KR102038766B1 - Polyolefin microporous membrane - Google Patents

Abstract

The present invention provides a polyolefin microporous membrane having a surface roughness (rms) of 65 nm or less on one or both sides and a coated porous membrane having a water-based coating layer formed on one or both surfaces of the polyolefin microporous membrane.

Description

Polyolefin microporous membrane {POLYOLEFIN MICROPOROUS MEMBRANE}

The present invention relates to a polyolefin microporous membrane, and more particularly to a polyolefin microporous membrane for battery separator having excellent waterborne coating properties.

Polyolefin-based microporous membranes are frequently used as separator materials for batteries. In particular, when used as separators for lithium ion secondary batteries, the performance of the porous membranes is closely related to battery characteristics, productivity, and safety. Therefore, appropriate permeability, mechanical properties, heat resistance, dimensional stability, shutdown properties, meltdown properties, and the like are required for the polyolefin microporous membrane.

Recently, in order to improve the safety of polyolefin-based microporous membranes used as battery separators, studies are being actively conducted to increase heat resistance with organic or organic / inorganic coatings. In particular, the demand for water-based coatings is increasing in consideration of the environment and cost, but studies on separators suitable for water-based coatings are insufficient.

An object of the present invention is to provide a polyolefin microporous membrane for battery separator having excellent waterborne coating properties.

The present invention provides a polyolefin microporous membrane having one or both surface roughness (rms) of 65 nm or less.

In addition, the present invention provides a coated porous membrane having a water-based coating layer formed on the aforementioned polyolefin microporous membrane.

In the polyolefin microporous membrane for battery separator according to the present invention, the surface roughness of the surface in contact with the coating layer is controlled to exhibit excellent waterborne coating properties. As a result, the battery separator prepared using the polyolefin microporous membrane according to the present invention has excellent heat resistance and adhesive strength while maintaining basic physical properties required for the separator, such as maintaining excellent strength and air permeability.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail. However, the present invention is not limited only to the following contents, and each component may be variously modified or optionally mixed as necessary. Therefore, it is to be understood that all changes, equivalents, and substitutes included in the spirit and scope of the present invention are included.

<Polyolefin microporous membrane>

The microporous membrane for the battery separator of the present invention is formed of a polyolefin resin, and conventional polyolefin resins known in the art may be used without limitation. One example may include polyethylene resin, polypropylene resin, or both. In addition, the microporous membrane may include one or more layers composed of the same or different polyolefin resin.

The content of the polyethylene resin may be, for example, 50 mass% or more, and in other examples 60 mass% or more, based on the total mass of the polyolefin resin. The polyethylene resin may have a weight average molecular weight (Mw), for example, in the range of 2.0 × 10 ⁵ to 4.0 × 10 ⁶ , and in another 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 kinds of different weight average molecular weights (Mw) or densities may be used.

Content of a polypropylene resin may be 50 mass% or less with respect to the total mass of a polyolefin resin, for example, 40 mass% or less. The polypropylene resin may have a weight average molecular weight (Mw), for example, in the range of 7.0 × 10 ⁴ to 3.2 × 10 ⁶ , and in another example, in the range 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 kinds of different weight average molecular weights (Mw) or densities may be used.

In order to improve the characteristics as a battery separator, the polyolefin resin may further include a polyolefin for imparting a shutdown function. As an example of the polyolefin which provides a shutdown function, there exist low density polyethylene (LDPE), polyethylene wax, etc. As the low density polyethylene (LDPE), at least one selected from the group consisting of branched LDPE, linear LDPE, and ethylene / α-olefin copolymers prepared by a single site catalyst may be used, and the amount thereof may be added in the art. It can adjust suitably within the range known to.

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

In the polyolefin microporous membrane according to the present invention, the surface roughness (rms) of one or both surfaces thereof is 65 nm or less, for example, 20 to 65 nm, and in another example, 30 to 65 nm. In one example, the surface roughness (rms) of the surface in contact with the coating layer of the polyolefin microporous membrane is 65 nm or less, for example 20 to 65 nm, in another example 30 to 65 nm. If the surface roughness is less than 20 nm, the effect of improving the adhesion with the coating layer may not be sufficient due to the control of the surface roughness. On the other hand, if the surface roughness exceeds 65 nm, the surface unevenness increases at the interface of the coating layer in contact with the separator. And adversely affect the performance of the battery. When the surface roughness is in the above-described range, the polyolefin microporous membrane exhibits excellent waterborne coating properties, and as a result, the separator for batteries manufactured using the polyolefin microporous membrane has excellent strength, air permeability, heat resistance, and adhesive strength.

When measuring the surface roughness of the microporous membrane using atomic force microscopy (AFM), the cross section cut at right angles to the surface shows a curved shape. The height from the lowest to the highest point of this curve is the highest roughness. The maximum roughness of the microporous membrane according to the present invention may be 600 nm or less.

The thickness of the polyolefin microporous membrane according to the present invention is not particularly limited, and may be, for example, 3 to 50 um, and other examples, 3 to 30 um.

The method for producing the polyolefin microporous membrane of the present invention is not particularly limited. For example, the method for producing a polyolefin microporous membrane of the present invention comprises the steps of (1) melt kneading a polyolefin and a plasticizer to prepare a polyolefin solution, and (2) extruding and cooling the polyolefin solution to form a sheet-like article. (3) a first drawing step of stretching the sheet-like article to form a film, (4) removing a plasticizer from the film, (5) drying the film from which the plasticizer has been removed, and (6) the drying It may comprise a second stretching step of re-stretching the film and (7) heat treatment step.

Hereinafter, each manufacturing process will be described in detail.

(1) Preparation Step of Polyolefin Solution

The polyolefin resin and the plasticizer are melt kneaded to prepare a polyolefin solution. Melt kneading a composition comprising a polyolefin-based resin and a plasticizer can use any conventional method known in the art without limitation. For example, a method of melt kneading a polyolefin 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. For example, the polyolefin-based resin may be included in an amount of 20 to 50 wt% based on the total weight of the composition including the polyolefin resin and the plasticizer, and may be included in another example 20 to 40 wt%.

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

Non-limiting examples of plasticizers that can be used include aliphatic or cyclic such as nonan, decane, decalin, liquid paraffin (LP), liquid paraffin (or paraffin oil), paraffin wax, etc. 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; Fatty acid alcohols having 10 to 20 carbon atoms, such as palmitic alcohol, stearic acid alcohol and oleic acid alcohol, or mixtures of two or more thereof. For example, the liquid paraffin of the plasticizer is harmless to the human body, has a high boiling point and less volatile components is suitable for use as a plasticizer in the wet method.

The amount of the plasticizer is not particularly limited. For example, the plasticizer may be included in an amount of about 50 wt% to about 90 wt%, and in another example, about 60 wt% to about 80 wt% based on the total weight of the composition including the polyolefin resin and the plasticizer. .

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

(2) Formation Process of Sheet Shape

The polyolefin solution prepared in step (1) is extruded into a die having an extruder and cooled to form a sheet-like article.

When the polyolefin solution is extruded from the die and cooled to form a gel-like sheet, surface roughness can be controlled, for example, by controlling the cooling rate. As an example, it may be cooled at a rate of 100 ° C./min or more, for example 100 to 650 ° C./min, and in another example 300 to 650 ° C./min. If the cooling rate is less than 100 ℃ / min, the size of the fibrils of the gel-like sheet may be increased due to the increase in the crystallinity of the polyolefin, which may increase the surface roughness, the gel is not suitable for stretching due to the increase in stiffness due to high crystallinity Sheets of phase may be formed. The cooling method is not particularly limited, and methods known in the art may be used, such as cooling by contact with a cooling roll, cooling by cold wind, cooling by impregnation with cold water, and the like.

(3) First drawing process

The sheet-like article obtained in the 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 the tenter method, the roll method, the inflation method, the rolling method, or a combination of these methods. In the case of biaxial stretching, biaxial stretching may be performed simultaneously or any of sequential stretching may be performed.

Although the draw ratio in the first stretching step varies depending on the thickness of the sheet-like article, for example, in biaxial stretching, at least 2 times 2 times or more in any direction is appropriate, for example, in a range of 3 to 10 times. Can be.

In the first stretching process, the structure and pore size of the fibrils 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 in the range of 100 to 130 ℃, for example, may be in the range of 110 to 125 ℃. When extending | stretching in the said range, it can extend | stretch to have appropriate air permeability and mechanical strength, without blocking the pore (pore) in a sheet | seat. When the first stretching temperature exceeds the above-mentioned temperature range, the fibrils may be melted to have a thicker thickness, and the air permeability may rapidly increase after the coating layer is formed, resulting in deterioration of battery performance. On the other hand, when the first stretching temperature is lower than the above-mentioned temperature range, not only the polyolefin microporous membrane is produced without the necessary physical properties (such as porosity and air permeability), but also causes the air permeability to increase even after the coating process is performed. This can be

(4) plasticizer removal process

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

In the present invention, as the washing solvent, any organic solvent capable of extracting the plasticizer is not particularly limited and may be used. For example, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, fluorocarbon system, 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, etc. can be used. For example, when using liquid paraffin as the plasticizer, methylene chloride can be used as the organic solvent.

Since the organic solvent used in the process of extracting the plasticizer is mostly volatile and toxic, water may 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 heating drying method, an air drying method, or the like can be used.

(6) 2nd drawing process

Subsequently, the dried film is again stretched at least in the uniaxial direction or more. In the present invention, the second stretching step can be performed by the tenter method or the like in the same manner as the first stretching step 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 range of more than the crystal dispersion temperature of the polyolefin resin constituting the microporous membrane, a crystal dispersion temperature of less than + 40 ℃, another example, the crystal dispersion temperature + 10 ℃ or more, crystal Dispersion temperature + 40 ° C or less. By adjusting a 2nd extending | stretching temperature to the above-mentioned range, the fall of air permeability and the case of extending | stretching in the horizontal direction (width direction: TD direction) can prevent generation | occurrence | production of the physical property deviation of the sheet width direction. By carrying out a 2nd extending | stretching temperature in the said range, especially generation | occurrence | production of the deviation of the extending | stretching sheet width direction of air permeability can be suppressed. The crystal dispersion temperature here means the value calculated | required by the temperature characteristic measurement of dynamic viscoelasticity based on ASTMD4065. 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 step in the above-described range, the polyolefin resin can be sufficiently softened, and can be stretched uniformly by preventing breakage. In one example, the second stretching temperature may be in the range of 90 to 140 ℃, another example may be in the range of 120 to 140 ℃.

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, it is 1.0 to 1.8 times in the longitudinal direction (machine direction: MD direction) or TD direction. In the case of biaxial stretching, it adjusts 1.0-1.8 times in MD direction and TD direction, respectively. In the case of biaxial stretching, each draw ratio in the MD direction and the TD direction may be the same or different. By adjusting the draw ratio in the above-mentioned range, the fall of permeability, electrolyte absorbency, and compression resistance can be prevented, and the fibril becomes too thin and the fall of heat shrink resistance can be prevented.

(7) heat treatment process

Then, the redrawn film is fixed and heat treated to obtain a microporous membrane. As the heat treatment method, a conventional method known in the art may be performed without limitation, and for example, a heat setting treatment and / or a heat relaxation treatment may be used.

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

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

In addition, 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 can be carried out in the range of 20% or less relaxation rate, for example in at least one direction, and the other example is performed in the range of 10% or less relaxation rate.

<Water-based coating porous film>

The present invention provides a coated porous membrane having an aqueous coating layer formed on the aforementioned polyolefin microporous membrane. The method of forming the coating layer is not particularly limited and may be prepared, for example, by the following method.

Coating and drying the coating liquid composition on one or both sides of the polyolefin microporous membrane according to the present invention to form a coating layer. At this time, a coating layer is formed on the surface whose surface roughness of a polyolefin microporous film is 65 nm or less.

The coating liquid composition according to the present invention includes a binder polymer and an aqueous solvent, and may further include inorganic particles.

Non-limiting examples of the binder polymer 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, polyvinylsulfo Nate, Chlorosulfonated Polyethylene, Perfluorinated Sulfonated Ionomer, Sulfonated Polystyrene, Styrene-Acrylic Acid Copolymer, Sulfonated Butyl Rubber, Polyvinylidene Floride (PVdF), Polyvinylidene Floride-hexafluoro Lopropylene (PVdF-HFP), Polyvinylpyrrolidone, Polyacrylonitrile, Polyvinylidene fluoride-trichloroethylene, Polyvinylidene fluoride-chlorotrifluoroethylene (PVdF-TFE), Polymethylmethacryl Latex, polyvinylacetate, ethylene-co-vinylacetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate moiety Latex, cellulose acetate propionate, cyano ethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile styrenebutadiene copolymer, polyimide or Mixtures of two or more thereof.

Non-limiting examples of inorganic particles that can be used include alumina, aluminum hydroxide, barium titanium oxide, magnesium oxide, magnesium hydroxide, Clay, titanium oxide, glass powder, boehmite, or a mixture 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 ㎛ for uniform film thickness and proper porosity.

The content of the inorganic particles may be, for example, in the range of 50 to 90% by weight, and in another example, in the range of 60 to 85% by weight based on the total weight of the coating layer. When the content of the inorganic particles falls within the above range, it is possible to achieve a heat resistance effect by using the inorganic particles.

After the coating solution composition is prepared, it is coated and dried on one or both sides of the polyolefin microporous membrane using the same. As a method of coating on the polyolefin microporous membrane using the coating liquid composition, conventional methods known in the art may be used. As an example, a dip coating method, a die coating method, a gravure roll coating method, or a comma coating method may be mentioned. Said coating method can be applied individually or in mixture of 2 or more methods.

In the drying step of the coating layer can be carried out without limitation to conventional methods known in the art, for example, a method of drying by hot air, hot air, low humidity wind or vacuum drying or irradiation with far infrared rays or electron beams can be used.

The thickness of the porous coating layer of the present invention formed according to the method described above may be, for example, 0.01 to 20 ㎛, another example may be 1 to 12 ㎛. Within the thickness range, excellent thermal stability and adhesion can be obtained, and the thickness of the entire separator can be prevented from becoming too thick, thereby suppressing an increase in the internal resistance of the battery.

According to a specific example of the present invention, the polyolefin microporous membrane includes a first coating layer and a second coating layer formed on the first and second surfaces of the porous membrane, respectively, wherein the first coating layer and the second coating layer are components, The thickness or both may be the same or different.

For example, one of the first coating layer and the second coating layer may be a coating layer including a binder polymer and inorganic particles, and the other may be a coating layer including a binder polymer. In addition, the first coating layer and the second coating layer may have a different thickness depending on its components.

The polyolefin microporous membrane for a battery separator according to the present invention exhibits excellent water-based coating properties by controlling the surface roughness of the surface in contact with the coating layer. As a result, the water-based coated porous membrane prepared using the polyolefin microporous membrane according to the present invention, when used as a battery separator, maintains the basic physical properties required for the separator, such as maintaining excellent strength and air permeability, while maintaining excellent heat resistance and adhesion. Indicates strength.

Hereinafter, the present invention will be described in more detail with reference to 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

Al ₂ O ₃ : Dispersant (3M, FC4430): Binder (Toyochem, CSB-130) = 100: 0.3 of the aqueous slurry consisting of a weight ratio of 3, to prepare a surface roughness (RMS) of 11 11 42 nm After coating in a gravure manner on the corresponding surface of the polyolefin microporous membrane of um thickness, hot air drying to prepare the aqueous coating porous membrane of Example 1.

Example 2

A water-based coated porous membrane of Example 2 was prepared in the same manner as in Example 1, except that an 11-um thick polyolefin microporous membrane having a surface roughness (RMS) of 56 nm was used.

Example 3

A water-based coated porous membrane of Example 3 was prepared in the same manner as in Example 1, except that a 12 μm thick polyolefin microporous membrane having a surface roughness (RMS) of 39 nm was used.

Example 4

A water-based coated porous membrane of Example 4 was prepared in the same manner as in Example 1, except that a 12 μm thick polyolefin microporous membrane having a surface roughness (RMS) of 61 nm was used.

Comparative Example 1

Aqueous coating porous membrane of Comparative Example 1 was prepared in the same manner as in Example 1, except that an 11 μm thick polyolefin microporous membrane having a surface roughness (RMS) of 70 nm was used.

Comparative Example 2

Aqueous coating porous membrane of Comparative Example 2 was prepared in the same manner as in Example 1, except that a 12 μm thick polyolefin microporous membrane having a surface roughness (RMS) of 72 nm was used.

Experimental Example: Evaluation of Physical Properties of Coating Film

Physical properties of the coated porous membranes prepared in Examples 1-4 and Comparative Examples 1-2 were performed as follows, and the results are shown in Table 1 below.

(1) protrusion strength (gf)

The maximum load when the coating film was punctured at a speed of 2 mm / sec was measured with a needle having a diameter of 1 mm whose end was spherical (curvature radius R: 0.5 mm).

(2) heat shrinkage (%)

When the coating film was maintained at 120 ° C. and 130 ° C. for 1 hour, the shrinkage ratios in the longitudinal direction and the lateral direction were measured three times, and then averaged.

(3) surface roughness (RMS, Ra)

The root mean square (RMS) and roughness average (Ra) values were obtained using the data measured by AFM (Hitachi, Inc.) and the following equation.

(4) airing

Air permeability refers to the time (Sec) it takes for 100 cc of air to pass through a certain area of the coating film under a constant pressure. It measured using the Oken type spectrometer (Asahi Seiko Co. Ltd., EGO-1T) according to JISP8117 measuring method.

(5) adhesive strength

An 18 mm double-sided tape (3M) was attached to the iron plate, and a test piece (18 mm × 100 mm) to measure peel strength was adhered to the tape. Thereafter, a tensile strength meter (Instron Co., Ltd.) was pulled at 180 ° at a speed of 300 mm / min to measure the strength when the adhesive tape and the coating layer were separated.

(6) tensile strength

Tensile strength was measured using a tensile tester (Instron Co., Ltd.) by pulling a test piece having a width of 10 mm and a length of 50 mm at a top / bottom clip interval of 20 mm and a tensile speed of 100 mm / min.

(7) Maximum heat shrinkage, maximum heat shrinkage temperature

Using TMA (SII Co., Ltd.), a test piece having a width of 3 mm and a length of 100 mm was heated to 200 ° C. under a load of 19.6 mN and a heating rate of 5 ° C./min, and the maximum heat shrinkage and maximum heat shrinkage temperature were measured.

As shown in Table 1, the coated porous membrane of Examples 1-4 formed from the polyolefin microporous membrane according to the present invention exhibited the same level of protrusion, tensile strength, and air permeability as those of the coated porous membranes of Comparative Examples 1 and 2. , Compared with the coated porous membranes of Comparative Examples 1 and 2 showed excellent heat resistance and adhesive strength. Specifically, when comparing the results of Examples 1 and 2 with the coating layer having a thickness of 2 um and Comparative Example 1, almost all physical properties measured in the case of the coating porous film of Comparative Example 1 whose surface roughness is outside the scope of the present invention It can be confirmed that the physical properties of the coated porous membranes of Examples 1 and 2 are inferior. In addition, when comparing the results of Examples 3 and 4 with the coating layer having a thickness of about 4 um and Comparative Example 2, the coated porous membrane of Comparative Example 2 whose surface roughness is outside the scope of the present invention has a particularly significant effect on the safety in the battery. It can be seen that the maximum heat shrinkage of the TMA is significantly worsened.

Claims (7)

A coating porous film having a water-based coating layer formed on one or both surfaces of a polyolefin microporous membrane having one or both surface roughness (rms) of 65 nm or less and a maximum roughness of 600 nm or less. delete The coated porous membrane of claim 1, wherein the polyolefin microporous membrane comprises a polyethylene resin, a polypropylene resin, or a mixture thereof. delete The coating porous film of claim 1, wherein the aqueous coating layer is formed from a coating liquid containing a binder polymer and an aqueous solvent. The coated porous film of claim 5, wherein the coating liquid further comprises inorganic particles. The method of claim 6, wherein the inorganic particles are alumina (Alumina), aluminum hydroxide (Aluminum Hydroxide), barium titanium oxide (Barium Titanium Oxide), magnesium oxide (Magnesium Oxide), magnesium hydroxide (Magnesium Hydroxide), clay (Clay), titanium oxide (Titanium Oxide), glass powder (Glass powder), boehmite (Coehmite) or a coating porous film of a mixture of two or more thereof.