JPH08134253A - Production of microporous film from high-molecular-weight polyethylene - Google Patents

Abstract

PURPOSE: To obtain a microporous film excellent in tensile strength, piercing strength, void content, and air permeability while using a solvent excellent in safety, environmental compatibility, and hygienic properties by subjecting a sheet made from a mixture of a high-mol.-wt. polyethylene with a paraffin wax to biaxial stretching, thermal treatment, and solvent removal. CONSTITUTION: A mixture of a high-mol.-wt. polyethylene with a paraffin wax is melt kneaded and solidified by cooling to give a sheet, which is then biaxially stretched at a temp. higher than the m.p. of the paraffin wax and lower then the m.p. of the polyethylene. The resulting stretched film is thermally treated at a temp. higher than the m.p. of the wax and lower than the m.p. of the polyethylene, is freed from the remaining wax with a lower alcohol solvent at a temp. lower that the m.p. of the wax, and is dried, thus giving a microporous high-mol.-wt. polyethylene film having an intrinsic viscosity [η]of 3.5dl/g or higher.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a high molecular weight polyethylene or an ethylene-α-olefin copolymer (hereinafter,
(Hereinafter simply referred to as high molecular weight polyethylene), and more specifically, it relates to a method for producing a microporous biaxially stretched film, and more specifically, a high molecular weight polyethylene (A) having an intrinsic viscosity [η] of 3.5 dl / g or more and paraffin. A wax (B) is melt-kneaded at a specific ratio and conditions to form a sheet, and then a sheet having a melting point higher than that of the paraffin wax (B) and lower than the melting point of the high-molecular-weight polyethylene (A) is used. Of the paraffin wax (B) is removed by the biaxial stretching in the presence of a part, heat treatment is performed in a fixed state in at least one direction, and then the paraffin wax (B) is substantially completely removed by the alcohol solvent (C). It relates to a method of producing a porous high molecular weight polyethylene film. The film obtained by the present invention is suitably used mainly as a separator for non-aqueous electrolyte secondary batteries, particularly as a separator for lithium ion batteries.

[0002]

2. Description of the Related Art In recent years, lithium ion battery separators have attracted attention as non-aqueous battery separators. The physical structural characteristics imposed on this separator are, in order to prevent passage of the dropout active material of the electrode, excellent puncture strength, excellent ion conductivity, that is, excellent air permeability, which is a substitute physical property, and electrolytic solution. Is required, that is, the porosity is large. Several prior arts have been proposed as a method for producing a film having such characteristics.

Japanese Unexamined Patent Publication (Kokai) No. 60-197752 discloses that ultra high molecular weight polyethylene having a viscosity average molecular weight of 400,000 or more should be used.
A flow improver, 99.5 to 45% by weight, which is a solid at room temperature selected from alcohols, ethers, ketones or esters having 15 or more carbon atoms and has a melting point lower than that of polyethylene, is uniformly added to 5 to 55% by weight. The mixed ultrahigh molecular weight polyethylene composition is molded into a sheet or film in a molten state without solidifying, and the flow improver component is extracted with lower alcohol or water or a mixture of lower alcohol and water during or after the molding. Disclosed is a method for producing an ultra high molecular weight polyethylene sheet or film.

Japanese Patent Laid-Open No. 3-236155 discloses that
Liquid paraffin, solid paraffin, stearyl alcohol, and cesyl alcohol are mixed as a plasticizer with ultra-high molecular weight polyethylene, and the composition is melt-kneaded with an extruder,
Form a sheet, and then add hydrocarbons such as pentane, hexane, heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorohydrocarbons such as trifluoroethane, alcohols such as methanol, ethanol, propanol, etc. A method of producing a microporous film by removing the plasticizer and then biaxially stretching the film in the machine direction 2 to 6 times and in the transverse direction 2 to 8 times is disclosed.

The core of these techniques is to use an aliphatic alcohol, which is solid at room temperature, as a plasticizer and remove it with a lower alcohol. The point is to use a lower alcohol, which has no danger of electrostatic ignition, as a solvent. Although it is an advantage, the method of extracting a higher alcohol after stretching, which has a small pore size and can obtain uniform microporosity, as disclosed in the above-mentioned JP-A-60-197752 has a gas permeability of a film that can be produced. There is a drawback that it is not enough.

Further, as disclosed in Japanese Patent Application Laid-Open No. 3-236155, a method in which the plasticizer is completely removed prior to stretching is such that the microporosity of the sheet is reduced when the plasticizer is removed from the sheet before stretching. It can be lost, and the film obtained at the same time as the work is difficult has a large vein-like pore size distribution,
It is not desirable to use it as a non-aqueous separator material due to the size of the maximum pore size.

Japanese Unexamined Patent Publication (Kokai) No. 2-94356 discloses a method in which a high molecular weight polyethylene is mixed with an inorganic fine powder such as fine silicic acid and an organic liquid such as dioctyl phthalate to form a film, and the inorganic powder is then treated with an alkali or a liquid organic substance. 1,1,1-
Disclosed is a method for producing a microporous film, which is removed by trichloroethane and stretched under specific conditions. The film obtained by this method has an advantage that it has large porosity and excellent air permeability, but 1,1,1-trichloroethane used here has no risk of ignition, explosion, etc., but it has carcinogenicity and environmental pollution. It has been pointed out that this is a solvent, and there is concern over the health problems of workers.

Further, Japanese Patent Application Laid-Open No. 3-105851 discloses a method for producing a microporous film in which a high molecular weight polyethylene and liquid paraffin are mixed to form a film, which is stretched under specific conditions and then the liquid paraffin is removed with methylene chloride. It is disclosed. The methylene chloride used here is a solvent that does not have a risk of ignition or explosion, like the above-mentioned solvent, but is a carcinogenic concern, and like the above, there are concerns about the health problems of workers. It

Conventionally, in a method of producing a biaxially stretched microporous film by using a high molecular weight polyethylene and a plasticizer, a plastic film satisfying safety, environmental compatibility, hygiene and high separator property of the film is used. There was no combination of agent and solvent. Therefore, from the viewpoint of flammability, the flash point is not extremely low, at least 0 ° C. or lower, the risk of electrostatic ignition is low, the environmental compatibility is satisfied, the safety from the viewpoint of hygiene, and It is desired to develop a production method using a solvent that increases the porosity.

Since chlorine-based solvents and fluorine-based solvents have extremely low risk of ignition and explosion, they are excellent in work safety and are widely used industrially. However, in recent years, a solvent that replaces the fluorine-based solvent (for example, CFC 113) has been sought from the viewpoint of environmental problems. Further, chlorine-based solvents (for example, 1,1,1-trichloroethane and methylene chloride) have not only environmental problems but also hygiene problems such as carcinogenicity, and other solvents that can replace them are being searched for. Recently, light boiling hydrocarbons (such as hexane and heptane) have been proposed as alternative solvents, but these solvents are flammable, have a low flash point, and have a high risk of electrostatic ignition and explosion. Compared with the case of using the above-mentioned fluorine-based or chlorine-based solvent, the technical aspect and equipment cost are large compared to the case of using the above-mentioned solvent, and even if these are cleared, there is a risk of ignition and explosion. The mental burden of dealing with a solvent having a certain property is great for the worker.

Further, even in the case of hydrocarbons, if the solvent has more than 10 carbon atoms, the flash point will be room temperature or higher, and the risk of flammable explosion will be relatively reduced, but the boiling point will be high, and drying will be extremely difficult. It does not meet its original purpose as a solvent. On the other hand, lower alcohol solvents have been attracting attention from the viewpoint of the above-mentioned problems. In the case of a lower alcohol solvent, it is a combustible substance, but it has a smaller amount of heat of combustion than hydrocarbons, has a very low risk of electrostatic ignition, and is an extremely excellent solvent from the viewpoint of environmental hygiene. In particular, in the case of a mixed system with a small amount of water, since the flammability is reduced, if these lower alcohol solvents can be used, not only in terms of safety and environmental hygiene, but also in terms of manufacturing equipment cost. Be beneficial.

[0012]

In recognition of such prior art, the present invention does not use a solvent having a low flash point and a risk of electrostatic ignition, or a solvent which may cause carcinogenicity or environmental pollution. In addition, tensile strength, puncture strength and porosity,
An object is to provide a method for producing a film having excellent air permeability. The film obtained by the production method of the present invention exhibits excellent suitability for use as a battery separator, particularly as a lithium ion battery separator.

[0013]

The present invention provides a method for producing a microporous polyethylene film as proposed in the prior art, which is a lower alcohol solvent or a lower alcohol solvent excellent in safety and environmental hygiene. A method for producing a microporous high-molecular-weight polyethylene film which is comparable to the purpose of use in comparison with the case of using a fluorine-based or chlorine-based solvent using a composition of water and water Is.

That is, according to the present invention, there is provided a method for producing a microporous film comprising a high molecular weight polyethylene (A) having an intrinsic viscosity [η] of 3.5 dl / g or more, which comprises the following steps: , Step 1. 1. A step of melt-kneading a mixture of high-molecular-weight polyethylene (A) and paraffin wax (B) to obtain a cooled and solidified sheet, Step 2. A step in which the sheet is biaxially stretched at a temperature not lower than the melting point of the paraffin wax (B) and not higher than the melting point of the high molecular weight polyethylene, with a part or all of the paraffin wax (B) remaining in the sheet; Step 3. After the biaxial stretching, a step of heat-treating the film in the presence of the paraffin wax (B) at a temperature not lower than the melting point of the paraffin wax (B) and not higher than the melting point of the high molecular weight polyethylene (A), Step 4. The paraffin wax (B) remaining from the film is removed using a lower alcohol solvent (C) at a temperature not lower than the melting point of the paraffin wax (B) and lower than the melting point of the high molecular weight polyethylene (A), and then dried. Accordingly, the method for producing a high-molecular-weight polyethylene microporous film is provided, which comprises the step of obtaining a microporous high-molecular-weight polyethylene film.

Further, according to the present invention, the lower alcohol solvent (C) is a solvent composed of isopropyl alcohol, normal propyl alcohol and isobutyl alcohol, alone or as a mixture of two or more kinds thereof, a high molecular weight polyethylene microporous film. A method of manufacturing the same is provided.

According to the present invention, the lower alcohol solvent (C) is normal butyl alcohol or isoamyl alcohol, and after the paraffin wax is removed, the alcohol solvent (C) is replaced with water. A method of making a high molecular weight polyethylene microporous film that is then dried is provided.

Further, according to the present invention, a paraffin wax (B) having a room temperature solid and a melting point of 50 ° C. or lower and a lower alcohol solvent (C) being ethanol are used to produce a high molecular weight polyethylene microporous film. A method is provided.

[0018]

DETAILED DESCRIPTION OF THE INVENTION Preparation of Microporous High Molecular Weight Polyethylene Film Below, how the microporous high molecular weight polyethylene film of the present invention should be prepared, that is, raw materials and preparation method (of the raw sheet) The method of preparation, the method of biaxial stretching, the method of heat treatment), and the physical properties and structure of the obtained film will be sequentially described.

<Raw material> The high-molecular-weight polyethylene (A) used in the present invention is a slurry polymerization of ethylene, or ethylene and a small amount of an α-olefin having 4 or more carbon atoms, for example, using a Ziegler type catalyst. It is polyethylene having an intrinsic viscosity [η] of 3.5 dl / g or more. Examples of the α-olefin having 4 or more carbon atoms include butene-1, pentene-1, 4-methylpentene-1, hexene-1, octene-1 and the like, or a combination of two or more thereof. It is preferable that an α-olefin having 6 or more carbon atoms such as methylpentene-1, hexene-1, and octene-1 is used as at least one component. The amount of α-olefin comonomer is preferably 5 mol% or less. Of these, preferred comonomers are propylene, butene-1,4-methyl-pentene-1.

High molecular weight polyethylene (A) used also
Has a molecular weight corresponding to the above-mentioned intrinsic viscosity [η], more preferably the intrinsic viscosity [η] is in the range of 5 to 15 deciliters / gram, and more preferably 5 to 10 deciliters / gram. belongs to. 3.
Below 5 deciliters / gram, the mechanical strength is low. If it exceeds 15 deciliters / gram,
Since the compatibility with the plasticizer becomes poor, the uniformity when formed into a film is poor.

Paraffin wax (B) in the present invention
Is used as a plasticizer, that is, as a fluidity improver, and the paraffin wax (B) has a boiling point higher than that of the high molecular weight polyethylene (A), and preferably has a high boiling point of the high molecular weight polyethylene (A). ) Melting point + 10 ℃
And the melting point is 110 ° C. or lower, 1
By melt-kneading at a temperature of 20 ° C. or higher, easily
It is a paraffin wax that is compatible with the high-molecular-weight polyethylene (A) and forms a uniform mixture, and is a room-temperature solid paraffin wax having a molecular weight of 2000 or less, and preferably has a molecular weight of 400 or more and 1000 or less from the viewpoint of dispersibility. .

Specific examples of the paraffin wax include n-alkanes having 22 or more carbon atoms such as docosane, tricosane, tetracosane, and triacontane, or a mixture of lower n-alkanes containing them as a main component, or separated and produced from petroleum. The so-called paraffin wax is used.

<Preparation of Original Sheet> The method for preparing the original sheet is as follows. High molecular weight polyethylene (A)
And the paraffin wax (B) have a composition ratio of 15 to 80% by weight of high molecular weight polyethylene (A) and 85 to 20% by weight of paraffin wax (B), preferably 20 to 60% by weight of high molecular weight polyethylene (A) and paraffin. Wax (B) 80 to 40% by weight, more preferably high molecular weight polyethylene (A) 25 to 50% by weight
And paraffin wax (B) in an amount of 75 to 50% by weight. These are uniformly blended, melt-mixed, and then cooled and solidified to obtain a raw sheet.

The high-molecular-weight polyethylene (A) and the paraffin wax (B) are melt-kneaded by mixing with a kneader such as a Henschel mixer, a V-blender, a ribbon blender, a tumbler blender, and then a uniaxial extruder,
A screw extruder such as a twin-screw extruder, a kneader, a Banbury mixer, or the like can usually be used at a temperature of not lower than the melting point and not higher than 300 ° C. Kneading at a temperature below the melting point of the ethylene / α-olefin copolymer results in a high viscosity of the mixture and cannot be uniformly mixed. Further, melt-kneading at a temperature exceeding 300 ° C. is not preferable because the ethylene / α-olefin copolymer is thermally deteriorated. Particularly preferable melt-kneading temperature is 180 to 2
It is in the range of 50 ° C.

For forming a raw sheet, extrusion molding using an extruder equipped with a T-die is usually preferable, and compression molding may be used although the productivity is poor. The melt-kneading may be performed in advance before forming the sheet, or may be performed by a continuous method in which the raw sheet is extruded from a die while melt-kneading with a screw extruder or the like. In the case of a screw extruder, a multi-screw extruder is preferable to a single-screw extruder for uniform kneading of the high molecular weight polyethylene (A) and the paraffin wax (B). In the case of compression molding, melt kneading is separately performed in advance, and the sheet-like shape is imparted by compression molding. The thickness of the original sheet is 0.05 mm to 5 because it is sandwiched by a chuck during stretching.
It is preferably in the range of mm.

<Biaxial Stretching> The raw sheet prepared as described above is then stretched. Although the removal of the paraffin wax (B) will be described in detail later, the stretching must be performed while at least part of the added paraffin wax (B) remains in the original sheet. When substantially all of the paraffin wax (B) is removed prior to stretching, the obtained porous biaxially oriented film does not have a uniform porous structure and has a vein-shaped non-uniform shape with a distribution in size. It is not preferable because it has a uniform porous structure. Further, it has a large average pore size, and is not suitable for a battery separator, which is a main application of the microporous film obtained by the film production method provided by the present invention, particularly a separator for a lithium ion battery.

The obtained sheet is preferably biaxially stretched at a temperature of 60 to 140 ° C. in the machine and transverse directions simultaneously or sequentially. The lower limit of the stretching temperature is 60 ° C. By stretching at a temperature within this range, the film has a uniform microporous structure. By stretching at a temperature of 60 ° C. or less, the attainable stretching ratio remains at a low value, and as a result, it is difficult to exhibit excellent puncture strength.
Further, the stretching stress is large, which is disadvantageous in the stretching operation.

When the stretching temperature is higher than 140 ° C., the high molecular weight polyethylene (A) is unlikely to be subjected to stretching stress, and as a result, it is also difficult to exhibit excellent tensile strength. When the stretching temperature is 160 ° C. or higher, a microporous structure is not formed and a dense structure is formed.
Is the upper limit of the stretching temperature. Examples of the method for biaxially stretching the prepared raw sheet include simultaneous or sequential biaxial stretching by a tenter method, or a sequential biaxial stretching method in which a longitudinal direction is stretched by a roll or the like and then a transverse direction is stretched by a tenter. To be

The preferred stretching ratio is 9 times or more in terms of areal magnification (longitudinal stretching ratio × horizontal stretching ratio), and more preferably 16 times or more. For the purpose of producing an ultrathin film, a stretching ratio of 20 times or more is preferable in both the machine direction and the transverse direction. When the stretching ratio exceeds 20 times, the thickness of the high molecular weight biaxially stretched film produced by stretching becomes 1/400 or less depending on the porosity and the composition of the raw sheet, so that it is ultra-thin. Although it is suitable for producing a film (film thickness of 1 μm or less), it can be said that the present invention is not suitable for producing a film as a separator, which is the main object of the present invention, except for a special case at present.

When the stretching ratio is 10 times or more in each of the machine direction and the transverse direction, multistage stretching is preferred. At this time, the stretching temperature may be increased from the former stretching step to the latter stretching step within a range not exceeding 160 ° C.
When the extruded raw sheet is stretched, when the molten sheet extruded from the die is cooled to the stretching temperature, there is also a method of stretching, but in the present invention, the sheet The molten mixture should be cooled and solidified once with the added paraffin wax (B), then heated again and stretched within the above stretching temperature in the presence of at least part of the molten paraffin wax (B).

Usually, the heat medium during stretching is air, but if the paraffin wax (B) is removed during stretching, it has a relatively high boiling point and is compatible with the high molecular weight polyethylene (A) at the stretching temperature. However, a lower alcohol solvent (C) described later, which is compatible with the paraffin wax (B), is used.

<Heat Treatment of Stretched Film (Heat Set)> The heat treatment of the stretched film is carried out in the presence of a part or the whole amount of the paraffin wax (B) after biaxial stretching. The heat treatment temperature is 60 ° C. or higher and the melting point of the paraffin wax (B) or higher and the melting point of the high molecular weight polyethylene (A) or lower, preferably the stretching temperature or higher and the melting point of the high molecular weight polyethylene (A) or lower. The preferred heat treatment temperature is 110 ° C to 130 ° C. At this time, the deformation applied to the film should not be at least on the stretch side. That is, vertical and horizontal fixed end processing is preferable. However, in the vertical direction,
The shrinkage may be performed during the heat treatment within a range that does not exceed 10% in the lateral direction.

<Removal of Paraffin Wax (B)> The paraffin wax (B) is finally removed from the film in a substantially total amount. What is important in the present invention is that the total amount of paraffin wax (B) must not be removed prior to stretching; All the paraffin wax should be removed. That is, a part of the paraffin wax (B) may be extracted and removed from the raw sheet prepared as described above prior to stretching, and then may be stretched. Alternatively, all the paraffin wax (B) remaining after stretching may be extracted and removed. However, from the viewpoint of extraction efficiency, it is most preferable to coexist a part or all of the paraffin wax (B) at the time of stretching, and to extract and remove all or all of the rest after stretching when the film becomes thin. The paraffin wax (B) is preferably removed at the stretching temperature in the stretching process and at a temperature of 80 ° C. or lower in other processes.

The first group of the solvent (C) for extracting and removing the paraffin wax (B) includes isopropyl alcohol, normal propyl alcohol and isobutyl alcohol, alone or as a mixture of two or more kinds. These solvents may contain water or other lower alcohol solvent within a range not exceeding 10% by weight. Especially in the case of water, the flash point increases, which is preferable.
From the viewpoint of ease of removing the paraffin wax (B),
The content of water is 10% by weight or less, preferably about 5% by weight.

Next, as the second group of the solvent (C),
Examples include normal butyl alcohol and isoamyl alcohol. When the paraffin wax (B) is removed using these, the porosity of the produced film is low probably because the drying speed is slow. In this case, after the paraffin wax (B) is removed, the solvent (C) is replaced with water and dried to prevent the decrease in the porosity. These solvents are 10% by weight like the first group of solvents.
Water or another lower alcohol solvent may be contained within a range not exceeding the above. Especially in the case of water, the flash point increases, which is preferable. Due to the ease of removing the paraffin wax (B), the content of water is 10% by weight or less in this case as well.
It is preferably about 5% by weight.

After treatment with a solvent selected from the first group,
As suggested by the second group, the replacement with water becomes complicated in terms of process, but it is not excluded because it can be expected to have better results for the physical properties of the film.

The amount of paraffin wax (B) remaining on the film after the extraction / removal treatment is a differential scanning calorimeter (DS) because the paraffin wax (B) is crystalline.
It can be confirmed by C) whether the melting point is large or small, and using a Soxhlet extractor, a suitable solvent that does not dissolve the high molecular weight polyethylene (A) and can dissolve the paraffin wax (B), for example, boiling It can also be confirmed from the decrease in the weight of the raw sheet by subjecting the raw sheet to an extraction treatment using something like n-hexane.

In the present invention, prior to the preparation of the film,
In addition to high molecular weight polyethylene (A) and paraffin wax (B), heat stabilizers, weather stabilizers, lubricants, antiblocking agents, slip agents, pigments, dyes, surfactants, inorganic fillers, etc. Various additives used as described above may be blended within a range not impairing the object of the present invention.

< Final heat treatment of film (heat set
G) Depending on the application, the obtained film can be further heat-treated after the completion of the deparaffin wax in order to improve dimensional stability, tensile modulus and tensile strength. The film after the stretching operation is once cooled to a temperature of 60 ° C. or lower, and then preferably in two directions (longitudinal direction and transverse direction), at least one direction (longitudinal direction or transverse method).
It is preferable that the treatment is carried out at a temperature in the range of 80 to 170 ° C. while restraining Prior to this stage, the paraffin wax (B) must have been substantially removed.

The heat medium for the final heat treatment is air,
A gas such as nitrogen gas or water that does not dissolve or modify polyethylene, or a liquid such as diethylene glycol or triethylene glycol is used. Suitable processing temperature is 120 ° C
However, the temperature is not limited to this temperature by selecting the treatment time. By the above treatment, the porosity is slightly reduced and the film becomes thinner than before treatment.

< Structure of Film Obtained by the Present Invention
And Physical Properties > The film of the present invention is semitransparent or opaque and has a glossy white color, although depending on the thickness.
The structure of the film is a paper-made structure in which fibrils of high molecular weight polyethylene are uniformly dispersed. This structure is easily recognized by observation with a scanning electron microscope at a magnification of 3000 to 10000 times.

The puncture strength of the film obtained in the present invention is 300 g or more, preferably 400 by the method described below.
g or more, more preferably 500 g or more. The thickness of the film obtained in the present invention is adjusted by the sheet thickness before stretching, the composition ratio with the plasticizer, and the stretching ratio, and the porosity of the obtained sheet. Seen from the ease of handling the film afterwards, 100
4 μm or less, preferably 1 μm or more, preferably 50 μm or less
m or more.

The porosity of the film obtained by the present invention is 40.
To 60%, preferably 45 to 55%. When the porosity of the film is less than 40%, good air permeability characteristic of the film of the present invention may not be obtained.
Further, when the porosity exceeds 60%, the above-mentioned tensile strength of the film cannot be obtained.

The air permeability of the film obtained in the present invention can be evaluated by measuring with a Gurley densometer. Air permeability is 50 to 1000 seconds / 100
should be in the range of ml, preferably 50 to 800
Second / 100 ml.

[0045]

Industrial Applicability According to the present invention, a high molecular weight polyethylene microporous material having excellent suitability as a separator for a non-aqueous electrolyte secondary battery using a solvent excellent in safety, environmental compatibility and hygiene. A method of making a film is provided.

The methods of measuring the physical properties used in this specification will be described below.

<Intrinsic Viscosity> The intrinsic viscosity in this specification is
It is a value measured at 135 ° C. in a decalin solvent. The measuring method is based on ASTM D4020.

<Melting Point> Melting point (° C.) used in the present specification
Is the peak top of the endothermic peak of the thermograph measured by a differential scanning calorimeter (DSC) according to ASTM D3417, unless otherwise specified.

<Measurement of film thickness> The film thickness was measured with a film thickness measuring instrument Miniax (model DH-150 type) manufactured by Tokyo Seimitsu Co., Ltd.

<Porosity> The sample weight was measured, the film density was set to 0.95 g / cm ³ , and the thickness of the dense film was calculated. The porosity was calculated in relation to the value measured by the above film thickness measuring device. . Here, T _o is the actual film thickness obtained by a film thickness measuring instrument, and T _w is the film thickness with a porosity of 0% calculated from the weight.

<Measurement of Air Permeability (Gurley Test)> AST
It is measured according to MD 726 by mounting the film on a standard Gurley Densometer. That is, the time (seconds) required for 100 ml of air to flow through an area of 1 square inch of the film with a standard differential pressure of water of 12.2 inches (pressure difference between both surfaces of the film)
To measure. This value was defined as air permeability.

<Puncture Strength> Using a Tensilon tensile tester (model RMT100 type) manufactured by Orientec Co., Ltd., 23
The measurement was performed at a crosshead speed of 100 mm / min at 0 ° C. The needle for piercing has a tip of 0.5 mmR,
This was done using a needle with a diameter of 1 mm. The puncture strength was defined as the force with which the needle pierces the film. The piercing force at this time naturally depends on the true thickness of the polyethylene of the film. The result obtained for that purpose was converted to the equivalent of the true thickness of polyethylene of 12.5 μm. That is, the weight of a film of a certain area is measured,
The thickness (true thickness) of polyethylene as a dense film was calculated from the polyethylene density, and the result was calculated to be 12.5μ by proportional calculation.
Standardized to correspond to m thickness.

Embodiments of the present invention will be described with reference to experimental examples. Unless otherwise specified, all percentages in the present specification and in the following Examples and Comparative Examples are weight standards (inner weight%).

<Experimental Example 1> Step 1. High molecular weight polyethylene (Intrinsic viscosity: [η] =
6.8 dl / g) powder and paraffin wax (melting point =
Powder at 69 ° C. and molecular weight = 460) was uniformly mixed.
Furthermore, 0.5% of 3,5-di-tachary-butyl-4-hydroxytoluene (BHT) was added to this mixture as a process stabilizer, based on the total amount of polyethylene. This mixture is a twin screw type melt kneader Labo Plastomill (manufactured by Toyo Seiki Seisakusho: Model 20R2
00 type) to form a uniform melt mixture. The conditions at this time were a melt-kneading temperature of 190 ° C. and a screw rotation speed of 50
Min, kneading time was 10 minutes. This molten mixture was taken out in a molten state, put between a pair of stainless steel press plates, the thickness was adjusted with a metal frame (spacer: thickness 1 mm), and the temperature was quickly set with a heat press machine at 190 ° C. Sandwiched between boards. After compressing for 5 minutes, it was taken out and sandwiched between cooling plates of a cooling press whose temperature was set to 20 ° C.
It was pressed for 0 minutes, cooled and solidified to obtain a pressed sheet.

Step 2. Using a tenter chuck type biaxial stretching machine heavy type manufactured by Toyo Seiki Seisaku-sho, the obtained press sheet was stretched at a stretching ratio of 6 × 6 (length × width) and a stretching temperature of 100.
C., stretching speed 1.5 m / min. Was biaxially stretched.

Step 3. Then, the obtained film was fixed in two directions (longitudinal and lateral) with a metal frame, and heat-treated (heat set) for 2 minutes in an air oven at 120 ° C.

Step 4. The film that has been heat treated,
Paraffin wax was completely removed in isopropanol (Wako Pure Chemical Industries, Ltd. reagent grade 1) which was fixed only in the MD direction and heated to 70 ° C. Then, it was dried at room temperature in an air stream.

The experimental conditions and the results are shown in Table 1 together with the case where the step 3 (heat treatment) is not performed. It can be seen that when step 3 is omitted, the porosity is low, and as a result, air permeability (Gurley second) is poor.

<Experimental Example 2> Using the same high molecular weight polyethylene as used in Experimental Example 1, a microporous film was prepared in the same manner as in Experimental Example 1. At this time, as a deparaffinization wax solvent, normal propanol heated to 80 ° C. (Wako Pure Chemical Industries Reagent Grade 1) was used. Similar to Experimental Example 1, the experimental conditions and results are shown in Table 2 together with the case where Step 3 (heat treatment) is not performed.

[0060]

<Experimental Example 3> Using the same high molecular weight polyethylene as used in Experimental Example 1, a microporous film was prepared in the same manner as in Experimental Example 1. At this time, as the deparaffin wax solvent, normal butanol (Wako Pure Chemical Industries, Ltd. first-grade reagent) heated to 80 ° C. was used. The drying of normalptanol was performed in an air stream heated to 60 ° C.
As in the case of Experimental Example 1, the experimental conditions and results are shown in Step 3 (heat treatment).
The results are shown in Table 3 together with the case of not performing.

[0062]

<Experimental Example 4> A microporous film was prepared in the same manner as in Experimental Example 1, using the same high molecular weight polyethylene as used in Experimental Example 1. At this time, as the deparaffin wax solvent, isobutanol (Wako Pure Chemical Industries, Ltd. first-grade reagent) heated to 80 ° C. was used. Drying of isobutanol was performed in an air stream heated to 60 ° C. Experimental conditions and results are shown in Table 4.

[0064]

<Experimental Example 5> Using the same high molecular weight polyethylene as used in Experimental Example 1, a microporous film was prepared in the same manner as in Experimental Example 1. At this time, stearyl alcohol (Wako Pure Chemical Industries, Ltd. reagent first grade) was used in place of paraffin wax, and as a destearyl alcohol solvent,
Isobutanol (Wako Pure Chemical Industries, Ltd. reagent first grade) heated to 80 ° C. was used. The drying of isoptanol was performed in an air stream heated to 60 ° C. Experimental conditions and results are shown in Table 5.

[0066]

<Experimental Example 6> Using the same high molecular weight polyethylene as used in Experimental Example 1, a microporous film was prepared in the same manner as in Experimental Example 1. At this time, as the deparaffin wax solvent, normal butanol (Wako Pure Chemical Industries, Ltd. first-grade reagent) heated to 80 ° C. was used. In Experiment No. 10, deparaffin wax was carried out with normal butanol as in Experiment 3, and its drying was carried out in an air stream heated to 60 ° C. In Experiment No. 11, after deparaffinizing wax, normal butanol was replaced with water at room temperature,
Drying was performed in an air stream heated to 60 ° C. Experiment number 12 is the same as experiment number 11, but water heated to 60 ° C. was used instead of room temperature water. The experimental conditions and results are shown in Table 6 together.

[0068]

<Experimental Example 7> Using the same high-molecular-weight polyethylene as that used in Experimental Example 1, a microporous film was prepared in the same manner as in Experimental Example 1. At this time, as a deparaffin wax solvent, a mixed solvent of normal butanol (Wako Pure Chemical Industries reagent grade 1) heated to 80 ° C. and water of 90:10 was used. In Experiment No. 13, deparaffin wax was performed with a mixed solvent of normal butanol and water in the same manner as in Experiment Example 3, and its drying was performed in an air stream heated to 60 ° C. In Experiment No. 14, after deparaffinizing wax,
The mixed solvent of normal butanol and water was replaced with water at room temperature, and drying was carried out in an air stream heated to 60 ° C.
The experimental conditions and results are shown in Table 7 together.

[0070]

<Experimental Example 8> Using the same high molecular weight polyethylene as used in Experimental Example 1, a microporous film was prepared in the same manner as in Experimental Example 1. At this time, as the deparaffin wax solvent, isoamyl alcohol heated to 80 ° C. (Wako Pure Chemical Industries reagent grade 1) was used. Experiment number 15
And 17 were deparaffinized wax with isoamyl alcohol, and the drying was performed in an air stream heated at 60 ° C. In Experiment Nos. 16 and 18, after deparaffin waxing, isoamyl alcohol was replaced with water at room temperature and drying was performed in an air stream heated to 60 ° C. The experimental conditions and results are shown in Table 8 together.

[0072]

<Experimental Example 9> A press sheet was prepared in the same manner as in Step 1 of Experimental Example 1. At this time, the composition of high molecular weight polyethylene and paraffin wax was 30:70. This sheet was fixed in two directions (longitudinal direction and lateral direction), deparaffinized wax was added with isobutanol (Wako Pure Chemical Industries, Ltd. reagent grade 1) heated to 80 ° C, and the airflow at 60 ° C was fixed. When dried in, the dried sheet became partially transparent and lost the microporous structure,
It was unsuitable as a sheet before stretching.

<Experimental Example 10> Using the same high molecular weight polyethylene as used in Experimental Example 1, a microporous film was prepared in the same manner as in Experimental Example 1. At this time, as a deparaffin wax solvent, a mixed solvent of ethanol (Wako Pure Chemical Industries, Ltd. reagent grade 1) heated to 60 ° C. and water of 95: 5 was used. In Experiment No. 21, paraffin wax having a melting point of 47 ° C. was used instead of the paraffin wax used in Experiment 1. The experimental conditions and the results are shown in Table 9.

[0075] When the residual amount of paraffin wax was examined, 36.4% remained in Experiment No. 19 and 26.3% in Experiment No. 20 based on the total weight of the film. However, in Experiment No. 21, it was 1.5%, and it was substantially completely removed.

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Claims (4)

[Claims] 1. A method for producing a microporous film made of a high-molecular-weight polyethylene (A) having an intrinsic viscosity [η] of 3.5 dl / g or more, which comprises the following steps: Method for producing a molecular weight polyethylene microporous film. Step 1. A step of melting and kneading a mixture of high molecular weight polyethylene (A) and paraffin wax (B) to obtain a cooled and solidified sheet. Step 2. A step of obtaining a film by biaxially stretching the sheet at a temperature not lower than the melting point of the paraffin wax (B) and not higher than the melting point of the high-molecular-weight polyethylene, with a part or all of the paraffin wax (B) remaining in the sheet. Step 3. After biaxial stretching, a step of heat-treating the film in the presence of paraffin wax (B) at a temperature not lower than the melting point of paraffin wax (B) and not higher than the melting point of high molecular weight polyethylene (A). Step 4. The paraffin wax (B) remaining from the film is removed using a lower alcohol solvent (C) at a temperature not lower than the melting point of the paraffin wax (B) and lower than the melting point of the high molecular weight polyethylene (A), and then dried. A step of obtaining a microporous high molecular weight polyethylene film. 2. The production method according to claim 1, wherein the lower alcohol solvent (C) is a solvent consisting of isopropyl alcohol, normal propyl alcohol and isobutyl alcohol, or a mixture of two or more thereof. 3. The lower alcohol solvent (C) is normal butyl alcohol or isoamyl alcohol, and after removing the paraffin wax, the alcohol solvent (C) is further replaced with water and then dried. 1
The manufacturing method described. 4. The method according to claim 1, wherein the paraffin wax (B) is a solid at room temperature and has a melting point of 50 ° C. or lower, and the lower alcohol solvent (C) is ethanol.