Cation exchange membrane and manufacturing method therefor

Abstract

Provided is a cation exchange membrane in which when a hydroxide aqueous solution showing strong alkalinity is produced by using an electrodialyzer, significant swelling in the cation exchange membrane in contact with the hydroxide aqueous solution is suppressed, destruction of a cation exchange resin due to polyvalent cations contained as impurities in a raw material inorganic salt is suppressed, and the capacity of the cation exchange membrane is hardly lowered.　This cation exchange membrane is characterized by having an electric resistance of 2.0-5.0 (Ω∙cm2), as measured at 25ºC in 0.5 mol / L of a NaCl aqueous solution, and a dimensional change rate of at most 1.2%.

Description

Cation exchange membrane and method for producing the same

[0001] The present invention relates to a cation exchange membrane used in electrodialysis for producing an alkaline solution, and a method for producing the same.

[0002] Ion exchange membranes are formed by forming a resin having ion exchange function into a film shape, but due to the cross-linked structure required to obtain good membrane properties, they are mechanically fragile. Therefore, to improve mechanical strength, ion exchange membranes use a porous substrate that functions as a reinforcing material, and the substrate is used as a core around which an ion exchange resin layer is provided. A known method for producing such ion exchange membranes is the so-called paste method. According to this paste method, an ion exchange membrane is produced by applying a polymerizable monomer paste having ion exchange groups or into which ion exchange groups can be introduced to a substrate that functions as a reinforcing material, polymerizing the monomer, and then optionally introducing ion exchange groups (see, for example, Patent Document 1).

[0003] Polyolefin resins or polyvinyl chloride are generally used as substrates that function as reinforcing materials. Polyolefin resins are superior to polyvinyl chloride in terms of alkali resistance, strength, and heat resistance. Therefore, polyolefin resins are used as substrates for ion exchange membranes used in areas where alkali resistance, strength, and heat resistance are required. Such polyolefin resin substrates are used in the form of nonwoven fabrics, woven fabrics, porous films, etc.

[0004] Ion exchange membranes are typically used in electrodialysis. Recently, development has progressed on technologies for producing high-purity alkaline solutions, particularly lithium hydroxide, using electrodialysis apparatuses that use bipolar membranes, each having a structure in which an anion exchange membrane and a cation exchange membrane are bonded together, and unipolar ion exchange membranes. Examples of such electrodialysis apparatuses include three-compartment electrodialysis apparatuses consisting of an acid chamber, a salt chamber, and an alkaline chamber, each using a bipolar membrane, a cation exchange membrane, and an anion exchange membrane, and two-compartment electrodialysis apparatuses in which a cation exchange membrane is positioned between two bipolar membranes. These electrodialysis methods produce lithium hydroxide from a raw inorganic salt, such as a lithium mineral acid salt, by electrodialysis (see, for example, Patent Documents 2 and 3). In the three-compartment electrodialysis apparatuses, the space between the anion exchange membrane side of the bipolar membrane and the cation exchange membrane forms an alkaline chamber. Hydroxide ions generated by dissociation of water within the bipolar membrane permeate the alkaline chamber, and lithium ions permeate from the salt chamber to the alkaline chamber, thereby enabling the direct production of a lithium hydroxide aqueous solution from a raw inorganic salt, such as a lithium mineral acid salt.

[0005] When an electrodialysis apparatus incorporating the bipolar membrane is used to produce a strongly alkaline hydroxide solution such as a lithium hydroxide solution, the cation exchange membrane in contact with the lithium hydroxide solution or the like swells significantly, causing problems such as leakage of the flowing liquid as the operation continues, a decrease in production efficiency due to membrane rupture, and trouble with reassembly after disassembly. This is thought to be due to the membrane swelling as the ion exchange groups in the cation exchange membrane are replaced by lithium ions with a large hydrated ionic radius and the membrane takes on a large amount of water. Calcium ions (Ca ２＋ ) and magnesium ions (Mg ２＋ ) precipitate as hydroxides inside the cation exchange membrane, causing destruction of the cation exchange resin and reducing the ability of the cation exchange membrane to function as a diaphragm over time.

[0006] Japanese Patent Application Laid-Open No. 6-329815 Japanese Patent Application Laid-Open No. 5367190 Japanese Patent Application Laid-Open No. 2009-269810

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a cation exchange membrane that, when a strongly alkaline hydroxide aqueous solution is produced using an electrodialysis apparatus, prevents significant swelling of the cation exchange membrane in contact with the hydroxide aqueous solution, and also prevents destruction of the cation exchange resin by polyvalent cations contained as impurities in the raw inorganic salt, so that the performance of the cation exchange membrane is hardly reduced.

[0008] Based on the above-mentioned problems, the present inventors have intensively investigated the type of substrate and the composition of the cation exchange resin for the cation exchange membrane used in an electrodialysis device incorporating a bipolar membrane. As a result, they have found that by using a cross-linked cation exchange membrane with a high degree of cross-linking, which is not normally adopted due to its high electrical resistance, swelling can be suppressed to a practical level even when contacted with a strongly alkaline hydroxide solution. Furthermore, they have found that there is a range of electrical resistance within which even such highly cross-linked cation exchange membranes can be used in an electrodialysis device incorporating a bipolar membrane. Furthermore, they have found that by using a porous film substrate for the cation exchange membrane with a pore size smaller than that of woven fabric or the like, destruction of the cation exchange resin can be effectively suppressed even when hydroxides of polyvalent cations are generated.

[0009] The present invention relates to a polymer having an electrical resistance of 2.0 to 5.0 (Ω cm) measured at 25°C in a 0.5 mol / L NaCl aqueous solution. ２ ) and a dimensional change rate of 1.2% or less.

[0010] The cation exchange membrane has a structure in which a substrate is coated with a cation exchange resin, and the substrate is preferably a porous film.

[0011] The porous film preferably has a porosity of 20 to 70%, an average pore diameter of 0.01 to 2.0 μm, and a thickness of 20 to 200 μm.

[0012] The material of the porous film is preferably polyolefin.

[0013] The polyolefin is preferably polyethylene.

[0014] The present invention also provides an electrodialysis apparatus for producing an alkaline solution using the cation exchange membrane.

[0015] Preferably, the electrodialysis device further uses a bipolar membrane.

[0016] Furthermore, the present invention provides a method for electrodialysis of a lithium salt solution using the cation exchange membrane.

[0017] Preferably, the electrodialysis method further uses a bipolar membrane.

[0018] Additionally, the present invention provides a method for producing lithium hydroxide using the electrodialysis method.

[0019] The present invention also provides a method for producing the cation exchange membrane, comprising the steps of: preparing a polymerizable composition containing monomer components including a monomer having a functional group capable of introducing a cation exchange group or a cation exchange group and a crosslinkable monomer; and a polymerization initiator; wherein the crosslinkable monomer accounts for 24 to 45 mass % of the monomer components; coating a substrate made of a porous film with the polymerizable composition; polymerizing the polymerizable composition; and, if necessary, introducing a cation exchange group.

[0020] The polymerizable composition preferably contains 1.0 to 15.0 parts by mass of a polymerization initiator relative to 100 parts by mass of the monomer component.

[0021] The cation exchange membrane of the present invention has an electrical resistance of 2.0 to 5.0 (Ω·cm) when measured in a 0.5 mol / L NaCl aqueous solution at 25° C. ２ ) and a dimensional change rate of 1.2% or less. Since the electrical resistance is within the above-mentioned range, the cation exchange membrane can be suitably used for electrodialysis. Furthermore, since the dimensional change rate is 1.2% or less, the cation exchange membrane does not swell even after long-term use and has high durability. Furthermore, electrodialysis devices and electrodialysis methods using the cation exchange membrane can be made more durable than conventional ones.

[0022] The electrical resistance of the cation exchange membrane of the present invention is 2.0 to 5.0 (Ω cm ２ ) and 2.3 to 4.5 (Ω cm ２ ), and preferably 3.0 to 4.3 (Ω cm ２ 2.0 (Ω cm) is more preferable. ２ If the resistance is less than 5.0 (Ω cm), swelling will increase when the film comes into contact with an aqueous hydroxide solution, making the film unsuitable for practical use. ２ ) may cause problems in terms of the power consumption rate in electrodialysis.

[0023] The cation exchange membrane of the present invention may swell significantly in an acidic aqueous solution, and the dimensional change rate indicates the rate of change in the dimension of the cation exchange membrane immersed in a 0.8 mol / L HCl aqueous solution, washed with water, and measured in water relative to the dimension of the cation exchange membrane immersed in a 0.5 mol / L NaCl aqueous solution, washed with water, and measured in water. The dimensional change rate is 1.2% or less, preferably 0.8% or less, more preferably 0.5% or less, and particularly preferably 0.3% or less. If it exceeds 1.2%, leakage of the flowing liquid due to continued operation of electrodialysis, reduced production efficiency due to membrane rupture, and problems with reassembly after disassembly may occur.

[0024] Preferred embodiments for carrying out the present invention will be described below, but the present invention is not limited to the following.

[0025] <Substrate> As explained in the Background Art section, polyolefin resins or polyvinyl chloride are generally used as the substrate of cation exchange membranes, but polyolefin resins are preferred because they are superior to polyvinyl chloride in terms of alkali resistance, strength, and heat resistance. Furthermore, as the polyolefin resin, polypropylene or polyethylene is preferred, and polyethylene is particularly preferred.

[0026] Furthermore, as explained in the Background Art section, when the substrate is used in the form of a woven fabric, polyvalent cations contained as impurities in the raw inorganic salt precipitate inside the cation exchange membrane, causing destruction of the cation exchange resin. Woven fabrics are generally fabrics woven using fibers as warp and weft threads. When a woven fabric is used as the substrate for a cation exchange membrane, the cation exchange resin is polymerized into the weave between the warp and weft threads. In particular, polyolefin-based woven fabric substrates have poor adhesion to the cation exchange resin, which makes it easy for crystals to precipitate at the interface between the substrate and the resin, accelerating destruction of the ion exchange resin due to crystal growth. Therefore, polyolefin-based woven fabric substrates polymerized at temperatures above 110°C, the melting temperature of the woven fabric surface, have good adhesion between the substrate and the resin, preventing destruction of the cation exchange resin. However, precise temperature control is required to avoid melting deep into the substrate, which would result in a loss of mechanical strength as a reinforcing material, resulting in increased manufacturing costs. Furthermore, when a cation exchange resin is polymerized to form the weave, which is the gap between the warp and weft threads, there is a limit to how small the weave can be, and since it has a certain size, it is not able to support the resin three-dimensionally like a porous film with very small, countless undulating tunnel-like pores, and the resin may easily break or fall off, thereby impairing its function as an ion exchange membrane.

[0027] Therefore, it is preferable to use a porous film, nonwoven fabric, or other form of substrate that can reduce gaps in the weave, etc., compared to woven fabric, and it is even more preferable to use a porous film, which makes it easier to control the gaps.

[0028] The porous film preferably has a porosity of 20 to 70%, an average pore diameter of 0.01 to 2.0 μm, and a thickness of 20 to 200 μm.

[0029] The porosity refers to the volume ratio of gaps to the total volume of the porous film. If it is less than 20%, the amount of cation exchange resin formed in the gaps will be too small, resulting in an excessive increase in the membrane resistance of the cation exchange membrane. If it exceeds 70%, the amount of cation exchange resin formed in the gaps will be too large, resulting in an increase in the dimensional change rate of the cation exchange membrane due to swelling and shrinkage of the cation exchange resin, and a decrease in mechanical strength, making it difficult for the cation exchange membrane to function as a reinforcing material. The porosity is preferably 35 to 65%, more preferably 40 to 60%.

[0030] The average pore diameter refers to the average value of the diameter of the pores formed on the surface of the porous film. If it is less than 0.01 μm, the filling ratio of the cation exchange resin formed in the pores will be insufficient, making it difficult to exhibit the performance of the cation exchange membrane. If it exceeds 2.0 μm, the pores will be too large, which may cause destruction or detachment of the ion exchange resin due to precipitation of multivalent ions during use in electrodialysis, and may easily impair the function as an ion exchange membrane. The average pore diameter is preferably 0.02 to 1.0 μm, more preferably 0.02 to 0.20 μm.

[0031] The durability of the cation exchange membrane of the present invention to polyvalent ions can be evaluated by assembling the membrane to be evaluated in an electrodialysis cell and measuring the "calcium ion durability" described below. The current efficiency value at the sixth run in this measurement method is preferably 50% or more, more preferably 60% or more, and particularly preferably 65% ​​or more.

[0032] If the thickness is less than 20 μm, the membrane is too thin, resulting in a decrease in mechanical strength and making it difficult to function as a reinforcing material, whereas if the thickness is more than 200 μm, the membrane is too thick, resulting in an increase in electrical resistance and making it difficult to exhibit the performance of a cation exchange membrane.The thickness is preferably 40 to 150 μm, and more preferably 60 to 130 μm.

[0033] <Cation Exchange Resin> The cation exchange resin of the present invention is a polymerizable composition containing monomer components including a monomer having a cation exchange group or a functional group capable of introducing a cation exchange group and a crosslinkable monomer, and a polymerization initiator, and the crosslinkable monomer accounts for 24 to 45 mass % of the monomer components.

[0034] The monomer component will be described in detail below. The monomer component contains at least one monomer selected from the group consisting of a monomer having a functional group capable of introducing a cation exchange group and a monomer having a cation exchange group, and a crosslinkable monomer.

[0035] Examples of the monomer having a functional group into which a cation exchange group can be introduced include styrene, chloromethylstyrene, vinyltoluene, vinylxylene, α-methylstyrene, vinylnaphthalene, and α-halogenated styrenes. When a cation exchange resin is polymerized using a polymerizable composition containing a monomer having a functional group into which a cation exchange group can be introduced, a step of introducing a cation exchange group into the functional group by sulfonation using concentrated sulfuric acid or the like is required after the polymerization step. Examples of the cation exchange group include a sulfonic acid group, a carboxylic acid group, a chlorosulfonyl group, and a fluorosulfonyl group, but it is preferable to use a sulfonic acid group because it is a strongly acidic group.

[0036] Furthermore, examples of the monomer having a cation exchange group include styrene-based monomers such as styrene sulfonic acid, carboxystyrene, chlorosulfonylstyrene, and fluorosulfonylstyrene, which have high mechanical strength as resins. However, styrene sulfonic acid is preferred because it has a strong acidic group and is easily introduced. When a cation exchange resin is polymerized using a polymerizable composition containing a monomer having a cation exchange group, a step of introducing a cation exchange group into a functional group is not required after the polymerization step. Furthermore, when carboxystyrene, chlorosulfonylstyrene, or fluorosulfonylstyrene is used, the carboxyl group, chlorosulfonyl group, or fluorosulfonyl group may be converted to a sulfonic acid group using a known means after polymerization of the cation exchange resin.

[0037] The monomer component may contain other known monomers as needed, such as acrylonitrile, vinyl chloride, acrolein, methyl vinyl ketone, maleic anhydride, maleic acid, its salts or esters, itaconic acid, its salts or esters, etc.

[0038] The crosslinkable monomer may be any crosslinkable monomer that has been conventionally used in the production of ion exchange membranes, without any particular limitation, and specific examples thereof include m-, p-, o-divinylbenzene, divinylsulfone, butadiene, chloroprene, isoprene, trivinylbenzenes, divinylnaphthalene, diallylamine, triallylamine, and divinylpyridines.

[0039] When the monomer component contains a large amount of styrene-based monomer, it is preferable to use m-, p-, or o-divinylbenzene because of its high copolymerizability with the styrene-based monomer. Furthermore, the crosslinkable monomer preferably accounts for 24 to 45 mass% of the monomer component, and more preferably 34 to 40 mass%. If the crosslinkable monomer content is less than 24 mass%, the crosslink density will be low, resulting in a large dimensional change rate of the cation exchange membrane due to swelling and shrinkage of the cation exchange resin, and it will be difficult to obtain the mechanical strength of the cation exchange resin. If the content exceeds 45 mass%, the crosslink density will be too high, which may make the cation exchange resin brittle. Here, when chloromethylstyrene, listed as a monomer having a functional group capable of introducing a cation exchange group, is copolymerized with a styrene-based monomer such as styrene, during the post-polymerization exchange group introduction reaction using an acid such as concentrated sulfuric acid or chlorosulfonic acid, the chloromethyl group in the chloromethylstyrene abstracts a hydrogen atom from the aromatic ring of the styrene-based monomer, forming a crosslinked structure with the styrene via a methylene bond. Therefore, when a cation exchange resin is polymerized using a polymerizable composition containing chloromethylstyrene together with the styrene-based monomer and then sulfonated, the chloromethylstyrene is treated as a crosslinkable monomer.

[0040] As the polymerization initiator, any conventionally known polymerization initiator can be used without any particular limitation. For example, radical polymerization initiators such as octanoyl peroxide, dilauroyl peroxide, t-butylperoxy-2-ethylhexanoate, benzoyl peroxide, t-butylperoxyisobutyrate, t-butylperoxylaurate, t-hexylperoxybenzoate, 1,1-di(tert-butylperoxy)cyclohexane, p-menthane hydroperoxide, diisopropylbenzene hydroperoxide, α,α'-bis(tert-butylperoxy-m-isopropyl)benzene, di-tert-butyl ... Examples of suitable initiators include 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexyne-3, cumene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, 2,5-dimethyl-2,5-dihydroperoxyhexane, and 2,5-dimethyl-2,5-dihydroperoxyhexane-3. The polymerizable composition preferably contains 1.0 to 15.0 parts by mass of the polymerization initiator per 100 parts by mass of the monomer components, and more preferably 1.2 to 10.0 parts by mass. If the amount of the polymerization initiator is less than 1.0 part by mass, the polymerization of the polymerizable composition may not proceed sufficiently. If the amount of the polymerization initiator is more than 15.0 parts by mass, the polymerization rate may become too high, resulting in heat generation and a decrease in the strength of the substrate. Here, the proportion of the crosslinkable monomer, etc. in the monomer components is expressed in mass %, while the proportion of the polymerization initiator, various additives, etc. in the polymerizable composition is expressed in mass parts relative to 100 mass parts of the total of the monomer components.

[0041] The polymerizable composition may contain, as necessary, various known additives such as a thickener, a plasticizer, a hydrochloric acid scavenger, etc. In particular, when the substrate is a woven fabric, it is preferable to contain 0 to 15 parts by mass of a thickener relative to 100 parts by mass of the monomer components of the polymerizable composition in order to improve the retention of the polymerizable composition on the woven fabric substrate.

[0042] Examples of thickeners include polyvinyl chloride, polyethylene, nitrile butadiene rubber and hydrogenated products thereof, polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer and acid-modified products thereof.

[0043] Examples of the plasticizer include dioctyl phthalate, dibutyl phthalate, tributyl phosphate, acetyl tributyl citrate, dibenzyl ether, or alcohol esters of aliphatic acids or aromatic acids. The inclusion of 30 parts by mass or less of the plasticizer per 100 parts by mass of the monomer components of the polymerizable composition is preferred because it facilitates the progress of the cation exchange group introduction step.

[0044] Examples of the hydrochloric acid scavenger include ethylene glycol diglycidyl ether, styrene oxide, etc. When a halogen-containing monomer such as chloromethylstyrene is used, these are preferred because they can capture hydrogen chloride generated by slight decomposition of the monomer.

[0045] <Production Process of Cation Exchange Membrane> The cation exchange membrane according to this embodiment is produced, for example, by the following process.

[0046] (1) Step of preparing a polymerizable composition A polymerizable composition is prepared, which contains monomer components including a crosslinkable monomer and a functional group capable of introducing a cation exchange group or a monomer having a cation exchange group, and a polymerization initiator, wherein the crosslinkable monomer accounts for 24 to 45% by mass of the monomer components.

[0047] (2) Step of Coating a Substrate with a Polymerizable Composition A substrate made of a porous film is coated with the polymerizable composition. There are no particular limitations on the coating method. For example, the porous film is immersed in a tank filled with the polymerizable composition, and the pores of the porous film are filled with the polymerizable composition. Alternatively, instead of immersion, the pores of the porous film can be filled with the polymerizable composition by a method such as spray coating or application using a doctor blade. The former method is preferred because it can reliably fill the pores with the polymerizable composition.

[0048] (3) Step of Polymerizing the Polymerizable Composition: After coating the substrate, the polymerizable composition is polymerized and cured by heating in a polymerization apparatus such as a heating oven to produce a cation exchange resin. This allows the cation exchange membrane of the present invention to be obtained. In this step, a porous film filled with the polymerizable composition is typically sandwiched between films such as polyester and heated from room temperature under pressure. Pressurization is typically carried out at a pressure of approximately 0.1 to 1.0 MPa using an inert gas such as nitrogen or a roll. This pressurization forces excess polymerizable composition present at the outer interface of the porous film into the voids in the porous film, thereby polymerizing the film and effectively preventing the formation of resin pools. For example, the substrate containing the polymerizable composition is coated on both sides with a release material made of polyester film and wound around a roller, followed by polymerization. The polymerization temperature is preferably set to approximately 20°C to 135°C. In the case of a polyolefin woven fabric, the final polymerization temperature is more preferably set to approximately 110°C to 125°C. The polymerization time varies depending on the polymerization temperature, but is generally approximately 3 to 20 hours. If the final polymerization temperature is less than 110° C., the desired adhesion between the substrate and the resin may not be obtained, and if it exceeds 135° C., the mechanical strength of the ion exchange membrane may be lost. If the polymerization time is less than 3 hours, the polymerization and curing of the polymerizable composition may be insufficient, and if it exceeds 20 hours, productivity may be poor, which is not preferable.

[0049] (4) Step of introducing cation exchange groups as needed When a monomer having a cation exchange group is used as a monomer component in the polymerizable composition, a cation exchange resin is formed by the above-mentioned "(3) step of polymerizing the polymerizable composition", and the desired cation exchange membrane is obtained at this stage. However, when a monomer having a functional group into which a cation exchange group can be introduced is used as the monomer component, the resin obtained by the above-mentioned "(3) step of polymerizing the polymerizable composition" does not have a cation exchange group, and therefore it is necessary to introduce a cation exchange group after the polymerization step, and therefore the step of introducing a cation exchange group is performed as needed.

[0050] The introduction of cation exchange groups is carried out by a method known per se, for example, by treatment such as sulfonation, chlorosulfonation, phosphoniumation, hydrolysis, etc. As already explained, sulfonation to introduce sulfonic acid groups is preferred.

[0051] The excellent effects of the present invention will be explained in the following examples. In the examples and comparative examples, the characteristics of the ion exchange membrane were determined by measuring the following physical properties.

[0052] <1. Electrical Resistance> A cation exchange membrane was sandwiched between two compartment cells having platinum black electrodes, and both sides of the cation exchange membrane were filled with a 0.5 mol / L NaCl aqueous solution. The resistance between the electrodes at 25°C was measured using an AC bridge (frequency 1000 cycles / second). The difference between the inter-electrode resistance and the inter-electrode resistance when no cation exchange membrane was installed was used to determine the membrane resistance (Ω cm ２ The cation exchange membrane used in the above measurement was previously equilibrated in a 0.5 mol / L NaCl aqueous solution.

[0053] <2. Dimensional Change Rate> The ion exchange membrane was cut into a rectangle, and the dimensional change rates were measured in two perpendicular directions by the following method. The average of these measurements was defined as the dimensional change rate in the present invention. The ion exchange membrane was immersed in a 0.5 mol / L-NaCl aqueous solution for 2 hours or more, and then the surface was thoroughly washed with ion-exchanged water. After immersion in ion-exchanged water for 2 hours, the dimensions of the ion exchange membrane were measured (L0). Thereafter, the ion exchange membrane was immersed in 0.8 mol / L-HCl ion-exchanged water for 2 hours or more, and then the surface was thoroughly washed with ion-exchanged water. After immersion in ion-exchanged water for 2 hours, the dimensions of the ion exchange membrane were measured (L1). The dimensional change rate was calculated using the following formula (1): Dimensional change rate (%) = (L1 / L0 - 1) × 100 (1)

[0054] <3. Calcium ion durability> 3-1. Electrodialysis under conditions containing calcium ions A four-compartment cell was used. The cell configuration was anode (Pt plate) / compartment 1: anode chamber / standard cation exchange membrane / compartment 2: salt chamber / test cation exchange membrane / compartment 3: alkaline chamber / standard cation exchange membrane / compartment 4: cathode chamber / cathode (Pt plate). The following solution was immersed in water at a current density of 8 A / dm2 at a liquid temperature of 25°C. ２The current was applied for 3 hours. ２ SO ４ +0.25mol / L H ２ SO ４ Aqueous solution 2 chambers: 0.75 mol / L Li ２ SO ４ +0.025mol / L H ２ SO ４ +250 ppm-Ca aqueous solution Chamber 3: 3.0 mol / L LiOH aqueous solution Chamber 4: 1.0 mol / L LiOH aqueous solution 3-2. Cleaning in place Without disassembling the cell, all chambers were cleaned by introducing ion-exchanged water, and then chambers 2 and 3 were cleaned with 0.05 mol / L H ２ SO ４ An aqueous solution was introduced, and cleaning in place was carried out for 30 minutes under stirring at a liquid temperature of 25° C. Steps 3-1 and 3-2 were repeated six times, after which a current efficiency test (similar to <4. Current efficiency test>) was carried out, and the current efficiency value at this time was taken as the calcium ion durability value.

[0055] <4. Current efficiency> A two-compartment cell sandwiching a cation exchange membrane was used. The cell configuration was anode (Pt plate) (0.5 mol / L-NaOH aqueous solution) / cation exchange membrane / (3.0 mol / L-NaOH aqueous solution) cathode (Pt plate), and the current density was 10 A / dm at a liquid temperature of 25°C. ２ After applying current at 1000 kJ / s for 1 hour, the solution on the anode side was recovered. The sodium hydroxide concentrations of the recovered solution and the initial solution were quantified using a sulfuric acid aqueous solution with a potentiometric titrator (AT-710, manufactured by Kyoto Electronics Manufacturing Co., Ltd.), and the current efficiency was calculated using the following formula: Current efficiency = (CB - CS) / (I × t / F) × 100 [%] In the above formula, CB is the concentration of the initial solution, CS is the concentration of the solution recovered after applying current (both in mol / L), I is the current value (A), t is the application time (sec), and F is the Faraday constant (96,500 C / mol).

[0056] <5. Membrane Thickness> After immersing the cation exchange membrane in a 0.5 mol / L NaCl aqueous solution for 4 hours or more, the moisture on the membrane surface was wiped off with tissue paper and the thickness was measured using a micrometer (MDE-25MX, manufactured by Mitutoyo Corporation).

[0057] 6. Ion Exchange Capacity and Water Content A cation exchange membrane was immersed in a 1 mol / L HCl aqueous solution for 10 hours or more. Then, the counter ions of the ion exchange groups were replaced with sodium ions from hydrogen ions using a 1 mol / L NaCl aqueous solution. The liberated hydrogen ions were quantified (Amol) using a potentiometric titrator (AT-710, manufactured by Kyoto Electronics Manufacturing Co., Ltd.) with a sodium hydroxide aqueous solution. The same ion exchange membrane was then immersed in a 1 mol / L NaCl aqueous solution for 4 hours or more and thoroughly rinsed with ion-exchanged water. The surface moisture was then wiped off with tissue paper, and the wet membrane mass (Wg) was measured. The membrane was then dried under reduced pressure at 60°C for 5 hours, and its dry weight (Dg) was measured. Based on the above measurements, the ion exchange capacity and water content of the cation exchange membrane were calculated using the following formula: Ion exchange capacity = A × 1000 / D [meq / g - dry mass] Water content = 100 × (W - D) / D [%]

[0058] <7. Thickness of porous film> A micro thickness measuring instrument (Type KBM, terminal diameter φ5 mm, measurement pressure 637 g cm) manufactured by Toyo Seiki Co., Ltd. ２ ) was used to measure the thickness of the porous film.

[0059] <8. Air permeability of porous substrate film> Measurement was performed using a Gurley air permeability meter conforming to JIS P-8117. In this specification, air permeability is expressed as air permeability. This measurement value is converted to a 100 μm thickness to determine the air permeability. Air permeability (sec / 100 mL) = Measured air permeability (sec / 100 mL) × 100 (μm) / Film thickness (μm)

[0060] 9. Porosity of Porous Substrate Film A rectangular sample of X cm x Y cm was cut out, and the porosity was calculated using the following formula: Porosity (%) = {1 - (10000 x M / ρ) / (X x Y x T)} x 100, where T is the sample thickness (μm), M is the sample weight (g), and ρ is the density of the resin (g / cm ３ )

[0061] 10. Average Pore Diameter of Porous Substrate Film Measurement was carried out by the half-dry method in accordance with ASTM-F316-86.

[0062] The polyethylene porous film and polyethylene woven fabric used in each of the examples and comparative examples are shown below.

[0063] (Examples 1 to 3, Comparative Example 2) Polyethylene porous film 1 (PE porous film 1) Thickness: 100 μm Porosity (void ratio): 45% Air permeability: 305 sec / 100 cc Average pore diameter: 0.13 μm

[0064] Comparative Example 1 Polyethylene porous film 2 (PE porous film 2) Thickness: 130 μm Porosity (void ratio): 49% Air permeability: 280 sec / 100 cc Average pore diameter: 0.13 μm

[0065] (Example 4) High-density polyethylene monofilament woven fabric (PE woven fabric, melt-type) Warp: 156 mesh - wire diameter 86 μm Weft: 100 mesh - wire diameter 86 μm Opening area (opening rate): 32% Thickness: 185 μm

[0066] (Comparative Example 3) High-density polyethylene monofilament woven fabric (PE woven fabric, non-melting type) Warp: 96 mesh - wire diameter 106 μm Weft: 76 mesh - wire diameter 122 μm Opening area (opening rate): 38% Thickness: 245 μm

[0067] (Comparative Example 4) High-density polyethylene monofilament woven fabric (PE woven fabric, non-melting type) Warp: 120 mesh - wire diameter 76 μm Weft: 120 mesh - wire diameter 76 μm Opening area (opening rate): 41% Thickness: 132 μm

[0068] The polymerization temperature conditions used in each of the Examples and Comparative Examples are shown below.

[0069] (Examples 1 to 3, Comparative Examples 1 and 2) The temperature was raised from 20°C to 50°C over 20 minutes, then from 50°C to 110°C over 60 minutes, and then from 110°C to 130°C over 80 minutes, and then held at 130°C for 180 minutes.

[0070] (Example 4) The temperature was raised from 20°C to 35°C over 25 minutes, then from 35°C to 90°C over 55 minutes, then from 90°C to 110°C over 40 minutes, then from 110°C to 120°C over 45 minutes, and held at 120°C for 225 minutes. Here, since 120°C, which is 110°C or higher, was used as the maximum temperature, the adhesion between the substrate and the resin was good, and thus the sample was designated as a "melt system."

[0071] (Comparative Example 3) The temperature was raised from 20° C. to 45° C. over 30 minutes, then from 45° C. to 95° C. over 50 minutes, then from 95° C. to 105° C. over 20 minutes, and held at 105° C. for 270 minutes. Here, since the maximum temperature used was 105° C., which is lower than 110° C., the adhesion between the substrate and the resin was poor, and therefore the sample was classified as a "non-melting system."

[0072] (Comparative Example 4) The temperature was raised from 20° C. to 30° C. over 30 minutes, then raised from 30° C. to 45° C. over 30 minutes, held at 45° C. for 45 minutes, then raised from 45° C. to 70° C. over 25 minutes, and held at 70° C. for 180 minutes. Here, since 70° C., which is lower than 110° C., was used as the maximum temperature, the adhesion between the substrate and the resin was poor, and therefore the sample was classified as a "non-melting system."

[0073] The sulfonation conditions used in each of the Examples and Comparative Examples are shown below.

[0074] (Examples 1 and 3) The reaction was carried out using chlorosulfonic acid at 40°C for 90 minutes.

[0075] Example 2, Comparative Example 1 Reaction was carried out using chlorosulfonic acid at 40° C. for 60 minutes.

[0076] Example 4 The reaction was carried out using chlorosulfonic acid at 40° C. for 65 minutes.

[0077] Comparative Example 2 The reaction was carried out using chlorosulfonic acid at 40° C. for 105 minutes.

[0078] Comparative Example 3 The reaction was carried out using chlorosulfonic acid at 40° C. for 110 minutes.

[0079] Comparative Example 4 A reaction was carried out using chlorosulfonic acid at 40° C. for 50 minutes.

[0080] Example 1 A polymerizable composition was prepared according to the following formulation. (Monomer Components) Chloromethylstyrene A (CMS-A) 36.5% by mass (meta:para = 5:95) Styrene (St) 43.5% by mass Acrylonitrile (AN) 20.0% by mass The following polymerization initiator was added to 100 parts by mass of the above monomer components: Di-tert-butyl peroxide 1.9 parts by mass Furthermore, the following additives were added to 100 parts by mass of the above monomer components: Acetyl tributyl citrate 10.0 parts by mass Ethylene glycol diglycidyl ether 2.9 parts by mass The mixture was then stirred for 2 hours to obtain a uniform polymerizable composition. Next, the polymerizable composition was applied to a polyethylene porous film 1 serving as a substrate, filling the pores of the porous film with the polymerizable composition. Then, polyester films were used as release films to cover both sides of the porous film, and polymerization was carried out under the polymerization temperature conditions described above. The obtained polymer membrane was sulfonated under the sulfonation conditions described above to obtain a cation exchange membrane, the details of which are shown in Table 1.

[0081] (Examples 2 to 4, Comparative Examples 1 to 4) As in Example 1, polymerization compositions having the formulations shown in Table 1 were used, and polymerization was carried out under the polymerization temperature conditions described above, followed by sulfonation under the sulfonation conditions described above, to obtain cation exchange membranes of Examples 2 to 4 and Comparative Examples 1 to 4.

[0082] The monomer components, polymerization initiators and various additives used in the formulations other than the polymerizable composition of Example 1 in Table 1 are shown below.

[0083] (Monomer components) Chloromethylstyrene B (CMS-B) (meta:para = 45-55:55-45) Divinylbenzene (DVB) Ethylvinylbenzene (EVB) α-methylstyrene (α-MeSt)

[0084] (Polymerization initiator) 1,1-di(tert-butylperoxy)cyclohexane Dilauroyl peroxide

[0085] (Various additives) ・Dibenzyl ether ・Styrene oxide ・Polyvinyl chloride-A ・Polyvinyl chloride-B (a polyvinyl chloride different from the above "Polyvinyl chloride-A") ・Polyethylene ・Polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymer ・Hydrogenated nitrile butadiene rubber

[0086] The results of measuring the physical properties of the cation exchange membranes obtained in Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 2. Here, Comparative Examples 3 and 4 in terms of calcium ion durability were marked as "NG" because destruction of the cation exchange membrane occurred during the test due to precipitation of calcium ions, making it impossible to measure the current.

[0087] In Examples 1 to 4, the electrical resistance was 2.1 to 4.6 (Ω cm ２ ) and the dimensional change rate was 0.2 to 0.6 (%), whereas in Comparative Examples 1 to 4, the electrical resistance was 1.2 to 1.9 or 10.0 (Ω cm ２ ) and in Comparative Examples 1 and 2, the dimensional change rate was 1.4 to 2.0 (%).

[0088]

[0089]

Claims

1. The electrical resistance measured at 25°C in a 0.5 mol / L NaCl aqueous solution is 2.0 to 5.0 (Ω cm ２ ) and a dimensional change rate of 1.2% or less.

2. The cation exchange membrane according to claim 1, wherein the cation exchange membrane has a structure in which a substrate is coated with a cation exchange resin, and the substrate is a porous film.

3. The cation exchange membrane according to claim 2, wherein the porous film has a porosity of 20 to 70%, an average pore diameter of 0.01 to 2.0 μm, and a thickness of 20 to 200 μm.

4. The cation exchange membrane according to claim 2, wherein the porous film is made of polyolefin.

5. The cation exchange membrane according to claim 4, wherein said polyolefin is polyethylene.

6. An electrodialysis apparatus for producing an alkaline solution using the cation exchange membrane according to any one of claims 1 to 5.

7. The electrodialysis device according to claim 6, wherein said electrodialysis device further uses a bipolar membrane.

8. A method for electrodialysis of a lithium salt solution using the cation exchange membrane according to any one of claims 1 to 5.

9. The electrodialysis method of claim 8, wherein said electrodialysis method further uses a bipolar membrane.

10. A method for producing lithium hydroxide using the electrodialysis method according to claim 9.

11. A method for producing a cation exchange membrane according to claim 1, comprising the steps of: preparing a polymerizable composition containing monomer components including a monomer having a functional group capable of introducing a cation exchange group or a cation exchange group and a crosslinkable monomer; and a polymerization initiator; wherein the crosslinkable monomer accounts for 24 to 45 mass % of the monomer components; coating a substrate made of a porous film with the polymerizable composition; polymerizing the polymerizable composition; and, if necessary, introducing a cation exchange group.

12. The method according to claim 11, wherein the polymerizable composition contains 1.0 to 15.0 parts by mass of a polymerization initiator per 100 parts by mass of the monomer component.

Images