Characterization Of Fatigue And Aging Properties Of Several High Performance Fibers Under Light And Heat

The excellent light, thermal stabilities and mechanical properties of the high-performance fiber are the basis for the application of special occasions. The used harsh conditions of high-performance fiber, and the durability and stability must be clear. Therefore, the aging, degradation and bending fatigue of high-performance fiber under thermal, light radiation, and the combined effect occasions were investigated. The corresponding aging mechanisms also were discussed. These studies provided some guidance for the preparation of such materials, their modification for high performance, and the characterization of theirs'light and thermal stabilities.The thermal degradations of the high-performance fibers, including Kevlar 49, Kevlar 129, Nomex, and PBO in the air and nitrogen were characterized by TGA-DTA/FTIR. The characteristic temperatures of the kinds of fibers in the two atmospheres were obtained. The heat resistance of PBO fiber was best and decomposition temperatures of the beginning and termination were the highest; despite the beginning decomposition temperatures of the Nomex fibers were the lowest and less than 450℃, the termination decomposition temperature was close to 800℃. The characteristic temperatures of the four kinds of fibers in the air were lower than the in nitrogen. The activation energy and reaction order in two different stages of the fibers were analyzed with the thermal dynamics theory. And it was found that, the activation energy of Nomex in the air was higher than in nitrogen in the first stage; the activation energy of the fibers in the air were lower than in the nitrogen in the other stages; and the reaction order were close in the two atmospheres. The gases released by the pyrolysis in air were mainly CO2, CO, H2O, NO, HCN, also containing a small amount of NH3, and the absorption peak of CO2 were the strongest. The gases released by the pyrolysis in nitrogen were mainly CO2, CO, H2O, NO2, NO, HCN, also containing a small amount of benzene compounds. The NO was exited in nitrogen, and the CO as pyrolysis product exited persistently. So the pyrolysis of these fibers was toxic based on the compositions of pyrolysis gas. The pyrolysis processes of Kevlar 49, Kevlar 129, Nomex, PBO were analyzed using Py-GC/MS spectrometer, respectively. It was found that the proportion of CO2 is large in pyrolysis products; and more small molecules containing benzene ring appeared in the pyrolysis products. There were more cleavage fragments of Kevlar 49, Kevlar 129 at 600℃and 700℃, and the cleavage fragments were reduced at 750℃. But there were still more cleavage fragments of Nomex fibers at 750℃.The pyrolysis temperature of PBO fiber was higher. So the proportion of the PBO repeat unit body and CO2 were large in the pyrolysis products at 750℃. The benzene contents of PBO were less than the other three fibers, and the pyrolysis of PBO was considerable safer.The in-situ measurements of flexible materials under the light—stress combined condition, light—thermal combined condition were achieved. The aging properties of aramid fiber under the two combined conditions were investigated, respectively. It was found that the maintain rate of the breaking strength and elongation were 75% and 69% under 10% stress and light combined condition. Which were significantly decreased compared with under the action of light alone. Meanwhile, the damage of the fiber morphology and fine structure were more seriously. The mechanical properties of aramid fibers under light and heat combined effect decreased faster than light or heat alone; and the fiber surface morphology and structure were more significant degradation too. So the redundancy design should be considered in the practical using process.The micro-nano-scale bending fatigue grip design and molding were realized using fixed bending fatigue analyser (FiBFAN) which was made in TMT team(Textile Materials and Technology Lab), and the bending fatigue of the fibers can be effectively evaluated. The bending fatigue performance of flexible materials under different temperature conditions can be measured in situ by the instrument. It also allows for observation of the bending fatigue fracture process and fiber fracture morphology in situ. The cycle stress in the bending fatigue process can be recorded in real time.The bending fatigue properties of several high-performance fibers were investigated using the fixed-point bending fatigue analysis apparatus. It was found that the pre-tension and bending angle were the most important factors which affected the bending fatigue life; the bending frequency also had certain effect on the bending fatigue life. Through the analysis of the bending fatigue morphology, it can be found that the bending fatigue fracture of Kevlar and PBO fibers was resulted in fibrillation split; the high strength polyethylene was because of the plastic accumulation; while the fracture-ends of Nomex fiber were the tongue shape of the tear fraqcture. The resistance to bending fatigue of high-strength polyethylene was the best compared with Kevlar, PBO, and Nomex fibers. The influences of heat, moisture, light on the bending fatigue of high-performance fibers were also studied. The fatigue life of high strength polyethylene fiber decreased when the temperature increased. The bending fatigue life at 100℃was only 30% of the bending fatigue life at 20℃. The moisture had little effect on the bending fatigue properties of the fiber. And the bending fatigue life of Kevlar 49 and PBO fiber in the wet environment is slightly lower. The light aging of the Kevlar 49 was greater, and after a certain time of light aging, the bending fatigue life of the fibers significantly reduced.Pure Kevlar fabric, Kevlar/aramid 1313 blended and Kevlar/aramid 1313 interwoven fabrics with three structures were obtained using the Kevlar and Nomex filament yarn as the raw materials. It was found that the warp strength of pure Kevlar fabric whose width was 2.5 cm was more than 2500N; and the weft strength was 3248N by experiments. With the similar density, the warp strength of Kevlar/aramid 1313blended fabric was only 1632N; and the weft strength was 1854N. The warp strength of Kevlar/aramid 1313interwoven fabric was 1300 N, which was higher than the weft direction; but the wear resistance of Kevlar/aramid 1313interwoven fabric was the best.. The warp damage of the fabrics was nearly 30% during processing, which was bigger than the weft direction. The reflection and transmittance of different structure Kevlar fabric was different, but the differences were not big according the measurements. There was not much difference of reflection and transmittance among these three kinds of fabrics.