| Epoxy resin itself has a linear structure and must exhibit its excellent performance through the cross-linking action of a curing agent.The hydroxyl groups in lignin molecules can react with epoxy groups to cure the epoxy resin,and the benzene ring structure in lignin molecules can increase the heat resistance of epoxy resin,so lignin has the potential as an epoxy curing agent.However lignin itself is a macromolecule,and its structure is relatively complex so that its utilization is low.Therefore,this article researches in using inexpensive,widely available sources of choline chloride(ChCl)and urea(U)as raw materials to prepare deep eutectic solvent(ChCl/U),and then used ChCl/U to modify enzymatic hydrolysis lignin(CEL),then directly used as a curing agent for epoxy resins without separation,to explore curing kinetic parameters and effect of curing performance of ChCl/U-CEL curing E-51 and mechanism of curing reaction,and the effect of DMP-30 accelerator on the performance of ChCl/U-CEL/E-51/DMP-30 system.The purpose is developing an epoxy resin curing agent that is non-toxic,non-polluting,simple in manufacturing process,low in viscosity,excellent in operability and usability,which not only reduces the use cost of epoxy resin,but also reduces waste of resources and reduces environmental pollution.The details are as follows:(1)The synthesis time of ChCl/U decreased with the increase of synthesis temperature.When the synthesis temperature was higher than 55°C,the synthesis time tended to be stable about 10 min.The combination of anions and cations in ChCl/U was that the active hydrogen on the amine group formed N—H?Cl hydrogen bond with the free Cl~-,and the O in the C=O bond formed O—H?O hydrogen bond with the H in—OH.(2)Treatment of CEL with ChCl/U gives ChCl/U-CEL,which could be used directly as an epoxy curing agent without separation and the optimum modification time is 6h,temperature is90°C and the addition of CEL was 3%of ChCl/U mass.Infrared analysis and thermal stability analysis of ChCl/U-CEL obtained under optimal modification conditions;The comparison of ultraviolet,infrared and thermal stability of CEL before and after modification of ChCl/U showed that CEL can be well dissolved in ChCl/U and ChCl/U-CEL has good thermal stability.(3)The n-stage curing kinetic equation of ChCl/U-CEL cured E-51 was obtained by a non-isothermal kinetics model with n stages.The optimum process of ChCl/U-CEL curing E-51was determined by extrapolation and tensile properties:after precuring at 140°C for 2 hours,it was heated to 160°C and then cured for 2 hours(140?/2h+160?/2h).(4)Comparing various properties of the cured resin after the three curing agents have respectively cured the E-51 by comparing the cured epoxy resin Curing Agents PA650,DDS and the self-made epoxy resin ChCl/U-CEL,getting the following conclusions:(1)The ChCl/U-CEL has the advantages of easy availability of raw materials,simple preparation process,low price and low viscosity,strong operability.(2)Surface morphology analysis of the fracture surfaces of cured resin can lead to the following conclusions:The fractures of ChCl/U-CEL/E-51 and DDS/E-51 are ductile fractures,and the fractures of PA650/E-51 is brittle fractures.(3)The tensile strength of ChCl/U-CEL/E-51 reaches 55.2MPa,the initial temperature of thermal decomposition is 340°C,and the glass transition temperature is as high as 124°C,which is obviously better than that property of PA650/E-51(the tensile strength is42.4MPa,heat decomposition temperature 310°C and glass transition temperature 70°C).(5)Through the study of the gel time of the cured epoxy resin system and the tensile strength and heat resistance of the cured resin when the amount of DMP-30 accelerator,found that DMP-30 can significantly promote the curing of epoxy resin E-51,so that the curing process of ChCl/U-CEL/E-51 system is reduced from 140°C/2h+160°C/2h to 80°C/0.5h+100°C/3h.The glass transition temperature of the cured resin of ChCl/U-CEL/E-51/DMP-30was 73.4%higher than that of the control group PA650/E-51 when the adding amount of the accelerator was 4 to 6%. |
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