SHORT FIBER-ELASTOMER COMPOSITES - EFFECTS OF MATRIX AND FIBER LEVEL ON SWELLING AND MECHANICAL AND DYNAMIC PROPERTIES

The effects of different types of elastomeric matrix (NR, SBR, CR, NBR) and several levels (10, 15, and 20 parts phr) of short fibers on mechanical properties of uncured and cured composites and on swelling behavior of composites in hydrocarbon solvent are studied. The variation of the dynamic properties, E', E", and tan delta is determined as a function of deformation amplitude, temperature, and vibration frequency in composite materials subjected to dynamic deformation. The increase of fiber level does not limit the orientation ability of the fibers, which in all materials seemed to be above 70%. The addition of fiber markedly reduces maximum swelling and entails an increase in material stiffness. In addition, the amount of dissipated energy is increased and hence transformable into heat upon fiber incorporation, which can reach up to 16 times the value corresponding to the matrix alone, in addition to an increase with strain amplitude. The effect is most pronounced in the presumed direction of fiber orientation. The marked reduction of elongation at break (up to values of 7-9% of those of unfilled samples) and the shape of stress-strain curves point to a good fiber-matrix adhesion. Dynamic glass transition temperature is displaced toward higher values as a consequence of matrix-fiber interaction, which increases proportionally to fiber level, thus proving a linear relationship between thermal displacement and the number of interactions between the two phases. By the same token, the apparent activation energy of the relaxation process is enhanced for fiber-containing materials as compared to the fiber-free matrix. The fiber composites present a less prominent yet broader transition zone.