CA1200158A - Surface modified polyamide membrane - Google Patents

Abstract

Abstract:

Surface modified, skinless, hydrophilic, micro-porous, polyamide membranes are formed by preparing a casting solution comprised of (A) a casting resin system comprised of (a) an alcohol-insoluble poly-amide resin, and (b) a cationic, water-soluble, qua-ternary ammonium, thermosetting, membrane surface modifying polymer, and (B) a solvent system in which the casting resin system is soluble; inducing nuclea-tion of the casting solution by controlled addition of a nonsolvent for the casting resin system under controlled conditions to obtain a visible precipitate of casting resin system particles, thereby forming a casting composition; spreading the casting composi-tion on a substrate to form a thin film; contacting and diluting the film of the casting composition with a liquid nonsolvent system for the casting resin system, thereby precipitating the casting resin sys-tem from the casting composition in the form of a thin, skinless, hydrophilic, surface modified, micro-porous, polyamide membrane; and washing and drying the membrane. The membranes of this invention are characterized by having fine pore ratings, the sur-face properties thereof being substantially controlled by cationic, quaternary ammonium groups of a modifying polymer thereby providing a positive zeta potential in alkaline media, and for those with moderate or low levels of surface modifying polymer present, a time to reach an effluent resistivity of 14 megaohms/cm under the Resistivity Test of 10 minutes or less.
They have greatly enhanced filtration efficiency over a broad pH range with a variety of contaminants, including very fine negatively charged particles, bacteria and endotoxins.

Description

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SURFACE ~IODIFIED POLYAMIDE MEMBRAN~

The pres~nt invention relates to microporous membranes, 5 their preparation and their use. Microporous membranes have been recognized for some time as useful for filtering fine particles from gas and liquid media. United States Patent Speci~ication No. 4,340,479, discloses a process for manufacturing microporous polyamide membranes with cert-10 ain desirable filtration characteristics. ~lembranes prepar-ed by this process are hydrophilic, have narrow pore size distributions and pore ratings as fine as about 0.04 micrometer. For many filtering requirements those membranes perform very effectively. For certain fine particulates, 15 e.~.,substantially below 0.1 micrometer in diameter, they are not effective. The reasons for this are rela~ed to the mechanisms by which filters work.

The function of a filter is the removal of suspended parti-20 culate material and the passage of the clarified fluid medium. A ilter membrane can achieve fluid clarification by different mechanisms. Particulate material can be rem-oved through mechanical sieving wherein all particles larger than the pore diameter of the filter membrane are 25 removed from the fluid. With this mechanism, filtration efficiency is controlled by the relative size of the cont-aminant and filter pore diameter and ~he efficient removal of very small particles, e.g., less than 0.1 micrometer, ~ in diameter, therefore requires filter membranes with very 30 ~mall pore sizes.

Such fine pore filter membranes tend to have the undesir-able characteristics of high pressure drop across the filter membrane, reduced dirt capacity and shortened filter life.

A filter may also remove suspended part-culate n.aterial by absorption onto the filter membrane surfaces. Removal of par.iculate material by this mechanism is controlled by the surface characteristics of (1) the suspended particulate material and (2) the filter membrane. Most suspended solids which are commonly subjected to removal by filtration are negatively charged in aqueous s~stems.
This feature has ].ong been recognized in water treatme~nt processes where cationic flocculating agents, Gpposltely charged to the suspended matter, are employed to improve settling efficiencies during ~ater clarification.

Colloid stability theor~ can be used to predict the interactions of electrostatically charged particles ànd surfaces. If the charges of suspended particle and the filter membrane surface are of like sign and witll zeta potentials of greater than about 2~nV~, mutual. repulsive forces will be suf~iciently strong to prevent capture by a~sorption. If the zeta potentials of the suspended particle and the filter mem~rane surface are small, or more desirably of opposite sign, particles will tend to adhere to the filter membrane surfaces with high capture efficiencies. Most suspensions of particles ~.,;Y
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encoun~ered in industrial practice llave a nega~ive zeta potential. Thus microporous filter membranes characterized by positive zeta potentials are capable in a large number of industrial applications of removing particles much 5 smaller than the pore diameters of the membrane through the mechanism of electrostatic capture.

The desirable hydrophilic properties of the polyamide membranes of U S. patent specification 4 340 479 are 10 believed to result in part from the high concelltration on the exposed membrane surfaces of amine and carboxylic acid end groups of ~he polyamide. The positioning of these gro-ups is also believed to provide these membranes with their unusual zeta potential versus pll profile. Tha~ ofile 15 positive at pHs below 6.5 becomes regati~e in alkallne media. Accordingl~ these nlembranes ha e limited ability to filter ver~ fine negatively charged partic~lates in alkaline media.

20 By modif~ing the surface characteristics of the h~drophilic membranes disclosed in U. S. patent specificati`on 4 340 479to for instance provide a strongly positive zeta potential over the alkaline range the spectrum of uses for these materials in filtration is substantially exparcled.

The present invention is directed then to ~he preparation and use of surface modified microporous h)~drophilic pol~amide membranes. The process of this invention provides s~

m~croporous membranes with narrow pore size distributions and fine pore ratings ranging :Erom 0.04 to 10 micrometers, preferably 0.1 to 5 micrometers, and filtration efficiencies ranging ~rom molecular dimensions (pyrogens) to particulates larger than the pore diameters.
The surface modified membranes of this invention with their strongly positive zeta potentials are use~ul for their greatly enhanced filtration efficiency over a broad pH range and with a wide variety of contaminants including ]o particulates, particularly very ~ine negatively charged particles, bacteria and endotoxins. ~;embranes o~ the present invention also are capable o~ delivering ultrapul^e effluent water rapidly after the onset of filtration. T~le ability to deliver such high purity effluent water, .~ree 15 from microparticulate and ionic contaminants, makes the products of this invention particularly desirable for the filtration of aqueous fluids employed in microelectronics manu~acture.

20 According to the present invention there is provided.a process for preparing a polyamide membrane that is readily wetted by water wllich process comprises the steps of:
(1) preparing a casting solution including (A) a 25 casting resin system comprised of (a) an alcohol-insoluble polyamide resin h~ving a ratio (CH2 :NHC0) of methylene - C~l2 to amide NHC0 groups within the range of Erom 5:1 to - 7:1, and (b) a cationic , water-solubi.e, quaternary ammonium, thermosetting, membrane-surface-modifying ~L20~

polymer and (B) a solvent system in WhiCIl said castin~ resin system is soluble;

(2) inducing nucleation of said castin~ solution by controlled addition of a non-solvent for said casting 5 resin system under controlled conditions of concentration, temperature, addition rate and degree of agitation to obtain a visible precipitate of casting resin system particles, thereby forming a casting composition;

(3) spreading said casting composition on a subs-10 tr~te to form a film thereof on the substrate;

(4) contacting and diluting the film oE saidcasting composition ~ith a liquid non-solvent system for said casting resin system comprised of a mixture o~ solvent alld non-solvent liquids, thereby precipltating on to the 15 s-lbstrate said casting resin system from said casting composition in the form of a thin, skinless, hydrophilic, surface-modified, microporous, polyamide membrane;

(5) ~ashing said membrane to remove the solvent;
and

(6) drying said membrane.

The surface modi~ied, alcohol-insoluble polyamide nlembralles in accordance with this invention have the unusual property of being hydrophilic, i.e., they are readily wetted by 25 water, have pore sizes (also referred to as pore ratings or pore diameters) of from 0.04 to 10 (prefer-ably 0.1 to 5 micrometers) or more, modified zeta potentials, i.e., strongly positive zeta potentials in ,.,~

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all~aline media~ filtration efficiencies ranging from molecular dimensions (pyro~ens) to particula~es larger than the pore diameters and, accordingly, are highly desirable as fil~er media, particularly for producing bacterially sterile filtrates, as well as for filtration of high purity water in microelectronics manuacture due to their ability to deliver ultrapure filtrate free from microparticulates and ionic contaminants.

T7-e membrane surface modifying polymers or resins useful in preparing the membranes in accordance with ~l~is invention are the cationic, water-soluble, q~la~elnar~
an~nonium, thermosetting polymers, Preerred pol!mers within this class are the epoxy-functional pol~amlde/
polyamido-epichlorohydrin resins The epox!-lut~ctional polyanine epichlorol-ydrin resins are partic~larly preferr-ed.

The sole figure is a graph of percent added surface modifying polymer versus time required to ob~ain 14 megaohms/cm effluent water from a surface modified membrane in accordance ~7ith this invention.

The subject invention is directed to surface modified, hydrophilic, microporous, polyamide membranes and a process for preparing them by th~ steps of (1) preparing ~ a casting solution comprised of (A) a resin casting system - comprised of ~a) an alcohol-insoluble polyamide resin having a ratio of CH2 :NHC0 of metllylene CH2 to amide ~CO groups within the range of from 5:1 to .

7:1 and (b) a membrane s~rface modifying polymer;
and (B) a solven~ system in which the casting resin system is soluble; (2) inducing nucleation of ~he casting solution by controlled addition of a non-solvent for the casting resin system under controlled conditions of concentration temperature addition rate and degree .of agi~ation to obtain a visible precipitate of casting resin s~stem particles which may or may not thereafter partially or completely redissolve thereby Iorming a casting composition; (3) preferabl~ filterirg tle casting c~mposition to remove visible precipitated particles;
(4) spreading the casting composition on a subs~rate to for.n a thin film tllereof on the subs~rate; (5) contactin~
and diluting the film of casting composition with a liquid non-solvent system conprised of a mixture of solvent and non-solvent liquid and containing 2 substantial prop-ortion of the solvent liquid but less than the proportion in the casting solution thereby precipitating tile casting resin system from the casting solution in the form of a thin skinle$s hydrophilic surace modifiecl microporous membrane; (6) washing the membrane to remove solvent; and (7) drying the membrane The polyam.de ~em~ranes of U. S. Fatent Specificatlcn 4 340 479 are prepared from alcohol-insoluble polyamide resins having a n-.ethylene to amide ratio in the range of 5:1 to : . 7:1 as are the surface modified membranes in accordance with this invention~ Membranes of this group include copolymers of hexa-methylene diamine and adipic acid (nylon 66) copolymers of hexamethylene . . ..... ~ . ... ..... , , , . , , _ ~Z~ 5~3 diamine and sebacic acid (nylon G10) homopolymers of poly-e-caprolactam (nylon 6) and copolymers of he~amethylene diamine and a~elaic acid (nylon 69). Nylon 66 is preferred.

5 In the process for manufacturing the membranes of U. S.
Patent specification 4,340,479, the polyamide resin is dissolved in a solvent, such as formic acid, and a non-solv-ent, such as water, is added under controlled conditions of agitation to achieve nucleation of the solution.
In inducing nucleation of the polyamide solution a visible precipitate is formed. This precipitate may partially or completely redissolve. Preferably, any visi.b].e particles which do not redissolve should be filtered out o the sys-tem, e.g., with a 10 micrometer filter, prior to casting the nucleated solution or casting composition.

The nucleated solution or casting com~osition is tllen cast onto a substrate, e.g., a porous polyester sheet of web 20 or a non-porous polyester sheet, in the form of a film and this film solution is then contacted with and diluted by a liquid non-solvent s,~stem which is a rllixture of a solvenL and a non-solvent for the polyamlide resin. A
preferred non-solvent liquid system for both the subject 25 in~ention and that o.~ U.S. Patent specification L~,3~0,L~79 is a solution of water and fonnic acid. For this invention, ..:
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the formic acid is preferabl~ present in an amount of from about 35% to about 60% by weight~ The polyamide resin thereupon precipitates from the solution, formin~
a hydrophilic membrane sheet on the substrate which can 5 be washed to remove the solvent. The membrane can then be s~ripped from the substrate and dried or, iE the substrate is porous, it can be incorporated in the membrane ~o serve as a permanent support, in which event it is dried with the membrane. I the subsTtrate is to be in-10 corporated into the membrane, it should be porous andcapable of being wetted and im~re~nated by the casting composition, e.~., a porous, fibrous, ~olyester sheet with an open structure. By appropriate control oE
process variables, membranes with thro~h poles of uniorm 15 si~e aTld sllape can be obtained. Con~ersel~y, i.f (~esired, tapered through pores, wider at one surace of the sheet and narrowing as they proceed toward the opposite surface of the sheet, can be obtained.

20 The same ~eneral proced-~re described above is follo~ed in manufacturin~ the surface modified Tnembranes in accordance ~ith this invention except that tlle membrane surace modif~in~ pol~rmers used in thc subject invention are combined with the polyamide resin,and the resulting 25combined modifying polymer/polyamide casting solution, after nucleation to form the casting composition, is cocast resultin~ in unique membranes with novel filtration properties extending the range of uses for microporous polyamide T~ranes.

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,, The novel properties of the filter membranes prepared by the process of U. S. Patent specification 4,340,479 are 5 believed to result in part from the high concentration on the membrane surfacesof amine and carboxylic acid end groups of the polyamide. These amine and carboxylic acid functions on the membrane surfaces result in unexpect-ed membrane properties, such as their unusual zeta potent-ial versus pH profile and their hydrophilic character, thatis beingreadily wetted by water, typically being wetted throu~h in 3 seconds or less, preferably 1 second or less, ~hen i~-nersed in water.

As previously stated, it has now been discovered that the surface modified membranes in accordance with this invention having unexpected and novel filtration properties can be prepared using the general procedure disclosed in U. S.
Patent specification 4,340,479 but with the addition of low levels of se]ected membrane surface modifying polymers to the polyamide membrane casting solutions. ~lus, surfac-e-modified, hydrophilic, microporous polyamide membranes with a stron`gly positive zeta potential in allialine media, having low levels of extractable matter, and havin~ the ability to deliver ultrapure water, free from micropart-iculate and ionic contaminants, quickly after the onset of filtration as required in microelectronics manufacture, are readily and economically prepared by the cocasting process in accordance with the ~resent invention.

Addition of as little as one weight percent, based on the polyan,ide resin, of the membrane-surface-n)odifying pol~er to the membrane casting solution has been found to producemicroporous hydrophilic membranes wl-ose surface 5 properties are substantially controlled by the modifyin~
polymer. It is the ability of relatively small amounts of the membrane surface modifying polymer to control the surface pro~erties of membranes in accordance with the present invention which provides the desir~ble character-10 istics of the sub~ect membranes. Accordingly, thefiltra~ior, cha-acteris ic and th_ physiochemical surface behaviour of these membranes are controlled b~ a surprisin~-ly low proi-ortion of the modifying polymer.

15 The (1~ sul-face modified, pol~amide membranes prepared using the cocasting process share certain characteristics with both (2) the base polyamidc membranes prepared using the same general casting process but without the modifying pol~mer present in the casting solution and (3) base 20 polvamide membranes prepared in the SaTne m~cnner as the membranes of (2) above but which have subsesuently been coated ~ith a cationic polymer or resin by contacting the finished base polyamide membrane with a solutioII of the cationic polymer or resin.
~5 A11 three types are skinless, hydrophilic (as herei.nbefol-e delil,ed all~ in U. S. Patent specification 4,340,479) and microporous. Each has desirable filtering character-istics for fine particulates derived in part from their 30 fine pore ratings and narrow pore size distributions. Un-~ like the base polyamide membranes of (2) above, llowever, . :~

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che surace modi.fied polyamide membranes of (1) above and the coated polyamide membranes of (3) above e~hibit a positive zeta potential in alkaline media which broadens the spectrum of uses of the membranes of (1) and (3).

Surface rnodified polyamide membranes in accordance with this invention and base polyamide membranes (2) also possess the ability to deliver ultrapure water, free from microparticulate and ionic contarninants, quickly after 10 the onset of filtration. Conversely, the coated pol~amide membranes (3) do not~have this ability to as great an extent as either (1) or (2). This ability, as described in detail hereinafter, is highl~ desirable in microelectr-onics rr,anufacturing applications. Since only the sur.race 15 modified polyamide membranes (1) in accordance wiLh LhiS
invention have the unique combination of a posiLive ~eta potential in allcaline media and the ability to del,ver ultrapure water, free from microparticulate and ionic con~aminants, quicl<ly upon the onset of filtration, the desirability of this class of mernbrane is manirest.
The highly desirable properties of rr,embranes iII accord-ance with the invention are believed to resul~ from the unique method of preparation in which the mo~lif~ g poly-mer becomes an interal part of the overall s~ructure of the membrane. The abilit~ to prepare ~hese n~embranes in a clean, straightforward, effi.cient and economic manner heightens ~heir desirability.

The membrane-surface-modifying polymers or resins (some-times herein referred to as "modifying polymer(s)") useful ~ 13 in processes in accordance with thls invention are the cationic, water-soluble, quaternary ammonium, tllermosettillg polymers. The preferred modifying polymers are those polymers which undergo cross-linking reactions through 5 reaction of epoxide groups. Epoxide functional cationic polymers or resins generally produce charge-modified sur-faces which, upon proper conversion to the cross-linked state by the application of heat, are found to be mechan-ically strong and chemically resistant to a wide range 10 of aggressive chemical environments.

Epoxide functional or epoxide-based cationic thcrlnosetting polymers are also preferred due to favourable interactions believed to occur between the amine and carboxvlic acid 15 end groups of the polyamide. ~mine functions alld carbo~-vlic acid functions are Icnown to co-react efficiently with epoxide f-lnctional polymers. It is bèlieved that the amine and carboxylic acid groups of the polyamide resin react ~ith the epoxide groups of the modifying polymer. While 20 it is believed that the reaction of these groups occurs throu~hout the membrane structure, the nature of the memb-rane forming process is believed to cause preferential orientation of the modifyin~ polymer towarcls the surfaces of the formed meulbrane. By this is meant tl)at as a 25 resu].t of the cocasting process in accordance with the invention,the modifyirlg polymer determines tlle s~lrface characteristics of the membrane. ~urther, the reaction of the groups is believed to result in intimate bonding of the modifying polymer and the polyamide resin forming 30 an integl~lstructure thereby reducing the amour-t of extractable matter,increased homogeneity of the surfaces and increased general stability of the membranes.

Another desired characteristic of the modifying poly~ers relates to the nature of their cationic char~e. Since ,~
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their cationicity stems from the presence of guaternary a~monium groups, t11ey maintain their positive charge in acid, neutral and al1~aline conditions of pH.

Surprisingly, during t11e membrane formation process, the lo~.~ added levels of modifying polymers appear to be preferentially orientated in such a manner as to result in membranes whose surface characteristics are substantia-lly controlled by the 7nodifying polymer~ This result is believed to re~lect both the membrane forming process and the hydrophilic nature of the modifying polymers. The combinatior. of their llydrophilicity, their apparellt stron~
interaction ~ith t11e polyamide end groups and the cocast-ing process are believed responsible f~r ~he a~parerlt preferential orientation of tlle modifyillg polylrers to-wards the membrane surface.

Also, surprisingly, while the modifyin~ ~olymers are llighly soluble in water, the~ are not leached out of t11e casting composition into the non-solvent liquid, wnich is used to precipitate the casting resin system. Apparent:lY, the scrong inter-action of the modifying pol~rmer with the polva~ide end groups coupled with the preferential orient-ation of the modifying ~olymer towards the membrane surfaces (both pore and external), perhaps under the in-fluence of the non-solvent liquid, combine to provide a membrane whose surface properties are substantially contr-olled by the cationic, quaternary ammonium groups of the modifying polymer. The unexpected result is hig111y desir-able.

The epoxy-functional or epoxy-based resins preferred fall into two classes: polyamido/polyamino-epichlorohydrin resins and polyamine-epichlorohydrin resins. The former are reaction products of epichlorohydrin with polyamides containing primary, secondary and tertiary amines in the backbone. Representative materials of ~I-is class are described in United States Patent Specifications 2,926,154, 3,332,901, 3,224,986, and 3,855,15S.

Preferred commercially available water-soluble, non-colloidal cationic thermosetting polymers of the polya-midolpolyamino-epichlorohydrin class, are ~Tmene 557 (Registered Trade Mark) and the Polycup ~Regis~el^ed Trade ~iark~ series of resins manu~actured by ~lercules lncorpor-ated. I~ is believed that these resins are preral^ed byreacting epichlorohydrin with 10~7 molecular weig~lt polyamides which contain amino groups in the polymer backbone. The reaction products have been described as containing quaternary ammonium groups present in the form of aze~idinium ions which are four-membered ring structures. Kymene 557 and the mernbers o~ ~he Polycup series have been described as being chemically and structurally similar but differing in ~heir molecular weight.

Polyamine-epichlorohydrin resins are condensation products of polyamines such as polyalkylene polyamines . "

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or their precursors with epichloroh~drin. They differ from the polyamido/polyamino-epichlorohy~ ~ resins in that the polyamine-epichlorohydrins contain no amide linkages as part of tlle backbone of the polymer. Commer-5 cially available compositions of this type are dîsclosedin (1) ~l.S. Patent Specification 3,885,158, ~hich dis-closes polymers prepared by reacting epichlordlydrin ~ith the condensation product of pol~alkylene polyamines and ethylene dichloride and in (2) U.S. Patent Specificat-10 ion 3,700,623 which discloses polymers made byreacting epichloro~,drin with polydiallymethylamine.
Compositions of the irst type are exemplified b~ Santo-res 31 (Registered Trade ~lark) (Monsallto, Xnc) and compositions of the second type are exempli~ied hy 15 Resin R4308 (Hercules Inc.):

Especially preferred are the epoxide functional water soluble cationic polymers which fall in the class of polyamine-epichlorohydrin resins and which bear 20 quaternary groups in the cured state. The fact tllat quaternary groups remain in the resin in the cross-linked state is important, since it affects the pH range over which the membrane can maintain a positive zeta potential.
A quaternary ammonium group is inherently cationic;
25 hence its positive charge is independent of its pH
environment. Resin R4308 and Santo-res 31 each bear quaternary ammonium groups in the cured state and bear a positive charge at alkaline pH.

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~lany of the modifying polymers require activation For the purpose of providing extended shelf life or storage stability to these resins, the epoxide groups are chemically-inactivated to prevent premature cross-link-ing of these polymers. Thus, prior to the use of these pol~rmers the polymers are activated into the reactive, thermo-setting state by regeneration of the epo~ide groups. Typically, activation entails adding sufficient aqueous caustic to a solution of the inactive polymer to chemical~y convert the inactive chlorohydrin ~orm to the cross-linking epoxide form. The parts by ~i7eigllt of aqueous caustic per weight polymer var~ with the product and are specified by the manufacturer. Complete activation is generall7~r achieved in about thirty mir-utes.

The preparation of membranes in accordance with this invention is carried out under controlled conditions including addition o~ the non-solvent, e.g., ~7ater, to a solution of the polyamide and the modifying polymer, control of the concentration of the constituents, control of the temperature and control oE the agitation of the system to induce the proper level of nucleation.

The detailed discussion in U.S. Patent speci~ication ~1O. 4,340, 479 concerning the relationship of the parameters set out above is gen~-ally applicable herein and will not be repeated ';

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Tlle manner and rate of addition of the non-solvent to induce nu~leation is interrelated with other process variables, such as intensity oE mixing, temperature and the concentration of the various components of the casting solution ~ The term "casting solution" is ~ced here to mean the solution made up of (A) the cast~ g resin system and (B) the solvent system, Addition of the non-solvent is conveniently carried out through an orifice at a rate sufficient to produce a visible precipitate which, preferably, at least in part subsequently redissol-ves. Maintaining all other parameters collstcnt~, casting compositions with quite diEferent cllaracteristics in terms of pore sizes of the resulting membranes will be o~tained b~ varying the diameter of ~he orlfice, The required degree of nucleation resulting fron non-solvent addition rate and orifice configuration is tllere~ore best established by trial and error for each given system, The controlled addition of non-solvent is discussed in detail in ~l,S, Patent s?e~ifica~on ~o. 4, ~ 79.
Prior to addition of the non-solvent to induce nucleation, tlle casting solution is prepared comprised o (~) a casting resin system comprised o (a) an alcohol-insolu~le polya-mide resin as hereinbefore described and (b~ a modifying polymer or resin and (B) a solvent system, The solvent system may simply be a solvent for the casting resin system, e,g,, formic acid, Alternatively, the solvent system contains an amount of a non-solvent, e,g,, water.
The amount of non-solvent present in the casting solution - ~.

5~3 is always less than the amount necessary to affect the stability of the solution.

Prior to casting nucleation of the casting soluLion 5 is initiated by controlled agitation and the controlled addition of non-solvent liquid. The amount and rate of addition of non-solvent is controlled along with the intensity of mixing of agitation. The ad~-antage of inciuding non-solvent as part of the solvent system in 10 making up the casting solution is that better control of the addition of non-solvent can be maintained during the inducement of nucleation because sr.~.aller amounts of non-sol~7er.t are needed due to the non-solvent alread~ present in the casting solution. As a result better control of the addition rate can be rnaintained ancl a more uniform product of any desired pore size can be obtained.

The casting resin s~stem includes (a) an alcohol-insoluble pol~amide resin having a methylene to amide ratio of i-;orl 5:1 tG 7:1 and (b) a surface modifyillg polymer or resin. All parts and percentages are by weigllt unless otherwise stated.

The proportion of modifying polymer to polyamide resin in the casting solution forr.led as the first step in the process based on the polyamide resin~ can vary from as much as 20 weight percent to as little as 0.1 weight percent that is 20 parts of modifying p~l~er to ~00 pl.tS polya~.i de `"!

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resin rangil-g to 0.1 part of modi~ying polymer to 100 parts polyamide resin. The generally preferred range of added modifying polymer is from about 1 weight percent to 5 weight percent. Addition levels of 5 1 to 2.0 weight percent have been found particularly desirable. It is believed that this moderate le~el of modifying polymer produces substantially complete membrane surface modification resulting in a membrane ~hose surface characteristics are substantially controlled by the cation-10 ic, quaternary, am~onium groups of the modifying polymer.Thus, for the purpose of membrane efficiency and production economy, the addition of about 1 to 2.0 weight percent of the modifying pol~ner, based on the polyamide resill, is preferred with the polyamide resin present in the castillg 15 solution in an amount of from 10% to 187c and tlle surface modifying polymer present in an amount of IlOm O.].% to 0.9%, (based on all components present in the solution).

20 The amount of solvent present in the casting solution for-med as the first step in the process will var~ dependent upon the polyamide resin and the modifying polymer used.
In general, the amount of solvent present will rallge from ~iO to ~0 percent (based on all componen~s present 25 in the solution).

It should be understood that the casting solution comprises both (1) the casting resin system , i.e., the polyamide resin and the modifying polymer or resin and (2) the 30 solvent system, i.e., a solvent for the polyamide resin/

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_ 21 modifying pol~ner casting resin system tsuch as formic acid) and, if desired 9 a minor amount of a non-solvent for the casting resin system (such as water), The amount of non-solvent present in the casting solution will in all cases be less than the amount in the liquid non-solvent system (membrane forming bath) used to precipitate the casting resin system from the casting composition, the casting composition being the composition formed from the in;tially prepared casting solution by inducing nucleation in that solution and, preferably, removing visible particles fro~ the resulting composition.
Gen-rally, when the non-solvent is water, it will be present in tl~ casting solution in an amount ranging from zero, preferably at least 5 percent, preferably 10 to 20 percent, and up to about 30 percerlt by wei~ht (a~ain based on all the components present in the solution).

For a preferred casting solution, a polyamine epichloro-hydrin resin or polymer, preferably Resin R4308, is pre-sent in the casting solution in an amo-lnt o from 0.1 to about 0.9 percent, polyhexamethylene adipamide is present in an amount of from 10 to 18 percent, formic acid is present in an amount of from 65 to 75 percent, and water is present in an amount of 10 to 20 percent, ~11 parts by weight and based on the total composition of the castin~ solution '~LZ~3`~

The temperature of the casting composition is not critical sc long as it is maintained at a constant value. 6-enerally, ho~ever a decrea5e in casting composition temperature produces a higher degree of 5 nucleation.

The intensity of mixing in a given system is a function of a large number of interrelated variables. For any ~o given system, the mixing intensity can be expressed in terms of the rotation rate of the agitator. Such equipment has many forms and designs commonly used in the mixing art and is difficult to quantify. Thus, trial and error experimentation involving custom6ry variables is necessary to establish the operable range oE mix;ng intensities suitable for a particular s~stem. Typically, using a 6.35cm rotor operating at ~ throughput of abut 500 to 1500 grams of solution per minute requires mixing speeds in the range of from 1500 to 4000 RP~I to 20 produce membranes witll pore rating in ~he range oE
interest.

The liquid non-solvent system used to dilute the film of casting composition and thereby precipitate the 25 casting resin system, typically ~y immersion in a bath of the liquid non-solvent system, can, and preferably ~ does, contain a substan~ial amount of a solvent for the casting resin system, preferably the one present in the casting solution. That is, the liquid non-solvent ~0~15t~
_ 23 system is comprised of a mixture of a non-solvent for the casting resin system, e.g., water, and 3 solvent for the casting resin system, e.g., formic acid. However9 on a percentage basis the amount of solvent present in the liquid non-solvent system will be less than the amount present in the casting solution. Typically, the liquid non-solvent system will be comprised of a non-solvent, eOg., water, present in an amount ranging rom about 65 to about 40 weight percent and a solvent for tThe casting resin system, e.g., formic acid, present in an amount ranging from about 35 to about 60 weight percent. The formic acid present in the casting solution may amount to from 60 to 80% and the water present therein from O to 30% by weight. ~tore normally expressed 15 ~he formic acid range is from 65 to 7S% and the water range from 10 to 20%. Preferably,:the bath of the liquid non-solvent s~Tstem is maintained at a substantially con-stant composition with respect to non-solvent and solvent by the addition of non-sol~ent to the bath, preferably continuously, in a quantity sufficient to compensate for solvent diffusion into the bath from the thin film of casting composition.

The solvent comprising at least part of the solvent s~s-tem used in the casting solutions can be an~T solvent for the casting resin system, .i.e., the combination of the polyamide resin and the modifying polymer. A preferr-ed solvent is formic acid. Other suitable solvents are 3~zll~L5~

_ 24 other liquid aliphatic acids, such as acetic acid and propionic acid; phenols 9 such as phenol, the cresols and their halogenated derivatives; inorganic acids, such as hydrochloric, sulfuric and phosphoric; saturated aqueous or alcohol solutions of alcohol-soluble salts~
such cs calcium chloride, magnesium chloride and lithium chloride; and hydroxylic solvents, including halogenated alcohols.

The only criteria in selecting a solvent are that (1) it should form a solution of the polyamide resin and the modifying polymer, (2) it should not react chemically with either the polyamide resin or the surface modifying polymer and (3) it should be capable of ready removal from the surface modified polyamide membrane. Practical considerations also are important, of course. ~or e~ample, inorganic acids are more hazardous to work with than are others of the named solvents and corrosion problems must be dealt with. Since fO rmic acid meets the criteria listed above and is a practical material as well r it is the solvent of choice. Due to economy and ease of handling, water is the non-solvent of choice for use in the solvent system when a non-solvent is used in the solvent system. In like manner, the preferred non-solvent added to the castin~ soiution to induce nucleation thereof is water~ And, the preferred non-solvent component of the liquid non-solvent system used to precipitate the casting resin system from the film of casting composition is also water for the same s~

reasons it is the non-solvent of choice in the solvent syste~

The membrane products are characterized by being hydroph-5 ilic, skinless,-microporous and alcohol-insoluble, with narrow pore size distributions, filtration efficiencies from molecular dimensions (pyrogens) up to particulates larger than the pore diameters, pore size ratin~s of from Ø04 to 10 micrometers, a preferred 10 ran~e being from 0.1 to 5 micrometers, film thicknesses in the range of from 0.01 to 1.5 millimeters, preferably from 0.025 to 0.8 mm, and by having a positive zeta potential over a broad pH range of from 3 to 10. The pores may extend substantially 15 uniformly from surface to surface both in size and sha?e or they may be wider at one surface than the other and thus taper bet~een the surfaces. Additionally, the membr-anes can be characterized as having ~urface properties which are substantially controlled by cationic quaternar~
20 ammonium ~roups of the cationic, quaternary a~monium, thermoset, surface modifying polymer. Surprisingly, the low added levels of quaternary an~onium, surface modifying pol~ers produce membranes whose sur~ace characteristics substantially reflect the presence of the 25 cationic quaternary all~onium groups. Along with their excellent pore structure and positive zeta potential, these membranes have ~ery low levels of extractable matter making them especially desirable in pharmaceutical and electronic filtration applications. Additionally, these ~ 30 membranes can be conveniently and economically 12~

prepared by a straightforward, continuous process as described hereinafter.

The surface modified membranes with moderate or low levels 5 of modifying polymer have rinse up times to deliver eff-luent water having a resistivity of 14 megaohms/crn of 10 minutes or less, and preferably less than 5 minutes, when tested using the Resistivity Test described hereina-fter.
Surface modified membranes particularly those ;ith moderate or low levels of surface modifying pol~ners present, are `believed to have quick rinse up tilTIes because of the apparent interaction between the surace 15 r,odif,~ing pol~er and the polyamide resin end groups.
Tilis i.nteraction and the resulting integral nature of mem-branes in accordance with the invention is belie~ed to lead to a red~ction in the extractables available for sloughing off and being carried through the filter and into 20 the effluent, a phenomenon belie~!ed to occur with coated membranes.

~lethod of Testing tile Sur~ace ~odified ~lembranes of the Followin~ Examples:

The properties of the membranes of tlle following examples were evaluated by a variety of test methods, as described below:
(a) Zeta Potential.
The zeta potentials of membranes were calculated from measurements of the streaming potential generated by flow i' j ~2~

of a 0.001 weight percent solution o~ KCl in distilled water through several layers of the membrane secured in a filter sheet or membrane holder. Zeta poten~ial is a measure of the net immobile electric charge on a membrane surface exposed to a fluid. It is related to the streamin~ potential generated when the fluid flows through the membrane by the following formula (J T.
Da~is et al, Interfacial Phenomena, Academic Press, ~ew ~ork, 1963):
Zeta Potential (mV) = 4 ~ . Es D P
Where ~is the viscosity of the flowing solution, D
is its dielectric contsant, A is its conductivit~, Es is the streaming potential and P is the pressure drop across the membranes during the period o ~low. In the ~ollo~ing examples, the quantity 4~?r~ was constant, having a value 2.052 x 10-2 , oDr when converted to Kg /Sq. M., the constant must be multiplied b~ the conversion factor 703.1 so tha~ the zeta potential can b~ expressed:
Zeta Potential (mV)=
14.43 . E~ (Volt) .~ nho/cm) P (Kg/cm2 ) (b) Capacity of Membrane for Latex Adsorption:
A 47mm diameter disc of ~er~rane was placed in a filter holder with a filtration area of 9 29cm2 and ~ was then challenged with a suspension of 0.01 weight percent monodisperse latex spheres in a 0.1 weight percent Triton X-100 in water solution (Triton X-100 is , l ~2~

an adduct of nonyl phenol with about lO moles nf ethylene oxide). The latex suspension was pumped through the membrane using a Sage Instrument Model 341 syringe pump at a rate of 2 milliliters per minute. The latex effluent ~as detected by 90 degrees scattering of 537 n~ as measured by a Brice-Phoenix BP 2000 li~ht scattering photometer using a flow cell.

The latex solution was pumped through the memb~ane until the light scattering of the effluent began to differ from that observed for a solution of 0.1% Triton X-lO0 alone, indicating the start o~ penetration of la~ex particles through the membrane. The latex adsorption capacity tLAC) was calculated by the following relation:

LAC (mg/cm2) = 10 x V
929.0 where V is the number of milliliters of 0.01 weight percent latex suspension which could be passed through the membrane until passa~e of latex was observed.

(c) Bacterial Titer Reduction:
Membranes sterilized by the autoclaving were placed in a suitable stainless steel filter holder and the filter holder with membrane in place was exposed to ~ steam at 123 degrees C. for 30 minutes followed by a 6G minute exhaust period in a ~ernitron/BetterBuilt Century 21 Model 222 laboratory sterilizer. They were then challen~ed with bacteria at four levels: 106~ 1o 101, 1012 bacteria per square metre .for a total challenge of about 1012 bac~eria per square metre of ~2~S~

membrane ~U.OS. P. Bacterial Titer Reduction Test).

The effluent was collected under aseptic conditions in a sterile glass v~ssel. The number of bacteria 5 in the influent and the effluent was determined by making serial dilutions of these suspensions and plating them on a 0.22 micrometer analytical membrane. These membranes were cultured as Muller-Hinton agar at 38 degrees C. for 24 hours to grow colonies of Serratia Marcescens 10 and for 48 hours to grow colonies of Pseudo~,onas diminuta.

The colonies growing on the cultured membranes were counted and the number of colonies observed was assumed equivalent to the number of bacteria in the solution plated.
As in the case of latex particle adsorption titer reduction, TR. is defined as the ratio of influent bacterial count to effll!ent bacterial count:
0 TR = influent count effluent count.

d), Endotoxin Titer Reduction Test ~lethod:
A 47 D.m , diameter disc of test membrane was pre-wetted with isopropyl alcohol and placed in a depyrogenat-25 ed 47 m.u , disc holder of filtration area 0~0009~9m2 ,(0.01 ft2) 9 which had been previously depyrogenated by heating in an oven at 250 degrees C. for 1 hour. 50ml pyrogen-free I

L2~ Sl~

water was passed through the test membrane and the last 3~4ml were collected in pyr~en-free glassware and saved as a control for the system~ The membrane was then challenged with successiv~ 10 ml aliquots of E. Coli 055:B5 purified endotoxin at a flow rate of 5 ml/min/
9~'3 ~: and the effluent collec~ed and saved as above.
The first aliquot was at a concentrati~n of lng/ml~ each succecsive portion was 10 t;mes the concentration o~ the previous one up to a ~aximum of 100 mg/ml. The influent and e~fluent solutions were diluted w;th pyrogen-free ~ater as required and analyzed for the presence of endotoxin by the Limulus Amebocyte Lysate Test ~United States Pharmocopeia X~, 1980 page 888).

(e) Resis~ivity of Effluent Water:

Prel;minary Preparation:

Surface modified, microporous, hydrophilic polyamide membranes were converted to standard cartridge form by conventional procedures to form cartidges havin~ 0.~
sq. m. (7.5 sq. ft.) of ~iltering area. The cartridges were then flushed with o.2. molar ammonium hydroxide at 3.5 Kg/sq. cm. (50 psi) pressure across the cartridge for 6 minutes in order to convert the surface modifyin~
polymer to the hydroxide form. ~he cartrid~es were then flushed with 1.5 liters of deionized water to remove - residual I

~z~s~

3]

ammonium hydroxide and then dried for 12 hours at 79.4 C.
(175 F). The cartridges were then ready for testing for their ability to deliver high purity water effluent by the flowing Resistivity Test Method.
s Resistivity Test.

Water of near theoretical resistivity was generated by passing tap water through a ~lodel ~ 18090 deionizing bed (Penfie~lInc~) and through two Unibed iOl- e~change beds (Culligan Inc.) Test membranes in the form o standard cartridges were mounted in a cartridge housing of common design and subjected to a flow of a,pproximately 10. ~ liters per square metre membrane area per minute using the water from the deionizing system. The effluent water from the elements was monitored for resistivity with a Model 3418 conductivity cell (Yellow Springs Instrument Company), The conductivity cell was connected to a Model 31 conductivity bridge (Yellow Springs Instrument Com~)an)r) which allowed the direct measurement of effluent resistivity as a function of water flow time.
The time in minutes required to reach an effluctlt resis-tivity of 14 megaohms per centimeter, the generally accepted water quality limit by the electronics industry, was determined.

General Method I for the Preparation By Continuous Casting of the Membranes of the Followin~ Examples 1-12:

N,ylon 66 resin pellets we-^e cissolvec, in ~o.5~', or~ic acid ~!

~LZO~L5~

Sufficient activated Resin R4308, as a 5 weight percent solution in water, was added to bring the relative proportion of Resin R4308 to nylon 66 to the desired value. This homogeneous 5 casting solution comprised of (1) the casting resin sys-tem, i.e., the nylon 66 plu5 R4308, and ~2) the solvent system, i.e., formic acid plus water, was tested for its viscosity at 30~ C on a Rion Viscotester with a number 1 rotor (Model VT-04, available from Extech International 10 Corp., Boston, Mass. U.S.A.) operating at 63.8 r.p.m.
The viscosity ~as found to be about 6000 centipoise.
After viscosity testing, the castin~ solution ~as deli~er-ed by ~etering pump, at flow rates from 250 ~rams per minute to about 1500 grams per minute, into an in-line 15 mixer of conventional design having a 6.35 cm rotor whose mixing intensity was controlled over a range of speeds. Simultaneously a non-sol~7ent, water, was added, as indicated for each example, to the mixer b~- metered injeccion to produce the desired ratio of formic acid to 20 water and to induce nucleation of the casting solution and obtain a visible precipitate.

Upon exiting the mixer, the resulting casting composition was filtered through a 10 micrometer filter to remove 25 vicible resin particles and was then formed into ~ thin film on a moving, porous, fibrous, polyester, non wover.
27.3 cm wide, continuous web by a doctor blade with about ~ 0.02G cm spacing. Within less than 3 seconds the coated web was immersed into a membrane forming bath of 30 the liquid non-solvent system comprised of a mixture of formic acid and water, as specified for each example, for approximately 1 to 3 minutes. The bath concentration was maintained constant by the con~inuous addition`of water to the bath at the required rate to compensate for solvent diffusion I

~Z~3~ s~

into the bath from the film of the casting composition.

The nylon membrane so formed on the non-woven, porous polyester support was washed with water for from about 3 to about 6 minutes to remove residual formic acid.
Excess water was removed from the nylon membrane by passing it between tensioned rubber wiper blades and the membrane was wound into suitably sized rolls for storage or further processing. ~or filtration application or testing of nylon membrane in flat sheet form, the membrane sheet was mounted in a restraining frame to prevent shrinkage in any direction and the membrane was oven dried to 143.3 degrees C. for about 5 minutes. The membranes were also converted to filter cartridges by ~nown methods and subjected to testing or filtration application in cartridge form. All parts and percentages herein are by weight unless otherwise specified.

Example 1.
The continuous casting method, described under General Method I above, was employed to produce a surface modif-;ed, microporous, hydrophilic polyamide membrane. The casting solution was prepared by mixing about 549 parts of formic acid, 92.2 parts of a 5 weight percent activated Resin R4308 in water and 108.1 p~rts of nylon 66 resin pellets. The mixture was mechanically agitated until homogeneous.
I

~L2~ 5~3 The casting solution was pumped into an in-line mixer operating at 3600 RPM at the rate of 1000 grams per minute while water was injected into the ~ixer at the rate of 36.4 grams per minute. The resulting casting composition was then passed through a 10 micrometer filter to remove visible particles. The casting composition was maintained at a temparature of 48.5 degrees C. and was delivered onto the polyester web by means of a doctor blade with an approximately 0.020 cm spacing. The web was passed by the doctor blade at approximately 15,2 metres per minute and then into a bath containing 50.1 weight percent fol~ic acid and the balance water. The membrane was ~hen further processed as outlined in Method I, that is, it was washed and then dried in a restraining frame for 5 n~inutes at 140 degrees C. and was then ~eady for filter application or testing.

Example 2.

The method of Example 1 was repeated but with the casting solution made up of about 374.5 parts o formic acid, 53.0 parts of 5.82% activated solution of Resin R4308 in water~ and 72.5 parts of nylon 66 resin. The casting solution was pumped at a rate of 500 grams per minute into an in-line mixer op~rating at 2718 RPM
while water was injected into it at a ra~e of 24.9 grams per minute. The resulting casting composition was maintained at 54.0 degrees C. and was cast onto the ~2~: L51~

_ 35 polyester web moving at 10.1 metres per minute. The web was then immersed in a bath containing 55 percent formic acid, the balance water, and then further process-ed as described previously until ready for filter applica-tion or testing.

Example 3.

The method of Example 2 was repeated except that the operating speed of the in-line mixer was 2717 RPM, and that the water was injected into the mixer at a rate Of 17.9 grans per minute. The casting composition temperature was controlled at 54 7 degrees C. The coated web was passed into a bath containing 55.4 weight percent formic acid, balance water, at a rate of 10.1 metres per minute. ~he membrane was then further processed as previously described until ready for filter ~plication or testing.

Example 4.

A casting solution composed of about 73 2 weight percent formic acid, 12.3 weight water, 14.02 weight percent nylon 66 and 0.43 weight percent activated resin R4308 was prepared by hereinbefore described procedures. The casting solution was pumped into the in-line mixer, operating at 3600 RPM, at the rate of 1000 gramC per minute. Water was injected into the mixer ~2~

at the rate of 32.9 grams per minute and the resulting casting composition was maintained at a temperature of 47.6 degrees C. The polyester web was passed by the doctor blade with a 0.02Q cm spacing at the rate of 16. ~ metres per minute and into the bath containing 50.1 weight percent formic acid, balance water. The membrane was then further processed as hereinbe~ore described until ready for filter application or testing.

Example 5.

The membrane forming process of Example 4 was repeated except that the relative weight nylon 66 and Resin R4308 was adjusted to 2 weight percent Resin R4308 based on the nylon 66, i.e., there were 2 parts of Resin R4308 for 100 parts of nylon 66. The casting sol-ution was delivered to the mixer at the rate of 1000 grams per min~te and the resulting casting composition temperature wa~ maintained at 47.5 de~rees C. The web, moving at 16. 8 metres per minute, was immersed in a bath containing 49,6 weight percent formic acid, balance water. The membrane was then further processed as hereinbefore described until ready for testing or filter application.

Example 6.

The membrane forming process of Example 4 was repeated except that the ratio of Resin R4308 to nylon 66 in the ~%~

casting solution was adjusted to 1.5 weight percent, i.e., 1.5 parts Resin R4308 to 100 parts nylon 66. Water was introduced into the in-line mixer at the rate of 28.7 grams per minute and the casting composition temperature was maintained at 47.1 degrees C. The web, moving at 18. 3 metres per minute, was immersed into a bath contain-ing 50.1 weight percent formic acid and the balance water.
The membrane was then further processed as previously described until ready for testing or filter application Example 7 The membrane forming process of Example 4 ~as repeated except that the ratio of Resin R43L8 to nylon 66 was ad-15 justed to 1.25 weight percent in the casting solution, i.e., 1.25 parts Resin R4308 to 100 parts nylon 66.
Water was introduced into the mixer at 28.7 grams per minute and the casting composition temperature was maintained at 48.0 degrees C. The web, mo~ing at 16.5 20 metres per minute, was immersed into a bath contaîning so.2 weight percent formic acid, balance water, and then further processed as previously described until ready for testing or filter application.

25 Example 8.

The method of Example 1 was repeated, but with the casting solution made up of about 487 parts of formic acid, 58.9 parts of a 2 weight percent activated solution ~2~S~

_ 38 of Resin R4308 in water, 9.9 parts of water and 94,0 parts of nylon 66 resin. The casting solution was pum~ed at a rate of 500 grams per minute into an in-line mixer operatin~ at 2648 RPM while water was injected into it at a rate o~ 25.3 grams per minute. The resulting casting c~mposition was maintained at 53.7 degree C and cast onto the polyester web movin~ at 10. 4 metres per minute. The web was then immersed into a bath containing 54 percent formic acid, the balance 1~ water, and then further processed as previously described until ready for filter application or testing.

Example 9.

The method of Example 8 was repeated except that the speed of the in-line mixer was 2631 RPM and water was injected into the mixer at a rate of 12.0 grams per minute. The resulting casting composition was maintained at 55.9 degrees C. before casting onto the polyester web moving at 9. 1 metres per minute. The web ~as then i~ersed into a bath containing 54 percent ormic acid, the balance water, and then processed as herein-before described until ready for ilter application or testing.

Example 10.

The method of Example 8 was repeated except that the speed of the in-line mixer was 2719 RPM and water was 15~

injected into the mixer at a rate of 4.4. grams per minute. The resulting casting composition was maintained at 5~7 degrees C. and cast onto the polyester web moving at 6.7 metres per ninute. The web was immersed into a bath containing 54 percent formic acid, the balance water, and then further processed as hereinbeore described until ready for filter application or testing, Example 11.

The method of Example 8 was repeated except that the speed of the in-line was 2651 RPM and water was injected into the mixer at a rate of 6.5 grams per minute. The resulting casting composition ~as maintained at 57.4 degrees C and cast onto the polyester web moving at 7.9 metres per minute. The web was immersed into a bath containing 54 percent formic acid, the balance water and then fur~ er proc~ssed as hereinbefore described until ready for filter application or testingO

Example 12.

The method of Example 2 was repeated except that the speed of the in-line was 2719 RPM and water was injected into the mixer at a rate of 7.5 grams per minute, The resulting casting composition was maintained at a temp-erature of 57.3 degrees C. and cast onto the polyester 3(~

web moving at 6.7 metres per minute. The web was immers-ed into a bath containing 55 percent formic acid, the balance water, and was then further processed as described hereinbe~ore until ready for filter .Ipplication or testing.

The pore diameters of the membranes of Examples 1-12 and a Control membrane, prepared from nylon 66 without added membrane surface modifying polymer, were determined by ~L measurement as described in U. S. Patent ~pecification 4,~,479 with the results set out in Table 1 belo~. The zeta potentials and adsorption capacities of the membranes were also determined, by tests (a) and (b) above respectively, with the results set out in Table 1.

Adsorption Capacity Added Weight Percent Calculated Zeta Potential For 0.038 Micrometer Membrane of Resin R4308 Relative Pore Dlameter In Millivolts Latex In Milligrsms Example To Nylon 66 Resin Micrometers At pH = 7. 5 Per Square Metre~
4.1 0.1 +15 1033 2 4.2 0.2 +22 1141 3 4.1 0.45 +19 936.
4 3.0 0.1 +13 743 2.0 0.1 +13 ~36 6 1.5 0.1 +12 377 7 1.25 0.1 +13 56

8 1.25 0.2 +11 226.

9 1.25 . 0.8 +18 312 1.25 3 +12 86 ~l~
11 1.25 102 +16 24~
a2 4,2 1.2 +25 452.
Control None 0.1 -20 0 ~2t~a~15~3 The data in Table 1 above demonstrate that the present process of preparing surface modified, microporous, hydrophilic membranes produces membranes with positive zeta potentials in alkaline pH. Furthermore, the data 5 in the table demonstrate that membranes with widely differing physical pore diameters can be prepared by this process. Moreover 3 the listed adsorption capacities for 0.03~ latex spheres, particles whose diameter is much smaller than the pore size (pore diameter) of

10 these membranes, demonstrates the greatly enhanced particulate removal efficiencies of these membranes comp-ared to the control membrane, a microporous, hydrophilic nylon 66 membrane made b~ the process of U. S. Patent Specification number 4,340,4/9 Conseq~ently, the 15 membranes are superior to unmodified membranes in ultra-fine filtration applications.

The membrane of Example 12 was also tested for its ability to remove the bacterium Serratia marcescens from aqueous 20 suspension by the previously described Bacterial Titer Reduction Test Method (test (c~ above). For comparative purposes, an identical membrane, but prepared without ~he added membrane surface modifying polymer, was included in the test evaluation and is designated as 25 Control in Table II.

TABLE lI.

Serratia Marcescens Challen~e Membrane of Challenge Organisms Titer Example Per Square Metre Membrane Reduction 12 1ol2 5.5 x 106 Control lol2 ~.5 x 10 The results in the above table demonstrate the greatly enhanced (by nearly 100,000 fold) bacterial removal efficiency of filter membranes prepared by the addition of the modifying polymer when compared to a similar Control Membrane but without surface modification.

The membrane of Example 3 was tested for its ability to rem~ e E. Coli endotoxin from aqueous suspension by the previously desc~ibed Endotoxin Titer Reduction lest Method (test (d)). This endotoxin is believed to be of molecular dimensions and exis~ in rod forms of abou~
0.001 micrometers in diameter. For comparative purposes, a similar membrane prepared without the added membrane surface modifying polymer was included in the test evaluations and is listed as Control in Table III.

TABLE III.

25 Membrane E~ Coli Endotoxin Concentration, in of Nanograms Per Milliliter, Required Example For Positive Effluent.
3 100,000 Control ~ 2~

4~ -Surprisingly, the membranes in accordance with the F.resent invention show extremely large improvements in the removal efficiency of bacterial endotoxins when compared to unmodified mem~ranes. The presence of low levels of membrane surface modifying polymer produces about a 100900Q fold increase in the endotoxin removal efficiency of the membrane.

Unexpectedly, in addition to being able to remove unwanted materials of biOlogical activity, the membranes are able to decrease the adsorptive removal of certain desirable components of filterable pharmaceutical preparations, For example, membranes prepared by the method of Exa~ple 2 were tested for their ability to pass a solution of ben-zalkonium chloride, a commonly used preservative inpharmaceutical products, without undue reduction in the concentration of this substance. An aqueous solution of 0.004 percent by weight of ben~alkonium chlorlde was passed through two layers of 47mm diameter discs at a rate of 0.7 litres per minute per 92~ cm2 and the concentration of the preservative in the effluent relative to that of the influent was determined as a function of throughput volume. For the purpose of comparison~ a commerical nylon 66 membrane of the same pore size rating, designated here as Control, was similarly tested.

TABLE IV.

Throughput Required to Obtain Percent of Influent Membrane Pore Size ('oncentration (Liters 5 of Example (Micrometers) ~er 929 cm2) 90% 95%
2 0.2 1.5 3.2 Control 0.2 4 5 The data in Table IV above illustrates that the effluent of filter membranes reaches acceptable levels substant-ially before the effluent of the Control. This is of great benefit when filtering such pharmaceutical prepar-ations because there is less wasta~e of the required amount f preservative.

General Method II: Preparation By Batch Procedure of the Membranes of Examples 13 and 14:

In the following e~amples, polyamide membranes were prepared containing different membrane surface modifyin~
polymers using the following batch procedure. Membrane casting resin solutions were prepared by dissolving nylon 25 66 resin pellets of the same nylon 66 as used in Examples 1 - 12 ( or other polyamide as specified in the examples) in a solution of formic acid and the desired surface modifying poly~er. Dissolution took place with stirring LS~

at about 500 RPM in a jacketed resin kettle maintained at 30 degrees C. When dissolution was complete (usually within 3 hours), a non-solvent, water, was added to the solution in an amount sufficient to adjust the final concentration of materials to that given in each example.
The water was pumped in at a rate of about 2 ml/min through an orifice about 1 mm in diameter located under the surface of the solution at a point about lcm from the stirring blade. Stirring was ~,aintained at about 500 RPM during addition of the water to induce nucleation.

The casting composition was filtered through a 10 micro-meter filter, after which about 40 grams of the resulting casting composition ~as spread out onto a clean glass plate by means of an adjustable gap doctor blade. The film was then promptly immersed in a bath containing formic acid and ~ater in the amounts given in the ~xamples below.

The membranes were held immersed in the bath ~or several minutes and were then stripped from the glass plate.
The membranes were washed in water to remove residual formic acid and were oven-dried for 15 minutes at 96 degrees C. while restr~ined in a fr~me to prevent shrinkage. The ~lat membrane sheets were then used for filter applications or for testin~.

~o~

Example 13.

A membrane was prepared according to General Method II
with the surface modifying polymer being Polycup (register-ed Trade Mark) 1884, a polyamido/polyamino-epichlorohydrin as described above having (1) a specific gravity of 1.12 and (2) 3 viscosity of 325 centipoise as a 35% aqueous solut-ion. The casting solution contained about 74,2 weight percent forTnic acid, 10.0 percent water, 14.3 weight percent nylon 66 and 1.43 weight percent Polycup;
(registered trade mark) 1884~ The casting composition was spread as a film 0.038 cm: thick on a glass plate and irrlrnersed in a bath containing 54% by weight fol~ic acid, the balance being water, The membrane ~as then further processed as described above under General ~lethod II.

The membrane prepared was instantly wetted upon contact with water (less than 1 second) and had a pore size of about 1 micrometer, as determined by KL measurement.
The zeta potential of the mernbrane was found to be +2.8 mV at a pH of 8Ø

Example 14.

A membrane was prepared accordin~ to General Method II
with the polyamide resin being poly(hexamethylene azelearnide),(nylon 6,9), and the surface modi~ying L5~3 polymer being activated Resin R4308. The casting solution contained about 65 4. weight percent formic acid, 17.7 weight percent water, 16.0 weight percent nylon 6,9 resin and 0.8 weight percent R4308 resin. The casting composition was spread as a film 0.053 cms thick on a glass plate and was immersed in a bath containing 60 percent by weight formic acid, the balance being water The membrane was then further processed as described above under General Method ~T, The membrane of Example 14 was completely wetted immed-iately upon contact with water (less than 1 second) and had a pore size of ~ micrometers as determined by KL measurement. The membrane had a zeta potent;al of ~-3 m~l at a pH of 8Ø Thus, the membrane prepared by this method was microporous, hydrophilic and exhibited a positive zeta potential at alkaline pH.

The continuous membrane preparation procedure (General Method T described above) was used to prepare a number of membranes, each with a pore size of 0.1 micrometer and containing various levels of added Resin R430S. The preparation of ~hese membranes is described in Examples 1,4,5,6 and 7. The membranes were prepared under ident-ical conditions from casting solutions containing fromabout 4 weight percent added Res;n R4308 to as li~tle as about 1 weight percent. A similar membrane, but without added Resin R4308 was also prepared by the process of U. S. Patent Specification ~'P 4,340,479 as a comparative example and is designated as Control in the discussion below.

5 These membranes were tested for their zeta potential at pH = 7.5. All of the membranes prepared with added Resin R4308 (from 1.25 to 4.1 weight percent) had a strongly positive zeta potential. The Control membrane, prepared without added Resin R4308, had a strong negative lO zeta potential under the same measurement conditions.
Thus, even low levels added Resin R4308 were found to produce membranes with strong positive zeta potential and improved filtration efficiency toward negatively charged particulates in aqueous suspension.

The membranes of Examples 1,4, 5,6, and 7 were also converted into filter cartridges by methods known in the art. These Filter cartridges were then flushed with o.2. molar ammonium hydroxide at 3.5~ kg per square cm 20 pressure across the cartridges for a period of 6 minutes, followed by flushing with 1.5 liters of deionized water.
They were then dr;ed for 12 hours at 79.4 degrees C. The filter cartridges were then tested for their ability to deliver, within a short time onset of filtration, high ~5 purity effluent water of extremely low ionic content, a reguirement for the filtration of electronics grade water.
For comparative purposes, a filter cartridge containing the Control membrane, prepared under similar conditions but without the added surface modifying poly-(~'~ after) ~20~ 5~3 mer, was included in the test evaluation and is designed Control in Ta~le V. The times for the effluent of these filter cartridges to reach a resistivity of 14 megaohms, as measured by the Resistivity Test described, above, along with the zeta potentials and particulates adsorption capacities for the filter membranes are also listed in Table V.

TABLE V.

Adsorption Cspacity of Tlme in Mlnutes for Welght Percent 0.038 Micrometer Latex Effluent ~o Reach 14 Membrane of R4308 Added to Zeta Potential in Spheres in Milligrams Megaohms Per Centi-Example , Nylon 66 Resin Millivolts at pH=7 Per Square Metre Meter Resistivity 1 4.1 +15 1033 22 4 3.0 ~13 743 15 2,0 +13 936 7.5 ' 6 1.5 +12 377 2.
7 1.25 +13 560 2,5 Control None -20 0 2,5 C~

~2~

The results in the table show that the membranes have novel propert;es useful in electronics water filtra-tion when compared to prior art membranes. The present surface modified membranes have positive ~eta potentials in alkaline media, vastly improved removal efficiencies for ultrafine particulates and the ability to deliver purified effluent of extremely low ;onic content in rapid fashion after the onset of filtration.

The relationship between (1) the time interval, from the onset of filtration, required to produce the re~uired filtrate water resisitivity of 14 megaohms/cm ~rinse up time as designated in the ~esistivity Test described above~ and (2) the percent added Resin R 4308 is shown in the sole Figure; a plot of rinse up time versus percent added Resin R4308 for each of the above filter cartridges. The Figure illustrates that the rinse up time diminishes linearly with decreasing levels of added Resin R4308. The data in Table V show that a~
about one to one and one-half percent added Resins R4308 the resulting membrane has a strong positive ~eta potential and a rinse up time substantially identical to that of an unmodified membrane. This behavi~ r is highly desirable since this membrane delivers high resistivity water efficiently and yet provides enhanced filtration efficiency through electros~ic effectsO

5~
~ 53 ll'LE 15 A surfàce modified, microporous, hydrophilic polyamide membrane prepared according to the process of tl-e subject invention from the nylon 66 as used in Examl~les 1-12 and Resin R430S and having a pore rating of 2 micrometers was tested for its ability to remove haze and haze precursors from commercial cherry brandy comprised of 40 percent alcohol by volume. Prior to filtration, the brandy was chilled to about 0 de~rees C. at which tem~erature it had a distinct, turbid appearance, indicating the presence of an insoluble, dispersed phase of finely divided hazeO The test was carried out by passin~ the chilled cherry brandy tllrough a filter media comprised of two layers of the microporous membrane described above, That is, a disc, ~i7 millimcLers in diameter, and comprised of tt~o layers of the membrane described above was mounted in a membrane holding device and the chilled cherry brandy was then passed througll tl~is filter medium at a rate of 0.5 liters minute per 929cm (per square foot~ of membrane surface area.

~e initial pressure drop across the filter medium was 0.3 2 kg/sq.cm (4.5 psi). ~fter 5 hours onstream, the pressure drop had increased to 0.51 Itg~/s~,cm.(7.2 psi).
Over the 5 hours of filtration~ the filter effl~lent had a crystal clear appearance, without evidence o:E any haze, l'he total volume of cherry brandy filtered over the 5 hour period corresponded to 125 liters per 929 cm2 (per square foot) of filter medium. After filtration, the cherry brandy was allowed to warm up to ambient temper-ature. No haze developed. Further, even after recoolin~, the cherry brandy remained haze freeO

This Exa~ple demonstrates that a membrane of the subject _ ., .,.
.

:~2~

-- 5~!

invention is useful for treating alcoholic beverages to render them haze free and stable against haze forrnation.

Example 16 To further demonstrate the ability of the membranes in accordance with the subject invention to ~perate in a clean manner 2S required in certain filtrati~n applications, such as the manufacture of near theoretical resistivity water for electronics manufac~ure, a series of elements A-D, as described below, were tested for"extractables"
by the process described below, In this series of tests, corrugated filter elements of conventional design having an effective surface area of about 0~46 squ metre ~5 square feet) were prepared by conventional means rom three different microporous membranes. These elements, labelled A through D in Table VI below, were prepared from membranes which themselves were initially prepared by the processes indicated below:
Element A ~ hvdrophilic, microporous polyamide membrane with a pore rating of 0,2 microme~ers was prepared b!~ the general process described in 2~ UOS~ Patent specification ~i,340,479 from the sa~ne nylon 66 as used in Examples 1-12. The formed membrane was coated with Resin R4308 by impregnating the membrane with a 3 percent by weight solution of R4308 in water, the Resin R4308 having been a~ti-~ ,.~

`~

vated according to the manufacturer'~
recommendati~ns, ~llowing which the membrane was wiped t~ remove excess res;n and thereafter formed into the filter element denoted as element A
in Table YI below.

B ~he membrane of element B was pre-pared by the same process as des-cribed above with regard to the membrane of element A. This mem-brane also had a pore rating of 0.2 micrometers.

C This membrane was prepared by the cocasting process of the subject invent;on from (1) the same nylon 66 as ~he membranes of elements A and B
above and (2) Resin R430B. The resulting membrane also ha~ a pore rating of 0.2 micro~eters and was comprised o~ 98 percent polyamide and 2 percent by weight Resin R430B.

~5 D ~Control) This Control membrane was prepared by the process described in U. S.
~atent 4,340,479 from the same nylon 66 as elements A and B above. The resulting membrane alsc had a pore rating of 0.2 micrometers~ It did not contain a mr~ifying polymer as e~ther (l) an integral part of the ~ 6tructure (as ~id element C) or (23 a coating comp~nent of the membrane (as in the case o~ elements A and B~.

The filter elements A through D as described above were tested as follows:
Elements A and B were each (separately) subjected to a leaching step by passing 1.8962 litres (0.5 U.S.gallons) per minute per element of room temperature, deionized water through the respectiveelement for the time specified in Table VI below. This leaching step ~as carried out in an effort to remove as much soluble material from elements A
and B as possible. Neither element C nor element ~ was give the benefit of this treatment~

After the water leaching step carried out with elements A
and B, each of the elements was individuallv subjected to a room teDperature, deionized water flush at the pressure and for the time specified in Table VI below. Note that the flushing times and pressures resulted in a total flow of ~ater through each individual element of about 189 litres (~0 U.S.gallons) (elements A and B) and 227 litres (60 U~S. gallons) (elements C and D)~
After the deionized water flushing step, the elements were dried at 96. C (205 degrees F) for about 12 hours, autoclaved with steam at 121 degrees C. for about 1 hour and then extracted(again, each element separately) with deionized water, The extraction step was carried out by plugging the bottom of each element and then placing each element in its own bath of one and one-half litres of deionized water, following which each of the elements was reciprocated in an up and down manner (with the top of the filter element rising about ~ centimeters above the upper level of thebath on the up-stro~e) for 4 hours.

In each case, the water in the bath was then evaporated and the non-volatile residue remaining behind weighed to deter-a~ s~

mine the extractable material in each filter element~ The values are set out in Table VI below.

TA~LE VI
Deion- Deion- Extract- Zeta ized ~ater ized Water able Mat- Poten-Element Leach Flush erial (mg) ti~ (mv) A 30 ~in. 5 minD at 96 18-20 1.4 kg/sq.
cm (20 psi) B 60 min. 5 min, at 68 18-20 1.4 kg/sq~
cm (20 psi) C None 3 min, at 27 18-20 3.5 kg/sq, cm.(50 psi) D None 3 min. at ~5 -18 3~5 kg/sqO
cm.(50 psi) As can be seen from Table VI, elements C and D had substantially reduced extractables compared with the elements prepared from coated polyamide membranes (A and B)o This was the case e~en though ele~ents A and B were given a deioni~ed water leach (of 30 and 60 minutes respectively). As can also be seen from Table VI, element C, prepared by the cocast process in accordance with this invention, had a low level of extractables comparable to the Control element D. However, element C combines - 30 the desirable positive zeta potential at pH 7 (as well as at higher pH levels) with the low extractables of the Control D which has, for many purposes, the undesirable negative zeta potential at pH7(and at hi~her pH values as well)~

~3~ 5~3 5$

Industrial Applicability:

The surface modiEied membranes hereinbefore described h~ve been demonstrat~d to be superior in many important filtr-5 ation related properties to untreated prior art membranes.They are also superior in many respects to coated memb-ranes, e.g.~ in improved efficiency in utilization of the surface modi~ying polymer and in certain surface properties of the comparative end products. They car.
10 be used for filtering applications in their manufactured form, with or without the incorporation of the substrate upon which they are found. Two or more membranes can be combined or secured to each other to form mul~iple layer membrane filter sheets or they may be converted into filter elements by kno~m n-ethods and emplo)ed in filter cartridges, e.g., as filter elements in the form of a corrugated sheet supported within a conventional cartridge.

20 The membranes display positive zeta potentials over a broad pH range of fr~m about 3 to about 10 and show greatly enhanced removal efficlencies toward negatively charged particles in aqueous suspension. Furthermore, they have enhanced ef~iciency to remove bacteria and endotoxins from aqueous fluids. Moreover, the irnproved physical and chemical properties~coupled with their ability to quickly deliver high purity effl~ent water, free from microparticulate and ionic contamlnantsg makes them particularly desirable for use in microelectronics - s (' manufacture, These me~branes find use in industry and the medical field for treatment of wcter supplies for critical applications such as water for injection into humans, in microelectronics manufacture for the reasons discussed above, for the filtration of blood serum to help achieve sterility, for filtration of parental fluids, and generally for any use where c~ ion containing fluid must be filtered to a high degree of clarity~

Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a polyamide membrane that is readily wetted by water which process com-prises the steps of:
(1) preparing a casting solution including (A) a casting resin system comprised of (a) an alcohol-insoluble polyamide resin having a ratio (CH2:NHCO) of methylene CH2 to amide NHCO groups within the range of from 5:1 to 7:1, and (b) a cationic, water-soluble quaternary ammonium, thermosetting, membrane-surface-modifying polymer and (B) a solvent system in which said casting resin is soluble;
(2) inducing nucleation of said casting solution by controlled addition of a non-solvent for said cast-ing resin system under controlled conditions of concen-tration, temperature, addition rate and degree of agitation to obtain a visible precipitate of casting resin system particles, thereby forming a casting composition;
(3) spreading said casting composition on a substrate to form a film thereof on the substrate;
(4) contacting and diluting the film of said casting composition with a liquid non-solvent system for said casting resin system comprised of a mixture of solvent and non-solvent liquids, thereby precipitat-ing onto the substrate said casting resin system from said casting composition in the form of a thin, skin-less, hydrophilic, surface-modified, microporous, polyamide membrane;
(5) washing said membrane to remove the solvent;
and (6) drying said membrane.

2. A process according to claim 1, wherein said precipitated casting resin system particles are redissolved before spreading said casting composition on said substrate.

3. A process according to claim 1, wherein the precipitated casting resin system particles are filtered out before spreading said casting composition on said substrate.

4. A process according to any one of claims 1 to 3, wherein said polyamide resin in polyhexamethylene adipamide, poly-e-caprolactam, or polyhexamethylene sebacamide.

5. A process according to claim 1, 2 or 3, wherein said solvent system for said casting resin system comprises formic acid, and said non-solvent added to induce nucleation is water.

6. A process according to claim 1, 2 or 3, wherein said solvent system for said casting resin system comprises formic acid and water.

7. A process according to claim 1, 2 or 3, wherein said membrane-surface-modifying polymer is a polyamine epi-chlorohydrin polymer, or a polyamido/polyamido-epichlorohydrin polymer.

8. A process according to claim 1, wherein said casting composition is continuously spread onto said substrate, said film of said casting composition is continuously immersed in a bath of said liquid non-solvent system, and the bath is maintained at a substantially constant composition with respect to non-solvent and solvent by the addition of non-solvent to the bath in a quantity sufficient to com-pensate for solvent diffusion into the bath from said film of said casting composition.

9. A process according to claim 8 wherein said substrate is a porous web having an open structure which is wetted and impregnated by the casting com-position, thereby forming a membrane film having the porous web incorporated as a part thereof.

10. A surface-modified, skinless, hydrophilic, microporous, alcohol-insoluble polyamide membrane derived from an alcohol-insoluble, hydrophobic, poly-amide resin having a ratio (CH2:NHCO) of methylene CH2 to amide NHCO groups within the range of from 5:1 to 7:1, said membrane having (1) the surface properties thereof substantially controlled by cationic, quater-nary ammonium groups of a cationic, quaternary ammon-ium, thermoset, surface-modifying polymer, thereby providing a positive zeta potential in alkaline media, and (2) a time to reach an effluent resistivity of 14 megaohms/cm under the Resistivity Test (as herein-before defined) of 10 minutes or less.

11. A membrane according to claim 10 having through pores extending from surface to surface that are substantially uniform in shape and size.

12. A membrane according to claim 10 having through pores extending from surface to surface that are tapered, being wider at one surface of the sheet and narrowing as they proceed toward the opposite surface of the membrane.

13. A membrane according to claim 10, 11 or 12, wherein said polyamide resin is polyhexymethylene adipamide.

14. An assembly comprising membranes according to claim 10, 11 or 12, wherein two or more of said membranes are secured to each other and form a multiple layer membrane filter sheet.

15. An integral, surface-modified, skinless, hydro-philic, microporous, alcohol-insoluble polyamide membrane derived from about 80 to about 99.9% of an alcohol-insoluble hydrophobic polyamide resin having a ratio CH2:NHC0 of methy-lene CH2 to amide NHC0 groups within the range of from about 5:1 to about 7:1 and from about 20 to about 0.1% of a cationic, quaternary ammonium, thermoset, surface-modifying polymer, the surface properties of the membrane being substantially control-led by cationic, quaternary ammonium groups of said modifying polymer, thereby providing a positive zeta potential in alka-line media.