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 (T_g). 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.