JPH0521050A - Uniform micro porous film made of polyethylene and its manufacture - Google Patents

Abstract

PURPOSE:To provide high strength while protruding an adequate rupture ductility and pore diameter by having a three-dimensional mesh structure, containing a polyethy lene of a predetermined super high molecular weight, and selecting a porosity, a permea bility, and a rupture ductility, etc., of a predetermined value, respectively. CONSTITUTION:A micro porous film containing a polyethylene of a super high molecular weight has a three-dimensional mesh structure, and a super high molecular weight polyethylene of at least 2 million in a viscosity average molecular weight is contained by at least 30wt.% of the micro porous film. The micro porous film is designed to have a porosity of not less than 40%, a permeability of not above 450sec/100cc, an elasticity in a mechanical direction of not less than 4000kg/cm<2>, a rupture ductility both in the mechanical direction and in the perpendicular direction of not less than 400%, a bubble point in an ethyl alcohol of 2 to 10kg/cm<2>, and the ratio of a maximum pore diameter to an average pore diameter of not less than 1.6. Whereby, a micro porous film can be provided such that it has an adequate elongation in its width direction while having high strength in the mechanical direction, and furthermore has an adequate pore diameter while being homogeneous.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a microporous membrane containing ultra high molecular weight polyethylene. In particular, the invention relates to microporous membranes containing homogeneous ultra high molecular weight polyethylene having a three dimensional network structure.

[0002]

Microporous membranes are used as separators for batteries, separators for electrolytic capacitors, microfiltration membranes and the like. For separators for batteries and separators for electrolytic capacitors, strong and homogeneous microporous membranes are desired from the viewpoints of assembly workability and reliability of batteries and electrolytic capacitors, and especially for non-rechargeable batteries such as lithium batteries. A separator for water-electrolyte batteries is required to have a strong, uniform, and microporous membrane with low electric resistance.

Further, for the microfiltration membrane, a strong and homogeneous microporous membrane having an appropriate pore size is desired. As one of the methods for producing a microporous membrane, there is a method of making it porous with a resin and a solvent. For example, JP-A-2-21559 and JP-A-3-105851 are disclosed.
According to the method disclosed in Japanese Patent No. 1559, a high-strength microporous membrane cannot be obtained. Further, the microporous membrane disclosed in Japanese Patent Laid-Open No. 3-105851 is not suitable as a microfiltration membrane for filtering a large amount of water in a water purifier or the like because it has a small average through-hole diameter, and also has a thin film thickness. Therefore, there is a problem in terms of safety when used as a separator of a lithium battery. Further, there is a problem that the molding sheet is significantly shrunk by the extraction of the plasticizer and must be biaxially stretched, so that it does not have an appropriate breaking elongation in at least one direction.

Another method for producing a microporous membrane is a method in which after phase separation of a mixture of a resin, a plasticizer and a filler, the plasticizer or the plasticizer and the filler are extracted and removed to make them porous. For example, as described in JP-A-55-165573, as described in the specification, a microporous membrane formed according to a general procedure and materials has high electric resistance, that is, permeation resistance of water or the like. It is also considered that it is not suitable for a separator because it does not have an appropriate pore size, is not suitable as a microfiltration membrane at least, and has a high electric resistance.

Further, Japanese Patent Application Laid-Open No. 57-49629 discloses a method for producing a microporous membrane in which the plasticizer is particularly limited, and this method also provides a microporous material having high strength and an appropriate pore size. No film is obtained.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a homogeneous microporous membrane which has a high strength and has an appropriate breaking elongation and pore size. Microporous membranes composed of a mixture of ultra-high molecular weight polyethylene and high molecular weight polyethylene, which are used in the present invention, are known, but they are microfiltration membranes for filtering large amounts of water, and separators for lithium batteries that require safety. A microporous membrane having the desired performance desired for a device, etc. could not be obtained by the conventional method.

[0007]

Means for Solving the Problems The present inventor has found that a microporous membrane containing at least an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more has high strength in the machine direction, but also in the width direction (machine direction). The present invention has been completed by finding a microporous membrane having a suitable elongation in the direction (perpendicular direction), a suitable pore size, and a homogeneous method, and a method for producing the same.

That is, the present invention is a microporous membrane having a three-dimensional network structure and having a viscosity average molecular weight of at least 2
At least 30 wt% of the microporous membrane containing at least 30% by weight of ultra-high molecular weight polyethylene and having a porosity of at least 40%,
Air permeability is 450 sec / 100 cc or less, elastic modulus in the machine direction is 4000 kg / cm ² or more, breaking elongation in the direction perpendicular to the machine direction is 400% or more, bubble point in ethyl alcohol is 2 kg / cm ^{2 to} 10 kg / cm ^2. And the ratio of the average pore diameter to the maximum pore diameter is 1.6 or less, which is a homogeneous polyethylene porous membrane.

The polyethylene or the ultrahigh molecular weight polyethylene as referred to in the present invention is a crystalline homopolymer obtained by polymerizing ethylene or 10 mol% of ethylene.
The following copolymers with propylene, 1-butene, 4-methyl-1-pentene and 1-hexene are mentioned. The three-dimensional network structure refers to a structure in which a resin has a three-dimensional network structure, and the network structures of the resin form holes as communication holes.

The porosity, when used as a separator for a battery, is from the viewpoint of the impregnation property of the electrolytic solution and the internal resistance of the battery.
It is preferably 40% or more, and more preferably 50% or more. When the porosity is 80% or more, the strength of the microporous membrane is reduced, which causes a trouble in handling. Air permeability is 450 sec / 100 cc or less, preferably 3
00 sec / 100 cc or less, more preferably 200
It is sec / 100 cc or less. Air permeability is 450sec
When it is used as a separator for batteries, the internal resistance of the battery becomes too high and the battery characteristics are deteriorated, and when it is used as a microfiltration membrane, the water permeability is low and the filtration efficiency is poor.

The elastic modulus in the machine direction (longitudinal direction) is 400.
0 kg / cm ² or more, preferably 5000 kg / cm ²
Above, more preferably 6000 kg / cm ² .
When used as a battery separator, 4000 kg / c
When it is less than m ^2, it is considered that the assembling workability and the productivity are poor and the practicality is low. The breaking elongation in the direction perpendicular to the machine direction (width direction) is 400% or more, preferably 450% or more, more preferably 500% or more. Although the reason is not clear, when the breaking elongation is 400% or less, the microporous membrane is likely to tear in the machine direction when it is pleated with a nonwoven fabric.

The bubble point in ethyl alcohol is 2 kg / cm ^{2 to} 10 kg / cm ² , preferably 3 kg / cm ^{2.
kg / cm 2 ~9kg / cm 2} , more preferably 4k
g / cm ^{2 to} 8 kg / cm ² . When the bubble point in ethyl alcohol is 2 kg / cm ² or less, the pore size of the microporous membrane is large, and when used as a microfiltration membrane,
There is a concern that the bacteria may leak, especially when used as a separator for a lithium battery, which may cause an internal short circuit. Also, the bubble point of ethyl alcohol is 10k.
When it is more than g / cm ² , the pore size of the microporous membrane is too small, and when it is used as a microfiltration membrane, the air permeability becomes small and the filtration efficiency is poor, and when it is used as a battery separator, it is impregnated with an electrolyte solution. There is a risk of deterioration of sex.

When used as a microfiltration membrane, a homogeneous microporous membrane is desired in order to ensure reliable sterilization. Especially when used as a separator for lithium batteries, the internal resistance of the battery is uniform. It is considered that the homogeneity is necessary for ensuring safety, and similarly, a homogeneous microporous membrane is desired, and if the ratio of the average pore diameter to the maximum pore diameter is 1.6 or more, there is concern about the homogeneity. It is preferably 1.5 or less, more preferably 1.4 or less.

In the present invention, the film thickness is not particularly specified, but from the viewpoint of reliability as a microfiltration membrane and reliability as a battery separator, it is preferably 15 μm to 60 μm. The thickness is more preferably 20 μm to 50 μm, further preferably 25 μm to 45 μm. If the thickness is 15 μm or less, the filtration accuracy (depth effect) in the film thickness direction becomes poor, and in the battery separator, there is a risk of internal short circuit.
When it is 60 μm or more, the water permeability of the microfiltration membrane decreases and the internal resistance of the battery separator increases, which is not preferable.

A microporous membrane having a three-dimensional network structure as disclosed in the present invention, having a viscosity average molecular weight of 200.
It contains at least 30 wt% of ultra-high molecular weight polyethylene of at least 30% by weight of the microporous membrane and has a thickness of 15 μm to 60 μm.
μm, porosity 40% or more, air permeability 450 sec / 1
00cc or less, elastic modulus in machine direction is 4000kg / cm
² or more, breaking elongation in the direction perpendicular to the machine direction is 400% or more, bubble point in ethyl alcohol is 2 kg
/ Cm ^{2 to} 10 kg / cm ² and the ratio of the average pore diameter to the maximum pore diameter is 1.6 or less, a homogeneous ultrahigh molecular weight polyethylene microporous membrane is used as a microporous membrane for lithium batteries. Optimal.

Further, it is considered that the shape of the pores formed on the surface of the microporous membrane is preferably elliptical. This is because when used as a microfiltration membrane, if the pores on the surface of the microporous membrane are elliptical, they are less likely to be clogged and have a longer filtration life than the circular shape. Is preferably 1.3 or more, more preferably 1.5 or more. The microporous membrane of the present invention comprises a mixture of an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more and a high molecular weight polyethylene having a viscosity average molecular weight of 500,000 or less, an inorganic fine powder and a plasticizer while kneading and heating and melting the mixture. In the production method for obtaining a microporous membrane by extracting and drying the inorganic fine powder and the plasticizer respectively after molding into a tongue shape and stretching in a uniaxial direction only, the SP value of the plasticizer is 7.5 to At least two kinds of mixed plasticizers of 8.4 and 8.5 to 9.5 are used, and the amount of the plasticizer having an SP value of 7.5 to 8.4 is 1 to 50 of the weight of the microporous membrane.
It is manufactured by a manufacturing method characterized by being less than or equal to%.

Furthermore, a microporous membrane having a strong and appropriate pore size cannot be obtained by simply selecting two or more kinds of mixed plasticizers. By using two or more kinds of plasticizers selected in particular according to the present invention and limiting the amount of at least one kind of plasticizer, the molding process is easy, and stretching is carried out only in the uniaxial direction to obtain high strength and an appropriate pore size. It is possible to obtain a homogeneous polyethylene microporous membrane suitable for a microfiltration membrane, a battery separator, etc. Having a proper pore size by stretching only in the uniaxial direction,
And the fact that a particularly high-strength microporous film can be obtained in the stretching direction is not only highly economical in industrial production, but also has an appropriate breaking elongation in the width direction, which makes it difficult to tear in the machine direction. Have.

Specifically, it is produced by mixing polyethylene, a plasticizer, and an inorganic fine powder, molding, extracting, drying, and further stretching. As the inorganic fine powder, finely divided silicic acid,
Calcium silicate, aluminum silicate, calcium carbonate,
Fine talc, etc. can be raised. For example, the mixing composition of polyethylene, inorganic fine powder, and plasticizer is 10 to 40% by weight, 5 to 35% by weight, and 20 to 80% by weight, respectively, and the mixture is mixed by an ordinary mixer such as a Henschel mixer, and then an extruder. And the like, and the resulting kneaded product is molded into a thickness of 80 μm to 600 μm by extrusion molding or the like. Further, the plasticizer is extracted and removed from the molded product with a solvent, the inorganic fine powder is subsequently extracted with a solvent for extracting the inorganic fine powder, and then stretched only in the uniaxial direction to obtain a microporous membrane.

Solvents for the plasticizer include alcohols such as methanol and ethanol, ketones such as acetone and MEK, chlorine-based hydrocarbons such as 1,1,1-trichloroethane, and other general organic compounds. A solvent is used. S used in the present invention
Examples of the plasticizer having a P value of 7.5 to 8.4 include liquid paraffin and mineral oil such as process oil.

Further, as the plasticizer having an SP value of 8.5 to 9.5, DBP, DOP, DNP, DBS, TBP, etc. can be mentioned. The amount of the plasticizer having an SP value of 7.5 to 8.4 is 1% to 50% of the weight of the microporous membrane, preferably 2% to 40.
%, More preferably 3% to 30%, most preferably 3%
% To 20%. When the amount of the plasticizer is 50% or more,
A microporous membrane having an appropriate pore size cannot be obtained, and if it is 1% or less, moldability is poor and production is difficult.

The amount of the mixed plasticizer is such that the mixture (polyethylene / plasticizer /
Inorganic fine powder) 50% to 180% by weight, preferably 55
% To 150%, more preferably 60% to 120%. When the amount of the mixed plasticizer is 50% or less, a microporous membrane having an appropriate pore size cannot be obtained, and when it is 180% or more, the mixing property of polyethylene / plasticizer / inorganic fine powder is deteriorated and molding process becomes difficult.

The content of ultra high molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more is 30 w of polyethylene weight.
It is at least t%, preferably at least 40 wt%, more preferably at least 50 wt%. If it is 30 wt% or less, a high-strength microporous film cannot be obtained. In addition to ultra high molecular weight polyethylene, it is desirable to mix high molecular weight polyethylene in order to improve moldability. The viscosity average molecular weight of the high molecular weight polyethylene is 1,000,000 or less, preferably 800,000 or less, more preferably 500,000 or less.

The content of high molecular weight polyethylene is 70 wt% or less, preferably 60 wt% or less, and more preferably 50 wt% or less based on the weight of polyethylene. Further, the stretching needs to be performed only in the uniaxial direction. As a matter of course, it is possible to perform biaxial stretching, but there is a problem that an appropriate breaking elongation cannot be obtained in the width direction.

[0024]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. The measuring method is shown below. (1) It was measured with a dial gauge having a minimum film thickness scale of 1 μm. (2) A sample having a porosity of 10 cm square was cut out, and the weight of the sample when it was wet, the weight when it was absolutely dried, and the film thickness were measured and determined from the following formula.

Porosity = (pore volume / microporous membrane volume) × 100 (%) Pore volume = (weight of water [g] -weight of absolutely dry weight [g]) / density of water [g / cm ³ ] Porous membrane volume = 100 × film thickness [cm] (3) Bubble point The bubble point in ethanol was measured according to ASTM E-128-61. (4) Air permeability In accordance with JIS P-8117, a B-type galley type densometer manufactured by Toyo Seiki was used to measure the time required for the marking line scale 0 to 100 with a stopwatch. (5) Modulus of elasticity Using a model Autograph AG-A manufactured by Shimadzu, a tensile test was conducted at a test piece size of width 10 mm x length 100 mm, a chuck distance of 50 mm, and a pulling speed of 200 mm / min. The elastic modulus was measured.

The cross-sectional area was calculated by (film thickness × film width measured in item (1)). (6) Tensile rupture elongation Using a model Autograph AG-A manufactured by Shimadzu, a tensile test was performed at a test piece size of 10 mm width × 100 mm length, a chuck distance of 50 mm, and a pulling speed of 200 mm / min. Do the following to determine the amount of elongation from chart to fracture,
It was calculated by the following formula.

Tensile elongation at break = (elongation before breaking [mm] -50 [m
m]) / 50 [mm] × 100 [%] (7) Average Pore Size Obtained by the Half Dry method according to ASTM F-316-70. (8) Maximum pore size Calculated from the bubble point in ethanol according to ASTM E-128-61. (9) Viscosity average molecular weight Using decalin, the viscosity was measured at a measurement temperature of 135 ° C. with an uberose type viscosity type, and the viscosity average molecular weight was determined by the Chang's formula.

[0028]

Example 1 11% by weight of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 3,000,000, 6% by weight of high molecular weight polyethylene having a viscosity average molecular weight of 300,000, 22% by weight of finely divided silicic acid, 57% by weight of dioctyl phthalate and 4 of liquid paraffin. Wt% is mixed with a Henschel mixer, and the mixture is φ30 mm.
The film was formed into a film having a thickness of 150 μm by a film manufacturing apparatus in which a T-die having a width of 450 mm was attached to a twin-screw extruder.

The formed film is dipped in 1,1,1-trichloroethane for 10 minutes to extract dioctyl phthalate and then dried, and further placed in 25% caustic soda at 60 ° C. for 60 minutes. It was dipped to extract fine silicic acid and then dried. Further, the microporous film was stretched by a uniaxial roll stretching machine heated at 125 ° C. so that the film thickness was 30 μm to 40 μm, and heat-treated at 115 ° C. for 5 seconds.

The characteristics of the obtained microporous membrane are shown in Table 1.

[0031]

Example 2 The procedure of Example 1 was repeated, except that ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 was used. The results are shown in Table 1.

[0032]

Example 3 Example 1 was repeated except that 44% by weight of dioctyl phthalate and 15% by weight of liquid paraffin were used.
The results are shown in Table 1.

[0033]

Example 4 13.5% by weight of ultra high molecular weight polyethylene having a viscosity average molecular weight of 3,000,000, 3.5% by weight of high molecular weight polyethylene having a viscosity average molecular weight of 300,000, 22% by weight of finely divided silicic acid, 53% by weight of dioctyl phthalate, and a flow. Example 1 was repeated except that 8% by weight of paraffin was used. The results are shown in Table 1.

[0034]

Example 5 5.5% by weight of ultra high molecular weight polyethylene having a viscosity average molecular weight of 3,000,000, 11.5% by weight of high molecular weight polyethylene having a viscosity average molecular weight of 300,000, 22% by weight of finely divided silicic acid and 53% by weight of dioctyl phthalate. And liquid paraffin 8
The same procedure as in Example 1 was performed except that the weight% was mixed in a Henschel mixer. The results are shown in Table 1.

[0035]

COMPARATIVE EXAMPLE 1 An attempt was made in the same manner as in Example 1 except that 24% by weight of dioctyl phthalate and 35% by weight of liquid paraffin were used, but the mixing properties in the Henschel mixer were poor, and molding processing could not be achieved. .

[0036]

Comparative Example 2 The procedure of Example 1 was repeated except that only 59% by weight of liquid paraffin was used as the plasticizer. The results are shown in Table 1.

[0037]

[Comparative Example 3] Dioctyl phthalate alone as a plasticizer 6
An attempt was made in the same manner as in Example 1 except that 5% by weight was used, but molding could not be performed.

[0038]

Comparative Example 4 An attempt was made in the same manner as in Example 1 except that acetyl tri-n-butyl citrate having an SP value of about 10.0 was used as the plasticizer instead of dioctyl phthalate. However, it could not be molded.

[0039]

Comparative Example 5 An attempt was made in the same manner as in Example 1 except that di-n-octyltetrahydrophthalate having an SP value of about 7.2 was used as the plasticizer instead of liquid paraffin. Could not be molded.

[0040]

Comparative Example 6 A microporous membrane was obtained by the method according to Example 1 of JP-A-2-21559. The results are shown in Table 1.

[0041]

[Table 1]

[0042]

According to the above structure, a microporous membrane having high strength and an appropriate pore size can be obtained. Particularly, as a microfiltration membrane,
As a homogeneous and highly reliable filtration membrane, especially as a separator for non-aqueous electrolyte batteries, as a separator with high workability and safety, low internal resistance, and uniform low internal resistance. Applicable.

Claims (2)

[Claims] 1. A microporous membrane having a three-dimensional network structure.
Ultra high molecular weight with a viscosity average molecular weight of 2,000,000 or more
30% by weight or more of polyethylene in the microporous membrane
Contains, porosity of 40% or more, air permeability of 450 sec /
100 cc or less, machine direction elastic modulus is 4000 kg / c
m²Above, breaking elongation in the direction perpendicular to the machine direction is 400% or less
Above, bubble point 2kg in ethyl alcohol
/ Cm ²-10kg / cm²And mean pore size and maximum pore
Diameter ratio is 1.6 or less
High molecular weight polyethylene microporous membrane 2. A polyethylene mixture containing an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more, an inorganic fine powder and a plasticizer are kneaded and heat-melted to form a sheet, and then the inorganic fine powder and In the production method in which the plasticizer is extracted and removed, and dried, and stretched only in the uniaxial direction to obtain a microporous membrane, the SP value of the plasticizer is 7.5 to 8.
4 and 8.5-9.5, and the amount of the plasticizer having an SP value of 7.5-8.4 is 1% to 50% of the weight of the polyethylene mixture. Characteristically, porosity is 40% or more, air permeability is 450 sec / 1
00cc or less, elastic modulus in machine direction is 4000kg / cm
² or more, breaking elongation in the direction perpendicular to the machine direction is 400% or more, bubble point in ethyl alcohol is 2 kg
/ Cm ^{2 to} 10 kg / cm ² , and a method for producing a homogeneous ultrahigh molecular weight polyethylene microporous membrane having a ratio of average pore diameter to maximum pore diameter of 1.6 or less.