JPWO2012169510A1 - Porous polypropylene film and method for producing the same - Google Patents

Abstract

The present invention is a porous polypropylene film containing a polypropylene resin and having a deviation of 3% shrinkage temperature in the film width direction represented by the following formula (1) of less than 0.05, and a method for producing the same, and the width of the film Excellent uniformity of thermal dimensional change in direction. Deviation of 3% shrinkage temperature in the film width direction = (Tmax−Tmin) / Tave (1) Tmax: The highest temperature among the temperatures showing 3% shrinkage in the measurement points of the shrinkage curve in the film width direction, Tmin: the lowest temperature among the 3% shrinkage temperature measurement points in the film width direction, Tave: average temperature at all measurement points of the film width direction shrinkage curve, measurement of the film width direction shrinkage curve Point: A position in the center in the film width direction and every 30 mm toward both ends with the center as a base point.

Description

  The present invention relates to a porous polypropylene film excellent in the uniformity of thermal dimensional change in the film width direction and a method for producing the same.

  Polypropylene films are used in various applications such as industrial materials, packaging materials, optical materials, and electrical materials due to their excellent mechanical, thermal, electrical, and optical properties. Since this porous polypropylene film is porous and porous, it has excellent properties such as permeability and low specific gravity in addition to the properties of a polypropylene film. Development into a wide range of applications such as separation membranes, clothing, moisture-permeable waterproof membranes in medical applications, reflectors for flat panel displays, and thermal transfer recording sheets is under consideration.

  Various proposals have been made as methods for making a polypropylene film porous. Porous methods can be broadly classified into wet methods and dry methods.

  In the wet method, polypropylene is used as the matrix resin, the extractables to be extracted after sheeting are added and mixed, and only the additives are extracted using the good solvent of the extractables, creating voids in the matrix resin. Various methods have been proposed (for example, see Patent Document 1).

  On the other hand, as a dry method, for example, by adopting low temperature extrusion and high draft ratio at the time of melt extrusion, the lamellar structure in the film before stretching formed into a sheet is controlled, and the lamella is stretched uniaxially in the longitudinal direction. There has been proposed a method (so-called lamellar stretching method) in which cleavage at the interface is generated to form voids (see, for example, Patent Document 2).

  On the other hand, as a porous polypropylene film that is a dry method and can be produced in a wide and large area because it is formed by biaxial stretching, α-type crystal (α crystal) and β Many proposals have been made of a so-called β crystal method in which voids are formed in a film by utilizing the difference in crystal density and crystal transition of a type crystal (β crystal) (see, for example, Patent Documents 3 to 5). .

For the purpose of improving the uniformity in the width direction, a method for improving the thickness uniformity by controlling the lamellar orientation in the stretching process (see, for example, Patent Document 6), the casting conditions, the longitudinal stretching conditions, and the transparency by controlling the winding. A method (for example, refer to Patent Document 7) for improving the temper, porosity, and thickness uniformity has also been proposed.
Japanese Patent Laid-Open No. 55-131028 Japanese Patent Publication No.55-32531 JP-A 63-199742 JP-A-6-100720 Japanese Patent Laid-Open No. 9-255804 International Publication No. 2002/066233 JP 2010-242060 A

  However, any of the methods disclosed in Patent Document 1 and Patent Document 2 have difficulty in production efficiency, such as being difficult to manufacture in a wide and large area and increasing costs.

  The methods disclosed in Patent Documents 3 to 5 can form a porous film having excellent air permeability with a wide width, a large area and high productivity, but the width of the porous polypropylene film is also stretched in the width direction. In some cases, the thickness of the direction, the air permeability, and the uniformity of the porosity were inferior.

  In the methods disclosed in Patent Document 6 and Patent Document 7, heat fixation and relaxation treatment are insufficient in the heat treatment process, and thus the reduction and uniformity of the thermal dimensional change in the width direction are insufficient. When the slit is cut to the width used for the separator, the thermal dimension unevenness occurs at each slit position, and when it is used as a storage device separator, the battery performance of the storage device is insufficiently uniform, causing the manufacturing yield to deteriorate. There was a case.

  An object of the present invention is to solve the above-described problems. That is, an object of the present invention is to provide a porous polypropylene film excellent in uniformity of thermal dimensional change in the film width direction and a method for producing the same.

In order to solve the above problems, the porous polypropylene film of the present invention has the following configuration. That is,
It is a porous polypropylene film containing a polypropylene resin and having a deviation of 3% shrinkage temperature in the film width direction represented by the following formula (1) of less than 0.05.

Deviation of 3% shrinkage temperature in the film width direction = (Tmax−Tmin) / Tave (1)
here,
Tmax: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Tmin: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Low temperature Tave: Average temperature at all measurement points of the shrinkage curve in the film width direction Measurement points of the shrinkage curve in the film width direction: The center in the film width direction, and the position at every 30 mm toward both ends with the same center as the base point The manufacturing method of the porous polypropylene film of this invention has the following structure. That is,
A polypropylene resin is melt-extruded on a support to form a polypropylene resin sheet, and the polypropylene resin sheet is biaxially stretched and then subjected to a heat treatment to produce a porous polypropylene film, the heat treatment comprising a tension treatment and a relaxation treatment. A multi-stage heat treatment step having a plurality of steps, and the multi-stage heat treatment step includes at least a step having a relaxation treatment in which the total relaxation rate exceeds 15% and the relaxation rate in the width direction is 5 to 15%. A method for producing a porous polypropylene film having two steps and having a heat treatment temperature in a multistage heat treatment step of not less than the stretching temperature and not more than the melting point Tm of the film.

  In the porous polypropylene film of the present invention, the 3% shrinkage temperature in the film width direction at each measurement point of the shrinkage curve in the film width direction is preferably 130 ° C. or higher.

  The porous polypropylene film of the present invention preferably has a β crystal forming ability of the porous polypropylene film of 60% or more.

  In the method for producing a porous polypropylene film of the present invention, the heat treatment temperature of the first step in the multistage heat treatment step is not less than the transverse stretching temperature and not more than the melting point Tm of the film, and the heat treatment temperature after the second step is not less than the heat treatment temperature of the immediately preceding step. It is preferable that it is below melting | fusing point Tm of a film.

  Since the porous polypropylene film of the present invention is excellent in the uniformity of thermal dimensional change in the film width direction, for example, when used as a separator for an electricity storage device, a battery excellent in battery performance uniformity can be obtained.

  The present invention is a porous polypropylene film containing a polypropylene resin and having a deviation of 3% shrinkage temperature in the film width direction represented by the following formula (1) of less than 0.05.

Deviation of 3% shrinkage temperature in the film width direction = (Tmax−Tmin) / Tave (1)
here,
Tmax: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Tmin: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Low temperature Tave: Average temperature at all measurement points of the shrinkage curve in the film width direction Measurement points of the shrinkage curve in the film width direction: The center in the film width direction, and the position at every 30 mm toward both ends with the same center as the base point The above measurement is performed under the condition that the temperature is increased from 25 ° C. to 160 ° C. at a temperature increase rate of 4 mm width × measured length 15 mm, film width direction (TD) load 0.15 MPa, 5 ° C./min by Thermal Mechanical Analysys (TMA). In the width direction (TD).

  In the porous polypropylene film of the present invention, when the deviation of the 3% shrinkage temperature in the film width direction (TD) is 0.05 or more, thermal shrinkage unevenness is likely to occur in the width direction of the film roll. As a result, not only when the film roll width is used as a power storage device separator, but also when the film roll is slit to the width of the power storage device separator, for example, physical property irregularities occur in each slit roll, and part of the battery performance test occurs. Cause thermal deterioration and lead to deterioration of characteristics, and insufficient battery performance uniformity. From the above viewpoint, the deviation of the 3% shrinkage temperature in the film width direction (TD) is preferably less than 0.04, more preferably less than 0.03. The lower limit is 0.001. To make the deviation within the above range can be realized by appropriately controlling the heat treatment conditions at the time of manufacture, details will be described later.

  The porous polypropylene film of the present invention has pores that penetrate both surfaces of the film and have air permeability (hereinafter referred to as through-holes). The through holes are preferably formed in the film by at least uniaxial or biaxial stretching, and are preferably formed by a β crystal method from the viewpoint of achieving high productivity, uniform physical properties, and thinning.

  In order to form through holes in the film using the β crystal method, it is preferable that the β crystal forming ability of the polypropylene resin is 60% or more. If the β-crystal forming ability is within this preferable range, the amount of β-crystals is sufficient at the time of film production, so that the number of voids formed in the film using the transition to α-crystal becomes sufficient, and as a result, the permeability is improved. An excellent film is obtained. On the other hand, the upper limit of the β-crystal forming ability is not particularly limited, but it exceeds 99.9% by adding a large amount of the β-crystal nucleating agent described later or the stereoregulation of the polypropylene resin to be used. The industrial practical value is low, for example, the film forming stability is deteriorated. Industrially, β-crystal forming ability is preferably 65 to 99.9%, particularly preferably 70 to 95%. Similarly, the β-crystal forming ability of the porous polypropylene film is preferably 60% or more.

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 various pigment compounds and amide compounds. In particular, amide compounds disclosed in JP-A-5-310665 can be preferably used. The addition amount of the β crystal nucleating agent is preferably 0.05 to 0.5% by mass, more preferably 0.1 to 0.3% by mass, based on the whole polypropylene resin. If the added amount of the β crystal nucleating agent is 0.05% by mass or more, the formation of β crystals is sufficient, and the air permeability of the porous polypropylene film can be increased. If the addition amount of the β crystal nucleating agent is 0.5% by mass or less, a coarse void is not formed, and safety can be improved when it is used for a separator for an electricity storage device.
The porous polypropylene film of the present invention is substantially composed of a polypropylene resin, and when the total polypropylene resin constituting the film is 100% by mass, it is 80% by mass or more of the polypropylene resin, and the thermal dimensional stability of the film. In view of the above, it is preferably 85% by mass or more, more preferably 90% by mass or more.

  The polypropylene resin constituting the porous polypropylene film of the present invention preferably has a melt flow rate (hereinafter referred to as MFR, measurement conditions: 230 ° C., 2.16 kg) in the range of 2 to 30 g / 10 min. When the MFR is 2 g / 10 min or more, the melt viscosity of the resin does not become too high, high-precision filtration is possible, and the high quality of the film can be maintained. When the MFR is 30 g / 10 min or less, the molecular weight does not become too low, the film is hardly broken during stretching, and high productivity can be maintained. More preferably, the MFR is 3 to 20 g / 10 min.

  Moreover, it is preferable that the polypropylene resin which comprises the porous polypropylene film of this invention is an isotactic polypropylene resin. When isotactic polypropylene resin is used, the isotactic index is preferably 90 to 99.9%. When the isotactic index is within this preferred range, the resin has high crystallinity and it is easy to achieve high air permeability.

  As the polypropylene resin used in the present invention, it is possible to use a homopolypropylene resin, as well as from the viewpoint of stability in the film-forming process, film-forming properties, and uniformity of physical properties, polypropylene with an ethylene component or butene, Resins obtained by copolymerizing α-olefin components such as hexene and octene in an amount of 10% by mass or less, more preferably 5% by mass or less, and further preferably 2.5% by mass or less can also be used. The form of introduction of the comonomer (copolymerization component) into polypropylene may be either random copolymerization or block copolymerization.

  In addition, it is preferable that the above-mentioned polypropylene resin contains a high molecular weight polypropylene, a low melting point polypropylene, a high melt tension polypropylene or the like in a range not impairing the effects of the present invention from the viewpoint of improving safety and improving film forming property. Here, the high molecular weight polypropylene is a polypropylene having an MFR of 0.1 to 2 g / 10 min, and the low melting point polypropylene is a polypropylene having a melting point lower than a resin melting point 153 ° C. (for example, ethylene component, butene, hexene, octene, etc. Polypropylene copolymerized with an α-olefin component), and high melt tension polypropylene is a mixture of a high molecular weight component or a component having a branched structure in a polypropylene resin, or copolymerization of a long-chain branched component with polypropylene. Polypropylene resin with increased tension in the molten state.

  The polypropylene resin used in the present invention is composed of 80 to 99% by mass of polypropylene and ethylene / α-olefin from the viewpoint of improving void formation efficiency during biaxial stretching, and improving air permeability by expanding the hole diameter and the hole diameter. It is preferable to use a mixture of 20% by mass or less of the copolymer. 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) (type names: 8411, 8452, 8100, etc.) manufactured by Dow Chemical.

  From the viewpoint of improving air permeability, the copolymer polyethylene resin is preferably contained in an amount of 1 to 10% by mass when the entire polypropylene resin constituting the film of the present invention is 100% by mass. More preferably, it is 1-7 mass%, More preferably, it is 1-5 mass%.

  The polypropylene resin forming the porous polypropylene film of the present invention preferably has a cold xylene soluble component (CXS) of less than 2% by mass. More preferably, it is less than 1.5 mass%. When the CXS is less than 2% by mass, the low molecular weight component is small and the mechanical properties of the porous polypropylene film are excellent. In order to make CXS less than 2% by mass, a method of polymerizing with a polymerization catalyst system capable of reducing CXS, a method of removing atactic polymer by providing a washing step after the polymerization reaction, or the like can be used.

  In the polypropylene resin forming the porous polypropylene film of the present invention, the amount of hydrotalcite in the polypropylene resin is preferably 0.01% by mass or less. More preferably, it is 0.005 mass% or less, More preferably, it is 0.001 mass% or less. Hydrotalcite may inhibit β-crystal formation, and if the amount of hydrotalcite is 0.01% by mass or less, the air permeability of the porous polypropylene film can be maintained high.

  The polypropylene resin forming the porous polypropylene film of the present invention preferably has an ash content in the polypropylene resin of 0.01% by mass or less. When the ash content is 0.01% by mass or less, the withstand voltage is high and the battery life is long when used for a separator for an electricity storage device.

  In the polypropylene resin forming the porous polypropylene film of the present invention, an antioxidant, a heat stabilizer, a neutralizing agent, an antistatic agent and a lubricant composed of inorganic or organic particles, as long as the effects of the present invention are not impaired, May contain various additives such as anti-blocking agents, fillers, and incompatible polymers. In particular, it is preferable to add an antioxidant for the purpose of suppressing the oxidative deterioration due to the heat history of the polypropylene resin, but the amount of the antioxidant added is preferably 2% by mass or less with respect to 100% by mass of the polypropylene resin. More preferably, it is 1 mass% or less, More preferably, it is 0.5 mass% or less.

  The porous polypropylene film of the present invention preferably has a porosity of 35 to 80% from the viewpoint of achieving both ionic conductivity and safety when used as a separator. When the porosity is 35% or more, the electrical resistance can be reduced when used as a separator. On the other hand, when the porosity is 80% or less, it is excellent in safety when used in a separator for a large-capacity battery such as an electric vehicle. From the viewpoint of developing excellent battery characteristics, the porosity of the film is more preferably 40 to 75%, and even more preferably 40 to 70%.

  The porous polypropylene film of the present invention preferably has an air permeability resistance of 50 to 1,000 sec / 100 mL. More preferably, it is 80-600 sec / 100mL, More preferably, it is 80-400 sec / 100mL. When the air permeation resistance is 50 sec / 100 mL or more, the mechanical strength of the film is not lowered and the handling property is not lowered, and the safety is not lowered when it is used for a separator. When the air permeation resistance is 1,000 sec / 100 mL or less, the output characteristics do not deteriorate when used for a separator.

  In the method for producing a porous polypropylene film of the present invention, the heat treatment after biaxial stretching is performed under specific conditions as will be described later, so that the porous material has excellent uniformity of thermal dimensional change in the film width direction. Can be obtained. As the conditions for the heat treatment, a polypropylene resin is melt-extruded on a support to obtain a polypropylene resin sheet, and when the polypropylene resin sheet is biaxially stretched and then subjected to a heat treatment to produce a porous polypropylene film, the heat treatment is a tension treatment. Including a multi-stage heat treatment process having a plurality of steps including a relaxation treatment as a set. The multi-stage heat treatment process has a relaxation treatment in which the total relaxation rate exceeds 15% and the relaxation rate in the width direction is 5 to 15%. It is necessary to have at least two steps and that the heat treatment temperature in the multistage heat treatment step is not less than the stretching temperature and not more than the melting point Tm of the film.

In the above, biaxial stretching is preferably performed in the longitudinal direction (longitudinal direction, MD) using a stretching roll or the like and then stretched in the transverse direction (width direction, TD) using a tenter or the like. At this time, the tenter can be divided into three processes, a preheating process, a lateral stretching process, and a heat treatment process. As described above, the tensioning process and the relaxation process are a set of steps for the heat treatment process. It is necessary to include a multistage heat treatment step having a plurality of the above. Here, the tension treatment refers to heat treatment with a fixed length in the width direction of the film, and the relaxation treatment refers to heat treatment while reducing the length in the width direction by 1% or more. Each of the steps described above preferably includes a relaxation process in which the relaxation rate in the width direction is 5 to 15%, and it is necessary to have at least two such steps. Furthermore, the total relaxation rate in the multi-stage heat treatment step needs to exceed 15% from the viewpoint of obtaining the uniformity effect of the thermal dimensional change in the width direction. When the total relaxation rate is 15% or less, the stress relaxation caused by stretching becomes insufficient, and the film tends to be inferior in the uniformity of the thermal dimensional change in the width direction. A more preferable total relaxation rate is 17% or more, and further preferably 20% or more. The upper limit of the total relaxation rate is not particularly limited, but is preferably 50%. If it exceeds 50%, the film flatness may deteriorate. Here, the total relaxation rate is defined as follows. The distance between the tenter clips after transverse stretching is the width direction length (L ₀ ), and the distance between the tenter clips after the relaxation treatment at the first step is the width direction length (L ₁ ). Thereafter, the second step, the third step, ... When the n-th step is (L ₂ ), (L ₃ ), ... (L _n ), the relaxation rate at each step is represented by the following equation (1),
Formula (1):
First step relaxation rate (Rx ₁ ) = {(L ₀ ) − (L ₁ )} / (L ₀ )
Relaxation rate of the second step (Rx ₂ ) = {(L ₁ ) − (L ₂ )} / (L ₀ )
Relaxation rate at the third step (Rx ₃ ) = {(L ₂ ) − (L ₃ )} / (L ₀ )
Relaxation rate at the n-th step (Rx _n ) = {(L _n−1 ) − (L _n )} / (L ₀ )
The total relaxation rate with n steps is expressed by the following equation (2).

  Formula (2):

  Moreover, the heat treatment temperature in the above-described multistage heat treatment step is required to be not less than the stretching temperature and not more than the melting point Tm of the film in any step.

  When the heat treatment process is one step or when the relaxation rate of each step is less than 5%, the stress relaxation caused by transverse stretching cannot be sufficiently uniformly performed in the plane, and the heat dimension in the film width direction If the uniformity of change is insufficient, or if the heat treatment process is one step and the relaxation rate exceeds 15%, the tenter outlet width becomes extremely narrow when two or more steps are applied according to the present invention. In some cases, productivity is inferior because the final product area is small. As described above, by providing two or more steps including a relaxation treatment in which the relaxation rate in the width direction is 5 to 15% in a multistage heat treatment process, partial residual stress that could not be relaxed in one step is released. And a film having a uniform thermal dimensional change in the width direction can be obtained.

  Here, the rate at which the relaxation treatment is performed (relaxation rate) is preferably 50 to 1,000% / min. When the relaxation rate is 50% / min or more, there is no need to slow down the film forming speed or increase the tenter length, and the productivity is excellent. If the relaxation speed is 1,000% / min or less, the speed at which the film shrinks will not be slower than the speed at which the rail width of the tenter shrinks, and the film flutters in the tenter and is not flat. It does not occur. The relaxation rate is more preferably 100 to 800% / min.

  In the production method of the porous polypropylene film of the present invention, from the viewpoint of further improving the uniformity of the thermal dimensional change in the film width direction, the heat treatment temperature of the first step in the multistage heat treatment step is set to the transverse stretching temperature or more and the film melting point Tm or less. It is preferable that the heat treatment temperature after the second step is not less than the heat treatment temperature of the previous step and not more than the melting point Tm of the film. When the heat treatment temperature in the first step (first step) is equal to or higher than the transverse stretching temperature, the stress relaxation in the width direction is sufficient and the thermal contraction rate can be reduced. On the other hand, when the heat treatment temperature in the first step (first step) is not higher than the melting point Tm of the film, the polymer around the hole does not melt and the air resistance is not increased. Further, when the heat treatment temperature after the second step is equal to or higher than the heat treatment temperature of the immediately preceding step, the residual stress that could not be relaxed at the first step is sufficiently released, and the uniformity in the width direction of the thermal dimensional change can be maintained high. . On the other hand, if the heat treatment temperature after the second step is not more than the melting point Tm of the film, the polymer around the hole will not melt and the air resistance will not increase. Here, the difference in heat treatment temperature between the first step and the last step after the second step is preferably less than 15 ° C. If the difference in the heat treatment temperature is 15 ° C. or more, the polymer around the pores may melt due to excessive heat during the heat treatment, and the air resistance may increase. In the present invention, the difference in heat treatment temperature between the first step and the last step after the second step is more preferably 10 ° C. or less, from the viewpoint of achieving appropriate air resistance and uniformity of thermal dimensional change. The following is more preferable.

  Further, the heat treatment time in each step in the above-described multi-stage heat treatment process is 1 sec or more and 30 sec or less from the viewpoint of achieving uniformity of the thermal dimensional change in the width direction while having air permeability resistance suitable as a separator. More preferably, it is 5 sec or more and 30 sec or less, and further preferably 10 sec or more and 30 sec or less. If the heat treatment time in each step is 1 sec or longer, the state is not substantially unheated, and the uniformity in the width direction of the thermal dimensional change can be maintained high. On the other hand, if the heat treatment time in each step is 30 seconds or less, the polymer around the pores will not melt due to excessive heat quantity and the air permeability resistance will not increase, and it is also necessary to slow down the film forming speed or increase the tenter length There is no productivity.

  The step of the multistage heat treatment process needs to be 2 steps or more, and preferably 3 steps or more. The upper limit of the number of steps is not particularly limited, but it is preferable to set the upper limit to 5 steps from the viewpoint of obtaining an air resistance suitable for the separator because the polymer around the pores is not melted due to excessive heat treatment.

  The porous polypropylene film of the present invention preferably has a 3% shrinkage temperature in the film width direction at each measurement point of 130 ° C. or more from the viewpoint of thermal dimensional stability. When the 3% shrinkage temperature in the film width direction is 130 ° C. or higher, for example, when the temperature of the battery rises when the separator is used, the separator is difficult to shrink and short-circuiting hardly occurs. When used for a separator for a large-capacity battery such as for an electric vehicle, further heat resistance is required, and the 3% shrinkage temperature in the film width direction is more preferably 135 ° C. or more, and further preferably 140 ° C. or more. In order to set the 3% shrinkage temperature in the film width direction within the above range, the number of steps in the multi-stage heat treatment process is 2 steps or more, the total relaxation treatment rate is over 15%, and the heat treatment time in each step is in the range of 1 to 30 seconds. It is preferable to set.

  The porous polypropylene film of the present invention preferably has a film thickness of 5 to 50 μm. When the film thickness is 5 μm or more, the film is not broken at the time of use, and when it is 50 μm or less, the volume ratio of the porous film in the electric storage device does not become too high, and a high energy density can be obtained. The film thickness is more preferably 7 to 30 μm, and further preferably 10 to 25 μm.

  The example of the manufacturing method of the porous polypropylene film of this invention is demonstrated concretely below. In addition, the manufacturing method of the film of this invention is not limited to this.

  First, as a polypropylene resin, 99.6% by mass of a commercially available homopolypropylene resin is added to 0.3% by mass of N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide, which is a β-crystal nucleating agent, and further an antioxidant is 0%. The raw material is supplied from the weighing hopper to the twin screw extruder so that 1% by mass is mixed at this ratio, melt kneaded at 300 ° C., discharged from the die in a strand shape, and cooled and solidified in a 25 ° C. water tank. Then, it is cut into chips to produce a polypropylene composition (I).

  Next, 100% by mass of the polypropylene composition (I) is supplied to a uniaxial melt extruder, and melt extrusion is performed at 200 to 230 ° C. And after removing a foreign material, a modified polymer, etc. with the filter installed in the middle of the polymer pipe | tube, it discharges on a cast drum from T-die, and an unstretched sheet is obtained. The cast drum for obtaining an unstretched sheet preferably has a surface temperature of 105 to 130 ° C. from the viewpoint of controlling the β crystal fraction in the unstretched sheet to be high. 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 by coextrusion using a some extruder and pinol.

  Next, the obtained unstretched sheet is biaxially stretched to form pores (through holes) in the film. As a biaxial stretching method, use a sequential biaxial stretching method in which the film is stretched in the width direction after stretching in the film longitudinal direction, or a simultaneous biaxial stretching method in which the longitudinal direction and the width direction of the film are stretched almost simultaneously. However, it is preferable to apply the sequential biaxial stretching method in that it is easy to obtain a highly permeable film. Even in the case of applying the simultaneous biaxial stretching method, as in the sequential biaxial stretching method, the heat treatment process is performed in a state of tension gripping after stretching, but any step in the multistage heat treatment in which the tension process and the relaxation process are performed as one step. Also, it is preferable that the temperature is not less than the stretching temperature and not more than the melting point Tm of the film.

  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., and particularly preferably 123 to 130 ° C. from the viewpoint of film characteristics and uniformity. The draw ratio is 4 to 6 times, more preferably 4.5 to 5.8 times. Further, the higher the stretching ratio, the higher the porosity. However, when the film is stretched within the above preferred range, the film is hardly broken in the next lateral stretching step.

  Next, the uniaxially stretched polypropylene film is introduced into the tenter type stretching machine by gripping the film end. And preferably, it heats to 130-155 degreeC, More preferably, it is 145-153 degreeC, 4-12 times in the width direction, More preferably, it is 6-11 times, More preferably, 6.5-10 times is stretched. The transverse stretching speed at this time is preferably 500 to 6,000% / min, more preferably 1,000 to 5,000% / min.

  Next, heat treatment is performed in the tenter as it is, but in order to obtain a film excellent in uniformity of thermal dimensional change in the width direction while having air permeability suitable as a separator of the present invention, the above-described multistage heat treatment is performed. Preferably, the operating conditions are set in the range of the number of steps, relaxation rate, and heat treatment time. Moreover, the film excellent in planarity can be obtained by performing the tension | tensile_strength process for 1 to 30 seconds at the heat processing temperature of the last step or less and melting | fusing point Tm of a film, keeping the distance between clips after a multistage heat processing process.

  The film after the heat treatment step is removed by slitting the ears gripped by the tenter clip, and wound around the core with a winder to form a film roll. This film roll may be re-slit to a desired width and length.

  The porous polypropylene film of the present invention is excellent in uniformity of thermal dimensional change in the film width direction, and is suitable from the viewpoint of excellent battery performance uniformity when used as a separator for an electricity storage device. Here, examples of the electricity storage device include a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery, and an electric double layer capacitor such as a lithium ion capacitor. Since such an electricity storage device can be repeatedly used by charging and discharging, it can be used as a power supply device for industrial devices, household equipment, electric vehicles, hybrid electric vehicles, and the like. In particular, an electricity storage device using a separator using the porous polypropylene film of the present invention is excellent in output characteristics, and therefore can be suitably used for a non-aqueous electrolyte secondary battery for an electric vehicle.

Hereinafter, the present invention will be described in detail by way of examples. The characteristics were measured and evaluated by the following methods.
(1) β crystal forming ability 5 mg of a polypropylene composition or a porous polypropylene film was sampled in an aluminum pan and measured using a differential scanning calorimeter (RDC220 manufactured by Seiko Instruments Inc.). First, the temperature is first run from room temperature to 260 ° C. at 10 ° C./min under a nitrogen atmosphere, held for 10 min, and then cooled to 20 ° C. at 10 ° C./min. After maintaining for 5 min, the melting peak observed when the temperature is again increased at 10 ° C./min (second run) is the melting peak of the β crystal at 145 ° C. to 157 ° C. The melting of the α crystal is the melting peak of the α crystal, the melting peak of the α crystal is taken as the melting peak of the base, and the area of the region surrounded by the peak drawn from the flat portion on the high temperature side. When the heat of fusion of ΔHα and β crystals is ΔHβ, the value calculated by the following formula is β crystal forming ability.

  The heat of fusion was calibrated using indium.

β crystal forming ability (%) = [ΔHβ / (ΔHα + ΔHβ)] × 100
In addition, the β crystal fraction in the state of the sample can be calculated by similarly calculating the existence ratio of the β crystal from the melting peak observed in the first run.
(2) Film melting point (Tm)
The porous polypropylene film was measured by the same method as the method for measuring the β crystal forming ability, and the melting peak temperature of 158 ° C. or higher observed in the first run was defined as the film melting point (Tm).
(3) 3% shrinkage temperature Using the Thermal Mechanical Analysys (TMA / SS6000) manufactured by Seiko Instruments Inc., the following temperatures at the center position in the film width direction and the position at 30 mm intervals from the same position to both ends. The shrinkage curve in the film width direction under a constant load in the film width direction (TD) was determined by the program. The measurement direction was the film width direction (TD).

  From the shrinkage curve obtained, the temperature when shrinking 3% from the original sample length was read.

Temperature program 25 ℃ → (5 ℃ / min) → 160 ℃ (hold 5min)
Load 0.15 MPa
Sample size Sample length (measurement length) 15mm x width 4mm
Also, the deviation of the 3% shrinkage temperature in the film width direction was calculated from the following formula: Deviation of the 3% shrinkage temperature in the film width direction = (Tmax−Tmin) / Tave
here,
Tmax: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Tmin: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Low temperature Tave: Average temperature at all measurement points of the shrinkage curve in the film width direction Measurement points of the shrinkage curve in the film width direction: the center in the film width direction, and the position (30 mm) toward the both ends from the same center ) Air permeability resistance A square having a size of 100 mm x 100 mm was cut out from a porous polypropylene film and used as a sample. Using a JIS P 8117 (1998) type B Gurley tester, the permeation time of 100 mL of air was measured at 23 ° C. and a relative humidity of 65%. The measurement was performed three times by changing the sample at the center of the film, and the average value of the permeation time was defined as the air permeability of the film. In addition, it can confirm that the through-hole is formed in the film that this air permeability value is a finite value.
(Example 1)
As a polypropylene resin, 99.45% by mass of homopolypropylene FLX80E4 manufactured by Sumitomo Chemical Co., Ltd. and N, N′-dicyclohexyl-2,6-naphthalenedicarboxyamide (Shin Nippon Rika Co., Ltd. Nu) as a β crystal nucleating agent -100, hereinafter simply expressed as β crystal nucleating agent) 0.3% by mass, and “IRGANOX” (registered trademark) 1010, “IRGAFOS” (registered trademark) manufactured by Ciba Specialty Chemicals Co., Ltd. ) 0.18% by mass and 0.1% by mass of 168 are fed to the twin screw extruder from the weighing hopper so as to be mixed at this ratio, melt-kneaded at 300 ° C., and stranded from the die It was discharged, solidified by cooling in a 25 ° C. water tank, and cut into chips to obtain a polypropylene composition (A).

Supply the resulting polypropylene composition (A) to a uniaxial melt extruder, melt extrude at 220 ° C, remove foreign matter with a 20 µm cut sintered filter, and control the surface temperature from the T die to 120 ° C. The undrawn sheet was obtained by discharging to a cast drum and casting indirect for 15 seconds on the drum. Next, preheating was performed using a ceramic roll heated to 125 ° C., and the film was stretched 5 times in the longitudinal direction of the film. Next, the end portion was introduced into a tenter-type stretching machine by holding it with a clip, and stretched in the width direction at 150 ° C. by 8.5 times at a stretching speed of 1,500% / min. The distance between the clips in the width direction at the entrance of the tenter was 120 mm, and the distance between the clips after lateral stretching with the tenter (L ₀ ) was 1,020 mm.

  In the subsequent heat treatment step, multi-stage heat treatment was performed. Specifically, in the first step, after the tension treatment at 150 ° C. for 3 seconds while maintaining the distance between the clips after stretching, a relaxation treatment is performed for 3 seconds at a relaxation rate of 10%, and then in the second step, one step. While maintaining the distance between clips after eye heat treatment, after tensing treatment at 155 ° C. for 3 seconds, relaxation treatment is performed for 3 seconds at a relaxation rate of 10%, and in the third step, the distance between clips after the second heat treatment is set. After maintaining the tension at 158 ° C. for 3 seconds, the tension was applied for 3 seconds at a relaxation rate of 10%. Finally, the tension was applied at 158 ° C. for 3 seconds while maintaining the distance between the clips after relaxation. The relaxation process in each step was performed at a rate of 120% / min.

  Thereafter, the ears of the film held by the tenter clip were removed by slitting, and a porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound around the core by 500 m with a winder.

The film forming conditions and film characteristics are shown in Table 1.
(Examples 2 and 3)
A porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound around a core by 500 m in the same manner as in Example 1 except that the multistage heat treatment conditions in the heat treatment step were changed to the conditions shown in Table 1.
(Comparative Examples 1 and 2)
The width is 600 mm in the same manner as in Example 3 except that the heat treatment process is only one step, the treatment temperature, the treatment time, and the tension treatment conditions after treatment are as shown in Table 1 and the relaxation treatment speed is 240% / min. A porous polypropylene film having a thickness of 25 μm was wound around the core by 500 m.
(Comparative Example 3)
A porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound on a core by 500 m in the same manner as in Example 3 except that the multistage heat treatment conditions in the heat treatment step were changed to the conditions shown in Table 1.
Example 4
69.75% by mass of homopolypropylene FLX80E4 manufactured by Sumitomo Chemical Co., Ltd. as the polypropylene resin, and ethylene-octene-1 copolymer as the copolymerized PE resin (“ENGAGE” (registered trademark) manufactured by Dow Chemical Co., Ltd.) 8411, melt index: 18 g / 10 min) is added to 30% by mass, and further, “IRGANOX” (registered trademark) 1010 and “IRGAFOS” (registered trademark) 168 manufactured by Ciba Specialty Chemicals Co., Ltd. are used as antioxidants. The raw material is supplied from the weighing hopper to the twin screw extruder so that 0.15% by mass and 0.1% by mass are mixed at this ratio, melt kneaded at 240 ° C., and discharged from the die in a strand shape. It was solidified by cooling in a water bath at 25 ° C. and cut into chips to obtain a polypropylene composition (B). Next, 90% by mass of the polypropylene composition (A) prepared in Example 1 and 10% by mass of the polypropylene composition (B) were dry-blended and supplied to a single-screw melt extruder, melt-extruded at 220 ° C., and 20 μm. After removing the foreign matter with a cut sintered filter, it was discharged from a T-die to a cast drum whose surface temperature was controlled at 120 ° C., and cast on the drum so as to be indirect for 15 seconds to obtain an unstretched sheet. The conditions of the biaxial stretching and heat treatment steps were the same as in Example 1, and a porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound around the core by 500 m.
(Example 5)
A film stretched in the longitudinal direction in the same manner as in Example 1 was introduced into a tenter-type stretching machine by gripping the end with a clip, and it was 6.5 times at 150 ° C. and a width at a stretching speed of 1,600% / min. Stretched in the direction. The distance between the clips in the width direction at the entrance of the tenter was 150 mm, and the distance between the clips after transverse stretching with the tenter (L ₀ ) was 975 mm.

  In the subsequent heat treatment step, multi-stage heat treatment was performed. Specifically, in the first step, after the tension treatment at 150 ° C. for 10 seconds while maintaining the distance between the clips after stretching, a relaxation treatment is performed for 10 seconds at a relaxation rate of 10%, and then in the second step, one step. Tension treatment at 155 ° C. for 10 seconds while maintaining the distance between clips after eye heat treatment, relaxation treatment for 10 seconds was performed at a relaxation rate of 10%, and finally, the distance between clips after the second step heat treatment was maintained. Tensile treatment was performed at 155 ° C. for 10 seconds. The relaxation process in each step was performed at a rate of 60% / min.

  Thereafter, the ears of the film held by the tenter clip were removed by slitting, and a porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound around the core by 500 m with a winder.

The film forming conditions and film characteristics are shown in Table 1.
(Example 6)
The film stretched in the longitudinal direction in the same manner as in Example 1 was introduced into a tenter-type stretching machine by gripping the ends with clips, and the film was stretched 6.5 times at 150 ° C. and the width at a stretching speed of 2,650% / min. Stretched in the direction. The distance between the clips in the width direction at the entrance of the tenter was 150 mm, and the distance between the clips after transverse stretching with the tenter (L ₀ ) was 975 mm.

  In the subsequent heat treatment step, multi-stage heat treatment was performed. Specifically, in the first step, after the tension treatment at 150 ° C. for 6 seconds while maintaining the distance between the clips after stretching, the relaxation treatment is performed for 6 seconds at a relaxation rate of 10%, and then in the second step, one step. After maintaining the distance between the clips after the eye heat treatment, the tension treatment was performed at 155 ° C. for 6 seconds, and then the relaxation treatment was performed at a relaxation rate of 10% for 6 seconds. Finally, the distance between the clips after the second step heat treatment was maintained. Tension treatment was performed at 155 ° C. for 6 seconds. The relaxation process in each step was performed at a rate of 100% / min.

  Thereafter, the ears of the film held by the tenter clip were removed by slitting, and a porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound around the core by 500 m with a winder.

The film forming conditions and film characteristics are shown in Table 1.
(Examples 7 and 8 and Comparative Example 4)
The same procedure as in Example 5 except that the temperature of the tension treatment performed while maintaining the distance between the clips after the first step, the second step, and finally the second step heat treatment of the multi-step heat treatment was the conditions shown in Table 1. A porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound around the core by 500 m.

The film forming conditions and film characteristics are shown in Table 1.
(Comparative Example 5)
The same procedure as in Example 3 except that the conditions of the tension treatment performed while maintaining the distance between the clips after the first step, the second step, and finally the second step heat treatment of the multi-step heat treatment were the conditions shown in Table 1. A porous polypropylene film having a width of 600 mm and a thickness of 25 μm was wound around the core by 500 m.

  In the examples satisfying the requirements of the present invention, since the air flow resistance suitable for the separator is excellent and the uniformity of the thermal dimensional change in the width direction is excellent, the battery performance is uniform and suitable as a separator for an electricity storage device. It is thought that it can be used for. On the other hand, in the comparative example, the uniformity of the thermal dimensional change in the film width direction was insufficient.

  The porous propylene film of the present invention is a porous polypropylene film excellent in the uniformity of thermal dimensional change in the film width direction, and is preferable because it has excellent battery performance uniformity when used, for example, as a separator for an electricity storage device. Can be used.

Claims (5)

A porous polypropylene film containing a polypropylene resin and having a deviation in 3% shrinkage temperature in the film width direction represented by the following formula (1) of less than 0.05.
Deviation of 3% shrinkage temperature in the film width direction = (Tmax−Tmin) / Tave (1)
here,
Tmax: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Tmin: The highest temperature among the temperatures showing the shrinkage of 3% in the measurement point of the shrinkage curve in the film width direction. Low temperature Tave: Average temperature at all measurement points of the shrinkage curve in the film width direction Measurement points of the shrinkage curve in the film width direction: Position in the center of the film width direction and every 30 mm from the same center toward both ends The porous polypropylene film according to claim 1, wherein the 3% shrinkage temperature in the film width direction at each measurement point of the shrinkage curve in the film width direction is 130 ° C or higher. The porous polypropylene film according to claim 1, wherein the β-crystal forming ability of the porous polypropylene film is 60% or more. A polypropylene resin is melt-extruded on a support to form a polypropylene resin sheet, and the polypropylene resin sheet is biaxially stretched and then subjected to a heat treatment to produce a porous polypropylene film, the heat treatment comprising a tension treatment and a relaxation treatment. A multi-stage heat treatment step having a plurality of steps, and the multi-stage heat treatment step includes at least a step having a relaxation treatment in which the total relaxation rate exceeds 15% and the relaxation rate in the width direction is 5 to 15%. A method for producing a porous polypropylene film having two steps and having a heat treatment temperature in a multistage heat treatment step of not less than the stretching temperature and not more than the melting point Tm of the film. The heat treatment temperature of the first step in the multistage heat treatment step is not less than the transverse stretching temperature and not more than the melting point Tm of the film, and the heat treatment temperature in the second step and after is not less than the heat treatment temperature of the immediately preceding step and not more than the melting point Tm of the film. The manufacturing method of the porous propylene film of description.