Study On Wet-Spinning Process Of High Molecular Weight PAN Precursor

In order to prepare high performance PAN-based carbon fiber, the strength of carbon fiber precursor must be improved at first for overall improvement of carbon fiber performance. In this study, from angle of increase of polyacrylonitrile (PAN) molecular weight, high molecular weight PAN was synthesized by suspension polymerization, and spinnable spinning solution for wet spinning was then prepared by controlling the temperature, proportion of mixed solvents and other parameters. Moreover, according to the studies of the effects of coagulation in wet spinning, stretching in boiling water, densification, stretching in steam and other related factors (temperature, stretch ratio, time, etc.) on the properties of PAN precursor, the optimal parameters in the production of PAN precursor were determined and high performance PAN precursor was prepared in this research.According to the measurement of falling ball viscosity test, the mixed solvents (DMSO and micro amount of water) can minimize the viscosity spinning solution; the viscosity of spinning solution decreased with increasing temperature, while increased exponentially as the increase of PAN weight concentration. The results suggested that under the conditions of 1.5% (w) water content,75℃temperature,10.5% PAN concentration,1300s-1 shear rate,2h dissolution time, the viscosity of spinning solution was lower than 100Pa*s.By scanning electron microscopy, X-ray diffraction, fiber strength tester and other tests, the cross sectional morphology of PAN fiber coagulated in wet spinning, degree of crystallization, mechanical properties were analyzed, and the results showed that the concentration of coagulation bath had greater influence. With lower temperature, smaller negative draw ratio, the coagulation rate was smaller, the cross sectional shape of filament was circular, the density was relatively large, the amount of holes in fiber was fewer. The optimal coagulation parameters were:75% coagulation bath concentration,50℃coagulation bath temperature,-15% negative draw ratio in the first coagulation bath.The properties of PAN fiber during densification were studied by X-ray diffraction, fiber strength testing. The results showed that with increasing temperature and time in densification, there was a best value of PAN fiber performance, and under the conditions of 130℃temperature,100s densification time, the PAN fiber quality was best.Characterized by a variety of means, the relationship between draw ratio in the process of stretching in boiling water and steam and the degree of crystallization, diameter, tensile power was studied. The results showed that with the draw ratio increase, the diameter and denier of fiber became smaller with a different change rate under different conditions, at the same time, the degree of crystallization and crystal size became larger, the strength and tensile power had an optimum value. Under the conditions of 4.5 times draw ratio in boiling water,2.4 times draw ratio in steam, the physical and mechanical properties of high molecular weight PAN precursor was best. Finally, high performance PAN precursor fiber with 9.5 cN/dtex tensile strength, circular cross sectional shape, few deep groove surface,9μm diameter was prepared.