Design And Preparation Of γ-ray-induced Degradable Epoxy Resins And Their Composites

Advanced thermoset composites are widely used in aerospace,transportation,wind energy and other fields,while degradation and recycling of waste composites restrict their development and application.Under this background,γ-Ray radiation degradation technology emerges,affording a promising solution with a low energy consumption,environmental protection and high efficiency for th e degradation of thermoset materials.However,for high-energyγ-Ray radiation,the uncontrollable radicals generated by the chain breaking lead to complex chemical changes such as cross-linking and polymerization,which hinder the radiation degradation.T herefore,reducing the probability of cross-linking and polymerization is the key to realize the controlled degradation.In this paper,in view of the key scientific problems in the degradation of thermoset materials,such as the performance and stability cannot trade-off,the degradation conditions are harsh,and the efficiency is low,and taking epoxy thermoset as the research object,the molecular structure design,synthesis and preparation method,processing process and efficient degradation mechanism o fγ-Ray-degradable epoxy composites were studied.Through the innovative structural design,phenylimine conjugated N-N bond was introduced into the two aromatic amine curing agents,which cured with bisphenol A-type epoxy resin（EP,0.51 mol/100 g of epoxy value）.Two epoxy thermosets with excellent serviced properties,stability andγ-Ray radiation degradation properties were developed.γ-Ray-degradable epoxy resins exhibited excellent mechanical and thermal properties,and their tensile strength and glass transition temperature have～79 MPa and～76 MPa,～180℃and～180℃,which are higher than those of the traditional epoxy EP-DDM（4,4’-diaminodiphenylmethane）.The two epoxy thermosets were degraded byγ-Ray radiation at 40 k Gy.Theγ-Ray-sensitive behavior can be attributed to the controlled cleavage of phenylimine conjugated N-N bonds.N-N bonds can provide the easy degradation sites,and the phenylimine conjugated groups can stabilize the radical fragments generated by radiation and inhibit the cross-linking,thus realizing the controllable degradation of materials.In addition,a radical scavenger can reduce the uncontrolled recombination of the degraded chain segment and further improve the degradation efficiency of the materials（complete degradation can be achieved at a dose of 30k Gy）.Finally,carbon fiber/γ-Ray-degradable epoxy laminates were prepared by molding process,and the bending strength and interlaminar shear strength of the materials were～739 MPa and～45 MPa,respectively.Under the radiation dose of120 k Gy,the resin components in the composites were completely degraded,and the carbon fibers with regular braided structure were obtained.The physical,chemical properties and mechanical properties of the recycled carbon fiber have no obvio us changes,which is expected to be used in a new generation of composites.By further optimizing the structural design,phenylimine conjugated N-N bond and S-S bond were introduced into aromatic amine curing agent.Through the cascade synergy of radiation-sensitive groups,the epoxy thermoset with high radiation-degradable efficiency was developed.The tensile strength and glass transition temperatures of the material are～72 MPa and～167℃,respectively,which are close to those of the traditional epoxy resin EP-DDM.Underγ-Ray radiation,epoxy thermoset can be completely degraded at 10 k Gy.The mechanism analysis shows that the phenylimine conjugated structure can stabilize the radical fragments generated by N-N bond cleavage,thus inhibiting the cross-linking and recombination.In addition,N-N bond cleavage can produce a large number of phenylimine conjugated radicals,which then attack S-S bond and lead to S-S bond cleavage,promoting the subsequent spontaneous degradation and maximizing the degradation efficiency.Finally,the degradation strategy ofγ-Ray radiation was applied to advanced epoxy composites,and its bending strength and interlaminar shear strength of the materials were～730 MPa and～43 MPa,respectively.At the radiation dose of 80 k Gy,the degradation of epoxy matrix was triggered by radiation to realize the nondestructive recycling of carbon fiber.The reaction of vanillin with p-hydroxybenzoyl hydrazide was selected to prepare bisphenol compound containing phenylimine conjugated structure to replace petroleum-based material（bisphenol A）.Through the further functional group conversion reaction of epichlorohydrin,radiation-sensitive biomass epoxy resin was prepared.By cross-linking with the curing agent containing radiation-sensitive groups,a bio-based epoxy thermoset with high content of radiation-sensitive groups was developed.Benefits from the design of phenylimine conjugate structure,the epoxy thermoset exhibits excellent mechanical properties and high carbon residue,and its tensile strength and Young’s modulus are～70 MPa and～2.0 GPa;and C_y800 is44%at 800℃,which is much higher than that of EP-DDM.Underγ-Ray radiation,the bio-based epoxy thermoset was completely degraded at 10 k Gy,and the efficient degradation behavior could be attributed to the controllable cleavage with high content of phenylimine conjugated N-N structure.Overall,the design of radiation-sensitive phenylimine conjugated N-N bond provides a general chemical platform for the design of thermoset and their composites with excellent radiation degradation and service performance.