KR100873851B1 - Manufacturing method of polyolefin microporous membrane - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polyolefin microporous membrane, and to a method for producing a polyolefin microporous membrane having improved dimensional stability at high temperature and freely adjusting permeability without loss of strength.

In the method for producing a polyolefin microporous membrane of the present invention, stretching before extraction and stretching after extraction are used in combination, and when stretching after extraction, stretching in the longitudinal direction and the width direction is performed by a separate process.

Description

Manufacturing method of polyolefin microporous membrane {METHOD OF MANUFACTURING POLYOLEFIN MICROPOROUS FILMS}

The present invention relates to a method for producing a polyolefin microporous membrane suitable for separators used in battery materials such as various cylindrical batteries, square batteries, thin batteries, button batteries, and electrolytic capacitors.

Microporous membranes have been widely used as materials for filtration materials such as water purifiers, garment material applications requiring air permeability, and separators for batteries and separators for electrolytic capacitors. In particular, in recent years, as the demand for the use of lithium ion secondary batteries increases, high performance is also required for separators, accompanied by high energy density of the batteries.

Since a lithium ion secondary battery uses an agent such as an electrolyte solution and a positive electrode active material, a polyolefin-based polymer is generally used in consideration of chemical resistance, and inexpensive polyethylene and polypropylene are particularly used. Separators for nonaqueous electrolyte batteries such as lithium ion secondary batteries have been required for basic performance such as electrode short circuit prevention function, high ion permeability, assembly processability during battery winding, battery safety and reliability. . In addition, in order to meet the needs of diversified battery grades, there is an urgent need to develop technologies for freely controlling permeability such as pore structure and porosity and for controlling dimensional stability at high temperature.

Electrode short circuit prevention means that the separator serves as a fusion wall to prevent internal short-circuiting between the positive and negative poles. In order to prevent internal short-circuit, high strength of separator, small pore size and proper thickness of membrane are required. In a secondary battery, in order to expand an electrode inside by charging / discharging, in some cases, a pressure of about several tens of kg / cm 2 may be applied to the separator. In addition, the electrode surface is not smooth, and active material particles of various sizes become protrusions. Even in such a case, a high strength that does not break is required for the separator. In addition, when a separator is used for a square battery and a thin battery, it is necessary to compress and casing the coil which laminated | stacked the electrode and the separator, and the demand for high strength is further urgent.

The ion permeability of the separator means the ability to permeate only the ions and the electrolyte without permeating the active material particles. In general, high porosity, low air permeability, and low electrical resistance are required to reduce resistance loss and increase discharge efficiency. However, in recent years, fine separators to cope with the high air permeability may be required for applications that do not require large current discharge and applications in which the demand for battery stability is particularly high. Therefore, it is useful to develop a flexible molding technology capable of freely adjusting the permeability for any demand.

The assemblability of the separator means that when the coil is wound together with the electrode by applying a constant tension in the machine direction, the separator should not extend in the machine direction, should not have a dimensional change in the width direction, and high elastic modulus. Battery safety means a function that suppresses overheating and explosion of a battery by stopping the heat generation by automatically cutting off the current when the battery heats up due to a trouble such as an external short circuit or an overcharge. When the temperature inside the battery rises to the melting point temperature of the resin constituting the separator, the separator shuts off the heat flow and closes the fine pores by thermal deformation or heat shrinkage, or the resin is absorbed on the electrode surface to form a so-called shutdown function. Expression. At this time, the lower the temperature at which the shutdown function is expressed, the more preferable since it is capable of blocking current at low temperatures and suppressing heat generation. In addition, the wider the temperature range in the shutdown state, the longer the time for interrupting the current is preferable, so that it can withstand a temperature rise environment due to extreme heat generation.

Dimensional stability at high temperature is to evaluate the degree of dimensional deformation of the separator due to heat shrinkage when the internal temperature of the battery rises due to any high temperature treatment or trouble during battery production.

In the former case, when the separator shrinks and deforms in the width direction, the electrode is exposed and the internal short circuit occurs, so the dimensional stability in the width direction at the high temperature is particularly important. In the latter case, if the dimensional stability in the width direction cannot be maintained when the battery is overheated, even the shutdown function becomes meaningless, which is a very serious problem. Thus, the manufacturing technology of the separator which can endure high temperature use without damaging a shutdown function is not established.

In addition, with the diversified battery applications, the characteristics required for the separator are also diversified. However, in the prior art, it is impossible to adjust the range from a relatively low region to a high region.

In the manufacturing technology of a microporous membrane, a microporous membrane manufacturing method is known by applying the process of extracting and removing a plasticizer and extending | stretching thin film from the composition which consists of a polymer and a plasticizer by a phase-separation process, and then removes a plasticizer. . In the above known art, depending on whether the stretching step is performed before or after the extraction process, the stretching before the extraction or the extraction after the extraction, respectively.

Japanese Patent Laid-Open No. 06-240036 discloses a technique for combining stretching before extraction and stretching after extraction in order to improve the permeation performance and control the pore size of the polyolefin microporous membrane containing an ultra high molecular weight component, but only transverse Only extending | stretching of the direction is proposed, and there exists a problem in the dimensional stability of the width direction at high temperature. There is also a problem in that the strength in the longitudinal direction is insufficient.

Japanese Patent Laid-Open No. 11-060789 proposes a lateral stretching and a biaxial stretching method, but the simultaneous biaxial stretching method cannot change a wide range of stretching ratios, and the range of the permeability of the adjustable microporous membrane is limited. There is a point.

In the conventional method for producing a microporous membrane, there is a difficulty in controlling the permeability freely without impairing the strength of the microporous membrane. That is, in the case of performing the stretching only before the extraction, the orientation can be efficiently given and the strength can be increased, but there is a disadvantage that there is no degree of freedom in terms of permeability. On the other hand, in the case where only stretching fixing is performed after extraction, interfacial fracture by stretching is mainly performed to obtain a microporous membrane having high permeability, but there is a problem that control of permeability is difficult and strength is lowered.

In addition, as disclosed in Japanese Patent Laid-Open Publication No. 06-240036, the use of pre-extraction stretching and transverse stretching after extraction can improve the permeation performance, but improves the dimensional stability in the width direction at high temperatures, and thermal shrinkage. It was impossible to obtain a microporous membrane.

In addition, as disclosed in Japanese Patent Laid-Open No. 11-060789, it was possible to improve the permeation performance and improve the dimensional stability in the width direction at a high temperature by using a combination of pre-extraction and simultaneous biaxial stretching after extraction. There exists a problem that the permeability range of a possible microporous membrane is limited.

Accordingly, there remains a problem in the art of establishing a technique for producing an excellent microporous membrane which can be adjusted over a wide range from a relatively low permeability to a high range and further excellent in dimensional stability at high temperature.

Accordingly, the present inventors have studied closely to solve the above problems, and as a result, the present invention can simultaneously use stretching before extraction and stretching after extraction, and also perform heat treatment to freely adjust the permeation performance without compromising strength and at a high temperature. The present invention was completed by devising a method for producing a polyolefin microporous membrane having excellent dimensional stability at.

It is an object of the present invention to provide a method for producing a polyolefin microporous membrane that can freely control permeability without compromising strength and excellent dimensional stability at high temperature.

Another object of the present invention is to provide a polyolefin microporous membrane, which is optimized in physical properties by a separator for a lithium ion secondary battery prepared in the above method.

The present invention is melt kneaded and extruded a composition consisting of a polyolefin resin and a plasticizer, molded into a cooling solidified sheet, and at least one stretching in at least one axial direction, and then the plasticizer is extracted and the plasticizer is extracted in the longitudinal direction. It relates to a method for producing a polyolefin microporous membrane, characterized in that to perform the stretching step.

In the first method of melt kneading the polyolefin resin and the plasticizer of the present invention, the polyolefin resin is introduced into a resin kneading apparatus such as an extruder, the plasticizer is introduced at any ratio while the resin is heated and melted, and the polyolefin resin and the plasticizer are made of A method of obtaining a uniform solution by kneading a composition. The form of the polyolefin resin to be added may be any of powder, granule, and pellet. Moreover, when kneading by the said method, it is preferable that the form of a plasticizer is normal temperature liquid. The extruder may be a single screw extruder, a biaxial bidirectional screw extruder, a biaxial coaxial screw extruder, or the like.

The second method of melt kneading a polyolefin resin and a plasticizer is a method of obtaining a uniform solution by mixing and mixing a mixed composition obtained by mixing and dispersing a resin and a plasticizer at room temperature in advance in a resin kneading apparatus such as an extruder. In the form of the mixed composition to be added, the plasticizer is in the form of a slurry when it is a room temperature liquid, and when the plasticizer is a room temperature solid, it is preferable that it is a powder form. In the first and second methods, in both methods, it is very important to knead the polyolefin and the plasticizer in a kneading apparatus such as an extruder so as to obtain a uniform solution, thereby improving productivity.

The first method of producing a microporous membrane precursor on a sheet by extruding and cooling and solidifying is to extract a homogeneous solution of a resin and a plasticizer between the T-die and extract it onto the sheet, contact with a thermal conductor to cool to a temperature sufficiently lower than the crystallization temperature of the resin. Is performed. As the heat conductor used in the present invention, metal, water, air, or plasticizer itself may be used, but a method of cooling by bringing it into contact with a metal roll is particularly preferred because of its high efficiency of heat conduction.

The second method for producing a microporous membrane precursor on a sheet is a method in which a homogeneous solution of a resin and a plasticizer is sandwiched between a circular die, extruded through a through phase, and subsequently processed into a sheet.

In the method of extracting the plasticizer, the microporous membrane is continuously introduced into the tank filled with the extraction solvent, and soaked in the tank and immersed in the tank for a sufficient time for the plasticizer to be removed. The extraction efficiency can be increased by applying a known method such as a multi-stage method to inject the microporous membranes into the respective tanks in turn and a counterflow method to estimate the concentration gradient by supplying the extraction solvent in the opposite direction to the running direction of the microporous membrane. desirable. It is very important to substantially remove the plasticizer from the microporous membrane. In addition, if the temperature of the extraction solvent is added within the range below the boiling point of the solvent, the diffusion of the plasticizer and the solvent can be promoted, so that the extraction efficiency can be increased, which is preferable.

In the manufacturing method of this invention, extending | stretching performed before an extraction process is called extending | stretching before extraction, and at least 1 extending | stretching operation is essential in at least one axial direction. The term "at least uniaxial direction" includes machine direction uniaxial stretching, width direction uniaxial stretching, simultaneous biaxial stretching and sequential biaxial stretching. In addition, at least 1 time means a 1st stage extending | stretching, a multistage stretching, and many times extending | stretching.

In the present invention, stretching before extraction is performed in a state in which the plasticizer is stretched in a high order dispersed in the micropores, crystal gaps, and amorphous portions of the microporous membrane, so that the stretchability is improved by the plasticizing effect and the increase in porosity of the microporous membrane is suppressed. As a result, high strength capable of high magnification stretching can be realized. Furthermore, biaxial stretching is preferred to achieve high strength, and more preferably, simultaneous biaxial stretching performs the process, thereby simplifying the process.

The stretching temperature is preferably at least 50 ° C lower than the melting point (Tm) lower than the melting point (Tm) of the polyolefin microporous membrane, and more preferably 5 ° C below the melting point (Tm) at 40 ° C lower than the melting point (Tm) of the polyolefin microporous membrane. It is carried out below the low temperature. At this time, when extending | stretching temperature is less than 50 degreeC lower than melting | fusing point (Tm), since extending | stretching property falls and the strain component after extending | stretching remains, dimensional stability at high temperature falls, it is unpreferable. If the stretching temperature is equal to or higher than the melting point (Tm), the microporous membrane melts and impairs the permeation performance.

Although the draw ratio can be set at any magnification, in the case of uniaxial stretching, it is preferable to carry out at 2 to 20 times, more preferably 4 to 10 times. Further, the area ratio in the biaxial direction is preferably 2 to 400 times, more preferably 4 to 100 times.

In the production method of the present invention, the stretching performed after the extraction step is called stretching after extraction, and is stretched in the longitudinal direction. In addition, in the stretching carried out after the extraction step, it is more preferable to perform stretching in the width direction in a separate process from the stretching in the longitudinal direction described above. Since the stretching after extraction is performed in a state where the plasticizer is substantially removed from the microporous membrane, the stretching process predominantly causes breakage of the polymer interface, thereby increasing the porosity of the microporous membrane. Therefore, if only stretching is performed after extraction without carrying out the pre-extraction stretching, which is essential in the present invention, excessively increasing the porosity unnecessarily causes the stretching orientation to be imparted to the microporous membrane, resulting in low strength. .

In contrast, in the case of the production method of the present invention in which stretching before extraction and stretching after extraction, the porosity can be increased without impairing the strength of the microporous membrane. At this time, the stretching temperature is preferably at least 50 ° C. or lower than the melting point (Tm) lower than the melting point (Tm) of the polyolefin microporous membrane, and more preferably at least 40 ° C. lower than the melting point (Tm) of the polyolefin microporous membrane. It is carried out below 5 ° C low temperature. If the stretching temperature is lower than the temperature lower by 50 ° C than the melting point (Tm) of the polyolefin microporous membrane, the stretchability is inferior, the strain component after stretching remains, and the dimensional stability at high temperature is not preferable. Although the stretching magnification can be set at any magnification, less than 5 times is preferable at the magnification in the uniaxial direction, and less than 20 times is preferable at the area magnification in the biaxial direction.

Stretching in the longitudinal direction can increase the permeation performance without impairing the dimensional stability in the width direction at high temperature. Moreover, when used together with the width direction extending process, the permeation | transmission performance can be improved and the intensity | strength balance between a longitudinal direction and the width direction can be improved.

At this time, by stretching in the longitudinal direction and in the width direction in a separate process, each of the stretching ratio can be changed widely. Therefore, by carrying out the above process, the permeation performance can be controlled in a wide range from a relatively low region to a high region.

The stretching in the longitudinal direction may be carried out by adopting a roll stretching machine. Here, it is important to prevent shrinkage in the width direction due to residual stress after stretching before extraction. It is preferable that the width of the microporous membrane is mechanically constrained by means of strongly pulling both ends of the microporous membrane by using a thermal state after stretching the microporous membrane on the roll before extraction. It is preferable to carry out the longitudinal stretching quickly after the fever. The draw ratio of the roll type stretching machine can be widely changed, and at the same time, the thermal relaxation process described later can be performed.

In addition, when the microporous membrane is thermally heated on the roll, if the width of the microporous membrane is not mechanically constrained, shrinkage can not be prevented only by friction between the roll and the microporous membrane, and the microporous membrane contracts in the width direction to provide permeability. It gets worse. After completion of the residual heat, residual stress in the width direction is alleviated, so that the shrinkage can be prevented only by friction between the roll and the microporous membrane.

Mechanically constrain the width of the microporous membrane by attaching a clip to the end of the roll and inserting the microporous membrane with the roll and the clip, pressing the microporous membrane with a roll with a belt-shaped object, and fixing by sucking the end of the microporous membrane. And the like can be mentioned.

As a method of extending | stretching in the width direction, the method of using a tenter type drawing machine is mentioned. The draw ratio of a tenter type drawing machine can be changed widely, and the heat relaxation mentioned later can also be performed.

In the manufacturing method of the present invention, the fifth step of heat treatment is preferably performed after the stretching after extraction, or after the stretching after extraction. Here, heat treatment includes heat fixation or heat relaxation. The heat setting in the above means that the heat treatment is carried out in a tension state while maintaining or restraining the set draw ratio at the time of stretching after extraction. On the other hand, thermal relaxation is to perform heat treatment in a relaxed state to relax the restrained state. Both the heat setting and the heat relaxation remove the residual stress or deformation component which may be thought to occur during stretching, thereby improving the dimensional stability at high temperature and controlling the permeability represented by the porosity and the permeability. .

In the manufacturing method of the present invention, as long as it does not interfere with the characteristics of the microporous membrane of the present invention, namely, after improving the dimensional stability at high temperature and adjusting the permeability, Processing can be performed. Examples of the post-treatment include hydrophilization treatment with a surfactant and crosslinking treatment with ionizing radiation. The polyolefin resins used in the present invention are resins used in conventional extrusion, injection, inflation, and blow molding, and include ethylene, propylene, 1-butene, 1-butene, 4-methyl-1-pentene, 1-hexene and 1 Homopolymers and copolymers of octenes can be used, and mixed forms of the homopolymers and copolymers are also preferred. 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, polybutylene, ethylene propylene rubber, and the like. More preferably, high density polyethylene is used. When the microporous membrane obtained by the manufacturing method of this invention is used as a battery separator, it is preferable to use resin which is a low melting point resin, and at the same time from a high-strength required performance especially a high density polyethylene.

The average molecular weight of the polyolefin resin used in the present invention is preferably 50,000 or more and less than 1 million, more preferably 100,000 or more and less than 700,000, and most preferably 200,000 or more and less than 500,000. The mean molecular weight refers to the weight average molecular weight obtained by GPC (gel filtration chromatography) measurement, etc., but since the average molecular weight of resins exceeding one million is difficult to accurately measure GPC, the viscosity average molecular weight can be adjusted as an alternative. Can be. At this time, if the average molecular weight is less than 50,000, it is not preferable at the time of melt molding because the melt tension is lost and the moldability is reduced, or the stretchability is lowered and the strength is lowered. On the other hand, when the average molecular weight exceeds 1 million, since it is difficult to obtain a uniform resin composition, it is preferable not to use it.

Moreover, the molecular weight distribution of the polyolefin resin used for this invention is 1 or more and less than 10, More preferably, it is 2 or more and less than 9, Most preferably, it is 3 or more and less than 8. The said molecular weight distribution is represented by the ratio (Mw / Mn) between 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 be deteriorated, resulting in partial distribution of thickness and reduction in strength.

The plasticizer used in the present invention can be used as long as it is a non-volatile solvent that is easy to form a homogeneous solution in the melting point of the polyolefin resin when mixed with the polyolefin resin. For example, hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl butyric acid or dibutyl butyric acid, higher alcohols such as oleyl alcohol or stearyl alcohol can be used.

The mixing ratio between the polyolefin resin and the plasticizer used in the present invention is sufficient to cause fine phase separation and form a microporous membrane precursor on a sheet, and at this time, productivity should not be reduced. More specifically, the weight fraction of the polyolefin resin in the composition consisting of the polyolefin resin and the plasticizer is preferably 20 to 70%, more preferably 30 to 60% of the polyolefin resin. At this time, if the weight fraction of the polyolefin resin is less than 20%, the melt stress during the melt molding is insufficient, the moldability is inferior. Although the weight fraction of polyolefin resin can also be performed in less than 20%, in this case, since ultra-high molecular weight polyolefin must be mixed in large quantity in order to raise melt stress, uniform dispersibility falls and it is unpreferable.

The extraction solvent used in the present invention should be less soluble in polyolefin, at the same time excellent in solubility in plasticizer, and have a boiling point lower than the melting point of the polyolefin microporous membrane. Such preferred 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, diethylethane and tetrahydro Ethers, such as furan, ketones, such as acetone and 2-butanone, can be used. In consideration of environmental adaptability, safety and hygiene, it is preferable to use alcohols and ketones in the solvent.

The composition used in the present invention can be mixed with additives such as antioxidants, nucleating agents, antistatic agents, flame retardants, lubricants, and ultraviolet absorbers within a range that satisfies the purpose without impairing the properties of the desired microporous membrane. The microporous membrane of the present invention means a porous sheet or film substantially composed of polyolefin, and is used as a battery material such as a separator. Although the form of a battery is not specifically limited, For example, it can apply to uses, such as a rectangular battery, a thin battery, a button type battery, an electrolytic capacitor, including a cylindrical battery.

The present invention provides a polyolefin microporous membrane prepared in the above production method.

As for the film thickness of the microporous film manufactured by the manufacturing method of this invention, 1-500 micrometers is preferable, More preferably, it is 5-100 micrometers. At this time, if the film thickness is less than 1 µm, the mechanical strength is insufficient, and if the thickness exceeds 500 µm, the occupancy volume of the separator increases, which is disadvantageous for high battery capacity.

The air permeability of the microporous membrane prepared according to the production method of the present invention is preferably 3000 seconds / 100 Pa / 25 μm or less, more preferably 1000 seconds / 100 Pa / 25 μm or less. The air permeability is defined by the ratio between the air permeation time and the film thickness, but if the air permeability is greater than 3000 sec / 100 μs / 25 μm, the ion permeability is lowered and the pore diameter is extremely small, so that the permeability is eventually reduced.

As for the porosity of the microporous membrane manufactured by the manufacturing method of this invention, 20 to 80% is preferable, More preferably, it is 30 to 70%. At this time, if the porosity is less than 20%, the ion permeability associated with air permeability or electrical resistance becomes insufficient, and if it exceeds 80%, the strength related to the piercing strength, the tensile strength, and the like is insufficient.

As for the puncture strength of the microporous membrane manufactured by the manufacturing method of this invention, 300 g / 25 micrometers or more are preferable, More preferably, it is 400 g / 25 micrometers or more. The puncture strength is defined by the ratio of the maximum load and the film thickness in the puncture test. If the puncture strength is less than 300 g / 25 μm, defects such as short circuit defects increase when winding the battery, are not preferable.

The thermal contraction rate of the microporous membrane prepared according to the production method of the present invention is an index for evaluating the dimensional stability at a high temperature of the microporous membrane, preferably 10% or less, more preferably 5% or less with respect to the width direction of the microporous membrane. At this time, if the heat shrinkage exceeds 10%, it is not preferable because it causes a safety problem such as a short circuit inside the battery.

Hereinafter, the present invention will be described in detail by way of examples.

The following examples are merely illustrative of the present invention, but the scope of the present invention is not limited to the following examples. The test method performed in the Example is as follows.

(1) film thickness

It measured on the dial gauge (PEACOCK NO.25 by Ozaki Corporation).

(2) porosity

A 20 cm square sample was cut out of the microporous membrane, and the volume (cm 3) and weight (g) of the sample were measured, and the porosity (%) was calculated using the following equation from the result obtained.

(3) speculation

Based on JIS P-8117, the air permeation time (seconds / seconds) and film thickness (μm) obtained by measuring with a Gull type air permeability meter are calculated by converting the film thicknesses by the following equation (seconds / seconds / 25 seconds). Μm) was calculated.

(4) stone strength

Using a compression tester (KES-G5, manufactured by Katotech), the sticking test was conducted under the condition of a bending radius of 0.5 mm and a sticking speed of 2 mm / sec. From the equation, the film thickness was converted to calculate the puncture strength (g / 25 mu m).

(5) breaking strength

It cut | disconnected to width 15mm and measured the test piece based on ASTMD882 about the narrow paper-shaped test piece.

(6) heat shrinkage

A 20 cm square sample was cut out of the microporous membrane and placed in a hot air circulating oven heated to 100 ° C. without restraining all four sides of the sample, and heated for 2 hours. The dimensions in the machine direction and the width direction of the microporous membrane were measured before and after heating to calculate the heat shrinkage percentage (%) based on the following equation.

(7) Average molecular weight and molecular weight distribution

According to the following conditions, GPC (gel filtration chromatography) measurement was performed to obtain a weight average molecular weight (Mw) and a number average molecular weight (Mn), and Mw was calculated as the average molecular weight or Mw / Mn was determined as the molecular weight distribution. .

Device: WATERS 150-GPC

Temperature: 140 ℃

Solvent: 1,2,4-trichlorobenzene

Concentration: 0.05% (injection amount: 500µl)

Column: 1 Shodex GPC AT-807 / S,

2 x Tosoh TSK-GELGMH6-HT

Melting condition: 160 ℃, 2.5 hours

Calibration curve: 3rd calculation using polyethylene conversion constant 0.48 for polystyrene standard samples

(8) melting point

About 7 mg of sample was left to stand in nitrogen atmosphere using the differential scanning calorimeter DSC-210 by Seiko Electronics Co., Ltd., and it heated up at the rate of 10 degree-C / min at room temperature, and evaluated by the endothermic peak temperature.

(9) relaxation rate

About the microporous membrane dimension after extending | stretching after extraction, the relaxation rate (%) was defined from the difference of the setting magnification at the time of extending | stretching after extraction, and the setting magnification at the time of heat processing as follows.

Relaxation rate = (stretching set magnification after extraction-heat treatment set magnification) × 100

< Example  1>

Dry blending high density polyethylene (weight average molecular weight 300,000, molecular weight distribution 7, density 0.956) and 0.3 parts by weight of 2,6-di-tert-butyl p-cresol with respect to the polyethylene using a HENSCHEL MIXER. Into a 35 mm twin screw extruder. Next, a sheet having a thickness of 1.4 mm was injected into the extruder on a cooling roll in which fluid paraffin (flow viscosity 75.9 cSt at 37.78 ° C.) was poured, melt kneaded at 200 ° C., and passed through a die to maintain a surface temperature of 40 ° C. Got. At this time, the composition ratio was adjusted to 30% by weight of polyethylene and 70% by weight of liquid paraffin. The obtained sheet 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 dried methylene chloride was removed. Subsequently, using both ends of the microporous membrane, the roll stretching machine was extracted and stretched in the longitudinal direction, followed by heat treatment while being relaxed in the longitudinal direction. In addition, heat was fixed using a tenter type heat stabilizer.

Hereinafter, formation conditions are described in Table 1, and the physical properties of the obtained microporous membrane are described in Table 2. At this time, the melting point of the obtained polyethylene microporous membrane was 133.4 ° C.

< Example  2>

The polyethylene microporous membrane was obtained in the same manner as in Example 1 except that the stretching and heat treatment conditions after extraction were performed as described in Table 1. The physical properties of the obtained polyethylene microporous membrane are shown in Table 2.

< Example  3>

After the sheet was obtained in the same manner as in Example 1, the sheet was stretched before extraction using a tenter type simultaneous biaxial drawing machine, and then immersed in methylene chloride to extract and remove the liquid paraffin, and then the attached methylene chloride was dried out. . Subsequently, extraction and stretching were carried out in the longitudinal direction using a roll stretching machine that was heated in a state where both ends of the microporous membrane were held tight, and then heat-treated while relaxing in the longitudinal direction. Moreover, it extracted after extending | stretching in the width direction using the tenter type drawing machine, and then heat-processing, relaxing in the width direction.

Hereinafter, formation conditions are described in Table 1, and the physical properties of the obtained microporous membrane are described in Table 2.

< Example  4>

The polyethylene microporous membrane was obtained in the same manner as in Example 2 except that the stretching and heat treatment conditions after extraction were performed as described in Table 1. The physical properties of the obtained polyethylene microporous membrane are shown in Table 2.

< Comparative example  1>

After obtaining a sheet 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 dried methylene chloride was removed. Thereafter, heat setting was performed using a tenter type heat stabilizer.

Hereinafter, formation conditions are described in Table 1, and the physical properties of the obtained microporous membrane are described in Table 2.

< Comparative example  2>

After obtaining a sheet as in Example 1, it was stretched before extraction using a tenter type simultaneous biaxial stretching machine, and subsequently dipped in methylene chloride to extract and remove liquid paraffin, and then dried methylene chloride was removed. Moreover, it extract | stretched after extending | stretching in the width direction using the tenter type | mold drawing machine, and heat-processing, relaxing in the width direction continuously.

Hereinafter, formation conditions are described in Table 1, and the physical properties of the obtained microporous membrane are described in Table 2.

< Comparative example  3>

The polyethylene microporous membrane was obtained in the same manner as in Comparative Example 2 except that the stretching and heat treatment conditions after extraction were performed as described in Table 1. Table 2 shows the physical properties of the obtained microporous membrane.

< Comparative example  4>

After obtaining a sheet as in Example 1, it was stretched before extraction using a tenter type simultaneous biaxial stretching machine, and subsequently dipped in methylene chloride to extract and remove liquid paraffin, and then dried methylene chloride was removed. Subsequently, extraction and stretching were carried out in the longitudinal direction using a roll stretching machine that was heated in a free state without restraining both ends of the microporous membrane, followed by heat treatment while relaxing in the longitudinal direction. In addition, heat was fixed using a tenter type heat stabilizer.

As described above, according to the manufacturing method of the polyolefin microporous membrane of the present invention, the dimensional stability at high temperature can be improved, and at the same time, there is also a flexible advantage of freely adjusting the permeation performance. In addition, the balance of the strength in the longitudinal direction and the width direction of the microporous membrane can be adjusted. Therefore, the microporous membrane prepared according to the present invention can produce a microporous membrane having various cell permeability, in order to meet the needs of the diversified battery, while also having high battery safety, especially when used as a battery separator.

Although the present invention has been described in detail only with respect to the embodiments described, it will be apparent to those skilled in the art that various modifications and variations are possible within the technical spirit of the present invention, and such modifications and variations belong to the appended claims. .

Claims (6)

Melt kneading and extruding a composition composed of a polyolefin resin and a plasticizer to form a sheet on a cooling solidified sheet, and performing stretching at least once before extraction in at least one axial direction, After the plasticizer is extracted and the plasticizer is extracted, the stretching is performed before the extraction, and the stretching process is performed in the longitudinal direction in a state in which the width of the sheet is mechanically constrained using a thermal state. The manufacturing method of the polyolefin microporous membrane made into. Melt kneading and extruding a composition composed of a polyolefin resin and a plasticizer to form a sheet on a cooling solidified sheet, and performing stretching at least once before extraction in at least one axial direction, After extracting the plasticizer and extracting the plasticizer, the drawing is performed in the lengthwise direction while the width of the sheet is mechanically constrained by stretching after pre-extraction, and the stretching process is separately performed in the width direction. A method for producing a polyolefin microporous membrane, characterized by heat treatment in a relaxed manner. delete 3. The stretching process according to claim 1 or 2, wherein the longitudinal stretching process after the plasticizer extraction is thermally performed while mechanically constraining the width of the film on the roll, and then the stretching process is performed on the roll. Method for producing a polyolefin microporous membrane. delete The polyolefin microporous membrane prepared according to claim 1 or 2, wherein the thermal shrinkage in the width direction satisfies 10% or less of the heat shrinkage calculated by the following formula, thereby ensuring dimensional stability during high temperature treatment. Polyolefin microporous membrane for lithium ion secondary battery separator.