KR102139884B1 - Polyolefin Microporous Membrane - Google Patents

Abstract

The present invention relates to a polyolefin microporous membrane. More specifically, the present invention relates to the polyolefin microporous membrane having excellent sliding properties and handling properties by controlling the pore size and a static friction coefficient and having excellent electrode adhesion after coating. According to the present invention, the polyolefin microporous membrane satisfies formula 1, wherein formula 1 is 0.059 <= μ_s x (A/P)/t <= 0.095.

Description

Polyolefin microporous membrane

The present invention relates to a polyolefin microporous membrane, and more particularly, to a polyolefin microporous membrane having excellent slipperiness and handling property and excellent electrode adhesion after coating by controlling the size and static friction coefficient of the pores.

With the rapid development of portable information and communication devices and electric vehicles, lithium-ion secondary batteries having a relatively high energy density are attracting much attention as power sources, and studies to improve battery performance by making lithium-ion secondary batteries have a higher energy density Development continues.

During the process of manufacturing a microporous membrane used as a separator for a secondary battery (for example, a winding process), there is a problem that wrinkles or tears occur on the membrane. In order to solve this problem, Japanese Patent Publication No. Hei 11-314333 proposes to control the roughness or the friction coefficient by disposing particles on the film surface. However, even with the above method, not only wrinkle avoidance was not sufficiently achieved, but the problem of tearing the film was not sufficiently improved.

The present invention provides a polyolefin microporous membrane having excellent slipperiness and handling properties and excellent electrode adhesion after forming a coating layer.

In addition, the present invention provides a lithium ion secondary battery comprising the polyolefin microporous membrane.

The present invention provides a polyolefin microporous membrane that satisfies Expression 1 below.

[Equation 1]

0.050 ≤ μ _s × (A/P)/t ≤ 0.095

In the above formula,

P is the porosity (%) of the polyolefin microporous membrane,

A is the air permeability (sec/100cc) of the polyolefin microporous membrane,

t is the thickness of the polyolefin microporous membrane (um),

μ _s is the static friction coefficient (uS) of one surface of the polyolefin microporous membrane.

The polyolefin microporous membrane according to the present invention controls the thickness and structure of the fibril by controlling the size of the pores and the static friction coefficient of the surface of the porous membrane, thereby controlling the thickness and structure of the fibril, thereby exhibiting excellent slipperiness and handling in the product processing stage. In addition, when the coating layer is formed on the polyolefin microporous membrane according to the present invention, excellent coating adhesion may be maintained to improve durability of the lithium ion secondary battery including the same.

Hereinafter, the present invention will be described in detail. However, the present invention is not limited only to the following contents, and each component may be variously modified or selectively mixed as necessary. Therefore, it should be understood to include all modifications, equivalents, or substitutes included in the spirit and scope of the present invention.

<Polyolefin microporous membrane>

As the polyolefin resin constituting the polyolefin microporous membrane of the present invention, conventional polyolefin resins known in the art can be used without limitation. Examples may include polyethylene resins, polypropylene resins, or mixtures thereof.

The content of the polyethylene resin may be, for example, 50% by mass or more, and 60% by mass or more, with respect to the total mass of the polyolefin resin. The polyethylene resin may have a weight average molecular weight (Mw) in the range of 2.0×10 ⁵ to 4.0×10 ⁶ , for example, in the range of 2.5×10 ⁵ to 3.5×10 ⁶ . As the polyethylene, high density polyethylene (HDPE), medium density polyethylene (MDPE) or low density polyethylene (LDPE) may be used without limitation. Alternatively, two or more polyethylenes having different weight average molecular weights (Mw) or densities may be mixed.

The content of the polypropylene resin may be, for example, 50% by mass or less, and 40% by mass or less, with respect to the total mass of the polyolefin resin. The polypropylene resin may have a weight average molecular weight (Mw) of, for example, 7.0×10 ⁴ to 3.2×10 ⁶ and another example of 9.5×10 ⁴ to 3.0×10 ⁶ . As the polypropylene, high density polypropylene (HDPP), medium density polypropylene (MDPP), or low density polypropylene (LDPP) may be used without limitation. Alternatively, two or more polypropylenes having different weight average molecular weights (Mw) or densities may be mixed.

In order to improve the properties as a battery separator, the polyolefin resin may further include a polyolefin that provides a shutdown function. Examples of polyolefins that provide a shutdown function include low density polyethylene (LDPE) or polyethylene wax. As the low-density polyethylene (LDPE), one or more selected from the group consisting of branched LDPE, linear LDPE, and ethylene/α-olefin copolymers prepared by a single site catalyst can be used, and the amount added is sugar It can be appropriately adjusted within a range known in the art.

If necessary, conventional additives known in the art, for example, antioxidants, pore formers, etc., may be included in a range that does not impair the effects of the present invention.

The polyolefin microporous membrane according to the present invention satisfies Expression 1 below.

[Equation 1]

0.050 ≤ μ _s × (A/P)/t ≤ 0.095

In the above formula,

P is the porosity (%) of the polyolefin microporous membrane,

A is the air permeability (sec/100cc) of the polyolefin microporous membrane,

t is the thickness of the polyolefin microporous membrane (um),

μ _s is the static friction coefficient (uS) of one surface of the polyolefin microporous membrane.

In the present invention, μ _s × (A/P)/t value may be 0.050 to 0.095, for example, 0.060 to 0.071. When the μ _s × (A/P)/t value falls within the aforementioned range, wrinkles and sagging may be prevented and coating adhesion may be improved.

The polyolefin microporous membrane according to the present invention may satisfy Equation 2 below.

[Equation 2]

2.0 ≤ r × μ _s × (A/P)/t ≤ 5.0

In the above formula,

P is the porosity (%) of the polyolefin microporous membrane,

A is the air permeability (sec/100cc) of the polyolefin microporous membrane,

t is the thickness of the polyolefin microporous membrane (um),

r is the maximum pore size (nm) of the polyolefin microporous membrane,

μ _s is the static friction coefficient (uS) of one surface of the polyolefin microporous membrane.

In the present invention, the r × μ _s × (A/P)/t value may be 2.0 to 5.0, for example 2.6 to 4.0. When the value of r × μ _s × (A/P)/t falls within the above-mentioned range, the thickness and structure of the fibrils on the film surface are controlled to improve the slipperiness and handling properties in the processing step of the microporous membrane, As a result, wrinkles and sagging can be prevented. In addition, it is possible to improve the electrode adhesion after coating.

The diameter of the fibrils constituting the polyolefin microporous membrane according to the present invention may be 0.01 to 9.0 μm, for example, 0.03 to 5.0 μm, and another example may be 0.1 to 2.5 μm. The average pore size of the polyolefin microporous membrane of the present invention may be 20 to 40 nm, for example 25 to 35 nm, and the maximum pore size may be 35 to 55 nm, for example 40 to 50 nm. When the diameter of the fibril and the pore size of the microporous membrane are out of the above-described range, wrinkles and sagging may occur due to insufficient sliding and handling properties.

The coating adhesion of the polyolefin microporous membrane according to the present invention may be 60 gf/25 mm or more, for example, 80 to 180 gf/25 mm.

The polyolefin microporous membrane according to the present invention may have a multi-layer structure in which the microporous membrane is stacked in multiple layers.

In the polyolefin microporous membrane according to the present invention, a coating layer may be formed on one side or both sides. The coating layer may include inorganic particles or crosslinked polymer particles.

Non-limiting examples of usable inorganic particles include alumina, aluminum hydroxide, barium titanium oxide, magnesium oxide, magnesium hydroxide, and the like. Clay, titanium oxide, glass powder, boehmite, or mixtures of two or more thereof.

The size of the inorganic particles is not limited, but may be in the range of 0.001 to 10 μm for uniform film formation and proper porosity.

The content of the inorganic particles may be, for example, 50 to 90% by weight based on the total weight of the coating layer, and another example may be 60 to 85% by weight. When the content of the inorganic particles falls within the above-described range, a heat resistance effect due to the use of the inorganic particles can be achieved.

Non-limiting examples of crosslinked polymer particles that can be used include polyacrylic acid, polymethacrylic acid, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, butadiene-acrylic acid copolymer, butadiene-methacrylic acid copolymer, polyvinyl Sulfonate, chlorosulfonated polyethylene, perfluorinated sulfonated ionomer, sulfonated polystyrene, styrene-acrylic acid copolymer, sulfonated butyl rubber, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexa Fluoropropylene (PVdF-HFP), polyvinylpyrrolidone, polyacrylonitrile, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-TFE), polymethyl meta Acrylate, polyvinyl acetate, ethylene-co-vinyl acetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butylate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose , Cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile styrene-butadiene copolymer, polyimide, or mixtures of two or more thereof.

The thickness of the polyolefin microporous membrane according to the present invention is not particularly limited, and may be 1 to 30 μm, for example, 3 to 25 μm, and another example, 4 to 20 μm.

The method for manufacturing the polyolefin microporous membrane of the present invention is not particularly limited, for example, (1) preparing a polyolefin solution by melt-kneading a polyolefin and a plasticizer, and (2) extruding the polyolefin solution and cooling it to form a sheet. Forming, (3) a first stretching step of stretching the sheet-like material to form a film, (4) removing a plasticizer from the film, (5) drying the film from which the plasticizer is removed, ( 6) A second stretching step for re-stretching the dried film, (7) heat treatment, and (8) slitting.

Hereinafter, each manufacturing process will be described in detail.

(1) Process for preparing polyolefin solution

The polyolefin resin and the plasticizer are melt-kneaded to prepare a polyolefin solution. Melting and kneading a composition comprising a polyolefin-based resin and a plasticizer may be performed without limitation using conventional methods known in the art. As an example, a method of melt kneading a polyolefin-based resin and a plasticizer at a temperature of 100 to 250° C. and using a twin-screw extruder can be used.

The content of the polyolefin-based resin is not particularly limited, and for example, may be included in an amount of 20 to 50% by weight relative to the total weight of the composition containing the polyolefin-based resin and a plasticizer, and may be included in another example 20 to 40% by weight.

In the present invention, as the plasticizer, conventional components known in the art may be used without limitation, and for example, may be any organic compound that forms a single phase with the polyolefin-based resin at an extrusion temperature.

Non-limiting examples of plasticizers that can be used include non-, non-, decane, decalin, liquid paraffin (Liquid paraffin, LP), such as liquid paraffin (or paraffin oil), aliphatic or cyclic, such as paraffin wax hydrocarbon; Phthalic acid esters such as dibutyl phthalate and dioctyl phthalate; Fatty acids having 10 to 20 carbon atoms such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; And fatty acid alcohols having 10 to 20 carbon atoms, such as palmitic acid alcohol, stearic acid alcohol, and oleic acid alcohol, or mixtures of two or more thereof. As an example, among the plasticizers, liquid paraffin is harmless to the human body and has a high boiling point and low volatile components, and is suitable for use as a plasticizer in a wet method.

The content of the plasticizer is not particularly limited, for example, it may be included, for example, 50 to 90% by weight, based on the total weight of the composition containing the polyolefin-based resin and plasticizer, and may be included as another example 60 to 80% by weight .

In addition to the above-mentioned polyolefin-based resin, other conventional resins, inorganic particles, additives, and the like in the art may be included.

(2) Forming process of sheet

The polyolefin solution prepared in step (1) is extruded into a die equipped with an extruder and cooled to form a sheet.

When the polyolefin solution is extruded from the die and cooled to form a gel-like sheet, the surface roughness can be controlled, for example, by controlling the cooling rate. For example, it may be cooled at a rate of 100° C./min or more, for example, 100 to 650° C./min, and another example, 300 to 650° C./min. When the cooling rate is less than 100°C/min, the fibril size of the gel-like sheet increases due to an increase in crystallinity of the polyolefin, thereby increasing the surface roughness, and an unsuitable gel due to an increase in stiffness due to high crystallinity. A sheet of top may be formed.

The cooling method is not particularly limited, and methods known in the art, such as cooling by contact with a cooling roll (Chill), cooling with cold air, or impregnating with cold water, may be used.

(3) First stretching process

The sheet-like article obtained in step (2) is biaxially stretched one or more times to form a film.

In the present invention, the first stretching process may be performed at a predetermined magnification by conventional methods in the art, such as a tenter method, a roll method, an inflation method, a rolling method, or a combination of these methods. In the case of biaxial stretching, biaxial stretching may be performed at the same time, or any of sequential stretching may be performed.

In the first stretching step, the stretching ratio differs depending on the thickness of the sheet-like material, but for example, in biaxial stretching, it is appropriate to perform at least 2 times Ｘ 2 times or more in any direction, for example, in the range of 3 to 10 times. Can.

In the first stretching process, the structure, pore size, diameter, and friction force of the fibril can be controlled by adjusting the stretching temperature and the air volume.

In the first stretching process of the present invention, the stretching temperature may be, for example, in the range of 100 to 130°C, and in another example, in the range of 110 to 125°C. In the case of stretching in the above range, it can be stretched to have appropriate air permeability and mechanical strength without blocking pores (pores) in the sheet. When the first stretching temperature exceeds the above-described temperature range, the fibril is melted, the thickness thereof becomes thick and the frictional force may be increased, thereby sliding property (sliding) may be deteriorated. In addition, since the size of the pores becomes very large, adhesive strength may decrease after coating. On the other hand, when the first stretching temperature is lower than the above-mentioned temperature range, the polyolefin microporous membrane is not only produced without having the necessary properties (for example, porosity and air permeability), but also reduces the frictional force as the fibril becomes thinner, resulting in sliding properties ( Slippage) can be too high.

(4) Plasticizer removal process

Using a cleaning solvent, a plasticizer is extracted from the stretched film. Since the polyolefin phase is phase-separated from the plasticizer, removing the plasticizer provides a porous membrane having a large number of pore structures. As a method for removing the plasticizer, conventional methods known in the art can be used without limitation.

In the present invention, as the cleaning solvent, any organic solvent capable of extracting a plasticizer may be used without particular limitation. For example, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbon, which have high extraction efficiency and are easy to dry; hydrocarbons such as n-hexane and cyclohexane; Alcohols such as ethanol and isopropanol; Ketones, such as acetone and 2-butanone, can be used. For example, when using liquid paraffin as a plasticizer, methylene chloride can be used as an organic solvent.

Since the organic solvent used in the process of extracting the plasticizer is highly volatile and most toxic, water can be used to suppress volatilization of the organic solvent, if necessary.

(5) Drying process

The polyolefin microporous membrane obtained by plasticizer removal can be dried using conventional drying methods known in the art. As an example, a heat drying method, an air drying method, or the like can be used.

(6) Second stretching process

Subsequently, the dried film is again stretched at least in the uniaxial direction or more. In the present invention, the second stretching process may be performed by a tenter method or the like in the same manner as the first stretching process while heating the film. At this time, it may be uniaxial stretching or biaxial stretching.

The temperature of the second stretching process may be, for example, a crystal dispersion temperature of the polyolefin resin constituting the microporous membrane, a crystal dispersion temperature of + 40° C. or less, and another example, a crystal dispersion temperature + 10° C. or more, crystal The dispersion temperature may be in the range of +40° C. or less. By adjusting the second stretching temperature in the above-described range, it is possible to prevent deterioration of air permeability and occurrence of physical property variations in the sheet width direction when stretching in the transverse direction (width direction: TD direction). By setting the second stretching temperature within the above range, it is possible to suppress the occurrence of variations in the width direction of the stretched sheet, particularly of air permeability. Here, the crystal dispersion temperature refers to a value obtained by measuring the temperature characteristics of dynamic viscoelasticity based on ASTM D4065. When the polyolefin resin is polyethylene, the crystal dispersion temperature is generally 90 to 100°C.

In addition, by adjusting the temperature of the second stretching process in the above-described range, the polyolefin resin can be sufficiently softened, and it is possible to uniformly stretch by preventing rupture. In one example, the second stretching temperature may range from 90 to 140°C, and another example may range from 120 to 140°C.

The magnification of the second stretching in the uniaxial direction may be, for example, 1.0 to 1.8 times, and another example may be 1.2 to 1.6 times. For example, in the case of uniaxial stretching, 1.0 to 1.8 times in the longitudinal direction (machine direction: MD direction) or in the TD direction. In the case of biaxial stretching, it is adjusted to 1.0 to 1.8 times in the MD direction and the TD direction, respectively. In the case of biaxial stretching, each stretching magnification in the MD direction and the TD direction may be the same or different. By adjusting the stretching magnification to the above-described range, it is possible to prevent deterioration of permeability, electrolyte absorbency, and compressibility, and prevent fibril from becoming too thin and heat shrinkage resistance.

(7) Heat treatment process

Subsequently, the redrawn film is fixed and heat treated. As the heat treatment method, a conventional method known in the art may be performed without limitation, and heat treatment and/or heat relaxation treatment may be used as an example.

In particular, by performing the heat setting treatment, a network structure composed of fibrils formed by the second stretching process is maintained, and the crystals of the porous membrane are stabilized, whereby the microporous diameter is appropriately adjusted and a fine porous membrane having excellent strength can be produced.

In the present invention, the heat setting treatment is performed within a temperature range of a crystal dispersion temperature of the polyolefin resin constituting the microporous membrane and a melting point or lower. At this time, the heat setting treatment may be performed by a tenter method, a roll method or a rolling method.

Further, the thermal relaxation treatment may be performed by a tenter method, a roll method or a compression method, or may be performed using a belt conveyor or a floating roll. The thermal relaxation treatment may be performed, for example, in a range of 20% or less relaxation rate in at least one direction, and in another example, in a range of 10% or less relaxation rate.

(8) Slitting step

Subsequently, a slitting process is performed in which the microporous membrane obtained through the heat treatment process is cut by adjusting the width in the width and length directions. As the slitting process, a conventional method known in the art may be performed without limitation, and for example, a slitting machine having a winding-out part and a winding-up part may be used.

For example, the tension of the unwinding portion may be 30 to 85 N/m, for example, 40 to 75 N/m, and the tension of the winding portion may be 10 to 70 N/m, for example, 15 to 60 N/m. Can. In addition, the difference between the unwinding portion and the winding portion tension may be 0 N/m or more, for example, 0 to 30 N/m. When the tension of the unwinding portion and the winding portion is out of the above-mentioned range, the sliding property is too low, resulting in meandering, cutting and wrinkles of the film, or, on the contrary, the sliding property is too high, causing the film to sag and the film end to rise. Can. When the tension of the unwinding portion and the winding portion satisfies the above-described range, it is possible to secure an appropriate sliding property to prevent the above problems as well as to enable a stable slitting process.

<Lithium ion secondary battery>

The present invention provides a secondary battery including the aforementioned polyolefin microporous membrane, for example, a lithium ion secondary battery.

The lithium ion secondary battery of the present invention can be manufactured according to a conventional method known in the art, except that the polyolefin microporous membrane according to the present invention is used as a separator. For example, a separation membrane may be interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte may be introduced to produce it.

The lithium ion secondary battery of the present invention includes a negative electrode, a positive electrode, a separator, and an electrolyte as a battery component, wherein the components of the positive electrode, the negative electrode, the electrolyte, and other additives, if necessary, other than the above-described separator are conventionally known in the art. It conforms to the elements of the lithium ion secondary battery.

For example, the positive electrode may be manufactured using a positive electrode active material for a lithium ion secondary battery known in the art, and non-limiting examples thereof include lithium transition metal composites such as LiMxOy (M = Co, Ni, Mn, CoaNibMnc). Oxides (for example, lithium manganese composite oxides such as LiMn ₂ O ₄ , lithium nickel oxides such as LiNiO ₂ , lithium cobalt oxides such as LiCoO ₂ , and other common transition metals or parts of manganese, nickel, and cobalt of these oxides) Aluminum, or lithium-containing vanadium oxide, etc., or chalcogen compounds (eg, manganese dioxide, titanium disulfide, molybdenum disulfide, etc.).

For example, the negative electrode may be prepared using a conventional negative electrode active material for a lithium ion secondary battery known in the art, and a non-limiting example is a material capable of intercalating/deintercalating lithium ions, Examples include lithium metal or lithium alloys, coke, artificial graphite, natural graphite, organic polymer compound burners, carbon fibers, silicones, and tins.

Non-aqueous electrolytes include electrolyte components commonly known in the art, such as electrolyte salts and electrolyte solvents.

The electrolyte salt is ^{(i) Li +, Na +} , K + cations and (ii) selected from the group consisting of _{^{PF 6 -, BF 4 -,}} Cl -, Br -, I -, ClO 4 -, AsF 6 -, CH ₃ CO ₂ ^- , CF ₃ SO ₃ ^- , N (CF ₃ SO ₂ ) ₂ ^- , C (CF ₂ SO ₂ ) ₃ ^-It may be made of a combination of anions selected from the group consisting of, a lithium salt is preferred . Specific examples of lithium salts include LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiN(CF ₃ SO ₂ ) ₂ . These electrolyte salts may be used alone or in combination of two or more.

As the electrolyte solvent, cyclic carbonate, linear carbonate, lactone, ether, ester, acetonitrile, lactam, and ketone may be used.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluorethylene carbonate (FEC), and examples of the linear carbonate include diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). Examples of the lactone include gamma-butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxy ethane. Examples of such esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl pivalate, and the like. In addition, the lactam is N-methyl-2-pyrrolidone (NMP) and the like, and the ketone is polymethylvinyl ketone. In addition, a halogen derivative of the organic solvent may also be used, but is not limited thereto. In addition, the organic solvent may also be used for glyme, diglyme, triglyme, and tetraglyme. These organic solvents may be used alone or in combination of two or more.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only to aid the understanding of the present invention, and the scope of the present invention is not limited to the examples in any sense.

[Example 1-5]

Polyolefin composition comprising 15% by weight of ultra high molecular weight polyethylene (UHMWPE) having a weight average molecular weight of 2,000,000 g/mol, 25% by weight high density polyethylene (HDPE) having a weight average molecular weight of 500,000 g/mol, and 60% by weight of a plasticizer (fluid paraffin) Was prepared.

28 parts by weight of the resulting polyolefin composition was added into a steel-blended twin screw extruder having an inner diameter of 58 mm and L/D of 42, and 72 parts by weight of liquid paraffin (50 cst at 40°C) was fed to the twin screw extruder through a side feeder. Did. Melt-blending was performed at 210° C. and 200 rpm to prepare a polyethylene solution. This polyethylene solution was extruded from the T-die mounted on the twin screw extruder. The extrudate was cooled while passing through a cooling roll to form a cooled extrudate, that is, a gel-like sheet.

Using a tenter-stretcher, the gel-like sheet material was simultaneously biaxially stretched at 114°C in both the longitudinal and transverse directions 6 times. The stretched film was repeatedly immersed in a methylene chloride section adjusted to 25°C to remove liquid paraffin, and dried by air flow at room temperature. The dried membrane was re-stretched by a batch stretching machine at 129.8°C in the transverse direction at a magnification of 1.4 times. The re-stretched membrane was fixed in a batch stretching machine, heat-set at 125°C for 10 minutes, and slitting to prepare a microporous membrane of Example 1-5.

The thickness of the microporous membrane according to Example 1-5, the static friction coefficient, the porosity, the air permeability, the pore size, the unwinding section and the unwinding section tension are shown in Table 1 below.

[Comparative Example 1-4]

Fine, according to Comparative Example 1-4 in the same manner as in Example 1-5, except that the thickness, static friction coefficient, porosity, air permeability, pore size, unwinding and unwinding tension were adjusted as shown in Table 2 below. A porous membrane was prepared.

[Experimental Example-Property evaluation]

The physical properties test of the microporous membrane was performed as follows, and the results are shown in Table 3 below.

<Method for measuring physical properties>

Wrinkles and sagging

It was visually evaluated whether wrinkles and sagging occurred in the microporous membrane prepared according to Example 1-5 and Comparative Example 1-4.

(○: Excellent, △: Normal, ×: Poor)

coating

(1) Preparation of first coating liquid composition

A polyvinylidene fluoride-hexafluoropropylene (hereinafter,'PVdF-HFP') copolymer (Solvay) having a weight average molecular weight of 700,000 g/mol was added to acetone at 10% by weight, using a stirrer at 25°C. The polymer solution was prepared by stirring for 4 hours. Alumina was added to acetone at 25% by weight, and milled at 25°C for 3 hours using a ball mill to prepare an inorganic dispersion. The prepared polymer solution: an inorganic dispersion: a solvent (acetone) was mixed in a weight ratio of 1:3:6, and stirred at 25°C for 2 hours with a power mixer to prepare a first coating solution composition.

(2) Preparation of second coating liquid composition

The polymer solution of the first coating solution composition was used as the second coating solution composition.

(3) Preparation of coating separator

The first coating liquid composition and the second coating liquid composition were coated on the first and second surfaces of the polyolefin microporous membranes of Examples 1-5 and Comparative Example 1-4, respectively, and dried to dry them to obtain 7 μm on both sides. A coating separator having a coating layer was prepared. At this time, the thickness of the first coating layer was 6.5 μm, and the thickness of the second coating layer was 0.5 μm.

Coating adhesion

The microporous membranes according to Examples 1-5 and Comparative Examples 1-4, which were subjected to the coating process, were folded and laminated on a release polyethylene terephthalate (PET), respectively, and then bonded using a roll laminator at 100°C. After cutting the bonded microporous membrane to a width of 25 mm and a length of 130 mm, a force (gf/25 mm) required to detach the bonded microporous membrane was measured using a tensile strength measuring device.

As shown in Table 3, it was confirmed that the polyolefin microporous membrane of Examples 1-5 according to the present invention exhibits excellent effects in all measured physical properties. On the other hand, in the microporous membrane according to Comparative Example 1-4, wrinkles and sagging occurred, and thus it was confirmed that the coating adhesion decreased.

Claims (5)

A polyolefin microporous membrane satisfying the following formula (1).
[Equation 1]
0.059 ≤ μ _s × (A/P)/t ≤ 0.095
(In the above formula,
P is the porosity (%) of the polyolefin microporous membrane,
A is the air permeability (sec/100cc) of the polyolefin microporous membrane,
t is the thickness of the polyolefin microporous membrane (um),
μ _s is the static friction coefficient (uS) of one surface of the polyolefin microporous membrane) The polyolefin microporous membrane according to claim 1, which satisfies Expression 2 below.
[Equation 2]
2.0 ≤ r × μ _s × (A/P)/t ≤ 5.0
(In the above formula,
P is the porosity (%) of the polyolefin microporous membrane,
A is the air permeability (sec/100cc) of the polyolefin microporous membrane,
t is the thickness of the polyolefin microporous membrane (um),
r is the maximum pore size (nm) of the polyolefin microporous membrane,
μ _s is the static friction coefficient (uS) of one surface of the polyolefin microporous membrane) The polyolefin microporous membrane according to claim 1, wherein the average pore size of the polyolefin microporous membrane is 20 to 40 nm, and the maximum pore size is 35 to 55 nm. The polyolefin microporous membrane of claim 1, wherein the coating adhesion of the polyolefin microporous membrane is 80 to 180 gf/25 mm. A lithium ion secondary battery comprising the microporous polyolefin membrane according to any one of claims 1 to 4.