Stress whitening effects in transparent structural silicone adhesives

Stress whitening is a common 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 effect for the transparent structural silicone adhesive (TSSA), this effect occurs in different forms depending on the applied deformation. The intensity and appearance of whitening in TSSA depends decisively on the type of loading, i.e. under isochoric deformation a spot-wise whitening can be observed, 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 uniaxial cyclic tensile tests and constrained tensile tests. The special feature of the uniaxial cyclic tensile tests is that they are performed in a miniature tensile testing machine, which is positioned under a light microscope. This makes it possible to observe stress whitening during cyclic deformation at a micro-scale. Furthermore, for the observation of whitening during constrained tensile tests, so-called pancake test are investigated and compared with the results of the uniaxial cyclic tensile tests. Since the whitening effect in the pancake test is in clear contrast to the uniaxial tensile tests, differences of both results are presented. Finally, the causes of both forms of whitening are defined and characterized.