JP2003238720A - Polyolefin porous membrane - Google Patents

Abstract

(57) [Problem] To provide a polyolefin-based porous membrane which is easy to produce and contains a small amount of filler. A polyolefin-based porous membrane comprising a polyolefin containing particles having an average particle diameter of 100 nm or less in an amount of 1% by weight or more and 30% by weight or less, wherein the polyolefin-based porous film is stretched and made porous. Porous membrane.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin porous membrane, and more particularly to a polyolefin porous membrane suitable for a battery separator, a filter, a water vapor permeable film, and the like. Membrane. [0002] A porous membrane made of polyolefin is used as a separator of a battery such as a lithium ion secondary battery.
It is widely used as a water vapor permeable film used for diapers and sanitary products, or as a filter. It is known that this polyolefin-based porous membrane is manufactured by the following method. (1) A porous membrane manufactured by utilizing phase separation. That is, a porous membrane obtained by mixing a polyolefin and a solvent and heating a solution to form a film, and then performing an extraction treatment with a solvent. (2) A porous membrane produced by cleaving a polyolefin crystal interface by stretching. That is, after the polyolefin crystal is grown by performing a heat treatment on the thin film obtained by stretching the polyolefin, the crystal interface is cleaved by stretching at a low temperature, and then the cleavage is performed by stretching at a high temperature. A porous membrane obtained by enlarging. (3) A porous membrane produced by stretching a polyolefin to which a filler has been added. That is, a porous film obtained by adding a filler such as calcium carbonate or barium sulfate to a polyolefin and stretching the same to peel off the interface between the filler and the polyolefin (for example, Japanese Patent Application Laid-Open No. 60-129).
240, JP-A-63-210144). [0004] The porous membrane (1) uses a large amount of a solvent to be extracted after being mixed with a polyolefin to make it porous, and a solvent to extract this solvent. Therefore, the manufacturing process becomes complicated. In addition, the porous film (2) requires a long-time heat treatment step for promoting crystallization after forming the polyolefin thin film, which complicates the manufacturing process. The above porous membrane (3) is suitable as a porous membrane which can be produced at a low cost with a simple production process, and is used as a water vapor permeable membrane which is widely used in diapers and sanitary goods. well known. In the porous membrane (3), a large amount of filler is mixed with the polyolefin. For example, in JP-A-60-129460, 50 to 500 parts by weight of barium sulfate and in JP-A-63-210144, 30 to 70 parts by weight of calcium carbonate are mixed with polyolefin. As described above, when a large amount of filler is mixed with polyolefin, the strength of the porous membrane tends to decrease, and the pore diameter of the porous membrane tends to increase, so that it is difficult to obtain a porous membrane having a fine pore diameter. As a porous film using a filler, there is a porous film in which silica is dispersed in polyolefin by utilizing a sol-gel reaction. For example, 199
Published in 9th year Appl. Polym. Sci. From page 1489 to page 1494 of Vol. 72, tetraethoxysilane is added when polypropylene is melt-molded with an extruder, silica particles are generated in the polypropylene by a sol-gel reaction, and the polypropylene in which silica is dispersed is extruded, A method of obtaining a porous film by stretching the film has been proposed. However, this porous membrane
The production equipment and operation are likely to be complicated because of the danger of the tetraethoxysilane used and the generation of ethyl alcohol by the sol-gel reaction. [0007] There is a demand for a porous membrane obtained by mixing a small amount of a filler with a polyolefin. However, with a conventionally proposed porous polyolefin membrane mixed with a filler, Nothing was satisfactory. An object of the present invention is to provide a polyolefin porous membrane which uses a small amount of filler and can be easily manufactured. Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventor has found that a polyolefin is mixed with particles having an average particle diameter of 100 nm or less and then stretched to form a porous film. It has been found that the converted porous membrane becomes a porous membrane in which the proportion of the filler contained in the polyolefin is as small as 1% by weight or more and 30% by weight or less, and the present invention has been accomplished. That is, the present invention is as follows. 1% to 30% by weight of particles having an average particle diameter of 100 nm or less
What is claimed is: 1. A polyolefin-based porous membrane comprising a polyolefin contained below, which is stretched and made porous. BEST MODE FOR CARRYING OUT THE INVENTION
This will be specifically described below. The particles used in the present invention are particles having an average particle diameter of 100 nm or less, and it is preferable to use inorganic particles having an average particle diameter of 100 nm or less. Further, the inorganic particles are desirably insulating. Further, it is more preferable to use silicon oxide or alumina particles whose surfaces have been subjected to a hydrophobic treatment with an alkyl group. The lower limit of the particle diameter is preferably at least 1 nm, more preferably at least 3 nm. The average particle diameter of the present invention is a value of an average particle diameter obtained by an average particle diameter measuring method generally called a specific surface area measuring method, and is measured by a gas adsorption method generally called a BET method. It is a value of the average particle diameter obtained by the calculation formula 6 / (Sρ) using the specific surface area (S) of the particles and the density (ρ) of the particles. If the average particle size exceeds 100 nm, the strength of the polyolefin-based porous membrane to which inorganic particles are added tends to be small. In addition, if particles having an average particle size exceeding 100 nm are mixed with polyolefin and then stretched and made porous, the pore size becomes too large, and when used as a battery separator, it tends to be unsuitable. The proportion of particles contained in the polyolefin-based porous membrane is from 1% by weight to 30% by weight based on the total amount of the porous membrane.
% By weight, more preferably 1% by weight or more and 20% by weight or less. When the content ratio of the particles is less than 1% by weight, it is difficult to obtain the effect of increasing the rupture temperature as compared with a polyolefin-based porous membrane containing no particles. On the other hand, when the content ratio of the particles exceeds 30% by weight, In addition, the strength of the polyolefin-based porous membrane is reduced, and aggregates of particles are easily generated in the porous membrane. In the present invention, the term "polyolefin" refers to homopolymers of olefins such as ethylene and propylene and blends thereof, ethylene and olefins such as propylene, 1-butene, 4-methyl-1-pentene and 1-hexene. And blends of these copolymers with ethylene homopolymers, and blends of polyolefins with polyamides and modified polyphenylene ethers. From the viewpoint of securing the strength of the polyolefin porous membrane, JIS K
It is preferable to use a polyolefin having a melt flow rate of 2 g / 10 minutes or less, as measured by the method described in 7210, and more preferably a melt flow rate of 1 g / min.
A polyolefin of 10 minutes or less, more preferably a high-density polyethylene having a melt flow rate of 1 g / 10 minutes or less is used. In order to uniformly disperse the particles in the polyolefin, the porous polyolefin membrane of the present invention contains a fatty acid such as stearic acid and erucic acid, a fatty acid amide compound such as stearic acid amide and erucamide, calcium stearate and stearic acid. Fatty acid metal salts such as zinc acid,
An acid-modified polyolefin obtained by reacting an organic acid such as maleic acid with a polyolefin may be added. When a fatty acid, a fatty acid amide compound or a fatty acid metal salt is used, the addition ratio to the polyolefin is 0.05 to 5% by weight,
Preferably it is 0.1 to 4% by weight. When an acid-modified polyolefin is used, the addition ratio to the polyolefin is 5 to 30% by weight, preferably 5 to 20% by weight. The polyolefin used in the present invention may optionally contain various additives such as an antioxidant and a nucleating agent. The polyolefin-based porous membrane of the present invention has a thickness of 5 to 5 from the viewpoint of securing strength as a battery separator.
The thickness is preferably 100 μm, more preferably 10 to 30 μm, and the porosity is 30 to 60% from the viewpoint of securing strength as a battery separator, preventing short circuit inside the battery, and securing appropriate electric resistance. Preferably even 35
The air permeability is preferably from 50 to 1000 sec / 100 cc, and more preferably from 70 to 600 sec / 100 cc, from the viewpoint of securing the performance of the battery. It is preferably 3N or more, more preferably 4N or more, from the viewpoint of reducing the defective rate. The polyolefin-based porous membrane of the present invention contains particles having an average particle diameter of 100 nm or less in an amount of 1% by weight to 3% by weight.
It can be produced by a method in which a polyolefin composition obtained by melt-mixing a polyolefin mixed at a ratio of 0% by weight or less is stretched and made porous. The polyolefin has an average particle diameter of 100 nm.
The polyolefin composition in which the following particles are melt-mixed can be produced by a method in which the polyolefin and the particles are heated to a temperature higher than the melting point of the polyolefin and kneaded using a mixing device such as a kneader or a twin-screw extruder. . The polyolefin has an average particle diameter of 100 nm.
The polyolefin composition obtained by heating and kneading the following particles can be cooled and formed into a sheet or a tube. For example, a heat-kneaded polyolefin composition is sandwiched between cooled metal plates and quenched to form a sheet, or the heat-kneaded polyolefin composition is heated and kneaded using an extruder equipped with a sheet forming die at its tip. Can be formed into a sheet by taking out the material extruded from the cooling roll, or extruding the heated and kneaded polyolefin composition from the tubular die using an extruder equipped with a tubular die at the tip to form a tube. Polyolefin and average particle diameter 100 nm
The stretching for making the polyolefin composition obtained by heating and kneading the following particles porous can be performed by uniaxial stretching, biaxial stretching, or a combination of uniaxial stretching and biaxial stretching. The stretching for making the film porous is preferably performed at a temperature of from 20 ° C. to the melting point of the polyolefin, and more preferably from 20 ° C. to the melting point of the polyolefin, −20 ° C. In the case of uniaxial stretching, it can be performed using a stretching machine such as a roll stretching machine or a tenter. In the case of biaxial stretching, it can be carried out using a sequential stretching apparatus combining a stretching machine such as a roll stretching machine or a tenter or a simultaneous stretching apparatus using a simultaneous biaxial tenter. Further, the stretching may be performed in combination with the sequential stretching apparatus and the simultaneous stretching apparatus. The stretching for porosity is performed by first stretching at a low temperature to generate a crack at the interface between the polyolefin in the thin film and the particles having an average particle diameter of 100 nm or less, and then stretching at a higher temperature. Can grow and crack. Before performing the stretching for making the film porous, the polyolefin and the particles having an average particle size of 100 nm or less may be stretched in order to form a thin film of the polyolefin composition which is melt-mixed. When stretching to make a thin film,
It is preferable to carry out the stretching at a higher temperature than the stretching for making it porous, and furthermore, the melting point of the polyolefin −40 ° C. or more and the melting point + 10 ° C.
It is more preferable to perform stretching below. In addition, heat treatment may be performed to increase the degree of crystallinity of the polyolefin before performing the stretching for making the film porous. The present invention will be described based on examples. The method for evaluating the physical properties of the porous film in the examples is as follows. (A) Dial gauge manufactured by Thickness Ozaki Seisakusho (trademark "PEACOK N
o. 25 ". (B) Porosity The volume of the sample was determined from the thickness and area, the mass was measured, and the porosity was determined using the following equation. As the density, a value obtained by calculation from the density and the mixing ratio of the used polyolefin and the layered mineral particles was used. Porosity (%) = (1− (mass / density) / volume) × 100 (c) Air permeability Measured using a Gurley-type air permeability meter based on JIS P-8117. (D) Piercing strength A needle with a radius of curvature of 0.5 mm at the tip was attached to a compression tester KES-G5 made by Kato Tech, and a piercing test was performed at a piercing speed of 2 mm / sec. did. EXAMPLES Example 1 83% by weight of high-density polyethylene having a melting point of 134 ° C., a melt flow rate of 0.3 g / 10 minutes, and a density of 0.95 g / cm ³ , and 2% by weight of stearic acid %, And silicon oxide particles having an average particle diameter of 7 nm were mixed at a ratio of 15% by weight, and the mixture was heated and mixed using a Plastmill C type manufactured by Toyo Seiki Seisaku-Sho, Ltd. After heating and mixing for 10 minutes while setting the temperature of the plastomill at 200 ° C. and the number of revolutions at 50 rpm,
The molten mixture was removed from the plast mill and cooled.
The cooled and solidified mixture is sandwiched between two metal plates,
The sheet was compressed at a pressure of 10 MPa using a hot press machine set at 00 ° C. to form a sheet having a thickness of 0.8 mm. The obtained sheet was heated at a temperature of 1 using a biaxial stretching machine (Iwamoto Seisakusho).
The film was biaxially stretched at 28 ° C. in both the longitudinal and transverse directions at a stretch ratio of 6 times to prepare a biaxially stretched thin film having a thickness of 21 μm. The obtained thin film is stretched 1.3 times in one direction at a temperature of 70 ° C. (this stretching direction is defined as a longitudinal direction) using a biaxial stretching machine (manufactured by Iwamoto Seisakusho), and then perpendicular to this direction. The film was stretched twice at a temperature of 105 ° C. (the stretching direction was defined as a transverse direction) to form a polyolefin-based porous membrane. The resulting polyolefin-based porous membrane has a thickness of 20
μm, porosity 37%, air permeability 490 sec / 100c
c, the puncture strength was 4.1N. Example 2 The mixing ratio of the silicon oxide particles of Example 1 was 5% by weight, and the mixing ratio of the high-density polyethylene was 93%.
A polyolefin-based porous membrane was prepared under the same conditions as in Example 1 except that the amount was changed to% by weight. The obtained polyolefin-based porous membrane has a thickness of 23 μm, a porosity of 43%,
The air permeability was 570 seconds / 100 cc, and the puncture strength was 5.0 N. Comparative Example 1 The same conditions as in Example 1 except that the mixing ratio of the silicon oxide particles in Example 1 was changed to 0.5% by weight and the mixing ratio of the high-density polyethylene was changed to 99.5% by weight. Thus, an attempt was made to produce a polyolefin-based porous membrane. The obtained film had a thickness of 21 μm, a porosity of 1%, and an air permeability of 10,000 seconds / 100 cc or more, and showed no permeability. [Comparative Example 2] A polyolefin-based polyolefin was used under the same conditions as in Example 1 except that the mixing ratio of silicon oxide particles was changed to 40% by weight and the mixing ratio of high-density polyethylene was changed to 60% by weight. An attempt was made to make a porous membrane. The resulting polyolefin-based porous membrane had a small thickness of 24 μm, an air permeability of 320 sec / 100 cc, and a puncture strength of 1.1 N. The porous membrane of the present invention provides a polyolefin-based porous membrane which is easy to produce and contains a small amount of filler, and is extremely useful as a polyolefin-based porous membrane. .

Continuation of front page    F term (reference) 4D006 GA01 HA21 HA41 LA06 MA02                       MA03 MA24 MA31 MB03 MB16                       NA21 NA66 PA01 PB65                 4F074 AA17 AA24 AC20 AC32 AE01                       CA03 CA06 DA02 DA08 DA10                       DA23 DA49                 4J002 BB021 BB111 CH072 CL002                       DE146 DJ016 GQ02                 5H021 BB05 EE04 HH01 HH03

Claims (1)

Claims: 1. A particle having an average particle diameter of 100 nm or less
A polyolefin-based porous membrane comprising a polyolefin containing from 30% by weight to 30% by weight, wherein the polyolefin-based porous membrane is stretched and made porous.