JP2001348439A - Ion-exchange membrane of fluororesin having widely ranging ion-exchange capacity, and process for preparation thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ion-exchange membrane of a fluororesin having a widely ranging ion-exchange capacity, especially an ion-exchange membrane for fuel cells. SOLUTION: The polymer ion-exchange membrane comprises a fluorine-based polymer material having a main body of a long chain-branched polytetrafluoroethylene structure bonded with a polystyrene graft side chain having sulfonic acid groups and also has an ion-exchange capacity of 0.5 to 4 meq/g and a tensile strength at break in the hydrous state of ranging from 1 to 25 MPa. The process for the preparation thereof comprises forming a long chain-branched polytetrafluoroethylene membrane which is irradiated in the range of a radiation dose of 5 to 300 kGy by using a radiation of an electron ray or gamma ray at a specific temperature and under a specific oxygen partial pressure, irradiating again the radiation of an electron ray or gamma ray onto the above membrane, introducing a polystyrene graft side chain by the grafting of styrene onto the irradiated polytetrafluoroethylene membrane in a liquid styrene monomer or in a styrene monomer solution diluted with a solvent, and introducing sulfonic acid groups onto the above polystyrene graft side chain.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluorine resin ion exchange membrane having a performance not only as a solid polymer electrolyte membrane suitable for use in a fuel cell but also as a membrane, and a method for producing the same.

[0002]

2. Description of the Related Art A fuel cell using a solid polymer electrolyte type ion exchange membrane has a low operating temperature of 100 ° C. or less and a high energy density, so that it is widely used as a power source for electric vehicles and a simple auxiliary power source. Expected. In this fuel cell, an ion exchange membrane which is a solid polymer electrolyte,
There are important element technologies related to a platinum-based catalyst, a gas diffusion electrode, and a polymer electrolyte membrane-electrode assembly. However, among them, development of a polymer ion exchange membrane having good characteristics as a fuel cell is one of the most important technologies.

In a polymer ion exchange membrane fuel cell, a gas diffusion electrode is compounded on both sides of the ion exchange membrane, and the membrane and the electrode have a substantially integral structure. For this reason, the ion exchange membrane acts as an electrolyte for conducting protons, and also has a role as a diaphragm for preventing the fuel or hydrogen or methanol from directly mixing with the oxidant even under pressure. Such an ion-exchange membrane is required to have a high proton exchange rate as an electrolyte, a high ion-exchange capacity, and a constant and high water retention to keep the electric resistance low. On the other hand, from the role of the membrane, the membrane has high mechanical strength, excellent dimensional stability, excellent chemical stability for long-term use, hydrogen gas and oxidation as fuel. It is required not to have excessive gas permeability to oxygen gas as an agent.

In the early polymer ion exchange membrane fuel cells, an ion exchange membrane of a hydrocarbon resin produced by copolymerization of styrene and divinylbenzene was used. However, this ion-exchange membrane has very low durability and is therefore impractical. For this reason, a fluororesin-based perfluorosulfonic acid membrane “Nafion (registered trademark)” developed by DuPont and the like are generally used thereafter. Has been used.

[0005] However, conventional fluororesin-based ion exchange membranes such as "Nafion" are excellent in chemical durability and stability, but have a small ion exchange capacity of about 1 meq / g, so that the electric charge of the membrane is low. High output due to high resistance, insufficient water retention and drying of the ion exchange membrane, resulting in reduced proton conductivity, and hindering the gas reaction of fuel gas and oxidant at the electrode catalyst There was something to do.

Further, since the fluororesin-based ion-exchange membrane starts from the synthesis of a monomer, the number of production steps increases,
Therefore, it is expensive and poses a major obstacle when the proton exchange membrane fuel cell is mounted on an automobile and put to practical use. For this reason, efforts have been made to develop a low-cost electrolyte membrane that replaces the aforementioned “Nafion” and the like.

In the radiation graft polymerization method closely related to the present invention, an attempt has been made to produce a solid polymer electrolyte membrane by introducing a side chain capable of introducing a sulfonic acid group into a fluororesin-based membrane. It has been done. However, in the case of a polymer film in which the polymer forming the main chain is only a fluorocarbon polymer containing no olefin hydrocarbons, when a radiation such as an electron beam or γ-ray is irradiated to perform a graft reaction, a remarkable film strength is obtained. A decrease is observed. Further, after the sulfonation reaction, the film strength further decreases. For this reason, when a membrane having ion conductivity is produced from a polymer membrane in which all of the main chain is fluorinated by the grafting method using radiation, the membrane is very brittle, and the membrane is formed during battery production or battery operation. Deterioration and destruction of the battery occurred, and the production of the battery was practically impossible.

For example, polytetrafluoroethylene (P
In the case of TFE), PTFE-propylene hexafluoride copolymer (FEP) or PTFE-perfluoroalkylvinyl ether copolymer (PFA), etc., it is known that remarkable cleavage of the polymer main chain occurs upon irradiation with radiation. ing. PTFE-propylene hexafluoride copolymer (F
In a battery prepared by using a solid polymer electrolyte membrane obtained by subjecting styrene to radiation graft polymerization of EP) and introducing a sulfonic acid group to the styrene, sulfonic acid groups are desorbed due to decomposition of the membrane immediately after the battery operation. As a result, it has been reported that the internal resistance of the battery increases, and that the battery performance significantly decreases even after several tens of hours of operation (Electroch).
emica Acta, 40, 345 (199
5)).

Further, in a fluororesin-based ion exchange membrane,
Due to the difficulty in introducing the crosslinks, the ion exchange capacity could not be increased. That is, when an attempt is made to introduce a large amount of graft side chains or sulfonic acid groups in order to increase the ion exchange capacity, the membrane is greatly swollen by water because of no cross-linking structure in the polymer chain, and the membrane strength is remarkably reduced. Will be damaged. Therefore, it is necessary to suppress the amount of graft side chains and sulfonic acid groups in a fluororesin-based ion exchange membrane to such an extent that the strength and dimensions of the membrane are maintained. Therefore, only a relatively small ion exchange capacity can be obtained. This does not provide the required performance as an ion exchange membrane when a large current flows as an electrolyte membrane of a fuel cell or the like.

On the other hand, when the polymer forming the main chain is a fluorocarbon polymer containing an olefinic hydrocarbon,
Radiation does not cause backbone breakage. For example, according to JP-A-9-102322, an ion-exchange membrane synthesized by introducing styrene, which is a hydrocarbon-based monomer, into an ethylene-tetrafluoroethylene copolymer resin film by radiation graft polymerization and then sulfonating the same is disclosed in Functions as an ion exchange membrane for fuel cells. However, a disadvantage is that the olefinic hydrocarbon is contained in the main chain, so that the water content becomes extremely high, and accordingly, the film strength is remarkably reduced. Furthermore, when this ion exchange membrane is used as a solid electrolyte membrane, if the catalyst layer of the gas diffusion electrode does not have sufficient water repellency, the electrode is excessively wet, particularly in a positive electrode in which water is generated in a fuel cell reaction. It has been pointed out that there is a problem that the output is reduced (Japanese Patent Laid-Open No. 11-11131).
0).

On the other hand, regarding radiation modification of fluororesin,
In recent years, the following patents have been reported. Conventionally, when the polymer main chain is only a fluorocarbon polymer, for example,
Polytetrafluoroethylene (PTFE) or propylene hexafluoride copolymer (FEP) or PTFE-
Perfluoroalkyl vinyl ether copolymer (PF
A) and others have been considered to be degradable polymers typical for radiation. However, even in these fluorine-based resins, it is said that crosslinking occurs when irradiation is performed under specific conditions at or above the crystal melting point. JP-A-6-1164
No. 23 has PTF which has rubber elasticity by radiation crosslinking.
It is described that E is obtained. Further, Japanese Unexamined Patent Application Publication No.
According to JP-A-49867, PTFE-propylene hexafluoride propylene copolymer (FEP) or PTFE-perfluoroalkylvinyl ether copolymer (PFA) is also crosslinked when irradiated at a temperature near the crystal melting point.

[0012]

As described above, in an ion exchange membrane mainly composed of a polymer having a main chain of an all-fluorinated carbon skeleton, particularly polytetrafluoroethylene (PTFE), radiation or crosslinking Since it was difficult to introduce a crosslinked structure with the agent, the ion exchange capacity could not actually be increased. In addition, when these films were irradiated with radiation such as γ-rays for grafting, a significant decrease in film strength was observed. Further, after the graft reaction and the sulfonation reaction, the film strength is further reduced. For this reason, when a film having high ion conductivity is produced using a fluororesin by a radiation grafting reaction, the film is very brittle and breaks during production of the fuel cell or operation of the fuel cell. Further, when an ion-exchange membrane having a large membrane strength is to be produced, the ion-exchange capacity is small and does not have sufficient performance as an electrolyte membrane or a diaphragm. From these facts, it has been considered that it is practically impossible to produce an ion exchange membrane for a fuel cell mainly composed of a fluororesin, particularly PTFE, using a radiation grafting method.

The present invention has been made in order to solve and solve the problems of the prior art. In a fluororesin ion exchange membrane, it has excellent characteristics as a solid polymer electrolyte, and has an excellent function as a diaphragm. It is intended to provide an inexpensive ion exchange membrane that does not impair the performance. That is, in the ion exchange membrane, the ion exchange capacity can be controlled in a wide range from 0.5 to 4 meq / g using a fluororesin;
The purpose is to provide an ion exchange membrane as a solid polymer electrolyte by maintaining the mechanical strength of the membrane even at a high ion exchange capacity, and controlling the electrical conductivity and water content within appropriate ranges. I have.

[0014]

[Means for Solving the Problems] Polytetrafluoroethylene (PTFE) was modified by irradiation with high-temperature radiation, and the results of research on the introduction of side chains and the introduction of sulfonic acid groups by radiation grafting reaction on this were continued. It was possible to solve all of the biggest drawbacks of the PTFE ion exchange membrane, such as a small ion exchange capacity and a significant decrease in membrane strength when containing water. Further, they have invented a fluororesin ion exchange membrane capable of controlling various properties including water content and electric conductivity within an appropriate range.

The present invention relates to a polymer ion exchange membrane which is excellent in characteristics as a solid polymer electrolyte and does not impair the performance as a membrane.
It is composed of a fluorine-based polymer mainly composed of a TFE structure and having a polystyrene graft side chain having a sulfonic acid group bonded thereto, and the ion exchange capacity of the ion exchange membrane is 0.1%.
A fluororesin ion exchange membrane characterized in that the membrane has a tensile strength at break of 5 to 4 meq / g in a water-containing state within a range of 1 to 25 MPa.

Here, the long-chain branched PTFE is a fluorine-containing polymer having a repeating unit of the following formula (1); a fluorine-containing polymer having a repeating unit of the following formula (2); (2) a fluorine-based polymer having a mixture of both repeating units; and a mixture of two or more of these fluorine-based polymers.

[0017]

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[0018]

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This is because when PTFE is irradiated and reacted at a high temperature such as a molten state, a tertiary carbon is obtained from a solid-state high-resolution ¹⁹ F nuclear magnetic resonance measurement.

[0020]

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The number of PTFE chains was considered to be the number of cross-linking points between PTFE chains (Radiation).
Physics and Chemistry, p1
65, 54 , 1999). However, from 50 kGy to 1
In a creep test of PTFE irradiated with γ-rays of MGy at a high temperature of 340 ° C., the permanent distortion (deformation amount) of 1MGy-irradiated PTFE is larger than that of non-irradiated PTFE. It was found that the larger the irradiation dose, which is considered to be higher, the larger the permanent distortion (Example 1). In general, a crosslinked polymer has a smaller distortion (deformation amount) of the material by a creep test than an uncrosslinked polymer, and the higher the crosslink density of a crosslinked material, the smaller the distortion. Therefore, PTFE generated by such high-temperature irradiation is not a network structure formed by cross-linking in which PTFE chains are directly bonded to each other in a molecular chain, but a long-chain branched structure. It can be seen that the bond is formed at the terminal.
In the molten state of PTFE, the cleavage and binding reaction of PTFE caused by radiation occur uniformly throughout the PTFE, and tertiary carbon in solid-state high-resolution ¹⁹ F nuclear magnetic resonance measurement.

[0022]

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From the amount, PTFE irradiated with 100 kGy has one long chain branch per 2500 CF ₂ units. From these facts, the proton exchange type ion exchange membrane mainly composed of the fluororesin in the present invention has a chemical structure in which the unit represented by the formulas (1) and (2) is a repeating unit. Such a structure of PTFE is referred to as “long-chain branched polytetrafluoroethylene (PTFE)” in this specification.

In this long-chain branched PTFE, the entanglement of long-chain branches and the introduction of a bond at the terminal of the branched chain enable
The ion exchange capacity can be increased without substantially reducing the strength of the fluororesin membrane. That is, the present inventors considered that the limitation that the ion exchange capacity, which is the greatest defect of the fluororesin-based ion exchange membrane, cannot be increased can be solved. The conventional grafting method simply involves irradiating the PTFE membrane with radiation and grafting styrene. On the other hand, the method for producing a fluororesin ion exchange membrane of the present invention has a 300
Irradiate the PTFE membrane with radiation at a temperature of
The FE was introduced with a long-chain branch and a bond at the branch end, and then irradiated with radiation near room temperature to graft styrene. In this case, a new discovery was found that the graft ratio of the long-chain branched PTFE was significantly increased at the same radiation dose as compared with ordinary PTFE (see FIG. 1). This is considered to be due to the fact that the field of the graft reaction in the PTFE film was increased by the irradiation at a high temperature.

As described above, in the present invention, the graft ratio (percentage of the graft chain portion to the main chain portion when dried, where the long chain branched PTFE structure is the main chain portion and the polystyrene side chain is the graft chain portion) is 5.の も の 150 wt% can be easily produced. As described above, in the present invention, since a fluororesin having a high graft ratio of polystyrene is obtained, the amount of sulfonic acid groups introduced, that is, the ion exchange capacity can be easily increased. As the ion exchange membrane into which the sulfonic acid group is introduced, the ion exchange capacity of the fluororesin ion exchange membrane of the present invention is 0.5 to 4 meq / g, preferably 1.0 to 4 meq / g, and more preferably 1.3 to 3 me.
q / g. Here, the ion exchange capacity of the ion exchange membrane is the amount of exchange groups (meq / g) per 1 g of the weight of the dry ion exchange membrane.

The fluororesin ion exchange membrane according to the present invention comprises:
A fluoropolymer having a repeating unit of the above formula (1);
A fluoropolymer having a repeating unit of the above formula (2);
And a fluorinated polymer having a mixture of both repeating units of the above formulas (1) and (2); and a mixture of two or more of these fluorinated polymers; by grafting styrene and introducing a sulfonic acid group. The resulting fluoropolymer having a repeating unit of the following formula (5); a fluoropolymer having a repeating unit of the following formula (6); and both repeating units of the following formulas (5) and (6): An ion-exchange membrane comprising: a fluorine-containing polymer having a mixture; and a mixture of two or more of these fluorine-containing polymers.

[0027]

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[0028]

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Also, 45 kGy was applied to the long-chain branched PTFE membrane.
The film produced by irradiation and grafting styrene became a film having sufficient strength to withstand use when wet, but the film produced by irradiating 45 kGy to normal PTFE and grafting styrene was used when wet. Very weak,
It turned out that it cannot be handled as an ion exchange membrane. That is, despite the high ion exchange capacity of the fluororesin according to the present invention, the entanglement of PTFE long-chain branches and the binding at the branch end also suppress the increase in water content due to swelling of the membrane, and maintain an appropriate membrane strength. I found it. Here, the water-containing state of the membrane is a state in which the ion-exchange membrane is stored in purified water at room temperature for 24 hours or more, and the water content is the weight of the ion-exchange membrane stored in the water. It is a percentage calculated from the weight of the film when the film was vacuum-dried at 60 ° C. for 16 hours by the formula (10) described in Examples.

Conventionally, ion exchange capacity of a fluororesin-based ion exchange membrane is 1 in terms of mechanical strength and dimensional stability of the membrane.
Only those having meq / g were practically usable. This is because it is generally difficult to introduce crosslinks with a fluorine-based resin, especially PTFE, and in this ion exchange membrane, the membrane strength is mainly maintained by the crystal part of PTFE. For this reason, when a large amount of graft side chains or sulfonic acid groups are introduced, the strength of the PTFE resin is rapidly reduced, and the resin cannot be used. In contrast, the long-chain branched PT of the present invention
Fluorine resin with FE structure has mechanical properties and dimensional stability even when a large amount of grafted side chains or sulfonic acid groups are introduced up to 4 meq / g due to entanglement of long-chain branches or binding of long-chain branches. Since the properties are maintained, it can be put to practical use (for example, see Examples 10 and 11). Although it is possible to produce a membrane having an ion exchange capacity of 4 meq / g or more, the mechanical properties of the membrane deteriorated, and it was judged that the membrane could not withstand practical use. From these facts, the fluororesin ion exchange membrane of the present invention has the above-mentioned ion exchange capacity, and has a tensile rupture strength of 1 to 2 in a water-containing state.
It is characterized by being 5 MPa, more preferably 5 to 20 MPa. At this time, the film material has a tensile elongation of 15% or more, more preferably 30% or more. These mechanical strengths are sufficient for practical use as an ion exchange membrane. Further, from the mechanical properties of the film, the graft ratio is characterized by being 5 to 150 wt%, more preferably 20 to 70 wt%.

The fluororesin ion-exchange membrane according to the present invention is, as a modified embodiment thereof, one of the above polystyrene side chains in the polystyrene structure.
２５25 wt% α-methylstyrene, divinylbenzene copolymer or crosslinked structure, and polyfunctional cross-linking aid such as triallyl isocyanurate or triallyl cyanurate in a polystyrene graft chain in an amount of 1 to 15 wt% based on polystyrene Characterized by having a crosslinked structure.

In the fluororesin ion exchange membrane of the present invention, the amount of the introduced long-chain branch, the amount of the sulfonic acid group, and the —OH where the double bond at the molecular terminal in the formulas (1) and (2) reacts. The water content of the fluororesin can be controlled by the amount of the group. When this membrane is used as an ion exchange membrane for a fuel cell, if the water content is too low, the output voltage decreases when the pressure of oxygen or hydrogen is low or when air is used as an oxygen source, resulting in high current density and high output. Cannot be maintained. Also, a slight change in the operating conditions undesirably changes the electric conductivity and the gas permeability coefficient. Therefore, it is necessary that the ion-exchange membrane hardly becomes a dry state and the change in gas permeability coefficient and electric conductivity is relatively small. The water content of the ion exchange membrane of the present invention can be arbitrarily controlled at 10 to 150 wt%. In general, as the ion exchange capacity increases, the water content also increases. However, by controlling the introduction amount of long-chain branches by irradiating the PTFE membrane in advance, even if the ion exchange membranes have the same exchange capacity, The moisture content can be varied. Since the water content of the ion exchange membrane of the present invention can be greatly changed, the water content of the membrane is 10 to 150 wt%, preferably 20 wt%.
-100 wt%.

The higher the electrical conductivity of the polymer ion exchange membrane for a fuel cell, the lower the electrical resistance and the higher the performance as an electrolyte membrane. When the electrical conductivity of the ion exchange membrane at 25 ° C. is 0.05 Ω ⁻¹ · cm ⁻¹ or less, the output performance as a fuel cell often decreases significantly,
Electrical conductivity of the ion-exchange membrane is 0.05? ^-1 · cm ^-1 or more, the higher performance of the ion exchange membrane 0.15Ω ^-1 · cm
Must be ^-1 or more. On the other hand, it is known that the strength of the ion exchange membrane is reduced when the electric conductivity of the ion exchange membrane at 25 ° C. is 0.12 Ω ⁻¹ · cm ⁻¹ or more in a normal fluororesin ion exchange membrane. That is, if the exchange capacity of the ion-exchange membrane is increased and the electric conductivity is made too high, there is a disadvantage that the strength of the membrane is reduced (for the electric conductivity of the membrane, see the formula (11) shown in the embodiment). But,
In the ion exchange membrane according to the present invention, the exchange capacity at 25 ° C. is 2.8 meq / g, and the electric conductivity is 0.166 Ω ⁻¹ · cm ^−1.
It was clarified by the present inventors that the film strength did not decrease so much in Example 8 (Example 8). This is due to the fact that the polytetrafluoroethylene membrane is previously modified into a long-chain branched type by radiation, and is a very important invention for practical use of a high ion exchange capacity membrane.
The ion exchange membrane of the present invention has an electrical conductivity of 0.2 at 25 ° C.
0.5 to 0.3 Ω ^-1 · cm ^-1 , preferably 0.15 to 0.
It is characterized by being 3Ω ^-1 · cm ^-1 .

The ion-exchange membrane of the present invention has the above-mentioned properties, an ion exchange capacity of 0.5 to 4 meq / g, a membrane strength of 1 to 25 MPa, a water content of 10 to 150 wt%, and 25 ° C.
Can be manufactured by controlling the electric conductivity in each of the numerical ranges of 0.05 to 0.3 Ω ^-1 · cm ^-1 . It is also a feature of the present invention that the characteristics can be arbitrarily controlled within these limited ranges.

It is conceivable to reduce the thickness of the ion exchange membrane in order to improve the characteristics of the ion exchange membrane. However, at present, an ion exchange membrane that is too thin is easily damaged, and it is difficult to manufacture the ion exchange membrane itself. Further, since the absolute amount of water contained in the ion exchange membrane is also reduced, the ion exchange membrane is likely to dry, and it may be impossible to maintain high performance for a long time. Therefore, usually 50
Ion exchange membranes in the range of ~ 300 [mu] m thickness are used.
In the case of the present invention, the film thickness is not particularly limited, but is 50 μm to 3 μm.
Those having a range of 00 μm are preferred.

The present invention relates to an embodiment of the production of a fluororesin-based ion exchange membrane, in which a long-chain branched PTFE membrane is produced according to the method described below, followed by introduction of a polystyrene graft side chain, The method is characterized by a method for producing a fluororesin-based ion-exchange membrane comprising the introduction of an acid group.

First, in producing a long-chain branched PTFE membrane,
PTFE having a film thickness of 30 to 300 μm at 330 to 350 ° C.
Irradiation of electron beams or γ-rays in an inert gas within a range of 10 Torr or less of oxygen or under reduced pressure at an arbitrary constant temperature between 300 and 365 ° C. or at a variable temperature between 300 and 365 ° C.
It is manufactured by irradiation with kGy. By selecting the irradiation temperature and dose within this range, the long-chain branched PTF generated
The characteristics of the E film can be changed. For example, 200kG
If the dose of y is selected, a long-chain branched PTFE film having a larger number of long-chain branches than that of 5 kGy irradiation, that is, a relatively large film strength can be obtained. This is because a long-chain branched PTFE membrane having high strength can be obtained by entanglement of branches due to a large number of long-chain branches and chemical bonding between the branches.

Subsequently, for the introduction of a graft side chain, the above film is again irradiated with γ-rays or electron beams at room temperature at 5 to 100 kGy (so-called pre-irradiation), and then subjected to a temperature range below the boiling point of the styrene monomer and the solvent, usually 40 ° C. Grafting reaction is performed at -60 ° C in a solution obtained by diluting styrene alone or a styrene monomer with a solvent such as benzene to introduce a polystyrene graft side chain. Since the presence of oxygen inhibits the graft reaction, these series of operations must be performed in an inert gas such as an argon gas or a nitrogen gas, or in a state where oxygen in the liquid has been removed with an argon or a nitrogen gas. The amount of grafting is proportional to the dose (the number of active species), with higher doses producing more graft side chains. Temperature 40-60
After the reaction at 1 ° C. for 1 to 24 hours, the graft membrane is taken out of the reaction solution and immersed in toluene at 40 ° C. overnight to extract and remove styrene and styrene homopolymer. Thereafter, vacuum drying is performed at 40 ° C.

In the grafting reaction, α-methylstyrene and divinylbenzene are added together with styrene monomer in an amount of 1 to 25% by weight as a comonomer to form a graft side chain in a copolymerized or crosslinked structure. % Of triallyl cyanurate or triallyl isocyanurate is also effective.

Further, in the introduction of a sulfonic acid group, an ion exchange membrane is prepared by introducing a sulfonic acid group into a polystyrene side chain of the graft membrane. Sulfonation conditions are from room temperature to 6
0 ° C., 2 to 24 hours, tetrachloromethane, chloroform, dichloromethane, 1,2-dichloroethane, 1,
Using 1,2,2-tetrachloroethane or the like as a solvent, 0.2 to 0.5 mol / L of chlorosulfonic acid is reacted. After reacting for a predetermined time, the membrane is taken out and sufficiently washed with water to remove the solvent and unreacted chlorosulfonic acid. At this time, hydrolysis occurs and sulfonic acid groups are formed to form an ion exchange membrane.

When styrene or the above comonomer is grafted, if a long-chain branched PTFE film irradiated with a heavy ion beam instead of a γ-ray or an electron beam is used, radicals are generated along the tracks of the heavy ions. Here, dense graft chains are formed. Since these tracks penetrate the membrane, ions move more efficiently through ion-exchange groups attached to the graft chains formed along the tracks than when using γ-rays or electron beams with the same absorbed dose. Conductivity increases.
For a film having a thickness of 50 μm, helium ions are 8 MeV or more, carbon ions are 40 MeV or more, and neon ions are 80 MeV or more.
MeV or more, argon ion is 180 MeV or more. Similarly, if the film is 100 μm thick, helium ion is 12 MeV or more, carbon ion is 62 MeV or more, neon ion is 130 MeV or more, and argon ion is 300 M
eV or more is desirable. The irradiation amount is 1 × 10 ¹⁰ to 1 × 10 ¹³
Pcs / cm ² is preferred.

The fluororesin ion exchange membrane according to the present invention can be manufactured by the above-described series of operations and reactions.

[0043]

EXAMPLES The present invention will be described below with reference to some examples and comparative examples, which are given for illustrative purposes only, and the scope of the present invention is not limited to the specific examples shown in these examples. It is not limited to a simple matter.

Example 1 and Comparative Example 1 A polytetrafluoroethylene (PTFE) sheet having a thickness of 500 μm and a number average molecular weight of 1 × 10 ⁷ was 10 cm × 10
cm pieces into a heating type irradiation container, and the inside of the container is 10 ^-3 T
It was degassed to about orr and replaced with argon gas. Then, the temperature of the PTFE sheet is increased by 33 with an electric heater.
Assuming a temperature of 5 to 345 ° C, an electron beam of 2 MeV is applied at a dose of 50;
0; 300; 500 kGy or 1 MGy. After the irradiation, the container was cooled and the high-temperature irradiated PTFE sheet was taken out.

Table 1 shows the creep characteristics of the obtained sheet. Unirradiated (0 dose) for comparison purposes in this test
The characteristics of the sample were also measured (Comparative Example 1). The measurement method is AST
M-D621-64 (1996). Comparative Example 1
In the irradiated sample of 1 MGy as compared with the non-irradiated sample indicated by, the permanent distortion was rather large. In addition, dose 100k
In the irradiation of Gy or more, the permanent strain increases as the dose increases. In general, a crosslinked polymer having a crosslinked polymer has a smaller permanent set in a creep test than an uncrosslinked polymer, and has a smaller permanent set as the crosslink density is higher (as the irradiation dose is higher). Therefore, PTFE generated by such high-temperature irradiation is not a network structure formed by cross-linking in which PTFE chains are directly bonded to each other in a molecular chain, but a long-chain branched structure. It can be seen that the bond is formed at the terminal. As described above, in the long-chain branched PTFE, the long-chain branches bonded to the polymer main chain are entangled with each other, and the bonds are formed at the ends of the branches. The degree decreases. For this reason, the strain amount in the creep test increases.

[0046]

[Table 1]

Example 2-3 and Comparative Example 2 Two 10 cm × 4 cm sheets of PTFE (number average molecular weight: 1 × 10 ⁷ ) having a thickness of 50 μm were placed in a heating type irradiation vessel, and the inside of the vessel was placed at 10 ⁻³ Torr. It was degassed to a degree and replaced with argon gas. At this time, a case where a very small amount of oxygen was intentionally added was also prepared. Thereafter, the sample was heated by an electric heater, the temperature of the PTFE sheet was set to 340 ° C., and ⁶⁰ Co-γ
The line was irradiated at 60 kGy (dose rate 3 kGy / h for 20 hours). After irradiation, the container was cooled and a sheet of high-temperature irradiated PTFE was taken out.

Table 2 shows the oxygen atom content in the PTFE irradiated at a high temperature. The oxygen atom content was obtained by cutting a PTFE sheet into a circle having a diameter of about 40 mm and fixing it to a sample holder of a fluorescent X-ray analyzer (manufactured by Rigaku Denki Kogyo). An electron beam is applied to a sheet having an area of 30 mm in diameter at a tube voltage of 40 kV and a tube current of 7 mm.
At 0 mA, the content of each atom was determined from the intensity of the intrinsic X-ray of the atom emitted from the sample. At this time, for comparison purposes, the element content in the unirradiated PTFE sheet sample was also measured (Comparative Example 2).

By introducing oxygen into the atmosphere at the time of irradiation, oxygen atoms are introduced into the PTFE irradiated at a high temperature.
It is believed that the oxygen atom attaches to the end of the PTFE backbone that has been cut by radiation and is introduced at the base of the long chain branch or at the branch end.

[0050]

[Table 2]

Example 4 A long-chain branched PTFE film (thicknesses of 50 μm and 100 μm) obtained by irradiating ⁶⁰ Co-γ rays with 0 to 210 kGy was degassed.
Again, γ-rays were pre-irradiated at 15 to 45 kGy to graft styrene at 60 ° C. The relationship between the grafting rate and the grafting time obtained under various grafting conditions is shown graphically in FIG. The graft ratio was determined by the following equation (7). As is apparent from this figure, the conventional PTF having no long-chain branch introduced therein.
Graft ratio to E film (0 kGy) is 20% even in 22 hours
Although the degree was large, the film irradiated with 135 kGy increased remarkably to 90%. However, the dose was increased to 210kG
The irradiation with y makes the bonds between the long-chain branched PTFE chains denser and increases the strength of the membrane, but the styrene hardly enters the membrane, so that the initial grafting speed is greatly suppressed.
When the pre-irradiation dose for the graft reaction is increased from 15 kGy to 45 kGy, the graft ratio increases with the pre-irradiation dose. In this example, the non-hot irradiated P
A graft experiment of TFE (0 kGy) is referred to as Comparative Example 3.

Assuming that the graft ratio is a long-chain branched polytetrafluoroethylene structure as a main chain and a polystyrene chain as a graft side chain, the weight ratio (wt%) of the graft side chain to the main chain is generally It is expressed as a graft ratio (X _dg ) of the following formula (7).

X _dg (wt%) = 100 · (W _t −W ₀ ) / W ₀ (7) W ₀ : Weight of PTFE film before grafting (g) W _t : PTFE film after grafting (dry state) 5 ) Examples 5 to 11 and Comparative Examples 4 to 6 The above graft membranes were immersed in 0.2 M to 0.5 M chlorosulfonic acid (1,2-dichloroethane solvent),
The sulfonation reaction was performed at 0 ° C. for 6 to 24 hours. afterwards,
This membrane was washed with water to obtain sulfonic acid groups. The reaction is 50
At 20 ° C. for 20 hours, the degree of sulfonation reached almost 100%.
That is, it was found from the ion exchange capacity and the graft ratio that almost one sulfonic acid group was bonded to one grafted styrene unit. The degree of sulfonation at 60 ° C. for 6 hours was around 50%. Table 3 shows the ion exchange capacity, water content, film thickness, and electrical conductivity (specific conductivity) of the membrane obtained in this example.

The ion exchange capacity of the ion exchange membrane obtained in this example is as high as 1 to 3 meq / g. Further, the mechanical properties of the membrane when it contains water, in particular, the tensile strength is 8.4 MPa for an ion exchange capacity of 1.1 meq / g (a styrene graft ratio of 66% and a water content of 13%), and an ion exchange capacity of 2.
6 meq / g (styrene grafting ratio 68%, water content 6
8%) was 7.1 MPa.

As described above, when 1,2-dichloroethane is used as the solvent, the sulfonation rate X _ds (%) is 60 ° C.
Approximately 50% in 6 hours, approximately 100% in 50 ° C, 20 hours
Met. In selecting a solvent, swellability of the styrene graft membrane in the solvent, sulfonation reaction rate, ease of handling such as boiling point, and the like are important. The sulfonation reaction rate varied depending on the solvent, but no significant difference was observed. Since solvents having one carbon atom such as tetrachloromethane, chloroform, and dichloromethane all have a low boiling point, when the temperature is increased, the vapor pressure increases, and there is a difficulty in handling. In particular, chloroform is excellent in both swellability and sulfonation reaction rate, but has a low boiling point and is difficult to handle. When 1,2-dichloroethane, which is a solvent having two carbon atoms, is compared with 1,1,2,2-tetrachloroethane, the swellability of the styrene graft membrane is larger in the latter, but the sulfonation reaction rate is higher. The former was larger, indicating that the swellability and the sulfonation reaction rate did not always correspond.

The sulfonation rate X_ds(%) Is the following formula
It is represented by (8). X_ds(%) = 100 · n (SO_ThreeH)_obs/ N (SO_ThreeH)_cal (8) n (SO_ThreeH)_obs, N (SO_ThreeH)_calAre
The measured and calculated values of the phonic acid group concentration (mmol, mM)
You. Calculated value n (SO_ThreeH)_calIs the grafted styrene
Assume that one sulfonic acid group is in all para positions
And the graft ratio X _dgAnd weight W of the membrane after grafting_tFor
And is expressed by the following equation (9).

N (SO ₃ H) _cal (mM) = X _dg · W _t / M (X _dg +100) (9) M = 104.15 (molecular weight of styrene) Measurement of measured value n (SO ₃ H) _obs Is 5 in 1M sulfuric acid solution.
It was immersed at 0 ° C. for 4 hours to make all the exchange groups —SO ₃ H type. Thereafter, it was immersed in a ₃ M aqueous NaCl solution at 50 ° C. for 4 hours to form a —SO ₃ Na type, and the substituted proton (H ⁺ ) was neutralized and titrated with 0.2 N NaOH to determine the sulfonic acid group concentration. Also, the dry weight W of the sulfonated membrane
_d is expressed by the following equation (10) when the -SO ₃ H form. W _d (SO ₃ H) _cal (g) = W _t + n (SO ₃ H) _cal · 80 = W _t [1 + 80 · X _dg / M (X _dg +100)] (10) Therefore, ion exchange of the ion exchange membrane The capacity (mM / g) can be easily predicted from the graft ratio X _dg using the following equation (11).

N (SO ₃ H) _cal / W _d (SO ₃ H) _cal = 1000 / [100M / X _dg + (M + 80)] (11) When the graft ratio is 10, 30, 50, 100, 150% , The ion exchange capacities are 0.82 and 1.8, respectively.
8, 2.55, 3.47, 3.94 mM / g.

As described above, it is apparent from the above calculation that the ion exchange capacity of the ion exchange membrane depends on the amount of styrene side chains (graft ratio) and the amount of sulfonic acid groups.
The results obtained by the prediction and the experiment agreed very well, and it was confirmed that the control could be arbitrarily performed in the range of 0.5 to 4 meq / g.

The measured value of the ion exchange capacity was determined by measuring the sulfonic acid group concentration and the dry weight of the ion exchange membrane, as is apparent from the equation (11). The SO ₃ H type ion-exchange membrane stored in water at room temperature was taken out of the water and gently wiped, and the weight of the membrane was designated as W _s (g). Thereafter, the membrane was vacuumed at 60 ° C. for 16 hours. Weight of membrane W _d when dried
(G) was taken as the dry weight. The ion exchange capacity is n (SO ₃
H) _obs / W _d (meq / g).

From the measured values W _s and W _d , the water content can be obtained by the following equation (12). Water content (wt%) = 100 · (W _s −W _d ) / W _d (12) The electric conductivity of the ion exchange membrane was measured by an AC method (New Experimental Chemistry Course 19, Polymer Chemistry <II>). , P.99
2, Maruzen), the film resistance was measured using an ordinary film resistance measurement cell and an LCR meter (E-4925A) manufactured by Hewlett-Packard. A cell was filled with a 1 M aqueous sulfuric acid solution, and the resistance between the platinum electrodes (distance 0.5 mm) was measured depending on the presence or absence of the membrane. The electrical conductivity (specific conductivity) of the membrane was expressed by the following equation (1).
Calculated using 3).

Κ = (1 / R _m ) · (d / S) (Ω ⁻¹ cm ⁻¹ ) (11) κ = specific conductivity (Ω ⁻¹ cm ⁻¹ ) d = thickness of ion exchange membrane (cm) S) Current carrying area of the ion exchange membrane (cm ² ) For comparison, Mark W.C. Verb
Rugge, Robert, F.R. Hill et al.
Electrochem. Soc. , 137 , pp
3770-3777 (1990)) and a potentiostat, a function generator. Good correlation was found between the measured values. Table 3 shows the values of the former AC method. The ion exchange capacity, water content, film thickness, and electrical conductivity (specific conductivity) measured for Dow 800, Nafion 115, and Nafion 117 shown in Table 3 are for comparison purposes. Are Comparative Examples 4 to 6.

[0063]

[Table 3]

[0064]

The fluororesin ion exchange membrane of the present invention is mainly composed of a long-chain branched polytetrafluoroethylene structure,
The ion exchange capacity is 0.5 to 4 meq / g, the tensile rupture strength of the membrane material in a water-containing state is 1 to 25 MPa, the water content is 10 to 150 wt%, and the electrical conductivity at 25 ° C. is 0.
An ion-exchange membrane having an excellent characteristic within the range of 0.5 to 0.3 Ω ^-1 cm ^-1 . This is a fluororesin ion exchange membrane having a wide range of ion exchange capacities that can be produced at low cost, especially a high ion exchange capacity and strong membrane strength that have never been seen before. The ion exchange membrane of the present invention is particularly suitable for fuel cell applications.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of a pre-irradiation dose on the graft ratio of styrene to a high-temperature irradiated and unirradiated polytetrafluoroethylene film (see Example 4 and Comparative Example 3). The first dose in the legend for each symbol is the hot dose, the later dose is the pre-dose for the graft reaction.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01M 8/02 H01M 8/02 P 8/10 8/10 // C08L 51:06 C08L 51:06 F term (Reference) 4F071 AA22X AA27 AA27X AA77X AA78X AF06Y AF16Y AF37Y AH15 FA01 FA05 FB01 FB05 FB06 FB07 FC01 FD02 FD04 4F073 AA06 AA32 BA16 BB01 CA41 CA42 CA51 CA65 EA31 EA32 BA02 DB01 GA03 GA06 GA08 5G301 CD01 CE00 CE01 5H026 AA06 CX05 EE18 EE19

Claims (7)

[Claims] 1. Ion exchange of 0.5 to 4 meq / g consisting of a long chain branched polytetrafluoroethylene structure and a fluorinated polymer material having a polystyrene graft side chain having a sulfonic acid group bonded thereto. Has the capacity,
A polymer ion exchange membrane having a tensile breaking strength in the range of 1 to 25 MPa in a water-containing state. 2. An electric conductivity at a water content of 10 to 150% by weight (wt%) and a temperature of 25 ° C. is 0.05 to 0.3Ω ⁻¹ ·.
2. The polymer ion exchange membrane according to claim 1, wherein the molecular weight is within a range of cm ^-1 . 3. The method according to claim 1, wherein α-
The copolymer according to claim 1 or 2, wherein the copolymer has one or more of copolymerization with methylstyrene, copolymerization with divinylbenzene, a crosslinked structure by triallyl cyanurate and a crosslinked structure by triallyl isocyanurate. Polymer ion exchange membrane. 4. A polytetrafluoroethylene film having a thickness of 300
A long-chain branched polytetrafluoroethylene film is obtained by irradiating a long-chain branched polytetrafluoroethylene film by using an electron beam or a γ-ray radiation within a dose range of 5 to 300 kGy under an atmosphere condition of a temperature range of up to 365 ° C. and an oxygen partial pressure range of 10 Torr or less. After that, the film is irradiated again with an electron beam or γ-ray radiation in an inert gas at a dose range of 5 to 100 kGy in an inert gas, and in a temperature range from room temperature to a boiling point of the styrene monomer or lower in a liquid styrene monomer, Or a styrene monomer in a solution diluted with a solvent in a temperature range from room temperature to the boiling point of the solvent or less, to introduce a polystyrene graft side chain by performing a styrene graft reaction on the irradiated polytetrafluoroethylene film,
A method for producing an ion exchange membrane, further comprising introducing a sulfonic acid group into the polystyrene graft side chain. 5. The ion exchange according to claim 4, wherein when introducing the polystyrene graft side chain, 1 to 25% by weight of α-methylstyrene and / or divinylbenzene with respect to the styrene monomer is used as a comonomer. Manufacturing method of membrane. 6. The method according to claim 4, wherein when introducing the polystyrene graft side chain, 1 to 15 wt% of triallyl cyanurate and / or triallyl isocyanurate relative to the styrene monomer is used as a crosslinking aid. 6. The method for producing an ion exchange membrane according to 5. 7. Instead of irradiation by electron beam or gamma ray for graft reaction of styrene monomer, helium,
A heavy ion beam of a carbon ion, a nitrogen ion, an oxygen ion, a neon ion, or an argon ion, the irradiation amount of which is a heavy ion beam having energy sufficiently penetrating the thickness of the long-chain branched polytetrafluoroethylene film. according to any one of claims 4-6, characterized in that irradiation with long chain branched polytetrafluoroethylene membrane with the irradiated film surface 1 cm ² per 1 × ¹⁰ 10 ~1 × 10 ¹³ pieces of range A method for producing an ion exchange membrane.