Design And Performance Study Of Thermal Latent Curing Agent For Imidazole-based Epoxy Resins

As an important thermosetting resin widely used in composite materials,epoxy resin has excellent properties.Long storage time,rapid curing and low viscosity of one-component EPoxy resin is ideal for pr EParing composite materials by vacuum assisted resin transfer molding(VARTM).For this sake,we developed and synthesized a new imidazole thermal latent curing agent,4,7-diimidazol-2,1,3-benzothiadiazole(BTD-MZ₂)with a yield of about 75%.The curing behavior,thermal stability and storage time of the prepared EPoxy resin(EP)system was studied by differential scanning calorimeter(DSC),thermal gravimetric analyzer(TGA),modular compact rheometer(MCR),respectively.DSC results showed that EP/17.5wt%BTD-MZ₂had a good latent period at low temperature,and can rapidly cure EP when heated to 150?.TGA results show that BTD-MZ₂promoted the formation of carbon residue and the residual carbon rate of EP/17.5wt%BTD-MZ₂is as high as 37.9%.MCR results show that the storage time of EP/17.5wt%BTD-MZ₂at 60?can reach 850 min.In addition,it could play a role of catalyst used as co-curing agents with amines for EP.We prepared two epoxy/amine/BTD-MZ₂curing systems:EP/DDM(/BTD-MZ₂-5)and EP/H256(/BTD-MZ₂-5).DSC test showed that the addition of 5wt%BTD-MZ₂reduced the activation energy(E_a)of EP/DDM and promoted the curing reaction.Through DSC test,the E_aof the two curing systems was calculated by Kissinger and Ozawa equation,and the E_aof the curing system with 5wt%BTD-MZ₂decreased.The decrease of the E_aproves that the addition of BTD-MZ₂catalyzes the curing reaction.The thermal stability of the two cured systems was tested by using a TGA.The addition of BTD-MZ₂hardly caused a significant decrease in the thermal stability of the cured system.We also tested the mechanical properties of both cured systems.Compared with the EP/DDM curing system,the storage modulus and glass transition temperature of the EP/DDM/BTD-MZ₂-5 curing system were increased by 10.8%;6.5%,the flexural strength and modulus also increased from 127 MPa,3025 MPa to 147 MPa and 3734 MPa,an increase of 15.7%and 24.3%,respectively.Similarly,the mechanical properties of the EP/H256/BTD-MZ₂-5 curing system have also been greatly improved compared with the EP/H256 curing system.The storage modulus and glass transition temperature of EP/H256/BTD-MZ₂-5 increased by68.1%and 15.9%,respectively,and the flexural strength and modulus increased by 4.9%and 24.9%,respectively.Finally,we made two kinds of carbon fiber composite materials with EP/H256/BTD-MZ₂curing system and unidirectional carbon fiber cloth by vacuum-assisted resin transfer molding technology(VARTM):EP/H256/CF and EP/H256/BTD-MZ₂-5/CF.Through the universal testing machine,we have tested the bending,tensile and interlaminar shear of the two composite materials,and the mechanical properties have been improved to a certain extent.The flexural strength,tensile strength and interlaminar shear were improved by 10.1%,39.8%and 10.1%,respectively.Through the SEM image of the cross-section,we can find that the addition of BTD-MZ₂improves the interfacial bonding ability between the resin matrix and the carbon fiber.