JP2015041458A - Battery separator - Google Patents

Abstract

An object of the present invention is to provide a battery separator in which a wet non-woven fabric is subjected to a hydrophilic treatment, which has good production stability of the non-woven fabric, reduces a defect occurrence rate due to a short circuit during battery production, and has excellent liquid retention. It is to provide a battery separator having a long life. SOLUTION: The content of the polyolefin-based core-sheath type composite fiber having a fiber diameter of 5 μm and having a crimp is more than 85% by mass of the fiber constituting the nonwoven fabric, and 25% by mass of the fiber constituting the nonwoven fabric. % Of the fiber diameter is more than 5 μm and less than 8 μm, the wet nonwoven fabric is hydrophilically treated, and the sulfonation treatment is more preferably used as the hydrophilic treatment of the nonwoven fabric. [Selection diagram] None

Description

  The present invention relates to a battery separator that can be suitably used for alkaline secondary batteries such as nickel-cadmium batteries, nickel-zinc batteries, and nickel-hydrogen batteries.

  Alkaline secondary batteries such as nickel-cadmium batteries and nickel-hydrogen batteries have excellent charge / discharge characteristics and overcharge / discharge characteristics, and can be used repeatedly with a long service life. Therefore, they can be used in small electronic devices such as cordless phones, laptop computers, and audio equipment. In addition, it is widely used for small power applications such as electric tools and electric bicycles, and large power applications such as hybrid cars and electric cars. The role of the battery separator used in the alkaline secondary battery is to separate the positive electrode and the negative electrode, prevent short circuit, absorb and retain the electrolyte (high concentration alkaline aqueous solution), and prevent the gas generated by the electrode reaction. For example, transmission.

  Conventionally, a nonwoven fabric has generally been used as a battery separator. In a nickel-cadmium battery, a nonwoven fabric made of a polyamide fiber that is easily wetted by an electrolyte, has a large amount of liquid retention, and has a low electrical resistance in a state containing the electrolyte has been used. However, in nickel-hydrogen batteries, the decomposition products of polyamide fiber hydrolysis promote self-discharge, so nonwoven fabrics mainly composed of polyolefin fibers with excellent alkali resistance and oxidation resistance are mainly used. ing.

  On the other hand, since nonwoven fabrics mainly composed of polyolefin fibers have low hydrophilicity, they are generally subjected to sulfonation treatment, hydrophilic monomer grafting treatment, corona discharge treatment, surfactant application treatment, and the like. .

  Of these, the sulfonation treatment is a method of introducing sulfonic acid groups into the nonwoven fabric with fuming sulfuric acid, concentrated sulfuric acid, sulfur trioxide gas or the like. The nonwoven fabric subjected to the sulfonation treatment is excellent in the ability to absorb and retain the electrolyte, and has the effect of suppressing the self-discharge reaction of the battery.

  As the nonwoven fabric used for the sulfonation treatment, a dry nonwoven fabric or a wet nonwoven fabric is used. However, a nonwoven fabric excellent in uniformity of formation is required in order to increase the battery capacity and prevent internal short circuit. Considering good uniformity of formation, wet nonwoven fabrics are generally superior to dry nonwoven fabrics. Polyolefin fibers are excellent in alkali resistance and oxidation resistance, and are effective in extending the life of batteries. However, it is relatively difficult to produce a nonwoven fabric made only of polyolefin fibers. In particular, in the production of wet nonwoven fabrics, there are problems such as surface cracks during heating and drying, resulting in uneven formation and poor production stability. For this reason, attempts have been made to improve production stability by using polyolefin fibers and ethylene-vinyl alcohol copolymer fibers excellent in wet heat adhesion (see, for example, Patent Document 1). However, the sulfonic acid group introduced into the ethylene-vinyl alcohol copolymer fiber tends to deteriorate in the strong alkaline electrolyte. Therefore, attempts have been made to improve the production stability of nonwoven fabrics by controlling the thermal behavior of polyolefin fibers (see, for example, Patent Document 2).

  In addition, polyolefin nonwoven fabrics mainly composed of polyolefin core-sheath composite fibers are over-dried, and if the heat-fusible polyolefin core-sheath composite fibers are excessively melted, the stretchability and tear strength of the nonwoven fabrics decrease. When used as a separator, when the battery is manufactured, the burrs of the electrode plate may penetrate the separator and cause a short circuit, or the separator may tear at the part where the end of the electrode plate and the separator are in contact with each other. Occurs, and the yield of battery manufacturing is reduced.

JP 7-1442047 A JP 2011-210701 A

  An object of the present invention relates to a battery separator obtained by hydrophilizing a wet nonwoven fabric, the manufacturing stability of the nonwoven fabric is good, the occurrence rate of defects due to a short circuit during battery manufacturing is reduced, the liquid retention is excellent, and the life is long. An object is to provide a battery separator.

  As a result of intensive studies to solve this problem, the following means for solving were found.

(1) The content of the polyolefin-based core-sheath composite fiber having a fiber diameter exceeding 5 μm and having crimps is more than 85 mass% of the fibers constituting the nonwoven fabric, and 25 mass% of the fibers constituting the nonwoven fabric A battery separator characterized by hydrophilizing a wet nonwoven fabric having a fiber diameter of more than 5 μm and less than 8 μm,
(2) A battery separator, wherein the hydrophilic treatment of the nonwoven fabric is a sulfonation treatment.

  In the present invention, the polyolefin core-sheath composite fiber having a fiber diameter exceeding 5 μm and having crimps is contained in an amount exceeding 85 mass% of the fibers constituting the nonwoven fabric, and 25 mass% or more of the fibers constituting the nonwoven fabric. By making the fiber into a fiber that exceeds 5 μm and less than 8 μm, surface cracking of the nonwoven fabric during heat drying in wet papermaking can be suppressed, and by hydrophilizing the obtained wet nonwoven fabric, A battery separator that has excellent liquid retention, reduces the short-circuit rate due to breakage of the separator in the battery manufacturing process, and has a long life can be obtained.

  As a combination of the core component and the sheath component in the polyolefin-based core-sheath composite fiber having crimps used in the present invention, the core component is polypropylene, the sheath component is a combination of high-density polyethylene, the core component is polypropylene, and the sheath component is medium. Combination of density polyethylene, core component of polypropylene, sheath component of low density polyethylene, core component of polyethylene, sheath component of linear low density polyethylene, core component of polypropylene, sheath component of ethylene-propylene copolymer , A core component is polypropylene, and a sheath component is polybutene-1. Among these polyolefin-based core-sheath type composite fibers having crimps, a core-sheath type composite fiber in which the core component is a combination of polypropylene and the sheath component is high-density polyethylene is preferable because it has excellent papermaking properties and a high strength nonwoven fabric is obtained. Used.

  In the polyolefin core-sheath composite fiber in which the core component is polypropylene and the sheath component is high-density polyethylene, the core component polypropylene adjusts the physical properties of the fiber, so that other than high-density polyethylene, polymethylpentene, etc. The polyolefin can be mixed. The mixing ratio of the polyolefin other than polypropylene is preferably 10% by mass or less of the core component. In addition, for the high density polyethylene of the sheath component, other polyolefins such as polypropylene and ethylene-propylene copolymer can be mixed as necessary in order to adjust the fiber physical properties. The mixing ratio of the polyolefin other than the high-density polyethylene is preferably 10% by mass or less of the sheath component.

  The fiber length of the crimped polyolefin-based core-sheath composite fiber used in the present invention is not particularly limited, but a fiber having a fiber diameter exceeding 5 μm is used. The fiber length is preferably 1 mm or more and 20 mm or less from the nonwoven fabric strength and manufacturability. When the fiber length is less than 1 mm, sufficient mechanical strength of the nonwoven fabric may not be obtained. If the fiber length exceeds 20 mm, formation may be poor and a good nonwoven fabric may not be formed. In particular, in a wet nonwoven fabric, abnormal entanglement between fibers at the time of dispersion may occur, and a uniform dispersion state may not occur, resulting in poor formation. Although it will not specifically limit if a fiber diameter exceeds 5 micrometers, it exceeds 5 micrometers and 20 micrometers or less are preferable.

  The number of crimps and the crimp rate of the polyolefin-based core-sheath composite fiber having crimps used in the present invention are not particularly limited, but the number of crimps is preferably 1 to 25 per 25 mm fiber, More preferably, 3 or more and 15 or less per 25 mm. The crimp rate is preferably 1% to 20%, and more preferably 5% to 15%. The “crimp number” and “crimp rate” used in the present invention represent “crimp number” and “crimp rate” defined by JIS L 1015.

  Examples of other fibers that can be used in combination with the crimped polyolefin-based sheath-core composite fiber used in the present invention include polyolefin fibers composed of a single component such as polyethylene fibers and polypropylene fibers, and two or more different types. Side-by-side type, splittable composite type polyolefin fiber that is a suitable combination of mixed polyolefin fiber made of a mixture of polyolefins, copolymerized polyolefin fiber made of a copolymer of two or more different olefins, polyethylene, polypropylene, copolymerized polyolefin, etc. , Aliphatic polyamide fiber, aromatic polyamide fiber, semi-aromatic polyamide fiber, ethylene-vinyl alcohol copolymer fiber, polyvinyl alcohol fiber, polyethylene terephthalate fiber, polybutylene terephthalate fiber, polyacrylonitrile Polyolefin fiber consisting of a single component such as polyethylene fiber or polypropylene fiber, mixed polyolefin fiber consisting of a mixture of two or more different polyolefins, copolymer consisting of a copolymer of two or more different olefins Polyolefin fibers such as side-by-side and splittable composite polyolefin fibers, which are suitably combined with resins such as polyolefin fibers, polyethylene, polypropylene, and copolymerized polyolefins, are preferred.

  The fiber length of the polyolefin fiber that can be used in combination with the polyolefin core-sheath composite fiber having these crimps is not particularly limited, but a fiber having a fiber diameter exceeding 5 μm is used. The fiber length is preferably 1 mm or more and 20 mm or less from the nonwoven fabric strength and manufacturability. Further, the splittable conjugate fiber can be used after being refined by a refiner. When the fiber length is less than 1 mm, sufficient mechanical strength of the nonwoven fabric may not be obtained. If the fiber length exceeds 20 mm, formation may be poor and a good nonwoven fabric may not be formed. In particular, in a wet nonwoven fabric, abnormal entanglement between fibers at the time of dispersion may occur, and a uniform dispersion state may not occur, resulting in poor formation. Although it will not specifically limit if a fiber diameter exceeds 5 micrometers, it exceeds 5 micrometers and 20 micrometers or less are preferable.

  The fiber diameter and fiber length of fibers other than polyolefin fibers that can be used in combination with the polyolefin core-sheath composite fibers having these crimps are not particularly limited, but the fiber diameter is 1 μm from the strength of nonwoven fabric and manufacturability. The fiber length is preferably 20 μm or less, and the fiber length is preferably 1 mm or more and 20 mm or less. Further, the splittable conjugate fiber can be used after being refined by a refiner. When the fiber length is less than 1 mm, sufficient mechanical strength of the nonwoven fabric may not be obtained. If the fiber length exceeds 20 mm, formation may be poor and a good nonwoven fabric may not be formed. In particular, in a wet nonwoven fabric, abnormal entanglement between fibers at the time of dispersion may occur, and a uniform dispersion state may not occur, resulting in poor formation.

  In the present invention, the polyolefin core-sheath composite fiber having a fiber diameter exceeding 5 μm and having crimps is contained in excess of 85% by mass of the fibers constituting the nonwoven fabric. The fiber diameter is characterized in that 25% by mass or more of all the fibers constituting the nonwoven fabric including the crimped polyolefin core-sheath composite fiber are fibers having a fiber diameter of more than 5 μm and less than 8 μm. The content of the polyolefin-based core-sheath composite fiber that exceeds 5 μm and has crimps is more preferably 95% by mass or more of the fibers constituting the nonwoven fabric.

  In the present invention, as the wet papermaking method for forming the fiber web, there are a plurality of conventionally known methods such as a horizontal long net method, an inclined wire type short net method, a circular net method, and a short net / circular net combination method. A combination combination method is available. In the present invention, when a wet nonwoven fabric is used as a battery separator, in order to prevent pinholes and defects, a combination system combining a short net / round net or a round net / round net is preferable.

  As a method for producing a nonwoven fabric from a fiber web, a wet fiber web containing a polyolefin core-sheath composite fiber having crimps, which is a heat-bonded fiber, is heated and dried, and at the same time, heat-bonding of the fibers occurs. Let As this heat drying method, a method combining a Yankee dryer and a hot air hood dryer is used.

  The battery separator of the present invention imparts affinity with an electrolytic solution by subjecting the wet nonwoven fabric obtained as described above to a hydrophilic treatment. Examples of the hydrophilic treatment include surfactant application treatment, corona discharge treatment, fluorine gas treatment, hydrophilic monomer graft polymerization treatment, sulfonation treatment, and the like.

  As the surfactant application treatment, for example, an anionic surfactant or a nonionic surfactant is immersed in a nonwoven fabric, or hydrophilicity is imparted by applying or spraying the solution onto the nonwoven fabric. be able to.

  For corona discharge treatment, an appropriate interval is provided between the electrode connected to the high voltage generator and the metal roll covered with silicon rubber, chlorosulfonated polyethylene rubber, ethylene propylene rubber, etc., and several thousand to tens of thousands V at high frequency. The high voltage corona is generated, the nonwoven fabric is run at an appropriate speed during this interval, and ozone or nitrogen oxide generated in the corona is reacted with the nonwoven fabric to react with the carbonyl group, carboxyl group, hydroxyl group, peroxide group. Can be generated to impart hydrophilicity.

  As the fluorine gas treatment, for example, a mixture of fluorine gas diluted with an inert gas (for example, nitrogen gas, argon gas, etc.) and at least one gas selected from oxygen gas, carbon dioxide gas, and sulfur dioxide gas is used. By exposing the nonwoven fabric to gas, a sulfonic acid group, a carbonyl group, a carboxyl group, a sulfofluoride group, a hydroxyl group, or the like can be introduced to the fiber surface of the nonwoven fabric to impart hydrophilicity.

  In the graft polymerization treatment of the hydrophilic monomer, for example, acrylic acid, methacrylic acid, acrylic ester, methacrylic ester, vinyl pyridine, vinyl pyrrolidone, or styrene can be used as the hydrophilic monomer. As a method for polymerizing these hydrophilic monomers, a method in which a nonwoven fabric is immersed in a solution containing these hydrophilic monomers and a polymerization initiator and heated, a hydrophilic monomer is applied to the nonwoven fabric and radiation is applied. Examples include a method of irradiating, a method of irradiating a nonwoven fabric with radiation and bringing it into contact with a hydrophilic monomer, and a method of applying a hydrophilic monomer solution containing a sensitizer to a nonwoven fabric and irradiating ultraviolet rays.

  Although it does not specifically limit as a sulfonation process, For example, the liquid phase (solution) method which immerses a nonwoven fabric in the solution which consists of fuming sulfuric acid, hot concentrated sulfuric acid, chlorosulfuric acid, etc., introduces a sulfonic acid group, sulfur dioxide gas, There is a gas phase (gas) method in which sulfur trioxide gas or the like is exposed to a nonwoven fabric to introduce a sulfonic acid group.

  In the battery separator of the present invention, sulfonation treatment by a gas phase treatment method is more preferable. The sulfonation treatment by the liquid phase treatment method has a problem that it is difficult to set reaction conditions, and when the reaction time is excessively long or the temperature is excessively high, the nonwoven fabric is easily carbonized, contracted, and formed into a film. There is also a problem that a large amount of strongly acidic waste liquid is produced.

  In the battery separator of the present invention, in order to further improve the affinity with the electrolytic solution, the non-woven fabric after the sulfonation treatment can be subjected to a corona discharge treatment or a surfactant can be added. Surfactants used include alkyl sulfates, polyoxyethylene alkyl ether sulfates, alkylbenzene sulfonates, dialkyl sulfosuccinates, alkyl diphenyl ether disulfonates, alkane sulfonates, phosphate esters, long chains Anionic surfactants such as fatty acid salt, sodium salt of β-naphthalenesulfonic acid formalin condensate, sodium salt of special aromatic sulfonic acid formalin condensate, special polycarboxylic acid type polymer surfactant; polyoxyethylene alkyl ether , Polyoxyalkylene derivatives, polyoxyalkylene alkenyl ethers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, glycerin Fatty acid esters, polyoxyethylene fatty acid esters, nonionic surfactants of polyoxyethylene hardened castor oils and the like. These surfactants can be applied to the nonwoven fabric by impregnation, coating, spraying and drying, and the amount of the surfactant applied is 0.1% by mass relative to the nonwoven fabric after the sulfonation treatment. The content is preferably 1.0% by mass or less.

In the battery separator of the present invention, the thickness is adjusted by super calendering or thermal calendering as necessary. The basis weight of the battery separator of the present invention is preferably in the range of 30 g / m ^{2 to} 100 g / m ² , and the thickness is preferably in the range of 60 μm to 250 μm. The basis weight and thickness of the battery separator can be appropriately selected according to the characteristics of the applied battery. Here, the basis weight represents the basis weight defined in JIS P 8124, and the thickness represents the thickness defined in JIS P 8118. The maximum pore diameter of the battery separator of the present invention is preferably in the range of 1 μm to 50 μm. When the maximum pore diameter is larger than 50 μm, short-circuiting is likely to occur, and the defect rate during battery manufacture may increase. When the maximum pore diameter is less than 1 μm, oxygen gas permeability and ionic conductivity may be lowered. Here, the maximum pore diameter represents the maximum pore diameter according to the bubble point method defined in JIS K3832.

  EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to a following example.

Example 1
Polyolefin core-sheath type composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimpability (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp rate 10%) 70 mass Polyolefin core-sheath type composite fiber having a core and a core component of polypropylene and a sheath component of high-density polyethylene and having crimpability (fineness 0.3 dtex, fiber length 5 mm, number of crimps 3, crimp rate 5%) 30 parts by mass was disaggregated and dispersed in water of a pulper, and gently stirred with an agitator to prepare a uniform papermaking slurry. A web is formed from the papermaking slurry by a wet papermaking method using a circular paper machine, and a Yankee dryer set at 138 ° C. and hot air at 140 ° C. are blown into a hot air hood and dried. The sheath portion of the mold composite fiber was heat-melt bonded.

The nonwoven fabric thus obtained was sulfonated for 25 seconds in 75 ° C. dry air containing sulfur trioxide gas, neutralized with a 2.5% by mass aqueous sodium hydroxide solution, Wash thoroughly with exchange water, then spray-apply sodium alkyldiphenyl ether disulfonate as a surfactant to 0.3% by mass with respect to the non-woven fabric after sulfonation treatment, and after drying, by super calender treatment The thickness was adjusted to obtain a battery separator having a sulfur content of 0.62% by mass, a thickness of 120 μm, and a basis weight of 53.5 g / m ² .

(Example 2)
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp ratio 10) %) Polyolefin core-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.3 dtex, fiber length of 5 mm, crimp number of 3, crimps) 5% by mass) 25 parts by mass of the core component is polypropylene, the sheath component is high-density polyethylene, and there is no crimped polyolefin-based core-sheath composite fiber (fineness 0.3 dtex, fiber length 5 mm) 5 parts by mass A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.60% by mass, a thickness of 121 μm, and a basis weight of 53.6 g / m ² .

Example 3
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp ratio 10) %) Polyolefin core-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.3 dtex, fiber length of 5 mm, crimp number of 3, crimps) 5%) Polyolefin core-sheath type composite fiber (fineness 0.3 dtex, fiber length 5 mm) 10 parts by mass with 20 parts by mass and polypropylene as the core component, high density polyethylene as the sheath component and no crimp A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.63% by mass, a thickness of 119 μm, and a basis weight of 53.2 g / m ² .

Example 4
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp ratio 10) %) Polyolefin core-sheath type composite fiber (fineness 0.3 dtex, fiber length 5 mm, crimp number 3, crimp) 5%) 26 parts by mass, and the core component is polypropylene, the sheath component is high-density polyethylene, and there is no crimped polyolefin-based core-sheath composite fiber (fineness 0.8 dtex, fiber length 5 mm) 14 parts by mass A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.65 mass%, a thickness of 120 μm, and a basis weight of 53.5 g / m ² .

(Example 5)
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp ratio 10) %) Polyolefin core-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.3 dtex, fiber length of 5 mm, crimp number of 3, crimps) A non-woven fabric was produced in the same manner as in Example 1 except that 20 parts by mass) and 10 parts by mass of polypropylene single fiber (fineness 0.3 dtex, fiber length 5 mm) having no crimp were used. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.65 mass%, a thickness of 120 μm, and a basis weight of 53.5 g / m ² .

(Example 6)
Polyolefin-based sheath-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.8 dtex, fiber length of 5 mm, number of crimps of 15, crimp rate of 15) %) Polyolefin core-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.3 dtex, fiber length of 5 mm, crimp number of 15, crimps) 15%) Polyolefin core-sheath type composite fiber (fineness 0.3 dtex, fiber length 5 mm) 10 parts by mass with 20 parts by mass and polypropylene as the core component, high density polyethylene as the sheath component and no crimp A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.61% by mass, a thickness of 120 μm, and a basis weight of 53.1 g / m ² .

(Example 7)
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 2, crimp ratio 4) %) Polyolefin core-sheath type composite fiber (fineness 0.3 dtex, fiber length 5 mm, crimp number 3, crimp) 5%) 26 parts by mass, and the core component is polypropylene, the sheath component is high-density polyethylene, and there is no crimped polyolefin-based core-sheath composite fiber (fineness 0.8 dtex, fiber length 5 mm) 14 parts by mass A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.65 mass%, a thickness of 120 μm, and a basis weight of 53.5 g / m ² .

(Example 8)
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.8 dtex, fiber length of 5 mm, number of crimps of 17, crimp rate of 20) %) Polyolefin core-sheath type composite fiber (fineness 0.3 dtex, fiber length 5 mm, crimp number 3, crimp) 5%) 26 parts by mass, and the core component is polypropylene, the sheath component is high-density polyethylene, and there is no crimped polyolefin-based core-sheath composite fiber (fineness 0.8 dtex, fiber length 5 mm) 14 parts by mass A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.62% by mass, a thickness of 120 μm, and a basis weight of 53.3 g / m ² .

Example 9
The nonwoven fabric obtained in Example 3 was immersed in a treatment solution consisting of 50 parts by mass of acrylic acid, 0.5 parts by mass of benzophenone, 1 part by mass of polyoxyethylene lauryl ether, and 48.5 parts by mass of ion-exchanged water, and then removed. After carrying out graft polymerization treatment by irradiating short wavelength ultraviolet rays of 185 nm and 254 nm for 1 minute using a low pressure mercury lamp under oxygen, this grafted nonwoven fabric is washed thoroughly with water and dried, and then the thickness is adjusted by super calender treatment. Thus, a battery separator having a graft ratio of 7.5%, a thickness of 120 μm, and a basis weight of 55.2 g / m ² was obtained.

(Example 10)
The nonwoven fabric obtained in Example 3 was introduced into a mixed gas processing device, and after the inside of the processing device was vacuum replaced, 25 ° C., gas composition was 2% by volume of fluorine gas, 8% by volume of oxygen gas, and 8% by volume of sulfur dioxide gas. Then, a mixed gas consisting of 82% by volume of nitrogen gas was introduced into the processor, and the fluorine gas treatment was performed for 90 seconds at 1 atm. This non-woven fabric subjected to the fluorination treatment was sufficiently washed with water and dried, and then the thickness was adjusted by supercalender treatment to obtain a battery separator having a thickness of 121 μm and a basis weight of 52.6 g / m ² .

(Comparative Example 1)
The composition of the fiber was changed to 100 parts by mass of a polyolefin core / sheath composite fiber (fineness 0.8 dtex, fiber length 5 mm) having a core component of polypropylene and a sheath component of high density polyethylene and having no crimp. A nonwoven fabric was produced in the same manner as in Example 1. The nonwoven fabric was subjected to sulfonation treatment in the same manner as in Example 1 to obtain a battery separator having a sulfur content of 0.60% by mass, a thickness of 121 μm, and a basis weight of 53.6 g / m ² .

(Comparative Example 2)
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp ratio 10) %) Polyolefin core-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.3 dtex, fiber length of 5 mm, crimp number of 3, crimps) (Rate 5%) 14 parts by mass, and the core component is polypropylene, the sheath component is high-density polyethylene, and there is no crimped polyolefin core-sheath composite fiber (fineness 0.8 dtex, fiber length 5 mm) 16 parts by mass A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was subjected to graft polymerization treatment in the same manner as in Example 9, and the thickness was adjusted by supercalender treatment to obtain a battery separator having a graft ratio of 7.8%, a thickness of 120 μm, and a basis weight of 55.4 g / m ^2. It was.

(Comparative Example 3)
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp ratio 10) %) Polyolefin core-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.3 dtex, fiber length of 5 mm, crimp number of 3, crimps) 5% by mass) and 5 parts by mass of a core component of polypropylene, a sheath component of high-density polyethylene, and 10 parts by mass of a polyolefin-based core-sheath type composite fiber (fineness 0.3 dtex, fiber length 5 mm) having no crimp. A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was treated with fluorine gas in the same manner as in Example 10, and the thickness was adjusted by supercalendering to obtain a battery separator having a thickness of 119 μm and a basis weight of 53.2 g / m ² .

(Comparative Example 4)
Polyolefin core-sheath composite fiber having a core composition of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness 0.8 dtex, fiber length 5 mm, crimp number 5, crimp ratio 10) %) Polyolefin core-sheath composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene and having crimps (fineness of 0.15 dtex, fiber length of 3 mm, crimp number of 3, crimps) 5% by mass) and 5 parts by mass of a core component of polypropylene, a sheath component of high-density polyethylene, and 10 parts by mass of a polyolefin-based core-sheath type composite fiber (fineness 0.3 dtex, fiber length 5 mm) having no crimp. A nonwoven fabric was produced in the same manner as in Example 1 except that. The nonwoven fabric was treated with fluorine gas in the same manner as in Example 10, and the thickness was adjusted by supercalendering to obtain a battery separator having a thickness of 120 μm and a basis weight of 54.1 g / m ² .

  A method for measuring [sulfur content] in each example and comparative example will be described. A sample with a diameter of 35 mm was taken from the sulfonated battery separator, washed twice in 200 mL of ion exchange water for 10 minutes, and dried at 60 ° C. for 10 minutes to prepare a measurement sample. This sample was set in a holder, and all elemental measurements were performed with a fluorescent X-ray apparatus (device name: ZSX Primus II, Rh target, 50 kV-50 mA, manufactured by Rigaku Corporation). The sulfur content was calculated by calculating the measured value by SQX, which is a semi-quantitative analysis method, and the sulfonation treatment amount was estimated by mass%.

A method for measuring [graft rate] in each example and comparative example will be described. A sample piece of 200 mm × 250 mm was taken from the nonwoven fabric before the graft polymerization treatment, and the mass (P ₁ ) in a water equilibrium state was measured. Next, a sample piece of 200 mm × 250 mm was taken from the battery separator after the graft polymerization treatment, and the mass (P ₂ ) in a water equilibrium state was measured. The graft ratio was calculated from the following formula (1) and used as an index of the efficiency of the graft polymerization treatment.

Graft ratio (%) = (P ₂ −P ₁ ) / P ₁ × 100 (1)

<Evaluation>
The following evaluation was performed about the nonwoven fabric and battery separator obtained by the Example and the comparative example, and the result was shown in Table 1.

[Evaluation of surface cracks]
Evaluation of the surface cracking of the nonwoven fabric during heat drying in wet papermaking was performed as follows. Inspecting the rolled-up non-woven fabric of 1500 m, “◎” indicates that there is no surface crack in 1500 m, and “○” indicates that there are 1 to 3 surface cracks in a length of less than 50 mm in 1500 m. “△” indicates that there are 1 or more and 3 or less surface cracks in a length of 50 to 100 mm, and “x” indicates that there are 4 or more surface cracks in a length of 1500 m regardless of the length. Expressed in

[Evaluation of tensile strength and elongation at break]
Ten samples having a flow direction of 250 mm and a width direction of 50 mm were cut out from the nonwoven fabric before the sulfonation treatment in a water equilibrium state, and a tabletop material testing machine (apparatus name: STA-1150, ( Tensile strength and breaking elongation were measured, and the average value of 10 sheets was taken as the tensile strength and breaking elongation of the nonwoven fabric.

[Evaluation of liquid retention]
A 100 mm square test piece is taken from the battery separator whose thickness has been adjusted, and after measuring the mass (W ₁ ) in a water equilibrium state, it is immersed in an aqueous potassium hydroxide solution having a specific gravity of 1.3 for 30 minutes. Then, pulled up from aqueous potassium hydroxide solution, the mass after 10 minutes (W ₂₎ was measured to calculate the liquid retention ratio by the following equation (2).

Liquid retention (%) = (W ₂ −W ₁ ) / W ₁ × 100 (2)

[Battery fabrication and short rate evaluation]
As the electrode current collector, a paste type nickel hydroxide positive electrode (40 mm width) using a foamed nickel base material and a hydrogen storage alloy negative electrode (40 mm width) using a nickel-plated punching metal base material are used one by one. A battery separator obtained in the 43 mm-wide Example and Comparative Example was interposed between the electrodes, and wound up using a battery construction machine to produce a spiral electrode group. After the spiral electrode plate group is stored in a cylindrical metal case, a certain amount of alkaline electrolyte mainly composed of 7N potassium hydroxide aqueous solution containing 1N lithium hydroxide is injected, and then a sealing lid with a safety valve is attached. 2000 AA sealed nickel-metal hydride batteries each having a nominal capacity of 1.7 Ah were produced. Thereafter, a voltage of 240 V was applied between the positive electrode and the negative electrode, and those having an electric resistance exceeding 1 kΩ were regarded as normal, those having an electric resistance of 1 kΩ or less were regarded as defective, and the short-circuit rate at the time of battery production was determined. Less than 0.05% is “◎”, 0.05% or more and less than 0.10% is “◯”, 0.10% or more and less than 0.15% is “△”, and 0.15% or more is “×”. evaluated.

[Evaluation of self-discharge characteristics]
Of the batteries produced as described above, 10 normal batteries were selected for each battery separator. Charge and discharge 4 times at 25 ° C for 15 hours at a current of 170 mA (0.1 C) and discharge at 1.7 A (1 C) until the terminal voltage reaches 0.8 V for battery formation. Repeated. Using the obtained 10 formed batteries, charging at 25 ° C. with a current of 1.7 A (1 C), reaching full charge, and when the battery voltage drops by 10 mV, charging is suspended for 1 hour, then final voltage at a current of 340 mA (0.2 C) to the measured _{Q 1} the discharge capacity when discharged until 1.0 V. Similarly, the battery was charged with a current of 1.7 A (1 C), then stored in a thermostatic bath at 60 ° C. for 7 days, then allowed to cool at 25 ° C. for 6 hours, and similarly 340 mA (0.2 C) of and Q ₂ to measure the discharge capacity when discharged at a current was calculated capacity retention rate from the following equation (3). It shows that self-discharge characteristic is excellent, so that the value of a capacity | capacitance maintenance factor is large.

Capacity maintenance ratio (%) = Q ₂ / Q ₁ × 100 (3)

[Evaluation of battery life]
Of the batteries produced as described above, 10 normal batteries were selected for each battery separator. Charge and discharge 4 times at 25 ° C for 15 hours at a current of 170 mA (0.1 C) and discharge at 1.7 A (1 C) until the terminal voltage reaches 0.8 V for battery formation. Repeated. Using the obtained 10 converted batteries, the battery is charged with a current of 1.7 A (1 C) at 40 ° C. for 1.2 hours, and then the final voltage is 1.0 V with a current of 1.7 A (1 C). The battery life was evaluated by repeating the charge / discharge cycle of discharging until the battery was discharged. Less than 300 cycles are represented by “x”, 300 cycles or more and less than 500 cycles are represented by “Δ”, 500 cycles or more and less than 750 cycles are represented by “◯”, and 750 cycles or more are represented by “◎”.

  From the results of Examples 1 to 10, the fiber diameter exceeds 5 μm, the content of the crimped polyolefin-based core-sheath composite fiber exceeds 85 mass% of the fibers constituting the nonwoven fabric, and constitutes the nonwoven fabric. By making 25% by mass or more of the fibers to be processed exceed the fiber diameter of 5 μm and less than 8 μm, it is possible to suppress surface cracks in the heat drying of the nonwoven fabric during wet papermaking, and the separator obtained by hydrophilizing the obtained wet nonwoven fabric By using this, the short-circuit rate at the time of battery production can be reduced, the liquid retention is excellent, and the life can be extended. Among them, Examples 1 and 2 are excellent because the content of the polyolefin-based core-sheath composite fiber having crimps is in a more preferable range.

  When Example 4 is compared with Examples 7 and 8, Example 7 has a low number of crimps and a low crimp rate of the polyolefin core-sheath type composite fiber having crimps. Rate and liquid retention are slightly inferior. On the other hand, in Example 8, the number of crimps and the crimp rate of the polyolefin core-sheath composite fiber having crimps are high. Compared with Example 4, the formation at the time of papermaking is slightly worse, and the surface cracking and short circuit rate are lower. Somewhat inferior. In addition, comparing Example 3 with Examples 9 and 10, the capacity of Example 3 in which the hydrophilization treatment is sulfonation treatment is higher than that of Example 9 in which grafting treatment is performed and Example 10 in which fluorination treatment is performed. The rate is high and the battery life is also excellent. When Example 9 and Example 10 are compared, Example 9 which is a grafting process has a higher liquid retention rate and capacity retention rate and is superior.

  On the other hand, from the results of the comparative examples, when the conditions of the present invention were not satisfied, various performances were inferior. For example, in Comparative Example 1 consisting only of a polyolefin-based core-sheath type composite fiber having no crimp, surface cracks frequently occurred, the short-circuit rate was high, and the liquid retention rate was low. In Comparative Example 2, the content of the polyolefin-based core-sheath composite fiber having crimps, and the content of fibers exceeding 5 μm and less than 8 μm do not satisfy the conditions of the present invention. For example, compared with Example 9, The results were inferior in surface cracking, short-circuit rate, and liquid retention rate. In Comparative Examples 3 and 4, the fiber content of more than 5 μm and less than 8 μm does not satisfy the conditions of the present invention, and, for example, the surface cracking, short circuit rate, and battery life are inferior as compared with Example 10. Furthermore, in Comparative Example 4, the polyolefin core-sheath composite fiber having a crimp with a fiber diameter of 5 μm or less was contained, the fibers were twisted during preparation of the papermaking slurry, and the nonwoven fabric was deteriorated.

  Examples of utilization of the present invention include separators for alkaline secondary batteries such as nickel-cadmium secondary batteries and nickel-hydrogen secondary batteries. It can also be used for lithium ion battery separators, capacitor separators, and molten salt battery separators.

Claims (2)

  The content of the polyolefin core-sheath composite fiber having a fiber diameter exceeding 5 μm and having crimps exceeds 85 mass% of the fibers constituting the nonwoven fabric, and 25 mass% or more of the fibers constituting the nonwoven fabric are fibers. A battery separator characterized by hydrophilizing a wet nonwoven fabric having a diameter of more than 5 μm and less than 8 μm.   The battery separator according to claim 1, wherein the hydrophilic treatment of the nonwoven fabric is a sulfonation treatment.