AbstractCompressive stress softening (Mullins effect) in black‐filled vulcanizates of the blended system natural : nitrile : brominated butyl rubber has been investigated. The results are expressed in terms of parameters derived from successive stress‐strain cycles. The ratio of the work done on loading in the first and second cycles is called the load work ration,Rw. A similar quantity,Rh, is defined for the loss or hysteresis work. Both load and loss work ratios show a compositional dependence similar to other properties of blends. The variation ofRwandRhwith composition agrees with the recent thesis of Mullins, Harwood, Payne and others that the Mullins effect is a property of the elastomer. Sudden variation ofRhwith elongation has been interpreted in light of recent results of R. W. Smith as indicative of vacuole format
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