Stress Whitening in Polyester Melamine Coatings

Stress whitening is a long-standing problem and scientific work has focused on evaluating causes of this in bulk polymer systems. In this paper we focus on this optical defect exhibited by a complex thermosetting polyester melamine coating system used extensively in the pre-coated metal industry. There are several mechanisms proposed for how stress whitening occurs and hence there is uncertainty over the causes in the systems mentioned. The most likely explanation given to date is that a number of proposed micro-mechanisms exist, which one is occurring is entirely dependent on the system being investigated. The work presented shows that the presence of dissimilar particles is the cause of the stress whitening. The proposed mechanism for whitening and its disappearance in this case is a time and temperature dependent change in density, i.e. cracking or voiding, where the cracks are outside the range that scatters light with an increase in temperature.