Synthesis And Properties Of Recyclable Epoxy Resins Containing Dynamic Covalent Bonds

Epoxy resins,as a typical class of thermosetting polymers,have widespread industrial applications such as adhesives,coatings,electronic encapsulants,and polymer matrices for advanced composites,owing to their excellent mechanical performance,adhesive capacity,dimensional stability,heat and chemical resistance.However,conventional epoxy resins cannot be reprocessed,repaired or dissolved due to their permanent crosslinked structures,which make it a longstanding challenge to recycle the ever-increasing epoxy and epoxy-based composite wastes.Therefore,both economic and environmental factors are driving the development of recyclable epoxy resins.In this dissertation,a series of recyclable epoxy resins containing dynamic covalent bonds were designed and synthesized.The thermal,mechanical and recycling properties as well as the structure-property relationship of these epoxy resins were systematically investigated.Some of synthesized epoxy resins were used as matrices to obtain recyclable carbon fiber reinforced epoxy composites(CFRCs).The research contents of this dissertation are as follows:1.An epoxy resin containing disulfide bonds was prepared for recyclable,weldable,reprocessable,catalyst-free,high-performance epoxy resin.The cured epoxy resin had a tensile strength of 33 MPa and a glass transition temperature(T_g)of 116 oC,which were comparable to conventional epoxy resins.The dynamic nature of the disulfide bonds allowed the cured epoxy resin to be welded like thermoplastics,and the welded joints had strength that matched the resin's cohesive strength.It could also be reprocessed with 90 % tensile strength retention after three reprocessing cycles,allowing the parts to be repaired instead of being replaced.In addition,the cured resin was chemically degradable in a thiol-based solvent,allowing it to be recycled in closed-loop processes.2.Two imine-containing hardeners,IH-VAN and IH-HB,were synthesized by condensation of isophorone diamine(IPDA)and two different aldehydes,i.e.,bio-based vanillin and petroleum-based p-hydroxy benzaldehyde,respectively.The chemical structures of these hardeners were characterized by FTIR,1H-NMR,13C-NMR and HRMS.The epoxy resins cured by these two hardeners show high T_g(>120 oC),tensile strength(>60 MPa),Young's modulus(>2500 MPa)and good solvent resistance.The cured epoxy resins are re-processable and degradable due to the incorporation of imine dynamic covalent bond.Notably,the chemical degradation products can be reused to prepare new epoxy resins,thus achieving a closed-loop recycling process.Both the reprocessed and chemically recycled epoxy resins exhibit high percentage retention of thermal and mechanical properties.Finally,the epoxy resin cured by the vanillin-based hardener shows comparable thermal,mechanical and recycling properties compared to the epoxy resin cured by the petroleum-based counterpart.3.To endow CFRCs with reparability and recyclability,a bio-based epoxy resin containing imine dynamic covalent bonds was designed and prepared.First,an imine-containing epoxy hardener(ICH)was synthesized by condensation of lignin-derived vanillin and methylcyclohexanediamine.The epoxy resin(vitrimer)cured by ICH has sufficient T_g(? 131 oC),tensile strength(? 82 MPa)and solvent resistance.The incorporated dynamic imine bonds allow the epoxy resin to be reprocessable and degradable.The reprocessed resin has slightly increased T_g and around 90% retention rate of tensile strength.Besides,the imine-containing epoxy resin can be chemically recycled in a closed-loop manner by two different methods.Among them,one recycled epoxy resin almost obtains a full recovery in terms of tensile strength and T_g.The CFRCs based on the ICH cured epoxy resin exhibit comparable flexural properties compared to those based on conventional epoxy resin.The vitrimer feature of the ICH cured epoxy resin enables the CFRCs to be repairable.Especially,92% strength recovery was achieved for the repaired CFRCs after interlaminar shear failure.Furthermore,nondestructive carbon fibers were recovered from CFRCs by degrading the matrix resin in an amine solvent,and the degradation products could be reused to prepare new epoxy resin,thus achieving a full recycling process for CFRCs.4.To investigate the effects of crosslinking degree on the properties of dynamic epoxy networks,a series of mixed epoxies composed by different ratios of di-and tri-functional epoxies were cured by an imine-containing hardener.With the increase of crosslinking density,the epoxy resins show increased curing reaction enthalpy and T_g.The results of stress relaxation tests indicate that the activation energy(Ea)of imine exchange reactions monotonously increases with increasing the crosslinking density of epoxy resins.All the epoxy resins are reprocessable;however,the strength retention efficiency decreases with the increase of crosslinking density.