Physical interpretation of the Mullins softening in a carbon-black filled SBR

•Physical source of the Mullins softening has been investigated in a filled SBR.•Damage in the rubber matrix was evaluated by swelling.•Damage of the filler network structure was tested by electrical conduction.•Adsorption at the filler-rubber interface was modified by heating in vacuo.•Changes in the microstructure are linked to the mechanical behavior.

A 40 phr carbon-black filled styrene butadiene rubber has been submitted to several experiments in order to identify the physical damage responsible for the mechanical softening recorded upon first stretch. Damage in the rubber matrix was determined by swelling. The filler structure alteration was monitored by electrical conductivity measurements. Both damages are shown to be of minor importance compared to the substantial mechanical softening undergone by the material. Degradation at the rubber-filler interface may be recovered by exposing the material at high temperatures in vacuo. The chain mobility in such storage conditions promotes free chain adsorption at the filler surface. The existence of a layer of polymer whose movements are hindered adds to the filler reinforcement and its desorption creates Mullins softening.

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