KR20140085728A - Polyolefin microporous membrane and method of manufacturing the same - Google Patents

Abstract

Disclosed is a polyolefin microporous membrane having excellent hydrophilicity, high mechanical strength and porosity, and no difficulties in a preparing process, while the polyolefin microporous member can sufficiently manage a rapid increase in temperature inside a battery. The porous membrane comprises 0.1-1 wt% of cellulose with respect to polyolefin resin composed of 90-97 wt% of polyethylene and 3-10 wt% of polypropylene.

Description

TECHNICAL FIELD The present invention relates to a polyolefin microporous membrane,

The present invention relates to a polyolefin microporous membrane, and more particularly, to a polyolefin microporous membrane used for a separator or the like of a battery.

Polyolefin microporous films are widely used for various battery separators, separating filters, and microfiltration membranes due to their chemical stability and excellent physical properties. In the secondary battery separator, there is an increasing demand for the safety of the battery, and the thermal stability of the separator becomes very important in accordance with the high capacity and high output of the secondary battery. Particularly, in the case of a lithium secondary battery, if the thermal stability of the separator is lowered, damage or deformation of the separator due to overheating of the battery and short-circuit between the electrodes may occur, which may lead to overheating or fire of the battery.

On the other hand, in order to improve the heat resistance, a method of blending polyethylene and polypropylene at 10 wt% or more has been proposed. However, after the porous film is melted due to overheating, fracture occurs while the porous film is flowing and lack of compatibility of polyethylene and polypropylene And does not bring satisfactory thermal stability. Accordingly, there is a demand for a microporous membrane excellent in compatibility with polyethylene and polypropylene and having high resistance to heat shrinkage and having excellent thermal stability.

A problem to be solved by the present invention is to provide a polyolefin microporous membrane having excellent compatibility between polyethylene and polypropylene and preventing shrinkage due to heat to prevent internal short-circuiting of the battery, thereby improving battery stability.

The polyolefin microporous membrane for solving the problems of the present invention comprises 0.1 to 1% by weight of cellulose relative to a polyolefin resin consisting of 90 to 97% by weight of polyethylene and 3 to 10% by weight of polypropylene. At this time, the amount of the cellulose is more preferably 0.3 to 0.7% by weight. The cellulose may be in the form of fibers and may have a length of 20 to 500 mu m.

According to the polyolefin microporous membrane of the present invention, by adding 0.1 to 1% by weight of cellulose to a polyolefin resin composed of 90 to 97% by weight of polyethylene and 3 to 10% by weight of polypropylene, heat shrinkage is suppressed, And the mechanical strength and the porosity can be increased.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments described below can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

An embodiment of the present invention relates to a process for producing a polyolefin resin comprising 3 to 10% by weight of polypropylene by adding cellulose to polyolefin resin with good compatibility between polyethylene and polypropylene and suppressing heat shrinkage, And shows a polyolefin microporous membrane having high mechanical strength and high porosity. For this purpose, a process for producing a microporous membrane by adding cellulose to polyolefin will be described, and characteristics of the finished microporous membrane will be described. The microporous membrane of the present invention can be produced by a wet and dry method, and in the embodiment of the present invention, a wet method is exemplified.

The embodiment of the present invention is intended to lower heat shrinkage of the microporous membrane at high temperature, which can be confirmed by shrinkage of heat. The heat shrinkage ratio is [(L ₀ -L) / L ₀ ] x 100 (%) where L ₀ is the initial specimen length and L is the specimen length after heat treatment at 150 ° C for 1 hour. At this time, the embodiment of the present invention aims at lowering the heat shrinkage rate to 5% or less. On the other hand, a conventional polyethylene microporous membrane has a heat shrinkage of not less than 40% under the above-mentioned heat treatment condition, and thus it is difficult to use it as a separator for a battery.

Cellulose is a molecular formula (C ₆ H ₁₀ O ₅ ) _n , which is a saccharide that constitutes a major component of cell walls of higher plants, and is also called fibrin. The physical properties of cellulose are mainly crystalline, and the flexibility and swelling properties are mainly due to the amorphous region. In the crystal region, glucose residues in the form of isoform CI were supported in hydrogen bonds, glucoside bonds and van der Waals forces in the directions of a-axis, b-axis and C-axis respectively so that a = 8.35 Å, b = 10.3 Å, c = 7.9 A unit cell exists. The proper properties of the cellulose, such as strength, elongation, elasticity and wettability, are determined by the appropriate arrangement of the crystal region and the amorphous region.

The cellulose used in the present invention has less shrinkage than polyolefins, has excellent heat resistance, and has high wettability. On the other hand, when the shrinkage percentage is smaller than that of the polyolefin, the porosity can be increased when the microporous membrane is produced, and when the heat resistance is good, the thermal characteristics such as the shutdown characteristic are excellent. When the wettability is high, the electrolyte solution is easily absorbed, and if the mechanical strength is high, the microporous membrane can be firmly supported. Thus, cellulose is effective for improving the physical properties of the polyolefin microporous membrane.

In the embodiment of the present invention, 30 to 70% by weight of a resin consisting of 0.1 to 1% by weight of cellulose is added to the entire polyolefin resin consisting of 90 to 97% by weight of polyethylene and 3 to 10% by weight of polypropylene, and the remainder is a liquid or wax- By weight of a composition containing at least one selected from a polymer, a polyester plasticizer or an epoxy plasticizer, a fluorine surfactant and other additives in an amount of 70 to 30% by weight is formed into a sheet shape and then stretched. For convenience of explanation, liquid or wax-like hydrocarbon polymers, polyester plasticizers, epoxy plasticizers, fluorosurfactants and other additives are collectively referred to as additives.

Polyethylene means a unit derived from 50% or more repeating ethylene, preferably a polyethylene homopolymer and a polyethylene copolymer wherein at least 85% of the repeating units are ethylene units. Polypropylene means a unit derived from more than 50% repeating propylene, preferably a polypropylene homopolymer and a polypropylene copolymer wherein at least 85% of the repeat units are propylene units.

At this time, the content of the cellulose fiber is 0.1 to 1% by weight, preferably 0.3 to 0.7% by weight, based on the polyolefin resin. That is, when the content of cellulose is smaller than the predetermined ratio, the heat shrinkage ratio of the microporous membrane to be implemented in the present invention is not improved. On the contrary, when the content is larger than the predetermined ratio, cellulose is added more than necessary to improve heat shrinkage. The cellulose is preferably in a fiber form, and the length is preferably 20 to 500 mu m. If the length is less than 20 탆, the effect of improving the physical properties of the microporous membrane to be implemented in the present invention is weakened. If the length is more than 500 탆, the surface of the microporous membrane may become rough. When cellulose is used in a fiber form, the fiber reinforcing effect can be imparted to increase the strength of the microporous membrane.

Examples of the liquid or wax-like hydrocarbon polymer include saturated or unsaturated hydrocarbons such as polybutadiene, polybutene, and polyisoprene; compounds having a hydrogen group at the terminals of the saturated or unsaturated hydrocarbon; and hydrogenated polyhydroxy saturated or unsaturated hydrocarbons And the like. However, it is preferable to check the solubility of the cell in the organic electrolyte solution in advance and use one having a low solubility. Examples of the polyester plasticizer used in the present invention include dibasic acid or tribasic acid such as sebacic acid, adipic acid, phthalic acid, azelaic acid and trimellitic acid, and ethylene glycol, propylene glycol, butyleneglycol, A polyester compound composed of adipic acid or sebacic acid and propylene glycol, butylene glycol or long-chain alkylene glycols is preferably used as the polyester compound composed of tricol recol and long-chain alkylene glycol. As the epoxy-based plasticizer, epoxidized soybean oil and epoxidized linseed oil may be used alone or in combination.

 The plasticizer and the surfactant can easily obtain a good microporous membrane by the action of imparting the homogeneous dispersibility of the cellulose fiber or the uniform stretching property of the film. In order to obtain a more uniform porosity distribution and a more uniform pore diameter distribution, it is preferable to use the above-mentioned plasticizer and a liquid-phase or wax-like hydrocarbon polymer.

Examples of the fluorine-based surfactant include a fluorocarbon compound in which the hydrogen atom of the hydrophobic group of the hydrocarbon-based surfactant is entirely or partly replaced with a fluorine-based atom. Examples of such a compound include an oxonium acid, a perfluoroalkylsulfonic acid, a perfluoroalkylsulfonic acid, Perfluoroalkyl quaternary ammonium iodides, perfluoroalkylpolyoxyethylene ethers, fluorinated alkyl esters of nonionic type, and the like, which are composed of potassium perborate, potassium perborate, perfluoroalkyl quaternary ammonium iodide, and perfluoroalkyl quaternary ammonium iodide, Iodide or a fluorocarbon compound comprising a fluorinated alkyl ester is preferably used.

The composition comprising the polyolefin resin and the cellulose fiber is generally prepared by using a high-speed stirrer such as a super mixer or a Henschel mixer, for example, adding a liquid or wax-like hydrocarbon polymer or a fluorosurfactant to a filler, The mixture and the polyolefin resin powder or the pelletized product are mixed and kneaded by a common uniaxial or biaxial screw extruder. In such a process, additives such as a colorant, a lubricant, a processing aid, an antioxidant, and an antistatic agent may be mixed together at the same time or separately as required.

The method of molding the above polyolefin-based resin composition into a sheet shape is not particularly limited, but an extrusion molding method using a T-die is generally preferable. Next, the sheet molded product can be generally prepared by stretching 2 to 10 times in general by axial stretching or simultaneous biaxial stretching, then extracting a plasticizer or the like, and stretching it 1.5 to 3 times in the transverse direction while drying and heat setting. The stretching temperature is generally preferably not higher than the melting point of the polyolefin resin at room temperature or lower, and more preferably 10 to 70 ° C lower than the melting point.

(Example)

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples. The properties and determination of the films shown in Examples and Comparative Examples show the values measured or determined by the following methods.

1) Air permeability (air permeability)

- Measuring instrument: Gurley Type Densometer Model G-B2C from Toyoseiki of Japan.

- Measurement method: Measured in seconds (sec) / 100 ml, in which 100 ml is passed (permeated) in accordance with JIS P8117.

- Measurement conditions: temperature 23 ± 2 ℃, humidity 50 ± 5% RH.

The results are shown in Table 1. Results of measurement: ⊚ is (200 to 400 sec) / ml, ∘ is (400 to 600 sec) / ml, Δ is (600 to 1,000 sec) X is not suitable for use as battery separator.

2) Heat shrinkage

- Measuring instrument: Oven Model OF-22GW of J-OTEC CO., LTD.

- Measurement method: After leaving in an oven at 150 ℃ for 1 hour, measure MD / TD shrinkage.

※ shrinkage of heat; %) = [(L ₀ -L) / L ₀ ] x 100

Where L ₀ is the initial specimen length and L is the specimen length after heat treatment.

- Measurement results: ⊚ is not more than 10%, ∘ is 10 to 40%, △ is 40 to 70%, X is more than 70%, and X and X are not suitable for battery separator.

3) Breaking characteristics

- Measuring instrument: Oven Model OF-22GW of J-OTEC CO., LTD.

Measuring method: A film of 5 cm * 5 cm was sandwiched by blocks each having a square opening of 2 cm * 2 cm, and the film in the rectangular opening melted and broken during heating at a temperature-rising rate of 5 ° C / min in an oven Temperature.

- Measurement results: A is 180 DEG C or higher, O is 180-160 DEG C, DELTA is 160-150 DEG C, X is 150 DEG C or lower, and DELTA and X are not suitable for battery separator.

≪ Example 1 >

50% by weight of a resin composed of 0.5% by weight of a cellulose fiber (Alba-Fibre, C-200P) was added to a polyolefin resin consisting of 90% by weight of polyethylene (Korean Oil Company, VH035) and 10% by weight of polypropylene , A polyester-based plasticizer or an epoxy-based plasticizer and a fluorine-based surfactant, and if necessary, a liquid-phase or wax-like hydrocarbon-based polymer and other additives in an amount of 50% by weight, The plasticizer was extracted with MC (the residual ratio of the plasticizer was 0.02% or less), and then transversely stretched by 2 times while drying and heat setting. The heat set was annealed at 120 ~ 140 ℃ in the relaxed tension for stretching.

≪ Example 2 >

50% by weight of a resin composed of 0.5% by weight of a cellulose fiber (Alba-Fibre, C-200P) was added to a polyolefin resin consisting of 95% by weight of polyethylene (Korea Emulsified Industry, VH035) and 5% by weight of polypropylene Was prepared in the same manner as in Example 1 to prepare a microporous membrane.

≪ Example 3 >

50 wt% of a resin composed of 0.5 wt% of a cellulose fiber (Alba-Fibre, C-200P) was added to a polyolefin resin consisting of 97 wt% of polyethylene (Korea Emulsification Industry Co., VH035) and 3 wt% of polypropylene (Korea Emulsifier Industry, HF5103) Was prepared in the same manner as in Example 1 to prepare a microporous membrane.

<Example 4>

50% by weight of a resin composed of 0.3% by weight of a cellulose fiber (Alba-Fibre, C-200P) was added to a polyolefin resin consisting of 95% by weight of polyethylene (Korean Oil Company, VH035) and 5% by weight of polypropylene Was prepared in the same manner as in Example 1 to prepare a microporous membrane.

&Lt; Example 5 >

50% by weight of a resin composed of 0.7% by weight of a cellulose fiber (Alba-Fibre, C-200P) was added to a polyolefin resin consisting of 95% by weight of polyethylene (Korean Oil Company, VH035) and 5% by weight of polypropylene Was prepared in the same manner as in Example 1 to prepare a microporous membrane.

&Lt; Comparative Example 1 &

A microporous membrane was prepared in the same manner as in Example 1 by adding and mixing 50% by weight of a polyolefin resin consisting of 80% by weight of polyethylene (Korea Emulsified Oil Industry, VH035) and 20% by weight of polypropylene .

&Lt; Comparative Example 2 &

A microporous membrane was prepared in the same manner as in Example 1, except that 50 wt% of a polyolefin resin consisting of 95 wt% of polyethylene (Korea Petrochemical Industry Co., Ltd., VH035) and 5 wt% of polypropylene (Korea Petroleum Industry, HF5103) .

&Lt; Comparative Example 3 &

50% by weight of a resin composed of 2% by weight of a cellulose fiber (Alba-Fibre, C-200P) was added to a polyolefin resin consisting of 95% by weight of polyethylene (Korean Oil Company, VH035) and 5% by weight of polypropylene Was added and mixed to prepare a microporous membrane in the same manner as in Example 1.

The properties of the porous separator of Examples 1 to 5 and Comparative Examples 1 to 3 thus prepared are shown in Table 1.

division Specularity Heat shrinkage Fracture
characteristic Example 1 △ ○ ○ Example 2 ○ ○ ○ Example 3 ○ ○ ○ Example 4 ○ ○ ○ Example 5 ○ ◎ ◎ Comparative Example 1 X X X Comparative Example 2 ○ X X Comparative Example 3 ○ X X

According to Table 1, the porous separator according to the embodiment of the present invention is superior to the comparative example particularly in heat shrinkage and breaking characteristics. This means that the cellulose fiber plays an important role in improving the heat shrinkage and fracture characteristics. Compared with the cellulose fibers, the MD / TD shrinkage was 70% or more after being left in an oven at 150 ° C. for 1 hour, which was impossible to use as a battery separator. The temperature at which the film in the rectangular opening melted and broken was not more than 150 캜 and was unsuitable as a battery separator.

On the contrary, in Examples 1 to 4 of the present invention, all the MD / TD shrinkage rates after being left in an oven at 150 ° C. for 1 hour were 10 to 40%, and while heating in an oven at a temperature-rising rate of 5 ° C./min The temperature at which the film within the rectangular opening melted and broken was suitable as a battery separator at 180-150 캜. Particularly, in the case of Example 5, the heat shrinkage was 10% or less and the breaking property was 180 ° C or more.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but many variations and modifications may be made without departing from the scope of the present invention. It is possible.

Claims (3)

     From 0.1 to 1% by weight of cellulose to a polyolefin resin consisting of 90 to 97% by weight of polyethylene and 3 to 10% by weight of polypropylene.  The polyolefin microporous membrane according to claim 1, wherein the cellulose is 0.3 to 0.7 wt%.  The polyolefin microporous membrane according to claim 1, wherein the cellulose has a fiber length of 20 to 500 탆.