Process for preparing polyamide membrane filter media and products thereof

Abstract: A process is provided for preparing skinless hydrophilic alcohol-insoluble polyamide membranes by preparing a solution of an alcohol-insoluble polyamide resin in a polyamide solvent, inducing nucleation of the solution by controlled addition to the solution of a nonsolvent for the polyamide resin, under controlled conditions of concentration, temperature, addition rate, and degree of agitation to obtain a visible precipitate of polyamide resin particles which may or may not thereafter partially or completely redissolve, thereby forming a casting solution; spreading the casting solution on a substrate to form a thin film thereof on the substrate; contacting and diluting the film of casting solution with a mixture of solvent and nonsolvent liquids containing a substantial proportion of the solvent liquid, but less than the proportion in the casting solution, thereby precipitating polyamide resin from the casting solution in the form of a thin skinless hydrophilic membrane; and washing and drying the resulting membrane; the alcohol-insoluble polyamide membranes obtained by this process have the unusual property of being hydrophilic, i.e., readily wetted by water, have absolute particle removal capabilities of the order of 0.1 to 5 µ M or more, and are useful as filter media, particularly for producing bacterially sterile filtrates.

Description: This invention relates to a process for the manufacture of microporous membranes of polyhexamethylene adipamide and to membranes made by that process. For many practical applications it is necessary that a membrane should be both skinless and hydrophilic. One process for preparation of membrane sheets starts from a solution of the resin or cellulose derivative, casting a film of the solution on a support, and then forming the membrane by precipitation upon immersion of the film solution in a nonsolvent for the resin. This process results in a skinned membrane, with surface portions having fewer or very much smaller pores, or even zero pores, and an interior portion with larger pores, the outer skinned portions having higher apparent density than the interior portions. Skinned membranes are non-uniform with respect to particle removal; for example, certain membranes now used for reverse osmosis are effective in accomplishing such tasks as 90% or better salt rejection, thus functioning in the 2 to 5 Angstroms (0.002 to 0.005 µm) range, but are incapable of providing sterility in the effluent, allowing bacteria with diameter in the range of 2000 Angstroms (0.2 µm) to pass. Such membranes are poorly suited when absolute removal of particulate material such as bacteria is needed. Thus, for example. US-A-3 615 024 describes the formation of anisotropic membranes having pores of from 1 to 1000 µm in diameter from a variety of synthetic resins by: (1) forming a casting dope of a polymer in an organic solvent. (2) casting a film of said casting dope. (3) preferentially contacting one face of said film with a diluent characterized by a high degree of miscibility with the organic solvent and a sufficiently low degree of compatibility with said casting dope to effect rapid precipitation of said polymer, and (4) maintaining said diluent in contact with said membrane until substantially all said solvent has been replaced with said diluent. The sub-microscopically porous anisotropic membranes of US-A-3 615 024 consist of an integral macroscopically thick film of porous polymer, usually more than 50 micrometers (about 0.05 mm (0.002 inch) and less than about 0.27 mm (0.050 inch)) in thickness. One surface of this film has an exceedingly thin but relatively dense barrier layer or "skin" of from about 0.1 to 5.0 microns thickness of microporous polymer in which an average pore diameter is in the millimicron (or nanometer) range, for example from 1.0 to 1000 millimicrons, i.e., about one-tenth to one-hundredth the thickness of the skin. The balance of the integral film structure is a support layer comprised of a much more coarsely porous polymer structure through which fluid can pass with little hydraulic resistance. By "integral film" is meant continuous, i.e., a continuing polymer phase. When such a membrane is employed as a "molecular filter" with the "skin-face" in contact with fluid under pressure, virtually all resistance to fluid flow through the membrane is encountered in the "skin", and molecules or particles of dimensions larger than the pores in the "skin" are selectively retained. Because the skin layer is of such extraordinary thinness, and because the transition from the skin layer to the macroporous support structure is so abrupt, normally less than about one-half thickness of the barrier layer or less than one micron, the overall hydraulic resistance to fluid flow through the membrane is very low; that is, the membrane displays surprisingly high permeability to fluids in proportion to its pore size. The membranes of US-A-3 615 024 are all skinned, and moreover, while the membranes are water-wettable as long as they are kept wet, once dried they are all hydrophobic, and difficult to wet with water, except with the aid of surface-active agents or other wetting aids. US-A-4 032 309 discloses the preparation of polycarbonate resin membranes described as hydrophobic, evidently of very small pore size, in the ultrafiltration range, and intentionally having a skin. US-A-2 783 894 and US-A-3 408 315 both disclose a process for producing alcohol-soluble polyamide membrane sheets using Nylon 4, poly-ε-butyrolactam. A solution of nylon can be cast as a liquid film and then converted to a solid film which presents a microporous structure when dried. An alcohol-water solution containing nylon is prepared and adjusted to the point of incipient precipitation. The solution is brought to the point of incipient precipitation by adding to the solution a solvent-miscible non-solvent which decreases the solubility of the nylon. The nylon solution is cast as a liquid film on an optically smooth surface of a solid base and then converted to a solid film by exposure to an atmosphere containing a constantly maintained concentration of exchangeable non-solvent vapors, that is, vapors of a liquid in which nylon is not soluble but which are exchangeable with vapors of the solvent for the nylon. The resulting membranes are, of course, soluble in alcohol, as well as in a considerable number of other solvents, and may not be steam sterilized, which limits the scope of their usefulness. US-A-3 876 738 describes a process for producing microporous membrane sheets from alcohol-soluble and alcohol-insoluble polyamides by casting a solution of the polymer on a substrate and then precipitating the membrane, both steps being carried out sequentially or concurrently in a quenching bath of non-solvent liquid. The nylon solution after formation is diluted with a non-solvent for nylon and the non-solvent employed is miscible with the nylon solution. This specification discusses polymer molecule aggregation in solution and the assertion is made that the "tightest or most nonporous polymer film is produced from a solution in which there is no aggregate formation". Film porosity is alleged to be determined by "control of the aggregation tendency in the casting solution. This is accomplished . . . by the addition of non-solvent or other additives to change the solvent power of the solution, hence influencing and controlling the aggregation tendency of the polymer molecules. The interaction of these aggregates in determining the resulting film structure is further influenced by the various process variables previously maintained". The product of US-A-3 876 738 has not been commercialized, and is unavailable. The formation, as proposed, of a polymer film by direct immersion of the casting resin into a quench bath is difficult, if not impossible, and tests carried out show that, as might be expected since it is not a stated objective, the membranes are all skinned. Moreover, again as might be expected since the specification refers to the addition of surfactants, the membranes prepared almost in accordance with this prior proposal are hydrophobic. A number of polyamide resin membranes have been used for reverse osmosis and ultrafiltration, but all have pore sizes below 0.1 µm, and therefore provide flow rates below the range useful in particulate and bacteria filtration. Although the pores are small enough to remove microorganisms, such as bacteria, such membranes are not suited for this purpose due to imperfections or defects which impair quantitative removal of bacteria, but instead accomplish such tasks as reverse osmosis and ultrafiltration, which are not quantitative, and which can tolerate the imperfections which characterize skinned nylon membranes. US-A-3 980 605 provides semi-permeable membranes made from mixtures of polyamides, especially N-alkoxyalkyl polyamides, and water-soluble polyvinyl alcohols. The membranes are useful for separating chemicals from their mixtures by techniques using an aqueous liquid barrier and complex-forming metals, e.g., for the separation of ethylenically unsaturated hydrocarbons such as ethylene from close-boiling hydrocarbons, but such membranes have pore sizes too small and porosities too low to provide flow rates useful in particulate and bacteria filtration. It is an unfortunate fact that most available membrane sheets are hydrophobic, i.e. not readily wetted by water. Synthetic resin membrane sheet has almost invariably been made of hydrophobic synthetic resin, and retains the hydrophobic characteristic of the polymer from which it has been made. Conventional cellulose ester membranes are also hydrophobic. Of the available membrane sheets useful in the particle separation range only regenerated cellulose sheet and alcohol-soluble polyamide membrane sheet are hydrophilic, i.e., wettable by water. Since the bulk of filter applications for membrane sheet is in the filtration of aqueous media, it is essential to obtain an adequate wetting of the sheet to facilitate filtration, but this is not easy to accomplish. Surface-active agents can be added to the medium being filtered, to enable the medium to wet the sheet sufficiently to penetrate it for filtration. However, the addition of foreign materials such as surface-active agents is not possible or desirable in many applications, as for example, in assaying bacteria, since some bacteria are killed by surface-active agents. In other applications, filtering media cannot be adulterated by the addition of surface-active agents without deleterious consequences. According to the present invention there is provided a process for preparing a microporous skinless, hydrophilic, micro-porous membrane of polyhexamethylene adpiamide, comprising the steps of preparing a solution of the polyamide in formic acid, adding a prescribed quantity of water with mixing in an in-line rotary mixer thereby inducing nucleation of the solution, forming a film of the nucleated solution on a substrate, immersing with as little delay as possible the film and substrate in a quench bath containing formic acid and water, removing the resultant membrane from the bath, washing to remove the formic acid from the membrane and finally drying the skinless, hydrophilic micro-porous membrane, wherein the substrate, unless porous, is wettable by the solution and the quench bath, and formic acid concentration in the quench bath is not less than 37% and not more than 50% and formic acid concentration in the casting solution is 63% to 72%. The membrane can be stripped from the substrate and dried, or if the substrate is porous, it can be incorporated in the membrane or attached to the membrane to serve as a permanent support, and is then dried with the membrane. The conditions under which the polyamide resin is precipitated determine the skinless nature of the membrane, as well as its physical characteristics., i.e., the size, length and shape of the through pores of the membrane. Under certain conditions, a membrane is formed which has through pores extending from surface to surface that are substantially uniform in shape and size. Under other conditions, the through pores are tapered, being wider at one surface and narrowing towards the other surface of the membrane. Thus, by control of the process by which the casting resin is nucleated, and of the precipitation conditions, it is possible to obtain hydrophilic polyamide resin membranes with through pores of desired characteristics, either uniform from face to face, or tapered, with larger pores on one face transitioning to finer pores on the other face. The formation of a polyamide membrane having uniform pores or tapered pores without a skin on either surface is also remarkable. As shown by US-A-3 615 024 and US-A-3 876 738 precipitation of a polyamide resin membrane in a non-solvent is known to result in a skinned membrane. The formation of a hydrophilic skinless polyamide resin membrane by the process in accordance with the invention has not previously been achieved. The process can provide membranes having absolute particle removal ratings of 0.10 micrometer to 5 micrometers or larger. Two membranes made by the process in accordance with the invention can be combined to form a dual layer and then dried together while under constraint to prevent shrinkage. The membranes thus attached can have the same or differing porosities, and the membrane layers can be selected from membranes having tapered pores and membranes having uniform pores, in any combination, supported or unsupported. The combined membranes can also be obtained from a single roll of filter medium, and when combined with matching faces in contact to form a sheet which is symmetrical, and which provides equal filtration characteristics regardless of which face is upstream. The membranes preferably have absolute removal ratings within the range from 0.1 micrometer to 5 micrometers, and a thickness within the range from 0.025 mm to 0.8 mm. The membranes may be supported by the substrate on which the membrane is formed, either imbedded therein, or having the substrate attached to one face thereof. The hydrophilicity of membranes made by the process in accordance with the invention is a most remarkable property, inasmuch as the alcohol-insoluble polyamide resin from which the sheet is made is hydrophobic. The reason why the polyamide resin membrane sheet is hydrophilic is not at present understood, but it appears to be due to a spatial orientation of the hydrophilic groups of the polymer chain that is fixed in the solid polymer membrane surface as a result of a precipitation process. It may be related to crystal structure or to solid structure, or to some spatial form of the NH and/or CO groups on the surface of the membrane sheet, facilitating its being wetted by water. The fact is that a drop of water placed on a dry polyamide resin membrane sheet made by the process in accordance with the invention will penetrate into the sheet and disappear within a few seconds. A sheet of the dry membrane placed on the surface of a body of water will be wetted through and may even sink in the water within a few seconds. If the membrane is completely immersed in water the membrane is wetted through in less than a second. The capability of a membrane's or substrate's being wetted by water is determined by placing a drop of water on the membrane or substrate surface. The angle of contact provides a quantitative measure of wetting. A very high angle of contact indicates poor wetting, while a zero angle of contact defines complete or perfect wetting. The polyamide resin from which the membranes are made have a high angle of contact. The wettability of the membranes made by the process in accordance with this invention is not a function of retained water. Membrane specimens dried at 177°C for 72 hours in an inert atmosphere, in vacuum, and in air, are unchanged with respect to wettability by water. If, however, they are heated to a temperature just below the softening temperature of the membrane (to heat at a higher temperature would, of course, destroy the membrane, since it would melt), the membrane reverts to a hydrophobic material. This suggests that the hydrophilicity is a function of solid structure, and is obtained by the process of membrane formation, probably during precipitation of the membrane in the course of the process. It may be associated with crystal structure or it may only be associated with noncrystalline or amorphous solid structure, but it does appear to be related to a physical orientation of the hydrophilic groups in the polyamide chain, which orientation is lost when the membrane film is heated to a high enough temperature to permit reorientation to a normal configuration, in which the material is hydrophobic. It follows, of course, that during processing and drying it is important not to heat the membrane above this temperature. A further important characteristic of the polyhexamethylene adipamide membrane sheets made by the process in accordance with this invention is their high flexibility. In the normal thickness range in which they are useful, in the absence of an extreme state or dryness, they can be folded back and forth on themselves several times without harm, and without the addition of a plasticizer. To determine the effectiveness of membranes made by the process in accordance with the invention, a procedure for quantitative characterization of uniform pore membranes for their ability to provide sterile effluent when challenged by a stated number of a given microorganism has been devised. This procedure is also applicable to uniform pore distribution membranes made of other than polyamide resins, and using other processes; it is not part of the claimed invention. The invention will now be further described and explained with reference to the accompanying drawings in which: Figure 1 is a graph showing in a qualitative manner the relationship between the degree of nucleation of the casting resin solution made in accordance with the process according to the invention and the pore diameter of the resulting membranes; Figure 2 is a graph showing the relationship for a uniform pore membrane between titer reduction, defined as the ratio of Pseudomonas diminutiae bacteria contained in the influent liquid to that obtained in the effluent liquid, and the number of layers of uniform pore filter medium through which the bacteria laden liquid is passed; Figure 3 is a graph showing the relationship obtained when a wetted membrane is pressurized by a gas and the ratio is plotted against air pressure applied. The quantity K L is defined by the broken line of Figure 3. Figure 4 is a graph showing the relation between T R1 and K L , where T R1 =T R 1/t (or Log T R1 =1/t Log T R ) where t is the thickness, in millimeters of the uniform pore membrane which shows a titer reduction, as defined above, equal to T R ; T R1 is the calculated titer reduction for a 0.025 mm thick membrane of equal pore size; K L is the pressure, measured in Kg/m² at which air flow through the water wet membrane increases very sharply (see Figure 3); and K L5 is the value of K L corrected to correspond to that of a membrane 0.13 mm thick, using the empirically determined correction factors listed in Table I. Measured thickness mm Correction factor 0.051 1.10 .076 1.044 0.10 1.019 0.13 1.000 0.15 0.985 0.20 0.962 0.25 0.946 0.38 0.920 The curve of Figure 4 represents the results of measuring the K L and T R for forty-five different specimens made by the process of this invention. Figure 5 is a scanning electron micrograph (SEM) at 1500× magnification of a membrane with uniform pores, made by the process of this invention, having a K L5 of 3.3 × 10⁴ kg/m² t=0.094 mm, and an estimated T R of 3 × 10¹⁸ for Pseudomonas diminutiae organism. The center portion of this micrograph shows a section through the thickness of the membrane, in which the pore sizes are seen to be uniform from surface to surface. The upper and lower micrographs show the upper and lower surfaces respectively adjacent to the section, and the pore size at each of these surfaces may also be seen to be equal, Figure 6 is a scanning electron micrograph at 1000× magnification of another membrane with uniform pores made by the process of this invention, having a K L5 of 2.8 × 10⁴ kg/m² t=0.140 mm, and an estimated T R of 8 × 10¹⁵ for Pseudomonas diminutiae organism. Similar to Figure 5, the center portion is a section through the membrane showing uniform pore size from surface to surface, and the upper and lower views show the adjacent upper and lower surfaces, again indistinguishable with respect to pore size, Figure 7 is a scanning electron micrograph at 1000× magnification of a membrane with tapered pores made by the process of this invention. This membrane is 81 micrometers (.081 mm) thick, and the upper portion of the section may be seen in the central part of the SEM to be considerably smaller in pore diameter than the adjacent material, with the pore diameter gradually transitioning to the larger size. Comparing the top and bottom views, the pore diameters in the upper surface are substantially smaller than those on the lower surface, Figure 8 is a scanning electron micrograph at 1500× magnification of a lightly skinned membrane, of the type obtained when baths outside of the range of this invention are employed, Figure 9 is a similar micrograph of a more heavily skinned membrane; and Figure 10 is a graphical representation of the relationship between (a) K L5 , a particle removal rating parameter of membranes made by the process of this invention; (b) the mixing intensity, expressed as revolutions per minute (rpm) of the in-line mixer used to process a 15.5% solution of resin in 98.5% formic acid to obtain the casting solution used to produce the membranes; and (c) the formic acid concentration of the resulting casting solution. The process of the invention is applied to polyhexamethylene adipamide (Nylon 66), and this readily produces skinless hydrophilic alcohol-insoluble polyamide resin membranes. For reasons which are not understood, this polyamide resin is quite susceptible to precipitation under the process conditions of the invention to form microporous, hydrophilic skinless membrane sheets. The formation of a hydrophilic membrane appears to be related to the chemical composition of the polymer, i.e., to the arrangement of the units composing the polymer chain. Polymer free of additives is generally preferred, but the addition of antioxidants or similar additives may have benefit under some conditions. The solvent for the polyhexamethylene adipamide is formic acid at any temperature from freezing to boiling, this mixture will be referred to as the starting resin solution. If the starting resin solution is to be stored for more than a few hours, water in excess of 1 to 2% should not be present, as otherwise a slow hydrolysis of the resin takes place, resulting in an undesirable reduction in molecular weight. In general, the amount of water should be less than 2%, and preferably the solution is water-free. The casting resin solution is prepared from the starting resin solution by diluting it with water, or with a mixture of formic acid and water. The state of nucleation of the resulting casting resin solution is affected by the following factors: 1) Concentration and temperature of the starting resin solution. 2) Temperature of the water, or of the water/formic acid mixture. 3) The rate at which the water, or the water/formic acid mixture, is added. 4) The intensity of mixing during the addition. 5) The geometry of the apparatus in which the mixing is accomplished; and 6) the temperature of the resulting casting resin solution. The casting resin solution so prepared is then formed into a thin film by casting it onto an appropriate substrate, and the film is immersed with minimum delay (preferably within 5 minutes to 60 minutes from the time of addition of the water) into a bath containing water, together with a substantial proportion i.e. not less than 37% and no more than 50% of formic acid. This amount of formic acid is desired to prevent formation of a skinned membrane, which occurs at lower concentrations of formic acid. Casting resin solution prepared using a continuous in-line mixer, which can produce a membrane of equal characteristics, tends to be stable for a period of an hour or more unless maintained at elevated temperature. The addition of the non-solvent is accompanied by the appearance of a visible precipitate, in order to produce a useful, properly nucleated casting resin solution. Useful membranes are those with uniform or tapered pore structures, skinless, with permeabilities to air and water such that substantial quantities of fluids can be passed at low pressure differentials, while providing a required degree of filtration. A convenient index of usefulness may be obtained by considering the permeabilities to air and to water of uniform pore cellulose ester membranes now on the market made by the so-called dry (evaporative) process. These are shown in Table II under, together with typical permeabilities of similar range media made in accordance with the process of this invention. "Nucleation" and "state of nucleation" are believed to account for the facts that (a) casting resin solutions can be prepared with a variety of ratios of polyhexamethylene adpiamde to formic acid solvent, which yield identical or nearly identical membranes; and (b) casting resin solutions can be prepared, which have equal resin, solvent and non-solvent concentrations, which are then cast at equal temperatures into the same bath, yet yield very different membranes; the resulting membranes extend from "not useful" in the sense of having significantly lower flows-by factors of 2 to 5 or more-compared with Table I, through the range from 0.1 µm or coarser, producing membranes in all those ranges with good flow capacities, for example, equal to those listed in Table II. Since the preparation of casting resin solutions capable of producing membranes with flow properties in the useful range has been observed to invariably be accompanied by the local precipitation and at least partial redissolution of solid resin, and since if is well known to those familiar with the chemical arts that the characteristics of a solid precipitated from solution can be greatly influenced by the presence or absence of submicroscopic nuclei, we have chosen to use the term "state of nucleation" to distinguish casting solutions having equal composition, but diverse results, as described in paragraph (b) above, and to account as well for the observation of paragraph (a). The assumption that nucleation accounts for the differences in behavior of membranes made from casting resin solutions of equal composition is confirmed by the results of an experiment in which a stable casting resin solution was prepared, with the degree of nucleation controlled to yield a 0.4 µm absolute membrane. A portion of the casting resin solution was subjected to fine filtration to determine whether nucleation behavior would be affected, and the properties of membranes cast from the two lots of casting resin solution were compared. Examples 58 and 59 show the results of this experiment; product characteristics are greatly altered by fine filtration; the finally filtered resin solution produces a membrane with a very poor ratio or flow capacity to removal rating; the Δp of Example 59 is more than three times higher than that for a similar membrane made using a properly nucleated casting resin solution. This result supports the theory that resin nuclei are developed during the controlled-condition dilution used to prepare the casting resin solution, whose number, size, or other characteristics strongly influence the characteristics of the membrane generated by that casting resin solution, and that at least a portion of these nuclei were removed by fine filtration. The control of the precipitation so as to obtain the formation of a microporous, hydrophilic membrane sheet of desired flow characteristics and pore size requires that the casting resin solution be controlled with respect to the characteristic referred to herein as "nucleation". The variables that must be controlled include the concentration of the resin in the starting polyamide resin solution, temperatures of all components, the quantity and mode of addition of water, including rate of addition, intensity' of mixing during addition, and the geometry of the apparatus, the latter including especially size and location of the nozzle through which the water is added. The effect of these variables on the degree of nucleation is qualitatively stated in Table III. Variables affecting degree of nucleation Type of variable Variable Direction of change to obtain a higher degree of nucleation Mixing conditions Temperature Decrease Rate of water addition Increase Size of inlet opening through which the water is fed Increase Distance of the inlet opening from actual mixing area Increase Intensity of mixing Decrease Concentration of the components in the casting solution % of resin Increase % of water Increase Degree of non-solvency of the nonsolvent Increase In Table III, the concentration of formic acid is not included, as it is defined by the concentration of the resin and water. It will be appreciated that the intensity of mixing in a given system is a function of a large number of variables. However, for a given system the relative intensity of mixing can be expressed in terms of the rotation rate in the in-line mixer and in a suitably designed multiblade mixer about 0.2-1.5K w hp is required to produce about 30 kg per hour of 2000 m.Pa.s casting resin solution at a rotation rate between about 200 to 2000 rpm. Such equipment can take diverse forms, and can take any of a number of the designs commonly used in the mixing art, since the various mixing principles can all lead to similar results. The importance of mixing intensity and of the other conditions related to mixing cannot be overemphasized. For example, a series of casting resin solutions with the same concentrations of the same resin, formic acid, and water, and the same temperature and viscosity can be produced by simply changing the mixer rpm. The most highly nucleated of these casting resin solutions, made using the slowest mixer speed, will then produce a membrane having an absolute pore rating of 0.1 µm; the next more highly agitated casting solution, cast into the same bath, will, if the mixing rate was correctly chosen, produce a 0.2 µm absolute membrane, and similarly by using successively higher mixing rates membranes can be made with absolute ratings of 0.4, 0.6 or 0.8 µm, etc. The nozzle diameter through which water is delivered during preparation of the casting resin solution is also very important. It is at this nozzle that the precipitate forms, which at least in part subsequently redissolves, and the formation and complete or partial redissolution of the precipitate appears to play an essential role in the preparation of the casting resin solutions in accordance with this invention. With all other parameters maintained equal, a casting resin solution of quite different characteristics, in terms of the pore size of the resulting membrane, will be obtained by simply varying the diameter of the nozzle. We have used nozzle diameters varying from 0.33 mm to 3.175 mm (0.013-0.125 inch) diameter, but smaller or larger nozzles could be used with successful results. The relationship between the degree of nucleation and the absolute particle removal rating of the resulting membrane is shown in Figure 1, which shows an inverse relationship between the pore diameter of the membrane sheet and the degree of nucleation, i.e. to obtain small pore diameter, a high degree of nucleation is required. Reference to the graph of Figure 1 shows that in Region A, where the degree of nucleation is very small, the pore size tends to become non-reproducible. In addition, the pressure drop at a given pore diameter is high. Membranes made on the assumption that the concentrations of the components are the controlling factors, and without nucleation, for example, by the process disclosed in US-A-3 876 738 fall into this range, and tend to be of relatively poor quality. In Region B, the pore size decreases in a regular, though not necessarily linear fashion, as the degree of nucleation increases. In Region C, the casting resin solution becomes increasingly populated by particles of resin which have not redissolved, but still produces good quality membrane if these are removed by filtration prior to casting; and in Region D, the resin solution from which these lumps have been removed by filtration becomes unstable, and prone to early local or overall gelation before the film can be cast. The very high degree of nucleation in Region D is sometimes manifested by an opalescent appearance, suggesting that the nucleation procedure has resulted in an excessive number and/or excessively large nuclei. Because methods of achieving a required intensity of mixing vary so greatly among the various types of equipment used in the mixing art, it is not possible to quantify this characteristic. Consequently, any given apparatus must initially be used on a "cut and try" basis to produce casting solutions of the desired characteristics, applying the principles tabulated in Table III. Once the parameters of mixer rate, concentrations, temperatures, flow rates, etc. have been established, casting resin solutions having quite reproducible characteristics can be produced in the B and C Region of Figure 1, on successive days or weeks of operation. The viscosity of the casting resin solution is preferably adjusted to between about 500 m.Pa.s and 5000 m.Pa.s at the temperature existing at the time it is cast as a film. Viscosities below about 500 m.Pa.s allow some of the cast film to float off as a liquid to the surface of the bath, when it forms a filmy precipitate, thereby adversely affecting cast membrane properties and fouling the bath. Viscosities much above 5000 m.Pa.s. for example, 100,000 m.Pa.s. are not needed to obtain a smooth, coherent cast film, but are helpful in casting membranes where no substrate is used, for example, hollow fibers, or unsupported film. Solutions of a viscosity well above 5000 m.Pa.s at the casting temperature can be cast without difficulty; however, the preferred viscosity limit is about 5000 m.Pa.s, since at higher viscosities the energy input to the mixture when water is blended with the polyamide resin solution is very high, with the result that the solution can reach excessively high temperature, with ensuing operating problems. Moreover, the pumping of the hexamethylene adipamide solution to the casting operation becomes progressively more difficult, as viscosity increases. Also, manipulation of the casting resin solution within the reservoir from which the resin is cast as a film on the substrate becomes troublesome, if the viscosity is very high. When a porous substrate is used, with the intention of completely impregnating it with casting resin solution, viscosities much above 3000 m.Pa.s can cause improper penetration, and the resulting product has undesirable voids. The temperature of the casting resin solution is not critical, and useful membranes have been made over the range from 85°C downward. Under some circumstances, somewhat higher flow rates relative to removal rating are obtained by reducing the resin temperature to a lower value prior to casting the films. After the cast film of liquid enters the quenching bath a precipitation process occurs, whose mechanism is not completely understood. The formic acid/water mixture of the bath diffuses into the cast film, and the solvent mixture in the casting resin solution diffuses out of the film into the bath, but it is not understood why this results in a uniform pore size throughout the thickness of the film when the bath solvent: non-solvent ratio is held within certain limits. If the bath contains only water or water with a small proportion of solvent (e.g. water with less than 15 to 20% of formic acid precipitation occurs very rapidly, and a solid membrane is formed within a few seconds, typically in less than 1 to 10 seconds. Membranes made in this manner are heavily skinned, regardless of the mode of preparation of the casting resin solution. If the bath contains 43 to 47% of formic acid in aqueous solution, and the casting resin solution is properly nucleated as described herein, the resulting membrane will be uniform in pore structure from face to face, provided only that if cast on a solid substrate, the surface of the substrate be wetted by the casting resin solution and by the bath solution. The time required for the film to form under these circumstances is a function of the following: (a) Casting resin solutions which produce membranes which have high K L values (e.g. in excess of 7 × 10⁴ kg/m²) set very rapidly, e.g. in less than 10 seconds. Less highly nucleated casting resin solutions, producing membranes with K L values of 2.8 to 3.5 × 10⁴ kg/m² will typically set in the 10 to 20 second range, and the setting time continues to increase as K L decreases, such that membranes about 0.15 mm thick with K L values of under 1.40 × 10⁴ kg/m² require about 5 minutes or more to set, and still lower K L 's require still longer periods. (b) Thickness of the cast film is an important parameter, setting times being shorter for thin films. (c) Use of lower casting resin solution temperatures results in faster setting. (d) Setting is faster at the low end of the 43 to 47% range, and can be further speeded by use of bath concentrations of less than 43% formic acid, at the cost of only slight deviation from pore uniformity. As the bath concentration decreases to and below the 40 to 43% range, the membranes become progressively more asymmetric, progressing from uniform as shown in Figures 5 and 6, to tapered pore as shown in Figure 7, to skinned as shown in Figure 8, to heavily skinned as shown in Figure 9. Operation at formic acid concentrations much lower than those producing tapered pores as exemplified by Figure 7 is undesirable. It is important that the casting resin solution be clear, and free from suspended material, before being spread upon the substrate to form a film. If suspended material is present, such as undissolved resin particles, these are removed by screening or filtration before casting. If a non-supported membrane film is the desired product, then the substrate should have a surface to which the membrane does not adhere, and from which the membrane film can readily be stripped at the conclusion of the drying operation. Strippability usually requires that the substrate surface be smooth-surfaced, and non-porous. As both water and formic acid have relatively high surface tension, it is important that the non-porous surface on which the film is cast be wettable, i.e., have zero or near zero angle of contact, when contacted by the casting resin solution, and preferably also by the bath as well. Failing this condition, a skin will form on the membrane on the substrate side. Such temporary substrate can be glass, metal or ceramic. Plastics, such as polyethylene, polypropylene, polyester, synthetic and natural rubber, polytetrafluoroethylene, polyvinyl chloride, and similar materials are not inherently suitable, as they are not wetted by the casting resin and water, but these can be rendered suitable by application of an appropriate oxidative or similar surface treatment. A corona discharge can, for example, be used to treat Mylan® (polyester) film, and polypropylene. The substrate can be made of or merely surfaced with such materials. If the substrate is to form a part of the final membrane film, as a permanent supporting layer, then it should be of porous material that preferably is wetted by the casting resin solution, so that the casting resin solution will penetrate during the casting of the solution on the substrate, and become firmly attached thereto during precipitation of the membrane film. It is not essential however that the substrate be wetted: if it is not wetted, the film will be largely confined to the surface of the support, but is nonetheless adherent thereto. Such substrates can, for example, be of fibrous material, such as non-woven mats and bats, and woven textiles and cloth, as well as netting, including extruded plastics filament netting, papers, and similar materials. As permanent supports which are not wetted by the casting resin solution, fine-pored non-woven webs can be used, made from fibers with poor wetting characteristics, such as, for example, polypropylene or polyethylene. The resin solution is cast as a film onto the non-woven web, and since it does not wet the fibers of the web, it is carried on its surface. The substrate carrying the casting resin solution film on its lower surface is plunged into a bath of formic acid and water or allowed to float on the surface of the bath, and the membrane film is precipitated onto the substrate. The resulting film has good adhesion to the substrate, and the substrate has very little or no effect on the pressure drop. In the case of permanent supports which are wetted by the casting resin solution, the fibers of which the substrate is made should have a relatively high critical surface energy, such that the casting resin solution film will completely permeate the supporting web, and the resulting membrane precipitates in and around the fibrous material, and is permanently supported thereby, since the material of the support is embedded in the membrane. The resulting membrane has a somewhat higher pressure drop when tested with flowing fluid but the increase compared with the unsupported membrane is small, if the supporting web has an open structure. Suitable wetted substrates that can serve as permanent supports for the membrane include polyesters, as a non-woven fibrous web or a woven web, using monofilament or multifilament yarn, the monofilaments being preferable in terms of open structure and lower pressure drop; also polyimide fiber woven webs, woven and non-woven webs of aromatic polyamides or Nomex, and other relatively polar fibrous products such as cellulose, regenerated cellulose, cellulose esters, cellulose ethers, glass fiber, and similar materials. Cellulosic and synthetic fiber filter papers can be used, as well as perforated plastics sheets, and open mesh expanded plastics. If the substrate is relatively coarse or in a very open weave structure, even if the fibers are not well wetted by the resin solution, the substrate may nonetheless be embedded or embraced by the membrane material in the final supported membrane product; such relatively poorly wetted materials as polypropylene and polyethylene can function as embedded substrates if they have a sufficiently open structure. If a polyolefin substrate has a relatively smaller pore size, for example, about 30 micrometers, the casting resin solution will not penetrate into it, but will instead form a membrane external to, but adhered to, the polyolefin substrate. In a continuous process, the substrate can be in the form of an endless belt, which circulates through the entire film-forming operation, from casting of the casting resin solution film into and through a precipitating bath of water and formic acid, and then through the bath liquid removal step. A corrosion resistant metal drum, or endless metal belt can be used, but the surfaces on which the film is cast should be treated or coated so as to make them wettable. The nucleated casting resin solution can be cast or spread upon the substrate in the desired film thickness using a conventional doctor blade or roll, kissing or squeeze rolls or other conventional devices, and then contacted with the bath liquid with as little delay as possible. Following precipitation, the membrane film is washed to remove formic acid. Water is suitable, but any volatile liquid in which the solvent is soluble and that can be removed during drying can be used as the washing liquid. One or several washes or baths can be used as required to reduce solvent content to below the desired minimum. In the continuous process, wash liquid flow is countercurrent to the membrane, which can, for example, be passed through a series of shallow washing liquid baths at the washing stage. The amount of washing required depends upon the residual solvent content desired in the membrane. Residual formic acid can cause hydrolysis during storage, with a consequent reduction in molecular weight; therefore, the washing should be continued until the formic acid level is low enough to prevent any significant hydrolysis during the anticipated storage period. The drying of the washed membrane film requires a technique that takes into account the tendency of the membrane to shrink linearly when dried unsupported, with the result that the dried membrane film is warped. In order to obtain a flat uniform film, the membrane must be restrained from shrinkage during drying. One convenient way to do this, is to roll up a continuous web on a core, with a high degree of tension so as to obtain a tight roll, then firmly wrap this with a rigid but porous outer wrap, and then dry the assembly. Other methods of preventing shrinkage, such as tentering, or drying in drums under felt, are also satisfactory. Individual membrane sheets of a selected size can be dried to produce flat sheets free of warpage by clamping the sheets in a frame restraining the sheet from shrinkage on all four sides, and then heating the framed membrane at elevated temperature until ii has been dried. Two or more equally sized membrane sheets can be placed in contact and dried together in a frame to prevent shrinkage. When this is done, the contacting layers adhere to each other, and can thereafter behave as though they were a single sheet. When the individual starting sheets are relatively thin, e.g. under 0.127 mm thick, and are of the unsupported (substrate free) type, they may be subsequently cut to size, for example, by steel rule dies, and are thereafter for practical purposes a single sheet or disc of filter medium. The membranes can be dried in any of the ways described above, and then corrugated, seamed to provide a closed cylinder, and end capped. This process can be greatly simplified, while producing a superior product, by corrugating the filter medium while it is still wet, together with upstream and downstream layers of dry porous material, this material being chosen to be relatively rigid, and subject to no more than a small shrinkage during the drying operation. The corrugated pack so formed is lightly compressed, so that the corrugations are in firm close contact, while being held in a holding jig, preferably one perforated to allow free access for heating and escape of vapour, and placed in an oven to dry. The resulting dried corrugated assembly shows only slight shrinkage, and the corrugated membrane so obtained is free of warpage, with well formed smooth corrugation crests, and flat faces between. When formed into a filtering element by side seaming and end capping, the porous support layers provide flow spaces for access of upstream (dirty) fluid and passage out of the element for downstream (clean) fluid. If the filter cartridge is made using two or more thin layers of the polyamide membrane, these will be firmly adhered to each other at the conclusion of the drying operation, and behave mechanically as though they were a single layer. A favourable condition for producing membranes having low pressure drops and particle removal ratings covering a wide range utilizes a starting resin containing 15.5% of 42,000 molecular weight Nylon 66 resin, 83.23% of formic acid, and 1.27% of water. When this starting resin solution is diluted using the conditions of Examples 1 to 39, the results depicted in Figure 10 are obtained. The product range of K L5 is such that membranes are obtained with absolute particle ratings ranging from about 0.1 micron (for example, a 0.3 mm thick membrane with K L5 = 7.0 × 10⁴ kg/m²) to about 1 µm (for example, a 0.025 mm thick membrane with K L5 = 1.9 × 10⁴ kg/m²). The curves of Figure 10 are obtained using a specific in-line mixer configuration, in which the rotor was 6.35 cm in diameter. The same results can be obtained by using other mixers, and the RPM needed to produce these results may vary; however, it is within the ability of a person familiar with the art to determine by test the conditions required with his apparatus to duplicate the intensity of mixing represented, for example, by the 1950 RPM and 400 RPM conditions of Figure 10, and once this has been accomplished the conditions for making membranes covering the whole range of Figure 10 will be apparent to him. This same correlation of mixing conditions would then be equally applicable to the other Examples using an in-line mixer. As previously described, three types of substrates are used: (a) non-porous, for example, commercial polypropylene or other plastics film or glass; (b) porous, not wetted by the casting resin solution; and (c) porous, wetted by the casting resin solution. The concentration of the water and formic acid in the precipitation bath have an important effect on the properties of the resulting membrane. If the concentration of formic acid is zero, or at low level, for example, below 20%, a heavily skinned membrane (outside the invention as claimed) will be obtained. If the concentration is adjusted to one of the preferred ranges of this invention-43 to 47% of formic acid, the resulting membrane has uniform pores from one face to the other. If the bath concentration is 43 to 47, and the substrates used are of types (b) or (c), the pores will always be uniform through the thickness of the polyamide membrane. However, if the film is cast on a non-porous substrate of type (a), it is important that the substrate surface be wettable by the casting resin, and by the bath fluid. Glass, and similar surfaces, are naturally so wetted; however, synthetic plastics, such as polyethylene, polypropylene, polyvinyl chloride, and polyester are not, and if the casting solution is spread on such a substrate, and immersed into a 45% formic acid 55% water quench bath, it will form a film with open pores on the face in contact with the bath, the pores being uniform throughout most of the body of the film, but with a dense skin on the substrate face. If such plastics films are rendered more wettable, for example, by surface oxidative processes such as chromic acid treatment or corona discharge treatment, the resulting membrane is skinless on both faces, and of uniform pore size throughout. In such a membrane, it is difficult if not impossible to determine by any manner of appraisal which face was in contact with the substrate. To obtain such skinless membrane sheets, a wide range of surfaces can serve as the substrate, provided that the critical surface energy is maintained at a sufficiently high value. This will vary somewhat depending on the concentration of formic acid in the resin solution and in the bath, and the temperature, and is best determined by trial-and-error treatment of the substrate surface for a given system. Critical surface tensions required are generally in the range from about 45 to about 60 dynes/cm, and most often in the range of from 50 to 56 dynes/cm. If a given casting resin solution is immersed as a film into a series of baths, each with slightly increasing water content, the characteristics of the membrane on the side facing the bath will gradually change, producing film which have finer pores at and near this face, compared with the balance of the thickness of the membrane. These finer pores show a gradual transition into the uniform pores of the balance of the membrane. Such membranes are described herein as "tapered pore membranes", and are useful in that, when filtering some suspensions, with flow from the coarser to the finer side, longer service life (higher dirt capacity) is obtained, with equal removal. Figure 7 shows scanning electron micrographs of a tapered pore membrane. The bath solvent concentration required to obtain any desired taper pore membrane varies considerably, depending, for example, on the state of nucleation of the casting resin solution, and should be determined for a given set of conditions by trial-and-error; however, in the case of a water-formic acid bath, it is never less than 15 to 25%, and usually is near to 30 to 35% of formic acid (conditions outside the range in the presently claimed process). As the bath water concentration increases, the membranes begin to form with increasingly heavier skins, and are characterized by high pressure drop, and poor pore size distribution characteristics. The uniform pore membranes made by the process of this invention, such as those shown in the scanning electron micrographs of Figures 5 and 6, are characterized by liquid displacement curves such as shown in Figure 3. When the membrane is immersed into water, its pores are filled by the water forming within the membrane a film of immobilized water, which remains in place when the membrane is removed from immersion. When air pressure is then applied across the membrane, there is noted a very small flow of air. This air flow when divided by the applied air pressure remains constant as the pressure is increased, when plotted as in Figure 3. From the thickness of the film, and the known diffusion constant of air in water, it can be calculated using Fick's law, that this flow is due to diffusion of air through the water film, and does not indicate flow through pores of the filter medium. At a sufficiently high pressure, the flow as plotted in Figure 3 is seen to increase suddenly, reflecting displacement of water from the largest pores, and flow of air through these pores, and the curve becomes nearly vertical. The sharpness of this rise will be appreciated by noting that in this region, the membranes made by the process of this invention require less than a 1% to 3% increase in pressure drop to accomplish a 5000 fold increase in air flow rate. The rapid transition from zero flow of air (except that due to diffusion) to a very steeply rising rate of flow for small changes in applied pressure, characterizes uniform pore media, which have sharply defined removal characteristics; such media will, for example, quantitatively remove one bacterium, but will allow an only slightly smaller organism to pass. Such membranes generally also have favorably low pressure drop, for a given removal. Skinned membranes behave very differently; when water wetted and their air flow-pressure drop relationship is determined, the curve is not flat initially, but slopes upward, indicating presence of large pores; transition to a more nearly vertical line is slow, with a large radius, and in the "vertical" area, instead of the sharp rise of Figure 3, a sloping line is obtained, reflecting a wide pore size range. Such membranes are poorly suited to obtain sterile filtrates when challenged by bacteria; either a nonsterile fluid is obtained, or if sterility is gotten, it is at the cost of very high pressure drop to achieve a low throughput rate. It is apparent from the preceding discussion that control within narrow limits of the concentration of formic acid in the water of the quench bath is desirable to obtain a uniform product. In a continuous process, this control is obtained by an appropriate feed of water to the bath, while simultaneously withdrawing some of' the bath liquid to maintain constant total bath volume. A relatively higher concentration of formic acid enters the bath from the casting resin solution, and the concentration of formic acid in the bath therefore tends to increase. Water is therefore constantly added to the bath to compensate. Accordingly, control of the rate of addition of water and of the rate of withdrawal of surplus bath solution will give the desired result: substantially constant concentration of formic acid in the solution, within the limits that give a membrane of the characteristics desired. Thus, Example 47 shows that in order to obtain a skinless membrane sheet having a uniform pore distribution, with fine enough pores to quantitatively remove all incident bacteria and particles over 0.2 µm a relatively highly nucleated casting resin solution is cast as a film and the membrane precipitated in a 46.4% aqueous formic acid solution as the bath liquid. To produce a membrane with tapered fine pores, a film of less highly nucleated casting resin solution is precipitated in membrane form by an aqueous 25% formic acid solution as the bath, as in Example 50 (conditions outside the presently claimed process). It is instructive to note that in the range of 0.2 µm and below, the uniformity from face to face of commercially available regenerated cellulose and cellulose ester membranes becomes quite poor, and such membranes are to some degree tapered pore types. In the same range, membranes made by the process of the invention remain uniform, or may be tapered, as desired. Thus, in the continuous production of membrane sheets, to obtain uniform characteristics in the membrane, the casting resin solution must be prepared under carefully controlled conditions and the bath liquid composition must remain constant. Such a liquid is referred to as an "equilibrium bath", i.e., a bath in which the concentration of ingredients remains constant, regardless of additions and withdrawals. To illustrate, consider a casting resin solution containing 13% resin and 69% formic acid with the balance water, continuously being cast in film form on a substrate, and then plunged into an aqueous nonsolvent bath containing 46% formic acid. As the resin membrane precipitates, a proportion of the solvent from the film of casting resin solution (which contains 69 parts of formic acid to 18 parts of water, or 79.3% formic acid) diffuses into the bath, thereby altering its composition. To counteract this, water is continuously added to the bath at a rate controlled, for example, by a device using density measurements to report formic acid concentration, at the 46% level, and bath liquid is withdrawn continuously to maintain total bath volume constant. Maintaining this equilibrium bath makes it possible to continuously produce a membrane sheet having uniform pore characteristics. When used continuously, at high production rates, the bath temperature will gradually increase; cooling by a heat exchanger may be used to maintain constant conditions. From the above-mentioned casting resin solution and bath, unsupported membrane sheets can be made by casting the resin solution onto an endless belt, or onto a plastics sheet unreeled from a roll, as a substrate to support the cast film. The membrane sheet has a tendency to adhere to the substrate surface on drying, and it is therefore important to remove the membrane sheet from the surface while it is still wet, and before it has been dried and developed adherency. Unsupported membrane sheets obtained by the process of the invention are quite strong, with water-wet tensile strengths in the range of 28 to 42 × 10⁴ kg/m² (400 to 600 psi) and elongations generally exceeding 40%. For some applications, even higher tensile strengths may be desired. In addition, unsupported membrane sheet requires special care to manipulate in the typical range of thicknesses from 0.05 to 0.25 mm. The unsupported filter membrane obtained at the conclusion of the membrane-forming process is wet with water, also contains a small amount of residual formic acid. This product can be dried in various ways. It can, for example, be collected on a roll on a suitable core in lengths from 15.2 to 30.4 metres (50-100 ft) and placed in an oven until dry. During drying, some shrinkage occurs, but an acceptable product is obtained. It is also possible to clamp a length of membrane in a frame holding all sides against shrinkage, and then dry the membrane by exposure to heat, as by infrared radiation, or in an oven in air. The resulting sheet is very flat, and when discs are cut from it, these are adapted for use in apparatus designed to accept disc filter membranes. The membrane discs are quite strong and flexible, and can be readily and reliably assembled in such apparatus. A similar product can be obtained with less hand labor by passing the wetted membrane sheet over a hot drum, against which it is firmly held by a tensioned felt web or other porous sheet, and the dry web collected as a roll. If two or more layers of wet unsupported membrane sheet are dried in contact with each other, using any of the drying methods described above, they adhere to each other, forming a multi-layer structure. No bonding agent or other adhesion technique is required. The resulting multi-layer membranes are useful in the manner of a single layer filter membrane. Since in manufacture a small proportion of undetected faults may occur, caused, for example, by bubbles of air entrained in the casting resin solution, using two layers instead of one neutralizes such areas, covering them over with a second layer of filter membrane that is also capable of providing the required removal rating; an extremely high degree of reliability is obtained in this manner. Very good adhesion of adjacent layers is also obtained if a layer of supported resin membrane and one not supported are dried in contact, using the same procedures. In this manner, filter media can be made in which a supported layer of uniform pore size is bonded to an unsupported tapered pore membrane layer, which provides efficient prefiltration. The fine face of the tapered pore layer would be about the same pore size or somewhat larger than the pore size of the supported layer, and this face would be adjacent to the unsupported layer. Supported filter membranes made by the process in accordance with the invention are particularly well suited to use on filter presses, where self-sealing characteristics are needed, and the filters are subjected to large stresses. They are also useful in making plain or corrugated filter cartridges for use at high differential pressures, or for impulse type service. The filter membranes made by the process of the invention are well suited for use as the filter media in filter cartridges. Filter cartridges are self-contained filter elements, provided with a filter sheet in tubular form capped off by end caps at each end. Either or both end caps can have a through opening for fluid circulation through the filter sheet in either direction. Filter cartridges are designed to be installed in and to be readily removable from filter assembly housings when replacement is necessary. A good filter cartridge has a filter sheet that is free of faults, and with removal characteristics that are relatively uniform with stated standards. Filter cartridges take many forms, including simple cylinders, corrugated cylinders, stacked discs, etc. Of these configurations, a favored form for the filter membranes made by the process of the invention is a corrugated cylinder. Such a cylinder is made by corrugating one or more layers of supported or unsupported wet membrane (two layers is preferred) sandwiched between two open porous or foraminous sheets which provide for fluid flow up and downstream of the contacting surfaces of the filter medium within the corrugations. The resulting corrugated structure can be dried while lightly restrained, in the course of which contacting membrane layers are bonded together, thus forming a more rigid, stronger structure, and then seamed closed along the contacting ends, using heat-sealing techniques similar to those used for sealing conventional thermoplastic filter materials. End caps are then attached in a leak-tight manner to each end of the resulting cylinder. The accurate determination of effective pore size for membrane filter media that is meaningful in its representation of expected effectiveness as a filter is difficult. When a uniform pore filter medium made by this process of this invention, or any of the currently marketed uniform pore membranes are examined using a scanning electron microscope, e.g. as is shown in Figure 5, and the apparent pore openings as seen on the micrograph are measured, a pore size is determined which is about three to five times the diameter of the largest particle which the filter will pass, as determined, for example, by bacteria challenge. Similarly, it has been attempted to ascertain the pore diameter from the K L value as determined by the procedure of applying air pressure to a wetted element, obtaining the K L value in the manner shown in Figure 3, and inserting the so determined pressure into the well known capillary rise equation; when this is done a diameter is determined which is about four times the absolute removal rating of the filter medium as determined by bacterial challenge. Such methods, upon reflection, appear to have little relevance to the capability of the membrane as a filter. What the user needs to know is not pore size; rather, it is the capability of the filter in removing particulates, bacteria, yeast, or other contaminants. Contrary to established thinking, we have determined by test that the effectiveness of membranes of structure similar to those made by the process of this invention as a filter media is dependent not only on pore size, but also on thickness. In the development of the process of the present invention, it has, for example, been demonstrated that of two membranes, one having small pores and quite thin, and the other having relatively larger pores and much thicker the membrane have the larger pores but the greater thickness may be more effective as a filter. Accordingly, the effectiveness of the membrane sheets made by the process in accordance with the invention as filter media is rated not in terms of pore size, but in terms of effectiveness in the removal of a contaminant of known dimensions. One of the principal applications of this type of filter membrane is to deliver a filtrate freed of all incident bacteria, hence bacterially sterile. A technique usually used in the industry to determine the ability of a filter to deliver bacterially sterile effluent is to challenge it with a suspension of Pseudomonas diminutiae , which is a small diameter relatively nonpathogenic bacterium referred to in abbreviated form by Ps . Filters which successfully meet such a challenge are generally accepted in the industry as being 0.22 micrometer absolute in filter rating, and in any event Pseudomonas diminutiae is a bacterium that represents the lower limit of bacterial dimensions. If no combination of challenge conditions can be found which will allow even a single organism of Pseudomonas diminutiae to pass, the filter can be regarded as capable of quantitatively removing all bacteria. The filtering effectiveness of the membranes made by the process according to this invention is measured by a standardized test based on Pseudomonas diminutiae removal that correlates such removal with air flow measurements through the wetted membrane and the thickness of the membrane, and is capable of providing a quite complete characterization of the removal characteristics of the membrane filter sheet being tested. The removal of Pseudomonas diminutiae is a function not only of pore size but also of thickness, and is expressed by the exponential relationship: or where T R is the titer reduction for the membrane and is the ratio of Pseudomonas diminutiae content in the influent to the content thereof in the effluent; the titer reduction achieved by a membrane of unit thickness; and t is the thickness of the membrane. As an example of the application of this formula, if a given membrane has a titer reduction of 10⁵, two layers of the membrane will have a titer reduction of 10¹⁰, three layers of 10¹⁵, etc. Since the incident test bacterium is monodisperse (i.e., of uniform dimensions), the applicability of this formula is self-evident. Its correctness has also been confirmed experimentally, by determining titer reductions for 1, 2, 3, 4 and 5 layers of the same membranes. As shown in Figure 2 the resulting plot of log T R vs. number of layers is linear, as predicted by the formula. It is known in the industry to measure air flow rates through a membrane which has been wetted by a liquid; such measurements yield useful information on the pore size characteristics of the membrane. We have used, in the course of this invention, a parameter designated as K L , K L is a form of abbreviation for the "knee location" of the curve of Figure 3. When the air flow/unit of applied pressure through a wetted membrane is plotted against increasing applied pressure, as in Figure 3, the initial air flow is very small and the flow per unit of applied pressure remains nearly constant, until a point is reached where a very small increment in pressure causes a very sharp rise in flow, such that the curve becomes nearly vertical. The pressure at which this occurs is designated as the K L for the membrane. K L has been measured for a group of forty-five membranes made by the process of this invention from polyhexamethylene adipamide; these membranes were selected to cover a range of thickness from 0.038 mm to 0.305 mm and with a wide range of pore diameters. These same membranes were then challenged with a suspension of Ps bacteria, and the number of influent bacteria was then divided by the number of effluent bacteria, thus determining the T R for each of the membranes. Thickness of each membrane was then measured, in cm, and using the formula log T R1 = 1/t log T R log T R1 was then calculated for each membrane. T R1 being the theoretical titer reduction for a .0025 cm membrane. K L values were measured, for both relatively coarse and relatively fine membranes, for a number of thin membranes. These same membranes were then laid up as 2, 3 and more layers, and the K L values again measured for the multilayers. In this way, a relationship between the thickness and K L value of equal pore size membranes was determined; this relationship is summarized in Table I. Using Table I, the K L values of the 45 membranes were connected to the K L which would apply to an equal pore size membrane 0.127 mm thick; these values are designated as K L5 . Log T R1 for each membrane was then plotted against K L5 for that membrane. All the results fell close to a single line, which is shown in Figure 4. Using Figure 4, the titer reduction (T R ) which can be expected to be obtained with any membrane made of hexamethylene adipamide by the process of this invention, can be calculated, using the measured K L and thickness (t) for that specimen. The procedure is as follows: (1) measure the K L and thickness for the specimen; (2) use Table I, determine K L5 ; (3) use K L5 to determine T R1 from Figure 4; and (4) calculate T R from the equation log T R = t log T R1 There is an upper limit to the number of bacteria which can be collected on a membrane; by the time that about 10¹⁴ Ps . have been collected per square metre of filter medium, flow through the filter has fallen t o less than 0.01 % of a normal starting flow rate of 21.5 to 54 liters/minute per square metre. This has been determined, by actual test, to be true for the membranes of made by the process of this invention, as well as for commercially available membranes, for the full range of T R from 10 to > 10³⁰. Thus, the figure of 10¹⁴/square metre may be taken as a practical upper limit of invading Pseudomonas diminutiae . This upper limit is taken in combination with the calculated T R to obtain assurance that a given membrane will yield sterility under all conditions of use. For example, a membrane may be selected with an estimated T R of 10²³; statistically, if a square metre of this membrane were challenged with 10¹⁴ Pseudomonas diminutiae , such a membrane would have to be so challenged for 10⁹ (or 1 billion) times, in order to produce a single effluent with one bacterium, and such a high ratio may be taken as adequate assurance of sterility, hence the filter can be considered to have an absolute removal capability at 0.2 µM. In practice, it is difficult to consistently produce a membrane with an estimated T R of exactly 10²³, but it is feasible to establish a permissible range, say 10²³ to 10²⁷, with 10²³ as the lower limit, and thus obtain assurance of consistency bacterially sterile filtrates. In a similar fashion, K L and thickness can be correlated with titer reduction for larger bacteria, yeasts of known size, and other particulate material, the latter assayed by particle detection methods, over a size range from under 0.1 µm or larger. The curve of Figure 4 is applicable to the membranes made by the process of this invention. The process by which this curve was developed can be applied to membranes made by other processes. The location of the curve for other membranes may shift somewhat, but we have done sufficient testing using currently marketed uniform pore dry process membranes to determine that the same principles are applicable. The horizontal portion of the curve of Figure 3 is truly horizontal only if the pore size is quite uniform. Uniform pore media are further characterized by a sharp change in slope to a nearly vertical course at the K L value. If the filter medium is relatively nonuniform in pore size, it will tend to have a distinct slope in the horizontal portion of the curve, and exhibits a relatively large radius for the change in slope to the more vertical portion of the curve, followed by a sloping rather than a nearly vertical portion. The lower or horizontal portion of the curve is a measure of the diffusion of air through the immobilized, liquid film which fills the pores of the membrane. The wetting liquid may be water, in which case a relatively low air flow is obtained in the horizontal part of the curve, or alcohol, in which case the diffusional air flow is higher. At the change in slope, the wetting liquid begins to be expelled from the pores, and in the vertical portion of the curve, a large number of nearly equal size pores begin to pass air. When the data of Figure 3 is plotted for a tapered pore membrane, that is, one with larger pores at one face, tapering to a smaller pore at the other face of the membrane, the curves obtained by reversing the direction of pressurization do not coincide. Instead, two distinct curves are obtained, one flat, and the other higher and sloping upward, of which the sloping curve with higher flow values is obtained when the more open side is upstream, and reflects the penetration of air partly into the coarser face of the membrane, thereby effectively decreasing the thickness of the liquid film, and hence increasing the air diffusion rate. Thus, by applying air pressure and measuring flow through a membrane successively in both directions, it is possible to determine whether it is a uniform or tapered pore membrane. If the flow-pressure curves are equal, or nearly so, in both directions, the pores are uniform, and the method described herein for relating K L and thickness to titer reduction for any given organism, or to a monodisperse particulate, may be applied to that membrane. The following Examples in the opinion of the inventor represent preferred embodiments of the invention: Examples 1 to 5 Nylon 66 resin pellets of molecular weight approximately 42,000 were dissolved in 98.5% formic acid, to yield a 35°C solution containing 15.5% of the resin. Without delay, this solution was delivered at a flow rate of 250 g/minute to an in-line mixer. Simultaneously, a controlled water flow at 31°C was delivered to the mixer, the quantity being such as to produce as the effluent a casting resin solution containing 70.2% of formic acid and 13.1% of the resin. The casting resin solution was filtered through a 10 µm filter to remove visible resin particles, and was then formed as a thin film by a doctoring roll with 0.22 mm inch spacing on a moving polyester sheet surface, which had been pretreated by corona discharge to improve its wettability; and in less than 3 seconds immersed into a bath containing 46.5% formic acid, balance water, for approximately 3 minutes. Bath concentration was maintained constant by adding water continuously. The nylon membrane so formed was washed with flowing water for 1 hour. Two layers of the membrane were removed from the polyester substrate sheet and oven dried in contact with each other, while restrained to prevent shrinkage of the length and width during drying. The rotation rate of the in-line mixer was varied from 400 to 1600 RPM during this run. Table IV shows the product characteristics obtained. In this Table, "uniform pores" means that the pore size was equal, as determined by SEM examination throughout the whole width (thickness) of the membrane. Examples 1 and 2 represent conditions in region A of Figure 1, in which the degree of nucleation is too low to produce a satisfactory product; in this zone pressure drops are high, and product characteristics lend to be non-reproducible. Example 5, in which mixer speed was 400 RPM falls into region D of Figure 1, and resulted in an unstable condition, with so much precipitating resin generated within the mixer, that it began to clog, such that casting resin solution could not be delivered. The wide variation in behavior and in product characteristics, for the same casting resin solution as defined by the concentration of its components, should be noted. Examples 6, 7 and 8 Casting resin solution was prepared and processed as for Example 4, except that it was heated by means of an in-line heat exchanger to respectively 53, 61 and 68°C prior to casting. The product characteristics were not significantly different from those of Example 4. This result confirmed previous test data indicating that temperature of the casting resin solution is not a significant parameter, except insofar as viscosity may be reduced to the point (below about 500 mPa. s or centipoises) where casting problems may be experienced. Examples 9 to 13 The membranes were prepared in the same way as Examples 1 to 5, except that the quantity of water added was such as to produce a casting resin containing 69.8% of formic acid and 13.0% of resin. The results are shown in Table V. The casting resin solution made at 1950 RPM mixer speed was insufficiently nucleated, resulting in a poor product with high pressure drop. Examples 14 to 18 These membranes were prepared in the same way as Examples 1 to 5, except that the quantity of water added was such as to produce a casting resin solution containing 69.0% of formic acid and 12.85% of resin. The results are shown in Table VI. Examples 19 to 39 These membranes were prepared in the same way as Examples 1 to 5, except that the quantities of water added were such as to produce casting resin solutions containing 71.4%, 67.5% and 66.0% of formic acid, and 13.3%, 12.55% and 12.41% respectively of resin. The results are shown in graphical form, along with the data of Examples 1 to 19, in Figure 10. Figure 10 includes only those membranes which fall in the regions B and C of Figure 1, and therefore have favorably low pressure drop in proportion to their thickness and particle removal capability, and are consistently reproducible. Examples 40 to 46 These membranes were prepared in the same way as Examples 1 to 5, except (A) Starting resin containing 14.5% of Nylon 66 was delivered to the mixer at 1510 litres/minute. (B) Water was added in various quantities, to obtain formic acid and resin concentrations as listed. (C) Doctor roll set at 0.56 mm. The results are listed in Table VII. Examples 47 to 50 Nylon 66 resin pellets of molecular weight approximately 42,000 were dissolved in 98.5% formic acid, to yield a 35°C solution containing 15.5% of the resin. Without delay, this solution was delivered at a flow rate of 944 litres/minute to an in-line mixer rotating at 1200 RPM. Simultaneously, a controlled water flow also at 30°C was delivered to the mixer, the quantity being such as to produce as the effluent a casting resin solution containing 69.0% of formic acid and 12.9% of resin. Temperature of the resulting casting resin solution was 57°C. The casting resin solution was without delay filtered through a 10 µm filter to remove visible resin particles, and was then formed into thin films by a doctoring blade with 0.25 mm (0.010 inch) spacing on glass plates, and in less than 10 seconds immersed into a bath containing formic acid and water, for approximately 5 to 10 minutes. The nylon membranes so formed were washed with flowing water for 1 hour. Two layers of the membrane were oven dried in contact with each other, while restrained to prevent shrinkage of the length and width during drying. Table VIII shows the product characteristics obtained, for various bath concentrations. Examples 51 to 57 Membranes were prepared exactly as Examples 47 to 50, except as follows: Mixer rate was 1600 RPM Casting resin solution temperature was 64°C Table IX shows the product characteristics. Examples 55 and 56 are not within the scope of this invention; they are included to illustrate the effect of using bath concentrations of less than about 20% formic acid. This group of Examples also illustrates the advantages of baths in the range near to 46.5% producing membranes with minimum pressure drop at a given particle removal rating. Examples 58 and 59 These membranes were prepared using the same procedure as Examples 47 to 50 except as follows: (a) Starting resin concentration was 17%. (b) Casting resin solution was prepared from 344.7 grams per minute of starting resin solution using as nonsolvent diluent a solution containing 32.8% formic acid in water, delivered to the mixer at a flow rate of 132.1 grams/minute. (c) Mixer speed was 1900 RPM. (d) Composition of the casting resin solution was: 12.1% resin, and 67.8% of formic acid. (e) After filtering through a 10 µm filter, one-half of the solution was further filtered through a filter with an approximate 0.05 to 0.10 µm removal rating. The two portions were then cast as films into a 46.5% formic acid bath, as Examples 58 (filtered 10 µm only) and 59 (filtered 10 µm and ∼0.05 to 0.10 µm). Data for the two, measured on a single thickness of each, are listed in Table X. The 295 mm pressure drop of Example 59, resulting from the fine filtration step, should be compared with that of a normal product of this invention with the same thickness and values, which would be approximately 89 mm of mercury. Examples 60 to 64 In these Examples, polyhexamethylene adipamide was formed into membrane sheets using a small batch procedure. A 20% starting resin solution was prepared by dissolving resin of molecular weight=34000 in 98.5% formic acid. A quantity of 500 grams of this solution was heated to 65°C in a glass jacketed vessel approximately 102 mm inside diameter by 204 mm high, fitted with a 50 mm (2 inches) diameter propellor-type agitator, and an externally operated flush valve at its bottom. A nonsolvent solution was prepared containing 12.77% formic acid, the balance being water. With the agitator rotating at 300 to 500 RPM, 241 g of this nonsolvent solution was pumped into the apparatus, at a constant rate, over a period of 2 minutes, the inlet nozzle being 2 mm inside diameter, and located 6.3 mm from the arc described by the rotating propellor. During the last portion of the two minute period, resin was seen to precipitate at the inlet nozzle, all of which subsequently redissolved except for a small quantity of lumps of resin about 3.2 mm in diameter. About 20 grams of the casting resin solution so formed was withdrawn through the bottom valve, passed through a 42 mesh screen to remove lumps, and without delay spread on a glass plate as a thin film, using a 0.254 mm doctor blade, and the film then promptly immersed in a bath containing formic acid and water, at 25°C. The membranes were allowed to set for several minutes, then were stripped from the glass plate, washed in water and dried by exposure to infrared heat. The properties of the resulting membranes are shown in Table XI. Examples 60, 61 and 62 illustrate the effect of degree of nucleation on product characteristics. Examples 60 and 61 are properly nucleated, and yield products with favorably low pressure drop, for their removal ratings. In Example 62, the higher rotation rate resulted in a casting solution with too low a degree of nucleation, and as a consequence, a relatively high pressure drop.

Claims: A process for preparing a skinless, hydrophilic, microporous membrane of polyhexamethylene adipamide, comprising the steps of preparing a solution of the polyamide in formic acid, adding a prescribed quantity of water with mixing in an in-line rotary mixer thereby inducing nucleation of the solution, forming a film of the nucleated solution on a substrate, immersing with as little delay as possible the film and substrate in a quench bath containing formic acid and water, removing the resultant membrane from the bath, washing to remove the formic acid from the membrane and finally drying the skinless, hydrophilic, micro porous membrane, wherein the substrate, unless porous, is wettable by the solution and the quench bath, and formic acid concentration in the quench bath is not less than 37% and not more than 50% and the formic acid concentration in the solution is 63 to 72%. A process according to claim 1 in which precipitated resin particles are redissolved or filtered out before spreading the casting solution on a substrate. A process according to claim 1 in which part of the precipitated resin particles are redissolved and part are filtered out before spreading the casting solution on a substrate. A process according to claim 3 in which the bath comprises 43 to 47% of formic acid. A process according to claim 1 in which the resin concentration in the casting solution is 10 to 18% by weight, and the formic acid concentration is 63 to 72%. A process according to claim 1 in which the polyamide resin concentration of the casting solution is 12 to 18% and the water is added at a fixed intensity of mixing. A process according to any of claims 1-6 in which the casting resin is continuously spread onto the substrate, the thin film of casting solution is continuously immersed in the quench bath, and the bath is maintained at a substantially constant composition by continuous addition of water to the bath in a quantity to compensate for solvent diffusion into the bath from the film of casting solution. A process according to claim 7 in which the substrate is a non-porous synthetic polymer film having a surface that is wetted by the casting solution and the quench bath. A process according to claim 8 in which the substrate is a web of a fibrous polyester sheet, the web having an open structure which is wetted and impregnated by the casting solution, thereby forming a membrane film having the porous web incorporated as a part thereof. A process according to claim 8 in which the substrate is a porous web which is not wetted by the casting solution, thereby forming a membrane film having the porous web attached to one surface thereof. A process according to any of claims 1-10 in which the resin solution has a viscosity within the range from 5.000 to 50.000 mPa. s at the operational temperature. A process according to any of claims 1-11 in which the casting and precipitating temperatures are within the range from 10°C to the boiling temperature of the lowest boiling solvent or non-solvent component present. A process according to any of claims 1-12 in which the casting resin solution is cleared, and freed from suspended material, before being spread upon the substrate to form a film. A process according to any of claims 1-13 in which the membrane is stripped from the support after washing and before drying. A process according to any of claims 1-14 in which the substrate is not stripped from the resin membrane before drying, and after drying remains attached to resin membrane. A process according to any of claims 1-15 in which the substrate is of polypropylene resin or polyester resin. A process according to any of the claims 1-16 in which the casting solution has a viscosity within the range from 500 to 100,000 mPa . s. A process according to claim 17 in which the viscosity is 5000 mPa . s. A process according to any of claims 1-18 in which a multilayer skinless hydrophilic alcohol-insoluble polyamide membrane is obtained by preparing two individual said membranes, placing the membranes after the washing step and while still wet in contact with one another and then drying the juxtaposed membranes while maintaining such contact, thereby obtaining a multilayer membrane in which the separate membranes are integral layers thereof. A process according to claim 19 in which the separate membranes are stripped from the substrate before being placed in contact with one another and dried. A process according to claim 19 in which the separate membranes are contacted and dried while supported on the substrate, which thereby becomes an integral part of the multilayer membrane. A process according to any of claims 1 to 21 in which the membranes are dried under restraint to limit dimensional change after they had been corrugated.

Inventors: PALL DAVID B, PALL, DAVID B.
Applicants: PALL CORP, PALL CORPORATION
Application year: 1979
Main classification: B01D61
IPC codes: B01D65, C08J9, C08J5, B01D61, B01D67, B32B27, B01D71