JP2003105122A - Polyolefin minute porous film and method of its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyolefin minute porous film having high strength to piercing even with a thin film of 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 thermal contraction in mechanical direction (MD) and transverse direction (TD) of not greater than 8% when exposed at 105 deg.C for 8 hours, a strength to piercing of not lower than 14, 700 mN/25 μm, and an air permeability of 20-1,000 second/100 ml.

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 required level for battery characteristics such as a lithium battery is becoming higher 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. In particular, from the viewpoint of improving the battery capacity, it is required to reduce the thickness of the microporous film from the previous 25 to 30 μm to about 10 μm, and the puncture strength is 14700 mN /
It is in a situation where it cannot be used unless it is about 25 μm (1500 gf / 25 μm).

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). The puncture strength is improved by simply increasing the draw ratio, but conversely the heat shrinkage ratio is deteriorated, and it is not possible to sufficiently satisfy the strict requirements demanded for the 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 piercing strength is 9800mN / 25μm (1000gf / 2
Although it is as high as 5 μm), 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 dendrite growth easily occurs due to heat generation. Therefore, the 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. Further, the permeability is about 800 seconds / 100 ml, and the flexibility is as high as 2.5 to 7.0, so 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 microporous polyolefin membrane having a high puncture strength, a low thermal shrinkage and a good permeability even in a thin film having a thickness of about 10 μm. Is.

[0010]

Means for Solving the Problems As a result of intensive studies in view of the above objects, the present inventors have found that a melt-kneaded product obtained from a solvent containing a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent is used. The gel sheet obtained by extrusion from a die and cooling was biaxially stretched with a temperature distribution in the film thickness direction, and the residual solvent had a surface tension (25 ° C) of 24 mN / m.
Removed using the following washing solvent (A), by further stretching and heat treatment at a predetermined temperature, puncture strength even in a thin film having a thickness of about 10μm, a microporous film excellent in thermal shrinkage and permeability. The present invention was discovered and it came to 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.
Thermal shrinkage in both machine direction (MD) and transverse direction (TD) is 8% or less and puncture strength is 14700m when exposed at 8 ℃ for 8 hours.
N / 25 μm or more and air permeability of 20 to 1000 seconds / 100 ml
Is characterized in that.

The first method for producing a polyolefin microporous membrane of the present invention is to melt-knead a polyolefin having a weight-average molecular weight of 5 × 10 ⁵ or more and polyethylene as an essential component, and a solvent to obtain a melt-kneaded product. After extruding from a die and 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 then the surface tension at 25 ° C. is 24 mN / m or less as a washing solvent (A ) Is used to remove the solvent from the biaxially stretched stretched product, and the resulting microporous film is stretched again at a temperature not lower than the crystal dispersion temperature of the polyolefin and lower than the melting point, and higher than the crystal dispersion temperature of the polyolefin and lower than the melting point. It is characterized in that it is heat-treated at the temperature of.

The second method for producing a microporous polyolefin membrane of the present invention is to melt-knead a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent, and obtain the melt-kneaded product. After extruding from a die and 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 then the surface tension at 25 ° C. is 24 mN / m or less as a washing solvent (A And a microporous film (A) obtained by removing the solvent from the biaxially stretched product using (1), and the microporous film (A) at a temperature not lower than the melting point and higher than the crystal dispersion temperature of the polyolefin. At least two layers are laminated with the microporous film (B) obtained by stretching and heat-treating the polyolefin at a temperature higher than the crystal dispersion temperature and lower than the melting point.

The third method for producing a microporous polyolefin membrane of the present invention is to melt-knead a polyolefin containing polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent, and obtain the melt-kneaded product. After extruding from a die and 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 then the surface tension at 25 ° C. is 24 mN / m or less as a washing solvent (A ) Is used to remove the solvent from the biaxially stretched stretched product, and at least two or more microporous membranes (A) obtained are laminated, and then stretched again at a temperature equal to or higher than the crystal dispersion temperature of the polyolefin and lower than the melting point. In addition, the heat treatment is performed at a temperature not lower than the crystal dispersion temperature of the polyolefin and lower than the melting point.

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

The characteristics of the microporous polyolefin 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 even 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.

[0018]

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 may 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, so that ultrahigh molecular weight polyethylene and polyethylene (weight average molecular weight of 1 × 10 ⁴ or more and 5 × 10 5) can be used. ⁽ 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) Physical Properties The polyolefin microporous membrane of the present invention has a heat shrinkage ratio (105
8% or less in both machine direction (MD) and transverse direction (TD), preferably 5% or less, more preferably 3%
It is the following. 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. Puncture strength is 14700mN / 25μm (1500gf
/ 25 μm) or more, preferably 15680 mN / 25 μm (160
0 gf / 25 μm) or more, more preferably 19600 mN / 25 μm (20
00gf / 25μm) or more. Puncture strength is 14700mN / 25μm
If it is lower than the above range, pinholes are likely to occur when incorporated in an electrode as a lithium battery. The average winding ratio is not particularly limited, but is preferably 1.2 to 2.5, more preferably 1.2.
~ 2.2. If the tortuosity is too high, the ion permeability will decrease significantly, and if it is too low, the ion permeability will improve.
When used in a lithium battery separator, short circuits are likely to occur. The air permeability is 20 to 1000 seconds / 100 ml. If it exceeds 1000 seconds / 100 ml, the battery capacity will decrease when used in a lithium battery, and if it is less than 20 seconds / 100 ml, the strength will be insufficient.

[2] Production Method (A) First Production Method The first production method of the polyolefin microporous membrane of the present invention is
A polyolefin containing a polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent are melt-kneaded, and the obtained melt-kneaded product is extruded from a die and cooled to form a gel-like sheet. Was biaxially stretched with a temperature distribution in the film thickness direction, and then the surface tension at 25 ° C was 24 mN / m.
The solvent is removed from the stretched product biaxially stretched using the following washing solvent (A), the resulting microporous film is stretched again at a temperature of the crystal dispersion temperature of the polyolefin or lower than the melting point, and further the crystal dispersion temperature or higher melting point. It is carried out by heat treatment at a temperature below.

(A) Process for Preparing Polyolefin Solution The above solvent is preferably one which is in a state of being mixed with the polyolefin in a melt-kneaded state. Nonane as a preferred solvent,
Aliphatic or cyclic hydrocarbons such as decane, decalin, paraxylene, undecane, dodecane, liquid paraffin, mineral oil fractions having boiling points corresponding to these, dioctyl phthalate (DOP), dibutyl phthalate (DBP), etc., Examples thereof include phthalic acid esters which are liquid at room temperature. A non-volatile solvent such as liquid paraffin is particularly preferable for obtaining a gel-like molded product having a stable solvent content. A mixed solvent may be used as long as it does not depart from the gist of the present invention.

The blending ratio of the polyolefin and the solvent is 1 to 50% by weight, preferably 20 to 40% by weight, with the total amount of both being 100% by weight. 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 antiblocking 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 method of melt kneading 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 (primarily stretched) 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 of 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) and 3 times or more in the transverse direction (TD), and the area ratio is 20 times or more. 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. 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) depends on the temperature and can be lowered as the use temperature is raised, 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, and is a value measured based on JIS K 3362.

As the washing solvent (A), a fluorine compound 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 aliphatic propionate ethyl 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.

In the final washing step, the washing solvent (A)
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 flammable 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.

Examples of the aromatic hydrocarbon having 6 or more carbon atoms in which at least a part of hydrogen atoms are replaced with halogen atoms are 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 with 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 by 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 the permeability of the microporous membrane can be improved while effectively removing the liquid solvent.

The washing with the washing solvent (A) alone or with the washing solvent (A) and the washing solvent (B) may be performed 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.

The content of the cleaning solvent (B) remaining in the microporous polyolefin membrane is preferably 5% by weight or less by the drying treatment (the weight of the membrane 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 microporous membrane after drying is stretched again (secondary stretching).
To do. The temperature for re-stretching is preferably not lower than the melting point of the polyolefin crystal (usually about 90 ° C.) and lower than the melting point. By stretching at such a temperature, the permeability can be improved. If the temperature is lower than the crystal dispersion temperature, sufficient stretching cannot be performed. As the secondary stretching, both uniaxial stretching and biaxial stretching can be applied, and in the case of biaxial stretching, both simultaneous biaxial stretching and sequential biaxial stretching can be applied. The draw ratio is preferably 1.001 to 9 times in the uniaxial direction.
It 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 obtained film is heat-treated at a temperature higher than the crystal dispersion temperature of polyolefin and lower than the melting point. 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 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. This makes it possible to reduce the heat shrinkage without reducing the air efficiency and the air permeability. 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). By performing the thermal relaxation treatment at a temperature not higher than the melting point of the polyolefin, preferably not lower than the crystal dispersion temperature of the polyolefin and not higher than the melting point, the heat shrinkage rate can be significantly relaxed. Thermal relaxation preferably maintains 80% or more of the length immediately before relaxation in both the machine direction (MD) and the transverse direction (TD), and may be relaxed in only one direction depending on the application. 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.

(B) Second Manufacturing Method The second manufacturing method of the polyolefin microporous membrane of the present invention is
A polyolefin containing a polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent are melt-kneaded, and the obtained melt-kneaded product is extruded from a die and cooled to form a gel-like sheet. Was biaxially stretched with a temperature distribution in the film thickness direction, and then the surface tension at 25 ° C was 24 mN / m.
A microporous film (A) obtained by removing the solvent from the stretched product biaxially stretched using the following washing solvent, and the microporous film (A) is further recrystallized at a temperature not lower than the melting point and higher than the crystal dispersion temperature of the polyolefin. It is carried out by laminating at least two layers with the microporous membrane (B) obtained by stretching and heat treatment at a temperature not lower than the crystal dispersion temperature of the polyolefin and lower than the melting point.

Among these, (a) the weight average molecular weight is 5 × 10 ⁵
The above-mentioned step of melt-kneading a polyolefin containing polyethylene as an essential component and a solvent, (b) extruding the melt-kneaded product from a die, and cooling to form a gel-like sheet, (c) the gel-like sheet in the film thickness direction Biaxially stretching with temperature distribution on the surface, (d) Washing solvent with surface tension (25 ℃) of 24mN / m or less
The step of removing the solvent from the stretched product biaxially stretched using (A) is the same as in the first manufacturing method. This (a) ~
The microporous membrane (A) is obtained by only performing the step (d) without stretching after solvent removal and subsequent heat treatment. The obtained microporous membrane (A) is stretched again at a temperature not lower than the crystal dispersion temperature of the polyolefin and lower than the melting point in the same manner as in the first method, and further heat-treated at a temperature not lower than the crystal dispersion temperature of the polyolefin and lower than the melting point. To produce a microporous membrane (B). 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 method of re-stretching and heat treatment (heat setting treatment and heat relaxation treatment) is the same as in the case of the first manufacturing method.

The microporous membrane (A) and the microporous membrane (B) are laminated to produce a multi-layered polyolefin microporous membrane. Stacking is A / B
It may be a two-stage structure like, or 3 like B / A / B.
A step structure may be used, and the number of stacked steps is not particularly limited. Although not limited, it is preferable to use the microporous membrane (B) as the surface layer. By doing so, it is possible to improve the transparency. Lamination is usually performed by fusion using a known method such as roll or embossing. The fusion is preferably performed at a temperature higher than the crystal dispersion temperature and lower than the melting point.

(C) Third Production Method The third production method of the polyolefin microporous membrane of the present invention is
A polyolefin containing a polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent are melt-kneaded, and the obtained melt-kneaded product is extruded from a die and cooled to form a gel-like sheet. Was biaxially stretched with a temperature distribution in the film thickness direction, and then the surface tension at 25 ° C was 24 mN / m.
The solvent was removed from the stretched product biaxially stretched using the following washing solvent (A), and at least two or more microporous membranes (A) obtained were laminated, and then the laminated microporous membrane was subjected to polyolefin crystal dispersion. It is carried out by re-stretching at a temperature equal to or higher than the temperature and lower than the melting point, and further heat-treated at a temperature equal to or higher than the crystal dispersion temperature of the polyolefin and lower than the melting point.

Of these, (a) the weight average molecular weight is 5 × 10 ⁵
The above-mentioned step of melt-kneading a polyolefin containing polyethylene as an essential component and a solvent, (b) extruding the melt-kneaded product from a die, and cooling to form a gel-like sheet, (c) the gel-like sheet in the film thickness direction Biaxially stretching with temperature distribution on the surface, (d) Washing solvent with surface tension (25 ℃) of 24mN / m or less
The step of removing the solvent from the stretched product biaxially stretched using (A) is the same as in the first manufacturing method. This (a) ~
The microporous membrane (A) is obtained by only performing the step (d) without stretching after solvent removal and subsequent heat treatment. At least two or more obtained microporous membranes (A) are laminated. As a method for laminating the microporous film, the same method as the second manufacturing method can be used. By laminating in this way, stretching after solvent removal is facilitated.

The laminated microporous membrane is stretched again (secondary stretching).
To do. The stretching temperature is preferably higher than the crystal dispersion temperature of polyolefin (usually about 90 ° C.) and lower than the melting point. By stretching at such a temperature, the permeability can be improved. If the temperature is lower than the crystal dispersion temperature, stretching cannot be sufficiently performed. This stretching ratio is preferably 1.01 times or more and less than 9 times in terms of area ratio. If it is 9 times or more, film rupture easily occurs, which is not preferable. The stretching method may be a tenter method, a roll method, a rolling method or a combination thereof.

The obtained film is heat-treated at a temperature not lower than the crystal dispersion temperature of polyolefin and lower than the melting point. 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 thermal relaxation treatment can significantly reduce the thermal contraction rate. Due to thermal relaxation, the machine direction (MD) and transverse direction (TD) are
It is preferably maintained at 80% or more, and may be relaxed in only one direction. The method of heat fixation treatment and heat relaxation treatment is the first
This is the same as the case of the manufacturing method of.

(D) Other Treatments The polyolefin microporous membranes obtained by the first to third production methods were subjected to a machine direction (M
D) and transverse direction (TD) both have a heat shrinkage of 8% or less, a puncture strength of 14700 mN / 25 μm or more, and an air permeability of 20 to 1000 seconds / 100 ml. Fulfill. 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 It is preferable that the microporous membranes obtained by the first to third methods are subjected to a crosslinking treatment by ionizing radiation. Α-rays, β-rays, γ-rays and electron beams are used as ionizing radiation.
It can be performed at 0.1 to 100 Mrad and acceleration voltage of 100 to 300 kV. This can improve the meltdown temperature. The crosslinking treatment is preferably performed before the heat treatment.

(B) Hydrophilization treatment The microporous membranes obtained by the first to third methods can also 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 the permeability, it is preferable to perform heat treatment while preventing shrinkage or stretching at a temperature equal to or lower than the melting point of the polyolefin microporous membrane.

(C) Impregnation treatment The microporous membranes obtained by the first to third methods are impregnated with an electrolytic solution. The electrolytic solution is prepared by dissolving an electrolyte in an organic solvent. As the organic solvent, ethylene carbonate, propylene carbonate, an organic solvent having a high boiling point and a high dielectric constant such as γ-butyrolactone, or a low boiling point and a low boiling point of tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, dioxolane, dimethyl carbonate, diethyl carbonate, etc. A viscous organic solvent is 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 , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃
SO ₂ ) ₂ , LiC (CF ₃ SO 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. Ion permeability can be imparted to the microporous membrane by impregnating it with the electrolytic solution.

[0085]

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.

Example 1 30 parts by weight of ultra high molecular weight polyethylene (weight average molecular weight 2
X 10 ⁶ ) and 70 parts by weight of high-density polyethylene (weight average molecular weight 3.5 × 10 ⁵ ) in total of 100 parts by weight, 0.25 parts by weight of an antioxidant (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.
Liquid paraffin (35 cSt / 40 ° C.) was charged from a side feeder provided in the twin-screw extruder so as to be 80 parts by weight with respect to 20 parts by weight of the composition, and melt-kneaded at 200 ° C. This is extruded from a T-die and cooled to 0 ℃
After cooling at a rate of 50 ° C / min, preheating is performed at 95 ° C, then it is introduced into a stretching furnace at 105 ° C and a tenter stretching machine is used to provide a temperature distribution in the film thickness direction and machine direction (MD). To 7
And biaxially stretched 7 times in the transverse direction (TD). The obtained stretched product was washed with C ₄ F ₉ OC ₂ H ₅ / n-heptane [90/10 (weight ratio), surface tension (25 ° C) 13.6mN / m / 19.7mN / m] washing solvent (A).
After washing at 25 ℃, C ₄ F ₉ OC ₂ H ₅ [surface tension (25 ℃) 13.6mN
/ m] to remove liquid paraffin and dry with hot air at 70 ° C. to obtain a microporous membrane (A). Then, the obtained film was roll-stretched 1.4 times in the machine direction (MD) at 115 ° C.,
It was stretched twice in the transverse direction (TD) with a tenter at 115 ° C. Then 95% in machine direction (MD) compared to the last dimension
And 11 by a tenter so that it becomes 95% in the lateral direction (TD).
Relaxation treatment is performed for 10 seconds at 0 ℃, and then 120 with a tenter.
A heat setting treatment was performed at 15 ° C. for 15 minutes to obtain a microporous membrane (B).

The physical properties of the obtained microporous membrane (B) are 10 μm in thickness.
m, porosity 45%, air permeability 270 seconds / 100ml, puncture strength 16072mN
/ 25μm (1640gf / 25μm), heat shrinkage 2.5% (MD) and
It was 2.9% (TD) and the average curving rate was 2.1. Using the obtained microporous membrane (B) as a battery separator, a lithium battery was prepared using lithium perchlorate as an electrolyte in a 1: 1 (weight ratio) mixed solvent of propylene carbonate and diethyl carbonate. When an overcharge test was carried out in, no defect occurred.

Example 2 Microporous membrane (A) and microporous membrane prepared in the same manner as in Example 1
(B) and (B) were laminated by fusing with a calendar roll at 110 ° C. The physical properties of the laminated microporous membrane are 25μ in thickness.
m, porosity 47%, air permeability 180 seconds / 100ml, puncture strength 15190mN
/ 25μm (1550gf / 25μm), heat shrinkage 2.8% (MD) and
It was 3.0% (TD) and the average curving rate was 2.0.

Example 3 Two microporous membranes (A) prepared in the same manner as in Example 1 were superposed and heat-sealed with a calender roll at 110 ° C., and the roll was rolled 2.0 times in the machine direction (MD) at 110 ° C. The film was stretched and then stretched by a tenter in the transverse direction (TD) at 115 ° C. by a factor of 2.2. Further, heat relaxation treatment was carried out by a tenter at 115 ° C for 15 seconds so that the machine direction (MD) was 90% and the transverse direction (TD) was 90%, and then the heat setting treatment was carried out by the tenter at 120 ° C for 15 minutes. The physical properties of the obtained porous film are as follows: film thickness 21 μm, porosity 51%, air permeability 250 seconds / 100 ml, puncture strength 17640 mN / 25 μm (1800 gf / 25
μm), heat shrinkages of 2.8% (MD) and 3.0% (TD), and an average curving rate of 2.1.

Example 4 As a polyolefin, 60 parts by weight of ultra-high molecular weight polyethylene (weight average molecular weight 1 × 10 ⁶ ) and 40 parts by weight of high density polyethylene (weight average molecular weight 3 × 10 ⁵ ) were added in total of 100 parts by weight of polyethylene ( A polyolefin microporous membrane was produced in the same manner as in Example 1 except that Mw / Mn = 10, melting point 134.8 ° C., crystal dispersion temperature 90 ° C.) was used. The physical properties of the obtained microporous membrane are as follows: film thickness 10 μm, porosity 45%, air permeability 260 seconds / 100 ml, puncture strength 14950 mN / 25 μm (1525 gf / 25 μm), thermal shrinkage 2.4.
% (MD) and 2.8% (TD), and the average curving rate was 2.2.

Example 5 100 parts by weight of ultrahigh molecular weight polyethylene as a polyolefin (weight average molecular weight 8 × 10 ⁵ , Mw / Mn = 7, melting point 135.0)
A polyolefin microporous membrane was produced in the same manner as in Example 1 except that the temperature was 0 ° C. and the crystal dispersion temperature was 90 ° C.). The physical properties of the obtained microporous membrane are as follows: film thickness 10 μm, porosity 46%, air permeability 220
Second / 100ml, puncture strength 17250mN / 25μm (1760gf / 25μ
m), the heat shrinkages were 2.9% (MD) and 2.8% (TD), and the average curving rate was 2.2.

Example 6 A microporous membrane (A) was obtained in the same manner as in Example 1 except that preheating was carried out at 90 ° C. and then introduced into a stretching furnace at 110 ° C. Three sheets of this microporous membrane (A) were laminated, and stretched twice at 110 ° C in the machine direction (MD) and then stretched at 115 ° C by a tenter in the transverse direction (TD) 2.1 times. Furthermore, compared with the immediately preceding dimension, heat relaxation treatment was performed at 110 ° C for 10 seconds by a tenter so that the machine direction (MD) was 90% and the transverse direction (TD) was 90%, and then heat setting treatment was performed at 120 ° C for 15 minutes by a tenter. A microporous membrane was obtained. The physical properties of the obtained microporous membrane are as follows:
Porosity 45%, Air permeability 450 seconds / 100ml, Puncture strength 15190mN / 2
5μm (1700gf / 25μm), thermal shrinkage 2.2% (MD) and 2.1
% (TD), and the average curving rate was 2.1.

Comparative Example 1 A stretched product (primary stretched product) was obtained in the same manner as in Example 1. The obtained stretched product was treated with methylene chloride (surface tension at 25 ° C: 27 mN /
m) was washed at 25 ° C to remove liquid paraffin, and dried by air drying. Then, the obtained film was simultaneously biaxially stretched at 110 ° C. by a tenter so as to be 1.5 times in the machine direction (MD) and 2 times in the transverse direction (TD). Furthermore, the microporous membrane is heat-relaxed at 110 ° C for 10 seconds by a tenter so that it is 95% in the lengthwise direction and 95% in the widthwise direction as compared with the size immediately before, and then heat set at 120 ° C for 15 minutes by a tenter. Got The physical properties of the obtained microporous membrane are 10 μm in thickness and 40 in porosity.
%, Puncture strength 10200 mN / 25 μm, air permeability 600 seconds / 100 ml,
The heat shrinkage was 1.4% (MD) and 1.5% (TD), and the average curving rate was 1.7.

Comparative Example 2 A microporous membrane was produced in the same manner as in Example 1 except that the heat setting treatment in the final step was omitted. The physical properties of the obtained microporous film are as follows: film thickness 10 μm, porosity 48%, puncture strength 15680 mN / 25 μm
(1600gf / 25μm), Air permeability 400sec / 100ml, Thermal shrinkage 9.7
% (MD) and 8.8% (TD), and the average curving rate was 1.7.

[0096]

[Table 1]

[0097]

As described above, the polyolefin microporous membrane of the present invention is obtained by extruding a melt-kneaded product obtained from a polyolefin containing a polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component and a solvent, and cooling it. After the obtained gel-like sheet is biaxially stretched with a temperature distribution in the film thickness direction, the remaining solvent is washed solvent with a surface tension (25 ° C) of 24 mN / m or less (A).
Since it is produced by stretching and heat setting at a temperature lower than the crystal dispersion temperature of the polyolefin, the puncture strength is high, the heat shrinkage rate is low, and the permeation rate is low even in a thin film with a thickness of about 10 μm. Good property. Therefore, it has excellent physical properties especially as a battery separator.

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Claims (6)

[Claims] 1. A microporous polyolefin membrane comprising polyethylene having a weight average molecular weight of 5 × 10 ⁵ or more as an essential component, and having a machine direction (MD) and a transverse direction (TD) when exposed at 105 ° C. for 8 hours. Both heat shrinkage rate is 8% or less, piercing strength
14700mN / 25μm or more and air permeability of 20 to 1000 seconds /
A microporous polyolefin membrane characterized by being 100 ml. 2. 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 then the surface tension at 25 ° C. is 24 mN / m or less and the washing solvent (A) is used from the biaxially stretched stretched product. The solvent is removed, and the resulting microporous film is stretched again at a temperature of the crystal dispersion temperature of the polyolefin or higher and lower than the melting point, and further heat treated at a temperature of the crystal dispersion temperature of the polyolefin or higher and lower than the melting point. Method for manufacturing porous membrane. 3. A gel sheet is formed 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, 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 then the surface tension at 25 ° C. is 24 mN / m or less and the washing solvent (A) is used from the biaxially stretched stretched product. A microporous film obtained by removing the solvent (A), the microporous film (A) is stretched again at a temperature of the crystal dispersion temperature of the polyolefin or higher and lower than the melting point, and further the crystal dispersion temperature of the polyolefin or lower than the melting point. A method for producing a polyolefin microporous membrane, comprising laminating at least two layers with the microporous membrane (B) obtained by heat treatment at the temperature of. 4. A melt-kneaded polyolefin and a solvent having a weight-average molecular weight of 5 × 10 ⁵ or more as an essential component of polyethylene, and the melt-kneaded product is extruded from a die and cooled to form a gel-like sheet, The gel-like sheet is biaxially stretched by providing a temperature distribution in the film thickness direction, and then the surface tension at 25 ° C. is 24 mN / m or less, and the washing solvent (A) is used to draw the solvent from the biaxially stretched product. Microporous membrane obtained by removal
After laminating at least two sheets of (A), it is stretched again at a temperature not lower than the crystal dispersion temperature of the polyolefin and lower than the melting point,
Further, the method for producing a microporous polyolefin membrane is characterized by heat-treating at a temperature not lower than the crystal dispersion temperature of the polyolefin and lower than the melting point. 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.