Effect Of Spinning Diffusion Process On Structure And Properties Of PAN Fiber

Studying the development history of carbon fiber in China,it was believed that the PANprecursorwas one of the main reasons restricting the development of carbon fiber in China,and the quality of PANprecursorlargely dependedon the process relatedwith material diffusion during the spinning process.After years of development,Hengshen had broken through the manufacturing technology of PANprecursor for high-performance carbon fiber.It was still optimizing the process of coagulation,washing,drying and densification,which would be of great significance to improve the preparation level of carbon fiber and reduce production costs.In this paper,the influencing factors of the spinning diffusion process and the effects of diffusion on the structure and properties of PAN precursorwere studied on the specific PAN precursor industrial production line of Hengshen Company.Using X-ray diffraction(XRD),optical microscope(OM),scanning electron microscope(SEM)combined with a variety of detection methods such as bulk density,swelling and physical and mechanical properties,the diffusion rule of DMSO and non-solvent H2O and the influence on PAN fiber structure and performancewere studied in coagulation,water washing and drying densification process.The following research results were obtained:1.During the coagulation process of wet spinning,the solvent(DMSO)in the PAN nascent fiber is continuously reduced,the coagulant(H2O)is continuously increased,and the double diffusion gradually reaches equilibrium.The diffusion rate of DMSO and H2O is positively correlated with the concentration of the polymerization solution,the temperature of the coagulation bath,and the draw ratio,and negatively correlated with the concentration of the coagulation bath.In order to guide the design of the spinning process parameters,that is,to reduce the coagulation capacity of the coagulation bath by reasonably lowering the temperature,and then select the appropriate coagulation bath concentration to control the coagulation of the surface layerin PAN nascent fiber,ease the diffusion speed,and finally increase the coagulation draft ratioappropriately in order to increase the orientationdegree ofthe nascent fiber.2.In the wet spinning process,the effects of coagulationdiffusion on the cross-section,surface grooves,porosity and properties of PAN precursorare studied.When the draft is increased,the PAN fiber cross section tends to be round.Within a certain range,increasing theconcentration and reducing the temperatureof the coagulation bath are conducive to the deepening of the grooves on the surface ofPAN fiber.As the concentration of PAN solution and the concentration of the coagulation bath decrease and the temperature of the coagulation bath increases,the diffusion rate increases,porosityof PAN fiber increases,and the density decreases.There is an optimal combination of PAN solution concentration,coagulation bath temperature,concentration,and apparent draft rate.In wet spinning,PAN contentof polymer solution is 20 wt%,the coagulation bath temperature is 53℃,and the coagulation bath concentration is 65 wt%.When the apparent draft is 75%,PAN precursor with round cross-section,excellent performance and small dispersion can be obtained.3.In the wet spinning process,the influence of the solvent diffusion in the washing process on the residual solvent content,fiber swelling,crystalline structure and performance of PAN precursoris studied.The results showthat:increasing the washing temperature,the amount of water and using drawing first and washing with water can effectively improve the washing effect of fiber and reduce the residual solvent content in PAN precursor.The degree of swelling decreases with the increase of the washing effect,which is beneficial to the uniformity of oiling.The higher residual solvent in the PAN fiber increases the solubility of the PAN fiber during the heating process,destroys the cross-sectional morphology and crystal structure of the PAN fiber,and reduces the tensile properties of the PAN fiber.Under the premise of comprehensive consideration of operating costs,a process with a temperature of 75～95℃,a water volume of 12 times the total output,drawing first and washing with water is carried out.The residual solvent content in PAN precursor is 300ppm,which does not affect the subsequent pre-oxidation and carbonizationprocess.4.In the wet spinning process,the effects of temperature and time on the moisture escape and the structure and properties of PAN precursor during the drying and densification process are studied.Within a certain range,increasing the temperature and extending the time can promote the PAN fiber moisture diffusion,improve the grain size and crystallinity of the fiber,reduce the porosity and increase the density of the fiber,increase the tensile strength of PAN precursor,and reduce the elongation at break.Finally,the technology of drying and densification process is determined.Densification at 135-145℃for 40s-60s could obtain PAN precursor with a porosity of 6.35%,a bulk density of 1.18g/cm3,and excellent performance.5.For Hengshen wet spinning production line,solution polymerization is used,PAN content of polymer solution is 20wt%,the coagulation bath temperature is 53℃,the coagulation bath concentration is 65wt%,the apparent coagulationdraftis 75%,and the temperature range of water washing from 75 to 95℃,the washing water volume is 12 times of the total output,and the washing draft is 4.8 times.It can be densified at 135-145℃ for 40s-60s,and can obtain PAN precursor with round cross-section and excellent performance.Using this process,high-quality strands with a tensile strength of 7.85cN/dtex(cv is 3%)and an elongation at break of 13%are obtained,and carbon fibers with a tensile strength of 5.0GPa are produced from PAN precursor.And the spinning line runs smoothly at a speed of 100m/min,PAN precursor production rate and fixed length rate are greater than 95%,and the final PAN precursor qualification rate is greater than 99%.In the process of pre-oxidation and carbonization for carbon fiber,the engineering passability is good,and the qualification rate of carbon fiber is greater than 90%.