Novel Application of Fluorosurfactants

For the past several decades, the focus of interior latex paint performance has been on improved hiding power, scrub resistance and color acceptance properties. Driven by regulatory initiatives, new paint formulations are required to meet low-VOC regulations. These regulations have forced paint formulators to seek practical solutions to overcome additional challenges, such as poor wetting, flow, foaming, block resistance and stain resistance that result from the reduction in VOC content in paints. For flat to low-sheen eggshell paints, washability (or easy-clean) has become the most difficult property to achieve. Softer resins, lack of coalescent aids, longer film forming time, lower film forming temperatures, and overuse of surfactants and additives are contributing factors to poor washability. Easy-clean and stain resistance are not only highly desired by consumers, but these properties can also extend paint service life and reduce recoat cycles for environmental protection. These benefits provide long-term value to our society and future generations.

Stains that adhere to interior latex paint films are divided into two groups: hydrophilic and hydrophobic. Hydrophilic stains, often called common household stains, include soy sauce, tea, coffee, red wine, colored markers, etc. Hydrophobic stains mainly come from oily hands or fingerprints typically found around door frames and light switches. Other hydrophobic stains are crayons, pencils, or tomato sauce.

The washability of hydrophilic stains depends on the water resistance of the film. The most effective way to increase water resistance of the paint is to use higher water-resistant binders and a lower pigment volume concentration (PVC) ratio to form a compact film. The use of hydrophobic additives, such as silicone oils or waxes, can also boost hydrophilic stain washability. However, the latter approach can lead to poor oil resistance as well as surface defects, such as pinholes, craters and fisheyes.

Hydrophobic stain washability depends largely on additives if the binder type and PVC level are set. Fluorosurfactants (FS) have proven to be the only chemical in the additive class that can provide oil resistance through migration to the paint surface when the paint is wet, and cure into it when the paint is dry. Due to latex paint formulation complexity, simply adding a fluorosurfactant to the formulation is less effective than using it to optimize the total formulation.

Formulating a well-balanced easy-clean paint is never a simple job.1,2 Easy-clean performance is related to the overall formulation design, as well as the selection of the individual components. Binder type, PVC ratio and the additives package are considered as three key factors for easy-clean performance. Among them, the resin is the most dominant component. However, even with a few newly developed easy-clean targeted binder systems, the choice is still somewhat limited due to availability and cost.

The second most important factor to an easy-clean paint is its PVC ratio. Super-high PVC paints have a PVC ratio exceeding Critical PVC (CPVC), which means that it does not have a continuous film at the surface. Some interior paints have higher than 70% PVC. These paints are highly porous,3 and stains can easily penetrate into the pores. As a result, it is extremely difficult to wash off the stains. To achieve good easy-clean performance, it is necessary to have a continuous film at the surface. In other words, the PVC level of the paint should be controlled below CPVC. This PVC level control will help block initial stain penetration and maintain scrub resistance to sustain repeated washes in the stain removal process. Poor scrub resistance can be misleading for a superficial easy-clean paint. The truth is that the cleanability is due to the loss of surface film by abrasion. Common side effects are change of gloss level, loss of hiding and the need for touch-up or recoating.

The additive package also plays a very important role in easy-clean paint performance, especially with surfactants and defoamers. Optimized additive packages can deliver cost-effective solutions for the paint formulation. Hydrophobic additives, while effective against hydrophilic stains, have no positive impact on cleanability of hydrophobic stains. An oily-stain-resistant additive is needed to balance overall easy-clean performance of the paint.

Historically, fluorinated chemicals have been used as typical surface treatments for carpet, stone and textile surfaces to provide water and oil repellency.4,5 In our previous studies, we demonstrated that fluorosurfactants can be used as multi-functional additives to provide wetting, leveling, anti-blocking, and oil repellency in low-VOC latex architectural paints and water-based wood coatings.6,7 In this article we focus on the easy-clean property of paints. Three binders and two PVC levels were selected to evaluate the performance with the addition of two fluorosurfactants. The fluorosurfactants are commercially available products based on new short-chain fluorochemical technology with a superior performance and environmental profile. The basic chemical structure of these fluorosurfactants contains a non-polar fluorocarbon component and polar functionalities. Fluorosurfactants are surface active, and have a lock-in mechanism to ensure durable performance. Minor adjustments of hydrocarbon surfactants, defoamers and rheology modifiers were included in the formulation design.

Performance data of Leneta stain (representing oily stains), household stain cleanability, and water and oil contact angle are discussed in detail. We also selectively checked the impact of the fluorosurfactants on hiding power, gloss and scrub resistance. The study demonstrates that optimizing the paint formulation through the use of fluorosurfactants can improve latex paint easy-clean performance.

Experimental

Coating Formulation and Panel Preparation

Three binders: styrene acrylic (SA), pure acrylic (PA) and ethylene vinyl acetate emulsion (VAE), and two PVC levels (40% and 55%) were used to design the base formulations. Two surfactant packages were selected to investigate easy-clean performance. One surfactant package contained a nonionic hydrocarbon surfactant (HCS), which was used as a pigment wetting agent in the grind stage. The second surfactant package contained an anionic fluorosurfactant (FS) added as a pigment wetting agent with reduced defoamer in the grind stage, and a polymeric fluorosurfactant added in the letdown stage as an easy-clean additive. Generic formulations for PVC 40% and 55% are listed in Tables 1 and 2. Fourteen paint formulations were made, and their formulation matrix by function is shown in Table 3.

Film Preparation

Formulated paints were mixed thoroughly and aged for 48 h to achieve maximum dispersion of the fluorosurfactant additives. Paint films were prepared on black Leneta Mylar® drawdown cards using a BYK-Gardner automatic drawdown machine and a bird applicator drawdown blade. The blade clearances were 5 mil for the cleanability test and 7 mil for the wet scrub test. The drawdown speed was 30 mm per second to produce even film thicknesses. The film was allowed to dry at room temperature for 7 days prior to evaluation.

Hiding Power and Gloss

The hiding power of paint measures its opacity or its ability to cover under-layer color so as to obscure or prevent the original color from showing through. Hiding power is determined by contrast ratio on standard black and white Leneta cards.

Contrast ratio=Rb/Rw

Rb: reflectance over black substrate

Rw: reflectance over white substrate

Contrast ratio and gloss of the panels were measured by a spectrocolorimeter supplied by Sheen.

Wet Scrub Test

Wet scrub resistance was tested per ASTM D2486 Method A. The dried panel was placed on the top of a shimmed plate in the Sheen wet abrasion scrub tester to start the test. The number of cycles, at which one continuous thin line of substrate was observed, was recorded.

Leneta Stain Cleanability

This test is a modified version of ASTM D3450. The prepared panels were cut into 40 x 100 mm sections for testing. The stain was applied to a Mylar card through a template with an 8 x 2 mm window to make a clear-edged staining area (Figure 1). The stain was allowed to remain on the panel for 1 h. The excess stain was gently scraped off and the stained areas were carefully wiped with a clean paper towel until no visible stain could be wiped off (Figure 2). The washing block of the abrasion tester was covered with eight layers of cheesecloth. Then 20 mL of cleaning solution (1% mild detergent solution) was added to the cheesecloth and the washability tester (Figure 3) was run for 25 cycles (50 wipes) over the stained drawdown panel. After rinsing with water and drying for 2 h, the panel was rated. Unstained paint, unwashed stained paint and washed stained paint were measured three times in different areas to report average L value by a colorimeter. Leneta stain cleanability rating can be calculated by the following formula:

(L washed paint – L unwashed stained paint) ×10 /

(L unstained paint – L unwashed stained paint)

Household Stain Cleanability

The household stain cleanability method measures the relative degree of common household stains removed by washing with a 1% mild detergent solution. Dried paint panels were divided into 20 mm width area for different stains. All stains were allowed to dry at room temperature for 1 h before washing. After the test, the panel was rinsed with water and dried for 2 h. Remaining stains were judged visually on a scale of zero to 10 to reflect 0% to 100% of stain removal.

Water and Oil Repellency

 The water and oil contact angles of panels were measured with a Goniometer using a high-speed CCD camera for image capturing. Water or n-hexadecane (oil) was dropped onto the surface by a syringe. The computer-aided Goniometer software measures a droplet’s advancing contact angle by assuming the droplet fits the geometry of a sphere when the drop is stable after 1 min. A high water or oil contact angle indicates high hydrophobicity or oleophobicity of the coating. 

Results and Discussion

Stain Resistance and Washability

The stain resistance rating data of 14 investigated formulations is shown in Table 4. For hydrophobic stains like Leneta stain, crayon and lipstick, fluorosurfactants containing Formulations (2, 4, 6, 8, 11 and 13) showed better performance than HCS-containing Formulations (1, 3, 5, 7, 10 and 12). In the formulation group of 55% PVC and pure acrylic binder (7, 8 and 9), Formulation 8 with a FS package has a rating of 8, 6 and 7 for Leneta stain, crayon and lipstick vs. Formulation 7 with the HCS package that has a rating of 4, 2 and 6, respectively. Incorporation of wax in Formulation 9 did not significantly improve the performance.

For hydrophilic stains, the cleanability of marker, ink, tea and coffee was not significantly improved among all formulations. This confirms that hydrophilic stains are very hard to clean in low-gloss interior paints. The addition of FS has minimum impact on their cleanability.

Effect of Binder Type

Formulations 7, 10 and 12 were formulated with pure acrylic, styrene acrylic and VAE binders and with 55% PVC and a HCS package. The total cleanability rating of all stains was 41, 45 and 52, as shown in Figure 4. The hydrophobic stain cleanability ratings were 20, 22 and 35. The VAE binder system showed better stain resistance performance than the other two binder systems in removing hydrophobic stains (Leneta, crayon, lipstick and pencil). However, the differentiation of the binders only delivers some degree of stain resistance. The formulation needs to be further optimized with respect to PVC level and surfactant packages.

Effect of PVC

In pure acrylic binder systems with HCS, two PVC level formulations (40% and 55%) were tested for stain cleanability, and the data is shown in Figure 5. The results show that the stain cleanability rating of most stains declined with the increase of PVC level. Formulating a good easy-clean paint requires a minimum PVC level. When PVC is too high, the stain cannot be cleaned off the porous surface. Even a good binder cannot aid the performance as expected.

Effect of Surfactant

Two classes of surfactants were used to evaluate their impact on stain cleanability: fluorosurfactants and hydrocarbon surfactants. With a pure acrylic or styrene acrylic binder and a 55% PVC ratio, good oil-stain cleanability was achieved by the use of the FS package represented by Formulations 8 and 11, shown in Figures 6 and 7. For the PA system, Formulation 8, containing an anionic FS and a polymeric FS, significantly improved the oil-stain cleanability rating to 30 from 20 for Formulation 7. For the SA system, Formulation 11 with FS improved the oil-stain rating to 29 vs. 22 for Formulation 10. The combination of anionic FS and wax was less effective than the combination of anionic and polymeric FS in achieving oil-stain cleanability, shown in Figure 6. Anionic FS serves as a multi-functional wetting agent in the grind stage to provide superior wetting power with less foam. As a result, less defoamer was needed, which improves paint oil repellency. In addition, the polymeric FS added in let-down can further improve the cleanability when cured in the paint surface with low surface energy.

Water and Oil Repellency

The water and oil contact angle data of formulated paints are listed in Figure 8. Water contact angle represents film water resistance, which is related to hydrophilic stain resistance and washability if surface porosity is not too high. In PA and SA systems, FS use slightly decreased the paint’s water contact angle. In VAE paints, FS use improved the paint’s hydrophobicity. The oil contact angles of formulations with HCS are near zero, which means the oily stain or oily chemical can easily wet-out on the film and will have negative effect on oil-stain cleanability. The oil contact angles of formulations with a FS package are significantly increased to more than 50°. Oil beading up on the paint film is demonstrated in Figure 9. The addition of FS can offer excellent oil repellency to paint films to resist oil stains. In the VAE system, the addition of anionic FS added both in the grind and letdown stage enhanced the water contact angle from 33° to 83°, which leads to hydrophobicity improvement in the VAE paint film.

Hiding Power

The contrast ratios of six paints with 55% PVC are listed in Table 5. Change in hiding power was not observed by altering the surfactant packages in specified PVC ratios and binder systems. The addition of FS did not affect the hiding power of the paint formulations. It is important to maintain hiding power when optimizing the formulation for other desired performance characteristics.

Wet Scrub

In this study, 55% PVC formulations were only checked for wet scrub resistance, since higher-PVC paints are more likely to have lower than desired scrub resistance. According to the data collected in Table 6, wet scrub resistance of PA, 55% PVC formulation is better than a SA, 55% PVC and a VAE, 55% PVC formulation. The scrub resistances mainly depend on the resin system and PVC level, and did not change significantly with the change of surfactant type. FS do not have significant impact on the wet scrub performance of paint.

Gloss

Gloss data of 40% and 55% PVC formulations in various binder systems are listed in Table 7. The gloss is mainly determined by PVC level. FS do not have significant impact on the gloss of paint.

Conclusions

Easy-clean properties of interior low-sheen paint are highly formulation dependent. Binder, PVC ratio and additive package all play important roles in final performance. Formulating an easy-clean paint is more challenging in high-PVC paints due to the high porosity of the paint film. Based on this study, the key points of formulation design are summarized below.

1. For binder selection, VAE binders have better oil-stain cleanability compared with PA and SA systems. The lack of hydrophobicity of a VAE binder can be improved by the use of proper additives. For PA and SA systems, higher hydrophobicity can contribute to hydrophilic stain resistance and cleanability. The PA and SA system’s lack of oil stain repellency can be improved by fluoroadditives in formulation design.
2. For PVC level, the increase of PVC ratio will decrease overall stain resistance and cleanability of the paint film. Easy-clean paints need to have the lowest possible PVC ratio while maintaining the sheen.
3. With respect to the surfactant package, this study revealed that a combination of small molecule and polymeric FS with reduced defoamer additives has the best overall effect in formulating easy-clean paints. The FS can migrate and concentrate at the liquid-air interface due to the oleophobic nature of fluorocarbons. At the surface, the FS provides oil repellency to prevent the penetration of oily stains. In the grind stage, anionic FS can partially or totally replace the nonionic HCS, and thus reduce the need for defoamer. Since most defoamers attract oil and decrease oily-stain resistance, the reduction (or elimination) of defoamers will further enhance the oily-stain resistance and cleanability.

In summary, fluorosurfactants can help a well-designed interior flat paint to achieve overall easy-clean properties by adding oil repellency to the paint. 

References

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