects — Fill and CPVC
From an engineering standpoint, the dry paint film is reasonably considered to be a mixture of discrete pigments having different sizes and shapes set in a more or less continuous polymeric matrix. The special relationship of pigment volume to the total volume of paint is a most crucial parameter on which many characteristics of the applied film depend. It may reasonably be argued that classification of a paint film in terms of its PVC/CPVC ratio is as sensible and pertinent to its usage as is any classification in terms of binder type.Asbeck and van Loo introduced the concept of critical pigment volume (CPV) to the coatings industry in the late 1940s.3 The relationship of actual PVC to this theoretical value was shown to play a paramount role in the behavior of paint films. Even before this, certainly since the introduction of titanium dioxide (TiO2) as the primary white pigment, it was realized that the extensive use of such “prime pigments” at levels beyond those levels required to achieve adequate hiding, color or UV absorption was a blatant waste of money. Extender pigments, rather than prime pigments, were thus preferred for adjusting the PVC/CPVC ratio to that value consistent with the optimization of film properties.
In using extenders in this manner, the oil (or binder) absorption value and specific gravity were realized to be crucial to this relationship. The more oil (or binder) that it takes to wet out a given weight of pigment, the higher the oil absorption value and the lower the CPVC. If PVC/CPVC ratio is crucial to usage, as it most certainly is, then the CPVC value will dictate the PVC at which the paint is formulated. As oil (or binder) absorption is related to surface area and surface area to particle size, the importance of particle size and particle size distribution is a corollary of the realization of the importance of oil absorption. Extenders vary in particle size from 0.01-44 microns. Super-fine silica particles are extremely small and have a very large surface area (per unit weight of product), with very high absorbency. This results in a dramatically minimized CPVC, so that in sufficient loadings, the surface of the applied film that bears such extenders may be disrupted as the particles of pigment protrude through the surface of the film and scatter incident light, so reducing gloss. In this way, fine silicas and/or other highly absorbent extenders (diatomaceous silicas, for example) act to flatten the paint.
While extender pigments have a wide range of particle sizes, the smaller particles may actually fit into the interstices between the larger particles of extender, thus taking up volume originally filled by the binder with pigment volume, and elevating CPVC. In this case, the additional pigment does not compromise the mechanical properties of the film. Where extender pigments did not provide the necessary range in particle size required to achieve a high degree of pigment packing, two extenders of similar (or different) chemical make up were (and are) often used to maximize such packing.
Making matters more complex, extenders come in all shapes and sizes, which can confuse any attempt to theoretically calculate any optimum PVC/CPVC ratio from the absorption characteristics and specific gravities of the individual pigments making up the mix. So complex and variable are the geometrics in which pigment of mixed size and shape can lie within the polymeric matrix that mathematical reduction of this spatial arrangement within the film defies even the most sophisticated computers. This forces the formulator to rely on more empirically determined changes in performance, in order to achieve optimum film characteristics.
On the other hand, differences in shape and size of different extenders can lead to many practical benefits. At first, the selection of extender systems to fill the system or to achieve this necessary relationship was for the most part determined empirically — almost casually, in fact. The make up of the extender package was largely predicated on cost reduction. Since then, if only slowly, we have come to understand that care in apportioning the available extender volume between the various extenders available to the formulator can result in films with enhanced mechanical and even performance characteristics. Different coating types having different usage and performance requirements benefit from different types of extenders.
Absorptivity is also, of course, related to the porosity of the pigment particle. Pigment binder may be absorbed into the pigment surface as well as adsorbed onto the surface. This is the reason why highly absorbent materials such as diatomaceous silica have high oil absorption values. As noted in Table 1, these materials, which are derived from the skeletal remains of primordial diatoms, are also widely used as flatting agents. Extenders, however, are used by today’s chemist for far more reasons than just for fill and flatting (see Table 2).
Particle Shape — Nodular Extenders
As a pigment system is adjusted so that more binder is absorbed (and the CPVC decreased), the solid, relatively binder-depleted system inevitably increases in viscosity. To compensate for this, the formulator must add solvent, which reduces both volume solids and increases VOC — neither a desirable consequence in these days of cost control and increasingly restrictive regulations. There are, therefore, very real advantages in seeking extender packages that are made up of extenders having lower oil absorption. This has become a primary requirement in the selection of extender systems. Some of the most effective fillers in terms of low oil absorption are the nodular extenders such as whiting, barytes and amorphous silica (as opposed to the fumed and diatomaceous types). These materials provide little value in aesthetics, however, as do the china clays, which are used effectively in combination with TiO2 to enhance opacity. This is achieved by optimization of the manner in which the film scatters light, and is discussed below. That being said, nodular pigments contribute little to the mechanical properties of the film.
Particle Shape — Flat Platy Extenders
Flat, platy extenders such as mica and some talcs and clays are also highly absorbent, again because of their very high surface area per unit weight compared to nodular extenders such as silica, barytes or calcium carbonate. These extenders tend to enhance the mechanical properties of the film by lateral reinforcement. It has also been reported that platy extenders of this type not only reinforce the film but minimize the accretion of internal stress by allowing better dissipation of stress along the plane of the pigment.4 The author (Hare) has confirmed the beneficial stress dissipation characteristics of films bearing mica. Flat, platy pigmentation orientated parallel to the substrate also tends to decrease the porosity of the applied film, sealing it and reducing the tendency of high PVC primer films to bubble when recoated. Mica and other flat platy extenders have shown some considerable value in reducing the transmission properties of the film. They provide a barrier by overlapping platelets, acting to minimize the transmission of water, oxygen and ionic solutions in much the same manner as venetian blinds in blocking out light.
Particle Shape — Acicular Extenders
Also of value in the enhancement of film strength is the uniquely acicular extender wollastonite. In this case, this pigment reinforces the film in the same manner that chopped fiberglass reinforces gel coat polyester lay ups used for boats, bathroom fixtures, etc. (Wollastonite is in fact used in these applications as well as a chopped fiberglass extender!)Film reinforcement by flat, platy pigments — especially acicular (rod-shaped) extenders like wollastonite — is valuable in many types of coatings that require reinforcement to prevent cracking in service. This loss of cohesive strength is particularly dangerous when it occurs as a result of film embrittlement with age or exposure to UV or other degradative forms of radiation. Oxidizing systems, which tend to crosslink on aging will crack and check in service unless properly reinforced. Asphalt roof coatings, which have poor resistance to oxidizing conditions, are rapidly affected by exposure to UV and will alligator badly as the surface embrittles. These types of effects may be controlled by reinforcing the system with fibrous, acicular or platy extenders.
Rods have lower surface area than plates or fibers, and therefore it is not surprising that reinforcement with the acicular wollastonite has less downside effect on reduced viscosity and allows lower VOCs than does the use of platy pigments, which is a definite plus in today’s tough regulatory environment.
Particle Shape — Fibrous Extenders
Fibrous extenders, almost exclusively asbestos, were for many years used as reinforcing fillers. In this respect, they functioned in much the same manner as the acicular species. Fibrous asbestos was widely used in light-sensitive (asphalt) finishes, especially in roof coatings, where the fibers held the coating together, preventing cracking and alligatoring as the surface of the film crosslinked on exposure to UV light. Since the abandonment of asbestos by the industry (because of toxic considerations) non-mineral-based extenders have been used. Fibrous pulps of polyolefin, Kevlar®, acrylics and cellulosics are examples of such fillers. All of the fibrous materials tend to induce heavy, short thixotropic consistency, giving high build, high bridging characteristics, and non-sagging systems generally having texture. Applications for fibrous extenders include block fillers, joint compounds, textured and special-effects paints, and roof coatings. Combinations of pulps such as cellulosic materials and high aspect ratio, microfiber extenders (wollastonite) have been used as reinforcing agents instead of asbestos in more modern asbestos free asphalt roof coatings. Mica, which has sealing properties as well as reinforcing properties, has also been widely used in joint compounds for wallboards.Strain Mitigation in the Case of the Surface-Treated Wollastonite
Strain mitigation may be enhanced by the engineered wollastonite pigments treated with reactive silanes. Here strain dissipation is achieved in another way.5 In the presence of moisture, this is accomplished by the reversible hydrolysis of the siloxane bond at the pigment surface, followed by slippage of the resultant silanol along the surface and the reformation of a second siloxane linkage at an area of lower strain. Pigment/binder adhesive strength is thus re-established without any permanently negative side effects. As the treated wollastonites are also less absorbent than the untreated materials, these advantages are achieved without decreasing CPVC and without increasing viscosity or VOC — an unfortunate corollary of strain dissipation that occurs by way of the use of flat, platy pigments. Wollastonite has a natural surface chemistry that is particularly amenable to such silane treatments, and these engineered products have achieved widespread use by the coatings industry.Hardness
Another important property is the hardness of the mineral. The hardest extenders are predominantly silicas, especially quartz type crystalline silicas. These materials are used to improve the hardness and wear resistance of coatings, giving optimum scrub and burnishing resistance in interior latex paints and abrasion resistance in floor finishes and maintenance coatings. Coarse grades, which may protrude above the surface of the film, will give excellent “tooth” to primers enhancing recoatability. “Amorphous” silicas, which are generally micronized versions of the same crystalline materials used in floor finishes, are also used in this way. Where slippage is likely, extra coarse crystalline silicas may be actually broadcast onto the wet film before it has dried, the excess “unbound” material being swept from the floor after the ground coat has dried. However, the toxicological hazards of free silica have, in recent years, begun to limit the use of this type of material in coatings.Only slightly less hard than silica are nepheline syenite and wollastonite. These materials might be considered suitable replacements for silica, and wollastonite has recently been growing in importance in traffic paint.
At the other end of the hardness (Mohs) scale are the very soft extenders such as the talc and china clay (calcined clays are somewhat harder than the normal water washed hydrated aluminum silicate). These materials are preferred for their good sanding properties. Both talcs and china clays are platy and this enhances their applicability in sanding sealers with good barrier properties.
Chemical Effects
While the physical aspects of extenders are of particular importance to their use in coatings, so is their chemical make up. Extenders were at one time classified as inert pigments. Today we know that this is not entirely true. Some products such as calcium carbonate or whiting are readily reactive with acids. As such, they must be avoided in the pigmentation of alkali-sensitive systems (carboxylated vinyls, waterborne alkyds, etc.) with which they may react, or systems intended for acidic environments (which may intensify the deterioration of these pigment-sensitized coatings). Acid-catalyzed systems will require some adjustment in catalyst level if such alkaline pigments are used. Care should also be taken to avoid combining alkaline extenders with alkaline-sensitive color pigments such as iron blue. Films containing alkaline pigments are also susceptible to staining by soluble iron and copper compounds. Dissolution of the extenders and the subsequent leaching of their salts from the film by acidic environments (including acid rain) will tend to weaken the film. This may also lead to other defects, such as frosting, where the sulfate salts of calcium (from acidic dew) tend to collect as a white crystalline deposit on dark colored films in locations where soluble salt removal with rain is not possible.However, whiting may be of value as a pigment for anti-corrosive paints, where its natural basicity will tend to reduce the levels of oxygen required to establish passivity on the sub-film metal. Larger particle sized pigments, which are favored for metal primers, give coarser films with better flow and tooth for improved intercoat adhesion. Fine, wet ground and precipitated carbonates are used in gloss finishes and enamels, while intermediate sized pigments are favored for semi-gloss pigments and flats. Whiting of intermediate fineness is also used in exterior house paints, where the pigments improve mildew resistance, reduce chalking as well as assist in controlling cost. Larger sized grades of whiting are said to give good color retention in exterior house paints. Whiting is a nodular pigment, however, and little reinforcement of the film may be achieved with any grade of this pigment.
Entirely more useful as a corrosion resistant auxiliary pigment is, again, wollastonite.5 Although quite basic, the pigment is neither as soluble nor as acid sensitive as the carbonate. Wollastonite has been successfully used as an extender in epoxy systems for acid service, but is soluble enough to provide the required degree of pH control in metal primers for improved passivity. In this regard, wollastonite has been used successfully as an auxiliary pigment in inhibitive systems (where it has been used to reduce the levels of the more expensive inhibitive pigments) necessary to achieve passivation. Also, in combination with flat, platy (highly impermeable) prime pigments such as aluminum flake, wollastonite finds use in barrier systems.
In both barrier primers and inhibitive primers, the wollastonite markedly elevates the corrosion resistance of the system with some considerable cost savings because of the reduction in the costly primes that the use of the wollastonite allows. It is now widely used in these applications. Still further improvement in acid resistance without any deterioration in value as an anti-corrosive auxiliary pigment, is achieved by surface treating the wollastonite with the same organo-terminated silanes noted above. Engineered pigments of this type are being widely used in both anti-corrosive primers and in chemically resistant finishes. Some care is, however, necessary in the selection of the pigment grade as mutual reactions of the treated pigment and the binder is predicated on the nature of the silane used.
Mica has also been used as an extender for aluminized barrier systems. As much as 25% by weight of aluminum flake has been replaced with mica without deleterious effects on corrosion resistance. As mica is more chemical resistant than aluminum, this device is particularly valuable where the film is exposed to conditions of extreme pH. In barrier systems, the degree of hydrophilicity must be controlled as “water loving” pigments such as china clays will tend to attract water into the film. Thus mica and platy talcs and chlorites are much more desirable than are the clays in barrier systems.
On the other hand, clays may be used more effectively in inhibitive systems. China clays that are platy like the talc, will also tend to reinforce the film, and are said to improve the sanding of industrial primers.
Some materials that have been used in coatings in the past have relatively high water solubles. Anhydrous calcium sulfate falls into this category. It is now little used, but was sold and employed for many years as a complex with TiO2.
In addition, chemical reactivity and corrosion resistance is often influenced by the soluble salt content of the extender. Materials having high solubles tend to reduce corrosion resistance and increase film blistering over both wood and metal. Solubles are equally problematic in both naturally derived materials, precipitated products and post engineered materials. Common thixotropes such as the bentonite clays for example, may contain higher than desirable levels of chloride from surface treatment with quaternary ammonium salts. These may substantially reduce corrosion resistance when such materials are used to excess.
Optimal chemical resistance is afforded by those extenders that are completely or substantially inert. These materials include silica (both micronized crystalline silicas and amorphous silicas) and barytes, which are only attacked under the most extreme conditions, at which attack on the polymer itself is likely. Silica is now being phased out by many manufacturers because of the dangers of free silica content. Low silica bearing feldspathic extenders such as nepheline syenite, which are almost as inert as silica itself, are taking up the slack in many applications where silica was once predominant. Like both barytes and silica, nepheline syenite is a cheap nodular pigment with an oil absorption that is in the same range as amorphous silica.
Barytes is a small inert nodular pigment that packs well and gives excellent enamel holdout properties. Barytes is available in two forms, naturally ground barytes and the less common but whiter precipitated grade, which is usually finer and higher in oil absorption than the natural product. The softer precipitated grade known as “blanc fix,” disperses easily and is widely used in automobile surfaces and undercoats for gloss systems. Barytes is the heaviest extender with the lowest oil absorption characteristics. It is widely used in low VOC systems, but needs some additional support in order to control settling.
Aesthetics — The Effect of Extenders on Opacity
Extenders do not enhance the true opacity or color of the coating film. Opacity is a physical effect of light rays bending and scattering within the film at the interfaces between the pigment and the binder. The more the light is bent, the less likely will be the passage of light through the film and the greater the opacity. Light is bent to the greatest extent when there is a large refractive index difference between that of the pigment and that of the binder on both sides of this interface. All extenders have refractive indexes that fall in a range between 1.45 and 1.65. This is not markedly different from the values found in most dry polymer films (1.45–1.7), and so extenders give neither opacity nor much color when they are added into clear coatings. This should be compared to true prime pigments, such as TiO2, which have refractive indexes near 2.75.If we introduce air voids into the film, either by pigmenting at an exceedingly high PVC/CPVC ratio or using such devices as opaque (hollow) polymeric spheres, we replace many of the pigmentary binder interfaces with pigmentary air interfaces. In this way we introduce some degree of hiding because of the larger differential across the pigmentary air or polymer air interface compared to the pigmentary/binder interface (the refractive index of the extender in air is greater than the refractive index of the extender in binder). The device improves hiding, but often at the expense of other film properties.
More complex is the use of delaminated kaolin (calcined china clay) platelets to separate adjacent particles of TiO2 in order to achieve highest hiding efficiency. For this to happen, it is necessary for each adjacent TiO2 particle to intercept the light wavelength at the same point in the wave pattern. As the wavelength of light is on average somewhat longer (0.4–0.7m) than the diameter of an optimally dispersed particle of TiO2 (0.25m), it is necessary to use spacers to boost the center to center spacing between the two adjacent particles of TiO2 so that it equals one full wavelength. While complex, the device works, and is used in many interior architectural paints.
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