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The Mechanical and Electrical Behaviours of Carbon Black Filled Elastomers under Strain

The aim of this work is to try and understand the mechanisms of structural change in carbon black filled and unfilled elastomers under strain. Carbon black filled elastomers consist of two fully interpenetrating networks, namely the filler and polymer networks. The electrical and mechanical properties of the filled elastomer depend upon the structure of these networks as well as the interactions between them. In carbon black filled elastomers a standard technique to evaluate the extent of the filler network is to measure the material's electrical resistivity. Here the electrical resistivity of carbon black filled natural rubber specimens is measured as a function of static and dynamic strain in a wide range of loading configurations. The breakdown of the carbon black network under initial loading reduces the number of conduction paths in the unstrained state and results in an increase in the resistivity of the elastomer. At higher extensions the resistivity is reduced due to the orientation of the highly structural carbon black aggregates.

Electrical resistivity of the elastomer swollen by an organic solvent is also investigated. As a result of the large increase in resistivity with modest extents of swelling it is proposed that the swelling liquids migrate preferentially to the polymer-filler interface. Parallel mechanical measurements indicate that viscoelastic changes at the polymer-filler interface mostly determine response of the filled elastomer.

The creep and recovery behaviour are investigated at moderate stresses in tension to examine the time dependent structural changes that occur in elastomers under strain. A method based upon the Boltzmann superposition principle is used to compare the creep compliance with a measurement of its set recovery after release from a range of constant loads held for different times. The set recovery data is seen to be reduced onto a single recovery curve for any given applied tensile stress for a range of loading times using the Boltzmann superposition principle. The differences between the relative rates of the creep and the set recovery can be partially attributed to the material non-linearity in the stress strain behaviour in tension.