Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
2000-10-08
2003-08-19
Gulakowski, Randy (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
C429S147000
Reexamination Certificate
active
06607859
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an alkaline battery separator and a process for producing the same.
BACKGROUND ART
In an alkaline battery, a separator is used to separate a positive electrode and a negative electrode from each other to prevent a short circuit therebetween, and further, to hold an electrolyte thereon and provide a smooth electromotive reaction.
Recently, in electronic equipment, the space allotted for the alkaline battery has been reduced due to the need for miniaturization and weight-saving. Nevertheless, the performance requirement for such a smaller alkaline battery is the same as or higher than that for a conventional battery, and therefore, it is necessary to increase the capacity of the, battery, and also increase the amounts of active materials in the electrodes. Thus, the volume allotted in the battery for the separator must be reduced, the thickness of the separator must be reduced thinner, and electrodes and separators must be placed in close contact to prevent the formation of a space therebetween when an electrodes-group is produced. Nevertheless, when thinner separators are used, and a strong tension is applied to ensure the close contact between the electrodes and separators, any flash occurring at the electrode can puncture the separator and cause a short circuit, an edge of the electrode might then tear the separator, and thus lower the yield.
Japanese Unexamined Patent Publication (Kokai) No. 7-29561 or Japanese Unexamined Patent Publication (Kokai) No. 8-138645, for example, disclose that fine fibers having a small diameter, for example, approximately 5 &mgr;m, are used to make the separator thinner. Nevertheless, the separator obtained from such fine fibers lacks stiffness and is easily wrinkled when an electrodes-group is produced. Because the separator has a high density, it is difficult to diffuse a poured electrolyte into the separator, and thus, the handling characteristics are poor when an electrodes-group is produced.
The inventors of the present invention made an intensive investigation of an alkaline battery separator which can remedy the above disadvantages of prior art, and can be manufactured in a good yield with a good workability. As a result, the inventors found that when a separator mainly comprising hydrophilicity-imparted, relatively thick and stiff polyolefin fibers having a fiber diameter of 8 &mgr;m or more wherein a part of the hydrophilicity-imparted polyolefin fibers of high-strength fibers having a tensile strength of 5 g/d or more is used, a short circuit caused by a puncturing of the separator by a flash occurring at the electrode is avoided, and the separator is not torn by an edge of an electrode. Further, a battery can be stably produced in a good yield with a good workability, while the separator has the desired stiffness and is not wrinkled. Further, the present inventors also found that the separator mainly comprising hydrophilicity-imparted, relatively thick and stiff polyolefin fibers having a fiber diameter of 8 &mgr;m or more can provide spaces sufficient for holding an electrolyte, and the property required to diffuse a poured electrolyte is excellent.
Accordingly, the object of the present invention is to provide an alkaline battery separator which can be produced in a good yield with a good workability, and a process for producing the same.
DISCLOSURE OF INVENTION
The present invention relates to an alkaline battery separator characterized in that the separator comprises a fiber sheet mainly comprising hydrophilicity-imparted polyolefin fibers having a fiber diameter of 8 &mgr;m or more is contained, and a part of the hydrophilicity-imparted polyolefin fibers is composed of high-strength fibers having a tensile strength of 5 g/d or more. The alkaline battery separator of the present invention will be sometimes referred to as simply a “separator” hereinafter.
Further, the present invention also relates to a process for producing an alkaline battery separator comprising the steps of:
forming a fiber sheet from polyolefin fibers which have a fiber diameter of 8 &mgr;m or more and contain polyolefin high-strength fibers having a fiber diameter of 8 &mgr;m or more and a tensile strength of 5 g/d or more; and then imparting a hydrophilic property to the resulting fiber sheet.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention will be explained in detail hereinafter.
A fiber sheet forming the alkaline battery separator of the present invention mainly comprises polyolefin fibers which have a fiber diameter of 8 &mgr;m or more and are treated to impart a hydrophilic property (i.e., hydrophilicity-imparted polyolefin fibers). Because the fiber sheet mainly comprises hydrophilicity-imparted polyolefin fibers, it has a high alkali resistance. Further, the fiber sheet has an excellent electrolyte-holding capacity, because it is treated to impart a hydrophilic property. The expression “mainly comprise” as used herein means that the subject fibers, i.e., hydrophilicity-imparted polyolefin fibers having a fiber diameter of 8 &mgr;m or more, account for more than 75 mass %, preferably 80 mass % or more, more preferably 85 mass % or more, more preferably 90 mass % or more, more preferably 95 mass % or more, most preferably 100 mass %, with respect to constituent fibers of the fiber sheet.
The polyolefin fibers before the treatment for imparting a hydrophilic property may contain a resin component, for example, polymers from monomers, such as propylene, ethylene, butene, or methyl pentene, copolymers of two or more kinds of the monomers as above, or copolymers of the above monomers and vinyl alcohol, acrylic acid or methacrylic acid, such as ethylene-vinyl alcohol copolymers, ethylene-acrylic acid copolymers, or ethylene-methacrylic acid copolymers. The polyolefin fiber may be composed of one kind of a resin component as above, or may be a composite fiber comprising two or more resin components as above. As the composite fiber, there may be mentioned, for example, a composite fiber whose sectional shape is a sheath-core type, a side-by-side type, an eccentric type, an islands-in-sea type, an orange type or a multiple bimetal type.
A fiber diameter of each fiber of the hydrophilicity-imparted polyolefin fibers contained as the main component in the fiber sheet forming the alkaline battery separator of the present invention is not limited, as long as it is 8 &mgr;m or more. However, it is preferably 9 &mgr;m or more, more preferably 10 &mgr;m or more, more preferably 10.5 &mgr;m or more, more preferably 11 &mgr;m or more, more preferably 12 &mgr;m or more, more preferably 12.5 &mgr;m or more, most preferably 13 &mgr;m or more. For a fiber having a non-circular sectional shape, a diameter of a circle having an area the same as that of the non-circular sectional shape is regarded as a diameter in the present specification.
Because the fiber sheet forming the alkaline battery separator of the present invention mainly comprises relatively thick hydrophilicity-imparted polyolefin fibers as above, i.e., hydrophilicity-imparted polyolefin fibers having a required stiffness, the fiber sheet has a good stiffness, a good workability, and an excellent compression elasticity. Therefore, a short circuit caused between electrodes by a puncturing of the separator by a flash occurring on the electrode is avoided, a separator is not torn by an edge of an electrode, and a battery can be stably produced in a good yield. Further, the fiber sheet forming the alkaline battery separator of the present invention mainly comprises relatively thick, i.e., stiff, hydrophilicity-imparted polyolefin fibers having a fiber diameter of 8 &mgr;m or more, and can maintain spaces sufficient for holding an electrolyte. Therefore, the fiber sheet has an excellent property required to diffuse a poured electrolyte, that is, the poured electrolyte can be rapidly diffused all over the battery. The upper limit of the fiber diameter is not limited, but preferably is approximately 30 &mgr;m. When the fiber diameter is 30 &mgr;m or less, t
Hirooka Masaki
Takase Toshiaki
Tanaka Masanao
Darby & Darby
Gulakowski Randy
Japan Vilene Company Ltd.
Wills M.
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