JPH1192587A - Preparation of polyolefin microporous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a polyolefin microporous film having good characteristics from a polyolefin solution of a high concentration with low stress. SOLUTION: A solution comprising from 10 to 50 wt.% of (1) an ultra-high- molecular-weight polyolefin having a weight average molecular weight of 7×10<5> or larger and the ratio of the weight average molecular weight to the number average molecular weight of from 10 to 300 or (2) a polyolefin composition which contains 1 wt.% or more polyolefin component having a weight average molecular weight of 7×10<5> or larger and has the ratio of the weight average molecular weight to the number average molecular weight of from 10 to 300 and from 50 to 90 wt.% nonvolatile solvent, is prepared and the above-mentioned solution is extruded from a die. Then, the solution is cooled to form a gel- formed sheet and the above-mentioned gel-formed sheet is biaxially oriented at a temperature 10 deg.C higher than the melting point of the above-mentioned ultra-high-molecular-weight polyolefin (1) or the above-mentioned polyolefin composition (2) or lower. Then, the residual nonvolatile solvent is removed through extraction by an easily-volatilizable solvent. Here, the ratio (A/B) of the tensile break strength of the biaxially oriented sheet before (A) and after (B) extraction is 0.75 or smaller.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a microporous membrane of an ultrahigh molecular weight polyolefin or a polyolefin composition containing the same.

[0002]

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

Conventional methods for producing a microporous polyolefin membrane include, for example, a mixed extraction method in which a pore-forming agent composed of a fine powder of a different polymer or the like is mixed and dispersed in a polyolefin, and then the pore-forming agent is extracted. Phase separation method to make a porous structure by micro phase separation of solvent with solvent, and by applying strain such as stretching to polyolefin molded body in which heterogeneous solids are micro-dispersed, interfacial breakdown between heterogeneous solids and voids are generated For example, a drawing method for making the film porous is performed. However, in these methods, since a polyolefin having a molecular weight of less than about 500,000 is usually used, there is a limit to a thinning and high strength by stretching.

Recently, ultrahigh molecular weight polyolefins that can be formed into high strength and high elasticity 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 liquid paraffin, a gel such as a fiber or a film is formed from the solution, and the gel containing the liquid paraffin is extracted with a volatile solvent. A method of heating and stretching is disclosed. However, a gel having a porous structure highly swollen with liquid paraffin cannot be stretched in a high orientation even if it is stretched in two directions. There is a drawback that the hole is small and the diameter of the formed hole is large. On the other hand, in the gel dried after extracting the liquid paraffin 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 shrinkage densification increases the density. There is a drawback that stretching at a magnification cannot be performed.

On the other hand, a gel-like sheet is formed from a solution obtained by heating and dissolving an ultra-high molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more in a solvent, and the amount of the solvent in the gel-like sheet is adjusted by desolvation treatment. Then, after heating and stretching,
Various methods have been proposed for producing a microporous ultra-high molecular weight polyolefin membrane by removing the residual solvent.

For example, Japanese Patent Application Laid-Open No. 60-242035 discloses that a gel-like sheet is formed from a solution obtained by heating and dissolving ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 5 × 10 ⁵ or more in a solvent. Desolvation treatment to 10-80% by weight, then heating and stretching and removing the residual solvent to reduce the thickness
10 μm or less, breaking strength 200 kg / cm ² or more, porosity 30
% Of ultra high molecular weight polyethylene is disclosed.

JP-A-61-195132 discloses that a gel-like sheet is formed from a solution of a polymer of an α-olefin having a weight average molecular weight of 5 × 10 ⁵ or more, and at least 10% by weight of a solvent is removed therefrom. After the proportion of the α-olefin polymer contained in the shaped sheet is 10 to 90% by weight, the film is stretched at a temperature equal to or lower than the melting point of the α-olefin polymer + 10 ° C., and the residual solvent contained in the obtained stretch molded product is removed. A method for producing a microporous membrane characterized by being removed is disclosed.

Japanese Patent Application Laid-Open No. 61-195133 discloses an α-olefin polymer having a weight average molecular weight of 5 × 10 ⁵ or more, an average pore diameter of 0.001 to 1 μm, a porosity of 30 to 90%, and a uniaxial structure. Disclosed is a microporous membrane characterized by being stretched by 2 times or more in the direction and 20 times or more in area ratio.

On the other hand, JP-A-63-39602 discloses that a gel-like sheet is formed from a polyethylene solution having a weight average molecular weight of 5 × 10 ⁵ or more, and the amount of the solvent in the gel-like sheet is more than 80% by weight and 95% by weight or less. And then stretched at least 120 times in uniaxial direction at a temperature of 120 ° C or less and at least 20 times in area ratio, and by removing residual solvent, 100 L / m ² · hr · atm or more of pure water A method is disclosed for producing a microporous polyethylene membrane having a permeation rate and a rejection of 50% or more for γ-globulin. This polyethylene microporous membrane has excellent water permeability and a fine pore diameter suitable for separating a protein solution or the like.

Japanese Unexamined Patent Publication (Kokai) No. 63-273651 discloses that a solution of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more is prepared, and the solution is rapidly cooled to a gelling temperature or less and extruded from a die to form a gel. A method comprising setting the content of the ultrahigh molecular weight polyolefin in the sheet to 10 to 90% by weight, stretching the film at a temperature not higher than the melting point of the ultrahigh molecular weight polyolefin + 10 ° C, and then removing the residual solvent. I have. According to this method, the microporous film can be manufactured to have a thickness of more than 10 μm, so that a microporous film suitable for applications requiring high strength and pressure resistance can be obtained.

[0011]

However, in any of the above documents, the ratio between the tensile strength at break of the gel sheet after stretching and before the extraction treatment and the tensile strength at break of the microporous membrane as the final product is within a predetermined range. There is no description that the selection of a solvent that falls within the range improves the mechanical strength of the microporous membrane. Furthermore, in the method for producing a microporous membrane described in the above document, it is necessary to prepare a dilute solution of polyolefin in order to biaxially stretch a gel-like sheet composed of only ultra-high-molecular-weight polyolefin. When forming a sheet, the swell and neck-in are large at the die exit, and the sheet contains excessive solvent, so even if it is stretched as it is, the desired microporous membrane cannot be obtained. Thus, there is a problem in productivity, for example, it is necessary to adjust the amount of solvent in the sheet.

Accordingly, an object of the present invention is to provide a method for producing a microporous polyolefin membrane having excellent properties at low stress from a high concentration solution of polyolefin.

[0013]

Means for Solving the Problems As a result of intensive studies in view of the above objects, the present inventors have found that the weight average molecular weight / number average molecular weight can be obtained by blending an ultra high molecular weight polyolefin alone or a polyolefin having a relatively low molecular weight. Using a polyolefin composition adjusted to a predetermined range, if a high-concentration solution is prepared with a non-volatile solvent, after biaxially stretching a gel-like sheet made of this solution, by removing the solvent with an easily volatile solvent, The present inventors have discovered that a polyolefin microporous membrane having excellent properties at low stress and good mechanical strength can be produced, and reached the present invention.

That is, in the first method for producing a microporous polyolefin membrane of the present invention, an ultrahigh molecular weight polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more and a weight average molecular weight / number average molecular weight of 10 to 300 is 10 to 50 weight%. % And the non-volatile solvent 50 ~
A solution consisting of 90% by weight is prepared, the solution is extruded from a die, cooled to form a gel-like sheet, and the gel-like sheet is biaxially stretched at a temperature not higher than the melting point of the ultrahigh molecular weight polyolefin + 10 ° C. A method for producing a microporous polyolefin membrane by extracting and removing the remaining non-volatile solvent with an easily volatile solvent, wherein the tensile breaking strength (A) of the biaxially stretched sheet before extraction and the tensile breaking strength after extraction The ratio (A / B) to the strength (B) is 0.75 or less. Further, the second production method is a polyolefin composition containing, in place of the polyolefin, a polyolefin component having a weight average molecular weight of 7 × 10 ⁵ or more at 1% by weight or more and a weight average molecular weight / number average molecular weight of 10 to 300. Is used.

[0015]

DETAILED DESCRIPTION OF THE INVENTION The method of the present invention will be described in detail below. [1] Raw material The polyolefin microporous membrane has (a) a weight average molecular weight of 7 ×
10 ⁵ or more and weight average molecular weight / number average molecular weight is 10 to 300
(1) a polyolefin component having a weight-average molecular weight of 7 × 10 ⁵ or more containing 1% by weight or more, and a weight-average molecular weight / number-average molecular weight of 10 to 300; It consists of a polyolefin composition.

(1) In the case of using only the ultrahigh molecular weight polyolefin The ultrahigh molecular weight polyolefin used in the present invention is obtained by multistage polymerization, and has a weight average molecular weight of 7 × 10 ⁵ or more, and a weight average molecular weight / number average molecular weight. Are from 10 to 300.

Such ultra-high molecular weight polyolefins include ethylene, propylene, 1-butene, 4-methyl-1-
A crystalline homopolymer or copolymer obtained by polymerizing pentene, 1-hexene or the like may be used. Of these, ultrahigh molecular weight polyethylene, particularly high density ultrahigh molecular weight polyethylene, is preferred.

(2) In the case of a polyolefin composition (a) Molecular weight distribution (weight average molecular weight / number average molecular weight) 1% by weight or more of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more and 99% by weight of another polyolefin % Or less, the weight-average molecular weight / number-average molecular weight of 10 to 300, preferably 12 to 250 (Mw /
Mn). If the weight-average molecular weight / number-average molecular weight (Mw / Mn) of the polyolefin composition is less than 10, the average molecular chain length is large and the entanglement density of the molecular chains upon dissolution becomes 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 (Mw
/ Mn) is a parameter 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 a composition composed of polyolefins having different weight average molecular weights, the larger the ratio of the molecular weights of the composition, the larger the difference in the weight average molecular weight of the polyolefin to be blended, and the smaller the ratio, the smaller the difference in the weight average molecular weight.

Weight average molecular weight of polyolefin composition /
The number average molecular weight (Mw / Mn) is set to 10 to 300, which is larger than the weight average molecular weight / number average molecular weight (Mw / Mn) (usually about 6) of the ultrahigh molecular weight polyolefin itself. As a result, the molecular weight distribution spreads to the lower molecular weight side, so that a high-concentration polyolefin solution can be prepared.

(B) Content of ultrahigh molecular weight polyolefin The content of the ultrahigh molecular weight polyolefin 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 the desired high concentration of the polyolefin solution, which is not preferable. The preferred blending ratio of the ultrahigh molecular weight polyolefin is 10 to 50% by weight.

(C) Other polyolefin As the polyolefin other than the ultrahigh molecular weight polyolefin, those having a weight average molecular weight of less than 7 × 10 ⁵ are used.
In this specification, other polyolefins will be referred to as “relatively low molecular weight polyolefins”, but the lower limit of the molecular weight of relatively low molecular weight polyolefins is 1 × 10 ⁴
The above is preferred. When a polyolefin having a weight-average molecular weight of less than 1 × 10 ⁴ is used, breakage easily occurs during stretching,
It is not preferable because a good microporous film cannot be obtained. In particular, it is preferable to blend a polyolefin having a weight average molecular weight of 1 × 10 ⁵ or more and less than 7 × 10 ⁵ with the ultrahigh molecular weight polyolefin. The relatively low molecular weight polyolefin is preferably the same as the ultrahigh molecular weight polyolefin, and a combination of ultrahigh molecular weight polyethylene and high density polyethylene is particularly preferable.

(3) Other components Various additives such as an antioxidant, an ultraviolet absorber, a lubricant, an antiblocking agent, a pigment and a dye are added to the polyolefin composition, if necessary, without impairing the object of the present invention. It can be added in a range.

[2] Method for Producing Polyolefin Microporous Membrane The production conditions are the same for the ultra-high molecular weight polyolefin obtained by the multi-stage polymerization alone and for the polyolefin composition. I will explain.

(1) Heat dissolution of the polyolefin composition A high concentration solution of the polyolefin composition is prepared by heating and dissolving the above-mentioned polyolefin composition in a nonvolatile solvent. As the non-volatile solvent, for example, nonane, decane, undecane, dodecane, aliphatic or alicyclic hydrocarbons such as liquid paraffin, or a mineral oil fraction such as liquid paraffin having a boiling point corresponding thereto, and the like,
Liquid paraffin is preferred to obtain a gel-like molded product having a stable solvent content.

Heat dissolution of the polyolefin composition is carried out with stirring at a temperature at which the polyolefin composition is completely dissolved in the non-volatile solvent. The heating temperature varies depending on the type of the polyolefin and the nonvolatile solvent used. For example, in the case of a polyethylene composition composed of ultrahigh molecular weight polyethylene and high density polyethylene + liquid paraffin, 140 to 2
Preferably it is in the range of 50 ° C.

(2) Concentration of Polyolefin Composition Solution The concentration of the polyolefin composition solution is 10 to 50% by weight, preferably 10 to 40% by weight. When the concentration of the polyolefin composition is less than 10% by weight, not only is the amount of the non-volatile solvent used too uneconomical, but also the swell and neck-in at the die exit are large at the time of forming a sheet, making it difficult to form the sheet. . On the other hand, when the concentration exceeds 50% by weight, it becomes difficult to prepare a uniform solution. In addition, at the time of heating and melting, it is preferable to add an antioxidant to prevent oxidation of the polyolefin.

(3) Extrusion of Polyolefin Composition Solution A heated solution of the polyolefin composition is extruded from a die. As the die, a sheet die having a generally rectangular die shape is used. The die gap of the sheet die is usually 0.1 to 5 mm, and is 140 to 250 mm during extrusion.
Heat to ° C. Extrusion speed is 20-30cm / min to 7-8m
/ Min.

(4) Formation of Gel Sheet The solution extruded from the die is cooled and formed into a gel sheet. Cooling is preferably performed at a rate of 50 ° C./min or more at least until the gelling temperature or less. 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. The solution extruded from the die may be taken at a take-up ratio of 1 to 10, preferably 1 to 5, before or during cooling. When the take-up ratio is 10 or more, neck-in becomes large, and breakage tends to occur during stretching, which is not preferable.

(5) Stretching of Gel Sheet (a) Batch Stretching Method After stretching, the gel sheet can be stretched at a predetermined magnification by a batch type tenter. Either vertical or horizontal simultaneous stretching or sequential stretching may be used, but simultaneous biaxial stretching is particularly preferred.

(B) Continuous stretching method When a gel-like sheet is continuously biaxially stretched, a biaxial stretching apparatus as shown in FIG. 1 is used. The biaxial stretching device includes a pair of guide rails 1a and 1b that expand in the traveling direction.
And conveyor members 2a and 2b that move along the guide rails 1a and 1b, and grip members 4a and 4b provided at the distal ends of the link members 3a and 3b connected to the conveyor members 2a and 2b. Conveyor member 2
As a and b move in the direction of travel, the distance between adjacent conveyor members 2a and 2b increases. Since the pair of guide rails 1a and 1b are expanded in the traveling direction, the distance between the pair of left and right grip members 4a and 4b is a gel-like sheet.
Gradually increases as 10 progresses.

The gel-like sheet 10 is added to the biaxial stretching apparatus having the above-described structure.
Is inserted, both ends 10a and 10b of the gel-like sheet 10 are gripped by the grip members 4a and 4b. The width of the gel-like sheet 10 at that time and B _1, P ₁ the distance between adjacent gripping members 4a, 4b in the guide rails 1a, the 1b
And As the gel-like sheet 10 advances in the direction of the arrow, the width of the gel-like sheet 10 whose both ends are gripped by both grip members 4a and 4b gradually increases, and the distance between the grip members 4a and 4b adjacent in the longitudinal direction. Also gradually increases. As a result, the gel-like sheet 10 is simultaneously biaxially stretched as shown by four arrows in FIG. The width of the gel-like sheet 10 at the time of biaxial stretching is complete and B _2, the guide rails 1a, the gripping member 4a in 1b, and distance 4b and P _2. Then, the stretching ratio S _{MD in} the machine direction (MD) can be represented by P ₂ / P ₁ , and the stretching ratio S _{TD in} the transverse direction (TD) can be represented by B ₂ / B ₁ .

The stretching ratios S _MD and S _TD vary depending on the thickness of the gel-like sheet, but are preferably at least 2 times or more in any direction and the area ratio is 10 times or more. More preferably 3-20 times, respectively S _MD and S _TD, the area magnification and 20 to 400 times. If the areal magnification is less than 10 times, stretching is insufficient and a highly elastic and high-strength microporous polyolefin membrane cannot be obtained. On the other hand, if the areal magnification exceeds 400 times, restrictions are imposed on the stretching apparatus, stretching operation, and the like.

(C) Stretching temperature In both the batch system and the continuous system, the stretching temperature is preferably equal to or lower than the melting point of the polyolefin composition + 10 ° C., and more preferably in the range from the crystal dispersion temperature to less than the crystal melting point.
For example, in the case of ultra-high molecular weight polyethylene + high-density polyethylene, the temperature is preferably 90 to 140 ° C, more preferably 100 to 130 ° C.
It is in the range of ° C. If the stretching temperature exceeds the melting point + 10 ° C., the resin melts and the molecular chains cannot be oriented by stretching. 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 the stretching, and high-magnification stretching cannot be performed. The “crystal dispersion temperature” is a temperature at which micro-Brownian motion of molecular chains in a crystal becomes active, and is determined by dynamic viscoelasticity measurement. The dynamic viscoelasticity is determined by giving a steady sine wave strain to the viscoelastic body and measuring the sine wave stress, or measuring the strain with respect to the stress. The crystal melting point is a temperature at which the polyolefin changes from a crystalline state to a liquid state, and is determined by differential scanning calorimetry.

(6) Making the porous film microporous by removing the nonvolatile solvent The obtained biaxially stretched film is washed with a volatile solvent, and the remaining nonvolatile solvent is extracted and removed. As the volatile solvent, hydrocarbons such as pentane, hexane and heptane,
Examples include chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as ethane trifluoride, and ethers such as diethyl ether and dioxane. The washing method can be performed by a method of immersing in a volatile solvent for extraction, a method of showering the volatile solvent, a method of a combination thereof, or the like. The above-mentioned washing is performed until the residual nonvolatile solvent in the biaxially stretched film becomes less than 1% by weight. Thereafter, the biaxially stretched film is dried by a heat drying method or an air drying method. It is desirable that the dried biaxially stretched film is heat-set at a temperature in the range of the crystal dispersion temperature to the melting point.

[3] Characteristics of Microporous Polyolefin Membrane (1) General Characteristics The porosity of the microporous polyolefin membrane produced as described above is 35 to 95%, and the average through-pore diameter is 0.001 to 0.2 μm.
m and the breaking strength of 15 mm width is 0.2 kg or more. The thickness of the polyolefin microporous membrane can be appropriately selected according to the application, but is preferably, for example, 5 to 50 μm, and more preferably 10 to 30 μm. For microporous polyolefin membrane,
If necessary, a hydrophilic treatment may be performed by plasma irradiation, surfactant impregnation, surface grafting, or the like.

(2) Ratio of Tensile Rupture Strength Tensile Rupture Strength (A) of the biaxially stretched film before extraction of the non-volatile solvent and tensile strength (B) of the biaxially stretched film (microporous film) after extraction ) Is 0.75 or less. Specifically, the biaxially stretched film before the extraction treatment has a tensile strength at break (A) of about 100 to 600 kgf / cm ² , and the microporous membrane has a tensile strength at break (B) of 600 to 1900 kgf / cm ^{2.
About 2} . As described above, the strength of the biaxially stretched film before the extraction treatment is low, but the strength is extremely increased by the extraction treatment.

[0038]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.
The test method in each example is as follows.

(1) Weight average molecular weight (Mw) and molecular weight distribution (Mw / Mn): A gel permeation chromatography (GPC) device manufactured by Waters Co., Ltd., and GMH-6 manufactured by Tosoh Co., Ltd. was used as a column. The measurement was performed by the GPC method using O-dichlorobenzene as a nonvolatile solvent at a temperature of 135 ° C. and a flow rate of 1.0 ml / min. (2) Film thickness: The cross section was measured by a scanning electron microscope. (3) Tensile breaking strength: Measured in accordance with ASTM D882 (unit: kgf / cm ² ). (4) Tensile elongation at break: measured in accordance with ASTM D882 (unit:%). (5) Pore size: Using OMNISORP360 (manufactured by Coulter), liquid nitrogen was used as a medium, and the average pore size was determined.

Example 1 A weight average molecular weight (Mw) of 1 × 10 ⁶ and a molecular weight distribution (Mw)
(Mn) of 10 was mixed with a liquid paraffin (64 cst / 40 ° C.) as a non-volatile solvent, and heated to 200 ° C. to prepare a 30% by weight ultrahigh molecular weight polyethylene solution. In addition, 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 ultrahigh molecular weight polyethylene solution were mixed with tetrakis [methylene-3- (3, 0.25 parts by weight of 5-di-t-butyl-4-hydroxyphenyl) -propionate] methane (“Irganox 1010”, Ciba-Geigy) was added as an antioxidant.

The obtained solution was extruded from a T-die by an extruder to form a 1.5 mm thick gel-like sheet. After cooling the gel-like sheet, it was mounted on a batch-type biaxial stretching machine,
The film was simultaneously biaxially stretched 5 × 5 times at ° C. The tensile breaking strength and tensile breaking elongation of the obtained biaxially stretched film (thickness: 61 μm) were measured. Next, the biaxially stretched film was washed with methylene chloride to extract and remove residual liquid paraffin, and then dried to produce a microporous film having a thickness of 32 μm. The tensile rupture strength and tensile rupture elongation of the obtained microporous membrane were measured. Table 1 shows the measurement results together with other characteristics of the microporous membrane.

EXAMPLE 2 A multistage ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 2 × 10 ⁶ and a molecular weight distribution (Mw / Mn) of 12 was used, and the polyethylene concentration of the solution was adjusted to 20% by weight. A biaxially stretched film having a thickness of 76 μm was formed under the same conditions as in Example 1 except that
A μm microporous membrane was produced. The tensile strength at break and tensile elongation at break were measured. Table 1 shows the measurement results together with other characteristics of the microporous membrane.

Example 3 20% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 2 × 10 ⁶ and a weight average molecular weight (Mw) of 4 × 10 ⁵
Liquid paraffin (64 cst / 40 ° C.) as a non-volatile solvent was mixed with 80% by weight of the high-density polyethylene, and heated to 200 ° C. to prepare a polyethylene composition solution having a concentration of 30% by weight. Molecular weight distribution (Mw) of polyethylene composition
/ Mn) was 14. The same antioxidant as in Example 1 was added to the polyethylene composition solution.

A biaxially stretched film having a thickness of 49 μm was formed from the polyethylene composition solution under the same conditions as in Example 1, and then a 26 μm thick microporous film was prepared by extracting liquid paraffin. The tensile strength at break and tensile elongation at break were measured. Table 1 shows the measurement results together with other characteristics of the microporous membrane.

Example 4 A thickness of 59 μm was obtained under the same conditions as in Example 3 except that the continuous biaxial stretching machine shown in FIG. 1 was used instead of the batch type biaxial stretching machine.
Was formed, and then a microporous film having a thickness of 29 μm was produced under the same conditions as in Example 3. The tensile strength at break and tensile elongation at break were measured. Table 1 shows the measurement results together with other characteristics of the microporous membrane.

Comparative Example 1 A biaxially stretched film having a thickness of 60 μm was formed under the same conditions as in Example 1 except that paraffin wax was used as the non-volatile solvent, and then a microporous film having a thickness of 45 μm was formed under the same conditions as in Example 1. A film was prepared. The tensile strength at break and tensile elongation at break were measured. Table 1 shows the measurement results together with other characteristics of the microporous membrane.

Comparative Example 2 20% by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight (Mw) of 2 × 10 ⁶ and a weight average molecular weight (Mw) of 4 × 10 ⁵
Comparative Example 1 except that 80% by weight of high-density polyethylene was used.
A biaxially stretched film having a thickness of 50 μm was formed under the same conditions as described above, and then a microporous film having a thickness of 38 μm was prepared under the same conditions as in Example 1. The tensile strength at break and tensile elongation at break were measured. Table 1 shows the measurement results together with other characteristics of the microporous membrane.

Table 1 Example No. Example 1 Example 2 Example 3 Raw material ultrahigh molecular weight polyethylene (Mw) 1,000,000 2,000,000 2,000,000 Content (% by weight) 100 100 20 High-density polyethylene Weight average molecular weight (Mw) Mw)--400,000 content (% by weight) 0 0 80 Types of non-volatile solvent ⁽¹⁾ FP FP FP solution concentration ⁽²⁾ 30 20 30 Biaxial stretching condition Stretching method Batch type Batch type Batch type Stretching ratio 5 × 5 5 × 5 5 × 5 Tensile breaking strength A of the biaxially stretched film before extraction ⁽³⁾ MD 255 231 378 TD- ^* - ^* - ^* Tensile breaking elongation ⁽⁴⁾ MD 244 267 222 TD- ^* - ^* - ^* Characteristic tensile strength at break of microporous membrane B ⁽³⁾ MD 1279 1302 1108 TD- ^* - ^* - ^* Tensile strength at break ⁽⁴⁾ MD 220 155 189 TD- ^* - ^* - ^* A / B MD 0.2 0.18 0.34 TD- ^* - ^* - ^* Average pore size (μm) 0.020 0.009 0.030 Note: * Not measured. (1) FP represents liquid paraffin, and PW represents paraffin wax. (2) The concentration (% by weight) of ultrahigh molecular weight polyethylene or polyethylene composition in the solution. (3) The unit is kgf / cm ² . (4) Unit is%.

Table 1 (continued) Example No. Example 4 Comparative Example 1 Comparative Example 2 Raw material ultra-high molecular weight polyethylene Weight average molecular weight (Mw) 2,000,000 1,000,000 2,000,000 Content (% by weight) 20 100 20 High-density polyethylene Weight Average molecular weight (Mw) 400,000-400,000 Content (% by weight) 80 0 80 Type of non-volatile solvent ⁽¹⁾ FP PW PW Solution concentration ⁽²⁾ 30 30 30 Biaxial stretching condition Stretching method Continuous batch Batch Formula Stretching magnification 5 × 5 5 × 5 5 × 5 Characteristics of biaxially stretched film before extraction A ⁽³⁾ MD 488 1135 1028 TD 364- ^* - ^* Tensile breaking elongation ⁽⁴⁾ MD 183 139 144 TD 243- ^* - ^* Characteristic tensile rupture strength B of microporous membrane ⁽³⁾ MD 1336 1448 1303 TD 1058- ^* - ^* Tensile rupture elongation ⁽⁴⁾ MD 150 147 147 TD 339- ^* - ^* A / B MD 0.37 0.78 0.79 TD 0.34- ^* - ^* Average pore size (μm) 0.031 0.028 0.020 Note: * Not measured. (1)-(4) Same as above.

As is apparent from the above results, when the microporous film is formed by simultaneously biaxially stretching the gel-like sheet, the tensile breaking strength (A) of the biaxially stretched film before the extraction treatment of the nonvolatile solvent is performed. Ratio (A / B) between the tensile strength at break (B) of the biaxially stretched film (microporous membrane) after the extraction treatment
In Examples 1 to 4 of 0.75 or less, regardless of the stretching method, a microporous membrane having low stress and excellent characteristics can be obtained.

[0051]

As described above in detail, according to the method of the present invention, the ratio between the tensile strength at break of the gel-like sheet after the stretching and before the extraction treatment and the tensile strength at break of the microporous membrane is within a predetermined range. Since a solvent that can be used is selected, a microporous polyolefin membrane having low stress and excellent characteristics can be obtained.

The microporous membrane produced by the method of the present invention has a high strength and is excellent in handleability and workability when laminated with a nonwoven fabric or the like. Furthermore, since it has excellent water permeability, molecular substances having a molecular weight of tens of thousands to hundreds of thousands can be fractionated. The microporous polyolefin membrane according to the method of the present invention is suitable for various uses such as a battery separator, a diaphragm for an electrolytic capacitor, an ultrafine filtration membrane, an ultrafiltration membrane, various filters, and a porous membrane for moisture-permeable waterproof clothing.

[Brief description of the drawings]

FIG. 1 is a schematic view of a biaxial stretching apparatus used for a continuous stretching method.

[Explanation of symbols]

1a, 1b ... guide rails 2a, 2b ... conveyor members 3a, 3b ... link member 4a, 4b ... gripping member 10 ... gel-like sheet B _1, B ₂ ... gel-like sheet Width P ₁ , P ₂ ... Grip member spacing

Claims (5)

[Claims] 1. An ultrahigh molecular weight polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more and a weight average molecular weight / number average molecular weight of 10 to 300, or (b) a polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more. A solution comprising 10 to 50% by weight of a polyolefin composition containing 1% by weight or more of components and having a weight average molecular weight / number average molecular weight of 10 to 300, and 50 to 90% by weight of a non-volatile solvent; Is extruded from a die, cooled to form a gel-like sheet, and the gel-like sheet is biaxially stretched at a temperature not higher than the melting point of the ultra-high molecular weight polyolefin (a) or the polyolefin composition (b) + 10 ° C, A method for producing a microporous polyolefin membrane by extracting and removing the remaining nonvolatile solvent with a volatile solvent, wherein the tensile strength at break (A) of the biaxially stretched sheet before extraction and the tensile strength at break after extraction (B The ratio of (A / B)
Is 0.75 or less. 2. The method for producing a microporous polyolefin membrane according to claim 1, wherein the non-volatile solvent is a good solvent for the ultra-high molecular weight polyolefin (a) or the polyolefin composition (b), and An ultrahigh molecular weight polyolefin (a) or a polyolefin composition (b) which becomes a gel when cooled, and is a liquid at room temperature and an aliphatic or alicyclic compound having a boiling point of 170 ° C or higher. Or a mineral oil fraction. 3. The method for producing a microporous polyolefin membrane according to claim 2, wherein the non-volatile solvent is liquid paraffin. 4. The method for producing a microporous polyolefin membrane according to claim 1, wherein the ultrahigh molecular weight polyolefin (a) is an ultrahigh molecular weight polyolefin obtained by multistage polymerization. And how. 5. The method for producing a microporous polyolefin membrane according to claim 1, wherein the polyolefin composition (b) comprises: (a) an ultra-high molecular weight polymer having a weight average molecular weight of 7 × 10 ⁵ or more. A method comprising a mixture of a polyolefin and (b) polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 7 × 10 ⁵ .