| The development of high-performance epoxy resin and its application technology is changing with each passing day.The advantages of excellent processing performance,bonding performance and high mechanical strength make the application of epoxy resin expand from the field of coatings to the fields of aerospace composite materials.Medium-temperature hot-melt epoxy prepreg is currently a common intermediate material for composite structural parts.However,due to the existence of a large number of polar groups in the epoxy resin cured product,the water absorption rate of the resin is high,which leads to the high water absorption of epoxy resin composites.The moisture and heat resistance performance is poor,so it is necessary to carry out research on the moisture and heat resistance modification of epoxy resin.In this paper,the general bisphenol A,phenolic and glycidylamine epoxy resins are used as the resin matrix,and the commonly used dicyandiamide for medium-temperature hot-melt epoxy prepregs is used as the curing agent.The effects of the type of accelerator,curing system and molar ratio of active hydrogen to epoxy group(Hy/Ep)on epoxy/dicyandiamide resin were discussed.According to the influence law of system structure and performance,the curing process and resin formula with the best comprehensive performance are optimized.The specific research contents and results are as follows:(1)In the case of epoxy/dicyandiamide resin system Hy/Ep of 0.33 and 0.55,dicyandiamine organic urea accelerator UR500,2-undecylimidazole and 2,4-diamino-6-[2-(2-Methyl-1-imidazolyl)ethyl]-1,3,5-triazine was used as accelerator.The methods of water absorption and mechanical properties were used to discuss the types of accelerators and Hy/The effect of Ep on the performance of the resin system,the results show that compared with the type of accelerator,the Hy/Ep of the resin system has a greater impact on the water absorption.When the Hy/Ep is 0.33 and the accelerator is UR500,the comprehensive performance of the resin system is excellent.Among them,the water absorption rate(95o C/7day)is 3.24%,the impact strength is 32.1 k J/m~2,the bending strength is 120 MPa,and the bending modulus is 2.57 Gpa.(2)In the case where the accelerator is UR500,in order to analyze the influence of Hy/Ep on the structure and performance of the resin system in detail,a resin system with Hy/Ep of0.8,0.6,0.4,0.2,and 0.1 was designed.The curing kinetics of different Hy/Ep resin systems were studied with the non-isothermal DSC test method,and different curing regimes were set according to the curing kinetic parameters to study the effects of different curing regimes on the structure and properties of resin castings.The results show that the curing process of the epoxy/dicyandiamide resin system conforms to the two-parameter autocatalytic model,and with the decrease of Hy/Ep,the proportion of autocatalytic reaction of the resin system increases;refer to the characteristic temperature and other parameters in the curing kinetics,respectively.Set the curing system to 120~oC/4 h,120~oC/2 h+135~oC/1.5 h+150~oC/0.5 h and120~oC/4 h+135~oC/4 h+150~oC/4 h,the infrared spectrum shows that the curing system is120~oC/4 h+135~oC/4 h+150~oC/4 h,the content of ether bonds in the resin network was the highest.At this time,the resin casting had the lowest water absorption rate of 2.45%(95~oC/7day)and the highest impact strength of 37.5 k J/m~2.(3)Under the premise of optimizing the type of accelerator and curing system,the evolution of the structure and properties of the resin system before and after hydrothermal aging was studied by using FTIR,DMA,DSC,SEM and other characterization methods.Before hydrothermal aging,with the decrease of Hy/Ep in the resin system,the results show that the content of ether bonds in the resin casting body increases and the content of cyano groups decreases;the glass transition temperature of the resin casting body first increases and then decreases.When it is 0.6,the glass transition temperature is as high as 175~oC;the impact strength first increases and then decreases,and when Hy/Ep is 0.2,the impact strength is as high as 37.5 k J/m~2;the flexural strength and modulus first decrease and then increase.After hydrothermal aging in 95~oC deionized water for 7 days,with the decrease of Hy/Ep of the resin system,the water absorption rate and bound water ratio of the resin casting body first decreased and then increased.When the Hy/Ep was 0.2,the water absorption rate was as low as 2.48%;The proportion of residual cyano hydrolysis reaction in the casting body decreased;the glass transition temperature and flexural property retention of the resin casting body first increased and then decreased.When Hy/Ep was 0.2,the retention rate of flexural strength and modulus was as high as 86%.The above results show that when Hy/Ep is 0.2,the resin casting body exhibits the most excellent comprehensive properties. |
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