Method for manufacturing multi-layer separator

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 uniformize the structural properties of each layer constituting the multilayer separator and improve the mechanical properties required of the separator. [Background technology]

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

[0003] Recently, as the energy density of lithium secondary batteries has increased, the load on the batteries has increased, and separators are also required to have high safety. As a result, the importance of not only the mechanical properties of separators but also heat resistance, which can ensure safety, has become apparent.

[0004] Polyolefin-based microporous membranes have traditionally been used as separators in lithium secondary batteries. Among polyolefin-based microporous membranes, those made of polyethylene resins are known to have an excellent shutdown function, in which the micropores in the membrane close, cutting off the flow of current, when the battery temperature rises. However, even after the shutdown function is activated, the battery temperature may continue to rise. In this case, the separator may melt (meltdown), causing a short circuit inside the battery. This generates a large amount of heat, posing risks such as smoke, fire, and explosion. Therefore, it is necessary to suppress the risk of short circuit even at temperatures higher than the shutdown temperature.

[0005] In order to achieve both shutdown and meltdown properties in the separator, methods have been proposed such as blending polyethylene and polypropylene, or laminating a microporous membrane made of a polyethylene-based resin with a microporous membrane made of a polypropylene-based resin.

[0006] For example, Patent Documents 1 and 2 disclose methods for producing multilayer separators by co-extrusion of two or more resin compositions, and disclose that the mechanical strength and heat resistance of the resulting multilayer separator can be improved in a balanced manner. However, because interlayer lamination is performed by co-extrusion before the pore structure is formed by stretching, i.e., stacking, problems arise in that the structural properties of each layer can be non-uniform during subsequent film-forming processes such as stretching and extrusion. Specifically, as the thickness deviation between the outermost layers of a multilayer separator increases, the mechanical properties of the separator itself can become non-uniform. In particular, when assembling a battery using the separator, areas that are significantly thinner than the surrounding areas can easily break or fracture. This problem is particularly pronounced in thin-film separators (thicknesses of approximately 15 μm or less, preferably approximately 10 μm or less) that are being developed to address the recent trend toward higher battery integration and capacity.

[0007] Patent Document 3 discloses a method for producing a multilayer separator by laminating two or more separators that have been completely formed through extrusion, stretching, extraction, and heat setting, as shown in Figure 1. However, in this case, sufficient interlayer bonding strength cannot be imparted, resulting in a problem that the individual layers easily peel off. To solve this problem, a process can be added in which two or more separators are laminated and then re-stretched and heat-set, but this poses a problem of reduced productivity and economic efficiency due to the additional equipment and processes. [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 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made to solve the problems of the prior art as described above, and an object of the present invention is to provide a method for producing a thin-film multilayer separator having a thickness of about 15 μm or less, preferably about 10 μm or less, which can uniformly structure the physical properties of each layer constituting the multilayer separator, stably ensure the mechanical properties required of the separator, and at the same time, improve productivity and cost-effectiveness. [Means for solving the problem]

[0010] One aspect of the present invention provides a method for producing a multilayer separator, comprising: (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 machine direction (MD) to produce first and second precursor films, respectively; (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 comprise one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations or copolymers of two or more thereof.

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

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

[0014] In one embodiment, 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 combinations of two or more thereof.

[0015] In one embodiment, the first and second precursor films may be pressed against each other during lamination, so that at least a portion of the interface between the first and second precursor films may be adhered to each other.

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

[0017] In the above formula, the thickness deviation is determined by a method including the steps of: cutting the multilayer separator to 100 mm x 100 mm (MD x TD), then dividing it into five equal parts in the machine direction (MD) to obtain five test pieces each measuring 20 mm x 100 mm (MD x TD); measuring the thickness of the outermost layer at the center of the test piece in the machine direction (TD); and calculating the thickness deviation using the formula based on the maximum and minimum thickness values.

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

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

[0020] A method for producing a multilayer separator according to one embodiment 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 machine direction (MD) to produce first and second precursor films, respectively; (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. This allows for uniform structural properties of each layer constituting the multilayer separator during the production of a thin-film multilayer separator having a thickness of about 15 μm or less, preferably about 10 μm or less, and ensures stable mechanical properties required for the separator, while also improving productivity and economic efficiency.

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

[0022] [Figure 1] 1 shows a method for manufacturing a multi-layer separator according to the prior art. [Figure 2] 1 illustrates a method for manufacturing a multi-layer separator according to one embodiment of the present invention. [Figure 3] 1 illustrates a method for measuring thickness variation of a multi-layer separator according to one embodiment of the present invention. [Figure 4] 1 illustrates a method for manufacturing a multilayer separator according to a comparative example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described below. However, the present invention can be realized in various different forms and is not limited to the embodiments described herein. In addition, in order to clearly explain the present invention in the drawings, parts that are not relevant to the description are omitted, and similar parts are designated by similar reference numerals throughout the specification.

[0024] Throughout the specification, when a part is said to be "connected" to another part, this includes not only "directly connected" but also "indirectly connected" through an intervening member. Furthermore, when a part is said to "comprise" a certain component, this does not mean that it excludes other components, but that it may further comprise other components, unless otherwise specified. Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0025]

[0013] Figure 2 shows a method for producing a multilayer separator according to one embodiment of the present invention. Referring to Figure 2, the method for producing a multilayer separator according to one embodiment of the present invention may include: (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 machine direction (MD) to produce first and second precursor films, respectively; (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.

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

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

[0028] The weight-average molecular weight of each of the first and second polyolefins may be 300,000 to 2,000,000, and the molecular weight distribution (Mw / Mn) may be 3 to 7. The first and second polyolefins may be the same and / or of the same quality, or may be different and / or of different qualities, as needed. If the molecular weight distribution of the first and second polyolefins is less than 3, dispersibility with the first and second pore-forming agents may decrease, which may result in a decrease in the uniformity of the manufactured multilayer separator. If the molecular weight distribution is greater than 7, the mechanical properties of the multilayer separator may decrease.

[0029] When the first and second polyolefins are different, the first polyolefin may be a high-density polyethylene (HDPE) having a weight-average molecular weight of 300,000 to 800,000, and the second polyolefin may be an ultra-high molecular weight polyethylene (UHMWPE) having a weight-average molecular weight of 1,000,000 to 2,000,000. Generally, the high-density polyethylene contributes to the separator's mechanical properties, such as tensile strength, tensile elongation, and puncture resistance, and the UHMWPE contributes to the separator's heat resistance. Therefore, the combination of the high-density polyethylene and the UHMWPE can complement each other to improve the separator's mechanical properties and heat resistance. However, these effects are not necessarily complementary, and in some cases, the mechanical properties and / or heat resistance may 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 more than 2, the deviations in structural, mechanical properties, and / or heat resistance of the layers derived from the first and second compositions in the multilayer separator may fall outside the allowable range, resulting in reduced compatibility with the battery.

[0031] The first and second pore-forming agents may each be paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C. The first and second pore-forming agents may be the same type and / or of the same quality, or, if necessary, different types and / or of different qualities. If the kinematic viscosity at 40°C of the first and second pore-forming agents is outside the above range, the viscosity of the first and second compositions may be too low or too high, which may result in reduced processability and dispersibility. In addition to the paraffin oil, the first and second pore-forming agents may each be 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 combinations of two or more 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 pore-forming agent.

[0032] The first or second composition may further contain a hydrophilic polymer. Separators made solely of the first or second polyolefin are inherently hydrophobic, but a predetermined hydrophilicity can be imparted to the separator by melting and kneading a certain amount of hydrophilic polymer with the polyolefin during separator production. In this case, by optimizing the molecular weight of the polyolefin and the content of the hydrophilic polymer in the separator to determine and combine variables that achieve the required level of electrolyte impregnation, it is possible to achieve a balance between the productivity of the process of 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 further containing 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 wt %, which can be achieved by adjusting the content of the hydrophilic polymer in the first or second composition to the range of 0.1 to 5 wt %.

[0034] In the layer, the hydrophobic region composed of the first or second polyolefin and the hydrophilic region composed of the hydrophilic polymer can constitute a continuous phase and a discontinuous phase, respectively. In the layer, the hydrophilic region is uniformly dispersed in a matrix composed of the hydrophobic region, imparting substantially uniform hydrophilicity to the entire area and / or thickness of the layer, thereby improving the layer's impregnation with the electrolyte. As used herein, the term "matrix" refers to a component that constitutes 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, within which the hydrophobic region containing the hydrophilic polymer may exist dispersed as a discontinuous phase.

[0035] The content of the hydrophilic region in the layer may be 0.1 to 7.5 wt%, preferably 1 to 6 wt%, and more preferably 3 to 6 wt%. A hydrophilic region content of 0.1 wt% may not achieve the required level of electrolyte impregnation, while a content greater than 7.5 wt% may further improve electrolyte impregnation, but may degrade the mechanical properties and heat resistance of the separator that can be achieved through the polyolefin. Furthermore, a hydrophilic region content greater than 7.5 wt% may result in a decrease in the dispersibility of the hydrophilic polymer, leading to an increase in the number of surface defects (having a different brightness from the surrounding area and measuring 2 mm or larger) on the surface of the layer, degrading the appearance quality, and a sudden change in resistance at sites and / or regions where the hydrophilic polymer randomly 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, and the ratio of the content of the hydrophilic region to the weight-average molecular weight of the high-density polyethylene is 0.1 × 10 -5 ~1.1×10 -5 , preferably 0.2 x 10 -5 ~1×10 -5 , more preferably 0.5 × 10 -5 ~1×10 -5 may be.

[0037] The ratio of the content of the hydrophilic region to the weight average molecular weight of the high-density polyethylene is 0.1 × 10 -5 If the thickness is less than 1.1 × 10, the required level of electrolyte impregnation cannot be achieved. -5 If the ratio of the content of the hydrophilic region to the weight average molecular weight of the high density polyethylene is larger than 1.1×10, the electrolyte impregnation property may be reduced, and the mechanical properties and heat resistance of the separator may be reduced. -5 If the size is larger, the dispersibility of the hydrophilic polymer may decrease, the number of surface defects having a different brightness from the surrounding area and a size of 2 mm or more may increase on the surface of the separator, and the appearance quality may decrease.In addition, the resistance may suddenly change at sites and / or regions where the hydrophilic polymer randomly aggregates on the surface and / or inside of the separator, which may adversely affect the electrochemical properties of the battery.

[0038] In addition, when 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 -5 ~0.75×10 -5 , preferably 0.15 x 10 -5 ~0.6×10 -5 , more preferably 0.3 × 10 -5 ~0.6×10 -5 may be.

[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 the thickness is less than 0.75×10, the required level of electrolyte impregnation cannot be achieved. -5 If the ratio of the content of the hydrophilic region to the weight average molecular weight of the ultra-high molecular weight polyethylene is larger than 0.75×10, the electrolyte impregnation property may be reduced, and the mechanical properties and heat resistance of the separator may be reduced. -5 If the thickness is larger, the dispersibility of the hydrophilic polymer will decrease, the number of surface defects on the surface of the layer that have a different brightness from the surrounding area and are 2 mm or larger in size will increase, and the appearance quality will decrease.In addition, the resistance will suddenly change at sites and / or regions where the hydrophilic polymer randomly aggregates on the surface and / or inside of the layer, which may have an adverse effect on 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 combinations of two or more 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 wt%, but is not limited thereto. If the vinyl acetate content of the ethylene vinyl acetate is less than 15 wt%, the mechanical properties and hydrophilicity of the separator may be reduced. If it is more than 30 wt%, the processability and therefore the dispersibility of the hydrophilic polymer may be reduced. In addition to the above-mentioned hydrophilic polymers, 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 may also be used as the hydrophilic polymer.

[0041] Among the hydrophilic polymers, ethylene vinyl acetate imparts soft properties, i.e., a predetermined flexibility, to the first and second polyolefins, thereby improving the tensile elongation of each layer derived from the first and second compositions and the multilayer separator including the same. It also increases the interlayer bonding strength of each layer constituting the multilayer separator, thereby contributing to improving mechanical properties such as puncture strength.

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

[0043] The first and second stretching machines 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. The stretching direction of the first and second sheets by the first and second stretching machines can be defined as the machine direction (MD, mechanical direction). The first and second stretching machines may each be a roll stretching machine. The roll stretching machine includes multiple rolls along the transport direction of the sheet, and can stretch the sheet at a predetermined stretch ratio along the machine direction (MD) by rotating the rolls located at the rear more quickly than the rolls located at the front. The stretching ratios of the first and second sheets may each be 2 to 20 times, preferably 5 to 10 times. The stretching ratios of the first and second sheets may be the same or different. Preferably, the stretching ratios are set to be the same in consideration of the adhesive strength of the first and second precursor films produced in step (c) and the resulting interlayer bonding strength, but are not limited thereto.

[0044] In step (d), the first and second precursor films are laminated to obtain a laminate. The lamination can be performed using a predetermined laminator. The laminator is a facility or device that laminates the first and second precursor films produced by the first and second stretching machines. During lamination, the first and second precursor films are pressed against each other, resulting in at least a portion, preferably the entirety, of the interface between the first and second precursor films. The adhesion can be achieved by physical means such as heat, ultrasound, high frequency, or laser applied to at least a portion, preferably the entirety, of the surfaces of the first and second precursor films. This adhesion can improve the running stability of the laminate as it passes through facilities or devices for subsequent stretching (TD), extraction, and heat setting, thereby effectively eliminating structural non-uniformities, such as excessive variations in the thickness and porosity of the outermost layer, that appear in conventional multilayer separators.

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

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

[0047] Thereafter, a predetermined extraction solvent is applied to the laminate, and the first and second pore-forming agents can be simultaneously and selectively extracted and removed from the laminate, specifically, from the first and second precursor films that constitute the laminate. The first and second pore-forming agents can be extracted and removed by immersing the laminate in an impregnation tank containing a solution containing the extraction solvent for a predetermined period of time.

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

[0049] In addition, the laminate from which the first and second pore-forming agents have been extracted and removed may be heated to remove the extraction solvent remaining in the laminate. Some of the extraction solvent applied in step (e) may remain on the surface and / or inside the laminate. The remaining extraction solvent may deteriorate the properties of subsequent processes and the separator manufactured therethrough. Therefore, the extraction solvent remaining in the laminate may be removed by appropriately heating the laminate at a temperature above the boiling point of the extraction solvent.

[0050] The method for manufacturing the multilayer separator may further include a step (e') of heat-setting the laminate after step (e). The heat-setting refers to a process of applying heat to the laminate in a fixed state to forcibly fix the laminate that is prone to shrinkage and remove residual stress. A high heat-setting temperature is advantageous for reducing the shrinkage rate, but if the temperature is too high, the laminate may partially melt, closing the formed pores and reducing the permeability.

[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 a heat setting temperature within this range can prevent problems such as insufficient rearrangement of polyolefin molecules within the laminate, resulting in ineffective residual stress relief in the film, and partial melting, resulting in pore closure and reduced transmittance. 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 produced by the above production 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) x 100

[0053] Conventional methods for manufacturing multilayer separators involve interlayer lamination (i.e., stacking) by coextrusion prior to the formation of pore structures by stretching, which can lead to problems with non-uniform structural properties of each layer during subsequent film-forming processes such as stretching and extrusion. Specifically, as thickness deviations between regions of the outermost layer of a multilayer separator increase, the mechanical properties of the multilayer separator itself can become non-uniform, leading to problems such as the increased susceptibility to breakage or fracture in areas significantly thinner than surrounding regions during the assembly of a battery using the multilayer separator. These problems are particularly pronounced in thin-film separators (thicknesses of approximately 15 μm or less, preferably approximately 10 μm or less) that address the recent trend toward increased battery integration and capacity.

[0054] In contrast, the multilayer separator is manufactured by laminating the first and second precursor films, which have been stretched in the machine direction (MD) to form a predetermined pore structure, and then stretching the resulting laminate in the transverse direction (TD), followed by extracting and removing the first and second pore-forming agents. This method makes the structural properties of each layer constituting the multilayer separator uniform and ensures stable mechanical properties, while minimizing the number of stretching machines required to manufacture the multilayer separator, thereby improving productivity and economy.

[0055] Figure 3 shows a method for measuring the thickness deviation of a multi-layer 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 piece measured using the following method and the above formula. In the above formula, the thickness deviation is determined by a method including the steps of: cutting the multi-layer separator into 100 mm x 100 mm (MD x TD), then dividing it into five equal parts in the machine direction (MD) to obtain five test pieces each measuring 20 mm x 100 mm (MD x TD); measuring the thickness of the outermost layer at the center of the test piece in the cross direction (TD); and calculating the thickness deviation using the above formula based on the maximum and minimum thickness values.

[0056] The multilayer separator can also satisfy at least one, and preferably all, of the following conditions (i) to (vi):

[0057] (i) thickness of 1 to 15 μm, preferably 5 to 12 μm; (ii) puncture strength of 600 gf or more, preferably 700 to 1,000 gf, more preferably 750 to 900 gf; (iii) tensile strength in the machine direction (MD) of 1,300 to 2,000 kgf / cm 2 , preferably 1,400 to 1,800 kgf / cm 2 (iv) Transverse direction (TD) tensile strength: 3,000 to 6,000 kgf / cm 2 , preferably 3,500 to 5,500 kgf / cm 2 , and more preferably 4,000 to 5,400 kgf / cm 2 (v) Machine direction (MD) tensile elongation of 150 to 450%, preferably 200 to 400%, more preferably 230 to 350%, and (vi) Transverse direction (TD) tensile elongation of 30 to 100%, preferably 40 to 90%. Hereinafter, embodiments of the present invention will be described in detail.

[0058] Example 1 A first composition, which was a mixture of 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 extruded material was discharged 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 70°C to produce a first sheet with a thickness of 800 μm.

[0059] The second composition, which was the same as the first composition, was fed into a twin-screw extruder (second extruder, inner diameter 58 mm, L / D = 56) that had the same configuration as the first extruder and was arranged in parallel. The extruded material was discharged 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 70°C to produce a second sheet having a thickness of 800 μm.

[0060] The first and second sheets were fed into first and second stretching machines (roll stretching machines) arranged parallel to each other and stretched 8 times in the machine direction (MD) at 120°C to produce first and second precursor films. Here, the first and second stretching machines refer to stretching machines installed downstream of the first and second extruders, respectively.

[0061] The first and second precursor films were fed into a laminator installed downstream of the first and second stretching machines, and the first and second precursor films were laminated together to obtain a laminate.

[0062] The laminate was placed in a third stretching machine (tenter stretching machine) and stretched 9 times in the transverse direction (TD) at 126°C, then immersed in a dichloromethane leaching tank at 25°C for 1 minute to extract and remove the paraffin oil, and dried for 5 minutes at 38°C. The laminate was stretched 1.5 times in the transverse direction (TD) at 138°C, then relaxed 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) having a weight average molecular weight of 600,000 in the second composition was replaced with ultra high molecular weight polyethylene (UHMWPE) having 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 wt %, and the contents (parts by weight) of the high-density polyethylene, ethylene vinyl acetate, and paraffin oil in the second composition were 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) having a weight average molecular weight of 600,000 in the second composition was replaced with an ultra high molecular weight polyethylene (UHMWPE) having 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 an ultrasonic wave of 50 kHz was applied to the entire surface of the laminate during lamination to bond (fuse) the interface of the laminate.

[0067] Example 6 A separator was manufactured in the same manner as in Example 2, except that an ultrasonic wave of 50 kHz was applied to the entire surface of the laminate during lamination to bond (fuse) the interface of the laminate.

[0068] Example 7 A separator was manufactured in the same manner as in Example 3, except that an ultrasonic wave of 50 kHz was applied to the entire surface of the laminate during lamination to bond (fuse) the interface of the laminate.

[0069] Example 8 A separator was manufactured in the same manner as in Example 4, except that an ultrasonic wave of 50 kHz was applied to the entire surface of the laminate during lamination to bond (fuse) the interface of the laminate.

[0070] Comparative Example 1 25 parts by weight of a mixture containing 60% by weight of ultra-high molecular weight polyethylene having a weight average molecular weight of 2,000,000 and 40% by weight of high-density polyethylene having a weight average molecular weight of 560,000, and 75 parts by weight of paraffin oil having a kinematic viscosity of 35 cSt at 40°C were charged 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 230°C to produce a first polyolefin solution.

[0071] 30 parts by weight of a mixture 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 charged 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 fed from the first and second extruders to a three-layer T-die, respectively, and extruded so that the layer thickness ratio of the first / second / first polyolefin solutions was 35 / 30 / 35. The extrudate was cooled while being taken up by a cooling roll (500 mm diameter) at 37°C to produce a three-layer gel sheet. The three-layer gel sheet was simultaneously biaxially stretched 5x5 times (MDxTD) at 114°C and then immersed in a dichloromethane leaching tank at 25°C for 5 minutes to extract and remove the paraffin oil, producing a porous membrane. The porous membrane was placed in a tenter stretching machine and re-stretched 1.4 times in the transverse direction (TD) at 124°C, followed by heat setting 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). The crosslinking catalyst, dibutyltin laurate, was pre-dispersed in the paraffin oil and added via the twin-screw extruder's side injector at 0.5 wt% based on the total weight of the material passing through the twin-screw extruder. The material was discharged from the twin-screw extruder through a 300 mm T-die at a screw rotation speed of 40 rpm and 200°C, and then passed through a casting roll at 40°C to produce a base sheet with a thickness of 800 μm.

[0074] The base sheet was stretched 6 times in the machine direction (MD) using a roll stretcher at 110°C and 7 times in the transverse direction (TD) using a tenter stretcher at 125°C to produce a film. The film was immersed 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. The film was then heated at 125°C using a tenter stretcher, stretched 1.45 times in the transverse direction (TD), relaxed, and heat-set to 1.25 times its original stretching ratio. The film was crosslinked for 72 hours in a constant temperature and humidity tank at 85°C and 85% humidity to produce a separator.

[0075] Comparative Example 3 30 parts by weight of 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 were mixed with 70 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C and 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 200°C, and then passed through a casting roll heated to 40°C to produce a base sheet with a thickness of 800 μm.

[0076] The base sheet was stretched 6 times in the machine direction (MD) using a roll stretcher at 110°C, and then stretched 7 times in the transverse direction (TD) using a tenter stretcher at 125°C to produce a film. The film was immersed in a dichloromethane leaching tank at 25°C for 1 minute to extract and remove the paraffin oil, and then dried at 50°C for 5 minutes to produce a porous film. The film was then heated at 125°C using a tenter stretcher, stretched 1.45 times in the transverse direction (TD), relaxed, and heat-set to 1.25 times its original stretching size, producing 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 wt% of high-density polyethylene (HDPE) with a weight-average molecular weight (Mw) of 600,000 and 58 wt% of ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight (Mw) of 1,500,000 was mixed with 70 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C, and the mixture was introduced 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 charged into a twin-screw extruder (first extruder, inner diameter 58 mm, L / D = 42). 250 parts by weight of paraffin oil was supplied via a side injector of the first extruder, and the mixture was melt-kneaded at 210°C and 100 rpm to produce a 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 fed via a 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 multiblock (layering device) of an extruder to alternately layer the first and second polyethylene solutions, and then extruded through a T-die to form a multilayer structure consisting of 20 layers.

[0081] The multilayer structure was passed through a cooling roll adjusted to 40°C to produce a gel-like base sheet with a thickness of 1,100 μm. The base sheet was simultaneously biaxially stretched 8x8 times (MDxTD) at 124°C, then immersed in a dichloromethane leaching tank at 25°C for 10 minutes to extract and remove the paraffin oil, and dried at 50°C for 5 minutes to produce a porous membrane. The porous membrane was placed in a tenter stretching machine and re-stretched 1.4 times in the transverse direction (TD) at 125°C, followed by heat setting to produce a separator.

[0082] Comparative Example 6 A separator was manufactured using the method shown in Figure 4 as follows. A first composition, which was a mixture of 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 extruded material was discharged 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 heated to 70°C to produce a first sheet with a thickness of 800 μm.

[0083] The second composition, which was the same as the first composition, was fed into a twin-screw extruder (second extruder, inner diameter 58 mm, L / D = 56) that had the same configuration as the first extruder and was arranged in parallel. The extruded material was discharged 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 70°C to produce a second sheet having a thickness of 800 μm.

[0084] The first and second sheets were fed into a laminator installed downstream of the first and second extruders, where the first and second sheets were laminated together to produce a laminate. The laminate was then fed into a stretching machine (roll stretching machine) and stretched 8 times in the machine direction (MD) at 120°C to produce a precursor film. The precursor film was then fed into a tenter stretching machine and stretched 9 times in the transverse direction (TD) at 126°C, after which it was immersed in a dichloromethane leaching tank at 25°C for 1 minute to extract and remove the paraffin oil, and then dried at 38°C for 5 minutes. The laminate was stretched 1.5 times in the transverse direction (TD) at 138°C, relaxed 10%, and heat-set to produce a separator.

[0085] Experimental Example 1 Each separator manufactured in the above examples and comparative examples was cut to 100 mm x 100 mm, and then the cut separator test pieces were divided into 5 equal parts in the machine direction (MD) to obtain test pieces 1 to 5 of 20 mm x 100 mm (MD x TD).

[0086] The cross section of each specimen was photographed using an SEM, and the thickness (μm) of the outermost layer was measured at the center of the transverse direction (TD) of each specimen. Based on this, the thickness deviation defined by the following formula was calculated and shown in Table 1 below. <expression> Thickness deviation (%) = {(maximum thickness) - (minimum thickness)} / (minimum thickness) x 100

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

[0088] [Table 1]

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

[0090] - Thickness (μm): The thickness of the support test piece was measured using a micro thickness measuring instrument. - Puncture strength (gf): Using a puncture strength tester, 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 punctured was measured.

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

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

[0093] [Table 2]

[0094] The above description of the present invention is for illustrative purposes only, and those skilled in the art will understand that the present invention may be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. Therefore, the above-described embodiments should be understood to be illustrative in all respects and not restrictive. For example, each component described as a single component may be implemented in a distributed form, and similarly, each component described as a distributed component may be implemented in a combined form.

[0095] The scope of the present invention is defined by the claims that follow, and it should be understood that all modifications and variations that fall within the meaning and scope of the claims and their equivalents are included within the scope of the present invention.

Claims

1. (a) extruding a first composition comprising a first polyolefin and a first pore former to produce a first sheet; (b) extruding a second composition comprising a second polyolefin and a second pore former to produce a second sheet; (c) stretching the first and second sheets in the machine direction (MD), respectively, to produce first and second precursor films; (d) laminating the first and second precursor films to obtain a laminate; and (e) removing the first and second pore formers from the laminate after stretching the laminate in the transverse direction (TD).

2. 2. The method for producing a multilayer separator according to claim 1, wherein the first and second polyolefins each comprise one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, and combinations or copolymers of two or more thereof.

3. 3. The method for producing a multilayer separator according to claim 2, wherein the weight average molecular weight of each of the first and second polyolefins is 300,000 to 2,000,000.

4. 2. The method for manufacturing a multi-layer separator according to claim 1, wherein the first and second pore-forming agents are each paraffin oil having a kinematic viscosity at 40° C. of 50 to 100 cSt.

5. The method for manufacturing a multilayer separator according to claim 1 , wherein the first or second composition further comprises a hydrophilic polymer.

6. 6. The method for producing a multilayer separator according to claim 5, wherein the content of the hydrophilic polymer in the first or second composition is 0.1 to 5 wt %.

7. 7. The method for producing a multilayer separator according to claim 6, 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 combinations of two or more thereof.

8. 2. The method for producing a multilayer separator according to claim 1, wherein the first and second precursor films are pressed against each other during lamination, and at least a portion of the interface between the first and second precursor films is adhered to each other.

9. A multilayer separator manufactured by the manufacturing method according to any one of claims 1 to 8, A multi-layer separator having an outermost layer with a thickness deviation of 10% or less as measured by the following formula: <Formula> Thickness deviation (%) = {(maximum thickness) - (minimum thickness)} / (minimum thickness) x 100 In the above formula, the thickness deviation is determined by a method including the steps of: cutting the multilayer separator to a size of 100 mm x 100 mm (MD x TD), then dividing it into five equal parts in the machine direction (MD) to obtain five test pieces each having a size of 20 mm x 100 mm (MD x TD); measuring the thickness of the outermost layer at the center of the test piece in the machine direction (TD); and calculating the thickness deviation using the formula based on the maximum and minimum thickness values.

10. The multilayer separator of claim 9, wherein the multilayer separator satisfies at least one of the following conditions (i) to (vi): (i) a thickness of 1 to 15 μm; (ii) Puncture strength of 600 gf or more, (iii) Machine direction (MD) tensile strength: 1,300 to 2,000 kgf / cm ２ , (iv) Transverse (TD) tensile strength: 3,000 to 6,000 kgf / cm ２ , (v) a machine direction (MD) tensile elongation of 150 to 450%, and (vi) Transverse direction (TD) tensile elongation of 30 to 100%.

Images