JP2008088393A - Method for producing polyolefin microporous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means of achieving high-speed continuous production, suppressing shrinkage during drying and thereby enhancing quality and the uniformity of the quality in a method for producing a polyolefin microporous film. <P>SOLUTION: A composition composed of a polyolefin resin and a plasticizer is melt kneaded, extruded, cooled, solidified and formed into the shape of a sheet. The resultant sheet is stretched in at least the uniaxial direction at least once and the plasticizer is then extracted with a solvent. The film or sheet is dried in such a state in which the width of the film or sheet is mechanically restrained after the extraction to thereby remove the solvent. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention provides a suitable means for manufacturing separators used for battery materials such as various cylindrical batteries, square batteries, thin batteries, button batteries, and electrolytic capacitors.

  Microporous membranes have been used in the past as materials for filter media such as water purifiers, breathable apparel, battery separators and electrolytic capacitor separators. In recent years, the demand for lithium ion secondary battery applications has been increasing, and high-speed production of separators has been strongly desired.

  In addition to the high energy density, high output, and mass production of batteries, there has been a strong demand for improved quality uniformity in addition to high quality for separators.

  Since lithium ion secondary batteries use chemicals such as electrolytes and positive and negative electrode active materials, the separator material is generally a polyolefin polymer in consideration of chemical resistance, and is particularly inexpensive. Polyethylene and polypropylene are used. For separators for non-aqueous electrolyte batteries such as lithium ion secondary batteries, electrode short-circuit prevention, high ion permeability, assembly workability during battery winding, battery safety, reliability, etc. are conventional. It has been required as a basic performance. Furthermore, in recent years, in order to meet the needs for higher energy density, higher output, and mass production of batteries, in addition to the aforementioned quality improvement, development of technology that can dramatically improve the uniformity of quality has become an urgent task. .

  Further, with the mass production of batteries, high-speed production of separators has been strongly desired. In the manufacturing technology of the microporous membrane, a microporous membrane precursor is formed from a composition comprising a polymer and a plasticizer by a phase separation process, and the stretching process is applied to extract the plasticizer with a solvent after stretching. A technique for forming a microporous membrane by removing the solvent by drying is known.

  In such a known technique, since the solvent is dried on the roll, in the drying step, the shrinkage in the width direction of the film cannot be prevented, and the permeability is deteriorated. However, the shrinkage increased and the microporous film with uniform quality could not be obtained.

Furthermore, there is a problem that wrinkles are generated in the film due to shrinkage during drying, causing incomplete drying, the drying rate cannot be increased, and the production rate is limited.
In the conventional microporous membrane manufacturing technology, it was impossible to continuously produce a microporous membrane of uniform quality at high speed. That is, since the solvent is dried on a roll, it was impossible to continuously produce at a high speed due to generation of soot. Furthermore, shrinkage in the width direction of the film could not be prevented, the permeability of the film deteriorated, and a uniform quality microporous film could not be obtained. Thus, the establishment of a technique for continuously producing a microporous membrane of uniform quality at a high speed has been left as an issue in the industry.
As a result of diligent research to solve the above problems, the present inventor has adopted a method of performing drying in a state where the width of the film or sheet is mechanically constrained in the drying step after extraction, thereby enabling high-speed continuous production. As a result, it has been found that the quality and uniformity of quality can be improved by suppressing shrinkage during drying, and the present invention has been made.

JP 60-089333 A JP 60-228122 A Japanese Patent Laid-Open No. 60-242035 Japanese Patent Laid-Open No. 62-132944 Japanese Patent Laid-Open No. 06-016862 Japanese Patent Laid-Open No. 06-240036 Japanese Patent Laid-Open No. 06-336535 Japanese Patent Laid-Open No. 11-060789 Korean Patent No. 371390 Specification Korean Patent No. 409019 Korean Patent No. 263919 Specification Korean Open Patent 2000-51312 Specification Korean Published Patent No. 2000-51313

  The objective of this invention is providing the manufacturing method of the polyolefin microporous film which can improve the quality and the uniformity of quality by suppressing high speed continuous production and shrinkage | contraction at the time of drying.

  In the present invention, a composition comprising a polyolefin resin and a plasticizer is melt-kneaded, extruded, cooled and solidified to form a sheet, and stretched at least once in at least one uniaxial direction. The present invention relates to a method for producing a polyolefin microporous membrane, wherein the solvent is removed by extraction and drying in a state where the width of the film or sheet is mechanically restricted after extraction.

  The first method of melt-kneading the polyolefin resin and plasticizer of the present invention is to introduce the polyolefin resin into a resin kneading apparatus such as an extruder, introduce a plasticizer at an arbitrary ratio while heating and melting the resin, This is a method of obtaining a uniform solution by kneading a composition comprising a plasticizer. The polyolefin resin to be introduced may be in any form of powder, granules, and pellets. Moreover, when knead | mixing by such a method, it is preferable that the form of a plasticizer is a normal temperature liquid. As an extruder, a single screw type extruder, a biaxial different direction screw type extruder, a biaxial same direction screw type extruder, etc. can be used.

  The second method of melt-kneading a polyolefin resin and a plasticizer is to mix and disperse the resin and the plasticizer at room temperature in advance, and put the obtained mixed composition into a resin kneading apparatus such as an extruder and knead. This is a method for obtaining a uniform solution. The form of the mixed composition to be added may be a slurry when the plasticizer is a liquid at room temperature, and may be a powder or the like when the plasticizer is a solid at room temperature. In the first and second methods, it is important to obtain a uniform solution by kneading polyolefin and a plasticizer in a kneading apparatus such as an extruder, thereby improving productivity. it can.

  The first method of producing a sheet-like microporous membrane precursor by extruding and cooling and solidifying is to extrude a uniform solution of resin and plasticizer into a sheet form via a T-die, etc. It is carried out by cooling to a temperature well below the crystallization temperature. As the heat conductor to be used, metal, water, air, or the plasticizer itself can be used. In particular, a method of cooling by contacting with a metal roll has the highest heat conduction efficiency and is preferable.

  The second method for producing a sheet-like microporous membrane precursor is a method in which a uniform solution of a resin and a plasticizer is extruded into a cylindrical shape via a circular die and then processed into a sheet shape.

  The stretching method requires at least one stretching operation in at least one axial direction. The at least uniaxial direction refers to machine direction uniaxial stretching, width direction uniaxial stretching, simultaneous biaxial stretching, and sequential biaxial stretching. Further, at least one stretching operation refers to one-stage stretching, multi-stage stretching, and multi-stage stretching.

  The stretching temperature is preferably 50 ° C. lower than the melting point Tm of the polyolefin microporous membrane and less than Tm, more preferably 40 ° C. lower than the melting point Tm of the polyolefin microporous membrane and less than 5 ° C. lower than Tm. If the stretching temperature is less than 50 ° C. lower than Tm, the stretchability is deteriorated, the strain component after stretching remains, and the dimensional stability at high temperature is lowered, which is not preferable. If the stretching temperature is Tm ° C. or higher, the microporous membrane melts and impairs the permeation performance, which is not preferable.

  The draw ratio can be set to an arbitrary ratio, but it is preferably 2 to 20 times, more preferably 4 to 10 times in terms of the uniaxial direction, and preferably 2 to 400 times, more preferably in terms of the area ratio in the biaxial direction. Is 4 to 100 times. In order to achieve high strength, biaxial stretching is preferable, and simultaneous biaxial stretching is particularly preferable because the process can be simplified.

  The method of extracting the plasticizer is to continuously feed the microporous membrane into a tank filled with the extraction solvent, soak it in the tank for a sufficient time to remove the plasticizer, and then adhere to it. This is done by drying the solvent. At this time, the inside of the tank is divided into multiple stages, and a multistage method in which the microporous membrane is sequentially fed to each tank having a concentration difference, or an extraction solvent is supplied from the opposite direction to the traveling direction of the microporous film to create a concentration gradient. Therefore, it is preferable to apply a known means such as a countercurrent method for increasing the extraction efficiency. It is important to substantially remove the plasticizer from the microporous membrane. Further, it is more preferable to heat the extraction solvent within the range below the boiling point of the solvent, since the diffusion between the plasticizer and the solvent can be promoted, and the extraction efficiency is increased.

  In the production method of the present invention, a method of drying in a state where the width of the microporous membrane is mechanically restricted in the drying step after extraction of the polyolefin microporous membrane is adopted. By constraining the width of the membrane mechanically, shrinkage of the microporous membrane during drying can be prevented. The occurrence of soot will be completely prevented. Thereby, high-speed drying becomes possible. Moreover, the deterioration of permeability can be prevented by preventing shrinkage of the microporous membrane during drying.

  Methods for mechanically constraining the width of the microporous membrane include a method in which a clip is provided at the end of the roll and the microporous membrane is sandwiched between the roll and the clip, a method in which the microporous membrane is pressed against the roll with a belt-like one, Examples include a method of fixing the end of the membrane by suction.

  Furthermore, it is preferable to perform stretching or heat treatment after drying. Here, the heat treatment refers to any one of stretching, heat setting, and thermal relaxation. Thermal fixation means a step of performing heat treatment in a tensioned state while being constrained. In contrast, thermal relaxation means a step of performing heat treatment in a relaxed state. Both heat fixation and thermal relaxation remove residual stress and strain components that are considered to occur during stretching, improve dimensional stability at high temperatures, and adjust the permeation performance represented by porosity and air permeability. It is what has.

  In the production method of the present invention, post-processing may be performed. Examples of the post-treatment include a hydrophilic treatment with a surfactant and the like, and a crosslinking treatment with ionizing radiation and the like. The polyolefin resin used in the present invention refers to a resin used for normal extrusion, injection, inflation, and blow molding. Ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1 -Octene homopolymers and copolymers can be used. In addition, polyolefins selected from the group of these homopolymers and copolymers can be mixed and used. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultra high molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber, and the like. . When the microporous membrane obtained by the production method of the present invention is used as a battery separator, it is preferable to use a resin having a high melting point polyethylene as a main component because it is a low melting point resin and high strength required performance. .

  The average molecular weight of the polyolefin resin used in the present invention is preferably from 50,000 to less than 1,000,000, more preferably from 100,000 to less than 700,000, and most preferably from 200,000 to less than 500,000. This average molecular weight refers to a weight average molecular weight obtained by GPC (gel permeation chromatography) measurement or the like, but it is generally difficult to accurately measure GPC for resins having an average molecular weight exceeding 1,000,000. Therefore, as an alternative, the viscosity average molecular weight determined by the viscosity method can be applied. If the average molecular weight is less than 50,000, the melt tension at the time of melt molding is lost and the moldability is deteriorated, and the stretchability is deteriorated and the strength is lowered. If the average molecular weight exceeds 1,000,000, it tends to be difficult to obtain a uniform resin composition.

  The molecular weight distribution of the polyolefin resin used in the present invention is preferably from 1 to less than 10, more preferably from 2 to less than 9, and most preferably from 3 to less than 8. The molecular weight distribution is represented by the ratio (Mw / Mn) of the weight average molecular weight (Mw) and the number average molecular weight (Mn) obtained by GPC measurement. When the molecular weight distribution exceeds 10, the stretchability tends to deteriorate, and there is a risk of local distribution of film thickness and strength reduction.

  The plasticizer used in the present invention may be any non-volatile solvent that can form a uniform solution above the melting point of the polyolefin resin when mixed with the polyolefin resin. Examples thereof include hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol.

  The ratio of the polyolefin resin and the plasticizer used in the present invention is a ratio sufficient to cause microphase separation and form a sheet-like microporous membrane precursor, and does not impair productivity. I just need it. Specifically, the weight fraction of the polyolefin resin in the composition comprising the polyolefin resin and the plasticizer is preferably 20 to 70%, more preferably 30 to 60%. When the weight fraction of the polyolefin resin is less than 20%, the melt tension at the time of melt molding is insufficient and the moldability is poor. Although it is possible to carry out the polyolefin resin at a weight fraction of less than 20%, in this case, in order to increase the melt tension, it becomes necessary to mix a large amount of ultra-high molecular weight polyolefin, and uniform dispersibility Is unfavorable because it decreases.

The extraction solvent used in the present invention is preferably a poor solvent for polyolefin and a good solvent for plasticizer, and its boiling point is preferably lower than the melting point of the polyolefin microporous membrane. Examples of such extraction solvents include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, alcohols such as ethanol and isopropanol, diethyl ether, Examples include ethers such as tetrahydrofuran and ketones such as acetone and 2-butanone. Furthermore, in consideration of environmental adaptability, safety, and hygiene, alcohols and ketones are preferable among the solvents.
The composition used in the present invention may further contain additives such as an antioxidant, a crystal nucleating agent, an antistatic agent, a flame retardant, a lubricant, and an ultraviolet absorber depending on the purpose. The microporous membrane of the present invention refers to a porous sheet or film substantially composed of polyolefin, and is used as a battery material such as a separator, for example. The form of the battery is not particularly limited, and is suitable for use in, for example, a cylindrical battery, a square battery, a thin battery, a button battery, an electrolytic capacitor, and the like.

  When manufacturing a microporous film using the manufacturing method of this invention, it is preferable that the film thickness of a microporous film shall be 1-500 micrometers, and it is more preferable to set it as 5-100 micrometers. If the film thickness is smaller than 1 μm, the mechanical strength becomes insufficient, and if it is larger than 500 μm, the occupied volume of the separator increases, which is disadvantageous in terms of increasing the capacity of the battery.

  When producing a microporous membrane using the production method of the present invention, the air permeability of the microporous membrane is preferably 3000 sec / 100 cc / 25 μm or less, more preferably 1000 sec / 100 cc / 25 μm or less. preferable. The air permeability is defined by the ratio between the air permeability time and the film thickness. If the air permeability is larger than 3000 seconds / 100 cc / 25 μm, the ion permeability is deteriorated or the pore diameter is extremely small.

  When producing a microporous membrane using the production method of the present invention, the porosity of the microporous membrane is preferably 20 to 80%, and more preferably 30 to 70%. When the porosity is less than 20%, ion permeability represented by air permeability and electrical resistance is insufficient, and when it is greater than 80%, strength represented by piercing strength and tensile strength is insufficient.

  When the microporous membrane is produced using the production method of the present invention, the puncture strength of the microporous membrane is preferably 300 g / 25 μm or more, and more preferably 400 g / 25 μm or more. The piercing strength is defined by the ratio between the maximum load and the film thickness in the piercing test. When the piercing strength is smaller than 300 g / 25 μm, defects such as short circuit failure increase when winding the battery, which is not preferable.

  According to the method for producing a polyolefin microporous membrane of the present invention, high-speed continuous production can be realized, and the quality and uniformity of the quality of the microporous membrane can be enhanced by suppressing shrinkage during drying.

EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, these Examples are illustrated in order to demonstrate this invention more concretely, and are not for limiting the scope of the present invention.
(1) Film thickness It measured with the dial gauge (PEACOCK NO.25 by Ozaki Seisakusho).
(2) Porosity A 20 cm square sample was cut from the microporous membrane, and the volume (cm ³ ) and weight (g) of the sample were measured. From the obtained results, the porosity (%) was calculated using the following equation. Calculated.
Porosity = 100 × (1−weight ÷ (resin density × volume))

(3) Air permeability Based on JISP-8117, from the air permeability time (seconds / 100 cc) and the film thickness (μm) obtained by measuring with a Gurley type air permeability meter, the film thickness is as follows: Converted to air permeability (seconds / 100 cc / 25 μm).
Air permeability = air permeability time × 25 ÷ film thickness (4) Puncture strength Using a compression tester (Kato Tech KES-G5), puncture was performed with the needle tip having a radius of curvature of 0.5 mm and a puncture speed of 2 mm / sec. A test was conducted, and the film thickness was converted from the maximum piercing load (g) and film thickness (μm) according to the following formula to obtain the piercing strength (g / 25 μm).
Puncture strength = maximum puncture load × 25 ÷ film thickness

(5) Average molecular weight and molecular weight distribution Under the following conditions, GPC (gel permeation chromatography) measurement is performed to determine the weight average molecular weight (Mw) and the number average molecular weight (Mn). The distribution was assigned Mw / Mn.
Equipment: WATERS 150-GPC
Temperature: 140 ° C
Solvent: 1,2,4-trichlorobenzene concentration: 0.05% (injection amount: 500 μl)
Column: One Shodex GPC AT-807 / S,
Tosoh TSK-GELGMH 6 -HT 2 dissolution conditions: 160 ° C., 2.5 hours Calibration curve:
3rd order calculation using polyethylene conversion constant 0.48 against polystyrene standard sample (6) Film width The width of the film was measured with an object.

<Example 1>
High density polyethylene (weight average molecular weight 300,000, molecular weight distribution 7, density 0.956) and 0.3 part by weight of 2,6-di-t-butyl-p-cresol with respect to the polyethylene using a Henschel mixer Dry blended and put into a 35 mm twin screw extruder. Furthermore, liquid paraffin (kinematic viscosity 75.9 cSt at 37.78 ° C.) is injected into the extruder, melted and kneaded at 200 ° C., and extruded and cast on a cooling roll controlled to a surface temperature of 40 ° C. through a coat hanger die. Thus, a sheet having a thickness of 1.1 mm was obtained. Here, the ratio of the composition was adjusted to be 70% by weight of liquid paraffin with respect to 30% by weight of polyethylene. The obtained sheet was stretched 5 × 5 times at 119 ° C. using a tenter type simultaneous biaxial stretching machine, and subsequently immersed in methylene chloride to extract and remove liquid paraffin, and then a clip was provided at the end. Drying was performed while blowing warm air in a state where the width of the microporous membrane was mechanically constrained on a heating roll.
Further, it was heat-set at 121 ° C. using a tenter-type heat fixing machine. Table 1 shows the molding conditions, the measurement results of the film width before extraction removal and the film width after drying, and the dry state. Table 2 shows the physical properties of the obtained microporous film. Samples were taken from three locations, the central portion of the membrane and the portion 150 mm inside from the left and right ends, and the properties of the obtained microporous membrane were measured.

<Example 2>
A microporous membrane was obtained in the same manner as in Example 1 except that the molding conditions were changed to those described in Table 1.
The measurement results of the film width before extraction removal and the film width after drying and the dry state are shown in Table 1, and the physical properties of the obtained microporous film are shown in Table 2.

<Example 3>
A sheet having a weight average molecular weight of 800,000, a molecular weight distribution of 7, a density of 0.950 and a thickness of 1.4 mm was obtained from high density polyethylene. Here, the ratio of the composition was adjusted to 80% by weight of liquid paraffin with respect to 20% by weight of polyethylene. Other than that was carried out similarly to Example 1, and obtained the microporous film.
Table 1 shows the molding conditions, the measurement results of the film width before extraction removal and the film width after drying, and the dry state. Table 2 shows the physical properties of the obtained microporous film.

<Comparative Example 1>
After obtaining a sheet in the same manner as in Example 1, it was stretched before extraction using a tenter-type simultaneous biaxial stretching machine, and subsequently immersed in methylene chloride to extract and remove liquid paraffin, and then microporous on a heating roll. Drying was performed while blowing warm air without mechanically constraining the width of the membrane. Further, it was heat-set at 121 ° C. using a tenter-type heat fixing machine. Table 1 shows the molding conditions, the measurement results of the film width before extraction removal and the film width after drying, and the dry state. Table 2 shows the physical properties of the obtained microporous film.

<Comparative Example 2>
A microporous membrane was obtained in the same manner as in Comparative Example 1 except that the molding conditions were changed to those described in Table 1. The measurement results of the film width before extraction removal and the film width after drying and the dried state are shown in Table 1, and the physical properties of the obtained microporous film are shown in Table 2.

<Comparative Example 3>
After obtaining a sheet in the same manner as in Example 3, it was stretched before extraction using a tenter type simultaneous biaxial stretching machine, and subsequently immersed in methylene chloride to extract and remove liquid paraffin, and then microporous on a heating roll. Drying was performed while blowing warm air without mechanically constraining the width of the membrane. Further, it was heat-set at 121 ° C. using a tenter-type heat fixing machine.
Table 1 shows the molding conditions, the measurement results of the film width before extraction removal and the film width after drying, and the dry state. Table 2 shows the physical properties of the obtained microporous film.

Claims (4)

  A composition comprising a polyolefin resin and a plasticizer is melt-kneaded, extruded, cooled and solidified, formed into a sheet, and stretched at least once in at least one axial direction. Then, the plasticizer is extracted with a solvent and extracted. A method for producing a polyolefin microporous membrane, wherein the solvent is removed later by drying in a state where the width of the film or sheet is mechanically restricted.   A composition comprising a polyolefin resin and a plasticizer is melt-kneaded, extruded, cooled and solidified, formed into a sheet shape, stretched at least once in the biaxial direction or width direction, and then the plasticizer is extracted with a solvent. A method for producing a polyolefin microporous membrane, wherein the solvent is removed by drying in a state where the width of the film or sheet is mechanically restricted after extraction.   The method for producing a polyolefin microporous membrane according to claim 1 or 2, wherein in the drying step after extraction, the film is dried in a state where the width of the film is mechanically restricted on a roll.   A drying machine that dries in a state where the width of the film or sheet of claim 1 is mechanically restricted.