Composite membranes solution coated

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

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. 

A coating composition was prepared by dissolving the step 1 product (3.00 g), and 1,3,5,7-adamantanetetracarboxylic acid (0.552 g) in /V,/V-dimcthylacctamidc (20.13 g) and then filtering through 0.2 pm membrane. The coating solution was then spin coated onto an 8-inch silicon wafer and heated to 300°C for 30 minutes and then further heated to 400°C for an additional 30 minutes. The film that formed had a thickness of 298 nm, a density of 1.05 g/cm3, and a dielectric constant of 2.3. 

Solution-coated, composite membranes. To prepare these membranes, one or more thin, dense polymer layers are solution coated onto the surface of a microporous support. 

Another important group of anisotropic composite membranes is formed by solution-coating a thin (0.5-2.0 xm) selective layer on a suitable microporous support. Membranes of this type were first prepared by Ward, Browall, and others at General Electric [52] and by Forester and Francis at North Star Research [17,53] using a type of Langmuir trough system. In this system, a dilute polymer solution in a volatile water-insoluble solvent is spread over the surface of a water-filled trough. 

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. 

Materials of interest include metals and alloys, semiconductors, ceramics and ionic solids, concrete, dielectrics and polymers, composites, biological materials including proteins and enzymes, membranes and coatings, aqueous and nonaqueous solvents and solutions, molten salts, catalytic materials, colloids, surfactants and inhibitors, and emulsions and foams. 

Fabrication of a thin film composite membrane is typically a more expensive route to reverse osmosis membranes because it involves a two-step process versus the one-step nature of the phase inversion film casting method. However, it offers the possibility of each individual layer being tailor-made for maximum performance. The semipermeable coating can be optimized for water flux and solute rejection characteristics. The microporous sublayer can be optimized for porosity, compression resistance and strength. Both layers can be optimized for chemical resistance. In nearly all thin film composite reverse osmosis membranes, the chemical composition of the surface barrier layer is radically different from the chemical composition of the microporous sublayer. This is a common result of the thin film composite approach. 

Strathmann prepared an all-polyimide composite membrane-both bottom and top layers.97 A microporous asymmetric film of the polyamic acid intermediate was cast by quenching in acetone, then dried and thermally cyclized to the polyimide at 300°C. The microporous polyimide sheet was then overcoated with a dilute solution of the same polymer, which was allowed to evaporate to give a 300-angstrom-thick coating. This was also cyclized to the polyimide to generate a fully solvent resistant reverse osmosis membrane. 

Asymmetric membranes are made from solution in the form of a hollow fiber, but the process used to form a thin, pore free dense layer on these hollow fibers is not disclosed.45 46 However, US patent 4,440,64312 describes a unique process for producing pore-free polyimide composite membranes. An asymmetric polyimide porous substrate is prepared from solution. When fully imi-dized, the substrate is insoluble. The substrate can now be coated with a poly-amic acid from dilute solution (— 1 %). When fully imidized, the resultant polyimide coating becomes the separating layer. This process allows use of the same or different polyimides for the substrate and the separating membrane. While the examples in the reference describe preparation of flat sheet membranes, this process could be used to prepare hollow fiber membranes. 

Silver-polymer complexes were prepared by dissolving a silver salt, AgCF3S03 or AgBF4, in a polar polymer, POZ or PVP. For the permeation experiments, an RK Control Coater was used to coat the silver-pol)mier complex solution (20 wt.% in water) onto a microporous membrane support to give composite membranes. The composite membranes were first dried at 40 °C for a day in a light-protected convection oven under nitrogen, then subsequently in a vacuum oven for a day at room temperature. The membrane samples were then cut into... 

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