Crosslinked polyolefin separator film and method for producing the same

Abstract

An aspect of the present invention provides a separator film and a method for manufacturing the same, the separator film including: fibrils including a polyolefin; and a bonding structure formed by a photo-reactive material, the bonding structure being formed by causing at least a portion of a first radical formed on a surface of the fibrils and a second radical formed in the photo-reactive material to react.

Description

Technical Field

[0001] This invention relates to a cross-linked polyolefin separator and its preparation method, and more specifically, to a photocross-linked polyolefin separator and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used as power sources for various electronic products that require miniaturization and lightweight design, such as smartphones, laptops, and tablets. As their applications expand to smart grids and medium-to-large-sized batteries for electric vehicles, there is a need to develop lithium-ion batteries with high capacity, long lifespan, and high stability.

[0003] As a means to achieve the above objectives, research and development are actively underway on microporous separators that use a porous separator to prevent internal short circuits by separating the positive and negative electrodes and to facilitate the movement of lithium ions during charging and discharging. Among these, microporous separators made of polyolefins, such as polyethylene, which facilitate pore formation through thermally induced phase separation and are economical and meet the physical properties required for the separator, are being actively researched and developed.

[0004] However, separators made of polyethylene with a low melting point of approximately 135°C may shrink and deform at temperatures equal to or above their melting point due to battery heating. If a short circuit occurs due to this deformation, the battery may experience thermal runaway, leading to safety issues such as ignition. To address these problems, methods have been proposed to improve the heat resistance of separators made of cross-linked polyolefins.

[0005] Japanese Patent Application Publication Nos. 11-144700 and 11-172036 disclose inventions using silane-modified polyolefins to manufacture crosslinked separators to improve heat resistance. However, the physical properties of the manufactured separators are as follows: a thickness of 25 μm, an air permeability of 900 sec / 100 ml, and a puncture strength of 200 gf. This is significantly inferior to the physical properties of currently commercially available separators, which have a thickness of 12 μm or less, an air permeability of 150 sec / 100 ml or less, and a puncture strength of 250 gf or greater, making commercialization practically impossible.

[0006] Japanese Patent No. 4583532 discloses a method for manufacturing a separator membrane by mixing ultra-high molecular weight polyethylene (UHMWPE) with a weight-average molecular weight of 500,000 or greater with silane-modified polyolefin. However, the aforementioned UHMWPE has the disadvantage of poor dispersibility with the aforementioned silane-modified polyolefin. As a result, the manufactured separator membrane exhibits deviations, a high waste rate, and the silane-crosslinkable polyolefin is concentrated in certain areas, thus failing to obtain a separator membrane with uniform physical properties.

[0007] Korean Patent No. 1536062 discloses a microporous separator for secondary batteries, which is composed of a resin composition containing 0.01 to 1 part by weight of a photoinitiator and 0.001 to 5 parts by weight of a coupling agent relative to 100 parts by weight of a polyolefin resin. The microporous separator is prepared by a dry process and has the following problems: its mechanical properties, such as tensile strength and elongation, and its heat resistance, represented by the melt-down temperature, are significantly lower than those of separators prepared by a wet process.

[0008] Korean Patent No. 1955911 discloses a method for preparing a separator membrane by crosslinking a silane-modified polyolefin contained in a porous membrane, and the separator membrane prepared therefrom. However, the crosslinking according to the above method is carried out in the presence of moisture, and the crosslinking time required is at least 10 minutes, thus limiting the productivity of achieving such a high level. Summary of the Invention

[0009] The problem the invention aims to solve

[0010] The present invention is proposed to solve the problems of the prior art mentioned above. The purpose of the present invention is to provide a separator and its preparation method that can significantly improve productivity by reducing the time required for crosslinking while achieving a balance between mechanical properties and heat resistance.

[0011] Solution for solving the problem

[0012] One aspect of the present invention provides a separating membrane, characterized in that it comprises: fibrils containing polyolefins; and a bonding structure formed by reacting at least a portion of a first free radical formed on the surface of the fibrils and a second free radical formed on the photoreactive material with a photoreactive material.

[0013] In one embodiment, the polyolefin may have a weight-average molecular weight (Mi) of 250,000 to 800,000. w ) and molecular weight distribution of 3-7 (M w / M n ).

[0014] In one embodiment, the polyolefin may be selected from the group consisting of polyethylene, polypropylene, polybutene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations or copolymers of two or more thereof.

[0015] In one embodiment, the aforementioned photoreactive material may be a hydrogen-substituted photoinitiator.

[0016] In one embodiment, the aforementioned hydrogen-substituted photoinitiator may be selected from the group consisting of benzophenones, camphorquinones, anthraquinones, thiozanthones, α-hydroxyketones, bisphosphonates, α-aminoketones, phenylglyoxylates, monoacylphosphinoxides, benzildimethylketals, or their substitutes or derivatives, and combinations of two or more thereof.

[0017] In one embodiment, the above-mentioned bonding structure may further include a coupling agent selected from the group consisting of divinylbenzene, bisphenol A dimethacrylate, bisphenol A epoxy diacrylate, triallyl cyanurate, triallyl isocyanurate, pentaerythritol triallyl ether, butylene diacrylate, ethylene glycol diacrylate, and combinations of two or more thereof.

[0018] In one embodiment, the content of the bonding structure in the above-mentioned separator membrane can be 0.001 to 10% by weight.

[0019] In one embodiment, the above-mentioned separator membrane can satisfy one or more of the following conditions (i) to (v): (i) a melting temperature of 170°C or higher; (ii) a longitudinal (MD) tensile strength of 700 to 3,000 kgf / cm. 2 (iii) Transverse (TD) tensile strength is 700–3,000 kgf / cm². 2 (iv) Longitudinal (MD) tensile elongation of 30–150%; and (v) Transverse (TD) tensile elongation of 30–150%.

[0020] Another aspect of the present invention provides an electrochemical device including the above-described separator, preferably a lithium secondary battery or a lithium-ion battery.

[0021] Another aspect of the present invention provides a method for preparing a separating membrane, characterized by comprising the following steps: (a) feeding a composition comprising a polyolefin and a pore-forming agent into an extruder, forming and stretching it into a sheet; (b) extracting the pore-forming agent from the stretched sheet to prepare a porous membrane; (c) coating or impregnating the porous membrane with a solution containing a photoreactive material; and (d) irradiating the porous membrane with light to cause at least a portion of a first free radical formed by the photoreactive material and a second free radical formed in the photoreactive material to react and generate a bonded structure.

[0022] The effects of the invention

[0023] According to one aspect of the invention, the isolation membrane can improve mechanical properties and heat resistance in a balanced way by forming a bonded structure on the surface of the polyolefin-containing fibril constituting the porous isolation membrane by reacting at least a portion of a plurality of free radicals formed by photoreactive materials.

[0024] Furthermore, according to another aspect of the invention, the method for preparing the separator membrane can shorten the crosslinking time to 1 minute or less after coating or impregnating a solution containing a photoreactive material onto a porous separator membrane and irradiating it with light to crosslink the photoreactive material, thereby significantly improving productivity.

[0025] The effects of this invention are not limited to those described herein, and should be understood to include all effects inferred from the structure of the invention as set forth in the detailed description or claims. Attached Figure Description

[0026] Figure 1 The structure of the isolation membrane according to an embodiment of the present invention is shown.

[0027] Figure 2 These are the results of thermomechanical analysis (TMA) of the separators in the embodiments and comparative examples of the present invention.

[0028] Figure 3 and Figure 4 These are photographs showing the rupture temperatures of the isolation membranes of embodiments and comparative examples of the present invention when exposed to high temperatures. Detailed Implementation

[0029] The present invention will now be described with reference to the accompanying drawings. The present invention can be implemented in many different ways and is not limited to the embodiments described in this specification. Furthermore, for the purpose of clearly illustrating the present invention, parts unrelated to the description have been omitted from the drawings, and throughout the specification, the same or similar structural elements are given the same reference numerals.

[0030] Throughout the specification, when it is stated that a part is "connected" to other parts, this includes not only "direct connections" but also "indirect connections" where other components are placed in between. Furthermore, when it is stated that a part "includes" a structural element, unless otherwise stated otherwise, this means that other structural elements are not excluded, but rather may be included.

[0031] In the following, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0032] An embodiment of the present invention may include: fibril comprising polyolefin; and a bonding structure formed by reacting at least a portion of a first free radical formed on the surface of the fibril and a second free radical formed on the photoreactive material with a photoreactive material.

[0033] As used in this specification, the term "fibril" refers to the portion of a polyolefin porous separator membrane other than the pores, and can be interpreted as all areas on the surface and inside of the separator membrane that are in contact with external air.

[0034] Figure 1 The structure of the separator membrane according to an embodiment of the present invention is shown. (Refer to...) Figure 1 When a certain amount of light is irradiated, oxygen atoms contained in photoreactive materials (e.g., benzophenone) extract hydrogen atoms from the CH bonds contained in the polyolefin chain, so that carbon free radicals (first free radicals) can be generated in the polyolefin chain. At the same time, H bonds with oxygen atoms in the aforementioned photoreactive materials, so that free radicals (second free radicals) can be generated on the carbonyl carbon.

[0035] A system containing different types of free radicals (first free radical and second free radical) can be transformed into Figure 1 At least one of the bonding structures in portions (a) to (c). First, carbon radicals (first radicals) generated in adjacent polyolefin chains can be crosslinked to generate C-C bonds ((a)). Benzophenone can be reacted with a radical (second radical) generated on the carbonyl carbon to generate benzopinacol ((b)). The radical (second radical) generated on the carbonyl carbon in benzophenone and the carbon radical (first radical) generated in the polyolefin chain can react to bond benzophenone to the polyolefin chain ((c)).

[0036] Especially in Figure 1 In the bonding structure shown, part (a) refers to the cross-linking between adjacent polyolefin chains, which can significantly improve the mechanical properties and heat resistance of the separator. In part (b), although it is not chemically bonded to the polyolefin chain, the benzidine alcohol produced by the reaction of two benzophenone molecules that generate free radicals on the carbonyl carbon plays a substantial role in binding the adjacent polyolefin chains, which can thus contribute to the mechanical properties and heat resistance of the separator.

[0037] The aforementioned polyolefins can have a weight-average molecular weight (Mi) of 250,000 to 800,000. w ) and molecular weight distribution of 3-7 (M w / M n ).

[0038] When the weight-average molecular weight of the aforementioned polyolefin is less than 250,000, the melt viscosity is too low, and the dispersibility of the pore-forming agent is extremely poor. Depending on the circumstances, phase separation or layer separation may occur between the aforementioned polyolefin and the aforementioned pore-forming agent. When the weight-average molecular weight of the aforementioned polyolefin exceeds 800,000, the melt viscosity becomes high, the processability deteriorates, and uneven mixing may occur during melt mixing. The molecular weight distribution of the aforementioned polyolefin (M...) w / M n The molecular weight distribution of the polyolefin can be 3 to 7. When the molecular weight distribution of the polyolefin is less than 3, the dispersibility with the pore-forming agent may decrease, thereby reducing the uniformity of the prepared separator. When the molecular weight distribution of the polyolefin is greater than 7, the mechanical strength of the final separator may decrease.

[0039] The aforementioned polyolefin may be selected from the group consisting of polyethylene, polypropylene, polybutene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations or copolymers of two or more thereof. Preferably, it may be polyethylene, but the present invention is not limited thereto.

[0040] As used in this specification, the term "photoreactive material" can be interpreted as a general term for materials whose structure, activity, etc., change with light. For example, the aforementioned photoreactive material can be a photoinitiator. The aforementioned photoinitiator refers to a material that absorbs energy from a light source, especially a UV light source, to initiate a polymerization reaction.

[0041] As for the aforementioned photoinitiators, such as hydrogen-substituted, direct cleavage, and ionic reactive types, there are no particular restrictions on their type as long as they are materials that can be excited and cause photopolymerization by light irradiation. However, in this invention, hydrogen-substituted photoinitiators are preferred.

[0042] The aforementioned hydrogen-substituted photoinitiators refer to substances that induce the formation of polymer free radicals required for photocrosslinking reactions by removing hydrogen from the main chain of a polymer, etc., under photoexcitation. Hydrogen-substituted photoinitiators generate polymer free radicals more effectively than other initiators, thereby improving the photocrosslinking effect. These hydrogen-substituted photoinitiators can be selected from one of the following groups: benzophenones, camphorquinones, anthraquinones, thiozanthones, α-hydroxyketones, bisphosphonates, α-aminoketones, phenylglyoxylates, monoacylphosphinoxides, benzildimethylketals, or their substitutes or derivatives, or combinations thereof.

[0043] The aforementioned isolation membrane may have at least one bonded structure generated by the reaction between free radicals produced in the aforementioned photoreactive material under certain conditions without including existing coupling agents or crosslinking agents. However, as needed, the aforementioned bonded structure may also include a coupling agent selected from the group consisting of divinylbenzene, bisphenol A dimethacrylate, bisphenol A epoxy diacrylate, triallyl cyanurate, triallyl isocyanurate, pentaerythritol triallyl ether, butylene diacrylate, ethylene glycol diacrylate, and combinations of two or more thereof.

[0044] The aforementioned coupling agent plays a secondary role in improving the mechanical properties of the separator through photocrosslinking and suppressing thermal shrinkage, and may contain two or more vinyl groups.

[0045] The content of the aforementioned bonding structure in the separator can be from 0.001 to 10% by weight. The content of the bonding structure can be adjusted based on the concentration of the solution containing the photoreactive material to be coated and / or impregnated during the preparation of the separator, and the amount of solution to be coated and / or impregnated per unit area. When the content of the bonding structure in the separator is less than 0.001% by weight, the desired melting temperature cannot be achieved; when the content of the bonding structure in the separator is greater than 10% by weight, the brittleness of the separator increases, leading to a decrease in tensile strength and elongation.

[0046] The aforementioned separator can satisfy one or more of the following conditions (i) to (v), preferably all of them: (i) melting temperature is 170°C or higher, preferably 170°C to 350°C, more preferably 210°C to 350°C; (ii) longitudinal (MD) tensile strength is 700 to 3,000 kgf / cm. 2 Preferably 2,000 to 2,800 kgf / cm³ 2 More preferably, it is 2,150 to 2,800 kgf / cm³. 2 (iii) Transverse (TD) tensile strength is 700–3,000 kgf / cm². 2 Preferably 2,000 to 2,800 kgf / cm³ 2 More preferably, it is 2,150 to 2,800 kgf / cm³. 2 (iv) Longitudinal (MD) tensile elongation of 30-150%, preferably 50-100%; and (v) Transverse (TD) tensile elongation of 30-150%, preferably 50-100%.

[0047] Another aspect of the present invention provides a method for preparing a separating membrane, characterized by comprising the following steps: (a) feeding a composition comprising a polyolefin and a pore-forming agent into an extruder, forming and stretching it into a sheet; (b) extracting the pore-forming agent from the stretched sheet to prepare a porous membrane; (c) coating or impregnating the porous membrane with a solution containing a photoreactive material; and (d) irradiating the porous membrane with light to cause at least a portion of a first free radical formed by the photoreactive material and a second free radical formed in the photoreactive material to react and generate a bonded structure.

[0048] In step (a) above, the material containing a weight-average molecular weight (M) can be extruded. w The molecular weight distribution is 250,000–800,000 and the molecular weight distribution (M) is... w / M n A composition of polyolefin with a content of 3 to 7 and a pore-forming agent is discharged through a T-die and then stretched to prepare a substrate.

[0049] The aforementioned polyolefin may be selected from the group consisting of polyethylene, polypropylene, polybutene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations or copolymers of two or more thereof. Preferably, it may be polyethylene, but the present invention is not limited thereto.

[0050] The above composition may contain 10-40 weight percent of the above-mentioned polyolefin and 60-90 weight percent of the above-mentioned pore-forming agent. When the content of the above-mentioned polyolefin in the above composition is less than 10 weight percent, the melt viscosity of the extruded melt decreases, making it difficult to form or cast suitable sheets, and the mechanical strength of the stretched porous membrane may decrease. When the content of the above-mentioned polyolefin in the above composition is greater than 40 weight percent, the melt viscosity of the extruded melt increases, and die-swelling becomes severe after exiting the T-die, making it difficult to form or cast sheets, and difficult to form a suitable microporous structure in the porous membrane.

[0051] The aforementioned pore-forming agent may be selected from paraffin oil, paraffin wax, mineral oil, solid paraffin wax, soybean oil, rapeseed oil, palm oil, palm oil, di-2-ethylhexyl phthalate, dibutyl phthalate, diisononyl phthalate, diisodecyl phthalate, bis(2-propylheptyl) phthalate, naphthenic oil, and combinations thereof. Preferably, it may be paraffin oil, more preferably, it may be paraffin oil with a kinematic viscosity of 50-100 cSt at 40°C, but the present invention is not limited thereto.

[0052] In step (a) above, the stretching can be performed by known methods such as uniaxial stretching or biaxial stretching (sequential or simultaneous biaxial stretching). In the case of sequential biaxial stretching, the stretching ratio in the transverse (TD) and longitudinal (MD) directions can be 4 to 20 times, respectively, and the surface magnification can be 16 to 400 times.

[0053] In step (c) above, a solution containing a photoreactive material is coated or impregnated onto the porous membrane from which the pore-forming agent has been extracted and removed, such that the solution is coated on at least a portion of the surface of the polyolefin fibrils contained in the porous membrane. This coating can be achieved by known methods such as roll coating, bar coating, or spray coating.

[0054] The solution may contain a photoreactive material, and may further contain the coupling agent if desired. The content of the photoreactive material in the solution may be 0.01 to 20% by weight, preferably 0.01 to 15% by weight. By adjusting the concentration of the solution containing the photoreactive material to be coated and / or impregnated on the porous membrane to the above range, the content of the bonding structure in the separator membrane can be adjusted to the range of 0.001 to 10% by weight. Furthermore, when the content of the photoreactive material in the solution is less than 0.01% by weight, the required melting temperature cannot be achieved; when the content of the photoreactive material in the solution is greater than 20% by weight, the brittleness of the separator membrane may increase, leading to a decrease in tensile strength and elongation.

[0055] In step (d) above, light is irradiated onto the porous membrane, thereby causing at least a portion of the first free radical formed from the photoreactive material and the second free radical formed from the photoreactive material to react and generate a bonded structure. Regarding the bonded structure, [refer to reference...] Figure 1 The description is the same. The light mentioned above can be ultraviolet light, i.e., UV, and the irradiation time required to produce the above-mentioned bonding structure can be 1 minute or less, preferably 30 seconds or less, more preferably 1 to 30 seconds. When the irradiation time is less than 1 second, the desired level of bonding structure cannot be produced. When the irradiation time exceeds 1 minute, the production of the bonding structure converges to the desired level, which is disadvantageous in terms of economic efficiency and productivity.

[0056] The embodiments of the present invention will be described in detail below.

[0057] Example 1

[0058] By using 35 parts by weight of weight-average molecular weight (M) w The molecular weight distribution (M) is 350,000.w / M n High-density polyethylene (HDPE) with a viscosity of 5% was mixed with 65 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 discharged from the twin-screw extruder through a 300 mm wide T-die at a screw speed of 40 rpm and a temperature of 200°C. The mixture was then passed through a casting roll at a temperature of 40°C to produce a substrate with a thickness of 800 μm.

[0059] The substrate was stretched 6 times in the longitudinal (MD) direction in a roller stretching machine at 110°C and 7 times in the transverse (TD) direction in a tenter frame at 125°C to prepare a membrane. The membrane was then immersed in a dichloromethane leaching bath at 25°C for 1 minute to extract and remove paraffin oil, followed by immersion in an impregnation bath containing a dichloromethane solution with benzophenone concentration adjusted to 1% by weight, and then dried at 50°C for 5 minutes. Subsequently, a porous membrane was prepared by heat-fixing by relaxing the membrane by 10% in the transverse (TD) direction at 130°C.

[0060] Using a 120W ultraviolet lamp at 2J / cm 2 The porous membrane was irradiated with energy for 10 seconds on both sides to prepare a porous isolation membrane.

[0061] Example 2

[0062] By using 35 parts by weight of weight-average molecular weight (M) w The molecular weight distribution (M) is 350,000. w / M n High-density polyethylene (HDPE) with a viscosity of 5% was mixed with 65 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 discharged from the twin-screw extruder through a 300 mm wide T-die at a screw speed of 40 rpm and a temperature of 200°C. The mixture was then passed through a casting roll at a temperature of 40°C to produce a substrate with a thickness of 800 μm.

[0063] The substrate was stretched 6 times in the longitudinal (MD) direction in a roller stretching machine at 110°C and 7 times in the transverse (TD) direction in a tenter frame at 125°C to prepare a film. The film was then immersed in a dichloromethane leaching bath at 25°C to extract and remove paraffin oil for 1 minute. A dichloromethane solution with benzophenone concentration adjusted to 1% by weight was used at 100 g / m³. 2A certain amount of material was sprayed onto both sides of the membrane after removing the paraffin oil and dried at 50°C for 5 minutes. Subsequently, a porous membrane was prepared by heat-fixing by relaxing it by 10% in the transverse direction (TD) at 130°C.

[0064] Using a 120W ultraviolet lamp at 2J / cm 2 The porous membrane was irradiated with energy for 10 seconds on both sides to prepare a porous isolation membrane.

[0065] Example 3

[0066] By using 35 parts by weight of weight-average molecular weight (M) w The molecular weight distribution (M) is 350,000. w / M n High-density polyethylene (HDPE) with a viscosity of 5% was mixed with 65 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 discharged from the twin-screw extruder through a 300 mm wide T-die at a screw speed of 40 rpm and a temperature of 200°C. The mixture was then passed through a casting roll at a temperature of 40°C to produce a substrate with a thickness of 800 μm.

[0067] The substrate was stretched 6 times in the longitudinal (MD) direction in a roller stretching machine at 110°C and 7 times in the transverse (TD) direction in a tenter frame at 125°C to prepare a membrane. The membrane was then immersed in a dichloromethane leaching bath at 25°C to extract and remove paraffin oil for 1 minute. The membrane was then immersed in an impregnation bath containing a dichloromethane solution with benzophenone and divinylbenzene concentrations adjusted to 0.5 wt% and 1 wt%, respectively, and dried at 50°C for 5 minutes. Subsequently, a porous membrane was prepared by heat-fixing by relaxing the membrane by 10% in the transverse (TD) direction at 130°C.

[0068] Using a 120W ultraviolet lamp at 2J / cm 2 The porous membrane was irradiated with energy for 10 seconds on both sides to prepare a porous isolation membrane.

[0069] Example 4

[0070] By using 35 parts by weight of weight-average molecular weight (M) w The molecular weight distribution (M) is 350,000. w / M nHigh-density polyethylene (HDPE) with a viscosity of 5% was mixed with 65 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 discharged from the twin-screw extruder through a 300 mm wide T-die at a screw speed of 40 rpm and a temperature of 200°C. The mixture was then passed through a casting roll at a temperature of 40°C to produce a substrate with a thickness of 800 μm.

[0071] The substrate was stretched 6 times in the longitudinal (MD) direction in a roller stretching machine at 110°C and 7 times in the transverse (TD) direction in a tenter frame at 125°C to prepare a film. The film was then immersed in a dichloromethane leaching bath at 25°C to extract and remove paraffin oil for 1 minute. A dichloromethane solution containing benzophenone and divinylbenzene, with concentrations adjusted to 0.5 wt% and 1 wt%, respectively, was then prepared at 100 g / m³. 2 A certain amount of material was sprayed onto both sides of the membrane after removing the paraffin oil and dried at 50°C for 5 minutes. Subsequently, a porous membrane was prepared by heat-fixing by relaxing it by 10% in the transverse direction (TD) at 130°C.

[0072] Using a 120W ultraviolet lamp at 2J / cm 2 The porous membrane was irradiated with energy for 10 seconds on both sides to prepare a porous isolation membrane.

[0073] Example 5

[0074] Except for changing the concentration of benzophenone in the dichloromethane solution in the impregnation tank to 5% by weight, the porous separator membrane was prepared in the same manner as in Example 1 above.

[0075] Example 6

[0076] Except for changing the concentration of benzophenone in the dichloromethane solution in the impregnation tank to 10% by weight, the porous separator membrane was prepared in the same manner as in Example 1 above.

[0077] Example 7

[0078] Except for replacing benzophenone in the dichloromethane solution in the impregnation tank with anthraquinone, the porous separator membrane was prepared in the same manner as in Example 1 above.

[0079] Example 8

[0080] Except for changing the concentration of benzophenone in the dichloromethane solution in the impregnation tank to 11% by weight, the porous separator membrane was prepared in the same manner as in Example 1 above.

[0081] Comparative Example 1

[0082] By using 30 parts by weight of weight-average molecular weight (M) w The molecular weight distribution (M) is 350,000. w / M n High-density polyethylene (HDPE) with a viscosity of 5% was 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 discharged from the twin-screw extruder through a 300 mm wide T-die at a screw speed of 40 rpm and a temperature of 200°C. The mixture was then passed through a casting roll at a temperature of 40°C to produce a substrate with a thickness of 800 μm.

[0083] The substrate was stretched 6 times in the longitudinal (MD) direction in a roller stretching machine at 110°C and 7 times in the transverse (TD) direction in a tenter frame at 125°C to prepare a membrane. The membrane was then immersed in a dichloromethane leaching bath at 25°C to extract and remove paraffin oil for 1 minute, and dried at 50°C for 5 minutes to prepare a porous membrane. Subsequently, it was heat-fixed by relaxing 10% in the transverse (TD) direction at 130°C to prepare a porous separator membrane.

[0084] Comparative Example 2

[0085] 100 parts by weight of polypropylene with a melt index of 3.0 g / 10 min, 0.05 parts by weight of benzophenone as a photoinitiator, and 2 parts by weight of divinylbenzene as a coupling agent were kneaded together using a twin-screw extruder to prepare granules. These granules were then extruded using a single-screw extruder with a T-die to prepare a precursor film. After heat treatment in a convection oven at 155°C, the film was stretched 20% at room temperature, stretched 100% at 150°C, and then relaxed 20% at 150°C for heat fixation in a longitudinal (MD) single-screw stretching mill. A porous separator membrane was prepared by exposing the resulting porous membrane to a 6,000 W UV lamp (lmax ~ 250 nm) for 10 seconds.

[0086] Comparative Example 3

[0087] Except for replacing polypropylene with high-density polyethylene with a melt index of 1 g / 10 min, and heat-treating in a convection oven at 125°C, and stretching and heat-fixing at 120°C, the porous separator membrane was prepared in the same manner as in Comparative Example 2 above.

[0088] Comparative Example 4

[0089] The porous separator was prepared in the same manner as in Comparative Example 2 above, except that benzil dimethylketal was used instead of benzophenone and bisphenol-A epoxydiacrylate was used instead of divinylbenzene.

[0090] Comparative Example 5

[0091] Except for replacing polypropylene with high-density polyethylene with a melt index of 3.0 g / 10 min, not using a coupling agent, and heat-treating in a convection oven at 155°C, and stretching and heat-fixing at 150°C, the porous separator membrane was prepared in the same manner as in Comparative Example 3 above.

[0092] Comparative Example 6

[0093] By using 29.5 parts by weight of weight-average molecular weight (M) w The molecular weight distribution (M) is 350,000. w / M n A mixture of 5 parts by weight of high-density polyethylene (HDPE), 0.5 parts by weight of silane-modified HDPE, and 70 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C was fed into a twin-screw extruder (inner diameter: 58 mm, L / D = 56). Dibutyltin dilaurate, acting as a crosslinking catalyst, was pre-dispersed in a portion of the paraffin oil and fed through a side injector of the twin-screw extruder at a weight percentage of 0.5% relative to the total weight of the material passing through the extruder. The mixture was discharged from the twin-screw extruder into a 300 mm wide T-die at a screw speed of 40 rpm and a temperature of 200°C, and then passed through casting rolls at 40°C to produce a substrate with a thickness of 800 μm.

[0094] The substrate was stretched 6 times in the longitudinal (MD) direction in a roller stretching machine at 110°C and 7 times in the transverse (TD) direction in a tenter frame at 125°C to prepare a stretched film. The stretched film was then immersed in a dichloromethane leaching bath at 25°C to extract and remove paraffin oil for 1 minute. The paraffin oil-free stretched film was then dried at 50°C for 5 minutes. Subsequently, after heating to 125°C in a tenter frame, the film was stretched 1.45 times in the transverse (TD) direction, then relaxed and heat-fixed to a ratio of 1.25 times to its original length. The film was then crosslinked in a constant temperature and humidity bath at 85°C and 85% humidity for 72 hours to prepare a porous separator membrane.

[0095] Comparative Example 7

[0096] Except for changing the concentration of benzophenone in the dichloromethane solution in the impregnation tank to 0.005% by weight, the porous separator membrane was prepared in the same manner as in Example 1 above.

[0097] Comparative Example 8

[0098] 35 parts by weight of high-density polyethylene with a melt temperature of 135°C and a weight-average molecular weight of 350,000, 65 parts by weight of paraffin oil with a kinematic viscosity of 70 cSt at 40°C, 2 parts by weight of trimethoxyvinylsilane, 2 parts by weight of dibutyltin dilaurate, and 0.04 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane were mixed and fed into a twin-screw extruder (inner diameter: 58 mm, L / D = 56). The mixture was reactive extruded in the twin-screw extruder at 200°C and a screw speed of 30 rpm to prepare a silane-modified polyolefin composition. The mixture was discharged through a 300 mm wide T-die and then passed through a casting roll at 40°C to prepare a substrate with a thickness of 800 μm. The substrate was stretched 5.5 times in the longitudinal (MD) direction in a roller stretching machine at 108°C and stretched 5.5 times in the transverse (TD) direction in a tenter frame at 123°C to prepare a stretched film. The stretched film was then immersed in a dichloromethane leaching bath at 25°C to extract and remove paraffin oil for 10 minutes. The paraffin oil-free film was heat-fixed at 127°C to prepare a porous separator membrane. The separator membrane was then crosslinked for 24 hours in a constant temperature and humidity bath at 80°C and 90% humidity to prepare the porous separator membrane.

[0099] Experimental Example 1

[0100] The test methods for the various physical properties measured in this invention are as follows. Unless otherwise specified regarding temperature, measurements are performed at room temperature (25°C).

[0101] Thickness (μm): The thickness of the separator sample was measured using a micrometer thickness gauge.

[0102] - Porosity (%): The porosity of a 25 mm diameter membrane sample was measured using a capillary porosometer manufactured by PMI in accordance with ASTM F316-03.

[0103] -Tensive strength (kgf / cm) 2 ): A tensile strength tester was used to apply stress to a 20×200mm sample of the release membrane to measure the stress applied until the sample broke.

[0104] - Tensile elongation (%): Stress is applied to a 20×200mm release membrane sample using a tensile strength tester and the maximum elongation is measured until the sample breaks. The tensile elongation is then calculated using the following formula.

[0105] Elongation at break (%) = (l1-l2) / l1×100

[0106] (In the above formula, 11 is the length of the sample in the transverse or longitudinal direction before elongation, and 12 is the length of the sample in the transverse or longitudinal direction before fracture.)

[0107] - Puncture strength (gf): The force applied when the sample is punctured is measured by using a puncture strength tester (model name: KES-G5) from Japan Kado Technology Co., Ltd. to apply force at a speed of 0.05 cm / s to a 100×50 mm release membrane sample through a 0.5 mm diameter stick.

[0108] - Melting temperature (°C): The deformation of the separator sample was measured by applying a force of 0.01 N and heating it at a rate of 5°C / min using thermomechanical analysis (TMA). The temperature at which the sample ruptured was defined as the melting temperature.

[0109] - Heat shrinkage rate (%): Place a 200×200mm release film sample between A4 sheets of paper in an oven at 120℃ and leave for 1 hour. Then cool at room temperature, measure the shrinkage length of the sample in the transverse and longitudinal directions, and calculate the heat shrinkage rate using the following formula.

[0110] Heat shrinkage rate (%) = (l3-l4) / l3×100

[0111] (In the above formula, 13 is the length of the sample in the transverse or longitudinal direction before shrinkage, and 14 is the length of the sample in the transverse or longitudinal direction after shrinkage.)

[0112] The physical properties of the separators prepared according to the above embodiments and comparative examples were measured, and the results are shown in... Figure 2 See Tables 1 and 2 below.

[0113] Table 1

[0114]

[0115]

[0116] Table 2

[0117]

[0118]

[0119] Experiment Example 2

[0120] After the isolation membranes prepared in Example 1, Comparative Example 1 and Comparative Example 6 were fixed to a frame (outer diameter: 15cm × 15cm, inner diameter: 10cm × 10cm) with polyimide tape, they were exposed to a convection oven while the temperature was increased from 80°C at a rate of 3°C / min.

[0121] Figure 3 and Figure 4 The rupture temperatures of the separators prepared according to Example 1, Comparative Example 1, and Comparative Example 6 are shown. First, refer to... Figure 3 The separating film of Comparative Example 1 completely melted and ruptured at 152°C, but the separating film of Example 1 almost maintained its original (opaque) transparency at the beginning of the temperature rise. Furthermore, referring to... Figure 4 It was observed that the isolation film of Comparative Example 6 was completely transparent at 203°C, and some of the outer part of the isolation film melted and cracked, while the isolation film of Example 1 almost maintained its original (non)transparency at the beginning of the temperature rise.

[0122] The above description of the invention is illustrative, and those skilled in the art should understand that other specific forms can be readily derived without altering the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as exemplary and not limiting in all respects. For example, the constituent elements described in a single form can be implemented separately, and similarly, the separately described constituent elements can be implemented in a combined form.

[0123] Therefore, the scope of this invention should be defined by the claims, and all changes and modifications derived from the meaning, scope, and equivalent concepts of the claims should be interpreted as being fully included within the scope of this invention.

Claims

1. A separating membrane, characterized in that, include: fibrils, including polyolefins; as well as A bonding structure is formed by reacting at least a portion of a first free radical formed on the surface of the aforementioned fibrils and a second free radical formed on the aforementioned photoreactive material with a photoreactive material. The aforementioned bonding structure includes bonds formed by the reaction of the first free radicals generated in adjacent polyolefin chains. The aforementioned photoreactive materials react with each other to bind the adjacent polyolefin chains. The aforementioned photoreactive material is a hydrogen-substituted photoinitiator. The aforementioned hydrogen-substituted photoinitiators are benzophenones or anthraquinones. The above-mentioned separator is prepared by a method including the following steps: (a) A composition comprising a polyolefin and a pore-forming agent is fed into an extruder, formed and stretched into a sheet; (b) Extracting a pore-forming agent from the stretched sheet to prepare a porous membrane; (c) After coating or impregnating the porous membrane with a solution containing a photoreactive material, the porous membrane is then heat-fixed; and (d) Irradiate the porous membrane with light for 1 second to 1 minute to cause at least a portion of the first free radical formed by the photoreactive material and the second free radical formed in the photoreactive material to react and generate a bonded structure. The content of the above-mentioned photoreactive material in the above solution is 0.01 to 0.5% by weight. The above-mentioned separator membrane satisfies the following conditions (i) to (v): (i) The melting temperature is 210–350 °C; (ii) Longitudinal tensile strength is 2,360–3,000 kgf / cm² 2 ; (iii) Transverse tensile strength is 2,280–3,000 kgf / cm² 2 ; (iv) Longitudinal tensile elongation of 72–150%; and (v) Transverse tensile elongation is 69–150%.

2. The separator membrane according to claim 1, characterized in that, The aforementioned polyolefins have a weight-average molecular weight of 250,000 to 800,000 and a molecular weight distribution of 3 to 7.

3. The separator membrane according to claim 1, characterized in that, The aforementioned polyolefin is selected from the group consisting of polyethylene, polypropylene, polybutene, polymethylpentene, ethylene vinyl acetate, ethylene butyl acrylate, ethylene ethyl acrylate, and combinations or copolymers of two or more of them.

4. The separator according to claim 1, characterized in that, The aforementioned bonding structure also includes a coupling agent selected from the group consisting of divinylbenzene, bisphenol A dimethacrylate, bisphenol A epoxy diacrylate, triallyl cyanurate, triallyl isocyanurate, pentaerythritol triallyl ether, butylene diacrylate, ethylene glycol diacrylate, and combinations of two or more thereof.

5. The separator membrane according to claim 1, characterized in that, The content of the aforementioned bonding structure in the above-mentioned separator is 0.001 to 10% by weight.

6. An electrochemical device, characterized in that, Includes the separator membrane according to any one of claims 1 to 5.

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