Alkaline battery separator and process for producing the same

Abstract

An alkaline battery separator which enables the preparation of a battery with a good yield and workability is provided. The alkaline battery separator of the present invention comprises a fiber sheet mainly comprising hydrophilicity-imparted polyolefin fibers having a fiber diameter of 8 μm or more, 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.

Description

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
wherein R₀ is an electrical resistance (unit=mΩ·100 cm²/sheet) of the separator to be examined, R₁ is a resistance (unit=mΩ) obtained when the separator to be examined was placed, and R is a resistance (unit=mΩ) obtained when the separator to be examined was not placed.

The specimen used was cut from approximately central portion of the separator to be examined, and had a size of about 70×70 mm. The specimen was dipped in a potassium hydroxide solution (specific gravity=1.3 at 20° C.) at 25±2° C. for 5 hours before the above test procedures. When a separator to be examined was too small to prepare a specimen of about 70×70 mm, a specimen was prepared from the original separator at a rate of one specimen per about 400 cm². As the current electrode in the test equipment, a nickel plate (length=70 mm, width=70 mm, thickness=1 mm) composed of one or more kinds of nickel metals defined in JIS H2105 (nickel metal) was used. The voltage electrode used was prepared by dipping a cadmium bar (diameter=about 5 mm, length=about 50 mm or more) composed of one kind of a cadmium metal defined in JIS H 2113 in potassium hydroxide (specific gravity=1.3 at 20° C.) at an ordinary temperature for 24 hours or more. The test battery container used was an alkali-resistant vessel. When a space was generated upon which the specimens were placed, the measurement was carried out after the specimens were fixed by an alkali-resistant fixing spacer having a shape the same as that of the position where a specimen is placed. In this case, the resistance in the absence of the separator to be examined was measured, while the alkali-resistant fixing spacer was inserted.

The results are shown in Table 2.

(6) Ratio of Maintaining Thickness and Ratio of Non-conforming Batteries Produced During the Battery Assembly

The separators of the present invention prepared in Examples 8 and 9 were examined for a ratio of maintaining thickness and a ratio of non-conforming batteries produced during the battery assembly in accordance with the methods as described above, respectively. The results are shown in Table 2.

	TABLE 2
	C	F	G	H	J	K	L
	Example 8	105	0.1	43	90	98	15	0.0015
	Example 9	102	0.5	45	74	97	25	0.0010

[In Table 2, “C” denotes a ratio of maintaining a thickness (unit=%), “F” denotes a ratio of non-conforming batteries produced during the battery assembly (unit=%), “G” denotes a maximum pore size (unit=μm), “H” denotes a 5% modulus strength (unit=N/5 cm width), “J” denotes a ratio of pores having a pore size of 30 μm or less in the whole pores (unit=%), “K” denotes an air permeability (unit=cm/sec), and “L” denotes an electrical resistance (unit=mΩ·100 cm²/sheet)]

INDUSTRIAL APPLICABILITY

In the alkaline battery separator of the present invention, a short circuit caused by a puncturing of the separator due to the occurrence of a flash on the electrode is avoided, and the separator is prevented from being torn by an edge of an electrode, and further a good workability is provided, and the separator is not wrinkled. Therefore, a battery can be stably produced in a good yield. The alkaline battery separator of the present invention has an excellent property required to diffuse a poured electrolyte.

As above, the present invention was explained with reference to particular embodiments, but modifications and improvements obvious to those skilled in the art are included in the scope of the present invention.

Claims

1. An alkaline battery separator comprising a fiber sheet comprising more than 75 mass percentage of hydrophilicity-imparted polyolefin fibers having a fiber diameter of 9 μm or more, based upon 100% total weight of constituent fibers of the fiber sheet, wherein a part of said hydrophilicity-imparted polyolefin fibers is comprised of fibers having a tensile strength of at least 5 g/d.

2. The alkaline battery separator according to claim 1, wherein said fiber sheet contains fibers having a tensile strength of 5 g/d or more and fusible fibers, as said hydrophilicity-imparted polyolefin fibers having a fiber diameter of at least 9 μm.

3. The alkaline battery separator according to claim 2, wherein the mass ratio of non-fusible fibers having a tensile strength of at least 5 g/d and said fusible fibers is 10:90 to 50:50.

4. The alkaline battery separator according to claim 1, wherein the maximum pore diameter of said fiber sheet is less than 50 μm.

5. The alkaline battery separator according to claim 1, wherein a 5% modulus strength with respect to at least a direction of said fiber sheet is at least 60 N/5 cm width.

6. The alkaline battery separator according to claim 1, wherein pores having a pore diameter of at least 30 μm or less in said fiber sheet accounts for at least 95% of the whole of pores.

7. The alkaline battery separator according to claim 1, wherein an air permeability of said fiber sheet is at least 4 cm/sec.

8. The alkaline battery separator according to claim 1, wherein an electrical resistance of said fiber sheet is less than 5 mΩ·100 cm²/sheet.

9. The alkaline battery separator according to claim 1, wherein polyethylene fibers whose surface consists substantially of a polyethylene resin account for at least 60 mass % of said hydrophilicity-imparted polyolefin fibers which are the major component of said fiber sheet.

10. The alkaline battery separator according to claim 1, wherein said fiber sheet is a non-woven fabric.

11. A process for producing an alkaline battery separator comprising a fiber sheet comprising more than 75 mass percentage of hydrophilicity-imparted polyolefin fibers having a fiber diameter of at least 9 μm, comprising steps of: forming a fiber sheet from polyolefin fibers which have a fiber diameter of at least 9 μm, said polyolefin fibers containing polyolefin fibers having a fiber diameter of at least 9 μm and a tensile strength of at least 5 g/d; and then imparting a hydrophilic property to the resulting fiber sheet.