Effect of ambient temperature and cure state on the age stiffening of a cured NR/BR blend

Cured natural rubber and polybutadiene elastomers undergo an increase in modulus with time, even at benign room temperature storage conditions. The increase in modulus of typical rubber compounds was studied at room temperature and at temperatures moderately above room temperature in order to enable the evaluation of the modulus after shelf aging. The rate of modulus increase was found to correlate well with storage temperature by using an Arrhenius activation energy approach. The marked dependence on temperature in the range of 20-70 °C suggests the possibility of significant acceleration of the timescale of experiments using very mild conditions. Data are presented for isothermal and step-isothermal aging protocols, which are shown to be reducible to a single curve. The effect of the initial cure cycle used to produce the specimens is also investigated.