Kopplung von zyklischen Betriebsfestigkeitsuntersuchungen und Materialcharakterisierung zur Untersuchung der Elastomeralterung
Combination of cyclic fatigue testing and materials characterisation to investigate ageing of elastomers
Cyclic fatigue testing and elastomer characterization were combined to study changes of material properties and network structure of elastomers by thermal aging. Natural rubber containing a typical additive package with carbon black was studied as model material. The samples were aged at different temperatures in air or under nitrogen atmosphere. The fatigue life in number of cycles (S-N-curves) was determined from force and displacement controlled fatigue tests on specimen after different times of thermal aging. Changes of mechanical properties and crosslink density were studied during and after thermal aging by tensile tests, dynamic mechanical analysis, stress relaxation experiments, measurements of compression setting, swelling measurements, and solid-state NMR. Changes in network density during thermal aging are related to the interplay between the formation of new crosslinks and chain scission. The average molecular mass of the network chains was found to be a sui table parameter for comparison of different characterization methods. An initial decrease in the molecular mass between two crosslinking points by post-curing is followed by an increase due to chain scission. A similar trend was found for fatigue life in number of cycles to failure (N) in force controlled fatigue tests: After an increase of N for short times of aging a decrease at longer aging times occurred.