JP2001011223A - Preparation of porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preparing a porous film having a high strength and an excellently uniform thickness while having a void content suitable as a separator for various batteries, particularly for a battery for an electric car. SOLUTION: There is provided a method for preparing a porous film comprising a step of melt-kneading a resin composition containing a polyolefin and a solvent, a step of extruding the resultant melted and kneaded product into a sheet, a step of forming it into a sheet by passing through a cooled sizing dice and a step of stretching and solvent removal, wherein a desired thickness of a sheet is attained by adjusting the width of a gap of the sizing dice.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a porous film. More specifically, the present invention relates to a method for producing a porous film which is preferably used as a battery separator or the like which is arranged between a positive electrode and a negative electrode of a battery to isolate them.

[0002]

2. Description of the Related Art Various types of batteries have been put to practical use, but in recent years, in order to cope with the cordless use of electronic equipment, etc., batteries are lightweight, have high electromotive force and high energy, and are self-contained. Lithium batteries with low discharge are attracting attention. For example, a large number of cylindrical lithium ion secondary batteries have been produced for use in mobile phones and notebook computers. It is also expected to be used as a battery for electric vehicles in the future.

In these batteries, a separator is interposed to prevent a short circuit between the positive electrode and the negative electrode. As such a separator, a porous film having a large number of micropores formed in order to secure ion permeability between the positive electrode and the negative electrode. used. The porous film for a separator is required to have many required characteristics related to battery characteristics, and particularly to be high in strength and high in porosity. The higher strength contributes to the improvement of the workability of the battery and the reduction of the internal short-circuit failure rate, and the capacity can be expected to be increased by making the separator thinner. High porosity improves ion permeability,
The charge / discharge characteristics, particularly at high rates, are advantageous.

Conventionally, a porous film has been produced by preparing a gel-like sheet from a solution obtained by heating and dissolving an ultra-high-molecular-weight polyolefin or an ultra-high-molecular-weight polyolefin and another polyolefin resin in a solvent, stretching the film, and removing the residual solvent. Various methods have been proposed.

A method for producing such a porous film for a separator is described in, for example, JP-A-9-87413.
In the publication, the ultra-high molecular weight polyolefin resin is dissolved by heating in an appropriate solvent, and then formed into a gel-like sheet, stretched, and then subjected to a desolvent treatment to remain in the sheet. A method for producing a porous film by removing a solvent is disclosed. However,
According to this manufacturing method, since the cooling and crystallization of the kneaded sheet is performed by a conventional cooling roll, there is a disadvantage that the thickness of the raw material cannot be increased due to the balance of the cooling rate. If the raw fabric thickness is small, not only the film thickness after the stretching and desolvation treatment becomes thin, but also the draw ratio cannot be increased, so that the film strength cannot be increased.

On the other hand, Japanese Patent Publication No. 5-75011 discloses a method for producing a thick material by extruding a solution of ultrahigh molecular weight polyethylene from a die cooled to 90 ° C. or less. However, in this method, the shape of the die is complicated and the control becomes difficult, and the workability is poor. In addition, the cooling of the gel-like sheet cannot be sufficiently performed, so that the average pore size becomes large and the film strength cannot be increased. was there.

Therefore, in order to use a porous film obtained by using an ultrahigh molecular weight polyolefin resin practically as a separator of a battery for an electric vehicle, the porous film has a higher strength and a higher porosity. There is a strong demand for excellent electrolyte retention properties, and further excellent ion permeability and high-speed charge / discharge characteristics.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a porous film which has a high porosity while having an appropriate porosity as a separator for various batteries, particularly for an electric vehicle, and which is excellent in uniformity of thickness. It is to provide a manufacturing method of.

[0009]

The gist of the present invention is to melt-knead a resin composition containing a polyolefin and a solvent,
After extruding the obtained melt-kneaded material into a sheet, formed into a sheet through a cooled sizing die, a method for producing a porous film having a step of performing stretching and desolvation treatment, wherein the sizing die The present invention relates to a method for producing a porous film, wherein a desired sheet thickness is obtained by adjusting a gap thickness.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, a resin composition containing a polyolefin and a solvent is melt-kneaded, and the obtained melt-kneaded material is extruded into a sheet and then formed into a sheet through a cooled sizing die. This is a method for producing a porous film having a step of performing stretching and desolvation treatment. By adjusting the gap thickness of the sizing die, a porous film having a desired sheet thickness can be obtained.

The polyolefin used in the present invention includes ethylene, propylene, 1-butene, 4-methyl-
Examples thereof include homopolymers and copolymers of olefins such as 1-pentene and 1-hexene, and blends thereof. In the present invention, in order to improve the film strength of the obtained porous film, a weight average It is desirable to contain an ultrahigh molecular weight polyolefin having a molecular weight of 1 × 10 ⁶ or more, preferably 2 × 10 ⁶ or more. The content of the ultra-high molecular weight polyolefin is preferably 1% by weight or more, more preferably 10% by weight or more, in order to obtain excellent film strength by sufficient entanglement of the polymer chains of the polyolefin.
１００100% by weight.

The solvent that can be used in the present invention is not particularly limited as long as it is excellent in solubility of polyolefin. For example, Nonan, Decane, Undecane,
Examples thereof include aliphatic or cyclic hydrocarbons such as dodecane, decalin, and liquid paraffin, mineral oil fractions having a boiling point corresponding thereto, and mixtures thereof. Among these, nonvolatile solvents such as liquid paraffin are preferable.

The mixing ratio of the polyolefin and the solvent varies depending on the type, solubility, kneading temperature and the like of the polyolefin, and thus cannot be unconditionally determined. However, the resulting slurry resin composition can be melt-kneaded and formed into a sheet. The degree is not particularly limited as long as the degree is about. For example, the polyolefin is preferably 10 to 30% by weight of the resin composition, and
More preferably, it is 5 to 25% by weight. Further, the solvent is preferably 70 to 90% by weight, more preferably 75 to 85% by weight of the resin composition. When the compounding amount of the polyolefin is 10% by weight or more, the strength of the obtained porous film can be improved. When the compounding amount of the polyolefin is 30% by weight or less, the polyolefin and the solvent are uniformly kneaded. Thus, a porous film free of thickness unevenness and characteristic unevenness can be obtained.

The above resin composition may contain, if necessary, an antioxidant, an ultraviolet absorber, a dye, a nucleating agent, a pigment,
Additives such as antistatic agents can be added in a range that does not impair the purpose of the present invention.

The melt-kneading of the resin composition is preferably performed by applying a high shearing force to the resin composition in order to obtain a sufficient entanglement of the polyolefin polymer chains. Therefore, for melt-kneading the resin composition in the present invention, usually, a kneader or a twin-screw kneader capable of giving a strong shearing force to the mixture is preferably used.

The temperature at which the resin composition is melt-kneaded may be any suitable temperature condition, and is not particularly limited.
15-185 ° C is preferred. The temperature at the time of melt-kneading is preferably 115 ° C. or higher, in order to sufficiently knead the resin composition and obtain sufficient entanglement of the polymer chain of the polyolefin, with an appropriate viscosity, and sufficient shearing force for the resin composition. Is preferably 185 ° C. or lower.

Next, the obtained melt-kneaded material is subjected to, for example, T
A sheet extruded into a sheet using a die is obtained. Then, this is formed into a sheet shape through a cooled sizing die. The thickness when the melt-kneaded product is extruded into a sheet is not particularly limited, but is usually preferably about 5 to 20 mm.

In the present invention, the thickness of the sheet-like molded product is controlled by adjusting the gap thickness of the sizing die so as to operate the sheet-like molded product at a take-up speed adapted to each thickness, regardless of the thickness of the extruded sheet. Can be easily adjusted. Therefore, a gel-like molded article having a desired thickness can be prepared without changing the extrusion conditions (that is, without changing the thickness of the extruded sheet). It is an excellent method.

The temperature of the sizing die is desirably -15 ° C or less, preferably -20 ° C or less, in order to form the extruded sheet into a sheet while stabilizing the movement of the solvent.

The method of cooling the sizing die is not particularly limited. For example, there is a method in which a pipe is provided in the sizing die and an antifreeze liquid diluted with water is circulated in the pipe at a constant circulation amount. In order to further improve the cooling capacity, a method may be used in which a cooling tank is provided in the sizing die, and the antifreeze diluted with the water is circulated in the cooling tank. Whichever method is employed, it is preferable to minimize the temperature difference between the inlet and outlet of the cooling water as much as possible to eliminate temperature irregularities and temperature gradients and to increase the cooling efficiency.

[0021] The thickness of the sheet-like molded product thus obtained is usually preferably 0.5 to 20 mm.

In the present invention, not only the surface layer of the sheet-like molded product obtained, but also the polyolefin is finely crystallized up to the center, and a porous film having fine and uniform fibrils and having a large curvature is formed. In order to obtain a porous film having a structure, the melt-kneaded product and the extruded sheet are preferably rapidly cooled and formed into a sheet. This is a solution state, that is, when the cooling rate at the time of forming the sheet from the melt-kneaded material through the extruded sheet is slow, it is stretched by melt-kneading, the entangled fibrils return to the shape of a hair, and become thick. This is to form fibrils.
However, since the gel-like sheet-shaped molded product generally does not have high thermal conductivity, the portion closer to the center than the surface layer is less likely to be cooled. In particular, this tendency is remarkable in a sheet-like molded product having a thickness of 5 mm or more, and the surface layer is cooled down to a temperature close to the cooling medium in several tens of seconds. If the temperature is not so low, the polyolefin cannot be finely crystallized by rapidly cooling to the center. Therefore, when a sheet-like molded product having a thickness of about 0.5 to 20 mm is obtained, when cooled by a commonly used cooling roll, uneven cooling occurs in the sheet-like molded product, and the degree of crystallinity, surface shape and the like are uniform. They tend to lack sex. However, as in the present invention, by using a cooled sizing die, due to the effect of heat conduction by metal, it is possible to suppress uneven cooling of the melt-kneaded material, and to pass through a space with high precision at a predetermined pressure. In addition, the cooling efficiency and the shape stability of the obtained sheet-like molded product can be significantly improved. As described above, in the present invention, even when a thick sheet-like molded product is obtained, it can be efficiently cooled, so that it not only has a porous structure having a dense and uniform pore structure, but also will be described below. By increasing the total stretching ratio, a porous film having higher strength can be prepared.

That is, generally, when a crystalline polymer is crystallized, a lamella crystal is formed. The thickness of the lamella crystal largely depends on the crystallization temperature. The larger the difference between the melting point and the crystallization temperature, the larger the lamella crystal. The thickness of the lamella crystal becomes smaller. Therefore, the greater the difference between the melt-kneading temperature and the cooling temperature when forming into a sheet, the smaller the thickness of the lamella crystal can be. Is divided into microcrystals, and it is considered that a porous film having a microfibril structure having a small fiber diameter and uniform fibril fibers and having a very large curvature can be obtained.

The sheet-like molded product thus obtained is stretched by melt-kneading, and the entangled fibril fibers return to a spherical shape to form thick fibers, and a large through-hole is formed in the sheet-like molded product. In order to prevent the formation of a polyolefin, it is preferable to immediately carry out the stretching treatment described later or store the polyolefin at a temperature lower than the freezing point of the solvent used to maintain the polyolefin crystal structure.

Next, the sheet-like molded product is stretched and desolvated. The stretching method is not particularly limited, and may be an ordinary tenter method, a roll method, an inflation method, or a combination of these methods.
In addition, any method such as uniaxial stretching and biaxial stretching can be applied. In the case of biaxial stretching, either vertical or horizontal simultaneous stretching or sequential stretching may be used, but vertical and horizontal simultaneous stretching is preferred.

The temperature during the stretching treatment is preferably not lower than the crystal dispersion temperature of the polyolefin and not higher than the melting point of the polyolefin + 5 ° C. For example, when ultrahigh molecular weight polyethylene is used as the polyolefin, the temperature is preferably 100 to 125 ° C, more preferably 110 to 120 ° C. As other stretching treatment conditions, commonly used known conditions can be adopted.

Further, in the present invention, before and after the stretching treatment,
Rolling of the sheet-like molded product may be performed by press rolling, roll rolling, or the like.

In the present invention, the total stretching ratio of the obtained porous film is adjusted to 50 to 400, preferably 100 to 300 by appropriately adjusting the rolling ratio and the stretching ratio in the rolling and stretching. desirable. The total stretching ratio of the porous film is preferably 50 or more in order to form a high-strength microporous film by forming a polyolefin with a high microfibril structure. Is preferably 400 or less.

In the present specification, the rolling ratio and the stretching ratio are the thickness of the sheet-like molded product before rolling (the thickness of the sheet-shaped molded product before rolling / the thickness of the sheet-shaped molded product before rolling / thickness, respectively). The ratio of the thickness of the sheet-like molded article before stretching to the thickness of the sheet-like molded article after stretching) (the thickness of the sheet-like molded article before stretching / the sheet-like molded article after stretching). ), And the total stretching ratio is a value obtained by integrating the rolling ratio and the stretching ratio.

The ratio of the stretching ratio to the rolling ratio (stretching ratio / rolling ratio) is preferably 1 or less, more preferably 0.1 to 1, from the viewpoint of the uniformity of the thickness of the sheet-like molded product at the time of stretching.

The desolvation treatment is a step of forming a porous structure by removing a solvent from a sheet-like molded product.
It can be carried out by washing the sheet-like molded product with a solvent to remove the residual solvent. The solvent can be appropriately selected according to the solvent used in the preparation of the resin composition.Specifically, hydrocarbons such as pentane, hexane, heptane, and decane, and halogenated compounds such as methylene chloride and carbon tetrachloride are used. Examples include volatile solvents such as hydrocarbons, ethers such as diethyl ether and dioxane, and these can be used alone or in combination of two or more. The method of desolvation treatment using such a solvent is not particularly limited, and examples thereof include a method of immersing a sheet-like molded product in a solvent to extract the solvent, a method of showering the solvent to the sheet-like molded product, and the like. .

In the present invention, the solvent removal treatment may be appropriately performed before and after stretching. For example, the sheet-like molded article may be subjected to a desolvation treatment and then subjected to a stretching treatment, or the sheet-like molded article may be subjected to a stretching treatment and then a desolvation treatment. Alternatively, the solvent may be removed before the stretching, and the solvent may be removed again after the stretching to remove the residual solvent.

In the present invention, the porous film thus obtained may be subjected to a heat setting treatment or the like for reducing the heat shrinkage of the film, if necessary, to fix the shape. The temperature at the time of the heat setting treatment is preferably a temperature not lower than the temperature of the polyolefin crystal dispersion temperature −10 ° C. and not higher than the melting point. Known methods such as a method of bringing the porous film into contact with a heating roll and a method of leaving it in a dryer can be used without particular limitation as a method of the heat setting treatment. From the viewpoint of prevention, it is preferable to fix the entire periphery of the porous film.

The thickness of the porous film thus obtained is preferably 1 to 60 μm, more preferably 15 to 60 μm.
50 μm, porosity 30-60%, piercing strength 50
0 gf / 25 μm or more, air permeability 500 sec / 100 cc
It is desirable that:

The porous film obtained according to the present invention comprises:
Although the air permeability is good, the specific surface area is high, the fine fibrils are densely arranged, and the average pore size is small, so that short-circuiting due to dendrites in the overcharge test hardly occurs.
Therefore, it can be suitably used as a high-performance separator having excellent stability and durability in various batteries, particularly batteries for electric vehicles.

[0036]

The present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples. In addition, various characteristics are measured in the following manner.

(Weight average molecular weight) A gel permeation chromatograph "GPC-150C" manufactured by Waters Co., Ltd. was used, o-dichlorobenzene was used as a solvent, and "Shodex-80M" manufactured by Showa Denko KK was used as a column. 13
Measure at 5 ° C. Data processing is performed using a data processing system manufactured by TRC. The molecular weight is calculated based on polystyrene.

(Melting point) Using a differential scanning calorimeter “DSC-200” manufactured by Seiko Denshi Kogyo Co., Ltd.
Measure at a rate of ° C./min, and let the onset temperature be the melting point.

(Film thickness) It is measured from a 1 / 10,000 mm thickness gauge and a 10,000 × scanning electron micrograph of the cross section of the porous film.

(Puncture Strength) The maximum load is measured from a load displacement curve obtained by measurement using a handy compression tester “KES-G5” manufactured by Kato Tech Co., Ltd., and is defined as the puncture strength. The needle is 0.75m in diameter
m, a tip having a radius of curvature of 0.5 mm is used at a pushing speed of 2 mm / sec. All values are converted to 25 μm.

(Porosity) The porosity is calculated from the weight W per unit area S, the average thickness t and the density d by the following formula.

Porosity (%) = (1- (10 ⁴ × W / S /
t / d)) × 100

(Air permeability) Measured according to JIS P8117.

Example 1 Ultra high molecular weight polyethylene (weight average molecular weight: 2 × 1)
0 ^6, crystal dispersion temperature: 84 ° C., the melting point: a 136 ° C.) 15 parts by weight of kinematic viscosity at liquid paraffin (40 ° C. as a solvent: 59Cst) and 85 parts by weight were uniformly mixed into a slurry, the resulting resin composition The product is fed to a twin-screw extruder (cylinder diameter 40 mm, L / D = 42) at a processing rate of 20 kg / hr, heated to a temperature of 160 ° C., dissolved, melt-kneaded and mixed with ultra-high molecular weight polyethylene and a solvent. Was obtained. Next, using a T-die (width 100 mm, gap thickness 15 mm) attached to the tip of the twin screw extruder, 1
After extruding the melt-kneaded material at 60 ° C. into a sheet,
The resultant was passed through a sizing die (width 100 mm, gap thickness 15 mm, length 300 mm) cooled to 0 ° C., and cooled and crystallized while being formed to a thickness of the gap, to obtain a gel-like sheet-like molded product. The obtained sheet-like molded product (thickness: 15 mm) was rolled by a heat press at a temperature of 120 ° C. until the sheet thickness became 0.8 mm, and simultaneously biaxially stretched at a temperature of 120 ° C. to obtain a thickness of 73 μm. To remove the solvent. Then, the obtained porous film was heat-set at 134 ° C. for 20 minutes. The porous film thus obtained (thickness: 22 μm)
Are shown in Table 1.

Example 2 A porous film was obtained in the same manner as in Example 1 except that the gap thickness of the sizing die was changed to 10 mm. The porous film (thickness: 23.2 μm)
Table 1 shows the characteristics of m).

Example 3 A porous film was obtained in the same manner as in Example 1, except that the gap thickness of the sizing die was changed to 5 mm. The porous film thus obtained (having a thickness of 25.4 μm)
Table 1 shows the characteristics of m).

[0047]

[Table 1]

Comparative Example 1 Ultra high molecular weight polyethylene (weight average molecular weight: 2 × 1)
0 ^6, crystal dispersion temperature: 84 ° C., the melting point: a 136 ° C.) 15 parts by weight of kinematic viscosity at liquid paraffin (40 ° C. as a solvent: 59Cst) and 85 parts by weight were uniformly mixed into a slurry, the resulting resin composition The product is fed to a twin-screw extruder (cylinder diameter 40 mm, L / D = 42) at a processing rate of 20 kg / hr, heated to a temperature of 160 ° C., dissolved, melt-kneaded and mixed with ultra-high molecular weight polyethylene and a solvent. Was obtained. Next, using a T-die (width 100 mm, gap thickness 15 mm) attached to the tip of the twin screw extruder, 1
After extruding the melt-kneaded material at 60 ° C. into a sheet, a cooling roll (roll diameter 500 mm) previously cooled to −15 ° C.
mm, minimum nip 20 mm). But,
Since the cooling by the cooling roll was not sufficiently performed, the extruded sheet necked in, and a uniform gel-like sheet-like molded product could not be obtained.

From the above results, in Examples 1 to 3, by adjusting the gap thickness of the sizing die, a sheet-like molded product having a desired thickness can be obtained without changing the extrusion conditions of the melt-kneaded product. Further, it can be seen that the obtained porous film is excellent in all of the properties of piercing strength, porosity and air permeability.

[0050]

According to the present invention, there is provided a process for producing a porous film which has a high porosity while having an appropriate porosity as a separator for various batteries, particularly for a battery for an electric vehicle, and which is excellent in thickness uniformity. Became possible.

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Claims (3)

[Claims] 1. A resin composition containing a polyolefin and a solvent is melt-kneaded, and the obtained melt-kneaded product is extruded into a sheet, then formed into a sheet through a cooled sizing die, and subjected to stretching and desolvation treatment. A method for producing a porous film, comprising: adjusting a gap thickness of the sizing die to a desired sheet thickness. 2. The resin composition has a weight average molecular weight of 1 × 1.
0 ⁶ or more and 10 to 30 wt% polyolefin containing 1 wt% or more of ultra-high molecular weight polyolefin, a solvent 70
The method according to claim 1, comprising 90% by weight. 3. The method according to claim 1, wherein the sizing die is cooled to -15 ° C. or lower.