JPWO2018181330A1 - Wound body of microporous membrane - Google Patents

Abstract

An object of the present invention is to provide a wound body of a microporous membrane which is suitable for long-term storage by preventing discoloration unevenness. The present invention is a microporous film rolled by winding a microporous film containing a fluororesin, and covered with a polymer film having a light transmittance of 365% or less at a wavelength of 365 nm. [Selection diagram] None

Description

  The present invention relates to a microporous membrane wound body.

  A microporous membrane containing a fluororesin has an adhesive property to an electrode, and is therefore widely used as an adhesive secondary battery separator (Patent Document 1). Further, a fluororesin such as polyvinylidene fluoride is inferior in mechanical strength, and thus may be used by being laminated on a microporous polyolefin membrane (Patent Document 1).

Japanese Patent No. 4988973

  In contrast to the problem that a microporous membrane containing a fluororesin may cause uneven discoloration when stored for a long period of time, the present invention provides a rolled microporous membrane suitable for long-term storage that prevents uneven discoloration. With the goal.

  The present invention has been made as a result of intensive studies to solve the above problems, and as a result, when producing a microporous membrane containing a fluororesin, the inventor derived from a polyene structure or the like in the presence of specific impurities or in a high-temperature environment. The present inventors have found that the fluororesin is discolored and the microporous membrane obtained in this way is stored for a long period of time, and that the fluororesin discolors, resulting in uneven discoloration of the microporous membrane.

That is, the present invention
(1) A microporous membrane rolled by a microporous membrane containing a fluororesin and covered with a polymer film having a light transmittance of 365% or less at a wavelength of 365 nm.
(2) In the wound body of the microporous membrane of the present invention, the polymer film preferably has a light transmittance at a wavelength of 550 nm of 10% or more.
(3) In the wound body of the microporous membrane of the present invention, the polymer film preferably has a bag shape.
(4) In the wound body of the microporous membrane of the present invention, the fluororesin is preferably polyvinylidene fluoride or a derivative thereof.
(5) The wound body of the microporous film of the present invention is preferably a laminated microporous film in which the microporous film has a porous layer containing a fluororesin on at least one surface of the polyolefin microporous film.

  ADVANTAGE OF THE INVENTION According to this invention, the discoloration nonuniformity of the microporous membrane containing a fluororesin can be prevented and the wound body of a microporous membrane suitable for long-term storage can be provided.

The polymer film will be described below.
The light transmittance of the polymer film at a wavelength of 365 nm is 20% or less. By setting the light transmittance at a wavelength of 365 nm to 20% or less, discoloration of the fluororesin can be prevented. The light transmittance at a wavelength of 365 nm is preferably 15% or less, most preferably 13% or less. The lower limit of the light transmittance at a wavelength of 365 nm is 0%.

  By setting the transmittance of light at a wavelength of 550 nm to 10% or more, the polymer film can visually recognize the contents while suppressing discoloration of the fluororesin, and is excellent in quality controllability. The light transmittance at a wavelength of 550 nm is preferably at least 20%, most preferably at least 70%. There is no particular upper limit for the light transmittance at a wavelength of 550 nm, but generally it does not exceed 100%.

  The polymer film can be produced by a known method. For example, it can be manufactured by adding low-density polyethylene as a main component, adding an ultraviolet absorber, a dye, or a pigment, melt-kneading, extruding, and adjusting the thickness so as to fall within the range of the present invention. . In another method, for example, a polymer film containing a UV absorber, a dye, a pigment and the like is applied to a commercially available polypropylene transparent film and dried to produce a polymer film.

  The polymer film is not particularly limited as long as it is within the scope of the present invention, and may be a film obtained by depositing a metal such as aluminum or laminating a metal foil.

Next, the microporous membrane will be described below.
The microporous membrane may be a single membrane or a laminated microporous membrane having a porous layer containing a fluororesin on at least one surface of a substrate. The microporous membrane may be used as a battery separator. From the viewpoint of adhesiveness to the electrode, it is preferable that the fluororesin be contained as a main component in the entire resin contained in the single film or the porous layer containing the fluororesin.

  The fluororesin contained in the microporous film is important for exhibiting adhesiveness to the positive electrode and the negative electrode. As the fluororesin, one kind of fluororesin selected from the group consisting of vinylidene fluoride homopolymer, vinylidene fluoride / fluorinated olefin copolymer, vinyl fluoride homopolymer, and vinyl fluoride / fluorinated olefin copolymer It is preferable to use the above, and polyvinylidene fluoride resin and polyvinylidene fluoride-hexafluoropropylene copolymer are particularly preferable.

  Particles may be added to the microporous membrane for the purpose of improving the meltdown temperature during excessive heating. The particles are suitably used for both organic particles and inorganic particles. As inorganic particles, calcium carbonate, calcium phosphate, amorphous silica, crystalline glass filler, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, zeolite , Molybdenum sulfide, mica, boehmite and the like. In particular, titanium dioxide, alumina, and boehmite are preferred from the viewpoint of crystal growth, cost, and availability of the fluororesin.

  The substrate may be any porous film used for a battery separator, and a polyolefin microporous film can be particularly preferably used. Examples of the polyolefin include homopolymers of ethylene and propylene, ethylene, propylene, butene, hexene, pentene, methylpentene, octene, vinyl acetate, methyl methacrylate, and copolymers composed of olefin copolymers such as styrene and mixtures thereof. Is exemplified. It is preferable to use polyethylene, polypropylene, or a copolymer of polyethylene and polypropylene because the shutdown temperature can be controlled to about 120 ° C. to 150 ° C.

  As the polyethylene, high-density polyethylene, medium-density polyethylene, ultra-high-molecular-weight polyethylene (UHMwPE), branched low-density polyethylene, linear low-density polyethylene, and a mixture thereof can be used. Among them, high-density polyethylene, ultra-high-molecular-weight polyethylene, and a mixture thereof are preferred from the viewpoint of controlling the pore structure of the microporous membrane and the thermal properties and strength of the membrane.

  Polyolefin is a range which does not impair the effects of the present invention, for example, an amorphous heat-resistant resin, an antioxidant, a heat stabilizer, an antistatic agent, an ultraviolet absorber, an antiblocking agent and a filler, a crystal nucleating agent And various additives such as a crystallization retarder.

  The microporous polyolefin membrane may be a single membrane or a multilayer membrane having a layer configuration composed of two or more layers having different molecular weights or average pore diameters. Various production methods are known for the polyolefin microporous membrane, and any of the production methods may be used. For the multilayer film, for example, each polyolefin constituting the a layer and the b layer is melt-kneaded with a molding solvent, and the obtained molten mixture is supplied from a respective extruder to one die, and a gel sheet constituting each component is integrated. It can be produced by either a method of co-extrusion and a method of laminating separately prepared layers and heat-sealing. The co-extrusion method is more preferable because it is easy to obtain a high adhesive strength between the layers, and it is easy to form a communication hole between the layers, so that high permeability is easily maintained and the productivity is excellent.

  When the microporous film is a laminated microporous film provided with a porous layer containing a fluororesin on at least one surface of the substrate, the method of laminating the fluororesin on the substrate is not particularly limited, and is known. It can be laminated by a method. For example, a fluororesin solution (hereinafter, sometimes referred to as "varnish") dissolved in a solvent that is soluble in a fluororesin and is miscible with water is applied to a substrate by a known application method, and a water bath (coagulation bath) is used. ) To solidify and dry the fluororesin, thereby forming a porous layer. At this time, the solvent may contain impurities or the fluororesin may be colored due to a polyene structure or the like due to drying at a high temperature.

  Examples of the method of applying the varnish include dip coating, reverse roll coating, gravure coating, kiss coating, roll brushing, spray coating, air knife coating, Meyer bar coating, and pipe doctor. Method, blade coating method, die coating method, and the like. These methods can be performed alone or in combination. It is important that the varnish is kept tightly closed so that it does not come into contact with the outside air as far as possible.

  In the coagulation bath, the fluororesin solidifies in a three-dimensional network by phase separation. The immersion time in the coagulation bath is preferably 3 seconds or more. If the time is less than 3 seconds, the resin may not be sufficiently coagulated. There is no upper limit, but 10 seconds is sufficient. Further, the above uncleaned microporous membrane is immersed in an aqueous solution containing 1 to 20% by weight, more preferably 5 to 15% by weight, of a good solvent for the fluororesin constituting the porous layer, and washed with pure water. Through a step and a drying step using hot air of 100 ° C. or lower, a microporous membrane (a battery separator) can be obtained.

  The battery separator of the present invention is used as a battery separator for a secondary battery such as a nickel-hydrogen battery, a nickel-cadmium battery, a nickel-zinc battery, a silver-zinc battery, a lithium ion secondary battery, and a lithium polymer secondary battery. Although it can be used, it is particularly suitably used as a separator of a lithium ion secondary battery.

Hereinafter, the present invention will be described more specifically with reference to examples.
The method for measuring physical properties and the method for evaluating effects in the present invention were performed according to the following methods.

1. Measurement of discoloration According to JIS Z8781-4 (2013), using a portable spectral colorimeter CM-2600d (manufactured by Konica Minolta Co., Ltd.), light source: D65, irradiation diameter 11 mmΦ, measurement diameter 8 mmΦ, viewing angle 10 ° Measured the coordinates of the SCE (excluding specularly reflected light) CIELAB color space. Next, the color difference ΔE * ab was calculated based on the two color space coordinate measurement results to be compared, and the following determination was made.
ΔE * ab ≦ 0.4… ○
0.4 <ΔE * ab ≦ 0.8 △
0.8 <ΔE * ab... ×.

2. Light transmittance Using a spectrophotometer UV-2450 (manufactured by Shimadzu Corporation), the absorption spectrum of the polymer film at a wavelength of 200 nm to 900 nm was measured by a transmission method, and the transmittance at a wavelength of 365 nm and a wavelength of 500 nm was determined. Was.

3. Visibility A polymer film cut to 75 mm × 75 mm is superimposed on a laminated microporous film cut to 50 mm × 50 mm, and “○” indicates that the laminated microporous film can be confirmed from the polymer film side; Was determined.

(Example 1)
Using polyvinylidene fluoride resin KF polymer W9300 (manufactured by Kureha Corporation) as the fluorine resin, N-methylpyrrolidone was added thereto to prepare a fluorine resin solution having a solid content of 3% by weight. Next, a fluorocarbon resin dissolving solution is applied on both sides using a 7 μm polyethylene microporous film (Setira (registered trademark)) as a base material using a die coater and immersed in a water tank for 10 seconds, and a phase separation step is performed. After a drying step at 1 ° C. for 1 minute, a laminated microporous film having a total thickness of 11 μm was obtained. Table 1 shows CIELAB color space coordinates of the laminated microporous film.

  Next, a sharp cut filter SC-37 (manufactured by FUJIFILM Corporation) was prepared as a polymer film. The light transmittances of the polymer film at wavelengths of 365 nm and 550 nm were 20% and 93%, respectively.

  Next, a laminated microporous film and a polymer film were superimposed in order of 3.0 m from a fluorescent lamp (FLR40S EX-W / M / 36, manufactured by Hitachi Appliances Co., Ltd.), and the fluorescent lamp was used for 1,000 hours. Irradiation was performed, and CIELAB color space coordinates of the laminated microporous film were measured.

(Examples 2 to 5)
Except that the polymer film was changed to SC-38, SC-56, SC-60 (manufactured by FUJIFILM Corporation) and VM-CPP (manufactured by Toray Film Processing Co., Ltd., thickness 30 μm, grade 2403) The test was conducted in the same manner as in Example 1, and the results are shown in Table 1.

(Comparative Example 1)
The test was performed in the same manner as in Example 1 except that the polymer film was not used, and the results are shown in Table 1.

(Comparative Examples 2 to 4)
Tests were performed in the same manner as in Example 1 except that the polymer film was changed to ND-0.5, CCPG-100Y (manufactured by Fuji Film Co., Ltd.), and rose LD / LLD polyethylene standard bags (manufactured by Sankyo Co., Ltd.). And the results are shown in Table 1.

  As is clear from Table 1, in Examples 1 to 5, the change in color before and after the irradiation of the fluorescent lamp is small. On the other hand, in Comparative Examples 1 to 4, it can be seen that the color change was large and the color was faded white.

  That is, by using the wound body of the present invention, the discoloration unevenness of the microporous membrane containing the fluororesin can be prevented even during long-term storage, and the microporous film covered with the polymer film having a light transmittance of 20% or less at a wavelength of 365 nm can be used. A wound body of a porous membrane can be provided.

Claims (5)

A roll of a microporous film, which is obtained by winding a microporous film containing a fluororesin and covered with a polymer film having a light transmittance of 365% or less at a wavelength of 365 nm. The wound body of a microporous membrane according to claim 1, wherein the light transmittance of the polymer film at a wavelength of 550 nm is 10% or more. The wound body of a microporous membrane according to claim 1, wherein the polymer film has a bag shape. The wound body of a microporous membrane according to any one of claims 1 to 3, wherein the fluororesin is polyvinylidene fluoride or a derivative thereof. The wound body of a microporous membrane according to any one of claims 1 to 4, wherein the microporous membrane is a laminated microporous membrane having a porous layer containing a fluororesin on at least one surface of the polyolefin microporous membrane.