Mechanical properties and scratch recovery of nanoclay/polyester composite coatings for pre-coated metal (PCM) sheets

Scratch resistance is crucial for the polyester coating on pre-coated metal (PCM) used in domestic appliances and automotive parts to prevent aesthetic degradation and corrosion of the steel substrate. Incorporating nanoclay filler into a polyester coating has demonstrated the potential to enhance scratch resistance. In this study, LucentiteTM (LU) nanoclay with hydrophilic surface modification was added into polyester to fabricate LU/ polyester nanocomposite coatings. Nanoindentation and scratch were applied to evaluate the hardness and scratch resistance of the polyester-based coatings, respectively. The result showed that as the LU loading increased from 0.0 to 5.0 wt%, the coating hardness decreased from 187 +/- 3 MPa to 107 +/- 11 MPa. This softening effect is ascribed to the interactions between the modified clay surface and crosslinker, resulting in reduced degree of crosslinking. Despite the decrease in hardness, adding LU resulted in impressive self-healing abilities of the coatings, attributed to both the reduced crosslinking density and the rotation of the LU platelets. The magnitude of recovery increased with the LU loading. When subjected to scratch loads between 5 and 40 mN, coatings with 5.0 wt% LU demonstrated full recovery after 100 min, while the pure polyester only exhibited 80%-63 % recovery 1000 min after scratch. This unique ability can provide a significant advantage for utilizing these polyester-based composite coatings on PCM sheet.