JPS638583B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a battery separator, and particularly to a battery separator using an alkaline aqueous solution as an electrolyte.

Conventional separators used in alkaline batteries, especially sealed silver oxide batteries in which the anode is made of silver oxide or silver peroxide and the cathode is made of bad lead or cadmium, are designed to prevent the transfer of silver oxide ions dissolved from the anode to the cathode. In order to prevent this, semipermeable membrane-type membranes with extremely fine pores, such as regenerated cellulose membranes, crosslinked polyvinyl alcohol membranes, and polyethylene-acrylic acid graft polymer membranes, are used. These membranes have a common property of being swollen by an alkaline electrolyte, thereby providing ionic conductivity. Due to these properties, materials with fine pores or materials that swell with electrolyte were able to meet the requirements of these batteries.
Among these materials, polyethylene film in which acrylic acid is graft-polymerized has excellent alkali resistance and high oxidation resistance against anode active materials, silver acid ions, etc., and can be made into a thin film. It also has various features such as excellent ionic conductivity.

However, there is a problem when actually assembling it into a battery. After punching out a circular separator made of polyethylene-acrylic acid graft copolymer and placing it on the electrode plate, an alkaline electrolyte is dripped onto the separator. When absorbed, the degree of swelling of the separator is directional, so it deforms into an elliptical shape, and
The separator would warp before it was completely absorbed, and in severe cases, the liquid-retaining paper placed above the separator would fall out of the battery container. Deformation of the separator due to absorption of the electrolyte makes the outer can insufficiently fastened and promotes leakage of the electrolyte. Causes of these deformations and warping include non-uniformity of the graft ratio and directionality based on the film forming method of the base polyethylene film. In particular, polyethylene films are generally produced by the inflation method and T-die extrusion method, and in either case, the film has directionality, so no matter how uniformly the graft polymerization is carried out, the electrolyte solution It has been difficult to improve the anisotropy of dimensional changes during swelling.

The present invention eliminates the above-mentioned conventional drawbacks, and the synthetic resin film does not swell in the alkaline electrolyte in the plane direction, or even if it does, it has at least the same degree of swelling in the vertical and horizontal directions, so that it does not swell in the alkaline electrolyte. The present invention provides a separator that does not deform or warp when it comes into contact with the separator. In this method, the base film is biaxially stretched to eliminate the directionality caused by the manufacturing method, and a monomer having a dissociable group is graft-polymerized to this film. This eliminates the directionality of the film and provides a separator that, when swollen in an alkaline electrolyte, has the same degree of swelling in at least two axial directions.

When stretching a film, increasing the degree of stretching increases the thermal shrinkage rate. Therefore, if a film with a high degree of stretching is used, the separator obtained by graft polymerization will shrink during high-temperature storage of the battery, causing damage to the negative and anode plates. is smaller than the area required to isolate the
It no longer functions as a separator. Therefore, the degree of stretching is 3 or less, preferably 2.5 or less.

The graft polymerization method in the present invention is a so-called pre-irradiation method in which a base film is irradiated with ionizing radiation and then immersed in or brought into contact with a monomer solution.
This method is preferable because the uniformity of graft polymerization and productivity are good. However, a simultaneous irradiation method in which the base film is immersed in or in contact with a monomer solution and irradiated with ionizing radiation to carry out graft polymerization can also be used.

In the pre-irradiation method, the irradiation dose of ionizing radiation is
50 Mrad or less, preferably 1 to 30 Mrad,
In the simultaneous irradiation method, 10 Mrad or less is preferable.

Examples of monomers having electrolytic groups include acrylic acid, methacrylic acid, and styrene sulfonic acid. Furthermore, more uniform graft polymerization can be achieved by containing an organic solvent that swells the base film, such as xylene, carbon tetrachloride, ethylene dichloride, benzene, toluene, etc., in the reaction solution. The organic solvent is not necessarily essential, and an aqueous solution of the monomer can also be used. Furthermore, in the grafting reaction, the grafting rate can be determined by adjusting the temperature and time of immersion or contact with the monomer solution.

EXAMPLES Next, the present invention will be explained in more detail with reference to Examples, but these are not intended to limit the scope of the present invention in any way.

Example 1 A film made of high-density polyethylene (Sanrak B-180 manufactured by Asahi Kasei Corporation) having a melt index of 2.2 and a density of 0.955 and having a thickness of 80 μm was formed by the inflation method and was stretched to 2.1 and 2.0 times in two axes at 70°C. A film with a thickness of 19μ was obtained. This film was irradiated with 30 Mrad of electron beam, and 15 parts of methacrylic acid and 85 parts of water, which had dissolved oxygen below 0.1 ppm, were used.
The sample was immersed in a solution containing 0.16% Mohr's salt at 25°C for 5 hours. Then, a washing treatment with hot water was performed to obtain a separator with a grafting rate of 68% and an electrical resistance of 120 mΩ·cm ² . When this separator was immersed in a 40% KOH aqueous solution, its elongation due to swelling was 6% in the longitudinal direction, 6% in the transverse direction, and 15% in the thickness direction. Additionally, the battery installation work could be carried out without any problems.

On the other hand, a film with a thickness of 20μ was formed using the inflation method.
For a separator with a grafting rate of 75% obtained by grafting onto a high-density polyethylene film without the above-mentioned stretching process but under the same conditions as above, the elongation when immersed in a 40% KOH aqueous solution was as follows: It was 13% in the direction, 17% in the lateral direction, and 16% in the thickness direction.

Claims (1)

[Scope of Claims] 1. A film made of a graft copolymer obtained by graft copolymerizing a monomer having a dissociable group onto a synthetic resin film, wherein the synthetic resin film is biaxially stretched at a stretching degree of 3 or less. A battery separator characterized in that it has a property of not swelling in the planar direction when it comes into contact with an alkaline electrolyte, or a property of swelling with the same degree of swelling in the planar direction even if it swells. 2. Claim 1, wherein the synthetic resin film is made of tetrafluoroethylene-ethylene copolymer, polytetrafluoroethylene, polypropylene, polyamide, polyimide, or polyethylene.
The battery separator described in section. 3. The battery separator according to claim 1, wherein the monomer having a dissociable group is comprised of acrylic acid, methacrylic acid, or styrene sulfonic acid.