Separator manufacturing method

Abstract

One aspect of the present invention provides a method for producing a separator, comprising the steps of: (a) extruding and stretching a composition comprising a crosslinkable polyolefin in which a silane-based compound has been grafted onto a first polyolefin, a non-crosslinkable polyolefin consisting of a second polyolefin, and a pore-forming agent to produce a base film; (b) applying a solution containing a crosslinking catalyst and an extraction solvent to the base film to coat the crosslinking catalyst on a surface of the base film while extracting the pore-forming agent from the base film; and (c) applying heat and steam to the base film to crosslink the crosslinkable polyolefin while removing the extraction solvent remaining in the base film.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a separator, and more particularly to a method for manufacturing a porous separator for a lithium secondary battery. [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] As a means to achieve this goal, research and development is being actively conducted on separators with micropores that separate the positive and negative electrodes to prevent internal shorts and facilitate the movement of lithium ions during charging and discharging. In particular, microporous separators made of polyolefins such as polyethylene are advantageous in forming pores through thermally induced phase separation, are economical, and easily satisfy the physical properties required for separators.

[0004] However, separators made of polyethylene, which has a low melting point of around 135°C, can shrink and deform at temperatures above the melting point due to heat generated by the battery. If this deformation causes a short circuit, it can lead to thermal runaway in the battery, resulting in safety issues such as fire. To solve this problem, a method has been proposed in which polyolefin separators are crosslinked to improve their heat resistance.

[0005] Patent Documents 1 and 2 disclose inventions that improve heat resistance by producing a crosslinked separator using a silane-modified polyolefin. However, the physical properties of the produced separator are 25 μm thick, 900 sec / 100 ml air permeability, and 200 gf puncture strength, which are significantly inferior to the physical properties of currently commercially available separators, which are 12 μm thick or less, 150 sec / 100 ml or less air permeability, and 250 gf or more, making them virtually unusable for commercial use.

[0006] Patent Document 3 discloses a method for manufacturing a separator by mixing ultra-high molecular weight polyethylene (ULHMWPE) with a silane-modified polyolefin, but the ultra-high molecular weight polyethylene has a drawback in that it has poor dispersibility with the silane-modified polyolefin, which results in uneven distribution of the manufactured separator, resulting in a high waste rate, and uneven distribution of the silane-crosslinked polyolefin in certain regions, making it difficult to obtain a separator with uniform physical properties.

[0007] Patent Documents 4 and 5 disclose a method for producing a polyolefin separator by feeding a polyolefin, a diluent, an alkoxy group-containing vinylsilane, and an initiator into an extruder, mixing them, and then reactively extruding the mixture to produce a silane-grafted polyolefin solution, which is then stretched, extracted, and crosslinked. However, this method has the problem that the alkoxy group-containing vinylsilane is grafted to the diluent during reactive extrusion, making it impossible to regenerate the diluent, and unreacted alkoxy group-containing vinylsilane elutes during extraction, resulting in poor productivity and economic efficiency.

[0008] In particular, Patent Document 5 discloses that productivity can be improved by performing a series of in-line processes to reduce the time and cost required for crosslinking a separator, including applying a crosslinking catalyst to a film from which the pore-forming agent has been extracted and removed, and then adding water and / or steam to crosslink the film. However, because the individual processes and equipment that make up this in-line process are independent of each other, it is difficult to improve productivity beyond a certain level, and the time required for crosslinking to achieve a predetermined meltdown temperature is unclear, limiting the ability to quantify the degree of productivity improvement.

[0009] Patent Document 6 discloses a method for manufacturing a separator by crosslinking a silane-modified polyolefin contained in a porous membrane and a separator manufactured by the method. However, the crosslinking process is carried out in the presence of moisture, and the crosslinking time required is a minimum of 10 minutes, limiting productivity beyond that. Furthermore, the trade-off between the mechanical properties and heat resistance of the separator cannot be adequately resolved. Furthermore, when manufacturing such a separator, side reactions, including the crosslinking reaction between the silane-modified polyolefin and the matrix resin, can cause a decrease in the surface quality of the product. These side reactions can impede ion mobility between the battery's driving components, resulting in a decrease in the battery's electrochemical properties. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 11-144700 [Patent Document 2] Japanese Patent Application Publication No. 11-172036 [Patent Document 3] Patent No. 4583532 [Patent Document 4] Korean Patent Registration No. 1857156 [Patent Document 5] Korean Patent Publication No. 10-2021-0001785 [Patent Document 6] Korean Patent Registration No. 1955911 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention has been made to solve the above-mentioned problems of the conventional art, and an object of the present invention is to provide a method for manufacturing a separator that can achieve a balanced combination of mechanical properties and heat resistance while shortening the time required for crosslinking and other processes to achieve the required levels of mechanical properties and heat resistance, thereby significantly improving productivity. [Means for solving the problem]

[0012] One aspect of the present invention provides a method for producing a separator, comprising: (a) extruding and stretching a composition containing a crosslinkable polyolefin in which a silane-based compound has been grafted onto a first polyolefin, a non-crosslinkable polyolefin consisting of a second polyolefin, and a pore-forming agent to produce a base film; (b) applying a solution containing a crosslinking catalyst and an extraction solvent to the base film to extract the pore-forming agent from the base film while coating the surface of the base film with the crosslinking catalyst; and (c) applying heat and steam to the base film to remove the extraction solvent remaining in the base film while crosslinking the crosslinkable polyolefin.

[0013] In one embodiment, the weight average molecular weights (Mw) of the first and second polyolefins may be 1,000 to 300,000 and 300,000 to 2,000,000, respectively, and the ratio of the weight average molecular weight (Mw) of the first polyolefin to the weight average molecular weight (Mw) of the second polyolefin may be 0.0005 to 1.

[0014] In one embodiment, the first and second polyolefins may each be one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations of two or more thereof.

[0015] In one embodiment, the silane-based compound may be a vinyl silane containing an alkoxy group.

[0016] In one embodiment, the content of the crosslinkable polyolefin in the crosslinkable polyolefin and the non-crosslinkable polyolefin may be 5 to 90% by weight.

[0017] In one embodiment, the content of the silane compound in the crosslinkable polyolefin may be 10 to 50% by weight.

[0018] In one embodiment, in step (c), while applying heat by contacting at least one surface of the base film with a heating element having a first temperature, steam having a second temperature can be applied to at least one surface of the base film using an injection element.

[0019] In one embodiment, the first temperature may be 40 to 70°C, and the second temperature may be 80 to 200°C.

[0020] In one embodiment, the injection member may be provided at least one of inside and outside the heating member.

[0021] Another aspect of the present invention provides a separator manufactured by the above method, wherein the meltdown temperature of the separator is 200 to 300°C, as measured one hour after the start of step (c) by applying a force of 0.01 N to the separator in the machine direction (MD) and the machine direction (TD), and then increasing the temperature at a rate of 5°C / min. [Effects of the Invention]

[0022] In one aspect of the present invention, a method for manufacturing a separator includes a step of applying a crosslinking catalyst and an extraction solvent to a base film, and a step of drying and crosslinking the base film, all in a single process. This allows for balanced mechanical properties and heat resistance while shortening the time required for crosslinking and other processes required to achieve the required levels of mechanical properties and heat resistance, thereby significantly improving productivity.

[0023] 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]

[0024] [Figure 1] 3 illustrates a method for manufacturing a separator according to one embodiment of the present invention. [Figure 2] 1 illustrates a single step for drying and crosslinking according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described below with reference to the accompanying drawings. 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.

[0026] 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.

[0027] 1 shows a method for manufacturing a separator according to one embodiment of the present invention. Referring to FIG. 1, the method for manufacturing a separator according to one embodiment of the present invention may include the steps of: (a) extruding and stretching a composition containing a crosslinkable polyolefin in which a silane-based compound is grafted onto a first polyolefin, a non-crosslinkable polyolefin consisting of a second polyolefin, and a pore-forming agent to produce a base film; (b) applying a solution containing a crosslinking catalyst and an extraction solvent to the base film to extract the pore-forming agent from the base film while coating the crosslinking catalyst on the surface of the base film; and (c) applying heat and steam to the base film to crosslink the crosslinkable polyolefin while removing the extraction solvent remaining in the base film.

[0028] In the step (a), a composition including a crosslinkable polyolefin in which a silane compound is grafted onto a first polyolefin, a non-crosslinkable polyolefin consisting of a second polyolefin, and a pore-forming agent is extruded, discharged through a T-die, and then stretched to produce a base film.

[0029] The crosslinkable polyolefin can be produced by reacting a first polyolefin having a weight average molecular weight (Mw, g / mol) of 1,000 to 300,000, preferably 1,000 to 200,000, and more preferably 4,000 to 100,000 and a molecular weight distribution (Mw / Mn) of 3 to 7, a silane compound, and an initiator, and the amounts and ratios of the reactants can be adjusted so that the content of the silane compound in the crosslinkable polyolefin is 10 to 50 wt %.

[0030] The non-crosslinkable polyolefin may be a second polyolefin having a weight average molecular weight (Mw) of 300,000 to 2,000,000, preferably 300,000 to 1,000,000, and more preferably 300,000 to 700,000, and a molecular weight distribution (Mw / Mn) of 3-7.

[0031] If the molecular weight distribution of the first and second polyolefins is less than 3, the dispersibility with the pore-forming agent may decrease, and the uniformity of the separator may decrease. If the molecular weight distribution is greater than 7, the mechanical properties of the separator may decrease.

[0032] The ratio of the weight average molecular weight (Mw) of the first polyolefin to the weight average molecular weight (Mw) of the second polyolefin may be 0.0005 to 1, preferably 0.001 to 0.7, and more preferably 0.001 to 0.5. If the ratio of the weight average molecular weight (Mw) of the first polyolefin to the weight average molecular weight (Mw) of the second polyolefin is outside the above range, the compatibility and dispersibility of the crosslinkable polyolefin and the non-crosslinkable polyolefin may decrease, the deviation in mechanical properties between regions of the separator may increase, and the reliability and reproducibility of the product may decrease.

[0033] The first and second polyolefins may each be one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations of two or more thereof, preferably polyethylene and / or polypropylene, and more preferably polyethylene, but are not limited thereto.

[0034] The silane compound may be a vinylsilane containing an alkoxy group, which may be, for example, one selected from the group consisting of trimethoxyvinylsilane, triethoxyvinylsilane, triacetoxyvinylsilane, 3-(methacryloxypropyl)trimethoxysilane, and a combination of two or more thereof, preferably trimethoxyvinylsilane, but is not limited thereto.

[0035] The pore-forming agent may be one selected from the group consisting of paraffin oil, 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, preferably paraffin oil, and more preferably paraffin oil having a kinematic viscosity of 50 to 100 cSt at 40°C, but is not limited thereto.

[0036] The composition may contain 1 to 40 wt% of the crosslinkable polyolefin, 1 to 40 wt% of the non-crosslinkable polyolefin, and 40 to 80 wt% of the pore-forming agent. If the content of the crosslinkable polyolefin in the composition is less than 1 wt%, the crosslinking reaction is inhibited and the required level of mechanical properties cannot be achieved. If the content is more than 40 wt%, it may be difficult to achieve the properties required for a commercial separator.

[0037] The stretching can be carried out by a known method such as uniaxial stretching or biaxial stretching (sequential or simultaneous biaxial stretching). In the case of sequential biaxial stretching, the stretching ratio may be 4 to 20 times in each of the transverse direction (MD) and the longitudinal direction (TD), and the resulting areal stretching ratio may be 16 to 400 times.

[0038] In step (b), a single solution containing a crosslinking catalyst and an extraction solvent is applied to the base film, thereby extracting the pore-forming agent from the base film and simultaneously applying the crosslinking catalyst to the surface of the base film (the surfaces of the internal and external pores).

[0039] The extraction solvent can selectively extract and remove the pore-forming agent from the base film, and examples of the extraction solvent include, but are not limited to, methyl ethyl ketone, hexane, and dichloromethane.

[0040] The pore-forming agent can be extracted and removed by immersing the base film in an impregnation bath containing a solution containing the extraction solvent for a predetermined time. After extraction, the content of the pore-forming agent remaining on the surface and / or inside of the base film may be 1 wt % or less.

[0041] The solution may contain the extraction solvent and the crosslinking catalyst. The solution may further contain a crosslinking catalyst to promote the crosslinking reaction in the subsequent step (c). Examples of such crosslinking catalysts include carboxylates of metals such as tin, zinc, iron, lead, and cobalt, organic bases, inorganic acids, and organic acids. For example, the crosslinking catalyst may be inorganic acids such as dibutyltin dilaurate, dibutyltin diacetate, stannous acetate, stannous caprylate, zinc naphthenate, zinc caprylate, cobalt naphthenate, ethylamine, dibutylamine, hexylamine, pyridine, sulfuric acid, and hydrochloric acid; organic acids such as toluenesulfonic acid, acetic acid, stearic acid, and maleic acid; or 1,8-diazabicyclo(5,4,0)undec-7-ene, preferably, but not limited to, dibutyltin dilaurate.

[0042] The crosslinking catalyst has been applied by adding it during the preparation of the crosslinkable polyolefin or by coating a separately prepared crosslinking catalyst solution on the base film. However, these conventional methods make it difficult to uniformly disperse the crosslinking catalyst in the crosslinkable polyolefin. As mentioned above, the crosslinkable polyolefin contained in the base film must be uniformly dispersed before crosslinking, and the crosslinking catalyst involved in the crosslinking reaction of the crosslinkable polyolefin must also be uniformly dispersed.

[0043] In contrast, by premixing the crosslinking catalyst with the extraction solvent to prepare a single solution, the pore-forming agent is extracted and removed from the base film, and the crosslinking catalyst is then applied to the base film. This allows the crosslinking catalyst to be in-situ filled not only on the surface of the base film but also inside the pores formed by the extraction and removal of the pore-forming agent, thereby effectively penetrating and being introduced into the base film. The crosslinking catalyst uniformly introduced into the surface and inside of the base film can uniformly carry out the crosslinking reaction of the crosslinkable polyolefin in all directions and across the entire area of ​​the separator, thereby contributing to improving the heat resistance and mechanical properties of the separator.

[0044] The content of the crosslinking catalyst in the solution may be 0.01 to 5% by weight. If the content of the crosslinking catalyst is less than 0.01% by weight, the crosslinking reaction cannot be promoted to the required level, whereas if the content is more than 5% by weight, the reaction rate converges to the required level, which is disadvantageous from the viewpoints of economy and productivity.

[0045] The time required for extracting and removing the pore-forming agent and applying the crosslinking catalyst in step (b) can be determined depending on the thickness and porosity of the base film, and when the thickness and porosity of the base film are 10 to 30 μm and 40 to 60% by volume, respectively, the time may be 10 minutes or less, preferably 5 minutes or less, and more preferably 1 minute or less.

[0046] In step (c), the pore-forming agent is extracted, and heat and steam are applied to the base film coated with the crosslinking catalyst to remove the extraction solvent remaining in the base film, thereby crosslinking the crosslinkable polyolefin.

[0047] A portion of the extraction solvent applied in step (b) may remain on the surface and / or inside of the base film. The remaining extraction solvent may deteriorate the properties of subsequent processes and the separator manufactured therethrough. Therefore, the base film may be appropriately heated at a temperature equal to or higher than the boiling point of the extraction solvent to remove the remaining extraction solvent from the base film.

[0048] The steam applied to at least one surface of the base film together with heat in step (c) can activate the crosslinking catalyst applied to the base film in step (b) and initiate and accelerate the crosslinking reaction of the crosslinkable polyolefin contained in the base film. In addition, the inherent heat of the steam can also contribute to removing the extraction solvent remaining in the base film.

[0049] The heat and steam simultaneously applied in step (c) can remove the extraction solvent remaining in the base film, and the steam can activate the crosslinking catalyst applied to the base film to initiate and promote the crosslinking reaction of the crosslinkable polyolefin contained in the base film.

[0050] In particular, by simultaneously removing the extraction solvent and activating the crosslinking catalyst, it is possible to prevent a portion of the crosslinking catalyst from being arbitrarily lost along with the extraction solvent due to the heat, and to induce at least 95 wt % of the crosslinking catalyst applied to the surface of the base film (the surface of the internal and external pores) in step (b) to participate in the crosslinking reaction of the crosslinkable polyolefin. In other words, since the removal of the extraction solvent and the crosslinking of the crosslinkable polyolefin can be performed simultaneously in a single step in step (c), the heat resistance and productivity of the separator can be improved in a balanced manner.

[0051] 2 shows a single process for drying and crosslinking according to one embodiment of the present invention. Referring to FIG. 2, in step (c), a heating element 100 having a first temperature is brought into contact with at least one surface of the base film 10 to apply heat to the base film 10, while a spray element 200 is used to apply steam S having a second temperature to at least one surface of the base film 10.

[0052] The heating member 100 may be a thermally conductive member that contacts at least one surface of the base film 10 and can transfer a predetermined amount of heat to the base film 10. The thermally conductive member may be, for example, a fixed panel or a rotating roll made of a metal material, but is not limited thereto. When the heating member 100 is a roll, the rotation speed of the roll may be adjusted so that the base film 10 moves in the machine direction (MD) at a speed of 5 to 100 m / min, preferably 10 to 80 m / min, and more preferably 15 to 50 m / min.

[0053] The extraction solvent remaining in the base film 10 can be evaporated and removed by heat transferred from the base film 10. The temperature of the heating member 100 required to remove the extraction solvent, i.e., the first temperature, may be 40 to 70°C.

[0054] The injection member 200 may be provided inside or outside the heating member 100, and may include a heater that heats water to generate steam having the second temperature and a nozzle that injects the generated steam at a predetermined position. The second temperature may be adjusted to a range necessary to activate the crosslinking catalyst and initiate a crosslinking reaction of the crosslinkable polyolefin, for example, 80 to 200°C, preferably 100 to 150°C.

[0055] 2(a), the injection member 200 may be separately provided outside the heating member 100. The steam S may be injected toward at least one surface of the base film 10 and directly applied to at least one surface of the base film 10, or may be retained in a predetermined space so that at least one surface of the base film 10 is exposed to the retained steam, or may be applied to at least one surface of the base film 10 by a combination of these methods.

[0056] 2(b), the injection member 200 may be built into the heating member 100. In this case, the steam S may be injected toward at least one surface of the base film 10 and directly applied to the contact surface between the heating member 100 and the base film 10. The steam S applied to the contact surface of the base film 10 may permeate the base film 10 in the thickness direction through the pores of the base film 10. During this process, the crosslinking catalyst applied to the internal pores of the base film 10 may also be properly activated by the steam S, thereby further improving the heat resistance and mechanical properties of the manufactured separator.

[0057] The separator manufactured by the method may include a crosslinked product of the crosslinkable polyolefin and the non-crosslinkable polyolefin. The crosslinked product may be uniformly dispersed in the non-crosslinkable polyolefin, and may be formed by crosslinking at least a portion of the silane-based compounds grafted onto the main chain of the crosslinkable polyolefin under certain conditions.

[0058] The content of the crosslinkable polyolefin in the crosslinkable polyolefin and non-crosslinkable polyolefin constituting the separator is 5 to 90 wt%, preferably 5 to 50 wt%, and more preferably 10 to 50 wt%. If the content of the crosslinkable polyolefin is less than 5 wt%, the required meltdown temperature cannot be achieved, and if it is more than 90 wt%, the brittleness of the separator increases and the tensile strength may decrease.

[0059] The content of the silane compound in the crosslinkable polyolefin may be 10 to 50 wt %. If the content of the silane compound in the crosslinkable polyolefin is less than 10 wt %, the required meltdown temperature cannot be achieved, and if it is more than 50 wt %, the brittleness of the separator increases, the tensile strength decreases, and excessive oil vapor is generated during the production of the separator, which may reduce processability and workability.

[0060] One hour after the start of the step (c), a force of 0.01 N is applied to the separator in the machine direction (MD) and the transverse direction (TD) using a thermomechanical analyzer (TMA), and the separator is then heated at a rate of 5°C / min. The meltdown temperature of the separator measured at the temperature at which the separator melts and fractures may be 200 to 300°C.

[0061] In the separator, the crosslinked product of the crosslinkable polyolefin and the non-crosslinkable polyolefin can constitute a discontinuous phase and a continuous phase, respectively. In the separator, the crosslinkable polyolefin is crosslinked in the continuous phase matrix made of the non-crosslinkable polyolefin, and firmly supports and fixes the matrix, thereby improving the heat resistance and mechanical properties of the separator.

[0062] The term "matrix" as used herein refers to a component that constitutes a continuous phase in a separator containing two or more components, i.e., the non-crosslinkable polyolefin including the second polyolefin may exist as a continuous phase in the separator, and the crosslinked product of the crosslinkable polyolefin may exist as a discontinuous phase therein.

[0063] In the past, in the process of manufacturing a separator containing a polyolefin and a crosslinkable compound, for example, a silane-based compound, attempts have been made to produce a crosslinked separator by coating the silane-based compound on a base sheet before extracting a pore-forming agent, followed by grafting, or by pre-mixing the polyolefin and silane-based compound, followed by preparing a graft and base sheet. While these methods have achieved satisfactory physical properties for a commercial separator, they have the disadvantage of grafting the silane-based compound with other components and / or compositions in addition to the polyolefin, which require disposal of these components and / or compositions after each process, resulting in a sharp increase in manufacturing costs.

[0064] Since the crosslinking of the crosslinkable polyolefin is carried out simultaneously with drying in a series of processes including extrusion, stretching, extraction, drying, etc., it is necessary to adjust the distribution of the crosslinkable polyolefin uniformly in the porous film into which the pore-forming agent has been extracted, and to adjust the distribution of the crosslinked product uniformly throughout the separator. If the crosslinking reaction of the crosslinkable polyolefin occurs preferentially in a certain region of the separator, the required level of mechanical properties cannot be achieved, and in particular, the deviation of mechanical properties depending on the region of the separator becomes large, which may significantly reduce the reliability and reproducibility of the product.

[0065] As described above, in a separator having a structure in which a crosslinked product of the crosslinkable polyolefin, in which a silane compound is grafted onto the first polyolefin, is dispersed in the non-crosslinkable polyolefin containing the second polyolefin, by adjusting the content of the crosslinked product in the separator, the content of the silane compound in the crosslinkable polyolefin, and / or the weight average molecular weights and ratios of the first and second polyolefins, or by performing the steps (b) and (c) of applying a crosslinking catalyst and an extraction solvent to the base film and the steps of drying and crosslinking the base film in a single step, it is possible to achieve a balanced level of mechanical properties and heat resistance while shortening the time required for crosslinking and other steps to achieve the required levels of mechanical properties and heat resistance, thereby significantly improving productivity.

[0066] For example, one hour after the start of step (c), a force of 0.01 N is applied to the separator in both the machine direction (MD) and the transverse direction (TD), and the temperature is increased at a rate of 5°C / min using a thermomechanical analyzer (TMA), and the meltdown temperature of the separator, measured at the temperature at which the separator melts and fractures, may be 200 to 300°C, preferably 205 to 280°C, and more preferably 230 to 260°C. That is, the meltdown temperature of the separator can be increased to 200°C or higher while shortening the time required for the drying and crosslinking steps in step (c) to within one hour, thereby achieving a balanced combination of separator productivity, heat resistance, and mechanical properties.

[0067] The separator can also satisfy one or more of the following conditions (i) to (viii): (i) a tensile strength in the machine direction (MD) of 2,000 kgf / cm 2 or more, preferably 2,000 to 2,500 kgf / cm 2 (ii) Transverse direction (TD) tensile strength 2,000 kgf / cm 2 or more, preferably 2,000 to 2,500 kgf / cm 2(iii) a tensile elongation in the machine direction (MD) of 90% or more, preferably 90 to 150%; (iv) a tensile elongation in the transverse direction (TD) of 100% or more, preferably 100 to 150%; (v) a heat shrinkage rate in the machine direction (MD) at 130°C of 10% or less, preferably 2 to 10%; (vi) a heat shrinkage rate in the transverse direction (TD) at 130°C of 10% or less, preferably 2 to 10%; (vii) a puncture strength of 300 gf or more, preferably 340 gf or more; (viii) a number of surface defects (white dots and / or black dots) on the surface of the separator that have a brightness different from the surrounding area and a size of 2 mm or more of 20 / m 2 Less than or equal to 15 pieces / m 2 below.

[0068] After step (c), the method may further include a step (d) of heat-setting the crosslinked base film. Heat-setting involves fixing the film and applying heat to forcibly hold the film in place while it shrinks, thereby removing residual stress. A high heat-setting temperature is advantageous for reducing shrinkage, but if the temperature is too high, the film may partially melt, closing the pores that have formed, resulting in reduced permeability.

[0069] The heat setting temperature is preferably selected within a range in which 10 to 30% by weight of the crystalline portion of the film melts, which can prevent problems such as insufficient rearrangement of polyolefin molecules in the film, resulting in ineffective removal of residual stress in the film, and partial melting, resulting in pore closure and reduced permeability.

[0070] When the crosslinkable polyolefin is crosslinked, the crystallinity of the separator decreases, which leads to an increase in the amorphous region, and the heat setting temperature may be lower than the conventional range. 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. Hereinafter, examples of the present invention will be described in detail.

[0071] Manufacturing Example 1 High-density polyethylene with a weight-average molecular weight (Mw) of 4,000 g / mol, density of 0.98 g / mL, crystallinity of 85%, and melting point of 126°C was gradually added to a 2 L round-bottom flask preheated to 200°C while stirring to melt it into a liquid state, and then trimethoxyvinylsilane was gradually added. The ratio (weight ratio) of high-density polyethylene to trimethoxyvinylsilane was 90:10, respectively.

[0072] After preparing a solution in which high density polyethylene and trimethoxyvinylsilane were completely melted, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane was gradually added as an initiator for the grafting reaction of trimethoxyvinylsilane and the mixture was stirred for 1 hour to prepare high density polyethylene grafted with trimethoxyvinylsilane (hereinafter referred to as "silane-modified PE").

[0073] Manufacturing Example 2 High-density polyethylene with a weight-average molecular weight (Mw) of 8,000 g / mol, density of 0.97 g / mL, crystallinity of 84%, and melting point of 127°C was gradually added to a 2 L round-bottom flask preheated to 200°C while stirring until melted and liquid, and then trimethoxyvinylsilane was gradually added. The weight ratio of high-density polyethylene to trimethoxyvinylsilane was 90:10.

[0074] After preparing a completely molten solution of high-density polyethylene and trimethoxyvinylsilane, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane was gradually added as an initiator for the grafting reaction of trimethoxyvinylsilane while stirring for 1 hour to prepare high-density polyethylene grafted with trimethoxyvinylsilane (hereinafter, "silane-modified PE"). 1 part by weight of the initiator was added per 100 parts by weight of trimethoxyvinylsilane. The silane-modified PE was cooled sufficiently to room temperature and crushed to prepare silane-modified PE powder.

[0075] Manufacturing Example 3 High-density polyethylene with a weight-average molecular weight (Mw) of 11,000 g / mol, density of 0.97 g / mL, crystallinity of 83%, and melting point of 128°C was gradually added to a 2 L round-bottom flask preheated to 200°C while stirring to melt it into a liquid state, and then trimethoxyvinylsilane was gradually added. The ratio (weight ratio) of high-density polyethylene to trimethoxyvinylsilane was 90:10, respectively.

[0076] After preparing a solution in which high-density polyethylene and trimethoxyvinylsilane were completely melted, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane was gradually added as an initiator for the grafting reaction of trimethoxyvinylsilane and the mixture was stirred for 1 hour to prepare high-density polyethylene grafted with trimethoxyvinylsilane (hereinafter, "silane-modified PE"). At this time, 1 part by weight of the initiator was added based on 100 parts by weight of the trimethoxyvinylsilane.

[0077] Production Example 4 54.5 parts by weight of high-density polyethylene with a weight-average molecular weight (Mw) of 100,000 and a molecular weight distribution (Mw / Mn) of 5, 30 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C, 15 parts by weight of trimethoxyvinylsilane, and 0.5 parts by weight of dicumyl peroxide were mixed and charged into a twin-screw extruder (inner diameter 58 mm, L / D = 56, twin screw extruder). The mixture was extruded from the twin-screw extruder at 200°C and a screw rotation speed of 160 rpm into a 5 mm diameter multi-strand die and passed through a water bath and a pelletizer to produce high-density polyethylene grafted with trimethoxyvinylsilane (hereinafter "silane-modified PE").

[0078] Manufacturing Example 5 54.5 parts by weight of high-density polyethylene with a weight-average molecular weight (Mw) of 200,000 and a molecular weight distribution (Mw / Mn) of 5, 30 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C, 15 parts by weight of trimethoxyvinylsilane, and 0.5 parts by weight of dicumyl peroxide were mixed and charged into a twin-screw extruder (inner diameter 58 mm, L / D = 56, twin screw extruder). The mixture was extruded from the twin-screw extruder at 200°C and a screw rotation speed of 160 rpm into a 5 mm diameter multi-strand die and passed through a water bath and a pelletizer to produce high-density polyethylene grafted with trimethoxyvinylsilane (hereinafter "silane-modified PE").

[0079] Manufacturing Example 6 54.5 parts by weight of high-density polyethylene with a weight-average molecular weight (Mw) of 300,000 and a molecular weight distribution (Mw / Mn) of 5, 30 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C, 15 parts by weight of trimethoxyvinylsilane, and 0.5 parts by weight of dicumyl peroxide were mixed and charged into a twin-screw extruder (inner diameter 58 mm, L / D = 56, twin screw extruder). The mixture was extruded from the twin-screw extruder at 200°C and a screw rotation speed of 160 rpm into a 5 mm diameter multi-strand die and passed through a water bath and a pelletizer to produce high-density polyethylene grafted with trimethoxyvinylsilane (hereinafter "silane-modified PE").

[0080] Example 1 38 parts by weight of high-density polyethylene with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, 2 parts by weight of the silane-modified polyethylene obtained in Preparation Example 1, and 0.5 parts by weight of an antioxidant were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56, twin screw extruder) using a metering feeder, and 59.5 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C was added using a side injector. The mixture was extruded from the twin-screw extruder through a 400 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 1,000 μm.

[0081] The base sheet was stretched 6 times in the machine direction (MD) using a roll stretching machine at 110°C, and then stretched 7 times in the transverse direction (TD) using a tenter stretching machine at 125°C to produce a base film. The base film was immersed in an impregnation bath containing a dichloromethane solution with a dibutyltin dilaurate concentration of 0.5 wt% and a temperature of 40°C, respectively, to extract and remove paraffin oil from the base film, while simultaneously applying dibutyltin dilaurate to the surface of the base film (the surface of the internal and external pores).

[0082] One side of the dibutyltin dilaurate-coated base film was brought into contact with a rotating roll preheated to 50°C to remove any remaining dichloromethane from the base film. At the same time, 110°C steam was sprayed toward the other side of the base film from a nozzle installed opposite the roll to crosslink the silane-modified PE in the base film. The rotation speed of the roll was adjusted so that the base film moved at a speed of 20 m / min in the machine direction (MD). The crosslinked base film was heated to 125°C in a tenter stretching machine, stretched 1.2 times in the transverse direction (TD), relaxed, and heat-set to 0.9 times its original stretching ratio to produce a separator.

[0083] Example 2 A separator was manufactured in the same manner as in Example 1, except that the silane-modified PE obtained in Preparation Example 1 was replaced with the silane-modified PE obtained in Preparation Example 2.

[0084] Example 3 A separator was manufactured in the same manner as in Example 1, except that the silane-modified PE obtained in Preparation Example 1 was replaced with the silane-modified PE obtained in Preparation Example 3.

[0085] Example 4 A separator was manufactured in the same manner as in Example 1, except that the silane-modified PE obtained in Preparation Example 1 was replaced with the silane-modified PE obtained in Preparation Example 4.

[0086] Example 5 A separator was manufactured in the same manner as in Example 1, except that the silane-modified PE obtained in Preparation Example 1 was replaced with the silane-modified PE obtained in Preparation Example 5.

[0087] Example 6 A separator was manufactured in the same manner as in Example 1, except that the silane-modified PE obtained in Preparation Example 1 was replaced with the silane-modified PE obtained in Preparation Example 6.

[0088] Experimental Example 1 The test methods for each physical property measured in the present invention are as follows: Unless otherwise specified, measurements were made at room temperature (25°C). -Thickness (μm): The thickness of the separator test piece was measured using a micro thickness measuring device. Porosity (%): The porosity of a separator specimen having a radius of 25 mm was measured using a Capillary Porometer manufactured by PMI Corporation in accordance with ASTM F316-03.

[0089] -Air permeability (Gurley, sec / 100ml): Using Asahi Seiko's Gurley Densometer EGO2-5 model, the time it takes for 100ml of air to pass through a separator test piece with a diameter of 29.8mm was measured at a measurement pressure of 0.025MPa. -Tensile strength (kgf / cm 2 ): Using a tensile strength tester, stress was applied to a separator test piece measuring 20 x 200 mm, and the stress applied until the test piece broke was measured.

[0090] Tensile elongation (%): Using a tensile strength tester, stress was applied to a separator test piece measuring 20 x 200 mm, and the maximum length elongated until the test piece broke was measured. The tensile elongation was calculated using the following formula. Tensile elongation (%) = (l1-l2) / l1 x 100 (In the above formula, l1 is the horizontal or vertical length of the test piece before stretching, and l2 is the horizontal or vertical length of the test piece just before fracture.)

[0091] - Puncture strength (gf): Using a puncture strength tester KES-G5 model manufactured by KATO TECH, a force was applied to a separator test piece having a size of 100 x 50 mm with a stick having a diameter of 0.5 mm at a speed of 0.05 cm / sec, and the force applied when the test piece was punctured was measured.

[0092] Meltdown temperature and shutdown temperature (°C): Using a thermomechanical analyzer (TMA), a tension of 0.01 N was applied to the separator specimen in the machine direction (MD) and the transverse direction (TD), and the temperature was increased at a rate of 5°C / min to measure the deformation of the specimen.

[0093] Heat shrinkage rate (%): A separator test piece having a size of 200 x 200 mm was placed between A4 sheets of paper and left in an oven at 130°C for 1 hour, and then cooled to room temperature. The length of the test piece that had shrunk in the horizontal and vertical directions was measured, and the heat shrinkage rate was calculated using the following formula. Heat shrinkage rate (%) = (l3-l4) / l3 x 100 (In the above formula, l3 is the horizontal or vertical length of the test piece before shrinkage, and l4 is the horizontal or vertical length of the test piece after shrinkage.)

[0094] - Number of surface defects (ea / m 2 ): The number of uneven microscopic dots (surface defects) with a size of 2 mm or more on the separator surface that were significantly different in brightness from the surrounding area was visually counted. The properties of the separators prepared in the above examples were measured, and the results are shown in Table 1 below.

[0095] [Table 1]

[0096] Example 7 A separator was manufactured in the same manner as in Example 5, except that the amounts of high-density polyethylene and silane-modified polyethylene, each with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, were changed to 36 parts by weight and 4 parts by weight, respectively.

[0097] Example 8 A separator was manufactured in the same manner as in Example 5, except that the amounts of high-density polyethylene and silane-modified polyethylene, each with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, were changed to 28 parts by weight and 12 parts by weight, respectively.

[0098] Example 9 A separator was manufactured in the same manner as in Example 5, except that the amounts of high-density polyethylene and silane-modified polyethylene, each with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, were changed to 20 parts by weight and 20 parts by weight, respectively.

[0099] Example 10 A separator was manufactured in the same manner as in Example 5, except that the amounts of high-density polyethylene and silane-modified polyethylene, each with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, were changed to 4 parts by weight and 36 parts by weight, respectively.

[0100] Comparative Example 1 A separator was manufactured in the same manner as in Example 5, except that the amounts of high-density polyethylene and silane-modified polyethylene, each with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, were changed to 39 parts by weight and 1 part by weight, respectively.

[0101] Comparative Example 2 A separator was manufactured in the same manner as in Example 5, except that the amounts of high-density polyethylene and silane-modified polyethylene, each with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, were changed to 2 parts by weight and 38 parts by weight, respectively.

[0102] Comparative Example 3 36 parts by weight of high-density polyethylene with a weight-average molecular weight (Mw) of 350,000 and a molecular weight distribution (Mw / Mn) of 5, 4 parts by weight of the silane-modified polyethylene obtained in Preparation Example 5, and 0.5 parts by weight of an antioxidant were mixed and fed into a twin-screw extruder (inner diameter 58 mm, L / D = 56, twin screw extruder) using a metering feeder, and 59.5 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C was added using a side injector. The mixture was discharged from the twin-screw extruder through a 400 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 1,000 μm.

[0103] 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 base film. The base film was immersed in a dichloromethane impregnation bath at 25°C for 1 minute to extract and remove the paraffin oil, and then immersed in an impregnation bath containing a dichloromethane solution adjusted to a dibutyltin dilaurate concentration of 1 wt% to coat the surface of the base film (the surface of the internal and external pores) with dibutyltin dilaurate, and then dried at 50°C for 5 minutes.

[0104] The dried base film was heated at 125°C in a tenter stretching machine, stretched 1.2 times in the transverse direction (TD), relaxed, and heat-set to 0.9 times the original stretching ratio. The base film was crosslinked in a constant temperature and humidity chamber at 80°C and 90% humidity for 1 hour to produce a separator.

[0105] Comparative Example 4 High-density polyethylene (VH035, Daehan Oil & Chemical Co., Ltd., melting point 135°C) with a weight-average molecular weight of 300,000 was prepared as the polyolefin, and liquid paraffin oil (LP350, Kukdong Oil & Chemical Co., Ltd., kinematic viscosity 40 cSt at 40°C) was prepared as the diluent. 3-Acryloxypropyltrimethoxysilane (KBM-5103, Shin-Etsu Co., Ltd.) was prepared as the alkoxy group-containing vinylsilane. 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (Luperox 101, Sigma-Aldrich Co., Ltd.) was prepared as the initiator.

[0106] The polyolefin and diluent were prepared in a weight ratio of 35:65, the alkoxy group-containing vinylsilane was prepared in an amount of 0.3 parts by weight per 100 parts by weight of the total content of the polyolefin and diluent, and the initiator was prepared in an amount of 2 parts by weight per 100 parts by weight of the alkoxy group-containing vinylsilane.

[0107] Polyolefin, diluent, alkoxy group-containing vinylsilane and initiator were fed into a twin-screw extruder with an L / D of 56 and kneaded to prepare a polyethylene composition, which was reactively extruded at 200°C.

[0108] The extruded composition was passed through a die and a cooling casting roll, and then biaxially stretched in the machine direction (MD) and the transverse direction (TD) using a tenter-type sequential stretching machine. The MD stretch ratio and TD stretch ratio were both 5.5 times. The stretching temperatures were 118°C for MD stretching and 123°C for TD stretching.

[0109] Dichloromethane was prepared as an organic solvent to extract the diluent from the sheet, and the sheet was immersed in the dichloromethane bath to extract the diluent and prepare a silane-grafted porous membrane.

[0110] Dibutyltin dilaurate was prepared as a crosslinking catalyst, and acetone was prepared as a solvent for dissolving the crosslinking catalyst. The crosslinking catalyst was dissolved in a solvent to prepare a crosslinking catalyst solution with a concentration of 0.05 wt %. The crosslinking catalyst solution was then immersed in an immersion bath, and the temperature of the crosslinking catalyst solution was adjusted to 45°C. The silane-grafted porous membrane was passed through the immersion bath containing the crosslinking catalyst solution, and the crosslinking catalyst solution was applied to the silane-grafted porous membrane. The residence time of the porous membrane in the crosslinking catalyst solution immersion bath was 2 to 3 seconds. The amount of crosslinking catalyst applied to the silane-grafted porous membrane through this process was calculated to be 0.05 parts by weight based on 100 parts by weight of the polyolefin, diluent, and crosslinking initiator combined.

[0111] Next, water at 60°C was applied to the silane-grafted porous membrane for 1 hour using a spray, and then the porous membrane was heat-set at 124°C to prepare a separator.

[0112] Comparative Example 5 A separator was prepared in the same manner as in Comparative Example 4, except that the silane-grafted porous membrane was placed in a chamber equipped with a spray gun and 110°C steam was applied instead of 60°C water.

[0113] Experimental Example 2 The test methods for each of the physical properties measured in the present invention are as described above. Unless otherwise specified, the measurements were performed at room temperature (25°C). The properties of the separators prepared in the examples and comparative examples were measured, and the results are shown in Tables 2 and 3 below.

[0114] [Table 2]

[0115] [Table 3]

[0116] 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 limiting. 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.

[0117] 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. [Explanation of symbols]

[0118] 100 Heating element 200 Injection member 10 Base film S Steam

Claims

1. (a) a step of extruding and stretching a composition containing a crosslinkable polyolefin in which a silane-based compound is grafted onto a first polyolefin, a non-crosslinkable polyolefin composed of a second polyolefin, and a pore-forming agent to produce a base film; (b) applying a solution containing a crosslinking catalyst and an extraction solvent to the base film to coat the surface of the base film with the crosslinking catalyst while extracting the pore-forming agent from the base film; and (c) applying heat and steam to the base film to crosslink the crosslinkable polyolefin while removing the extraction solvent remaining in the base film; A method for producing a separator, wherein the content of the crosslinkable polyolefin in the crosslinkable polyolefin and the non-crosslinkable polyolefin is 5 to 90% by weight.

2. the weight average molecular weights (Mw) of the first and second polyolefins are 1,000 to 300,000 and 300,000 to 2,000,000, respectively; 2. The method for producing a separator according to claim 1, wherein a ratio of a weight average molecular weight (Mw) of the first polyolefin to a weight average molecular weight (Mw) of the second polyolefin is 0.0005 to 1.

3. 2. The method for producing a separator according to claim 1, wherein the first and second polyolefins are each one selected from the group consisting of polyethylene, polypropylene, polybutylene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations of two or more thereof.

4. The method for producing a separator according to claim 1 , wherein the silane compound is a vinylsilane containing an alkoxy group.

5. 2. The method for producing a separator according to claim 1, wherein the content of the silane compound in the crosslinkable polyolefin is 10 to 50% by weight.

6. In the step (c), 2. The method for manufacturing a separator according to claim 1, wherein steam having a second temperature is applied to at least one surface of the base film using an injection member while applying heat by contacting at least one surface of the base film with a heating member having a first temperature.

7. the first temperature is 40 to 70°C; The method for manufacturing a separator according to claim 6, wherein the second temperature is 80 to 200°C.

8. The method for manufacturing a separator according to claim 6 , wherein the injection member is provided at least one of inside and outside the heating member.

9. A separator manufactured by the manufacturing method according to any one of claims 1 to 8, A separator having a meltdown temperature of 200 to 300°C, as measured one hour after the start of the (c) step by applying a force of 0.01 N to the separator in the machine direction (MD) and the cross direction (TD) of the separator using a thermomechanical analyzer (TMA), and then increasing the temperature at a rate of 5°C / min, at which the separator melts and fractures.

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