In the past 50 years, coatings technology has evolved at a rapid pace. As demand, performance requirements and environmental regulations have changed, so has the industry. Some things, however, have not. Cost versus performance has always been, and will always be, a primary motivator among formulators, contractors and do-it-yourselfers.
Successful chemists and material manufacturers understand this key truth and design products accordingly. Vinyl acetate-based polymers have helped formulators develop decorative coatings that meet this balance since the 1960s. Because vinyl technology was introduced to the coatings industry nearly one-half century ago, some formulators may have a perception that it is “old technology,” with little new to offer in the continually changing landscape of cost-performance needs and regulatory requirements faced by coatings producers today. In fact, there are many advances in vinyl acetate-based polymers that allow these binders to continue as a preferred choice of coatings formulators.
New technologies have opened up new options for formulators, including conventional vinyl acrylic binders, vinyl acrylic binders engineered to form films without coalescent, and vinyl acetate/ethylene (VAE) binders also designed to form films without coalescent. These new technologies and choices allow emulsion producers to offer a range of vinyl acetate-based polymers that exhibit a variety of performance attributes in coatings that still meet applicable government and industry regulatory standards. Understanding how these newer products compare to other technologies on the market will help formulators and end users make more informed decisions based on the needs of specific products. This article discusses a wide range of attributes, including scrub resistance, washability and aesthetic properties, and how coatings made with vinyl acetate-based polymers help formulators develop cost-effective solutions that meet these needs.
Defining Consistent Parameters
|Figure 1 Click to enlarge|
The findings in this article are based on an extensive study comparing the properties of many different vinyl acetate-based binders. To ensure consistency, a standard protocol was used for preparing all paints:
- A common grind was prepared using all raw materials except for the binder, coalescent (if required) and adjustment water.
- Equal weights of the common grind were poured into individual cans, and the appropriate amount of binder was added to obtain the desired pigment volume concentration (PVC).
- Finally, water and, if necessary, VOC-free coalescent were added to achieve the desired volume and volume solids for the formulation.
The binders were divided into three categories:
1.Conventional vinyl acrylics (VACon)
- Glass transition temperature (Tg) range for polymers: 10 °C to 24 °C.
- All of these latexes require coalescent to form a film at 5 ºC.
2.Self-coalescing (low minimum film forming temperature [MFFT]) vinyl acrylics (VALow)
- Tg range for polymers: 2 °C to 6 °C.
- None of these latexes require coalescent to form a film at 5 ºC.
3.Vinyl acetate/ethylene (VAE)
- Tg range for polymers: 4 °C to 12 °C.
- None of these latexes require coalescent to form a film at 5 ºC.
VA-Based Materials Provide a Wide Range of Benefits to Meet Formulator Needs
In comparing these three materials, the study showed clear differences between these three sub categories. The results discussed below focus on how the three categories differ and show the performance range based on specific application needs. If the testing yielded no clear differences, the property is not discussed.
|Figure 2 Click to enlarge|
Freeze/thaw stability showed a better correlation across the binder categories. Figure 2 shows that conventional vinyl acrylics have a better chance of achieving freeze thaw stability across the PVC range. In evaluating the individual products within the polymer type across the paint formulas, tests show that all of the low-Tg technologies failed freeze/thaw for all paint systems. The conventional technologies showed a progression of more difficult freeze/thaw as the PVC of the paint system became lower. At very low VOC levels, the chance of having a paint that is freeze/thaw stable is directly related to the PVC of the paint and the Tg of the polymer. To achieve good freeze/thaw at low VOC, a substantial improvement in technology development is required.
Another factor that comes into play for freeze/thaw stability is the PVC of the paint system. There is a statistically significant difference between the PVCs tested here, showing that there is a much better chance to achieve freeze/thaw stability in higher-PVC paints. A plausible explanation for this is that at higher PVC levels, paints have lower levels of latex and higher levels of pigmentation, which aids in protecting the polymer particles from coming in contact with each other during freezing.
|Figure 3 Click to enlarge|
Gloss Development Gloss development of the respective paints was very similar in the 65 and 50 PVC paints, with the 20º/60º/85º gloss averaging 1.4/2.2/1.8 for the 65 PVC paints, and 1.4/2.5/2.0 for the 50 PVC paints. The gloss differential becomes the most apparent in the 25 PVC semigloss, where pigment interaction with the latex polymer influences gloss development (Figure 3). By category, the polymers having the best gloss development in each category are:
- Conventional vinyl acrylics;
- Low-Tg vinyl acrylics;
|Figure 4 Click to enlarge|
Contrast ratio (hiding) in paints is affected by PVC, level of TiO2 pigmentation, extender type, and rheology and film formation. In evaluating the data, the biggest influence on contrast ratio is the paint formulation. It should be noted that all three paints utilized the same TiO2 levels, thus increasing PVC results in creation of air voids and increased hiding. (Figure 4).
In further evaluation of the 25 PVC paint formulation by binder category it is possible to show a small but statistically significant difference in hiding for conventional VA over low-VOC VA and VAEs.
|Figure 5 Click to enlarge|
Tint strength generally correlates with the same variables as contrast ratio but is also affected by surfactant levels, particle size and many other factors. In evaluating all the paint formulations, no significant statistical differences in tint strength based on binder category were observed. On average, a 3% spread across Y value was observed for each of the different polymers evaluated (Figure 5).
Application/Film Formation Properties
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|Figure 7 Click to enlarge|
Scrub resistance was measured per ASTM D 2486 (method B), using a side-by-side drawdown against a known control. For the 50 PVC paints, the highest scrub-resistant polymers by category are: VAE, > VALow and then VACon; however, within each category there was variability in how the specific binders performed. The coalescent-free vinyl acrylics all scrubbed more or less equivalent, whereas the VAEs showed the broadest range with a standard deviation of +/- 1500 scrub cycles, and conventional vinyl acrylics had a standard deviation of +/-500 cycles (Figure 7). Although not shown, the 25 and 50 PVC paints showed the same general trends by category for scrub resistance.
Because the 65 PVC is past critical PVC (CPVC), a change in the relative performance by category for scrub resistance is expected. This was found to be true. In these formulations, the VAE and coalescent-free vinyl acrylics performed similarly and there was less of a performance gap with conventional vinyl acrylics (Figure 8). It is apparent that PVC and formulation become dominant factors in scrub resistance above CPVC, whereas below CPVC inherent binder properties play an important role.
|Figure 8 Click to enlarge|
Blending with Acrylics
It is common practice in the coatings industry to blend vinyl-based binders with acrylic binders to achieve a desired balance of cost and performance. Vinyl acrylics are a preferred choice in blended systems versus VAE products because they typically demonstrate better compatibility with acrylic systems and corresponding improved performance. As an example, shown in Figure 9, it was observed that in the 25 PVC paints blending of the VAE with acrylic technology resulted in a greater reduction in scrub resistance when compared to a low-VOC vinyl acrylic technology. This is an important consideration in binder selection.
|Figure 9 Click to enlarge|
Washability is similar to stain resistance in this study in that both have a series of stains applied and allowed to sit for a standard period of time before rinsing off. Washability goes further, in that a sponge and cleansing solution are used to try to rub the stain off the surface. The conventional wisdom is that acrylics will do better for washability, followed by vinyl acrylics, with VAEs being the worst for washability.
For the purposes of this study, the following stains were used:
- Yellow mustard;
- Red ketchup;
- Grape juice;
- Hot coffee;
- Fountain pen ink;
- No. 2 pencil;
- Blue crayola crayon;
- Red grease pencil;
- Black shoe heel;
- Crayola washable marker – black;
- E-190 Royal Red lipstick.
In analyzing the washability for each stain in each of the three paint formulations, there were a few stains that stood out as having significant differences. In the 25 PVC paint, pencil proved to be the most difficult to remove. At 25 PVC, the overall washability for these stains (average) was best for low-VOC vinyl acrylics (Figure 10).
|Figure 10 Click to enlarge|
In the 50 PVC paint, the technology having the most difficulty with stain removal was again the low-Tg vinyl acrylic. This was specifically focused around the removal of mustard and ketchup stains. In evaluating high-PVC paints (65), the technology that had the most difficulty removing stains was the conventional vinyl acrylic.
Approaching the critical PVC, a number of performance properties show a marked change in performance. In this study, that change occurred between the 50 and 65 PVC paints as they passed the CPVC point. Choice of pigmentation above CPVC will influence this performance property and improve washability by two mechanisms: rapid film erosion and increased hydrophobicity based on pigment choice.
|Table 1 Click to enlarge|
Vinyl acetate-based binders offer coatings formulators proven performance, and this technology continues to advance to meet the evolving needs of the coatings industry. Across the three categories outlined in this study, formulators of interior paints can find many of the attributes needed to meet both customer and industry standards. Additionally, within each category, many different options are available, each with their own unique balance of performance attributes.
Table 1 summarizes performance properties across the three categories. If freeze thaw is an important performance feature, conventional vinyl acrylic technology is the best choice today for achieving this property without substantial negative impact on performance. If scrub resistance in unblended systems is important, the formulator might want to consider a VAE. If blending with other technologies is important, along with excellent touch-up and scrub resistance, the formulator should consider conventional and low-Tg vinyl acrylics.
By understanding the benefits each provides, as well as the material’s inherent strengths and weaknesses, formulators can make more informed choices that balance product performance with the most cost-efficient solution.
The authors would like to acknowledge the assistance of Lisa Sullivan, Bill Schmitz and Katherine Weatherington for their help in collecting data. We would also like to thank Ron Grieb for his guidance and support.