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It is known that Young’s modulus of elastomers increase strongly due to high filler loadings, which is critical for practical applications and processing. An experimental study was performed in order to investigate the mechanical properties of silicone rubber, which was highly filled with alumina trihydrate (ATH) under uniaxial tensile tests. Different sizes of ATH particles having different surface conditions were used. The composites were prepared in a Z-blade mixer, and the particles were partly in situ modified. In preliminary investigations, the correct amount of silane for the in situ modification was determined and the effect of the mixing time on the mechanical properties of the cured rubber was studied. Long mixing times show generally a decrease in the modulus. This is explained by the increasing formation of bound rubber. A substantial dependence of the particle size and particle surface modification on the Young’s modulus was found. This effect can be simulated using a model based on a stiff particle surrounded by a soft shell, which has a higher modulus than the polymer. Smaller particles show a stronger increase of the modulus than larger ones, which is due to (1) higher volume of interphase area and (2) shorter distance between the particles increasing the modulus of the interphase.

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