Synthesis Of High Molecular Weight PAN Copolymer And Spinning Technology Of PAN Precursor By Wet Spinning

For its excellent performance, carbon fiber is widely used in aerospace, general industrial, leisure and sports fields. However, our national levels of high performance carbon fiber still have a great gap compared with international advanced level for various reasons. The inferior quality of PAN precursor is the most important factor which has been restricting our development of high performance carbon fiber. Currently, using high molecular weight PAN to produce ultra high strength and high modulus carbon fiber in foreign is a research hotspot which also encountered a number of difficulties, including how to prepare high performance PAN precursor from high molecular weight PAN. To solve this problem, the study on high performance carbon fiber precursor is critical to solve this embarrassing situation. Start from the synthesis of copolymerization of AN, MA and IA, this paper polymerized PAN with 560,000 viscosity-average molecular weight by precipitation polymerization in mixture solvent, and studied sequence structure of polymer, control of PAN molecular weight and distribution of PAN particle size in precipitation polymerization, coagulation process of high molecular weight PAN in wet spinning and the spinning technology of PAN precursor through comprehensive utilization of SEM, XRD, strength tester, density gradient tube and other analytical testing methods which provides fine detailed and reliable experimental data and theoretical basis for the production of high strength and high modulus carbon fiber. The main conclusions are as follows:From uniformity of the sequence structure of PAN, scale R1·R2 which is related to reactivity ratio of two monomers in copolymerization is introduced to be representative of uniformity degree of copolymer sequence structure. Through theoretical calculation and deduction, the relationship between solubility parameters and the scale of copolymer sequence is confirmed, which can be expressed as:In (r1r2)= k'(δ1-δ2)2. In addition, the control of sequence structure of PAN copolymer is also studied from the theoretical point of view.During the copolymerization process of PAN, the molecular weight of PAN can be effectively adjusted by changing the ratio of mixed solvent. When the mixed solvent is DMSO and H2O, the molecular weight and conversion rate increase with the increase of H2O content, which is due to the increase of polarity of the solvent. When the mixed solvent consists with DMSO and C2H5OH or C4H9OH, the molecular weight of resulting PAN decreases with the increase of the content of C2H5OH or C4H9OH.During the copolymerization of AN, MA and IA in mixed solvent, the solubility of monomer increases with the addition of DMSO, which makes better swelling of PAN particles and a loose structure of PAN. With the increase of DMSO components, PAN particles changes from irregular beads into a solid block.The uniformity scale r1*r2of the distribution of PAN particle size is introduced according to the Fourth Statistical Mechanics theory. A smaller r1*r2 value is corresponding to a more uniform distribution of PAN particle size and a larger r1*r2 value is corresponding to a more non-uniform distribution of PAN particle size. According to the calculation, in a mixed solvent with the content of DMSO or other solvent in equal portions, the distribution of resulting PAN particles size is more uniform. Through theoretical calculation and deduction, good linear relationship between logarithmic value of uniformity scale of the distribution of PAN particles size and the cohesive energy density of mixed solvent can be confirmed, which can be expressed as In r1*r2=A+BeCED. In addition, the relationship between surface tension and cohesive energy density is also determined, which is lnf/M=A0+K0eCDE.High molecular weight PAN solution exhibits a shear-thinning phenomenon which shows that it is a Non-Newtonian Fluids. With the increase of PAN concentration, viscosity of PAN solution decreases. Elasticity of high molecular weight PAN spinning solution is more obvious than its viscosity. In the solid content of 10%, spinning solution shows good physical stability which is suitable for preparing high-quality PAN precursor.In a certain concentration of coagulation bath, the diffusion coefficient of H2O and DMSO are increased with the increase of coagulation bath concentration, increased with the increase of coagulation bath temperature; the higher coagulation bath concentration is corresponding to a smaller coagulation diffusion activation energy which results in faster diffusion. Coagulation ability of coagulation bath has a significant impact on cross section and surface morphology of nascent fiber. To obtain a nascent fiber with good morphology, coagulation ability should be moderate. Too large or too small is easier to cause defects in cross section and surface morphology of nascent fiberThe order of the influence of coagulation bath concentration, coagulation temperature and coagulation negative draw ratio on the cross section of nascent fiber is:coagulation bath concentration> coagulation negative draw ratio> coagulation temperature. As the concentration of coagulation bath increased from 50% to 85%, De (degree of deviation from circular cross section) of nascent fiber decreases first and then increases, in 80% of concentration, De achieves the minimum value, which means the cross section of nascent fiber is closest to circular. With the apparent negative draw ratio decreases from -40% to 0, De of nascent fiber reduced, which means under 0 negative draw ratio, the cross section shape of nascent fiber is closest to a circular. As the coagulation bath temperature decreased from 70℃to 30℃, De increases first, then decreases, and then increases. At 40℃, PAN fiber has a cross-section morphology most closer to circular. PAN nascent fiber obtained in a coagulation bath concentration of 80%, the apparent negative draw ratio is 0 and the coagulation bath temperature is 40℃, not only has circular cross-section, but also has a good surface morphology and mechanical properties. To high molecular weight PAN, a draw ratio of 4.5 times can greatly improve the mechanical properties of fiber in drawing process of boiling water. But too large draw ratio will damage the fiber structure which results in reduction of mechanical properties of fiber. The condition that the temperature of dry densification at 130℃and drying time is 60s, is conducive to good mechanical properties of fiber. A draw ratio of 2.5 times in the steam stretch is favor to produce PAN precursor with optimal physical and mechanical properties. And the importance of drawing and drying densification process and exert tension in drying densification are also analyzed quantitatively according to the Fourth Statistical Mechanics theory. After optimization of the spinning process, the strength of final high molecular weight PAN precursor can reach 0.96GPa (8.2cN/dtex).