JPH09302120A - Production of finely porous membrane of polyolefin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing the subject mambrane high in strength, useful as a separator for batteries, etc., at a low cost in high productivity, by heating and drawing a solution of an ultrahigh molecular weight polyolefin. SOLUTION: A solution comprising 10-80% of a polyolefin composition containing >=1wt.% of an ultrahigh molecular weight polyolefin having >=7×10<5> weight-average molecular weight and 90-20wt.% of a solution is prepared. The solution is extruded from a die having 0.5-2.5mm die gap at 120-250 deg.C die temperature into a sheet shape, drawn at a draw ratio of 110-5,000% while cooling to form a gelatinous molding. The gelatinous molding is heated and drawn at least in the uniaxial direction at 5-50 times and then the residual solvent is removed. Preferably the solution is extruded from a die having 1.5-2.5mm die gap at 160-230 deg.C die temperature into a sheet shape and drawn at 110-1,000% draw ratio while cooling to form a gelatinous molding.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a microporous membrane composed of a polyolefin composition containing an ultrahigh molecular weight polyolefin, and particularly to produce a high-strength microporous polyolefin membrane at low cost and with high productivity. Regarding the method.

[0002]

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

Various methods for producing a high-strength microporous membrane using an ultrahigh molecular weight polyolefin have been proposed. For example, JP-A-60-242035 and JP-A-61-495312
JP-A-61-195133, JP-A-63-396
02, JP-A-63-273651, etc., a gel-like sheet is formed from a solution obtained by heating and dissolving a polyolefin composition containing an ultrahigh molecular weight polyolefin, and the gel-like sheet is heated and stretched, and the solvent is extracted and removed. Describes a method for producing a microporous membrane.

[0004]

However, in the above invention, a large number of fine pores are formed in the gel-like sheet by a sequential or simultaneous biaxial stretching method. There is a problem in cost and productivity because a microporous membrane having high strength, fine pores with a sharp pore size distribution cannot be obtained.

Therefore, the present invention solves the above-mentioned problems in the conventional method for producing a microporous membrane, and provides a high-strength polyolefin microporous membrane having fine pores at low cost and high productivity. It is to provide a method of manufacturing in.

[0006]

Means for Solving the Problems The present inventors have extruded a solution of an ultrahigh molecular weight polyolefin into a sheet form while performing shear crystallization under a specific condition, and obtain a gel form obtained at a specific take-up ratio. It was found that a microporous membrane with low cost, high productivity and high strength can be obtained by stretching a sheet in at least a uniaxial direction within a specific range, and conceived the present invention.

That is, the present invention has a weight average molecular weight of 7 ×
1% by weight of 10 ⁵ or more ultra high molecular weight polyolefin component
Polyolefin composition containing 10 to 80% by weight
And a solution consisting of 90 to 20% by weight of a solvent are prepared, and the solution is applied from a die having a die gap of 0.5 to 2.5 mm,
Extrude into a sheet at a die temperature of 120 to 250 ° C., take it off with cooling at a take-up ratio of 110 to 5000% to form a gel-like molded article, and heat the gel-like molded article 5 to 50 times in at least one axial direction. A method for producing a polyolefin microporous membrane, which comprises stretching and then removing the remaining solvent.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail below.

The polyolefin microporous membrane produced in 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. If the content of the component having a weight average molecular weight of 7 × 10 ⁵ or more in the polyolefin is less than 1% by weight, the entanglement of the molecular chains of the ultrahigh molecular weight polyolefin that contributes to the improvement of the stretchability is insufficient, so that the strength is sufficiently improved. Will be difficult. On the other hand, the upper limit of the content of the ultrahigh molecular weight component is not particularly limited, but if it exceeds 90% by weight, it is difficult to achieve the desired high concentration of the polyolefin solution and to stretch it, which is not preferable. In addition,
It is preferable that the polyolefin does not substantially contain a component having a molecular weight of 1 × 10 ³ or less. Further, the molecular weight distribution (weight average molecular weight / number average molecular weight) of the above polyolefin is preferably 300 or less, particularly preferably 5 to 50. When the molecular weight distribution exceeds 300, the low molecular weight component is broken during stretching and the strength of the entire film is reduced, which is not preferable. As the above-mentioned polyolefin, ethylene, propylene, 1
Examples include crystalline homopolymers obtained by polymerizing butene, 4-methyl-pentene-1, 1-hexene and the like, two-stage polymers, copolymers and blends thereof. Among these, polypropylene, polyethylene (particularly high-density polyethylene), and compositions thereof are preferred.

As long as the polyolefin has the above-mentioned molecular weight and molecular weight distribution, it may be one obtained by multi-stage polymerization or a composition of two or more kinds of polyolefins.

In the case of multi-stage polymerization, for example, multi-stage polymerization of olefins is carried out so that the ultra-high molecular weight component having a weight average molecular weight of 7 × 10 ⁵ or more is contained in an amount of 1 wt% or more and the molecular weight distribution is 300 or less. It can be manufactured.
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 kinds of polyolefins, a homopolymer or copolymer of the above-mentioned olefins, an ultrahigh molecular weight polyolefin having a weight average molecular weight of 7 × 10 ⁵ or more, and a weight average molecular weight of 7 × 10 5. It can be obtained by mixing an appropriate amount of a polyolefin having a molecular weight of less than ^{5 so} that the molecular weight distribution falls within the above range.

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

The lower limit of the molecular weight of the polyolefin other than the ultrahigh molecular weight polyolefin (polyolefin having a weight average molecular weight of less than 7 × 10 ⁵ ) in the polyolefin composition is preferably 1 × 10 ⁴ or more. When a polyolefin having a weight average molecular weight of less than 1 × 10 ⁴ is used,
It is not preferable because breakage easily occurs during stretching and the desired microporous membrane cannot be obtained. Therefore, the weight average molecular weight is 1
× preferably blended with 10 ⁵ or more 7 × 10 ⁵ less than the polyolefin ultra high molecular weight polyolefin. In addition, the polyolefin containing the ultrahigh molecular weight component as described above, if necessary, various additives such as antioxidants, ultraviolet absorbers, antiblocking agents, pigments, dyes, inorganic fillers of the present invention. It can be added in a range that does not impair the purpose.

The method for producing a microporous membrane of the present invention comprises heating and dissolving the above polyolefin composition in a solvent,
Prepare solution. Nonane, decane,
An aliphatic or cyclic hydrocarbon such as decalin, p-xylene, undecane, dodecane, liquid paraffin, or a mineral oil fraction having a boiling point corresponding thereto can be used. The viscosity of the solvent at 25 ° C. is preferably 30 to 500 cSt, particularly preferably 50 to 200 cSt. If the viscosity at 25 ° C. is less than 30 cSt, non-uniform discharge occurs, and kneading is difficult.
If it exceeds 00 cSt, it will not be easy to remove the solvent in the subsequent step.

The heating dissolution is carried out by stirring the polyethylene composition at a temperature at which it is completely dissolved in a solvent, or by uniformly mixing and dissolving it in an extruder. When dissolved in a solvent while stirring, the temperature varies depending on the polymer and solvent used, but in the case of a polyethylene composition, for example, it is in the range of 140 to 250 ° C. When producing a microporous membrane from a high-concentration solution of the polyolefin composition, it is preferable to dissolve it in an extruder.

In the case of melting in the extruder, first, the above-mentioned polyolefin is supplied to the extruder and melted. The melting temperature depends on the type of polyolefin used,
The melting point of polyolefin +30 to 100 ° C is preferable. For example, in the case of polyethylene, 160 to 230 ° C, especially 1
It is preferably 70 to 200 ° C, and in the case of polypropylene, 190 to 270 ° C, particularly 190 to 250 ° C. Next, a liquid solvent is supplied to the molten polyolefin from the middle of the extruder.

The blending ratio of the polyolefin and the solvent is 10 to 80% by weight, preferably 15 to 70% by weight, based on 100% by weight of the total of the polyolefin and the solvent.
% By weight, solvent 90 to 20% by weight, preferably 8
It is 5 to 30% by weight. When the content of polyolefin is less than 10% by weight (when the content of solvent exceeds 90% by weight), swell and neck-in are large at the die exit when forming into a sheet, and it becomes difficult to form the sheet. On the other hand, when the polyolefin content exceeds 80% by weight (when the solvent content is less than 20% by weight),
It is difficult to prepare a uniform solution.

The solvent must be supplied to the molten polyolefin from the middle of the extruder by using an extruder having a side feeder or the like in the middle. When the polyolefin containing the ultra high molecular weight polyolefin and the solvent are simultaneously supplied, the viscosity difference is too large to mix them, and the polyolefin and the screw of the extruder co-rotate, so that the solution cannot be prepared. Thus, by adding the solvent to the molten polyolefin and kneading it in the extruder, a highly concentrated solution of the polyolefin having a uniform concentration can be prepared in a short time.

Next, the melted and kneaded polyolefin heated solution is directly or through another extruder, or once cooled and pelletized, and then again through the extruder, a die or the like. To extrude and mold.

As the die, a sheet die having a rectangular die shape is usually used. The die gap is 0.5 to 2.5 mm, and preferably 1.5 to 2.5 mm for performing shear crystallization. If it is less than 0.5 mm, too much shear is applied to cause rough surface, and if it exceeds 2.5 mm, a sheet cannot be obtained. Extrusion temperature is 150
To 250 ° C, preferably 160 to 230 ° C. If the temperature exceeds 250 ° C, deterioration occurs, which is not preferable, and if the temperature is lower than 150 ° C, it is difficult to transfer the polyolefin solution and the discharge becomes unstable, which is not preferable. 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 cooled to form a gel-like molded product. Cooling is preferably performed at a rate of 50 ° C./min or more up to at least the gelation temperature. In general, when the cooling rate is low, the higher order structure of the obtained gel-like composition becomes coarse, and the pseudo cell unit that forms it becomes large. However, when the cooling rate is high, the cell unit becomes dense. When the cooling rate is less than 50 ° C./minute, the crystallinity increases and it is difficult to obtain a gel composition suitable for stretching. As a cooling method, a method of directly contacting with cold air, cooling water, or other cooling medium, a method of contacting with a roll cooled with a refrigerant, or the like can be used.

The gel-like sheet extruded from the die has 1
It is cooled and solidified while being collected at a high take-up ratio of 10 to 5000%, preferably 110 to 1000%, and sheared and crystallized. If the take-up ratio exceeds 5000%, neck-in becomes large, and breakage easily occurs during stretching, which is not preferable, and if it is less than 110%, shear crystallization does not sufficiently occur and a microporous membrane cannot be obtained. It is also possible to use a bank molding method when taking it back.

Next, this gel-like molded product is stretched in at least a uniaxial direction. As for the stretching, the gel-like molded product is heated and stretched 5 to 50 times, preferably 10 to 20 times by a normal tenter method. This stretching causes cleavage between crystals,
A hole is formed. When the magnification exceeds 50 times, there are restrictions in the stretching operation and the like. If the magnification is less than 5 times, sufficient cleavage between crystals is not performed, and a satisfactory microporous membrane cannot be obtained.

The stretching temperature is not higher than the melting point of polyethylene + 10 ° C., preferably in the range from the crystal dispersion temperature of polyethylene to less than the crystal melting point.

The obtained stretch molded product is washed with a solvent to remove the remaining solvent. Examples of cleaning solvents include hydrocarbons such as pentane, hexane and heptane, chlorinated hydrocarbons such as methylene chloride and tetrasalt carbon, fluorinated hydrocarbons such as trifluoroethane, ethers such as diethyl ether and dioxane. A volatile one can be used. These solvents are appropriately selected according to the solvent used for dissolving the polyolefin composition, 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 above-mentioned washing is carried out 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.

Further, the pore diameter can be controlled by stretching the obtained microporous membrane successively or simultaneously with a biaxial stretching machine at a ratio of 4-49 times, preferably 25-49 times.

The polyethylene microporous membrane produced as described above has a porosity of 35 to 95% and an average through pore diameter of 0.
001-0.5μ, and breaking strength is 500 kg / cm
² or more. The thickness of the polyethylene microporous membrane of the present invention may be appropriately selected depending on the application, but is generally 0.1 to
It can be 100 μ, and preferably 2 to 50 μ.

The obtained polyethylene microporous membrane is
If necessary, further plasma irradiation, surfactant impregnation,
Surface modification such as hydrophilic treatment such as surface grafting can be performed.

[0031]

EXAMPLES The present invention will be described below in more detail with reference to examples, but the present invention is not particularly limited to the examples. In addition, the test method in an Example is as follows. (1) Film thickness: The cross section was measured with a scanning electron microscope. (2) Air permeability: Measured according to JIS P8117. (3) Breaking strength: measured according to ASTM D882. (4) Average pore size: Omni soap 360 (Nikkiso Co., Ltd.)
Measured by.

Example 1 5.5 parts by weight of ultra high molecular weight polyethylene having a weight average molecular weight of 2 × 10 ⁶ , 24.5 parts by weight of high density polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ , and an antioxidant of polyethylene 0.375 parts by weight per 100 parts by weight was charged into a twin-screw extruder (58 mmφ, L / D = 42, strong kneading type). Further, 70 parts by weight of liquid paraffin was supplied from the side feeder of this twin-screw extruder and melt-kneaded to prepare a polyethylene solution in the extruder.

Subsequently, the T installed at the tip of the extruder.
A gel sheet was formed by extruding from a die at a die gap of 1.5 mm and a temperature of 180 ° C., and taking it with a cooling roll at a take-up ratio of 500%. Then, apply this gel-like sheet to 1
Lateral uniaxial stretching was performed 5 times at 15 ° C to obtain a stretched film. The stretched film obtained was washed with methylene chloride to extract and remove the residual liquid paraffin, followed by drying and heat treatment to obtain a polyethylene microporous film. Table 1 shows the results of evaluating the physical properties of this polyethylene microporous membrane.

Example 2 A microporous membrane was obtained in the same manner as in Example 1 except that the stretching ratio was 10 times in the lateral uniaxial direction. The microporous membrane obtained as described above had the physical properties shown in Table 1.

Example 3 A microporous membrane was obtained in the same manner as in Example 2 except that the take-up ratio was 1000%. The microporous membrane obtained as described above had the physical properties shown in Table 1.

Example 4 A microporous membrane was obtained in the same manner as in Example 3 except that 30 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 1 × 10 ⁶ was used as polyethylene. The microporous membrane obtained as described above had the physical properties shown in Table 1.

Example 5 In Example 3, 7 parts by weight of ultrahigh molecular weight polyethylene having a weight average molecular weight of 2.5 × 10 ⁶ was used as polyethylene, and 33 parts by weight of high-density polyethylene having a weight average molecular weight of 3.0 × 10 ^5. A microporous membrane was obtained in the same manner as in Example 3 except that 60 parts by weight of liquid paraffin was used as the solvent.
The microporous membrane obtained as described above had the physical properties shown in Table 1.

Example 6 In Example 1, the stretching ratio is 25 in the biaxial direction in terms of areal ratio.
A microporous membrane was obtained in the same manner as in Example 1 except that the number of times was doubled.
The microporous membrane obtained as described above had the physical properties shown in Table 1.

Example 7 A microporous membrane was obtained in the same manner as in Example 2 except that the die temperature was 200 ° C., the die gap was 2.5 mm, and 200% take-off was performed by bank molding. The microporous membrane obtained as described above had the physical properties shown in Table 1.

Comparative Example 1 In Example 1, the die temperature was set to 20 under the sheet forming conditions.
0 ℃, die gap 2.5mm, take-back ratio 100%
A microporous membrane was obtained in the same manner as in Example 1 except that the stretching was 10 times in the horizontal axis direction. The microporous membrane obtained as described above had the physical properties shown in Table 2.

Comparative Example 2 In Example 1, the die temperature was set to 20 under the sheet forming conditions.
0 ℃, die gap 0.3mm, take-back ratio 100%
A microporous membrane was obtained in the same manner as in Example 1 except that the stretching was 10 times in the horizontal axis direction. The microporous membrane obtained as described above had the physical properties shown in Table 2.

[0042]

[Table 1]

[0043]

[Table 2] As is clear from Tables 1 and 2, the take-up ratio is 110-110.
It can be seen that a high-strength microporous film can be obtained by setting the content to 5000%.

[0044]

As described in detail above, according to the method of the present invention, a gel-like sheet is prepared from a solution of a polyolefin composition containing an ultrahigh molecular weight polyolefin, with a specific die gap, a specific die temperature, and a specific take-up ratio. By molding with, a high-strength microporous membrane can be easily obtained.

Claims (2)

[Claims] 1. A polyolefin composition containing 10 to 80% by weight of an ultrahigh molecular weight polyolefin component having a weight average molecular weight of 7 × 10 ⁵ or more and 1% by weight or more, and a solvent of 90 to 20.
A solution consisting of 10 wt% is prepared, and the solution is applied from a die having a die gap of 0.5 to 2.5 mm and a die temperature of 120 to
Extruded into a sheet at 250 ° C., and with cooling at a take-up ratio of 110 to 5000%, a gel-like molded product is formed,
A method for producing a polyolefin microporous membrane, which comprises heat-drawing a gel-like molded article at least uniaxially by a factor of 5 to 50, and then removing the remaining solvent. 2. The method for producing a polyolefin microporous film according to claim 1, wherein the solution has a die gap of 1.
From the die of 5 to 2.5 mm, the die temperature is 160 to 230
Extruded into a sheet at ℃, take-up ratio 110 while cooling
A method for producing a polyolefin microporous membrane, characterized in that the gel-like molded product is formed by taking it out at ˜1000%.