Defining and Predicting Performance of SurfaceModifiers in Coatings
Wax-based surface modifiers are commonly used additives in the coatings industry. They are used by coatings formulators to optimize abrasion resistance, raise or lower coefficient of friction, or improve chemical resistance. Unfortunately, many formulators do not understand how and why surface additives perform, as well as the methods necessary to maximize performance of this class of additives. This paper examines the performance benefits derived from surface modifiers, the major surface modifiers available, how surface modifiers function and the decision-making processes necessary for maximum performance. The functions of surface modifiers can be listed as follows:
- Reduce the effect of friction/surface tension of
Improve abrasion resistance
Alter a coating’s “feel”
Modify surface appearance
Enhance other performance properties
Surface modifiers are used in paints and clear coatings to modify the appearance of a cured film, or to improve its performance characteristics. Typical performance features and benefits are described in Table 1.
How Waxes Perform in Coatings
For a wax to perform, the material must migrate to the surface and sometimes protrude out of the coating. Surface modifiers rely on two mechanisms to migrate.
Other variables that affect migration are raw material interaction, coating viscosity/specific gravity, curing conditions, additive chemistry and additive form.
Wax-Based Surface Modifier Chemistries
A wax is a low-melting-point organic material or compound that is solid at room temperature. Waxes may be hydrocarbons, alcohols or esters of fatty acids. They may or may not be soluble in organic solvents. Waxes melt above 40 deg C without decomposing to produce relatively low-viscosity, nonstringing liquids.
Waxes can be divided into synthetic (produced by polymerization), refined (from fossil fuels) and natural. Performance features based on wax chemistries can be predicted. Each wax chemistry may offer a particular feature or a blend of performance benefits.
Polyethylene (PE) waxes are produced by the polymerization of ethylene. The reaction conditions determine the molecular branching and, therefore, crystallinity and molecular weight. PE waxes have a broad range of physical characteristics, providing a wide variety of performance possibilities in coatings. Melting points for PE waxes are between 100 deg C and 130 deg C. The features and benefits of PE waxes include good slip and matting properties; improved scratch, mar and abrasion resistance; and reduced solubility compared to FT or microcrystallines. The disadvantages of PE waxes are that they lower gloss and may melt at high temperatures.
Polypropylene (PP) waxes are produced by polymerization of propylene. They have higher molecular weight and, therefore, a higher melting point than most other waxes. Some of their more plastic-like properties, e.g. their high elasticity and toughness, are made use of in coatings in blends with PE waxes. The features and benefits of PP waxes include improved scratch resistance, very good antiblocking properties, excellent abrasion resistance and good migration characteristics. Their disadvantages are cost and poor slip qualities.
Mono and bis-amide waxes are semi-synthetic waxes produced by reaction of fatty acids with amines and diamines. Although they have higher melting points (140-160 deg C) than most other waxes, they have relatively low penetration hardness and are relatively brittle.
The features and benefits of amide waxes include good matting, excellent sanding, enhances silky and soft feel, thickening of liquids, antisettling properties and good migration characteristics. Their disadvantages are reduced gloss, thickening of solventborne paints, and possible yellowing in light-colored thermoset coatings.
Carnauba wax is an ester of long-chain alcohols and acids. It is extracted from the leaves of the carnauba palm tree. Carnauba generally melts below 100 deg C. The features and benefits of carnauba wax include excellent slip and good mar resistance, hardness, excellent clarity, and FDA compliance. Its disadvantages are cost/availability due to crop variations, and the color may prohibit use in some applications.
PTFE is produced by the polymerization of C2F4. It is not a wax because it does not dissolve or melt at the normal temperatures used in coatings. However, it can function like one. PTFE produces a very low coefficient of friction in coatings and inks, and is usually used in blends with PE. The features and benefits of PTFE wax include excellent slip; antiblocking; improved stability against polishing; and improved abrasion, scratch, mar and scuff resistance. Its disadvantages are high cost, potential intercoat adhesion problems at high levels, possible outgassing of fluorine at high thermosetting temperatures, and poor migration properties.
In PE/PTFE-combination products, the PE often acts as a carrier for the PTFE particle to transport this product to the surface of the coating where it can provide the ultimate physical properties required.
Paraffin and microcrystalline waxes are both refined from petroleum. Paraffin waxes are lower-molecular-weight hydrocarbons of relatively high crystallinity. Microcrystalline waxes are higher-molecular-weight, more highly branched hydrocarbons with lower crystallinity. Their melting points vary between 50 deg C and 100 deg C, depending on structure.
Fisher-Tropsch (F-T) waxes, also known as polymethylenes, are chemically similar to polyethylene but are produced by the reaction of carbon monoxide and hydrogen. The types used in coatings melt at 100 deg C to 106 deg C.
The molecular weight of waxes lies between 500 (paraffin) and 6000 (polyethylene). Polypropylene and PTFE are not waxes but perform in a similar way and, hence, are included. Important properties such as melt point, melt viscosity, hardness and especially crystallinity depend on the molecular weight and molecular branching, as shown in Figure 4.
Surface Modifier Performance Based on Particle Size
As important as chemistry is to wax performance, particle size is equally important. Waxes are available in a number of forms.
Powders. Wax powders can be prepared by micronization or spray drying. In both cases, typical average particle size ranges from 5 to 25 microns. Because of their coarser particle size, dry powders typically offer the best matting, antiblocking and abrasion resistance.
Cold Dispersions. Cold dispersions are finer-particle dispersions produced by grinding equipment including ball mills, three roll mills or bead mills. The resulting particles normally range from 1 to 4 microns. As finer particles, these dispersions exhibit higher gloss and transparency, with a compromise on properties such as antiblocking or mar resistance.
Emulsions or Precipitations. These pastes and liquids are very-fine-particle-size dispersions manufactured by precipitation of molten material in a desired fluid carrier. Particle sizes normally are submicron, which yield the highest gloss, transparency and ease of incorporation.
Table 2 summarizes the performance of surface modifiers versus physical form.
Selecting the Correct Surface Modifier
In selecting surface modifiers, a number of questions must be answered:
1. What do you want to achieve? Each surface modifier is formulated to provide some level of properties.
2. Is your coating system solvent- or water-based? Each surface modifier is developed to perform in specific systems.
3. Is your system pigmented or clear? Particle size affects clarity, and clear systems may require a smaller-size particle.
4. What is the dry film thickness? This is required to determine optimum particle size of the surface modifier.
5. What are the gloss requirements? Particle size may impact gloss. A high-gloss system requires smaller-particle-size products.
6. What is the temperature and time cycle? Choose a surface modifier with an appropriate melt point to optimize properties
7. Will there be a need to recoat? At higher levels, some surface modifiers may affect recoat ability. Keep an eye on addition levels
8. How will you evaluate the performance of the surface modifier? This will be determined by the coating application requirements.
Case Study 1 – Optimizing Gloss and Transparency in a Solventborne Coating
In certain situations, the required performance is provided by specific surface modifier chemistries. To optimize gloss and transparency, a formulator is then forced to modify particle size. Table 3 shows how reducing particle size can dramatically improve transparency and specular gloss in a nitrocellulose lacquer.
PE/PP-D is a micronized polyethylene/polypropylene alloy. Taking this material and further reducing particle size via bead milling produces the PE/PP-D1 dispersion. This finer particle size improved the 60-deg gloss measurement by 12 units and reduced haze by 50%.3
Case Study 2 – Improving Scratch Resistance of a Waterborne Coating at Various Gloss Requirements
This study was performed using a waterborne acrylic/PU varnish that, without surface modifiers, exhibited poor antiblocking, water resistance and scratch/mar resistance. The 60 deg gloss of this varnish was 80+. The formulation is listed in Table 4.
The choices for a surface modifier to improve the properties of this varnish include a micronized PE, a PE dispersion and a PE emulsion. The micronized PE and PE dispersion will increase scratch and mar resistance, improve water resistance and antiblocking while providing uniform matting if a lower gloss varnish is required. The PE dispersion will be much easier to incorporate than a micronized PE. Similar performance properties at lower efficiency can be expected from the PE emulsion, but the gloss of the varnish will be maintained. Matting efficiencies of the three PE surface modifiers are shown in Figure 6.
Scratch-resistance tests were performed on the waterborne acrylic varnish with and without surface modifier, and the results are pictured below.
The addition of PE surface modifiers either in micronized, dispersed or emulsion form improved scratch and mar resistance, antiblocking and water resistance of the waterborne acrylic/PU varnish. The micronized and dispersed PE also provided uniform matting to the varnish.
Case Study 3 – Improving Slip without Compromising Gloss in Powder Coatings
Micronized surface modifiers normally reduce the gloss of all coatings into which they are incorporated. This is also the case with powder coatings. It is difficult to provide excellent reduction of COF (coefficient of friction) and improved slip properties without affecting the specular gloss of a powder coating. With a surface modifier like PE, the gloss is greatly affected at the levels of addition required to provide good slip properties. PTFE affects gloss less, but it is still reduced at PTFE levels greater than 1%, which are required to lower COF significantly.
A study was performed using a urethane powder coating, comparing a PE/PTFE surface modifier to a proprietary polymer especially developed to maintain gloss in powder coating formulations while providing excellent COF reduction. The formulation is listed in Table 5.
A gloss-stability study was performed comparing the PE/PTFE surface modifier to the proprietary polymer. The 60 deg gloss stability versus surface modifier loading is presented in Figure 7.
A COF study was performed utilizing the same powder coating formulation and surface modifiers. The COF results were obtained using an Altek unit, which is pictured in Figure 8. The data for this COF study is shown in Figure 9.
Comparing Figures 7 and 9, it is evident that to get COF values below 0.1, a formulator must use more than 1% PE/PTFE, which will lower the 60 deg gloss of the powder coating. With the proprietary polymer, COF values of 0.1 can be produced at levels below 1%, while maintaining the 60º gloss of the powder coating.
Case Study 4 – Matting UV Coatings with Minimal Viscosity Increase
UV coatings are increasingly being used as low-VOC (volatile organic compounds) alternatives for wood coatings. UV varnishes typically exhibit high gloss with excellent abrasion and chemical resistance. However, most consumers prefer a matted finish in their wood products. Traditionally, chemists use silica chemistries to reduce gloss of varnishes and coatings. For UV coatings, silica is not always effective due to excessive viscosity increase, which makes the coatings unsuitable for spray or roll application.
To overcome the above viscosity issue, the coatings formulator should consider alternative additive chemistries. As mentioned earlier, polypropylene waxes exhibit excellent matting properties combined with outstanding mar, scratch and abrasion resistance. A UV 100% solids formulation is listed in Table 6. This type of coating is typically used as a roller-applied top coating for furniture.
Films were applied at 12 microns dry film thickness. Performance data are presented in Table 7.
The polypropylene surface modifier at 5% treat effectively reduces gloss without dramatically affecting viscosity. These properties are achieved due to the lower surface area and reduced oil absorption of polypropylene compared to traditional silica matting agents.
Choosing the correct surface modifier requires knowledge of required end properties of the formulated coating along with that knowledge and understanding of the basic properties of surface modifier chemistry and the relationship of performance versus particle size. After the correct surface modifier is chosen, the last step is to optimize the treat rate to have the best of all worlds (cost versus benefit).
For more information, contact Lubrizol Corp., 29400 Lakeland Blvd., Wickliffe, OH 44092; phone 440/943.4200; fax 440/347.6590; or visit www.lubrizol.com.
1 Jansen, K. “The Role of Waxes in Coatings and Inks,” Paint and Resins, April 1989.
2 Shah, Vishu. Handbook of Plastics Testing Technology, New York: John Wiley and Sons Inc., 1984.
3 Rohr, E. “Surface Modifiers for Matting with Minimal Haze,” Modern Paint and Coatings, Atlanta, GA, Argus Incorporated, July 1995.
4 Rohr, E. “Defining and Predicting Performance of Surface Modifiers in Paints and Coatings,” 7th International Paint Congress, September 19-21, 2001, Sao Paulo, Brazil.