The glass transition temperature (Tg) is a very useful parameter that characterizes the temperature dependence of the physical and mechanical properties of coatings. Differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamical mechanical analysis (DMA) were used to determine the Tg of automotive coatings.
DMA analyses were done on coatings deposited on a steel substrate in the fixed frequency mode, at frequencies of 0.01, 0.1, 1.0, and 10 Hz. Specimens were heated in either step mode (2°C) or ramp mode (5°C/min). TMA analyses were done either on coatings deposited on steel substrate or on free films at 5°C/min. DSC analyses were done at 5°C/min either on free films or on specimens that were sanded out from steel panel. Indium metal was used for the temperature calibration of DSC, TMA, and DMA. For one-coat films, the correlation between the temperature of midpoint in DSC, the onset temperature of penetration by TMA, and the onset temperature of loss modulus by DMA at 0.1 Hz was good. However, for multicoat films the Tg's for individual layers were visible only if their Tg's were far apart. It was possible in some cases to remove the top layer by scraping and analyzing the residual layers. The use of DSC, DMA, and TMA to determine the Tg's of different automotive coatings on metal substrate has been demonstrated, which is very useful in research and development, and problem solving.