JP3508510B2 - Laminated porous film and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a laminated porous film useful as a separator for batteries, a diaphragm for electrolytic capacitors, and the like.

[0002]

2. Description of the Related Art Conventionally, polyolefin-based porous films have been used as separators for batteries and diaphragms for electrolytic capacitors. In particular, with the advancement of technology in recent years, high precision, high-performance separators have been required in lithium batteries and the like, and a separator of a laminated porous film has attracted attention in place of the conventional single-layer porous film. It's starting to happen.

Taking a battery as an example, the battery has both positive and negative polarities.
There is a separator to prevent short circuit between poles.
High energy density, high electromotive force, low self-discharge
Non-aqueous electrolyte batteries, especially lithium batteries.
The pond has been developed and put into practical use. As the negative electrode of a lithium battery
For example, metallic lithium, the combination of lithium and other metals
Adsorbs lithium ions such as gold, carbon and graphite
Ability to store or to occlude by intercalation
A carbon material with a lithium ion doped conductor
Electropolymer materials are known, and examples of positive electrodes are
For example (CF_x)_nFluorinated graphite, MnO₂, V
₂O_Five, CuO, Ag₂CrO_Four, TiO ₂, LiCo
O_Four, LiMn₂O_FourMetal oxides, sulfides, chlorides, etc.
It has been known.

As a non-aqueous electrolyte, ethylene carbonate is used.
, Propylene carbonate, γ-butyrolactone,
Acetonitrile, 1,2-dimethoxyethane, tetrahi
LiPF in organic solvents such as Drofran₆, LiBF_Four, L
iClO_Four, LiCF₃SO ₃Also dissolved electrolyte such as
Is used. However, lithium is particularly reactive
Therefore, if an abnormal current flows due to an external short circuit or incorrect connection, etc.
Battery temperature rises significantly, leading to an accident such as ignition.
Or thermal damage to equipment incorporating it.
I have a reminder. In order to avoid such a risk, conventional security
The following various porous films are used as a palletizer.
Is proposed.

A single layer porous film of a thermoplastic resin such as polyethylene or polypropylene (Japanese Patent Publication No. 46-401).
19, Japanese Patent Publication No. 55-32531, Japanese Patent Publication No. 5
9-37292, JP-A-60-23954, JP-A-2-75151, and US Pat. No. 3679.
No. 538, etc.). Porous film made of a polyethylene mixture having different molecular weights or a mixture of polyethylene and polypropylene (Japanese Patent Laid-Open No. 21559/1990, 33130/1993).
No. 6, etc.). Porous film using a thermoplastic resin or a non-woven fabric as a support (JP-A-3-245457, JP-A1-2583)
58, etc.). A laminated porous film in which a plurality of porous films of thermoplastic resins having different materials are laminated (JP-A-62-10857, JP-A-63-308866, JP-A-2-77).
108, JP-A-5-13062, and Japanese Patent Publication No. 3
-65776, JP-A-6-55629, JP-A-6-20671, JP-A-7-307146). The above-mentioned porous film is generally a stretching method in which an unstretched film is made porous by stretching, a filler that can be extracted, a filler, a plasticizer, and the like are extracted from the unstretched film with a solvent to extract the filler, the plasticizer, etc. And is uniaxially or biaxially stretched before or after extraction, if necessary.

The basic idea of using a single-layer or laminated porous film as a separator is to prevent short circuit between both electrodes,
While maintaining the battery voltage, etc., when the internal temperature of the battery rises above a certain temperature due to abnormal current, etc., the porous film is made non-porous, in other words, the pores are blocked and ions flow between the electrodes. To prevent this, increase the electric resistance and stop the battery function to prevent the danger of ignition due to excessive temperature rise and to ensure safety. The danger prevention function due to excessive temperature rise is an extremely important function as a battery separator, and is generally called non-porous or shutdown (abbreviated as SD).

In the battery separator, if the non-porous temperature is too low, there is a problem in practical use because the flow of ions is blocked by a slight temperature rise, and conversely, if it is too high, in a lithium battery or the like. Has a problem in terms of safety because it may cause a fire. It is generally recognized that a non-porous temperature of 110 to 160 ° C, preferably 120 to 150 ° C is suitable. In this specification, the upper limit of the temperature at which the non-porous state or the SD state is maintained is referred to as the heat-resistant temperature. However, in a battery using a porous film as a separator, the temperature inside the battery is the heat-resistant temperature of the porous film. If the temperature rises above the temperature, the film will melt and tear,
The non-porous state is lost, ions start to flow again, and the temperature rises further. Therefore, as a battery separator, it is required to have an appropriate non-porous temperature and to have a high heat resistance temperature, and to surely achieve non-porous in the non-porous temperature range.
Further, as the battery separator, in addition to the characteristics relating to the non-pore formation, low electric resistance, high tensile strength, high mechanical strength such as puncture strength, and small variations in thickness unevenness and electric resistance, etc. Is required.

[0008]

Various porous films have been proposed as mentioned above, but as a battery separator, there is a problem in the reliability of SD, which is an extremely important function.

Further, the single-layer porous film has a narrow temperature range between the non-porous temperature and the heat-resistant temperature, and it is difficult to achieve a reliable SD. Therefore, an improvement to solve this problem is studied. ing. In particular, the single-layer porous film has not only a low heat resistance temperature but also SD even in a temperature range below the heat resistance temperature.
It has been pointed out that when the temperature rises above the temperature, the shrinkage is large, a short circuit occurs due to the exposure of the electrode plate, and it becomes impossible to maintain the non-porous state.

Regarding a laminated porous film in which a plurality of porous films of thermoplastic resins of different materials are laminated and laminated, the film is preliminarily made porous by a stretching method, an extraction method or the like, and two kinds of materials and melting points are different. After manufacturing a porous film, it is laminated and stretched, pressed, or manufactured by adhesion with an adhesive, or by a method such as making a laminated film thermocompression-bonded with a polyolefin having a different melting point by a stretching method or the like. Has been done.

These laminated porous films are obtained by combining two kinds of porous films having different melting points,
Provided are a battery separator, an electrolytic capacitor diaphragm and the like, which have a non-porous temperature of about 135 ° C. and a heat resistant temperature of about 180 ° C. and are excellent in safety. Further, by combining two types of porous films having different mechanical strengths, it is possible to provide a battery separator or the like having high mechanical strength such as piercing strength, which basically has excellent characteristics as a battery separator or the like. is doing.

[0012] Further, the method for making the laminated porous film porous is roughly classified into a stretching method (dry method) and an extraction method (wet method). The wet method is a method of filling a thermoplastic resin having a different material or melting point with a filler. It is a method of producing a laminated film by coextruding a resin composition containing a plasticizer and a plasticizer, and then extracting the filler and the plasticizer from the film to make them porous, thereby obtaining a laminated porous film. In order to obtain a laminated porous film having a fine and uniform pore size, it is necessary to mix and extract a filler and a plasticizer, which not only complicates the operation process but also has a problem such as treatment of the extract. On the other hand, the stretching method is to co-extrude thermoplastic resins having different melting points, or stretch and permeate laminates obtained by separately extruding them, or to extrude and stretch thermoplastic resins having different melting points separately. It is manufactured by a method of laminating after making it porous. Since these stretching methods are dry processes that do not use any solvent at all, they are extremely simple and excellent in safety and are low-cost processes, and in addition, a porous membrane with a fine and uniform pore size can be obtained. It is superior to the wet method as a manufacturing method.

The laminated porous film, which is porously laminated by the stretching method having these excellent characteristics, is preferably used as a battery separator or an electrolytic capacitor diaphragm, but it has a low melting point that makes it nonporous at lower temperatures. Further improvement is desired in that the quality film is surely non-porous near the melting point and that the SD film is completely maintained in the vicinity of the melting point of the high melting point porous film.

The inventors of the present invention confirmed the certainty of SD when the laminated porous film became non-porous (SD) and then reached a higher temperature, and the non-porous temperature range of the laminated porous film and The present invention has been accomplished by focusing on the relationship with the shrinkage characteristics at the above temperatures.

[0015]

The present invention relates to a laminated porous film comprising a high melting point polyolefin film and a low melting point polyolefin film which have been made porous by a stretching method and have melting points different by 20 ° C. or more.
The film and the low melting point polyolefin film are
Separately melt-extruded, heat-treated, then laminated
And a laminated porous film made porous by a stretching method.
Thus, the high melting point polyolefin film after heat treatment has a birefringence of 15 × 10 ^{−3 to} 25 × 10 ⁻³ and a birefringence of 100.
% Elastic recovery rate at the time of extension is 80 to 94 percent, after the heat treatment
The low melting point polyolefin film has a birefringence of 30 × 1.
0 ^{-3 to} 48 x 10 ^-3 , and an elastic recovery rate at 50% elongation of 50 to 80%, and a laminated porous film.
The low melting point polyolefin film has a thickness of 20 to 80% of the total thickness of the laminated porous film, and the low melting point polyolefin film has an elastic modulus of 1 at the non-porous temperature.
0 ⁴ dyne / cm ² or more, and the shrinkage ratio in the longitudinal direction (stretching direction) of the laminated porous film in the unconstrained state in the temperature range of non-porous temperature to non-porous temperature + 50 ° C. or less is 5 to 7
It relates to a laminated porous film which is 0%. Further, in the present invention, the melting points made porous by the stretching method are different by 20 ° C. or more.
High melting point polyolefin film and low melting point polyolefin
Odor for manufacturing laminated porous film consisting of film
Te, In birefringence 15 × ^{10 -3} ~25 × ¹⁰ -3 obtained by heat treatment after melt-extrusion, and the refractory polyolefin film is and 100% elongation during elastic recovery ratio from 80 to 94%, The birefringence obtained by heat treatment after melt extrusion is 30 × 10 ^{−3 to} 48 × 10 ⁻³ ,
And, after laminating with a low melting point polyolefin film having an elastic recovery rate of 50 to 80% at 50% elongation, it is stretched at a low temperature of -20 to + 50 ° C for 5 to 200% at a low temperature, and then 70 to 130 ° C. 100-400% at temperature
Stretched at high temperature, the low melting point polyolefin film has a thickness of 20 to 80% of the total thickness of the laminated porous film,
The low-melting-point polyolefin film has an elastic modulus of 10 ⁴ dyne / cm ² or more at a non-porous temperature, and the laminated porous film has a longitudinal length in a non-constrained state in a temperature range of a non-porous temperature to a non-porous temperature + 50 ° C. or less. It relates to a method for producing a laminated porous film, which comprises obtaining a laminated porous film having a shrinkage in the direction (stretching direction) of 5 to 70%.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a high melting point polyolefin film and a low melting point polyolefin film are
If the difference in melting point is small, the non-porous maintenance temperature region becomes narrower,
Those whose melting points are different by 20 ° C. or more, preferably 30 ° C. or more are used.

In the present specification, the melting point of the polyolefin film is a differential scanning calorimeter (manufactured by Shimadzu Corporation, DSC).
-50) was used to measure about 10 mg of a sample under a nitrogen stream at a temperature rising rate of 10 ° C./minute from room temperature and measurement,
It means the endothermic peak temperature associated with melting.

As the high melting point polyolefin film,
For example, polypropylene, poly 4-methylpentene-1,
As the film such as poly-3-methylbutene-1 and the low melting point polyolefin film, films such as polyethylene, polybutene, ethylene propylene copolymer and the like are used. Polypropylene film is preferably used as the high-melting point polyolefin film, and polyethylene film is used as the low-melting point polyolefin film. Polypropylene having high stereoregularity is preferable, and polyethylene is preferably high-density polyethylene, but medium-density polyethylene may be used. The high melting point polyolefin film and the low melting point polyolefin film may optionally contain additives such as a surfactant, an antioxidant, a plasticizer, a flame retardant, and a coloring agent.

In the present invention, as the low melting point polyolefin film which becomes non-porous at a lower temperature, one having a relatively high elastic modulus is used in consideration of the film strength, the shape retaining property upon melting and the like. Generally, a material having an elastic modulus of 10 ⁴ dyne / cm ² or more in the non-porous temperature range is used.

It is preferable that the shrinkage ratio required for making non-porous is usually several% or more, preferably 5% or more at the non-porous temperature. It is preferable that the larger the shrinkage ratio, the larger the shrinkage force is, but if it is too large, the dimensional change becomes too large, so that it is generally preferably 90% or less. The non-porous temperature in the present invention means the resistance value of the porous polymer film in the electrolytic solution at room temperature of 10
The temperature is 0 times or more, and the measuring method and the resistance measuring device are as follows.

Measurement method: Electrolyte solution An equal volume mixture of propylene carbonate and dimethoxyethane in which 1 mol of lithium perchlorate was dissolved was used. Electrode area 1 cm ² Resistance measuring device: LCR HiTester (manufactured by Hioki Electric Co., Ltd.) Measuring frequency 1 kHz A porous polymer film sample was immersed in an electrolytic solution for 5 minutes and then set between electrodes, and a speed of 2 ° C./minute was set in an oven. The resistance value was measured while increasing the temperature.

The elastic modulus measuring method in the present invention is carried out by the following method. Measuring method of elastic modulus Measuring device: Dynamic Spectrometer RDS2
(Rheometrics Co., Ltd.) Measuring conditions: torsional mode dynamic measurement Frequency 10 rad / sec

When the elastic modulus of the low melting point polyolefin film is excessively small, the shape retention and film strength during melting are deteriorated. Therefore, porous polyethylene having an elastic modulus of 10 ⁴ dyne / cm ² or more is used. It If the elastic modulus is too high, the film becomes brittle,
Elastic modulus is 10 ¹¹ dyne / cm ² in non-porous temperature range
The following are preferably used.

In general, when a polymer material is heated to around its melting point, it does not suddenly show a fluidized state like a low molecular weight substance even if a crystal part melts, and there is a so-called rubbery region. Therefore, generally, in the non-porous temperature range around the melting point, the porous polymer film does not easily flow and it is difficult to achieve complete non-porous.

In the present invention, when porous polyethylene having an elastic modulus of 10 ⁴ dyne / cm ² or more in the non-pore forming temperature range is used as the porous polymer film, the film strength is high and the viscosity is relatively high. It is high and has an excellent shape-retaining property at the time of melting, and further, it is possible to achieve complete non-pore formation by substantially shrinking in the non-pore formation temperature range. In particular, due to the action of the contracting force, the porous polymer film, which is in a state where it does not easily flow, is contracted, and it is possible to achieve completely non-porous.

In the present invention, the low melting point polyolefin film has a thickness of 20 to 80% of the total thickness of the laminated porous film. If the thickness ratio of the low-melting-point polyolefin film becomes excessively small, it becomes non-porous, and the shape retention during melting when the temperature further rises is poor, making it difficult to achieve complete non-porous. If the thickness ratio of the low-melting point polyolefin film is excessively higher than 80%, the thickness ratio of the high-melting point polyolefin film becomes too small, and heat resistance, which is a role of the high-melting point polyolefin film, cannot be sufficiently secured.

The laminated porous film made porous by the stretching method has a shrinkage ratio of 5 in the longitudinal direction (stretching direction) of the laminated porous film in the temperature range from non-porous temperature to non-porous temperature + 50 ° C. or less. 70% is used. If the shrinkage in the longitudinal direction of the laminated porous film is excessively smaller than 5%, a sufficient shrinkage force does not work in the non-porous temperature range and it becomes difficult to achieve complete non-porous. On the other hand, if the shrinkage ratio in the longitudinal direction is excessively larger than 70%, the dimensional change of the laminated porous film becomes too large, which is not preferable.

The laminated porous film made porous by the stretching method, when heated to the non-porous temperature range in an unconstrained state,
The main shrinking direction is the longitudinal direction (stretching direction) of the laminated porous film. Little or little expansion in the transverse direction (direction perpendicular to the stretching direction). When used as a battery separator, the laminated porous film made porous by this stretching method is generally wound in the longitudinal direction together with the electrodes under constant tension. In this case, both ends of the laminated porous film in the vertical direction are restrained, and the contraction in the vertical direction is restricted. When heated above the non-porous temperature range in this state, the contraction in the longitudinal direction is restricted, so that the laminated porous film contracts in the lateral direction or the thickness direction. Longitudinal direction (stretching direction) in the unconstrained state of the laminated porous film in the temperature range of non-porous temperature to non-porous temperature + 50 ° C or less
If the shrinkage ratio of the above is in the range of 5 to 70%, the shrinkage ratio in the horizontal direction when both ends in the vertical direction are restrained can be reduced. If the shrinkage ratio in the lateral direction becomes large, the electrode plate is exposed due to the shrinkage, which is not preferable. When the electrode plate is exposed due to the lateral contraction of the porous film above the non-poration temperature range, a short circuit occurs and the non-perforation cannot be maintained and the safety of the battery cannot be ensured.

Although it depends on the melting point of the high-melting-point polyolefin film, the shrinkage ratio in the longitudinal direction of the laminated porous film in the unconstrained state is generally 5 in the temperature range of non-porous temperature to non-porous temperature + 50 ° C. or less. If it is in the range of 70%, perfect porosity is maintained even if the temperature rises above the porosity temperature.
Furthermore, if the shrinkage ratio in the thickness direction of the laminated porous film in the lengthwise both ends constrained state in the temperature range of non-pore forming temperature range to non-pore forming temperature + 50 ° C. or less is in the range of 20 to 80%,
In this temperature range, shrinkage in the width direction in a state where both ends of the laminated porous film in the vertical direction are restrained is suppressed, which is more preferable.

The laminated porous film can be produced by a known method (Japanese Patent Laid-Open No. 4-18).
1651, JP-A-7-307146, JP-A-8-222197, etc.). As a specific example of the production method, a method of melt-extruding a low-melting point polyolefin film and a high-melting point polyolefin film and then stretching and porosifying, a low-melting point polyolefin film and a high-melting point polyolefin film, each of which is melt-extruded and laminated and then stretched. There are a method of making it porous, a method of laminating a low-melting point polyolefin film and a high-melting point polyolefin film separately, respectively, stretching, making them porous, and then laminating them. The battery separator of the present invention can be produced by any method.

In the following, polypropylene is used as the high melting point polyolefin film and polyethylene is used as the low melting point polyolefin film. The melt extrusion method may be a melt extrusion molding method using a T die, an inflation method, or the like. For example, when a film is melt-molded by a T-die, the temperature is generally 20 to 60 ° C. higher than the melting temperature of each resin, and the draft ratio is 10 to 1000.
It is preferably performed at a draft ratio of 100 to 500, and the take-up speed is not particularly limited, but usually 10 to 100 m / min. The melt extruded film is heat treated to enhance its crystallinity and its orientation.

The heat-treated polypropylene film is
The birefringence is 15 × 10 ^{−3 to} 25 × 10 ⁻³ , preferably 17 × 10 ^{−3 to} 22 × 10 ⁻³ , and the elastic recovery rate at 100% elongation is 80 to 94%, preferably 84 to 92. It is preferably in the range of%. The polyethylene film is
Its birefringence is 30 × 10 ^{−3 to} 48 × 10 ⁻³ , preferably 35 × 10 ^{−3 to} 45 × 10 ⁻³ , and the elastic recovery rate at 50% elongation is 50 to 80%, preferably 60 to 75. It is preferably in the range of%. When the birefringence is out of these ranges, it is not suitable because the porosity cannot be sufficiently obtained even by stretching, and even when the elastic recovery ratio is out of the above range, the degree of porosity becomes insufficient, and lamination after stretching The quality of the porous film tends to vary due to the influence on the pore size and pore size distribution, the porosity, the delamination strength, the mechanical strength and the like.

In the present invention, birefringence is a value measured by using a polarizing microscope and a Berek compensator under crossed Nicols. The elastic recovery rate is calculated by the following equations (1) and (2). Formula (1) is for a polypropylene film, and formula (2) is for a polyethylene film. The polypropylene film has a temperature of 25 ° C,
Sample width 10 mm and length 50 mm at 65% relative humidity
And set it in the tensile tester at a speed of 50 mm / min for 10
After elongation to 0%, immediately after relaxing at the same speed, the polyethylene film was measured at 25 ° C and 65 ° C.
The sample was set in a tensile tester with a sample width of 15 mm and a length of 2 inches at 50% relative humidity, expanded to 50% at a speed of 2 inches / min, held in an expanded state for 1 minute, and then relaxed at the same speed. did.

[0034]

[Equation 1]

[0035]

[Equation 2]

The heat-treated film is laminated as it is or by thermocompression bonding and then stretched to make it porous. For example, in the case of stretching after being laminated, it is preferable that the stretching is performed at a low temperature and then at a high temperature. The low temperature drawing is usually carried out by the peripheral speed difference of the drawing rolls. Cold drawing temperature is -20
C. to plus 50.degree. C., particularly preferably 20 to 35.degree. If the stretching temperature is too low, the film is likely to break during the work, and if it is too high, the porosity is insufficient, which is not preferable. The draw ratio at low temperature is 5 to 200%, preferably 10
Is in the range of 100%. If the stretching ratio is too low, only the desired porosity is small, and if it is too high, the desired porosity and pore diameter cannot be obtained, so the above range is suitable. In the present invention, the low temperature draw ratio (E ₁ ) is according to the following formula (3). In the formula (3), L ₁ means the film size after the low temperature drawing, and L ₀ means the film size before the low temperature drawing.

(Formula 3) E ₁ = [(L ₁ −L ₀ ) / L ₀ ] × 100

The cold-stretched film is then hot-stretched. The high temperature drawing is usually carried out in a heated air circulating oven at a peripheral speed difference of a drawing roll. Hot drawing temperature is 7
0 to 130 ° C, particularly 100 to 128 ° C is preferable.
If it is out of this range, it is not suitable because it is not sufficiently porous. Further, the high temperature stretching is 40 to 100 ° from the temperature of the low temperature stretching.
It is preferable to carry out at a high temperature. The draw ratio for hot stretching is 1
The range is from 00 to 400%. If the draw ratio is too low,
The above range is preferable because the gas permeability is low, and if it is too high, the gas permeability becomes too high. In the present invention, the high temperature draw ratio (E ₂ ) is in accordance with the following formula (4). In the formula (4), L ₂ means the film size after high temperature drawing, and L ₁ means the film size after low temperature drawing.

(Formula 4) E ₂ = [(L ₂ −L ₁ ) / L ₁ ] × 100

In the present invention, after low-temperature stretching and high-temperature stretching, heat setting is performed at the temperature of high-temperature stretching. In the heat setting, the length of the film after stretching is 10 to 50% in advance in order to prevent the film from shrinking in the stretching direction due to residual stress applied during stretching.
It is carried out by a method of heat shrinking to such an extent that it decreases, or a method of heating by controlling so that the dimension in the stretching direction does not change. In order to set the longitudinal shrinkage of the laminated porous film in the temperature range of non-porous temperature to non-porous temperature + 50 ° C. to 5 to 70%, the film length after stretching is 10 to 50%.
It is preferable to heat-set while heat-shrinking to the extent that it decreases. If the rate of decreasing the length of the film after stretching is increased, the shrinkage rate in the longitudinal direction of the laminated porous film produced becomes smaller, and if the rate of decreasing the length of the film after stretching is decreased, the above shrinkage rate becomes smaller. Grows. By this heat setting, a separator having good dimensional stability and satisfying the intended problem can be obtained.

The laminated porous film of the battery separator produced in this manner has a porosity of 30 to 80%, preferably 35 to 60%, and a maximum pore diameter, although it varies somewhat depending on the selection of the production conditions. 0.02 to 2 μm, preferably 0.05 to 0.5 μm, the non-porous temperature is 120 to 140
° C, the non-porous maintenance upper limit temperature is 180 to 220 ° C. When the porosity is too low, the function when used as a battery separator is insufficient, and when it is too high, the mechanical strength becomes poor. If the maximum pore size is too small, the mobility of ions will deteriorate when used as a battery separator, and if the maximum pore size is too large, the ion migration will be too large, which is not suitable.

The gas permeation rate (Gurley value) of the battery separator made of a laminated porous film is 100 to 1500.
sec / 100cc, preferably 200-1000se
It is c / 100cc. When used as a battery separator, if the gas permeation rate is too slow, the flow of ions will be suppressed, and if it is too fast, the flow of ions will be too fast and the temperature rise upon failure will be increased, which is not suitable. The thickness of the battery separator film as a whole is 15 to 50 μm in view of mechanical strength, performance, and miniaturization, and further 20 to
40 μm is suitable.

The battery separator of the present invention is characterized in that the low melting point polyolefin film constituting the laminated porous film has a thickness of 20 to 80% of the total thickness of the laminated porous film. A method for obtaining the target thickness ratio can be easily achieved by adjusting the thickness of each layer to a predetermined ratio during melt extrusion molding.
When manufacturing a laminated film by coextrusion, by adjusting each width of the multi-layer die, and when each film is melt-extruded separately and then laminated, the thickness of each film before laminating is adjusted to the above ratio. By adjusting the above, it is possible to obtain a laminated film having desired layer thickness ratios. Also, in the case where each layer film is separately stretched and porous and then laminated, each porous film having a layer thickness ratio within a predetermined range may be laminated in the same manner.

[0044]

EXAMPLES Next, the laminated porous film of the battery separator of the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

Example 1 A T-die having a discharge width of 1000 mm and a discharge lip opening of 2 mm was used, and the number average molecular weight was 59000 and the weight average molecular weight was 42.
0000, melt index 3.9, melting point 166 ° C. polypropylene (F10 manufactured by Grand Polymer Co., Ltd.)
4) was melt extruded at 200 ° C. Discharge film is 90 ℃
After being guided to a cooling roll of No. 2 and cooled by being blown with cool air of 25 ° C., it was taken out at 35 m / min. This unstretched polypropylene film was heat-treated at 120 ° C. in a state where it was wound around a 3 inch paper tube for 3500 m.
Hot air circulating type oven (PS-made by Tabai Seisakusho)
(Type 222) and allowed to stand for 24 hours, then removed from the oven and cooled to room temperature. The birefringence of the heat-treated unstretched polypropylene film is 20.5 × 10 ^-3 (16.1 × 10 ^-3 before heat treatment), and the elastic recovery rate at 100% elongation is 90.5% (before heat treatment 77.7%). )Met.

Discharge width 1000 mm, discharge lip opening 2 m
1 high density polyethylene (Mitsui Petrochemical Co., Ltd., HIZEX 5202B) with a density of 0.964, a melt index of 0.33 and a melting point of 132 ° C.
Melt extruded at 73 ° C. The discharge film was guided to a cooling roll at 115 ° C., cooled by blowing cold air at 25 ° C., and then taken off at 20 m / min. This unstretched polyethylene film has 3 parts in order to heat-treat it.
Hot air circulation type oven (Tabay Manufacturing PS-222)
It was put in a mold and left for 24 hours, then taken out of the oven and cooled to room temperature. The birefringence of the heat-treated unstretched polyethylene film is 39.6 × 10 ⁻³ (3
6.9 × 10 ⁻³ ), elastic recovery rate at 50% elongation is 7
It was 4.8% (43.0% before heat treatment).

Then, a three-layer laminated film having a sandwich structure in which both outer layers were polypropylene and the inner layer was polyethylene was produced as follows. The heat-treated unstretched polypropylene film and unstretched polyethylene film were respectively unwound from three sets of original roll stands at an unwinding speed of 6.5 m / min, and introduced into a heating roll at a temperature of 134 ° C. and a linear pressure of 1.5 kg. It was thermocompression-bonded at a rate of / cm, and then it was guided to a cooling roll at 50 ° C. at the same speed and wound up. The speed at this time was 4.0 m / min, and the unwinding tension was 3 kg for polypropylene film and 0.
It was 9 kg.

The three-layer laminated film was stretched at a low temperature of 25% between nip rolls maintained at 35 ° C. At this time, the distance between the rolls is 350 mm, and the roll speed on the supply side is 2.
It was 8 m / min. Subsequently, the film was introduced into a hot air circulation oven heated to 124 ° C., drawn at a high temperature until the total drawing amount between rollers reached 180% by utilizing the difference in roll peripheral speed, and then drawn by a roll heated to 124 ° C. 17% of the length of the film was relaxed and heat-set for 25 seconds to continuously obtain a laminated porous film.

Each layer thickness and polyethylene film layer thickness ratio of the obtained laminated porous film, porosity, maximum pore diameter, gas permeation rate (Gurley value), lateral (width) direction shrinkage, thickness direction shrinkage Tables 1 and 2 show the measurement results of the longitudinal and transverse tensile strengths. No pinholes or curls were found in the laminated porous film. Among the constituent films of the obtained laminated porous film, the elastic modulus of the polyethylene film layer, which is a low melting point polyolefin, is
The unstretched polyethylene film was 6.7 × 10 ⁸ dyne / cm ² in the non-porous temperature range of 135 ° C., and the stretched porous polyethylene film was 1.
It was 9 × 10 ⁸ dyne / cm ² . The porosity,
The maximum pore size is measured with a mercury porosimeter (made by Yuasa Ionic Co.), and the gas permeation rate (Gurley value) is JIS.
It measured according to P8117.

The lateral shrinkage and the thickness shrinkage are 25 m.
A strip-shaped sample film of m (horizontal direction) x 100 mm (longitudinal direction) is left for 15 minutes in a hot air circulation oven whose temperature is regulated at a predetermined temperature with both ends of the longitudinal direction (stretching direction) restrained, and then cooled to room temperature. Then, the size of the sample before the oven treatment was measured and obtained. The longitudinal shrinkage was determined by measuring in the same manner as above in the unrestrained state. Further, the tensile strength (kg / cm ² ) in the machine direction and the transverse direction was measured. The results of each evaluation are shown in Tables 1 and 2.

Porosity and Maximum Pore Diameter Porosity and maximum pore diameter were determined from the maximum values of the pore distribution curve measured with a mercury porosimeter (manufactured by Yuasa Ionic Co., Ltd.). Specifically, a sample piece having an MD of 30 mm and a sample piece of TD of 300 mm was sampled, placed in a cell, and the porosity and the maximum pore size were determined from the amount of mercury and the pressure with respect to the pore size. Gurley value Measured according to JIS P8117. A B-type Gurley Densometer (manufactured by Toyo Seiki Co., Ltd.) was used as a measuring device. The sample piece is clamped in a circular hole having a diameter of 28.6 mm and an area of 645 mm ² . With the inner cylinder weight of 567 g, the air in the cylinder is passed outside the cylinder from the test circular hole. The air permeability (Gurley value) was measured by measuring the time taken for 100 cc of air to pass. Tensile strength was measured according to ASTM D-822.

Examples 2 to 4 Laminated porous films were obtained in the same manner as in Example 1 except that the length of the film after high temperature stretching was relaxed by 36% and the composition ratio of each film layer was changed. The measurement results of the film are shown in Tables 1 and 2.

Example 5 Example 1 except that a sandwich structure in which both outer layers are polyethylene and the inner layer is polypropylene is formed, and the film is stretched at a high temperature until the total stretching amount reaches 250%, and then the stretched film length is relaxed by 20%. A laminated porous film was obtained in the same manner as.
The measurement results of the film are shown in Tables 1 and 2.

Comparative Example 1 A T-die having a discharge width of 1000 mm and a discharge lip opening of 2 mm was used, and the density was 0.964 and the melt index was 0.3.
3. High-density polyethylene (HIZEX 5202B, manufactured by Mitsui Petrochemical Co., Ltd.) having a melting point of 132 ° C. was melt-extruded at 163 ° C. The discharge film is guided to a cooling roll at 125 ° C., cooled by blowing cool air at 25 ° C.,
It was collected at 10 m / min.

In order to heat this unstretched polyethylene film, it was placed in a hot air circulation type oven (PS-222 type manufactured by Tabai Seisakusho) which was kept at 125 ° C. in a state where it was wound 3500 m around a 3-inch paper tube. And heat-treated for 150 seconds under 10% tension. The heat treated film was then cold stretched 50% between nip rolls held at 35 ° C. At this time, the distance between the rolls was 350 mm, and the roll speed on the supply side was 1.2 m / min. 80 continuously
It was introduced into a hot air circulation oven heated to ℃, and after being stretched at a high temperature to a total stretching amount of 200% between the rollers while keeping a roll peripheral speed difference, it was relaxed by 25% by a roll heated to 108 ° C. After heat setting for 28 seconds, a polyethylene single layer porous film was continuously obtained. The measurement results of the film are shown in Tables 1 and 2.

[0056]

[Table 1]

[0057]

[Table 2]

[0058]

EFFECTS OF THE INVENTION According to the present invention, the film strength is high in the non-porous temperature range, the shape retention property upon melting is excellent, and the complete non-porous temperature range is achieved. It is possible to provide a laminated porous film that can be substantially shrunk in the non-porous temperature range when used as a battery separator, and can be surely made non-porous when an abnormality occurs.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI H01M 2/16 H01M 2/16 P (56) Reference JP-A-4-181651 (JP, A) JP-A-8-244152 ( JP, A) JP 7-304110 (JP, A) JP 7-307146 (JP, A) JP 9-219184 (JP, A) JP 9-117959 (JP, A) JP JP 9-171808 (JP, A) JP 10-100344 (JP, A) JP 11-115084 (JP, A) JP 10-50286 (JP, A) JP 10-316781 (JP , A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B32B 1/00-35/00 H01G 9/02 H01M 2/16

Claims (9)

(57) [Claims] 1. A melting point of 20 which is made porous by a stretching method.
℃ In the laminated porous film consisting of more different refractory polyolefin film and a low melting polyolefin film, wherein said high melting point polyolefin film low melting
Dot polyolefin film is melt extruded separately
Heat treatment, then laminated, and made porous by the stretching method.
The high melting point polyolefin film after heat treatment, which has a birefringence of 15 × 10 ⁻³ to 2
5 × 10 ⁻³ and elastic recovery rate at 100% elongation is 8
Is 0 to 94%, low melting polyolefin film after the heat treatment, birefringence 30 × ¹⁰ -3 ~48 × ^10
-3 , and the elastic recovery rate at 50% elongation is 50-80%
, And the and a 20-80% thickness of the thickness of the entire laminated porous film of the low melting polyolefin film in the laminated porous film, elastic modulus at no Anaka temperature of the low melting polyolefin film is 10 ⁴ dyne / cm
² or more, and the laminated porous film has a shrinkage rate of 5 to 70% in the longitudinal direction (stretching direction) in the unconstrained state in the temperature range of non-porous temperature to non-porous temperature + 50 ° C. or less of the multilayer porous film. Film. 2. The laminated porous film according to claim 1, which has been stretched by 100 to 400% and then contracted and heat-set for 10 to 50% of the stretched film length. 3. The shrinkage ratio in the thickness direction of the laminated porous film in the temperature range from non-porous temperature to non-porous temperature + 50 ° C. or less in the longitudinal direction both ends constrained state is 20 to 80%. The laminated porous film described. 4. The laminated porous film according to claim 1, wherein the high-melting-point polyolefin film is a polypropylene film. 5. The laminated porous film according to claim 1, wherein the low melting point polyolefin film is a polyethylene film. 6. A melting point of 20 which is made porous by a stretching method.
High melting point polyolefin film and low melting point
Laminated porous film consisting of Liolefin film
In the production method, a high melting point polyolefin film having a birefringence of 15 × 10 ^{−3 to} 25 × 10 ⁻³ , obtained by heat treatment after melt extrusion, and an elastic recovery rate at 80% elongation of 80 to 94%. , The birefringence obtained by heat treatment after melt extrusion is 30 × 10 ^{−3 to} 48 × 1.
0 ^-3, and 50% elastic recovery when stretched 50 to 80
% Of the low melting point polyolefin film and then 5 to 200 at a temperature of minus 20 ° C. to plus 50 ° C.
% Cold stretched, then 100 at a temperature of 70-130 ° C.
˜400% high temperature drawing, the low melting point polyolefin film has a thickness of 20 to 80 of the total thickness of the laminated porous film.
%, And the elastic modulus of the low melting point polyolefin film at the non-porous temperature is 10 ⁴ dyne / cm ² or more,
Non-porous temperature of laminated porous film to non-porous temperature + 50 ° C
A process for producing a laminated porous film, which comprises obtaining a laminated porous film having a longitudinal shrinkage (stretching direction) of 5 to 70% in an unrestrained state in the following temperature range. 7. The method for producing a laminated porous film according to claim 6, wherein 10 to 50% of the length of the stretched film is contracted by heat setting after stretching at a high temperature. 8. The method for producing a laminated porous film according to claim 6, wherein the high melting point polyolefin film is a polypropylene film. 9. The method for producing a laminated porous film according to claim 6, wherein the low melting point polyolefin film is a polyethylene film.