Separator for lead acid batteries

Abstract

[Problem] The present invention addresses the problem of providing a separator for lead acid batteries, the separator being composed of a microporous film that is mainly composed of a polyolefin-based resin, an inorganic powder, and a mineral oil, wherein penetration of dendrite is suppressed by the whole separator including a base part by means of a manufacturing process that has excellent economic efficiency, variation in short-circuit resistance performance is reduced, and short-circuit time is improved.　The present invention also addresses the problem of providing a separator for lead acid batteries, the separator being capable of suppressing corrosion in a lead acid battery even in cases where the separator is used in the lead acid battery in an overcharged state at high temperatures, and the separator being most suitable for a vehicle with start-stop system. [Solution] The present invention provides a separator for lead acid batteries, the separator being composed of a microporous film that is mainly composed of a polyolefin-based resin, an inorganic powder, and a mineral oil. This separator for lead acid batteries is characterized in that: the ash content is 65% or less; the short-circuit time is improved to be 110% or more with respect to a conventional separator that contains the same amount of the mineral oil; and the amount of a reducing substance that is released / dissolved after being electrolyzed at a DC current of 1.2 A and about 25°C for 24 hours with use of an electrolysis cell that is composed of the microporous film, a positive electrode plate, a negative electrode plate, and dilute sulfuric acid is 1.6 mL / 100 cm2 or less in terms of the consumption amount of N / 100 potassium permanganate per one 100 cm2 porous film.

Description

Lead-acid battery separator

[0001] The present invention relates to a separator for a lead-acid battery comprising a microporous membrane.

[0002] Conventionally, separators for lead-acid batteries have been made of polyethylene separators, which are usually made of 20 to 60% by weight of polyolefin resin (usually ultra-high molecular weight polyethylene) with a weight-average molecular weight of 500,000 or more and a specific surface area of ​​50 m. ２ / g or more of inorganic powder (usually silica powder), 0 to 30% by weight of mineral oil as a plasticizer that also functions as a pore-opening agent, 0 to 10% by weight of surfactant (solid content), and 0 to 5% by weight of additives (antioxidants, weather resistance agents, etc.).

[0003] The microporous membrane separator is typically obtained by extruding a raw material composition, which is a mixture of the polyolefin resin, the inorganic powder, the mineral oil (blended in a larger amount than in the separator composition), the surfactant, and the additives, while heating, melting, and kneading the material composition into a sheet, rolling the sheet to a predetermined thickness, and then extracting and removing all or a portion of the plasticizer. The microporous membrane separator has a base thickness of about 0.1 to 0.3 mm, an average pore diameter (mercury porosimetry) of about 0.01 to 0.5 μm, and a porosity (mercury porosimetry) of about 50 to 90% by volume.

[0004] In recent years, a new problem has emerged in lead-acid batteries using microporous membrane separators: osmotic shorting (dendrite shorting). This phenomenon occurs when dendrites (branching deposits of lead) penetrate the separator thickness, connecting the positive and negative electrodes and resulting in a short circuit. Osmotic shorting is accelerated by overdischarge of the battery, which causes the sulfuric acid electrolyte to approach neutrality. This phenomenon is due to the unique battery usage environments of recent automobiles, such as charge-controlled vehicles and idle-stop vehicles. Batteries are typically never fully charged, but are constantly charged and discharged, making them prone to a discharged state. This makes it difficult to expect the electrolyte stirring effect caused by gassing during overcharging, a common feature in conventional automotive batteries (lead-acid batteries), and electrolyte stratification (a phenomenon in which high-density sulfuric acid released from the electrodes during charging migrates to the bottom of the battery, resulting in electrolyte concentration differences between the top and bottom of the battery) more likely to occur. Furthermore, since the concentration of the electrolyte is low in the upper part of the battery where stratification has occurred, the generation and growth of dendrites is promoted under a discharge-like usage environment, and it is believed that even in separators made of the above-mentioned microporous membranes with an average pore diameter of about 0.01 to 0.5 μm, dendrite shorts that penetrate the separator occur.

[0005] Patent Document 1 proposes that in a separator for a flooded lead-acid battery made of a ribbed microporous film having main ribs for contacting electrode plates protruding from one side of a flat sheet, when the ribbed microporous film is divided into a rib portion and a base portion, by reducing the porosity, average pore diameter, and maximum pore diameter of the rib portion to 0.8 times or less than those of the base portion, or by reducing the surface opening ratio to 0.8 or less, it becomes difficult for dendrites to penetrate the main ribs for contacting electrode plates, and as a result, it is possible to suppress the occurrence of dendrite shorts in separators made of ribbed microporous film.

[0006] However, the technology of Patent Document 1 is primarily focused on preventing dendrite penetration through the rib portion, and it is necessary to densify or reduce the pore structure of only a small portion of the entire separator (total area) to make it difficult for dendrites to penetrate, which complicates the manufacturing process and increases the difficulty of manufacturing. Also, while this is effective in preventing dendrite penetration through the rib portion, it is not effective in cases where dendrites penetrate through the base portion and grow by creeping along the outside of the rib, or in cases where the base portion is in direct contact with the electrode plate due to the low height of the rib.

[0007] Furthermore, Patent Document 2 proposes that in a lead-acid battery separator made of a microporous film mainly composed of a polyolefin resin, silica powder, and a plasticizer, glass flakes having an average particle size of 20 to 800 μm and an average thickness of 0.2 to 8 μm and not having self-film-forming properties can be further added to the raw material composition to extend the growth path of dendrites, thereby making it possible to suppress the occurrence of dendrite shorts or to lengthen the time until dendrite shorts occur.

[0008] However, the technology of Patent Document 2 has problems such as the glass flakes being broken down and reduced in size when the raw material composition is heated, melted, and kneaded, and the glass flakes being randomly dispersed when the raw material composition is heated, melted, and kneaded, resulting in a microporous film with vertically arranged glass flakes when the raw material composition is extruded into a sheet, resulting in a large variation in the short-circuit resistance of the produced separators.

[0009] On the other hand, lead-acid batteries, even those for vehicles with idle-stop systems, may be temporarily used at high temperatures and under overcharged conditions, such as in summer operating environments. In such cases, reducing substances released or eluted from the separator may be converted into organic acids (e.g., volatile organic acids such as acetic acid) that dissolve lead due to the oxidizing power within the lead-acid battery. The organic acids cause corrosion within the lead-acid battery, reducing the cross-sectional area of ​​the cell, making it difficult to discharge large currents when large currents are required, such as during startup, and potentially shortening the battery's lifespan. Therefore, lead-acid battery separators are also required to suppress corrosion within the battery, even when used at high temperatures and under overcharged conditions, so that large currents can be discharged when starting from an idle-stop system. Patent Document 3 describes a lead-acid battery separator that prevents corrosion within the battery, even when used at high temperatures and under overcharged conditions, by suppressing the amount of reducing substances released or eluted from a synthetic resin porous membrane separator.

[0010] Japanese Patent Application Laid-Open No. 2013-211115 Japanese Patent Application Laid-Open No. 2016-059739 Japanese Patent Application Laid-Open No. 2005-243585

[0011] In view of the above-mentioned conventional problems, the present invention aims to provide a separator for a lead-acid battery, which is made of a microporous membrane mainly composed of a polyolefin resin, an inorganic powder, and a mineral oil, and which is manufactured by an economical process to suppress dendrite penetration throughout the entire separator, including the base, and which has little variation in short-circuit resistance and an improved short-circuit time. Another object of the present invention is to provide a separator for a lead-acid battery, which is optimal for vehicles with an idle-stop system, and which can suppress corrosion inside the lead-acid battery even when the lead-acid battery is used at high temperatures and in an overcharged state.

[0012] As a result of extensive research aimed at solving the above problems, the present invention has provided a lead-acid battery separator having the following characteristics: (1) A lead-acid battery separator comprising a microporous membrane primarily composed of a polyolefin resin, inorganic powder, and mineral oil, the separator having an ash content of 65% or less and exhibiting a short-circuit time improved by 110% or more compared to a conventional separator containing the same amount of mineral oil. (2) The lead-acid battery separator according to (1), wherein the polyolefin resin comprises an ultra-high molecular weight polyethylene resin having a weight-average molecular weight of 1.5 million or more and a high-density polyethylene resin having a melt flow rate of 1.2 g / 10 min or less. (3) The lead-acid battery separator according to (1) or (2), wherein the microporous membrane has a melting point of less than 135°C as measured by a differential scanning calorimeter. (4) The separator for a lead-acid battery according to any one of (1) to (3) above, characterized in that the inorganic powder is silica powder, and the weight ratio of the silica powder to the polyolefin-based resin in the raw material composition (silica powder / polyolefin-based resin) is less than 2.6. (5) The separator for a lead-acid battery according to any one of (1) to (4) above, characterized in that the porosity of the microporous membrane is 62% or less. (6) The separator for a lead-acid battery according to any one of (1) to (4) above, characterized in that the amount of reducing substances liberated and eluted after 24 hours of electrolysis at about 25°C with a direct current of 1.2 A using an electrolytic cell comprising the microporous membrane, a positive electrode plate, a negative electrode plate, and dilute sulfuric acid is 100% or less per 100 cm of porous membrane. ２ The equivalent amount of potassium permanganate consumed per sheet is 1.6 mL / 100 cm ２ The separator for a lead acid battery according to any one of (1) to (5) above, characterized in that:

[0013] As described above, the lead-acid battery separator of the present invention, which has an ash content of 65% or less, is a separator that has a short-circuit time that is improved by 110% or more compared to conventional products containing the same amount of mineral oil. A lead-acid battery separator made of a microporous membrane primarily composed of polyolefin resin, inorganic powder, and mineral oil can be provided using an economical manufacturing process that prevents dendrite penetration throughout the entire separator, including the base, and provides a separator with minimal variation in short-circuit resistance and improved short-circuit time. Furthermore, using an electrolytic cell consisting of the microporous membrane, positive and negative electrode plates, and dilute sulfuric acid, the amount of reducing substances liberated and eluted after 24 hours of electrolysis at a direct current of 1.2 A and approximately 25°C is measured per 100 cm of porous membrane. ２ The equivalent amount of potassium permanganate consumed per sheet is 1.6 mL / 100 cm ２ It is possible to provide a lead-acid battery separator that is optimal for use in idle-stop vehicles and that can suppress corrosion inside the lead-acid battery even when the lead-acid battery is used at high temperature and in an overcharged state.

[0014] The microporous membrane preferably has a total content of 90% by weight or more of polyolefin resin, inorganic powder, and mineral oil as a plasticizer, with a polyolefin resin content of 20 to 60% by weight, an inorganic powder content of 40 to 80% by weight, and a mineral oil content of 0 to 30% by weight. If the polyolefin resin content is less than 20% by weight or the inorganic powder content is more than 80% by weight, the mechanical strength, oxidation resistance, and sealing properties of the microporous membrane provided by the polyolefin resin are insufficient. If the polyolefin resin content is more than 60% by weight or the inorganic powder content is less than 40% by weight, it becomes difficult to ensure a large porosity or a fine and complex pore structure in the microporous membrane, making it difficult to maintain good electrical resistance characteristics in the separator made of the microporous membrane.

[0015] The polyolefin resin may be a homopolymer or copolymer of polyethylene, polypropylene, polybutene, polymethylpentene, or the like, or a mixture thereof. Among these, polyethylene is preferred as the main component in terms of moldability and economy. Polyethylene has a lower melt molding temperature than polypropylene, resulting in good productivity and reduced manufacturing costs. By adjusting the weight-average molecular weight of the polyolefin resin to 500,000 or more, the mechanical strength of the film can be ensured even in a microporous film containing a large amount of inorganic powder. Therefore, it is more preferable that the weight-average molecular weight of the polyolefin resin is 1,000,000 or more, or even 1,500,000 or more. Polyolefin resins also have good mixability with inorganic powders, and in microporous films, they maintain strength while bonding the skeleton of the inorganic powder as an adhesive functional material, while also being chemically stable and highly safe.

[0016] In the present invention, it is preferable to use a polyolefin resin by mixing an ultra-high molecular weight polyethylene resin having a weight-average molecular weight of 1.5 million or more with a high-density polyethylene resin in order to reduce the pressure when the raw material composition is extruded into a sheet while being heated, melted, and kneaded. In the present invention, the high-density polyethylene resin used by mixing with the ultra-high molecular weight polyethylene resin having a weight-average molecular weight of 1.5 million or more preferably has a melt flow rate of 0.01 g / 10 min or more, more preferably 0.02 g / 10 min or more, and even more preferably 0.03 g / 10 min or more. It is also preferably 1.2 g / 10 min or less, more preferably 1.0 g / 10 min or less, and even more preferably 0.6 g / 10 min or less. The mixing ratio of the ultra-high molecular weight polyethylene resin to the high-density polyethylene resin (ultra-high molecular weight polyethylene resin / high-density polyethylene resin) is preferably 9 / 1 to 1 / 9, more preferably 5 / 1 to 1 / 5, and even more preferably 3 / 1 to 1 / 3. If the mixing ratio is too small, the effect of reducing the pressure when raw material composition is extruded into sheet form while being heated and melted and kneaded cannot be obtained, and if the mixing ratio is too large, it is difficult to ensure the mechanical strength of microporous film.In this case, in order to obtain the effect of reducing the pressure when raw material composition is extruded into sheet form while being heated and melted and kneaded, the melting point of microporous film measured by differential scanning calorimetry is preferably less than 135 ° C, more preferably 133 ° C or less, and even more preferably 131 ° C or less.In addition, in order to ensure the mechanical strength of microporous film, the melting point of microporous film measured by differential scanning calorimetry is preferably 128 ° C or more, more preferably 129 ° C or more.

[0017] In the present invention, silica powder is preferably used as the inorganic powder, and silica having a fine particle size and a pore structure inside or on the surface is more preferably used. Among inorganic powders, silica has a wide range of powder properties such as particle size and specific surface area, is relatively inexpensive and easily available, and contains few impurities. The silica powder has a specific surface area of ​​100 m ２ / g or more, the pore structure of the microporous film is made finer (more dense) and more complex, thereby improving short-circuit resistance, increasing the electrolyte retention of the microporous film, and providing a large number of hydrophilic groups (—OH) on the powder surface, thereby improving the hydrophilicity of the microporous film, which is preferable. ２ It is more preferable that the specific surface area of ​​the silica powder is 400 m / g or more. ２ It is preferable that the specific surface area of ​​the silica powder is 400 m / g or less. ２ If the surface activity of the particles is higher than 1 / g, the particles have a high cohesive force, which makes it difficult for the silica powder to be uniformly dispersed in the microporous film, which is not preferable.

[0018] The mineral oils described above are preferably used as plasticizers because they are compatible with polyolefin resins and can be easily extracted with various solvents, etc., and mineral oils such as industrial lubricants made from saturated hydrocarbons (paraffins) are preferred because they can be easily reused.

[0019] As described above, the mineral oil used as a plasticizer is melt-kneaded and formed into a film-like material of a predetermined shape using a raw material composition primarily composed of a polyolefin resin, inorganic powder, and mineral oil, and then removed to create a porous structure. Incorporating an appropriate amount of mineral oil into a separator made of a microporous membrane contributes to improved oxidation resistance and also has a favorable effect on short-circuit resistance. From this perspective, it is more preferable that the mineral oil content in the separator be 5 to 30 wt %. However, since a high content of mineral oil in the separator reduces the porosity of the microporous membrane and deteriorates the electrical resistance of the separator made of a microporous membrane, it is more preferable that the mineral oil content in the separator be 20 wt % or less.

[0020] As the solvent used to extract and remove the mineral oil from the melt-kneaded film-like raw material composition, a saturated hydrocarbon organic solvent such as hexane, heptane, octane, nonane, or decane can be used.

[0021] If necessary, additives such as surfactants (hydrophilizing agents), antioxidants, ultraviolet absorbers, weathering agents, lubricants, antibacterial agents, antifungal agents, pigments, dyes, colorants, antifogging agents, and matting agents may be added (blended) or contained in the raw material composition or the microporous membrane, as long as the purpose and effect of the present invention are not impaired.

[0022] The base thickness of the microporous film is preferably 0.1 to 0.3 mm. If the thickness exceeds 0.3 mm, the electrical resistance deteriorates, and if the thickness is less than 0.1 mm, it becomes difficult to maintain good short-circuit resistance (the short-circuit here refers to both a permeation short-circuit known as a dendrite short-circuit and a normal short-circuit caused by holes or cracks due to localized weak spots in the substrate, high pressure, impact, or puncture from the protrusions of the electrode plate, or oxidative wear due to the oxidative power of the electrode plate). The base thickness is a term used, for example, when the microporous film has rib-like protrusions, to distinguish it from the total thickness including the rib-like protrusions, and refers to the film thickness excluding the height of the rib-like protrusions (when the rib-like protrusions are not provided).

[0023] The average pore diameter of the microporous membrane (measured by mercury porosimetry) is preferably 0.01 to 0.5 μm. If it exceeds 0.5 μm, the insulation between electrodes decreases, promoting the occurrence of permeation short circuits. If it is less than 0.01 μm, the electrical resistance becomes too high.

[0024] The porosity of the microporous membrane (mercury porosimetry) is preferably 45% by volume or more and less than 63% by volume, more preferably 48% by volume or more and 62% by volume or less, and even more preferably 50% by volume or more and 61% by volume or less. When the porosity of the microporous membrane (mercury porosimetry) is less than 63% by volume, the short-circuit resistance performance of the lead-acid battery separator is less varied, the short-circuit time can be improved, and this contributes to improving the performance of the lead-acid battery used. When the porosity of the microporous membrane (mercury porosimetry) is less than 45% by volume, the electrical resistance of the separator increases, and the battery performance of the lead-acid battery used deteriorates.

[0025] The ash content of the microporous membrane is preferably 40% by weight or more and 65% by weight or less, and more preferably 63% by weight or less. The lower the ash content of the microporous membrane, the longer the short-circuit time of the lead-acid battery separator. This is thought to be because a low ash content of the microporous membrane reduces the porosity of the microporous membrane, suppressing the generation and growth of dendrites. Therefore, by making the ash content of the microporous membrane 65% by weight or less, the short-circuit resistance performance of the lead-acid battery separator can be reduced and the short-circuit time can be improved, contributing to the high performance of the lead-acid battery used. If the ash content of the microporous membrane exceeds 65% by weight, the porosity of the microporous membrane does not decrease, and the effect of suppressing the generation and growth of dendrites cannot be obtained.

[0026] A preferred method for obtaining the microporous membrane is to melt-knead a raw material composition primarily composed of a polyolefin resin, an inorganic powder such as silica powder, and a mineral oil as a plasticizer, to form a membrane, and then remove some or all of the mineral oil. This results in a membrane with numerous uniform, fine, and intricately interwoven interconnected pores throughout the membrane. A specific example of the manufacturing method is shown below. First, a predetermined amount of raw materials, including a polyolefin resin, silica powder, and mineral oil, plus various additives (surfactants, antioxidants, weathering agents, etc.) as needed, are stirred and mixed in a mixer such as a Henschel mixer or a Lödige mixer to obtain a raw material mixture. Next, this mixture is fed into a twin-screw extruder equipped with a T-die at the tip, heated, melted, and kneaded to form a sheet, and then passed through a pair of forming rolls with predetermined grooves on one or both rolls to obtain a film-like product in which ribs of a predetermined shape are integrally formed on one or both sides of a flat sheet. Next, this film-like material is immersed in an appropriate solvent (e.g., n-hexane) to extract and remove a predetermined amount of mineral oil, and then dried to obtain the desired microporous membrane. Note that the raw material composition refers to a composition consisting of all raw materials brought into the melt-kneading process, and is intended to refer only to "all raw materials (composition)," and does not specifically refer to a raw material mixture or a melt-kneaded product.

[0027] In the present invention, in a separator made of a microporous membrane, in order to minimize variation in short-circuit resistance and improve short-circuit time, the weight ratio (silica powder / PE) of the inorganic powder (e.g., silica powder) to the polyolefin resin (e.g., polyethylene resin) in the raw material composition is preferably less than 2.6, more preferably 2.5 or less, and even more preferably 2.2 or less. In this case, the content of the polyolefin resin (e.g., polyethylene resin) in the raw material composition is preferably 10% by weight or more, more preferably 11% by weight or more, and even more preferably 12% by weight or more.

[0028] The mineral oil in the raw material composition is primarily absorbed by the inorganic powder (e.g., silica powder). The optimal amount of oil absorbed by the silica powder per weight is determined by the manufacturing process, and therefore the optimal amount of mineral oil in the raw material composition depends on the amount of silica powder blended. In the present invention, as described above, the weight ratio (silica powder / PE) of the inorganic powder (e.g., silica powder) to the polyolefin resin (e.g., polyethylene resin) in the raw material composition is preferably less than 2.6. Therefore, the optimal amount of mineral oil blended in a preferred raw material composition depends on the amount of silica powder blended, and the blending amount of mineral oil in the raw material composition is preferably 30 to 60 wt %. In the present invention, by setting the preferred blending amount of mineral oil in the raw material composition to 30 to 60 wt %, the porosity of the microporous film after mineral oil extraction is reduced, and the short-circuit resistance performance of the lead-acid battery separator is more consistent, resulting in improved short-circuit time.

[0029] In the present invention, the amount of reducing substances liberated and eluted after electrolysis for 24 hours at a direct current of 1.2 A and 25±2° C. using an electrolytic cell comprising the microporous membrane, a positive electrode plate, a negative electrode plate, and dilute sulfuric acid is ２ The equivalent consumption of N / 100 potassium permanganate solution per sheet is 1.6 mL / 100 cm ２ It is desirable to have the following: Even when used at high temperatures and in an overcharged state, corrosion inside the lead-acid battery can be suppressed.

[0030] The microporous membrane contains a large amount of inorganic powder such as silica powder, which has a large specific surface area and high hydrophilicity, and is therefore hydrophilic enough to be wettable by and permeable to the sulfuric acid electrolyte of a lead-acid battery, which is an aqueous solution. In addition, it is preferable that the microporous membrane contain 0.2 to 8 wt % of a surfactant (solid content) so that when sulfuric acid electrolyte is poured into a laminate in which electrode plates and separators are densely assembled in a battery case, the electrolyte is quickly absorbed into the voids in the separator and the voids in the separator are quickly replaced with the electrolyte.

[0031] Methods for incorporating the surfactant into the microporous membrane include adding the surfactant in a dispersed state to the raw material composition before film formation (internal addition method) and post-treating (adhesion treatment) the microporous membrane after film formation and removal of the plasticizer (external addition method). The method of adding the surfactant to the raw material composition beforehand (internal addition method) is preferred because it simplifies the manufacturing process and makes it less likely for the surfactant to leach out of the microporous membrane of the present invention. The content (required amount) of the surfactant (solid content) in the microporous membrane is 0.2 to 8 wt. %. Increasing the surfactant (solid content) content beyond this range does not significantly improve the hydrophilicity of the microporous membrane, but instead reduces the porosity of the microporous membrane, resulting in increased internal resistance (electrical resistance) and increased self-discharge when used as a lead-acid battery separator. Furthermore, the amount of reducing substances released and eluted from the separator increases, accelerating corrosion within the lead-acid battery. Therefore, the content of the surfactant (solid content) in the microporous membrane is more preferably 0.2 to 6.5% by weight, and even more preferably 0.2 to 5% by weight.

[0032] The surfactant may be any material capable of improving the hydrophilicity of the microporous membrane, and may be any of nonionic surfactants, cationic surfactants, and anionic surfactants. Examples of nonionic surfactants that can be used include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkylaryl ethers, fatty acid monoglycerides, and sorbitan fatty acid esters. Examples of cationic surfactants that can be used include aliphatic amine salts, quaternary ammonium salts, polyoxyethylene alkylamines, and alkylamine oxides. Examples of anionic surfactants that can be used include alkyl sulfonates, alkyl benzene sulfonates, alkyl naphthalene sulfonates, alkyl sulfosuccinates, and dodecyl benzene sulfonates. Among these, alkyl benzene sulfonates, alkyl sulfosuccinates, and dodecyl benzene sulfonates are preferred because they can impart high hydrophilicity to polyolefin resins with a small amount of addition and have relatively high heat resistance, allowing the surfactant to be added to the raw material composition in advance to produce a microporous membrane (production by hot melt molding).

[0033] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention.

[0034] Example 1: 100 parts by weight of a 3:2 blend of ultra-high molecular weight polyethylene (UHMWPE) powder with a weight-average molecular weight of 1.5 million and high-density polyethylene resin with a melt flow rate of 1.2 g / 10 min as a polyolefin resin, 120 parts by weight of silica powder as an inorganic powder, 264 parts by weight of paraffinic oil (a type of mineral oil), and 1.4 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane to extract and remove a predetermined amount of paraffinic oil. The resulting film was then dried to obtain a microporous membrane with a base thickness of 0.25 mm and containing 16% mineral oil. This was used as the lead-acid battery separator of Example 1.

[0035] Example 2: 100 parts by weight of a blend of ultra-high molecular weight polyethylene (UHMWPE) powder with a weight-average molecular weight of 1.5 million and high-density polyethylene resin with a melt flow rate of 1.2 g / 10 min in a weight ratio of 3:2 as a polyolefin resin, 150 parts by weight of silica powder as an inorganic powder, 330 parts by weight of paraffinic oil (a type of mineral oil), and 1.4 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane to extract and remove a predetermined amount of paraffinic oil. The resulting film was then dried to obtain a microporous membrane with a base thickness of 0.25 mm and containing 16% mineral oil. This was used as the lead-acid battery separator of Example 2.

[0036] Example 3: 100 parts by weight of a blend of ultra-high molecular weight polyethylene (UHMWPE) powder with a weight-average molecular weight of 1.5 million and high-density polyethylene resin with a melt flow rate of 1.2 g / 10 min in a weight ratio of 3:2 as a polyolefin resin, 180 parts by weight of silica powder as an inorganic powder, 396 parts by weight of paraffinic oil (a type of mineral oil), and 1.4 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane to extract and remove a predetermined amount of paraffinic oil. The resulting film was then dried to obtain a microporous membrane with a base thickness of 0.25 mm and containing 16% mineral oil. This was used as the lead-acid battery separator of Example 3.

[0037] Example 4: 100 parts by weight of a blend of ultra-high molecular weight polyethylene (UHMWPE) powder with a weight-average molecular weight of 1.5 million and high-density polyethylene resin with a melt flow rate of 1.2 g / 10 min in a weight ratio of 3:2 as a polyolefin resin, 180 parts by weight of silica powder as an inorganic powder, 396 parts by weight of paraffinic oil (a type of mineral oil), and 6.5 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane, a predetermined amount of paraffinic oil was extracted and removed, and the mixture was dried to obtain a microporous membrane with a base thickness of 0.25 mm and a mineral oil content of 16%. This was used as the lead-acid battery separator of Example 4.

[0038] Example 5: 100 parts by weight of a blend of ultra-high molecular weight polyethylene (UHMWPE) powder with a weight-average molecular weight of 1.5 million and high-density polyethylene resin with a melt flow rate of 1.2 g / 10 min in a weight ratio of 3:2 as a polyolefin resin, 220 parts by weight of silica powder as an inorganic powder, 484 parts by weight of paraffinic oil (a type of mineral oil), and 1.4 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane to extract and remove a predetermined amount of paraffinic oil. The resulting film was then dried to obtain a microporous membrane with a base thickness of 0.25 mm and containing 16% mineral oil. This was used as the lead-acid battery separator of Example 5.

[0039] Comparative Example 1: 100 parts by weight of ultra-high molecular weight polyethylene (UHMWPE) resin powder with a weight-average molecular weight of 1.5 million as a polyolefin resin, 260 parts by weight of silica powder as an inorganic powder, 572 parts by weight of paraffinic oil (a type of mineral oil), and 1.4 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane to extract and remove a predetermined amount of paraffinic oil. The film was then dried to obtain a conventional microporous membrane with a base thickness of 0.25 mm and a mineral oil content of 16%. This was used as the lead-acid battery separator of Comparative Example 1.

[0040] Comparative Example 2: 100 parts by weight of ultra-high molecular weight polyethylene (UHMWPE) resin powder with a weight-average molecular weight of 1.5 million as a polyolefin resin, 260 parts by weight of silica powder as an inorganic powder, 572 parts by weight of paraffinic oil (a type of mineral oil), and 11.0 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane, a predetermined amount of paraffinic oil was extracted and removed, and the resulting film was dried to obtain a microporous membrane with a base thickness of 0.25 mm and a mineral oil content of 16%. This was used as the lead-acid battery separator of Comparative Example 2.

[0041] Comparative Example 3: 100 parts by weight of ultra-high molecular weight polyethylene (UHMWPE) resin powder with a weight-average molecular weight of 1.5 million as a polyolefin resin, 350 parts by weight of silica powder as an inorganic powder, 770 parts by weight of paraffinic oil (a type of mineral oil), and 1.4 parts by weight of dialkyl sulfosuccinic acid sodium salt (solid content) as a surfactant were mixed in a mixer. The resulting mixture was heated, melted, and kneaded in a twin-screw extruder, extruded into a sheet from a T-die, and pressure-molded through a pair of forming rolls. Next, this film-like material was immersed in n-hexane, a predetermined amount of paraffinic oil was extracted and removed, and the resulting film was dried to obtain a microporous membrane with a base thickness of 0.25 mm and a mineral oil content of 16%. This was used as the lead-acid battery separator of Comparative Example 3.

[0042] [Test and Evaluation Methods] The lead-acid battery separators of the above Examples and Comparative Examples were evaluated under the following conditions, and the results are summarized in Table 1. Note that MD (machine direction) refers to the manufacturing direction of the manufactured sheet, and CD (crosstalk direction) refers to the direction perpendicular to the MD direction.

[0043] [Base Thickness (mm)] Using a dial gauge (Peacock G-6 manufactured by Ozaki Seisakusho Co., Ltd.), the thickness (mm) was measured at several arbitrary points on the microporous membrane (or at points not including the rib-like protrusions if the membrane has rib-like protrusions), and the average value was expressed.

[0044] [Tensile Strength CD (MPa), Elongation CD (%)] The tensile strength in the CD direction and the elongation in the CD direction were measured as follows. The separator was cut into a rectangular shape of 10 mm x 70 mm in the CD direction to prepare a test specimen. Using a Shopper tensile tester, the test specimen was attached with a grip distance (a) of approximately 50 mm, and a tensile test was performed at a pulling speed of 200 mm per minute to measure the tensile load (b) and distance (c) at which the test specimen broke. The tensile strength was calculated by dividing the tensile load (b) by the cross-sectional area of ​​the test specimen. The elongation was calculated by dividing the distance (c) by the grip distance (a) of the tester.

[0045] [Oil Amount (%)] The oil amount was measured as follows. The separator was cut into strips to prepare test specimens. The test specimens were dried in a dryer (105±5°C) for 30 minutes, then cooled in a desiccator (30 minutes), and their weight (W1, g) was measured. The test specimens were placed in an ultrasonic cleaner containing trichlene, and the oil was extracted by applying ultrasonic waves for 10 minutes. After extraction, the test specimens were dried in a dryer (105±5°C) for 30 minutes, then cooled in a desiccator (30 minutes), and their weight (W2, g) was measured. The oil amount was calculated using the following formula: Oil Amount (%) = [(W1-W2) / (W1)] x 100

[0046] [Ash content (%)] Ash content was measured as follows. The test piece (weight: W2, g) after oil extraction used in the measurement of [Oil content (%)] above was transferred to a constant-weight evaporating dish and incinerated in a muffle furnace (580°C). After confirming that the test piece had turned white, it was cooled to room temperature and its weight (W3, g) was measured. The ash content was calculated using the following formula: Ash content (%) = (W3 / W2) x 100

[0047] [Porosity (volume %)] The porosity was measured as follows: The pore volume of the separator (mercury porosimetry: P, cm ３ ) and volume (V, cm ３ ) and calculated using the following formula: Porosity (volume %) = [P / (P+V)] x 100

[0048] [Electrical Resistance (Relative Value)] Electrical resistance was measured as follows. The separator was cut into a 70 mm x 70 mm square to prepare a test specimen. After impregnating with ethanol, the specimen was replaced with a sulfuric acid aqueous solution and measured using a test device conforming to SBA S 0402. To approximate the state of sulfuric acid aqueous solution retention in the separator during actual battery use, the separator was first impregnated with ethanol, which has low surface tension, and then replaced with a sulfuric acid aqueous solution. This allowed the sulfuric acid aqueous solution to permeate the separator's pores, allowing the electrical resistance of the separator during actual battery use to be evaluated. The electrical resistance was expressed as a relative value, with the value of Comparative Example 1 (conventional product) set to 100.

[0049] [Melting Point (°C)] The melting point was measured as follows. Differential scanning calorimetry (measurement temperature range: 40°C to 350°C, heating rate: 10°C / min) was performed using a differential scanning calorimeter (DSC) (Shimadzu Corporation, DSC60) under a nitrogen atmosphere (flow rate: 100 ml / min). The melting point (°C) was determined from the obtained DSC curve.

[0050] [Short-Circuit Resistance (Relative Value)] Short-circuit resistance was measured and evaluated using a test device conforming to SBA S 0402 as follows. Separators were cut into 70 mm x 70 mm squares to prepare test specimens. The test specimens were impregnated with ethanol and then substituted with a saturated lead sulfate solution. The test specimens were sandwiched between two 50 mm x 50 mm square lead plates (made of pure lead, 3 mm thick). The test specimens were placed horizontally in a battery container, with the centers of the test specimen and the two lead plates aligned and the sides of the three squares parallel to each other. A 5 kg weight was then placed on the center of the three squares, and saturated lead sulfate solution was poured into the battery. A current of 5.0 mA was then applied to the lead plates, and the voltage change was continuously recorded. The voltage rose slightly after the start of current application and then gradually decreased. The time until the voltage decreased to 70% of its maximum voltage was measured. The short-circuit resistance was expressed as a relative value with the value of Comparative Example 1 (conventional product) being set at 100.

[0051] [Amount of reducing substance (mL / 100 cm ２The amount of reducing substance contained in the separator was measured according to the method described in Patent Document 3, and expressed as a converted value of N / 100 potassium permanganate consumption (mL / 100 cm ２ )

[0052] [Corrosion in Battery (Visual Inspection: "Yes" or "No")] Test batteries were prepared using the lead-acid battery separators of the above examples and comparative examples according to the method described in Patent Document 3. The test batteries were submerged in a water tank set at 75°C, and a light-load life test was conducted in accordance with the test conditions specified in JIS D5301-2006 9.5.5(a). However, to accelerate the evaluation of life in a high-temperature atmosphere, the test was started with the liquid level at the minimum liquid level line, and the liquid was replenished once a week based on the minimum liquid level line. After the test, the test batteries were disassembled and the cell welds were visually inspected. If corrosion was observed, it was rated as "Yes," and if not, it was rated as "No."

[0053]

[0054] The results in Table 1 reveal the following: (1) The results in Table 1 reveal that the lead-acid battery separators of Examples 1 to 5, which are made of a microporous membrane with an ash content of 65% or less, have a short-circuit resistance that is 110% or more better than the lead-acid battery separator of Comparative Example 1, which is a conventional product containing the same amount of mineral oil. (2) Furthermore, the lead-acid battery separators of Examples 1 to 5, which are made of a microporous membrane using a polyolefin-based resin that is a blend of an ultra-high molecular weight polyethylene resin (UHMWPE) with a weight-average molecular weight of 1.5 million and a high-density polyethylene resin with a melt flow rate of 1.2 g / 10 min in a weight ratio of 3:2, have a significantly better short-circuit resistance than the lead-acid battery separators of Comparative Examples 1 to 3, which use only an ultra-high molecular weight polyethylene resin (UHMWPE) with a weight-average molecular weight of 1.5 million as the polyolefin-based resin. (3) Furthermore, the lead-acid battery separators of Examples 1 to 5 made of the microporous membrane of the present invention were found to have a melting point of less than 135°C as measured by a differential scanning calorimeter because they used a blend of an ultrahigh molecular weight polyethylene resin (UHMWPE) with a weight-average molecular weight of 1.5 million or more and a high-density polyethylene resin with a melt flow rate of 1.2 g / 10 min or less as the polyolefin-based resin. (4) Furthermore, by setting the weight ratio of silica powder to polyolefin-based resin (silica powder / polyolefin-based resin) in the raw material composition to less than 2.6, the ash content of the lead-acid battery separators of Examples 1 to 5 made of the resulting microporous membrane could be reduced to 65% or less, and the short-circuit resistance was significantly improved compared to the lead-acid battery separators of Comparative Examples 1 to 3. (5) Furthermore, the lead-acid battery separators of Examples 1 to 5 made of a microporous membrane with a porosity of 62% or less were found to have significantly improved short-circuit resistance compared to the lead-acid battery separators of Comparative Examples 1 to 3. (6) In addition, the separator for lead-acid batteries of Example 4 has an increased amount of surfactant mixed therein, and the amount of reducing substances released and eluted is 1.6 mL / 100 cm ２ (equivalent to the consumption of N / 100 potassium permanganate), and no corrosion was observed in the cell welds when the battery was disassembled after the light load life test. On the other hand, by further increasing the amount of surfactant mixed, the amount of reducing substances released and eluted increased to 2.0 mL / 100 cm.２ In the lead-acid battery separator of Comparative Example 2, which was set at a concentration of 1.6 mL / 100 cm (equivalent to the amount of potassium permanganate consumed), corrosion was observed at the cell welds when the battery was disassembled after the light-load life test. From this, in order to be practically usable as a separator and a lead-acid battery, the microporous membrane of the present invention also needs to have a concentration of 1.6 mL / 100 cm. ２ (N / 100 equivalent value of potassium permanganate consumption) was found to be the critical value.

[0055] The separator is made of a microporous membrane primarily composed of polyolefin resin, inorganic powder, and mineral oil, and is manufactured using an economical manufacturing process that prevents dendrite penetration throughout the entire separator, including the base, resulting in a separator with minimal variation in short-circuit resistance and improved short-circuit time.

Claims

1. A separator for lead-acid batteries, which is made of a microporous membrane mainly composed of polyolefin resin, inorganic powder, and mineral oil, and which is characterized by having an ash content of 65% or less and an improved short circuit time of 110% or more compared to conventional products containing the same amount of mineral oil.

2. The lead-acid battery separator according to claim 1, wherein the polyolefin resin comprises an ultra-high molecular weight polyethylene resin having a weight-average molecular weight of 1.5 million or more, and a high-density polyethylene resin having a melt flow rate of 1.2 g / 10 min or less.

3. The separator for a lead-acid battery according to claim 1, wherein the melting point of the microporous film is less than 135° C. as measured by a differential scanning calorimeter.

4. The lead-acid battery separator according to claim 1, wherein the inorganic powder is silica powder, and the weight ratio of the silica powder to the polyolefin-based resin in the raw material composition (silica powder / polyolefin-based resin) is less than 2.

6.

5. The separator for a lead-acid battery according to any one of claims 1 to 4, wherein the porosity of the microporous film is 62% or less.

6. Using an electrolytic cell consisting of the microporous membrane, a positive electrode plate, a negative electrode plate, and dilute sulfuric acid, electrolysis is performed for 24 hours at a direct current of 1.2 A at about 25° C. The amount of reducing substances liberated and eluted from 100 cm of the porous membrane is ２ The equivalent amount of potassium permanganate consumed per sheet is 1.6 mL / 100 cm ２ 2. The separator for a lead acid battery according to claim 1, characterized in that:

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