Crosslinking characteristics of dual-curable blocked isocyanate with thermal radical initiator for lowering curing temperature of automotive clearcoats

A novel dual-curable blocked isocyanate crosslinker was designed for the simultaneous radical and urethane crosslinking of automotive clearcoats under low-temperature conditions. Three isocyanate groups on the three ends of the hexamethylene diisocyanate-based crosslinker were blocked using two different blocking agents: a thermal radical initiator (TRI) and dimethyl pyrazole. Clearcoats based on hydroxyl-functionalized urethane methacrylate oligomer resin and the hybrid blocked isocyanate crosslinker (BITD) were confirmed to be densely crosslinked by the simultaneous radial and urethane reactions occurring at the curing temperature of 130 °C, which temperature condition is lower than the conventional 150 °C. A comparison of the real-time curing behavior of clearcoat mixtures and the thermo-mechanical and surface hardness properties of the post-cured clearcoat films revealed that the crosslinking performance of a dual-curable BITD crosslinker was better than that of a mono-curable TRI and commercial blocked isocyanate crosslinker. This study demonstrated that through the correlation between crosslinking characteristics and coating performance, it is possible to actively implement the low-temperature curing technology of clearcoats by utilizing a dual-curable blocked isocyanate. • A hybrid crosslinker containing an isocyanate blocked with DMP and TRI was synthesized. • Crosslinking performance of the dual-curable crosslinker was demonstrated. • This crosslinker can contribute to low-temperature curing technology of automotive cleracoats.