JP3347835B2 - Method for producing microporous polyolefin membrane - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a microporous membrane made of polyolefin, and more particularly to a method for efficiently producing a microporous polyolefin membrane having an appropriate pore size.

[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 of 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 body 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, 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 of the formed pores becomes large. On the other hand, 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 in the film raw material, and the shrinkage and densification However, there is a drawback that stretching at a high 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 7 × 10 ⁵ or more in a solvent, and the amount of the solvent in the gel-like sheet is reduced by desolvation treatment. Various methods have been proposed for producing a microporous membrane of ultra-high molecular weight polyolefin (polyethylene) by preparing, then heat-stretching, and removing the residual solvent (JP-A-60-242035, JP-A-60-242035). 61-4
95132, JP-A-61-195133, JP-A-63-39602, JP-A-63-273651).

However, in the above method, the higher the concentration of the polyolefin solution, the longer it takes to prepare the solution, the more difficult it is to make the solution concentration uniform, and the more the solution tends to deteriorate. .

Accordingly, an object of the present invention is to provide a method for efficiently producing a microporous polyolefin membrane having an appropriate pore size, and to use a microporous membrane obtained by such a method.
To provide a battery separator and a filter .

[0008]

As a result of intensive studies in view of the above-mentioned object, the present inventors have melted a polyolefin containing an ultrahigh molecular weight component in an extruder and supplied a good solvent for the polyolefin to the melt. A solution of the polyolefin is thereby prepared, and the solution is extruded into a sheet from a die of an extruder, and then cooled to form a gel-like composition, and the gel-like composition is heated and stretched. By removing the solvent, a microporous polyolefin membrane having a moderately large pore size can be produced, and according to such a method, the time required for preparing a high-concentration solution of the polyolefin is significantly longer than before. The present inventors have found that it can be shortened, and that the economical efficiency is improved in a ripple manner, and arrived at the present invention.

That is, the method for producing a microporous polyolefin membrane of the present invention comprises the steps of: (a) melting a polyolefin containing 1% by weight or more of a component having a weight average molecular weight of 7 × 10 ⁵ or more in an extruder; with respect to the polyolefin 10 to 80 wt% of the state, and melt-kneaded by supplying (good solvent for the polyolefin) 90 to 20 wt% liquid solvent the molten state from the middle of the extruder, (c) to give Uniform polio
Olefin solution extruded from a die into a sheet form, cooled to form a gel-like sheet, and heating and drawing (d) is the gel-like sheet, and removing the solvent thereafter remaining (e) the polyolefin A method for producing a microporous membrane.

The present invention will be described in detail below. [1] Polyolefin The microporous polyolefin membrane produced by the method of the present invention comprises a polyolefin containing 1% by weight or more of a component having a weight average molecular weight of 7 × 10 ⁵ or more.

The polyolefin has a weight average molecular weight of 7 × 10
^If the content of ⁵ or more components is less than 1% by weight, the entanglement of the molecular chain of the ultra-high molecular weight polyolefin that contributes to the improvement of the stretchability becomes insufficient, so that it is difficult to sufficiently improve the strength. On the other hand, the upper limit of the content of the ultra-high molecular weight component is not particularly limited. The polyolefin preferably does not substantially contain a component having a weight average molecular weight of 1 × 10 ³ or less.

The molecular weight distribution (weight-average molecular weight / number-average molecular weight) of the polyolefin is 5 to 300, especially 5 to 300.
Preferably it is ~ 50. If the molecular weight distribution exceeds 300, low molecular weight components are broken during stretching, and the strength of the entire film is lowered, which is not preferable.

The polyolefin includes ethylene,
Propylene, 1-butene, 4-methyl-1-pentene, crystalline homopolymer obtained by polymerizing 1-hexene, etc., two-stage polymer,
Alternatively, a copolymer and a blend thereof may be used.
Among these, polypropylene, polyethylene (particularly high-density polyethylene), and compositions thereof are preferred.

If the polyolefin has the above-mentioned weight average molecular weight and molecular weight distribution, even if it is obtained by a reactor blend (a multi-stage polyolefin), it may have a molecular weight of 2 or less.
The composition may be any one or more of polyolefins.

[0015] In the case of the reactor blend, for example, a weight flat
Manufacturing by multi-stage polymerization so as to contain 1% by weight or more of an ultra-high molecular weight component having an average molecular weight of 7 × 10 ⁵ or more and a molecular weight distribution (weight average molecular weight / number average molecular weight) of 10 to 300. Can be. As the multistage polymerization method, it is preferable to adopt a method of producing a high molecular weight portion and a low molecular weight portion by two-stage polymerization.

In the case of a composition comprising two or more polyolefins, an ultrahigh molecular weight polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more, which is a homopolymer or copolymer of the olefin, and a weight average molecular weight of 7 × 10 ⁵ It can be obtained by mixing an appropriate amount of less than polyolefin such that the weight average molecular weight / number average molecular weight falls within the above range.

In the case of the composition, the content of the ultra-high molecular weight polyolefin in the polyolefin composition is preferably 1% by weight or more based on 100% by weight of the whole polyolefin composition. When the content of the ultrahigh molecular weight polyolefin is less than 1% by weight, the portion contributing to the improvement of stretchability is insufficient. On the other hand, the upper limit is not particularly limited.

The lower limit of the weight average molecular weight of the polyolefin (polyolefin having a weight average molecular weight of less than 7 × 10 ⁵ ) other than the ultrahigh molecular weight polyolefin in the polyolefin composition is preferably 1 × 10 ⁴ or more. It is not preferable to use a polyolefin having a weight average molecular weight of less than 1 × 10 ⁴ , since breakage is likely to occur during stretching and a desired microporous film cannot be obtained. Therefore, it is preferable to blend a polyolefin having a weight average molecular weight of 1 × 10 ⁴ or more and less than 7 × 10 ⁵ with the ultrahigh molecular weight polyolefin.

The polyolefin containing an ultrahigh molecular weight component as described above may contain various additives such as an antioxidant, an ultraviolet absorber, an antiblocking agent, a pigment, a dye, and an inorganic filler, if necessary. It can be added within a range that does not impair the purpose of the present invention.

[2] Production Method Next, a method of producing the microporous polyolefin membrane of the present invention using the above-described ultrahigh molecular weight component-containing polyolefin will be described.

First, the above-mentioned polyolefin is supplied to an extruder and melted. The melting temperature depends on the type of polyolefin used, but the melting point of polyolefin + 30
~ 100 ° C is preferred. Here, the melting point is based on JIS K 7121.
The value measured by DSC (hereinafter the same).
For example, in the case of polyethylene, 160-230 ° C., particularly 170 ° C.
~ 200 ° C, preferably in the case of polypropylene
It is preferably between 190 and 270 ° C, especially between 190 and 250 ° C.

Next, a solvent liquid in the molten state (a good solvent for the above-mentioned polyolefin) is supplied to the polyolefin in the molten state from the middle of the extruder. As the solvent, for example, an aliphatic or cyclic hydrocarbon such as nonane, decane, decalin, p-xylene, undecane, dodecane, and liquid paraffin, or a mineral oil fraction having a boiling point corresponding to these can be used. .

The solvent as described above preferably has a viscosity at 25 ° C. of 30 to 500 Cst, particularly 50 to 200 Cst. If the viscosity at 25 ° C. is less than 30 Cst, uneven discharge occurs and kneading is difficult. On the other hand, if the viscosity exceeds 500 Cst, it becomes difficult to remove the solvent in a subsequent step.

The mixing ratio of the polyolefin and the solvent is 10 to 80% by weight, preferably 15 to 70% by weight, with the total of the polyolefin and the solvent being 100% by weight, and the solvent is 90 to 20% by weight. Preferably it is 85 to 30% by weight. When the polyolefin content is less than 10% by weight (when the solvent content exceeds 90% by weight), when forming into a sheet, the swell and neck-in are large at the die outlet, making it difficult to form the sheet. On the other hand, if the polyolefin exceeds 80% by weight (the solvent is less than 20% by weight), it is difficult to prepare a uniform solution.

It is necessary to supply the solvent to the polyolefin in a molten state from the middle of the extruder using an extruder having a side feeder or the like in the middle. When a polyolefin containing an ultrahigh molecular weight polyolefin and a solvent are supplied at the same time, mixing is not possible because the viscosity difference is too large, and the polyolefin and the screw of the extruder co-rotate to prepare a solution.

By adding a solvent to the polyolefin in a molten state and kneading it in an extruder in this manner, a high-concentration polyolefin solution having a uniform concentration can be prepared in a short time.

Next, the heated solution of the polyolefin thus melt-kneaded is directly or through another extruder, or once cooled and pelletized, and then again through an extruder, into a die or the like. And extruded.
As the die, a sheet die having a generally rectangular base shape is used. When a sheet die is used, the die gap is usually 0.1 to 5 mm.
Heated to 0-250 ° C. At this time, the extrusion speed is usually
20 to 30 cm / min to 5 to 10 m / min.

The solution thus extruded from the die or the like is formed into a gel by cooling. Cooling is performed at least to a gelling temperature or lower. 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 extruded solution may be taken off 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.

Next, the gel composition is stretched. Stretching is performed at a predetermined magnification by heating the gel composition and subjecting it to a usual tenter method, roll method, rolling method or a combination of these methods. Biaxial stretching is preferred, and either vertical or horizontal simultaneous stretching or sequential stretching may be used, but simultaneous biaxial stretching is particularly preferred.

The stretching temperature is the melting point of polyolefin + 10 ° C.
Hereinafter, the range is preferably from the crystal dispersion temperature to less than the melting point. Here the crystal dispersion temperature and, based on A STM D 4065 soluble
It means the value obtained by the melt viscoelasticity measurement (hereinafter the same). For example, in the case of polyethylene, the temperature ranges from 90 to 140 ° C, more preferably from 100 to 130 ° C. Stretching temperature is melting point +10
If the temperature exceeds ℃, it is not preferable because effective orientation of molecular chains cannot be achieved by stretching due to melting of the resin. On the other hand, if the stretching temperature is lower than the crystal dispersion temperature, the resin is insufficiently softened, the film is easily broken in stretching, and high-magnification stretching cannot be performed.

Although the stretching ratio varies depending on the thickness of the raw material, it is at least twice or more, preferably from 3 to 1 in the uniaxial direction.
The magnification is 30 times and the area magnification is 10 times or more, preferably 15 to 400 times. If the area ratio is less than 10 times, stretching is insufficient and porosity is high, 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 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 is 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.
1 to 0.5 μm and a tensile strength at break of 500 g / cm ² 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 may be further subjected to surface modification such as plasma irradiation, surfactant impregnation, surface grafting and other hydrophilic treatments, if necessary.

[0036]

In the present invention, a polyolefin solution containing an ultrahigh molecular weight component is melted in an extruder, and a solvent is supplied thereto to prepare a polyolefin solution, which is extruded from a die into a sheet and cooled. Thus, a gel composition is formed, and the gel composition is heated and stretched, and then the remaining solvent is removed to produce a microporous polyolefin membrane. By such a method, it is possible to produce a polyolefin microporous membrane much more efficiently than in the past.

Although it is not always clear why such an effect is obtained, it takes time to prepare a polyolefin solution containing an ultrahigh molecular weight component.
There are problems such as difficulty in uniformizing the concentration of the solution and deterioration of the solution.However, a solvent is supplied to a polyolefin containing an ultrahigh molecular weight component in a molten state in an extruder, and a solvent is further kneaded. This is presumably because by preparing the solution, it is possible to prepare a uniform solution of high-concentration polyethylene in a short time.

[0038]

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
/ Min, measured by gel permeation chromatography (GPC). (2) Film thickness: The cross section was measured by a scanning electron microscope (unit: μm). (3) Average pore diameter: Measured by a nitrogen adsorption / desorption type pore diameter measuring instrument (manufactured by Nikkaki Co., Ltd.) (unit: μm). (4) Tensile breaking strength: Measured according to ASTM D882 (unit: kg / cm ² ). (5) Air permeability: Measured according to JIS P8117 (unit: seconds
/ 100cc).

Examples 1 to 5 17.6 parts by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 2.5 × 10 ^{6 and} a weight average molecular weight (Mw)
w) A raw material resin of Mw / Mn = 14.2 mixed with 82.4 parts by weight of 3.3 × 10 ⁵ polyethylene (PE) (melting point: 135 ° C., crystallization
Dispersion temperature 90 ° C) and 2,6-di-t-butyl-p as antioxidant
-0.125 parts by weight of cresol (BHT, manufactured by Sumitomo Chemical Co., Ltd.) and tetrakis [methylene-3- (3,5-di-t-butyl-4-
(Hydroxylphenyl) -propionate] methane (Irganox 1010, Ciba-Geigy) 0.25 parts by weight and dry-blended with a twin screw extruder (58 mmφ, L / D = 4).
2, strong kneading type). In addition, liquid paraffin (64 Cst / 40) is supplied from the side feeder of this twin-screw extruder.
° C) is the ratio (resin component +
Liquid paraffin = 100% by weight).
The mixture was melt-kneaded at 0 rpm to prepare a polyethylene solution in an extruder.

Subsequently, the T set at the tip of the extruder
The sheet was extruded from the die at 190 ° C. and taken up by a cooling roll to form a gel-like sheet. Next, this gel-like sheet is
Stretch film formation was performed under the conditions shown in Table 1. The obtained stretched membrane was washed with methylene chloride to extract and remove the remaining liquid paraffin, and then dried to obtain a polyethylene microporous membrane.

Table 1 shows the composition and production conditions of the microporous polyethylene membrane. The thickness, tensile strength at break, average pore size, and air permeability of the microporous polyethylene film were measured. The results are shown in Table 1.

Comparative Example 1 A microporous polyethylene membrane was produced in the same manner as in Example 1 except that liquid paraffin was added so that the concentration of the resin component (ultra high molecular weight polyethylene + polyethylene) became 85% by weight. It was difficult to make a uniform solution of the polyolefin in the extruder, and the sheet could not be formed.

Comparative Example 2 A microporous polyethylene membrane was produced in the same manner as in Example 1 except that liquid paraffin was added so that the concentration of the resin component (ultrahigh molecular weight polyethylene + polyethylene) was 5% by weight. The discharge in the extruder was not uniform, and the swell was large at the die exit, so that the sheet could not be formed.

Comparative Example 3 17.6 parts by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight (Mw) of 2.5 × 10 ^{6 and} a weight average molecular weight (Mw)
w) A raw material resin of Mw / Mn = 14.2 mixed with 82.4 parts by weight of 3.3 × 10 ⁵ polyethylene (PE) (melting point: 135 ° C., crystallization
Dispersion temperature 90 ° C) and 2,6-di-t-butyl-p as antioxidant
-0.125 parts by weight of cresol (BHT, manufactured by Sumitomo Chemical Co., Ltd.) and tetrakis [methylene-3- (3,5-di-t-butyl-4-
(Hydroxylphenyl) -propionate] methane (Irganox 1010, Ciba-Geigy) 0.25 parts by weight and liquid paraffin (64 Cst / 40 ° C.) at a resin component concentration of 15
% By weight (resin component + liquid paraffin = 100% by weight), put it into a vessel-type dissolution tank, and uniformly increase the temperature from 100 ° C to 180 ° C at 10 ° C / Hr under high stirring conditions. Attempt to prepare a suitable polyethylene solution,
It took time.

Subsequently, this solution was supplied to an extruder, extruded at 190 ° C. from a T-die provided at the tip of the extruder, and formed into a gel-like sheet while being taken up by a cooling roll. Subsequently, the gel-like sheet was subjected to a temperature of 115 ° C. and a stretching speed of 3.5 m /
Simultaneous biaxial stretching was performed 5 × 5 times per minute. The obtained stretched membrane was washed with methylene chloride to extract and remove the remaining liquid paraffin, and then dried to obtain a microporous polyethylene membrane having a thickness of about 25 μm.

Table 1 shows the composition and production conditions of the microporous polyethylene membrane. The thickness, tensile strength at break, average pore size, and air permeability of the microporous polyethylene film were measured. The results are shown in Table 1.

Comparative Example 4 In Comparative Example 3, the solution was charged into a vessel-type dissolution tank such that the concentration of the resin component was 30% by weight (resin component + liquid paraffin = 100% by weight), and 100 ° C. under high stirring conditions. To 10 ° C
When an attempt was made to prepare a polyethylene solution while raising the temperature to 180 ° C./Hr, it took 10 hours. However,
In the above solution, since the concentration of polyethylene was too high, the viscosity of the solution was too high, and a solution having a uniform polyethylene concentration could not be obtained.

[0048]

[0049]

As is apparent from Table 1, the microporous polyethylene membranes obtained by the methods of Examples 1 to 5 had microporosity and had good strength and air permeability. Although the same microporous membrane as that of the method of the present invention can be obtained by the method of Comparative Example 3, it takes as long as 10 hours to prepare a uniform polyethylene solution in this method. .

[0051]

As described in detail above, according to the present invention,
A polyolefin solution containing an ultrahigh molecular weight component is melted in an extruder, a solvent is supplied to the solution to prepare a polyolefin solution, and this solution is extruded into a sheet from a die and cooled to form a gel composition. Then, since the gel composition is heated and stretched, and then the remaining paraffin oil is removed to produce the microporous polyolefin membrane, it is possible to efficiently produce the microporous polyolefin membrane.

The microporous polyolefin membrane according to the method of the present invention can be used as a separator for a battery such as a lithium battery, a diaphragm for an electrolytic capacitor, an ultrafine filtration membrane, an ultrafiltration membrane, various filters, a porous material for a moisture-permeable waterproof clothing. Suitable for various applications such as membranes.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-163938 (JP, A) JP-A-4-261441 (JP, A) JP-A-5-94812 (JP, A) JP-A-5-94812 156058 (JP, A) JP-A-5-25305 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08J 9/00, 9/26

Claims (3)

(57) [Claims] 1. A (a) melting a weight average molecular weight 7 × 10 ⁵ or more polyolefins containing 1 wt% or more of the components in an extruder, to the polyolefin 10 to 80 wt% of (b) a molten state , said (good solvent for the polyolefin) 90 to 20 wt% liquid solvent in the molten state is supplied from the middle of the extruder were melt-kneaded, (c) homogeneous polyolefin solution obtained into sheet form from the die A method for producing a microporous polyolefin membrane, comprising: extruding and cooling to form a gel-like sheet ; (d) heating and stretching the gel-like sheet ; and (e) removing the solvent remaining thereafter. 2. A battery separator using a polyolefin microporous membrane produced by the method according to claim 1. 3. A filter using the microporous polyolefin membrane produced by the method according to claim 1.