KR101857156B1 - Crosslinked polyolefin separator and the method of preparing the same - Google Patents

Abstract

Preparing a silane grafted polyolefin solution using a polyolefin having a weight average molecular weight of 200,000 or more, a diluent, an alkoxy group-containing vinylsilane, and an initiator; Forming and stretching the silane grafted polyolefin solution into a sheet form; Extracting a diluent from the stretched sheet to produce a porous membrane; And crosslinking the porous membrane in the presence of water, and a crosslinked polyolefin separator.

Description

Crosslinked polyolefin separator and method of preparing same [0002]

The present invention relates to a crosslinked polyolefin separator having excellent heat resistance and a method for producing the same.

A secondary battery is a chemical battery that can be used semi-permanently by continuously repeating charging and discharging using an electrochemical reaction, and is classified into a lead acid battery, a nickel cadmium battery, a nickel hydrogen battery, and a lithium secondary battery. The lithium secondary battery is superior to other batteries in terms of high voltage and energy density characteristics, leading the secondary battery market. Depending on the type of electrolyte, a lithium ion battery using a liquid electrolyte and a lithium ion battery using a solid electrolyte Polymer battery.

The lithium secondary battery is composed of a cathode, an anode, an electrolytic solution and a separator. The lithium secondary battery separator is required to separate the anode and the cathode from each other and electrically insulate it from each other while enhancing the permeability of lithium ions based on the high porosity. . Polyolefins such as polyethylene, which are advantageous for pore formation and have excellent chemical resistance, mechanical properties, electrical insulation characteristics, and low cost, are mainly used as polymer materials of membranes which are generally used.

However, when the melting point of polyethylene is as low as about 130 ° C, heat generation of the battery causes shrinkage deformation without having dimensional stability at a temperature higher than the melting point. In this case, the anode and the cathode meet and cause an internal short circuit and thermal runaway phenomenon, which leads to ignition.

In order to improve such low thermal properties, methods of improving heat shrinkage at high temperature have been developed by using a method of coating inorganic or heat-resistant polymer on the surface of polyethylene.

Polyethylene has low thermal properties, but crosslinking improves heat resistance and can be used at high temperatures.

Such a method of cross-linking polyethylene is practically used for electric wire insulation, water pipes, and the like. As a method for crosslinking polyethylene, there are a crosslinking method using a peroxide initiator, a water treatment method using a silane material, and an electron beam crosslinking method. The crosslinking using a peroxide initiator is not suitable for the production of a separation membrane in which a stretching process is carried out, and the electron beam crosslinking method has a disadvantage of high facility investment cost.

Therefore, there is still a need to develop a method for improving the low thermal properties of polyolefins such as polyethylene.

Accordingly, an object of the present invention is to provide a crosslinked polyolefin separator having excellent heat resistance and a method for producing the same.

In order to solve the above problems, according to one aspect of the present invention,

Preparing a silane grafted polyolefin solution using a polyolefin having a weight average molecular weight of 200,000 or more, a diluent, an alkoxy group-containing vinylsilane, and an initiator;

Extruding and stretching the silane-grafted polyolefin solution to form and stretch into a sheet form;

Extracting a diluent from the stretched sheet to produce a porous membrane; And

And cross-linking the porous membrane in the presence of water to produce a crosslinked polyolefin separator.

The step of preparing the silane grafted polyolefin solution may include a step of adding a polyolefin, a diluent, an alkoxy group-containing vinyl silane, and an initiator to an extruder, mixing and then performing reactive extrusion.

The weight ratio of polyolefin to diluent in the silane grafted polyolefin solution may be 50:50 to 20:80.

Wherein the content of the alkoxy group-containing vinylsilane in the silane-grafted polyolefin solution is 0.1 to 10 parts by weight based on 100 parts by weight of the total amount of the polyolefin and the diluent, and the content of the initiator is 100 parts by weight of the alkoxy group- , And 0.2 to 100 parts by weight.

Wherein the polyolefin is polyethylene; Polypropylene; Polybutylene; Polypentene: polyhexene: polyolefin: copolymer of at least two of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; Or mixtures thereof.

Wherein the diluent is at least one selected from the group consisting of paraffin oil, mineral oil, wax, soybean oil, phthalic acid esters, aromatic ethers, fatty acids having 10 to 20 carbon atoms, fatty acid alcohols having 10 to 20 carbon atoms, and fatty acid esters . ≪ / RTI >

The alkoxy group-containing vinylsilane may be at least one selected from the group consisting of trimethoxyvinylsilane, triethoxyvinylsilane, and triacetoxyvinylsilane.

Wherein the step of forming and stretching the sheet comprises extruding the silane-grafted polyolefin solution through a die to form an extrudate; Cooling the extrudate to form a sheet; And biaxially stretching the sheet-shaped product in the longitudinal and transverse directions to form a stretched sheet.

The method may further include a step of opening and fixing the porous membrane before the crosslinking step.

The crosslinking step may be carried out at a temperature of 50 to 100 DEG C and a humidity of 50 to 100%.

According to another aspect of the present invention, there is provided a silane crosslinked polyolefin separator having a meltdown temperature of 150 DEG C or higher and a piercing strength of 10 to 50 gf / [mu] m.

The crosslinked polyolefin separator may have a degree of crosslinking of 50% or more.

According to another aspect of the present invention, there is provided a lithium secondary battery comprising the crosslinked polyolefin separator.

According to the present invention, it is possible to provide the high temperature stability of a lithium ion battery using the separator as a separator by improving the heat resistance characteristic through crosslinking of the polyolefin separator.

Particularly, in the present invention, there is no restriction in the manufacture of a separation membrane having a stretching process by using a silane number-imparting method in which a silane group is grafted on polyethylene and a final product is formed and crosslinked while remaining in the presence of water. And it is possible to provide a crosslinked polyolefin separator having excellent heat resistance and improved heat shrinkage even at a high temperature.

FIG. 1 is a TMA (Thermomechanical Analysis) curves of the membranes of Examples 1 and 2 and Comparative Examples 1 and 2. FIG.

Hereinafter, the present invention will be described in detail with reference to the drawings. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

A method for producing a crosslinked polyolefin separator according to one aspect of the present invention includes the steps of: preparing a silane grafted polyolefin solution using a polyolefin having a weight average molecular weight of 200,000 or more, a diluent, an alkoxy group-containing vinyl silane, and an initiator; Forming and stretching the silane grafted polyolefin solution into a sheet form; Extracting a diluent from the stretched sheet to produce a porous membrane; And crosslinking the porous membrane in the presence of moisture.

First, a step of preparing a silane grafted polyethylene solution using a polyolefin, a diluent, an alkoxy group-containing vinylsilane, and an initiator is performed.

Examples of the polyolefin include polyethylene; Polypropylene; Polybutylene; Polypentene: polyhexene: polyolefin: copolymer of at least two of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; Or a mixture thereof may be used.

Particularly, examples of the polyethylene include low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE). Of these, high density polyethylene having a high degree of crystallinity and a high melting point of the resin is the most preferable.

The weight average molecular weight of such a polyolefin is 200,000 or more, preferably 220,000 to 800,000, more preferably 250,000 to 500,000, and even more preferably 300,000 to 450,000. In the present invention, a high molecular weight polyolefin having a weight average molecular weight of 200,000 or more By using it as a starting material for the preparation of a separation membrane, properties such as strength and heat resistance of the finally obtained separation membrane can be greatly improved.

As the diluent, liquid or solid paraffin, wax, soybean oil and the like generally used for producing a wet separation membrane can be used.

As the diluent, a diluent capable of liquid-liquid phase separation with polyolefin may be used. Examples of the diluent include dibutyl phthalate, dihexyl phthalate, dioctyl phthalate, Phthalic acid esters; Aromatic ethers such as diphenyl ether and benzyl ether; 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; Mono-, di-, or triesters of palmitic acid, mono-, di-, or triesters of stearic acid. Saturated and unsaturated fatty acids having 4 to 26 carbon atoms in fatty acid groups such as oleic acid mono-, di-, or triesters, mono-, di-, or triesters of linoleic acid, or one saturated or unsaturated fatty acid in which the double bond of the unsaturated fatty acid is substituted with an epoxy Or fatty acid esters in which two or more fatty acids are ester-bonded with an alcohol having 1 to 8 hydroxyl groups and 1 to 10 carbon atoms.

The diluent may be a mixture containing two or more of the above-mentioned components.

The weight ratio of the polyolefin to the diluent in the silane grafted polyolefin solution may be 50:50 to 20:80, preferably 40:60 to 30:70. If the weight ratio is larger than 50:50, the porosity decreases and the pore size decreases. Since the interconnectivity between the pores is small and the permeability is greatly decreased, the viscosity of the polyolefin solution rises and the polyolefin solution is processed If the weight ratio is less than 20:80 and the content of the polyolefin is decreased, the kneading property between the polyolefin and the diluent is lowered, so that the polyolefin is extruded into a gel form without being thermally kneaded into the diluent, Or the like, and the strength of the produced separation membrane may be lowered.

As the alkoxy group-containing vinylsilane, trimethoxyvinylsilane, triethoxyvinylsilane, triacetoxyvinylsilane, etc. may be used alone or in a mixture of two or more. Such an alkoxy group-containing vinylsilane is grafted to a polyolefin by a vinyl group, and the alkoxyl group causes water cross-linking to proceed to crosslink the polyolefin.

The content of the alkoxy group-containing vinylsilane is 0.1 to 10 parts by weight, preferably 0.1 to 5 parts by weight, more preferably 0.5 to 2 parts by weight, based on 100 parts by weight of the total amount of the polyolefin and the diluent. When the content of the alkoxy group-containing vinylsilane is in this range, the graft ratio is lowered as the silane content is lowered, the degree of crosslinking is lowered, or the unreacted silane remains as the silane content is increased, It is possible to prevent the problem from being solved.

The initiator may be any initiator which is capable of radical generation, for example, benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, dicumyl peroxide, cumyl peroxide, Peroxides, potassium persulfates, and the like, but are not limited thereto.

The content of the initiator is 0.2 to 100 parts by weight, preferably 1 to 50 parts by weight, more preferably 2 to 10 parts by weight, based on 100 parts by weight of the alkoxy group-containing vinylsilane. When the content of the initiator satisfies this range, the silane graft ratio decreases as the content of the initiator decreases, and the problem of cross-linking of the polyethylene in the extruder as the content of the initiator increases can be prevented.

The silane-grafted polyethylene solution may further contain a crosslinking catalyst which accelerates crosslinking, that is, water flow, in the presence of water, if necessary. In addition, the composition may contain an oxidizing stabilizer, a UV stabilizer, an antistatic agent, nucleating agents may be further added.

The step of preparing the silane grafted polyolefin solution may be a step of charging the polyolefin, the diluent, the vinyl silane containing the alkoxy group, and the initiator all at once into the extruder, mixing and then performing the reactive extrusion.

Conventionally, a method of preparing a silane-grafted polyolefin through a reaction between a polyolefin and a vinylsilane compound before reaction extrusion, and then subjecting the resulting silane-grafted polyolefin to reaction extrusion together with a solvent such as a diluent, And the like.

However, according to one embodiment of the present invention, the silane grafted polyolefin, the diluent, the alkoxy group-containing vinyl silane, and the initiator are all put into the extruder at once, and the reaction of the polyolefin and the alkoxy group- To produce a silane grafted polyolefin solution. That is, the silane grafted polyolefin solution can be prepared by a single continuous process without the need of a pretreatment step of silane grafting of polyolefin, so that there is no need for additional facility investment, and it is very advantageous in cost and process.

Further, in the step of preparing the silane grafted polyolefin solution, a polyolefin and a diluent are included together as a starting material. Particularly, in the present invention, a high molecular weight polyolefin having a weight average molecular weight of 200,000 or more is used as the polyolefin, as described above. Unless a diluent is used in preparing the silane grafted polyolefin solution, the extrusion reaction of such a high molecular weight polyolefin itself is impossible . However, in the present invention, since a diluent is used together with a polyolefin or an alkoxy-containing vinylsilane as a starting material, the diluent acts as a lubricant at the time of extrusion reaction, so that a silane grafting reaction and extrusion can be performed on a polyolefin having a high molecular weight . At this time, the reaction extrusion conditions may be carried out at a temperature of 160 to 240 캜 for a few minutes by using a uniaxial or biaxial extruder apparatus. The alkoxy group-containing vinyl silane and the initiator may be injected into the side port of the extruder by respective metering pumps. For uniform reaction extrusion, the L / D (length / diameter) of the screw is preferably 30 or more.

Next, the silane grafted polyolefin solution is extruded and molded and stretched in a sheet form.

As the extrusion processing, a conventional single-screw extruder or a twin-screw extruder can be used. The extrusion condition, the stretching condition, and the heat setting condition are not different from the ordinary separation membrane processing condition range.

According to an embodiment of the present invention, the step of forming and stretching in the form of a sheet comprises extruding the silane-grafted polyolefin solution through a die to form an extrudate; Cooling the extrudate to form a sheet; And biaxially stretching the sheet-shaped product in the longitudinal and transverse directions to form a stretched sheet.

That is, the silane-grafted polyolefin solution obtained through the reaction extrusion is extruded by using an extruder equipped with a teddy or the like, and then the extruded product is extruded by a general casting or calendering method using water cooling or air cooling, .

Thereafter, the sheet is formed by stretching using a cooled extrudate.

In the case of forming a sheet using a high molecular weight polyolefin having a weight average molecular weight of 200,000 or more as in the present invention, a stretching treatment step can be performed. As a result, properties such as improved strength and puncture strength required for a secondary battery separator are imparted .

On the contrary, when a sheet is formed using a polyolefin having a low molecular weight, there is a problem that the sheet is broken at the time of the drawing process, and drawing at a desired magnification level is difficult.

According to one embodiment of the present invention, this stretching can be performed by a roll method or a tenter method sequential or simultaneous stretching. It is preferable that the stretching ratio is 3 times or more, preferably 5 to 10 times, and 20 to 80 times, respectively, in the longitudinal direction and the transverse direction. If the stretching ratio in one direction is less than 3 times, the orientation in one direction is not sufficient and the physical property balance between the longitudinal direction and the transverse direction is broken, and the tensile strength and puncture strength may be lowered. If the total stretching ratio is less than 20 times, undrawn stretching may occur and pores may not be formed. If the total stretching ratio is more than 80 times, breakage may occur during stretching and the shrinkage percentage of the final film may increase.

In this case, the stretching temperature may be varied depending on the melting point of the polyolefin used and the concentration and type of the diluting agent. Preferably, the stretching temperature is selected in a temperature range where 30 to 80% by weight of the crystalline portion of the polyolefin in the sheet melts It is appropriate.

If the stretching temperature is selected in a temperature range lower than the temperature at which 30 wt% of the crystalline portion of the polyolefin in the sheet is melted, the softness of the sheet will be poor and the stretchability will deteriorate. do. On the other hand, if the stretching temperature is selected in a temperature range higher than the melting temperature of 80 wt% of the crystalline portion, the stretching is easy and the unstretched occurrence is small, but the thickness deviation occurs due to the partial overextension, . On the other hand, the degree of melting of the crystalline portion with temperature can be obtained from differential scanning calorimeter (DSC) analysis of the sheet.

Then, the diluent is extracted from the stretched sheet thus obtained to produce a porous membrane. Specifically, the organic solvent is used to extract the diluent from the porous membrane, and then the organic solvent is dried. Any organic solvent that can extract the diluent used for resin extrusion is usable. For example, as the organic solvent, methyl ethyl ketone, methylene chloride, hexane, etc., which have a high extraction efficiency and are quick drying, are suitable.

As the extraction method, any general solvent extraction method such as an immersion method, a solvent spray method, an ultrasonic method or the like can be used individually or in combination. The content of the residual diluent after the extraction treatment is preferably not more than 1% by weight. If the content of the residual diluent exceeds 1% by weight, the physical properties are lowered and the permeability of the porous membrane is decreased. The content of the residual diluent may be influenced by the extraction temperature and the extraction time. In order to increase the solubility of the diluent and the organic solvent, the extraction temperature is preferably high. However, considering the safety problem due to the boiling of the organic solvent, . If the extraction temperature is below the freezing point of the diluent, the extraction efficiency is greatly lowered and must be higher than the freezing point of the diluent.

Further, the extraction time depends on the thickness of the porous membrane to be produced, but in the case of the porous membrane having a thickness of 10 to 30 탆, 2 to 4 minutes are suitable.

According to an embodiment of the present invention, the method may further include a step of extracting a diluent from the sheet to produce a porous membrane, and a step of heat-setting between the step of crosslinking the porous membrane.

That is, if the dried porous film is required to reduce the residual stress as in the case of a separator for a battery and to reduce the high-temperature shrinkage rate of the final separator to 5% or less in each of the longitudinal and transverse directions, a heat fixing step may be performed.

The hot fix is to fix the porous membrane and apply heat to force the porous membrane to shrink to remove the residual stress. If the heat setting temperature is high, it is advantageous for lowering the shrinkage rate, but if it is too high, the porous membrane may partially melt and the formed micropores may be clogged and the permeability may be lowered.

The preferred heat setting temperature is selected within a temperature range in which 10 to 30% by weight of the crystalline portion of the film is melted. If the heat-setting temperature is lower than a temperature at which 10 wt% of the crystalline portion of the porous film is melted, the polyolefin molecules in the film are not rearranged and the residual stress of the porous film is not removed. % Is selected at a temperature higher than the melting temperature, the micropores are clogged by partial melting and the permeability is lowered.

The heat fixation time should be relatively short when the heat fixation temperature is high, and relatively long when the heat fixation temperature is low. Preferably 5 seconds to 1 minute.

Next, the porous membrane produced by extracting the diluent is subjected to a crosslinking step in the presence of water. Such crosslinking may be carried out at a temperature of about 50 to 100 DEG C, more preferably at about 60 to 90 DEG C, and at a constant temperature and humidity chamber of 50 to 100%, more preferably 70 to 100%, or after immersing in hot or boiling water for several hours Or over several days.

When the temperature and the humidity at the time of crosslinking satisfy the above range, the crosslinking rate can be increased, and the problem that the film is deformed due to melting of some polyolefin crystals at a high temperature can be prevented.

In order to promote the crosslinking reaction, a crosslinking catalyst may be used. As such a crosslinking catalyst, carboxylic acid salts, organic bases, inorganic acids and organic acids of metals such as tin, zinc, iron, lead and cobalt can be used. Specific examples thereof include dibutyltin dilaurate, dibutyltin diacetate, stannous acetate, stannous caprylate, zinc naphthenate, zinc caprylate, cobalt naphthenate, ethylamine, dibutylamine, hexyl Amine, pyridine, sulfuric acid and hydrochloric acid, and organic acids such as toluenesulfonic acid, acetic acid, stearic acid and maleic acid.

Examples of the method of using the crosslinking catalyst include a method of adding a crosslinking catalyst at the time of producing a silane-grafted polyethylene solution, a method of applying a solution or dispersion of a crosslinking catalyst to a porous film, and the like.

When the catalyst is added during the production of the silane grafted polyethylene solution, the content of the crosslinking catalyst may be in the range of 0.0001 to 5% by weight based on the silane grafted polyethylene solution. Further, when the crosslinking catalyst is applied to the porous film, the concentration of the crosslinking catalyst may be adjusted in the range of 0.001 to 30 wt% with respect to the solution or dispersion of the crosslinking catalyst.

According to another aspect of the present invention, there is provided a crosslinked polyolefin separator film produced by the above-described production method.

According to another aspect of the present invention, there is provided a silane crosslinked polyolefin separator having a meltdown temperature of 150 DEG C or higher and a puncture strength of 10 to 600 gf / [mu] m.

Specifically, the crosslinked polyolefin separator has a meltdown temperature of 150 ° C or higher, preferably 160 to 220 ° C, more preferably 170 to 210 ° C, and still more preferably 180 to 200 ° C. The meltdown temperature refers to the temperature at which the membrane is heated at a rate of 5 ° C / minute by applying a load of 0.01 N to the separator using a TMA (thermomechanical analysis) Lt; / RTI >

Since the crosslinked polyolefin separator has a high melt-down temperature of 150 ° C or higher, the melt integrity of the separator can be maintained at a high temperature, and thus excellent dimensional stability can be obtained.

Further, the crosslinked polyolefin separator has a pore strength of 10 to 50 gf / [mu] m, preferably 15 to 40 gf / [mu] m, more preferably 17 to 37 gf / Lt; / RTI > At this time, the puncture strength is calculated by dividing the perforated membrane by the thickness of the perforated membrane by measuring the strength when the perforated membrane is broken by pressing at a rate of 2 mm / sec using a needle having a diameter of 1 mm (radius of curvature: 0.5 mm).

In addition, the crosslinked polyolefin separator according to an embodiment of the present invention has a degree of crosslinking of 50% or more, preferably 60 to 90%, more preferably 70 to 85%.

At this time, the degree of crosslinking is calculated as the ratio of the dry weight to the initial weight by measuring the remaining dry weight after boiling the separator in a decalin solution at 135 DEG C according to ASTM D 2765 for 4 hours.

According to another aspect of the present invention, there is provided a lithium secondary battery comprising the crosslinked polyolefin separator.

The lithium secondary battery includes a cathode, an anode, and a crosslinked polyolefin separator according to one aspect of the present invention disposed therebetween.

The cathode and the anode may be produced by binding an electrode active material to an electrode current collector according to a conventional method known in the art. Examples of the cathode active material include, but are not limited to, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a combination thereof A lithium complex oxide may be used. Examples of the anode active material include, but not limited to, carbonaceous materials such as natural graphite, artificial graphite, and the like; Lithium-containing titanium composite oxide (LTO); Metals (Me) which are Si, Sn, Li, Zn, Mg, Cd, Ce, Ni or Fe; An alloy composed of the metal (Me); An oxide of the metal (Me) (MeOx); And a composite of the metal (Me) and carbon, or a mixture of two or more of them may be used. Non-limiting examples of the cathode current collector include aluminum, nickel, or a combination thereof, and examples of the anode current collector include copper, gold, nickel, or a copper alloy or a combination thereof Foil to be manufactured, and the like.

The electrolyte used in the lithium secondary battery according to one aspect of the present invention is a salt having a structure such as A ⁺ B ^- , where A ⁺ is an alkali metal cation such as Li ⁺ , Na ⁺ , K ⁺ including consisting of ions and B ^- is _{^{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 - a salt containing an ion composed of such anions, or a combination thereof, and propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (G-butyrolactone) or a mixture thereof. However, it is not limited thereto. It is not fixed.

The electrolyte injection may be performed at an appropriate stage of the battery manufacturing process, depending on the manufacturing process and required properties of the final product. That is, it can be applied before assembling the cell or at the final stage of assembling the cell.

As a process for applying the crosslinked polyolefin separator according to one aspect of the present invention to a secondary battery, lamination, stacking and folding processes of a separator and an electrode other than a conventional winding process are possible.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the following examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the following embodiments. Embodiments of the invention are provided to more fully describe the present invention to those skilled in the art.

[Example 1]

High-density polyethylene having a weight average molecular weight of 300,000 was used as the polyolefin, and liquid paraffin oil was used as the diluent. The high-density polyethylene had a melting temperature of 135 ° C, and the liquid paraffin oil had a kinematic viscosity of 40 cSt at 40 ° C.

The weight ratio of the high-density polyethylene and liquid paraffin oil was 35:65. As the alkoxy-containing vinylsilane, trimethoxyvinylsilane was used, and the content of trimethoxyvinylsilane was 2 parts by weight based on 100 parts by weight of the total of high-density polyethylene and liquid paraffin oil. As an initiator, 2,5-dimethyl-2,5-di (tert-butylperoxy) hexane was added to 100 parts by weight of trimethoxyvinylsilane 2 parts by weight were added. The above components were fed into a twin screw extruder having an L / D of 56, kneaded to prepare a polyethylene solution, and reactively extruded at a temperature of 200 ° C to prepare a silane grafted polyethylene solution.

The silane-grafted polyethylene solution thus prepared was passed through a die and a cooling casting roll to form a sheet, and after the MD stretching, the biaxial stretching was performed by a TD stretching type tenter stretching stretching machine. Both the MD stretching ratio and the TD stretching ratio were 5.5 times. The stretching temperature was 108 占 폚 for MD and 123 占 폚 for TD.

The liquid paraffin oil was extracted with methylene chloride from the stretched sheet thus obtained and heat set at 127 DEG C to prepare a porous membrane. The obtained porous membrane was placed in a thermostatic wet room at 80 DEG C and 90% humidity for 24 hours to carry out crosslinking to prepare a crosslinked polyethylene separator.

[Example 2]

A crosslinked polyethylene membrane was prepared in the same manner as in Example 1, except that crosslinking was carried out by placing it in a constant temperature and humidity room for 72 hours.

[Comparative Example 1]

A separation membrane was prepared in the same manner as in Example 1, except that the opened and fixed membrane after extraction of the diluent was not placed in a constant temperature and humidity chamber but was placed in the air.

[Comparative Example 2]

A separator was prepared in the same manner as in Example 1, except that trimethoxyvinylsilane and an initiator were not used and the step of placing in a constant temperature and humidity chamber was not carried out.

[Comparative Example 3]

A separator was prepared in the same manner as in Example 1 except that low molecular weight polyethylene having a weight average molecular weight of 100,000 was used as the polyolefin.

However, after the sheet was formed into a sheet shape, it was broken at the stretching step, and separation membrane formation itself was impossible.

Property evaluation

The properties of the membranes prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were measured by the following methods and are shown in Table 1 below.

Bridging degree

The prepared membrane was immersed in a decalin solution at 135 ° C according to ASTM D 2765, boiled for 4 hours, and the dry weight of the remaining membrane was measured. The ratio of the dry weight of the membrane after boiling to the initial membrane weight was calculated, The degree of crosslinking was measured.

Meltdown temperature

To investigate the heat resistance characteristics, a 0.01 N load was applied to the separator using a thermomechanical analysis (TMA), and the degree of deformation was measured by raising the temperature at a rate of 5 ° C / min. As the temperature rises, it is shrunk and then stretched to measure the temperature at which it breaks. This temperature is defined as the melt-down temperature of the separator. The higher the temperature, the higher the melt integrity and the dimensional stability at higher temperatures.

Ventilation

Was measured using a Gurley-type air permeability meter according to JIS P-8117. At this time, the time when 100 ml of air passed through was measured with a diameter of 28.6 mm and an area of 645 mm 2.

The tensile strength

The strength and elongation at break of the specimen were measured when a specimen of 15 mm width was formed according to ASTM D882 and pulled at a speed of 500 mm / min.

Puncture strength

The fabricated porous membrane was pressed at a rate of 2 mm / sec using a needle having a diameter of 1 mm (radius of curvature: 0.5 mm), and the strength was measured.

Item Example 1 Example 2 Comparative Example 1 Comparative Example 2 Thickness (㎛) 18.0 14.5 13.5 13.4 Cross-linkability (%) 80 78 10 0 Melting-down temperature (캜) 186 190 148 142 Air permeability (s / 100cc) 440 430 405 112 MD Tensile strength (kg / cm ² ) 1250 1373 1530 1530 MD elongation (%) 227 210 230 224 TD Tensile Strength (kg / cm ² ) 1160 1280 1500 1660 TD elongation (%) 150 183 125 123 Perforation strength (gf / 탆) 18.3 33.7 32.7 34.9

Referring to Table 1, in the case of the crosslinked polyethylene separators of Examples 1 and 2 obtained by crosslinking the porous membrane of silane-grafted polyolefin in the presence of water, according to one embodiment of the present invention, the meltdown temperature was greatly improved, The stability is improved.

Also, referring to the TMA graph of FIG. 1, it can be seen that the meltdown temperature of the crosslinked polyethylene separator greatly increases with the crosslinking time in the presence of water.

Claims (13)

Adding a polyolefin having a weight average molecular weight of 200,000 or more, a diluent, an alkoxy group-containing vinyl silane, and an initiator to an extruder, mixing and extruding the mixture to produce a silane grafted polyolefin solution;
Forming and stretching the silane grafted polyolefin solution into a sheet form;
Extracting a diluent from the stretched sheet to produce a porous membrane;
Opening and fixing the porous membrane; And
And crosslinking the porous membrane in the presence of water. delete The method according to claim 1,
Wherein the weight ratio of the polyolefin to the diluent is 50:50 to 20:80 in the step of preparing the silane grafted polyolefin solution. The method according to claim 1,
Wherein the content of the alkoxy group-containing vinylsilane is 0.1 to 10 parts by weight based on 100 parts by weight of the total amount of the polyolefin and the diluent, and the content of the initiator is less than the content of the alkoxy group-containing vinylsilane in the step of producing the silane grafted polyolefin solution. Is 0.2 to 100 parts by weight based on 100 parts by weight of vinylsilane. The method according to claim 1,
Wherein the polyolefin is polyethylene; Polypropylene; Polybutylene; Polypentene: polyhexene: polyolefin: copolymer of at least two of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; Or a mixture thereof. ≪ RTI ID = 0.0 > 11. < / RTI > The method according to claim 1,
Wherein the diluent is at least one selected from the group consisting of paraffin oil, mineral oil, wax, soybean oil, phthalic acid esters, aromatic ethers, fatty acids having 10 to 20 carbon atoms, fatty acid alcohols having 10 to 20 carbon atoms, and fatty acid esters Wherein the cross-linked polyolefin separator comprises a polyolefin. The method according to claim 1,
Wherein the alkoxy group-containing vinylsilane is at least one selected from the group consisting of trimethoxyvinylsilane, triethoxyvinylsilane, and triacetoxyvinylsilane. The method according to claim 1,
Wherein the step of forming and stretching the sheet comprises extruding the silane-grafted polyolefin solution through a die to form an extrudate; Cooling the extrudate to form a sheet; And biaxially stretching the resultant product in the sheet form in the longitudinal and transverse directions to form a stretched sheet. delete The method according to claim 1,
Wherein the crosslinking step is carried out at a temperature of 50 to 100 DEG C and a humidity of 50 to 100%. delete delete delete

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