JP2009096821A - Cation exchange membrane for producing salt and method for manufacturing the membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cation exchange membrane, which is used for producing salt and the concentrating performance and mechanical strength of which are improved in comparison with the cation exchange membranes used up to now without increasing electric resistance. <P>SOLUTION: The cation exchange membrane for producing salt is manufactured by irradiating an ethylene-tetrafluoroethylene copolymer film with ionizing radiation to generate radicals in an ethylene-tetrafluoroethylene copolymer and performing graft polymerization in a polymerizable mixture comprising a polymerizable monomer having a functional group, into which a cation exchanging group can be introduced, and a cross-linking monomer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a cation exchange membrane used for salt production and a method for producing the same.

  An electrodialysis tank using positive and negative ion exchange membranes is used in the seawater concentration step in the ion exchange membrane salt production method. Improvements in the performance of ion exchange membranes used in electrodialysis tanks are demanded by membrane electrical resistance, concentration performance, durability, etc. In order to reduce salt production costs, the membrane electrical resistance is increased. It is necessary to improve the concentration performance without any problems. In addition, it is necessary to improve durability, particularly mechanical strength.

  A number of methods have been proposed for producing an ion exchange membrane for salt production (see, for example, Patent Documents 1 to 3), and a polymerizable monomer having a functional group or an ion exchange group into which an ion exchange group can be introduced ( Monomer), a crosslinkable monomer (crosslinking agent) and a mixture containing a polymerization catalyst as main components are coated on a woven cloth made of polyvinyl chloride and polymerized, and then an ion exchange group is introduced as necessary. Is widely known.

  However, the ion exchange membrane obtained by this method is difficult to improve the concentration performance without increasing the electrical resistance of the membrane, and the mechanical strength is not satisfactory.

  In order to solve this problem, after impregnating and supporting a polymerizable monomer on a polypropylene fiber base material, etc., a method of obtaining an ion exchange membrane by graft polymerization with ionizing radiation, or impregnating and supporting a polymerizable monomer on an olefin base material, etc. Then, a method is proposed in which the polymerization is completed by partially polymerizing with ionizing radiation, followed by heating in the presence of a polymerization initiator to obtain an ion exchange membrane (for example, Patent Documents 4 to 4). 6).

However, although either method can improve the mechanical strength of the membrane, no satisfactory result has been found in the concentration performance of the membrane.
Japanese Examined Patent Publication No. 39-27861 Japanese Patent Publication No.40-28951 Japanese Patent Publication No. 44-19253 JP-A-51-52489 JP 60-238327 A Japanese Patent Application Laid-Open No. 06-271687

  It is an object of the present invention to improve the concentration performance and mechanical strength of a cation exchange membrane used for salt production without increasing the electric resistance as compared with a conventionally used membrane. Is.

  As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention are formed after irradiating an ethylene-tetrafluoroethylene copolymer film with ionizing radiation, for example, by graft polymerization of a styrene monomer or the like. By introducing a sulfonic acid group into the graft side chain, the concentration performance was increased and the mechanical strength was improved without increasing the electrical resistance, compared with the conventionally used ion exchange membrane for salt production. It has been found that a membrane can be provided.

That is, the present invention has achieved the above object by adopting the following configuration.
(1) Polymerization having a functional group capable of introducing a cation exchange group after generating radicals in the ethylene-tetrafluoroethylene copolymer by irradiating the ethylene-tetrafluoroethylene copolymer film with ionizing radiation A cation exchange membrane for salt production obtained by graft polymerization in a polymerizable mixture containing a crosslinking monomer (monomer) and a crosslinking monomer (crosslinking agent).
(2) When the functional group capable of introducing the cation exchange group itself is not a cation exchange group, the salt production according to the above (1), which is treated with a compound capable of imparting a cation exchange group after graft polymerization Cation exchange membrane for use.
(3) The cation exchange membrane for salt production according to (1), wherein the functional group itself capable of introducing the cation exchange group is a cation exchange group.
(4) Polymerization having a functional group capable of introducing a cation exchange group after generating radicals in the ethylene-tetrafluoroethylene copolymer by irradiating the ethylene-tetrafluoroethylene copolymer film with ionizing radiation A method for producing a cation exchange membrane for salt production, wherein graft polymerization is carried out in a polymerizable mixture containing a polymerizable monomer (monomer) and a crosslinkable monomer (crosslinking agent).

As is clear from the above, the gist of the present invention resides in the following (1) and (2).
(1) A polymerizable monomer (monomer) having a functional group capable of introducing a cation exchange group after generating radicals by irradiating an ionizing radiation to an ethylene-tetrafluoroethylene copolymer, crosslinkability A cation exchange membrane production method in which graft polymerization is carried out in a polymerizable mixture containing a monomer (crosslinking agent) and a swelling solvent as main components, and sulfonic acid groups are introduced using chlorosulfonic acid or the like as necessary. is there.
(2) A cation exchange membrane obtained by the method described in (1) above.

  According to the present invention, it is possible to provide a cation exchange membrane having an increased concentration performance and an improved mechanical strength without increasing the electric resistance as compared with the cation exchange membrane currently used for salt production. , Can contribute to reducing salt production costs.

  The method for producing a cation exchange membrane according to the present invention comprises a polymerizable group having a functional group capable of introducing a cation exchange group after generating radicals by irradiating an ethylene-tetrafluoroethylene copolymer film with ionizing radiation. Graft polymerization is performed in a polymerizable mixture mainly composed of a monomer (monomer), a crosslinkable monomer (crosslinking agent), and a swelling solvent, and if necessary, sulfonic acid groups are formed using chlorosulfonic acid or the like. It is the feature to introduce.

Hereinafter, embodiments of the present invention will be described in detail.
As the polymer film substrate that can be used in the present invention, an ethylene-tetrafluoroethylene copolymer film that improves the durability of the obtained ion exchange membrane and suppresses the swellability can be used. It is preferable to use one having a thickness of 20 to 100 μm.
The form of the base material is a form of a membrane (film) from the viewpoint of utilization as an ion exchange membrane for salt production, and its size and thickness can be appropriately determined.
Examples of the ethylene-tetrafluoroethylene copolymer film according to the present invention include Asflex 1250NT (product name) manufactured by Asahi Glass Co., Ltd.

The polymerizable monomers (monomers) that can be used in the present invention are listed below, but are not limited thereto.
(1) A monomer having an aromatic ring into which a sulfonic acid group is easily introduced. For example, styrene, vinyl toluene and the like.
(2) A monomer having a carboxylic acid group or a nitrile group. For example, acrylic acid ester, methacrylic acid ester, acrylonitrile and the like.
The crosslinkable monomers (crosslinking agents) that can be used in the present invention are listed below, but are not limited thereto.
(3) A monomer capable of introducing a crosslinked structure (crosslinkable monomer). That is, one having at least two vinyl groups. For example, divinylbenzene, trivinylbenzene, divinyltoluene, divinylnaphthalene, ethylene glycol dimethacrylate, etc.
The proportion of the crosslinkable monomer used is 10% by weight or less, preferably 3% by weight or less, based on the total amount of monomers in the polymerizable mixture.

  In the present invention, the monomer may be diluted in a solvent. The diluting solvent is not particularly limited, but hydrocarbons such as benzene, xylene, toluene and hexane, alcohols such as methanol, ethanol and isopropyl alcohol, ketones such as acetone, methyl isopropyl ketone and cyclohexane, dioxane and tetrahydrofuran, etc. Ethers, esters such as ethyl acetate and butyl acetate, and solvents such as nitrogen-containing compounds such as isopropylamine, diethanolamine, N-methylformamide, N, N-dimethylformamide, and the like. You can select and use. When diluted in a solvent and used, the monomer concentration is not particularly limited, but is usually preferably 20 wt% or more.

  The graft polymerization of the above monomer onto the base material (ethylene-tetrafluoroethylene copolymer film) is a so-called pre-irradiation method in which the base material is subjected to a polymerization reaction with the monomer after irradiation with ionizing radiation, or simultaneously with the base material and monomer. Irradiation and polymerization reaction can be performed by any of the so-called simultaneous irradiation methods. The pre-irradiation method is preferably used because the amount of homopolymer that does not undergo graft polymerization on the substrate is small. There are two pre-irradiation methods: a polymer radical method that irradiates the substrate in an inert gas and a peroxide method that irradiates the substrate in an oxygen-existing atmosphere, both of which are used in the present invention. Can do.

An example of the pre-irradiation method will be described below.
First, an electron beam which is one of ionizing radiations is irradiated to a base material at −10 to 50 ° C., preferably near room temperature, for 5 to 100 kGy. If there is a large spatial or temporal difference between the irradiation of ionizing radiation and the graft polymerization, the substrate is inserted into an oxygen-impermeable plastic bag, and then oxygen is removed from the bag by nitrogen substitution or the like. Subsequently, the bag containing this base material is irradiated with ionizing radiation. By transporting the substrate after irradiation in an oxygen-impermeable plastic bag and transporting it in a dry ice-packed box or storing it in a freezer, due to spatial or temporal differences from ionizing radiation irradiation to graft polymerization, ionizing radiation irradiation The disappearance of the generated radical can be suppressed. Next, after the irradiated substrate is taken out in the atmosphere and transferred to a glass container, the container is filled with a monomer solution or a monomer mixed solution (solvent dilution). As the monomer liquid or the monomer mixed solution, one obtained by removing oxygen gas in advance by bubbling or freeze degassing with an inert gas containing no oxygen is used. Graft polymerization for introducing a polymer graft chain to an irradiated substrate is usually carried out at room temperature to 80 ° C, preferably 40 to 70 ° C.

  The graft ratio of the polymer thus obtained (that is, the weight percent of the graft chain relative to the base material before polymerization) is 10 to 300% by weight, more preferably 20 to 150% by weight. The graft ratio can be appropriately changed depending on the irradiation dose, polymerization temperature, polymerization time and the like.

  A cation exchange group such as a sulfonic acid group can be introduced into the base material into which the graft chain has been introduced as the next step. The introduction of sulfonic acid groups can be performed without any limitation by a wide range of methods conventionally used. Specific examples are shown below.

  The substrate after the graft reaction is immersed in a chlorosulfonic acid solution having a concentration of 0.2 to 1.5 mol / L using 1,2-dichloroethane as a solvent at 25 to 80 ° C. for 1 to 96 hours to be reacted. After reacting for a predetermined time, the membrane is thoroughly washed. Then, the sulfonation reaction is terminated by immersing in an aqueous sodium hydroxide solution having a concentration of 1 to 10% by weight for 1 to 24 hours, and the membrane is sufficiently washed with water. As the sulfonating agent necessary for the sulfonation reaction, concentrated sulfuric acid, sulfur trioxide, sodium thiosulfate and the like can be used, and there is no particular limitation as long as these sulfonic acid groups can be introduced.

  Hereinafter, the cation exchange membrane of the present invention and the production method thereof will be described in more detail based on examples. In addition, this invention is not limited to this Example.

Example 1
After an ethylene-tetrafluoroethylene copolymer film having a thickness of 50 μm is inserted into an oxygen-impermeable polybag, the inside of the bag is purged with nitrogen to remove oxygen in the bag. Next, the bag containing the substrate was irradiated with an electron beam at 25 ° C., an acceleration voltage of 250 keV, and an electron beam current of 10 mA at 25 kGy. Next, the irradiated substrate was taken out in the atmosphere and transferred to a glass container, and then bubbled with high-purity nitrogen, and charged with a polymerizable mixture from which oxygen gas had been removed in advance. The polymerizable mixture is a cyclohexane solution containing styrene as a polymerizable monomer and divinylbenzene as a crosslinkable monomer as monomers, and the total monomer concentration in the solution is 75% by weight, The proportion of the crosslinkable monomer in the body was adjusted to 3% by weight. After the filling, the graft polymerization was carried out at 50 ° C. for 120 minutes, and then the membrane was taken out from the glass container, washed with acetone and methanol in this order, vacuum dried, and the weight was measured. The graft ratio was 105%.

  The substrate after the grafting reaction was immersed in a chlorosulfonic acid solution having a concentration of 10% by weight using 1,2-dichloroethane as a solvent at room temperature for 24 hours, and then the membrane was thoroughly washed with water. Thereafter, it was immersed in an aqueous solution of sodium hydroxide having a concentration of 10% by weight for 24 hours. The obtained cation exchange membrane was thoroughly washed with water and stored in 0.5N-NaCl aqueous solution. The film thickness of the synthesized film was 96 μm. The rupture strength of the obtained cation exchange membrane was measured with a Murren burst strength tester.

Further, the cation exchange membrane and a commercially available anion exchange membrane (Asahi Glass Co., Ltd. ASA) were mounted on a small electrodialysis apparatus (membrane area 8 cm ² ), and a concentration test was performed. A 0.5 M sodium chloride aqueous solution was used as the feed solution under the concentration conditions of a desalting chamber flow rate of 6 cm / s and a current density of 3 A / dm ² .

  Membranes synthesized by a method different from that of Example 1 were used as Examples 2 to 20, and membranes currently used as cation exchange membranes for salt production were set as Comparative Examples 1 and 2, and together with Example 1, the synthesis conditions and membrane characteristics were Table 1 shows. In addition, the cation exchange membrane for salt production used in Comparative Example 1 is Selemion CSO manufactured by Asahi Glass Co., Ltd., and the cation exchange membrane for salt production used in Comparative Example 2 is Neoceptor CMX manufactured by Astom. .

FIG. 1 shows the relationship between the membrane resistance and the concentration of sodium chloride in the concentrated solution as a result of the concentration test.
As shown in Table 1, all the membranes produced according to the present invention showed high burst strength compared with the commercially available cation exchange membrane for salt production.
Further, the membrane resistance was almost the same as or lower than that of the commercially available membrane.
Furthermore, as shown in FIG. 1, any cation exchange membrane produced according to the present invention showed a concentration performance equal to or higher than that of a commercially available cation exchange membrane. In addition, the straight line shown in FIG. 1 is a straight line which shows the concentration performance equivalent to a commercially available ion exchange membrane, and it can be said that all the membrane performance shown above a straight line is higher concentration performance than a commercial membrane.

It is a graph showing the relationship between the resistance of a cation exchange membrane in the Example and comparative example of this invention, and the sodium chloride density | concentration of a concentrate.

Claims (4)

  A polymerizable monomer having a functional group capable of introducing a cation exchange group after irradiating an ethylene-tetrafluoroethylene copolymer film with ionizing radiation and generating radicals in the ethylene-tetrafluoroethylene copolymer; And a cation exchange membrane for salt production obtained by graft polymerization in a polymerizable mixture containing a crosslinkable monomer.   2. The salt-forming cation according to claim 1, wherein when the functional group capable of introducing the cation exchange group is not a cation exchange group, the cation exchange group is treated with a compound capable of providing a cation exchange group after graft polymerization. Ion exchange membrane.   The cation exchange membrane for salt production according to claim 1, wherein the functional group capable of introducing the cation exchange group itself is a cation exchange group.   A polymerizable monomer having a functional group capable of introducing a cation exchange group after generating radicals in the ethylene-tetrafluoroethylene copolymer by irradiating the ethylene-tetrafluoroethylene copolymer film with ionizing radiation. And a method for producing a cation exchange membrane for salt production, wherein graft polymerization is carried out in a polymerizable mixture containing a crosslinkable monomer.

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