CA1160007A - Process for preparing hydrophilic polyamide membrane filter media and product - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

_ A process is provided for preparlng skinless hydrophilic alcohol-insoluble polyamide membranes by preparing a solution in a polyamide solvent of an alcohol-insoluble polyamide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within the range from about 5:1 to about 7:1 inducing nucleation of the solution by controlled addition to the solution of a nonsolvent for the pplyamide 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 precipitat-ing polyamide resin from the casting solution in the form of a thin skinless hydrophilic membrane; and washing and drying the result-ing 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

~6~1~07 SPECI~IC ATION
Microporous membrane sheets are available which have absoiute particle removal capabilit~T in the range of about 0.1 micron and larger.
These are for the most part made of synthetic resins and cellulose 5 derivatives, and are used as filter media for removing suspended particles and micro-organisms from fluids.
Such membranes are made using the so-called 1'dry process" by casting a solvent solution of the resin or cellulose deri~ative on a temporary support or substrate as a thin film, after which the solvent is 10 removed or exchanged under carefully controlled condItions. Solvent remo~al and exchange are very slow, and while the process is adaptable for continuous operation, a very large supporting belt system is required as the substrate for laydown or casting of the film, and the drying set-up to carry out removal of the solvent. This Increases plant size and the 15 capital costs in plant construction, and ensures a high cost of manufacture.
Because of the very great length of material (solution or film) which is in process at any one time, adjustment of processing conditions for close con-trol of product characteristics is difficult. While the final product is being re-moved and tested for its characteristics, a very large volume of material is 20 already in process of being formed into a membrane, and past the point where an adjustment of the process parameters to modify product characteristics, however prornpt, could affect it. Thus, a considerable amount of out-of-specification membrane sheet is made before the result of a correction can be seen at the end of the production line. This results in a large proportion 25 of membrane sheet being out-of-specification, and a wide range of product .~

variation necessarily has to be accepted~ to keep rejec-tions at a minimum.
As a consequence of high production cost and high rejection rate, the price for such membrane sheet tends to be rather high.
Another process for preparation of membrane sheets also starts 5 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 Ln a nonso~ent for the resin. This process results in a skinned membrane, with surface portions having fewer or very much smaller pores, or even z~ro pores, and an interior portion with larger 10 pores, the outer skinned portions having higher apparent densit~ than the inter ior por tions .
Sl~inned membranes are nonuniform with respect to particle removal;
for example, the membranes now used for reverse ~smosis are effective in accomplishing such tasks as g~c or better salt rejection, thus functioning 15 in the 2 to 5 Angstroms (0. 002 to 0. 005 ,uM) range, but are incapable of providing sterility in the effluent, allowing bacteria in the range of aooo Angstroms(0. 2 ,uM) to pass. Such membranes are poorly suited when absol~te removal of particulate material as bacteria is needed.
Thus, for example, Michaels U.S. patentNo. 3,615,024, patented 20 October 26, 19719 describes the formation of anisotropic membranes having pores of from 1 to 1000 ~lM from a variety of synthetic resins by:
(1) forming a casting dope of a polymer in an o:rganic solvent,

(2) casting a film of said casting dope,

(3) preferentially contacting one side of said film with a diluent 25 characterized by a high degree of miscibility with said organic solvent and a ()7 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 submicroscopically porous a~nisotropic rnembranes consist o~
an integral macroscopically thick film of porous polymer9 usually more than about 0. 002 and less than about 0. 050 inch in th~ckness~ One surface of this film is an exceedingly thin but r elatively dense barrier layer or"skin" of ~om about Q. 1 to 5. 0 microns thickness of microporous 10 polymer in which an aYerage pore diameter is in the millimicron 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 ~ilm structure is a support layer comprised of a much more coarsely porous polymer structure through which fluid can pass with little hydraulic 15 resista~ce. By "integral film" is meant continuous, i. e. 9 a continuing polymer phase. When such a membra~e is employed as a "molecular f-lter"
with the "skin-side" 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 thall the pores in the "skin"
20 are selectively retalned. Becallse 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 the thick-ness of the barrier la~er or less than one micron, the over-all hydraulic resistance to fluid flow through the membrane i~ very low; that is, the 25 membrane displays surprisingly high permeability to fluids in proportion to its pore size.

~0'7 Michaels suggests that the formation of these aIlisotropic membranes appears to be rel~ted to certain diffusional and osrrlotic solvent-exchailge processes as described hereinbelow:
When a thin layer of polymer solution deposited on a suitable

5 substrate (to assure preferential contact of diluent with orle surface) is contacted with diluent on one sur-face, diluent and solvent interdiffuse in the outermost layer almost instantaneously. Thus gelation or precipitation of the polymer tal~es place almost instantaneously. In view of the rapidity of this process, the topmost layer of the cast film solidifies as an exceedingly 10 thin membrane skin whose porosity and pore-fineness are governed by the compatibility criteria developed above. As soon as this membrane skin is formed, however, the rate of penetration of diluent into the underlying region of the cast film, and ra~e of extraction of the solvent component, are greatly retarded. (Itmustnot, however, bestoppedentirely.3 Underthesecircum-15 stances, subsequent alteration in solution composition within the film occursquite slowly. As a result there i~ opportunity, when a suitable solvent is present,for slow phase-separation to occur to form a grossly microporous substructure consisting of large interconnected voids occupiedby solvent/
diluent solution, and an interstitial polymer matrix comprising consolidated, 20 nearly solvent-free polymer. Hence, the formation of a highly permeable, coarsely microporous substructure is in large part due to proper selection of a sol~ent system for film-casting dopes arld the selection ~ a proper diluent for coaction with the solvent system during the precipitation step~
Thus, the Michaels membranes are all skinned, and moreover, 25 while the membranes are water-wettable as long as they are kept wet, once

6~ 7 dried they are all h~drophobic, and difficult to wet with water, except with the aid of surface-active agents or other wetting aids.
SalemmeU.S. patentNo. 4,032,309, patentedJune28, 1977, prepares polycarbonate resin membranes described as hydrophobic, evidently 5 of very small pore size, in the ultrafiltration range. Salemme refers to Michaels U.S. patent No. 3,615,024 and Kimura U.S. patent No. 3,709,7q4, and st~tes that both Michaels and Kimura utilize the general procedure of preparing a casting solution of the polymer, casting a film thereof on a smooth substrate and immersing the substrate and film in an appropriate quenching 10 bath for the development of asymmetric structural cha~acteristics of the completed film.
These methods differ from each other in the manner in which some of the process steps are conducted. While the Michaels patent is particularly directed to the preparation of a membrane having a 15 microporous support layer and an integral microporous skin7 Kimura is primarily interested in a filrn structure presenting a p~rous region adjacent a very thin dense nonporous layer. Kimura specifically teaches the preparation of a casting solution consisting of the polymer and two mutually miscible solvents in which the polymer is soluble to su~stantially 20 differ2nt degrees. Both the Michaels and Kim-lra methods view the immersion (or mennbrane-forming) bath as one which Eunctions as a solvent for the casting solution solvent system, functioning thereby solely to remove casting solution solvent f~om the film structure.
Contrary to the Kimura process, ~alemme does not employ a 25 three-component (resin, good solvent, poor solvent) casting solution ~ 16~)7 and, in contrast to both Ximura and Michaels, Salemme utilizes an immersion (quenching) bath to initiate formation of the film that must provide a function neither disclosed nor contemplated in either Kimura or ~ichaels; namely, causing swelling of the polycarbonate resin 5 material at the same time as the casting solvent is removed ~om the film thereby.
The Salemme method for the preparation of porous polycarbonate alld other resin membranes comprises the steps of:
(a) preparing a casting solution at room temperature consisting 10 of polycarbonate resin material and a casting solvent compo~ed of one or more good solvents, the casting solution being stable at room temperature;
~ b) casting a layer of the casting solution so formed on a smooth, clean surface or suppor t;
(c) permitting desolvation to occur for a predetermined time 15 interval from said layer;
(d) immersing said layer and support in a quenching bath liquid, the quenching bath liquid being capable of dissolving the casting solvent and causing swelling of the polycarbonate resin content of the layer while being a non-solvent for the polycarbonate r~sin, the immersion step 20 initiating formation of a microporous membrane by entry of the quenchinD
bath liquid into said layer and exit of casting sol~ent theref~om;
(e) removing the microporous membrane from the quenchirlg bath;and (f) removing the remaining casting solvent and quenching bath liquid from the microporous membrane.

!07 The microporous films produced by the Exampl~s are said to be at least as effective for filtration as those produced in accordance with the prior art method of casting and maintaining in controlled atmosphere for extended periods. Generally, the films are said to exhibit better flow rates and to be 5 more readily wettable than the prior art films The response of these microporous films is measured in terms of the foam-all-over point, which is the pressure required to cause foam to develop over the surface of the film This method is commonly employed in this art, and i8 referred to as the Bubble Point. Moreover, the process 10 for manufacture of these membranes is not susceptible of adaptation for contlnuous production.
~ number of alcohol-insoluble polyamide resin membrane sheets have been described, but to our knowledge none has been marketed. Where sufficient information has been provided to permit duplication of the pro-15 duction of these membranes, they have all been heavily skinned Membranesof alcohol-soluble polyamides have been made which are skinless, but they have to be used with media which do not contain alcohol or a number of other solvents in which they are soluble. Further, such membranes are not capable of use after steam sterilization, a highly desirable attribute for 20 media used in large part for producing bacterially sterile filtrates. Hollow fiber membranes made of polyamide resin are marketed in commercially available equipment, but these are heavily skinned, and serve to accomplish partial separations in the reverse osmosis range.
LovelletalU S. patentNo 2,783,894, patented~arch5, 1957, and 25 PaineU.S patentNo. 3,408,315,patentedOctober29,1968,provideaprocess for producing alcohol-soluble polyamide membrane sheets using Nylon 4, poly-~-butyrolactam. The term " alcohol-soluble" is used by these patentees to refer to polyamide resins soluble in lower aliphatic alcohols ~uch as methanol or etha~ol, and is so us~ed in the présent specification 5 and claims. A solution of r~rlon can be cast as a liquid film and then con-verted 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 in-cipient precipitation by adding to the solution a solvent-miscible nonsolvent 10 which decreases the solubility of the nylon. This point is indicated when a small amount of nonsolvent added to a sample of the solution causes an obvious precipita~ion of nylon.
The nylon solution, adjusted to the point of incipient precipitation and containing the proper additlves, is cast as a liquid film on an opticaliy - 15 smooth surface of a solid base and then converted to a solid film by eg-posure to an atmosphere containing a constantly maintained concentration of e~cha~geable nonsolYent vapors, that is, vapors of a liquid in which nylon is not soluble but which are exchallgea~le with vapors of the solvent for the nylo~. The resulting membranes are7 of course, soluble in 20 alcohol, as well as in a considerable number of other solvents, and may not be steam sterili~ed, which limits the scope of their usefulness.
HiratsukaandHoriguchiU.S. patentNo. 3,746,668, patented ~uly 17, 1973, also prepares membranes from alcohol solutions of poly-amides which are alcohol-soluble, gelling the solution by addition of a 25 cyclic ether as a gelling agent, and drying the film. Alcohol-soluble rQlatively low molecular weight copolymers of Nylon 6 and Nylon 66, and of Nylon 67 Nylon 66 and Nylon 610 are used.
Marinaccio a~d Knight7 U. S. patent No. 3, 876, 738, patented April 8, 1975, describes a process for producing microporous membrane sheets from a~;ohol-soluble and alcohol-insoluble polyamicles such as Nylon 6, poly-~-capro-lactam, and Nylon 610, polyhexamethylene sebacamide, by casting a solution of the polymer on a substrate and then precipitating the membrane, both steps being carried out sequentially a concurrently in a ~uenching bath of nonsolvent liquid.
The nylon solution after formation is diluted with a nonsolvent for nylon, and the nonsolvent employed is miscible with the nylon solution.
Marinaccio et al discuss polymer molecule aggregation in solution, and assert that "the tightest or most nonporous polymer film is produced from a solution in which there is no aggregate formation. "
According to Marinaccio et al, " . . . the resulting film strength is primarily determined by the polymer concentration because of the larger number of chain entanglements occurring at higher polymer levels. In addition, for film cast from the ideal solution the "pore size" would increase slightly with polymer concentration because of the increasing aggregation 15 tendency at higher concentrations. Aggregation in solution re~sults in film porosity since the film as cast can be thought to consist of interacting aggregated spherical particles. The larger the spheres, the larger the voids in the film. Structurally $his is much like a box o~ tennis balls or other nonspherical geomeb~ics fused at their point of contact. "
20 - As a first step, then, ~arinaccio et al control film porosity by "control of the aggregation tendency in the casting solution. This is accom-plished. . .by the addition of nonsolvent or other additives to change the solvent power of the solution,hence influencing and controlling the aggregat~on tendency of the polymer molecules. The interaction of these aggregates in 25 determining the resulting film structure is further influenced by the various process variables previously maintained."

g ~l~L6~l~0~
'rhis is Marinaccio et al's theory, but it is not adequate to explain what actually occurs, and is in many respects not consistent without actual observations. Moreover, it differs from other more generally accepted theories advanced to explain polyrner membrane formation7 as for instance, Synthetic Polymeric Membranes, Kesting (McGraw Hill 1971) pp 117 to 15~.
Kesting's theory is more credible for a number of reasons; for e~ample, it accounts for the very high voids volume of the rnembranes, which Marinaccio's "tennis ball" theory fails to do;~ further it explains why only relatively polar polymers are susceptible to membrane formation, which 10 again Marinaccio does not.
Marinaccio et al then-assert: "The selection of a solvent for a selected film-forming polymer can be made on the basis of the foregoing information. Determination of optimum solvent systems as well as other process variables can then be made- on the basis of routine laboratory 15 experimentation. " However, dilution of the solution by addition of a nonsolvent has a limit: "dilution with nonsolvent can be effected up to the point of incipient precipitation of the nylon, but not beyond. " The casting solutions are stable enough to be subjected to ageing periods of as much as five to eight days, and indefinitely in some cases, but not so long that the 20 dissolved nylon separates.
The quenching bath may or may not be comprised of the same non-solvent selected for preparation of the nylon solution, and may also contain "slinall amounts'of the solvent employed in the nylon solution. However, the ratio of solvent to nonsolvent is lower in the quenching bath than in the polymer 25 solution, in order that the desired result be o~tained. The quenching bath may also include other nonsolvents , e. g., water. In all of the Examples, the solvent utilized for the solutions is formic acid, but none of the quench baths contained even a small amount of formic acid.

The Marinaccio et al process is said to di-ffer from conventional methods of preparing microporous films in using more simplified castin~
solutions, but more impor-tantly in eliminating the slow equilibration step o-f gelling in a high humidity atmosphere. In conventional processes this is a critical step in the formation of the desired film structure. In the Marinaccio et al process the film is cast directly in the quench bath, and immediately quenched. By controlling the casting solution formulation as discussed abo~e and controlling the quench bath variables including composition and temperature~film structure is said to be controlled. This technique forms the film structure"catastrophically"and is in direct contrast to the slow equilibrium technique needed in conventional proce~ses.
In some cases Marinaccio et al suggest it may be desirable to pass the cast film through a short air evaporation zone prior to the quench bath.
The technique could be used in those cases in which a graded cross-sectional structure is desired in the film.
The product o~ Marinaccio et al has not been commercialized, and is unavailable. The formation of a polymer film by direct immersion of the casting resin into a quench bath is dlfficult, and it has not been economically feasible to attempt to duplicate the Marinaccio et al process so that the characteristics of the product couldbe studied, since such a study would require constructing a rather elaborate apparatus. It is also nd:eworthy that none of Marinaccio et al's Examples include formation of the film in a quench bath, but instead are manually cast in individual laboratory tests onto glass plates.
Tests were run using the glass plate method described by Marinaccio et al, with delay periods between drawing the film and immersion in the bath ~ 6~ 7 varied from less than three seconds to as long as one minute; there was no significant di-Eerence in product characteristics. It may therefore be assumed that the film resulting from casting under the bath surface (representing e2~trapolation to zero time) will not be different. With this in mlnd, the casting 5 resins of his ~xamples were formed as thin films, and with minimum delay, always under one minute, so as to allow no significant loss of solvent by evaporation, immersed into the baths described; in all cases the films obtained ~vere heavily skinned.
A number of polyamide resin membranes have been used for reverse 10 osmosis and ultrafiltration, but all have pore sizes below 0. 1 ,u, 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 used for this purpose, l~ut instead accom~
plish sucb tasl~s as reverse osmosis and ultrafillration, which are not 15 quantitative, and which can tolerate the imperfections which characterize skinned nylon membranes.
Steigelmann and Hughes, U.S. patentNo. 3,9809605 patented September 14, 1976, provides semipermeable membranes made-from mi~tures of polyamides, especially N-alkoxyalkyl polyamides, and water-soluble polyvinyl 20 alcohols. The membranes are preEerably fvrmed as hollow fibers. The membranes can be made from compositions containing the polymer components and a di (lower alkyl) sulfoxide, e. g. ,dimethyl sulfoxide. The membranes may contain complex-forming metal components. The membranes are useful for separating chemicals from their mi}ctures by techniques using an aqueous 25 liquid barrier and cornple~-forming metals, e. g. ,for the separation of ethylen-ically unsaturated hydrocarbons such as ethylene from close-boiling hydro carbons, but such membranes have pore sizes too small to provide flow rates useful in particulate and bacteria filtration.
It is an unfortunate fact that most available membrane sheets are hydrophobic, i. e. 7 not readily wetted by water. Synthetic resin membrane 5 sheet has almost invariably been made of hydrophobic synthetic resin, and retains the hydrophobic characteristic of the polymer from which it has been made. The cellulose ester membranes ar e also hydrophobic. Of the available membrane sheets useful in the particle separation range only ~egenerated .

cellulose sheet and alcohol-soluble polyamide membrane sheet are hydrophîlic, 10 i. e., wettable by ~ater.
Brooks, Gaefke and Guilbault, U.S. patentNo. 3,9~1,810, proposed a way around this problem, by preparing ultrafil~ation membranes made from segmented polymers having distinct hydrophilic portions and hydrophobic portions. Brooks et al suggest that if the casting solvent be a bett~r solvent for 15 the hydrophilic polymer segments than for the hydrophobic segments, the resulting film or membrane will display a gross morphology in which the hydrophilic portion of the system exists as a continuous phase while the hydrophobic portion is present as a disperse phase. The membrane system will include segregated domains of hydrophobic segments dispersed in a back~
20 ground of the hydrophilic polymer segments. By the ~ame token, if a casting soluticnis selected such that it is a hetter solvent for the hydrophobic polymer segments than for the hydrophilic segments, the phase relationships in resulting films will be reversed and the film will not function as a membrane for a~ueous media but will behave more as a hydrophobic film displaying virtually no 25 water permeabilityO

However, this expedient merely utilizes combinations of hydrophilic and hydrophobic groups to achie~e water permeability~ and does not suggest a way of modifying normally hydrophobic groups to improve water permea~
bility of hydrophobic polymers. Polyamides are not referred to by Brooks 5 et al as acceptable membrane materials -Eor their in~rention.
Yamarichietal, U.S. patentNo. 4,073,~33, describeahydrophilic polyvinyl alcohol hollow fiber membrane with a relatively uniform pore size distribution in the range from 0. 02 to 2 nnicrons, but these pores are not interconnected, and the product serves for separation in the dialysis (high 10 molecular weight dissolved compound) range, rather than as a particle or bacterial filter. - -Since the bulk of filter applications for membrane sheet is in thefiltration-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 15 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 posei~le or desirable in many applications, as for example, in assaying bacteria, since some bacteria are killed by surface-active agents. In other applications, filtering 20 media cannot be adulterated by the addition of surface-active agent~ without deleterious consequences.
Membrane sheets made of cellulose esters, which currently account for over 95~c of all of the membrane sheet material sold,are inheren~y not water-wettable; hence ~urface-active agents are added for water service.
25 Further, these membranes tend to be brittle7 and to counteract this, glycerine ~L6()~07 is added as a plasticizer, but this is also undesirable, since it will leach into aqueous fluids, and poses a contamination problem which is unaccept-able in ma~y uses.
In accordance wLth the invention, alcohol-insoluble polyamide resin 5 membrane sheet is provided that is inherently hydrophilic. This is a most remarka~le properl;y, inasmuch as the alcohol-insolu~Le polyamide resin from which the sheet is made is hydrophobic. The phenomenon occurs only with a1cohol-insoluble polyamide resins having a ratio CH2: NHCO oP
methylene C~I2 to amide NHCO groups within the r~ge from about 5: ~ to 10 about 7:1. The reason why such polyamide resi~ membra~e sheet prepared in accordance with the process of the invention 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 fi~ed in the solid polymer membrane surface as a result of the precipitation process. It may be related 15 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, facUitating its being wetted by water. The fact is that a drop of water placed on a dry polyamide resin membrane sheet of the invention will penetrate into the sheet and disappear within a few seconds. ~ sheet of the dry membrane place on 20 ~e surface of a body o~ water will be wetted through and may even sir~{ in the water within a few seconds. If the membralle is complet~ly immersed in water, 1~e membralle is wetted through in less than a ~econd.
The capabUity of a membrane's or subs$ra~e's being wetted by water is detexmined by plaLcing a drop of water on the membra~e or substrate sur-25 face. The angle of contact pro~rides a quantitative measure of wetting. A veryhigh angle of contact indicates poor wetting, while a zero angle of contact defines complete or perfect wetting. The polyamide resin from which the membranes of this in~ention are made ha~7e a high angle of contact, andL
are not wetted by water.

The wettability of these membranes is not a function of retained water.
Membrane specimens dried at 350F for 72 hours in an inert atmosphere, in vacuum, and in air, are unchanged with respect to wettabili-ty by water. I-f, however,they are heated to a temperature just below the softening temperature 5 of the membrarle ~to heat at a higher temperature would of course destroy the membrane, since it would melt), the membrane reverts to a hydrophobic material, which is no longer wetted by water. This suggests that the hydro-philicity is a function of solid structure, and i~ obtained by the process of membrane formation, probably during precipitation of the membrane in the 10 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 losl; when the memhrane film is heated to a high enough temp~rature to permit reorientation to a normal configuration, 15 in which $he material is hydrophobic.
It follows, of course, that during processing and drying it is importa~
not to heat the ~embrane above this temperature.
A further important characteristic of the polyamide resin membrane sheets of the invention is their high fle~ibility. In the normal thickness range 20 in which they are useful, in the absence of an ext;reme state of dryness~ the~
can be folded back and for$h on themsel~es several times, without harm, and without the addition of a plasticizer.
In the proce~ of the invention, polyamide re5in havlng a ratLo C~2:N~ICO of methylene Cl-I2 to amide N~CO groups within the range from ~5 about 5:1 to a3~out q~ ; dissolved in a polyamide resill solvent, such as forr~ic acid; a nonsolvent is ad~ed under controlled con~itions to achieve a nucleated solution, and the resulting solution cast on a substrate in the form of a film, and this film of solution is contacted and diluted with a liquid which is a mixture of a solvent and a nonsolven~ for the polyamide resin. The polyamide resin thereupon precipitates from the solution, forming a 5 skir~ess hydrophilic membrane sheet on the sub~trate, and the sheet can then be wa~hed to remove the nonsolvent. The membrane can be stripped off of the substrate and dried, or i~ the substrate is porous7 it ca~ be in-corporated in the membrane or attached to the membralle to serve as a permanent support, and is then dried with the membra~e.
The conditions under which the polyamide resin is precipitated de$ermine the skinless nature of the membrane, as well as its physical characteristics, i.e., the size, length alld shape of ~he through pores of the membrane. Under certain conditions, a membrane is formed which has 1;hrough pores extending from surface to surface tha~ are subst~l~ially 15 uniform in shape and size. Under other oonditions, the through pores are tapered, being wider at one surface alld narrowing towards the other surface of the membrane.
Under conditions outside the scope of the ir~ve~tion, still another form of the membrane is obtained, having a dense skin penetrated by pores 2~ o~ smaller diameter $han the pores in the remainder of the sheet. This skin is normally on one side of the membrane sheet~ but it can be on both sides of the mem~rane sheet. Such skinned membranes are con~rentioTlal in the art, e~hibit relatiYely higher pressure drop and other poor filgration characteristics, and a~e undesirable.
Thus, by colltrol of the method by which the casting resin is nucleated, and of the precipitation c~nditions, it is possible to obtain ~ 7 hydrophilic polyamide resin membranes with through pores of desired characteristics, either uniform from face to face, or tapered, with larger pores on one face transNioning to finer pores on the other face.
The formation of a polyamide memb~ane ha~ing uni~orm pores 5 or tapered pores without a skin on either surface is also ~emarkable. As shown by l~ie Michaels pa$ent No. 3, 615, 024, and Marinaccio et al No. 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 s~ ess poly~nide resin membrane by this process has not 10 previousl~ been achieved.
The process of the inve~tion for preparing from hydrophobic polyamide resin a skir~ess microporous polyamide membrane having absolute particle removal ratings of 0.10 ,~M to 5 ,u~ or larger in a solid form that is hydrophilic and remains hydrophilic until heated to a tempera-15 ture just below its softening point comprises preparing a solution in apoly~n~ide sol~Tent of an alcohol-insoluble poly~mide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within the range from abou$ 5:1 to about 7:1, inducing nucleation by dilution of the solution with a nonsol~ent liquid under controlled 0nditl0ns o~ solvent and nonsolvent 20 and resin concen$ratioII9 temperature, mixing intensi~y, addition time and system geometry such that a visible precipitate of polyamide resin forms during the addition of the nonsolvent, with or without visibly complete redissolutlon of the precipitated polyamide resin; removing any undissolved resin by filtration; spreading the resulting solution on a 25 substrate to form a thin film thereof on the substrate; con~acting the film wi~ a mi~ture of nonsolvent liquid containing a substantial proportion of solvent for the polyamide resing thereby precipitating polyamide resin in ffle form of a thin skir~ess hydrophUic membrane; and washing and drying the resulting membrane.
In a preferred embodiment of this process, the solvent for the 5 polyamide resin solution is formic acid and the nonsolYeDt is water, aIld the polyamide resin solution film is contacted with the nonsolvent by immersing the film carried on the substrate in a bath of nonsolvent comprising water containin~ a substantial proportion of formic acid.
The invention in another preferred embodimen~ provides a process 10 for preparing ski~ess hydrophUic alcohol-insoluble polyamide membrane sheets having pores that are substantially uniform from surface to surface, which comprises preparing a solution in a polyamide solvent of an alcohol-insoluble polyamide resin ha~ing a ratio of CH2: NHCO of methylene ~H2 l:o amide N~ICO groups within the range from about 5:1 to about 7:1; inducing 15 ~cleation by dilution of ~e solution while controllin~ sol~ent and nonsolvent and resin concentration, temperature, mixing intensity, addition time and system geometry to obtain a ~Tisible precipitate of polyamide resin during the addition of the diluent,with or without visually complete redissolution of the precipitated polyamide resin, thereby forming a casting solution;
20 remo ~ing any undissolved resin by filtration, spreading the castin~
solution on a substrate which is nonporous and whose surface is wetted ~y ~e casting solution and preferably also by th~ nonsolvent-solve~ mi~ture to form a thin film ~hereof on the substrate, contacting the film with a mixture of nonsol~ent liquid containing a subs~antial proportion o~ solvent 25 for ~e polyamide resin, thereby precipitating polyamide resin in the form of a thin skinless ~rdrophilic membrane, a~l washing and drying the resulting membrane.

~i~L~ 307 Further, a continuous process is provided for preparing skinless hydrophilic alcohol-insolu~le polyamide membrane sheets which comprises preparing a solution in a polyamide solvent of an alcohol-insoluble poly-amide resin having a ratio CH2: N~ICO of methylene CH2 to amide NHCO
5 groups within the range from about 5:1 to about 7:1; inducing ~cleation by dilution of the solution with a nonsolvent while controlling solvent and nonsol~ent and resin concentration, temperature, mi~{ing intensity, addition time and system geomQtry to obtain precipitation of polyamide resin ~uring the addi$ion of the nonsolvent, with or without visually com-10 plete l edissolution of the precipitated polyamide resin, ~ereby forminga casting solution; removing any undissolved resin by filtration; spread-ing the casting solution on a substrate which is nonporous and whose surface is wetted by the casting solution and preferably also by the nonsol~Tent-solvent mixture to form a thin film thereof on the substrate;
15 contacting the film with a bath of nonsolvent liquid containing a substantial proportion of solven~ for the polyamide resin, thereby precipitating polyamide resin in the form of a thin skinless hydrophilic membrane;
aIld continuously washing and drying the resulting membr~ne, while maintaining constant the rel~tive proportion of solvent and nonsolverlt 20 liquid in the bath. In a preferred embodiment, the rates o withdrawal and addition of sol~ent and nonsolvent to and from the bath are maintained substantially const~nt.
The invention further provides a process for preparing skinless hydrophUic alcohol-insoluble polyamide membrane sheets having multi-25 membrane layers, which co~nprises preparîng at least two startingsolutions in a polya:mide solvent of alcohol-insoluhle polyamide resin 0~l - having a ratio CH2:NHCO of methyleIle CH2 to amide NHCO groups within the range from about 5:1 to about ~:1; inducing nucleation by dilution of the solutions wLth a nonsolven$ while controlling solvent and nonsolvent and resin concentration, temperature, mi~ing intensity, addition time and 5 system geometry to obtain a visible precipitate of polyamide resin d~ring the addition of the nonsolvent, with or without visibly complete redissolution of the precipitated polyamide resin; removing any undissolved resin by filtration; spreading the resulting solution on a substrate which is non-porous and whose surface is wetted by the casting solution and prefera~ly 10 also by the nonsol~rent-solvent mixture to form a thin film thereof on the substrate; contacting the film with a migture of nonsolvent liquid containing a substantial proportion of solvent for the polyamide resin, thereby pre-cipitating pol~amide resin in the form of a ~hin ski~ess hydrophilie mem-bran~; washing the resulting two membranes; combining the t~o mem-15 branes so ~ormed as a dual layer; and dryillg the dual layer underconditions of restrain to prevent more than minor reduction of the length and wid~:h of the membrane; the membranes so dried forming a single sheet with particle removal characteristics æuperior to those of the Individual layers.
The membranes thus attached can ha~re the sar~e or difering porosities, and the m~mbrane layers can be selected from membranes having tapered pores and mem~ranes having uniform pores, in alYy combination, supported or unsupportedO
The two combined mem~ranes can be obtained from a single 25 roll of filter medium, and when combined with matching faces in con-tact form a sheet which is symmetrical, and which provides equal filtration chaxacteristics regardless of which face is upstream.

The Invention also provides several types of polyamide resin membrane products. One prefelred embodiment is a hydrophilic micro-porous polyamide membrane comprising a normally hydrophobic polyamide resin having a ratio CH2:NHCO of methylene CH~ to amide NHCO groups 5 within the rangefrom about 5: 1 to about 7:1 in a solid structure that is hydrophilic, having absolute removal ratings wi$hin the range from about 0. 1 uM to about 5 ,uM, and a thicl~ness wi thin the range from about 0. 025 mm to about 0. 8 mm.
These hydrophilic microporous polyamide resin membranes can 10 have pores e~tending from surface to surface in a relati~ely uniform structure, or in a tapered pore structure.
Also provided are hydrophilic polyamide resîn membranes that are supported by the substrate on which the polyamide resin membrane is formed, either imbedded therein, or having the substrate attached to one 15 face thereof.
In addition, the invention provides microporous polyamide resin membrane composites having a plurality of polyamide resin membrane layers, formed of meml~ranes preparad separ~tely ~y precipitation on separate substrates and then bonded together by drying two or more 20 layers main$ained in close contact.
In all of these em~odiments, $he polyamide resins having a ratio C~2:NHCO of methylene CH2 to amide NHCO groups wifflin the range from a~out 5:1 to about q: 1 include polyhe~amethylene adipamide ~ylon 66), poly-E~caprolactam (Nylon 6)~ polyhe~amethylene sebacamide ~ylon 610), 25 poly-7-aminoheptarloamide (Nylon 7), polyhe~amethylene a~eleamide (Nylon 69), and mi~tures of two or more thereof, as well as mixtures thereof with higher polyamide homologues such as polyhe~methylene sebacamide ~Nylon 612) in proportion such that the mixhlre has an average of CH2:NHCO ratio within the stated range. The first three 5 polyamides, Nylon 66, Nylon 6 and Nylon 610, are preferred.
Another purpose of the invention is to pro~Tide a procedure for quantitati~Te characterization of uniform pore membranes for their abilit~ to provide sterile ef~luent when challenged by a stated number of a gLven microorganism. This procedure is applicable to uniform pore 10 distribution membranes made of other than polyamide resins, and using other processes.

In the drawings:

Figure 1 is a graph showing in a qualitative manner the relation-ship between the degree of nucleation of the casting resin ~olution alld 15 the pore diameter of the resulting membrane;
Figure 2 is a graph showing the relationship for a uniform pore membrane between titer reduction, defined as ~he ratio of Pseudomonas diminutiae bacteria contained in the inflNent liquid to ~at obtained in the effluent liquid, a~d the number of layers of uniform pore fUter medium 20 through which the bacteria laden liquid is passed.
Figure 3 is a graph showing ~e relationship obtained when a we~ted membrane is pressurizedby agas, and the ratio airaiprrflsure is plotted against air pressure applied. The qualltil~T KL is defined by ~e broken line of F_gure 3 .

~:~6~0a7 Fi~ure 4 is a graph showing the relation between T.~ and KL, where T
(or, Log TR ~ ~ Log TR) 10 where t is the thickness, in thousands of an inch, of the uniform pore membrane which shows a titer reduction, as ~defined above, equal to TR;and TR is the calculated titer reduction for a 0. 001 inch thick membrane of equal pore size~
KL is the pxessure, measured inpsi, atwhich air flow through the water wet membrane increases very sharply (see Fi~ure 3); and K is the ~ralue of K corrected to correspond to that of a membrane 0. 005 inch thick, using the empirically determined correction factors listed in Table 1.
TABLE I
:
Measured Thickness .
inch Corr ection ~actor 0.002 1.10 . 003 . -- 1. 044 . 00~ 1. 019 O. 005 ~L. 000 0.006 0.985 0. 008 0. ~62 O. ûlO 0. 9~6 0.015 0.920 - ~160~07 The curve of Fi~-~lre 4 represents the results of measuring the KL
and TR for forty-five different specimens made by the process of this invention.
Figure 5 is a scanning electron rnicrograph (SEM) at 1500~magnifi-- cation of a membrane with uniform pores, made by the process of this invention, having a KL of 47 psi, t = 0. 003~ inch, and an estimated TR of 3 x 10l8 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 $o surface. The upper and lower micrographs show the upper and lower surfaces respectively adjacent to 10 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 lOOQXmagnification of another membrane with unlform pores made by the process of this in-vention, having a KL of 40 psi, t = 5. 6 mils, and an estimated TR 8 ~ 5 15 for Pseudomonas diminutiae organism. Similar to Figure 5, the center portion is a section through the membrane sh~wing uniform pore size from surface to surface, and the upper and lower views show t~e adjacent upper and lower surfaces, again indistinguishable with respect to pore size.
Figure ~ is a scanning electron micrograph at lOOOXmagnification 20 of a membrane with tapered pores made by the process of this invention.
This membrane is 81 ,uM (0. 0032 inch) thick, and the upper portion of the section may be seen in the central part of the SE~ 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 25 vie~s, the pore diameters in the upper surface are substantially smal~er than those on the lower surface.

~L~6C~07 - Figure 8 is a scanning electron micrograph at 1500 X magnifi-cation of a lightly skinned membrane, of the l~pe obtained when baths outside of the range of this in~7ention are employed.
Figure 9 is a similar microgra~?h of a ~ re heavily skinned 5 mem~rane.
Figure 10 is a graphical representation of the relationship between (a) KL ~ a particle removal rating parameter of membranes made by the process of this invention, defined by this invention;
~3) the mixing intensity7 expressed as revolution per minute (rpm) of the in-line mixer used to process a 15. 5 Yc solution of resin in 98. 5~c formic acid to obtain the casting solution used to produce the membra~es; and (c) ~e formic acid concentration of t~e resulting casting solution.
~ 1i1hile the various polyamide resins are all copolymeræ of a diamine and a dicarbogylic acid7 or homopolymers of a lactam of an amino acid, they vary widely in crgstallinit~ or solid structure, melting point, and other physical properties. It has been determined in accordance 20 with the invention that the process of the invention is applicai)le oT~y to polyamides h~ving a ratio CH2:NHCO oX methylerle CH2 to amide N~ICC
groups within the raxlge from about 5:1 to a~out 7:1 exemplified by the polyamides enumerated pre~riously. The preferred members of this group, copolymers of he~amethylene diamine and adipic acid (~ylon 66), 25 to copolymers of he~nethyl0ne diamine alld sebacic acid ~Nylon 610), a~d to homopolymers of poly-~-c~prolactam ~Nylon 6), readily produce ~6 ~L~6(~(~0'7 skir~ess hydrophilic alcohol-insoluble polyamide resin membra~es. For reasons which are not understood, all members of this limited class of polyamide resins are quite susceptibl0 to precipitation under the process conditions of the invention to form hydrophilic membrane sheets.
These polymers are a~ailable in a wide variety o~ grades, which vary apprecia~ly with respect to molecular weight9 within the range from about 15, 000 to about 42, 000, and in other characteristics. The formation of a hydrophilic membrane appears to be a function not of these character~
istics, but of the chemical composition of the polymer, i. e., the spacing 10 and arrangement of the units composing the polymer chain. The especially pre~erred species of the units composing the polymer chain is polyhexa-me~ylene adipamide, and molecular ~eights in the range above about 30, 000 are preferred. Polymers free of additires are gen~rally pre-ferred, but the addition of antioxidants or simUar additives may have 15 benefi$ under some conditions; for e2~3mple, addition of the a~tioxidant E~yl 330 (1,3,5-trimethyl-2,4J6-tris [3~5-di-tert-butyl-4-~ydrox~
benzyl~ ben~ene) has been shown to e~tend the life of polyamide mem-~ranes exposed to extreme ogidative hydrolytic conditions.
The polyamide resin solution fr~m which the polyamide membrane 20 film is precipita~ed can be a solution in any sol~er~ for the polymer~ These solvents are well ~own, and are themselves no part of the instant in~ention.
~ preferred solven~ is formic acid at any temperature from its freezing point to its boiling pomt. O~er suitable solvents are: other liquid aliphatic acids such as acetic acid and propionic acid, and halogenated 25 aliphatic acids such as trichloroacetic, trichloropropionic, chloroacetic acid, dichloroacetic acids, phenols such as phenol, the cresols, and their 2~

halogenated derivatives; inorganic acids such as hydrochloric, sulfuric, hydrofluoric and phosphoric; saturated aqueous or alcohol solutions of alcohol-soluble salts such as calcium chlorid~, magnesium chloride and lithium chloride; hydro~ylic solvents including halogenated alcohols 5 (trichloroethanol, trifluoroethan~l), benzyl alcohol, and polyhydric alcohols such as ethylene glycol, propylene glycol7 and glycerol; and polar aprotic solvents such as ethylene carbon~tea diethyl succinate, dimethyl sulEoxide and dimethyl formamide.
The polyamide resin solution, hereafter refereIlced as the 10 starting resin solution, is prepared by dissolution of the polyamide resin to ~e used in the membrane in the desired solYent. The resin can be dissolved in the solvent at ambient $emperature, but a higher temperature may be used to accelerate dissolution.
If the starting resin solution is to be stored for more than a few 15 hours, water in eEcess of a~out 1 to 2~c should not be present, as other-wise a slow hydrolysis of the polyamide resin takes place, resulting in a~ undesirable reduction în molecular weight of the polyamide. In general, -the amount of water in this event should be less than 2~c, and preferably the solution is water-free. If water or formic acid-water mixture is 20 added to accomplish nucleation, it can be added just prior to casting, preferably within a~out five to six~y minutes of the casting oper~tion.
The casting resin solution is prepared from the starting resin solution by diluting 'It wiffi a nonsolvent, or with a mixture of solvent and nonsolvent.

~6~(~()7 The state of nucleation of the resulting casting resin solution is strongly affected by the following factors: -1) Concen~ation, temperature and molecular weight of the starting resin solution;
2) Composition and temperature of the nonsolvent, or of the nonsolvent-solvent mixture.
3) The ràte at which the nonsolvent, or nonsolYent-solvent mixture, is added.
4) The intensity of mixing during the addition.
5) The geometry of the appaxatus in which the mixing is accomplished.
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 into a bath containing a nonsolvent for the polyamide resin, 15 together with a substantial proportion of solvent for the resin. Tf ths nonsolvent in the bath is water, and i the solvent is formic acid, the presence of at least about 20~C and usually of at least 30 to 40~ of formic acid is desired to prevent formation of a skinned membrane, which occurs at lower concentra-tions of formic acid.
The stability of the casting ~esin solution varies greatly depending on the method used to prepare it. For example, casting resin solution prepared under small scale batch conditions tends to be relatively unstable; for example,the characteristics of the membranes it produces will be quite different if it is cast as long as five to ten minutes after it has been prepared, or it may transform to a noncastable semi-solid gel within 10 minutes or less. On .6~)7 the other hand, casting resin solution prepared using a continuous in~line mîxer, which can produce a membrane of equal characteristics, tends to be stable for a period of an hour or more. Casting resin solutions prepared in this way should, however, be used within an hour or less, particularly if 5 maintained at ele~rated temperature, to prevent substantial reduction in molecular weight of the polyamide resin which will otherwise occur due to the presence o water in the acid solutiong with resultant hydrolysis.
Eith~r of the above me$hods may be used to produce casting resin solutions which function equally when cast as membranes~ and regardless of 10 which is used the addition of the nonsolvent is accompanied by the appearance of a ~-isible polyamide resin precipitate, in order to produce a useul, properly nucleated casting resin solution. Casting resin solutions prepared by o~her means, for example, by dissolving the resin pellets in a solution of formic acid and water, or by adding the nonsolvent in a manner such as not to 15 produce such a precipitate, do not produce useful membranes~

IJseful 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 providinv a required dearee of filtration. A convenient index of useful~ess may be 20 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 sho~Yn in Table ll under, to~ether with typical permeabilities of similar range media made by the process of this invention.

~5 TABLE TI

Typical Flow Rates of IJseful MembraneS
~ - Polyamide mem-Absolute removal Flow per sq. ft. Commercial cellulo5e branes of this rating, micrometers per psid ester membranes invention - ~pm H2O 0.04 0.17 0.1 cfm air~ ~ ~.4 2.5 gpmHO 0.38 . 0.57 0. 2~ _ 2 cfm air 8. 0 8. 4 0. 45 cfm air 1 0 l j0 Membranes having significantl~ low~r flow capacities for equal 10 removal characteristics, when compared with currently marketed membranes, are not widely commercially acceptable, and have been defined, for purposT?s of this discussion, to be out of the useful range.
It is an important feature of this invention, that the conditions ar~ .
described for achieving a casting solution with controlled degree of nucleation 15 to make membranes with useful pressure drop characterîstics.
We use herein the terms "nucleation" and "~ta~e of n~lcleation" to account f~ the discovery that (a) casting resin solutions can be prepared with a wide variation of composition with respect to resin, solvent, and nonsolvent concenl:ra-tions, whic.h yield identical or nearly identical membranes; and (b) casting resin solutions can be prepared~ which have equal resin?solvent and nonsolvent concentrations, which are then ~ast at equal temperatures into the same bath, yet yield very different membranes;
in fact, the resulting membranes can run the gamut ~om "not usefulT' in the sense of ha~ing very significantly lower flows-by factors of 2 to 5 or more~ compared with Table I, through $he range ~om 0.1 ~M

absolute or coarser, producing membrane5 in all those ranges with good flow capacities, for example, equal to those listed on Table II.

~1 ~

~L~L6~ ;)7 Since the preparation of casting resin solutions capahle of producing membranes with flo~v properties in ~he useEul range has been observed to invariably be accompanied by the local precipitation and at least partial redissolution of solid resin, and since it is well known to those familiar with the chemical arts that the characteristics of a solid precipita~ed from solution can be greatly influenced by the presence or absence of submicro-scopic nuclei, we have chosen to use the tlerm "state of nucleation" to distinguish casting solutions having equa1 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 a degree of nucleation controlled to yield a 0. 4 ,u~
absolute membrane. A portion of the casting resin solution was subjected to fine filtration to determine whether nucleation beha~rior would be affected, andthe properties o 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 fins filtration; the finely filtered casting resin solution produces a membrane with a ver~ poor ratio or flow capacity to remo~al rating; $he ~p of sample No. 59 is more than three times higher than that for a similar membrane made using a properly nucleated casting resin solution o this invention.
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 ~y that casting resin solution, and that at least a po. L;on of these nuclei were removed by fine filtration.
It should, however, be understood that we have not unque~tionably 5 established that nucle~tion is the only explanation Eor the observed results, and that they could be caused by phenomena other than nucleation.
The viscosity of the casting resin solution is preferably ad3usted to between about 500 centipoises and 5000 centipoises at the temperature existing at the time it is cast as a film~ ~iscositi~s below about 500 cp allo~v some of 1~ the cast film to float off as a liquid to the surface of the bath, when it forms a filmy precipitate, thereby adversely af~ecting cast membrane propertie~s and fouling the bath. Viscosities much above 5000 cp, For example, 100, 000 cp, are not Ileeded to obtain a smooth, coherent cast film7 but are helpful in casting membranes where no substrate is used, for example, 15 hollow fibers, or unsupported film.
Solutions of a viscosity well above 5000 cp at the castLng temper~ture can be cast without difficulty7 however, the preferred vissosity limit is about 5000 cp, since at higher viscosities the energy input to the mixture when a nonsolvent is blended with the polyamide resin solution is very high, with the 20 result that the solution can l~each e2~cessively hîgh temperature, with ensuing operating problems. Moreover, the pumping of the startingrpolyamide resin solution to the casting operation becomes progressively more difficult, as viscosity increases~ Also, manipulation of the casting resin solution within $he 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 solu-tion, viscosities much above about 3000 cp can cause improper penetration, and the resulting product has undesirable voiAs.
The temperature of the casting resin solution is not sritical, and useful membranes have been made over the range from about 85C downward.
Under some circumstances, some~vhat highler 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 li~uid enters the bath, a precipitation process occurs, whose mechanism is not completely underslood. The nonsolvent ~nixture 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 notunderstood why this results in a uniform pore size throughout the thickness of the film ~vhen the bath solvent-nonsolvent ratio is held within certain limitsO
If the bath contains only nonsolvent (such as water, alcohols or organic esters), or nonsolvent with a small proportion of solvent ~e. g. water with less than 15 to 20% of formic acid) precipitation occurs very rapi~ly, and the 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, and are undesirable~
If the bath contains about 43 to 55% oE 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 that substrate be wetted by the casting resin solution and by the bath solution. The time required ~or the film to form under these circumstances is a function of the following:
(a) Casting resin solutions which produce membranes which ha~e high KL values (e.g. in excess of 100 psi) set very rapidly, e.g. in less than 10 seconds. Less highly nucleated casting resin solutions, producing membranes with KL values of about 40 to 50 psi will typically set in the 10 to 20 second range, and the setting time continues to increase as KL decreases, such that membranes about 0. 006 inch thick with KL ~ralues o under 20 psi require about 5 m;nutes or more to set, and still lower KL's require still longer periods.
O 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 aster settingO
(d) Setting is faster at the low end of the 43 to 55% recommended 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 a~d 6, to tapered pore as sho~Tn in Figure 7~ to skinned as shown in Figure 8? to heavily skinned as sh~wn in Figure 9. Operation at formic acid concentrations much lower than those 5 proc~ucing tapered pores as exemplified by Figure 7 is undesirable.
The formation of the membrane from a casting resin solution can be carried 3~

~ )7 out as an intermittent or batch operation or as a continuous or semicontinuous process. A small scale operation may be most conveniently carried out as a batch operation, while at high production rates a continuou~ or semicont~nuous operation is more convenient. In all types of processes7 it is important to 5 carefully control all of the operating parameters to ensure a uniform product, including operating temperatures, and relative proportions of resin solution and nonsolvent liquid. The control of conditions of non-solvent addition are particularly important, includlng the geometry of the ~pparatus, the rates of ~:low, and duration and intensity of mixing; also the interval between nonsoluent 10 addition and casting of the resin film must be controlled. Such controls can be established by trial and error e~perimentation without undue difficulty by those skilled in this art, taking into account the following considerations:
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.
15 If suspended material is present, such as undissolved resin particles, these are removed by screening or filtration before casting.
Any type of subs~ate or support can be used as a surface on which the casting resin solution is cast to foxm the solution film. If a nonsupported membrane film is the desired product, then the substxate should have a 20 sur-~ace to ~vhich ~he membrane does not adhere, and from which the 07 ) membrane film can readily be stripped at the conclusion of the drying operation. Strip~ability usually requires that the substrate surface be smooth-surfaced9 and nonporous. When the solvent is one with a relatively high surface tension, such as formic acid, and the nonsol~ent also has a 5 relatively hi,,h surface tension (as3 for example, water), it is important that the nonporous surface on which the filrm is cast be wettable, i. e, have zero or near zero allgle 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, with undesirable 10 effect on membrane properties~ Such temporary subs~ate or support surfaces can be of a suitable ~naterial, such as glass, metal or ceramic.
Plastics, such as polyethylene, polypropylene, polyester, synthetic and natural rubber, polytetrafluoroethylene, polyvinyl chloride7 and simUar materials ~re not inherently suitable, as they are not wetted by the casting 15 resin and nonsolvent, but these can be rendered suitable by application of an appropriate oxidative nr similar surface treatment. A corona discharge can, for e~ample, be used to treat Mylar ~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 20 permanent supporting layer, then it should be o~ porous material that preferably is wetted by the casting resin solution, so that the casting resin solution will penetrate it during the casting of the solution on the subs~ate, a~d become firmly attached thereto during precipitation of the polyamide membrane film.
It is not essential howe~er that the substrate be wetted; if it is not wetted, 25 the polyamide resin film will be largely confined to the surEace of the supportJ

* Trademark 37 ~ut is nonetheless aclherent thereto. Such substrates ca~, ~or example, be of nonwoven or woven fibrous material, such as nonwoven mats and bats, and woven te~tiles and cloth, as well as netting of various types, including extruded plastic filament netting, papers, and similar materials.
As permanent supports which are not wetted by the casting resin '6 solution, fine-pored nonwoven webs can be used, made from fibers with poor wetting characteristics, such as, for example, polypropylene or polyethylene.
The resin solution îs cast as a film onto the nollwoven web, and since it does not wet the fibers of the web, it is carried on its sur~ace. The substrate carrying the casting resin solution film on its lower ~urface is plunged into a bath of nonsol~rent liquid or allowed to float on the surface of the bath, and the membrane film precipitated onto the substrate. The resulting film has good adhesion to the substrate, and the substrate has very l~ttle or no effect on the pressure drop for fluid ~low through the membrane.
lS 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 tension, such that the casting resin sol7ltion film wili ~ompletely permea~e the suppor$ing web, and the resulting membrane precipitates in arld around the fibrous material, and is permanently supported thereb~, since the material of the support is embedded in the membrane. The resulting membrane has a somewhat higher pressure drop when tested with flowing fluid, but the increase compared with the unsupported membrane is small, if the supporting web has an open structure.
Suitable wetted substrates that can serve as permanent supports for the membrane include polyesters, as a nonwoven fibrous web or as a woven web, using monofilament or multifilament yarn, the monofilaments being preferable in terms OI open structure and lower pressure drop; also polyimide fiber woven webs, woven and nonwoven webs of aromatic polyamides or Nom~, and other relatively polar fibrous products such as cellulose, regenerated cellulose, cellulose Psters, cellulose ethers, glass fiber, and 5 similar materials.
Cellulosic and synthetic fiber filter papers can be used, as well as perforated plastic sheets, and open mesh expanded plastics such as Delnet or similar extruded and thereafter e~pandecl nefflngs. If the substrate is relatively coarse or in a very open w0ave s~ucture, even if the fibers are 10 not well wetted b~r the resin solution, the substrate may nonetheless be embedded or embraced by the membrane material in the ~Einal supported membrane productj such relatlvely poorly wetted materials as polypropylene and polyethylene can function as embedded substrates if they have a sufficiently open s~ucture. If a polyolefin substrate has a relatively smaller 15 pore size, for ~ample, about 30 microns, the casting resin solution wlll not penetrate in$o 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 pq~ecipitating bath of the nonsolvent liquid, 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 ~Eilm is cast should be treated or coated so as to make them wet~able.
The nucleated casting resin solution can be cast or spread out upon the substrate in the de9ired ~Eilm thickness using a conventional doctor blade * Trademarks ~6~l~1)7 or roll, kissing or squeeze rolls or other co~entional devices, and then contacted with the ~ath liquid with as little delay as possible.
The choice of nonsolvent liquid depends upon the sol~ent utilized.
The preferred nonsolvent for producing nucleation in the polyamide resin 5 solution is water or wa~er-formic acid mixtures. lIowever, any substance is suitable which is soluble in water and reduces the sur~ace tension of water. Other nonsolvents anclude formamides and acetamides, dimethyl sulfoxide, and similar polar solvents, as weli as polyols such as glycerol, glycols, polyglycols, and ethers and esters thereof, and mi}~tures of such 10 compounds. Salt~ can also be added.
~ ?ollowing precipitation, the membran0 film is washed to remove sol-vent. Water ls suitable, ~ut any ~olatile liquid in which the solvent is soluble and that can be removed during drying can be used as ~e washing liquid.
One or several washes or baths can be used as required to reduce 15 sol~ent content to below the desired minimum. In the continuous process, w~ash liquid flow is countercurrently to the membrane, which can, for example~
be passed through a series of shallow washing liquid baths in the washing stage.
The amount of washing required depends upon the residual sol~rent content desired in the mem~rane. If the solvent is an acid such as formlc 20 acid, residual formic acid can cause hydrolysis during st~rage of the poly~
amide of which the membrane is composed, wLth a conse~uent 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 fUm requires a technique that takes into account the tendency of the membrane to shri~ linearly when dried unsupported, wLth the Fesult that the dried membrane film is warped. In ~ ~601~07 order to obtain a flat uniorm film, the mernbrane must be restrained from shrinkage during drying. One convenient way to do this, is to roll up a continuous web on a plastic or metal core, with a high degree of tension so as to obtain a tight roll, then ~îrmly wrap this with a rigid but porous outer 5 wrap, and then dry the assembly. Other methods of preventing shrinka~e, 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 clampinD the sheets in a frame restraining the sheet from shri~age on all four sides, and then heating the 10 framed membrane at elevated temperature until it has been dried. We have discovered that two or more equally sized membrane sheets can be placed in contact a~d dried together in a frame $o 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 indi~idual starting sheets are 15 relatively thin, e.g. under 0.005 inch thick, and are of the ~supported ~subs~ate free) type, they ma~p be subsequently cut to size, for example, by steel rule dies, and are thereafter for practical purpose~ a single sheet or disc of filter medium.
The membranes can be dried in any of the ways described above, 2~ and then corrugated, seamed to provi~e a closed cylinder, and end capped.
We have discovered that this process can be greatly simplified, while producing a superior product, by corrugating tbe filter medium while it is still wet, together with upstream ~d dowllstream layers oE dry porous material, this material being chosed to be relatively rigid, and not subject ~5 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 oE vapor, and placed in an oven to dry. The resulting dried corrugated assembly shows only slight 5 shrir~age, and the corrugated polyamide mernbrane so obtained is free of ~varpage, 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 upst;ream (dirty) fluid and passage out o~ the element for downstream (clean) fluid.
If the filter cartridge is made using two or more thin layers of the polyamide membralle, these will be firmly adhered to each othe} at the conclus;on of the drying operation, and behave mechanicall~ as though they were a single layer.
The control of the precipitation so as to obtain the formation of a 15 hydrophilic polyamide membrane sheet of desired flow characteristics and pore size requires that the casting resin solution be controlledwith respect to a characteristic referred to herein as "nucleation". The variables that must be controlled include the choice of resin and of solvent and nonsolvent7 the concentration of the resin in the starting polyamide resin solu~ion, tempera-20 tures of all components, the quantity and mode of addition of nonsolvent,including rate of addition, intensity of mixing during addition, and the geome~ry of the apparatus, the latter including especially size and location of the nozzle through which the nonsolvent is added. For a given resin, solvent and non-solvent, the effect of these variables on the degree of nucleation is qualitatively 25 stated in Table lll.

~6(~007 TABLE Ill Variables af-fecting degree of nucleation Dir ection oE change to obtain a higher Type of Variable Variable degree of nucleation ~.
Mixing conditions Temperature De~rease Rate of nonsol~lent addition Increase Size of inlet opening through which the nonsolvemt is fed Increase Distance of the inlet opening from actual mixing area Increase Intensity of mixing Decrease Concentration of the ~c of resin Increase components in the 15 casting solution ~c of nonsolvent Increase _ Degree o~ nonsolvency of the nonsolvent Tncrease In Table Irr, the concentration of solvent is not included, as it is defined by the concentration of the resin and the nonsol~ent.
It will be appreciated that the intensity of mixing in a g~ven system is a function of a large number of variables. However, for a gi~en system the relative intensity of mixing can be expressed in terms of the rotation rate of the agitator~ or of the cutting blades of a homogenizer, etc. For a continclous production system (as opposed to a batch operation) an in-line mixer is requi~ed, and in a suitable designed multiblade ml~er about 1/4 to 2 hp is required to produce about 30 kg per hour of 2000 centipoise casting resin solution at a rotation rate between about 200 to 2000 rpm. Such equipment 0~ , can take diverse forms, and can take any oE a number of the designs commonly used in the mixing art, since the various mixing principles can all lead to similar results.
Because the intensity of mixing is diEficult to quantify, transfer of 5 manufacturing technology from batch systems, to continuous systems requires trial-and-error experimentation, varying the operating condition parameters ~mtil one obtains the desired membrane sheet, all of which is within the capability of one skilled in this art, since it involves manipulation of variables that are cus$omarily adjusted in chemical process industry manufacturing 10 processes.
The importance of mixing intensity and oE the other conditions related to mixing cannot be overemphasized. For example, a series of casting resin solutions with the same concentrations of the same resin~ solvent, and nonsolvent~ and the ~ame temperature and viscosity can be produced by 15 simply changing the mixer rpm. The most highly nucleated of these casting resin solutions, made using the slowest mixer speed, will then produce a membrane having an absolute pore ra$ing of 0.1 ,uM; the next more highly agitated casting solution, cast into the same bath, will, if the mi~ing rate was correctly chosen, produce a 0. 2 IlM absolute membrane, and similarly by using 20 successively higher mixing rates membranes can be made with absolute ratings of 0.4 ,uM, 0.6 ,uM, 0.8 ,uM, etc.
The nozzle diameter through which nonsolvent is delivered during pre-paration 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 25 the formation and complete or partial redissolution of the precipitate appears to play an essential role in the preparation of the casting resin solutions of 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. 013 mch to 0.125 inch diameter, but smaller or larger nozzles could be used with successful results.
Not only can a casting resin solution of given composition and tempera-ture be made by varying the mi~ing intensity and thereby the degree of nucleation 10 to produce greatly different membranes, but the converse is trueS namely, membranes of equal or nearly equal characteristics can be made usin~ a t5; ide variety of resin7 solvent, and nonsolvent concentration in the casting resin solutionj for example, an increase in water content will increase the degr ee of nucleation, but if the mi2~ing intensity is also increased~ a casting resin 15 solution will be obtained with the degree of nucleation unchanged, and the membrane cast from this casting resin solution will ha~e characteristics equal to that made from the lower water content casting resin solution.
The relationship between the degree of nucleation and the a~solute pa~ticle removal rating of the resulting membrane is graphed in Figure 1, 20 which shows an inverse relationship between the pore diarneter of the membrane sheet and the degree of nucleation, i. e. 9 to obtain small pore diameter, a high degree of nucleation is required.
Reference to the gx aph of Figure 1 shows that in Region A, where the degree of nucleation is very small, the pore size tends to become 25 nonreproducible. 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 of Marinaccio, fall into this range, and tend to be of relatively poor quality. In Region B, the pore size decreases in a regular, 5 though not necessarily linear fa~hion, 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 remoYed by filtration prior to casting; and in Region D, the resin solution i~om which these lumps ha~e been removed 10 by filtration becomes unstable, and prone to early local or orerall gelation before the film can be cast. The very high degree of nucleation in Region D
is sometime9 manifested by an opalescent ~ppe~rance, su~gesting that the nucleation procedure has resulted in an e~cessive number and/or e~cessively large nuclei.
Because methods of achieving a required intensity o ml~in~ vary so greatly among the various types of equipment used in the mixing aLrt, it is not possible to quantify this characteristic. Consequently, any given apparatus must initially be used on a "cut and try" basis to produce ~asting solutions o~ the desired characteristics, applying the principles tabulated 20 in TablelII. Once the parameters of mixer rate, concentration~, tempera- -tures, flow rates, etc. have been es~lblished, casting resin solutions having quite reproducible characteristics can be produced in the B and C 3~egion o-f Fi~ure 1, on successive days or weeks of operation.
A favorable condition for producing membranes having low pressure 25 drops and particle remoYal ratings covering a wide range utilizes a starting resin containing 15.5~c of 42,000 molecular weightNylon 66 resing 83.23~C

~J~ 3!07 OI formic acid, and 1. 27tC of water . When this starting resin solution is diluted using the conditions of Examples 1 to 39, the results obtained in Figw7ce lU
are obtained. The product range of I~L is such that membranes are obtained with absolute particle ratings ranging from about 0.1 micron (for e~ample, a O. 012 inch thick membrane with KL ~ 100 psi) to about 1 micron (for example, a 0. 001 inch thick membrarle with KL ~ 27 psi).
The curves of Fi~ure 10 were obtained usin~ a specific in~line mixer configuration, in which the rotor was 2-1/2 inch in diameter. The same results can be obtained by using other mixers, and theRPM needed to produce these results rnay 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 reprssented, for example, hy the 1950 RPM and 400 RPM conditions of Figure 10, and once this has been accomplished, the conditions for making mem~ranes covering the whole range of Figure 10 will be apparent to him.
This same correlation of mixing cvnditions would then be equally applicable to the other Examples of this invention, m which an in~line mixer was used.
The casting resin solution can be extrudedabove or under the surface of the nonsolvent bath, especially if used to make hollow fibers; this process is more easily realized in practice by using relatively high resin viscosities (e. g. 100, 000 cp) and rapidly setting casting resin solution in relatively lower formic acid concentration baths, e. g. in the 20 to 40~tc range.
As previously described, three types of substrates are used:

~7 . , ~ o~ .
(a) nonporous, for example, commercial polypropylene or other plastic film~ glass, etc.;
(b) porous, not wetted by the casting resin solution; and (c) porous, wetted by the casting resin solution.
The nonsolvent precipitation baths used in this invention contain a mi~ture of solvent and nonsolvent for the resin. The characteristic of the bath which has an important effect on the properties of the resulting membrane is the relative concentration of solvent and nonsolvent in the bath. If the con-centration of solvent is ~ero, or at low level, Eor example, below 20~C, a heavily 10 skinned membrane will be obtained~ If the concentration is adjusted to one of the preferred ranges of this irlvention (about 43 to 55~ of formic acid, in the case of a bath containing only water and formic acid) the reæul~ing membrane has uniform pores from one face to the other.

If the bath concentration is 43 t~ 55~c, and the su~strates used are 1~ of types (b) or (c) described above, the pores will always be uniform through the thickness of the polyamide membrane. However, if the film is cast on a nonporous 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 plastic film materials, 20 such as polyethylene, polypropylene, polyvinyl chloride, andpol~ester are not, and if the casting solution is spread on such a substrate, and immersed into a 45~'c formic acid 55~c water 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 film1 but with a dense skin on the substrate side. We have, 25 however, discovered that if such plastic films are rendered more wett~ble, O~
- for example, by surace oxidative processes such as chromic acid ~eatment or corona discharge treatment, the resulting membrane is skinless on both faces, and of uniform p~re size throughout. In such a membrane, it is difficult if not impossible to determine by any manner of appraisal which 5 side 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 tension is maintained at a sufficiently high value. This wi~ll vary somewhat depending on the concentration of formic acid in the resin solution and in the bath, and 10 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 afilm into a series of 15 baths, each with slightly increasing water content, the characteristics of the men~brane 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 21~ 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.
_gure 7 shows scanning electron micrographs of a tapered pore membrane.
The bath sol~ent concentration required to obtain any desired taper pore 25 membrane varies consiclerably, depending, for example, on the state of o~
nucleation oE the cas-ting resin solution, and shoulcl be determinedfor agiven set of condition~ by tria~and-error; however, in the case of a water-formic acid bath, it is never less than 15 to 25/3tC, and usually is near to 30 to 35~c of formic acid.
As the bath water concen~a.tion increases, the membranes begin to form with increasingly heavier skins, and are characterized b~r high pressure drop, and poor pore size distribution characteristics.
The uniform pore membranes made by the process of this inYention' such as those shown in the scanning electron micrographs of Figures 5 and 6, 10 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, wh~ch remains in place when the membrane is removed from immersion. When air pressure is then applied across the membrane, there is noted a very 5 small flow of air. This air flow when divided by the applied air pressure re;nains constant as the pressure is increasedS when plotted as in Figure 3.
From the thickness of the film, and the known di~fusion constant of air in water, it can be calculated using Fic~'s law, that this 1OW is due to diffusion of air through the water film, and does not indicate flow through pores of the 20 fater medium. At a sufficiently high pressure7 the flow as plotted in Figure 3 is seen to increase suddenly, reflecting displacement of water from the ~argest pores, and flow of air through these pores, and the curve becornes nearly vertical. The sharpness of this rise will be appreciated by noting that in this region, the membranes of this invention require less than a 1~c 25 to 3~Zc 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 difusion) to a v~ry steeply rising rate of Elow for small changes In applied pressure, characterizes uniform pore media, which have sharply defined removal characteristics; such mediawill, for example, quantitatively 5 remove one bacterium, but will allow an only slightly smaller organism to pass. ~uch membranes generally also have favorably low pressure drop, for a given removal.
Skinned membranes behave very dif~Eerently; when water wetted and their air flow-pressure drop relationship is determined, the curve is 10 not flat initially, but slopes upward, indicating presence of large pores;
transition to a more nearly vertiral line is slow, with a large radius, and in the "vertical" area, instead of the sharp rise of ~p~ure 3, a sloping line is obtained, reflecting a wide pore size range. Such membranes are poorly suited to obtain sterile fil~ates when challenged by bacteria; either a 15 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 concen~ation of formic acid in the nonsolvent liquid in the bath is desirable to obtain a uniform product. In a continuous process, 20 this control is obtained by an appropriate feed to the bath of nonsolYent liquid, 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 25 added to the bath to compensate. Accordingly, control of the rate of ~L~363 D7 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 47shows that in order to obtain a skinless membrane sheet havin,, a uniform pore distribution, with fine enough pores to quanti-tatively remove all incident bacteria and particles over 0. 2 ,uM, a relatively highly nucleated castin~ resin solution is cast as a film and the membrane precipitated in a 46. 4~c aqueous formic- acid solu~ion 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 ~orm by an aqueous 25~C formic acid solution as ~he bath, as in Example 60.
It is instructive to note that in the range o-E 0. 2 ,uM 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 sarne range, the membranes of the invention remain uniform, or ma~r be tapered, as desired.
Thus, in the continuous production o~ membrane sheets in ar-cordance with the invention, to obtain uniform ch~acteristics in the membraneS the castinD
20 resin solution must be prepared under carefully controlled conditions and the bath liquid compo9ition must remain constant. Such a liquid is referred to as an "equilibrium bath'~, i. e., a bath in which the concentratlon o~
ingredients remains con5tant~ regardless of additions and withdrawals.
To illustrate, consider a casting resin solutioncontainingl3~c resin and 25 69~c formic acid with the balance water, continuously being cast in film ~oool~
form on a substr~te, and then plunged into an aqueous nonsolvent bath containing 46~C formic acid. As the resin membrane precipitates, a pro-portion of the solvent from the film of casting resin solutlon ~Nhich contains 69 parts of formic acid to 18 parts of water, or 7~-3~c formic acid) diffuses 5 into the bath, thereby altering its composition. To counteract this, water is continuously added to the bath at a rate conltrolled, for example, by a device using density measurements to report ~Eormic acid concentration, at the 46~7C level, and bath liquid is withdrawn continuously to tnaintain total bath volume constant. MaiIItaining this equilibrium bath makes it possible 10 to continuously produce a membrane sheet having uniform pore character-is~ics.
When used continuously, at high production rates, the bath temperaturewill gradually increase; cooling by a heat exchanger ma~T 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 plastic sheet unreeled from a roll, as a substrate to support the cast film.
The membrane sheet has a tendency to adhere to the subs~ate surface on drying, and it i~ 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 stren~ths in the rage oE 400 to 25 600 lbs/sq. inch, and elongations generally exceeding 40~c.

~.G~oO7 For some applications, even higher tensile strengths may be desired.
In additi~n, unsupported membrane sheet requires special care to manipulate in the typical range of thicknesses from 0. 002 to 0. 010 inch in which it is normally manufactured. In such cases, a supported membrane sheet is 5 desired. Such membrane sheet is prepared by forming the film of resin solution on a substrate which adheres to the membrane sheet after it has been precipitated thçreon. Either of the two types of substrates can be used; those-which are not wetted by the resin solution, and those which are.
The unsupported filter ~nembrane obtained at the conclusion of the 10 !nembrane forming process is wet with water, also contains a small amoun~
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 len~ths from 50 to 100 linear feet and placed in an oven until dry. Durir~
drying, some shrinkage occurs, but an acceptable product is obtained.
It is also possible to clamp a léngth OI membrane in a frame holding all sides against shrinkage, and then dry the membrane by e~posure 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 20 strong and flexible, and can be readily and reliably assembled in such appa~atus.
A similar product can be obtained with less hand labor by passing the wetted membrane sheet Dver 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 25 in contact with each other~ using any of the drying methods described above, ;~ a6~0~

they adhere to each other, forming a multi-layer structure. No honding agent or other adhesion technique is required.
The resulting multi-layer membranes are useful in the manner of a single layer filter membrane. SinGe in manufacture a small proportion of 5 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 removàl rating; an extremely high degree of reliability is obtained in this manner.
Very good adhesion o 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 uniorm poxe size is bonded to an unsupported t~pered pore membrane layer, which provides e-fficient prefiltration. The fine face 15 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 in accordance with the invention are particularly well suited to use on filter presses, where self-sealing character-20 istics are needed, and the filters are subjected to large stresses. They arealso useful in making plain, or corrugated filter cartridges for use at high differential pressures, or for impulse type æervice .
The filter membranes of the invention are well suited for use as the filter media in filter cartridges. Filter cartridges are self-contained 25 filter elements, provided with a filter sheet in tubular form capped off by .

~6~ 07 end caps at each end. Either or both end caps can have a throuDh opening ~or 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 lthese configurations, a favored form for the filter membranes of the invention is a corrugated cylinder. Such a cylinder is made by corrugating one or more layers o~ supported or unsupported wet mernbrane (two layers is preferred) sandwiched between two open porous or foraminous sheets which provide for fluid ~low up and downstream of the contacting surfaces of the filter medium within the corru,,ations. 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-tiDFht manner to each end o~
the resulting cylinder. The preferred method is in a~cordance with U. S. patent No. 3, 45'?, 339, patented December 8, 1965, to Pall et al. The end cap material can be an~ of a wide range of thermoplastic synthetic resin materials, particularly polypropylene, polyamides, polyesters and polyethylene. Polyester end caps, particularly polyethylene terephthalate and polybutylene terephthalate, seal very well to polyamide membrane materials, and have the advantage that 5Ç

~1~6~

the assembled carh idge is rapidly wetted by water, permitting a test using the standardized procedures of the invention to verify the integrity of the assembled filter cartridge.
In the manufacture of corrugated cylindrical filter cartridDes, a seam must be made joining the ends o-f the corrugated structures. Since the polyamides used to mal~e the membranes of this invention are thermo-plastic, heat sealing may be used to close the seam, and is for many or most purposes an acceptable method. Heat sealing does have some disadvantages, however:
(a) in order to make the seal, it is, practically, necessary to bend the last leaf of each outerrnost corruga$ion to an angle of 90, which is sometimes difficult to accomplish without weakening or other injury to the filter medium at bend;
(b~ the temperature used and duration of the sealing operation need to be changed to accommodate changes in thickness of the filter medium layers usedj and (c) a weakening of the structure occurs due to the introduction of a stress concentration at the edge of the seal a:rea; ~ highly stressed, the filter will fail at this edge, in preference to any other part of the assembly.
All these disadvantages are overcome by a novel joining technique.
We have discovered that a solution of trifluoroethanol containing 3 to 7~c of Nylon 66 in solution can be applied to the outermost face of each end corrugation, and the two surfaces then lightly clamped-together, and the fluoroethanol allowed to evaporate. Other solutions may be used, for example, 6~

a 33~c solution of Nylon 66 in formic acid, similarly solutions of polyamide resins in hexafluoroisopropanol or he~a-~luoroacetone sesquihydrate. An excellent seal results, free of all the disadvantages enumerated above; indeed the seal area is now s~onger than ~he remalning corrugations.
The quantity and concentration of the resin solution are quite noncritical, and good seals have been made with as little as zero percent or as much as 9~ of Nylon B6 resin in the ~ ifluoroethanol solution9 but Ul this solvent solutions in the neighborhood of 5~Yc are preferred, being sta~le, and having a convenient viscosity if a high molecular weight resin is used to 10 prepare the solution. Solution9 in formic acid have also been successfully used.
The accurate determination of effective pore size for membrane filter media that is meaningful in its representation of expected e~ectiveness as a ilter is difficult. When a uniform pore filter medium of this invention, 15 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 apparen~ pore openin~s as seen on the micrograph are measured, a pore size is determined which is about three to five times the diameter OI
the largest particle which the filter wlll pass, as determined~for example, 20 by bacteria challenge. Similarly, it has been attempted to a~certain the pore diame~er from the KL valu~ as determined by the procedure of applying air pressure to a wetted element, obtaini~g the KL value in the manner ~hown in Figur0 3, and inserting the so determined pressure into the well known capillary rise equation; when this is done a diameter is 25 determined which is a~out four times the absolute removal rating of the filter medium as determined by bacterial challenge.

~8 ~16~0~7 Such methods, upon reflection, appear to haYe 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 conta~ninants.
Contrary to established thinking, we have determined by test that the effectiveness of membranes of structure silrlilar to those of this illvention asfilter media is dependent not only on pore size, but also on thickness. In the development of the present invention, it has, for example, been demonstrated that of two memb.; anes, 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 in accordance with the invention as filter media is rated not in terms of pore size, but in terms of effectiveness in the remo~ral of a contaminant of known dimPnsions.
One of the principal applications of this type of filter membrane is to deliv~
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 Pseudonnon_s diminutiae, which is a small diameter relatively nonpatho~,enic bacterium referred to in abbreviated form by Ps. Filters which successfully meet such a challengre 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.

()7 This invention employs a standardized test based on Pseudomonas diminutiae removal that correlates such removal ~vith air flow measurements through the wetted membrane and the thickness of the membrane, and is capable of providing a quite complete characterization of the rernoval 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:
TR TE~l 9 or log T~ ~ t log TR
where TR is the titer reduction for the membrane and is the r~itio of Pseudomonas diminutiae content in the influent to the content thereo in the effluent;
TR the titer reduction achieved by a membrane of unit thickness; and t is the thickness of the membrane.
As an example o~ the application of this formula, if a given membrane has a titer reduction of 105, two layer~ of the membrane will have a titer reduction of 101 ~ three layers of 1015, etc.
Since the incident test bæterium is monodisperse (i. e. ~ of uniform dimensions), the applicability of this formula is self-evident. Its carrectness has also been confirmed experimentally, by determining titer reductions for 1, 2, 3, 4 and 5 layers of the same membranes. As shown in ~
the resulting plot of log TR vs. number of layers is linear, as predicted by the formula.

~L~60V()7 It is known in the industry to measure air flow rates through a membrane which has been wetted by a liquicl; such measurements yield useful information on the pore size characteristics of the membrane. We have used, in the course of this invention7 a parameter designated as KL.
KL 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 Fi~ure 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 verticalr The pressure at which this occurs is designated as the KL for the membrane.
KL 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. 0015 inch to 0- 012 inch, and with a wide range of pore diameters. These same membranes were then challenged with a suspension of Ps bacteria, and the num~er of in~uentbacteria was then divided by the number of effluent bacteria, ~hus determining the TR for each of the membranes. Thickness of eacb membrane was then measured, in mils (one mil - 0. 001 inch),and using the formula log T~ = t log TR
log TR was then calculated for each membrane, T bein~ the theoretical titer ~1 reduction for a 1 mil membrane.
KL 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 Kl~, values again measured for the multilayers. In this way, a relationship between the thickness and KL

value of equal pore size membranes was determined; this relationship is summarized in Table 1. IJsing Table 1, the KL values of the 45 membranes were corrected to the KL which would apply to an equal pore size membrane 0. 005 inch (5 mils) thick; these values are dlesignated as KL .
Log TR for each membrane was then plotted against KL for that membrane. All the results fell close to a single line, which is shown in Figure 4.
Using Figure 4, the titer reduction (TR) which can be expected to be obtained with any membrane made of hexamethylamine adipamide by the lO process of this invention, can be calculated, using the measured Kh and thickness ~t) for that specimen. The procedure is as ~ollows:
(1) measure the KL and thickness for the specimen;
(2) use Table 1, determine KL
~3) use ~L to determine TR i~rom Figure 4; and (4) calculate TR from the equation TR = TRt .
There is an upper limit to the number of bacteria which can be collected on a membrarle; by the time that about 1013 Ps. have been collected per square foo~ of filter medium, flow through the filter has fallen to less than 0. Ol'~C of a normal starting flow rate of 2 to 5 liters/minute per square 20 foot. This has been determined, by actual test, to be true for the membranes of this invention, as well as for commercially available membranes, for the full range of TR from 10 to ~103~. Thus, the figure of 1013/square Eoot may be taken as a practical upper limit of invading Pseudomonas diminutiae.
This upper limit is taken in combination ~ith the calculated TR to 25 obtain assurance that a given membrane will yield 5terility under all conditions ~2 of use. For example, a membrane may be selected with an estirnated TR f 10'3; statistically, if challenged with 1013 PseudomonaS ~iminutiae, such a membrane would have to be so challenged for 101 (or lO billion) times7 in order to produce a single effluent wi~h one bacterium, and such a high ratio 5 may be taken as adequate assurance of sterility, hence the filter canbe considered to have an absolute removal capability at 0. 2 ,uM. In practice, it is difficult to consistently produce a membrane with an estimated TR f exactly 10~3, 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 lO bacterially sterile filtrat~s.
In a similar fashion, KL and thickness can be correlated with titer reduction for larger bacteria7 yeasts of known size, and other particulate material, the Iatter assayed by particle detection methods, over a size ran;,e from under 0. l IlM 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 curre~tly marketed-uniform pore dry process membranes to determine 20 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 character~
ized by a sharp change in slope to a nearly vertical course at the ~L value.
If the filter medium is relatively nonuniform in pore size, it will tend to have 25 a distinct slope in the ho:rizontal portion of the curve, and exhibits a relatively .

~ 6~007 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 5 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 slope7 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, taperingr to a smaller pore at the other face of the membrane, the curves obtained by reversing $he direction of pressurization do not coincide. Instead, two distinct curves are obtained?
one Ilat, and the other higher and sloping upward, of which the sloping curve 15 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 throug~ a 20 membrane successively in both directions, it is possible to determine wbether 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 describ`ed herein for relating KL and thickness to titer reduction for any given organism, or to a rnonodisperse particulate, may be applied to 25 that membrane.

.... .

The following Examples in the opinion of the inventor represent preferred embodiments of the invention:
E~AMPLES 1 to 5 .
Nylon 66 resin pellets of molecular weight approximately 42, 000 5 were dissolved in 98. 5% formic acid, to yield a 35C solution containing 15. 5% of the resin. Without delay, this solution was delivered at a now rate of 250 g/minute to an in-line mixer. Simultaneously, a controlled water flow at 31C was delivered to the mixer, the quantit~ being such as to produce as the e~fluent a casting resin solution containing70. 2% of formic acid and 13.1'3~ of 10 the resin. The casting resin solution was filtered through a 10 ,uMfUter to remove visible resin particles, and wa~ then formed as a thin film by a doctorlng roll with 0. 0085 inch spacing on a moving polyester sheet surface, which had been pretreated by corona discharge to improve its wettabilit~, and in less than 3 seconds immersed into a bath containing 46. 5% formic acid) balance 15 water, for approximately 3 minutes. Ba~h concentration was maintained constant by adding water continuously, in the amount required. The nylon membrane so ~ormed was washed with flowing water for 1 hour. Two layers of the membrane were removed from t~}e polyester substrate sheet and ove dried in contact with each other, while restrained to prevent shrinkdge of 20 the length and wiclth 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 of the membrane. Examples 25 1 and 2 represent conditions in region A of ~igure 1, in which the degree of 31 ~6~
nucleation is too low to produce a satisfactory product; in this zone pressure drops are high, and product characteristics tend to be nonreproducibleD
Example 5, in which mixer speed was 400 ~PM falls into region D oE
Figure 1, and resulted in an unstable condition, with so much precipitating 5 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 somponents, should be noted.
XAMPLES 6, r? and 8 Casting resin solution was prepared and processed as for E~ample 4, except that it was heated by means of an in-line heat exchanger to respectively 53, 61 and 68C prior to casting. The product characteristics were not sig-nificantly different from those of Example 4. This result confirmed previous 15 test ~lata indicating that temperature ~f the casting resin solution is not a signific~t parameter, except insofar as viscosi~ may be reduced to the point (below about 500 cp) where casting problems may be experienced.
EXAMPLES g to 13 The membranes were prepared in the same way as Examples 1 to 5, 20 except that the quantity of water added was such as to produce a casting resin containing 69. 8~c of formic acid and 13. ~c of resin. The results are shown in Ta~le V. The casting resin solution made at 1950 RPM mixer speed was insufficiently nucleated, resulting in a pOOl product with high pressure drop.

~.6a~07 ~MPLES 14 to 18 , These membranes were prepared in the same way as Examples 1 to 5, except that the quantity oE water added was such as to produce a casting resin solution containing 69.0~C of formic acid and 12.85~C of resin. The results 5 are shown in Table VI.
E~?l,ES 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 solutibns containing 71.4~c, 67-5'3~c and 66.0~C of formic acid, and 13.3~ZC, 12.55~C
10 and 12. 41% respecti~Tely of resin.
The results are shown in graphical form, along wi~h 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 there~ore have favorably low pressure drop in proport.ion to their thickness and particle removal l5 capaoility, and -e con-islently reproducible.

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0~?~)7 E~AMpLES 40 to 46 These membranes were prepared in the same way as Examples 1 to 5, except (A) Starting resin containing 14. 5~O of Nylon ~6 was delivered to 5 the mixer at 400 g/minute.
~) Water was added in various quantities, to obtain formic acid and resin concentrations as listed.
- ~C) Doctor roll set at 0O 022 inch.
The results are listed in Table VII.

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ao7 E~AMPLES 47 to 5 0 .. :
Nylon 66 resin pellets of molecular weight approximatel~r 42, ooo were dissolved in 98. 5~3~ formic acid, to yielld a 35C 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 rotating at 1200 RPM~ Simultaneously, a controlled water flow also at 30C 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 resu]ting casting resin solution was 57C. The casting resin solution was without delay filtered 10 through a ~0 ,u M filter to remove visible resin particles~ alld was then formed into thin films by a doctoring blade with 0. 010 inch spacing on glass plates, and in less than 10 seconds immersed into a bath containin~ formic acid and water, for appro}~imately 5 to 10 minutes. The nylon membranes so formed were washed with flowing water for 1 hour. Two layers of the membralle were 15 oven dried in contact with each other, while restrained to prevent shririkage of the length and width during drying.
Table vm shows the product characteristics obtained, for various bath concentrations.

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X~MPLES 51 to 57 Membranes were prepared exactly as Examples 47 to 50, e:xcept as follo~vs:
Mixer rate was 1600 RPM
Casting resin solution temperature was 64C.
Table IX shows the product characteristics., Examples 56 and 57 are not within the scope of this in~entionj they are included to illustrate the effect of using bath concentrations of less than about 20~C formic acid.
This ~oup of Exa.mples also illustrates the advantages of baths in the range neal to 46. 5~c in producing membranes with minimum pressure drop at a given particle remo~al rating.

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~6~(~07 E~AMPLES 58 and 59 _ .
These membranes were prepa~ed using the same procedure as E~amples 4~ to 50 except as follows:
~a) Starting resin concentration was 17~c.
(b) ~astin~ r esin solution was prepared from 3~4. 7 grams per minute of star$ing resin solution using as nonsolvent diluent a solution containing 32. 8~c formic acid in water, delivered to the mixer at a flow rate of 1~2.1 grams/
minute.
(c) Mixer speed was lgOb RPM.
(d) Composition of the ca~ting resin solution was: 12. l~c resin, and 67. 8'3~ of formic acid.
te) After filtering through a 10 ,uM filter, one-half of the so~ution was further filtered through a filter with an approximate 0~ 05 to 0~10 ,uM
removal rating. The two portlons were then cast as films into a 46. 5% formic acid bath, as Exarnples 58 (Eiltered 10 ,uM only) and 59 (~iltered lOIlM and ^~ O. 05 to 0O 10 IlM ) Data for the two, measured on a single thickness of each, are listed in Table X.
- TABLE ~
Degree of filtration ~p, Estimated Absolute 20 ~3xa~ple of casting resin,~L5 inches t Tp~ particle No. ,uM PSI ~f Hg mils ~?s.) rating, ,u~
. _ _ 58 10 33.0 q.1 ~.~ 9 9x108 0~4 59 0 10 27.2 11.6 7.0 61 8XlOg û.65 The 11. 6 inch pressure drop of Example 59, resulting from the -ine 25 iltration step, should be compared with that of a normal product of this invention with the same thickness and KL values, which would be appro~ima~ely 3. 5 inches of mercury.

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EXAMPLES 60 to 64 In these Examples, polyhexamethylene adipamide (Nylon 66) was formed into membrane sheets usin~ a small batch proced lre. ~ 20% starting resin solution was prepared by dissolving resin oE molecular weight =34000 in 5 ~8. 5% formic acidO A quantity of 500 grams of this solution was heated to 65C in a glass jacketed vessel approximately 4 inches inside diameter by

8 inches high, fitted with a two-inch diameter propellor-t~pe agitator, and an externally operated flush val~e at its bottomD
- A nonsolvent solution was prepared containing 121 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 ~ mm inside diameter, and located 1/4 inch from the arc described by the rotating propeller. During the last portion of the two minute period, resin was seen to precipitate at the 15 inlet nozzle, all of which subsequently redissolved except for a small quantity of lumps of resin about l/8 inch in diameter.
About 20 grams of the casting resin solution so formed was with~rawn through the bottom valve, passed through a 42 mesh screen to remove lumps, and without delay spread on a glass plate as a thin -Eilm, using a 0/ 010 inch doctor 20 blade, and the film then promptly immersed in a bath containing formic acid and water, at 25C~
The membranes were allowed to set for several minutes, then were stripped from the glass plate, washed in water and dried by exposure to înfrared heat. The properties of the resulting membranes are shown in Table X1O

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~L6~(~07 Examples 60, 61 and 62 illustrate the effect o~ degree of nucleation on product characteristics. Examples 60 and 61 are properly nucleated, and ~ield products with favorably low pressure drop, for their removal ratings.
In Exarnple 62, the higher rotation rate resulted in a casting solution with too5 low a degree of nucleation, and as a consequence, a relatively high pressure drop.

, C~7 EXAMPLES 65 to 68 In these Ega~ples, the polyamide resins shown in Table g~I
below were formed into membrane sheets using a small batch procedure.
A 20% starting resin solution was prepared hy dissolving resin of molecular 5 weight = 34000 in g8. 5% formic acid. A quantity of 500 grams of this casting solution was held at ambient tempera~ re in a glass jacketed vessel approximately 4 inches inside di~meter by 8 inches high, fitted with a two-inch diameter propellor-type agitator, and an extelnally operated flush valve at its bottom.
As the nonsolvent wa~er was used in Egamples 65 to 67, while in Example 68 dimethylformamide was used. With the agitator rotatin~ a~
500 RPM, the nonsol~Tent was pumped into the apparatus a~ a constant rate over a period of 2 minutes, the iT~et nozzle being 2 mm inside diameter, and located 1/4 inch from the arc described ~y $he 15 rotating propeller. During the last portion of the two minute period, resin was seen to precipitate at the i~et nozzle, all of which subsequently re-dissolved e~cept for a small qua~tity of lumps of resin about 1j8 inch in diameter.
A~bout 20 grams of the casting resin solution so formed was wîth-20 drawn through the bottom valve7 passed through a 42 mesh screen to removelumps, and without delay spread on a glass plate as a thin film, using a O. 010 inch doctor blade, and the film then promptly immersed in a bath containing formic acid and wa;ter, at ambient temperature.
The membranes were allo~ed to set for several minutes, then 25 were stripped from the glass plate, washed in water and dried by ex3?osure to i~rared heat. The properties of the resulting membranes are shown in Table Xll.

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Claims (162)

Having regard to the foregoing disclosure, the following is claimed as inventive and patentable embodiments thereof.

1. A process for preparing skinless hydrophilic alcohol-insoluble polyamide membranes that are readily wetted by water which comprises preparing a solution of an alcohol-insoluble polyamide resin selected from the group consisting of polyhexamethylene adipamide polyhexamethylene sebacate, and poly-e-caprolactam in a polyamide resin 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, 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 nonsolvent liquid for the polyamide resin, thereby precipitating polyamide resin from the casting solution in the form of a thin skinless membrane; washing the membrane to remove solvent; and drying the resulting membrane.

2. A process according to claim 1 in which precipitated polyamide resin particles are redissolved before spreading the casting solution on a substrate.

3. A process according to claim 1 in which precipitated polyamide resin particles are filtered out before spreading the casting solution on a substrate.

4. A process according to claim 1 in which part of the precipi-tated polyamide resin particles are redissolved and part are filtered out before spreading the casting solution on a substrate.

5. A process according to claim 1 in which the nonsolvent used to contact and dilute the casting solution is a mixture of solvent and nonsolvent liquids containing a substantial proportion of the solvent liquid, but less than the proportion in the casting solution.

6. A process according to claim 1 in which the polyamide resin is polyhexamethylene adipamide.

7. A process according to claim 1 in which the polyamide resin is poly -e-caprolactam.

8. A process according to claim 1 in which the polyamide resin is polyhexamethylene sebacamide.

9. A process according to claim 1 in which the polyamide resin is polyhexamethylene adipamide, the solvent for the polyamide resin solution is formic acid, and the nonsolvent added for dilution is water.

10. A process according to claim 1, in which the polyamide resin solution film is contacted with the nonsolvent by immersing the film carried on the substrate in a bath of nonsolvent liquid.

11. A process according to claim 10 in which the bath comprises both solvent and nonsolvent liquids.

12. A process according to claim 11 in which the bath comprises an amount within the range from about 20% to about 55% of a solvent for the resin; washing the resulting membrane substantially free of solvent;

and drying the membrane.

13. A process according to claim 9 in which the polyamide resin is polyhexamethylene adipamide, the solvent is formic acid, and the nonsolvent is water, and the polyamide resin concentration in the casting solution is within the range from about 10 to about 18% by weight, and the formic acid concentration is within the range from about 63 to about 72%.

14. A process according to claim 13 in which the polyamide resin concentration of the casting solution is within the range from about 12 to about 18%, and the nonsolvent is added at a fixed intensity of mixing.

15. A process according to claim 1 in which the casting resin is continuously spread onto the substrate, the thin film of casting solution is continuously immersed in a bath of nonsolvent liquid, and the bath is maintained at a substantially constant composition with respect to nonsolvent and solvent by continuous addition of nonsolvent to the bath in a quantity to compensate for solvent diffusion into the bath from the thin film of casting solution.

16. A process according to claim 15 in which the substrate is a nonporous synthetic polymer film having a surface that is wetted by the casting solution and the bath.

17. A process according to claim 15 in which the substrate is a porous 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.

18. A process according to claim 17 in which the substrate is a fibrous polyester sheet.

19. A process according to claim 15 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.

20. A process according to claim 1 in which the polyamide resin solution has a viscosity within the range from about 5,000 centipoises to about 50,000 centipoises at the operating temperature.

21. A process according to claim 1 in which the casting and precipitating temperatures are within the range from about 10°C to the boiling temperature of the lowest boiling solvent or nonsolvent component present.

22. A process according to claim l in which the casting resin solution is clear, and free from suspended material,before being spread upon the substrate to form a film.

23. A process according to claim 1 in which the membrane is stripped from the support after washing and before drying.

24. A process according to claim 1 in which the substrate is not stripped from the polyamide resin membrane before drying, and after drying remains attached to the polyamide resin membrane.

25. A process according to claim 1 in which the substrate is of polypropylene resin.

26. A process according to claim 1 in which the substrate is of polyester resin.

27. A process according to claim 1 in which the casting solution has a viscosity within the range from about 500 centipoises to about 100, 000 centipoises at the temperature existing at the time it is cast as a film.

28. A process according to claim 1 in which the casting solution has a viscosity within the range from about 500 centipoises to about 5000 centipoises at the temperature existing at the time it is cast as a film.

29. A process for preparing skinless hydrophilic alcohol-insoluble polyhexamethylene adipamide resin membranes having a substantially zero contact angle with water and having pores that are substantially uniform from surface to surface which comprises preparing a flowable solution of the alcohol-insoluble polyhexamethylene adipamide resin in a concentration within the range from about 10% to about 18% by weight in an aqueous formic acid solution containing from about 63% to about 72% formic acid by weight; inducing nucleation of the resin solution by adding water thereto while controlling resin and formic acid concen-tration, temperature, rate of addition of water and degree of agitation to obtain a visible precipitate of resin particles, thereby forming a casting solution; spreading the casting solution on a polyester resin substrate to form a thin film of resin solution thereon; contacting and diluting the film of casting resin solution with an aqueous solution containing from 37 to 55% formic acid and thereby precipitating the polyamide resin from the casting resin solution as a thin skinless hydrophilic membrane;
washing the membrane to remove solvent; and drying the membrane.

30. A process according to claim 29 in which the membrane is stripped from the support after washing and before drying.

31. A process according to claim 29 in which precipitated polyamide resin particles are redissolved before spreading the casting solution on a substrate.

32. A process according to claim 29 in which precipitated polyamide resin particles are filtered out before spreading the casting solution on a substrate.

33. A process according to claim 29 in which part of the precipitated polyamide resin particles are redissolved and part are filtered out before spreading the casting solution on a substrate.

34. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet of alcohol-insoluble hydrophobic polyamide resin selected from the group consisting of polyhexamethylene adipamide, polyhexamethylene sebacate, and poly-e-caprolactam, and capable when completely immersed in water of being wetted through within no more than one second, and reverting when heated to a temperature just below the softening temperature of the membrane to a hydrophobic material which is no longer wetted by water.

35. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 having through pores extending from surface to surface that are substantially uniform in shape and size.

36. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 having through pores extending from surface to surface that are tapered, wider at one surface and narrowing towards the other surface of the membrane sheet.

37. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 having absolute particle removal ratings of 0.10 µM to 5 µM.

38. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 in which the polyamide resin is polyhexamethylene adipamide.

39. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 in which the polyamide resin is poly- e-caprolactam.

40. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 in which the polyamide resin is polyhexamethylene sebacamide.

41. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 having two membrane layers adherent to each other and forming a single membrane sheet with particle removal characteristics superior to those of the individual membrane layers.

42. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 41 in which the two membrane layers have the same porosities.

43. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 41 in which the two membrane layers have differing porosities.

44. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 41 in which the membranes have tapered pores.

45. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 41 in which the membranes have uniform pores.

46. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 41 in which the membranes are unsupported.

47. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 41 in which the membranes are unsupported.

48. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 having a thickness within the range from about 0. 025 to about 0. 8 mm.

49. A filter element comprising a hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 34 formed in a tubular configuration with the ends of the tube sealed to end caps of which at least one end cap has a central aperture giving access to the interior of the tube, and with the sides of the sheet lapped and sealed together, all seals being fluid-tight.

50. A filter element according to claim 49 in which the sheet is corrugated.

51. A filter element according to claim 49 in which at least one of the exterior faces of the sheet is adhered to a porous support layer.

52. A filter element according to claim 51 in which both the membrane sheet and the support layer are corrugated.

53. A filter element according to claim 49 comprising a multilayer membrane sheet, the layers being adhered together as one sheet.

54. A filter element according to claim 53 in which the layers of membrane are separated by a porous support layer to which each membrane layer is adhered.

55. A filter element according to claim 49 in which the end caps are of polyester resin and the filter element including the end caps is hydrophilic and rapidly wetted by water.

56. A filter element according to claim 55 in which the polyester is polybutylene glycol terephthalate.

57. A filter element according to claim 55 in which the polyester is polyethylene glycol terephthalate.

58. A process for preparing multilayer skinless hydrophilic alcohol-insoluble polyamide membranes that are readily wetted by water which comprises preparing a solution of an alcohol-insoluble polyamide resin selected from the group consisting of polyhexamethylene adipamide, polyhexamethylene sebacate, and poly-.epsilon.-caprolactam in a polyamide resin 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, 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 nonsolvent liquid for the polyamide resin, thereby precipitating polyamide resin from the casting solution in the form of a thin skinless membrane; washing the membrane to remove solvent; placing the washed membrane while still wet in con-tact with at least one other wet washed membrane; and then drying the juxtaposed membranes while maintaining such contact, thereby obtaining a multilayer membrane in which the separate membranes are integral layers thereof.

59. A process according to claim 58 in which the separate membranes are stripped from the substrate before drying.

60. A process according to claim 58 in which the separate membranes are dried while supported on the substrate, which thereby becomes an integral part of the multilayer membrane.

61. A process according to claim 50 in which the substrate is a porous fibrous web.

62. A process according to claim 61 in which the fibrous web is of polyester resin.

63. A process according to claim 61 in which the fibrous web is of polypropylene resin.

64. A process according to claim 58 in which the polyamide resin is polyhexamethylene adipamide.

65. A process according to claim 58 in which the polyamide resin is poly-.epsilon.-caprolactam.

66. A process according to claim 58 in which the polyamide resin is polyhexamethylene sebacamide.

67. A process according to claim 58 in which the membranes are dried under restraint to limit dimensional change.

68. A process according to claim 58 in which the membranes are corrugated and then dried.

69. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet of alcohol-insoluble hydrophobic polyamide resin selected from the group consisting of polyhexamethylene adipamide, polyhexamethylene sebacate, and poly-.epsilon.-caprolactam, and capable when completely immersed in water of being wetted through within no more than one second, and reverting when heated to a tempera-ture just below the softening temperature of the membrane to a hydrophobic material which is no longer wetted by water, and having at least two membrane layers integrally adhered together.

70. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 69 in which the membrane layers are supported on a substrate.

71. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 70 in which the substrate is a porous fibrous web.

72. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 70 in which the fibrous web is of polyester resin.

73. A multilayer hyrdrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 70 in which the fibrous web is of polypropylene resin.

74. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 70 in which the polyamide resin is polyhexamethylene adipamide.

75. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 70 in which the polyamide resin is poly-.epsilon.-caprolactam.

76. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 70 in which the polyamide resin is polyhexamethylene sebacamide.

77. A process for preparing skinless hydrophilic alcohol-insoluble polyamide membranes that are readily wetted by water which comprises preparing a solution in a polyamide solvent of an alcohol-insoluble polyamide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within the range from about 5:1 to about 7:1;
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, 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 nonsolvent liquid for the polyamide resins, thereby precipitating polyamide resin from the casting solution in the form of a thin skinless hydrophobic membrane; washing the membrane to remove solvent; and drying the resulting membrane.

78. A process according to claim 77 in which precipitated polyamide resin particles are redissolved before spreading the casting solution on a substrate.

79. A process according to claim 77 in which precipitated polyamide resin particles are filtered out before spreading the casting solution on a substrate.

80. A process according to claim 77 in which part of the pre-cipitated polyamide resin particles are redissolved and part are filtered out before spreading the casting solution on a substrate.

81. A process according to claim 77 in which the nonsolvent used to contact and dilute the casting solution is a mixture of solvent and nonsolvent liquids containing a substantial proportion of the solvent liquid, but less than the proportion in the casting solution.

82. A process according to claim 77 in which the polyamide resin is polyhexamethylene adipamide.

83. A process according to claim 77 in which the polyamide resin is poly-.epsilon.-caprolactam.

84. A process according to claim 77 in which the polyamide resin is polyhexamethylene sebacamide.

85. A process according to claim 77 in which the polyamide resin is poly-7-aminoheptanoamide.

86. A process according to claim 77 in which the polyamide resin is polyhexamethylene azeleamide.

87. A process according to claim 77 in which the polyamide resin is a mixture of polyamide resins of which at least one has a CH2:NHCO ratio within the range from about 5:1 to about 7:1 and at least one has a CH2:NHCO ratio exceeding 7:1, in proportions such that the average CH2:NHCO ratio is within the range from about 5:1 to about 7:1.

88. A process according to claim 87 in which the polyamide resin having a CH2:NHCO ratio exceeding 7:1 is polyhexamethylene undecanediamide.

89. A process according to claim 87 in which the polyamide resin having a CH2:NHCO ratio exceeding 7:1 is poly-11-aminoundecanoic acid.

90. A process according to claim 77 in which the polyamide resin is polyhexamethylene adipamide, the solvent for the polyamide resin solution is formic acid, and the nonsolvent added for dilution is water.

91. A process according to claim 77 in which the polyamide resin solution film is contacted with the nonsolvent by immersing the film carried on the substrate in a bath of nonsolvent liquid.

92. A process according to claim 91 in which the bath comprises both solvent and nonsolvent liquids.

93. A process according to claim 92 in which the bath comprises an amount within the range from about 20% to about 55% of a solvent for the resin; washing the resulting membrane substantially free of solvent;
and drying the membrane.

94. A process according to claim 90 in which the polyamide resin is polyhexamethylene adipamide, the solvent is formic acid, and the nonsolvent is water, and the polyamide resin concentration in the casting solution is within the range from about 10 to about 18% by weight, and the formic acid concentration is within the range from about 63 to about 72%.

95. A process according to claim 94 in which the polyamide resin concentration of the casting solution is within the range from about 12 to about 18%, and the nonsolvent is added at a fixed intensity of mixing.

96. A process according to claim 77 in which the solvent is formic acid, and the polyamide resin concentration in the casting solution is within the range from about 10 to about 22% by weight, and the formic acid concentration is within the range from about 60 to about 72%.

97. A process according to claim 77 in which the casting resin is continuously spread onto the substrate, the thin film of casting solution is continuously immersed in a bath of nonsolvent liquid, and the bath is maintained at a substantially constant composition with respect to nonsolvent and solvent by continuous addition of nonsolvent to the bath in a quantity to compensate for solvent diffusion into the bath from the thin film of casting solution.

98. A process according to claim 97 in which the substrate is a nonporous synthetic polymer film having a surface that is wetted by the casting solution and the bath.

99. A process according to claim 97 in which the substrate is a porous 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.

100. A process according to claim 99 in which the substrate is a fibrous polyester sheet.

101. A process according to claim 97 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.

102. A process according to claim 77 in which the polyamide resin solution has a viscosity within the range from about 5,000 centipoises to about 50,000 centipoises at the operating temperature.

103. A process according to claim 77 in which the casting and precipitating temperatures are within the range from about 10°C to the boiling temperature of the lowest boiling solvent or nonsolvent component present.

104. A process according to claim 77 in which the casting resin solution is clear, and free from suspended material, before being spread upon the substrate to form a film.

105. A process according to claim 77 in which the membrane is stripped from the support after washing and before drying.

106. A process according to claim 77 in which the substrate is not stripped from the polyamide resin membrane before drying, and after drying remains attached to the polyamide resin membrane.

107. A process according to claim 78 in which the substrate is of polypropylene resin.

108. A process according to claim 77 in which the substrate is of polyester resin.

109. A process according to claim 77 in which the casting solution has a viscosity within the range from about 500 centipoises to about 100,000 centipoises at the temperature existing at the time it is cast as a film.

110. A process according to claim 77 in which the casting solution has a viscosity within the range from about 500 centipoises to about 5000 centipoises at the temperature existing at the time it is cast as a film.

111. A process for preparing skinless hydrophilic alcohol-insoluble polyamide resin membranes having pores that are substantially uniform from surface to surface which comprises preparing a flowable solution of the alcohol-insoluble polyamide resin in a concentration within the range from about 10% to about 22% by weight in an aqueous formic acid solution containing from about 60% to about 72% formic acid by weight; inducing nucleation of the resin solution by adding water thereto while controlling resin and formic acid concentration, temperature, rate of addition of water and degree of agitation to obtain a visible precipitate of resin particles, thereby forming a casting solution; spreading the casting solution on a polyester resin substrate to form a thin film of resin solution thereon; contacting and diluting the film of casting resin solution with an aqueous solution containing from 37 and 55% formic acid and thereby precipitating the polyamide resin from the casting resin solution as a thin skinless hydrophilic membrane; washing the membrane to remove solvent; and drying the membrane.

112. A process according to claim 111 in which the membrane is stripped from the support after washing and before drying.

113. A process according to claim 111 in which precipitated polyamide resin particles are redissolved before spreading the casting solution on a substrate.

114. A process according to claim 111 in which precipitated polyamide resin particles are filtered out before spreading the casting solution on a substrate.

115. A process according to claim 111 in which part of the precipitated polyamide resin particles are redissolved and part are filtered out before spreading the casting solution on a substrate.

116. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet of alcohol-insoluble hydrophobic polyamide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within the range from about 5:1 to about 7:1; capable when completely immersed in water of being wetted through within no more than one second, and reverting when heated to a temperature just below the softening tempera-ture of the membrane to a hydrophobic material which is no longer wetted by water.

117. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 having through pores extending from surface to surface that are substantially uniform in shape and size.

118. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 having through pores extending from surface to surface that are tapered, wider at one surface and narrowing towards the other surface of the membrane sheet.

119. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 having absolute particle removal ratings of 0.10 µM to 5 µM.

120. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 in which the polyamide resin is polyhexamethylene adipamide.

121. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 in which the polyamide resin is poly-.epsilon.-caprolactam.

122. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 in which the polyamide resin is polyhexamethylene sebacamide.

123. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 in which the polyamide resin is poly-7-aminoheptanoamide.

124. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 in which the polyamide resin is polyhexamethylene azeleamide.

125. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 having two membrane layers adherent to each other and forming a single membrane sheet with particle removal characteristics superior to those of the individual membrane layers.

126. A hydrophilic skinless alcohol-insoluble polyamide resi membrane sheet according to claim 125 in which the two membrane layers have the same porosities.

127. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 125 in which the two membrane layers have differing porosities.

128. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 125 in which the membranes have tapered pores.

129. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 125 in which the membranes have uniform pores.

130. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 125 in which the membranes are supported.

131. A hydrophilic skinless-alcohol insoluble polyamide resin membrane sheet according to claim 125 in which the membranes are unsupported.

132. A hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 having a thickness within the range from about 0.025 to about 0.8 mm.

133. A filter element comprising a hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 116 formed in a tubular configuration with the ends of the tube sealed to end caps of which at least one end cap has a central aperture giving access to the interior of the tube, and with the sides of the sheet lapped and sealed together, all seals being fluid-tight.

134. A filter element according to claim 133. in which the sheet is corrugated.

135. A filter element according to claim 133 in which at least one of the exterior faces of the sheet is adhered to a porous support layer.

136. A filter element according to claim 135 in which both the membrane sheet and the support layer are corrugated.

137. A filter element according to claim 136 comprising a multilayer membrane sheet, the layers being adhered together as one sheet.

138. A filter element according to claim 137 in which the layers of membrane are separated by a porous support layer to which each membrane layer is adhered.

139. A filter element according to claim 133 in which the end caps are of polyester resin and the filter element including the end caps is hydrophilic and rapidly wetted by water.

140. A filter element according to claim 139 in which the polyester is polybutylene glycol terephthalate.

141. A filter element according to claim 139 in which the polyester is polyethylene glycol terephthalate.

142. A process for preparing multilayer skinless hydrophilic alcohol-insoluble polyamide membranes that are readily wetted by water which comprises preparing a solution in a polyamide resin solvent of an alcohol-insoluble polyamide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within the range from about 5:1 to about 7:1;
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, 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 nonsolvent liquid for the polyamide resin, thereby precipitating polyamide resin from the casting solution in the form of a thin skinless membrane; washing the membrane to remove solvent;
placing the washed membrane while still wet in contact with at least one other wet washed membrane; and then drying the juxtaposed membranes while maintaining such contact, thereby obtaining a multilayer membrane in which the separate membranes are integral layers thereof.

143. A process according to claim 142 in which the separate membranes are stripped from the substrate before drying.

144. A process according to claim 142 in which the separate membranes are dried while supported on the substrate, which thereby becomes an integral part of the multilayer membrane.

145. A process according to claim 144 in which the substrate is a porous fibrous web.

146. A process according to claim 145 in which the fibrous web is of polyester resin.

147. A process according to claim 145 in which the fibrous web is of polypropylene resin.

148. A process according to claim 142 in which the polyamide resin is polyhexamethylene adipamide.

149. A process according to claim 142 in which the polyamide resin is poly-.epsilon.-caprolactam.

150. A process according to claim 142 in which the polyamide resin is polyhexamethylene sebacamide.

151. A process according to claim 142 in which the polyamide resin is poly-7-aminoheptanoamide.

152. A process according to claim 142 in which the polyamide resin is polyhexamethylene azeleamide.

153. A process according to claim 142 in which the membranes are dried under restraint to limit dimensional charge.

154. A process according to claim 142 in which the membranes are corrugated and then dried.

155. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet of alcohol-insoluble hydrophobic polyamide resin having a ratio CH2:NHCO of methylene CH2 to amide NHCO groups within the range from about 5:1 to about 7:1, capable when completely immersed in water of being wetted through within no more than one second, and reverting when heated to a temperature just below the softening temperature of the membrane to a hydrophobic material which is no longer wetted by water, and having at least two membrane layers integrally adhered together.

156. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 155 in which the membrane layers are supported on a substrate.

157. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 156 in which the substrate is a porous fibrous web.

158. A multilayer hydrophilic skinless alcohol-insolubls polyamide resin membrane sheet according to claim 156 in which the fibrous web is of polyester resin.

159. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 156 in which the fibrous web is of polypropylene resin.

160. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 155 in which the polyamide resin is polyhexamethylene adipamide.

161. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 155 in which the polyamide resin is poly-.epsilon.-caprolactam.

162. A multilayer hydrophilic skinless alcohol-insoluble polyamide resin membrane sheet according to claim 155 in which the polyamide resin is polyhexamethylene sebacamide.