Microporous polysulfone

Figure 3a. SEM photomicrographs of composite membranes surface structure of microporous polysulfone support material.

More recently, composite membranes have been made by interfacial polymerization or by in situ polymerization A representative case is illustrated in F. 8. Here, a microporous polysulfone membrane is used as a substrate. This membrane is soaked in a dilute aqueous solution of a low molecular weight polyethylenimine (PEI). Without drying, this membrane is then contacted with a crosslinking agent such as toluene diisocyanate (TDI) or isophthaloyl chloride dissolved in hexane, after which the membrane is cured in an oven. A highly crosslinked, salt-rejecting interfacial layer is formed in this way. A summary of the properties of three of the more important composite membranes is presented in Table 10. 

Klein E, Eichholz E, and Yeager D. Affinity membranes prepared from hydrophiUc eatings on microporous polysulfone hollow fibers, J. Membr. Sci. 1994 90 69-80. 

The spiral wound membranes tested for extraction of impurity-free NaSCN from aqueous process solution were polyamide (PA-300), CTA-700, PERMA-400, and PERMA-250. PA-300 was prepared by interfacial polymerization technique, while the PERMA membranes were prepared by coating a novel proprietary copolymer onto a microporous polysulfone substrate followed by cross-linking of the top layer. Thus, the morphology of these membranes was TFC. CTA-700 was asymmetric in nature and was prepared by solution casting and phase inversion method. 

Figure 9.2 shows the scanning electron microscopic (SEM) image of a cross-section of the membrane on the GE E500Amicroporous polysulfone support. It can be seen that the membrane consisted of two portions. The top portion was a dense active layer, which provided separation, and the bottom portion was a microporous polysulfone support, which provided mechanical strength. This composite structure minimizes the mass transfer resistance while maintaining sufficient mechanical strength. 

Figure 9.2. SEM image of a cross-section of the membrane synthesized (on GE E500A microporous polysulfone support).

Grant No. 342-0561), and the Ohio State University. We would like to thank Chris Plotz and BHA Technologies, and Debbie de la Cruz and GE Infrastructure for giving us the BHA microporous Teflon support and GE E500A microporous polysulfone support, respectively, used in this work. 

A schematic diagram of a typical commercial composite membrane is presented in Figure 1. The microporous polysulfone support film is cast on a woven or nonwoven backing material, usually made from polyester fibers. The polysulfone support is approximately 50 urn (two mils) in thickness. About half of this thickness sits above the polyester backing material, and about half of it is embedded in the fibrous carrier web. The thickness of the applied barrier layer may range from 20 to over 500 nm, depending upon the composition of the barrier layer. 

The initial microporous support films used in the work were made from cellulose acetate by a modification of the Loeb-Sourirajan procedure. Later work showed that several types of the membrane filters manufactured by Millipore Corporation and Gelman Sciences, Inc., performed as well and allowed higher flux. A continued search for a more compression-resistant support film led to the development of polycarbonate, polyphenylene oxide and polysulfone microporous films in 1966 to 1967 (8). Of these, microporous polysulfone film proved to have the best properties. The polysulfone support was made by casting a liquid layer of a 12.5 to 15 percent solution of Union Carbide Udel P35OO polysulfone in dimethylformamide onto a glass plate at 4 to 7 mils (100-175 pm) thickness, then coagulating the film in water. 

Research. It is formed by interfacially crosslinking a monomeric amine on a microporous polysulfone support membrane. 

In the fall of 1966, researchers at North Star Research Institute began a search for compression-resistant microporous substrates.19 This effort resulted in the development of microporous sheets of polycarbonate (Lexan) and poly-sulfone (Udel).20 Figure 5.4 shows a graph comparing the flux levels and flux stability for three membranes made at that time (a) float-cast cellulose acetate on microporous polysulfone, (b) float-cast cellulose acetate on a mixed cellulose ester microfilter support and (c) a standard asymmetric cellulose acetate membrane. The improvement in membrane fluxes was readily apparent, when switching from cellulosic substrates to the microporous polysulfone substrate. 

Figure 5.5 Cross section and surface of a microporous polysulfone sheet used in composite reverse osmosis membranes (a) total cross section of a polysulfone sheet cast on a nonwoven polyester fabric, then delaminated prior to freeze-fracture for SEM (note fiber trecks on backside of the sheet) (b) backside of sheet showing cellular structure, which extends through 85% of the sheet thickness (c) transition region from cellular to nodular structure near film surface (d) dense nodular structure at the surface (e) high magnification of the extreme top surface cross section (f) high magnification view of the surface structure showing tha texture of the top surface.

The NS-100 membrane (initially designated as NS-1) was the first noncellu-losic composite membrane to appear in the published literature and have an impact on the reverse osmosis scene.22/23 This membrane, invented by Cadotte,24 consisted of a microporous polysulfone sheet coated with polyethylenimine, then interfacially reacted with either 2,4-toluenediisocyanate (TDI) or with isophthaloyl chloride (IPC). In the first case, a polyurea is formed in the second case, a polyamide. The chemistry of this membrane is as follows ... 

The initial studies by Cadotte on interfacially formed composite polyamide membranes indicated that monomeric amines behaved poorly in this membrane fabrication approach. This is illustrated in the data listed in Table 5.2, taken from the first public report on the NS-100 membrane.22 Only the polymeric amine polyethylenimine showed development of high rejection membranes at that time. For several years, it was thought that polymeric amine was required to achieve formation of a film that would span the pores in the surface of the microporous polysulfone sheet and resist blowout under pressure However, in 1976, Cadotte and coworkers reported that a monomeric amiri piperazine, could be interfacially reacted with isophthaloyl chloride to give a polyamide barrier layer with salt rejections of 90 to 98% in simulated seawater tests at 1,500 psi.4s This improved membrane formation was achieved through optimization of the interfacial reaction conditions (reactant concentrations, acid acceptors, surfactants). Improved technique after several years of experience in interfacial membrane formation was probably also a factor. 

Several composite membranes have been prepared based on sulfonated polymers. These are typically formed on microporous polysulfone supports by solu-... 

When furfuryl alcohol was added as a comonomer to the THEIC, water fluxes were increased tenfold. In addition, the extremely high salt rejections characteristic of NS-200 were obtained, while the high organic rejections characteristic of the isocyanurate moiety were retained. A typical patent example of membrane fabrication uses a water solution of 1 2 2 1 weight percent THEIC fur-furyl alcohol sulfuric acid dodecyl sodium sulfate, deposited on microporous polysulfone and cured at 150°C for 15 minutes. This membrane, possessing a thin active layer 100 to 300 angstroms thick, showed 99.9% rejection and 12 gfd flux under seawater test conditions at 1,000 psi. 

Knoell T., Safank J., Cormack T., Riley R., Lin S.W., Ridgway H. (1999), Biofouling potentials of microporous polysulfone membranes containing a sulfonated polyether-ethersulfone/polyethersulfone block copolymer correlation of membrane surface properties with bacterial attachment, J Membrane Science, 157, 117-138. 

Sulfonated polysulfone seems to also play an important role in nanofiltration and reverse osmosis membranes commercialized by Desal. According to Petersen [34] the Desal-5 membrane appears to consist of three layers a microporous polysulfone, a sulfonated overlay and a top ultrathin layer based on polypipera-zineamide. 

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