JP2657430B2 - Polyolefin microporous membrane and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microporous membrane comprising a polyolefin containing an ultrahigh molecular weight component and a method for producing the same, and more particularly to a microporous polyolefin membrane having a moderate pore size and a sharp pore size distribution. And its manufacturing method.

[0002]

2. Description of the Related Art Microporous membranes include separators for batteries, diaphragms for electrolytic capacitors,
It is used for various applications such as various filters, moisture-permeable waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes, and microfiltration membranes.

[0003] Conventionally, as a method for producing a microporous polyolefin membrane, for example, a pore-forming agent composed of fine powder of a different polymer or the like is mixed with polyolefin and micro-dispersed.
A mixed extraction method for extracting a pore-forming agent, a phase separation method for forming a porous structure by separating a polyolefin phase into a microphase with a solvent, and applying a strain such as stretching to a polyolefin molded article in which different solids are micro-dispersed, An elongation method or the like has been used in which a heterogeneous solid is broken down at the interface to generate pores and make the pores porous. However, in these methods, the molecular weight is usually
Since a polyolefin of less than about 500,000 is used, there is a limit to thinning and high strength by stretching.

Recently, ultrahigh molecular weight polyolefins that can be formed into high-strength and high-elastic films have been developed, and various productions of high-strength microporous membranes have been proposed. For example, JP-A-58-5228 discloses that an ultrahigh molecular weight polyolefin is dissolved in a non-volatile solvent, a gel such as a fiber or a film is formed from the solution, and the gel containing the solvent is extracted with a volatile solvent. A method of heating and stretching is disclosed. However, a gel having a porous structure highly swollen with a non-volatile solvent cannot be stretched in a high orientation even if it is stretched in two directions, and is easily broken due to expansion of a network structure, and the obtained film has low strength. In addition, there is a disadvantage that the pore size distribution to be formed becomes large. On the other hand, in the gel dried after extracting the non-volatile solvent with the volatile solvent, the network structure shrinks and densifies, but the non-uniform evaporation of the volatile solvent easily causes warpage of the film raw material, and the shrinking and densification causes However, there is a drawback that stretching at a high magnification cannot be performed.

On the other hand, the weight average molecular weight is 5 × 10 ⁵
Forming a gel-like sheet from a solution obtained by heating and dissolving the above ultra-high molecular weight polyolefin in a solvent, preparing a solvent amount in the gel-like sheet by desolvation treatment, and then heating and stretching to remove a residual solvent. Various methods for producing microporous membranes of ultra-high molecular weight polyolefin (polyethylene) have been proposed (JP-A-60-242035, JP-A-61-21).
-495132, JP-A-61-195133, JP-A-63-39602,
JP-A-63-273651). However, in any of the above methods for producing a microporous ultrahigh molecular weight polyolefin (polyethylene) film, it is necessary to prepare a dilute solution of the polyolefin in order to biaxially stretch the ultrahigh molecular weight polyolefin. Since the swell and neck-in at the exit of the die for forming the sheet are large, it is difficult to form the sheet.Moreover, since the sheet contains an excessive amount of solvent, the desired microporous membrane can be obtained even if it is stretched as it is. However, there is a problem in productivity such as the necessity of removing the solvent to adjust the amount of solvent in the sheet.

[0006] In order to solve such a problem, the present inventors contain an ultrahigh molecular weight polyolefin,
A method for producing a microporous polyolefin membrane using a composition having a value of (weight average molecular weight / number average molecular weight) within a specific range was proposed (Japanese Patent Application No. 1-201785). In this way,
It becomes possible to produce a microporous polyolefin membrane from a polyolefin composition which has good stretchability and can be made into a high-concentration solution.

However, when the pore size of the polyolefin microporous membrane was examined, it was possible to obtain an average through-pore size in the range of 0.001 to 0.2 μm for the polyolefin microporous membrane by any of the above methods. 0.5 μm
Sufficient filtration efficiency cannot always be exerted when separating a component having a size as large as possible. Therefore, outer diameter 0.1 ~
In order to efficiently and quickly separate components having a size of about 0.5 μm, a microporous membrane having a pore diameter in the range of 0.05 to 0.2 μm and having a somewhat sharp pore diameter distribution has been desired. .

Accordingly, it is an object of the present invention to provide a microporous polyolefin membrane having a moderate pore size and a sharp pore size distribution.

It is another object of the present invention to provide a method for producing a microporous polyolefin membrane having a moderate pore size and a sharp pore size distribution.

[0010]

Means for Solving the Problems As a result of intensive studies in view of the above object, the present inventors have found that a polyolefin solution containing an ultrahigh molecular weight component and having a wide molecular weight distribution (weight average molecular weight / number average molecular weight is large). Is formed into a sheet shape, and the microporous film obtained by subjecting the gel-like sheet obtained by quenching to primary and secondary stretching at least in a uniaxial direction at a specific temperature has a pore size of an appropriate size. And found that the pore size distribution was sharp, and reached the present invention.

That is, the microporous polyolefin membrane of the present invention contains 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, and is composed of a polyolefin having (weight average molecular weight / number average molecular weight) of 10 to 300, Rate is 35-95%, average through-hole diameter is 0.05-0.2 μm, breaking strength of 15 mm width is 0.2 kg or more, and value of pore size distribution (maximum pore diameter / average through-hole diameter) is 1.5 or less. Features.

The first method for producing the microporous polyolefin membrane of the present invention comprises the step of adding a component having a molecular weight of 7 × 10 ⁵ or more to 1
% By weight or more (weight average molecular weight / number average molecular weight)
Is 10 to 300% by weight of a polyolefin and 10 to 300% by weight of a solvent.
A solution consisting of 90% by weight is prepared, the solution is extruded from a die, and cooled to form an unstretched gel-like composition. Stretching at least uniaxially at a temperature, then removing the residual solvent, and then stretching the resulting stretched product at least uniaxially at a temperature not higher than the melting point of the polyolefin −10 ° C. .

Further, a second method for producing the microporous polyolefin membrane of the present invention comprises the step of adding a component having a molecular weight of 7 × 10 ⁵ or more to 1
% By weight or more (weight average molecular weight / number average molecular weight)
Is 10 to 300% by weight of a polyolefin and 10 to 300% by weight of a solvent.
A solution consisting of 90% by weight is prepared, the solution is extruded from a die, and cooled to form an unstretched gel-like composition, and the gel-like composition is formed at least at a temperature not higher than the crystal dispersion temperature of the polyolefin. The film is stretched in a uniaxial direction by 1.2 to 10 times, subsequently stretched in a uniaxial direction at a temperature of from the crystal dispersion temperature to the melting point + 10 ° C. by at least 1.2 to 10 times, and then the remaining solvent is removed. I do.

The present invention will be described in detail below. The polyolefin microporous membrane of the present invention comprises a polyolefin having a molecular weight of 7 × 10 ⁵ or more, 1% by weight or more, and a molecular weight distribution (weight average molecular weight / number average molecular weight) of 10 to 300.

The weight average molecular weight / number average molecular weight of the polyolefin is 10 to 300, preferably 12 to 250.
If the weight-average molecular weight / number-average molecular weight is less than 10, the average molecular chain length is large and the entanglement density of the molecular chains during dissolution is high, so that it is difficult to prepare a high-concentration solution. Also 300
If it exceeds, the low molecular weight component is broken at the time of stretching, and the strength of the entire film is reduced.

The weight-average molecular weight / number-average molecular weight is:
It is used as a measure of the molecular weight distribution, and the width of the molecular weight distribution increases as the ratio of the molecular weights increases.
That is, in the case of a composition composed of polyolefins having different weight average molecular weights, the larger the ratio of the molecular weights of the composition is, the larger the difference in the weight average molecular weight of the polyolefin to be blended is,
The smaller the weight average molecular weight, the smaller the difference. In the case of a single polyolefin, the ratio of the molecular weights indicates the spread of the distribution, and the larger the value, the wider the distribution.

In the present invention, the weight-average molecular weight / number-average molecular weight of the polyolefin is set to 10 to 300, which is larger than the weight-average molecular weight / number-average molecular weight of the ordinary ultrahigh-molecular-weight polyolefin itself (generally about 6). . As a result, the molecular weight distribution spreads to the lower molecular weight side,
Preparation of a high-concentration polyolefin solution becomes possible.

The polyolefin has a molecular weight of 7 × 10
^When the content of ⁵ or more components is less than 1% by weight, the entanglement of the molecular chain of the ultrahigh molecular weight polyolefin which contributes to the improvement of the stretchability is hardly formed, and a high-strength microporous film cannot be obtained. On the other hand, the upper limit of the content of the ultrahigh molecular weight component is not particularly limited. However, if the content exceeds 90% by weight, it becomes difficult to achieve a high concentration of the target polyolefin solution, which is not preferable.

The polyolefin may be either a single polyolefin (not a mixture) or a composition comprising two or more polyolefins as long as it has the above-mentioned molecular weight and molecular weight distribution.

In the case of a single polyolefin, for example, it contains 1% by weight or more of an ultrahigh molecular weight component having a molecular weight of 7 × 10 ⁵ or more, and has a molecular weight distribution (weight average molecular weight / number average molecular weight) of 10%.
It can be produced by multi-stage polymerization so as to give a molecular weight of up to 300. As the multistage polymerization, it is preferable to produce a high molecular weight portion and a low molecular weight portion by two-stage polymerization.

In the case of a polyolefin composition (mixture), an ultrahigh molecular weight polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more and a polyolefin having a weight average molecular weight of less than 7 × 10 ⁵ have a weight average molecular weight / number average molecular weight. It can be obtained by mixing an appropriate amount so as to be within the above range.

In the case of the composition, the ultrahigh molecular weight polyolefin has a weight average molecular weight of 7 × 10 ⁵ or more, preferably 1 × 10 5
⁶ to 15 × 10 ⁶ . The weight average molecular weight is 7 × 10 ⁵
If it is less than 1, the maximum stretching ratio is low, and the desired microporous membrane cannot be obtained. On the other hand, the upper limit is not particularly limited, but those exceeding 15 × 10 ⁶ are inferior in moldability in forming a gel-like molded product.

Such ultra-high molecular weight polyolefins include ethylene, propylene, 1-butene, 4-methyl-1-
Examples include a crystalline homopolymer, a two-stage polymer, or a copolymer of pentene, 1-hexene, or the like, and a blend thereof. Of these, ultrahigh molecular weight polyethylene, particularly high density ultrahigh molecular weight polyethylene, is preferred.

The content of the ultrahigh molecular weight polyolefin in the polyolefin composition is 1% by weight or more, based on 100% by weight of the whole polyolefin composition. If the content of the ultra-high-molecular-weight polyolefin is less than 1% by weight, the entanglement of the molecular chain of the ultra-high-molecular-weight polyolefin that contributes to the improvement in stretchability is hardly formed, and a high-strength microporous film cannot be obtained. On the other hand, the upper limit is not particularly limited, but if it exceeds 90% by weight, it is difficult to achieve a high concentration of the target polyolefin solution, which is not preferable.

The polyolefin other than the ultrahigh molecular weight polyolefin in the polyolefin composition has a weight average molecular weight of less than 7 × 10 ⁵ , and the lower limit of the molecular weight is preferably 1 × 10 ⁴ or more. It is not preferable to use a polyolefin having a weight average molecular weight of less than 1 × 10 ⁴ , since breakage is likely to occur during stretching and a desired microporous film cannot be obtained. In particular, the weight average molecular weight is 1 × 10 ⁵ or more and 7 × 10
^It is preferred to incorporate less than ⁵ polyolefins into the ultra-high molecular weight polyolefin.

Such polyolefins include ethylene, propylene, 1-butene, 4-methyl-1-pentene,
Examples include a crystalline homopolymer obtained by polymerizing 1-hexene or the like, a two-stage polymer, a copolymer, and a blend thereof. In particular, high-density polyethylene which is a polymer mainly composed of ethylene is preferable.

The above-mentioned polyolefin may be added to an antioxidant, an ultraviolet absorber, a lubricant,
Various additives such as an anti-blocking agent, a pigment, a dye, and an inorganic filler can be added as long as the object of the present invention is not impaired.

Next, a first method for producing a microporous polyolefin membrane of the present invention using the above-described polyolefin will be described.

In the present invention, a high concentration solution of polyolefin as a raw material is prepared by heating and dissolving the above-mentioned polyolefin in a solvent.

The solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin. For example,
Nonane, decane, undecane, dodecane, aliphatic or cyclic hydrocarbons such as paraffin oil, or a mineral oil fraction having a boiling point corresponding thereto, and the like, in order to obtain a gel-like molded product having a stable solvent content. Is preferably a non-volatile solvent such as paraffin oil.

Heat dissolution is carried out with stirring at a temperature at which the polyolefin is completely dissolved in the solvent. The temperature varies depending on the polymer and the solvent used, but for example, in the case of polyethylene, it is in the range of 140 to 250 ° C. The concentration of the polyolefin solution is 10 to 50% by weight, preferably 10 to 50% by weight.
40% by weight. If the concentration is less than 10% by weight, not only is it economical to use a large amount of solvent, but also when forming into a sheet, the swell and neck-in at the exit of the die are large, making it difficult to form the sheet. On the other hand, if the concentration exceeds 50% by weight, it becomes difficult to prepare a uniform solution. In addition, at the time of heating and dissolving, it is preferable to add an antioxidant to prevent oxidation of the polyolefin.

Next, the heated solution of the polyolefin is extruded from a die and molded. As the die, a sheet die having a rectangular base shape is usually used, but a double cylindrical inflation die or the like can also be used. The die gap when using a sheet die is usually 0.1 to
5 mm and heated to 140-250 ° C during extrusion. At this time, the extrusion speed is usually 20-30 cm / min.
3 m / min.

The solution thus extruded from the die is formed into a gel by cooling. Cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature. When the cooling rate is low, the crystallinity increases, and it is difficult to form a gel 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 by a refrigerant, or the like can be used. Note that the solution extruded from the die may be 1 to 10, preferably 1 to 5, before or during cooling.
May be picked up at the pick-up ratio. When the take-up ratio is 10 or more, neck-in becomes large, and breakage tends to occur during stretching, which is not preferable.

Next, the gel is stretched (hereinafter referred to as primary stretching). The stretching is performed at a predetermined magnification by heating the gel-like molded product and using a usual tenter method, a roll method, an inflation method, a rolling method or a combination of these methods. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either vertical or horizontal simultaneous stretching or sequential stretching may be used.
Axial stretching is preferred.

The stretching temperature is from the crystal dispersion temperature of the polyolefin to the crystal melting point + 10 ° C. or lower, preferably from the crystal dispersion temperature to less than the crystal melting point. For example, in the case of a polyethylene composition, the temperature ranges from 90 to 140 ° C, more preferably from 100 to 130 ° C. When the stretching temperature exceeds the melting point + 10 ° C., the molecular chains cannot be oriented by stretching due to the melting of the resin. On the other hand, if the stretching temperature is lower than the crystal dispersion temperature, the resin is insufficiently softened, the film is easily broken in stretching, and high-magnification stretching cannot be performed.

Although the stretching ratio varies depending on the thickness of the raw material, it is at least twice, preferably from 3 to 1, in the uniaxial direction.
The magnification is 30 times and the area magnification is 10 times or more, preferably 15 to 400 times. If the areal magnification is less than 10 times, stretching is insufficient and a highly elastic and high-strength microporous film cannot be obtained. On the other hand, when the area magnification exceeds 400 times, restrictions are imposed on a stretching apparatus, a stretching operation, and the like.

The molded article stretched in this way is subsequently washed with a solvent to remove the residual solvent. Examples of the cleaning solvent include hydrocarbons such as pentane, hexane, and heptane; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride; fluorinated hydrocarbons such as ethane trifluoride; and ethers such as diethyl ether and dioxane. Volatile ones can be used. These solvents are appropriately selected according to the solvent used for dissolving the polyolefin, and used alone or as a mixture. The washing method can be performed by a method of immersing in a solvent for extraction, a method of showering the solvent, a method of a combination thereof, or the like.

The washing as described above is performed until the residual solvent in the stretch molded product becomes less than 1% by weight. After that, the washing solvent is dried. The washing solvent can be dried by a method such as heating and drying. The dried stretch molded product is
It is desirable to heat set in a temperature range from the crystal dispersion temperature to the melting point.

In the first method of the present invention, the obtained stretch-formed product is heated to a melting point of -10 ° C. or lower and stretched again (hereinafter referred to as “secondary stretching”). Stretching can be performed by a method similar to the above-described primary stretching. The stretching method may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either vertical or horizontal simultaneous stretching or sequential stretching may be used.

The stretching temperature is the melting point of polyolefin−10 ° C.
Or lower, preferably lower than the crystal dispersion temperature. The lower limit is preferably room temperature or higher. For example, in the case of a polyethylene composition, the temperature is preferably 115 ° C. or lower. Stretching temperature is 11
If it exceeds 5 ° C., local stretching occurs, and the film thickness becomes uneven.

The stretching ratio varies depending on the thickness of the raw material, but is 1.1 to 5 times, preferably 1.2 to 2.5 times, in one axis direction.
The magnification is 1.2 to 5 times, preferably 1.5 to 3 times. When the area magnification exceeds 5 times, the thickness of the film after stretching becomes uneven, which is not preferable.

It is desirable that the obtained microporous polyolefin membrane is heat-set at a temperature within a range from the crystal dispersion temperature to the melting point. Further, if necessary, hydrophilic treatment can be performed by plasma irradiation, surfactant impregnation, surface grafting, or the like.

The microporous polyolefin membrane produced as described above has a porosity of 35 to 95% and an average through-pore diameter of 0.05 to 0.05%.
The breaking strength of 0.2 to 0.2 μm and 15 mm width is 0.2 kg or more, preferably 0.5 kg or more. Furthermore, the pore size distribution (maximum pore size /
(Average through-hole diameter) is as sharp as 1.5 or less, and precise filtration can be performed efficiently. In the pore size distribution, the maximum pore size is a value calculated based on the value when the rejection of the pullulan solution is 90% using Flory's theory. The thickness of the polyolefin microporous membrane of the present invention can be appropriately selected depending on the application, but is generally 0.1 to 50 μm.
m, and preferably 2 to 40 μm.

Next, a second method for producing the microporous polyolefin membrane of the present invention will be described. The microporous polyolefin membrane produced by the second method of the present invention also comprises the same polyolefin as in the first method of the present invention described above.

The steps from the preparation of a high-concentration solution of polyolefin as a raw material to the step of producing a gel-like molded product are the same as those in the first production method described above.

Next, the obtained gel-like molded product is stretched (hereinafter, referred to as primary stretching). The stretching is performed at a predetermined magnification by heating the gel-like molded product and using a usual tenter method, a roll method, an inflation method, a rolling method or a combination of these methods. The stretching may be uniaxial stretching or biaxial stretching.
In the case of biaxial stretching, either vertical or horizontal simultaneous stretching or sequential stretching may be used.

The temperature of the primary stretching is not higher than the polyolefin crystal dispersion temperature and not lower than room temperature. For example, in the case of polyethylene, the temperature is 90 ° C. or less. When the primary stretching temperature is higher than 90 ° C., the composition does not undergo a change that causes fine cracks, and the pore size of the porous film cannot be increased by the secondary stretching described below.

The stretching ratio at the time of the primary stretching varies depending on the thickness of the raw material, but at least 1.2 to 10 in the uniaxial direction.
And preferably 1.5 to 5 times. If the stretching ratio is less than 1.2 times, the stretching is insufficient, so that a change that causes a fine crack does not occur, and the pore size of the porous film cannot be increased by the secondary stretching described later. If it exceeds 10 times, the film strength will decrease. In the case of biaxial stretching, the area magnification is 1.5
It is about 15 times, more preferably 2 to 12 times. When the stretching ratio exceeds 15 times in area ratio, a porous membrane having a sharp pore size distribution and a large pore size cannot be obtained.

In the second method of the present invention, the molded article stretched in this manner is subsequently stretched (hereinafter referred to as secondary stretching). Stretching is performed at a predetermined magnification by the same method as in the primary stretching. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either vertical or horizontal simultaneous stretching or sequential stretching may be used.
Axial stretching is preferred.

The secondary stretching temperature is from the crystal dispersion temperature of the polyolefin to the crystal melting point + 10 ° C., preferably from the crystal dispersion temperature to a temperature lower than the crystal melting point. For example, in the case of polyethylene, the temperature ranges from 90 to 140 ° C, more preferably from 100 to 130 ° C. When the stretching temperature exceeds the melting point + 10 ° C., the molecular chains cannot be oriented by stretching due to the melting of the resin. On the other hand, if the stretching temperature is lower than the crystal dispersion temperature, the resin is insufficiently softened, the film is easily broken in stretching, and high-magnification stretching cannot be performed.

Although the stretching ratio varies depending on the thickness of the raw material, it is at least 1.2 to 10 times, preferably 2 to 10 times in the uniaxial direction.
~ 5 times. If the stretching ratio exceeds 10 times, a porous membrane having a sharp pore size distribution and a large pore size cannot be obtained.
Also, the total area ratio of primary stretching and secondary stretching is 10 times or more,
Preferably it is 10 to 300 times. If the total area ratio is less than 10 times, stretching is insufficient, and a highly elastic and high-strength microporous film cannot be obtained. On the other hand, if the total area magnification exceeds 400 times, restrictions are imposed on the stretching apparatus, stretching operation, and the like.

Subsequently, the thus obtained stretched product is
Wash with solvent to remove residual solvent. The washing can be performed by the same method as the first method described above. Thereafter, the cleaning solvent is dried. The method for drying the cleaning solvent is the same as the above-described first method.

The microporous polyolefin membrane thus obtained can be further subjected to a hydrophilic treatment by plasma irradiation, surfactant impregnation, surface grafting, etc., if necessary.

The microporous polyolefin membrane produced as described above has a porosity of 35 to 95% and an average through-pore diameter of 0.05 to 0.05%.
The breaking strength of 0.2 to 0.2 μm and 15 mm width is 0.2 kg or more, preferably 0.5 kg or more. Furthermore, the pore size distribution (maximum pore size /
The average diameter of through-holes is relatively sharp compared to the pore size distribution of the conventional polyolefin microporous membrane. In the pore size distribution, the maximum pore size is a value calculated by using Flory's theory based on the value in the case where the rejection of the pullulan solution is 90%. The thickness of the microporous polyolefin membrane of the present invention can be appropriately selected depending on the application, but is generally about 0.1 to 50 μm, preferably 2 to 40 μm.

[0055]

According to the first method for producing a microporous polyolefin membrane of the present invention, a polyolefin solution containing an ultrahigh molecular weight component and having a wide molecular weight distribution (high weight average molecular weight / number average molecular weight) is formed into a sheet. Then, the gel-like sheet obtained by quenching is cooled at a temperature of crystal dispersion temperature to melting point + 10 ° C,
After stretching in at least one axis direction, removing and drying the residual solvent, and then stretching again in at least one axis direction at a temperature not higher than the melting point of the polyolefin −10 ° C., a microporous film is produced. The resulting microporous membrane has a moderate pore size and a sharp pore size distribution.

Although the reason why such an effect is obtained is not necessarily clear, usually, if the stretching temperature is increased in order to increase the pore size, the film strength decreases and the film becomes impractical. When the temperature is lowered to near the dispersion temperature, the porosity decreases. On the other hand, it is conceivable to increase the stretching ratio, but problems such as deformation due to stretching also occur in the thickness direction of the film or fibrillation further proceeds. Therefore, in the present invention, the stretching is first performed at a temperature from the crystal dispersion temperature to the melting point + 10 ° C. to form microporous particles, and after removing the solvent, the pore size is increased by further stretching at a temperature equal to or lower than the melting point −10 ° C. This is considered to be due to the uniformization.

In the second method for producing a microporous polyolefin membrane of the present invention, a polyolefin solution containing an ultrahigh molecular weight component and having a wide molecular weight distribution (high weight average molecular weight / high number average molecular weight) is formed into a sheet. The gel-like sheet obtained by molding and quenching is stretched at least uniaxially by a factor of 1.2 to 10 at a temperature not higher than the crystal dispersion temperature, and subsequently at least uniaxially at a temperature of from the crystal dispersion temperature to the melting point + 10 ° C. 1.
Since the microporous membrane is manufactured by stretching 2 to 10 times and then removing and drying the remaining solvent, the obtained microporous membrane has a pore size of an appropriate size and a pore size distribution. It is sharp.

Although the reason why such an effect is obtained is not necessarily clear, microporous particles are usually formed between fibrils formed from polyolefin. By performing stretching at a specific magnification at a temperature, a fine microcrack is generated while suppressing fibrillation, and then the microcracking is performed at a temperature not higher than the crystal dispersion temperature to the melting point + 10 ° C. This is considered to be because the diameter of the hole is enlarged by enlarging the hole and the hole is made uniform.

[0059]

Examples of the present invention will be described below. In addition, the test method in an Example is as follows. (1) Molecular weight and molecular weight distribution: GP manufactured by Waters Corporation
Using C apparatus, GMH-6 manufactured by Tosoh Corp.
Using O-dichlorobenzene, temperature 135 ° C, flow rate 1.0 ml
Per minute, measured by gel permeation chromatography (GPC). (2) Film thickness: The cross section was measured with a scanning electron microscope. (3) Air permeability: Conforms to JIS P8117. (4) Permeability: A microporous membrane is incorporated into a flat membrane module, subjected to a hydrophilization treatment by passing a distilled water / ethanol mixture (volume ratio of 50/50), washed sufficiently with distilled water, and then dried at 380 mmHg.
Was determined by measuring the permeation amount of the filtrate when the water pressure was applied. (5) Average pore size: contained in the filtrate when a 0.05% by weight aqueous solution of pullulan (manufactured by Showa Denko KK) was circulated under a differential pressure of 380 mmHg using the module described in (4) above. The concentration of pullulan was determined from the differential refractive index measurement. Then, the pore diameter was converted from the value of the molecular weight of pullulan at which the rejection calculated by the following formula was 50%, using Flory's theory as described later. Pullulan rejection = {1-(Pullulan concentration in filtrate / Pullulan concentration in stock solution)} x 100 The chain polymer in the form of a solution is in the form of a spherical thread, and its diameter d is the both ends of the molecular chain. It can be considered that there is an approximate relationship of [d / 2] ² = <γ ² > (1) with respect to the mean square distance <γ ² >. According to Flory's theory of viscosity and molecular chain spread in a polymer solution, [η] M = 2.1 × 10 ²¹ <γ ² > ^3/2 ... (2) holds regardless of the type of polymer. From formulas (1) and (2), the diameter d of the chain polymer can be calculated from the measured value of the intrinsic viscosity [η] and the molecular weight M at which the rejection becomes 50%. This d was defined as the average pore size of the polyethylene microporous membrane. (6) Pore size distribution: In the measurement according to (5) above, the rejection
Similarly, the pore size is converted from the value of the molecular weight of pullulan, which is 90%, to obtain the maximum pore size, and the maximum pore size is calculated by using the value of the maximum pore size / the average pore size. (7) Porosity: Measured with a mercury porosimeter. (8) Breaking strength: ASTM D for a 15 mm wide strip specimen
Measured according to 882.

Example 1 2 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 2.5 × 10 ⁶ and a weight average molecular weight (Mw) of 3.
Mw mixed with 13 parts by weight of 7 × 10 ⁵ polyethylene
/ Mn = 11 raw material resin polyethylene composition
Crystal dispersion temperature 90 ° C., melting point 134.0 ° C.) and 85 parts by weight of liquid paraffin (64 cst / 40 ° C.) were mixed to prepare a solution of a polyethylene composition. Next, 0.125 parts by weight of 2,6-di-t-butyl-p-cresol (“BHT”, manufactured by Sumitomo Chemical Co., Ltd.) and 100 parts by weight of the solution of the polyethylene composition were mixed with 0.125 parts by weight of tetrakis [methylene-
3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane ("Irganox 1
010 "(manufactured by Ciba Geigy) as an antioxidant. This mixture was filled in an autoclave equipped with a stirrer and stirred at 200 ° C. for 90 minutes to obtain a uniform solution.

This solution was extruded from a T-die by an extruder having a diameter of 45 mm, and a gel-like sheet was formed while being taken up by a cooling roll. Subsequently, the gel-like sheet was heated at 115 ° C.
Simultaneous biaxial stretching was performed 5 × 5 times at a stretching speed of 0.5 m / min. The obtained stretched membrane was washed with methylene chloride to extract and remove the remaining liquid paraffin, and then dried to obtain a polyethylene microporous membrane. This polyethylene microporous membrane is heated to 90
The film was stretched 1.5 times in the transverse direction at a temperature of 0.5 ° C. and a stretching speed of 0.5 m / min. Table 1 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, water permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. Second result
It is shown in the table.

Example 2 A microporous polyethylene membrane was produced in the same manner as in Example 1 except that the stretching in the transverse direction was doubled. Table 1 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, water permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. Second result
It is shown in the table.

Example 3 In Example 1, a two-stage polymerized polyethylene (weight average molecular weight 7.5 × 10 ⁵ , weight average molecular weight / number average molecular weight = 43.0, component having a molecular weight of 7 × 10 ⁵ or more) was used as a raw material resin. 32% by weight , crystal dispersion temperature 90 ° C., melting point 13
4.5 ° C. ) except that a microporous polyethylene membrane was produced. Table 1 shows the production conditions for the microporous polyethylene membrane. Also, the thickness of the polyethylene microporous membrane,
Air permeability, water permeability, porosity, breaking strength, average pore size and pore size distribution were measured. The results are shown in Table 2.

Example 4 In Example 1, a two-stage polymerized polyethylene (weight average molecular weight 8.2 × 10 ⁵ , weight average molecular weight / number average molecular weight = 28.8, molecular weight 7 × 10 ⁵ or more) was used as the raw material resin. 40% by weight , crystal dispersion temperature 90 ° C., melting point 13
(4.7 ° C. ) except that a microporous polyethylene membrane was produced. Table 1 shows the production conditions for the microporous polyethylene membrane. Also, the thickness of the polyethylene microporous membrane,
Air permeability, water permeability, porosity, breaking strength, average pore size and pore size distribution were measured. The results are shown in Table 2.

Comparative Example 1 A microporous polyethylene membrane was produced in the same manner as in Example 1, except that the stretching in the transverse direction was 1.5 times at a temperature of 125 ° C. (the temperature exceeding the melting point of the polyethylene composition −10 ° C.). did. Table 1 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, water permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. The results are shown in Table 2.

Comparative Example 2 A microporous polyolefin membrane was produced in the same manner as in Example 1 except that the primary stretching temperature was 125 ° C., the stretching ratio was 10 × 10, and the secondary stretching was not performed. Table 1 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, water permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. The results are shown in Table 2.

[0067]

Table 2 Film thickness Air permeability Permeability Average pore size Example of pore size distribution No. (μm) (sec / 100cc) (*) (μm) Example 1 15 180 650 0.08 1.2 Example 2 15 120 750 0.10 1.2 Example 3 15 180 650 0.08 1.2 Example 4 15 150 700 0.07 1.2 Comparative Example 1 13 to 19 270 or less 350 or less 0.05 2.3 Comparative Example 2 15 300 500 0.06 2.8 Note) *: Unit: liter / m ² · hr · atm

[0069]

As is apparent from Table 2, the microporous polyethylene membranes of Examples 1 to 4 according to the first method of the present invention were:
As compared with the microporous polyethylene membrane of Comparative Example 1, the membrane thickness was stable, the filtration efficiency was good, and the pore size distribution was sharp. Also, the pore size distribution is remarkably sharper than that of Comparative Example 2 according to the conventional method.

Example 5 2 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 2.5 × 10 ⁶ and a weight average molecular weight (Mw) of 3.
Mw mixed with 13 parts by weight of 7 × 10 ⁵ polyethylene
/ Mn = 11 raw material resin polyethylene composition
Crystal dispersion temperature 90 ° C., melting point 134.0 ° C.) and 85 parts by weight of liquid paraffin (64 cst / 40 ° C.) were mixed to prepare a solution of a polyethylene composition. Next, 0.125 parts by weight of 2,6-di-t-butyl-p-cresol (“BHT”, manufactured by Sumitomo Chemical Co., Ltd.) and 100 parts by weight of the solution of the polyethylene composition were mixed with 0.125 parts by weight of tetrakis [methylene-
3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane ("Irganox 1
010 "(manufactured by Ciba Geigy) as an antioxidant. This mixture was filled in an autoclave equipped with a stirrer and stirred at 200 ° C. for 90 minutes to obtain a uniform solution.

This solution was extruded from a T-die using an extruder having a diameter of 45 mm, and a gel-like sheet was formed while being taken up by a cooling roll. The obtained sheet was cut into small pieces of 9 cm × 9 cm, set on a biaxial stretching machine, and simultaneously biaxially stretched 3 × 3 times at a primary stretching temperature of 80 ° C. and a primary stretching speed of 0.3 m / min.
Subsequently, the obtained film was simultaneously biaxially stretched 2 × 2 times at a secondary stretching temperature of 118 ° C. and a primary stretching speed of 0.3 m / min.
The obtained stretched membrane was washed with methylene chloride to extract and remove the remaining liquid paraffin, and then dried to obtain a polyethylene microporous membrane. Table 3 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, porosity, breaking strength, average pore size and pore size distribution of the polyethylene microporous membrane were measured. The results are shown in Table 4.

Example 6 A microporous polyethylene membrane was produced in the same manner as in Example 1 except that the secondary stretching ratio was changed to 2.5 × 2.5. Table 3 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, porosity, breaking strength, average pore size and pore size distribution of the polyethylene microporous membrane were measured.
The results are shown in Table 4.

Example 7 A microporous polyethylene membrane was produced in the same manner as in Example 1 except that the secondary stretching ratio was changed to 4 × 4. Table 3 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, porosity, breaking strength, average pore size and pore size distribution of the polyethylene microporous membrane were measured.
The results are shown in Table 4.

Example 8 In Example 5, a two-stage polymerized polyethylene (weight average molecular weight 7.5 × 10 ⁵ , weight average molecular weight / number average molecular weight = 43.0, and a component having a molecular weight of 7 × 10 ⁵ or more) was used as a raw material resin. 32% by weight , crystal dispersion temperature 90 ° C., melting point 13
4.5 ° C. ) except that a microporous polyethylene membrane was produced. Table 3 shows the production conditions for the microporous polyethylene membrane. Also, the thickness of the polyethylene microporous membrane,
Air permeability, porosity, breaking strength, average pore size and pore size distribution were measured. The results are shown in Table 4.

Example 9 In Example 5, two-stage polymerized polyethylene (weight average molecular weight 8.2 × 10 ⁵ , weight average molecular weight / number average molecular weight = 28.8, molecular weight 7 × 10 ⁵ or more) was used as a raw material resin. Component ratio 40% by weight , crystal dispersion temperature 90 ° C, melting point 13
(4.7 ° C. ) except that a microporous polyethylene membrane was produced. Table 3 shows the production conditions for the microporous polyethylene membrane. Also, the thickness of the polyethylene microporous membrane,
Air permeability, porosity, breaking strength, average pore size and pore size distribution were measured. The results are shown in Table 4.

Comparative Example 3 A microporous polyethylene membrane was produced in the same manner as in Example 5 except that the primary stretching ratio was changed to 4 × 4. Table 3 shows the production conditions for the microporous polyethylene membrane. Also, the thickness, air permeability, porosity, breaking strength,
The average pore size and the pore size distribution were measured. The results are shown in Table 4.

Comparative Example 4 A microporous polyethylene membrane was produced in the same manner as in Example 5 except that the primary stretching temperature was changed to 118 ° C. (a temperature exceeding the crystal dispersion temperature of polyethylene). Table 3 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, porosity, breaking strength, average pore size and pore size distribution of the polyethylene microporous membrane were measured. The results are shown in Table 4.

Comparative Example 5 A microporous polyethylene membrane was produced in the same manner as in Example 5 except that the stretching conditions were changed to a stretching temperature of 125 ° C. and a stretching ratio of 10 × 10 without performing the primary stretching. Table 3 shows the production conditions for the microporous polyethylene membrane. Further, the thickness, air permeability, porosity, breaking strength, average pore size and pore size distribution of the polyethylene microporous membrane were measured. The results are shown in Table 4.

[0080] Table 3 Primary stretching conditions secondary stretching conditions Example No. stretching temperature stretch ratio stretching temperature stretching ratio Example 5 80 ° C. 3 times × 3 times 118 ° C. 2-fold × 2 times Example 6 80 ° C. 3 times × 3 Example 118 80 ° C 3 times × 3 times 118 ° C 4 times × 4 times Example 8 80 ° C 3 times × 3 times 118 ° C 2 times × 2 times Example 9 80 ° C 3 times × 3 times 118 ° C 2 times × 2 times Comparative Example 3 80 ° C. 4 times × 4 times 118 ° C. 2 times × 2 times Comparative Example 4 118 ° C. 3 times × 3 times 118 ° C. 2 times × 2 times Comparative Example 5--125 ° C 10x10x

Table 4 Film thickness Air permeability Average pore size Pore size distribution Porosity Breaking load example No. (μm) ( sec / 100cc ) (μm) (%) (kg / 150mm width ) Example 5 10 120 0.09 1.3 50 0.8 Example 6 10 180 0.08 1.4 47 1.0 Example 7 10 200 0.07 1.4 45 1.1 Example 8 10 120 0.09 1.4 50 0.8 Example 9 10 150 0.08 1.4 50 0.8 Comparative Example 3 10 150 0.07 2.5 57 1.0 Comparative Example 4 10 120 0.08 2.8 55 1.0 Comparative Example 5 10 200 0.06 2.9 60 0.2

As is evident from Table 4, the microporous polyethylene membranes of Examples 5 to 9 according to the method of the present invention have a sharper pore size distribution than the microporous polyethylene membranes of Comparative Examples 3 to 5.

[0083]

As described in detail above, according to the first method of the present invention, a polyolefin solution containing an ultrahigh molecular weight component and having a wide molecular weight distribution (high weight average molecular weight / number average molecular weight) can be obtained. The gel-like sheet obtained by forming into a sheet and quenching is stretched at least uniaxially at a temperature of from the crystal dispersion temperature to the melting point + 10 ° C., and after removing and drying the remaining solvent, the melting point of the polyolefin is lowered by 10 ° C. The microporous membrane is manufactured by stretching again at least in a uniaxial direction at a temperature of not more than ° C.

According to the second method of the present invention, the gel-like sheet is stretched at least 1.2 to 10 times in one axial direction at a temperature equal to or lower than the crystal dispersion temperature.
A microporous membrane is produced by stretching the film at a temperature of ° C. at least 1.2 to 10 times in one axis direction, and then removing and drying the remaining solvent.

The microporous polyolefin membrane obtained by such a method has an appropriate pore size and a sharp pore size distribution.

The polyolefin microporous membrane according to the method of the present invention can be used for various types of separators for batteries, diaphragms for electrolytic capacitors, ultra-fine filtration membranes, ultrafiltration membranes, various filters, porous membranes for moisture-permeable waterproof clothing and the like. Suitable for use.

Claims (3)

(57) [Claims] 1. A composition containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, wherein (weight average molecular weight / number average molecular weight) is 10 to 30.
It has a porosity of 35 to 95%, an average through hole diameter of 0.05 to 0.2 μm, and a 15 mm width breaking strength of 0.
A microporous polyolefin membrane having a pore size of 2 kg or more and a pore size distribution (maximum pore size / average through pore size) of 1.5 or less. 2. A composition containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, wherein (weight average molecular weight / number average molecular weight) is 10 to 30.
0 to 10% by weight of polyolefin and 50 to 90% by weight of solvent
To prepare a solution consisting of, extruding the solution from a die,
An unstretched gel composition is formed by cooling, and the gel composition is dispersed at a crystal dispersion temperature of the polyolefin.
Stretching at least uniaxially at a temperature of + 10 ° C. of the melting point, removing the remaining solvent, and then stretching the obtained stretched product at least at a temperature not higher than the melting point of the polyolefin of −10 ° C. in at least a uniaxial direction. A method for producing a microporous polyolefin membrane, characterized in that: 3. A composition containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, wherein (weight average molecular weight / number average molecular weight) is 10 to 30.
0 to 10% by weight of polyolefin and 50 to 90% by weight of solvent
To prepare a solution consisting of, extruding the solution from a die,
Forming an unstretched gel-like composition by cooling, stretching the gel-like composition at least in a uniaxial direction at a temperature equal to or lower than the crystal dispersion temperature of the polyolefin by a factor of 1.2 to 10;
Subsequently, at a temperature of from the crystal dispersion temperature to the melting point + 10 ° C., the film is stretched at least 1.2 to 10 times in one axial direction, and thereafter, the remaining solvent is removed.