Modeling of asymmetric membrane formation by dry-casting method (original) (raw)
Related papers
Dense polymer film and membrane formation via the dry-cast process Part I. Model development
1994
The dry-cast membrane-formation process is a major phase-inversion technique by which asymmetric membranes are manufactured. In this paper a fully predictive model which incorporates coupled heat and mass transfer is developed to describe the evaporation of both solvent and nonsolvent from an initially homogeneous polymer/solvent/nonsolvent system. This unsteady-state, one-dimensional, coupled heat-and mass-transport model allows for local film shrinkage owing to excess volume of mixing effects as well as evaporative solvent and nonsolvent loss. The model can predict composition paths into the ternary phase diagram which determine the onset of phase transition. The ternary phase diagram is predicted using the Flory-Huggins theory allowing for composition-dependent interaction parameters. The model is applied to the well-characterized cellulose acetate/acetone/water system for which suffrcient experimental data are available to permit determination of the friction coefficients in the ternary mass-transport model. The model is solved numerically using a software package based on the method of lines which is capable of handling moving boundary problems. The modeling studies indicate that for a given polymer/solvent/ nonsolvent/support system, the most influential parameters are the gas-phase mass transport, initial cast film thickness, and initial composition. Of particular importance, the model can predict the general morphological characteristics associated with the formation of dense polymer films and symmetric as well as asymmetric membranes.
Mass transfer modeling of asymmetric membrane formation by phase inversion
Journal of Polymer Science Part B: …, 1990
A derivation is presented of a ternary diffusion model to describe the mass transfer processes associated with the quench bath period of the phase inversion process for membrane formation. The complete governing equations, initial conditions, and boundary conditions in the casting film and coagulation bath are presented. Equations for ternary chemical potentials and diffusion coefficients are consistently based on constant specific volume formulations. The model is applied to the analysis of mass transfer paths and their effects on membrane structure formation. Precipitation times are determined for given sets of conditions by superposing calculated mass transfer paths on the ternary phase diagram and observing when the miscibility gap is crossed. Comparisons are made with an earlier reported study on the membrane-forming system: water-acetone-cellulose acetate (CA). Agreement between predicted and measured precipitation times is found to be excellent. The polymer film composition profile a t the moment of precipitation is shown to be a useful indicator of both skin and sublayer structures, allowing distinctions to be made between conditions leading to spongelike and fingerlike morphologies. The influence of model parameters on the mass transfer paths and associated polymer profiles is also discussed.
Journal of Membrane Science, 1981
A numerical analysis describing acetone evaporation from a cellulose acetate casting solution is reported. Profiles of acetone density and acetone density gradient within the membrane are calculated as a function of evaporation time for three evaporation rates. The analysis indicates that, for a given composition of the casting solution, there are optimum evaporation times for the formation of the asymmetric skin, according to the composition of the casting atmosphere. These data make it possible to define a membrane "productivity factor", which has a maximum value under conditions when the polymer is precipitating and the solvent density gradient is maximized at the evaporating surface, and a minimum value when the solvent concentration throughout the membrane is below a critical value for polymer precipitation.
Membrane formation by dry-cast process
J Membrane Sci, 2005
Asymmetric membranes were prepared by dry-cast phase inversion technique from a cellulose acetate, acetone, water solution in order to assess the validity of the mathematical model recently developed by us. Based on the model predictions, general structural characteristics of the membranes were determined by plotting the composition paths on the ternary phase diagram and polymer concentration profile at the first
Journal of Membrane Science, 2006
A new model is developed for the evaporative or dry-casting process for polymeric membrane formation that incorporates convective transport owing to density changes; the latter effect has been ignored in all prior evaporative casting models. Densification inevitably occurs during the evaporative casting process owing to the removal of solvent that in turn causes the polymer molecules to assume a more
Journal of Membrane Science, 2010
A new model is developed for wet-casting polymeric membranes that address how the concentrations at the interface between the casting solution and nonsolvent bath adjust from initial nonequilibrium to equilibrium values on the binodal. Properly describing the evolution of the interface concentrations enables this new model to predict vitrification, which has been observed experimentally but not predicted heretofore. This new model also incorporates densification-induced convection that arises owing to density changes associated with the concentration gradients and contributes to the mass-transfer fluxes. The predictions for the cellulose acetate, acetone, and water system indicate that densificationinduced convection can increase the mass-transfer flux by nearly two orders-of-magnitude shortly after initiating wet-casting. This increased mass-transfer flux can have a marked effect on the properties of the functional layer of asymmetric membranes that is formed early in the casting process. The predictions for initial casting-solution thicknesses of 75 and 125 m are markedly different. When densificationinduced convection is included, the 125 m film is predicted to enter well into the metastable region, thereby allowing supersaturation that promotes macrovoid defects. Hence, this new model provides an explanation for the effect of casting-solution thickness on the occurrence of macrovoids.
A diffusion-controlled model for wet-casting membrane formation
Journal of Membrane Science, 1991
A simplified analytical treatment of membrane formation mechanism, following the Paul method, is proposed using a casting solution immersed in a coagulation bath. This model assumes a discontinuous system consisting of parallel layers in the membrane with each layer proceeding rapidly to phase inversion. It allows the measurement of local composition variation at each layer depending on the flux ratio of solvent to nonsolvent. It is difficult to test the model unless a direct method of estimating the flux ratio at each layer is developed. However, some conclusions can be drawn about the applicability of the suggested model according to the type of membrane. For a homogeneous membrane, the flux ratio is independent of position. For an asymmetric membrane, the flux ratio has two values: the larger value for the skin layer and the other for the sublayer. Experimental results are compared with the values predicted by this pseudobinary mathematical model, and show fairly good agreement.
Journal of Membrane Science, 1994
In order to validate the dry-cast model developed by us, a multifaceted experimental approach was undertaken whereby three process variables can be followed independently in real time. Since it is extremely difficult to determine experimentally the concentration and temperature profiles within the cast polymer solution, experiments were designed so as to provide information on the coupled mass-and heattransfer processes by measuring other process variables. The experimental data-acquisition technique combined gravimetric, inframetric, and light-reflection analyses which provided information on the overall mass change, surface temperature, and the onset and duration of phase separation, respectively. Structural studies were conducted using scanning electron microscopy. These studies revealed that macrovoids or "fingers" can be formed in dry-cast membranes. A hypothesis for the formation of fingers based on the model predictions and experimental observations is proposed.
Journal of Membrane Science, 2001
This note describes an improved algorithm for the solution of the governing equations describing ternary mass transfer during the quench-period in the formation of immersion precipitation membranes. The algorithm is applied to the model developed by Reuvers et al. (J. Membr. Sci. 34 (1987) 45) for the water-acetone-cellulose acetate system. The improved algorithm developed in this work numerically simulates the multi component unsteady state diffusion process in immersion precipitation membrane formation without the necessity of user intervention in terms of initial guesses for the interfacial composition. Phase separation is assumed to take place at the spinodal. The algorithm is presented in detail for three component systems, however, it is easily extendable to four component systems. Numerical simulations using the improved algorithm are in good agreement with those of Reuvers et al. (J. Membr. Sci. 34 (1987) 45), however, experimental studies are required to validate some of the assumptions relating to the polymer phase separation. Published by Elsevier Science B.V.