JP7787142B2 - Polyethylene resin powder for secondary battery separator, its manufacturing method and secondary battery separator containing the same - Google Patents

Description

This article relates to polyethylene resin powder for secondary battery separators, its manufacturing method, and secondary battery separators containing the same.

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 its high rigidity, chemical resistance, and abrasion resistance due to its high molecular weight. Due to its excellent chemical resistance and battery properties, VHMWPE is widely used as a battery separator for various types of batteries.

As described in U.S. Patent Publication US4972035A, ultra-high molecular weight polyethylene, which has such excellent properties, is difficult to process due to its high molecular weight. 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 separation membrane products not only in the process of manufacturing ultra-high molecular weight polyethylene but also in the extrusion process. To process this ultra-high molecular weight polyethylene to produce separation membranes, it is injected into an extruder like oil and melt-processed while mixed with oil. However, due to its high molecular weight, it has low fluidity even when mixed and melted like oil, posing various processing challenges.

In recent years, the trend toward thinner battery separators to increase the capacity of secondary batteries has created an essential need to increase the separator's mechanical strength. Increasing mechanical strength generally requires the use of raw materials with higher molecular weights. However, the use of relatively high-molecular-weight polyethylene makes it more difficult to ensure a uniform melting state in the extruder, which can result in unmelted powder particles remaining in the final molded product, the separator. In this case, unmelted gel can remain and appear as a defect in the separator's appearance.

One embodiment provides a polyethylene resin powder for secondary battery separators that allows smooth feeding during extrusion, improves extrusion processability, and suppresses the formation of unmelted gel.

Another embodiment provides a method for producing the polyethylene resin powder for secondary battery separators.

In yet another embodiment, a secondary battery separator containing the polyethylene resin powder for secondary battery separators provides a separator with high mechanical strength and improved appearance defects.

One embodiment provides a polyethylene resin powder for secondary battery separators that contains 1.0 wt% or less of large particles with a particle size of 500 μm or more and 1.0 wt% or less of fine particles with a particle size of 50 μm or less.

The average particle size of the polyethylene resin powder may be 100 μm to 200 μm.

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

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

The high-load 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.

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

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

Another embodiment provides a method for producing a polyethylene resin powder for a separator for a secondary battery, the method comprising: polymerizing ethylene in the presence of a polyethylene polymerization catalyst under conditions of a pressure of 2 kgf/ ^cm2 to 5 kgf/cm2, ^a temperature of 70°C to 80°C, and a residence time of 2 to 3 hours to produce a polyethylene resin powder; and obtaining a polyethylene resin powder having a controlled particle size and distribution from the polyethylene resin powder using a sieve, wherein the polyethylene polymerization catalyst is obtained by 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 the metal chloride and a carbonyl compound to prepare a catalyst, and washing the catalyst with a hydrocarbon solvent, wherein the carbonyl compound is represented by the following Chemical Formula 1 or 2:

[Chemical formula 1]
_R1 (CO) _R2

[Case 2]
R ₃ (CO)OR ₄

(In the above formulas 1 and 2, R ₁ to 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.)

The washing step may be repeated 5 to 8 times.

Another embodiment provides a secondary battery separator containing the polyethylene resin powder for secondary battery separators.

When using polyethylene resin powder for secondary battery separators according to one embodiment, particle fluidity is improved, allowing for smooth feeding during extrusion, improving extrusion processability, and suppressing the formation of unmelted gel. This ensures high mechanical strength during the formation of secondary battery separators while improving appearance defects, making it useful for secondary battery separators.

FIG. 1 is a scanning electron microscope (SEM) photograph of the polyethylene resin powder for a secondary battery separator according to Example 1.

The following detailed description of the embodiments will be provided so that those skilled in the art can easily implement them. However, the embodiments may be embodied in a variety of different forms and are not limited to the embodiments described herein.

The polyethylene resin powder for secondary battery separators according to one embodiment has an average particle size within a predetermined range, and contains 1.0 wt% or less of large particles with a particle size of 500 μm or more and fine particles with 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 with a particle size of 500 μm or more and fine particles with a particle size of 50 μm or less.

When using polyethylene resin powder with controlled particle size and distribution, the fluidity of the particles is improved and injection from the extruder hopper to the extruder is stable, resulting in smooth feeding during extrusion, improved extrusion processability, and suppressed formation of unmelted gel. This allows for high mechanical strength during the formation of a secondary battery separator while also providing improved appearance defects.

Specifically, the particle size of the macroparticles is 500 μm or more, and may be, 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. Furthermore, the particle size of the microparticles is 50 μm or less, and may be, 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, and may be, 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. Additionally, the polyethylene resin powder may contain 1.0 wt % or less of fine particles having the above particle size ranges, 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 polyethylene resin powder with particle size and distribution controlled within the above ranges is used, it is possible to smoothly supply the particles during extrusion, improve extrusion processability, and suppress the formation of unmelted gels, thereby obtaining a secondary battery separator with high mechanical strength and improved appearance defects.

The average particle size of the polyethylene resin powder may be 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 this range, the flowability in the extruder hopper is improved, the bulk density is increased, and extrusion defects are prevented, ensuring excellent productivity.

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 this range, it has uniform melting properties within the extruder, preventing the formation of unmelted gels and improving fluidity in the hopper.

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

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

(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 bottom 10%, respectively.)

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

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 unmelted gel can be prevented.

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 this range, the final molded porous film, i.e., the separator, has excellent mechanical strength and improved extrusion processability.

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, improves extrusion processability, and can prevent the formation of unmelted gels.

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, improves extrusion processability, and prevents the formation of an unmelted gel.

Hereinafter, a method for manufacturing the polyethylene resin powder for secondary battery separators will be described in accordance with another embodiment.

According to one embodiment, polyethylene resin powder for secondary battery separators can be obtained by polymerizing polyethylene resin using a polyethylene polymerization catalyst to obtain a powder, and then subjecting it to a particle size control process.

Method for Producing a Catalyst for Polyethylene Polymerization The catalyst for polyethylene polymerization may be produced in the following steps.

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

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 be, for example, a magnesium chloride such as magnesium dichloride (MgCl ₂ ), and the alkoxy magnesium compound may be, for example, diethoxy magnesium.

The type of alcohol is not particularly limited, but may be, for example, a C1 to C20 alcohol, such as a C4 to C20 alcohol, such as normal butanol.

The hydrocarbon 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 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.

The alcohol may be mixed in an amount of 3 to 7 parts by weight per 1 part by weight of the magnesium-containing compound, for example, 3 to 5 parts by weight per 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 per 1 part by weight of the magnesium-containing compound, for example, 10 to 15 parts by weight per 1 part by weight of the magnesium-containing compound.

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 during catalyst particle formation.

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

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.

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.

The reaction in the second stage may involve cooling 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.

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 can play a role in determining the particle shape, size, and pore characteristics within the catalyst during catalyst particle formation. In other words, when the stirring speed is within this range, excellent particle characteristics can be achieved during particle formation of the polyethylene polymerization catalyst.

Then, the mixture is centrifuged and the supernatant liquid is removed to obtain the solid catalyst precursor.

The third step involves reacting the catalyst precursor with a metal chloride and a carbonyl compound to produce a polyethylene polymerization catalyst. The reaction can also be carried out in the presence of a hydrocarbon solvent.

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

The carbonyl compound is represented by the following formula 1 or 2:

[Chemical formula 1]
_R1 (CO) _R2

[Case 2]
R ₃ (CO)OR ₄

(In the above Chemical Formulas 1 and 2,
The R ₁ to 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.

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

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

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.

The reaction in the third stage may involve mixing the catalyst precursor, the hydrocarbon solvent, the metal chloride, and the carbonyl compound, then heating the mixture 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.

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

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 polyethylene resin powder with particle size and distribution adjusted within a predetermined range.

The catalyst washing process not only removes unreacted materials and by-reactants, but also removes fine catalyst particles that did not properly form catalyst particles, thereby pre-cleaning fine particles generated during the polymerization process. However, since this process involves sinking the solid catalyst components 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.

Production of Polyethylene Resin Powder 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.

The organometallic compound may be represented by the general formula MRn, where M is a metal from 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.

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

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.

Of these, for example, triethylaluminum may be used as the organometallic compound.

The polymerization reaction can be carried out by gas-phase or bulk polymerization in the absence of an organic solvent, or by liquid slurry polymerization in the presence of an organic solvent. These polymerization methods are carried out in the absence of oxygen, water, and other compounds that can act as catalyst poisons.

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.

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, a polyethylene resin can be produced in the reactor by reacting a polyethylene polymerization catalyst, the organometallic compound cocatalyst, the organic solvent (i.e., an inert hydrocarbon solvent), and ethylene.

The polymerization reaction may be carried out under conditions of ^a pressure of ² kgf/cm to 5 kgf/cm, for example, ³ kgf/cm to ⁴ kgf/cm, a temperature of 70°C to 80°C, for example, 76°C to 78°C, and a residence time of 2 to 3 hours. When the polymerization reaction is carried out under conditions within 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, the powder particles are uniform, the bulk density is increased, and productivity is improved. 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.

The polymerization reaction may be carried out in multiple reactors connected in series, with different concentrations of hydrogen and ethylene adjusted to control the molecular weight distribution of the polyethylene resin.

This forms a slurry by mixing the polymerized ultra-high molecular weight polyethylene resin with the inert hydrocarbon solvent. The resulting slurry is then transferred to a degassing process via a transfer pipe. The inert hydrocarbon solvent and ultra-high molecular weight polyethylene are then separated in a separation process, and finally, a drying process is performed to produce the ultra-high molecular weight polyethylene resin in powder form.

The produced polyethylene resin powder may be passed through a particle sorter, for example, using a sieve to remove most of the large particles with a particle size of 500 μm or more and most of the fine particles with a particle size of 50 μm or less. In other words, this process results in polyethylene resin powder with a controlled particle size and distribution. 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 is suppressed, and a final secondary battery separator without appearance defects is obtained.

According to yet another embodiment, a secondary battery separator containing the aforementioned polyethylene resin is provided.

The secondary battery separator may be in the form of a porous film and may be manufactured by methods known in the art.

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 oil to powder ratio is 5.5:4.5 to 7:3.

A sheet-like porous film with a certain thickness can 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 from the produced film can be passed through a water bath containing, for example, methyl chloride (MC) and then dried to produce the final porous film separator.

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

The secondary battery includes a positive electrode, a negative electrode, and a separator membrane positioned between the positive electrode and the negative electrode and formed using the aforementioned polyethylene resin.

The structure, materials, and manufacturing methods of the secondary battery are widely known in the field, so a detailed description will be omitted.

Specific examples of the present invention are 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 here can be fully inferred by those skilled in the art, and therefore will not be explained further.

Example 1
(Production of polyethylene polymerization catalyst)
First step: After replacing the atmosphere in a 1 L reactor equipped with a mechanical stirrer with nitrogen, 25 g of magnesium dichloride (MgCl ₂ ), 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.

Second step: The temperature of the prepared magnesium halide compound solution was cooled to 40°C, and 70 ml _{of TiCl4} was slowly injected for 1 hour. After the injection was completed, the temperature of the reactor was raised to 60°C for 1 hour with stirring at 350 rpm, and then aged for another 1 hour. After the entire process was completed, the reactor was stopped, the solid components were completely submerged, the supernatant was removed, and the solid components in the reactor, i.e., the catalyst precursor, were washed with 200 ml of hexane.

Third step: 200 ml of hexane, 60 ml _{of TiCl4} , and 8 ml of ethyl benzoate were added to the prepared 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 completing all steps, the reactor was stopped, the solid components were completely submerged, and the supernatant was removed.

Fourth step: The solid component, i.e., the polyethylene polymerization catalyst, was washed six times with 200 ml of hexane.

(Production of polyethylene resin powder)
A 150-liter CSTR reactor equipped with an internal temperature controller, pressure regulator, and agitator was used. 3 kg/hr of ethylene, 22.5 kg/hr of hexane, and the prepared polyethylene polymerization catalyst were continuously injected at a rate of 0.1 kg/hr to 0.2 kg/hr depending on the activity, and the reactor was stirred at 220 rpm. Hydrogen for molecular weight control was injected at an injection rate adjusted by measuring the high load melt index (HLMI). 11 wt% triethylaluminum dissolved in hexane was used as a cocatalyst. The discharge volume and liquid level of the reactants 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 granular ultra-high molecular weight polyethylene resin powder was then produced through a continuous drying process.

The polymerization reaction was carried out at a constant temperature of 80°C, a pressure of 3 kgf/ ^cm2 , and a residence time of 2.5 hours. 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.

The resulting polyethylene resin powder was sieved using a sieve with 500 μm mesh size (35 MESH) to remove powder particles with a particle size of 500 μm or more. The remaining content of flake particles with a particle size of 500 μm or more (i.e., large particles) and the remaining content of fine particles with a particle size of 50 μm or less were then measured.

The particle size distribution of the final polyethylene resin powder was measured using a laser particle analyzer (Mastersizer X, Malvern Instruments). The fluidity 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 and measuring the relative viscosity according to ISO 1628 Part 3, and then extrapolating this relative viscosity value to the value when the concentration was zero.

(Forming of porous film for secondary battery)
The ultra-high molecular weight polyethylene resin powder was mixed uniformly using a Henschel mixer with 500 ppm of a primary antioxidant, Iganox 1010, 500 ppm of a secondary antioxidant, Iganox 1680, and 2000 ppm of a neutralizer, Ca-stearate, and then processed using a twin-screw extruder. A fixed amount of powder was fed from a hopper via a feeder, and oil was injected into the front end of the extruder in proportion to the powder feed amount, so that the oil to powder weight ratio was 7:3. The extruder had a length/diameter (L/D) of 56 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 through a casting roll at the rear of the die. The sheet then passed through a machine direction orientation (MDO) unit, where the roll speed was adjusted to stretch the sheet six times. The sheet then passed through a transverse direction orientation (TDO) unit, where it was stretched four times in the transverse direction. The final stretched film thickness was 12 μm. During this process, the oil and resin phase-separated, forming pores in the film. The oil in the produced film was removed by passing it through a water bath containing methylene chloride (MC), and the film underwent a drying process to complete the final porous separator film.

The film was analyzed using an appearance analysis device (camera) to measure and quantify the number of defects per area. The locations of the defects were confirmed, and a scanning electron microscope (SEM) was used to confirm that they differed from the surrounding normal pore formation areas.

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

Example 2
The process for producing a polyethylene polymerization catalyst was the same as in Example 1. A polyethylene resin powder was produced in the same manner as in Example 1, except that the polymerization pressure in Example ¹ was changed from 3 kgf/ ^cm² to 4 kgf/cm², and flake particles having a particle size of 600 µm or more were removed from the obtained polyethylene resin powder using a sieve with 600 µm mesh (30 MESH).

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 five times with 200 ml of hexane, the polymerization reaction pressure in Example 1 was changed from 3 kgf/ ^cm² to 4 kgf/ ^cm² , and the temperature was changed from 80°C to 75°C, and flake particles having a particle size of 600 µm or more were removed from the obtained polyethylene resin powder using a sieve with 600 µm sieve openings (30 MESH).

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 having 1000 μm sieve openings (18 MESH).

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 having 1000 μm mesh (18 MESH).

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 1,400 μm or more were removed from the polyethylene resin powder obtained in Example 1 using a sieve having 1,400 μm mesh (14 MESH).

Evaluation: Measurement of Physical Properties of Polyethylene Resin Powder The following physical properties were measured for the polyethylene resin powders produced in Examples 1 to 3 and Comparative Examples 1 to 3, and the results are shown in Table 1 below.

High Load Melt Flow Index (HLMI)
Measured at 190°C under a load of 21.6 kg in accordance with ASTM D1238.

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, and the value obtained by dividing the viscosity of a polymer by the viscosity and concentration of the solution is called the specific viscosity. 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 mainly affected by the size of the polymer, and therefore has a high correlation with the molecular weight. In the case of ultra-high molecular weight polyethylene, the following Margolis equation is widely used:

Mv= ^5.37x104x [η]1.49

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

Intrinsic viscosity (IV)
After dissolving in a decalin solution at 135°C for 70 minutes, the measurement was carried out in accordance with ISO 1628-1.

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

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

Powder flowability: In accordance with ISO 6186:1998, a certain amount of sample was poured into a hopper, and the time required 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.

Unmelted gel
The prepared porous separator was measured using a film surface analyzer FSA-100 manufactured by OCS Co. The number of gel defects was counted from gels of 50 μm or more, and the average value of the entire measured area was calculated.

Referring to Table 1 above, it can be seen that the polyethylene resin powder of Examples 1 to 3 according to one embodiment contains large particles with a particle size of 500 μm or more and fine particles with a particle size of 50 μm or less, all at 1.0 wt% or less, unlike Comparative Examples 1 to 3. In this way, Examples 1 to 3 have significantly fewer defects in the molded film than Comparative Examples 1 to 3.

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 significantly lower.

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 manufacturing process increases the content of fine particles with a particle size of 50 μm or less, resulting in poor powder fluidity.

While the above describes preferred embodiments of the present invention, the present invention is not limited to these. Various modifications and variations are possible within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and naturally fall within the scope of the present invention.

Claims (2)

The method includes the steps of: polymerizing ethylene in the presence of a polyethylene polymerization catalyst under conditions of a pressure of 2 kgf/cm ² to 5 kgf/cm ² , 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 secondary battery separator; and 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;
The catalyst is obtained by reacting the catalyst precursor with a metal chloride and a carbonyl compound to prepare 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]
_R1 (CO) _R2
[Case 2]
R ₃ (CO)OR ₄
(In the above formulas 1 and 2, R ₁ to 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. The method of claim 1 , wherein the washing step is performed 5 to 8 times.