Mullins effect of foamed liquid silicone elastomers

Silicone elastomers filled with silica exhibit a softening effect under cyclic loading, also known as the Mullins effect. There are various theories as to the origin of this effect. One theory cites the breaking of secondary bonds as the reason, while another theory cites molecular slipping and the loosening of entanglements as the reason for the softening. In contrast to previous studies, foamed silicone rubbers are investigated here. In addition to the silica, the samples investigated here contain thermally expandable microspheres, resulting in two types of filler (silica and the microspheres as thermoplastic particles). Two different kinds of these microspheres with different particle sizes and expansion temperatures are used. To study the effect, hysteresis tensile tests (Mullins tests) as well as FTIR analyses, density measurements and microscopy images were carried out. The Mullins tests were evaluated for their curve shape, their hysteresis area of the first cycle and the residual strain. The residual strain shows a correlation with the porosity, as the larger microspheres have a higher porosity and a higher residual strain. The area of the first hysteresis correlates with the number of microspheres, but not with the porosity. The larger microspheres have a smaller hysteresis area but a higher porosity. The FTIR analysis showed that the stabilizer used to produce the microspheres is based on silicon oxide. As a result, there are secondary bonds between the microspheres and the silicone elastomer, which are broken at the beginning.