Determination of degradation kinetics of two polyester thermosetting powder coatings using TGA and colorimetric analysis

Thermal stability and thermal degradation kinetics of two different polyester thermosetting powder coatings used as decorative finish in outdoor architectural applications, one standard durable and one superdurable, were investigated. The activation energies of the main stages of the degradation processes were determined by applying Kissinger's and Ozawa's equations on TGA data, which were obtained by using a thermobalance at different rates of heating in a mixed atmosphere of oxygen and nitrogen. The superdurable sample has an activation energy higher than that of the standard durable coating. Both the samples showed greater stability with respect to carboxylated polyesters with TGIC as cross-link agent and epoxy resins with dicyandiamide as reticulating agent. The results were compared to those obtained from aging experiment carried out in a oven at three different isothermal temperatures. The activation energy values were obtained from colorimetric measurements of the aged samples. Data from different approaches resulted closely comparable and it was possible to estimate the activation energies of the degradation processes from total chromatic changes with an error lower than 5% with respect to TGA. This last result highlights the possibility of obtaining kinetic data on these coatings also by using the colorimetric technique.