JPH06234875A - Finely porous polyolefin film and its production - Google Patents

Abstract

PURPOSE:To provide a finely porous polyolefin film excellent in drawability, pore size distribution sharpness, etc., useful for battery separators, electrolytic capacitors, etc., consisting of a polyolefin having each specific pore characteristics, breaking strength and molecular weight characteristics. CONSTITUTION:The porous film consisting of a polyolefin having the following characteristics: (1) porosity: 35-95%; (2) mean penetrated pore diameter: 0.001-0.02mum; (3) breaking strength (15mm width): >=0.2kg; (4) pore size distribution (max. pore diameter/means penetrated pore diameter): <=1.5; (5) content of the components >=7X10<5> in molecular weight: >=1wt.%; and (6) weight-average molecular-weight/number-average molecular weight: 10-300. This porous film can preferably be obtained by the following process: a solution composed of 10-50wt.% of a polyolefin having the above-mentioned molecular weight characteristics and 50-90wt.% of a solvent is first prepared and then extruded through a die and cooled into a gel-like composition, which is then heat set at a temperature between the crystal dispersion temperature and the melting point of the polyolefin followed by drawing at a temperature not higher than the melting point plus 10 deg.C and then desolvation.

Description

Detailed Description of the Invention

[0001]

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

[0002]

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

Conventionally, as a method for producing a microporous polyolefin membrane, for example, a pore-forming agent composed of a fine powder of a different polymer or the like is mixed with polyolefin and microdispersed,
A mixed extraction method for extracting a pore-forming agent, a phase separation method for forming a porous structure by microphase-separating a polyolefin phase with a solvent, and imparting strain such as stretching to a polyolefin molded body in which a heterogeneous solid is microdispersed, A stretching method or the like is used in which interfaces between different kinds of solids are destroyed to generate pores to make them porous. However, in these methods, the molecular weight is usually
Since polyolefin of less than 500,000 is used, there is a limit to thinning and high strength by stretching.

Recently, an ultra-high molecular weight polyolefin which can be formed into a high-strength and high-elasticity film has been developed, and various methods for producing a high-strength microporous membrane have been proposed. For example, JP-A-58-5228 discloses that ultra-high molecular weight polyolefin is dissolved in a non-volatile solvent, a gel such as a fiber or a film is molded from this solution, and the gel containing this solvent is subjected to extraction treatment with a volatile solvent. A method of heating and stretching is disclosed. However, a gel having a porous structure swollen highly in a non-volatile solvent cannot be stretched in a high orientation even when it is stretched in two directions, and is easily broken due to expansion of a network structure, and the resulting film has low strength. Also, there is a drawback that the distribution of pore diameters formed is large. On the other hand, in the gel dried after extracting the non-volatile solvent with the volatile solvent, the network shrinks and densifies, but uneven evaporation of the volatile solvent tends to cause warpage in the original film, and shrinkage and densification However, there is a drawback that stretching at a high magnification cannot be performed.

On the other hand, the weight average molecular weight is 7 × 10 ^5.
Molding a gel-like sheet from a solution obtained by heating and dissolving the above ultra-high molecular weight polyolefin in a solvent, adjusting the amount of the solvent in the gel-like sheet by a desolventizing treatment, and then drawing by heating and removing the residual solvent. Have proposed various methods for producing a microporous membrane of ultra-high molecular weight polyolefin (polyethylene) (JP-A-60-242035 and JP-A-61).
-495132, JP-A-61-195133, JP-A-63-39602,
JP-A-63-273651). However, in any of the above methods for producing an ultrahigh molecular weight polyolefin (polyethylene) microporous membrane, it is necessary to prepare a dilute solution of the polyolefin in order to biaxially stretch the ultrahigh molecular weight polyolefin, and thus the obtained solution is The swell and neck-in are large at the die exit for forming the sheet, making it difficult to form the sheet. Further, since the sheet contains an excessive amount of solvent, the target microporous membrane can be obtained even by stretching as it is. Since there is no solvent, it is necessary to remove the solvent to adjust the amount of solvent in the sheet.

In order to solve such a problem, the inventors of the present invention contain an ultra high molecular weight polyolefin,
We proposed a method for producing a microporous polyolefin membrane using a composition having a (weight average molecular weight / number average molecular weight) value within a specific range (Japanese Patent Application No. 1-201785). By this method,
It is possible to produce a polyolefin microporous film from a polyolefin composition that 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 make the average pore size in the range of 0.001 to 0.2 μm for the polyolefin microporous membranes by any of the above-mentioned methods, but it was possible to obtain fine components (for example, Outer diameter
(Not more than 0.02 μm) is not suitable for accurate separation. Therefore, in order to exert a precise filtration function for fine components, a microporous membrane having a pore diameter in the range of 0.001 to 0.02 μm and having a somewhat sharp pore diameter distribution has been desired.

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

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

[0010]

As a result of earnest research in view of the above object, the present inventors have found that a solution of a polyolefin containing an ultrahigh molecular weight component and having a wide molecular weight distribution (weight average molecular weight / large number average molecular weight). Is formed into a sheet, and the gel sheet obtained by rapid cooling is subjected to heat setting at a temperature between the crystal dispersion temperature and the melting point to increase the crystallinity, and then the microporous film obtained by stretching is It was found that the present invention has a fine pore size and a sharp pore size distribution, and has conceived the present invention.

That is, the polyolefin microporous membrane of the present invention comprises 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, (weight average molecular weight / number average molecular weight) of 10 to 300, and has pores. The ratio is 35-95%, the average through-hole diameter is 0.001-0.02μm, and the breaking strength of 15mm width is 0.2 kg.
Above, and pore size distribution (maximum pore size / average through pore size)
The value of is less than 1.5.

The method for producing a polyolefin microporous membrane of the present invention contains 1 to 10% by weight of a component having a molecular weight of 7 × 10 ⁵ or more, and (weight average molecular weight / number average molecular weight) 10 to 300 polyolefin 10 to 50 % By weight, a solution consisting of 50 to 90% by weight of a solvent is prepared, the solution is extruded from a die, and cooled to form a gel composition, and the gel composition is formed from the crystal dispersion temperature of the polyolefin to the melting point. It is characterized in that it is heat-set at the temperature of 10 seconds or more for 10 seconds or more, and then stretched at a temperature of melting point + 10 ° C. or less, and then the residual solvent is removed.

The present invention is described in detail below. The polyolefin microporous membrane of the present invention comprises 1% by weight or more of a 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 to 300.

The above polyolefin has a weight average molecular weight / number average molecular weight of 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 becomes high during dissolution, making it difficult to prepare a high-concentration solution. Again 300
When it exceeds, the breakage of the low molecular weight component occurs during stretching, and the strength of the entire film decreases.

The weight average molecular weight / number average molecular weight is
It is used as a measure of the molecular weight distribution, and the larger the ratio of the molecular weights, the wider the width of the molecular weight distribution.
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 compositions, the larger the difference in the weight average molecular weights of the polyolefins to be blended,
Further, the smaller the value, the smaller the difference in the weight average molecular weight. Further, in the case of a single polyolefin, the molecular weight ratio shows the spread of the distribution, and the larger the value is, the wider the distribution is.

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 ordinary ultrahigh molecular weight polyolefin itself (usually about 6). . As a result, the molecular weight distribution spreads toward the lower molecular weight side,
A highly concentrated polyolefin solution can be prepared.

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

The polyolefin may be either a single polyolefin (not a mixture) or a composition composed of 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, it contains, for example, 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 carrying out multi-stage polymerization so as to obtain ~ 300. As the multi-stage polymerization, it is preferable to produce the high molecular weight portion and the low molecular weight portion by the two-stage polymerization.

Further, 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 of It can be obtained by mixing an appropriate amount 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 . Weight average molecular weight is 7 × 10 ⁵
If it is less than the above, the maximum draw ratio is low and the desired microporous membrane cannot be obtained. On the other hand, the upper limit is not particularly limited, but if it exceeds 15 × 10 ⁶ , moldability is poor in forming a gel-like molded product.

Such ultra-high molecular weight polyolefins include ethylene, propylene, 1-butene, 4-methyl-1-
Examples thereof include crystalline homopolymers obtained by polymerizing pentene, 1-hexene and the like, two-stage polymers, copolymers and blends thereof. Of these, ultra high molecular weight polyethylene, particularly high density ultra high 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 entire polyolefin composition. When the content of the ultrahigh molecular weight polyolefin is less than 1% by weight, the entanglement of the molecular chains of the ultrahigh molecular weight polyolefin, which contributes to the improvement of the stretchability, is hardly formed, and a high-strength microporous membrane 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 the desired high concentration of the polyolefin solution, which is not preferable.

The polyolefin other than the ultra-high 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. Use of a polyolefin having a weight average molecular weight of less than 1 × 10 ⁴ is not preferable because breakage easily occurs during stretching and the desired microporous membrane cannot be obtained. Especially the weight average molecular weight is 1 × 10 ⁵ or more 7 × 10
^It is preferred to blend a polyolefin of less than ⁵ with the ultra high molecular weight polyolefin.

Examples of such polyolefin include ethylene, propylene, 1-butene, 4-methyl-1-pentene,
Examples thereof include crystalline homopolymers obtained by polymerizing 1-hexene and the like, two-stage polymers, copolymers and blends thereof. High density polyethylene, which is a polymer mainly composed of ethylene, is particularly preferable.

If necessary, the above-mentioned polyolefin may be added with an antioxidant, an ultraviolet absorber, a lubricant,
Various additives such as anti-blocking agents, pigments, dyes and inorganic fillers can be added within a range that does not impair the object of the present invention.

Next, a method for producing the polyolefin microporous membrane of the present invention using the above-mentioned 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,
Aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane, and paraffin oil, or mineral oil fractions having boiling points corresponding to these are used to obtain a gel-like molded product having a stable solvent content. A non-volatile solvent such as paraffin oil is preferred.

The heating 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 solvent used, but in the case of polyethylene, for example, 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
40% by weight. When the concentration is less than 10% by weight, not only is the amount of solvent used large and it is not economical, but also when forming into a sheet, swell and neck-in are large at the die outlet, 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, in heating and melting, it is preferable to add an antioxidant in order to prevent the oxidation of the polyolefin.

Next, this heated solution of polyolefin is extruded from a die to be molded. As the die, a sheet die having a rectangular mouthpiece shape is usually used, but a double cylindrical inflation die or the like can also be used. When using sheet dies, the die gap is usually 0.1-
It is 5 mm and is heated to 140 to 250 ° C during extrusion. At this time, the extrusion speed is usually 20 to 30 cm / min to 2 to
It is 3 m / min.

The solution thus extruded from the die is cooled to be formed into a gel. Cooling is preferably performed at a rate of 50 ° C./min or more up to at least the gelation temperature. When the cooling rate is slow, the degree of crystallinity increases and it is difficult to form a gel-like material suitable for stretching. As a cooling method, a method of directly contacting with cold air, cooling water, or other cooling medium, a method of contacting with a roll cooled with a refrigerant, or the like can be used. The solution extruded from the die should be 1-10, preferably 1-5 before or during cooling.
You may collect at the collection ratio of. When the take-up ratio is 10 or more, neck-in becomes large, and breakage easily occurs during stretching, which is not preferable.

Next, heat setting is performed on this gel-like material. The temperature of heat setting is a crystal dispersion temperature to a melting point.
Specifically, in the case of a polyethylene composition, 90 to 140 ° C,
It is preferably in the range of 100 to 130 ° C. When the temperature of heat setting is lower than the crystal dispersion temperature, it is difficult to increase the crystallinity of the gelled material, while when it exceeds the melting point, the polyolefin is melted and molding becomes difficult. The heat setting time is preferably 10 seconds or more, more preferably 1 minute or more. If the heat setting time is less than 10 seconds, it becomes difficult to increase the crystallinity of the gelled material. The upper limit is not particularly limited as long as oxidative deterioration does not occur. As a method of heat setting, a method of blowing hot air, a method of contacting with a heating roll, a method of immersing in a heating medium, or the like can be used.

After the heat setting is performed in this way, stretching is performed. The stretching is carried out by heating the gel-like molded product and using a usual tenter method, roll method, inflation method, rolling method or a combination of these methods at a predetermined magnification. Biaxial stretching is preferred, and either longitudinal / transverse simultaneous stretching or sequential stretching may be used, but simultaneous biaxial stretching is particularly preferred.

The stretching temperature is the melting point of polyolefin + 10 ° C.
Below, it is preferably within the range from the crystal dispersion temperature to below the crystal melting point. For example, in the case of polyethylene at 90-140 ℃,
More preferably, it is in the range of 100 to 130 ° C. If the stretching temperature is higher than the melting point + 10 ° C, the resin cannot be oriented due to the melting of the resin. On the other hand, if the stretching temperature is lower than the crystal dispersion temperature, the softening of the resin is insufficient, the film is easily broken during stretching, and high-stretching cannot be performed.

The stretching ratio varies depending on the thickness of the raw fabric, but is at least 2 times or more in the uniaxial direction, preferably 3 to.
The surface magnification is 30 times, 10 times or more, preferably 15 to 400 times. If the surface magnification is less than 10 times, the stretching is insufficient and a highly elastic and high-strength microporous membrane cannot be obtained. On the other hand, if the areal magnification exceeds 400 times, there are restrictions on the stretching apparatus and the stretching operation.

The stretched product thus obtained is washed with a solvent to remove the residual solvent. Examples of cleaning solvents include hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorohydrocarbons such as ethane trifluoride, and ethers such as diethyl ether and dioxane. A volatile one 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 cleaning method can be carried out by a method of immersing in a solvent for extraction, a method of showering the solvent, or a combination thereof.

The above-mentioned washing is carried out until the residual solvent in the stretch-molded product is less than 1% by weight. After that, the washing solvent is dried. The washing solvent can be dried by the same method as the heat setting method described above. It is desirable that the dried stretched molded product be heat-set in the temperature range of the crystal dispersion temperature to the melting point.

The obtained microporous polyolefin membrane can be further hydrophilized by plasma irradiation, surfactant impregnation, surface grafting, etc., if necessary.

The polyolefin microporous membrane produced as described above has a porosity of 35 to 95% and an average through pore diameter of 0.005%.
It has a breaking strength of 1 to 0.02 μm and a width of 15 mm of 0.2 kg or more. Furthermore, the value of the pore size distribution (maximum pore size / average through-pore size) is less than 1.5, which is surprisingly sharp compared to the pore size distribution (about 2.0) of the conventional polyolefin microporous membrane. In the pore size distribution, the maximum pore size and the average through-pore size are values calculated by using the theory of Flory based on the value of the transmittance of the pullulan solution. The thickness of the polyolefin microporous film of the present invention may be appropriately selected depending on the application, but is generally about 0.1 to 50 μm, preferably 2 to 40 μm.

[0041]

In the present invention, an ultra high molecular weight component is contained,
After forming a solution of polyolefin with a wide molecular weight distribution (weight average molecular weight / large number average molecular weight) into a sheet and quenching it, the gel-like sheet is heat-set at a temperature between the crystal dispersion temperature and the melting point. Since the polyolefin microporous membrane is produced by stretching, the obtained microporous membrane has a fine pore size and a sharp pore size distribution.

Although the reason why such an effect is obtained is not necessarily clear, the gel-like sheet usually has amorphous portions between the lamellae, which tends to cause nonuniformity in the microstructure of the membrane, which is the reason for the pore size distribution. However, in the present invention, a polyolefin containing a predetermined amount or more of an ultra-high molecular weight component and having a molecular weight distribution in a predetermined range is used, and the gel-like polyolefin has a crystal dispersion temperature of By heat setting at a temperature between the melting points,
The crystallinity in the polyolefin is increased, and the amorphous portion between the lamellas is reduced accordingly. It is considered that when this polyolefin is stretched, the through holes are formed mainly by the cleavage between the lamellas, so that it becomes possible to obtain a microporous film having fine holes with a narrow distribution width.

[0043]

EXAMPLES Examples of the present invention will be shown below. The test method in the examples is as follows. (1) Molecular weight and molecular weight distribution: GP manufactured by Waters Co., Ltd.
Using C instrument, GOH-6 manufactured by Tosoh Corporation as a column and as a solvent
Using O-dichlorobenzene, temperature 135 ℃, flow rate 1.0 ml
/ Min, measured by gel permeation chromatography (GPC) method. (2) Film thickness: The cross section was measured with a scanning electron microscope. (3) Air permeability: Conforms to JIS P8117. (4) Average pore size: When a flat membrane module was used to circulate an aqueous solution of 0.05 wt% pullulan (manufactured by Showa Denko KK) under a differential pressure of 380 mmHg, the concentration of pullulan contained in the filtrate was determined. It was determined from the measurement of the differential refractive index. Then, the pore diameter was converted from the value of the molecular weight of pullulan at which the rejection rate calculated by the following equation was 50%, using the Flory's theory described later. Retention rate of pullulan = {1- (pullulan concentration in filtrate / pullulan concentration in undiluted solution)} x 100 The chain-like polymer in solution has a spherical thread shape and its diameter d is at 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 root-mean-square distance <γ ² > of. According to Flory's theory of viscosity and molecular chain spread in polymer solution, [η] M = 2.1 × 10 ²¹ <γ ² > ^3/2 (2) holds regardless of the type of polymer. From equations (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 is 50%. This d was defined as the average pore diameter of the polyethylene microporous membrane. (5) Pore size distribution: In the measurement according to (4) above, the rejection rate is
Similarly, the pore diameter was converted from the value of the molecular weight of pullulan to be 90% to obtain the maximum pore diameter, and the value of the maximum pore diameter was used to calculate the value of maximum pore diameter / average pore diameter. (6) Porosity: Measured with a mercury porosimeter. (7) Breaking strength: ASTM D88 using a strip test piece with a width of 15 mm
Measured according to 2.

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 3.7 × 10
Mw / Mn = 11 mixed with 13 parts by weight of polyethylene of ⁵
The raw material resin was mixed with 85 parts by weight of liquid paraffin (64 cst / 40 ° C.) to prepare a polyethylene composition solution. Next, to 100 parts by weight of the solution of this polyethylene composition,
0.125 parts by weight of -t-butyl-p-cresol ("BHT", Sumitomo Chemical Co., Ltd.) and tetrakis [methylene-3- (3,5-
0.25 part by weight of di-t-butyl-4-hydroxylphenyl) -propionate] methane (“Irganox 1010”, manufactured by Ciba Geigy) was added as an antioxidant and mixed.
Fill this mixture into an autoclave with a stirrer and
Stir at 90 ° 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 by taking it out with a cooling roll. Then, this gel-like sheet is moved at a speed of 0.5 m /
Heat setting was performed by exposing to 120 ° C. hot air for 15 minutes at a feed rate of minutes.

The obtained sheet was set in a biaxial stretching machine and simultaneously biaxially stretched 5 × 5 times at a temperature of 115 ° C. and a stretching speed of 0.5 m / min. The obtained stretched membrane was washed with methylene chloride to remove the residual liquid paraffin by extraction, and then dried to obtain a polyethylene microporous membrane having a thickness of 25 μm. The production conditions of this polyethylene microporous membrane are shown in Table 1. Further, the air permeability, film thickness, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. Second result
Shown in the table.

Example 2 In Example 1, the raw material resin was two-stage polyethylene (weight average molecular weight 8.2 × 10 ⁵ , weight average molecular weight / number average molecular weight = 28.8, ratio of components having a molecular weight of 7 × 10 ⁵ or more 40 weight %) Was used to produce a polyethylene microporous membrane in the same manner. The production conditions of the obtained polyethylene microporous membrane are shown in Table 1. Also, the air permeability of the polyethylene microporous membrane,
The film thickness, porosity, breaking strength, average pore size and pore size distribution were measured. The results are shown in Table 2.

Comparative Example 1 A polyethylene microporous membrane was produced under the same conditions as in Example 1 except that the gel sheet was not heat set. The production conditions of the obtained polyethylene microporous membrane are shown in Table 1. Further, the air permeability, film thickness, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. Second result
Shown in the table.

Table 1 Example No. Heat setting temperature Heat setting time Stretching ratio Stretching temperature Stretching temperature Example 1 120 ° C. 15 minutes 5 times × 5 times 115 ° C. Example 2 120 ° C. 15 minutes 5 times × 5 times 115 ° C. Comparative Example 1 --5 times x 5 times 115 ° C

Table 2 Air Permeability Film Thickness Average Pore Size Pore Size Distribution Porosity Breaking Strength Example No. (sec / 100cc ) (μm) (μm) (%) (Kg / 15mm width ) Example 1 450 25 0.01 1.2 60 2.2 Example 2 400 25 0.01 1.2 60 2.4 Comparative Example 1 450 25 0.01 2.4 55 2.2

As is clear from Table 2, the polyethylene microporous membranes of Examples 1 and 2 have air permeability, film thickness, and average pore diameter that are larger than those of Comparative Example 1 produced by the conventional method. , But the width of the pore size distribution was narrow.

[0052]

As described in detail above, according to the present invention,
A gelled sheet obtained by molding a solution of polyolefin containing an ultra-high molecular weight component and having a wide molecular weight distribution (weight average molecular weight / large number average molecular weight) into a sheet, and then rapidly cooling the solution between the crystal dispersion temperature and the melting point. Since the polyolefin microporous membrane is manufactured by heat-setting at a temperature and then stretching, the obtained microporous membrane has a fine pore size and a sharp pore size distribution.

The polyolefin microporous membrane produced by the method of the present invention can be used in various kinds of separators for batteries, diaphragms for electrolytic capacitors, ultraprecision filtration membranes, ultrafiltration membranes, various filters, porous membranes for moisture-permeable waterproof clothing, etc. Suitable for use.

Claims (2)

[Claims] 1. Containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, and (weight average molecular weight / number average molecular weight) is 10 to 30.
It consists of 0 polyolefin and has a porosity of 35 to 95%, an average through hole diameter of 0.001 to 0.02 μm, and a breaking strength of 15 mm width.
A microporous polyolefin membrane having a weight distribution of 0.2 kg or more and a pore size distribution (maximum pore size / average through pore size) of 1.5 or less. 2. Containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, and (weight average molecular weight / number average molecular weight) is 10 to 30.
0-50% by weight polyolefin and 50-90% by weight solvent
To prepare a solution consisting of, and extruding the solution from a die,
The gel composition is cooled to form a gel composition, the gel composition is heat-set at a temperature from the crystal dispersion temperature to the melting point of the polyolefin, and then stretched at a temperature not higher than the melting point + 10 ° C., and then the residual solvent is removed. A method for producing a microporous polyolefin membrane, comprising: