Rolling friction of elastomers: role of strain softening
We study the temperature and velocity dependency of rolling friction. Steel and PMMA cylinders are rolled on sheets of Nitrile Butadiene Rubber (NBR), with and without filler, and flouroelastomer (FKM) with filler. Measurements of the rolling friction is performed for temperatures between −40○ C and 20○ C, and for velocities between 5 μm/s and 0.5 cm/s. For the unfilled NBR a smooth rolling friction master curve is obtained using the bulk viscoelastic frequency–temperature shift factor aT . For the filled rubber compounds a small deviation from the bulk vidscoelastic shift factor is observed at low temperatures. The experimental data is analyzed using an analytic theory of rolling friction. For the filled compounds good agreement with theory is obtained when strain softening is included, which increases the rolling friction with a factor ∼ 2 for the filled FKM and ∼ 3 for filled NBR compounds. For the unfilled NBR the maximum of the rolling friction occur at higher sliding speeds than predicted by the theory. We discuss the role of the adhesive (crack-opening) contribution to the rolling friction, and the role of "frozen-in" elastic deformations as the rubber is cooled down below the rubber glass transition temperature.