Simulation for Dry Spinning Process with New Hybrid Expression of Diffusion Coefficient Dependent on Solvent Concentration and Temperature

Abstract

The optimum processing conditions for dry spinning process of segmented poly(urethane-urea) (SPUU) – solvent (dimethylformamide (DMF) or dimethylacetamide (DMAc)) system have been desired to be predicted by computer simulations because of high cost and difficulties in experiment. Previous simulation model with a diffusion coefficient obtained from Moiré Method has the issue that simulated residual solvent concentration is somewhat lower than practical residual solvent concentration for SPUU-DMF system. As DMAc has been substituted for DMF because of its lower toxicity, it is desire to newly measure diffusion coefficient of SPUU-DMAc system. Recently, the authors have newly measured the diffusion coefficients of DMAc for SPUU-DMAc system with Film (diffusion in the solid state) and Dope (diffusion in the liquid state) methods, and tried to simulate the spinline behavior during dry spinning with them. The simulation has led to the following results that residual solvent concentration simulated by Dope method is almost same as that by Moiré method. Residual solvent concentration simulated by Film method is much higher and that by Dope method is somewhat lower than practical residual solvent concentration, respectively. Though a spinline changes from the liquid state to the solid state in a practical dry spinning process, Moiré pattern, Dope and Film methods cannot express the change of state from liquid to solid. Therefore, Hybrid method has been proposed, which can express the change of state with the fixed state transition points under the assumption that each state transition of liquid-semisolid-solid occurs at only given solvent concentrations independently of temperature. Residual solvent concentration simulated by Hybrid method is higher than practical one. The difference between the simulated result and practical one is assumed to be caused by the state transition points of fixed residual solvent concentrations only independently of temperature. The state of spilnline continuously changes with both solvent concentration and spinline temperature from liquid to solid via semisolid in a practical process. The state transition points from liquid to semisolid and from semisolid to solid depend on both solvent concentration and temperature. In the semisolid state between the liquid and solid states, the diffusion coefficient greatly changes with solvent concentration. It is necessary to express the diffusion coefficient as a continuous function of both solvent concentration and temperature. In order to remove the restriction in fixed state transition points of Hybrid method, a new method which is called “Logistic method” is proposed. Logistic method can express the diffusion coefficient of solvent in a spinline continuously changing from liquid to solid with solvent concentration and temperature by a logistic function. Logistic method can predict the closer residual solvent concentration to a practical one than Hybrid method, but the residual solvent concentration simulated by Logistic method is slightly lower than that obtained by practical plant. This small difference is very important for the design of dry spinning chamber and the grasp of optimum dry spinning conditions. It can be estimated that the difference between simulated result and practical one is caused by the imperfect expression of diffusion coefficient as a function of solvent concentration and temperature because of the difficulties in accurately determining the parameters in a logistic equation. In order to predict the residual solvent concentration closer to that in a practical process, new Hybrid method which can accurately express the state transition points changing with solvent concentration and temperature is proposed in this study. This method expresses the dependence of diffusion coefficient on both solvent concentration and temperature in the spinline, where the whole dry spinning process is divided into three sections depending on the state of spinline such as liquid state (near the spinneret), semi-solid state and solid state (near the take-up position). The differences of simulated residual concentration among new Hybrid method and other ones will be discussed in this study.