The Characterization Of Ppta Fibrid Structure And The Study On Its Paper Based Composite Performance

Poly( para-phenylene terephthalamide)(PPTA) fiber is a kind of high-tech fiber with high strength, high modulus, light weight, thermal resistance and corrosion resistance. PPTA paper based composite are made of para-aramid fibres using papermaking wet forming and calendering operations. Owing to their attractive combination of properties including exceptional mechanical properties, excellent dielectric strength, stable thermal properties and flexibility of designing, PPTA paper based composite are widely used as high-temperature insulating materials, high performance electronic materials, and honeycomb structure materials with application in the aerospace, traffic, electricity,communication and defense industries. At the same time, as a new indispensable material in national economic development and defense construction, PPTA paper based composite has broad market prospect and remarkable economic benefits.PPTA fibrid, manufactured in recent years, was a new type of aramid fiber,which was formed when aramid solution are precipitated in a non-solvent condition under high shear. Because of different preparation method, PPTA fibrid has the same mechanical properties as PPTA fiber, but a larger surface area and result in excellent compound effect, which indicated that PPTA fibrid is suitable for making PPTA paper based composite. However, there is no reports on PPTA fibrid and its application in PPTA paper based composite.In order to investigated the structures and properties of PPTA fibrid and its application in PPTA paper based composite, the analytical procedures in modern material science was adopted. The optimum technical parameters of wet forming and calendering process using to make high peroformance PPTA paper based composite was obtained by orthogonal test. The structures of aramid paper based material before and after calendering were characterized, which is beneficial to study the mechanism of hot calendering. Fractal dimension was used in evaluation of pore structure and physical properties of PPTA paper basedcomposite that contributed to present a complete prediction and evaluation method. The pyrolysis characteristics and the thermal degradation mechanism of PPTA paper based composite were analyzed.PPTA fibrid exhibited small sized pieces with wrinkled surface, uniform size and highly degree of fine fibrosis while chopped fiber was smooth and nearly cylindrical in shape, and their weighted length were 0.552 mm, 5.290 mm,respectively. PPTA fibers exhibited secondary amide absorption bands. The crystallinity of chopped fiber was higher than that of PPTA fibrid with non-crystal structure, and their crystalline were 77.9% and 28.6%, respectively.The initial decomposition temperature of chopped fiber and PPTA fibrid were530 ℃ and 490 ℃, respectively, in addition, their TG10% were 550 ℃ and 535 ℃,respectively, which revealed that two PPTA fibers exhibited excellent thermal properties.In the papermaking wet forming process, the beating degree of PPTA fibrid,the length of chopped fiber and the ratio of PPTA fibrid to chopped fiber were the main factors of PPTA paper based composite properties, and the dosage of PEO was the secondary factor. The optimum process of PPTA paper based composite was as follow: the beating degree of fibrid, the length of chopped fiber,the ratio of fibrid to chopped fiber and the dosage of PEO were 55 °SR, 5 mm,0.3% and 7:3, respectively. The properties of obtained paper based composite included tensile index, tear index and dielectric strength with 39.36 N·m·g-1,19.14 m N·m2·g-1 and 22.18 k V/mm, respectively.In the calendering operation, the calendering pressure, the calendering temperature and the calendering time were the main factors of PPTA paper based composite properties, and the roller speed was the secondary factor. The optimum process of PPTA paper based composite was as follow: the calendering pressure, the calendering temperature, the calendering time and the roller speed were 0.4 MPa, 240 ℃, 1.5 m/min and 2 times, respectively. The properties of obtained paper based composite included tensile index, tear index and dielectric strength with 54.27 N·m·g-1, 24.63 m N·m2·g-1 and 23.81 k V/mm, respectively.Within the novel PPTA paper based composite, PPTA fibers were evenly distributed and intertwined, which form a structure similar to reinforced concrete.PPTA paper based composite trended to compact after calendering, its porosityand pore diameter were 18.33% and 14.64 μm, respectively. Moreover, paper based material had obvious two phase structure made of crystalline and amorphous. Importantly, the interface binding between chopped fibers and fibrids was physical binding.The pore structure of PPTA paper based composite has obvious fractal characteristics. With increasing of fractal dimension, porosity, average diameter and special surface area, pore structure were degraded and mechanical properties and dielectric strength decreased correspondingly in PPTA paper-based composite. Therefore, fractal dimension can be employed as a comprehensive parameter to evaluate the characteristic of pore structure, and it has good correlation with mechanical and dielectric performance of PPTA paper based composite.The initial decomposition temperature and TG10% of PPTA paper based composite was 535 ℃ and 560 ℃, respectively, and pyrolysis peak can only be found in the temperature rising process, which exhibited excellent thermal properties. HCN、NO2、NH3、NO、CO、CO2 and H2 O were formed in the thermal degradation process. The initial and decomposition temperature of PPTA paper based composite were elevated while raising the heating rate. The thermal degradation process was investigated by means of Coats-Redfern method, and it was found that the reaction order was 1 at the organic pyrolysis region, the activation energy E and the apparent factor A was correlated with heating rate.