Study On The Effect Of Surface Modification And Properties Of High-performance Aramid Fibers On The Properties Of Composites

Because of the high crystallinity and the relatively smooth surface of aramid fiber,the chemical inertness of aramid fiber is produced,and the interface adhesion between aramid fiber and matrix is poor,which makes the reinforcement effect of aramid fiber difficult to be well reflected.At present,the grafting modification of aramid fiber can only improve the surface polarity of the fiber,and it is difficult to form an interface layer on the surface of the fiber.Modifiers with certain viscosity,such as dopamine,can form a coating layer on the surface of the fiber by virtue of strong physical adhesion and self-assembly reaction,and enhance the interface adhesion between the modified fiber and the matrix by improving the roughness and surface polarity.However,there is no chemical bond between dopamine and aramid fiber,which is only a kind of physical coating.This physical coating is not outstanding in the processing of fiber/rubber composite.In order to effectively solve the above problems,this paper is based on the oxidation treatment of aramid fiber,and then the aramid fiber is modified by glycidyl poss and polyamide acid respectively.Under the condition of no damage to the fiber body,an interface layer is formed on the surface of the fiber through the interface grafting reaction,so as to achieve the purpose of modifying aramid fiber.The effects of the properties of the interface layer on the interfacial adhesion of polar（epoxy resin）and non-polar matrix（SBR）composites were explored,which provided theoretical guidance for the modification of aramid fiber and the preparation of fiber-reinforced composites.The research content of this paper mainly includes the following aspects:（1）The surface polarity of aramid fiber was increased by air oxidation at high temperature,which laid a foundation for subsequent grafting modification.The results show that the surface molecular chain of aramid fiber is oxidized at 200-300℃in air,and the molecular chain begins to decompose at 300℃.The oxidation of aramid fiber starts from the main chain benzene ring,and the benzene ring C-H is oxidized to-Oh,-COOH.The hydrogen bond on the surface of aramid fiber begins to be affected obviously at 250℃,and the surface molecular chain can be rearranged to enhance the hydrogen bond.After heat treatment at 250℃for 1 h,the average molecular grain size of the surface layer of aramid fiber increased,and the crystallinity reached 86.22%,13.2%higher than that of the untreated fiber.However,with the increase of temperature,the change of crystallinity of the fiber surface is not obvious.In the process of heat oxidation treatment,250℃treatment for 1 hour is the most favorable for the comprehensive properties of aramid fiber.（2）The surface properties of aramid fiber treated with supercritical CO₂（50℃,10MPa）were studied.The results showed that the molecular structure of aramid fiber cortex did not change under the action of supercritical CO₂（50℃,10MPa）.Meanwhile,supercritical CO₂ could diffuse to the fiber cortex and destroy the fiber cortex.After 30 minutes of treatment,the effect of supercritical treatment on the fiber is less.Compared with the untreated fiber,after 30 minutes of supercritical CO₂treatment（50℃,10 MPa）,the crystallinity of the fiber surface was 77.12%,decreased by 6.53%;the tensile strength of the single fiber was 22.6 cn.dtex^-1,decreased by5.04%.（3）The group reaction between glycidyl-POSS and aramid fiber was studied by surface coating with high temperature treatment.The results show that the relative intensities of（110）and（200）diffraction peaks are basically the same,but the stacking density of the fiber cortex increases and the half peak width of the crystal decreases.The"perfection"of the crystal morphology is essentially the effect of the dense glycidyl-POSS coating on the fiber surface.At high temperature,the end group of glycidyl-POSS can react with the oxidized aramid fiber to form the grafting interface layer.When the amount of glycidyl-POSS is less than 5%,it can be grafted or coated uniformly,and the coating can improve the thermal stability of aramid fiber.（4）At high temperature,glycidyl-POSS can react with oxidized aramid fiber to form the grafting interface layer.When the amount of glycidyl-POSS is less than 5%,it can be grafted or coated uniformly,and the grafted polymer can improve the thermal stability of aramid fiber.At the same time,it can reduce the damage of supercritical CO₂ on the fiber.2E4MZ,as a curing accelerator,promoted the ring opening reaction and grafting（or self polymerization）of glycidyl-POSS on the fiber surface,which was conducive to improving the mechanical properties of the fiber.Compared with untreated fiber,the tensile strength of aramid fiber treated by 5%glycidyl-POSS assisted with supercritical CO₂increased by 8%,reaching25.7cn·dtex^-1.（5）After PAA grafting modification,the surface crystal spacing and the crystallinity of aramid fiber increases slightly,and the microcrystalline bulk density decreases,.In the process of PAA grafting,polyimide（PI）was formed by self polymerization.At the same time,a part of-COOH in PAA molecular chain does not undergo amidation at 250℃,and the-COOH enhances the polarity of the fiber.The crystallinity of aramid fiber treated with 5 wt%and 10 wt%PAA was 81.6%and 84.5%respectively.When the amount of PAA is too large（10 wt%）,a large amount of PAA adheres to the fiber surface,forming a rigid coating,which greatly increases the fiber roughness.（6）Assisted with supercritical CO₂,PAA adheres to the surface of the fiber more evenly and penetrates into the surface of the fiber at the same time.After 5%PAA treatment,PAA can be uniformly coated and grafted on the surface of the fiber.At the same time,the coating can further reduce the damage of supercritical CO₂ on the surface structure of the fiber,and the tensile strength of the treated fiber increases by 9.6%.（7）After glycidyl-POSS processing assisted with supercritical CO₂,the thickness of grafted coating on the surface of the fiber was about 50 nm.However,glycidyl-POSS was easy to assemble,and there was obvious aggregation of glycidyl-POSS on the surface of the modified fiber.After PAA processing assisted with supercritical CO₂,the coating layer on the fiber surface is uniform,and the thickness of the coating layer is more than 100 nm.In the process of grafting and coating modification of macromolecules,it is easier to form uniform coating layer,and the effect of interface modification is better than that of grafting and coating modification of micromolecules.（8）After modifing by glycidyl-POSS,the surface roughness of aramid fiber was improved,and the compatibility between aramid fiber and epoxy resin was further improved by terminal group and Si-O structure of glycidyl-POSS.After modifing by glycidyl-POSS assisted with supercritical CO₂,the interfacial shear strength（IFSS）between the modified fiber and epoxy resin increased by 40.7%.After supercritical CO₂ assisted PAA treatment,PAA was grafted onto the surface of aramid fiber,and some PAA was thermally imidized to form a rigid coating.At the same time,-COOH in PAA without imidization can effectively react with epoxy resin to improve the interface adhesion.The interfacial shear strength of the modified fiber and epoxy resin increased by 53.8%.（9）For rubber composites（AF/CSBR=2/50/100）,the end group of glycidyl-POSS and the unimidized-COOH in PAA can interact with sulfur after modifing of glycidyl-POSS and PAA assisted with supercritical CO₂.At the same time,PAA,as an acid substance,delayed the vulcanization process of rubber and increased the positive vulcanization time.After supercritical CO₂ assisted modification of glycidyl-poss and PAA,the grafted layer on the surface of the fiber increased the binding of the rubber molecular chain and hindered the movement of the rubber molecule under the shear force.After vulcanization of the modified aramid fiber/C/SBR blend,the crosslinking network was formed.In the small strain region,the G’and G"of the vulcanizate increased more obviously after PAA treatment assisted with supercritical CO₂.After supercritical CO₂ assisted modification by glycidyl-POSS and PAA,the values of the relative interface slip energy between the modified aramid fiber and SBR were 6.07and 6.94,respectively,which were 23.87%and 41.63%higher than that of the composite interface.Accordingly,the tensile strength and 100%elongation of the composite modified by supercritical CO₂ assisted glycidyl poss increased by 10.77%and 19.58%respectively,while the tensile strength and 100%elongation of the composite modified by supercritical CO₂ assisted PAA increased by 15.13%and25.77%respectively.