CA1313734C - Felted pad based on fibers of cellulose and of organic microporous membrane - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

There is described a felted pad comprising fibers of cellulose and fibers of an organic micro-porous membrane having a surface modifying agent bonded to substantially all of the wetted surfaces thereof.

Description

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RELATED APPLICATIONS
This application is related to U.S. Pa~ent No. 4,473,474 issued September 25, 1984, entitled "Charge Modified Microporous Membrane, Process for Charge Modifying Said Membrane and Process for Filtration of Fluid", to Ostreicher.
This application is also related to United S~a~es Pa~ent iYo. 4,473,475 issued September 25, 1984, entitled "Charge Modified Microporous Membrane, Process for Charge Modifying Said Membrane, and Process for Filt a~ion of Fluid", to Barnes, ~r. et al.
This application is further related to U.S. Patent No. 4,604,208 issued August 5, 1986, entitled "Anionic Charge Modified Microporous Membrane, Process for Charge Modifying Said Micro-porous Membrane and Fil~ration of Fluid", to Chu et al.

- This application is a division of Canadian Application No. 455,077 filed October 10, 1984.

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BACKGROU~'D OF ~HE INVENTION
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l. Field of the Invention This invention relates to microporous membranes, and more particularly to surface modified microporous mem-branes suitable for the filtration of aqueous fluids, such as biolo~ical liquids.
2. Prior Art Micro~orous membranes are well known in the art.
For example, U.S. Patent ~o. 3,876,738 to Marinaccio et al.
(1975) describes a process for preparing a microporous mem-brane, for e~a.~ple, by quenching a solution of a film forming polymer in a non-solvent system for the polymer. European Patent Application O OOS 536 to Pall (1979) describes a similar process.
Commercially available microporous membranes, for example, made of nylon, are available from Pall Corporation, Glen Cove, New York under the trademark ULTIPOR N65". Such membranes are advertised as useful for the sterile filtration of pharmaceuticals~ e.~. removal of microorganisms.
Various studies in recent years, in particular Wall-hausser, Journal o~ Parenteral Drug Association, June, 1979, Vol. 33, ~3, pp. 156-170, and Howard et al, Journal of the Parenteral Drug Association, March-April, 1980, Volume 34, ~2, pp. 94-102, have reported the phenomena of bacterial bre~kthrough in filtration media, in spite of the fact that the media had a low microrneter rating. For example, commer-clally available membrane filters for bacterial removal are ty~ically rated as havin~ an effective micrometer rating for ~he microreticulate membranes structure of 0.2 micrometers or less, yet such mem~ranestypically have only a 0.357 effec-tive micrometer rating for spherical contaminant particles, even when rated as absolute for Ps. diminuta, the conven-Iional tes~ for bacterial retention. This problem of pas-s~ge Or a few microorganisms under certain conditions have ~een rendered more severe as the medical uses of filter ~nembranes has increased.

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One method of addressing this problem is to prepare a ti~hter fllter having a sufficiently small effective pore dimension to capture microorganisms, etc., by mechanical sieving. Such microporous membranes of 0.1 micrometer rating or less may be readily prepared but flow rates at conventional ~ressure drops are prohibitively low. Increasing the pressure dro~ to provide the desired flow rate is not ~enerally feasi-ble because pressure drop is an inverse function of the fourth power of pore diameter.
It has lon~ been recognized that adsorptive effects can enhance the capture of particulate contaminants. For example, Wen~, "Electrokinetic and Chemical Aspects of ~Yater Filtration", Filtration and Separation, May/June 1974, indi-cates that surfactants, pH, and ionic strength may be used in various ways to improve the efficiency of a filter by mo-difyin~ the char~e characteristics of either the suspension, filter or both.
It has also been sug~ested that adsorptive seques-tration (particle capture within pore channels), may some-times be more important in sterile filtration than bubble point characterization of internal geometry (representing tne "lar~est pore"). See, e.g., Tanny et al, Journal of the Parenteral Dru~ Association, November-December 1978, Vol.
~1, #6, pp. 258-267 and January-February, 1979, Vol. 33, ~1, p~. 40-51 and Lu~aszewicz et al, Id., July-August, 1979, Vol. 33, ~4, pp. 187-194.
Pall et al, Colloids and Surfaces 1 (1980), pp.
~35-256, indicates that if the zeta potential of the pore walls of a melnbrane, e.g. nylon 66, and of the particles are bolh low, or if they are oppositely charged, the particles wlll tend to adhere to the pore walls, and the result will ~e removal of particles smaller than the pores of the filter.
Pall et al su~est the use of membranes of substantially smaller pore size to increase the probability of obtaining microbial sterillty in filtering fluids.

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Zierdt, Applied and Environmental Microbiology, Dec.
1979, pp. 1166-1172, found a stron~ adherence by bacteria, yeast, erythrocytes, leukocytes, platele~s, spores and polystyrene spheres to membrane materials during filtration through membranes with pore-size diame~ers much larger than the particles themselves.
Zierdt found that cellulose membranes adsorbed more bacteria, blood cells and other particles than did polycaxbonate filters. Of lesser adsorptive capacity were vinyl acetate, nylon, acrylic and TeflonTM membranes. Zierdt additionally found that solvent cast membrane filter materials, e.g. nylon, had strong surface cha ges, whereas ordinary fibrous cellulose materials which are not solvent cast do not.
Attempts to increase the short life of filter media due to pore blockage and enhance flow rates through filter media having small pores have been made by charge modifying the media by various means to enhance capture potential of the filter. For example, U.S. Patents 4,007,113 and 4,007,114 to Ostreicher, describe the use of a melamine formaldehyde cationic colloid to charge modify fib.ous and particulate filter elements; U.S. Patent No. 4,305,782, to Ostreicher et al describes the use of an inorganic cationic colloidal silica to charge modify such elements. None of these references teaches or suggests charge modifying a synthetic organic polymeric microporous membrane, nor do any of the filtration media described therein, e.g. fiber and/or particulate, provide the advantages of such a membrane.
Similarly, U.S. Patent No. 3,242,073 (1966) and 3,352,424 (1967) to Guebert et al, describe removal of microorganisms from fluids by passage through a filter medium of conventional anionic type filter aid, e.g. diatomaceous earth, , ~3~37~

~a~er filter pulp, fullers earth, charcoal, etc., having an adsor~ed cationic, organic, polyelectrolyte coating. The coated filter aid media is said to possess numerous cationic sites which are freely available to attract and hold parti-cles bearin~ a ne~ative surface charge.
U.S. Patent No. 4,178,438 to Hasset et al (1979) describes ~ process for the purification of industrial efflu-ent using cationically modified cellulose containing material, e.O., bleached or unbleached pine sulphite cellulose, kraft sul~hate cellulose, paper, card~oard products, textiles fibers made of cotton, rayon staple, jute, woodfibers, etc.
The cationic substituent is bonded to the cellulose via a ~roupin~ -0-C~2-N- , where the nitrogen belongs to an amide grou~ of the cationic part and the oxygen to the cellulose part.
There are numerous references which describe the treatment of porous membranes for various objects. U.S.
Patent No. 3,556,305 to Shorr (1971) describes a tripartite ~embrane for use in reverse osmosis comprising an anisotropic ~orous substrate, an ultra-thin adhesive layer over the porous substrate, and a thin diffusive membrane formed over the adhesive layer and bound to the substrate by the adhesive layer. Such a~isotropic porous membranes are distinguished from isotropic, homogeneous membrane structures used for microfiltration whose flow and retention properties are in-dependent of flow direction and which do not function properly when subs~i_u-'~ed in the invention of ~x~-~.
U.S. Patent ~o. 3,556,99 to .~assuco (1971) describes another anisotro~ic ultra-filtration membrane having thereon an adhering coating of irreversibly compressed gel.
U.S. Patent No. 3,808,305 to Gregor (1974) describes a charged membrane of ~.acrosco~ic homogeneity prepared by pro-vidin~ a solution containin~ a matrix polymer, polyelectro-lytes (for charge) and a crosslinking agent. The solvent is 1313~

evaporated fron~ a cast film which is then chemically cross-linked. The membranes are used for ultrafiltration.
U.S. Patent No. 3,944,485 (1976) and 4,045,352 (1977) to Rembaum et al describe ion exchange hollow fibers produced by introducing into the wall of the pre-formed fiber, poly-merizable liquid monomers ~hich are then polymerized to form solid, insoluble, ion e~;chan~e resin particles embedded within the wall of the fiber. The treated fibers are useful as membranes in ~rater treatment, dialysis and generally to separate ionic solutions. See also U.S. Patent No. 4,014,798 to Rembaum (1977).
U.S. Patent ~o. 4,005,012 to Wrasidlo (1977) describes a process for producing a semi-permeable anisotropic membrane useful in reverse osmosis processes. The membranes are pre-pared by for~ing a polymeric ultra-thin film, possessing semi-permeable properties by contacting an amine modified polyepi-halohydrin with a polyfunctional agent and depositing this film on the external surface of a microporous sllbstrate.
Preferred semi-permeable membranes are polysulfone, polysty-rene, cellulose butyrate, cellulose nitrate and cellulose acetS~te.
U.S. ~atent No. 4,125,462 to Latty (1978) describes a coated semi-permeable reverse osmosis membrane having an external layer or coating of a cationic polyelectrolyte pre-ferably poly(vinylimidazoline) in the bi-sulfate form.
U.S. Patent No. 4,214,020 to ~'ard et al (1980) de-scribes a novel method of coating the e~teriors of a bundle of hollow-fiber semi-permeable membranes for use in fluid separations. Typical polymers coated are polysulfones, poly-st~renes, polycarbonates, cellulosic polymers, polyamides and polyimides. Numerous depositable materials are listed, see col. 10, lines 55 - col. 12, for example, poly(epichlor-hydrin) or polyamides.
U.S. Paten~ No. 4,239,714 to Spar~s et al (1980) descri~es a method of modifyin~ the pore size distribution 131 37~ `

of a separation media to provide it with a sharp upper cut-off of a preselected molecular size. This is accomplished by effectively blocking the entrances to all of the pores larger than a pre-selected desired cut-off size, but leaving unchanged the smaller pores. The separation media may be in the form of polymeric membranes, e.g. cellulose acetate, cellulose nitrate, poly-carbonates, polyolefins, polyacrylics, and polysulfones. The pores are filled with a vola~ile liquid which is evaporated to form voids at the pore entrances and a concentrated solution of a cross-linkable or polymerizable pore blocking agent, such as protein, enzyme, or polymeric materials is then applied to the surface of the membrane.
U.S. Patent No. 4,250,029 to Kiser et al (1981) describes coated membranes having two or re external coatings of polyelectrolytes with at least one oppositely charged adjacent pair separated by a layer of material which is substantially charge neutralized. Kiser et al is primarily directed to the use of charged membranes to repel ions and thereby prevent passage through the membrane pores. The coated membranes are described as ordinary semi-permeable membranes used for ultrafiltration, reverse osmosis, electrodialysis or other filtration processes. A microscopic observation of the coated membranes shows microscopic hills and valleys of polyelectrolyte coating formed on the original external smooth skin of the membrane. The mmbranes are particularly useful for deionizing aqueous solutions. Preferred membranes are organic polymeric me~branes used for ultrafiltration and reverse osmosis processes, e.g., polyimide, polysulfone, aliphatic and aromatic nylons, polyamides, etc. Preferred membranes are anisotropic hollow fiber membranes having an apparent pore diameter of from about 21 to about 480 angstroms.
Charge modified microporous filter membranes are disclosed in Canadian Patent No. 1,044,537 of Ostreicher, issued 1 3 ~ 3 r~ ~ ~

December 19, 1978, (corresponding to Japanese Patent No. 923,649).
As disclosed therein, an isotropic cellulose mixed ester membrane, was treated with a cationic colloidal melamine-formaldehyde resin to provide charge functionality. The membrane achieved only marginal charge modification. Additionally, the membrane was discolored and embrittled by the treatment, extractables exceeded desirable limits for certain critical applications, and the membrane was not thermally sanitizable or sterilizable. Ostreicher also suggests such treatment for the nylon membranes prepared by the methods described in U.S. Patent No. 2,783,894 to Lovell (1957) and U.S. Patent No. 3,408,315 to Paine (1968). It has been demonstrated that nylon microporous membranes treated according to said Ostreicher reference would also demonstrate marginal charge modification, high extractables and/or inability to be thermally sanitizable or sterilizable.
The aforesaid Ostreicher U.S. Patent No. 4,473,474 (published as European 0050804 on May 5, 1982) generally describes a novel cationic charge modified microporous membrane comprising a hydrophilic organic polymeric microporous membrane and a charge modifying amount of a primary cationic charge modifying agent bonded to substantially all of the internal microstructure o$ the membrane. The primary charge modifying agent is a water-soluble organic poly~ner having a molecular weight greater than about 1,000 wherein each monomer thereof has at least one epoxide group capable of bonding to the surface of the membrane and at least one tertiary amine or quaternary ammonium group. Preferably, a portion of the epoxy groups on the organic polymer are bonded to a secondary charge modifying agent selected from the group consisting of:
i) aliphatic amines having at least one primary amino or at least two secondary amino groups; and 9` 13i3~3~`

ii) aliphatic amines having at least one secondary amino and a carboxyl or hydroxyl substituent.

The membrane is made by a process for cationically charge modifying a hydrophilic organic polymeric microporous membrane by applying to the membrane the aforesaid charge modifying agents, preferably by contacting the membrane with aqueous solutions of the charge modifying agents. The preferred microporous membrane is nylon, the preferred primary and secondary charge modifying agents are, respectively, polyamido-polyamine epichlorohydrin and tetraethylene pentamine. The charge modified microporous membrane may be used for the filtration of fluids, particularly parenteral of biological liquids. The membrane has low extract~bles and is sanitizable or sterilizable.
m e aforesaid Chu et al U.S. Patent No. ~,604,208 generally describes a novel anionic charge ~odified microporous m~mbrane comprising a hydrophilic organic polymeric microporous membrane and a charge modifying amount of anionic charge modifying agent bonded to substantially all of the membrane microstructure.
m e anionic charge modifying agent is preferably a water-soluble polymer having anionic functional groups, e.g. carboxyl, phos-phonous, phosphonic and sulfonic groups. The charged n~mbrane is made by a process of applying the anionic charge modifying agent to the membrane, preferably by contacting the membrane with aqueous solutions of the charge modifying agent.
The just described Patents describe a comparatively complex treatment of a preformed membrane requiring treatment, rinse and drying steps which involve complicated equipment and expensive capital inves~nent.

OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to provide a process for surface ncdifying a hydrophilic organic polymeric 131373~

micro~orous me!~brane so as to provide a novel surface modi-fied micro~orous membrane, part cularly suitable for the microfiltralion of biolo~ical or parenteral liquids.
It is another object of this inventon to provide an isotropic, surface modified microporous membrane which pre-ferably has low extractables suitable for the microfiltration of biolo~ical or parenteral liquids.
It is yet another object of this invention to pre-pare a sanitizable or sterilizable microporous membrane for the efficient removal of bacteria, viruses and pyrogen from contaminated liquids.
A still further object of this invention is to pro-vide a process for enhancing the filtration, adsorptive and/or capacity of microporous membranes without affecting the internal microreticulate structure.
It is still a further object of this invention to provide a process for producin~ a microporous membrane cap-a~le of capturin~ anionic or cationic particulate contaminant o~ a size smaller than the effective pore size of the membrane.
These and other objects of this invention are attained by a process for surface modifying a hydrophilic organic poly-meric microporous membrane by forming the membrane from a composition con~æinin~ surface modifying agents. The pre-ferred microporous membrane is nylon, the preferred surface modifyin~ agents are polyamido-polyamine epichlorohydrin, e~hylene diamine ~etraacetic acid, carbon, silica and other cnromato~raphic additives, poly (styrene sulfonic acid) and ~oly (acrylic acid).
The surface modified microporous membrane produced by tnis invention may be used for the microfiltration of fluids, particularly parenteral or biological liquids.

3RIEF DESCRI?TION OF T~E FIGUP.ES
Figure 1 is a time vs. transmittance graph of mem-~ranes described in E~ample V.

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DETAIT.F.l- DESCRIPTION OF T~IE INVENTlON
-m e process of -this invention produces a hydrophilic surface modified organic polymeric microporous membrane.
By the use of the term "microporous membrane" as used herein is meant a skinless ("symmetric"), isotropic or anisotropic porous membrane having a pore size of at least .05 microns or larger, or an initial bubble point (IBP), as that term is used herein, in water of less than 120 psi. A maximum pore size useful for this invention is abou~ 1.2 micron or an IBP of grea-~er ~han about 10 psi. By "isotropic" it is meant that the pore structure is substantially the same throughout the cross-sectional structure of the membrane. By "anisotropic" is meant tha~ the pore size differs from one surface to the other. m ere are a nu~ber of commercially available membranes not encompassed by the tenm "microporous membrane~ or ~microfiltration membrane" such as those having one side formed with a very light thin skin layer (skinned, i.e. asymmetric) which is supported by a much more porous open structure which are typically used for reverse osmosis, ultra-filtration and dialysis. Thus, by the term "microporous membrane"
or "microfiltration membrane" are meant membranes suitable for the re val of suspended solids and particulates from fluids and which do not function as ultrafiltration or reverse osmosis membranes but which may have adsorptive and/or sequestration capacity.
By "surface modified microporous membranes" are meant microporous membranes which provide surface adsorption and/or sequestration effects in addition to the microfiltration effects of the membranes per se. By adsorptive surface, it is meant a surface that has controlled molecular geometry and/or surface functionality that allows species to be attacned to the surface by means of ionic, covalent, hydrogen and/or Van Der Walls bonding and/or molecular geometric effects, e.g. ionic exchange, affinity, frontal, size exclusion and the like.

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By Ihe use of the term `'hydrophilic" in describing tne microporous membrane, it is meant a membrane which ad-sorbs or a~sor~s water. Generally, such hydrophilicity is ~roduced by a sufficienl amount of hydroxyl (OH-), carboxyl ( 2)~ (-C-NH-), and/or similar functional ~rou~s on the surface of the membrane. Such groups assist in the adsor~tion and/or absorption of the water onto the membrane. Such hydrophilicity of the membrane and internal ~icrostructure of the surface modified membrane of this invention is preferred in order to render the membrane more useful for the treatmen~ of aqueous fluids.
Preferred microporous membranes are produced from nylon. The term "nylon" is intended to embrace film forming ~olyamide resins includin~ copolymers and terpolymers which include the recurrin~ amido groupin~.
While, ~enerally, the various nylon or polyamide resins are all copolymers of a diamine and a dicarbo~ylic acia, or homopolymers of a lactam of an amino acid, they v~ry widely in crystallinity or solid structure, meltin~
~oint, and other physical properties. Preferred nylons for use in this invention are copolymers of hexamethylene dia- -mine and adipic acid (nylon 66), copolymers of hexamethylene diamine and se~acic acid (nylon 610) and homopolymers of poly-o-ca~rolactam (nylon 6).
Alternatively, these preferred polyamide resins have a ratio of methylene (CH2) to amide (~HCO) ~-oups within the ran~e about 5:1 to about 8:1, most preferably acout 5:1 to about 7:1. ~ylon 6 and nylon 66 each have a ratio of 6:1, whereas nylon 610 has a ratio of 8:1.
The nylon polymers are available in a wide variety of ~rades, which vary a~reciably with respect to molecular wel~ht, within the ran~e from about lS,OOO to about a2,000 and in otner characteristics.

13 1 3 ~ 3 7 3ll ' The highly preferred species of the unites composing the polymer chain is polyhexamethylene adipamide, i.e. nylon 66, and molecular weights in the range above about 30,000 are preferred.
To the extent that commercially available polymers contain additives such as antioxidants and the like, such additives are included within the term "polymer" as used herein.
The membrane substrates can be produced by modifying the method disclosed in U.S. Patent No. 3,876,738 to Marinaccio et al or described in European Patent ~pplication No. 0 005 536 to Pall.
I`he Marinaccio et al process for producing membrane develops a unique fine internal microstructure through the quench technique described therein, offering a superior substrate for filtration. Broadly, Marinaccio et al produces micropor~us films by casting or extruding a solution of a film-forming polymer in a solvent system into a quenching bath comprised of a non-solvent system for the polymer. Although the non-solvent system may comprise only a non-solvent, the solvent system may consist of any combination of materials provided the resultant non-solvent system is capable of setting a film and is not deleterious to the formed film. For example, the non-solvent system may consist of materials such as water/salt, alcohol/salt or other solvent-chemical mixtures. The Marinaccio et al process is especially effective for producing nylon films. More specifically, the general steps of the process involve first forming a solution of the film-forming polymer, casting the solution to form a film and quenching the film in a bath which includes a non-solvent for the polymer.
The nylon solutions which can be used in the Marinaccio et al process inciude solutions of certain nylons in various solvents, such as lower alkanols, e.g., methanol, 131373(~

ethanol and butanol, including mixtures thereof. It is known that other nylons ~ill dissolve in solutions of acids in which they ~ehave as a polyelectrolyte and such solutions are useful.
Re~resentative acids include, for example, formic acid, citriG
acid, acetic acid, maleic acid and similar acids which react ~ith nylons throu~h pro~onation of nitrogen in the amide group characteristic of nylon.
The nylon solutions after formation are diluted with non-solvent for nylon and the non-solvent employed is miscible with the nylon solution. Dilution with non-solvent may, accordin~ to Marinaccio et al, be effected up to the point of incipient precipitation of the nylon. The non-solvents are selected on the basis of the nylon solvent utilized.
For example, when water-miscible nylon solvents are employed, water can oe employed. Generally, the non-solvent can be methyl forma~e, aqueous lower alcohols, such as methanol and ethanol~ polyols such as ~lycerol, glycols, polyglycols and ethers and esters thereof, water and mixtures of such com-~ounds. Moreover, saits can also be used to control solu-tion pro~erties.
The 4uenchi~0 bath may or may not be comprised of the same non-solvent selected for preparation of the nylon solution and may also contain small amounts of the solvent em~loyed in the nylon solulion. However, the ratio of sol-vent to non-solvent is lower in the quenching bath than in the polymer solu~ion in order that the desired result be o~tained. The o,uenchin~ bath may also include other non-solvents, e.g. WQter, The formation of the polymer film can be accomplished ~y any of the recognized methods familiar to the art. The ~referred method is casting using a knife edge which controls tne thickness of the cast film. The thickness o the film will be dictated by the intended use of the microporous ~roduc~. In ~eneral, the films will be cast at thicknesses in the range of from about 1 mil to about 20 mils, prefer-a~ly from about 1 to about 10 mils.

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Preferably, the polymer solution is cast and simul-ta~eously 4uenched, although it may be desirable to pass the cast film throu~h a short air evaporation zone prior to the quench bath. This la~ter technique is, however, not pre-ferred.
After the polymer solution is cast and quenched, it is removed from the quench bath and preferably washed free of solvent and/or non-solvent. Subsequently the film can be at least partially dried.
Pall's aforementioned European Patent Application No. 0 005 536 describes another similar method for the con-version of polymer into microporous membrane which may be used. Broadly, Pall provides a process for preparing skin-less hydrophilic alcohol-insoluble polyamide membranes by ~re~arin~ a solution of an alcohol-insoluble polyamide resin in a polyamide solvent. ~ucleation of the solution is in-duced by the controlled addition to the solution of a non-solvent for the polyamide resin, under controlled conditions of concentration, tem~erature, addition rate, and degree of agitation to obtain a visible precipitate of polyamide resin ~articles (which may or may not partially or completely re-dissolve) ~hereby forminJ a casting solution.
The castin~ solution is then spread on a substrate to form a thin film. The film is then contacted and diluted with a mixture of solvent and nonsolvent liquids containing a substantial proportion of the solvent liquid, but less tnan the proportion in the casting solution, thereby preci-~itatinO polyamide resin from the casting solution in the form of a ~hin skinless hydrophilic membrane. The resulting mem~rane is then washed and dried.
In Pall's preferred embodiment of the process, the sol~ent for the polyamide resin solution is formic acid and tne nonsolvent is water. The polyamide resin solution film is contact~d with the nonsolvent by immersing the film, car-ried on the substrate, in a bath of nonsolvent comprising water containing B substantial proportion of formic acid.

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The nylon membranes described in Marinaccio et al and Pall are characterized ~y hydrophilic isotropic stl~cture, having a high effective surface area and a fine internal microstructure of con-trolled pore dimensions with narrow pore size distribution and adequate pore volume. For example, a representative 0.22 nucrometer rated nylon 66 men~rane (polyhexarnethylene adiparnide) exhibits an initial bubble point (IBP) of about 45 to 50 psid, a foam all over point (FAOP) of about 50 to 55 psid, provides a flow of from 70 to 80 ml/min of water at 5 psid (47 mm. diameter discs), has a surface area (BET, nitrogen adsorption) of about 13 m2/g and a thickness of about 4.5 to 4.75 mils.
As will be apparent from the foregoing description, both the Marinaccio and Pall processes involve the fonnation of a nylon polymer solution or dope which is then diluted with a non-solvent, cast on a suitable su~strate surface and contacted ~ith additional non-solvent to cause precipitation of the polyamide resin from the dope solution in -~he fonn of a thin skinless hydrophilic m~nbrane.
In thc aforementioned Patents to Ostreicher et al, Barnes et al and Chu et al, the res~llting melllbrane is charge modified by contacting the formed membrane with a charge modifying amowlt of a charge modifying agent. In the present invention, the surface modifying agent (which can be a cationic or anionic charge modifying agent) is incorporated into the polymer solution or dope before the membrane is precipitated. The men~rane can thereafter be fonned by the casting technique described in Marinaccio et al and Pall or alternatively, the dope can be introduced into the quenching bath of the non-solven-t under shear to produce fibers of the surface modified n~mbrane which can be formed into a sheet material similarly to the fonnation of paper from fibers, e.g. as described in U.S. Patent 4,309,247 to ~lou et al (1982) or made into hollow fibers to produce surface modified hollow fibers.

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The surface modifying agent is bound to the internal microstructure, preferably substantially all of the internal microstructure, of the microporous membrane. By the use of the term "bound" is meant that the surface modifying agent is sufficiently attached to or incorporated into the membrane so that it will not be significantly extracted under the intended conditions of use. By the use of the term "substantially all of the internal microstructure" as used herein, is meant substantially all of the external surface and internal pore surfaces. ~`ypically by this is meant the surfaces which are wetted by a fluid, e.g., water, passing through the membrane or in which the membrane is immersed .
The term "surface difying agent" means a compound, material or composi-tion which when bound to ~he m~nbrane, alters its capacity to remove a desired entity from a fluid being filtered and which is compatible with the dope. By the use of the term "charge modifying agent", is meant a compound or composition that when bound to the microporous filter membrane alters the "zeta potential" of the membrane (see Knight et al, "Measuring the Electrokinetic Properties of Charged Filter Media," Filtration and Separation, pp. 30-34, Jan./Feb. 1981).
The cationic charge modifier is a compound or composition which is capable of being bound to the membrane microstructure and provides a more positive zeta potential 'co the membrane micro-structure. Preferably, such cationic charge modifier is a water-soluble compound having substituents capable of binding to the membrane and substituents which are capable of producing a more positive "zeta potential" in the use enviroml~nt (e.g. aqueous) or cationic functional groups. Most preferably, the agent may be a water-soluble organic polymer capable of becoming a non-extractable constituent of the membrane.

1~3 131373~x The cationic charge modifying agent can also be cross-linked ~o itself or to the membrane polymer through a cross-linking agent, for example, an aliphatic polyepoxide having a m~lecular weight of less than about 500.
The cationic charge modifying agent may have either a high or low charge density, or anything between these extremes, however, high charge density is preferred.
The preferred cationic charge difier is selected from the class of polyamido-polyamine epichlorohydrin cationic resins, in particular, those described in the following U.S. patents:
2,926,116 to Keim;
2,926,154 to Keim;
3,224,986 to Butler et al;
3,311,594 to Earle, Jr.;
3,332,901 to Keim;
3,382,096 to Boardman; and 3,761,350 to ~lunjat et al.
Broadly, these preferred charge modifiers (hereinafter "polyamido-polyamine epichlorohydrin") are produced by reacting a long chain polyamide with epichlorohydrin, i.e. 1 - chloro-2,3 epoxypropane having the formula:

~.
2 CH CH2Cl The polyamide may be derived from the reaction of a polyalkylene polyamine and a saturated aliphatic dibasic carboxylic acid containing from abou-t 3 to 10 carbon atoms. The polyamide produced is water-soluble and con~ains the recurring groups:

-NH(CnH2nHN)X-CORCO-~3~3~

where n and x are each 2 or more and R is the divalent hydr~-carbon radical of the dicarboxylic acid. This polyamide is then reacted with epichlorohydrin to form the preferred wa~er-soluble charge modifiers used in its invention.
The dicarboxylic acids which may be used in preparing the polyaMides are the saturated aliphatic dicarboxylic acids containing from 3 to 10 carbon atoms each as malonic, succinic, glutaric, adipic, azelaic and the like. Blends of two or more of the saturated carboxylic acids may also be used. -A variety of polyalkylene polyamines including poly-ethylene polyamines, polypropylene polyamines, polybutylene polyamides and so on may be employed. More specifically, the polyalkylene polyamines are polyamines containing two primary amine groups and at least one secondary amine group in which the nitrogen atoms are linked together by groups of the formula - CnH2n_, where n is a small integer greater than unity and the number of such groups in the molecule ran~es from two up to about eight. The nitrogen atoms may be attached to adjacent carbon atoms in the group -CnH2n_ or to carbon atoms further apart, but not to the same carbon atom. Polyamines such as diethylenetriamine, triethylene-tetramine, tetraethylene-pentamine, dipropylenetriamine, and the li~e, and mixtures thereof may be used. Generally, these yolyalkylene polyamines have the general formula:
H2[(CnH2n)NH]yCnH2nNH2 wherein n is an integer of at least 2 and y is an integer of 1 to 7.
In carrying out the reaction of the polyalkylene polyamine with tbe acid, it is preferred to use an amount of dicarboxylic acid sufficient to react substantially completely witn the primary amine groups of the polyalkylene polyamine but insufficient to react with the secondary amine groups to any subst~ntial extent. The polyamide produced is then re-acted with Ihe epichlorohydrin to form the preferred poly-amido-polyamine epichlorohydrin charge modifying agent.

1313~

Typically, in the polyamide-epichlorohydrin reaction it is preferred to use sufficient epichlorohydrin to convert all of the secondary amine groups to tertiary amine groups, and/ox quaternary ammonium groups (including cyclic structures). Generally, however, from about 0.5 mol to about 1.8 moles of epichlorohydrin for each secondary amine group of -the polyamide may be used.
The epichlorohydrin may also be reacted with a polyamino-ureylene containing tertiary amine nitrogens to produce the primary charge modifying agents which may be utilized in this invention (see for example the aforementioned Earle, Jr.).
Other suitable charge modifying agents of the foregoing type may be produced by reacting a heterocyclic dicarboxylic acid with a diamine or polyalkylene polyamine and reacting the resultant product with epichlorohydrin (see for example the aforementioned Munjat et al.) The polyamido-polyamine epichlorohydrin cationic resins are available commercially as Polycup 172, 1884, 2002 or S2064 (Hercules; Cascamide Resin pR-420 (Borden); or Nopcobond 35 (Nopco). Most preferably, the polyamido-polyamine epichlorhydrin resin is Polycup 1884 or Hercules R4308, wherein the charged nitrogen atom forms part of a heterocyclic grouping and is bonded through methylene to a depending, reactive epoxide group. The terms Polycup, Cascamide, Nopcobond and Hercules are all trade-marks.

13~373~ ` `

. Each monomer grou~ in R 4308 has the general formula:
r~
L 2 ~~
~ CH2_ CH CH .~
'.

~+

. . `~ Cl-_ . i CH3 CH2-CH-CH2 O __ Polycup 172, 2002 and 1884, on the other hand, have J~onomer groups of the general formula:

R C1- ~' (C~2)4 --- CNHCH2 -- CH2 -- NCH2 -- CH2NH

!_ O

wherein R is methyl or hydrogen (Polycup 172 and 2002, R=H;
and Polycup 1884, R=CH3).
A secondary charge modifying agent may be used to enhance the cationic charge of the primary charge modifying agent and/or enhance the bonding of the primary charge modi-fying a~en~. The secondary charge modifying agent may be selecled from the group consisting of:
~ i) aliphatic amines having at least one pri-mary amino or at least two secondary amino groups;
and .- .

131373 `~

(ii) aliphatic amines having at least one secondary amine and a carboxyl or hydroxyl sub-stituent.
Preferably, ~he secondary charge modifying agent is a polyamide havin~ the formula:
H

H2N-(Rl-N-)~-R2-NH2 wherein Rl and R2 are alkyl of 1 to 4 carbon atoms and ~
is an integer from O to 4. Preferably, Rl and R2 are both ethyl.
Preferred polyamines are:
Ethylene diamine H2N-(cH2)2-NH2-NH2 ~iethylenetriarnine H2N-(cH2)2-NH-(c~2)2-NH2 Triethylenetetrarnine H2N-(CH2-CH2-NH)2-CH2-CH2-NH2 Tetraethylenepen'amine H2N-(CH2-C~2-N~)3-CH2-CH2-NH2 The hi~hly preferred polyamine is tetraethylene pentamine.
Alternatively, aliphatic amines used in this inven-tion may have at least one secondary amine and a carbo~yl or hydroxyl substituent. Exemplary of such aliphatic amines are gamlna-amino-butyric acid (H2NCH2CH2CH2COOH) and 2-amino-ethanol (H2Nc~2c~2oH).
The secondary charge modifying agent is bonded to ~he micro~orous membrane by bondin~ to a portion of the epo~ide substituents of the polymeric primary charge modifying agent.
Tne amount of primary and secondary cationic charge modifyin~ a~ens utilized is an amount sufficient to enhance the electropositive c~p~ure potential of the microporous membrane. Such an amount is ni~hly dependent on the speci-fic charge modifying~ ents utilized. For general guidance, nowever, it has been Iound that a weight ratio of primary to secondary char~e modi~yin~ agenl of from about 2:1 to about 5~:1, preferably from about 20:1 to about 75:1 is generally sufficient.
In another embodiment of the present invention, the foregoinD "secondary" charge mod~fyin~ a~ent can be used as , ~3137~

tne cnar~e modifyin~ agent by the cojoint employment of an aliyhatic yolyepoxide crosslinking agent having a molecular weight of less than about 500. Preferably, the polyepoxide is a di- or tri- epoxide having a molecular weight of from about 146 ~o about 300. Such polyepo~ides have viscosities (undiluted) of less than about 200 centipoises at 25C. Due to the necessity of the epo~ide to act as a crosslinking a~ent, monoepo~ides, e.g. glycidyl ethers, are unsuitable.
Similarly, it is theorized that a polyepo~ide offerin~
~reater than three epoxy ~roups offers no benefit and in fact may limit the couplin~ reactions of the polyepoxide by steric hindrance. Additionally, the presence of unreacted eyoxide grouys in the cationically charge modified micropor-ous membrane may be undesirable in the finished product.
Highly preferred yolyepo~ides have the formula:
'- R(0-CH2-CH~cH2)n wherein R is an alkyl of 1 to 6 carbon atoms and n is from 2 to 3. The limitation that the number of carbon atoms in the non-e~o~ide portion --(R)-- be less than 6 is so that the polyeyoxide will be soluble in water or ethanol-water mix-tures, e.g. uy to 20~ ethanol. While higher carbon content materials are functionally suitable, their application would involve the use of polar organic solvents with resulting ~roblems in to~icity, flammability and vapor emissions.
The anionic charge modifying agent is a compound or cornyosition which is capable of bonding to the membrane microstructure without substantial pore size reduction or yore blockage and provides an anionic charge or negative zeta yotential to the membrane microstructure. ?referably, such anionic charge modifier is a water-soluble compound havin~ substituents capable of binding to the membrane and substituents which are capable of producing a more negative "zet~ potential" in the use environrnent (e.g. aqueous) or anionic functional grou~s.

13137~

Preferred anionic functional groups may be carboxyl, phosphonous, phosphonic and sulfonic. Preferably, the anionic charge modifying agent may be a water-soluble organic polymer or polyelectrolyte having a molecular weight greater than about 2,000 and less than about 500,000 and capable of becoming a non-extractable constituent of the membrane.
m e anionic charge modifying agent may have either a high or low charge density, or anything between these extremes, however high charge density is preferred. Specific preferred anionic charge modifying agents useful herein are poly (styrene sulfonic) acid, poly (toluene sulfonic) acid, poly (vinyl sulfonic) acid and poly (acrylic) acid. Other anionic charge modifying agents are poly (methacrylic acid), poly (itaconic acid), hydrolyzed poly (styrene/maleic anhydride) and poly (vinyl phosphonic acid).
Additionally, the alkali and alkaline earth m~al salts of all of the foregoing may be utilized.
Highly preferred anionic charge modifying agents are poly (styrene sulfonic) acids having a molecular weight between 2,000 and 300,000 and poly (acrylic acid~ having a molecular weight between 2,000 and 300,000.
The anionic charge modifying agent may also be cross-linked to the microporous membrane structure or itself in the same manner as the cationic agents using the same aliphatic polyepoxi~e cross-linking agent having a molecular weight of less than about 500. In addition to the preferred polyepoxides described above, certain diglycidyl ethers of aliphatic diols, Cl-l -CH_C~2-O-R-o-cH2-cH~H2 b o may be used. Examples are 1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol. 1'he preferred diglycidyl ether of 1,4-butanediol is commercially available from Ciba-Geigy, Inc. as RD-2 and from Celanese Corp. as Epi-Rez 5022 and Polyscience. The terms RD-2, Epi-Rez and Polyscience are trademarks.

13~3~3`~ `

~ ther higher carbon diglycidyl ethers may be used as the polyepoxide cross-linkin~ a~ent, for example 5-pen-tauediol di~lycidyl ether. However, the appropriate polar or~anic solvents must be used for diluting such polyepoxides.
Tri~lycidyl ethers, i.e. tri-epoxides may also be utilized as the polyepoxide cross-linking agent. The tri-eyoxides have the followin~ formula:
C~2-CH-CH2_0_CH2_CH_CH2_0_CH2-CH-CH2 o C~2 . .
CH
~ O

The tri~lycidyl ether of glycerol is available from Shell, Inc. as Epon 812 and Celanese Corp. as Epi-Rez 5048.
Another prei'erred cross-linkin~ agent is ~ethylated urea formaldehyde resin, commercially available from American Cyanalnid; i'or examplé, Beetle 65, and melamine formaldehyde, e.~., Cymel 303 from American Cyanamid.
Other water-soluble polymers havinO polar groups can also be employed in this invention as the charge modi-f~in~ a~ent. E~amples include sodium alginate, ethylene diamine tetraacetic acid, diethylene triamine tetraacetic acid, tetraethylene pentamine tetraacetic acid, quaternized ~olyethylenei~ine, quaternized vinyl pyridine, quaternized diethylaminoethylmethacrylate and the like. The molecular weight of the char~e modifyin~ agent does not appear to be sl~nificant so lon~ as the agent is soluble in the polymer "~o~e". Thus, sodium alginate which has a molecular weight above lO,OOV and ethylene diamine tetra acetic acid which nas a molecular wei~ht below 10,000 are equally employable.
Tne ~olyamido-polyamine epichlorohydrin cationic resins enerally have a molecular wei~ht above 10,000. For example, Polycu~ 18~4 has a molecular wei~ht of about 300,000 and ~4308 has ~ molecular wei~ht of about 530,000.

13137~`~

Other surface modifying agents which are soluble or suspendable in aqueous solvents are such materials as carbon, diatornaceous earth, barium ferrite, iodine, aluminum, alumina, silica, kaolin, molecular sieves, carbohydrates, perlite, clays, vermiculite, asbestos, bentonite, casein and the like.
Broadly, the process of this invention is directed to surface modifying a hydrophilic organic polymeric rnicroporous men~rane, e.g. nylon. The process comprises forrning a dope solution of nylon polymer, water-soluble or water-suspendable membrane surface modifying agent and a solvent, diluting the resulting dope solution with a miscible non-solvent for the nylon polyrner and contacting the diluted dope solul:ion with sufficient non-solven. for the nylon polyrmer ~:o precipitate said nernbrane therefrom. The dilution of the dope solution is preferably carried out up to the point of incipient precipitation of the nylon but should any precipitation occur, the solids can be eliminated by filtration or can be redissolveâ by adding additional solvent to the diluted dope solution. ~en cast films are prepared, the diluted dope solution is spread on a substrate surface prior to contact with the non-solvent for precipitation. When fibers are being prepared, the contacting step is conducted by extruding the dope into a quenching bath and/or with the application of shear.
Ln order to provlde the surface modifying amount of surface rnodifying agent ~o the rnembrane, it is preferred tha-t the polyrner dope solution contain at least about 0.0196 surface modifying agent, by ~eight of total solids. The rnaximum amount of surface modifying agent in the solution is limited by economic and solubility-suspendability lirnitations. For example, an excess of rnodifying agent which does not becorne bonded to the microporous membrane will not be economically utilized and will constitute an undesirable extractive from the membrane. It has been found that the amount of surface modifying agen~ in the dope should not exceed about 75~6 by weight of total solids 13137~

After the microporous membrane has been prepared, it is then dried and cured, preferably in a restrained condition to prevent shrinkage.
Drying of the membrane under restraint is described in the Assignee's defensive publication T 103,602 to Repetti, published ~ovember 1, 1983. Generally, any suitable restraining technique may be used while drying, such as winding the membrane tightly about a drying surface, e.g. a drum. Biaxial control is preferred and tensioning the membrane on a stretching frame is considered the most preferred. Preferably, the restraining imposed effects no reduction in dimensions.
Final drving and curing temperatures should be to dry and cure the treated membranes, preferably from abou~ 120& to 140C
for minimization of drying times wi~hou~ ernbrittlement or other detrimental effects to the membrane.
m e completed membrane may be rolled and stored for use under ambient conditions. It will be understood that the treated membrané may be supplied in any of the usual commercial fonns, for example, as discs or pleated cartridges.
The present invention provides an integral, coherent microporous membrane of retained internal pore geometry. m e surface modified membrane has an improved effective filtration rating relative to the untreated micro-reticulate polymer structure.
For so-called sterile filtrations involving biological liquids, the filter is sanitized or sterilized by autoclaving or hot water flushing. Accordingly, the surface modified mernbrane rnust be resistant to this type treatrnent, and must retain its integrity in use. Any modification to the filter structure, especially brought about by chemical agents which m~y be unstable under conditions of treatment and use, rnust be scrutinized with care to minimize the prospect of extractables contaminating the filtrate, interfering 13i373`~.

with analyses and potentially introducing harmful toxins to a patient. Specifically, any such filter must meet the test standards in the industry, e.g. ASTM D 3861-79, and generally prove less than S mg. of extractables in 250 ml solvent (water at 80C.i 35~ ethanol at room temperature) for a 293 mm diameter disc.
Biological liquids as that term is employed in the specification and claims, is a liquid system which is derived from or amenable to use with living organisms. Such liquids are ordinarily handled and processed under sanitary-or sterile conditions and therefore require sanitized or sterilized media for filtration. Included within such term are isotonic solutions for intermuscular (im) or intravenous (iv) administration, solutions designed for administration per os, as well as solutions for topical use, biological wastes or other biological f~uids which rrlay comprise filterable bodies such as impurities, e.g., bacterial, viruses or pyrogens which are desirably isolated or separated for examination or disposal by i D bilization or fixation upon or entrapment within filter rnedia.
Filter rnembranes in accordance with this invention may be er~ployed alone or in combination with other filter media to treat pharmaceuticals such as antibiotics, saline solutions, dextrose solutions, vaccines, blood plasma, serums, (e.g. to remove hormones or toxins), sterile water or eye washesi beverages, such as cordials, gin, vodka, beer, scotch, whiskey, sweet and dry wines, champagne or brandy; cosmetics such as mouthwash, perfume, sham~oo, hair tonic, face cream or shaving lotion; food products such as vinegar, vegetable oils; chemical such as antiseptics, insecti-cides, photographic solutions, electroplating solutions, cleaning compounds, solvent purification and lubrication oils, cutting oils for r~noval of me,allic fines (e.g. where the ferrite modifying agent has been magnetized); and the like for retention of submicronic particles, removal of bacterial contaminants and _ 1313~

2~

resolution of colloidal hazes. Illustratively, in hospital usage, membrane filters are employed to concentrate abnormal exfoliated cells from a vaginal rinse, to isolate blood ~arasites from peripheral blood, or bacteria from serum or leucocytes and casts from urine.
In the case of ~re~aration for use in sterile fil-tration, the membrane is thermally sterilized or~sanitized as by treatment in an autoclave at 121C. under 15 psig. for 1 hour, or hot water flushing at 85F. for l hour.
The membranes and fibers, etc. of this invention can also be used to provide a bactericide (e.g. where the modifying agent is iodine) or bacteriostatic treatment to fluids, to remove contaminants such as chlorine or phenol from fluids, in molecular separation columns, in bioreactors where cells, etc. are immobilized thereon, as cigarette fllters, and for many other uses.
Havin~ now generally described this invention, the s~me will become better understood by reference to certain s~ecific eamples, which are included herein for the purposes of illustration only and are not intended to be limiting of the invention.
EXAMPLES
The following are the measurement and test procedures utilized in all the Examples.
Thickness The dry me~brane thickness was measured with a l/2 inch (1.27 cm) diameter platen dial indicator thickness gauge.
Gauge accuracy was +O.OOOOa inches (+.05 mils).
Initial Bubble Point (IBP) and Foam-All-_ver-Point (FAOP) Tests A 47 mm diameter disc of the membrane sample is ~laced in a special test holder which seals the edge of the disc. Above the membrane and direc~ly in contact with its u~er face, is a perforated stainless steel support screen which prevents the membrane from deforming or rupturing when .

~3~373l~

air ~ressure ~s ap?lied tO its bottom face. Above the mem-brane and support screen, the holder provides an inch deep cavity into which distilled water is introduced.
~ re~ulated air pressure is increased until a first stream of air bubbles is emitted by the water wetted membrane into quiescent pool of water. The air pressure at which this first stream of air bubbles is emitted is called the Initial bubble Point (IBP) of the lar~est pore in that membrane s~mple - see AST~I D-2499-66T.
Once the Initial Bubble Point pressure has been determined and recorded, the air pressure is further in-creased until the air flow throu~h the wetted membrane sam-ple, as measured by a flow meter in the line between the re~ulator and the sample holder, reaches 100 cc/min. The air pressure at this flow rate is called the Foam-All-Over-Point (FAOP), and is directly proportional to the mean pore diameter of the sample mem~rane. In this series of tests, these two parameters (IBP and FAOP) are used to determine if any chan~e has occurred in the maximum or mean pore size of the mem~rane sample as a result of the charge modifying process utilized.
Flow Rate Test A 47 mm diameter disc of the membrane sample is placed in a test housin~ which allows pressurized water to flow through the membrane. Prefiltered water is passed through the membrane sample at a pressure differential of 5 psid. A graduate cylinder is used to measure the volume of water passed by the membrane sample in a one minute period.
In this series of tests this parameter is used in conjunction with the IBV and FAOP to determine if any reduction in pore size or ~ore blockage has occurred as a result of the charge modif~ing process utilized.
Dye Adsor~tion Test A 47 mm diameter disc of the membrane sample is placed in a test housin~ which allows pressurized water flow 31 i~37~

through the m~nbrane. The challenge solution consists of distilled water at a pH of 7.0, and Metanil Yellow dye (color index CI#13065:
CAS587-98-4) for cationically charged membranes and methylene blue (color index CI#52015: CAS61-73-4) for anionically charged membranes. The dye inlet concentration is adjusted to produce a 76 percent transmittance at a waveleng~h of 430 nm, as measured on a Perkin-Elmer Model 295 Spectrophotometer for cationic membranes or 34 percent at 660 nm as measured on a Bausch & L~nb Spectronic 710 Spectrophotometer for anionic membranes. By means of a peristaltic pump the challenge solution is flowed through the me~brane sample at a flow rate of 28 ml/min. m e transmittance of the effluent is measured by passing it through a constant flow cell in the aforementioned spectrophotometer. The effluent transmittance and pressure drop across the membrane is measured and recorded as a function of time. m e test is terminated when the effluent transmittance increases to 85 percent for cationic membranes or 45 percent for anionic membranes of the inlet transmittance. In this series of tests, the length of time that it takes to reach the 85 or 45 percent, ~ransmittance in the effluent is called the "breakthrough" time. Since the Metanil Yellow and methylene blue are low molecular weight charged dyes incapable of being mechan-ically removed (filtered) by the membrane, -~his breakthrough time is proportional to the charge adsorptive capaci-~y of the membrane sample. This test is therefore used to determine the effectiveness of the charge modification technique.
Extractables (ASTM D-3861-79) Extractables were determined by ASTM D-3861-79. l`he quantity of water-soluble extractables present in the membrane filters was determined by immersing the preweighed membrane in boiling reagent grade water for an extended time and then drying and reweighing the membrane. A control membrane was employed to eliminate weighing errors caused by balance changes or changing moisture content of the membrane in the weighing procedures.
Weight changes of the control 131373~

membrane were ~plied as a correction factor to the weight change of the test membrane filters.
EXA~IPLE I
A. reparation of ~licroporous Membrane A re~resentative nylon 66 membrane of 0.22 micrometer nominal ratin~, having a nominal surface area of about 13 m2/g, an Initi~l Bubble Point of about 47 psi, a Foam-All-Over-Point of about 52 ~si was prepared by the method of Marinaccio et al, U.S. Patent 3,87~,738, utilizing a dope composition of 16 per-cent ~y weight nylon 6~ (Monsanto Vydyne 66B), 7.1~ methanol and 76.9~ formic acid, a quench bath composition of 25Z metha-nol, 75~ water by volume (regenerated as required by the method of Kni~ht et al, U.S. Patent 3,928,517) a casting speed of 24 inches/minute (61 cm/min), and a quench bath temperature of 20C. The membrane was cast just under the surface o~
the quench bath by a~plication to a casting drum rotating in the bath (9 to 10 mils as cast wet, to obtain 4.5 to 5.5 mils dry) and allowed to separate from the drum about 90 of arc from the point of application, the self-supporting mem-brane forming a shallow catenary to takeup. A portion of the uniform opaque film was dried (in restrained condition to resist shrinka~e) in a forced air oven at 89-90C. for 30 mi nu tes.
B. Preparatior. of Charge Modified MicroDorous Membrane - Post-Treatment 1. Membrane samples (dried and undried) were dipped in a bath of ~ercules 1~4 polyamido-polyamine epichloro~ydrin resin (4~ solids by weight), and allowed to attain adsorption e~uilibrium. The treated membrane samples were washed to re-rnove e~cess resin and dried in restrained condition on a drum at a tem~erature of 110~C. for a period of about 3 minutes.
The Ireated memr)rane samples were compared for flow and bubble point characteristics as follows, and found to be essenlially identical for treated and untreated samples, evidencing retention of pore and surface geometry. The results are set forth in Table I.

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TABLE I
Control (No Undried Dried Treatment~ Membrane Membrane Thickness (mils) 4.25 4.58 4.83 Initial ~u~ble Point (psi) 43.7 44.7 44.7 Foam-All-Over-Point (psi)55.0 54.0 54-7 Thlckness Normalized Flow Rate (cc. mil/min. cm2 psi) 7.1 7.2 7.0 BET, N2 adSorption 13.12 - 13.58 Thus, in terms of the morphological and hydrodynamic parameters that control mechanical sieving,the filtration charac-teristics of the treated membrances were essentially identical with the untreated nylon membrane.
2. Similar characterizations were conducted on an-other membrane sample, similarly prepared, but treated with 2~o ~ercules K4308 resin (a free radical polymerized resin based u~on diallyl nitrogen-containing materials, reacted with epi-chlorohydrin) in a bath adjusted to pH 10.5, overcoated with V.lZ tetraethylene pentamine, dried, cured, washed and redried.
The results are set forth in Table II.
TABLE II
Control (No Treatment~ Dried Membrane Tensile Stren~th (Dsi) Wet S28 6~5 Vry 860 960 Elon~ation (Z) Wet 140 100 ~ry 95 40 Surface area of the treated and untreated membranes remained essentially unchan~ed; tensile stren~th increased with treatment with some loss in elon~ation. The treated .

- .
-~ ' .
,~

13137~

sneet was mor~ flexible; creasing of the untreated sheet resulted in crac~ing and splitting.
C. Filtration Tests The ~ercules 1884 treated membrane samples (Example I.B.l.) were subjected to the filtraton tests indicated below:
Pyro~en Removal Purified E. coli endotoxin was added to a 0.9~ NaCl solution, pH 6.7 and passed through test filters mounted in a 25 mm diameter stainless steel holder. Inlet and effluent endotoxin levels were determined by standard L.A.L. analysis.
Results are set forth in Table III.
TABLE III
Inlet Endotoxin Effluent Endotoxin Level (pg/ml) Filter Level (~g/ml) 10 ml. 50 ml 100 ml Dried, treated llembrane 15000 1000 lOC0 1000 Control -Untreated 15000 10000 10000 10000 (Pg is "picogram") Virus Xemoval ~ S-2 bacleriophage was added to ~ouston Texas (U.S~A.) zay waler to produce a concentration of 3.4 x 105 ?FU/ml (PFU
is "Plaque Forming Unit"), and 10 ml was passed through each of the test filters mounted in a 25 mm diameter stainless steel holder. Effluents were analyzed for viral content by standard ~echni~ues. Results are set forth in Table I~':
TABLE IV
- Total Viral PFU Virus Removal Filter in Filtrate Efficiency t~o) ~ried, trealed ~lembrane 100 99.997 Control - un~reated 250000 26.4 ~onodis~erse Latex Filtration The test filters were challenged with a 10 ~TU dis-~ersion (NTU is "nephlometric turbidity units") of 0.109 13137~

micrometer monodisperse latex (~DL) particles at a flow rate of 0.5 ~m/ft.2 (.002 lpm/cm2), pH 7.0, R=21000-ohm-cm.
Effluent turbidities (~TU) were monitored and filtration efficiencies were calculated from equilibrium effluent tur-bidities. Results are set forth in Table V.
TABLE V
Filter MDL Removal Efficiency Undried, treated 97.3 Control-untreated 10%
Dye Removal Efficiency The test filters were challenged with a solution of ~lue food coloring dye (FD ~ C ~o. 1). The solution had a light transmittance of 62.5~ at 628 nm. The light trans-mittance of the effluent was monitored and removal ef~icien-cies determined (based on distilled water light transmit-tance - 100Z). Results are-set forth in Table VI.
TABLE VI
Throughput (litres) to 90X
Transmittance Undried, treated 1.99 ~ried, treated 1.76 Control-untreated o EXA~PLE II
The cationically charged microporous membrane of Example I.B. 1. is prepared by repeating the procedure of Example I.~. and incorporating the Hercules 1884 resin into the do~e composilion.
EXAMPLE III
A nylon dope solution was prepared containin~ 10 nylon, 85.3~ formic acid and 4.7~ methanol. About 28~ of ~ercules 1884 resin based on the wei~ht of the nylon was introduced into the dope solution. The resulting dope solu-tion ~as extruded throu~h an orifice which was in near proxi-13137~ .~

mity tO a recirculating quench bath stream of about 25~ v/va~ueous ~ethanol. The recirculating strearn produces a mo-derate shear on the dope solution entering the bath, thereby producin~ fine fibrils from the dope solution. The resulting fibers were blended at a ratio of 1:1 with coho cellulose fi~er and 4.3 ~rams of the resulting mi~ture was felted into pads. The electrokinetic status of the pad was determined usin~ streamin~ potential techniques (~night and Ostreicher, Measurin~ the Electrokinetic Properties of Charged Filter .~edia, Filtration and Separation, January/February, 1981, pp. 30-34). The pad had a slope Mv/Ft H2O of -6.8, an in-tercept of -90.70 and an a~parent zeta potential of +0.33.
EXAI~IPLE IV
Approximately 1 litre of a mixture of methanol and formic acid in a weight ratio of 0.04 was prepared and allowed tO equilibriate for 1 hour. Then to four separate flasks, 150 ml of the solution was added. Thereafter, Hercules resin 1~84 (35~ solids) were added in amounts of 1, 5, 10 and 15 millilitres and allowed to equilibriate in a water bath at 4UC. for one hour with a~itation.
A sufficient quantity of nylon was added to bring the wei~ht percentage of the nylon to 8~ based on the weight of the methanol and acid and the flasXs were shaken in a water bath at 40C. until the nylon dissolved. The composi-tions of tAe resultin~ doped solutions were:
_ Percentage ~lethanol 4.1 4.1 4.0 4.0 Formic .~cid 87.887.2 86.4 85.7 ~ylon 8 7.9 7.8 7.8 1~4 0.2 0.9 1.8 2.5 Cationically modified microporous membranes are pro-duced repeating the procedure of Example I. A.

131373 '~

EXAMPLE V~
Four dope compositions containin~ 39 grams of Nylon 6~ and the following other ingredients were prepared:
Do~e Fonmic Acid Grams Water Grams 4308 Resin Grams Pentamine Gra~s 1 231.36 ~.6~ 0 0 2 231.36 16.916 10.263 2.46 3 231.36 4.~3 20.526 4.92 4 231.36 24.719 0 4.92 Dope 2 contains one equivalent wei~ht of 4308 Resin and triethyler.epentamine per weight nylon, formulation 3 contains two equivalent weights of both resin and pentamine per wei~ht nylon Pnd dope 4 contains two equivalent weights of the pentamine alone. The dopes were placed in a jar mill roller bath at 20C. until full dissolution. Following the procedure of EXAMPLE I. A., two membranes were cast from each do~e just under the surface of a quench bath (30~ meth-anol, ~ water by volume) by application to a casting drum rotatin~ in the bath usin~ an 8 mil blade to drum depth.
The membranes made ~rom each dope were separated from the castin~ drum and rinsed in two successive wash baths of distilled water. The membrane sheets were then doubled over on top of themselves while wet and mounted in restrained condition to resist shrin~a~e and placed in a forced draft oven at 80C. for one-half hour. The membranes were then subJected to .~.~etanil Yellow dye absorption tests, the results of which are shown in Figure 1. Thereafter, the membranes were subjected to flow, IBP and FOAP tests and the following results obtained:
:: ~

' .

.
:: : : . . .

i3i37~'~

Do~,e Sam~LeFlo~ (Ml/~lin) IBP (psi) FAOP (psi) l 1 72 53.5 85 2 78 45.5 82.5 2 1 39 59 53.8 2 59.5 41.5 53 4 1 6 90+ 90+
2 11 85.5 90+

EXAllPLE VI
To 253.6 ~rams of a ~ylon 66 membrane dope for a membrane of a 0.45 micron nominal rating containing 40.576 ~rams of Nylon 66, methanol and formic acid (16~ solids) was aaded 1.159 ~rams of Hercules 1884 resin (35% solids) to ~ive l~ resin based on the nylon and the resulting mixture was a~itated urltil a clear solution was obtained. Membranes were ~re~ared followin~ the procedure of E~ample I.A., using the do~e witnout the 1884 resin and the dope with the resin.
The membranes were dried under restrained conditions for 30 minutes at 85C. and their properties were measured using test water which had been prefiltered through a 0.2 micro-meter nominal rating memorane. The results are shown in the following table:

Flow cc/Min.-b~embrane Thickness psi-cm2IBP (PSi) FAOP (Psi) Do~e wit.~out resin 4.13 2.~7 41.3 47.5 ~o~e wi~h resm 4.4 2.44 38.4 45.3 The membrane prepared with the dope which did not contain the cationic 1884 resin had an IBP/FAOP ratio of 0.8~ wnile Ihe membrane prepared with the resin had a ratio of 0.848. -~ 3~373~

EXAMPLE VII
A membrane dope was prepared by combining 180S.5~arts of ~ylon 66 with 9479 parts of a mixture of methanol an~l formic acid to obtain a 16~ solids nylon dope. The mix-ture was heated with agitation at 30C. for about 4 hours.
A quantity of Polycup 1884 was added to the dope in a qUanlity such that the concentration of the cationic char~e modifyin~ resin was about laO based on the weight of the nylon. Cast membranes were then prepared using the pro-cedure described in Example I.A. A portion of the resulting wet membrane was dried in restrained condition as a single layer in an oven at 85C. for 1~ minutes. The resultin~
nominal 0.22 micrometer rated membrane had a thickness of 4.1 mils. Another portion of the ~vet membrane was folded back onto i~self and dried under restrained conditions in the 85C. oven for 60 minutes.- The resulting membrane was 7.8 mils thick. Prior to dryin~, the wet membrane had a thickness of about 6.1-6.4 mils. The nominal pore size of the membr~ne was 0.3 micron.

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13~ 373l~

- EXAI~PLE VI I I
Followin~ ~he procedure of Example III, pads were ~roduced usin~ ~ther surface modifyin~ agents. The agent, blend ratio, num~er of ~rams felted and electrokinetic status Or the pads are shown in the following table:

Fiber to GramsSlope Apparent A~ent COH0 Ratio Felted Mv/Ft H20Interce~t Zeta Pot Alon 0.53 1.632.9 0.69 - 1.60 .4sbestosl 0.83 2.55.1 -32.90 - 0,25 Asbestos2 1.00 4.414.2 32.30 - 0.69 Asbestos3 1.00 3.625.1 -35.00 - 1,22 Asbestos4 0.97 2.917.4 -33.50 - 0.84 Casein 1.00 3.220.1 59.00 - 0.97 Silica 1.00 3.025.0 20.66 - 1.21 - Cabosil q~ 1.00 8.032.2 -53.50 - 1.56 Se~hadex (G-75)~1.00 5.56.3 -67.24 - 0.30 Bentonite 1.00 5.446.3 64.30 - 2.25 Diat~ ceous Ear~h D.E. 215 1.00 5.0 27.9 26.65 - 1.35 ~aolin 1.00 6.062.4 -70.~0 - 3.02 Na-Al,sinate 1.00 5.;: 18.5 -15.82 - 0.89 Aluminum 1.00 7.8-181.8 - 5.49 + 8.82 Carbon l.00 4.457.5 -10.14 - 2.78 Car~on/18~4 Resin 1.00 5.1 0.3 32.00 - 0.01 L)~215/1884 Resin 1.00 5.6 7.6 -26.30 - 0.37 Aluminum (1~) 1.00 3.6- 6.2 -50.99 1 0.30 1884/5A ,~lolecular Sieve 0.67 2.0-29.2 -30.70 + 1.42 arium Ferrite1.00 8.653.2 -89.50 - 2.58 EL~rA l.OO 5.519.6 - 4.34 - 0.95 Loaine (Tinc~ ure) 1.00 4.3 33.1 50.03 - 1.60 ~: :

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131 373~

1: Arizona - not acid washed 2: Canadian - not acid washed 3: ~rizoua - acid washed 4: Canadian - acid washed EXAMPLE IX
Followin~ the procedure of Example III, fibers were pre~ared from a 60 ml dope solution containing 4.8g nylon with and without lOg powdered activated carbon. The fibers were exposed for 16 hours to 150 ml of distilled water which had been chlorinated to 4~0 ppm chlorine. The chlorine con-tent of the water was then determined to be 360 ppm for the water treated with the non-carbon containing fibers and 0.4 ppm for the water treated with the carbon containing fibers.
EXAMPLE X
Anionically charged microporous membranes are prepared by repeatin~ the procedure of Example I.A. and incorporating the followin~ into the dope composition:
4~ polystyrene sulfonic acid and 2.7~ ethylene glycol di~lycidal ether;
1.3~ polyacrylic acid;
0.88Z polyacrylic acid and 0.l2Z polyo~yethylene-polyoxypropylene glycol;
3.6Z polyacrylic acid (mw 104,000) and 1.3Z he~a-methoxy methylmelamine resin.
EXAMPLE XI
Into a polymer dope solution containin~ about 8Z
nylon 66, was suspended activated carbon (6?~Z of total solids).
The suspen~ion was allowed to flow by gravity into a 75Z/25Z
by volume waler/methanol non-solvent through a small orifice.
The resultin~ fibrils were harvested, washed and then tested for chlorine and ~henol removal from water. In both cases, : .

1~37~ ~

the ca~acity o~ the fibrils was about 90_95a of the particu-l~te carbon per se and at equivalent mass transfer rates.
The fibrils did not manifest the same degree of problems encountered with finely powdered carbon which has very poor hydrodynamic characteristics, is difficult to retain and tends to mi~rate.
EXAMPLE XII
Following the procedure of Example I, unmodified and post-formation modified microporous membranes were prepared.
The post-formation modified microporous membranes were made by dippin~ one of the unmodified membranes into a 2 wZ solu-tion of ~ercules Polycup 172 resin (0.24~ solids). The same dope was modified by the addition of 7 w~ of the Polycup 172 resin (0.84~ solids) and duplicate microporous membranes prepared. When removed from the quench bath, the membranes were air dried and then dried in a forced air oven at 40C.
for 1~ hours. The five membrane~ were analyzed for integrity by determirlin~ bubble point, FOAP and then challenged with ~etanil Yellow dye. The results are shown in the following table:
Bubble PSI Dye Ret. Time `lembrane Poirt FOAP Initial Final (min.) Unmodified 40 46 3.0 3.6 7 3~ 44 1.9 2.9 7 A,lodified -Post Treatment 44 SO 3.9 5.0 ~4 .!~lodified Dope 31 52 3.9 12.0 77 31 53 4.3 14.0 73 Various changes and modifications can be made in the process of the present invention witnout departing from ~ne s~irit and scope thereof. The various embodiments which have been described herein were for the purpose of further illustrating the invention but were not intended to limit it.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A felted pad comprising fibers of cellulose and fibers of an organic microporous membrane having a surface modifying agent bonded to substantially all of the wetted surfaces thereof.

2. The felted pad of claim 1 , wherein said micro-porous membrane is a nylon membrane.

3. The felted pad of claim 2, wherein the surface-modifying agent is a charge modifying agent.

4. The felted pad of claim 3, wherein the charge modifying agent is a water-soluble cationic surface modifying agent.

5. The felted pad of claim 3, wherein the charge modifying agent is a water-soluble anionic surface modifying agent.

6. The felted pad of claim 2, wherein the charge modifying agent is a water suspendable surface modifying agent.