KR101964055B1 - A porous separator and a method for manufacturing the same - Google Patents

Abstract

An embodiment of the present invention is a multilayer structure in which a first layer of a first polyethylene mixture and a second layer of a second polyethylene mixture are alternately laminated, wherein the composition of the first and second polyethylene blend differ, Wherein the multi-layer structure comprises 4 to 100 layers, and a method for producing the same.

Description

TECHNICAL FIELD [0001] The present invention relates to a porous separator and a method of manufacturing the porous separator.

The present invention relates to a porous separator and a method for producing the same, and more particularly, to a porous separator having improved mechanical properties and high-temperature dimensional stability by alternately stacking two types of polymer bases having different compositions will be.

Lithium secondary batteries are widely used as power sources for various electric appliances that require miniaturization and light weight such as smart phones, notebooks, and tablet PCs. As the application fields of smart grid, electric vehicles, Development of a lithium secondary battery which is large, long in life, and high in stability is required.

Particularly, research and development have been actively conducted on separators in which micropores are formed to prevent internal short-circuiting by separating the positive and negative electrodes and to facilitate the movement of lithium ions during charging and discharging.

Korean Patent No. 0739337 and No. 0754746 disclose that a porous support is coated with a slurry composed of an inorganic material and a polymer binder to improve the compressive strength and stability of high-temperature dimensions of the separator. In this case, however, There is a problem that the manufacturing cost of the separator increases due to the coating process, the polar organic solvent contained in the slurry may cause environmental pollution or harm the health of the operator, and the toughness of the separator is small. Toughness refers to the strain energy per unit volume and is known as one of the properties related to the stability and durability of the battery. Specifically, when an impact is applied to the battery from the outside, if the toughness of the separator in the battery is small, the positive electrode and the negative electrode are exposed and an internal short circuit can easily occur.

On the other hand, in order to increase the productivity and yield of the membrane, it has been widely produced. However, as the width increases, the transverse direction (TD) may cause deflection. This sagging phenomenon is analyzed to be caused by the minute deformation of the polymer structure accumulated in the high temperature extrusion and drawing process for the membrane production. To solve this problem, a separate blocking layer made of a filler is formed in the separation membrane, or inorganic particles are introduced And the like. However, even in this case, the filler may be desorbed from the battery, the tensile strength and tensile elongation of the separator may be lowered, and the inorganic particles may promote internal short circuit.

Disclosure of Invention Technical Problem [8] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a porous separator excellent in physical properties such as toughness, tensile strength and puncture strength and high-

One aspect of the present invention is a multilayer structure comprising a multilayer structure in which a first layer of a first polyethylene mixture and a second layer of a second polyethylene mixture are alternately laminated and wherein the composition of the first and second polyethylene blend differ, Wherein the multi-layer structure comprises 4 to 100 layers.

In one embodiment, the first and second polyethylene blends may each comprise ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000 and high density polyethylene having a weight average molecular weight of 300,000 to 500,000.

In one embodiment, the content of the ultra high molecular weight polyethylene in the first polyethylene mixture may be from 60 to 95% by weight.

In one embodiment, the content of the high density polyethylene in the second polyethylene mixture may be at least 90% by weight.

In one embodiment, the porosity of the porous separator may be 30 to 50% by volume.

In one embodiment, the average pore size of the porous separator may be 20 to 50 nm.

In one embodiment, the porous separator may have a thickness of 3 to 20 탆.

In one embodiment, the toughness of the porous separator may be greater than 0.9 N / mm.

In one embodiment, the tensile strength of the porous separator may be 1,700 to 3,000 Kgf / cm ² .

In another aspect of the present invention, there is also provided a process for producing a polyurethane foam, comprising: (a) co-extruding a first and a second polyethylene mixture comprising two or more types of polyethylene and a pore-forming agent to form a bilayer structure; (b) co-extruding the bilayer structure 1 to 50 times to form a multilayer structure comprising 4 to 100 layers; (c) cooling the multilayer structure to produce a base sheet; And (d) stretching the base sheet and dipping the base sheet in a solvent to remove the pore-forming agent.

In one embodiment, the polyethylene may comprise ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000 and high density polyethylene having a weight average molecular weight of 300,000 to 500,000.

In one embodiment, the first and second polyethylene blends may comprise 150-300 parts by weight of the pore former relative to 100 parts by weight of the polyethylene.

In one embodiment, the pore former may be selected from the group consisting of paraffin oil, alkyl phthalate, and mixtures of two or more thereof.

In one embodiment, in the step (b), the co-extrusion may be performed in such a manner that the double-layer structure is divided into first and second structures along the extrusion direction and the first and second structures are laminated .

In one embodiment, the separation and lamination can be performed continuously.

In one embodiment, in step (c), the cooling may be performed at a temperature of 10 to 60 캜.

In one embodiment, in the step (d), the stretching may be performed such that the elongation in the transverse and longitudinal directions of the base sheet is 400 to 1,000%, respectively.

In one embodiment, in step (d), the solvent may be selected from the group consisting of pentane, hexane, benzene, dichloromethane, carbon tetrachloride, methyl ethyl ketone, acetone, and mixtures of two or more thereof.

In one embodiment, the step (d) may further include: (e) thermally fixing the base sheet at a temperature of 110 to 135 ° C for 10 seconds to 10 minutes.

According to one aspect of the present invention, since the porous separation membrane has a fine multilayer structure in which two kinds of polyethylene mixtures are extruded and divided into four to 100 layers, the physical properties and high-temperature dimensional stability of the separation membrane can be improved, It is possible to prevent a deflection phenomenon, which is one of the defects that are likely to occur in the separation membrane manufactured by the method.

In addition, the lithium secondary battery to which the porous separator is applied is safe under mechanical misuse conditions such as impact, drop, and vibration, and is excellent in thermal stability under abnormal high temperature conditions. Thus, the internal short circuit between the anode and the cathode due to contraction of the separator, (internal short) can be prevented, and the durability and safety of the resulting lithium secondary battery can be improved.

It should be understood that the effects of the present invention are not limited to the effects described above, but include all effects that can be deduced from the description of the invention or the composition of the invention set forth in the claims.

1 is a schematic view illustrating a method of manufacturing a porous separation membrane according to an embodiment of the present invention.
2 is a photograph of a sagging phenomenon of a porous separator according to a comparative example of the present invention.
3 is a graph showing the toughness measurement results of the porous separator according to Examples and Comparative Examples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

In the specification, when a part is referred to as being "connected" to another part, it includes not only "directly connected" but also "indirectly connected" . Also, when an element is referred to as "comprising ", it means that it can include other elements, not excluding other elements unless specifically stated otherwise.

Porous membrane

One aspect of the present invention is a multilayer structure comprising a multilayer structure in which a first layer of a first polyethylene mixture and a second layer of a second polyethylene mixture are alternately laminated and wherein the composition of the first and second polyethylene blend differ, Wherein the multi-layer structure comprises 4 to 100 layers.

The thickness of the single layer (first and second layers) made of the first or second polyethylene mixture included in the multi-layer structure may be 0.01 to 1.0 탆, and the thickness of the porous separator having the multi-layer structure may be 3 to 20 Lt; / RTI > Since the single layers are laminated to form the porous separation membrane, the thickness of the single layer determines the thickness of the porous separation membrane. If the thickness of the single layer is less than 0.01 탆, the physical properties of the separator may be deteriorated. If the thickness of the single layer is more than 1.0 탆, the separator may be thickened to reduce permeability and compatibility with the battery.

Wherein the first and second polyethylene mixtures each comprise ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000 and a molecular weight distribution of 3 to 4 and high density polyethylene having a weight average molecular weight of 300,000 to 500,000 and a molecular weight distribution of 4 to 7 And may further contain a phosphorus-based, hydrocarbon-based antioxidant, if necessary.

Generally, the wider the molecular weight distribution (Mw / Mn), the lower the shear stress and the lower the viscosity, so the processability is improved but the physical properties are lowered. The narrower the molecular weight distribution, the lower the workability but the better the physical properties. Thus, even if two or more polymeric materials are kneaded and used in the form of a composition, physical properties and processability can not be harmonized if their molecular weight distributions are similar. Therefore, the first and second polyethylene mixtures, in which the ultra-high molecular weight polyethylene and the high-density polyethylene having different molecular weight distributions are mixed at different composition ratios, respectively form the first and second layers of the porous separation membrane, And the second layer are alternately stacked, physical properties and processability can be more harmoniously realized on the layered structure (vertical structure) of the separation membrane.

For example, the content of the ultra high molecular weight polyethylene in the first polyethylene mixture may be from 60 to 95% by weight. The content of the high density polyethylene in the second polyethylene mixture may be 90 wt% or more. That is, since the main component of the first polyethylene mixture is an ultra-high molecular weight polyethylene having a narrow molecular weight distribution, the physical properties are good, and the main component of the second polyethylene mixture is a high molecular weight polyethylene having a high molecular weight distribution, And physical properties can be complemented.

Further, the molecular weight distribution may have a different value depending on the kind of the catalyst used in polymer polymerization. For example, when the polyethylene is polymerized with a Ziegler-Natta catalyst, the molecular weight distribution is 6 to 10, while when polymerized with a metallocene catalyst, it can be 3 to 7. However, when the molecular weight distribution is excessively wide, the physical properties of the single sites may be uneven in the cross-sectional structure (horizontal structure) of the single layer. Therefore, in order to uniformize the uniformity of the physical properties, a metallocene catalyst having a relatively narrow molecular weight distribution It is preferable to use polyethylene.

In addition, the molecular weight distribution can have a great influence on toughness as well as typical physical properties such as tensile strength and tensile elongation of the separator. As used herein, the term "toughness" is interpreted as the sum of the energy absorbed by the separator from the moment the separator is deformed until it is finally broken. It is distinguished from the tensile strength, which is the value of the instantaneous stress when the membrane breaks, and the strength must be high and ductile at the same time for the toughness of the material to be high.

For example, in the case of nonflexible materials such as ceramics, the toughness is not high because the strength is high but the flexibility is not sufficient and the stress-strain behavior is rapidly destroyed at high strength. In addition, toughness is not high even in the case of a material having a low strength, such as a packaging wrap, but a high flexibility. Therefore, it must be able to withstand high stresses and high strains at the same time, that is, high strain energy per unit volume can be evaluated as good toughness.

Therefore, when the weight average molecular weight and the molecular weight distribution of the ultrahigh molecular weight polyethylene and the high density polyethylene contained in the first and second polyethylene mixtures satisfy the above ranges, the toughness of the porous separator is 0.9 N / mm or more, Preferably 0.9 to 3.0 N / mm.

When the toughness of the separator is less than 0.9 N / mm, when the impact is applied to the battery from the outside, the toughness of the porous separator tends to be affected by the stability and durability of the battery as compared with the tensile strength. An internal short circuit can easily occur due to exposure.

The porosity of the porous separator may be 30 to 50% by volume, and the average pore size of the porous separator may be 20 to 50 nm.

The porous separator has a tensile strength of 1,700 to 3,000 Kgf / cm ² , a tensile elongation of 80 to 150%, a puncture strength of 300 to 700 gf, a heat shrinkage of 5% or less at 1 hour exposure at 105 ° C, Time exposure can be less than 20%.

Manufacturing method of porous separator

Another aspect of the present invention is a process for preparing a polyethylene composition, comprising: (a) co-extruding a first and a second polyethylene mixture comprising at least two types of polyethylene and a pore-former to form a bilayer structure; (b) co-extruding the bilayer structure 1 to 50 times to form a multilayer structure comprising 4 to 100 layers; (c) cooling the multilayer structure to produce a base sheet; And (d) stretching the base sheet and dipping the base sheet in a solvent to remove the pore-forming agent.

In the step (a), the first and second polyethylene blends are prepared by melt-kneading two or more types of polyethylene and a pore-forming agent in a separate extruder at a temperature of 180 to 250 ° C, Can be co-extruded to form a bilayer structure.

The polyethylene may include ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000 and a molecular weight distribution of 3 to 4 and high density polyethylene having a weight average molecular weight of 300,000 to 500,000 and a molecular weight distribution of 4 to 7. The composition ratio and physical properties of the ultrahigh molecular weight polyethylene and the high density polyethylene are as described above.

On the other hand, unlike the porous separator, which is the final product, the first and second polyethylene blends used in step (a) may further include a pore-forming agent. This is because the pore-forming agent is extracted and removed by the solvent in the subsequent step, specifically step (d), and is no longer present in the final product. That is, the pore-forming agent is a substance which is dispersed in the first and second polyethylene mixture, exhibits heterogeneity in the polymer base material of the porous separation membrane produced through extrusion, drawing and the like, and is subsequently removed from the polymer base material. Therefore, the portion where the pore-forming agent is present in the polymer matrix remains as pores.

The pore-forming agent may be a liquid or a solid material in the extrusion process. For example, the pore former may be alkyl phthalate, paraffin, or a mixture thereof, preferably a liquid paraffinic oil, but is not limited thereto.

The pore former may be included in an amount of 150 to 300 parts by weight based on 100 parts by weight of the polyethylene in consideration of porosity and physical properties of the porous separation membrane.

The first and second polyethylene blends melt-kneaded at the composition ratios described above are co-extruded to form a bilayer structure serving as the base material of the base sheet having a multilayer structure to be described later.

In the step (b), the double layer structure may be co-extruded once to 50 times to form a multi-layer structure including 4 to 100 layers.

FIG. 1 schematically illustrates a co-extrusion process of step (b) in a method of manufacturing a porous separation membrane according to an embodiment of the present invention. Referring to FIG. 1, the co-extrusion may be performed in such a manner that the double-layer structure is divided into first and second structures having the same layered structure along the extrusion direction, and the first and second structures are laminated.

The extrusion direction refers to a direction in which the multi-layer structure of 4 to 100 layers is formed by the co-extrusion, and is interpreted in the same manner as a machine direction (MD) to be described later.

The separation and lamination can be carried out continuously by means of a layer multiplier. Referring to FIG. 1 again, first, in the region (i), the bilayer or bilayer structure formed in the step (a) is introduced into the multilayered device, and (ii) After they are separated, they are coextruded to form a four-layer structure. Thereafter, in the region (iii), the four-layer structure is divided into two structures having the same layered structure along the extrusion direction, and then they are co-extruded to form an eight-layer structure. This process can be similarly performed in (iv) one or more subsequent regions, which are not shown, so that a base sheet having 16 layers or more of a multilayer structure can be produced.

The separation and lamination in the regions (i) to (iv) can be carried out continuously by a single or a plurality of layer multipliers, so that the productivity of the multi-layer structure in which the two polymer mixtures are alternately stacked Can be improved. The multi-layer structure formed in the step (b) may be discharged in a gel state through a tie die, and the thickness of the discharged multi-layer structure may be 1,000 to 6,000 μm.

In the step (c), the multi-layer structure discharged in a gel state is cooled and crystallized by passing through at least one casting roll and a nip roll whose surface temperature is controlled at 10 to 60 캜 A base sheet can be produced.

Generally, the casting rolls are about 1.5 times larger in diameter than the nip rolls, so that passing the gel-like sheet therebetween causes the nip rolls to rotate significantly faster than the casting rolls, resulting in unnecessary deviations in the sheet cooling function of both rolls do.

In this case, when a counter gradient is formed in which the diameter of the intermediate portion is smaller than the diameter of both ends by 1/1500,000 to 1/500,000 times as much as the diameter of the nip roll, the surface area of the nip roll is relatively widened, The same cooling function as the casting roll can be given.

In the step (d), the base sheet may be stretched and dipped in a solvent to remove the pore-forming agent.

First, after reheating the base sheet, the base sheet was biaxially stretched so as to have an elongation in transverse direction (TD) and machine direction (MD) of 400 to 1,000%, respectively, to form a porous separator having a thickness of 3 to 20 μm Can be obtained.

The biaxial stretching may be carried out simultaneously or sequentially. For example, a clip-type biaxial stretching machine can be used for simultaneous biaxial stretching. A roll stretching method using the difference in speed of rolls in the longitudinal direction (MD) can be used for the biaxial stretching in the sequential manner, and a method in which the stretching machine is passed through the clip type stretcher in the transverse direction (TD).

The stretching may be performed at a temperature of 100 to 130 ° C. If the temperature at the time of stretching is less than 100 ° C, the separation membrane may be broken. If the temperature is more than 130 ° C, the separation membrane is partially melted and micropores are difficult to form.

Thereafter, the stretched separation membrane can be immersed in an extraction tank having a solvent to extract and remove the pore-forming agent. The extraction temperature may be 25 to 40 占 폚, and the extraction time may be 1 to 10 minutes. The solvent may be selected from the group consisting of pentane, hexane, benzene, dichloromethane, carbon tetrachloride, methyl ethyl ketone, acetone, and mixtures of two or more thereof.

In addition, when the pore-forming agent is removed, rapid shrinkage stress is generated inside the separation membrane. Therefore, it is necessary to apply and maintain tension greater than the shrinkage stress in the transverse and longitudinal directions of the separation membrane.

After the step (d), (e) the base sheet may be thermally fixed at a temperature of 110 to 135 ° C for 10 seconds to 10 minutes. As used herein, the term " hest setting " refers to a process of high-temperature treatment in order to semi-permanently retain the shape of the sheet after stretching the sheet, The heat shrinkage ratio and thus the high temperature stability can be greatly improved.

The heat setting can be performed at a temperature of 110 to 135 캜. If the temperature is less than 110 ° C, the heat shrinkage of the separator may be lowered. If the temperature is more than 135 ° C, the micropores may be partially closed to increase the gurley value.

Further, the heat setting can be performed for 10 seconds to 10 minutes, and the transverse direction (TD) elongation of the separator upon heat setting can be first adjusted to 100 to 200% and then readjusted to 50 to 150% .

Hereinafter, embodiments of the present invention will be described in detail.

Example  One

90 weight% of ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 1,000,000 and a molecular weight distribution of 3, and 10 weight% of high density polyethylene (HDPE) having a weight average molecular weight of 380,000 and a molecular weight distribution of 4 , 0.5 part by weight of a phosphorus antioxidant and 0.5 part by weight of a hydrocarbon antioxidant were mixed and charged into a twin-screw extruder (inner diameter: 58 mm, L / D = 42, Twin Screw Extruder) I. 250 parts by weight of paraffin oil was fed through the side feeder of the twin screw extruder I and melt kneaded at 210 DEG C and 100 rpm to prepare a first polyethylene mixture.

100 parts by weight of a polyethylene mixture comprising 10% by weight of ultrahigh molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 1,000,000 and a molecular weight distribution of 3, and 90% by weight of high density polyethylene (HDPE) having a weight average molecular weight of 380,000 and a molecular weight distribution of 4 0.5 part by weight of a phosphorus antioxidant and 0.5 part by weight of a hydrocarbon antioxidant were mixed and charged into a twin-screw extruder (inner diameter: 58 mm, L / D = 42, Twin Screw Extruder) II. 250 parts by weight of paraffin oil was fed through a side feeder of the twin screw extruder II and melt kneaded at 210 DEG C and 100 rpm to prepare a second polyethylene mixture.

The first and second polyethylene mixtures were passed through a multi-block (multilayered device) of an extruder to alternately laminate the first and second polyethylene mixtures, and then a multi-layer structure consisting of 20 layers was discharged through a tie die.

The multi-layer structure was passed through a cooling roll controlled at 40 ° C to prepare a gel-like base sheet having a thickness of 1,100 μm. The base sheet was heated to 120 DEG C in a biaxial film stretching machine and then simultaneously biaxially stretched so that elongation in both longitudinal direction (MD) and transverse direction (TD) was 8 times at a rate of 50 mm / min.

A part of the stretched base sheet was cut out and fixed to a SUS frame, and then immersed in a dichloromethane leaching tank adjusted to 25 ° C to extract and remove paraffin oil for 10 minutes. The base sheet from which the paraffin oil had been removed was dried at room temperature, fixed to a tenter, and heat-settled at 125 DEG C for 10 minutes to prepare a porous separator having a thickness of 20 mu m.

Example  2

Except that in the first polyethylene mixture, ultra high molecular weight polyethylene (UHMWPE) was used having a weight average molecular weight and a molecular weight distribution of 1,500,000 and 4 respectively, and the content of ultra high molecular weight polyethylene in the first polyethylene mixture was changed to 80% A porous separator having a thickness of 20 μm was prepared in the same manner as in Example 1.

Example  3

In the first polyethylene mixture, ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight and a molecular weight distribution of 2,000,000 and 4 respectively and high density polyethylene (HDPE) having a weight average molecular weight of 300,000 were used. In the first polyethylene mixture A porous separator having a thickness of 16 탆 was prepared in the same manner as in Example 1 except that the content of ultrahigh molecular weight polyethylene was changed to 70% by weight.

Comparative Example  One

0.5 parts by weight of a phosphorus-based antioxidant and 0.5 parts by weight of a hydrocarbon-based antioxidant were mixed with 100 parts by weight of high-density polyethylene (HDPE) having a weight average molecular weight of 380,000 and a molecular weight distribution of 4, Twin Screw Extruder). 250 parts by weight of paraffin oil was fed through a side feeder of the twin screw extruder and melted and kneaded at 210 DEG C and 100 rpm to prepare a polyethylene mixture.

The polyethylene mixture was passed through an extruder to discharge a single layer structure, and then passed through a cooling roll controlled at 40 ° C to prepare a base sheet having a thickness of 1,100 μm. The base sheet was heated to 120 DEG C in a biaxial film stretching machine and then simultaneously biaxially stretched so that elongation in both longitudinal direction (MD) and transverse direction (TD) was 8 times at a rate of 50 mm / min.

A part of the stretched base sheet was cut out and fixed to a SUS frame, and then immersed in a dichloromethane leaching tank adjusted to 25 ° C to extract and remove paraffin oil for 10 minutes. The base sheet from which the paraffin oil had been removed was dried at room temperature, fixed to a tenter, and heat-settled at 125 DEG C for 10 minutes to prepare a porous separator having a thickness of 20 mu m.

Experimental Example  One

The tensile strength, elongation, porosity, piercing strength, thickness, heat shrinkage, deflection length, Gurley number, etc. of the porous separator prepared in Examples and Comparative Examples were measured and evaluated according to the following methods. Respectively.

- Gurley (sec / 100 ml): The time of passage of 100 ml of air through a membrane sample of 30 * 30 mm in size was measured using a Gurley meter.

- Tensile Strength (Kgf / cm ² ): Stress applied until a fracture occurs in transverse direction (TD) and machine direction (MD) for a membrane specimen of 20 * 200 mm size using a tensile strength tester Were measured.

- Tensile elongation (%): The tensile strength tester was used to measure the elongation of the specimen until the fracture occurred in the transverse and longitudinal directions for the 20 * 200 mm separator specimen.

- Perforation strength (gf): The force applied until the sample was punctured by applying a force to a separator sample having a size of 100 * 50 mm using a puncture strength meter.

Heat Shrinkage (%): The 200 * 200 mm separator specimen was heated in an oven at 120 to 150 ° C for 1 hour and then cooled at room temperature to measure the shrunk length of the specimen in the transverse and longitudinal directions, Was calculated.

Porosity (volume%): The porosity of the porous separator specimen with a radius of 30 mm was measured according to ASTM F316-03.

- Deflection Length (mm): When the lengths of the porous membrane in the transverse and longitudinal directions were 600 mm or more and 1,000 mm or more, the distance from the top to the bottom in the transverse and longitudinal directions was measured.

division unit Example 1 Example 2 Example 3 Comparative Example 1 Gully value sec / 100ml 156 163 140 162 Toughness N / mm 2.3 2.1 2.5 0.7 Tensile Strength (MD) Kgf / cm ² 2,653 2,276 2,203 1,685 Tensile Strength (TD) Kgf / cm ² 2,365 2,013 1,987 1,365 Tensile elongation (MD) % 89 102 105 76 Tensile elongation (TD) % 92 106 120 65 Puncture strength gf 539 450 356 402 Porosity volume% 36 35 38 35 Heat shrinkage (105 ° C) % 3.5 3 3 13 Heat shrinkage (130 ° C) % 18 17 16 32 Deflection length mm 5 6 5 25

Experimental Example  2

The tensile strength and tensile elongation in the transverse and longitudinal directions of the porous separator specimens having a size of 20 mm * 60 mm were measured according to the above method. Based on the measured load (N) and displacement (mm), the displacement / gauge distance and the load / width were plotted as x and y values, respectively, and the toughness was calculated by integrating them. Toughness values of porous separator samples according to Examples and Comparative Examples are shown in Table 2 and FIG.

division unit Example 1 Example 2 Example 3 Comparative Example 1 Toughness (MD) N / mm 2.3 2.1 2.5 0.7

It will be understood by those skilled in the art that the foregoing description of the present invention is for illustrative purposes only and that those of ordinary skill in the art can readily understand that various changes and modifications may be made without departing from the spirit or essential characteristics of the present invention. will be. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. For example, each component described as a single entity may be distributed and implemented, and components described as being distributed may also be implemented in a combined form.

The scope of the present invention is defined by the appended claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

Claims (19)

Layer structure in which a first layer of a first polyethylene mixture and a second layer of a second polyethylene mixture are alternately laminated,
Wherein the composition of the first and second polyethylene mixtures is different,
Wherein the multilayer structure comprises 4 to 100 layers,
The thickness of the layer is 0.01 to 1.0 탆,
A porous separator having a toughness of 0.9 N / mm or more. The method according to claim 1,
Wherein the first and second polyethylene blends each comprise ultra high molecular weight polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000 and high density polyethylene having a weight average molecular weight of 300,000 to 500,000. 3. The method of claim 2,
Wherein the content of the ultrahigh molecular weight polyethylene in the first polyethylene mixture is 60 to 95 wt%. 3. The method of claim 2,
Wherein the content of the high density polyethylene in the second polyethylene mixture is 90 wt% or more. The method according to claim 1,
Wherein the porous separator has a porosity of 30 to 50% by volume. The method according to claim 1,
Wherein the porous separator has an average pore size of 20 to 50 nm. The method according to claim 1,
Wherein the porous separator has a thickness of 3 to 20 占 퐉. delete The method according to claim 1,
Wherein the porous separator has a tensile strength of 1,700 to 3,000 Kgf / cm ² . (a) co-extruding a first and a second polyethylene mixture comprising at least two polyethylenes and a pore-former to form a bilayer structure;
(b) co-extruding the bilayer structure 1 to 50 times to form a multilayer structure comprising 4 to 100 layers;
(c) cooling the multilayer structure to produce a base sheet; And
(d) stretching the base sheet and dipping the base sheet in a solvent to remove the pore forming agent,
In the step (b), the co-extrusion is performed in a manner of separating the double-layer structure into first and second structures along the extrusion direction and laminating the first and second structures,
Wherein the thickness of the layer is 0.01 to 1.0 占 퐉. 11. The method of claim 10,
Wherein the polyethylene comprises an ultrahigh molecular weight polyethylene having a weight average molecular weight of 1,000,000 to 3,000,000 and a high density polyethylene having a weight average molecular weight of 300,000 to 500,000. 11. The method of claim 10,
Wherein the first and second polyethylene blends comprise 150-300 parts by weight of the pore former relative to 100 parts by weight of the polyethylene. 11. The method of claim 10,
Wherein the pore former is one selected from the group consisting of paraffin oil, alkyl phthalate, and a mixture of two or more thereof. delete 11. The method of claim 10,
Wherein said separation and lamination are carried out continuously. 11. The method of claim 10,
In the step (c), the cooling is performed at a temperature of 10 to 60 캜. 11. The method of claim 10,
Wherein the stretching is performed so that elongation in the transverse direction and the transverse direction of the base sheet is 400 to 1,000%, respectively, in the step (d). 11. The method of claim 10,
In the step (d), the solvent is selected from the group consisting of pentane, hexane, benzene, dichloromethane, carbon tetrachloride, methyl ethyl ketone, acetone, and mixtures of two or more thereof. 11. The method of claim 10,
Further comprising the step of (e) thermally fixing the base sheet at a temperature of 110 to 135 DEG C for 10 seconds to 10 minutes after the step (d).

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