JP2011208144A - Polyethylene microporous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-density polyethylene microporous film having excellent physical properties and uniform void structure for improving problems due to increased in molecular weight and allowing use as a porous film for a battery.SOLUTION: The polyethylene microporous film is formed of a high-density polyethylene, which has weight average molecular weight 2×10to 5×10and contains 5 wt.% or less of molecules with molecular weight 1×10or less. In the polyethylene microporous film, tensile strength in each of the lateral direction and the longitudinal direction is 1,100 kg/cmor more, perforation strength is 0.22 N/μm or more, a gas permeability constant (Darcy's permeability constant) is 1.3×10Darcy or more, and a shrinkage factor in each of the longitudinal direction and the lateral direction is 5% or less.

Description

  The present invention relates to a high-density polyethylene microporous membrane. More specifically, the present invention relates to a high-density polyethylene microporous membrane that not only has excellent extruding kneadability and stretchability but also high productivity, and can improve the performance and safety of a battery using the same.

  Polyolefin microporous films are widely used in various battery separators, separation filters, and microfiltration membranes because of their chemical safety and excellent physical properties.

  There are three main methods for producing microporous membranes from polyolefins. The first is a method of making a microporous membrane in the form of a non-woven fabric by making the polyolefin from a thin fiber, and the second is to make a thick polyolefin film and then drawing it at a low temperature to It is a dry method in which micro cracks are induced between thin plates (lamella) as crystal parts to form micro voids. The third is a wet method in which polyolefin is kneaded with a diluent at a high temperature to form a single phase, and after the polyolefin and diluent are phase separated in the cooling process, the diluent is extracted to create voids in the polyolefin. .

  Among these, the third wet method can produce a thin film with a thin and uniform film thickness compared to the other two methods, and it has excellent physical properties and is a separator for secondary batteries such as lithium ion batteries. Widely used for.

  A general method for producing a porous film by a wet method is described in US Pat. No. 4,247,498. This patent uses polyethylene and compatible liquids to mix these mixtures at high temperatures to form a thermodynamic single-phase solution, which is then cooled and cooled with polyethylene during the cooling process. A technique for producing a polyolefin porous membrane by using a mixed solvent as a solid / liquid or liquid / liquid and performing phase separation is described.

  In US Pat. No. 4,335,193, an organic liquid compound such as dioctylphthalate or liquid paraffin and an inorganic filler are added to a polyolefin and processed, and then the organic liquid compound and the inorganic substance are removed. In addition, techniques for producing polyolefin porous membranes are described, and these techniques are also described in US Pat. No. 5,641,565.

  However, since such a method uses an inorganic material such as silica, it is difficult to input and knead the inorganic material, and a process for extracting and removing this is added in the subsequent process, which complicates the process and hardly increases the stretch ratio. There are disadvantages.

  U.S. Pat. No. 4,539,256 also describes a basic method for producing a microporous film by extruding and drawing a polyethylene and a hybrid liquid compound and then drawing.

  Along with the full-scale use of secondary batteries, efforts to improve the productivity of microporous membranes and the characteristics of the films have continued. Typically, there are methods such as using ultra-high molecular weight polyolefin (UHMWPO) having a weight average molecular weight of about 1 million or mixing to increase the molecular weight of the composition to increase the strength of the porous film.

  In this context, US Pat. Nos. 4,588,633 and 4,873,034 use a polyolefin that has a weight average molecular weight of 500,000 or more and a solvent that can dissolve polyolefins at high temperatures, and are microporous through a two-stage solvent extraction process and a stretching process. The process of making a film is introduced. However, in this method, in order to improve the kneadability and extrudability with the solvent, which is a disadvantage of ultra-high molecular weight polyolefin, it is extracted once after being drawn and stretched in one step using an excessive amount of solvent in the extruding process. It goes through the stage to do.

  US Pat. No. 5,051,183 contains 10 to 50% by weight of a polyolefin containing 1% or more of ultrahigh molecular weight polyolefin having a weight average molecular weight of 700,000 or more, 90 to 50% by weight of a solvent such as mineral oil, and the polydispersity index (weight). A polyethylene microporous membrane using a composition having an average molecular weight / number average molecular weight) of 10 to 300 has been introduced. In the method of forming voids, the composition is squeezed to form a gel-like sheet, stretched at a temperature between the melting point of the composition and the melting point + 10 ° C., and then the solvent is extracted to form a porous film. .

  However, in this method, the ultra-high molecular weight polyolefin is mixed, and at the same time, the molecular weight distribution is widened to overfeed the polyolefin having a large molecular weight. If this happens, the chain entanglement due to these molecules is severely generated and the stretchability is greatly reduced. That is, a breakage at a high draw ratio and a fast draw speed and an unstretched phenomenon at a low draw ratio occur.

  The method of solving this is to obtain the same effect as increasing the stretching temperature to make the composition soft when stretching, or increasing the stretching speed to increase the temperature of the composition. However, if this is done, on the contrary, when stretching, the orientation of the resin is reduced and the stretching effect is reduced, resulting in a problem that the physical properties of the final porous film are lowered.

  Also, a film made of a resin having a wide molecular weight distribution has more defects due to molecules having a lower molecular weight than a film made of a resin having a narrow molecular weight distribution. Strength, puncture strength, etc. are disadvantageous.

  Such a phenomenon is no exception even in a microporous film, and if the molecular weight distribution is widened, one of the most important physical properties of the microporous film is not sufficiently high. That is, the effect of the ultrahigh molecular weight polyolefin added for improving the physical properties does not sufficiently appear. Such problems also exist in similar technologies such as Japanese Unexamined Patent Publication Nos. 06-234876 and 06-212006 and US Pat. No. 5,786,396.

  On the other hand, Japanese Patent Application Laid-Open No. 09-3228 uses a similar resin composition and introduces a method for complementing physical properties by matching the stretch ratio balance in the machine direction (MD) and the transverse direction (TD). .

  In Japanese Patent Laid-Open No. 09-259858, the shutdown temperature of a polyethylene microporous membrane (if the temperature inside the battery rises due to abnormal operation of the battery, the porous membrane is melted and voids are formed to prevent accidents such as ignition and explosion) In order to reduce the temperature that hinders the flow of electric current), 70 to 99% by weight of polyethylene having a weight average molecular weight of 500,000 or more and 1 to 30% by weight of low molecular weight polyethylene having a weight average molecular weight of 1,000 to 4,000. Introducing a method of producing 10% to 80% by weight resin composition and 20 to 90% solvent by weight, extruding with a die and freezing to form a gel composition and extracting the residual solvent after stretching Has been.

  This technique is characterized by lowering the shutdown temperature using low molecular weight polyethylene having a weight average molecular weight of 1,000 to 4,000. However, there are two problems with this technology.

That is, the addition of a low molecular weight molecule lowers the molecular weight and broadens the molecular weight distribution, thus lowering the physical properties. In the examples, it was shown that the tensile strength of the polyethylene porous membrane produced by such a technique was about 1,000 to 1,200 kg / cm ^2, which was a relatively low level.

  In addition, the process of kneading polyolefin with a diluent or solvent requires a high technique. Commercially used are twin screw extruders, kneaders and banbury mixers.

  When the resin is mixed with a solvent, as in the technique described above, the viscosity is significantly different (ultra high molecular weight polyethylene having a weight average molecular weight of 500,000 or more and low molecular weight polyethylene having a weight average molecular weight of 1,000 to 4,000). In addition to the kneading problem between the resin / solvent, there also occurs a kneading problem between two resins having different molecular weights (the viscosities in the molten state differ greatly).

  In this case, a fine gel (fine gel or fish eye) is generated in the final film, and the quality of the film may be lowered. A method for solving this problem is to increase the residence time of the melt in the extruder, but there is a disadvantage that the productivity is lowered.

  In US Pat. No. 5,830,554, the weight average molecular weight is between 500,000 and 2500,000 in order to solve the problems of physical properties and stretchability degradation caused by resins having a wide molecular weight distribution. A method for producing a polyolefin microporous film by extruding-stretching-extracting a solution containing 5 to 50% by weight of a resin having a molecular weight ratio of 10 or less is described.

This method uses a large amount of solvent (preferably 80 to 90% by weight) in order to solve the problem of pressure variation due to the increase in viscosity generated by the pressure of ultrahigh molecular weight resin, so that the porosity becomes high and porous. The physical properties were not significantly improved when the tensile strength of the adhesive film was 800 kg / cm ² or more (Example 950-1200 kg / cm ² ).

  In US Pat. No. 6,566,012, a resin composition containing 10 to 40 wt% and 90 to 60 wt% solvent containing an ultra high molecular weight polyolefin having a weight average molecular weight of 500,000 or more and an ultra high molecular weight polyolefin having a weight average molecular weight of 500,000 or more is used. A method for producing a polyolefin microporous membrane suitable for a battery separation membrane by extrusion-molding-stretching-extraction-heat setting is described.

  As described above, the conventional technology uses a resin having a large molecular weight in order to increase the physical properties. However, the increase in the molecular weight of the resin used additionally increases the pressure load, decreases the kneadability with the solvent, and stretches. This causes problems such as an increase in stretching load, occurrence of unstretching, a decrease in productivity due to a stretching speed and a decrease in the stretching ratio.

  Here, as a result of repeating extensive research to solve the problems of the prior art as described above, the present inventors adjusted the content of low molecular weight polyethylene molecules contained in polyethylene below a specific content. Thus, the present invention has been completed by paying attention to the fact that defects can be prevented from being formed in polyethylene.

  Accordingly, an object of the present invention is to provide a high-density polyethylene microporous membrane having excellent physical properties and a uniform void structure that can be used in a microporous membrane for a battery while improving the problem caused by an increase in molecular weight.

In order to achieve the above object, the polyethylene microporous membrane according to the present invention is a high density polyethylene having a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ^{5 having} a molecular weight of 1 × 10 ⁴ or less and a content of molecules of 5% by weight or less. Manufactured from a composition comprising 20 to 50% by weight of (Component I) and 80 to 50% by weight of diluent (Component II), and has a tensile strength of 1,100 kg / cm ² or more in the transverse and longitudinal directions, respectively. The strength is 0.22 N / μm or more, the gas permeability (Darcy's permeability constant) is 1.3 × 10 ⁻⁵ Darcy or more, and the shrinkage is 5% or less in the horizontal and vertical directions, respectively. And

  As shown in the following examples, the high-density polyethylene microporous membrane of the present invention is easy to extrude and stretch and can improve productivity with stable product production. The produced product has high gas permeability and is tensile. Since it has excellent strength and perforation strength and has a low shrinkage rate, it can be used effectively in battery separators and various filters. All simple variations and modifications of the present invention belong to the scope of the present invention, and the specific scope of protection of the present invention is defined by the claims.

  Hereinafter, the present invention will be described in more detail as follows. As described above, the present invention provides a high-density polyethylene microporous membrane excellent in extrusion kneadability and stretchability so that it can be applied to a microporous membrane for a battery while improving the problem due to an increase in molecular weight. The basic theory for making a polyethylene microporous membrane from polyethylene used in the present invention is as follows. A low molecular weight organic substance (hereinafter referred to as a diluent) having a molecular structure similar to polyethylene forms a thermodynamic single phase with polyethylene at a high temperature at which polyethylene dissolves. If these polyethylene and diluent solutions that form these thermodynamic single phases are frozen at room temperature, phase separation between polyethylene and diluent occurs during the cooling process.

  Each phase to be phase-separated consists of a polyethylene rich phase centered on a lamella, which is a crystalline part of polyethylene, and a small amount of polyethylene and diluent that are dissolved in the diluent at room temperature. It consists of a diluent rich phase. If the diluted solution is extracted with an organic solvent after cooling, a polyethylene porous membrane can be produced.

  Therefore, the basic structure of the microporous membrane is determined by the phase separation process. That is, the size and structure of the microporous membrane are finally determined by the size and structure of the diluted liquid-rich phase produced after phase separation. In addition, the basic physical properties of the microporous membrane are affected by the crystal structure of polyethylene produced from the process of extracting the diluent.

  As a result of the study by the present inventors, the following facts were found. In other words, in order to produce an excellent microporous membrane, as little polyethylene as possible should be present in the dilute rich phase, and no defects should be created in the process of extracting the dilute. It is the low molecular weight polyethylene molecules contained in polyethylene that have the greatest effect on this.

  As a result of producing products using polyethylene with a low low molecular weight substance based on this, a polyethylene microporous film having excellent physical properties and a uniform void structure can be made even with a resin having a lower molecular weight than the conventional invention, and processed. The characteristics were also greatly improved.

The high-density polyethylene microporous membrane according to the present invention has a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ⁵ and a molecular weight of 1 × 10 ⁴ or less. After forming a sheet-form molded product by melt-extruding a composition composed of 20 to 50% by weight of component I) and 80 to 50% by weight of diluent (component II), the sheet was stretched. After the film is formed and the diluted solution is extracted, it is manufactured by drying and heat setting.

In particular, the high-density polyethylene microporous membrane of the present invention has a tensile strength of 1,100 kg / cm ² or more in the transverse and longitudinal directions, a perforation robbery of 0.22 N / μm or more, and a gas permeability (Darcy's permeability constant) of 1. .3 × 10 ⁻⁵ Darcy or more and shrinkage is 5% or less in the horizontal direction and the vertical direction, respectively.

In general, polyethylene used commercially has a molecular weight distribution, and some polyethylene having a weight average molecular weight of more than 1 × 10 ⁶ has a molecular weight of several thousand. In applications such as blown film and blow molding, which are common uses of polyethylene commercially, low molecular weight molecules play a role in improving the processability of resins with large molecular weights, so these low molecular weight materials are intentionally used during the polyethylene production process. It has been made.

  However, in the process of making a polyethylene microporous film, these low molecular weight substances reduce the completeness of the thin plate that is a polyethylene crystal part in the polyethylene-rich phase, and the number of tie molecules that connect the thin plates is reduced. Drop the strength of the entire polyethylene.

In addition, it has a high affinity with the diluent and is present in a large amount in the diluent-rich phase, and is present in the interface portion of the void after extraction, which causes the void interface to be incomplete and voids. Acts as a factor to reduce the degree. Such a phenomenon appears in molecules having a molecular weight of 1 × 10 ⁴ or less and appears when the content exceeds 5% by weight.

  There are various types of polyethylene (such as low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, and high-density polyethylene) and polypropylene that are commonly used in the prior art.

  However, in the case of polyethylene and polypropylene excluding high-density polyethylene, the structural regularity of the polymer is dropped to reduce the thin plate completeness of the crystal part of the resin itself, thereby reducing the thickness. In addition, if the comonomer is present during the polymerization reaction, the reactivity of the comonomer is lower than that of ethylene, and many low molecular weight molecules are produced.

Therefore, the high-density polyethylene used in the present invention has a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ⁵ and a content of molecules having a molecular weight of 1 × 10 ⁴ or less is 5% by weight or less.

  The high-density polyethylene desirably has a comonomer content of 2% by weight or less. As the comonomer, alpha olefins such as propylene, butene-1, hexene-1, 4-methylpentene-1, octene-1, etc. can be used. Hexene-1 or 4-methylpentene-1 are suitable.

  On the other hand, it is better to use high-density polyethylene with a high molecular weight for the final physical properties of the porous film. However, when the molecular weight is high, the load on the presser increases due to an increase in viscosity during the press-out process, and the viscosity with the diluent increases. The kneadability deterioration due to the difference is generated, and the surface shape of the extruded sheet becomes rough.

  The solution to this problem is to increase the extrusion temperature or to increase the shear rate of the screw structure of the screw compound, or in this case, the resin has a deterioration and the physical properties are increased. Will be reduced.

  In addition, the increase in the molecular weight of the resin may increase the forcing of the sheet due to the increase in kneading between molecules, thereby increasing the stretching load and causing slippage in the clip that fixes the sheet. Further, unstretching may occur at a low stretch ratio due to an increase in sheet strength. However, it is difficult to increase the stretch ratio because the shrinkage rate of the produced microporous membrane increases and the load on the stretcher clip also increases.

  A method for solving this has the same effect as increasing the stretching temperature to make the stretched sheet soft, or slowing the stretching speed to increase the temperature of the composition. However, if it does in this way, the orientation (orientation) of stretched resin will decrease and the stretching effect will fall and the physical property of a final porous film will fall. Decreasing the drawing speed is undesirable because it also reduces productivity.

Accordingly, in the present invention, when high density polyethylene having a sufficiently low low molecular weight portion of the high density polyethylene, that is, a portion having a molecular weight of 1 × 10 ⁴ or less is 5% by weight or less, even if the weight average molecular weight is 5 × 10 ⁵ or less. It has been found that it has sufficient physical properties to be used for battery separation membranes.

Here, if the weight average molecular weight of the high density polyethylene is less than 2 × 10 ⁵ , sufficient physical properties cannot be obtained, so the high density polyethylene used in the present invention has a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ^5. Is selected to have

  High-density polyethylene having such a molecular weight and molecular weight distribution has excellent stretchability and an excellent void structure even at low magnification stretching. Since the load of the stretching clip is small, stretching is facilitated and productivity is increased.

  The diluent used in the present invention can be any organic liquid that forms a single phase with the resin at the extrusion temperature. Examples include nonane, decane, decalin, paraffin oil and other aliphatic (aliphatic) or cyclic hydrocarbons, dibutyl phthalate, and phthalic acid. There are phthalic acid esters such as dioctyl phthalate.

  A paraffin oil that is harmless to the human body, has a high boiling point, and has a small amount of volatile components is suitable. More preferably, a paraffin oil having a kinetic viscosity at 40 ° C. of 20 to 200 cSt is suitable. .

  If the kinematic viscosity of paraffin oil exceeds 200 cSt, the kinematic viscosity in the extruding process may be high, which may cause problems such as increased load, surface defects of sheets and films, and extraction becomes difficult in the extraction process. And a problem such as a decrease in permeability due to residual oil may occur. If the kinematic viscosity of the paraffin oil is less than 20 cSt, kneading during the extrusion process becomes difficult due to the difference in viscosity from the molten polyethylene in the extruder.

  It is desirable that the high-density polyethylene and the diluent used are 20 to 50% by weight of the high-density polyethylene and 80 to 50% by weight of the diluent. If the content of the high-density polyethylene exceeds 50% by weight (that is, if the diluent is less than 50% by weight), the porosity is reduced and the size of the voids is reduced. (Interconnection) is reduced and the transmittance is greatly reduced.

  On the other hand, if the content of the high-density polyethylene is less than 20% by weight (that is, if the diluent exceeds 80% by weight), the kneadability between the polyethylene and the diluent is reduced, and the polyethylene is added to the diluent. It becomes impossible to knead and is extruded into a gel form, and when stretched, problems such as breakage and variation in thickness occur.

  If necessary, general additives for improving specific functions such as an oxidation stabilizer, a UV stabilizer, and an antistatic agent may be further added to the composition.

  The composition is melt-extruded using a biaxial compound, a kneader, or a banbury mixer designed for kneading the diluent and polyethylene to form a sheet-like molded product. Polyethylene and oil can each be fed from a blender that has been pre-blended and fed into the compound or separated. In general, a method such as casting or calendaring is used as a method for producing a sheet-like molded product from a melt.

  Next, the stretching process is preferably performed by a tenter type simultaneous stretching. Roll type stretching has drawbacks such as scratches on the film surface during the stretching process. Here, the stretching ratio is preferably 3 times or more in the longitudinal direction and the transverse direction, and the total stretching ratio is preferably 25 to 50 times.

  When the stretch ratio in one direction is less than 3 times, the orientation in one direction is not sufficient, and at the same time, the physical property balance between the longitudinal direction and the transverse direction is broken, and the tensile strength, the piercing strength, and the like are lowered. In addition, if the total stretch ratio is less than 25 times, unstretching occurs, and if it exceeds 50 times, there is a high possibility of breakage during stretching, and there is a disadvantage that the shrinkage rate of the film eventually increases. .

  At this time, the stretching temperature varies depending on the melting point of the polyethylene used and the concentration and type of the diluent. The optimum stretching temperature is preferably selected within a temperature range in which 30 to 80% by weight of the polyethylene crystal portion in the film molding is melted.

  If the stretching temperature is selected in a temperature range lower than the temperature at which 30% by weight of the crystalline portion of polyethylene in the film molding is melted, there is no softness of the film. At the same time, unstretched occurs.

  On the other hand, if the stretching temperature is selected in a temperature range higher than the temperature at which 80% by weight of the crystal part is melted, it is easy to stretch and the occurrence of unstretching is small. The orientation effect is reduced and the physical properties are greatly reduced. On the other hand, the degree of melting of the crystal portion due to temperature can be obtained from differential scanning calorimetry (DSC) analysis of the film molding.

  The film thus stretched is extracted and dried using an organic solvent. The organic solvent that can be used in the present invention is not particularly limited, and any solvent that can extract the diluent used for resin extrusion can be used, but preferably methyl ethyl ketone having high extraction efficiency and quick drying, Methylene chloride, hexane, etc. are desirable.

  As the extraction method, all common solvent extraction methods such as an immersion method, a solvent spray method and an ultrasonic method can be used individually or in combination. At the time of extraction, the content of residual diluent must be 1% by weight or less. If the residual diluent exceeds 1% by weight, the physical properties are lowered and the transmittance of the film is reduced.

  The amount (extraction rate) of the residual diluted solution greatly depends on the extraction temperature and the extraction time. The extraction temperature should be high in order to increase the solubility of the diluent and the solvent, but is preferably 40 ° C. or less in view of safety problems due to boiling of the solvent.

  If the extraction temperature is lower than or equal to the freezing point of the diluent, the extraction efficiency will drop greatly, so it must be higher than the freezing point of the diluent. Although the extraction time varies depending on the thickness of the film to be produced, 2 to 4 minutes is suitable for producing a general microporous film having a thickness of 10 to 30 μm.

  The dried film is finally subjected to a heat setting step in order to remove residual stress and reduce the shrinkage of the final film. In the heat setting, the residual stress is removed by fixing the film, applying heat, and forcibly removing the film to be shrunk. A higher heat setting temperature is advantageous for lowering the shrinkage rate. However, when the heat setting temperature is excessively high, the fine pores formed by partially dissolving the film are blocked and the permeability is lowered.

  The desired heat setting temperature is preferably selected within a temperature range in which 10 to 30% by weight of the crystalline portion of the film is melted. If the heat setting temperature is selected within a temperature range lower than the temperature at which 10% by weight of the crystalline portion of the film is melted, there is no effect of removing the residual stress of the film due to insufficient reorientation of polyethylene molecules in the film. If the temperature is selected in a temperature range higher than the temperature at which 30% by weight of the crystal portion is melted, the fine porosity is blocked by partial melting and the permeability is lowered.

  Here, the heat setting time must be shortened when the heat setting temperature is high, and can be increased when the heat setting temperature is low. Most preferably, 5 to 20 minutes is suitable for the temperature range in which 10 to 20% by weight of the crystalline portion of the film is melted, and 1 to 5 minutes is suitable for the temperature range in which 20 to 30% by weight is melted.

The high-density polyethylene microporous membrane of the present invention produced as described above has the following physical properties.
(1) The tensile strength is 1,100 kg / cm ² or more in the transverse direction and the longitudinal direction, respectively.
When the tensile strength is less than 1,100 kg / cm ² , the porous membrane may break during the process of using the microporous membrane. In particular, when used in battery separators, the porous membrane breaks due to the tension applied during the high-speed battery assembly process. The polyethylene microporous membrane according to the present invention has a tensile strength of 1,100 kg / cm ² or more and can minimize breakage during the assembly process.

(2) The perforation strength is 0.22 N / μm or more.
The perforation strength expresses the strength of the film against sharp materials, and when used in battery separators, if the perforation strength is not sufficient, the film is torn due to electrode surface abnormalities or dendrite generated during battery use. A short circuit occurs.
When battery separators for commercial use have a weight at break of 350 g or less, safety due to short circuit becomes a problem. The film having a perforation strength of more than 0.22 N / μm according to the present invention has a breakage weight of 350 g when the thinnest film of 16 μm thickness is used among the separator films widely used at present. So it can be used safely.

(3) Gas permeability (Darcy's permeability constant) is 1.3 * 10 < ^-5 > Darcy (Darcy) or more.
When the gas permeability is 1.3 × 10 ⁻⁵ Darcy or less, the efficiency as a porous membrane is greatly reduced. In particular, when the gas permeability does not exceed 1.3 × 10 ⁻⁵ Darcy, when used as a battery separator, the charge / discharge characteristics of the battery deteriorate and the life is shortened. A film having a gas permeability of 1.3 × 10 ⁻⁵ Darcy or more according to the present invention has excellent charge / discharge characteristics such as high-efficiency charge / discharge of the battery, low-temperature characteristics, and life.

(4) The shrinkage rate is 5% or less in each of the horizontal direction and the vertical direction.
The shrinkage rate is measured after leaving the film at 105 ° C. for 10 minutes. If the shrinkage rate is high, the film cannot be used in high temperature applications. In particular, in the case of a battery separator, if the shrinkage rate exceeds 5%, the separator shrinks due to the heat generated by the battery itself, and the electrodes come into contact with each other, causing a short circuit.

  The film according to the present invention can be safely used as a battery separator with a shrinkage rate of 5% or less in each of the horizontal and vertical directions. In addition to these physical properties, the high-density polyethylene microporous membrane of the present invention is excellent in extrusion kneadability and stretchability.

  Hereinafter, the present invention will be described more specifically with reference to the following examples. However, the scope of the present invention is not limited to these examples.

[Example]
The molecular weight and molecular weight distribution of polyethylene were measured by high temperature GPC (Gel Permeation Chromatography) manufactured by Polymer Lab. The viscosity of the diluted solution was measured with a Canon CAV-4 automatic viscometer.

  The polyethylene and diluent were kneaded with a biaxial compounder with φ = 30 mm. The extrusion temperature was 160-240 ° C., and the residence time was 3 minutes. The extruded melt was formed into a sheet having a thickness of 600 to 1200 μm by a casting roll using a T-shaped die, and these were used for stretching.

In order to determine the presence or absence of gels due to poor melting and kneading, a film having a thickness of 200 μm was separately manufactured and the number of gels in an area of 2000 cm ² was counted. In order not to affect the quality of the microporous membrane, the number of gels per 2000 cm ² must be 50 or less.

  The formed sheets were subjected to differential scanning calorimetry (DSC) analysis in order to analyze the phenomenon that the crystal part melts due to temperature. The analysis conditions were a sample weight of 5 mg and a scanning rate of 10 ° C./min.

  Sheets were stretched simultaneously from a tenter type lab stretcher (lab. Stretcher) by changing the stretching ratio, stretching temperature, and stretching speed. The stretching temperature was determined in the temperature range in which 30 to 80% by weight of the polyethylene crystal portion in the film molding was dissolved with reference to the results of a differential scanning calorimeter (DSC).

  Five sheets were stretched to measure the success rate of stretching without slipping and stretching breakage. Extraction of the diluted solution was performed by a deposition method using methylene chloride. The heat setting was performed by drying the film from which the diluted solution was extracted in air, fixing the film to a frame, and changing the temperature and time in a hot air oven (convection oven).

The produced film was measured for the most important physical properties of the microporous membrane, namely tensile strength, perforation strength, gas permeability and shrinkage, and the results are shown in Table 1 below.
* Physical property measurement method (1) The tensile strength was measured by ASTM D882.
(2) The perforation strength was measured as the strength when a pin having a diameter of 0.5 mm breaks the film at a speed of 120 mm / min.
(3) The gas permeability was measured with a void meter (PFP CFP-1500-AEL). In general, the gas permeability is expressed as a Gurley number, but since the influence of the film thickness is not corrected, the relative permeability due to the void structure of the film itself is difficult to understand. To solve this, the present invention uses Darcy's permeability constant. Darcy's permeability constant can be obtained from Equation 1 below. In the present invention, nitrogen was used.
(Equation 1)
C = (8FTV) / (πD ² (P ² −1))
Where C = Darcy's permeability constant, F = flow velocity, T = sample thickness, V = gas viscosity (0.185 for N ₂ ), D = sample diameter, P = pressure. In the present invention, the average value of Darcy's permeability constant was used in the range of 100 to 200 psi.
(4) Shrinkage was measured by the percentage of shrinkage in the vertical and horizontal directions after the film was left at 105 ° C. for 10 minutes.

[Example 1]
A high-density polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ and a content of molecules having a molecular weight of 10 ⁴ or less of 4.2% by weight and containing no comonomer was used. For component II, paraffin oil having a kinematic viscosity at 40 ° C. of 95 cSt was used. The contents of component I and component II were 30% by weight and 70% by weight, respectively.

  Stretching was performed at 115 ° C., where 30% of the polyethylene crystal part was melted, at a stretch ratio of 36 times (longitudinal direction × transverse direction = 6 × 6), and at a stretching speed of 2.0 m / min. The film from which the diluted solution was extracted passed through a drying process in the air, and was then fixed to a frame and heat-fixed at 120 ° C. for 15 minutes, at which the crystal portion of the film was dissolved by 20% by weight. The final film thickness obtained was 20 ± 2 μm.

[Example 2]
A weight average molecular weight of 4.0 × 10 ⁵ in component I, at a content of molecular weight of 10 ⁴ or less molecules 3.9 wt%, high density butene-1 was used 0.5 wt% comonomer The same procedure as in Example 1 except that polyethylene was used. As in Example 1, the stretching temperature was set to 114.5 ° C. in order to match the ratio of the stretched crystal portion melted at 30% by weight. Also, the heat setting temperature was set to 119 ° C. in order to adjust the degree of crystal melting at 20 wt% as in Example 1.

[Example 3]
A weight average molecular weight of 4.7 × 10 ⁵ in component I, the content of molecular weight of 10 ⁴ or less molecules with 1.2 wt% high-density polyethylene containing no comonomer was used. The stretching was performed in the same manner as in Example 1 except that the stretching was performed at 117 ° C., which is a temperature at which 40% by weight of the polyethylene crystal part was melted.

[Example 4]
A weight average molecular weight of 3.0 × 10 ⁵ in component I, at a content of molecular weight of 10 ⁵ or less molecule 4.5 wt%, high density butene-1 was used 1.5 wt% comonomer Polyethylene was used. As component II, paraffin oil having a kinematic viscosity at 40 ° C. of 70 cSt was used. The contents of component I and component II were 40% by weight and 60% by weight, respectively. Except this, the procedure was the same as in Example 1. As in Example 1, the stretching temperature was set to 116 ° C. in order to match the proportion at which the stretched crystal portion was dissolved at 30% by weight. Further, the heat setting temperature was set to 118 ° C. in order to adjust the degree of crystal melting as in Example 1 to 20 wt%.

[Example 5]
For component I, high-density polyethylene having a weight average molecular weight of 4.7 × 10 ⁵ , a molecular weight of 10 ⁴ or less and a content of 1.2% by weight and no comonomer was used as in Example 3. . For component II, paraffin oil having a kinematic viscosity at 40 ° C. of 120 cSt was used. The contents of component I and component II were 40% by weight and 60% by weight, respectively.

  Stretching was performed at 117 ° C., which is a temperature at which 30% by weight of the crystalline portion of polyethylene was melted, at a stretch ratio of 36 times (longitudinal direction × transverse direction = 6 × 6), and at a stretching speed of 2.0 m / min. The heat setting was carried out for 15 minutes at 115 ° C., which is the temperature at which the crystal part of the film melts 10%.

[Example 6]
For component I, high-density polyethylene having a weight average molecular weight of 4.7 × 10 ⁵ and a molecular weight of 10 ⁴ or less was 1.2% by weight and containing no comonomer, as in Example 3. . For component II, paraffin oil having a kinematic viscosity at 40 ° C. of 30 cSt was used. The contents of Component I and Component II were 20% by weight and 80% by weight, respectively.

  Stretching was performed at 113 ° C., a temperature at which 30% by weight of the polyethylene crystal part was melted, at a stretch ratio of 49 times (longitudinal direction × transverse direction = 7 × 7), and at a stretching speed of 2.0 m / min. The heat setting was carried out for 5 minutes at 120 ° C., which is a temperature at which the crystal part of the film melts 20%.

[Example 7]
The stretching temperature was 122 ° C., which is the temperature at which 60% by weight of the polyethylene crystal melts, and the same procedure as in Example 3 was performed except that the stretching ratio was 25 times (longitudinal direction × lateral direction = 5 × 5).

[Example 8]
Component I has the same weight average molecular weight of 3.5 × 10 ^{5 as} in Example 4, the content of molecules having a molecular weight of 10 ⁴ or less is 4.5% by weight, and comonomer is 1.5% by weight of butene-1. The used high density polyethylene was used. For component II, paraffin oil having a kinematic viscosity at 40 ° C. of 160 cSt was used.

  The contents of Component I and Component II were 30% by weight and 70% by weight, respectively. Stretching was performed at 120 ° C., a temperature at which 50% by weight of the crystalline portion of polyethylene was melted, at a stretch ratio of 25 times (longitudinal direction × transverse direction = 5 × 5), and at a stretching speed of 2.0 m / min. The heat setting was carried out for 15 minutes at 118 ° C., which is the temperature at which the crystalline part of the film melts 20% by weight.

[Example 9]
Components I and II were used equally as in Example 3. The contents of Component I and Component II were 50% by weight and 50% by weight, respectively. Stretching was performed at 119 ° C., where 30% by weight of the crystalline portion of polyethylene was melted, at a stretch ratio of 36 times (longitudinal direction × transverse direction = 6 × 6), and at a stretching speed of 10.0 m / min. The heat setting was carried out for 5 minutes at 123 ° C., which is a temperature at which 30% by weight of the crystalline portion of the film was dissolved.

[Example 10]
Component I has the same weight average molecular weight of 3.5 × 10 ^{5 as} in Example 4, the content of molecules having a molecular weight of 10 ⁴ or less is 4.5% by weight, and comonomer is 1.5% by weight of butene-1. The used high density polyethylene was used. For component II, paraffin oil having a kinematic viscosity at 40 ° C. of 95 cSt was used. The contents of Component I and Component II were 20% by weight and 80% by weight, respectively. Stretching was performed at 112 ° C., a temperature at which 30% by weight of the crystalline portion of polyethylene was melted, at a stretch ratio of 36 times (longitudinal direction × transverse direction = 6 × 6), and at a stretching speed of 2.0 m / min. The heat setting was carried out for 15 minutes at 119 ° C., which is a temperature at which 25% by weight of the crystalline portion of the film was melted.

[Example 11]
Components I and II and their contents were used in the same manner as in Example 3. Stretching was performed at 115 ° C., which is a temperature at which 30% by weight of the crystalline portion of polyethylene was melted, at a stretch ratio of 49 times (longitudinal direction × transverse direction = 7 × 7), and a stretching speed of 2.0 m / min. The heat setting was carried out for 20 minutes at 115 ° C., which is a temperature at which 10% by weight of the crystalline portion of the film was melted.

[Comparative Example 1]
A high density of component I having a weight average molecular weight of 1.8 × 10 ⁵ , a molecular weight of 10 ⁴ or less, 22.0% by weight, and 0.5% by weight of butene-1 as a comonomer Performed identically to Example 1 except that polyethylene was used. As in Example 1, the stretching temperature was 114.5 ° C. in order to match the ratio of the crystal part melted during stretching at 30% by weight, and the heat setting temperature was 20% by weight to the extent that the crystal melted as in Example 1. 119 ° C.

[Comparative Example 2]
A high density of component I having a weight average molecular weight of 2.1 × 10 ⁵ , a molecular weight of 10 ⁴ or less, and a content of 15.0% by weight and 1.5% by weight of butene-1 as a comonomer The same procedure as in Example 1 except that polyethylene was used. As in Example 1, the stretching temperature is adjusted to be 114 ° C. in order to match the ratio of the crystal part melted during stretching at 30% by weight, and the heat setting temperature is adjusted to 20% by weight to adjust the degree of crystal melting as in Example 1. Therefore, the temperature was set to 118 ° C.

[Comparative Example 3]
Component I has a weight average molecular weight of 5.7 × 10 ⁵ , a molecular weight of 10 ⁴ or less, a content of 9.0% by weight, and a comonomer of 0.8% by weight of butene-1 Performed identically to Example 1 except that polyethylene was used. As in Example 1, the stretching temperature was adjusted to 114.5 ° C. in order to match the proportion of the crystal melted during stretching at 30% by weight, and the heat setting temperature was set at 20% by weight to the extent that the crystal melted as in Example 1. It was 119 ° C. to match.

[Comparative Example 4]
Example 3 with the exception that the content of component I is 60% by weight, the content of component II is 40% by weight, and the stretching temperature is 120 ° C. where the proportion of the crystalline portion that dissolves during stretching is 30%. Performed identically.

[Comparative Example 5]
The procedure was the same as that of Comparative Example 4 except that the content of component I was 13% by weight and the content of component II was 87% by weight. The stretching temperature was 112 ° C. in order to match the proportion of the stretched crystal portion dissolved at 30% by weight.

[Comparative Example 6]
The stretching was performed in the same manner as in Example 3 except that the stretching temperature was 110 ° C., which is a temperature at which 5% by weight of polyethylene crystals were dissolved.

[Comparative Example 7]
The drawing was performed in the same manner as in Example 3 except that the drawing temperature was 125 ° C., which was a temperature at which 85% by weight of polyethylene crystals were dissolved.

[Comparative Example 8]
Weight average molecular weight of 5.1 × 10 ⁵ in component I, the content of molecular weight of 10 ⁴ or less molecules was 9.4% by weight, high density polyethylene containing no comonomer was used. The same operation as in Example 1 was performed except that the draw ratio was 16 times (longitudinal direction × transverse direction = 4 × 4).

[Comparative Example 9]
Weight average molecular weight of 5.1 × 10 ⁵ in component I, the content of molecular weight of 10 ⁴ or less molecules was 9.4% by weight, high density polyethylene containing no comonomer was used. The same procedure as in Example 1 was performed except that the stretch ratio was 56.25 times (longitudinal direction × transverse direction = 7.5 × 7.5).

[Comparative Example 10]
The heat setting was carried out in the same manner as in Example 3 except that the heat setting was carried out for 15 minutes from 127 ° C., which is a temperature at which 35% by weight of the crystalline portion of the film was dissolved. The experimental conditions of the examples and comparative examples and the results obtained therefrom are shown in Tables 1 to 4 below.

Claims (3)

20 to 50% by weight of high-density polyethylene (component I) having a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ⁵ having a molecular weight of 1 × 10 ⁴ or less and a content of molecules of 5% by weight or less;
A dilute solution (component II) 80 to 50% by weight,
The tensile strength is 1,100 kg / cm ² or more in the transverse direction and the longitudinal direction, the perforation strength is 0.22 N / μm or more, and the gas permeability (Darcy's permeability constant) is 1.3 × 10 ⁻⁵ Darcy or more. A high-density polyethylene microporous membrane having a shrinkage rate of 5% or less in each of the horizontal and vertical directions.   2. The high density polyethylene microporous structure according to claim 1, wherein the component I contains 2% by weight or less of a comonomer, and the comonomer is propylene, butene-1, hexene-1 or 4-methylpentene-1. film.   The high-density polyethylene microporous membrane according to claim 1, wherein the component II contains paraffin oil having a kinematic viscosity at 40 ° C of 20 to 200 cSt.