Method for manufacturing polypropylene for secondary battery separator having excellent mechanical properties and thermal properties, and secondary battery separator produced therefrom

Abstract

Disclosed is a method for manufacturing polypropylene for a secondary battery separator having maximized mechanical properties while improving thermal properties and processability compared to existing materials in the manufacture of polypropylene for a secondary battery dry separator. The present invention provides a method for manufacturing polypropylene for a secondary battery separator by subjecting propylene monomers to a polymerization reaction in the presence of a Ziegler-Natta catalyst, wherein the propylene monomer polymerization reaction comprises the steps of: a) obtaining a high molecular weight polypropylene having a weight average molecular weight of 550,000 to 750,000 g / mol in a first reactor; and b) obtaining a low molecular weight polypropylene having a weight average molecular weight of 200,000 to 400,000 g / mol in a second reactor, and a co-catalyst and an external electron donor are added in a molar ratio of 2 to 14 to prepare polypropylene for a secondary battery separator.

Description

Method for manufacturing polypropylene for secondary battery separator with excellent mechanical and thermal properties and secondary battery separator manufactured therefrom

[0001] The present invention relates to a method for producing polypropylene and a secondary battery separator produced therefrom, and more particularly, to a method for producing polypropylene for a secondary battery dry separator and a secondary battery separator produced therefrom.

[0002] This application claims priority to and the benefit of Republic of Korea Patent Application No. 10-2023-0171498, filed November 30, 2023, which is incorporated herein by reference in its entirety.

[0003] Since the commercialization of lithium secondary batteries, the development of new thin-film separators that exhibit superior mechanical and electrical properties has continued. Currently, microporous separators using polyolefin-based materials are the mainstream separators for lithium secondary batteries. While polyolefin-based microporous separators have traditionally been used with a thickness of approximately 15 to 25 μm (based on commercialization standards), the use of separators with a thickness of 7 to 9 μm is increasing recently to maximize energy density. Several battery companies are currently conducting preliminary performance evaluations of batteries using 5 μm prototype separators.

[0004] Likewise, commercialization of 5 μm-class separators is expected in the future. With the recent proliferation of electric vehicles, battery safety is significantly increasing due to issues such as battery fires. Therefore, the development of secondary battery separators that can exhibit high strength despite their thin thickness is essential.

[0005] Polymer bases commonly used in the manufacture of secondary battery separators include polyethylene and polypropylene, which are advantageous for pore formation and are inexpensive. Polypropylene, compared to polyethylene, exhibits both low thermal shrinkage and superior mechanical properties. U.S. Patent Nos. 5,385,777 and 5,480,745, and Korean Patent Publication No. 2003-0080007 disclose methods for manufacturing polypropylene films for secondary battery separators using a dry process, which exhibit excellent heat resistance and competitive pricing. However, these existing methods fail to achieve satisfactory results in terms of both mechanical and thermal properties.

[0006] Meanwhile, the applicant of the present invention has proposed a polypropylene having excellent properties related to stereoregularity, crystallinity, melting temperature, and flexural modulus by applying a catalyst manufactured by a specific method and a bimodal mode process as a polypropylene for a high heat-resistant and high-strength secondary battery separator in Korean Patent No. 1711261, but there are limitations in maximizing mechanical properties and thermal characteristics.

[0007] In addition, the applicant of the present invention has proposed a method for manufacturing polypropylene for secondary battery separators that maximizes mechanical and thermal properties compared to existing materials while improving the flowability of the resin in Korean Patent Publication No. 10-2023-0073831. However, improvements are needed to realize high strength characteristics at a thin thickness in the future.

[0008] The present invention aims to provide a method for manufacturing polypropylene for a secondary battery separator, which improves thermal properties and processability compared to existing materials, and in particular, maximizes mechanical properties, and a secondary battery separator manufactured thereby.

[0009] In order to solve the above problem, the present invention provides a method for producing polypropylene for a secondary battery separator by polymerizing a propylene monomer in the presence of a Ziegler-Natta catalyst, wherein the propylene monomer polymerization reaction comprises the steps of: a) obtaining a high molecular weight polypropylene having a weight average molecular weight of 550,000 to 750,000 g / mol in a first reactor, and b) obtaining a low molecular weight polypropylene having a weight average molecular weight of 200,000 to 400,000 g / mol in a second reactor, wherein a) a cocatalyst (TEAL) and an external electron donor (Donor) are added in a molar ratio of 2 to 14, thereby providing a method for producing polypropylene for a secondary battery separator.

[0010] In addition, the cocatalyst is at least one selected from the group consisting of trimethylaluminum, triethylaluminum, triisobutylaluminum, and trioctylaluminum, and the external electron donor is cyclohexylmethyldimethoxysilane, dicyclopentyldimethoxysilane, diisopropyldimethoxysilane, vinyltriethoxysilane, triethylmethoxysilane, trimethylethoxysilane, dicyclopentyldiethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, diphenyldiethoxysilane, phenylpropyldimethoxysilane, phenyltrimethoxysilane, tert-butyltrimethoxysilane, cyclohexylethyldimethoxysilane, cyclohexylmethyldimethoxysilane, cyclopentyltriethoxysilane, diisobutyldiethoxysilane, isobutyltriethoxysilane, A method for manufacturing a polypropylene for a secondary battery separator is provided, characterized in that the polypropylene is at least one selected from the group consisting of normalpropyltrimethoxysilane, isopropyltrimethoxysilane, cycloheptylmethyldiethoxysilane, and dicycloheptyldiethoxysilane.

[0011] In addition, the present invention provides a method for manufacturing a polypropylene for a secondary battery separator, wherein the polypropylene for a secondary battery separator has a weight average molecular weight of 400,000 to 600,000 g / mol as measured by the following method, a molecular weight distribution (Mw / Mn) of 6 or more, and a xylene soluble content of less than 2 wt%.

[0012] [Method for measuring molecular weight characteristics]

[0013] Weight-average molecular weight and molecular weight distribution (Mw / Mn) were measured using gel permeation chromatography (GPC, Agilent) according to ASTM D3536;

[0014] [Method for measuring xylene availability]

[0015] Dissolve the sample in boiling xylene according to ASTM D5492, crystallize the insoluble portion from the solution, and separate the soluble portion for measurement.

[0016] In order to solve the above-mentioned further problem, the present invention provides a secondary battery separator comprising the manufactured polypropylene.

[0017] In addition, the present invention provides a secondary battery separator characterized in that the separator has a meltdown temperature of more than 166°C as measured by the following method, a tensile strength of more than 1,700 kgf / ㎠, a puncture strength of more than 280 gf, and a region in which the lamella thickness is more than 45 nm in an amount of more than 25% and a region in which the lamella thickness is less than 10 nm in an amount of more than 10%.

[0018] [Method for Measuring Meltdown Temperature]

[0019] The polypropylene for the above separator is extruded in a T-die manner at 220 to 250°C with a twin-screw extruder to form a sheet, and then sequentially stretched in MD and TD directions in a stretching machine to manufacture a single-layer porous film having a thickness of 15 μm, and a lithium ion secondary battery manufactured using the manufactured porous film is heated in an oven at a rate of 2°C / min while measuring the resistance in real time, and the temperature at which the resistance exceeds 10,000 Ω is evaluated as the shutdown temperature, and the temperature at which the resistance rapidly decreases after the shutdown temperature is evaluated as the meltdown temperature;

[0020] [Tensile strength measurement method]

[0021] The porous film specimen manufactured above (ASTM D638 Type IV standard) was measured under 50 mm / min conditions using a universal materials testing machine according to ASTM D638;

[0022] [Method of measuring perforation strength]

[0023] After cutting a 15 ㎛ thick membrane into a size of 50 × 50 ㎜, the specimen is placed on a plate with a 10 ㎜ diameter circular hole formed, and the force at the point where the specimen is pierced is measured when lowered at a speed of 0.05 cm / sec with a probe with a diameter of 1 mm (radius of curvature 0.5 mm);

[0024] [Method for measuring lamella thickness distribution]

[0025] Using the SSA (Successive Self Nucleation and Annealing) of the DSC equipment, the melting point corresponding to each temperature is measured by annealing at each temperature for 30 minutes at 5℃ intervals from 180℃ to 140℃, and the melting point corresponding to each temperature is converted into the lamella thickness according to the Gibbs-Thomson equation of the following mathematical formula 1. In the SSA, the polypropylene is heated to 180℃ using a DSC (differential scanning calorimeter) and cooled to 140℃, and the melting point corresponding to each temperature is measured by repeating heating-annealing-quenching while annealing at each temperature for 30 minutes at 5℃ intervals, and the melting point corresponding to each temperature is converted into the lamella thickness according to the Gibbs-Thomson equation of the following mathematical formula 1;

[0026] <Mathematical Formula 1>

[0027]

[0028] In mathematical equation 1, l is the lamellar thickness, σ is the basal plane surface free energy (49.6Х10 -3 (Jm -2 )), ΔH v is the melting enthalpy per unit area (1.84Х10 8 (Jm -3 ), T m Silver melting point, T m 0 is the equilibrium melting point (460 K) of an infinite crystal.

[0029] According to the present invention, in the production of polypropylene for a secondary battery dry separator, a bimodal mode process is applied, ideal conditions are set for the input ratio of a cocatalyst and an external electron donor, and in particular, the molecular weight characteristics of the resin are controlled during the bimodal mode process to improve thermal characteristics and processability, while a secondary battery separator having maximized mechanical properties can be produced, and a secondary battery separator produced thereby can be provided.

[0030] Figure 1 is a graph showing the results of measuring the lamella thickness distribution for films manufactured according to Example 1 and Comparative Example 5.

[0031] Hereinafter, the present invention will be described in detail through preferred embodiments. Prior to this, it should be noted that the terms and words used in this specification and claims should not be interpreted as limited to their conventional or dictionary meanings. Based on the principle that the inventor can appropriately define the concept of a term to best explain his or her invention, they should be interpreted as meanings and concepts that conform to the technical concept of the present invention. Therefore, the configuration of the embodiments described in this specification is only the most preferred embodiment of the present invention and does not represent the entire technical concept of the present invention. Therefore, it should be understood that various equivalents and modified examples may exist as of the time of filing this application.

[0032]

[0033] The present inventors, in a situation where it is necessary to develop a material with maximized mechanical properties while improving thermal properties and processability compared to the prior art in the production of polypropylene for dry separators of secondary batteries, have confirmed that it is possible to produce polypropylene for separators of secondary batteries with maximized mechanical properties while improving thermal properties and processability by applying a bimodal mode process and controlling the input ratio of a specific cocatalyst and an external electron donor and the molecular weight characteristics of the resin between processes, and thus have reached the present invention.

[0034] Accordingly, the present invention discloses a method for producing polypropylene for a secondary battery separator by polymerizing a propylene monomer in the presence of a Ziegler-Natta catalyst, wherein the propylene monomer polymerization reaction comprises the steps of a) obtaining a high molecular weight polypropylene having a weight average molecular weight of 550,000 to 750,000 g / mol in a first reactor, and b) obtaining a low molecular weight polypropylene having a weight average molecular weight of 200,000 to 400,000 g / mol in a second reactor, wherein a cocatalyst and an external electron donor are added in a molar ratio of 2 to 14 in step a), thereby producing a polypropylene for a secondary battery separator.

[0035] In the present invention, the Ziegler-Natta catalyst is a solid catalyst produced by, for example, reacting a magnesium compound with an alkanediol having 3 to 15 carbon atoms substituted or unsubstituted with an alkyl group having 1 to 5 carbon atoms and a benzoyl halide compound to produce a magnesium compound solution, reacting the magnesium compound solution with a transition metal compound to produce a support, and reacting the support with the transition metal compound. With respect to a specific production method, Patent No. 1711261 in the name of the present applicant is incorporated by reference.

[0036] The production of polypropylene for a secondary battery separator according to the present invention involves polymerizing a propylene monomer in the presence of the solid catalyst, and applying a bimodal mode process. That is, the propylene monomer polymerization reaction includes a) a step of obtaining a high molecular weight polypropylene having a weight average molecular weight of 550,000 to 750,000 g / mol in a first reactor, and b) a step of obtaining a low molecular weight polypropylene having a weight average molecular weight of 200,000 to 400,000 g / mol in a second reactor, thereby producing a high crystallinity polypropylene.

[0037] In the present invention, the propylene polymerization reaction can be carried out in the gas phase, liquid phase, or solution phase. When the polymerization reaction is carried out in the liquid phase, a hydrocarbon solvent can be used, and propylene itself can also be used as the solvent. The polymerization reaction temperature can be 0 to 200°C, and preferably 50 to 150°C. If the reaction temperature is below 0°C, the activity of the catalyst may be reduced, and if it exceeds 200°C, the stereoregularity may be reduced. The pressure condition during the polymerization can be carried out at 1 to 100 atm, and preferably 2 to 30 atm. If the pressure exceeds 100 atm, it is not desirable from an industrial and economic perspective. The polymerization reaction can be carried out in any of batch, semi-continuous, and continuous methods, but in the present invention, a continuous bimodal mode process is applied, and in the first reactor, a high molecular weight polypropylene having a weight average molecular weight of 550,000 to 750,000 g / mol and in the second reactor, a low molecular weight polypropylene having a weight average molecular weight of 200,000 to 400,000 g / mol are polymerized in a weight ratio of 8:2 to 4:6, preferably in the first reactor, a high molecular weight polypropylene having a weight average molecular weight of 600,000 to 700,000 g / mol and in the second reactor, a low molecular weight polypropylene having a weight average molecular weight of 220,000 to 350,000 g / mol are polymerized in a weight ratio of 7:3 to 5:5, more preferably in the first reactor, a high molecular weight polypropylene having a weight average molecular weight of 620,000 to 750,000 g / mol and in the second reactor, a low molecular weight polypropylene having a weight average molecular weight of 200,000 to 400,000 g / mol are polymerized in a weight ratio of 7:3 to 5:5, more preferably in the first ... A high molecular weight polypropylene having a weight average molecular weight of 680,000 g / mol and a low molecular weight polypropylene having a weight average molecular weight of 230,000 to 270,000 g / mol are produced in a weight ratio of 6.5:3.5 to 5.5:4.5 in a second reactor, so that the final polypropylene has a weight average molecular weight of 400,000 to 600,000 g / mol and a molecular weight distribution (Mw / Mn) of 6 to 10, preferably a weight average molecular weight of 450,000 to 550,000 g / mol and a molecular weight distribution (Mw / Mn) of 6.5 to 8, more preferably a weight average molecular weight of 470,000 to 530,000 g / mol and a molecular weight distribution (Mw / Mn) of 6.5 to 7 can be manufactured. At this time, the polypropylene finally manufactured may have a molecular weight fraction of 1,000,000 g / mol or more of 10% or more and a molecular weight fraction of 10,000 g / mol or less of 2.5% or more, and preferably a molecular weight fraction of 1,000,000 g / mol or more of 10% to 15% and a molecular weight fraction of 10,000 g / mol or less of 2.5% to 5%.

[0038] That is, polypropylene of specific low molecular weight and high molecular weight is included in an ideal content ratio, and a molecular weight fraction of 1,000,000 g / mol or more and a molecular weight fraction of 10,000 g / mol or less in the final polypropylene is composed in an ideal ratio, so that the mechanical properties can be maximized while maintaining the improved thermal properties and processability of polypropylene by applying the specific cocatalyst / external electron donor input ratio described later.

[0039] Here, in the first reactor, a cocatalyst and an external electron donor are introduced, and as the transition metal compound is reduced in the solid catalyst, a portion of the internal electron donor present in the solid catalyst is removed, and the external electron donor binds to this empty space to allow the polymerization reaction to proceed. In the present invention, when the ratio of the external electron donor to the introduced cocatalyst is applied at a lower level than in the past, it was confirmed that the thermal characteristics and processability of polypropylene manufactured by the bimodal mode process are improved, and the mechanical properties are maximized.

[0040] That is, in the present invention, the molar ratio of the cocatalyst and the external electron donor added in step a) is 2 to 14, preferably 3 to 10, and more preferably 4 to 6.

[0041] The above cocatalyst and external electron donor are not particularly limited as long as they are components used in the production of polypropylene for a conventional secondary battery dry separator, and for example, trimethylaluminum, triethylaluminum, triisobutylaluminum, trioctylaluminum, etc. can be used as the cocatalyst, and the external electron donor can be cyclohexylmethyldimethoxysilane, dicyclopentyldimethoxysilane, diisopropyldimethoxysilane, vinyltriethoxysilane, triethylmethoxysilane, trimethylethoxysilane, dicyclopentyldiethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, diphenyldiethoxysilane, phenylpropyldimethoxysilane, phenyltrimethoxysilane, tert-butyltrimethoxysilane, cyclohexylethyldimethoxysilane, cyclohexylmethyldimethoxysilane, Cyclopentyltriethoxysilane, diisobutyldiethoxysilane, isobutyltriethoxysilane, normalpropyltrimethoxysilane, isopropyltrimethoxysilane, cycloheptylmethyldiethoxysilane, dicycloheptyldiethoxysilane, etc. can be used, and preferably, triethyl aluminum (TEAL) can be used as the cocatalyst, and dicyclopentyldimethoxysilane can be used as the external electron donor.

[0042] Generally, the crystallinity of general-purpose polypropylene is about 50 to 52%, but the polypropylene manufactured according to the present invention is a highly crystalline polypropylene with a crystallinity of 55% or more. In addition, because the stereoregularity is very high, at 97% or more, it has a melting temperature of 167°C or higher, exhibits high crystallization temperature characteristics, and possesses excellent mechanical and thermal properties.

[0043] The above polymerized polypropylene can further have antioxidants, neutralizing agents, nucleating agents, etc. added during the mixing process, and the extruded sheet can be stretched to produce a porous film through a dry process. A method used to produce a membrane using the above-mentioned polypropylene using a dry process may be a method in which the polymer crystal portions are oriented in a certain direction, and then relatively weak amorphous portions are ruptured through cold stretching to form pores. In addition, it can be produced due to differences in the polymer crystal shape and crystallization temperature, and when produced through this method, a nucleating agent is added. Therefore, it is believed that the characteristics of the microporous membrane produced are determined not only by the degree of orientation of the polymer crystal portions and the polymer crystal morphology, but also by the added additives and modifiers.

[0044] For example, in the present invention, the separator is manufactured by forming a sheet by extruding the polypropylene using a twin-screw extruder at a temperature of 200 to 230°C in a T-die manner, and then simultaneously and sequentially stretching the film in the MD and TD directions in a stretching machine to form a porous film. In order to be used as a separator for secondary batteries, the film's porosity must be at least 30%, and mechanical properties such as puncture strength and tensile strength are important factors that determine whether it can be used. As confirmed in the following examples, the polypropylene for a secondary battery separator according to the present invention improves the thermal properties and flowability (processability) of a film used as a material for a separator for secondary batteries, exhibits an improved meltdown temperature, and, in particular, dramatically improves the mechanical properties.

[0045] Specifically, the polypropylene for a secondary battery separator according to the present invention has a meltdown temperature of more than 166°C, preferably 168°C, as measured by the following method, and a xylene soluble content of less than 2 wt%, preferably 1.5 wt% or less, and more preferably 1.2 wt% or less, and the secondary battery separator manufactured using the polypropylene has a meltdown temperature of more than 166°C, as measured by the following method, a tensile strength of more than 1,700 kgf / cm2, preferably more than 1,900 kgf / cm2, a puncture strength of more than 280 gf, preferably more than 330 gf, and an area where the lamellar thickness is 45 nm or more may be 25% or more and an area where the lamellar thickness is less than 10 nm may be 10% or more.

[0046] [Method for Measuring Meltdown Temperature]

[0047] The polypropylene for the above separator is extruded in a T-die manner at 220 to 250°C with a twin-screw extruder to form a sheet, and then sequentially stretched in MD and TD directions in a stretching machine to manufacture a single-layer porous film having a thickness of 15 μm, and a lithium ion secondary battery manufactured using the manufactured porous film is heated in an oven at a rate of 2°C / min while measuring the resistance in real time, and the temperature at which the resistance exceeds 10,000 Ω is evaluated as the shutdown temperature, and the temperature at which the resistance rapidly decreases after the shutdown temperature is evaluated as the meltdown temperature;

[0048] [Tensile strength measurement method]

[0049] The porous film specimen manufactured above (ASTM D638 Type IV standard) was measured under 50 mm / min conditions using a universal materials testing machine according to ASTM D638;

[0050] [Method of measuring perforation strength]

[0051] After cutting a 15 ㎛ thick membrane into a size of 50 × 50 ㎜, the specimen is placed on a plate with a 10 ㎜ diameter circular hole formed, and the force at the point where the specimen is pierced is measured when lowered at a speed of 0.05 cm / sec with a probe with a diameter of 1 mm (radius of curvature 0.5 mm);

[0052] [Method for measuring lamella thickness distribution]

[0053] Using the SSA (Successive Self Nucleation and Annealing) of the DSC equipment, the melting point corresponding to each temperature is measured by annealing at each temperature for 30 minutes at 5℃ intervals from 180℃ to 140℃, and the melting point corresponding to each temperature is converted into the lamella thickness according to the Gibbs-Thomson equation of the following mathematical formula 1. In the SSA, the polypropylene is heated to 180℃ using a DSC (differential scanning calorimeter) and cooled to 140℃, and the melting point corresponding to each temperature is measured by repeating heating-annealing-quenching while annealing at each temperature for 30 minutes at 5℃ intervals, and the melting point corresponding to each temperature is converted into the lamella thickness according to the Gibbs-Thomson equation of the following mathematical formula 1;

[0054] <Mathematical Formula 1>

[0055]

[0056] In mathematical equation 1, l is the lamellar thickness, σ is the basal plane surface free energy (49.6Х10 -3 (Jm -2 )), ΔH v is the melting enthalpy per unit area (1.84Х10 8 (Jm -3 ), T m Silver melting point, T m 0 is the equilibrium melting point (460 K) of an infinite crystal.

[0057] Hereinafter, the present invention will be described in more detail through specific examples and comparative examples.

[0058]

[0059] Example

[0060] A 2 liter pressure-resistant glass reactor equipped with a stirrer and an oil circulation heater was sufficiently ventilated with nitrogen, and anhydrous magnesium dichloride, 2,4-pentanediol, and decane were charged under a nitrogen atmosphere, and stirred at a rotation speed of 500 rpm at 130°C. After the magnesium compound was completely dissolved to form a homogeneous solution and aged for 1 hour, benzoyl chloride was added for 30 minutes, aged at 130°C for 1 hour, and the temperature of the reactor was lowered to 25°C to prepare a magnesium compound solution. Thereafter, a pressure-resistant glass reactor equipped with a stirrer and an oil circulation heater was sufficiently ventilated with nitrogen, and 800 ml of hexane and 800 ml of titanium tetrachloride were charged under nitrogen reflux, and the temperature of the reactor was lowered to -20°C while stirring at 300 rpm, and a mixed solvent was prepared. The prepared magnesium dichloride solution was added to the reactor containing the titanium compound dispersed in the hexane solvent over a period of 4 hours. A magnesium compound solution was added and maintained for 1 hour, and then the temperature of the reactor was increased at a rate of 0.25 ℃ / min until the temperature reached 20 ℃. Once the temperature of the reactor reached 20 ℃, it was aged for 1 hour, and the temperature of the reactor was increased at a rate of 1 ℃ / min to 73 ℃. After reaching 73 ℃, the aging process was carried out for 2 hours, and the supernatant except for the precipitated solid in the reactor was removed to produce a solid support. After that, titanium tetrachloride was added to the manufactured solid support, stirred, and the temperature was increased at a rate of 1 ℃ / min, and 2,2'-diisobutyl phthalate and 1,3-diethyl ether mixed in a 1:1 molar ratio were added at 110 ℃. After that, the reactor was aged for 1 hour, the solid catalyst was precipitated again, and the supernatant was removed. The solid catalyst from which the supernatant had been removed was further washed once with titanium tetrachloride, then cooled to 63°C and washed seven times with 1 L of hexane to obtain a final slurry solid catalyst. The final catalyst slurry was dried with nitrogen to obtain a solid catalyst for polypropylene polymerization in a solid state.

[0061] Polypropylene polymerization was performed using a bimodal mode process using the solid catalyst and propylene as a solvent in a bulk polymerization method. First, a 2 L nitrogen atmosphere was created in the first reactor heated to 120°C. The temperature of the reactor was lowered to 25°C under the nitrogen atmosphere and the reactor was vented with propylene to maintain a propylene atmosphere. 2 mmol of triethylaluminum (TEAL) diluted in decane solvent at a 1 molar concentration was added to the reactor maintained in a propylene gas atmosphere, and dicyclopentyldimethoxysilane (Donor) diluted in decane solvent was added at a molar ratio (dicyclopentyldimethoxysilane / triethylaluminum) of 5. The catalyst was diluted in decane solvent and added at a rate of 0.005 g, followed by 1,000 mL of hydrogen, 500 g of propylene, and the prepolymerization was performed for 5 minutes using a stirrer. After the pre-polymerization, the temperature of the reactor was heated to 70℃ and polymerized at 70℃ for 1 hour. After that, unreacted propylene was discharged to the atmosphere and the temperature of the reactor was lowered to room temperature to produce high molecular weight polypropylene having a weight average molecular weight of about 650,000 g / mol and a relatively low melting index. In the second reactor, the same method as above was performed, but the polymerization time was adjusted to produce low molecular weight polypropylene having a weight average molecular weight of about 250,000 g / mol and a relatively low melting index, and the produced high molecular weight polypropylene and low molecular weight polypropylene were polymerized at a weight ratio of about 6:4. The produced polypropylene was obtained after drying in a vacuum oven at 50℃ for 10 hours.

[0062]

[0063] Comparative Examples 1 to 4

[0064] Polypropylene was manufactured in the same manner as in Example 1, except that the polymerization process and TEAL / Donor ratio were adjusted as shown in Table 1 below. In the case of the monomodal mode process, only the first reactor was used to obtain the average molecular weight shown in Table 1 below.

[0065]

[0066] Comparative Example 5

[0067] In Example 1, a high molecular weight polypropylene having a weight average molecular weight of about 550,000 g / mol was produced in a first reactor, and in a second reactor, the same method was performed, but the polymerization time was controlled to produce a low molecular weight polypropylene having a weight average molecular weight of about 210,000 g / mol and a relatively low melting index, and the polypropylene was obtained in the same manner, except that the produced high molecular weight polypropylene and low molecular weight polypropylene were polymerized in a weight ratio of about 6:4.

[0068]

[0069] Exam example

[0070] The polypropylene manufactured above was extruded in a T-die manner at 220 to 250°C with a twin-screw extruder to form a sheet, and then sequentially stretched in the MD and TD directions in a stretching machine to manufacture a single-layer porous film having a thickness of 15 μm. The molecular weight characteristics, meltdown temperature, xylene availability, tensile strength, puncture strength, and processability of the polypropylene and the film were measured or evaluated according to the following methods, and the results are shown in Table 1 below. The lamella thickness distribution of the films manufactured according to Example 1 and Comparative Example 5 was measured according to the following method, and the results are shown in Fig. 1.

[0071] [measurement method]

[0072] (1) Molecular weight characteristics

[0073] Weight-average molecular weight, molecular weight fraction, and molecular weight distribution (Mw / Mn) were measured using gel permeation chromatography (GPC, Agilent) according to ASTM D3536. Polystyrene was used as a standard in chloroform solvent.

[0074] (2) Available xylene

[0075] According to ASTM D5492 standard, the sample was dissolved in boiling xylene, the insoluble portion was crystallized from the solution, and the soluble portion was separated and measured.

[0076] (3) Membrane processability

[0077] The polypropylene manufactured above was extruded at 220°C using a twin-screw extruder in a T-die manner, and the pressure applied to the extruder when passing the extrudate through the T-die was measured and relatively evaluated.

[0078] (4) Meltdown temperature

[0079] The lithium ion secondary battery manufactured using the porous film manufactured above was heated in an oven at a rate of 2 ℃ / min, and the resistance was measured in real time. The temperature at which the resistance exceeded 10,000 Ω was evaluated as the shutdown temperature, and the temperature at which the resistance decreased rapidly after the shutdown temperature was evaluated as the meltdown temperature. 'Shutdown' refers to a characteristic in which some melting occurs in the film above a certain temperature to close the pores, and a short circuit in which the positive and negative electrodes of the secondary battery are directly connected due to thermal deformation of the film at high temperatures is the main cause of explosion and fire accidents, but this can be prevented through shutdown. At this time, 'meltdown' (or 'film rupture') refers to a phenomenon in which melting occurs in the film above a certain temperature, causing some of the film to be damaged. If this phenomenon occurs, an accident such as a fire or explosion may occur due to a short circuit in which the positive and negative electrodes of the secondary battery are connected.

[0080] (5) Tensile strength

[0081] The specimen (ASTM D638 Type IV standard) was measured using a universal testing machine according to ASTM D638 at 50 mm / min.

[0082] (6) Perforation strength

[0083] After cutting a 15 ㎛ thick membrane into a size of 50 × 50 ㎜, the specimen was placed on a plate with a 10 ㎜ diameter circular hole formed therein, and the force at the point where the specimen was pierced was measured when the specimen was lowered at a speed of 0.05 cm / sec using a probe with a diameter of 1 mm (radius of curvature of 0.5 mm).

[0084] (7) Lamella thickness distribution

[0085] Using the SSA (Successive Self Nucleation and Annealing) of the DSC equipment, the melting point corresponding to each temperature was measured by annealing at 5℃ intervals from 180℃ to 140℃ for 30 minutes, and the melting point was converted into lamella thickness according to the Gibbs-Thomson equation in the following mathematical equation 1.

[0086] This is a method that utilizes the SSA (Successive Self Nucleation and Annealing) of DSC equipment to gradually lower the temperature and rapidly cool at the end of each step, thereby preserving the crystals crystallized at the corresponding temperature at each step.

[0087] That is, after heating the polyolefin to completely melt it, it is annealed at each temperature from 180℃ to 140℃ at 5℃ intervals for 30 minutes, and the lamellae that are not stable at the temperature are still melted, and only the stable lamellae are crystallized. At this time, the stability for the temperature (T) depends on the lamella thickness, and the lamella thickness depends on the chain structure. By performing this heat treatment step by step, the lamella thickness and its distribution according to the polymer chain structure can be quantitatively measured, and accordingly, the distribution of each melting peak area can be measured.

[0088] In the present invention, SSA uses DSC (differential scanning calorimetry) to heat the polypropylene to 180°C and cool it to 140°C, and repeat heating-annealing-quenching at each temperature for 30 minutes at 5°C intervals to measure the melting point corresponding to each temperature, and convert it into lamella thickness according to the Gibbs-Thomson equation of the following mathematical equation 1.

[0089] <Mathematical Formula 1>

[0090]

[0091] In mathematical equation 1, l is the lamellar thickness, σ is the basal plane surface free energy (49.6Х10 -3 (Jm -2 )), ΔH v is the melting enthalpy per unit area (1.84Х10 8 (Jm -3 ), T m Silver melting point, T m 0 is the equilibrium melting point (460 K) of an infinite crystal.

[0092] As the proportion of weight-average molecular weights of 1,000,000 g / mol or more increases, the proportion of lamellar thicknesses of 45 nm or more increases, the rigidity increases, and also as the proportion of molecular weights of 10,000 g / mol or less increases, the proportion of lamellar thicknesses of less than 10 nm increases, which reduces the pressure during extrusion, improves processability, and increases productivity.

[0093]

[0094] Unit Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Polymerization method Polymerization mode - Bimodal Monomodal Monomodal Monomodal Monomodal Bimodal Teal / Donor mole ratio - 55 15 15 5 Polypropylene resin properties Xylene soluble weight % 1.2 1.4 2.0 2.1 1.3 1.1 Weight average molecular weight g / mol 50 4,000 495 000 497 000 50 1,000 395 000 420 000 10 6 g / mol or more%12.411.612.211.78.79.610 4 g / mol or less %2.72.31.92.52.83.2Molecular weight distribution-6.84.66.54.54.66.8Membrane processabilityExtruder pressurebar353934383229Thickness㎛151515151515Membrane mechanical propertiesMeltdown temperature℃168168166166168168Tensile strength(MD)kgf / ㎠1,9321,8501,8001,7501,4501,500Puncture strengthgf340320310290235240TEAL: Cocatalyst, triethylaluminumDonor: External electron donor, dicyclopentylmidethoxysilane

[0095]

[0096] Referring to Table 1 and Figure 1, in the production of polypropylene for a secondary battery dry separator, when the bimodal mode process is applied according to the present invention and the specific cocatalyst and external electron donor input ratio and the molecular weight characteristics of the resin between the processes are controlled (Example 1), it can be confirmed that the mechanical properties are maximized while improving the thermal properties and processability at a similar molecular weight level (Comparative Examples 1 and 3) compared to when the existing monomodal mode process is applied.

[0097] In this regard, when polypropylene is manufactured through the monomodal mode process (Comparative Examples 1 and 3), it can be seen that the molecular weight distribution is narrow, which reduces resin flowability and increases the extrusion load (pressure), resulting in poor processability. At this time, even when the monomodal mode process is applied, it can be seen that when the molecular weight of the final polypropylene is lowered (Comparative Example 4), the decline in processability can be prevented, but the mechanical properties are significantly reduced.

[0098] In addition, even when polypropylene is manufactured through a bimodal mode process, it can be seen that when the TEAL / Donor input ratio does not reach a certain level (Comparative Example 2), the thermal properties deteriorate, such as a decrease in the meltdown temperature along with a decrease in the mechanical properties.

[0099] In addition, it can be seen that even when polypropylene is manufactured at a certain level of TEAL / Donor input ratio using a bimodal mode process, the mechanical properties are significantly deteriorated when the molecular weight fraction of 1,000,000 g / mol or more does not reach a certain level (Comparative Example 5).

[0100]

[0101] The preferred embodiments of the present invention have been described in detail above. The description of the present invention is provided for illustrative purposes only, and those skilled in the art will readily appreciate that other specific modifications can be readily made without altering the technical spirit or essential features of the present invention.

[0102] Accordingly, the scope of the present invention is indicated by the claims described below rather than the detailed description above, and all changes or modifications derived from the meaning, scope and equivalent concepts of the claims should be interpreted as being included in the scope of the present invention.

Claims

1. A method for producing polypropylene for a secondary battery separator by polymerizing a propylene monomer in the presence of a Ziegler-Natta catalyst. The above propylene monomer polymerization reaction comprises a) a step of obtaining a high molecular weight polypropylene having a weight average molecular weight of 550,000 to 750,000 g / mol in a first reactor, and b) a step of obtaining a low molecular weight polypropylene having a weight average molecular weight of 200,000 to 400,000 g / mol in a second reactor, A method for manufacturing a polypropylene for a secondary battery separator, wherein a cocatalyst and an external electron donor are added in a molar ratio of 2 to 14 in step a) above, and the molecular weight fraction of 1,000,000 g / mol or more is 10% or more and the molecular weight fraction of 10,000 g / mol or less is 2.5% or more.

2. In paragraph 1, The above-mentioned cocatalyst is at least one selected from the group consisting of trimethylaluminum, triethylaluminum, triisobutylaluminum, and trioctylaluminum, and the above-mentioned external electron donor is at least one selected from the group consisting of cyclohexylmethyldimethoxysilane, dicyclopentyldimethoxysilane, diisopropyldimethoxysilane, vinyltriethoxysilane, triethylmethoxysilane, trimethylethoxysilane, dicyclopentyldiethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, diphenyldiethoxysilane, phenylpropyldimethoxysilane, phenyltrimethoxysilane, tert-butyltrimethoxysilane, cyclohexylethyldimethoxysilane, cyclohexylmethyldimethoxysilane, cyclopentyltriethoxysilane, diisobutyldiethoxysilane, isobutyltriethoxysilane, A method for manufacturing polypropylene for a secondary battery separator, characterized in that the polypropylene is at least one selected from the group consisting of normalpropyltrimethoxysilane, isopropyltrimethoxysilane, cycloheptylmethyldiethoxysilane, and dicycloheptyldiethoxysilane.

3. In paragraph 1, A method for manufacturing a polypropylene for a secondary battery separator, wherein the polypropylene for a secondary battery separator has a weight average molecular weight of 400,000 to 600,000 g / mol as measured by the following method, a molecular weight distribution (Mw / Mn) of 6 or more, and a xylene soluble content of less than 2 wt%: [Method for measuring molecular weight characteristics] Weight-average molecular weight and molecular weight distribution (Mw / Mn) were measured using gel permeation chromatography (GPC, Agilent) according to ASTM D3536; [Method for measuring xylene availability] Dissolve the sample in boiling xylene according to ASTM D5492, crystallize the insoluble portion from the solution, and separate the soluble portion for measurement.

4. A secondary battery separator comprising polypropylene manufactured by any one of claims 1 to 3.

5. In paragraph 4, The above separator is a secondary battery separator characterized in that the meltdown temperature measured by the following method is greater than 166℃, the tensile strength is greater than 1,700 kgf / ㎠, the puncture strength is greater than 280 gf, and the area where the lamella thickness is greater than 45 nm is 25% or more and the area where the lamella thickness is less than 10 nm is 10% or more: [Method for measuring meltdown temperature] The polypropylene for the above separator is extruded in a T-die manner with a twin-screw extruder at 220 to 250°C to form a sheet, and then sequentially stretched in the MD and TD directions in a stretching machine to manufacture a single-layer porous film having a thickness of 15 μm, and a lithium ion secondary battery manufactured using the manufactured porous film is heated in an oven at a rate of 2°C / min while measuring the resistance in real time, and the temperature at which it exceeds 10,000 Ω is evaluated as the shutdown temperature, and the temperature at which the resistance rapidly decreases after the shutdown temperature is evaluated as the meltdown temperature; [Tensile strength measurement method] The porous film specimen manufactured above (ASTM D638 Type IV standard) was measured using a universal materials testing machine according to ASTM D638 under the condition of 50 mm / min; [Method for measuring perforation strength] After cutting a 15 ㎛ thick membrane into a size of 50 × 50 ㎜, the specimen is placed on a plate with a 10 ㎜ diameter circular hole formed, and the force at the point where the specimen is pierced is measured when the probe with a diameter of 1 mm (radius of curvature of 0.5 mm) is lowered at a speed of 0.05 cm / sec; [Method for measuring lamella thickness distribution] Using the SSA (Successive Self Nucleation and Annealing) of the DSC equipment, the melting point corresponding to each temperature is measured by annealing at each temperature for 30 minutes at intervals of 5℃ from 180℃ to 140℃, and the result is converted into the lamella thickness according to the Gibbs-Thomson equation of the following mathematical formula 1. For the SSA, the polypropylene is heated to 180℃ using a DSC (differential scanning calorimeter) and cooled to 140℃, and the melting point corresponding to each temperature is measured by repeating heating-annealing-quenching while annealing at each temperature for 30 minutes at intervals of 5℃, and the result is converted into the lamella thickness according to the Gibbs-Thomson equation of the following mathematical formula 1; <Mathematical formula 1> In mathematical expression 1, l is the lamella thickness, σ is the basal plane surface free energy (49.6Х10 -3 (Jm -2 )), ΔH v is the melting enthalpy per unit area (1.84Х10 8 (Jm -3 ), T m Silver melting point, T m 0 is the equilibrium melting point of an infinite crystal (460 K).

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