KR20000051284A - Microporous membrane and method of preparing the same - Google Patents

Abstract

PURPOSE: A preparation of producing of micropore membrane is provided which has excellent in porosity and air permeability and the process is simply, so that the membrane has homogeneous size and shape. CONSTITUTION: A process comprises the steps of: preparing a precursor film using an extruder adhered T-die polyolefin such as high density polyethylene, medium density polyethylene, low density polyethylene, linear low density polyethylene and polypropylene; annealing the prepared film carry out heat treatment not more than melting point of the polyolefin in a dry oven; scanning that insert the film in a high-vacuumed vacuumer, followed by injecting ion forming gas into ion gun to form a particle having energy, followed by scanning ionized radiation in which the particle has energy of 0.01 to 10 keV on one side or both sides of the film which is formed out of the polyolefin as ion-beam current is made to change; stretching the scanned film using a roll or a biaxial flat to form micropore at a high temperature, that is melting point of the polymer; and heat setting during the fixed time not more than melting point of the polymer under condition tension. Thereby, the membrane has excellent air permeability and increased tensile strength and Young's modules.

Description

MICROPOROUS MEMBRANE AND METHOD OF PREPARING THE SAME

[Industrial use]

The present invention relates to a method for producing a microporous membrane and a microporous membrane for a separator membrane of a battery produced by the method, in detail, to a method for producing a microporous membrane capable of producing a microporous membrane having excellent mechanical strength in a simple process. It is about.

[Prior art]

Microporous membranes are widely used in various fields, and in particular, they have recently been used as separators for lithium secondary batteries. Among lithium secondary batteries, particularly, lithium ion batteries use highly active organic solvents as electrolytes, and thus, microporous membranes having low reactivity with organic solvents and made of inexpensive polyolefin resins are mainly used as separators.

The method for producing a microporous membrane used as a separator of a battery using a polyolefin resin is to prepare a precursor film, annealing the original film, and then stretching to form micropores, followed by hot setting. )). The method mainly used as a method of manufacturing the original film includes a wet method using a filler or a wax and a solvent and a dry method using no solvent. The double dry method is mainly used in that the production of a wide disc film is relatively easy, the production process is relatively easy, and because it does not use a solvent, it can have an excellent manufacturing environment, and mass production is easy. The conventional method of manufacturing a microporous membrane using such a dry method is as follows.

U.S. Patent Nos. 3,679,538 and 3,801,692 describe the formation of micropores by continuously hot stretching a film of high crystallinity and elasticity through a continuous cold stretching process, followed by thermal A method of forming a microporous membrane by heat setting is described. U.S. Patent Nos. 3,843,761 and 4,238,459 describe a method of initially cold drawing annealing annealed films continuously and then multistage hot drawing. In addition, US Pat. No. 5,013,439 discloses a method for producing membranes having reduced pore size and increased pore density by using a multi-step cold draw process in continuous cold draw and hot draw processes. It is.

U.S. Pat.Nos. 5,385,777 and 5,480,745 present the preparation of microporous membranes through the continuous multi-step cold and hot stretching processes described above using polyethylene / polypropylene blends to improve the safety of lithium ion batteries. Doing. In addition, patents introducing a method of manufacturing a lithium ion battery separator by lamination of polyethylene and polypropylene using a dry method include European Patent Nos. 715,364, 718,901, 723,304, and US Pat. Nos. 5,240,655, 5,342,695. No. 5,472,792 and Japanese Patent Laid-Open No. 4-181651.

As described above, in the dry method for preparing a separation membrane using a crystalline polymer as a material, the relatively weak amorphous portion of the membrane is ruptured through cold drawing, thereby forming pores. Since this manufacturing process uses only pure polymers, it has the advantage of being a clean process without any problems such as solvent contamination. However, the separation membrane prepared by this process has a problem that the porosity and pore size of the membrane are somewhat reduced, so that the electrolyte and ions cannot easily pass when the membrane is used as a separator of the battery. In addition, the membrane prepared by the process is difficult to uniformly control the pore size and shape, and there is a problem in improving the porosity because there is a limit to increase the elongation to maintain the shape of the separator. In addition, the above-described methods are complicated because the low-temperature stretching and the two-step stretching of high temperature stretching must be carried out. In addition, US Pat. No. 4,994,335 has been reported to form a microporous membrane by performing a drawing process at a high temperature without performing a low temperature drawing. This method can form pores of uniform size and shape in the separator, but there is a restriction that pores can be formed only when the drawing speed is very slow.

The present invention is to solve the above problems, an object of the present invention is to provide a method for producing a microporous membrane that can form pores of uniform size and shape in the separation membrane.

Another object of the present invention is to provide a method for producing a microporous membrane which can easily form the pores of the uniform size and shape as described above in the separation membrane.

Still another object of the present invention is to provide a method for producing a microporous membrane capable of forming pores having a uniform size and shape in a separation membrane without deterioration in stretching speed.

Still another object of the present invention is to provide a microporous membrane for a separator of a battery having the above-described physical properties and useful as a battery separator.

1 is an electron micrograph of a microporous membrane prepared according to an embodiment of the present invention.

[Means for solving the problem]

In order to achieve the above object, the present invention comprises the steps of annealing a film formed of a polymer; Irradiating particles having an energy of 0.01 to 10 keV on the surface of the annealed film; Stretching the film irradiated with the particles; It provides a method for producing a microporous membrane comprising the step of heat fixing the stretched film.

The present invention also provides a fine pore membrane for the separator of the battery produced by the above method.

Hereinafter, the present invention will be described in more detail.

Irradiating ionic particles with energy to the surface of the polymer under vacuum can reduce the contact angle or increase the adhesion of the surface of the polymer (Korean Patent No. 2456). The present invention uses this principle, and relates to a method for producing a microporous membrane which forms pores by forming fine cracks by irradiating particles with energy on the surface of the polymer film prior to the stretching process. Hereinafter, the method for producing the microporous membrane of the present invention will be described in more detail.

1. Preparation of Precursor Film

Polyolefins, such as high density polyethylene, medium density polyethylene, low density polyethylene, linear low density polyethylene, and polypropylene, are made into films using an extruder with a T-die.

2. Annealing

An annealing process is performed in which the prepared film is heat treated at a temperature below the melting point of the polyolefin in a drying oven. By performing the annealing process, it is possible to increase the crystallinity and elastic recovery rate of the manufactured original film.

3. ionizing irradiation

Next, the film is introduced into a vacuum chamber maintained at high vacuum. The ion generating gas is injected into the ion gun to generate particles with energy, and the ion beam current is changed to generate particles with energy. Irradiate one or both sides of the film formed of polyolefin. At this time, by controlling the power supply connected to the ion gun so that the energy of the particles with energy is 0.01 to 10 keV. The ion generating gas may be any gas capable of generating ions, and representative examples thereof may include hydrogen, nitrogen, argon, oxygen, air, krypton, N ₂ O, and the like.

The film surface is subjected to hydrophilic or hydrophobic treatment by injecting the reactive gas injector with the ion beam irradiation while changing the amount of the reactive gas to 0.5 to 20 ml / min. Surface modification steps, such as hydrophilic or hydrophobic treatment, may be performed separately after the preparation of the microporous membrane.

4. Stretch

The ion-irradiated film is stretched uniaxially or biaxially at a temperature of room temperature (melting point-10 ° C.) using a roll or a biaxial stretching machine to form micropores having a desired size. Imparts mechanical properties to the membrane.

5. heat setting

After stretching, heat fixation for a certain period of time under tension under the melting point of the polymer.

The above steps describe the overall process for the preparation of the membrane with optimal properties, and some steps may be omitted depending on the final properties desired.

The microporous membrane prepared by this process can be usefully used as a separator of the battery.

EXAMPLE

Hereinafter, preferred embodiments of the present invention will be described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

(Example 1)

High density polyethylene was used, with a melt index of 0.3 g / 10 min and a density of 0.964 g / cc. A high density polyethylene disc film was prepared using a single screw extruder and a take-up device attached with a T-die. The extrusion temperature was 180 ° C., the cold roll temperature of the winding device was 50 ° C. and the winding speed was 35 m / min, with the winding ratio 70. The prepared original film was annealed at 110 ° C. for 1 hour in a drying oven. After the annealing, the original film was placed in a vacuum chamber maintained at 10 ⁻⁵ to 10 ⁻⁶ torr, and then argon particles (Ar ⁺ ) were irradiated on both sides of the original film using an ion gun to form fine cracks. At this time, the energy of the ion beam was 1 keV, and the ion irradiation amount was 10 ⁷ ions / cm 2. After the ion beam irradiation was finished, uniaxial stretching was performed at a stretching ratio of 150% with respect to the initial length at 100 ° C. using a roll stretching method. After stretching, the microporous membrane was prepared by heat-fixing for 2 minutes under tension using an annealing roll fixed at 115 ° C. The electron micrograph of the obtained microporous membrane is shown in FIG.

(Example 2)

Using an ion gun, 4 ml of oxygen, a reactive gas, was injected around the film using a reactive gas injector while irradiating argon particles to both sides of the original film under conditions of ion beam energy of 1 keV and ion dosage of 5 x 10 ¹⁶ ions / cm 2. A microporous membrane was prepared in the same manner as in Example 1 except that the surface treatment was performed by injection.

(Comparative Example 1)

After the annealed film was used to form fine cracks using normal temperature stretching, the microporous membrane was formed in the same manner as in Example 1 except that the high temperature stretching process was performed and heat-set. Various physical properties of the prepared membrane are shown in Table 1.

(Comparative Example 2)

The microporous membrane was prepared in the same manner as in Example 1 except that the annealed film was formed by cracking using high temperature stretching and then heat-set.

The thickness and puncture resistance of the microporous membranes prepared by the method of Example 1-2 and Comparative Example 1-2 were measured, and the results are shown in Table 1 below. In addition, the air permeability, pore size, tensile strength and tensile modulus, puncture resistance and water absorption speed of the pore membrane are as follows. Measured by the standard and method, the results are shown in Table 1 below.

1) Breathability: JIS P8117

2) Pore size: SEM, TEM

3) Tensile Strength and Tensile Modulus: ASTM D882

4) Penetration strength: The maximum value at which the film resists rupture at a puncture rate of 2 mm / sec was measured using a needle having a radius of curvature of 0.5 mm.

5) Absorption rate: JIS L-1096

In Table 1 below, MD represents a stretching direction in a mechanical direction, and TD represents a perpendicular direction of the stretching direction in a transverse direction.

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Thickness [㎛] 25 25 25 25 Breathability [sec / 100cc] 410 460 515 790 Pore Size (MD × TD) [μm × μm] 0.2 × 0.05 0.2 × 0.05 0.25 × 0.07 0.1 × 0.03 Tensile Strength (MD / TD) [kgf / cm 2] 1250/130 1300/145 900/85 1100/105 Young's modulus (MD / TD) [kgf / cm 2] 9500/3600 9800/3800 6200/2300 8500/3200 Piercing strength 293 302 280 320 Absorption Rate [sec] 5.5 3.4 6.2 8.4

As shown in Table 1, it can be seen that the microporous membrane prepared by the method of Example 1-2 has superior air permeability and uniform pore size than the microporous membrane prepared by the method of Comparative Example 1-2. . In addition, in Table 1, it can be seen that the absorption rate of Example 2 is high, indicating that the hydrophilicity of the microporous membrane surface-treated with the hydrophilic reactive gas together with the ion beam irradiation is increased.

In the present invention, before the stretching process, the ion beam is irradiated onto the disc film to form fine cracks, thereby forming pores. Show physical properties In addition, since the stretching process is performed only once at a constant temperature, the process is simple, and thus it is possible to economically form pores having a uniform size and shape. In addition, an increase in tensile strength and Young's Modulus in the machine direction (MD) and in the vertical direction (TD) could be observed.

Claims (6)

Annealing a film formed of a polymer; Irradiating the surface of the polymer with particles having an energy of 0.01 to 10 keV on the annealed film; Stretching the film irradiated with the particles; Heat fixing the stretched film Method for producing a microporous membrane comprising a. The method of claim 1, The drawing step is carried out at a temperature of room temperature-(melting point of the polymer-10 ℃). The method of claim 1, Wherein said irradiating step is carried out in the presence of a reactive gas. The method of claim 1, The polymer is a polyolefin production method. The method of claim 4, wherein The polyolefin is selected from the group consisting of polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene and laminates thereof. Annealing a film formed of a polymer; Irradiating the surface of the polymer with particles having an energy of 0.01 to 10 keV on the annealed film; Stretching the film irradiated with the particles; Heat fixing the stretched film Fine pore membrane for battery separator prepared by.