Alleviating Molecular-Scale Damages in Silica-Reinforced Natural Rubber Compounds by a Self-Healing Modifier

Abstract

The property retentions of silica-reinforced natural rubber vulcanizates with various contents of a self-healing modifier called EMZ, which is based on epoxidized natural rubber (ENR) modified with hydrolyzed maleic anhydride (HMA) as an ester crosslinking agent plus zinc acetate dihydrate (ZAD) as a transesterification catalyst, were investigated. To validate its self-healing efficiency, the molecular-scale damages were introduced to vulcanizates using a tensile stress–strain cyclic test following the Mullins effect concept. The processing characteristics, reinforcing indicators, and physicomechanical and viscoelastic properties of the compounds were evaluated to identify the influences of plausible interactions in the system. Overall results demonstrate that the property retentions are significantly enhanced with increasing EMZ content at elevated treatment temperatures, because the EMZ modifier potentially contributes to reversible linkages leading to the intermolecular reparation of rubber network. Furthermore, a thermally annealing treatment of the damaged vulcanizates at a high temperature, e.g., 120 °C, substantially enhances the property recovery degree, most likely due to an impact of the transesterification reaction of the ester crosslinks adjacent to the molecular damages. This reaction can enable bond interchanges of the ester crosslinks, resulting in the feasibly exchanged positions of the ester crosslinks between the broken rubber molecules and, thus, achievable self-reparation of the damages.

A Self-Healing System Based on Ester Crosslinks for Carbon Black-Filled Rubber Compounds

“…Therefore, these results support well the elucidation of extra crosslinks generated by the EMZ modifier in a sulfur-crosslinked network. of the compounds, which is consistent with a finding reported in previous work [3]. This is attributed to EMZ which contains a strong acidic substance that has the capability of peptizing rubber molecules resulting in shortened molecular chains or reduced molecular weight of the rubber matrix.…”

“…Furthermore, the composites with the higher contents of the EMZ modifier show a better crack resistance. This improvement in the crack propagation resistance can be attributed to the increment of sacrificial bonds (i.e., hydrogen and ionic bonds) due to the addition of the EMZ modifier, as verified in previous work [3]. The sacrificial bonds are defined as the weak crosslinks that rapture before the failure of strong covalent bonds during deformation [42].…”

“…ENR-50, hydrolyzed maleic anhydride (HMA), DMI, and ZAD at 100, 3, 5, and 5 phr (parts per hundred rubber by weight), respectively, were mixed in an internal mixer for 15 min with a rotor speed of 70 rpm at a starting mixer temperature of 40 • C, following the protocol described in previous work [3,21,22]. The obtainable rubber masterbatch was named "EMZ" (epoxidized natural rubber modified with hydrolyzed maleic anhydride plus zinc acetate dihydrate) as an acronym in this context.…”

“…where w 1 is the weight of a swollen sample, w 2 is the weight of the sample after drying, and ρ s and ρ r are the densities of the solvent used (i.e., 0.886 g/cm 3 for toluene) and of the rubber (i.e., 0.92 g/cm 3 for natural rubber), respectively.…”

“…Thereafter, the cracks propagate to a macroscopic level, leading to the failure of the sidewalls [1,2]. In other words, as the damages start from a molecular scale, the repeated dynamic operations of rubbers result in the progressive damages of rubber molecular networks, that grow to a larger defect scale, called macroscopic damages [3]. These initiated damages are cumulative under service circumstances and will not disappear at later stages even when an operation of rubbers, e.g., a running vehicle, stops.…”