Stress whitening is an aesthetic defect, commonly seen in formed coil coated samples. Currently stress whitening introduced during forming at temperatures below ambient can be removed by heating the object concerned which is time consuming and costly. A novel solution will be considered by reformulating the paint using a systematic variation of the constituents, including selected additions. Before the formulation can be altered with functional agents a study of the mechanical properties of the existing paint system was undertaken to gain a fundamental understanding of the factors that influence stress whitening and this is the focus of this article. Theory suggests that stress whitening is a form of micro-cracking, initiated at points where particles that possess dissimilar properties to that of the bulk resin meet forming an interface. Paints contain various additives and pigments which can be seen in microscopy as separate. A coatings system which consistently stress whitened, was selected. By systematically breaking down the formula into different stages and bending coated samples over a conical mandrel, it was determined one additive caused the onset of stress whitening whilst others had no effect. Microhardness tests were carried out. There was no appreciable difference in a coatings overall hardness with or without the particles present, however when the particles were tested on their own the microhardness results show a significantly lower hardness. Infra-Red (IR) Spectroscopy had a similar result, bulk coatings with and without additives in had the same spectra, whilst the additives were different. IR Microscopy is being investigated to confirm if particles seen in the image are the same, so far the formulation breakdown supports this. Microscopy studies are currently being undertaken to isolate and image micro-cracks and to determine if initiation point is the dissimilar particles. So far imaging has shown these particles are present heterogeneously with the bulk coating. The stress whitening is proving to be a evasive phenomenon because of its tendency to disappear on warming.

Investigation into stress whitening production caused by coatings additives

253rd National Meeting (2017)

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