Green and Catalyst-Free Cross-Linking of Bio-Based Elastomers toward Robust, Reprocessable Abilities and Improved Thermal Aging Resistance Enabled by β-Hydroxyl Esters

The bio-based epoxidized natural rubber (ENR) is considered a promising platform to design and fabricate sustainable and high-performance rubber materials. The epoxide sites in the ENR chains contribute to designing a green cross-linking strategy to reduce the release of toxic volatile organic compounds and achieve the recycling of end-of-life rubbers. However, it is still challenging to achieve a catalyst-free and effective cross-linking strategy and obtain a cross-linked ENR with mechanically robust properties. Herein, a series of carboxylic acids with different chain lengths and degrees of functionality were synthesized through the catalyst-free and solvent-free alcoholysis of maleic anhydride and served as cross-linkers for ENR. We demonstrated that the increased chain lengths and degrees of functionality of carboxylic acids were conducive to increasing the rate and efficiency of the epoxy-acid cross-linking reaction and improving the mechanical properties of ENR compared with those of commercial maleic acid. Hence, ENR could be facilely and effectively cross-linked by the modified carboxylic acids without additional additives, producing beta-hydroxy ester linkages. The mechanical properties of ENR/ carbon black composites could be facilely adjusted via changing the structure and content of the cross-linkers. Due to the introduction of the exchangeable beta-hydroxy ester linkages, the covalently cross-linked networks could be able to achieve topological rearrangements via transesterifications, thus conferring the cross-linked ENR with good reprocessability. Moreover, the carboxylic acid-cured ENR showed the improved thermal-oxidative aging resistance compared to sulfur-curd one because the formed ester cross-links are thermally stable. This work provides a catalyst-free, efficient and green cross-linking strategy for the epoxidized elastomer, which could open a broad application scenario in vitrimer-like rubbers.

Automated summary

A series of carboxylic acids with different chain lengths and degrees of functionality were synthesized through the catalyst-free and solvent-free alcoholysis of maleic anhydride and served as cross-linkers for ENR. The mechanical properties of ENR/carbon black composites could be adjusted via changing the structure and content of the cross-linkers. The carboxylic acid-cured ENR showed improved thermal-oxidative aging resistance compared to sulfur-cured ENR due to the formation of thermally stable ester cross-links.