An approach to assess the thermal aging effects on the coupling between inelasticity and network alteration in filled rubbers

Abstract

The development of a constitutive representation for predicting the durability of filled rubbers upon aggressive environmental conditions is crucial for numerous technical examples but still remains largely an open issue. This paper presents a constitutive model for the prediction of thermal aging effects on the inelastic response of filled rubbers. A network alteration theory is proposed for the thermally driven network degradation by considering a collection of chemical and physical changes occurring inside the rubber-filler material system. The network decomposition considers scission-rebound mechanisms of links (between filler aggregates and between elastically active cross-linked chains) and changes in movements of the free chains superimposed into the (newly born and original) chemically linked network. The material kinetics is designed using multi-step (physical) stress relaxation experiments performed at room temperature on a sulfur-vulcanized styrene–butadiene rubber containing different amounts of carbon-black fillers, and previously submitted to an accelerated chemical stress relaxation procedure, i.e. aging at constant stretch for various temperatures and exposure times. The time–temperature equivalence principle is used to construct master curves of the alteration kinetics for both the cross-linked chains and the superimposed free chains. The amplified effect of the fillers is also considered in our network decomposition to account for the cross-linked chains/fillers interactions and the free chains/fillers interactions. The capabilities of the proposed network alteration based constitutive model to describe and predict the material response evolution in terms of stiffening, hysteresis area increase and physical relaxation increase are shown. The chemically induced “plastic-like” deformation and stress relaxation are predicted using the developed model. The results show the importance of the inelastic effects on the prediction of long-term material response due to thermally activated degradation. From an applicative point of view, the model makes possible to estimate the operational life of pre-strained rubber mechanical components with respect to loss of pressure and flexibility.