Method of Making Novel Two Component Bipolar Ion Exchange Membranes

In recent years bipolar membranes have gained prominence as "water-splitters" and "acid-base" generators in electrodialysis operations. The main purpose of the bipolar membranes in such processes is to promote the dissociation of water into hydrogen and hydroxyl ions under the influence of an applied electrical potential. The commercial feasibility of all such processes requires the membranes to operate with low potential drops across them and with high current efficiencies at practical current densities and electrolyte concentrations.

Bipolar membranes consist of two oppositely charged ion exchange layers or membranes, either in close proximity or joined together. Each component is composed of a semi-permeable matrix material, for example, polyethylene, initimately containing a charge-bearing portion, for example, functionalized, cross-linked polystyrene. Usually strong acidic (sulfonic acid) and strong basic (quaternary ammonium) groups are introduced into the separate membranes. Each component is capable of transporting ions of opposite charge and hence also conducting an electric current. Under the influence of a direct current, cations or hydrogen ions present in the cationic-permeable component migrate toward the cathode and anions or hydroxyl ions in the anion-permeable component migrate toward the anode. This soon creates a depletion of ions in the vicinity of the interface and continuation of the flow of current is maintained only by the dissociation of water into hydrogen ions and hydroxyl ions. In order that the process is sustained, water must diffuse into the membrane to replenish that consumed by the dissociation.

The potential drop across the bipolar membrane is the sum of the individual potential drops of each component and the interface plus that required to split water (E.sub.m H.sup.+,OH.sup.- .apprxeq.0.8 v) E.sub.m =E anionic+E cationic+E interface+E.sub.m H.sup.+,OH.sup.- The component membrane potentials may be minimized by increasing their exchange capacities, while lowering interface potential drops depends upon the means one uses to join the components.

Individual component membranes of the type discussed here are subject of a patent by J. F. A. Hazenberg and B. F. Knol (U.S. Pat. No. 3,133,889, May 19, 1964). Their joining together into a bipolar membrane by a hydraulic press is disclosed in U.S. Pat. No. 3,372,101. Bipolar membranes so prepared however are found to possess relatively high potential drops across them, evidently produced by the dislocation of fixed charges during fusion and the creation of microthin areas of essentially pure polyethylene. To avoid this drawback various means have been attempted such as by adding ion exchange resin mixtures between the components prior to fusion to increase the charge concentration at the interface to avoid this difficulty. In so far as is known, membranes made in this manner have not yielded practically suitable products. In any event there is a need for a durable bipolar membrane with a relatively low potential drop which would be attractive in various chemical process operations such as, for example, the treatment of effluent streams e.g., removal of SO.sub.2 and the conversion of H.sub.2 SiF.sub.6 to HF.

The present invention discloses a method by which additional lowering of the potential drop of the interface may be accomplished. More specifically it has been demonstrated that interface potentials may be minimized by more completely filling the interface with a monobead layer of ion exchange resin, either as a mixture of anion and cation resins, or separately. It has been found that more complete interface covering may be accomplished by using (a) selected small size (micro fine) resins and (b) resins with narrow particle size distributions, either alone or mixed with oppositely charged resins with selected particle size distributions. The membrane potentials so obtained (1-2 volts at 100 amp./ft.sup.2) are commercially attractive and the membranes perform with high current efficiencies at realistic current densities and electrolyte concentrations.

Reference is made to TABLE I wherein a series of membranes, sampled according to the procedure disclosed below, are recorded. Resulting potential drops across bipolar membranes which were prepared under various conditions are set forth.

Commercially available ion exchange membranes were employed, e.g. Ionac Chemical Company's strong base (MA3475R) (strongly basic, anionic, heterogeneous, permselective anion exchange membranes) and strong acid (MC3470) (strongly acidic, heterogeneous, permselective cation exchange membranes) membranes and AMF's strong base (A100) (an anionic membrane having a polyethylene backbone containing polyelectrolytes of quaternized ammonium type) and strong acid (C100) (a cationic membrane having polyethylene backbones containing polyelectrolytes of sulfonic acid type) membranes. Likewise, commercially available ion exchange resins, e.g. Ionac Chemical Company's strong base (CGA541 (beads of strong base--alkyl quaternary ammonium chloride 200-400 mesh -7% crosslinked polystyrene--divinylbenzene (S-DVB) matrix); MFA-6 (micro fine (<35 microns) resin beads of alkyl quaternary ammonium chloride or sulfate -6% crosslinked S-DVB matrix)) and strong acid (CGC241, (beads of strong acid, as the sodium salt of sulfonic acid 8% crosslinked S-DVB matrix) MFC-6, (micro fine resin as the sodium Salt of Sulfonic Acid -6% crosslinked S-DVB matrix) MFC-10 (micro fine resin as the sodium salt of sulfonic acid -10% crosslinked S-DVB matrix)) resins and Dow Chemical Company's strong base (Dowex Ix16, (strongly basic beads of 20-50 mesh resin -16% crosslinked S-DVB matrix) Dowex Ix8 (strongly basic beads of 20-50 mesh resin, 8% crosslinked S-DVB matrix)) and strong acid (Dowex 50Wx8 (strongly acid beads of 20-50 mesh resin, 8% crosslinked S-DVB matrix), Dowex 50Wx12 (strongly acid beads of 20-50 mesh resin, 12% S-DVB matrix)) resins, were also employed. Both component membranes were first converted into their salt form and vacuum dried overnight over P.sub.2 O.sub.5 at 40.degree. C. Then 21/4".times.21/4" specimens were coated, each on one side only, with a dimethyl formamide or methanol slurry of the desired resin mixture, usually a 50/50 mixture of the anionic and cationic resins. In addition to the resin solvents dimethyl formamide and methanol, various other solvents which are preferably a partial solvent for the membrane may be employed, such as carbon tetrachloride, chloroform, methylene chloride, 1,2-dichloroethane, 1,1,2 trichloroethane, 1,1,2,2-tetrachloroethane, dimethyl acetamide, decalin tetralin and cyclohexane and the like and various mixtures thereof, may be used. Ten minutes were allowed for the excess solvent to evaporate before sandwiching the two membranes between two pieces of 3 mil aluminum foil. They were next inserted between two pre-heated, 6" diameter steel discs, at 131.degree.-150.degree. C. The whole ensemble was then placed on a hydraulic press and the membranes were fused together for 2.5 minutes at 131.degree.-150.degree. C. at 1500-3000 psi. It will be understood that any suitable known means in addition to fusion by a hydraulic press may be used such as, for example, sonic bonding, thermal impulse, pneumatic press and the like. After cooling in the press to room temperature, the aluminum foil was peeled from the membrane, which was then equilibrated in 1 N KF for 48 hours. Its potential drop was measured in an electrodialysis cell while mounted between 1 N HCl and 1 N KOH. The foregoing procedure was used when fusing together polyethylene matrix membranes, such as AMF's A100 and C100. For Ionac MC3470 and MA3475R membranes, however, it was found that good fusion was obtained after 2.5 minutes at 93.degree.-95.degree. C. and 900-2000 psi if DMF was used as the resin slurry media.

The first membrane in Table I illustrates the high potential drop resulting when components are fused without ion exchange resins between them, despite the fact that good bonding was achieved. Membranes 2 to 4 show the potential drop decrease one obtains by (a) fusing a resin mixture between the components and (b) the additional decline due to the use of smaller particle size (microfine) resins. However, potential drops of 4.8 volts (at 72 amp/ft.sup.2) are of no commercial interest. The same sort of effect was observed with AMF component membranes as seen with membranes numbered 5 to 7. The latter membrane was prepared with a mixture of Dowex 50 Wx8 and Dowex Ix8 resins which were previously pulverized at liquid nitrogen temperatures in a freezer mill to particle sizes of 2-28.mu.. The lowering of the potential drop to about 2 volts illustrates again the importance of particle size in bipolar membrane preparation.

Ionac microfine resins (.ltoreq.40.mu.) were used in the preparation of membranes 8 to 11, also with AMF component membranes. In specimen 8, both resins distributions ranged from 4-40.mu.. The resulting potential drop was 2.11 volts at 72 amp/ft.sup.2. It is seen however, that further lowering of the potential drop was obtained by the use of the combination of Ionac MFC-10 with the MFA-6 resin. The former resin was found to have the narrow particle size distribution of 7-14.mu.. The resulting potential drops of 1.40-1.78 volts were among the lowest obtained. Microscopic examination of similarly prepared membranes containing the same resin mixtures which had been previously dyed (MFC-10 resins with methylene blue and MFA-6 with methyl orange) clearly illustrates the following points:

(a) the resin is fused in a somewhat mono-bead layer, bead-to-bead, in a random distribution of anionic and cationic beads, between the membranes.

(b) normal to the film surface the membrane is almost completely covered randomly with anionic and cationic beads, with the smaller cationic beads, 7-14.mu., very effectively filling the voids between the larger sized anionic beads, 4-40.mu.. This closely-packed arrangement of beads has repeatedly been obtained with the combination of the MFC-10/MFA-6 mixture and has yielded the lowest potential drop membranes of this type.

Further evidence of the effect of particle size upon membrane potential is seen with membranes 12 to 15. The MFA-6 and MFC-6 resins were first classified into particles (a) greater than 38.mu., (b) greater than 22.mu., (c) from 12-22.mu. and finally (d) less than 12.mu.. The four categories of anionic and cationic resins were then prepared as 50/50 mixtures and applied to the membranes as DMF slurries prior to fusion, by the previously described methods. Once more it is seen that membranes (specimen 15) prepared from the smallest size resins had the lowest potential drop. In specimen 16, the MFC-10 (7-14.mu.) resin was used alone and a fairly low potential drop also resulted. Evidently the narrow particle size distribution of the resin also allows it to form a closely-packed, somewhat mono-bead layer, which effectively fills the interface across the entire membrane. A similar result was obtained with specimen 17, where a three-component bipolar membrane was prepared. Here, between the anionic and cationic components was placed a polyethylene film containing 75% by weight pulverized MFC-6 polyethylene resin (4-38.mu.). In addition to polyethylene other polyolefins or halogenated polyolefins may be used such as polypropylene, copolymers of polyethylene and polypropylene, polytrichlorofluoroethylene and the like may be employed. The film was prepared on a roller mill, then hydraulically pressed to a thin film, at 170.degree. C. and 60 tons, which had a potential drop of 0.4 volts at 72 amp/ft..sup.2 in. When fused between two component membranes, the bipolar membrane produced had a potential drop of 2.27 v at 72 amp/ft..sup.2 and microscopically, was found to have a closely packed, mono-bead layer at its interface.

In Table II are recorded the potential drops measured on bipolar membranes prepared with the MFC-10/MFA-6 resin mixture fused between experimental (non-commercial) anionic and cationic component membranes. The component membranes ranged from 2.4 to 5.8 mils thick, containing from 15-25% polystyrene with 2-10% divinylbenzene. They had individual resistances and current efficiencies comparable to commercial anionic and cationic membranes. The final bipolar membranes were found to have potential drops ranging from 1.26-1.87 volts at 72 amp/ft.sup.2, but operated successfully in electrodialysis cells at higher current densities, e.g. 100-153 amp/ft.sup.2, and both higher acid, 8.4-10.1% HF, and base, 3.8-7.1% KOH, concentrations, than those previously discussed,

The individual component membranes were prepared from polystyrenated high density polyethylene films which were prepared by the following method. A 5".times.11" sheet of HDPE film was tared and suspended in a stainless steel trough containing a mixture of 83.5-97.5 parts freshly distilled styrene, 2-10 parts commercial (55%) divinylbenzene, and from 0.1-0.5 parts benzoyl peroxide at the desired impregnation temperature from 70.degree.-80.degree. C. After equilibrium swelling of the film was obtained (15-45 minutes) the film was withdrawn, excess liquid polymerizate squeezed from its surface, and it was then clamped firmly between two pre-heated, aluminum foil (3 mil) covered, glass plates. This ensemble was then completely submerged in a saturated sodium sulfate salt bath maintained from 75.degree.-85.degree. C. and polymerized over night (18 hours). The film was next removed and excess polystyrene removed from its surface. The film was rinsed in distilled water and vacuum dried over P.sub.2 O.sub.5 at 45.degree. C. and 1 mm. Hg. The films contained from 15-25% polystyrene and if higher polystyrene contents were desired the entire process was repeated.

The styrenated sheets were converted into anionic membranes by immersion into chloromethylmethylether containing 2.5 weight percent SnCl.sub.4 and refluxing (59.degree. C.) for six hours before removing and quaternizing by immersion in a 25% trimethylamineacetone solution for 18 hours at 25.degree.-30.degree. C. The membranes were then equilibrated for 48 hours in IN KF.

Cationic membranes were prepared by first pre-swelling the styrenated films in carbon tetrachloride for 18-24 hours at 25.degree.-30.degree. C. before immersion in a 65/35 chlorosulfonic acid/carbon tetrachloride mixture at room temperature, with stirring, under a dry nitrogen atmosphere for 3 hours. The sulfonyl chloride groups so introduced were hydrolyzed by immersion into IN H.sub.2 SO.sub.4 at 45.degree. C. for 48 hours.

The polystyrene content of the component anoinic or cationic membrane may range from 15 to 50 percent by weight.

The cross-linking agent employed is equivalent to that produced by a divinylbenzene-styrene swelling solution containing from 1 to 20 weight percent commercial 55% divinylbenzene.

The invention will be further illustrated from the following examples in which, unless expressly stated otherwise, parts are parts by weight and temperatures are in degrees Celsius.

A fused bipolar member was prepared by applying a 50/50 mixture of pulverized Dowex 50 W8, Dowex Ix8 resin mixture (2-28.mu. particle size) as a DMF slurry, to one side of AMF A100 and C100 membranes. After allowing excess solvent to evaporate for 10 mixtures, the membranes were placed between two 3 mil aluminum foil sheets, with the resin coated faces toward one another, clamped between two 6" diameter pre-heated discs, and fused together on a hydraulic press for 2.5 minutes at 135.degree. C. and 1500 psi. The product was equilibrated in 1 N KF for 48 hours and found to have a potential drop of 2.05 volts at 72 amp/ft.sup.2. Performance test in an electrodialysis cell with the membrane mounted between 3% HF and 0.7% KOH and with a current density of 93 amp/ft.sup.2, yielded a current efficiency of 75% for base generation and a current efficiency of 72% for acid generation.

A fused bipolar membrane was prepared by applying a 50/50 mixture of Ionac MFC-10 (7-14.mu.) and MFA-6 (4-40.mu.) resins as a methanol slurry to one side of both an AMF A100 and C100 ion exchange membrane. After allowing 10 minutes for the excess solvent to evaporate, the membranes were placed with the resin coated faces toward one another between two 3 mil sheets of aluminum foil. The sandwich was then placed between two preheated 6" diameter steel plates and the membranes were fused together by placing in a hydraulic press for 2.5 minutes at 136.degree. C. at 1500 psi. After cooling to room temperature, the membrane was removed and equilibrated in 1 N KF for two days. It was found to have a potential drop of 1.47 volts at 72 amp/ft.sup.2. When mounted between 6% HF and 0.6% KOH in an electrodialysis cell, it was found to operate at an acid current efficiency of 85% and a base current efficiency of 91% at a current density of 93 amp/ft.sup.2.

A fused bipolar membrane was prepared by first preparing a composition containing 75% pulverized Ionac MFC-6 ion exchange resin (4-38.mu.) and 25% high density polyethylene (HDPE) film on a roller mill at 140.degree. C. and then pressing to a thin film on a hydraulic press at 170.degree. C. and 60 tons. The film had a potential drop of 0.4 volts across it at 72 amp/ft.sup.2. The resin containing film was then fused between and A100 and a C100 membrane on a hydraulic press for 2.5 minutes at 136.degree. C. and 1500 psi. After equilibration for 48 hours in 1 N KF the resulting bipolar membrane was found to have a voltage drop of 2.27 volts at 72 amp/ft.sup.2. It performed in an electrodialysis cell between 2.3% HF and 0.6% KOH at an acid current efficiency of 85% and a base current efficiency of 93% at a current density of 74 amp/ft.sup.2.

A fused bipolar membrane was prepared by fusing a 50/50 Ionac MFC-10 (7-14.mu.)/MFA-6 (4-38.mu.) ion exchange resin mixture, applied as a DMF slurry, between anionic and cationic component membranes, on a hydraulic press for 0.5 minutes at 170.degree. C. and 1500 psi. The anionic membrane was prepared from 5.6 mil HDPE film and contained 20% polystyrene with 2% DVB. The cationic membrane was prepared also from 5.6 mil HDPE film and contained 15% polystyrene with 5% DVB. The methods previously described in the specification were used to styrenate and functionalize these components. The resulting bipolar membrane was found to have a potential drop across it of 1.42 volts. It performed in an electrodialysis cell when mounted between 9.2% HF and 4.0% KOH, at 152 amp/ft.sup.2, at a base current efficiency of 75% and an acid current efficiency of 81%. At the same current density, between 8.7% HF and 6.3% KOH it had a base current efficiency of 75% and an acid current efficiency of 78%.

The procedure of Example 4 is repeated using instead the composition set forth in Table III, employed in styrenation temperature of 75.degree. C. and a fusion temperature of 140.degree. C. for a period of 2.5 minutes.

It will be apparent to those skilled in the art that various modifications may be in the details provided by way of illustrations and the invention is not to be limited thereby except as may be recited in the claims which follow.