Scratch and recovery characteristics of automotive clearcoats containing blocked polyisocyanate crosslinkers

Scratch characteristics and self-recovery behaviors of automotive clearcoats including newly designed silane-modified blocked polyisocyanate (SMBI)1 as an organic–inorganic hybrid crosslinker were compared with those with the commercially well-known crosslinkers such as blocked HDI-based and blocked IPDI-based polyisocyanates. To extensively scrutinize the effects of various crosslinkers with different chemical structures on the chemical and mechanical properties of clearcoats themselves, rigid-body pendulum tester analysis, creep-recovery analysis, and FTIR analysis were performed, resulting in a noticeable variation in curing features and crosslinking networks. Employing the overall coating systems by depositing clearcoats with different crosslinkers above the same undercoats on galvanized steel, the scratch behaviors on the surface of the outermost clearcoat layer were examined via the nano-scratch tester for scratch depth profiles and atomic force microscopy for three-dimensional scratch images, under various self-reflow temperatures and duration time periods. The results demonstrated that the SMBI crosslinker induced a considerably higher degree of crosslinked networks, through the reaction of urethane bonds and silanol bonds in clearcoats, in comparison with the blocked HDI and IPDI polyisocyanates. Also, the recoverable behaviors of scratched clearcoats containing different blocked polyisocyanates were affected by the intrinsic chemical structures of crosslinkers, as well as scratch-recovery conditions such as external temperatures and duration times.