Feasible estimation of weathering stability for free film acrylic coatings via dynamic mechanical analysis as the sole characterization method

Abstract

Weathering stability is a highly relevant materials property, especially for exterior coatings. Assessment of artificially or naturally weathered coatings is however still mostly subjective without distinct correlation between the formulation of a coating and its weathering stability. An aggravating issue in the industrial context is the often very sparse information on the exact chemical composition of materials by the manufacturers. In this study, a quantitative method to examine weathering-induced changes in coatings with limited chemical information is described, which is based solely on dynamic mechanical analysis (DMA). This approach is applied to free films to eliminate possible influences of the substrate during the measurement. The effects of artificial weathering on three exemplary acrylic waterborne wood coatings are then examined and compared to the results of state of the art artificial weathering. Precisely, the impact of additional light stabilizer on mechanical properties, molecular weight, and T_g is monitored. For the exemplary systems, it could be shown that the molecular weight of the polymers decreases during artificial weathering, where the most prominent decrease was observed in the first 300 h. In the same time frame, E′ and E″ also decrease, while the glass transition temperature increased dependent of the formulation by 6–15°C. Remarkably, for coatings without light stabilizer an apparent two-staged degradation mechanism is observed, which leads to an increase of E′ and E″ after 600 h of artificial weathering, accompanied with an increased brittleness of the free film. The primary goal of this approach is to offer a tool for quick and systematic investigation of new coating formulations in the industrial context.