JP2002194132A - Polyolefin fine porous film and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyolefin fine porous film excellent in smoothness and compression characteristics. SOLUTION: A polyolefin composition comprises a polypropylene and a polyethylene as essential ingredients, wherein (1) the polypropylene comprises a powdered polypropylene as essential ingredients, and/or (2) the polypropylene has a MFR of 2.0 or below and is contained a content of 20 wt.% or below, and the polyethylene has a weight average molecular weight/number average molecular weight of 8-100. The polyolefin composition provides a polyolefin fine porous film in which (a) film thickness variation ratio is 1.5 or below, and (b) a ratio (A)/(B) of the air permeability (A) after pressing for 60 seconds with a load of 3 MPa at a temperature of 100 deg.C and the air permeability (B) before the press is 4.0 or below.

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 more particularly, to a polyolefin microporous membrane having excellent smoothness and compression 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.

In particular, since separators for lithium ion batteries are required to have excellent permeability, mechanical properties, dimensional stability, shutdown properties, meltdown properties, etc., various studies have been made on polyolefin microporous membranes and methods for producing the same. Has been done.

In general, a microporous membrane made of polyethylene alone has a low meltdown temperature and low mechanical strength, and a microporous membrane made of polypropylene alone has a high shutdown temperature. In many cases, a microporous membrane is produced using a material composed of polypropylene and polypropylene. Further, it is more effective to include an ultrahigh molecular weight polyolefin for improving mechanical strength. For example, the present inventors have used 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 as a high strength and high elasticity polyolefin microporous membrane. And a method of manufacturing by a process that does not perform a desolvent treatment (Japanese Patent Laid-Open No. 3-64).
No. 334). In addition, JP-A-4-126352, JP-A-5-234
No. 578 and the like propose a microporous membrane comprising a composition containing an ultrahigh molecular weight polyolefin as an essential component.

However, microporous membranes made of polyethylene and polypropylene, and among them, microporous membranes containing ultrahigh molecular weight polyolefin, are often inferior in surface smoothness to microporous membranes made only of polyethylene. For example,
JP-A-6-96753 and JP-A-6-336535 propose a method for producing a microporous membrane comprising high-molecular-weight polyethylene and high-molecular-weight polypropylene and having high strength and excellent in various physical properties without using ultra-high-molecular-weight polyethylene. However, it is described in each of the comparative examples that when ultrahigh molecular weight polyethylene is added to these compositions, the surface of the microporous film becomes shark-skinned and the film cannot be formed well.

Further, the present inventors have disclosed the above-mentioned Japanese Patent Application Laid-Open No.
By using the composition described in JP-A-4-57835 or the composition described in JP-A-4-126352, the composition described in JP-A-3-64334 is slightly produced. It was confirmed that the surface of the porous film had a shark skin shape, and that the film could not be formed properly. That is, in the case of the composition containing the ultrahigh molecular weight polyolefin, the moldability of the film may be deteriorated.

[0007] If the surface of the microporous membrane is poor in smoothness, the adhesiveness to the electrode when used as a battery separator is poor, resulting in an increase in resistance.

In recent years, battery characteristics such as cycle characteristics and high-temperature storability tend to be emphasized. Therefore, as the mechanical properties desired for improving the battery life, it is preferable that not only the tensile breaking strength, elongation, and piercing strength, which have been conventionally evaluated, but also the compressive properties be excellent. Poor compression characteristics often lead to battery capacity shortage (deterioration in cycle characteristics) when used as a battery separator, and thus improvement in compression characteristics is desired.

[0009] However, there has been no example studied so far for the purpose of improving both the smoothness and the compression characteristics of a microporous polyolefin membrane for a battery separator.

Accordingly, an object of the present invention is to solve the above-mentioned disadvantages of the prior art and to provide a microporous polyolefin membrane having excellent smoothness and compression properties required for a battery separator and a method for producing the same.

[0011]

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 composition comprising polypropylene and polyethylene as essential components, and polypropylene comprising powdered polypropylene as an essential component, and / or Or the above problem is solved by using a polyolefin composition in which the polypropylene has an MFR of 2.0 or less, the content of the polypropylene is 20% by weight or less, and the polyethylene has a weight-average molecular weight / number-average molecular weight of 8 to 100. The present inventor has found out what can be done and arrived at the present invention.

That is, the microporous polyolefin membrane of the present invention comprises a polyolefin composition containing polypropylene and polyethylene as essential components, and (a) the film thickness variation ratio is 1.
5 or less, and (b) The ratio (A) / (B) of the air permeability (A) after pressing for 60 seconds at a load of 3 MPa and a temperature of 100 ° C. to the air permeability (B) before pressing is 4.
It is characterized by being 0 or less.

In the method for producing a microporous polyolefin membrane according to the present invention, a solution comprising a polyolefin composition containing polypropylene and polyethylene as essential components and a solvent is melt-kneaded, extruded, and cooled to obtain a gel-like substance. Then, in the step of removing the solvent from the gel-like substance before and / or after the stretching, using a polyolefin composition in which polypropylene is a powdery polypropylene as an essential component (hereinafter referred to as a production method), or polypropylene MFR is 2.0 or less, polypropylene content is 20
It is characterized by using a polyolefin composition having a weight percentage of not more than 8 and a polyethylene (weight average molecular weight / number average molecular weight) of 8 to 100 (hereinafter referred to as a production method).

In order for the above-mentioned microporous polyolefin membrane to obtain more excellent properties, the polyolefin composition must meet the following conditions:
It is preferable to satisfy (1) to (8). (1) Polypropylene has a weight average molecular weight of 3 × 10 ⁵ to 1 × 1
0 is ⁶ . (2) The polypropylene is a propylene homopolymer. (3) The polyethylene has a weight average molecular weight of 1 × 10 ^{4 to} 5 × 10 ⁶
It is. (4) The polyethylene is composed of an ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more. (5) The polyethylene is an ultra-high molecular weight polyethylene 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
Less than ⁵ high-density polyethylene. (6) The polyethylene is an ultrahigh molecular weight polyethylene 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
^A polyethylene composition comprising a high-density polyethylene of less than ⁵ , and a weight average molecular weight / number average molecular weight (hereinafter, referred to as “Mw / Mn”) of the polyethylene composition is 8
~ 100. (7) The polyethylene is at least one selected from the group consisting of ultrahigh molecular weight polyethylene, high density polyethylene, medium density polyethylene, low density polyethylene, and linear low density polyethylene. (8) polyethylene, comprising a composition comprising ultra-high molecular weight polyethylene, high-density polyethylene and a polymer imparting a shutdown function, wherein the polymer having the shutdown function is low-density polyethylene (LDPE) produced by a medium-pressure method; Linear low density polyethylene (LLDPE) manufactured by low pressure method, weight average molecular weight 1 × 10 ³ -4 × 1
0 ³ low molecular weight polyethylene, but at least one kind selected from the group consisting of ethylene · alpha-olefin copolymer prepared using a single site catalyst.

In order for the microporous polyolefin membrane produced by the production method to obtain more excellent properties, the polyolefin composition preferably satisfies the following conditions (9) to (12). (9) Assuming that the whole polypropylene is 100% by weight, the polypropylene contains 20 to 100% by weight of powdered polypropylene. (10) The average particle size of the powdery polypropylene contained in the polypropylene is 100 to 2000 μm, and the particle size distribution is 50 to 30.
00. (11) The polyethylene contains 20 to 100% by weight of powdered polyethylene based on 100% by weight of the entire polyethylene. (12) The average particle size of the powdered polyethylene contained in the polyethylene is 100 to 2000 μm, and the particle size distribution is 50 to 4000.

The polyolefin composition preferably satisfies the following condition (13) so that the above-mentioned microporous polyolefin membrane produced by the production method can obtain more excellent properties. (13) The polyethylene has an Mw / Mn of 10 to 50.

In order for the microporous polyolefin membrane produced by the production method or the production method to obtain more excellent compression properties, mechanical strength and dimensional stability, the following conditions (14) to (14)
It is preferable to satisfy (16). (14) The solvent is removed after stretching. (15) After removing the solvent, drying is performed, and the obtained microporous membrane is heat-treated. (16) The heat treatment is performed by a treatment selected from any of stretching, fixing and shrinking, or a combination of these treatments.

The microporous polyolefin membrane according to a preferred embodiment of the present invention has a thickness variation ratio of 1.5 or less and a load of 3 MPa ·
The ratio (A) / (B) of the air permeability (A) after pressing at a temperature of 100 ° C. for 60 seconds to the air permeability (B) before pressing is 4.0 or less, and the air permeability is 1000 seconds / 100.
cc or less, porosity 30 ~ 95%, puncture strength 3000mN / 25μm or more, tensile breaking strength 70MPa or more, heat shrinkage 10% or less,
Shutdown temperature is 120 ~ 140 ℃, meltdown temperature is
Excellent characteristics of 160 ° C or higher can be satisfied.

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

[0020]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a microporous polyolefin membrane comprising polypropylene and polyethylene as essential components, wherein (a) the thickness variation ratio is 1.5 or less, and (b) the load is 3 MPa.
The present invention relates to a microporous polyolefin membrane in which the ratio (A) / (B) of the air permeability (A) after pressing at a temperature of 100 ° C. for 60 seconds to the air permeability (B) before pressing is 4.0 or less.

The film thickness variation ratio is a measure of the smoothness. If this value is large, the adhesion to the electrode when used as a battery separator is poor, and the resistance is increased. further,
The ratio (A) / (B) between the air permeability (A) after pressing and the air permeability (B) before pressing at a load of 3 MPa and a temperature of 100 ° C. for 60 seconds is a measure of the compression characteristics, and this value is large. In addition, when used as a battery separator, the battery capacity is often insufficient (deterioration in cycle characteristics), which is not preferable.

Incidentally, the film thickness variation ratio is as follows.
The maximum value and the minimum value of the film thickness were measured at three points of the m microporous polyethylene membrane (the production method is described in detail in Examples), and the average value of the difference was used as the film thickness variation d as a reference. The maximum value and the minimum value of the film thickness were measured at three points for each of the microporous films, and the average value of the difference was defined as the film thickness variation d ', and d' /
Defined as d. The smaller this value is, the better the smoothness is.

[1] Method for producing microporous polyolefin membrane (a) Production method In the production method, a polyolefin composition comprising polypropylene as an essential component is used. (1) Polypropylene It is necessary that polypropylene be a powdered polypropylene as an essential component. In the absence of powdered polypropylene,
The resulting microporous film has a skin-like appearance and cannot be formed well. In addition, all the components of the polypropylene may be in the form of powder, but a part of the polypropylene may be other than powder. In addition,
It is preferable to use a pellet-like material other than the powder-like material, but it is preferable that the powdery polypropylene accounts for 20% by weight or more of the polypropylene component. The powdery polypropylene preferably has an average particle size of 100 to 2000 μm and a particle size distribution of 50 to 3000. Average particle size 2000
If the average particle diameter is less than 100 μm, handling becomes difficult. If the particle size distribution exceeds 3,000, mixing with polyethylene tends to be insufficient, which is not preferable. Here, the average particle size and the particle size distribution are measured based on JISK-0069.

The weight average molecular weight of the polypropylene is not particularly limited, but is usually preferably 3 × 10 ⁵ to 1 × 10 ⁶ , more preferably 3.5 × 10 ⁵ to 1 × 10 ⁶ , and still more preferably 4 × 10 ⁵ to 1 × 10 ⁶ . × 10 ⁵ to 1 × 10 ⁶ .

The type of the polypropylene is not particularly limited, and a propylene homopolymer or a copolymer of propylene and another α-olefin can be used. As the copolymer, any of a random copolymer and a block copolymer can be used, and as the α-olefin, ethylene, butene-1, pentene-1, 4-methylpentene-1,
Examples thereof include octene, vinyl acetate, methyl methacrylate, and styrene. Among them, ethylene is preferable. Among these, a propylene homopolymer is preferred. Use of a propylene homopolymer is effective in improving the balance between mechanical strength and permeability. In addition, these polypropylenes may be two or more of propylene homopolymers, random copolymers, block copolymers and different molecular weights, or propylene homopolymer, random copolymer, Two or more selected from block copolymers may be used.

(2) Polyethylene The weight average molecular weight of polyethylene is not particularly limited.
No, but usually 1 × 10 ^Four~ 5 × 10⁶Is preferred, more preferred
Or 5 × 10^Four~ 3 × 10⁶, More preferably 1 × 10^Five~
1 × 10⁶It is.

Although the molecular weight distribution Mw / Mn of polyethylene is not limited, it is preferably from 8 to 100, more preferably from 10 to 100. When Mw / Mn is less than 8, the high molecular weight component becomes too large and melt extrusion becomes difficult. When Mw / Mn exceeds 100, the low molecular weight component becomes too large and the strength is reduced.

The type of polyethylene includes ultrahigh molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene. These 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, vinyl acetate, methyl methacrylate, styrene and the like are preferred. In addition, polyethylene may be used alone or as a mixture of two or more. That is, two or more kinds of ultra-high molecular weight polyethylenes having different weight average molecular weights, two or more kinds of high density polyethylenes having different weight average molecular weights, two or more kinds of medium density polyethylenes having different weight average molecular weights, two or more kinds of weights Low-density polyethylenes having different average molecular weights may be used, or two or more polyethylenes selected from ultra-high-molecular-weight polyethylene, high-density polyethylene, medium-density polyethylene, and low-density polyethylene may be mixed. Among them, polyethylene is weight average molecular weight
5 × 10 ⁵ or more ultra high molecular weight polyethylene, or a composition comprising polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more ultra high molecular weight polyethylene and a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ polyethylene It is preferred that As the polyethylene having a weight-average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , any of high-density polyethylene, medium-density polyethylene, and low-density polyethylene can be used, but high-density polyethylene is particularly preferable. As the polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , two or more polyethylenes having different molecular weights may be used, or two or more polyethylenes having different types (density) may be used.

The polyethylene may be composed of a composition comprising ultra-high-molecular-weight polyethylene, high-density polyethylene and a polymer having a shutdown function, and the polymer having a shutdown function may be a composition composed of low-density polyethylene. Can be. Examples of low-density polyethylene include branched polyethylene (LDPE) produced by the high-pressure method, linear low-density polyethylene (LLDPE) produced by the low-pressure method, and ethylene-α-olefin copolymer produced using a single-site catalyst. At least one selected material or the like can be used, and these can be used alone or as a mixture. It should be noted that a low molecular weight polyethylene having a weight average molecular weight of 1 × 10 ³ to 4 × 10 ³ may be used as the polymer having the shut-down function. As the low molecular weight component increases, the strength is reduced as described above.Therefore, the addition amount of the low molecular weight polyethylene having a weight average molecular weight of 1 × 10 ³ to 4 × 10 ³ is 20 when the polyethylene component is 100 parts by weight. It is preferred that the amount be not more than part by weight.

Preferably, the polyethylene comprises powdered polyethylene. This is more effective in improving the smoothness of the microporous membrane surface. In addition, all the components of the polyethylene may be in a powder form, but a part may be other than a powder form. It is preferable to use a pellet-like material other than the powder-like material, but it is preferable that the powder-like polyethylene accounts for 20% by weight or more of the polyethylene component. The powdered polyethylene preferably has an average particle size of 100 to 2000 μm and a particle size distribution of 50 to 4000.

(3) Other Components In addition to polyethylene and polypropylene, other polyolefins such as polybutene-1 and poly-4-methylpentene-1 can be added without departing from the scope of the present invention. . These components make up the entire polyolefin composition.
When the amount is 0 parts by weight, the amount is preferably 30 parts by weight or less.

(B) Production method The production method is such that the MFR of polypropylene is 2.0 or less, the content of polypropylene is 20% by weight or less, and the Mw /
A polyolefin composition having Mn of 8 to 100 is used. (1) Polypropylene The MFR (230 ° C, 2.16 kg) of polypropylene must be 2.0 or less. When the MFR exceeds 2.0, the polyethylene and polypropylene undergo phase separation during sheet formation to form a shark skin, microscopic irregularities on the surface of the microporous membrane become large, fluctuations in the film thickness are large, and the desired physical properties are not fine. A porous membrane cannot be obtained. Therefore, it cannot be used as a battery separator.

Furthermore, the content of polypropylene must be not more than 20% by weight of the whole polyolefin composition. If polypropylene is not present, the meltdown temperature cannot be improved, and if the content of polypropylene exceeds 20% by weight, the sheet becomes shark-skin-like, the film thickness fluctuates greatly, and the microporous film with the desired physical properties Can not be obtained.

The weight average molecular weight of the polypropylene is not particularly limited, but is usually preferably 3 × 10 ⁵ to 1 × 10 ⁶ , more preferably 3.5 × 10 ⁵ to 1 × 10 ⁶ , and still more preferably 4 × 10 ⁵ to 1 × 10 ⁶ . × 10 ⁵ to 1 × 10 ⁶ .

The type of polypropylene is not particularly limited, and a propylene homopolymer or a copolymer of propylene and another α-olefin can be used. As the copolymer, any of a random copolymer and a block copolymer can be used, and as the α-olefin, ethylene, butene-1, pentene-1, 4-methylpentene-1,
Examples thereof include octene, vinyl acetate, methyl methacrylate, and styrene. Among them, ethylene is preferable. Among these, a propylene homopolymer is preferred. In addition, these polypropylenes may be two or more of propylene homopolymers, random copolymers, block copolymers and different molecular weights, or propylene homopolymer, random copolymer, Two or more selected from block copolymers may be used.

(2) Polyethylene The molecular weight distribution Mw / Mn of polyethylene must be 8 to 100. More preferably, it is 10-100. When Mw / Mn is less than 8, the high molecular weight component becomes too large and melt extrusion becomes difficult. When Mw / Mn exceeds 100, the low molecular weight component becomes too large and the strength is reduced.

The weight average molecular weight of polyethylene is not particularly limited, but is usually 1 × 10 ^{4 to} 5 × 10 ^6, preferably 5 × 10 ^{4 to} 3 × 10 ⁶ , more preferably 1 × 10 ⁵ to 1
× 10 ⁶ .

Examples of the type of polyethylene include ultrahigh molecular weight polyethylene, high density polyethylene, medium density polyethylene, and low density polyethylene. These 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, vinyl acetate, methyl methacrylate, styrene and the like are preferred.

The polyethylene may be used alone or as a mixture of two or more. That is, two or more kinds of ultra-high molecular weight polyethylenes having different weight average molecular weights, two or more kinds of high density polyethylenes having different weight average molecular weights, two or more kinds of medium density polyethylenes having different weight average molecular weights, two or more kinds of weights Low-density polyethylenes having different average molecular weights may be used, or two or more polyethylenes selected from ultra-high-molecular-weight polyethylene, high-density polyethylene, medium-density polyethylene, and low-density polyethylene may be mixed. Among them, it polyethylene weight average molecular weight 5 × 10 ⁵ or more ultra-high molecular weight polyethylene, or polyethylene weight average molecular weight 5 × 10 ⁵ or more ultra-high-molecular-weight polyethylene and the weight average molecular weight ^of 1 × 10 ⁴ or more less than 5 × 10 ⁵ Preferably, the composition comprises polyethylene. As the polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , high density polyethylene, medium density polyethylene,
Any of low density polyethylene can be used,
It is particularly preferable to use high-density polyethylene. In addition,
As the polyethylene having a weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ , two or more kinds having different molecular weights may be used, or two or more kinds having different kinds (density) may be used.

As the polyethylene, a composition comprising an ultra-high molecular weight polyethylene, a high-density polyethylene and a polymer having a shutdown function is used, and as the polymer having the shutdown function, a composition comprising a low-density polyethylene is used. Can be. Examples of low-density polyethylene include branched polyethylene (LDPE) produced by the high-pressure method, linear low-density polyethylene (LLDPE) produced by the low-pressure method, and ethylene-α-olefin copolymer produced using a single-site catalyst. At least one or more selected ones can be used, and these can be used alone or in combination. It should be noted that a low molecular weight polyethylene having a weight average molecular weight of 1 × 10 ³ to 4 × 10 ³ may be used as the polymer having the shut-down function. As the low molecular weight component increases, the strength is reduced as described above.Therefore, the addition amount of the low molecular weight polyethylene having a weight average molecular weight of 1 × 10 ³ to 4 × 10 ³ is 20 when the polyethylene component is 100 parts by weight. It is preferred that the amount be not more than part by weight.

(C) Production step (1) Step of adding a solvent to a polyolefin composition and melt-kneading to prepare a polyolefin solution In the production method of the present invention, first, the above [1] (a) or [1] ( An appropriate solvent is added to the polyolefin composition described in b) and melt-kneaded to prepare a polyolefin solution. Antioxidant, UV absorber,
Various additives such as an anti-blocking agent, a pigment, a dye, and an inorganic filler can be added 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 preferably, usually 160 to 230 ° C., and preferably 180 to 230 ° C.
More preferably, it is ~ 220 ° 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.

The mixing ratio of the polyolefin composition and the solvent in the polyolefin solution is preferably 1 to 50% by weight, more preferably 20 to 40% by weight, with the total of both being 100% by weight. If the polyolefin composition 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, and the swell and neck-in will increase at the die outlet, making molding difficult. on the other hand,
If it exceeds 50% by weight, the moldability of the gel-like molded product will be 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.

The heated solution extruded from the die lip is cooled to form a gel-like molded product. 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 article and removing the solvent The gel-like molded article is stretched after heating 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 vertical or horizontal simultaneous 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 microporous polyolefin membrane having high elasticity and high strength 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 the melting point of the polyolefin composition +
The temperature is preferably set to 10 ° C. or lower, 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. On the other hand, if the stretching temperature is lower than the crystal dispersion temperature, the resin is insufficiently softened. In the present invention, the stretching temperature is usually 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 removal of the solvent can be performed 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 appropriately selected according to the solvent used for dissolving the polyolefin composition, 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 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. Also, because of its high boiling point, it is easy to condense,
It is easy to collect and recycle.

As the above-mentioned non-aqueous solvent, it is preferable to use a solvent which is compatible with the residual solvent and which is not compatible with the polyolefin composition. For example, paraffinic compounds having a boiling point of 100 ° C. or higher and a flash point of 0 ° C. or higher, aromatics, alcohols, esters, ethers, ketones and the like can be mentioned.

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, a method of a combination thereof, or the like. The above-described cleaning is performed until the residual solvent becomes less than 1% by weight.

(4) Step of Drying the Obtained Film and Performing a Cross-linking Treatment by Ionizing Radiation In the present invention, it is preferable to perform a cross-linking treatment by ionizing radiation after drying. Thereby, the meltdown temperature can be improved. Ionizing radiation can be performed before stretching, during the stretching process, or before or after the heat treatment, but is preferably performed after drying because the film properties can be easily controlled.

As the 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 300 kV and an electron dose of 0.1 to 100 Mrad. If the accelerating voltage is less than 100 kV, the degree of cross-linking in the thickness direction changes greatly, and if it exceeds 300 kV, the substrate (film)
Are undesirably shrunk by heat. 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 100 Mrad, the film is deteriorated and the piercing strength is greatly reduced, so that the use is limited. When using an ultra-high molecular weight polyolefin,
It is preferable to irradiate with an electron dose of 10 to 100 Mrad, and 30 to 1
Irradiation at 00 Mrad is more preferred. Thereby, the meltdown temperature can be improved.

(5) Step of heat treatment After the film obtained by stretching and solvent removal is dried by a heat drying method, an air drying method or the like, it is preferable to perform a heat treatment. The heat treatment stabilizes the crystal and makes the lamella layer uniform.

As the heat treatment, any of a heat stretching treatment, a heat fixing 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 between 60 ° C and -10 ° C.

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 2.0 times, more preferably 1.01 to 1.5 times.

The heat setting 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, a large number of the above-described heat stretching, heat setting, and heat shrinking treatments may be combined.

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

(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. In addition, it is preferable to perform such a hydrophilization treatment after ionizing radiation.

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, it is preferable to perform the heat treatment while preventing or shrinking at a temperature not higher than the melting point of the polyolefin microporous membrane −10 ° C. so that the permeability is not significantly reduced.

[2] Polyolefin Microporous Membrane The physical properties of the polyolefin microporous membrane produced as described above are generally such that the film thickness variation ratio is 1.5 or less, the air permeability after pressing for 60 seconds at a load of 3 MPa and a temperature of 100 ° C. for 60 seconds. The ratio (A) / (B) between the degree (A) and the air permeability (B) before pressing is 4.0 or less, and the air permeability is 1000 seconds / 100cc
Below, the porosity is 30-95%, the piercing strength is 3000mN / 25μm or more, the tensile breaking strength is 70MPa or more, the heat shrinkage is 10% or less,
Shutdown temperature is 120 ~ 140 ℃, meltdown temperature is
160 or more excellent properties can be satisfied. The thickness of the polyolefin microporous membrane can be appropriately selected according to the intended use.
It is preferable to set it to 200 μm.

As described above, the microporous polyolefin membrane of the present invention is particularly excellent in smoothness and compression properties, and also has excellent air permeability, porosity, mechanical strength, dimensional stability, shutdown properties, and meltdown properties. Because of its excellent balance, it is suitable as a battery separator, and can be suitably used for various filters and the like utilizing its permeability.

[0071]

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. Measurement method of film thickness variation ratio The microporous film used as the reference for the film thickness variation ratio was a microporous film made of ultrahigh molecular weight polyethylene and high density polyethylene. First, 30 weight parts of ultra high molecular weight polyethylene (UHMWPE) with a weight average molecular weight of 2.0 × 10 ⁶
A resin composition comprising 70 parts by weight of 3.5 × 10 ⁵ high-density polyethylene (HDPE) (Mw / Mn = 18, melting point 135 ° C., crystal dispersion temperature 9
0 ° C), tetrakis [methylene-3- (3,
5-Ditert-butyl-4-hydroxyphenyl) -propionate] methane is added at 0.375 per 100 parts by weight of the resin composition.
Parts by weight. 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 injected was such that the concentration of high-density polyethylene was 30% by weight based on 100% by weight of high-density polyethylene + liquid paraffin. The inside of the twin-screw extruder was depressurized by a vacuum pump to prevent air from being mixed therein, and melt-kneaded at 200 ° C. and 200 rpm to prepare a polyethylene solution. Subsequently, the biaxially stretched film was extruded from a T-die installed at the tip of the extruder so that the biaxially stretched film became 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 at 115 ° C. to 5 × 5 times using a batch stretching machine to obtain a stretched film. Next, the stretched product was washed with methylene chloride to extract and remove the remaining liquid paraffin. Further, the obtained membrane was dried and heat-set at 125 ° C. for 30 seconds to produce a microporous polyethylene membrane. The average thickness of this polyethylene microporous membrane was 25 μm. The difference between the maximum value and the minimum value of the film thickness of the three prepared microporous polyethylene membranes was within the range of 0.8 to 1.5 μm, and the average value was 1.0 μm. The average value was used as a reference for the film thickness variation d, and the maximum value and the minimum value of the film thickness were measured for each of three points of the polyolefin microporous film composed of the resin composition of polypropylene and polyethylene described below. The film thickness variation ratio d '/ d was calculated with the average value as the film thickness variation d'. A smaller value means better results.

Example 1 30% by weight of powdery ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.0 × 10 ⁶ and a weight average molecular weight of 3.0 × 1
0 ⁵ powdery high density polyethylene (HDPE) 65% by weight and (ultra high molecular weight polyethylene and polyethylene Mw / Mn = 20.5 consisting of a high density polyethylene), pellets of a propylene homopolymer with MFR of 0.5 (wt Average molecular weight 6.0 × 10 ⁵ )
5% by weight of a resin composition (melting point 165 ° C., crystal dispersion temperature 90 ° C.) was added to tetrakis [methylene-
3- (3,5-ditert-butyl-4-hydroxyphenyl)-
Propionate] methane is added in an amount of 0.1 parts by weight per 100 parts by weight of the resin composition.
375 parts by weight were added. This is a twin screw extruder (φ58mm, L / D = 4
2. 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 depends on the resin composition +
Liquid paraffin was taken as 100% by weight, and the amount was such that the resin composition had a concentration of 30% by weight. The inside of the twin-screw extruder is depressurized by a vacuum pump to prevent air from
-The mixture was melt-kneaded at 200 rpm to prepare a polyolefin solution. Subsequently, this polyolefin solution was extruded from a T-die installed at the tip of the extruder so that the biaxially stretched film became about 10 mm, and was taken up by a cooling roll adjusted to 50 ° C. to form a gel-like sheet. . The obtained gel-like sheet was biaxially stretched using a batch stretching machine at 120 ° 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 the remaining liquid paraffin. Further, the obtained membrane was dried and heat-set at 125 ° C. for 30 seconds to prepare a polyolefin microporous membrane.

Example 2 A powdery high-density polyethylene (HDPE, Mw / Mn = 7.1) having a weight average molecular weight of 3.0 × 10 ⁵ (95% by weight) and a powdery propylene homopolymer having an MFR of 0.5 (weight average molecular weight) 7.0 × 10 ⁵ ,
Resin composition consisting of 5% by weight (average particle size: 80 to 1000 μm, particle size distribution: 600 to 2000) (melting point: 163 ° C., crystal dispersion temperature: 90 ° C.)
A microporous polyolefin membrane was prepared in the same manner as in Example 1, except that was used.

Example 3 A powdery ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.0 × 10 ⁶ (UHMWPE) of 19% by weight and a weight average molecular weight of 3.0 × 1
0 ⁵ (polyethylene Mw / Mn = 18.3 consisting of ultra high molecular weight polyethylene and high density polyethylene) powdered high density polyethylene (HDPE) 76% by weight, the pellets of the propylene homopolymer with MFR of 0.5 (wt Average molecular weight 6.0 × 1
0 ⁵⁾ 5% by weight of the resin composition (melting point 165 ° C., except for using crystal dispersion temperature 90 ° C.), in the same manner as in Example 1 to prepare a polyolefin microporous membrane.

Example 4 95% by weight of a powdery high-density polyethylene (HDPE, Mw / Mn = 10.5) having a weight average molecular weight of 4.3 × 10 ⁵ and a propylene homopolymer in a pellet form having an MFR of 0.5 (weight average molecular weight) 6.0 × 1
0 ⁵⁾ 5% by weight of the resin composition (melting point 165 ° C., except for using crystal dispersion temperature 90 ° C.), in the same manner as in Example 1 to prepare a polyolefin microporous membrane.

Example 5 Except that a propylene homopolymer having an MFR of 0.5 and a powdery form (weight average molecular weight 7.0 × 10 ⁵ , average particle size 800 to 1000 μm, particle size distribution 600 to 2000) was used. In the same manner as in Example 1, a microporous polyolefin membrane was produced.

Comparative Example 1 A powdery ultra-high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.0 × 10 ⁶ and a weight average molecular weight of 3.0 × 1 was 25% by weight.
0 ⁵ (polyethylene Mw / Mn = 20.7 consisting of ultra high molecular weight polyethylene and high density polyethylene) powdered high density polyethylene (HDPE) 50% by weight, the pellets of the propylene homopolymer with MFR of 0.5 (wt Average molecular weight 7.7 × 1
0 ⁵⁾ resin composition comprising 25 wt% (melting point 165 ° C., except for using the crystal dispersion temperature 90 ° C.), as in Example 1 to prepare a polyolefin microporous membrane.

Comparative Example 2 70% by weight of powdery ultrahigh molecular weight polyethylene (UHMWPE, Mw / Mn = 7) having a weight average molecular weight of 3.0 × 10 ⁶ and propylene homopolymer in the form of pellets having an MFR of 0.5 (weight average) Molecular weight 7.
0 × 10 ⁵ ) 30% by weight of a resin composition (melting point: 162 ° C.,
An attempt was made to produce a polyolefin microporous film in the same manner as in Example 1 except that the crystal dispersion temperature was 93 ° C.), but the surface of the microporous film was shark-skinned and could not be formed.

Comparative Example 3 30% by weight of powdery ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2.0 × 10 ⁶ and a weight average molecular weight of 3.0 × 1
0 ⁵ high density polyethylene (HDPE) 50% by weight (polyethylene Mw / Mn = 21.1 consisting of ultra high molecular weight polyethylene and high density polyethylene), pellets of a propylene homopolymer with MFR of 2.5 (weight average molecular weight 4.0 × 10 ⁵ ) 20% by weight
(Melting point 165 ° C, crystal dispersion temperature 91
C)), except that a microporous polyolefin membrane was prepared in the same manner as in Example 1.

The physical properties of the microporous polyolefin membranes obtained in Examples 1 to 5 and Comparative Examples 1 to 3 were measured by the following methods. -Average film thickness: measured with a contact thickness meter. The length of the measured part is 10 cm, and the number of measurement points includes the maximum and minimum values.
point. -Porosity: Measured by a gravimetric method. -Air permeability: Measured according to JIS P8117.・ Puncture strength: 1mm (0.5mm R) diameter 25μm microporous membrane
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. 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.・ Film thickness variation ratio: above ・ Air permeability ratio (compression characteristics): pressure 3MPa ・ 60 at 100 ℃
Air permeability ratio before and after pressing for 2 seconds 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. Table 1 shows the measurement results.

[0081]

[Table 1]

As shown in Table 1, the microporous polyolefin membrane of the present invention is particularly excellent in smoothness and compressive properties, as well as air permeability, porosity, mechanical strength, dimensional stability, shutdown properties, and Excellent balance of meltdown characteristics. On the other hand, in the polyolefin composition of Comparative Example 1, the composition ratio of polypropylene exceeded 20% by weight, and in the polyolefin composition of Comparative Example 3 the MFR exceeded 2, so that the film thickness variation ratio was large and the smoothness was high. Inferior. The microporous membrane of Comparative Example 2 used a composition in which the Mw / Mn of polyethylene was less than 8 and the composition ratio of polypropylene exceeded 20% by weight, so that the surface of the microporous membrane became shark skin-like and molding was impossible. It was possible.

[0083]

As described above, the microporous polyolefin membrane of the present invention is particularly excellent in smoothness and compressive properties, as well as air permeability, porosity, mechanical strength, dimensional stability, shutdown properties, Because of its excellent balance of meltdown characteristics, it is suitable as a battery separator, and can be suitably used for various filters utilizing the permeability.

 ──────────────────────────────────────────────────続 き Continuation of the front page (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 GA47 MA21 MA31 MC22 MC22X MC23 MC23X MC88 NA22 NA64 NA66 PC80 4F074 AA17A AA18A AA19A AA20A AA24A AA24B CA02 CA03 CC02Y CC04X CC04Z CC27Z CC28Z CC29Z DA38 DA43 DA47 DA49 4J05 BB03 BB03 BB03 BB03 BB03B01 BB03W01 HH06 HH07

Claims (7)

[Claims] 1. A polyolefin composition comprising polypropylene and polyethylene as essential components, wherein (a) a film thickness variation ratio is 1.5 or less, and (b) air permeability after pressing at a load of 3 MPa at a temperature of 100 ° C. for 60 seconds. Ratio (A) between (A) and air permeability (B) before pressing
/ (B) is 4.0 or less. 2. The microporous polyolefin membrane according to claim 1, wherein the MFR of the polypropylene is 2.0 or less, the content of the polypropylene is 20% by weight or less, and the weight average molecular weight / number average molecular weight of the polyethylene is 8 to 8. 100. A microporous polyolefin membrane comprising a polyolefin composition of 100. 3. A solution comprising a polyolefin composition containing polypropylene and polyethylene as essential components and a solvent is melt-kneaded, extruded, and cooled to form a gel-like substance.
In the method for producing a microporous polyolefin membrane, wherein the solvent is removed from the gel material before and / or after the stretching,
A method for producing a microporous polyolefin membrane, wherein the polypropylene uses a polyolefin composition containing powdered polypropylene as an essential component. 4. A solution comprising a polyolefin composition containing polypropylene and polyethylene as essential components and a solvent is melt-kneaded, extruded, and cooled to form a gel-like substance.
In the method for producing a microporous polyolefin membrane, wherein the solvent is removed from the gel material before and / or after the stretching,
A polyolefin microporous membrane, wherein a polyolefin composition having an MFR of the polypropylene of 2.0 or less, a content of the polypropylene of 20% by weight or less, and a weight average molecular weight / number average molecular weight of the polyethylene of 8 to 100 is used. Manufacturing method. 5. A battery separator using the microporous polyolefin membrane according to claim 1 or 2. 6. A filter using the microporous polyolefin membrane according to claim 1 or 2. 7. A battery using the microporous polyolefin membrane according to claim 1 or 2 as a battery separator.