JPH0430142B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a diaphragm for batteries, and more specifically, it is made of a microporous polyfluorinated olefin film, has excellent chemical resistance, low ion permeation resistance and low electrical resistance, and has excellent mechanical strength. The present invention relates to a method for manufacturing a diaphragm for alkaline batteries. 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, causing 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. It is required to have the following. Cellophane membranes, which have conventionally been widely used as diaphragms for batteries, generally satisfy the above-mentioned properties, but tend to lack the above-mentioned chemical stability. That is, it is susceptible to oxidative deterioration due to the oxidizability of the anode active material in an alkaline electrolyte or under alkaline conditions, 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 extremely low surface energy and satisfies the above-mentioned required properties, except that it is water repellent. Therefore, after activating the surface of this microporous polyfluorinated olefin film by irradiating it with ionizing radiation such as an electron beam, it is brought into contact with a hydrophilic polymerizable monomer and graft-polymerized to form a film. It is considered to make it hydrophilic. However, the C-F bond energy in polyfluorinated olefin is
441J/mol, C-C bond energy
Ionizing radiation, because it is greater than 348 J/mol;
In particular, when irradiated with an electron beam, crosslinking radicals are generated in some of the fluorine-substituted carbons, and even though the polymerizable monomer is effectively grafted onto these radicals, the majority of the electron beam destroys the polymer chain. It acts to cleave and modify the polymer into a low molecular weight product. 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 film could not be avoided from having a lower molecular weight, thereby impairing the desired physical properties of the polyfluorinated olefin film. On the other hand, when low-dose electron beams are irradiated to avoid the above-mentioned scission of polymer chains in polyfluorinated olefin films, the amount of crosslinking radicals generated is insufficient for graft polymerization of polymerizable monomers. or because the amount of electron beam transmission is small,
Crosslinking radicals are generated only on the very surface of the film and immediately react with oxygen in the atmosphere and lose activity, making it difficult to effectively graft polymerize the polymerizable monomer. Furthermore, polyfluorinated olefin molded products have extremely low surface energy and poor wettability. For this reason, for example, a method is conventionally known in which an alkali metal-naphthalene complex is formed in an organic solvent and a polyfluorinated olefin film is immersed in the complex to improve its surface wettability. In order to solve the above-mentioned problems in making the microporous polyfluorinated olefin film hydrophilic by grafting, the present inventors conducted intensive research on the surface treatment of the microporous polyfluorinated olefin film prior to graft polymerization. By pre-treating the film with alkali metals as described above, unexpectedly, the untreated film has a low dose of ionizing properties which makes it substantially difficult to graft polymerize the polymerizable monomers. By irradiating a molded article with radiation, an amount of crosslinking radicals necessary for graft polymerization of a polymerizable monomer is generated 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-aromatic hydrocarbon complex solution or a liquid ammonia solution of metallic sodium, and then irradiating it with ionizing radiation. The film is then brought into contact with a hydrophilic polymerizable monomer to graft-polymerize the monomer onto the film. In the present invention, polyfluorinated olefins include polyvinyl fluoride, polyvinylidene fluoride, vinyl fluoride-vinylidene fluoride copolymer, polychlorotrifluoroethylene, polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, It includes vinyl fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene copolymer, etc., but preferably a microporous film made of polytetrafluoroethylene is used. used. As such microporous films, for example, "NTF" (Nitto Electric Industries, Ltd.) and "Polyflon Paper" (Daikin Industries, Ltd.), which are made porous by stretching polytetrafluoroethylene film, are commercially available. It can be obtained as a product. 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 naphthalene, 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 such 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. Such treatment of a polyfluorinated olefin microporous film with an alkali metal solution is usually carried out by immersing it in the above solution at room temperature for several tens of 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, COOH, CHO, etc.
Observation using ESCA confirms that polar groups such as OH are generated. Further, such polar groups can also be generated by leaving the film in the air after immersion treatment. It is known that when 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. According to the above, since the polyfluorinated olefin is modified in this way and has the above-mentioned active polar groups at least on the surface, even when irradiated with a low dose of radiation, the polymerizable monomer can be absorbed into the polyfluorinated olefin. The amount of crosslinking radicals necessary for graft polymerization will be generated. 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, but according to the method of the present invention, In the case of a polyfluorinated olefin film that has been previously treated with an alkali metal-aromatic hydrocarbon complex or an alkali metal liquid ammonia solution, the polymerizable monomer can be effectively graft-polymerized, and the irradiated radiation dose is low. As a result, substantially no polymer chain scission occurs. In the present invention, as the ionizing radiation, for example, α rays, β rays, γ rays, neutron beams, X rays, electron beams, etc. are used, and preferably electron beams are used. The amount of electron beam irradiation to the polyfluorinated olefin film is in the range of 0.1 to 50 megarads, preferably 0.5 to 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 methacrylic acid are particularly preferably used. In order to bring such a polymerizable monomer into contact with the polyfluorinated olefin film after irradiation with ionizing radiation, the film is placed in the monomer itself or in an aqueous or organic solution of the monomer. It may be immersed or brought into contact with vapor of a polymerizable monomer. In the present invention, the above-mentioned microporous film is rolled under heat as necessary to adjust the diameter of the micropores so that the selective permeability of ions as a battery diaphragm is further improved. can do. Although not necessarily limited, in the present invention, the microporous film has a pore diameter of 0.05 to
It is preferable to have micropores of about 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, in the case of an untreated film, even when irradiated with a low dose of ionizing radiation that does not substantially cause graft polymerization of polymerizable monomers, the amount of radical properties necessary for graft polymerization can be generated. resulting in effective graft polymerization of vinyl polymerizable monomers without polymer chain scission,
In addition to being hydrophilic, the microporous film thus obtained has excellent chemical stability, especially alkali resistance, low electrical resistance, and excellent selective ion permeability. It can be used as an excellent 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 Polyflon paper, which has many micropores with a pore diameter of 40 μm and a thickness of 500 μm, was placed in a dimethoxyethane solution of a metal sodium-naphthalene complex for 30 minutes at room temperature.
After being immersed for a second, it was thoroughly washed with acetone and then water, and then dried. The film treated in this way is then heated to
It is rolled to a predetermined thickness at 170°C, and then heated to an oxygen concentration of 500ppm using an electron curtain beam (CB-150 model manufactured by Sony Trading Co., Ltd.).
After being irradiated with a 5 megarad electron beam under the conditions of 165 kV and 3 mA in a nitrogen stream of The mixture was left standing for a while to allow graft polymerization. The acrylic acid homopolymer and remaining unreacted monomers were extracted from the graft-polymerized film thus obtained using a Soxhlet extractor and dried. Table 1 shows the grafting rate and electrical resistance of the treated film thus obtained. The grafting rate is defined as [(film weight after graft polymerization - film weight before grafting)/film weight before grafting] x 100 (%). In addition, in measuring the electrical resistance, a 1 kHz alternating current was applied to the film, and the current value was measured with an alternating current ammeter, completely eliminating 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 therein before measurement. Example 2 In Example 1, “Polyflon paper” was replaced with

[Table] A diaphragm was obtained in the same manner using "NTF5205" which has many micropores with a pore diameter of 0.5 μm, and the electrical resistance was measured for this membrane. The results are shown in Table 1. Comparative Example 1 The same "Polyflon paper" as used in Example 1 was rolled to a predetermined thickness under the same conditions as Example 1 without being immersed in a dimethoxyethane solution of metal sodium-nafrethane complex, and Next, after irradiation with an electron beam, graft polymerization of acrylic acid was performed. Table 2 shows the grafting rate and electrical resistance of the film thus obtained after graft polymerization.

[Table] It is clear that in each of the examples, the obtained films had significantly lower electrical resistance than the untreated films. Also, the film according to the comparative example has a lower electrical resistance than the untreated film in Table 1 above, but a higher electrical resistance than the film according to the invention.

Claims (1)

[Claims] 1. A microporous film made of polyfluorinated olefin is treated with an alkali metal-aromatic hydrocarbon complex solution or a liquid ammonia solution of metallic sodium, then irradiated with ionizing radiation, and then treated with a hydrophilic polymerizable monomer. A method for producing a diaphragm for a battery, characterized in that the monomer is graft-polymerized on the film by contacting the film with the monomer.