Creep and recovery of unvulcanized natural rubber

Abstract

The creep and recovery behaviour of unvulcanized natural rubber, both filled and unfilled, is investigated under low stresses in tension at room temperature. A method based on the Boltzmann superposition principle has been used to predict the creep function from measurements of its recovery after release from a range of constant loads each held for various lengths of time ( t 1). For both gum and carbon black-filled rubbers of a given Mooney viscosity, the technique resurrects a master creep curve which is found to be independent of t 1. Although this has the same general shape as the experimental creep curve the two differ significantly, except for the filled rubber at small stresses. The discrepency is ascribed to non-linear effects mainly associated with the tendency of the unvulcanized rubbers (especially the gum rubber) to flow at high applied tensile stresses and long times. Under such conditions the superposition principle is no longer valid. This non-linear effect due to flow can be approximately corrected for by substracting the experimentally measured permanent set from the creep and recovery data before treating the remainder with the Boltzmann superposition principle. This method of correction yields good agreement between the revised experimental creep and the derived creep compliances for gum rubbers up to σ=0.50 kg cm −2, and for black-filled rubbers up to σ=0.78 kg cm −2.