Impact of pre-strain on dynamic-mechanical properties of carbon black and silica filled rubbers

The effect of pre-strain on dynamic-mechanical properties of reinforced S-SBR composites is investigated for a variety of fillers and filler concentrations. Two types of industrial fillers, carbon black and silica, are considered. Furthermore, the impact of polymer-filler coupling agent (silane) for the silica composites is analyzed. Dependent on pre-strain and sample composition, pronounced differences of the stress–strain cycles are observed. The various dynamic stress–strain cycles are shown to be closely related to the stress softening cycles observed for quasi-static deformations up to large strain. Accordingly, they are analyzed by referring to a recently developed micro-mechanical model of stress softening and filler induced hysteresis, which is based on the cyclic breakdown and re-aggregation of tender filler clusters in the stress field of the rubber matrix. The obtained microscopic material parameters provide information on the mean size and distribution width of filler clusters, the tensile strength of filler-filler bonds and the polymer network chain density. Significant implications of pre-straining are often ignored, if the data treatment focuses on the evaluation of the complex modulus, only. In the past, this has led to confusions about the role of filler networking in explaining the Payne effect. Nevertheless, for a qualitative analysis of the various effects it appears reasonable to discuss the data in the modulus picture. Therefore, an energy based definition of the complex modulus is introduced that allows for a proper consideration of the strongly nonlinear stress–strain behavior of filler reinforced rubbers at larger strains. The obtained results are discussed in the framework of microscopic structure-property relationships of filler networking.