Synthesis and characterization of acrylic-grafted polyester coatings for automotive pre-coated metal

摘要

An automotive pre-coated metal (PCM) system has been investigated to remove the wet coating process, such as pre-treatment, dip coating and spray coating for environmental regulations. However, automotive pre-coated metal sheets must have high flexibility and stiffness to overcome the harsh conditions such as encountered in cutting, press and the stamping process. For these reasons, a series of acrylic-grafted polyester coatings (i.e., AGP-0, AGP-10, AGP-20, AGP-30) were designed to satisfy both the surface hardness and the formability for an automotive PCM. The characteristics of the resins were measured by GPC, FT-IR and ~1H NMR. The viscoelastic behavior and flexibility was evaluated using DMA and UTM. The physical properties such as pendulum hardness, pencil hardness and adhesion were measured to define the effect of incorporating acrylates. A cylindrical deep drawing tester was used to evaluate the formability of coatings. With increasing acrylate content, the storage modulus and T_g of the coatings increased because of increasing entanglement from long chain branches of acrylates and the cross-link network between a hydroxyl function of 2-HEMA and the polyester backbone. In terms of surface hardness, AGP-30 has the maximum hardness because of the high hardness of MMA. According to the tensile strength test, the result was similar to that of DMA with increasing acrylate content. From the result of formability, some defects occurred after the 30 mm drawing test, excepting that of AGP-10. In the case of the AGP-0 system, the coating film was destroyed by die pressure because of that the maximum tensile strength is lower than 5 MPa. The large defects occurred when cross-link density of the coating increased, in this case the destruction of cross-link networks is hardly occurred. Consequently, acrylic-grafting is a powerful method affecting both the surface hardness and the formability of the polyester coatings for an automotive PCM.