Stress-Whitening Effects in Transparent Structural Silicone Adhesives

Stress whitening is a frequent effect in polymers where an increase in brightness or an increased opacity of the material can be observed under mechanical loading. Investigating the stress whitening for the transparent structural silicone adhesive (TSSA), this effect occurs in different forms depending on the applied deformation. The intensity and appearance of the whitening in TSSA depends decisively on the type of loading, i.e. under isochoric deformation a spot-wise whitening is formed, whereas under volumetric loading a very dense, cloud-like whitening becomes visible. In order to clarify why the stress whitening effect occurs at all, experimental investigations are carried out on cyclically loaded tensile tests. The special feature of the tensile tests is that they were performed in a miniature tensile testing machine, which was positioned in a light microscope. This made it possible to observe stress whitening during cyclic deformation. Furthermore, special tests for the observation of whitening in the so-called pancake test are shown and compared with the results of the cyclic tensile tests. Since the whitening in the pancake test is clearly different from the classical tensile tests, cyclic pancake tests are also presented to decide whether the whitening effect is real damage or even reversible.