Propylene-based multilayer film for battery cell pouch having excellent adhesion and formability

Abstract

Disclosed is a specific layering composition of a multilayer film that is mainly used as a sealant for a battery cell pouch, has excellent adhesion properties, and at the same time satisfies formability in subsequent processing. The present invention provides a polypropylene-based multilayer film comprising: a skin layer comprising a propylene-based binary copolymer (a), a modified polyolefin (f), and an anti-blocking agent (e); a core layer comprising an ethylene propylene block copolymer (b1) and an amorphous propylene rubber (c); and a seal layer comprising a propylene-based binary copolymer (a), an ethylene propylene block copolymer (b2), a slip agent (d), and an anti-blocking agent (e).

Description

Technical Field

[0001] This invention relates to a polypropylene multilayer film, and more specifically to a polypropylene multilayer film for use in battery cell pouches.

[0002] This application claims priority and benefit to Korean Patent Application No. 10-2020-0159121, filed on November 24, 2020, the entire contents of which are incorporated herein by reference. Background Technology

[0003] Secondary batteries generally refer to lithium secondary batteries, which are used in portable terminal devices such as laptops, smartphones, tablet PCs, and cameras, as well as electric vehicles including hybrid vehicles and smart grids for energy storage. In order to achieve miniaturization, lightweighting, and thinning while overcoming various environmental factors such as harsh thermal environments and mechanical shocks, relevant research is underway.

[0004] As a packaging material used in this type of lithium battery, a soft pack for secondary batteries, which is an outer packaging material composed of a multi-layer structure (such as an inner resin layer, an aluminum layer, and an outer resin layer), has the advantage of being able to freely deform the shape of the battery, unlike conventional can-type packaging materials. The soft-pack films commonly used in secondary batteries are composed of a multilayer film structure, which sequentially includes: an inner resin layer consisting of an adhesive layer formed from polyolefins such as polyethylene (PE), cast polypropylene (cPP), and polypropylene (PP), or copolymers thereof, which have thermal adhesion and act as a sealant; an aluminum layer serving as a metal foil layer, which functions as a substrate to maintain mechanical strength and as a barrier layer against moisture and oxygen; and an outer resin layer formed from a functional polymer film such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), nylon, or liquid crystal polymer (LCP) to protect the battery cells from external impacts.

[0005] Previously, packaging materials were mainly made by stamping metal, especially aluminum, into cylindrical or parallelepiped shapes. However, in the case of such metal can packaging materials, the outer wall of the container is rigid, and the shape of the battery itself depends on the shape constraints of the metal can packaging material.

[0006] To overcome this limitation, packaging material technology consisting of multilayer plastic films has been developed. For example, Korean Patent Publication No. 2003-0029141 discloses a battery cell soft pack, which consists of a substrate layer, an adhesive layer, a barrier layer, a dry laminate layer, and a sealant layer, wherein the sealant layer is composed of a low-flow polypropylene layer and a high-flow polypropylene layer. Additionally, Korean Patent Publication No. 2002-0030737 discloses a battery cell soft pack formed by laminating biaxially stretched nylon, polyethylene terephthalate (PET), and polyolefin resin onto a substrate film and a surface protective layer, as well as a technique for secondary processing by coating the substrate film with fluorinated, silicone, or acrylic resins.

[0007] Pouch cells offer the advantages of flexibility in form and the ability to achieve the same capacity with a smaller volume and weight. However, unlike can-type cells, pouch cells use a flexible pouch as a container, making them susceptible to damage during various processes. For example, during the housing of electrode components within the pouch, protruding parts such as electrode tabs or leads can cause cracks or other damage to the PP or cPP layers inside the pouch. When the aluminum layer is exposed due to this damage, it can react with the electrolyte, leading to side reactions. This exposed aluminum layer has the following problems: it may chemically react with the electrolyte, oxygen, or moisture that has permeated or diffused into the battery, causing corrosion and generating corrosive gases that result in swelling of the battery. Specifically, lithium hexafluoride phosphate (LiPF6) may react with water and oxygen to generate the corrosive gas hydrogen fluoride (HF). This hydrofluoric acid may also react with aluminum, causing a rapid heating reaction. If it is adsorbed onto the aluminum surface and penetrates into the tissue due to a secondary reaction, it will increase the brittleness of the tissue. Even a small impact can cause cracks in the soft film, leading to electrolyte leakage, which may cause lithium to react with the atmosphere and ignite.

[0008] Korean Patent No. 1499740 describes the following: for a polymer film for battery cell packaging materials containing 1 to 30% by weight of low-density polyethylene (LDPE), 50 to 98% by weight of polypropylene (PP), and 1 to 20% by weight of crosslinking resin, LDPE is used to reduce the electrolyte penetration rate, PP is used to compensate for usability and heat resistance, and crosslinking resin is used to improve the compatibility of LDPE and PP, but does not specifically mention the accompanying changes in physical properties such as molecular weight, density, melting point, crystallinity, and comonomer content.

[0009] Korean Patent No. 1485523 discloses an aluminum pouch film for secondary batteries, which includes an outer resin layer made of polyethylene terephthalate, nylon, etc., a first adhesive layer, an aluminum layer, a second adhesive layer, and an inner resin layer made of polyolefin. However, the patent only describes the overall layer structure of the pouch and does not mention the specific composition of the polyolefin that makes up the inner resin layer.

[0010] Korean Patent Publication No. 2019-0047104 discloses a polypropylene composite film comprising: a layer A, which is mainly composed of a random copolymer of propylene and polyethylene or butene; a layer B, which is composed of 60-90% by weight of propylene polymer with a comonomer (ethylene or butene) content of 0-1.5% by weight and 10-40% by weight of propylene polymer with a comonomer (ethylene or butene) content of 10-40% by weight; and a layer C, which is composed of a random copolymer of propylene and ethylene or butene. The film is characterized by containing 100-2000 ppm of fatty acid amide lubricant in layer B and 0-20% by weight of low-density polyethylene based on a metallocene catalyst in layer B. However, the specific details of the other raw materials besides those in layer B are not mentioned. Summary of the Invention

[0011] The purpose of this invention is to provide a specific stacked composition of a multilayer film, which is mainly used as a sealing adhesive layer for a battery cell soft pack, and has excellent adhesive properties while also satisfying the moldability requirements in subsequent processing.

[0012] Furthermore, the object of the present invention is to provide a battery cell soft pack that uses the multilayer film as a sealing layer.

[0013] To address the aforementioned issues, the present invention provides a polypropylene multilayer film comprising: a skin layer comprising an propylene-based binary copolymer a, a modified polyolefin f, and an anti-blocking agent e; a core layer comprising an ethylene-propylene block copolymer b1 and an amorphous propylene rubber c; and a sealing layer comprising an propylene-based binary copolymer a, an ethylene-propylene block copolymer b2, a slip agent d, and an anti-blocking agent e.

[0014] Furthermore, a polypropylene multilayer film is provided, characterized in that the propylene binary copolymer a contains 2.0 to 6.5% by weight of ethylene, the ethylene-propylene block copolymer b1 contains 2.0 to 6.0% by weight of ethylene, the ethylene-propylene block copolymer b2 contains 6.0 to 10.0% by weight of ethylene, the amorphous propylene rubber c contains 10 to 60% by weight of ethylene or butene, the slip agent d is an amide slip agent, the anti-blocking agent e is spherical silica, and the modified polyolefin f is grafted with maleic anhydride in a content of 1 to 10% by weight.

[0015] Furthermore, a polypropylene multilayer film is provided, characterized in that the melt flow index (MI, 230°C, 2.16 kg load) of the propylene binary copolymer a is 2-12 g / 10 min, the melt flow index (MI, 230°C, 2.16 kg load) of the ethylene-propylene block copolymer b1 is 1-5 g / 10 min, the melt flow index (MI, 230°C, 2.16 kg load) of the ethylene-propylene block copolymer b2 is 1-5 g / 10 min, the melt flow index (MI, 230°C, 2.16 kg load) of the amorphous propylene rubber c is 1-5 g / 10 min, the slip agent d is a mixture of two or more slip agents from amide-type slip agents with 14-22 carbon atoms, and the anti-blocking agent e is a particle with an average particle size of 1.5-3.0 μm and a water content of less than 0.5% by weight, and the bulk density of the particles is 0.8-1.0 g / cm³. 3 .

[0016] Furthermore, a polypropylene multilayer film is provided, characterized in that the skin layer comprises 80-99 parts by weight of propylene-based binary copolymer a, 1-20 parts by weight of modified polyolefin f, and 0.1-1 parts by weight of anti-blocking agent e; the core layer comprises 50-90 parts by weight of ethylene-propylene block copolymer b1 and 10-50 parts by weight of amorphous propylene rubber c; and the sealing layer comprises 20-60 parts by weight of propylene-based binary copolymer a, 40-80 parts by weight of ethylene-propylene block copolymer b2, 0.1-1.0 parts by weight of slip agent d, and 0.1-1 parts by weight of anti-blocking agent e.

[0017] Furthermore, a polypropylene multilayer film is provided, characterized in that the coefficient of friction (ASTM D1894, thickness 40 μm, based on the surface of the sealing layer) of the multilayer film is 0.5 or less, and the aluminum peel strength measured by the following method is 8 N / 15 mm or more, and the hot tack strength measured by the following method is 60 N / 15 mm or more.

[0018] [Methods for measuring aluminum peel strength]

[0019] A laminated film is prepared by laminating the multilayer film (40 μm thick), aluminum foil (50 μm thick, laminated on the skin layer of the multilayer film), and nylon (15 μm thick, laminated on the aluminum foil) at 130°C and curing at 50°C for 14 days. The laminated film is cut to a width of 15 mm and the peel strength at a peel angle of 180° is measured at 23°C and a peel speed of 50 mm / min.

[0020] [Methods for measuring hot tack strength]

[0021] With the multilayer films in the laminated film facing each other, at 2 kgf / cm 2 The sample was made by thermally bonding under pressure for one second, and the sample was cut into a width of 15 mm. The peel strength at a peel angle of 180° was measured at 23°C using a tensile strength measuring instrument at a peel speed of 100 mm / min.

[0022] To address the aforementioned issue, the present invention provides a battery cell pouch, comprising: a sealant layer formed of the film; a barrier layer formed on the sealant layer; and an outer resin layer formed on the barrier layer.

[0023] According to the present invention, a specific laminated composition of a polypropylene multilayer film can be provided, wherein the polypropylene multilayer film comprises a skin layer, a core layer and a sealing layer of a specific composition, which is used as a sealing adhesive layer for a battery cell soft pack, has excellent adhesive properties and simultaneously meets the requirements for formability in subsequent processing, and also has excellent mechanical and physical properties, surface characteristics and transparency. Detailed Implementation

[0024] The present invention will now be described in detail through preferred embodiments. Before proceeding, it should be noted that the terms and vocabulary used in this specification and claims should not be limited to their general or dictionary meanings. Based on the principle that "inventors may appropriately define the concepts of terms in order to best illustrate their invention," the aforementioned terms and vocabulary should be interpreted as conforming to the meaning and concept of the technical concept of the present invention. Therefore, the structures of the embodiments described in this specification are merely one preferred embodiment of the present invention and do not represent all the technical concepts of the present invention. It should be understood that at the time of this application, various equivalents and modifications could be used to replace them.

[0025] The inventors, recognizing that the specific composition of film materials used as sealing layers in battery cell pouches that simultaneously meet the required adhesive properties and formability in subsequent processing was not yet clear, repeatedly studied numerous layered compositions and discovered that a specific composition of the skin layer, core layer, and sealing layer centered on propylene-based binary copolymers, when used as a sealing layer for battery cell pouches, can achieve excellent adhesive properties while simultaneously meeting the requirements for formability in subsequent processing, and also possesses excellent mechanical and physical properties, surface characteristics, and transparency, thus obtaining the present invention.

[0026] Therefore, the present invention discloses a polypropylene multilayer film comprising: a skin layer comprising an propylene-based binary copolymer a, a modified polyolefin f, and an anti-blocking agent e; a core layer comprising an ethylene-propylene block copolymer b1 and an amorphous propylene rubber c; and a sealing layer comprising an propylene-based binary copolymer a, an ethylene-propylene block copolymer b2, a slip agent d, and an anti-blocking agent e.

[0027] In this invention, the propylene-based binary copolymer 'a' is the main component of the skin layer and sealing layer of the multilayer film. It is a copolymer of propylene and an α-olefin other than propylene, preferably a binary copolymer formed by the simultaneous injection and polymerization of propylene and ethylene. The polymerization method of the copolymer is a conventional method known in the art and is not particularly limited in this invention.

[0028] The propylene-based binary copolymer a contributes to improved aluminum peel strength by including propylene, the same component as olefin adhesives, as a constituent. This propylene-based binary copolymer a is a crystalline copolymer, not an amorphous propylene-ethylene copolymer. Preferably, it contains 2-6.5% by weight of ethylene, more preferably 2.5-3.5% by weight of ethylene. The melting point (Tm) is preferably 125-155°C, more preferably 140-154°C. When the ethylene content of the propylene-based binary copolymer a is less than 2% by weight, it lacks flexibility, which may lead to whitening and edge cracking during forming, and reduced wetting during lamination with the adhesive. Furthermore, when the ethylene content exceeds 6.5% by weight, it lacks heat resistance, which may cause appearance problems (deformation and twisting) due to heat generation during use.

[0029] On the other hand, the melt flow index (MI, 230°C, 2.16 kg load) of the propylene-based binary copolymer a can be 2–12 g / 10 min, preferably 2–8 g / 10 min, and more preferably 5–8 g / 10 min. When the melt flow index is below 2 g / 10 min, productivity may be reduced, and when the melt flow index exceeds 12 g / 10 min, the improvement effect on aluminum peel strength may be weak.

[0030] In this invention, the ethylene-propylene block copolymer b1 is the main component of the core layer of the multilayer film, and its constituent materials are propylene and ethylene. This is the same as the constituent materials of the propylene-based binary copolymer a, therefore, due to the affinity between the two phases, interlayer delamination will not occur. The polymerization method of the ethylene-propylene block copolymer b1 is a conventional method known in the art and is not particularly limited in this invention.

[0031] The ethylene-propylene block copolymer b1 preferably contains 2-6% by weight of ethylene, more preferably 4-6% by weight of ethylene. When the ethylene content of the ethylene-propylene block copolymer b1 is less than 2% by weight, the impact resistance improvement effect may be weak; when the ethylene content of the ethylene-propylene block copolymer b1 exceeds 6% by weight, a large number of fish eyes due to undispersed particles may be generated.

[0032] On the other hand, the melt flow index (MI, 230°C, 2.16 kg load) of the ethylene-propylene block copolymer b1 can be 1 to 5 g / 10 min, preferably 2 to 5 g / 10 min. When the melt flow index is below 1 g / 10 min, a large number of fish eyes due to lack of dispersion may be generated, and when the melt flow index exceeds 5 g / 10 min, the effect of improving impact resistance may be weak.

[0033] In this invention, the ethylene-propylene block copolymer b2 is the main component of the sealing layer of the multilayer film, and its constituent materials are propylene and ethylene. This is the same as the constituent materials of the propylene-based binary copolymer a. Therefore, through the affinity between the two phases, the compatibility within the layer is excellent, improving the uniformity of quality. The polymerization method of the ethylene-propylene block copolymer b2 is a conventional method known in the art and is not particularly limited in this invention.

[0034] The ethylene-propylene block copolymer b2 may contain 6 to 10% by weight of ethylene, more preferably 6 to 8% by weight of ethylene. When the ethylene content of the ethylene-propylene block copolymer b2 is less than 6% by weight, it cannot exhibit sufficient hot-tack strength during thermal bonding. When the ethylene content of the ethylene-propylene block copolymer b2 exceeds 10% by weight, it has the disadvantage that the hot-tack strength decreases due to thermal fusion between the hot-tack layers during thermal bonding.

[0035] The melt flow index (MI, 230°C, 2.16 kg load) of the ethylene-propylene block copolymer b2 can be 1–5 g / 10 min, preferably 2–5 g / 10 min. When the melt flow index is below 1 g / 10 min, a large number of fish eyes may be generated due to lack of dispersion. When the melt flow index exceeds 5 g / 10 min, the effect of improving impact resistance may be weak.

[0036] In this invention, the amorphous propylene rubber c is a secondary component of the core layer of a multilayer film. It is a copolymer of propylene and an α-olefin other than propylene, preferably simultaneously infused with propylene and ethylene or propylene and butene. It preferably contains 10-60% by weight of ethylene or butene, more preferably 10-30% by weight of ethylene or 20-40% by weight of butene. When the content of ethylene or butene is less than 10% by weight, the impact resistance improvement effect may be weak. When the content of ethylene or butene exceeds 60% by weight, due to the presence of a large amount of low molecular weight molecules, it becomes sticky during film molding, which may lead to surface inhomogeneity and potentially reduce tensile strength. The polymerization method of the copolymer is a conventional method known in the art and is not particularly limited in this invention.

[0037] In this invention, the slip agent d and the anti-blocking agent e are secondary components of the sealing layer of the multilayer film. As for the slip agent d, over time, the corresponding organic compounds in the sealing layer migrate to the surface of the substrate to impart sliding properties, thereby reducing the coefficient of friction and improving formability and workability in subsequent processing steps such as deep drawing. As for the anti-blocking agent e, it forms unevenness on the film surface, thereby alleviating sticky (blocking) phenomena and improving formability and workability in subsequent processing steps such as deep drawing.

[0038] The lubricant d is preferably an amide organic compound synthesized from fatty acids, which has a high melting point and a neutral structure due to the hydroxyl group being replaced by an amino group.

[0039] The aforementioned lubricant d is a mixture of two or more lubricants from the amide class of lubricants having 14 to 22 carbon atoms, and more preferably a mixture of tetradecanamide, hexadecanamide, octadecanamide and cis-13-docosenamide, which are saturated and unsaturated amide organic compounds having 14 to 22 carbon atoms.

[0040] The anti-blocking agent e has spherical particles. Preferably, the anti-blocking agent (e) is a silica anti-blocking agent with uniform particle distribution, and more preferably, the particles have a moisture content of less than 0.5% by weight when dried (at 105°C for 60 min), an average particle size of 1.5–3.0 μm, preferably 2–3 μm, and a bulk density of 0.8–1.0 g / cm³. 3 .

[0041] The modified polyolefin f is a by-component of the skin layer of the multilayer film, and may be modified polypropylene grafted with maleic anhydride at a content of 1-10% by weight, preferably 3-5% by weight. When the maleic anhydride content of the modified polypropylene is less than 1% by weight, the adhesion between it and the aluminum foil may decrease. When the maleic anhydride content of the modified polypropylene exceeds 10% by weight, fish-eye defects and low molecular weight defects may occur due to the large number of polar functional groups, resulting in a decrease in surface tension after corona treatment and a decrease in adhesion between it and the aluminum foil.

[0042] The components a to f mentioned above constitute the main and secondary components of the skin layer, core layer and sealing layer. When used as the sealing adhesive layer of the battery cell soft pack, in order to achieve the desired adhesive performance and formability in subsequent processing, the components a to f mentioned above can be included in a specific content ratio.

[0043] That is, in this invention, the outer skin layer may contain 80-99 parts by weight of propylene-based binary copolymer a, 1-20 parts by weight of modified polyolefin f, and 0.1-1 parts by weight of anti-blocking agent e; the core layer may contain 50-90 parts by weight of ethylene-propylene block copolymer b1 and 10-50 parts by weight of amorphous propylene rubber c; the sealing layer may contain 20-60 parts by weight of propylene-based binary copolymer a, 40-80 parts by weight of ethylene-propylene block copolymer b2, 0.1-1.0 parts by weight of slip agent d, and 0.1-1 parts by weight of anti-blocking agent e. Preferably, the outer skin layer may contain 87-93 parts by weight of propylene-based binary copolymer a, 7-13 parts by weight of modified polyolefin f, and 0.1-0.9 parts by weight of anti-blocking agent e; the core layer may contain 65-85 parts by weight of ethylene-propylene block copolymer b1 and 15-35 parts by weight of amorphous propylene rubber c; the sealing layer may contain 25-55 parts by weight of propylene-based binary copolymer a, 45-75 parts by weight of ethylene-propylene block copolymer b2, 0.1-0.4 parts by weight of slip agent d, and 0.1-0.3 parts by weight of anti-blocking agent e.

[0044] On the other hand, regarding the resin composition of each layer of the polypropylene multilayer film constituting the present invention, as long as it does not impair the purpose of the present invention, it may further contain one or more of various additives such as antioxidants, catalyst neutralizers, pigments, dispersants, weathering agents, antistatic agents, ultraviolet stabilizers, and talc powder, as needed. The amount of such additives used can be adjusted within a range known to be suitable for preparing the polypropylene multilayer film without affecting its properties, taking into account the total amount prepared and the preparation process.

[0045] In this invention, the preparation of the multilayer film using the aforementioned components can be carried out according to conventional methods known in the art. For example, the components constituting each layer can be added to a Henschel mixer in the required amounts for mixing, then extruded into granules at 180–240°C using an extruder, and finally melt-formed into a film using a multilayer film forming machine to prepare a multilayer film.

[0046] When the polypropylene multilayer film of the present invention is used as a sealing layer for battery cell pouches, it can provide excellent adhesion while simultaneously satisfying the moldability requirements in subsequent processing. Furthermore, it possesses excellent mechanical and physical properties, surface characteristics, and transparency. Specifically, the tensile strength (ASTM D638) of the polypropylene multilayer film of the present invention is 140 MPa or more, preferably 142 MPa or more; the surface treatment strength (ASTM D2578-09, based on the surface of the skin layer) is 40 dyne, preferably 42 dyne or more; the coefficient of friction (ASTM D1894, thickness 40 μm, based on the surface of the sealing layer) is 0.5 or less, preferably 0.3 or less; the haze (based on thickness 40 μm) is 20% or less, preferably 18% or less; the aluminum peel strength measured according to the following method is 8 N / 15 mm or more, preferably 10 N / 15 mm or more; and the hot tack strength measured according to the following method is 60 N / 15 mm or more, preferably 73 N / 15 mm or more.

[0047] [Methods for measuring aluminum peel strength]

[0048] A laminated film is prepared by laminating the multilayer film (40 μm thick), aluminum foil (50 μm thick, laminated on the skin layer of the multilayer film), and nylon (15 μm thick, laminated on the aluminum foil) at 130°C and curing at 50°C for 14 days. The laminated film is cut to a width of 15 mm and the peel strength at a peel angle of 180° is measured at 23°C and a peel speed of 50 mm / min.

[0049] [Methods for measuring hot tack strength]

[0050] With the multilayer films in the laminated film facing each other, at 2 kgf / cm 2 The sample was made by thermally bonding under pressure for one second, and the sample was cut into a width of 15 mm. The peel strength at a peel angle of 180° was measured at 23°C using a tensile strength measuring instrument at a peel speed of 100 mm / min.

[0051] The polypropylene multilayer film of the present invention can be mainly used for the molding of battery cell pouches, but is not limited thereto. It can also be used for the molding of, for example, packaging films, retortable pouches, and pouches for electronic materials.

[0052] The present invention will now be described in more detail through specific preparation examples, embodiments, and comparative examples.

[0053] Preparation Example

[0054] (1) Preparation of propylene-based binary copolymer a

[0055] Propylene and ethylene are injected into a reaction apparatus and subjected to bulk polymerization to prepare a propylene-based binary copolymer, wherein the ethylene content in the copolymer is 3% by weight.

[0056] (2) Preparation of ethylene-propylene block copolymer b1

[0057] Propylene is injected into a reaction apparatus and polymerized. Then, propylene and ethylene are injected into a gas-phase reactor and subjected to bulk polymerization to prepare a propylene block copolymer, wherein the ethylene content in the block copolymer is 5.5% by weight.

[0058] (3) Preparation of ethylene-propylene block copolymer b2

[0059] Propylene is injected into a reaction apparatus and polymerized. Then, propylene and ethylene are injected into a gas-phase reactor and subjected to bulk polymerization to prepare a propylene block copolymer, wherein the ethylene content in the block copolymer is 6% by weight.

[0060] Examples 1-4 and Comparative Examples 1-4

[0061] Following the composition ratios in Table 1 below (unit: parts by weight, with the addition of commonly added phenolic antioxidants, phosphate antioxidants, and hydrotalcite as a catalytic neutralizer), propylene copolymer mixed resin compositions were prepared by melt mixing at 220°C using a uniaxial extruder to form granules, and then layered accordingly. The prepared mixed resin compositions were then melt-molded at 220°C using a multilayer film forming machine to form a film, thereby preparing a multilayer film with a thickness of 40 μm (skin layer: core layer: sealing layer thickness ratio of 1:3:1).

[0062] Test case

[0063] Test pieces were made using the prepared particles and films, and their physical properties were measured and evaluated according to the following methods. The results are shown in Table 1 below.

[0064] [Methods for Measuring and Evaluating Physical Properties]

[0065] (1) Melt Index

[0066] Measurements were taken according to ASTM D1238 conditions at 230°C and a 2.16 kg load.

[0067] (2) Melting point (Tm)

[0068] Using a differential scanning calorimeter (DSC, manufactured by Perkin-Elmer Co.), 10 mg of the test sample was pre-melted for five minutes in a nitrogen atmosphere at 220°C, and then the temperature was lowered to 40°C at a cooling rate of 5°C. The temperature was then increased at a heating rate of 5°C, and the peak temperature of the resulting endothermic melting curve was defined as the melting point (Tm). Furthermore, the melting point of indium (In), measured using the aforementioned measuring device at a heating rate of 5°C, was 156.6°C.

[0069] (3) Ethylene content

[0070] Using Fourier Transmission Infrared Spectroscopy (FT-IR) and utilizing 720 cm⁻¹ -1 730cm -1 The characteristic peaks are used to measure ethylene content.

[0071] (4) Tensile strength

[0072] The measurements were performed according to ASTM D638.

[0073] (5) Surface treatment strength

[0074] The measurements were performed according to ASTM D2578-09.

[0075] (6) Coefficient of friction

[0076] For films with a thickness of 40 μm, measurements were performed according to ASTM D1894.

[0077] (7) Haze

[0078] For films with a thickness of 40 μm, measurements were performed according to the ASTM D1003 method.

[0079] (8) Aluminum peel strength

[0080] A laminated film is prepared by laminating the multilayer film (40 μm thick), aluminum foil (50 μm thick, laminated on the skin layer of the multilayer film), and nylon (15 μm thick, laminated on the aluminum foil) at 130°C and curing at 50°C for 14 days. The laminated film is cut to a width of 15 mm and the peel strength at a peel angle of 180° is measured at 23°C and a peel speed of 50 mm / min.

[0081] (9) Thermal adhesion strength

[0082] With the multilayer films in the laminated film facing each other, at 2 kgf / cm 2 The sample was made by thermally bonding under pressure for one second, and the sample was cut into a width of 15 mm. The peel strength at a peel angle of 180° was measured at 23°C using a tensile strength measuring instrument at a peel speed of 100 mm / min.

[0083] (10) Electrolyte resistance

[0084] After immersing the test piece used in the hot tack strength measurement in an electrolyte at 85°C for 75 hours, the peel strength is measured using the same method as the hot tack strength measurement. A peel strength after electrolyte immersion of 90 parts or more compared to the peel strength before electrolyte immersion is marked with "◎", 70 parts or more but less than 90 parts is marked with "△", and less than 70 parts is marked with "X". The electrolyte composition is [EC / DEC / DMC = 1 / 1 / 1 (v / v parts) + LiPF6 (1 mol / L) + H2O 300 ppm], where EC is ethylene carbonate, DEC is diethyl carbonate, and DMC is dimethyl carbonate.

[0085] (11) Molding performance

[0086] The laminated film is placed on a frame-shaped metal mold (10cm×10cm) with the nylon side facing the edge of the mold. The multilayer film is then pulled by applying pressure from the top of the four-corner pyramid until a minimum depth of 9mm is reached. Zero cracks at the corners of the laminated film are marked with "◎", one to three are marked with "△", and four are marked with "X".

[0087] [Table 1]

[0088]

[0089] Referring to Table 1, it can be confirmed that when a polypropylene multilayer film comprising a skin layer, a core layer, and a sealing layer according to the present invention is used as a sealing layer for a battery cell pouch (see Aluminum Peel Strength and Thermal Adhesion Strength), it exhibits excellent adhesive properties while simultaneously satisfying the moldability requirements of subsequent processing (see Molding Performance), and also achieves excellent mechanical and physical properties, surface characteristics, and transparency. However, regarding aluminum peel strength, it can be seen that when the skin layer is composed of a mixture of propylene-based binary copolymer a, anti-blocking agent e, and modified polyolefin f, the case of a mixture of a relatively large amount of modified polyolefin f (Examples 1, 3, and 4) is more preferable than the case of a mixture of a relatively small amount of modified polyolefin f (Example 2), and is even more preferable than the case where propylene-based binary copolymer a replaces modified polyolefin f (Comparative Example 1).

[0090] In contrast, when the main component of the sealing layer is replaced by a single propylene-based binary copolymer a instead of the mixed propylene-based binary copolymer a and ethylene-propylene block copolymer b2 (Comparative Example 3), the thermal tack strength decreases as the tensile strength of the film decreases. Furthermore, when the amorphous propylene rubber c, a secondary component of the core layer, is replaced by ethylene-propylene block copolymer b1 (Comparative Example 2), the rigidity of the film increases and the tensile strength improves, but the moldability in subsequent processing decreases significantly. Moreover, when the sealing layer does not contain slip agent d and anti-blocking agent e (Comparative Example 4), the moldability in subsequent processing decreases, which may reduce processability during production.

[0091] Therefore, it can be seen that in the multilayer film used as the sealing layer of the battery cell soft pack, there exists a specific laminate composition that satisfies excellent mechanical and physical properties, surface characteristics and transparency together with adhesive performance and formability in subsequent processing, and it is confirmed that the polypropylene multilayer film of the present invention, which includes a skin layer, a core layer and a sealing layer, can fully satisfy these characteristics.

[0092] The preferred embodiments of the present invention have been described in detail above. The description of the present invention is for illustrative purposes, 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.

[0093] Therefore, the scope of this invention is defined less by the content of the above detailed description and more by the content of the claims. All modifications or variations derived from the meaning, scope and equivalent concepts of the claims should be interpreted as being included within the scope of this invention.

Claims

1. A polypropylene multilayer film, comprising: The outer skin layer comprises an propylene-based binary copolymer a, a modified polyolefin f, and an anti-blocking agent e; The core layer comprises an ethylene-propylene block copolymer b1 and an amorphous propylene rubber c; and The sealing layer comprises propylene binary copolymer a, ethylene-propylene block copolymer b2, slip agent d, and anti-blocking agent e. The propylene-based binary copolymer a contains 2.0 to 6.5% by weight of ethylene. The ethylene-propylene block copolymer b1 contains 2.0 to 6.0% by weight of ethylene. The ethylene-propylene block copolymer b2 contains 6.0 to 10.0% by weight of ethylene. The amorphous propylene rubber c contains 10.0 to 60.0% by weight of ethylene or butene. The lubricant d is an amide-based lubricant. The anti-blocking agent e is spherical silicon dioxide. The modified polyolefin f is grafted with maleic anhydride in a content of 1.0 to 10.0% by weight.

2. The polypropylene multilayer film according to claim 1, characterized in that, The propylene-based binary copolymer a has a melt flow index (MI) of 2–12 g / 10 min at 230°C and a load of 2.16 kg. The ethylene-propylene block copolymer b1 has a melt flow index (MI) of 1–5 g / 10 min at 230°C and a load of 2.16 kg. The ethylene-propylene block copolymer b2 has a melt flow index (MI) of 1–5 g / 10 min at 230°C and a load of 2.16 kg. The amorphous propylene rubber c has a melt flow index (MI) of 1–5 g / 10 min at 230°C and a load of 2.16 kg. The slip agent d is a mixture of two or more slip agents from the amide class of slip agents with 14 to 22 carbon atoms. The anti-blocking agent e consists of particles with an average particle size of 1.5–3.0 μm and a water content of less than 0.5% by weight, and the bulk density of the particles is 0.8–1.0 g / cm³. 3 .

3. The polypropylene multilayer film according to claim 1, characterized in that, The skin layer comprises 80-99 parts by weight of propylene-based binary copolymer a, 1-20 parts by weight of modified polyolefin f, and 0.1-1 parts by weight of anti-blocking agent e. The core layer comprises 50-90 parts by weight of ethylene-propylene block copolymer b1 and 10-50 parts by weight of amorphous propylene rubber c. The sealing layer comprises 20-60 parts by weight of propylene-based binary copolymer a, 40-80 parts by weight of ethylene-propylene block copolymer b2, 0.1-1.0 parts by weight of lubricant d, and 0.1-1.0 parts by weight of anti-blocking agent e.

4. The polypropylene multilayer film according to claim 1, characterized in that, The multilayer film has a coefficient of friction of 0.5 or less under the conditions of ASTM D1894, a thickness of 40 μm, and based on the surface of the sealing layer, and an aluminum peel strength of 8 N / 15 mm or more as measured by the following method, and a hot tack strength of 60 N / 15 mm or more as measured by the following method. [Methods for measuring aluminum peel strength] A laminated film was prepared by laminating a 40 μm thick multilayer film, a 50 μm thick aluminum foil laminated on the skin layer of the multilayer film, and a 15 μm thick nylon laminated on the aluminum foil at 130°C and curing at 50°C for 14 days. The laminated film was cut to a width of 15 mm and its peel strength was measured at a peel speed of 50 mm / min at 23°C and a peel angle of 180°. [Methods for measuring hot tack strength] With the multilayer films in the laminated film facing each other, at 2 kgf / cm 2 The sample was made by thermally bonding under pressure for one second, and the sample was cut into a width of 15 mm. The peel strength at a peel angle of 180° was measured at 23°C using a tensile strength measuring instrument at a peel speed of 100 mm / min.

5. A battery cell pouch, comprising: The sealant layer is formed from the film described in any one of claims 1 to 4; A barrier layer is formed on the sealant layer; and An outer resin layer is formed on the barrier layer.

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