Among short fiber reinforced composites, those with rubbery matrices have gained great importance due to the advantages they have in processing and low cost, coupled with high strength. These composites combine the elastic behavior of rubbers with strength and stiffness of fibers. Aramid fibers have been chosen because of their significantly higher modulus and strength, compared to other commercial fibers. Compounds based on NR and EPDM are prepared. Short aramid fibers with different kinds of surface treatments, standard finish, and resorcinol formaldehyde latex (RFL)-coating result in different rubber–fiber interfaces. The reinforcing effect of these short aramid fibers is characterized by mechanical and viscoelastic experiments, and by studying the fracture surfaces with electron microscopy techniques. Related to the fiber coating and rubber curing system, sulfur- or peroxide-based, different reinforcement mechanisms are observed, where the combination of peroxide-cured EPDM with RFL-treated fibers is the only case showing clear signs of chemical adhesion. In all other combinations there are only indications of mechanical interactions of the fibers with the rubber matrices, due to bending/buckling of fibers, dog-bone shaped fiber ends, and surface roughness due to the RFL-coating.

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