JP2003253026A - Method for producing microporous polyolefin membrane and membrane obtained thereby - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a microporous polyolefin membrane having an excellent balance among void content, air permeability, thrust strength and degree of heat shrinkage. <P>SOLUTION: The method is as follows: A gel molding is obtained by melt kneading a polyolefin and a liquid solvent followed by extruding the obtained molten mixture through a die and cooling it. The gel molding is then drawn uniaxially more than three fold and further drawn vertically to the direction of the more than three fold drawing under a temperature higher than that at the more than three fold drawing, thereafter the liquid solvent is removed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyolefin microporous membrane and a polyolefin microporous membrane obtained by the production method, and in particular, it has an excellent balance of porosity, air permeability, puncture strength and heat shrinkage. The present invention also relates to a method for producing a polyolefin microporous membrane and a polyolefin microporous membrane obtained by the method.

[0002]

BACKGROUND OF THE INVENTION Polyolefin microporous membranes include separators for electrolytic capacitors, including separators for batteries used in lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, etc. It is widely used in various filters such as reverse osmosis filtration membranes, ultrafiltration membranes and microfiltration membranes, moisture-permeable waterproof clothing, and medical materials. When the polyolefin microporous membrane is used as a battery separator, particularly as a lithium ion battery separator, its performance is deeply related to battery characteristics, battery productivity and battery safety. Therefore, excellent air permeability, mechanical properties, dimensional stability, shutdown characteristics,
Meltdown characteristics are required.

For example, as a method for increasing the strength of a microporous membrane, JP-A-60-242035, JP-A-60-255107, and JP-A-60-255107 are known.
63-273651 proposes a method for producing a microporous membrane using an ultrahigh molecular weight polyolefin. These methods are methods in which an ultrahigh molecular weight polyolefin and various plasticizers or solvents are melt-kneaded, the resulting melt-kneaded product is extruded to form a gel-like sheet, and then stretched. However, since ultra-high molecular weight polyolefin is used in these methods, it is necessary to use a large amount of a plasticizer or a solvent in order to extrude the melt-kneaded product, and therefore it takes time to remove the plasticizer or the solvent. In addition to problems with productivity, the strength of the microporous membrane obtained was not sufficient.

On the other hand, JP-A-3-064334 proposes a method of using a polyolefin composition containing ultra-high molecular weight polyolefin and having a value of (weight average molecular weight / number average molecular weight) within a specific range. ing. According to this method
It is possible to increase the concentration of the melt-kneaded product, that is, to reduce the amount of solvent used, and the microporous membrane obtained has both excellent strength and water permeability.

However, when the raw material contains ultrahigh molecular weight polyethylene, if a plasticizer is used and then stretched, the pore size of the obtained microporous membrane becomes relatively small, and it is not easy to control the pore size. For this reason, there has been a problem that a microporous membrane having a large pore size on the negative electrode surface cannot be obtained, which is required for a recent lithium battery separator. On the other hand, WO00 / 20493 proposes a method for producing a microporous membrane having a large pore size on one side or both sides and a small pore size inside by immersing the stretched membrane in a heated solvent. However, the piercing strength could not be made sufficient by this method.

There is also a method of laminating a microporous membrane having a large pore diameter and a microporous membrane having a small pore diameter, but this method inevitably complicates the process. Further, JP 2001-002813A,
JP-A-2001-002826, JP-A-2001-019791 and JP-A-2001-1
No. 64018 describes a method of extruding a molten mixture of polyethylene and paraffin wax, sequentially stretching a gel-like molded product obtained by cooling, removing the paraffin wax, and then heat-treating it. For example, a microporous film having a high porosity can be obtained. However, in these methods, since paraffin wax having a relatively high melting point is solidified, it is difficult to perform high-magnification stretching at a low temperature. It was difficult to improve the degree in a balanced manner.

Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a method for producing a microporous polyolefin membrane which is excellent in the balance of porosity, air permeability, puncture strength and heat shrinkage, and a method for producing the same. It is to provide a polyolefin microporous membrane.

[0008]

As a result of earnest research in view of the above objects, the present inventors obtained by melt-kneading a polyolefin and a liquid solvent, extruding the obtained melt-kneaded product from a die, and cooling. The gel-like molded product is uniaxially
Stretching is performed so as to exceed the stretching ratio, and then the obtained stretched product is stretched again at a temperature higher than the stretching temperature of the stretching of more than 3 times and at least in a direction perpendicular to the direction of stretching of more than 3 times. Then, they found out that the above problems can be solved by removing the solvent, and have conceived the present invention.

That is, in the method for producing a polyolefin microporous membrane of the present invention, in the step of stretching a gel-like molded article, it is stretched uniaxially so as to exceed 3 times, and then the obtained stretched article is subjected to the above-mentioned 3 The stretching is performed at a temperature higher than the stretching temperature of the stretching of more than 3 times, and the stretching is performed again in a direction perpendicular to at least the stretching direction of more than 3 times.

The polyolefin microporous film of the present invention obtained by such a manufacturing method has a total light transmittance deviation of 5
%, The porosity is 35 to 95%, and the air permeability at 25 ° C. at a film thickness of 25 μm is 10 to 300 seconds / 100 cc.

By using a liquid solvent when preparing the melt-kneaded product, it becomes possible to perform high-magnification stretching in the primary stretching, and the total light transmittance of the resulting microporous film is relatively low and the total light transmittance is relatively low. The deviation becomes smaller. Further, the melt-kneaded product is extruded from a die, and the gel-like molded product obtained by cooling is stretched in the uniaxial direction so as to exceed 3 times (primary stretching) to expand the pore size, and as a result, the porosity and Permeability is improved (good air permeability is obtained). Further, the stretched product obtained by the primary stretching is stretched again at least at a temperature higher than the stretching temperature of the primary stretching in the direction perpendicular to the direction of the primary stretching (secondary stretching), thereby piercing strength and heat shrinkage. The rate balance is improved.
As a result, a polyolefin microporous film having an excellent balance of porosity, air permeability, puncture strength and heat shrinkage can be obtained.

The temperature of the primary stretching is generally preferably not more than the crystal dispersion temperature of the raw material polyolefin + 20 ° C.,
It is more preferable that the temperature is not higher than the crystal dispersion temperature. Especially when the raw material polyolefin is composed of polyethylene or a composition containing polyethylene, the temperature of the primary stretching is preferably 40 to 105 ° C, more preferably 70 to 90 ° C. By performing the primary stretching at a crystal dispersion temperature of + 20 ° C or lower, the porosity and the permeability are further improved. The temperature difference between the primary stretching and the secondary stretching is preferably 5 to 80 ° C, more preferably 5 to 40 ° C. The temperature of the secondary stretching is generally preferably from the crystal dispersion temperature of the raw material polyolefin to (melting point + 10 ° C.), more preferably from the crystal dispersion temperature of the raw material polyolefin to the melting point. Especially when the raw material polyolefin is composed of polyethylene or a composition containing polyethylene, the temperature of the secondary stretching is preferably 80 to 130 ° C, and 90 to 120 ° C.
Is more preferable. The total of the primary stretching and the secondary stretching is preferably 5 times or more, more preferably 10 times or more, and further preferably 20 times or more in terms of surface magnification.

In order for the microporous polyolefin membrane to obtain more excellent properties, the polyolefin preferably satisfies the following conditions (1) to (11). (1) The polyolefin includes polyethylene or polypropylene. (2) The polyolefin described in (1) above has a weight average molecular weight of 5
Includes × 10 ⁵ or more polyethylene. (3) The polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more described in (2) above is an ultrahigh molecular weight polyethylene. (4) The weight average molecular weight of the ultrahigh molecular weight polyethylene described in (3) above is 1 × 10 ⁶ to 15 × 10 ⁶ . (5) The weight average molecular weight of the ultra high molecular weight polyethylene described in (3) above is 1 × 10 ^{6 to} 5 × 10 ⁶ . (6) The polyolefin described in (2) above is a composition containing other polyolefin as an optional component. (7) The other polyolefin described in (6) above is a polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more to less than 5 × 10 ⁵ , 1 ×
10 ⁴ to 4 × 10 ⁶ polypropylene, weight average molecular weight 1 × 1
0 ⁴ -4 × 10 ⁶ polybutene-1, weight average molecular weight 1 × 10 ³
It is at least one selected from the group consisting of a polyethylene wax having a weight of 1 to less than 1 × 10 ⁴ and an ethylene / α-olefin copolymer having a weight average molecular weight of 1 × 10 ⁴ to 4 × 10 ⁶ . (8) The polyolefin described in (2) above is a composition of ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more and polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ . (9) The polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ^{5 in} the polyethylene composition described in (8) above is selected from high density polyethylene, medium density polyethylene, low density polyethylene and linear low density polyethylene. At least one selected from the group consisting of (10) The polyolefin composition described in (8) or (9) above is an ultra high molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more and a high density polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more to less than 5 × 10 ^5. Consists of. (11) The polyethylene composition according to any one of (6) to (10) has an Mw / Mn of 5 to 300.

As for the physical properties of the microporous polyolefin membrane prepared by the method of the present invention, the puncture strength is usually 3920 mN / 25.
μm or more, preferably 4900 mN / 25 μm or more, and the heat shrinkage rate (105 ° C / 8 hr) is 8% or less in both MD and TD directions, preferably 5% or less, more preferably 3% The following characteristics are satisfied.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION [1] Polyolefin The porosity used in the production of the polyolefin microporous membrane of the present invention.
Liolefins include polyethylene or polypropylene.
It is preferable that the weight average molecular weight is 5 × 10.^Five More po
It is preferred to include polyethylene. Weight average molecular weight is 5 × 1
0^FivePolyethylene including the above polyethylene is super
High-molecular-weight polyethylene is mentioned, and its weight average molecular weight
Is 1 × 10⁶ ~ 15 × 10⁶Is preferred and 1 × 10⁶ ~
5 x 10⁶Is more preferable.

The polyethylene to be used may be a composition containing other polyolefin as an optional component as long as it contains polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more. Such other polyolefins, weight average molecular weight ^of 1 × 10 ⁴ or more to 5 × 10 ⁵ less than polyethylene, 1 × 10 ⁴ to 4 × 10 ⁶ polypropylene, the weight average molecular weight ^of 1 × 10 ⁴ to 4 × 10 ⁶ Polybutene-1, a polyethylene wax having a weight average molecular weight of 1 × 10 ³ to less than 1 × 10 ⁴ and an ethylene / α-olefin copolymer having a weight average molecular weight of 1 × 10 ⁴ to 4 × 10 ⁶ are selected. At least one selected from the above can be used. The amount of the other polyolefin added is 80 parts by weight or less based on 100 parts by weight of the entire polyolefin composition.

The polyolefin composition includes ultra high molecular weight polyethylene and a weight average molecular weight of 1 × 10 ⁴ or more to 5 × 10 5.
Compositions consisting of less than ⁵ polyethylene are preferred. This composition can easily control the molecular weight distribution (Mw / Mn) depending on the application. Mw / of polyolefin composition
Although Mn is not limited, it is preferably 5 to 300, more preferably 5 to 100. Weight average molecular weight is 1 x 10 ⁴ or more ~ 5 x 10
^As polyethylene less than ⁵ , high-density polyethylene,
Either medium density polyethylene or low density polyethylene can be used, and not only ethylene homopolymer but also copolymer containing a small amount of other α-olefins such as propylene, butene-1, and hexene-1. May be. The polyolefin composition is not limited, but a composition composed of an ultra high molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more and a high density polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more to less than 5 × 10 ⁵ is used. It is suitable.

[2] Method for producing a polyolefin microporous membrane The method for producing a polyolefin microporous membrane of the present invention comprises (a) a step of adding a liquid solvent to the above polyolefin and melt-kneading the mixture to prepare a polyolefin solution; ) Extruding the polyolefin solution through a die and cooling it to form a gel-like molded article, (c) primary stretching step and secondary stretching step, (d) liquid solvent removal step, (e) drying the obtained film Including steps. Furthermore, after the steps (a) to (e), (f) heat treatment, (g) cross-linking treatment by ionizing radiation, (h) hydrophilization treatment and the like may be performed, if necessary.

(A) Preparation Step of Polyolefin Solution First, a solvent which is liquid at room temperature is added to polyolefin and melt-kneaded to prepare a polyolefin solution. If necessary, various additives such as an antioxidant, an ultraviolet absorber, an antiblocking agent, a pigment, a dye and an inorganic filler can be added to the polyolefin solution within a range that does not impair the object of the present invention. For example, finely divided silicic acid can be added as a pore-forming agent.

It is essential to use a liquid solvent which is liquid at room temperature as a solvent for preparing the polyolefin solution. By using a liquid solvent, it is possible to perform high-strength stretching in the primary stretching, and the total light transmittance of the obtained microporous film is relatively low and the total light transmittance deviation is small. Here, the total light transmittance is a value measured according to JIS K7105, and the total light transmittance deviation is a standard deviation calculated from the total light transmittance measured for 20 samples. Generally, when the pore size is large, the transparency is good and the total light transmittance tends to be low. The smaller the deviation of the total light transmittance, the smaller the unevenness of the polyolefin microporous film and the better the mechanical strength. Liquid solvents include nonane, decane, decalin, paraxylene, undecane, dodecane, liquid paraffin and other aliphatic or cyclic hydrocarbons, and mineral oil fractions having boiling points corresponding to these, and room temperature such as dibutyl phthalate and dioctyl phthalate. Can use liquid phthalate ester. In order to obtain a gel-like molded product having a stable liquid solvent content, it is preferable to use a non-volatile liquid solvent such as liquid paraffin. Although it is in a state of being mixed with the polyolefin in the heat-melting and kneading state, a solid solvent which is solid at room temperature may be mixed with the liquid solvent. As such a solid solvent, stearyl alcohol, ceryl alcohol, paraffin wax, etc. can be used. If only the solid solvent is used, uniaxial stretching exceeding 3 times cannot be performed in the primary stretching described below, or uneven stretching occurs even if uniaxial stretching exceeding 3 times can be performed, resulting in a total light transmittance deviation. Getting worse.

The viscosity of the liquid solvent is 30 to 500 at 25 ° C.
It is preferably cSt, more preferably 50 to 200 cSt. If the viscosity at 25 ° C is less than 30 cSt, foaming is likely to occur, making kneading difficult. On the other hand, if it exceeds 500 cSt, it becomes difficult to remove the liquid solvent.

The method of melt kneading is not particularly limited, but it is usually carried out by uniformly kneading in an extruder. This method is suitable for preparing concentrated solutions of polyolefins. Melting temperature is the melting point of polyolefin +10 ℃ ~ +100
C is preferred. Therefore, the melting temperature is generally 160-230 ° C.
Is preferable, and 170 to 200 ° C. is more preferable. Here, the melting point refers to a value obtained by differential scanning calorimetry (DSC) based on JIS K7121. The liquid solvent may be added before the start of kneading or may be added from the middle of the extruder during the kneading, but it is preferable to add the liquid solvent before the start of the kneading and form a solution in advance. Upon melt-kneading, it is preferable to add an antioxidant in order to prevent the oxidation of the polyolefin.

In the polyolefin solution, the blending ratio of the polyolefin and the liquid solvent is 1 to 50% by weight, preferably 20 to 50% by weight, with the total amount of both being 100% by weight.
40% by weight. When the amount of the polyolefin is less than 1% by weight, swell and neck-in increase at the die outlet when forming a gel-like molded product, and the moldability and self-supporting property of the gel-like molded product deteriorate. On the other hand, if it exceeds 50% by weight, the moldability of the gel-like molded article is deteriorated.

(B) Step of forming gelled product The melt-kneaded polyolefin solution is extruded from the die directly or through another extruder, or once cooled and pelletized, and again through the extruder. As the die, a sheet die having a rectangular die shape is usually used, but a double-cylindrical hollow die, an inflation die, or the like can also be used. In the case of sheet dies, the die gap is usually 0.1 to 5 mm.
Heat to 250 ° C. Extrusion speed of heated solution is 0.2 ~ 15
It is preferably m / min.

A gel-like molded article is formed by cooling the solution thus extruded from the die. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature. It is also preferable to cool to 25 ° C or lower. In this way, the phase separation structure in which the polyolefin phase is microphase-separated by the solvent can be fixed. Generally, if the cooling rate is slow, the higher-order structure of the obtained gel-like molded article becomes coarse and the pseudo-cell units forming it become large, but if the cooling rate is fast, the cell units become dense. When the cooling rate is less than 50 ° C / min, the crystallinity increases,
It is difficult to obtain a gel-like molded product suitable for stretching. As a cooling method, a method of directly contacting with cold air, cooling water, or another cooling medium, a method of contacting with a roll cooled with a refrigerant, or the like can be used.

(C) Primary stretching step and secondary stretching step The gel-like molded product obtained by cooling the extruded solution is stretched. In the present invention, the stretching is carried out by stretching in the uniaxial direction so as to exceed 3 times (primary stretching), and then again stretching at a temperature higher than the stretching temperature of the primary stretching and at least in a direction perpendicular to the primary stretching direction. (Secondary stretching)
To do. By the primary stretching, the cell structure of the gel-like sheet is destroyed and minute cracks are generated, so that the pore size is expanded. The secondary stretching improves the balance between puncture strength and heat shrinkage. As a result, porosity, air permeability,
A polyolefin microporous film having an excellent balance of puncture strength and heat shrinkage can be obtained. In the primary stretching and the secondary stretching, it is preferable to stretch with a temperature distribution in the film thickness direction. As a result, a microporous membrane that is generally excellent in mechanical strength can be obtained. As a method of providing a temperature distribution in the film thickness direction and stretching, for example, the method disclosed in JP-A-7-188440 can be applied.

The primary stretching needs to be carried out so as to exceed 3 times in the uniaxial direction. The primary stretching is preferably performed at a magnification of 4 times or more. When the uniaxial stretching is performed at a magnification of 3 times or less, the porosity and the permeability are not sufficiently improved. However, it is not preferable to carry out so as to exceed 50 times because the membrane tends to be broken. The primary stretching can be performed by a tenter method, a roll method or a rolling method.

Primary stretching is generally preferably carried out at a temperature not higher than the crystal dispersion temperature of the raw material polyolefin + 20 ° C.,
It is more preferable to carry out at a temperature not higher than the crystal dispersion temperature of the raw material polyolefin. For example, the crystal dispersion temperature of polyethylene is generally 90 ° C. The lower limit of the primary stretching temperature is not particularly limited, but it is preferably -20 ° C or higher for ease of use. Especially when the raw material polyolefin is composed of polyethylene or a composition containing polyethylene, the temperature of the primary stretching is 40
The temperature is preferably ~ 105 ° C, more preferably 70-90 ° C. Stretching at a temperature above the crystal dispersion temperature + 20 ° C cannot be expected to increase the pore size. Here, the crystal dispersion temperature means a value obtained by measuring the temperature characteristic of dynamic viscoelasticity based on ASTM D 4065.

In the secondary stretching, it is necessary to stretch at a temperature higher than the stretching temperature of the primary stretching and at least in a direction perpendicular to the direction of the primary stretching. The temperature difference between the secondary stretching and the primary stretching is not limited, but is preferably 5 to 80 ° C, more preferably 5 to 40 ° C. However, the secondary stretching is preferably performed at a melting point of the raw material polyolefin + 10 ° C. or less. If the secondary stretching temperature exceeds (melting point + 10 ° C), the resin melts and the molecular chains cannot be oriented by stretching. Secondary stretching is generally the crystal dispersion temperature of the raw material polyolefin.
It is preferable to carry out at a temperature of (melting point + 10 ° C.), more preferably at a temperature of crystal dispersion temperature of the raw material polyolefin to a melting point. Especially when the raw material polyolefin is composed of polyethylene or a composition containing polyethylene, the temperature of the secondary stretching is preferably 80 to 130 ° C, more preferably 90 to 120 ° C.

The secondary stretching may be uniaxial stretching or sequential biaxial stretching. For example, it may be stretched in a direction perpendicular to the direction of primary stretching and then stretched in the same direction as the primary stretching. By performing such successive biaxial stretching, the effect of improving the puncture strength is enhanced. Secondary stretching can be performed by a tenter method, a roll method, a rolling method, or a combination thereof.

The total of primary stretching and secondary stretching is preferably 5 times or more, more preferably 10 times or more, and further preferably 20 times or more in terms of surface magnification. The puncture strength is further improved by setting the total surface magnification to 5 or more. On the other hand, if the total areal magnification exceeds 400 times, there are restrictions on the stretching device, stretching operation, and the like. Further, the areal magnification of the primary stretching is preferably the same as that of the secondary stretching or higher than that of the secondary stretching. When the area ratio of the secondary stretching is higher than that of the primary drawing, the pore diameter of the microporous membrane becomes small, and the porosity and the permeability may not be sufficiently improved.

(D) Liquid solvent removal step The liquid solvent is removed (washed) from the film obtained by the primary stretching and the subsequent secondary stretching. Since the polyolefin phase is microphase-separated by the solvent, a porous membrane can be obtained by removing the liquid solvent. Liquid solvent removal (cleaning)
It can be performed using a known washing solvent. Known washing solvents include, for example, methylene chloride, chlorinated hydrocarbons such as carbon tetrachloride, pentane, hexane, hydrocarbons such as heptane, fluorocarbons such as trifluoroethane, diethyl ether, ethers such as dioxane, Examples include easily volatile solvents such as methyl ethyl ketone.

As the washing solvent for removing the liquid solvent, in addition to the above-mentioned known solvents, the surface tension at 25 ° C. is 24
A washing solvent (A) having a mN / m or less, preferably 20 mN / m or less can be used. By using such a washing solvent (A), gas generated inside the micropores during drying after washing-
It is possible to suppress the shrinkage and densification of the network structure caused by the surface tension of the liquid interface, and as a result, the porosity and permeability of the microporous membrane are further improved, and in addition, the porosity / permeability and heat shrinkage This is preferable because it improves the balance. The washing solvent (A) is preferably one that is incompatible with the thermoplastic resin. The surface tension of the washing solvent decreases as the temperature at which it is used increases,
The usable temperature range is limited to the boiling point or lower. Here, "surface tension" means the tension generated at the interface between gas and liquid,
It is measured based on JIS K 3362.

As the washing solvent (A), fluorine compounds such as hydrofluorocarbon, hydrofluoroether, cyclic hydrofluorocarbon, perfluorocarbon and perfluoroether, normal paraffin having 5 to 10 carbon atoms, isoparaffin having 6 to 10 carbon atoms, Such as aliphatic ethers having 6 or less carbon atoms, cycloparaffins such as cyclopentane, aliphatic ketones such as 2-pentanone, methanol, ethanol, 1-propanol, 2-propanol, tertiary butanol, isobutanol, 2-pentanol, etc. Aliphatic alcohols, propyl acetate, tertiary butyl acetate, secondary butyl acetate, ethyl acetate, isopropyl acetate, isobutyl acetate, methyl formate, ethyl formate, isopropyl formate, isobutyl formate, aliphatic ester of propionate, etc. It can be mentioned.

Examples of the fluorine compound include C ₅ H ₂ F ₁₀
A chain hydrofluorocarbon represented by the composition formula of, for example, a hydrofluoroether represented by the composition formula of C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , for example, represented by the composition formula of C ₅ H ₃ F _7. Cyclic hydrofluorocarbons such as C ₆ F ₁₄ and C ₇ F
_At least one selected from the group consisting of perfluorocarbons represented by the composition formula ₁₆ and perfluoroethers represented by the composition formulas C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ is preferable. Since the surface tension of these fluorine compounds is 24 mN / m or less at 20 ° C., the effect of suppressing shrinkage and densification of the network structure due to the surface tension is high. Also, the boiling point is 100 ℃
It is easy to remove the washing solvent because it is below. Furthermore, because it has no ozone depletion, it can reduce the load on the environment, and its flash point is 40 ° C or higher (some compounds have no flash point), so the risk of flammable explosion during the drying process is low.

As the normal paraffin having 5 to 10 carbon atoms, normal pentane, normal hexane, normal heptane, normal octane, normal nonane and normal decane are preferable. These have a surface tension of 20 ° C
It is less than 24mN / m. Among them, normal pentane, normal hexane, and normal heptane, which have a boiling point of 100 ° C. or less and are easily dried, are more preferable.

2 as an isoparaffin having 6 to 10 carbon atoms
-Methylpentane, 3-methylpentane, 2,2-dibutylbutane, 2,3-dibutylbutane, 2-methylhexane, 3-methylhexane, 3-ethylpentane, 2,2-dimethylpentane, 2,3-dimethyl Pentane, 2,4-dimethylpentane, 3,
3-dimethylpentane, 2,2,3-trimethylbutane, 2-methylheptane, 3-methylheptane, 2,2-dimethylhexane, 2,3-dimethylhexane, 2,5-dimethylhexane, 3,
4-dimethylhexane, 2,2,3-trimethylpentane, 2,2,
4-trimethylpentane, 2,3,3-trimethylpentane, 2,
Preference is given to 3,4-trimethylpentane, 2-methyloctane, 2,2,5-trimethylhexane, 2,3,5-trimethylhexane, 2-methylnonane and 2,3,5-trimethylheptane. Among them, the surface tension is 24 mN / m or less at 20 ° C., and the boiling point is 100 ° C. or less, 2-methylpentane, 3-methylpentane, 2,2-dibutylbutane, 2,3-dibutylbutane,
2-methylhexane, 3-methylhexane, 3-ethylpentane, 2,2-dimethylpentane, 2,3-dimethylpentane, 2,
More preferred are 4-dimethylpentane and 3,3-dimethylpentane.

As the ether having 6 or less carbon atoms, diethyl ether, butyl ethyl ether, dipropyl ether and diisopropyl ether are preferable. These have a surface tension of 24 mN / m or less at 20 ° C. and a boiling point.
It is below 100 ℃.

Cyclopentane, methanol, ethanol, 1-propanol and 2-propanol have a surface tension of 24 mN / m or less at 20 ° C. and a boiling point of 100 ° C.
The following is preferable.

Among the above aliphatic esters, the surface tension is
Tertiary butyl acetate, secondary butyl acetate, isopropyl acetate, isobutyl acetate, ethyl formate, isopropyl formate, and isobutyl formate, which are 24 mN / m or less at 20 ° C., are preferable. Further, the boiling point is 100 ° C. or lower, but tertiary butyl acetate, ethyl formate and isopropyl formate are more preferable.

As the washing solvent (A), it is also possible to use a mixture of water and an aliphatic alcohol having 3 or less carbon atoms, which is blended so as to have a surface tension of 24 mN / m or less at 25 ° C.

The above-mentioned washing solvent (A) can be used as a mixture with another solvent. In this case, the mixing ratio is 25 ℃
The surface tension at is less than 24mN / m. For example, a chain hydrofluorocarbon represented by the composition formula of C ₅ H ₂ F ₁₀ , for example, a hydrofluoroether represented by the composition formula of C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , such as C ₅ H ₃ F ₇
A cyclic hydrofluorocarbon represented by the composition formula, for example, a perfluorocarbon represented by the composition formula of C ₆ F ₁₄ and C ₇ F _{16, and} a composition formula of, for example, C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F _5. A mixture of at least one solvent selected from the group consisting of the perfluoro ethers shown and a hydrocarbon solvent such as paraffin can be used.

The washing with the washing solvent (A) is preferably performed in two or more steps, and a step using a washing solvent (washing solvent (B)) other than the washing solvent (A) may be included. In this case, the washing solvent (A) may be used in at least one step. As the cleaning solvent (B), those not compatible with the thermoplastic resin are preferable, and for example, known cleaning solvents such as methylene chloride described above can be used, and the boiling point is 100 ° C. or higher and the flash point is 0. A non-aqueous solvent having a temperature of 0 ° C. or higher can also be used. Such a washing solvent (B) is appropriately selected according to the solvent used for dissolving the polyolefin composition, and is used alone or as a mixture.

By such a washing process of two or more stages,
Sufficient cleaning can be performed while suppressing shrinkage and densification of the network structure caused by the surface tension of the cleaning solvent. Preferably, the washing solvent (A) is used at least in the final step.
To process. This makes it possible to remove the washing solvent (B) used for washing (hereinafter referred to as “rinsing treatment”), and prevent shrinkage and densification of the network structure that occurs during removal of the washing solvent. As a result, it is effective in improving the porosity and permeability of the microporous polyolefin membrane.

Treatment with the washing solvent (A) in the final step
In particular, treat with a cleaning solvent (A) with a boiling point of 100 ° C or less.
If so, the efficiency of the washing solvent removal step is improved. Further analogy above
If C_FiveH ₂F_TenA chain hydrofluorocarbon represented by the composition formula
Carbon, for example C_FourF₉OCH₃And C_FourF₉OC₂H_FiveThe composition formula of
Hydrofluoroethers such as C_FiveH₃F₇Composition formula
A cyclic hydrofluorocarbon represented by, for example, C₆F
₁₄And C₇F₁₆Perfluorocarbo represented by the composition formula of
, And for example C_FourF₉OCF₃And C_FourF₉OC₂F_FiveThe composition formula
Fluorinated compounds such as perfluoroether used
Then, as described above, the load on the environment during the manufacturing process is reduced.
Can be lowered. Especially as a cleaning solvent (B) with a boiling point of 150 ° C or higher
If a solvent is used, its removal will take time and its effect
However, the porosity and permeability may decrease, but the boiling point is 100.
The problem can be solved by using a cleaning solvent (A) below ℃.
It can be erased.

The non-aqueous solvent having a boiling point of 100 ° C. or higher and a flash point of 0 ° C. or higher which can be used as the cleaning solvent (B) is hardly volatile, has a low load on the environment, and may cause a fire explosion in the cleaning solvent removal step. It is safe to use due to its low properties.
In addition, since it has a high boiling point, it is easy to condense, easy to collect, and easy to recycle. In the present specification, "boiling point" refers to a boiling point at 1.01 x 10 ⁵ Pa, and "flash point" refers to a value measured based on JIS K 2265.

As the non-aqueous solvent, for example, a boiling point of 100 ° C.
Examples thereof include paraffin compounds having a flash point of 0 ° C. or higher, aromatic compounds, alcohols, esters, ethers and ketones. The flash point thereof is preferably 5 ° C. or higher, more preferably 40 ° C. or higher. However, it is not preferable to make the non-aqueous solvent into an aqueous solution because the solvent cannot be sufficiently removed.

As the non-aqueous solvent, normal paraffins having 8 or more carbon atoms, normal paraffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms,
Isoparaffins having 8 or more carbon atoms, cycloparaffins having 7 or more carbon atoms, cycloparaffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms, aromatic hydrocarbons having 7 or more carbon atoms, and at least hydrogen atoms Aromatic hydrocarbons having 6 or more carbon atoms partially substituted with halogen atoms, alcohols having 5 to 10 carbon atoms in which some hydrogen atoms may be substituted with halogen atoms, and some hydrogen atoms being halogen atoms At least one selected from the group consisting of esters and ethers having 7 to 14 carbon atoms which may be substituted with and ketones having 5 to 10 carbon atoms is preferable.

As the normal paraffin having 8 or more carbon atoms, normal octane, normal nonane, normal decane, normal undecane and normal dodecane are preferable, and normal octane, normal nonane and normal decane are more preferable.

Examples of normal paraffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms include 1-chloropentane, 1-chlorohexane, 1-chloroheptane, 1-chlorooctane and 1-bromopentane. , 1-bromohexane, 1-bromoheptane, 1-bromooctane, 1,
5-dichloropentane, 1,6-dichlorohexane and 1,7-dichloroheptane are preferable, and 1-chloropentane, 1-chlorohexane, 1-bromopentane and 1-bromohexane are more preferable.

2, as an isoparaffin having 8 or more carbon atoms,
3,4-trimethylpentane, 2,2,3-trimethylpentane,
2,2,5-Trimethylhexane, 2,3,5-trimethylhexane, 2,3,5-trimethylheptane and 2,5,6-trimethyloctane are preferable, and 2,3,4-trimethylpentane and 2,2 , 3-trimethylpentane, 2,2,5-trimethylhexane and 2,3,
5-Trimethylhexane is more preferred.

Cycloparaffins having 7 or more carbon atoms include cycloheptane, cyclooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethylcyclohexane,
Also, cis- and trans-1,4-dimethylcyclohexane are preferable, and cyclohexane is more preferable.

As the cycloparaffin having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms, chlorocyclopentane and chlorocyclohexane are preferable, and chlorocyclopentane is more preferable.

As the aromatic hydrocarbon having 7 or more carbon atoms,
Toluene, ortho-xylene, meta-xylene and para-xylene are preferable, and toluene is more preferable.

Examples of the aromatic hydrocarbon having 6 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms are chlorobenzene, 2-chlorotoluene, 3-chlorotoluene,
4-chlorotoluene, 3-chloroorthoxylene, 4-chloroorthoxylene, 2-chlorometaxylene, 4-chlorometaxylene, 5-chlorometaxylene and 2-chloroparaxylene are preferred, and chlorobenzene, 2-chlorotoluene,
More preferred are 3-chlorotoluene and 4-chlorotoluene.

Alcohols having 5 to 10 carbon atoms in which a part of hydrogen atoms may be replaced by halogen atoms include isopentyl alcohol, tertiary pentyl alcohol, cyclopentanol, cyclohexanol and 3-methoxy-1-. Butanol, 3-methoxy-3-methyl-1-butanol, propylene glycol normal butyl ether and
5-chloro-1-pentanol is preferred, 3-methoxy-1-butanol, 3-methoxy-3-methyl-1-butanol, propylene glycol normal butyl ether and 5-chloro-1.
-Pentanol is more preferred.

Examples of the ester having 7 to 14 carbon atoms in which a part of hydrogen atoms may be replaced with halogen atoms are diethyl carbonate, diethyl maleate, normal propyl acetate,
Normal butyl acetate, isopentyl acetate, 3-methoxybutyl acetate, 3-methoxy-3-methylbutyl acetate, ethyl normal butyrate, ethyl normal valerate, 2-chloroethyl acetate are preferable, isopentyl acetate, 3-methoxybutyl acetate, 3-acetate More preferred are methoxy-3-methylbutyl, ethyl normal butyrate and 2-chloroethyl acetate.

As the ether having 7 to 14 carbon atoms in which a part of hydrogen atoms may be replaced by a halogen atom, dipropylene glycol dimethyl ether, normal butyl ether, diisobutyl ether and bischloroethyl ether are preferable, and dipropylene glycol dimethyl ether and Bischloroethyl ether is more preferred.

As the ketone having 5 to 10 carbon atoms, 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, cyclopentanone and cyclohexanone are preferable, and 2-pentanone and 3-pentanone are more preferable.

The washing solvent (B) as described above may be used as a mixture, but the washing solvent (B) may be used as an optional component (C).
For example, a chain hydrofluorocarbon represented by the composition formula of C ₅ H ₂ F ₁₀ mentioned as the cleaning solvent (A), for example, C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H _{5 represented by} the composition formula hydrofluorocarbon. Ethers, for example cyclic hydrofluorocarbons having the composition formula C ₅ H ₃ F ₇ , such as perfluorocarbons having the composition formula C ₆ F ₁₄ or C ₇ F ₁₆ , and for example C ₄ F ₉ OCF ₃ and C ₄ F _A mixture of at least one solvent selected from the group consisting of perfluoroethers represented by the composition formula ₉ OC ₂ F ₅ may be used. In this case, the washing solvent (B) and the optional component (C) are preferably mixed in a ratio such that the surface tension is 24 mN / m or less at any temperature of 20 to 80 ° C. Generally, the optional component (C) is 2 to 98 parts by weight, preferably 5 to 50 parts by weight in 100 parts by weight of the mixed solvent. By including 2 to 98 parts by weight of the optional component (C), sufficient washing can be performed while suppressing shrinkage and densification of the network caused by the surface tension of the washing solvent.

The washing solvent (B) has a surface tension of 20 to 80 ° C.
It is preferable to use one having a temperature of 24 mN / m or less at any of the temperatures. Examples of such a washing solvent (B) include normal pentane, hexane, heptane, ethane trifluoride, diethyl ether, 2-methylpentane, 3-methylpentane, cyclohexane, cyclopentane, acetone, methyl ethyl ketone and the like.

Washing solvent used in the first step of washing below
Preferred combinations of (B) and the washing solvent (A) used in the second step are shown below. However, as described below, the washing solvent (A)
Since the washing with the washing solvent (B) can be performed in three or more steps, these are not intended to be limited to two steps. For example, washing solvent (B) / washing solvent (A) =
Methylene chloride / C ₄ F ₉ OCH ₃ , methylene chloride / C ₆ F ₁₄ , methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane,
Methylene chloride / normal hexane, methylene chloride / diethyl ether, ether / hydrofluoroether,
Ether / cyclic hydrofluorocarbon, ether /
Alcohol, mixture of ether / alcohol and water, normal paraffin / hydrofluoroether, normal paraffin / cyclic hydrofluorocarbon, normal paraffin / alcohol, mixture of normal paraffin / alcohol and water, isoparaffin / hydrofluoroether, isoparaffin / cyclic hydro Fluorocarbon, isoparaffin / alcohol, mixture of isoparaffin / alcohol and water, cycloparaffin / hydrofluoroether, cycloparaffin / cyclic hydrofluorocarbon, cycloparaffin / alcohol, mixture of cycloparaffin / alcohol and water, ketone / hydrofluoroether, Ketone / cyclic hydrofluorocarbon, ketone / alcohol, and ketone / alcohol with water Mixtures thereof. Preferably, the washing solvent (B) / the washing solvent (A) = methylene chloride / C ₄ F ₉ OC
H ₃ , methylene chloride / C ₆ F ₁₄ , methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane, methylene chloride / normal hexane, methylene chloride / diethyl ether, normal heptane / C ₄ F ₉ OCF ₃ , and normal heptane / C ₆ F ₁₄ By using such a combination, the solvent can be effectively removed, and the porosity and permeability of the microporous membrane can be improved.

The washing solvent (A) alone or the washing solvent (A) and the washing solvent (B) may be washed in three or more steps. The one-step or two-step treatment is effective when the solvent cannot be sufficiently removed and the physical properties of the obtained polyolefin microporous film are deteriorated. In this case, at least the final step may be treated with the washing solvent (A),
The number of washings is not particularly limited, but is usually 3 to 5 steps, preferably 3 to 4 steps. Also, even if treated with the same washing solvent in each stage, the production process simply becomes long, the space of the polyolefin microporous membrane production equipment is expanded, and the efficiency of solvent removal is reduced. It is preferable to use a solvent. However, it is not limited to using different cleaning solvents. Therefore, for example, in the case of three-step treatment, it is possible to use the same washing solvent in the first step and the second step, and to use a washing solvent different from the first and second steps in the third step.

The washing method may be a method of immersing in the washing solvent (A) and / or the washing solvent (B) for extraction, a method of showering the washing solvent (A) and / or the washing solvent (B), or a combination thereof. It can be performed by a method or the like. Also the washing solvent (A)
The washing solvent (B) is preferably used in an amount of 300 to 30,000 parts by weight per 100 parts by weight of the gel-like molded product. When the washing solvent (A) and the washing solvent (B) are used for washing in two or more steps, the amount of the washing solvent (A) used is 100 parts by weight of the washing solvent (B). It is preferably 50 to 200 parts by weight. The washing with the washing solvent (A) alone or with the washing solvent (A) and the washing solvent (B) is preferably performed until the residual solvent is less than 1% by weight based on the amount added.

The washing temperature with the washing solvent (B) is the washing solvent (B).
Depends on the boiling point of. When the boiling point of the washing solvent (B) is 150 ° C. or less, washing at room temperature is possible, and heating washing may be performed if necessary, and washing at 20 to 80 ° C. is generally preferable. Further, when the boiling point of the washing solvent (B) is 150 ° C. or higher, the penetrability into the inside of the membrane is poor at room temperature, so it is preferable to wash by heating.

The washing temperature and / or the rinsing temperature with the washing solvent (A) depend on the surface tension of the washing solvent (A). Specifically, it is preferable to perform the washing and / or rinsing treatment at a temperature or higher at which the surface tension of the washing solvent (A) becomes 24 mN / m or less.
The washing solvent (A) has a surface tension of 2 at 25 ° C at the highest.
Since it becomes 4 mN / m or less, heating is not required in most cases, and washing and / or rinsing treatment can be performed at normal room temperature. If the ambient temperature is less than the temperature at which the surface tension of the cleaning solvent (A) becomes 24 mN / m or less, wash solvent until the surface tension becomes 24 mN / m or less, if necessary.
Heat (A).

(E) Drying Step of Membrane The membrane obtained by stretching and removing the liquid solvent can be dried by a heat drying method, an air drying method or the like. The drying temperature is preferably equal to or lower than the crystal dispersion temperature of the polyolefin, and particularly preferably 5 ° C. or more lower than the crystal dispersion temperature.

The content of the washing solvent remaining in the microporous polyolefin membrane is preferably 5% by weight or less by the drying treatment (the weight of the dried membrane is 100% by weight), 3
It is more preferable that the content be less than or equal to weight%. If the drying is insufficient and a large amount of the washing solvent remains in the film, the porosity decreases in the subsequent heat treatment and the permeability deteriorates, which is not preferable.

(F) Heat Treatment Step It is preferable to perform heat treatment after removing the washing solvent. The heat treatment stabilizes the crystals and makes the lamella layer uniform. As the heat treatment method, any of heat stretching treatment, heat setting treatment, and heat shrinking treatment can be used, and these are appropriately selected depending on the physical properties required for the microporous membrane. These treatments are carried out below the melting point of the microporous polyolefin membrane, preferably above 60 ° C and below -10 ° C.

The hot stretching treatment is carried out by a tenter system, a roll system or a rolling system which is usually used, and it is preferable to carry out at a stretching ratio of 1.01 to 2.0 in at least one direction.
It is more preferable to carry out at a ratio of 01 to 1.5.

The heat setting treatment is carried out by a tenter system, a roll system or a rolling system. The heat shrinkage treatment may be performed by a tenter method, a roll method, a rolling method, or a belt conveyor or a floating method. The heat shrinkage treatment should be performed in at least 50
% Or less, and more preferably 30% or less.

A large number of combinations of the above heat stretching treatment, heat setting treatment and heat shrinking treatment may be carried out. In particular, it is preferable to perform the heat shrinking treatment after the heat stretching treatment because a microporous film having a low shrinkage ratio and high strength can be obtained.

(G) Crosslinking Treatment Step of Membrane It is preferable that the membrane obtained by stretching and solvent removal is dried by a heat drying method, an air drying method or the like, and then subjected to a crosslinking treatment by ionizing radiation. Α rays, β as ionizing radiation
Ray, γ ray, electron beam, etc. are used, and the electron dose is 0.1 to 100 Mr.
It can be performed at ad and accelerating voltage of 100-300 kV. This can improve the meltdown temperature.

(H) Hydrophilization treatment step The obtained microporous membrane can be used after being hydrophilized. As the hydrophilic treatment, a monomer graft, a surfactant treatment, a corona discharge treatment or the like is used. The monomer graft treatment is preferably performed after ionizing radiation.

When a surfactant is used, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants can be used, but nonionic surfactants can be used. Is preferred. In this case, the surfactant is made into an aqueous solution or a solution of a lower alcohol such as methanol, ethanol or isopropyl alcohol, and is hydrophilized by a method such as dipping and a doctor blade.

The obtained hydrophilized microporous membrane is dried. At this time, in order to improve the permeability, it is preferable to perform heat treatment while preventing shrinkage or stretching at a temperature equal to or lower than the melting point of the polyolefin microporous membrane.

[3] Polyolefin Microporous Membrane The polyolefin microporous membrane according to the preferred embodiment of the present invention has the following physical properties. (1) The total light transmittance is 33% or less, preferably 30% or less. If the total light transmittance exceeds 33%, the transmittance deteriorates. (2) Total light transmittance deviation is within 5%, preferably 4%
Within. If the total light transmittance deviation exceeds 5%, the mechanical strength decreases. (3) Air permeability is 10 to 300 seconds / 100 cc (25 μm film thickness conversion). As a result, the battery capacity is increased and the cycle characteristics of the battery are improved. If the air permeability exceeds 300 seconds / 100 cc, the battery capacity will decrease when the polyolefin microporous membrane is used as a battery separator. Meanwhile, 10 seconds / 10
If it is less than 0cc, shutdown will not be performed sufficiently when the temperature inside the battery rises. The microporous membrane of the present invention is 80 ° C / 1.96MPa
Even if it is heat-treated for 5 minutes, the air permeability does not change significantly, and is 1000 seconds / 100 cc or less. Therefore, when the microporous polyolefin membrane of the present invention is used as a battery separator, a battery with less deterioration in capacity characteristics can be obtained. (4) The piercing strength is 3920 mN / 25 μm or more, preferably
4900 mN / 25 μm or more. Puncture strength is 3920 mN / 25 μm
When the amount is less than the range, a short circuit may occur when the microporous polyolefin membrane is incorporated into a battery as a battery separator. (5) Thermal shrinkage after exposure at 105 ° C for 8 hours is in the machine direction (MD).
And the transverse direction (TD) are both 8% or less, preferably 5% or less, and more preferably 3% or less. When the heat shrinkage ratio exceeds 8%, when the polyolefin microporous film is used as a separator for a lithium battery, when the heat is generated, the end portion of the separator shrinks and a short circuit is likely to occur. (6) The porosity is 35 to 95%, preferably 40 to 90%, and more preferably 40 to 85%. Porosity is 3
If it is less than 5%, good air permeability cannot be obtained. If it exceeds 95%, the balance between battery safety and impedance becomes unbalanced.

As described above, the polyolefin microporous film obtained by the production method of the present invention is excellent in the balance of porosity, air permeability, puncture strength and heat shrinkage, so that it can be used as a battery separator, a filter or the like. It can be preferably used.
The film thickness of the polyolefin microporous film can be appropriately selected depending on the application, but when it is used as a battery separator, for example, it is preferably 5 to 200 μm.

[0079]

EXAMPLES The present invention will be explained in more detail by the following examples, but the present invention is not limited to these examples.

Example 1 20% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.0 × 10 ⁶ and 80% by weight of high density polyethylene (HDPE) having a weight average molecular weight of 3.5 × 10 ⁵ were used. Mn
= 16 for a polyethylene composition (melting point: 135 ° C, crystal dispersion temperature: 90 ° C) with tetrakis [methylene] as an antioxidant.
-3- (3,5-ditert-butyl-4-hydroxyphenyl)
-Propionate] Methane was dry-blended per 100 parts by weight of the polyethylene composition to obtain a polyethylene composition. 30 parts by weight of the obtained polyethylene composition was charged into a twin-screw extruder (inner diameter 58 mm, L / D = 42, strong kneading type), and liquid paraffin (50 cst (40 ° C. )) 70 parts by weight were supplied and melt-kneaded at 200 ° C. and 200 rpm to prepare a polyethylene solution in an extruder. Subsequently, this polyethylene solution was extruded from a T-die installed at the tip of the extruder and cooled while being drawn by a cooling roll whose temperature was adjusted to 0 ° C to form a gel-like sheet. The resulting gel-like sheet was uniaxially stretched (primary stretching) at 90 ° C in the longitudinal direction (MD) to 5 times using a tenter stretching machine, and then transversely at 120 ° C using the same tenter stretching machine. Uniaxial stretching (secondary stretching) is performed in the direction (TD) 5 times (sequential biaxial stretching),
A stretched film was obtained. The obtained stretched membrane was fixed to a 20 cm x 20 cm aluminum frame and a washing tank containing methylene chloride (surface tension 27.3 mN / m (25 ° C), boiling point 40.0 ° C) temperature-controlled at 25 ° C It was immersed in the solution and washed by rocking at 100 rpm for 3 minutes. The obtained membrane was air-dried at room temperature and then heat-set at 125 ° C. for 10 minutes while holding the membrane in a tenter to prepare a polyethylene microporous membrane.

Example 2 A microporous polyethylene membrane was prepared in the same manner as in Example 1 except that the heat setting treatment was carried out at 128 ° C.

Example 3 A microporous polyethylene membrane was prepared in the same manner as in Example 1 except that the heat setting treatment was carried out at 130 ° C.

Example 4 A stretched membrane was prepared by adjusting the temperature of the film to 25 ° C. with ethyl perfluorobutyl ether [C ₄ F ₉ OC ₂ H ₅ , manufactured by Sumitomo 3M HFE-720].
0, surface tension 13.6 mN / m (25 ° C), boiling point 76 ° C, no flash point (same below)] / n-decane [surface tension 23.4 mN / m
(25 ° C), boiling point 174 ° C] = 90/10 (wt / wt) After washing in the first washing tank, the temperature of HFE-7 was adjusted to 25 ° C.
A microporous polyethylene membrane was produced in the same manner as in Example 1 except that the rinse treatment was performed by immersing the substrate in a second cleaning tank (rinse tank) containing 200 for 1 minute, and the heat setting treatment was performed at 130 ° C.

Example 5 A microporous polyethylene membrane was produced in the same manner as in Example 4, except that the primary stretching was carried out at 40 ° C.

Comparative Example 1 Paraffin wax (melting point: instead of liquid paraffin)
A gel-like sheet was formed in the same manner as in Example 1 except that (61 ° C.) was used. MD of the obtained gel-like sheet at 40 ℃
However, the stress limit of the stretching machine was exceeded.

Comparative Example 2 A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the stretching was carried out at 120 ° C. by simultaneous biaxial stretching of MD 5 times and TD 5 times.

Comparative Example 3 Paraffin wax (melting point: instead of liquid paraffin)
Primary stretching at 70 ° C and washing with 2-propanol [surface tension 20.9 mN / m (25 ° C), boiling point 82.4
[° C.], and a polyethylene microporous membrane was produced in the same manner as in Example 1 except that the heat setting treatment was performed at 120 ° C.

Comparative Example 4 A microporous polyethylene membrane was produced in the same manner as in Example 1 except that the primary stretching was performed 2.5 times and the biaxial stretching was performed 10 times.

Comparative Example 5 Except that the primary stretching and the biaxial stretching were performed twice in the same direction,
A microporous polyethylene film was prepared in the same manner as in Example 1.

The physical properties of the polyolefin microporous membranes obtained in Examples 1-5 and Comparative Examples 1-5 were measured by the following methods. -Film thickness: Measured with a contact thickness meter. -Porosity: Measured by a gravimetric method.・ Puncture strength: 25 μm thick microporous membrane with a diameter of 1 mm (0.5 mm
The maximum load at the time of piercing at a speed of 2 mm / sec was measured using the R) needle.・ Heat shrinkage ratio: When the microporous membrane was exposed at 105 ° C for 8 hours
The shrinkage rates of MD and TD were measured respectively. -Average total light transmittance: Measured according to JIS K7105, and an average value was calculated for 20 samples. -Total light transmittance deviation: The standard deviation was calculated from the total light transmittance values measured for 20 samples. -Air permeability: Measured according to JIS P8117 (film thickness 25 μm conversion). -Heat treatment characteristics: A heat treatment test was performed at 80 ° C / 1.96 MPa / 5 minutes, and the air permeability after the test was examined. -Primary stretching tension ratio: The tension required for 5-fold stretching at 120 ° C was defined as 100%, and the tension required for primary stretching was measured.

[0091]

[Table 1]

As shown in Table 1, the polyethylene microporous membranes of Examples 1 to 5 produced by the method of the present invention have porosity,
It can be seen that it has an excellent balance of air permeability, puncture strength and heat shrinkage and has heat treatment characteristics that can withstand use. On the other hand, in Comparative Examples 1 and 3, a melt-kneaded product is prepared using a solid solvent, in Comparative Example 2, simultaneous biaxial stretching is carried out, and in Comparative Example 4, primary stretching is carried out at 3 times or less. In Example 5, since the primary stretching and the biaxial stretching were performed in the same direction and the primary stretching was performed at 3 times or less, the stretching was impossible in Comparative Example 1, and compared with Examples 1 to 5, In Comparative Example 2, the air permeability and the air permeability after the heat treatment are inferior, and in Comparative Example 3, the heat shrinkage, the average total light transmittance, the total light transmittance deviation, the air permeability and the air permeability after the heat treatment are low. Inferior, Comparative Example 4 is inferior in TD direction thermal shrinkage, average total light transmittance and air permeability, and Comparative Example 5 is in MD direction thermal shrinkage, average total light transmittance, air permeability and The air permeability after the heat treatment test was poor. Furthermore, in Comparative Example 3, the primary stretching tension ratio is high, and it is expected that high-magnification stretching is difficult due to mechanical restrictions.

[0093]

As described above in detail, the polyolefin microporous membrane of the present invention is a gel obtained by melt-kneading a polyolefin and a liquid solvent, extruding the obtained melt-kneaded product through a die, and cooling. The shaped product is primarily stretched in a uniaxial direction so as to exceed 3 times, and then the obtained stretched product is stretched at a temperature higher than the stretching temperature of the primary stretching in at least a direction perpendicular to the direction of the primary stretching. Subsequent stretching, since it is produced by removing the solvent thereafter, to produce a polyolefin microporous membrane having excellent porosity, air permeability, puncture strength and heat shrinkage balance, and excellent heat treatment characteristics. You can By using the obtained polyolefin microporous membrane as a battery separator, not only battery characteristics such as capacity characteristics, cycle characteristics, and discharge characteristics at low temperatures but also battery productivity and battery safety are all improved. Will be possible.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kotaro Takita             116 Setogaya-cho, Hodogaya-ku, Yokohama-shi, Kanagawa             -3 Higami Palais Stage Minamidai 405 (72) Koichi Kono, inventor             3-29-10-40, Mihara, Asaka City, Saitama Prefecture F-term (reference) 4F074 AA17 AA24 AB01 CA02 CC22X                       CC32Y DA02 DA10 DA24

Claims (2)

[Claims] 1. A gel-like molded product obtained by melt-kneading a polyolefin and a liquid solvent, extruding the obtained melt-kneaded product through a die and cooling, and drawing the solvent from the obtained stretched product. In the method for producing a polyolefin microporous membrane including a step of removing, the gel-like molded product is stretched uniaxially so as to exceed 3 times, and then the obtained stretched product is stretched at a stretching temperature of more than 3 times. A method for producing a polyolefin microporous membrane, which comprises stretching again at a higher temperature in a direction perpendicular to at least the stretching direction of at least 3 times. 2. A polyolefin microporous film obtained by the production method according to claim 1, which has a total light transmittance deviation of 5% or less, a porosity of 35 to 95%, and 25 ° C.
The air permeability in terms of film thickness of 25 μm at 10 to 300 seconds / 100 cc
A microporous polyolefin membrane characterized by: