JPH11130900A - Finely porous polyethylene membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject finely porous membrane that has higher ion permeability than that of conventional membrane and can retain the membrane thickness and porosity that are needed for manifesting membrane strength. SOLUTION: This membrane is a finely porous one made of polyethylene that has a flex ratio of >=1.0-1.5, a porosity of 10-80% and the average pore diameter of 0.01-0.3 μm. In addition, the thickness of this membrane is 1-500 μm. preferably 5-200 μm. In the production of this finely porous membrane, for example, a polyethylene (preferably with a weight-average molecular weight of 1,000,000-4,000,000) is dissolved in a plasticizer at a higher temperature than its melting point, cooled down lower than the crystallization temperature to form polymer gel. Then, the gel is oriented, the plasticizer is removed from the gel by extraction, and the gel is oriented again, then heat-set or heat-relaxed. The resultant finely porous membrane is useful as a separator for lithium ion secondary battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a microporous polyethylene membrane and a battery separator comprising the same.

[0002]

2. Description of the Related Art Microporous polyethylene membranes are used in microfiltration membranes, breathable clothing applications, battery separators, condenser separators, and the like. Among them, when used as a battery separator, particularly a lithium ion battery separator, particularly high ion permeability is required to correspond to an organic electrolyte having low electric conductivity.

[0003] As a method of improving the ion permeability of a microporous membrane, thinning and increasing the porosity have been conventionally known. However, in these methods, the microporous membrane is replaced with an improvement in the ion permeability. Its use was naturally limited due to the reduced strength. Therefore, there has been a demand for a microporous polyethylene membrane capable of maintaining a film thickness and a porosity required for strength development and exhibiting higher ion permeability than conventional membranes.

Japanese Patent Application Laid-Open No. 7-228718 discloses a microporous polyolefin membrane for improving the internal short circuit of a lithium secondary battery, but does not pay attention to ion permeability and is not sufficient.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a microporous polyethylene membrane which maintains a film thickness and a porosity required for strength development and has high ion permeability.

[0006]

As a result of intensive studies on the above problems, it has been found that a microporous polyethylene membrane having a specific pore structure has higher ion permeability than a conventional membrane having the same porosity. Was reached. That is, according to the present invention, the bending rate is 1.0 to 1.5 and the porosity is 10%.
To 80% and an average pore diameter of 0.01 to 0.3 μm. Further, the present invention relates to a battery separator comprising the polyethylene microporous membrane.

In the present invention, the bending ratio is defined as the ratio of the length of the capillary to the film thickness when the pores inside the microporous membrane are replaced by an equivalent set of cylindrical capillaries. Here, the bending rate of 1.0 means that the ion movement distance is equal to the film thickness, and the microporous membrane at this time shows the highest ion permeability among the microporous membranes having the same porosity. obtain. On the other hand, if the bending ratio is larger than 1.0, the moving distance of the ions becomes longer than the film thickness, and the ion permeability decreases as the bending ratio increases. This is also shown in the equation of the electrical conductivity of the film λ _eff = (porosity / flexibility ² ) × λ (λ = electric conductivity of the solvent), where the electrical conductivity is inversely proportional to the square of the flexural modulus. ing.

Since the conventional microporous polyethylene membrane has a flexural modulus of more than 1.5, it is necessary to increase the porosity in order to obtain high ion permeability. On the other hand, in the present invention, the ion permeability is improved by changing the bending ratio. Polyethylene used in the present invention,
High-density polyethylene which is a crystalline polymer mainly composed of ethylene, or propylene for ethylene units,
A copolymer (linear copolymer polyethylene) containing units of α-olefin such as butene, pentene, hexene and octene at a ratio of 4 mol% or less may be used. Further, a polyolefin such as polypropylene, medium-density polyethylene, linear low-density polyethylene, low-density polyethylene, and EPR may be blended with these at a ratio of 30% or less.

The weight average molecular weight of the raw material polyethylene is 10
It is from 10,000 to 4,000,000, preferably from 200,000 to 700,000, and more preferably from 250,000 to 500,000. If the weight average molecular weight is less than 100,000, it tends to break during stretching, and if it is more than 4,000,000, it becomes difficult to produce a hot solution, which is not preferable. Further, the weight average molecular weight may be adjusted to a preferable range by means such as blending or multi-stage polymerization.

The thickness of the microporous membrane is 1 to 500 μm, preferably 5 to 200 μm, more preferably 10 to 50 μm.
It is. When the film thickness is smaller than 1 μm, the mechanical strength of the film is not sufficient. The porosity in the present invention is 10 to 8
0%, preferably 20-70%, more preferably 30%
~ 70%. If the porosity is less than 10%, the permeability is not sufficient, and if the porosity is more than 80%, sufficient mechanical strength cannot be obtained.

[0011] The average pore diameter and the flexural modulus can be determined by the "gas-liquid method" based on the combined measurement of gas and liquid permeation. Average pore size in the present invention is 0.01 to 0.3μ
m, preferably 0.02 to 0.2 μm. If the average pore size is smaller than 0.01 μm, the permeability is not sufficient, and if the average pore size is larger than 0.3 μm, there is a concern that a dendrite deposited or a collapsed active material may cause a short circuit, so that it is suitable for use as a battery separator. Absent. Further, in order to prevent an internal short circuit due to falling off or penetration of the active material, the maximum pore diameter is preferably 0.5 μm or less. Here, the maximum pore size is defined as F from the value of the molecular weight of pullulan showing a rejection of 90% when a 0.05% by weight aqueous solution of pullulan is permeated.
It refers to the pore size converted using the theory of lowy. This rejection can be obtained by a GPC (gel permeation chromatography) measurement method.

The bending ratio in the present invention is 1.0 or more.
It is less than 5, preferably 1.0 to 1.47, more preferably 1.0 to 1.4. Higher ion permeability cannot be obtained as the flexural modulus exceeds 1.5. Piercing strength is 1
It is 50 g or more, preferably 250 g or more. If the puncture strength is smaller than 150 g, the separator may be short-circuited by the dropped active material or the like.

Next, a method for producing the microporous polyethylene membrane of the present invention will be described. A polyethylene microporous membrane is obtained by dissolving polyethylene in a solvent called a plasticizer at a temperature higher than its melting point, cooling the same to a temperature lower than a crystallization temperature to form a polymer gel, and further stretching the polymer gel to form a plasticizer. It is manufactured by extracting and removing the film, and then performing stretching again, and then preferably performing heat treatment such as heat fixing or thermal relaxation. These steps are called a film forming step, a stretching step before extraction, an extraction step, a stretching step after extraction, and a heat treatment step, respectively.

The term "plasticizer" as used herein refers to an organic compound capable of forming a uniform solution with polyethylene at a temperature not higher than its boiling point. Specifically, decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, Oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, paraffin oil and the like are mentioned. Of these, paraffin oil, dioctyl phthalate and decalin are preferred.
The weight fraction of the plasticizer in the polymer gel is not particularly limited, but is 20% to 90%, preferably 50% to 70%. If it is less than 20%, it is difficult to obtain a microporous film having an appropriate porosity, and if it is more than 90%, the viscosity of the hot solution is reduced, making it difficult to continuously form a sheet.

[0015] The film forming method is not particularly limited. For example, a polyethylene powder and a plasticizer are supplied to an extruder, melt-kneaded, and then cast from a usual hanger coat die onto a cooling roll for several tens of μm.
Sheets from m to several mm can be formed continuously. In the present invention, since ultra-high molecular weight polyethylene is not an essential component, no special heating and melting equipment is required, and it is possible to adjust a homogeneous sheet extremely simply by adding polyethylene and a plasticizer to the extruder. It is.

Next, the obtained sheet is stretched in at least one axial direction to form a stretched film. The stretching method is not particularly limited, but a tenter method, a roll method, a rolling method, or the like can be used. Of these, simultaneous 2 by the tenter method
Axial stretching is preferred. The stretching temperature is in the range from room temperature to the melting point of the polymer gel, preferably 80 to 130C, more preferably 100 to 125C. The stretching ratio is 4 to 400 times, preferably 8 to 200 times, and more preferably 16 to 100 times in area ratio. If the stretching ratio is lower than 4 times, the strength as a separator is insufficient,
If the number exceeds twice, not only stretching is difficult, but also adverse effects such as a decrease in porosity of the obtained microporous membrane are likely to occur.

Next, a plasticizer is extracted and removed from the stretched film to obtain an extracted film. As the extraction solvent, for example, n
-Hydrocarbons such as hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, acetone and 2-butanone And the like. Although there is no particular limitation on the extraction method, when paraffin oil or dioctyl phthalate is used, it can be removed by extracting with the above-mentioned extraction solvent and then heating and drying at a temperature below the temperature at which the pores of the obtained extraction membrane are closed. I can do it. When a low boiling point compound such as decalin is used as the plasticizer, it can be removed only by heating and drying at a temperature lower than the temperature at which the pores of the microporous membrane are closed. In any case, it is preferable to restrain the film in order to prevent a decrease in physical properties due to contraction of the film.

Finally, the extraction membrane is stretched again in at least one axial direction. The stretching method is not particularly limited,
A tenter method, a roll method and the like can be used. The stretching temperature ranges from room temperature to the melting point of the polymer gel, preferably from 80 to 1
The temperature is 30 ° C, more preferably 100 to 125 ° C. The stretching ratio is 1.0 to 10 times, preferably 1.1 to 6 times, and more preferably 1.2 to 4 times in area ratio.
If the stretching ratio is lower than 1.0, the improvement of the separator bending ratio is insufficient, and if it is higher than 10, the stretching is difficult.

In general, stretching after extraction tends to increase the porosity.
It is possible to suppress the increase in the porosity and to reduce only the bending rate. Further, in order to improve the permeability and enhance the dimensional stability, it is preferable to perform a heat treatment such as heat fixing or thermal relaxation subsequent to or after stretching after extraction.

[0020]

Next, the present invention will be further described by way of examples.
This will be described in detail. The test method shown in the examples is
It is as follows. (1) Film thickness dial gauge (Ozaki Seisakusho: PEACOCK No. 2)
Measured in 5). (2) Porosity A sample of 20 cm square is cut out from the microporous membrane, and its body is cut out.
Calculate the product and weight, and calculate
Was. Porosity (%) = (Volume (cm^Three) -Weight (g) / density)
/ Volume (cm^Three) × 100 (3) Puncture strength Use KAT-G5 Handy Compression Tester manufactured by Kato Tech
The radius of curvature at the tip of the needle is 0.5 mm and the piercing speed is 2 mm /
A piercing test is performed under the conditions of sec.
Degree (g). The piercing strength is 25 (μm) / film thickness.
(Μm) to obtain a 25 μm converted piercing strength
Was. (4) Air permeability Measured using a Gurley air permeability meter based on JIS P-8117.
Specified. The pressure at this time is 0.01276 atm, and the film surface
Product is 6.424cm^Two, The amount of permeated air is 100cc
You. Further, the air permeability (sec) is 25 (μm) / film thickness (μm).
multiplied by m) to convert the air permeability into 25 μm (sec)
And (5) Water permeability In a stainless steel liquid-permeable cell with a diameter of 42 mm,
Set the microporous membrane soaked in alcohol,
After washing the alcohol with water, with a differential pressure of about 0.5 atm
Permeate water after passing water for 120 seconds (c
m^Three), The permeability per unit time, unit pressure, unit area
Calculate the amount of water and calculate the water permeability (cm^Three/ Cm ^Twosec
atm). (6) Pore size and bending rate Fluid inside the capillary is determined by the mean free path of the fluid.
When the diameter is smaller than the diameter of the pillar, the flow of poiseuille
It is known to follow the Knudsen flow
You. Here, for air permeability measurement in accordance with JIS P-8117,
The air flow is Knudsen's flow, water permeability at room temperature
Assuming that the flow of water in the measurement follows the flow of Poiseuille
Then, the pore diameter d (m) and the bending rate τ are determined by the air transmission velocity
Number R_gas, Water transmission rate constant R_liq, Water viscosity η (Pa
 sec), standard pressure Ps (101325 Pa), pores
Rate ε (dimensionless), film thickness L (m), gas molecular velocity ν (m
/ Sec) from the following equation.

D = 2ν (R _liq / R _gas ) (16η /
3) (1 / Ps) τ ² = dεν / (3L Ps R _gas ) Here, R _gas is obtained from the air permeability (sec) using the following equation. R _gas (m ³ / m ² sec Pa) = 0.0001 / air permeability / 0.0006424 / (0.01276 × 101)
325) Also, R _liq is the water permeability (cm ³ / cm ² sec at
m) using the following equation.

R _liq (m ³ / m ² sec Pa) = water permeability / 1,000,000 / 0.0001 / 101325 Further, ν is a gas constant R (8.314) and an absolute temperature T
(K), pi, average molecular weight of gas M (kg / mo)
From l), it can be obtained using the following equation. ν ² = 8RT / πM (7) DC electric resistance It was measured with an electric resistance tester based on JIS C-2313.

Further, the reciprocal of the value obtained by dividing the electric resistance value by the film thickness is defined as a film-conversion electric conductivity, and the value obtained by dividing the film-conversion electric conductivity by a porosity is defined as a film-porosity-converted electric conductivity. It was used as an index of ion permeability.

[0024]

Example 1 High-density polyethylene (weight average molecular weight 25
10,000, molecular weight distribution 7, density 0.956) 50 parts by weight, liquid paraffin 50 parts by weight and 0.1 part by weight of the polyethylene.
3 parts by weight of tetrakis- [methylene-3- (3 ′, 5 ′)
-Di-t-butyl-4'-hydroxyphenyl) propionate] methane using a 35 mm twin screw extruder.
Knead at 0 ° C. to prepare polymer solution, 1.7 m between lips
The resulting polymer solution was cast from a hanger coat die on a cooling roll to obtain a sheet having a thickness of 1.7 mm. The obtained sheet was stretched by a simultaneous biaxial tenter stretching machine at a stretching temperature of 120 ° C. 7 × 7 times before extraction, and then immersed in methylene chloride to extract and remove liquid paraffin. Furthermore, after stretching 1.8 times in the width direction after stretching at a stretching temperature of 120 ° C. using a tenter stretching machine, the stretching in the width direction was reduced by 17%.
Heat treatment was performed while relaxing. The physical properties of the film are shown in Tables 1 and 2.

[0025]

Example 2 Two sheets from which liquid paraffin had been extracted were rolled and stretched at a stretching temperature of 120 ° C. and a stretching ratio of 1.8 times in a stretching step after the extraction, and then stretched at a stretching temperature of 120 ° C. using a tenter stretching machine. After stretching 1.7 times in the width direction, a microporous film was obtained in the same manner as in Example 1 except that the film was fixed in the width direction and heat-treated. The physical properties of the obtained film are shown in Tables 1 and 2.

[0026]

Example 3 High density polyethylene (weight average molecular weight 40
10,000 parts, molecular weight distribution 8, density 0.950) 45 parts by weight, liquid paraffin 55 parts by weight and 0.1 part by weight of the polyethylene.
3 parts by weight of tetrakis- [methylene-3- (3 ′, 5 ′)
-Di-t-butyl-4'-hydroxyphenyl) propionate] methane using a 35 mm twin screw extruder.
The polymer solution was adjusted by kneading at 0 ° C., and cast on a cooling roll from a hanger coat die having a lip of 1.7 mm to obtain a sheet having a thickness of 1.7 mm. Simultaneous sheet 2 obtained
7 × 7 at a stretching temperature of 125 ° C. using an axial tenter stretching machine
It was stretched twice before extraction and then immersed in methylene chloride to extract and remove liquid paraffin. Further, using a tenter stretching machine, the film was stretched after extracting 1.8 times in the width direction at a stretching temperature of 120 ° C., and then heat-treated while relaxing the stretching in the width direction by 22%. The physical properties of the film are shown in Tables 1 and 2.

[0027]

Example 4 High-density polyethylene (weight average molecular weight 25
10,000, molecular weight distribution 7, density 0.956) 40.5 parts by weight,
Linear copolymerized polyethylene (melt index 0.01
7, density 0.930 propylene content 1.6 mol%)
4.5 parts by weight, 55 parts by weight of liquid paraffin and 0.3 parts by weight based on the polyethylene of tetrakis- [methylene-3- (3 ′, 5′-di-tert-butyl-4′-hydroxyphenyl) propionate ] Methane is kneaded using a 35 mm twin screw extruder to prepare a polymer solution, and the polymer solution is cast on a cooling roll from a hanger coat die with a lip between 1.8 mm, and then cast to a thickness of 1.8 mm. I got a sheet. The obtained sheet was stretched before extraction at a stretching temperature of 120 ° C. by 7 × 4 times using a simultaneous biaxial tenter stretching machine, and then immersed in methylene chloride to extract and remove liquid paraffin. Further, using a tenter stretching machine, a stretching temperature of 11
After extracting 2.8 times in the width direction at 0 ° C. and then stretching, heat treatment was performed while relaxing the stretching in the width direction by 35%. The physical properties of the film are shown in Tables 1 and 2. The maximum pore size of the obtained membrane is
It was 0.25 μm.

[0028]

Example 5 High-density polyethylene (weight average molecular weight 25
28 parts by weight, molecular weight distribution 7, density 0.956), linear copolymer polyethylene (melt index 0.017,
Density 0.930 Propylene content 1.6 mol%) 12 parts by weight, liquid paraffin 60 parts, and 0.3 parts by weight based on the polyethylene of tetrakis- [methylene-3-
(3 ', 5'-di-t-butyl-4'-hydroxyphenyl) propionate] stretch before extraction of methane by 7 × 7
, And after elongation after extraction was 1.87, the relaxation rate was 10%.
A microporous membrane was obtained in the same manner as in Example 1 except for the above. The physical properties of the obtained film are shown in Tables 1 and 2.

[0029]

Comparative Example 1 A microporous membrane was obtained in the same manner as in Example 1 except that stretching and heat treatment were not performed after extraction. The physical properties of the obtained film are shown in Tables 1 and 2.

[0030]

[Comparative Example 2] Except not performing stretching and heat treatment after extraction,
A microporous membrane was obtained in the same manner as in Example 2. The physical properties of the obtained film are shown in Tables 1 and 2.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

As described above, the present invention provides a separator having a low flexural modulus of 1.0 to 1.5 without impairing the strength, thickness and porosity values conventionally required for a separator. The present invention provides a microporous polyethylene membrane having improved ion permeability required as battery characteristics.

Claims (2)

[Claims] 1. A flexural modulus of 1.0 to 1.5 and a porosity of 10
% To 80% and an average pore diameter of 0.01 μm to 0.3 μm. 2. A battery separator comprising the microporous membrane according to claim 1.