Thermoplasic powders melt and flow when heat is applied but they continue to have the same chemical composition once they cool and solidify.
They are generally applied to a surface that has been preheated to a temperature significantly higher than the melting point of the powder. As a thermoplastic powder material is applied to the hot surface it will melt and "fusion bond" to the surface and then "flow out" into a strong, continuous film. As the film cools it develops its physical properties.
Thermoset powder coatings also melt when exposed to heat, but they then chemically cross-link within themselves or with other reactive components. The cured coating then has a different chemical structure than the basic resin. Thermosetting coatings are heat-stable and, unlike thermoplastic powders, will not soften back to liquid phase when re-heated.
Thermosetting powders are derived from four generic types of resins: epoxy, acrylic, polyester and fluoropolymer. From these resin types, several coating systems are derived. Resins used in thermosetting powders can be ground into fine particles necessary for spray application and a thin film finish. Most of the technological advancements in recent years have been with thermosetting powders.