The mechanical properties of polyester-based coil coatings. Correlations with chemical structure

Abstract

The present work studies the effect of changes in the formulation of coil coatings on their mechanical properties. The paint systems investigated were generally based on polyester binders that were heat-cured with hexa(methoxymethyl)melamine (HMMM). The chemical structure of the polyester, as well as the concentrations of the materials present in the formulations, was systematically changed and the mechanical properties of free-films of the paints were studied at a variety of different temperatures. Changes in the polyester back-bone resulted in significant changes in the glass transition temperature, Tg, of the paints. For example, the substitution of phthalic acid in the polyester, with iso-phthalic acid, resulted in a decrease in Tg from 36 to 8 °C. At the same time, the respective effect on the mechanical properties was modest. On the other hand, changes in cross-link density resulted in dramatic changes in the mechanical properties of the paints. For example, the maximum strain at failure decreased from 180% to 30% when the concentration of the HMMM cross-linker in the paint was increased from about 5 wt% to about 30 wt%. The effects of temperature and loading rate on the mechanical properties of the free-films of the paints were also investigated. Multi-frequency dynamic mechanical analysis (DMA) was used to obtain the correspondence between time and temperature, at low strains, in a quantitative way. This correspondence was extended to large strains, when tensile data under a range of different temperatures and loading rates were considered. Modelling studies were also performed, where a hybrid visco-elastic/hyper-elastic model was used to predict the tensile behaviour of the paints, with good agreement between the predictions and the measured data. Finally, steel panels coated with a selection of the paint systems were tested in bending, using a T-bend test at 0T. An important finding was the increase in the amount of damage in the coating with time after bending, even though the panels were not deformed further. It was generally found that panels coated with paints which showed large values of strain to failure and toughness when tested as free-films, and also with little tendency for elastic recovery, suffered the least amount of damage when tested in bending.