As the 1990s come to a close, the U.S. powder coating industry continues to grow at more than 10% each year.¹ The environmental advantages powder holds over conventional coatings continue to be amplified, as the costs of regulatory compliance continue to increase for metal finishers. Perhaps just as importantly, raw-material suppliers have continued to develop novel materials, thereby allowing powder coatings to compete in market niches, previously the exclusive domain of liquid coating technology. Two examples are so-called low-temperature and UV-curable powders for use on thermally sensitive substrates.

Another niche area in powder technology is that of matte finish powders. Matte powder coatings are popular due to their excellent physical and aesthetic properties. They can hide defects or irregularities in metal surfaces, and add value to consumer products. Applications for these powders include lighting fixtures, automotive trim parts, lawn furniture and architectural extrusions.

Many formulating techniques and chemistries have been developed for matte powder coatings over the last 15 years that allow for a range of appearance and film characteristics.

Epoxy and epoxy-polyester matte powders have been commercialized and have a strong track record in metal finishing.² However, their poor weathering resistance has previously limited their use to indoor applications. This article will emphasize new developments in weatherable matte finishes, with a focus on “ultra-durable” low-gloss powder coatings.

Weatherable Matte Powder Coatings

Polyurethane chemistry is used for weatherable matte finishes. Low-gloss powders are produced by blending two or more OH-functional polyester resins, which have widely varying functionalities.³ These resins are cured with a blocked isocyanate, e.g., isophorone diisocyanate (IPDI) blocked with x-caprolactam. This approach is limited in practice in that the gloss may change markedly depending upon the formulator’s choice of isocyanate curative.⁴

Matte Polyester/Acrylic Technology

A matte finish curing mechanism has been recently commercialized by Reichhold using a bifunctional polyester (reactive COOH and OH groups) and glycidyl methacrylate (GMA) acrylic resin. The GMA acrylic crosslinks the polyester’s carboxyl groups, while isocyanate curative cures the hydroxyl groups, as before.⁵

The difference in cure speeds between the two reactions creates a micro wrinkling of the film, and the resulting matte effect. A simplified diagram of this three component system is shown in Figure 1.

The polyester/acrylic system can be formulated consistently into a matte finish with excellent mechanical properties. Tables 1 and 2 show a formulation and application data for a black powder coating.

Even with this novel crosslinking system and its excellent physical properties, further optimization can be realized. Two areas of technical interest are improvements in burnish resistance and exterior durability.

Burnish Resistance

In many finishing applications, handling and packaging of painted products can cause marring of the painted surface, creating rejects and increasing the cost to the metal finisher and ultimately the consumer. In matte-finished products, marring or burnishing usually result in a gloss increase.

Figures 2 and 3 show SEM photographs of a cured polyester/acrylic matte powder coating before and after a burnishing event. Although the naked eye can easily view an unacceptable burnish area, surprisingly, there is virtually no change in the coating surface structure when viewed with the microscope. This powder coating with a relatively high glass transition temperature (Tg = 57°C), may be too brittle to be resistant to scratching. While this polyester/acrylic retains other desirable properties such as impact and chemical resistance, it may not be suitable for applications that demand burnish resistance.

If softer, lower Tg polymers are substituted for the standard materials, one can observe quite different results, both visually and microscopically.

Figures 4 and 5 display SEM photographs of Tg-modified polyester/acrylic matte powders, before and after the burnishing event. While it is easy to see a change in the film structure under the microscope (circled area denotes damage, see Figure 5), virtually no damage can be seen by the observer.

The burnish resistance of the modified powder is excellent. Just as importantly, other polymer physical properties are retained. Modifications to the polyester and acrylic polymers are contrasted to the “first-generation” polymers in Table 3.

Exterior Durability

Automotive and architectural powder coating applications have demanding weatherability requirements. Particularly in the architectural arena, “standard” exterior weatherable powders often do not meet these stringent specifications.⁶ “Superdurable” powders, while meeting weathering requirements, can be difficult to matte.

Special monomers are required to develop polyester powder resins with excellent weatherability. When introduced into resins, these materials typically cause reduced flexibility. The new matte finish system does not display this problem; the acrylic polymer has been altered to help retain coating flexibility.

Accelerated testing indicates this new-generation matte system features better exterior durability than the matte powders being used now. Figure 6 illustrates the difference in color retention between currently available matte powders and the new generation polyester/acrylic. Approximately a three-fold decrease in color shift is observed in QUV-340A accelerated testing.

Lower Temperature Cure

As mentioned earlier, low-temperature cure powders are an area of new-development in powder coatings. Use of polyurethane technology has been limited by the high temperatures required to unblock the isocyanate groups. Novel isocyanate curatives have been recently introduced that offer the potential for reduced cure temperatures. Some of these compounds have been tested for the possibility of lowering the cure temperature of the bifunctional polyester/GMA acrylic. The combination, as demonstrated in Table 4, helps deliver matte finish and cure properties as low as 250°F (see Figure 7).

GMA Compatibility

Concerns about compatibility of GMA acrylic resins with other powder chemistries have persisted. However, empirical studies show that differences between two powders’ melt viscosity and surface tension are more important to good compatibility than simply chemistry differences.⁷

The polyester/acrylic matte illustrates this point. The majority of the powder formula is a polyester resin. The polyester’s melt viscosity dictates the powder viscosity. Since the resin viscosity is in the same range of other typical powder resins, contamination of other powders from the polyester/acrylic should not be expected beyond what would occur when any two other powder chemistries are mixed. Lab tests conducted according to Powder Coating Institute Test Number 2 reveal no cratering, pinholing or significant surface defects resulting from contamination of the polyester/acrylic in other powder chemistries.⁸

Conclusion

A new-generation polyester/acrylic powder coating matte offers several interesting features, including the following.

• Excellent reproduceability of gloss,

• Excellent resistance to burnishing,

• Excellent weathering resistance without sacrifice of mechanical properties,

• Possibility of low temperature cure and

• Good compatibility with other powder chemistries.

This development should have a wide application range in the growing exterior durable powder coating market.

Acknowledgements

Several Reichhold team members contributed to the success of this development, especially Allen Burbank, Gail Harrison, Goro Iwamura, Alan Toman and Andrew Woo.

For more information on powder coating resins technology, call Reichhold at 800/431.1920 or 919/990.7500 outside the U.S. www.Reichhold.com; or Circl.

Links

• Reichhold