Recovery of surface defects on epoxy coatings and implications for the use of accelerated weathering

Surface roughness, arising from photodegradation, increases overall during weathering but may relax and diminish during episodes when exposure is limited. Different ambient temperatures will change the balance between photodegradation defect size and recovery, depending on the value of the glass transition temperature of the polymer. Epoxy coatings were exposed to periods of ultraviolet irradiation, after which the recovery of the surface roughness was monitored at several temperatures, above and below their glass transition temperatures. Atomic force microscopy, as well as following the increase in roughness with exposure, showed that increased exposure made phase separated domains more distinct. Recovery of nanoindentation on un-damaged coatings produced a similar value of the glass transition temperature to that deduced from the degradation roughness recovery. This was significantly lower at the surface of the epoxy coatings than was measured for the bulk. Confocal Raman spectroscopy was unable to detect any chemical difference between the surface of any films and deeper in their bulk. This evidence suggests that the low glass transition temperature is not due to different curing chemistry at the surface of the coating, but hints that the surface of these crosslinked coatings may relax differently to the bulk or have a different physical structure. These results lead to questions about how to change accelerated testing to better serve the needs of coatings’ technology and how to make progress in the overall goal of service lifetime prediction.