Cation exchanger.

0001The present invention relates to an ion exchanger capable of adsorbing or permselectively separating a certain component from a fluid mixture. More paticularly, it relates to a cation exchanger having excellent workability, which is useful, for example, as a thin cation exchange membrane having a low electric resistance useful as a separator for electric cells or for electrodialysis for concentrating sea water, as a hollow fiber-type cation exchange membrane useful for dialysis, or as a porous cation exchange membrane providing good permeability for high molecular cations.

0002Among cation exchangers reported in many literatures and patent documents, cation exchangers of sulfonated styrene divinylbenzene copolymers may be mentioned as most practical and useful cation exchangers. Such sulfonated cation exchangers have been developed and various kinds of products have been synthesized in various fields of applications by virtue of their chemical resistance and heat resistance and their adaptability such that their ion exchange properties and selective permeability can be controlled by changing the content of the divinylbenzene as the cross linking agent.

0003However, there is a drawback that the conventional styrene-divinylbenzene type cation exchangers can not satisfy the needs for super low resistance cation exchange membranes in new applications such as an application to concentrate sea water to produce sodium chloride at a cost as low as the cost for industrial salt, and an application as a separator for redox flow cells or methanol fuel cells. Namely, in order to reduce the resistance, it is necessary to reduce the membrane thickness. However, the styrene divinylbenzene type resin has a difficulty in the mechanical strength, particularly in brittleness, and it is thereby impossible to obtain an ion exchange membrane having a thickness of not more than 100»m. Further, the styrene-divinylbenzene type resin is poor not only in the mechanical properties but also in the workability and thus has a drawback that it is thereby difficult to obtain a fabricated membrane such as a hollow fiber membrane or a porous cation exchange membrane.

0004On the other hand, a polysulfone membrane having excellent mechanical strength and workability is used as a membrane for ultrafiltration or reverse osmosis. A sulfonated polysulfone membrane is being studied for improving the permeability of ultrafiltration or reverse osmosis through the polysulfone membrane.

0005For example, US Patent 3,709,841 discloses a sulfonated product of a polysulfone having a repeating unit of the formula: <chemistry id="chem0001" num="0001"><img file="EP0330187B1_D0001.tif" /></chemistry>    Further, Japanese Unexamined Patent Publications No. 99973/1975, No. 146379/1976 and No. 4505/1986 disclose semipermeable membranes having such a sulfonated polysulfone laminated on an asymmetric ultrafilter.

0006However, these sulfonated polysulfone membranes are not cross linked and thus are water-soluble when the ion exchange capacity is 2.0 meq/g dry resin or higher. Even when the ion exchange capacity is low, the water absorptivity is high, and the ion permselectivity is low. Thus, they are not qualified as a substitute for the conventional styrene-divinylbenzene-type cation exchange membranes.

0007It is an object of the present invention to solve the above-mentioned drawbacks inherent to the conventional techniques and to provide a novel cation exchanger having excellent workability and high ion permselectivity.

0008Another object of the present invention is to provide a cation exchange membrane useful for electrodialysis of an energy saving type or useful as a separator for electric cells, or useful for a hollow fiber dialysis module which is compact and convenient for maintenance, which can not be accomplished by the conventional techniques.

0009The above objects of the present invention can be accomplished by a cation exchanger of polysulfone polymer consisting essentially of a sulfonated product of a polysulfone polymer having at least one chloromethyl (-CH₂Cl) per molecule, or a cured product thereof, and having an ion exchange capacity of from 0.5 to 3.5 meq/g dry resin.

0010Now, the present invention will be described in detail with reference to the preferred embodiments.

0011The present invention provides a process for preparing a cation exchanger having an ion exchange capacity of from 0.5 to 3.5 meq/g dry resin, selected from on of the following methods: <ul id="ul0001" list-style="none"><li>(1) a chloromethylated polysulfone is molded, cross-linked in the presence of a Lewis acid, and sulfonated by a sulfonating agent; or</li><li>(2) a molded chloromethylated polysulfone polymer is sulfonated and crosslinked simultaneously by concentrated sulfuric acid.</li></ul>

0012As mentioned above, the conventional sulfonated polysulfone polymers are not cross-linked and tend to be water-soluble polymers when the ion exchange capacity exceeds 2.0 meq/g dry resin, and even if the ion exchange capacity is not higher than 2.0 meq/g dry resin, e.g. at a level of 1.0 meq/g dry resin, they have high water absorptivity and consequently have a low fixed ion concentration in the membranes, whereby the permselectivity is low.

0013Whereas, the cation exchanger of the present invention using a polysulfone polymer containing at least one chloromethyl group in its molecule, has excellent mechanical properties, moldability and ion exchange properties.

0014As the cation exchanger of the second embodiment of the present invention, any polysulfone polymer may be used so long as it contains at least one chloromethyl group per molecule.

0015The polysulfone polymer containing such a chloromethyl group can be obtained by contacting a polymer containing an aromatic polysulfone of the formula: <chemistry id="chem0002" num="0002"><img file="EP0330187B1_D0002.tif" /></chemistry> wherein Ar is <chemistry id="chem0003" num="0003"><img file="EP0330187B1_D0003.tif" /></chemistry> each of X, Y and Z which may be the same or different, is a single bond, -O-, -S-, -SO₂-, <chemistry id="chem0004" num="0004"><img file="EP0330187B1_D0004.tif" /></chemistry> each of R₁ to R₉ which may be the same or different, is a hydrocarbon group having from 1 to 8 carbon atoms, each of a to d is an integer of from 0 to 4, e is an integer of from 0 to 3, (f + g) is from 0 to 7, (h + i) is from 0 to 5, each of R₁₀ and R₁₁ is a hydrogen atom or a hydrocarbon group having from 1 to 6 carbon atoms, and m/n = 100/0 - 1/100, with a chloromethylating agent such as chloromethyl methyl ether, 1,4-bis(chloromethoxy)butane, 1-chloromethoxy-4-chlorobutane or a nucleophilic chloromethylating agent such as formalin-hydrogen chloride or parafromaldehyde-hydrogen chloride in the presence of a catalyst.

0016Such a polysulfone polymer may be a polysulfone polymer of the formula (a), (b) or (c): <chemistry id="chem0005" num="0005"><img file="EP0330187B1_D0005.tif" /></chemistry> or a block copoymer of the above polysulfone unit with other unit such as the following unit (d) to (h): <chemistry id="chem0006" num="0006"><img file="EP0330187B1_D0006.tif" /></chemistry><chemistry id="chem0007" num="0007"><img file="EP0330187B1_D0007.tif" /></chemistry>    According to the present invention, it is possible to obtain a cation exchange membrane providing excellent performance than ever by using the polysulfone (a) which is readily available as a commonly employed polysulfone. However, it is preferred to use a polysulfone block copolymer in view of the mechanical properties of the sulfonated polymer thereby obtained and the easiness in the control of the ion exchange capacity.

0017The reason why the block copolymer provides excellent mechanical properties, has not yet been clearly understood, but may be explained as follows.

0018In the present invention, the presence of the chloromethyl group is necessary to cross-link the polymer and thereby to reduce the water absorptivity of the ion exchanger and to improve the permselectivity. However, by the cross linking, the polymer is more or less prevented from the molecular motion or from swelling upon absorption of water, whereby the internal strain of the polymer increases. Therefore, when the polysulfone homopolymer is used, such action prevails throughout the molecules, whereby the polymer tends to be hard and brittle. However, in the block copolymer, the existence of the segments in which chloromethyl group as an ion exchange group and as a cure site, is not present or very little, whereby the excellent mechanical properties specific to the polysulfone polymer may not be impaired.

0019In the present invention, an aromatic polysulfone/polythioethersulfone copolymer comprising units (a) to (c) and units (e) may, for example, be mentioned as a preferred block copolymer, since such a copolymer has a high molecular weight, and the composition of the copolymer can easily be controlled, and it provides excellent moldability, mechanical strength and chemical resistance.

0020For the introduction of the chloromethyl group to the above polysulfone polymer, a method may be used in which a granular polymer or a molded product in the form of a membrane is contacted with the above-mentioned chloromethylating agent. However, from the view point of the uniformity of the reaction and with a view to obtaining to a polymer having good moldability, it is preferred to dissolve the reactants in a solvent which is stable against the chloromethylating agent and capable of dissolving the polysulfone polymer and to conduct the reaction in a liquid state. As such a solvent, a halogenated hydrocarbon such as trichloroethane or tetrachloroethane, may be employed.

0021Thus, it is possible to obtain a chloromethylated polysulfone polymer having a desired content of chloromethyl groups by adding a chloromethylating agent and a catalyst such as tin chloride to a polysulfone polymer solution and properly selecting the reaction temperature and the reaction time.

0022The content of chloromethyl groups varies depending upon the desired ion exchange capacity in the subsequent sulfonation treatment. It is usually preferred to use a chloromethylated polysulfone polymer having an content of chloromethyl groups of from 0.01 to 3.5 meq/g dry resin, which contains chloromethyl groups in an amount corresponding to from 1 to 100% equivalent of the ion exchange capacity, more preferably from 5 to 50% equivalent of the ion exchange capacity.

0023The chloromethylated polysulfone polymer thus obtained is then dissolved in a single solvent such as trichloroethane, tetrachloroethane, dimethylacetamide, dimethylformamide, dimethylsulfoxide, triethylphosphate or N-methylpyrrolidone, or in a solvent mixture such as a water-acetone mixture, a methanol tetrahydrofuran mixture, and then cast into a desired shape, followed by drying to obtain a molded product. It is also possible to incorporate a Lewis acid such as SnCl₄, ZnCl₄ or H₂SO₄ to said solution and to cross-link the chloromethyl group during the casting, drying and molding.

0024The molded product of the chloromethylated polysulfone polymer or the cross-linked product thereof, thus obtained, may be sulfonated by a known sulfonating agent. However, when the molded product is not cross-linked, sulfonation and a cross linking reaction may be carried out simultaneously by concentrated sulfuric acid to obtain the cation exchanger of the present invention.

0025In another embodiment, the present ivnention provides a cation exchanger consisting essentially of a polymer obtained by mixing the above-mentioned chloromethylated polysulfone polymer and a sulfonated product of the same or different polysulfone polymer in the presence of a Lewis acid.

0026As such a Lewis acid, the ion exchange group of the sulfonated polysulfone polymer in the form of -SO₃H may be used. Thus, a cured product insoluble in a solvent can be obtained simply by casting a mixture of the chloromethylated polysulfone polymer and the sulfonated polysulfone polymer, such as a solution mixture of the two polymers in a polar solvent such as dimethylacetamide, dimethylformamide or dimethylsulfoxide as the common solvent, then drying it to obtain a molded product, and heating the molded product at a temperature of from 180 to 300°C, preferably 220 to 250°C.

0027The cation exchanger of the present invention may be treated in a suitable solution such as an aqueous sodium chloride solution to hydrate ion exchange groups and then used. The cation exchanger of the present invention in the form of a membrane having a thickness of not more than 100 »m, may be used as a membrane for electrodialysis, as a separator for electric cells or as a separating membrane such as for diffusion dialysis.

0028Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples.

REFERENCE EXAMPLE 1

0029In the same manner as the synthesis disclosed in Japanese Unexamined Patent Publication No. 168629/1986, 4,4'-diphenol and a dihalodiphenylsulfone were reacted to obtain a precursor comprising aromatic polysulfone units. Then, the precursor, a dihalodiphenylsulfone and sodium sulfide were reacted to obtain an aromatic polysulfone/polythioethersulfone copolymer A of the fomula: <chemistry id="chem0008" num="0008"><img file="EP0330187B1_D0008.tif" /></chemistry>    Intrinsic viscosity: 0.65    Then, the copolymer A was dissovled in 1,1,2-trichloroethane and then reacted with a trichloroethane solution containing a complex of sulfuric anhydride triethylphosphate in a molar ratio of 2/1 at 25°C, for 100 hours so that one unit of the copolymer A is contacted with two equivalents of the complex. Then, the reaction solution was neutralized with sodium hydroxide, washed and dried. The obtained sulfonated copolymer A had an ion exchange capacity of 2.05 meq/g dry resin.

0030100 parts by weight of the sulfonated copolymer A thus obtained and 25 parts by weight of hexamethoxymethylmelamine were dissolved in N-methylpyrrolidone to obtain a solution having a solid content of 20% by weight. Then, the polymer solution was cast on a glass sheet, dried and heat-treated at 300°C for one hour to obtain a membrane having a thickness of 25 »m.

0031The membrane thus obtained was immersed in a 0.5N NaCl soltuion, and then the alternating current resistance and the transport number of Na ions by a membrane potential method were obtained.    Alternate current resistance    (0.5N NaCl 100Hz): 0.4 Ω·cm²    Na+ transport number    (0.5M NaCl/1M NaCl membrane potential): 0.90

COMPARATIVE EXAMPLE 1

0032Bisphenol A and a dihalodiphenylsulfone were reacted to obtain an aromatic polysulfone polymer C. <chemistry id="chem0009" num="0009"><img file="EP0330187B1_D0009.tif" /></chemistry>    Intrinsic viscosity: 0.56    Then, the above polymer C was sulfonated with the sulfuric anhydride/triethylphosphate complex at 25°C for 60 hours in the same manner as in Example 1 to obtain a sulfonated polymer C having an ion exchange capacity of 2.4 meq/g dry resin.

0033Then, in the same manner as in Example 4, a cation exchange membrane having a thickness of 25 »m was obtained from a solution comprising 100 parts by weight of the sulfonated polymer C of sodium salt type and 25 parts by weight of hexamethoxymethylmelamine. The membrane was immersed in a 0.5N NaCl solution, and it was attempted to measure the resistance, but the membrane raptured during the handling. The membrane was boiled in pure water, whereby the sulfonated copolymer C was dissolved and disintegrated.

EXAMPLE 1

0034The polymer C obtained from bisphenol A and a dihalodiphenylsulfone, used in Comparative Example 1, was dissolved in 1,1,2,2-tetrachloroethane. Then, chloromethyl methyl ether and anhydrous tin chloride were added thereto. The mixture was reacted at 50°C for 4 hours and then precipitated with methyl alcohol. The precipitates were washed to obtain a chloromethylated copolymer C wherein the chloromethyl group content was 1.2 meq/g dry resin.

0035The chloromethylated copolymer C thus obtained was dissolved in tetrachloroethane to obtain a 10 wt% solution. Then, the polymer solution was cast on a glass sheet and then dried under heating at 150°C for two hours to obtain a cast membrane having a thickness of 25 »m.

0036Then, the cast membrane of the chloromethylated copolymer C was sulfonated with 98% by weight of concentrated sulfuric acid at 90°C to obtain a cation exchange membrane.

0037The cation exchange membrane thus obtained had an ion exchange capacity of 2.1 meq/g dry resin. The alternating current resistance in 0.5N NaCl was 0.1 Ω·cm², and the Na ion transport number was 0.78.

COMPARATIVE EXAMPLE 2

0038A cast membrane of the copolymer C was obtained in the same manner as in Example 1 except that no chloromethylating reaction was conducted.

0039Then, the cast membrane was sulfonated with 98 wt% concentrated sulfuric acid. However, the membrane was dissolved, and no cation exchange membrane was obtained.

EXAMPLE 2

0040The copolymer A as used Reference Example 1 was treated in the same manner as in Example 1 to obtain a cast membrane of a chloromethylated copolymer A1 having a thickness of 25 »m wherein the content of chloromethyl groups was 0.5 meq/g dry resin.

0041Then, the cast membrane of the chloromethylated copolymer A1 was sulfonated in the same manner as in Example 1 to obtain a cation exchange membrane.

0042The cation exchange membrane thus obtained had an ion exchange capacity of 1.5 meq/g dry resin. The alternating current resistance in 0.5N NaCl was 0.3 Ω·cm², and the Na ion transport number was 0.90.

EXAMPLE 3

0043A chloromethylated copolymer A2 was obtained in the same manner as in Example 1 except that the copolymer A as used in Reference Example 1 was chloro-methylated at 108°C for 4 hours. The chloromethyl group content of the chloromethylated copolymer A2 was 2.2 meq/g dry resin.

0044On the other hand, a sulfonated copolymer A having an ion exchange capcity of 2.05 meq/g dry resin was obtained in the same manner as in Reference Example 1 except that no neutralization treatment was conducted.

0045Then, 75 parts by weight of the sulfonated copolymer A thus obtained and 25 parts by weight of the above-mentioned chloromethylated copolymer A2 were dissolved in N-methylpyrrolidone to obtain a solution having a solid concentration of 20% by weight. Then, the polymer solution was cast on a glass sheet and heat-treated at 260°C for one hour to obtain a cation exchange membrane having a thickness of 25 »m.

0046With the membrane thus obtained, the alternating current resistance in a 0.5N NaCl solution was 0.5 Ω·cm², and the Na⁺ transport number was 0.95.

0047Then the membrane was immersed in N-methylpyrrolidone, but did not dissolve.

EXAMPLE 4

0048A cation exchange membrane was obtained in the same manner as in Example 3 except that the sulfonated copolymer was neutralized to a sodium type.

0049With the membrane thus obtained, the alternating current resistance in a 0.5N NaCl solution was 0.5 Ω·cm², and the Na⁺ transper number was 0.94.

0050The membrane was immersed in N-methylpyrrolidone, whereby it was dissolved.

EXAMPLE 5

0051An aqueous isopropyl alcohol solution was added to an N-methylpyrrolidone solution containing 100 parts by weight of the sulfonated copolymer A obtained in Reference Example 1 and 25 parts by weight of hexamethoxymethylmelamine and having a solid content concentration of 20% by weight, to obtain a solution having a surface tention of not higher than 28 dyne/cm. The solution was coated on a porous body of polytetrafluoroethylene having a pore diameter of 0.2 »m, a porosity of 80% and membrane thickness of 20 »m and heat-treated at 300°C for one hour to obtain a composite membrane having a deposition of 2 g/m². The alternating current resistance of the membrane in a 0.5N NaCl solution was 0.1 Ω·cm².

0052The cation exchanger of the present invention is characterized in that it consists essentially of a cross linkable sulfonated polysulfone. Therefore, it is possible to obtain a cation exchange membrane capable of controlling the fixed ion concentration i.e. having an optimum permselectivity depending upon its use, by selecting the type of the curing agent, the amount of the curing agent and the curing conditions. Further, the resistance against an organic solvent can be improved.

0053Especially when an aromatic polysulfone/polythioethersulfone copolymer is employed, a cation exchange membrane having excellent moldability and mechanical strength can be obtained. Further, during the sulfonation, sulfonic acid groups are introduced to certain specific sites due to the difference in the reactivity, whereby a block copolymer comprising hydrophilic segments having ion exchange groups introduced and hydrophobic segments having no ion exchange groups introduced may be obtained. There is a feature that even when the ion exchange capacity is high, a tough membrane having high mechanical strength can be obtained.

0054Further, a membrane may be prepared by casting a solution containing a sulfonated copolymer and a curing agent, whereby a thin ion exchange membrane can be obtained, and by coating on a porous base material or on other polymer membrane, followed by drying, a double layered or composite ion exchange membrane can readily be prepared.