JP2006111712A - Polyolefin microporous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polyolefin microporous film suitably used as a separator for a battery and to provide a method for producing the same. <P>SOLUTION: The polyolefin microporous film comprises a polyolefin composition containing ≥1.0 wt.% of an ultra high molecular weight polyolefin having ≥5×10<SP>5</SP>viscosity average molecular weight and has 25-60% porosity and a porous structure of a branch and trunk-like fibril. The area of the porous part observed on the surface by a scanning electron microscope amounts to ≥80% and, in the porous part, the average diameter of trunk-like fibril is 0.01-0.05 μm and the average diameter of branch-like fibril crossing the trunk is 0.01-0.03 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polyolefin microporous membrane containing ultra-high molecular weight polyolefin, and more particularly to a polyolefin microporous membrane suitably used as a battery separator such as a lithium ion secondary battery, and a method for producing the same.

  Various polyolefin microporous membranes containing ultra-high molecular weight polyolefin have been proposed, and as a production method thereof, a solution comprising an ultra-high molecular weight polyolefin composition and a solvent is prepared, an extrusion gel composition is formed from a die, and then There is a method of producing by stretching and then removing the residual solvent (see Patent Document 1).

  These methods require studies on the control of the amount of solvent in a solution comprising an ultrahigh molecular weight polyolefin composition and a solvent, and it has also been proposed to prepare a polymer solution by combining volatile and non-volatile solvents ( Patent Document 2).

  Various polyolefin microporous membranes containing various ultra-high molecular weight polyolefins have been proposed as films suitable for use as battery separators (Patent Document 3, etc.), but thinner and higher strength films are required.

JP 7-138404 A JP-A-6-228333 JP-A-6-325747

  The present invention relates to a polyolefin microporous membrane containing ultra-high molecular weight polyolefin, and relates to a polyolefin microporous membrane having a uniform microporous structure and suitably used as a battery separator, and a method for producing the same.

The present invention comprises a polyolefin composition containing 1% by weight or more of an ultrahigh molecular weight polyolefin having a viscosity average molecular weight of 5 × 10 ⁵ or more, has a porosity of 25 to 60%, and has an open pore structure made of branched fibrils. Polyolefin microporous film having an area occupied by the aperture observed on the surface with a scanning electron microscope is 80% or more, and the average diameter of the trunk fibrils is 0.01 to 0.05 μm in the aperture The polyolefin microporous membrane is characterized in that the average diameter of the branched fibrils intersecting the trunk is 0.01 to 0.03 μm.

  Since the polyolefin microporous membrane of the present invention has a uniform microporous structure, and has a low porosity and appropriate permeability, it is suitably used as a high-strength battery separator even if it is extremely thin.

  Hereinafter, the present invention will be described in detail. In addition, these Examples etc. and description illustrate this invention, and do not restrict | limit the scope of the present invention.

The polyolefin microporous membrane of the present invention comprises a polyolefin composition containing at least 1% by weight of an ultrahigh molecular weight polyolefin having a viscosity average molecular weight of 5 × 10 ⁵ or more.

Ratio of viscosity average molecular weight 5 × 10 ⁵ 10,000 or more ultra-high molecular weight polyolefin in the polyolefin composition is 1 wt% or more, preferably 10 to 90 wt%. Examples of the polyolefin include crystalline homopolymers or copolymers obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Of these, polyethylene and copolymers thereof are preferable.

  The ultra-high molecular weight polyolefin may be a combination of two or more types, that is, a combination of different types of ultra-high molecular weight polyolefins, or a combination of the same type but different in molecular weight range and molecular weight distribution.

Examples of components other than the ultra-high molecular weight polyolefin in the polyolefin composition include polyolefins having a viscosity average molecular weight of less than 5 × 10 ⁵ . The polyolefin having a viscosity average molecular weight of less than 5 × 10 ⁵ may be the same type of polyolefin as that of an ultra high molecular weight or a different type of polyolefin. Examples of the polyolefin include crystalline homopolymers or copolymers obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like. Of these, polyethylene and copolymers thereof are preferable. Further, linear polyethylene is preferable, and high density polyethylene is more preferable.

  Various other additives such as an ultraviolet absorber, a lubricant, an antiblocking agent, a pigment, a dye, and an inorganic filler can be added to the other polyolefin composition as necessary without departing from the object of the present invention.

  The polyolefin microporous membrane of the present invention is extremely thin and has a thickness of 20 μm or less, preferably 0.1 to 16 μm. The porosity determined by the apparent density is 25 to 60%, more preferably 25 to 50%.

  The polyolefin microporous membrane of the present invention is characterized by having an open pore structure composed of branched fibrils and uniform micropores. That is, the area occupied by the aperture when the surface is observed with a scanning electron microscope is 80% or more, and the average diameter of the stem-like fibrils is 0.01 to 0.05 μm in the aperture, and the branches intersect with the stem. The average diameter of the fibrils is 0.01 to 0.03 μm.

  When the surface is observed with a scanning electron microscope, a branch-like fibril group is observed in one direction, and a branch-like fibril group is observed in a direction substantially perpendicular to the trunk-like fibril group. Refers to the structure. As for the area occupied by the aperture, take a random number of 2,000 times photographs of a scanning electron microscope, close the aperture (the part that appears black and the closed state is observed when enlarged), and other apertures. It can be determined from the area ratio to the hole (the part that appears white). The area occupied by the opening is more preferably 90% or more.

  The average diameter of the stem fibrils and branch fibrils in the opening is determined from the average value of the diameters of the branch fibrils and the stem fibrils observed from the 50,000 times the scanning electron micrograph of the aperture. Can do.

  The membrane of the present invention preferably has an average pore size measured by a Coulter porometer of 0.01 to 0.1 μm, more preferably 0.03 to 0.1 μm.

  The membrane of the present invention preferably has a pore volume measured with a Coulter porometer of 0.3 to 1 ml / g, more preferably 0.5 to 1 ml / g.

  The film of the present invention preferably has a breaking strength of 60 MPa to 200 MPa. That is, the membrane of the present invention is characterized by high strength despite its large pore volume. The film of the present invention preferably has a Gurley value of 150 to 1000 seconds / 100 cc.

  As described above, the polyolefin microporous membrane of the present invention has an open pore structure composed of branched fibrils, expresses a uniform microporous structure, and has appropriate permeability even at a low porosity. Even if it is thin, it is suitably used as a battery separator having high strength and good battery performance. Moreover, it is used suitably for battery separators, such as a lithium ion secondary battery and a lithium ion polymer battery.

Next, a method for producing the polyolefin microporous membrane of the present invention will be described. The polyolefin microporous membrane of the present invention comprises 5 to 70 parts by weight of a polyolefin composition containing at least 1% by weight of a polyolefin having a viscosity average molecular weight of 5 × 10 ⁵ or more, a solvent (A) having a boiling point of 200 ° C. or higher at atmospheric pressure, and 200 A solution containing 30 to 95 parts by weight of a mixed solvent composed of a volatile solvent (B) having a temperature of less than 0 ° C. is prepared. The solution is extruded from a die at a temperature of the melting point of the polyolefin composition + 60 ° C. or less, cooled, and gelled. It can be produced by forming a composition, removing the solvent (B) from the gel composition and then stretching, and then removing the solvent (A).

  The polyolefin composition is as described above, and a polyolefin solution is prepared from the polyolefin composition and a mixed solvent. The concentration of the polyolefin solution is preferably 1 to 30% by weight, more preferably 10 to 25% by weight. When the concentration of the polyolefin solution is less than 1% by weight, the gel-like molded product obtained by cooling and gelation is highly swollen with a solvent, so that it may be easily deformed and hinder handling. On the other hand, if it is 30% by weight or more, the pressure at the time of extrusion becomes high, so the discharge amount becomes low and the productivity may not be improved. Moreover, the orientation in the extrusion process proceeds, and stretchability and uniformity may not be ensured.

  Here, 50 parts by weight or more of the solvent (A) having a boiling point of 200 ° C. or higher at atmospheric pressure is used with respect to 100 parts by weight of the polyolefin composition. It is preferable to use 100 parts by weight or more of the solvent (A) with respect to 100 parts by weight of the polyolefin composition.

  The mixing ratio of the solvent (A) having a boiling point of 200 ° C. or more and the solvent (B) having a boiling point of less than 200 ° C. is 5 to 95%, more preferably 10 to 60%.

  The difference in boiling point between the solvent (A) having a boiling point of 200 ° C. or more and the solvent (B) having a boiling point of less than 200 ° C. is 15 ° C. or more, preferably 25 ° C., more preferably 35 ° C. or more. Preferably there is.

  In addition, the solvent (A) and the solvent (B) are miscible at the temperature at which the polyolefin solution is produced and processed, and thereby a homogeneous solution can be obtained.

  The solvent (A) having a boiling point of 200 ° C. or higher at atmospheric pressure is not particularly limited as long as it can sufficiently dissolve the polyolefin. Hereinafter, the boiling point of the solvent at atmospheric pressure is shown in parentheses. Preferred examples of the solvent include paraffin (> 300 ° C., 17 or more carbon atoms), paraffin oil (230-300 ° C.), some mineral oils (˜ 300 ° C.), tetralin (206 ° C.), ethylene glycol (> 300 ° C.), and glycerin (290 ° C.). Of these, paraffin is preferable. Here, the boiling point at atmospheric pressure is shown in parentheses.

  The solvent (B) having a boiling point of less than 200 ° C. at atmospheric pressure is not particularly limited as long as it can sufficiently dissolve the polyolefin. For example, toluene (110 ° C), xylene (138-144 ° C), alkane having 9 to 11 carbon atoms (151-196 ° C), or decalin (187-196 ° C), diethylenetriamine (107 ° C), ethylenediamine ( 116 ° C.) or dimethyl sulfoxide (189 ° C.). Of these, decalin is preferred.

  The ultrahigh molecular weight polyolefin solution (suspension) is kneaded with a single screw extruder, preferably a twin screw extruder, and the thickened fluid is extruded with a T die or an I die at a temperature of melting point + 60 ° C. or lower, and then a chill roll or cooling. It is preferable to pass through a bath, rapidly cool below the gelation temperature and gel.

  Next, the solvent (B) is removed from the gel-like molded product. Examples of the solvent removal treatment include a method of evaporating and removing by heating or the like. By removing the solvent (B), the amount of the solvent contained in the gel-like molded product is preferably 10 to 80% by weight. More preferably, it is 30 to 70% by weight.

  The heating temperature for removing the solvent (B) by heating is 50 to 100 ° C, preferably 70 to 90 ° C. Thereby, the solvent (B) is removed from the gel-like molded product, and the solvent (A) remains in the gel-like molded product.

  When the amount of the solvent (A) in the gel molded product is 5% by weight or less, the pore diameter of the micropores formed in the stretched molded product is intended for a battery such as a lithium ion secondary battery as an object of the present invention. This is not preferable because it is larger than that required for the separator. On the other hand, when it exceeds 95% by weight, there is a problem in handling due to a large amount of solvent oozing with stretching.

  After removing the solvent (B), a stretching treatment is performed. Here, a relaxation treatment may be performed before the stretching treatment.

  In the stretching treatment, the gel-like molded product is heated and biaxially stretched at a predetermined magnification by a normal tenter method, roll method, rolling method, or a combination of these methods. Biaxial stretching may be simultaneous or sequential. Moreover, it can also be set as longitudinal multistage extending | stretching, 3 or 4 steps | stretching.

  It is preferable that extending | stretching temperature is 90 to less than melting | fusing point, More preferably, it is 100 to 120 degreeC. When the heating temperature exceeds the melting point, it cannot be stretched because the gel-like molded product is dissolved. On the other hand, when the heating temperature is less than 90 ° C., the gel-like molded product is not sufficiently softened, and the film is likely to be broken during stretching, making it difficult to stretch at a high magnification.

  Moreover, although a draw ratio changes with thickness of an original fabric, it carries out by at least 2 times or more in a uniaxial direction, Preferably it is 4-20 times.

  The microporous membrane after stretching is immersed in an extraction solvent to extract the solvent (A). Extraction solvents include hydrocarbons such as pentane, hexane, heptane, cyclohexane, decalin and tetralin, chlorinated hydrocarbons such as methylene chloride, carbon tetrachloride and methylene chloride, fluorinated hydrocarbons such as ethane trifluoride, diethyl ether, Easily volatile substances such as ethers such as dioxane can be used. These solvents are appropriately selected according to the solvent used for dissolving the polyolefin composition, and can be used alone or in combination. Solvent extraction removes the solvent in the microporous membrane to less than 1% by weight.

  Thereafter, a polyolefin microporous thin film having stable target performance can be obtained by performing heat setting in a temperature range of 80 to 150 ° C. as necessary.

  The polyolefin microporous membrane is preferably subjected to hydrophilization in applications that require hydrophilization such as water treatment. Examples of the hydrophilic treatment include various known methods such as chemical methods such as impregnation with hydrophilic compounds such as glycerin, ethylene glycol and alcohol, and physical treatments such as corona treatment, plasma treatment, and UV treatment. .

Examples of the present invention are shown below, but the present invention is not limited thereby. In addition, the test method in an Example is as follows.
(1) Thickness measurement An Anritsu probe (5 mmφ cylindrical probe with a weight of 1 g) was used for Mitutoyo Lytematic and measured with a contact pressure of 9.8 × 10 ⁻³ N.
(2) Porosity The apparent density ρ _ap is calculated from the thickness d (μm) measured in (1) and the basis weight BW (g / m ² ) measured with an electronic balance, and the following formula is calculated from the polyethylene density ρ _PE. Obtained by substituting for (I).
ρ _ap = BW / d
Porosity = (1-ρ _ap / ρ _PE ) × 100% (I)
(3) Average pore diameter and pore volume Using 100 cx manufactured by Beckman Coulter, the sample was dried at 50 ° C. for 3 hours, measured, and the average pore diameter and pore volume were determined.
(4) The strength at break was measured according to ASTM D882.
(5) The air permeability was measured according to JISP8117.
(6) Opening degree and average diameter of fibrils The opening degree was measured by randomly taking 10 2,000 times photographs of a scanning electron microscope, and the closed part (shown in black and enlarged when the closed state was observed). The ratio was calculated from the area) and the area of the aperture.
The average diameter of the stem fibrils and the branch fibrils was taken at random from 50,000 times the photomicrograph of the electron micrograph of the aperture, and the vertical and horizontal dimensions in the field image of 1.8 μm x 2.3 μm Five uniform lines including the upper, lower, left and right ends were drawn, and the average diameter of the intersecting trunk fibrils and branch fibrils was determined to calculate the average value.
The scanning microscope used S-5200 manufactured by Hitachi High-Technologies, and platinum was sputtered to a thickness of about 2 nm to coat the surface, and the acceleration voltage was measured at 2 kV. Image dimensions were calibrated with a reference sample.

[Example 1]
30 parts by weight of ultra high molecular weight polyethylene having a viscosity average molecular weight of 2 × 10 ⁶ (“Hi-Zex Million” 240S manufactured by Mitsui Chemicals, Inc.), 70 parts by weight of high-density polyethylene of 3.5 × 10 ⁵ , 244 parts by weight of decalin and liquid paraffin (A viscosity at 40 ° C. is 68 × 10 ⁻⁶ m ² / s) 244 parts by weight are dissolved at 180 ° C. using a twin-screw kneading extruder to form a sol, and the sol product is 190 ° C. through a flat film extrusion die. And extruded into water to form a gel sheet. Subsequently, the gel-like sheet was heated at 80 ° C. to remove decalin to 1% by weight or less. Next, the film was stretched 5 times in the longitudinal direction at 115 ° C. and then 10 times in the transverse direction at 115 ° C. to obtain a stretched film. The stretched membrane is heat-set at 130 ° C. (heat setting temperature at which the melting point of 1st DSC after solvent removal has a double peak including the shoulder), and the resulting stretched membrane is washed with hexane to remove residual liquid paraffin and then dried. And a heat treatment was performed to obtain a polyethylene microporous membrane.
Table 1 shows the physical properties of this polyethylene microporous membrane and the results of surface observation by a scanning electron microscope.

[Examples 2 to 5]
A polyethylene microporous membrane was obtained in the same manner as in Example 1 using ultrahigh molecular weight polyethylene, high density polyethylene, and a solvent as described in Table 1. Table 1 shows the physical properties of the obtained polyethylene microporous film and the results of surface observation by a scanning electron microscope.

[Comparative Examples 1-4]
A polyethylene microporous membrane was obtained in the same manner as in Example 1 using ultrahigh molecular weight polyethylene, high density polyethylene, and a solvent as described in Table 2. Table 2 shows the physical properties of the obtained polyethylene microporous film and the results of surface observation by a scanning electron microscope.

2 is a 2,000 times photograph of a scanning electron microscope of the film obtained in Example 1. FIG. 2 is a 50,000 times photographic view of an electron micrograph of an aperture portion of the film obtained in Example 1. FIG.

Claims (9)

A polyolefin fine particle comprising a polyolefin composition containing 1% by weight or more of an ultrahigh molecular weight polyolefin having a viscosity average molecular weight of 5 × 10 ⁵ or more, having a porosity of 25 to 60%, and having an open pore structure composed of branched fibrils It is a porous film, and the area occupied by the aperture observed on the surface with a scanning electron microscope is 80% or more, and the average diameter of the stem fibrils is 0.01 to 0.05 μm in the aperture, A polyolefin microporous membrane, wherein the average diameter of the branched fibrils intersecting with each other is 0.01 to 0.03 μm.   The polyolefin microporous membrane according to claim 1, wherein an average pore diameter measured by a Coulter porometer is 0.01 to 0.1 µm.   The polyolefin microporous membrane according to claim 1 or 2, wherein the pore volume measured by a Coulter porometer is 0.3 to 1.0 ml / g. 5 to 70 parts by weight of a polyolefin composition containing at least 1% by weight of a polyolefin having a viscosity average molecular weight of 5 × 10 ⁵ or more, a solvent (A) having a boiling point at atmospheric pressure of 200 ° C. or higher, and a volatile solvent (B And a solution containing 30 to 95 parts by weight of a mixed solvent, and extruding the solution from a die at a temperature not higher than the melting point of the polyolefin composition + 60 ° C. and cooling to form a gel composition. The method for producing a polyolefin microporous membrane according to any one of claims 1 to 3, wherein the solvent (B) is removed from the composition and then stretched, and then the solvent (A) is removed.   The method for producing a polyolefin microporous membrane according to claim 4, wherein 50 parts by weight or more of the solvent (A) having a boiling point of 200 ° C or higher at atmospheric pressure is used with respect to 100 parts by weight of the polyolefin composition.   The method for producing a polyolefin microporous membrane according to claim 4 or 5, wherein the solvent (A) is used in an amount of 100 parts by weight or more based on 100 parts by weight of the polyolefin composition.   The method for producing a polyolefin microporous membrane according to any one of claims 4 to 6, wherein the solvent (A) is at least one selected from the group consisting of paraffin, castor oil, and mineral oil.   The method for producing a polyolefin microporous membrane according to any one of claims 4 to 7, wherein the solvent (B) is at least one selected from the group consisting of decalin, tetralin, and hexane.   Use of the polyolefin microporous membrane according to any one of claims 1 to 3 as a battery separator.

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