Filler-polymer interaction investigated using graphitized carbon blacks: Another attempt to explain reinforcement

Abstract

Many theories on the origin of rubber reinforcement have been presented over the last decades. None of them explains beyond reasonable doubt why the dispersion of carbon black into a polymer matrix induces an immense improvement of ultimate properties.We present a throughout analysis of ethylene-propylene-diene rubber (EPDM), filled with carbon blacks treated at temperatures between 900 °C and 2500 °C. The fillers are investigated extensively using static gas adsorption, transmission-electron microscopy and elemental analysis. Afterwards, correlations between filler surface properties of the filler and properties of the compounds are drawn with a special focus on large-strain softening effects (Mullins’ softening). The latter successively vanishes with temperature treatment of carbon black. Moreover, the softened samples do not recover at temperatures below 100 °C or by swelling. A very simple model involving a stress-limiting process at the polymer-filler interface is derived, which reproduces the experimental results well. Equilibrium hysteresis is found to be originated in physical interaction only. It turns out that the softening-generating effect (“reinforcement”) is best explained by chemical filler-polymer bonds, successively breaking down during stretching, and low-strain modulus and equilibrium hysteresis by physical compatibility.