Effect of bulk deformation on rubber adhesion

In general, rubber friction is divided into two parts; the bulk hysteresis and the contact adhesive term. These two contributions are regarded to be independent of each other, but this is only a simplified assumption. If the adhesive force is solely a function of the surface free energy, it has been assumed that this adhesive force per unit area should be constant during any bulk (surface) deformation. The surface free energy is a function of both internal energy and entropy, and so it should change if the internal energy and/or entropy are changing due to any bulk deformation. In order to ascertain the effect of the bulk deformation on rubber adhesion, this effect on the surface free energy of two rubber compounds filled with various loading fractions of carbon black, natural rubber (NR) and styrene butadiene rubber (SBR) was studied. The surface free energy was determined from measured contact angles on rubber samples at constant applied strain. It was found that the surface free energy increases significantly upon applying a deformation. However, with increasing the carbon black loading fraction the surface tension increase becomes less pronounced. The effect of the surface free energy on the surface damage generated by scratch tests was also discussed. It was found that roll debris formation occurs only in the case of the rubber compounds whose apparent surface free energy increases greatly after applying large bulk deformations.