JPH0576331B2 - - Google Patents

Description

[Detailed description of the invention]

BACKGROUND OF THE INVENTION Technical Field The present invention relates to filtration membranes. These membranes can be used to produce particle and bacteria free water or solutions. Such membranes are particularly useful in the electronics and pharmaceutical industries. BACKGROUND OF THE INVENTION In aqueous microfiltration, for example in the production of particle-free water or solutions in the electronics and pharmaceutical industries, it is highly desirable to have membranes with as little leachable material as possible. It is also usually desirable that the membrane be easily wetted by water or an aqueous solution. Therefore, membranes that are inherently wettable are often preferred over hydrophobic membranes that are post-treated with a wetting agent. That is, it is common to produce hydrophilic membranes by adding a wetting agent, such as a surfactant, to a preformed hydrophobic membrane. However, when using treated hydrophobic membranes it is always possible that the wetting agent can be leached by the filtered solution. Such leaching can also result in contamination of the filtrate. Currently, there are very few inherently wettable membranes recognized. Generally these membranes are nylon membranes. An example of such a membrane is U.S. Pat. No. 3,876,738 to Marinoccio et al.
issue (issued April 8, 1975) and Pall
No. 4,340,479 issued July 20, 1982. Polymers that are highly hydrophilic in nature are difficult to manufacture or have other disadvantages. Therefore, cellulose acetate membranes are limited in their thermal and hydrolytic stability. Nylon membranes must be made in solvent systems that are hazardous and difficult to handle. Polymers that are more hydrophilic, such as polyvinyl alcohol, polyvinylpyrrolidone, etc., are difficult or impossible to coagulate under practically possible conditions solely due to their high affinity for water. Therefore, it is generally not convenient to produce hydrophilic membranes from highly hydrophilic polymers. Therefore, the ideal polymer for forming microfiltration membranes is one that is hydrophobic in nature to facilitate membrane formation and provide the desired mechanical and thermal properties. However, membranes made from polymers may also be inherently or easily wettable to facilitate filtration of water without contaminating the filtrate. The present invention relates to a class of polymers which surprisingly fulfill these apparently contradictory requirements. Polyamide, polyimide, polysulfone and polyethersulfone polymers were used in the production of membranes. For example, U.S. Pat. No. 3,816,303 (issued June 11, 1974) to Wrasidlo describes a process for desalination of salt water by reverse osmosis using a membrane containing a film of poly(N-amide)imide having a specific formula. Disclosed. Urasidro's patent describes a reverse osmosis membrane made from a specific polyimide structure. Reverse osmosis membranes have a significantly different porosity than that found in microporous membranes. Purely aromatic polyamides have been used in the manufacture of reverse osmosis membranes as shown by US Pat. No. 3,172,741 to Jolley. Polyimide was developed for gas separation membranes as disclosed in JP58-08514. Polyimide polymers are usually insoluble and can be processed in situ (i.e., the membrane needs to be synthesized).
It is necessary to form the polyamic acid form and then heat treat it to form the final polyimide film. Aromatic polyamides are not readily available as pure bulk polymers and therefore typically require synthesis from membrane precursors. Unlike the above polymers, polyamide-imides are commercially available at reasonable cost and are soluble in commonly used solvents. U.S. Patent No. to Lester et al.
No. 3,719,640 (issued March 6, 1973) discloses linear polymers of polyamide-imide having specific groups containing quaternizable nitrogen atoms. Quaternization of nitrogen is PH dependent. When the nitrogen is quaternized, the polymer is wettable and can be used as a separation membrane in processes such as desalination. US Patent No. for Bourganel
No. 3,855,122 (issued December 17, 1974) and U.S. Pat. has been done. The Borganel patent discloses an asymmetric membrane having porosity in the reverse osmosis/ultrafiltration range. The Borganel patent simply discloses a membrane made from sulfonated polysulfone. The Yoshida et al. patent also discloses an asymmetric ultrafiltration membrane made from polysulfone. U.S. Patent No. to Iwama et al.
No. 4240914 (published December 23, 1980) discloses membranes made from aliphatic polyimide polymers and methods for their preparation. None of the above patents contain microporous
Inherently hydrophilic membranes with a range of porosity are not disclosed. Japanese Patent No. 54-26283 discloses a method for producing an apparently microporous membrane from a polysulfone polymer and high molecular weight polyethylene glycol as an additive. Conventional microfiltration membrane property tests (eg, bubble point, bacterial retention) are not reported and do not differentiate between the various possible polysulfones. The present invention provides membranes made of hydrophobic polymers that are PH independent and externally wettable in a microporous membrane structure. SUMMARY OF THE INVENTION According to the present invention, a membrane is (1) essentially hydrophobic in bulk form and (2) hydrophilic as a polymerized membrane or polymerized when containing 1 to 6% by weight of polyethylene glycol. Provided is a microporous membrane comprising a polymer selected from the group of polymers characterized by being hydrophilic. The present invention further includes the steps of dissolving the polymer in a solvent to form a polymer solution, forming a thin film of the polymer solution, precipitating the polymer as a microporous film, and drying the microporous film. Provided is a method for producing a microporous membrane comprising: DETAILED DESCRIPTION OF THE INVENTION The solubility parameter of a polymer is a measure of cohesive energy per unit volume and is generally related to polarity. The more polar the functional groups of the polymer, the higher the solubility parameter. Therefore, polymers with higher solubility parameters are more wettable (water,
(with low contact angles with highly polar liquids) is usually observed. However, this order appears to be reversed in some of the subject polymers that characterize this invention. For example, US patent no.
4387187 (published June 7, 1983), a membrane made of polyether sulfone with a solubility parameter of 20.6 MJ/m ³ has a solubility parameter of 21.7 MJ/m ³
It is more wettable than polysulfone membranes with solubility parameters of . The present invention is the first example of a microfiltration membrane application that identifies a clear advantage of one of these polymers over another. It is another surprising aspect of the invention that this advantage is exhibited only at porosity in the microporous range. Membranes according to the present invention have microporous porosity and are (1) essentially hydrophobic in bulk form and (2) hydrophilic when precipitated as a membrane or contain between 1 and 6 weights of polyethylene glycol. % of polymers selected from the group of polymers characterized in that they are hydrophilic and the membrane has porosity within the microfiltration range. The polymer can be characterized as hydrophobic in bulk form. Polymers in bulk or powder form are non-wetting or "water repellent." These membranes are hydrophilic or naturally wettable when precipitated into membrane forms within the microfiltration range, or hydrophilic or naturally wettable when containing 1 to 6% by weight of polyethylene glycol. This provision is not a limitation on the properties of the membrane, as other reagents will accomplish a similar function to polyethylene glycol if required. Rather, this limitation is useful in identifying the polymer from which the membrane is made. However, when the polyethylene glycol is completely extracted from the membrane, the membrane remains as a fully functional membrane, but it is sometimes no longer hydrophilic. On the other hand, similar amounts of polyethylene glycol in membranes made from structurally similar polymers such as polysulfone do not result in wettable membranes. Therefore, the properties of membranes made according to the present invention are completely unexpected. A common measurement of polymer hydrophobicity is ASTM D570.
(Standard Method for Measuring Water Uptake by Polymers) by the bulk polymer within 24 hours or at equilibrium. However, the definition of hydrophobic and hydrophilic polymers is generally inconsistent. The inventor defines a hydrophobic polymer as one that absorbs less than 0.5% of its weight of water in 24 hours, with an equilibrium of 4% or less. The surface of a solid piece of such a polymer is typically non-wetting, and a drop of water placed on its sloped surface will roll without trailing. Literature data for water absorption for a number of polymers are shown in the table below.

[Table] According to the above regulations, polyetherimide, polysulfone, and polyethersulfone are hydrophobic, and nylon 66, aromatic polyamide, and cellulose acetate are hydrophilic. The weights used in the membranes of the invention are identified by a specific set of properties. They are hydrophobic according to the above definition, but they form hydrophilic or naturally wettable microporous membranes, or the membranes are made to contain polyethylene glycol or other polymer adducts. When mixed, a hydrophilic, naturally wettable microporous membrane is formed. U.S. Pat. No. 4,413,074 to Brunswick Corp. describes post-treatment of preformed porous polysulfone membranes to render them water-wettable. It is emphasized in that patent that the treatment is particularly suitable for polysulfones. In fact, treatment with a polysulfone membrane, i.e. addition of substantial amounts of surfactants, is absolutely necessary for wettability. However, in the context of the present invention, a milder and simpler method with polyether sulfone can be used to make the membrane wettable. For example, it may be the inclusion of some polyethylene glycol in the membrane or the inclusion of a small amount of polyvinylpyrrolidone in the membrane matrix. The inventors define microporous membranes as membranes with pore size ratings of 0.02 to 20 μm. Such membranes do not exclude salts from the feed solution, nor do they retain dissolved high molecular weight substances such as proteins.
The inventor defines wettability as the spontaneous absorption of water into at least 50% of the exposed membrane area within 10 seconds when the membrane is placed over the surface of stagnant water. The inventors have observed that polymers meeting the requirements of the present invention typically have a 24 hour water absorption of about 0.2-0.4% and an equilibrium water absorption of about 2% or more and about 4% or less. More specifically, in order to determine whether a polymer is suitable and falls within the provisions of the present invention, the following tests useful for membrane validation are applied (hereinafter referred to as test methods). The hydrophobic polymer according to the above definition is dissolved in a polar aprotic solvent such as dimethylformamide, dimethylacetamide or n-methylpyrrolidone. Add polyethylene glycol 400 (PEG),
Stir the mixture to homogenize. The concentration is that the polymer is about 10-13% of the mixture and the solvent is about 17-20%
and adjust the PEG to be approximately 65-70%. The mixture is cast onto a clear glass plate to a thickness of about 10 mils and exposed to air at about 50-80% relative temperature at room temperature until the film becomes opaque. The membrane is then soaked in ambient water overnight to wash out the solvent and dried in ambient air for 2 hours and in the open at 70°C for 1 hour. Membranes made in this manner have porosity in the microporous range and typically contain 2-6% residual PEG. The polymer according to the invention confirms that the microporous membrane produced as described above is naturally wettable. The above test method shows surprising differences between apparently similar polymers. After checking the table, nylon 66
It is not surprising that the membrane is inherently wettable.
However, it is surprising and unexpected that polymers with lower water absorption form wettable films. Even more surprisingly, polymers with very similar chemical composition and water absorption give membranes with different wetting properties. Polymers according to the invention include aromatic polyethers, preformed polyimides,
and polyamides derived from fully aromatic acids.
Including, but not limited to, imides. One of the preferred polymers of the present invention is polyether sulfone (sold under the trade name Victrex). its molecular structure,

[Formula] is the structure of polysulfone (sold under the trade name Udel),

Very similar to . The table shows that the two polymers have quite similar water absorption properties in bulk form and are both hydrophobic. Nevertheless, only polyethersulfones give wettable microporous membranes according to the above test method, whereas polysulfones give hydrophobic membranes. Polysulfones and polyethersulfones are often mentioned in the public and patent literature together or in a similar context, especially in the context of ultrafiltration membranes. It is shown in these references that membranes from both polymers behave similarly in the ultrafiltration range of porosity. For example, many methods have been devised to make polysulfone and polyethersulfone ultrafiltration membranes wettable. In this more stringent range of porosity, no membrane is wettable. Key references are U.S. Patent No. 3651030 to Desauliners (issued March 21, 1972) and U.S. Patent No. 3632404 to Desauliners.
No. (issued January 4, 1972); German Patent No. 2829630
No. 8250507 and Japanese Patent No. 8250507. Recent publications have pointed out the good thermal stability of polyethersulfone ultrafiltration membranes [Kai et al.
AVCS Syrup Series
Series) 281, p.281, p.21, 1985]. Similar to the relationship between polyethersulfone and polysulfone, polyetherimides, polyamide-imides and thermoplastic polyimides have similar water absorption properties in bulk form, but only the latter two can be used in hydrophilic microfiltration trays. give a film. Apparent subtle differences in the hydrophobicity of polymers become visible very quickly when made into high surface forms such as microporous membranes. Such a difference has not been previously observed and is the basis for the test method of the present invention. To specifically explain the above group of polymers:
Preferably the aromatic polyether has repeating groups,

It is a polyether sulfone having the formula: This particular polyether sulfone can be purchased under the trade name "Victrex". The polyether sulfones are generally resistant to aliphatic alcohols, some chlorinated hydrocarbons, and some aromatic hydrocarbons. Preferably the polyimide has repeating groups,

[ka] (In the formula, 10 to 90% of the R groups are

【formula】 and the remaining R groups are

[expression] or

[Formula] ) is included. Such polyimides are manufactured by the Upjohn Company under the trade name "Polyimide".
2080”. polyimide 2080
is resistant to many organic solvents and dilute acids. However, polyimide is not recommended for long-term exposure to caustic solutions, including ammonia and hydrazine. Preferably the polyamide-imide is a compound sold under the trade name "Torlon" by Amoco Corporation.
The molecular structure of the polymer consists of the following repeating units,

[Formula] (wherein R is primarily aromatic) consisting essentially of: Polyamide-imides are available at reasonable cost and are soluble in commonly used solvents such as N,N-dimethylformamide (DMF) or N-methylpyrrolidone. Generally polyamide-imides are not attacked by aliphatic and aromatic hydrocarbons, chlorinated and fluorinated hydrocarbons, and many acids and bases. It is attacked by some hot caustic and hot acid systems. Other polymeric additives can be used in addition to polyethylene glycol. With polyethersulfone, the inventors have found that polyethersulfone membranes can be made wettable by a variety of simple methods. Therefore, in addition to the membrane produced, a small amount of polyvinylpyrrolidone (for example up to 1% of the weight of polyethersulfone) can be mixed into the membrane according to the test method. Microporous membranes made in this way remain fully wettable even after extensive extraction with water or alcohol or prolonged heat treatment. Very high concentrations of polyvinylpyrrolidone are required to make the polysulfone membrane wettable. The present invention further provides polymers selected from the group of polymers that are essentially hydrophobic in bulk form and hydrophilic as a 1-6% PEG-containing polymeric membrane with a porosity in the microfiltration range. A method of making a microporous membrane is provided, comprising dissolving a polymer in a solvent to form a polymer solution. A thin film is produced from the polymer solution. The polymer is precipitated as a microporous membrane and dried. More particularly, the polymer is dissolved in a suitable solvent, such as dimethylformamide. A suitable plasticizer can be added to the polymer solution. Examples of suitable plasticizers are glycerin or polyethylene glycol. For example, the mixing ratio of the components can be: Polymer 12% PEG/Glycerin 68% Solvent 20%. Polyethylene glycol can also be used as a pore former. Generally, the polymer solution is cast onto a moving belt and subjected to controlled air velocity, belt temperature, and relative humidity conditions. The liquid film of polymer absorbs sufficient water from solution to effect initial precipitation of the polymer. The final precipitation forming the microporous membrane occurs in a quench bath containing a strong non-solvent such as water. The formed microporous biomembrane can then be dried in the open. As further illustrated, polyamide-imide membranes made to contain pores with size ratings within the micron filtration range of about 0.02 to about 20 microns are inherently wettable and exhibit excellent water penetration. speed and has unusual thermal stability. This is surprising since the polymer itself is hydrophobic in bulk resin form. However, as shown in the examples,
These formed membranes exhibit water flow rates comparable to or better than the best commercially available microfiltration membranes. Thermal stability is far superior to available membranes, thus offering new potential applications for microfiltration membranes. Microfiltration membranes made according to the present invention are very useful for water filtration. If necessary, the membrane can also be rendered hydrophobic by post-treatment, for example by application of silicon compounds. The post-treatment of the silicon compound is over the entire pore surface of the membrane, so that the membrane becomes hydrophobic on its internal pore surface as well as on the external surface of the membrane. These microporous membranes can be exposed to temperatures of 250° C. and above for extended periods of time without major changes in filtration performance. Polyamide-imide membranes can be manufactured to include pores with size ratings in the ultra- or reverse osmosis range of 0.002 microns or smaller. These membranes were found to exhibit reasonable protein retention. The membrane can potentially be used in temperature and chemical environments that are outside the range of most commonly available ultrafiltration membranes. Preferably the solution contains 11-15% by weight polyamide-imide, 40-45% DMF and 44-48% PEG. Preferably, the solution for the microfiltration membrane is cast onto a flat surface to a thickness of 10-12 mils. The solution is capable of condensation under ambient air at 60-70% relative humidity. Once coagulation is complete, the solvent is removed by immersing the membrane in a non-solvent to allow leaching of the solvent. Setting can also be completed during this time. The non-solvent can be water and other polar solvents that keep the polymerized membrane stable and dissolve the PEG and DMF. After coagulation and leaching of excess solvent, the membrane is dried. Preferably, the membrane is dried at a temperature within the range of room temperature to 70°C. Polyamide-imide microporous membranes exhibit excellent water flow versus bubble point properties. The mechanical and hydrolytic properties of these membranes can be substantially improved by thermal curing of the membranes. Curing the film at 260°C for 4 to 20 hours increases the tensile strength by at least 50%.
Additionally, hydrolytic stability is improved to the extent that the membrane can withstand repeated autoclaving and steaming. Alternatively, curing can be performed on the resin before forming it into a film with similar results. Thermal curing has little effect on the flow properties of the membrane. Membranes can be made from cured or uncured polymers. Any type of membrane can be made into devices such as capsules and cartridges by plating and sealing the membrane. By the above method, membranes can be produced on a small or large scale. Large scale manufacturing can be accomplished by casting on an endless belt and leaching the solvent from the formed membrane in a large water bath. The following examples illustrate the invention in greater detail. The following examples are intended to illustrate the invention and should not be construed as limiting the invention in any way. Example Definitions Water Bubble Point: Water Bubble Point is a test that measures the maximum pore size of a filter and is based on the air thickness required to pass liquid through the pores of a wet filter. The larger the pore, the less pressure there is to force it to exit. The passage of air through the empty pores is detected as bubbles. The pressure differential that produces the first bubble is measured as the bubble point. The relationship between bubble point pressure and large pore diameter is: P=Bγcosθ/D, where B is a constant, γ is the liquid-air surface tension, θ is the liquid-solid contact angle, and D is the pore diameter). Air flow: Air flow mainly depends on the pressure differential and the total porosity and area of the filter. The total amount of air that can be filtered is also a function of contamination in the stream. The Gurley and Fragier test methods are two common measurements of filter air flow. Water Flow: The water flow/flow rate test measures the rate at which water flows through the filter, variable pressure differential, porosity, and filtration area. Flow rates are typically gallons/gal at a given pressure.
It is expressed in minutes per square foot or milliliters per minute per square centimeter. Example 1 Polyethersulfone membrane 0.2μ Polyethersulfone (Victrex 5200), dimethylformamide and polyethylene glycol 400 were mixed in a ratio of 13:18:69. The mixture was stirred to homogeneity and cast onto glass or stainless steel to 10-12 mil. Reduce it to 60-70% until it becomes opaque
Exposure to ambient air at relative humidity. The film was then soaked in water for 2-12 hours to terminate coagulation and to leach out excess solvent. It was then dried at room temperature to 70°C. The resulting membrane was naturally water wettable. It is Pseudomonas deimiunitae (Pseudomonas
dimunitae) showed 100% bacterial retention when tested at 10 ⁷ /cm ² . The membrane had the following flow characteristics: Kerosene bubble point 22 psi Water bubble point 53 psi Air flow 2.7/cm ² ·min, 10 psi Water flow 23 ml/cm ² ·min, 10 psi Nuclear magnetic resonance of the dissolved membrane Contains polyethylene glycol. Comparative Example 1 Polysulfone Membrane A 0.2μ polysulfone (Uden 3500) membrane was made by substantially the same procedure as in Example 1. Nuclear magnetic resonance of the dissolved membrane showed that it contained 6% polyethylene glycol. but,
It was quite hydrophobic. Membrane performance was: Kerosene bubble point 34 psi Water flow 17.8 ml/cm ² ·min, 10 psi. (after pre-wetting with ethanol). Example 2 Polyethersulfone Membrane 0.45μ A casting solution was prepared by mixing 11.5% polyethersulfone (Victrex 5200) with 25% N-methylpyrrolidone, 68% polyethylene glycol and 0.5% glycerin. The membrane was cast and set as in Example 1. The resulting membrane was naturally water wettable. The flow characteristics were: kerosene bubble point 17 psi, water bubble point 36 psi, air flow 4.8/cm ² ·min, 10 psi water flow 42 ml/cm ² ·min, 10 psi. Example 3 Polyethersulfone membrane 0.1μ Casting solution: Polyethersulfone (Victrex 5200) 15% dimethylformamide
18%, polyethylene glycol 66.5% and glycerin 0.5%. The membrane was cast and formed as described in Example 1. The resulting membrane was naturally water wettable. The performance was as follows. Kerosene bubble point 37.5psi Water bubble point 88.8psi Air flow 1.7/cm ²・min, 10psi Water flow 9ml/cm ²・min, 10psi Example 4 Polyamide-imide membrane 0.2μ Polyamide-imide “Toilon”
4000TF), N,N-dimethylformamide and polyethylene glycol were mixed in a ratio of 11.5:40:48.5. Stir the mixture until homogeneous and spread it on a glass or stainless steel belt as in Example 1.
Cast to 12 mil thickness. The cast solution was exposed to ambient air at 60-70% relative humidity until phase separation occurred. The membrane is then immersed in water to fully condense,
Excess solvent was leached out. The membrane is then heated to room temperature to 70°C.
It was dried. The membrane self-wetted when placed on the surface of stagnant water. It showed the following performance characteristics. Kerosene Bubble Point 23psi Water Bubble Point 54psi Air Flow 2.3/cm ²・min, 10psi Water Flow 9ml/cm ²・min, 10psi Example 5 Polyamide-imide membrane 0.45μ Casting mixture with polyamide-imide (Torlon 4000TF) 11.5% dimethylformamide
44% and polyethylene glycol 400, 44.5%
It was prepared by mixing with The membrane was cast and condensed substantially as described in Example 4.
The resulting membrane was inherently wettable and exhibited the following properties: Kerosene Bubble Point 13 psi Water Bubble Point 30 psi Air Flow 4.5/cm ² ·min, 10 psi Water Flow 36 ml/cm ² ·min, 10 psi Examples 6 Polyamide-imide membrane 3μ Polyamide-imide (Torlon 4000TF) 8%,
A casting mixture was prepared consisting of 45% dimethylformamide and 47% polyethylene glycol 400. The membrane was cast and set substantially as described in Example 4. The resulting membrane was hydrophilic in nature and exhibited the following properties: Kerosene bubble point 2.4 psi Water bubble point 6.2 psi Air flow 219/cm ² ·min, 10 psi Water flow 144.6 ml/cm ² ·min, 10 psi Comparative Example 6 Polyetherimide Membrane 3μ A casting mixture was prepared as in Example 6, but polyetherimide (Ultem 1000) was used in place of the polyamide-imide. The kerosene bubble point was 3.5 psi and the membrane was completely hydrophobic. Example 7 Thermoplastic polyimide film Thermoplastic polyimide "Upjohn"
2080D], 40% dimethylformamide, 6% ethylene glycol, 8% tetrahydrofuran, and 27% N-methylpyrrolidone (by weight). The solution was cast and set as in Example 1. The resulting membrane exhibited the following wettability properties: Kerosene bubble point 20.8 psi Air flow 2.0/cm ² ·min, 10 psi Water flow 9.9 ml/cm ² ·min, 10 psi Example 8 Thermoplastic polyimide membrane Thermoplastic Polyimide (Apuzion 2080D) 13
%, dimethylformamide 46%, ethylene glycol 6%, tetrahydrofuran 2% and N-methylpyrrolidone 33% (by weight). A membrane was prepared as in Example 1. The resulting membrane was water wettable and exhibited the following performance characteristics: Kerosene Bubble Point 14.6 psi Air Flow 3.8/cm ² ·min, 10 psi Water Flow 23.7 ml/cm ² ·min, 10 psi Example 9 Polyamide Thermal Stability of Imide Films Film samples made according to the present invention were subjected to heat treatment in an oven at various temperatures for various times. Performance was measured before and after treatment, and the results were as follows.

[Table] Between
Example 10 Polyamide-imide ultrafiltration membrane Torlon 4000TF in dimethylformamide
Ultrafiltration membranes were manufactured by casting a 15% solution onto a glass plate and coagulating it in water. Scanning electron microscopy showed a typical ultrafiltration structure with a dense outer layer and a porous structured lower layer. The membrane showed more than 50% protein preservation when tested with myoglobin solution. Example 11 Effect of Heat Curing on Polyamide-Imide Film Properties A 0.45 μm film was heat cured at 269° C. for 20 hours. It was then repeatedly autoclaved at 118° C. for 20 minutes and typical performance and mechanical properties were determined. The results summarized in the following table show that the membranes are largely unaffected by autoclaving.

Table: Example 12 Blended Polyethersulfone Membrane A membrane was prepared as in Example 1, but polyvinylpyrrolidone was substituted for 0.7% of the polyethersulfone weight. The resulting membrane was essentially wettable and, unlike the membrane of Example 1, retained its wettability upon extensive extraction with isopropanol or extended exposure to 170° C. in an oven. Polysulfone membranes required substantially higher concentrations of polyvinylpyrrolidone to induce similar wettability. Example 13 Post-treated Polyethersulfone Membrane A membrane was prepared as in Example 1. The membrane was then post-treated with a dilute aqueous solution of polyvinyl alcohol and then cross-linked. This membrane remained wettable after prolonged extraction in isopropanol and extended heat treatment as in Example 12. The results showed that the invention provided high flow and throughput; it was also observed that the invention provided good temperature characteristics and handling, resulting in high practicality of use. It is to be understood that the invention has been described by way of example and that the terminology used is intended to be in the nature of words of description rather than limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims (1)

Claims: 1. Contains hydrophobic polymers and polymer additives in bulk form, having an equilibrium water absorption in the range of 12 to 4%, and containing dissolved protein or dissolved protein from an aqueous feed solution. A membrane characterized in that it has a pore size greater than 0.01 μm in a range such that the membrane does not retain or reject salts, and is essentially water-wettable and hydrophilic when dried. 2 The polymer additive is polyethylene glycol,
A membrane according to claim 1, selected from ethylene glycol and polyvinylpyrrolidone. 3 The hydrophobic polymer is polyether, polyimide,
2. Membrane according to claim 1, selected from polyamideimide. 4. The polymer according to claim 1, wherein the polymer comprises a polyether sulfone having the following general formula: (where n=50 to 150) and a polymer additive containing polyvinylpyrrolidone film. 5 The polymer has the following general formula: 2. The membrane according to claim 1, which is a polyimide of ). 6 A compound in which the polymer has the following general formula [Formula] [where q=50 to 150; R' is -Ar(X)-] (However, Ar is [Formula] [Formula] or [Formula] ; X is H, halogen, alkyl or alkoxy. 7. In a method for producing a microporous, essentially water-wettable dry membrane, (a) a hydrophobic polymer having an equilibrium water absorption of 2 to 4% is dissolved in a polar aprotic solvent; ) the membrane has a pore size in a range such that it does not retain or reject dissolved protein or dissolved salts from the aqueous feed solution, and the membrane is essentially water-wettable when formed and dried; (c) casting the solution into a thin layer; (d) subjecting the solution to a relative humidity of 50° C. until opaque. ~80
% of air through the solution to sufficiently humidify the solution so that the membrane precipitates, and then (e) drying the membrane. 8. The manufacturing method according to claim 7, which comprises adding to the solution any of the following polymer additives (a) to (c): (a) 40 to 70% of the total solution weight; (b) a mixture of polyethylene glycol ranging from 40 to 70% by weight of the total solution and polyvinylpyrrolidone ranging from 0.1 to 1% by weight of the hydrophobic polymer dissolved in the solvent; or (c) ethylene glycol in an amount equal to 6% of the weight of the total solution. 9. The manufacturing method according to claim 7, wherein the hydrophobic polymer dissolved in the solvent is selected from polyether, polyimide, and polyamideimide. 10 The hydrophobic polymer dissolved in the solvent is a polyether sulfone of the following general formula, [Chemical formula] (wherein n=50 to 150), a polyether sulfone of the following general formula [Chemical formula] (however, m=50 to 150) (wherein 10 to 90% of R is [Formula] and the remaining R groups are [Formula] or [Formula]), and the following general formula, [Formula] ( 8. The manufacturing method according to claim 7, wherein q=50 to 150; R is a compound represented by the following formula: polyamideimide.