Polyethylene resin powder for secondary battery separator, its manufacturing method and secondary battery separator containing the same

Abstract

To provide: a polyethylene resin powder for secondary battery separation membranes which is smoothly fed during extrusion so as to improve extrudability and inhibit formation of unmelted gel; a method for producing the same; and a secondary battery separation membrane including the same.SOLUTION: A polyethylene resin powder for secondary battery separation membranes contains 1.0 wt.% or less of giant particles of 500 μm or more in particle diameter, and 1.0 wt.% or less of fine particles of 50 μm or less in particle diameter.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a polyethylene resin powder for a secondary battery separator, a method for producing the same, and a secondary battery separator containing the same. [Background technology]

[0002] Very high molecular weight polyethylene (VHMWPE) is polyethylene with a viscosity average molecular weight of 200,000 g / mol to 2.5 million g / mol, and is characterized by high rigidity, chemical resistance, and abrasion resistance due to its high molecular weight. Of these, VHMWPE is widely used as a battery separator for various batteries due to its excellent chemical resistance and battery properties.

[0003] Ultra-high molecular weight polyethylene (ULHMWPE), which has such excellent properties, is difficult to process due to its high molecular weight, as described in U.S. Patent Publication No. US4972035A. Unlike general-purpose polyethylene, it cannot be pelletized and is instead produced and sold in powder form after the polymerization process. In this case, the particle characteristics of the powder are extremely important. Among the particle characteristics of the powder, particle size, particle size distribution, bulk density, and fine powder content are important factors that affect the quality of separator membrane products not only during the UHMWPE manufacturing process but also during the extrusion process. To process such UHMWPE to produce separator membranes, it is injected into an extruder like oil and melt-processed in a mixed state with oil. However, due to its high molecular weight, it has poor fluidity even when mixed and melted like oil, making it difficult to process.

[0004] In recent years, the trend toward thinner battery separators to increase the capacity of secondary batteries has created a need to increase the mechanical strength of the thinner separators. Generally, to increase mechanical strength, a raw material with a higher molecular weight is used. However, the use of relatively high-molecular-weight polyethylene makes it more difficult to ensure a uniform melting state in the extruder, and unmelted powder particles may remain in the final molded product, the separator. In this case, the remaining unmelted gel may appear as an external defect in the separator. Summary of the Invention [Problem to be solved by the invention]

[0005] One embodiment provides a polyethylene resin powder for a separator of a secondary battery, which has smooth feeding during extrusion, improved extrusion processability, and suppresses the formation of unmelted gel.

[0006] Another embodiment provides a method for manufacturing the polyethylene resin powder for a separator of a secondary battery.

[0007] In yet another embodiment, a separator for a secondary battery is provided that contains the polyethylene resin powder for a separator for a secondary battery, thereby having high mechanical strength and improved appearance defects. [Means for solving the problem]

[0008] In one embodiment, a polyethylene resin powder for a separator of a secondary battery is provided, which contains 1.0 wt % or less of large particles having a particle size of 500 μm or more and 1.0 wt % or less of fine particles having a particle size of 50 μm or less.

[0009] The polyethylene resin powder may have an average particle size of 100 μm to 200 μm.

[0010] The particle size distribution (SPAN) of the polyethylene resin powder may be 0.7 to 1.3.

[0011] The fluidity of the polyethylene resin powder may be 12 sec / 100 g to 20 sec / 100 g.

[0012] The polyethylene resin powder may have a high load melt flow index (190°C, 21.6 kg) of 0.1 g / 10 min to 5.0 g / 10 min.

[0013] The viscosity average molecular weight of the polyethylene resin powder may be 200,000 g / mol to 2,500,000 g / mol.

[0014] The bulk density of the polyethylene resin powder may be 0.40 g / cc to 0.50 g / cc.

[0015] Another embodiment is a polyethylene polymerization catalyst in the presence of 2 kgf / cm 2 ~5kgf / cm 2 a pressure of 1000 kJ / cm2, a temperature of 70 to 80°C, and a residence time of 2 to 3 hours to produce a polyethylene resin powder, and a step of obtaining a polyethylene resin powder having a controlled particle size and distribution from the polyethylene resin powder using a sieve; the polyethylene polymerization catalyst is obtained by mixing a magnesium-containing compound, an alcohol, and a hydrocarbon solvent to produce a magnesium-containing compound solution, reacting the magnesium-containing compound solution with a metal chloride to produce a catalyst precursor, reacting the catalyst precursor with a metal chloride and a carbonyl compound to produce a catalyst, and washing the catalyst with a hydrocarbon solvent, wherein the carbonyl compound is represented by Chemical Formula 1 or Chemical Formula 2 below.

[0016] [C1] R1(CO)R2

[0017] [Case 2] R3(CO)OR4

[0018] (In the above chemical formulas 1 and 2, R1 to R4 are each independently a C2 to C10 linear alkyl group, a C6 to C14 cycloalkyl group, or a C6 to C14 aryl group.)

[0019] The washing step may be performed 5 to 8 times.

[0020] Yet another embodiment provides a secondary battery separator including the polyethylene resin powder for secondary battery separators. [Effects of the Invention]

[0021] When using polyethylene resin powder for secondary battery separators according to an embodiment, the fluidity of particles is improved, which allows smooth feeding during extrusion, improving extrusion processability and suppressing the formation of unmelted gels. As a result, high mechanical strength is ensured during the formation of secondary battery separators, while appearance defects are improved, making the powder useful for secondary battery separators. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a scanning electron microscope (SEM) photograph of the polyethylene resin powder for a secondary battery separator according to Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0023] Although the present invention will be described in detail below so that those skilled in the art can easily implement the present invention, it should be understood that the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0024] The polyethylene resin powder for a separator of a secondary battery according to one embodiment has an average particle size within a predetermined range, and contains 1.0 wt% or less of large particles having a particle size of 500 μm or more and fine particles having a particle size of 50 μm or less. In other words, the polyethylene resin powder according to one embodiment has no or only minimal residual content of large particles having a particle size of 500 μm or more and fine particles having a particle size of 50 μm or less.

[0025] When a polyethylene resin powder with controlled particle size and distribution is used, the fluidity of the particles is improved and the injection from the extruder hopper to the extruder is stable, resulting in smooth feeding during extrusion, improved extrusion processability, and suppressed formation of unmelted gels. As a result, a secondary battery separator can be obtained that has high mechanical strength and reduced appearance defects when formed.

[0026] Specifically, the particle size of the macroparticles is 500 μm or more, for example, 500 μm to 2000 μm, 500 μm to 1800 μm, or 500 μm to 1400 μm, but is not limited to these exemplary ranges. The particle size of the microparticles is 50 μm or less, for example, 1 μm to 50 μm, 5 μm to 50 μm, or 10 μm to 50 μm, but is not limited to these exemplary ranges. The polyethylene resin powder contains macroparticles having the above particle size ranges at 1.0 wt% or less, for example, 0 wt% to 1.0 wt%, 0.001 wt% to 1.0 wt%, 0.01 wt% to 1.0 wt%, or 0.1 wt% to 1.0 wt%, but is not limited to these exemplary ranges. Furthermore, the polyethylene resin powder may contain 1.0 wt% or less of fine particles having the above particle size range, for example, 0 wt% to 1.0 wt%, 0.001 wt% to 1.0 wt%, 0.01 wt% to 1.0 wt%, or 0.1 wt% to 1.0 wt%, but is not limited to these exemplary ranges. The above contents are based on the total weight of the polyethylene resin powder. When a polyethylene resin powder having a particle size and distribution controlled within each of the above ranges is used, it is possible to smoothly feed the fine particles during extrusion, improve extrusion processability, and suppress the formation of unmelted gel, thereby obtaining a secondary battery separator having high mechanical strength and improved appearance defects.

[0027] The polyethylene resin powder may have an average particle size of 100 μm to 200 μm, for example, 100 μm to 150 μm or 110 μm to 140 μm. When the average particle size of the polyethylene resin powder is within the above range, the flowability in the extruder hopper is improved, the bulk density is increased, and extrusion defects are prevented, ensuring excellent productivity.

[0028] The particle size distribution (SPAN) of the polyethylene resin powder may be 0.7 to 1.3, for example, 0.7 to 1.1. When the particle size distribution (SPAN) of the polyethylene resin powder is within the above range, the powder has uniform melting properties in the extruder, thereby preventing the formation of unmelted gel and improving the flowability in the hopper.

[0029] The particle size distribution (SPAN) can be defined by the following equation 1, where a smaller SPAN value means a narrower distribution.

[0030] [Number 1] SPAN=(D(v,0.9)-D(v,0.1)) / D(v,0.5)

[0031] (In Equation 1, D(v,0.5) is the average particle size of the samples in the bottom 50% of particle sizes, and D(v,0.9) and D(v,0.1) are the average particle sizes of the samples in the bottom 90% and 10% of particle sizes, respectively.)

[0032] The fluidity of the polyethylene resin powder particles may be 12 sec / 100 g to 20 sec / 100 g, for example, 15 sec / 100 g to 18 sec / 100 g. When the fluidity of the polyethylene resin powder particles is within this range, feeding during extrusion is smooth, extrusion processability is improved, and the formation of an unmelted gel can be prevented.

[0033] The high-stress melt flow index (190°C, 21.6 kg) of the polyethylene resin powder may be 0.1 g / 10 min to 5.0 g / 10 min, for example, 0.2 g / 10 min to 3.0 g / 10 min. When the high-stress melt flow index of the polyethylene resin powder is within the above range, feeding during extrusion is smooth, extrusion processability is improved, and the formation of an unmelted gel can be prevented.

[0034] The melting temperature of the polyethylene resin powder measured by differential scanning calorimetry (DSC) may be 130° C. to 140° C., for example, 130° C. to 135° C. or 132° C. to 135° C. When the melting temperature of the polyethylene resin powder is within the above range, the final molded porous film, i.e., the separation membrane, has excellent mechanical strength and improved extrusion processability.

[0035] The viscosity average molecular weight of the polyethylene resin powder may be 200,000 g / mol to 2,500,000 g / mol, for example, 200,000 g / mol to 2,000,000 g / mol or 200,000 g / mol to 1,500,000 g / mol. When the viscosity average molecular weight of the polyethylene resin powder is within this range, the final molded porous film, i.e., the separator, has excellent mechanical strength, and extrusion processability is improved, and the formation of an unmelted gel can be prevented.

[0036] The bulk density of the polyethylene resin powder may be 0.40 g / cc to 0.50 g / cc, for example, 0.42 g / cc to 0.48 g / cc. When the bulk density of the polyethylene resin powder is within this range, the final porous film, i.e., the separator, has excellent mechanical strength, and extrusion processability is improved, and the formation of an unmelted gel can be prevented.

[0037] Hereinafter, a method for preparing the polyethylene resin powder for a separator of a secondary battery will be described according to another embodiment.

[0038] According to an embodiment, the polyethylene resin powder for a separator of a secondary battery may be obtained by polymerizing a polyethylene resin using a polyethylene polymerization catalyst to obtain a powder, and then by subjecting the powder to a particle size control process.

[0039] Method for producing polyethylene polymerization catalyst The polyethylene polymerization catalyst may be prepared in the following steps.

[0040] In the first step, a magnesium-containing compound, an alcohol, and a hydrocarbon solvent are mixed to prepare a magnesium-containing compound solution. In the second step, the magnesium-containing compound solution is reacted with a metal chloride to prepare a catalyst precursor. In the third step, the catalyst precursor is reacted with a metal chloride and a carbonyl compound to prepare a catalyst. In the fourth step, the prepared catalyst is washed with a hydrocarbon solvent.

[0041] In the first step, the magnesium-containing compound may include a magnesium halide compound, an alkoxy magnesium compound, or a combination thereof. The magnesium halide compound may include, for example, magnesium chloride, such as magnesium dichloride (MgCl2), and the alkoxy magnesium compound may include, for example, diethoxy magnesium.

[0042] The type of the alcohol is not particularly limited, and may be, for example, a C1 to C20 alcohol, such as a C4 to C20 alcohol, for example, normal butanol.

[0043] The hydrocarbon solvent may be a C1-C20 aliphatic hydrocarbon, a C3-C20 alicyclic hydrocarbon, a C6-C30 aromatic hydrocarbon, a halogenated hydrocarbon, or the like. Examples of the aliphatic hydrocarbon include pentane, hexane, heptane, octane, decane, and kerosene. Examples of the alicyclic hydrocarbon include cyclopentane, methylcyclopentane, cyclohexane, and methylcyclohexane. Examples of the aromatic hydrocarbon include benzene, toluene, xylene, ethylbenzene, cumene, and cymene. Examples of the halogenated hydrocarbon include dichloropropane, dichloroethylene, trichloroethylene, carbon tetrachloride, and chlorobenzene. The hydrocarbon solvent may be, for example, an aromatic hydrocarbon such as toluene.

[0044] The alcohol may be mixed in an amount of 3 to 7 parts by weight relative to 1 part by weight of the magnesium-containing compound, for example, 3 to 5 parts by weight relative to 1 part by weight of the magnesium-containing compound. The hydrocarbon solvent may be mixed in an amount of 9 to 16 parts by weight relative to 1 part by weight of the magnesium-containing compound, for example, 10 to 15 parts by weight relative to 1 part by weight of the magnesium-containing compound.

[0045] The mixing ratio of the magnesium-containing compound to the alcohol, and the mixing ratio of the magnesium-containing compound to the hydrocarbon solvent, determine the viscosity of the total magnesium-containing compound solution, and such viscosity can play a role in determining the particle morphology, size, and pore characteristics within the catalyst in the formation of catalyst particles.

[0046] The reaction in the first stage may be carried out by mixing and stirring the magnesium-containing compound, hydrocarbon solvent, and alcohol, then heating the mixture to 60°C to 70°C for 30 minutes to 90 minutes, and then maintaining the mixture for 1 hour 30 minutes to 2 hours 30 minutes.

[0047] In the second step, the metal chloride may include titanium tetrachloride, zirconium chloride, hafnium chloride or a combination thereof, for example, titanium tetrachloride may be used.

[0048] In the second step, the metal chloride may be used in an amount of 3 to 10 parts by weight per 1 part by weight of the magnesium-containing compound solution, for example, 5 to 10 parts by weight per 1 part by weight of the magnesium-containing compound solution.

[0049] The reaction in the second stage may be carried out by cooling the temperature of the magnesium-containing compound solution to 30°C to 50°C, injecting the metal chloride and stirring, then raising the temperature to 50°C to 70°C for 30 to 90 minutes, and aging for 30 to 90 minutes.

[0050] In this case, the reaction between the magnesium-containing compound solution and the metal chloride may be carried out at a stirring speed of 300 rpm to 400 rpm, for example, 330 rpm to 370 rpm. The stirring may play a role in determining the particle shape, size, and pore characteristics within the catalyst during catalyst particle formation. That is, when the stirring speed is within this range, excellent particle characteristics can be obtained during particle formation of the polyethylene polymerization catalyst.

[0051] Thereafter, the mixture is centrifuged to remove the supernatant, thereby obtaining a solid catalyst precursor.

[0052] The reaction in the third step is to produce a polyethylene polymerization catalyst by reacting the catalyst precursor with a metal chloride and a carbonyl compound, and the reaction can be carried out in the presence of a hydrocarbon solvent.

[0053] The metal chloride may be used in an amount of 3 to 10 parts by weight, for example, 5 to 10 parts by weight, relative to 1 part by weight of the catalyst precursor.

[0054] The carbonyl compound is represented by the following formula 1 or 2.

[0055] [C1] R1(CO)R2

[0056] [Case 2] R3(CO)OR4

[0057] (In the above chemical formulas 1 and 2, The R1 to R4 are each independently a C2 to C10 linear alkyl group, a C6 to C14 cycloalkyl group, or a C6 to C14 aryl group.

[0058] The carbonyl compound may be, for example, the compound represented by Chemical Formula 2 above, or may be ethyl benzoate.

[0059] The carbonyl compound may be used in an amount of 0.1 to 0.5 parts by weight relative to 1 part by weight of the catalyst precursor.

[0060] The hydrocarbon solvent used in the third step is not limited to its type, but may be, for example, a C1 to C10 linear hydrocarbon solvent such as hexane.

[0061] The reaction in the third step may be carried out by mixing the catalyst precursor, the hydrocarbon solvent, the metal chloride, and the carbonyl compound, then raising the temperature to 60°C to 80°C for 30 to 90 minutes while stirring, and then aging for 1 to 3 hours. The catalyst is then obtained by centrifuging and removing the supernatant. The catalyst may be a magnesium-supported titanium catalyst.

[0062] Next, in the fourth step, the produced catalyst is washed with a hydrocarbon solvent to obtain a polyethylene polymerization catalyst.

[0063] In this case, the produced catalyst may be washed with a hydrocarbon solvent 5 to 8 times, for example 6 to 7 times. When washed within this range, the catalyst may be useful in a process for obtaining a polyethylene resin powder having particle size and distribution adjusted within a predetermined range.

[0064] The catalyst washing process removes not only unreacted and by-reacted materials but also fine catalyst particles that did not properly form catalyst particles, thereby preliminarily removing fine particles generated during the polymerization process. However, since this process involves sinking the solid catalyst component and then removing the supernatant, it can be difficult to remove large catalyst particles that are larger than the desired size and form flakes during polymerization.

[0065] Manufacture of polyethylene resin powder A polyethylene resin powder may be produced by carrying out a polymerization reaction using the polyethylene polymerization catalyst and an organometallic compound as a co-catalyst.

[0066] The organometallic compound may be represented by the general formula MRn, where M is a metal of Group II or III of the periodic table, such as magnesium, calcium, zinc, boron, aluminum, or gallium. R is a C1-C20 alkyl group, such as methyl, ethyl, butyl, hexyl, octyl, or decyl. n represents the valence of the metal component.

[0067] Specifically, the organometallic compound may be an organometallic compound having at least one C1 to C6 alkyl group, such as trialkylaluminum such as triethylaluminum or triisobutylaluminum, which may be used alone or as a mixture.

[0068] The organometallic compound may also be an organoaluminum compound having at least one C1-C6 alkyl group and further containing at least one halogen or hydride substituent, such as ethylaluminum dichloride, diethylaluminum chloride, ethylaluminum sesquichloride, diisobutylaluminum hydride, or a mixture thereof.

[0069] Among these, for example, triethylaluminum may be used as the organometallic compound.

[0070] The polymerization reaction can be carried out by gas phase or bulk polymerization in the absence of organic solvents, or by liquid slurry polymerization in the presence of organic solvents, which are carried out in the absence of oxygen, water, and other compounds that can act as catalyst poisons.

[0071] The organic solvent may be a C1 to C20 aliphatic hydrocarbon, a C3 to C20 alicyclic hydrocarbon, a C6 to C30 aromatic hydrocarbon, a halogenated hydrocarbon, or a mixture thereof. Examples of the aliphatic hydrocarbon include pentane, hexane, heptane, n-octane, and isooctane. Examples of the alicyclic hydrocarbon include cyclohexane and methylcyclohexane. Examples of the aromatic hydrocarbon include toluene, xylene, ethylbenzene, isopropylbenzene, ethyltoluene, n-propylbenzene, and diethylbenzene. Examples of the halogenated hydrocarbon include chlorobenzene, chloronaphthalene, and o-dichlorobenzene. The organic solvent may be a C4 to C10 aliphatic hydrocarbon such as hexane.

[0072] The polymerization reaction may be carried out in a reactor equipped with an internal temperature control device, a pressure control device, and an agitator. Specifically, the polyethylene polymerization catalyst, the organometallic cocatalyst, the organic solvent, i.e., the inert hydrocarbon solvent, and ethylene may be reacted in the reactor to produce a polyethylene resin.

[0073] The polymerization reaction was carried out at a pressure of 2 kgf / cm 2 ~5kgf / cm 2 pressure, e.g., 3kgf / cm 2 ~4kgf / cm 2 The polymerization reaction may be carried out under the following conditions: a pressure of 1000 kJ / cm2, a temperature of 70°C to 80°C, for example, a temperature of 76°C to 78°C, and a residence time of 2 to 3 hours. When the polymerization reaction is carried out under these pressure, temperature, and residence time ranges, activity is increased, the desired particle size of the polyethylene resin is obtained, localized overpolymerization heat generation is prevented, and the powder particles become uniform, increasing bulk density and improving productivity. That is, when the polymerization reaction is carried out under these conditions, a polyethylene resin powder having a particle size and distribution controlled within a predetermined range according to one embodiment is obtained, which allows smooth supply during extrusion, improves extrusion processability, and suppresses the formation of unmelted gel, resulting in a secondary battery separator with high mechanical strength and improved appearance defects.

[0074] The polymerization reaction may be carried out in a plurality of series-connected reactors, each having different concentrations of hydrogen and ethylene, to control the molecular weight distribution of the polyethylene resin.

[0075] This forms a slurry by mixing the polymerized ultra-high molecular weight polyethylene resin with the inert hydrocarbon solvent, and the formed slurry is transferred to a degassing step via a transfer pipe. Thereafter, in a separation step, the inert hydrocarbon solvent and the ultra-high molecular weight polyethylene are separated, and finally, through a drying step, the ultra-high molecular weight polyethylene resin is produced in powder form.

[0076] The produced polyethylene resin powder may be passed through a particle selector, for example, using a sieve to remove most of the large particles with a particle size of 500 μm or more and the fine particles with a particle size of 50 μm or less. That is, through this process, polyethylene resin powder with a controlled particle size and distribution is obtained. When most of the large particles and fine particles are removed, the fluidity of the polyethylene resin powder is improved, the formation of unmelted gel during extrusion processing is suppressed, and a final secondary battery separator without appearance defects is obtained.

[0077] According to yet another embodiment, there is provided a secondary battery separator including the polyethylene resin.

[0078] The secondary battery separator may be in the form of a porous film and may be prepared by a method known in the art.

[0079] For example, the polyethylene resin powder, primary antioxidant, secondary antioxidant, and neutralizer may be uniformly mixed and then processed using a twin-screw extruder. The polyethylene resin powder may be fed in a fixed amount from a hopper through a feeder, and oil may be injected in proportion to the amount fed. The oil may be injected at the front end of the extruder so that the ratio of oil to powder is 5.5:4.5 to 7:3.

[0080] A sheet-like porous film having a certain thickness may be produced by stretching the film while adjusting the extrusion speed in the extruder. During this process, phase separation occurs between the oil and polyethylene resin, forming pores in the film. The oil in the produced film may be passed through a water bath containing, for example, methyl chloride (MC), and then subjected to a drying process to produce the final porous film separator.

[0081] According to yet another embodiment, a secondary battery including the secondary battery separator is provided.

[0082] The secondary battery includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode and formed using the polyethylene resin.

[0083] The structure, materials and manufacturing method of the secondary battery are well known in the art, and therefore, the description thereof will be omitted.

[0084] Specific examples of the present invention will be presented below. However, the following examples are merely intended to specifically illustrate or explain the present invention, and are not intended to limit the present invention. Furthermore, details not described herein can be fully inferred by those skilled in the art, and therefore, a detailed description thereof will be omitted.

[0085] Example 1 (Production of polyethylene polymerization catalysts) First step: After replacing the nitrogen atmosphere in a 1 L reactor equipped with a mechanical stirrer, 25 g of magnesium dichloride (MgCl2), 300 ml of toluene, and 100 ml of normal butanol were added and stirred. The temperature was raised to 65°C for 1 hour, and then maintained at this temperature for 2 hours to obtain a uniform magnesium halide compound solution.

[0086] Second step: The temperature of the magnesium halide compound solution prepared above was cooled to 40°C, and 470 ml of TiCl was slowly injected for 1 hour. After the injection was completed, the temperature of the reactor was raised to 60°C for 1 hour while stirring at 350 rpm, and then the reactor was aged for another 1 hour. After the entire process was completed, the reactor was stopped, the solid components were completely submerged, and the supernatant was removed. The solid components in the reactor, i.e., the catalyst precursor, were then washed with 200 ml of hexane.

[0087] Third step: 200 ml of hexane, 460 ml of TiCl, and 8 ml of ethyl benzoate were added to the catalyst precursor, and the temperature of the reactor was raised to 70°C over 1 hour while stirring at 350 rpm, followed by aging for 2 hours. After the entire process was completed, the reactor was stopped, the solid components were allowed to settle, and the supernatant was removed.

[0088] Fourth step: The solid component thus prepared, ie, the polyethylene polymerization catalyst, was washed six times with 200 ml of hexane.

[0089] (Production of polyethylene resin powder) A 150-liter CSTR reactor equipped with an internal temperature controller, pressure regulator, and agitator was used. Ethylene (3 kg / hr), hexane (22.5 kg / hr), and the prepared polyethylene polymerization catalyst were continuously injected at 0.1–0.2 kg / hr depending on the activity, with stirring at 220 rpm. Hydrogen was injected to control molecular weight, with the injection amount adjusted based on the high load melt index (HLMI). Triethylaluminum dissolved in 11 wt% hexane was used as a cocatalyst. The discharge volume and liquid level in the reactor were controlled to maintain a volume of 70 liters. The continuously discharged hexane slurry was subjected to degassing and separation processes to produce a wet cake-like ultra-high molecular weight polyethylene resin. A continuous drying process then produced a granular ultra-high molecular weight polyethylene resin powder.

[0090] At this time, the polymerization reaction takes place at a temperature of 80°C and a pressure of 3 kgf / cm 2 The temperature and residence time were kept constant at 2.5 hours, and ethylene was injected 2 hours after the catalyst and cocatalyst were injected. The polymerization activity was 20 kg / g (20 kg polyethylene / g catalyst). The polymerization activity was calculated as the weight ratio of polyethylene produced per amount of catalyst used.

[0091] The obtained polyethylene resin powder was sieved using a sieve with 500 μm sieve openings (35 mesh) to remove powder particles with a particle size of 500 μm or more. Then, the residual content of flake particles with a particle size of 500 μm or more, i.e., large particles, and the residual content of fine particles with a particle size of 50 μm or less were measured.

[0092] The particle size distribution of the final polyethylene resin powder was measured using a laser particle analyzer (Mastersizer X, Malvern Instruments). The flowability of the polyethylene resin powder was evaluated by pouring 100 g of a sample into a hopper and measuring the time it took for the total amount to flow out. The intrinsic viscosity of the polyethylene resin powder was calculated by dissolving the polymer in decahydronaphthalene solvent according to ISO 1628 Part 3, measuring the relative viscosity, and then extrapolating the relative viscosity value to the value when the concentration was zero.

[0093] (Forming of porous films for secondary batteries) The ultra-high molecular weight polyethylene resin powder was mixed uniformly using a Henschel mixer with 500 ppm of primary antioxidant Iganox 1010, 500 ppm of secondary antioxidant Iganox 1680, and 2000 ppm of neutralizer Ca-stearate. The mixture was then processed using a twin-screw extruder. A constant amount of powder was added from a hopper via a feeder, and oil was injected into the front end of the extruder in proportion to the amount added, maintaining a 7:3 weight ratio of oil to powder. The extruder had a length / diameter (L / D) of 56 mm and a die width of 400 mm. The processing temperature was 210°C. The extrusion speed was 0.65 m / min, and the sheet thickness was adjusted to a constant value via a casting roll at the rear of the die. The sheet was then stretched six times by adjusting the roll speed via a machine direction orientation (MDO) unit. The film then passes through a TDO (transverse direction orientation) unit, where it is stretched four times in the transverse direction. The final stretched film has a thickness of 12 μm. During this process, the oil and resin undergo phase separation, forming pores in the film. The oil in the film is removed by passing it through a water bath containing methylene chloride (MC), and the film undergoes a drying process to complete the final porous separator film.

[0094] The number of defects per area was measured and quantified using an appearance analysis device (camera). The locations of the defects were confirmed, and a scanning electron microscope (SEM) was used to confirm that they were different from the surrounding normal pore formation areas.

[0095] Figure 1 is a scanning electron microscope (SEM) photograph of the polyethylene resin powder for a separator of a secondary battery according to Example 1. Referring to Figure 1, defects in the polyethylene resin powder prepared according to Example 1 are shown, and it can be seen that the defects are the fewest compared to the other Examples and Comparative Examples, as shown in Table 1 below.

[0096] Example 2 The process for producing the polyethylene polymerization catalyst is the same as in Example 1. In Example 1, the pressure of the polymerization reaction was 3 kgf / cm 2 to 4kgf / cm 2 A polyethylene resin powder was produced in the same manner as in Example 1, except that the mixture was changed to the above, polymerization was carried out, and flake particles having a particle size of 600 μm or more were removed from the obtained polyethylene resin powder using a sieve having 600 μm sieve openings (30 mesh).

[0097] Example 3 In the process of producing the polyethylene polymerization catalyst in Example 1, the polyethylene polymerization catalyst was washed five times with 200 ml of hexane, and the polymerization reaction pressure in Example 1 was 3 kgf / cm 2 from 4kgf / c 2 A polyethylene resin powder was produced in the same manner as in Example 1, except that the polymerization temperature was changed from 80°C to 75°C, and the obtained polyethylene resin powder was passed through a sieve with 600 μm sieve openings (30 mesh) to remove flake particles with a particle size of 600 μm or more.

[0098] Comparative Example 1 A polyethylene resin powder was produced in the same manner as in Example 1, except that flake particles having a particle size of 1000 μm or more were removed from the polyethylene resin powder obtained in Example 1 using a sieve with 1000 μm sieve openings (18 mesh).

[0099] Comparative Example 2 A polyethylene resin powder was produced in the same manner as in Example 1, except that in the process of producing the polyethylene polymerization catalyst in Example 1, the polyethylene polymerization catalyst was washed four times with 200 ml of hexane, and flake particles having a particle size of 1000 μm or more were removed from the polyethylene resin powder obtained in Example 1 using a sieve with 1000 μm sieve openings (18 mesh).

[0100] Comparative Example 3 A polyethylene resin powder was produced in the same manner as in Example 1, except that in the process of producing the polyethylene polymerization catalyst in Example 1, the polyethylene polymerization catalyst was washed four times with 200 ml of hexane, and flake particles having a particle size of 1400 μm or more were removed from the polyethylene resin powder obtained in Example 1 using a sieve with 1400 μm sieve openings (14 MESH).

[0101] Evaluation: Measurement of the physical properties of polyethylene resin powder The polyethylene resin powders produced in Examples 1 to 3 and Comparative Examples 1 to 3 were measured for the following physical properties, and the results are shown in Table 1 below.

[0102] High Load Melt Flow Index (HLMI) Measured at 190°C under a 21.6 kg load according to ASTM D1238.

[0103] Viscosity average molecular weight (Mv) The viscosity average molecular weight (Mv) was calculated from the intrinsic viscosity [η] according to ASTM D4020. In the case of polymers, viscosity can provide useful information in dilute solutions; the specific viscosity is the value obtained by dividing the viscosity of the polymer by the viscosity and concentration of the solution; when the polymer concentration becomes zero, the extrapolated value of the specific viscosity is defined as the intrinsic viscosity (IV). For linear polymers, the intrinsic viscosity value is primarily affected by the size of the polymer, so it has a high correlation with the molecular weight; in the case of ultra-high molecular weight polyethylene, the Margolies equation below is widely used.

[0104] Mv=5.37x10 4 x[η]1.49

[0105] The Mv is the viscosity average molecular weight (unit: g / mol), and the [η] is the intrinsic viscosity (unit: dL / g).

[0106] Intrinsic viscosity (IV) After dissolving in decalin solution at 135°C for 70 minutes, the measurement was carried out in accordance with ISO1628-1.

[0107] Average particle size and particle size distribution (SPAN) The average particle size of polyethylene resin powder was measured using a polymer particle analyzer (MALVERN MASTER SIZE X PARTICLE ANALYSER) in accordance with ISO 13320-2. The average particle size was expressed as D(v,0.5), and the particle size distribution (SPAN) was expressed as (D(v,0.9)-D(v,0.1)) / D(v,0.5). Here, D(v,0.5) is the average particle size of the bottom 50% of the sample, while D(v,0.9) and D(v,0.1) are the average particle sizes of the bottom 90% and 10% of the sample, respectively. A smaller particle size distribution (SPAN) number indicates a narrower distribution.

[0108] Bulk Density (BD) Measured according to ASTM D1895-96.

[0109] Powder fluidity According to ISO 6186:1998, a certain amount of sample was poured into a hopper, and the time it took for the total amount to flow out was measured. The time was then divided by the measured weight of the sample and expressed as time (s) per 100 g.

[0110] Unmelted gel The fabricated porous separator was measured using a film surface analyzer FSA-100 from OCS. Gel defects were counted from gels larger than 50 μm and the average value of the entire measured area was calculated.

[0111] [Table 1]

[0112] Referring to Table 1, the polyethylene resin powder of Examples 1 to 3 according to one embodiment contains large particles having a particle size of 500 μm or more and fine particles having a particle size of 50 μm or less all at 1.0 wt % or less, unlike Comparative Examples 1 to 3. In such cases, it can be seen that the number of defects in the formed film is significantly less in Examples 1 to 3 than in Comparative Examples 1 to 3.

[0113] Specifically, in Example 1, in which flake particles with a particle size of 500 μm or more were removed, the number of defects was smaller than in Examples 2 and 3, in which flake particles with a particle size of 600 μm or more were removed, i.e., the number was very low.

[0114] In addition, in Comparative Examples 1 to 3, increasing the size of the mesh used to remove flake particles significantly increases the content of flake particles with a particle size of 500 μm or more, resulting in a significant increase in the number of defects in the formed separator film. Furthermore, further reducing the number of washings during the catalyst production process increases the content of fine particles with a particle size of 50 μm or less, resulting in poor powder fluidity.

[0115] Although the preferred embodiment of the present invention has been described above, the present invention is not limited to this, and various modifications can be made within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and it is natural that these modifications fall within the scope of the present invention.

Claims

1. 2 kgf / cm in the presence of a polyethylene polymerization catalyst ２ ~5 kgf / cm ２ a pressure of 1000 kJ / cm, a temperature of 70°C to 80°C, and a residence time of 2 to 3 hours to prepare a polyethylene resin powder for a separator for a secondary battery; obtaining a polyethylene resin powder for a secondary battery separator having a controlled particle size and distribution from the polyethylene resin powder for a secondary battery separator using a sieve; The polyethylene resin powder for secondary battery separators, which has a controlled particle size and distribution obtained from the polyethylene resin powder for secondary battery separators using the sieve, contains 0.35 wt % or less of large particles having a particle size of 500 μm or more and 0.5 wt % or less of fine particles having a particle size of 50 μm or less, The polyethylene resin powder for secondary battery separators is The average particle size is 100 μm to 200 μm, The particle size distribution (SPAN) is 0.7 to 1.3; The fluidity is 12 sec / 100 g to 20 sec / 100 g, a high load melt flow index (190°C, 21.6 kg) of 0.1 g / 10 min to 5.0 g / 10 min; The melting temperature is 132°C to 135°C, The polyethylene polymerization catalyst is mixing a magnesium-containing compound, an alcohol, and a hydrocarbon solvent to prepare a magnesium-containing compound solution; reacting the magnesium-containing compound solution with a metal chloride to prepare a catalyst precursor; reacting the catalyst precursor with a metal chloride and a carbonyl compound to produce a catalyst; and washing the catalyst with a hydrocarbon solvent; The carbonyl compound is represented by the following formula 1 or 2. [Chemical formula 1] R １ (C)R ２ [Chemical 2] R ３ (CO)OR ４ (In the above-mentioned Chemical Formulas 1 and 2, the R １ ~R ４ are each independently a C2 to C10 linear alkyl group, a C6 to C14 cycloalkyl group, or a C6 to C14 aryl group.

2. 2. The method of claim 1, wherein the washing step is performed 5 to 8 times.

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