JP2009142814A - Fluorine-containing cation exchange membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently and stably manufacture an aqueous sodium hydroxide solution from a low concentration to a high concentration. <P>SOLUTION: A cation exchange membrane has at least two layers of a first layer comprising a fluorine-containing polymer having a sulfonic acid group and a second layer comprising a fluorine-containing polymer having a carboxylic acid group arranged at its cathode side. In the fluorine-containing cation exchange membrane, thickness of the second layer is larger than 15 μm, and difference of water content in the 25 mass% of the aqueous sodium hydroxide solution and water content in the 40 mass% sodium hydroxide solution of the second layer is 3.5% or less. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fluorine-containing cation exchange membrane and a salt electrolysis method.

  An ion exchange membrane method and an alkali chloride electrolysis method are known in which an alkali chloride aqueous solution is electrolyzed using a fluorine-containing cation exchange membrane as a diaphragm to produce alkali hydroxide and chlorine. In salt electrolysis by this method, in order to maintain good operating performance for a long time, it is important to adjust and maintain the catholyte concentration in the cathode chamber within a certain range and uniformly. The catholyte concentration usually exhibits a high current efficiency of 95% or more in the range of 30 to 35% by mass.

  A general method for adjusting the catholyte concentration is a method of supplying water to the cathode chamber. However, this method has the following problems. First, when the liquid circulation in the cathode chamber is insufficient, the catholyte concentration in the vicinity where water is supplied decreases, and conversely, the catholyte concentration in a portion that is difficult to dilute increases, resulting in a decrease in current efficiency. Further, when the supply of water to the cathode chamber is stopped due to some trouble, the concentration of the catholyte rapidly increases and the current efficiency is remarkably lowered. On the other hand, since the electroosmotic water is increased in the portion where the liquid circulation in the anode chamber is insufficient, the concentration of the catholyte is locally reduced and the current efficiency is lowered.

  As a fluorine-containing cation exchange membrane, a method of producing an alkali hydroxide in a wide concentration range using a porous layer made of an inorganic substance and a cation exchange resin on the cathode side is known (see Patent Document 1). .) Further, it is known that a high concentration alkali hydroxide can be produced by using a film modified by irradiation with beta rays or the like (see Patent Document 2). However, all of these methods have a problem that current efficiency is low, a process of forming a porous layer or irradiation of beta rays is required, and the manufacturing process of the film becomes complicated.

JP-A-2-54791 (Claim 1) JP-A-6-16842 (Claim 1)

  An object of the present invention is to provide a fluorine-containing cation exchange membrane capable of stably expressing high current efficiency with a wide range of catholyte concentrations in electrolysis of alkali chloride.

  The present invention relates to a cation exchange membrane having at least two layers: a first layer comprising a fluorinated polymer having a sulfonic acid group and a second layer comprising a fluorinated polymer having a carboxylic acid group disposed on the cathode side thereof. The film thickness of the second layer is larger than 15 μm, and the difference between the water content of the second layer in the 25% by mass sodium hydroxide aqueous solution and the water content in the 40% by mass sodium hydroxide aqueous solution is A fluorine-containing cation exchange membrane having a content of 3.5% or less is provided.

  The present invention is high in a wide range of sodium hydroxide concentrations by using a fluorinated cation exchange membrane in which the change in water content of the second layer having a carboxylic acid group is small with respect to the change in sodium hydroxide concentration. It has been made by finding that current efficiency can be expressed.

  By using the fluorine-containing cation exchange membrane of the present invention for sodium chloride electrolysis, high current efficiency can be stably expressed in a wide range of sodium hydroxide concentration, so an aqueous sodium hydroxide solution can be efficiently and stably reduced from a low concentration to a high concentration. Can be manufactured.

Example of X-ray diffraction pattern of fluorine-containing cation exchange membrane

  In the present invention, the second layer forming the fluorine-containing cation exchange membrane has a difference between the water content in the 25% by mass sodium hydroxide aqueous solution and the water content in the 40% by mass sodium hydroxide aqueous solution being 3.5% or less. is there.

The water content difference W in the present invention is obtained by the following equation 2.
W (%) = ((W ₁ −W ₂ ) / W ₂ ) × 100 Expression 2
In Formula 2, W ₁ is obtained by immersing a film made of an ion exchange resin constituting the second layer in a 25% by mass or 40% by mass sodium hydroxide aqueous solution at 90 ° C. for 16 hours, and then removing the film from the surface. It is the weight measured by wiping off the adhered aqueous sodium hydroxide solution at room temperature. W ₂ is a value obtained by immersing the film after measuring W ₁ in 90 ° C. water for 16 hours and then vacuum-drying the film at 130 ° C. for 16 hours, then taking it out and measuring it at room temperature.

  Examples of the method of setting the difference in water content of the second layer to 3.5% or less include a method of controlling the degree of crosslinking of the fluoropolymer having a carboxylic acid group. It can also be achieved by setting the crystallinity obtained from the X-ray diffraction pattern of the second layer to 18 to 22%. If the degree of crystallinity exceeds 22%, the change in water content due to the change in alkali hydroxide concentration may increase, and if it is less than 18%, the current efficiency during electrolysis may decrease.

Here, the crystallinity refers to the area of the crystalline peak (Ic) and the area of the amorphous halo (Ia) obtained from the X-ray diffraction pattern of the dried film in which the counter ion of the ion exchange group is Na ion. ) Based on the following formula 3.
Xc = (Ic / (Ic + 0.661 × Ia)) × 100 Equation 3
Three examples of X-ray diffraction patterns of the film are shown in FIG. As shown in the figure, a peak near 18 ° (crystalline peak) due to the crystalline portion and a wide peak (noncrystalline halo) with a peak near 16 ° due to the amorphous portion are recognized. It is done. Therefore, peak separation is performed on the assumption that only these two types of peaks exist in the range of 2Θ of the diffraction pattern of 11 to 24 °, and Ic and Ia are obtained. In FIG. 1, the crystallinity decreases as it goes from top to bottom. Further, 0.661 in the equation is a constant known from experience.

  In the present invention, the thickness of the second layer is greater than 15 μm. A thickness of 15 μm or less is not preferable because the concentration of alkali chloride in the cathode that is transmitted from the anode side increases, and the quality of the alkali hydroxide product is impaired. In addition, when the thickness of the second layer is too large, the resistance of the film becomes high. Therefore, the thickness of the second layer is preferably 50 μm or less. A more preferable thickness of the second layer is 17 to 30 μm.

As the fluorine-containing polymer constituting the second layer, a polymer having a repeating unit based on the monomer represented by Formula 1 and having a carboxylic acid group precursor group converted to a carboxylic acid is used. Is preferred.
CF ₂ = CFO (CF ₂ ) _m Y Formula 1
In Formula 1, m is an integer of 2 to 5, Y is hydrolyzed in an alkaline solvent, and is a precursor group that can be converted into a carboxylic acid group (indicated by -COOM, M represents hydrogen or an alkali metal atom). Represents. As this precursor group, -COOR (carboxylic acid ester group, R is a lower alkyl group having 1 to 4 carbon atoms), -CN (cyano group), and -COZ (acid halide, Z is a halogen atom) ).

  By using a polymer having a repeating unit based on the monomer represented by Formula 1, the degree of crystallinity is increased, and the change in water content due to the sodium hydroxide concentration can be reduced.

In the case where the second layer is made of a polymer including a long side chain, a side chain having a branched structure, and a number of side chains having two or more ether bonds, for example, CF ₂ = CF (OCF ₂ CF (CF ₃ )) _{When a} large amount of monomer such as _n OCF ₂ CF ₂ CO ₂ CH ₃ (n is 1 or more) is contained, the crystallinity is lowered, and there is a possibility that the change in water content with respect to the change in alkali hydroxide concentration is increased. . Also, in the case of a ternary copolymer including a large number of monomers having no functional group such as CF ₂ = CFOCF ₂ CF ₂ CF ₃ , the degree of crystallinity tends to decrease.

As the polymer having a repeating unit based on the monomer represented by the formula 1, a copolymer of the monomer represented by the formula 1 and CF ₂ = CF ₂ is particularly preferable, and in particular, CF ₂ = A copolymer of CF ₂ and CF ₂ ═CFO (CF ₂ ) ₃ CO ₂ CH ₃ or a copolymer of CF ₂ ═CF ₂ and CF ₂ ═CFO (CF ₂ ) ₂ CO ₂ CH ₃ is preferred.

  The ion exchange capacity of the polymer used in the second layer is preferably 0.80 to 1.10 mmol / g dry resin, although it depends on the structure of the side chain. When the ion exchange capacity exceeds the above range, the moisture content change with respect to the alkali hydroxide concentration change tends to be large, and when the ion exchange capacity is less than the above range, the membrane resistance increases and the current efficiency may decrease. The ion exchange capacity is particularly preferably 0.85 to 1.05 mmol / g.

  The first layer forming the fluorinated cation exchange membrane is made of a fluorinated polymer having a sulfonic acid group. The first layer may be a laminate of two or more fluoropolymer layers each having a sulfonic acid group.

  The fluorinated polymer having a sulfonic acid group comprises a copolymer of at least one monomer represented by the following formula 4 and at least one monomer represented by the following formula 5, The thing which converted the precursor group into the sulfonic acid group is mentioned.

CF ₂ = CX ¹ X ² Formula 4
(In Formula 4, X ¹ and X ² each independently represent a fluorine atom, a chlorine atom, a hydrogen atom, or a trifluoromethyl group.)
^{_{CF 2 = CF (OCF 2 CFX}} 3) s O (CF 2) t -A ··· formula 5
(In Formula 5, X ³ represents a fluorine atom or a trifluoromethyl group, t is an integer of 1 to 3, s is 0, 1 or 2, and A is a sulfonic acid group (hydrolyzed in an alkaline solvent ( represented by -SO 3 _M, precursor groups M may be converted to represent.) hydrogen or an alkali metal atom.)
As the monomer represented by Formula 4, CF ₂ = CF ₂ , CF ₂ = CF (CF ₃ ), CF ₂ = CH ₂ , CF ₂ = CFH, and CF ₂ = CFCl are preferable. As the polymer, the following are preferable.

  The thickness of the first layer is preferably 20 to 300 μm. When the thickness is less than 20 μm, the strength of the film may be reduced, and when it exceeds 300 μm, the resistance of the film increases. A more preferable thickness of the first layer is 30 to 100 μm. Moreover, it is preferable that the ion exchange capacity of the polymer used for the first layer is 0.8 to 1.2 mmol / g dry resin.

  The fluorine-containing cation exchange membrane of the present invention can be used as it is, but is preferably a treatment for releasing chlorine gas on at least one surface of the cation exchange membrane, particularly preferably at least on the anode side surface of the cation exchange membrane. To improve the long-term stability of the current efficiency.

  As a method for performing a gas release treatment on the surface of the cation exchange membrane, a method of giving fine irregularities on the membrane surface (Japanese Patent Publication No. 60-26495), a liquid containing an iron compound, zirconium oxide, etc. in an electrolytic cell To provide a gas release coating layer containing hydrophilic inorganic particles on the membrane surface (Japanese Patent Laid-Open No. 56-152980), and a porous layer containing particles having no gas and liquid permeable electrode activity Examples of such a method are JP-A-56-75583 and JP-A-57-39185. In addition to the effect of improving the long-term stability of the current efficiency, the gas release coating layer on the surface of the cation exchange membrane can further reduce the voltage under electrolysis.

  The fluorine-containing cation exchange membrane of the present invention can be reinforced by laminating with a woven fabric, a nonwoven fabric fibril, a porous body, or the like preferably made of a fluorine-containing polymer such as polytetrafluoroethylene, if necessary.

  The fluorine-containing cation exchange membrane of this invention can be manufactured, for example with the following method. First, a fluorine-containing polymer having a precursor group of a carboxylic acid group and a fluorine-containing polymer having a precursor group of a sulfonic acid group are respectively synthesized, and these polymers are molded by a coextrusion method to form a film. obtain. Next, the obtained film is laminated with a reinforcing woven fabric, another film made of a fluorinated polymer having a sulfonic acid group precursor group, if necessary, by a roll press to obtain a laminated film. The obtained laminated film is immersed in an alkaline solvent to hydrolyze the precursor group of the carboxylic acid group and the precursor group of the sulfonic acid group, thereby obtaining a fluorine-containing cation exchange membrane.

  By using the fluorine-containing cation exchange membrane of the present invention as a diaphragm between the anode chamber and the cathode chamber, salt electrolysis can be performed stably for a long period of time. The electrolytic cell at this time may be a monopolar type or a bipolar type. The material constituting the electrolytic cell is resistant to saline and chlorine in the case of the anode chamber, such as titanium, and in the case of the cathode chamber, stainless steel or nickel resistant to sodium hydroxide and hydrogen, etc. Is used. In the case where electrodes are disposed in the present invention, the cathode may be disposed in contact with the ion exchange membrane or may be disposed at an appropriate interval.

Moreover, the salt electrolysis using the fluorine-containing cation exchange membrane of the present invention can be carried out under known conditions. For example, by operating at a temperature of 50 to 120 ° C. and a current density of 1 to 6 kA / m ² , a concentration of 20 to A 40% by weight aqueous sodium hydroxide solution can be produced.

  Examples of the present invention (Examples 1 to 3) and comparative examples (Examples 4 and 5) will be described below.

[Example 1]
Resin A having an ion exchange capacity of 0.95 mmol / g after hydrolysis comprising a copolymer of CF ₂ = CF ₂ and CF ₂ = CFO (CF ₂ ) ₃ CO ₂ CH ₃ , and CF ₂ = CF ₂ Resin B having an ion exchange capacity of 1.10 mmol / g after hydrolysis and comprising a copolymer of CF ₂ ═CFOCF ₂ CF (CF ₃ ) OCF ₂ CF ₂ SO ₂ F was synthesized. Next, the resin A and the resin B were molded by a coextrusion method to obtain a film A having a two-layer structure in which the resin A layer had a thickness of 25 μm and the resin B layer had a thickness of 65 μm. Moreover, resin B was shape | molded by the melt extrusion method, and the 20-micrometer-thick film B was obtained.

  On the other hand, after rapidly stretching a polytetrafluoroethylene (PTFE) film, six monofilament PTFE yarns obtained by slitting to a thickness of 100 denier and 6 denier polyethylene terephthalate (PET) fibers are aligned and twisted. The multifilament PET yarn was plain woven with an alternating arrangement of two PET yarns for one PTFE yarn to obtain a reinforced woven fabric with a yarn density of 30 yarns / cm. This woven fabric was flattened using a roll press so that the thickness of the woven fabric was about 80 μm.

  The obtained woven fabric and film are laminated in the order of film B, woven fabric, film A (so that the resin A layer is on the side of the release PET film), and release PET film (thickness 100 μm) in this order. Used to laminate. Then, the release PET film was peeled off to obtain a reinforced laminated film.

Next, a paste composed of 29.0% by mass of zirconium oxide having an average particle diameter of 1 μm, 1.3% by mass of methylcellulose, 4.6% by mass of cyclohexanol, 1.5% by mass of cyclohexane, and 63.6% by mass of water, The film was transferred to the film B side of the laminated film by a roll press, and a gas release coating layer was adhered. The amount of zirconium oxide deposited at this time was 20 g / m ² .

Next, the obtained membrane was immersed in an aqueous solution of 30% by mass of dimethyl sulfoxide and 15% by mass of potassium hydroxide at 90 ° C. for 30 minutes to hydrolyze —CO ₂ CH ₃ groups and —SO ₂ F groups, Converted to ion exchange groups.

Further, a dispersion in which 13% by mass of zirconium oxide having an average particle diameter of 5 μm was dispersed in an ethanol solution containing 2.5% by mass of the acid type polymer of resin B was prepared, and this dispersion was used as the film A of the laminated film. The gas releasing coating layer was deposited by spraying to the side. The adhesion amount of zirconium oxide at this time was 10 g / m ² .

  Separately from the above, the resin A is formed into a film and hydrolyzed in the same manner as described above, and the crystallinity, the moisture content in 25% by mass sodium hydroxide, and the moisture content in 40% by mass sodium hydroxide are measured. As a result, the crystallinity was 19.3%, the water content in 25% by mass sodium hydroxide was 5.7%, and the water content in 40% by mass sodium hydroxide was 2.5%. Was 3.2%.

The fluorine-containing cation exchange membrane thus obtained was placed in an electrolytic cell so that the film A faces the cathode, and electrolysis of an aqueous sodium chloride solution was performed. In the electrolysis, an electrolytic cell (height 5 cm, width 30 cm) having an effective energization area of 1.5 dm ² is used in the cathode chamber frame so that the catholyte hardly circulates in the cathode chamber frame. The generated sodium hydroxide aqueous solution outlet was arranged in the upper part of the cathode chamber. A titanium punched metal (4 mm short diameter, 8 mm long diameter) coated with a solid solution of ruthenium oxide, iridium oxide and titanium oxide is used as the anode, and a punched metal made of SUS304 (5 mm short diameter, 10 mm long diameter) is used as the cathode. The electrodeposited Raney nickel containing ruthenium was used.

The electrolysis is performed in such a manner that the cathode side is pressurized so that the anode and the membrane are in contact with each other, and 290 g / L sodium chloride aqueous solution is supplied to the anode chamber and water is supplied to the cathode chamber. When the electrolysis was conducted for 1 week at a temperature of 90 ° C. and a current density of 4 kA / m ² while maintaining the sodium hydroxide concentration discharged from the cathode chamber at 32 mass%, the current efficiency was 97.3. % Was kept almost constant. Thereafter, electrolysis was performed for 1 week under the same conditions as described above while maintaining the sodium hydroxide concentration at 25% by mass, and the current efficiency was kept at a constant level of 95.8%. Furthermore, when electrolysis was performed for 1 week under the same conditions as described above while maintaining the sodium hydroxide concentration at 40% by mass, the current efficiency was maintained at 96.3% and was almost constant. Thus, high current efficiency of 95.5% or more was expressed in a wide range of sodium hydroxide concentration of 25 to 40% by mass.

[Example 2]
A fluorine-containing cation exchange membrane was obtained in the same manner as in Example 1 except that the ion exchange capacity of Resin A was 0.90 mmol / g, and sodium chloride was electrolyzed under the same conditions as in Example 1. Resin A formed into a film has a crystallinity of 21.0%, a moisture content in 25 mass% sodium hydroxide is 5.0%, and a moisture content in 40 mass% sodium hydroxide is 2.2%. The difference in moisture content was 2.8%.
The current efficiency at each sodium hydroxide concentration is 97.5% for 32% by mass, 96.3% for 25% by mass, and 95.8% for 40% by mass, All exhibited high current efficiency of 95.5% or more.

[Example 3]
Fluorine-containing positive electrode was obtained in the same manner as in Example 1 except that resin A was a copolymer of CF ₂ ═CF ₂ and CF ₂ ═CFO (CF ₂ ) ₂ CO ₂ CH ₃ having an ion exchange capacity of 1.05 mmol / g. An ion exchange membrane was obtained and electrolysis was performed under the same conditions as in Example 1. Resin A formed into a film has a crystallinity of 21.7%, a moisture content in 25 mass% sodium hydroxide of 4.9%, and a moisture content in 40 mass% sodium hydroxide of 2.4%, The difference in water content was 2.5%.
The current efficiency at each sodium hydroxide concentration is 96.8% for 32% by mass, 96.5% for 25% by mass, and 95.5% for 40% by mass, All exhibited high current efficiency of 95.5% or more.

[Example 4 (comparative example)]
Except that resin A was a copolymer of CF ₂ ═CF ₂ and CF ₂ ═CFOCF ₂ CF (CF ₃ ) OCF ₂ CF ₂ CO ₂ CH ₃ having an ion exchange capacity of 0.95 mmol / g, the same as Example 1 Thus, a fluorine-containing cation exchange membrane was obtained, and electrolysis was performed under the same conditions as in Example 1. Resin A formed into a film has a crystallinity of 14.8%, a moisture content in 25% by mass sodium hydroxide of 8.0%, and a moisture content in 40% by mass sodium hydroxide of 2.6%. The difference in moisture content was 5.4%.
The current efficiency at each sodium hydroxide concentration was 96.8% at 32% by mass, 92.3% at 25% by mass, and 94.8% at 40% by mass. .

[Example 5 (comparative example)]
Terpolymer of the resin A and _CF 2 = CF ₂ of the ion exchange capacity _{1.00mmol / g CF 2 = CFO (} CF 2) 2 CO 2 CH 3, CF 2 = CFOC 3 F 7 (CF 2 = CFO (CF ₂ ) ₂ CO ₂ CH ₃ / CF ₂ = CFOC ₃ F ₇ = 21/79 molar ratio), a fluorinated cation exchange membrane was obtained in the same manner as in Example 1, and electrolysis was performed under the same conditions as in Example 1. Went. Resin A formed into a film has a crystallinity of 6.5%, a moisture content in 25% by mass sodium hydroxide is 8.0%, and a moisture content in 40% by mass sodium hydroxide is 2.8%. The difference in moisture content was 5.2%.
The current efficiency at each sodium hydroxide concentration was 96.2% for 32% by mass, 87.6% for 25% by mass, and 88.2% for 40% by mass. .

Claims (4)

  A cation exchange membrane having at least two layers of a first layer composed of a fluorinated polymer having a sulfonic acid group and a second layer composed of a fluorinated polymer having a carboxylic acid group disposed on the cathode side, The thickness of the second layer is larger than 15 μm, and the difference between the water content in the 25% by mass sodium hydroxide aqueous solution and the water content in the 40% by mass sodium hydroxide aqueous solution is 3.5%. The following fluorine-containing cation exchange membrane.   The fluorine-containing cation exchange membrane according to claim 1, wherein the crystallinity obtained from the X-ray diffraction pattern of the second layer is 18 to 22%. The fluorine-containing cation exchange membrane according to claim 1 or 2, wherein the second layer is made of a polymer having a repeating unit based on the monomer represented by Formula 1.
CF ₂ = CFO (CF ₂ ) _m Y Formula 1
In Formula 1, m is an integer of 2 to 5, Y is hydrolyzed in an alkaline solvent, and is a precursor group that can be converted into a carboxylic acid group (indicated by -COOM, M represents hydrogen or an alkali metal atom). Represents.   A salt electrolysis method using the cation exchange membrane according to any one of claims 1 to 3 as a diaphragm for an anode chamber and a cathode chamber.

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