DK145564B - PROCEDURE FOR PREPARING AN ANISOTROP POLYMER MEMBRANE - Google Patents

Description

(19) DENMARK

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DIRECTORATE OF THE PATENT AND TRADEMARKET SYSTEM

(21) Application No. 4553/68 (51) IntCf. » B 01 D 13 / 0k (22) Filing day 20 Sep. 1968 '(24) Race day 20 Sep. 1968 (41) Aim. available Mar 22 1969 (44) Presented 13 · d.ec. 1982 (86) International Application No. - (86) International Filing Day (86) Continuation Day - (62) Master Application No. -

(30) Priority Sep 21 1967, 669648, US

(71) Applicant AMICON CORPORATION, Lexington, US.

(72) Inventor Alan Sherman Michaele, US.

(74) Clerk Th. Ostenfeld Patentbureau A / S.

(54) Process for preparing an anisotropic polymer membrane.

The present invention relates to a process for preparing an anisotropic polymer membrane.

In membrane separation processes such as ultrafiltration and reverse osmosis, a solvent is removed from a solution, subjected to such pressure that the solvent is forced to flow through a membrane. Ultrafiltration is a term OQ preferably used to describe such pressure-activated separations involving solutions of a solute having molar or cold dimensions significantly greater than the molecular dimensions of the solvent in which the substance is dissolved. i.e. a dissolved crude having a molecular weight of approx. 500 and above. The term is also commonly used for processes involving, instead of dissolved molecules Q, colloid-sized particles. Reverse osmosis is a term commonly used for pressure-activated processes which include solutes of smaller molecular size, e.g. molecules of the same order of magnitude as the solvent molecules.

The particular advantage of the membrane modulated separation processes described above lies in their potential speed, mild operating conditions and low operating costs compared to various other separation processes such as evaporation, dialysis, ultra-centrifugation, chemical precipitation and the like. These advantages are particularly critical when treating thermally unstable or biologically active materials or when relatively large volumes of solvent are present in the solution to be treated.

Most of the favorable membrane modulated separation processes depend on the characteristics of the membrane used. Among the desired characteristics are the following: (1) High hydraulic permeability to the solvent: The membrane must be capable of transferring fluid at high speeds per minute. membrane area unit under moderate pressure.

(2) Sharp retention cut-off: The membrane must be capable of completely or almost completely withholding all solutes with a molecular weight or size above a first specified value and allowing passage of all solutes with a molecular weight or size below another value that should be as close as possible to the first value.

(3) Good mechanical durability under chemical and thermal operating conditions: A membrane should preferably be suitable for use in an extensive chemical and thermal environment.

(4) A minimal dependence on the solvent permeability of the solvent type concentration.

(5) High pollution resistance.

So far, two types of basic membranes have been available. The first is an isotropic, sometimes called homogeneous, microporous membrane, whose flow and retention properties depend on the flow direction. Such structures are typically produced in the form of sheets having a thickness of from 2.54 mm to 0.254 mm. Such membranes are analogous to conventional filters and are in fact not retentive for solutes U5564 3 having a molecular weight less than ca. one million. In attempting to produce such membranes capable of retaining much smaller molecules, a great deterioration of the hydraulic permeability occurs. Such degradation results in too slow a flow rate of the solvent through the membrane, or the use of these isotropic membranes is limited to very few or no practical applications. Furthermore, isotropic membranes are clogged relatively easily by retained solutes. Such polymeric isotropic membranes are e.g. known from Swiss Patent Specification No. 400,096.

The second type of membrane used in ultrafiltration processes is the diffusion membrane. In these membranes, the solvent is transported by molecular diffusion under the influence of a concentration or activity gradient. They differ from the aforementioned filter-like, microporous membranes in that the migration of a molecule of solvent from one site to another across the membrane practically depends on the availability of a number of sites in the polymer matrix for the molecules being transported. The mass transferability of such membranes is highly temperature dependent, with an activation energy. Such membranes contain few or no pores and are not suitable for obtaining high flow rates. This is true even when these membranes are in the form of anisotropic membranes with a very thin barrier layer and, therefore, do not apply to industrial treatment of large volumes.

In addition, the polymeric material from which such membranes are made must have a high sorption capacity for the solvent to be transported. An example is cellulose acetate which absorbs 15-20 $ water and has found some use in the aforementioned diffusion membranes for separation processes, cf. U.S. Patent No. 3,290,286. Anisotropic cellulose acetate membranes are also known from Ulrich Mertens "Desalination by Reverse Osmosis", 1964, pages 58 and 63.

The object of the present invention is to provide novel, anisotropic, fluid-permeable membranes for use in separation processes, which are characterized by unusually high hydraulic permeability through virtually permanent microscopic or submicroscopic pores, surprising contamination resistance, enhanced "retention cut-off" characteristics that it has not been possible to achieve at such unusually high hydraulic permeabilities so far, and improved physical characteristics.

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In particular, the membranes provided should be suitable for use in ultrafiltration whereby, unlike cellulose acetate membranes, they retain their favorable mechanical properties after drying.

This object has been achieved according to the present invention by a process for producing an anisotropic polymer membrane with a microporous barrier layer and integrally with the barrier layer on one side thereof a relatively large pore-size support layer, the barrier layer and the support layer together constituting a continuous uninterrupted synthetic organic polymer phase, which is characterized in that a film is prepared from a casting coating comprising a synthetic organic polymer in an organic solvent, thereby contacting one side of the film with a diluent having a high degree of compatibility with the organic solvent and a sufficiently low degree of compatibility with the polymer to induce immediate precipitation of the polymer from the solution upon contact with it, keeping the diluent in contact with the formed membrane until virtually all solvent has been replaced with the diluent wherein the polymer is a such as absorb less than 10% moisture at a temperature of 25 ° C and a relative humidity of 100%.

The present anisotropic membranes are preferably made from the known crystalline and glassy thermoplastic polymers.

By crystalline and vitreous polymers is meant those materials which possess from ca. 5 to 90% by weight crystallinity, as measured by known X-ray diffraction technique, and having a glass transition temperature (Tg) of at least approx. 20 ° C. For the membranes, low water sorption polymers are used which, unlike the cellulose acetate materials known in the membrane technology, can withstand drying during storage without losing their good mechanical and processing characteristics. These polymers are, as mentioned, those with water absorption coefficients of less than ca. 10% by weight of moisture at 25 ° C and 100% relative humidity.

The present anisotropic membranes consist of an integral, macroscopically thick porous polymeric film, usually having a thickness of more than about 100 microns. 0.051 mm and less than 1.27 mm. On one surface of this film, there is an extremely thin, but relatively dense, barrier layer or skin, preferably from 0.1 to 5.0 microporous polymer, in which the pore diameter is in the range of 1- 1000 iryu, ie about one-tenth to one-hundredth of the thickness of the skin. In particular, it is preferred that the barrier layer has a pore size of 1 to 500 mp, which is achieved by following the preferred casting conditions set forth herein. The remainder of the integral film structure is constituted by a support layer consisting of a polymer structure with very coarse pores through which fluids can pass under low hydraulic resistance. By the term "integral film" is meant a film having a continuous polymer phase. When such a membrane is used as a molecular filter with the skin in contact with fluid under pressure, virtually all of the radiation resistance across the membrane is in the skin, and molecules or particles with dimensions larger than the pores of the skin are selectively retained.

Due to the exceptional thickness of the skin and the transition from the skin or the barrier layer to the support layer is so sharp, usually less than approx. half of barrier layer thickness or less than approx. l / i, the total hydraulic flow resistance across the diaphragm is very small, i. the membrane exhibits surprisingly high permeability to fluids. Furthermore, there is a surprisingly low tendency for such membranes to become clogged or contaminated by molecules or particles.

The fact that such anisotropic membrane structures can be prepared by casting and leaching of specially formulated polymer solutions under controlled conditions is believed to be a consequence of the unusual interaction of diffusion and polymer precipitation phenomena that occur during a controlled leaching process.

By diluting a solution of a film-forming polymer with a diluent which is miscible with the solvent of the polymer but which is not a solvent of the polymer, there will always be a critical solvent-diluent relationship, forming two phases, where one is rich in polymer and poor in diluent and the other is poor in polymer and rich in diluent. The polymer-rich phase can be liquid, quasi-liquid, solid or gel-like, depending on the amount of solvent present in the phase. It is generally advantageous for the formation of microporous, anisotropic membranes that the polymer-rich phase formed initially by dilution with a non-solvent is relatively solid or gel-like and thus relatively poor in solvent. This condition has been found to be satisfactory when the diluent selected for the precipitation process is highly incompatible with the polymer, ie. not able to dissolve it and / or the solvent is much more compatible with the diluent than with the polymer.

The fine structure of the two-phase system formed under the above conditions depends on (1) the volume fraction of polymer in the mixture of the polymer and solvent, and (2) the rate at which the composition of the mixture changes to produce precipitation. As for (1), the polymer-rich phase will tend to be discontinuous (i.e., in the form of small droplets or particles) if the polymer concentration is low, while the polymer-poor phase will be continuous. If the polymer concentration is high, e.g. on the other hand, of the order of 50% or higher, the polymer-rich phase will be continuous and the polymer-poor phase will be discontinuous, ie. available in the form of small drops. However, at polymer concentrations between 10 and 40 percent, it is often the case that both the polymer-rich and the polymer-poor phase are continuous, ie. forming a polymer matrix containing a coherent gap configuration filled with polymer-poor liquid. It is under these latter precipitation conditions that anisotropic microporous membranes of appropriate structure can be formed if the rate of formation of the skin layer is carefully controlled.

As for (2), it has been found that the faster a liquid mixture is imparted to precipitation conditions, the finer the degree of dispersion in the precipitating phase. The faster a suitable concentrated polymer solution is diluted with non-solvent, the less pores will form in the polymer matrix at phase separation. In addition, in any region of a polymer solution where the dilution rate of non-solvent is constant up to the precipitation point, the size of all the pores formed in the precipitated matrix will be virtually identical. Practically uniform pore size in the surface of the thin barrier layer is considered to be an important property for the formation of the most useful permselective and anisotropic microporous membranes, as such uniformity will ensure the sharpest size delimitation in the membrane.

However, when a continuous two-phase polymer / liquid structure is formed under the above conditions, it has been found that the interfacial forces acting in the polymer-liquid interface cause capillary stresses acting on the (solid) polymer phase, which tend to cause breakdown of the polymer matrix and compound or elimination of the liquid phase filling the pore space. The magnitude of these capillary U5564 7 forces is assumed to follow the Young / LaPlace equation / j P = 2 y PL · r which / [P represents it on the polymer acting tensile stress, γ PL means the polymer-liquid interface stress, and r is the radius of the pore. From this it is clear that the finer the pore structure, ie. the smaller is r, the higher the voltage of the 1 polymer. The ability of the polymer to withstand the impact without permanent deformation, ie. its creep stress depends on its elastic modulus and / or its creep relaxation modulus. This, in turn, depends on (1) the polymer's glass transition temperature or crystallinity and (2) its solvent plasticization degree. If the polymer phase is soft or viscoelastic (corresponding to a low modulus, as is often found at the glass transition temperature or in very low-level or crystalline polymers, and / or in high-resolution plasticization polymers) when the precipitation occurs, and / or if the gap size in the polymer is extremely small, the polymer will creep or flow, either with complete elimination of the gap or forming much larger spaces at reduced volume fractions. In such a case, either the ability of the polymer to be permeable to fluid at high velocity will be lost, or its ability to induce one-molecule separation can be lost. Therefore, if highly porous, highly permeable molecular permselective anisotropic membranes are to be formed in this precipitation process, it has been found and desirable that the polymer selected for the purpose have either a high glass transition temperature or high crystalline or both.

It has been found that the lower the interface voltage γ PL, the smaller the tendency for collapse and compression of the polymer matrix will be. Therefore, if the non-solvent used to induce precipitation is a polar liquid such as water, fine-pore structures will be formed if the polymer used is relatively polar, i.e. low γ LP, or if a surfactant or other solvent modifier is added to the non-solvent to reduce γ LP.

The preferred way of contacting the thin layer of polymer solution with the diluent can be done in many different ways.

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An appropriate method is to cast the film on an impermeable surface, e.g. a glass or metal surface. However, definitely a permeability is not necessary and paper support layers which exhibit an appropriate flow resistance or diluent resistance are also applicable. Particularly advantageous are permeable, non-woven, non-woven sheets. Eg. polyolefin fiber felt is not easily wetted by water that. is the most commonly used diluent. In addition, it has good porosity and excellent mechanical and chemical properties, which is useful in the thin membranes requiring a porous backing layer.

If a thin layer of polymer solution is cast with one side sealed against an impermeable plate and this plate is immersed in a bath of a non-solvent, an immediately directed diffusion of solvent and non-solvent occurs over the interface between the molded film and the bath. . Initially, the concentration gradients of solvent and non-solvent in the immediate vicinity of the interface are very large, and according to Pick1's diffusion law, the rate of solvent and non-solvent transfer at the interface is huge. Thus, it has been found that the rate at which the polymeric phase precipitates from the solution in the interface is enormous and the pore structure of the polymer becomes extremely fine very close to the interface. Further inside the polymer solvent layer, the rate at which the solvent / non-solvent ratio changes becomes necessarily slower with time.

Therefore, as the diluent penetrates deeper into the molded film layer, the rate of change of concentration decreases very rapidly with time, and therefore the rate at which the polymer precipitation takes place will also decrease. As a result (provided the polymer used has a suitably high modulus and rigidity to support very small pores), a structure will form in the form of a very anisotropic, microporous membrane, in which the membrane pore size varies with the distance from the skin or the extruded surface to the surface of the support layer. The pore size is much smaller in the skin and grows rapidly with increasing distance from the skin.

In order for the skin to have a thickness of the order of 0.1-5 µm, it must be formed within 10 ° - 2.5 seconds, with the most H R mm 1 beneficial membranes forming over 10 ° to approx. io seconds.

Thus, it is clear why a film made in this way possesses the characteristic anisotropy so important for the preparation of highly permeable, molecular bead selective, microporous membranes. It should be noted that, if the selected polymer has a relative glass transition temperature or is highly solvent plasticized when the precipitation starts, the capillary forces in the small pores of the skin layer may be sufficient to induce breakdown and compression of this layer, thereby forming a skin of a non- porous polymer. Such a membrane will therefore act as a so-called diffusive anisotropic permselective membrane, i.e. a membrane over which flow is primarily determined by the chemical characteristics of the polymer and not by the hydro-dynamic flow through micropores.

Film-forming polymers which can be used in the process of the invention are e.g. following:

Polycarbonates, i.e. linear polyesters of carboxylic acids, wherein the carbonate groups are repeated in the polymer chain by phosgenation of an aromatic dihydroxy compound such as bisphenol-A. Such materials are marketed under the designation Lexan by General Electric Company.

Polyvinyl chlorides. Such material is sold under the trade name Geon 121 by B.G. Goodrich Chemical Company.

Polyamides such as polyhexamethylene adipamide and other such polymers popularly called nylon. A particularly advantageous material is sold under the trade name '' NOMEX '' by E.I. du Pont de Nemours Inc.

Modacrylic copolymers, such as the one sold under the trade name DYNEL · and made of polyvinyl chloride ($ 60) and aorylonitrile ($ 40), styrene acrylic acid copolymers and the like.

Polysulfones such as those of the type characterized by diphenylene sulfone groups in the linear chain are useful. Such materials are available from Union Carbide Corporation under the trade designation P-1700.

Halogenated polymers such as polyvinylidene fluoride sold under the trade name Kynar by Pennsalt Chemical Corporation, polyvinyl fluoride sold under the trade name Tedlar by E.I. du Pont de Nemours & Co., and the polyfluorohalogen carbon, which is traded under the trade name Aclar by Allied Chemical Corporation.

Polychlorethers, such as those sold under the trade name Penton by Hercules Incorporated and other such thermoplastic polyethers.

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Acetal polymers such as the polyformaldehyde sold under the trade name Delrin by E.I. du Pont de Nemours & Co., and the like.

Other polymers may also be used, such as polyurethanes, polyimides, polybenzimidazoles, aromatic and aliphatic polyethers and the like.

The large number of copolymers that can be formed by reaction in various ratios of monomers from which the above polymers are synthesized can also be used to prepare the membranes of the invention. Of course, this only applies to those copolymers whose crystallinity and / or glassy characteristics are suitable for fabricating the novel membranes described herein.

As can be seen from the above discussion of the polymers which can be used for the membranes of the invention, relatively polar polymeric materials are usually preferred. This is primarily due to the fact that it is an easier task to select usable systems of harmless solvents, co-solvents and economical washing fluids when using polar polymers. Generally, nonpolar polymers, such as polyethylene, require a more rare solvent system and therefore cannot be suitably adapted to an economical and safe manufacturing process. Nevertheless, they can be used when membranes with special properties are required.

As previously mentioned, the preferred polymers are those which have moderate crystallinity at ambient temperature, e.g. between approx. 5% and 90% crystallinity measured by X-ray diffraction analysis and / or those having relatively high glass transition temperatures, e.g. of at least 20 ° C and preferably higher. Polymers meeting these requirements usually provide membranes with good mechanical strength, resistance to collapse at elevated pressure and good long-term stability at elevated temperatures.

The solvents will usually be selected for their ability to form a film-forming casting resin with the polymer from which the membrane is to be prepared. Usually, a solubility of at least 5% by weight of the polymer in the solvent is required. Thickening agents may be added to the casting lacquer to provide the viscosity required for the casting, but such agents will usually affect the rate of fluid flow through the formed membrane.

On the other hand, some thickening agents such as pyrogenic silica and the like will result in membranes with improved strength characteristics.

There are known a number of useful methods for selecting particular solvent systems for particular polymers,

The Polymer Handbook edited by Brandrup and Immergut (John Wiley and Sons, New York 1966) contains some particularly useful chapters. In particular, reference should be made to the chapters entitled "Solvents and Nonsolvents for Polymers" by Klaus Meyerson and "Solubility Parameter Values" by H. Burreli and B. Immergut in connection with a large amount of other data in Section IV of this work. Further assistance in the selection of suitable polymer-solvent mixtures can be found in the Journal of Paint Technology, Vol. 58, May 1, 1966 by Crowley et al. in an article entitled "A Three-Dimensional Approach to Solubility" and in the Journal of Paint Technology, Volume 39, No. 505 * February 1967 by Hansen in an article entitled "The Three-Dimensional Solubility Faraméter-Key to Paint Component Affinities ".

A more detailed review of the referenced works will provide one skilled in the art with information on the large number of solvents that can be selected for cohesion energy density (as defined by a so-called solubility parameter), hydrogen bonding tendency, and polarity for use with a given polymer system. It can generally be stated that the greater the solubility of a given polymer system, the higher the flux rates can be obtained with the cast membranes from a casting coating of a given concentration.

In addition to the following specific examples described, Table I lists some of the many polymer solvent systems which have been found to be useful in the manufacture of mold varnishes.

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Table I

System No. Polymer Solvent 1 acrylonitrile (40) -vinyl-N, N1-dimethyl chloride (60) copolymer (Dynel) formamide (DMP) 2 polyacrylonitrile Dimethyl acetate amide (DMaC)

5 polyvinyl chloride DMP

4 polycarbonate DMP

5 polystyrene DMP

6 poly (N-butyl methacrylate) DMP

7 acrylonitrile copolymer (orlon) 70 $ ZnClg (aqueous)

All of the above-mentioned polymer solvent systems provide film-forming mold varnishes with a concentration range of 5-20% by weight of polymer. All of these moldings can be converted into the fluid-permeable, anisotropic membranes of the invention exhibiting extremely high flow rates in relation to their retention characteristics, i.e. their effective ultrafiltration pore size.

Molding varnishes made from the above polymers and solvents can be used directly, and the preparation is usually carried out at very moderate temperatures (25-90 ° C) to mold useful highly selective membranes.

Examples of these are the polyvinyl chloride polymers, polycarbonate polymers and acrylonitrile vinyl chloride polymers, each of which is converted into a molding coating with DMP. Usually, however, the pore structure of the membranes can be further modified by the addition of a solvent modifier and / or by the further moderate increase in temperature of the casting and washing operations, and / or by changes in the polymer concentration in the casting paint.

Solvent modifiers are often advantageously selected to enhance the overall solvent system solvation effect. The notification of such a solvent modifier will be detached, ie. decreasing the retention efficiency of a membrane at a given level of molecular size cut-off. By increased solvating effect is meant an increase in the compatibility or degree of approximation to an ideal solution.

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Conversely, a solvent modifier which decreases the solvating effect of the overall solvent system will tend to increase the retention efficiency but to decrease the flow rate in a membrane at a given level of molecule size cut-off.

This is illustrated with respect to the preparation of a diluent membrane with water as diluent and DMF as the primary solvent: DMF has a solubility parameter (calories / cc) of 12.1, is a strong to moderate hydrogen bonding solvent, and has a dipole moment of 2. Water has a solubility parameter of is a strong hydrogen bonding solvent and has a dipole moment of approx.

1.8. Therefore, it is expected that a solvent modifier used in the process of the invention which has a solubility parameter of 10.0, moderate hydrogen bonding tendency and a dipole moment of 2.9, νϋ decreases the solvation effect on Dynel and thus tends to "seal" This is the case with acetone used as a solvent modifier, for example in an amount of 5% by weight, attributed to the total weight of the solvent Tetrahydrofuran is an example of another such modifier.

In contrast, a solvent modifier having about the same dipole moment as DMF and a strong affinity for water will have an effect more similar to that of a ZnClg inorganic salt, which will be described in greater detail below and have a sufficiently greater compatibility with water than DMF to "loosen" the membrane, ie. increase the achievable flux across the membrane at a given pressure.

This is the case with formamide used as a solvent modifier, e.g. in an amount of 5% by weight, attributed to the total solvent. This despite the fact that the mere reference to the solubility parameter of formamide F will lead one to assume that its use would result in a poorer solvent for Dynel and therefore a more retentive membrane.

Usually, a large number of such solvent modifiers can be selected for a given polymer solvent system. The selection can be made not only from the classic lists of organic solvents, but also from solid organic compounds which can be made soluble in the primary solvents.

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Another class of solvent modifiers are those inorganic electrolytes which can dissociate into inorganic solutions, e.g. many halides, nitrates and the like. Some of these compounds are PeCl-j, LiBr, LiCl, A12 (NO2) 2, CuNOg and NaCNS and the like. In solution, these materials tend to act as solvating agents on polar polymers and to increase μ opnå the achievable flux rate with membranes cast out for solutions into which these substances are incorporated as solvent modifiers. Some particularly useful inorganic electrolyte solution modifiers in the systems described in Table I are listed below in Table II.

Table II

System Solvent Modifier Ϊ ZnCl2 1 FeCl-, s> ... 1 LiBr 1 (ai) 2 (no2) ^

1 NaCNS

1 CuNO2 2 LiCl; 4 ZnClg

The action of these salts, which act as solvents for polymers, is quite the opposite when incorporated into the diluent.

Organic and other liquid solvents which are particularly useful in the systems described in Table I include those exemplified below in Table III.

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Table III

System Solvent Modifier 1 Yinic Acid 1 H2O 1 HClO22 1 Dioxane

As previously stated, the diluent must be compatible with the solvent modifier and the primary solvent which together form the total solvent system to be leached from the molded membrane. Yand, which is the most commonly used diluent, will usually be used in all systems where possible. Occasionally, a mixture of water and an organic solvent will provide a more suitable diluent. In this case, the organic solvent for the dilution mixture can often be selected from the solvent modifier or the primary solvent system solvent or a mixture of the two. However, it is suggested that some polymer lacquers, e.g. those with incorporated nylon can be better washed with an organic solvent such as methanol, fusible oil, gasoline or the like, while the polymeric varnishes in which polyvinyl chloride incorporated in methyl isobutyl ketone is incorporated should be washable with naphtha.

It is often possible, by means of a quick qualitative analytical test, to determine whether a particular diluent will be suitable for use with a particular casting solution. If the addition of a few drops of the particular diluent to the casting solution induces approximately instantaneous precipitation of the polymer, good membranes can usually be prepared.

The polymer solids in the casting solution will usually amount to approx. 5- ^ 0 ^ of the polymer-solvent mixture. The exact polymer solids concentration must be sufficiently high to form a good film-forming mold varnish, and sufficiently low that the precipitated membrane has some pore volume in the barrier layer. If a given solution produces an impermeable membrane, a decrease in concentration will usually make it possible to produce a permeable microporous membrane. Film cast varnishes advantageously have a viscosity of approx. 50 to 5000 centipoise. When solvent modifiers are used, these are usually present at concentrations of up to ca. 10 # of the polymer solvent mixture. Usually the concentration is approx. 1-6 $.

A small amount of a non-solvent for the polymer, e.g. a material such as water or other diluent will sometimes be a useful additive in the molding formulation.

Such a liquid in the casting lacquer can, in many formulations, act as a thickener in a system in which the liquid is not highly compatible. Another function of such a non-solvent may be to some extent to prevent any unwanted early compression of the gel structure.

Particularly advantageous process steps in the preparation of mold varnishes by the process of this invention include clearing the mold varnish by centrifugation prior to extracting films therefrom. This clearance need not be so complete that e.g. The thin-valley effect is eliminated from the casting paint containing inorganic salts. As an alternative to this process, it is sometimes possible to obtain this clearance by changing the pH of the casting paint. For example, when ZnClg is used as an inorganic electrolyte solvent, if some zinc oxychloride and / or hydroxide is present in the casting paint. A few drops of hydrochloric acid will greatly dissolve salts and greatly reduce the magnitude of the Tyndal effect in the casting paint. It is also desirable to keep the casting paint under stirring from its manufacture to its use in the film-making. It has e.g. It has been found that when the casting lacquer is held in a laboratory ball mill from the manufacture to the extraction of the membranes, membranes of considerably more uniform nature are obtained from it. one extension to the other.

Illustrative Examples of the Invention.

The following are the illustrated membranes and their preparation by a number of examples. Most of these examples illustrate membranes with pore sizes between 10 and 500 m / h, i.e. the most advantageous size in polymer fractionation processes.

In the examples, the flux rate means the rate at which distilled water is found to pass through the membrane when no solute is present, unless otherwise stated.

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Example 1.

A 15 %% solution, ie a casting varnish is prepared by dissolving powdered poly (bisphenol A carbonate) in N, Nt-dimethylformamide (DMF). The solution is prepared at a temperature of approx. 60-70 ° C. The solution has a viscosity of approx. 20 centipoise (measured at 23 ° C with a Brookfield Spindle at 60 rpm), and the solution is drawn into a 0.3556 mm film on a glass support. Prior to pulling, tape is applied to the glass along the intended membrane edges to ensure continued membrane retention to the glass plate during washing.

This retention is necessary to avoid the deposition of a barrier layer on the back of the membrane. In addition, the tape helps to minimize membrane shrinkage in subsequent process steps. The extracted film is covered with an intermediate glass plate to protect the film from dust and the like while placed horizontally for approx. 1 minute. Then, the extracted film is washed by immersion in a water bath, i.e. the diluent, for 15 minutes at 20 ° C, forming a porous polycarbonate membrane. The membrane has an anisotropic structure, i.e. there is a relatively dense thin skin on the upper surface, and a much more open or porous structure behind this skin.

When testing the membrane, 400 cc of an aqueous solution containing 1% by weight of Dextran 110 (a trade name for a dextran having a molecular weight of 110,000) is placed in a pressurized cell. The first 10 cm - 1 filtrate coming through the membrane is discarded. The next 10 cm 2 of filtrate is used to determine the initial performance of the membrane. At a pressure of 3.51 kg / cra 2, a water flow rate of 1.54 cm / min is obtained and all dextran passes through the membrane.

An albumin solution having a concentration of 0.03% by weight is also subjected to ultrafiltration with the membrane in question.

90% of the albumin is retained at a flow rate through the membrane of approx. 0.202 cm 2 / min-cm 2.

This test is performed at approx. 25 ° C. Unless otherwise stated, the same test is used to determine the water flux in all of the following examples.

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Example 2.

A non-plasticized powdered polyvinyl chloride resin with a specific viscosity of 0.66 (sold under the trade designation, Geon 121) is dissolved in DMF at a temperature of approx. 65 ° C. The solution contains 8 parts of resin per 100 cm 2 DMF. A 0.508 mm thick membrane is extracted on a glass plate as described in Example 1. After washing with water at room temperature for 15 minutes, a portion of the formed membrane is tested and found to be anisotropic. The water flow rate is 1.03 cm 2 / minute-cm 2 at 25 ° C and a pressure applied of 3.51 kg / cm.

When testing the luxury and retention properties, the following results are obtained:

Pressure in 3 2 Reten- kg / cm2 Solvent in water Flux in cmVmin-cation 3.51 1.0 $ Dextran 110 0.36 0.0 1.76 0.03 $ Albumin 0.14 95 $ 1.76 0.15 $ Globulin 0.065 $ 95

Mr methyl alcohol is used as the washing solution, ie. diluent, instead of water, has the following retention properties:

Pressure in 2 Retenkg / cm 2 Solvent in water Flux in circulation 3.51 1.0 $ Dextran 110 0.71 0 1.76 0.3 $ Albumin 0.17 17 $ 1.76 0.15 $ Globulin 0.057 88 $

The flux rate of distilled water over the methanol washed membrane is 1.22 cm 2 / minute cm 2.

Example 3 ·

The procedure of Example 2 is repeated except that 4% by weight of ZnClg as solvent modifier is incorporated into the casting paint. The prepared membranes are tested as in Example 2. The retention properties of the membranes are practically the same, but the flux rates during the treatment of the dextran and albumin solutions increased from 0.36 and 0.14 rpm to 1.5 and 0.23 cra ^ / mlnutter-cm 2. However, the flux rate during the globulin retention decreased to approx. 0.05 cm 2 / min-about 2.

The water flux at 3.51 kg / cm 2 and 25 ° C is 1.68 cm 2 / minute-cm 2.

Example 4

A casting lacquer is made from 13 g of modular aryl fiber, which is marketed under the name Dynel, 87 g of DMF and 5 g of ZnClg. A 0.254 mm extension is made on a glass plate which is placed horizontally for 60 seconds and washed with water at approx. 20 ° C.

. The membrane retention properties are tested with a 0 3 # rs p albumin solution at a pressure of 1.76 kg / cm. A complete retention of albumin is obtained. The flow rate of distilled water through the membrane is approx. 1.14 cm 2 / minute-cm 2 at 7 in 02 kg / cm 2.

Example 5

The formulation of Example 4 is modified so that the polymer concentration is decreased.

9 parts Dynel

91 parts DMF

5.0 parts of ZnCl2

The flow rates for distilled water are 18.5 cm 2 / minute-p-cm. Dextran 110 is not retained, but albumin 2 is completely retained when the membrane is tested at a pressure of 7.02 kg / cm.

The formulation of Example 4 is further modified so as to increase the polymer concentration.

15 parts Dynel 85 parts DMF 5 parts ZnClg

The flow rate of distilled water drops to approx.

3.9 cm 2 / minute-cm 2 at a pressure of 7.02 kg / cm 2. However, B-lactoglobulin with molecular weights of 35,000, Dextran 110 and albumin are all practically complete. Thus, it is clear that variation in the polymer solids in a given system can be used to yield denser or looser membranes depending on the purpose of the membrane.

145564 20

Examples 6-9.

The following castings are made:

Example 6 Example 7 Example 8 Example 9

Dyhel 13 13 17.5 14.5 DMSG 87 87 20.0 20

ZnCl2 2.5

Phenylphosphonic acid - 5 -

Formamide - - 10.0 10.0

Acetone - - 52.5 55.5

Flux at p 7.02 kg / cm p 1 cm-ymin-ciri 3.7 3.7 0.29 0.94

Example 10.

5 g of polysulfone, obtained under the trade designation P1700, are dissolved in 45 g of N-methyl-2-pyrrolidone at 30 ° C. The film is placed horizontally for 1 minute, then immersed in a water bath at 25 ° C for 5 'minutes.

The resulting anisotropic membrane has an average water flux at 7.02 kg / cm 1.31 cm / min for a test period of 40 minutes. This membrane retains 77.4 $ Dextran 110 at 3.51 kg / cm2 and 25 °. The flux rate during this retention is on average 0.091 cnrVminut-cm2.

Other permeable polysulfone membranes are prepared using. (l) Ν, ΪΡ-dimethylpropionamide as the solvent and methanol as the diluent, (2) cyclohexanone as solvent and methanol as diluent.

21 U5564

Example 11

Although the main application of the microporous anisotropic membranes of the invention is believed to be in separations involving the transport of liquid across the membrane, the following examples demonstrate the use of membranes in separating gas mixtures.

An anisotropic membrane prepared as described above from a modacrylic polymer (Dynel) is used in this example.

The relative permeability of such a membrane to oxygen and carbon dioxide is determined at J5 ° C using an atmospheric driving pressure across the membrane. The supply side of the membrane, ie. the side where the oxygen and carbon dioxide are supplied are held in two atmospheres and the outlet side of the membrane is held in one atmosphere. The permeability of each gas is given below in terms of about 1 second at standard p · ..

temperature and pressure conditions per cm membrane surface.

Relative permeability to oxygen and carbon dioxide.

Q

Oxygen 8.25 x 10 "°

Carbon dioxide 1.77 x 10 “^

The porosity of the film with respect to carbon dioxide is seen to be twice that of the film's permeability to oxygen.

Example 12.

10 g of polyacrylonitrile fibers sold under the name Orlon are dissolved, with stirring 1, a solution of 70 g of zinc chloride hydrate and 50 g of water at ca. 95 ° C.

This solution is allowed to stand at room temperature so that gas bubbles which have entered the solution during stirring are avoided. A 0.178 mm extract of this solution is made on a glass plate which is left to stand for J minutes, then washed for 15 minutes in a water bath at a temperature of 70 ° C.

The anisotropic membrane formed exhibits a water flux of 1.18 cm 2 / min-cm 2 at 25 ° C and a pressure of 7.72 kg / cm 2. When testing with an aqueous solution containing 5000 parts per million albumin, a 90% retention of albumin is achieved at 7.72 kg / cm and 25 ° C.

2

Claims (5)

145564 22 A process for preparing an anisotropic polymer membrane suitable for use in ultrafiltration, with a microporous barrier layer and integral with the barrier layer on one side thereof a relatively large pore-size support layer, wherein the barrier layer and the support layer together constitute a continuous, uninterrupted synthetic organic CHARACTERISTICS characterized in that a film is prepared from a molding coating comprising synthetic organic polymer in organic solvent, thereby contacting one side of the film with a diluent having a high degree of compatibility with the organic solvent and a sufficiently low degree of compatibility with the polymer to induce immediate precipitation of the polymer from the solution upon contact with it, whereupon the diluent is kept in contact with the formed membrane until virtually all solvent has been replaced by diluent, the polymer being one which absorbs less than 10 % humidity at a temperature of 25 ° C and a relative humidity of 100%. 2. A method according to claim 1, characterized in that the conditions are chosen such that the thickness of the barrier layer is from 0.1 to 0.5 µ. 3. A method according to claim 1 or 2, characterized in that the conditions are chosen such that the transition from the barrier layer to the support layer takes place at a distance of less than half the thickness of the barrier layer. 4. A method according to claims 1-3, characterized in that the conditions are selected such that the barrier layer achieves a pore size from 1 to 500 µm. Process according to any one of the preceding claims, characterized in that the polymer is a polysulfone, a modacrylic copolymer, polyvinylidene fluoride or a polyamide.