JPS6032247A - Manufacture of diaphragm for battery - Google Patents

Abstract

PURPOSE:To obtain a diaphragm having a good chemical resistance, a small ion transmission resistance, a small electric resistance and an excellent mechanical strength by subjecting a microporous film consisting of a polyfluoroolefin to alkali metal treatment, then irradiating ionizing radiation upon the film before hydrophilic monomers are subjected to graft polymerization. CONSTITUTION:A microporous polyfluoroolefin film preferably a polytetrafluoroethylene film is subjected to alkali metal treatment so as to introduce active polar groups which can easily produce cross-linking radicals onto the surface of the film. When an untreated film is used, radicals of an amount necessary for graft polymerization are produced by the irradiation of ionizing radiation of such a low dose as not to cause graft polymerization, thereby effectively graft- polymerizing the monomers to make the film hydrophilic. The thus obtained microporous film has excellent chemical stabilities especially a good alkali resistance. Moreover, it has a small electric resistance and a high selective permeability for ions. Therefore the microporous film can be used as a superior diaphragm for an alkaline battery.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a diaphragm for batteries, and more particularly, it is made of a microporous polyfluorinated olefin film, has excellent chemical resistance, and has low ionic permeation resistance and low electrical resistance. The present invention also relates to a method for producing a diaphragm for alkaline batteries that has excellent mechanical strength.

In general, alkaline batteries such as mercury and silver batteries, which are small high-performance batteries, have a cathode mixture and an anode mixture separated by a diaphragm, and only the electrolyte moves between the negative and anode electrodes via the diaphragm to carry out the battery reaction. Therefore, the diaphragm for alkaline batteries must be chemically stable, have good ion permeability, have low electrical resistance, be non-conductive, and have sufficient mechanical strength. etc. are required.

Although cellophane membranes that have conventionally been widely used as diaphragms for batteries generally satisfy the above-mentioned properties, they tend to lack the above-mentioned chemical stability.

That is, the battery is susceptible to oxidative deterioration due to the oxidizing properties of the anode active material in the alkaline electrolyte or the alkali, and the cathode and anode active materials come into contact with each other during battery storage, resulting in internal short circuit and self-discharge. Battery life will be shortened.

On the other hand, a microporous film made of polyfluorinated olefin has an extremely small surface energy and satisfies the above-mentioned required characteristics, except that it is water repellent.

Therefore, after irradiating this microporous polyfluorinated olefin film with ionizing radiation such as an electron beam to activate the surface, a hydrophilic polymerizable monomer is contacted and graft polymerized. , it is considered to make the film hydrophilic. However, because the C-F bond energy in polyfluorinated olefins is 1; l'441J1 mole, which is larger than the C-C bond energy -348J1 mole, when irradiated with ionizing radiation, especially electron beams, some Even if a crosslinking radical is generated in the fluorine-substituted carbon and the polymerizable monomer is effectively graft-polymerized to this radical, the majority of the electron beam acts to sever the polymer chain, causing the polymer to degrade. Denatures into molecular weight products. That is, conventionally, even when a polyfluorinated olefin film, particularly a polytetrafluoroethylene film, is irradiated with ionizing radiation and brought into contact with a polymerizable monomer, effective graft polymerization of the monomer is difficult to occur, and There was a problem in that the polyfluorinated olefin could not be avoided from having a low molecular weight, and the desirable physical properties of the polyfluorinated olefin film were impaired.

On the other hand, when irradiating a polyfluorinated olefin film with a low dose of electron beam to avoid the above-mentioned scission of polymer chains, the amount of crosslinking radicals generated is insufficient for graft polymerization of polymerizable monomers. Or, because the amount of electron beam transmission is small, cross-linking radicals are generated only on the very surface of the film, and the polymerizable monomer is effective because it does not immediately react with oxygen in the atmosphere and lose its activity. It was difficult to make graft polymers.

Furthermore, polyfluorinated olefin molded articles have extremely low surface energy and poor wettability. For this purpose, for example, an alkali metal-naphthalene complex is formed in an organic solvent, and a polyfluorinated olefin film is immersed in the complex.
Methods for improving the surface wettability have been known for some time.

In order to solve the above-mentioned problems in making microporous polyfluoroolefin films hydrophilic by grafting, the present inventors conducted intensive research on surface treatment of microporous polyfluoroolefin films prior to polymerization (Graph 1). As a result, the above-described prior alkali metal treatment of the film unexpectedly results in a low dose of ionization that would make it substantially difficult to graft-polymerize the polymerizable tli form in the case of an untreated film. By irradiating the molded article with sexual radiation, a sufficient amount of crosslinking radicals are generated to graft-polymerize the polymerizable monomer without substantially cutting the polymer chain of the polyfluorinated olefin. The present invention was achieved by discovering that this can be done.

The method for producing a battery diaphragm according to the present invention includes treating a microporous film made of polyfluorinated olefin with an alkali metal, irradiating it with ionizing radiation, and then bringing the film into contact with a hydrophilic polymerizable monomer. It is characterized by graft polymerizing the Rffi body described above.

In the present invention, polyfluorinated olefins include polyvinyl fluoride, polyvinylidene fluoride, vinyl fluoride-vinylidene fluoride copolymer, polychlorotrifluoroethylene, polytetrafluoroethylene, and tetrafluoroethylene-hexafluoropropylene copolymer. , vinyl fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, etc., but preferably micropores made of polytetrafluoroethylene. A transparent film is used. As a loose microporous film,
For example, rNTF J (Nitto Electric Industries@) and "Polyflon Vapor" (Daikin Industries@), which are made porous by stretching a polytetrafluoroethylene film, are available as commercial products.

The alkali metal-aromatic hydrocarbon complex solution used in the present invention is already known, and the alkali metal used is sodium or lithium, and the aromatic hydrocarbon used is naphthazine, phenanthrene, anthracene, etc. It is obtained by dispersing metallic sodium in a mixture of group hydrocarbons and a solvent such as tetrahydrofuran or dimethoxyethane and reacting the mixture. Some of the Gagaru complex solutions are already commercially available. Further, in the present invention, a liquid ammonia solution of metallic sodium can also be used in place of the alkali metal-aromatic hydrocarbon complex solution.

Treatment of polyfluorinated olefin microporous films with such alkali metal solutions is typically carried out at room temperature with the above i.
This is done by immersing it in a 1'8 liquid for several minutes to several hours. Thereafter, the film is washed with a suitable solvent and dried. In this case, especially when cleaning with a polar solvent such as water or alcohol, the surface of the polyfluorinated olefin molded article may be
Observation with EscA confirms that polar groups such as 00II, Cll01011, etc. are generated. Also,
Such polar groups can also be generated by leaving the film in the air after dipping. It is known that when a polyfluorinated olefin is treated with an alkali metal, the fluorine in the polyfluorinated olefin is eliminated as an alkali metal fluoride and modified into a polyacetylene type polymer. For example, since the polyfluorinated olefin is modified in this way and has active polar groups as described above at least on the surface, even when irradiated with a low dose of radiation, the polymerizable monomer is It will generate the necessary amount of crosslinking radicals to enable polymerization. For example, even if an untreated polyfluorinated olefin film is irradiated with an electron beam of about 3 megarads and then brought into contact with a polymerizable monomer, it will not substantially undergo graft polymerization; In the case of polyfluorinated olefin films treated with alkali metal-aromatic hydrocarbon complexes or liquid ammonia solutions of alkali metals, the polymerizable monomers are effectively graft-polymerized, and the radiation dose is low. , substantially no polymer chain scission occurs.

- In the present invention, the ionizing radiation includes, for example, α
rays, β-rays, γ-rays, neutron beams, X-rays, electron beams, etc., and preferably electron beams are used. In addition, the amount of electron beam irradiation to the polyfluorinated olefin film was 0.1
-50 Megarads, preferably 0.5-20 Megarads.

Further, as the hydrophilic polymerizable monomer, a vinyl polymerizable monomer having a carboxyl group or a hydroxyl group is preferably used, and acrylic acid and meccrylic acid are particularly preferably used. In order to bring such an r1-combinable monomer into contact with the polyfluorinated olefin film after irradiation with ionizing radiation, the monomer itself or an aqueous or organic solution of the monomer may be used, and the film may be added to the monomer itself or in an aqueous or organic solution. Alternatively, the polymerizable R-mer vapor may be brought into contact with the polymerizable R-mer.

In the present invention, in order to further improve the selective permeability of ions as a battery diaphragm, the above-mentioned microporous film is rolled under heat as necessary to reduce the diameter of the micropores it has. Can be adjusted. Although not necessarily limited, in the present invention, the microporous film preferably has micropores with a pore diameter of about 0.05 to 50 μm.

As described above, according to the method of the present invention, a microporous polyfluorinated olefin film, particularly a polytetrafluoroethylene film, is treated with an alkali metal in advance to introduce active polar groups that easily form crosslinking radicals on its surface. Therefore, even if an untreated film is irradiated with a low ljl amount of ionizing radiation that does not substantially cause graft polymerization of polymerizable monomers, the amount of radicals necessary for graft polymerization will be generated. In this way, the vinyl polymerizable monomer can be effectively graft-polymerized to make it hydrophilic without cutting the polymer chain, and the resulting microporous film has excellent chemical stability, especially It has excellent alkali resistance, and
Since it has low electrical resistance and excellent selective ion permeability, it can be used as an unprecedented diaphragm for alkaline batteries.

The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way.

Example 1 It has many micropores with a pore diameter of 40 μm and a thickness of 5 mm.

"Polyflon Paper", which is .mu., was immersed in a dimethoxyethane solution of metal sodium-naphthalene complex for 30 seconds at room temperature, thoroughly washed with acetone, then water, and dried.

Next, the film thus treated was rolled to a predetermined thickness at a temperature of 170°C, and then rolled in a nitrogen stream with an oxygen concentration of 5°Oppm using an electron curtain beam (Model McB-150, Sony Trading Co., Ltd.). ,1
After being irradiated with a 5 megarad electron beam under conditions of 65 kV and 3 mA, the tube was immediately placed in a glass tube filled with a 50% aqueous solution of acrylic acid, replaced with nitrogen, sealed, and allowed to stand at 60°C for 14 hours. Then, graft polymerization was carried out.

The acrylic acid homopolymer and remaining unreacted monomers were extracted from the thus obtained graft-polymerized film using a Soxhlet extractor and dried.

The grafting rate and electrical resistance of the treated film thus obtained are shown in the table. The grafting rate is [(film weight after graft polymerization - film weight before grafting)/
It is defined as the film weight before grafting] x 100 (%). Further, in measuring the electrical resistance, an alternating current of IKIIz was applied to the film, and the current value was measured with an alternating current ammeter, completely excluding the influence of the resistance of the electrolyte itself. In addition, a saturated potassium chloride aqueous solution was used as the electrolytic solution, and the film was sufficiently equilibrated in a rice cooker before measurements were taken.

Example 2 In place of “Polyflon paper” in Example 1,
rNTF5205J with many micropores with a pore diameter of 0.5 μm
A diaphragm was obtained in the same manner using a diaphragm, and the electrical resistance of the membrane was measured. The results are shown in the table.

In both examples, it is clear that the resulting films have significantly lower electrical resistance than untreated films.

Claims (1)

[Claims] (1) After a microporous film made of polyfluorinated olefin is treated with Alta metal, it is irradiated with ionizing radiation,
A method for producing a battery diaphragm, comprising: then bringing the film into contact with a hydrophilic polymerizable monomer to graft-polymerize the monomer onto the film.