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EFFECTS OF DUAL CURE AND SURFACE TREATMENTS ON COATING ADHESION TO SMC SUBSTRATES

INTRODUCTION Sheet molded compound (SMC) is a highly filled, glass fiber reinforced, thermoset material used in trim and body panel automobile parts. It is beneficial over traditional steel in that it imparts not only lighter weight, but also improved stiffness and shatter resistance. Some problems are however associated with SMC-substrates when they are to be coated with conventional thermally cured automotive paints. As a consequence of inherent SMC porosity and entrapped volatiles, popping paint defects (“paint pops”) appear during the curing process. The defects in the final finish require significant resources to detect and repair. As an alternative coating, UV-curable primers provide a reduction of paint defects by an order of magnitude in SMC coatings. This reduction is achieved by sealing in volatiles contained within the substrate and preventing outgassing from affecting subsequent coats. UV-curable coatings however have poor adhesion attributed to the low surface free energy of polymer substrates and coating shrinkage. The present study investigates a dual-cure approach to address both the popping defect problem and the poor adhesion of UV-curable primers. An initial UV-cure will seal the SMC-surface and subsequent thermal cure will improve the adhesion. The effect of different pre-treatments of the SMC-substrate is furthermore studied with respect to coating adhesion.