JP2013040231A - Method of producing propylene resin microporous film, propylene resin microporous film, separator for lithium-ion cell, and lithium-ion cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a propylene resin microporous film, by which: a high-end lithium-ion cell can be manufactured which is excellent in lithium-ion permeability; and short-circuiting between a positive electrode and a negative electrode can be prevented from occurring due to dendrite.SOLUTION: The method of producing the propylene resin microporous film includes: a first cooling step of cooling a propylene resin film extruded from a lower die at equal to or lower than a temperature lower by 60 to 1°C than the melting point of the propylene resin; a masking step of masking the propylene resin film obtained after the first cooling step for ≥1 minute at a temperature lower by 100 to 5°C than the melting point of the propylene resin; a second cooling step of cooling the propylene resin film obtained after the masking step until the surface of the propylene resin film is cooled to less than a temperature lower by 100°C than the melting point of the propylene resin; an extension step of uniaxially extending the propylene resin film obtained after the second cooling step; and an annealing step of annealing the propylene resin film obtained after the extension step.

Description

  The present invention relates to a method for producing a propylene-based resin microporous film used for a separator of a lithium ion battery, a propylene-based resin microporous film obtained thereby, a battery separator comprising the propylene-based resin microporous film, and a battery separator incorporated therein It is related with the battery which consists of.

  Conventionally, lithium ion batteries have been used as power sources for portable electronic devices. This lithium ion battery generally includes a positive electrode formed by applying lithium cobaltate or lithium manganate on the surface of an aluminum foil, a negative electrode formed by applying carbon on the surface of a copper foil, and a short circuit between the positive electrode and the negative electrode. In order to prevent this, a separator that partitions the positive electrode and the negative electrode is disposed in the electrolyte.

  The lithium ion battery is charged and discharged by discharging lithium ions from the positive electrode and entering the negative electrode during charging and discharging lithium ions from the negative electrode and moving to the positive electrode during discharging. Therefore, the separator used for the lithium ion battery needs to be able to transmit lithium ions well.

  When the lithium ion battery is repeatedly charged and discharged, lithium dendrite (dendritic crystals) is generated on the end face of the negative electrode, and this dendrite breaks through the separator, causing a minute internal short circuit (dendritic short) between the positive electrode and the negative electrode. There is a problem that the capacity deteriorates.

  In order to improve the safety of the lithium ion battery, a porous film of an olefin resin mainly composed of polyethylene is used for the separator. This is because when the lithium ion battery abnormally generates heat due to a short circuit or the like, the polyethylene constituting the porous film melts in a temperature range of around 130 ° C. and the porous structure is blocked (shutdown). This is because abnormal heat generation can be stopped to ensure safety.

  In recent years, large batteries such as lithium-ion batteries for automobiles have been increasing in output, and a rapid temperature rise exceeding 130 ° C. is possible. Sex is regarded as important. Further, in order to increase the output of a lithium ion battery, a reduction in resistance when lithium ions pass through the separator is required, and the separator needs to have high air permeability. Further, in the case of a large-sized lithium ion battery, it is important to ensure long life and long-term safety.

  Various separators using a porous film of polypropylene having high heat resistance have been proposed. For example, Patent Document 1 discloses a melt extrusion of a polymer having a higher melting crystallization temperature than polypropylene and polypropylene and a composition serving as a β crystal nucleating agent. There has been proposed a method for producing a polypropylene microporous film characterized by at least uniaxial stretching after being formed into a sheet at a high temperature.

  However, the polypropylene microporous film obtained by the method for producing a polypropylene microporous film has low air permeability, insufficient lithium ion permeability, and is used for a lithium ion battery requiring high output. Have difficulty.

  Patent Document 2 includes a porous layer having a thickness of 0.2 μm or more and 100 μm or less containing an inorganic filler or a resin having a melting point and / or a glass transition temperature of 180 ° C. or more on at least one surface of a polyolefin resin porous membrane. A multilayer porous membrane having an air permeability of 1 to 650 seconds / 100 cc has been proposed, but the multilayer porous membrane also has insufficient lithium ion permeability and is used for a lithium ion battery requiring high output. Have difficulty.

  Further, Patent Document 3 discloses a non-aqueous electrolyte battery comprising a negative electrode made of light metal, a separator impregnated with a non-aqueous electrolyte, and a positive electrode, in which polyethylene fine powder is pre-attached to the separator. An electrolyte battery has been proposed, and a highly heat-resistant polypropylene non-woven fabric suitable for high-power applications is used as a separator.

  However, since the separator has a large pore diameter of about several μm, it is expected that a fine short-circuit is likely to occur. In addition to the problem that the life and long-term safety of the separator are not sufficient, a nonwoven fabric is used. Since it is used, there is a problem that it is difficult to reduce the thickness of the separator.

  Patent Document 4 proposes a method for producing a microporous separator using a polymer film obtained by an inflation molding method. Specifically, a cylindrical polymer film in a molten state is extruded, a low-temperature fluid is allowed to flow and solidify on the inner surface and outer surface of the cylindrical polymer film, and then the polymer film is annealed and stretched to obtain fine porosity. A separator has been manufactured. However, in the manufacturing method of Patent Document 4 using the inflation molding method, it is difficult to uniformly control the temperature and thickness of the cylindrical polymer film during extrusion and cooling of the cylindrical polymer film in a molten state. Crystallization of the polymer film cannot be performed highly and uniformly. Therefore, the microporous separator produced by the method of Patent Document 4 also has low air permeability, insufficient lithium ion permeability, and is difficult to use for a lithium ion battery that requires high output.

  In Patent Document 5, an unstretched sheet is obtained by melting and kneading a polypropylene resin and then extruding, and the unstretched sheet is slowly cooled on a cooling roll and then biaxially stretched to produce a porous polypropylene film. A method is disclosed. However, since the polypropylene resin in the slowly stretched unstretched sheet is not sufficiently oriented, even if such an unstretched sheet is biaxially stretched, many through holes are uniformly formed in the resulting porous polypropylene film. Can not be generated. Therefore, the porous polypropylene film produced by the method of Patent Document 5 also has low air permeability, insufficient lithium ion permeability, and is difficult to use for a lithium ion battery that requires high output.

JP-A 63-199742 JP 2007-273443 A JP-A-60-52 JP 2000-340207 A JP 2010-242060 A

  The object of the present invention is to provide a lithium ion battery that is excellent in lithium ion permeability and can be used for high-power applications. Incorporating a method for producing a propylene-based resin microporous film that is unlikely to occur, a propylene-based resin microporous film produced by the above method, a battery separator using the propylene-based resin microporous film, and the battery separator It is to provide a battery.

The method for producing the propylene-based resin microporous film of the present invention includes the following steps:
Propylene-based resin is supplied to an extruder, and the propylene-based resin is melt-kneaded at a temperature that is 20 ° C. higher than its melting point and 100 ° C. higher than the melting point of the propylene-based resin, and attached to the extruder An extrusion step of obtaining a propylene-based resin film by extruding from a T-die,
The propylene-based resin film extruded from the T-die is cooled until the surface temperature is 60 ° C. lower than the melting point of the propylene-based resin and 1 ° C. lower than the melting point of the propylene-based resin. A first cooling step to perform,
The surface temperature of the propylene-based resin film after the first cooling step is lower than the surface temperature of the propylene-based resin film cooled in the first cooling step, and is 100 ° C. higher than the melting point of the propylene-based resin. A curing step of curing for 1 minute or more at a temperature lower than the low temperature and lower than 5 ° C. lower than the melting point of the propylene resin;
A second cooling step for cooling the propylene-based resin film after the curing step until the surface temperature is less than 100 ° C. lower than the melting point of the propylene-based resin;
A stretching step of forming micropores in the propylene-based resin film by uniaxially stretching the propylene-based resin film after the second cooling step;
An annealing step for annealing the propylene-based resin film after the stretching step;
It is characterized by including. Hereinafter, the method of the present invention will be described in order.

  FIG. 1 shows an example of a manufacturing apparatus for carrying out the method of the present invention. In the manufacturing apparatus of FIG. 1, a T die 12 is attached to the tip (front end) of the extruder 11. A first cooling means is disposed below the T die 12, and further, a curing means, a second cooling means, and a winding means are sequentially arranged from the first cooling means to the front.

  The first cooling means includes a first cooling roll 21, a pressing belt 24 that is pressed against the rear peripheral surface of the first cooling roll 21, and a first nip roll 22 that spans the pressing belt 24 in an endless manner. The rotating belt 23 and the belt surface of the pressing belt 24 pressed against the first cooling roll 21 are configured to run in the same direction at the same speed in synchronization with the rotation of the first cooling roll 21. ing. The surface temperature of the first nip roll 22 and the pressing belt 24 is preferably heated so as to be the same as the surface temperature of the first cooling roll 21. The T die 12 is disposed vertically above the opposing surface of the first cooling roll 21 and the first nip roll 22, and the extrusion port 12 a opened downward of the T die 12 is provided to these first cooling rolls. It faces between 21 and the first nip roll 22.

  The curing means disposed in front of the first cooling means is constituted by a first curing furnace 31. Inside the first curing furnace 31, a plurality of upper and lower guide rolls 32 are propylene. The guide resin 32 is arranged in a zigzag manner in the transport direction of the resin film A, and a nip roll 33 is provided in contact with each of the guide rolls 32. Further, the second cooling means disposed in front of the curing means is constituted by a second cooling roll 51, and the winding means is constituted by a winding roll 61. Next, a method for manufacturing the propylene-based resin film A by the manufacturing apparatus configured as described above will be described.

[Extrusion process]
In the method of the present invention, a propylene-based resin film A is obtained by first supplying a propylene-based resin to the extruder 1, melt-kneading, and then continuously extruding the film from the T die 2 attached to the extruder 1. Perform the extrusion process.

(Propylene resin)
Examples of the propylene-based resin include a propylene homopolymer, a copolymer of propylene and another olefin, and the like. Propylene-type resin may be used independently, or 2 or more types may be used together. The copolymer of propylene and another olefin may be a block copolymer or a random copolymer.

  Examples of the olefin copolymerized with propylene include α such as ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-nonene and 1-decene. -Olefin and the like.

  When the weight average molecular weight of the propylene-based resin is small, the formation of the micropores of the propylene-based resin microporous film may be nonuniform, and when it is large, the film formation may become unstable. Since there exists a possibility that a hole may become difficult to form, 250,000-500,000 are preferable and 280,000-480,000 are more preferable.

  When the molecular weight distribution (weight average molecular weight Mw / number average molecular weight Mn) of the propylene-based resin is small, the surface opening ratio of the propylene-based resin microporous film may be low. 7.5-12.0 is preferable, 8.0-11.5 is more preferable, and 8.0-11.0 is particularly preferable.

  Here, the weight average molecular weight and the number average molecular weight of the propylene-based resin are polystyrene-converted values measured by a GPC (gel permeation chromatography) method. Specifically, 6-7 mg of propylene-based resin was sampled, and the collected propylene-based resin was supplied to a test tube, and then 0.05 wt% BHT (dibutylhydroxytoluene) o-DCB (ortho) was added to the test tube. Dichlorobenzene) solution is added to dilute the propylene resin concentration to 1 mg / mL to prepare a diluted solution.

  The dilution liquid is shaken for 1 hour at a rotation speed of 25 rpm at 145 ° C. using a dissolution filter, and the propylene resin is dissolved in the BHT o-DCB solution to obtain a measurement sample. Using this measurement sample, the weight average molecular weight and the number average molecular weight of the propylene-based resin can be measured by the GPC method.

The weight average molecular weight and number average molecular weight in the propylene-based resin can be measured, for example, with the following measuring apparatus and measurement conditions.
Product name “HLC-8121GPC / HT” manufactured by TOSOH
Measurement conditions Column: TSKgelGMHHR-H (20) HT x 3
TSK guard column-HHR (30) HT x 1
Mobile phase: o-DCB 1.0 mL / min
Sample concentration: 1 mg / mL
Detector: Bryce refractometer
Standard substance: polystyrene (Molecular weight: 500-8420000, manufactured by TOSOH)
Elution conditions: 145 ° C
SEC temperature: 145 ° C

  When the melting point of the propylene-based resin is low, the mechanical strength of the propylene-based resin microporous film at high temperatures may be lowered. When the melting point is high, film formation may become unstable. 160-165 ° C is more preferable.

  When the heat of fusion obtained by differential scanning calorimetry (DSC) in the propylene resin is small, the orientation of the propylene resin is lowered, and in the stretching process of the propylene resin film, uniform pores are formed in the propylene resin film. Part may not be formed, so 85 mJ / mg or more is preferable, and 90 mJ / mg or more is more preferable.

  The melting point of the propylene-based resin and the heat of fusion obtained by DSC are values measured in the following manner. First, 10 mg of propylene-based resin is collected. Next, the propylene-based resin is heated from 0 ° C. to 250 ° C. at a heating rate of 10 ° C./min, and held at 250 ° C. for 3 minutes. Next, the propylene-based resin is cooled from 250 ° C. to 0 ° C. at a temperature decrease rate of 10 ° C./min and held at 0 ° C. for 3 minutes. Subsequently, the propylene-based resin is reheated from 0 ° C. to 250 ° C. at a heating rate of 10 ° C./min, the melting peak top temperature in this reheating step is taken as the melting point, and the total area of the melting peak is calculated to calculate the heat of fusion. And The DSC of the propylene-based resin can be performed using, for example, DSC220C manufactured by Seiko Instruments Inc.

(Melt kneading)
If the temperature of the propylene resin when the propylene resin is melt-kneaded by the extruder 2 is low, the thickness of the resulting propylene resin microporous film becomes uneven or the surface smoothness of the propylene resin microporous film is low. If it is low and high, the orientation of the propylene-based resin may be lowered. Therefore, the temperature is limited to a temperature that is 20 ° C. higher than the melting point of the propylene-based resin and 100 ° C. higher than the melting point of the propylene-based resin. However, the temperature is preferably 25 ° C. higher than the melting point of the propylene-based resin and 80 ° C. higher than the melting point of the propylene-based resin.

Further, the extrusion rate R ₀ of the propylene-based resin film A at the extrusion port 12a of the T die 2 is preferably 0.05 to 40 m / min in consideration of the film formation stability of the propylene-based resin film A, and 0.1 to 20 m / min is more preferable, and 0.1 to 5 m / min is particularly preferable.

The extrusion rate R ₀ (m / min) of the propylene-based resin film is determined by the extrusion amount E (kg / min) of the propylene-based resin extruded from the T die 2, the density D (kg / m ³ ) of the propylene-based resin, and the T die. 2 can be calculated by the following formula from the cross-sectional area A (m ² ) of the extrusion port 2a.
R ₀ (m / min) = E (kg / min) / [D (kg / m ³ ) × A (m ² )]

[First cooling step]
Next, in the method of the present invention, the surface temperature of the propylene-based resin film A extruded from the T-die 12 is 60 ° C. lower than the melting point of the propylene-based resin and is 1 higher than the melting point of the propylene-based resin. A first cooling step is performed in which the cooling is performed until the temperature is lower than or equal to ° C.

  In this first cooling step, as shown in FIG. 1, the propylene-based resin film A extruded from the T-die 12 is a first cooling roll 21 that is a first cooling means that is adjusted to a predetermined surface temperature and a first cooling roll 21. Rotating the first cooling roll 21 while supplying the propylene-based resin film A between the opposite surfaces of the pressing belt 24 and the first cooling roll 21. The propylene-based resin film A is transferred according to the above, and the propylene-based resin film A is cooled until the surface temperature thereof becomes the surface temperature of the first cooling roll 21 while passing between the pressing belt 24 and the first cooling roll 21. Thus, after the lamella is generated in the propylene resin constituting the propylene resin film A, the propylene resin film A is fed from the first cooling roll 21 to the front curing means. In this way, the propylene-based resin film A cooled by the first cooling roll 21 is formed with a lamellar structure in which crystallized portions (lamellar) and amorphous portions are alternately stacked in the transport direction.

  The propylene-based resin film A supplied onto the peripheral surface of the first cooling roll 21 is pressed by the first cooling roll 21 via the pressing belt 24 by the first nip roll 22 and the first propylene-based resin film A and the first cooling roll 21. The occurrence of air accumulation at the interface with the cooling roll 21 is suppressed. Further, by using the pressing belt 24, the propylene-based resin film A is conveyed while being pressed between the pressing belt 24 and the first cooling roll 21 while being positively pressed against the surface of the first cooling roll 21. The propylene-based resin film A can be reliably cooled so as to have a desired surface temperature.

  Further, in order to suppress the occurrence of air accumulation at the interface between the propylene-based resin film A and the first cooling roll 21, instead of the first nip roll 22 and the pressing belt 24, as shown in FIG. Using a knife 25, air can be blown from the air knife 25 onto the propylene resin film A to bring the propylene resin film A into close contact with the surface of the first cooling roll 21. When the air knife 25 is used, the air blown from the air knife 25 is preferably the same temperature as the surface temperature of the first cooling roll 21.

  In the first cooling step, as described above, the surface temperature of the propylene-based resin film A extruded from the T-die 12 is 60 ° C. lower than the melting point of the propylene-based resin and 1 than the melting point of the propylene-based resin. By cooling to a temperature lower than or equal to a temperature lower than 0 ° C., the first cooling step allows the propylene-based resin constituting the propylene-based resin film extruded from the T die to form a lamellar structure.

  The surface temperature of the propylene-based resin film A cooled in the first cooling step is limited to a temperature that is 60 ° C. lower than the melting point of the propylene-based resin and 1 ° C. lower than the melting point of the propylene-based resin, The temperature is preferably 45 ° C. lower than the melting point of the propylene resin and 1 ° C. lower than the melting point of the propylene resin, preferably 15 ° C. lower than the melting point of the propylene resin and higher than the melting point of the propylene resin. A temperature lower than 5 ° C. is more preferable. If the surface temperature of the propylene-based resin film A cooled in the first cooling step is low, the propylene-based resin constituting the propylene-based resin film A may not be able to generate lamellae sufficiently. In addition, even if the surface temperature of the propylene resin film A cooled in the first cooling step is high, the propylene resin is a lamella because the molecular orientation of the propylene resin constituting the propylene resin film A is relaxed. May not be generated sufficiently.

The ratio (R ₁ / R ₀ ) of the peripheral speed R ₁ (m / min) of the first cooling roll 21 to the extrusion speed R ₀ (m / min) of the propylene-based resin film A at the extrusion port 12a of the T die 2 is It is preferably 40 or more. By setting the ratio (R ₁ / R ₀ ) of the peripheral speed R ₁ (m / min) of the first cooling roll 21 to the extrusion speed R ₀ (m / min) of the propylene-based resin film A to 40 or more, the propylene type When the resin is extruded from the extruder into a film, tension is applied to the propylene resin, and the propylene resin constituting the propylene resin film A extruded from the extruder can be oriented in the transport direction. Thus, by orienting the propylene-based resin constituting the propylene-based resin film A in advance, the portion where the propylene-based resin is oriented can promote the formation of lamellae in the cooling step.

Further, when the ratio (R ₁ / R ₀ ) of the peripheral speed R ₁ (m / min) of the first cooling roll 21 to the extrusion speed R ₀ (m / min) of the propylene-based resin film A is large, the propylene-based resin Although the molecular orientation is high, the film formation stability of the propylene-based resin film A is lowered, and the thickness accuracy and width accuracy of the resulting propylene-based resin film may be lowered. Therefore, the ratio (R ₁ / R ₀ ) of the peripheral speed R ₁ (m / min) of the first cooling roll 21 to the extrusion speed R ₀ (m / min) of the propylene-based resin film A is preferably 40 to 300, 45-250 are more preferable and 50-100 are more preferable.

  The peripheral speed (m / min) of the first cooling roll 21 means a distance (m) that an arbitrary point on the outer peripheral surface of the first cooling roll 21 moves per unit time (minute). The measurement of the peripheral speed (m / min) of the 1st cooling roll 21 can be performed using a non-contact-type tachometer (for example, DT-105N by Nidec Shinpo Co., Ltd.).

[Curing process]
Next, the propylene-based resin film after the first cooling step is fed into the first curing furnace 31 as the curing means, and the surface temperature of the propylene-based resin film cooled in the first cooling step is It moves to the curing process of curing for 1 minute or more at a temperature lower than the temperature and 100 ° C. lower than the melting point of the propylene resin and lower than 5 ° C. lower than the melting point of the propylene resin. Hereinafter, the surface temperature of the propylene-based resin film in the curing process is also simply referred to as “curing temperature”.

  The curing process will be described in detail. The propylene-based resin film A fed from the first cooling roll 21 is supplied into the first curing furnace 31 and arranged in a staggered manner in the first curing furnace 31 in the forward direction. It extends over the upper and lower guide rolls 32, 32, and the moving distance in the first curing furnace 31 is increased so that the surface temperature of the propylene-based resin film A during the passage of these guide rolls 32 is the above-mentioned Maintained for a predetermined time at a temperature lower than the surface temperature of the propylene-based resin film cooled in one cooling step, not less than 100 ° C. lower than the melting point of the propylene-based resin and lower than 5 ° C. lower than the melting point of the propylene-based resin. The lamella of propylene resin produced in the first cooling process is grown to increase the thickness of the lamella and improve the crystallinity of the propylene resin. It can be.

  The propylene-based resin film A is pressed on the guide roll 32 by the nip roll 33 in the first curing furnace 31, thereby suppressing the propylene-based resin film A from loosening between the upper and lower guide rolls 32, 32. In addition, the propylene-based resin film A can be reliably conveyed.

  If the curing temperature of the propylene resin film in the curing process is low, crystallization of the lamella of the propylene resin film may not be promoted. If it is high, the orientation of the propylene resin molecules of the propylene resin film is relaxed and the lamella Since the structure may collapse, it is limited to a temperature that is 100 ° C. lower than the melting point of the propylene-based resin and less than 5 ° C. lower than the melting point of the propylene-based resin, and a temperature that is 80 ° C. lower than the melting point of the propylene-based resin. Above and below a temperature lower by 10 ° C. than the melting point of the propylene-based resin is preferable, more preferably a temperature not lower than 45 ° C. lower than the melting point of the propylene-based resin and lower than 20 ° C. below the melting point of the propylene-based resin.

  Furthermore, if the curing time of the propylene-based resin film is short in this curing step, crystallization of the lamella of the propylene-based resin film may not be promoted, so it is preferable to secure a sufficient time. Therefore, the curing time of the propylene-based resin film A is limited to 1 minute or longer, but is preferably 1 minute to 3 hours, and more preferably 1 minute to 30 minutes.

  Moreover, it is preferable to implement a curing process in multiple steps. The curing process is divided into a plurality of times, for example, a treatment for gradually reducing the surface temperature of the propylene-based resin film A within the range of the curing temperature described above, or the surface temperature of the propylene-based resin film A within the range of the curing temperature described above. By performing the process which raises in steps, the growth of the lamella of the propylene-based resin can be further promoted.

  When the curing process is performed in a plurality of times, as shown in FIG. 2, the first curing furnace 31 is used as the first curing furnace 31, and the curing process by the first curing furnace 31 is used as the first curing process. A second curing furnace 31 'having the same structure as that of the first curing furnace 31 is disposed continuously to the curing furnace 31, and the second curing process by the second curing furnace 31' is continuously performed. preferable. In this way, the curing process is divided into a plurality of times (two times in the figure), and the process of gradually decreasing the surface temperature of the propylene-based resin film A or the process of increasing the surface temperature of the propylene-based resin film A in stages. By doing so, the growth of the lamella of the propylene-based resin can be further promoted.

  When the curing process is divided into two steps as described above and the surface temperature of the propylene-based resin film A is lowered stepwise, the propylene-based resin film A that has undergone the first cooling process is subjected to the surface treatment. The temperature is lower than the surface temperature of the propylene-based resin film A cooled in the first cooling step, is 50 ° C. lower than the melting point of the propylene-based resin, and 5 ° C. than the melting point of the propylene-based resin. A first curing step for curing for 1 minute or more at a temperature lower than a low temperature, and a propylene-based resin film that has undergone the first curing step at a temperature that is 100 ° C. lower than the melting point of the propylene-based resin and higher than the melting point of the propylene-based resin It is preferable to sequentially perform the second curing step of curing for 1 minute or more at a temperature lower than 50 ° C.

  Then, using the manufacturing apparatus shown in FIG. 2 described above, the propylene-based resin film A fed from the first cooling roll 21 is supplied into the first curing furnace 31 and arranged in a staggered manner in the first curing furnace 31. On the way to be detoured by the provided guide roll 32, the surface temperature of the propylene-based resin film A is lower than the surface temperature of the propylene-based resin film A cooled in the first cooling step, and propylene A first curing step is performed in which the temperature is 50 ° C. lower than the melting point of the resin and less than 5 ° C. lower than the melting point of the propylene resin, and the surface temperature is maintained for a predetermined period of time for transfer. The propylene-based resin film A delivered from the curing furnace 31 is supplied into the second curing furnace 31 ′, and the propylene-based resin film A is being transported in the first curing furnace 31. The surface temperature is not less than 100 ° C. lower than the melting point of the propylene-based resin and less than 50 ° C. lower than the melting point of the propylene-based resin. 2 Carry out the curing process.

  Also in the second curing furnace 31 ′, the propylene-based resin film A is pressed onto the guide roll 32 ′ by the nip roll 33 ′, and thereby, between the upper and lower guide rolls 32 ′ and 32 ′. Thus, the propylene-based resin film A can be reliably conveyed while suppressing the loosening of the propylene-based resin film A.

  The curing temperature of the propylene-based resin film A in the first curing step in the first curing furnace 31 is not less than a temperature that is 50 ° C. lower than the melting point of the propylene-based resin and 5 ° C. lower than the melting point of the propylene-based resin. The temperature is preferably 40 ° C. or more lower than the melting point of the propylene resin and more preferably 10 ° C. or lower than the melting point of the propylene resin.

  Further, if the curing time of the propylene-based resin film A in the first curing step is short, crystallization of the lamella of the propylene-based resin film A is not promoted, and in the first stretching step described later, minute penetration between lamellae is not achieved. Since it becomes difficult to form holes, it is preferable to secure a sufficient time. Therefore, the curing time of the propylene-based resin film A is preferably 1 minute or longer, more preferably 1 minute to 3 hours, and particularly preferably 1 minute to 30 minutes.

  Further, the curing temperature of the propylene-based resin film A in the second curing step in the second curing furnace 31 'is not less than 100 ° C lower than the melting point of the propylene-based resin and 50 ° C higher than the melting point of the propylene-based resin. It is preferably less than a low temperature, more preferably a temperature that is 80 ° C. lower than the melting point of the propylene resin and less than 50 ° C. lower than the melting point of the propylene resin.

  Further, if the curing time of the propylene-based resin film in the second curing step is short, there is a possibility that crystallization of the lamella of the propylene-based resin film A may not be promoted, and it is preferable to secure a sufficient time. Therefore, the curing time of the propylene-based resin film A is preferably 1 minute or longer, more preferably 1 minute to 3 hours, and particularly preferably 1 minute to 30 minutes.

  In this way, by performing the first curing process and the second curing process and curing the propylene resin film A while gradually reducing the surface temperature of the propylene resin film A, the growth of the lamella of the propylene resin film is increased. It can be promoted more.

  In the above description, the curing process is divided into two times, and the method of curing the propylene-based resin film A while gradually decreasing the surface temperature of the propylene-based resin film A has been described. However, the method is limited to such a method. Not. For example, the curing process is divided into three or more times, and the surface temperature of the propylene-based resin film A is lower in the subsequent curing process than in the previous curing process, and the propylene-based resin film A is lowered step by step. You may be cured.

  In addition, when the curing process is divided into two steps as described above and the surface temperature of the propylene-based resin film A is increased stepwise, the surface temperature of the propylene-based resin film A that has undergone the first cooling process is increased. Is cured at a temperature not lower than 100 ° C. lower than the melting point of the propylene-based resin and lower than 20 ° C. lower than the melting point of the propylene-based resin for 1 minute or longer, and propylene that has undergone the first curing step. A second 'curing step in which the resin film is cured at a temperature of 20 ° C. or more lower than the melting point of the propylene resin and less than 5 ° C. lower than the melting point of the propylene resin for 1 minute or more. preferable. In addition, it is preferable that the surface temperature of the propylene-type resin film in a 1 'curing process shall be less than the surface temperature of the said propylene-type resin film A cooled in the said 1st cooling process.

  Then, using the manufacturing apparatus shown in FIG. 2 described above, the propylene-based resin film A fed from the first cooling roll 21 is supplied into the first curing furnace 31 and arranged in a staggered manner in the first curing furnace 31. On the way to be detoured by the provided guide roll 32, the surface temperature of the propylene-based resin film A is 100 ° C lower than the melting point of the propylene-based resin and 20 ° C lower than the melting point of the propylene-based resin. The first curing step is performed in which the surface temperature is maintained while maintaining the surface temperature for a predetermined time, and the propylene-based resin film A fed from the first curing furnace 31 is supplied into the second curing furnace 31 '. Then, the propylene-based resin film A has a surface temperature of 20 ° C. lower than the melting point of the propylene-based resin during transportation in the first curing furnace 31 and the propylene-based resin. On which a 5 ° C. temperature lower than the melting point of, by transfer while maintaining the surface temperature given time, to implement the second 'and curing process.

  The curing temperature of the propylene-based resin film A in the first curing step 31 in the first curing furnace 31 is a temperature that is 100 ° C. lower than the melting point of the propylene-based resin and 20 ° C. lower than the melting point of the propylene-based resin. The temperature is preferably 40 ° C. or more lower than the melting point of the propylene-based resin, and more preferably less than 20 ° C. lower than the melting point of the propylene-based resin.

  Further, the curing time of the propylene-based resin film A in the first 1 curing step is preferably 1 minute or more, more preferably 1 minute to 3 hours, and particularly preferably 1 minute to 30 minutes.

  Further, the curing temperature of the propylene-based resin film A in the second ′ curing step in the second curing furnace 31 ′ is not less than a temperature lower by 20 ° C. than the melting point of the propylene-based resin and 5 than the melting point of the propylene-based resin. The temperature is preferably lower than the lower temperature by 15 ° C., more preferably lower than the temperature lower by 15 ° C. than the melting point of the propylene resin and lower by 5 ° C. than the melting point of the propylene resin.

  In addition, the curing time of the propylene-based resin film in the second ′ curing step is preferably 1 minute or more, more preferably 1 minute to 3 hours, and particularly preferably 1 minute to 30 minutes.

  Thus, the propylene-based resin is cured in the curing process by performing the first 'curing process and the second' curing process and curing the propylene-based resin film A while gradually increasing the surface temperature of the propylene-based resin film A. While suppressing the occurrence of uneven thickness in the film A, the lamella of the propylene resin can be grown, and the thickness of the propylene resin film A after the curing process can be made uniform. Such a propylene-based resin film A can be stretched uniformly in the stretching step to form many fine through holes, and can provide a propylene-based resin microporous film B having excellent air permeability.

  In the above description, the curing process is divided into two times, and the method of curing the propylene-based resin film A while gradually increasing the surface temperature of the propylene-based resin film A has been described. However, the method is limited to this method. Not. For example, the curing process is divided into three or more times, and the surface temperature of the propylene-based resin film A is higher in the subsequent curing process than in the previous curing process, and the propylene-based resin film A is gradually raised. You may be cured.

[Second cooling step]
Next, in the method of the present invention, a second cooling step is performed in which the propylene-based resin film A after the curing step is cooled until the surface temperature becomes less than 100 ° C. lower than the melting point of the propylene-based resin. By this second cooling step, the lamellae of the propylene resin constituting the propylene-based resin film A can be further grown, and the propylene-based resin film can be cooled and solidified. In the propylene-based resin film A after the second cooling step, a sufficiently grown thick lamella is formed. Therefore, by stretching the propylene-based resin film A in a stretching process described later, a crack is generated not between the lamellas but between the lamellas, and a minute through hole (micropore part) is formed starting from this crack. Can do.

  In the second cooling step, as shown in FIG. 1, the propylene-based resin film A fed from the first curing furnace 31 is supplied to the surface of the second cooling roll 51, and the peripheral surface of the second cooling roll 51 By making the transition in contact with the top, the propylene-based resin film A can be cooled until its surface temperature reaches a predetermined temperature. The propylene-based resin film A sent out from the second cooling roll 51 is continuously wound around the winding roll 61.

  The surface temperature of the propylene-based resin film A cooled in the second cooling step is limited to a temperature lower by 100 ° C. than the melting point of the propylene-based resin, but a temperature lower by 145 to 110 ° C. than the melting point of the propylene-based resin. Preferably, a temperature lower by 140 to 120 ° C. than the melting point of the propylene-based resin is more preferable. When the surface temperature of the cooled propylene-based resin film A is higher than the temperature lower by 100 ° C. than the melting point of the propylene-based resin, not only the propylene-based resin film A cannot be cooled and solidified but also the propylene-based resin film A There exists a possibility that the propylene resin which comprises may not fully grow a lamella.

  In the method of the present invention, a propylene-based resin film having a high molecular orientation of the propylene-based resin is obtained by extruding the melt-kneaded propylene-based resin in the extrusion step, and the propylene-based resin film is cooled in the first cooling step, thereby propylene-based resin. After the lamella is generated in the resin film, the lamella can be grown in the curing process and the second cooling process of the propylene-based resin film. Therefore, the propylene-based resin film obtained after the second cooling step described above is obtained by differential scanning calorimetry (DSC) of the propylene-based resin film by sufficiently growing the lamella and improving the crystallinity. The amount of heat of fusion produced can be 110 mJ / mg or more, in particular 112 mJ / mg or more.

  And in the propylene-type resin film whose heat of fusion obtained by DSC is 110 mJ / mg or more, since the lamella of propylene-type resin is fully formed as mentioned above, such a propylene-type resin film is uniaxially stretched. By doing so, the micropores can be formed uniformly and finely dispersed in the propylene-based resin film.

  In addition, the heat of fusion obtained by DSC of a propylene-type resin film says the value measured in the following way. First, a 10 mg test piece is prepared by cutting a propylene-based resin film into a predetermined size. Next, the test piece is heated from 0 ° C. to 250 ° C. at a heating rate of 10 ° C./min, and the total area of the melting peak is calculated to be the heat of fusion of the propylene-based resin film. DSC of the propylene-based resin film can be performed using, for example, DSC220C manufactured by Seiko Instruments Inc.

[Stretching process]
Next, in the method of the present invention, the stretching step and the annealing step are sequentially performed on the propylene-based resin film A after the second cooling step. In the stretching step, the propylene-based resin film is preferably uniaxially stretched in the transport direction. In this stretching step, by separating the lamellae by stretching the propylene-based resin film A, a fine crack is efficiently generated independently in the amorphous part between the lamellae, and this crack is reliably used as a starting point. A large number of micropores can be formed. This stretching step preferably includes a first stretching step and a second stretching step following the first stretching step.

  FIG. 3 shows an example of stretching means and annealing means for carrying out the stretching process and the annealing process in the present invention. The stretching means is a propylene-based system fed from the second cooling roll 51 (not shown). The 1st extending | stretching roll 71 arrange | positioned at the back of the winding roll 61 by which the resin film A was wound up, and the 2nd extending | stretching arrange | positioned under this 1st extending | stretching roll at predetermined intervals. A primary stretching means comprising a roll 72 and a plurality of stretching rolls 82 in the heating furnace 81 are arranged in a staggered manner in the up-down direction and in the transport direction of the propylene-based resin film A. Secondary stretching means, and is arranged in a state where the first nip roll 73 is in contact with the front peripheral surface of the first stretching roll 71 in the primary cooling means, and on the rear peripheral surface of the second stretching roll 72. The second nip roll 74 is in contact It is arranged in a state. In addition, the 1st extending | stretching roll 71 and the 1st nip roll 73, the 2nd extending | stretching roll 72, and the 2nd nip roll 74 are a pair of rolls which each send out the propylene-type resin film A, respectively. Further, the first stretching roll 71 and the second stretching roll 72 can be independently driven by controlling the peripheral speed, and the first and second nip rolls 73 and 74 are respectively driven by the first stretching roll 71 and the second stretching roll. The roller 72 is driven and rotated.

  Similarly, a nip roll 83 is disposed in contact with each of the stretching rolls 82 constituting the secondary stretching means. The stretching roll 82 and the nip roll 83 are a pair of rolls that are sent out with the propylene-based resin film A interposed therebetween. The stretching roll 82 can be independently driven by controlling the peripheral speed, and the nip roll 83 can be in contact with the stretching roll 82 and driven to rotate. Annealing means is disposed adjacent to the front of the secondary stretching means. In this annealing means, several guide rolls 92 are arranged in a staggered manner in the conveying direction of the propylene-based resin film A with a predetermined interval in the vertical direction inside the annealing furnace 91. Each is provided with a nip roll 93 in contact therewith. The guide roll 92 and the nip roll 93 are each a pair of rolls that are fed out with the propylene-based resin film A interposed therebetween, and the guide rolls 92 can be independently driven by controlling the peripheral speed. It abuts on the guide roll 92 and is driven to rotate.

(First stretching step)
In the first stretching step, as shown in FIG. 3, the propylene-based resin film A is continuously unwound from the take-up roll 61 and passed over the first stretching roll 71, and then below the first stretching roll 71. It is carried over the second stretching roll 72 arranged in the above. At this time, the circumferential speed of the second stretching roll 72 is made smaller than the circumferential speed of the first stretching roll 71, and the first stretching roll 71 and the second stretching roll 72 are rotated, respectively. The first stretching step can be performed by stretching the propylene-based resin film A with the second stretching roll 72.

  In the first stretching step using the first stretching roll 71, if the surface temperature of the propylene-based resin film A is low, the propylene-based resin film A may be broken at the time of stretching. Since it becomes difficult to generate | occur | produce a crack, -20-100 degreeC is preferable and 0-80 degreeC is more preferable.

  Furthermore, in the first stretching step, if the draw ratio of the propylene-based resin film A is small, micropores are hardly formed in the amorphous part between lamellae, and if large, the micropores are formed in the propylene-based resin microporous film. May not be formed uniformly, it is preferably 1.05 to 1.6 times, more preferably 1.10 to 1.5 times.

  In addition, in this invention, the draw ratio of the propylene-type resin film A means the value which remove | divided the length of the extending direction in the propylene-type resin film A after extending | stretching by the length of the propylene-type resin film A before extending | stretching.

  If the stretching speed in the first stretching step of the propylene-based resin film A is small, it is difficult to form micropores uniformly in the non-crystalline portion between the lamellae, so 20% / min or more is preferable. Since the resin film A may be broken, it is more preferably 20 to 3000% / min, and particularly preferably 20 to 70% / min.

In the present invention, the stretching speed (% / min) of the propylene-based resin film A is the length in the stretching direction of the propylene-based resin film A before stretching, L ₀ (mm), and the propylene-based resin after stretching. It is a value calculated based on the following formula when the length of the film A in the stretching direction is L ₁ (mm) and the time required for stretching is T (minutes).
Stretching rate (% / min) = [(L ₁ −L ₀ ) / (L ₀ × T)] × 100

(Second stretching step)
Next, in the propylene-based resin film A after uniaxial stretching in the first stretching step, preferably, the surface temperature of the propylene-based resin film A is higher than the surface temperature of the propylene-based resin film A during uniaxial stretching in the first stretching step. The propylene-based resin film A is stretched in the heating furnace 81 at a temperature that is higher and lower than the melting point of the propylene-based resin by 10 to 100 ° C., and uniaxially stretched in the transport direction. As described above, the propylene-based resin film A is stretched in the same direction as that in the first stretching step by the second stretching step at a surface temperature higher than the surface temperature of the propylene-based resin film A in the first stretching step. Thus, a large number of micropores formed in the propylene-based resin film in the first stretching step can be grown.

  In this second stretching step, as shown in FIG. 3, the propylene-based resin film A fed from the second stretching roll 72 is supplied into the heating furnace 81, and the propylene-based resin film A is stretched in the heating furnace 81. By rotating the rolls 82 in a zigzag manner and rotating the peripheral speeds of the stretching rolls 82 so as to increase sequentially in the transport direction of the propylene-based resin film A, there is a certain interval in the vertical direction. This is performed by stretching the propylene-based resin film A between the facing stretching rolls 82 and 82.

  If the surface temperature of the propylene-based resin film A is low in the second stretching step, the micropores formed in the propylene-based resin film A in the first stretching step are difficult to grow, and the air permeability of the propylene-based resin microporous film is low. If this is high, if it is high, the micropores formed in the propylene-based resin film A in the first stretching step may be blocked, and instead the air permeability of the propylene-based resin microporous film may be reduced. The temperature is preferably higher than the surface temperature of the propylene-based resin film A in the first stretching step and 10 to 100 ° C. lower than the melting point of the propylene-based resin, and higher than the surface temperature of the propylene-based resin film A in the first stretching step. And the temperature below 15-80 degreeC lower than melting | fusing point of propylene-type resin is more preferable.

  In the second stretching step, if the stretching ratio of the propylene-based resin film A is small, the micropores formed in the propylene-based resin film A during the first stretching step are difficult to grow. The air permeability may be reduced. If the air permeability is large, the micropores formed in the propylene-based resin film A in the first stretching step may be blocked, and instead the air-permeability of the propylene-based resin microporous film may be reduced. Therefore, 1.05 to 3 times is preferable, and 1.8 to 2.5 times is more preferable.

Further, in the second stretching step, if the stretching speed of the propylene-based resin film A is large, micropores may not be uniformly formed in the propylene-based resin film A, so 500% / min or less is preferable, and 400% / Min or less is more preferable, and 60% / min or less is particularly preferable. Further, in the second stretching step, if the stretching speed of the propylene-based resin film A is small, it is difficult to form micropores uniformly in the non-crystalline portions between lamellas, so it is preferable to be 15% / min or more. .

  The method for stretching the propylene-based resin film A in the second stretching step is not particularly limited as long as the propylene-based resin film can be uniaxially stretched only in the transport direction. For example, the propylene-based resin film is uniaxially stretched. And a method of uniaxial stretching at a predetermined temperature.

[Annealing process]
Next, the propylene-based resin microporous film B is obtained by feeding the propylene-based resin film A, which has been uniaxially stretched in the second stretching step, into the annealing furnace 91 and performing an annealing treatment. In this annealing step, the residual strain generated in the propylene-based resin film A due to stretching applied in the above-described stretching step is alleviated, and the resulting propylene-based resin microporous film B is prevented from being thermally contracted by heating. To be done.

  In this annealing step, as shown in FIG. 3, the propylene-based resin film A sent out from the heating furnace 81 is hung up and down on each of the guide rolls 92 in the annealing furnace 91 in a zigzag manner in the conveying direction. The propylene-based resin film A can be annealed by being transported and heating the propylene-based resin film A for a predetermined time. The propylene-based resin microporous film B thus obtained is continuously wound around the winding roll 63 after being sent out from the annealing furnace 91.

  If the surface temperature of the propylene-based resin film A in the annealing process is low, the distortion remaining in the propylene-based resin film A is insufficiently relaxed, and the resulting propylene-based resin microporous film B is dimensionally stable when heated. If it is high, there is a possibility that the micropores formed in the stretching process may be clogged. Therefore, the temperature is higher than the surface temperature of the propylene-based resin film A in the second stretching process, and is propylene-based. A temperature lower by 10 ° C. than the melting point of the resin is preferred. In addition, when performing an extending | stretching process in multiple times, the surface temperature of the propylene-type resin film A in an annealing process has the highest surface temperature of the propylene-type resin film A among the surface temperatures of the propylene-type resin film A in all the extending processes. It is preferable that the temperature be higher than the temperature.

  If the shrinkage ratio of the propylene-based resin film A in the annealing process is large, the propylene-based resin film A may sag and cannot be annealed uniformly, or the shape of the micropores may not be maintained. % Or less is preferable. The shrinkage ratio of the propylene-based resin film A is obtained by dividing the shrinkage length of the propylene-based resin film A in the stretching direction at the annealing step by the length of the propylene-based resin film A in the stretching direction after the second stretching step. And the value multiplied by 100.

[Propylene resin microporous film]
The propylene-based resin microporous film B obtained by the method of the present invention described above has a large number of micropores formed through the film front and back surfaces, and has excellent air permeability. Therefore, when the propylene-based resin microporous film B is used as, for example, a separator of a lithium ion battery, lithium ions can pass through the propylene-based resin microporous film B smoothly and uniformly. Exhibits excellent battery performance.

  Furthermore, since the propylene-based resin microporous film B is formed with a large number of micropores independently, it maintains the above-described excellent air permeability and allows lithium ions to pass smoothly and uniformly. It is difficult to generate dendrites, and also has excellent mechanical strength. Even if lithium dendrites are generated on the negative electrode end face due to charging / discharging of the lithium ion battery, the dendrites are propylene resin microporous film B Thus, dendrite short-circuiting can be reliably prevented, and problems such as battery capacity degradation can be prevented in advance.

  When the air permeability of the propylene-based resin microporous film B is small, the lithium ion permeability of the propylene-based resin microporous film B decreases, and the battery performance of the lithium ion battery using the propylene-based resin microporous film B When it is large, the film strength of the propylene-based resin microporous film B is lowered. Therefore, 40 to 400 s / 100 mL is preferable, and 200 to 380 s / 100 mL is more preferable.

  In addition, the air permeability of the propylene-based resin microporous film B refers to a value measured at 23 ° C. and a relative humidity of 65% in accordance with JIS P8117.

  The surface opening ratio of the propylene-based resin microporous film is a main factor for controlling the air permeability, and is determined by the size of the micropores and the number of micropores per unit area. In order to increase the air permeability of the propylene-based resin microporous film, the size of the micropores may be increased or the number of micropores may be increased. In the porous film, the film having a higher air permeability tended to have a larger micropore size. As a result, the conventional microporous film has problems such as an increase in resistance value due to local lithium ion movement, generation of dendrites, and a decrease in film strength. Therefore, in the propylene-based resin microporous film of the present invention, the size of the micropores can be ensured to be low, and the micropores per unit area of the propylene-based resin microporous film can be maintained at a level at which dendrites are hardly generated. The propylene-based resin microporous film having the above predetermined air permeability was successfully obtained by increasing the number, and the surface area ratio is preferably 25 to 55%, more preferably 30 to 50%.

  In addition, the surface opening ratio of a propylene-type resin microporous film can be measured in the following way. First, in a desired portion of the surface of the microporous propylene-based resin film, a measurement portion having a plane rectangular shape of 9.6 μm in length and 12.8 μm in width is defined, and this measurement portion is photographed at a magnification of 10,000 times.

Next, each micropore formed in the measurement portion is surrounded by a rectangle whose one of the long side and the short side is parallel to the extending direction. The rectangle is adjusted so that both the long side and the short side have the minimum dimension. Let the area of the said rectangle be an opening area of each micropore part. The total opening area S (μm ² ) of the micropores is calculated by summing the opening areas of the micropores. This is the total opening area S of the minute hole ([mu] m ²⁾ of 122.88μm ² (9.6μm × 12.8μm) surface porosity values multiplied by 100 and divided by the (%). In addition, about the micropore part which exists across the measurement part and the part which is not a measurement part, only the part which exists in a measurement part among micropores is set as a measuring object.

  If the maximum long diameter of the opening end of the microporous portion in the propylene-based resin microporous film B is large, there is a possibility that a dendrite short circuit may occur due to local movement of lithium ions. Since there exists a possibility that mechanical strength may fall, 1 micrometer or less is preferable, 100 nm-900 nm are more preferable, 500 nm-900 nm are especially preferable.

  If the average major axis of the open ends of the micropores in the propylene-based resin microporous film B is large, a dendrite short circuit may occur. Therefore, the average major axis is preferably 500 nm or less, more preferably 10 nm to 400 nm, and particularly preferably 200 nm to 400 nm. preferable.

  In addition, the maximum major axis and the average major axis of the opening end of the micropores in the propylene-based resin microporous film B are measured as follows. First, the surface of the propylene-based resin microporous film B is carbon-coated. Next, 10 arbitrary positions on the surface of the propylene-based resin microporous film B are photographed at a magnification of 10,000 using a scanning electron microscope. The shooting range is a plane rectangular range of 9.6 μm long × 12.8 μm wide on the surface of the propylene-based resin microporous film B.

  The major axis of the opening end of each micropore appearing in the obtained photograph is measured. Among the long diameters of the opening ends in the microhole portion, the maximum long diameter is set as the maximum long diameter of the opening end of the microhole portion. The arithmetic mean value of the major axis of the open end in each micropore is defined as the average major axis of the open end of the micropore. The major axis of the open end of the microhole is defined as the diameter of a perfect circle having the smallest diameter that can surround the open end of the microhole. Micropores that exist across the imaging range and the non-imaging range are excluded from the measurement target.

The pore density of the propylene-based resin microporous film B is such that the lower limit value is naturally constrained in order to satisfy the above air permeability range and the maximum major axis and average major axis of the opening end of the microporous part satisfy the above range. small as, propylene resin microporous film B in physical properties since no longer satisfied (air permeability, the average major axis of the opening end of the maximum diameter or microporous portion of the open end of the minute hole), 15 / [mu] m ² or more , and more preferably 17 to 50 pieces / [mu] m ^2.

The pore density of the propylene-based resin microporous film B is measured as follows. First, in an arbitrary part of the surface of the propylene-based resin microporous film B, a measurement part having a plane rectangular shape of 9.6 μm × 12.8 μm is determined, and this measurement part is photographed at a magnification of 10,000 times. Then, the number of micropores is measured in the measurement part, and the pore density can be calculated by dividing this number by 122.88 μm ² (9.6 μm × 12.8 μm). In addition, the micropore part which exists over the measurement part and the part which is not a measurement part is counted as 0.5 pieces.

  According to the method of the present invention, a propylene-based resin microporous film having excellent air permeability can be easily produced. According to such a propylene-based resin microporous film, for example, when used as a separator of a lithium ion battery, the lithium ion battery allows smooth and uniform passage of lithium ions, and the lithium ion battery has excellent battery performance. In addition, the occurrence of a dendrite short circuit can be generally prevented, and a lithium ion battery having stable battery performance over a long period of time can be configured.

  In particular, by using a propylene-based resin microporous film obtained by the method of the present invention as a separator, it has high-performance battery performance in which a sudden decrease in discharge capacity and occurrence of dendrite shorts are generally prevented even in high output applications. A lithium ion battery can be constructed.

It is the model side view which showed an example of the manufacturing apparatus used for the extrusion process in the method of this invention, a 1st cooling process, a curing process, and a 2nd cooling process. It is the model side view which showed another example of the manufacturing apparatus used for the extrusion process in the method of this invention, a 1st cooling process, a curing process, and a 2nd cooling process. It is the model side view which showed an example of the manufacturing apparatus used for the extending process and annealing process in the method of this invention. It is the model side view which showed the manufacturing apparatus used for the extrusion process in the comparative example 1, a 1st cooling process, and a 2nd cooling process. It is the model side view which showed the manufacturing apparatus used for the extrusion process in the comparative example 2, a 1st cooling process, and a 2nd cooling process.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

[Example 1]
(Extrusion process)
The cooled homopolypropylene film A was obtained using the manufacturing apparatus shown in FIG. Specifically, homopolypropylene having the weight average molecular weight, number average molecular weight, molecular weight distribution, melting point, and heat of fusion obtained by DSC shown in Table 1 is supplied to the extruder 1 and melt kneaded at a resin temperature of 195 ° C. The homopolypropylene film A was extruded from the extrusion port 12a of the T die 2 attached to the front end of the extruder 1 at an extrusion speed (R ₀ ) of 0.37 m / min.

(First cooling step)
The homopolypropylene film A extruded from the T-die 2 is supplied between the first cooling roll 21 and the pressing belt 24 that spans the first nip roll 22 and the rotating roll 23. While holding the homopolypropylene film A on the surface facing the pressing belt 24, the homopolypropylene film A is moved according to the rotation of the first cooling roll 21, and the homopolypropylene film A passes through between the first cooling roll 21 and the pressing belt 24. The polypropylene film A was cooled.

The surface temperature of the first cooling roll 21 was kept at 100 ° C. Further, the surface temperature of the pressing belt 24 was set to the same temperature as the surface temperature of the first cooling roll 21. The first cooling roll 21 is rotated so that its peripheral speed (R ₁ ) is 15 m / min, and the peripheral speed of the first cooling roll 4 with respect to the extrusion speed (R ₀ ) of the propylene-based resin film A at the outlet of the T die 2. by the ratio of (R ₁₎ a (R ₁ / R ₀₎ and 48, a homopolypropylene film a extruded from the T-die 2 was actively tensioning the first cooling roll 21. And the surface temperature of the propylene-type resin film A immediately after peeling from the 1st cooling roll 21 was 155 degreeC.

(Curing process)
The homopolypropylene film A fed from the first cooling roll 21 is supplied into the first curing furnace 31 and conveyed while being passed over the upper and lower guide rolls 32, 32 disposed in the first curing furnace 31, While passing through the upper and lower guide rolls 32, 32, the surface temperature of the homopolypropylene film A was set at 152 ° C. for 15 minutes.

(Second cooling step)
By supplying the homopolypropylene film A fed from the first curing furnace 31 to the surface of the second cooling roll 51 and moving it in contact with the peripheral surface of the second cooling roll 51, the surface temperature of the homopolypropylene film A is increased. The homopolypropylene film A was cooled until the temperature reached 25 ° C. Thereafter, the homopolypropylene film A fed from the second cooling roll 51 was continuously wound around the winding roll 61.

(First stretching step)
Next, the homopolypropylene microporous film was obtained using the manufacturing apparatus shown in FIG. Specifically, the homopolypropylene film A was continuously unwound from the take-up roll 61, and sequentially passed over the first drawing roll 71 and the second drawing roll 72 and conveyed. At this time, the surface temperature of the homopolypropylene film A is 23 ° C., and the peripheral speed of the second stretching roll 72 is rotated so as to be larger than the peripheral speed of the first stretching roll 71, so that the homopolypropylene film A is 50%. The film was uniaxially stretched only in the conveying direction at a stretching ratio of 1.2 times at a stretching speed of / min.

(Second stretching step)
The homopolypropylene film A fed out from the second stretching roll 72 is supplied into the heating furnace 81, and the homopolypropylene film A is moved up and down in the heating furnace 81 in a zigzag manner in each of the stretching rolls 82 in the conveying direction. It was transported as it was rolled over. At this time, the surface temperature of the homopolypropylene film A is set to 120 ° C., and the respective peripheral speeds of the drawing rolls 82 are rotated so as to be gradually decreased toward the conveying direction of the homopolypropylene film A, thereby making the homopolypropylene film A 42 The film was uniaxially stretched only in the conveying direction at a stretching ratio of 2 at a stretching speed of% / min.

(Annealing process)
Then, the homopolypropylene film A fed from the heating furnace 81 is supplied into the annealing furnace 91, and in this annealing furnace 91, the homopolypropylene film A is staggered up and down on each of the guide rolls 92 in the conveying direction. The homopolypropylene film A is annealed for 10 minutes so that the surface temperature is 130 ° C. and no tension is applied to the homopolypropylene film A Film B was obtained. The homopolypropylene microporous film B thus obtained was continuously wound around the winding roll 63 after being sent out from the annealing furnace 91. In addition, the shrinkage rate of the homopolypropylene film A in the annealing process was 20%.

[Example 2]
(Extrusion process)
The cooled homopolypropylene film A was obtained using the manufacturing apparatus shown in FIG. Specifically, homopolypropylene having the weight average molecular weight, number average molecular weight, molecular weight distribution, melting point, and heat of fusion obtained by DSC shown in Table 1 is supplied to the extruder 1 and melt kneaded at a resin temperature of 200 ° C. The homopolypropylene film A was extruded from the extrusion port 12a of the T die 2 attached to the front end of the extruder 1 at an extrusion speed (R ₀ ) of 0.34 m / min.

(First cooling step)
After the homopolypropylene film A extruded from the T-die 2 is brought into close contact with the surface of the first cooling roll 21 by the air knife 25, the homopolypropylene film A is moved according to the rotation of the first cooling roll 21, and the homopolypropylene film A Cooled.

The surface temperature of the first cooling roll 21 was kept at 120 ° C. The air blown from the air knife 25 was set to the same temperature as the surface temperature of the first cooling roll 21. The first cooling roll 21 is rotated so that its peripheral speed (R ₁ ) is 20 m / min, and the peripheral speed of the first cooling roll 4 with respect to the extrusion speed (R ₀ ) of the propylene-based resin film A at the outlet of the T die 2. by a ratio of (R ₁ / R ₀₎ 70 of the (R _1), it was actively tensioning by T-die homopolypropylene film a extruded from the 2 first cooling roll 21. And the surface temperature of the propylene-type resin film A immediately after peeling from the 1st cooling roll 21 was 155 degreeC.

(First curing process)
The homopolypropylene film A fed from the first cooling roll 21 is supplied into the first curing furnace 31 and conveyed while being passed over the upper and lower guide rolls 32, 32 disposed in the first curing furnace 31, While passing through the upper and lower guide rolls 32, 32, the homopolypropylene film A was cured for 10 minutes so that the surface temperature of the homopolypropylene film A was 130 ° C.

(Second curing process)
The homopolypropylene film A fed from the first curing furnace 31 is supplied into the second curing furnace 31 'and is passed over the upper and lower guide rolls 32' and 32 'disposed in the second curing furnace 31'. Then, while passing through the upper and lower guide rolls 32 ′ and 32 ′, the homopolypropylene film A was cured for 10 minutes so that the surface temperature of the homopolypropylene film A became 110 ° C.

(Second cooling step)
The homopolypropylene film A fed from the second curing furnace 31 ′ is supplied to the surface of the second cooling roll 51, and brought into contact with the peripheral surface of the second cooling roll 51, thereby transferring the surface of the homopolypropylene film A The homopolypropylene film A was cooled until the temperature reached 25 ° C. Thereafter, the homopolypropylene film A fed from the second cooling roll 51 was continuously wound around the winding roll 61.

(First stretching step)
Next, the homopolypropylene microporous film was obtained using the manufacturing apparatus shown in FIG. Specifically, the homopolypropylene film A was continuously unwound from the take-up roll 61, and sequentially passed over the first drawing roll 71 and the second drawing roll 72 and conveyed. At this time, the surface temperature of the homopolypropylene film A is 23 ° C., and the peripheral speed of the second stretching roll 72 is rotated so as to be larger than the peripheral speed of the first stretching roll 71, so that the homopolypropylene film A is 50%. The film was uniaxially stretched only in the conveying direction at a stretching ratio of 1.2 times at a stretching speed of / min.

(Second stretching step)
The homopolypropylene film A fed out from the second stretching roll 72 is supplied into the heating furnace 81, and the homopolypropylene film A is moved up and down in the heating furnace 81 in a zigzag manner in each of the stretching rolls 82 in the conveying direction. It was transported as it was rolled over. At this time, the surface temperature of the homopolypropylene film A is set to 120 ° C., and the respective peripheral speeds of the drawing rolls 82 are rotated so as to be gradually decreased toward the conveying direction of the homopolypropylene film A, thereby making the homopolypropylene film A 42 The film was uniaxially stretched only in the conveying direction at a stretching ratio of 2 at a stretching speed of% / min.

(Annealing process)
Then, the homopolypropylene film A fed from the heating furnace 81 is supplied into the annealing furnace 91, and in this annealing furnace 91, the homopolypropylene film A is staggered up and down on each of the guide rolls 92 in the conveying direction. The homopolypropylene film A is annealed for 10 minutes so that the surface temperature of the homopolypropylene film A becomes 130 ° C. and no tension is applied to the homopolypropylene film A by carrying it over and transporting. B was obtained. The homopolypropylene microporous film B thus obtained was continuously wound around the winding roll 63 after being sent out from the annealing furnace 91. In addition, the shrinkage rate of the homopolypropylene film A in the annealing process was 20%.

[Example 3]
Using the homopolypropylene having the weight average molecular weight, number average molecular weight, molecular weight distribution, melting point, and heat of fusion obtained by DSC shown in Table 1, the temperature at which the homopolypropylene is melt-kneaded in the extrusion step, the first in the first cooling step A homopolypropylene microporous film B was obtained in the same manner as in Example 2 except that the surface temperature of the cooling roll and the surface temperature of the homopolypropylene film in the first curing process and the second curing process were changed as shown in Table 1, respectively.

[Comparative Example 1]
(Extrusion process)
The cooled homopolypropylene film A was obtained using the manufacturing apparatus shown in FIG. Specifically, homopolypropylene having the weight average molecular weight, number average molecular weight, molecular weight distribution, melting point, and heat of fusion obtained by DSC shown in Table 1 is supplied to the extruder 1 and melt kneaded at a resin temperature of 200 ° C. The homopolypropylene film A was extruded from the extrusion port 12a of the T die 2 attached to the front end of the extruder 1 at an extrusion speed (R ₀ ) of 0.37 m / min.

(First cooling step)
After the homopolypropylene film A extruded from the T die 2 is supplied between the first cooling roll 21 and the rubber roll 26, the homopolypropylene film A is brought into close contact with the surface of the first cooling roll 21 by the rubber roll 26, The homopolypropylene film A was moved according to the rotation of the first cooling roll 21, and the surface temperature of the homopolypropylene film A was cooled.

The surface temperature of the first cooling roll 21 was kept at 25 ° C. The surface temperature of the rubber roll 26 was the same as the surface temperature of the first cooling roll 21. The first cooling roll 21 is rotated so that its peripheral speed (R ₁ ) is 15 m / min, and the peripheral speed of the first cooling roll 4 with respect to the extrusion speed (R ₀ ) of the propylene-based resin film A at the outlet of the T die 2. by the ratio of (R ₁₎ a (R ₁ / R ₀₎ and 48, a homopolypropylene film a extruded from the T-die 2 was actively tensioning the first cooling roll 21. And the surface temperature of the propylene-type resin film A immediately after peeling from the 1st cooling roll 21 was 30 degreeC.

(Second cooling step)
By supplying the homopolypropylene film A fed from the first cooling roll 21 to the surface of the second cooling roll 51 and making contact with the peripheral surface of the second cooling roll 51, the surface temperature of the homopolypropylene film A is changed. The homopolypropylene film A was cooled until the temperature reached 25 ° C. Thereafter, the homopolypropylene film A fed from the second cooling roll 51 was continuously wound around the winding roll 61.

(First stretching step)
Next, the homopolypropylene microporous film was obtained using the manufacturing apparatus shown in FIG. Specifically, the homopolypropylene film A was continuously unwound from the take-up roll 61, and sequentially passed over the first drawing roll 71 and the second drawing roll 72 and conveyed. At this time, the surface temperature of the homopolypropylene film A is 23 ° C., and the peripheral speed of the second stretching roll 72 is rotated so as to be larger than the peripheral speed of the first stretching roll 71, so that the homopolypropylene film A is 50%. The film was uniaxially stretched only in the conveying direction at a stretching ratio of 1.2 times at a stretching speed of / min.

(Second stretching step)
The homopolypropylene film A fed out from the second stretching roll 72 is supplied into the heating furnace 81, and the homopolypropylene film A is moved up and down in the heating furnace 81 in a zigzag manner in each of the stretching rolls 82 in the conveying direction. It was transported as it was rolled over. At this time, the surface temperature of the homopolypropylene film A is set to 120 ° C., and the respective peripheral speeds of the drawing rolls 82 are rotated so as to be gradually decreased toward the conveying direction of the homopolypropylene film A, thereby making the homopolypropylene film A 42 The film was uniaxially stretched only in the conveying direction at a stretching ratio of 2 at a stretching speed of% / min.

(Annealing process)
Then, the homopolypropylene film A fed from the heating furnace 81 is supplied into the annealing furnace 91, and in this annealing furnace 91, the homopolypropylene film A is staggered up and down on each of the guide rolls 92 in the conveying direction. The homopolypropylene film A is annealed for 10 minutes so that the surface temperature of the homopolypropylene film A becomes 130 ° C. and no tension is applied to the homopolypropylene film A by carrying it over and transporting. B was obtained. The homopolypropylene microporous film B thus obtained was continuously wound around the winding roll 63 after being sent out from the annealing furnace 91. In addition, the shrinkage rate of the homopolypropylene film A in the annealing process was 20%.

[Comparative Example 2]
(Extrusion process)
The cooled homopolypropylene film A was obtained using the manufacturing apparatus shown in FIG. Specifically, homopolypropylene having the weight average molecular weight, number average molecular weight, molecular weight distribution, melting point, and heat of fusion obtained by DSC shown in Table 1 is supplied to the extruder 1 and melt kneaded at a resin temperature of 200 ° C. The homopolypropylene film A was extruded from the extrusion port 12a of the T die 2 attached to the front end of the extruder 1 at an extrusion speed (R ₀ ) of 0.34 m / min.

(First cooling step)
After the homopolypropylene film A extruded from the T-die 2 is brought into close contact with the surface of the first cooling roll 21 by the air knife 25, the homopolypropylene film A is moved according to the rotation of the first cooling roll 21, and the homopolypropylene film A The homopolypropylene film A was cooled until the surface temperature became the surface temperature of the first cooling roll 21.

The surface temperature of the first cooling roll 21 was maintained at 110 ° C. The air blown from the air knife 25 was set to the same temperature as the surface temperature of the first cooling roll 21. The first cooling roll 21 is rotated so that its peripheral speed (R ₁ ) is 20 m / min, and the peripheral speed of the first cooling roll 4 with respect to the extrusion speed (R ₀ ) of the propylene-based resin film A at the outlet of the T die 2. by a ratio of (R ₁ / R ₀₎ 70 of the (R _1), it was actively tensioning by T-die homopolypropylene film a extruded from the 2 first cooling roll 21. And the surface temperature of the propylene-type resin film A immediately after peeling from the 1st cooling roll 21 was 150 degreeC.

  The homopolypropylene film A fed from the first cooling roll 21 is subjected to the second cooling step, the first stretching step, the second stretching step, and the annealing step in the same manner as in Comparative Example 1 to perform a homopolypropylene microporous film. B was obtained.

[Comparative Example 3]
Using the homopolypropylene having the weight average molecular weight, number average molecular weight, molecular weight distribution, melting point, and heat of fusion obtained by DSC shown in Table 1, the temperature at which the homopolypropylene is melt-kneaded in the extrusion step, the first in the first cooling step A homopolypropylene microporous film B was prepared in the same manner as in Comparative Example 1 except that the surface temperature of the cooling roll 21 was changed as shown in Table 1. However, in the first cooling step, the homopolypropylene film A is wound around the first cooling roll 21, and the homopolypropylene film A cannot be peeled off from the first cooling roll 21, and the homopolypropylene microporous film B can be obtained. There wasn't.

  The thickness of the homopolypropylene film A after the curing process, the birefringence, the air permeability of the homopolypropylene microporous film B, the surface aperture ratio, the maximum major axis and the average major axis of the open end of the micropore, and the pore density are as described above. The results are shown in Table 1.

[Comparative Examples 4 and 5]
The temperature and extrusion speed at which the homopolypropylene is melt-kneaded in the extrusion step using the homopolypropylene having the weight average molecular weight, number average molecular weight, molecular weight distribution, melting point, and heat of fusion obtained by DSC shown in Table 1, and the first cooling step In the same manner as in Example 1 except that the surface temperature and the peripheral speed of the first cooling roll in and the surface temperature of the homopolypropylene film in the first curing step were changed as shown in Table 1 and the second curing step was not performed. Homopolypropylene microporous film B was obtained.

11 Extruder
12 T-die
12a extrusion port
21 Cooling roll
22 Nip roll
23 Rotating roll
24 Press belt
25 Air knife
26 Rubber roll
31 Curing furnace
32 Guide roll
33 Nip roll
42 Guide roll
43 Nip roll
51 Cooling roll
61 Winding roll
63 Winding roll
71 First draw roll
72 Second draw roll
73 Nip roll
74 Nip roll
81 furnace
82 Stretch roll
83 Nip roll
91 Annealing furnace
92 Guide roll
93 Nip roll A Propylene resin film B Propylene resin microporous film

Claims (9)

Propylene-based resin is supplied to an extruder, and the propylene-based resin is melt-kneaded at a temperature that is 20 ° C. higher than its melting point and 100 ° C. higher than the melting point of the propylene-based resin, and attached to the extruder An extrusion step of obtaining a propylene-based resin film by extruding from a T-die,
The propylene-based resin film extruded from the T-die is cooled until the surface temperature is 60 ° C. lower than the melting point of the propylene-based resin and 1 ° C. lower than the melting point of the propylene-based resin. A first cooling step to perform,
The surface temperature of the propylene-based resin film after the first cooling step is lower than the surface temperature of the propylene-based resin film cooled in the first cooling step, and is 100 ° C. higher than the melting point of the propylene-based resin. A curing step of curing for 1 minute or more at a temperature lower than the low temperature and lower than 5 ° C. lower than the melting point of the propylene resin;
A second cooling step for cooling the propylene-based resin film after the curing step until the surface temperature is less than 100 ° C. lower than the melting point of the propylene-based resin;
A stretching step of forming micropores in the propylene-based resin film by uniaxially stretching the propylene-based resin film after the second cooling step;
An annealing step for annealing the propylene-based resin film after the stretching step;
The manufacturing method of the propylene-type resin microporous film characterized by including.   The first stretching step in which the stretching step stretches the propylene-based resin film after the second cooling step by 1.05 to 1.60 times in the transport direction at a surface temperature of -20 to 100 ° C, and the first stretching. 1. The propylene-based resin film after the process has a surface temperature of not less than the surface temperature of the propylene-based resin film at the time of the first stretching step and a temperature lower by 10 to 100 ° C. than the melting point of the propylene-based resin. The manufacturing method of the propylene-type resin microporous film of Claim 1 including the 2nd extending process extended | stretched 05 to 3 times.   In the annealing step, the propylene-based resin film after the stretching step is annealed at a temperature not lower than the surface temperature of the propylene-based resin film in the stretching step and not more than 10 ° C. lower than the melting point of the propylene-based resin. The manufacturing method of the propylene-type resin microporous film of any one of Claims 1-3 characterized by these.   A propylene-based resin microporous film produced by the method according to any one of claims 1 to 3.   5. The propylene-based resin microporous film according to claim 4, wherein the air permeability is 40 to 400 s / 100 mL and the surface opening ratio is 25 to 55%.   6. The propylene-based resin microporous film according to claim 4, wherein the maximum major axis of the opening end of the micropore part is 1 μm or less and the average major axis is 500 nm or less. The propylene-based resin microporous film according to claim 4, wherein the pore density is 15 / μm ² or more.   A separator for a lithium ion battery, comprising the propylene-based resin microporous film according to any one of claims 4 to 7.   A lithium ion battery comprising the battery separator according to claim 8 incorporated therein.

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