Influence of fiber type and coating on the composite properties of EPDM compounds reinforced with short aramid fibers

There is a renewed interest in the application of short aramid fibers in elastomers because of the considerable improvement in mechanical and dynamic properties of the corresponding rubber composites. Possible applications of short aramid fiber–reinforced elastomers are tires, dynamically loaded rubber seals, diaphragms, engine mounts, transmission belts, conveyer belts, and hoses. Our studies are related to the investigation of dispersion, length distribution, and the fiber–matrix interaction of two types of short aramid fibers, standard coated and resorcinol formaldehyde latex (RFL) coated, in ethylene–propylene–diene rubber (EPDM). Because the detection of the polymer fiber morphology in rubber compounds is hampered in the presence of carbon black, which is typically used in industrial elastomer compounds, fiber length, fiber length distribution, and dispersion are investigated in corresponding carbon black–free model compounds. Optical methods, scanning electron microscopy, and tensile testing are employed to explore the short aramid fiber–reinforced elastomer composites. The effects of morphology and fiber–matrix interaction on the mechanical properties of composites are discussed. Regarding fiber type, it is shown that co-poly-(paraphenylene/3,4′-oxydiphenylene terephthalamide) (PP/ODPTA) fibers end up with a higher final length than does poly(para-phenylene terephtalamide) (PPTA), which results in considerably higher mechanical properties of corresponding rubber compounds. For each fiber type, the higher final length as a result of RFL coating and the interaction with the rubber matrix are the key factors that overcome even the negative effect of poorer dispersion of RFL-coated fibers. The differences between the short aramid fibers and aramid cords regarding the RFL coating are also discussed.