Thermal analysis methods (differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical thermal analysis) were used to characterize the nature of polyester-melamine coating matrices prepared under nonisothermal, high-temperature, rapid-cure conditions. The results were interpreted in terms of the formation of two interpenetrating networks with different glass-transition temperatures (a cocondensed polyester-melamine network and a self-condensed melaminemelamine network), a phenomenon not generally seen in chemically similar, isothermally cured matrices. The self-condensed network manifested at high melamine levels, but the relative concentrations of the two networks were critically dependent on the cure conditions. The optimal cure (defined in terms of the attainment of a peak metal temperature) was achieved at different oven temperatures and different oven dwell times, and so the actual energy absorbed varied over a wide range. Careful control of the energy absorption, by the selection of appropriate cure conditions, controlled the relative concentrations of the two networks and, therefore, the flexibility and hardness of the resultant coatings.
The effect of postcure high energy (␥), ultraviolet (UV) and thermal treatment on the properties of polyestermelamine clearcoats of a range of compositions has been investigated. Two initial cure conditions were used, of which one was "optimally" cured and the other undercured. It was found that postcure treatments, particularly ␥ and UV, led to coatings of similar mechanical and thermal properties irrespective of initial cure, although the change in properties on postcure treatment was greater for the under-cured samples. The results were interpreted in terms of the effect of the treatments on the structure of the crosslinked matrices. The study suggests the possibility of the development of a dual-cure process for polyester-melamines, whereby cure optimization and property improvement can be achieved. This could also be used to "correct" for small variations in thermal cure levels brought about by adventitious online fluctuations in cure oven conditions. POLYM. ENG.
SCI., 46:532-539, 2006. FIG. 2. Effect of posttreatments (⅐ ⅐ ⅐ ⅐, untreated; ---, 15 kGy ␥; ---8 h UV) on thermal decomposition (DTGA) of thermally undercured samples (cure condition D) with different HMMM concentrations.
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