JP2011210701A - Separator for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for batteries which is preferably applicable to alkaline secondary batteries such as nickel-cadmium batteries, nickel-zinc batteries, and nickel-hydrogen batteries, is manufactured with high nonwoven fabric manufacturing stability, and is excellent in fabric uniformity and self-discharge minimizing effect.SOLUTION: A separator for a battery which is produced by sulfonating a polyolefin nonwoven fabric that includes core-sheath-type composite fibers each comprising polypropylene as a core component and high-density polyethylene as a sheath component is characterized in that, in a DSC curve obtained by the differential scanning calorimetry of the core-sheath-type fibers and the separator, the ratio of the melting peak area (A) derived from the high-density polyethylene and appearing on the lower melting point side to the melting peak area (B) derived from the polypropylene and appearing on the higher melting point side (i.e., A/B) is 1.00 to 2.50 inclusive.

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. .

  Among these, the sulfonation treatment is a method of introducing sulfonic acid groups into the nonwoven fabric by fuming sulfuric acid or concentrated sulfuric acid (see, for example, Patent Documents 1 to 3). 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.

  Here, the reason why the self-discharge reaction of the battery is promoted is that the nitrogen-containing compound present as an impurity in the battery is decomposed, and the generated ammonia is oxidized on the positive electrode to become nitrate ion, and this nitrate ion moves, There has been proposed the theory that a repetitive reaction (shuttle mechanism) that is reduced to ammonia is generated to accelerate self-discharge. It is considered that by introducing into the battery a battery separator made of a nonwoven fabric subjected to sulfonation treatment, ammonia generated by the sulfonic acid group introduced into the fiber is captured and the self-discharge reaction of the battery is suppressed. .

  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 non-woven fabrics consisting only of polyolefin fibers. There is a problem that surface cracks are likely to occur, the formation is uneven, and the production stability is poor. 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 4). However, when a sulfonation treatment is applied to a nonwoven fabric that uses both polyolefin fibers and ethylene-vinyl alcohol copolymer fibers, the ethylene-vinyl alcohol copolymer fibers are preferentially sulfonated. However, there is a problem that the separator is easily deteriorated and the sustainability of the self-discharge suppressing effect is shortened.

JP 58-175256 A Japanese Patent Application Laid-Open No. 64-57568 JP-A-6-140018 JP 2002-134090 A

  The subject of this invention is related with the battery separator which performed the sulfonation process to the nonwoven fabric, It is providing the battery separator which was excellent in the manufacturing stability of a nonwoven fabric, and was excellent in formation uniformity and the self-discharge suppression effect. .

  As a result of intensive studies to solve this problem, the present inventors have determined that a core-sheath type composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component is added to a polyolefin-based nonwoven fabric to be sulfonated. The present inventors have found that a battery separator satisfying the above-mentioned problems can be provided by controlling the thermal behavior in differential scanning calorimetry of the core-sheath composite fiber.

  That is, the present invention provides a battery separator obtained by sulfonating a polyolefin-based non-woven fabric containing a core-sheath composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component. In the DSC curve obtained by analysis, the ratio (A / B) of the melting peak area (A) derived from the high-density polyethylene on the low melting point side to the melting peak area (B) derived from the polypropylene on the high melting point side is 1.00 or more. 2. A battery separator characterized by being not more than 2.50.

  Furthermore, it is preferable that the heat shrinkage ratio after the heat treatment at 130 ° C. for 30 minutes of the core-sheath type composite fiber is 8.0% or less. More preferably, it is a nonwoven fabric.

  In the DSC curve obtained by differential scanning calorimetry, the battery separator of the present invention has a melting peak area (A) derived from high-density polyethylene on the low melting side and a melting peak area (B) derived from polypropylene on the high melting point side. By subjecting a polyolefin nonwoven fabric containing a core-sheath type composite fiber having a core component of polypropylene having a ratio (A / B) of 1.00 to 2.50 to a core component and high-density polyethylene as a sheath component to a sulfonation treatment The production stability of the nonwoven fabric can be improved, the uniformity of formation as a separator can be ensured, the self-discharge of the battery can be suppressed, and the battery capacity can be maintained over a long period of time. Moreover, by making the heat shrinkage rate of the core-sheath type composite fiber by heat treatment at 130 ° C. for 30 minutes to 8.0% or less, the production stability of the nonwoven fabric and the uniformity of the formation as a separator are further improved. The polyolefin-based non-woven fabric can be made into a battery separator with even more excellent sheet strength such as tensile strength by making it a wet non-woven fabric produced by a drying process using a hot plate compression method represented by a Yankee dryer. it can.

  The battery separator in the present invention is obtained by sulfonating a polyolefin-based nonwoven fabric containing a core-sheath type composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component, and differential scanning of the core-sheath type composite fiber. In the DSC curve obtained by calorimetric analysis (DSC), the ratio (A / B) of the melting peak area (A) derived from the high melting point polyethylene on the low melting point side and the melting peak area (B) derived from the polypropylene on the high melting point side is It is 1.00 or more and 2.50 or less.

  Differential scanning calorimetric analysis of the core-sheath composite fiber using polypropylene as the core component and high-density polyethylene as the sheath component is performed according to JIS K 7121, and the area of the melting peak is determined according to JIS K 7122. In the differential scanning calorimetry, the melting peak derived from each of the core component and the sheath component may be single or may be a series of a plurality of overlapping peaks. In the case of a series of a plurality of overlapping peaks, the peak area is obtained as one melting peak, and the larger peak height is defined as the melting point of the core component and the sheath component.

  In the present invention, the melting peak derived from high-density polyethylene on the low melting point side in the DSC curve obtained by differential scanning calorimetry is an endothermic peak having the largest peak height in the range of 120 ° C. or higher and 140 ° C. or lower. The melting peak derived from polypropylene on the high melting point side is an endothermic peak having a peak with the largest peak height in the range of 150 ° C. or higher and 180 ° C. or lower.

  In the present invention, the melting peak area (A) derived from the high melting point polyethylene on the low melting point side and the melting peak derived from the polypropylene on the high melting point side obtained from the DSC curve obtained by differential scanning calorimetry of the core-sheath type conjugate fiber. The area (B) ratio (A / B) is in the range of 1.00 to 2.50, more preferably 1.20 to 2.20, and even more preferably 1.40 to 2.00. . When A / B is less than 1.00, the bonding strength between the fibers due to the high-density polyethylene which is a heat-sealing component is not sufficient, and the strength of the nonwoven fabric is lowered. Further, due to the decrease in density and crystallinity, sulfonation near the fiber surface is likely to proceed, and the content of sulfonic acid groups inside the fiber is relatively decreased, so that the self-discharge suppressing effect is likely to decrease. On the other hand, when A / B exceeds 2.50, the shrinkage ratio of the fibers increases, and cracks and wrinkles are likely to occur during the production of the nonwoven fabric, resulting in reduced uniformity and manufacturing stability. Moreover, when density and crystallinity become high and sulfonation becomes difficult to proceed, it is considered that the content of sulfonic acid groups decreases and the self-discharge suppressing effect decreases.

  In the present invention, the molecular weight, the density, the crystallinity, the component ratio of the core component and the sheath component, the draw ratio of the core-sheath conjugate fiber, the polypropylene as the core component of the core-sheath conjugate fiber and the high-density polyethylene as the sheath component. By appropriately changing, the thermal behavior in the differential scanning calorimetry can be controlled. For example, when the density of polypropylene or high-density polyethylene is increased, the heat of fusion increases, and the melting peak area in the differential scanning calorimetry analysis also increases. Also, increasing the degree of crystallinity generally increases the amount of heat of fusion and increases the melting peak area, so A / B can be changed even if the constituent ratio of the core component and the sheath component is the same. .

  The content ratio of the core component and the sheath component constituting the core-sheath composite fiber may be any ratio that can form the core-sheath composite fiber, but the core / sheath mass ratio is 40/60 to 60/40. More preferably, it is 45/55 to 55/45.

In the present invention, the core-sheath type composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component has a heat shrinkage ratio of preferably not more than 8.0% after 130 ° C. for 30 minutes, 7.0% The following is more preferable, and 6.0% or less is still more preferable. Here, the heat shrinkage rate is calculated by the following method. That is, the core-sheath composite fiber conditioned at a temperature of 23 ° C. and a humidity of 50% for 24 hours is put into a glass capillary tube, photographed with a digital microscope, and the fiber length (L ₁ ) before heating is measured. Next, after heating at 130 ° C. for 30 minutes and allowing to cool for 1 hour at a temperature of 23 ° C. and a humidity of 50%, the fiber length (L ₂ ) after heating is measured in the same manner as before heating. The ratio of the difference in fiber length before and after heating (L ₁ -L ₂ ) to the fiber length before heating (L ₁ ), expressed as a percentage, is defined as the thermal shrinkage rate (%).

  The core-sheath type composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component used in the present invention is melt-spun using a melt-spinning machine and using a core-sheath type composite spinning die. The spinning temperature is a temperature at which the high density polyethylene as the sheath component does not change, and the polymer is extruded at a spinning temperature of 200 ° C. or higher and 300 ° C. or lower to produce a spinning filament having a predetermined fineness. The spinning filament is subjected to a stretching treatment as necessary. The stretching treatment is performed at a temperature at which the high-density polyethylene as the sheath component is not fused. For example, when the stretching temperature is 50 ° C. or more and 100 ° C. or less and the treatment is performed at a draw ratio of 2 times or more, the fiber strength is preferably improved. . The obtained filament is provided with a fiber treatment agent as necessary, and after controlling the hydrophilicity and dispersibility, it is cut into a predetermined length and used as a core-sheath type composite fiber for producing a nonwoven fabric.

  Polypropylene is used as the core component constituting the core-sheath composite fiber, but polyolefin such as high-density polyethylene and polymethylpentene can be mixed as necessary in order to adjust fiber properties. The mixing ratio of the polyolefin is preferably 10% by mass or less of the core component. Moreover, the resin additive used for normal polyolefin can be added as needed. Examples of the resin additive include various antioxidants, neutralizers, light stabilizers, ultraviolet absorbers, nucleating agents, lubricants, antistatic agents, and the like. It is used in the range of 0.01% by mass or more and 1.0% by mass or less.

  Next, as the sheath component constituting the core-sheath type composite fiber, high-density polyethylene is used, but in order to adjust the fiber physical properties, polyolefin such as polypropylene or ethylene-propylene copolymer is mixed as necessary. be able to. The mixing ratio of the polyolefin is preferably 10% by mass or less of the sheath component. Moreover, the resin additive used for normal polyolefin can be added as needed. Examples of the resin additive include various antioxidants, neutralizers, light stabilizers, ultraviolet absorbers, nucleating agents, lubricants, antistatic agents, and the like. It is used in the range of 0.01% by mass or more and 1.0% by mass or less.

  The polyolefin-based nonwoven fabric used in the present invention contains at least a core-sheath type composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component. Examples of polyolefin fibers that can be used in combination with the core-sheath type composite fibers include fibers composed of a single component such as polyethylene fibers and polypropylene fibers, mixed polyolefin fibers composed of a mixture of two or more different polyolefins, 2 Examples thereof include a core-sheath type, a side-by-side type, an eccentric type, and a splittable composite fiber, which are appropriately combined with a copolymer polyolefin fiber composed of copolymers of different types of olefins, polyethylene, polypropylene, copolymer polyolefin, and the like. . The fiber length and fiber diameter of these polyolefin fibers are not particularly limited, but the fiber diameter is preferably 1 μm or more and 20 μm or less, and the fiber length is preferably 1 mm or more and 20 mm or less from the strength of the nonwoven fabric and manufacturability. Further, the splittable composite fiber can be used after being subdivided by hydroentanglement or 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, a polyolefin-based fiber is a single resin or copolymer resin obtained by polymerizing one or more monomers having one or more double bonds in the molecule and carbon and hydrogen as constituent elements. A single resin or copolymer resin obtained by polymerizing monomers containing constituent elements other than carbon and hydrogen, such as polyvinyl alcohol fiber and ethylene-vinyl alcohol copolymer fiber This does not include fibers that have been melt-spun.

  In the polyolefin-based nonwoven fabric used for the battery separator of the present invention, examples of the fibers that can be suitably used in combination with the polyolefin-based fibers include semi-aromatic polyamide fibers and wholly aromatic polyamide fibers, particularly semi-aromatic polyamides. Fiber is more preferred. The semi-aromatic polyamide fiber is a fiber made of a semi-aromatic polyamide obtained by condensing an aromatic dicarboxylic acid and an aliphatic diamine as a main component, and 60 mol% or more of the dicarboxylic acid component is an aromatic carboxylic acid, 60 mol% or more of the diamine component is an aliphatic alkylenediamine having 6 to 12 carbon atoms. Aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,4-phenylenedioxydiacetic acid, 1,3 -Phenylenedioxyacetic acid, 2,2'-biphenyldicarboxylic acid, 2,4'-biphenyldicarboxylic acid, 4,4'-biphenyldicarboxylic acid, 4,4'-oxydibenzoic acid, diphenylmethane-4,4'-dicarboxylic Acid, diphenylsulfone-4,4'-dicarboxylic acid is used. Examples of the aliphatic diamine include 1,6-hexanediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine, 1,11-undecanediamine, 1,12-dodecanediamine, and 2-methyl. -1,5-pentanediamine, 3-methyl-1,5-pentanediamine, 2,2,4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine, 2 -Methyl-1,8-octanediamine, 5-methyl-1,9-nonanediamine and the like.

  The content of the core-sheath type composite fiber having polypropylene as the core component and high-density polyethylene as the sheath component in the battery separator of the present invention is in the range of 40% by mass to 100% by mass with respect to the total fibers. More preferably, it is 60 mass% or more and 100 mass% or less, More preferably, it is the range of 80 mass% or more and 100 mass% or less.

  As a manufacturing method of the nonwoven fabric used for the battery separator of the present invention, any general nonwoven fabric manufacturing method can be used. By forming a fiber web and adhering, fusing, and intertwining the fibers in the fiber web Can be manufactured. Examples of the method for producing the fiber web include a wet papermaking method, a dry method such as a card method and an airlaid method. However, dry methods such as the card method and the airlaid method can use fibers having a long fiber length, but it is difficult to form a uniform fiber web, and the formation is generally inferior to the wet papermaking method. There is.

  On the other hand, the wet papermaking method has an advantage that the production rate is higher than that of the dry method, and fibers having different fiber diameters or a plurality of types of fibers can be uniformly mixed at an arbitrary ratio in the same apparatus. In other words, there are a wide selection of fiber forms such as staple and pulp, and the usable fiber diameter can be used from ultrafine fibers to thick fibers. It is done. For these reasons, the nonwoven fabric used for the battery separator of the present invention is preferably a wet nonwoven fabric obtained by a wet papermaking method.

  As a method for producing a nonwoven fabric from a fibrous web, a hydroentanglement method, a needle punch method, a binder adhesion method, or the like can be used. In particular, when the wet papermaking method is used with an emphasis on uniformity, it is preferable to add heat-bonded fibers to the nonwoven fabric and bond them by a binder bonding method, whereby a uniform nonwoven fabric is formed.

  In addition, in the wet papermaking method, when forming a non-woven fabric by a binder bonding method by heat fusing of heat fusing fibers, a process of causing heat fusing at the same time as heat drying of the wet fiber web is used. Examples thereof include a hot plate pressure bonding method typified by a Yankee dryer, a band-type through dryer, and a hot air ventilation method typified by an air through dryer. In the present invention, heat drying by a hot plate pressure bonding method is more preferable. In the hot plate compression method, a core-sheath composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component has a high heat fusion efficiency, a high uniformity of formation, and a nonwoven fabric with improved strength can be obtained. it can.

  In the battery separator of the present invention, a polyolefin nonwoven fabric is subjected to sulfonation treatment, and sulfonic acid groups are introduced into the inside from the surface of the fiber. As the sulfonation treatment, a gas phase treatment method using sulfur dioxide gas, sulfur trioxide gas, or the like, a liquid phase treatment method using hot concentrated sulfuric acid, fuming sulfuric acid, chlorosulfuric acid, or the like can be used. In the battery separator of the present invention, sulfonation treatment by a gas phase treatment method is 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, it is preferable to add a surfactant to the nonwoven fabric after the sulfonation treatment in order to further improve the affinity with the electrolytic solution. As the surfactant used, alkyl sulfate ester salt, polyoxyethylene alkyl ether sulfate ester salt, alkylbenzene sulfonate, dialkyl sulfosuccinate, alkyl diphenyl ether disulfonate, alkane sulfonate, long chain fatty acid salt, β- Sodium salt of naphthalenesulfonic acid formalin condensate, sodium salt of special aromatic sulfonic acid formalin condensate, anionic surfactant such as special polycarboxylic acid type polymer surfactant, polyoxyethylene alkyl ethers, 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, more preferably 5 μm to 40 μm, and still more preferably 10 μm to 35 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. If the maximum pore diameter is less than 1 μm, oxygen gas permeability and ionic conductivity may be reduced. 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
The ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 1.03, and the heat shrinkage rate is 6.5. %, 100 parts by mass of a core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) whose core component is polypropylene and whose sheath component is high-density polyethylene Then, the mixture was disaggregated and dispersed, and gently stirred with an agitator to prepare a uniform papermaking slurry. This slurry for paper making is made by a wet paper making method using a circular paper machine, and dried by a hot air hood attached to a Yankee dryer that is a hot plate pressure bonding method set at 135 ° C. A non-woven fabric having a width of 500 mm was produced by hot-melt bonding the sheath portion of the above. This non-woven fabric was sulfonated for 25 seconds in 75 ° C. dry air containing sulfur trioxide gas, neutralized with a 2.5 mass% sodium hydroxide aqueous solution, thoroughly washed with ion-exchanged water, As a surfactant, sodium alkyldiphenyl ether disulfonate is applied by spraying to 0.3% by mass with respect to the non-woven fabric after the sulfonation treatment, and after drying, the thickness is adjusted to 120 μm with a super calendar, and the basis weight is obtained. A battery separator of 53.6 g / m ² was obtained.

(Example 2)
In Example 1, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 1.23, and the heat shrinkage A core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) in which the rate is 6.2%, the core component is polypropylene, and the sheath component is high-density polyethylene was used. Except for the above, a battery separator having a basis weight of 53.8 g / m ² was obtained in the same manner as in Example 1.

(Example 3)
In Example 1, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 1.52, and the heat shrinkage A core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) having a rate of 5.8%, a core component of polypropylene, and a sheath component of high-density polyethylene was used. Except for the above, a battery separator having a basis weight of 54.0 g / m ² was obtained in the same manner as in Example 1.

Example 4
In Example 1, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 1.68, and the heat shrinkage A core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) having a rate of 4.7%, a core component of polypropylene, and a sheath component of high-density polyethylene was used. Except for the above, a battery separator having a basis weight of 53.8 g / m ² was obtained in the same manner as in Example 1.

(Example 5)
In Example 1, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 1.95, and heat shrinkage A core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) in which the rate is 6.2%, the core component is polypropylene, and the sheath component is high-density polyethylene was used. Except for the above, a battery separator having a basis weight of 54.2 g / m ² was obtained in the same manner as in Example 1.

(Example 6)
In Example 1, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 2.17, and the heat shrinkage A core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) having a rate of 7.6%, a core component of polypropylene, and a sheath component of high-density polyethylene was used. Except for the above, a battery separator having a basis weight of 53.6 g / m ² was obtained in the same manner as in Example 1.

(Example 7)
In Example 1, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 2.45, and the heat shrinkage A core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) having a rate of 7.9%, a core component of polypropylene, and a sheath component of high-density polyethylene was used. Except for the above, a battery separator having a basis weight of 53.4 g / m ² was obtained in the same manner as in Example 1.

(Example 8)
In Example 3, the drying method at the time of paper making is the same as in Example 3 except that an air-through dryer set at 135 ° C. is used instead of the hot air hood attached to the Yankee dryer that is a hot plate pressure bonding method. A battery separator of 53.7 g / m ² was obtained.

Example 9
In Example 3, the drying method at the time of papermaking was the same as in Example 3 except that a band-type through dryer set at 135 ° C. was used instead of the hot air hood attached to the Yankee dryer that is a hot plate pressure bonding method. A battery separator having a basis weight of 54.1 g / m ² was obtained.

(Example 10)
The ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 1.65, and the thermal contraction rate is 5.8. %, A core component is polypropylene, and a sheath component is a high density polyethylene, a core-sheath composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) 70 parts by mass and polypropylene 30 parts by mass of one fiber (fineness 0.8 dtex, fiber length 5 mm) was mixed, disaggregated and dispersed in water of a pulper, and gently stirred with an agitator to prepare a uniform papermaking slurry. This slurry for paper making is made by a wet paper making method using a circular paper machine, and dried by a hot air hood attached to a Yankee dryer that is a hot plate pressure bonding method set at 135 ° C. A non-woven fabric having a width of 500 mm was produced by hot-melt bonding the sheath portion of the above. This non-woven fabric is sulfonated for 25 seconds in 75 ° C. dry air containing sulfur trioxide gas, neutralized with a 2.5 mass% sodium hydroxide aqueous solution, thoroughly washed with ion-exchanged water, and then interfaced. As an activator, polyoxyethylene lauryl ether sodium sulfate is spray-applied so as to be 0.3% by mass with respect to the non-woven fabric after sulfonation treatment, and after drying, the thickness is adjusted to 120 μm with a super calendar, A battery separator having a basis weight of 53.9 g / m ² was obtained.

(Example 11)
In Example 10, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry was 1.70, and the heat shrinkage The core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) whose ratio is 6.5%, the core component is polypropylene, and the sheath component is high-density polyethylene is 70 parts by mass. A battery separator having a basis weight of 54.2 g / m ² was obtained in the same manner as in Example 10 except that 30 parts by mass of a single fiber (fineness 0.8 dtex, fiber length 5 mm) made of polypropylene was used.

(Example 12)
In Example 10, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry was 1.85, and the heat shrinkage The core-sheath composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) 70% by mass with a ratio of 7.5%, the core component being polypropylene, and the sheath component being high-density polyethylene A battery separator having a basis weight of 53.6 g / m ² was obtained in the same manner as in Example 10, except that 30 parts by mass of single fiber (fineness 0.8 dtex, fiber length 5 mm) made of polypropylene was used.

(Example 13)
The ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 2.06, and the thermal contraction rate is 7.8. A core-sheath type composite fiber (core / sheath mass ratio 52/48, fineness 1.7 dtex, fiber length 10 mm) having a core component of polypropylene and a sheath component of high-density polyethylene, and a single unit comprising polypropylene. 20 parts by mass of one fiber (fineness 0.8 dtex, fiber length 5 mm) and 20 parts by mass of ultrafine fiber (fineness 0.08 dtex, fiber length 5 mm) made of a mixture of polypropylene and ethylene-propylene copolymer were mixed, Dispersed and dispersed in water, and gently stirred with an agitator to prepare a uniform papermaking slurry. This slurry for paper making is made by a wet paper making method using a circular paper machine, and dried by a hot air hood attached to a Yankee dryer that is a hot plate pressure bonding method set at 135 ° C. A non-woven fabric having a width of 500 mm was produced by hot-melt bonding the sheath portion of the above. This non-woven fabric was sulfonated for 25 seconds in 75 ° C. dry air containing sulfur trioxide gas, neutralized with a 2.5 mass% sodium hydroxide aqueous solution, thoroughly washed with ion-exchanged water, As a surfactant, polyoxyethylene dodecyl phenyl ether is applied by spraying to 0.3% by mass with respect to the non-woven fabric after the sulfonation treatment, and after drying, the thickness is adjusted to 120 μm with a super calender, A battery separator having a basis weight of 54.1 g / m ² was obtained.

(Example 14)
In Example 13, the basis weight is the same as in Example 13 except that an air-through dryer set at 135 ° C. is used in place of the hot air hood attached to the Yankee dryer that is a hot plate crimping method as the drying method during paper making. A battery separator of 53.8 g / m ² was obtained.

(Example 15)
In Example 13, the drying method at the time of papermaking was the same as in Example 13 except that a band-type through dryer set at 135 ° C. was used instead of the hot air hood attached to the Yankee dryer that is a hot plate pressure bonding method. A battery separator having a basis weight of 53.6 g / m ² was obtained.

(Example 16)
In Example 13, instead of the wet papermaking method, a web was prepared by an airlaid method, and a band-type through dryer set at 135 ° C. was used to heat-melt and bond the sheath component of the core-sheath type composite fiber, and the width was 500 mm. A battery separator having a weight per unit area of 54.3 g / m ² was obtained in the same manner as in Example 13 except that a non-woven fabric was prepared.

(Example 17)
In Example 16, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry was 2.36, and the heat shrinkage The core-sheath type composite fiber (core / sheath mass ratio 52/48, fineness 1.7 dtex, fiber length 10 mm) whose ratio is 8.3%, the core component is polypropylene, and the sheath component is high-density polyethylene 60 parts by mass 20 parts by mass of a single fiber (fineness 0.8 dtex, fiber length 5 mm) made of polypropylene and 20 parts by mass of an ultrafine fiber (fineness 0.08 dtex, fiber length 5 mm) made of a mixture of polypropylene and ethylene-propylene copolymer A battery separator having a basis weight of 54.5 g / m ² was obtained in the same manner as in Example 16 except that it was used.

(Example 18)
In Example 10, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry was 1.75, and the heat shrinkage The core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 1.7 dtex, fiber length 10 mm) 85% by mass is 6.2%, the core component is polypropylene, and the sheath component is high-density polyethylene In addition, the basis weight was 53.8 g / m in the same manner as in Example 10 except that the amount was 15 parts by mass of semi-aromatic polyamide fiber (fineness 0.7 dtex, fiber length 5 mm) using 1,9-nonanediamine and terephthalic acid as monomers. ² battery separators were obtained.

(Example 19)
In Example 10, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 1.35, and the heat shrinkage The core-sheath type composite fiber (core / sheath mass ratio 40/60, fineness 1.7 dtex, fiber length 10 mm) whose ratio is 7.3%, the core component is polypropylene, and the sheath component is high-density polyethylene 85 parts by mass In addition, the basis weight was 53.7 g / m in the same manner as in Example 10 except that 15 parts by mass of semi-aromatic polyamide fiber having a monomer of 1,9-nonanediamine and terephthalic acid (fineness 0.7 dtex, fiber length 5 mm) was used. ² battery separators were obtained.

(Example 20)
In Example 10, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 2.12, and the heat shrinkage The core-sheath type composite fiber (core / sheath mass ratio 60/40, fineness 1.7 dtex, fiber length 10 mm) having a rate of 6.8%, the core component being polypropylene, and the sheath component being high-density polyethylene 85 parts by mass In addition, the basis weight was 53.9 g / m in the same manner as in Example 10, except that the amount was 15 parts by mass of semi-aromatic polyamide fiber (fineness 0.7 dtex, fiber length 5 mm) using 1,9-nonanediamine and terephthalic acid as monomers. ² battery separators were obtained.

(Comparative Example 1)
In Example 1, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry is 2.55, and the heat shrinkage 100% by mass of a core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) whose ratio is 8.5%, the core component is polypropylene, and the sheath component is high-density polyethylene A battery separator having a basis weight of 53.7 g / m ² was obtained in the same manner as in Example 1 except that was used.

(Comparative Example 2)
In Example 10, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry was 2.65, and the heat shrinkage The core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) whose ratio is 9.5%, the core component is polypropylene, and the sheath component is high-density polyethylene 70 parts by mass A battery separator having a basis weight of 54.6 g / m ² was obtained in the same manner as in Example 10 except that 30 parts by mass of a single fiber (fineness 0.8 dtex, fiber length 5 mm) made of polypropylene was used.

(Comparative Example 3)
In Example 10, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry was 2.55, and the heat shrinkage The core-sheath type composite fiber (core / sheath mass ratio 50/50, fineness 0.8 dtex, fiber length 5 mm) 70% by mass with a ratio of 8.5%, the core component being polypropylene, and the sheath component being high-density polyethylene A battery separator having a basis weight of 54.2 g / m ² was obtained in the same manner as in Example 10 except that 30 parts by mass of ethylene-vinyl alcohol copolymer fiber (fineness 0.8 dtex, fiber length 5 mm) was used.

(Comparative Example 4)
In Example 16, the ratio (A / B) of the melting peak area (A) on the low melting point side to the melting peak area (B) on the high melting point side in the DSC curve of differential scanning calorimetry was 0.94, and the heat shrinkage 60% by mass of a core-sheath type composite fiber (core / sheath mass ratio 52/48, fineness 1.7 dtex, fiber length 10 mm) in which the rate is 7.2%, the core component is polypropylene, and the sheath component is high-density polyethylene And Example 16 except that 20 parts by mass of single fiber (fineness 0.8 dtex, fiber length 5 mm) made of polypropylene and 20 parts by mass of ultrafine single fiber (fineness 0.08 dtex, fiber length 5 mm) made of polypropylene were used. Similarly, a battery separator having a basis weight of 53.9 g / m ² was obtained.

<Evaluation>
The battery separators obtained in Examples and Comparative Examples were evaluated as follows, and the results are shown in Table 1.

[Evaluation of peak area ratio (A / B)]
A core-sheath composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component was washed with ethanol, dried at 80 ° C. for 30 minutes, and then 10 mg of a sample conditioned at 23 ° C. and 50% humidity for 24 hours. The sample is sealed in an Al sample container, subjected to differential scanning calorimetry specified in JIS K 7121, a DSC curve is obtained, and the melting peak area on the low melting point side derived from high-density polyethylene is obtained by the method specified in JIS K 7122. The melting peak area (B) on the high melting point side derived from (A) and polypropylene was calculated, and the peak area ratio (A / B) was obtained from the following formula (1).
Peak area ratio (A / B) = melting peak area (A) / melting peak area (B) (1)

[Evaluation of heat shrinkage]
A sample prepared by conditioning a core-sheath type composite fiber having a core component of polypropylene and a sheath component of high-density polyethylene at a temperature of 23 ° C. and a humidity of 50% for 24 hours is placed in a glass capillary tube, and the length of the fiber before heating. (L ₁ ) was photographed with a digital microscope (product name: VHX-900, manufactured by Keyence Corporation) and measured in a distance measurement mode. Next, after heating at 130 ° C. for 30 minutes and allowing to cool at 23 ° C. and 50% humidity for 1 hour, the length (L ₂ ) of the fiber after heating was measured in the same manner as before heating. The thermal contraction rate was obtained by the following formula (2).
Thermal contraction rate (%) = (L ₁ −L ₂ ) / L ₁ × 100 (2)

[Evaluation of nonwoven fabric production stability]
The production stability in the production of nonwoven fabric was evaluated according to the following criteria. When manufacturing a non-woven fabric 5000m, it can be produced without causing any problems such as sheet breakage, cracking, grain, shading unevenness, curly wrinkles, etc. The case where the above-mentioned minor trouble frequently occurs is indicated by △, and the case where a large trouble such as sheet breakage or crack formation frequently occurs is indicated by “X”.

[Evaluation of separator uniformity]
A 500 mm square sheet is cut out from the battery separator whose thickness has been adjusted, and 100 sheets of 50 mm square air permeability measurement samples are produced from the sheet, and conform to the air permeability A method (Fragile method) prescribed in JIS L 1096. Then, the air permeability was measured with a breathability tester (device name: KES-F8-AP1, manufactured by Kato Tech Co., Ltd.), and the average value (Q ₁ ) and standard deviation (Q ₂ ) of the air permeability of 100 samples. And the coefficient of variation was determined from the following equation (3). The smaller the coefficient of variation, the higher the uniformity of the separator formation.
Coefficient of variation (%) = standard deviation of air permeability (Q ₂ ) / average value of air permeability (Q ₁ ) × 100 (3)

[Evaluation of tensile strength]
Ten samples having a winding direction of 250 mm and a width direction of 50 mm were cut from the battery-adjusted separator, and a desktop material testing machine (apparatus name: STA-1150, Oriente Co., Ltd.) according to JIS P8113. The tensile strength of the separator was measured, and the average value of 10 sheets was taken as the tensile strength of the separator.

[Evaluation of maximum pore size]
For the battery separator, the maximum pore diameter was determined by the bubble point method specified in JIS K3832.

[Evaluation of sulfur content]
A sample having a diameter of 35 mm was collected from the battery separator subjected to the sulfonation treatment, washed twice in 200 mL of ion-exchanged 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%.

[Production of battery]
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. AA type sealed nickel metal hydride batteries 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.

[Evaluation of capacity maintenance rate]
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 to measure the discharge capacity when end voltage was discharged until 1.0V with a current of 340 mA (0.2 C), and _{C 1.} 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 a C ₂ to measure the discharge capacity when discharged at a current was calculated capacity retention rate from the following equation (4). It shows that self-discharge characteristic is excellent, so that the value of a capacity | capacitance maintenance factor is large.
Capacity maintenance rate (%) = C ₂ / C ₁ × 100 (4)

  In the DSC curves obtained by differential scanning calorimetry, the battery separators shown in Examples 1 to 20 have melting peak areas (A) derived from low-melting-side high-density polyethylene and melting derived from high-melting-side polypropylene. The ratio of peak area (B) (A / B) is 1.00 or more and 2.50 or less, and sulfone is added to a polyolefin-based nonwoven fabric containing core-sheath composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component. The non-woven fabric has good production stability and is excellent in separator uniformity and self-discharge suppression effect.

  When Examples 1 to 7 are compared, Examples 2 to 6 have a melting peak area ratio (A / B) in the range of 1.20 or more and 2.20 or less and exhibit good characteristics. In Nos. 3 to 5, the melting peak area ratio is in the range of 1.40 or more and 2.00 or less, and the manufacturing stability of the nonwoven fabric, the separator uniformity, and the capacity retention rate are further excellent. In Examples 3 and 4, the heat shrinkage of the core-sheath fiber is 6.0% or less, and the maximum pore diameter is small and particularly excellent.

  When Example 3 and Example 8 and Example 9, Example 13 and Example 14 and Example 15 are compared, Example 3 and Example 13 are the wet nonwoven fabrics heat-dried by the hot-plate press bonding system, and the same Even if it is a wet nonwoven fabric, it is superior in separator uniformity, strong tensile strength, and maximum pore diameter from Examples 8 and 14 using an air-through dryer and Examples 9 and 15 using a band-type through dryer. Also small and excellent.

  Comparing Examples 13 to 17, Example 16 and Example 17 using a dry nonwoven fabric compared to Examples 13 to 15 using a wet nonwoven fabric, separator uniformity, tensile strength, maximum pore diameter, capacity maintenance. The result is somewhat inferior. In addition, in Example 17, the heat-shrinkage rate of the core-sheath composite fiber exceeded 8.0%, and the separator uniformity, tensile strength, and maximum pore diameter were slightly inferior to those in Example 16. .

  On the other hand, the battery separator obtained in the comparative example does not satisfy the conditions of the present invention. For example, Comparative Example 1 and Comparative Example 2 are significantly inferior in manufacturing stability and separator uniformity of the nonwoven fabric. In Comparative Example 3, an ethylene-vinyl alcohol copolymer is used in combination, and the capacity retention rate of the battery is significantly reduced. In Comparative Example 4, compared with Examples 16 and 17, the separator uniformity was inferior, and in particular, the tensile strength was greatly reduced.

  As an application example of the present invention, an alkali capable of maintaining the battery capacity over a long period of time with good manufacturing stability of the nonwoven fabric, good uniformity of formation as a separator, suppressing self-discharge of the battery It can be suitably used as a separator for a secondary battery.

Claims (3)

  A DSC obtained by differential scanning calorimetry of a core-sheath composite fiber in a battery separator obtained by sulfonating a polyolefin-based nonwoven fabric containing a core-sheath composite fiber containing polypropylene as a core component and high-density polyethylene as a sheath component The ratio (A / B) of the melting peak area (A) derived from the high-density polyethylene on the low melting point side to the melting peak area (B) derived from the polypropylene on the high melting point side in the curve is from 1.00 to 2.50. There is a battery separator characterized by that.   The battery separator according to claim 1, wherein the heat-shrinkage ratio of the core-sheath composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component after heat treatment at 130 ° C is 8.0% or less.   The battery separator according to claim 1 or 2, wherein the polyolefin-based nonwoven fabric containing a core-sheath type composite fiber having polypropylene as a core component and high-density polyethylene as a sheath component is a wet nonwoven fabric dried by a hot plate compression method.