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Ind. Eng. Chem. Prod. Res. Dev. 1983,22, 320-326
Polysulfone Ultrafiltration Membranes T. A. Tweddle, 0. Kutowy, W. L. Thayer, and S. SourIraJan" Dlvlsion of Chemistry, Natlonal Research Council of Canada, Offawa, Canada, K I A OR9
Three types of polysulfone polymers (Udel, Radel, and Victrex) were used for making flat ultrafiltration membranes. Several film casting variables were investigated. Detailed studies with Victrex polymer, using NMP as the solvent as the gelation medium, showed that variations and PVP as the additive in the casting solution and H,SO,-H,O in the composition of the film casting sdution and the concentration of sulfuric acid in the gelation medium together offered a means of improving the productfvity of the resulting membranes for direct use in ultrafiltration applications.
Introduction In view of their chemical, mechanical, thermal, and hydrolytic stability, polysulfone polymers are of practical interest as membrane materials for a wide variety of ultrafiltration (UF) and reverse osmosis (RO) applications. Under the polysulfone family of polymers, three kinds of materials are commercially available under the trade names Udel (polysulfone, grades 1700 and 3500), Radel (polyphenylsulfone, grade 5000), and Victrex (polyethersulfone, grades KM1, 100P, 200P and 300P); the first two kinds are supplied by Union Carbide and the third kind is supplied by ICI. The chemical structures of the above three kinds of polymers are as follows (Ballintyn, 1982). Udel
1
Victrex
@+I+{ 0
n
In the technical literature (Dreger, 1978), along with other properties, the equilibrium water absorption data at 20 OC for Udel, Radel, and Victrex are given as 0.85%, 1.3%, and 2.1 % , respectively, which may be considered as a measure of the relative polar nature of the polymers involved. While polysulfone materials have long been used for making porous supports for reverse osmosis membranes, they are now increasingly being used for making membranes for direct use as UF/RO membranes. This work, which stems from the latter application of the polysulfone materials, is concerned with details for making flat polysulfone membranes for direct use as UF/RO membranes. Among the several published papers on the subject, those 0196-4321/83/1222-0320$01.50/0
of Cabasso (1980), Cabasso et al. (1975, 1976, 1977), Cadotte et al. (1976), Kesting (1979), and Nishimura et al. (1977) are of interest in this work. The work of Cabasso et al. (1976, 1977) is mainly concerned with the production of polysulfone hollow fiber membranes by the dry/wet spinning technique. They used Udel as the polymer, dimethylacetamide (DMA, preferred) or N,N-dimethylformamide (DMF) as the solvent, and poly(vinylpyrro1idone) PVP, preferred) or polyethylene glycol (PEG) as the additive in the casting solution mainly to increase its viscosity for supporting the fiber in the spinning process. The smallest pore size reported in the fibers produced is 0.25 pm. They found that increasing the polymer content in the spinning solution decreased the hydraulic permeability of the resulting fibers; the substitution of PVP for polysulfone in the spinning solution produced more permeable fibers, and a decrease or increase of surface pore size could be accomplished by passing an already gelled polysulfone hollow fiber membrane through concentrated sulfuric acid. The work of Cadotte et al. (1976) is mainly concerned with underwater extrusion process for microporous polysulfone sheeting used as support for their NS-200 membranes. They used Udel as the polymer and DMF as the solvent. They noted that their polymer solutions are not shelf-stable; they became cloudy in one or two weeks a t room temperature, indicating phase separation or formation of suspended insoluble matter; if any water was absorbed into the solution from air, the phase separation was accelerated. The clarity of the polysulfone casting solution was, however, restored through simply heating the container of the solution for an hour in a 150 " C circulating air oven. Clarity of casting solution was found to be a necessity for good membrane formation. Storage of the polymer solutions in a 65 "C oven for one or two weeks appeared to stabilize them adequately for membrane casting. Kesting (1979) observed that high-boiling solvents such as DMF strongly solvated the polysulfone material, and their complete removal by immersion in cold water in the membrane making process was extremely difficult. Even traces of such solvents could cause varying degrees of densification of the "washed" polysulfone membranes rendering their RO/UF performance irreproducible. Hot water washing was suggested as a means of removing the last traces of solvent in the finished polysulfone membrane. Nishimura et al. (1977) studied in detail the preparation and performance of flat polysulfone (Udel) membranes for UF applications. The effects of casting solution composition, membrane thickness, and evaporation period on the performance of resulting membranes were studied. N-
Published 1983 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 321
Methyl-2-pyrrolidone (NMP) was used as the solvent for the polymer in the casting solution. The membrane formed by casting the polymer for a 15 w t % solution and solvent evaporation at 60 "C for 15 min gave a water flux of 102 L/m2.h at 20 "C and 97.2% separation for PEG (mol wt 20 OOO) at the operating pressure of 390 kPag (57 psig). The membrane performance was stable even after it was kept standing in 1N HC1 or NaOH solution for more than one month. The objects of this paper are to explore the effects of some film-casting variables on the performance of resulting membranes and to make some .detailed studies with Victrex as membrane material for RO/UF applications. Experimental Section The general film casting procedure used was as follows. The film casting solution consisted of the polymer and the solvent, with or without the nonsolvent additive, at the specified composition. The temperature of the film casting solution and that of the film casting atmosphere were both the lab temperature (23-25 "C). The solvent evaporation period prior to gelation was 1 min. The gelation medium was either water or aqueous acid solution at 2 to 5 O C , as indicated. The residence period of membrane in the gelation bath was usually 5 to 15 min, and the film was subsequently kept immersed in an ice-water bath for 1h and then stored in lab temperature water which was replaced frequently. The film was either hand cast or machine cast in flat form, usually on a glass plate unless otherwise specified. The data on membrane performance were obtained in UF experiments using 200 ppm PEG60Wwater feed solutions at the operating pressure of 345 Wag (50 psig) at room temperature using thin channel flow cells and a feed flow rate of 2000 cm3/min; these are referred in this paper as "the standard test conditions". For each experiment, data on pure water permeation rate (PWP), membrane permeated product rate (PR), and solute separation V, calculated from the relation f = [(solute ppm in feed) (solute ppm in product)] /(solute ppm in feed) were obtained at the preset operating conditions. The solute concentrations in the feed and product solutions were determined with a Beckman total carbon analyzer. Because of the very low feed concentrations involved, the data on PWP and PR were essentially identical. The effective area of the membrane used in the cell was 14.5 cm2 in all cases. Results and Discussion Preliminary Experiments. Several flat films were hand cast on glass plates with casting solutions containing 12 to 30 wt % of polymer (Udel, Radel, or Victrex) in one of the high boiling water-miscible solvents from the group DMA, DMF, DMSO (dimethyl sulfoxide), and NMP, using ice-cold water as the gelation medium. These films were greatly nonuniform both in appearance and performance, which was not surprising in view of the following observations. With DMF as the solvent, the homogeneity of the film casting solution was changing constantly as a function of time resulting in precipitation of the polymer segments from the casting solution when left standing for several days; to a lesser degree, this was probably happening with the other solvents also. The polymer-polymer attraction seemed so high that the film always tended to shrink linearly to different extents during membrane formation in the gelation medium. Further, during the gelation process, even though surface precipitation occurred immediately on immersion of the cast film in water, the solvent remaining underneath the surface layer of the film was
leached out only very slowly; after short gelation periods (
