Powder coating is the application of a protective film to an item by spraying it with a cloud of ground up organic plastic through a special gun. The powder cloud is electrostatically charged by the spray gun so it is attracted to and adheres to the substrate.
The item is then baked in an oven to melt the powder film during the baking process the chemistry of the powder will cross link and ‘cure’ to form a tough durable coating.
The powders are typically organic coatings made of resins such as polyester, polyurethane, epoxy or mixtures of these called hybrid powders. Each resin will have strengths and weaknesses; polyester has good UV resistance so it is usually used in powders for exterior use. Polyurethane has good chemical resistance so it is used in anti-graffiti coatings on component for trains etc.
Since the baking process usually occurs at around 200 degrees Celsius the substrates coated are usually metals such as steel and aluminium. The advent of low bake powders has allowed the technology to be used on other substrates such as MDF timber and plastics.
In order to provide a durable coating for metal products used outside it is necessary to ‘pre-treat’ the metal. The pre-treatment process is a chemical reaction on the surface of the metal to clean it of oils, grease and dirt and then convert it into a chemical layer that is resistant to corrosion. Powder coating alone will not provide a corrosion resistant coating on metal.
Typical pre-treatments are zinc or iron conversion coatings for steel products and chromate conversion for aluminium. New pre-treatment technologies such as silan and zirconium based conversion coatings are coming into use as they are more environmentally friendly and produce better results.
They are also better for the environment as they discharge no heavy metals or dangerous by-products into the waste discharge and require less energy to heat the solutions.