JP2657431B2 - Polyolefin microporous membrane and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microporous membrane comprising a polyolefin, particularly a multistage polyolefin containing an ultrahigh molecular weight component, and a method for producing the same, and more particularly to a polyolefin microporous membrane having good productivity and a method for producing the same. .

[0002]

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

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

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

On the other hand, the weight average molecular weight is 7 × 10 ⁵
Forming a gel-like sheet from a solution obtained by heating and dissolving the above ultra-high molecular weight polyolefin in a solvent, preparing a solvent amount in the gel-like sheet by desolvation treatment, and then heating and stretching to remove a residual solvent. Various methods for producing microporous membranes of ultra-high molecular weight polyolefin (polyethylene) have been proposed (JP-A-60-242035, JP-A-61-21).
-495132, JP-A-61-195133, JP-A-63-39602,
JP-A-63-273651).

However, in any of the above methods for producing a microporous ultrahigh molecular weight polyolefin (polyethylene) film, it is necessary to prepare a somewhat dilute solution of the polyolefin in order to biaxially stretch the ultrahigh molecular weight polyolefin. The resulting solution has a large swell and neck-in at the exit of the die for forming the sheet, making it difficult to form the sheet. Further, since the sheet contains an excessive amount of solvent, even if it is stretched as it is, the desired fineness can be obtained. Since a porous membrane cannot be obtained, a large amount of solvent is required, for example, it is necessary to remove the solvent to adjust the amount of solvent in the sheet, and there is a problem in productivity. Therefore, it has been desired to produce a microporous polyolefin membrane from a high-concentration solution of an ultrahigh molecular weight polyolefin without deteriorating the stretchability.

Accordingly, an object of the present invention is to provide a polyolefin microporous membrane comprising a polyolefin, which has good stretchability and can be used as a high-concentration solution.

Another object of the present invention is to provide a method for efficiently producing a microporous polyolefin membrane from a high-concentration polyolefin solution without impairing stretchability.

[0009]

Means for Solving the Problems As a result of intensive studies in view of the above objects, the present inventors have found that a multistage polyolefin containing an ultrahigh molecular weight component and having a wide molecular weight distribution (high weight average molecular weight / number average molecular weight). It has been found that a high-concentration solution can be prepared by using, and a polyolefin microporous membrane having various characteristics can be efficiently produced from this solution.

That is, the microporous polyolefin membrane of the present invention comprises a multi-stage polyolefin having a weight-average molecular weight / number-average molecular weight of 10 to 300, containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more. 0.1 to 50μm thickness
With a porosity of 35 to 95% and an average through-hole diameter of 0.001 to 0.2
μm and a breaking strength of 15 mm width is 0.2 kg or more.

Further, the method of the present invention for producing such a polyolefin microporous membrane, the molecular weight 7 × 10 ⁵ or more ingredients 1
% By weight or more (weight average molecular weight / number average molecular weight)
Is 10 to 300, and 10 to 50% by weight of a multistage polymerized polyolefin,
A solution comprising 50 to 90% by weight of a solvent is prepared, the solution is extruded from a die, cooled to form a gel-like molded product, and the gel-like molded product is stretched at a temperature not higher than the melting point of the polyolefin + 10 ° C or lower. Thereafter, the remaining solvent is removed.

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

The weight-average molecular weight / number-average molecular weight of the multistage polyolefin is 10 to 300, preferably 12 to 250.
It is. If the weight average molecular weight / number average molecular weight is less than 10,
Since the average molecular chain length is large and the entanglement density of the molecular chains at the time of dissolution increases, it is difficult to prepare a high-concentration solution.
On the other hand, when it exceeds 300, the low molecular weight component is broken at the time of stretching, and the strength of the whole 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 width of the molecular weight distribution increases as the ratio of the molecular weights increases.
That is, the ratio of the molecular weights of the multi-stage polymerized polyolefin shows the spread of the distribution, and the larger the value, the wider the distribution.

In the present invention, the weight-average molecular weight / number-average molecular weight of the multistage polymerized polyolefin is set to 10 to 300, which is larger than the weight-average molecular weight / number-average molecular weight of ordinary ultra-high-molecular-weight polyolefin (usually about 6). I have. As a result, the molecular weight distribution spreads to the lower molecular weight side, so that a high-concentration polyolefin solution can be prepared.

If the component having a molecular weight of 7 × 10 ⁵ or more is less than 1% by weight in the multistage polymerized polyolefin, the entanglement of the molecular chain of the ultrahigh molecular weight polyolefin which contributes to the improvement of the stretchability is hardly formed, and the high-strength polyolefin has a high strength. A 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. However, if the content exceeds 90% by weight, it becomes difficult to achieve a high concentration of the target polyolefin solution, which is not preferable.

In the present invention, the multi-stage polyolefin refers to ethylene, propylene, 1-butene, 4-methyl-1-
It is a homopolymer or copolymer obtained by polymerizing pentene, 1-hexene, or the like in multiple stages. Particularly, a multistage polymer mainly composed of ethylene is preferable. The multi-stage polymerization polyolefin,
For example, 1% by weight of an ultrahigh molecular weight component having a molecular weight of 7 × 10 ⁵ or more
It can be produced by polymerizing a high molecular weight portion and a low molecular weight portion in multiple stages so that the above-mentioned components are contained and the molecular weight distribution (weight average molecular weight / number average molecular weight) is 10 to 300. As the multi-stage polymerization, a two-stage polymerization in which a high molecular weight component is polymerized in the first stage and then a low molecular weight component is polymerized in the second stage is preferable.

For example, in a two-stage polymerized polyethylene, a raw material gas of ethylene / hydrogen is vapor-phased using a Ziegler-Natta type catalyst (for example, a polymerization catalyst comprising a catalyst component in which titanium tetrachloride is supported on a magnesium compound and triethylaluminum). Or, in the presence of a solvent, perform polymerization of a desired high-molecular-weight portion in the first-stage reactor, and then change the conditions such as hydrogen concentration, temperature, and cocatalyst, and the degree of polymerization differs in the second-stage reactor. It can be produced by polymerizing a low molecular weight portion. By polymerizing in such a manner, while containing a component having a molecular weight of 7 × 10 ⁵ or more in the above-described range, a molecular weight distribution of a molecular weight distribution to a relatively low molecular weight portion of less than 7 × 10 ⁵ to 1 × 10 ⁴ . Spread polyethylene can be obtained.

In addition, an antioxidant, an ultraviolet absorber,
Various additives such as a lubricant, an antiblocking agent, a pigment, a dye, and an inorganic filler can be added within a range that does not impair the purpose of the present invention.

Next, a method for producing a microporous polyolefin membrane of the present invention using the above-described polyolefin will be described. In the present invention, a high-concentration solution of a polyolefin as a raw material is prepared by heating and dissolving the above-mentioned multistage polymerized polyolefin in a solvent.

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

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

Next, the heated solution of the multistage polymerized polyolefin is extruded from a die and molded. As the die, a sheet die having a rectangular base shape is usually used.
A heavy cylindrical hollow die, an inflation die and the like can also be used. When a sheet die is used, the die gap is usually 0.1 to 5 mm.
Heated to 140-250 ° C. At this time, the extrusion speed is usually 20 to 30 cm / min to 2 to 3 m / min.

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

Next, this gel-like molded product is stretched. The stretching is performed at a predetermined magnification by heating the gel-like molded product and using a usual tenter method, a roll method, an inflation method, a rolling method or a combination of these methods. Biaxial stretching is preferred, and any of vertical and horizontal simultaneous stretching or sequential stretching may be used.
Particularly, simultaneous biaxial stretching is preferable.

The stretching temperature is equal to or lower than the melting point of the multistage polymerized polyolefin + 10 ° C., preferably in the range from the crystal dispersion temperature to less than the crystal melting point. For example, in the case of multi-stage polymerized polyethylene, the temperature is from 90 to 140 ° C, more preferably from 100 to 130 ° C. When the stretching temperature exceeds the melting point + 10 ° C., the molecular chains cannot be oriented by stretching due to the melting of the resin. Also,
If the stretching temperature is lower than the crystal dispersion temperature, the softening of the resin is insufficient, the film is easily broken in the stretching, and the stretching at a high magnification cannot be performed.

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

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

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

The microporous polyolefin membrane produced as described above has a porosity of 35 to 95% and an average through-pore diameter of 0.00
The breaking strength of 1 to 0.2 μm and 15 mm width is 0.2 kg or more. The thickness of the polyolefin microporous membrane of the present invention,
Although it can be appropriately selected according to the application, it is generally 0.1 to 50 μm, preferably 2 to 40 μm. The obtained microporous polyolefin membrane can be subjected to a hydrophilization treatment by plasma irradiation, surfactant impregnation, surface grafting or the like, if necessary.

[0031]

Examples of the present invention will be described below. In addition, the test method in an Example is as follows. (1) Molecular weight and molecular weight distribution: GP manufactured by Waters Corporation
Using C apparatus, GMH-6 manufactured by Tosoh Corp.
Using O-dichlorobenzene, temperature 135 ° C, flow rate 1.0 ml
Per minute, measured by gel permeation chromatography (GPC). (2) Tensile breaking strength: The breaking strength of a 15 mm wide strip
Measured according to ASTM D882. (3) Film thickness: The cross section was measured with a scanning electron microscope. (4) Air permeability: Conforms to JIS P8117. (5) Water permeability: After incorporating the microporous membrane into the flat membrane module, hydrophilizing with a mixed solution of distilled water / ethanol (50/50 volume ratio), washing thoroughly with distilled water, and increasing the water pressure to 380 mmHg. The permeation amount of the filtrate when applied was measured and determined. (6) Hole diameter: 38 using the module described in (5) above
When a 0.05% by weight aqueous solution of pullulan (manufactured by Showa Denko KK) was circulated under a differential pressure of 0 mmHg, the concentration of pullulan contained in the filtrate was determined by differential refractive index measurement. Then, the pore diameter was converted from the value of the molecular weight of pullulan at which the rejection calculated by the following formula was 50%, using Flory's theory as described later. Pullulan rejection = {1-(Pullulan concentration in filtrate / Pullulan concentration in stock solution)} x 100 The chain polymer in the form of a solution is in the form of a spherical thread, and its diameter d is the both ends of the molecular chain. It can be considered that there is an approximate relationship of [d / 2] ² = <γ ² > (1) with respect to the root mean square distance <γ ² >. According to Flory's theory of viscosity and molecular chain spread in a polymer solution, [η] M = 2.1 × 10 ²¹ <γ ² > ^3/2 ... (2) holds regardless of the type of polymer. From formulas (1) and (2), the diameter d of the chain polymer can be calculated from the measured value of the intrinsic viscosity [η] and the molecular weight M at which the rejection becomes 50%. This d was defined as the average pore size of the polyethylene microporous membrane. (7) Pore size distribution: In the measurement according to (6) above, the rejection
Similarly, the pore size is converted from the value of the molecular weight of pullulan, which is 90%, to obtain the maximum pore size, and the maximum pore size is calculated by using the value of the maximum pore size / the average pore size. (8) Porosity: Measured with a mercury porosimeter.

Example 1 The weight average molecular weight (Mw) was 9.0 × 10 ⁵ , the molecular weight distribution (weight average molecular weight / number average molecular weight) = 195, and the ratio of the components having a molecular weight of 7 × 10 ⁵ or more was 21% by weight. 15 parts by weight of a two-stage polymerized polyethylene (melting point: 132 ° C.) having 21% by weight of a component having a molecular weight of 7 × 10 ⁵ or more and 85 parts by weight of liquid paraffin (64 cst / 40 ° C.) A solution of step polymerized polyethylene was prepared. Then, 2,6-di-t-
0.125 parts by weight of butyl-p-cresol (“BHT”, manufactured by Sumitomo Chemical Co., Ltd.) and tetrakis [methylene-
3- (3,5-di-t-butyl-4-hydroxyphenyl) -propionate] methane ("Irganox 1
010 "(manufactured by Ciba Geigy) as an antioxidant. This mixture was filled in an autoclave equipped with a stirrer and stirred at 200 ° C. for 90 minutes to obtain a uniform solution.

This solution was extruded from a T-die using an extruder having a diameter of 45 mm, and was formed into a gel-like sheet while being taken up by a cooling roll.

The obtained sheet was set on a biaxial stretching machine, and simultaneously biaxially stretched 7 × 7 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 extract and remove the remaining liquid paraffin, and then dried to obtain a polyethylene microporous membrane.

Weight average molecular weight and molecular weight distribution of the two-stage polymerized polyethylene used for the obtained microporous polyethylene membrane
Table 1 shows (weight average molecular weight / number average molecular weight (Mw / Mn)). The tensile strength at break, film thickness, air permeability, water permeability, and average pore size of the microporous polyethylene membrane were measured.
The results are shown in Table 2.

Example 2 In Example 1, the weight average molecular weight (M
w) is 7.5 × 10 ⁵ , the molecular weight distribution (weight average molecular weight / number average molecular weight) = 43.3, and the proportion of components having a molecular weight of 7 × 10 ⁵ or more is 32% by weight of a two-stage polymerized polyethylene (fused).
(133 ° C.) to produce a microporous polyethylene membrane.

Weight average molecular weight and molecular weight distribution of the two-stage polymerized polyethylene used for the obtained microporous polyethylene membrane
(Weight average molecular weight / number average molecular weight (Mw / Mn)
It is shown in the table. In addition, the tensile rupture strength of polyethylene microporous membrane,
The film thickness, air permeability, water permeability, and average pore size were measured. The results are shown in Table 2.

Example 3 In Example 1, as a two-stage polyethylene solution, the weight average molecular weight (Mw) was 7.8 × 10 ⁵ , the molecular weight distribution (weight average molecular weight / number average molecular weight) = 16.7, and the molecular weight was 7 The same procedure was performed except that a mixture of 10 parts by weight of a two-stage polymerized polyethylene (melting point: 134 ° C.) having a ratio of × 10 ⁵ or more of 47% by weight and 90 parts by weight of liquid paraffin (64 cst / 40 ° C.) was used. Thus, a polyethylene microporous membrane was produced.

Weight average molecular weight and molecular weight distribution of the two-stage polymerized polyethylene used for the obtained microporous polyethylene membrane
(Weight average molecular weight / number average molecular weight (Mw / Mn)
It is shown in the table. In addition, the tensile rupture strength of polyethylene microporous membrane,
The film thickness, air permeability, water permeability, and average pore size were measured. The results are shown in Table 2.

Comparative Example 1 In Example 1, the weight average molecular weight (M
w) is 4.3 × 10 ⁵ , molecular weight distribution (weight average molecular weight / number average molecular weight) = 350, and the ratio of components having a molecular weight of 7 × 10 ⁵ or more is 15% by weight. Thus, a polyethylene microporous membrane was produced, but the obtained microporous membrane had remarkably inferior breaking strength of the membrane.

Comparative Example 2 In Example 1, the weight average molecular weight (M
Let's prepare a solution of polyethylene using polyethylene having a w) of 2.5 × 10 ⁶ , a molecular weight distribution (weight average molecular weight / number average molecular weight) = 6.0, and a ratio of components having a molecular weight of 7 × 10 ⁵ or more being 95% by weight. However, a uniform solution could not be obtained.

Table 1 No. Ratio of weight average molecular weight Mw / Mn molecular weight of 7 × 10 ⁵ or more Example 1 9.0 × 10 ⁵ 195 21% by weight Example 2 7.5 × 10 ⁵ 43.3 32% by weight Example 3 7.8 × 10 ⁵ 16.7 47% by weight Comparative Example 1 4.3 × 10 ⁵ 350 15% by weight Comparative Example 2 2.5 × 10 ⁶ 6.0 95% by weight

Table 2 Breaking load Film thickness Air permeability Permeability Average pore diameter Example of porosity No. (kgf / 15mm width ) (μm) (sec / 100cc ) (*) (μm) (%) Example 1 0.40 5 450 115 0.03 50 Example 2 0.52 5 400 175 0.03 55 Example 3 0.75 5 450 190 0.03 60 Note) *: Unit is liter / m ² · hr · atm

As is apparent from the present example, the microporous polyethylene membranes of Examples 1 to 3 had a large value of the tensile strength at break and good air permeability and water permeability. On the other hand, Comparative Example 1 using a two-stage polymerized polyolefin having a molecular weight distribution of more than 300 lacks practicability as a microporous membrane, and is uniform when polyethylene having a molecular weight distribution of less than 10 is used. A highly concentrated solution could not be obtained.

[0045]

As described in detail above, according to the present invention,
The polyolefin microporous membrane is manufactured using multi-stage polymerized polyolefin that contains ultra-high molecular weight components and has a wide molecular weight distribution (high weight-average molecular weight / number-average molecular weight), enabling high-concentration polyolefin solutions. Therefore, it is not necessary to adjust the amount of the solvent before stretching, the amount of the solvent used can be reduced, and the microporous polyolefin membrane can be efficiently obtained. Further, the microporous polyolefin membrane of the present invention has high strength and is also excellent in water permeability.

The microporous polyolefin membrane of the present invention can be used as a separator for a battery, a diaphragm for an electrolytic capacitor,
It is suitable for various uses such as ultra-fine filtration membranes, ultrafiltration membranes, various filters, and porous membranes for moisture-permeable and waterproof clothing.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Mamoru Tsuneyoshi 1-3-1 Nishitsurugaoka, Oimachi, Iruma-gun, Saitama

Claims (2)

(57) [Claims] 1. A composition containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, wherein (weight average molecular weight / number average molecular weight) is 10 to 30.
0 multi-stage polymerized polyolefin having a thickness of 0.1 to 50
μm, porosity 35-95%, average through-hole diameter 0.001-
A microporous polyolefin membrane having a thickness of 0.2 μm and a breaking strength of 15 mm width of 0.2 kg or more. 2. A composition containing 1% by weight or more of a component having a molecular weight of 7 × 10 ⁵ or more, wherein (weight average molecular weight / number average molecular weight) is 10 to 30.
0 to 10 to 50% by weight of a multistage polymerized polyolefin and 50 to 50% by weight of a solvent.
A solution consisting of 90% by weight is prepared, the solution is extruded from a die, cooled to form a gel-like composition, and the gel-like composition is stretched at a temperature not higher than the melting point of the polyolefin + 10 ° C. A method for producing a microporous polyolefin membrane, comprising removing a residual solvent.