Structure And Properties Of Polyimide Fiber Weft Knitted Fabrics For Rubber Composite

High performance rubber composite has the characteristics of high specific streng th and high deformation,which has great application potential in aerospace,transporta tion and other fields.The insulating layer of solid rocket engine combustion chamber i s made of rubber composites,which can closely fit the inner wall of combustion cham ber and bear the load together.High performance fiber weft knitted fabrics are used as reinforcements of rubber composite can improve the failure problem of the solid rock et engine insulation material caused by deformation incompatibility between the reinfo rced fabric and rubber matrix under loading.In this work,the structure and mechanica l properties of Polyimide fiber weft knitted fabric reinforced nitrile rubber(NBR)com posites were studied from the aspects of fiber properties and optimization,fabric struct ure design and preparation,NBR composites preparation and composites mechanical p roperties test.Firstly,the tensile and hook properties of PI fiber bundle were evaluated.Results showed that the tensile strength and hook strength of PI fiber are higher than Kevlar fi ber and Vectran fiber among the four high performance fibers tested.The hook strengt h retention rate of PI fiber was up to 92%,and the damage of loop bending was less.F or the weft knitted fabric,PI fiber without any treatment on machine knitting performa nce is great,the loop presents a skewed state.Secondly,based on the rib structure,the rib fabric with four structural parameters and the warp insertion fabric with four binding structures were designed.Compared w ith other warp insertion fabrics,the loop-shifting process is to move two rows and one needle distance.The coating deformation of the warp insertion fabric U4 with two co nsecutive shifts is the best,and its overall deformation rate is 12.8 %.U4 fabric is sele cted as the reinforcement of rubber composite materials.The weft knitted fabric reinfo rced NBR composites with parallel layer structure and orthogonal layer structure were prepared by molding process.Finally,the tensile,interlaminar shear and low velocity impact properties of weft knitted fabric were tested.The results show that the rib fabric reinforced composites with large tow fiber,long loop length and orthogonal lay-up structure have higher tensile strength and impact resistance.Under interlaminar shear,the strength of the specimens with small tow fiber and long loop length is higher,and the interlaminar shear strength is less affected by the lay-up structure.The damage morphologies of rib fabric reinforced composites is mainly caused by rubber matrix cracks along the loop direction,and large plastic deformation in the thickness direction without delamination cracking.For the warp insertion fabric reinforced composites,the tensile,interlaminar shear and impact resistance of the orthogonal lay-up specimens are better than those of the parallel lay-up.The tensile damage morphologies is mainly that the rubber matrix cracks along the warp yarn direction,and the interlaminar shear and impact damage morphology are not obvious.Compared with rib fabric reinforced composites,the strength of the warp insertion fabric reinforced composite increases obviously and the deformation decreases slightly.