Evaluating aging of coatings and sealants: Mechanisms

Photodegradation mechanisms of a model coating/sealant system based upon styrene-butadiene-styrene triblock copolymer have been investigated. The model system was exposed to ultraviolet-visible radiation using an integrating sphere-based exposure chamber at 30 °C and <1% relative humidity. Chemical degradation was monitored by transmission Fourier transform infrared (FTIR) spectroscopy and changes in mechanical properties via dynamic mechanical thermal analysis (DMTA). It was found that cross-linking was the dominant degradation process, causing substantial increases in storage modulus and glass transition temperature of the butadiene units with increasing exposure. The styrene units were relatively stable as indicated by the lack of significant changes in the FTIR bands or in the glass transition temperature associated with the styrene units. The present study clearly has shown that chemorheological investigations provide useful tools to characterize the mechanisms of environmental attack of polymers. The ability to investigate both chemical and mechanical-rheological properties provides the opportunity to make modifications to the material and, hence, improve its chemical and mechanical-rheological properties.