In reinforced rubber, filler and rubber molecules interact, forming additional “effective crosslinks”. During deformation, rubber chains slide (disbond /rebond) over the filler surface. This causes energy dissipation and stress softening, which make it difficult to study filler-rubber interactions. In this thesis, stress-softening was depressed using controlled swelling techniques.
Natural rubber compounds filled with different levels of carbon black and silica were made, and cure characteristics determined. Cured samples were swollen to various degrees under controlled conditions. Dynamic mechanical thermal analysis (DMTA), tensile tests and torsion tests were performed using unswollen and swollen samples. Dynamic modulus, Young’s modulus, shear modulus and stiffness were compared. At a critical degree of swelling, stiffness and strength dropped greatly, indicating loss of reinforcement. With further swelling, dynamic and tensile moduli were independent of strain, and stress-strain curves started to superimpose. This is hypothesized to result from the disruption of filler-filler and filler-rubber bonds, an important source of filler reinforcement.
Cut growth tests were performed on swollen black-filled natural rubber vulcanizates at different strain rates. The effects of swelling and strain rate on cut growth were studied, and crack patterns were observed. Critical cut size was independent of the degree of swelling and strain rate. When equilibrium swelling was reached, crack deviation, due to marked anisotropy of strength in stretched rubber, was suppressed. These observations support the hypothesis that sufficient swelling disrupts the filler network.