Study On Hygrothermal Aging Properties And Modification Of Aramid Fiber/Epoxy Resin Composites

Aramid fiber/epoxy resin composite possesses excellent mechanical properties,good chemical corrosion resistance,high specific strength and specific modulus,low specific density.It is an ideal shell material for missile and aircraft engine,and is widely used in important fields such as national defense and military industry.However,the "skin core" structure of aramid fiber leads to its transverse strength far lower than its longitudinal strength,and the aramid fiber has less polar groups on the surface,smooth surface,and suffers from a lack of chemical active points,so that it shows poor interfacial compatibility with resin matrix.There are also residual stresses due to resin curing when compounding with resin,which will lead to weak interfacial bonding performance between fiber and matrix resin.Especially after the heat and moisture aging treatment,the interface properties of the composites decrease more significantly,which seriously limits its use in harsh areas.Therefore,this paper studies the modification of aramid fiber to improve the interfacial performance of aramid fiber,enhance the bonding ability of aramid fiber skin and core layer,and promote the interface bonding performance between fiber and resin matrix,especially in the environment of hygrothermal aging.Firstly,the properties of Kevlar49 and F-12 aramid fibers and their composites were studied under hygrothermal aging conditions.The results of molecular dynamics simulation showed that the aggregation structure of the two fibers changed little during the degradation process,but that of the resin changed before and after hygrothermal aging.Then,the water molecules on the fiber/resin interface are more difficult to exist after surface modification,and some hydrogen bonding and van der Waals forces were replaced by chemical bond forces in the surface modification,so as to enhance the interfacial bonding strength and improve the mechanical properties of aramid fiber/resin composites.Therefore,the surface modification of aramid fiber was the key means to improve the hygrothermal aging properties of composites.The surface modification of aramid fiber was realized by hexanedioylchloro/benzene solution impregnation.The effect of the modification on the interfacial properties was investigated by orthogonal experiment.The performance changes of aramid fiber after modification were systematically analyzed by means of IR,XPS,XRD,SEM and surface energy test.It was found that the concentration of adicyl chloride has little influence on the bulk strength of aramid fiber,followed by the treatment time,and the treatment temperature impacted greatly on the bulk strength of aramid fiber.Moreover,high temperature was more likely to destroy the hydrogen bonding force between the molecular chains of aramid fiber,which made the reaction activity of N-H bond increase,so that it was conducive to increase the grafting amount of adicyl chloride on the surface of aramid fiber.The results of hygrothermal aging test showed that the hexanedioylchlorobenzene solution impregnation could improve the hygrothermal aging resistance of aramid fiber.The IFSS and ILSS values of the modified aramid fiber were higher than those of the unmodified aramid fiber before and after the hygrothermal aging treatment.Although the above-mentioned modification method could effectively introduce polar groups on the fiber surface and improve the interfacial adhesion between the fiber and the matrix,the only drawback was that the bulk strength of the fiber would be damaged.In order to solve this problem,the aramid fiber was treated with 1,4-dichlorobutane in supercritical carbon dioxide,and the surface and interior of aramid fiber were modified simultaneously.XPS and IR were used to characterize the surface and internal element distribution and surface functional groups of the fiber before and after treatment;XRD was adopted to analyze the change of fiber structure;The surface morphology is observed by SEM and AFM;Surface energy of the fiber was also testded;monofilament tensile test was used to characterize the change of the tensile strength of the fiber before and after treatment.The indentation diameter test of composite fiber indirectly indicated the change of crosslinking degree in the fiber,and the mechanical properties before and after hygrothermal aging were also analyzed.The results showed that the surface strength of the fiber could be improved significantly by activating the fiber and improving the thermal resistance of the fiber.