This article discusses the role of fluorosurfactants in waterborne wood coatings and how they can be used as cost-effective additives to help improve surface tension reduction, wetting, leveling, oil repellency and chemical resistance in thin film applications.
Coatings on wood surfaces are mainly for protective and decorative purposes. In recent years, manufacturers of wood coatings have devoted their research and development resources to improving waterborne coatings due to their huge environmental advantage.(1) Waterborne coatings offer finishers outstanding environmental benefits such as VOC compliance, lower odor and fewer harmful organic vapors, lower flammability, a safer and healthier workplace with fewer allergies and less skin irritation, and reduced fire hazards. Along with these come economic advantages such as lower insurance and storage costs, and lower hazardous waste disposal costs. They also allow for application advantages such as the ability to use existing application equipment to apply at varying thicknesses and reduced labor for clean up.
However, the change from solventborne to waterborne coatings led to the development of more complex coating systems, containing more components. The surface tension of water (72 dyne/cm) is much higher than solvents. Simple VOC reduction without a proper surfactant package will cause many surface defects such as craters and orange peel, so wetting, leveling, foam control and surface repellency of waterborne wood coating are becoming ever challenging tasks for formulators. One effective solution is to add suitable additives to improve the performance. Surfactants in wood coatings provide two major functions; they ensure smooth application of the wet film and improve film substrate adhesion; and provide surface energy modification to improve the film’s repellency and chemical resistance.
In a previous study, we demonstrated that fluorosurfactants can be used in low-VOC latex architectural paints as multi-functional additives to provide wetting, leveling, anti-blocking, oil repellency and dirt pick up resistance properties.(2) In this article, we discuss the role of fluorosurfactants in waterborne wood coatings and how they can be used as cost-effective additives to help improve surface tension reduction, wetting, leveling, oil repellency and chemical resistance in thin film applications.
|Table 1 Click to enlarge|
Almost all types of resins are now available to meet the specific needs in waterborne formulations like acrylics, epoxies, alkyds and urethanes.(3) For waterborne wood coatings, the main coating systems include one-pack acrylic latex or dispersion, one-pack polyurethane dispersion (PUD) or PUD acrylic hybrid, one-pack alkyd, and two-pack waterborne polyurethane (polyol and isocyanate). The acrylic binders are generally produced by emulsion co-polymerization of various monomers to form colloidal stabilized solid particles in water (latex). Film formation of acrylic dispersions consists of water evaporation, followed by coalescence and finally by inter-diffusion of the polymer particles to form a film.(4) Water-reducible polyurethane dispersions (PUDs) and polyurethane-acrylic hybrids offer near-urethane performance with the application ease of a one-component coating. In this chemistry, modified isocyanate molecules are stabilized in water and reacted with polyols before they reach the end user to form a waterborne polyurethane dispersion.(3)
This study used a waterborne self-crosslinking acrylic dispersion as the binder because waterborne, acrylic dispersion, air-dry coatings are widely used in wood finishing where a fast, low-temperature cure is needed. The films are hard and very fast drying.(5) It is important that the use of fluorosurfactants will not cause recoatability issues in wood coatings, and our data confirms this.
Coating Formulation and Application
For our experiment, the binder used was a medium-hard, self-crosslinking acrylic dispersion with MFFT of 113
Results and Discussion
|Figure 1 Click to enlarge|
The surfactants were diluted with water to the same dosage in the paint formulations, and foam was generated by hand shaking. The volume of foam is shown in Figure 1, which was taken 15 sec after shaking. The figure shows that the four fluorosurfactants all generated less foam than the silicone surfactant. Zonyl® FS-610, Capstone® FS-61 and Capstone® FS-63 fluorosurfactants are especially low-foaming products, as there is almost no foam after shaking.
Surface Tension Reduction
Surface tension of fluorosurfactants in deionized water and commercial wood coating binders is shown in Table 2. Water-reducible polyurethane dispersion (PUD) is also widely used in one-component wood coating systems due to its near-urethane performance. The reduction of surface tension was listed to compare with the studied acrylic resin. Compared to silicone surfactants, fluorosurfactants have the most power to reduce surface tension in water and water-based binders, especially in PUD resin. In four fluorosurfactants, the new-generation Capstone® surfactants showed better surface reduction abilities than Zonyl® products.
|Table 2 Click to enlarge|
Wetting and Leveling
Coatings were sprayed on oak planks; the surface appearances of different formulations are shown in Figure 2.
Optical microscopy photos (8x magnification) of the formulation using fluorosurfactant and silicone surfactant are shown in Figure 3. The formulations using Capstone® FS-61 and Capstone® FS-63 achieved good wetting and leveling performance in a low dosage of defoamer. The formulation using Zonyl® FSP had air foam in the surface, and serious surface defects such as cratering and orange peel were observed in the formulation using silicone surfactant. It is demonstrated that Zonyl® FS-610, Capstone® FS-61 and
|Figure 2 Click to enlarge|
Capstone® FS-63 fluorosurfactants are helpful in improving wetting and leveling in waterborne wood coatings.
Wetting is the action of the liquid coating as it spreads across a surface, and is essential for successful coating application. Wetting performance can be improved by two methods: 1) raising the surface energy of the substrate by cleaning, removal of contamination and pretreatment of substrate; 2) lowering the surface tension of the coating by the use of surfactants. If there is no change to the substrate, then it is crucial to lower the surface tension of the coating to achieve good wetting. Compared with silicon surfactants, fluorosurfactants are more effective in lowering the surface tension, which leads to better wetting performance.
|Figure 3 Click to enlarge|
Leveling is the ability to achieve a smooth and uniform surface. It is a highly demanded attribute in high-gloss wood coatings and finishes. During film formation, numerous problems can occur, such as inadequate wetting of the substrate, poor flow, crater formation and pigment floating. These problems all influence the leveling performance. In general, leveling performance of coatings can be controlled by surface tension, rheology, film thickness, the evaporation rate of solvent and pigment dispersion. In our testing, factors such as thickener, dry film thickness and co-solvent are the same, and the difference is the surface tension of the paint.
|Table 3 Click to enlarge|
So it is reasonable to consider that better surface tension reduction of fluorosurfactants leads to better leveling performance compared to silicon surfactants. Fluorosurfactants promote substrate wetting to prevent craters, fisheyes and crawling, and the elimination of these surface defects is very helpful to achieve good leveling. Fluorosurfactants also can establish a uniform surface tension over the entire surface area due to their fast migration and good compatibility, which also are positive to leveling. Overall, the use of fluorosurfactants in waterborne wood coatings can help retain good gloss and even reflection, and smooth formation of the film.
|Figure 4 Click to enlarge|
The water and oil contact angles of different paint films are listed in Table 3, and Figures 4 and 5 show water and oil contact angles of different formulations.
|Figure 5 Click to enlarge|
The water contact angle of the formulation using silicone surfactant is slightly higher than the formulations using fluorosurfactants. However, the oil contact angle of the silicone surfactant formulation is very small (near zero), meaning the oil stain or oil chemical can almost spread completely on the film surface.
The oil contact angles of fluorosurfactant formulations are much higher than silicone surfactant formulations, and oil beads up on the surface. This indicates the addition of fluorosurfactant can offer excellent oil repellency to paint film.
Tack testing results are shown in Table 4. The addition of -Capstone® fluorosurfactant in formulations A, B, C and D offers better performance than silicone surfactant for formulation E.
|Table 4 Click to enlarge|
Adhesion, after three layers of recoating, was measured by the crosscut method using 3M scotch tape. The results are listed in Table 6. The adhesion of the formulations with fluorosurfactant was 5B, and the formulation with silicone surfactant was only 3B. This indicates that there is no negative effect of fluorosurfactant on recoat adhesion.
|Table 5 Click to enlarge|
It has long been known that the surface tension of a liquid is the most important factor influencing wetting performance. In general, the lower the surface tension, the better the wetting. This becomes particularly important with the introduction of water-based coatings. In water-based systems, due to the high surface tension of water, it is usually necessary to incorporate suitable additives to lower the surface tension, ensuring good wetting of the substrate. Wetting is especially difficult on substrates with low surface energy, such as plastics or on substrates contaminated by grease or dirt. Fluorosurfactants are the most efficient in lowering the surface tension to improve wetting and prevent surface defects. The elimination of these surface defects is also positive for flow and the improvement of leveling performance.
|Table 6 Click to enlarge|
One of the features of fluorosurfactants is that they are oleophobic or oil repellent. The use of fluorosurfactants can dramatically reduce film tackiness and enhance oil repellency, and improve chemical resistance. This multi-functional property is directly related to the improvement of dirt pick up resistance of exterior wood coatings. With respect to concerns about recoating, our study showed that there is no recoating issue when using fluorosurfactants in waterborne wood coatings.
Compared to other classes of additives, fluorosurfactants have many advantages in waterborne wood coatings. Fluorosurfactants can:
- Most effectively reduce liquid coating’s surface tension;
- Offer excellent wetting and leveling to the paint film;
- Help the formulator simplify the formulation by reducing the need of defoamer and improve overall film quality;
- Reduce film tackiness and enhance oil repellency, and improve the chemical resistance.
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This paper was presented at the 2011 38th Annual Waterborne Symposium in New Orleans. © 2011 DuPont.