Study On Two-Step Preparation Of Pan-Based Carbon Fiber Precursors

This article mainly aims at the dissolution and fiber spinning in the preparation process of polyacrylonitrile (PAN) fibers. The intensive study on evolution of structure and properties during the preparing process and its correlation with techniques has significance in improving performance of PAN precursors and carbon fibers. Most of the experiments were carried out in homemade fiber spinning line. Rheometer, ultraviolet-visible spectrophotometer, X-ray diffractometer, surface area and pore size analyzer, stereo microscope, polarizing microscope, field emission scanning electron microscopy, fiber strength and elongation tester, fiber fineness tester were used to characterize the performance change of PAN solutions and fibers. Influence of drawing technology was studied for establishing molecular deformation structure model.In the aspect of preparation of PAN spinning solution, the influence of agitating speed, temperature and solvent on the change of rheology for PAN solution was discussed. In normal dissolving process, shear viscosity change will present certain rule:the viscosity firstly declines with the dispersion of powder, then increases slightly with the dissolution of macromolecular, and lastly declines again with the orientation of molecular chain in the solution. Speeding up agitating, increasing temperature and using composite solvent could promote the dissolution. Among them, speeding up agitating had a role of increasing dissolution rate, and increasing temperature could improve solubility, whereas using composite solvent could integrate advantages of both components, making the solution has better dispersibility and solubility. Under same concentration, the solution with composite solvent (DMAC/DMSO) had a lowest viscosity. Change of light absorption properties of the solution during dissolution process was analyzed by ultraviolet-visible spectrophotometer. It was found that the solution had a strong absorption peak near300nm wavelength in ultraviolet region. With the dissolution, the peak gradually strengthened, and the higher was the solution temperature, the stronger the peak was. Finally, the structure of coagulated PAN membranes was compared, and it was found that the solution with better solubility had more uniform structure, and the orientation of molecular chains was presented as raised net structure in the membrane surface.A detailed study on coagulation and drawing deformation of PAN fibers was made during spinning process. The coagulation of PAN solution is a phase separation process which is affected by double diffusion, determining the structure of PAN nascent fibers. In this paper, mass transfer coefficient obtained from the second Fick’s law was utilized to represent double diffusion rate. According to the diffusion rate in the solidification process, the diffusion process was divided into initial stage, middle stage and later stage. The influence of temperature, concentration difference and solution concentration on the diffusion velocity was analyzed, and it was found that the mass transfer coefficient of the initial stage (K1) increased with temperature and concentration difference, and was basically in proportion to concentration difference and T1/2. The mass transfer coefficient of the middle stage (K2) was still proportion to T1/2, and the lower was the temperature the better the proportional relationship was, as it increased first then decreased with the increase of the concentration difference. The diffusion rate of the last stage decreased slightly with the concentration difference and increased slightly with temperature. The diffusion rate basically declined linearly with the increase of polymer concentration, expect the medium and later mass transfer coefficient slightly deviation linear relationship. The influence of coagulation conditions on the crystallization and mechanical performance of PAN nascent fibers was discussed, and it was found that reducing temperature could inhibit crystallization and improve drawing performance, when the change of coagulation bath temperature made a more obvious effect than spinning solution temperature. Speeding up drawing could promote the plastic drawing deformation of PAN nascent fiber, and increasing draw ratio could refine aggregated fibrillar and reduce crystallinity, helpful for making fine-denier precursors.The influence of solvent content, tensile speed on the drawing performance of PAN nascent fibers was discussed. It was found that yield strength increased linearly, breaking strength increased gradually, and elongation at break increased quickly first and then decreased slightly with solvent content decreasing. The fibers dried for10hours had the lowest yield platform and high elongation, providing the best drawing performance at room temperature. Speeding up drawing can promote deformation of nascent fibers with high solvent content, but not for fibers dried sufficiently. Drawing performance of PAN nascent fibers with the different first coagulation bath draw ratio under different temperature was discussed. It was found that the nascent fibers with small diameter tended to achieve maximum draw ratio in lower temperature, and increasing the first coagulation bath draw ratio could promote thinning fibers, when increasing the high temperature draw ratio was helpful for improving the strength of fibers.The drawing deformation behavior of the second coagulation bath pre-drawing, high temperature air drawing, collapsing drawing and steam drawing during spinning process was discussed. It was found that the nascent fibers with the first coagulation bath draw ratio of5×could be drawn nearly23.5times in the80%and70℃second coagulation bath under a tensile speed of50mm/min. Afterwards the draw ratio of pre-drawn fibers increased first and then decreased in250～400℃during high temperature air drawing, achieving maximum draw ratio at350℃, and fiber strength increased greatly and elongation at break fell sharply with the increase of drawing ratio. Imposing1.05times drawing could make fibers keep good plasticity at the same time enhance densification effect in the collapsing process, improving strength and density sufficiently. Steam drawing could greatly reduce fiber denier and improve strength at the same time. And increasing steam pressure could promote deformation during steam drawing. According to the evolution of the fibers’ structure and performance during drawing deformation, a molecular structure model was constructed for explaining the drawing deformation mechanism.Evolution of the fibers’ structure and performance during various stage of spinning process was discussed. It was found that, along with the spinning, the strength increased while elongation at break decreased gradually. The birefringence and polarizing light transmittance had a good corresponding relation with mechanical strength of the fibers. The diffraction peak at nearly2θ≈17°strengthened gradually, when the degree of crystallinity had a good corresponding relation with the density of the fibers. In the comparison of dry-jet wet spinning precursors P1, quasi dry-jet wet spinning precursors P2and wet spinning precursors P3, it was found that in aspects of crystallinity, density, strength:P1> P2> P3, in aspects of grain size, denier:P1<P2<P3, indicating that it was easier to making fine denier, high strength and density PAN precursors by dry-jet wet spinning technology.