JP2010265414A - Microporous film, method for producing the same and battery separator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microporous film excellent in resistance to film-breakdown, having high film-breakdown temperature; and a battery separator using the film. <P>SOLUTION: This microporous film comprises a polypropylene resin composition having 500,000-750,000 weight-average molecular weight, and has air permeability of 10 to 5,000 sec/100 cc and elongation viscosity of 30,000 to 60,000 Pa s. The method of production includes a cold-stretch step of stretching the film comprising the polypropylene resin composition at a temperature not lower than -20°C and lower than 90°C; and a hot-stretch step of stretching the resultant film at a temperature not lower than 90°C and lower than 150°C. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a microporous film, a method for producing the same, and a battery separator.

  Microporous films, particularly polyolefin-based microporous films, are used in microfiltration membranes, battery separators, capacitor separators, fuel cell materials, and the like, and are particularly suitably used as lithium ion battery separators. . In recent years, lithium ion batteries have been used for small electronic devices such as mobile phones and notebook personal computers, while being applied to hybrid electric vehicles and the like.

  The battery separator provided in the lithium ion battery is required to have a shutdown function in order to ensure safety. The shutdown function is a function of stopping the battery reaction and preventing further temperature increase by rapidly increasing the electric resistance of the battery separator when the temperature inside the battery rises excessively. As an expression mechanism of the shutdown function, for example, in the case of a battery separator made of a microporous film, when the internal temperature of the battery rises to a predetermined temperature, the porous structure is lost to make it nonporous, and ion permeation is blocked. Can be mentioned. However, even if the pores are made non-porous in this way and ion permeation is blocked, if the temperature further rises and the entire film melts and breaks, the electrical insulation cannot be maintained. The temperature at which the film cannot maintain its shape and cannot block ion permeation is called the membrane breaking temperature. The higher the membrane breaking temperature, the better the battery separator. Moreover, it can be said that the greater the difference between the film breaking temperature and the temperature at which shutdown starts, the better the safety.

For the purpose of providing a microporous film serving as a separator that can cope with such circumstances, for example, Patent Document 1 discloses a laminated structure in which a polypropylene microporous film is laminated on a conventional polyethylene microporous film. A composite microporous film (battery separator) has been proposed.
Patent Document 2 discloses a laminated microporous film formed from polypropylene having a specific weight average molecular weight.

Japanese Patent No. 3003830 Japanese Patent No. 3939778

However, any of the microporous films described in Patent Documents 1 and 2 still has room for improvement from the viewpoint of achieving a high film breaking temperature.
This invention is made | formed in view of such a situation, and makes it a subject to provide the microporous film excellent in the tear resistance which has a high tear temperature, and the battery separator using the same. .

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a microporous film having a specific elongational viscosity is difficult to break even at high temperatures. As a result, the inventors have found that a microporous film having a high film breaking temperature and having a film breaking resistance suitable as a separator for a lithium ion secondary battery can be obtained, and the present invention is completed. It came to.

That is, the present invention is as follows.
[1]
A microporous film comprising a polypropylene resin composition, having an air permeability of 10 seconds / 100 cc to 5000 seconds / 100 cc, and an elongational viscosity of 30000 Pa · s to 60000 Pa · s.
[2]
The microporous film according to the above [1], wherein the polypropylene resin composition has a weight average molecular weight of 500,000 to 750,000.
[3]
A battery separator comprising the microporous film according to the above [1] or [2].
[4]
A method for producing a microporous film according to the above [1] or [2], comprising the following steps (A) and (B):
(A) a cold drawing step of drawing a film comprising a polypropylene resin composition at a temperature of -20 ° C or higher and lower than 90 ° C;
(B) A hot stretching process in which the film stretched in the cold stretching process is stretched at a temperature of 90 ° C. or higher and lower than 150 ° C.

  According to the present invention, it is possible to provide a microporous film having a high film breaking temperature and excellent film breaking resistance, and a battery separator using the same.

  Hereinafter, a mode for carrying out the present invention (hereinafter simply referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiment, and can be implemented with various modifications within the scope of the gist.

  The microporous film of the present embodiment is made of a polypropylene resin composition (hereinafter referred to as “polypropylene resin composition Ac”), and has an air permeability of 10 to 5000 seconds / 100 cc and an extensional viscosity of 30000 to 60000 Pa · s. It has been adjusted.

The polypropylene resin composition Ac is a concept that includes only a polypropylene resin, and may be a mixture of a polypropylene resin and another resin, and may further contain any additive.
The “polypropylene resin” refers to a polymer whose main component is propylene. Here, the “main component” means a monomer occupying 50% by mass or more with respect to the total amount of monomers constituting the polypropylene resin, preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably. It means a monomer showing 95% or more, still more preferably 98% or more, particularly preferably 100% by mass (ie the total amount).

  The polypropylene resin of this embodiment is a polymer containing propylene as a monomer component, and may be a homopolymer or a copolymer. From the viewpoint of air permeability and membrane breaking temperature, a homopolymer is preferable. When it is a copolymer, it may be a random copolymer or a block copolymer. In the case of a copolymer, the copolymerization component is not particularly limited, and examples thereof include ethylene, butene, hexene and the like.

  When the polypropylene resin is a copolymer, the copolymerization ratio of propylene is 50% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more.

  A polypropylene resin can be used 1 type or in mixture of 2 or more types. The polymerization catalyst is not particularly limited, and examples thereof include a Ziegler-Natta catalyst and a metallocene catalyst. Further, there is no particular limitation on the stereoregularity, and isotactic polypropylene or syndiotactic polypropylene can be used.

  Examples of the other resins include polyethylene resins and olefin elastomer resins. Examples of the polyethylene resin include a high density polyethylene resin, a low density polyethylene resin, and a linear low density polyethylene resin. These can be used alone or in combination of two or more.

  The blending ratio of the polypropylene resin and the other resin is preferably 20% by mass or less, more preferably 10% by mass as the ratio of the other resin in the total amount of the polypropylene resin and the other resin. % Or less, more preferably 5% by mass or less, or 0% by mass.

  In addition to the above components, the polypropylene resin composition Ac of the present embodiment may contain other additional components as necessary within the range that does not impair the features and effects of the present invention, for example, an antioxidant, metal deactivation. Agent, heat stabilizer, flame retardant (organophosphate ester compound, ammonium polyphosphate compound, aromatic halogen flame retardant, silicone flame retardant, etc.), fluorine polymer, plasticizer (low molecular weight polyethylene, large epoxidation) Bean oil, polyethylene glycol, fatty acid esters, etc.), flame retardant aids such as antimony trioxide, weathering (light) improvers, polyolefin nucleating agents, slip agents, inorganic or organic fillers and reinforcing materials (polyacrylonitrile) Fiber, carbon black, titanium oxide, calcium carbonate, conductive metal fiber, conductive carbon black, etc.), various colorants, release agents, etc. It may be. The total content of these additional components is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and still more preferably 5 parts by mass or less with respect to 100 parts by mass of the polypropylene resin composition Ac. is there.

  The MFR (melt flow rate) of the polypropylene resin composition Ac of the present embodiment is preferably 0.01 to 20 g / 10 minutes, more preferably 0.1 to 10 g / 10 minutes, and still more preferably 0. .3 to 1.0 g / 10 min. If the MFR is 0.01 g / 10 min or more, the elongation at the time of melting tends to be high and the film formability tends to be good, and if it is 20 g / 10 min or less, drawdown hardly occurs, and the film formability is high. Tends to be good. MFR of polypropylene resin composition Ac is measured according to the method described in the following Example.

  Although it does not specifically limit as a manufacturing method of the microporous film of this embodiment, (A) The film (henceforth "raw film Af") consisting of polypropylene resin composition Ac is -20 degreeC or more and less than 90 degreeC. It is preferable to include a cold stretching step of stretching at a temperature of (B) and a thermal stretching step of stretching the film stretched in the cold stretching step at a temperature of 90 ° C. or higher and lower than 150 ° C.

  As a method for producing the raw film Af made of the polypropylene resin composition Ac of the present embodiment, a sheet molding method such as T-die extrusion molding, inflation molding, calendar molding, and Skyf's method can be employed. Among these, T-die extrusion molding is preferable from the viewpoint of physical properties and applications required for the microporous film of the present embodiment.

  In the manufacturing method of the raw film Af, the draw ratio after extrusion, that is, the film winding speed (unit: m / min) is the extrusion speed of the polypropylene resin composition Ac (the line in the flow direction of the molten resin passing through the die lip). The value divided by the speed (unit: m / min) is preferably 10 to 500, more preferably 100 to 400, and still more preferably 150 to 350. Moreover, the winding speed of the film when winding the raw film Af is preferably about 2 to 400 m / min, more preferably 10 to 200 m / min. Setting the draw ratio in the above range is preferable from the viewpoint of improving the air permeability of the obtained microporous film.

  The raw film Af is preferably subjected to heat treatment (annealing) as necessary. As the annealing method, for example, a method of bringing the original film Af into contact with a heated roll, a method of exposing it to a heated gas phase before winding, a method of winding the original film Af on a core body, and a heated gas phase or heating liquid The method of exposing in a phase and the method of combining these are mentioned. The annealing conditions are preferably, for example, annealing at a heating temperature of 100 ° C. to 150 ° C. for 10 seconds to 100 hours. If the heating temperature is 100 ° C. or higher, the air permeability of the microporous film obtained later tends to be further improved, and if it is 150 ° C. or lower, the raw film Af is annealed in a state wound on the core. However, the films tend not to be fused. A more preferable heating temperature range is 120 ° C to 140 ° C.

  In the cold stretching step, the raw film Af made of the polypropylene resin composition Ac is cold stretched in at least one direction, preferably 1.05 times to 2.0 times, in a state where the original film Af is held at −20 ° C. or higher and lower than 90 ° C. To do.

  The stretching temperature for cold stretching in the cold stretching step is -20 ° C or higher and lower than 90 ° C, preferably 0 ° C or higher and 50 ° C or lower. If it extends | stretches above -20 degreeC, it will become the tendency for the original film Af to become difficult to fracture | rupture, and if it extends below 90 degreeC, it will become the tendency for the air permeability of the microporous film obtained to become more favorable. Here, the stretching temperature for cold stretching refers to the surface temperature of the film in the cold stretching step.

  The draw ratio of cold drawing in the cold drawing step is preferably 1.05 times or more and 2.0 times or less, more preferably 1.2 times or more and 1.7 times or less. When the draw ratio is 1.05 times or more, a microporous film having good air permeability tends to be obtained, and when it is 2.0 times or less, a microporous film having a uniform film thickness tends to be obtained. It is in. The cold stretch of the raw film Af may be performed in at least one direction and may be performed in two directions. Preferably, the original film Af is uniaxially stretched only in the film extrusion direction (hereinafter referred to as “MD direction”).

  In the manufacturing method of the present embodiment, in the cold stretching step, the raw film Af is preferably uniaxially stretched 1.1 to 2.0 times in the MD direction at a temperature of 0 ° C. or higher and 70 ° C. or lower.

Next, the heat stretching process will be described.
The method for producing a microporous film in this embodiment is preferably 1.05 times or more and 5.0 times in at least one direction in a state where the film stretched in the cold stretching step is held at 90 ° C. or higher and lower than 150 ° C. The film is hot stretched below.

  The stretching temperature for hot stretching is not particularly limited as long as it is higher than the stretching temperature for cold stretching. Further, the stretching temperature of the thermal stretching is a temperature of 90 ° C. or higher and lower than 150 ° C., preferably 110 ° C. or higher and 140 ° C. or lower. If it is hot-stretched at 90 ° C. or higher, the film is difficult to break, and if it is heat-stretched below 150 ° C., the air permeability of the microporous film obtained is improved. Here, the stretching temperature of the heat stretching indicates the surface temperature of the film in the heat stretching step.

  The draw ratio of the hot drawing in the hot drawing step is preferably 1.05 to 5.0, more preferably 1.1 to 5.0, and still more preferably 2.0 to 4. 0 times or less. When the draw ratio in the heat drawing step is 1.05 times or more, a microporous film having good air permeability tends to be obtained, and when it is 5.0 times or less, the film thickness is uniform. Tends to be obtained. The hot stretching may be performed in at least one direction and may be performed in two directions, but is preferably performed in the same direction as the cold stretching direction, more preferably uniaxial stretching only in the same direction as the cold stretching direction. Do.

  In the manufacturing method of the present embodiment, in the hot stretching process, the film that has been cold-drawn through the cold-drawing process is uniaxial in the MD direction at 2.0 times to 5.0 times at a temperature of 90 ° C. or higher and lower than 150 ° C. It is preferable to stretch.

  The manufacturing method of the microporous film of the present embodiment includes a two-stage stretching process including a cold stretching process and a hot stretching process from the viewpoint of good air permeability and use required for the microporous film. When the manufacturing method of a microporous film is a method which performs an extending | stretching process in 1 step, the obtained microporous film becomes difficult to satisfy | fill the required favorable air permeability. In addition, the manufacturing method of the microporous film of this embodiment may include the further extending process in addition to each above-mentioned extending process.

  The method for producing a microporous film according to the present embodiment includes a heat setting step in which heat setting is preferably performed at a temperature of 100 ° C. or higher and 150 ° C. or lower with respect to the microporous film obtained through the heat stretching step. Is preferred. This heat setting method includes a method of heat shrinking to such an extent that the length of the microporous film after heat setting is reduced by 3 to 50% with respect to the length of the microporous film before heat setting (hereinafter referred to as this method). The method is referred to as “relaxation”), and heat setting is performed so that the dimension in the stretching direction does not change.

  The heat setting temperature is preferably 100 ° C. or higher and 150 ° C. or lower, and more preferably 130 ° C. or higher and 140 ° C. or lower. Here, the heat setting temperature indicates the surface temperature of the microporous film in the heat setting process.

  In each step of the cold drawing step, the hot drawing step, other drawing steps and the heat setting step in the method for producing the microporous film of the present embodiment, the drawing or heat setting is performed in one step by a roll, a tenter, an autograph, or the like. Or it can carry out to a uniaxial direction and / or a biaxial direction in two steps or more. In particular, from the viewpoint of physical properties and applications such as air permeability and porosity required for the obtained microporous film, it is preferable to perform uniaxial stretching / fixing in two or more stages with a roll in at least one step.

  Next, the physical property of the microporous film in this embodiment is demonstrated.

  The elongational viscosity of the microporous film of this embodiment is 30,000 to 60000 Pa · s, preferably 40000 to 50000 Pa · s. When the elongational viscosity is 30000 Pa · s or more, the film breaking temperature of the microporous film is good, and when it is 60000 Pa · s or less, it is difficult to break during film forming.

The elongational viscosity of the microporous film is a value measured under conditions of a temperature of 200 ° C. and an elongation strain rate of 10 s ^−1. The inflow pressure loss method using a twin capillary rheometer is used, and Cogswell theory [Polymer Engineering Science, 12 , 64 (1972)].

  The inventors of the present invention have found that the membrane breaking temperature is remarkably improved by setting the elongation viscosity of the microporous film within the above range. This factor has not been fully elucidated. In the prior art, in order to improve the film breaking temperature, it was considered necessary to use a resin having a higher melting point. However, the present inventor has found that even if the resins have the same melting point, the film breaking temperature of the microporous film can be dramatically improved by setting the elongational viscosity within the above range.

  In addition, as a method for increasing the extensional viscosity of the microporous film, for example, a method using a polypropylene resin composition containing a long chain branch, or melt kneading the polypropylene resin composition with a crosslinking agent such as an organic peroxide, A method of appropriately crosslinking is mentioned.

  It is preferable that the weight average molecular weight (Mw) of the polypropylene resin composition of this embodiment is 300,000-2 million, More preferably, it is 500,000-1 million, More preferably, it is 500,000-750,000. If Mw is 300,000 or more, the membrane breaking temperature of the microporous film tends to be improved, and if it is 2 million or less, the film formability of the microporous film tends to be good. If Mw is 500,000-750,000, the thermal shrinkage rate of the microporous film tends to be lower. When the thermal shrinkage rate is low, when a microporous film is used as a battery separator, even if the battery temperature rises abnormally, the separator does not easily shrink, and there is a risk that the positive and negative electrodes come into contact and short-circuit. It tends to decrease.

  The molecular weight distribution of the polypropylene resin composition is a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (hereinafter referred to as “Mw / Mn”) and is 2.0 to 20.0. More preferably, it is 3.0-10.0, More preferably, it is 5.0-7.0. If Mw / Mn is 2.0 or more, heat generation during molding of the polypropylene resin composition Ac is suppressed, and resin degradation tends to hardly occur. If it is 20.0 or less, it is derived from a high molecular weight component. There is a tendency for unmelted material to decrease. Mw and Mn are obtained from gel permeation chromatography (hereinafter referred to as “GPC”) of a microporous film using polystyrene as a standard sample, and measured in detail according to the method described in the following examples. Is done.

  The porosity of the microporous film of this embodiment is preferably 20% to 80%, more preferably 30% to 70%, and still more preferably 40% to 60%. By setting the porosity to 20% or more, when a microporous film is used as a battery separator, sufficient ion permeability tends to be ensured. On the other hand, by setting the porosity to 80% or less, the microporous film tends to ensure sufficient mechanical strength.

The porosity of the microporous film can be adjusted to the above range by appropriately setting the composition of the polypropylene resin composition Ac, the stretching temperature in each stretching step, the stretching ratio, and the like. For example, in order to increase the porosity, the draw ratio at the time of forming the raw film Af may be increased, or the stretch ratio may be increased. Moreover, the porosity of a microporous film cuts out the sample of 10 cm x 10 cm square from the film, the volume V (cm < ³ >) and mass M (g) of the sample, and the density of resin composition Ac which comprises a film is calculated using the following formula from the d (g / cm ^3).
Porosity (%) = {(VM−d / V) × 100

  The air permeability of the microporous film is 10 seconds / 100 cc to 5000 seconds / 100 cc, preferably 50 seconds / 100 cc to 1000 seconds / 100 cc, more preferably 100 seconds / 100 cc to 300 seconds / 100 cc. . When the air permeability is 5000 seconds / 100 cc or less, the microporous film can ensure sufficient ion permeability. On the other hand, when the air permeability is 10 seconds / 100 cc or more, a more uniform microporous film having no defects can be obtained.

  The air permeability of the microporous film can be adjusted to the above range by appropriately setting the composition of the polypropylene resin composition Ac, the stretching temperature in each stretching step, the stretching ratio, and the like. For example, in order to increase the air permeability, the stretch ratio may be increased or the relaxation ratio in heat setting may be decreased. The air permeability of the microporous film is measured using a Gurley type air permeability meter according to JIS P-8117.

  The film thickness of the microporous film is preferably 5 to 40 μm, more preferably 10 to 30 μm.

  The microporous film in this embodiment is suitably used as a battery separator, more specifically as a lithium ion secondary battery separator. The battery separator has a known configuration except that it includes the microporous film of the present embodiment, and may be manufactured by a known method. In addition, the microporous film of this embodiment is also used as various separation membranes.

  Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist. In addition, the evaluation method of various characteristics is as follows.

(1) MFR
MFR is synonymous with melt index, and the MFR of the polypropylene resin composition was measured under conditions of a temperature of 230 ° C. and a load of 2.16 kg in accordance with JIS K7210. The unit of MFR is g / 10 minutes.

(2) Melting point The melting point of the polypropylene resin composition was measured by a method based on JIS K-7121. The unit of melting point is ° C.

(3) Molecular weight, molecular weight distribution (Mw / Mn)
The molecular weight distribution of the resin in the polypropylene resin composition is a value of the ratio Mw / Mn of the weight average molecular weight (Mw) and the number average molecular weight (Mn) obtained from gel permeation chromatography (GPC). The GPC measurement was performed using a GPS device (trade name “HLC-8121GPC / HT”) manufactured by Tosoh Corporation. Using the trade name “TSKgel GMHHR-H (20)” (two) manufactured by Tosoh Corporation as the column, mobile phase o-dichlorobenzene (o-DCB), column temperature 155 ° C., flow rate 1.0 mL / min, sample concentration The test was performed under the conditions of 0.5 mg / mL (o-DCB), injection amount 500 μL, sample dissolution temperature 160 ° C., and sample dissolution time 3 hours. The molecular weight was calibrated with polystyrene, and Mw and Mn were obtained from the polystyrene-equivalent molecular weight to derive the molecular weight distribution.

(4) Elongation Viscosity The elongation viscosity of the microporous film was obtained by measuring according to Cogswell's theory [Polymer Engineering Science, 12, 64 (1972)] using the inflow pressure loss method. As a measuring apparatus, a twin capillary rheometer manufactured by Rosand was used, and for the orifice, the following long die and short die were used, and measurement was performed under the conditions of a temperature of 200 ° C. and an elongation strain rate of 10 s ⁻¹ .
Long die: length 16mm, diameter 1mm, inflow angle 180 °
Short die: Length 0.25mm, diameter 1mm, inflow angle 180 °

(5) Film thickness (μm)
The film thickness of the microporous film was measured using a dial gauge (manufactured by Ozaki Seisakusho, trade name “PEACOCK No. 25”).

(6) Porosity (%)
The porosity of the microporous film was determined by cutting out a 10 cm × 10 cm square sample from the microporous film, the volume V (cm ³ ) and mass M (g) of the sample, and the density d of the resin composition constituting the film. It calculated using the following formula from (g / cm ³ ).
Porosity (%) = {(VM−d / V) × 100

(7) Air permeability (sec / 100cc)
The air permeability of the microporous film was measured with a Gurley type air permeability meter based on JIS P-8117. In addition, the value converted into the value when the film thickness of a microporous film was 20 micrometers was made into the air permeability of the microporous film.

(8) Thermal Shrinkage A 12 cm × 12 cm square sample was cut out from the film, two marks were made at 10 cm intervals in the MD direction of the sample, and the sample was sandwiched between papers and allowed to stand still in an oven at 100 ° C. for 60 minutes. I put it. After removing the sample from the oven and cooling, the length (cm) between the marks was measured, and the thermal shrinkage in the MD direction was calculated by the following formula.
Thermal contraction rate (%) = (10−length between marks after heating (cm)) / 10 × 100

(9) Shutdown temperature A microporous film was attached to a 40 mm square holder in a constrained state and left in a hot air circulation oven set at 120 ° C. for 60 minutes. Next, the sample was taken out of the sample oven, cooled to 20 ° C. in a restrained state, and the air permeability was measured. If the air permeability is 10000 seconds / 100 cc or less, a new microporous film is prepared, the temperature of the oven is increased by 5 ° C., the same test is performed, and the air permeability becomes 10000 seconds / 100 cc or more. Was the shutdown temperature.

(10) Film breakage temperature A microporous film was attached to a 40 mm square holder in a constrained state and left in a hot air circulation oven set at 170 ° C. for 60 minutes. Subsequently, it was taken out from the sample oven and it was judged by visual observation whether the film was torn. If the membrane was not torn, a new microporous film was prepared, the oven temperature was raised by 5 ° C., the same test was performed, and the temperature at which the membrane was broken was defined as the membrane breaking temperature.

In addition, the used polypropylene resin composition is as follows.
(Ac-1) E111G made of propylene homopolymer and prime polymer
(Ac-2) Propylene homopolymer containing long chain branch, SH9000 manufactured by Nippon Polypro
(Ac-3) F113G made of propylene homopolymer and prime polymer
(Ac-4) Propylene homopolymer containing long chain branches, PF814 made by Sun Allomer

[Example 1]
As shown in “Composition of the polypropylene resin composition” in Table 1, 80 parts by mass of the polypropylene resin composition (Ac-1) and 20 parts by mass of the polypropylene resin composition (Ac-3) were mixed, and two screws having a screw diameter of 40 mm were mixed. Using a shaft extruder, extrusion was performed while kneading under conditions of a cylinder temperature of 200 ° C. and an extrusion rate of 40 kg / hour to obtain a polypropylene resin composition (Ac-5). About the obtained polypropylene resin composition (Ac-5), MFR and melting | fusing point were measured. The results are shown in Table 1.

  The polypropylene resin composition (Ac-5) was introduced into a single-screw extruder set at 220 ° C. having a diameter of 20 mm and L / D = 30 via a feeder, and a lip thickness of 4.0 mm set at the tip of the extruder. Extruded from the die. The extruded polypropylene resin composition (Ac-5) after extrusion was immediately applied with cold air of 25 ° C., and then wound with a cast roll cooled to 95 ° C. under a draw ratio of 200 and a winding speed of 15 m / min. An original film (Af-1) was obtained.

The obtained raw film (Af-1) was annealed at a temperature of 130 ° C. for 3 hours in the state of being wound on the core, cooled to 25 ° C., and 1.3 ° in the MD direction at a temperature of 25 ° C. Uniaxially stretched twice (cold stretching step), then uniaxially stretched 2.5 times in the MD direction (thermal stretching step) at a temperature of 120 ° C, and further relaxed 0.9 times at a temperature of 145 ° C. Then, heat setting was performed to obtain a microporous film.
The obtained microporous film was measured for molecular weight, molecular weight distribution, elongational viscosity, film thickness, porosity, air permeability, shutdown temperature, membrane breaking temperature, and heat shrinkage rate. The results are shown in Table 1.

[Examples 2-8, Comparative Examples 1-4]
A microporous film was obtained in the same manner as in Example 1 except for the conditions described in Table 1 (for Comparative Example 2, a microporous film was not obtained).
Various physical properties of the obtained microporous film were evaluated. The results are shown in Table 1.

As is clear from the results in Table 1, all of the microporous films of Examples 1 to 8 exhibited a high film breaking temperature and good air permeability (low air permeability). In addition, the difference between the shutdown temperature and the film breaking temperature was also large.
On the other hand, Comparative Example 1, Comparative Example 3, and Comparative Example 4 having an extensional viscosity smaller than 30000 Pa · s had a low membrane breaking temperature, and the difference between the shutdown temperature and the membrane breaking temperature was also small. Further, in Comparative Example 2 having an extensional viscosity exceeding 60000 Pa · s, it was difficult to form a raw film, and a microporous film could not be obtained.

  The microporous film of the present invention has industrial applicability as a battery separator, more specifically as a lithium secondary battery separator.

Claims (4)

  A microporous film comprising a polypropylene resin composition, having an air permeability of 10 seconds / 100 cc to 5000 seconds / 100 cc, and an elongational viscosity of 30000 Pa · s to 60000 Pa · s.   The microporous film according to claim 1, wherein the polypropylene resin composition has a weight average molecular weight of 500,000 to 750,000.   A battery separator comprising the microporous film according to claim 1. It is a manufacturing method of the microporous film of Claim 1 or 2, Comprising: The manufacturing method of a microporous film including each process of the following (A) and (B):
(A) a cold drawing step of drawing a film comprising a polypropylene resin composition at a temperature of -20 ° C or higher and lower than 90 ° C;
(B) A hot stretching process in which the film stretched in the cold stretching process is stretched at a temperature of 90 ° C. or higher and lower than 150 ° C.