JP2012072380A - Porous polypropylene film and electricity storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous polypropylene film having good thermal dimensional stability, and exhibiting excellent characteristics when used as a separator.SOLUTION: The porous polypropylene film is obtained by using a polypropylene resin having β-crystal formability of 40-90%, and the thermal shrinkage rate at 135°C for 60 min in either one direction of longitudinal and width directions of the film is 0-10%.

Description

  The present invention relates to a porous polypropylene film. More particularly, the present invention relates to a highly safe and high-power porous polypropylene film that can be suitably used for a non-aqueous solvent battery or a separator used in a capacitor.

  A non-aqueous solvent battery such as a lithium battery or a lithium ion battery has a problem that the electrolytic solution used is an organic solvent and is inferior in safety against heat generation of the battery as compared with an aqueous solvent of an aqueous battery. Therefore, conventionally, in order to improve the safety of non-aqueous solvent batteries, particularly lithium ion batteries having a large energy density, a separator using a microporous porous film of an olefin-based material mainly composed of polyethylene has been used. Polyethylene is mainly used because polyethylene can be used in an organic solvent, and when the battery abnormally generates heat due to a short circuit, etc., the polyethylene melts at an appropriate temperature (around 130 ° C) and the porous structure is blocked. This is because safety can be ensured by doing (shutdown).

  However, in recent years, large batteries such as hybrid vehicle (HEV) batteries and tool batteries have been increased in output and rapidly rise to temperatures higher than 130 ° C. The function of shutting down is not always required, and heat resistance is required. Further, when used as a battery for HEV in a high temperature region, the battery ambient temperature may rise to 135 ° C., it is important that the thermal shrinkage rate at a high temperature of 135 ° C. is low and the dimensional stability is good. I came.

  Further, in order to increase the output of the battery and to use it for a lithium ion capacitor, it is required that the average pore diameter of the separator alone is large in order to improve the ionic conductivity. Further, it is important for HEV batteries to be able to guarantee a long life of 10 years and more stringent safety. Since the film of the present invention has high safety and high output as a separator, it can be suitably used as a separator for large-sized lithium ion batteries such as electric vehicles and distributed power supplies.

  In Patent Document 1, production of a microporous membrane containing an ultrahigh molecular weight polyolefin and having excellent thermal dimensional stability and a large average pore diameter by adjusting the ratio of elastic modulus in the length direction and the width direction to a specific range. A method is disclosed. However, the shrinkage rate at a high temperature such as 135 ° C. is large, and it cannot be said that the dimensional stability is sufficient. Furthermore, in this method, since the solvent contained in the entire porous membrane is removed by washing with an organic solvent for washing, a large amount of organic solvent is required, which is not preferable from an environmental viewpoint.

  Patent Document 2 discloses a method for producing a microporous film having excellent thermal dimensional stability by laminating a polyethylene resin and a polypropylene resin. However, since a polyethylene resin is used as a matrix, if the temperature exceeds 120 ° C., the film itself melts and the pores are blocked, so that there is a problem that usable applications are limited.

  In patent document 3, the proposal which uses the polypropylene nonwoven fabric excellent in a thermal dimensional stability and suitable for high output uses like a large sized battery for a separator is made. However, in this case, since the base material is a non-woven fabric composed of fibers, it has a large average pore diameter of about several μm, suggesting that a fine short circuit is likely to occur. Such long life and even more stringent safety requirements cannot be fully compensated. Furthermore, as long as the nonwoven fabric is used, the film thickness becomes large and the volume increase is inevitable, and there is also a problem that goes against the trend of the era of battery miniaturization and weight reduction.

  Patent Document 4 discloses a method for producing a microporous membrane having excellent thermal dimensional stability by coating particles on the film surface. However, in the method of coating particles on the film surface, when particles are dropped from the inside during handling of the film, or particles are released in the electrolyte during use as a separator and move onto the electrode. And there was a possibility of insulation. In addition to the cost of the particles themselves, there is a problem that the film manufacturing process becomes complicated and the cost increases.

  Patent Document 5 discloses a method for producing a microporous membrane having excellent thermal dimensional stability and high porosity by adjusting the transverse stretching temperature, heat setting temperature, and relaxation rate to specific ranges. However, the shrinkage rate at a high temperature such as 135 ° C. is large, and it cannot be said that the dimensional stability is sufficient.

  In patent document 6, the manufacturing method of the microporous film which is excellent in thermal dimensional stability is disclosed by heat-processing a porous film roll in high temperature oven. However, the shrinkage rate at a high temperature such as 135 ° C. is large, and it cannot be said that the dimensional stability is sufficient.

  Patent Document 7 discloses a method for producing a microporous membrane having excellent heat resistance and cycle characteristics by combining a film-like separator and a ceramic separator, and adjusting the average roughness of the film surface to a predetermined range. Yes. However, in this case, since the ceramic material is contained, the weight is increased, and there is also a problem that goes against the trend of the era of battery miniaturization and weight reduction. In addition to the cost of the ceramic material itself, there is a problem that the film manufacturing process becomes complicated and the cost increases.

  As described so far, there has been known a method for increasing the output characteristics by increasing the average pore diameter or setting the surface roughness of the film within a predetermined range. In addition, a technique for improving heat resistance by a technique such as the inclusion of a heat-resistant resin or inorganic particles or off-annealing has been known. However, this is a microporous polypropylene film that is substantially free of heat-resistant resin and inorganic particles, meets the requirements for battery size reduction and weight reduction, and has both output characteristics and heat resistance at 135 ° C. Did not exist until.

JP 2010-7053 A JP 2010-111095 A JP-A-60-52 JP 2008-123996 A JP 2008-248231 A JP 2010-1000084 A JP 2009-238752 A

  An object of the present invention is to solve the above-described problems. That is, the present invention has an object of providing a porous polypropylene film having high safety and high output when used as a separator because it has an appropriate pore size while having good thermal dimensional stability. .

  The above-mentioned problem is that the β-crystal-forming ability is 40 to 90% polypropylene resin, and the heat shrinkage rate at 135 ° C. for 60 minutes in any one of the longitudinal direction and the width direction of the film is 0 to 10%. This can be achieved with certain porous polypropylene films.

The present invention can provide a porous polypropylene film having high safety and high output when used as a separator because the through-hole has an appropriate hole diameter while having good thermal dimensional stability. In addition, since the electricity storage device using the film of the present invention as a separator can be suitably used as a battery for a hybrid car or an electric vehicle, the use of gasoline or the like can be suppressed, and thus the emission of CO ₂ can be suppressed. Is possible.

  The porous polypropylene film of the present invention will be described below. Polypropylene is a polymer compound obtained by addition polymerization of propylene. The porous polypropylene film of the present invention is preferably composed mainly of a polypropylene resin. Here, having a polypropylene resin as a main component means that the proportion of the polypropylene resin component in the total resin component is preferably 50% by mass or more, more preferably 75% by mass or more, and still more preferably 95% by mass. That's it. In addition, as long as the object of the present invention is not impaired, a random copolymer or a block copolymer obtained by copolymerizing propylene with a monomer other than propylene may be used, or the copolymer may be blended with polypropylene. Good. Examples of the monomer constituting such a copolymer component or blend include, for example, ethylene, 1-butene, 1-pentene, 3-methyl-1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 5-ethyl-1-hexene, 1-octene, 1-decene, 1-dodecene, 1-heptene, 1-nonene, vinylcyclohexene, styrene, allylbenzene, cyclopentene, norbornene, 5- Examples include, but are not limited to, methyl-2-norbornene, acrylic acid, and derivatives thereof. In the laminated film, the main component of at least one layer is preferably a polypropylene resin, and the main component of the laminated film as a whole is more preferably a polypropylene resin.

  For the polypropylene resin used in the present invention, it is preferable to use an isotactic polypropylene resin having a melt flow rate (hereinafter referred to as MFR) of 2 to 30 g / 10 min from the viewpoint of extrusion moldability and uniform formation of pores. . Here, MFR is an index indicating the melt viscosity of a resin defined in JIS K 7210 (1995), and is a physical property value indicating the characteristics of a polyolefin resin. In the present invention, it refers to a value measured at 230 ° C. and 2.16 kg.

  In the present invention, the isotactic index of the polypropylene resin is preferably in the range of 90 to 99.9%. When the isotactic index is less than 90%, the crystallinity of the resin is lowered, and the film forming property may be deteriorated or the strength of the film may be insufficient. The porous polypropylene film of the present invention has a plurality of through holes that penetrate both surfaces of the film and have air permeability.

  As a method for forming the through-hole in the film, either a wet method or a dry method may be used, but a dry method is desirable because the process can be simplified.

  The porous polypropylene film of the present invention preferably has a heat shrinkage rate of 0 to 10% at 135 ° C. for 60 minutes in any one of the longitudinal direction and the width direction of the film. It is more preferably 0 to 8%, and further preferably 0 to 5%. When the thermal shrinkage rate at 135 ° C. for 60 minutes exceeds 10% in both the longitudinal direction and the width direction, when the porous polypropylene film is used as a lithium battery separator, the separator shrinks due to the heat generated by the battery. A short circuit may occur at the end.

  The porous polypropylene film of the present invention preferably has a thermal shrinkage of 0 to 10% at 135 ° C. for 60 minutes in both the longitudinal direction and the width direction of the film. It is more preferably 0 to 8%, and further preferably 0 to 5%. When used as a separator for a wound battery, even if the thermal contraction rate in the longitudinal direction of the film exceeds 10%, it is difficult to cause a problem, but when used as a separator for a laminated battery, among the longitudinal direction and the width direction, If either thermal shrinkage rate (135 ° C., 60 minutes) exceeds 10%, the separator may shrink due to the heat generated by the battery, and a short circuit may occur at the end.

  As a method for controlling the heat shrinkage rate at 135 ° C. for 60 minutes within the above preferred range, for example, heat treatment is performed at a high temperature while stretching at a low speed after transverse stretching. When this method is used, since the shrinkage of the holes due to heat can be suppressed, high permeability and roughening of the film can also be achieved.

  The porous polypropylene film of the present invention preferably has an average pore diameter of 80 to 200 nm, more preferably 80 to 180 nm, and even more preferably 100 to 180 nm. When the average pore diameter is 80 nm or more, the ion permeability is particularly excellent, which is preferable from the viewpoint of battery output. If the average pore diameter is less than 80 nm, the output of the battery may be lowered because the movement of ions is inhibited. In addition, when the average pore diameter is larger than 200 nm, dendrites generated by repeated charging and discharging, and electrode active materials that have fallen off during the electrode preparation or battery assembly process easily penetrate through the holes of the separator, causing internal short circuit. It may be a cause.

  The porous polypropylene film of the present invention preferably has an air permeability resistance of 10 to 1,000 seconds / 100 mL. More preferably, it is 50-300 seconds / 100 mL, More preferably, it is 60-250 seconds / 100 mL, More preferably, it is 80-200 seconds / 100 mL. If the air permeation resistance is less than 10 seconds / 100 mL, the puncture strength of the film, which is an index for dendrite resistance, becomes too low, which may be inferior in safety. When the air permeation resistance exceeds 1,000 seconds / 100 mL, the output characteristics may deteriorate particularly when used as a separator for a high-power battery.

  As a method of controlling the air permeability resistance to the above preferable range, for example, by performing heat treatment at a high temperature while performing ultra-low speed stretching after transverse stretching, pore formation between the fibrils progresses, and the air resistance is reduced. Can be controlled.

  In the porous polypropylene film of the present invention, the air resistance after heat treatment at 135 ° C. for 60 minutes is preferably not more than twice the air resistance before heat treatment. More preferably, it is 1.7 times or less, and further preferably 1.5 times or less. If it is more than twice, the air permeability may deteriorate in the drying step before assembling the battery or capacitor, resulting in poor performance, or the output characteristics may deteriorate when used in a high temperature environment.

  As a method for controlling the air resistance increase rate after the heat treatment within the above preferable range, for example, heat treatment is performed at a high temperature while performing ultra-low speed stretching after transverse stretching. When this method is used, since the shrinkage of the holes due to heat can be suppressed, high permeability and roughening of the film can also be achieved.

  As for the porous polypropylene film of this invention, it is preferable that arithmetic mean roughness Ra of the roughness of the surface of at least one side is 0.4-3.0 micrometers. More preferably, it is 0.5-1.5 micrometers. When the thickness is larger than 3.0 μm, the flatness may be deteriorated when used as a thin separator that requires thickness accuracy. On the other hand, if the thickness is less than 0.4 μm, the portion where the electrolyte accumulates on the surface of the battery separator may decrease, and the output of the battery may decrease.

  The porous polypropylene film of the present invention preferably has a ten-point average roughness Rz of at least one surface roughness of 5.0 to 10.0 μm. More preferably, it is 6.0-8.0 micrometers. When it is larger than 10.0 μm, when used as a thin separator that requires thickness accuracy, planarity may be deteriorated. On the other hand, when the thickness is less than 5.0 μm, the portion where the electrolyte accumulates on the surface of the battery separator may decrease, and the output of the battery may decrease.

  As a method for controlling Ra and Rz within the above preferable range, for example, by performing heat treatment at a high temperature while performing ultra-low speed stretching after transverse stretching, pore formation between fibrils proceeds, and the fibril parts and The irregularities in the pores become large. The Ra and Rz values can be controlled by controlling the opening of the pores by controlling the drawing speed in the ultra-low-speed drawing heat treatment step.

  In the porous polypropylene film of the present invention, the average interval Sm of the roughness of at least one surface is preferably 2 to 20 μm, more preferably 7 to 18 μm, and further preferably 10 to 15 μm. The peak-valley average interval Sm is considered to be an index of how many irregularities of the surface roughness are generated. When Sm is less than 2 μm, the output characteristics of the battery may be deteriorated. On the other hand, if it exceeds 20 μm, the thermal shrinkage rate may increase.

  As a method for controlling Sm to the above preferable range, for example, by performing heat treatment at a high temperature while performing ultra-low speed stretching after transverse stretching, pore formation between fibrils progresses, and the distance between fibrils is controlled. be able to. The value of Sm can be controlled by controlling the distance between fibrils by controlling the stretching conditions before ultra-low speed stretching and the ultra-low speed stretching ratio.

  It is preferable that Ra, Rz, and Sm are simultaneously controlled within the above range. If one or two of these three roughness parameters are out of the preferred range, the output characteristics may deteriorate.

  In general, in order to increase the thermal dimensional stability of a biaxially stretched film, a method of heat fixing at a high temperature after transverse stretching or a method of giving relaxation is used in order to reduce stretching stress. However, in the porous film, the pores shrink due to heat fixation or relaxation and the average pore diameter becomes small, causing deterioration of air permeability and lowering the surface roughness, or the film melts and the pores are closed. In some cases, the output characteristics deteriorated when used in a separator.

  As a method to achieve both low heat shrinkage and high permeability of the film and roughening of the film surface, and to achieve both output characteristics and safety when used in a separator, heat treatment at a high temperature for a long time in the heat treatment process after stretching. And in addition, the film surface pores are not shrunk by heat and stretched in at least one of the film longitudinal direction and width direction at a very low stretching speed so that no stretching stress remains. It can be controlled with. A specific method will be described later.

  From the viewpoint of efficiently forming voids in the porous polypropylene film of the present invention, the polypropylene resin used preferably has a β-crystal forming ability of 40 to 90%. 60 to 85% is more preferable, and 65 to 80% is more preferable. If the β crystal forming ability is less than 40%, the amount of β crystal is small at the time of film production, so the number of voids formed in the film is reduced by utilizing the transition to α crystal, and as a result, the film has poor permeability. There is a case. On the other hand, in order to make the β crystal forming ability exceed 90%, it is necessary to add a large amount of a β crystal nucleating agent described later, or to make the stereoregularity of the polypropylene resin to be used extremely high. Industrial practical value is low, such as deterioration of stability.

  The β crystal fraction is an existing ratio of β crystals formed in the polypropylene resin. The β crystal fraction is preferably 40% or more. In order to achieve a β crystal fraction of 40% or more, it is preferable to use an additive having a function of assisting the formation of β crystals by adding to a polypropylene resin, generally called a β crystal nucleating agent.

  Examples of the β crystal nucleating agent include alkali or alkaline earth metal salts of carboxylic acids such as sodium benzoate, calcium 1,2-hydroxystearate, magnesium succinate, and N, N′-dicyclohexyl-2,6-naphthalene. Amide compounds represented by carboxyamide, tetraoxaspiro compounds such as 3,9-bis [4- (N-cyclohexylcarbamoyl) phenyl] -2,4,8,10-tetraoxaspiro [5.5] undecane Preferred examples include aromatic sulfonic acid compounds such as sodium benzenesulfonate and sodium naphthalenesulfonate, imide carboxylic acid derivatives, phthalocyanine pigments, and quinacridone pigments. Among these, amide compounds represented by the following chemical formulas (1) and (2) represented by N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide can be mentioned as particularly preferable β crystal nucleating agents. .

R ² —NHCO—R ¹ —CONH—R ³ (1)
Here, R ¹ in the formula is a saturated or unsaturated aliphatic dicarboxylic acid residue having 1 to 24 carbon atoms, a saturated or unsaturated alicyclic dicarboxylic acid residue having 4 to 28 carbon atoms, or 6 carbon atoms. Represents an aromatic dicarboxylic acid residue having ˜28, and R ² and R ³ are the same or different cycloalkyl groups having 3 to 18 carbon atoms, cycloalkenyl groups having 3 to 12 carbon atoms, or derivatives thereof.

R ⁵ —CONH—R ⁴ —NHCO—R ⁶ (2)
Here, R ⁴ in the formula is a saturated or unsaturated aliphatic diamine residue having 1 to 24 carbon atoms, a saturated or unsaturated alicyclic diamine residue having 4 to 28 carbon atoms, or 6 to 12 carbon atoms. A heterocyclic diamine residue or an aromatic diamine residue having ⁶ to 28 carbon atoms, wherein R ⁵ and R ⁶ are the same or different cycloalkyl groups having 3 to 12 carbon atoms and cycloalkenyl groups having 3 to 12 carbon atoms. Or these derivatives.

  Specific examples of such particularly preferred β crystal nucleating agent or β crystal nucleating agent-added polypropylene include β crystal nucleating agent “NJESTER” (type name: NU-100, etc.) manufactured by Shin Nippon Rika Co., Ltd.

When the porous polypropylene film of the present invention is biaxially stretched to form through-holes, the polyethylene exemplified below is from the viewpoint of improving void formation efficiency at the time of stretching and improving air permeability by increasing the pore diameter. It is preferable to contain a resin or an elastomer. Low density polyethylene (LDPE), (linear) low density polyethylene (LLDPE), very low density polyethylene (VLDPE), ethylene butene rubber (EBR), ethylene propylene rubber (EPR), propylene butene rubber (PBR), ethylene Vinyl acetate (EVA), ethylene ethacrylate (EEA), ethylene methyl methacrylate (EMMA), ethylene propylene diene copolymer (EPDM), isoprene rubber (IR), styrene butadiene rubber (SBR), hydrogenated styrene Examples thereof include butadiene rubber (H-SBR), styrene / butylene / styrene copolymer (SBS), and styrene / ethylene / butylene / styrene copolymer (SEBS). Among them, the ethylene / α / -olefin copolymer obtained by the metallocene catalyst method is highly effective, and an ultra-low density polyethylene having a melting point of 60 to 90 ° C. is preferable, and a copolymer obtained by copolymerizing octene-1 is preferable. “Engage (registered trademark)” (type names: 8411, 8452, 8100, etc.) manufactured by Dow Chemical can be used.
As content of the said polyethylene-type resin and an elastomer, it is preferable to set it as 10 mass% or less with respect to the whole porous polypropylene film.

  The porous polypropylene film of the present invention has an antioxidant, a heat stabilizer, an antistatic agent, a lubricant composed of inorganic or organic particles, an anti-blocking agent, a filler, a non-phase as long as the effects of the present invention are not impaired. Various additives such as a soluble polymer may be contained. In particular, it is preferable to add an antioxidant for the purpose of suppressing the oxidative deterioration due to the thermal history of the polypropylene resin, but the content of the antioxidant is 100 parts by mass of the polypropylene composition constituting the porous polypropylene film. The amount is preferably 2 parts by mass or less, more preferably 1 part by mass or less, and still more preferably 0.5 parts by mass or less.

  The method for producing a porous polypropylene film in the present invention will be specifically described below.

  First, the polypropylene resin constituting the base film is supplied to an extruder and melted at a temperature of 200 to 320 ° C., passed through a filtration filter, extruded from a slit-shaped base, cast into a cooling metal drum, and formed into a sheet. Cool and solidify to make an unstretched sheet.

  Here, in order to produce a large amount of β crystals in the unstretched sheet, the melt extrusion temperature is preferably low, but if it is less than 200 ° C., an unmelted material is generated in the molten polymer discharged from the die. , It may cause a process failure such as tearing in the subsequent stretching process. Moreover, when it exceeds 320 degreeC, the thermal decomposition of a polypropylene will become intense and it may be inferior to the film characteristics, for example, a Young's modulus, breaking strength, etc. of the porous polypropylene film obtained.

  The temperature of the cooling metal drum is set to 60 to 130 ° C., and β crystals are produced in large quantities and uniformly to obtain a highly permeable porous polypropylene film after stretching. If the temperature of the metal drum for cooling is less than 60 ° C, the β crystal fraction of the first run of the obtained unstretched sheet may be reduced. If the temperature exceeds 130 ° C, the solidification of the sheet on the drum is insufficient. Thus, it may be difficult to uniformly separate the sheet from the cooling metal drum. Here, the amount of β crystals in the unstretched sheet corresponds to the β crystal fraction obtained from the first run caloric curve obtained by using the unstretched sheet as a sample and using a differential scanning calorimeter. In the case of a highly permeable porous polypropylene film, the cooling metal drum temperature is preferably 100 to 125 ° C.

  The time for the unstretched sheet to contact the cooling metal drum (hereinafter referred to as the contact time to the drum) is preferably 6 to 60 seconds. Here, the contact time with the drum is the time required until the unstretched sheet peels from the drum, with the start time (= 0 seconds) when the molten polymer first lands on the drum in the casting step. Means. In addition, when the casting process is composed of a plurality of drums, the total time when the unstretched sheet comes into contact with these drums is the contact time with the cooling metal drum. When the contact time with the cooling metal drum is less than the above range, although depending on the temperature, the unstretched sheet sticks at the time of peeling, or there are few β crystals generated in the unstretched sheet (β crystals of the unstretched sheet) Therefore, the porosity of the film after biaxial stretching may be lowered to an insufficient level. When the contact time with the cooling metal drum exceeds the above range, although depending on the size of the cooling metal drum, the peripheral speed of the cooling metal drum is unnecessarily low, and the productivity may be significantly deteriorated. Usually, the contact time of 10 minutes or more may not be substantially removed. The contact time with the metal drum is more preferably 7 to 45 seconds, and further preferably 8 to 40 seconds.

  In addition, as an adhesion method to the cooling metal drum, any one of an electrostatic application (pinning) method, an adhesion method using the surface tension of water, an air knife method, a press roll method, an underwater casting method, etc. is used. However, as a method for obtaining the porous polypropylene film of the present invention, it is preferable to use an air knife method or an electrostatic application method that has good thickness uniformity and the cooling rate can be controlled by the temperature of the blowing air. . Here, in the air knife method, air is blown from the non-drum surface (the surface that does not contact the drum), and the temperature is preferably 10 to 200 ° C., and the surface is controlled by controlling the cooling rate of the surface. It is possible to control the amount of β crystal and thus the surface porosity, that is, to control the permeability of the resulting porous polypropylene film.

  In addition, in the case of forming a laminate in which the second and third layers are coextruded and laminated on at least one surface of the porous polypropylene film, each of the desired resins is prepared as necessary in addition to the above-described polypropylene. The resin is supplied to separate extruders, melted at a desired temperature, passed through a filtration filter, merged in a short tube or a die, extruded from a slit-like die at each desired laminated thickness, and a cooling metal drum It can be cast into a non-laminated stretched sheet by cooling and solidifying into a sheet.

  Next, the obtained unstretched sheet is biaxially stretched to form pores (through holes) in the film. As a biaxial stretching method, the film is stretched in the longitudinal direction of the film and then stretched in the width direction, or the sequential biaxial stretching method of stretching in the longitudinal direction after stretching in the width direction, or the longitudinal direction and the width direction of the film are stretched almost simultaneously. The simultaneous biaxial stretching method can be used, but it is preferable to adopt the sequential biaxial stretching method in that it is easy to obtain a highly air-permeable film, and in particular, it is possible to stretch in the width direction after stretching in the longitudinal direction. preferable.

  As specific stretching conditions, first, the unstretched sheet is controlled to a temperature at which it can be stretched in the longitudinal direction. As a temperature control method, a method using a temperature-controlled rotating roll, a method using a hot air oven, or the like can be adopted. As the stretching temperature in the longitudinal direction, it is preferable to employ a temperature of 110 to 140 ° C., more preferably 120 to 135 ° C., from the viewpoint of film characteristics and uniformity. The draw ratio is preferably 1.1 to 8 times, more preferably 1.5 to 6 times, and still more preferably 2 to 5 times. If the draw ratio is less than 1.1 times, the air permeability may deteriorate, and the productivity may decrease. As the draw ratio is increased, the air permeability is improved. However, if the draw ratio is more than 8 times, the film may be easily broken in the next transverse drawing step.

  Next, the uniaxially stretched polypropylene film is introduced into a tenter stretcher while gripping the film end, and stretched in the width direction to obtain a biaxially stretched film. The stretching temperature is preferably 130 to 155 ° C, and more preferably 145 to 150 ° C because high air permeability is obtained. The draw ratio in the width direction is preferably 1.1 to 10 times, more preferably 1.5 to 6 times. If the draw ratio is less than 1.1 times, the porosity may be lowered and the battery characteristics may be deteriorated, and the productivity may be lowered. Further, the higher the draw ratio, the better the air permeability, but if the film is stretched more than 10 times, the film may be easily broken. The transverse stretching speed at this time is preferably 500 to 6,000% / min, more preferably 1,000 to 5,000% / min. It is particularly preferable that the stretching speed is as low as 2,000% / min or less.

  Subsequently, as described above, in order to achieve both low thermal shrinkage, low air permeability resistance, and roughening of the film surface, the following heat treatment process (longitudinal and / or widthwise ultra-low speed stretching process) is performed. Introduce.

  As the heat treatment step, it is preferable to heat-treat for 10 to 600 seconds while stretching at an ultra-low speed in the longitudinal direction of the film in a stretching speed range of 0.01 to 5% / second, more preferably 0.1 to 3% / second. . The stretching temperature (heat treatment temperature) at this time is in the temperature range of Tm-5 to Tm + 7 ° C. when the melting point of the polypropylene resin used is Tm, and the stretching ratio is preferably 1.1 to 5 times. .

  When said heat processing temperature (stretching temperature) is less than Tm-5 degreeC, the effect of heat processing is inadequate and the improvement of the thermal dimensional stability of the longitudinal direction at high temperature may not be achieved. Moreover, when it exceeds Tm + 7 degreeC, there exists a possibility that a porous polypropylene film may melt | dissolve and air permeability may deteriorate.

  When the stretching rate in the heat treatment step is less than 0.01% / second, the pores shrink due to the heat treatment, and thus the air permeability may be deteriorated and the film surface roughness may be lowered. On the other hand, if the stretching speed in the heat treatment step exceeds 5% / second, there may be a case where improvement of the thermal dimensional stability in the longitudinal direction at a high temperature cannot be achieved due to residual stretching stress.

  When the heat treatment time is less than 10 seconds, the effect of the heat treatment is insufficient, and the improvement of the thermal dimensional stability in the longitudinal direction at high temperatures may not be achieved. If the heat treatment time exceeds 600 seconds, the heat treatment may cause the pores to shrink.

  If the draw ratio in the longitudinal direction of the film in the heat treatment step is less than 1.1 times, the pores shrink due to the heat treatment, so that the air permeability tends to deteriorate and the roughness of the film surface tends to decrease. On the other hand, when the draw ratio exceeds 5 times, there is a case where improvement of the thermal dimensional stability in the longitudinal direction at a high temperature cannot be achieved due to residual drawing stress.

  Also in the width direction, the film is stretched in the film width direction in the film width direction within the range of 0.01 to 5% / second, more preferably in the range of 0.1 to 3% / second in the same manner as the heat treatment for ultra-low speed stretching in the longitudinal direction. However, it is preferable to heat-treat for 10 to 600 seconds. In addition, the extending | stretching temperature at this time is a temperature range of Tm-5-Tm + 7 degreeC, when the melting point of the polypropylene resin to be used is set to Tm, and it is preferable that a draw ratio is 1.1-5 times.

  When the heat treatment temperature is lower than Tm−5 ° C., the effect of the heat treatment is insufficient, and the improvement of the thermal dimensional stability in the width direction at high temperatures may not be achieved. Moreover, when it exceeds Tm + 7 degreeC, a porous polypropylene film may melt | dissolve and air permeability worsens and the roughness of the film surface may fall.

  When the stretching speed in the heat treatment step is less than 0.01% / second, the pores are shrunk by the heat treatment, so that the air permeability may be deteriorated or the film surface roughness may be lowered. On the other hand, if the stretching speed in the heat treatment step exceeds 5% / second, the improvement of the thermal dimensional stability in the width direction at high temperature may not be achieved due to residual stretching stress.

  When the heat treatment time is less than 10 seconds, the effect of the heat treatment is insufficient, and the improvement of the thermal dimensional stability in the width direction at high temperatures may not be achieved. If the heat treatment time exceeds 600 seconds, the heat treatment may cause the pores to shrink.

  If the draw ratio in the film width direction in the heat treatment step is less than 1.1 times, the pores shrink due to the heat treatment, so that the air permeability may be deteriorated or the film surface roughness may be lowered. On the other hand, when the draw ratio exceeds 5 times, there may be a case where improvement of the thermal dimensional stability in the width direction at high temperature cannot be achieved due to residual drawing stress.

  As a method for improving the thermal dimensional stability in the longitudinal direction and the width direction of the film, in the heat treatment step, after the ultra-low speed stretching in the film longitudinal direction, the ultra-low speed stretching in the width direction, or the ultra-low speed stretching in the width direction, the longitudinal direction A sequential biaxial stretching method in which the film is stretched at a very low speed in the direction or a simultaneous biaxial stretching method in which the longitudinal direction and the width direction of the film are stretched at a very low speed almost simultaneously can be used. It is preferable.

  Since the polypropylene porous film of the present invention has an appropriate surface roughness and an appropriate pore diameter while having good thermal dimensional stability, it has high safety and high output when used as a separator. An electricity storage device can be configured. Moreover, it can use preferably for a lithium ion battery, a lithium ion capacitor, an electrical double phase capacitor, and an electrolytic capacitor among electrical storage devices. In addition to separator applications, it can also be used for packaging products, sanitary products, agricultural products, building products, medical products, separation membranes, light diffusion plates, and reflective sheets.

  Hereinafter, the present invention will be described in detail by way of examples. The characteristics were measured and evaluated by the following methods.

(1) Thermal contraction rate The film was cut into a rectangle 150 mm long × 10 mm wide in the longitudinal direction and the width direction to obtain a sample. Marks were drawn on the sample at intervals of 100 mm, a weight of 3 g was applied to the lower end of the sample, and the sample was placed in a hot air oven heated to 135 ° C. for 60 minutes for heat treatment. After the heat treatment, it was allowed to cool, the weight was removed, the distance between the marked lines was measured, the thermal shrinkage was calculated from the change in the distance between the marked lines before and after heating, and used as an index of dimensional stability. For each film, five samples were carried out in the longitudinal direction and the width direction for each film, and the average value was evaluated.

(2) β crystal fraction, β crystal forming ability 5 mg of resin or film was sampled, loaded into an aluminum pan as a sample, and measured using a differential scanning calorimeter (DSC, RDC220 manufactured by Seiko Electronics Co., Ltd.). did. First, the temperature is raised from room temperature to 240 ° C. at 10 ° C./min (first run) in a nitrogen atmosphere, held for 10 minutes, and then cooled to 30 ° C. at 10 ° C./min. About the melting peak observed when the temperature is raised (second run) again at 10 ° C./min after holding for 5 minutes, the melting having a peak in the temperature range of 145 to 157 ° C. is the melting peak of β crystal, 158 ° C. or more The melting at which the peak is observed is defined as the melting peak of the α crystal, and the amount of heat of fusion is determined from the base line drawn on the high temperature side flat portion of the DSC curve and the area of the region surrounded by the peak. Assuming that the heat of fusion of the α crystal is ΔHα and the heat of fusion of the β crystal is ΔHβ, the value calculated by the following formula is the β crystal forming ability. In addition, calibration of the heat of fusion was performed using indium.

β crystal forming ability (%) = [ΔHβ / (ΔHα + ΔHβ)] × 100
In addition, the value which computed the abundance ratio of (beta) crystal similarly from the melting peak observed by a first run has shown the (beta) crystal fraction in the state of the sample.

(3) Air permeability resistance A square having a size of 100 mm × 100 mm was cut out from a porous polypropylene film and used as a sample. Using a B-type Gurley tester of JIS P 8117 (1998), the permeation time of 100 mL of air was measured at 23 ° C. and a relative humidity of 65%.

(4) Permeability increase rate after heat treatment The sample used in the air resistance measurement of (3) above was left in a hot air oven heated to 135 ° C for 60 minutes for heat treatment. The sample was allowed to cool after heat treatment, and the permeation time of 100 mL of air was measured at 23 ° C. and 65% relative humidity using a JIS P 8117 (1998) B-shaped Gurley tester. From the change, the rate of increase in air resistance after heat treatment was calculated using the following formula. For the measurement location, the vicinity of the measurement location of the air resistance measurement of (3) was taken as the measurement location.

Increase rate of air resistance after heat treatment = Air resistance after heat treatment (second / 100 mL) / Air resistance before heat treatment (second / 100 mL)
(5) Average hole diameter of through holes POROUS MATERIALS, Inc. This was measured using an automatic pore size distribution measuring device “PERM-POROMETER”. Measurement conditions are as follows.

Test solution: 3M “Fluorinert” FC-40
Test temperature: 25 ° C
Test gas: Air Analysis software: Capwin
Measurement conditions: Automatic measurement based on default conditions of Capillary Flow Porometry-Wet up, Dry down Note that the following relational expression is established between the pore diameter (pore diameter) and the test pressure.

d = Cγ / P × 10 ³
[However, d: pore diameter (nm), C: constant, γ: surface tension of fluorinate (16 mN / m), P: pressure (Pa). ]
Here, based on the above, the average pore diameter was calculated from the 1/2 half-wetting curve using the data analysis software attached to the apparatus. However, the measurement limit was set to 37 nm due to the problem of the upper limit of pressure during measurement. The same measurement was performed for the same sample five times at different locations, and the average value of the average pore diameters obtained was taken as the average pore diameter of the through holes in the sample.

(6) Yamatani average interval Sm
Measured according to JIS B0601-1994. First, a battery separator is cut out by 10 mm width × 50 mm length. Double-sided tape (Nitto Denko's double-sided adhesive tape, No. 501F, 5 mm width) in which the cut-out battery separator was attached to a glass plate (Matsunami Glass Industry Co., Ltd., Micro Slide Glass S1225, 76 mm × 26 mm) at a distance of 15 mm or more in parallel. × 20m)

  The sample was measured and analyzed in the following order using a laser microscope (VK-8510 manufactured by Keyence Corporation) and an objective lens (CF IC EPI Plan manufactured by Nikon Corporation).

  (1) The stage was lowered to the lowest limit, and the origin was found using a 10x objective lens.

  (2) Set RUN MODE to black and white ultra-deep, VIEW MODE to color raw image, and put the sample on the stage.

  (3) The objective lens was changed to 50 times, the laser was turned on, and the focusing handle was adjusted to focus.

  (4) Press the measurement start button and coarsely adjust the auto gain.

  (5) The light reception gain was adjusted so that the amount of light was in the range of 240-250.

  (6) The beam attenuator was closed and the offset was adjusted so that the amount of light was in the range of 30-40.

  (7) The beam attenuator was opened, and the light receiving gain was adjusted again so that the amount of light entered the range of 240-250.

  (8) Using the lens position move button, the lens was raised until it was out of focus, and the upper limit of the measurement range was set. Similarly, the lower limit of the measurement range was set by lowering the lens.

  (9) PITCH was set to 0.1 μm and measured and stored.

  (10) The image measurement / analysis software VK-H1W was started up, the measurement image was read, and the line roughness in the longitudinal and width directions was measured. The measurement range was 110 μm in the longitudinal direction and 150 μm in the width direction, DCL was set to 0, BCL was set to 255, and smoothing and inclination correction were not performed.

  (11) The average value of the peak-valley average intervals Sm at five places in the longitudinal direction and five places in the width direction was defined as Sm.

(7) Arithmetic average roughness Ra, Ten-point average roughness Rz
It measured based on JIS B0601-1994. First, a battery separator is cut out by 10 mm width × 50 mm length. Double-sided tape (Nitto Denko's double-sided adhesive tape, No. 501F, 5 mm width) in which the cut-out battery separator was attached to a glass plate (Matsunami Glass Industry Co., Ltd., Micro Slide Glass S1225, 76 mm × 26 mm) at a distance of 15 mm or more in parallel. × 20m)

  The sample was measured and analyzed in the following order using a laser microscope (VK-8510 manufactured by Keyence Corporation) and an objective lens (CF IC EPI Plan manufactured by Nikon Corporation).

  (1) The stage was lowered to the lowest limit, and the origin was found using a 10x objective lens.

  (2) Set RUN MODE to black and white ultra-deep, VIEW MODE to color raw image, and put the sample on the stage.

  (3) The objective lens was changed to 50 times, the laser was turned on, and the focusing handle was adjusted to focus.

  (4) Press the measurement start button and coarsely adjust the auto gain.

  (5) The light reception gain was adjusted so that the amount of light was in the range of 240-250.

  (6) The beam attenuator was closed and the offset was adjusted so that the amount of light was in the range of 30-40.

  (7) The beam attenuator was opened, and the light receiving gain was adjusted again so that the amount of light entered the range of 240-250.

  (8) Using the lens position move button, the lens was raised until it was out of focus, and the upper limit of the measurement range was set. Similarly, the lower limit of the measurement range was set by lowering the lens.

  (9) PITCH was set to 0.1 μm and measured and stored.

  (10) Image measurement / analysis software VK-H1W was started up, a measurement image was read, and the surface roughness was measured. The measurement range was 110 μm in the longitudinal direction × 150 μm in the width direction, DCL was set to 0, BCL was set to 255, and smoothing and inclination correction were not performed.

  (11) The average values of the arithmetic average roughness Ra and the ten-point average roughness Rz at five locations in the measurement image were Ra and Rz, respectively.

(8) Evaluation as separator (8-1) Production of battery The positive electrode is a lithium cobalt oxide (LiCoO ₂ ) thickness of 40 μm manufactured by Hosen Co., Ltd., and punched out into a square of 47 mm each. Made.

  The negative electrode was manufactured by using a graphite negative electrode having a thickness of 50 μm manufactured by Hosen Co., Ltd., and punching it into a square of 50 mm each.

In the electrolyte solution, a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) as an organic solvent at a volume ratio of 30:70 is used, and LiPF ₆ 1.0 mol / L is used as a supporting salt. Was dissolved and used.

  The positive electrode and the negative electrode were arranged so that the active material layers of the positive electrode and the negative electrode face each other, and were placed in a battery cell made of a laminate cell with a separator interposed therebetween. Here, the separator film of each Example / Comparative Example was used as a separator, and the shape was made into two types of rectangles having different dimensions so that the area was wider than both the positive electrode and the negative electrode. The tip of each tab was pulled out of the battery laminate cell. Next, after filling the battery cell with the electrolyte solution, the laminate cell was sealed by vacuum degassing to obtain a single-layer laminate type lithium ion secondary battery. Each battery is a battery A using a separator whose dimensions are 55 mm in the film longitudinal direction and 52 mm in the width direction, and battery B is a battery using a separator whose dimensions are 55 mm in the film width direction and 52 mm in the longitudinal direction. Two batteries were prepared for each, and one battery was used for each of the heating test and output characteristic evaluation described below.

(8-2) Heat test (battery surface temperature)
The fabricated battery was charged to 4.2 V at 30 mA in an atmosphere of 25 ° C. for 3.5 hours and then left at 130 ° C. for 1 hour to evaluate the temperature and state of the battery surface.

    ○: Surface temperature of 135 ° C. or lower.

(Triangle | delta): Surface temperature exceeding 135 degreeC and less than 140 degreeC X: Surface temperature 140 degreeC or more, cell damage (8-3) Output characteristics About each produced secondary battery, charge is 30 mA in 25 degreeC atmosphere. A charge / discharge operation was performed for 3.5 hours up to 2 V and discharge up to 2.7 V at 30 mA, and the discharge capacity was examined. Further, a charge / discharge operation was performed in which charging was performed at 30 mA up to 4.2 V for 3.5 hours, and discharging was performed at 300 mA up to 2.7 V, and the discharge capacity was examined.

  [(Discharge capacity at 300 mA) / (Discharge capacity at 30 mA)] × 100 The value obtained by the calculation formula was evaluated according to the following criteria. Of battery A and battery B, the value with the lower discharge capacity was evaluated.

☆: 95% or more ◎: 90% or more and less than 95% ○: 85% or more and less than 90% △: 80% or more and less than 85% ×: less than 80% (Example 1)
As raw material resin for porous polypropylene film, 95 parts by mass of homopolypropylene FLX80E4 (MFR: 7.5 g / 10 min, melting point: 163 ° C.) manufactured by Sumitomo Chemical Co., Ltd., Dow Chemical 843 (MFR: 18 g / 10 min) added to 5 parts by mass, β crystal nucleating agent N, N′-dicyclohexyl-2,6-naphthalenedicarboxamide (manufactured by Shin Nippon Rika Co., Ltd., Nu-100) is further mixed from the weighing hopper so that 0.2 parts by weight of the antioxidant and IRGANOX 1010 and IRGAFOS 168 manufactured by Ciba Specialty Chemicals are mixed at a ratio of 0.15 and 0.1 parts by weight, respectively. The raw material is supplied to the shaft extruder, melt kneaded at 300 ° C, discharged from the die into strands, and a 25 ° C water tank Cooling and solidifying Te and was a chip raw material and cut into chips.

  This chip is supplied to a single screw extruder and melt extruded at 220 ° C. After removing foreign matter with a 30 μm cut sintered filter, it is discharged from a T-die to a cast drum whose surface temperature is controlled at 120 ° C. An unstretched sheet was obtained by casting for 15 seconds. Next, preheating was performed using a ceramic roll heated to 100 ° C., and the film was stretched 3 times in the longitudinal direction of the film. Once cooled, the end is then held in a tenter-type stretching machine with a clip, and stretched in the width direction of the film at a stretch rate of 1,500% / min. A porous polypropylene film was obtained. Next, the produced porous polypropylene film was cut into a 200 mm square in the film longitudinal direction and the width direction, and the following heat treatment process was performed. Set the sample on a KARO-IV film stretcher manufactured by Bruckner, set the oven temperature to 165 ° C, and stretch the film in the longitudinal direction and the width direction at 1.67 times at a stretching rate of 1% / sec. The film was heat treated for 67 seconds to obtain a film having a final thickness of 19.8 μm in which the final magnification in the film longitudinal direction and the film width direction was 5 times, respectively.

  135 ° C., heat shrinkage rate for 60 minutes, air permeability resistance, air resistance increase rate after heat treatment at 135 ° C. for 1 hour, average pore diameter, mean valley interval Sm, arithmetic average roughness Ra, ten-point average roughness The thickness Rz, heating test, and battery characteristics were measured, and the results are shown in Table 1.

(Example 2)
In Example 1, the biaxially stretched porous polypropylene film was obtained by setting the stretching ratio in the width direction with a tenter to 4 times, and then a heat treatment step was performed. First, a low-speed stretching heat treatment is performed for 42 seconds until reaching 1.42 times at a stretching speed of 1% / second only in the film longitudinal direction, followed by 1.67 times in the film longitudinal direction and 1.25 times in the width direction. The same operation as in Example 1 was performed except that a film having a final magnification of 5 times in the film longitudinal direction and the width direction was obtained by performing low-speed stretching heat treatment at a stretching rate of 1% / second for 25 seconds, A film having a final thickness of 19.6 μm was obtained, and each physical property value is shown in Table 1.

(Example 3)
In the heat treatment step of Example 1, the same operation as in Example 1 was performed except that the ultra-low speed was 3% / second in the film longitudinal direction and the width direction to obtain a film having a final thickness of 20.2 μm. The physical property values are shown in Table 1.

Example 4
In the heat treatment step of Example 1, the same operation as in Example 1 was performed except that the set temperature of the oven at the time of heat treatment was 169 ° C. to obtain a film having a final thickness of 20.0 μm. Indicated.

(Example 5)
In Example 1, after the biaxially stretched porous polypropylene film was obtained by setting the longitudinal stretch ratio in the tenter to 5 times, a heat treatment step was performed. Except for obtaining a film with a final magnification of 5 times in the film longitudinal direction and width direction by performing a low-speed stretching heat treatment for 67 seconds until reaching 1.67 times at a stretching rate of 1% / second only in the film width direction. The same operation as in Example 1 was performed to obtain a film having a final thickness of 20.4 μm.

(Comparative Example 1)
As raw material resin for porous polypropylene film, 95 parts by mass of homopolypropylene FLX80E4 manufactured by Sumitomo Chemical Co., Ltd. and 5 parts by mass of Engage 8411 (MFR: 18 g / 10 min) manufactured by Dow Chemical which is an ethylene / α-olefin copolymer are used. In addition to 0.2 parts by mass of N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide (Shin Nippon Rika Co., Ltd., Nu-100), which is a β crystal nucleating agent, with an antioxidant A raw material is supplied from a measuring hopper to a twin screw extruder so that IRGANOX1010 and IRGAFOS168 manufactured by Ciba Specialty Chemicals are mixed at a ratio of 0.15 and 0.1 parts by mass, respectively, and melt kneaded at 300 ° C., It is discharged from the die in the form of a strand, cooled and solidified in a 25 ° C. water bath, cut into a chip shape, and a chip raw material It was.

  This chip is supplied to a single screw extruder and melt extruded at 220 ° C. After removing foreign matter with a 30 μm cut sintered filter, it is discharged from a T-die to a cast drum whose surface temperature is controlled at 120 ° C. An unstretched sheet was obtained by casting for 15 seconds. Next, preheating was performed using a ceramic roll heated to 100 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Once cooled, the end is then held in a tenter-type stretching machine with a clip, and stretched at 150 ° C. five times at a stretching speed of 1,500% / min. A heat treatment was performed for 7 seconds at 155 ° C. while relaxing to obtain a biaxially stretched porous polypropylene film having a width of 1 m and a final thickness of 19.9 μm. The physical property values are shown in Table 1.

(Comparative Example 2)
In the heat treatment step of Example 1, the same operation as in Example 1 was performed except that the ultra-low speed stretching rate was 10% / second in the film longitudinal direction and the width direction to obtain a film having a final thickness of 19.7 μm. The physical property values are shown in Table 1.

(Comparative Example 3)
As a polypropylene resin, 94 parts by mass of homopolypropylene WF836DG3 (MFR: 7 g / 10 min, isotactic index: 97%) manufactured by Sumitomo Chemical Co., Ltd., high melt tension homopolypropylene Pro-fax PF814 (MFR: 2) manufactured by Basell .5 g / 10 min, isotactic index: 97%) 1 part by mass, ethylene / α-olefin copolymer Engage 8411 (melt index: 18 g / 10 min) made by Dow Chemical Co. Then, 0.2 parts by mass of N, N′-dicyclohexyl-2,6-naphthalenedicarboxamide (Nu-100 manufactured by Shin Nippon Rika Co., Ltd.), which is a β crystal nucleating agent, was added to the twin screw extruder. Supplied, melt-kneaded at 220 ° C, extruded into a strand, cooled and solidified in a 25 ° C water bath. To obtain a polyolefin resin material and cut into chips.

  This polyolefin resin is supplied to a single screw extruder, melt extruded at 220 ° C., foreign matter is removed by a sintered filter, and then discharged from a T-die onto a cast drum whose surface temperature is controlled at 120 ° C. To give an unstretched sheet. Next, the unstretched sheet was heated using a roll heated to 95 ° C., and stretched 5 times in the longitudinal direction. Once cooled, the film is stretched 5 times in the width direction at 145 ° C. at a stretching speed of 1,500% / min with a stenter-type transverse stretching machine, heat-fixed at 155 ° C. for 5 seconds, and then 140 ° C. with a relaxation rate of 10 % For 5 seconds to obtain a porous polypropylene film having a final thickness of 21.2 μm. The physical properties are shown in Table 1.

(Comparative Example 4)
As raw material resin for porous polypropylene film, 94 parts by mass of homopolypropylene FSX80E4 manufactured by Sumitomo Chemical Co., Ltd., 1 part by mass of Basel polypropylene PF-814 which is a high melt tension polypropylene resin, and ethylene-octene-1 copolymer N Dow Chemical Engage 8411 (MFR: 18 g / 10 min) is added to 5 parts by mass, and N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide (Shin Nihon Rika Co., Ltd.) is a β crystal nucleating agent. , Nu-100), 0.2 parts by weight, and an antioxidant, Ciba Specialty Chemicals IRGANOX 1010 and IRGAFOS 168 are mixed at a ratio of 0.15 and 0.1 parts by weight, respectively. The raw material is fed to the twin screw extruder and melt kneaded at 300 ° C. It was discharged from a die in a mold, cooled and solidified in a 25 ° C. water bath, cut into a chip, and used as a chip raw material.

  This chip is supplied to a single screw extruder and melt extruded at 220 ° C. After removing foreign matter with a 30 μm cut sintered filter, it is discharged from a T-die to a cast drum whose surface temperature is controlled at 120 ° C. An unstretched sheet was obtained by casting for 15 seconds. Next, preheating was performed using a ceramic roll heated to 100 ° C., and the film was stretched 5 times in the longitudinal direction of the film. After cooling, the end portion was introduced into a tenter-type stretching machine by holding it with a clip, and stretched at 150 ° C. five times at a stretching rate of 1,500% / min. As it was, heat treatment was carried out at 155 ° C. for 7 seconds while relaxing 5% in the width direction to obtain a porous polypropylene film having a final thickness of 20.4 μm. The produced porous polypropylene film was slit to a width of 700 mm, and a 500 m winding film roll was obtained on a plastic core having a winding tension of 30 N / m and an inner diameter of 152.4 mm and an outer diameter of 172.4 mm. Next, the film roll was put in an oven at a temperature of 80 ° C. and a humidity of 10% RH while holding both ends of the core, and heat-treated for 50 hours. The physical properties of the obtained porous film are shown in Table 1. .

  As shown in Table 1, regarding the output characteristics, the average pore diameter, air permeability resistance, and surface roughness parameters all show extremely good output characteristics when they fall within the scope of the claims. Then, it can be seen that when one or two of the three parameters Ra, Rz, and Sm are out of the preferred range, the output characteristics are degraded.

  The porous polypropylene film of the present invention can be provided as a porous film having excellent thermal dimensional stability and good battery characteristics.

Claims (8)

A porous polypropylene film using a polypropylene resin having a β-crystal forming ability of 40 to 90% and having a thermal shrinkage rate of 0 to 10% at 135 ° C. for 60 minutes in any one of the longitudinal direction and the width direction. 2. The porous polypropylene film according to claim 1, wherein the thermal shrinkage rate at 135 ° C. for 60 minutes is 0 to 10% in both the longitudinal direction and the width direction. The arithmetic average roughness Ra of at least one surface is 0.4 to 3.0 μm, the ten-point average roughness Rz is 5.0 to 10 μm, and the peak and valley average interval Sm is 2 to 20 μm. 3. The porous polypropylene film according to 1 or 2. The porous polypropylene film in any one of Claims 1-3 whose average hole diameter of a through-hole is 80-200 nm. The porous polypropylene film according to any one of claims 1 to 4, wherein the air resistance is 10 to 1,000 seconds / 100 mL. The porous polypropylene film according to any one of claims 1 to 5, wherein the air resistance after heat treatment at 135 ° C for 60 minutes is 2 times or less than the air resistance before heat treatment. The separator for electrical storage devices which uses the porous polypropylene film in any one of Claims 1-6. The electrical storage device using the separator for electrical storage devices of Claim 7.