Control of Mullins stress softening in silicone elastomer composites by rational design of fumed silica fillers

The influence of fumed silica as reinforcement filler is investigated on the Mullins stress softening in composite silicone elastomers. Especially, the impact of surface area, loading level, surface chemistry, and structure modification of fumed silica is evaluated on the fatigue behavior of high consistency silicone rubber. It is observed that high loading level and large surface area of fumed silica increase the energy loss during initial loading cycles which is related to disruptions in filler-polymer interactions. Addition of a processing aid or hydrophobization of the silica surface decrease the brittleness and energy loss of the compounds. A close correlation between Shore A hardness and energy loss is found confirming the contribution of the filler-polymer interactions on Mullins stress-softening. At the same time, the permanent set remains almost independent of the surface hydrophobicity of the filler indicating a breakdown of the structured silica agglomerates during initial deformation. To improve the fatigue resistance of the silicone elastomer, a structure modified and hydrophobic silica, AEROSIL® R 8200, was used leading to the lowest permanent set even at high loading levels in combination with good mechanical reinforcement. The reduction in energy loss and permanent set is attributed to the combination of hydrophobic surface and low aggregate structure of the silica preventing agglomerate breakdown and chain disentanglements at the silica surface. This study provides fundamental understanding on the relation between filler-polymer interactions, filler morphology and Mullins stress softening for the rational design of future filler particles in reinforced elastomer composites with improved fatigue resistance.