Materials Science of Synthetic Membranes - American Chemical Society

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Microporous Membranes via Upper Critical Temperature Phase Separation 1

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W. C. ΗΙΑΤΤ , G. H. VITZTHUM , Κ. B. WAGENER , K. GERLACH , and C. JOSEFIAK 1

Membrana Research Department, Akzona Inc., American Enka Research, Enka, NC 28728 AKZO Research Institute, Obernburg, Federal Republic of Germany

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Flat stock microporous membranes can be made using a variety of polymer types by dissolving the polymer in a solvent at an elevated temperature, followed by casting this solution on a temperature controlled flat surface, provided that the system, polymer/solvent, shows a miscibility gap. Extraction of the solvent, called "pore former", is done with low molecular weight alcohols. Decidedly different pore structures can result depending upon, among other factors, polymer concentration and rate of solution cooling. Pore dimensions can be varied widely. The pore structures have been well characterized, and these data are presented. The l i t e r a t u r e describes numerous manufacturing methods f o r syn thetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from m i c r o f i l t r a t i o n to d e s a l i nation to gas separation. In t h i s paper, a thermal phase-separation technique of preparing membranes i s presented. The method i s a development of an invention described i n US Patent 4,247,498 by Anthony J . Castro (2). This technique i s similar i n many respects to the c l a s s i c a l phase-inversion methods; however, the additional consideration of thermal s o l u b i l i t y c h a r a c t e r i s t i c s of the poly mer/solvent p a i r offers new p o s s i b i l i t i e s to membrane production. W. Henné, M. Pelger, K. Gerlach, and J . T r e t z e l (3) described the same process of dissolving polypropylene i n an amine for producing microporous hollow f i b e r s to be used i n plasmapheresis. The development of solvent resistant commercial microporous membranes v i a thermal phase separation began with an unexpected laboratory discovery. The early workers were searching for a way to make a concentrate of an a n t i - s t a t i c agent i n polypropylene that 3

Current address: Department of Chemistry, University of Florida, Gainesville, FL 32611.

0097-6156/85/0269-0229$06.00/0 © 1985 American Chemical Society In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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would be easier to handle. In doing so, they discovered that a single-phase solution could be formed at high temperatures, but, that as the solution cooled, a two-phase system developed. Thermal phase separation had occurred. The resulting structure of the polymer matrix appeared to be dependent on a number of controllable process variables including polymer concentration, solution temperature, and cooling rate. Of these, the most s i g n i f i c a n t variable for c o n t r o l l i n g membrane structure i s the cooling rate. This discovery of thermal phase separation led to commercialization i n two main areas: controlled release and m i c r o f i l t r a t i o n . Flat stock, hollow f i b e r , and hollow-tube membranes are a l l produced commercially using this technology. This paper covers several aspects of the membrane manufacturing process, membrane s t r u c t u r a l c h a r a c t e r i s t i c s , and current a p p l i c a tions. In addition, this paper b r i e f l y discusses applications of the manufacturing process to two polymers and a v a r i e t y of solvents. Phase Separation i n Membrane Formation Broens, Altena, and Smolders (4) describe the t h e o r e t i c a l considerations for phase separation from concentrated polymer solutions by chemically induced means. The authors discuss three separate phenomena: l i q u i d - l i q u i d phase separation, c r y s t a l l i z a t i o n of the polymer, and gelation. They also address phase separation for some c l a s s i c a l polymer/solvent/non-solvent systems; s p e c i f i c a l l y , systems using cellulose acetate, polysulfone, p o l y a c r y l o n i t r i l e or p o l y d i methylphenyleneoxide are considered. In t h e i r discussions, extensive use i s made of ternary phase diagrams similar to that schemati c a l l y represented i n Figure 1. Key features are the equilibrium phase curve or binodal, the spinodal, the stable one-phase region, the metastable region between binodal and spinodal, and the instable two-phase region. A more detailed discussion of such ternary diagrams i s presented by Strathmann (5). Broens, et a l , describe two types of l i q u i d - l i q u i d phase separation: nucleation followed by growth and spinodal decomposit i o n . The nucleation and growth process i s i n i t i a t e d by a small amount of nonsolvent entering the homogeneous polymer solution which causes the l o c a l i z e d concentration to move from the single-phase region across the binodal curve. A nucleus i s formed which contains almost no polymer. This nucleus grows, and the polymer-rich regions surround the polymer-poor regions, ultimately r e s u l t i n g i n a c e l l u l a r structure. In the metastable region between the binodal and spinodal curves, phase separation has to occur by the mechanism of nucleation and growth. In t h i s region, the one-phase-state i s indeed stable against small concentration fluctuations but unstable against separation into two phases of more d i f f e r e n t concentrations. Phase transformations i n one-component systems l i k e condensation, evaporat i o n or s o l i d i f i c a t i o n as well as the c r y s t a l l i z a t i o n of solutes from solvents occur by the nucleation and growth mechanism. The w e l l known phenomena of oversaturation and hindered-phase transformation can be explained by discussing the nucleation as an e q u i l i brium reaction with the creation of the " c r i t i c a l nucleus" (6, 7).

In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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10.

Figure 1.

C l a s s i c a l Binodal/Spinodal Phase Diagram f o r Polymer/Solvent/ Non-Solvent System


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In low molecular weight systems, no hindrance of phase separation during liquid-liquid-demixing could occur by the spinodal mechanism. Spinodal decomposition i s the second mechanism of l i q u i d - l i q u i d phase separation. Here, the solution spontaneously separates into interconnected regions of high and low polymer concentration with intertwined networks. The authors report that for chemically induced systems (polymer/solvent/nonsolvent), the nucleation and growth k i n e t i c s are much too rapid to permit spinodal decomposition. It i s possible that the mechanism of the upper c r i t i c a l temperature phase-separation process i s d i f f e r e n t from c l a s s i c a l l i q u i d l i q u i d phase separation as described by Broens, et a l (4). Class i c a l phase separation i s induced by nonsolvent d i f f u s i n g into the homogeneous polymer solution, thus inducing l o c a l composition changes and causing a s h i f t from the one-phase to the two-phase area of the phase diagram. D i f f u s i o n through a polymer solution i s r e l a t i v e l y slow compared with the nucleation and growth k i n e t i c s which predominate i n the metastable region of the phase diagram. In the upper c r i t i c a l temperature phase-separation process, the spinodal curve can be rapidly passed since l i q u i d - l i q u i d phase separation i s induced by thermal rather than chemical means. Two major types of structure can occur, designated Type I and Type I I , whereby the primary determining factor i n the development of these two structures i s the rate of cooling. At r e l a t i v e l y low cooling rates, the Type I structure results as shown i n Figure 2. Type I materials are characterized by a polymeric matrix of i n t e r connecting pores or passageways with pores ranging i n size from 10 micrometers to approximately 0.02 micrometers. Type II structure develops at higher cooling rates (Figure 3) and has a lacy appearance. Both of these structures are characterized by a high s p e c i f i c surface area, 90 m /g, which i s almost independent of solvent concentration. The void volume i s approximately equal to the solvent-volume f r a c t i o n i n the solution. The Type I structure i s believed to be the result of binodal decomposition by the nucleation and growth phenomena r e s u l t i n g from r e l a t i v e l y low cooling rates, whereas Type II structure i s a product of r e l a t i v e l y high cooling rates and i s thought to be the product of true spinodal decomposit i o n . This difference i s important because the nucleation and growth structure (Type I) i s characterized by the membranes having extremely low a i r or l i q u i d flows due primarily to the r e l a t i v e l y small passageways between the open c e l l s . The polymer/solvent combination polypropylene/N,N-bis-(2-hydroxyethyl) tallow amine has received the most attention i n these investigations. The amine and polypropylene form a complete solut i o n at temperatures above the melting point of the polypropylene. Upon cooling, the mixture undergoes a complete phase separation on a microscopic l e v e l wherein each of the two phases i s e s s e n t i a l l y free of the other component. The s o l i d phase i s a l l polypropylene, and the l i q u i d phase i s a l l amine; thus, the amine solvent can subsequently be extracted leaving a microporous structure. As mentioned, the c l a s s i c a l description of a polymer/solvent system uses a ternary diagram with a nonsolvent as the t h i r d component. This representation i s v a l i d at a constant temperature. In the case of thermally induced phase separation, t h i s description i s 2



10. HIATT ET A L .

Figure 2.

Figure 3.


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Type I "Open Cell"Structure of Polypropylene Formed at Low Cooling Rates (2400X)

Type II "Lacy" Structure of Polypropylene Formed at High Cooling Rates (2000X)


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expanded to include a temperature axis of the diagram. Thus, a family of curves can be developed to describe a system. In practice i t i s most useful to maintain a constant nonsolvent concentration, often zero; so c o n t r o l l i n g solution concentration, temperature, and cooling rate becomes the means of achieving the desired structure and properties. Figure 4 shows a representation of these concepts.


Membrane Properties Membrane preparation consists of three major steps: solution preparation, membrane formation or casting, and aftertreatment. Solution preparation can be either batch or continuous. Uniformity of feed materials as well as process conditions are extremely c r i t i c a l . The process can be described further using the simple flow sheet as shown i n Figure 5. The solvent i s added to a large j a c keted vessel and heated to the needed temperature, then polymer i s added slowly and the mixture i s agitated under a nitrogen blanket u n t i l solution i s achieved. The solution i s f i l t e r e d and then cast onto a water-cooled r o l l (known as the c h i l l r o l l ) . The membrane i s taken up and trimmed on conventional film-winding equipment, f o l lowed by extraction and inspection. A number of solvents, such as methanol, ethanol, isopropanol, and methylene chloride, are suitable for extraction, which i s accomplished i n a straightforward multistation fashion. Inspection consists of using a laser device capable of detecting and marking the location of pinholes, t h i n spots, d i r t y areas, etc. From the foregoing description, i t becomes apparent that several aspects of t h i s process (8) are important (Table I ) . The r a t i o of the weight of amine to the weight of polypropylene i s c r i t i c a l , as further reduction i n the amount of polypropylene r e s u l t s i n a membrane with poor mechanical properties, such as a low burst strength (measured as burst pressure). If the amount of polypropylene i s increased too much, the result may be unacceptably low a i r and/or water flow rates. As mentioned above, the cooling rate of the solution of amine and polypropylene i s c r i t i c a l inasmuch as i t a f f e c t s the structure type and the maximum pore size of the membranes. A rapid cooling rate generally produces a small pore s i z e , and a slower cooling rate r e s u l t s i n a larger pore s i z e . As the cooling rate i s related to the length of time i t takes the solution to s o l i d i f y a f t e r the phase separation has occurred, the primary t o o l i n c o n t r o l l i n g the cooling rate i s the temperature of the c h i l l r o l l . However, i f the temperature of the c h i l l r o l l i s too low, the result w i l l be a substantial formation of skin on the membrane side i n contact with the c h i l l r o l l surface. A skin i s simply a region within the membrane which has an apparent polymer density d i f f e r e n t from that of the remainder of the membrane. If the c h i l l r o l l temperature i s too warm, proper s o l i d i f i c a tion may not occur i n time to remove a s o l i d i f i e d sheet from the c h i l l r o l l on a continuous basis. Also, as stated, a warm c h i l l r o l l w i l l r e s u l t i n a slower cooling rate and a possibly too large pore diameter.



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SOLUTION PREPARATION

LASER INSPECTION

CASTING ROLL

EXTRACTION AND AFTERTREATMENT

SOLVENT

Figure 5.

Simplified Process Flowsheet



3

2

C h i l l Roll Temperature i n °C 60 65 78 75 60 62 70 70 Thickness in Micrometers 140 140 190 168 86 165 173 81 1

Maximum Pore S i z e in Micrometers 0.46 0.58 0.77 0.54 0.27 0.35 0.37 0.34 129 101 154 162 83

(14.7) (22.4) (23.6) (12.0)

(18.8)

2

Burst Pressure kPa (psi) 77 (11.2)

Determined by the bubble point method (8) (adapted from ASTM Method F361). Determined by measuring the maximum a i r pressure (8) when the membrane bursts See (8).

Solution Composition Amine/Polypropylene i n Percent 79/21 79/21 79/21 75/25 70/30 70/30 70/30 70/30

3

2

A i r Flow at L/min cm @ 69 kPa (10 p s i ) 0.96 1.86 1.64 1.16 0.87 0.51 0.51 1.10

Table I. Processing Conditions and Properties of Microporous ACCUREL Polypropylene Membranes


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The thickness of the membrane, of course, a f f e c t s the flow rate properties of the membrane: the thicker the membrane, the lower the flow rate. However, a thin membrane can have poor mechanical properties such as a low burst strength.


Microporous Polypropylene Membranes The products of the thermal phase-separation membranes form a wide range of styles and configurations. Three pore sizes are currently i n commercial production i n polypropylene f l a t stock, rated at 0.45, 0.2, and 0.1 micrometers, having maximum pore sizes of about 1.0, 0.55, and 0.3 micrometers, respectively. The l a s t two are p a r t i c u l a r l y a t t r a c t i v e for depyrogenation work which has been described by J. R. Robinson, et a l (9). A similar membrane-manufacturing process i s also used for making hollow f i b e r s and tubes which are especially useful i n cross-flow applications (10) and plasmapheresis (3). Polypropylene membranes of this type can be used where the chemical resistance of polypropylene and tight control of pore size are needed. These polypropylene membranes, known as ACCUREL, are more chemically resistant than the remaining polypropylene f i l t e r cartridge components due to a proprietary process used during membrane manufacture and have replaced poly(tetrafluoroethylene) membranes i n many applications. Though polypropylene membranes do have many applications, a need also exists for microporous membranes that have a higher degree of chemical and heat resistance than polypropylene while s t i l l retaining the high flow rates and good s e l e c t i v i t y of these membranes. Microporous membranes of polymers such as p o l y t e t r a fluoroethylene (PTFE) are available. However, such membranes have t y p i c a l l y been made by sintering processes, and the r e s u l t i n g membranes have lacked the good uniformity and flow rates needed i n m i c r o f i l t r a t i o n applications. Microporous Polyvinylidene Fluoride Membranes Polyvinylidene fluoride (PVDF) possesses many desirable properties as a polymer. For example, i t has a reasonably high melting point and good temperature resistance. I t i s highly resistant to oxidation and to the effects of gamma radiation. Due to i t s cryst a l l i n e nature, i t offers a satisfactory degree of solvent r e s i s tance. I t s oxidation resistance makes these membranes reusable by cleaning them with aqueous sodium or calcium hypochlorite or hydrogen peroxide. Also, PVDF membranes t y p i c a l l y possess superior ageing resistance as w e l l as good resistance to abrasion. For these reasons, i t i s desirable to have microporous PVDF membranes which also possess suitable properties f o r f i l t r a t i o n applications. Several methods for producing microporous PVDF membranes are disclosed i n the patent l i t e r a t u r e . U.S. Patent No. 3,642,668 (11) discloses a method for making a microporous membrane from polyvinylidene f l u o r i d e by heating and dissolving the polymer i n d i methylsulfoxide or dimethylacetamide. The solution i s cooled, coated onto a glass plate using a doctor blade, immersed i n methanol, and f i n a l l y a i r dried. Thus, a chemically induced phase-



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separation process i s u t i l i z e d . U.S. Patent No. 3,518,332 (12) discloses a method for making microporous polymer sheet material from PVDF. In t h i s process a mixture i s formed of p a r t i c l e s of the polymer, a leachable p a r t i c u l a t e metallic s a l t , and a p a r a f f i n wax. A sheet i s formed from the mixture, the wax removed with a solvent, and the sheet heated to sinter the PVDF p a r t i c l e s . Subsequently, the metallic s a l t pore-former i s removed with a leaching solvent. Japanese Patent No. 51-6268 (13) discloses a method for making porous structures of PVDF with a closely controlled solution (or gel) process with cyclohexanone. Solutions (or gels) of ten to t h i r t y percent polymer are cast at elevated temperatures during a non-equilibrium state. The patent states that when the solution i s prepared by heating, the v i s c o s i t y of the system increases with time, reaches a maximum value, and then gradually decreases u n t i l an equilibrium value i s reached. I t i s c r i t i c a l that the casting takes place during t h i s period of descending v i s c o s i t y . The patent states that upon drying, a g e l r e s u l t s , from which the cyclohexanone can be removed by evaporation or extraction. None of the membranes made by these processes have found substantial commercial success, and the need f o r a suitable PVDF microporous membrane continues to e x i s t . Microporous membranes of PVDF-containing polymers can be made using the upper c r i t i c a l temperature phase-separation technique. Either a homopolymer of vinylidene f l u o r i d e or a copolymer of vinylidene fluoride with copolymerizable monomers, such as v i n y l f l u o r i d e , tetrafluoroethylene, or trifluoroethylene chloride have been found useful. T y p i c a l l y the copolymerizable content w i l l be less than about 15% by weight. Compatible solvents which are useful i n t h i s process include cyclohexanone, butyrolactone, propylene carbonate, and c a r b i t o l acetate. These l i q u i d s are known to be latent solvents for polyvinylidene f l u o r i d e , latent solvents being defined as l i q u i d s which do not dissolve or substantially swell the polymer at room temperature. Other compounds which are known as being latent solvents for PVDF include butyl acetate, diacetone alcohol, d i i s o b u t y l ketone, tetraethyl urea, isophorone, t r i e t h y l phosphate, and dimethyl phthalate. Tetramethyl urea i s also believed to be a latent solvent. PVDF i s a t y p i c a l example f o r polymers with r e l a t i v e l y low molecular weights, resulting i n r e l a t i v e l y low v i s c o s i t i e s of polymer solutions needed for membrane production. Because of the lower v i s c o s i t i e s of these polymer solutions, phase separation occurs faster than i n the higher molecular weight polypropylene solutions. For a given cooling rate predetermined by the process, the solvents used f o r PVDF have to show a small temperature range between l i q u i d - l i q u i d demixing and s o l i d i f i c a t i o n of the polymer phase. Choosing d i f f e r e n t solvents usually means choosing d i f f e r e n t temperature gaps between the beginning of phase separation and s o l i d i f i c a t i o n . The f i n a l membrane structure i s e s s e n t i a l l y determined when the s o l i d i f i c a t i o n temperature i s passed during cooling. This s o l i d i f i c a t i o n temperature i s e a s i l y determined experimentally. To achieve comparable structures and pore sizes with the high molecular weight polypropylene, solvents must be chosen which provide a remarkably broader temperature range between phase separation and s o l i d i f i c a t i o n . Because the f i n a l membrane properties,


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especially a i r and water flows, are strongly dependent on polymer concentration (flows decrease with increasing polymer concentration) , the v i s c o s i t y of the PVDF solutions cannot be "corrected" by increasing the PVDF concentration. Depending upon the concentration of the polymer i n solution and upon how quickly the solution i s cooled, one of two membrane structures may be formed, each being c l e a r l y d i s t i n c t from the Type I and Type II structures found for polypropylene (14).


Table I I .

Experimental Data for Forming Polyvinylidene Fluoride Microporous Membranes

Solvent Cyclohexanone Butyrolactone Propylene Carbonate C a r b i t o l Acetate

Polymer Concentration 20 to 40 30 to 50 20 to 40 10 to 50

(%)

Temperatures (°C) Solution/Casting Structure Type Roll III 185/RT III 164/RT III 165/RT III & IV 180/19-60

Type I I I (Figures 6 and 7) membrane consists of a r e l a t i v e l y tight skin supporting spheres of polymer, whereas Type IV (Figures 8 and 9) contains "leaves" of polymer stacked upon a more open skin. Depending upon the cooling conditions and the polymer concentration, the same membrane can show a t r a n s i t i o n between Type III and Type IV structures within i t s cross-section (Figures 10 and 11). Typical membrane properties are found i n Table I I I . Table I I I .

Membrane Properties for Two Types of PVDF Membranes

Property Maximum Pore Size, Micrometers Water Flow Rate, mL/min cm @ 69 kPa (10 psi) 2

Type I I I 0.2 - 0.5 0.2 - 0.7

Type IV - 2.0 1.0 20 - 50

Structure Type I I I , although quite i n t r i g u i n g i n i t s appearance, has l i t t l e to o f f e r i n terms of being a membrane because of the tight skin that exists on the surface. Flow properties are too low to be useful. However, the presence of so many spheres on the surface lends to the suggestion that fine powders could be generated from the membrane, powders that might be useful i n controlled release applications. Type III structures are created at higher polymer concentrations and with slower cooling rates; experimental conditions which promote nucleation and thus growth of polymer i n the s o l i d state from solution. Using c a r b i t o l acetate as the solvent, Type III structures could be generated either by using a polymer concent r a t i o n minimum of 30% or by maintaining the c h i l l - r o l l temperature above 40°C. Variations i n skin thickness were present when changing the casting r o l l temperature with an apparent increase i n thickness occurring when lowering the temperature.



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Figure 6.

Type I I I Structure of Polyvinylidene Fluoride Membrane Formed at Low Cooling Rates and Higher Polymer Concentrations (503X)

Figure 7.

Type I I I Structure of Polyvinylidene Fluoride Membrane at Higher Magnification (5,000X; compare Figure 6)



10.

HIATTETAL.

Figure 8.

Figure 9.


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Type IV Structure of Polyvinylidene Fluoride Membrane Formed at Higher Cooling Rate and Low Polymer Concentration (Split Image Photograph to Show Both Edges; 503X)

Type IV Structure of Polyvinylidene Fluoride Membrane at Higher Magnification (5,000X; compare Figure 8)



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Figure 10. Transition Between Type I I I (Spherical) and Type IV (LeafLike) Structure of Polyvinylidene Fluoride (252X)

Figure 11. SEM Picture of Skin of Polyvinylidene Membrane Shown i n Figure 10 (503X; Right Side)



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Type IV structures were formed from c a r b i t o l acetate solutions by maintaining a cold c h i l l r o l l (less than 35 °C ) and keeping the polymer concentration below 30 weight %. This process i s demonstrated i n the following example, A homogeneous solution of 20 weight % PVDF and 80 weight % c a r b i t o l acetate i s prepared i n a r e s i n k e t t l e at 150°C i n a dry atmosphere. This solution can be formed into a microporous embodiment simply by allowing i t to cool. Microporous membranes can be made by feeding the solution into a heated casting box. (Membranes can also be made by using a heated doctor blade and glass plate arrangement.) The solution i s then cast onto a r o l l maintained at room temperature. The membrane can be removed from the r o l l and has s u f f i c i e n t mechan i c a l strength for a l l needed handling. The membrane i s extracted with isopropyl alcohol and has the following properties: Bubble Point (IPA) Maximum Pore Size Water Flow A i r Flow

43 kPa (6.3 p s i ) 1.4 micrometers 30.4 mL/min cm @ 69 kPa (10 p s i ) 2.5 L/min cm @ 69 kPa (10 p s i ) 2

2

Summary The exploration of the thermal phase-separation process has added a new dimension to the science of microporous membranes. New and useful products can be made from a large spectrum of polymers ranging from hydrophobic polymers, l i k e polypropylene or PVDF, to hydrophilic ones, l i k e nylon which we have not touched upon i n t h i s article. Acknowledgments The authors would l i k e to thank Dr. T. Castro and Dr. D. Frank of Akzo Chemie America, Dr. D. Lloyd from the University of Texas, and Dr. C. Smolders from TNO, The Netherlands, for t h e i r suggestions to this work and Mrs. Ruth Sherlin and Mrs. Cathey T u r b y f i l l f o r patiently preparing t h i s manuscript. Literature Cited 1. 2. 3.

4. 5. 6. 7.

Pusch, W.; Walch A. Angew. Chem. Int. Ed. Engl. 1982, 21, 660-685. Castro, A. J . US Patent 4,247,498, 1981. Henne, W.; Pelger, M.; Gerlach, K; and Tretzel, J . in "Plasma Separation and Plasma Fractionation," M. J. Lysaght and H. J. Gurland, Eds;, S. Karger AG: Basel, 1983; pp. 164-179. Broens, L.; Altena, F. W.; Smolders, C. A. Desalination, 1980, 32, 33-45. Strathmann, H. in "Material Science of Synthetic Membranes", D. R. Lloyd, Ed.; American Chemical Society: Washington, DC. Volmer, Μ., "Kinetik der Phasenbildung," Steinkopff: Dresden und Leipzig 1939. Zettlemayer, A. C . , (Ed.); "Nucleation," M. Decker, New York 1969.


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8. 9. 10.


11. 12. 13. 14.

Davis, M. Α.; Vitzthum, G. H. US Patent Application. Robinson, J . R.; Barnes, T . ; Genovesi, C . ; O'Dell, M.; Takacs, J . "Task Force Report on Depyrogenation," Parenteral Drug Association, No. 1982, Chap. 6. Klein, W. Filtration & Separation, March/April 1982, 130. Schneider, K. and Klein W. Desalination, 1982, 41, 263-275. Bailey, J . L.; McCune, R. F. US Patent 3,642,668, 1972. Sklarchuk, J . C . ; Kobie, W. F. US Patent 3,518,332, 1970. Murayama, Nahiro; et a l . Japanese Patent 51-6268, 1976. Coggins, M. D.; Vitzthum, G. H . ; Wagener, Κ. B. US Patent Application.

RECEIVED July 16, 1984


Materials Science of Synthetic Membranes - American Chemical Society

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