Abstract
The authors investigate the friction properties of crosslinked butadiene-acrylonitrile, butadiene-styrene copolymers and natural rubber on polished steel in vacuum over a temperature range −100° to + 100°C, which takes in the glassy and high-elastic states of rubbery polymers, and also the transition range between these states. A sharp difference is observed between the friction properties of these polymers below 0°C, investigated in vacuum and under atmospheric conditions. Above the glass-transition temperature, i.e. in the range of high elasticity and in the transition range, the mechanical losses in friction play an insignificant part, and therefore they cannot be used to explain the maximum on the curves of the dependence of the friction force on temperature or on the logarithm of the rate of slip. The alteration in the friction properties on the transition from high to low temperatures and from low to high rates of slip is explained by the alteration in the elastic properties in the rubbery polymers in the glass-transition region, and also by the transition from the molecular-kinetic mechanism of friction to the mechanism of friction characteristic of rigid polymers (below the glass-transition temperature the coefficient of friction is practically independent of the rate of slip and the normal load). In the range of high elasticity the friction properties of polymers are explained by the adhesion molecular-kinetic theory, proposed in 1954 by one of the present authors. Working from this theory the authors introduce the critical rate of slip v k and characteristic temperatures T k and T o , and an explanation is given for their physical meaning. An assessment is made of the influence, the type of polymer and of the normal load on the friction properties of vulcanised rubber. It is shown that the friction force alters in an analogous fashion on reduction in the temperature and raising of the logarithm of the rate of slip, which indicates the applicability of the principle of temperature-time equivalence to the friction properties of polymers.
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