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

Abstract

An automotive pre-coated metal system (PCM) has been investigated to replace wet coating process, such as pre-treatment, dip coating and spray coating, to address environmental regulations. However, automotive pre-coated metal sheets must have high flexibility and stiffness to overcome harsh conditions such as cutting, press and stamping processes. For these reasons, elastomeric polyester coatings were designed to improve scratch resistance and to impart reflow characteristic for an automotive PCM. The characteristics, curing behavior and viscoelastic property of the resins were measured by 1H NMR, GPC, RPT and DMA. The flexibility was evaluated using a texture analyzer. A nano scratch tester, equipped with an optical microscope was used to measure the scratch resistance of coatings and the scratched surface morphology. An Amtec laboratory car wash test and a sharp knife were used to evaluate the reflow property of the coatings. Change of the damaged surface morphology was measured using a mini-SEM.With increasing 1,4-cyclohexanediol content, the final frequency decreased and the storage modulus and Tg of the coatings increased. This is related to the high stiffness of the cyclohexane structure of 1,4-cyclohexanediol. In terms of scratch resistance, EP-6, one of the formulations examined, has high scratch resistance due to the restoring force of the stiff structure of 1,4-cyclohexanediol. Consequently, the gloss recovery after a heat treatment was increased with increasing the content of cyclohexane structure in the car wash test. In the case of a deep scratch, the reflow property of coating was demonstrated using a heat treatment. Therefore, a cyclohexane structure is a good factor for improving the scratch resistance and reflow property of the polyester coatings.