Thermoset polyurethane (PUR) was commonly used for domestic bumper covers in the 1980s and 1990s, but has since been largely replaced by polypropylene. However, PUR is still occasionally used.

Polyurethane is a thermoset plastic, so it cannot be melted. The solid is formed by reacting two liquid components which cross-link in the mold. Because PUR doesn't melt, a nitrogen welder cannot be used to make a fusion weld. Instead, the airless plastic welder is used to make the repair. When repairing urethane with an airless welder, the base material isn’t melted; instead, the welding rod is used like hot melt glue, similar to a brazing process.