The Effect of Hardness and Surface Roughness of Elastomer on Friction between Shoe Soles and Particle-contaminated Floors

Nearly one third of workplace injuries results from slip- and trip-induced falls. Solid particles are among the most common floor contaminants in both occupational and outdoor environments, reducing shoe–floor friction and increasing slip risk. This study investigates how rubber hardness and surface roughness affect the frictional behaviour of shoe soles on smooth, particle-contaminated floors. Coefficient of friction (COF) measurements and post-test surface wear analyses were conducted using nitrile rubbers with hardness between 57.9 and 84.0 ShA and varied surface roughness. Samples were slid against smooth epoxy flooring in a pin‑on‑plate test simulating the heel-strike phase of walking. The floor was either clean or uniformly covered with corundum particles (40–50 µm, 120–140 µm, or 280–315 µm). On clean floors, increasing rubber hardness and roughness significantly decreased COF (p < 0.0001) due to reduced real contact area. Under contaminated conditions, softer and rougher rubbers yielded higher COF values (p < 0.0001). Higher COF correlated with greater floor wear, showing long scratches and grooves, suggesting slip occurs mainly at the particle–floor interface. Rubber hardness and surface roughness primarily influence the strength of the particle–elastomer interface; greater particle–elastomer strength suppresses particle rolling and thereby leads to an increase in COF. These findings indicate that, on particle-contaminated smooth floors, slip resistance is governed more by particle–floor interactions than by rubber adhesion. Optimising outsole roughness and hardness can reduce the adverse effects of particle rolling in the contact and improve the frictional properties of the outsole.