Effect of ageing on the ability of natural rubber to strain crystallise

Experiments were carried out in a deformation calorimeter on unfilled NR cured using three vulcanization systems: peroxide, efficient sulphur and conventional sulphur systems, before and after different periods of air ageing. The internal energy (deduced form the measured deformation work minus the heat evolved) was found to increase approximately linearly at small to moderate strains. At a sufficiently high elongation, of the order of 300% but depending on the particular sample, the internal energy in all cases reached a maximum and then rapidly fell. This is ascribed to the evolution of latent heat of crystallisation. The amount of crystallinity was deduced from the difference in directly calculated internal energy and that estimated by extrapolating the initially linear energy-elongation relationship. It is concluded that the ability to strain crystallise is diminished with ageing, but that this may not be the only cause of the loss in strength.