April 3 - 9, 2008 - Plastics Flammability is the measure of the extent to which plastic will support combustion. Flame retardant plastics are compounded with certain chemicals to reduce or eliminate the tendency to burn. For polyethylene and similar resins, chemicals such as antimony trioxide and chlorinated paraffins are useful. UL flame ratings group materials into categories based on their flammability. UL 94 covers two types of testing: vertical burn and horizontal burn.