GB1561200A

GB1561200A - Permselective n-linked condensation polymer membrane

Info

Publication number: GB1561200A
Application number: GB43796/77A
Authority: GB
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1976-10-21
Filing date: 1977-10-20
Publication date: 1980-02-13
Also published as: FR2368289B1; DE2747408C2; FR2368289A1; CA1107424A; JPS5351184A; DE2747408A1; NL7711594A; JPS6127085B2; IT1143711B

Description

(54) PERMSELECTIVE N-LINKED CONDENSATION POLYMER MEMBRANE (71) We, E.I. DU PONT DE NEMOURS AND COMPANY, a Corporation organised and existing under the laws of the State of Delaware, United States of America of Wilmington, State of Delaware, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described and and by the following statement: This invention relates to improved membranes for the selective separation of components of aqueous compositions by reverse osmosis or ultrafiltration and to a process for preparing the membranes.

Permeation selective (i.e. permselective) membranes which preferentially pass certain components of liquid mixtures while retaining other components, have long been known, as has the principle of reverse osmosis, wherein a hydrostatic pressure in excess of the equilibrium osmotic pressure of a liquid mixture is applied to the mixture to force the more permeating components of the mixture, usually water, through the membrane in preference to the less permeating components, usually a salt, contrary to normal osmotic flow. Recent research in this field has been directed primarily toward the development of membranes for the reverse osmosis desalination of brackish and sea waters on a practical scale.

It is well known that complete separation of the more permeating from the less permeating components of liquid mixtures is never obtained with permselective membranes in practical use. All components of a mixture permeate to some degree through any membrane which has a practical permeation flux rate for the more permeating components.

A principal goal with such membranes has been the production of membranes with economically attractive optimum balances of high flux rates for the more permeating components and high rejection efficiencies for the less permeating salt components of liquid mixtures. Aromatic nitrogen-linked synthetic organic polymers; such as those described in Richter and Hoehn U.S. 3,567,632, have been found useful in this respect, and continuing effort has been made to improve upon balance between flux rate and salt rejection properties of such membranes. For example, salt rejection is improved by contacting the membranes with hydrolyzable tannins as disclosed in U.S. Patent Specification No.

3886066; or by contacting the membranes with a hydrous heavy metal composition as disclosed by Ganci U.S. 3,853,755; or by contacting the membranes with a selected ether as disclosed by Ganci, Jensen and Smith U.S. 3,808,303 (U.K. Patent Specification No.

1416457). As is seen, the effort has been directed toward enhancing the salt rejection properties by treatment of the membrane after it has been prepared.

In contrast, in the present invention, water flux rate, rather than salt rejection properties, has been improved over the rate in membranes not subjected to the treatment of this invention. In addition, in the present invention, water flux rate is enhanced, not by treatment of the membrane, but rather by treatment of the solution used to prepare the membrane.

This invention provides a solution for preparing a permselective membrane, which solution comprises a water-misicible organic polar aprotic solvent; a substantially linear, aromatic, synthetic, organic, nitrogen-linked, condensation polymer present in the solution in an amount of between 12% and 40% by weight based on the total weight of said solution; at least one salt which is soluble in the solvent and is present in an amount of between 10% and 100% by weight based on weight of polymer; and a surfactant present in an amount of between 100 ppm and 10,000 ppm based on weight of polymer. The surfactant has a molecular weight between 200 and 7000, and contains (a) at least one hydrophobic moiety having a molecular weight of between 100 and 400 which is a hydrocarbyl group that may be substituted with halogen (F, Cl, Br or I), -NO2 or -OH, and (b) at least one hydrophilic moiety. The surfactant can be i a nonionic surfactant, or ii an anionic surfactant, or iii an ampholytic surfactant of the formula

wherein Y is hydrocarbyl of between 6 and 20 carbon atoms which can be substituted with halogen, -NO2 or -OH.

The invention also provides a process for preparing a permselective membrane which process comprises (a) disposing the solution defined above into a desired shape for a permselective membrane,and (b) removing the organic polar aprotic solvent and salt from the solution in a manner and at a rate sufficient to solidify the membrane in the shape and disposed in step (a).

Still another aspect of this invention is a permselective membrane consisting essentially of a substantially linear, aromatic, synthetic, organic, nitrogen-linked, condensation polymer and a surfactant (defined above) distributed throughout the polymer in an amount of between 10 ppm and 10,000 ppm based on weight of polymer.

Permselective membranes desirably have high flux rates for one or more components of the mixture to be separated and high rejection efficiencies for one or more other components. The improved membranes of this invention have flux rates higher than similar membranes prepared without the surfactant present in the solution used to prepare the membranes. Particularly beneficial results can be obtained by combining the flux rate improvement of the present invention with the methods for improving the salt rejection that are described hereinbefore in respect of the prior art.

In the permselective membrane aspect of the invention, the membranes can be in any of several forms, such as thin coatings on porous substrates, thin films supported by porous substrates or thin-walled hollow fibers. The porous substrates in turn can be shaped as tubes supporting either internal or external membranes, as, for example, flat plates or corrugated sheets. Typical apparatus for employing the membranes in reverse osmosis separation applications are described in Richter and Hoehn U.S. 3,567,632, particularly in the apparatus depicted in Figures 1, 2 5 and 9 thereof.

As used herein, the term "permselective" means the ability preferentially to pass one or more components of a liquid mixture while simultaneously restraining permeation by one or more other components. The flux rates of permselective membranes are conveniently expressed in terms of the quantity of a component of a feed mixture which passes in a given time through a membrane of a given size under a specified pressure.

Solute rejection efficiencies of reverse osmosis membranes for water permeation are conveniently expressed in terms of the percentage of the salt in the water feed mixture which is passed by the membrane. The concentrations of salts in the feed mixture and in the liquid passing through the membrane may be determined conductometrically or by chemical analysis.

Preferably, the permselective membranes of this invention are employed where the solute to be preferentially rejected is a dissociated salt, such as sodium chloride, sodium sulfate, and calcium chloride, and the salt is preferentially rejected from an aqueous solution while water passes through the membrane counter to the normal osmotic direction of flow under the influence of a pressure greater than the osmotic pressure of the solution.

(1) The polymers The polymers employed herein are substantially linear aromatic synthetic organic nitrogen-linked condensation polymers having the general formula fL-R 1), in which (i) each -L- group, as it occurs along the polymer chain, is independently a linking grop.

(ii) each -R- group, as it occurs along the polymer chain, is independently an organic radical, (iii) the degree of polymerization is indicated by n, an integer sufficiently large to provide film-forming molecular weight. The terminal groups will depend on the L and R groups present.

By the term "independently" is meant that each -L- or -R- group may be the same as or different from each other -L- or -R- group along the same polymer backbone chain.

"Condensation polymers" contain a backbone chain composed of alternating -L- groups and -R- groups which is formed by a condensation polymerization reaction as contrasted to a free-radical polymerization reaction. Polymers are useful which have molecular weights of sufficient magnitude so that they are film-forming or fiber-forming and have a non-tacky surface at room temperature when dry. Polymers with an inherent viscosity above 0.6 are useful and polymers whose inherent viscosity is between 0.8 and 3.0 are preferred.

"Nitrogen-linked" polymers contain nitrogen atoms in the polymer chain as linking parts of at least 50 percent of the -L- groups. They can also contain other nitrogen atoms either as part of or attached to the -R- groups. Any remaining linking groups can be other functional groups formed by condensation reactions, such as ether and ester groups.

"Synthetic organic" polymers are "man-made" in the usual connotation and are composed substantially of carbon, hydrogen, oxygen, nitrogen, and sulfur. These polymers can also contain minor amounts of other atoms.

"Aromatic" polymers are polymers in which at least 50 percent of the -R- groups contain a 5-membered or 6-membered ring system subject to resonance bonding and which can contain hereto atoms such as oxygen and nitrogen.

"Substantially linear" polymers are substantially straight chain ones which exhibit the general solubility and melting properties characteristic of linear polymers as contrasted to highly cross-linked polymers but can contain minor amounts of cross-linked and chain branched structures.

The permselective membranes useful in accordance with this invention can be composed of polymers containing repeated tL-Rtn units of a single type or of polymers containing repeat units of two or more different types. Repeat units of different types may result from the presence of different -L- groups, from the presence of different -R- groups, or from both. When the polymers contain different -L- groups and different -R- groups, they can be in an ordered sequence or in a random sequence. The membranes can also be composed of compatible physical mixtures of polymers of any of the above-described types.

(a) Linking group L: The -L- groups in the general formula fL-Rtn are preferably independently chosen so that at least 50 percent of the -L- groups in each polymer backbone chain contain at least one of each of the structures

in any sequence such that no one structure of either of these types is adjacent to more than one other structure of the same type. It should be understood that the structures of the linking groups recited herein are given without regard to the direction in which the structures are read; that is, these linking groups can appear both as recited and as the reverse structure in a single polymer chain.

In one class of polymers useful in membranes of this invention, each "X" in the

structure can be indepenently oxygen or sulfur, and is preferably oxygen, and each "Z" in the

structures can be independently hydrogen, a one to four carbon alkyl, or phenyl, and preferably at least one-fourth of all the "Z's" are hydrogen. Typical examples of -L- groups of this class of polymers are

(the amide group)

(the acyl hydrazide group)

(the diacyl hydrazide group)

(the semicarbazide group), L can also be

in which the fourth valences of the carbonyl carbon atoms are linked vicinally to an aromatic ring in the polymer chain structure so that the complete unit forms an imide structure of the type

In preferred polymers of this type, two such units are combined in a structure of the type

in which E is a tetravalent aromatic radical which can be a monocarbocyclic, monoheterocyclic, fused carbocyclic, or fused heterocyclic radical or of the formula

in which p is zero or one and Y is a divalent radical such as -CO-, -O-, -S-, -SO2-, -NH-, and lower alkylene.

In another class of polymers useful in membranes of this invention, each

structure in the -L- groups can be a

group in which the third valence of the nitrogen atom is linked to an aromatic ring which is also separated from the

group in the polymer chain by an

structure linked to the aromatic ring vicinally to the

structure so that the complete unit forms a. benzimidazole structure of the type

In the preferred polymers of this type, two such units are combined in a structure of the type

in which Z and E are radicals as defined above.

Preferably L is

(amide).

(b) The organic group R ?he organic radical -R- groups in the general formula+L-Rin are preferably independently chosen so that at least 50 percent of the groups is each polymer backbone chain are aromatic radicals which can be monocarbocyclic, monoheterocyclic, fused carbocyclic, or fused heterocyclic or of the formula

in which p is zero or one and Y is a divalent group as defined above. These aromatic radicals can be unsubstituted or can have substituents which do not change the fundamental characteristics of the polymer. The most preferred substituents are the sulfonic acid group and the carboxyl group Any remaining -R- groups can be saturated aliphatic, carbocycloaliphatic or heterocycloaliphatic radicals with non-vicinal points of attachments or alkylene radicals having less than six carbon atoms between points of attachment.

Preferably the membranes useful as taught herein are made of polymers which contain two or more different phenylene -R- groups. A particularly preferred class of polymers are those in which 50 to 90 percent of the -R- groups are metaphenylene groups and 10 to 50 percent of the -R- groups are paraphenylene.

(c) "n" in the formula fL-Rin is the degree of polymerization and is an integer sufficiently large to provide a film-forming molecular weight.

The polymers useful herein can generally be prepared as described in Richter and Hoehn U.S. 3,567,632.

(d) Preferred Polymer Classes Preferred aromatic polyamides suitable for use in membranes of this invention include those of the recurring structural group H HO O rs n -N-Arl-N-C-Ar2-C in the polymer chain, where Arl and Ar2 are substituted or unsubstituted divalent aromatic radicals wherein the chain-extending bonds are oriented meta or para to each other and any substituents attached to the aromatic nucleus are not condensed with reactants during polymerization.

In an especially preferred class of polymers, substantially all the -L- groups are amide groups and the -R- groups are phenylene groups.

Polyimides preferred for use in membranes of this invention include those obtained by the action of heat and, optionally, of chemicals upon polyamide-acids as taught for example, by Koerner et al. in U.S. Pat. 3,022,200 and in the other patents and applications mentioned by Dinan in U.S. Pat. 3,575,936. Useful polyamide-acids include those of the AB type formed by self-condensation of an amino aromatic dicarboxylic acid anhydride or acid salt thereof as well as those of the AA-BB type formed by reaction of an aromatic tricarboxylic acid anhydride or acid halide thereof, or an aromatic tetracarboxylic acid dianhydride, with an organic diamine. The preferred polyamide-acids are characterized by the structure.

wherein < denotes isomerism, R is a tetravalent organic radical containing at least two carbon atoms, no more than two carbonyl groups of each polyamide-acid unit being attached to any one carbon atom of the tetravalent radical, and R' is a divalent radical containing at least two carbon atoms, the amide groups of adjacent polyamide-acid units each being attached to separate carbon atoms of the divalent radical. Either the R of the tetracarboxylic acid dianhydride or the R' of the organic diamine can be an aromatic radical, an aliphatic radical, or a combination or aromatic and aliphatic bridged radicals wherein the bridge is carbon, oxygen, nitrogen, sulfur, silicon, or phorphorus, and substituted groups thereof, so long as at least 50 percent of these radicals contain 5-membered and 6-membered ring systems subject to resonance bonding.

Aromatic polyhydrazides preferred for use in membranes of this invention embrace high molecular weight aromatic condensation polymers derived from hydrazine which are filmand fiber-forming. Preferably they are characterized by the recurring structural unit 0 0 -Ar-C-NH-NH-C in the polymer chain, where Ar is a divalent aromatic radical having at least three nuclear atoms between points of attachment, at least 35 mole percent of the aromatic radicals in any polyhdrazide being other than paraphenylene radicals. Polymers with this structure indue the condensation products of hydrazine or aromatic dihydrazides, e.g. a 50:50 weight ratio mixture of isophthalic dihydrazide and ethylene bis-4-benzoyl hydrazide, and a mixture of aromatic diacid chloride, e.g. a mixture of isophthaloyl chloride and terephthaloyl chloride.

Poly(amide-hydrazides) preferred for use in membranes of this invention include polymers containing both amide and hydrazide linking groups. Preferred polymers exhibiting this structure include those obtained by condensation of one or more diacid chlorides, for example a mixture of 50 to 90 percent by weight of isophthaloyl chloride and the balance terephthaloyl chloride, with a mixture of metaphenylenediamine with at least one dihydrazide, for example ethylene-l-(3-oxybenzoic)-2(4-oxybenzoic) dihydrazide. A particularly preferred polymer for use in the membranes in accordance with this invention is the polymer synthesized from a mixture of 80 mole percent of 3-aminobenzhydrazide and 20 mole percent of 4-aminobenzhydrazide and a mixture of 70 mole percent isophthaloyl chloride and 30 mole percent terephthaloyl chloride. The preparation of such polymers is described by Culbertson and Murphy in Polymer Letters, vol. 5, pages 807-812 (1967).

Aromatic polybenzimidazoles preferred for use in membranes in accordance with this invention are characterized in recurring structural units of the type

in which -R'- is a divalent radical as previously described and E is a tetravalent aromatic radical such as those of the types

in which p is zero or one and Y is a divalent radical as defined above. These aromatic polybenzimidazoles can be prepared, for example, by the condensation of one or more aromatic tetramines of the type.

with one or more dicarboxylic acid chlorides of the formula OO Cl-C-R-C-Cl as disclosed by Marvel et al. in U.S. Reissue Pat. 26,065, based upon U.S. Pat. 3,174,947, and also in articles by Marvel et al. in the Journal of Polymer Science, vol. 50, pages 511-529 (1961) and in the Journal of Polymer Science, part A, vol. 1, pages 1531-1541 (1963). Polymers of the same general type can also be derived from bis-(3,4diaminophenyl) compounds of the type

as described by Foster and Marvel in the Journal of Polymer Science, part A, vol. 3, pages 417-421 (1965). Other tetraamino compounds suitable for use in making such polymers are described by Brinkley et al. in U.S. Pat. 2,895,948.

The polymers useful in membranes as taught herein are preferably soluble in certain water-miscible dipolar aprotic solvents so that they can be put readily into membrane form as described by Richter and Hoehn in U.S. Pat. 3,567,632. The polymers preferably have a solubility of at least 10 percent by weight at 250C. in a medium consisting essentially of 0 to 3 percent by weight of lithium chloride in a solvent selected from dimethylacetamide, dimethyl sulfoxide, N-methylphrrolidone, hexamethylphosphoramide, and mixtures thereof.

(2) Solvent The solvent in the solution used to prepare the membrane is a water miscible polar aprotic organic solvent. By water miscible is meant any solvent which is capable of mixing with water in all proportions without separation. By polar aprotic is meant any solvent which has a dielectric constant greater than 15, and, although it can contain hydrogen atoms, cannot donate suitable labile hydrogen atoms to form strong hydrogen bonds with an appropriate species. Especially preferred water miscible polar aprotic organic solvents include N.N-dimethylformamide, dimethyl sulfoxide, tetramethylurea, Nmethylpyrrolidone, dimethylacetamide, tetramethylene sulfone, and hexamethylphosphoramide.

The preferred solvents can be represented by the formula

where R3, R4, and R5 may be the same or different and are 1 to 4 carbon alkyl or any two of R3, R4 and R5 taken together are alkylene so chosen that the total numer of carbon atoms in all of R3, R4, and R5 is not more than 6, a is 1 or 2, b is 0 or 1, T is

and the sum of a + b is such as to satisfy the valences of T. While R3, R4 and R5 as indicated can be separate alkyl groups, any two of these groups can be present in combination as an alkylene group, thus forming a heterocyclic ring structure. When such a heterocyclic ring is present, the ring must contain 5 or 6 nuclear atoms in all.

(3) The Salt The salt is soluble in the polymer and can be present in the solution used for preparing the membrane in an amount between 10% and 100% by weight based on the weight of the polymer. For hollow fiber preparation, 10-40% salt is employed because greater amounts affect spinning performance.

The salts usually increase the water permeability of the final membrane at least roughly in proportion to the volume percent of the salt present, based on the volume of the polymer.

The volume fraction of the salt present can be calculated from the weights of the salt and polymer and their respective densities. The densities of many suitable salts are listed in the "Handbook of Chemistry and Physics", published by the Chemical Rubber Publishing Company. Although the densities of different polymers vary somewhat, it has been found that a value of 1.31 grams per cubic centimeter can be used, without substantial error in calculating the volume fraction of salts, as the density of any polymer useful in making the membranes described herein.

The type of salt present influences the permeability and separation effectiveness of a membrane obtained therefrom. Contemplated soluble salts include the salts of Groups IA and IIA metals of the Periodic Table and are preferably highly dissociated, are soluble in the amount present, and are chemically inert toward the other materials involved in the process. Suitable salts include lithium chloride, lithium bromide, lithium nitrate, calcium nitrate, and calcium chloride. A desired balance of properties can frequently be obtained by the optimum choice of the type and amount of polymer solubilizing salt in the solution.

Mixtures of two or more salts are preferred for preparing hollow fiber membranes.

Particularly preferred are salts containing mixtures of lithium nitrate and lithium chloride between 5 percent and 25 percent lithium nitrate and 5 percent to 15 percent lithium chloride in which the combined amounts of lithium nitrate an lithium chloride are between 10 percent and 40 percent, all based on weight of polymer.

(4) The surfactant The surfactant is defined as set forth herein. Generally, surfactants, sometimes called surface active agents, form micelles and lower the surface tension at relatively low concentrations in aqueous solution.

The term "nonionic" means that the surfactants hydrophilic and hydrophobic moieties are not positively or negatively charged.

The term anionic means that the surfactant contains the hydrophobic moiety in a group that is negatively charged.

The term "ampholytic" means that the surfactant contains both an acidic and a basic function in the hydrophilic moiety.

The greatest increase in water flux is observed with ampholytic and nonionic surfactants.

Of these two classes, the nonionic surfactant is preferred. The salt rejection efficiency may decrease in the resulting membrane but this decrease can be overcome by treatment of the membrane after preparation by procedures described hereinbefore.

The nonionic surfactants preferably are the reaction products of ethylene oxide and optionally propylene oxide with other components which impart hydrophobic moieties to the resultant surfactant, such as alcohols, acids and alkyl phenols. The most preferable nonionic surfactants can be presented by the formulae: (1) R [ O(A)nH ]x wherein (A) is the group fC2H40in or a mixture of the groups fC2H4Oia and fC3H60)b, wherein n in each instance is an integer of from 4 to 150 and preferably 6 to 18, b is an integer of 0 to 30, and a is an integer of at least 2, a + b being equal to n; xis an integer of 1, 2 or 3; and R is an aliphatic hydrocarbon group which can be saturated or unsaturated, straight-chain, branched, or cyclic, or combinations thereof and will generally contain from 8 to 24 carbon atoms, preferably from 8 to 18 carbon atoms; examples of R groups include stearyl, lauryl, decyl and the groups derived from aliphatic glycols and triols; (2) R'-C6H4O(B)mH, wherein B is the group fC2H4Oi or a mixture of the groups flC2H4OiC and fC3H6Oid, wherein m in each instance in an integer of from 4 to 150 and preferably 8 to 20, d is an integer of 0 to 30, c is an integer of at least 2, c + d being equal to m; R' is a monovalent aliphatic and usually saturated radical containing 4 to 20 carbon atoms and preferably 8 to 12 carbon atoms; R2 l (3) R3 - CON [ (CH2CH2O)pH]z, wherein p is an integer of 2 to 150, z is an integer of 1 or 2, R3 is an alkyl group containing 1 to 8 carbon atoms, R2 is a chemical bond to a group+CH2CH2O)pH when z is 2 and an alkyl group of 1 to 8 carbon atoms when z is 1, with the proviso that at least 5 carbon atoms are provided by R2 + R3, (4) The polyalkylene oxide block copolymers of the formula HO(C2H40)e (C3H6o)f(c2H4o)gH wherein f is an integer of from 15 to 65 and e and g are integers sufficiently large that e + g total 20 to 90 percent of the total weight of the polymer. Additional specific surfactants include (5) ClIs CH3 (CH2)6CH2(OCH2CH2)3OH; (6) CH3 (CH2)l0CH2(OCH2CH2)l2 (OCH(CH3)CH2)5OH; (7) CH3(CH2)8CH2(OCH2CH2)l0OH; (8) CH3 (CH2)8CH2(OCH2CH2)^OH; and

Most preferably the nonionic surfactant is one in which the formula is (R[O (A)nH] wherein R is acyl (R'CO-) of 8-20 carbon atoms, (A) is fCH2CH2Ot, and n is a cardinal number of between 8 to 18, and R' is alkyl, aryl, aralkyl or alkaryl. Preferably R' is alkyl.

The anionic surfactant is preferably one of the formula AM where M is a cation, such as Na+, Li+ or NH+4 and A is an anion containing a hydrophobic hydrocarbyl group of 8-20 carbon atoms. Preferably A will have the formula R"A'e wherein R" is alkyl of 8-20 carbon atoms and A' is -COO0, 5030 or -OSO36.

The ampholytic surfactant is a betaine of the formula

wherein Y is preferably alkyl of 8-15 carbon atoms.

Representative specific surfactants include

n-decyl polyethylene glycol, polyethylene glycol monolaurate, polyethylene glycol monostearate having a molecular weight in the glycol portion of 400, 600 800, 1000 or 6000

and CH3(CH2)11-O-SO3Na The amount of surfactant in the solution will be between 100 p.p.m. and 10,000 p.p.m.

based on polymer present. Preferably the amount will be between 300 and 1000 p.p.m. and most preferably between 400 and 880 p.p.m.

The effect of the surfactant is believed to be to open the membrane structure during solidification. Sometimes the surfactant is adsorped onto the membrane which causes the flux rate to be lowered. To counteract any adsorption tendency, the amount of surfactant employed can be lowered.

Because the surfactant is in solution, it will be distributed throughout the resulting membrane formed from the solution. During extraction of salts and solvent (described further below), some, but not all of the surfactant will be removed from the membrane.

Thus, the amounts of surfactant remaining in the membrane after extraction will ran the salt, is miscible with the solvent and is practically chemically inert toward the polymer and is practically a nonsolvent for the polymer. Thus, the rinse medium extracts most of the solvent and the salt from the protomembrane to form the membrane. Suitable rinse media include water, methanol and ethanol. The protomembrane should be contacted with the rinse for a time sufficient to extract at least 75% of the salt or at least 75% of the solvent.

Preferably, substantially all the salt and solvent are removed. The temperature of the rinse medium should be between 0 C. and 40"C.

Extraction of the salts, any remaining solvent, and other materials from protomembranes in preparing permselective membranes can be carried out continuously or batchwise. The membranes are cooled and partially extracted by flooding with water or with recycled water containing the extract immediaXly after formation of the protomembrane.

In order to observe the enhancement of water flux properties in hollow fiber membranes, it is advantageous to anneal the fibers after extraction of solvent and salt. The fibers can conveniently be annealed by subjecting them to water at 40-80"C. for 1/2 to 1-1/2 hours.

The following Examples illustrate this invention. The parts of materials recited therein are by weight unless otherwise indicated. The percentage of polymers in solution are based on the total weights of the solutions. The percentages of salts and other materials present in polymer solutions are based on the weights of polymers in the solutions unless otherwise indicated. Polymer inherent viscosities are determined with 0.5 grams of polymer in a solution of 4 grams of lithium chloride in 100 milliliters of dimethylacetamide unless otherwise indicated.

COMPARATIVE EXAMPLE A This comparative example illustrates the properties of fibers spun from a solution containing no surfactant, using a quench temperature of 13"C.

A polyamide was prepared from a 67/33 mixture of metaphenylenediamine and metaphenylenediamine 4-sulfonic acid calcium salt, a 70/30 mixture of isophthaloyl chloride and terephthaloyl chloride, as described in Example I of U.S. 3,775,361. The inherent viscosity of the polymer was between 1.2 and 1.3. It was neutralized with calcium hydroxide, washed three times by stirring with water, and dried at 1400C.

The polymer flake was redissolved in dimethylacetamide after which lithium chloride and lithium nitrate salts were added. The pH, as measured by a glass electrode, was adjusted to 6 to 7 by the addition of an aqueous slurry of lithium hydroxide. The resulting solution contained 24 percent polymer, 6 percent lithium chloride and 15.5 percent lithium nitrate.

The solution was filtered and concentrated to 28.5 percent polymer for spinning by applying heat and vacuum.

The concentrated solution, i.e. the spinning solution, was spun through a spinneret of 150 annular holes of the type described by Burke et al. in U.S. Pat. 3,397,427. The solution temperature was 125"C.

The spun fibers were passed through a 19 foot by 9 inch diameter cell maintained at 160-180"C. and contacted concurrently with an inert aspiration gas (nitrogen) at 1850C.

On leaving the cell, the partially dried fiber was quenched with a liquor at 13"C. It was then piddled at 127 yards per minute into a container while spraying the liquor at 130C. into the container. The liquor for the quench and piddler was a 4.2 percent dimethylacetamide in water as measured by refractive index.

The piddled fiber was then annealed, using a two-stage counter current extraction system by recycling liquor from each stage, through a spray, over the fiber. Temperature was 50"C.

Time on each stage was 8 hours. Pure water was fed into the last stage. The liquor from the first stage was used to maintain the quench liquor at 4.2 percent dimethylacetamide as measured by refractive index.

Extracted fibers were characterized by constructing permeators containing them and testing the permeators. From the results, the following data was calculated: The fiber outside diameter was obtained by measuring the volume of water displaced by a given length and number of fibers, and using the formula

where: V = volume of water displaced in cubic centimeter, L = fiber length in centimeters, N = number of filaments, OD = fiber outside diameter in microns.

The fiber inside diameter was calculated from the equation

where: ID = fiber inside diameter in microns, F = permeator flux in U.S. gallons per day, Lp = pot length of permeator in feet, La = active length of fiber in permeator in feet, Pd = dead end tube pressure, pounds per square inch gauge, N = number of filaments in permeator.

This equation is valid as long as the dead end tube pressure is less than two-thirds the feed pressure.

The percent salt passage, SP, was calculated from the equation SP = Concentration of salt in permeate x 100.

Concentration of salt in feed The water permeability constant Kw, for films is defined as F = Kw x A x Pe where: F = permeation rate in U.S. gallons per day A = area of the film through which permeation occurs, square feet.

Pe = the effective pressure in pounds per square inch = hydraulic pressure drop across the film minus the difference in osmotic pressure across the film.

Kw water permeability constant. U.S. gallons per square feet per day per pounds per square inch.

The corresponding equation used for fiber, using the same definition for water permeability constant as for films, was 1.031 x 10-5 x ID4 x Kw x OD x N x La x Ps ID4 + 7.236 x 105 x Kw x OD x La x (Lp + La/3) where Ps = feed pressure minus the osmotic pressure difference between solution on outside of fiber and permeate. The other terms are previously defined. The restriction on this equation is the same as that for the equation for the fiber inside diameter. In terms of Kw. the equation becomes Kw = OD x La x (1.031 x 10 ID4 x F x OD x La x x x 10-5 X Ps x ID4 x N - 7.236 x 105 x F x (Lp + Lea"3)) which was used to calculate the water permeability constant.

Each permeator was made as a double ended permeator from a single strand of fibers (150 filaments) but tested as a single ended permeator. A 150 filament skein of hollow fibers was doubled to obtain 300 parallel fibers and while wet with water, was inserted into a rigid tube fitted with two side tubes at one end. The two ends of the 150 filament skein were placed in the two side tubes and sealed with epoxy resin. The loose ends outside the side tubes were cut to open the hollow fibers for fluid flow. To one side tube was attached a pressure gauge to measure the dead end tube pressure. The permeator was 30 inches long in the filament-containing portion and had a pot length of 4 inches. The permeators were tested at 250C with shell side feed at 400 psig and a conversion, (i.e., permeation rate over feed rate x 100) of 4 to 6 percent using 5,000 parts per million of sodium chloride in water.

The average fiber properties for this Comparative Example A (where no surfactant was present in the solution to be spun) were: OD = 87.8 microns ID = 40.1 microns Kw = 12.2 x 10-3 U.S. gallons per square foot per day SP = 4.5% COMPARATIVE EXAMPLE B This example illustrates properties of a fiber spun from a solution containing no surfactant, using a quench temperature of 10 C.

Polymer flake and fiber were prepared as in Comparative Example A except the liquor temperature to the quench tanks and piddle was 10 C. From the test data, the following fiber properties were calculated: OD = 87.2 microns ID = 40.6 microns Kw = 11.2 x 10-3 U.S. gallons per square foot per day SP = 4.0% INVENTION EXAMPLE 1 This example illustrates properties of fibers spun from a solution containing 857 parts per million, based on polymer, of polyethylene glycol monostearate (as surfactant) present in the spinning solution. The molecular weight of the polyethylene glycol unit of the monostearate was 1000.

The surfactant was made up as a 10 percent solution in dimethylacetamide. This solution was added to about one-fourth of the dimethylacetamide which was to be used in redissolving the polymer flake described in Comparative Example A. Polymer flake solution and fiber were prepared as in Comparative Example A. Quench and piddle temperature was 13"C. From the test data the following fiber properties were calculated: OD = 87.5 microns ID = 40.3 microns Kw = 16.4 x 10-3 U.S. gallons per square foot per day SP = 5.2% It can be seen that the Kw is significantly higher than in Comparative Examples A and B.

Using 894 parts per million, based on polymer, of a polyethylene glycol monostearate in which the molecular weight of the polyethylene glycol unit of the monostearate was 600, the following properties were calculated: OD = 87.5 microns ID = 41.8 microns Kw = 16.4 x 10-3 U.S. gallons per square foot per day SP - 4.7% Using 440 parts per million, based on polymer, of polyethylene glycol monostearate in which the molecular weight of the polyethylene glycol unit of the monostearate was 600, the following properties were calculated: OD = 87.2 microns ID = 41.3 microns Kw = 14.9 x 10-3 U.S. gallons per square foot per day SP = 4.5% INVENTION EXAMPLE 2 This example illustrates properties of fibers spun from a solution containing 801 parts per million, based on polymer, of C-cetyl betaine (as surfactant) present in the spinning solution. The quench temperature was 10 C.

The surfactant was obtained as 100 percent active ingredient and made up as a 10 percent solution in dimethylacetamide. This solution was added to about one-fourth of the dimethylacetamide to be used in redissolving the polymer flake described in Comparative Example A. Polymer flake and solution were prepared as in Comparative Example A.

Fiber was spun as in Comparative Example B. Average fiber properties were: OD = 86.2 microns ID = 41.7 microns Kw = 14.4 x 10-3 U.S. gallons per square foot per day SP = 7.9% INVENTION EXAMPLE 3 This example illustrates properties of fibers spun from a solution containing 816 parts per million, based on polymer, of the sodium salt of stearyl sulfate (as surfactant), containing some other long chain impurities, present in the spinning solution. The quench temperature was 10 C.

The surfactant employed was a 47 percent aqueous solution which was diluted with dimethylacetamide to 10 percent. This solution was added to about one-fourth of the dimethylacetamide to be used in redissolving the polymer flake described in Comparative Example A. Polymer flake and solution were prepared as in Comparative Example A.

Fiber was prepared as in Comparative Example B. Average fiber properties were: OD = 86.2 microns ID = 41.3 microns Kw = 12.6 x 10-3 U.S. gallons per square foot per day SP = 8.4% The fiber properties in the Invention Examples were obtained by preparing permeators and carrying out tests, as described in Comparative Example A.

WHAT WE CLAIM IS: 1. A permselective membrane consisting essentially of: (a) a substantially linear, aromatic, synthetic organic, nitrogen-linked, condensation polymer; and (b) a surfactant distributed throughout the polymer in an amount of between 100 ppm and 10,000 ppm, based on the weight of the polymer (a), said surfactant having a molecular weight between 200 and 7000, and containing (i) at least one hydrophobic moiety having a molecular weight of between 100 and 400 and being a hydrocarbyl group or such a hydrocarbyl group substituted with halogen (F, Cl, Br or I), -NO2 or -OH; and (ii) at least one hydrophilic moiety, said surfactant being i) a nonionic surfactant or, ii an anionic surfactant, or iii) an ampholytic surfactant of the formula

2. A membrane according to claim 1 wherein the surfactant is a nonionic surfactant which is the reaction product of (1) ethylene oxide and optionally propylene oxide, with (2) an alcohol, carboxylic acid or alkyl phenol.

3. A membrane according to claim 2 wherein the nonionic surfactant is of the formula: 0 R C O fCH2CH2OinH wherein R is alkyl of between 8 and 20 carbon atoms and n is a cardinal number of between 8 and 18.

4. A membrane according to claim 3 wherein the nonionic surfactant is polyethylene glycol monostearate present in the membrane in an amount of between 300 ppm and 1000 ppm based on weight of the polymer (a).

5. A membrane according to claim 1 wherein the surfactant in an anionic surfactant of the formula:

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. dimethylacetamide to be used in redissolving the polymer flake described in Comparative Example A. Polymer flake and solution were prepared as in Comparative Example A. Fiber was spun as in Comparative Example B. Average fiber properties were: OD = 86.2 microns ID = 41.7 microns Kw = 14.4 x 10-3 U.S. gallons per square foot per day SP = 7.9% INVENTION EXAMPLE 3 This example illustrates properties of fibers spun from a solution containing 816 parts per million, based on polymer, of the sodium salt of stearyl sulfate (as surfactant), containing some other long chain impurities, present in the spinning solution. The quench temperature was 10 C. The surfactant employed was a 47 percent aqueous solution which was diluted with dimethylacetamide to 10 percent. This solution was added to about one-fourth of the dimethylacetamide to be used in redissolving the polymer flake described in Comparative Example A. Polymer flake and solution were prepared as in Comparative Example A. Fiber was prepared as in Comparative Example B. Average fiber properties were: OD = 86.2 microns ID = 41.3 microns Kw = 12.6 x 10-3 U.S. gallons per square foot per day SP = 8.4% The fiber properties in the Invention Examples were obtained by preparing permeators and carrying out tests, as described in Comparative Example A. WHAT WE CLAIM IS:

1. A permselective membrane consisting essentially of: (a) a substantially linear, aromatic, synthetic organic, nitrogen-linked, condensation polymer; and (b) a surfactant distributed throughout the polymer in an amount of between 100 ppm and 10,000 ppm, based on the weight of the polymer (a), said surfactant having a molecular weight between 200 and 7000, and containing (i) at least one hydrophobic moiety having a molecular weight of between 100 and 400 and being a hydrocarbyl group or such a hydrocarbyl group substituted with halogen (F, Cl, Br or I), -NO2 or -OH; and (ii) at least one hydrophilic moiety, said surfactant being i) a nonionic surfactant or, ii an anionic surfactant, or iii) an ampholytic surfactant of the formula

AM where M is a cation and A is an anion containing a hydrophobic hydrocarbyl group of between 8 and 20 carbon atoms.

6. A membrane according to claim 5 wherein M is Na+, Li+ or NH+ A is the anion R"A' 8 wherein R' is alkyl of between 8 and 20 carbon atoms and A' is -cr300, S030 or -OS03e), and the surfactant is present in the membrane in an amount of between 300 ppm and 1000 ppm based on weight of polymer (a).

7. A membrane according to claim 1 wherein the surfactant is an ampholytic surfactant of the formula:

8. A membrane according to claim 7 wherein the surfactant has the formula:

cooO Y - CH wherein Y is alkNyf 315 carbon atoms, and the surfactant is present in the membrane in an amount of between 300 ppm and 1000 ppm based on weight of the polymer (a).

9. A membrane according to any one of the preceding claims wherein the condensation polymer has the formula: fL-Rlln wherein OH n L is -C=N R is phenylene which can be substituted with a carboxyl or sulfonic acid group, and n is an integer sufficiently large to provide a film-forming molecular weight.

10. A membrane according to any one of the preceding claims in the form of a hollow fibre.

11. A membrane according to claim 1 substantially as described in Example 1, 2 or 3.

12. A solution for use in preparing a membrane as claimed in any one of the preceding claims, the solution comprising: (A) a water-misicible organic polar aprotic solvent; (B) the condensation polymer (a) in an amount between 12% and 40% by weight based on the total weight of the solution; (C) at least one salt which is soluble in the solvent (A) and is present in an amount between 10% and 100% by weight based on the weight of the polymer (a); and (D) the surfactant (b) in an amount between 100 ppm and 10,000 ppm based on the weight of the polymer (a).

13. A solution according to claim 12 wherein the solvent (A) has the formula

wherein R39 R4 and R5 independently are 1 to 4 carbon alkyl, or any two of R3, R4 or R5 taken together can be alkylene so chosen that the total number of carbon atoms in all of R3, R4 and Rs is not more than 6, a is 1 or 2, b is 0 or 1, T is

and the sum of a + b is such to satisfy the valences of T.

14. A solution according to claim 12 substantially as described in Example 1, 2 or 3.

15. A process for preparing a permselective membrane as claimed in any one of claims 1 to 11 which comprises (a) disposing a solution as claimed in claim 12 into a desired shape for a permselective membrane, and (b) removing the organic polar aprotic solvent and salt from the solution in a manner and at a rate sufficient to solidify the membrane in the shape disposed in step (a).