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

Abstract

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. 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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“…Therefore, the softening of the compound after damage and the drop of conductivity can be correlated and associated to the carbon black structure and conductive network breakdown, as seen in Figure 5c [51,52]. Once the repair process has taken place, the material conductivity remains low, indicating that the alteration of the filler network is not recovered [43]. It seems that the repair protocol used in this research was not effective enough to form again the contact and alignment of the conductive particles.…”

“…Consequently, for the unfilled SBR compound, the decrease in E could mainly be due to the broken chains transformed into dangling chains, which do not contribute to the material stiffness [41]. Instead, the decrease in E for the SBR/GTR compound can be a consequence of the disentanglement of chains and breakage of the rubber network, together with the fracture of filler aggregates and/or rupture of the carbon black network [43]. After the thermal healing protocol application, an increase and thus recovery of E is observed.…”

“…In the case of the SBR/10GTR compound, there is an almost complete recovery of stiffness with temperature (η = 96%), which suggests that heating the samples increases the molecular mobility and free chains adsorbed at the CB aggregates surface, creating stronger rubber-filler interactions. New entanglements and the layer of bonded rubber at the rubber-filler interface are believed to play a key role, resulting in the recovery of the stiffness, and hence the healing of the SBR/10GTR compound [43,44].…”

A fully equilibrated microsphere model with damage for rubberlike materials

“…The microsphere approach provides a convenient framework to apply directional damage, allowing for instance to modify the chain property in each direction independently. Mullins softening appears to result from the debonding and unravelling of the layer of rubber at the surface of the filler particles (Diaz et al, 2014). Therefore, links that were mechanically inactive due to restricted motions become active resulting in an increase of the number of chain links noted N i .…”

“…Nonetheless, such filled rubbers exhibit a substantial softening upon first stretch, known as Mullins softening (Mullins, 1969). At the macroscopic scale, the Mullins softening was shown to be a directional damage reactivating when exceeding the maximum load already applied (Merckel et al, 2012), probably due to debonding an unraveling of the bounded layer of polymer chains at the gum/particle interface (Diaz et al, 2014). The damage and induced anisotropy resulting from the Mullins softening, have been well reproduced by introducing directional damage in affine microsphere models , Diani et al, 2006, but have never been taken into account within a non-affine three-dimensional microsphere model framework.…”