Use of the unified theory of rubber friction for slip-resistance analysis in the testing of footwear outsoles and outsole compounds

It has been shown theoretically and experimentally that the friction properties of rubber are controlled by multiple parameters, such that coefficient of friction (COF) calculations for rubber differ from the traditional, metallic-based laws of friction. A previously formulated hypothesis details a unified theory of rubber friction that incorporates adhesion forces, surface deformation hysteresis, bulk deformation hysteresis and cohesion losses from rubber wear. This paper discusses experimental data that support the theory of surface deformation hysteresis (FHs) and the implications for footwear testing. The experimental results indicate that surface deformation hysteresis on smooth test surfaces can contribute 6.5-42% of the total frictional forces depending on the applied load and outsole compound. This contribution is specific to the experimental dataset reported, and can vary with the rubber compound used. The data also show that FHs is a constant, resulting in a decreasing contribution to the total COF (mu T) with increasing normal load. The combination of these two facts has implications in the establishment of standard test methods and the application of minimum COF requirements for footwear.