Research On Surface Modification Of Paraaramid And Exploration Of Its Properties

Poly-p-phenylene terephthamide(PPTA) fibers are a kind of high-tech fibers. PPTA fibers are commonly used to be as a reinforcement of composite materials due to their superior properties, such as high strength, high modulus, anti-high temperature and low density. However, because of its smooth surface, low surface free energy, poor wettability and less active functional group, PPTA fibers are easily pulled out from resin matrix to induce that composites cannot take advantage of fibers and resin matrix fully. Therefore, it’s necessary to take effective modification methods to improve surface performance so as to increase the interfacial shear strength between fibers and resin matrix. In this paper, methods of enzyme-mediated grafting and plasma-mediated grafting of glycidyl methacrylate(GMA) to PPTA fibers are taken to improve the interfacial shear strength(IFSS), the paper also compares experimental results of two methods; Furthermore, Raman spectroscopy technique is utilized to analyse interfacial micromechanical behaviors(stress distribution and transfer) of fiber/microdroplets.Firstly, influence of enzyme-mediated grafting of GMA to PPTA fibers on the interfacial shear strength between fibers and resin matrix was discussed, optimal processing conditions were explored through the orthogonal experiment. The changes of surface elements and functional groups of untreated and treated fibers were analysed by Fourier transform infrared spectroscopy(FTIR) and X-ray photoelectron spectroscopy(XPS), surface topography and roughness were examined by Scanning electron microscope(SEM) and Atomic force microscope(AFM), the micro-debonding method was adopted to characterize IFSS, the creep properties, the mechanical properties, crystallization properties and thermal performance were also researched. The results show that a new functional group, epoxy functional group, formed on the surface of PPTA fibers; there was increasement of oxygen and improvement of roughness after treatment; optimum conditions: temperature was 48 ℃, enzyme concentration was 0.01 mg/mL, monomer concentration was 5 %, oxidant concentration was 0.03 g/mL, pH value was 6.9, IFSS increased by 34 %; creep elongation declined by 33.5 %; the mechanical properties, crystallization properties and thermal performance were not affected.Secondly, the method of BDB plasma-mediated grafting of GMA to PPTA fibers was taken to improve the interfacial shear strength, the paper explored how processing conditions(treatment temperature, monomer concentration, treatment time) affect IFSS, utilized the orthogonal experiment to determine optimal processing conditions, characterized surface elements, functional groups, surface topography, roughness, the mechanical properties, crystallization properties and thermal performance by some test instruments, for example FTIR, XPS, SEM, AFM, XRD and TG, compared experimental result of two metheds of enzyme-mediated grafting(first method) and BDB plasma-mediated grafting(second method) of GMA to PPTA fibers. The results indicated that: compared with first method, after treatment through second method, there were more epoxy functional groups, oxygen atoms and bulges on the surface of fibers; optimum conditions: treatment temperature was 120 ℃, monomer concentration was 18 % and treatment time was 30 min, IFSS was improved by 43 % when treatment conditions were optimal; second method was more helpful to improve IFSS, but it also easily damaged crystallinity, thermal properties and mechanical properties, for example, tensile strength declined by 16 %.Finally, interfacial micromechanical behaviors of PPTA fiber/microdroplet were investigated through micro-debonding test and Raman spectroscopy technique. Fiber/resin composites were made through embedding single fiber with the droplet of epoxy resin. The relationship between stress and Raman shifts was explored when fiber was subjected to different stress. Then the paper studied stress distribution along single fiber in droplet and interface stress transfer, also investigated interfacial shear stress(ISS) along fibers. The results showed that Raman shifts were linear with stress, The Raman shift sensitivity was about 3.59 cm-1/GPa after linear fitting; when fiber/microdroplet composites bear 0 GPa stress, stress distribution along fiber in droplet exhibited “M” shape; when fiber/microdroplet composites bear 0.36 GPa stress and 0.51 GPa stress, stress distribution along fiber in droplet exhibited “U” shape; when fiber/microdroplet composites bear same stress, stress along treated fiber was always lower than stress along untreated fiber, which indicated more excellent bonding property existed between treated fiber and microdroplet; ISS along fibers was an anti symmetric distribution, ISS along treated fiber was always lower than ISS along untreated fiber.