The Research On Wet-Spinning Process Of PAN Synthesized By Homogeneous And Heterogeneous Polymerization

Ultra-fine high strength and high modulus carbon fiber is a essential special fiber materials applied in national aviation and aerospace fields, and high-performance carbon fiber can be prepared only by high-performance precursor. The three most important performance paramrters of PAN precursor are fine denier, high strength and high modulus. In this article, from the standpoint of how to prepare high-performance PAN precursor, one-step wet-spinning process of common molecular weight PAN synthesized by homogeneous solution polymerization and two-step wet-spinning process of HMWPAN synthesized by suspension polymerization in mixed solvent were investigated,respectively. The main works in the study are as follows:1. According to the design parameters of equipment, the optimum formulation of raw materials used for homogeneous solution polymerization and heterogeneous suspension polymerization were determined. In the stretching process to make precursor fine denier, the calculation and setting methods of rotation rate of stretching rolls were discussed in detail. Reasonable distribution of the stretch ratio in the various stages of spinning process and better control of precursor productive process, have very important significance in preparing high-performance carbon fiber precursor.2. PAN precursor fibers were produced via one-step wet-spinning process, and effects of polymerization and spinning processes, especially the stretching process, were investigated on mechanical properties and micro-morphologies of precursor fibers. An increase in molecular weight, dope solid and densification and a decrease in surface defects were possible by controlling polymerization temperature, the number of heating rollers for densification and the jet stretch ratio, which improved the mechanical properties of precursor fibers. The curves for strength, modulus, tensile power and diameter as a function of stretch ratio can be divided into three stages:steady change area, little change area and sudden change area. With the increase of stretch ratio, the fiber diameter became smaller, the degree of crystallization increased and the structure of precursor fibers became compact and homogeneous, which resulted in the increase of strength, modulus and tensile power of precursor fibers. Empirical relationship between fiber strength and stretch ratio was studied by using the sub-cluster statistical theory. It was successfully predicted when the strengths were 0.8GPa and 1.0GPa under a certain technical condition, the corresponding stretch ratio of the fiber were 11.16 and 12.83 respectively. 3. HMWPAN precursor was manufactured via two-step wet-spinning process. Firstly, the synthetic technology of HMWPAN by suspension polymerization in mixed solvent were investigated, and the optimum process parameters were determined as follows:AN/MA/IA=96/3/1, monomer concentration 30% (wt.),0.1% dispersant (PVA) out of total monomer mass, 1% initiator (AIBN) out of total monomer mass, H2O/DMF=85/15, reaction temperature 60℃, reaction time 2.5h. The obtained PAN copolymer possessed a higher molecular weight (Mη,456,000) and a higher isotactic degree (28.1%) than solvent polymerization and the particles were loose and easily dissolved in DMSO to get homogeneous and stable spinning solution to meet the requirements of wet-spinning process. Then the surface and cross section of as-spun fiber were observed by SEM, the results shows that the as-spun fibers of HMWPAN have a circular cross section and few surface defects with concentration of coagulation bath of 80%, jet stretch minus ratio of 0.9 and coagulation bath temperature of 40℃, which can be stretched further to obtain high-performance precursor. The PAN fibers after dried densification were stretched under dry-heat and steam atmosphere, respectively, and found that the fiber can be stretched better in steam at a lower temperature due to plasticization of water molecules, the obtained precursor possessed a higher strength than dry-heat stretching.4. Evolution of structure and properties of HMWPAN precursor during wet-spinning process was investigated. The results showed that:with the spinning process proceeding, the diameter of HMWPAN fiber decreased and strength increased as a whole. The stretching in hot water, dry densification and the stretching in high temperature-pressure steam are the most critical processes to improve the strength of high molecular weight PAN fiber. The high performance precursor can be odtained with strength of 0.96GPa, elongation of 10.3%, diameter of 9.88μm and boiling water shrinkage rate of 3.9%. However, there are many stripes and deep grooves in the surface of HMWPAN precursor fiber, and it should be studied continuely how to reduce grooves and surface defects further, so that a higher-performance HMWPAN precursor can be manufactured by wet-spinning process.