The effect of thermal radical initiator derived from O-imino-isourea on thermal curing characteristics and properties of automotive clearcoats

Abstract

Curing characteristics of melamine-based automotive clearcoats with or without thermal radical initiator (TRI) derived from an O-imino-isourea have been examined by means of their real-time rheological properties during a thermal curing process. By changing functionalities of reactive sites at hydroxyl groups and unsaturated double bonds in hydroxyl-functionalized urethane methacrylate oligomer as a main binder, the effect of TRI on various properties of thermally cured clearcoats has been compared by means of rheological, rigid-body pendulum, ultra nano-indentation, and nano-scratch tests. It was found that a small portion of TRI significantly enhanced the final elastic modulus of clearcoats with the lower content of hydroxyl groups and considerably shortened curing time, implying the formation of denser crosslinked networks inside clearcoats by the free-radical polymerization at relatively lower curing temperature. These results were directly linked with the improvement of their mechanical properties such as indentation hardness and scratch resistance. TRI plays a key role in desirably controlling the rheological and mechanical properties of clearcoats under insufficient thermal curing conditions.