Polyolefin resin blend, polyolefin microporous membrane prepared by using the same, method for preparing the polyolefin microporous membrane, and secondary battery comprising the same

Abstract

The present invention relates to a polyolefin resin blend comprising a polyolefin resin and an additive having an ethylene-vinyl acetate polar group, wherein the content of the additive is greater than 0.05 weight% and less than 0.2 weight% based on the total amount of the polyolefin resin blend, a polyolefin microporous membrane manufactured using the same, a method for manufacturing the same, and a secondary battery comprising the same.

Description

Technology Field

[0001] The present invention relates to a polyolefin resin blend, a polyolefin microporous membrane manufactured using the same, a method for manufacturing the same, and a secondary battery comprising the same. Background Technology

[0002] Ultra-high molecular weight polyethylene (UHMWPE) has a molecular weight of 10 6 It generally refers to polyethylene with a g / mol or higher, and this is further classified as 3 x 10⁻⁶ 6 UHMWPE is defined as having a molecular weight of g / mol or more, while VHMWPE is defined as having a molecular weight of less than that.

[0003] Ultra-high molecular weight polyethylene resin has superior characteristics such as stiffness, wear resistance, environmental stress resistance, uniformity, self-lubrication, chemical resistance, and electrical properties compared to general-purpose polyethylene due to its higher molecular weight.

[0004] Due to their excellent chemical resistance and battery properties, ultra-high molecular weight polyethylene resins are widely used for various battery separators. Ultra-high molecular weight polyethylene resins used for battery separators typically have a molecular weight of 250,000 to 2,500,000 g / mol.

[0005] However, as disclosed in U.S. Patent Publication No. 4972035, ultra-high molecular weight polyethylene resin is difficult to process due to its low flowability even in a completely molten state caused by its high molecular weight. Consequently, it cannot be pelletized like general-purpose polyethylene and is produced and sold in the form of powder generated after the polymerization process.

[0006] The produced powder is mixed with oils for processing into a battery separator and uniformly melt-kneaded in a twin-screw extruder to form a single-phase sheet. Subsequently, a porous film is formed through stretching processes in the MD and TD directions, and the phase-separated oil is removed to complete the battery separator.

[0007] Among the components of a secondary battery, the separator comprises a porous polymer membrane located between the positive and negative electrodes. The separator serves to isolate the positive and negative electrodes, prevent electrical short circuits between the two electrodes, and allow the electrolyte and ions to pass through. Although the separator itself does not participate in the electrochemical reactions of the battery, its physical properties—such as electrolyte impregnation, degree of porosity, and thermal shrinkage rate—significantly influence the battery's performance and safety.

[0008] Among these factors, the electrolyte impregnation rate is a critical element affecting battery lifespan and capacity; a higher impregnation rate is advantageous for manufacturing superior batteries. Furthermore, improving the electrolyte impregnation speed is one of the key factors that can accelerate the overall battery production process.

[0009] Therefore, there is a need for technology to provide a separator for secondary batteries with an improved impregnation rate into the electrolyte using polyethylene resin. Prior art literature

[0010] U.S. Patent Publication No. 4972035 The problem to be solved

[0011] The present invention, aimed at solving the problems described above, provides a polyolefin resin blend with improved affinity with an electrolyte by adding an additive having an ethylene-vinyl acetate polar group to the polyolefin resin.

[0012] In addition, the present invention aims to provide a polyolefin microporous membrane manufactured using the polyolefin resin blend.

[0013] In addition, the present invention aims to provide a method for manufacturing the above-mentioned polyolefin microporous membrane.

[0014] In addition, the present invention aims to provide a secondary battery comprising the polyolefin microporous membrane.

[0015] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by a person skilled in the art from the description below. means of solving the problem

[0016] One embodiment of the present invention for solving the above-mentioned problem provides a polyolefin resin blend comprising a polyolefin resin and an additive having an ethylene-vinyl acetate polar group, wherein the content of the additive is greater than 0.05 weight% and less than 0.2 weight% based on the total amount of the polyolefin resin blend.

[0017] The above polyolefin resin blend may be a dry mixture of the above polyolefin resin and the above additive having an ethylene-vinyl acetate polar group.

[0018] The above polyolefin resin comprises olefin-derived units, and the olefin may be one or more selected from the group consisting of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene, 1-undecene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-eicocene.

[0019] The above polyolefin resin may have a weight-average molecular weight (Mw) calculated by Formula 1 below of 500,000 g / mol to 800,000 g / mol.

[0020] [Equation 1]

[0021] Mw = 5.37 X 10 4 Х [η] 1.49

[0022] In the above Equation 1, η is the intrinsic viscosity of the polyethylene resin.

[0023] The average particle size (Dv50) of the above polyolefin resin may be 110 μm to 140 μm.

[0024] The additive having the ethylene-vinyl acetate polar group may be one or more selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, and polymer composites thereof.

[0025] In addition, one embodiment of the present invention for solving the above-described problem provides a polyolefin microporous membrane manufactured using the above-described polyolefin resin blend.

[0026] The above polyolefin microporous membrane may have a thickness of 5 μm to 20 μm.

[0027] The above polyolefin microporous membrane may have a pin puncture of 430 gf or more.

[0028] The above polyolefin microporous membrane may have a surface contact angle of 115° to 125°.

[0029] The above polyolefin microporous membrane may have a weight increase rate of 465% or more calculated by the following Equation 2.

[0030] [Equation 2]

[0031] (W after -W before ) / W before x 100

[0032] In Equation 2 above, W before is the weight of the polyolefin microporous membrane before electrolyte impregnation, and W after is the weight of the polyolefin microporous membrane after electrolyte impregnation.

[0033] The above polyolefin microporous membrane may have an ion conductivity of 1.290 mS / cm or higher.

[0034] In addition, one embodiment of the present invention for solving the above-described problem provides a method for manufacturing a polyolefin microporous membrane comprising the steps of: supplying the above-described polyolefin resin blend and oil to an extruder and extruding a mixture of the two to obtain a sheet-shaped extruder; and stretching the extruder.

[0035] The above oil may be a paraffinic oil.

[0036] The weight ratio of the polyolefin resin blend supplied during the above extrusion and the oil may be 1.5:8.5 to 4.5:5.5.

[0037] The temperature at which the above extrusion is performed may be 150 ℃ to 250 ℃, and the pressure may be 15 bar to 25 bar.

[0038] The step of stretching the extruded material may include a first stretching step of stretching the extruded material by 4 to 10 times in the machine direction (MD) at 105 ℃ to 115 ℃; and a second stretching step of stretching the extruded material stretched in the first stretching step by 4 to 10 times in the width direction (TD) at 116 ℃ to 125 ℃.

[0039] After the step of stretching the above discharge, a step of removing oil may be further included.

[0040] In addition, one embodiment of the present invention for solving the above-described problem provides a secondary battery comprising the above-described polyolefin microporous membrane.

[0041] The above polyolefin microporous membrane may be a separator of the above secondary battery. Effects of the invention

[0042] A secondary battery comprising a polyolefin microporous membrane manufactured using a polyolefin resin blend according to the present invention, wherein the polyolefin resin and an additive having ethylene-vinyl acetate polar groups are used as a separator, can have improved affinity with the electrolyte.

[0043] The effects of the present invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by a person skilled in the art from the description below. Specific details for implementing the invention

[0044] The advantages and features of the present invention and the methods for achieving them will become clear by referring to the embodiments described in detail below. However, the present invention is not limited to the embodiments disclosed below but can be implemented in various different forms. These embodiments are provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the present invention, and the present invention is defined only by the scope of the claims.

[0045] Prior to the explanation, the meanings of the terms used in this specification are briefly explained. However, since the explanation of terms is intended to aid in understanding this specification, it should be noted that they are not used to limit the technical scope of the invention unless explicitly stated to be a limiting factor.

[0046] The terms used herein are for describing the embodiments and are not intended to limit the invention. In this specification, the singular form includes the plural form unless specifically stated otherwise in the text. As used in this specification, "comprises" and / or "comprising" do not exclude the presence or addition of one or more other components in addition to the components mentioned. Throughout the specification, "and / or" includes each of the components mentioned and all combinations of one or more. Although terms such as "primary," "secondary," etc., are used to describe various components, these components are not limited by these terms. These terms are used merely to distinguish one component from another.

[0047] Unless otherwise defined, all terms used herein (including technical and scientific terms) may be used in a meaning commonly understood by those skilled in the art to which the present invention pertains. Additionally, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless explicitly and specifically defined otherwise.

[0048] Hereinafter, embodiments of the present invention will be described in detail.

[0050] Polyolefin resin blend

[0051] According to one embodiment, the present invention provides a polyolefin resin blend comprising a polyolefin resin and an additive having an ethylene-vinyl acetate polar group. By mixing the additive having an ethylene-vinyl acetate polar group with the polyolefin resin, polar groups can be imparted to the surface of a polyolefin microporous membrane manufactured using the same, thereby improving affinity with an electrolyte.

[0052] In one embodiment, the polyolefin resin blend may be in the form of a powder in which the polyolefin resin and the additive having an ethylene-vinyl acetate polar group are dry blended.

[0053] In one embodiment, the polyolefin resin may include olefin-derived units.

[0054] The above olefin may be one or more selected from the group consisting of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene, 1-undecene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-eicocene. As a specific example, the polyolefin resin may be a single polyolefin composed of one of the above olefins, or a copolymer of two or more different olefins.

[0055] The above polyolefin resin may be an ultra-high molecular weight polyolefin resin having a weight-average molecular weight (Mw) calculated by Formula 1 below of 500,000 g / mol to 800,000 g / mol, 550,000 g / mol to 750,000 g / mol, or 550,000 g / mol to 650,000 g / mol. A polyolefin microporous membrane manufactured using a polyolefin resin blend comprising the above polyolefin resin having a weight-average molecular weight within the aforementioned range has excellent mechanical strength and excellent extrusion processability, and can prevent the formation of a gel in the polyolefin microporous membrane.

[0056] [Equation 1]

[0057] Mw = 5.37 X 10 4 Х [η] 1.49

[0058] In Equation 1 above, η is the intrinsic viscosity measured according to ISO 1628-1.

[0059] The average particle size (Dv50) of the above polyolefin resin may be 110 μm to 140 μm, 115 μm to 130 μm, or 120 μm to 130 μm. When the average particle size of the above polyolefin resin is within the aforementioned range, flowability in the extruder hopper is improved, and the decrease in bulk density (BD) can prevent reduced productivity and extrusion defects.

[0060] The average particle size (Dv50) of the above polyolefin resin may be measured according to ISO13320-2 using a polymer particle analyzer (MALVERN MASTER SIZE X PARTICLE ANALYSER).

[0061] In one embodiment, the additive having the ethylene-vinyl acetate polar group may be a copolymer having the ethylene-vinyl acetate polar group. The copolymer having the ethylene-vinyl acetate polar group may be, for example, one or more selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, and polymer composites thereof.

[0062] The content of the above additive may be greater than 0.05 wt% and less than 0.2 wt%, 0.08 wt% to 0.18 wt%, or 0.1 wt% to 0.15 wt% based on the total amount of the polyolefin resin blend. A polyolefin resin blend containing the above additive within the aforementioned range can improve affinity with the electrolyte by imparting ethylene-vinyl acetate polar groups to the surface of the porous membrane, while preventing a decrease in ion conductivity due to excessive ethylene-vinyl acetate polar groups.

[0064] Polyolefin Microporous Membrane

[0065] According to one embodiment, the present invention provides a polyolefin microporous membrane manufactured using the aforementioned polyolefin resin blend.

[0066] In one embodiment, the polyolefin microporous membrane may have a thickness of 5 μm to 20 μm, 8 μm to 17 μm, or 10 μm to 15 μm. The polyolefin microporous membrane has a constant thin thickness within the aforementioned range and can improve the stability of a secondary battery including it as a separator.

[0067] In one embodiment, the polyolefin microporous membrane may have a pin puncture of 430 gf or more, 440 gf or more, or 440 gf to 500 gf. The polyolefin microporous membrane having a pin puncture within the aforementioned range can prevent the polyolefin microporous membrane from breaking due to external forces during the secondary battery assembly process or due to internal foreign matter after assembly.

[0068] In one embodiment, the polyolefin microporous membrane may have a surface contact angle of 115° to 125°, 115° to 123°, 115° to 120°, or 115° to 118°. By having a surface contact angle within the aforementioned range, the polyolefin microporous membrane has excellent affinity with the electrolyte, thereby improving the electrolyte impregnation rate and thus improving production efficiency and battery performance during the battery manufacturing process.

[0069] The above polyolefin microporous membrane may have a weight increase rate calculated by the following Equation 2 of 465% or more, 470% or more, 470% to 500%, or 470% to 480%. For example, the weight increase rate may be calculated by measuring the weight of the polyolefin microporous membrane before impregnating it with the electrolyte and measuring the weight after impregnating it with the electrolyte for 1 hour and 30 minutes, and then calculating the weight increase rate using the following Equation 2. By having a weight increase rate within the aforementioned range, the polyolefin microporous membrane has excellent affinity for the electrolyte, so the impregnation speed may be fast.

[0070] [Equation 2]

[0071] (W after -W before ) / W before x 100

[0072] In Equation 2 above, W before is the weight of the polyolefin microporous membrane before electrolyte impregnation, and W after is the weight of the polyolefin microporous membrane after electrolyte impregnation.

[0073] The above polyolefin microporous membrane may have an ionic conductivity of 1.290 mS / cm or higher, 1.300 mS / cm or higher, 1.300 mS / cm to 1.500 mS / cm, or 1.305 mS / cm to 1.400 mS / cm. The polyolefin microporous membrane according to the present invention may have excellent ionic conductivity by being manufactured using a polyolefin resin blend containing an appropriate amount of an additive having an ethylene-vinyl acetate polar group.

[0075] According to one embodiment, a method for manufacturing a polyolefin microporous membrane is provided, comprising the steps of: supplying the aforementioned polyolefin resin blend and oil to an extruder to knead a molten mixture and extruding it to obtain a sheet-shaped extruder; and stretching the extruder.

[0076] In one embodiment, the polyolefin resin blend is prepared by dry mixing a polyolefin resin and an additive having an ethylene-vinyl acetate polar group, and may have a powder form.

[0077] The above polyolefin resin may include olefin-derived units. The olefin may be one or more selected from the group consisting of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene, 1-undecene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-eicocene. As a specific example, the above polyolefin resin may be a single polyolefin composed of one of the above olefins, or a copolymer of two or more different olefins.

[0078] The above polyolefin resin may be an ultra-high molecular weight polyolefin resin having a weight-average molecular weight (Mw) calculated by Formula 1 below of 500,000 g / mol to 800,000 g / mol, 550,000 g / mol to 750,000 g / mol, or 550,000 g / mol to 650,000 g / mol. A polyolefin microporous membrane manufactured using a polyolefin resin blend comprising the above polyolefin resin having a weight-average molecular weight within the aforementioned range has excellent mechanical strength and excellent extrusion processability, and can prevent the formation of a gel in the polyolefin microporous membrane.

[0079] [Equation 1]

[0080] Mw = 5.37 X 10 4 Х [η] 1.49

[0081] In Equation 1 above, η is the intrinsic viscosity measured according to ISO 1628-1.

[0082] The average particle size (Dv50) of the above polyolefin resin may be 110 μm to 140 μm, 115 μm to 130 μm, or 120 μm to 130 μm. When the average particle size of the above polyolefin resin is within the aforementioned range, flowability in the extruder hopper is improved, and the decrease in bulk density (BD) can prevent reduced productivity and extrusion defects.

[0083] The average particle size (Dv50) of the above polyolefin resin may be measured according to ISO13320-2 using a polymer particle analyzer (MALVERN MASTER SIZE X PARTICLE ANALYSER).

[0084] The additive having the ethylene-vinyl acetate polar group may be a copolymer having the ethylene-vinyl acetate polar group. The copolymer having the ethylene-vinyl acetate polar group may be, for example, one or more selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, and polymer composites thereof.

[0085] The content of the above additive may be greater than 0.05 wt% and less than 0.2 wt%, 0.08 wt% to 0.18 wt%, or 0.1 wt% to 0.15 wt% based on the total amount of the polyolefin resin blend. A polyolefin resin blend containing the above additive within the aforementioned range can improve affinity with the electrolyte by imparting ethylene-vinyl acetate polar groups to the surface of the porous membrane, while preventing a decrease in ion conductivity due to excessive ethylene-vinyl acetate polar groups.

[0086] The oil supplied to the extruder when manufacturing the above melt may be a paraffinic oil. The paraffinic oil may be any paraffinic hydrocarbon oil commonly used in the industry without limitation.

[0087] In one embodiment, the weight ratio of the polyolefin resin blend and the oil supplied during the extrusion may be 1.5:8.5 to 4.5:5.5, 2:8 to 4.5:5.5, or 3:7 to 4.5:5.5. When the polyolefin resin blend and the oil are supplied to an extruder and mixed within the aforementioned ranges, the mixing properties and extrusion processability are improved, and pores can be appropriately formed in the polyolefin microporous membrane being manufactured.

[0088] The temperature and pressure conditions for performing the extrusion above can be adjusted to conditions favorable for extrusion processing, such that the polyolefin resin melts and has excellent mixability with oil. The temperature for performing the extrusion above may be, for example, 150°C to 250°C, 180°C to 250°C, or 190°C to 220°C, and the pressure may be 15 bar to 25 bar, 17 bar to 23 bar, or 19 bar to 20 bar.

[0089] In one embodiment, the step of stretching the extruded material can be performed through a biaxial stretching process. In the biaxial stretching process, a stable biaxial stretching process can be performed on the extruded material in the machine direction (MD) and width direction (TD) through a tenter frame process. Through this, the extruded material can be stretched with a high elongation rate to achieve sufficient crystal orientation and form a polyolefin microporous membrane of uniform thickness, thereby obtaining the desired effect of improving physical properties.

[0090] It may include a first stretching step of stretching the extruded material by 4 to 10 times in the machine direction (MD) at 105 ℃ to 115 ℃; and a second stretching step of stretching the extruded material stretched in the first stretching step by 4 to 10 times in the width direction (TD) at 116 ℃ to 125 ℃. For example, in the first stretching step, the extruded material may be stretched by 6 to 8 times in the machine direction (MD), and in the second stretching step, the extruded material stretched in the first stretching step may be stretched by 6 to 8 times in the width direction (TD).

[0091] During the above stretching step, the polyolefin resin and oil undergo phase separation, thereby forming pores in the finally manufactured polyolefin microporous membrane.

[0092] In one embodiment, the oil removal process is a process for removing oil from a polyolefin microporous membrane manufactured through the extrusion and stretching process, and may include the step of dissolving oil present in the polyolefin microporous membrane in an organic solvent; and the step of drying and removing the organic solvent in which the oil is dissolved.

[0093] The above organic solvent may be any conventional organic solvent used in the industry to dissolve oil without limitation, and for example, the organic solvent may be methylene chloride.

[0095] Secondary Battery

[0096] According to one embodiment, a secondary battery comprising the polyolefin microporous membrane described above is provided. More specifically, the secondary battery may be a lithium-ion secondary battery.

[0097] The above secondary battery may be of various forms, such as electrolytic, stacked, and wound types, and as a specific example, a separator comprising the above polyolefin microporous membrane may be used as a separator for a wound secondary battery.

[0098] The aforementioned secondary battery is a term commonly used in the industry and refers to a power storage system that provides excellent energy density capable of converting electrical energy into the form of chemical energy for storage. Unlike primary batteries, which are non-rechargeable, secondary batteries are rechargeable and are widely used in IT devices such as smartphones, cellular phones, laptops, and tablet PCs. Recently, interest in electric vehicles has increased due to the need to prevent environmental pollution, and consequently, high-capacity secondary batteries are being adopted in electric vehicles.

[0099] In the above secondary battery, the polyolefin microporous membrane according to the present invention may be included as a separator, and components other than the separator may include those commonly used in the art.

[0100] The four major materials that are key components of the above lithium-ion secondary battery are the positive electrode, the negative electrode, the separator, and the electrolyte. Among these, the separator contains micropores, which allows lithium ions to move smoothly between the positive and negative electrodes during the charging and discharging process of the lithium-ion secondary battery, while preventing physical contact (short circuit) between the positive and negative electrodes to prevent thermal runaway.

[0101] The separator in a lithium-ion secondary battery serves to isolate the positive and negative electrodes, prevent electrical short circuits between the two electrodes, and allow the electrolyte and ions to pass through. Although the separator itself does not participate in the electrochemical reactions of the battery, its physical properties—such as electrolyte impregnation, degree of porosity, and thermal shrinkage rate—significantly affect the battery's performance and safety.

[0102] Among these factors, the electrolyte impregnation rate is a critical element affecting battery lifespan and capacity; a higher impregnation rate is advantageous for manufacturing superior batteries. Furthermore, improving the electrolyte impregnation speed is one of the key factors that can accelerate the overall battery production process.

[0103] In this regard, the present invention can improve productivity and enhance the performance of the secondary battery by using a polyolefin microporous membrane with excellent properties manufactured using the polyolefin resin blend described above as a separator for the secondary battery.

[0105] Specific embodiments of the present invention are presented below. However, the embodiments described below are merely for the purpose of specifically illustrating or explaining the present invention and should not be limited thereby. Furthermore, details not described herein can be sufficiently technically inferred by a person skilled in the art, so such description is omitted.

[0107] [Examples and Comparative Examples]: Preparation of Polyolefin Microporous Membranes

[0108] Example 1

[0109] As a polyolefin resin, an ultra-high molecular weight polyethylene resin having the characteristics shown in Table 1 below was used, and an ethylene-vinyl acetate copolymer (using dried Chemipearl V200 product) was uniformly mixed with the ultra-high molecular weight polyethylene resin powder using a Henschel mixer as an additive having ethylene-vinyl acetate polar groups to prepare a polyolefin resin blend. At this time, the content of the additive was adjusted to 0.1 wt% based on the total amount of the polyolefin resin blend.

[0110] Subsequently, 2,000 ppm of Iganox 1010, a primary antioxidant, and 600 ppm of Ca-stearate, a neutralizing agent, were added to the above polyolefin resin blend and evenly mixed using the same Henschel mixer, after which the mixture was processed using a twin-screw extruder. A fixed amount of powder was fed from the hopper through a metering feeder, and oil was injected in proportion to the amount fed, with an oil-to-powder ratio of 7:3 at the front of the extruder. At this time, the oil used was the Kukdong Oil & Chemical LP-350F product. The extruder had an L / D (length / diameter) of 40 and a die length of 1,230 mm. The processing temperature was controlled at 200 ℃, the extrusion speed was 0.65 m / min, and the thickness of the sheet was uniformly controlled as it passed through the casting roll at the rear of the die.

[0111] Subsequently, the film was stretched 7 times in the MD (Machine direction) direction at 118 ℃ using a separate biaxial stretching machine, and then stretched 7 times in the TD (transverse direction) direction at 123 ℃. During this process, the oil and resin undergo phase separation and form pores in the film.

[0112] The oil in the manufactured 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 polyolefin microporous membrane.

[0114] weight-average molecular weight

[0115] The weight-average molecular weight was calculated from the intrinsic viscosity [η] according to ASTM D 4020. For polymers, viscosity in a dilute solution can provide useful information. The value obtained by dividing the viscosity of the polymer by the viscosity and concentration of the solution is called specific viscosity, and the extrapolated value of specific viscosity as the concentration of the polymer approaches zero is defined as intrinsic viscosity (IV). Since the intrinsic viscosity of linear polymers is mainly influenced by the size of the polymer, it has a high correlation with molecular weight. For the weight-average molecular weight of ultra-high molecular weight polyethylene, the measurement method using the Margoliesequation of Equation 1 below is widely used.

[0116] [Equation 1]

[0117] Mw = 5.37 X 10 4 Х [η] 1.49

[0118] In Equation 1 above, Mw represents the weight-average molecular weight (g / mol), and η represents the intrinsic viscosity (dl / g). Here, the intrinsic viscosity is the intrinsic viscosity measured after dissolving in a decalin solvent at 135°C for 70 minutes using a viscometer in accordance with ISO 1628-1.

[0120] Average particle size (Dv0.5)

[0121] The average particle size of the above polyolefin resin was measured according to ISO13320-2 using a polymer particle analyzer (MALVERN MASTER SIZE X PARTICLE ANALYSER). The above average particle size (Dv50) represents the particle size when the cumulative volume reaches 50% from a sample with small particle size.

[0123] Extrusion temperature (°C) and pressure (bar)

[0124] Using the SM Platek TEK-30 twin extruder, the temperature and pressure gauge readings attached to the extruder were recorded.

[0126] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Ultra-high molecular weight polyethylene properties Additive content (weight%) 0.1 0.15 0 0.05 0.2 Weight-average molecular weight (x 10 6 g / mol) 0.6 0.6 0.6 0.6 0.6 Average particle size (㎛) 128 124 125 124 123 Extrusion conditions Resin / Oil weight ratio 3:7 3:7 3:7 3:7 3:7 Extruder temperature (°C) 200 200 200 200 200 Extruder pressure (bar) 19.8 20 19.6 20.1 20.0 Continuous conditions MD stretching temperature (°C) 118 118 118 118 118 MD Stretch Ratio (Double) 7 7 7 7 7 TD stretching temperature (°C) 123 123 123 123 123 TD Stretch Ratio (Double) 7 7 7 7 7

[0128] Evaluation: Measurement of physical properties of polyolefin microporous membranes

[0129] The following properties were measured for the polyolefin microporous membranes prepared in Examples 1-2 and Comparative Examples 1-3 above, and the results are shown in Table 2 below.

[0131] thickness

[0132] For the polyolefin microporous membranes of each example and comparative example, a VL-50 instrument from Mitutoyo, Japan was used, and the thickness was measured according to ASTM D374.

[0134] Pin Puncture

[0135] Using the KES-G5 instrument from Kato Tech, Japan, the strength (gf) was measured at a speed of 10 mm / sec using a tip with a tip diameter of 1 mm in accordance with ASTM D-4833.

[0137] Contact angle

[0138] For the polyolefin microporous membranes of each example and comparative example, the contact angle (°) was measured according to ASTM D 5946 using an MSA-S instrument from Kruss, Germany.

[0140] Weight growth rate

[0141] For each example and comparative example, the weight of the polyolefin microporous membrane was measured before and after immersion in the electrolyte for 1 hour and 30 minutes, and the electrolyte affinity was evaluated by calculating the weight increase rate (%) using the following Equation 2.

[0142] [Equation 2]

[0143] (W after -W before ) / W before x 100

[0144] In Equation 2 above, W beforeis the weight of the polyolefin microporous membrane before electrolyte impregnation, and W after is the weight of the polyolefin microporous membrane after electrolyte impregnation.

[0146] Ion Conductivity (Electrochmical Impedance Spectroscopy)

[0147] An ion conductivity cell with SUS (D: 16 mm) electrodes on both sides of the polyolefin microporous membrane (D: 18 mm) of each example and comparative example was prepared, and AC Impedance measurements were performed, and the ion conductivity (mS / cm) was measured using Equation 3 below.

[0148] [Equation 3]

[0149]

[0151] Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Thickness (㎛) 12.4 12.1 12.1 12.5 12 Rotation strength (gf) 454 447 445 457 443 Weight before impregnation (g) 6.96 7.04 5.39 5.7 7.59 Weight after impregnation (g) 40.1 40.3 30.27 30.27 42.46 Weight increase rate (%) 476.1 472.4 461.6 431.1 459.4 Ionic conductivity (mS / cm) 1.355 1.307 1.111 1.030 1.284 Contact angle (°) 117.96 117.92 126.5 119.88 117.02

[0152] As confirmed from Table 2 above, the polyolefin microporous membrane of the example prepared using a polyolefin resin blend containing an appropriate amount of an additive having an ethylene-vinyl acetate polar group as in the present invention has polarity on its surface, and thus has excellent electrolyte affinity and excellent ion conductivity.

[0153] On the other hand, in the case of Comparative Example 1, which does not include the additive according to the present invention, polarity was not imparted to the surface of the polyolefin microporous membrane, and in the case of Comparative Examples 2 and 3, in which the content of the additive falls outside the range of the present invention, the affinity for the electrolyte was not significantly improved.

[0155] Although embodiments of the present invention have been described above, those skilled in the art will understand that the present invention may be implemented in other specific forms without altering its technical concept or essential features. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

Claim 1 A polyolefin resin blend in which a polyolefin resin and an additive having ethylene-vinyl acetate polar groups are dry-mixed, wherein the content of the additive is 0.1% to 0.15% by weight based on the total amount of the polyolefin resin blend, and the polyolefin resin has a weight-average molecular weight (Mw) calculated by the following Formula 1 of 550,000 g / mol to 750,000 g / mol. Polyolefin resin blend: [Formula 1] Mw = 5.37 × 10 4 Х[η] 1.49 In the above Equation 1, η is the intrinsic viscosity of the polyethylene resin. Claim 2 delete Claim 3 A polyolefin resin blend according to claim 1, wherein the polyolefin resin comprises olefin-derived units, and the olefin is one or more selected from the group consisting of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene, 1-undecene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-eicocene. Claim 4 delete Claim 5 A polyolefin resin blend according to claim 1, wherein the average particle size (Dv50) of the polyolefin resin is 110 μm to 140 μm. Claim 6 In claim 1, the additive having an ethylene-vinyl acetate polar group is a polyolefin resin blend comprising one or more selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, and polymer composites thereof. Claim 7 A polyolefin microporous membrane manufactured using a polyolefin resin blend according to any one of claims 1, 3, 5 and 6. Claim 8 In claim 7, the polyolefin microporous membrane is a polyolefin microporous membrane having a thickness of 5 μm to 20 μm. Claim 9 In claim 7, the polyolefin microporous membrane is a polyolefin microporous membrane having a pin puncture of 430 gf or more. Claim 10 In claim 7, the polyolefin microporous membrane is a polyolefin microporous membrane having a surface contact angle of 115° to 125°. Claim 11 In claim 7, the polyolefin microporous membrane is a polyolefin microporous membrane having a weight increase rate calculated by the following Equation 2 of 465% or more: [Equation 2](W after -W before ) / W before x 100 In the above Equation 2, W before is the weight of the polyolefin microporous membrane before electrolyte impregnation, and W after is the weight of the polyolefin microporous membrane after electrolyte impregnation. Claim 12 In claim 7, the polyolefin microporous membrane is a polyolefin microporous membrane having an ion conductivity of 1.290 mS / cm or higher. Claim 13 A method for manufacturing a polyolefin microporous membrane comprising: a step of extruding a molten mixture mixed by supplying a polyolefin resin blend and oil according to any one of claims 1, 3, 5 and 6 to an extruder to obtain a sheet-shaped extruder; and a step of stretching the extruder. Claim 14 In paragraph 13, the above oil is a paraffin-based oil, a method for manufacturing a polyolefin microporous membrane. Claim 15 A method for manufacturing a polyolefin microporous membrane according to claim 13, wherein the weight ratio of the polyolefin resin blend supplied during extrusion and the oil is 1.5:8.5 to 4.5:5.

5. Claim 16 A method for manufacturing a polyolefin microporous membrane according to claim 13, wherein the temperature at which the extrusion is performed is 150 ℃ to 250 ℃ and the pressure is 15 bar to 25 bar. Claim 17 A method for manufacturing a polyolefin microporous membrane according to claim 13, wherein the step of stretching the extruded material comprises: a first stretching step of stretching the extruded material by 4 to 10 times in the machine direction (MD) at 105 ℃ to 115 ℃; and a second stretching step of stretching the extruded material stretched in the first stretching step by 4 to 10 times in the width direction (TD) at 116 ℃ to 125 ℃. Claim 18 A method for manufacturing a polyolefin microporous membrane according to claim 13, further comprising a step of removing oil after the step of stretching the extruded material. Claim 19 A secondary battery comprising a polyolefin microporous membrane according to claim 7. Claim 20 In claim 19, the polyolefin microporous membrane is a secondary battery that is a separator of the secondary battery.

Images