JPH11268118A - Polyolefin porous film, production thereof, and separator film for battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a film suitably used in a non-aqueous electrolyte and a separator for an electrolyte battery so as to have specific closing temp. and heat-resistant temp. and performing crosslinking so as to obtain a specific gel ratio. SOLUTION: An inflation film made of a high mol.wt. polyethylene having a thickness of 14 μm is produced. This film 31 is held between a pair of stainless steel mold frames 33 and four sides thereof are fixed between the upper and lower mold frames by using screws 32. In this state, the whole is immersed in a tank filled with heated paraffin oil. The film fixed to the mold frames is taken out of a heat treatment tank to be immersed in a tank filled with 1,3-dichloro-1,1,2,2,3-pentafluoropropane in this state. This film is taken out to be dried by air and the dried film is detached from the mold frames. The obtained film is perforated and irradiated with electron beam of 2 Mrad to perform crosslinking. This perforated film has a gel ratio of 59 and also has a closing temp. of 131 deg.C and heat-resistance temp. of 175 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin porous film excellent in obstruction, heat resistance and puncture strength, a method for producing the same, and a separator film for a battery. More specifically, the present invention relates to a leaf-like and / or mesh-like form. A polyolefin porous film having a fibril structure and having a gel fraction of 50% or more, a method for producing the same, and a battery separator as a preferred use thereof.

[0002]

2. Description of the Related Art In recent years, various measures have been taken for large capacity batteries such as lithium ion batteries from the viewpoint of ensuring safety. As one of the measures, the separator film is also required to have a function for ensuring battery safety. For example, when the temperature inside the battery rises due to an external short circuit or the like, there is a risk of explosion due to ignition or decomposition gas of the electrolytic solution. In order to prevent this, when the battery internal temperature rises and reaches a temperature near the melting point of the porous film, the porous film is made nonporous and closed, thereby increasing the resistance value inside the battery and reducing the internal temperature. There is a need for a function of preventing the above-mentioned danger by suppressing a chemical reaction inside the battery, that is, a function of shutting down the battery.

As a separator having such a function, a porous film made of polyethylene or polypropylene having a low melting point has been conventionally used. Normally, even if the pores of the porous film are closed and non-porous, the temperature inside the battery does not drop rapidly, but after a slight temperature rise, it tends to gradually decrease, so the battery has a melting point or higher. At a temperature for a long time. In this state, if a defect occurs in the nonporous separator film, a short circuit occurs again and the temperature starts to rise, leading to ignition or rupture.

[0004] Therefore, various attempts have been made to produce a separator having both a low blocking temperature and a high heat resistance temperature and also having improved other properties such as strength. As one of them, there is a method utilizing a crosslinked structure of polyolefin, for example,
JP-A-63-205048 proposes using a microporous membrane of cross-linked polyethylene as a separator. The crosslinked polyethylene separator generates heat due to short-circuit current, so that even if the battery temperature rises, the microporous membrane melts and closes the microporous membrane, and the crosslinked polyethylene separator has a viscosity during softening and melting. And the shape retention is excellent, so that there is an advantage that short-circuiting at a high temperature is unlikely to occur. However, since the separator made of the crosslinked polyethylene porous film has a high melt viscosity and poor fluidity, the pore closing speed is low even when the melting point of the polyethylene is reached or higher, and the temperature at which the current is interrupted under a temperature rising condition. Is high.

In order to solve this problem, Japanese Patent Laid-Open Publication No. Hei 3-59
947 and JP-A-3-245457 have proposed a separator film in which a crosslinked polyethylene porous film and a non-crosslinked polyethylene porous film are laminated. In these separators, when the internal temperature of the battery rises due to abnormal current, (1) the uncrosslinked polyethylene porous film first melts and flows, and its melt viscosity is low, so that the pores of the crosslinked polyethylene porous film are quickly filled. (2) The crosslinked polyethylene porous film has a high viscosity when softened and melted, has excellent shape retention and is excellent in both positive and negative polarities. An advantage is that a short circuit between them is unlikely to occur.

[0006] However, in this technique, it is necessary to laminate two or more kinds of porous films after they are prepared. However, simply laminating as this laminating method may cause troubles in battery production. Further, when laminating by slightly crushing with a nip roll or the like, there is a possibility that a part of the hole is crushed and the uniformity is partially deteriorated.

Japanese Patent Application Laid-Open No. Hei 9-216964 proposes a method in which a porous film is molded from an uncrosslinked polyolefin into which an active group has been introduced and polyethylene, followed by a crosslinking reaction. According to this method, an excellent closing temperature and heat-resistant temperature can be obtained without previously laminating a plurality of porous films, and the uniformity of the porosity is not easily impaired. Therefore, this method is a very useful method. However, the operations involved in the preparation, such as introduction of silane active groups into the raw materials, preparation of the master batch, dry blending, and crosslinking treatment after molding, are very complicated.

[0008]

SUMMARY OF THE INVENTION Accordingly, the present inventors
As a result of intensive studies to solve the above problems, the introduction of a cross-linked structure into a porous film consisting of only a polyolefin of a specific molecular weight and having a specific structure has excellent closing temperature and heat resistance temperature, and also has the strength required when incorporating the battery. Have been found that a porous film and a battery separator having the same can be obtained, and have completed the present invention.

[0009] That is, an object of the present invention is to provide a polyolefin porous film excellent in closing temperature, heat resistance temperature and strength by introducing a crosslinked structure into a specific polyolefin porous film. Further, another object of the present invention is to form a porous film having a specific structure without laminating two or more porous films like a crosslinked porous film and a non-crosslinked porous film, and thereafter,
An object of the present invention is to provide a method for producing a porous film having an excellent closing temperature, heat resistance temperature and strength by using a single porous film by introducing a crosslinked structure by a simple method.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been proposed to achieve the above object, and comprises a plasticizer and / or a plasticizer.
Or intrinsic viscosity [η] substantially containing no solvent: 4 dl /
g or more high molecular weight polyolefin impermeable film,
A porous polyolefin film characterized by introducing a crosslinked structure so as to have a gel fraction of 50% or more after a heat treatment and, if necessary, a stretching treatment to make it porous, a method for producing the same, and a preferred use thereof. The present invention also provides a battery separator.

That is, according to the present invention, the gel fraction is 5
A polyolefin porous film is provided which is crosslinked so as to be 0% or more and has the following properties. (1) The blocking temperature is 140 ° C. or less; (2) The heat-resistant temperature is 170 ° C. or more;

Further, according to the present invention, the gel fraction is 7
Provided is a polyolefin porous film that is greater than 0%.

According to the present invention, the porous film has fibrils having a vein shape and / or a network shape as main components, and the fibrils are composed of stretched chain crystals and plate-like crystals. The above polyolefin porous film, which is a fibril composed of a helical crystal, is provided.

Further, according to the present invention, there is provided the above-mentioned polyolefin porous film having the following characteristics. (1) film thickness of 100 μ or less; (2) porosity of 20 to 80%; (3) air permeability of 50 to 2000 seconds / 100 ml; (4) shrinkage of 10% or less; Is 250 g or more;

Further, according to the present invention, there is provided the above polyolefin porous film, wherein the polyolefin is polyethylene.

Further, according to the present invention, an intrinsic viscosity [η] substantially not containing a plasticizer and / or a solvent is 4 dl / g.
A method for producing a porous polyolefin film, comprising subjecting the high-molecular-weight polyolefin impervious film to microporous treatment by heat treatment, followed by electron beam crosslinking so that the gel fraction becomes 50% or more. You.

Further, according to the present invention, there is provided the method for producing a polyolefin porous film, wherein the high molecular weight polyolefin impermeable film is a film obtained by inflation molding.

Further, according to the present invention, there is provided a battery separator film obtained from the above polyolefin porous film.

Further, according to the present invention, there is provided a battery separator film wherein the separator film has the following characteristics. (1) film thickness of 100 μ or less; (2) porosity of 20 to 80%; (3) air permeability of 50 to 2000 seconds / 100 ml; (4) shrinkage of 10% or less; Is 250 g or more; (6) the blocking temperature is 140 ° C. or less; (7) the heat-resistant temperature is 170 ° C. or more;

In addition, according to the present invention, the maximum pore diameter is set to 0.1.
There is provided a porous film as described above, which is greater than 1 micron.

Further, according to the present invention, the maximum pore diameter is set to 0.1.
The battery separator film having a thickness exceeding 1 μm is provided.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, regarding the polyolefin porous film of the present invention and the method for producing the same, the raw materials, the method for forming the film before the porous treatment, the method for the porous treatment, the crosslinking treatment, and the characteristics of the obtained film will be described.

<Raw Materials> The polyolefin used in the present invention includes ethylene, propylene and α having 4 to 8 carbon atoms.
-Refers to those obtained by polymerizing olefins alone or in combination of two or more, for example, by slurry polymerization using a Ziegler catalyst. Preferred copolymers are
Ethylene and small amount of propylene or carbon number 4-8
Or a copolymer of a combination of two or more α-olefins. In the case of an ethylene copolymer, the amount of the comonomer is preferably 5 mol% or less. Particularly preferred among these are ethylene homopolymers.

The molecular weight of the raw material polyolefin is not particularly limited as long as it does not hinder film formation, but the intrinsic viscosity [η] is preferably 4 dl / g or more, more preferably 4 to 25 dl / g. In particular, for the purpose of obtaining a high-strength porous film, the intrinsic viscosity [η] is 5 to 20 dl.
/ G, preferably 8 to 20 dl / g. If the intrinsic viscosity [η] exceeds 25 dl / g,
As described below, when the pre-processed film is formed, the melt viscosity tends to be too high and the blown film moldability tends to be poor.

<Film before Porous Treatment> The film before porous treatment made of a polyolefin obtained by a film forming method such as an inflation film forming method or a T-die film forming method substantially consists of a polyolefin. Substantially consisting of polyolefin means that the raw material polyolefin does not contain a large amount of solvent or plasticizer at the time of film formation. Therefore, various additives used in addition to the usual polyolefins such as heat stabilizers, weather stabilizers, lubricants, anti-blocking agents, slip agents, pigments, dyes, etc. are blended within a range that does not impair the object of the present invention. However, the upper limit is preferably 10% or less, more preferably 5% or less in total amount.

It is preferable that the pre-porous film is plane-oriented. In the case of the T-die film forming method, since the film formed by melt stretching is uniaxially oriented, the film must be plane-oriented by post-processing after forming. The film can be plane-oriented at the time of film formation by selecting the above.

Among polyolefins, intrinsic viscosity [η]
If less than 5 dl / g, it can be formed by an ordinary blown film forming method. For details of the method of forming blown film, see “Polyethylene extrusion molding and its application” [Keiji Sawada: Published by Seibundo Shinkosha (1966)]. General methods as described above.

A preferred condition for forming the film before porous treatment of the present invention into an inflation film is to make the draft ratio and the expansion ratio large. The draft ratio is the ratio of the take-off speed of the cooled and solidified tube film to the outflow speed (linear speed) of the film resin at the lip exit of the blown film die. The ratio of the diameter of the solidified tube film. In the case of the usual blown film forming method, the draft ratio is appropriately adjusted in a range of 2 or more, but a preferable range is 3 or more, and the expansion ratio is appropriately adjusted in a range of 1.1 to 10.

5 dl / g or more in intrinsic viscosity [η], 25 d
In the case of a high molecular weight polyolefin of 1 / g or less, a film before porous treatment can be obtained as follows. The polyolefin is melted by a screw extruder, and then the mandrel is extruded from a tube die with at least L / D of 5 which rotates with the rotation of the screw or independently, and is taken up at a predetermined draft ratio while the tube is in a molten state. A gas is blown into the interior of the film, expanded in the range of 1.1 to 20 in the expansion ratio, cooled, and formed into a film by an inflation molding method.

The preferred draft ratio is 5 or more, particularly preferably 8 or more. A preferred expansion ratio is 5 or more, and particularly preferably 8 or more. Here, L is the length of the tube die composed of the mandrel and the outer die, and D is the inner diameter of the outer die at the screw die outlet. An embodiment relating to a blown film forming apparatus is disclosed in Japanese Patent Publication No. 6-5 / 1993 filed by the present applicant.
It is described in detail in Japanese Patent No. 5433.

In any method, the obtained film before the porous treatment has an intrinsic viscosity [η] of 4 to 25 dl.
/ G, are plane-oriented, have a crystallinity of preferably 40% or more, a tensile strength in the machine direction of 0.04 GPa or more, and a tensile strength in a direction perpendicular to the machine direction of 0.04 GPa.
This is an impermeable film having a moisture permeability coefficient of 0.45 g · mm / m ² · 24 hr or less under the conditions of a temperature of 40 ° C. and a humidity of 90%. The air-impermeable film has a gas permeability of 10,000 seconds / 100 ml in a gas permeability test described later.
The above is the film. The thickness of the film before processing obtained is not particularly limited, but is preferably 5 to 500 μm, more preferably 5 to 100 μm for the convenience of handling in the subsequent processing steps.

The crystallinity of the unprocessed film determined by the differential scanning calorimeter (DSC) from the heat of crystal fusion is preferably 60% or more, more preferably 60 to 70% in the case of polyethylene. In the case of a polyolefin other than polyethylene, the crystallinity of the film before treatment is preferably 40% or more, more preferably 50% or more.

The film before treatment of the present invention is preferably plane-oriented. The plane orientation in the present invention means that the crystal is biaxially oriented. The fact that the film is biaxially oriented means that among the unit crystals of the polyolefin in the film plane, one of the a-axis and the b-axis other than the c-axis corresponding to the molecular chain direction is mainly perpendicular to the film plane. It refers to a state in which it exists and a state in which, for example, the c-axis other than the axis is almost non-oriented in the film plane. The axis that is perpendicular to the film surface is polyethylene,
Usually, it is a-axis, and in the case of other polyolefins, it is usually b-axis.

This state can be confirmed as follows by observation with an X-ray diffractometer. That is, when the film is arranged in the equator direction from the end (END) direction of the film and X-rays are incident thereon and the diffraction pattern is observed, the orientation coefficient fa for polyethylene is at least (fb for other polyolefins). 0.2 or more, and arranged so that the machine axis direction of the film is the meridian direction, and X from the through (THROUGTH) direction.
When the diffraction pattern is observed by incident light, the orientation coefficient f
This is a state where c is not less than -0.2 and not more than 0.2.

The method for obtaining and calculating the orientation coefficients fa, fb, and fc is described in “X-ray diffraction of polymer (top)” (LERO).
YE. ALEXANDER, edited by Ichiro Sakurada, Chemical Doujinshi), as described in the section on the preferred orientation. In particular, in the case of a film before treatment in which fc is more than 0.2 (c-axis oriented state) or fa is less than 0.2, even if the crystallinity satisfies the above condition, the film can be made porous by heat treatment. May not be possible. In the case of a film before treatment having an intrinsic viscosity [η] of less than 4.0 dl / g, the film may be porous depending on the conditions, but may not be satisfactory in terms of tensile strength and piercing strength.

<Porosification treatment> The above-mentioned porosity treatment of the film before the porosity treatment varies depending on the state of the atmosphere.
For example, in the case of polyethylene, usually 100 to 14
It is preferable to perform the heat treatment at a temperature of 5 ° C. for 1 minute or more, under such a condition that the crystallinity after the treatment is increased by about 10 to 20% as compared with that before the treatment. At this time, the unprocessed film is preferably fixed in at least one direction, most preferably in two orthogonal directions, so as to prevent shrinkage. If shrinkage is required, the preferred range of shrinkage is 10% or less in the length and width directions.

The heat treatment is preferably carried out in a first liquid having a suitable affinity for the high molecular weight polyolefin. Having an appropriate affinity with high molecular weight polyolefin means that when a high molecular weight polyolefin unprocessed film is formed and immersed in the first liquid at the processing temperature, it has almost no effect on the crystal part of the unprocessed film. Instead, it mainly penetrates into the amorphous portion, selectively melts or dissolves it, and when cooled, crystallizes a part thereof, thereby increasing the crystallinity as a whole. Therefore, a solvent having remarkably excellent affinity and dissolving polyolefin crystals in a heat treatment temperature range is excluded.

It should be noted that having the affinity for the high molecular weight polyolefin means that the liquid is sufficiently adapted to the high molecular weight polyolefin film, and can be rephrased as having a small surface tension. And the scale is 10 in contact angle.
0 degrees or less, preferably 90 degrees or less, more preferably 80 degrees or less.
(The surface tension can be measured by a conventional method using a commercially available automatic contact angle meter).

The term “liquid that does not dissolve the high molecular weight polyolefin crystals in the heat treatment temperature range” refers to, for example, a differential scanning calorimeter (DSC) equipped with a solution cell, which is used to determine the melting point of the high molecular weight polyolefin in the presence of the liquid. When observed in, compared to the melting point of high molecular weight polyolefin alone,
It is a liquid that does not lower its melting point by 20 ° C. or more. Since the affinity of the liquid for the high-molecular-weight polyolefin varies depending on the treatment temperature, a suitable affinity can be obtained by selecting the treatment temperature and the type of the liquid, and the effect of the porous structure can be maximized.

Examples of the first liquid include lower aliphatic alcohols such as ethanol, propanol, butyl alcohol and amyl alcohol; lower aliphatic ketones such as acetone, methyl ethyl ketone and cyclohexanone; ethyl formate, acetic acid Lower aliphatic esters such as butyl, etc .; halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene, perchloroethylene, chlorobenzene, etc .; hydrocarbons such as heptane, cyclohexane, octane, decane, dodecane, etc .; pyridine, formamide, Nitrogen-containing organic compounds such as dimethylformamide and the like; methyl ether, ethyl ether, dioxane,
Ethers such as butyl cellosolob. Further, glycols such as monoethylene glycol, diethylene glycol, triethylene glycol and the like, and silicon oil generally used as a heating medium are also preferable liquids. These liquids can also be used as a mixture of two or more. Warm water or hot water to which a surfactant is added is also effective, but benzene, xylene,
Tetralin is not preferred because it dissolves high molecular weight polyolefins at the heat treatment temperature.

Suitable first liquids for polyethylene and polypropylene are octane, decane, dodecane,
It is a liquid of paraffin oil, molten paraffin wax, a liquid containing these as a main component, and at least one or more of these compositions. The heat treatment temperature depends on the type of polyolefin and the type of liquid. For example, as described above, in the case of polyethylene, the temperature is usually 100 ° C. to 145 ° C.
Preferably it is 115 ° C to 140 ° C. The processing temperature in the case of polyolefins other than polyethylene is usually 50 ° C to 170 ° C, preferably 80 ° C to 16 ° C.
0 ° C.

Generally, the processing time is 10 seconds to 10 minutes, preferably 3 seconds after the film before processing reaches the processing temperature.
The time is from 0 seconds to 5 minutes, and the processing time can be shortened as the processing temperature increases. In addition, it is preferable to avoid an unnecessary processing time because there is a possibility that the strength of the porous film may be reduced.

The film subjected to the heat treatment in the first liquid is subjected to a drying treatment. Depending on the type of liquid used in the treatment, the treatment liquid may be dried directly with warm air or hot air as long as it is fixed in two directions so as to prevent film shrinkage. In the case of a slow liquid, it is preferable to immerse and dry in a second liquid that is compatible with the first liquid, has a lower boiling point than the liquid, and has a lower affinity for polyolefin than the liquid. Further, when drying, the treated film is preferably fixed in at least one direction, most preferably in two orthogonal directions, so as to suppress shrinkage. If shrinkage is required, the preferred range of shrinkage is 10% or less in the length and width directions.

Examples of the second liquid which can be used include low-boiling hydrocarbons such as hexane and heptane, chlorine-substituted low-boiling hydrocarbons such as methylene chloride, and 1,2-dichloro-2,2,2. -Trifluoroethane, 1,1-dichloro-1-fluoroethane, 1,3-dichloro-1,
1,2,2,3-pentafluoropropane, 2,2
Chlorine-fluorine-substituted low-boiling hydrocarbons such as 3,3,3-pentafluoropropanol are exemplified. As the immersion temperature and the immersion time, the lowest temperature and the shortest time are selected among the conditions under which the liquid replacement is completely performed at a temperature lower than the heat treatment temperature.

The dried porous film may be heat-set for removing wrinkles of the film, adjusting the porosity and film thickness, and reducing the surface friction coefficient of the film. The conditions at the time of heat setting are appropriately selected such as the temperature and the processing time in a gas (air) atmosphere.

In order to adjust the porosity and the pore diameter, the film may be stretched simultaneously with the heat treatment, or may be stretched before and after the heat treatment. Stretching is performed at a temperature equal to or lower than the melting point of the film before processing. The lower limit of the stretching temperature depends on the type of the high-molecular-weight polyolefin, but is around the melting point of the film before treatment -40 ° C. The stretching ratio is 150% or more, preferably 15%, in the case of uniaxial stretching.
0 to 500%.

In the case of uniaxial stretching, uniaxial stretching with a constant width is preferred. In the case of biaxial stretching, the area ratio is 150% or more, preferably 150 to 2500%. Stretching may be performed in an air atmosphere, or, as described in the heat treatment section above, has a suitable affinity with a high molecular weight polyolefin, and does not dissolve the film before polyolefin treatment at the stretching treatment temperature. It may be performed in contact with a liquid.

The polyolefin porous film obtained by this method has fibrils in the form of veins and / or mesh as main components. The vein-like and / or mesh-like fibrils refer to a state in which the fibrils forming the film have a thick trunk fiber and thin fibers connected to the outside thereof, and the thin fibers form a complex network structure. Say. As fibrils forming the film,
There are fibrils (A) composed of stretched chain crystals and plate crystals and fibrils (B) composed of spiral crystals.

As an example of the structure and morphology of fibrils in a polymer material, studies have been made on a solution crystallized product under stress. For example, AJPennings, AAKiel, Koll
oidZ., 205 , p160 (1965); A. Keller, J. Machin, J. Macromol.
Sci., B1 , p41 (1967), etc., or K. Kobayashi, T. Nag
asawa, J. Macromol. Sci. Phys., B3 , p153 (1970); T. Nagasaw
a, Y.Shimomura, J.Polymer Sci.Polymer Phys.Ed., 12 , p2
291 (1974).

In these documents, two structural models are proposed as fibril structures. The former is
It has a structure consisting of an extended chain crystal formed at the center of the fibril and a plate-like crystal formed by a molecular chain hanging from the crystal.The latter has a spiral transition in the crystal nucleus consisting of the folded chain in the flow direction. This is a model that becomes a fibril shape for orientation, and does not necessarily require the extended chain crystal existing in the former.

The fibril (A) composed of a stretched chain crystal and a plate-like crystal described in the present specification corresponds to the former of the above structural model, and is a crystal generally called a shish-kebab structure. This is a fiber in which a fiber-like stretched chain crystal is present in the center, and a folded chain crystal (plate-like crystal) is periodically formed with the stretched chain crystal as a nucleus. , FIG. 3 and FIG. It is apparent from a comparison between FIG. 3 and FIG. 4 that this structure does not change even when a crosslinking treatment described later is performed.

On the other hand, the fibril (B) composed of a helical crystal described in this specification corresponds to the latter of the above structural model, and the fibril crystal at the center is hardly or not observed at all. And a plate-like crystal composed of a folded chain is formed in a spiral shape.

As the conditions for forming the above two types of fibrils, when the same air-impermeable film is used, the lower the heat treatment temperature, the easier the fibrils of (A) are formed,
As the heat treatment temperature increases, the fibrils of (B) are more likely to be generated. Further, when [η] of the raw material is high (for example, about 15 dl / g), fibrils of (A) are generated in most heat treatment temperature ranges. Conversely, when the [η] of the raw material is low (for example, about 5 dl / g), the heat treatment temperature range at which the fibrils of (B) are generated becomes relatively wide. The resulting film has a tensile strength of 0.05 GPa or more, preferably 0.08 GPa or more.

<Cross-Linking Treatment> In the present invention, the cross-linking treatment is performed to increase the gel fraction of the obtained polyolefin porous film to 50% or more. From the viewpoint, it is preferably more than 70%. The upper limit of the gel fraction is not particularly limited as long as the strength required for the intended use is maintained. The crosslinking treatment can be performed on the uncrosslinked porous film by chemical crosslinking or radiation crosslinking.

Chemical cross-linking is a method of treating a film by a chemical reaction in the presence of a cross-linking agent, but the following radiation cross-linking is preferable from the viewpoint of little change in physical properties of the film and uniform cross-linking. Radiation crosslinking is to crosslink by directly irradiating the surface of the film with an electron beam or γ-ray, and it can also penetrate into the inside of the film to cause a crosslinking reaction. Among these, electron beam irradiation that can be performed by general-purpose equipment is preferable.

In the case of the electron beam crosslinking treatment, usually 0.1 Mra
Irradiation of about 100 Mrad from d is performed. However, when the irradiation dose is small, the heat resistant temperature may be deteriorated.
It is preferably at least 1 Mrad, more preferably at least 2 Mrad, and particularly preferably at an irradiation dose exceeding 10 Mrad. The irradiation can be performed in the air, but preferably in a nitrogen atmosphere in order to prevent an oxidation reaction.

<Characteristics of Obtained Film> The characteristics of the polyolefin porous film (hereinafter, referred to as "crosslinked film") obtained by the above treatment are shown below. The gel fraction of the crosslinked film is an index indicating the degree of crosslinking, and is 50%.
% Or more. Further, when the gel fraction exceeds 70%, a high heat-resistant temperature can be obtained, which is preferable. The blocking temperature is determined in relation to the safety of the battery used as the separator, and is preferably 140 ° C. or lower, more preferably 138 ° C. or lower, and particularly preferably 135 ° C. or lower. The heat-resistant temperature as the upper limit of the temperature at which the film is not broken is 170 ° C. or higher, preferably 1 ° C.
The temperature is at least 75 ° C, more preferably at least 180 ° C.

The crosslinked film has a vein shape and / or
Alternatively, mesh-like fibrils are the main components, and the structure is the same as that described for the porous treatment. As a preferred embodiment of the crosslinked film, the film thickness is 100 μm or less, preferably 10 to 100 μm, more preferably 20 to 8 for easy handling.
0 μm. The porosity is 20 to 80%, preferably 40 to 80%, and more preferably 50 to 80%. The air permeability is 50 to 2000 seconds / 100 ml, and preferably 50 to 1500 seconds / 100 ml. The shrinkage at 105 ° C. is 10% or less, and the piercing strength is 250 g or more.

The tensile strength of the crosslinked film is usually at least 7 MPa in at least one direction, preferably at least 10 MPa in at least one direction, more preferably at least 30 MPa in at least one direction. In addition, in order to use this cross-linked film as a battery separator film, a non-aqueous electrolyte battery is used as it is, and an aqueous electrolyte battery is subjected to a hydrophilization treatment or the like, and in any case, the following physical properties are provided. Is important. (1) film thickness of 100 μ or less; (2) porosity of 20 to 80%; (3) air permeability of 50 to 2000 seconds / 100 ml; (4) shrinkage of 10% or less; Is 250 g or more; (6) the blocking temperature is 140 ° C. or less; (7) the heat-resistant temperature is 170 ° C. or more;

The maximum pore diameter preferably exceeds 0.1 μm. Preferably it is more than 0.11μ and 10μ or less, more preferably more than 0.13μ and 1μ or less, further preferably more than 0.15μ and 0.5μ or less. For example, when used as a separator for a lithium ion battery, it is required that the lithium ions move through the pores as much as possible without causing a short circuit between the positive electrode and the negative electrode. Therefore, it is desirable that the battery has a large pore diameter as long as the safety of the battery can be ensured. If the maximum pore diameter exceeds 0.1μ, the problem when used as a battery separator, that is, the charge / discharge characteristics at low temperature is deteriorated, large current cannot be passed, and clogging is caused by a reactant such as an electrolytic solution. It is less likely to cause problems such as shortening the cycle life of the battery. Further, if the maximum pore diameter does not exceed the upper limit of the above-mentioned preferable range, it is preferable because the blocking temperature becomes high and safety cannot be secured.

The above characteristics in the present invention are measured by the following methods. <Intrinsic viscosity> In the present specification, the intrinsic viscosity is a value measured at 135 ° C in a decalin solvent. The measurement method is AST
Performed based on MD4020.

<Gel Fraction> The gel fraction was determined by weighing a porous film (W), adding about 200 ml of p-xylene to 0.2 g of the film weight, heating the mixture under reflux at the boiling point for 2 hours, and then measuring the undissolved content. After filtration and drying, the weight (W ₁ ) was measured and calculated from the following equation. Gel fraction (%) = W ₁ / W × 100

<Measurement of Film Thickness> The film thickness was measured with a film thickness measuring instrument Miniax (model DH-150) manufactured by Tokyo Seimitsu Co., Ltd.

<Porosity> The sample weight was measured, and the density of the film was determined to be 0.95 g / cm ³ , and the thickness as a dense film was determined by calculation. . Here, T _O is the actual film thickness determined by a film thickness measuring instrument, and T _W is the film thickness assuming a porosity of 0% calculated from the weight.

<Measurement of Air Permeability (Gurley Test)> AST
According to MD726, the film was measured with a standard Gurley Densometer (Gurley Densometer: Type B Gurley Densometer manufactured by Toyo Seiki Seisaku-sho, Ltd.).

<Measurement of Melting Point> The melting point in the present invention is a value measured by a differential scanning calorimeter (DSC) according to ASTM D3417.

<Clogging temperature> 1 mol of anhydrous lithium perchlorate was previously stored in a dry nitrogen atmosphere (water content: 50 ppm or less).
A solution of le / liter was prepared using propylene carbonate dehydrated with a molecular sieve (manufactured by Wako Pure Chemical Industries, Ltd .: 4A) as a solvent, and this solution was impregnated into a film using a reduced pressure operation. This film is sandwiched between nickel electrodes, and an impedance meter (made by Mita Radio Laboratory: Model D)
-52S), the volumetric electrical efficiency of the film was measured. The instrument and measurement method are described in Raman et al.'S report (FCLaman et al., JE.
Soc., Vol. 140, 51-53 (1993)).
The resistivity per volume at normal temperature (23 ° C.) is determined by the volume electric resistivity of the film (usually, this porous film has a resistivity of 20
0 Ωcm ² or less), and the temperature is raised to make the electric resistance 1
The temperature that reached 000 Ωcm ² was defined as the closing temperature.

<Shrinkage ratio> A sample of 10 × 10 cm was
It was left in an air oven (manufactured by Tabai) at 05 ° C. for 1 hour, and the shrinkage was calculated.

<Tensile strength> Tensileon (Model RTM100) manufactured by Orientec Co., Ltd. at room temperature (23 ° C.)
I went in. Method A of ASTM D882 (sample width 15m
m) and calculated.

<Puncture Strength> Using a Tensilon tensile tester (model: RTM100 type) manufactured by Orientec, at 23 ° C.
At a crosshead speed of 100 mm / min. The needle for piercing was performed using a needle having a diameter of 1 mm having a needle tip of 0.5 mmR, and the force when the needle pierced the film was defined as the piercing strength.

<Film Breaking Temperature> The temperature at which film breakage or pinholes occur when the porous film is left for 3 minutes in an air oven at a predetermined temperature with the film fixed between a pair of circular fixed frames having an inner diameter of 5 cmφ. Show. The symbols in the table indicate 〇: no rupture, △: pinhole generation, ×: rupture. The upper limit of the temperature at which the film was not broken was set as the heat resistant temperature.

<Maximum pore diameter> The pore diameter was measured using a mercury porosimeter (Autoscan 33 manufactured by Yuasa Ionics). The maximum value of the pore size distribution was adopted as the maximum pore size.

[0073]

The porous polyolefin film provided by the present invention has a specific fibril structure and is excellent in air permeability, tensile strength, and piercing strength by being crosslinked so as to have a gel fraction of 50% or more. In addition, the membrane rupture temperature increases while maintaining a low blocking temperature, and is suitably used for applications such as non-aqueous electrolytes such as alkaline batteries, lithium-ion batteries, lead-acid batteries, nickel-metal hydride batteries, and nickel cadmium batteries, and separators for electrolyte batteries. In particular, it is used for a high-capacity battery having a volume energy density of 200 Wh / 1 or more. Further, since the porous film has high strength, it can be applied to various filters and air-permeable films.

[0074]

The present invention will be described below with reference to examples. In the following examples, “%” is “% by weight” unless otherwise specified.

< Examples 1 to 4 and Comparative Examples 2 to
4 ><Preparation of Film Before Treatment> In the apparatus for producing a blown film shown in FIG. 1, a blown film made of high molecular weight polyethylene was produced using an apparatus having the following specifications. Apparatus Specifications First screw outer diameter of the extruder 50 mmφ; screw effective length 1100 mm; flight pitch 30 mm constant; screw compression ratio 1.8; screw die effective length 1490 mm (L / D = 28);
Die exit outer die inner diameter 53 mmφ, die exit mandrel outer diameter 45 mmφ; S1 / S2 = 1.17; S2 / S
3 = 3.14; second screw outer diameter of screw die 7
0 mmφ; second screw effective length 238 mm; flight pitch 25 mm constant; second screw compression ratio 1.0;
Outer diameter of stabilizer rod: 39 mmφ; stabilizer rod length: 600 mm;

An 8 mmφ gas flow path, a stabilizer, a pinch roll, and a product winder extending inside the second screw, inside the mandrel, and inside the stabilizer bar shaft are provided. Here, S1 is a cross-sectional area of the resin flow path of the second screw tip portion 20A, and S2 is a screw die intermediate portion 20B.
5 is the cross-sectional area of the resin flow path, and S3 is the cross-sectional area of the resin flow path at the screw die outlet 20C.

<Production of blown film> High molecular weight polyethylene [η]: 7.5 dl / g, melting point: 13
6 ° C., using a powder resin having a bulk density of 0.45 g / cm ³ ,
Extruder, joint (J) shown in FIG. 1, die base (D
1) Set the temperature of the die tip (D2) to 200
℃, 180 ℃, 160 ℃, 160 ℃, the first screw rotation speed is 21 rpm, the second screw rotation speed is 4.5.
rpm, while taking in with a pinch roll at a speed of 7.7 m / min, blowing compressed air from an 8 mmφ gas flow path extending inside the second screw, the mandrel and the stabilizer rod shaft, Inflate the parison to 8.5 times the inner diameter of the outer die 53 mmφ, and set the fold width to 710.
A high-molecular-weight polyethylene blown film having a thickness of 14 mm and a thickness of 14 μm was produced stably. The air ring was appropriately changed to one having an appropriate inner diameter according to the expansion ratio. Table 1 shows the molding conditions and the characteristics of the obtained film.

[0078]

[Table 1]

<Porosification> The obtained blown film was subjected to a heat treatment as follows. FIG.
The inflation film 31 was sandwiched between a pair of stainless steel frames 33 shown in FIG. In this state, it was put into a tank filled with the heated liquid for heat treatment (first liquid) and immersed for a predetermined time.

<Second liquid immersion and drying> The film fixed to the metal frame removed from the heat treatment tank was put into a tank filled with the second liquid in that state, and immersed. This was taken out and air-dried at room temperature (23 ° C.), and then the film was removed from the metal frame to obtain a measurement sample. The processing conditions were as shown in Table 2 below for all samples.

[0081]

[Table 2]

<Crosslinking treatment> Using the porous film obtained in the above example, an electron beam was irradiated under the conditions shown in Table 3.
Crosslinking of the porous film was performed. For the electron beam irradiation, use “EBC300-60” manufactured by Nissin High Voltage Co., Ltd.
The test was performed under a nitrogen atmosphere at an acceleration voltage of 200 kV.
After the cross-linking treatment, the film was allowed to stand in a vacuum at 60 ° C. for 20 hours, and various physical properties of the obtained porous film were measured. Comparative Example 1
Various physical properties were similarly measured for a sample that had not been subjected to a crosslinking treatment. Table 3 shows the results.

[0083]

[Table 3]

< Examples 5 and 6 and Comparative Example 6 > A blown film made of high molecular weight polyethylene was produced using the same apparatus as in the above example.

<Production of blown film> High molecular weight polyethylene [η]: 8.9 dl / g, melting point: 13
6 ° C., using a powder resin having a bulk density of 0.45 g / cm ³ ,
Extruder, joint (J) shown in FIG. 1, die base (D
1) Set the temperature of the die tip (D2) to 200
℃, 180 ℃, 160 ℃, 160 ℃, the first screw rotation speed is 21 rpm, the second screw rotation speed is 4.5.
rpm, and while taking in with a pinch roll at a speed of 4.3 m / min, compressed air was blown from an 8 mmφ gas flow path extending inside the second screw, the mandrel, and the inside of the stabilizer rod shaft, Inflate the parison to 8.5 times the inner diameter of the outer die 53 mmφ, and set the fold width to 710.
A blown film made of high molecular weight polyethylene having a thickness of 27 μm and a thickness of 27 μm was produced stably. The air ring was appropriately changed to one having an appropriate inner diameter according to the expansion ratio. Table 4 shows the molding conditions and the properties of the obtained film.

[0086]

[Table 4]

<Porosification> The obtained inflation film was subjected to a heat treatment in the same manner as in the above example.

<Second liquid immersion and drying> The film fixed to the metal frame taken out of the heat treatment tank was put into a tank filled with the second liquid in that state, and immersed. This was taken out and air-dried at room temperature (23 ° C.), and then the film was removed from the metal frame to obtain a measurement sample. The processing conditions were as shown in Table 5 below for all samples.

[0089]

[Table 5]

<Crosslinking Treatment> The porous film thus obtained was irradiated with an electron beam under the conditions shown in Table 6 to crosslink the porous film. The electron beam irradiation was performed under the condition of an acceleration voltage of 200 kV in a nitrogen atmosphere using “EBC300-60” manufactured by Nissin High Voltage Co., Ltd. After the cross-linking treatment, the film was allowed to stand in a vacuum at 60 ° C. for 20 hours, and various physical properties of the obtained porous film were measured. In addition, as Comparative Example 5, various physical properties were similarly measured for a sample that had not been subjected to a crosslinking treatment. Table 6 shows the results.

[0091]

[Table 6]

< Examples 7, 8 and Comparative Example 7 > Using the blown film obtained in Table 1, a porous film heat-treated under the conditions shown in Table 7 was subjected to a crosslinking treatment. Table 8 shows the properties of the obtained porous film.

[0093]

[Table 7]

[0094]

[Table 8]

[Brief description of the drawings]

FIG. 1 is a front sectional view showing an example of a molding apparatus for producing a porous film of the present invention.

FIG. 2 is a view showing an example of a metal frame for fixing a film before stretching in the present invention when heat-treating the film.

FIG. 3 is an electron micrograph (magnification: 10,000 times) of a polyolefin porous film obtained in Example 4 of the present invention after electron beam irradiation.

FIG. 4 is a magnification of 100 of a polyolefin porous film not irradiated with an electron beam obtained in Comparative Example 1 of the present specification.
It is an electron microscope photograph of 00 times.

FIG. 5 is an electron micrograph (magnification: 10,000) showing a state of the polyolefin porous film of FIG. 3 after nonporous occlusion.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 extruder 2 grooved cylinder 3 first screw 10 torpedo 11 pressure gauge 20 screw die 20A second screw tip 20B screw die middle part 20C screw die outlet 21 second screw 22 outer die 23 mandrel 24 gas flow path 25 air ring 26 Stabilizing bar 27 Windproof tube 30 Parison 31 Inflation film 32 Screw 33 Metal frame

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI B29K 23:00 105: 04 105: 24 B29L 7:00

Claims (11)

[Claims] 1. A polyolefin porous film which is crosslinked to have a gel fraction of 50% or more and has the following characteristics. (1) The blocking temperature is 140 ° C. or less; (2) The heat-resistant temperature is 170 ° C. or more; 2. The polyolefin porous film according to claim 1, wherein the gel fraction exceeds 70%. 3. The porous film has fibrils in the form of veins and / or mesh as main components, and the fibrils are fibrils composed of stretched chain crystals and plate-like crystals and / or fibrils composed of spiral crystals. The polyolefin porous film according to claim 1 or 2, wherein 4. The polyolefin porous film according to claim 1, which has the following characteristics. (1) film thickness of 100 μ or less; (2) porosity of 20 to 80%; (3) air permeability of 50 to 2000 seconds / 100 ml; (4) shrinkage of 10% or less; Is 250 g or more; 5. The polyolefin porous film according to claim 1, wherein the polyolefin is polyethylene. 6. A high molecular weight polyolefin impervious film having a limiting viscosity [η] of 4 dl / g or more, which is substantially free of a plasticizer and / or a solvent, is microporous by heat treatment, and then has a gel fraction. A method for producing a polyolefin porous film, comprising crosslinking an electron beam so that the content becomes 50% or more. 7. The method for producing a polyolefin porous film according to claim 6, wherein the high molecular weight polyolefin impermeable film is a film obtained by inflation molding. 8. A battery separator film obtained from the polyolefin porous film according to any one of claims 1 to 5. 9. The battery separator film according to claim 8, wherein the separator film has the following characteristics. (1) film thickness of 100 μ or less; (2) porosity of 20 to 80%; (3) air permeability of 50 to 2000 seconds / 100 ml; (4) shrinkage of 10% or less; Is 250 g or more; (6) the blocking temperature is 140 ° C. or less; (7) the heat-resistant temperature is 170 ° C. or more; 10. The porous film according to any one of claims 1 to 5, wherein the maximum pore size exceeds 0.1 μm. 11. The battery separator film according to claim 8, wherein the maximum pore diameter exceeds 0.1 μm.