JP7746341B2 - Polyethylene resin for secondary battery separator, method for producing same, secondary battery separator and secondary battery including same - Google Patents

Description

This article relates to a polyethylene resin for secondary battery separators, a method for producing the same, and secondary battery separators and secondary batteries containing the same.

Ultra-high molecular weight polyethylene (UHMWPE) generally refers to polyethylene with a molecular weight of 10 ^g /mol or more, which is further classified as UHMWPE with a molecular weight of 3× ¹⁰ g/mol or more and VHMWPE (very high molecular weight polyethylene) with a molecular weight of less than 3×10 g/mol. Because UHMWPE resin has a higher molecular weight than general-purpose polyethylene, it is characterized by excellent rigidity, abrasion resistance, environmental stress resistance, uniformity, self-lubrication, chemical resistance, and electrical properties.

Among these, ultra-high molecular weight polyethylene is widely used as a battery separator for various batteries due to its excellent chemical resistance and electrical properties. Products with a molecular weight of 250,000 g/mol to 2.5 million g/mol are typically used for battery separators.

However, as described in US Patent Publication No. US4972035A, ultra-high molecular weight polyethylene resin is difficult to process because of its low flowability even when fully molten due to its high molecular weight characteristics. Therefore, unlike general-purpose polyethylene, it cannot be pelletized and is instead produced and sold in powder form after the polymerization process.

To be processed into battery separators, the produced powder is mixed with oils and uniformly melted and kneaded in a twin-screw extruder to form a single phase. It is then stretched in the machine direction (MD) and transverse direction (TD) to form a porous film, and the phase-separated oil is then removed to complete the battery separator. In this battery separator manufacturing process, stable operation of the melt extrusion process is a key factor in producing separators with uniform physical properties.

In addition, with recent increasing demands for improved stability in secondary battery separators, the importance of maintaining the separator's physical properties and appearance through stable operation during the extrusion stage is increasing. If there are problems with the physical properties, the separator will not function properly, and if there are problems with the appearance, the defective part is likely to be the main cause of a short circuit in the battery. For this reason, in addition to developing resin products with optimized processability and rigidity, many extruder conditions (uniform feeding speed, extruder temperature, extruder screw, oil injection temperature, extruder load, etc.) must be optimized.

One embodiment provides a polyethylene resin for secondary battery separators that improves processability in the extruder and maintains the rigidity of the final separator film by optimizing the content of low-molecular-weight polyethylene and high-molecular-weight polyethylene.

Another embodiment provides a method for manufacturing the polyethylene resin for a separator of a secondary battery.
In yet another embodiment, there is provided a separator for a secondary battery, comprising the polyethylene resin for a secondary battery separator.

Another embodiment provides a secondary battery including the separator for a secondary battery.

One embodiment provides a polyethylene resin for secondary battery separators, which has a weight-average molecular weight (Mw) value of 350,000 g/mol to 500,000 g/mol as measured by gel permeation chromatography. The integrated area of the low molecular weight region is 1.3% to 2.4% of the total integrated area of the gel permeation chromatography graph, and the integrated area of the high molecular weight region is 9.5% to 13.5%. The low molecular weight region is the portion of the gel permeation chromatography graph where the log M (where M corresponds to the molecular weight of the polymer passing through the column in the gel permeation chromatography) value is 4 or less, and the high molecular weight region is the portion of the graph where the log M value is 6 or more.

The polyethylene resin may be in powder form.
The polyethylene resin may be a particle formed of a low molecular weight polymer having a viscosity average molecular weight of 180,000 g/mol to 700,000 g/mol and a high molecular weight polymer having a viscosity average molecular weight of 1,500,000 g/mol to 2,500,000 g/mol.

The polyethylene resin may have a viscosity average molecular weight of 200,000 g/mol to 2,500,000 g/mol.
The polyethylene resin may have a melting temperature of 130°C to 138°C.

The polyethylene resin may have an average particle size of 110 μm to 140 μm.
The polyethylene resin may have a particle size distribution (SPAN) of 1.0 or less.
The polyethylene resin may have a molecular weight distribution (MWD) value of 4-6.

Another embodiment provides a method for producing a polyethylene resin for a separator for a secondary battery, including the steps of: injecting ethylene, a catalyst, and an organic solvent into a first reactor; injecting hydrogen into the first reactor; injecting a slurry produced by polymerization in the first reactor into a second reactor; injecting additional ethylene and an organic solvent into the second reactor; and injecting hydrogen into the second reactor so that the ratio of the injection rate of the hydrogen to the injection rate of the ethylene into the second reactor is 0.01 to 0.30.

The method for producing a polyethylene resin for a secondary battery separator may further include injecting hydrogen into the first reactor so that the ratio of the injection rate of hydrogen to the injection rate of ethylene into the first reactor is 0.02 to 0.35.

The organic solvent may be an alkane including pentane, hexane, heptane, n-octane, isooctane, or a combination thereof; a cycloalkane including cyclohexane, methylcyclohexane, or a combination thereof; an alkyl aromatic including toluene, xylene, ethylbenzene, isopropylbenzene, ethyltoluene, n-propylbenzene, diethylbenzene, or a combination thereof; a halogenated aromatic including chlorobenzene, chloronaphthalene, orthodichlorobenzene, or a combination thereof; or a combination thereof.

Yet another embodiment provides a separator for a secondary battery, comprising the polyethylene resin.
Yet another embodiment provides a secondary battery including: a positive electrode; a negative electrode; and the separator for a secondary battery located between the positive electrode and the negative electrode.

When using the polyethylene resin for secondary battery separators according to one embodiment, optimizing the content of low molecular weight polyethylene and high molecular weight polyethylene improves processability within the extruder, allowing for the production of separator products with a uniform surface, while also maintaining the rigidity of the final separator.

FIG. 1 is a gel permeation chromatography graph of a polyethylene resin for a secondary battery separator according to an embodiment, illustrating a low molecular weight region and a high molecular weight region of the polyethylene resin 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.

One embodiment provides a polyethylene resin for use in a separator for a secondary battery, which has a weight-average molecular weight (Mw) value of 350,000 g/mol to 500,000 g/mol as determined by gel permeation chromatography. The integrated area of the low molecular weight region is 1.3% to 2.4% of the total integrated area of the gel permeation chromatography graph, and the integrated area of the high molecular weight region is 9.5% to 13.5%. The low molecular weight region is the portion of the gel permeation chromatography graph where the log M value is 4 or less, and the high molecular weight region is the portion of the gel permeation chromatography graph where the log M value is 6 or more. Here, M corresponds to the molecular weight of the polymer passing through the column in the gel permeation chromatography.

Figure 1 is a gel permeation chromatography graph illustrating the low molecular weight region and high molecular weight region of a polyethylene resin for a secondary battery separator according to Example 1, which is an embodiment of the polyethylene resin for a secondary battery separator. Referring to Figure 1, the portion of the graph where the log M value on the horizontal axis is 4 or less corresponds to the low molecular weight region, and the portion of the graph where the log M value is 6 or more corresponds to the high molecular weight region.

In one embodiment, the polyethylene resin for secondary battery separators may have an integrated area of a low molecular weight region of 1.3% to 2.4%, for example, 1.5% to 2.3%, for example, 1.7% to 2.3%, of the total integrated area of a gel permeation chromatography graph. The low molecular weight region is the portion of the gel permeation chromatography graph where the log M value is 4 or less. If the integrated area of the low molecular weight region is within this range, the processing load in the extruder does not increase when producing a separator using the polyethylene resin for secondary battery separators, improving processability and allowing for the production of a separator product with a uniform surface, while maintaining the rigidity of the final separator product.

The polyethylene resin may have an integrated area of a high molecular weight region of 9.5% to 13.5%, for example, 9.8% to 13.2%, for example, 10.0% to 13.0%, of the total integrated area in a gel permeation chromatography graph. The high molecular weight region is the graph portion where the log M value is 6 or more. If the integrated area of the high molecular weight region is within this range, the polyethylene resin for secondary battery separators can be used to manufacture separators, improving processability in an extruder and producing separator products with a uniform surface. Furthermore, the rigidity of the final separator product is not reduced, resulting in a separator of excellent quality.

The polyethylene resin may be in powder form.
The polyethylene resin may be a mixture of a low molecular weight polymer having a viscosity average molecular weight of 180,000 g/mol to 700,000 g/mol and a high molecular weight polymer having a viscosity average molecular weight of 1,500,000 g/mol to 2,500,000 g/mol, which are formed as a single particle. Specifically, polymer particles polymerized in a first reactor are transferred to a second reactor and polymerized continuously so that the low molecular weight polymer formed in a first reactor and the high molecular weight polymer polymerized in a second reactor form a single particle. In this case, hydrogen may be injected as a molecular weight modifier into each reactor to achieve a target molecular weight.

The viscosity average molecular weight of the low molecular weight polymer may be 180,000 g/mol to 700,000 g/mol, for example, 200,000 g/mol to 680,000 g/mol. When the viscosity average molecular weight of the low molecular weight polymer is within this range, the processing load in the extruder does not increase when manufacturing a separator using the polyethylene resin for secondary battery separators, improving processability and allowing for the production of separator products with a uniform surface, and preventing a decrease in the rigidity of the final separator product.

The viscosity average molecular weight of the high molecular weight polymer may be 1,500,000 g/mol to 2,500,000 g/mol. When the viscosity average molecular weight of the high molecular weight polymer is within this range, the processability in the extruder when manufacturing a separator using the polyethylene resin for a secondary battery separator is improved, allowing for the production of a separator product with a uniform surface, and the rigidity of the final separator product is not reduced.

The viscosity average molecular weight of the polyethylene resin may be 200,000 g/mol to 2,500,000 g/mol, for example, 400,000 g/mol to 1,000,000 g/mol, for example, 400,000 g/mol to 800,000 g/mol. When the viscosity average molecular weight of the polyethylene resin is within this range, processability within the extruder is improved and gel formation within the final separation membrane product is reduced, ensuring a separation membrane product with a uniform surface and preventing a decrease in the mechanical strength of the separation membrane.

The polyethylene resin may have a melting temperature of 130°C to 138°C, for example, 133°C to 136°C. The melting temperature may be measured using a differential scanning calorimeter (DSC). When the melting temperature of the polyethylene resin is within this range, extrusion processability is improved and the mechanical strength of the final molded separator is not reduced.

The polyethylene resin may have an average particle size of 110 μm to 140 μm. Specifically, the polyethylene resin powder may have an average particle size of 110 μm to 140 μm, for example, 110 μm to 130 μm. When the average particle size is within this range, the flowability in the extruder hopper does not decrease, and reduced productivity and extrusion defects due to reduced bulk density do not occur.

The polyethylene resin may have a particle size distribution (SPAN) of 1.0 or less. Specifically, the polyethylene resin powder may have a particle size distribution of 1.0 or less, which results in uniform melting characteristics within the extruder, reducing the formation of unmelted gel and improving flowability in the hopper.

The molecular weight distribution (MWD) of the polyethylene resin may be 4 to 6. The molecular weight distribution (MWD) is defined as Mw/Mn (where Mw corresponds to the weight average molecular weight and Mn corresponds to the number average molecular weight).

Another embodiment provides a method for producing a polyethylene resin for a separator for a secondary battery, including the steps of: injecting ethylene, a catalyst, and an organic solvent into a first reactor; injecting hydrogen into the first reactor; injecting a slurry produced by polymerization in the first reactor into a second reactor; injecting additional ethylene and an organic solvent into the second reactor; and injecting hydrogen into the second reactor so that the ratio of the injection rate of the hydrogen to the injection rate of the ethylene into the second reactor is 0.01 to 0.30.

The method for producing a polyethylene resin for a secondary battery separator may include injecting ethylene, a catalyst, and an organic solvent into a first reactor and injecting a slurry produced by polymerization in the first reactor into a second reactor. Specifically, the first and second reactors are connected in series, and ethylene, a catalyst, and an organic solvent are injected into the first reactor. The slurry produced through the polymerization process in the first reactor is injected into the second reactor through a transfer pipe to ensure that the polymer polymerized in the first reactor and the polymer polymerized in the second reactor form a single particle. Then, in the second reactor, additional ethylene and an organic solvent are injected through another pipe, and continuous polymerization occurs, in which the polymerization process is repeated.

In the first reactor, a polyethylene polymerization reaction can be carried out using a polyethylene polymerization catalyst as a catalyst and an organometallic compound as a cocatalyst. The polyethylene polymerization catalyst can be a solid complex titanium catalyst, for example, a magnesium-supported titanium catalyst.

The organometallic compound cocatalyst may be represented by the general formula MRn. M may be, for example, a Group II metal in the periodic table, including magnesium and calcium, a Group IIIA metal in the periodic table, including boron, aluminum, and gallium, or zinc. R represents an alkyl group having 1 to 20 carbon atoms, and n represents the valence of the metal component.

The organometallic compound may be, for example, an organometallic compound having one or more alkyl groups having 1 to 6 carbon atoms, such as trialkylaluminums including triethylaluminum and triisobutylaluminum, which may be used alone or in combination.

The organometallic compound may also be an organoaluminum compound having one or more alkyl groups having 1 to 6 carbon atoms and further containing one or more halogen or hydride groups, such as ethylaluminum dichloride, diethylaluminum chloride, ethylaluminum sesquichloride, diisobutylaluminum hydride, or a combination thereof. For example, the organometallic compound may be triethylaluminum.

The method for producing the polyethylene resin for secondary battery separators may include injecting hydrogen as a molecular weight regulator into each of the first and second reactors to obtain a target molecular weight in each reactor.

When hydrogen is injected into the second reactor, the ratio of the hydrogen injection rate to the ethylene injection rate into the second reactor may be 0.01 to 0.30, for example, 0.01 to 0.25. When the ratio of the hydrogen injection rate to the ethylene injection rate into the second reactor is within this range, a high molecular weight polymer having a viscosity average molecular weight of 1,500,000 g/mol to 2,500,000 g/mol may be formed in the second reactor.

When hydrogen is injected into the first reactor, the ratio of the hydrogen injection rate to the ethylene injection rate into the first reactor may be 0.02 to 0.35, for example, 0.02 to 0.30. When the ratio of the hydrogen injection rate to the ethylene injection rate into the first reactor is within this range, a low molecular weight polymer having a viscosity average molecular weight of 180,000 g/mol to 700,000 g/mol may be formed in the first reactor.

The method for producing a polyethylene resin for a secondary battery separator further includes the steps of transferring the slurry produced by polymerization in the second reactor to a degassing process; and drying the slurry transferred to the degassing process to obtain a powdered polyethylene resin. That is, the slurry produced by polymerization in the second reactor is then transferred to a degassing process through a pipe to discharge the final polyethylene resin produced in the second reactor. After the degassing process, the organic solvent and polyethylene resin are separated in a separation process, and a final drying process is performed to produce an ultra-high molecular weight polyethylene resin, for example, a powdered ultra-high molecular weight polyethylene resin.

In the method for producing a polyethylene resin for a secondary battery separator, the polymerization reactions in the first and second reactors can be carried out using a liquid slurry polymerization method in the presence of an organic solvent. These polymerization methods can be carried out in the absence of oxygen, water, and other compounds that may act as catalyst poisons.

The organic solvent may be an alkane including pentane, hexane, heptane, n-octane, isooctane, or a combination thereof; a cycloalkane including cyclohexane, methylcyclohexane, or a combination thereof; an alkyl aromatic including toluene, xylene, ethylbenzene, isopropylbenzene, ethyltoluene, n-propylbenzene, diethylbenzene, or a combination thereof; a halogenated aromatic including chlorobenzene, chloronaphthalene, orthodichlorobenzene, or a combination thereof; or a combination thereof. For example, the organic solvent may be a hydrocarbon solvent having 4 to 6 carbon atoms, such as hexane.

Yet another embodiment provides a separator for a secondary battery, comprising the polyethylene resin.
In yet another embodiment, a secondary battery is provided, comprising: a positive electrode; a negative electrode; and a separator for the secondary battery disposed between the positive electrode and the negative electrode. The structure, materials, and manufacturing methods of the secondary battery are well known in the art, and therefore will not be described here.

The following provides specific examples of the present invention. However, the examples described below are merely intended to specifically illustrate or explain the present invention and should not be construed as limiting the scope of the present invention. Furthermore, details not described herein can be easily inferred by those skilled in the art, and therefore will not be described here.

(Production of polyethylene resin)
Example 1
Phase 1
A 1 L reactor equipped with a mechanical stirrer was purged with nitrogen, and then 25 g of magnesium chloride (MgCl ₂ ), 300 ml of toluene, and 100 ml of n-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.

In the second step, the temperature of the magnesium halide compound solution prepared above was cooled to 40°C, and 70 ml of titanium tetrachloride ( _TiCl4 ) was slowly added over a period of 1 hour. After the addition was completed, the temperature of the reactor was raised to 60°C for 1 hour while stirring at 350 rpm, and the mixture was aged for an additional 1 hour. When the entire process was completed, the reactor was stopped, the solid components were completely submerged, and the supernatant liquid was removed. The solid component, i.e., the catalyst precursor, in the reactor was then washed with 200 ml of hexane.

200 ml of hexane, 60 ml of titanium tetrachloride ( _TiCl4 ), and 8 ml of ethyl benzoate were added to the prepared catalyst precursor, and the reactor temperature was raised to 70°C over 1 hour while stirring at 350 rpm, followed by aging for 2 hours. Upon completion of the entire process, the reactor was stopped, the solid component was completely submerged, and the supernatant was removed. The prepared solid component, polyethylene polymerization catalyst (A), was washed six times with 200 ml of hexane.

Two 150-liter continuous stirred tank reactors (CSTRs) (the first and second reactors) equipped with third-stage internal temperature and pressure regulators and agitators were connected in series to conduct continuous polymerization. Ethylene (2.5 kg/hr), hexane (22.5 kg/hr), and the polyethylene polymerization catalyst (A) prepared above were continuously injected into the first reactor at rates ranging from 0.1 g/hr to 0.2 g/hr depending on activity, and the mixture was stirred at 180 rpm. The hexane injection rate was adjusted to ensure a reaction residence time of 2 hours. Hydrogen was injected to control molecular weight, and the injection rate was adjusted by measuring the high load melt index (HLMI). Triethylaluminum dissolved in 11 wt% hexane was used as a cocatalyst. The discharge volume and liquid level of the reactants in the first reactor were controlled to maintain a volume of 70 liters. The continuously discharged hexane slurry was transferred to the second reactor connected in series, where the liquid level was controlled to maintain the same reaction volume and residence time. Ethylene was injected at 1.5 kg/hr to continue polymerization. The reaction temperatures were maintained at 80°C for the first reactor and 78°C for the second reactor, and the reaction pressures were maintained at 3.8 kgf/ ^cm² and 3.0 kgf/ ^cm² , respectively. Hydrogen was injected into the first reactor so that the ratio of the hydrogen injection rate to the ethylene injection rate into the first reactor was 0.28, and hydrogen was injected into the second reactor so that the ratio of the hydrogen injection rate to the ethylene injection rate into the second reactor was 0.014. The mixing ratio of the slurries in each reactor was maintained at 53:1 for the first reactor and 47:1 for the second reactor. The slurry solution continuously discharged by controlling the liquid level was subjected to a degassing process and a separation process to produce a wet cake-type ultra-high molecular weight polyethylene powder, which was then subjected to a continuous drying process to produce a particulate ultra-high molecular weight polyethylene resin in the form of a powder.

Example 2
A polyethylene resin was produced in the same manner as in Example 1, except that in the third step of Example 1, the polymerization reaction was carried out with the hydrogen injection rate ratios in the first and second reactors set to 0.025 and 0.22, respectively, and the slurry mixing ratios in the first and second reactors set to 47 and 53, respectively.

Comparative Example 1
A polyethylene resin was produced in the same manner as in Example 1, except that in the third step of Example 1, a 150-liter CSTR reactor equipped with an internal temperature controller, a pressure controller, and an agitator was used, and the polymerization reaction was carried out at a polymerization temperature of 80°C, a polymerization pressure of 4.7 kgf/ ^cm2 , and a hydrogen injection rate ratio of 0.103.

Comparative Example 2
A polyethylene resin was prepared in the same manner as in Example 1, except that in the third step of Example 1, the polymerization reaction was carried out with the hydrogen injection rate ratios of the first reactor and the second reactor set to 0.36 and 0.006, respectively.

Comparative Example 3
A polyethylene resin was produced in the same manner as in Example 1, except that in the third step of Example 1, the polymerization reaction was carried out with the hydrogen injection rate ratios in the first and second reactors set to 0.22 and 0.004, respectively, and the slurry mixing ratios in the first and second reactors set to 59 and 41, respectively.

Evaluation: Measurement of Physical Properties of Polyethylene Resins The following physical properties were measured for the polyethylene resins prepared in Examples 1 and 2 and Comparative Examples 1 to 3, and the results are shown in Table 1 below.

Reaction pressure (kgf/cm ² )
The pressure was measured by a pressure gauge installed in the reactor.
High load melt index (HLMI) (g/10 min)
Measured according to ASTM D1238 at 190°C under a load of 21.6 kg.

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

Viscosity average molecular weight = 5.37 x 10 ⁴ x [η] ^1.49
[η] = intrinsic viscosity (dl/g)
Melting temperature (Tm) (°C)
The melting temperature (Tm) was measured by differential scanning calorimetry (DSC). After heating to 200°C and maintaining an isothermal state for 10 minutes, the temperature was lowered to 30°C at a rate of 10°C per minute to remove the thermal history, and then the temperature was raised to 200°C at a rate of 10°C per minute to measure the melting temperature (Tm).

Average Particle Size and Distribution (SPAN)
The average particle size of the polymer was measured using a polymer particle analyzer (MALVERN MASTER SIZE X PARTICLE ANALYSER) according to ISO 13320-2. The average particle size is D(v,0.5), and the particle size distribution (span) is expressed as (D(v,0.9)-D(v,0.1))/D(v,0.5). Here, D(v,0.5) represents the particle size of 50% of the sample, and D(v,0.9) and D(v,0.1) represent the particle sizes of 90% and 10% of the sample, respectively. A smaller distribution number indicates a narrower distribution.

Bulk density (g/cc)
Measurements were made in accordance with ASTM D1895-96.
Melting time (s)
3 g of powder, 7 g of oil, and a magnetic bar were placed in a 50 ml vial, and the melting state was monitored over time using a magnetic stirrer set at 180° C. and 1800 rpm to measure the time until complete melting.

Molecular weight distribution (MWD) and integrated area (%) of low/high molecular weight region
The molecular weight distribution (MWD) was defined as Mw/Mn (where Mw corresponds to weight average molecular weight and Mn corresponds to number average molecular weight) from the measurement data obtained through gel permeation chromatography (GPC) analysis. Based on the GPC analysis data, the ratio of the integrated area of the low molecular weight region to the integrated area of the high molecular weight region was calculated to calculate the low/high molecular weight region range (see Figure 1). The equipment used for GPC analysis was an Agilent PS-GPC220. The solvent used was 1,2,4-TCB (1,2,4-trichlorobenzene), and the measurement temperature was 160°C.

Extrusion temperature (°C) and pressure (bar)
An SM Platek TEK-30 twin screw extruder was used and temperature and pressure gauge readings attached to the extruder were recorded.

Sheet Appearance Grade The surface of a sheet extruded through a T-die using a TEK-30 twin screw extruder manufactured by SM Platek was visually observed and graded.

Grade: 0 to 10, 0: very good, 10: very bad Grades 3 and above are considered to be defective products and cannot be used.
Pin puncture strength (g)
Using a KES-G5 instrument manufactured by Kato Tech, Japan, the puncture strength was measured at a speed of 10 mm/sec using a tip with a terminal diameter of 1 mm according to ASTM D-4833.

Looking at Table 1, it can be seen that while Examples 1 and 2 show lower appearance ratings than Comparative Examples 1 to 3, the separation membrane sheets made with the polyethylene resins of Examples 1 and 2 show a much improved appearance than the separation membrane sheets made with the polyethylene resins of Comparative Examples 1 to 3. In other words, the separation membrane sheets made with the polyethylene resins of Comparative Examples 1 to 3 show a sheet appearance rating of 3 or higher, making them unusable due to product defects, while the separation membrane sheets made with the polyethylene resins of Examples 1 and 2 show a sheet appearance rating of less than 3, indicating that they can be used as separation membranes without any problems.

Referring to Figure 1, this is because the polyethylene resins of Examples 1 and 2 satisfy the range of physical properties of the polyethylene resin for secondary battery separators according to one embodiment, including the ratio of the integrated area of the low molecular weight region and the integrated area of the high molecular weight region to the total integrated area of the gel permeation chromatography graph, thereby increasing the melting rate of the polyethylene resin in the extruder. In other words, this is because the time it takes for the powder-form polyethylene resin to melt into oil during extrusion is reduced, resulting in rapid and uniform melting and reduced gel formation within the sheet.

Therefore, when using the polyethylene resin for secondary battery separators according to one embodiment, optimizing the content of low molecular weight polyethylene and high molecular weight polyethylene improves processability within the extruder, allowing for the production of separator products with a uniform surface.

Furthermore, with regard to the strength of the separation membrane, Examples 1 and 2 exhibited slightly higher puncture strength values than Comparative Examples 1 to 3. In other words, the separation membranes made with the polyethylene resins of Examples 1 and 2 exhibited slightly higher rigidity or mechanical strength than the separation membranes made with the polyethylene resins of Comparative Examples 1 to 3, without being reduced in rigidity or mechanical strength.

In the case of polyethylene resins, a broader molecular weight distribution generally results in excellent processability but poor rigidity. However, in the case of the polyethylene resins of Examples 1 and 2, the molecular weight distribution was broadened by having an optimal ratio of integrated areas of the low molecular weight region and the high molecular weight region, and by maintaining an optimal low/high molecular weight content, it was found that a decrease in the rigidity of the final product, the separator membrane, was prevented.

Therefore, when using the polyethylene resin for secondary battery separators according to one embodiment, optimizing the content of low molecular weight polyethylene and high molecular weight polyethylene improves processability within the extruder, allowing for the production of separator products with a uniform surface, while also maintaining the rigidity of the final separator.

While the above describes preferred embodiments of the present invention, the present invention is not limited to these and can be implemented in various modified forms within the scope of the claims, the detailed description of the invention, and the accompanying drawings, and it goes without saying that these also fall within the scope of the present invention.

Claims (7)

A method for producing a polyethylene resin for a secondary battery separator,
injecting ethylene, hexane, and a polyethylene polymerization catalyst into a first reactor;
injecting hydrogen and a cocatalyst into the first reactor;
injecting the slurry produced by polymerization in the first reactor into a second reactor;
additionally injecting ethylene, hexane, and a polyethylene polymerization catalyst into the second reactor;
maintaining the reaction temperature and reaction pressure in the first reactor and the second reactor at predetermined values;
injecting hydrogen into the first reactor so that the ratio of the injection rate (kg/hr) of hydrogen to the injection rate (kg/hr) of ethylene injected into the first reactor is 0.02 to 0.35;
injecting hydrogen into the second reactor so that the ratio of the injection rate (kg/hr) of hydrogen to the injection rate (kg/hr) of ethylene into the second reactor is 0.01 to 0.30;
The polyethylene resin for secondary battery separator has a weight average molecular weight (Mw) of 350,000 g/mol to 500,000 g/mol as determined by gel permeation chromatography;
the integrated area of the low molecular weight region is 1.3% to 2.4% and the integrated area of the high molecular weight region is 9.5% to 13.5% of the total integrated area of the gel permeation chromatography graph;
The low molecular weight region is a graph portion in which the log M (where M corresponds to the molecular weight of the polymer passing through the column in the gel permeation chromatography) value is 4 or less in the gel permeation chromatography graph, and the high molecular weight region is a graph portion in which the log M value is 6 or more.
A manufacturing method for polyethylene resin for secondary battery separators. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator is in a powder form. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has a viscosity average molecular weight of 200,000 g/mol to 2,500,000 g/mol. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has a melting temperature of 130 ° C to 138 ° C. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has an average particle diameter of 110 μm to 140 μm. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the polyethylene resin for a secondary battery separator has a particle size distribution (SPAN) of 1.0 or less. 2. The method for producing a polyethylene resin for a secondary battery separator according to claim 1, wherein the molecular weight distribution (MWD) value of the polyethylene resin for a secondary battery separator is 4 to 6.