| Epoxy resin,a typical type of thermosetting polymers with excellent mechanical properties,heat resistance and solvent resistance,is an ideal matrix resins for high-performance fiber-reinforced composites.However,the stable and irreversible cross-networking structure formed after curing makes the epoxy resin insoluble and non-melting,which cause problems for the reforming or recycling of the material.The presence of exchangeable dynamic covalent bonds in the cross-linked network of epoxy vitrimers allow them to reform or recycle similar to thermoplastic polymers under certain conditions,while maintaining the mechanical properties of traditional thermosetting epoxy resins.Epoxy resins cured by anhydride or carboxylic acid contain ester bonds which potentially can go through ester exchange.These systems are widely studied due to the relative simplicity of the preparation process and their high practical application value.There are two main types of catalysts typically used to promote ester exchange reactions,i.e.salts or complexes of metals such as zinc,lead,tin and titanium,and organo-azacyclic compounds such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene(TBD).However,metal-organic catalysts are often toxic and the organic nitrogen heterocyclic compounds are not as effective,and both inevitably affect the physical and chemical properties of the material.In this thesis,a new dynamic exchange catalyst,1-cyanoethyl-2-ethyl-4-methylimidazole(CEMI),was discovered and a series of reformable,degradable and recyclable epoxy vitrimers are prepared,and their thermal mechanical and recyclable properties are systematically investigated.The carbon fiber composites prepared from these epoxy resins also have good mechanical properties and are suitable for use in industrial applications.The research in this paper is divided into the following sections.(1)The curing behavior,thermal properties and reprocessing properties of epoxy resins with three imidazole catalysts were investigated and compared with typical ester exchange catalyst TBD.It was found that the CEMI-catalyzed epoxy resins were reprocessable at low temperature(200 °C),compared to that for the TBD-catalyzed resin(230 °C).In addition,this epoxy has mechanical properties comparable to epoxy resins catalyzed by conventional imidazole catalysts.(2)To further investigate the conditions and mechanism CEMI-catalyzed ester exchange,the effects of different epoxy/anhydride equivalent ratios and CEMI loadings on the curing behaviour,thermal properties,mechanical properties and repetitive processing of the epoxy resins were investigated.It was found that free hydroxyl groups in the resins was necessary to induce CEMIcatalyzed ester exchange at a CEMI content of 2.0 phr.At 2.0 phr CEMI content and an epoxy/anhydride equivalent ratio of 2/1,the vitrimer had the best reprocessing properties.In addition,the stress relaxation analysis at different temperatures yielded the activation energy for the dynamic exchange reaction is 104 k J/mol.(3)Two CEMI-catalyzed epoxy resins were used to prepare carbon fibre composites,namely a conventional epoxy resin with an epoxy/anhydride equivalent ratio of 1(1-BMC-3)and an epoxy vitrimer with an epoxy/anhydride equivalent ratio of 2(D-BMC-3),using a vacuum assisted infusion process.Their tensile,short beam shear,bending and compression properties were tested.The results show that,except the bending properties,the other properties of the carbon fibre composites prepared from the vitrimer matrix were better than those of the conventional epoxy resin matrix. |
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