Asymmetric membrane manufacture

The Loeb-Sourirajan process often is referred to as diffusion induced phase separation (DIPS) to reflect the role of diffusion in forming the asymmetric structure. Liquid-liquid phase separation and the resulting asymmetric structure arise from diffusion of a solvent (acetone) out of the film and diffusion of a nonsolvent (water) into the film. This physical interpretation provided the basis for the development of asymmetric membrane manufacturing processes for other polymer - solvent - non-solvent systems. 

These solvents include tetrahydrofuran (THF), 1,4-dioxane, chloroform, dichioromethane, and chloroben2ene. The relatively broad solubiHty characteristics of PSF have been key in the development of solution-based hoUow-fiber spinning processes in the manufacture of polysulfone asymmetric membranes (see Hollow-fibermembranes). The solvent Hst for PES and PPSF is short because of the propensity of these polymers to undergo solvent-induced crysta11i2ation in many solvents. When the PES stmcture contains a small proportion of a second bisphenol comonomer, as in the case of RADEL A (Amoco Corp.) polyethersulfone, solution stabiHtyis much improved over that of PES homopolymer. 

AG Thombre, JR Cardinal, AR DeNoto, SM Herbig, KL Smith. Asymmetric membrane capsules for osmotic drug delivery. I. Development of manufacturing process. J Controlled Release 57 55-64, 1999. 

The value of X found here appears to agree well with the values 5.0 and 2.3mequiv dm 3 absorbed water found by Oemisch and Pusch (4) by analytical and electro-chemical measurements respectively in cellulose acetate from a different manufacturer. The agreement is deceptive however because their value is based on the total water held in an asymmetric membrane. A value of M estimated from their data is almost ten times our M. It may be noted that their electrochemical method and their assumptions are quite different from ours. 

Two different RO membrane types were evaluated in this study. The first was a standard cellulose acetate based asymmetric membrane. The second type, a proprietary cross-linked polyamine thin-film composite membrane supported on polysulfone backing, was selected to represent potentially improved (especially for organic rejection) membranes. Manufacturer specifications for these membranes are provided in Table III. Important considerations in the selection of both membranes were commercial availability, high rejection (sodium chloride), and purported tolerance for levels of chlorine typically found in drinking water supplies. Other membrane types having excellent potential for organic recovery were not evaluated either because they were not commercially... 

Membranes manufactured by Spectrum Separations, Inc., a subsidiary of SEPAREX CORPORATION, are of the cellulose acetate type. They are similar to those made for reverse osmosis except they must be dried for gas separation use. A proprietary process is used to accomplish this so that the membrane does not collapse and lose its asymmetric character upon removal of the water. 

Thombre, A. G., Cardinal, J. R., DeNoto, A. R., Herbig, S. M., and Smith, K. L. (1999), Asymmetric-membrane capsules for osmotic drug delivery. Part I Development of a manufacturing process,/. Controlled Release, 57,55-64. 

Most UF membranes are asymmetric, having a thin separating layer or skin layer with small pores on one side of the membrane, and a much thicker layer with larger pores below the membrane which provides structural support with minimum flow resistance. Asymmetric membranes are manufactured by wet phase inversion casting. In this process, a casting solution of a polymer in a water-miscible solvent is spread in a thin layer onto a flat surface and then immersed in water. The water causes extraction of solvent and precipitation of the polymer as a porous flat sheet. The skin layer is formed on the upper surface that was in direct contact with water, and the underlying... 

These types of cellulose acetate composite membranes are of historical interest only. During the period when this research was done the composite membranes made using very thin cellulose acetate barrier layers (under 100 nm) looked attractive for their high flux properties. However, later optimization of the asymmetric cellulose acetate membrane process improved flux and, in general, outdistanced composite CA types for practical, low cost membrane manufacture. 

In the last few years, however, the outstanding development of single molecule manipulation and observation, particularly by fluorescence spectroscopy, has thrown some light on the operational mechanism of several biological machines. For example, it was shown that ATP synthase, the enzyme that manufactures adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (Pi) is powered by a rotary motor fueled by a proton gradient (Fig. 1). The asymmetric membrane-spanning Fq portion of the enzyme contains a proton channel, and the soluble F] portion contains three catalytic sites that cooperate in the... 

Thombre AG, Cardinal JR, DeNoto AR, Herbig SM, Smith KL. Asymmetric membrane capsnles for osmotic drug delivery. I— Development of a manufacturing process. J. Control. Release 1999 57 55-64. 

Porons membranes contain interconnected homogeneous sized pores. Symmetric porous membranes thus may present either a high permeability or a high separation factor towards small molecules, but not both. This drawback was overcome with the development of asymmetric membranes, in which there is a continuous shift in the pore size in one direction. Several subtypes are manufactured [168]. 

Although TEC membranes are widely used in today s market, several significant advantages of asymmetric membranes have kept them competitive. With respect to the manufacturing cost, asymmetric membrane is relatively cheaper than composite membrane. Furthermore, the surface properties of asymmetric NF are also less sensitive to chlorine disinfectants, which minimize the chances of surface deterioration such as that encountered with composite membranes. 

Solubility of the three commercial polysulfones follows the order PSF > PES > PPSF. At room temperature, all three of these polysulfones as well as the vast majority of other aromatic sulfone-based polymers can be readily dissolved in a handful of highly polar solvents to form stable solutions. These powerful solvents include NMP, DMAc, pyridine, and aniline. 1,1,2-Trichloroethane and 1,1,2,2-tetrachloroethane are also suitable solvents but are less desirable because of their potentially harmful health effects. In addition to being soluble in the aforementioned list, PSF is also readily soluble in a host of less polar solvents by virtue of its lower solubility parameter. These solvents include tetrahydrofuran (THF), 1,4 dioxane, chloroform, dichloromethane, and chlorobenzene. The relatively broad solubility characteristics of PSF have been key in the development of solution-based hollow-fiber spinning processes in the manufacture of polysulfone asymmetric membranes (see Membrane Technology). The solvent list for PES and PPSF is short because of the propensity of these polymers to undergo solvent-induced crystallization in many solvents. When the PES structure contains a small proportion of a second bisphenol comonomer, as in the case of RADEL A (British Petroleum) polyethersulfone, solution stability is much improved over that of PES homopolymer. 

Dialysis membranes must be soaked overnight prior to use. Most membrane manufacturers sell prehydrated packed specimens. Some membranes exhibit an asymmetric response, i.e. their transfer efficiency change depending on the side where the transfer originates. 

There seems to be a historical reason for the difference in these two approaches. The solution-diffusion approach was established for the permeation of liquid and gas through the membrane before reverse osmosis membranes of practical usefulness were developed by the phase-inversion technique. Dense membranes without asymmetricity were prepared from polymeric materials, and their transport properties were measured, assuming that the membranes were defect-free and the transport parameters so produced were the values intrinsic to the material. The membrane with the highest separation capacity for a given polymer was believed to be that which could exhibit the transport properties intrinsic to the polymer. The goal of membrane production engineering was to ensure the membrane intrinsic transport properties of the polymeric material. This approach is still popular in the membrane manufacturing industry. 

MV growth is an example of several questions that can be addressed by membrane processing in low gravity. It is quite possible that entirely new membrane structures may be created in low-g. Indeed, such low-g studies may lead to modified manufacturing practices that can be used to control asymmetric membrane morphologies. 

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