Another group of powder coating materials that make up about 5–10 percent of the powder market are thermoplastic powder coatings. Thermoplastic powder coatings differ from thermosetting powder coatings mainly in the fact that no chemical reaction occurs during curing. Thus thermoplastic materials can be reclaimed and re-melted to be applied again.
Thermoplastic powder coatings are most frequently applied by fluidized bed application—that is, dipping parts into a container of powder coating that is fluidized by a flow of air blown up through the container. These types of coatings have a much coarser particle size than thermosetting powder coating and are therefore normally not sprayed with powder coating guns.
After application, thermoplastic powder coatings melt in the curing oven and develop into a solid film upon cooling. Repair of thermoplastic powder coatings is relatively easy and can be accomplished by simply re-heating and re-melting the coating. As mentioned, no chemical reaction occurs during curing, allowing the powder coating to be re-melted. One of the drawbacks of thermoplastic powder coatings is relatively low surface hardness.
Most applications for thermoplastic powder coatings are in aggressive chemical environments. One particular area of use is valves. Following is information on some of the more common thermoplastic powder coatings used in today’s market:
Nylon-based powder coatings. Nylon is still the most common thermoplastic powder coating because of its resistance to abrasion and chemical exposure. Applications can be found in medical instruments, where cleaning by autoclave is required, as well as dishwasher baskets and so on.
Polyolefin-based coatings are based on polypropylene or polyethylene resins. They have a soft, almost waxy feel. Adhesion is an issue for these coatings even on sandblasted surfaces, and outdoor application is not recommended. Copolymers are used to improve adhesion and, with the correct primer, applications can be found in the pipeline business as insulation coatings.
Vinyls. PVC-based coatings have a good water resistance and, with a proper primer, can be used in such applications as dishwasher baskets. Surface hardness is limited, and the surface scratches easily.
PVDF (polyvinylidene fluoride) resins are normally used in liquid systems to create very high-performing architectural coatings. In powder coatings, PVDF resins are mainly used in applications requiring very high chemical resistance. Valves, pipelines and other equipment in the chemical industry are often coated with PVDF powder coatings. A primer is recommended for adhesion.