Study On Preparation And Properties Of Polyimide Fiber Reinforced Resin Matrix Composites

High performance fiber reinforced resin based composites play a more and more important role in aeronautical satellites,motor vehicles,motor device.In recent years,the comprehensive performance of resin based composites is becoming a hot topic.The good interface compatibility between the fiber reinforcements and the resin matrix of the high performance resin matrix composites is the key to determining the performance of the composites.So appropriate modification to the fiber can improve the interface structure of the composites,and thus optimize the composites' macro properties.In addition,the existence of fiber in the form,distribution,uniformity also affect the composite properties,there are some problems in the traditional fiber directly reinforced resin,such as low fiber effective length,too many pores,uneven distribution,unstable properties and other issues.The purpose of the study is to use the effective method to functionalize the surface of the polyimide fiber to improve the performance of the fiber surface,the use of wet-laid paper-making technology to get the fiber paper reinforcement,the preparation of good interface compatibility,excellent mechanical properties of the resin based composites.In this paper,the polyimide fibers were used as raw materials,NaOH,HCl and H₂O₂ as wet chemical fluid.The specific changes of the fineness,mechanical properties,thermogravimetric properties,chemical structure,wettability,surface morphology and microscopic state of aggregation structure of the polyimide fibers in three kinds of fluid were studied.The results showed that the fineness and mechanical properties of the fibers decreased in the wet chemical fluid,and with the increased of the concentration and temperature of the wet chemical fluid,processing time extended,the fineness of the fiber becomes smaller and the mechanical properties decreased even more.The polyimide fiber opened loop and hydrolysised to polyamic acid or its salt under the action of OH-,H+,the molecular chain of imide ring part of the C-N fracture,resulting in thermogravimetric properties,chemical structure and microscopic state of aggregation structure changed,SEM showed the fiber surface became rough,degree of roughness increasing,localized by the fluid.The use of appropriate treatment process can contribute to the functionalization of polyimide fiber surface modification.Combining the structure and properties of polyimide fibers in NaOH,HCl and H₂O₂,the fibers were modified by HNO₃/H₂SO₄ mixed acid.The surface of the modified polyimide fibers appeared to the trench,which meant that the specific surface area increases and the surface became rough.The ratio of O/C to the surface of the fiber was 0.6268,0.4133,and 0.3497,when the volume of HNO₃/H₂SO₄ was 1: 2,1: 1,2: 1 and the 25 min oxidation time.The wettability improved with the increase of the ratio of H₂SO₄,the processing time prolonged,and the improvement of the wettability was accelerated.At the same time,the fineness of the fiber rised because of swelling,mechanical properties decreased,the quality increased,and the changes of chemical structure were not obvious.The crystalline structure of the fiber surface was transformed into an amorphous region,and the orientation structure and supramolecular structure of the fiber molecules were affected during the transformation.The molecular chain of the polymer was relaxed and the rigidity decreased,and the orderly structure of the microscopic state of aggregation structure was destroyed.The increase in the ratio of the amorphous region also contributed to the fiber wettability.The mechanical properties of the composites were tested by using hot-press method to prepare the polypropylene resin based composites with the modified fiber as the reinforced body and polypropylene as the matrix.The results showed that the mechanical properties of the composites were determined by the ratio of O/C ratio,roughness,wettability and fiber strength.The interfacial shear strength of the composites were improved by 97.74%,25.37%,5.82% and flexural strength increased by 28.03%,26.26%,24.15%,respectively;tensile strength decreased by 54.39%,increased by 22.27%,decreased by 2.75%.In order to solve the problem that the fiber bundles exist in the discontinuous fiber reinforced composites,the pore size distribution is not uniform,the effective length is low and the material properties is not high.In this paper,from the perspective of reinforcement structure,stress transfer and molding method,fiber was made into fiber paper by using wet-laid paper-making technology,fiber paper reinforced form effectively overcame the drawbacks in the fiber directly reinforced.Compared with the direct reinforcement of short fibers,the fiber bundles in the fiber paper were less,the fiber pore size was small and the distribution was uniform,and the effective length of the fiber was improved.Fiber paper as a loose porous structural materials,which was advantageous for the matrix resin to be compounded with the fiber under hot pressing.The fiber and fiber reinforced polyimide resin based composites were prepared by hand-paste molding and hot-pressing process,and the fiber mass fraction was controlled at 40%.Compared with the fiber reinforced composites,the tensile strength of the fiber reinforced composites was improved by 130%,flexural strength increased by 108%,interlaminar shear strength increased by 34.5%.The papermaking process conditions of the fiber paper also had an effect on the mechanical properties of the composites: 6mm unbeated fiber paper had better mechanical properties,and the tensile strength of the 6mm unbeated fiber paper reinforced composites increased by 23.53% compared to 3mm unbeated fiber paper,the flexural strength was improved by 47.02% and the interlaminar shear strength did not changed obviously.Compared to unbeated fiber paper reinforced composite,the tensile strength of the beated fiber paper reinforced composites increased by 59.97%,the flexural strength was increased by 39.14%,and the interlaminar shear strength decreased by 5.39%.And with the increase of the basis weight of fiber paper,the flexural strength and interlaminar shear strength of the composites increased all the way,and the tensile strength increased at first,and then decreased,the maximum value was 6.9346 MPa at 100g/m2.