JP2015071723A - Production method of porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous film which suppresses increase of air permeability resistance and improves mechanical strength, and exhibits excellent output characteristics when being used as a separator.SOLUTION: When a porous sheet is subjected to press working for producing a porous film having a through hole, the press working is performed to a thickness direction of the porous sheet under conditions of (1) and (2), (1) press temperature is 100°C or more, and (2) press pressure is 1-100 kgf/cm in linear pressure.

Description

  The present invention relates to a porous film excellent in air resistance and mechanical strength. Specifically, the porosity is reduced by suppressing the increase in air resistance during pressing by high-temperature pressing of a porous sheet, and the porosity suitable for a separator for lithium ion batteries, which achieves both air resistance and mechanical strength, is particularly suitable. The present invention relates to a film manufacturing method.

  Porous film has various uses such as various separators such as batteries and electrolytic capacitors, various separation membranes (filters), absorbent articles, moisture permeable waterproof members for clothing and medical use, heat sensitive paper members, ink receptor members, etc. Polyolefin-based porous films are mainly used. The porous polyolefin film is particularly used as a separator for an electricity storage device because of its characteristics such as high permeability and high porosity.

  Energy storage devices are one of the most important electrical devices that support today's ubiquitous society because they can take out electrical energy whenever and wherever they are needed. On the other hand, energy storage devices (especially secondary batteries) with the spread of mobile devices The need for high capacity, small size and light weight is increasing year by year. In particular, lithium ion batteries have a high volume of energy density per unit mass and output density as compared with other power storage devices, and therefore, the demand for power storage devices satisfying the above needs is greatly increasing.

  When a porous film is used as a separator for a lithium ion battery, high productivity and low cost are required, but in addition to this, excellent air permeability resistance, mechanical strength, etc. are required as film characteristics. In particular, in order to improve the mechanical strength, it is necessary to increase the amount of resin per thickness of the film. However, since the porosity decreases relative to this, it becomes difficult to achieve both air resistance and resistance. As a method of increasing the amount of resin per thickness of the porous film, a method of pressing a previously formed porous film in the thickness direction has been proposed (for example, Patent Documents 1 to 3). Although these production methods have the effect of reducing the porosity by pressurization and improving the impregnation property of the electrolytic solution, the gas permeability is lowered and the mechanical strength is not compatible.

JP 2000-344930 A JP 2009-149710 A JP 2009-249477 A

  The present invention provides the above-mentioned problem, that is, to provide a method for producing a porous film capable of exhibiting excellent output characteristics when used as a separator by achieving both mechanical strength and air permeability resistance. An object of the manufacturing method is to press the porous sheet at a high temperature to suppress an increase in air resistance and achieve an improvement in mechanical strength by reducing the porosity.

  The present invention for solving the above-described problems and achieving the goals has the following characteristics.

  When a porous sheet is pressed to produce a porous film having through-holes, the porous film is subjected to the pressing in the thickness direction of the porous sheet under the following conditions (1) and (2). Production method.

(1) Press temperature is 100 ° C. or higher (2) Press pressure is linear pressure 1-100 kgf / cm

  The porous film produced according to the present invention achieves a low porosity while suppressing an increase in the amount of resin per thickness and an accompanying increase in air resistance, and provides high output characteristics and safety when used as a separator. Show.

  Below, the manufacturing method of the porous film of this invention is demonstrated.

  The porous film used in the present invention is preferably made of a thermoplastic resin as a main component, and examples thereof include resins such as polyester, polyamide, and polyolefin. Among these, from the viewpoint of cost, productivity, and performance as a separator of the obtained film, a porous film mainly composed of polyolefin, particularly polypropylene, is preferable. The main component described here refers to the component having the largest mass% among the components constituting the porous film.

  Furthermore, the porous film of the present invention has holes (through holes) penetrating both surfaces of the film and exhibits a finite air resistance. There are wet and dry methods for forming the through-holes, but the dry method is preferred for simplification of the process. Among them, the β crystal method is particularly preferable from the viewpoint of achieving high productivity, uniform physical properties, and thinning. The β crystal method is a method of obtaining a porous film by forming β-crystals formed in a cast sheet into fibrils oriented in the film forming direction by longitudinal stretching and forming a network while cleaving the fibrils by transverse stretching. It is.

  As the resin constituting the porous film of the present invention, isotactic polypropylene is preferable, but the isotactic index of this isotactic polypropylene is preferably 90 to 99.9%. If the isotactic index is less than 90%, the crystallinity of the resin is low, and it may be difficult to achieve low air permeability resistance. A commercially available isotactic polypropylene can be used.

  In order to form a through-hole in a film using the β crystal method, it is preferable that the β crystal forming ability of a raw material resin containing polypropylene (hereinafter sometimes referred to as polypropylene resin) is 60% or more. If the β-crystal forming ability is less than 60%, 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, only a film with low permeability is obtained. It may not be possible. On the other hand, the upper limit of β-crystal forming ability is not particularly limited, but in order to exceed 99.9%, a large amount of a β-crystal nucleating agent described later is added, or the stereoregulation of the polypropylene resin to be used The industrial practical value is low, for example, the film forming stability is lowered. Industrially, β-crystal forming ability is preferably 65 to 99.9%, particularly preferably 70 to 95%.

  In order to control the β crystal formation ability to 60% or more, a polypropylene resin with a high isotactic index is used, or a β crystal is selectively formed by adding it to a polypropylene resin called a β crystal nucleating agent. The crystallization nucleating agent to be used is preferably used as an additive. Examples of the β crystal nucleating agent include alkali or alkaline earth metal salts of carboxylic acids such as calcium 1,2-hydroxystearate and magnesium succinate, and N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide. Amide compounds, tetraoxaspiro compounds such as 3,9-bis [4- (N-cyclohexylcarbamoyl) phenyl] -2,4,8,10-tetraoxaspiro [5.5] undecane, benzenesulfonic acid Preferable examples include aromatic sulfonic acid compounds such as sodium and sodium naphthalene sulfonate, imide carboxylic acid derivatives, phthalocyanine pigments, and quinacridone pigments. Particularly, amides disclosed in JP-A-5-310665 Compounds can be preferably usedThe addition amount (content) of the β crystal nucleating agent is preferably 0.05 to 0.5% by mass, preferably 0.1 to 0.3% by mass, based on the whole polypropylene resin. More preferable. If it is less than 0.05% by mass, the formation of β crystals becomes insufficient, and the air resistance of the porous film may increase. When it exceeds 0.5 mass%, when a coarse hole is formed and it uses for the separator for electrical storage devices, safety | security may fall.

  In the case of using a polyolefin resin, the porous film of the present invention can use homopolypropylene as well as ethylene in polypropylene from the viewpoint of stability in the film forming process, film forming property, and uniformity of physical properties. A resin obtained by copolymerizing the component and an α-olefin component such as butene, hexene, and octene in an amount of 5% by mass or less, more preferably 2.5% by mass or less can also be used. The form of the comonomer introduced into the polypropylene may be either random copolymerization or block copolymerization. When the polyolefin resin is biaxially stretched to form a through hole, the second component is exemplified below from the viewpoint of improving the void formation efficiency at the time of stretching and reducing the air resistance due to the expansion of the hole diameter. It is preferable to contain a polyolefin component that is incompatible with the first component or an elastomer component. Examples of the polyolefin component include high-density PE (PE: polyethylene, the same applies hereinafter), medium-density PE, low-density PE, LLDPE (linear low-density polyethylene), ultra-low-density PE, polybutene, and the like, and the elastomer component is ethylene. -Vinyl acetate copolymer, ethylene-acrylic copolymer, ethylene propylene rubber, ethylene butylene rubber, SEBS (styrene-ethylene-butylene-styrene block copolymer), HSBR (hydrogenated styrene-butadiene rubber) and the like. As such a component, for example, an ethylene / α-olefin copolymer is preferably contained. The enlargement of the pore diameter by the addition of the second component is also preferable in order to suppress an increase in air resistance due to press working described later.

  Here, examples of the ethylene / α-olefin copolymer include linear low-density polyethylene and ultra-low-density polyethylene, and among them, copolymerized octene-1 copolymerized polyethylene having a melting point of 60 to 90 ° C. A resin (copolymerized PE resin) can be preferably used. Examples of the copolymer polyethylene include commercially available resins such as “Engage” (registered trademark) manufactured by Dow Chemical (type names: 8411, 8452, 8100, etc.).

  The ethylene / α-olefin copolymer is preferably contained in an amount of 10% by mass or less from the viewpoint of low air permeation resistance when the entire polypropylene composition is 100% by mass. From the viewpoint of the mechanical strength of the film, it is more preferably 1 to 7% by mass, and more preferably 1 to 4% by mass.

  The dispersant used in the present invention is not limited as long as it can increase the dispersibility of the ethylene / α-olefin copolymer in the polypropylene resin, but as described in WO2007 / 046225, the polypropylene resin and the ethylene · The compatibility of the α-olefin copolymer is good. For example, an ethylene / propylene random copolymer generally used as a compatibilizer for polypropylene resin and polyethylene resin is used as a dispersant for homogenizing the pore structure in the present invention. Does not work. As the dispersant of the present invention, block copolymers each having a segment having high compatibility with polypropylene (for example, polypropylene segment, ethylene butylene segment) and a segment having high compatibility with polyethylene (for example, polyethylene segment) are preferable. As a resin having such a structure, a commercially available resin, for example, olefin crystal, ethylene butylene, olefin crystal block polymer (hereinafter referred to as CEBC) manufactured by JSR “DYNARON” (Dynalon) (registered trademark) (type name) : 6100P, 6200P, etc.). The addition amount of the dispersant is preferably 1 to 50% by mass and more preferably 5 to 33% by mass with respect to 100% by mass of the ethylene / α-olefin copolymer. From the viewpoint of improving the dispersibility of the ethylene / α-olefin copolymer in polypropylene resin and improving the uniformity of pore formation, the melting point of the dispersant is higher than the melting point of the ethylene / α-olefin copolymer. It is preferably 0 to 60 ° C. and more preferably 15 to 30 ° C.

  Hereinafter, although the manufacturing method of the porous film in this invention is demonstrated taking the case of using polyolefin resin as an example, the manufacturing method of the film of this invention is not limited to this.

  First, the polyolefin 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, and cast into a cooling metal drum to form 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 resin will become intense, and it may be inferior to the film characteristics, for example, an elasticity modulus, breaking strength, etc. of the obtained porous polyolefin film.

  The temperature of the cooling metal drum is preferably 105 to 130 ° C., and β crystals are produced in a large amount and uniformly, and a highly permeable porous polyolefin film is preferably formed after stretching. If the temperature of the cooling metal drum is lower than 105 ° C, the β-crystal fraction of the first run of the resulting unstretched sheet may decrease. 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. At this time, in particular, since the forming of the end portion of the sheet affects the subsequent stretchability, it is preferable that the end portion is sprayed with spot air to be in close contact with the drum. Further, air may be blown over the entire surface using an air knife as necessary based on the state of close contact of the entire sheet on the drum. Moreover, you may laminate | stack by co-extrusion using a some extruder.

  Next, the obtained unstretched sheet is biaxially stretched (initial stretching) to form pores (through holes) in the film. As the biaxial stretching method, after stretching in the machine direction, stretching in the width direction, or sequentially stretching in the width direction and then stretching in the machine direction, or stretching the film machine direction and the width direction almost simultaneously. The simultaneous biaxial stretching method can be used, but it is preferable to adopt the sequential biaxial stretching method from the viewpoint that it is easy to obtain a low air permeability resistance film, particularly stretching in the width direction after stretching in the machine direction. Is preferred.

  As specific initial stretching conditions, first, an unstretched sheet is controlled to a temperature at which it can be stretched in the machine 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 machine direction, it is preferable to employ a temperature of 110 to 140 ° C., more preferably 115 to 135 ° C., from the viewpoint of film characteristics and uniformity. The initial draw ratio in the machine direction is 1.1 to 8 times, preferably 4.2 to 7 times, more preferably 5 to 6.5 times.

  Next, the uniaxially stretched porous polyolefin film is introduced by gripping the film end with a tenter stretching machine and stretched in the width direction to obtain a biaxially stretched film.

  About the extending | stretching of the width direction, 133-158 degreeC is preferable and the preheating temperature and extending | stretching temperature have more preferable 143-158 degreeC. When the preheating temperature and the stretching temperature are less than 133 ° C., the stress at the time of fibril cleavage becomes large, and the cause of film breakage and the porosity are too high, which may be inferior in safety. Moreover, when preheating temperature and extending | stretching temperature are higher than 158 degreeC, a porosity may fall and output characteristics may fall.

  The preheating time is preferably 5 to 70 seconds, more preferably 10 to 50 seconds. If the preheating time is less than 5 seconds, the film may not be sufficiently warmed and may cause film breakage. Moreover, when preheating time exceeds 70 second, productivity may be inferior.

  The initial draw ratio in the width direction is preferably 1.1 to 15 times, more preferably 2.5 to 10 times, and further preferably 6.5 to 9.5 times. When the draw ratio is less than 1.1 times, the porosity may be lowered, battery characteristics may be lowered, and productivity may be lowered. Moreover, although air permeability resistance falls, so that a draw ratio is made high, when it exceeds 15 times, a film tear may occur easily. The stretching speed in the width direction at this time is preferably 500 to 6,000% / min, more preferably 1,000 to 5,000% / min.

  For the purpose of relaxing the stretching stress and improving the elastic modulus by improving the crystallinity, a heat treatment step may be performed in the tenter following the stretching in the width direction (before re-stretching described later). The heat treatment temperature is preferably not less than the stretching temperature in the width direction and not more than 170 ° C. If the temperature is lower than the initial stretching temperature in the width direction, heat setting may not be sufficient, and the mechanical strength in the width direction may decrease. When the temperature exceeds 170 ° C., the polymer around the pores melts due to the high temperature, the air resistance increases, and the output characteristics may deteriorate. It is more preferable if it is higher than the stretching temperature in the width direction and lower than 167 ° C. from the viewpoint of both output characteristics and safety. The heat treatment time is preferably 0.1 second or longer and 10 seconds or shorter, more preferably 3 seconds or longer and 8 seconds or shorter from the viewpoint of achieving both the mechanical strength in the width direction and the productivity. Let the film obtained by the process so far be an initial stretched film.

  Subsequently, in order to achieve both the mechanical strength and the air resistance of the porous film, it is preferable to redraw the initially stretched film in the machine direction. The expansion of the hole diameter of the hole extending in the film surface direction by this re-stretching suppresses increase in air resistance due to hole blockage by press working. As specific re-stretching conditions, first, the initial stretched film is controlled to a temperature at which it can be re-stretched in the machine direction. As a temperature control method, a method of running a film on a temperature-controlled rotating roll, a method of using a hot air oven, or the like can be adopted. The stretching temperature in the machine direction is preferably 130 ° C. or higher, more preferably 150 ° C. or higher, from the viewpoints of film properties and uniformity. More preferably, it is 160 degreeC or more. If it is less than 130 ° C., the stretching stress increases, so the number of film breaks may increase. In addition, the normal temperature shrinkage in the machine direction may increase, and the flatness may decrease. If the re-stretching temperature is too high, the polymer around the pores melts due to the high temperature and air permeability resistance increases, and the output characteristics may decrease, and thickness spots and air resistance resistance spots may increase. is there.

  The redrawing ratio in the machine direction is preferably 1.02 to 2.0 times, more preferably 1.05 to 1.7 times, and still more preferably 1.1 to 1.4 times. If it is less than 1.05 times, the effect of redrawing may not be exhibited. If it exceeds 2.0 times, the amount of normal temperature shrinkage in the film machine direction becomes large, and when the porous polyolefin film is wound as a roll, the tightening becomes strong and the flatness may be lowered.

  The film may be heat-treated following re-stretching in the machine direction. The heat treatment method of the film can be selected from a method of passing through a roll and a method of introducing the film end by gripping the end of the film with a tenter type stretching machine. The heat treatment temperature is preferably a film temperature of 140 ° C. or higher, more preferably 150 ° C. or higher, and further preferably 160 ° C. or higher. If it is less than 140 degreeC, relaxation of a film will become inadequate and planarity may fall because the normal temperature shrinkage amount of a machine direction becomes large. Moreover, the shrinkage | contraction amount to the width direction becomes large, and the short circuit between positive and negative electrodes may occur inside a battery. If the heat treatment temperature is too high, the polymer around the pores melts due to the high temperature and the air permeability resistance increases, and the output characteristics may decrease, or the thickness spots and air resistance resistance spots may increase, so 170 ° C is the upper limit. . The heat treatment time is preferably 0.1 second or more and 120 seconds or less, more preferably 3 seconds or more and 60 seconds or less from the viewpoint of achieving both stress relaxation and productivity.

  Subsequently, when re-stretching in the width direction in the tenter type stretching machine, the preheating temperature and the stretching temperature are preferably 110 to 165 ° C, and more preferably 120 to 150 ° C. When the preheating temperature and the stretching temperature are less than 110 ° C., the stress at the time of fibril cleavage is too large, and the cause of film breakage or the porosity is too high, which may be inferior in safety. Moreover, when preheating temperature and extending | stretching temperature are higher than 165 degreeC, a porosity may fall and output characteristics may fall.

  The preheating time is preferably 5 to 70 seconds, more preferably 10 to 50 seconds. If the preheating time is less than 5 seconds, the film may not be sufficiently warmed and may cause film breakage. Moreover, when preheating time exceeds 70 second, productivity may be inferior.

  The redraw ratio in the width direction is preferably 2.0 times or less, more preferably 1.8 or less. If it exceeds 2.0 times, film tearing may occur easily. The lower limit is 1.0 times. The stretching speed in the width direction 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, it is preferable to perform a heat treatment step in the tenter. The heat treatment temperature is preferably not less than the re-stretching temperature in the width direction and not more than 170 ° C. If the temperature is less than the re-stretching temperature in the width direction, heat setting may not be sufficient, and the tensile strength in the width direction may decrease. When the temperature exceeds 170 ° C., the polymer around the pores melts due to high temperature and the air resistance increases, and the output characteristics may deteriorate, and the thickness spots and air resistance resistance spots may increase. From the viewpoint of achieving both output characteristics and safety, it is more preferable if it is not less than the re-stretching temperature in the width direction and not more than 165 ° C. The heat treatment time is preferably 0.1 second or longer and 10 seconds or shorter, more preferably 3 seconds or longer and 8 seconds or shorter from the viewpoint of achieving both mechanical strength and productivity.

  Following the re-stretching in the width direction, or after the heat treatment step after the re-stretching in the width direction, a relaxation treatment may be performed. When relaxing, the relaxation rate is preferably 0 to 35% in the width direction. If the relaxation rate exceeds 35%, the air permeability resistance may be reduced to deteriorate the output characteristics, and the thickness unevenness and flatness in the width direction may be deteriorated. From the viewpoint of achieving both output characteristics and safety, it is more preferably 13 to 25%. The relaxation temperature is preferably 155 to 170 ° C. When the relaxation temperature is less than 155 ° C., the shrinkage stress for relaxation is lowered, the above-described high relaxation rate cannot be achieved, and the thermal shrinkage rate in the width direction may be increased. When the temperature exceeds 170 ° C., the polymer around the pores melts due to high temperature, and the air resistance may decrease, or the thickness spots and air resistance resistance spots may increase. From the viewpoint of output characteristics and safety, it is preferably not less than the initial stretching temperature in the width direction and not more than 167 ° C.

  In the present invention, the porous sheet obtained as described above is pressed under the following conditions (1) and (2) to obtain a porous film having through-holes.

(1) Press temperature is 100 ° C. or higher (2) Press pressure is linear pressure 1-100 kgf / cm
In the production method of the present invention, it is preferable to perform pressing in a state where the film is heated to reduce the stress against deformation. If this is not enough, it is necessary to set the press pressure high in order to deform it in the film thickness direction until a predetermined porosity is reached, but press working at high pressure closes the through hole and increases the air resistance. There is a fear. Therefore, the press temperature is 100 ° C. or higher, preferably 110 ° C. or higher, more preferably 130 ° C. or higher. Furthermore, it is preferable to carry out below the melting point of the polymer used from the concern of pore blockage due to melting of the film. In addition, the press temperature in this specification means the film temperature just before a press.

  The porosity of the porous film obtained by the production method of the present invention is preferably 30% or more, more preferably 40% or more in order to provide sufficient permeability. % Or less is preferable, and more preferably 65% or less. Further, the porosity is more preferably 30 to 75%.

  The rate of increase in air resistance in the present invention refers to a value obtained by dividing the difference between the air resistance of the porous film and the air resistance of the porous sheet by the air resistance of the porous sheet (see the following formula). In order to maintain the above, it is preferable to suppress this value to 10% or less. From the viewpoint of the insulating properties of the film, this value is preferably -50% or more. The air permeability resistance of the porous film is 50 to 500 seconds, preferably 100 to 300 seconds, from the viewpoint of compatibility between permeability when used as a separator and insulation.

Air permeability resistance increase rate (%) = ((air resistance of porous film−air resistance of porous sheet) / air resistance of porous sheet) × 100
(Note that the “porous sheet” in the present invention refers to a film that has not undergone pressing and has undergone an initial stretching step, and the “porous film” is an unstretched sheet in the above-described porous sheet manufacturing process. (This means a film that has been pressed after the process. In addition, the same applies to the present invention.)
The film thickness deformation rate in the present invention is a value obtained by dividing the film thickness of the porous film reduced by the press work by the thickness of the porous sheet, while suppressing the increase in air resistance due to the blockage of the pores, and reducing the porosity. In order to obtain the effect, it is preferably 1 to 50%, more preferably 2 to 30%.

  In addition, it is preferable to preheat before pressing a porous sheet. As a temperature control method, a method of running a film on a temperature-controlled rotating roll, a method of using a hot air oven, or the like can be adopted. The preheating temperature is set to 50 ° C. or higher, preferably 70 ° C. or higher. If it is lower than this, there is a concern that the film is not sufficiently heated at the time of pressing, and wrinkles are caused by being rapidly heated at the time of pressing. On the other hand, the preheating temperature is set to [(pressing temperature) −20] (° C.) or less because of concern about deterioration of flatness due to thermal shrinkage of the film. The preheating time is 1 second or longer, preferably 3 seconds or longer for sufficient preheating of the film, and 100 seconds or shorter, preferably 60 seconds or shorter, due to the concern of deterioration of flatness due to prolonged heating.

  The porous sheet to be pressed in the present invention has an excellent surface openness, and the porosity can be reduced without crushing holes (holes extending in the surface direction of the film) necessary for the permeation of ions and gases by pressing. Can be lowered. The surface open area ratio of the porous sheet is preferably 30 to 90%, more preferably 40 to 80%, and if lower than 30%, the air resistance increases by pressing, while if higher than 90%, The mechanical strength of the film tends to be low.

  The porous sheet is pressed at least once between a flat plate press or a pair of rotating rolls, and a method of pressing between rolls is more preferable from the viewpoint of productivity.

  A metal, resin, ceramic film roll or the like can be applied to the pair of rolls used, and one of the rolls is preferably resinous. The hardness of the resin roll is preferably 50 or more in a type A rubber hardness tester according to JIS K 6253 (2012) in order to suppress roll wear, and type D due to the possibility of uneven press due to low cushioning properties. The rubber hardness meter is more preferably 100 or less.

  The surface roughness (Ra) defined by JIS B 0601 (2013) is 50 μm or less by a measuring method based on JIS B 0633 (2001) because of concern about the decrease in smoothness of the film surface after pressing. It is preferable to use rolls, more preferably a pair of rolls having a thickness of 20 μm or less.

  The press speed in the machine direction of the film (transport direction of the porous sheet) is 1 to 150 m / min or less from the viewpoint of productivity that heating of the film does not become insufficient because the contact time with the roll is shortened. Preferably, it is 5-100 m / min.

  The press pressure is not particularly limited in order to obtain the predetermined film pressure deformation rate and porosity, but it is also necessary to suppress the increase in air resistance due to the blockage of the hole and to obtain the effect of reducing the porosity by pressing. The pressure is preferably 1 to 100 kgf / cm, more preferably 1 to 80 kgf / cm.

  By the above press working, the puncture strength of the porous film is increased as compared with the porous sheet. This is not only due to compression in the thickness direction, but the detailed cause is unknown, but it is thought that the puncture strength is further increased by changing the complicated hole structure.

  This press working may be incorporated after any film forming step after forming a cast sheet. In order to obtain the effect of reducing the porosity, it is preferably after the initial stretching in the machine direction in which holes are formed in the film, more preferably after the initial stretching in the width direction in which through-holes are formed. The “porous sheet” described in the present invention refers to a film that has not been subjected to press working and has undergone an initial stretching process, and the “porous film” refers to an unstretched sheet in the above-described porous sheet manufacturing process. It means a film that has been subjected to press working after the process of forming.

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

(1) Film thickness, rate of change in film thickness A total of 10 places in any place are contact-type film thickness meter Mitutoyo's Lightmatic VL-50A (10.5 mmφ super hard spherical measuring element, measuring load 0.06 N). The average value was measured as the film thickness of the porous film. Furthermore, the rate of change in film thickness due to press working was defined as [(film thickness of porous film−film thickness of porous sheet) × 100 / film thickness of porous sheet].

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

(3) Porosity The following formula was used to calculate the porosity.

Porosity = {1− (film thickness when porosity is 0% / film thickness of porous film)} × 100
The film thickness when the porosity was 0% was calculated from the mass of the porous film cut out in a 100 mm square with the density of the polymer portion being 0.91 g / cm ³ . As the film thickness of the porous film, the value calculated by the above-described film thickness measurement method was used.

(4) Puncture strength (gf / 18μm)
Measurement was performed at 25 ° C. according to JIS Z 1707 (1997) using a universal testing machine (Autograph AG-IS manufactured by Shimadzu Corporation). The maximum stress when the pin penetrates when the tip of the pin was pierced at a constant speed with respect to the test piece at the time of measurement was read, and the average value was calculated 5 times for each sample. The puncture strength in terms of 18 μm was calculated by multiplying this value by 18 (μm) / film thickness (μm). As the film thickness, the value calculated by the above-described film thickness measurement method was used.

(5) Surface open area ratio A porous film is ion-coated using an IB-5 type ion coater manufactured by Eiko Engineering, and the surface of the film is photographed using a field emission scanning microscope (JSM-6700F) manufactured by JEOL Ltd. It was observed at 1,000 times. The obtained image data (the image of only the observation part without the display of the scale bar or the like) was transferred to HALCON Ver. Image analysis was performed using 10.0, and the surface area ratio (%) was calculated. As an image analysis method, first, an 11-pixel average image A and a 3-pixel average image B were respectively generated for a 256-tone monochrome image, and the area (Area_all) of the entire image B was calculated.

  Next, the image A was removed from the image B as a difference, an image C was generated, and a region D where the luminance ≧ 10 was extracted. The extracted region D was divided into chunks, and a region E with an area ≧ 100 was extracted. For the region E, a region F subjected to a closing process with a circular element having a radius of 2.5 pixels is generated, and a region G subjected to an opening process with a rectangular element of 1 × 5 pixels is generated, whereby a vertical size <5 The pixel portion was removed. And the area | region G was divided | segmented for every lump and the fibril area | region was extracted by extracting the area | region H used as area> = 500.

  Further, a region I where the image ≧ 5 is extracted from the image C, the region I is divided into chunks, and a region J where the area ≧ 300 is extracted. An area K is subjected to an opening process with a circular element having a radius of 1.5 pixels and then subjected to a closing process with a circular element having a radius of 8.5 pixels. L was extracted. In the region L, an unopened region other than the fibrils was extracted by generating a region M in which a dark portion with an area ≧ 4,000 pixels was filled with a bright portion.

  Finally, the sum area N of the area H and the area M was generated, and the area of the sum area N (Area_closed) was calculated to obtain the area of the unopened portion. In addition, the calculation of the surface open area ratio was calculated by the following formula.

Surface open area ratio (%) = (Area_all−Area_closed) / Area_all
(6) Evaluation of air permeability resistance of post-press film (porous film) The rate of increase in air resistance due to pressing is the ratio of air resistance increase (%) = ((air resistance of porous film−air permeability of porous sheet) Resistance) / air permeability resistance of porous sheet) × 100, and the influence on the air resistance due to pressing was evaluated from the rate of increase in air resistance according to the following criteria.

○: Air permeability resistance increase rate 10% or less and air resistance 500 seconds or less Δ: Air resistance increase rate 10% or less or air resistance 500 seconds or less X: Air resistance increase rate 10% or more and air resistance Exceeding 500 seconds (7) Evaluation of the puncture strength of the post-press film (porous film) The puncture strength increase rate by pressing is calculated as the puncture strength increase rate (%) = ((puncture strength of porous film−puncture strength of porous sheet ) / Puncture strength of porous sheet) × 100, and the influence on the puncture strength by pressing was evaluated from the rate of increase in puncture strength according to the following criteria.

  A: The puncture strength increase rate is 15% or more.

  Δ: Increase rate of puncture strength is 10% or more and less than 15%.

  X: The increase rate of puncture strength is less than 10%.

(Example 1)
As raw material resin for porous resin film, 99.5% by mass of homopolypropylene FLX80E4 manufactured by Sumitomo Chemical Co., Ltd., N, N'-dicyclohexyl-2,6-naphthalenedicarboxyamide (β-nucleating agent) Co., Ltd. (Nu-100) 0.3% by mass, and further, 0.1% and 0.1% by mass of IRGANOX1010 and IRGAFOS168 manufactured by Ciba Specialty Chemicals, which are antioxidants, are mixed in this ratio. The raw material is fed from the weighing hopper to the twin screw extruder, melt kneaded at 300 ° C., discharged from the die in a strand shape, cooled and solidified in a 25 ° C. water bath, cut into a chip shape, and the chip raw material A did. 64.8% by mass of this chip raw material A, 30% by mass of ethylene-octene-1 copolymer (Dow Chemical Engage 8411, melt index: 18 g / 10 min) as copolymer PE resin, and manufactured by JSR as a dispersant Olefin crystal, ethylene butylene, olefin crystal block polymer (CEBC) “DYNARON (registered trademark)” (type name: 6200P) is 5% by mass, and an antioxidant, IRGAFOS168, manufactured by Ciba Specialty Chemicals, is set to 0.0. The raw material is supplied from the weighing hopper to the twin screw extruder so that 0.1% by mass of IRGANOX1010 manufactured by Ciba Specialty Chemicals is mixed at this ratio, and melt kneaded at 230 ° C. And then solidify by cooling in a 25 ° C water bath Then, a chip material B was obtained by cutting into chips. Further, 79.9% by mass of chip raw material A is mixed with 20% by mass of chip raw material B and 0.1% by mass of IRGANOX1010 manufactured by Ciba Specialty Chemicals, supplied from a measuring hopper to a twin screw extruder, melt kneaded at 230 ° C., strand It was discharged from the die in a shape, cooled and solidified in a water bath at 25 ° C., cut into a chip shape, and chip raw material C was obtained.

  This chip raw material C is supplied to a single-screw extruder and melt-extruded at 220 ° C. After removing foreign matter with a 60 μm cut sintered filter, it is discharged from a T-die onto a cast drum whose surface temperature is controlled at 120 ° C. An unstretched sheet was obtained by casting the drum indirectly for 15 seconds. Next, the film was heated to 120 ° C., preheated using a ceramic roll, and stretched 5 times in the longitudinal direction of the film. After cooling, the end was then held by a clip with a clip and introduced into a tenter type stretching machine and stretched 6.5 times at 145 ° C. As it was, heat treatment was performed at 160 ° C. for 6 seconds while relaxing 10% in the width direction.

  Subsequently, re-stretching in the machine direction was performed. A porous sheet was obtained by re-stretching 1.2 times in the machine direction on a roll whose temperature was controlled at 150 ° C., and heat setting at 150 ° C.

  Subsequently, this porous sheet is preheated to 100 ° C. by running on a heated HCr roll, and is 70 with a heated HCr roll and a type A rubber hardness meter, and the surface roughness (Ra) is 0.4 or less. A porous film was obtained by passing between silicon nip rolls at a press temperature of 150 ° C. and a linear pressure of 10 kgf / cm for pressing.

(Example 2)
The same procedure as in Example 1 was performed except that re-stretching in the machine direction and subsequent heat treatment (heat setting) were not performed.

(Example 3)
Chip raw material A is supplied to a single screw extruder, melt extruded at 190 ° C., foreign matter is removed by a 25 μm cut sintered filter, and then discharged from a T die to a cast drum whose surface temperature is controlled at 125 ° C. for casting. Thus, an unstretched sheet was obtained. Next, preheating was performed using a ceramic roll heated to 125 ° C., and the film was stretched 5.3 times in the longitudinal direction of the film. After cooling, the end portion was introduced into a tenter type stretching machine by holding the clip with a clip, and stretched 9.2 times at 150 ° C. As it was, heat treatment was performed at 160 ° C. while relaxing 17% in the width direction to obtain a porous sheet having a thickness of 18 μm.

  Subsequently, this porous sheet is preheated to 100 ° C. by running on a heated HCr roll, and is 70 with a heated HCr roll and a type A rubber hardness meter, and the surface roughness (Ra) is 0.4 or less. Pressing was performed by passing between silicon nip rolls at a press temperature of 150 ° C. and a linear pressure of 10 kgf / cm.

Example 4
A porous sheet formed under the same conditions as in Example 1 was used. Subsequently, the porous sheet is preheated to 60 ° C. by running on a heated HCr roll, and is 70 with a heated HCr roll and a type A rubber hardness meter, and the surface roughness (Ra) is 0.4 or less. Pressing was performed by passing between silicon nip rolls at a press temperature of 150 ° C. and a linear pressure of 10 kgf / cm.

(Comparative Example 1)
A porous sheet formed under the same conditions as in Example 1 was used. This porous sheet is preheated to 80 ° C. by running on a heated HCr roll, and is made of silicon having a heated HCr roll and a type A rubber hardness meter of 70 and a surface roughness (Ra) of 0.4 or less. The nip rolls were pressed at a press temperature of 80 ° C. and a linear pressure of 10 kgf / cm.

(Comparative Example 2)
A porous sheet formed under the same conditions as in Example 1 was used. This porous sheet is preheated to 100 ° C. by running on a heated HCr roll, and is made of silicon having a heated HCr roll and a type A rubber hardness meter of 70 and a surface roughness (Ra) of 0.4 or less. The nip rolls were pressed at a press temperature of 150 ° C. and a linear pressure of 150 kgf / cm.

(Comparative Example 3)
A porous sheet formed under the same conditions as in Example 1 was used. This porous sheet is preheated to 100 ° C. by running on a heated HCr roll, and is made of silicon having a heated HCr roll and a type A rubber hardness meter of 70 and a surface roughness (Ra) of 0.4 or less. The nip rolls were pressed at a press temperature of 150 ° C. and a linear pressure of 0.5 kgf / cm.

In an embodiment that satisfies the requirements of the present invention, the increase in air permeability resistance is suppressed, and the puncture strength is improved by reducing the porosity, and it can be suitably used as a separator for an electricity storage device. On the other hand, in the comparative example, air resistance is not increased or puncture strength is not improved, and it is difficult to use as a separator for an electricity storage device.

  Since the porous film produced according to the present invention can increase the mechanical strength while maintaining low air permeability, it is suitable as a separator for an electricity storage device, particularly a lithium ion battery that is a nonaqueous electrolyte secondary battery. Can be used.

Claims (6)

When a porous sheet is pressed to produce a porous film having through-holes, the porous film is subjected to the pressing in the thickness direction of the porous sheet under the following conditions (1) and (2). Production method.
(1) Press temperature is 100 ° C. or higher (2) Press pressure is linear pressure 1-100 kgf / cm The method for producing a porous film according to claim 1, wherein the porosity of the porous film is 30 to 75%. The method for producing a porous film according to claim 1 or 2, wherein the rate of increase in air resistance calculated by the following formula is 10% or less, and the air resistance of the porous film is 50 to 500 seconds. .
Air permeability resistance increase rate (%) = ((air resistance of porous film−air resistance of porous sheet) / air resistance of porous sheet) × 100 The method for producing a porous film according to any one of claims 1 to 3, wherein the porous sheet is preheated at [(pressing temperature) -20] (° C) or less before pressing the porous sheet. The manufacturing method of the porous film in any one of Claims 1-4 whose main component of a porous film is a polypropylene. The manufacturing method of the porous film in any one of Claims 1-5 which press-processes the porous sheet whose surface open area rate is 30 to 90%.