Enabling reprocessability of ENR-based vulcanisates by thermochemically exchangeable ester crosslinks

Abstract

A reprocessable elastomeric vulcanisate based on Epoxidised Natural Rubber with 50 mol-% epoxide content (ENR-50) was evloved by applying a self-assembled network based on thermochemically exchangeable ester crosslinks to the system. Hydrolysed Maleic Anhydride (HMA) as crosslinking substance in the presence of 1,2-DiMethylImidazole (DMI) as esterification accelerator and Zinc Acetate Dihydrate (ZAD) as transesterification catalyst was employed to generate the exchangeable ester crosslinking system. A sulphur-cured ENR-50 vulcanisate possessing a permanent sulphide crosslinking network was prepared as reference. Based on the results from cure characteristic and chemical structure analyses, the ENR-50 crosslinked with HMA behaves as a dynamic network because of a transesterification reaction catalysed by ZAD, promoting an exchangeable crosslinking network in the system. This dynamic network contributes to an intermolecular rearrangement of the ester crosslinking bonds at elevated temperatures, enabling interfacial self-adhesion and so reprocessability of the vulcanisates. The obtainable vulcanisates can be reprocessed, yielding relatively high retention of mechanical properties compared to their pristine counterpart. The interfacial self-adhesion and reprocessability of the vulcanisates have shown to significantly be improved with a higher loading of ZAD and elevated temperatures. This concept essentially shows a prospect towards developing e.g. novel recyclable and selfhealing systems for elastomers.

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

“…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.…”

“…ZAD functions as a transesterification catalyst with efficiency to promote the transesterification reaction at elevated temperatures, shuffling the formed ester bonds and enabling network rearrangement [20]. This mechanism allows the crumbled rubber to again reform into a coherent vulcanized sheet [21,22]. Hence, this material is considered a self-healing modifier, as it would potentially promote the intermolecular self-reparation of molecular damages in vulcanizates.…”

“…Hence, this material is considered a self-healing modifier, as it would potentially promote the intermolecular self-reparation of molecular damages in vulcanizates. The material structure, cure characteristics, and other technical properties of this modifier were recently reported in [21,22].…”

“…The catalyst initiates a transesterification reaction that introduces a dynamic network to the system, enabling the self-adhesion of ENR vulcanizates at elevated temperatures. Moreover, our previous publications [21,22] recently demonstrated that the rubber pellets crushed from a vulcanizate based on ENR crosslinked with an ester crosslinking system, i.e., a dicarboxylic acid, that is, hydrolyzed maleic anhydride, can be remolded, giving again a coherent rubber sheet. This is because the ester bonds at the interfaces of the vulcanizate pellets can exchange their bonding sites.…”

Self-Healable Biocomposites Crosslinked with a Combination of Silica and Quercetin

“…The chain of ENR consists of two types of functional moieties, namely oxirane rings and a double bonding; thus, such a polymer has more possible ways to be cured than natural rubber or conventional synthetic elastomers [ 5 ]. On the one hand, double bonds enable ENR to be cured using classic vulcanization systems, including sulfur assisted with activators and accelerators, where sulfidic crosslinks between the rubber molecules are formed, or by utilizing dicumyl peroxide (DCP) and creating carbon–carbon crosslinks between the rubber chains [ 6 ]. On the other hand, reactive oxirane groups present in the structure allow curing by multifunctional nucleophilic reagents, including amines most effectively in combination with phenol or bisphenol A as a catalyst [ 7 ], and by carboxylic acids with imidazole as an accelerator [ 8 ], as well as by anhydrides [ 9 ].…”