JP2002128943A - Polyolefin microporous film and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyolefin microporous film excellent in the balance among gas permeability, porosity, fine pore size, compression characteristics, mechanical strengths, dimensional stability, shut-down characteristics and melt- down characteristics, and its manufacturing method. SOLUTION: The polyolefin microporous film comprises a polyolefin composition (A) comprising as essential ingredients a polyolefin (A1) having a weight- average molecular weight of at least 5×105 and a polyolefin (A2) having a weight-average molecular weight of at least 1×104 and less than 5×105, where the polyolefin (A2) is a linear polyolefin having a weight-average molecular weight/number-average molecular weight ratio of at least 1 and less than 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microporous polyolefin membrane and a method for producing the same, and relates to air permeability, porosity, fine pore size, compression characteristics, mechanical strength, dimensional stability, shutdown characteristics, and meltdown. The present invention relates to a microporous polyolefin membrane having an excellent balance of properties and a method for producing the same.

[0002]

2. Description of the Related Art Microporous polyolefin membranes include battery separators, diaphragms for electrolytic capacitors, various filters, moisture-permeable waterproof clothing, reverse osmosis filtration membranes, ultrafiltration membranes and microfiltration membranes. Used for various purposes.

When a microporous polyolefin membrane is used as a battery separator, particularly as a separator for a lithium ion battery, its performance is closely related to battery characteristics, battery productivity, and battery safety.

[0004] Regarding battery characteristics, it is desired to improve discharge characteristics in a low temperature range and increase output, and therefore, a separator is required to have high permeability. In addition, cycle characteristics,
It is also desired to improve the characteristics related to the battery life such as high-temperature preservability.

With regard to battery productivity, it is desired to increase the efficiency of the battery assembling process and to prevent internal minute short circuits caused by the pressure of impurities mixed on the electrodes. For this reason, separators are required to have high mechanical strength. .

When the microporous polyolefin membrane is used for a battery separator such as a lithium battery separator,
Its safety is of paramount importance. The battery separator melts the separator to prevent accidents such as fire when the temperature inside the battery rises during high charge or overheating preservation tests, etc., the function to shut off the current by clogging the holes (shutdown function) It is necessary that the temperature (shutdown temperature) be lower. In addition, immediately after the shutdown of the battery reaction due to the shutdown, the temperature inside the battery continues to rise instantaneously. Therefore, it is necessary to maintain the shape of the separator at a high temperature and prevent a short circuit of the electrode. That is, it is preferable that the breakage temperature (meltdown temperature) of the microporous polyolefin membrane is higher. Therefore, it is desired that the shutdown temperature is high and the meltdown temperature is high, and that the temperature range between the shutdown temperature and the meltdown temperature is large.

As described above, the lithium ion battery separator is required to have excellent permeability, mechanical properties, dimensional stability, shutdown properties, meltdown properties, and the like. Various proposals have been made for the manufacturing method.

For example, as a microporous polyolefin membrane having high strength and high elasticity, it is produced by using a composition containing an ultrahigh molecular weight polyolefin and having a value of (weight average molecular weight / number average molecular weight) within a specific range. (Japanese Patent Laid-Open No. 3-64334).

Further, as a microporous polyolefin membrane having high permeability, a nucleating agent is blended with a polyolefin containing an ultrahigh molecular weight component and having a value of (weight average molecular weight / number average molecular weight) within a specific range. A method for producing a composition by using a composition comprising the above-described compositions has been proposed (JP-A-5-222236, JP-A-5-222237, and JP-A-8-12799).

Further, as a method for producing a microporous polyolefin membrane having a low shutdown temperature and a high meltdown temperature, a wide temperature range between the shutdown temperature and the meltdown temperature, and excellent in permeability and mechanical strength,
(A) a method of using a composition obtained by adding a low-molecular-weight polyethylene and a specific polypropylene to the ultra-high-molecular-weight polyethylene or its composition (Japanese Patent Application Laid-Open No. 10-298325), and (b) a composition containing the ultra-high-molecular-weight polyethylene Proposed a method using a composition containing a substantially linear ethylene-α-olefin copolymer produced using a single-site catalyst having a specific melting point (Japanese Patent Application Laid-Open No. H11-269289). .

[0011] However, at present, the above-mentioned individual physical properties (permeability, mechanical properties, dimensional stability, shutdown properties, meltdown properties, etc.) are further improved, and the polyolefin microporous material is excellent in the balance of each physical property. A membrane is desired.

For example, recently, with regard to battery characteristics, characteristics related to battery life, such as cycle characteristics and high-temperature storage stability, tend to be emphasized. Therefore, as the mechanical properties desired for improving the battery life, it is preferable to have excellent compression properties as well as tensile strength / elongation and piercing strength which have been conventionally evaluated. Furthermore, it is necessary to suppress a voltage drop due to a minute short circuit due to dendrite growth, and from this viewpoint, it is preferable to have a fine through-hole diameter.

However, there has been no example in which a study focused on improving the compression characteristics of a microporous polyolefin membrane for a battery separator has been made so far. In addition, when the diameter of the through-hole is small, generally excellent mechanical strength and dimensional stability can be obtained, but the permeability is not sufficient.

Accordingly, an object of the present invention is to solve the above-mentioned disadvantages of the prior art, and to provide air permeability, porosity, fine pore size, compression characteristics, mechanical strength, dimensional stability, shutdown characteristics, and meltdown characteristics. To provide a microporous polyolefin membrane excellent in the balance of the above and a method for producing the same.

[0015]

Means for Solving the Problems As a result of intensive studies in view of the above problems, the present inventors have found that a polyolefin (A1) having a weight average molecular weight of 5 × 10 ⁵ or more and a weight average molecular weight of 1 × 10 ⁴ or more and 5 × 10 ⁵
A polyolefin composition (A) containing less than polyolefin (A2) as an essential component, wherein the polyolefin (A2) has a weight average molecular weight / number average molecular weight (hereinafter, referred to as Mw / Mn) of 1 or more and less than 10; The present inventors have found that the above problem can be solved by using a linear polyolefin, and have reached the present invention.

That is, in the microporous polyolefin membrane of the present invention, the polyolefin (A2) having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ is a linear polyolefin and its weight average molecular weight / number average molecular weight (hereinafter, referred to as “weight average molecular weight / number average molecular weight”). , Mw / Mn) is 1 or more and less than 10.

The polyolefin microporous membrane of the present invention exhibits more excellent properties when the polyolefin composition satisfies the following conditions (1) to (6). (1) The polyolefin (A1) has a weight average molecular weight of 1 × 10 ⁶
It is preferably a polyethylene or polypropylene of up to 15 × 10 ⁶ . (2) The polyolefin (A2) is preferably linear polyethylene or polypropylene. (3) The weight average molecular weight / number average molecular weight (hereinafter, referred to as “Mw / Mn”) of the polyolefin composition (A) is preferably from 5 to 300. (4) The polyolefin composition (A) is a polyolefin (A1)
Is an ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more, and the polyolefin (A2) has a weight average molecular weight of 1 × 10 ⁴
It is preferable that the composition is a polyethylene composition having a composition of a linear high-density polyethylene of 5 × 10 ^{5 or} less. (5) As the polyolefin composition (A), the composition according to (4) above, a branched low-density polyethylene, a linear low-density polyethylene, an ethylene-α-olefin copolymer produced using a single-site catalyst. Combined or molecular weight 1 × 10 ³ -4
It is preferable to use a composition to which at least one selected from low-molecular-weight polyethylene of × 10 ³ is added, since it has an effect of lowering the shutdown temperature when used as a battery separator. (6) As the polyolefin composition (A), a polyolefin
(A1) is an ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more, and polyolefin (A2) is a linear high density polyethylene and polypropylene each having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ . With a composition that is a mixture,
When used as a battery separator, it is preferable because it has an effect of improving the meltdown temperature.

Further, in the production method of the present invention, by satisfying the following conditions (7) to (9), more excellent compression characteristics,
A microporous polyethylene membrane exhibiting mechanical strength and dimensional stability is obtained. (7) The removal of the solvent is preferably performed after stretching. (8) It is preferable to dry after removing the solvent, and to heat-treat the obtained microporous membrane. (9) The heat treatment may be a treatment selected from any one of stretching, fixing and shrinking, or may be a combination of these treatments.

The microporous polyolefin membrane according to a preferred embodiment of the present invention has an air permeability of 10 to 2000 sec / 100 cc, an average through-hole diameter of 0.01 to 0.1 μm, a porosity of 25 to 80%, and a puncture strength.
3920mN / 25μm or more, tensile strength at break 50MPa or more, preferably 80MPa or more, tensile elongation at break 50% or more, heat shrinkage 13
% Or less, preferably 7% or less, and a shutdown temperature of 120%
140140 ° C., the meltdown temperature satisfies 160-190 ° C., and the air permeability after pressing at a pressure of 30 kPa and a temperature of 100 ° C. for 60 seconds can be 400% or less of that before pressing.

The microporous polyolefin membrane of the present invention can be suitably used as a battery separator and filter.

[0021]

DETAILED DESCRIPTION OF THE INVENTION [1] Polyolefin composition (1) Polyolefin (A1) The polyolefin (A1) having a weight average molecular weight of 5 × 10 ⁵ or more used in the present invention has a weight average molecular weight of 1 × 10 ⁶ to 15
× 10 ⁶ is preferred. If the weight average molecular weight is less than 5 × 10 ⁵ , the strength and the meltdown temperature of the microporous membrane will not be sufficiently high, and it will be difficult to obtain a suitable microporous membrane. Also, by setting the weight average molecular weight to 15 × 10 ⁶ or less,
Melt extrusion can be facilitated. Polyolefin (A
As 1), ultrahigh molecular weight polyethylene or ultrahigh molecular weight polypropylene is preferable, and ultrahigh molecular weight polyethylene is more preferable. The ultrahigh molecular weight polyethylene may be not only a homopolymer of ethylene but also a copolymer containing a small amount of another α-olefin. Other α-olefins other than ethylene include propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene,
Examples thereof include vinyl acetate, methyl methacrylate, and styrene. If the copolymerization amount of the other α-olefin is too large, the moldability, mechanical strength, and permeability are reduced. Therefore, care must be taken so that the amount is 20 mol% or less.

(2) Polyolefin (A2) The polyolefin (A2) having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ^{5 is} a polyolefin having a Mw / Mn of 1 or more and less than 10 and a substantially linear polyolefin. Need to be. As a result, excellent dimensional stability and mechanical strength can be obtained together with appropriate air permeability and porosity and a fine through-hole diameter, and particularly, compression characteristics can be improved.

On the other hand, when a polyolefin having Mw / Mn of 10 or more is used, the balance between mechanical strength and permeability can be maintained even if the weight average molecular weight is 1 × 10 ⁴ or more and less than 5 × 10 ^5. In addition to the need for improvement, since the compression characteristics are not good, when used as a battery separator, the cycle characteristics are not sufficient, and there is a high possibility that the performance of the battery deteriorates quickly.

If a non-linear polyolefin is used, the piercing strength tends to decrease.

As the polyolefin (A2), an olefin homopolymer such as linear polyethylene or polypropylene is preferable. Examples of polyethylene include high-density polyethylene, low-density polyethylene, and medium-density polyethylene. These are ethylene homopolymers. The use of a linear olefin homopolymer is more effective in improving the balance between mechanical strength and permeability. In addition, a copolymer containing a small amount of another α-olefin as an optional component may be used. Other α-olefins other than ethylene include propylene, butene-1, hexene-1,
Pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like are preferred.

As the polypropylene, a linear propylene homopolymer is preferable, but a block copolymer or a random copolymer of propylene and another α-olefin as an optional component can also be used. . Other α-olefins include ethylene, butene-1,
Hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, styrene and the like can be used. Preferred as the other α-olefin is ethylene.

The above-mentioned linear polyethylene used as the polyolefin (A2) may be a mixture of different kinds of polyethylene, and the above-mentioned linear polypropylene may be a mixture of different kinds of polypropylene. . Further, it may be a mixture of polyethylene selected from at least one of various polyethylenes and polypropylene selected from at least one of various polypropylenes.

(3) Compounding ratio The most preferred polyolefin composition (A) is an ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more, and a polyolefin (A2) having a weight average molecular weight of 1 × 10 ⁵ or more. × 10 ⁴ or more and less than 5 × 10 ⁵ and Mw / Mn is 1 or more and 10
A polyethylene composition comprising a composition that is less than and linear in high density polyethylene.

The content of the ultra-high molecular weight polyethylene in the above polyethylene composition is 100
It is preferably 1% by weight or more as a percentage by weight, and
More preferably, it is% by weight. If it exceeds 50% by weight,
The viscosity of the raw material solution increases and the moldability deteriorates. By using ultrahigh molecular weight polyethylene, the melt viscosity at a high temperature can be increased, and the meltdown temperature is improved.

When used in a battery separator, the weight average molecular weight is 5 × 10 ⁵ to reduce the shutdown temperature.
The above ultra high molecular weight polyethylene and weight average molecular weight 1 × 10
^A composition comprising ⁴ or more and less than 5 × 10 ⁵ and a linear high-density polyethylene having Mw / Mn of 1 or more and less than 10 is added to a low-density polyethylene or a weight average molecular weight of 1 as another polyolefin.
It is preferable to add a low molecular weight polyethylene of × 10 ³ to 4 × 10 ³ . The low-density polyethylene does not need to be linear, but is a branched low-density polyethylene (LDPE) by the high-pressure method, a linear low-density polyethylene (LLDPE) by the low-pressure method, and ethylene-α- using a single-site catalyst.
Olefin copolymers and the like can be mentioned, and at least one of them can be selected, and these can be used as a mixture. The low-density polyethylene and low-molecular-weight polyethylene used here necessarily have Mw / Mn of 1 or more and 10 or more.
It need not be less than. LDPE and LLDPE may be homopolymers or copolymers with other α-olefins. The other α-olefin and the ethylene-α-
The α-olefin in the olefin copolymer is at least one selected from propylene, butene-1, hexene-1, pentene-1, 4-methylpentene-1, octene, vinyl acetate, methyl methacrylate, and styrene. Can be used. When the copolymerization amount of these α-olefins is large, the effect of lowering the shutdown temperature is increased, but on the other hand, the moldability, mechanical strength, and permeability are reduced, so that
It is preferable that the copolymerization amount be 20 mol% or less.

When used for a battery separator,
In order to improve the tilt-down temperature,
Quantity 5 × 10^FiveUltra high molecular weight polyethylene and weight average molecule
Quantity 1 × 10^Four5 × 10 or more^FiveMw / Mn less than 1 and less than 10
Polyp to a composition consisting of linear high-density polyethylene
It is preferred to add propylene. This polypropylene
It is preferable that Mw / Mn is 1 or more and less than 10,
It may be the above. In addition, as polypropylene,
In addition to homopolymers, other α-olefins other than propylene
Uses block copolymer or random copolymer with resin
can do. Etylene as other α-olefin
, Butene-1, hexene-1, pentene-1, 4-methylpen
Ten-1, octene, vinyl acetate, methyl methacrylate,
Styrene or the like can be used. Also, its weight average
The molecular weight is weight average molecular weight 1 × 10 ^Four5 × 10 or more^FiveLess than
This is preferable because stretching becomes easy.

It is also possible to add both polyethylene selected from at least one of the various polyethylenes for lowering the shutdown temperature and polypropylene selected at least one from the various polypropylenes for improving the meltdown temperature.

At least one kind of polyethylene selected from the above-mentioned various kinds of polyethylene for lowering the shutdown temperature, at least one kind of polypropylene selected from the above-mentioned various kinds of polypropylene for improving the meltdown temperature, or a mixture thereof (hereinafter collectively referred to as “ Content of other polyethylene and / or polypropylene ”) is 3 to 50% by weight of the entire polyethylene composition as 100% by weight.
%, More preferably 5 to 40% by weight. Assuming that the entire polyethylene composition is 100% by weight, ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more, linear high density polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , and other polyethylenes And / or polypropylene is preferably 1 to 94% by weight, 1 to 94% by weight, 3 to 50% by weight, preferably 10 to 50% by weight,
More preferably, it is 20 to 80% by weight and 10 to 30% by weight.
If the content of other polyethylene and / or polypropylene is less than 3% by weight, the effect of lowering the shutdown temperature or improving the meltdown temperature will not be sufficient, and if it exceeds 50% by weight, the film-forming properties and the permeability and strength of the film will be reduced.

The polyolefin composition (A) of the present invention
Has a molecular weight distribution Mw / Mn of preferably from 5 to 300, more preferably from 5 to 100. When Mw / Mn is less than 5, the solution viscosity becomes too high, and the melt extrudability and stretchability deteriorate.
If it exceeds 0, the low molecular weight component becomes too large, which causes a decrease in strength. When Mw / Mn is 5 to 30, it is particularly effective for improving the transmittance.

[2] Method for Producing Polyolefin Microporous Membrane (1) Step of Adding a Solvent to Polyolefin Composition and Melting and Kneading to Prepare Polyolefin Solution In the production method of the present invention, first, polyolefin (A1) and polyolefin ( A suitable solvent is added to the polyolefin composition (A) containing A2) and melt-kneaded to prepare a polyolefin solution. If necessary, various additives such as an antioxidant, an ultraviolet absorber, an antiblocking agent, a pigment, a dye, and an inorganic filler can be added to the polyolefin solution as long as the object of the present invention is not impaired.

As the solvent, aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane and liquid paraffin, and mineral oil fractions having a boiling point corresponding to these can be used. In order to obtain a gel-like molded product having a stable solvent content, it is preferable to use a non-volatile solvent such as liquid paraffin.

The viscosity of the solvent at 25 ° C. is preferably 30 to 500 cSt, more preferably 50 to 200 cSt.
If the viscosity at 25 ° C. is less than 30 cSt, ejection from a non-uniform die lip occurs, and kneading is difficult. If it exceeds 500 cSt, solvent removal becomes difficult.

The method of melt kneading is not particularly limited, but is usually carried out by uniformly kneading in an extruder. This method is suitable for preparing a highly concentrated solution of polyolefin. Melting temperature is the melting point of polyolefin + 30 ℃ ～ ＋ 100 ℃
Is preferred, usually 160 ~ 230 ℃, preferably 170
More preferably, it is -200 ° C. Here, the melting point is JIS
A value determined by differential scanning calorimetry (DSC) based on K7121 (the same applies hereinafter). The solvent may be added before the start of kneading or may be added from the middle of the extruder during kneading, but it is preferable to add the solvent before kneading and to form a solution in advance. In melt kneading, it is preferable to add an antioxidant to prevent oxidation of the polyolefin.

In the polyolefin solution, the mixing ratio of the polyolefin composition (A) and the solvent is 100% by weight of the total of both.
The polyolefin composition (A) is preferably 1 to 50% by weight, more preferably 20 to 40% by weight. If the polyolefin composition (A) is less than 1% by weight, the solution viscosity will be low, the self-supporting property of the gel-like molded product will be reduced, the swell and neck-in will increase at the die exit, and molding will be difficult. On the other hand, when the content exceeds 50% by weight, the moldability of the gel-like molded product is reduced.

(2) Step of extruding the polyolefin solution from the die lip and cooling to form a gel-like molded product The melt-kneaded polyolefin solution is directly or through another extruder, or once cooled and pelletized. Extrude from the die lip again through the extruder. As the die lip, a sheet die lip having a generally rectangular base shape is used, but a double cylindrical hollow die lip, an inflation die lip, or the like can also be used. For die lip for sheet, the gap of die lip is usually 0.1
5050 mm and heated to 140-250 ° C. during extrusion.
The extrusion rate of the heated solution is preferably from 0.2 to 15 m / min.

By cooling the heated solution extruded from the die lip in this way, a gel-like molded product is formed. The cooling is preferably performed at a rate of 50 ° C./min or more at least up to the gelation temperature or less. In general, when the cooling rate is low, the higher-order structure of the obtained gel-like molded product becomes coarse, and the pseudo cell unit forming the same becomes large. However, when the cooling rate is high, the cell unit becomes dense. If the cooling rate is less than 50 ° C./min, the crystallinity increases, and it is difficult to obtain a gel-like molded product 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.

(3) Step of stretching the gel-like molded product and removing the solvent After the heating, the gel-like molded product is stretched to a predetermined temperature by a usual tenter method, a roll method, an inflation method, a rolling method or a combination of these methods. Perform at magnification. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferred. In the case of biaxial stretching, either simultaneous longitudinal or transverse stretching or sequential stretching may be used, but simultaneous biaxial stretching is particularly preferred.

Although the stretching ratio varies depending on the thickness of the gel-like molded product, it is preferably at least 2 times, more preferably 3 to 30 times for uniaxial stretching. In biaxial stretching, at least 2 times or more in any direction, preferably 10 times or more in area ratio,
More preferably, it is 15 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 area magnification exceeds 400 times,
There are restrictions on the stretching apparatus, stretching operation, and the like.

The stretching temperature is preferably not higher than the melting point of the polyolefin composition (A) + 10 ° C., and more preferably in the range from the crystal dispersion temperature to lower than the crystal melting point. If the stretching temperature exceeds the melting point + 10 ° C., the resin melts and the molecular chains cannot be oriented by stretching. When the stretching temperature is lower than the crystal dispersion temperature, the resin is insufficiently softened, and the film is easily broken in stretching,
Unable to stretch at high magnification. In the present invention, the stretching temperature is usually
The temperature is 100 to 140 ° C, preferably 110 to 120 ° C. Here, the crystal dispersion temperature means a value obtained by measuring temperature characteristics of dynamic viscoelasticity based on ASTM D 4065 (the same applies hereinafter).

The solvent can be removed before and / or after the stretching, but is preferably performed after the stretching, and it is preferable to extract and remove the residual solvent by washing with a volatile solvent. Examples of the volatile 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. Can be used. These solvents are used in polyolefin compositions.
It is appropriately selected according to the solvent used for dissolving (A), and used alone or as a mixture.

For removing the solvent, a non-aqueous solvent having a boiling point of 100 ° C. or more and a flash point of 0 ° C. or more can also be used. The non-aqueous solvent has low load on the environment because of its low volatility, and has low risk of explosion in the drying step, and is safe in use. Further, since it has a high boiling point, it is easily condensed, easily collected, and easily recycled.

As the above-mentioned non-aqueous solvent, it is preferable to use a solvent which has compatibility with the residual solvent and does not have compatibility with the polyolefin composition (A). For example, boiling point 100
Examples include paraffinic compounds having a flash point of 0 ° C. or higher, aromatics, alcohols, esters, ethers, ketones and the like.

The washing method can be carried out by a method of immersing in a volatile solvent or a non-aqueous solvent for extraction, a method of showering a volatile solvent or a non-aqueous solvent, or a combination thereof. The above-described cleaning is performed until the residual solvent becomes less than 1% by weight.

(4) Heat Treatment Step In the present invention, heat treatment may be performed after drying. The heat treatment stabilizes the crystal and makes the lamella layer uniform.

As the heat treatment, any of a heat stretching treatment, a heat setting treatment, and a heat shrink treatment can be used. These treatments are carried out below the melting point of the polyolefin, preferably
Perform at a temperature of 60 ° C or higher and the melting point or lower.

The hot stretching is carried out by a commonly used tenter method, roll method, or rolling method, and is preferably performed in at least one direction at a draw ratio of 1.01 to 4.0 times, more preferably 1.8 to 3.0 times.

The heat setting process is performed by a tenter method, a roll method, and a rolling method.

The heat shrinkage treatment can be performed by a tenter method, a roll method, or a rolling method, or in particular, by performing a heat treatment without fixing the film. For example, it can be performed using a belt conveyor or floating. Further, when the film is wound by using a roll, heat may be applied to the roll. in this case,
The heat shrinkage can be improved. The heat shrinkage treatment is preferably performed in at least one direction in a range of 50% or less, more preferably in a range of 30% or less.

In the present invention, the above-described heat stretching, heat setting, and heat shrinking may be performed in combination.

In particular, it is preferable to perform a heat shrinkage treatment after the heat stretching treatment since a microporous polyolefin film having a low heat shrinkage, high mechanical strength and excellent compression properties can be obtained.

(5) Step of drying the obtained film and performing a crosslinking treatment by ionizing radiation After drying the film obtained by stretching and removing the solvent by a heat drying method, an air drying method or the like, a crosslinking treatment is performed by ionizing radiation. Is preferred. Thereby, the meltdown temperature can be improved. Ionizing radiation can be performed before stretching, during the stretching step, or before or after the heat treatment, but is preferably performed after drying because the film properties can be easily controlled.

As ionizing radiation, α-rays, β-rays, γ-rays,
Electron beams (accelerated electrons), neutron beams, X-rays and the like can be mentioned.
Of these, electron beams are preferred because they are easy to handle and can be efficiently crosslinked without using additives. Further, ultraviolet irradiation may be performed, and in that case, a photosensitizer is preferably added.

When an electron beam is used, at room temperature, the acceleration voltage
The irradiation is preferably performed at 100 to 5000 kV and an electron dose of 0.1 to 100 Mrad. If the acceleration voltage is less than 100 kV, the degree of cross-linking in the thickness direction changes greatly, and if it exceeds 5000 kV, the substrate (film) shrinks due to heat, which is not preferable. On the other hand, if the electron dose is less than 0.1 Mrad, the degree of crosslinking of the polyolefin and the gel fraction will be low, and the meltdown temperature will not be sufficiently high. On the other hand, if it exceeds 100Mrad,
Since the deterioration of the film progresses and the piercing strength is greatly reduced, its use is limited.

(6) Step of Hydrophilizing The obtained microporous membrane can be used after hydrophilizing. As the hydrophilic treatment, monomer grafting, surfactant treatment, corona discharge treatment, or the like is used.

When a surfactant is used, any of a nonionic surfactant, a cationic surfactant, an anionic surfactant and a zwitterionic surfactant can be used. Is preferred.

In this case, the surfactant is converted into an aqueous solution or a solution of a lower alcohol such as methanol, ethanol or isopropyl alcohol, and hydrophilized by a method such as dipping and doctor blade.

The obtained microporous hydrophilized membrane is dried. Here, heat treatment is preferably performed at a temperature lower than the melting point of the polyolefin microporous film while preventing or shrinking so that the permeability is not significantly reduced.

[3] Polyolefin Microporous Membrane The physical properties of the polyolefin microporous membrane produced as described above are usually such that the air permeability is 10 to 2000 sec / 100 cc, the average through-hole diameter is 0.01 to 0.1 μm, and the pores are Rate 25-80%, puncture strength 39
20mN / 25μm or more, tensile breaking strength is 50MPa or more, preferably 8
0 MPa or more, tensile elongation at break of 50% or more, heat shrinkage of 13% or less, preferably 7% or less, shutdown temperature of 120 to 140
℃, meltdown temperature 160 ~ 190 ℃, pressure 30kP
a. It can have a compression property that the air permeability after pressing at 100 ° C. for 60 seconds is 400% or less of that before pressing. The thickness of the polyolefin microporous membrane can be appropriately selected according to the intended use. For example, when used as a battery separator, the thickness is preferably 5 to 200 μm.

As described above, the microporous polyolefin membrane of the present invention has an excellent balance among air permeability, porosity, fine pore size, compression characteristics, mechanical strength, dimensional stability, shutdown characteristics, and meltdown characteristics. Therefore, it is suitable as a battery separator, and can also be suitably used for various filters utilizing the permeability.

When heat treatment is performed, 105 ° C./8
After heat treatment, the heat shrinkage rate is calculated in the machine direction (MD) / vertical direction (T
D) = 7/6 or less (porosity 40%).

[0066]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1 20% by weight of ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.5 × 10 ⁶ and a weight average molecular weight of 3.0 × 10 ⁵ and Mw /
80% by weight of linear high density polyethylene (HDPE) with Mn = 6
To a polyethylene composition (melting point 135 ° C, crystal dispersion temperature 90 ° C) containing tetrakis [methylene-
3- (3,5-ditert-butyl-4-hydroxyphenyl)-
Propionate] methane was added in an amount of 0.375 part by weight per 100 parts by weight of the polyethylene composition to obtain a polyethylene composition.
This was charged into a twin-screw extruder (φ58 mm, L / D = 42, strong kneading type), and liquid paraffin (135 cSt / 25 ° C.) was injected from a side feeder of the extruder by a pump. The amount of liquid paraffin to be injected is 30% by weight of polyethylene composition with 100% by weight of polyethylene composition + liquid paraffin.
Was determined to be the amount that resulted in the concentration of The inside of the twin-screw extruder is depressurized by a vacuum pump to prevent air from being mixed in.
The mixture was melt-kneaded at rpm to prepare a polyethylene solution. Subsequently, this polyethylene solution was placed at the tip of the extruder
The biaxially stretched film was extruded from a T-die so as to have a thickness of about 10 mm, and a gel-like sheet was formed while being taken up by a cooling roll adjusted to 50 ° C. The obtained gel-like sheet was biaxially stretched using a batch stretching machine at 112 ° C. so as to be 5 × 5 times to obtain a stretched film. Next, the stretched product was washed with methylene chloride to extract and remove residual liquid paraffin. The obtained membrane was further dried and heat-set at 118 ° C. to a thickness of 2
A 5 μm polyethylene microporous membrane was prepared.

Example 2 Example 2 was repeated except that the heat setting temperature was 123 ° C.

Example 3 30% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.5 × 10 ⁶ and 70% by weight of 3.0 × 10 ⁵ linear high density polyethylene (HDPE) with Mw / Mn = 6 (Melting point: 136 ° C., crystal dispersion temperature: 90 ° C.) and the heat setting temperature was changed to 122 ° C., and the same procedure as in Example 1 was carried out.

Example 4 40% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.5 × 10 ⁶ and a weight average molecular weight of 3.0 × 10 ⁵ and Mw /
Mn = 6 linear high density polyethylene (HDPE) 60% by weight
(Melting point: 136 ° C., crystal dispersion temperature: 90 ° C.) and the heat setting temperature was changed to 122 ° C., and the same procedure as in Example 1 was carried out.

Example 5 The same procedure as in Example 3 was carried out except that the amount of the liquid paraffin to be injected was 25% by weight of the polyethylene composition.

Example 6 Ultra high molecular weight polyethylene having a weight average molecular weight of 2.5 × 10 ⁶ (UHMWPE) 20% by weight and a weight average molecular weight of 3.0 × 10 ⁵ and Mw /
Mn = 6 linear high density polyethylene (HDPE) 60% by weight
And polypropylene (PP; Mw / Mn = 5.0, weight average molecular weight)
4.5 × 10 ⁵ ) A polyolefin composition (melting point 165 ° C., crystal dispersion temperature 90 ° C.) consisting of 20% by weight was used, and the heat setting temperature was 122.
The procedure was performed in the same manner as in Example 1 except that the temperature was changed to ° C.

Example 7 20% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.5 × 10 ⁶ and a weight average molecular weight of 3.0 × 10 ⁵ and Mw /
Mn = 6 linear high density polyethylene (HDPE) 60% by weight
And branched low density polyethylene (LDPE; weight average molecular weight 2.
5 × 10 ⁵ ) 20% by weight of a polyethylene composition (melting point 13
(4 ° C., crystal dispersion temperature 90 ° C.) and the same procedure as in Example 1 except that the heat setting temperature was 112 ° C.

Example 8 20% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.5 × 10 ⁶ and Mw having a weight average molecular weight of 3.0 × 10 ⁵
Polyethylene composition comprising 60% by weight of linear high density polyethylene (HDPE) / Mn = 6 and 20% by weight of linear low density polyethylene (LLDPE; weight average molecular weight 2.5 × 10 ⁵ ) (melting point 134 ° C., crystal (Dispersion temperature 90 ° C) and heat setting temperature 115
The procedure was performed in the same manner as in Example 1 except that the temperature was changed to ° C.

Example 9 20% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.5 × 10 ⁶ and Mw having a weight average molecular weight of 3.0 × 10 ⁵
/ Mn = 6 linear high-density polyethylene (HDPE) 60% by weight and an ethylene-octene copolymer produced using a metallocene catalyst (octene 8.0 mol%, weight average molecular weight 2 ×
10 ⁵ ) 20% by weight of a polyethylene composition (melting point 134
C. and a crystal dispersion temperature of 90 ° C.), and the same procedure as in Example 1 was carried out except that the heat setting temperature was 115 ° C.

Example 10 Ultra high molecular weight polyethylene having a weight average molecular weight of 2.5 × 10 ⁶ (UHMWPE) 20% by weight and a weight average molecular weight of 3.0 × 10 ⁵ and Mw
/ Mn = 6 linear high-density polyethylene (HDPE) 60% by weight and polyethylene wax (weight average molecular weight 2 × 10 ³ ) 20
The procedure was performed in the same manner as in Example 1 except that a heat-setting temperature was set to 115 ° C. using a polyethylene composition (wt.%, Melting point 135 ° C., crystal dispersion temperature 90 ° C.).

Example 11 The microporous membrane obtained in Example 1 was fixed with its ends fixed.
Heat treatment was performed in an oven at 80 ° C. for 10 minutes.

Comparative Example 1 A high-density polyethylene (HDPE) having Mw / Mn = 12 and a weight average molecular weight of 3.0 × 10 ⁵ was used, and the procedure was performed in the same manner as in Example 1 except that the heat setting temperature was 120 ° C.

Comparative Example 2 The same procedure as in Example 1 was carried out except that high-density polyethylene (HDPE) having a weight average molecular weight of 3.0 × 10 ^{5 and} Mw / Mn = 20 was used and the heat setting temperature was set to 120 ° C.

Comparative Example 3 The same operation as in Example 7 was carried out except that high-density polyethylene (HDPE) having a weight average molecular weight of 3.0 × 10 ^{5 and} Mw / Mn = 12 was used and the heat setting temperature was 110 ° C.

Comparative Example 4 The same operation as in Example 8 was carried out except that high-density polyethylene (HDPE) having a weight average molecular weight of 3.0 × 10 ^{5 and} Mw / Mn = 12 was used and the heat setting temperature was set at 113 ° C.

[0082] Comparative Example 5 weight-average molecular weight in Mw / Mn = 12 is using the 3.0 × 10 ⁵ high density polyethylene (HDPE), except that the heat setting temperature was 110 ° C. were performed in the same manner as in Example 9.

The physical properties of the polyethylene microporous membranes obtained in Examples 1 to 11 and Comparative Examples 1 to 5 were measured by the following methods. -Film thickness: The cross section is measured by a scanning electron microscope. -Air permeability: Measured according to JIS P8117.・ Average through-hole diameter: Omnisoap 360 (manufactured by Coulter)
Was measured by -Porosity: Measured by a gravimetric method.・ Puncture strength: 25 μm thick microporous membrane 1 mm in diameter (0.5 mm R)
The maximum load at the time of piercing at a speed of 2 mm / sec was measured using the above-mentioned needle.・ Tensile rupture strength: Tensile rupture strength of a 10 mm wide strip specimen
Measured according to ASTM D882.・ Tensile rupture elongation: Tensile rupture elongation of a 10 mm wide strip specimen
Measured according to ASTM D882. Heat shrinkage: Shrinkage in the machine direction (MD) and vertical direction (TD) when the microporous membrane was exposed at 105 ° C. for 8 hours were measured. Shutdown temperature: Measured as a temperature at which the air permeability becomes 100,000 seconds / 100 cc or more by heating to a predetermined temperature. Meltdown temperature: Measured as the temperature at which the film melts and breaks when heated to a predetermined temperature.・ Change in air permeability (compression characteristics): Pressure 30kPa ・ Temperature 60 at 100 ℃
Table 1 shows the measurement results of the air permeability change after pressing for 2 seconds.

[0084]

[Table 1]

As shown in Table 1, the microporous polyolefin membrane of the present invention has the following properties: air permeability, porosity, fine pore size, compression characteristics, mechanical strength, dimensional stability, shutdown characteristics, and meltdown characteristics. Excellent balance. On the other hand, the microporous membranes of Comparative Examples 1 to 5 had a weight average molecular weight of 1 × 10 ⁴ or more and 5 ×
Since the Mw / Mn of the polyolefin of less than 10 ⁵ is 10 or more, the compression properties and the air permeability are poor, and the balance of various physical properties is poor.

[0086]

As described above, the microporous polyolefin membrane of the present invention has the following advantages: air permeability, porosity, fine pore size, compression characteristics, mechanical strength, dimensional stability, shutdown characteristics,
Because of its excellent balance between meltdown characteristics and its properties, when used as a separator for a lithium ion battery, a battery having excellent battery characteristics, battery productivity, and battery safety can be produced.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme court ゛ (Reference) // B29K 23:00 B29K 23:00 105: 04 105: 04 B29L 7:00 B29L 7:00 (72) Inventor Kotaro Takita 4-16-24 Okamura, Isogo-ku, Yokohama-shi, Kanagawa Prefecture (72) Inventor Koichi Kono 3-29-10-404 Mihara, Asaka-shi, Saitama F-term (reference) 4D006 GA02 GA47 MA22 MA24 MA31 MB03 MB15 MB16 MB20 MC22 QW31 4J002 BB03W BB03X BB05W BB05X BB06W BB06X BB07W BB07X BB10W BB10X BB12W BB12X BB15W BB15X 5H021 BB01 BB05 BB13 EE04

Claims (5)

[Claims] 1. A polyolefin composition (A) comprising a polyolefin (A1) having a weight average molecular weight of 5 × 10 ⁵ or more and a polyolefin (A2) having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ as essential components. A microporous polyolefin membrane, wherein the polyolefin (A2) is a linear polyolefin having a weight average molecular weight / number average molecular weight of 1 to less than 10. 2. The polyolefin composition according to claim 1,
The method for producing a microporous polyolefin membrane in which a solution comprising (A) and a solvent is melt-kneaded and extruded, and a gel-like molded product obtained by cooling is subjected to solvent removal before and / or after stretching, wherein the polyolefin (A2 A) a method for producing a microporous polyolefin membrane, wherein a linear polyolefin having a weight average molecular weight / number average molecular weight of 1 or more and less than 10 is used. 3. A battery separator using the microporous polyolefin membrane according to claim 1. 4. A filter using the microporous polyolefin membrane according to claim 1. 5. A battery using the microporous polyolefin membrane according to claim 1 as a battery separator.