JP3121047B2 - Homogeneous polyethylene microporous membrane and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Microporous membranes are used as separators for batteries, separators for electrolytic capacitors, microfiltration membranes, and the like. A strong and uniform microporous film is desired for a battery separator / electrolytic capacitor separator from the viewpoint of the processability and reliability of the battery / electrolytic capacitor. A separator for a water electrolyte battery is required to have a strong and uniform microporous membrane having a small electric resistance.

Further, a strong and homogeneous microporous membrane having an appropriate pore size is desired for a microfiltration membrane. As one of the methods for producing a microporous membrane, there is a method in which the membrane is made 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 microporous membrane with high strength cannot be obtained. Further, the microporous membrane disclosed in JP-A-3-105585 is not suitable as a microfiltration membrane for filtering a large amount of water, such as a water purifier, because of a small average through-hole diameter, and has a small thickness. For this reason, there is a problem in terms of safety when used as a separator for a lithium battery. Further, there is a disadvantage that the molded sheet is largely shrunk by the extraction of the plasticizer, must be biaxially stretched, and does not have an appropriate breaking elongation in at least one direction.

[0004] As another method for producing a microporous membrane, there is a method in which a plasticizer or a plasticizer and a filler are extracted and removed after the phase separation of a mixture of a resin, a plasticizer and a filler to make the membrane porous. For example, Japanese Patent Application Laid-Open No. 55-165573, as described in the specification, a microporous membrane formed according to general procedures and materials has a high electric resistance, that is, a permeation resistance such as water. It is considered that it does not have an appropriate pore diameter, is at least not suitable as a microfiltration membrane, and is not suitable for a separator due to its high electric resistance.

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

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a homogeneous microporous membrane having high strength and an appropriate breaking elongation and pore size. Microporous membranes comprising a mixture of ultra-high molecular weight polyethylene and high molecular weight polyethylene, as practiced in the present invention, are known, but microfiltration membranes for large-volume water filtration and separators for lithium batteries requiring safety are required. Microporous membranes having the desired performance desired for such devices cannot be obtained by conventional methods.

[0007]

Means for Solving the Problems The present inventor has developed a microporous membrane containing ultrahigh molecular weight polyethylene having at least a viscosity average molecular weight of 2,000,000 or more, while having high strength in the machine direction, while maintaining high strength in the machine direction (in the machine direction). The present inventors have found a microporous membrane which has an appropriate elongation in a direction (at right angles), has an appropriate pore diameter, and is homogeneous, and a method for producing the same, and has completed the present invention.

[0008] That is, the present invention relates to a microporous membrane having a three-dimensional network structure, which has a viscosity average molecular weight of at least 2.
At least 30% by weight or more of ultra-high molecular weight polyethylene of at least 30% by weight of the microporous membrane, and a porosity of 40% or more;
Air permeability of 450 sec / 100 cc or less, elastic modulus in the machine direction of 4000 kg / cm ² or more, elongation at break in the direction perpendicular to the machine direction of 400% or more, bubble point in ethyl alcohol of 2 kg / cm ^{2 to} 10 kg / cm ² Wherein the ratio of the average pore diameter to the maximum pore diameter is 1.6 or less.

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

When the porosity is used as a battery separator, the porosity is determined from the viewpoint of the impregnation of the electrolyte and the internal resistance of the battery.
It is preferably at least 40%, more preferably at least 50%. When the porosity is 80% or more, the strength of the microporous membrane is reduced, which causes trouble in handling. Air permeability is 450 sec / 100 cc or less, preferably 3
00 sec / 100 cc or less, more preferably 200
sec / 100 cc or less. Air permeability is 450 sec
If it is more than / 100 cc, the internal resistance of the battery becomes too high when used as a battery separator, and the battery characteristics are degraded. When 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
The above is more preferably 6000 kg / cm ² .
4000kg / c when used as a battery separator
If it is less than m ^2, it is considered that assembly workability and productivity are inferior and practicality is low. The elongation at break in the direction perpendicular to the machine direction (width direction) is 400% or more, preferably 450% or more, and more preferably 500% or more. Although the reason is not clear, if the elongation at break is 400% or less, the microporous membrane tends to tear in the machine direction, for example, when the microporous membrane 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 bacteria may leak, and particularly when used for a separator of a lithium battery, an internal short circuit may occur. In addition, the bubble point in ethyl alcohol is 10k.
If it is more than g / cm ² , the pore size of the microporous membrane is too small, and when used as a microfiltration membrane, the air permeability becomes low and the filtration efficiency is poor. There is a possibility that the property may be reduced.

When used as a microfiltration membrane, a uniform microporous membrane is desired in order to ensure reliable sterilization. Especially when used as a separator for a lithium battery, the internal resistance of the battery is uniform. Is considered to be necessary for ensuring safety. Similarly, a homogeneous microporous membrane is desired, and when the ratio of the average pore size to the maximum pore size becomes 1.6 or more, there is concern about homogeneity. Preferably it is 1.5 or less, more preferably 1.4 or less.

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

A microporous membrane having a three-dimensional network structure disclosed in the present invention, wherein the viscosity average molecular weight is 200
At least 30% by weight of the microporous membrane and a film thickness of 15 μm to 60 μm.
μm, porosity of 40% or more, air permeability of 450 sec / 1
00cc or less, elasticity in machine direction is 4000kg / cm
² or more, the elongation at break in the direction perpendicular to the machine direction is 400% or more, and the 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. Optimal.

Further, it is considered that the shape of the holes formed in the surface of the microporous membrane is preferably elliptical. This is because, when used as a microfiltration membrane, it is thought that if the pores on the surface of the microporous membrane are elliptical, they are less likely to be clogged than circular and have a longer filtration life. Is preferably 1.3 or more, more preferably 1.5 or more. The microporous membrane of the present invention has a viscosity-average molecular weight of 2,000,000 or more ultra-high molecular weight polyethylene and a viscosity-average molecular weight of 1,000,000 or less, a mixture of a high-molecular-weight polyethylene, an inorganic fine powder and a plasticizer. After forming into a shape, the inorganic fine powder and the plasticizer are each extracted and removed, and dried, and stretched in only one axis direction to obtain a microporous film,
The SP value of the plasticizer is 7.5 to 8.4 and 8.5 to 9.5.
And a plasticizer having an SP value of 7.5 to 8.4 is used as a microporous membrane (polyethylene).
(Inorganic fine powder) It is manufactured by a manufacturing method characterized by being 1 to 50% or less of the weight.

Furthermore, a strong and moderately porous microporous membrane cannot be obtained with two or more selected plasticizers. By using two or more types of plasticizers selected in particular and limiting the amount of at least one type of plasticizer disclosed in the present invention, molding processing is easy and stretching in only one axis direction provides high strength and moderate pore size. A homogeneous microporous polyethylene membrane suitable for microfiltration membranes, separators for batteries, etc. having Having a moderate pore size by stretching only in the uniaxial direction,
The fact that a microporous membrane with particularly high strength in the stretching direction is obtained is not only highly economical in industrial production, but also has a moderate breaking elongation in the width direction, so that it is difficult to tear in the machine direction. Having.

Specifically, it is produced by mixing, molding, extracting and drying a polyethylene, a plasticizer, and an inorganic fine powder, and further stretching. As the inorganic fine powder, fine silica powder,
Calcium silicate, aluminum silicate, calcium carbonate,
Fine talc and the levator up. For example, the mixture composition of polyethylene, inorganic fine powder, and plasticizer is set to 10 to 40% by weight, 5 to 35% by weight, and 20 to 80% by weight, respectively, and mixed with a normal mixer such as a Henschel mixer, and then extruded. And the obtained kneaded material is formed into a thickness of 80 μm to 600 μm by extrusion molding or the like. Furthermore, a plasticizer is extracted and removed from the molded product using a solvent to form a microporous membrane (polyethylene / inorganic fine powder) ,
Subsequently, the inorganic fine powder is extracted from the microporous membrane with an inorganic fine powder extraction solvent, and then stretched in only one axis direction to obtain a microporous membrane.

Examples of the solvent for the plasticizer include common organic compounds such as alcohols such as methanol and ethanol, ketones such as acetone and MEK, and chlorinated hydrocarbons such as 1,1,1-trichloroethane. 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, mineral oil such as process oil, and the like.

As plasticizers having an SP value of 8.5 to 9.5, dibutyl phthalate ( DBP ) , dioctyl phthalate
Tallate ( DOP ) , dinonyl phthalate ( DNP ) ,
Dibutyl sebacate ( DBS ) , tributyl phosphate
Doo (TBP) or the like can be mentioned up. SP value is 7.5 to 8.
Plasticizer No. 4 is a microporous membrane (polyethylene, inorganic fine powder)
Body) 1% to 50% by weight, preferably 2% to 40%,
More preferably, it is 3% to 20%. 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, in order to have a miscibility and moderate pore size is also 50% to 180% of the heavy <br/> amount of the sum of Po Riechiren and inorganic fine powder, preferably 55% 150%,
More preferably, it is 60% to 120%. When the amount of the mixed plasticizer is 50% or less, a microporous film having an appropriate pore size cannot be obtained. When the amount of the mixed plasticizer is 180% or more, the mixing property of polyethylene, plasticizer, and inorganic fine powder is reduced, and molding processing becomes difficult.

The content of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more is 30 w
It is at least t%, preferably at least 40 wt%, more preferably at least 50 wt%. If it is less than 30 wt%, a high-strength microporous film cannot be obtained. In order to improve moldability other than ultrahigh molecular weight polyethylene, it is desirable to mix high molecular weight polyethylene. 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 the high molecular weight polyethylene is at most 70 wt%, preferably at most 60 wt%, more preferably at most 50 wt% of the weight of the polyethylene. In addition, it is necessary to stretch only in a uniaxial direction. Naturally, biaxial stretching is possible, 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 measurement method is described below. (1) Film thickness It was measured with a dial gauge having a minimum scale of 1 μm. (2) Porosity A sample having a size of 10 cm square was cut out, and the weight of the sample when wet, the weight when completely dried, and the film thickness were measured, and were obtained from the following equation.

Porosity = (pore volume / microporous membrane volume) × 100 (%) Pore volume = (water content [g] −absolute weight [g]) / water density [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 According to JIS P-8117, the time required for the mark scale 0 to 100 was measured with a stopwatch using a Toyo Seiki B-type Galley-type densometer. (5) Elastic Modulus Using a Shimadzu type autograph AG-A type, a tensile test was performed at a test piece size of 10 mm wide × 100 mm long, a distance between chucks of 50 mm, and a tensile speed of 200 mm / min. The elastic modulus was measured.

The cross-sectional area was calculated from the film thickness × film width measured in the section (1). (6) Tensile rupture elongation Using a Shimadzu Model Autograph AG-A type, a tensile test was performed at a test piece size of 10 mm wide x 100 mm long, a distance between chucks of 50 mm and a tensile speed of 200 mm / min. Perform, and determine the amount of elongation from chart to break,
It was calculated by the following equation.

Tensile elongation at break = (elongation to break [mm] −50 [m
m]) / 50 [mm] × 100 [%] (7) Average pore diameter It was determined by a half-dry method based on ASTM F-316-70. (8) Maximum pore diameter Calculated from the bubble point in ethanol according to ASTM E-128-61. (9) Viscosity-average molecular weight The viscosity was measured using decalin at a measurement temperature of 135 ° C. using a Ubbelose type viscometer, and the viscosity-average molecular weight was determined by Chiang'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 silica, 57% by weight of dioctyl phthalate, and liquid paraffin 4 % By weight 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 was immersed in 1,1,1-trichloroethane for 10 minutes, extracted with dioctyl phthalate, dried, and further immersed in 25% caustic soda at 60 ° C. for 60 minutes. After immersion, fine silica powder was extracted and dried. Further, the microporous film was stretched by a uniaxial roll stretching machine heated at 125 ° C. so as to have a film thickness of 30 μm to 40 μm, and heat-treated at 115 ° C. for 5 seconds.

Table 1 shows the properties of the obtained microporous membrane.

[0031]

Example 2 The same procedure as in Example 1 was carried out except that an ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 was used. Table 1 shows the results.

[0032]

Example 3 The same procedure as in Example 1 was carried out except that dioctyl phthalate was 46 % by weight and liquid paraffin was 15% by weight.
Table 1 shows the results.

[0033]

Example 4 13.5% by weight of ultrahigh 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 dityl phthalate and fluidity The procedure was performed in the same manner as in Example 1 except that 8% by weight of paraffin was used. Table 1 shows the results.

[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 carried out except that the weight% was mixed with a Henschel mixer. Table 1 shows the results.

[0035]

Comparative Example 1 An attempt was made in the same manner as in Example 1 except that 26 % by weight of dioctyl phthalate and 35% by weight of liquid paraffin were used. However, the mixing property with a Henschel mixer was poor, and no molding was performed.

[0036]

Comparative Example 2 The same procedure as in Example 1 was carried out except that only 61 % by weight of liquid paraffin was used as a plasticizer. Table 1 shows the results.

[0037]

Comparative Example 3 Only dioctyl phthalate as a plasticizer 6
An attempt was made to carry out in the same manner as in Example 1 except that 1 % by weight was used, but molding could not be performed.

[0038]

Comparative Example 4 The same procedure as in Example 1 was carried out except that acetyl tri-n-butyl citrate having an SP value of about 10.0 was used instead of dioctyl phthalate as a plasticizer. However, molding could not be performed.

[0039]

Comparative Example 5 The same procedure as in Example 1 was carried out except that di-n-octyltetrahydrophthalate having an SP value of about 7.2 was used instead of liquid paraffin as a plasticizer. 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. Table 1 shows the results.

[0041]

[Table 1]

[0042]

According to the above construction, a microporous membrane having a high strength and an appropriate pore size can be obtained.
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.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01M 2/14-2/18 B01D 71/26 C08J 9/00 CES C08J 9/26 101

Claims (2)

(57) [Claims] 1. A microporous membrane having a three-dimensional network structure.
An ultra-high molecular weight having a viscosity average molecular weight of 2,000,000 or more
Polyethylene at least 30 wt% of the microporous membrane
Containing, having a porosity of 40% or more and an air permeability of 450 sec /
100cc or less, elastic modulus in machine direction 4000kg / c
m^TwoAs described above, the elongation at break in the direction perpendicular to the machine direction is 400% or less.
Above, the bubble point in ethyl alcohol is 2kg
/ Cm ^Two-10 kg / cm^TwoThe average pore size and the maximum pore size
A uniform super-characteristic having a diameter ratio of 1.6 or less
Microporous membrane made of high molecular weight polyethylene 2. A kneaded, heated and melted polyethylene mixture containing an ultra-high molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more, and a plasticizer. Are extracted and dried, and stretched in only one axis direction to obtain a microporous film, wherein the plasticizer has an SP value of 7.5 to 8.4.
And at least two types of mixed plasticizers of 8.5 to 9.5, and an SP value of 7.5 to 8.4 .
1% to 50% of the weight of the membrane (polyethylene, inorganic fine powder)
Method for manufacturing ultra-high molecular weight polyethylene microporous membrane you wherein a is