Coil-coated steel sheets measured by DMA using an immersion testing cell

The effect of humidity on the glass transition temperature of coatings is well known and analyzed. In this study, an analysis method is introduced for analyzing coil-coated metal sheets submerged in water and water/methyl ethyl ketone (MEK) by using an immersion cell coupled to a DMA. For this purpose, a model polyurethane acrylate coating was applied to zinc/magnesium-coated steel plates and measured before and after immersion in water and water/MEK by DMA and immersion cell. The results show a shift of glass transition temperature from 138 to 13 degrees C of the coating by storing the coated steel plates under water and immersion testing. A time-dependent diffusion of water from the cell into the coating until saturation can be observed. An increasing concentration of MEK within the immersion cell results in a greater shift of the glass transition temperature. Overall, the immersion cell seems to be a useful tool for in situ characterization of the behavior of coil coating varnishes in liquid environment (Foster et al. in Prog Organ Coat 51:244-249, 2004, van der Wel et al. in Prog Organ Coat 37:1-14, 1999).