Synthesis And Properties Of Biobased Recyclable Epoxy Resins With Dynamic Covalent Cross-Linked Networks

Epoxy thermosets have been widely used as anticorrosive coatings,structural adhesives,electronic encapsulation materials and polymer matrices for fiber reinforced composites,due to their excellent mechanical properties,adhesive capacity,dimensional stability,corrosion resistance and electric insulation.However,conventional epoxy thermosets are produced from unrenewable petroleum-based resources and cannot be reused due to their permanently crosslinked structures.Developing epoxy thermosets with inherent recyclability from biobased feedstocks is a great importance from the views of resource conservation and environmental protection.This thesis is concerned with the design and synthesis of a couple of biobased and recyclable epoxy thermosets containing dynamic covalent bonds.The comprehensive performances of prepared epoxy thermosets were investigated in terms of curing behaviors,heat resistance,mechanical properties,reprocessability and degradability.The relationship between the structure and recyclability was analyzed emphatically.This thesis covers the following research findings:1.A fully biobased carboxyl-containing epoxy hardener（CF-quercetin）was prepared from quercetin and succinic anhydride（starch/sugar derivative）through a mild one-step reaction.The epoxy resin cured by CF-quercetin shows tensile strength（92 MPa）comparable with commercial counterparts and high storage modulus（3800 MPa）at room temperature.Owing to the high content of dynamic ester bonds in the epoxy networks,the cured epoxy resin can be repeatedly reprocessed like thermoplastics,and the recovery of tensile strength from the first reprocessing cycle can achieve 91%.Meanwhile,the glass-transition temperature and thermal stability of the reprocessed epoxy resin remain almost constant compared with the original epoxy resin.Furthermore,the epoxy resin cured by CF-quercetin presents rapid degradation in alkali aqueous solution（Na OH,1 M）within 6 h at room temperature,but it is fully nondegradable in acidic and neutral solutions over a period of 120 days.The cured epoxy resin also shows acceptable stability in many conventional organic solvents.2.Two biobased epoxy hardeners containing imine bonds（VAN-BAC and SYR-BAC）were prepared from lignin-derived aromatic aldehydes（vanillin and syringaldehyde）.The epoxy resins cured by these two hardeners showed comparable mechanical performances in comparison to traditional epoxy resins.Owing to the dynamic feature of incorporated imine bonds,both cured epoxy resins could be repeatedly reprocessed via hot pressing like thermoplastics,and were degradable in the amine solution.It is noteworthy that a closed-cycle chemical recycling process could be realized by utilizing the degradation products to prepare fresh epoxy resins.Impressively,the flexural strength and modulus of the chemically recycled and thermally reprocessed epoxy resins were nearly 100%recovered.In addition,both cured epoxy resins showed acceptable glass-transition temperature,thermal stability and chemical resistance.3.A novel recyclable biobased curing agent（VIBCA）was produced from lignin-based vanillin in a single-stage process.However,this curing agent was fully solid and difficult to process,requiring large amounts of solvent during processing.Since the viscosity of 4-methyl-1,3-cyclohexendiamine（HTDA）is low,it can be used as a co-curing agent at various amounts（0-100%）to make a viscous liquid curing agent for Diglycidyl Ether Bisphenol-F（DGEBF）epoxy resin as mentioned VH01 to VH05 samples.The curing behaviors,thermomechanical and mechanical properties of epoxy resins cured with the mixed curing agents were evaluated along with their self-healing ability.The results reveal that the viscosity and activation energy of the epoxy systems are reduced as the weight ratio of HTDA increases,whereas the flexural strength（106～122 MPa）and moduli（2365～2587 MPa）,T_g values（95～110℃）increase gradually.In addition,the heat-resistant temperature（T_s）values and char residue at 700℃ of the cured epoxy resins continuously decreased.Overall 40%of HTDA co-curing agent sample VH03 showed a suitable viscous form,mechanical and thermal properties.Subsequently,the flexural strength retention and T_g of the reprocessed VH03 sample were still 80%and 115℃,respectively,and it also demonstrated absolute solvent resistance at room temperature.4.Two fully biobased epoxy imine bonds containing hardeners（V-BDA and V-HDA）were synnthesized from biomass feedstocks vanillin,butanediamine and hexanediamine and their chemical structures were ensured by FTIR,¹H-NMR and¹³C-NMR.The two novel hardeners were used to cure epoxy resins,rendering vitrimers with good reprocessability,self-healing,recyclability and solvent resistance due to the revesible imine bonds.The flexural strengths and modulus of these cured resins were comparable to epoxy resins cured by conventional amine hardeners.The cured resins further maintained 100%of their T_g and flexural properties after being reprocessed up to three times.It was demonstrated that the cured epoxy vitrimers could be completely degraded in special acidic solution capable of bond exchanging reactions within 12 h at 50℃,allowing the thermoset matrix to be chemically recycled and the monomers regenerated.