JPH06240036A - Microporous polyolefin film and its production - Google Patents

Abstract

PURPOSE:To obtain a microporous film having suitable pore sizes and a sharp pore size distribution by extruding and cooling a specific polyolefin soln. to give a get-like compsn. and stretching, drying, and again stretching the compsn. CONSTITUTION:A soln. is obtd. by dissolving, in 50-90wt.% solvent, 10-50wt.% poly olefin which contains at least 1wt.% component with a mol.wt. of 7X10<5> or higher and has a ratio of wt. average mol.wt. to number average mol.wt. of 10-300. The soln. is heated to 140-250 deg.C, extruded through a die, and cooled to give a gel-like compsn. The compsn. is stretched 2-10 times at least monoaxially at a temp. between the crystal dispersion temp. and the temp. 10 deg.C higher than the m.p. of the polyolefin, heated to remove the residual solvent, and stretched again 1.1-5 times at least monoaxially at a temp. at least 10 deg.C lower than the m.p., giving a microporous polyolefin film having a void content of 35-95%, a mean size of interconnecting pore of 0.05-0.2mum, a breaking strength of 0.2kg/15mm-width or higher, and a pore size distribution (a ratio of max. pore size to mean interconnecting pore size) of 1.5 or lower.

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 pore size of an appropriate size and a sharp pore size distribution. And a manufacturing method thereof.

[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 5 × 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 diameter of the polyolefin microporous membrane was examined, it was possible to obtain an average through-pore diameter in the range of 0.001 to 0.2 μm in the polyolefin microporous membrane by any of the above methods, but especially the outer diameter of 0.1 to 0.2 μm. 0.5 μm
It is not always possible to exert sufficient filtration efficiency when separating components of a moderate size. Therefore, the outer diameter is 0.1 ~
In order to efficiently and promptly separate components having a size of about 0.5 μm, a microporous membrane having a pore size within the range of 0.05 to 0.2 μm and having a somewhat sharp pore size distribution has been desired. .

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

Another object of the present invention is to provide a method for producing a polyolefin microporous membrane having a pore size of an appropriate size and having 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-like sheet obtained by rapid cooling is subjected to primary and secondary stretching in at least a uniaxial direction at a specific temperature. The microporous membrane has a pore size of an appropriate size. The inventors have found that the pore size distribution is sharp, and have arrived at 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 0.05-0.2 μm, the breaking strength of 15 mm width is 0.2 kg or more, and the value of pore size distribution (maximum pore size / average through-hole diameter) is 1.5 or less. Characterize.

The first method for producing the polyolefin microporous membrane of the present invention is one in which a component having a molecular weight of 7 × 10 ⁵ or more is used.
Contains more than weight% (weight average molecular weight / number average molecular weight)
10 to 50% by weight of polyolefin with 10 to 300 and 50 to 50% of solvent
90 wt% to prepare a solution, extruding the solution from a die, to form an unstretched gel composition by cooling, the gel composition, the crystal dispersion temperature of the polyolefin ~ melting point + 10 ℃ It is characterized in that it is stretched at least uniaxially at a temperature, then the residual solvent is removed, and then the obtained stretched product is stretched again at least uniaxially at a temperature not higher than the melting point of the above-mentioned polyolefin -10 ° C. .

Further, the second method for producing the polyolefin microporous membrane of the present invention is to add 1 component having a molecular weight of 7 × 10 ⁵ or more.
Contains more than weight% (weight average molecular weight / number average molecular weight)
10 to 50% by weight of polyolefin with 10 to 300 and 50 to 50% of solvent
A solution consisting of 90% by weight is prepared, the solution is extruded from a die, and an unstretched gel composition is formed by cooling, and the gel composition is at least at a temperature not higher than the crystal dispersion temperature of the polyolefin. It is characterized in that it is stretched 1.2 to 10 times in the uniaxial direction, and subsequently stretched 1.2 to 10 times in the uniaxial direction at a temperature of the crystal dispersion temperature to the melting point + 10 ° C., and then the residual solvent is removed. To do.

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 weight average molecular weight / number average molecular weight of the above 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 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 two-step 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 above 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.

Such polyolefins 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, the first 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 form 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, the gel is stretched (hereinafter referred to as primary stretching). 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. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either longitudinal / transverse simultaneous stretching or sequential stretching may be used.
Axial stretching is preferred.

The stretching temperature is from the crystal dispersion temperature of polyolefin to the crystal melting point + 10 ° C. or less, preferably from the crystal dispersion temperature to less than the crystal melting point. For example, in the case of a polyethylene composition, the temperature is 90 to 140 ° C, more preferably 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 molded product thus stretched is subsequently 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 heating, air drying or the like. The dried stretched molded product is
It is desirable to heat-set in the temperature range of the crystal dispersion temperature to the melting point.

In the first method of the present invention, the stretched molded product obtained is heated to a melting point of -10 ° C or lower and stretched again (hereinafter referred to as secondary stretching). The stretching can be performed by the same method as the above-mentioned primary stretching. The stretching method may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either longitudinal simultaneous stretching or sequential stretching may be performed.

The stretching temperature is the melting point of polyolefin-10 ° C.
The temperature is preferably below 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 fabric, but is 1.1 to 5 times in the uniaxial direction, preferably 1.2 to 2.5.
The surface magnification is 1.2 to 5 times, preferably 1.5 to 3 times. If the areal magnification exceeds 5 times, the thickness of the film after stretching becomes uneven, which is not preferable.

The obtained polyolefin microporous film is preferably heat-set in the temperature range of crystal dispersion temperature to melting point. Furthermore, if necessary, hydrophilic treatment can be performed by plasma irradiation, surfactant impregnation, surface grafting, or the like.

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

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

The preparation of the high-concentration solution of the polyolefin as the raw material and the manufacturing process of the gel-like molded product are the same as in the first manufacturing method described above.

Then, the obtained gel-like molded product is stretched (hereinafter referred to as primary stretching). 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. The stretching may be uniaxial stretching or biaxial stretching.
In the case of biaxial stretching, either longitudinal simultaneous stretching or sequential stretching may be performed.

The primary stretching temperature is not higher than the crystal dispersion temperature of the polyolefin and not lower than room temperature. For example, in the case of polyethylene, the temperature is 90 ° C or lower. When the primary stretching temperature exceeds 90 ° C., the composition does not undergo changes such as generation of fine cracks, and the pore size of the porous membrane cannot be increased by the secondary stretching described below.

The stretching ratio in the primary stretching depends on the thickness of the raw fabric, but is at least 1.2-10 in the uniaxial direction.
Times, preferably 1.5 to 5 times. If the stretching ratio is less than 1.2 times, the stretching is insufficient, so that changes that cause fine cracks do not occur, and the pore size of the porous membrane cannot be increased by the secondary stretching described below. 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. If the draw ratio exceeds 15 times in terms of areal magnification, a porous membrane having a sharp pore size distribution and a large pore size cannot be obtained.

The molded product thus stretched is subsequently stretched in the second method of the present invention (hereinafter referred to as secondary stretching). Stretching is performed at a predetermined ratio by the same method as in primary stretching. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either longitudinal / transverse simultaneous stretching or sequential stretching may be used.
Axial stretching is preferred.

The secondary stretching temperature is from the crystal dispersion temperature of polyolefin to the crystal melting point + 10 ° C., preferably from the crystal dispersion temperature to below the crystal melting point. For example, in the case of polyethylene, the temperature is 90 to 140 ° C, and more preferably 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.

Although the draw ratio varies depending on the thickness of the material, it is at least 1.2 to 10 times, preferably 2 times in the uniaxial direction.
~ 5 times. If the draw ratio exceeds 10 times, a porous membrane having a sharp pore size distribution and a large pore size cannot be obtained.
In addition, the total surface magnification of primary stretching and secondary stretching is 10 times or more,
It is preferably 10 to 300 times. If the total 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 total areal magnification exceeds 400 times, there will be restrictions in terms of the stretching device and stretching operation.

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

The polyolefin microporous membrane thus obtained 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 0.05.
The breaking strength in a width of 15 μm is 0.2 kg or more, preferably 0.5 kg or more. Furthermore, pore size distribution (maximum pore size /
The value of the average through-hole diameter) is relatively sharp as compared with the pore size distribution of the polyolefin microporous membrane obtained by the conventional method. The maximum pore size in the pore size distribution is a value calculated by using the theory of Flory based on the value when the rejection rate of the pullulan solution is 90%. 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.

[0055]

In 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 (weight average molecular weight / number average molecular weight is large) 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.
Since a microporous membrane is produced by stretching at least uniaxially, removing and drying the residual solvent, and then stretching again at least uniaxially at a temperature not higher than the melting point of the polyolefin of −10 ° C. The obtained microporous membrane has a pore size of an appropriate size and has a sharp pore size distribution.

Although the reason why such an effect is obtained is not always clear, when the stretching temperature is usually increased to increase the pore size, the film strength is lowered and it becomes unpractical. When the temperature is lowered to around the dispersion temperature, the porosity decreases. On the other hand, increasing the draw ratio is conceivable, but it causes problems such as deformation due to drawing occurring in the thickness direction of the film and further progress of fibrillation. Therefore, in the present invention, the stretching is first carried out at a temperature of the crystal dispersion temperature to the melting point + 10 ° C to form micropores, the solvent is removed, and further stretching is performed at a temperature of the melting point -10 ° C or less to enlarge the pore diameter. It is considered that this is because the homogenization is also attempted.

Further, 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 (weight average molecular weight / number average molecular weight is large) is formed into a sheet. The gel-like sheet obtained by molding and quenching is stretched 1.2 to 10 times at least uniaxially at a temperature below the crystal dispersion temperature, and then at a temperature from the crystal dispersion temperature to the melting point + 10 ° C at least uniaxially. 1.
Since the microporous membrane is produced by stretching it 2 to 10 times and then removing and drying the residual solvent, the obtained microporous membrane has a pore size of an appropriate size and a pore size distribution of It's sharp.

Although the reason why such an effect is obtained is not always clear, micropores are usually formed between fibrils formed from a polyolefin. By stretching at a specific ratio at a temperature, fine microcracks are generated while suppressing fibrillation, and subsequently stretching is performed at a temperature of crystal dispersion temperature to melting point + 10 ° C. or less to generate these microcracks. It is considered that this is because the pore diameter is expanded and the pores are made uniform by enlarging.

[0059]

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) Water permeability: A microporous membrane was installed in a flat membrane module, water was passed through with a mixture of distilled water / ethanol (volume ratio 50/50) to make it hydrophilic, and after thoroughly washing with distilled water, 380 mmHg
It was determined by measuring the amount of permeation of the filtrate when the water pressure was applied. (5) Average pore size: Included in the filtrate when 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 produced was determined by measuring 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 polymer in the solution state is a spherical thread, 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 >. 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. (6) Pore size distribution: In the measurement according to (5) 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. (7) Porosity: Measured with a mercury porosimeter. (8) Breaking strength: ASTM D for a strip test piece with a width of 15 mm
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 3.7 × 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 with an extruder having a diameter of 45 mm, and a gel-like sheet was formed while being taken up by a cooling roll. Then, the gel-like sheet, a temperature of 115 ℃,
Simultaneous biaxial stretching was carried out at a stretching speed of 0.5 m / min and 5 × 5 times. The stretched film obtained was washed with methylene chloride to remove residual liquid paraffin by extraction, and then dried to obtain a polyethylene microporous film. This polyethylene microporous membrane is heated to a temperature of 90.
At a temperature of 0.5 ° C. and a stretching speed of 0.5 m / min, the film was laterally stretched 1.5 times to obtain a polyethylene microporous membrane having a thickness. The production conditions of this polyethylene microporous membrane are shown in Table 1. Further, the film thickness, air permeability, water permeability, 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 A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the transverse stretching was doubled. The production conditions of this polyethylene microporous membrane are shown in Table 1. Further, the film thickness, air permeability, water permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. Second result
Shown in the table.

Example 3 In Example 1, as a raw material resin, two-stage polymerized polyethylene (weight average molecular weight 7.5 × 10 ⁵ , weight average molecular weight / number average molecular weight = 43.0, ratio of components having a molecular weight of 7 × 10 ⁵ or more 32)
A polyethylene microporous membrane was produced in the same manner except that the (% by weight) was used. The production conditions of this polyethylene microporous membrane are shown in Table 1. Further, the film 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.

Example 4 In Example 1, as a raw material resin, two-stage polymerized 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
A polyethylene microporous membrane was produced in the same manner except that the (% by weight) was used. The production conditions of this polyethylene microporous membrane are shown in Table 1. Further, the film 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 1 A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the transverse stretching was performed 1.5 times at a temperature of 120 ° C. (the melting point of the polyethylene composition was higher than −10 ° C.). The production conditions of this polyethylene microporous membrane are shown in Table 1. Further, the film 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 polyolefin microporous 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 times × 10 times, and the secondary stretching was not performed. The production conditions of this polyethylene microporous membrane are shown in Table 1. Further, the film 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.

Table 1 Primary stretching conditions Secondary stretching conditions Example No. Stretching temperature Stretching ratio Stretching temperature Stretching ratio (transverse direction) Example 1 115 ° C. 5 times × 5 times 90 ° C. 1.5 times Example 2 115 ° C. 5 times × 5 times 90 ° C. 2.0 times Example 3 115 ° C. 5 times × 5 times 90 ° C. 1.5 times Example 4 115 ° C. 5 times × 5 times 90 ° C. 1.5 times Comparative Example 1 115 ° C. 5 times × 5 times 120 ° C. 1.5 times Comparative Example 2 125 ℃ 10 times × 10 times − −

Table 2 Film Thickness Air Permeability Water Permeability Average Pore Size Pore Size Distribution Example 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 is liter / m ² · hr · atm

[0069]

As is clear from Table 2, the polyethylene microporous membranes of Examples 1 to 4 according to the first method of the present invention are:
Compared with the polyethylene microporous membrane of Comparative Example 1, the membrane thickness was stable, the filtration efficiency was good, and the pore size distribution was sharp. In addition, the pore size distribution is significantly sharper than that of Comparative Example 2 using 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 3.7 × 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. The obtained sheet was cut into small pieces of 9 cm × 9 cm, set in 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 stretched film obtained was washed with methylene chloride to remove residual liquid paraffin by extraction, and then dried to obtain a polyethylene microporous film. The production conditions of this polyethylene microporous membrane are shown in Table 3. Further, the film thickness, air permeability, porosity, breaking strength, average pore diameter and pore diameter 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 2.5 × 2.5. The production conditions of this polyethylene microporous membrane are shown in Table 3. Further, the film thickness, air permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured.
The results are shown in Table 4.

Example 7 A polyethylene microporous membrane was produced in the same manner as in Example 1 except that the secondary draw ratio in Example 5 was set to 4 × 4. The production conditions of this polyethylene microporous membrane are shown in Table 3. Further, the film thickness, air permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured.
The results are shown in Table 4.

Example 8 In Example 5, as the raw material resin, two-stage polymerized polyethylene (weight average molecular weight 7.5 × 10 ⁵ , weight average molecular weight / number average molecular weight = 43.0, ratio of components having a molecular weight of 7 × 10 ⁵ or more 32)
A polyethylene microporous membrane was produced in the same manner except that the (% by weight) was used. The production conditions of this polyethylene microporous membrane are shown in Table 3. Further, the film thickness, air permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. The results are shown in Table 4.

Example 9 In Example 5, as the raw material resin, polyethylene of two-stage polymerization (weight average molecular weight 8.2 × 10 ⁵ , weight average molecular weight /
Number average molecular weight = 28.8, ratio of components with a molecular weight of 7 x 10 ⁵ or more
Polyethylene microporous membrane was produced in the same manner except that 40% by weight) was used. The production conditions of this polyethylene microporous membrane are shown in Table 3. Also, the thickness of the polyethylene microporous membrane,
The 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 polyethylene microporous membrane was produced in the same manner as in Example 5, except that the primary draw ratio was 4 × 4. The production conditions of this polyethylene microporous membrane are shown in Table 3. The polyethylene microporous film 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 polyethylene microporous membrane was produced in the same manner as in Example 5 except that the primary stretching temperature was 118 ° C. (temperature above the crystal dispersion temperature of polyethylene). The production conditions of this polyethylene microporous membrane are shown in Table 3. Further, the film thickness, air permeability, porosity, breaking strength, average pore diameter and pore diameter distribution of the polyethylene microporous membrane were measured. The results are shown in Table 4.

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

Table 3 Primary stretching conditions Secondary stretching conditions Example No. Stretching temperature Stretching ratio Stretching temperature Stretching ratio Example 5 80 ° C. 3 times × 3 times 118 ° C. 2 times × 2 times Example 6 80 ° C. 3 times × 3 Double 118 ° C 2.5 ×× 2.5 × Example 7 80 ° C 3 ×× 3 118 ° C 4 ×× 4 × Example 8 80 ° C 3 ×× 3 118 ° C 2 ×× 2 Example 9 80 ° C 3 × 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. 10 times x 10 times

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 clear from Table 4, the polyethylene microporous membranes of Examples 5 to 9 produced by the method of the present invention have a sharper pore size distribution than the polyethylene microporous membranes of Comparative Examples 3 to 5.

[0083]

As described in detail above, according to the first method of the present invention, 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 prepared. The gel-like sheet obtained by forming into a sheet and quenching is stretched in at least one axial direction at a temperature of crystal dispersion temperature to melting point + 10 ° C to remove residual solvent and dry, and then to a melting point of polyolefin-10 A microporous membrane is produced by re-stretching in at least a uniaxial direction at a temperature of ℃ or less.

According to the second method of the present invention, the gel-like sheet is stretched 1.2 to 10 times at least uniaxially at a temperature equal to or lower than the crystal dispersion temperature, and then the crystal dispersion temperature to the melting point +10.
A microporous membrane is produced by stretching at least uniaxially 1.2 to 10 times at a temperature of ° C, and then removing and drying the residual solvent.

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

The polyolefin microporous membrane produced by the method of the present invention can be used in various types 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 (3)

[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 is made of 0 polyolefin and has a porosity of 35 to 95%, an average through hole diameter of 0.05 to 0.2 μm, and a breaking strength of 15 mm width of 0.
A microporous polyolefin membrane having a weight distribution of 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,
An unstretched gel-like composition is formed by cooling, and the gel-like composition is dispersed at the crystal dispersion temperature of the polyolefin.
Stretching at least uniaxially at a temperature of melting point + 10 ° C, then removing the residual solvent, and then stretching the obtained stretched product again at least uniaxially at a temperature not higher than the melting point of the polyolefin-10 ° C. A method for producing a polyolefin microporous membrane, comprising: 3. 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,
An unstretched gel composition is formed by cooling, and the gel composition is stretched 1.2 to 10 times in at least one axial direction at a temperature not higher than the crystal dispersion temperature of the polyolefin,
Then, the method for producing a microporous polyolefin membrane is characterized in that it is stretched 1.2 to 10 times at least uniaxially at a temperature of crystal dispersion temperature to melting point + 10 ° C., and then residual solvent is removed.