Polyacrylonitrile reverse osmosis

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. 

Polyacrylonitrile (PAN) has been used in the preparation of UP membranes for a long time [82, 83] due to its superior resistance to hydrolysis and oxidation. PAN is highly crystalline and relatively hydrophilic and is usually copolymerized with more hydrophilic monomers to improve processability and to make it less brittle. Hollow fibers can be prepared from PAN dissolved in nitric acid [84]. Preparation of PAN membranes by phase inversion from solutions in DMAC, DMF or NMP is also possible. An example is shown in Fig. 4.4. A Sumitomo patent [85] discloses the preparation of membranes from copolymers containing 89% acrylonitrile and 11% ethyl acrylate dissolved in DMF and formamide and coagulated in water. A microporous membrane is obtained. In order to make the membranes suitable for reverse osmosis, they were submitted to a plasma treatment in the presence of 4-vinyl pyridine. 

Polyacrylonitrile (PAN) is a synthetic polymer with the hnear formula (C3H3N) . Although it is thermoplastic it does not melt under normal conditions, since it degrades before melting at above 300 °C. Almost all PAN resins are copolymers made from mixtures of monomers, with acrylonitrile as the main component. It is a versatile polymer used to produce a large variety of products, including fibers for textiles, ultra-filtration membranes, hollow fibers for reverse osmosis, etc. PAN fibers are the precursor in the production of high-quality carbon fibers. 

Polymeric membranes utilized for nano- and ultrafiltration applications are usually produced by the phase inversion process. Unlike in reverse osmosis, there is no continuous thin dense skin on the membrane surface. The membrane, however, has a tighter pore surface than the bulk of the membrane to allow the desired separation. The bulk of the membrane is much more open, to provide membrane support while limiting the resistance to water flux. Polymers employed in these membranes include polysulfone, poly(ether sulfone), polyacrylonitrile, poly(vinylidene fluoride), aromatic polyamides, sulfonated poly(ether sulfone), and cellulose acetate. 

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