In this applicative study, the ratio of active and inactive filler loadings was the prime factor for determining the dynamic–mechanical behaviour of ethylene–propylene–diene monomer rubbers. Scanning electron microscopy was used to study the structure of reinforced dense and microcellular elastomeric materials. The effects of filler and blowing agent content on the morphology of composites were investigated. Microcellular samples cured in salt bath show smaller cells and uniform cell size compared with samples cured in hot air. Dynamic–mechanical thermal analysis showed appreciable changes in the viscoelastic properties by increasing active filler content, which could enable tailoring the material properties to suit sealing applications.