Vacuole Formation and the Mullins Effect in SBR Blends with Polybutadiene

Electron microscopic examination of stretched thin films of black loaded vulcanizates of SBR and polybutadiene blends shows that vacuole formation is prevalent when the polybutadiene content drops below one-third. The vacuoles that formed on stretching these stocks did so at small coherent carbon black agglomerates rather than at individual carbon black particles. It is felt that the mixing characteristics were such that stocks with more polybutadiene experienced more grinding action, which resulted in destroying the small agglomerates that caused vacuoles in stocks with less polybutadiene. Hysteresis data on the same series indicated that for the vacuole forming stocks the ratio of the areas of the first to the second hysteresis loop reflects the fact that vacuoles are being formed. The overall Mullins' effects displayed by all members of the series was similar, and a mechanism was proposed that slightly modifies the hysteresis criteria of Harwood, Mullins, and Payne. It is suggested that this mechanism can also be used to explain the effects that various particle size carbon blacks have on extensions following the initial one. The effect of vacuole formation on the overall Mullins stress softening was minor and equivalent to disrupture of filler structure.