Table 1

Introduction

Fluorosurfactant use in coatings dates back to the 1970s.2 They were used as wetting agents and to improve leveling for many years in various coatings systems.3 In the past decade, more demanding performance requirements were expected from coatings users. In addition, the trend to lower solvent use levels was creating new challenges for the coatings chemist. Fluorosurfactants are useful additives in solving low-VOC coating-related problems in architectural coatings, such as anti-crater, anti-block and shorter open times in addition to their traditional role as wetting agents.

Telomer-based fluorosurfactants contain perfluoroalkyl chains and various functional groups. They have unique properties in significantly reducing surface tension in all aqueous, solvent and neat systems. They are air-liquid interface active, providing wetting, re-wet and oil repellency while meeting surface tension needs in coatings. They have been used in solving low-VOC formulation problems mentioned above. Until very recently, fluorosurfactants were used to address formulation problems near the end of the coating development cycle. Our recent research and practice have demonstrated that fluorosurfactants are most useful and cost effective due to the multiple functions they provide in formulating a low- or VOC-free architectural coating. The formulator can reduce other ingredients and create high-performance paints by minimizing the use of other coating additives, such as defoamers, wetting agents, leveling aids, anti-block additives and open-time extenders. Oil repellency and other functions that fluorosurfactants provide have also made them attractive to formulators in developing dirt pickup-resistance exterior paints.

Figure 1

Surface Tension Reduction

Table 1 shows selected Zonyl® fluorosurfactants’ surface tension in deionized water (DI), measured by tensiometer. Compared to hydrocarbon and silicone-based surfactants, fluorosurfactants have the most power to reduce surface tension. Not only can they reach 20 dynes/cm surface tension, but their CMCs are very low.

While fluorosurfactants are perceived by many as foam generators, certain types of fluorosurfactants act as defoamers in coatings when used at low levels. The grades we recommend for most architectural paint applications decrease foam when the formulation also minimizes the use of other foam-causing surfactants. In most cases, when fluorosurfactants are used other defoaming agent usage can be decreased or eliminated.

Fluorosurfactants also work synergistically with hydrocarbon surfactants in resin synthesis during emulsion polymerization. Their main functions are stabilizing the emulsion, controlling particle size and allowing the reduction of hydrocarbon emulsifier surfactants. Reducing hydrocarbon surfactants can also help reduce foaming in the paint formulation. After the polymerization, fluorosurfactants are carried over to the final coating formulation to provide the other properties provided by fluorosurfactants.

Figure 1 shows surface tension reduction in DI water of hydrocarbon surfactants alone and mixtures of hydrocarbon surfactants with Zonyl at an equal cost basis. This experiment was run with two hydrocarbon surfactants and one fluorosurfactant. At $0.02 per pound cost of surfactants, the hydrocarbon surfactants alone were less effective than using $0.01 of hydrocarbon surfactant and $0.01 of Zonyl FSO. With $0.02 of Zonyl FSO alone, surface tension was reduced below 20 dynes/cm.

Figure 2

Anti-Block

In a developmental deep-base semigloss paint, we were able to improve the block rating (ASTM D 4946) from 0 to 5 by adding 0.1% (a.i.) Zonyl 9361. Later study showed that if the colorant dispersant is reduced, the block rating was further improved with 0.05% (a.i.) Zonyl 9361. The addition of the fluorosurfactant did not change other properties of the coating.

Figure 2 showed the impact on block resistance with fluorosurfactants in two low-VOC paints. In both cases, multiple additives were replaced with one fluorosurfactant to achieve block resistance.

Table 2

Open-Time Extension

Figure 3

Dirt Pickup Resistance (DPR)

One approach is to make the paint surface highly hydrophobic, measured by high water contact angle. This approach often uses a silicone-modified resin or silicone additives in the formulation. The challenge with this approach is that if the paint does not have high enough water repellency, which has a water contact angle above 130-145°, dirt stains will appear as dark streaks on the wall that are not removed by rain.

A second approach is to make the paint surface hydrophilic, so it will allow rain to rinse the surface. If this surface is also static dissipative it will repel airborne particles from the painted surface. Fluorosurfactants work well with the second approach. Fluorosurfactants serve three functions: allow the reduction of other oil-attracting additives; allow the reduction of hydrophobic additives; and add oil repellency to the paint. Because fluorosurfactants migrate to the film surface and enrich the top 25% depth of the paint surface, the amount needed to achieve oil repellency at the surface is very low. Figure 3 demonstrates that with selected Zonyl products, a commercial high gloss, low-VOC paint film maintained its water contact angle and significantly increased its oil contact angle. (In this example no other additives were removed, which would help in further reducing water contact angle.)

Figure 4 shows an example of formulating an exterior paint utilizing Zonyl 9361 as a wetting, leveling, anti-blocking and oil-repellent agent. In the formulation, a silicone-based leveling agent and a wax-based anti-blocking agent were removed, and a mineral oil-based defoamer was reduced by 20%. The overall result is that the paint with fluorosurfactants performed better in DPR after nine months outdoor exposure in a southern U.S. test field. The liquids in the picture are water (upper) and hexadecane (lower). They were used to determine the retention of surface repellency after exposure.

Figure 4

Summary