The different types of powder coating powder can be separated into two major categories: thermoplastic and thermosetting. Thermoplastic powders are applied to the work surface and heated, or cured, during which process they melt and flow into a smooth sheet that hardens upon cooling. Thermosetting powders work much the same way, except that the curing process alters their chemical structure. Both types are used as an alternative to wet paint application, especially for such items as household appliances, automobiles and motorcycles, and both provide a durable, attractive finish. In addition, both types of powder coating powder are used as coverings for metal objects not visible but in need of protection from corrosion and oxidation.
Thermoplastic powder coating powder includes such substances as nylon, poly-vinyl chloride (PVC), polyolefin and polyester. They each melt at different temperatures, and upon re-exposure to that temperature, will melt again. This facilitates recycling, but limits the use of thermoplastic powders to environments where the temperature isn’t likely to reach their melting point. These coatings are typically used in indoor and outdoor applications, such as appliances, automotive surfaces and patio furniture. In addition, most of these substances have special properties that make them especially appropriate for certain applications. Nylon, for example, is frequently used as a coating on gears and conveyor components because of its low friction coefficient. Polyester, on the other hand, is abrasion-resistant, has a pleasing appearance and weathers well, making it ideal for outdoor furniture and exposed automotive parts.
Thermosetting powder coating powder, once melted into a smooth coating on a workpiece, does not melt upon re-exposure to the original melting temperature, because the chemical change it undergoes, called cross-linking, chemically bonds the separate components together during the curing process to create new compounds. In addition to having a greater heat resistance than thermoplastic powders, they’re also generally harder. Some of the different thermosetting materials are epoxy, acrylic, epoxy-polyester hybrids and silicon-based powders. Their heat resistance makes them ideal for higher-temperature environments such as exhaust-system automotive components, the inside walls of ovens, and racks for ovens and barbecues. Some, like epoxy, react poorly to ultraviolet (UV) rays, found in sunlight, limiting their utility to environments where they won’t be exposed to the sun, such as underground utility pipes.
Powder coating powder is created by one basic process: granules are mixed together with hardeners and pigments, heated and extruded into a sheet, which is then broken up into chips that are then milled into a fine powder. Unlike wet paint, which requires a solvent in which paint particles are suspended, powder coating powder is dry when applied. Some of the powders are sprayed onto the surfaces to be painted, while others are set in a bath into which the work piece is dipped. The powder itself has no adherence properties, requiring that the work piece either be primed or electrostatically charged for the powder to adhere to the surface prior to the heating, or curing process.
Powder coating is often used as an acceptable alternative to metal plating. Interestingly, while metal plating deposits an extremely thin layer of metal onto a work piece, sometimes only thousandths of an inch thick, powder coating an object will result in a bright, hard, durable surface, sometimes as much as a quarter-inch to half-inch thick (0.635 cm - 1.27 cm). In fact, the thicker the powder coating, the smoother it will be on the coated object; very thin coatings tend to be pebbly in texture rather than smooth. Electroplated coatings, on the other hand, will be very smooth no matter how thinly they’re applied, as long as the work piece itself has been polished smooth.