JP2003105123A - Polyolefin minute porous film and method of its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyethylene minute porous film having high strength to piercing even with a thin film of about 10 μm thick, a low thermal contraction and a good permeability. SOLUTION: This polyolefin minute porous film contains as an essential component a polyethylene having a weight average molecular weight of not lower than 5×10<5> , and has a strength to piercing of not lower than 9,800 mN/25 μm, and air permeability of 20-1,000 s/100 ml, and an average pore diameter of the polyolefin minute porous film is varied along the direction of film thickness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin microporous membrane having excellent puncture strength, heat shrinkage and permeability, a method for producing the same, and a battery separator and a battery using the polyolefin microporous membrane.

[0002]

Microporous polyolefin membranes include separators for batteries such as lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries and polymer batteries, separators for electrolytic capacitors, ultrafiltration membranes and microfiltration membranes. It is widely used in various filters, clothing materials, medical materials, etc.

In order to increase the strength of such a microporous polyolefin membrane, it has been studied to produce a microporous membrane using ultrahigh molecular weight polyethylene. For example, a method of kneading ultra-high molecular weight polyethylene with various plasticizers, solvents, etc., and then removing these plasticizers, solvents, etc., and stretching if necessary is disclosed in JP-A-60-242035 and JP-A-60-255107. Issue, JP 63
-273651 etc.

However, these methods require the use of a large amount of various plasticizers, solvents, etc., and it not only takes a long time to remove the plasticizers, solvents, etc., but also the strength of the obtained microporous membrane is sufficient. Was not. In order to improve this, by adding other polyethylene with a weight average molecular weight of 1 × 10 ⁴ or more to ultra-high molecular weight polyethylene, the molecular weight distribution is controlled to have excellent strength and water permeability, and the amount of solvent used is increased. An invention that can be reduced is disclosed in Japanese Patent Laid-Open No. 3-64334.

However, the level required for battery characteristics such as lithium batteries is increasing day by day, and further improvement in puncture strength, permeability and heat shrinkability is required. However, if the permeability is increased, the porosity generally increases, and as a result, the puncture strength of the microporous membrane tends to decrease. If the draw ratio is increased to increase the puncture strength, the heat shrinkage ratio will be significantly increased. For this reason, what satisfies these three physical properties at the same time is not yet known. Especially from the viewpoint of improving the battery capacity, the thickness of the microporous membrane is 20 to 30 μm.
Therefore, it is required to reduce the film thickness to about 10 μm, which requires a piercing strength higher than before. Specifically, if a puncture strength of 3920 mN (400 gf), which is permissible for a film with a thickness of 25 μm, is required for a film with a thickness of 10 μm, it will be 9800 mN (1000
Needle strength of about gf) is required.

The microporous membrane described in JP-A-7-188440 was obtained by melt-kneading a solution consisting of ultra-high molecular weight polyethylene or a composition containing the same and a solvent, and then extruding the kneaded product and cooling. It is obtained by subjecting the gel-like molded product to temperature distribution in the film thickness direction, stretching, and solvent removal treatment. This microporous membrane is
Good puncture strength and breaking strength, and air permeability at a level that can be used as a battery separator (520-700 seconds / 100m
l). However, the piercing strength is still 5880mN / 25μm
(600gf / 25μm). Although strength is improved simply by increasing the draw ratio, conversely, the heat shrinkage ratio is decreased, and in the end it is not possible to sufficiently satisfy the strict requirements demanded for a lithium battery separator.

Further, in JP-A-2000-248094, a gel-like molded product obtained by kneading ultrahigh molecular weight polyethylene and liquid paraffin is cooled to 0 ° C., then rolled at 120 ° C. and further stretched, A method of performing heat treatment after desolvation is described. In this case, the puncture strength is 9800mN / 25μm (1000g / 25
However, the heat shrinkage rate is as high as 9.8 to 13.4%. Therefore, although it has the advantage of improving the permeability and strength at the same time by rolling before stretching, when used as a lithium battery separator, short circuit due to separator shrinkage easily occurs due to heat generation. , Long-term cycle characteristics of secondary batteries such as lithium batteries deteriorate.

Further, in Japanese Patent Laid-Open No. 2001-081221, a lateral direction (T
Although a polyolefin microporous membrane having improved contraction force, improved puncture strength, and improved tensile strength of D) is disclosed,
The heat shrinkage ratio is not improved in all directions. The permeability is about 800 seconds / 100 ml,
Furthermore, since it has a high flexibility of 2.5 to 7.0, further improvement is required in terms of ion permeability.

[0009]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a polyolefin microporous membrane having high puncture strength, low thermal shrinkage and good permeability even in a thin film having a thickness of about 10 μm, and a method for producing the same. Is to provide.

[0010]

Means for Solving the Problems In view of the above object, as a result of intensive research, a melt-kneaded product obtained from a solvent and a polyolefin containing a polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as essential components and a solvent is extruded from a die and cooled. The remaining solvent is removed from the obtained gel-like sheet, and after stretching in at least uniaxial direction, a microporous film (A) obtained by heat treatment at a temperature not lower than the melting point of the crystal dispersion of the polyolefin (A), The gel-like sheet is stretched at least uniaxially at a temperature lower than the crystal dispersion temperature of the polyolefin, and then stretched at least uniaxially at a temperature higher than the crystal dispersion temperature of the polyolefin and lower than the melting point, and the solvent is removed from the obtained stretched product. By laminating with the microporous membrane (B) obtained as described above, the porosity in which the average pore diameter of one surface is larger than the average pore diameter of the inside or the other surface. Fin microporous membrane is obtained,
The present inventors have discovered that such a microporous membrane is excellent in ion permeability and also in physical properties such as puncture strength and heat shrinkage rate, and arrived at the present invention.

That is, the polyolefin microporous membrane of the present invention is a polyolefin microporous membrane containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and having a puncture strength of 9800 mN / 25 μm or more and an air permeability of 20-1000 seconds / 100 ml
And the average pore diameter of the polyolefin microporous film is changed in the film thickness direction.

It is preferable that at least one surface of the polyolefin microporous membrane has an average pore diameter larger than the internal average pore diameter, or one surface has an average pore diameter larger than the other surface.

The method for producing a polyolefin microporous membrane of the present invention comprises melt-kneading a solvent containing a polyolefin having a weight average molecular weight of 5 × 10 ⁵ or more and polyethylene as an essential component.
The obtained melt-kneaded product is extruded from a die, and after cooling to form a gel-like sheet, the gel-like sheet is biaxially stretched by providing a temperature distribution in the film thickness direction, and at 25 ° C. from the obtained stretched product. Surface tension is 24mN / m or less to remove the solvent using a washing solvent (A), then after stretching in at least uniaxial direction,
Further microporous film (A) obtained by heat treatment at a temperature below the crystal dispersion temperature of the polyolefin and below the melting point, and stretching the gel-like sheet in at least one axial direction at a temperature below the crystal dispersion temperature of the polyolefin, and then It is characterized in that the polyolefin is biaxially stretched at a temperature equal to or higher than the crystal dispersion temperature and lower than the melting point, and the microporous membrane (B) obtained by removing the solvent from the obtained stretched product is laminated.

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

The characteristics of the polyolefin microporous membrane are more excellent.
In order to obtain the above, the polyolefin must meet the following conditions (1) to (11).
It is preferable to fill. (1) Weight average molecular weight contained in the above polyolefin 5 × 1
0^FiveThe above polyethylenes are ultra high molecular weight polyethylenes
It (2) The weight average of the ultra high molecular weight polyethylene described in (1) above.
Average molecular weight is 1 x 10⁶~ 15 × 10⁶Is. (3) of the ultra high molecular weight polyethylene according to (1) or (2) above
Weight average molecular weight is 1 x 10 ⁶~ 5 x 10⁶Is. (4) The ultrahigh molecular weight polyester according to any one of (1) to (3) above.
Tylene is produced by multistage polymerization. (5) The ultra high molecular weight polyester according to any one of (1) to (4) above.
The Mw / Mn of chilen is 5 to 300. (6) The polyolefin has a weight average molecular weight of 5 × 10^Fivethat's all
Ultra high molecular weight polyethylene and weight average molecular weight 1 × 10^FourSince
Top 5 × 10^FiveComposition with less than a polyolefin. (7) Weight average in the polyolefin composition according to the above (6)
Average molecular weight 1 × 10^Four5 × 10 or more^FiveHigher than polyolefin
Density polyethylene, medium density polyethylene, low density polyethylene
Selected from the group consisting of ethylene and linear low density polyethylene
It is at least one kind. (8) The polyolefin composition according to (6) or (7) above is
Weight average molecular weight 5 × 10^FiveWith the above ultra high molecular weight polyethylene
Weight average molecular weight 1 × 10^Four5 × 10 or more^FiveLess than high density poli
It consists of chilen. (9) The polyolefin set according to any one of (6) to (8) above.
The Mw / Mn of the product is 5 to 300. (10) The above polyolefin is any of the above (1) to (5)
Ultra high molecular weight polyethylene according to, or the above (6) ~ (9)
The polyolefin composition according to any one of the above,
Function (to prevent accidents such as ignition when the temperature inside the battery rises)
To stop, the microporous membrane melts and clogs the micropores.
As a polyolefin that imparts the function of blocking the current)
Branched low density polyethylene, linear low density polyethylene
Ethylene produced using a single-site catalyst
α-olefin copolymer, and molecular weight 1 × 10³~ 4 x 10³
Selected from the group consisting of low molecular weight polyethylene
Both are polyolefin compositions to which one kind is added. (11) The polyolefin is any one of (1) to (5) above.
Ultra high molecular weight polyethylene according to, or (6) to (10) above
The polyolefin composition according to any one of
Temperature (breaking temperature of thermoplastic microporous membrane)
Composition with polypropylene added for
Is.

In order to obtain more excellent properties of the microporous polyolefin membrane, the above-mentioned first to third production methods are carried out under the following conditions (1
It is preferable to satisfy 2) to (20). (12) The melt-kneading of a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent is performed at a melting point of the polyolefin of + 10 ° C. to + 100 ° C. (13) The stretching (primary stretching) of the gel-like sheet obtained by extrusion molding is performed at a temperature not lower than the crystal dispersion temperature of the polyolefin and not higher than the melting point. (14) The washing process for removing the solvent is performed in two or more steps. (15) Use the washing solvent (A) having a surface tension of 24 mN or less at 25 ° C in at least the final washing step. (16) The washing solvent (A) is a fluorinated hydrocarbon, having 5 to 10 carbon atoms.
At least one selected from the group consisting of normal paraffins, isoparaffins having 6 to 12 carbon atoms, aliphatic ethers having 6 or less carbon atoms, cycloparaffins such as cyclopentane, cycloparaffins, aliphatic ketones, aliphatic alcohols and fatty acid esters. Is. (17) A step of using the washing solvent (B) in addition to the washing solvent (A) may be included. The cleaning solvent (B) is an easily volatile solvent (hydrocarbon, chlorinated hydrocarbon, fluorohydrocarbon, ether, ethyl methyl ketone, etc.), or a boiling point of 100 ° C or higher and a flash point of 0 ° C.
It is the above non-aqueous solvent. (18) Before and after the heat setting treatment, heat relaxation treatment is performed as necessary. The thermal relaxation treatment is performed at a temperature not lower than the crystal dispersion temperature of the polyolefin and not higher than the melting point. (19) The thermal relaxation treatment may be carried out by maintaining 60% or more of the length immediately before relaxation in both the machine direction (MD) and the transverse direction (TD), and relaxation may be performed in only one direction. (20) The lamination of the microporous membrane is carried out by fusion using a method such as roll or embossing.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION [1] Polyolefin Microporous Membrane (A) Polyolefin The polyolefin used in the present invention contains polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component. Examples of such polyethylene include ultra-high molecular weight polyethylene, preferably having a weight average molecular weight of 1 × 10 ⁶ to 15 × 1.
0 ⁶ , more preferably 1 × 10 ^{6 to} 5 × 10 ⁶ .

The polyolefin to be used may be a composition containing other polyolefin as an optional component as long as polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more is an essential component. As such another polyolefin, polyethylene (weight average molecular weight 1 × 10 ⁴
Above 5 × 10 ⁵ ), polypropylene (weight average molecular weight 1 × 10 ⁴ to 4 × 10 ⁶ ), polybutene-1 (weight average molecular weight 1 × 10 ⁴ to 4 × 10 ⁶ ), polyethylene wax (weight average molecular weight 1 ×) 10 ³ or more and less than 1 × 10 ⁴ ) and ethylene / α-olefin copolymer (weight average molecular weight 1 × 10 ⁴ to 4 × 10 ⁶ ) can be used.

These optional polyolefins can be used up to 80% by weight of the total material used.

As the polyolefin composition, the molecular weight distribution (weight average molecular weight / number average molecular weight) can be easily controlled depending on the application, and therefore ultrahigh molecular weight polyethylene and polyethylene (weight average molecular weight of 1 × 10 ⁴ or more and 5 × 10 5 ⁽ Less than ⁵ ) can be preferably used.
The molecular weight distribution of the polyolefin composition is preferably 5 to 300, more preferably 5 to 100.

As the type of polyethylene (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. In addition to ethylene homopolymer, It may be a copolymer with other olefins such as propylene, butene and hexene. Among them, a composition composed of ultra high molecular weight polyethylene (weight average molecular weight of 5 × 10 ⁵ or more) and high density polyethylene (weight average molecular weight of 1 × 10 ⁴ or more and less than 5 × 10 ⁵ ) can be particularly preferably used. It is not limited to.

(B) Structure and Physical Properties The polyolefin microporous film of the present invention has a heat shrinkage ratio (105
Although not particularly limited, the machine direction (MD) and the transverse direction (TD) are preferably 8% or less, more preferably 5% or less, and most preferably 3% or less. When it exceeds 8%, when it is used as a separator for lithium batteries, a film breakage occurs at the time of heat generation, which easily causes a short circuit. The puncture strength is 9800 mN / 25 μm (1000 gf / 25 μm) or more, preferably 11760 mN / 25 μm (1200 gf / 25 μm) or more, and more preferably 14700 mN / 25 μm (1500 gf / 25 μm) or more.
If the puncture strength is lower than 9800 mN / 25 μm, pinholes are likely to occur when incorporated into the electrode as a lithium battery. The average curvature ratio is not particularly limited, but is preferably 1.2 to 2.5, more preferably 1.2 to 2.2. If the tortuosity is too high, the ion permeability will be significantly reduced, and if it is too low, the ion permeability will be improved, but a short circuit will easily occur when used in a lithium battery separator. The air permeability is 20 to 1000 seconds / 100 ml, preferably 20 to 500 seconds / 100 ml, and most preferably 20 to 300 seconds / 100 ml. If it exceeds 1000 seconds / 100 ml, it will lead to a decrease in battery capacity when used in a lithium battery, and if it is less than 20 seconds / 100 ml, it will not be fully shut down during heat generation. The average pore diameter varies in the film thickness direction, and it is preferable that the average pore diameter of at least one surface is larger than the internal average pore diameter, or that the average pore diameter of one surface is larger than the average pore diameter of the other surface. Specifically, the average pore size on one surface is preferably 0.005 to 0.1 μm,
It is more preferably 0.01 to 0.08 μm, and the average pore size of the inside or the other surface is preferably 0.01 to 0.2 μm, more preferably 0.02 to 0.15 μm. The ratio of the average pore diameter of one surface / the average pore diameter of the inside (or the average pore diameter of the other surface) is preferably 1.1 to 5, more preferably 1.5 to 3. With such a structure, the ion permeability can be improved.

[2] Method for Producing Microporous Membrane The method for producing a polyolefin microporous membrane of the present invention is obtained by melt-kneading a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent. The resulting melt-kneaded product is extruded from a die, and after cooling to form a gel-like sheet, the gel-like sheet is biaxially stretched by providing a temperature distribution in the film thickness direction, and the surface at 25 ° C. from the obtained stretched product. Remove the solvent using a washing solvent (A) with a tension of 24 mN / m or less,
Then, after stretching in at least uniaxial direction, further microporous membrane obtained by heat setting treatment at a temperature of melting point or more and less than the melting point of the crystal of the polyolefin (A), and the gel-like sheet less than the crystal dispersion temperature of the polyolefin. At least uniaxially stretched at a temperature of, then at least uniaxially stretched at a temperature less than the melting point above the crystal dispersion temperature of the polyolefin, further obtained by removing the solvent from the stretched product obtained microporous membrane (B ) And are laminated.

(A) Method for producing microporous membrane (A) + (a) Step for preparing polyolefin solution The above solvent is preferably one which is in a state of being mixed with polyolefin in a melt-kneaded state. Nonane as a preferred solvent,
Decane, decalin, paraxylene, undecane, dodecane, liquid paraffin and other aliphatic or cyclic hydrocarbons, mineral oil fractions with boiling points corresponding to these, dioctyl phthalate (DOP), dibutyl phthalate (DBP), etc. at room temperature The liquid phthalate ester and the like can be mentioned. A non-volatile solvent such as liquid paraffin is particularly preferable for obtaining a gel-like molded product having a stable solvent content. A solvent which is in a state of being mixed with a polyolefin in a melt-kneaded state and which is solid at room temperature can also be used by being mixed with a liquid solvent. Examples of such a solid include stearyl alcohol, ceryl alcohol, paraffin wax, dicyclohexyl phthalate (DCHP), and the like, and dicyclohexyl phthalate (DCHP) is particularly preferable because the pore size can be easily controlled.

The blending ratio of the polyolefin and the solvent is 1 to 50% by weight, preferably 20 to 40% by weight, based on 100% by weight of the total of both. When the amount of the polyolefin is less than 1% by weight, swell and neck-in increase at the die outlet when forming a gel-like molded product, and the moldability and self-supporting property of the gel-like molded product deteriorate. On the other hand, if it exceeds 50% by weight, the moldability of the gel-like molded article is deteriorated.

If necessary, various additives such as an antioxidant, an ultraviolet absorber, an anti-blocking agent, a pigment, a dye and an inorganic filler may be added to the melt-kneaded product within a range not impairing the object of the present invention. You can

The melt-kneading method is not particularly limited, but it is usually carried out by uniformly kneading the polyolefin and the solvent with a twin-screw extruder. The melting temperature is preferably the melting point of polyolefin + 10 ° C to + 100 ° C. Specifically, 160-250
C. is preferable, and 170 to 230.degree. C. is more preferable. Here, the melting point refers to a value obtained by differential scanning calorimetry (DSC) according to JIS K 7121. In the melt-kneading, a heated solution of polyolefin and a solvent may be charged in advance in the extruder, or the polyolefin may be charged first and the solvent may be added halfway.

(B) Molding step of gel-like sheet The kneaded product obtained by melt-kneading is extruded directly or through another extruder, or once cooled and pelletized, then again through the extruder. . The extrusion molding can be performed by a known method such as a T-die method and an inflation method. The obtained molded product is cooled into a gel sheet. Cooling rate is 50 ℃ /
It is preferable that the temperature is 25 ° C. or lower by quenching for more than a minute. In this way, the phase separation structure in which the polyolefin phase is microphase-separated by the solvent can be fixed. Generally, if the cooling rate is slow, the higher-order structure of the obtained gel-like molded article becomes coarse and the pseudo-cell units forming it become large, but if the cooling rate is fast, the cell units become dense. If the cooling rate is less than 50 ° C./minute, the crystallinity increases and it is difficult to form 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 with a refrigerant, or the like can be used.

(C) Biaxial Stretching Step Next, the gel-like sheet is biaxially stretched (primary stretching) by providing a temperature distribution in the film thickness direction. Stretching is performed by a tenter method, a roll method, a rolling method, or a combination thereof, and both simultaneous biaxial stretching and sequential biaxial stretching can be used. The stretching temperature is preferably above the crystal dispersion temperature of polyolefin (usually about 90 ° C.) and below the melting point. Here, the crystal dispersion temperature means a value obtained by measuring temperature characteristics of dynamic viscoelasticity by ASDM D 4065.

The method described in JP-A-7-188440 is preferably adopted as the method for biaxially stretching by providing a temperature distribution in the film thickness direction. That is, the gel-like sheet is preheated to a relatively low temperature (preferably the melting point of the polyolefin is −40 ° C.).
~ Melting point of polyolefin -10 ° C, more preferably melting point of polyolefin -30 ° C ~ melting point of polyolefin -10 ° C)
And a uniform temperature, and then a relatively high temperature (preferably polyolefin melting point -20 ° C to polyolefin melting point +10
℃, more preferably the melting point of polyolefin -15 ℃ ~ melting point of polyolefin + 10 ℃) method of stretching the surface, pre-heating the gel-like sheet at a relatively high temperature (preferably polyolefin melting point -20 ℃ ~ polyolefin Melting point + 10 ° C, more preferably polyolefin melting point -15 ° C
~ Polyolefin melting point + 10 ℃, and after heating to a uniform temperature, relatively low temperature (preferably polyolefin melting point -40 ℃ ~ polyolefin melting point -10 ℃, more preferably polyolefin melting point -30 ℃ ~ polyolefin Method of stretching the surface by cooling the surface to a melting point of -10 ° C), preheating the gel-like sheet, and then heating air from one of the upper and lower sides of the gel-like sheet (preferably polyolefin melting point -20 ° C to polyolefin melting point +10). ℃, more preferably the melting point of polyolefin -15 ℃ ~ melting point of polyolefin +
Stretching while heating by 10 ° C (when the preheating temperature is higher than the temperature of the heating air, the temperature of the heating air is preferably the melting point of the polyolefin-40 ° C to the melting point of the polyolefin-10 ° C, more preferably the polyolefin. If the temperature of preheating is lower than the temperature of the heating air, the temperature of the heating air is preferably −20 ° C. of the melting point of polyolefin to the melting point of + 10 ° C. of the polyolefin, more preferably Is the melting point of polyethylene −15 ° C. to the melting point of polyolefin + 10 ° C.).

The stretching ratio is not particularly limited, but it is preferably 3 times or more in the machine direction (MD), 3 times or more in the transverse direction (TD) and 20 times or more in area magnification. By doing so, the puncture strength can be improved.

(D) Washing step The solvent is removed (washed) from the film obtained by the primary stretching. Since the polyolefin phase is microphase-separated by the solvent, a porous film can be obtained by removing the solvent. To remove the solvent, the washing solvent (A) having a surface tension at 25 ° C. of 24 mN / m or less, preferably 20 mN / m or less is used. 24mN /
Treatment with a solvent exceeding m is not preferable because the structure of the microporous membrane becomes too dense and the permeability becomes insufficient. The washing solvent (A) is preferably one that is not compatible with polyolefin. By using such a washing solvent (A), it is possible to suppress the shrinkage and densification of the network structure caused by the surface tension of the gas-liquid interface generated inside the micropores during drying after washing. As a result, the porosity and air permeability of the microporous membrane can be improved. The surface tension of the washing solvent (A) can be lowered as the temperature at which it is used is increased, but the usable temperature range is limited to the boiling point or lower. Here, "surface tension" means the tension generated at the interface between gas and liquid,
It is a value measured based on JIS K 3362.

As the cleaning solvent (A), fluorine compounds such as hydrofluorocarbon, hydrofluoroether, cyclic hydrofluorocarbon, perfluorocarbon, perfluoroether, normal paraffin having 5 to 10 carbon atoms, isoparaffin having 6 to 12 carbon atoms, Such as aliphatic ethers having 6 or less carbon atoms, cycloparaffins such as cyclopentane, aliphatic ketones such as 2-pentanone, methanol, ethanol, 1-propanol, 2-propanol, tertiary butanol, isobutanol, 2-pentanol, etc. Aliphatic alcohol, propyl acetate, tertiary butyl acetate, secondary butyl acetate, ethyl acetate, isopropyl acetate, isobutyl acetate, methyl formate, ethyl formate,
Examples thereof include isopropyl formate, isobutyl formate, and ethyl propionate aliphatic ester.

As the fluorine-based compound, a chain hydrofluorocarbon represented by a composition formula of C ₅ H ₂ F ₁₀ , C ₄ F ₉ OCH ₃
And C ₄ F ₉ OC ₂ H ₅ hydrofluoroether represented by the composition formula, C ₅ H ₃ F ₇ cyclic hydrofluorocarbon represented by the composition formula, C ₆ F ₁₄ and C ₇ F _{16 represented by} the composition formula At least one selected from the group consisting of perfluorocarbons and perfluoroethers represented by the composition formula of C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ is preferable. Since the surface tension of these fluorine compounds is 24 mN / m or less at 20 ° C., the effect of suppressing shrinkage and densification of the network structure due to the surface tension is high. Further, since the boiling point is 100 ° C or lower, it is easy to dry. Furthermore, because it has no ozone depletion, it can reduce the load on the environment, and its flash point is 40 ° C or higher (some compounds have no flash point), so the risk of flammable explosion during the drying process is low.

As the normal paraffin having 5 to 10 carbon atoms, normal pentane, normal hexane, normal heptane, normal octane, normal nonane and normal decane are preferable. These have a surface tension of 24 at 20 ° C.
mN / m or less. Among them, normal pentane, normal hexane, and normal heptane, which have a boiling point of 100 ° C. or less and are easily dried, are more preferable.

As the isoparaffin having 6 to 12 carbon atoms, 2-
Methylpentane, 3-methylpentane, 2,2-dibutylbutane, 2,3-dibutylbutane, 2-methylhexane, 3-methylhexane, 3-ethylpentane, 2,2-dimethylpentane,
2,3-dimethylpentane, 2,4-dimethylpentane, 3,3-dimethylpentane, 2,2,3-trimethylbutane, 2-methylheptane, 3-methylheptane, 2,2-dimethylhexane, 2,
3-dimethylhexane, 2,5-dimethylhexane, 3,4-dimethylhexane, 2,2,3-trimethylpentane, 2,2,4-trimethylpentane, 2,3,3-trimethylpentane, 2,3, 4-Trimethylpentane, 2-methyloctane, 2,2,5-trimethylhexane, 2,3,5-trimethylhexane, 2-methylnonane and 2,3,5-trimethylheptane are preferred. Among them, the surface tension is 24 mN / m or less at 20 ° C., and the boiling point is 100 ° C. or less 2-methylpentane, 3-methylpentane, 2,2-dibutylbutane, 2,3-dibutylbutane, 2- Methylhexane, 3-methylhexane, 3-ethylpentane, 2,
2-Dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane and 3,3-dimethylpentane are more preferred.

As the ether having 6 or less carbon atoms, diethyl ether, butyl ethyl ether, dipropyl ether and diisopropyl ether are preferable. These have a surface tension of 24 mN / m or less at 20 ° C. and a boiling point.
It is below 100 ℃.

Cyclopentane is used as the cycloparaffin, and methanol, ethanol, 1-propanol and 2-propanol are used as the aliphatic alcohol, both having a surface tension of 24 mN / m or less at 20 ° C. and a boiling point of 100.
It is preferable because the temperature is not higher than ° C.

The aliphatic ester has a surface tension of 20 ° C.
In the above, tert-butyl acetate, secondary butyl acetate, isopropyl acetate, isobutyl acetate, ethyl formate, isopropyl formate and isobutyl formate having 24 mN / m or less are preferable. Further, tert-butyl acetate, ethyl formate and isopropyl formate having a boiling point of 100 ° C. or lower are more preferable.

As the washing solvent (A), it is also possible to use a mixture of water and an aliphatic alcohol having 3 or less carbon atoms, which is blended so as to have a surface tension of 24 mN / m or less at 25 ° C.

The washing solvent (A) can be used as a mixture with other solvents. In this case, the mixing ratio is 25
The surface tension should be 24mN / m or less at ℃. For example, a chain hydrofluorocarbon represented by the composition formula of C ₅ H ₂ F _10, a hydrofluoroether represented by the composition formula of C ₄ F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , and C ₅ H ₃ F ₇ Cyclic hydrofluorocarbon represented by the composition formula, perfluorocarbon represented by the composition formula of C ₆ F ₁₄ and C ₇ F ₁₆ , and perfluorocarbon represented by the composition formula of C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F _5. It is possible to use a mixture of at least one solvent selected from the group consisting of fluoroethers and a hydrocarbon solvent such as paraffin.

The washing is preferably performed in two or more washing steps, and a step of using a washing solvent (washing solvent (B)) other than the washing solvent (A) may be included. In this case, the washing solvent (A) may be used in at least one step. Washing solvent
As (B), those which are not compatible with polyolefin are preferable, for example, known pentane, hexane, hydrocarbons such as heptane, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, ethane trifluoride, etc. Fluorohydrocarbons, ethers such as diethyl ether and dioxane, and volatile solvents such as methyl ethyl ketone can be used. The boiling point is 100 ° C
It is also possible to use a non-aqueous solvent having a flash point of 0 ° C. or higher. The washing solvent (B) is appropriately selected according to the solvent used for dissolving the polyolefin composition, and is used alone or as a mixture.

By such a washing process of two or more stages,
Sufficient cleaning can be performed while suppressing shrinkage and densification of the network structure caused by the surface tension of the cleaning solvent. Preferably, a washing solvent is used at least in the final washing step.
Process with (A). As a result, when the washing solvent (B) is used, the washing solvent (B) can be removed (hereinafter referred to as “rinsing treatment”), and shrinkage and densification of the network that occurs during drying after washing can be prevented. As a result, the porosity and air permeability of the polyolefin microporous film are improved.

Wash solvent (A) in the final washing step
When the treatment is performed with a washing solvent (A) having a boiling point of 100 ° C. or less, the efficiency of the drying step is improved. Furthermore, the above-mentioned C ₅ H ₂ F ₁₀
A chain hydrofluorocarbon represented by the composition formula of C ₄
Hydrofluoroether represented by the composition formula of F ₉ OCH ₃ and C ₄ F ₉ OC ₂ H ₅ , cyclic hydrofluorocarbon represented by the composition formula of C ₅ H ₃ F ₇ , composition of C ₆ F ₁₄ and C ₇ F ₁₆ . Perfluorocarbon represented by the formula, and C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅
The use of a fluorine-based compound such as perfluoroether represented by the above composition formula also has an effect of further reducing the environmental load in the manufacturing process as described above. Especially washing solvent
When using a solvent having a boiling point of 150 ° C. or higher as (B), it takes time to dry simply by drying with hot air, and the porosity and the air permeability may decrease in the subsequent heat treatment due to the influence, but the boiling point The problem can be solved by using the washing solvent (A) at 100 ° C or lower.

The non-aqueous solvent having a boiling point of 100 ° C. or higher and a flash point of 0 ° C. or higher, which can be used as the washing solvent (B), is hardly volatile, has a low environmental load, and has a risk of ignition and explosion in the drying process. Since it is low, it is safe to use. In addition, since it has a high boiling point, it is easy to condense, easy to collect, and easy to recycle. The "boiling point" means the boiling point at 1.01 x 10 ⁵ Pa, and the "flash point" means the temperature measured according to JIS K 2265.

As the non-aqueous solvent, for example, a boiling point of 100 ° C.
Examples thereof include paraffin compounds having a flash point of 0 ° C. or higher, aromatic compounds, alcohols, esters, ethers and ketones. The flash point thereof is preferably 5 ° C or higher, more preferably 40 ° C or higher. However, it is not preferable to make the non-aqueous solvent into an aqueous solution because the solvent cannot be sufficiently removed.

As the non-aqueous solvent, normal paraffins having 8 or more carbon atoms, normal paraffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms,
Isoparaffins having 8 or more carbon atoms, cycloparaffins having 7 or more carbon atoms, cycloparaffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms, aromatic hydrocarbons having 7 or more carbon atoms, and at least hydrogen atoms Aromatic hydrocarbons having 6 or more carbon atoms partially substituted with halogen atoms, alcohols having 5 to 10 carbon atoms in which some hydrogen atoms may be substituted with halogen atoms, and some hydrogen atoms being halogen atoms At least one selected from the group consisting of esters and ethers having 7 to 14 carbon atoms, which may be substituted with, and ketones having 5 to 10 carbon atoms is preferable.

As the normal paraffin having 8 or more carbon atoms, normal octane, normal nonane, normal decane, normal undecane and normal dodecane are preferable, and normal octane, normal nonane and normal decane are more preferable.

Examples of normal paraffins having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms include 1-chloropentane, 1-chlorohexane, 1-chloroheptane, 1-chlorooctane and 1-bromopentane. , 1-bromohexane, 1-bromoheptane, 1-bromooctane, 1,
5-dichloropentane, 1,6-dichlorohexane and 1,7-dichloroheptane are preferred, and 1-chloropentane, 1-chlorohexane, 1-bromopentane and 1-bromohexane are more preferred.

2, as an isoparaffin having 8 or more carbon atoms,
3,4-trimethylpentane, 2,2,3-trimethylpentane,
2,2,5-Trimethylhexane, 2,3,5-trimethylhexane, 2,3,5-trimethylheptane and 2,5,6-trimethyloctane are preferred, and 2,3,4-trimethylpentane, 2,2 , 3-
Trimethylpentane, 2,2,5-trimethylhexane and 2,
More preferred is 3,5-trimethylhexane.

Cycloparaffins having 7 or more carbon atoms include cycloheptane, cyclooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexane, cis- and trans-1,3-dimethylcyclohexane and cis- and trans. -1,4-Dimethylcyclohexane is preferable, and cycloheptane is more preferable.

As the cycloparaffin having 5 or more carbon atoms in which at least a part of hydrogen atoms are substituted with halogen atoms, chlorocyclopentane and chlorocyclohexane are preferable, and chlorocyclopentane is more preferable.

As the aromatic hydrocarbon having 7 or more carbon atoms,
Toluene, ortho-xylene, meta-xylene and para-xylene are preferable, and toluene is more preferable.

Aromatic hydrocarbons having 6 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms include chlorobenzene, 2-chlorotoluene, 3-chlorotoluene,
4-chlorotoluene, 3-chloroorthoxylene, 4-chloroorthoxylene, 2-chlorometaxylene, 4-chlorometaxylene, 5-chlorometaxylene and 2-chloroparaxylene are preferred, and chlorobenzene, 2-chlorotoluene,
More preferred are 3-chlorotoluene and 4-chlorotoluene.

Alcohols having 5 to 10 carbon atoms in which a part of hydrogen atoms may be replaced by halogen atoms include isopentyl alcohol, tertiary pentyl alcohol, cyclopentanol, cyclohexanol and 3-methoxy-1-. Butanol, 3-methoxy-3-methyl-1-butanol, propylene glycol normal butyl ether and
5-chloro-1-pentanol is preferred, 3-methoxy-1-butanol, 3-methoxy-3-methyl-1-butanol, propylene glycol normal butyl ether and 5-chloro-1.
-Pentanol is more preferred.

Examples of the ester having 7 to 14 carbon atoms in which a part of hydrogen atoms may be replaced with halogen atoms are diethyl carbonate, diethyl maleate, normal propyl acetate, normal butyl acetate, isopentyl acetate, 3-methoxybutyl acetate. , 3-methoxy-3-methylbutyl acetate, ethyl normal butyrate, ethyl normal valerate and 2-chloroethyl acetate are preferred, and isopentyl acetate, 3-methoxybutyl acetate, 3-methoxy-3-methylbutyl acetate, ethyl normal butyrate and acetic acid 2 -Chloroethyl is more preferred.

Diether glycol dimethyl ether, normal butyl ether, diisobutyl ether and bischloroethyl ether are preferable as the ether having 7 to 14 carbon atoms in which a part of hydrogen atoms may be replaced by a halogen atom, and dipropylene glycol dimethyl ether and Bischloroethyl ether is more preferred.

As the ketone having 5 to 10 carbon atoms, 2-pentanone, 3-pentanone, 2-hexanone, 3-hexanone, cyclopentanone and cyclohexanone are preferable, and 2-pentanone and 3-pentanone are more preferable.

The washing solvent (B) as described above may be used as a mixture, but the washing solvent (B) may be used as an optional component (C).
For example, a chain hydrofluorocarbon represented by the composition formula of C ₅ H ₂ F ₁₀ listed as the cleaning solvent (A), C ₄ F ₉ OCH ₃ and C ₄ F ₉
Hydrofluoroether represented by the composition formula of OC ₂ H ₅ , C
₅ H ₃ F ₇ compositional formula of cyclic hydrofluorocarbon, C ₆ F ₁₄ and C ₇ F ₁₆ compositional formula of perfluorocarbon, C ₄ F ₉ OCF ₃ and C ₄ F ₉ OC ₂ F ₅ composition A mixture of at least one solvent selected from the group consisting of perfluoroethers represented by the formula may be used. In this case, the washing solvent (B) and the optional component (C) have a surface tension of 20 to 80 ° C.
It is preferable to mix them at a ratio of 24 mN / m or less at the temperature. Specifically, the optional component (C) is preferably 2 to 98 parts by weight, more preferably 5 to 50 parts by weight in 100 parts by weight of the mixed solvent. By including 2 to 98 parts by weight of the optional component (C), sufficient washing can be performed while suppressing shrinkage and densification of the network caused by the surface tension of the washing solvent.

The washing solvent (B) has a surface tension of 20 to 80 ° C.
It is preferable to use a material having a temperature of 24 mN / m or less at the temperature. For example, normal pentane, hexane, heptane, ethane trifluoride, diethyl ether, 2-methyl pentane, 3-methyl pentane, cyclohexane, cyclopentane, acetone, methyl ethyl ketone and the like.

Washing solvent used in the first step of washing here
Preferred combinations of (B) and the washing solvent (A) used in the second step are shown below. However, as described below, the washing solvent (A)
Since the washing with the washing solvent (B) can be performed in three or more steps, these are not intended to be limited to two steps. For example, washing solvent (B) / washing solvent (A) =
Methylene chloride / C ₄ F ₉ OCH ₃ , methylene chloride / C ₆ F ₁₄ , methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane,
Methylene chloride / normal hexane, methylene chloride / diethyl ether, ether / hydrofluoroether,
Ether / cyclic hydrofluorocarbon, ether /
Alcohol, mixture of ether / alcohol and water, normal paraffin / hydrofluoroether, normal paraffin / cyclic hydrofluorocarbon, normal paraffin / alcohol, mixture of normal paraffin / alcohol and water, isoparaffin / hydrofluoroether, isoparaffin / cyclic hydro Fluorocarbon, isoparaffin / alcohol, mixture of isoparaffin / alcohol and water, cycloparaffin / hydrofluoroether, cycloparaffin / cyclic hydrofluorocarbon, cycloparaffin / alcohol, mixture of cycloparaffin / alcohol and water, ketone / hydrofluoroether, Ketone / cyclic hydrofluorocarbon, ketone / alcohol, and ketone / alcohol with water Mixtures thereof. More preferably, the washing solvent (B) / the washing solvent (A) = methylene chloride / C ₄ F ₉
OCH ₃ , methylene chloride / C ₆ F ₁₄ , methylene chloride / C ₇ F ₁₆ , methylene chloride / normal heptane, methylene chloride / normal hexane, methylene chloride / diethyl ether, normal heptane / C ₄ F ₉ OCF ₃ , and normal heptane / C ₆ F ₁₄
Is. By using such a combination,
The porosity and air permeability of the microporous membrane can be improved while effectively removing the liquid solvent.

The washing solvent (A) alone or the washing solvent (A) and the washing solvent (B) may be washed in three or more washing steps. The one-step or two-step treatment is effective when the liquid solvent cannot be sufficiently removed and the physical properties of the obtained polyolefin microporous film are deteriorated. In this case, at least in the final washing step, it may be treated with the washing solvent (A), and the number of washings is not particularly limited, but usually 3 steps to
There are five stages, preferably three to four stages. Further, even if treatment is performed with the same washing solvent in each stage, the production process is simply lengthened, so that the space of the polyolefin microporous membrane production equipment is expanded and the efficiency of solvent removal is reduced. For this reason, it is preferable to use different washing solvents at each stage, but the present invention is not limited to this. Therefore,
For example, in the case of three-step treatment, the same washing solvent can be used in the first step and the second step, and a different washing solvent from the first and second steps can be used in the third step.

The washing method may be a method of immersing in the washing solvent (A) and / or the washing solvent (B) for extraction, a method of showering the washing solvent (A) and / or the washing solvent (B), or a combination thereof. It can be performed by a method or the like. Also wash solvent
The (A) and the washing solvent (B) are preferably used in an amount of 300 to 30,000 parts by weight per 100 parts by weight of the gel-like molded product. When the washing solvent (A) and the washing solvent (B) are used for washing in two or more steps, the amount of the washing solvent (A) is 100 parts by weight of the washing solvent (B). It is preferably 50 to 200 parts by weight. The washing with the washing solvent (A) alone or with the washing solvent (A) and the washing solvent (B) is preferably performed until the residual solvent is less than 1% by weight based on the amount added.

The washing temperature with the washing solvent (B) is the washing solvent (B).
Depends on the boiling point of. When the boiling point of the washing solvent (B) is 150 ° C. or less, washing can be performed at room temperature, and heating washing may be performed as necessary, and washing at 20 to 80 ° C. is generally preferable.
When the boiling point of the cleaning solvent (B) is 150 ° C. or higher, the permeability to the inside of the membrane is poor at room temperature, and therefore heating cleaning is preferable.

The washing temperature and / or the rinsing temperature with the washing solvent (A) depend on the surface tension of the washing solvent (A). Specifically, it is preferable to perform the washing and / or rinsing treatment at a temperature or higher at which the surface tension of the washing solvent (A) becomes 24 mN / m or less. If the ambient temperature is lower than the temperature at which the surface tension of the washing solvent (A) becomes 24 mN / m or less, the surface tension is heated to a temperature at which the surface tension becomes 24 mN / m or less, if necessary. Washing solvent
Since the surface tension of (A) is 24 mN / m or less at 25 ° C. at most, heating is not required in most cases, and washing and / or rinsing treatment can be performed at ordinary room temperature.

(E) Drying, re-stretching and heat treatment steps The microporous membrane thus obtained is dried to substantially completely remove the solvent. The drying method is a heat drying method, an air drying method, or the like. The drying temperature is preferably a temperature below the crystal dispersion temperature of the polyolefin, and particularly preferably a temperature lower than the crystal dispersion temperature by 5 ° C. or more.

By the drying treatment, the content of the cleaning solvent (B) remaining in the polyolefin microporous film is preferably 5% by weight or less (the film weight after drying is 100% by weight), 3% by weight The following is more preferable. When the drying is insufficient and the washing solvent (B) remains in a large amount in the film, the porosity is lowered by the subsequent heat treatment and the air permeability is deteriorated, which is not preferable.

The dried microporous film is stretched again (secondary stretching).
To do. The temperature in the case of stretching again is not particularly limited,
It is preferably above the crystal dispersion temperature of the polyolefin and below the melting point. Specifically, the temperature is preferably 90 to 130 ° C, more preferably 95 to 125 ° C. Secondary stretching, stretching in at least uniaxial direction,
Biaxial stretching can also be applied. In the case of biaxial stretching, both simultaneous biaxial stretching and sequential biaxial stretching can be applied. The stretching ratio is preferably 1 to 3 times in the uniaxial direction, and the area ratio is preferably 1.01 to 9 times. If the area magnification exceeds 9 times, film rupture is likely to occur, which is not preferable. As the stretching method, any of a tenter method, a roll method, a rolling method and a combination thereof can be used.

The film obtained after the stretching again is heat-treated.
The heat treatment includes heat fixation treatment and heat relaxation treatment, and either one treatment may be performed or both treatments may be performed. The temperature of the heat setting treatment is equal to or higher than the crystal dispersion temperature of the polyolefin and lower than the melting point, preferably 90 to 130 ° C, more preferably 95 to 125 ° C. The heat setting treatment is a heat treatment that is fixed in both the machine direction (MD) and the transverse direction (TD) so that there is no dimensional change in both directions. As a result, the strain is relaxed and the heat shrinkage rate can be reduced. The heat setting treatment is performed by either a tenter method, a roll method, or a rolling method.

The thermal relaxation treatment is a heat treatment performed while shrinking in the machine direction (MD) and / or the transverse direction (TD). The temperature of the heat relaxation treatment is not less than the crystal dispersion temperature of the polyolefin and less than the melting point, preferably 90 to 130 ° C, more preferably 95 to 125 ° C.
Is. The heat relaxation treatment can significantly reduce the heat shrinkage rate. The heat relaxation treatment may be performed by a fixed method such as a tenter method, a roll method, a rolling method, or a free method using a belt conveyor method, a mesh drum (rotary drum) method, a floating method, or the like. The free method is preferable from the viewpoint of uniform physical properties in the width direction and lengthening of the winding length.

The heat setting treatment or heat relaxation treatment preferably maintains 80 to 100% in both the machine direction (MD) and the transverse direction (TD), more preferably 90 to 100%, as compared with the dimension of the film immediately before the treatment. (If both MD and TD are 100%, it means heat setting). The microporous membrane (A) is obtained by the above steps.

(B) Method for Producing Microporous Membrane (B) The method for producing the microporous membrane (B) comprises (a) a weight average molecular weight of 5 × 1.
Regarding the step of melt-kneading a polyolefin having a polyethylene of ⁵ or more as an essential component and a solvent and extruding the obtained melt-kneaded product from a die, (b) forming a gel-like sheet obtained by cooling, It is the same as the method for producing the microporous membrane (A). Therefore, (c) at least uniaxially stretched at a temperature lower than the crystal dispersion temperature of the polyolefin, (d) then stretched at least uniaxially at a temperature not lower than the crystal dispersion temperature of the polyolefin and (e) further obtained. The step of removing the solvent from the stretched product to obtain the microporous membrane (B) will be described.

The steps (c) to (e) are described in, for example, Japanese Patent Laid-Open No.
The method described in No. 240036 can be adopted. That is, first, it is stretched at least uniaxially at a temperature below the crystal dispersion temperature of the polyolefin. The stretching temperature is preferably a crystal dispersion temperature of -20 ° C or higher, and the stretching ratio is preferably at least 1.2 to 10 times in the uniaxial direction, more preferably 1.5 to 8 times. In the case of biaxial stretching, the area ratio is preferably 1.5 times or more. When narrowing the pore size distribution, it is preferable to set the area magnification to 15 times or less, but not limited to this. The stretching method may be a tenter method, a roll method, a rolling method, or a combination thereof.

Next, the polyolefin is stretched at least uniaxially at a temperature not lower than the crystal dispersion temperature and lower than the melting point. The stretching temperature is preferably 90 to 130 ° C, more preferably 90 to 125 ° C.
The stretching ratio is preferably at least 2 times or more in the uniaxial direction, and the area ratio is preferably 4 times or more. The stretching method is the same as the method performed below the crystal dispersion temperature.

Further, the solvent remaining in the obtained stretched product is extracted. The washing solvent used for extraction is surface tension (25 ° C)
Is preferably 24 mN / m or less, but may be a chlorinated hydrocarbon such as methylene chloride or carbon tetrachloride. As the washing solvent having a surface tension (25 ° C.) of 24 mN / m or less, the washing solvent (A) used in the method for producing the microporous membrane (A) can be used. The washing step can be the same as the washing step for the microporous membrane (A). The solvent-removed film is dried to obtain a microporous film (B). If necessary, stretching, heat setting, heat relaxation and the like can be performed after drying. The temperature of the stretching treatment, heat setting treatment and heat relaxation treatment is preferably equal to or higher than the crystal dispersion temperature of polyethylene and lower than the melting point. The methods of these stretching treatment, heat setting treatment and heat relaxation treatment are the same as those used in the method for producing the microporous membrane (A).

(C) Laminating Method Microporous Membrane (A) and Microporous Membrane (B) Produced by the Manufacturing Method Above
And are stacked. Lamination is usually preferably carried out by thermocompression bonding, and specifically, it is preferably carried out by calendering, embossing or the like. As the processing device, devices such as heat seal, high frequency seal, and ultrasonic seal can be used.
The processing temperature is preferably about 70 to 140 ° C. By laminating the microporous membrane (A) and the microporous membrane (B) in this way, the microporous membrane of the present invention is obtained. The lamination may have a two-stage structure such as A / B or a three-stage structure such as B / A / B, and the number of layers to be laminated is not particularly limited.

(D) Other Treatments The polyolefin microporous membrane obtained by the above-mentioned production method has a puncture strength of 9800 mN / 25 μm or more and an air permeability of 20 to 1000.
Second / 100 ml, and since the average pore diameter of the polyolefin microporous membrane changes in the film thickness direction, the physical properties of the polyolefin microporous membrane of the present invention are satisfied. The resulting microporous membrane can be further imparted with functionality by being subjected to various treatments such as the following crosslinking treatment, hydrophilic treatment and impregnation treatment.

(A) Crosslinking Treatment The obtained microporous membrane is preferably subjected to crosslinking treatment by ionizing radiation. As ionizing radiation, α rays, β rays, γ
Ray and electron beam are used, and the electron dose can be 0.1 to 100 Mrad and the acceleration voltage can be 100 to 300 kV. This can improve the meltdown temperature. The crosslinking treatment is preferably performed before the heat treatment.

(B) Hydrophilization treatment The obtained microporous membrane can be used after being hydrophilized. As the hydrophilic treatment, a monomer graft, a surfactant treatment, a corona discharge treatment or the like is used. The hydrophilic treatment is preferably performed after ionizing radiation.

When a surfactant is used, any of nonionic surfactants, cationic surfactants, anionic surfactants and amphoteric surfactants can be used, but nonionic surfactants are also usable. Is preferred. In this case, the surfactant is made into an aqueous solution or a solution of a lower alcohol such as methanol, ethanol or isopropyl alcohol, and is hydrophilized by a method such as dipping and a doctor blade.

The obtained hydrophilized microporous membrane is dried. At this time, in order to improve air permeability, it is preferable to perform heat treatment while preventing or stretching shrinkage at a temperature equal to or lower than the melting point of the microporous polyolefin membrane.

(C) Impregnation treatment By impregnating the obtained microporous membrane with an electrolytic solution, ion permeability can be imparted to the microporous membrane. The electrolytic solution is prepared by dissolving an electrolyte in an organic solvent. As the organic solvent, ethylene carbonate, propylene carbonate,
Organic solvents having a high boiling point and a high dielectric constant such as γ-butyrolactone and organic solvents having a low boiling point and a low viscosity such as tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate and diethyl carbonate are preferably used. Since an organic solvent having a high dielectric constant has a high viscosity and an organic solvent having a low viscosity has a low dielectric constant, both are often mixed and used. As the electrolyte, LiPF ₆ , L
iClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ S
O ₂ ) _{3 and the} like are preferably used. The impregnation treatment is usually performed by immersing the microporous membrane in an electrolytic solution at room temperature.

[0083]

The present invention will be described in more detail by the following examples, but the present invention is not limited thereto.
The test method in the examples is as follows.

(1) Film thickness: Measured with a contact thickness meter. (2) Porosity: Measured by a gravimetric method. (3) Air permeability: Measured according to JIS P 8117 (film thickness 20 μm
Conversion). (4) Puncture strength: 25 μm thick microporous membrane with a diameter of 1 mm (0.5 mmR)
The maximum load was measured when the needle was pierced at a speed of 2 mm / sec. (5) Heat shrinkage ratio: When the microporous film was exposed at 105 ° C for 8 hours
MD and TD shrinkage were measured respectively. (6) Average Curvature Ratio: Obtained by the following approximate equation (Kozeny-Carman equation) that introduced a three-dimensional orientation model of the fiber. k≈ [4 ⁵ ε / (3π ² δ ₀ ² S _p ² )] × (L ₀ / L) ² where k is the Kozeny constant (usually 5.0), ε is the porosity, and δ
₀ is the fibril diameter, S _p is the surface area (specific surface area) per unit volume of the solid of the porous body only, L is the apparent permeation distance (film thickness), L ₀ is the true permeation distance (the length of the penetration path), And L ₀ /
L represents the winding rate. (7) Average Pore Size: Measured with Omni Soap 360 (manufactured by Coulter).

[0085]Example 1 (1) Fabrication of microporous membrane (A) 30 parts by weight of ultra high molecular weight polyethylene (weight average molecular weight 2
× 10⁶) And 70 parts by weight of high density polyethylene (weight average
Child quantity 3.5 × 10^Five) Oxidation of 0.25 parts by weight to a total of 100 parts by weight
Inhibitor (tetrakis [methylene-3- (3,5-di-t-butyl-4
-Hydroxyphenyl) -propionate] methane)
Added composition (Mw / Mn = 16, melting point 135.0 ° C, crystal dispersion temperature
90 ° C) was dry blended and charged into a twin-screw extruder.
Flow from the side feeder installed in the twin-screw extruder
Paraffin (35cSt / 40 ℃) was added previously composition 20 times
Add to 80 parts by weight relative to the parts, melt at 200 ℃
Kneaded This is extruded from the T-die and cooled with a cooling roll.
Form a gel-like sheet by cooling to 50 ° C / min at a rate of 50 ° C / min.
After that, preheating is performed at 90 ° C and then in a drawing furnace at 105 ° C.
Introduce the temperature distribution in the film thickness direction using a tenter stretching machine.
Provided 7 times in the machine direction (MD) and 7 times in the transverse direction (TD)
Was biaxially stretched. The obtained stretched product is C_FourF₉OC₂H _Five/ N-Hep
Tan [90/10 (weight ratio), surface tension (25 ℃) 13.6mN / m / 1
[9.7 mN / m] washing solvent (A) at 25 ° C, then C_FourF₉O
C₂H_FiveWash only with [surface tension (25 ℃) 13.6mN / m] and apply
The raffin was removed and dried with hot air at 70 ° C. Then
Hold the obtained film in a tenter and machine direction (MD) at 115 ℃.
In the transverse direction (TD) and doubled in the transverse direction (TD). Then the machine
Direction (MD) and transverse direction (TD) and ratio of the previous dimension respectively
By the tenter so that it is 0.95 times and 0.95 times compared to
Perform thermal relaxation treatment at 115 ° C for 10 seconds, then use a tenter.
The microporous membrane (A) was obtained by heat setting at 120 ° C for 10 minutes.
It was

(2) Preparation of Microporous Membrane (B) After forming a gel-like sheet in the same manner as the microporous membrane (A), the membrane was held in a tenter, and the gel sheet was placed in a machine direction (MD) at 80 ° C.
And biaxially stretched 3 times in the transverse direction (TD). Then 11
It was biaxially stretched twice at 8 ° C in the machine direction (MD) and twice in the transverse direction (TD). The obtained stretched membrane was washed with methylene chloride to remove residual liquid paraffin, and then dried with hot air at 70 ° C to obtain a microporous membrane (B).

The microporous membrane (A) and the microporous membrane (B) were laminated one by one by calendering and fusing at 115 ° C. After that, heat setting treatment was performed at 120 ° C. for 30 seconds. The physical properties of the obtained microporous membrane are as follows: film thickness 25 μm, porosity 45%, air permeability 330 seconds / 100 m
l, Puncture strength 11760mN / 25μm (1200gf / 25μm), thermal shrinkage 3.8% (MD) and 4.3% (TD), and average curving rate 2.1
Met. The obtained microporous membrane has an average pore diameter of 0.05 μm on the microporous membrane (A) side and an average pore diameter of 0.09 μm on the microporous membrane (B) side.
The average pore diameter on the side of the microporous membrane (B) was larger than that on the side of the microporous membrane (A).

The performance as a battery separator was evaluated. Lithium perchlorate (1M) was added to a mixed solvent of propylene carbonate and diethylene carbonate at a ratio of 1: 1 (electrolyte), a carbon electrode was used as the negative electrode, and LiCoO ₂ was used as the positive electrode.
Was used, and the above-mentioned laminated microporous membrane was used as a battery separator. As a result of repeating charging / discharging at 4.2V for 700 cycles, the capacity did not decrease.

Example 2 A microporous membrane (A) and a microporous membrane (B) were prepared in the same manner as in Example 1. The microporous membrane (B), the microporous membrane (A), and the microporous membrane (B) were laminated in this order by calendering at 115 ° C. After that, heat setting treatment was performed at 120 ° C. for 30 seconds. The physical properties of the obtained microporous membrane are as follows: thickness 35 μm, porosity 51%, air permeability 210
Second / 100ml, Puncture strength 10780mN / 25μm (1100gf / 25μ
m), the heat shrinkages were 2.6% (MD) and 4.1% (TD), and the average curving rate was 2.1. The average pore size on the surface side (microporous membrane (B)) is 0.09 μm, and the average pore size on the inside (microporous membrane (A)) is 0.05 μm
It was m.

The performance of the battery separator was evaluated in the same manner as in Example 1. As a result, the capacity was not reduced even after 700 cycles.

Example 3 A microporous membrane (A) was prepared in the same manner as in Example 1. The microporous membrane (B) was prepared in the same manner as in Example 1 and then held in a tenter, and the machine direction (MD) at 115 ° C.
To 1.2 times, 1.4 times in the transverse direction (TD), 0.95 times in the machine direction (MD) and 0.90 times in the transverse direction (TD) with a tenter at 115 ° C for 10 seconds The treatment was performed to obtain a microporous membrane (B). The microporous film (A) and the microporous film (B) thus obtained were calendered and fused and laminated at 115 ° C. Furthermore, the physical properties of the microporous membrane obtained by heat setting treatment at 120 ° C for 30 seconds are: film thickness 25 μm, porosity 49%, air permeability 250 seconds / 100 ml, puncture strength 14210 mN / 25 μm (1490 gf / 25 μm),
The heat shrinkage was 2.2% (MD) and 2.4% (TD), and the average curving rate was 2.1. The resulting microporous membrane has an average pore diameter of 0.05 μm on the microporous membrane (A) side and an average pore diameter of 0.09 on the microporous membrane (B) side.
μm, the average pore size on the side of the microporous membrane (B), the microporous membrane (A)
It was larger than the average pore size on the side.

The performance of the battery separator was evaluated in the same manner as in Example 1. As a result, the capacity was not reduced even after 700 cycles.

[0093]Example 4 In the manufacturing process of the microporous membrane (B), using methylene chloride
C instead of extraction process_FourF ₉OC₂H_Five/ N-heptane [90/10 (heavy
Volume ratio), surface tension (25 ℃) 13.6mN / m / 19.7mN / m] cleaning
After washing with solvent (A) at 25 ° C, C_FourF₉OC₂H_Five[Surface tension (25
℃) 13.6mN / m] only
Microporous membrane in the same manner as in Example 1 except that
(Laminate of microporous membrane (A) and microporous membrane (B)) was produced. Profit
The physical properties of the obtained microporous film are as follows: film thickness 25 μm, porosity 54%, permeability
Temperament 190 seconds / 100ml, Puncture strength 14210mN / (1490gf / 25μ
m), heat shrinkage 2.0% (MD) and 1.8% (TD), and flat
The uniform road ratio was 2.1. The obtained microporous membrane is microporous.
Membrane (A) side average pore size is 0.05 μm, microporous membrane (B) side average pore size
The diameter is 0.09 μm, and the average pore size on the microporous membrane (B) side is microporous.
It was larger than the average pore size on the membrane (A) side.

The performance of the battery separator was evaluated in the same manner as in Example 1. As a result, no decrease in capacity was observed even after 700 cycles.

Example 5 60 parts by weight of ultra-high molecular weight polyethylene (weight average molecular weight 1 × 10 6) was used as the polyolefin used for producing the microporous membrane (B).
⁶ ) and 40 parts by weight of high density polyethylene (weight average molecular weight 5 × 10 ⁵ ) (Mw / Mn = 10, melting point 135.0 ° C., crystal dispersion temperature 90 ° C.) were used in the same manner as in Example 1. Thus, a microporous film (a laminated body of the microporous film (A) and the microporous film (B)) was produced. The physical properties of the obtained microporous film are 27 μm in thickness and 46 in porosity.
%, Air permeability 250 seconds / 100 ml, puncture strength 11760 mN / 25 μm (12
00gf / 25μm), heat shrinkage 4.5% (MD) and 4.7% (T
D), and the average curving rate was 2.1. The obtained microporous membrane has an average pore size of 0.05 μm on the microporous membrane (A) side,
The average pore size on the (B) side was 0.09 μm, and the average pore size on the microporous membrane (B) side was larger than the average pore size on the microporous membrane (A) side.

The performance of the battery separator was evaluated in the same manner as in Example 1. As a result, the capacity was not decreased even after 700 cycles.

Example 6 The polyolefin used for the production of the microporous membrane (B) was the ultrahigh molecular weight polyethylene alone (Mw having a weight average molecular weight of 8 × 10 ⁵ (Mw)).
/ Mn = 7, melting point 135.0 ° C., crystal dispersion temperature 90 ° C.) except that the microporous membrane (a laminated body of the microporous membrane (A) and the microporous membrane (B)) was obtained in the same manner as in Example 1. It was The physical properties of the obtained microporous membrane are as follows: film thickness 27 μm, porosity 46%, air permeability 200 seconds / 100 ml,
Puncture strength 10780mN / 25μm (1100gf / 25μm), heat shrinkage
It was 4.8% (MD) and 4.5% (TD), and the average curving rate was 2.1. The resulting microporous membrane has an average pore diameter of 0.05 μm on the microporous membrane (A) side, an average pore diameter of 0.09 μm on the microporous membrane (B) side, and an average pore diameter of the microporous membrane (B) side. It was larger than the average pore diameter on the microporous membrane (A) side.

The performance of the battery separator was evaluated in the same manner as in Example 1. As a result, no decrease in capacity was observed even after exceeding 700 cycles.

Comparative Example 1 After forming a gel-like sheet in the same manner as in Example 1, 110
A temperature distribution was provided in the film thickness direction using a tenter stretching machine at 7 ° C., and biaxial stretching was performed 7 times in the machine direction (MD) and 7 times in the transverse direction (TD). The resulting stretched product was treated with methylene chloride (25
Liquid paraffin was removed by washing with a surface tension of 27 mN / m at ℃, and dried by air drying. Then the obtained film is 110 ℃
1.5 times in the machine direction (MD) and in the transverse direction (T
Simultaneous biaxial stretching is performed so as to double in D), then 95% in the machine direction (MD) and transverse direction (T
D) was 90% heat-treated at 115 ° C for 10 seconds, and then heat-set at 120 ° C for 15 minutes by a tenter. The physical properties of the obtained microporous film are as follows: film thickness 10 μm, porosity 40%, puncture strength 8330 mN / 25 μm (850 gf
/ 25μm), Air permeability 600 seconds / 100ml, Thermal shrinkage 2.4% (MD)
And 2.5% (TD), and an average curving rate of 1.7. The average pore size was 0.03 μm both on the surface and inside.

The performance of the battery separator was evaluated in the same manner as in Example 1. As a result, it was possible to use it for up to 700 cycles, but it was possible to charge up to a voltage of 2.7 V, and the capacity was clearly reduced.

Comparative Example 2 A microporous membrane was produced in the same manner as in Example 1 except that the heat setting treatment at 120 ° C. for 15 minutes was not performed in the production of the microporous membrane (A). The physical properties of the obtained microporous membrane are 25 μm in thickness.
m, Porosity 60%, Air permeability 250 seconds / 100ml, Puncture strength 9800mN
/ 25μm (1000gf / 25μm), heat shrinkage 9.1% (MD) and
It was 9.9% (TD) and the average curving rate was 2.1. The obtained microporous membrane had an average pore diameter on the microporous membrane (A) side of 0.07 μm and an average pore diameter on the microporous membrane (B) side of 0.09 μm.

The performance of the battery separator was evaluated in the same manner as in Example 1. As a result, charging could not be performed in 450 cycles. When the battery was disassembled, a membrane rupture was observed.

[0103]

[Table 1]

[0104]

As described above, the microporous polyolefin membrane of the present invention is a gel-like sheet obtained by melt-kneading an ultrahigh molecular weight polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more and a solvent and extruding. From the stretching, washing, heat treatment, etc., the microporous membrane (A) and the microporous membrane (B) obtained by changing the conditions are laminated, so that the average pore diameter changes in the thickness direction, and the ion permeability is improved. In addition to being excellent, it is also excellent in physical properties such as puncture strength and heat shrinkage. Therefore, it has excellent physical properties as a battery separator.

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) C08L 23:00 C08L 23:00 F term (reference) 4F074 AA16 AA17 AB01 AD01 AD11 CB34 CB43 CC02X CC02Z CC04X CC05X CC22X CC27Y CC29Y CC32X CC32Y CC32Z DA08 DA10 DA49 4F212 AA03 AA04 AG01 AG03 AG20 UA15 UN29 5H021 BB01 BB02 BB05 BB13 CC04 CC05 EE01 EE04 HH00 HH03 HH06 HH07 HH09 5H029 AJ02 AJ05 AJ12 AK03 AL06 AM03 AM05 AM07 CJ02 CJ08 CJ12 DJ04 EJ11 EJ12 HJ00 HJ06 HJ14

Claims (6)

[Claims] 1. A microporous polyolefin membrane having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component of polyethylene, having a puncture strength of 9800 mN / 25 μm or more and an air permeability of 20 to 1000.
The polyolefin microporous membrane is characterized in that the average pore diameter of the polyolefin microporous membrane changes in the film thickness direction. 2. The polyolefin microporous membrane according to claim 1, wherein the average pore diameter of at least one surface of the polyolefin microporous membrane is larger than the internal average pore diameter. 3. The polyolefin microporous membrane according to claim 1, wherein the average pore diameter of one surface of the polyolefin microporous membrane is larger than the average pore diameter of the other surface. 4. A gel sheet is formed by melt-kneading a polyolefin having polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent, extruding the obtained melt-kneaded product through a die, and cooling. After that, the gel-like sheet is biaxially stretched by providing a temperature distribution in the film thickness direction, and the solvent is removed from the obtained stretched product by using a washing solvent (A) having a surface tension of 24 mN / m or less at 25 ° C. , Then, after stretching in at least uniaxial direction, further microporous membrane obtained by heat treatment at a temperature below the melting point of the crystal dispersion of the polyolefin (A),
The gel-like sheet is stretched at least uniaxially at a temperature lower than the crystal dispersion temperature of the polyolefin, and then stretched at least uniaxially at a temperature not lower than the crystal dispersion temperature of the polyolefin and a solvent from the obtained stretched product. A method for producing a polyolefin microporous membrane, which comprises laminating a microporous membrane (B) obtained by removing 5. A battery separator using the polyolefin microporous membrane according to claim 1. 6. A battery using the microporous polyolefin membrane according to claim 1 as a battery separator.