Studies on the development of improved reverse osmosis membranes from cellulose acetate–acetone–formamide casting solutions

Improved membranes from cellulose acetate–acetone–formamide casting solutions have been prepared for low-pressure reverse osmosis applications. The film-casting details for one such type of membranes (Batch 400) are as follows. Casting solution composition: cellulose acetate (E-398-3), 17 wt-%, acetone, 56 wt-%, formamide, 27 wt-%; temperature of casting solution, 24°C; temperature of casting atmosphere, 24°C; casting atmosphere, ambient air in contact with 30 wt-% acetone in aqueous solution; solvent evaporation period, 30 sec; gelation medium, ice-cold water. Using aqueous feed solutions containing 3500 ppm of NaCl, the product rates obtained with the above membranes at 95, 90, and 60% levels of solute separation were 15.9, 22.1, and 58.7 gallons/(day ft²), respectively, at 250 psig under feed flow conditions corresponding to a mass transfer coefficient of 45 × 10^?4 cm/sec on the high-pressure side of the membrane. The effects of casting solution composition, presence of acetone in the casting atmosphere, evaporation period, evaporation rate constant, and the remoteness of casting solution composition from the corresponding phase boundary composition on membrane performance and shrinkage temperature profile were found to be similar to those reported earlier for membranes obtained from cellulose acetate–acetone–aqueous magnesium perchlorate casting solutions. The results illustrate the practical utility of the approach based on the solution structure–evaporation rate concept for creating more productive reverse osmosis membranes.     

An interpolymer anionic composite reverse osmosis membrane derived from poly(vinyl alcohol) and poly(styrene sulfonic acid)

An interpolymer anionic composite membrane for reverse osmosis was prepared from poly(vinyl alcohol) and poly(styrene sulfonic acid). The effects of composition of a casting solution, heat-curing periods, and casting thickness on the reverse osmosis performance of resulted membranes have been examined. A mixture of water and ethyl alcohol (12/7, wt %) was found to be a proper solvent for casting an interpolymer membrane on the supporter. The composite membrane was formed by casting the polymer solution in ultrathin film on a microporous polypropylene supporter, evaporating the solvent, and heat-curing at 120°C for a proper period. the optimum composition of a casting solution was as follows: wt % of poly(vinyl alcohol)/poly(styrene sulfonic acid)/solvent was 3/2/95. The membrane heat-cured at 120°C for 2 h has a good performance for reverse osmosis, viz., water flux of 9.1–28.4 L/m².h at salt rejection level of 88.1–93.4% under applied pressure of 80 kg/cm² with 0.5% NaCl aqueous solution. The formation mechanism of a water-insoluble membrane was discussed.     

Effect of casting conditions on the performance of porous cellulose acetate membranes in reverse osmosis

Several sets of porous cellulose acetate membranes were made using the same casting solution composition and gelation conditions but varying the casting solution temperature and solvent evaporation conditions. The films were tested in reverse osmosis experiments at 250 psig using aqueous feed solutions containing 3500 ppm NaCl. The results show that the product rate obtained at a given level of solute separation is independent of evaporation time in the range tested and, for a given casting solution composition, the temperature of the casting solution and conditions of solvent evaporation during film formation together constitute an important interconnected variable governing the porous structure of the resulting membranes. These results offer a new approach to the problem of developing more productive reverse osmosis membranes and have led to a new class of porous cellulose acetate membranes capable of giving product rates 100% to 150% higher than those of the best membranes reported, at any given level of solute separation under the experimental conditions used. These results are of practical importance in low-pressure reverse osmosis applications.     

Improvement of porous cellulose acetate reverse osmosis membranes by change of casting conditions

The effects of temperature of casting solution in the range ?10° to 15°C, that of casting atmosphere in the range 10° to 30°C, relative humidity of casting atmosphere in the range 35% to 75%, and solvent evaporation period in the range 0.5 to 3 min were studied on shrinkage temperatures, solute separations, and product rates of Loeb-Sourirajan-type cellulose acetate membranes in reverse osmosis experiments. The composition of casting solution used was as follows: cellulose acetate, 17; acetone, 69.2; magnesium perchlorate, 1.45; and water, 12.35 wt-%. Best performance was obtained with membranes cast under the following conditions: temperature of casting solution, 10°C; temperature of casting atmosphere, 30°C; relative humidity of casting atmosphere, 65%; and solvent evaporation period, 1 min. For a 90% level of solute separation, the productivities of the above type of membranes were 22.9, 61.4, and 64.5 gallons/day-ft² at 250, 600, and 1500 psig using 3500 ppm NaCl–H₂O, 5000 ppm NaCl–H₂O, and 28395 ppm NaCl–H₂O feed solutions, respectively. In all cases, the feed flow rates corresponded to a mass transfer coefficient of 45 × 10^?4 cm/sec on the high-pressure side of the membrane. The general specifications of the above type of membranes are given for the operating pressures of 250, 600, and 1500 psig. The effects of the above casting condition variables on the surface pore structure during film formation are discussed.     

Adsorption of water on cellulose acetate membrane

This study was performed in order to obtain some information on the effect of variables of cellulose acetate membrane procedure on its reverse osmosis properties by adsorption of water vapor. It was found that the rate of adsorption of vapor was slower for the membrane prepared at casting condition of longer evaporation period. This variation corresponded to that of water permeability by reverse osmosis performance. It seems from the isotherm data that the overall porosity of membrane is determined during the evaporation step. This provides additional support for membrane properties depending on casting conditions.     

Preparation and performance of poly(vinyl butyral) membrane for ultrafiltration

Ultrafiltration membrane was prepared from poly(vinyl butyral). The effects of membrane thickness, polymer concentration, evaporation time, and evaporation temperature, etc., on the performance of the resulting membranes have been studied. Dimethylacetamide was used as a casting solvent. The membrane formed by casting the polymer from a 15 wt % solution and evaporation at 25°C for 30 s had a flux value of 250 cm³ / cm² h (4.8 kg/cm², 26°C) at 92.9% rejection level for dextran sodium sulfate (average mol. wt. 550,000) separation. © 1993 John Wiley & Sons, Inc.     

The Effect of Skin Layer Composition and Operating Parameters on the Performance of Sulfonated Polysulfone-Poly(Vinyl Alcohol) Composite Reverse Osmosis Membrane

Abstract

A composite membrane for reverse osmosis was prepared from sulfonated polysulfone and poly(vinyl alcohol). The effects of overall polymer solution concentration, composition of casting solution, and heat-curing periods on the reverse osmosis performance of the resulting membrane have been examined. The composite membrane was formed by casting the polymer solution as an ultrathin film on a microporous polysulfone supporter, evaporating the solvent, and heat curing at 120°C for a proper period. The influence of different operating parameters on the performance of the resulting membrane was examined. The results showed that the flux of water increased and salt rejection increased with an increase in the ratio of sulfonated polysulfone to poly(vinyl alcohol) (SPSf/PVA). The flux of water is proportional to the operating pressure of the formed membrane with an SPSf/PVA ratio of 3/3–4/2. The flux of water was found to be a highly nonlinear function of the operating pressure of the formed membrane with an SPSf/PVA ratio of 2/4–0/6.     

Effect of secondary additives in casting solution on the performance of porous cellulose acetate reverse osmosis membranes

The solution structure and evaporation rate constant can be varied by changing the temperature of the casting solution and the temperature of the casting atmosphere for a given film-casting solution composition. The effects of the two temperature changes can be simulated (without changing the two temperatures) by replacing a small part of the solvent (acetone) by a secondary additive in the casting solution. The effect of 20 different secondary additives in the batch 316-type casting solution has been studied and is discussed. Porous cellulose acetate reverse osmosis membranes, capable of giving a 20% to 25% increase in productivity at a 90% level of solute separation for a 3500 ppm NaCl–H₂O feed solution at 250 psig, have been produced using 5 wt-% ethyl ether as the secondary additive in the above casting solution. The use of secondary additives offers a new flexibility in the choice of film-casting conditions and in the general development of reverse osmosis membranes.     

Effects of Membrane-Making Conditions and Shrinkage Treatment on Morphology and Performance of Cellulose Acetate Butyrate Membranes

Abstract

The functions of additives in cellulose acetate butyrate (CAB) membrane casting solution, effect of thermal shrinkage treatment on porous CAB membranes, and the changes of CAB membrane surface morphology during the solvent evaporation step have been investigated. Additives (glycerol and lactic acid) in CAB membrane casting solution function only as pore number promoting agents when used at low concentration and function both as pore number and pore size promoting agents when used at higher concentrations. Triethyl phosphate in CAB membrane casting solution functions both as a pore number promoting agent and as a secondary solvent for CAB. Three distinct phases can be observed in the solvent evaporation step in making CAB membranes. With the increase in solvent evaporation time, the number of pores in the first pore size distribution increases in the initial small pore-forming phase and decreases in the large pore-forming phase, and the number of pores in the second pore size distribution always increases with solvent evaporation time. These changes in pore numbers, pore sizes, and pore number ratio in two pore size distributions as well as the membrane skin layer thickness together govern the ultimate membrane performance and result in a maximum solute separation which, in the case of CAB/ acetone membranes, falls at 60 seconds of solvent evaporation time. Significant improvement of the performance of a porous CAB membrane can be achieved by thermal shrinkage treatment. Equally high CAB membrane performance can also be achieved by using a lower concentration of additives in the membrane casting solution.     

Asymmetric reverse osmosis and ultrafiltration membranes prepared from sulfonated polysulfone

The concept of solubility parameter, developed by Hansen, has been applied to the determination of solubility diagram of polysulfone and sulfonated polysulfone. Data showing the effect of various parameters such as thermodynamic quality of the solvent, casting solution composition and viscosity, solvent evaporation period and precipitation bath composition, on membrane structure and performance have been presented. By adjusting these parameters a wide variety of sulfonated polysulfone membranes useful for both ultrafiltration and reverse osmosis applications could be obtained.     

An approach to the development of cellulose acetate ultrafiltration membranes

The film-casting solution consisted of a mixture of cellulose acetate, acetone, and aqueous magnesium perchlorate [Mg(ClO₄)₂:H₂O = 1:8.5], designated as polymer P, solvent S, and nonsolvent N, respectively. Using the composition P:S:N = 17: 69.2: 13.8 as reference, films were obtained from 19 different casting solutions in which the weight ratios S/P, N/S, and N/P were varied in different directions. The casting solution temperature was 0°C, and solvent evaporation period during film formation was minimum in most cases. The effects of variations of casting solution temperature and solvent evaporation period were also briefly studied. Reverse osmosis experiments with resulting membranes were carried out at 100 psig using 200 ppm NaCl–H₂O as the feed solution. Decrease in S/P, increase in N/S, and increase in N/P in the casting solution, decrease in temperature of the casting solution, and increase in solvent evaporation period tend to increase the size of pores on the surface of resulting membranes in the ascast condition. Increase in S/P in the casting solution, and increase in the temperature of the casting solution tend to increase the effective number of pores on the membrane surface. These results offer definitive physicochemical criteria in terms on solution structure–evaporation rate concept for developing useful cellulose acetate ultrafiltration membranes.     

Effect of surface properties of polysulfone support on the performance of thin film composite polyamide reverse osmosis membranes

In this work, influence of initial conditions and surface characteristics of porous support layer on structure and performance of a thin film composite (TFC) polyamide reverse osmosis (RO) membrane was investigated. The phase inversion method was used for casting of polysulfone (PSf) supports and interfacial polymerization was used for coating of polyamide layer over the substrates. The effect of PSf concentrations that varied between 16 wt % and 21 wt %, and kind of the solvent (DMF and NMP) used for preparation of initial casting solution were investigated on the properties of the final RO membranes. SEM imaging, surface porosity, mean pore radius, and pure water flux analysis were applied for characterization of the supports. The substrate of the membrane, which synthesized with 18 wt % of PSf showed the most porosity and the synthesized RO membrane had the lowest salt rejection. In case of the solvents, the membranes synthesized with DMF presented better separation performance that can be attributed to their lower thickness and sponge‐like structure. The best composition of support for TFC RO membranes reached 16 wt % PSf in DMF solvent. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44444.     

Effect of ethanol–water mixture as gelation medium during formation of cellulose acetate reverse osmosis membranes

By using ethanol–water mixtures in a wide range of alcohol concentrations and temperatures, cellulose acetate membranes with a wide range of surface porosities can be obtained. Two different casting solution compositions were used, involving cellulose acetate, acetone, and aqueous magnesium perchlorate (composition I) or formamide (composition II). All reverse osmosis experiments were carried out at 250 psig using a 3500 ppm NaCl–H₂O feed solution at laboratory temperature. The effective area of film surface was 12 cm² in all cases. With composition I, with pure water gelation medium at 0°C, the resulting membrane gave a solute separation of 5% and product rate of 220 g/hr, whereas with 95% alcohol as gelation medium, the resulting membrane gave a solute separation of ～1% and product rate of 1240 g/hr under otherwise identical experimental conditions. With composition II membranes, the maximum product rate of 360 g/hr with the corresponding minimum solute separation of ～1% was obtained with 71.2% alcohol–water gelation medium at 0°C. Increase in the temperature of the gelation medium in the range 12° ?25°C tends to increase the average size of pores on the membrane surface. These results offer a basis for the development of cellulose acetate ultrafiltration membranes.     

Cellulose acetate ultrafiltration membranes

Twenty-three different casting solution compositions involving cellulose acetate E-938-3 (polymer, P), acetone (solvent, S), and aqueous magnesium perchlorate (Mg(ClO₄)₂:H₂O = 1:8.5) (nonsolvent, N) were studied for making ultrafiltration membranes. These compositions (expressed in weight units) involved N/P ratios of 0.817 to 1.3 and S/P ratios of 3.5 to 5.33. It was found that by adjusting the values of S/P and N/P ratios in the casting solution composition, temperature of the casting solution, temperature of the casting atmosphere, and solvent evaporation period during film formation, a wide variety of cellulose acetate membranes useful for both ultrafiltration and reverse osmosis applications could be obtained. The results of a continuous test run at 50 psig for a period of 250 hr with a typical set of membranes, and their separation characterstics for a group of solutes ranging in molecular weight from 58 to 160,000, are presented.     

Physicochemical processes occurring during the formation of cellulose diacetate membranes. Research of criteria for optimizing membrane performance. IV. Cellulose diacetate-acetone-organic additive casting solutions

Asymmetric membranes were prepared from cellulose diacetate using a mixture of acetone and formamide, DMF, DMSO, NMP, dioxane or acetic acid as a basic solvent and water as the precipitation agent. The membrane water flux and salt rejection were determined by reverse osmosis tests. The change in mobility of the polymer chains during the evaporation and coagulation phases was studied by fluorescence depolarization. The main parameters such as casting solution composition, evaporation time, water concentration required for polymer precipitation, speed of coagulation and their effect on membrane structure and performance are discussed.     

Understanding and guiding the phase inversion process for synthesis of solvent resistant nanofiltration membranes

Since its introduction in membrane technology in the 1960's, phase inversion by means of immersion precipitation has been widely studied for the preparation of membranes to be applied in the fields of microfiltration (MF) and ultrafiltration (UF). However, much less knowledge is available about this process in terms of integrally skinned asymmetric nanofiltration membranes, especially for more hydrophobic polymers applied in solvent resistant nanofiltration (SRNF). This review focuses on the preparation aspects of integrally skinned asymmetric membranes to be applied in the field of SRNF via phase inversion. It starts with the explanation of the basic principles of the phase inversion process, covering both thermodynamic and kinetic aspects. Further, it summarizes the parameters that significantly influence final membrane performance and morphology, including polymer type and concentration, casting solvent, additives, evaporation time, and temperature, humidity, membrane thickness, composition, and temperature of coagulation bath and post‐treatment. Literature contained within this review constitutes the core references in the field of SRNF, but also several references on preparation of MF, UF, aqueous NF, and reverse osmosis (RO) membranes have been included to better clarify or illustrate certain aspects of the process. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42130.     

高通量聚醚砜纳滤膜的制备及对染料浓缩脱盐

以聚醚砜（PES）为膜材料,以嵌段式聚醚 Pluronic F127为添加剂,利用特制刮膜设备通过相转化法制备出高通量PES/Pluronic F127复合纳滤膜,并将其用于染料的浓缩脱盐。研究了添加剂含量、溶剂蒸发温度和蒸发时间对膜结构和膜性能的影响,考察了不同操作压力和操作温度下膜对染料的分离性能。扫描电镜 （SEM）、接触角、孔隙率数据和蛋白吸附测试结果表明,Pluronic F127改善了膜孔结构,提高了孔隙率,并且显著提高了膜的抗污染性能。纯水通量、截留率以及膜表面孔径表征结果表明,当Pluronic F127含量为3%、溶剂蒸发温度为90℃、 蒸发时间为18 s时,膜的分离性能最佳。在0.6 MPa下该膜对低分子量染料的截留率可达99.9%,且通量达到110.2 L·m^-2·h^-1,对NaCl的截留率仅为5.5%。在12 h的染料浓缩脱盐中,膜对染料的通量维持在较高水平且截留率始终保持在99%左右,具有良好的稳定性和抗污染性。     

On the structure of concentrated cellulose acetate solutions

The supermolecular structure of cellulose acetate membrane casting solutions was determined by measuring their rheological behavior. The method was applied to both clear and turbid casting solutions. The resulting data on activation energy of viscous flow are shown to depend on the cellulose acetate content of the casting solution and on the nature of solvent used. The separation properties of the prepared asymmetric membranes determined by reverse osmosis testing indicate a good correlation with the data obtained by the rheological measurements of the casting solutions. The latter can, therefore, be used as a practical tool for the investigation of early stages in the asymmetric membrane formation process.     

醋酸纤维素－丙烯腈接枝改性反渗透干膜

本文介绍了醋酸纤维素－丙烯腈接枝改性反渗透膜的铸液组成、溶剂、致孔剂的选择及其交互影响，也详细介绍了成膜条件及膜干燥条件对膜性能的影响，简单介绍了膜的中试放大情况及膜的耐酸碱和抗微生物侵蚀的性能     

Preparation and structure of cross-linked asymmetric membranes based on polystyrene and divinylbenzene

Asymmetric polystryene-based membranes were prepared by photocross-linking with divinylbenzene (DVB) combined with phase-inversion steps. The primary variables examined were original polystrene concentration in the membrance casting solution and degree of cross-linking. The effects of these upon membrane morphology and reverse osmosis performance were determined. At higher DVB concentration, skin was formed at the top due to phase separation and at the bottom due to preferential polymer absorption at the support interface. An attempt was made to increase membrane ductility by incorporating low levels of liquid polybutadiene or styrene-butadiene block copolymer, but no significant change was measured. © 1994 John Wiley & Sons, Inc.