JPS598028B2 - Battery separator and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a battery separator, particularly a battery separator using an alkaline aqueous solution as an electrolyte, and a method for producing the same. It is characterized by graft copolymerization of monomers having hydrophilic groups such that they have different hydrophilic groups.

5. In batteries that use an alkaline aqueous solution as the electrolyte, especially in so-called silver oxide batteries that use silver oxide or silver peroxide as the anode and zinc or cadmium as the cathode, silver oxide, which is the anode active material, is present in the alkaline aqueous solution, albeit in a small amount. dissolve.

Its solubility is about 40% in an aqueous solution with 0 hydroxide power.
×IOJN's stannate ions finally diffuse toward the cathode due to the concentration gradient in the electrolyte. reaches the cathode. At the cathode, stannate ions are reduced and self-discharge occurs, and the deposited silver forms a bridge with the anode, causing a short circuit. In order to prevent this, a semipermeable membrane having extremely fine pores was used to prevent the movement of stannate ions to the cathode. Regenerated cellulose membranes have conventionally been used as semipermeable membranes, but the regenerated cellulose membranes have deteriorated significantly (due to oxidation of the anode and silvery acid ions) and have been unable to function satisfactorily as separators. In order to improve this problem, a graft copolymer film has recently been proposed in which a monomer having a hydrophilic group is graft-copolymerized onto a synthetic resin film having chemical resistance, such as a polyethylene film. Although this improves the oxidative deterioration of the separator, this graft copolymer film has a high electrical resistance and is unsuitable for use as a battery separator.Therefore, it is necessary to increase the grafting ratio in order to lower the electrical resistance. It was necessary. Increasing the grafting ratio impairs the density of the membrane and increases the porosity. As a result, the function of inhibiting the movement of silver acid ions deteriorates, and self-discharge increases. Although it is effective to make the film denser by lowering the grafting ratio, it has the disadvantage of increasing electrical resistance. The present invention eliminates the above-mentioned conventional drawbacks, and is characterized in that the graft copolymer membrane has a surface layer with a low grafting rate (compared to other parts).

That is, by providing a thin layer with a low graft ratio in a part, an increase in electrical resistance due to a decrease in the graft ratio is prevented, and movement of silver acid ions by this dense layer is suppressed. One embodiment of the production method of the present invention involves irradiating two synthetic resin films that will become the backbone polymer with an electron beam, deactivating one side of the film, and then using a monomer having a hydrophilic group such as acrylic acid, methacrylic acid, styrene, etc. Grafting is carried out by contacting the synthetic resin by immersion in a solution consisting of sulfonic acid and an organic solvent that swells the synthetic resin, such as dioxane, ethylene dichloride, xylene, carbon tetrachloride, or benzene.

This method separates the irradiation process and the grafting process, so of the radicals generated by irradiating the film with an electron beam, the radicals 5 on the surface layer on one side are deactivated in advance before grafting. This makes it possible to suppress the polymerization reaction on one side during contact with the monomer solution. That is, a dense and uniform surface layer can be formed and has sufficient performance as a separator.
This will be explained in detail in the following figures and examples. FIG. 1 shows the distribution of the graft ratio in the cross section of the battery separator according to the present invention.

FIG. 2 shows the distribution of the graft ratio in the cross section of a membrane obtained by general graft copolymerization not according to the present invention. The horizontal axis shows the membrane thickness, and the vertical axis shows the grafting rate. Membranes produced by general graft copolymerization have insufficient diffusion of monomer into the membrane and form a fairly thick and dense layer (low grafting rate) in the center of the membrane, resulting in extremely high electrical resistance). Since the separator according to the present invention shown in FIG. 1 has deactivated radicals on one side, grafting can be carried out sufficiently on the other side, and it has a dense thin surface layer on one side. On the other side, a layer with a high graft ratio, that is, a high porosity, is formed relatively uniformly deep inside.

On the other hand, in the film shown in FIG. 2, layers with high porosity exist on both sides of the film.

In the deep part, the monomer does not diffuse sufficiently into the interior, so the grafting rate decreases. Example 1 Melt index 20 density 0.922 high pressure polyethylene (F-2135 manufactured by Asahi Tau Co., Ltd.) was molded into a 25 μm thick film by the inflation method.

After this film is irradiated with an electron beam of 7 Mrad at an accelerating voltage of 1.7 MeV and a current of 50 μA, a polypropylene film is laminated on one side of the irradiated film using polyethyleneimine, which is insoluble in the following monomer solvent and soluble in water. This film is immersed in a monomer solution consisting of 50 parts of acrylic acid, 10 parts of xylene, 30 parts of ethylene dichloride, and 0.25 parts of Mohr's salt at a temperature of 20 DEG C. for one hour in a CN2 atmosphere.
Thereafter, it is immersed in water, the polypropylene film is peeled off, and then dried. The obtained separator thickness was 29μ,
The average grafting rate was 72%. As a result of observing the distribution state of the grafting rate in the cross section using an interference microscope, the maximum value of the grafting rate was 75%, the minimum value was 20%, and the thickness of the layer between 20% and 71% was 2μ, and 71 to 75%. % thickness was 27μ. The electrical resistance is 1 in a 40% potassium hydroxide aqueous solution.
It was 35 mΩ・i. The same average and
The electrical resistance of the graft copolymer membrane having a grafting rate and the entire membrane having this grafting rate was 115 mΩ,d. The electrical resistance of the graft copolymer film in which the minimum grafting rate of 20% in this example is distributed throughout the film is 1.2Ω.・
It is as high as d. Incidentally, the grafting ratio of the separator according to the present invention was calculated by the following formula. Example 2 High-pressure polyethylene with a melt index of 2.0 and a density of 0.923 (Sumikasen F-2041 manufactured by Sumitomo Chemical Co., Ltd.)
A film with a thickness of 20 μm was prepared using the T-guid extrusion method.

This film was subjected to electron beam irradiation in the same manner as in Example 1. One side of this irradiated film was heated to a temperature of 9 ClO for seconds, and then immersed in a monomer solution in the same manner as in Example 1. The obtained separator had an average grafting rate of 75%, with a maximum grafting rate of 78 (fl) and a minimum grafting rate of 18 fl. The thickness of the dense layer with a low grafting rate was 2 μm. In this example, by heating from one side, radicals in the heated portion are deactivated, and the grafting ratio of the surface layer can be reduced. Example 3 An accelerating voltage of 1 was applied to a 25μ thick film made of tetrafluoroethylene-ethylene copolymer (Aflon, manufactured by Asahi Glass Co., Ltd.).
．． Electron beam was applied at 5MeV current 15μA in N2 atmosphere for 1
0 Mra? I shot it.

A polypropylene film was pasted on one side of this film in the same manner as in Example 1. Methacrylic acid 40% Ethylene dichloride 20 parts Acetone 5 parts Xylene 26 parts Mohr's salt 0.2
Immerse for 1 hour at a temperature of 40° C. in a monomer solution consisting of 5 parts. After the reaction is completed, the polypropylene film is peeled off in the same manner as in Example 1. The obtained separator had an average grafting rate of 55%, a maximum grafting rate of 65%, a minimum grafting rate of 15%,
The thickness of the dense layer with a low grafting rate on one side was 1.5 μm. The electrical resistance was 220 mΩ·d. Comparative Example The separator of the present invention shown in Example 1 and Example 2,
In order to make a battery separator, 100 JISGS-12 type silver oxide batteries (Ag2O/Zn) were each made using comparative separators that were uniformly grafted over the entire membrane with the same grafting rate as in Example 1. Assembled one by one.

The results of the storage characteristics test for this battery are shown in Table 1. Table-1
As can be seen, the examples of the present invention are significantly superior in the storage performance of the separator.

When the test battery was disassembled and the separator was observed under a microscope,
In the example separator, a thin dense surface layer with a low graft ratio prevents the diffusion and movement of silver acid ions, and since the dense layer is extremely thin, there is little increase in electrical resistance. It was found that in the comparison separator that had been stored for three months, silver acid ions reached the cathode, causing self-discharge.
In the present invention, the electrical resistance is proportional to the thickness of the dense layer with a low grafting rate, so it needs to be thin, and the thickness is preferably 10 μm or less, preferably 5 μm or less.

The average grafting rate of the membrane is 50 to 100.
%, and in this case, the grafting ratio of the dense layer is preferably 30% or less, preferably 10 to 25%. The layer with a high grafting rate preferably has a grafting rate in the range of 55 to 110%. When the grafting ratio exceeds 110 f1), the membrane begins to take on a gel-like appearance and is difficult to handle. To the manufacturing method of the present invention? The method of pre-irradiating the film with an electron beam and then polymerizing it prevents the monomers in the bath during the polymerization reaction from forming a homopolymer, and the properties of the solution in the bath remain unchanged. Uniform grafting can be obtained by uniformly diffusing the particles.

In addition, this manufacturing method is separated into an irradiation step and a grafting step, and in between, there are pretreatments to form a dense layer, such as a method of heating the radicals generated by irradiation, and a method of covering the radical surface with another film ( That is, in the former case, the radical activity is controlled, and in the latter case, the radicals on one side are deactivated by controlling the polymerization reaction, and the grafting ratio is made low. The solvent for the monomer is preferably an organic solvent that swells the synthetic resin film and is effective for uniformly dispersing the monomer into the film, and may also be a mixture of an organic solvent and water.

As the monomer having a hydrophilic group, methacrylic acid, acrylic acid, styrene sulfonic acid, etc. are effective, and the concentration during polymerization is preferably in the range of 10 to 50%. The grafting rate can be adjusted by appropriately selecting the electron beam irradiation amount, monomer concentration, polymerization time, and reaction temperature. The electron beam irradiation amount is 5 to 10
Mrad is suitable to obtain a grafting rate of 50% or more. In order to increase the strength of the separator, the electron beam irradiation amount can be increased to 20 Mrad or more to crosslink the inside of the film.

The main polymers include polyethylene film, polypropylene film, polytetrafluoroethylene, and tetrafluoroethylene.
Ethylene copolymers and tetrafluoroethylene-hexafluoropropylene copolymers can be used. As described above, the battery separator according to the present invention has excellent electrical properties and an excellent ability to inhibit the movement of harmful substances, and is of high industrial value.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the distribution of graft ratios in the cross section of a battery separator according to the present invention, and FIG. 2 is a diagram showing the distribution of graft ratios in the cross section of a separator in which both sides are grafted at the same ratio not according to the present invention. 1. Dense layer, 2. Layer with high graft ratio, 3.
- Graft distribution curve.

Claims (1)

[Scope of Claims] 1. A battery separator characterized in that a monomer having a hydrophilic group is graft-polymerized onto a synthetic resin film so that the grafting ratio varies in the thickness direction. 2. The battery separator according to claim 1, in which one surface layer of the separator has a lower grafting rate than the other surface layers. 3. The battery separator according to claim 1, wherein the surface layer on one or both sides of the separator has a lower grafting rate than the inside. 4. The battery separator according to claim 1, wherein the synthetic resin is a polyolefin resin. 5. The battery separator according to claim 1, wherein the synthetic resin is a fluorine resin. 6. The battery separator according to claim 1, wherein the monomer having a hydrophilic group is acrylic acid, methacrylic acid, or styrene sulfonic acid. 7 After irradiating the synthetic resin film with an electron beam and deactivating one side of the film, the synthetic resin is swollen and immersed in a monomer solution containing the monomer in an organic solvent that is compatible with the monomer, and then dried. A method for manufacturing a battery separator characterized by: 8. The method for producing a battery separator according to claim 7, which comprises irradiating the synthetic resin film with an electron beam and then attaching another film to one side of the film to deactivate it. 9. The method for producing a battery separator according to claim 7, which comprises irradiating the synthetic resin film with an electron beam and then heating one side of the film to deactivate it.