| As a result of its excellent properties,carbon fiber reinforced composites(CFRP)hold great promise in aerospace,wind power blades,automobile industry.As CFRP reaches its service life,the amount of waste increases year by year.However,the three-dimensional cross-linked network structure of thermosetting resin makes it difficult to be recycled,which has become a bottleneck problem of the CFRP industry.Aiming at the recycling of CFRP,the design and synthesis of covalently cross-linked degradable epoxy resin based on thermally reversible Diels-Alder structure modification were reported.The covalent cross-linking degradation mechanism and the recycling effects of epoxy resin were analyzed.The thermally reversible CFRP were prepared and the mild condition recycling method of CFRP was achieved.(1)Covalently cross-linked degradable epoxy resin(EP-DA)with DielsAlder structure was synthesized by molecular structure design,and the structure of EP-DA and the reaction temperature of r-DA were characterized.The covalent cross-linking degradation mechanism of EP-DA resin was explored by stress relaxation,creep and FTIR.The effects of recycling times on the chemical structure,mechanical properties and thermo-mechanical properties of EP-DA resin were explored by hot press recycling method and solvent recycling method.The results showed that the r-DA reaction temperature of EP-DA is between120 ℃ and 140 ℃.The stress relaxation time was gradually decreased with the increase of temperature,and the creep residual strain was gradually increased.The peak intensity of infrared characteristic of the DA structure was gradually decreased with the increase of temperature.The reason was that the epoxy resin with the Diels-Alder structure underwent r-DA reaction at high temperature,and the cross-linked structure was dissociated into linear molecular segments,which absorbed heat and moved faster,resulting in the rearrangement of the topological network structure.The mechanical properties of EP-DA resin with three time recycle by solvent are better than those obtained by hot press recycling.With the growth of recycling times,the storage modulus,glass transition temperature and crosslinking density of epoxy resin were increased for hot press recycling and solvent recycling.(2)The thermally reversible carbon fiber reinforced composite was prepared from T300 carbon fiber fabric and EP-DA resin.The composite was reprocessed by hot press recycling method.The effect of recycling times on the mechanical properties and microstructure of thermally reversible carbon fiber reinforced composites were characterized by hot press recycling and solvent recycling.The surface morphology and chemical structure differences between solvent-recovered carbon fibers and original carbon fibers were analyzed.The results showed that the cured prepregs can be reprocessed into composites by hot pressing recycling,and the retention rates of bending strength and bending modulus reached 93% and 92%.The interlaminar shear strength(ILSS)of CFRP was gradually decreased with the increase of recovery times,which was recovered by hot pressing,and the ILSS retention rate could still reach more than 80% after recovery for three times.The retention rate of bending strength,bending modulus and ILSS reached more than 95% after three solventrecovered cycles.The surface micromorphology of solvent-recovered carbon fiber was not significantly different from original carbon fiber,and the degree of graphitization of the surface was slightly decreased with the increase of recycling times.Compared with the original carbon fiber surface O/C of 0.277,the O/C was 0.284 after the solvent-recovered for three times,indicating that the elemental composition on the surface of the recycled carbon fiber did not change significantly.Compared with virgin carbon fiber,the tensile strength and modulus of recycled carbon fiber monofilament remained above 98%. |
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