JP2002200670A - Method for producing porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a porous film suitable for a battery separator. SOLUTION: A primary film is manufactured from a resin composition comprising a thermoplastic resin and 15-85 pts.vol. of a filler 1 μm or below in average particle size per 100 pts.vol. of the thermoplastic resin and stretched at least in one direction at a distortion speed of 1-300%/s. By stretching the primary film under the above conditions, a porous film suitable for a battery separator can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a porous thermoplastic resin film. More specifically, the present invention relates to a method for producing a porous thermoplastic resin film which can be preferably used as a separator for an electrolytic capacitor, a lithium battery, a fuel cell, a battery and the like.

[0002]

2. Description of the Related Art As a porous thermoplastic resin film having air permeability, a porous film obtained by stretching a polyolefin resin film containing a filler is known.
Such a porous film is used as a material for sanitary articles such as diapers and bed covers, and clothing such as gloves. As a method for producing such a porous film, a composition obtained by kneading a polyolefin resin and inorganic particles or resin particles such as calcium carbonate, titanium oxide, calcined clay, and diatomaceous earth is processed into a film, and then the film is stretched. There is known a method for obtaining a porous film.

For example, JP-A-9-176352 discloses a polypropylene resin having an average particle size of 0.01 to 10 μm.
m, a method of processing a polypropylene composition comprising resin particles and a β-nucleating agent into a film and stretching the film with a roll.
It is disclosed that a porous film having a μm and a Gurley value of 10 to 30,000 seconds / 100 cc can be obtained.

However, although the reason for the porous film obtained by the above method is not clear, when it is used as a battery separator (particularly, a lithium battery separator), the internal resistance of the battery becomes high. It did not work well.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a porous film suitable as a battery separator.

[0006]

Means for Solving the Problems In the process of developing a porous film that can suppress the internal resistance of a battery when used as a battery separator,
By forming a resin composition containing a specific amount of a filler of a specific size into a film and stretching the film in at least one direction at a specific strain rate, a porous film suitable for a battery separator can be obtained. Heading,
The present invention has been completed.

That is, according to the present invention, a primary film is prepared from a resin composition comprising a thermoplastic resin and a filler having an average particle diameter of 15 to 85 parts by volume per 100 parts by volume of the thermoplastic resin and having an average particle diameter of 1 μm or less. A method for producing a porous film, comprising stretching a film in at least one direction at a strain rate of 1 to 300% / sec.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the present invention, first, a resin composition comprising a thermoplastic resin and a filler having an average particle size of 1 μm or less is processed into a film. This film is substantially non-porous and is referred to in the present invention as a "primary film".
Called.

The thermoplastic resin applicable to the method of the present invention includes homopolymers of olefins such as ethylene, propylene, butene and hexene, copolymers of two or more olefins, and one or more olefins. And a polyolefin resin such as a copolymer of at least one polymerizable monomer polymerizable with the olefin, an acrylic resin such as polymethyl acrylate, polymethyl methacrylate, and ethylene-ethyl acrylate copolymer, and a butadiene-styrene copolymer. Polymer, acrylonitrile-styrene copolymer, polystyrene, styrene-butadiene-
Styrene resin such as styrene copolymer, styrene-isoprene-styrene copolymer, styrene-acrylic acid copolymer, vinyl chloride resin, polyvinyl fluoride resin such as polyvinyl fluoride and polyvinylidene fluoride, 6-nylon ,
Amide resins such as 6,6-nylon and 12-nylon,
Saturated ester resins such as polyethylene terephthalate and polybutylene terephthalate, polycarbonate, polyphenylene oxide, polyacetal, polyphenylene sulfide, silicone resin, thermoplastic urethane resin, polyetheretherketone, polyetherimide, various thermoplastic elastomers and crosslinked products thereof Is mentioned.

[0010] Among the above thermoplastic resins, polyolefin resins are preferably used. The olefins constituting the polyolefin resin include ethylene, propylene,
Butene, hexene and the like. Specific examples of the polyolefin resin include low-density polyethylene, linear polyethylene (ethylene-α-olefin copolymer), polyethylene resins such as high-density polyethylene, polypropylene, polypropylene resins such as ethylene-propylene copolymer, Examples include poly (4-methylpentene-1), poly (butene-1) and ethylene-vinyl acetate copolymer.

In particular, when a polyolefin resin containing a polyolefin having a molecular chain length of 2850 nm or more is used, a porous film which is excellent in strength and which gives a battery having a lower internal resistance when used as a separator can be obtained. . Polyolefin resins have a molecular chain length of 2
It is preferable that the polyolefin of 850 nm or more is contained in an amount of 10% by weight or more, more preferably 20% by weight or more, and even more preferably 30% by weight or more.

The resin composition used for preparing the primary film contains a filler having an average particle size of 1 μm or less, and the average particle size of the filler is preferably 0.05 μm to 1 μm, more preferably 0.1 μm to 1 μm. 0.6 μm is more preferred. When the average particle diameter of the filler is 1 μm or more, undesirably large pores are formed in the finally obtained porous film, and the undesirably high internal volume when used as a battery separator. A porous film that produces resistance is obtained. From the performance aspect of the obtained porous film, the average particle diameter of the filler is 0.05 μm
Such fillers are usually
Difficult to prepare and difficult to obtain. The average particle size of the filler used in the present invention is as follows. The resin and the filler are kneaded with a mixer, formed into a film by a press machine, stretched into a porous film, and then subjected to scanning electron microscope (SEM). The average value of the diameters of the particles obtained by observing the surface of the porous film and measuring all the particles observed in a visual field having a size of 10 μm × 10 μm.

The filler may be an inorganic filler or an organic filler. Examples of the inorganic filler include calcium carbonate, magnesium carbonate, barium carbonate, talc, clay, mica, kaolin, silica, hydrotalcite, diatomaceous earth,
Calcium sulfate, magnesium sulfate, barium sulfate, aluminum hydroxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, alumina, zinc oxide, zeolite, glass powder, etc., especially calcium carbonate, hydrotalcite, sulfuric acid Barium, magnesium hydroxide and alumina are preferred.

Various resin particles can be used as the organic filler. Among them, styrene, vinyl ketone,
Homopolymer of acrylonitrile, methyl methacrylate, ethyl methacrylate, glycidyl methacrylate, glycidyl acrylate or methyl acrylate, copolymer of two or more monomers selected from the above monomer group, polycondensation of melamine, urea, etc. Resin particles are preferred.

The content of the filler in the resin composition used for preparing the primary film is preferably 15 to 85 parts by volume, more preferably 25 to 70 parts by volume, per 100 parts by volume of the thermoplastic resin. By setting the content of the filler within such a range, the film can be prevented from being broken at the time of stretching, and a porous film that can preferably function as a battery separator can be efficiently obtained.

In the present invention, as long as the effects of the present invention are not significantly impaired, the resin composition used for producing the primary film may include a stretching aid such as a fatty acid ester or a low molecular weight polyolefin resin, a stabilizer, and an antioxidant. And additives such as ultraviolet absorbers, flame retardants, and nonionic surfactants.

The above resin composition is prepared by mixing, for example, a predetermined amount of a thermoplastic resin and a filler and, if necessary, a desired additive such as a nonionic surfactant with a mixing device, for example, a roll, a Banbury mixer, a single screw extruder, It can be prepared by mixing using a twin screw extruder or the like.

For example, a resin composition comprising a thermoplastic resin containing 10% by weight or more of a polyolefin having a molecular chain length of 2850 nm or more and a filler has a weight average molecular chain length of 2%.
850 nm or more polyolefin [A] and weight average molecular weight 700-6000 polyolefin wax [B]
[A] / [B] = 90/10 to 50/50 in a weight ratio, and a predetermined amount of filler is further added. It can be prepared by kneading using a kneading device provided with a screw in the barrel, which is constituted by combining two types of segments of the loading block. In particular,
The overall length of the screw is L (mm) and the inner diameter of the barrel is D (m
m), the total length of the full flight screw is L _f (m
m), the total length of the kneading block is L _n (mm)
When L / D is 30 or more, L _f / D is 3 or more,
And, L _n / D is preferred to use 5 or more kneading apparatus. Furthermore, when the flight angle of the full flight screw is α (degree) and the depth of the screw groove of the full flight screw is M (mm), α is 35 or more and 60 or less, and the value of M / D is 0. It is preferable to use an apparatus having a size of not less than .15 and not more than 0.25.

In the present invention, the molecular chain length, the weight average molecular chain length, the molecular weight and the weight average molecular weight of the polyolefin are measured by GPC (gel permeation chromatography), and the polyolefin having a specific molecular chain length range or a specific molecular weight range is measured. Can be determined by integrating a molecular weight distribution curve obtained by GPC measurement.

The molecular chain length of the polyolefin is a molecular chain length in terms of polystyrene measured by GPC (gel permeation chromatography), and is more specifically a parameter determined by the following procedure. That is, GP
As a mobile phase for C measurement, a solvent that can dissolve both an unknown sample to be measured and standard polystyrene with a known molecular weight is used. First, a plurality of types of standard polystyrene having different molecular weights are subjected to GPC measurement to determine the retention time of each standard polystyrene. The molecular chain length of each standard polystyrene is determined using the Q factor of the polystyrene, whereby the molecular chain length of each standard polystyrene and the corresponding retention time are known. The molecular weight, molecular chain length and Q
The factors have the following relationship. Molecular weight = molecular chain length × Q factor Next, GPC measurement of an unknown sample is performed to obtain a retention time-eluting component amount curve. In the GPC measurement of the standard polystyrene, when the molecular chain length of the standard polystyrene having the retention time T is L, the “molecular chain length in terms of polystyrene” of the component having the retention time T in the GPC measurement of the unknown sample is L.
And Using this relationship, the polystyrene-equivalent molecular chain length distribution (the relationship between the polystyrene-equivalent molecular chain length and the eluted component amount) of the unknown sample is determined from the retention time-eluted component amount curve of the unknown sample.

Using the resin composition prepared as described above, a primary film can be prepared by a film forming method such as inflation processing, calendar processing, and T-die extrusion processing.

The thickness of the primary film is not particularly limited, but is preferably set to about 5 μm to 300 μm, more preferably about 10 μm to 100 μm.

In the method of the present invention, the primary film produced as described above is stretched in at least one direction to form a porous film having a large number of fine communication holes. Stretching of the primary film is performed by stretching the primary film in at least one direction at a strain rate in the range of 1 to 300% / sec. The strain rate during stretching is more preferably in the range of 10 to 300% / sec. More specifically, at least 1 to 300% in the MD direction of the primary film.
Preferably, the primary film is stretched at a strain rate of 10 / sec, especially at a strain rate of 10-300% / sec. By stretching the primary film at such a strain rate, tearing of the film at the time of stretching is well prevented, and the obtained porous film is very suitable for a battery separator.
That is, even when a large current flows, the internal resistance of the battery can be suppressed to a small value. On the other hand, when the strain rate exceeds 300% / sec, the battery including the obtained porous film as a separator has a high internal resistance at a large current. In the production of a stretched resin film, the stretching is usually performed at a strain rate of 1000% / sec or more, with emphasis on production efficiency and the like, whereas the method of the present invention is 1 to 300% /
There is a great feature in that stretching is performed at a low strain rate of seconds, and by adopting such stretching conditions, it is possible to produce a porous film which is very suitable for a battery separator which cannot be obtained under conventional stretching conditions. Became possible.

In the present invention, the "strain rate" at the time of stretching is defined by the following equation. Strain rate (% / sec) = (L ₁ / L ₀ ) × 100Δt where L ₁ is the length of the film in the stretching direction after stretching, and L ₀ is the film in the stretching direction before stretching. And t is the time (seconds) required to stretch the film from the length L ₀ to the length L ₁ in the stretching direction.

The stretching of the primary film is performed at least in a uniaxial direction, preferably in a uniaxial or biaxial direction, by a roll stretching machine, a tenter stretching machine or the like. Stretch ratio is 2-1
It is preferably 0 times, more preferably 3 to 8 times. When the film is stretched at a stretching ratio of less than 2 times, a porous film which has a high Gurley value and gives a battery having high internal resistance when used as a separator may be obtained. When the film is stretched at a stretching ratio of more than 10 times, it is difficult to obtain a film having a uniform thickness, and at the same time, the film is easily broken at the time of stretching.

In the present invention, the temperature (stretching temperature) of the primary film at the time of stretching is not particularly limited, but the film is stretched smoothly without breaking the film, and has a good air permeability as a battery separator. In order to efficiently form a polymer, the temperature is preferably equal to or lower than the melting point or softening point of the thermoplastic resin. For example, when the thermoplastic resin is a polyolefin resin, the temperature is preferably equal to or lower than the melting point of the polyolefin resin, and particularly preferably in the range of 50 to 150 ° C.

The method of the present invention can be carried out by any of the following continuous method and discontinuous method. The continuous method is a method in which the produced primary film is subjected to a stretching step without being wound, and is generally performed as follows. Using a manufacturing device that can continuously perform from the formation of the primary film to the winding of the porous film,
A primary film is formed and stretched at a predetermined strain rate without being wound once to produce a porous film. In such a continuous method, if necessary, the primary film may be heated before stretching. On the other hand, the discontinuous method is a method in which a manufactured primary film is once wound up, and then subjected to a stretching step while unwinding the wound primary film, and is roughly performed as follows. First, a primary film is manufactured, and this is once wound up. Next, while unwinding the primary film at a desired time, the unwound primary film is heated to a temperature at which it can be stretched to be softened and stretched at a predetermined strain rate. In the method of the present invention, the produced porous film can be appropriately subjected to steps such as cooling, cutting, and winding.

The porous film produced by the method of the present invention can be further subjected to irradiation to crosslink the thermoplastic resin constituting the film. By performing this treatment, a porous film having better heat resistance and strength than a porous film made of a non-crosslinked thermoplastic resin can be obtained.

When the porous film according to the method of the present invention is used as an ion permeable membrane, it is effective that the porous film is a thin film having a thickness of about 3 to 50 μm so as to achieve excellent ion conductivity. In this case, it is more effective that the thermoplastic resin constituting the porous film is cross-linked by irradiation with radiation. Usually, when the porous film is made thinner, there is a problem that the film strength is reduced. On the other hand, a porous film produced by the method of the present invention, whose film thickness is about 3 to 50 μm, and whose thermoplastic resin is crosslinked by irradiation with radiation, It can be an ion permeable membrane having excellent ion conductivity and high strength.

The type of radiation applied to the porous film for crosslinking is not particularly limited, but gamma rays, alpha rays, electron beams and the like are preferably used, and electron beams are particularly preferred in view of production speed and safety. As the radiation source, an electron beam accelerator having an acceleration voltage of 100 to 3000 kV is preferably used. If the accelerating voltage is smaller than 100 kV, the penetration depth of the electron beam is not sufficient, and if it is larger than 3000 kV, the apparatus is large and is not preferable in cost. Examples of the radiation irradiation device include an electron beam scanning type device such as a Van de Graaff type, and an electron beam fixed / conveyor moving type device such as an electron curtain type. The absorbed dose of radiation is preferably from 0.1 to 100 Mrad, and more preferably from 0.5 to 50 Mrad.
When the absorbed dose is less than 0.1 Mrad, the effect of crosslinking the resin is not sufficient, and when the absorbed dose is more than 100 Mrad, the strength is significantly reduced, which is not preferable. The irradiation atmosphere when irradiating the porous film of the present invention with radiation may be air, but an inert gas atmosphere such as nitrogen is preferable.

Further, when irradiating with radiation, the porous film of the present invention may be mixed or impregnated with another monomer compound or polymer, followed by irradiation and reaction to effect cross-linking or graft polymerization. As the compound to be mixed or impregnated into the porous film of the present invention, styrene, divinylbenzene, acrylic acid, acrylic acid ester, methacrylic acid, methacrylic acid ester, fluorine compound, a homopolymer or a copolymer thereof, the monomer or Sulfonic acid derivatives of polymers,
Phosphate derivatives and the like can be mentioned.

[0032]

According to the study of the present inventors, the Gurley value of a porous film used as a battery separator is 40.
~ 3000 sec / 100cc, preferably 6
It is particularly preferred that it is in the range of 0 to 1000 seconds / 100 cc. Further, the average pore size of the porous film for a separator is preferably in the range of 0.04 to 0.4 μm, and particularly preferably in the range of 0.04 to 0.2 μm. According to the method of the present invention, such a condition can be satisfied, and a porous film suitable for a battery separator can be obtained. In particular, according to the method of the present invention, the thickness is set to Y (μ
m), _Gurley value is T _GUR (sec / 100cc), average pore size is d
([Mu] m) as it is possible to produce a porous film suitable for battery separator A porous film X _R is less than 5, which is defined by the following equation. X _R = 25 × T _GUR × d ² ÷ Y The average pore diameter of the porous film in the present invention is determined by, for example, using a Perm-Porometer (manufactured by PMI) according to the bubble point method (ASTM F316-86). Measure.

[0033]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. The physical properties of the porous films shown in Examples and Comparative Examples were measured by the following methods.

_Gurley value: The _Gurley value T _GUR (second / 100 cc) of the film was measured by a B-type densometer (manufactured by Toyo Seiki Seisaku-sho, Ltd.) according to JIS P8117.

Evaluation of internal resistance (load characteristics): In order to evaluate the internal resistance, electrodes for a charge / discharge test and a flat test cell were prepared by the following method.

89% by weight of lithium cobaltate powder, 1% by weight of acetylene black and 5% by weight of flaky artificial graphite
Was added to an N-methylpyrrolidone solution containing 5% by weight of polyvinylidene fluoride, and the mixture was sufficiently kneaded to obtain a paste. The paste was applied to a 20 μm-thick aluminum foil as a current collector, dried, and roll-pressed to obtain a positive electrode sheet. The positive electrode sheet prepared as described above and metallic lithium as a negative electrode are laminated via a separator made of a porous film, and the volume ratio of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 30: 35: 3.
An electrolytic solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / liter in the mixed solvent of No. 5 was added to prepare a flat test cell.

The flat test cell thus obtained was subjected to a charging / discharging test with constant current / constant voltage charging and constant current discharging under the following conditions to measure a discharge capacity and evaluate load characteristics. Charge / discharge A was performed at a maximum charging voltage of 4.3 V, a charging time of 8 hours, a charging current of 0.5 mA / cm ² , a minimum discharging voltage of 3.0 V, and a discharging current of 0.5 mA / cm ² . Discharge B is charged up to a voltage 4.3 V, 8 hours charging time, the charging current 0.5 mA / cm ^2, discharge minimum voltage 3.0 V, was conducted at a discharge current of 6.7mA / cm ^2. Charge / discharge C is a maximum charge voltage of 4.3 V, a charge time of 8 hours, and a charge current of 0.5 mA.
/ Cm ² , minimum discharge voltage 3.0 V, discharge current 16.7 m
A / cm ² was used.

The load characteristic I is (discharge capacity of charge / discharge B) /
(Discharge capacity of charge / discharge A). Load characteristics II
It is defined as (discharge capacity of charge / discharge C) / (discharge capacity of charge / discharge A). Here, the load characteristic is a ratio of the electric capacity that can be taken out when a large current flows to the electric capacity that can be taken out when a weak current flows, and the load characteristic increases as the internal resistance of the battery decreases. When the internal resistance is 0, the load characteristic is 100%.
In particular, this is an important property in a secondary battery such as a lithium ion battery.

(Example 1) 35 parts by volume of hydrotalcite (DHT-4A, manufactured by Kyowa Chemical Co., Ltd.) having an average particle diameter of 0.4 μm and a polypropylene resin (FS201, manufactured by Sumitomo Chemical Co., Ltd.)
1D) After kneading 65 parts by volume with a biaxial kneader (L / D = 60, manufactured by Plastics Engineering Laboratory), it was processed into a primary film having a thickness of about 60 μm by a T-die. The obtained primary film was subjected to a strain rate of 286% /
The film was stretched about 4 times at a stretching temperature of 100 ° C. for 2 seconds to obtain a porous film having a thickness of 31 μm. The Gurley value of the obtained porous film was measured, and the performance as a battery separator (load characteristic evaluation) was evaluated. The results are shown in Tables 1 and 2.

Example 2 The primary film produced in Example 1 was stretched about 5 times at a stretching rate of 130 ° C. and a strain rate of 10% / sec by a tenter stretching machine. The Gurley value of the obtained porous film was measured, and the performance as a battery separator (load characteristic evaluation) was evaluated. Tables 1 and 2 show the results.
Shown in

Comparative Example 1 Polymethyl methacrylate beads having an average particle size of 1 to 2 μm (Nippon Shokubai, Eposter M)
A1001) 30 parts by volume and 70 parts by volume of a polypropylene resin (FS2011D manufactured by Sumitomo Chemical Co., Ltd.) are kneaded by a twin-screw kneader (L / D = 60, manufactured by Plastics Engineering Laboratory), and then a primary die having a thickness of about 100 μm is formed by a T-die. Processed into film. The obtained primary film was stretched about 6 times by using a roll stretching machine at a strain rate of 1250% / sec and a stretching temperature of 120 ° C. Measurement of Gurley value of the obtained porous film and evaluation of performance as a battery separator (load characteristic evaluation)
Was done. The results are shown in Tables 1 and 2.

Comparative Example 2 The primary film prepared in Example 1 was stretched about 4 times at a stretching rate of 1250% / sec and a stretching temperature of 100 ° C. using a roll stretching machine. The Gurley value of the obtained porous film was measured, and the performance as a battery separator (load characteristic evaluation) was evaluated. The results are shown in Tables 1 and 2.

[0043]

[Table 1]

[0044]

[Table 2] "-" In the column of discharge capacity and load characteristics indicates that the internal resistance was too high to evaluate.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) B29K 105: 04 B29K 105: 04 105: 16 105: 16 B29L 7:00 B29L 7:00 C08L 23:00 C08L 23:00 (72) Inventor Takeshi Yamada 2-10-1, Tsukahara, Takatsuki-shi, Osaka F-term in Sumika Plastics Co., Ltd. AA90 CA01 CC02 DA43 DA49 4F210 AA03 AA11 AB11 AB16 AG01 AG20 AH33 AR08 QC02 QC06 QG01 QG18 5H021 BB05 CC08 EE02 EE04 EE31 HH00 HH01 HH03 HH10

Claims (3)

[Claims] 1. A primary film is prepared from a resin composition comprising a thermoplastic resin and a filler having an average particle diameter of 15 to 85 parts by volume per 100 parts by volume of the thermoplastic resin and having a mean particle size of 1 μm or less. 1 to 300% in one direction
A method for producing a porous film, wherein the film is stretched at a strain rate of / sec. 2. The method according to claim 1, wherein the thermoplastic resin is a polyolefin resin. 3. A polyolefin resin having a molecular chain length of 28.
The method according to claim 2, wherein the polyolefin having a thickness of 50 nm or more is contained in an amount of 10% by weight or more.