Used primarily as a radical initiator in polymerization reactions, especially in the curing of unsaturated polyester resins. It enables the hardening of resins used in fiberglass-reinforced plastics, commonly found in marine, automotive, and construction applications. Frequently employed in room-temperature curing systems, it is mixed with accelerators to initiate cross-linking. Also used in some plastic casting, laminating, and molding processes where controlled decomposition generates free radicals to start the reaction. Due to its instability, handling requires strict temperature control and safety precautions.