JP2013073737A - Separator for electric power storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for an electric power storage device which includes a microporous film formed by melting, mixing and kneading raw material constituents mainly consisting of a polyolefin-based resin, inorganic powder, and a plasticizer into a film, and then removing, by extraction, part or all of the plasticizer from it, and which serves to prevent a detection failure in detecting a defect and the like with an image-detection type inspection apparatus even in a case where the film is arranged so as to contain an adequate amount of carbon black for imparting weather resistance thereto.SOLUTION: The separator for an electric power storage device comprises: a microporous film which contains 20-80 wt.% of a polyolefin-based resin, 20-80 wt.% of inorganic powder, and 0-30 wt.% of a plasticizer, and which has an average pore diameter of 1 μm or less, a porosity of 50-95%, and a base thickness of 10-500 μm. The raw material constituents of the film include carbon black having an average primary particle diameter of 100-500 nm. The microporous film contains 0.5-5 wt.% of the carbon black, and has a luminosity of 5-8.

Description

  The present invention relates to a separator for an electricity storage device such as a lead storage battery, a lithium ion secondary battery, a polymer lithium secondary battery, an electric double layer capacitor, a lithium ion capacitor, and an aluminum electrolytic capacitor.

  A raw material composition mainly composed of a polyolefin resin such as a polyethylene resin, an inorganic powder such as silica fine powder, and a plasticizer such as mineral oil is melt-kneaded to form a part or all of the plasticizer with an organic solvent after film formation. A microporous film obtained by extraction and removal is used as a separator for a lead storage battery, for example. Such a polyolefin microporous film separator, as disclosed in Patent Document 1, contains a small amount of carbon black to provide weather resistance in order to prevent deterioration of the polyolefin-based resin due to ultraviolet rays. I am doing so. That is, a microporous film is obtained by mixing an appropriate amount of carbon black in the raw material composition.

JP 07-130348 A

In the case of the polyolefin microporous film containing carbon black, the semi-finished product sheet before extraction and removal of the plasticizer melted and kneaded (before making porous) has a black color, and the plasticizer The microporous film sheet extracted and removed to have a porosity of 50 to 95% has a blue color.
The microporous film separator detects defects such as adhesion of foreign matter (insects, oils, etc.) in the process of visual inspection of the finished sheet after the plasticizer is extracted and removed in the manufacturing process. Although it is performed by an inspection machine, there is a problem that the defect detection accuracy is not 100% in the dark blue color, and the defect cannot be detected. In addition, in the process of processing the completed microporous film separator into a bag shape so as to wrap the electrode plate, the microporous film separator cut into a rectangle is folded in the middle in the longitudinal direction, with the folding side down. The side ends (left and right sides) of the lever are pressed and pressure-bonded by applying a pressing force of about 50 to 100 kg / cm ² by passing the work base through a pressurizing jig composed of a pair of meshing gears. However, in the process of determining whether the sealing process (sealing process) is good or bad as an inspection after the sealing process, the gear seal part and The gear seal portion is detected by discriminating the color difference from the non-gear seal portion to determine whether the gear seal processing is good / bad. Since the gear seal part is darker (blackish) as the pressure is applied than the non-gear seal part, the gear seal part is imaged to identify the gear seal part, and the belt-like line of a predetermined width that is the gear seal part is a separator. By seeing whether the seals are normally present at both ends, it is seen whether the seal has been performed normally. However, if the dark color system of the original microporous film separator is strong, the color difference between the gear seal part and the non-gear seal part is reduced, and there is a problem that the gear seal part cannot be detected normally. The gear seal part is pressurized and darkened (the color is darker), rather than because the voids are crushed, rather than the carbon black that exists in a dispersed state inside the porous separator being collected in a compressed state. It is assumed that the apparent packing density increases and the carbon color becomes stronger.

  Therefore, in view of the above-mentioned conventional problems, the present invention extracts a part or all of the plasticizer after melt-kneading a raw material composition mainly composed of a polyolefin resin, an inorganic powder, and a plasticizer. Even when an appropriate amount of carbon black for imparting weather resistance is included in the separator for an electricity storage device made of a microporous film that has been removed, it is possible to detect defects in the manufacturing process and to detect the gear seal part in the processing process. An object of the present invention is to provide an electricity storage device separator that can prevent a detection failure when it is performed by an image detection type inspection machine.

The present invention has obtained the following knowledge as a result of intensive studies to achieve the above object.
In the case of the polyolefin microporous film containing carbon black, the semi-finished product sheet before extraction and removal of the plasticizer melted and kneaded (before making porous) has a black color, and the plasticizer The microporous film sheet extracted and removed to have a porosity of 50 to 95% has a blue color.
First, the effect of the amount of carbon black added on the color was investigated. However, since carbon black is added for the purpose of providing weather resistance to the microporous film, it was necessary to add 0.5% by weight or more of carbon black in order to provide weather resistance to the microporous film. . Therefore, the influence of the addition amount of carbon black was investigated in the range of 0.5% by weight or more. For the semi-finished product sheet before extraction and removal of the plasticizer, the black color did not change strongly even when the amount of carbon black added was changed. For sheets with a porosity of 50-95% by extracting and removing the plasticizer, the lightness in blue color (Munsell color system hue is about 3 PB, saturation is about 2) when the amount of carbon black added is lowered There was a tendency that (Munsell color system) gradually increased. However, even if the carbon black content is as small as 0.5% by weight, the lightness of the blue color does not exceed 5.5, and the defects due to the image detection type inspection machine and the detection failure of the gear seal part are completely eliminated. The level to prevent is not reached.
Next, the influence of the properties of carbon black on the color was investigated, and the influence on the color was investigated by paying attention to the particle size of the carbon black. Conventional carbon black has an average primary particle diameter of about 20 to 80 nm, but when special carbon black having an average primary particle diameter of 100 nm or more is used, the carbon black content is as small as 0.5% by weight. However, it has been found that the lightness of the blue color is 5.5 or more, and it is possible to completely prevent the defects caused by the image detection type inspection machine and the poor detection of the gear seal portion.

The present invention has been made on the basis of such knowledge, and as described in claim 1, after forming a film by melt-kneading a raw material composition mainly comprising a polyolefin resin, an inorganic powder, and a plasticizer. , Including 20 to 80% by weight of the polyolefin-based resin, 20 to 80% by weight of the inorganic powder, and 0 to 30% by weight of the plasticizer obtained by extracting or removing a part or all of the plasticizer, In an electricity storage device separator comprising a microporous film having an average pore size of 1 μm or less, a porosity of 50 to 95%, and a base thickness of 10 to 500 μm, the average primary particle size is 100 in the raw material composition. -500 nm carbon black is contained, 0.5-5 wt% of the carbon black is contained in the microporous film, and the lightness (Munsell color system) of the microporous film is 5-8. It is a sign.
The electricity storage device separator according to claim 2 is the electricity storage device separator according to claim 1, wherein the carbon rack has an average primary particle diameter of 150 to 500 nm.
The electricity storage device separator according to claim 3 is the electricity storage device separator according to claim 1 or 2, wherein the lightness (Munsell color system) of the microporous film is 5.5 to 7.5. It is characterized by.
The power storage device separator according to claim 4 is the power storage device separator according to any one of claims 1 to 3, wherein the carbon black is added in an amount of 0.5 to 2% by weight in the microporous film. It is characterized by including.

  According to the present invention, a microporous film obtained by melt-kneading a raw material composition mainly composed of a polyolefin resin, an inorganic powder, and a plasticizer, and then extracting and removing part or all of the plasticizer. Even when an appropriate amount of carbon black for imparting weather resistance is included in the separator for an electricity storage device, the defect detection in the manufacturing process and the gear seal part detection in the processing process are detected by an image detection type inspection machine. It is possible to prevent detection failure when performing.

  The separator for an electricity storage device of the present invention is a polyolefin obtained by melting and kneading a raw material composition mainly composed of a polyolefin resin, an inorganic powder, and a plasticizer, and then extracting and removing part or all of the plasticizer. Base resin thickness including 20-80% by weight of resin, 20-80% by weight of inorganic powder, 0-30% by weight of plasticizer, average pore diameter of 1 μm or less, porosity of 50-95% Is a microporous film having an average primary particle size of 100 to 500 nm in the raw material composition, and 0.5 to 5 wt% of carbon black in the microporous film. The lightness (Munsell color system) of the microporous film is from 5 to 8.

  If the lightness of the microporous film is less than 5, in the detection of defects and gear seals by the image detection type inspection machine, since the microporous film body is a dark color system, the defects (such as insect adhesion, oil adhesion, etc.) ) And the gear seal part (crimp part) are not conspicuous, and detection failure may not be completely prevented. In addition, when the lightness of the microporous film is more than 8, for the detection of defects by the image detection type inspection machine, since the microporous film body is light-colored, there are defects (such as insect adhesion and oil adhesion). ) Is conspicuous and can completely prevent detection failure. However, for the detection of the gear seal part by the image detection type inspection machine, the microporous film body is light-colored, but the gear seal part (crimp part) is also This is unsuitable because it is not a dark color system and the gear seal portion is not conspicuous and detection failure may not be completely prevented. The reason why the gear seal part does not become dark color is that when the microporous film body is light color, the carbon black content is low, and the carbon black present in a dispersed state is collected inside the separator and apparently filled Even if the density is increased, it is still a very small amount, and it seems that the gear seal part to which the microporous film is pressure-bonded is also unlikely to have a dark carbon color. Therefore, the lightness of the microporous film is more preferably 5.5 to 7.5.

  As described above, the microporous film of the present invention is a separator for use in an electricity storage device whose performance is increasing, and meets the requirements of a small thickness, a small pore diameter, and a high porosity. It is a microporous film having a thickness of 10 to 500 μm, an average pore diameter of 1 μm or less, and a porosity of 50 to 95%. Since the microporous film of the present invention is obtained by melt-kneading the raw material composition as described above and extracting and removing the plasticizer after film formation, the average pore diameter is 1 μm or less and the porosity is 50 to 50%. It has a high microporosity of 95%. In addition, if the base thickness is less than 10 μm, it is difficult to exert the isolation effect as a separator, which adversely affects the life performance of the electricity storage device, which is not suitable. On the other hand, when the base thickness exceeds 500 μm, the electrical resistance of the separator is increased, and the internal resistance of the electricity storage device is increased. When rib-like projections are provided on a microporous film, such as when used as a lead-acid battery separator, the base thickness is preferably 100 to 500 μm according to the shape and dimensions of the rib-like projections. More preferably, it is 100-300 micrometers. When the microporous film is not provided with rib-like projections, such as when used as a separator for lithium ion secondary batteries, electric double layer capacitors, etc., the base thickness is preferably 10 to 100 μm. More preferably, it is 10-50 micrometers. The base thickness is a term used to distinguish from the total thickness including the rib-like projections when the microporous film has rib-like projections, for example, excluding the height of the rib-like projections (ribs) (When no protrusion is provided)

  As described above, the method for obtaining the microporous film of the present invention is by melting and kneading a raw material composition mainly composed of a polyolefin resin, an inorganic powder, and a plasticizer, and removing the plasticizer. Thereby, the film | membrane in which the countless communicating hole which has the complicated path | route which was uniform, fine, and complicated was formed in the whole film | membrane is obtained. An example of a specific manufacturing method is shown below. First, a raw material in which carbon black and, if necessary, a hydrophilizing agent (surfactant) are added to a polyolefin resin, inorganic powder, or plasticizer is stirred and mixed with a mixer such as a Henschel mixer or a Ladige mixer. To obtain a raw material mixture. Next, this mixture is put into a twin-screw extruder having a T-die attached at the tip, extruded into a sheet while heating and melting and kneading, and formed into a sheet having a predetermined thickness by secondary processing such as rolling and stretching. Next, if this sheet is immersed in a suitable solvent (for example, n-hexane), a predetermined amount of the plasticizer is extracted and dried, and heat-treated as necessary, the desired microporous film can be obtained. . The stretching process may be performed in the preceding process of the plasticizer extraction process, in the subsequent process, or in the preceding and subsequent processes.

  As the polyolefin resin, homopolymers or copolymers such as polyethylene, polypropylene, polybutene, polymethylpentene, and mixtures thereof can be used. Of these, polyethylene is the main component in terms of moldability and economy. Polyethylene has a melt molding temperature lower than that of polypropylene, has good productivity, and can suppress production costs. When the polyolefin resin has a weight average molecular weight of 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, the polyolefin resin preferably has a weight average molecular weight of 1 million or more. Polyolefin-based resins are well mixed with inorganic powders, are in a microporous film, maintain strength while bonding the inorganic powder framework as an adhesive functional material, and are chemically stable and safe. Is expensive. Polyolefin resin has a high melting point or high softening point such as polymethylpentene (4-methyl-1-pentene) and cyclic polyolefin (ethylene norbornene) in the case where the microporous film requires heat resistance. It is preferable to use a resin together.

As the inorganic powder, one or more kinds of silica, alumina, titania, calcium silicate, kaolin clay, talc, clay, glass fine powder, etc., which have a fine particle size and a pore structure on the inside or surface are used. it can. Among these, silica is preferable because it has a wide selection range of various powder properties such as particle diameter and specific surface area, is relatively inexpensive and easily available, and has few impurities. When the inorganic powder has a specific surface area of 100 m ² / g or more, the pore structure of the microporous film is refined and complicated to increase the short circuit resistance, and the electrolyte holding power of the microporous film is increased. Since the hydrophilicity of a microporous film is improved by providing many hydrophilic groups (-OH) on the body surface, it is preferable. Therefore, the specific surface area of the inorganic powder is more preferably 150 m ² / g or more. The specific surface area of the inorganic powder is preferably 400 m ² / g or less. When the specific surface area of the inorganic powder exceeds 400 m ² / g, the surface activity of the particles is high and the cohesive force is strong, so that it is difficult to uniformly disperse the inorganic powder in the microporous film.

  As the plasticizer, it is preferable to select a material that can be a plasticizer of a polyolefin resin, and various organic liquids that are compatible with the polyolefin resin and can be easily extracted with various solvents can be used. Further, mineral oil such as industrial lubricating oil made of saturated hydrocarbon (paraffin), higher alcohol such as stearyl alcohol, ester plasticizer such as dioctyl phthalate, and the like can be used. Among these, mineral oil is preferable because it can be easily reused. The plasticizer is preferably blended in an amount of 30 to 70% by weight in the raw material composition mainly composed of polyolefin resin, inorganic powder, plasticizer, and carbon black.

  As described above, the plasticizer melts and kneads the raw material composition mainly composed of polyolefin resin, inorganic powder, and plasticizer to form a sheet, and then removes it to make the sheet porous. However, depending on the purpose of the separator, the entire amount thereof may be removed or only a part thereof may be removed. When a microporous film is used as a lead-acid battery separator, it contributes to the improvement of oxidation resistance by containing an appropriate amount of mineral oil as a plasticizer.

  As a solvent (solvent) used for extracting and removing the plasticizer, a saturated hydrocarbon organic solvent such as hexane, heptane, octane, nonane and decane can be used.

  Carbon black has an average primary particle diameter of 100 to 500 nm as described above. When the average primary particle size of the carbon black is less than 100 nm, the lightness in the blue color of the microporous film is 0.5% by weight which is the minimum content for imparting weather resistance to the microporous film. May be less than 5, and may not be able to be completely prevented from the drawbacks caused by the image detection type inspection machine and the detection failure of the gear seal portion. Moreover, if the average primary particle diameter of carbon black is less than 100 nm, the particles are likely to aggregate and become highly conductive, which is not suitable. In the microporous film separator responsible for electrical insulation between the electrodes, the development of conductivity must be avoided. Further, if the average primary particle diameter of carbon black exceeds 500 nm, the material becomes expensive and the weather resistance imparting effect may be inferior. Therefore, the average primary particle diameter of carbon black is more preferably 150 to 500 nm, and further preferably 200 to 450 nm. Carbon black is a fine particle of carbon generated mainly by gas phase pyrolysis or incomplete combustion of natural gas or hydrocarbon gas. Depending on the production method, carbon black, such as channel black, furnace black, thermal black, acetylene black, etc. However, it is preferable because thermal black is easy to obtain with a large primary particle size.

  In addition to the raw material composition or microporous film, a surfactant (hydrophilic agent), antioxidant, ultraviolet absorber, weathering agent, lubricant, antibacterial agent, antifungal agent, pigment, dye as necessary In addition, additives such as a colorant, an antifogging agent, and a matting agent may be added (blended) or contained within a range that does not impair the object and effect of the present invention.

  As described above, the microporous film of the present invention has a polyolefin resin of 20 to 80% by weight, an inorganic powder of 20 to 80% by weight, a plasticizer of 0 to 30% by weight, and carbon black of 0.1%. Contains 5 to 5% by weight. If the content of the polyolefin resin is less than 20% by weight, the polyolefin resin cannot be uniformly dispersed throughout the microporous film, and it is not suitable because sufficient mechanical strength of the microporous film cannot be ensured. Even when the content of the inorganic powder exceeds 80% by weight, the content of the polyolefin resin is less than 20% by weight, which is not suitable for the same reason. Further, if the content of the inorganic powder is less than 20% by weight, the effect of improving the dimensional stability of the microporous film (the dimensional shrinkage of the sheet after the plasticizer is extracted and removed, the dimension when the microporous film is exposed to high temperature) The effect of suppressing shrinkage, that is, the effect of a microporous film as a skeletal material), the effect of miniaturizing and complicating the pore structure of the microporous film to prevent short circuit, the effect of retaining the electrolyte of the microporous film, etc. Is not suitable because it cannot be fully exhibited, and the blending amount of the plasticizer is reduced, so that the porosity of the microporous film is lowered and the internal resistance of the electricity storage device is increased. Even when the content of the polyolefin resin exceeds 80% by weight, the content of the inorganic powder is less than 20% by weight, which is not suitable for the same reason. Therefore, the content of the polyolefin resin in the microporous film is more preferably 20 to 60% by weight, and the content of the inorganic powder is more preferably 40 to 80% by weight. As described above, the microporous film of the present invention is obtained by melt-kneading a raw material composition mainly composed of a polyolefin resin, an inorganic powder, and a plasticizer to form a film and removing the plasticizer. However, the composition ratio between the polyolefin resin and the inorganic powder in the raw material composition and the composition ratio between the polyolefin resin and the inorganic powder in the microporous film are basically the same. Further, if the carbon black content is less than 0.5% by weight, it is not suitable because the weather resistance is not sufficiently imparted to the microporous film. When the content of carbon black exceeds 5% by weight, the effect of imparting weather resistance will reach its peak, while the content of polyolefin resin and inorganic powder, which are the main constituent materials, will decrease and the strength of the microporous film will decrease. This is unsuitable because of the tendency to occur, etc., and is also unsuitable because carbon black tends to agglomerate and may increase conductivity, and the lightness is less than 5 in the blue color of the microporous film. This is not suitable because there is a possibility that the defect by the image detection type inspection machine and the detection failure of the gear seal portion may not be completely prevented. Therefore, the content of carbon black in the microporous film is more preferably 0.5 to 2% by weight.

Next, examples of the present invention will be described in detail together with comparative examples.
Example 1
100 parts of a high density polyethylene resin powder having a weight average molecular weight of 1,500,000 as a polyolefin resin (parts by weight, the same applies hereinafter), 260 parts of silica fine powder having a specific surface area of 200 m ² / g and an average particle size of 5 μm A biaxial extruder with 590 parts of paraffinic oil which is a kind of mineral oil as plasticizer and 2 parts of thermal black with an average primary particle diameter of 280 nm as carbon black mixed with a Henschel mixer and attached with a T die at the tip. The sheet was extruded into a sheet while being melted and kneaded, and then rolled with a forming roll to obtain a sheet having a thickness of 200 μm. Next, this sheet is immersed in an n-hexane tank to extract and remove a predetermined amount of the plasticizer, followed by heating and drying to obtain 23.5% by weight of polyethylene resin, 61.0% by weight of fine silica powder, and paraffinic oil. An electricity storage device separator (suitable as a lead acid battery separator) made of a microporous film having a thickness of 200 μm composed of 15.0 wt% and carbon black 0.5 wt% was obtained.

(Example 2)
Except that the blending amount of carbon black was 8 parts, the same procedure as in Example 1 was carried out, but 23.1% by weight of polyethylene resin, 60.0% by weight of fine silica powder, 15.0% by weight of paraffinic oil, 1. An electricity storage device separator made of a microporous film having a thickness of 200 μm and comprising 8% was obtained.

(Example 3)
Except that the blending amount of carbon black was 20 parts, the same procedure as in Example 1 was carried out, however, polyethylene resin 22.4% by weight, silica fine powder 58.2% by weight, paraffinic oil 15.0% by weight, carbon black 4. An electrical storage device separator made of a microporous film having a thickness of 200% and comprising 5% was obtained.

Example 4
23.5 wt% polyethylene resin and 61.0 wt% silica fine powder in the same manner as in Example 1 except that thermal black having an average primary particle diameter of 405 nm was used as carbon black instead of thermal black having an average primary particle diameter of 280 nm. A separator for an electricity storage device comprising a microporous film having a thickness of 200 μm composed of 15.0% by weight of paraffinic oil and 0.5% by weight of carbon black was obtained.

(Example 5)
Except that the blending amount of carbon black was 8 parts, the same procedure as in Example 4 was carried out, but 23.1% by weight of polyethylene resin, 60.0% by weight of fine silica powder, 15.0% by weight of paraffinic oil, 1. An electricity storage device separator made of a microporous film having a thickness of 200 μm and comprising 8% was obtained.

(Example 6)
Except for using 20 parts of carbon black in the same manner as in Example 4, 22.4% by weight of polyethylene resin, 58.2% by weight of fine silica powder, 15.0% by weight of paraffinic oil, 4. An electrical storage device separator made of a microporous film having a thickness of 200% and comprising 5% was obtained.

(Example 7)
23.5 wt% polyethylene resin and 61.0 wt% silica fine powder in the same manner as in Example 1 except that furnace black having an average primary particle diameter of 120 nm was used instead of thermal black having an average primary particle diameter of 280 nm as carbon black. A separator for an electricity storage device comprising a microporous film having a thickness of 200 μm composed of 15.0% by weight of paraffinic oil and 0.5% by weight of carbon black was obtained.

(Comparative Example 1)
23.5 wt% polyethylene resin and 61.0 wt% silica fine powder in the same manner as in Example 1 except that furnace black having an average primary particle diameter of 29 nm was used instead of thermal black having an average primary particle diameter of 280 nm as carbon black. A separator for an electricity storage device comprising a microporous film having a thickness of 200 μm composed of 15.0% by weight of paraffinic oil and 0.5% by weight of carbon black was obtained.

(Comparative Example 2)
Except that the blending amount of carbon black was 8 parts, the same procedure as in Comparative Example 1 was carried out, 23.1% by weight of polyethylene resin, 60.0% by weight of fine silica powder, 15.0% by weight of paraffinic oil, 1. An electricity storage device separator made of a microporous film having a thickness of 200 μm and comprising 8% was obtained.

(Comparative Example 3)
Except that the blending amount of carbon black was 20 parts, 22.4% by weight of polyethylene resin, 58.2% by weight of silica fine powder, 15.0% by weight of paraffinic oil, 4. An electrical storage device separator made of a microporous film having a thickness of 200% and comprising 5% was obtained.

(Comparative Example 4)
23.5 wt% polyethylene resin and 61.0 wt% silica fine powder in the same manner as in Example 1 except that furnace black having an average primary particle diameter of 70 nm was used as carbon black instead of thermal black having an average primary particle diameter of 280 nm. A separator for an electricity storage device comprising a microporous film having a thickness of 200 μm composed of 15.0% by weight of paraffinic oil and 0.5% by weight of carbon black was obtained.

(Comparative Example 5)
Except that the blending amount of carbon black was 8 parts, the same procedure as in Comparative Example 4 was carried out, 23.1% by weight of polyethylene resin, 60.0% by weight of silica fine powder, 15.0% by weight of paraffinic oil, 1. An electricity storage device separator made of a microporous film having a thickness of 200 μm and comprising 8% was obtained.

(Comparative Example 6)
22.4% by weight of polyethylene resin, 58.2% by weight of silica fine powder, 15.0% by weight of paraffinic oil, and carbon black 4. An electrical storage device separator made of a microporous film having a thickness of 200% and comprising 5% was obtained.

Next, the separators obtained in Examples 1 to 7 and Comparative Examples 1 to 6 were evaluated by the following test methods. The results are shown in Table 1. In the following, MD (direction) refers to the flow direction during production of a film sheet produced in a continuous band shape, and CD (direction) refers to the direction orthogonal to MD.
<Average primary particle size of carbon black>
Arithmetic average value obtained by taking a photo of several tens of thousands of times with an electron microscope having a resolution of at least 1.5 to 2.0 nm and directly measuring it.
<Base thickness>
Using a dial gauge (Peacock G-6 manufactured by Ozaki Seisakusho Co., Ltd.), arbitrary points and several points of the separator were measured.
<Tensile strength, elongation>
A test piece is cut from the separator in the MD and CD directions to a size of 10 mm × 70 mm. Using a shopper type with a capacity of 294 N or less or a tensile tester equivalent to this, set the interval between the grips of the tester to about 50 mm, attach the test piece, perform at a pulling speed of 200 mm per minute, and pull when the test piece is cut Read the load and distance. The tensile load is divided by the cross-sectional area to obtain the tensile strength, and the elongation is the distance divided by the distance between the grips of the tester.
<Porosity>
From the pore volume (mercury intrusion method) and true density (immersion method) of the microporous film, the following formula was used.
Porosity = Vp / ((1 / ρ) + Vp)
Where Vp: pore volume (cm ³ / g), ρ: true density (g / cm ³ )
<Average pore diameter>
The pore size distribution was calculated from the pressure and mercury volume at the time of mercury intrusion. The pore diameter at the time when mercury having a volume of 50% of the total pore volume was injected was defined as the average pore diameter (median diameter).
<Electric resistance>
The separator was cut into a size of 70 mm × 70 mm to obtain a test piece, and measured with a test apparatus based on SBA S0402.
<Weatherability>
The separator was irradiated with ultraviolet rays for 5 days in an environment of a temperature of 65 ° C. and a humidity of 90% RH, and then the elongation (%) in the CD direction was measured, and this was regarded as weather resistance.
<Color tone>
A color difference meter (CR210 manufactured by Minolta Co.) was used to measure the Munsell color system (hue, brightness, saturation) on the separator surface.
<Image detectability>
In order to wrap the electrode plate with the separator, the separator cut into a rectangular shape is folded in the middle of the longitudinal direction, and the side edges (left and right sides) of the folded side are down, and a gear seal (workpiece substrate is paired with a pair of substrates) By passing between pressurizing jigs composed of meshing gears, a pressing force of about 50 to 100 kg / cm ^{2 was} applied to press and pressurize, and a physical action was applied to form a bag. Next, as an inspection after the sealing process, it was determined whether the sealing process (sealing process) was good or bad. This is to determine the color difference between the gear seal part and the non-gear seal part, and after detecting (detecting) the gear seal part, it is checked whether the detected (detected) gear seal part is normal or not. Compared with the gear seal part, the color becomes darker (blackish) as the pressure is applied, so this is detected by an image detection type inspection machine, the gear seal part is specified, and the gear seal part has a predetermined width and length By checking whether the strip-shaped line is normally present at both end portions (left and right sides) of the separator, it is confirmed whether the sealing process has been normally performed. For example, on one of the left and right sides, there is no band-shaped line that is a gear seal part (no sealing process is performed), or a part of the band-shaped line is missing (having a predetermined width and a predetermined length). If it is not: Sealing is performed but not normal), it is determined that the sealing process was defective. However, in this case, for example, when there is no belt-shaped line that is a gear seal portion, the gear seal portion is not detected (detected). The case where it cannot be detected is also included.
For each of the separators of Examples 1 to 7 and Comparative Examples 1 to 6, 100 separator sheets cut as described above were sealed, and the separator sheets after the sealing process were passed through an image detection type inspection machine to detect the gear seal portion. It was determined whether or not (detection) was normally performed, and the ratio of those normally detected (detected) in 100 sheets was obtained.

From the results in Table 1, the following was found.
(1) Both the separators of Examples 1 to 7 and Comparative Examples 1 to 6 have the same film thickness, porosity, and average pore diameter manufactured by the same manufacturing method. It is a microporous film composed of 4 to 23.5% by weight, silica powder 58.2 to 61.0% by weight, mineral oil 15.0% by weight, and carbon black 0.5 to 4.5% by weight. .
(2) In Examples 1 to 7 using special carbon black having an average primary particle diameter of 100 nm or more as carbon black, the brightness of the separator (Munsell color system) is 5.0 to 7.8, and the image The detectability is 97% or more, and the detection failure of the gear seal portion can be almost completely prevented. In particular, in Examples 1, 2, 4, 5, and 7 in which the average primary particle size of carbon black is 150 nm or more and the content of carbon black is 2% by weight or less, the brightness of the separator (Munsell color system) is 5. 5 or more, and the image detectability is 100%.
(3) On the other hand, in Comparative Examples 1 to 6 using conventional carbon black having an average primary particle size of less than 100 nm as carbon black, the brightness of the separator (Munsell color system) is 2.6 to 5.3. The image detectability is 0 to 99%. In the case of Comparative Example 4 in which the average primary particle diameter of carbon black is 70 nm and the content of carbon black is 0.5% by weight, which is the minimum content for imparting weather resistance, the brightness of the separator (Munsell color system) ) Is 5.3 and the image detectability is 99%, but in the other Comparative Examples 1 to 3 and 5 to 6, the brightness of the separator (Munsell color system) is 2.6 to 4. 6 and the image detectability is 0 to 88%, and many detection defects of the gear seal portion occur. In Comparative Example 4 in which the brightness of the separator (Munsell color system) was 5.3 and the image detectability was as good as 99%, the average primary particle size of carbon black was less than 100 nm. Therefore, it is not practical to use as a separator for an electricity storage device because there is a risk that the electrical insulation between the electrodes is lowered.

Claims (4)

  20 to 80% by weight of the polyolefin resin obtained by melting and kneading a raw material composition mainly composed of a polyolefin resin, inorganic powder, and a plasticizer, and then extracting and removing a part or all of the plasticizer. And 20 to 80% by weight of the inorganic powder and 0 to 30% by weight of the plasticizer, the average pore diameter is 1 μm or less, the porosity is 50 to 95%, and the base thickness is 10 to 500 μm. In the electricity storage device separator comprising a certain microporous film, the raw material composition contains carbon black having an average primary particle size of 100 to 500 nm, and the carbon black is contained in the microporous film in an amount of 0.5 to 5 wt. %, And the brightness (Munsell color system) of the microporous film is 5 to 8.   2. The electricity storage device separator according to claim 1, wherein the carbon black has an average primary particle size of 150 to 500 nm.   The separator for an electricity storage device according to claim 1 or 2, wherein the lightness (Munsell color system) of the microporous film is 5.5 to 7.5.   The separator for an electricity storage device according to any one of claims 1 to 3, wherein the carbon black is contained in the microporous film in an amount of 0.5 to 2% by weight.