Study On Spinning Formation Mechanism And Processing Of Polyacrylonitrile Fibers

As the first step of polyacrylonitrile spinning process, the coagulation formation process is the key factor which determine the quality of polyacrylonitrile (PAN)-based carbon fibers during the preparation of carbon fibers. In order to obtain high quality carbon fibers, it is important to carry out deeply studies on the coagulation formation process of PAN fibers. The reaserches on the physical and chemical changes, structure evolution, as well as the relationship between formation processing, structure and properties during coagulation can provide academic help for the preparation of the high-quality carbon fibers. In this work, a series of coagulation and spinning experiments were conducted on a spinning line by using home coploymers and spinning solutions as the raw material fibers. Several technologies, such as the rheometer, differential scanning calorimetry (DSC), thermal gravimetry (TG), Fourier transform infared spectroscopy (FTIR), elemental analyzer (EA), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro-analyser (EPMA), high resolution transmission electron microscopy (HRTEM) and dynamic mechnics analyser (DMA) were used to systemically charaterize the structure and feature of the samples. At the same time, the influences of processsing conditions on the changes of the properties of nascent fibers and PAN fibers were also discussed.The investigation on the PAN spinning solution rheology indicates that the viscosity, elasticity and stability of spinning solutions depended on the molecular weight of copolymers and the spinning solution concentration at a certain extent. The molecular weight was higher, the solution concentration was bigger, the viscosity was bigger, the structure viscous index was higher, the Non-Newtonian index was smaller, the physical stability was worse, the elastic modulus was higher, the elastic network formed was more compact. During the actual spinning solution flowing temperature range from 60℃to 80℃, the effect of change of temperature on the viscosity of PAN solutions with low molecualr weight (23×10⁴ and 50×10⁴ ) was biggish, and it was not sensitive to temperature for spinning solutions with high molecular weight (100×10⁴).The coagulation formation process of PAN nascent fibers is effected by the double diffusion of DMSO and H₂O. The diffusion equation of PAN/DMSO system was solved by use of Bessel funtion, and the diffusion coefficients under different coagultion conditions were received. The experimental results showed that the diffusion coefficients of solvent DMSO and precipitator H₂O hoisted with the rise of coagulation bath temperature, decreased with the rise of coagulation bath concentration, increased with the rise of coagulation drawing ratios, decrased with the rise of spinning solution concentrations. Diffusion relaxative degree descended with the rise of coagulation bath temperature, changed a little with the rise of coagulation bath concentration, increased with the rise of coagulation drawing ratios, and increased with the rise of spinning solution concentrations.Structure changes of PAN nascent fibers were further clarified and the excellent coagulation bath conditions for wet spinning were acquired in the forming process by use of XRD, SEM, fiber fineness and fiber strength and elongation analysers. With the increase of coagulation bath concentration, cross-sectional shapes of nascent fibers became round, and the cross-sectional shapes had little change when the concentration was 60-70wt%. With the increase of coagulation bath concentration, crystallinity of nascent fibers and corresponding PAN precursors firstly increased and then reduced, the crystallinity got the maximum when the concentration was 65wt%, and the residual solvent content got the minimusm. With the increase of the coagulation bath temperature, crystallinity of nascent fibers and corresponding PAN precursors increased, and the residual solvent content in nascent fibers became lower, and the residual solvent content hardly had any changes when the temperature was higher than 55℃. During the process of coagulation bath temperature varying from 50℃to 60℃, the cross-sectional shapes of nascent fibers gradually became from the kidney shapes to circular shapes. When the temperature was 70℃, circular cross-sectional shapes of the fibers collapsed. As the rise of minus stretch ratios, crystallinity of the nascent fibers and corresponding PAN precursors and the residual solvent contents in nascent fibers continuously reduced. When the minus stretch ratio reached -20%, the residual solvent contents had smaller changes, cross-sectional shapes of nascent fibers became regular and homogeneous, and nascent fibers showed better toughness. But higher drawing was not favorable for producing high-performance PAN precursor, especially the minus stretch ratio reached about 0%. With the increase of coagulation bath length from 15cm to 90cm, crystallinity of the nascent fiber increased, and residual solvent content reduced. When the coagulation bath length was over 75cm, changes of crystallinity and residual solvent content was no longer obvious, and cross-sectional shapes of the nascent fibers tended to become circular, whose diameters became lower and the dispersion coefficient became smaller. In wet spinning, when solution polymerization was used, viscosity-average molecular weight was 16×10⁴, spinning solution concentration was 21wt%, and the drawing ratios of coagulation bath with concentration of 65wt% in 60℃was -10%, the excellent performance PAN fibers with circular cross-section were obtained, whose fineness was about 1.04dtex with strength up to 7.50cN/dtex, and the indensity of corresponding carbon fiber was about 3.86Gpa.The existence of air gap improved double diffusion of spinning solution droplets at the coagulation bath during dry-jet wet spinning, which inhibited the rapid diffusion of water, and it was in favor of nascent fiber formation with uniform structure. With the increase of air gap distance, coagulation bath drawing ratios and the length of coagulation bath, crystallinity of nascent fibers increased continuously. With the increase of coagulation bath concentration, especially when it reached 80wt%, the nascent fibers showed loose and symmetrical cross-section, and the cross-section shape presented regular circle. With the rise of coagulation bath drawing ratio from +5% to +105%, the diameter of nascent fiber continuous decreased and the cross section progressively tended to compact. With increase of coagulation bath drawing ratios, the fracture intensity of nascent fibers continuously increased and the fracture elongation decreased, showing a better toughness, higher density, lower porosity, and the corresponding PAN precursor fiber showed the similar rules. In dry-jet wet spinning, when aqueous precipitation polymerization was used, viscosity-average molecular weight was 23×10⁴ , spinning solution concentration was 18wt%, air gap distance was 3mm, and drawing ratio of coagulation bath with concentration of 80wt% in 20℃was +130%, we could also get the excellent performance nascent fibers with circular cross-section, whose PAN fiber fineness was about 1.01 dtex with intensity up to 7.52cN/dtex.The thermal properties, chemical structure changes and fracture behavior of nascent fibers under different coagulation conditions were studied by use of the DSC, TG, FTIR, EPMA. The results showed that the nascent fiber heat release increased and thermal stability improved with the increase of coagulation bath drawing ratios and the air gap distance during dry-jet wet spinning. And the nascent fibers showed higher heat release with the rise of coagulation bath length, but when the length increased to a certain value (>75cm), the nascent fibers' structure stabilized and the heat release changed a little. In wet spinning and dry-jet wet spinning, the nascent fibers showed obvious necking phenomenon, but the wet-spinning fiber's structure was more dense and showed skin-core structure which is difficult to eliminate, and dry-jet wet spinning fibers showed a more loose but uniform microstructure. Nascent fibers showed obvious ductile fracture characteristics at a slower rate, and the fracture characteristics were not obvious at a faster rate. Judging from the chemical structure, the wave number near 950cm^-1 peak was S=O characteristic peak in DMSO, which was used to indicate the changes of DMSO in the follow-up process based on nascent fibers.The effect of spinning process to the structure and properties of the PAN fiber was analyzed, and the parameters were optimized by the use of DMA, SEM and XRD etc. The study found that the improving of drawing was conducive to get the stable orientation of the PAN fibers' internal structure, and improved its mechanical properties. The improving of densification temperature was helpful to the rise of crystallinity. With the spinning process to be carried out, the crystallinity of PAN fiber increased, residual solvent content became lower and lower. Comparing the normal water bath fiber with the nascent fiber washed directly, the former had the lower residual solvent content and is not obvious in FTIR, the latter had the higher residual solvent content and its structure was looser, which was not conducive to the following preoxidation process.