Establishment Of Coagulation Model For Polyacrylonitrile(PAN) Wet Spinning

Carbon fiber has the advantages of high strength,high modulus,low density and corrosion resistance.It is often used as a reinforcing material for composite materials and is widely used in various fields.Among the main products of carbon fiber,polyacrylonitrile(PAN)-based carbon fiber is the most widely used and has the best comprehensive performance.China ’s carbon fiber technology mainly differs from foreign countries in the production quality of the precursor,so it is of great significance to study the forming of the precursor.The solidification process is an important part of the precursor forming,involving the mutual influence and restriction of diffusion,phase equilibrium and phase separation,which determines the uniformity and compactness of the precursor.Therefore,this paper establishes and calculates the solidification process model based on the double diffusion theory,in order to optimize and determine the solidification conditions more accurately,and apply it to the subsequent washing process.(1)Firstly,the mass transfer kinetics of wet spinning diffusion process is studied.Based on Fick ’s second law,a binary model is established.The binary model is programmed by MATLAB and the double diffusion coefficient is solved.During the experiment,it was found that there was volume shrinkage in the solidified filaments.Therefore,on the basis of the binary model,the volume shrinkage was related to gelation.The gelation rate constant and the volume shrinkage rate constant were introduced into the binary model to establish a quaternary model.The model was programmed and the double diffusion coefficient was solved to study the gelation kinetics.The coefficients obtained by the two models were analyzed and the effects of coagulation bath concentration and temperature on the diffusion coefficient were studied.Finally,based on the known diffusion coefficient,the diffusion coefficient combined with Fick ’s second law is used to calculate and design the solidification process,which is the theoretical basis for guiding production.(2)Washing is an important step to remove the residual solvent in the fiber.The principle is the same as the double diffusion theory in the solidification process.Therefore,the diffusion model established in the solidification stage is also applicable to the washing process.Considering that there will also be volume shrinkage in the washing stage,the quaternary model is used to calculate the washing diffusion coefficient of solvent and non-solvent.The effects of washing temperature and solidification structure on the diffusion coefficient were studied,and the gelation rate constant in the solidification stage and the washing stage were compared and discussed.Finally,the washing process was designed by using the diffusion coefficient.(3)The solidification process can be divided into solution diffusion stage and gel diffusion stage.In the actual solution process,the solution diffusion coefficient and gel diffusion coefficient are not solved separately,only the average value of the two is calculated,because too many parameters lead to excessive calculation.In order to reduce the amount of calculation,the washing stage is regarded as the diffusion in the pure gel structure,and the diffusion coefficient is calculated and then substituted into the solidification process.The solidification process is divided into two parts: solution and gel to obtain the diffusion coefficient of the solution layer.The model is established under certain assumptions,so there will be some deviations from the actual situation.By analyzing the fitting results,the main reasons for the deviation are found in order to improve the model and establish a more accurate model.