KR100506159B1 - Microporous Polyethylene film for battery separator and Method of preparing the same - Google Patents

Abstract

The present invention relates to a porous polyethylene film for a battery separator and a method for manufacturing the same, more specifically, 20 to 40% by weight of a high density polyethylene resin having a melt flow index of 0.2 to 0.5 g / 10 min and a density of 0.960 to 0.969 g / cm ³ , and melting ⁴ to 20% by weight of a high density polyethylene resin having a flow index of 0.02 to 0.1 g / 10 min and a density of 0.950 to 0.958 g / cm3, 40 to 70% by weight of paraffin oil and 5 to 15% by weight of DOP, 0.1 to 0.5 weight of nucleating agent Percentage and 0.1 to 0.5% by weight of the antioxidant relates to a porous polyethylene film for a battery isolation film and a method for producing the same. The porous film thus obtained has a tensile strength of 1,000 to 1,300 kg / cm ² , an impact strength of 500 to 700 g / μm, a fine pore heat closing temperature of 130 ° C. to 135 ° C., a hot melting temperature of 145 ° C. to 155 ° C., and a porosity. 30 ~ 70%, average pore size of 0.05 ~ 0.1㎛, especially good surface smoothness, it is used as a separator, filter for separation and microfiltration of various batteries, especially suitable as a lithium ion battery separator. .

Description

Microporous polyethylene film for battery separator and method of preparing the same

The present invention relates to a porous polyethylene film for battery isolation membrane and a method for manufacturing the same, and more particularly, separator, separation filter, and microfiltration of various batteries due to excellent mechanical strength, chemical resistance, minimization of ion conductivity, etc. The present invention relates to a polyethylene film having a fine porous structure that can be used as a membrane and a method for manufacturing the same.

As a separator of a conventional primary battery and a secondary battery, nonwoven fabrics made of polyamide-based and polypropylene-based, polyolefin-based porous sheets or films, and the like have been variously used. In particular, the lithium ion battery maintains safety because the micropores are closed at the polyethylene melting point, and the porous polyolefin film for lithium ion battery isolation membrane can be largely divided into a dry method and a wet method according to the production method thereof.

The method for producing a porous polyethylene film by the wet method was already known by US Patent No. 4,247,498 in 1981. The patent uses a polyolefin and a plasticizer, and the polyolefin porous film by solid / liquid phase separation or liquid / liquid phase separation method. Processes for preparing the same are described.

Meanwhile, in the 1990s, Japanese Patent Application Laid-Open No. 06-263930, Japanese Patent Application Laid-Open No. 05-222237, Japanese Patent Application Laid-Open No. 05-222236, and U.S. Patent No. 5,051,183 used ultra-high molecular weight polyethylene having a molecular weight of 700,000 or more. A method for producing a porous polyethylene film for separators is disclosed. In addition, Japanese Patent Application Laid-Open No. 05-9332, Japanese Patent Application Laid-Open No. 05-234578, and US Pat. No. 5,641,565 describe a method for producing a porous polyethylene film for separator using ultra high molecular weight polyethylene having a molecular weight of 1 million or more.

As described above, although the mechanical strength of the separator film can be improved by using a mixture of an ultra high molecular weight polyethylene resin with a general polyethylene resin, processing is very difficult. Particularly, the ultra high molecular weight polyethylene resin has a very high molecular weight, so that blending with a general polyethylene resin requires a long kneading process in a batch kneader such as an autoclave. In the case of kneading in a twin extruder, which is a conventional continuous kneader, since the molecular weight difference between ultra high molecular weight polyethylene resin and general polyethylene resin is very large, a compound that is uniformly kneaded cannot be obtained and the film for separation membrane is processed. The smoothness of the surface of the film is very poor.

Therefore, in order to obtain a uniform film by mixing an ultra high molecular weight polyethylene resin with a general polyethylene resin, a kneading time of about 0.5 to 2 hours is required in a kneading machine that can be kneaded for a long time such as a thermal stirrer. There is a disadvantage in that productivity is reduced due to such a long kneading process, and also because the use of expensive ultra-high molecular weight polyethylene resin is a factor of cost increase.

In order to overcome this problem, the present inventors have conducted extensive research, and have excellent mechanical properties by blending and processing two general polyethylenes having different molecular weights and densities without mixing ultra high molecular weight polyethylene resins used in conventional wet methods. While it was found that the porous polyethylene film composition for battery separator can be easily processed, the surface smoothness is good, and the cost can be reduced, the present invention has been completed based on this.

Accordingly, it is an object of the present invention to provide a porous polyethylene film for battery isolation membrane that can have excellent mechanical properties, easy processing, good surface smoothness, and cost reduction.

Another object of the present invention to provide a method for producing the porous polyethylene film.

The composition of the present invention for achieving the above object is 20 to 40% by weight of the melt flow index of 0.2 to 0.5g / 10min and density 0.960 to 0.969g / cm ³ high density polyethylene resin, 0.02 to 0.1g / 10min of melt flow index of 0.950~0.958g / cm ³ at a high density polyethylene resin 4-20% by weight, 40 to 70% by weight of paraffin oil and DOP 5~15 mixing a plasticizer, a nucleating agent, 0.1~0.5% by weight and 0.1~0.5 wt% of an antioxidant by weight Is done.

In addition, the manufacturing method of the present invention for achieving another object is 20 to 40% by weight of the melt flow index of 0.2 to 0.5g / 10min and density 0.960 to 0.969g / cm ³ high density polyethylene resin, 0.02 to 0.1g / melt flow index ⁴ to 20% by weight of high density polyethylene resin, 10 to 40% by weight of paraffin oil and 5 to 15% by weight of DOP, 0.1 to 0.5% by weight of nucleating agent and 0.1 to 0.5 of antioxidant, 10 min and density 0.950 to 0.958 g / cm ³ Adding a weight% to a twin-screw kneader with T-die and kneading for 1-3 minutes to form a sheet of 500 to 1,000 mu m; Simultaneously stretching the sheet in the transverse and longitudinal axes with a film of 20-30 μm in a biaxial stretching machine; Extracting paraffin oil and DOP from the film using an organic solvent and drying at 50 to 70 ° C .; And heat treating the film at 110 to 130 ° C. for 1 minute.

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

As described above, the porous polyethylene film of the present invention does not use an ultra high molecular weight polyethylene resin in a conventional manner, but has a melt flow index of 0.2 to 0.5 g / 10 min (30 to 450,000 in terms of GPC weight average molecular weight). And 20-40% by weight of high density polyethylene (HDPE 1, hereinafter referred to as "HDPE 1") having a density of 0.960 to 0.969 g / cm 3 and a melt flow index of 0.02 to 0.1 g / 10 min (in terms of GPC weight average molecular weight). ), ⁴ to 20% by weight of high density polyethylene (HDPE 2, hereinafter referred to as "HDPE 2"), 40 to 70% by weight of paraffin oil, 5 to 15% by weight of DOP (dioctyl phthalate) with a density of 0.950 to 0.958 g / cm ³ It consists of a mixed plasticizer, 0.1-0.5 weight% of nucleating agents, and 0.1-0.5 weight% of antioxidant.

Since ultra high molecular weight polyethylene resin is not used, it is not necessary to knead for a long time in an autoclave or a mixer, but in a conventional twin extruder to obtain a uniformly kneaded compound. Because of this, the final film having good surface smoothness can be formed.

If the melt flow index of the used HDPE 1 is 0.2g / 10min or less, the weight average molecular weight is too large, so that it is difficult to form a uniform sheet, and if it is 0.5g / 10min or more, the mechanical strength becomes weak. Even if the melt flow index is less than 0.02g / 10min, uniform sheet forming is difficult, and if it is more than 0.1g / 10min, the mechanical strength becomes weak. On the other hand, when the composition of the used HDPE 1 is 20% or less, the heat resistance of the film is weakened, the charge or discharge characteristics are lowered, and when 40% or more, the mechanical properties become weak. In addition, when the composition of HDPE 2 is 4% or less, the workability of the film is lowered, and when 20% or more, a problem of very weak heat resistance occurs.

In addition, in the present invention, a mixture of paraffin oil and DOP is used as a plasticizer, and the kinematic viscosity of the paraffin oil used here has a viscosity of about 40 to 450 cSt (40 ° C.). If the kinematic viscosity is 450cSt (40 ℃) or more has a disadvantage in that it takes a long time to extract from the organic solvent, if it is 40cSt (40 ℃) or less there is a difficulty in sheet molding. In addition, when the paraffin oil is less than 40% by weight, it is difficult for the film to have a porosity of 30% or more, and when it exceeds 70% by weight, swell and neck-in phenomenon appear during sheet molding in the extruder. There is. In addition, if the DOP is less than 5% by weight, the size of the pores on the surface of the film is too small, and if it exceeds 15% by weight, the compatibility with the polyethylene resin is lowered and the smoothness of the sheet surface becomes very poor. Such a mixed plasticizer should have a boiling point at 230 ° C. or higher higher than the extruder processing temperature. If the mixed plasticizer has a boiling point lower than the extruder processing temperature, the plasticizer is decomposed to obtain a uniform sheet surface and fine pores cannot be obtained.

Meanwhile, the present invention provides a uniform pore diameter distribution by adding a nucleating agent. If the pore size is too small, the flow of ions is interrupted and the life of the battery is short when it is used in the battery. If the pore size is too large, safety problems occur when the battery is abnormal due to excessive flow of ions. Therefore, the present invention uses a small amount of K-sterate as a nucleating agent, that is, 0.1 to 0.5% by weight to make the crystal structure of polyethylene uniformly, and to uniformly adjust the pore size in the film, as shown in FIG. By narrowing the degree of distribution, the battery life and safety can be improved. The porous separator thus obtained has excellent mechanical properties and exhibits excellent battery safety and low electrical resistance when used as a lithium ion battery isolation membrane.

In addition, the present invention is 20 to 40% by weight of HDPE 1, 4 to 20% by weight of HDPE 2, 40 to 70% by weight of paraffin oil, 5 to 15% by weight of DOP, plasticizer of 0.1 to 0.5% by weight of nucleating agent and 0.1 to 0.5% of antioxidant. Adding 0.5% by weight to a twin-screw kneader with a T-die and kneading for 1 to 3 minutes to form a sheet of 500 to 1,000 mu m;

Simultaneously stretching the sheet in the transverse and longitudinal axes with a film of 20-30 μm in a biaxial stretching machine;

Extracting paraffin oil and DOP from the film using an organic solvent and drying at 50 to 70 ° C .; And

The film is subjected to a heat treatment step at 110 to 130 ° C. for 1 minute. In general, a kneading method may be a twin screw extruder, a single screw extruder, a banbury kneader, or the like, but the present invention is characterized by using a twin screw kneader. Sheet forming is obtained by cooling a sheet of 500 µm to 1,000 µm in a quench roll in a twin screw extruder with a T-die. Moreover, it is preferable that the said sheet is obtained by 600 micrometers. This can adjust the thickness of a final film to 20-30 micrometers when it biaxially stretches 5 to 6 times on the horizontal axis and the vertical axis.

In addition, the stretching method may be used uniaxial stretching, simultaneous biaxial stretching, sequential stretching and the like, but in the present invention is characterized in that the simultaneous biaxial stretching to make a spherical form of pores, stretching to 20 ~ 30㎛ which is a lithium ion battery It is the thickness of the film most used for the separator of, and 25 micrometers is preferable.

In addition, the film thus obtained is extracted with paraffin oil and DOP in organic solvents methyl ethyl ketone, methyl chloride, etc., dried at 50 ~ 70 ℃ to remove the remaining organic solvent and plasticizer. .

Since the film obtained as described above undergoes heat shrinkage, it is possible to maintain dimensional stability against temperature change by performing heat treatment in advance. The heat treatment temperature is the largest heat treatment effect at 110 ~ 130 ℃ the softening point of the polyethylene resin, the heat treatment time can be from 30 seconds to 10 minutes depending on the heat treatment temperature, 1 minute is most suitable in the above temperature range.

The porous film thus obtained is easier to process than the conventional products and has a uniform pore size and distribution, and thus exhibits excellent characteristics when used as a lithium ion battery separator. Particularly, the tensile film has a tensile strength of 1,000 to 1,350 kg / cm ² and an impact strength of 500. ~ 700g / ㎛, micropore shut down temperature is 125 ℃ ~ 135 ℃, melt down temperature is 145 ℃ ~ 155 ℃, porosity is 30 ~ 70%, average pore The average pore diameter has the property of 0.05 to 0.1 탆 and also has excellent surface smoothness.

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

Physical properties of the film were measured by the following method.

Melt Flow Rate (ASTM D-1238)

Tensile Stength at Break (ASTM-D-882)

-Impact Resistance (Puncture Resistance, ASTM D-3763)

Porosity (ASTM-D-1622)

-Gel Permeation Chromatograph

-Shut Down Temperature

: Unfold the film of constant width and length in the longitudinal direction and fix the upper and lower parts to the frame and then use 80 ℃, 90 ℃, 100 ℃, 110 ℃, 120 ℃, 130 ℃, 140 ℃, 150 ℃ in the oven. After heating for 15 minutes at each temperature, and cooled to room temperature, the electrical resistance of the separator was measured at each temperature, and the heat closing temperature was measured by graphing the heating temperature and the electrical resistance to obtain a temperature at which the electrical resistance rapidly increased. .

-Melt Down Temperature

: Unfold the film of constant width and length in the longitudinal direction and fix the upper and lower parts to the frame, and then use the oven oven at 100 ℃, 110 ℃, 120 ℃, 130 ℃, 140 ℃, 150 ℃, 160 ℃. The temperature at which the film breaks when heated for 15 minutes at the temperature was determined to determine the thermal melting temperature.

Air Permeation (ASTM-D-726)

Initial electrical resistance measurement

: The electrolyte LiPF ₆ (1 mole / L) was dissolved in a mixture of ethylene carbonate and diethyl carbonate 1: 1 as an electrolyte solution, and an initial resistance was obtained by applying an alternating current of 1 Khz to an ohmmeter.

R = (R ₁ -R ₀ ) × A

Where R _o : electrical resistance measurement of electrolyte

R ₁ : Install isolation membrane and measure electrical resistance

A: surface area of the separator

Example 1

The melt flow index of 0.3g / 10min, a density of 0.964g / ㎤ of high density polyethylene (HDPE 1) 36% by weight, melt flow index of 0.04g / 10min, density 0.956g / cm ³ High density polyethylene (HDPE 2) 4 parts by weight In a mixed plasticizer consisting of 50% by weight of flowable paraffin oil and 10% by weight of DOP in a% and kinematic viscosity of 80 cst (40 ° C), 0.2% by weight of nucleating agent K-sterite and Irganox 1010 (Shiba-Geigi) antioxidant 0.1% by weight of 0.15% by weight of antioxidant Iganox 1076 (Shiba-Geigy) was added and kneaded in a twin screw extruder, followed by forming a sheet having a thickness of 600 μm in a twin screw extruder with a T-die, and then sufficiently After cooling, the film was drawn at a longitudinal axis of 6 times and a horizontal axis of 6 times in a simultaneous biaxial drawing machine, and then extracted with paraffin oil and DOP in methyl ethyl ketone, which is an organic solvent, dried at 60 ° C, and then heat treated at 120 ° C for 1 minute. A film was obtained. Mechanical and electrical properties of the porous film thus obtained were measured and shown in Table 1 below.

Example 2

Except for adding 28% by weight of HDPE 1, 12% by weight of HDPE 2 was carried out in the same manner as in Example 1, it was shown in Table 1 to measure the mechanical and electrical properties of the porous film thus obtained.

Example 3

Except for adding 24% by weight of HDPE 1, 16% by weight of HDPE 2 was carried out in the same manner as in Example 1, it was shown in Table 1 to measure the mechanical and electrical properties of the porous film thus obtained.

Example 4

20% by weight of HDPE 1 and 20% by weight of HDPE 2 were added in the same manner as in Example 1, and the mechanical and electrical properties of the thus obtained porous film were measured and shown in Table 1 below.

Comparative Example 1

The melt flow index of 0.3g / 10min, density 0.964g / cm ³ of high density polyethylene resin 36% by weight, the molecular weight (Mw) 100 all men ultra high molecular weight polyethylene resin 4% by weight, and the liquid paraffin has a kinematic viscosity of 50 80cSt (40 ℃) To the mixed plasticizer consisting of a weight percent and 10% by weight of DOP was carried out in the same manner as in Example 1 by adding 0.1% by weight of antioxidant Iganox 1010, 0.1% by weight of antioxidant Iganox 1076, Mechanical and electrical properties were measured and shown in Table 2 below. As a result, the smoothness of the surface of the separator film is obtained very poorly as compared with Examples 1, 2, 3, and 4.

Comparative Example 2

36 wt% high density polyethylene resin with a melt flow index of 0.04 g / 10 min, density 0.956 g / cm ³ , 4 wt% ultra high molecular weight polyethylene resin with a molecular weight (Mw) of 1 million, and flowable paraffin oil 50 with kinematic viscosity of 80 cSt (40 ° C.) Mechanical properties and electrical properties of the porous film obtained in the same manner as in Example 1 by adding 0.1% by weight of antioxidant Iganox 1010 and 0.1% by weight of antioxidant Iganox 1076 to a mixed plasticizer consisting of a weight% and 10% by weight of DOP Physical properties were measured and shown in Table 2 below. Compared with Example 1, 2, 3, 4, the smoothness of the surface of a separator film is obtained very poorly.

Comparative Example 3

Antioxidant Iganox in a mixed plasticizer composed of 40% by weight of a high density polyethylene resin having a melt flow index of 0.8g / 10min, a density of 0.954g / cm ³ , a kinematic viscosity of 80cSt (40 ° C), fluidic paraffin and 50% by weight of DOP. 0.110% by weight of 1010, 0.176% by weight of antioxidant Iganox 1076 was added to measure the mechanical and electrical properties of the porous film obtained in the same manner as in Example 1 shown in Table 2 below. Compared with Examples 1, 2, 3, and 4, the tensile and impact strengths of the separator film are inferior.

Comparative Example 4

Composed of 0.3g / 10min melt flow index, 32% by weight high density polyethylene resin with density of 0.964g / cm ³ , 8% by weight ultra high molecular weight polyethylene with 1 million molecular weight, and 60% by weight of 80cSt (40 ° C) flowable paraffin oil 0.1 wt% of antioxidant Iganox 1010 and 0.1 wt% of antioxidant Iganox 1076 were added to the mixture, and the mechanical and electrical properties of the porous film obtained in the same manner as in Example 1 were measured and shown in Table 2 below. . Compared with Example 1, 2, 3, 4, the smoothness of the surface of a separator film is obtained very poorly.

Example 1 Example 2 Example 3 Example 4 Polyethylene Composition High density polyethylene: HDPE 1 (MFR: 0.3, density: 0.964) 36 28 24 20 High Density Polyethylene: HDPE 2 (MFR: 0.04, Density: 0.956) 4 12 16 20 Drawing condition Elongation ratio 6 * 6 6 * 6 6 * 6 6 * 6 Drawing temperature 115 ℃ 115 ℃ 115 ℃ 115 ℃ Properties Tensile Strength (Kg / cm ² at Break) 1220 1310 1200 1100 % Of acid 70 85 75 70 Impact strength (g / ㎛) 655 650 670 700 Air permeability (sec / 100cc) 550 545 542 552 Porosity (%) 42 43 43 42 Pore size (㎛) 0.07 0.07 0.08 0.08 Electric resistance (Ωcm ² ) 2.3 2.2 2.3 2.4 Heat closing temperature (℃) 135 130 127 125 Hot Melt Temperature (℃) 155 152 150 145 Surface smoothness Good Good Good Good Film thickness (㎛) 25 25 25 25

Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Polyethylene Composition High Density Polyethylene (MFR: 0.3, Density: 0.964) 36 - - 32 High Density Polyethylene (MFR: 0.04, Density: 0.956) - 36 - - High Density Polyethylene (MFR: 0.8, Density: 0.954) - - 40 - Ultra high molecular polyethylene (molecular weight Mw: 1 million) 4 4 - 8 Drawing condition Elongation ratio 6 * 6 6 * 6 6 * 6 6 * 6 Drawing temperature 115 ℃ 115 ℃ 115 ℃ 115 ℃ Properties Tensile Strength (Kg / cm ² at Break) 1204 1310 917 1327 % Elongation 84 80 99 82 Impact strength (g / ㎛) 770 800 460 816 Air permeability (sec / 100cc) 510 530 560 515 Porosity (%) 46 44 40 45 Pore size (㎛) 0.08 0.08 0.09 0.09 Electric resistance (Ωcm ² ) 2.3 2.4 2.5 2.4 Heat closing temperature (℃) 155 148 145 155 Hot Melt Temperature (℃) 165 155 155 160 Surface smoothing Bad Bad Good Bad Film thickness (㎛) 25 25 25 25

As can be seen from the above examples and comparative examples, the porous polyethylene film according to the present invention has excellent tensile strength, impact strength, and the like, even when compared to a film prepared using ultra high molecular weight polyethylene in physical properties. You can get it. In addition, since the porous polyethylene film of the present invention does not use an ultra high molecular weight polyethylene resin, its processing is easy, and it can be carried out in a conventional twin screw extruder without having to knead it for a long time in a heat stirrer, a mixer, etc. to obtain a uniformly kneaded compound. The film with favorable smoothness of the film surface can be shape | molded.

1 is a graph comparing the pore size distribution of the film according to the present invention and the pore size distribution of the film according to the conventional method without the addition of the nucleating agent.

Claims (5)

Melt flow index 0.2~0.5g / 10min and high density polyethylene resin with a density 0.960~0.969g / high density polyethylene resin, 20 to 40% by weight of cm ^3, melt flow index 0.02~0.1g / 10min and a density 0.950~0.958g / cm ³ 4-20% by weight, paraffin oil 40-70% by weight and DOP 5-15% by weight of mixed plasticizer, nucleating agent 0.1-0.5% by weight and antioxidant 0.1-0.5% by weight porous polyethylene film . The porous polyethylene film of claim 1, wherein the nucleating agent is K-sterate. The film according to claim 1, wherein the film has a tensile strength of 1,000 to 1,300 kg / cm ² , an impact strength of 500 to 700 g / μm, a fine pore heat closing temperature of 130 ° C. to 135 ° C., a heat melting temperature of 145 ° C. to 155 ° C., and a porosity of 30. Porous polyethylene film, characterized in that ~ 70%, the average pore size of 0.05 ~ 0.1㎛. High density polyethylene resin with melt flow index of 0.2 to 0.5 g / 10 min and density 0.960 to 0.969 g / cm ³ 20 to 40% by weight, high density polyethylene resin with melt flow index 0.02 to 0.1 g / 10 min and density 0.950 to 0.958 g / cm ³ 4 to 20% by weight, 40 to 70% by weight of paraffin oil and 5 to 15% by weight of DOP, 0.1 to 0.5% by weight of nucleating agent and 0.1 to 0.5% by weight of antioxidant are added to a twin-screw kneader with T-die. Kneading for 1-3 minutes to form a sheet of 500-1,000 mu m; Simultaneously stretching the sheet in the transverse and longitudinal axes with a film of 20-30 μm in a biaxial stretching machine; Extracting paraffin oil and DOP from the film using an organic solvent and drying at 50 to 70 ° C .; And Method for producing a porous polyethylene film for a battery isolation membrane, characterized in that the film was heat-treated at 110 to 130 ° C. for 1 minute. The method according to claim 4, wherein the paraffin oil has a kinematic viscosity of 40 to 450 cSt at 40 ° C.