JP5005387B2 - Method for producing polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a method for producing a polyolefin microporous membrane and a polyolefin microporous membrane obtained thereby.

Microporous membranes have various pore diameters, pore shapes, and numbers of pores, and are used in a wide range of fields because of their characteristics that can be manifested by their unique structures. For example, separation membranes used for water treatment and concentration utilizing the sieving effect due to the difference in pore diameter, adsorption sheets used for water absorption, oil absorption, deodorizing materials utilizing large surface area and porous space due to microporosity, difference in molecular size Moisture permeable waterproof sheet using the feature that allows air and water vapor to pass through but not water, multi-functionalized by filling various materials in the porous space, polymer electrolyte membrane and humidifying membrane useful for fuel cells, etc. Furthermore, they are used as liquid crystal materials and battery materials.
In the separation membrane field, ensuring selective permeability and maintaining the initial permeation amount are always required issues. For this reason, optimization of the pore shape and affinity control between the membrane substrate and the filtrate have been studied in the past, but with the diversification of the filtrate and filtration method, further improvements have been required for the membrane substrate. Yes.
In recent years, research and development of energy storage devices such as lithium ion secondary battery (LIB), lithium ion capacitor (LIC), electric double layer capacitor (EDLC), and application development have been promoted from the viewpoint of energy saving and resource saving. Consideration is being actively conducted. In these electricity storage devices, a porous film holding an electrolytic solution called a separator having a function of preventing contact between positive and negative electrodes and transmitting ions is provided between the positive and negative electrodes. Various performances are required depending on the intended use of the electricity storage device.

For example, high output characteristics and high safety for automotive applications, etc., and higher capacity and higher energy density for applications such as personal computers and mobile phones, uninterruptible power supply (UPS) and As development for power storage system applications and the like, high capacity and high reliability are required for power storage devices. The separator, which is one of the device constituent members, is strongly required to achieve both improved electrical characteristics and safety / reliability. However, this is a trade-off relationship and is not always satisfactory. In addition, compatibility with other constituent materials (positive electrode active material, negative electrode active material, electrolytic solution, etc.) is also an important factor in determining battery performance. For example, in LIB, in the positive electrode active material, composite oxides such as Li (NiCoMn) O ₂ and Li (NiCoAl) O ₂ , olivine type crystal compounds of LiMPO ₄ (M = Fe, Mn, Co, Ni, etc.), etc. With regard to the negative electrode active material, alloy compounds with Sn, Si, and the like have been energetically studied, and a highly productive method for producing a porous film capable of flexibly controlling the pore shape is required as needed.

A method of manufacturing a microporous membrane that can meet these various requirements has been proposed.
Patent Document 1 discloses a method for producing a microporous membrane in which a polyolefin resin, inorganic particles, and a plasticizer are melt-kneaded and formed into a sheet shape, biaxially stretched at a high magnification, and the plasticizer is extracted. The microporous membrane disclosed in the present technology is said to be suitable as a separator for a storage battery because it has a high puncture strength and a short circuit resistance at high temperatures and is excellent in electrolyte impregnation.
Patent Document 2 discloses a method for producing a separator for a non-aqueous battery comprising a porous film composed of a polyolefin resin and an inorganic powder. In this technique, low shrinkage is achieved by heat treatment at a relaxation rate of 5 to 50% in the stretching direction at a temperature equal to or higher than the melting point of polyolefin.

Patent Document 3 discloses a method for producing a high-strength microporous membrane in which a gel composition comprising a polyolefin and a solvent is stretched at a melting point + 10 ° C. or less, and then the residual solvent is removed.
Patent Document 4 discloses a method for producing a microporous film excellent in high-temperature dimensional stability, in which a polyethylene powder sintered sheet is melt-drawn at a temperature equal to or higher than the melting point.
WO2006-25323 JP 2001-266831 A Japanese Patent No. 3549311 WO2004-24809

However, in the above prior art, it is difficult to control the hole shape in a flexible manner, and the resulting membrane is not sufficient in terms of permeability. In the examples of Patent Document 1, since the stretching temperature is not higher than the melting point of the base polyethylene, it is not easy to control the hole shape to flexivir by stretching, and it is not always sufficient in terms of permeability. I couldn't. The film obtained by the heat treatment at the above-mentioned Patent Document 2, that is, the melting point or higher and the relaxation rate of 5 to 50% has reduced permeability, and when used as a power storage separator, for example, electrical characteristics such as output characteristics It becomes a film that remains uneasy. In Patent Document 3, since the gel-like composition, which is a non-porous sheet containing a solvent, is stretched, it is difficult to control the pore shape. Moreover, there is no description about extending | stretching more than melting | fusing point after solvent removal. The above-mentioned patent document 4 does not achieve porosity by melt stretching, but by applying a separate porous means after melt stretching (for example, a step of extracting and removing the plasticizer after swelling with a plasticizer), This is a method for obtaining a microporous membrane excellent in high-temperature dimensional stability, and the obtained membrane is not sufficient in terms of high permeability.
An object of the present invention is to provide a method for producing a highly porous microporous membrane, which is capable of providing a highly permeable microporous membrane and can control the pore shape flexibly.

The present inventor has found that high permeability and pore shape can be flexibly controlled by stretching a porous sheet having a three-dimensional network structure formed from a polyolefin resin and fine particles at a temperature higher than the melting point of the polyolefin resin. Invented the invention.
That is, the present invention is as follows.
(1) A method for producing a polyolefin microporous membrane comprising stretching a porous sheet having a three-dimensional network structure containing a polyolefin resin and fine particles at a temperature higher than the melting point of the polyolefin resin.
(2) The polyolefin according to (1), wherein the fine particles are inorganic particles, organic particles having a melting point higher than the melting point of the polyolefin resin, or organic particles having no melting point and a glass transition point higher than that of the polyolefin resin. A method for producing a microporous membrane.
(3) The method for producing a polyolefin microporous membrane according to (1) or (2), wherein the primary particle size of the fine particles is 1 nm or more and less than 1 μm.
(4) The method for producing a polyolefin microporous membrane according to any one of the above (1) to (3), wherein the puncture strength of the polyolefin microporous membrane is 3 N / 20 μm or more.
(5) The method for producing a polyolefin microporous film according to any one of (1) to (4), wherein the porosity of the porous sheet is 25% or more.
(6) The method for producing a polyolefin microporous membrane according to any one of the above (1) to (5), wherein the film is stretched at least once at a temperature not higher than the melting point of the polyolefin resin before being stretched at a temperature higher than the melting point of the polyolefin resin.
(7) (a) a step in which a polyolefin resin, inorganic particles and a plasticizer are melt-kneaded to obtain a melt,
(B) a step of forming the melt into a sheet to obtain a sheet-like molded product, (c) a step of drawing the sheet-like molded product at a temperature below the melting point of the polyolefin resin to obtain a stretched sheet, (d) A method for producing a polyolefin microporous membrane comprising: a step of obtaining a porous sheet by extracting a plasticizer from the stretched sheet; and (e) a step of stretching the porous sheet at a temperature higher than the melting point of the polyolefin resin.
(8) The method for producing a polyolefin microporous membrane according to any one of (1) to (7), wherein the polyolefin microporous membrane is for a storage battery separator.
(9) A polyolefin microporous membrane obtained by the production method of any one of (1) to (7) above.

  According to the present invention, a microporous membrane having high permeability can be provided. In addition, it is possible to provide a method for producing a highly productive microporous membrane capable of flexibly controlling the pore shape.

The method for producing a microporous membrane of the present invention will be described in detail below, particularly focusing on its preferred form.
The production method of the present invention is characterized in that a three-dimensional network structure porous sheet containing a polyolefin resin and fine particles is stretched at a temperature higher than the melting point of the polyolefin resin.
Polyolefin resins include polyolefin resins that can be used for normal extrusion, injection, inflation, blow molding, and the like, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-hexene. Homopolymers and copolymers such as octene, multistage polymers and the like can be used. In addition, polyolefins selected from the group of these homopolymers, copolymers, and multistage polymers can be used alone or in combination. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. . When the microporous membrane of the present invention is used as a battery separator, it is a low melting point resin and has high strength required performance, so that high density polyethylene is the main component (for example, 10 parts by mass or more in 100 parts by mass of polyolefin resin). It is preferable to use a resin to be used. From the viewpoint of heat resistance, it is more preferable to use a mixture of high density polyethylene and polypropylene. Specifically, it is preferable to mix polypropylene in a range of 16 to 80 parts by mass, more preferably 20 to 60 parts by mass in 100 parts by mass of the polyolefin resin. When the polypropylene is 16 parts by mass or more, the heat resistance is drastically improved. If it is 80 mass parts or less, the drawability in the composition containing a high density polyethylene and a fine particle becomes still better, and it is preferable.

The viscosity average molecular weight of the polyolefin resin used in the present invention or the matrix polymer of the microporous membrane obtained by the production method of the present invention is preferably 50,000 or more and less than 10 million, more preferably 200,000 or more and less than 3 million, more preferably Is between 400,000 and less than 1 million. A viscosity average molecular weight of 50,000 or more is preferable because the melt tension at the time of melt molding is increased and the moldability is easily improved, and sufficient entanglement is easily imparted and the strength is easily increased. If the viscosity average molecular weight is 10 million or less, it tends to be easy to obtain uniform melt kneading, and it is preferable because it tends to be excellent in sheet formability, particularly thickness stability.
Polyolefin resins include phenolic, phosphorus and sulfur-based antioxidants, metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light as necessary, as long as the advantages of the present invention are not impaired. Additives such as stabilizers, antistatic agents, antifogging agents, and coloring pigments can be mixed and used.

  The production method of the present invention stretches a porous sheet having a three-dimensional network structure containing a polyolefin resin and fine particles. When a porous sheet of a three-dimensional network structure made of a single polyolefin resin that does not contain fine particles is stretched at a temperature higher than the melting point of the polyolefin resin, the fibrils forming the three-dimensional network structure melt and flow easily. It becomes a hole-like sheet or a sheet with extremely low permeability. On the other hand, when a three-dimensional network structure porous sheet containing a polyolefin resin and fine particles is stretched at a temperature higher than the melting point of the polyolefin resin as in the present invention, the fine particles are dispersed in the polyolefin resin. Even if the temperature is higher than the temperature at which the fibrils melt, the thickening effect by the fine particles dispersed in the polyolefin resin allows the fibrils to retain their shape and be deformed by stretching without making the three-dimensional network structure nonporous. It is considered to be.

A porous sheet having a three-dimensional network structure is, for example, a method in which a polyolefin resin and a plasticizer are melt-kneaded and then cooled, and the plasticizer is extracted to make it porous (so-called wet phase separation method), or a polyolefin resin and an inorganic filler. After melt-kneading and molding on a sheet, a method of making the porous structure by peeling the interface between the polyolefin resin and the inorganic filler by stretching, the polyolefin resin is dissolved and then immersed in a poor solvent for the polyolefin resin. It can be produced by a method of making it porous by removing the solvent simultaneously with solidification. It is preferably obtained by a wet phase separation method.
The content of fine particles in the porous sheet is desirably 20% by mass or more and 80% by mass or less with respect to the total mass of the polyolefin resin and the fine particles. More preferably, it is 20 mass% or more and 60 mass% or less, More preferably, it is 40 mass% or more and 60 mass% or less. When the content is 20% by mass or more, the effect of thickening at the time of melting is sufficiently large, and even when stretched at a temperature higher than the melting point of the polyolefin resin, a microporous film having good permeability is easily obtained. If it is 80 mass% or less, since a high draw ratio is possible and a hole shape can be controlled more flexibly, it is preferable.

The fine particles preferably have a particle size of 1 nm or more from the viewpoint of dispersibility in the polyolefin resin. The thickness is preferably less than 1 μm from the viewpoint that the thickening effect by the fine particles is further improved, and that the interfacial peeling between the polyolefin resin and the fine particles hardly occurs at the time of stretching, and the hole shape can be flexibly controlled. The fine particles are more preferably 1 nm or more and less than 100 nm. The particle size can be measured with a scanning electron microscope or a transmission electron microscope.
It is preferable that the fine particles have substantially no internal surface area inside the primary particles, that is, the primary particles themselves have substantially no fine pores. When such fine particles are used, for example, when used as a separator for a non-aqueous electrolyte battery, there is a tendency that performance deterioration such as capacity reduction is difficult to occur. The reason is not clear, but if it does not substantially have fine pores inside the primary particles, it is easy to remove adsorbed water etc. in the normal drying process, so it is difficult to cause capacity reduction due to moisture mixing. Guessed.

  The fine particles are preferably those which are inactive with the polyolefin resin, the plasticizer, the additive and the like at a temperature not higher than the stretching temperature of the porous sheet and do not melt. Specifically, inorganic particles, organic particles having a melting point higher than that of the polyolefin resin, or organic particles having no melting point and a glass transition point higher than that of the polyolefin resin are preferable. When the organic particles do not have a melting point, the glass transition point may be higher than the melting point of the polyolefin resin. Specifically, the inorganic particles are preferably oxides and nitrides such as silicon, aluminum, titanium and magnesium, and carbonates and sulfates such as calcium and barium. Moreover, the inorganic particle which performed the surface treatment suitably can be used. For example, when producing microporous membranes for filtration applications using aqueous solvents and separators for aqueous electrolyte storage devices, hydrophilic particles are suitable, and separators for nonaqueous electrolyte storage devices are manufactured. In this case, inorganic particles that have been subjected to hydrophobic treatment are suitable. As the organic particles, particles such as homopolymers such as methyl methacrylate, ethyl methacrylate, methyl acrylate, acrylonitrile, styrene and the like, copolymers selected from two or more types of monomers, and crosslinked products thereof are preferable.

In the production method of the present invention, a porous sheet having a three-dimensional network structure is stretched at a temperature higher than the melting point of polyolefin. The three-dimensional network structure is a structure in which fibrous fibrils are randomly spread in a three-dimensional direction. Aggregates made of polyolefin and fine particles may be contained between the fibrils. For example, when a sintered sheet of resin powder that does not have fibrils is stretched, the entanglement points of the polyolefin molecular chains on the contact surface of the sintering are stretched, so the hole shape can be controlled flexibly. Absent. When a non-porous sheet is stretched, the shape of the hole cannot be controlled flexibly, and the permeability is low.
The porosity of the porous sheet is preferably from 25% to 90%, more preferably from 30% to 70%, and even more preferably from 35% to 60%. If it is 25% or more, interfacial peeling from the polyolefin resin hardly occurs at the time of stretching, and the hole shape can be controlled more flexibly. If it is 90% or less, there is little concern that the porosity will unnecessarily increase due to stretching, and the obtained microporous film is preferable because it tends to have higher strength.

  In the present invention, the porous sheet is stretched at a temperature higher than the melting point of the polyolefin. By stretching at a temperature higher than the melting point, the pore shape can be controlled flexibly, and a method for producing a microporous membrane with remarkably excellent permeability can be proposed. In addition, as described above, the stretched base material is a three-dimensional network porous sheet containing a polyolefin resin and fine particles, and has excellent permeability even when stretched at a temperature higher than the melting point of polyolefin. This is a necessary condition for producing a microporous membrane. The film is preferably stretched at a temperature 50 ° C. higher than the melting point (melting point + 50 ° C.) or lower, more preferably 20 ° C. higher than the melting point (melting point + 20 ° C.) or lower. A melting point of + 50 ° C. or lower is preferable because sufficient film strength can be easily obtained. The melting point referred to in the present invention is a peak top temperature of a melting endothermic curve measured with a differential scanning calorimeter (DSC). When a single polyolefin resin or two or more types of polyolefin resins are used and there are two or more melting endothermic peaks, the peak top temperature having the largest melting endotherm is regarded as the melting point in the present invention. Stretching is a step of changing the surface magnification to 1 times or more, and the stretching magnification is preferably 1.01 to 50 times, more preferably 1.1 to 10 times in terms of surface magnification. If it is 1.01 times or more, good permeability can be obtained. If it is 50 times or less, the durability of the fibril is excellent.

The production method of the present invention is preferably a method including the following steps (A) to (E).
(A) Step of melt-kneading polyolefin resin, fine particles and plasticizer (B) Step of forming melt into sheet (C) Step of drawing below melting point of polyolefin resin (D) Step of extracting plasticizer ( E) Step of stretching at a temperature higher than the melting point of the polyolefin resin There is no particular limitation on the order and number of times of these steps, but it is preferable to perform step (C) in a step prior to step (E). The most preferable form is a manufacturing method performed in the order of (A) → (B) → (C) → (D) → (E). That is,
(A) a step of melt-kneading a polyolefin resin, inorganic particles and a plasticizer to obtain a melt,
(B) forming the melt into a sheet and obtaining a sheet-like molded body,
(C) a step of stretching the sheet-like molded body at a temperature below the melting point of the polyolefin resin to obtain a stretched sheet;
(D) extracting a plasticizer from the stretched sheet to obtain a porous sheet;
(E) A method for producing a polyolefin microporous membrane comprising a step of stretching the porous sheet at a temperature higher than the melting point of the polyolefin resin is preferred. This order is preferable because the pore shape can be controlled flexibly and a microporous membrane having low shrinkage and high strength in addition to high permeability can be obtained.

  The plasticizer added in the step (a) (or step (A)) is preferably a non-volatile solvent capable of forming a uniform solution at a temperature equal to or higher than the melting point of the polyolefin resin when mixed with the polyolefin resin. Examples thereof include hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol. In particular, when the polyolefin resin is polyethylene, liquid paraffin is preferable because it has high compatibility with polyethylene and is less likely to cause interfacial peeling between the resin and the plasticizer during stretching, so that uniform stretching can be easily performed.

The amount ratio of the fine particles is preferably 20% by mass to 80% by mass with respect to the total mass of the polyolefin resin and the fine particles. More preferably, it is 20 mass% or more and 60 mass% or less, More preferably, it is 40 mass% or more and 60 mass% or less. When the content is 20% by mass or more, the effect of thickening at the time of melting is sufficiently large, and even when stretched at a temperature higher than the melting point of the polyolefin resin, a microporous film having good permeability is easily obtained. If it is 80 mass% or less, since a high draw ratio is possible and a hole shape can be controlled more flexibly, it is preferable.
The ratio of the polyolefin resin, fine particles, and plasticizer is a ratio that enables uniform melt-kneading, is a ratio that is sufficient to form a sheet-like microporous membrane precursor, and does not impair productivity. It ’s fine. Specifically, the mass fraction of the plasticizer in the composition comprising the polyolefin resin, fine particles, and the plasticizer is preferably 30 to 80 mass%, more preferably 40 to 70 mass%. When the plasticizer has a mass fraction of 80% by mass or less, the melt tension at the time of melt molding is hardly insufficient and the moldability tends to be improved, which is preferable. On the other hand, a mass fraction of 30% by mass or more is preferable because it becomes thinner in the thickness direction as the draw ratio increases and it becomes easier to obtain a thin film. In addition, since the plasticizing effect is sufficient, the crystalline folded lamellar crystal can be efficiently stretched, and when stretched at a high magnification, the polyolefin chain is not broken and a uniform and fine pore structure is obtained and the strength is easily increased.

  The method of melt-kneading polyolefin resin, fine particles and plasticizer is to introduce polyolefin resin and fine particles into a resin kneading apparatus such as an extruder or kneader, and introduce a plasticizer at an arbitrary ratio while heating and melting the resin, Furthermore, a method of obtaining a uniform solution by kneading a composition comprising a resin, fine particles and a plasticizer is preferred. As a more preferable method, a polyolefin resin, fine particles, and a plasticizer are previously kneaded at a predetermined ratio using a Henschel mixer or the like, and the kneaded product is put into an extruder and plasticized at an arbitrary ratio while being heated and melted. Introducing an agent and further kneading.

  In the step (b) (or (B) step), the melt is extruded and cooled and solidified to produce a sheet-like microporous membrane precursor (sheet-like molded body). Sheeting is performed by extruding a uniform solution of polyolefin resin, fine particles, and plasticizer into a sheet shape via a T-die, etc., and contacting a heat conductor to cool it to a temperature sufficiently lower than the crystallization temperature of the resin. Preferably it is done. As the heat conductor used for cooling and solidification, metal, water, air, plasticizer itself, or the like can be used. In particular, a method of cooling by contacting with a metal roll has the highest heat conduction efficiency and is preferable. Further, it is more preferable that the metal roll is sandwiched between the rolls because the heat conduction efficiency is further increased, the sheet is oriented and the film strength is increased, and the surface smoothness of the sheet is also improved. The die lip interval when extruding into a sheet form from a T die is preferably 400 μm or more and 3000 μm or less, and more preferably 500 μm or more and 2500 μm. A die lip spacing of 400 μm or more is preferable because it reduces the meander and the like, has less influence on the film quality such as streaks and defects, and prevents film breakage in the subsequent stretching process. A thickness of 3000 μm or less is preferable because the cooling rate is high and uneven cooling can be prevented and the thickness stability can be maintained.

  (C) The extending | stretching temperature in a process (or (C) process) is below melting | fusing point. When the stretching temperature is not higher than the melting point, sufficient orientation can be imparted and a high-strength microporous film can be easily obtained. The stretching direction is at least uniaxial stretching. When the film is stretched at a high magnification in the biaxial direction, it has a stable structure that is difficult to tear because of molecular orientation in the plane direction, and a high puncture strength is obtained. The stretching method may be simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, multi-stretching, etc., either alone or in combination, but if the stretching method is simultaneous biaxial stretching, the piercing strength is increased. And from the viewpoint of uniform film thickness. Here, the simultaneous biaxial stretching is a method in which stretching in the MD direction and stretching in the TD direction are performed simultaneously, and the deformation rate in each direction may be different. Sequential biaxial stretching is a technique in which stretching in the MD direction or TD direction is performed independently. When stretching is performed in the MD direction or TD direction, the other direction is unconstrained or fixed. It is in a state of being fixed to the length. The draw ratio is preferably in the range of 20 times to less than 200 times, more preferably in the range of 20 times to 100 times, and more preferably in the range of 25 times to 50 times. The stretching ratio in each axial direction is preferably 4 to 10 times in the MD direction, preferably 4 to 10 times in the TD direction, 5 to 8 times in the MD direction, and 5 to 8 times in the TD direction. The range of is more preferable. When the total area magnification is 20 times or more, sufficient strength can be imparted to the film, and when it is less than 200 times, film breakage is prevented and high productivity is obtained, which is preferable. When the film is stretched 4 times or more in the MD direction and 4 times or more in the TD direction, it is preferable because a product with small film thickness unevenness is easily obtained in both the MD direction and the TD direction.

In step (d) (or step (D)), a plasticizer is extracted from the sheet-like molded body. The method of extracting the plasticizer may be either a batch type or a continuous type. However, the plasticizer is extracted by immersing the microporous membrane in an extraction solvent and sufficiently dried, so that the plasticizer is substantially removed from the microporous membrane. It is preferable to remove. In order to suppress the shrinkage of the microporous membrane, it is preferable to constrain the end of the microporous membrane during a series of steps of immersion and drying. Further, the residual amount of plasticizer in the microporous membrane after extraction is preferably less than 1% by mass.
It is desirable that the extraction solvent is a poor solvent for the polyolefin resin and fine particles and a good solvent for the plasticizer, and has a boiling point lower than the melting point of the polyolefin microporous membrane. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, and non-chlorine-based solvents such as hydrofluoroether and hydrofluorocarbon. Examples thereof include halogenated solvents, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, and ketones such as acetone and methyl ethyl ketone.

In step (e) (or step (E)), the porous sheet is stretched at a temperature higher than the melting point of the polyolefin resin. Thereby, the pore shape can be controlled flexibly, and a method for producing a microporous membrane with extremely excellent permeability can be proposed. The stretching direction can be appropriately selected according to the hole shape. As the stretching method, any method such as simultaneous biaxial stretching, sequential biaxial stretching, multistage stretching, and multiple stretching may be used alone or in combination.
It is preferable to add a heat treatment step such as heat fixation and heat relaxation after each stretching step within a range not impairing the advantages of the present invention, since it further has an effect of further suppressing the shrinkage of the microporous membrane.

As long as the advantages of the present invention are not impaired, the present invention may be carried out using a multi-layer porous sheet, or the microporous film obtained in the present invention may be multilayered.
You may post-process in the range which does not impair the advantage of this invention. Examples of the post-treatment include hydrophilization treatment with a surfactant and the like, cross-linking treatment with ionizing radiation, and the like, coating a thermoplastic resin, inorganic particles, and the like on one side or both sides.
Next, the preferable form of the microporous film manufactured in the manufacturing method of this invention is described.
The content of fine particles in the microporous membrane is preferably 20% by mass or more and 80% by mass or less with respect to the total mass of the polyolefin resin and the fine particles. More preferably, it is 20 mass% or more and 60 mass% or less, More preferably, it is 40 mass% or more and 60 mass% or less. When the content is 20% by mass or more, the effect of thickening at the time of melting is sufficiently large, and even when stretched at a temperature higher than the melting point of the polyolefin resin, a microporous film having good permeability is easily obtained. If it is 80 mass% or less, since a high draw ratio is possible and a hole shape can be controlled more flexibly, it is preferable.

The final film thickness of the microporous membrane is preferably in the range of 2 μm to 100 μm, more preferably in the range of 5 μm to 40 μm, and still more preferably in the range of 5 μm to 35 μm. When the film thickness is 2 μm or more, the mechanical strength is sufficient, and when the film thickness is 100 μm or less, the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the capacity of the battery.
The porosity is preferably in the range of 25% to 90%, more preferably 40% to 80%, and still more preferably 50% to 80%. When the porosity is 25% or more, the permeability is hardly lowered, while when the porosity is 90% or less, there is little possibility of self-discharge when used as a battery separator, which is preferable.

The air permeability ranges from 1 second to 500 seconds, more preferably from 5 seconds to 200 seconds, still more preferably from 10 seconds to 100 seconds. When the air permeability is 1 second or more, self-discharge is small when used as a battery separator, and when it is 500 seconds or less, good output characteristics are obtained, which is preferable.
The puncture strength is preferably 3 N / 20 μm or more. When the thickness is 3N / 20 μm or more, film breakage due to the dropped active material or the like during battery winding can be suppressed. In addition, there is little fear of short-circuiting due to the expansion and contraction of the electrode accompanying charging and discharging. The upper limit is not particularly limited, but is preferably 20 N / 20 μm or less from the viewpoint that width shrinkage due to orientation relaxation during heating can be reduced. More preferably, they are 4N / 20micrometer or more and 20N / 20micrometer or less, More preferably, they are 5N / 20micrometer or more and 10N / 20micrometer.

The heat shrinkage at high temperature is preferably 30% or less. Both the MD (Machine Direction) direction and the TD (Transverse Direction) direction are preferably small, but when used as a battery separator, it is preferably low shrinkage in the TD direction. The MD direction is the traveling direction when manufacturing the microporous membrane, that is, the length direction, and the TD direction is the direction perpendicular to the MD direction, that is, the width direction. The shrinkage in the TD direction at a high temperature (for example, 150 ° C.) is preferably 30% or less, more preferably 25% or less, and further preferably 20% or less from the viewpoint of ensuring the safety of the battery. Although a minimum in particular is not restrict | limited, 1% or more is preferable from an adhesive viewpoint with an electrode.
The pore diameter of the microporous membrane can be flexibly controlled according to the purpose. The major axis / minor axis ratio of the pore diameter is preferably 1.01 or more and 100 or less, more preferably 1.5 or more and 50 or less, and further preferably 2 or more and 20 or less. If it is 1.01 or more, when used as a filtration membrane, there is little reduction in permeability due to permeability clogging, and if it is 100 or less, the durability of fibrils is excellent. The direction of the major axis and the minor axis can be selected from the MD direction, the TD direction, and any direction depending on the purpose of use of the microporous membrane.

EXAMPLES Next, although an Example demonstrates this invention further in detail, these do not restrict | limit the scope of the present invention. The test methods in the examples are as follows.
<Evaluation of microporous membrane>
(1) Viscosity average molecular weight 2,6-di-tert-butyl-4-methylphenol was dissolved in decahydronaphthalene to prevent deterioration of the sample to a concentration of 0.1 w%, and this (hereinafter abbreviated as DHN) ) As a sample solvent. The microporous membrane is dissolved in DHN at 150 ° C. to a concentration of 0.1 w%. The solution is filtered, fine particles are removed and used as a sample solution. Alternatively, a microporous film in which the fine particles are extracted and removed first by immersing the microporous film in a solution in which the fine particles dissolve but the polyolefin resin does not dissolve or react may be used. 10 ml of the prepared sample solution is sampled, and the number of seconds passing through the marked line (t) at 135 ° C. is measured with a Canon Fenceke viscometer (SO100). Further, after heating the DHN to 0.99 ° C., and 10ml collected to measure the number of seconds passing between marked lines of the viscometer in the same manner _{(t B).} The intrinsic viscosity [η] was calculated by the following conversion formula using the obtained passing seconds t and t _B.
[Η] = ((1.651 t / t _B −0.651) ^0.5 −1) /0.0834
From the obtained [η], the viscosity average molecular weight (Mv) was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67

(2) Measurement was performed using a primary particle size scanning electron microscope (SEM) “Model S-4800, manufactured by HITACHI”. The sample used was osmium-deposited and observed at an acceleration voltage of 1.0 kV.
(3) Film thickness It measured at room temperature 23 degreeC using the micro thickness measuring device (type KBM by Toyo Seiki).
(4) Porosity A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were obtained, and calculated from these and the film density (g / cm ³ ) using the following formula: did.
Volume (cm ³ ) = 10 × 10 × film thickness (μm) / 10000
Porosity (%) = (volume-mass / density of mixed composition) / volume × 100
In addition, the value calculated | required by calculation from the density and mixing ratio of each used polyolefin resin and a fine particle was used for the density of a mixed composition.

(5) Air permeability It measured with the Gurley type air permeability meter (made by Toyo Seiki) based on JIS P-8117.
(6) Puncture strength Using a handy compression tester KES-G5 (trademark) manufactured by Kato Tech, a microporous membrane was fixed with a sample holder having a diameter of 11.3 mm at the opening. Next, the center of the fixed microporous membrane is subjected to a piercing test in a 25 ° C. atmosphere at a needle radius of curvature of 0.5 mm and a piercing speed of 2 mm / sec. (N) was obtained. By multiplying this by 20 (μm) / film thickness (μm), a 20 μm film thickness equivalent puncture strength (N / 20 μm) was calculated.
(7) Shrinkage rate A sample of MD120 mm × TD120 mm square was cut out from the microporous membrane and marked with an oil-based pen at three locations at intervals of TD100 mm. The microporous film was sandwiched between copy paper (manufactured by KOKYUYO) having an A4 size, a basis weight of 64 g / m ² , and a paper thickness of 0.092 mm, and the sides of the copy paper were stapled. It was placed horizontally in an oven at 150 ° C. and left for 1 hour. Then, it air-cooled and measured TD length (mm) between marks. The shrinkage rate was calculated from the average value at three locations.
Shrinkage rate (%) = (1-TD length (mm) / 100) × 100

(8) MD / TD pore size ratio The pore size of the microporous membrane was measured using a scanning electron microscope (SEM) “Model S-4800, manufactured by HITACHI”. The sample used was osmium-deposited and observed at an acceleration voltage of 1.0 kV. Observe and print at a magnification of about 10 to 300 holes per 5 cm x 5 cm of the surface photograph, count the MD diameter and TD diameter of the holes present from the surface photograph, and calculate the MD / TD hole diameter ratio from the average value. Was calculated. In Example 1, it was observed and measured at a magnification of 30000 times.
(9) Melting point It measured using DSC60 by Shimadzu Corporation. A perforated sheet was punched into a circle with a diameter of 5 mm, and several sheets were overlapped to give 3 mg as a measurement sample. This was spread on an aluminum open sample pan having a diameter of 5 mm, and a clamping cover was placed thereon and fixed in the aluminum pan with a sample sealer. Under a nitrogen atmosphere, the temperature was increased from 30 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min to obtain a melting endothermic curve. The peak top temperature of the obtained melting endotherm curve was defined as the melting point (Tm [° C.]).

[Example 1]
22.5 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with Mv of 2 million 15 parts by mass, silica “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having a primary particle diameter of 15 nm, and liquid paraffin “Smoyl P-350P” (trademark, Matsumura Co., Ltd.) as a plasticizer. 37.5 parts by mass of Petroleum Research Institute Co., Ltd. and 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant This was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the total amount of liquid paraffin in the entire mixture melt-kneaded and extruded was 60 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 40 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1480 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter was 121 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried to obtain a porous sheet having a three-dimensional network structure. The resulting porous sheet had a melting point of 137 ° C. Further, the porous sheet was guided to a transverse tenter with the exit magnification set to 1.2 times, stretched in the TD direction, and wound to obtain a microporous film. At this time, the set temperature of the TD stretched portion was 145 ° C. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 2]
17.5 parts by weight of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with an Mv of 2 million 12.5 parts by mass, alumina “AluC” (trademark, manufactured by Degussa) having a primary particle size of 13 nm is 37 parts by mass, and liquid paraffin “Smoyl P-350P” (trademark, Matsumura Oil Co., Ltd.) as a plasticizer. Made by adding 33 parts by mass of a laboratory) and 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant A microporous membrane was obtained in the same manner as in Example 1 except that the preliminary mixing was performed using a super mixer. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 3]
Example 2 except that titania “TTO-55 (S)” (Ishihara Sangyo Co., Ltd.) having a primary particle size of 40 nm was used instead of alumina “AluC” (trademark, manufactured by Degussa) of Example 2. In the same manner, a microporous membrane was obtained. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 4]
A microporous membrane was obtained in the same manner as in Example 1 except that the TD stretch ratio of the porous sheet was changed to 1.6 times. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 5]
Instead of TD stretching with the horizontal tenter of Example 1, MD stretching with a roll stretching machine was performed. A microporous membrane was obtained in the same manner as in Example 1 except that the winding speed / feeding speed ratio was set to 2.0 times and MD stretching was performed at a roll temperature of 145 ° C. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 6]
22.5 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with Mv of 2 million 15 parts by mass, silica “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having a primary particle diameter of 15 nm, and liquid paraffin “Smoyl P-350P” (trademark, Matsumura Co., Ltd.) as a plasticizer. 37.5 parts by mass of Petroleum Research Institute Co., Ltd. and 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant This was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the total amount of liquid paraffin in the entire mixture melt-kneaded and extruded was 60 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 40 ° C. to obtain a sheet-like polyolefin composition having a thickness of 2200 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter is 124 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. Furthermore, it led to the horizontal tenter which set the exit magnification | multiplying_factor to 0.96 times, and it heat-relaxed in TD direction and wound up and obtained the porous sheet of a three-dimensional network structure. At this time, the set temperature of the TD relaxation part was 150 ° C. The melting endothermic curve of the obtained porous sheet showed two peak tops. The melting point determined from the peak top with the larger melting endotherm was 130 ° C. Next, the porous sheet was MD-stretched using a roll stretching machine to obtain a microporous film. At this time, the winding speed / feeding speed ratio was set to 2.5 times, and the roll temperature was 145 ° C. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 7]
17.5 parts by weight of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with an Mv of 2 million 12.5 parts by mass, alumina “AluC” (trademark, manufactured by Degussa) having a primary particle size of 13 nm is 37 parts by mass, and liquid paraffin “Smoyl P-350P” (trademark, Matsumura Oil Co., Ltd.) as a plasticizer. Made by adding 33 parts by mass of a laboratory) and 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant A microporous membrane was obtained in the same manner as in Example 6 except that the preliminary mixing was performed using a super mixer. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 8]
18 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with an Mv of 2 million 12 parts by mass, 7.5 parts by mass of homopolypropylene “H-100M” (manufactured by Prime Polymer) having an Mv of 400,000, and 25 parts by mass of silica “DM10C” (trademark, manufactured by Tokuyama Corporation) having a primary particle size of 15 nm 37.5 parts by weight of liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer and pentaerythrityl-tetrakis- [3- (3,5- What added 1.0 part by mass of [di-t-butyl-4-hydroxyphenyl) propionate] was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the total amount of liquid paraffin in the entire mixture melt-kneaded and extruded was 60 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 40 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1480 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter was 121 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried to obtain a porous sheet having a three-dimensional network structure. The melting endothermic curve of the obtained porous sheet showed two peak tops (137 ° C. and 163 ° C.). The melting point determined from the peak with the largest melting endotherm was 137 ° C. Next, the porous sheet was MD-stretched using a roll stretching machine to obtain a microporous film. At this time, the winding speed / feeding speed ratio was set to 1.5 times, and the roll temperature was 145 ° C. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Example 9]
14.4 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with Mv of 2 million 9.6 parts by mass, Mv 400,000 Homopolypropylene “H-100M” (Prime Polymer Co., Ltd.) 6 parts by mass, and alumina “AluC” (trademark, manufactured by Degussa) whose primary particle size is 13 nm is 37 parts by mass. Part, 33 parts by mass of liquid paraffin “Smoyl P-350P” (trademark, manufactured by Matsumura Oil Research Co., Ltd.) as a plasticizer, and pentaerythrityl-tetrakis- [3- (3,5-di-) as an antioxidant t-butyl-4-hydroxyphenyl) propionate] was added except that 0.3 parts by mass was premixed with a super mixer. 8 and to obtain a microporous membrane in a similar manner. Table 1 shows the film forming conditions and the characteristics of the obtained microporous film.

[Comparative Example 1]
22.5 parts by mass of high-density polyethylene “SH800” (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene “UH850” (trademark, Asahi Kasei Chemicals Corporation) with Mv of 2 million 15 parts by mass, silica “DM10C” (trademark, manufactured by Tokuyama Co., Ltd.) having a primary particle diameter of 15 nm, and liquid paraffin “Smoyl P-350P” (trademark, Matsumura Co., Ltd.) as a plasticizer. 37.5 parts by mass of Petroleum Research Institute Co., Ltd. and 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant This was premixed with a super mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the total amount of liquid paraffin in the entire mixture melt-kneaded and extruded was 60 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 40 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1480 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter was 121 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried to obtain a microporous membrane. The melting point of the obtained sample was 137 ° C. Table 2 shows the film forming conditions and the characteristics of the obtained microporous film.

[Comparative Example 2]
The microporous membrane obtained in Comparative Example 1 was guided to a horizontal tenter with the exit magnification set to 1.6 times and stretched in the TD direction, and then wound to obtain a microporous membrane. The set temperature of the TD stretch portion was 130 ° C. Table 2 shows the film forming conditions and the characteristics of the obtained microporous film.

[Comparative Example 3]
The microporous film obtained in Comparative Example 1 was MD stretched with a roll stretching machine. At this time, the winding speed / feeding speed ratio was set to 2.0 times, and the roll temperature was 120 ° C. Table 2 shows the film forming conditions and the characteristics of the microporous film.

[Comparative Example 4]
The microporous membrane obtained in Comparative Example 1 was guided to a horizontal tenter with the exit magnification set to 0.92 times and subjected to thermal relaxation in the TD direction to obtain a microporous membrane. The set temperature of the TD relaxation part was 150 ° C. Table 2 shows the film forming conditions and the characteristics of the microporous film.

[Comparative Example 5]
60 parts by mass of high-density polyethylene "SH800" (trademark, manufactured by Asahi Kasei Chemicals Corporation) with a viscosity average molecular weight (Mv) of 270,000, ultra high molecular weight polyethylene "UH850" (trademark, manufactured by Asahi Kasei Chemicals Corporation) with an Mv of 2 million 40 parts by mass, 0.3 parts by mass of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added, and a tumbler blender was used. Dry blending was performed to obtain a polymer mixture. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the total amount of liquid paraffin in the entire mixture to be melt kneaded and extruded was 65 parts by mass. The melt-kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 180 rpm, and a discharge rate of 12 kg / h. Subsequently, the melt-kneaded product was extruded through a T-die between cooling rolls controlled at a surface temperature of 40 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1480 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter is 123 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried to obtain a porous sheet having a three-dimensional network structure. The resulting porous sheet had a melting point of 137 ° C. Furthermore, the obtained porous sheet was led to a horizontal tenter having an exit magnification set to 1.2 times, stretched in the TD direction, and wound to obtain a microporous film. At this time, the set temperature of the TD stretched portion was 145 ° C. Table 2 shows the film forming conditions and the characteristics of the obtained microporous film.

  As apparent from Tables 1 and 2, in Examples 1 to 8, the hole shape can be easily controlled by the stretching direction and magnification, whereas in Comparative Examples 1 to 3, the hole shape is stretched. The change is small. In Comparative Examples 4 and 5, the permeability is remarkably inferior. In this invention, it can be said that it is excellent in permeability | transmittance by extending | stretching at high temperature from melting | fusing point of polyolefin resin, and can be said to be a manufacturing method of the microporous film | membrane which can control a hole shape flexibly.

  According to the present invention, a microporous membrane having high permeability can be provided. Furthermore, the hole shape can be controlled flexibly. Therefore, as a method for producing a microporous membrane particularly suitable as a microporous membrane suitable for a filtration membrane or the like that requires permeation amount and selective permeability, or a storage battery separator having excellent electrical characteristics such as output characteristics and productivity. Available.

Claims (8)

A method for producing a polyolefin microporous membrane in which a porous sheet having a three-dimensional network structure containing a polyolefin resin and fine particles is stretched at a temperature higher than the melting point of the polyolefin resin, wherein the fine particles are formed of inorganic particles from the melting point of the polyolefin resin. A method for producing a polyolefin microporous membrane, which is an organic particle having a high melting point, or an organic particle having no glass melting point and a glass transition point higher than that of the polyolefin resin . The method for producing a polyolefin microporous membrane according to claim 1, wherein the primary particle diameter of the fine particles is 1 nm or more and less than 1 μm. The method for producing a polyolefin microporous membrane according to claim 1 or 2 , wherein the puncture strength of the polyolefin microporous membrane is 3 N / 20 µm or more. The method for producing a polyolefin microporous membrane according to any one of claims 1 to 3 , wherein the porosity of the porous sheet is 25% or more. The method for producing a polyolefin microporous membrane according to any one of claims 1 to 4 , wherein the film is stretched at least once at a temperature not higher than the melting point of the polyolefin resin before being stretched at a temperature higher than the melting point of the polyolefin resin. (A) a step of melt-kneading a polyolefin resin, inorganic particles and a plasticizer to obtain a melt,
(B) forming the melt into a sheet and obtaining a sheet-like molded body,
(C) a step of stretching the sheet-like molded body at a temperature below the melting point of the polyolefin resin to obtain a stretched sheet;
(D) extracting a plasticizer from the stretched sheet to obtain a porous sheet;
(E) A method for producing a polyolefin microporous membrane comprising a step of stretching the porous sheet at a temperature higher than the melting point of the polyolefin resin. The method for producing a polyolefin microporous membrane according to any one of claims 1 to 6 , wherein the polyolefin microporous membrane is for a storage battery separator. The polyolefin microporous film obtained by the manufacturing method as described in any one of Claims 1-6 .