JP4925238B2 - Method for producing polyolefin microporous membrane - Google Patents

Description

  The present invention relates to a method for producing a polyolefin microporous membrane which is used as a separator for a secondary battery, and particularly preferably used as a separator for a lithium ion secondary battery.

Along with the remarkable development of information-related equipment such as mobile phones and notebook computers in recent years, small and light batteries with high energy capacity are required. Among them, the market for lithium-ion batteries is rapidly expanding, and accordingly, the level of demand for microporous polyolefin membranes used as separators is increasing.
The quality and performance of polyolefin microporous membranes greatly affect not only the performance of lithium ion batteries but also their productivity. For example, if the thickness of the microporous membrane is not uniform, troubles such as winding slippage may occur in the battery winding process. In addition, if the strength of the microporous membrane is not sufficient, the probability of battery failure due to film breakage at the time of winding or piercing of foreign matter increases.

In Patent Document 1, a polyolefin microporous material that is prepared by preparing a solution comprising a specific polyolefin and a solvent, extruding it into a sheet, taking it at a constant take-up ratio while cooling, and then heating and drawing at least in a uniaxial direction to perform extraction. Membrane manufacturing methods have been proposed. Although it is expected that a high-strength microporous membrane can be produced at low cost by this method, a microporous membrane with good thickness uniformity cannot be stably obtained by the disclosed method.
JP-A-9-302120

  An object of the present invention is to provide a method for producing a polyolefin microporous membrane that can stably obtain a polyolefin microporous membrane having high strength, low shrinkage and high thickness uniformity.

The present invention solves the above problems. That is, the present invention
(1) Polyolefin composition and plasticizer are melt-kneaded with a plasticizer content ratio in the melt-kneaded product of 20 to 95 wt%, extruded from a T-die to form a bank (resin pool), and cast molding In the method for producing a microporous membrane made of polyolefin in which a sheet is prepared by stretching and extracting a plasticizer, the width of the bank is 60% or more with respect to the total width of the kneaded material sandwiched between rolls, A method for producing a polyolefin microporous film, characterized in that the temperature of the roll on the surface side forming the bank is higher by 1 ° C. or more than the temperature of the roll on the surface side where the bank is not formed,
(2) The method for producing a polyolefin microporous membrane according to (1), wherein the roll temperature is 30 ° C. lower than the melting point of the constituting polyolefin, 0 ° C. or higher,
(3) The method for producing a polyolefin microporous membrane according to (1) or (2), wherein polyethylene is used in an amount of 50 wt% or more as the polyolefin composition.

  According to the method for producing a polyolefin microporous membrane of the present invention, a microporous membrane having high thickness uniformity, high strength and low shrinkage can be stably obtained over a long period of time. Further, by using the microporous membrane of the present invention for a lithium ion battery, it is possible to produce a high performance and high safety lithium ion battery with high productivity.

  The present invention will be specifically described. The polyolefin composition used in the present invention is a single or composition comprising a homopolymer or copolymer of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene. It is preferable to use 50 wt% or more of polyethylene (meaning an ethylene homopolymer and a copolymer mainly composed of ethylene). There is no restriction | limiting in the catalyst used for manufacture of polyolefin, For example, a Ziegler-Natta catalyst, a Philips catalyst, a metallocene catalyst, etc. are mentioned. Moreover, there is no restriction | limiting also in the superposition | polymerization form, As an example, 1 step | paragraph polymerization and multistage polymerization of 2 steps | paragraphs or more are mentioned. The preferred viscosity average molecular weight (Mv) of polyolefin is 5 to 15 million, more preferably 10 to 5 million.

From the viewpoint of preventing thermal deterioration during melt kneading and quality deterioration due thereto, it is preferable to add an antioxidant. The concentration of the antioxidant is preferably 0.3 wt% or more, more preferably 0.5 wt% or more based on the total weight of the polyolefin. Moreover, 5.0 wt% or less is preferable and 3.0 wt% or less is more preferable.
As the antioxidant, a phenolic antioxidant which is a primary antioxidant is preferable, and 2,6-di-t-butyl-4-methylphenol, pentaerythrityl-tetrakis- [3- (3,5- Di-t-butyl-4-hydroxyphenyl) propionate], octadecyl-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate, and the like. Secondary antioxidants can also be used in combination, such as tris (2,4-di-t-butylphenyl) phosphite, tetrakis (2,4-di-t-butylphenyl) -4,4- Examples thereof include phosphorus antioxidants such as biphenylene-diphosphonite and sulfur antioxidants such as dilauryl-thio-dipropionate.

  Polymers other than polyolefin, other organic materials, and inorganic materials can also be blended within a range that does not impair the requirements and effects of the present invention. Furthermore, if necessary, known additives such as metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments are also included in the requirements and It can be used as a mixture as long as the effect is not impaired.

  The plasticizer used in the present invention is an organic material capable of forming a uniform solution with polyolefin at a temperature below the boiling point, specifically decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol. Decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, paraffin oil and the like. Of these, paraffin oil and dioctyl phthalate are preferred. The proportion of the plasticizer in the kneaded product with the polyolefin composition is preferably 20 to 95 wt%, more preferably 30 to 80 wt%, from the viewpoint of membrane permeability and film-forming property.

  Examples of the melt-kneading method include a method of mixing with a Henschel mixer, a ribbon blender, a tumbler blender or the like and then melt-kneading with a screw extruder such as a single screw extruder or a twin screw extruder, a kneader, or a Banbury mixer. As a method of melt-kneading, an extruder capable of continuous operation is preferable for production, and a twin-screw extruder is particularly preferable from the viewpoint of excellent kneadability. The plasticizer may be mixed with the raw material polymer by the Henschel mixer or the like, or may be directly fed to the extruder during melt kneading. The temperature at the time of melt kneading is preferably in the range of 150 to 300 ° C. Further, by performing melt kneading in a nitrogen atmosphere, it is possible to effectively prevent oxidative degradation.

Next, the kneaded material is extruded from a T-die, formed into a sheet by forming a bank (resin pool) and cast molding. As schematically shown in FIG. 1, the bank (resin pool) means an elongated resin pool that does not bite between two rolls in cast molding and stays on one of the roll surfaces.
In order to form a bank, for example, the thickness of the kneaded product before being sandwiched between two rolls is set to be larger than the roll gap dimension, and the formation status of the bank is visually observed. It can be obtained by adjusting the roll rotation speed.
The present invention is characterized in that the width of the bank is 60% or more of the total width of the kneaded product to be cast. Thereby, since a uniform sheet having a uniform thickness can be stably obtained, the subsequent stretching also proceeds uniformly, and as a result, a microporous film having a high performance and a uniform thickness can be obtained. For example, the bank width is confirmed by photographing with a digital still camera after being visually adjusted. It is also possible to control the bank by feeding back information obtained by the confirmation to an auto die or the like.

  In order to obtain a homogeneous sheet, the temperature of the two rolls to be sandwiched is preferably 0 ° C. or lower and 30 ° C. or lower, more preferably 0 to 100 ° C., more preferably 10 to 90 ° C. More preferably. Further, in order to obtain a uniform sheet having a uniform thickness, it is preferable that the temperature of each roll does not fluctuate, and specifically, it is preferable to keep the temperature within ± 2 ° C. with respect to the set temperature. Examples of the method for controlling the temperature of the roll include a method using dielectric heating and a method using heat solvent heating, but a method using heat solvent heating using a fluid having good heat conductivity such as oil or water is preferable.

  In the present invention, the roll temperature on the surface side where the bank is formed is preferably higher than the roll temperature on the surface side where the bank is not formed, from the viewpoint of stabilizing the bank for a long time, and a temperature difference of 1 ° C. or more is provided. It is more preferable to provide a temperature difference of 5 ° C. or more. The plasticizer that exudes from the sheet adheres to the roll surface after a long period of operation, but a new bank may be generated starting from this adhered plasticizer, which causes variations in thickness. Become. By increasing the roll temperature on the side where the bank is formed, the plasticizer adhering to the roll on the side where the bank is not formed is relatively reduced, and as a result, generation of new banks is prevented and stable. The nature is considered to have improved.

Although the thickness of the sheet | seat obtained by this invention is based also on a draw ratio and the thickness of the target microporous film, 0.1-3 mm is preferable.
Next, the stretching in the present invention is performed once or more using a uniaxial stretching machine or a biaxial stretching machine, but it is preferable to stretch using a simultaneous biaxial tenter. The stretching temperature is preferably in the range from room temperature to the melting point of the polyolefin mainly constituting the membrane, more preferably from 80 to 135 ° C, and even more preferably from 100 to 130 ° C. The draw ratio is preferably 4 to 400 times, more preferably 8 to 200 times, and still more preferably 16 to 100 times in terms of area ratio.

  In the extraction in the present invention, the extraction solvent is preferably a poor solvent for polyolefin and a good solvent for plasticizer, and has a boiling point lower than the melting point of polyolefin. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, alcohols such as methanol, ethanol and isopropanol, ketones such as acetone and methyl ethyl ketone, ethers such as tetrahydrofuran, methylene chloride, 1, And organic solvents such as halogenated hydrocarbons such as 1,1-trichloroethane. It selects from these and is used individually or in mixture. The plasticizer is extracted by immersing the film obtained by stretching in these extraction solvents, and then sufficiently dried. It is preferable that the residual amount of plasticizer in the film is less than 1 wt% by extraction.

Furthermore, after the plasticizer extraction, one or more stretching may be performed using a uniaxial stretching machine or a biaxial stretching machine. In this case, the stretching temperature is preferably in the range from room temperature to the melting point of the polyolefin mainly constituting the membrane, more preferably from 80 to 135 ° C, and even more preferably from 100 to 130 ° C. The draw ratio is preferably 50 times or less, more preferably 10 times or less, and further preferably 5 times or less in terms of area magnification.
Further, heat setting may be performed after the plasticizer is extracted and dried or / and stretched. Thermal fixation is an operation for reducing the shrinkage of the membrane by placing the microporous membrane in a high temperature environment while performing dimension fixing or relaxation operation. The relaxation operation is a reduction operation of the film to MD (machine direction) and / or TD (direction perpendicular to the machine direction). The heat setting can be performed with a tenter or a roll stretching machine. When heat setting is performed after the stretching process, it can be performed with the same machine as the stretching process. The heat setting temperature is preferably 80 ° C. or higher, more preferably 100 ° C. or higher, and further preferably 110 ° C. or higher from the viewpoint of the heat setting effect. Further, from the viewpoint of preventing deterioration of permeability, the melting point of the polyolefin mainly constituting the membrane is preferably not higher than 135 ° C., and more preferably 135 ° C. or lower.

As long as the effects of the present invention are not impaired, surface treatment such as electron beam irradiation, plasma irradiation, surfactant coating, and chemical modification can be performed as necessary.
Next, preferred physical properties of the polyolefin microporous membrane obtained in the present invention will be described.
The thickness is preferably 3 μm or more, more preferably 5 μm or more from the viewpoint of film strength. Moreover, 100 micrometers or less are preferable from a viewpoint of an air permeability, and 50 micrometers is more preferable.
The thickness variation is preferably 5 μm or less, more preferably 3 μm or less, from the viewpoint of battery productivity when used in a battery.
The TD maximum shrinkage force is preferably 0 to 1.5 MPa, more preferably 0 to 1.0 MPa, from the viewpoint of battery safety when used in a battery.

The TD maximum contraction force variation is preferably 30% or less, and more preferably 20% or less, from the viewpoint of battery safety when used in a battery.
The porosity is preferably 20% or more, more preferably 30% or more from the viewpoint of permeability. Further, from the viewpoint of film strength, 95% or less is preferable, 80% or less is more preferable, and 60% or less is more preferable.
The air permeability is preferably 1 sec or more, and more preferably 50 sec or more. Moreover, 2000 sec or less is preferable from a viewpoint of the battery performance at the time of using for a battery, and 1000 sec or less is further more preferable.
The puncture strength is preferably 0.5 to 25.0 N / 25 μm, and more preferably 1.0 to 25.0 N / 25 μm, from the viewpoint of battery productivity and battery safety when used for a battery.

Various physical properties in the present invention were determined by the following methods.
(1) Viscosity average molecular weight Mv
Based on ASTM-D4020, the intrinsic viscosity [η] at 135 ° C. in a decalin solvent is determined. Mv of polyethylene was calculated by the following formula.
[Η] = 6.77 × 10 ⁻⁴ Mv ^0.67
(2) Thickness (μm)
The measurement was performed at a room temperature of 23 ° C. using a micro thickness gauge “KBM” (trademark) manufactured by Toyo Seiki Co., Ltd.
(3) Porosity (%)
A sample of 10 cm × 10 cm square was cut out from the microporous membrane, its volume (cm ³ ) and mass (g) were determined, and calculated from these and the film density (g / cm ³ ) using the following formula.
Porosity = (volume−mass / film density) / volume × 100
The film density was calculated at a constant 0.95.

(4) TD maximum contraction force (MPa)
Using a thermo-mechanical analyzer “TMA-50” (trademark) manufactured by Shimadzu Corporation, a sample cut into a width of 3 mm and a length of 15 mm so that the length direction is TD, the distance between chucks is 10 mm. Fix it to the chuck so that The initial load was 1.0 g, the probe was heated from 30 ° C. to 200 ° C. at a rate of 10 ° C./min, and the TD contraction load (g) generated at that time was measured. The maximum TD contraction force was calculated from the maximum TD contraction load obtained (TD maximum contraction load (g)) using the following formula.
TD maximum contractile force = (TD maximum contractile load / (3 × t)) × 9.81
t: Sample thickness (μm)

(5) Air permeability (sec)
Based on JIS P-8117, it measured with the Gurley type air permeability meter "G-B2 type" (trademark) by Toyo Seiki Co., Ltd.
(6) Puncture strength (N / 25μm)
Using a handy compression tester “KES-G5” (trademark) manufactured by Kato Tech Co., Ltd., perform a puncture test in a 25 ° C. atmosphere with a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. Thus, the maximum piercing load (N) was obtained. By multiplying this by 25 (μm) / film thickness (μm), the puncture strength (N / 25 μm) in terms of film thickness of 25 μm was calculated.
(7) Thickness variation (μm)
Thickness measurement was performed at 5 points at regular intervals on TD and 5 times at intervals of about 500 m on MD, and the difference between the maximum value and the minimum value was calculated to obtain thickness variation.

(8) TD maximum contraction force variation (%)
TD maximum contraction force was measured by cutting out a sample with a width of 3 mm and a length of 15 mm so that the length direction would be TD from a total of 25 locations at 5 locations at regular intervals on TD and 5 times at intervals of about 500 m on MD. The average value, the maximum value, and the minimum value were obtained. A value obtained by dividing the difference between the maximum value and the minimum value by the average value and multiplying by 100 was defined as TD maximum contraction force variation.

The present invention will be described based on examples.
[Example 1]
Using a tumbler blender, 42.5 wt% of homopolyethylene having an Mv of 300,000, 42.5 wt% of homopolyethylene having an Mv of 700,000, and 15 wt% of polyethylene copolymerized with propylene having an Mv of 120,000 were dry blended. 0.5 wt% of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added to 99.5 wt% of the obtained pure polymer mixture, By dry blending again using a tumbler blender, a polymer mixture was obtained. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.

The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 65 wt%. The melt kneading conditions were a set temperature of 200 ° C., a screw rotation speed of 280 rpm, and a discharge rate of 15 kg / h.
Subsequently, after passing through a T-die in which the kneaded material melt-kneaded by the extruder was adjusted to a thickness direction clearance of 2000 μm, the first roll (2 roll in FIG. 1) was 45 ° C. and the second roll (3 roll in FIG. 1). ) Was extruded onto two rolls controlled at 25 ° C. Each roll temperature control means is a system in which oil is heated to a set temperature by a heater with each hot solvent heating type mold temperature controller and sent to the roll (can be held at ± 1 ° C. with respect to the set temperature). The rotation speeds of the first roll and the second roll were synchronized so as to be the same speed. The gap between the rolls was set to 1900 μm, the roll speed was adjusted, and casting was performed while forming a bank on the first roll side to obtain a 1860 μm sheet. At this time, the width of the bank was adjusted to 70% (140 mm) with respect to the width of 200 mm of the kneaded material sandwiched between the rolls. The ratio of bank width was set by visual estimation, and then confirmed by photographing with a digital still camera.

Next, it led to the simultaneous biaxial tenter stretching machine and biaxial stretching was performed. The stretching conditions were an MD magnification of 7.0 times and a TD magnification of 6. 4 times, temperature 120 ° C. Next, it was led to a methyl ethyl ketone bath and sufficiently immersed in methyl ethyl ketone to extract and remove the plasticizer, and then methyl ethyl ketone was removed by drying.
Next, after leading to a TD tenter and stretching 1.3 times the width of the tenter at 118 ° C., the heat fixing is performed by relaxing to 123 ° C. and returning to 1.1 times the width of the tenter. A microporous membrane was obtained. Table 1 shows the physical properties of the obtained microporous membrane.
Moreover, although the above setting was continuously performed for 10 hours, the state of the bank was not substantially different from the initially set state. Table 1 shows the variation in physical properties obtained.

[Example 2]
0.5 wt% of pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] as an antioxidant was added to 99.5 wt% of homopolyethylene having an Mv of 300,000, By dry blending again using a tumbler blender, a polymer mixture was obtained. The obtained mixture of polymers and the like was substituted with nitrogen and then fed to the twin-screw extruder with a feeder under a nitrogen atmosphere. Further, liquid paraffin (kinematic viscosity at 37.78 ° C .: 7.59 × 10 ⁻⁵ m ² / s) was injected into the extruder cylinder by a plunger pump.

The feeder and pump were adjusted so that the liquid paraffin content ratio in the total mixture melt-kneaded and extruded was 55 wt%. The melt-kneading conditions are the same as in Example 1.
Subsequently, after passing through a T die in which the kneaded material melt-kneaded by the extruder was adjusted to a thickness direction gap of 1600 μm, the first roll (2 roll in FIG. 1) was 80 ° C. and the second roll (3 roll in FIG. 1). ) Was extruded onto two rolls whose temperature was controlled at 70 ° C. by the same means as in Example 1. The rotation speeds of the first roll and the second roll were synchronized so as to be the same speed. The gap between the rolls was set to 1500 μm, the roll speed was adjusted, and casting was performed while forming a bank on the first roll side to obtain a 1450 μm sheet. At this time, the width of the bank was adjusted to 70% (140 mm) with respect to the width of 200 mm of the kneaded material sandwiched between the rolls. The ratio of bank width was set by visual estimation, and then confirmed by photographing with a digital still camera.

Next, it led to the simultaneous biaxial tenter stretching machine and biaxial stretching was performed. The stretching conditions were an MD magnification of 7.0 times and a TD magnification of 6. 4 times, temperature 123 ° C. Next, it was led to a methyl ethyl ketone bath and sufficiently immersed in methyl ethyl ketone to extract and remove the plasticizer, and then methyl ethyl ketone was removed by drying.
Next, it was led to a TD tenter and stretched by 1.4 times with respect to the width including the tenter at 118 ° C., and then heat-fixed at 129 ° C. by relaxing by returning to 1.1 times with respect to the width including the tenter. A microporous membrane was obtained. Table 1 shows the physical properties of the obtained microporous membrane.
In addition, although the above setting was continuously performed for 10 hours, the state of the bank was almost the same as the initially set state. Table 1 shows the variation in physical properties obtained.

[ Comparative Example 4 ]
A microporous membrane was obtained in the same manner as in Example 1 except that both roll control temperatures were 45 ° C. in the casting step. Table 1 shows the physical properties of the obtained microporous membrane.
In addition, the above setting was continuously performed for 10 hours. The bank was slightly affected by the plasticizer adhering to the roll, but the set width was maintained. Table 1 shows the variation in physical properties obtained.

[Comparative Example 1]
A microporous membrane was obtained in the same manner as in Example 1 except that the width of the bank was adjusted to 50% (100 mm) with respect to the width of 200 mm of the kneaded material sandwiched between the rolls. Table 1 shows the physical properties of the obtained microporous membrane.
In addition, the above setting was continuously performed for 10 hours. The bank was affected by the plasticizer adhered to the roll, and the width of the bank varied between 30 and 70%. Table 1 shows the variation in physical properties obtained.

[Comparative Example 2]
A microporous membrane was obtained in the same manner as in Example 1 except that the two roll control temperatures were 45 ° C. in the casting step, and casting was performed without forming a bank. Table 1 shows the physical properties of the obtained microporous membrane.
Further, although the above setting was continuously performed for 10 hours, the film formation was stable. Table 1 shows the variation in physical properties obtained.

[Comparative Example 3]
A microporous membrane was obtained in the same manner as in Example 2 except that the two roll control temperatures were set to 70 ° C. in the casting step, and casting was performed without forming a bank. Table 1 shows the physical properties of the obtained microporous membrane.
In addition, the above setting was continuously performed for 10 hours. In spite of the fact that the bank was not formed, the small bank intermittently repeated the formation / disappearance as time passed, affected by the plasticizer that gradually adhered to the roll. Table 1 shows the variation in physical properties obtained.

  INDUSTRIAL APPLICABILITY The present invention is used as a method for producing a microporous membrane made of polyolefin, which is used for separation of substances, selective permeation, separators, and the like, and particularly suitably used as a separator for lithium ion batteries and the like.

It is a schematic diagram of cast molding in the present invention.

Explanation of symbols

1 T-die 2 1st roll 3 2nd roll 4 Bank (resin pool)
5 Kneaded product 6 Cooled solidified gel sheet

Claims (3)

By melt-kneading a polyolefin composition and a plasticizer with a plasticizer content in the melt-kneaded product of 20 to 95 wt%, extruding from a T-die, forming a bank (resin pool) and cast molding In the method for producing a microporous polyolefin membrane in which a sheet is drawn and then stretched to extract a plasticizer, the width of the bank is 60% or more with respect to the total width of the kneaded material sandwiched between rolls,
A method for producing a polyolefin microporous film, characterized in that the temperature of the roll on the surface side forming the bank is higher by 1 ° C. or more than the temperature of the roll on the surface side where no bank is formed.   The method for producing a polyolefin microporous membrane according to claim 1, wherein the temperature of the roll is not higher than 30 ° C lower than the melting point of the constituting polyolefin and not lower than 0 ° C.   The method for producing a microporous polyolefin membrane according to claim 1 or 2, wherein polyethylene is used in an amount of 50 wt% or more as the polyolefin composition.

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