JPH059332A - Homogeneous microporous film made of ultrahigh-molecular weight polyethylene and its production - Google Patents

Abstract

PURPOSE:To obtain the subject microporous film having a high strength and a moderate pore diameter by kneading and melting a mixture of ultrahigh-molecular weight polyethylene with inorganic fine powder and a specific plasticizer, molding the mixture into a sheetlike shape, extracting and removing the additives, drying the sheet and uniaxially drawing the dried sheet. CONSTITUTION:A mixture of ultrahigh-molecular weight polyethylene having >=2000000 viscosity-average molecular weight with inorganic fine powder and a plasticizer is molded into a sheetlike form while being kneaded and thermally melted. The inorganic fine powder and the plasticizer are respectively extracted and removed and the resultant sheet is dried and then drawn only in one axial direction to produce a microporous film. In the process, a mixed plasticizer of two kinds having 7.5-8.4 SP value and 8.5-9.5 SP value of plasticizers is used and the plasticizer having the 7.5-8.4 SP value is used in an amount of 10-150% based on the weight of the polyethylene to afford the objective microporous film having >=40% porosity, <=450sec/100cc air permeability, >=4000kg/cm<2> modulus of elasticity in the machine direction, >=400% breaking elongation in the direction perpendicular to the machine direction, 2-10kg/cm<2> bubble point in ethyl alcohol and <=1.6 ratio of the maximum pore diameter to the average pore diameter.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a microporous membrane made of ultra high molecular weight polyethylene. In particular, the present invention relates to a homogeneous ultrahigh molecular weight polyethylene microporous membrane 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. In order to produce a strong microporous membrane, a method of using ultra-high molecular weight polyethylene and stretching is considered in the method of making it porous by a resin and a solvent. For example, JP-A-60-24203
However, it is not suitable as a microfiltration membrane for filtering a large amount of water such as a water purifier because it has a high average strength but has a small average through-hole diameter.
When used as a battery, the impregnation property of the electrolytic solution is poor. In addition, since the molding sheet is significantly shrunk by the extraction of the plasticizer and it is necessary to be biaxially stretched, the elongation at break in at least one direction is small, and the assembly workability of the battery is considered to be poor.

Further, as disclosed in JP-A-60-136161, since the film thickness is thin, there is a problem in safety when it is used as a separator for a lithium battery. Further, Japanese Patent Laid-Open No. 63-273651 discloses that
Although a method for producing a relatively thick microporous membrane made of ultra-high molecular weight polyethylene is disclosed, it is similarly strong, but does not have an appropriate pore size, has a low water permeability, and has a large amount of water. Is not suitable as a microfiltration membrane for treating

As another method for producing a microporous membrane, there 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 to be high, and it does not have an appropriate pore size, is at least not suitable as a microfiltration membrane, and is also considered not suitable as a separator because of its high electrical resistance.

Further, Japanese Patent Publication No. 45-32097,
Although there is JP-A No. 2-94356 and the like, none of them discloses a method for producing an ultrahigh molecular weight microporous membrane having a viscosity average molecular weight of 2,000,000 or more. This is because the higher the ultra-high molecular weight, the more difficult the molding process becomes, and the microporous membrane could not be manufactured by a general method.

Further, a microporous membrane made of a mixture of ultra-high molecular weight polyethylene and high molecular weight polyethylene is disclosed in JP-A-57 / 57.
Japanese Patent Laid-Open No. 49296/1990, Japanese Patent Laid-Open No. 21559/1990, and the like are disclosed, but as compared in Japanese Patent Laid-Open No. 60-242035 and Japanese Patent Laid-Open No. 3-105851,
Generally, when it becomes a mixture, the molding processability is improved, but the strength of the microporous membrane is significantly reduced, and in order to obtain a strong microporous membrane,
Not the preferred method.

[0008]

Although high strength can be achieved by using ultra-high molecular weight polyethylene, it can be used as a microfiltration membrane for filtration of a large amount of water or a lithium battery separator that requires safety. There is a demand for a microporous membrane having an appropriate pore size and an appropriate fracture elongation in the width direction (direction perpendicular to the machine direction). However, the desired microporous membrane has not been obtained due to problems such as low processability of ultrahigh molecular weight polyethylene.

[0009]

The present inventors have found that the ultra high molecular weight
Polyethylene microporous membrane with high strength and moderate
Microporous membrane having pore size and homogeneity and method for producing the same
Then, the present invention was completed. That is, the present invention is
Ultra high molecular weight polyethylene with a viscosity average molecular weight of 2 million or more
It is a microporous membrane with a three-dimensional network structure made of ren.
, Porosity of 40% or more, air permeability of 450 sec / 100
cc or less, elastic modulus in machine direction is 4000 kg / cm ²Since
The breaking elongation in the direction perpendicular to the machine direction is 400% or more.
Bubble point in chill alcohol is 2kg / cm
²-10kg / cm²And the ratio of average pore size to maximum pore size
Ultra-high molecular weight, characterized in that is less than 1.6
It is a polyethylene microporous membrane.

The polyethylene in the present invention has a viscosity average molecular weight of 2,000,000 or more and is a crystalline homopolymer obtained by polymerizing ethylene or ethylene and 10 mol% or less of propylene, 1-butene and 4-methyl. -1-
Examples thereof include a copolymer with pentene and 1-hexene. 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 battery separator, is from the viewpoint of the impregnation property of the electrolyte and the internal resistance of the battery.
It is preferably 40% or more, and more preferably 50% or more. When the porosity is 85% or more, the strength of the microporous membrane is reduced, which causes a problem 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.

When the elongation at break is 400% or less,
The internal short circuit of the battery tends to increase, and the reason is not clear, but it is considered that the battery does not easily tear in the machine direction and the hole does not become large because the battery has an appropriate elongation in the width direction during assembly. Ethyl alcohol - Bubble point in Le ^{is, 2kg / cm 2 ~10kg / cm} 2, preferably ^{3kg / cm 2 ~9kg / cm 2} , more preferably ^{4kg / cm 2 ~8kg / 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, especially when used as a separator for lithium batteries. Worry about the occurrence of internal short circuit. Further, when the bubble point in ethyl alcohol is 10 kg / cm ² or more, the pore diameter of the microporous membrane is too small and when used as a microfiltration membrane, the water permeability becomes small and the filtration efficiency is poor, resulting in a battery separator. When used,
There is a possibility that the impregnation property of the electrolytic solution may be deteriorated.

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 composed of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more, as disclosed in the present invention, having a thickness of 15
μm-60 μm, porosity 40% or more, air permeability 450
sec / 100cc or less, machine direction elastic modulus is 4000
kg / cm ² or more, breaking elongation in the direction perpendicular to the machine direction is 4
100% or more, ethyl alcohol - Bubble point in Le a is ^{2kg / cm 2 ~10kg / cm 2} , the ratio of the average pore size and maximum pore size, homogeneous ultra high molecular weight polyethylene, characterized in that more than 1.6 The microporous membrane is optimal as a lithium battery separator.

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 has a viscosity average molecular weight of 2,000,000 or more, and a mixture of ultra-high molecular weight polyethylene, an inorganic fine powder and a plasticizer is kneaded and heat-melted to form a sheet, and then the inorganic fine powder and the plasticizer are formed. In the production method for obtaining a microporous film by extracting and drying each of the above, and stretching the film in only a uniaxial direction, the SP value of the plasticizer is at least two types of 7.5 to 8.4 and 8.5 to 9.5. Of the mixed plasticizer, and SP value of 7.5
Manufactured by a manufacturing method characterized in that the amount of the plasticizer of ˜8.4 is 200% or less of the weight of the polyethylene.

Further, with two or more kinds of mixed plasticizers simply selected, a microporous membrane having a strong and proper pore size cannot be obtained. 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 ultra-high molecular weight polyethylene microporous membrane suitable for a microfiltration membrane, a battery separator, etc. Not only is it economical in industrial production that a microporous membrane with an appropriate pore size and a particularly high strength in the stretching direction can be obtained by stretching only in the uniaxial direction, as well as in the width direction (direction perpendicular to the stretching direction). Since it has an appropriate elongation at break, it has a characteristic that it is difficult to tear in the machine direction.

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 with an ordinary mixer such as a Henschel mixer, and then an extruder or the like. The melt-kneading device is used to knead, and the obtained kneaded product is molded into a thickness of 80 to 600 μm by extrusion molding or the like. Further, the plasticizer is extracted and removed from the molded product by using a solvent, the inorganic fine powder is extracted with a solvent for extracting the inorganic fine powder, and then the fine porous film is obtained by stretching only in the uniaxial direction.

As the solvent for the plasticizer, general organic solvents such as alcohols such as methanol and ethanol, ketones such as acetone and MEK, and chlorinated hydrocarbons such as 1,1,1-trichloroethane are 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 plasticizer having an SP value of 7.5 to 8.4 is 5% to 200%, preferably 15% of the weight of polyethylene.
˜150%, more preferably 35% to 100%, most preferably 50% to 100%. The amount of the plasticizer is 20
If it is 0% or more, a microporous membrane having an appropriate pore size cannot be obtained, and if it is 5% 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.

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) The thickness was measured with a dial gauge having a minimum 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) × 1
00 (%) Pore volume = (weight of water [g] -dry weight [g]) / density of water [g / cm ³ ] Microporous membrane volume = 100 x membrane thickness [cm] (3) Bubble point ASTM The bubble point in ethanol was measured according to E-128-61. (4) Air permeability In accordance with JIS P-8117, a B-type Gurley 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 5 mm, and a pulling speed of 200 mm / min. The rate was measured.

The cross-sectional area was calculated by (film thickness × film width measured in item (1)). (6) Tensile Rupture Elongation Using a Shimadzu model Autograph AG-A type, 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. , Calculate the elongation from the chart to the rupture,
It was calculated by the following formula.

Tensile breaking elongation = (elongation until breaking [m
m] -50 [mm]) / 50 [mm] × 100 [%] (7) Average pore size It was determined 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 with an Ubbelose type viscosity type at a measurement temperature of 135 ° C., and the viscosity average molecular weight was determined by the Chang's formula.

[0028]

Example 1 17% by weight of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 3,000,000, 18% by weight of finely divided silicic acid, 50% by weight of dioctyl phthalate and 15% by weight of liquid paraffin were mixed with a Henschel mixer, and the mixture was Φ30 mm.
The film was formed into a film having a thickness of 200 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, dried and then dipped in 25% caustic soda at 60 ° C. for 60 minutes to obtain finely divided silicic acid. Was extracted and 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 The procedure of Example 1 was repeated except that dioctyl phthalate was 35% by weight and liquid paraffin was 30% by weight.
The results are shown in Table 1.

[0033]

Example 4 Example 4 was repeated except that 57% by weight of dioctyl phthalate and 8% by weight of liquid paraffin were used. The results are shown in Table 1.

[0034]

Example 5 Same as Example 1 except that 18% by weight of ultra high molecular weight polyethylene having a viscosity average molecular weight of 3,000,000, 17% by weight of finely divided silicic acid, 48% by weight of dioctyl phthalate and 17% by weight of liquid paraffin were mixed in a Henschel mixer. Went to. 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 30% 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 65% 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 phthalate having an SP value of about 10.0 was used as the plasticizer instead of dioctyl phthalate. There wasn't.

[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, but molding was possible. There wasn't.

[0040]

Comparative Example 6 Example 1 was repeated except that the microporous membrane heated to 125 ° C. was biaxially stretched 3 times × 3 times using a tenter.
I went the same way. 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,
It can be applied as a homogeneous and highly reliable filtration membrane, particularly as a separator for non-aqueous electrolyte batteries, which has high workability and safety, low internal resistance, and uniform low internal resistance.

Claims (2)

[Claims] 1. A microporous membrane having a three-dimensional network structure made of ultrahigh molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more, having a porosity of 40% or more and an air permeability of 4 or less.
50sec / 100cc or less, machine direction elastic modulus is 40
00 kg / cm ² or more, the breaking elongation in the direction perpendicular to the machine direction is 400% or more, 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. Homogeneous ultra high molecular weight polyethylene microporous membrane characterized by being 6 or less 2. A mixture of ultra-high molecular weight polyethylene having a viscosity average molecular weight of 2,000,000 or more, an inorganic fine powder and a plasticizer is kneaded and heat-melted to form a sheet, then the inorganic fine powder and the plasticizer. In the production method for obtaining a microporous membrane by extracting and drying the above, respectively, and stretching in only a uniaxial direction, the SP value of the plasticizer is 7.5 to 8.4 and 8.5.
9.5 using at least two mixed plasticizers and
The amount of plasticizer having a P value of 7.5 to 8.4 is 10% to 150% of the polyethylene weight, the porosity is 40% or more, the air permeability is 450 sec / 100 cc or less,
The elastic modulus in the machine direction is 4000 kg / cm ² or more, the breaking elongation 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 k.
g / cm ² , and the ratio of the average pore diameter to the maximum pore diameter is 1.6.
The following is a method for producing a homogeneous ultrahigh molecular weight polyethylene microporous membrane