Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
1999-09-22
2002-07-23
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
C429S253000, C429S254000, C428S392000, C428S515000, C428S516000
Reexamination Certificate
active
06423445
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an alkaline battery separator, i.e., a separator for an alkaline battery.
2. Description of the Related Art
In an alkaline battery, a separator is installed between a positive electrode and a negative electrode to separate them from each other and prevent a short-circuit. The separator can hold an electrolyte to thereby smoothly carry out an electromotive reaction.
The separator is required to have an erosion resistance to an electrolyte, such as potassium hydroxide, and a separator containing polyolefin fibers having an excellent alkali resistance is preferably used. However, polyolefin fiber has a poor affinity for an electrolyte, and thus, a separator of the a polyolefin fibers has a disadvantage in its capacity for holding an electrolyte (i.e., the electrolyte-holding capacity).
A sulfonating treatment of the separator containing polyolefin fibers is known as a means of imparting an affinity for an electrolyte to such a separator. However, the sulfonating reaction of the separator containing polyolefin fibers cannot be efficiently carried out, and further, the reaction is carried out under dangerous condition wherein a fuming sulfuric acid is used. To efficiently carry out the sulfonating reaction under a moderate condition, methods for sulfonating a fiber sheet containing, as a polyolefin fiber, fibers carrying an acrylic acid/ethylene copolymer on a surface thereof are proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-121062, Japanese Unexamined Patent Publication (Kokai) No. 5-121063, Japanese Unexamined Patent Publication (Kokai) No. 5-307947, Japanese Unexamined Patent Publication (Kokai) No. 6-111801, and Japanese Unexamined Patent Publication (Kokai) No. 6-207321.
It is true that the fiber sheet containing fibers that carry an acrylic acid/ethylene copolymer on a surface thereof can be efficiently sulfonated. However, if the fiber sheet is used as a separator in a nickel-hydrogen battery, self-discharge thereof is very difficult to inhibit, and thus the battery capacity is often considerably reduced.
SUMMARY OF THE INVENTION
The inventors of the present invention found that an excellent effect of inhibiting a self-discharge can be obtained if a separator sheet contains, on at least a part of a surface thereof, a methacrylic/ethylene copolymer having a crystallinity of 25% or more, and that a hydrophilicity can be easily imparted to or enhanced for the methacrylic/ethylene copolymer. The present invention is based upon these findings.
Accordingly, the object of the present invention is to remedy the shortcomings in the prior art, and to provide an alkaline battery separator exhibiting an excellent effect of inhibiting a self-discharge, and is capable of easily being treated to obtain a hydrophilicity during the manufacturing process thereof.
Other objects and advantages will be apparent from the following description.
In accordance with the present invention, there is provided an alkaline battery separator comprising a gas-permeable sheet which contains a hydrophilic portion carrying a methacrylic/ethylene copolymer component on at least a part of a surface of the hydrophilic portion (and at the same time a surface of the gas-permeable sheet), a crystallinity of the methacrylic/ethylene copolymer being 25% or more.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The gas-permeable sheet for the alkaline battery separator according to the present invention may be, for example, a fiber sheet, such as a nonwoven fabric, a woven fabric, a knitted fabric, a composite fabric thereof, or a microporous film. The fiber sheet may contain the hydrophilic portion in the form of hydrophilic fibers carrying the methacrylic/ethylene copolymer component on at least a part of a surface thereof. Further, the fiber sheet may contain the hydrophilic portion in the form of hydrophilic powder materials containing the methacrylic/ethylene copolymer component on at least a part of a surface of the powder material and at the same time, at least a part of a surface of the constituent fibers. The microporous film may contain the hydrophilic portion in the form of a surface layer on the film. When such a microporous film is used, the film may preferably include a fibrous sheet on at least one side.
The hydrophilic fiber or powder material used in the present invention is particularly a hydrophilicity-enhancing fiber a powder material prepared by enhancing a hydrophilic property of the fiber or powder material containing the methacrylic/ethylene copolymer component on at least a part of a surface thereof and having a poor hydrophilicity, or a hydrophilicity-imparting fiber or powder material prepared by imparting a hydrophilic property to the fiber or powder material containing the methacrylic/ethylene copolymer component on at least a part of a surface thereof and having little hydrophilicity.
The hydrophilic portion-containing microporous film used in the present invention is particularly a hydrophilicity-enhancing microporous film prepared by enhancing a hydrophilic property of the microporous film containing the methacrylic/ethylene copolymer component on at least a part of a surface thereof and having a poor hydrophilicity, or a hydrophilicity-imparting microporous film prepared by imparting a hydrophilic property to the microporous film containing the methacrylic/ethylene copolymer component on at least a part of a surface thereof and having little hydrophilicity.
In one embodiment of the present invention, the alkaline battery separator is, for example, a single layer composed of the fiber sheet containing hydrophilic fibers as above. In another embodiment, the alkaline battery separator is, for example, a laminated sheet composed of a microporous film and a fiber sheet, or a laminated sheet composed of the microporous film sandwiched between two fiber sheets. In the latter cases where the microporous film is contained, the hydrophilic portion may be contained in the microporous film and/or the fiber sheet.
The present invention will be explained hereinafter mainly about the embodiment wherein the gas-permeable sheet and the fiber sheet comprising hydrophilic fibers containing the methacrylic/ethylene copolymer component.
The fiber sheet for the alkaline battery separator according to the present invention contains hydrophilic fibers, and the hydrophilic fibers carry the methacrylic/ethylene copolymer component on at least a part of a surface thereof.
The methacrylic/ethylene copolymer contained in the hydrophilic fiber may be prepared from a methacrylic acid comonomer or a methacrylic derivative comonomer and an ethylene comonomer. The methacrylic derivative may be, for example, a methacrylic acid ester with an alcohol, preferably an aliphatic saturated alcohol, such as an aliphatic saturated alcohol having 1 to 8 carbon atoms, or a methacrylic acid salt. The methacrylic acid ester is, for example, methyl methacrylate, ethyl methacrylate, butyl methacrylate, or 2-ethylhexyl methacrylate, and the methacrylic acid salt is, for example, an alkali metal salt, such as a potassium, sodium, or lithium salt. A preferred comonomer is methacrylic acid or potassium methacrylate.
The ratio of the methacrylic acid comonomer or a methacrylic derivative comonomer (hereinafter referred to as a methacrylic comonomer) and the ethylene comonomer is not particularly limited, so long as the crystallinity of the methacrylic/ethylene copolymer is 25% or more. Further, one or more other comonomers copolymerizable with the methacrylic comonomer and the ethylene comonomer, i.e., the third copolymerizable comonomer other than the methacrylic comonomer or the ethylene comonomer, may be present in the methacrylic/ethylene copolymer, so long as the crystallinity of the methacrylic/ethylene copolymer is 25% or more.
The methacrylic/ethylene copolymer may be an alternating copolymer, a random copolymer, a block copolymer, or a mixture thereof.
The methacrylic/ethylene copolymer contained in the hydrophilic fiber
Kato Koichi
Sato Kazuya
Tanaka Masanao
Darby & Darby
Japan Vilene Company Ltd.
Kalafut Stephen
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