Patent Application
20230349058
The present invention relates to an ion exchange membrane and an electrolyzer.
Fluorine-containing ion exchange membranes, which have excellent heat resistance, chemical resistance, and the like, are widely used as electrolytic membranes for, for example, alkali chloride electrolysis, ozone generation electrolysis, fuel cells, water electrolysis, and hydrochloric acid electrolysis in various applications, further extending to new applications.
Of these, in alkali chloride electrolysis for producing chlorine and alkali hydroxide, ion exchange membrane methods have been predominant recently. Additionally, in order to reduce the electric power consumption rate, natural-circulation zero-gap base electrolyzers including an ion exchange membrane, an anode, and a cathode in close contact one another have become predominant for alkali chloride electrolysis by ion exchange membrane methods. For ion exchange membranes used in alkali chloride electrolysis, required are various capabilities. For example, capabilities such as high mechanical strength of membranes are required. As an example to be contemplated from the viewpoint of strength, for example, Patent Literature 1 suggests an ion exchange membrane comprising a layer S comprising a fluorine-containing polymer having a sulfonic acid group, a layer C comprising a fluorine-containing polymer having a carboxylic acid group, and a plurality of reinforcing materials arranged inside the layer S and functioning as at least one of reinforcement yarn and sacrifice yarn, wherein A and B, both of which are defined below, satisfy formulas (1) and (2):
B≤240 μm (1)
2.0≤B/A≤5.0 (2)
wherein, when the ion exchange membrane is viewed from the top surface, A represents an average cross-sectional thickness of the membrane measured in pure water for a region, in which the reinforcing materials do not exist, and B represents an average cross-sectional thickness of the membrane measured in pure water for a region, in which strands of the reinforcement yarn cross each other, and for a region, in which the reinforcement yarn crosses the sacrifice yarn.
The ion exchange membrane described in Patent Literature 1 can reduce the electrolytic voltage in alkali chloride electrolysis using a natural-circulation zero-gap base electrolyzer while the high mechanical strength is retained. In contrast, studies conducted by the present inventors have revealed that, when the ion exchange membrane described in Patent Literature 1 is operated for electrolysis, its strength tends to significantly decrease before and after the operation. That is, the technique according to Patent Literature 1 still leaves room for further improvement, from the viewpoint of retention of the strength for a long period.
The present invention has been made in view of the above problem possessed by the conventional art, and it is an object thereof to provide an ion exchange membrane and the like that can retain the strength for a long period.
As a result of intensive studies to solve the problem, the present inventors have found that the problem described above can be solved by the strength change ratios before and after a predetermined electrolysis test of an ion exchange membrane having a predetermined structure being in a predetermined range, having completed the present invention.
That is, the present invention encompasses aspects as follows.
[1]
An ion exchange membrane comprising:
wherein, when the ion exchange membrane is viewed from a top surface, an average cross-sectional thickness A of the ion exchange membrane measured in pure water for a region, in which the reinforcing materials do not exist, is 20 μm or more and 75 μm or less, and
wherein a strength change ratio calculated from a strength S2 of the ion exchange membrane measured after the ion exchange membrane is subjected to an electrolysis test described below and a strength S1 of the ion exchange membrane measured before the ion exchange membrane is subjected to the electrolysis test, in terms of 100×S2/S1, is 85% or more and 120% or less:
A woven mesh formed by knitting a nickel fine wire having a diameter of 0.15 mm and coated with a cerium oxide and a ruthenium oxide as cathode catalysts in a sieve mesh size of 50 is used as a cathode, and a titanium expanded metal coated with a ruthenium oxide, an iridium oxide, and a titanium oxide as anode catalysts is used as an anode; the ion exchange membrane is arranged between the anode and the cathode, and further, in order to bring the cathode into close contact with the ion exchange membrane, a collector made of a nickel expanded metal is arranged on the cathode, and a mat formed by knitting a nickel fine wire is arranged between the collector and the cathode to provide a natural-circulation zero-gap electrolytic cell; four such zero-gap electrolytic cells are arranged in series for use as an electrolyzer; brine is supplied to an anode side of the electrolyzer while a concentration of the brine is adjusted to be 205 g/L, and water is supplied to a cathode side of the electrolyzer while the sodium hydroxide concentration is maintained at 32% by mass; and electrolysis is carried out for 7 days with a temperature of the electrolyzer set to 85° C., at a current density of 6 kA/m2 under a condition in which a liquid pressure of the cathode side of the electrolyzer is higher than the liquid pressure of the anode side by 5.3 kPa.
[2]
The ion exchange membrane according to [1], wherein, when the ion exchange membrane is viewed from the top surface, a value of A/B is 0.15 or more and 0.30 or less, wherein B represents an average cross-sectional thickness of the ion exchange membrane measured in pure water for a region, in which strands of the reinforcement yarn cross each other, and for a region, in which the reinforcement yarn crosses the sacrifice yarn.
[3]
The ion exchange membrane according to [1] or [2], wherein the strength S1 is 1.10 kgf/cm or more and 1.55 kgf/cm or less.
[4]
The ion exchange membrane according to any of [1] to [3], wherein a ratio of the thickness Tc of the layer C to the A, in terms of Tc/A, is 0.165 or more and 0.508 or less.
[5]
An electrolyzer comprising the ion exchange membrane according to any of [1] to [4].
According to the present invention, it is possible to provide an ion exchange membrane and the like that can retain the strength for a long period.
Hereinafter, an embodiment for carrying out the present invention (herein, also referred to as “the present embodiment”) will be described in detail. The present invention is not intended to be limited to the following present embodiment and may be variously modified and carried out within the spirit thereof. The positional relation such as up and down, left and right, or the like is based upon the positional relation shown in the figures unless otherwise indicated. Furthermore, a size ratio in the figures is not limited to the ratio illustrated.
The ion exchange membrane of the present embodiment is an ion exchange membrane comprising:
a layer S comprising a fluorine-containing polymer having a sulfonic acid group;
a layer C comprising a fluorine-containing polymer having a carboxylic acid group; and
a plurality of reinforcing materials functioning as at least one of reinforcement yarn and sacrifice yarn;
wherein, when the ion exchange membrane is viewed from a top surface, an average cross-sectional thickness A of the ion exchange membrane measured in pure water for a region, in which the reinforcing materials do not exist, is 20 μm or more and 75 μm or less, and
wherein a strength change ratio calculated from a strength S2 of the ion exchange membrane measured after the ion exchange membrane is subjected to an electrolysis test described below and a strength S1 of the ion exchange membrane measured before the ion exchange membrane is subjected to the electrolysis test, in terms of 100×S2/S1, is 85% or more and 120% or less:
A woven mesh formed by knitting a nickel fine wire having a diameter of 0.15 mm and coated with a cerium oxide and a ruthenium oxide as cathode catalysts in a sieve mesh size of 50 is used as a cathode, and a titanium expanded metal coated with a ruthenium oxide, an iridium oxide, and a titanium oxide as anode catalysts is used as an anode; the ion exchange membrane is arranged between the anode and the cathode, and further, in order to bring the cathode into close contact with the ion exchange membrane, a collector made of a nickel expanded metal is arranged on the cathode, and a mat formed by knitting a nickel fine wire is arranged between the collector and the cathode to provide a natural-circulation zero-gap electrolytic cell; four such zero-gap electrolytic cells are arranged in series for use as an electrolyzer; brine is supplied to an anode side of the electrolyzer while a concentration of the brine is adjusted to be 205 g/L, and water is supplied to a cathode side of the electrolyzer while the sodium hydroxide concentration is maintained at 32% by mass; and electrolysis is carried out for 7 days with a temperature of the electrolyzer set to 85° C., at a current density of 6 kA/m2 under a condition in which a liquid pressure of the cathode side of the electrolyzer is higher than the liquid pressure of the anode side by 5.3 kPa.
The ion exchange membrane of the present embodiment, as constituted described above, can retain the strength over a long period.
An ion exchange membrane 1 illustrated in
In the example of
As illustrated in
The average cross-sectional thickness of membrane A is calculated as follows.
A position represented by “◯” in
As for a method for measuring the thickness a, a slice having a width of about 100 μm may be cut off from a cross section of a target portion of the ion exchange membrane immersed in pure water in advance by means of a razor or the like, the slice may subsequently be immersed in pure water with its cross section facing upward, and then the thickness of the slice may be measured using a microscope or the like. Alternatively, a tomographic image of a target portion of the ion exchange membrane immersed in pure water observed using X-ray CT or the like may be used to measure the thickness.
The thickness a is measured at 15 points, and the thickness of the portion having the smallest thickness is taken as a (min).
a (min) is calculated at three different positions, and the average value thereof is the thickness A.
From the viewpoint of securing sufficient membrane strength, the thickness A is 20 μm or more, and from the viewpoint of reducing the electrolytic voltage, the thickness A is 75 μm or less. From the similar viewpoints as described above, the thickness A is preferably 40 μm or more and 70 μm or less, more preferably 50 μm or more and 60 μm or less.
The thickness A can be within the aforementioned preferred range by, for example, controlling the thickness each of the layer S and the layer C, or alternatively by setting production conditions (temperature conditions and extension ratio) on production of the ion exchange membrane (in particular, on lamination of the film and reinforcing material) within an appropriate range described below or the like. More specifically, when the film temperature on lamination is increased, the thickness A tends to be smaller. When the extension ratio on extension is reduced, the thickness A tends to be larger. The temperature conditions on lamination and the extension ratio on extension are not limited to those described above and preferably adjusted as appropriate, in consideration of the flow characteristics and the like of a fluorine-containing polymer to be used.
(Average cross-sectional thickness of membrane B)
The average cross-sectional thickness of membrane B is calculated as follows.
A position represented by “Δ” in
As for a method for measuring the thickness b, a slice having a width of about 100 μm may be cut off from a cross section of a target portion of the ion exchange membrane immersed in pure water in advance by means of a razor or the like, the slice may subsequently be immersed in pure water with its cross section facing upward, and then the thickness of the slice may be measured using a microscope or the like. Alternatively, a tomographic image of a target portion of the ion exchange membrane immersed in pure water observed using X-ray CT or the like may be used to measure the thickness.
The thickness b is measured at 15 points, and the thickness of the portion having the largest thickness is taken as b (max).
b (max) is calculated at three different positions, and the average value thereof is the thickness B.
In alkali chloride electrolysis using a zero-gap base electrolyzer, the distance between the electrodes may be affected by the thickness of the ion exchange membrane. Thus, when the average cross-sectional thickness of membrane B is smaller, the resistance between electrodes tends to decrease to thereby enable an increase in the electrolytic voltage to be prevented. From such a viewpoint, the thickness B is preferably a thickness of 240 μm or less, more preferably 230 μm or less.
The thickness B can be within the aforementioned preferred range by, for example, controlling the thickness each of the layer S and the layer C or alternatively by setting the yarn diameter of the reinforcing material and the production conditions (temperature conditions and extension ratio) on production of the ion exchange membrane (in particular, on lamination of a film and a reinforcing material) within an appropriate range described below or the like. More specifically, when the outside air temperature on lamination is lowered, the thickness B tends to be smaller. When the extension ratio on extension is reduced, the thickness B tends to be larger. The temperature conditions on lamination and the extension ratio on extension are not limited to those described above and preferably adjusted as appropriate, in consideration of the flow characteristics and the like of a fluorine-containing polymer to be used.
(Thickness ratio A/B)
A thickness ratio A/B is a value obtained by dividing the average cross-sectional thickness of membrane A by the average cross-sectional thickness of membrane B.
When A/B is smaller, the thickness of a window portion through which cations permeate tends to be smaller to thereby enable the electrolytic voltage to be reduced. Accordingly, in the ion exchange membrane of the present embodiment, A/B is preferably 0.30 or less. However, when A/B is excessively small, asperities on the surface of the membrane become large, and bubbles of the gas generated from the alkali chloride electrolysis may accumulate in the window portion, which is a recess. When gas adsorbs the surface of the ion exchange membrane, permeation of cations is prevented, and thus there is a tendency to lead to an increase in the electrolytic voltage. Accordingly, in the ion exchange membrane of the present embodiment, A/B is preferably 0.15 or more. That is, A/B is preferably 0.15 or more and 0.30 or less, more preferably 0.17 or more and 0.28 or less, still more preferably 0.19 or more and 0.26 or less, wherein, when the ion exchange membrane is viewed from the top surface, A represents an average cross-sectional thickness of the membrane measured in pure water for a region, in which the reinforcing materials do not exist, and B represents an average cross-sectional thickness of the membrane measured in pure water for a region, in which strands of the reinforcement yarn cross each other, and for a region, in which the reinforcement yarn crosses the sacrifice yarn.
(Average cross-sectional thickness of membrane C1)
The average cross-sectional thickness of membrane C1 is calculated as follows.
A position represented by “Δ” in
As for a method for measuring the thickness c1, a slice having a width of about 100 μm may be cut off from a cross section of a target portion of an ion exchange membrane immersed in pure water in advance by means of a razor or the like, the slice may subsequently be immersed in pure water with its cross section facing upward, and then the thickness of the slice may be measured using a microscope or the like. Alternatively, a tomographic image of a target portion of an ion exchange membrane immersed in pure water observed using MRI or the like may be used to measure the thickness.
The thickness c1 is measured 15 points, and the thickness of the portion having the largest thickness is taken as c1 (max).
c1 (max) is calculated at three different positions, and the average value thereof is the thickness C1.
Cations permeating the ion exchange membrane in the alkali chloride electrolysis have a property of preferentially permeating a window portion of the ion exchange membrane having a smaller thickness. When the thickness A is equivalent to or smaller than the thickness C1, cations tend to permeate the ion exchange membrane with no influence of a shadow portion, which is to be formed behind the reinforcement yarn non-permeable to ions to limit ion permeation. From the viewpoint of further reducing the electrolytic voltage in this manner, the thickness A is preferably equivalent to or smaller than the thickness C1.
That is, in the first ion exchange membrane, A and C1 preferably satisfy the following formula: wherein C1 represents the maximum value of the distance measured in pure water between the surface of the layer S and a reinforcing yarn most distant from the surface of the layer S, in the direction of the thickness of the membrane in a region in which strands of the reinforcement yarn cross each other.
20 μm≤A≤C1
The thickness C1 can satisfy the aforementioned relationship by, for example, setting the yarn diameter of the reinforcing material within an appropriate range described below.
The average cross-sectional thickness C2 of membrane is calculated as follows.
A position represented by “Δ” in
As for a method for measuring the thickness c2, a slice having a width of about 100 μm may be cut off from a cross section of a target portion of an ion exchange membrane immersed in pure water in advance by means of a razor or the like, the slice may subsequently be immersed in pure water with its cross section facing upward, and then the thickness of the slice may be measured using a microscope or the like. Alternatively, a tomographic image of a target portion of an ion exchange membrane immersed in pure water observed using MRI or the like may be used to measure the thickness.
The thickness c2 is measured 15 points, and the thickness of the portion having the largest thickness is taken as c2 (max).
c2 (max) is calculated at three different positions, and the average value thereof is the thickness C2.
In the ion exchange membrane of the present embodiment, the thickness A is preferably equivalent to or smaller than the thickness C2 because an effect of reducing the thickness of the membrane due to continuous holes formed by sacrifice yarn is effectively exerted.
The thickness C2 can satisfy the aforementioned relationship by, for example, setting the yarn diameter of the reinforcing material within an appropriate range described below.
In the ion exchange membrane of the present embodiment, C2 is preferably 130 μm or less. C2 within this range tends to enable the electrolytic voltage to be reduced by suppressing the influence of a shadow portion, which is to be formed behind reinforcement yarn which no ion permeate to limit permeation of cations through the ion exchange membrane. From the similar viewpoint, in the ion exchange membrane of the present embodiment, C2 is more preferably 100 μm or less.
In the ion exchange membrane of the present embodiment, the layer S contains a fluorine-containing polymer A having a sulfonic acid group. The fluorine-containing polymer A having a sulfonic acid group, constituting the layer S, is not limited to the following, and can be produced by copolymerizing monomers in a first group and monomers in a second group or homopolymerizing monomers in the second group, for example.
Examples of the monomer in the first group include, but not limited to, fluorinated vinyl compounds. The fluorinated vinyl compound is preferably a compound represented by the following general formula (1):
CF2═CX1X2 (1)
wherein X1 and X2 each independently represent F, Cl, H, or CF3.
Examples of the fluorinated vinyl compound represented by the above general formula (1) include, but not limited to, vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, and chlorotrifluoroethylene.
Particularly, when the ion exchange membrane of the present embodiment is used as a membrane for alkali electrolysis, the fluorinated vinyl compound is preferably a perfluoro monomer, more preferably a perfluoro monomer selected from the group consisting of tetrafluoroethylene and hexafluoropropylene. Tetrafluoroethylene (TFE) is more preferred.
Examples of the monomer in the second group include, but not limited to, vinyl compounds having functional groups that can be converted to sulfone-type ion exchange groups. As such vinyl compounds having functional groups that can be converted to sulfone-type ion exchange groups, those represented by the following general formula (2) are preferred:
CF2═CFO—(CF2YFO)a—(CF2)b—SO2F (2)
wherein a represents an integer of 0 to 2, b represents an integer of 1 to 4, Y represents F or CF3, and R represents CH3, C2H5, or C3H7.
Specific examples thereof include the monomers shown below;
CF2═CFOCF2CF2SO2F,
CF2═CFOCF2CF(CF3)OCF2CF2SO2F,
CF2═CFOCF2CF(CF3)OCF2CF2CF2SO2F,
CF2═CF(CF2)2SO2F,
CF2═CFO[CF2CF(CF3)O]2CF2CF2SO2F, and
CF2═CFOCF2CF(CF2OCF3)OCF2CF2SO2F.
Of these, CF2═CFOCF2CF(CF3) OCF2CF2CF2SO2F and CF2═CFOCF2CF(CF3)OCF2CF2SO2F are more preferred.
The types of combination of the monomers constituting the polymer A, ratios, and degree of polymerization thereof are not particularly limited. The polymer A contained in the layer S may be a single polymer or a combination of two or more polymers. The ion exchange capacity of the fluorine-containing polymer A having a sulfonic acid group can be adjusted by changing the ratio between monomers represented by the above general formulas (1) and (2). A specific example of the adjustment described above is not particularly limited to the following and includes copolymerization of monomers represented by the above general formula (1) and monomers represented by the above general formula (2) at 4:1 to 7:1.
The layer S may be a single layer or may be a two-layer structure. When the layer S is a single layer, its thickness is preferably 50 to 180 μm, more preferably 70 to 160 μm, from the viewpoint of sufficiently achieving electrolysis performance and resistance to damage of layer C on a conductive surface (hereinafter, may be simply referred to as “damage”). When the layer S has a two-layer structure, a layer to be in contact with the anode is referred to as a layer S-1, a polymer forming the layer S-1 as a fluorine-containing polymer A-1, a layer to be in contact with the layer C as a layer S-2, and a polymer forming the layer S-2 as a fluorine-containing polymer A-2. The thickness of the layer S-1 is preferably 10 to 60 μm from the viewpoint of sufficiently achieving electrolysis performance and resistance to the damage, and the thickness of the layer S-2 is preferably 30 to 120 μm, more preferably 40 to 100 μm from the viewpoint of sufficiently achieving electrolysis performance and resistance to the damage. From the viewpoint of retaining the strength of the membrane main body higher than a predetermined level, it is preferred to adjust the thickness of the layer S as mentioned above. The thickness of the layer S, which is a value obtained by measurement on an ion exchange membrane in a dry state by a routine method (e.g., an extruded film is sliced with a single-edged razor blade, and the sliced film is observed with an optical microscope), can be controlled to be in the range described above, for example, by employing preferable production conditions described below.
In the ion exchange membrane of the present embodiment, the layer C contains a fluorine-containing polymer B having a carboxylic acid group. The fluorine-containing polymer having a carboxylic acid group, constituting the layer C, is not limited to the following and can be produced by copolymerizing monomers in the first group described above and monomers in a third group described below or by homopolymerizing monomers in the third group, for example.
Examples of the monomer in the third group include, but not limited to, vinyl compounds having functional groups that can be converted to carboxylic acid-type ion exchange groups. As such vinyl compounds having functional groups that can be converted to carboxylic acid-type ion exchange groups, those represented by the following general formula (3) are preferred:
CF2═CF(OCF2CYF)c—O(CF2)d—COOR (3)
wherein c represents an integer of 0 to 2, d represents an integer of 1 to 4, Y represents F or CF3, and R represents CH3, C2H5, or C3H7.
In the above general formula (3), it is preferred that Y be CF3 and R be CH3.
Particularly, when the ion exchange membrane of the present embodiment is used as an ion exchange membrane for alkali electrolysis, it is preferred to use at least perfluoro monomers as monomers in the third group. However, the alkyl group in the ester group (see the above R) is eliminated from the polymer on hydrolysis, and thus, the alkyl group (R) may not be a perfluoro alkyl group in which all the hydrogen atoms are replaced by fluorine atoms. Of these, monomers shown below are more preferred, for example:
CF2═CFOCF2CF(CF3)OCF2COOCH3.
CF2═CFOCF2CF(CF3)O(CF2)2COOCH3.
CF2═CF[OCF2CF(CF3)]2O(CF2)2COOCH3,
CF2═CFOCF2CF(CF3)O(CF2)3COOCH3,
CF2═CFO(CF2)2COOCH3, and
CF2═CFO(CF2)3COOCH2.
The monomers in the third group may be used singly or two or more of these may be used in combination. In the latter case, monomers other than those described above may be used in combination. Examples thereof include those represented by the general formula (2). The mixing form of the monomers are not particularly limited. A fluorine-containing copolymer obtained by copolymerizing monomers in the first group and monomers in the third group and a fluorine-containing copolymer obtained by copolymerizing monomers in the first group and monomers not in the third group may be each simply mixed, or monomers in the first group, monomers in the third group, and monomers not in the third group may be copolymerized.
The types of combination of the monomers constituting the polymer B, ratios, and degree of polymerization thereof are not particularly limited. The polymer B contained in the layer C may be a single polymer or a combination of two or more polymers. The ion exchange capacity of the fluorine-containing polymer B having a carboxylic acid group can be adjusted by changing the ratio of the monomers represented by the above general formulas (1) and (3). More specifically, an example of adjustment includes copolymerization of monomers represented by the above general formula (1) and monomers represented by the above general formula (3) at 6:1 to 9:1.
As fluorine-containing polymers B that may be employed in the present embodiment, in addition to those mentioned above, various known configurations such as the fluorine-containing polymer described in Japanese Patent Laid-Open No. 2020-100816 may be employed.
In the ion exchange membrane of the present embodiment, the thickness of the layer C Tc is preferably 5 μm or more and 40 μm or less, more preferably 10 μm or more and 40 μm or less, still more preferably 10 μm or more and 20 μm or less, from the viewpoint of sufficiently achieving electrolysis performance and resistance to the damage. The thickness of the layer C Tc, which is a value obtained by measurement on an ion exchange membrane in a dry state by a routine method (e.g., an extruded film is sliced with a single-edged razor blade, and the sliced film is observed with an optical microscope), can be controlled to be in the range described above, for example, by employing preferable production conditions described below.
In the present embodiment, the ratio of the thickness of the layer C Tc to the average cross-sectional thickness of the membrane A, Tc/A, is preferably 0.165 or more and 0.508 or less, more preferably 0.190 or more and 0.406 or less, still more preferably 0.216 or more and 0.356 or less, from the viewpoint of retention of the strength for a long period.
As shown in
The height, shape, and arrangement density of the raised portions describe above each can be measured and checked by the following method. First, in an area of a 1000-μm square of the surface of the ion exchange membrane, a point having the smallest height is taken as the reference. Then, portions having a height of 20 μm or more from the reference point are taken as raised portions. The height of the raised portions is measured using a “Color 3D Laser Microscope (VK-9710)” manufactured by KEYENCE CORPORATION. Specifically, a piece of 10 cm×10 cm is optionally cut out from the ion exchange membrane in a dry state. The cathode side of the ion exchange membrane is fixed on a flat plate with double-sided tape, and the membrane is mounted on the measuring stage such that the anode side of the ion exchange membrane faces the measuring lens. The shape on the surface of the ion exchange membrane is measured in a 1000-μm square measuring area of each 10 cm×10 cm membrane. A point having the smallest height is taken as the reference, and the height from the reference is measured to thereby enable raised portions to be observed.
The arrangement density of raised portions is a value obtained by optionally cutting out three 10 cm×10 cm membranes from the ion exchange membrane, carrying out measuring at nine points across a 1000-μm square measuring area of each 10 cm×10 cm membrane, and averaging the measured values.
The shape of the raised portions is not particularly limited, and the raised portions preferably have at least one shape selected from the group consisting of conical, polygonally pyramidal, truncated conical, truncated polygonally pyramidal, and hemispherical shapes. The hemispherical shape referred to herein also includes a shape referred to as a centroclinal shape.
In the ion exchange membrane of the present embodiment, preferably, a plurality of opening portions 102 is formed on the surface of the layer S (10a), and continuous holes 104 for connecting the opening portions 102 with each other are formed inside the layer S (10a) (see
Opening portions are formed on the surface of the membrane and continuous holes for connecting the opening portions with each other inside the membrane are formed to thereby supply an electrolyte solution inside the ion exchange membrane on electrolysis. Since this changes the concentration of impurities inside the membrane, the amount of impurities accumulated inside the membrane tends to decrease. When metal ions generated from elution of the cathode or impurities contained in an electrolyte solution supplied to the cathode side of the membrane infiltrate inside the membrane, the impurities become likely to be emitted from the membrane because opening portions are formed on the surface of the membrane. Thus, the amount of impurities accumulated tends to decrease. That is, the ion exchange membrane of the present embodiment, when having a configuration as described above, tends to have improved resistance against impurities existing in the electrolyte solution on the anode side of the membrane and additionally against impurities generated on the cathode side of the membrane.
When the alkali chloride aqueous solution is not sufficiently supplied, the damage is known to occur on the layer near the cathode of the membrane. The opening portions in the present embodiment can improve the supplying performance of the alkali chloride aqueous solution and reduce the damage occurring on the cathode face of the membrane main body.
The opening portions 102 formed on the surface of the layer S (10a) are a portion of the continuous hole 104 that is open on one surface of the membrane main body 10. Being open referred to herein means that the continuous hole is open outward from the surface of the layer S (10a). For example, when the surface of the layer S (10a) is coated with a coating layer described below, an open-hole area on which the continuous hole 104 are open outward on the surface of the layer S (10a) from which the coating layer has been removed is referred to as an opening portion.
The opening portions 102 may be formed on the surface of the layer S (10a) and may be formed also on both the surfaces of the membrane main body 10 (that is, on the surface of the layer C (10b)). The arrangement interval and shape of the opening portions 102 on the surface of the layer S (10a) are not particularly limited, and appropriate and suitable conditions can be selected, in consideration of the shape and performance of the membrane main body 10 and operating conditions on electrolysis.
The continuous holes 104 are preferably formed so as to alternately penetrate through the layer S (10a) side (the (α) side in
Specifically, as shown in
The continuous holes 104, in
It is only required that continuous holes 104 connect at least two or more opening portions 102, and the positional relation between the opening portions 102 and the continuous holes 104 is not limited. One example of the opening portions 102 and continuous hole 104 is described herein using
In the region A1 in
In the region A2 in
In the region A3 in
The ion exchange membrane of the present embodiment has a reinforcing material. In the present embodiment, the reinforcing material is constituted of reinforcement yarn and sacrifice yarn. Examples thereof include, but not limited to, fabric formed by weaving reinforcement yarn and sacrifice yarn. The reinforcement yarn, which can stably exist inside the ion exchange membrane 1 by embedding the reinforcing material in the membrane, imparts desired mechanical strength and dimension stability to the ion exchange membrane. The sacrifice yarn is eluted in a step (5) described below to thereby form a continuous hole. The amount of the sacrifice yarn mix-woven is preferably 10 to 80% by mass, more preferably 30 to 70% by mass based on the total reinforcing material. The sacrifice yarn may be in a monofilament or multifilament form, preferably in a multifilament form. The sacrifice yarn preferably has a thickness of 20 to 50 deniers. The sacrifice yarn may be made of any raw material that is dissolved in the step (5) described below, and is preferably made of polyester such as polyethylene terephthalate (PET).
In the present embodiment, the arrangement of the reinforcing material is not particularly limited, and there is a tendency to enable the ion exchange membrane 1 to expand and contract within a desired range particularly by disposing the reinforcement yarn 12 inside the layer S (10a). Such an ion exchange membrane does not expand and contract more than required on electrolysis and the like and tends to be able to maintain excellent dimension stability for a long period.
The configuration of the reinforcement yarn 12 in the present embodiment is not particularly limited, and yarn formed by spinning reinforcement yarn can be used. Use of such yarn formed by spinning reinforcement yarn can impart further excellent dimension stability and mechanical strength to the ion exchange membrane 1.
The materials of the reinforcement yarn are not particularly limited and are preferably materials resistant to acid and alkali. From the viewpoint of imparting long-term heat resistance and chemical resistance, those containing a fluorine-containing polymer are more preferred. Examples of the fluorine-containing polymer include, but not limited to, polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoro alkyl vinyl ether copolymers (PFA), tetrafluoroethylene-ethylene copolymers (ETFE), tetrafluoroethylene-hexafluoropropylene copolymers, trifluorochlorethylene-ethylene copolymers, and vinylidene fluoride polymers (PVDF). Of these, polytetrafluoroethylene (PTFE) is preferred, from the viewpoint of heat resistance and chemical resistance.
The yarn diameter of the reinforcement yarn is not particularly limited and is preferably 20 to 150 deniers, more preferably 50 to 120 deniers. The weaving density (fabric count per unit length) of the reinforcement yarn is not particularly limited and preferably 5 to 50 strands/inch. The form of the reinforcement yarn is not particularly limited, and woven fabric, non-woven fabric, knitted fabric or the like is used, for example. Of these, it is preferred that the form be woven fabric. The thickness of the woven fabric is not particularly limited and is preferably 30 to 150 μm, more preferably 30 to 100 μm.
In the present embodiment, the reinforcement yarn 12 may be monofilament or multifilament. Additionally, such yarn, slit yarn or the like is preferably used.
The weaving method and arrangement for the reinforcement yarn 12 in the layer S (10a) are not particularly limited. An appropriately and suitably arrangement can be employed in consideration of the size and shape of the ion exchange membrane 1, physical properties required for the ion exchange membrane 1, an environment of usage and the like. For example, the reinforcement yarn 12 may be arranged along a predetermined direction of the layer S (10a). From the viewpoint of the dimension stability, it is preferred that a strand of the reinforcement yarn 12 be arranged along a predetermined first direction and another strand of the reinforcement yarn 12 be arranged along a second direction substantially perpendicular to the first direction (see
Particularly, the reinforcement yarn 12 is preferably arranged along both the MD (Machine Direction) and the TD (Transverse Direction) of the ion exchange membrane 1. That is, the reinforcement yarn 12 is preferably plain-woven in the MD and the TD. The MD herein refers to the direction in which the membrane main body and reinforcing material are carried (flow direction) in the production step of ion exchange membrane described below, and the TD refers to the direction substantially perpendicular to the MD. Yarn woven along the MD is referred to as MD yarn, and yarn woven along the TD is referred to as TD yarn. The ion exchange membrane used in electrolysis is usually rectangular. Thus, frequently, its longitudinal direction is the MD, and the width direction is the TD. By interweaving the reinforcement yarn 12 which is MD yarn into the reinforcement yarn 12 which is TD yarn, further excellent dimension stability and mechanical strength tend to be imparted in many directions.
The arrangement interval for the reinforcement yarn 12 is not particularly limited. The reinforcement yarn can be appropriately and suitably arranged in consideration of physical properties required for the ion exchange membrane 1, an environment of usage and the like.
As reinforcing materials that may be employed in the present embodiment, in addition to those mentioned above, various known configurations such as the reinforcing material described in Japanese Patent Laid-Open No. 2019-108607 may be employed.
In the ion exchange membrane of the present embodiment, the aperture ratio of the reinforcement yarn 12 is not particularly limited and is preferably 30% or more, more preferably 50% or more and 90% or less. The aperture ratio is preferably 30% or more from the viewpoint of the electrochemical properties of the ion exchange membrane 1 and preferably 90% or less from the viewpoint of the mechanical strength of the ion exchange membrane 1.
The aperture ratio referred to herein is a ratio of a total area of a surface through which substances such as ions (an electrolyte solution and cations contained therein (e.g., sodium ions)) can pass (B) to the projected area of either one surface of the membrane main body 10(A) (B/A). The total area of a surface through which substances such as ions can pass (B) can be the sum of the projected area of the region in the ion exchange membrane 1 in which the cations, electrolyte solution, and the like are not interrupted by the reinforcement yarn 12 included in the ion exchange membrane 1 or the like.
Aperture ratio=(B)/(A)=((A)−(C))/(A) (I).
Of these forms of reinforcement yarn 12, particularly preferred forms are preferably tape yarn and highly-oriented monofilaments containing PTFE from the viewpoint of heat resistance and chemical resistance. Specifically, the reinforcement yarn is more preferably formed by plain-weaving using 50 to 300 deniers of tape yarn obtained by slitting a high-strength porous sheet made of PTFE into a tape form or a highly-oriented monofilament made of PTFE at a weaving density of 10 to 50 strands/inch, having a thickness in the range of 50 to 100 μm. The aperture ratio of the ion exchange membrane including such reinforcement yarn is preferably 60% or more.
The shape of the reinforcement yarn is not particularly limited, and examples thereof include round yarn and tape yarn. These shapes are not particularly limited.
The ion exchange membrane 1 of the present embodiment preferably has a proportion of the total area of the opening portions 102 based on the area of the surface of the layer S (10a) on which the opening portions 102 are formed (opening area ratio) of 0.4 to 15%. When the opening area ratio is limited to such a range, impurities in the electrolyte solution have a minor influence on the electrolytic performance, and stable electrolytic performance can be exerted. When the opening area ratio is 0.4% or more, an increase in the electrolytic voltage, a decrease in the current efficiency, and a decrease in the purity of the product to be obtained, which are caused by infiltration of impurities contained in the electrolyte solution into the ion exchange membrane 1 and accumulation of the impurities inside the membrane main body 10, tend to be more reduced. When the opening area ratio of the present embodiment is 15% or less, a decrease in the strength of the membrane and exposure of the reinforcement yarn tend to be more reduced. That is, when the ion exchange membrane 1 of the present embodiment is adjusted to be in the range described above, an emission flow from the continuous holes 104 via the opening portions 102 to outside the membrane can be facilitated even when impurities are accumulated inside the membrane main body 10. Thus, the impurities have a minor influence on the electrolytic performance, and stable electrolytic performance can be exerted for a long period.
Particularly, in alkali chloride electrolysis, alkali chloride used as an anode liquid and alkali hydroxide used as a cathode liquid contain metal compounds, metal ions, and impurities such as organic substances. Thus, such impurities have a major influence on the electrolytic voltage and current efficiency in alkali chloride electrolysis. When the opening area ratio of the ion exchange membrane 1 of the present embodiment is adjusted to be in the range described above, however, the electrolyte solution is likely to be supplied inside the ion exchange membrane on electrolysis. This changes the concentration of the impurities inside the membrane, and the amount of the impurities accumulated inside the membrane can be reduced. When metal ions generated from elution of the cathode or impurities contained in the electrolyte solution supplied to the cathode side of the membrane infiltrate inside the membrane, the impurities described above are allowed to permeate via the opening portions 102 and the continuous holes 104 to outside the membrane main body 10 with no difficulty. For this reason, the influence of the impurities generated during alkali chloride electrolysis on the electrolytic performance can be reduced, and stable electrolytic performance can be maintained for a long period. Additionally, the concentration of the impurities (alkali chloride and the like) in alkali hydroxide, which is the product, also can be prevented from increasing. From the viewpoint of reducing the influence of the impurities on the electrolytic performance in the ion exchange membrane 1 of the present embodiment and maintaining a constant strength of the membrane, the opening area ratio of the opening portions 102 is more preferably 0.5 to 10%, still more preferably 0.5 to 5%. The opening area ratio described above can be checked by a method described in Examples and can be controlled to be in the range described above, for example, by employing preferable production conditions described below.
In the present embodiment, the opening area ratio of opening portions is the ratio of the area of the opening portions to the projected area, when the ion exchange membrane is viewed from the top surface, on the surface of the ion exchange membrane.
In the ion exchange membrane 1 of the present embodiment, the opening density of the opening portions 102 on the surface of the layer S (10a) is not particularly limited, and is preferably 10 to 1000 opening portions/cm2, more preferably 20 to 800 opening portions/cm2. The opening density referred to herein is the number of opening portions 102 formed on 1 cm2 of the surface of the layer S (10a) on which the opening portions 102 are formed. It should be noted that 1 cm2 of the surface of the layer S (10a) is the projected area when the layer S (10a) is viewed from the top surface. When the opening density of the opening portions 102 is 10 opening portions/cm2 or more, the average area per opening portion 102 can be appropriately smaller, and thus can be sufficiently smaller than the size of a hole (pinhole) from which a crack, which is a cause of a reduction in the strength of the ion exchange membrane 1, may occur. When the opening density of the opening portions 102 is 1000 opening portions/cm2 or less, the average area per opening portion 102 has a size large enough to allow metal ions and cations contained in the electrolyte solution to infiltrate the continuous holes 104, and thus, the ion exchange membrane 1 tends to supply metal ions and cations or to allow metal ions and cations to permeate more efficiently. The opening density described above can be controlled to be in the range described above, for example, by employing preferable production conditions described below.
Exposed area ratio (%)=(Sum of projected area of the exposed portions, which are portions of the reinforcement yarn exposed when the surface of the membrane main body is viewed from the top surface)/(Projected area of the surface of the membrane main body)×100.
In the present embodiment, the reinforcement yarn 22 preferably contains a fluorine-containing polymer such as polytetrafluoroethylene (PTFE). When the reinforcement yarn 22 constituted of a fluorine-containing polymer is exposed on the surface of the membrane main body 20, the surface of the exposed portion A5 may exhibit hydrophobicity. When electrolytically generated gas in a solution state and cations are adsorbed on the exposed portion, which is hydrophobic, membrane permeation of the cations may be inhibited. In such a case, the electrolytic voltage increases, and the concentration of chloride ions in alkali hydroxide to be obtained may also increase. In the present embodiment, setting the exposed area ratio to 5% or less enables the existence ratio of the hydrophobic exposed portion to be in an appropriate range, and the increase in the electrolytic voltage and the increase in chloride ions in alkali hydroxide described above tend to be effectively reduced.
Furthermore, electrolytically generated gas in a solution state and impurities in the electrolyte solution such as metal ions adsorb the exposed portions, infiltrate inside the membrane main body 20, and permeate the membrane, becoming impurities in sodium hydroxide. In the present embodiment, setting the exposed area ratio to 3% or less tends to more effectively reduce adsorption, infiltration, and permeation of the impurities, and thus, tends to enable more highly pure sodium hydroxide to be produced.
Particularly, in the ion exchange membrane 2 of the present embodiment, since the opening area ratio described above is 0.4 to 15% and the exposed area ratio described above is 5% or less, a decrease in the current efficiency due to impurities can be further reduced. In the case of alkali electrolysis, the concentration of the impurities in sodium hydroxide, which is a product, tends to be maintained lower. Furthermore, an increase in the electrolytic voltage is also reduced, and thus, there is a tendency to enable more stable electrolytic performance to be exerted.
In the present embodiment, the exposed area ratio of the exposed portions is the sum of the projected area of the exposed portions formed on the reinforcement yarn based on the sum of the projected area of the reinforcement yarn, when viewed from the top surface. The exposed area ratio will be an index that indicates how much the reinforcement yarn included in the ion exchange membrane is exposed. Accordingly, the exposed area ratio of the exposed portions can be directly calculated by determining the projected area of the reinforcement yarn and the projected area of the exposed portions, and also can be calculated using the aperture ratio described above by the following formula (II). A more specific description now will be given with reference to the drawings.
The exposed area ratio is Sa/Sb.
Herein, on the basis of the above formula (I), Sb=C=A—B=A(1−B/A)=A(1−aperture ratio) is established, and thus, resulting in
Exposed area ratio=Sa/(A(1−aperture ratio)) (II),
wherein
Sa: sum of the projected area of the exposed portions A5,
Sb: sum of the projected area of the reinforcement yarn 22,
A: projected area of the ion exchange membrane including the reinforcement yarn 22 arranged along the longitudinal direction and the reinforcement yarn 12 (22) arranged along the lateral direction (see
B: total area of the region through which substances such as ions can pass (B) (see
C: total area of the reinforcement yarn 22.
As shown in
Additionally, the opening portions 202, exposed portions, and raised portions 21 are more preferably formed on the surface of the layer S (20a). Usually, the ion exchange membrane is used in close contact with the anode for the purpose of reducing the electrolytic voltage. However, when the ion exchange membrane comes in close contact with the anode, the electrolyte solution (the anode liquid such as brine) becomes unlikely to be supplied. Then, since the raised portions have been formed on the surface of the ion exchange membrane, the close contact of the ion exchange membrane with the anode can be suppressed to thereby enable the electrolyte solution to be smoothly supplied. As a result, metal ions or other impurities can be prevented from accumulating in the ion exchange membrane, the concentration of chloride ions in alkali hydroxide to be obtained is reduced, and then, the damage of the cathode surface of the membrane can be reduced.
The ion exchange membrane of the present embodiment preferably further has a coating layer with which at least a portion of at least one surface of the membrane main body is coated, from the viewpoint of preventing adsorption of gas on the cathode side surface and the anode side surface on electrolysis.
The raised portions 31 and the opening portions 302 may or may not be completely coated with the coating layer 34a. That is, the raised portions 31 and the opening portions 302 may be visually observable from the surface of the coating layer 34a.
The materials constituting the coating layers 34a and 34b are not particularly limited and preferably contain minerals from the viewpoint of prevention of gas adsorption. The inorganic substance is not particularly limited, and examples include zirconium oxide and titanium oxide. As a method for forming the coating layers 34a and 34b on the surfaces of the membrane main body 30, known methods can be employed, without particular limitation. An example thereof is a method for applying a liquid prepared by dispersing fine particulates of inorganic oxide in a binder polymer solution by spraying or the like (spray method). Examples of the binder polymer include, but not limited to, vinyl compounds having functional groups that can be converted to sulfone-type ion exchange groups. The application conditions are not particularly limited, and spraying can be used at 60° C., for example. Examples of methods other than the spray method include, but not limited to, roll coating.
The coating layer 34a is laminated on the surface of the layer S (30a). In the present embodiment, the opening portions 302 are only required to be open on the surface of the membrane main body 30 and do not have to necessarily be open on the surface of the coating layer.
The coating layers 34a and 34b are only required to cover at least one surface of the membrane main body 30. Accordingly, for example, the coating layer 34a may be provided only on the surface of the layer S (30a), or the coating layer 34b may be provided only on the surface of the layer C (30b). In the present embodiment, each of the surfaces of the membrane main body 30 are preferably coated with the coating layers 34a or 34b from the viewpoint of prevention of gas adsorption.
The coating layers 34a and 34b are only required to cover at least a portion of a surface of the membrane main body 30 and may not necessarily cover the surface entirely. However, from the viewpoint of prevention of gas adsorption, it is preferred that the surfaces of the membrane main body 30 be entirely coated with the coating layers 34a and 34b.
The average thickness of the coating layers 34a and 34b is preferably 1 to 10 μm, from the viewpoint of prevention of gas adsorption and of an increase in the electrical resistance due to the thickness.
The ion exchange membrane 3 is a membrane formed by coating each of the surfaces of ion exchange membrane 1 shown in
At the exposed portion A5, the reinforcement yarn 42 is only required to be exposed on at least the surface of the layer S (40a) and is not required to be exposed on the surface of the coating layer 44a.
The ion exchange membrane 4 is a membrane formed by coating each of the surfaces of ion exchange membrane 2 shown in
As coating layers that may be employed in the present embodiment, in addition to those mentioned above, various known configurations such as the coating layer described in Japanese Patent Laid-Open No. 2019-108607 may be employed.
In the ion exchange membrane of the present embodiment, the ion exchange capacity of the fluorine-containing polymer refers to the equivalent of exchange groups per g of dry resin and can be determined by neutralization titration or infrared spectroscopic analysis. In the case of measurement by infrared spectroscopic analysis, the ion exchange capacity can be measured by a method described in Example described below. In the present embodiment, a value obtained by measuring a fluorine-containing polymer to be used (before hydrolysis treatment) by infrared spectroscopic analysis may be used as the ion exchange capacity, or a value obtained by measurement by neutralization titration after hydrolysis may be used as the ion exchange capacity.
The ion exchange capacity of the layer S is preferably 0.90 to 1.45 meq/g, more preferably 1.00 to 1.25 meq/g.
The ion exchange capacity of the layer C is 0.80 to 1.10 meq/g, preferably 0.80 to 1.00 meq/g, more preferably 0.83 to 0.98 meq/g. In the present embodiment, when the layer S and/or layer C are/is constituted of a plurality of layers, each of the layers preferably satisfies the aforementioned ion exchange capacity.
The strength change ratio 100×S2/S1, calculated from S2 and S1, is 85% or more and 120% or less, wherein S2 represents the strength of the ion exchange membrane measured after the ion exchange membrane of the present embodiment is subjected to the electrolysis test mentioned above, and S1 represents the strength of the ion exchange membrane measured before the ion exchange membrane is subjected to the electrolysis test. When the strength change ratio is less than 85% or more, retention of the strength for a long period is difficult. From the similar viewpoint as described above, the strength change ratio is preferably 90% or more and 120% or less, more preferably 95% or more and 120% or less.
The strength change ratio more specifically can be measured by a method described in Example described below.
The strength change ratio can be adjusted to be in the range described above, for example, by employing preferable production conditions described below. Especially, when a 2-stage saponification and salt exchange treatment to be mentioned below is conducted, the strength change ratio tends to be easily adjusted within the range described above.
The strength S1 before the ion exchange membrane of the present embodiment is subjected to the electrolysis test mentioned above is preferably 1.10 kgf/cm or more and 1.55 kgf/cm or less, more preferably 1.15 kgf/cm or more and 1.50 kgf/cm or less, still more preferably 1.15 kgf/cm or more and 1.45 kgf/cm or less, from the viewpoint of retention of the strength for a long period.
The method of producing an ion exchange membrane of the present embodiment is not particularly limited as long as an ion exchange membrane having the configuration mentioned above can be obtained. A suitable example of a method for producing an ion exchange membrane of the present embodiment includes a method including the following steps (1) to (5):
According to the method described above, in the step (4) of embedding, the membrane main body having desired raised portions formed can be obtained by controlling treatment conditions such as temperature, pressure, and time during embedding. Then, in the step (5), dissolution of the sacrifice yarn arranged inside the membrane main body enables continuous holes to be formed inside the membrane main body. This enables the ion exchange membrane to be obtained. Hereinafter, each of the steps will be described in more detail.
In the present embodiment, a fluorine-containing polymer having ion exchange groups or ion exchange group precursors, which may become ion exchange groups by hydrolysis, can be obtained by appropriately polymerizing the above monomers as mentioned above. In order to control the ion exchange capacity of the fluorine-containing polymer, it is only required that the mixture ratio of the raw material monomers and the like be adjusted in the production step as mentioned above.
In the step (2), adjustment of the shape and arrangement of the reinforcement yarn, sacrifice yarn and the like can control the opening area ratio, exposed area ratio, opening density, continuous hole arrangement and the like. For example, when the sacrifice yarn is made thicker, the sacrifice yarn is likely to be located near the surface of the membrane main body in the step (4) described below. The sacrifice yarn is dissolved in the step (5) described below, and opening portions are likely to be formed.
Controlling the number of strands of the sacrifice yarn also can control the opening density. Likewise, when the reinforcement yarn is made thicker, opening portion are likely to be formed.
Furthermore, the aforementioned aperture ratio of the reinforcement yarn can be controlled by adjusting the thickness of the reinforcement yarn and mesh, for example. That is, thicker reinforcement yarn tends to reduce the aperture ratio, and thinner reinforcement yarn tends to increase the aperture ratio. An increase of the mesh tends to reduce the aperture ratio, and less mesh tends to increase the aperture ratio. From the viewpoint of further increasing the electrolytic performance, the aperture ratio is preferably increased as described above, and from the viewpoint of achieving strength, the aperture ratio is preferably reduced.
Step (3): Step of film formation
In the step (3), a film is formed from the fluorine-containing polymer obtained in the step (1) by use of an extruder. The film may have a two-layer structure of a sulfonic acid layer and a carboxylic acid layer or may have a multilayer structure of three or more layers as described above. The method for forming a film is not particularly limited, and examples thereof include the following:
Coextrusion is preferred because of its contribution to an increase in the adhesive strength in the interface.
In the step (4), the reinforcing material obtained in the step (2) is embedded in the film obtained in the step (3) to obtain a membrane main body including the reinforcing material therein.
Examples of the embedding method include, but not limited to, a method in which the reinforcing material and the film are laminated in the order mentioned on breathable heat-resistant release paper on a flat plate or drum including a heat source and/or a vacuum source therein and having many pores on the surface thereof and integrated at a temperature at which the fluorine-containing polymer of the film melts while the air among each of the layers is evacuated by reduced pressure.
Examples of the embedding method in the case of a three-layer structure of two sulfonic acid layers and a carboxylic acid layer include, but not limited to, a method in which release paper, a reinforcing material, a film constituting a sulfonic acid layer, a film constituting a sulfonic acid layer, and a film constituting a carboxylic acid layer are laminated in the order mentioned on a drum and integrated, and a method in which release paper, a reinforcing material, a film constituting a sulfonic acid layer, and a composite film in which a sulfonic acid layer faces the reinforcing material side are laminated in the order mentioned and integrated.
An example of the embedding method in the case of a composite membrane having a multilayer structure of three or more layers includes, but not limited to, a method in which release paper, a reinforcing material, a plurality of films each constituting each of the layers, and a plurality of films each constituting each of the layers are laminated in the order mentioned on a drum and integrated. In the case of a multilayer structure of three or more layers, adjustment is preferably carried out such that the film constituting the carboxylic acid layer is laminated at the farthest position from the drum and the film constituting the sulfonic acid layer is laminated at a position near the drum.
The method including integration under a reduced pressure tends to make the third layer on the reinforcing material thicker than that of a pressure-application press method. A variety of laminations described herein is exemplary. After an appropriate and suitable lamination pattern (for example, combination of each of layers) is selected in consideration of the layer configuration and physical properties of a desired membrane main body, coextrusion can be carried out.
For the purpose of further improving the electric properties of the ion exchange membrane of the present embodiment, it is also possible to additionally interpose a layer containing both carboxylate functional groups and sulfonyl fluoride functional groups between the sulfonic acid layer and the carboxylic acid layer describe above or to use a layer containing both carboxylate functional groups and sulfonyl fluoride functional groups.
Examples of the method for producing a fluorine-containing polymer that forms this layer may include a method in which a polymer containing carboxylate functional groups and a polymer containing sulfonyl fluoride functional groups are separately produced and then mixed and a method in which both monomers containing carboxylate functional groups and monomers containing sulfonyl fluoride functional groups are copolymerized.
In the step (5), the sacrifice yarn included in the membrane main body is removed by dissolution in acid or alkali to form continuous holes in the membrane main body. The sacrifice yarn has solubility in acid or alkali in the step of producing an ion exchange membrane or under an electrolysis environment. Thus, elution of the sacrifice yarn in acid or alkali from the membrane main body allows continuous holes to be formed at corresponding sites. The ion exchange membrane including continuous holes formed in the membrane main body can be obtained in this manner. The sacrifice yarn may remain in the continuous holes, not completely dissolved and removed. The sacrifice yarn remaining in the continuous holes may be dissolved and removed by the liquid electrolyte when electrolysis is carried out.
The acid or alkali used in the step (5) is only required to dissolve the sacrifice yarn, and the types thereof are not particularly limited. Examples of the acid include, but not limited to, hydrochloric acid, nitric acid, sulfuric acid, acetic acid, and fluorine-containing acetic acid. Examples of the alkali include, but not limited to, potassium hydroxide and sodium hydroxide.
The step of forming continuous holes by eluting the sacrifice yarn will be now described in more detail.
If the sacrifice yarn is entirely dissolved in the step (5), as described in Japanese Patent No. 5844653, in the case where the ion exchange membrane is mounted in an electrolyzer and an alkali chloride aqueous solution is poured into the electrolyzer, the alkali chloride aqueous solution may leak out of the electrolyzer through the dissolution holes. Thus, it is preferred to leave the 30 to 80% of the yarn diameter of the sacrifice yarn.
The method described above is simple because interweaving of the reinforcement yarn 52 and sacrifice yarn 504a may be adjusted depending on the arrangement of the reinforcement yarn 52, continuous holes 504, and opening portions (not shown) inside the membrane main body of the ion exchange membrane.
In the step (5), it is also possible to introduce ion exchange groups into ion exchange group precursors by hydrolyzing the obtained membrane main body obtained in the step (4).
In the step (5), the hydrolysis treatment can be followed by salt exchange treatment.
Conditions for the hydrolysis are not particularly limited, and the hydrolysis can be performed, for example, in an aqueous solution of 2.5 to 4.0N potassium hydroxide (KOH) and 20 to 40% by mass DMSO (dimethyl sulfoxide) at 40 to 95° C. for 10 minutes to 24 hours. Conditions for the subsequent salt exchange treatment are also not particularly limited, and the salt exchange treatment can be performed, for example, under conditions of 40 to 95° C., using a 0.1 to 1.0N sodium hydroxide (NaOH) solution for 0.1 hours to 1 hour.
In the present embodiment, from the viewpoint of easily adjusting the strength change ratio of the ion exchange membrane to be obtained within the predetermined range, the hydrolysis treatment and the salt exchange treatment each are preferably performed in multiple stages, each are more preferably performed in 2 stages.
In such hydrolysis treatment that may be conducted in 2 stages, it is preferred to employ a condition under which the ion exchange membrane relatively swells (for example, a high-temperature condition) in the first stage and to employ, then in the second stage, a condition under which the ion exchange membrane relatively contracts (for example, a low-temperature condition). In this case, the gap from the subsequent salt exchange (the ion exchange membrane tends to contract more greatly in comparison with the hydrolysis.) can be reduced, and consequently an ion exchange membrane having a smaller change in the strength tends to be obtained.
In such salt exchange treatment that may be conducted in 2 stages, it is preferred to employ a condition under which the ion exchange membrane relatively contracts in the first stage, and to employ, then in the second stage, a condition under which the ion exchange membrane relatively swells. In this case, the gap from the preceding hydrolysis (the salt exchange tends to more greatly contract the ion exchange membrane.) can be reduced, and consequently, an ion exchange membrane having a smaller change in the strength tends to be obtained. In the salt exchange treatment, the condition under which the ion exchange membrane is likely to swell and the condition under which the ion exchange membrane is likely to contract can be appropriately adjusted by increasing or decreasing the sodium hydroxide concentration to be used and the temperature.
As described above, in the hydrolysis treatment, the conditions under which the ion exchange membrane is allowed to swell first and then contract are preferable, and in the salt exchange treatment, the conditions under which the ion exchange membrane is allowed to contract first and then swell are preferable. In this case, specific conducting conditions in each stage of the hydrolysis treatment and the salt exchange treatment, which may be conducted in 2 stages, are not particularly limited. That is, conditions such as temperature, time, and concentration in each stage of the hydrolysis treatment and the salt exchange treatment, which may be conducted in 2 stages, can be appropriately determined in consideration of the configuration and the like of the ion exchange membrane to be treated (for example, the polymer composition to constitute the ion exchange membrane).
In the ion exchange membrane of the present embodiment, raised portions may be formed on the surface of the membrane main body, and the method for forming the raised portions is not particularly limited. A known method also can be employed including forming raised portions on a resin surface. In the present embodiment, an example of the method for forming raised portions on the surface of the membrane main body specifically includes a method including subjecting the surface of the membrane main body to embossing. For example, when the film, reinforcing material and the like are integrated, the raised portions described above can be formed using embossed release paper embossed in advance.
According to the method of producing an ion exchange membrane of the present embodiment, opening portions and exposed portions are formed by polishing the membrane in a wet state after hydrolysis. For this reason, the polymer in the membrane main body is sufficiently flexible, and the shape of the raised portion does not come off. In the case where raised portions are formed by embossing, the height and arrangement density of the raised portions can be controlled by controlling the emboss shape to be transferred (shape of the release paper).
After the steps (1) to (5) are accomplished, the aforementioned coating layers may be formed on the surfaces of the ion exchange membrane obtained.
The ion exchange membrane of the present embodiment can be used in various electrolyzers. That is, the electrolyzer of the present embodiment includes the ion exchange membrane of the present embodiment. As illustrated in
Electrolysis conditions are not particularly limited, and the electrolysis can be carried out under known conditions. For example, with the anode chamber provided with 2.5 to 5.5N alkali chloride aqueous solution and the cathode chamber provided with water or diluted alkali hydroxide aqueous solution, electrolysis can be carried out under conditions including an electrolysis temperature of 50 to 120° C. and a current density of 5 to 100 A/dm2.
The configuration of the electrolyzer according to the present embodiment is not particularly limited and may be monopolar or bipolar, for example. Materials constituting the electrolyzer are not particularly limited. As materials for the anode chamber, titanium and the like, which are resistant to alkali chloride and chlorine, are preferred. As materials for the cathode chamber, nickel and the like, which are resistant to alkali hydroxide and hydrogen, are preferred. As for the arrangement of the electrodes, even when the ion exchange membrane and the anode are arranged with an appropriate gap therebetween or even when the anode is arranged in contact with the ion exchange membrane, the ion exchange membrane can be used without any problem. In a contact electrolyzer (zero-gap base electrolyzer), in which no gap is provided between the ion exchange membrane and the anode and between the ion exchange membrane and the cathode, the ion exchange membrane of the present embodiment achieves a greater effect.
Hereinafter, the present embodiment will be described in detail by means of examples. The present embodiment is not intended to be limited to the following examples.
The ion exchange membrane after the hydrolysis step was cut in the vertical direction from the layer C side or the layer S side to the surface of the layer to obtain a sample having a longer side of 6 mm or more and a shorter side of about 100 μm. At this time, as shown in
The ion exchange membrane after the hydrolysis step was cut in the vertical direction from the layer C side or the layer S side to the surface of the layer to obtain a sample having a longer side of 6 mm or more and a shorter side of about 100 μm. At this time, as shown in
The strength S1 of the ion exchange membrane (strength before the electrolysis test mentioned below is conducted) in Examples and Comparative Examples, which was breaking strength obtained by tensile testing, was measured on the ion exchange membrane swelled with pure water by the following method. Along the direction at an angle of 45 degrees with respect to the reinforcing yarn embedded in the ion exchange membrane, a sample having a width of 1 cm was cut from the ion exchange membrane immersed in pure water. Then, the breaking elongation of the sample was measured under conditions including a distance between chucks of 5 cm and a tensile speed of 100 mm/minute in compliance with JISK6732. The measurement sample was stored by immersion in pure water at 25° C. until immediately before measurement, and was measured within three minutes after the sample was taken out of pure water. Seven samples from the same ion exchange membrane were measured, and the average value of the seven breaking elongation values was taken as the strength of the ion exchange membrane S1.
[Measurement of Strength S2 and Voltage after Electrolysis Test]
The electrolyzer used for electrolysis was one in which four natural-circulation zero-gap electrolytic cells were arranged in series, each of which had a structure including an ion exchange membrane arranged between an anode and a cathode. As the cathode, woven mesh was used formed by knitting nickel fine wire having a diameter of 0.15 mm and coated with cerium oxide and ruthenium oxide as catalysts in a sieve mesh size of 50. To bring the cathode into close contact with the ion exchange membrane, a mat formed by knitting nickel fine wire was arranged between a collector made of nickel expanded metal and the cathode. As the anode, used was titanium expanded metal coated with ruthenium oxide, iridium oxide, and titanium oxide as catalysts. By use of the electrolyzer described above, brine was supplied to the anode side while the concentration was adjusted to be 205 g/L, and water was supplied to the cathode side while the sodium hydroxide concentration was maintained at 32% by mass. Electrolysis was carried out for 7 days with the temperature of the electrolyzer set to 85° C., at a current density of 6 kA/m2 under a condition in which the liquid pressure of the cathode side of the electrolyzer was higher than the liquid pressure of the anode side by 5.3 kPa.
After the electrolysis test described above, the strength S2 and the voltage were measured. That is, the strength of the ion exchange membrane after the electrolysis test S2 was measured by the same method as in [Measurement of strength S1 before electrolysis test] described above.
The strength change ratio due to the electrolysis test was calculated by the following formula.
Strength change ratio (%)=100×S2/S1
As the voltage after the electrolysis test, the average value of the voltage values of 5 days, 6 days, and 7 days after the start of the operation was used.
As a fluorine-containing polymer having ion exchange groups, about 1 g of a fluorine-containing polymer A-1, a fluorine-containing polymer A-2, or a fluorine-containing polymer B in each example described below was used and press-formed at a temperature about 30° C. higher than the pseudo-melting point of the polymer to obtain a film corresponding to each polymer. The obtained film was measured by a transmission infrared spectroscopic analyzer (FTIR-4200 manufactured by JASCO Corporation). From the height of each of the obtained infrared peaks CF2, CF, CH3, OH, and SO2F, the proportion of structural units having groups that can be converted into carboxylic acid functional groups or sulfonic acid functional groups was calculated. The proportion was taken as the proportion of structural units having carboxylic acid functional groups or sulfonic acid functional groups obtained by hydrolyzing the fluorine-containing polymer, and a calibration curve of a sample having a known ion exchange capacity calculated by a titration method was used to determine the ion exchange capacity.
Solution polymerization was performed in order to obtain a fluorine polymer C-1 (fluorine-containing polymer having a carboxylic acid group). The stirring blade used was anchor-shaped. First, 561.5 g of CF2═CFOCF2CF(CF3)O(CF2)2COOCH3 and 561.5 g of CF3CHFCHFCF2CF3 (hereinafter, “HFC-43-10mee”) were introduced into a 1 L stainless steel autoclave. The vessel was fully replaced with nitrogen, then further replaced with CF2═CF2 (hereinafter “TFE”), heated until the temperature inside the vessel became stable at 25° C., and pressurized by TFE to 0.411 MPa-G (gauge pressure). Then, 4.49 g of a 5% solution of (CF3CF2CF2COO)2 in HFC43-10mee as a polymerization initiator and 0.059 g of ethanol as a chain transfer agent were introduced therein to start the reaction. TFE was intermittently fed while stirring at 25° C., 0.059 g of ethanol was added in the process to lower the TFE pressure from 0.411 MPa-G initially to 0.387 MPa-G at the end, and 14 mL of methanol was added after 2.5 hours to terminate the polymerization. After unreacted TFE was discharged to the outside of the system, the liquid was removed by subjecting the resulting polymerization liquid to heating and pressure reduction to thereby obtain 68 g of the fluorine polymer C-1. The resulting fluorine polymer C-1 was kneaded in a LABO PLASTOMILL (model 4M150) of Toyo Seiki Seisaku-sho, Ltd. at a temperature of 260° C. and a number of revolutions of the blade of 50 rpm for 20 minutes. Thereafter, the EW of the fluorine polymer C-1 was determined by neutralization titration, and the ion exchange capacity was 0.89 meq/g.
Monomers represented by the following general formula (1) and monomers represented by the following general formula (2) were copolymerized to obtain a polymer having an ion exchange capacity of 1.05 meq/g, as a fluorine-containing polymer S-1 (fluorine-containing polymer having a sulfonic acid group).
CF2═CF2 (1)
CF2═CFO—CF2CF(CF3)O—(CF2)2—SO2F (2)
Monomers represented by the general formula (1) and monomers represented by the general formula (2) were copolymerized to obtain a polymer having an ion exchange capacity of 1.03 meq/g, as a fluorine-containing polymer S-2 (fluorine-containing polymer having a sulfonic acid group).
The fluorine polymer S-2 and the fluorine polymer C-1 were provided and coextruded by an apparatus equipped with two extruders, a T die for two layer extrusion, and a take-up apparatus to obtain a two-layer film (α) having a thickness of 50 μm. The observation result of the cross-section of the film obtained with an optical microscope showed that the thickness of the layer S-2 was 38 μm and the thickness of the layer C was 12 Additionally, a single-layer T die was used to obtain a single-layer film of a layer S-1 (b) having a thickness of 42 μm.
As reinforcement yarn, a yarn-like material prepared by twisting tape yarn made of polytetrafluoroethylene (PTFE) and having a yarn diameter of 100 deniers at 900 turns/m (hereinafter, referred to as PTFE yarn) was used. As weft sacrifice yarn, yarn prepared by twisting polyethylene terephthalate (PET) of 35 deniers and 8 filaments at 200 turns/m (hereinafter, referred to as PET yarn) was used. As weft sacrifice yarn, yarn prepared by twisting polyethylene terephthalate (PET) of 35 deniers and 8 filaments at 200 turns/m (hereinafter, referred to as PET yarn) was used. First, plain-weaving was carried out with the PTFE yarn arranged at 24 strands/inch and two strands of the sacrifice yarn arranged between adjacent strands of the PTFE yarns, and widening was carried out in the weft direction in heated air to obtain a reinforcing material 1 having a thickness of 100 μm.
On a drum including a heat source and a vacuum source therein and having many micropores on the surface thereof, embossed breathable heat-resistant release paper, the reinforcing material 1, the single-layer film (b), and the two-layer film (α) were laminated in the order mentioned (here, the layer S-2 of the two-layer film was arranged on the side of the single-layer film (b).) and integrated at a drum surface temperature of 230° C. and under a reduced pressure of −650 mmHg while the air among each of the materials was evacuated to obtain a composite membrane. In the integration step, during the period from feeding of the materials to contact of the materials with the drum, the extension ratio of the single-layer film and two-layer film in the running direction was controlled to be 4% or less. As the result of observation of the surface of the obtained membrane, it was observed that hemispherical projected portions having an average height of 60 μm constituted only by a polymer having ion exchange groups were formed on the anode-side film (b) at a density of 250 raised portions/cm2 and the total area of the raised portions was 0.2 cm2 per cm2. Saponification was conducted in 2 stages on the obtained composite membrane as follows. For former stage saponification, the composite membrane was immersed in an aqueous solution at 81° C. comprising 30% by mass of dimethyl sulfoxide (DMSO) and 15% by mass of potassium hydroxide (KOH) for 0.1 hours. Next, for latter stage saponification, the composite membrane was immersed in an aqueous solution at 60° C. comprising 30% by mass of dimethyl sulfoxide (DMSO) and 15% by mass of potassium hydroxide (KOH) for 0.6 hours.
Subsequent to the 2-stage saponification mentioned above, salt exchange treatment was conducted in 2 stages as follows. For former stage salt exchange treatment, the composite membrane was immersed in 0.6N NaOH at 90° C. for 0.3 hours. Next, for latter stage salt exchange treatment, the composite membrane was immersed in 0.1 N NaOH at 50° C. for 0.5 hours to replace the ion attached to the ion exchange group by Na.
Subsequent to the 2-stage salt exchange treatment mentioned above, the membrane was washed with water and further dried at 60° C. to obtain a membrane main body.
Additionally, a polymer (B3) as a dried resin, which was a copolymer of CF2═CF2 and CF2═CFOCF2CF(CF3)OCF2CF2SO2F and had an ion exchange capacity of 1.05 mg eq/g, was hydrolyzed and then converted into its acid form with hydrochloric acid. In a solution obtained by dissolving this acidic-form polymer (B3) at a proportion of 5% by mass in a 50/50 (mass ratio) mixed solution of water and ethanol, zirconium oxide particles having a primary particle size of 1.15 μm were added so as to achieve a mass ratio of the polymer (B3) to the total mass of the polymer (B3) and the zirconium oxide particles of 0.33. Thereafter, the zirconium oxide particles were dispersed with a ball mill until the average particle size in a suspension liquid reached 0.94 μm to thereby obtain the suspension liquid. Note that the zirconium oxide used was one obtained by pulverizing raw stones. Note that the average particle size described above was median diameter (D50) and was measured with a particle size analyzer (“SALD2200”, SHIMADZU CORPORATION).
The suspension liquid was applied to both the surfaces of the ion exchange membrane by a spray method. In the meantime, the average droplet size of the spray was adjusted to 46 μm. Additionally, the surface temperature of the membrane main body was adjusted to 57° C. to dry the surfaces, and then, an ion exchange membrane having a coating layer including the polymer (B3) and the zirconium oxide particles was obtained. In this ion exchange membrane, the content of the fluorine-containing polymer in the binder was 100% by mass. Note that the average droplet size means the volume average diameter D (4,3). For the measurement thereof, droplets located between the nozzle tip and a point 200 mm away therefrom in the droplet discharge direction were used as objects, using a “Spraytec” manufactured by Malvern Ltd. under a 25° C. atmosphere to determine the droplet size from the laser scattered light intensity. Hereinafter, average droplet sizes were determined in the same manner.
The average thickness, membrane strength, and electrolytic voltage of the ion exchange membrane obtained as described above were each measured, and the results are shown in Table 1.
Ion exchange membranes were obtained in the same manner as in Example 1 except that the saponification conditions and salt exchange treatment conditions were changed as described in Table 1. The average thickness, membrane strength, and electrolytic voltage of the obtained ion exchange membranes were each measured, and the results are shown in Table 1.
The polymerization ratio of the monomers in the fluorine polymer C-1 used in Example 1 was adjusted to obtain a polymer having an ion exchange capacity of 0.83 meq/g. The polymerization ratio of the monomers in the fluorine-containing polymer S-2 used in Example 1 was adjusted to obtain a polymer having an ion exchange capacity of 0.95 meq/g. A two-layer film (α) having a thickness of 50 μm (the thickness of the layer S-2 was 38 μm, and the thickness of the layer C was 12 μm.) was obtained in the same manner as in Example 1 except that these polymers were used. A composite membrane was obtained in the same manner as in Example 1 except that this two-layer film (α) was used. As the result of observation of the surface of the obtained composite membrane, it was observed that hemispherical projected portions having an average height of 60 μm constituted only by a polymer having ion exchange groups were formed on the anode-side film (b) at a density of 250 raised portions/cm2 and the total area of the raised portions was 0.2 cm2 per cm2.
Saponification was conducted on the obtained composite membrane as follows. That is, the composite membrane was immersed in an aqueous solution at 75° C. comprising 30% by mass of dimethyl sulfoxide (DMSO) and 15% by mass of potassium hydroxide (KOH) for 0.8 hours.
Subsequent to the saponification described above, salt exchange treatment was conducted in 2 stages as follows. For former stage salt exchange treatment, the composite membrane was immersed in 1 N NaOH at 90° C. for 0.5 hours. Next, for latter stage salt exchange treatment, the composite membrane was immersed in 0.3N NaOH at 50° C. for 1 hours to replace the ion attached to the ion exchange group by Na.
Subsequent to the 2-stage salt exchange treatment mentioned above, the membrane was washed with water and further dried at 60° C. to obtain a membrane main body. Thereafter, a coating layer was formed in the same manner as in Example 1 to obtain an ion exchange membrane.
The average thickness, membrane strength, and electrolytic voltage of the obtained ion exchange membranes were each measured, and the results are shown in Table 1.
Ion exchange membranes were obtained in the same manner as in Comparative Example 4 except that the saponification conditions and salt exchange treatment conditions were changed as described in Table 1. The average thickness, membrane strength, and electrolytic voltage of the obtained ion exchange membranes were each measured, and the results are shown in Table 1.
The polymerization ratio of the monomers in the fluorine polymer C-1 used in Example 1 was adjusted to obtain a polymer having an ion exchange capacity of 0.85 meq/g. This polymer and the fluorine polymer S-2 used in Example 1 (ion exchange capacity: 1.03 meq/g) were subjected to coextrusion using the same apparatus as that of Example 1 to obtain a two-layer film (α) having a thickness of 115 μm. The observation result of the cross-section of the film obtained with an optical microscope showed that the thickness of the layer S-2 was 100 μm and the thickness of the layer C was 15 μm.
On a drum including a heat source and a vacuum source therein and having many micropores on the surface thereof, embossed breathable heat-resistant release paper, the reinforcing material 1, and the two-layer film (α) having a thickness of 115 μm obtained above were laminated in the order mentioned and integrated at a drum surface temperature of 230° C. and under a reduced pressure of −650 mmHg while the air among each of the materials was evacuated to obtain a composite membrane. In the integration step, during the period from feeding of the materials to contact of the materials with the drum, the extension ratio of the two-layer film in the running direction was controlled to be 4% or less. As the result of observation of the surface of the obtained membrane, it was observed that hemispherical projected portions having an average height of 60 μm constituted only by a polymer having ion exchange groups were formed on the anode side at a density of 250 raised portions/cm2 and the total area of the raised portions was 0.2 cm2 per cm2.
Saponification was conducted on the obtained composite membrane as follows. That is, the composite membrane was immersed in an aqueous solution at 88° C. comprising 30% by mass of dimethyl sulfoxide (DMSO) and 15% by mass of potassium hydroxide (KOH) for 0.2 hours.
Subsequent to the saponification described above, salt exchange treatment was conducted in 2 stages as follows. For former stage salt exchange treatment, the composite membrane was immersed in 1 N NaOH at 50° C. for 0.6 hours. Next, for latter stage salt exchange treatment, the composite membrane was immersed in 0.4N NaOH at 90° C. for 0.6 hours to replace the ion attached to the ion exchange group by Na.
Subsequent to the 2-stage salt exchange treatment mentioned above, the membrane was washed with water and further dried at 60° C. to obtain a membrane main body. Thereafter, a coating layer was formed in the same manner as in Example 1 to obtain an ion exchange membrane.
The average thickness, membrane strength, and electrolytic voltage of the obtained ion exchange membranes were each measured, and the results are shown in Table 1.
An ion exchange membrane was obtained in the same manner as in Comparative Example 8 except that the saponification conditions were changed as described in Table 1. The average thickness, membrane strength, and electrolytic voltage of the obtained ion exchange membranes were each measured, and the results are shown in Table 1.
The polymerization ratio of the monomers in the fluorine polymer C-1 used in Example 1 was adjusted to obtain a polymer having an ion exchange capacity of 0.85 meq/g. This polymer and the fluorine polymer S-1 used in Example 1 (ion exchange capacity: 1.05 meq/g) were subjected to coextrusion using the same apparatus as that of Example 1 to obtain a two-layer film (α) having a thickness of 112 μm. The observation result of the cross-section of the film obtained with an optical microscope showed that the thickness of the layer S-1 was 100 μm and the thickness of the layer C was 12
On a drum including a heat source and a vacuum source therein and having many micropores on the surface thereof, embossed breathable heat-resistant release paper, the reinforcing material 1, and the two-layer film (α) having a thickness of 112 μm obtained above were laminated in the order mentioned and integrated at a drum surface temperature of 230° C. and under a reduced pressure of −650 mmHg while the air among each of the materials was evacuated to obtain a composite membrane. In the integration step, during the period from feeding of the materials to contact of the materials with the drum, the extension ratio of the two-layer film in the running direction was controlled to be 4% or less. As the result of observation of the surface of the obtained membrane, it was observed that hemispherical projected portions having an average height of 60 μm constituted only by a polymer having ion exchange groups were formed on the anode side at a density of 250 raised portions/cm2 and the total area of the raised portions was 0.2 cm2 per cm2.
Saponification was conducted on the obtained composite membrane as follows. That is, the composite membrane was immersed in an aqueous solution at 73° C. comprising 30% by mass of dimethyl sulfoxide (DMSO) and 15% by mass of potassium hydroxide (KOH) for 0.3 hours.
Subsequent to the saponification described above, salt exchange treatment was conducted in 2 stages as follows. For former stage salt exchange treatment, the composite membrane was immersed in 0.6N NaOH at 50° C. for 0.3 hours. Next, for latter stage salt exchange treatment, the composite membrane was immersed in 0.6N NaOH at 90° C. for 1 hours to replace the ion attached to the ion exchange group by Na.
Subsequent to the 2-stage salt exchange treatment mentioned above, the membrane was washed with water and further dried at 60° C. to obtain a membrane main body. Thereafter, a coating layer was formed in the same manner as in Example 1 to obtain an ion exchange membrane.
The average thickness, membrane strength, and electrolytic voltage of the obtained ion exchange membranes were each measured, and the results are shown in Table 1.
Ion exchange membranes were obtained in the same manner as in Comparative Example 10 except that the saponification conditions and salt exchange treatment conditions were changed as described in Table 1. The average thickness, membrane strength, and electrolytic voltage of the obtained ion exchange membranes were each measured, and the results are shown in Table 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-074574 | Apr 2022 | JP | national |
| 2023-068410 | Apr 2023 | JP | national |
