Catalytic effects of zirconium on scratch-healing and mechanical properties of urethane–acrylate automotive clearcoat

This work describes the effect of zirconium(IV) acetylacetonate (Zr(acac)4) on the self-healing and thermomechanical properties of urethane–acrylate copolymers, and which were compared with dibutyltin dilaurate (DBTDL). For prepolymer synthesis, glyceryl methacrylate (GlyMA) and 2-hydroxyethyl methacrylate (HEMA) were chosen and copolymerized with methyl methacrylate (MMA) and butyl acrylate (BA) at a concentration of 8 mol%. The prepolymers (G8 and H8) were then crosslinked with HDI trimer to prepare self-healing copolymers, and DBTDL or Zr(acac)4 was used as both crosslinking and self-healing catalysts. The self-healing performance of new carbonate bond formation by DBTDL and transesterification by Zr(acac)4 were studied and compared using single scratch test with atomic force microscopy (AFM). Owing to the secondary hydroxyl groups of GlyMA, DBTDL shows scratch-healing capability only in the GlyMA-based copolymer (G8) within 1 h at 55 and 75 °C. On the other hand, Zr(acac)4 shows scratch-healing capability in both copolymers (G8 and H8) within 1 h at 55 and 75 °C; however, transesterification may result in the changes of storage modulus and glass-transition temperature (Tg). This was confirmed from dynamic mechanical analysis (DMA) that a low temperature (∼55 °C) exhibited excellent self-healing performance with minimal hardness change by transesterification.