JP2008088392A - Method for producing polyolefin microporous film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means of improving dimensional stability at high temperatures and freely regulating permeation performances without deteriorating strength in a method for producing a polyolefin microporous film. <P>SOLUTION: A method, by which stretching before extraction and stretching after the extraction are used in combination and stretching processes in the longitudinal direction and width direction are carried out in the stretching after the extraction in separate steps, is adopted in the method for producing the polyolefin microporous film. <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, demand for lithium ion secondary battery applications has been increasing, and as the energy density of batteries has increased, separators have also been required to have high performance.

  Since lithium ion secondary batteries use chemicals such as electrolytes and positive and negative electrode active materials, the separator material is generally a polyolefin-based 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 diversifying needs of battery grades, there is an urgent need to develop a technique for freely adjusting the permeation performance such as the pore structure and porosity and a technique for adjusting the dimensional stability at high temperatures.

The electrode short-circuit prevention function means that a separator serves as a partition wall interposed between positive and negative electrodes to prevent an internal short circuit. In order to prevent an internal short circuit, the separator must have high strength, a small pore diameter, and an appropriate film thickness. In the secondary battery, the internal electrode expands due to charging / discharging, and in some cases, a pressure of several tens of kg / cm ² may be applied to the separator. Further, the electrode surface is not necessarily smooth, and active material particles of various sizes are protrusions. Even in such a case, the separator is required to have high strength that does not break. When the separator is used as a prismatic battery or a thin battery, the coil with the electrode and separator laminated and wound is compressed and casing, so that the demand for high strength can be said to be even stronger.

  Ion permeability means the ability of the separator to transmit only ions and electrolyte without transmitting active material particles. In general, in order to reduce ohmic loss and increase discharge efficiency, performance such as high porosity, low air permeability, and low electrical resistance is required. However, in recent years, for applications that do not require large current discharge, or for applications that require particularly high battery safety, there is a case in which a dense separator that matches the high permeability is required. Therefore, it is useful to develop a flexible molding technique that can freely adjust the permeability for any requirement.

  As for assembly workability, when the separator is wound with an electrode by applying a constant tension in the machine direction, the separator is required not to extend in the machine direction or to change in dimension in the width direction, and a high elastic modulus is required. Necessary. Battery safety is a function that suppresses battery runaway and explosion by automatically shutting off the current when the battery heats up due to trouble such as external short circuit or overcharge. Means. When the temperature inside the battery rises to the vicinity of the melting point of the resin constituting the separator, the separator closes the pores due to thermal flow, thermal deformation, or thermal contraction, or the resin is adsorbed on the electrode surface to form an insulating coating. By forming, a so-called shutdown function is exhibited. The lower the temperature at which this function is manifested, the more desirable it is because it has the ability to cut off current at low temperatures to suppress heat generation. Further, the wider the temperature range in the shutdown state, the longer the time during which the current is cut off. Therefore, it is desirable that it can withstand the temperature rise due to more intense heat generation.

The dimensional stability at high temperature is an evaluation of the degree to which the separator undergoes dimensional deformation due to thermal contraction or the like, assuming that the temperature inside the battery has risen due to some high-temperature treatment during manufacturing of the battery or due to trouble. In the former case, when the separator shrinks and deforms in the width direction, the electrodes are exposed and short-circuited internally, so that dimensional stability in the width direction at high temperatures is important. If the dimensional stability in the width direction cannot be maintained during battery runaway as in the latter case, even the shutdown function becomes meaningless, which is a very serious problem. Thus, no technology has been established for producing a separator that can withstand use at high temperatures without impairing the shutdown function.
Furthermore, the characteristics required of separators are diversifying due to diversifying battery applications, but it has been impossible to adjust over a wide range from the relatively low to high permeation performance in the prior art. .

  In microporous membrane manufacturing technology, a microporous membrane precursor is formed from a composition consisting of a polymer and a plasticizer by a phase separation process, and a plasticizer extraction removal process and a stretched thinning process are applied to this. A technique for forming a porous film is known. Among such known techniques, the stretching step is referred to as pre-extraction stretching or post-extraction stretching, respectively, depending on whether the stage of stretching is performed before or after the extraction process.

  JP 06-240036 discloses a technique combining stretching before extraction and stretching after extraction for the purpose of improving the permeation performance and pore size control of a polyolefin microporous membrane containing an ultra-high molecular weight component. This is a proposal for stretching only in the transverse direction, and there was a problem in dimensional stability in the width direction at high temperatures. There is also a problem that the strength in the vertical direction is insufficient.

  Japanese Patent Application Laid-Open No. 11-060789 proposes a simultaneous biaxial stretching method in addition to the stretching in the transverse direction. However, the simultaneous biaxial stretching method does not allow a wide range of stretching ratios to be changed and is adjustable. The range of the permeation performance of the porous membrane was limited.

  In the conventional microporous membrane manufacturing technology, it is difficult to freely adjust the permeation performance without impairing the strength of the microporous membrane. That is, in the case of a process using only pre-extraction stretching, it is possible to efficiently impart orientation and increase strength, but there is a drawback that there is no degree of freedom in terms of transmission performance. On the other hand, in the case of a process using only stretching after extraction, the interface fracture is dominantly progressed by stretching, so that it is possible to obtain a microporous membrane with high permeation performance, but it is difficult to adjust the permeation performance, and the strength However, there was a drawback of being low.

In addition, as disclosed in JP 06-240036 A, it is possible to improve the permeation performance by using the pre-extraction stretching and the post-extraction lateral stretching in combination, but the dimensional stability in the width direction at high temperatures. Thus, it was impossible to obtain a microporous film that hardly heat shrinks.
Furthermore, as disclosed in Japanese Patent Application Laid-Open No. 11-060789, if pre-extraction stretching and post-extraction simultaneous biaxial stretching are used in combination, the permeation performance is improved and the dimensional stability in the width direction at high temperatures is improved. However, the range of permeation performance of the adjustable microporous membrane was limited.
Thus, the establishment of a technology for obtaining a microporous membrane that can be adjusted over a wide range from a relatively low region to a high region and has excellent dimensional stability at a high temperature has been left as a problem in the industry. .

  As a result of diligent research to solve the above-mentioned problems, the present inventor has made it possible to freely adjust the permeation performance without sacrificing the strength by using both pre-extraction stretching and post-extraction stretching in combination, and further performing heat treatment. The inventors have found a method for producing a polyolefin microporous membrane having excellent dimensional stability at high temperatures, and have made the present invention.

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

An object of the present invention is to provide a method for producing a polyolefin microporous membrane that can freely adjust the permeation performance without impairing the strength and is excellent in dimensional stability at high temperatures.
Another object of the present invention is to provide a microporous polyolefin membrane manufactured by the above manufacturing method and having optimized properties with a separator for a lithium ion secondary battery.

  In the present invention, 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 at least one axial direction, and then the plasticizer is extracted. Further, the present invention relates to a method for producing a polyolefin microporous film, which is stretched in the length direction after extraction of a plasticizer.

  The first method of melting and kneading a polyacrylate-reolefin resin and a plasticizer is to introduce a polyolefin resin into a resin kneading apparatus such as an extruder, introduce a plasticizer at an arbitrary ratio while heating and melting the resin, and further polyolefin This is a method of obtaining a uniform solution by kneading a composition comprising a resin and 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 is most preferable because of high heat conduction efficiency.

  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 method for 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, stretching performed before the extraction step is referred to as pre-extraction stretching, and at least one stretching operation is essential 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.
In the stretch before extraction in the present invention, the plasticizer is stretched in a highly dispersed state in the micropores of the microporous membrane, in the crystal gaps, and in the amorphous part. It has the effect of suppressing the increase in the porosity of the microporous membrane, and can achieve high strength because high-stretching can be realized. Furthermore, biaxial stretching is preferable for achieving high strength, and simultaneous biaxial stretching is most preferable because the process can be simplified.

  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 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 the production method of the present invention, the stretching performed after the extraction step is called post-extraction stretching, and the stretching is performed in the length direction. In the stretching performed after the extraction step, it is more preferable to perform stretching in the width direction in a step different from the above-described stretching in the length direction. Since the stretching after the extraction is performed in a state where the plasticizer is substantially removed from the microporous membrane, the polymer interface breaks predominantly with the stretching, and has an effect of increasing the porosity of the microporous membrane. Therefore, if only the post-extraction stretching is performed without performing the pre-extraction stretching which is essential in the present invention, the porosity is excessively increased, and the stretching orientation cannot be imparted to the microporous film, resulting in low strength. End up.

  Compared to this, the production method of the present invention using both pre-extraction stretching and post-extraction stretching is useful because the porosity can be increased without impairing the strength of the microporous membrane. 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. The stretching ratio can be set to an arbitrary ratio, but it is preferably within 5 times in the uniaxial direction and within 20 times in the area ratio in the biaxial direction.

  The stretching in the length direction can improve the transmission performance without impairing the dimensional stability in the width direction at a high temperature. Furthermore, by combining with stretching in the width direction, the transmission performance can be made more favorable, and the balance between the strength in the length direction and the width direction can be improved.

  Furthermore, by making the stretching in the length direction and the stretching in the width direction as separate steps, each stretching ratio can be changed widely. Therefore, the transmission performance can be adjusted over a wide range from a relatively low region to a high region.

  Examples of the method of stretching in the length direction include a method using a roll type stretching machine. Here, it is important to prevent shrinkage in the width direction due to residual stress of stretching before extraction. When preheating the microporous membrane on the roll, it is necessary to mechanically constrain the width of the microporous membrane by gripping both ends of the microporous membrane. It is preferable to perform stretching in the length direction immediately after the preheating. The draw ratio of the roll-type stretching machine can be changed over a wide range, and at the same time, thermal relaxation described later can be performed.

  Here, if the width of the microporous film is not mechanically constrained when the microporous film is preheated on the roll, the shrinkage cannot be prevented only by friction between the roll and the microporous film, It shrinks in the width direction and deteriorates permeability. After the end of the preheating, the residual stress in the width direction is relaxed, and shrinkage can be prevented only by friction between the roll and the microporous film.

  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.

  Examples of the method of stretching in the width direction include a method using a tenter type stretching machine. The stretching ratio of the tenter type stretching machine can be changed over a wide range, and at the same time, thermal relaxation described later can be performed.

  In the production method of the present invention, it is more preferable that the heat treatment is performed following each post-extraction stretching or after the post-extraction stretching. Here, heat treatment refers to either heat fixation or heat relaxation. Heat setting means a process of performing heat treatment in a tensioned state while maintaining a set drawing ratio at the time of stretching after extraction or restraining, and thermal relaxation is heat treatment in a relaxed state. Means the process of performing. 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, within the range that does not impair the advantages of the present invention, that is, when improving the dimensional stability at high temperature and adjusting the permeation performance, post-treatment is performed as long as this is not impaired. You can go. 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, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. . 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.

The present invention provides a polyolefin microporous membrane produced by the production method.
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.

  When producing a microporous membrane using the production method of the present invention, the thermal shrinkage rate of the microporous membrane is an index for evaluating the dimensional stability of the microporous membrane at a high temperature, preferably in the width direction of the microporous membrane. Is preferably 10% or less, more preferably 5% or less. If the thermal shrinkage rate exceeds 10%, it causes a safety trouble such as a short circuit inside the battery, which is not preferable.

  According to the method for producing a microporous polyolefin membrane of the present invention, the dimensional stability at high temperatures can be improved, and at the same time, it has a flexible side surface that can freely adjust the permeation performance. Furthermore, the balance of strength in the length direction and width direction of the microporous membrane can be adjusted. Thus, when the microporous membrane produced by the present invention is used as a battery separator, the microporous membrane has high battery safety and various permeation performances to meet diversifying battery needs. Membranes can be manufactured.

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), a puncture test was carried out under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / second, and the maximum puncture load (g) and membrane The film thickness was converted from the thickness (μm) according to the following formula to obtain the piercing strength (g / 25 μm).
Puncture strength = maximum puncture load × 25 ÷ film thickness (5) Breaking strength A strip-shaped test piece having a width of 15 mm was measured according to ASTM D882.
(6) Thermal Shrinkage A 20 cm square sample was cut from the microporous membrane, placed in a hot air circulation oven heated to 100 ° C. without restraining the four sides of the sample, and heat-treated for 2 hours. The dimensions of the microporous membrane in the machine direction and the width direction were measured before and after heating, and the thermal shrinkage rate (%) was calculated according to the following formula.
Thermal shrinkage = 100 × (1- (dimension after heating ÷ dimension before heating))

(7) 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 average molecular weight is Mw and the molecular weight 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 for polystyrene standard sample

(8) Melting point Endothermic peak temperature when using a differential scanning calorimeter DSC-210, manufactured by Seiko Denshi Kogyo Co., Ltd., and placing about 7 mg of the sample in a nitrogen stream and raising the temperature from room temperature at a rate of 10 ° C / min. More evaluated.
(9) Relaxation rate The relaxation rate (%) is defined as the following equation based on the difference between the set magnification at the time of stretching after extraction and the set magnification at the time of heat treatment with respect to the dimension of the microporous membrane before stretching after extraction. did.
Relaxation rate = 100 × (stretching setting magnification after extraction−heat treatment setting magnification)

<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. As a result, a sheet having a thickness of 1.4 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 before extraction using a tenter type simultaneous biaxial stretching machine, and then immersed in methylene chloride to extract and remove liquid paraffin, and then the attached methylene chloride was removed by drying. Furthermore, using a roll type stretching machine that preheats while holding both ends of the microporous membrane, the film was extracted in the length direction and then stretched, followed by heat treatment while relaxing in the length direction. Furthermore, it heat-fixed using the tenter type | mold heat fixing machine.
Hereinafter, the molding conditions are shown in Table 1, and the physical properties of the obtained microporous film are shown in Table 2. In addition, melting | fusing point of the obtained microporous film was 133.4 degreeC.

<Example 2>
A microporous membrane was obtained in the same manner as in Example 1 except that the conditions for stretching after extraction and heat treatment were changed to the conditions described in Table 1. Table 2 shows the physical properties of the obtained microporous membrane.

<Example 3>
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, followed by immersion in methylene chloride to extract and remove liquid paraffin, and then the adhered methylene chloride was dried. Removed. Furthermore, using a roll type stretching machine that preheats while holding both ends of the microporous membrane, the film was extracted in the length direction and then stretched, followed by heat treatment while relaxing in the length direction. Furthermore, it extracted after extending | stretching in the width direction using the tenter type extending | stretching machine, Then, it heat-processed, making it ease in the width direction.
Hereinafter, the molding conditions are shown in Table 1, and the physical properties of the obtained microporous film are shown in Table 2.

<Example 4>
A microporous membrane was obtained in the same manner as in Example 2 except that the conditions for stretching after extraction and heat treatment were changed to the conditions described in Table 1. Table 2 shows the physical properties of the obtained microporous membrane.

<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, followed by immersion in methylene chloride to extract and remove liquid paraffin, and then the adhered methylene chloride was dried. Removed. Furthermore, it heat-fixed using the tenter type | mold heat fixing machine.
Hereinafter, the molding conditions are shown in Table 1, and the physical properties of the obtained microporous film are shown in Table 2.

<Comparative Example 2>
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, followed by immersion in methylene chloride to extract and remove liquid paraffin, and then the adhered methylene chloride was dried. Removed. Furthermore, it extracted after extending | stretching in the width direction using the tenter type extending | stretching machine, Then, it heat-processed, making it ease in the width direction.
Hereinafter, the molding conditions are shown in Table 1, and the physical properties of the obtained microporous film are shown in Table 2.

<Comparative Example 3>
A microporous membrane was obtained in the same manner as in Comparative Example 2 except that the conditions for stretching after extraction and heat treatment were changed to the conditions described in Table 1. Table 2 shows the physical properties of the obtained microporous membrane.

<Comparative Example 4>
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, followed by immersion in methylene chloride to extract and remove liquid paraffin, and then the adhered methylene chloride was dried. Removed. Furthermore, using a roll type stretching machine that preheats without holding both ends of the microporous membrane, the film was extracted in the length direction and then stretched, and subsequently heat treated while relaxing in the length direction. Furthermore, it heat-fixed using the tenter type | mold heat fixing machine.

Claims (5)

  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, and the plasticizer is extracted. A method for producing a polyolefin microporous membrane, which is stretched in the length direction later.   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, and the plasticizer is extracted. A method for producing a polyolefin microporous membrane, wherein the stretching in the length direction and the width direction is performed in separate steps later.   3. The polyolefin fine film according to claim 1, wherein in the step of stretching in the length direction after the plasticizer extraction, the film is stretched after preheating in a state where the width of the film is mechanically restricted. A method for producing a porous membrane.   In the step of stretching in the length direction after the plasticizer extraction, the film is stretched with a roll after preheating in a state where the width of the film is mechanically constrained on the roll. 2. A process for producing a microporous polyolefin membrane according to 2.   5. A heater that heats the sheet or film according to claim 3 or 4 in a state in which the width of the sheet or film is mechanically constrained.