Design,Preparation And Performance Of Novel And Green Cross-linked Elastomer

Covalently cross-linked elastomers are indispensable in many important fields due to their unique entropic elasticity.The covalent cross-linking of elastomers is the most important step for achieving high elasticity.However,there have been several inherent issues caused by traditional cross-linking methods.First,some curing additives(such as ZnO)have caused great trouble for the environment.Second,some curing additives,such as sulfur and accelerators,tend to release toxic volatile organic compounds(VOCs)during sulfur vulcanization process.Simultaneously,the VOCs are also accompanied by an unpleasant smell.Furthermore,the covalently cross-linked elastomers are inherently difficult to be recycled and reprocessed,resulting in serious environmental problems and resource waste at their end of life.To overcome the above-mentioned inherent issues,the concept and design strategy of green cross-linking system have been put forward.We have proposed that the cross-linking sites could be based on functional groups rather than double bonds,while bio-based and green molecules could serve as cross-linker instead of sulfur.The main research contents of this subject are as follows:(1)A green curing strategy is put forward to prepare EPDM/CB composites with high mechanical performance,efficient recyclability,and a shape memory effect by engineering(3-hydroxyl ester bonds into cross-linked networks.The epoxy-functionalized EPDM(EEPDM)was prepared using an in-situ epoxidation reaction and then cured with bio-based decanedioicacid(DA)through the reactions between the epoxy groups in EEPDM and the carboxylic groups in DA.Because of the existence of exchangeable β-hydroxyl ester,the covalently cross-linked networks in EEPDM/CB composites were able to rearrange their topological structure at high temperature,endowing the composites with recyclable and reshaped abilities.However,the mobility of rubber chains may be restricted because of the adsorption of the rubber chains on the CB surface,resulting in slowing of the transesterification rate.Meanwhile,the introduction of the uncured composites could significantly improve the recovery efficiency.Spcecailly,the tensile strength and elongation at break of the recycled EEPDM/CB composites are greater than 15 MPa and 200%,respectively.We envision that this work provides a successful paradigm that combines green curing chemistry,recyclability and reshaped ability into commercially available polyolefin rubber networks.(2)A simple,effective and green cross-linking strategy for epoxy-functionalized elastomers was provided to achieve the combination of green curing process,excellent mechanical properties and good recyclability.The epoxy-functionalized styrene-butadiene rubbers(ESBR-GMA)with different epoxy group contents were synthesized by introducing glycidyl methacrylate(GMA)during emulsion polymerization.This work highlights a green cross-linking strategy in which bio-based dicarboxylic acids serve as cross-linker to cross-link epoxy-functionalized elastomers without additional additives.By the reaction between the epoxy groups in ESBR-GMA chains and the carboxyl groups in dicarboxylic acids,the covalently cross-linking network containing ester linkages are formed.Because the epoxy groups in ESBR-GMA chains have high reactivity,ESBR-GMA with a small amount of dicarboxylic acids as cross-linker can achieve excellent cross-linking efficiency and rate.Additionally,the cross-linked elastomers exhibit excellent mechanical properties,which could be widely regulated by manipulating the type and content of dicarboxylic acids.By increasing the SA content and epoxy group content in ESBR-GMA,the cross-linking effeciency and density can be improved.Moreover,due to the incorporation of ester linkages into the cross-linking structure,we provide a facile and cost-effective chemical methodology for the recovery of end-of-life elastomers by selective cleavage of the ester linkages.We envision that the epoxy-functionalized elastomer may be a promising base material for green cross-linking strategy,which could be interesting candidates for technical applications.(3)A novel and green cross-linking strategy for carboxyl-functionalized elastomers using renewable bio-based epoxidized soybean oil(ESO)as the cross-linking agent.As a proof of concept,a commercial carboxylated nitrile rubber(XNBR)is efficiently cross-linked by a bio-based ESO,without any toxic additives.ESO exhibits an excellent plasticization effect and excellent scorch safety for XNBR.The cross-linking density and mechanical properties of the ESO-cured XNBR can be manipulated in a wide range by changing simply varying the content of ESO.In addition,zinc oxide(ZnO)performs as a catalyst to accelerate the epoxide opening reaction and improve the cross-linking efficiency,serving as reinforcement points to enhance the overall mechanical properties of the ESO-cured XNBR.Furthermore,the end-of-life elastomers demonstrate a closed-loop recovery by selectively cleaving the ester bonds,resulting in very high recovery of the mechanical performance of the recycled composites.This strategy provides an unprecedented green avenue to cross-link diene elastomers and a cost-effective approach to further recycle the obtained cross-linked elastomers at high efficiency.(4)Taking the most widely used NR and SBR in the rubber industry as an example,a simple and effective method for reducing zinc level is proposed in sulfur vulcanization system.First,in the zinc-free sulfur vulcanization system,the effects of the content of sulfur and sulfenamide accelerator on the vulcanization performance,mechanical properties and aging performance of NR were studied.It was found that the NR and SBR can be effectively cured with zinc-free sulfur vulcanization systems.Besides,the vulcanizing and mechanical properties of NR and SBR can be widely regulated by the content of accelerator and sulfur.Meanwhile,the properties could meet most of the requirements in the application.However,the influence of the sulfur vulcanization system on NR and SBR is quite different.For NR,the increase in accelerator content can significantly improve the vulcanization performance and mechanical properties;while sulfur content has little effect on the performance.For SBR,the increase in sulfur content can significantly improve the vulcanization performance and mechanical properties;however,accelerator content has little effect on the performance.In addition,the addition of a small content of ZnO or ZNSt2 can significantly improve the vulcanization performance and mechanical properties of NR and SBR,so its cross-linking efficiency and cross-linking density increase significantly.Finally,based on the sulfur vulcanization mechanism,a-H plays an important role in the formation of the cross-linking precursor and the initial cross-links,which leads to the difference in vulcanization mechanism and performance between NR and SBR.The dominant side reaction of NR is the formation of sulfur-containing cyclic structures combined with the rubber chains;however,the dominant side reaction of SBR is the formation of dangling group.