JPH08106893A - Separator film for alkaline battery - Google Patents

Abstract

PURPOSE: To provide a separator film for alkaline battery formed of a denaturated polyvinyl alcohol film containing 0.5-12 mole % of α-olefin units having four or less carbon atoms. CONSTITUTION: This separator film for alkaline battery has a durability resistant also to a long-time use as the degree of swelling to alkali aqueous solution is suppressed. A conventional separator paper by mixing of synthetic fiber and cellulose fiber has a problem in that the pore diameter between the fibers is so large that the active materials of both poles in the battery inner part can not be perfectly separated to each other, causing an internal short-circuit. However, the film according to this invention has a strong film physical property even when it is swollen to some degree by alkali aqueous solution to provide an excellent performance free from internal short-circuit. The volume of the battery inner part is minimized because of the film form, and the battery capacity can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator film used in various alkaline batteries such as alkaline manganese batteries, silver batteries, mercury batteries and zinc batteries. More specifically, the present invention relates to a separator film for an alkaline battery which has a small internal resistance and prevents an internal short circuit of the battery due to contact between the anode active material and the cathode active material.

[0002]

2. Description of the Related Art Usually, an alkaline battery uses a separator (paper or film) for separating active substances on both electrodes. The characteristics required for this separator have the effect of preventing short circuit due to contact between the active substances of the both electrodes, and have excellent durability that does not cause contraction or deterioration with respect to the alkali hydroxide solution or the depolarizer, It is desired that the electrolyte solution be sufficient to cause an electromotive reaction and that the volume occupied when incorporated in the battery is small.
Conventional separator paper is a separator paper in which copolymer fibers of vinyl chloride and vinyl acetate and rayon fibers are mixed and heat-bonded to each other by heat treatment, and polyvinyl alcohol fibers after mixing polyvinyl alcohol fibers and cellulose fibers. Partly melted with moisture and heat
Blended paper made of synthetic fibers such as fused separator paper and cellulose fibers has been used. Further, as a separator paper having a dense structure, one using ultrafine fibers having a fineness of 1 denier or less has been proposed (Japanese Patent Laid-Open No. 62-154559). As a separator film, polyvinyl alcohol (hereinafter referred to as PV
However, there is a problem that the durability against the electrolytic solution and the depolarizer is insufficient.

[0003]

The separator paper obtained by mixing and mixing conventional synthetic fibers and cellulose fibers has no practical problem with respect to durability and electrolyte retention, but since the separator paper has a large pore size. However, there is a problem in that it is impossible to completely prevent an internal short circuit due to contact between the bipolar active substances. Therefore, separator papers using fibers with small fineness as described above and separator papers having a two-layer structure have been studied, but the density of the obtained sheet becomes high, and the degree to which the fibers are heat-sealed or adhered to each other. Is larger, the smaller the denier, the larger the adhesive area,
There was a drawback that the electric resistance increased. Also 2
In the separator paper having a layered structure, there is a problem that the volume of the separator paper occupying the inside of the battery increases, the amount of the active substance inevitably decreases, and the battery capacity decreases.
Further, there is a problem in that the use of a fiber having a small fineness causes the cost to increase because the fiber becomes expensive. An object of the present invention is to solve the above problems and to provide a separator film for an alkaline battery, which has excellent durability, eliminates an internal short circuit, and keeps electric resistance low to ensure battery capacity.

[0004]

Means for Solving the Problems As a result of extensive studies to solve the above problems, the present invention has revealed that an alkaline battery separator comprising a modified PVA containing 0.5 to 12 mol% of an α-olefin unit having 4 or less carbon atoms. They found a film for use and completed the present invention.

The modified PVA containing 0.5 to 12 mol% of α-olefin units having 4 or less carbon atoms used in the present invention can be obtained by saponifying a copolymer of vinyl ester and α-olefin. it can. Examples of the vinyl ester include vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, and the like, and vinyl acetate, which is industrially produced in large quantities, is economically preferable. The α-olefin of the present invention has a carbon number of 4 or less, and examples thereof include ethylene, propylene, n-butene, and isobutene. However, ethylene does not change in the durability of the film and the electrolyte retention property over time. preferable. The content of α-olefin is 0.5 to 12 mol%, preferably 1 to 10 mol%, and more preferably 2 to 8 mol%. When the α-olefin content is less than 0.5 mol%,
If the above-mentioned remarkable effect is not observed, and if it exceeds 12 mol%, an aqueous solution of the modified PVA of the present invention cannot be obtained, so that a good film cannot be obtained and the electrolyte retention is small and the electric resistance is large. Therefore, it is not preferable. Modified PVA of the present invention
Can be further copolymerized with a monomer having another functional group within a range that does not impair the effects of the present invention. For example, as these monomers, carboxyl group-containing monomers such as maleic acid and itaconic acid, amide group-containing monomers such as acrylamide and N-vinylpyrrolidone are vinylamines,
Examples thereof include amino group-containing monomers such as allylamine and sulfonic acid group-containing monomers such as acrylamido-N-2-methylpropyl sulfonate and allylsulfonate.

The degree of saponification of the modified PVA of the present invention is important because it affects durability in the battery and retention of the electrolytic solution, and is preferably 70 to 100 mol%, more preferably 95 to 100 mol%. , 99 to 100 mol% is particularly preferable. When the saponification degree is lower than 70 mol%, the saponification reaction proceeds in the alkaline battery, the film shrinks or wrinkles, and the durability is also problematic. The degree of polymerization of the modified PVA is appropriately selected within the range of 100 to 8000, and preferably 500 to 4000.

As a method for producing the alkaline battery separator film of the present invention, a method of casting the aqueous solution or dispersion of the modified PVA of the present invention onto a hot roll is generally used. As a special method, the modified PVA obtained by adding water, a plasticizer or the like and heat-melting it may be formed into a film by inflation or melt extrusion. The film obtained by film formation is heat-treated at 100 to 180 ° C. to adjust the crystallinity in the film to prepare a film having excellent electrolytic solution retention and durability. The film of the present invention is preferably used alone, but in some cases, it can be used by laminating it with a separator paper obtained by mixing the synthetic fiber and the cellulose fiber by sticking or fusing. The modified PVA of the present invention is prepared alone as a film for a separator, but in some cases, a plasticizer such as glycerin or polyethylene glycol, a filler such as clay, titanium oxide, calcium carbonate or carbon black, glyoxal, and a polyvalent isocyanate compound. A film can also be prepared by blending a cross-linking agent such as a urea resin or a melamine resin prepolymer with the modified PVA.

[0008]

[Examples] Hereinafter, specific examples for producing the film for an alkaline battery separator according to the present invention will be described in detail. In the following, “%” means “% by weight” unless otherwise specified.

Example 1 100 parts of vinyl acetate and 30 parts of methanol were charged into a pressure reaction vessel equipped with a stirrer, nitrogen substitution was carried out, and then 2.7 parts of ethylene was injected under pressure. A methanol solution of 2,2'-azobisisobutyronitrile was prepared as an initiator and nitrogen substitution was performed. The temperature of the reaction vessel was raised, and when the internal temperature reached 60 ° C., the initiator solution was injected to initiate polymerization. After the polymerization for 4 hours, the polymerization was stopped by cooling when the polymerization rate reached 58%. Unreacted ethylene and vinyl acetate were removed to obtain a methanol solution of ethylene-vinyl acetate copolymer. A NaOH methanol solution was added to this to carry out a saponification reaction. The obtained ethylene-containing modified PVA has a polymerization degree of 120.
0, saponification degree 99.3 mol%, ethylene content 5.7
It was mol%. The modified PVA was Soxhlet extracted to remove residual sodium acetate. This transparent PVA solution was cast onto a hot roll at 70 ° C. to obtain a transparent film. The obtained film was heat-treated in a dryer at 150 ° C. for 10 minutes to prepare a separator film of the present invention. Table 1 shows the physical properties of the modified PVA. The physical properties of the film were measured by the following methods, and the results are shown in Table 2.

(1) Film thickness The thickness was measured at 5 points with a film thickness meter and taken as the average value. (2) Tensile strength The tensile strength of the film is as follows: after adjusting the humidity of the film at 65% RH, the width between chucks is 50 mm and the pulling speed is 500 mm / m.
In, the tensile elongation was measured by an autograph, and the maximum strength was measured. (3) Swelling degree The swelling degree was obtained by immersing the film in a 40% KOH aqueous solution for 30 minutes, measuring the thickness of the film before and after soaking with a film thickness meter, and determining the swelling degree by the following formula. Swelling degree (%) = (thickness after immersion−thickness before immersion) / thickness before immersion × 100 (4) Electric resistance Electric resistance was obtained by inserting a film between parallel platinum electrodes at an interval of 3 mm. The increase in the electric resistance between the electrodes due to this insertion was taken as the electric resistance of the film. 40% KO as electrode liquid
The electric resistance between the electrodes is 1000 Hz using H aqueous solution.
The frequency was measured using an ESR meter. (5) Battery Discharge Test Using the obtained separator film, an alkaline manganese battery (LR-
6) was prototyped, a discharge test was carried out at 75Ω for 100 hours, and the discharge voltage of the battery before and after the test was measured.

Examples 2 to 4 Three kinds of ethylene-containing modified PVA were prepared in the same manner as in Example 1 except that the charged composition of the monomers was changed. Then, a film for a separator was prepared in the same manner as in Example 1 for the film preparation and the heat treatment. The results are shown in Tables 1 and 2.

Example 5 An isobutylene-containing modified PV was prepared in the same manner as in Example 1 except that isobutylene was used in place of ethylene in Example 1.
A was prepared. Then, a film for a separator was prepared in the same manner as in Example 1 for the film preparation and the heat treatment. The results are shown in Tables 1 and 2.

Comparative Example 1 Commercially available PVA (trade name: PVA-11, manufactured by Kuraray Co., Ltd.)
7H) was used and a film was prepared and heat treated in the same manner as in Example 1. The results are shown in Table 1 and Table 2.
Shown in

Comparative Example 2 Modified PVA having different ethylene contents were prepared in the same manner as in Example 1, cast on a polyester film from a mixed solution of water / isopropyl alcohol, and air-dried to prepare a film. Heat treatment was performed in the same manner as in. The results are shown in Tables 1 and 2.

[0015]

[Table 1]

[0016]

[Table 2]

As shown in the results of Table 2, the separator film according to the present invention has a small degree of swelling and electric resistance due to PVA containing α-olefin and modified, and has a constant discharge even in a discharge test for 100 hours. It can be seen that the voltage is shown and the battery has excellent physical properties such that internal short circuit of the battery does not occur. On the other hand, the unmodified PVA of Comparative Example 1 has a small electric resistance but a very large swelling degree, and a partial deterioration of the film occurs in the discharge test for 100 hours.
The discharge voltage after the test is low. This is considered to be due to the deterioration of the film and the internal short circuit of the battery. In the present invention, the degree of swelling is suppressed and the durability and alkaline water resistance are significantly improved by copolymerizing 0.5 to 12 mol% of α-olefin. Therefore, it can be seen that the film does not deteriorate and there is no internal short circuit of the battery, and the film functions as an excellent separator film. When the ethylene content is modified beyond the range of the present invention (Comparative Example 2), the degree of swelling is small and the durability is excellent, but the electrical resistance is very large and the battery cannot be used.

[0018]

EFFECT OF THE INVENTION The separator film for an alkaline battery according to the present invention is suppressed in swelling degree in an alkaline aqueous solution and has durability to withstand long-term use. In addition, the conventional separator paper made from a mixture of synthetic fibers and cellulose fibers has a problem that the pore diameter between the fibers is large and it is difficult to completely separate the active substances of both electrodes inside the battery, which causes an internal short circuit. The film of the present invention has strong film physical properties even when it is swollen to some extent by an alkaline aqueous solution, and provides excellent performance without internal short circuit. Further, since it is a film, the internal volume of the battery is small and the battery capacity can be increased.

Claims (1)

[Claims] 1. A film for an alkaline battery separator comprising a modified polyvinyl alcohol film containing 0.5 to 12 mol% of an α-olefin unit having 4 or less carbon atoms.