Flexible clear coats—Effect of curing conditions

The effect of curing at different conditions was investigated for flexible model clear coat films for coil coating applications. Based on a commercial polyester binder two formulations with isocyanate based crosslinking agents (hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI)) were prepared. The clear coats were baked at laboratory (145 °C) and industrial processing (300 °C) conditions. While for high temperature curing the baking time was kept constant at 30 s, the curing time at 145 °C, which is above the deblocking temperature of the crosslinking agents, was varied from 10 to 90 min. The clear coat films were characterized as to their thermomechanical and mechanical properties by dynamic mechanical analysis (DMA) and by tensile testing. Regarding DMA loading in tensile and penetration mode was performed.By baking at 300 °C for 30 s and at 145 °C for at least 20–45 min similar thermomechanical and mechanical properties were obtained. Thus, it was shown that it is possible to prepare specimens at a moderate curing temperature under laboratory conditions, which are comparable to clear coat films baked at processing conditions. While HDI-cured films exhibited lower glass transition temperatures and a ductile to brittle transition depending on curing time at 145 °C, for the investigated IPDI crosslinked films about 20 °C higher glass transition temperatures and a brittle deformation behavior independent of curing time were obtained. Both clear coat formulations baked at 145 °C to free films exhibited a similar relationship between crosslinking density and curing time. For DMA in tensile and penetration mode a good agreement of the determined glass transition temperatures as a function of curing time was established. Whereas DMA in tensile mode was not applicable for rather brittle films, DMA in penetration mode proved to be a more appropriate testing method for the determination of glass transition temperatures of coat films or even coatings applied to a substrate.