CA1246799A - Anisotropic microporous supports impregnated with polymeric ion-exchange materials - Google Patents

Abstract

ANISOTROPIC MICROPOROUS SUPPORTS IMPREGNATED WITH
POLYMERIC ION-EXCHANGE OR COMPLEXING MATERIALS

ABSTRACT

Novel ion-exchange or complexing media (for both ionic and nonionic species) (FIGS. 1-3) are dis-closed, the media comprising polymeric anisotropic, microporous supports containing polymeric ion-exchange or complexing materials. The supports are anisotropic, having small exterior pores and larger interior pores, and are preferably in the form of beads ( FIG. 1), fibers (FIGS. 2 and 3), or sheets.

Description

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ANISOTROPIC MICROPOROUS SUPPORTS IMPREGNATED WITH
POLYMERIC ION-EXCHANGE OR COMPLEXING MATERIALS

BACKGROUND OF THE INVE~TION
This invention relates to novel ion-exchange S and complexing media comprising polymeric supports for incorporation of ion-exchange agents and complexing agents, the latter agents including agents for complexing both ionic and nonionic species.
There is currently a large research and development effort aimed at producing polymeric ion-exchange and ion-complexing materials (commonly known as ion-exchange materials) and materials for complexing nonionic species for a wide variety of applicatio~ns.
For the most part, these materials are resins compris-ing a cross-linked polymer (e.g., polystyrene cross-linked with divinylbenzene) that is substituted with an ion-exchange or complexing group either before or after polymerization. There are two major obstacles to the production of many different types of these resins with highly varied ion-exchange or complexing characteris-tics. First, the number of ion-exchange or complexing substituents that can be easily attached to reactive ites on the polymeric backbones of resins i~ limited.
Secondly, only a limited number of polymers are avail-able that exhibit suitable physical characteristicssuch as insolubility in water, resistance to abrasion, and resistance to osmotic swelling, and that also have chemically active sites for adding ion-exchange or complexing groups. For example, polytetrafluoethylene ; 30 and polypropylene have such favorable physical charac-teristics but are chemically unreactive toward addition of ion-exchange groups.
There are many polymeric materials that exhibit desirable ion-exchange or ion-complexing prop-erties (e.g., high selectivity and high capacities toextract metal ions) but that are not structurally suited for use in ion-exchange processes. See, for ~2~C~799

-2-example, J. Poly. Sci.: Polym. Let. Ed. 20(19~2) 291 and J. Chem Soc. Dalton (1981)1486. Thus, it has been found that certain water-soluble polymers such as poly(vinylbenzocrownether)s and poly(vinylbenzoglyme)s S are highly selective toward one metal ion over another, J. Pure Appl. Chem. 54(1982) 2129. Other copolymers such as bicyclo~2.2.1]hept-Sene-2,3-dicarboxylic anhydride with divinylbenzene and acrylic acid or acry-lonitrile and p-vinylbenzoylacetone and acrylamide or maleic acid, all of which show poor abrasion re~i~tance, are useful for separating Ca+2, Co~2, Cu+2 from other metal ions and for separatinq Cu+2 ions, Plaste Kautsch 29(1982)331, and for separating Cu+2 from other tran-~ition metal ions, J. Appl. Pol. Sci. 27(1982)811.
There are also many polymeric materials that exhibit desirable complexing of nonionic ~pecies.
Examples include certain sugars that are complexed by water-soluble polyelectrolytes such as the calcium salts of polystyrenesulfonic acid or polyacrylic acid and gases ~uch as hydrogen sulfide and carbon dioxide - which are complexed by polyamines or polyvinlypyr-rolidone. Ammonia may be complexed by macrocyclic ethers, while oxygen may be complexed by macrocyclic ; organometallic salts.
. There is thu~ a substantial need to exploit the favorable ion-exchange or complexing charac-teristics of such materials by incorporating them into media with favorable physical characteristics. To this end, in U.S. Patent Nos. 4,014,798, 4,045,352 and 30 4,187,333 there is disclosed hollow-fiber ion-exchange membranes comprising structurally sound porous hollow fibers containing a polymeric ion-exchange material within the fiber pore~. However, because the ion-exchange material was not held firmly within the par-ticular porous structure it was lost upon flu~hing the support with water.

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12~799 SUMMARY OF THE INVENTION
According to the present invention, there are provided novel ion-exchange and complexing media useful for highly efficient extraction of ions from aqueous solutions and for complexing nonionic species, the media comprising polymeric microporous supports, with an anisotropic pore structure of small pore~ at the surface and large pores in the interior, the large pores being filled with polymeric ion-exchange or complexing materials, and the small pores being ~uf-ficiently small to retain the ion-exchange or complex-ing materials, thereby preventing their loss from the support. Preferred forms of the microporous supports are beads, sheets and fibers.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cros~-section photomicrograph of an anisotropic microporous support in bead form, useful in the present invention.
FIG. 2 contains three cross-section photo-micrographs of different magnification of an aniso-tropic support similar to that shown in FIG. 1, but in lumen-containing or hollow fiber form.
FIG. 3 is a cross-~ection photomicrograph of an anisotropic support in fiber form.
FIGS. 4 and 5 are graphs ilustrating the extraction efficiency of the ion-exchange and complex-ing media of the present invention.

DETAILED DESCRIPTION OF THE INVENTION
The novel ion-exchange and complexing medium of the present invention comprises two component~.
These components may be broadly described as (1) a polymeric anisotropic microporous support, and (2) a polymeric ion-exchange or complexing material. The second component substantially fills the pores of the first component. Unlike the one-component ion-exchange .

7~9 materials of the prior art, the physical and chemical properties of each component can be varied indepen-dently. This allows both the physical and complexing and ion-exchange properties of the novel media of the present invention to be optimized. Thus, the present invention allows ion-exchange and complexing materials which are highly resiqtant to damage from heat, abra-sion, or o~motic swelling to be made by using aniso-tropic microporous support materialq made from tough thermoplastics such as polysulfone and polyvinylidene fluoride, while at the same time utiliæing the high ~electivity and high ion extraction and complexing capacity of polymeric ion-exchangers and complexing agentq which otherwise lack the necessary structural characteristics. Unlike the two-component ion-exchange material~ of the prior art, the microporous support of the present invention iq anisotropic with surface pores that are ~ufficiently small to prevent loss of the ion-exchange or complexing material held within the larger internal pores.
The microporous supports may be in virtually any ~hape with an anistropic pore structure, having very smal1 pores on the surface and relatively large pores in the interior. Preferred forms of the supports are beads, sheets and fibers with and without lumens, although any geometric ~hape will work. It is pref-erable to have surface poreq less than 0.1 micron in diameter and interior pores of from about 2 to 200 microns in diameter.
Exemplary polymeric compounds from which the microporous supportq of the present invention may be fabricated include polysulfone, polystyrene, poly-vinylchloride, polyacetonitrile, polyamides, polypheny-; lene oxide, polyvinylacetate, polyetherimide~, polyvinylidene fluoride and combinations thereof.
Aniqotropic microporous bead qupports of the present invention, as shown in FIG. 1, can be made by injecting droplets of a solution of the polymer through a stainless steel tube into a non-solvent bath where they are precipitated, the precipitation occurring more rapidly at the exterior surfaces than the interior, causing anisotropy with a graduation of pore sizes from very small (less than 0.1 micron) on the exterior to relatively large (20 to 200 microns) at the center.
Bead size may be varied between about 1 to about 5 mm by varying the tube diameter. The preferred bead size is 2 to 3 mm in diameter. After precipitation, the beads may be washed with water and air-dried. Smaller beads with diameters from 0.001 to 1.0 mm can be made by substantially the same method, except that the polymer solution is dispersed into droplets 0.001 to 1.0 mm in diameter by use of a spray atomizer or by breaking a qtream of polymer with an air jet and then allowing the droplets to fall into a water bath where the polymer precipitates.
Suitable anisotropic supports in the form of fibers with lumens, as shown in FIG. 2, are made by using the tube-in-orifice solution spinning technique.
This system utilizes a spinnerette that consists of two concentric tubes, the annular space between the tubes is the polymer solution orifice and the inner tube is the lumen-forming orifice. In practice, the polymer solution is forced down the outside of the inner, lumen-forming tube and through the polymer solution orifice, while the lumen-forming solution of water (or water and ~olvent) flows from the inner orifice of the - 30 needle. The fiber fallq through an air gap of up to 30 inches and collects in a water precipitation bath. A
hollow fiber forms as the polysulfone precipitates from the polymer solution. The lumen-forming solution pre-cipitates the inside fiber wall: air and then water precipitate the outside fiber wall. Precipitation occurs more rapidly on the surfaces of the hollow-fiber wall then on the interior, causing ani~otropy with a '` I

12~6799 graduation of pore sizes from very small (less than 0.1 micron) on the exterior to relatively large pore~ (2 to 20 microns) on the interior. The fibers can have an outside diameter from about 0.2 to about 1.0 mm and an outside diameter of from about 0.10 to about 0.95 mm.
- Anisotropic supports in the form of lumenless fibers, as shown in FIG. 3, are made by injecting a continuous stream of polymer solution through a stain-less steel tube into a water bath under conditions substantially similar to those used to fabricate aniso-tropic beads, except that the tube is submerged in the water bath. Fibers thus formed have surface pores less than 0.1 micron in diameter and interior pore~ from about S to about 100 microns in diameter.
Flat sheets are made by conventionally practiced casting procedures used in the production of anisotropic microporous polymeric membranes, as dis-closed, for example, in Adv. Chem. Serv. 38(1962)117, U.S. Patent No. 3,651,024 and Polym. Let. 11(1973)102.
As previously mentioned, the present inven-tion comprises the anisotropic microporous supports noted above, the pores of which are substantially filled with polymeric complexing agents, ion-exchange or ion-complexing materials. These are normally made by first loading the anisotropic microporous support with monomers or sufficiently low-molecular-weiqht pre-polymers to permit introduction through the small sur-face pores. These monomer~ or prepolymers are selected to yield a polymeric ion-exchange or complexing mate-rial under suitable reaction conditions. Polymeriza-tion within the support is then carried out. Once the polymer is formed in the support, it cannot escape through the small surface pores. Upon polymerization, the support with its polymeric ion-exchange or complex-ing material may be used, for example, for the extrac-tion of metal ions from aqueous solutions. Even structurally weak qels and poorly crosslinked, ':
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12~Ç~799 non-¢rosslinked, or low molecular weight polymers, includinq those that are water-~oluble can be used as ionexchange material as long as the size of the polymer is sufficient to prevent its escape through the small surface pores of the support. Thiq is an important aspect of the present invention, since it has been observed that highly selective ion-exchange materials can be made from structurally weak water-swelled gels, J. A~pl. Poly. Sci. 27(1982)811, from polymers with low cross-link density, J. Poly. Sci. Poly. Chem. Ed., 20(1982)1609, and from low molecular weight water-soluble polymers, J. Pure Appl. Chem., 54(1982)2129.
Ion-exchange or complexing monomers or~low molecular weight prepolymers suitable for polymer-ization within the anisotropic microporous supports ofthe present invention include compounds containing the following functional groups: amides, amines, beta-diketones, hydroxyoximes, alkylphosphate esters, hydroxyquinolines, thiophosphate esters, sulfonates, carboxylic acids, and macrocyclic ethers. They also include alkyl-, aryl-, halogen-, and amino-~ubstituted derivatives of such compounds and mixtures thereof.
; The monomers should al~o be chosen such that after polymerization the functional groups are available for ion (or nonionic) complexing. Acceptable monomers or prepolymers also include those that have little or no ion-exchange or complexing functionality but that deve-lop this functionality during polymerization. Examples include aziridines and epoxides. In the case of aziri-dines, upon polymerization, tertiary amine ion-exchange sites are formed. In the case of epoxides, hydroxyl or ether ion-complexing ~ites are formed upon polymeriza-tion. The monomers must be chosen such that the resulting polymer is sufficiently hydrophilic to allow migration of the ionic species into the polymer.
Polymerization and crosslinking of the mono-mers within the support are normally accomplished by , , I ;. ~

loading the microporous support, via the small ~urface pores, with a solution composed of monomers or prepoly-mers ~about 10 wt~ to about 99 wt%), solvent, and possibly a polymerization initiator (about 0.001 wt~ to about 1.0 wt%). Loading may be accompliqhed by sub-merging the support in the monomer solution and drawing a vacuum of 5 mmHg or less, and then alternately releas-ing and applying the vacuum until the pores are sub-stantially filled. Polymerization and crosslinking of the monomers take place by heating the monomer-filled support or by contact with external initiators.
Polymerization may also be initiated by thermal decomposition of an organic free-radical initiator. Suitable free-radical initiators include azo-nitriles and other azoderivatives, peroxide~ and peresters. Alternatively, polymerization and cross-linking can be carried out with reactive heterocyclic compounds that react by ring-opening reactions upon heat, examples include epoxide and aziridine derivatives.
Example 1 Anisotropic microporous ~upports in bead form substantially as shown in FIG. 1 were prepared by injecting dropwise a solution of 120 g polyqulfone in 1.0 L N,N-dimethylformamide (DMF) through a stainless steei tube with an inside diameter of 0.75 mm through an air gap of 2 inches and into a bath of water at 20C, thereby precipitating beads 2 to 3 mm in diameter with surface pores less than 0.1 micron in diameter and interior pores 100 to 200 microns in diameter. The beads were washed with water and allowed to air-dry.
Example 2 Microporous polysulfone lumen-containing fibers substantially as ~hown in FIG. 2 having surface pores less than 0.1 micron in diameter, interior pores of 2 to 20 microns, an average diameter of 0.625 mm, and an internal diameter of 0.50 mm were prepared by using the tube-in-orifice solution spinning technique.

12~6799 g This sy~tem utilizes a spinnerette, which consist~ of two concentric tubes, the annular space between the tubes is the polymer-solution orifice and the inner tube is the lumen-forming orifice. A polymer solution composed of 175 q polysulfone and 200 g 2-methoxyethanol per liter of DMF was forced down the outside of the inner, lumen-forming tube and through the polymer-solution orifice while a lumen-forming solution com-posed of 60 vol% DMF in water flowed through the lumen-forming tube. The two solutions fell through an air gap of 20 inches into a water precipitation bath at 20C. The hollow fiber thus formed was rinsed with water to remove residual DMF and 2-methoxyethanol. The fibers were air-dried for 48 hours and were then ready for use.
Example 3 Microporous polysulfone non-lumen-containing fibers substantially as shown in FIG. 3, having surface pores of less than 0.1 micron in diameter and interior pores 5 to 100 microns in diameter, were prepared by forcing a solution of 175 g polysulfone per liter of DMF through a stainless steel tube 0.75 mm in diameter submerged in a water bath at 20C. The polysulfone precipitated, forming a lumenless fiber as polymer ; 25 solution contacted the water solution. The fibers thus formed were rinsed in water and air-dried.
Example 4 Anisotropic microporous polysulfone fiber supports containing a polymeric hydrophilic methacry-late gel with pendant tertiary amine groups were pre-pared by first immersing the fibers of Example 3 in a solution of 47.5 wt% N,N-dimethylaminomethacrylate, 2.5 wt% tetraethyleneglycoldimethacrylate and 0.25 wt%
azo-bis-isobutylnitrile in methanol then alternately drawing a vacuum of about 5 mmHg and repressurizing to atmospheric pressure until the pores were substantially ; filled. The solution-filled fiberq were subjected to a : ' ~Z~799 temperature of 65C for 75 minutes, substantial comple-tion of t~he polymerization being indicated by the for-mation of a gel on the exterior of the fibers. ~he fibers were removed and the methanol solvent exchanged for water by soaking the fibers in water at 20C for 24 hours.
Example 5 The metal-ion capacity of the fibers of Example 4 was evaluated by contacting 0.85 g of the fibers w th 1 L of an aqueous solution containing 10 ppm uranium as uranyl sulfate at pH 3.0 (pH adjusted by a~dition of sulfuric acid) at 25C. After 16 hours the concentration of uranium in the aqueous solu~ion was reduced to 3.2 ppm uranium. This corresponds to a uranium content of the fibers of 0.80 g of uranium per 100 9 of fiber.
Example 6 Anisotropic microporous polysulfone fiber supports containing a polymeric ion-exchange material were prepared by first immersing the fibers of Example 3 in a solution of 50 wt% polyethyleneimine (CORCAT P-18*
sold by Cordova Chemical Company of Muskegon, Michigan) and 15 wt% epichlorohydrin in a 1:1 butanol-water mix-ture, then alternately drawing a vacuum of about 5 mmHg and repressurizing to atmospheric pressure until the pores were substantially filled. The solution-filled fibers were then subjected to a temperature of 60C for about 2 hours, substantial completion of the polymeri-zation being indicated by gellation of the exterior

3~ solution. The fibers were removed from the gelled solution and the butanol solvent exchanged for water by soa~ing the fibers in water at 20C for 24 hours.
Example 7 The ion-exchange capacity of the fibers of ; 35 E~am2le 6 was evaluated by cutting them into 10-cm-long bundles and contacting them with an aqueous feed solu-tion ccmprising 10 ppm uranium as uranyl sulfate and * Trade Mark 1~

10-g/L sulfuric acid (pH 1.0). Extraction of uranium ion was complete after about 24 hours. Uranium was stripped from the fibers by transferring them to a lS0-g/L sodiu~ carbonate solution (pH 11.5), and strip-ping was complete in about 2 hours. The loading/stripping cycle is shown in FIG. 4. The maximum dis-tribution coefficient, defined as the concentration of uranium ions in the fibers divided by the concentration of uranium ions in the aqueous feed solution, was approximately 750.
Example 8 Fibers of Example 6 were tested over 40 loadin~g/stripping cycles. During each loading/stripping cycle the fibers were contacted with an aqueous uranium feed solution comprising 10 ppm uranium as uranyl sulfate and 10-g/L sulfuric acid at pH 1.0 for 16 hours and then transferred to an aqueou~ stripping solution comprising 150-g/L sodium carbonate (pH 11.5) for 8 hours. The amount of uranium tran~ferred from the feed solution to the stripping solution in wt~ (defined as grams uranium per 100 grams fiber) is shown in FIG. 5 ; over 40 loading/stripping cycles. The results demon-strate that the fibers retain their ion-exchange characteri~tics over an extendeq period of operation.
The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expres~ions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention i8 defined and limited only by the claims which follow.

Claims (7)

I CLAIM:

1. An ion-exchange or complexing medium containing a polymeric ion-exchange or complexing material characterized in that said medium comprises a polymeric microporous support selected from polysulfone, polyvinylchlorides, polyacetonitriles, polyamides, polyphenylene oxides, polyvinylacetates, polyetherimi-des, polyvinylidene fluorides, and derivatives and mixtures thereof, having an anisotropic pore structure of small pores at the surface and large pores in the interior, said large pores containing said polymeric ion-exchange or complexing material and said small pores being sufficiently small to substantially prevent loss of said ion-exchange or complexing material from said support, said polymeric ion-exchange or complexing material being formed from polymerizable compounds that, following polymerization, contain a functional group selected from amides, amines, beta-diketones, hydroxyoximes, alkylphosphate esters, hydroxyquinolines, thiophosphate esters, hydroxyls, ethers, carboxyls, and macrocyclic ethers and amines and alkyl-, aryl-, halogen-, and amino-substituted derivatives and mixtures thereof.

2. The medium of claim 1 wherein the form of the anisotropic microporous support is selected from beads, fibers and sheets.

3. The medium of claim 1 wherein the anisotropic microporous support comprises beads having surface pores of less than 0.1 micron in diameter and interior pores from about 20 to about 200 microns in diameter.

4. The medium of claim 1 wherein the ani-sotropic microporous support comprises lumen-containing fibers having surface pores less than 0.1 micron in diameter and interior pores about 2 to 20 microns in diameter.

5. The medium of claim 1 wherein the ani-sotropic microporous support comprises fibers without lumens having surface pores less than 0.1 micron in diameter and interior pores about 5 microns to about 100 microns in diameter.

6. The medium of claim 3, 4 or 5 wherein said beads or fibers are polysulfone.

7. A process for making the medium of claim 1 comprising:

(a) introducing said ion-exchange or complexing material into said support through said small surface pores as a monomer or low molecular weight prepolymer: and (b) polymerizing said monomer or low molecu-lar weight prepolymer in situ.