The effects of run-in on rubber friction

Experimental measurement of the change in coefficient of friction with sliding distance of a carbon-black-reinforced rubber on either glass or Perspex surfaces increases monotonically to a constant value as required by Saibel's theory. However, this general stochastic model does not allow for a fully satisfactory physical interpretation of the effects of run-in on rubber friction. The present measurements for rubber on a glass surface agree well with those of Roth and co-workers reported many years ago. The observation of these effects on Perspex does not appear to have been reported previously. It is found that a material, probably stearic acid or zinc stearate, is deposited from the rubber onto a glass surface when the rubber slides on it.