Design And Synthesis Of Ionic Liquid Curing Agents For Fiber Reinforced Epoxy Resin

Fiber reinforced epoxy resin composites are widely used in aerospace.Epoxy resin curing agents are an important part of their development.Due to their unique physicochemical properties,ionic liquids as a kind of curing agent for epoxy resin has great application potential.Curing agents,the current research on ionic liquids as epoxy resin curing agents was limited to traditional structures of ionic liquids,so the development of new high-performance ionic liquid curing agents has become an increasingly urgent need.The amide functionalized ionic liquid was used as an epoxy resin curing agent for application research.The details were as follows:Nine cationic amide functionalized ionic liquids were synthesized by introducing amide groups on imidazole with Cl^-,N?CN?₂^-,and NTf₂^-as anions,respectively.The synthesized ionic liquids were characterized by 1H and 13C NMR,IR spectroscopy,elemental analysis?EA?and X-ray single crystal diffraction.Among the 9 ionic liquids,the anion was Cl^-having1-vinyl-3-?2-amino-2-oxoethyl-?imidazolium chloride?1?,1-methyl-3-?2-amino-2oxyethyl?Base-)imidazolium chloride?2?and 1-butyl-3-?2-amino-2-oxoethyl-?imidazolium chloride?3?.The anion was N?CN?₂^-having 1-vinyl-3-?2-amino-2-oxoethyl-?dicyanamide salt?4?,1-methyl-3-?2-amino-2-oxoethyl-And dicyanamide salt?5?and1-butyl-3-?2-amino-2-oxoethyl-?dicyanamide salt?6?.The anion was NTf₂^-having1-vinyl-3-?2-amino-2-oxoethyl-?bis?trifluoromethanesulfonimide?salt?7?,1-methyl-3-?2-amino-2 oxygen?Ethyl-?bis?trifluoromethanesulfonimide?salt?8?and1-methyl-3-?2-amino-2-oxoethyl-?bis?trifluoromethanesulfonimide?salt?9?.Their thermal decomposition temperature increases in the order of anions N?CN?₂^-<Cl^-<NTf₂^-and was very stable below 200?.The decomposition temperature ranged from 236.0??4?to409.9??9?.The ionic liquids 1 to 3 had an excessively high melting point?135.0 to185.0??,which causes inconvenience in handling and was not suitable as an epoxy resin curing agent.The melting points of ionic liquids 4,5,6 were relatively low?77.8¹05.5??.Especially for ionic liquids with NTf₂^-anion,7⁹ was liquid at room temperature,and its melting point ranged from-41.2??9?to-29.7??7?,which made the three ionic liquids of7⁹ were very easy to mix with the epoxy resin.The ionic liquid cured TDE-85 epoxy resin,and the tensile strength?72.1 to 115.3 MPa?and elongation at break?2.58 to 4.20%?of the thermosetting material were mostly significantly higher than those of DDS-cured TDE-85.These thermosets also exhibited good thermodynamic properties with a glass transition temperature?T_g?>190? and a thermal degradation temperature?T_d?>300?.Kinetic calculations show that E_a with N?CN?₂^-anion system is lower than E_a with NTf₂^-anion.The 7/TDE-85 system had the highest E_a?E_k was 105.8 kJ/mol,E_o was 108.6 kJ/mol?,which means that Compound 7 has the lowest curing reactivity.The reaction order was less than 1,indicating that the curing reaction was very complicated due to its polyfunctionality.The possible mechanisms of curing reactions were analyzed by using IR,and results indicated that curing reactions of TDE-85 with amide functionalized ionic liquids might include two steps,-NH₂ groups reacted with oxirane at lower temperatures,and then anions reacted with oxirane further to strengthen the cross-linking networks.This study provided informative guidelines for designing new curing agents.Finally,in order to investigate the universality of ionic liquid curing agent,it was cured with epoxy resin E-51,indicating that ionic liquid 6 was a high temperature curing agent for epoxy resin E-51,which was the best quality with E-51.The ratio was 10:100?6/E-51?,the optimum curing temperature range was 122¹56?,and the post curing temperature was178?.The cured product of 6 and E-51 has good heat resistance,and the thermal decomposition temperature was as high as 414?,but its mechanical properties need to be further improved.The curing reaction of 6 comprised two steps,the-CONH₂ group was first reacted with an epoxy group at a low temperature,and then the anion and the epoxy group were further reacted at a high temperature to finally achieve complete curing of the E-51.