| Poly(p-phenyleneterephthalamide) (PPTA) fiber, also called p-aramid fiber in China, is one of the aromatic polyamide fibers with high performances. It is shaped by liquid crystalline (LC) spinning process with high-level tenacity and modulus, and acted as structural materials instead of steal, titanium alloy and so on. Its heat resistance, unmelting in high temperature, is close to inorfil; it has strong self-extingushbility and fire-resistance; it can hold upper tenacity and preferable dimensional stability in high temperature. While PPTA fiber is flexible enough to spinning and waving, not breaking after bending, not frangibility after heating. So it has important applications in aerospace, defense and composite industries; it is one of the urgent needing fibers in the development of defense industry and country economy. We hope that, according to the research and development processes we joined in laboratory and industrialization, the studies on PPTA polymerization, PPTA-H2SO4 LC solution, LC spinning of PPTA, and its heat treatment would be usefulness to the preparation and industrialization of PPTA fiber.Molecular weight of PPTA polymer is one of the key factors in manufacturing its fiber with high tenacity and high modulus. In NMP/CaCl2 and NEP/CaCl2 solvent systems effects of monomer ratio, monomer concentration, water concentration in solvent system, initial temperature, weight of CaCl2, degree of monofunctioal group in TPC, and monomer purity on PPTA fiber were discussed to confirm the basic manufacturing conditions. The polycondensation, with and without the additive PVP in NMP/CaCl2 and NEP/CaCl2 solvent systems, were all the second order reactions. The addition of PVP advanced the dissolvability of PPTA in solvent systems, improved the reaction speed, and was unable to affect the chemical structure, crystallizability, and purity of PPTA. We used twin-screw extruder as the main reactor to try the continuous feed of TPC solution. The rather high molecular weight of PPTA was attempted by the continuous feed of TPC solution which was easily gauged and the nibbling twin-screw extruder which was the main reactor. The residual weight of TPC, weight concentration of TPC, dissolving temperature, depositing time, pre-condensation temperature, and temperature of the main reaction were all the key factors in influencing the inherent viscosity of PPTA. Structure combination and speed of screw had quite effects on depositing time, shearing rate, and mixing.Solution property is the foundation of spinning process. We characterized the PPTA LC solution by polarizing optical microscope (POM) that it was swelling first and then dissolving in the process. After a period of time, the LC was appeared and was more distinct under shear action. The blockage coefficients and the degradation degrees of these LC solutions under different conditions were measured. The concentrated sulfuric acid(H2SO4) in 100% weight concentration was the best solution in this paper. If the concentration of H2SO4 was a little low, the dissolvability and blockage coefficient of PPTA decreased and increased, respectively. The longer of the dissolving time, the smaller of the blockage coefficient. Within the weight concentration of PPTA in 18%-19%, it has no obvious impact on blockage coefficient. Increasing the weight concentration of PPTA in H2SO4 properly was propitious to enhance its heat-stability. The degradation degree increased with dissolving time and temperature increasing. Overheat partly of PPTA solution would be efficiently avoided and the dissolving temperature would be exactly controlled by using twin-screw when dissolving. However, PPTA degraded under the mechanical shear force of the twin-screw, and the higher the rotating speed, the higher the degradation degree. Effect of the dissolving process on the chemical structure of PPTA was also calculated semi-quantitively. The PPTA LC solution was researched with differential scanning calorimetry (DSC), either. The temperature of phase transformation was got more accurately with the testing temperature process of ascending, descending and ascending again. When warming, the endothermic peak moved to high temperature as the molecular weight and weight concentration of PPTA increased. When the weight concentration of PPTA was not larger than19.5%, the melting peak and the phase transformation peak of solution were superposed entirely in company with the phase transformation within the melting process; on the contrary, the phase transformation was undergoing above the melting temperature.The dynamical rheologies of PPTA/H2SO4 and PPTA/H2SO4/SWNTs LC solutions were studied. The PPTA-H2SO4 LC solution was the typical shear-thinning liquid. As shear frequency ascended, the complex viscosity descended rapidly, the loss modulus and the tangent of loss angle increased gradually and then slowly, and the storage modulus didn’t change almost. The curves of complex viscosity, storage modulus, loss modulus, and the tangent of loss angle moved up with the weight concentration and the molecular weight increasing, and moved down with the temperature increasing. PPTA-H2SO4 LC solution satisfied the Cross-Williamson equation. The zero shear viscosity and relaxation time, got by fitting with the equation, raised with ascending of weight concentration, descending of temperature, and increasing of molecular weight. The viscous activation energy of PPTA-H2SO4 LC solution was rather high, and increased slightly with increasing of weight concentration and molecular weight. The content of carboxylic acid in purified single-wall carbon nanotubes(SWNTs) increased obviously. The dissymmetric bending vibration peak of—CH, symmetric stretching vibration peak of—CH2—,dissymmetric stretching vibration peak of—CH2—were all disappeared. The phenomenon of nematic liquid crystal was exited distinctively in PPTA/SWNTS/H2SO4 solution system. The relation between the weight concentration of SWNTs and the complex viscosity was nonmonotonicity, i.e., the complex viscosity enhancing with the concentration increasing, falling quickly as it exceeding 0.1%, and enhancing again as it larger than 0.2%. The complex viscosity of PPTA/SWNT/H2SO4 LC solution was typical shear thinning in the range of the whole scan frequency and at different temperature, and it fell down linearly with the frequency raising. The slope on logarithmic plot between complex viscosity and shear frequency of solution was much at one,-0.91, at different temperature and with the different weight concentration of SWNTs. There were exited multi transformations, from solid state to liquid state and from isotropy to nematic phase, in the PPTA/SWNT/H2SO4 LC solution at the temperature scope of 75-85℃through DSC analysis. The PPTA/SWNT/H2SO4 LC solution satisfied the Cross-Williamson equation also. The zero shear viscosity and relaxation time, got by fitting with the equation, raised with ascending of weight concentration and descending of temperature. The viscous activation energy of PPTA/SWNT/H2SO4 LC solution was rather high, and increased slightly with increasing of weight concentration and molecular weight. The twin-screw was used to quick dissolve PPTA resin, and got PPTA LC spinning dope. Then PPTA fiber with high tenacity, high modulus, and uniform quality was got by dry-jet wet-spinning. The method of uniform design was applied in the spinning and heat-treating processes of PPTA fiber. The break tenacity was regarded as a quality index, and three main parameters:extrusion speed, draw ratio, and winding speed, in the spinning process were optimized. The initial modulus was regarded as a quality index, and three main parameters:tension, temperature, and time, in the heat-treating process were optimized. The regression equations were found, and their reliabilities were rather high by residual error analysis. Calculated with SUMT external point method, the best spinning conditions were:extrusion speed was 26.68m/min, draw ratio was 7.76, winding speed was 208.34m/min, and then the break tenacity was 35.67cN/dtex; the best spinning conditions were:tension was 20g/dtex, temperature was 300℃, time was 78.5s, and then the initial modulus was 1548.6cN/dtex.At last, dynamical thermal mechanical properties of PPTA fiber were analyzed, and two secondary transitions and a main transition were occurred. Theγtransition was insensible owing to the less of small groups connected between the molecular chain and the main chain of PPTA. The cold crystallization was happened, so theβtransition was visible, due to the local movement of—CO—NH—in the main chain at 226.8℃. The main glass transition was also distinct at 392.4℃.The innovations of this paper were as follows.(1) We prepared PPTA resin with high molecular weight in NEP/CaCl2 solvent system.(2) We discussed effects of water concentration of solvent system, degree of monofunctioal group in TPC, and monomer purity on PPTA fiber.(3) The nontoxic additive, PVP, enhanced the dissolving ability of solvent system, and increased the molecular weight of products.(4) We used twin-screw extruder as the main reactor to try the continuous feed of TPC solution. It may be useful in the further industrialization.(5) We studied the dynamical rheologies of PPTA/H2SO4 and PPTA/H2SO4/SWNTs LC solutions. The rheological data was curve fitted with Cross-Williamson equation, furthermore.(6) We used twin-screw to quick dissolve PPTA resin, and got PPTA LC spinning dope. Then PPTA fiber with high tenacity, high modulus, and uniform quality was got by dry-jet wet-spinning. That is helpful to the future continuous manufacture.(7) The regression model, designed by uniform distribution, optimized the main conditions, extrusion speed, draw ratio, and winding speed, in spinning process, and also optimized the main conditions, temperature, time, and tension, in heat-treatment process.(8) Dynamical thermal mechanical properties of PPTA fiber were analyzed. |
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