Catalyst-free curing and closed-loop recycling of carboxylated functionalized rubber by a green crosslinking strategy

Abstract

Current adopted rubber curing methods undergo large amounts of environmental issues, such as the release of toxic volatile organic compounds (VOCs) during curing process and difficulties in recycling of end-of-life rubbers. Many new curing reactions and cross-linking structures have been developed to solve these problems. A green curing strategy based on epoxy-acid reactions and ester linkages could be a potential alternative to the current curing ones. However, the curing reactions between epoxy and carboxyl groups generally require a catalysis to improve curing rate, and the recycling process requires toxic organic solution or high temperature. It is of great desired to develop a real environmentally friendly crosslinking strategy for rubber materials. In this paper, we report a facile and high-efficiency green crosslinking strategy for the carboxyl-functionalized rubber. N,N,N′,N'-tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) was selected as a multifunctional amine-type high-efficiency crosslinker for carboxylated nitrile rubber (XNBR). TGDDM-cured XNBR exhibit extremely fast crosslinking rate and high crosslinking efficiency due to the four epoxy terminated units and tertiary amine structure acting as an internal catalyst. Besides, the mechanical properties and crosslinking density of TGDDM-cured XNBR can be regulated by TGDDM content. Furthermore, the ester linkage formed by curing reaction could be selectively and efficiently cleaved in a mixed solution of ethanol and water under mild condition. The closed-loop recovered XNBR maintains chemical structure and molecular weight and exhibit good mechanical performance. This strategy offers a promising way for high-efficient crosslinking of diene rubber as well as a real cost-effective and environmentally friendly recovery of end-of-life rubber materials.