The Surface Modification Study Of Polyimide Fiber Preliminary

Aromatic polyimide fibers have been widely used in aerospace, nuclear industry, military industry and environmental protection due to their excellent thermal stability, chemical resistance and mechanical performance, but the ordered polyimide molecular orientation structure has made the surface of polyimide fibers very smooth, the fiber surface polarity is also very small, which makes fibers and resin infiltration difficult that to limit the applications of polyimide fibers in the field of advanced composite materials. In addition, polyimide fiber is prone to degrade under ultraviolet radiation, resulting in decreasing mechanical properties of the fiber products.In order to solve the two issues above, in this paper, phosphoric acid treatment, carbon nanotubes grafting and plasma treatment methods were used to modify polyimide fibers, the interfacial adhesion has been improved significantly between treated fibers and resin matrices, extnding the application of polyimide fibers in the field of advanced composite materials; meanwhile, sol-gel, a wet chemical method was used to form a proper TiO₂ coating on the surface of polyimide fibers to sheathe the surface of polyimide fiber as an anti-ager.After a series of experiments, major conclusions were summarized as following: polyimide fibers were treated by phosphoric acid in the conditions of phosphoric acid 20% at 45°C for 4h, the wetting property and polarity increased because new oxygen-containing groups were generated on the surface of fibers after treated by phosphoric acid. The untreated polyimide fiber system had an even lower interfacial shear strength value at 9.55 MPa. However, after treated by 20wt% phosphoric acid, the interfacial shear strength between fibers and resin improved by 60% to 15.1Mpa. Single-walled carbon nanotubes were treated by mixed acid （volume ratio: concentrated sulfuric acid: concentrated nitric acid = 3:1） in one hour, then, the carbon nanotubes of 4% （wt） were grafted to the treated polyimide fiber surface by the phosphoric acid, which made the modified fibers surface roughness increased, fiber and resin interface bonding performance enhanced. Modified fibers and resin interfacial shear strength was 18.9MPa, than unmodified fiber interfacial shear strength had increased 98%. Low temperature plasma treated polyimide fibers in the conditions of 30Pa pressure, 100W and 3min. The plasma treated fibers and epoxy resin interfacial shear strength was evidently improved, compared with untreated fiber increased by 54%, The moisture content of fiber increased by 63%, the wetting property between polyimide fibers and resin had been well improved.In addition, a stable and high transparency TiO₂ hydrosol has been prepared by using titanium alkoxide as precursor, glacial acetic acid as complexing agent, water and hydrochloric acid as solvent, it found that the appropriate formula as follows: the rate of TTB: H2O: HC1 was 1:150:0.15 （molar ratio）, and the treated temperature was 25°C, different sizes TiO₂ hydrosol was to prepare by changing the ratio of titanium alkoxide and glacial acetic acid, the sol aging time is 7-20 days which depend on the different formulations. This method was simple and easy to control, at the same time, this method avoided using alcohol as solvent. Then, the fibers surface was deposited TiO₂ hydrosol. The uniform coating was formed by heat-treated. The results showed that: the polyimide fibers previous was already broken while the TiO₂ coated polyimide fibers maintained its tensile strength at 42% after UV-radiation irradiating 312h, the coated fibers had no obvious fracture surface, so the coating to sheathe the surface of polyimide fibers as an anti-ager.