Combination of material characterization and cyclic fatigue testing for investigation of elastomer aging

Cyclic fatigue testing and elastomer characterization were combined to study changes in network structure and material properties of elastomers with and without filler by thermal aging. As a model material, a natural rubber (NR) with and without carbon black, and containing a typical additive package, was studied. The NR compounds were aged at different temperatures in air or under nitrogen atmosphere.The fatigue life in number of cycles (N) was determined from displacement and force controlled fatigue tests on specimens after different times of aging.Changes of mechanical properties and crosslink density were studied during and after thermal aging by stress relaxation experiments, dynamic mechanical analysis, measurements of compression setting, swelling measurements, and solid-state NMR. Chemical analysis of extracts was done by matrix-assisted laser desorption ionization-time of flight-mass spectrometry, nuclear magnetic resonance, and infrared spectroscopy. Pyrolysi s gas chromatography was used to study degradation of additives.The changes in crosslink densities were related to the interplay of formation of new crosslinks and chain scission. As a unique parameter derived from characterization methods the molar mass of the network chains showed an initial decrease followed by an increase at longer times. A similar trend was found for N in force controlled fatigue tests: an increase of N at short thermal aging times was observed, followed by a decrease at longer times.