Study On Structure And Properties Of Para-Aramid Fibers After Heat Treatment

Para-aramid fiber is a kind of high performance fiber with high strength, high modulus, high temperature resistant and chemical resistant. With its excellent mechanical and thermal properties, para-aramid fibers are widely used in military, aviation, tire cord and civil construction fields. Currently, PPTA fibers have been industrialized. However, it still lacks breakthrough in key technologies, such as mechanical properties. So it’s far from stable and large-scale production. In this paper, different post-treatment methods for PPTA fibers were designed. Combining with Synchrotron radiation X-ray diffraction techniques, the influence of different post-treatment on the structure and properties of PPTA fibers were studied to provide the theoretical guidance for the optimization of mechanical properties of PPTA fibers.Firstly, the domestic PPTA fiber was compared with Dupont’s Kevlar29fiber in terms of mechanical properties and crystal structure. Meanwhile, the differentia in crystallinity and crystal orientation between the skin and core layer of PPTA single fiber was studied as well as its kinetics of thermal decomposition. The result showed that the dispersion coefficient of strength for domestic PPTA fiber was greater than5%. The tensile modulus and breaking strength of domestic PPTA fibers were lower than those of Dupont’s Kevlar29fiber, so were the crystallinity and crystal orientation. For PPTA single fiber, it had higher orientation and crystallinity in the skin layer. The two active energy(E) of decomposition of PPTA fiber were33.08KJ/mol and67.84KJ/mol calculated by Friedman methond.Different kinds of post-treatment methods were designed, including heat treatment with free length, heat treatment under tension and thermal dried method for wet fibers under tension. Different post-treatment conditions, such as temperature, tension, time and moisture content of wet fiber, were subjected to study the changes in mechanical properties of PPTA fiber. The thermal performance, chemical structure, orientation and surface morphology of PPTA fiber after heat treatment were also investigated. The result showed that the fiber modulus goes through a maximum at400℃when the temperature increases for PPTA fiber heat-treated under tension. The fiber modulus increases with the increase of tension, while the strength was slightly decreased. For PPTA fiber heat-stretched at different ratio, its strength decreased more. However, its modulus could be up to845.64cN/dtex, nearly twice as high as the received fiber. When the moisture content of the wet PPTA fiber was greater than30%, its tensile modulus could be significantly increased when dried at110℃under tension. The orientation and thermal stability of PPTA fiber were both improved after heat-treatment. No chemical structure occurred after heat-treatment. The surface of PPTA fiber was smooth and became rougher after heat-treatment.Based on the experiment of heat-treatment under tension for PPTA fiber, we apply the as-spun PPTA fiber to different treatment conditions including different temperature and tension to carefully investigate all relative crystal structure parameters using simultaneous WAXD techniques. The fibril length in PPTA fiber was also studied in terms of Ruland streak method. The relationship between mechanical properties and crystal structure of PPTA fiber were discussed preliminarily. The result showed that the structure parameters including crystallinity, crystal size and crystal orientation evolved similarly to the modulus change. The c-dimension of unit cell was greatly affected by the tension and became larger after heat treatment, so was a-dimension. The fibril length in PPTA fiber increased and the misorientation decreased when the stretch ratio increased until5.3%. Certain correlations did exist for PPTA fiber under temperature and tension. The strength of the treated fiber decreased with increase in apparent crystal size. There was also an overall trend in that the modulus of treated fiber increased with increase in crystal orientation and c-dimension.