Polymeric composite membranes

A membrane designated "Solrox" made by Sumitomo Chemical Company is closely related to the above plasma polymerized composite membranes. A 1980 report by T. Sano described the Sumitomo process (31). A support film was cast from a polyacrylonitrile copolymer containing at least 40 mole percent acrylonitrile. The support film was dried and exposed to a helium or hydrogen plasma to form a tight cross-linked surface skin on the porous polyacrylonitrile support film. Data in a U.S. Patent issued in 1979 to Sano et al showed that the unmodified support film had a water flux of 87 gfd (145 L/ sq m/hr) at 142 psi (10 kg/sq cm). After the plasma treatment a reverse osmosis test using 0.55 percent NaCl at 710 psi (4895 kPa) showed 10.5 gfd (17.5 L/sq m/hr) flux at 98.3 percent salt rejection (32). This membrane appears to fall between a conventional asymmetric membrane and a composite membrane. If the surface skin is only cross-linked, one might call it a modified asymmetric membrane. However, if the surface skin is substantially modified chemically to make it distinct from the bulk of the membrane it could be considered as a composite type. 

According to Subramanian et al. [56], in a study of degumming of crude sunflower oil extracted by cold pressing, using polymeric composite membranes, obtained not only the retention of phospholipids by 100%, depending on the type of membrane, but also that of pigments and oxidation products. However, the permeate flux needs to be increased for an industrial application. 

Subramanian, R., Nakajima, M., and Kawakatsu, T. (1997) Processing of vegetable oils using polymeric composite membranes. Journal of Food Engineering 38,41-56. 

Zhang, K. Wu, X.Y. 2002, Modulated insulin permeation across a glucose-sensitive polymeric composite membrane , Journal of Controlled Release, vol. 80, no. 1-3, pp. 169-178. 

Benavente, 1. and Vazquez, M.l. 2004. Effect of age and chemical treatments on characteristic parameters for active and porous sublayers of polymeric composite membranes. J oUoidIrder e cL 3 5. ... 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

Although hydrophobic pervaporation membranes can be used to recover alcohol from fermentation broth, hydrophilic pervaporation membranes allow for the dehydration of water/alcohol mixmres (Figure 11.7). Hydrophilic pervaporation membranes can be applied to separate water from highly concentrated alcohol (>85%) (Abels et al., 2013). Two types of membranes have been explored in recent studies (1) zeoUte-based membranes and (2) polymeric composite membranes with polyvinylalcohol or polyimide as the active layer. 

Thomas et al. [16] used CSLM to observe in situ the formation of polyamide membranes and the measurements were used to study polymer precipitation kinetics. Turner and Cheng [17] applied CSLM and hydrophilic fluorescent probes of varying molecular weights to image the size distribution of poly(methacrylic acid) (PMAA) hydrogel domains in polydimethylsiloxane (PDMS)-PMAA interpenetrating polymer networks. The combination of CSLM with AFM, SEM and X-ray spectroscopy allowed characterization of the structure of stimuli-responsive polymeric composite membranes [18]. 

Nakanishi, S. C., Gon9alvesa, A. R., Rocha, G., Ballinas, M. L., and Gonzalez, G. 2011. Obtaining polymeric composite membranes from lignocellulosic components of sugarcane bagasse for use in wastewater treatment. Desalination Water Treat. 27 66-71. 

A typical polymeric composite membrane is composed of a porous film with a dense polymer barrier layer of a different material formed over the top. Composite membranes have the advantage over integral-asymmetric structures that different polymers may be used for the different layers, depending on their properties (e.g. a polymer with the correct selectivity for a separation problem can be used over a support structure of a different porous material). 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

An excellent review of composite RO and nanofiltration (NE) membranes is available (8). These thin-fHm, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-fHm composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-kniniscible solvent. 

Data for thermal movement of various bitumens and felts and for composite membranes have been given (1). These describe the development of a thermal shock factor based on strength factors and the linear thermal expansion coefficient. Tensile and flexural fatigue tests on roofing membranes were taken at 21 and 18°C, and performance criteria were recommended. A study of four types of fluid-appHed roofing membranes under cycHc conditions showed that they could not withstand movements of <1.0 mm over joiats. The limitations of present test methods for new roofing materials, such as prefabricated polymeric and elastomeric sheets and Hquid-appHed membranes, have also been described (1). For evaluation, both laboratory and field work are needed. 

Young, J.S., C02 Separations using High-Temperature Polymeric-Metallic Composite Membranes, 2nd Annual Conference on Carbon Sequestration, Alexandria, VA, May 2003. 

Kim, Y. S., Wang, R, Hickner, M., Zawodzinski, T. A. and McGrath, J. E. 2003. Fabrication and characterization of heteropolyacid (H3PWJ2O4Q)/directly polymerized sulfonated polyjarylene ether sulfone) copolymer composite membranes for higher temperature fuel cell applications. Journal of Membrane Science 2112 263-282. 

The membranes used for pervaporation are similar to reverse osmosis membranes, i.e. both are composite membranes consisting of a very thin dense permselective film on top of a nonselective porous support. In pervaporation, however, the membrane is highly swollen at the feed side and relatively dry at the permeate side. Two different types of pervaporation membranes based on polymeric materials were developed at about the same time in the early 1980s [31] ... 

Nanofiltration membranes usually have good rejections of organic compounds having molecular weights above 200—500 (114,115). NF provides the possibility of selective separation of certain organics from concentrated monovalent salt solutions such as NaCl. The most important nanofiltration membranes are composite membranes made by interfacial polymerization. Polyamides made from piperazine and aromatic acyl chlorides are examples of widely used nanofiltration membrane. Nanofiltration has been used in several commercial applications, among which are demineralization, oiganic removal, heavy-metal removal, and color removal (116). 

Figure 22 shows scanning electron micrographs of the composite membrane. The gel was chemically fixed on the porous glass by a radical polymerization. In the photograph, the gel layer of about 5 pm in thickness can be observed on the surface of the porous glass. 

Improved organic rejections result from the chemical nature of the newer polymeric materials in the nylon and thin-film composite membranes. Data by Chian and Fang (6) represented in Figure 2 illustrate different membrane rejections for a variety of organic com-... 

Although these composite fibers were developed lor reverse osmosis lltcir acceptance in the desalinatiun industry has been limited due to insufficient selectivity and oxidative stability. The concept, however, is extremely viable composite membrane fiat films made from interfacial polymerization have gained wide industry approval. Hollow libers using Ibis technique to give equivalent properties and life, yet lo be developed, should be market tested during the 1990s. 

H. Yasuda, Plasma Polymerization for Protective Coatings and Composite Membranes, 7. Membr. Sci. 18, 273 (1984). 

Since the discovery by Cadotte and his co-workers that high-flux, high-rejection reverse osmosis membranes can be made by interfacial polymerization [7,9,10], this method has become the new industry standard. Interfacial composite membranes have significantly higher salt rejections and fluxes than cellulose acetate membranes. The first membranes made by Cadotte had salt rejections in tests with 3.5 % sodium chloride solutions (synthetic seawater) of greater than 99 % and fluxes of 18 gal/ft2 day at a pressure of 1500 psi. The membranes could also be operated at temperatures above 35 °C, the temperature ceiling for Loeb-Sourirajan cellulose acetate membranes. Today s interfacial composite membranes are significantly better. Typical membranes, tested with 3.5 % sodium chloride solutions,... 

For a few years after the development of the first interfacial composite membranes, it was believed that the amine portion of the reaction chemistry had to be polymeric to obtain good membranes. This is not the case, and the monomeric amines, piperazine and phenylenediamine, have been used to form membranes with very good properties. Interfacial composite membranes based on urea or amide bonds are subject to degradation by chlorine attack, but the rate of degradation of the membrane is slowed significantly if tertiary aromatic amines are used and the membranes are highly crosslinked. Chemistries based on all-aromatic or piperazine structures are moderately chlorine tolerant and can withstand very low level exposure to chlorine for prolonged periods or exposure to ppm levels... 

An interesting group of composite membranes with very good properties is produced by condensation of furfuryl alcohol with sulfuric acid. The first membrane of this type was made by Cadotte at North Star Research and was known as the NS200 membrane [32], These membranes are not made by the interfacial composite process rather a polysulfone microporous support membrane is contacted first with an aqueous solution of furfuryl alcohol and then with sulfuric acid. The coated support is then heated to 140 °C. The furfuryl alcohol forms a polymerized, crosslinked layer on the polysulfone support the membrane is completely black. The chemistry of condensation and reaction is complex, but a possible polymerization scheme is shown in Figure 5.10. 

Although some nanofiltration membranes are based on cellulose acetate, most are based on interfacial composite membranes. The preparation procedure used to form these membranes can result in acid groups attached to the polymeric backbone. Neutral solutes such as lactose, sucrose and raffinose are not affected by the presence of charged groups and the membrane rejection increases in proportion to solute size. Nanofiltration membranes with molecular weight cut-offs to neutral solutes between 150 and 1500 dalton are produced. Typical rejection curves for low molecular weight solutes by two representative membranes are shown in Figure 5.13 [35],... 

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