Manufacturing method for multilayer separators

Abstract

To provide a method of manufacturing a multi-layer separator which, in manufacturing a thin film-type multi-layer separator, can stably ensure mechanical properties required for the separator and improve productivity and economic feasibility at the same time by uniformizing structural properties of all layers constituting the multi-layer separator.SOLUTION: A method of manufacturing a multi-layer separator includes (a) extruding a first composition including a first polyolefin and a first pore forming agent to produce a first sheet; (b) extruding a second composition including a second polyolefin and a second pore forming agent to produce a second sheet; (c) stretching each of the first sheet and the second sheet in the machine direction (MD) to produce a first precursor film and a second precursor film; (d) laminating the first precursor film and the second precursor film to obtain a stacked structure; and (e) stretching the stacked structure in the transverse direction (TD), and then removing the first pore forming agent and the second pore forming agent from the stacked structure.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a multilayer separator, and more particularly to a method for manufacturing a multilayer separator that can homogenize the structural properties of each layer constituting the multilayer separator and improve the mechanical properties required of the separator. [Background technology]

[0002] Lithium-ion batteries are widely used as power sources for various electrical products that require miniaturization and weight reduction, such as smartphones, laptops, and tablet PCs. As their application fields expand to include smart grids and medium- and large-sized batteries for electric vehicles, there is a growing demand for lithium-ion batteries with high capacity, long lifespan, and high stability.

[0003] Recently, with the increasing energy density of lithium-ion batteries, the load on the batteries has increased, and a high level of safety is required for the separators. This has highlighted the importance of not only the mechanical properties of the separators, but also their heat resistance, which is essential for ensuring safety.

[0004] Traditionally, polyolefin-based microporous membranes have been used as separators in lithium-ion batteries. Among polyolefin-based microporous membranes, those made of polyethylene resin are particularly known to have excellent shutdown functions, where the micropores of the membrane close and interrupt the flow of current when the battery temperature rises. However, even after the shutdown function is activated, the battery temperature may rise further. In this case, the separator may melt down, causing a short circuit inside the battery. This generates a large amount of heat, leading to dangers such as smoke, fire, and explosion. Therefore, it is necessary to suppress the risk of short circuits even at temperatures higher than the shutdown temperature.

[0005] To achieve both the shutdown and meltdown characteristics of the separator, methods have been proposed such as blending polyethylene and polypropylene, or laminating a microporous membrane made of polyethylene resin with a microporous membrane made of polypropylene resin.

[0006] For example, Patent Documents 1 and 2 disclose a method for manufacturing a multilayer separator by co-extruding two or more resin compositions, and disclose that the mechanical strength and heat resistance of the multilayer separator manufactured through this method can be improved in a balanced manner. However, since interlayer lamination is performed by co-extrusion, i.e., stacking is performed, in a step before the formation of a porous structure by stretching, there is a problem that the structural properties of each layer become non-uniform in subsequent film-forming steps such as stretching and extraction. Specifically, as the thickness deviation in different regions of the outermost layer of the multilayer separator increases, the mechanical properties of the separator itself may become non-uniform, and in particular, when assembling a battery using the separator, there is a problem that areas that are significantly thinner than the surrounding areas are easily damaged or fractured. Such problems have become even more noticeable recently in thin-film separators (thickness of about 15 μm or less, preferably about 10 μm or less) used to cope with the trend of battery integration and high capacity.

[0007] Furthermore, Patent Document 3 discloses a method for manufacturing a multilayer separator by laminating and stacking two or more separators that have been completely formed through extrusion, stretching, extraction, and heat fixing, as shown in Figure 1. However, in this case, sufficient interlayer bonding strength cannot be provided, resulting in the problem that each layer easily peels off. To improve this, a step of re-stretching and heat fixing can be added after laminating and stacking two or more separators, but this has the problem of reduced productivity and economic efficiency due to the added equipment and process. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Korean Patent Publication No. 10-2005-0120689 [Patent Document 2] Korean Patent Publication No. 10-2016-0094448 [Patent Document 3] Korean Patent Publication No. 10-2008-0028444 [Overview of the project] [Problems that the invention aims to solve]

[0009] The present invention aims to solve the problems of the prior art described above, and the object of the present invention is to provide a method for manufacturing a thin-film type multilayer separator with a thickness of about 15 μm or less, preferably about 10 μm or less, which can uniformize the structural properties of each layer constituting the multilayer separator, stably ensure the mechanical properties required for the separator, and improve productivity and economic efficiency. [Means for solving the problem]

[0010] One aspect of the present invention provides a method for manufacturing a multilayer separator, comprising the steps of: (a) extruding a first composition comprising a first polyolefin and a first pore-forming agent to produce a first sheet; (b) extruding a second composition comprising a second polyolefin and a second pore-forming agent to produce a second sheet; (c) stretching the first and second sheets in the longitudinal direction (MD) to produce first and second precursor films; (d) laminating the first and second precursor films to obtain a laminate; and (e) stretching the laminate in the transverse direction (TD) and then removing the first and second pore-forming agents from the laminate.

[0011] In one embodiment, the first and second polyolefins may each include one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and two or more combinations or copolymers thereof.

[0012] In one embodiment, the weight-average molecular weights of the first and second polyolefins may be 300,000 to 2,000,000, respectively. In one embodiment, the first and second pore-forming agents may each be paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C.

[0013] In one embodiment, the first or second composition may further contain a hydrophilic polymer. In one embodiment, the content of the hydrophilic polymer in the first or second composition may be 0.1 to 5% by weight.

[0014] In one embodiment, the hydrophilic polymer may be selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivatives, glycerol, and combinations of two or more thereof.

[0015] In one embodiment, during the lamination, the first and second precursor films are pressed so as to be in surface contact with each other, and at least a part of the interface between the first and second precursor films can be adhered.

[0016] Another aspect of the present invention provides a multilayer separator manufactured by the above manufacturing method, wherein the thickness deviation of the outermost layer measured by the following formula is 10% or less. <Formula> Thickness deviation (%) = {(maximum value of thickness) - (minimum value of thickness)} / (minimum value of thickness) × 100

[0017] In the above formula, the thickness deviation is determined by a method that includes: cutting the multilayer separator to 100 mm × 100 mm (MD × TD), dividing it into five equal parts in the longitudinal direction (MD) to obtain five test pieces having a size of 20 mm × 100 mm (MD × TD); measuring the thickness of the outermost layer at the center of the transverse direction (TD) of the test piece; and calculating the thickness deviation using the above formula based on the maximum and minimum values ​​of the thickness.

[0018] In one embodiment, the multilayer separator can satisfy at least one of the following conditions (i) to (vi).

[0019] (i) Thickness 1-15 μm, (ii) Piercing strength 600 gf or more, (iii) Longitudinal (MD) tensile strength 1,300-2,000 kgf / cm 2 (iv) Transverse (TD) tensile strength 3,000~6,000 kgf / cm 2 (v) Longitudinal (MD) tensile elongation of 150-450%, and (vi) Transverse (TD) tensile elongation of 30-100%. [Effects of the Invention]

[0020] A method for manufacturing a multilayer separator according to one aspect of the present invention includes the steps of: (a) extruding a first composition containing a first polyolefin and a first pore-forming agent to produce a first sheet; (b) extruding a second composition containing a second polyolefin and a second pore-forming agent to produce a second sheet; (c) stretching the first and second sheets in the longitudinal direction (MD) to produce first and second precursor films; (d) laminating the first and second precursor films to obtain a laminate; and (e) stretching the laminate in the transverse direction (TD) and then removing the first and second pore-forming agents from the laminate. By including these steps, when manufacturing a thin-film type multilayer separator with a thickness of about 15 μm or less, preferably about 10 μm or less, the structural properties of each layer constituting the multilayer separator can be made uniform, ensuring the mechanical properties required for the separator can be stably secured, while simultaneously improving productivity and economic efficiency.

[0021] The effects of the present invention are not limited to those described above, but should be understood to include all effects that can be inferred from the detailed description of the present invention or the configuration of the invention as described in the claims. [Brief explanation of the drawing]

[0022] [Figure 1] This document shows a conventional method for manufacturing a multilayer separator. [Figure 2] A method for manufacturing a multilayer separator according to one embodiment of the present invention is shown. [Figure 3] This invention provides a method for measuring the thickness deviation of a multilayer separator according to one embodiment of the present invention. [Figure 4] A method for manufacturing a multilayer separator according to one comparative example of the present invention is shown. [Modes for carrying out the invention]

[0023] The present invention will be described below. However, the present invention can be realized in various different forms and is therefore not limited to the embodiments described herein. Furthermore, in order to clearly illustrate the present invention in the drawings, parts unrelated to the description have been omitted, and similar parts have been given similar reference numerals throughout the specification.

[0024] Throughout the specification, when any part is said to be “connected” to other parts, this includes not only cases where they are “directly connected,” but also cases where they are “indirectly connected” through the interposition of other components. Furthermore, when any part is said to “include” a certain component, this does not mean that it excludes other components, but rather that it may further include other components, unless otherwise stated. Embodiments of the present invention will be described in detail below with reference to the attached drawings.

[0025] Figure 2 shows a method for manufacturing a multilayer separator according to one embodiment of the present invention. Referring to Figure 2, a method for manufacturing a multilayer separator according to one aspect of the present invention may include: (a) a step of extruding a first composition containing a first polyolefin and a first pore-forming agent to produce a first sheet; (b) a step of extruding a second composition containing a second polyolefin and a second pore-forming agent to produce a second sheet; (c) a step of stretching the first and second sheets in the longitudinal direction (MD) to produce first and second precursor films; (d) a step of laminating the first and second precursor films to obtain a laminate; and (e) a step of stretching the laminate in the transverse direction (TD) and then removing the first and second pore-forming agents from the laminate.

[0026] In steps (a) and (b) above, the first and second compositions, which may have the same or different compositions, can be extruded independently to produce the first and second sheets. The extrusion can be carried out using two independent extruders (first and second extruders) arranged in parallel. The structure, configuration, specifications, and capacity of the first and second extruders may be the same, and if necessary, the first and second extruders can be designed differently to optimize the composition and physical properties of the first and second compositions, and thereby the physical properties of each layer constituting the multilayer separator. The first and second compositions can be melted and kneaded in the first and second extruders, and the first and second sheets can be produced by extruding them to a predetermined thickness through a device or equipment such as a T-die installed downstream of the first and second extruders.

[0027] The first and second polyolefins may each be selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and two or more combinations or copolymers thereof, preferably polyethylene and / or polypropylene, and more preferably polyethylene, but are not limited thereto.

[0028] The weight-average molecular weights of the first and second polyolefins may be between 300,000 and 2,000,000, and the molecular weight distribution (Mw / Mn) may be between 3 and 7. The first and second polyolefins may be of the same type and / or of the same nature, or, if necessary, of different types and / or of different nature. If the molecular weight distribution of the first and second polyolefins is less than 3, the dispersibility with the first and second pore-forming agents may decrease, and the uniformity of the manufactured multilayer separator may decrease. If it is greater than 7, the mechanical properties of the multilayer separator may decrease.

[0029] If the first and second polyolefins are heterogeneous, the first polyolefin may be high-density polyethylene (HDPE) with a weight-average molecular weight of 300,000 to 800,000, and the second polyolefin may be ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 1,000,000 to 2,000,000. Generally, the high-density polyethylene can contribute to the mechanical properties of the separator, such as tensile strength, tensile elongation, and puncture strength, and the ultra-high molecular weight polyethylene can contribute to the heat resistance of the separator. Therefore, a combination of the two can complementarily realize the mechanical properties and heat resistance of the separator. However, these effects are not always complementary, and in some cases, the mechanical properties and / or heat resistance can be further improved.

[0030] The ratio of the weight-average molecular weight of the second polyolefin to the weight-average molecular weight of the first polyolefin may be 0.5 to 2, preferably 0.65 to 1.5. If the ratio is less than 0.5 or greater than 2, the structural, mechanical properties and / or heat resistance deviations of the layers derived from the first and second compositions in the multilayer separator may fall outside an acceptable range, resulting in reduced compatibility with the battery.

[0031] The first and second porosity-forming agents may each be paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C. The first and second porosity-forming agents may be of the same type and / or of the same substance, or, if necessary, of different types and / or of different substances. If the kinematic viscosity of the first and second porosity-forming agents at 40°C falls outside the above range, the viscosity of the first and second compositions may be too low or too high, which may reduce processability and dispersibility. In addition, the first and second porosity-forming agents may each be, in addition to the paraffin oil, one selected from the group consisting of paraffin wax, mineral oil, solid paraffin, soybean oil, rapeseed oil, palm oil, coconut oil, di-2-ethylhexyl phthalate, dibutyl phthalate, diisononyl phthalate, diisodecyl phthalate, bis(2-propylheptyl) phthalate, naphthenic oil, and two or more combinations thereof, but are not limited thereto. The first and second compositions may each contain 20 to 50% by weight of the first and second polyolefins and 50 to 80% by weight of the porosity-forming agent.

[0032] The first or second composition may further contain a hydrophilic polymer. A separator consisting solely of the first and second polyolefins is inherently hydrophobic, but a predetermined hydrophilicity can be imparted to the separator by melting and kneading the polyolefin with a certain amount of hydrophilic polymer during the manufacturing of the separator. In this case, by optimizing the molecular weight of the polyolefin and the content of the hydrophilic polymer in the separator to obtain variables that achieve the required level of electrolyte impregnation, and then combining them, it is possible to balance the productivity of the process for melting and kneading the hydrophilic polymer with the polyolefin, the hydrophilicity of the separator, and the resulting electrolyte impregnation.

[0033] The layer of the multilayer separator that further contains the hydrophilic polymer may have a hydrophobic region containing the polyolefin and a hydrophilic region containing the hydrophilic polymer dispersed in the hydrophobic region. The content of the hydrophilic region in the layer may be 0.1 to 7.5% by weight, and this can be achieved by adjusting the content of the hydrophilic polymer in the first or second composition to a range of 0.1 to 5% by weight.

[0034] In the layer, the hydrophobic region made of the first or second polyolefin and the hydrophilic region made of the hydrophilic polymer can each constitute a continuous phase and a discontinuous phase. In the layer, the hydrophilic region is uniformly dispersed in a matrix made of the hydrophobic region, thereby providing substantially uniform hydrophilicity to the entire area and / or thickness of the layer, thereby improving the impregnation of the layer with the electrolyte. As used herein, the term "matrix" means the components that constitute the continuous phase in a layer or separator containing two or more components. That is, in the layer, the hydrophobic region containing the polyolefin may exist as a continuous phase, and the hydrophobic region containing the hydrophilic polymer may be dispersed therein as a discontinuous phase.

[0035] The content of the hydrophilic region in the layer may be 0.1 to 7.5% by weight, preferably 1 to 6% by weight, and more preferably 3 to 6% by weight. If the content of the hydrophilic region is 0.1% by weight, the required level of electrolyte impregnation cannot be achieved, and if it is greater than 7.5% by weight, the electrolyte impregnation can be further improved, but the mechanical properties and heat resistance of the separator that can be achieved through the polyolefin may decrease. Furthermore, if the content of the hydrophilic region is greater than 7.5% by weight, the dispersibility of the hydrophilic polymer decreases, the number of surface defects having a different brightness from the surrounding area and a size of 2 mm or more on the surface of the layer increases, and the appearance quality may deteriorate. Also, the resistance may change abruptly in areas and / or regions where the hydrophilic polymer arbitrarily aggregates on the surface and / or inside the layer, which may adversely affect the electrochemical properties of the battery.

[0036] When the first or second polyolefin to be mixed with the hydrophilic polymer is high-density polyethylene (HDPE), the weight-average molecular weight of the polyethylene may be 200,000 to 800,000, preferably 250,000 to 600,000, and more preferably 300,000 to 500,000. The ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene may be 0.1×10 -5 ~1.1×10 -5 and preferably 0.2×10 -5 ~1×10 -5 and more preferably 0.5×10 -5 ~1×10 -5 .

[0037] If the ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene is less than 0.1×l〇 -5 it is impossible to achieve the required level of electrolyte impregnation. If it is greater than 1.1×10 -5 not only will the electrolyte impregnation decrease, but the mechanical properties and heat resistance of the separator that can be achieved through the high-density polyethylene may also decrease. Also, if the ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene is greater than 1.1×10 -5 the dispersibility of the hydrophilic polymer will decrease, the brightness on the surface of the separator will be different from the periphery, the number of surface defects with a size of 2 mm or more will increase, and the appearance quality may decrease. There may be a sudden change in resistance at the site and / or region where the hydrophilic polymer aggregates arbitrarily on the surface and / or inside of the separator, which may have an adverse effect on the electrochemical characteristics of the battery.

[0038] Furthermore, if the first or second polyolefin to be mixed with the hydrophilic polymer is ultra-high molecular weight polyethylene (UHMWPE), the weight-average molecular weight of the polyethylene may be 1,000,000 to 2,000,000, preferably 1,000,000 to 1,500,000, and the ratio of the content of the hydrophilic region to the weight-average molecular weight of the ultra-high molecular weight polyethylene is 0.1 × 10⁻⁶. -5 ~0.75 × 10 -5 Preferably, 0.15 × 10 -5 ~0.6 × 10 -5 More preferably, 0.3 × 10 -5 ~0.6 × 10 -5 That's fine.

[0039] The ratio of the content of the hydrophilic region to the weight-average molecular weight of the ultra-high molecular weight polyethylene is 0.1 × 10 -5 If it is less than 0.75 × 10, the required level of electrolyte impregnation cannot be achieved. -5 If the ratio is larger, not only will the electrolyte impregnation ability decrease, but the mechanical properties and heat resistance of the separator that can be realized through the ultra-high molecular weight polyethylene may also decrease. Furthermore, if the ratio of the content of the hydrophilic region to the weight-average molecular weight of the ultra-high molecular weight polyethylene is 0.75 × 10 -5 If the size is larger, the dispersibility of the hydrophilic polymer decreases, the number of surface defects with a different brightness from the surrounding area and a size of 2 mm or more on the surface of the layer increases, which may reduce the appearance quality, and the resistance may change abruptly in areas and / or regions where the hydrophilic polymer arbitrarily aggregates on the surface and / or inside the layer, which may adversely affect the electrochemical properties of the battery.

[0040] The hydrophilic polymer may be one selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivatives, glycerol, and two or more combinations thereof. Preferably, it may be ethylene vinyl acetate, and more preferably, it may be ethylene vinyl acetate with a vinyl acetate content of 15 to 30% by weight, but it is not limited thereto. If the vinyl acetate content of the ethylene vinyl acetate is less than 15% by weight, the mechanical properties and hydrophilicity of the separator may decrease, and if it is greater than 30% by weight, the processability and the resulting dispersibility of the hydrophilic polymer may decrease. In addition to the above, various types of polymers having hydrophilic functional groups such as amine groups, amide groups, hydroxyl groups, and carboxylic acid groups in the main chain and / or side chains can also be used as the hydrophilic polymer.

[0041] Furthermore, among the hydrophilic polymers, the ethylene vinyl acetate imparts soft properties, i.e., a predetermined degree of flexibility, to the first and second polyolefins, improving the tensile elongation of the respective layers derived from the first and second compositions and the multilayer separator containing them. In addition, it can also contribute to improving mechanical properties such as puncture strength by increasing the interlayer bonding strength of the layers constituting the multilayer separator.

[0042] In step (c) above, the first and second sheets can be stretched longitudinally (MD) to produce the first and second precursor films. The longitudinal (MD) stretching of the first and second sheets can be performed using first and second stretchers installed downstream of the first and second extruders, respectively.

[0043] The first and second stretchers are devices that stretch the first and second sheets discharged from the first and second extruders, respectively, along the transport direction of the first and second sheets in the process line, and the stretching direction of the first and second sheets by the first and second stretchers can be defined as the longitudinal direction (MD, mechanical direction). The first and second stretchers may each be roll stretchers. The roll stretchers include a plurality of rolls along the transport direction of the sheet, and can stretch the sheet along the longitudinal direction (MD) at a predetermined magnification by rotating the rolls located later than the rolls located earlier. The stretching magnification of the first and second sheets may be 2 to 20 times, preferably 5 to 10 times, respectively. Furthermore, the stretching magnifications of the first and second sheets may be the same or different, and preferably, these stretching magnifications can be set to be the same considering the adhesion force of the first and second precursor films manufactured in step (c) and the resulting interlayer bonding force, but are not limited thereto.

[0044] In step (d) above, the first and second precursor films are laminated to obtain a laminate. The lamination may be performed by a predetermined laminator, which is equipment or apparatus for laminating the first and second precursor films produced by the first and second stretchers to each other. During lamination, pressure is applied so that the first and second precursor films are in contact with each other, and at least a portion, preferably all, of the interface between the first and second precursor films can be adhered. The adhesion can be performed by physical means such as heat, ultrasound, high frequency, or laser applied to at least a portion, preferably all, of the surface of the first and second precursor films, and the running stability of the laminate as it passes through equipment or apparatus for subsequent stretching (TD), extraction, and heat fixing can be improved through the adhesion, thereby effectively eliminating structural non-uniformity that appears in the outermost layer of conventional multilayer separators, such as excessive deviations in the thickness and porosity of the outermost layer.

[0045] In step (e) above, the laminate can be stretched in the transverse direction (TD), and then the first and second pore-forming agents can be removed from the laminate. The transverse (TD) stretching of the laminate can be performed by a third stretching machine.

[0046] The third stretcher can stretch the laminate in the transverse direction (TD). The third stretcher may be a tenter stretcher. The tenter stretcher can stretch the laminate to a predetermined magnification by fixing both lateral ends of the laminate with predetermined members such as chucks and clips and separating the members in the transverse direction. The transverse (TD) stretching magnification of the laminate by the third stretcher may be 2 to 20 times, preferably 5 to 10 times, but is not limited thereto.

[0047] Subsequently, a predetermined extraction solvent can be applied to the laminate to selectively extract and remove the first and second pore-forming agents simultaneously from the laminate, specifically from the first and second precursor films constituting the laminate. The first and second pore-forming agents can be extracted and removed by immersing the laminate in an impregnation tank carrying a solution containing the extraction solvent for a predetermined time.

[0048] After extraction, the content of the porosity-forming agent remaining on the surface and / or inside the laminate may be 1% by weight or less. The extraction solvent may be, for example, methyl ethyl ketone, hexane, dichloromethane, etc., but is not limited thereto. The time required for extraction and removal of the first and second porosity-forming agents can be determined by the thickness and porosity of the laminate, but if the thickness and porosity of the laminate are 1 to 15 μm and 40 to 70 volume%, respectively, the time may be 10 minutes or less, preferably 5 minutes or less.

[0049] Furthermore, the laminate from which the first and second pore-forming agents have been extracted and removed can be heated to remove any remaining extraction solvent from the laminate. Some of the extraction solvent applied in step (e) may remain on the surface and / or inside the laminate. Since the remaining extraction solvent can degrade the physical properties of subsequent steps and the separator produced therethrough, the laminate can be appropriately heated to a temperature above the boiling point of the extraction solvent to remove any remaining extraction solvent from the laminate.

[0050] The method for manufacturing the multilayer separator may further include, after step (e), a step of (e') heat-setting the laminate. Heat setting means a step of applying heat to the laminate while it is fixed in place to forcibly fix the laminate that is about to shrink and to remove residual stress. A higher heat setting temperature is advantageous in reducing the shrinkage rate, but if the temperature is too high, the laminate may partially melt, the formed pores may close, and the permeability may decrease.

[0051] The heat-setting temperature is preferably selected within a range in which 10 to 30% by weight of the crystalline portion of the laminate melts. Selecting the heat-setting temperature within this range prevents problems such as insufficient rearrangement of polyolefin molecules within the laminate, resulting in no residual stress relief effect on the film, and partial melting causing pore closure and reduced permeability. For example, the heat-setting temperature may be 120 to 140°C, preferably 123 to 135°C, and the heat-setting time may be 5 seconds to 1 minute.

[0052] Another aspect of the present invention provides a multilayer separator manufactured by the above manufacturing method, wherein the thickness deviation of the outermost layer, measured by the following formula, is 10% or less, preferably 1 to 8.5% or 3 to 6.5%. <expression> Thickness deviation (%) = {(Maximum thickness) - (Minimum thickness)} / (Minimum thickness) × 100

[0053] Conventionally, in the manufacturing method of multilayer separators, interlayer lamination is performed by co-extrusion in a step before the formation of the porous structure by stretching, i.e., stacking can occur, which leads to the problem of non-uniform structural properties of each layer in subsequent film-forming steps such as stretching and extraction. Specifically, as the thickness deviation in different regions of the outermost layer of the multilayer separator increases, the mechanical properties of the multilayer separator itself may become non-uniform, and in particular, when assembling a battery using a multilayer separator, areas that are significantly thinner than the surrounding areas may easily break or fracture. Such problems have become even more pronounced recently in thin-film separators (thickness of about 15 μm or less, preferably about 10 μm or less) used to meet the trend of battery integration and high capacity.

[0054] In contrast, the multilayer separator is manufactured by laminating the first and second precursor films, which have been stretched in the longitudinal direction (MD) to form a predetermined pore structure, into a laminate, which is then stretched in the transverse direction (TD), and the first and second pore-forming agents are extracted and removed. This method makes it possible to homogenize the structural properties of each layer constituting the multilayer separator, ensure stable mechanical properties, and at the same time minimize the number of stretchers required to manufacture the multilayer separator, thereby improving productivity and economic efficiency.

[0055] Figure 3 shows a method for measuring the thickness deviation of a multilayer separator according to one embodiment of the present invention. Referring to Figure 3, the thickness deviation of the outermost layer can be calculated using the thickness of each test specimen measured by the following method and the formula above. The formula above determines the thickness deviation by a method including the steps of: cutting the multilayer separator to 100 mm × 100 mm (MD × TD), dividing it into five equal parts in the longitudinal direction (MD) to obtain five test specimens having a size of 20 mm × 100 mm (MD × TD); measuring the thickness of the outermost layer at the center of the transverse direction (TD) of the test specimen; and calculating the thickness deviation using the formula above based on the maximum and minimum values ​​of the thickness.

[0056] Furthermore, the multilayer separator can satisfy at least one, preferably all, of the following conditions (i) to (vi).

[0057] (i) Thickness 1 to 15 μm, preferably 5 to 12 μm; (ii) Piercing strength 600 gf or more, preferably 700 to 1,000 gf, more preferably 750 to 900 gf; (iii) Longitudinal (MD) tensile strength 1,300 to 2,000 kgf / cm² 2 Preferably, 1,400 to 1,800 kgf / cm² 2 (iv) Transverse (TD) tensile strength 3,000~6,000 kgf / cm 2 Preferably, 3,500 to 5,500 kgf / cm² 2 More preferably, 4,000 to 5,400 kgf / cm² 2 (v) Longitudinal (MD) tensile elongation of 150-450%, preferably 200-400%, more preferably 230-350%, and (vi) Transverse (TD) tensile elongation of 30-100%, preferably 40-90%. The following describes in detail embodiments of the present invention.

[0058] Example 1 A first composition, prepared by mixing 28 parts by weight of high-density polyethylene (HDPE) with a weight-average molecular weight of 600,000 and 72 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C, was fed into a twin-screw extruder (first extruder, inner diameter 58 mm, L / D = 56). The mixture was extruded from the first extruder through a 400 mm wide T-die at a screw rotation speed of 120 rpm and a temperature of 210°C, and then passed through a casting roll at a temperature of 70°C to produce a first sheet with a thickness of 800 μm.

[0059] The same second composition as the first composition was fed into a twin-screw extruder (second extruder, inner diameter 58 mm, L / D = 56) having the same configuration as the first extruder and arranged in parallel. The second composition was extruded from the second extruder through a 400 mm wide T-die at a screw rotation speed of 120 rpm and a temperature of 210°C, and then passed through a casting roll at a temperature of 70°C to produce a second sheet with a thickness of 800 μm.

[0060] The first and second sheets were fed into first and second stretchers (roll stretchers) arranged in parallel to each other, and stretched eight times in the longitudinal direction (MD) at 120°C to produce first and second precursor films. Here, the first and second stretchers refer to stretchers located downstream of the first and second extruders, respectively.

[0061] The first and second precursor films were fed into laminators installed downstream of the first and second stretching machines, and a laminate was obtained in which the first and second precursor films were laminated together in contact with each other.

[0062] The laminate was placed in a third stretching machine (tenter stretching machine), stretched nine times in the transverse direction (TD) at 126°C, then impregnated in a dichloromethane leaching tank at 25°C for 1 minute to extract and remove paraffin oil, and dried at 38°C for 5 minutes. The laminate was stretched 1.5 times in the transverse direction (TD) at 138°C, then relaxed by 10%, and heat-set to obtain a separator.

[0063] Example 2 A separator was manufactured in the same manner as in Example 1, except that the high-density polyethylene (HDPE) with a weight-average molecular weight of 600,000 in the second composition was replaced with ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 1,000,000.

[0064] Example 3 A separator was manufactured in the same manner as in Example 1, except that the second composition further contained ethylene vinyl acetate (EVA, HTC) with a vinyl acetate content of 28% by weight, and the content (parts by weight) of the high-density polyethylene, ethylene vinyl acetate, and paraffin oil in the second composition was changed to 30.4 parts by weight, 1.6 parts by weight, and 68.0 parts by weight, respectively.

[0065] Example 4 A separator was manufactured in the same manner as in Example 3, except that the high-density polyethylene (HDPE) with a weight-average molecular weight of 600,000 in the second composition was replaced with ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 1,000,000.

[0066] Example 5 A separator was manufactured in the same manner as in Example 1, except that a step was added in which 50 kHz ultrasonic waves were applied to the entire surface of the laminate during lamination to bond (fuse) the interfaces of the laminate.

[0067] Example 6 The separator was manufactured in the same manner as in Example 2, except that a step was added in which 50 kHz ultrasonic waves were applied to the entire surface of the laminate during lamination to bond (fuse) the interfaces of the laminate.

[0068] Example 7 The separator was manufactured in the same manner as in Example 3, except that a step was added in which 50 kHz ultrasonic waves were applied to the entire surface of the laminate during lamination to bond (fuse) the interfaces of the laminate.

[0069] Example 8 The separator was manufactured in the same manner as in Example 4, except that a step was added in which 50 kHz ultrasonic waves were applied to the entire surface of the laminate during lamination to bond (fuse) the interfaces of the laminate.

[0070] Comparative Example 1 A mixture containing 60% by weight of ultra-high molecular weight polyethylene with a weight-average molecular weight of 2,000,000 and 40% by weight of high-density polyethylene with a weight-average molecular weight of 560,000, along with 75% by weight of paraffin oil with a kinematic viscosity of 35 cSt at 40°C, was placed into a twin-screw extruder (first extruder, inner diameter 58 mm, L / D = 56) and melt-kneaded at a screw rotation speed of 250 rpm and a temperature of 230°C to produce the first polyolefin solution.

[0071] A mixture of 30 parts by weight containing 50% by weight of high-density polyethylene with a weight-average molecular weight of 560,000 and 50% by weight of polypropylene with a weight-average molecular weight of 1,600,000, and 70 parts by weight of paraffin oil with a kinematic viscosity of 35 cSt at 40°C were put into a twin-screw extruder (second extruder, inner diameter 58 mm, L / D=56) having the same configuration as the first extruder and arranged in parallel, and melt-kneaded under the same conditions to produce a second polyolefin solution.

[0072] The first and second polyolefin solutions were supplied from the first and second extruders, respectively, to a three-layer T-die, and extruded so that the ratio of the layer thicknesses of the first / second / first polyolefin solutions was 35 / 30 / 35. The extruded body was cooled while being taken up by a cooling roll (diameter 500 mm) at 37°C to produce a gel-like three-layer sheet. The gel-like three-layer sheet was simultaneously biaxially stretched 5 × 5 times (MD × TD) at 114°C, then impregnated in a dichloromethane leaching tank at 25°C for 5 minutes to extract and remove paraffin oil to produce a porous membrane. The porous membrane was put into a tenter stretcher, re-stretched 1.4 times in the transverse direction (TD) at 124°C, and then heat-set to produce a separator.

[0073] Comparative Example 2 29.5 parts by weight of high-density polyethylene with a weight-average molecular weight (Mw) of 350,000, 0.5 parts by weight of silane-modified high-density polyethylene, and 70 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56). Dibutylsulphurate, a crosslinking catalyst, was pre-dispersed in a portion of the paraffin oil and added to the twin-screw extruder via the side injector so that it constituted 0.5% by weight of the total weight of the material passing through the twin-screw extruder. After extrusion from the twin-screw extruder through a 300 mm wide T-die at a screw rotation speed of 40 rpm and a temperature of 200°C, the mixture was passed through a cast roll at a temperature of 40°C to produce a base sheet with a thickness of 800 μm.

[0074] The base sheet was stretched six times in the longitudinal direction (MD) using a roll stretcher at 110°C, and then seven times in the transverse direction (TD) using a tenter stretcher at 125°C to produce a film. The film was impregnated in a dichloromethane leaching tank at 25°C to extract and remove paraffin oil for 1 minute, and then dried at 50°C for 5 minutes to produce a porous film. Subsequently, the film was heated at 125°C using a tenter stretcher, stretched 1.45 times in the transverse direction (TD), and then relaxed to heat-set to 1.25 times its original size. The film was crosslinked in a constant temperature and humidity tank at 85°C and 85% humidity for 72 hours to produce a separator.

[0075] Comparative Example 3 A mixture containing 64% by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight (Mw) of 1,000,000 and 36% by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight (Mw) of 1,500,000 was mixed with 70% by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C. This mixture was then fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56). The mixture was extruded from the twin-screw extruder through a 300 mm wide T-die at a screw rotation speed of 40 rpm and a temperature of 200°C. After passing through a casting roll at a temperature of 40°C, a base sheet with a thickness of 800 μm was produced.

[0076] The base sheet was stretched six times in the longitudinal direction (MD) using a roll stretcher at 110°C, and then seven times in the transverse direction (TD) using a tenter stretcher at 125°C to produce a film. The film was impregnated in a dichloromethane leaching tank at 25°C for 1 minute to extract and remove paraffin oil, and then dried at 50°C for 5 minutes to produce a porous film. Subsequently, the film was heated at 125°C using a tenter stretcher, stretched 1.45 times in the transverse direction (TD), relaxed, and then heat-set to 1.25 times its original size to produce a separator.

[0077] Comparative Example 4 A separator was manufactured in the same manner as in Comparative Example 3, except that 30 parts by weight of a mixture containing 42% by weight of high-density polyethylene (HDPE) with a weight-average molecular weight (Mw) of 600,000 and 58% by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight (Mw) of 1,500,000, and 70 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56).

[0078] Comparative Example 5 100 parts by weight of a mixture containing 90% by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 1,000,000 and 10% by weight of high-density polyethylene (HDPE) with a weight-average molecular weight of 380,000 was fed into a twin-screw extruder (first extruder, inner diameter 58 mm, L / D = 42). 250 parts by weight of paraffin oil was supplied through the side injector of the first extruder, and the mixture was melt-kneaded at 210°C and 100 rpm to produce the first polyethylene solution.

[0079] 100 parts by weight of a mixture containing 10% by weight of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 1,000,000 and 90% by weight of high-density polyethylene (HDPE) with a weight-average molecular weight of 380,000 was fed into a twin-screw extruder (second extruder, inner diameter 58 mm, L / D = 42). 250 parts by weight of paraffin oil was supplied through the side injector of the second extruder, and the mixture was melt-kneaded at 210°C and 100 rpm to produce a second polyethylene solution.

[0080] The first and second polyethylene solutions were passed through a multi-block (multilayering device) of an extruder to alternately layer the first and second polyethylene solutions, and then a multilayer structure consisting of 20 layers was extruded via a T-die.

[0081] A gel-like base sheet with a thickness of 1,100 μm was produced by passing the aforementioned multilayer structure through a cooling roll adjusted to 40°C. The base sheet was simultaneously biaxially stretched 8 x 8 times (MD x TD) at 124°C, then impregnated in a dichloromethane leaching tank at 25°C for 10 minutes to extract and remove paraffin oil, and dried at 50°C for 5 minutes to produce a porous membrane. The porous membrane was placed in a tenter stretcher, re-stretched 1.4 times in the transverse direction (TD) at 125°C, and then heat-set to produce a separator.

[0082] Comparative Example 6 The separator was manufactured as follows using the method shown in Figure 4. A first composition, prepared by mixing 28 parts by weight of high-density polyethylene (HDPE) with a weight-average molecular weight of 600,000 and 72 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C, was fed into a twin-screw extruder (first extruder, inner diameter 58 mm, L / D = 56). The mixture was extruded from the first extruder through a 400 mm wide T-die at a screw rotation speed of 120 rpm and a temperature of 210°C, and then passed through a casting roll at a temperature of 70°C to produce a first sheet with a thickness of 800 μm.

[0083] The same second composition as the first composition was fed into a twin-screw extruder (second extruder, inner diameter 58 mm, L / D = 56) having the same configuration as the first extruder and arranged in parallel. The second composition was extruded from the second extruder through a 400 mm wide T-die at a screw rotation speed of 120 rpm and a temperature of 210°C, and then passed through a casting roll at a temperature of 70°C to produce a second sheet with a thickness of 800 μm.

[0084] The first and second sheets were fed into laminators installed downstream of the first and second extruders to obtain a laminate in which the first and second sheets were laminated in contact with each other. The laminate was fed into a stretcher (roll stretcher) and stretched eight times in the longitudinal direction (MD) at 120°C to produce a precursor film. The precursor film was fed into a tenter stretcher and stretched nine times in the transverse direction (TD) at 126°C, then impregnated in a dichloromethane leaching tank at 25°C for 1 minute to extract and remove paraffin oil, and dried at 38°C for 5 minutes. The laminate was stretched 1.5 times in the transverse direction (TD) at 138°C, then relaxed by 10% and heat-set to obtain a separator.

[0085] Experimental Example 1 After cutting each of the separators produced in the above examples and comparative examples into 100 mm x 100 mm pieces, the cut separator test pieces were divided into five equal parts in the longitudinal direction (MD) to obtain test pieces 1 to 5 of 20 mm x 100 mm (MD x TD).

[0086] A cross-section of each specimen was imaged using a scanning electron microscope (SEM), and the thickness of the outermost layer (μm) was measured at the center of each specimen in the transverse direction (TD). Based on this, the thickness deviation, defined by the following formula, was calculated and is shown in Table 1 below. <expression> Thickness deviation (%) = {(Maximum thickness) - (Minimum thickness)} / (Minimum thickness) × 100

[0087] In Examples 1-8 and Comparative Example 6, the layer derived from the first composition was selected as the outermost layer, and in Comparative Examples 1 and 5, the layer derived from the first polyolefin solution was selected as the outermost layer. The separators from Comparative Examples 2-4 were excluded from the measurement because they had a single-layer structure.

[0088] [Table 1]

[0089] Experimental Example 2 The thickness, puncture strength, tensile strength, and tensile elongation of the separators manufactured in the above examples and comparative examples were measured by the following method. Unless otherwise specified, measurements were taken at room temperature (25°C), and the results are shown in Table 2 below.

[0090] -Thickness (μm): The thickness of the support specimen was measured using a micro-thickness measuring instrument. - Perforation strength (gf): Using a perforation strength measuring device, force was applied with a stick to a separator test piece measuring 100 mm x 50 mm, and the force applied until the sample was perforated was measured.

[0091] - Tensile strength (kgf / cm 2 Using a tensile strength testing machine, the stress applied to a 20mm x 200mm separator specimen until fracture occurred was measured in both the longitudinal (machine direction, MD) and transverse (transverse direction, TD) directions.

[0092] -Tensile elongation (%): Using a tensile strength testing machine, the percentage of elongation of a 20mm x 200mm separator specimen until fracture occurred was measured in both the longitudinal (machine direction, MD) and transverse (transverse direction, TD) directions.

[0093] [Table 2]

[0094] The above description of the present invention is illustrative, and a person with ordinary skill in the art to which the invention pertains will understand that it can be easily modified in other specific forms without altering the technical idea or essential features of the invention. Therefore, the embodiments described above should be understood to be illustrative and not limiting in all respects. For example, each component described as a single type can be implemented in a distributed manner, and similarly, components described as distributed can be implemented in a combined manner.

[0095] The scope of the present invention is defined by the claims set forth below, and it should be understood that all modifications or altered forms derived from the meaning and scope of the claims and the concept of equivalents thereof are included within the scope of the present invention.

Claims

1. A multilayer separator comprising layers derived from a first composition containing a first polyolefin and a second composition containing a second polyolefin, A multilayer separator in which the thickness deviation of the outermost layer, measured by the following formula, is 10% or less. <Formula> Thickness deviation (%) = {(Maximum thickness) - (Minimum thickness)} / (Minimum thickness) × 100 In the above formula, the thickness deviation is determined by a method that includes the steps of: cutting the multilayer separator into 100 mm × 100 mm (MD × TD), dividing it into five equal parts in the vertical direction (MD) to obtain five test pieces having a size of 20 mm × 100 mm (MD × TD); measuring the thickness of the outermost layer at the center of the test piece in the horizontal direction (TD); and calculating the thickness deviation using the above formula based on the maximum and minimum values ​​of the thickness.

2. The multilayer separator according to claim 1, wherein the thickness of the multilayer separator is 1 to 15 μm.

3. The multilayer separator according to claim 1, wherein the perforation strength of the multilayer separator is 600 gf or more.

4. The multilayer separator according to claim 1, wherein the longitudinal (MD) tensile strength of the multilayer separator is 1,300 to 2,000 kgf / cm².

5. The multilayer separator according to claim 1, wherein the transverse (TD) tensile strength of the multilayer separator is 3,000 to 6,000 kgf / cm².

6. The multilayer separator according to claim 1, wherein the longitudinal (MD) tensile elongation of the multilayer separator is 150 to 450%.

7. The multilayer separator according to claim 1, wherein the lateral (TD) tensile elongation of the multilayer separator is 30 to 100%.

8. The first polyolefin and the second polyolefin each comprise one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and two or more combinations or copolymers thereof. The multilayer separator according to claim 1, wherein the weight-average molecular weights of the first polyolefin and the second polyolefin are each 300,000 to 2,000,000.

9. The first composition or the second composition further comprises a hydrophilic polymer, The multilayer separator according to claim 1, wherein the content of the hydrophilic polymer in the first composition or the second composition is 0.1 to 5% by weight.

10. The multilayer separator according to claim 9, wherein the hydrophilic polymer is one selected from the group consisting of ethylene vinyl acetate, ethylene vinyl alcohol, polyvinyl alcohol, polyacrylic acid, polyoxyethylene-polyoxypropylene block copolymer, polyethylene glycol, polyethylene oxide, polyvinylpyrrolidone, polyvinyl acetal, polyvinyl butyral, cellulose derivatives, glycerol, and two or more combinations thereof.

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