KR20200085676A - Crosslinked polyolefin separator and manufacturing method thereof - Google Patents

Abstract

Proposed are a method of preparing a crosslinked polyolefin separator and the crosslinked polyolefin separator, the method comprising the steps of: introducing a polyolefin, a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond, and a crosslinking catalyst into an extruder and blending the same in a blender unit, and extruding the resulting mixture through a die outlet via a gear pump, to form a silane-grafted polyolefin composition (S1); forming and drawing the extruded silane-grafted polyolefin composition into a sheet form (S2); extracting the diluent from the drawn sheet to form a porous film (S3); thermally fixing the porous film (S4); and water-crosslinking the thermally fixed porous film (S4), wherein in step (S1), a retention time lapsed for the polyolefin introduced into the extruder and blended to be transferred to the rear end of the gear pump is 3 minutes and 30 seconds to 6 minutes and 30 seconds. According to the present invention, the crosslinked polyolefin separator has an excellent exterior appearance and an excellent uniformity in porosity, and thus is suitable as a separator for secondary batteries.

Description

CROSSLINKED POLYOLEFIN SEPARATOR AND MANUFACTURING METHOD THEREOF}

The present invention relates to a crosslinked polyolefin separation membrane and a method for manufacturing the same.

Recently, interest in energy storage technology has been increasing. As the field of application extends to the energy of mobile phones, camcorders, notebook PCs, and even electric vehicles, efforts for research and development of electrochemical devices are becoming more concrete. The electrochemical device is the most attracting field in this aspect, and among them, the development of a secondary battery capable of charging and discharging has become a focus of interest, and recently, in developing such a battery, a new electrode to improve capacity density and specific energy. Research and development are being conducted on the design of the over battery.

Among the currently applied secondary batteries, lithium secondary batteries developed in the early 1990s have the advantage of higher operating voltage and higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries using aqueous electrolyte solutions. Is in the limelight.

The lithium secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, the separator has high ionic conductivity to increase permeability of lithium ions based on insulation and high porosity for electrically insulating the positive and negative electrodes. Is required.

In addition, in order to secure the safety of the lithium secondary battery, such a separator must have a wide gap between a shutdown temperature and a melt down temperature. In order to widen the interval between the shutdown temperature and the meltdown temperature, the shutdown temperature must be adjusted in a decreasing direction and the meltdown temperature in an increasing direction.

In order to control the melt-down temperature of the wet polyethylene (PE) separator, a method of increasing the melt-down temperature by mixing some of the polyolefins in the polyolefin, which is a main component of the separator, has been attempted, but the increase in the melt-down temperature is limited. The melt down temperature can be significantly increased using the concept of a crosslinking membrane, but it is difficult to control the crosslinking reaction if the reaction is not controlled in the extruder.

Accordingly, a problem to be solved by the present invention is to provide a method for producing a crosslinked polyolefin separation membrane having a good separation membrane appearance and a small air permeability variation, and a separation membrane produced thereby.

One aspect of the present invention provides a method for producing a crosslinked polyolefin separation membrane according to the following embodiments.

According to the first embodiment,

(S1) Polyolefin, diluent, initiator, alkoxy compound and carbon-carbon double bond group-containing silane compound, and crosslinking catalyst are introduced into an extruder and mixed in a kneading section, and then extruded through a gear pump to a die outlet to silane grafted polyolefin composition Preparing a;

(S2) forming and stretching the extruded silane-grafted polyolefin composition in a sheet form;

(S3) extracting a diluent from the stretched sheet to prepare a porous membrane;

(S4) heat-setting the porous membrane; And

(S4) the step of cross-linking the heat-set porous membrane; includes,

In the step (S1), a method for manufacturing a crosslinked polyolefin separation membrane having a residence time of 3 minutes 30 seconds to 6 minutes 30 seconds until the polyolefin is introduced into the extruder and mixed and transferred to the rear end of the gear pump is provided.

According to the second embodiment, in the first embodiment,

The extruder is a hopper containing a polyolefin, a diluent, an initiator, an alkoxy group and a silane compound containing a carbon-carbon double bond group, and a port for introducing a crosslinking catalyst, and transporting and mixing the material introduced from the hopper and port It is a twin-screw extruder having a kneading section, a gear pump for transporting the material through the kneading section to a die outlet, and a die outlet for extruding the material conveyed through the gear pump to the outside. ,

The gear is mixed with the diluent, an initiator, an alkoxy group and a silane compound containing a carbon-carbon double bond group, and a crosslinking catalyst in the kneading section from the time when the residence time is when the polyolefin is introduced into the hopper of the extruder. It may be a time required until the time when it is transferred to the rear end of the pump.

According to the third embodiment, in the first embodiment or the second embodiment,

The residence time can be controlled by the arrangement of a plurality of screw elements provided in the kneading section.

According to the fourth embodiment, in the second or third embodiment,

Among the advancing zones in which the kneading section sends the input material in the extruder in the direction of die exit (front), the stop zone in which the input materials in the extruder are mixed in place, and the reverse zone in which the input material in the extruder is sent in the hopper direction (back). It has two or more zones, each of which can independently comprise one or more screw elements.

According to the fifth embodiment, in any one of the first to fourth embodiments,

The content of the silane compound containing the alkoxy group and the carbon-carbon double bond group may be 0.1 to 3 parts by weight based on 100 parts by weight of the total amount of the polyolefin and the diluent.

The sixth embodiment according to any one of the first to fifth embodiments,

The alkoxy compound containing a alkoxy group and a carbon-carbon double bond group may include a compound represented by Formula 1 below:

[Formula 1]

In Formula 1, R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10 carbon atoms, an alkoxyalkoxy group having 2 to 10 carbon atoms, an alkylcarbonyloxy group having 1 to 10 carbon atoms, or an alkyl group having 1 to 10 carbon atoms, wherein R _{1 and} R _{2 ,} And At least one of R ₃ is an alkoxy group or an alkoxyalkoxy group;

R is a vinyl group, an acryloxy group, a methacryloxy group, or an alkyl group having 1 to 20 carbon atoms, wherein at least one hydrogen of the alkyl group is substituted with a vinyl group, an acryloxy group, or a methacryloxy group.

The seventh embodiment according to any one of the first to sixth embodiments,

The silane compound containing the alkoxy group and the carbon-carbon double bond group is vinyl trimethoxysilane, vinyl triethoxysilane, vinyl triacetoxysilane, (3-methacryloxypropyl) trimethoxysilane, (3-methacryloxy Propyl) triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or mixtures of at least two of them.

According to an aspect of the present invention, a crosslinked polyolefin separation membrane of the following embodiment is provided.

According to the eighth embodiment,

A crosslinked polyolefin separation membrane comprising a -Si-O-Si-bonding group between the main chains of polyolefins and having a variation in the number of gels of 0.15/m ² or less and the permeability of 10 sec/100cc or less is provided.

According to the ninth embodiment, in the eighth embodiment,

The number of gels may be 0.05 to 0.13/m ² , and the air permeability may be 6.4 to 8.2 sec/100cc.

According to the tenth embodiment,

In a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, a lithium secondary battery is provided, wherein the separator is a crosslinked polyolefin separator of the eighth or ninth embodiment.

Method for producing a crosslinked polyolefin separation membrane according to an embodiment of the present invention, in the step of preparing a silane-grafted polyolefin composition, the residence time until the polyolefin is mixed after being injected into the extruder and transferred to the rear end of the gear pump By controlling from 3 minutes 30 seconds to 6 minutes 30 seconds, the number of gels is small, the appearance of the separator is good, the air permeability is also small, the uniformity of the porosity of the separator is excellent, and a crosslinked polyolefin separator suitable as a separator for secondary batteries can be provided. .

Hereinafter, the present invention will be described in detail. The terms or words used in the specification and claims should not be interpreted as being limited to ordinary or dictionary meanings, and the inventor can appropriately define the concept of terms in order to best describe his or her invention. Based on the principle of being present, it should be interpreted as meanings and concepts consistent with the technical spirit of the present invention.

The present invention relates to a method for producing a crosslinked polyolefin separation membrane and a separation membrane produced thereby.

An electrochemical device such as a lithium secondary battery including a separator having a large difference between a shutdown temperature and a meltdown temperature has excellent safety. Accordingly, a separator having a low shutdown temperature and a high melt down temperature is required.

The present inventors prepared a crosslinked polyolefin porous membrane using a crosslinking reaction to prepare a separator having a high melt down temperature, but were still insufficient to lower the shutdown temperature.

In view of this, the present inventors have a high melt-down temperature and at the same time have a low shutdown temperature to improve heat resistance, and the residence time until the polyolefin is mixed after being introduced into the extruder and mixed and transferred to the rear end of the gear pump. It is to provide a crosslinked polyolefin separation membrane excellent in uniformity of porosity of the separation membrane and a method for manufacturing the same by improving the heat resistance according to a decrease in the shutdown temperature and an increase in the meltdown temperature, a good appearance of the separation membrane, and a small air permeability variation. .

Method for producing a crosslinked polyolefin separation membrane according to an aspect of the present invention,

(S1) Polyolefin, diluent, initiator, alkoxy compound and carbon-carbon double bond group-containing silane compound, and crosslinking catalyst are introduced into an extruder and mixed in a kneading section, and then extruded through a gear pump to a die outlet to silane grafted polyolefin composition Preparing a;

(S2) forming and stretching the extruded silane-grafted polyolefin composition in a sheet form;

(S3) extracting a diluent from the stretched sheet to prepare a porous membrane;

(S4) heat-setting the porous membrane; And

(S4) the step of cross-linking the heat-set porous membrane; includes,

In the step (S1), the residence time from 3 minutes 30 seconds to 6 minutes 30 seconds until the polyolefin is added to the extruder and mixed and transferred to the rear end of the gear pump.

Hereinafter, a method of manufacturing the separation membrane according to the present invention will be described in detail.

First, a polyolefin, a diluent, an initiator, an alkoxy group and a carbon-carbon double bond group-containing silane compound are added to an extruder, and a crosslinking catalyst is added and mixed in a kneading section, and then extruded through a gear pump to a die outlet to form a silane compound in the main chain of the polyolefin. A grafted polyolefin composition is prepared (S1).

When a polyolefin, an initiator, an alkoxy group and a silane compound containing a carbon-carbon double bond group, and a crosslinking catalyst were added to the extruder at one time as in the prior art, the melt down temperature could be increased, but the effect of reducing the shutdown temperature was still insufficient.

However, in order to increase the safety of the membrane, the gap between the shutdown temperature and the meltdown temperature must be high.

In one embodiment of the present invention, the polyolefin is polyethylene; Polypropylene; Polybutylene; Polypentene; Polyhexene; Polyoctene; Copolymers of two or more of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; Or mixtures thereof.

In particular, the polyethylene includes low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), etc. Among them, high-density polyethylene having high crystallinity and high melting point of the resin is most preferred.

In one embodiment of the present invention, the weight average molecular weight of the polyolefin may be 200,000 to 1,000,000 or 220,000 to 700,000 or 250,000 to 500,000. In the present invention, by using a high molecular weight polyolefin having a weight average molecular weight of 200,000 to 1,000,000 as a starting material for the production of a separator, a separator having excellent strength and heat resistance can be finally obtained while securing the uniformity and processability of the membrane.

In one embodiment of the present invention, the diluent may use liquid or solid paraffin oil, wax, soybean oil, and the like, which are generally used for preparing a wet separator.

In one embodiment of the present invention, a diluent capable of liquid-liquid phase separation with polyolefin may be used as the diluent, for example, dibutyl phthalate, dihexyl phthalate, di Phthalic acid esters such as octyl phthalate; 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 acid alcohol, stearic acid alcohol, and oleic acid alcohol; Palmitic acid mono-, di-, or triester, stearic acid mono-, di-, or triester. Saturated and unsaturated fatty acids having 4 to 26 carbon atoms in a fatty acid group such as mono-, di-, or triester, linoleic acid mono-, di-, or triester, or one double bond of an unsaturated fatty acid substituted with epoxy Or two or more fatty acids, 1 to 8 hydroxy groups, 1 to 10 carbon atoms, alcohols and ester-linked fatty acid esters; may include.

The diluent may be used alone or in a mixture containing at least two or more of the above components.

In one embodiment of the present invention, the total content of the diluent may be 100 to 350 parts by weight, or 125 to 300 parts by weight, or 150 to 250 parts by weight based on 100 parts by weight of the polyolefin. When the total content of the diluent satisfies the above numerical range, the porosity decreases and the pore size decreases as the polyolefin content increases, and the permeability decreases significantly due to less interconnection between pores, and the viscosity of the polyolefin composition increases, resulting in an increase in extrusion load. The problems that may be difficult to be processed may be reduced, and as the polyolefin content is small, the kneading property of the polyolefin and the diluent is lowered, and the polyolefin is not thermodynamically kneaded in the diluent and is extruded in the form of a gel. It can reduce problems such as.

In one embodiment of the present invention, the silane compound containing an alkoxy group and a carbon-carbon double bond group is a crosslinking agent that causes a silane crosslinking reaction, grafted to a polyolefin by a carbon-carbon double bond group, and a crosslinking reaction proceeds by an alkoxy group. It serves to crosslink the polyolefin. Accordingly, the melt-down temperature of the separator can be increased.

The carbon-carbon double bond group is a reactive group capable of being grafted to a polyolefin as described above, and is a substituent having a double bond between two carbons, such as vinyl group, acryloxy group, or methacryl. There may be a period of time.

In one embodiment of the present invention, the alkoxy compound containing the alkoxy group and the carbon-carbon double bond group may include a compound represented by Formula 1 below:

[Formula 1]

In Formula 1, R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10 carbon atoms, an alkoxyalkoxy group having 2 to 10 carbon atoms, an alkylcarbonyloxy group having 1 to 10 carbon atoms, or an alkyl group having 1 to 10 carbon atoms, wherein R _{1 and} R _{2 ,} And At least one of R ₃ is an alkoxy group or an alkoxyalkoxy group;

R is a vinyl group, an acryloxy group, a methacryloxy group, or an alkyl group having 1 to 20 carbon atoms, wherein at least one hydrogen of the alkyl group is substituted with a vinyl group, an acryloxy group, or a methacryloxy group.

According to an embodiment of the present invention, the R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 3 carbon atoms, an alkoxyalkoxy group having 2 to 5 carbon atoms, an alkylcarbonyloxy group having 1 to 3 carbon atoms, or an alkyl group having 1 to 3 carbon atoms, the alkoxy group, the alkylcarbo At least one hydrogen of a niloxy group, an alkoxyalkoxy group, or an alkyl group may be substituted with an alkoxy group having 1 to 10 carbon atoms, or an alkylcarbonyloxy having 1 to 10 carbon atoms. At this time, the alkoxyalkoxy group is a substituent defined as "-O-alkylene-O-alkyl", and the alkylcarbonyloxy group is a substituent defined as "-O-(CO)-alkyl".

In one embodiment of the present invention, the alkoxy compound containing the alkoxy group and the carbon-carbon double bond group is vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trime Oxysilane, (3-methacryloxypropyl)triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or mixtures of at least two or more thereof can do.

In one embodiment of the present invention, the content of the alkoxy compound containing the alkoxy group and the carbon-carbon double bond group is 0.1 to 3.0 parts by weight, or 0.15 to 2.0 parts by weight, or 0.2 based on 100 parts by weight of the total amount of the polyolefin and the diluent. It may be from 1.5 parts by weight. When the content of the alkoxy group and the carbon-carbon double bond group-containing silane compound satisfies the above numerical range, the content of the alkoxy group and the carbon-carbon double bond group-containing silane compound is small, resulting in a low graft rate and low crosslinking, or an alkoxy group and carbon- Since the content of the silane compound containing the carbon double bond group is high, unreacted silane remains, thereby preventing a problem that the appearance of the extruded sheet becomes poor, and by appropriate grafting with the polyolefin, the crystallization degree of the polyolefin can be lowered to lower the shutdown temperature. have.

In one embodiment of the present invention, the initiator may be used without limitation as long as it is an initiator capable of generating radicals. Non-limiting examples of the initiator, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (2,5-dimethyl-2,5-di(tert-butylperoxy)hexane) , (DHBP)), benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-ter-butyl peroxide, dicumyl peroxide, cumyl peroxide, hydrogen peroxide, potassium persulfate, and the like.

In one embodiment of the present invention, the content of the initiator is 0.1 to 20 parts by weight, or 0.5 to 10 parts by weight, or 1 to 5 parts by weight, based on 100 parts by weight of the alkoxy compound containing the alkoxy group and the carbon-carbon double bond group. It can be parts by weight. When the content of the initiator satisfies the above numerical range, the silane graft rate may decrease as the content of the initiator is low, or the crosslinking between polyolefins in the extruder may be prevented as the content of the initiator is high.

In one embodiment of the present invention, the crosslinking catalyst is added to promote a silane crosslinking reaction.

In one embodiment of the present invention, the crosslinking catalyst may be a carboxylic acid salt of a metal such as tin, zinc, iron, lead, or cobalt, an organic base, an inorganic acid and an organic acid. Examples of the crosslinking catalyst include, but are not limited to, dibutyl tin dilaurate, dibutyl tin diacetate, stannous acetate, stannous caprylate, zinc naphthenate, and zinc caprylate. And cobalt naphthenic acid. Examples of the organic base include ethylamine, dibutyl amine, hexyl amine, pyridine, etc., and the inorganic acids include sulfuric acid and hydrochloric acid, and the organic acids include toluene sulfonic acid, acetic acid, and stearic acid. And maleic acid. In addition, the crosslinking catalyst may be used alone or a mixture of two or more of them.

In one embodiment of the present invention, the content of the crosslinking catalyst is 0.1 to 20 parts by weight, 0.5 to 10 parts by weight, 1 to 5 parts by weight based on 100 parts by weight of the silane compound containing an alkoxy group and a carbon-carbon double bond group Can be When the content of the crosslinking catalyst satisfies the above numerical range, a desired level of silane crosslinking reaction may occur, and undesirable side reactions in the lithium secondary battery are not caused. In addition, there is no cost problem such as wasted crosslinking catalyst.

In one embodiment of the present invention, the silane-grafted polyolefin composition may further include general additives for improving specific functions, such as a surfactant, an oxidation stabilizer, a UV stabilizer, an antistatic agent, and a nucleating agent, if necessary. Can.

In one embodiment of the present invention, a single screw extruder or a twin screw extruder may be used as an extruder in the step of preparing the silane-grafted polyolefin composition.

After the polyolefin is added to the extruder in step (S1), the residence time until it is mixed and transferred to the rear end of the gear pump is 3 minutes 30 seconds to 6 minutes 30 seconds, and also 3 minutes 45 seconds to 6 minutes 15 Seconds, or 3 to 6 minutes. If the residence time is less than 3 minutes and 30 seconds, the desired silane grafting reaction may not proceed sufficiently, and if it is more than 6 minutes and 30 seconds, there is a risk that a gel is formed by the initiator reaction.

According to one embodiment of the invention, the extruder is a hopper (hopper), a diluent, an initiator, an alkoxy group containing a polyolefin and a silane compound containing a carbon-carbon double bond group, and a port for introducing a crosslinking catalyst, the injected It has a kneading section for transporting and mixing the material, a gear pump for transferring the mixed material to a die outlet, and a die outlet for extruding the material transferred by the gear pump to the outside. It may be a twin-screw extruder.

Here, the residence time in the extruder is from the time the polyolefin is introduced into the hopper of the extruder, the diluent, initiator, alkoxy group and silane compound containing a carbon-carbon double bond group, and a crosslinking catalyst in the kneading section It is defined as the time required until the time point of mixing and transferring to the rear end of the gear pump of the extruder.

The residence time in the extruder can be controlled by the arrangement of a plurality of screw elements provided in the kneading section.

According to one embodiment of the present invention, the extruder may have one or more kneading sections, each kneading section having three different zones, i.e. a forward zone, a stationary zone, and a reverse direction from the hopper to the die exit direction. It may consist of zones. Each of the zones can independently include one or more screw elements.

Specifically, the advance zone may include one or more screw elements designed to act to direct the input material in the extruder in the die direction (front); The stop zone may include one or more screw elements designed to rotate the input material in the extruder in place (parallel); The reverse zone may include one or more screw elements designed to return the input material in the extruder back to the hopper direction (back).

For example, according to one embodiment of the present invention, the forward zone has 2 to 5 forward screw elements, the stop zone has 2 to 5 neutral screw elements, and the reverse zone is 1 It may have up to five reverse screw elements.

According to one embodiment of the present invention, the kneading part of the extruder may be composed of a plurality of kneading parts of one or two or more according to the purpose, and furthermore, a conveying part including one or a plurality of forward screw elements at the end in the die direction. The screw structure of the whole extruder can be comprised by further including.

Next, the extruded silane-grafted polyolefin composition is molded and stretched in a sheet form (S2).

For example, the reaction extruded silane-grafted polyolefin composition is extruded using an extruder equipped with a T-die or the like, and then cooled extrudate using a general casting or calendering method using water cooling or air cooling. Can form.

In one embodiment of the present invention, it is possible to provide a separator having improved mechanical strength and puncture strength by going through the stretching step as described above.

In one embodiment of the present invention, the stretching may be performed by a rolling method or a tenter method sequential or simultaneous stretching. The stretching ratio may be 3 times or more, or 4 to 10 times, respectively, in the longitudinal and transverse directions, and the total stretching ratio may be 14 to 100 times. When the stretching ratio satisfies the above numerical range, the orientation in one direction is not sufficient, and at the same time, the property balance between the longitudinal and transverse directions is broken, thereby preventing the problem of a decrease in tensile strength and puncture strength, and the total stretching ratio is in the numerical range. As it satisfies, it is possible to prevent a problem that unstretched or pore formation does not occur.

In one embodiment of the present invention, the stretching temperature may vary depending on the melting point of the polyolefin used, the concentration and type of diluent.

In one embodiment of the present invention, for example, if the polyolefin used is polyethylene, the diluent is liquid paraffin, and the kinematic viscosity of the liquid paraffin is 50 to 150 cSt at 40°C, the stretching temperature is longitudinal stretching (MD, Machine Direction) may be 70 to 160°C, or 90 to 140°C, or 100 to 130°C, and for Traverse Direction (TD), 90 to 180°C, or 110 to 160°C or 120 to 150 It may be ℃, and when stretching in both directions at the same time, it may be 90 to 180 ℃, or 110 to 160 ℃, or 110 to 150 ℃.

When the stretching temperature satisfies the numerical range, as the stretching temperature has a low temperature range, there is no softness, so that fracture or non-stretching problems can be prevented, and the stretching temperature is high. This can prevent partial overstretching or differences in properties.

Thereafter, a diluent is extracted from the molded and stretched sheet to prepare a porous membrane (S3).

In one embodiment of the present invention, a diluent may be extracted from the porous membrane using an organic solvent and the porous membrane may be dried.

In one embodiment of the present invention, the organic solvent is not particularly limited as long as it can extract the diluent, but extraction efficiency is high and methyl ethyl ketone, methylene chloride, hexane and the like are suitable.

In one embodiment of the present invention, the extraction method may be any or a combination of all common solvent extraction methods, such as an immersion method, a solvent spray method, and an ultrasonic method. The content of the residual diluent after the extraction treatment should preferably be 1% by weight or less. When the content of the residual diluent exceeds 1% by weight, the physical properties decrease and the permeability of the porous membrane decreases. The content of the residual diluent may be influenced by the extraction temperature and the extraction time, and in order to increase the solubility of the diluent and the organic solvent, the extraction temperature is preferably high, but considering the safety problem caused by boiling of the organic solvent, 40°C or less is preferable. . If the extraction temperature is below the freezing point of the diluent, the extraction efficiency is greatly reduced, so it must be higher than the freezing point of the diluent.

In addition, the extraction time depends on the thickness of the porous membrane to be produced, but in the case of a porous membrane having a thickness of 5 to 15 μm, 2 to 4 minutes is suitable.

Thereafter, the porous membrane is opened and fixed (S4).

The heat setting is to fix the porous membrane and apply heat to forcibly hold the porous membrane to be contracted to remove residual stress.

In one embodiment of the present invention, when the polyolefin is polyethylene, the heat setting temperature may be 100 to 140°C, or 105 to 135°C, or 110 to 130°C. When the polyolefin is polyethylene, when the heat setting temperature satisfies the above numerical range, rearrangement of the polyolefin molecules may occur to remove residual stress of the porous membrane, and it is possible to reduce the problem of clogging the pores of the porous membrane due to partial melting. have.

In one embodiment of the present invention, the heat setting temperature may be 10 to 120 seconds, 20 to 90 seconds, and 30 to 60 seconds. In the case of heat setting at the above time, rearrangement of the polyolefin molecules may occur to remove residual stress of the porous membrane, and it is possible to reduce the problem of clogging of the pores due to partial melting.

Next, the heat-set porous membrane includes a step of crosslinking in the presence of moisture (S5).

In one embodiment of the invention, the crosslinking may be performed at 60 to 100 ℃, or 65 to 95 ℃, or 70 to 90 ℃.

In one embodiment of the present invention, the crosslinking may be performed for 6 to 50 hours at a humidity of 60 to 95%.

According to one aspect of the present invention, there is provided a crosslinked polyolefin separation membrane comprising -Si-O-Si- linkage groups between main chains of polyolefins, and having a variation in gel number of 0.15/m ² or less and air permeability of 10 sec/100cc or less. .

The silane compound containing an alkoxy group and a carbon-carbon double bond group in the main chain of the polyolefin is grafted via a carbon-carbon double bond group, and thereafter, the alkoxy group of the grafted silane undergoes a crosslinking reaction with alkoxy groups adjacent to each other. -Si-O-Si- bond (siloxane bond) groups are formed between the main chains of the polyolefin.

The number of gels is 0.15 pieces/m ² or less, and in one embodiment of the present invention, 0.01 to 0.15 pieces/m ² , or 0.05 to 0.13 pieces/m ² , or 0.05 to 0.12 pieces/m ² , or 0.05 to It can be 0.10 pcs/m ² . When the number of gels satisfies this range, the pore formation is uniform, and the thickness can be uniformly controlled without protrusions.

The number of gels can be obtained by cutting the prepared membrane to 1 m X 1 m to prepare a sample, and then measuring the number of gels (pieces/m ² ) having a long axis of 100 μm or more in the prepared sample.

The air permeability deviation is 10 sec/100cc or less, and according to an embodiment of the present invention, 5 to 10 sec/100cc, or 6.4 to 8.2 sec/100cc, or 6.5 to 8.0 sec/100cc, or 7.0 to 8.0 sec/100cc Can be If the air permeability deviation satisfies this range, the membrane resistance may be uniform in the surface direction.

The air permeability variation is a criterion for evaluating the uniformity of porosity of the crosslinked polyolefin separation membrane, and the air permeability of the following formula can be evaluated as the variation.

Air permeability deviation (sec/100cc) = maximum permeability-minimum permeability

At this time, the air permeability (Gurley) can be measured by the ASTM D726-94 method, as used herein, the resistance to the flow of air, can be measured by a Gurley densometer (densometer). The air permeability values described herein are expressed as the time in seconds it takes for 100 cc of air to pass through the cross section of the separator 1 in ² under a pressure of 12.2 inH ₂ O, that is, the aeration time.

In one embodiment of the present invention, the shutdown temperature of the separator is 140°C or less, and the meltdown temperature of the separator may be 175°C or more.

In one embodiment of the present invention, the difference between the shutdown temperature and the meltdown temperature of the separator may be 30°C or higher, or 35°C or higher, for example, 37.2 to 61.5°C.

The lithium secondary battery according to an aspect of the present invention includes a separator interposed between a cathode, an anode, and the anode and the cathode, wherein the separator is a crosslinked polyolefin separator according to one embodiment of the present invention described above. to be.

The lithium secondary battery may include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery or a lithium ion polymer secondary battery.

The positive electrode and the negative electrode to be applied together with the separator of the present invention are not particularly limited, and an electrode active material may be prepared in a form attached to an electrode current collector according to a conventional method known in the art. As a non-limiting example of the positive electrode active material among the electrode active materials, a conventional positive electrode active material that can be used for the positive electrode of a conventional electrochemical device can be used, in particular lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or combinations thereof. It is preferable to use one lithium composite oxide. As a non-limiting example of the negative electrode active material, a conventional negative electrode active material that can be used for the negative electrode of a conventional electrochemical device can be used, in particular lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, Lithium adsorbents such as graphite or other carbons are preferred. Non-limiting examples of the positive electrode current collector include aluminum, nickel, or a foil made of a combination thereof, and non-limiting examples of the negative current collector are copper, gold, nickel or copper alloys, or combinations thereof. And foils manufactured.

Electrolyte that may be used in the electrochemical device of the present invention is A ⁺ B ^- A salt of the structure, such as, A ⁺ comprises a Li ^+, Na ^+, an alkali metal cation or an ion composed of a combination thereof, such as K ⁺ 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} ^- anion, or a salt containing an ion composed of a combination of propylene carbonate (PC) such as, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl Carbonate (DPC), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethylcarbonate (EMC), gamma butyrolactone (g -Butyrolactone) or a mixture thereof, dissolved or dissociated in an organic solvent, but is not limited thereto.

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

Hereinafter, examples will be described in detail to specifically describe the present invention. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

Example 1

High-density polyethylene (Daehan Emulsion, VH035) 10.5kg/hr with a weight average molecular weight of 350,000 as a polyolefin in the extruder, liquid paraffin oil (Far East Emulsion, LP 350F, 68cSt) 13.65kg/hr as a diluent, alkoxy containing a carbon-carbon double bond As a silane, vinyl trimethoxysilane 450g/hr, and dibutyl tin dilaurate 6g/hr as a crosslinking catalyst were added and mixed. At this time, the carbon-carbon double bond group-containing alkoxy silane was introduced in a mixed state with a diluent.

Next, 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (2,5-dimethyl-2,5-di(tert-butylperoxy)hexane) as an initiator in the extruder, (DHBP)) 1 g/hr was added and mixed.

At this time, the extruder feeds a hopper containing a polyolefin, a diluent, an initiator, an alkoxy group and a silane compound containing a carbon-carbon double bond group, and a port through which a crosslinking catalyst is injected, and a material injected from the hopper and port A twin screw having a kneading section for mixing, a gear pump for transferring the material passing through the kneading portion to a die outlet, and a die outlet for extruding the material transferred through the gear pump to the outside. It was an extruder.

At this time, from the time when the polyolefin is put into the hopper of the extruder, the diluent, initiator, alkoxy group and silane compound containing a carbon-carbon double bond group, and a crosslinking catalyst are mixed in the kneading section to form a gear pump. The time required until the time of transfer to the rear end is defined as the residence time.

The residence time was controlled by the arrangement of a plurality of screw elements provided in a kneading section, and the extruder was provided with an extruder screw configuration for mixing by continuously configuring four kneading sections. Specifically, each kneading unit was configured such that the forward zone, the stop zone, and the backward zone were sequentially arranged. The forward zone serves to send the input material in the extruder in the direction of die exit (front), includes five forward screw elements, and the stop zone serves to mix the input material in the extruder in place, neutral screw element It includes three, and the reverse section serves to send the input material in the extruder in the hopper direction (back), and includes one reverse screw element. The extruder was followed by the four kneading sections, followed by a transport section with nine forward screw elements to transport the mixed material towards the T-die. At this time, the residence time was 5 minutes 30 seconds.

The silane-grafted polyethylene composition extruded by the extruder was molded into a sheet form after passing through a T-die and a cooling casting roll, and then biaxially stretched with a tenter-type sequential stretching machine of TD stretching (transverse stretching) after MD stretching (longitudinal stretching). . The MD stretching ratio and the TD stretching ratio were both 7.0 times. The stretching temperature was 110°C for MD and 125°C for TD.

The stretched sheet was prepared by extracting a diluent with methylene chloride and heat-setting at 126° C. to prepare a porous membrane. The porous membrane was crosslinked at 85° C. and 60% relative humidity for 24 hours to prepare a crosslinked polyethylene separator. The obtained crosslinked polyethylene separation membrane had a thickness of 9.0 µm.

The prepared crosslinked polyethylene separator has a silane compound grafted to the main chain of a polyolefin, contains a silane derived crosslinked structure, and the silane derived crosslinked structure includes a -Si-O-Si- linking group.

Example 2

The extruder consisted of three kneading sections continuously, followed by a transfer section with nine forward screw elements, and a crosslinked polyethylene in the same manner as in Example 1, except that the residence time was 3 minutes 30 seconds. A separator was prepared. That is, the configuration of each kneading portion among the three kneading portions was the same as in Example 1.

Example 3

A crosslinked polyethylene separator was prepared in the same manner as in Example 1 except that the forward screw element was reduced to 4 in the kneading section configuration of the extruder, the reverse screw element was increased to 2, and the residence time was 6 minutes and 30 seconds. .

Comparative Example 1

In the extruder, a cross-linked polyethylene separator was prepared in the same manner as in Example 1, except that the kneading part was not constituted, and only the conveying part having 9 forward screw elements was formed, and the residence time was 2 minutes.

Due to the lack of kneading, the prepared separator did not efficiently mix high-density polyethylene and liquid paraffin oil, so normal pores were not formed in the prepared separator.

Comparative Example 2

A crosslinked polyethylene separator was prepared in the same manner as in Example 1, except that the forward screw element was reduced to 1 in the kneading section configuration of the extruder, the reverse screw element was increased to 4, and the residence time was 11 minutes and 45 seconds. .

A gel was formed on the produced extruded sheet and could not be used as a separator.

Comparative Example 3

A crosslinked polyethylene separator was prepared in the same manner as in Example 1, except that the forward screw element was increased to 5 in the kneading section configuration of the extruder, the reverse screw element was increased to 2, and the residence time was 2 minutes and 50 seconds. .

The prepared separation membrane was unable to obtain a uniform separation membrane due to a difference in porosity by location of the extruded sheet due to insufficient kneading.

Comparative Example 4

Example 1, except that in the kneading section configuration of the extruder, the forward screw element is reduced to 3, the neutral screw element is increased to 4, and the reverse screw element is increased to 2, and the residence time is 7 minutes and 10 seconds. A crosslinked polyethylene separator was prepared in the same manner as.

A gel was partially formed on the produced extruded sheet, so it could not be used as a separator.

For the crosslinked polyethylene separation membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 4, the shutdown temperature, the melt down temperature, the number of gels, and the porosity uniformity (breathing airway deviation) were measured and shown in Table 1 Did.

(1) Shutdown temperature

The shutdown temperature measures the air permeability of the porous membrane while exposing the porous membrane to increasing temperature conditions (starting at 30°C and 5°C/min). The shutdown temperature of the porous membrane is defined as the temperature at which the air permeability (Gurley value) of the microporous membrane first exceeds 100,000 seconds/100 cc. At this time, the air permeability of the porous membrane can be measured using an air permeability meter (Asahi Seiko, EGO-IT) according to JIS P8117.

(2) Melt down temperature

Melt-down temperature is measured by thermomechanical analysis (TMA) after taking samples of the porous membranes in the manufacturing direction (MD) and the transverse direction (TD), respectively. Specifically, a sample having a length of 10 mm is put in a TMA equipment (TA Instrument, Q400) and exposed to an increasing temperature condition (starting at 30° C. and 5° C./min) while applying a tension of 19.6 mN. As the temperature rises, the length of the sample is accompanied, and the length is rapidly increased to measure the temperature at which the sample is cut off. MD and TD are measured respectively and the higher temperature is defined as the melt down temperature of the sample.

(3) Evaluation of the number of separator gels

The prepared separation membrane was cut to 1 m X 1 m to prepare a sample. Then, in the prepared sample, the number of gels having a long axis of 100 μm or more (pieces/m ² ) was measured.

(4) Uniformity of porosity (variation in air permeability)

Porosity uniformity was evaluated by the air permeability deviation of the following formula.

Air permeability deviation (sec/100cc) = maximum permeability-minimum permeability

At this time, the air permeability (Gurley) was measured by the ASTM D726-94 method. Gurley, as used herein, is the resistance to the flow of air and is measured by a Gurley densometer. The air permeability value described herein is the time (in seconds) for 100 cc of air to pass through the cross section of the separators 1 in ² prepared in Examples 1 to 3 and Comparative Examples 1 to 4 under a pressure of 12.2 inH ₂ O, that is, Expressed as aeration time.

Extruder
Residence time shut down
Temperature (℃) Melt down
Temperature (℃) Gel count
(Pcs/m ² ) Deviation in aeration
(sec/100cc) Membrane quality evaluation Example 1 5 minutes 30 seconds 139.1 196 0.11 7.1 Good (Can be used as a separator for lithium secondary batteries) Example 2 3 minutes 30 seconds 139.8 177 0.05 8.2 Good (Can be used as a separator for lithium secondary batteries) Example 3 6 minutes 30 seconds 139.5 201 0.13 6.4 Good (Can be used as a separator for lithium secondary batteries) Comparative Example 1 2 minutes Not applicable Not applicable 0.01 48.9 Porosity lack uniformity Comparative Example 2 11 minute 45 seconds 138.1 209 3.30 5.5 Gel formation Comparative Example 3 2 minutes 50 seconds Not applicable Not applicable 0.01 37.5 Poor uniformity Comparative Example 4 7 minutes 10 seconds 138.3 205 2.63 6.2 Gel formation

Referring to Table 1, the separation membranes of Examples 1 to 3 had a small number of gels, and thus, the appearance of the separation membrane was good, and the air permeability was small and the uniformity of the porosity of the separation membrane was excellent, and thus, it was suitable as a separation membrane for secondary batteries. It was confirmed that the separation membranes of 4 to 4 were difficult to apply as a separation membrane for secondary batteries, such as poor uniformity of porosity or a large number of gels, resulting in poor separation of the separation membrane.

Claims (10)

(S1) Polyolefin, diluent, initiator, alkoxy group and carbon-carbon double bond group-containing silane compound, and a crosslinking catalyst are introduced into an extruder and mixed in a kneading section, and then extruded through a gear pump to a die outlet to silane grafted polyolefin composition Preparing a;
(S2) forming and stretching the extruded silane-grafted polyolefin composition in a sheet form;
(S3) extracting a diluent from the stretched sheet to prepare a porous membrane;
(S4) heat-setting the porous membrane; And
(S4) the step of cross-linking the heat-set porous membrane; includes,
Method for producing a crosslinked polyolefin separation membrane having a residence time of 3 minutes 30 seconds to 6 minutes 30 seconds until the polyolefin is mixed in the extruder and then mixed and transferred to the rear end of the gear pump in step (S1). According to claim 1,
The extruder is a hopper containing a polyolefin, a diluent, an initiator, an alkoxy group and a silane compound containing a carbon-carbon double bond group, and a port for introducing a crosslinking catalyst, and a kneading unit for transporting and mixing the material input from the hopper and the port, the kneading It is a twin-screw extruder having a gear pump for conveying the material that has passed through the die to the outlet, and a die outlet for extruding the material transferred through the gear pump to the outside,
The residence time is mixed with the diluent, the initiator, the alkoxy group and the silane compound containing the carbon-carbon double bond group, and the crosslinking catalyst in the kneading section from the time when the polyolefin is introduced into the hopper of the extruder to the rear end of the gear pump. Method for producing a cross-linked polyolefin separation membrane, characterized in that it takes time until the point of transfer. According to claim 1,
The residence time is controlled by the arrangement of a plurality of screw elements provided in the kneading section, characterized in that the method for producing a crosslinked polyolefin separation membrane. According to claim 2,
Advance zone where the kneading unit sends the input material in the extruder in the die exit direction (front), a stop zone for mixing the input material in the extruder in place, and a reverse direction sending the input material in the extruder in the hopper direction (back) It has two or more zones,
Each of the zones independently comprises one or more screw elements. According to claim 1,
The content of the alkoxy compound containing the alkoxy group and the carbon-carbon double bond group is 0.1 to 3 parts by weight based on 100 parts by weight of the total amount of the polyolefin and the diluent. According to claim 1,
The alkoxy group and the carbon-carbon double bond group-containing silane compound is a method for producing a crosslinked polyolefin separation membrane comprising a compound represented by the following formula (1):
[Formula 1]

In Formula 1, R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10 carbon atoms, an alkoxyalkoxy group having 2 to 10 carbon atoms, an alkylcarbonyloxy group having 1 to 10 carbon atoms, or an alkyl group having 1 to 10 carbon atoms, wherein R _{1 and} R _{2 ,} And At least one of R ₃ is an alkoxy group or an alkoxyalkoxy group;
R is a vinyl group, an acryloxy group, a methacryloxy group, or an alkyl group having 1 to 20 carbon atoms, wherein at least one hydrogen of the alkyl group is substituted with a vinyl group, an acryloxy group, or a methacryloxy group. According to claim 1,
The silane compound containing the alkoxy group and the carbon-carbon double bond group is vinyl trimethoxysilane, vinyl triethoxysilane, vinyl triacetoxysilane, (3-methacryloxypropyl) trimethoxysilane, (3-methacryloxy Propyl) of a crosslinked polyolefin separator comprising triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of at least two of them. Manufacturing method. A crosslinked polyolefin separation membrane comprising a -Si-O-Si- linking group between main chains of polyolefins, and having a variation in gel number of 0.15/m ² or less and air permeability of 10 sec/100cc or less. The method of claim 8,
The number of gels is 0.05 to 0.13/m ² , and the crosslinked polyolefin separation membrane characterized in that the air permeability variation is 6.4 to 8.2 sec/100cc. A lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the crosslinked polyolefin separator of claim 8 or 9.