The Construction And Research Of A Novel Thermoreversible Chemical Crosslinking Network In Diene Rubber And Its Composite

Although the stable three-dimensional crosslinking networks formed by sulfur bonds provide conventional diene rubber materials and its composites with high elasticity and chemical stability,its irreversibility severely restricts waste rubber products from being effectively recycled,finally causing severe environmental burden.Hence,the conventional crosslinking technique has been the bottleneck problem of the development of rubber industry circular economy.Thus far,the polymeric networks based on reversible covalent bonds have caught the attention of researchers at home and abroad,which become frontier research hotpots of polymer science and technology worldwide.In this study,we have developed a novel strategy of applying a multifunctional nitroxide crosslinking agent to the construction of a thermoreversible chemical crosslinking network in diene rubber and its composites,which endows the crosslinked diene rubber materials with excellent reprocessing properties.The novel strategy provides a fresh thought and way to the green recycling of diene rubber materials,which has profound theoretical value and broad applied prospect.First,the tetrafunctional-nitroxide crosslinking agent QT is synthesized and applied to construct a thermoreversible crosslinking network in non-polar diene rubber styrene-butadiene rubber through conventional rubber processing technologies;then the thermoreversible styrene-butadiene rubber is prepared,namely QSBR.The structure of QT is successfully characterized.Via infrared spectroscopy and electron spin resonance spectroscopy,the construction of the thermoreversible crosslinking network is attributed to the reaction between nitroxides and carbon-carbon double bonds on styrene-butadiene rubber chains,in which alkoxyamines are formed.The thermoreversible mechanism of the network results from the alkoxyamine cleavage at elevated temperature and its rebonding at ambient temperature in QSBR.The tensible and tear strengths of QSBR are 2 MPa and 15 k N/m,respectively,while its permanent set is relatively low.After two recycling runs,the tensile strength of QSBR remains at least 80%of that of the original.Thermogravimetric analysis shows that QSBR has high thermal stability.The alkoxyamine thermoreversible crosslinking network can effectively established in Qc SBR with carbon black reinforced while the addition of carbon black promotes the QT dispersion in rubber matrix.Mooney-Rivilin and N_int-strain analysis shows that the interaction between carbon black and rubber matrix gradually enhanced with the increase in the QT content.Rubber processing analysis and electron spin resonance analysis upon temperature runs demonstrated that the formed network could be thermally rearranged.The tensile strength and strain-at-break of Qc SBR are 26 MPa and 478%,respectively,while those of Qc SBR both remain 70%after two recycling runs.Second,the nitroxide crosslinking agent QT is applied to estabish a thermoreversible crosslinking network in polar diene rubber nitrile-butadiene rubber,aiming to prepare thermoreversible nitrile-butadiene rubber QNBR.By infrared spectroscopy and differential scanning calorimetry,the chemical crosslinking network in QNBR is proved to be formed through the reaction between nitroxides with unsaturated carbon-carbon double bonds on nitrile-butadiene rubber chains.The thermoreversibility of the QNBR can be ascribed to the breakage of alkoxyamines at high temperatures and its recombination at room temperatures.The tensile strength of QNBR reaches at most 8 MPa.Via electron spin resonance spectrum and rubber processing analysis,the thermoreversibility of nitrile-butadiene rubber composites Qc NBR reinforeced by carbon black is confirmed.The tensile and tear strengths of Qc NBR are23 MPa and 34 k N/m,respectively,while those of Qc NBR remain 66%and 88%after recycling.