JPH0628153B2 - Method for manufacturing battery separator - Google Patents

Description

The present invention relates to a method for producing a battery separator by a radiation graft polymerization method, wherein a vinyl monomer solution is allowed to act on a synthetic resin film by an ionizing radiation irradiation method so that a dissociative group is added to the synthetic resin. When graft-polymerizing the vinyl monomer contained therein, a surfactant is added to the vinyl monomer solution to accelerate the polymerization rate and obtain a predetermined graft ratio in a shorter reaction time.

In recent years, as batteries have higher performance and longer life, separators having durability and low electric resistance are required. Particularly, a so-called graft membrane obtained by polymerizing a hydrocarbon polyolefin-based or fluorine-containing polyolefin-based sheet having excellent chemical resistance with a vinyl monomer having an ion-exchange property such as acrylic acid or methacrylic acid has been put into practical use as a separator. As a method for obtaining these graft films, as described in U.S. Pat. No. 3,427,206, a polyethylene film is pre-swelled with a solvent containing a monomer and then irradiated with electric field radiation in the presence of the monomer. Irradiation method, or as described in JP-A-53-84134, a polyethylene film is pre-irradiated with ionizing radiation to form an active site for polymerization on the film, and then contacted with a monomer. There is a pre-irradiation method for graft polymerization. In any of the methods, the trunk polymer film is a hydrocarbon polyolefin type which has durability in the battery electrolyte and has poor affinity with the monomer. For this reason, in the simultaneous irradiation method, it is necessary to swell the synthetic resin film with the solvent of the monomer in advance, and irradiation of radiation is performed after impregnating the inside of the film with the monomer, and the time required for this swelling is 24 hours or more. Is. Further, the monomer solvent used for the simultaneous irradiation is limited to the kind which swells the film which is the trunk polymer and is compatible with the monomer. On the other hand, in the pre-irradiation method, a graft reaction occurs due to a reaction active point generated by previously irradiating the polyethylene film with ionizing radiation, and a graft reaction occurs, and the graft ratio increases with the passage of time,
When a certain grafting rate (the grafting rate at this time is called the final grafting rate) is reached after a certain reaction time, the grafting rate no longer increases. In the pre-irradiation method, after irradiation with radiation, the reaction does not start immediately even if it is immersed in a monomer solution, and the reaction starts after a necessary time for the monomer to wet the synthetic resin film. For this reason, it takes time to reach the final graft rate,
The productivity of the separator was bad. Furthermore, the start time of the reaction varies depending on the dirt on the surface of the synthetic resin film used, such as oil and fat, and the grafting rate partially varies,
In order to prevent this, it is necessary to use a synthetic resin film which has been subjected to a preliminary degreasing treatment and the like, resulting in poor productivity.

The present invention eliminates the above-mentioned conventional defects and provides a battery separator having a uniform graft ratio and a low production cost by adding a surfactant to a monomer solution.

In the present invention, the detailed mechanism of action of the addition of the surfactant on the synthetic resin film and the vinyl monomer is not clear, but it is considered that the addition of the surfactant increases the affinity of the vinyl monomer solution for the synthetic resin film. To be That is, in the conventional method, the synthetic resin film has a low affinity for the acrylic acid solution, and the graft polymerization of the film surface does not start until the monomer wets the film surface, and the reaction takes a long time. However, by adding a surfactant to the monomer solution as in the method of the present invention, the affinity is remarkably improved and the reaction is immediately started. Then, the reaction proceeds from the film surface to the inside, but the surfactant promotes the diffusion of the monomer into the inside, and the reaction rate can be further increased.

The second effect of the addition of the surfactant is that it is possible to graft two kinds of monomers having different solubilities in the same solvent in the same solvent. Conventionally, when two kinds of monomers are both graft-polymerized or a branch polymer obtained by a certain kind of monomer is crosslinked with a different kind of monomer, different solvents are used, so that the polymerization is separately performed by a multistage polymerization method. For example, when acrylic acid is graft-polymerized and crosslinked with divinylbenzene, the former polymerization is carried out using industrially inexpensive water as a solvent, and the latter is insoluble in water, so expensive organic solvents such as benzene and triene are used. However, according to the present invention, acrylic acid is dissolved by adding a surfactant to water as a solvent and divinylbenzene is emulsified to coexist two kinds of monomers. It is possible to carry out copolymerization in a single reaction by irradiation with radiation. Although divinylbenzene is insoluble in water, it is emulsified by adding a surfactant and uniformly contacts the film surface in a microscopic phase to cause a polymerization reaction.

As described above, two or more kinds of monomers having different solubilities in the same solvent can be copolymerized at once in the same solvent.

In carrying out the present invention, surfactants added to the monomer solution include polyoxyethylene alkyl ether, sorbitan fatty acid ester, polyoxyethylene alkylphenol ether, polyoxyethylene amyl ester, sucrose fatty acid ester, and alkylallyl polyether sulfonate. It is preferable to use one or more selected from nonionic surfactants such as sodium salt, higher alcohol sulfate ester salt and stearyl dimethyl benzyl ammonium salt, or cationic and anionic surfactants. it can.

The addition of the surfactant is 0.1 to the monomer solution.
It is 5 wt%, preferably 0.3 to 1 wt%. The monomer solvent does not necessarily have to be compatible with the monomer, and water and organic solvents such as benzene, toluene,
Xylene, carbon tetrachloride, ethylene dichloride or the like can be used alone or in a mixture, and water is suitable especially on an industrial scale.

As the vinyl monomer having a dissociative group, acrylic acid, methacrylic acid, maleic acid, itaconic acid having a carboxy group, styrenesulfonic acid and vinylsulfonic acid as a monomer having a sulfo group, and vinylpyridine etc. as a monomer having an amino group. Is used. A suitable monomer concentration in the reaction solution is 10 to 50 wt%.

In the simultaneous irradiation method, the dose rate of r-ray is 5 × 10 ^{4 to} 5
The irradiation time was 5 to 24 hours, which was set to × 10 ⁵ rad / hr. In the pre-irradiation method, the irradiation dose is 1 to 50 Mrad, preferably 5 to 30 Mrad. Reaction temperature is 60 ℃
Below, the range of 20 to 30 ° C was preferable.

As a synthetic resin film, a hydrocarbon polyolefin resin having durability to a battery electrolyte, for example, polyethylene, polypropylene, polybran, fluorine-containing polyolefin resin, for example, polyvinyl fluoride, polyvinylidene fluoride, ethylene-tetrafluoroethylene Polymers, ethylene tetrafluoride and copolymers thereof are used.

Example 1 Melt index 2.2, density 0.922, thickness 25
A low density polyethylene of μ is irradiated with an electron beam in a nitrogen atmosphere at an acceleration voltage of 600 KV and a current of 10 mA at a dose of 20 Mard. Immediately, acrylic acid with an oxygen concentration of 0.2 wtPPm or less 5.
0 parts by weight, 50 parts by weight of water, 0.25 parts by weight of Mohr's salt,
Nonionic surfactant Trade name: Triton × 10
It is dipped in a solution (temperature 25 ° C.) consisting of 0.1 part by weight. After dipping for 15 minutes and pulling up, the graft ratio was measured and found to be 55%. This film is treated with a 25% KOH aqueous solution at 95 ° C. for about 5 minutes to convert the carboxyl group (—COOH) of the branch polymer into a potassium salt (—COOK). This is to facilitate dissociation in an alkaline electrolyte and improve ionic conductivity. 40% of this at 25 ℃
As a result of measuring the electric resistance in a KOH aqueous solution by the 1000 Hz AC method, it was 50 mΩ · cm ² . In order to compare with this, when the graft ratio was measured for the film which was subjected to the completely same treatment by irradiating the solution having a composition containing no surfactant in the monomer reaction solution with an electron beam under the same conditions as in this example, It was 5%. The figure is a correlation curve diagram of the graft ratio and the reaction time, where A is for this example and B is for the comparative example. As can be seen from this figure, A has a grafting speed 2 to 3 times that of B and the time to reach the final grafting rate is shortened to 1/2 to 1/3.

Example 2 Melt index 0.15, density 0.955, thickness 3
0μ high density polyethylene with 15 parts by weight of methacrylic acid,
80 parts by weight of benzene, 5 parts by weight of ethylene tetrachloride, anionic surfactant, trade name Triton × 400
Immerse in a solution consisting of 0.5 parts by weight, deoxidize with nitrogen gas for 5 minutes, and immediately irradiate with γ-rays containing Co ⁶⁰ as a radionuclide at a dose rate of 5 × 10 ⁴ rad / hr for 15 hours. After removing this, the homopolymer produced by homopolymerization of methacrylic acid was removed, and the graft ratio was measured to be 4
It was 8%. For comparison, the graft ratio of the graft membrane obtained in the same manner as in Example 2 without using the anionic surfactant was 23%. The respective electric resistances were measured by performing the same treatments as in Example 1 and found to be 60
mΩ · cm ^2, was 730mΩ · cm ^2. Further, in such a simultaneous irradiation method, a method using a solution containing a surfactant is more effective than a conventional method not using a surfactant.
It has become possible to significantly reduce the time for swelling the pre-formed film.

Example 3 Melt index 2.2, density 0.923, thickness 50
A low density polyethylene film of μ was irradiated with an electron beam of 20 Mrad in the same manner as in Example 1, and 45 parts by weight of acrylic acid deoxygenated with nitrogen gas in advance, 10 parts by weight of vinylbenzene, 45 parts by weight of water, Mr. Mohr. A reaction liquid consisting of 0.25 part by weight of salt and 3 parts by weight of nonionic surfactant was rotated at 350 rpm.
A solution in which divinylbenzene was emulsified by stirring with a 0 rpm trade name TK homomixer (made by Tokushu Kika Kogyo) (temperature 2
5 ° C), react for 30 minutes, then pull up
It was washed with water and dried. The graft ratio of this membrane was 57%, and the addition ratio of divinylbenzene was 11% of 57% from the measurement of the graft ratio and the ion exchange capacity.

The graft film according to each of the above examples also has low electric resistance,
It was suitable as a battery separator.

As described above, according to the present invention, since the surfactant is added to the vinyl monomer solution at the time of carrying out the graft polymerization, the affinity between the synthetic resin film and the vinyl monomer is improved, whereby the initiation of the graft reaction is started. It can accelerate and further accelerate the progress. Therefore, the productivity can be improved, the graft reaction can be carried out uniformly, and the graft ratio can be improved. Further, another monomer that is insoluble in the vinyl monomer solution can be emulsified and used together with the vinyl monomer. Therefore, what was conventionally carried out by multistage polymerization using different solvents when polymerizing two or more kinds of monomers can be carried out at one time using one solvent, and productivity can be improved and equipment can be improved. Can be simplified.

[Brief description of drawings]

FIG. 1 is a graph of a graft ratio-reaction time correlation curve of one example of the present invention and a comparative example. A: According to one embodiment of the present invention B: Comparative example

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuo Murata 6-6 Josaimachi, Takatsuki City, Osaka Prefecture Yuasa Battery Stock Company (72) Keiji Senoo 6-6 Josaimachi, Takatsuki City, Osaka Yuasa Battery In-house company (56) Reference JP 54-39833 (JP, A) JP 53-138490 (JP, A) "Chemical Dictionary 6" (1961-7-15) Kyoritsu Publishing P.P. 807

Claims (2)

[Claims] 1. A vinyl monomer used when a vinyl monomer solution is caused to act on a synthetic resin film to graft-polymerize a vinyl monomer having a dissociative group by an ionizing radiation irradiation method of simultaneous irradiation or pre-irradiation. A method for producing a battery separator, which comprises adding a surfactant to a solution. 2. The method for producing a battery separator according to claim 1, wherein at least one other monomer that is insoluble in the vinyl monomer solution is emulsified in the vinyl monomer solution.