KR20050118061A - Microporous high density polyethylene film and preparing method thereof - Google Patents

Abstract

The present invention relates to a high-density polyethylene microporous membrane that can be used as a battery separator, and a method of manufacturing the same. 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 is composed of high density polyethylene having a molecular weight of 1 × 10 ⁴ or less and a content of 5% or less by weight, and has a tensile strength. 1,100kg / cm2 or more in transverse and longitudinal directions, puncture strength of 0.22N / µm or more, Darcy's permeability constant of 1.3 × 10 ^-5 Darcy or more, and shrinkage of 5% or less in longitudinal and transverse directions, respectively Has characteristics. In particular, the high-density polyethylene microporous membrane according to the present invention is excellent in extrusion kneading property and stretchability, and at the same time improve the performance and stability of the battery using the same, it is possible to increase the productivity of the microporous membrane.

Description

High density polyethylene microporous membrane and its manufacturing method {Microporous high density polyethylene film and preparing method

The present invention relates to a high density polyethylene microporous membrane and a method of manufacturing the same. More specifically, the present invention relates to a high-density polyethylene microporous membrane and a method for manufacturing the same, which have not only excellent extrusion kneading property and stretchability but also high productivity and can improve the performance and stability of a battery using the same.

Polyolefin microporous films are widely used as battery separators, separation filters, and microfiltration membranes due to their chemical stability and excellent physical properties.

There are three main methods for making a microporous membrane from polyolefins. The first method is to make a microporous membrane in the form of non woven fabric by making polyolefin into a thin fiber, and the second method is to make a thick polyolefin film and stretch it at low temperature to form a lamellar crystal part of the polyolefin. The dry method of forming micro voids by inducing micro cracks. The third method is to knead polyolefins with diluent at high temperature to form a single phase. It is a wet method that extracts and creates voids in polyolefin. The third wet method is thinner and more uniform in thickness than the other two methods, and it is widely used for separators of secondary batteries such as lithium ion batteries because of its thin film and excellent physical properties.

A method for producing a general porous film by a wet method is described in US Pat. No. 4,247,498, which uses a polyethylene and a compatible compatible liquid to blend these mixtures at high temperature to thermodynamic single phase solutions. After cooling, it is cooled and the polyethylene and the miscible solvent are separated from the solid / liquid or liquid / liquid phase in the cooling process, and a polyolefin porous membrane using the same is described.

U.S. Patent No. 4,335,193 discloses an organic liquid compound and an inorganic filler, such as dioctylphthalate and liquid paraffin, to the polyolefin, followed by processing to remove the organic liquid compound and the inorganic material. The technique for producing a polyolefin porous membrane is described, which is also described in US Pat. No. 5,641,565 and the like. However, this method has a disadvantage that it is difficult to add and kneading the inorganic material because the inorganic material, such as silica, is added, and the process for extracting and removing it in a later process is complicated and the drawing ratio is difficult to increase.

U.S. Patent No. 4,539,256 also describes a basic method of extruding, stretching and extracting a polyethylene and miscible liquid compound to produce a microporous film.

With full-scale use of secondary batteries, efforts have been made to improve the productivity and film properties of microporous membranes. A typical method is to increase the molecular weight of the composition by increasing the molecular weight of the composition by using or blending ultra high molecular weight polyolefin (UHMWPO) of about 1 million weight average molecular weight.

In this regard, US Pat. No. 4,588,633 and US Pat. No. 4,873,034 use polyolefins having a weight average molecular weight of more than 500,000 and a solvent capable of dissolving polyolefins at high temperature, and then, through a two-step solvent extraction process and an extension process, a microporous membrane is formed. The making process is introduced. However, in order to improve the kneading and extrudability with the solvent, which is a disadvantage of UHMWPO, this method requires the step of using an excess solvent in the extrusion process, extracting and stretching it in one step, and then extracting it again.

U.S. Patent No. 5,051,183 contains 10-50% by weight of polyolefin containing 1% or more of UHMWPO with a weight average molecular weight of 700,000 or more, and 90-50% by weight of solvents such as mineral oil. Polyolefin microporous membrane using the composition of (10) -300 is introduced. The method for forming the voids is to extrude the composition to form a gel-like sheet, draw at a temperature between the melting point of the composition and the melting point + 10 ° C, and then extract the solvent to form a porous membrane. However, this method blends UHMWPO and at the same time widens the molecular weight distribution and contains an excessive amount of polyolefin having a large molecular weight. This results in severe chain entanglements by these molecules, resulting in very poor stretchability. That is, breakage at a high draw ratio and a high draw speed or unstretched phenomenon at a low draw ratio occurs.

The solution to this problem is to increase the stretching temperature to make the composition soft during stretching, or to achieve the same effect as raising the temperature of the composition by slowing down the stretching speed. However, in this case, on the contrary, the orientation of the resin during the stretching decreases, which lowers the stretching effect, thereby lowering the physical properties of the final porous membrane. In addition, 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, thereby resulting in impact strength and puncture strength. strength) is lowered. This phenomenon is not an exception even in the microporous membrane film, and when the molecular weight distribution is wide, the puncture strength, which is one of the most important physical properties of the microporous membrane film, is not high enough. That is, the effect of the added UHMWPO for improving the physical properties will not appear sufficiently. This problem also exists in similar technologies, such as Japanese Patent Application Laid-Open No. 06-234876, Japanese Patent Application Laid-Open No. 06-212006, US Patent No. 5,786,396, and the like.

On the other hand, Japanese Patent Laid-Open No. 09-3228 uses a similar resin composition and introduces a method of compensating physical properties by balancing the draw ratio in the longitudinal direction (MD) and the transverse direction (TD).

In Japanese Patent Application Laid-Open No. 09-259858, the shutdown temperature of a polyethylene microporous membrane (when the temperature inside the battery rises due to abnormal operation of the battery, the porous membrane is melted to prevent accidents such as ignition and explosion. 70 to 99% by weight of polyethylene with a weight average molecular weight of 500,000 or more and 10 to 80% by weight of a resin composition of 1 to 30% by weight of low molecular weight polyethylene having a weight average molecular weight of 1,000 to 4,000 in order to reduce the A method of preparing a gel composition by preparing a solution of 20 to 90% by weight of a solvent, extruding and cooling in a die, and extracting the remaining solvent after stretching is introduced. This technique is characterized by lowering the shutdown temperature by using low molecular weight polyethylene having a weight average molecular weight of 1,000 to 4,000. However, there are two problems with this technique. In other words, the addition of low molecular weight molecules lowers the molecular weight, broadens the molecular weight distribution, and lowers the physical properties. Looking at the embodiment it can be seen that the tensile strength of the polyethylene porous membrane prepared by this technique is relatively low level of about 1,000 ~ 1,200kg / ㎠. In addition, the process of kneading polyolefin and diluent or solvent requires high technology. Commercially, twin screw extruders, kneaders and banbury mixers are used. As in the above-mentioned technique, when blending a resin having a large viscosity (ultra high molecular weight polyethylene having a weight average molecular weight of 500,000 or more and a low molecular weight polyethylene having a weight average molecular weight of 1,000 to 4,000) with a solvent, not only the kneading problem between the resin and the solvent, There is also a kneading problem between two resins having different molecular weights (largely different in melt viscosity). In this case, a fine gel or fish eye may be generated in the final film, thereby degrading the quality of the film. The solution to this problem is to increase the residence time of the melt in the extruder, but there is a disadvantage that the productivity is low.

U.S. Patent No. 5,830,554 discloses a resin having a weight average molecular weight of 500,000 to 2.5 million and a ratio of weight average molecular weight and number average molecular weight of 10 or less in order to solve the problem of deterioration in physical properties and elongation caused by resin having a wide molecular weight distribution. Extrusion-stretching-extraction of a solution containing 5 to 50% by weight of a polyolefin microporous membrane is shown. This method used a large amount of solvent (preferably 80 to 90% by weight) to solve the extrusion nonuniformity problem caused by the increase in viscosity caused by the extrusion of the ultra high molecular weight resin, thus increasing the porosity and stretching the porous film. The strength was also 800 kg / cm 2 or more (Examples 950-1,200 kg / cm 2), and the physical properties were not significantly improved.

US Pat. No. 6,566,012 also applies extrusion-molding of 10-40 wt% and 90-60 wt% of a resin composition comprising an ultra high molecular weight polyolefin having a weight average molecular weight of 500,000 or more or an ultra high molecular weight polyolefin having a weight average molecular weight of 500,000 or more. Disclosed is a method for producing a polyolefin microporous membrane suitable for battery separator by stretching-extracting-heat-fixing.

As described above, all of the prior arts used a resin having a large molecular weight to increase physical properties, but the increase in the molecular weight of the used resin additionally increased the extrusion load, decreased the extrusion kneading with the solvent, and increased the drawer load during stretching. , It causes problems such as unstretched generation, lowered productivity due to draw speed and draw ratio reduction.

Accordingly, the present inventors have conducted extensive research in order to solve the above-mentioned problems of the related art, and as a result, by controlling the content of the low molecular weight polyethylene molecules contained in the polyethylene below a certain content, it is possible to prevent the formation of defects in the polyethylene. The present invention has been completed based on the facts.

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

Another object of the present invention is to provide a method for producing the high density polyethylene microporous membrane with high productivity through an economic process.

Polyethylene microporous membrane according to the present invention for achieving the above object is a high-density polyethylene (component I) having a weight average molecular weight of 2 × 10 ^{5 ~} 5 × 10 ⁵ , the molecular weight of 1 × 10 ⁴ or less of 5 A) 20 to 50% by weight and diluent (component II) 80 to 50% by weight, the tensile strength in the transverse direction and longitudinal direction, respectively 1,100kg / ㎠ or more, puncture strength of 0.22N / 탆 or more The gas permeability (Darcy's permeability constant) is 1.3 × 10 ^-5 Darcy or more, the shrinkage is characterized in that less than 5% in the transverse and longitudinal direction, respectively.

Method for producing a polyethylene microporous membrane according to the present invention for achieving the above another object:

(a) 20-50% by weight of high density polyethylene (component I) and diluent (components having a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ⁵ and a molecular weight of 1 × 10 ⁴ or less; II) melt-extruding the composition consisting of 80 to 50% by weight to form a sheet;

(b) in the temperature range in which 30 to 80% by weight of the crystal part of the sheet is melted, the sheet is drawn to a film by stretching at least three times in the longitudinal and transverse directions, respectively, and having a total draw ratio of 25 to 50 times in the tenter type simultaneous drawing. Molding;

(c) extracting diluent from the film; And

(d) heat-setting the film at a temperature in which 10 to 30% by weight of the crystal part of the film is melted

Including;

Tensile strength in the transverse and longitudinal directions, respectively 1,100kg / ㎠ or more, puncture strength in 0.22N / ㎛ or more, Darcy's permeability constant of 1.3 × 10 ^-5 Darcy or more, shrinkage in the transverse and longitudinal directions, respectively It is characterized by being 5% or less.

Looking at the present invention in more detail as follows.

As described above, the present invention provides a high-density polyethylene microporous membrane having excellent extrusion kneading property and stretchability and a method for producing the same, which can be applied to a battery microporous membrane while improving the problems caused by molecular weight increase.

The basic theory of making polyethylene microporous membrane from polyethylene used in the present invention is as follows.

Low molecular weight organic materials (hereinafter referred to as diluents) having a molecular structure similar to polyethylene form a thermodynamic single phase with polyethylene at a high temperature at which polyethylene is melted. When these thermodynamic single phase polyethylene and diluent solutions are cooled to room temperature, phase separation of polyethylene and diluent occurs during the cooling process. Each phase separated is a polyethylene rich phase composed mainly of lamellae, a crystalline part of polyethylene, and a diluent multi-containing phase composed of a small amount of polyethylene and diluent dissolved in diluent even at room temperature. diluent rich phase). After cooling, diluent is extracted with organic solvent to make polyethylene porous membrane.

Therefore, the basic structure of the microporous membrane is determined during the phase separation process. That is, the size and structure of the diluent multi-containing phase made after phase separation determine the pore size and structure of the final microporous membrane. In addition, the basic physical properties of the microporous membrane are affected by the crystal structure of polyethylene produced during the extraction of the diluent.

The present inventors have found the following facts as a result of long research. In other words, in order to make a good microporous membrane, a small amount of polyethylene should exist as much as possible in the diluent multi-containing phase, and defects should not be made in the polyethylene during the extraction of the diluent. It is true that the low molecular weight polyethylene molecules included.

As a result of producing a product using polyethylene having a low molecular weight material based on this, it is possible to make a polyethylene microporous membrane having excellent physical properties and uniform pore structure even with a resin having a lower molecular weight than the conventional invention. 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 20 to 50 weight of high density polyethylene (component I) having a content of 5 wt% or less It is prepared by melting and extruding a composition consisting of 80% to 50% by weight of the composition of% and the diluent (component II) to form a molded article in a sheet form, drawing the film, extracting the diluent, and then 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, a puncture strength of 0.22 N / µm or more, and a Darcy's permeability constant of 1.3 × 10 ⁻⁵ Darcy in the transverse and longitudinal directions, respectively As described above, the shrinkage rate is 5% or less in the transverse direction and the longitudinal direction, respectively.

In general, polyethylene used commercially inevitably has a molecular weight distribution, and there are some molecules having a molecular weight of several thousand even in polyethylene having a weight average molecular weight of 1 × 10 ⁶ . These low molecular weight materials have been intentionally made during the production process of polyethylene because low molecular weight molecules play a role in improving the processability of resins having high molecular weight in commercial applications such as blow film and blow molding, which are common uses of polyethylene. However, in the process of making polyethylene microporous membranes, these low molecular weight materials reduce the completeness of lamellar, which is a polyethylene crystal part on the polyethylene, and reduce the number of tie molecules connecting the lamellars, thereby reducing the strength of the entire polyethylene. Is degraded. In addition, due to the high affinity with the diluent, it is present in the multiluent multi-containing phase and thus exists in the interface portion of the pore after extraction, which acts as a factor of reducing the porosity by making the interface of the pore incomplete. do. This phenomenon occurs in molecules with molecular weights of 1 × 10 ⁴ or less, largely when their content exceeds 5% by weight.

Polyolefin microporous membranes commonly used in the prior art include various polyethylenes (low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, etc.) and polypropylene. However, in the case of polyethylene and polypropylene except high density polyethylene, the structural regularity of the polymer is lowered, thereby lowering the lamellar completion of the resin crystalline portion and making the thickness thinner. In addition, when a comonomer is present during the polymerization reaction, a low molecular weight molecule is produced because the reactivity of the comonomer is lower than that of ethylene.

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.

In addition, the high density polyethylene is preferably a content of comonomer is 2% by weight or less. As the comonomer, alpha olefins such as propylene, butene-1, hexene-1, 4-methylpentene-1, and octene-1 may be used, but more preferably, propylene, butene-1, and hexene are relatively highly reactive. -1 or 4-methylpentene-1 is suitable.

On the other hand, it is preferable to use high molecular weight polyethylene having a high molecular weight for the final physical properties of the porous membrane. However, when the molecular weight is large, the load of the extruder is increased due to the viscosity increase during extrusion, and the kneading property decreases due to the large viscosity difference with the diluent. Is generated, and the surface shape of the sheet to be extruded becomes rough. The solution to this problem is to increase the extrusion temperature or make the shear configuration of the screw configuration of the twin screw compound higher, but in this case, deterioration of the resin occurs, thereby reducing physical properties.

In addition, the increase in the molecular weight of the resin may increase the stiffness of the sheet due to the increase of intermolecular entanglement and thus increase the load at the time of stretching may cause slippage in the clip fixing the sheet. In addition, unstretched may occur at low draw ratios due to the increased strength of the sheet. However, the increase in the draw ratio is difficult to greatly increase because the shrinkage rate of the produced microporous membrane increases and the load of the drawer clip also increases. The solution to this problem is to obtain the same effect as raising the temperature of the composition by increasing the stretching temperature to make the sheet soft during stretching or slowing the stretching speed. However, in this case, the orientation of the resin during the stretching decreases, resulting in a problem that the stretching effect is lowered and the physical properties of the final porous membrane are lowered. Reducing the drawing speed is also undesirable because it also lowers the productivity.

Thus, in the present invention, when the low molecular weight portion of the high density polyethylene is sufficiently low, that is, when the high molecular weight polyethylene having a molecular weight of 1 × 10 ⁴ or less is 5% by weight or less, a weight average molecular weight of 5 × 10 ⁵ or less may be used for a battery separator or the like. It has been found that it can have sufficient physical properties. Here, since the weight average molecular weight of the high density polyethylene may not have sufficient physical properties at less than 2 × 10 ⁵ , the high density polyethylene used in the present invention is selected to have a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ⁵ . The high-density polyethylene having such a molecular weight and molecular weight distribution has excellent stretchability and excellent pore structure even at low magnification stretching, and can be easily stretched due to low load of the drawing clip, thereby increasing productivity.

The diluent used in the present invention may be any organic liquid that forms a single phase with the resin at the extrusion processing temperature. Examples include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin, and paraffin oils, dibutyl phthalate, and dioctyl phthalate ( phthalic acid esters such as dioctyl phthalate. Preferably, paraffin oils that are harmless to the human body, have high boiling points and low volatile components, and more preferably, paraffin oils having a kinetic viscosity at 40 ° C. of 20cSt to 200cSt. This is suitable. If the kinematic viscosity of paraffin oil exceeds 200 cSt, the kinematic viscosity in the extrusion process may be high, causing problems such as load rise and surface defects of sheets and films.In the extraction process, extraction becomes difficult, resulting in poor productivity and reduced permeability due to residual oil. Problems may occur. If the kinematic viscosity of paraffin oil is less than 20 cSt, kneading becomes difficult during extrusion due to the difference in viscosity with polyethylene melt in the extruder.

The composition of the high density polyethylene and diluent used in the present invention is preferably 20 to 50% by weight of high density polyethylene and 80 to 50% by weight of diluent. When the content of the high density polyethylene exceeds 50% by weight (ie, when the diluent is less than 50% by weight), the porosity decreases, the pore size becomes small, and the interconnection between the pores decreases so that the permeability is greatly reduced. On the other hand, if the content of the high-density polyethylene is less than 20% by weight (that is, when the diluent exceeds 80% by weight), the kneading property of the polyethylene and the diluent is lowered so that the polyethylene is not thermodynamically kneaded with the diluent. It can be extruded in the form of the film, causing problems such as fracture and thickness unevenness in stretching.

If necessary, general additives such as an oxidative stabilizer, a UV stabilizer, an antistatic agent, and the like may be further added.

The composition is melt-extruded using a biaxial compounder, kneader or short-barrier mixer designed for kneading the diluent and polyethylene to form a sheet-like molding. Polyethylene and oil may be pre-blended and introduced into the compounder, or from separate feeders, respectively. As a method of forming a sheet-shaped molding from the melt, both a general casting or calendering method may be used.

Next, the stretching process is preferably performed by simultaneous stretching of a tenter type. Roll type drawing may create defects such as scratches on the surface of the film during the drawing process. Here, the draw ratio is three times or more in the longitudinal direction and the transverse direction, respectively, and the total draw ratio is preferably 25 to 50 times. When the draw ratio in one direction is less than three times, the orientation in one direction is not sufficient, and at the same time, the balance of physical properties between the longitudinal direction and the transverse direction is broken, and the tensile strength and puncture strength are lowered. In addition, if the total draw ratio is less than 25 times unstretched, if more than 50 times the possibility of breaking during stretching is high, there is a disadvantage that the shrinkage of the final film is increased.

At this time, the stretching temperature depends on the melting point of the polyethylene used and the concentration and type of the diluent. The optimal stretching temperature is suitably selected in the temperature range in which 30 to 80% by weight of the crystalline portion of polyethylene in the film molding melts. If the stretching temperature is selected in the temperature range lower than the melting temperature of 30% by weight of the crystalline portion of the polyethylene in the film molding, there is no softness of the film, the stretchability is worsened, the breakage at the time of stretching is likely to occur and at the same time unstretched Occurs. On the other hand, when the stretching temperature is selected in the temperature range higher than the melting temperature of 80% by weight of the crystal portion, the stretching is easy and less unstretched, but the thickness deviation occurs due to partial overstretching, the orientation effect of the resin is less This will greatly fall. On the other hand, the degree of melting of the crystal portion with temperature can be obtained from differential scanning calorimeter (DSC) analysis of the film molding.

The stretched film is extracted and dried using an organic solvent. The organic solvent usable in the present invention is not particularly limited, but any solvent capable of extracting the diluent used in resin extrusion may be used, but preferably methyl ethyl ketone, methylene chloride, which has high extraction efficiency and rapid drying, Hexane and the like are suitable. As the extraction method, all general solvent extraction methods, such as an immersion method, a solvent spray method, and an ultrasonic method, may be used individually or in combination. The content of residual diluent should be less than 1% by weight during extraction. If the residual diluent exceeds 1% by weight, the physical properties are lowered and the transmittance of the film is reduced.

The amount of residual diluent (extraction rate) depends greatly on the extraction temperature and extraction time. The extraction temperature is preferably high in order to increase the solubility of the diluent and the solvent. However, the extraction temperature is preferably 40 ° C. or lower in consideration of safety problems due to the boiling of the solvent. If the extraction temperature is below the freezing point of the diluent, the extraction efficiency is greatly reduced, so it must be higher than the freezing point of the diluent. Extraction time depends on the thickness of the film produced, but 2 to 4 minutes is suitable when producing a general microporous membrane of 10 to 30㎛ thickness.

The dried film is finally subjected to a heat setting step to remove residual stresses to reduce the shrinkage of the final film. The heat setting is to fix the film and apply heat, to force the film to shrink and to remove the residual stress. The higher the heat setting temperature is advantageous to lower the shrinkage rate, but if it is too high, the permeability of the film is partially blocked due to partial melting of the film, thereby decreasing the permeability. The preferred heat setting temperature is preferably selected in the temperature range in which 10 to 30% by weight of the crystal part of the film is melted. If the heat setting temperature is selected in the temperature range lower than the melting temperature of 10% by weight of the crystal part of the film, the reorientation of polyethylene molecules in the film is insufficient, there is no effect of removing the residual stress of the film, the crystal part of the film If 30% by weight of is selected in the temperature range higher than the melting temperature, the microporous is blocked by partial melting, the permeability is reduced.

Here, the heat setting time may be relatively short when the heat setting temperature is high, and may be relatively long when the heat setting temperature is low. Preferably about 1 to 20 minutes are suitable. Most preferably, 5 minutes to 20 minutes in the temperature range where 10 to 20% by weight of the crystalline portion of the film is melted, and 1 to 5 minutes in the temperature range where 20 to 30% by weight is melted are suitable.

The high density polyethylene microporous membrane of the present invention prepared according to the above has the following physical properties.

(1) The tensile strength is at least 1,100 kg / cm 2 in the transverse and longitudinal directions, respectively.

If the tensile strength is less than 1,100kg / ㎠ may break the porous membrane during use of the microporous membrane. In particular, when used as a battery separator, breakage of the porous membrane occurs due to tension applied during a high-speed battery assembly process. Polyethylene microporous membrane according to the present invention can minimize the breakage in the assembly process with a tensile strength of 1,100kg / ㎠ or more.

(2) The puncture strength is 0.22 N / µm or more.

The puncture strength indicates the strength of the film against the pointed material. When used as a battery separator, if the puncture strength is not sufficient, the film is torn due to abnormality on the surface of the electrode or dendrite generated on the electrode surface during battery use. (short) may occur. Commercially used battery separators have a safety issue due to short circuiting when their breaking point weight is less than 350 g. The film having a puncture strength of more than 0.22 N / μm according to the present invention has a breaking point weight of 350 g or more when the film is used with the thinnest 16 μm thickness among the separator films currently widely used, and can be safely used for all applications. .

(3) The Darcy's permeability constant is 1.3 × 10 ⁻⁵ Darcy or higher.

If the gas permeability is 1.3 × 10 ⁻⁵ Darcy or less, the efficiency as the 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 become worse and the service life becomes short. The film having a gas permeability of 1.3 × 10 ⁻⁵ Darcy or more according to the present invention has excellent charge / discharge characteristics, low temperature characteristics, and long-life characteristics such as high rate charge / discharge of batteries.

(4) Shrinkage is 5% or less in the transverse and longitudinal directions, respectively.

Shrinkage is measured after leaving the film at 105 ° C. for 10 minutes. If the shrinkage rate is high, it cannot be used for high temperature applications. In particular, in the case of a battery separator, when the shrinkage exceeds 5%, the separator may be contracted by the self-heating of the battery, and the electrodes may come into contact with each other, which may cause a short circuit. The film according to the present invention can be safely used as a battery separator with shrinkage of 5% or less in the transverse and longitudinal directions, respectively.

In addition to these physical properties, the high density polyethylene microporous membrane of the present invention is excellent in extrusion kneading property and stretchability.

Hereinafter, the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited thereto.

EXAMPLE

The molecular weight and molecular weight distribution of polyethylene were measured by high temperature gel permeation chromatography (GPC) by Polymer Laboratory.

The viscosity of the diluent was measured with a CAV-4 Automatic Viscometer from Cannon.

Polyethylene and diluent were kneaded in a biaxial compounder having a diameter of 30 mm. The extrusion temperature was 160-240 ° C. and the residence time was 3 minutes. The extruded melt was extruded from a T-shaped die and formed into a sheet of 600 to 1,200 mu m thick by a casting roll, which was used for stretching. In order to know the presence of gel due to poor melting and kneading, a film having a thickness of 200 μm was separately prepared 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 should be 50 or less.

The molded sheet was subjected to DSC analysis to analyze the melting phenomenon of the crystal part with temperature. Analytical conditions were 5 mg of sample weight and 10 ° C./min scanning rate.

The stretching of the sheet was carried out simultaneously by changing the stretching ratio, stretching temperature, and stretching speed in a tenter-type lab stretching machine, and the stretching temperature was 30 to 80 in the polyethylene crystal part in the film molding based on the DSC result. The weight percent melting was determined in the temperature range. Five sheets were stretched to measure the elongation success rate at which the sliding and elongation at the clip did not occur.

Extraction of the diluent was carried out in an immersion manner using methylene chloride.

The heat setting was carried out by drying the film from which the diluent was extracted in air, then fixing the film to the frame and changing the temperature and time in a convection oven.

The prepared film measured tensile strength, puncture strength, gas permeability and shrinkage, which are the most important physical properties in the microporous membrane, and the results are shown in Table 1 below.

※ How to measure properties

(1) Tensile strength was measured by ASTM D882.

(2) The puncture strength was measured by the strength when a 0.5 mm diameter pin breaks the film at a speed of 120 mm / min.

(3) Gas permeability was measured from a porometer (CFP-1500-AEL manufactured by PMI). In general, the gas permeability is expressed as the Gurley number, but the Gurley number is difficult to know the relative permeability according to the pore structure of the film itself because the influence of the film thickness is not corrected. In order to solve this problem, Darcy's permeability constant was used in the present invention. Darcy's permeability constant is obtained from Equation 1 below and nitrogen is used in the present invention.

C = (8 FTV) / (πD ² (P ² -1))

Where C = Darcy Permeability Constant

       F = flow rate

       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 in% in the longitudinal and transverse shrinkage after leaving the film at 105 ℃ for 10 minutes.

Example 1

As component I, a high density polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ , a molecular weight of 10 ⁴ or less, and a molecular weight of 4.2 wt% and no comonomer was used. As a component II, paraffin oil having a kinematic viscosity of 40 ° C. and 95 cST Was used. The contents of components I and II were 30% by weight and 70% by weight, respectively.

The stretching was performed at 115 ° C., at which the crystal part of polyethylene melted at 30% by weight, with a draw ratio of 36 times (longitudinal x transverse direction = 6 x 6) and a drawing speed of 2.0 m / min. The diluent-extracted film was fixed in a frame after drying in air, and then heat-fixed at 120 ° C. for 15 minutes at a temperature at which 20% by weight of the crystalline portion of the film was melted. The thickness of the obtained final film was 20 ± 2 μm.

Example 2

Except for the use of high density polyethylene having a weight average molecular weight of 4.0 × 10 ⁵ as a component I, a molecular weight of 10 ⁴ or less, and a content of 3.9% by weight and 0.5% by weight of butene-1 as a comonomer. It carried out similarly to 1. As in Example 1, the stretching temperature was set to 114.5 ° C. in order to adjust the melting rate of the crystal part to 30 wt%. The heat setting temperature was also set to 119 ° C in order to adjust the degree of melting of the crystals to 20% by weight as in Example 1.

Example 3

As the component I, a high density polyethylene having a weight average molecular weight of 4.7 × 10 ⁵ , a molecular weight of 10 ⁴ or less, and 1.2% by weight of molecular weight and no comonomers were used. It carried out similarly to Example 1 except having performed at phosphorus 117 degreeC.

Example 4

As the component I, a high-density polyethylene having a weight average molecular weight of 3.5 × 10 ⁵ , a molecular weight of 10 ⁴ or less, 4.5% by weight of molecular weight, and 1.5% by weight of butene-1 as a comonomer was used. Paraffin oil with a 70 cSt was used. The contents of component I and component II were 40% by weight and 60% by weight, respectively. Except this, it was carried out in the same manner as in Example 1. As in Example 1, the stretching temperature was set to 116 ° C. in order to adjust the melting rate of the crystal part to 30% by weight, and the heat setting temperature was also set to 118 ° C. in order to adjust the melting degree of the crystal to 20% by weight as in Example 1. It was.

Example 5

As the component I, a high-density polyethylene having a weight average molecular weight of 4.7 × 10 ⁵ , a molecular weight of 10 ⁴ or less, 1.2% by weight, and no comonomers was used, and the component II had a kinematic viscosity of 40 ° C. Paraffin oil with a 120 cSt was used. The contents of component I and component II were 40% by weight and 60% by weight, respectively. The stretching was performed at 117 ° C., at which the crystal part of polyethylene melted at 30% by weight, with a draw ratio of 36 times (longitudinal x transverse direction = 6 x 6) and a drawing speed of 2.0 m / min. The heat setting was heat set at 115 ° C. for 15 minutes at which the crystalline portion of the film melted 10%.

Example 6

As the component I, a high-density polyethylene having a weight average molecular weight of 4.7 × 10 ⁵ , a molecular weight of 10 ⁴ or less, 1.2% by weight, and no comonomers was used, and the component II had a kinematic viscosity of 40 ° C. Paraffin oil with a 30 cSt was used. The contents of components I and II were 20% by weight and 80% by weight, respectively. The stretching was performed at 113 ° C., at which the crystalline portion of polyethylene melted at 30% by weight, with a draw ratio of 49 times (longitudinal x transverse direction = 7 x 7) and a drawing speed of 2.0 m / min. The heat setting was heat setting for 5 minutes at 120 ° C., at which the crystalline portion of the film melts 20% by weight.

Example 7

Stretching temperature was 122 degreeC which is the temperature which 60 weight% of polyethylene crystals melt | dissolved, and it carried out similarly to Example 3 except having an extending | stretching ratio 25 times (longitudinal x transverse direction = 5x5).

Example 8

As the component I, high density polyethylene having a weight average molecular weight of 3.5 × 10 ⁵ , a molecular weight of 10 ⁴ or less, 4.5% by weight of molecular weight and 1.5% by weight of butene-1 as a comonomer was used. As the component II, paraffin oil having a 40 ° C kinematic viscosity of 160 cSt was used. The contents of components I and II were 30% by weight and 70% by weight, respectively. Stretching was carried out at a stretching ratio of 25 times (longitudinal x transverse direction = 5 x 5) and a stretching speed of 2.0 m / min at 120 ° C, at which the crystal part of the polyethylene was melted at 50% by weight. The heat setting was heat set for 15 minutes at 118 ° C., at which the crystalline portion of the film melts 20 wt%.

Example 9

Component I and Component II were used in the same manner 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., at which the crystal part of polyethylene melted at 30% by weight, with a draw ratio of 36 times (longitudinal x transverse direction = 6x6) and a drawing speed of 10.0 m / min. The heat setting was heat setting for 5 minutes at 123 ° C., at which the crystalline portion of the film melted 30% by weight.

Example 10

As the component I, high density polyethylene having a weight average molecular weight of 3.5 × 10 ⁵ , a molecular weight of 10 ⁴ or less, 4.5% by weight of molecular weight and 1.5% by weight of butene-1 as a comonomer was used. As the component II, paraffin oil having a 40 ° C kinematic viscosity of 95 cSt was used. The contents of components I and II were 20% by weight and 80% by weight, respectively. The stretching was performed at 112 ° C., at which the crystalline portion of polyethylene melted at 30% by weight, with a draw ratio of 36 times (longitudinal x transverse direction = 6 x 6) and a drawing speed of 2.0 m / min. The heat setting was heat setting for 15 minutes at 119 ° C., at which the crystal part of the film melted 25% by weight.

Example 11

Components I and II and their contents were used in the same manner as in Example 3. The stretching was performed at 115 ° C., at which the crystal part of polyethylene melted at 30% by weight, with a draw ratio of 49 times (vertical direction × transverse direction = 7 × 7) and a drawing speed of 2.0 m / min. The heat setting was heat set at 115 ° C. for 20 minutes, at which the crystalline portion of the film melted 10% by weight.

Comparative Example 1

Except for the use of high density polyethylene having a weight average molecular weight of 1.8 × 10 ⁵ as a component I, a molecular weight of 10 ⁴ or less, 22.0% by weight, and 0.5% by weight of butene-1 as a comonomer. It carried out similarly to Example 1. As in Example 1, the stretching temperature was set to 114.5 ° C. in order to adjust the melting rate of the crystal parts at 30 wt%, and the heat setting temperature was set to 119 ° C. in order to adjust the melting degree of the crystal to 20 wt% as in Example 1. .

Comparative Example 2

Example 1 except that high density polyethylene having a weight average molecular weight of 2.1 × 10 ⁵ and a molecular weight of 10 ⁴ or less as a component I and a content of 15.0 wt% and 1.5 wt% of butene-1 as a comonomer were used. The same procedure was followed. As in Example 1, the stretching temperature was set to 114 ° C in order to adjust the melting rate of the crystal part to 30% by weight, and the heat setting temperature was also set to 118 ° C in order to adjust the melting degree of the crystal to 20% by weight as in Example 1. .

Comparative Example 3

Example 1, except that a high-density polyethylene having a weight average molecular weight of 5.7 × 10 ⁵ and a molecular weight of 10 ⁴ or less and 9.0% by weight of component I and 0.8% by weight of butene-1 as a comonomer was used. The same procedure was followed. As in Example 1, the stretching temperature was set to 114.5 ° C. in order to adjust the melting rate of the crystal part to 30% by weight, and the heat setting temperature was set to 119 ° C. to adjust the melting degree of the crystal to 20% by weight as in Example 1. .

[Comparative Example 4]

The content was carried out in the same manner as in Example 3, except that the content of component I was 60% by weight, the content of component II was 40% by weight, and the stretching temperature was 120 ° C. at which the crystal part melted at 30% by weight.

[Comparative Example 5]

It carried out similarly to the comparative example 4 except the content of component I being 13 weight%, and the content of component II being 87 weight%. The stretching temperature was 112 ° C. in order to adjust the ratio of the crystal parts melted at the time of stretching to 30% by weight.

Comparative Example 6

The drawing was carried out in the same manner as in Example 3 except that the drawing temperature was 110 ° C., at which the polyethylene crystals melt at 5% by weight.

Comparative Example 7

The stretching temperature was the same as in Example 3 except that the stretching temperature was 125 ° C, at which the crystals of polyethylene melted by 85% by weight.

Comparative Example 8

As component I, high molecular weight polyethylene with a weight average molecular weight of 5.1 × 10 ⁵ and a molecular weight of 10 ⁴ or less was 9.4% by weight, high density polyethylene without comonomer was used, and the draw ratio was 16 times (longitudinal x transverse = 4 x 4). Except that was performed in the same manner as in Example 1.

Comparative Example 9

As component I, a high molecular weight polyethylene having a weight average molecular weight of 5.1 × 10 ⁵ and a molecular weight of 10 ⁴ or less was 9.4% by weight, high density polyethylene without comonomer was used, and the draw ratio was 56.25 times (longitudinal x transverse direction = 7.5 × 7.5 Except that was performed in the same manner as in Example 1.

Comparative Example 10

The heat setting was carried out in the same manner as in Example 3 except that the crystallization of the film was carried out at 127 ° C. for 15 minutes at a temperature of 35% by weight.

Experimental conditions of the Examples and Comparative Examples and the results obtained therefrom are summarized in Tables 1 to 4 below.

Example Article Conditions unit One 2 3 4 5 6 High Density Polyethylene (Component I) Mw g / mol 3.0 × 10 ⁵ 4.0 × 10 ⁵ 4.7 × 10 ⁵ 3.5 × 10 ⁵ 4.7 × 10 ⁵ 4.7 × 10 ⁵ Mw≤10 ⁴ weight% 4.2 3.9 1.2 4.5 1.2 1.2 Comonomer weight% 0.0 0.5 0.0 1.5 0.0 0.0 content weight% 30 30 30 40 40 20 Paraffin Oil (Component II) Viscosity (40 ℃) cSt 95 95 95 70 120 30 content weight% 70 70 70 60 60 80 Stretch Temperature ℃ 115 114.5 117 116 117 113 Crystal recording weight% 30 30 40 30 30 30 Ratio (MD × TD) ratio 6 × 6 6 × 6 6 × 6 6 × 6 6 × 6 7 × 7 speed m / min 2.0 2.0 2.0 2.0 2.0 2.0 Heat setting Temperature ℃ 120 119 120 118 115 120 Crystal recording weight% 20 20 20 20 10 20 time min 15 15 15 15 15 5 Number of gels in the sheet # / 2000㎠ 5 15 17 14 12 25 Elongation success rate % 100 100 100 100 100 100 The tensile strength MD kg / ㎠ 1340 1450 1400 1520 1630 1550 TD kg / ㎠ 1200 1300 1350 1420 1520 1470 Film thickness Μm 22 20 20 18 20 19 Punching strength N / μm 0.22 0.24 0.28 0.26 0.29 0.27 Gas permeability Darcy (× 10 ^-5 ) 1.7 1.7 1.8 1.6 1.6 1.9 Shrinkage MD % 3.1 3.0 3.1 3.3 4.5 4.5 TD 1.3 1.6 2.3 2.9 3.5 3.9

Example Manufacture conditions unit 7 8 9 10 11 High Density Polyethylene (Component I) Mw g / mol 4.7 × 10 ⁵ 3.5 × 10 ⁵ 4.7 × 10 ⁵ 3.5 × 10 ⁵ 4.7 × 10 ⁵ Mw≤10 ⁴ weight% 1.2 4.5 1.2 4.5 1.2 Comonomer weight% 0.0 1.5 0.0 1.5 0.0 content weight% 30 30 50 20 30 Paraffin Oil (Component II) Viscosity (40 ℃) cSt 95 160 95 95 95 content weight% 70 70 50 80 70 Stretch Temperature ℃ 122 120 119 112 115 Crystal recording weight% 60 50 30 30 30 Ratio (MD × TD) ratio 5 × 5 5 × 5 6 × 6 6 × 6 7 × 7 speed m / min 2.0 2.0 10.0 2.0 2.0 Heat setting Temperature ℃ 120 118 123 119 115 Crystal recording weight% 20 20 30 25 10 time min 15 15 5 15 20 Number of gels in the sheet # / 2000㎠ 17 14 15 27 17 Elongation success rate % 100 100 100 100 100 The tensile strength MD kg / ㎠ 1250 1170 1750 1420 1650 TD kg / ㎠ 1180 1120 1570 1320 1580 Film thickness Μm 18 20 21 20 18 Punching strength N / μm 0.23 0.22 0.29 0.24 0.28 Gas permeability Darcy (× 10 ^-5 ) 1.9 1.9 1.5 1.8 1.7 Shrinkage MD % 1.6 1.8 4.0 3.2 4.6 TD 1.2 1.2 3.6 2.0 3.6

Comparative example Manufacture conditions unit One 2 3 4 5 High Density Polyethylene (Component I) Mw g / mol 1.8 × 10 ⁵ 2.1 × 10 ⁵ 5.7 × 10 ⁵ 4.7 × 10 ⁵ 4.7 × 10 ⁵ Mw≤10 ⁴ weight% 22.0 15.0 9.0 1.2 1.2 Comonomer weight% 0.5 1.5 0.8 0.0 0.0 content weight% 30 30 30 60 13 Paraffin Oil (Component II) Viscosity (40 ℃) cSt 95 95 95 95 95 content weight% 70 70 70 40 87 Stretch Temperature ℃ 114.5 114 114.5 120 112 Crystal recording weight% 30 30 30 30 30 Ratio (MD × TD) ratio 6 × 6 6 × 6 6 × 6 6 × 6 6 × 6 speed m / min 2.0 2.0 2.0 2.0 2.0 Heat setting Temperature ℃ 119 118 119 120 120 Crystal recording weight% 20 20 20 20 20 time min 15 15 15 15 15 Number of gels in the sheet # / 2000㎠ 15 12 75 15 150 Elongation success rate % 100 100 60 40 100 The tensile strength MD kg / ㎠ 980 950 1480 1770 1070 TD kg / ㎠ 800 850 1350 1650 830 Film thickness Μm 20 20 19 20 19 Punching strength N / μm 0.15 0.16 0.27 0.27 0.16 Gas permeability Darcy (× 10 ^-5 ) 0.8 0.9 0.8 0.2 2.0 Shrinkage MD % 2.8 2.4 4.7 7.5 3.3 TD 2.2 1.6 3.3 5.5 2.7

Comparative example Manufacture conditions unit 6 7 8 9 10 High Density Polyethylene (Component I) Mw g / mol 4.7 × 10 ⁵ 4.7 × 10 ⁵ 5.1 × 10 ⁵ 5.1 × 10 ⁵ 4.7 × 10 ⁵ Mw≤10 ⁴ weight% 1.2 1.2 9.4 9.4 1.2 Comonomer weight% 0.0 0.0 0.0 0.0 0.0 content weight% 30 30 30 30 30 Paraffin Oil (Component II) Viscosity (40 ℃) cSt 95 95 95 95 95 content weight% 70 70 70 70 70 Stretch Temperature ℃ 110 125 115 115 117 Crystal recording weight% 5 85 30 30 40 Ratio (MD × TD) ratio 6 × 6 6 × 6 4 × 4 7.5 × 7.5 6 × 6 speed m / min 2.0 2.0 2.0 2.0 2.0 Heat setting Temperature ℃ 120 120 120 120 127 Crystal recording weight% 20 20 20 20 35 time min 15 15 15 15 15 Number of gels in the sheet # / 2000㎠ 17 17 45 45 17 Elongation success rate % 20 20 100 60 100 The tensile strength MD kg / ㎠ 1730 700 1040 1570 1620 TD kg / ㎠ 1570 650 890 1320 1420 Film thickness Μm 19 20 21 22 18 Punching strength N / μm 0.26 0.12 0.14 0.26 0.23 Gas permeability Darcy (× 10 ^-5 ) 0.4 1.7 0.9 0.9 0.2 Shrinkage MD % 6.6 1.6 2.6 6.0 0.6 TD 4.8 0.4 1.8 5.6 0.4

As shown in Tables 1 to 4, the high-density polyethylene microporous membrane prepared according to the method of the present invention can be easily extruded and stretched to increase productivity by producing a stable product, and the produced product has high gas permeability and high tensile strength. Excellent strength and puncture strength and low shrinkage rate can be usefully used in battery separators and various filters.

All simple modifications and variations of the present invention fall within the scope of the present invention, and the specific scope of the present invention will be apparent from the appended claims.

Claims (8)

20 to 50% by weight of high density polyethylene (component I) and diluent (component II) having a weight average molecular weight of 2 x 10 ^{5 to} 5 x 10 ⁵ and a molecular weight of 1 x 10 ⁴ or less Prepared from a composition consisting of ˜50% by weight, the tensile strength in the transverse direction and longitudinal direction, respectively, 1,100kg / ㎠ or more, the puncture strength is 0.22N / ㎛ or more, Darcy's permeability constant is 1.3 × 10 ^-5 Darcy Above, the high-density polyethylene microporous membrane, characterized in that the shrinkage is 5% or less in the transverse and longitudinal directions, respectively. The high density polyethylene fine according to claim 1, wherein the component I contains up to 2% by weight of comonomer, and the comonomer is propylene, butene-1, hexene-1 or 4-methylpentene-1. Sclera. The high-density polyethylene microporous membrane according to claim 1, wherein the component II is a paraffin oil having a 40 ° C kinematic viscosity of 20 to 200 cSt. (a) 20-50% by weight of high density polyethylene (component I) and diluent (components having a weight average molecular weight of 2 × 10 ^{5 to} 5 × 10 ⁵ and a molecular weight of 1 × 10 ⁴ or less; II) melt-extruding the composition consisting of 80 to 50% by weight to form a sheet; (b) in the temperature range in which 30 to 80% by weight of the crystal part of the sheet is melted, the sheet is drawn to a film by stretching at least three times in the longitudinal and transverse directions, respectively, and having a total draw ratio of 25 to 50 times in the tenter type simultaneous drawing. Molding; (c) extracting diluent from the film; And (d) heat-setting the film at a temperature in which 10 to 30% by weight of the crystal part of the film is melted Including; Tensile strength in the transverse and longitudinal directions, respectively 1,100kg / ㎠ or more, puncture strength in 0.22N / ㎛ or more, Darcy's permeability constant of 1.3 × 10 ^-5 Darcy or more, shrinkage in the transverse and longitudinal directions, respectively A method for producing a high density polyethylene microporous membrane, characterized in that less than 5%. 5. The dense polyethylene fine as claimed in claim 4, wherein the component I contains up to 2% by weight of comonomer, and the comonomer is propylene, butene-1, hexene-1 or 4-methylpentene-1. Method of preparation of sclera. The method for producing a high density polyethylene microporous membrane according to claim 4, wherein the component II is a paraffin oil having a 40 ° C kinematic viscosity of 20 to 200 cSt. The method of claim 4, wherein the diluent content remaining in the film during the extraction of the step (c) is 1 wt% or less. The method of claim 4, wherein the heat setting time of the step (d) is 1 to 20 minutes.