Polyamide-based film having an improved coating adhesion property and preparation method thereof

Abstract

In a multilayer film having a primer layer composed of a specific composition containing a polyester-based resin on a polyamide-based substrate, the adhesion strength to a rigid coating layer and an OCA layer to be coated on its surface is excellent, as are the optical and mechanical properties.

Description

Polyamide-based thin films with improved coating adhesion properties and their preparation method This invention relates to a polyamide-based film with improved coating adhesiveness and a method for preparing the polyamide-based film. Background Art Display technology continues to evolve, driven by demands linked to the development of IT equipment. In recent years, flexible display devices, which can be flexibly bent or folded in response to an external force, have become preferred in the field of mobile devices that require both large screens and portability. In particular, a major advantage of a foldable display device is that it can be folded to a small size when not in use to enhance its portability, and unfolded to form a large screen when in use. A polymer film is preferred as a cover window for these flexible display devices. For example, polyamide-based films, such as polyamide-imide (PAI) films, are widely used because they are transparent, flexible, and have excellent mechanical properties. However, such polyamide-based films are susceptible to damage from external scratches. Furthermore, further improvements are needed regarding features required for window covers, such as anti-fingerprint and anti-static properties. For example, Korean Patent No. 2147367 discloses a film for a cover window, wherein a hard coating layer and an anti-fingerprint layer are formed on a base layer made of a polyamide imide (PAI) resin, and the film is applied to a flexible display. [ [Previous Technical Documents][Patent Documents] (Patent Document 1) Korean Patent No. 2147367 The contents of the invention Technical issues A polyamide-based film currently used as a cover for a flexible display device is either used without surface treatment or with a conventional primer. However, ensuring adhesion to various functional layers, such as a rigid coating and an optical adhesive (OCA) layer, that are coated or bonded to its surface is difficult. Therefore, as a result of the research conducted by the inventors in this case, it has been found that when a primer layer composed of a specific component containing a polyester-based resin is placed on a polyamide-based substrate layer, the adhesive strength to a functional coating layer (such as a rigid coating layer and an OCA layer) is improved, and such optical and mechanical properties are also enhanced. Therefore, the objective of this invention is to provide a polyamide-based film with improved coating adhesion through primer treatment and a method for preparing the polyamide-based film. Solutions to the problem According to a specific example, a multilayer film comprising the following: a base layer comprising a polyamide-based polymer; and a primer layer formed on the base layer, wherein the primer layer comprises a polyester-based resin comprising 60% to 95% by weight of the total weight of the primer layer. According to a specific example, the multilayer film is prepared by a method comprising: preparing a base layer comprising a polyamide polymer; and coating the base layer with a primer composition and drying it, wherein the primer composition comprises a polymer resin and a solvent, and the polymer resin comprises a polyester resin comprising 60% to 95% by weight of the total solids content of the primer composition. Advantages of the invention Because the multilayer film according to this specific example has a primer layer composed of a specific composition containing a polyester-based resin on a polyamide-based substrate layer, the adhesion strength to a rigid coating layer to be coated on its surface and an OCA layer is excellent, as are the optical and mechanical properties. Therefore, in order to achieve the features required for a cover window of a display device (such as a flexible display device), the multilayer film according to this specific example may be provided with various functional layers, such as a rigid coating layer, an anti-fingerprint layer, an antistatic layer, an anti-glare layer, an anti-reflective layer, and an optical adhesive layer. The best mode for carrying out the present invention In the following text, various specific examples and illustrative examples will be described in detail with reference to these diagrams. In the following specific examples, if it is determined that a detailed description of one of the related known components or functions may obscure the subject matter of this patent, its detailed description will be omitted. Furthermore, the dimensions of individual elements in these drawings may be exaggerated or omitted for illustrative purposes, and they may differ from their actual dimensions. In this specification, when a component is described as being formed on or under another component, or being connected or coupled to one another, it covers situations where these components are formed, connected, or coupled directly or indirectly through another component. Furthermore, it should be understood that designations regarding the upper / lower position of each component may vary depending on the orientation in which the object is observed. In this specification, the terms referring to the individual components are used to distinguish them from each other and are not intended to limit the scope of the specific example. Furthermore, in this specification, a singular expression is interpreted to also encompass a plural, unless otherwise specified in the context. In this specification, the term "comprising" is intended to specify a particular feature, area, step, method, element, and / or component. It does not exclude the presence or inclusion of any other feature, area, step, method, element, and / or component unless specifically stated to the contrary. In this specification, terms such as first, second, etc., are used to describe various different components. However, these components should not be limited by these terms. These terms are used for the purpose of distinguishing one element from another. The molecular weight of a compound or polymer described in this specification, for example, a number average molecular weight or a weight average molecular weight, is, as is well known, a relative mass based on carbon-12. Although its unit is not described, it may be understood, if necessary, as a mole mass (g / mole) having the same value. In this specification, the term "[substituted]" means to be substituted with at least one substituent group selected from the group consisting of: deuterium, -F, -Cl, -Br, -I, a hydroxyl group, a cyano group, a nitro group, an amino group, a amide group, a hydrazine group, a hydrazone group, an ester group, a ketone group, a carboxyl group, or a substituted or unsubstituted C group. 1-C 30 Alkyl groups, monosubstituted or unsubstituted C 2-C 30Alkenyl group, monosubstituted or unsubstituted C 2-C 30 alkynyl group, monosubstituted or unsubstituted C 1-C 30 Alkoxy group, monosubstituted or unsubstituted C 6-C 30 Alicyclic organic groups, monosubstituted or unsubstituted C 4-C 30 Heterocyclic groups, monosubstituted or unsubstituted C 6-C 30 aryl group and one substituted or unsubstituted C 4-C 30 Heteroaryl groups. Two adjacent substituents can be bonded together to form a ring. Multilayer thin films Figure 1 shows a cross-sectional view of one of the multilayer films according to a specific example. Referring to FIG1, the multilayer film (11) according to a specific example includes: a base layer (100) containing a polyamide polymer; and a primer layer (200) formed on the base layer (100). In one specific example, the primer layer (200) comprises a polyester-based resin comprising 60% to 95% by weight of the total weight of the primer layer. In addition, the primer layer may further comprise a polyurethane-based resin comprising 5% to 40% by weight of the total weight of the primer layer. Because, according to a specific example, the multilayer film has a primer layer composed of such a particular composition disposed on a polyamide-based substrate, the adhesion strength to a functional layer (such as a rigid coating layer and an OCA layer) to be coated on its surface is excellent, and such optical and mechanical properties can also be enhanced. On the other hand, if the composition of the substrate layer and the primer layer is outside the above-mentioned range, the surface energy of the primer layer can be reduced, which may deteriorate such optical and mechanical properties, as well as the adhesion strength to a functional layer (such as a rigid coating layer and an OCA layer) to be coated or adhered to its surface. The method for preparing a multilayer film includes: preparing a substrate layer comprising a polyamide-based polymer; and coating the substrate layer with a primer composition and drying it. In one specific example, the primer composition includes a polymer resin and a solvent, wherein the polymer resin comprises a polyester-based resin comprising 60% to 95% by weight of the total weight of the primer composition. As an example, the solvent may contain more than 75% by weight of water and less than 25% by weight of isopropanol, based on the total weight of the solvent. Because the primer layer has a certain level of surface energy, it has excellent adhesion strength to an adjacent layer. For example, the surface energy of the primer layer can be 10 dynes or higher, 20 dynes or higher, 30 dynes or higher, 35 dynes or higher, 40 dynes or higher, or 45 dynes or higher, and 100 dynes or lower, 90 dynes or lower, 80 dynes or lower, 70 dynes or lower, 60 dynes or lower, 55 dynes or lower, or 50 dynes or lower. As a specific example, the surface energy of the primer layer can be between 30 dynes and 70 dynes. As yet another specific example, the surface energy of the primer layer can be between 35 dynes and 70 dynes, 35 dynes and 65 dynes, 35 dynes and 60 dynes, 35 dynes and 55 dynes, 35 dynes and 50 dynes, or 35 dynes and 45 dynes, but it is not limited to these. The surface energy can be measured by a method known in the art, for example a non-drip method, but it is not particularly limited thereto. Referring to FIG6, in the non-drip method, the contact angle (Θ) of a test solution (3) with the surface of a sample surface (10c) is measured, and the surface energy of the sample is then calculated using the known surface tension of the test solution. Therefore, in the multilayer film according to a specific example, the adhesive strength between the primer layer and the substrate layer can be very excellent. For example, the surface finish of the primer layer may be 3B or higher according to the cross-cut adhesion test result of ASTM D3359. Specifically, the surface finish of the primer layer may be 4B or higher according to the cross-cut adhesion test result of ASTM D3359. More specifically, the surface finish of the primer layer may be 5B or higher according to the cross-cut adhesion test result of ASTM D3359. In this cross-cut test, the sample surface is cut at regular intervals to form a grid shape. Adhesive tape is applied and then peeled off, and the number of delaminated grid units is counted. According to ASTM D3359 (Method B), it is evaluated by classifying it into five grades (0B to 5B) based on the percentage of undelaminated grid units relative to the total number of grid units. A higher grade number indicates better adhesion between layers (see Figure 4). Figure 2 shows a cross-sectional view of one of the multilayer films according to another specific example. Referring to Figure 2, the multilayer film (12) may further include a functional layer (300). In the multilayer film (12), the primer layer (200) can be inserted between the base layer (100) and the functional layer (300). The functional layer may be a rigid coating, an optical adhesive layer, a printing coating layer, an anti-fingerprint layer, an antistatic layer, an anti-glare layer, an anti-reflective layer, or something similar, but it is not particularly limited to these. For example, the functional layer may be selected from at least one of the following: a rigid coating layer, an optical adhesive layer, and a printed coating layer. As a specific example, the multilayer film can have a structure consisting of a rigid coating layer, a primer layer and a base layer. As another specific example, the multilayer film can have a structure consisting of a printed coating layer, a primer layer and a base layer. As another specific example, the multilayer film can have a structure consisting of a base layer, a primer layer and an optical adhesive layer. Figure 3 shows a cross-sectional view of one of the multilayer films according to another specific example. Referring to FIG3, the multilayer film (13) may further include two or more functional layers (310, 320). In the multilayer film (13), a first primer layer (210) is inserted between the base layer (100) and the first functional layer (310), while a second primer layer (220) is inserted between the base layer (100) and the second functional layer (320). As a specific example, the multilayer film may have a structure comprising: a rigid coating layer, a first primer layer, a substrate layer, a second primer layer, and an optical adhesive layer. As a specific example, the multilayer film may have a structure comprising: a printed coating layer, a first primer layer, a substrate layer, a second primer layer, and an optical adhesive layer. As another specific example, the multilayer film may have a structure comprising: a rigid coating layer, a first primer layer, a substrate layer, a second primer layer, and a printed coating layer. The first primer layer and the second primer layer may have the same composition and characteristics. Alternatively, the first primer layer and the second primer layer may have a difference in composition and / or characteristics. For example, the first primer layer may contain a polyester-based resin, and the second primer layer may contain a polyurethane-based resin. Furthermore, the first primer layer may have a thickness, for example, from 0.03 μm to 1.0 μm, and the second primer layer may have a thickness, for example, from 0.03 μm to 0.3 μm. As a specific example, the first functional layer may be a rigid coating layer, while the second functional layer may be an optical adhesive layer or a printed coating layer, but they are not limited to these. In this multilayer film, the adhesive strength between the functional layer and the substrate layer can be enhanced by the primer layer. As an example, the rigid coating or printed coating formed on the primer layer can have further enhanced adhesion strength to the substrate layer via the primer layer. Specifically, the surface of the rigid coating or printed coating can be 3B or higher, or 4B or higher, more specifically, 5B or higher, according to the cross-cut test result of ASTM D3359 (see Figure 4). As another example, the optical adhesive layer, formed by laminating an optical adhesive (OCA) sheet onto the primer layer, can have further enhanced adhesion strength to the substrate through the primer layer. Specifically, the OCA layer can have a 180° peel strength to the substrate of 0.8 kgf / inch or higher according to the ASTM D903 standard. More specifically, the peel strength can be 0.9 kgf / inch or higher, 1.0 kgf / inch or higher, or 1.1 kgf / inch or higher. Furthermore, the upper limit of the peel strength is not specifically limited, but it can be, for example, 10 kgf / inch or lower, 5 kgf / inch or lower, 3 kgf / inch or lower, or 2 kgf / inch or lower. The peel strength can be obtained by measuring the applied load while peeling the two layers to be measured at a constant speed at 180°. The peel strength can be measured using a peel strength tester or a universal test machine. Figure 5 illustrates a 180° peel strength test method according to ASTM D903. Referring to Figure 5, the base layer (10a) of a sample is attached to an alignment plate (21) and secured using an upper clamp (22a), and the functional layer (10b) of the sample to be measured for peel strength is then secured to a lower clamp (22b). The upper clamp (22a) is then lifted at a constant speed to measure the load applied to obtain the peel strength as the functional layer (10b) of the sample is peeled off from the base layer (10a) at 180°. According to one specific example, the multilayer film is excellent in optical properties; therefore, it is advantageous to apply it to a cover window of one of the display devices. According to a specific example, the multilayer film may have a transmittance, for example, an average visible light transmittance at at least a certain level. For example, it could be 70% or higher, 75% or higher, 80% or higher, 82% or higher, 83% or higher, 85% or higher, 88% or higher, 89% or higher, or 90% or higher. Meanwhile, the upper limit of the transmittance range of the film is not specifically limited. It could be, for example, 100% or lower, 95% or lower, or 92% or lower. This transmittance can be measured, for example, using a haze meter NDH-5000W manufactured by Nippon Denshoku Kogyo according to the JIS K 7136 standard. Furthermore, according to a specific example, the multilayer film may have a haze level or lower. For example, it could be 5% or lower, 4% or lower, 3.5% or lower, 3% or lower, 2% or lower, 1.5% or lower, 1.2% or lower, or 1% or lower. Meanwhile, the lower limit of the haze range of the film is not particularly limited. It could be, for example, 0% or higher, or 0.5% or higher. This haze can be measured, for example, using a haze meter NDH-5000W manufactured by Nippon Denshoku Kogyo Co., Ltd. according to the JIS K 7136 standard. In addition, the multilayer film may have a yellow index (YI) of 5 or lower, for example, 4.5 or lower, 4 or lower, 3.8 or lower, 3.5 or lower, 3 or lower, 2.8 or lower, 2.5 or lower, or 2.3 or lower, but it is not limited to this. As a specific example, the multilayer film may have a haze of 3% or less, a transmittance of 85% or more, and a yellowness (YI) of 5 or less. In the following text, the constituent layers of the multilayer thin film according to this specific example will be described in detail. primer layer The primer layer is formed on the base layer. The primer layer contains a polymer resin, for example, a curable resin, specifically a thermosetting resin or a UV-curable resin. The polymer resin can be cured to form one of the solid structures of the primer layer. The primer layer may further comprise at least one polymer resin system derived from a polyester-based resin, a polyurethane-based resin, an acrylic-based resin, a silicone-based resin, and an epoxy-based resin. In one specific example, the primer layer contains a polyester-based resin. Depending on the total weight of the primer layer, the content of the polyester-based resin in the primer layer can be from 60% by weight to 95% by weight. For example, the content of the polyester-based resin in the primer layer can be 60% by weight or higher, 65% by weight or higher, 70% by weight or higher, or 75% by weight or higher, and 95% by weight or lower, 90% by weight or lower, 85% by weight or lower, or 80% by weight or lower. Specifically, depending on the total weight of the primer layer, the content of the polyester-based resin in the primer layer can be from 65% by weight to 95% by weight, 70% by weight to 95% by weight, or 70% by weight to 90% by weight, but it is not limited to these. The polyester-based resin can be a homopolymer resin or a copolymer resin in which a dicarboxylic acid and a diol are polycondensed. Furthermore, the polyester-based resin can be a blended resin in which such homopolymer resins or copolymer resins are mixed. Exemplary examples of this dicarboxylic acid include terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-naphthalene dicarboxylic acid, 2,6-naphthalene dicarboxylic acid, 1,4-naphthalene dicarboxylic acid, 1,5-naphthalene dicarboxylic acid, diphenylcarboxylic acid, diphenoxyethane dicarboxylic acid, diphenylsulfonic acid, anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, and 1,3-cyclohexanedicarboxylic acid. 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethyl malonic acid, succinic acid, 3,3-diethyl succinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid, trimethyladipic acid, pimelic acid, azelaic acid, sebacic acid, suberic acid, dodecadicarboxylic acid, and similar substances. Furthermore, exemplary examples of such diols include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, 1,10-decanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)sulfone, and similar substances. As a specific example, the polyester-based resin may comprise a monomer composition containing terephthalic acid and ethylene glycol. As another specific example, the polyester-based resin may comprise a monomer composition containing terephthalic acid and neopentyl glycol. The polyester-based resin may have a weight average molecular weight (Mw) of, for example, 10,000 to 100,000 or 10,000 to 50,000. The polyester-based resin may have a hydroxyl group or a carboxyl group as a functional group. According to another specific example, the primer layer may further comprise a polyurethane-based resin. For example, the content of the polyurethane-based resin in the primer layer may be 5% by weight or higher, 10% by weight or higher, 15% by weight or higher, or 20% by weight or higher, and 40% by weight or lower, 35% by weight or lower, 30% by weight or lower, 25% by weight or lower, 20% by weight or lower, or 15% by weight or lower. Specifically, depending on the total weight of the primer layer, the primer layer may further comprise the polyurethane-based resin in an amount ranging from 5% by weight to 35% by weight or from 10% by weight to 30% by weight, but it is not limited thereto. The polyurethane-based resin can be formed by reacting a diisocyanate compound with a polyol. Both the diisocyanate compound and the polyol may each comprise at least one of a linear, branched, alicyclic, or aromatic compound. Specifically, the diisocyanate compound may comprise at least one of: a linear, branched, or alicyclic diisocyanate compound having 4 to 12 carbon atoms, and an aromatic diisocyanate compound having 6 to 20 carbon atoms. The polyol may comprise two or more (for example, 2 to 4) hydroxyl groups (-OH). Specifically, the polyol may be a linear, branched, or alicyclic polyol compound having 4 to 12 carbon atoms, or an aromatic polyol compound having 6 to 20 carbon atoms. The polyurethane-based resin may contain a urethane acrylate-based compound. Exemplary examples of this urethane acrylate-based compound include a difunctional urethane acrylate oligomer having a weight average molecular weight of 1,400 to 25,000, a trifunctional urethane acrylate oligomer having a weight average molecular weight of 1,700 to 16,000, a tetrafunctional urethane acrylate oligomer having a weight average molecular weight of 500 to 2,000, a hexafunctional urethane acrylate oligomer having a weight average molecular weight of 818 to 2,600, and a compound having a... An ennea-functional urethane acrylate oligomer having a weight average molecular weight of 2,500 to 5,500, a deca-functional urethane acrylate oligomer having a weight average molecular weight of 3,200 to 3,900, and a pentakaideca-functional urethane acrylate oligomer having a weight average molecular weight of 2,300 to 20,000, but not limited thereto. The urethane acrylate-based compound may have a glass transition temperature (Tg) range of -80°C to 100°C, -80°C to 90°C, -80°C to 80°C, -80°C to 70°C, -80°C to 60°C, -70°C to 100°C, -70°C to 90°C, -70°C to 80°C, -70°C to 70°C, -70°C to 60°C, -60°C to 100°C, -60°C to 90°C, -60°C to 80°C, -60°C to 70°C, -60°C to 60°C, -50°C to 100°C, -50°C to 90°C, -50°C to 80°C, -50°C to 70°C, or -50°C to 60°C. The primer layer may further contain a photoinitiator. Examples of photoinitiators include 1-hydroxy-cyclohexyl-phenyl ketone, 2-hydroxy-2-methyl-1-phenyl-1-propanone, 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone, methylbenzoylformate, and α,α-dimethoxy-α-phenylacetophenone. α-phenylacetophenone), 2-benzoyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone, 2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone, diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide, and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, but it is not limited to these. In addition, commercially available products include Irgacure. TM 184. Irgacure TM 500, Irgacure TM 651. Irgacure TM 369. Irgacure TM 907, Darocur TM 1173, Darocur TM MBF, Irgacure TM 819, Darocur TM TPO, Irgacure TM 907 and Esacure TM KIP 100F. This photoinitiator can be used alone or in combination of two or more different forms. The photoinitiator may be applied to the primer layer in an amount of 1 to 10 or 3 to 7 parts by weight, based on 100 parts by weight of the total weight of the polymer resin. The primer layer is formed by applying a primer composition onto the base layer and then drying it. The primer coating composition contains a polymer resin and a solvent. In one specific example, the polymer resin comprises 60% to 95% by weight of a polyester-based resin, depending on the total solids content of the primer layer composition. Specifically, the polymer resin may comprise, depending on the total solids content of the primer layer composition, 65% to 95% by weight, 70% to 95% by weight, or 70% to 90% by weight of the polyester-based resin, but it is not limited thereto. Furthermore, depending on the total solids content of the primer layer composition, the polymer resin may further comprise 5% to 30% by weight of a polyurethane-based resin. Specifically, depending on the total solids content of the primer layer composition, the polymer resin may further comprise 10% to 30% by weight of the polyurethane-based resin, but it is not limited thereto. The amount of solvent is not particularly limited, as it can be adjusted in various ways within a range that does not deteriorate the physical properties of the primer composition. For example, the amount of solvent can be such that the solids content of the primer composition is 1 to 50% by weight, specifically 1 to 30% by weight, and more specifically 1 to 10% by weight. Examples of such solvents include: water; alcohol-based solvents such as methanol, ethanol, isopropanol, and butanol; alkoxy-based solvents such as 2-methoxyethanol, 2-ethoxyethanol, and 1-methoxy-2-propanol; ketone-based solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl propyl ketone, and cyclohexanone; and diethylene glycol monopropyl ether, diethyl glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, and diethylene glycol 2-ethylhexyl ether. Ether-based solvents such as glycol-2-ethylhexyl ether, and aromatic solvents such as benzene, toluene and xylene, can be used alone or in combination. As an example, the solvent contains water. Specifically, depending on the total weight of the solvent, the water content can be 50% by weight or higher, 60% by weight or higher, 70% by weight or higher, 75% by weight or higher, higher than 75% by weight, 80% by weight or higher, or 85% by weight or higher, and 100% by weight or lower, lower than 100% by weight, 99% by weight or lower, 95% by weight or lower, 90% by weight or lower, or 85% by weight or lower. More specifically, depending on the total weight of the solvent, the water content can be higher than 75% by weight to 100% by weight, higher than 75% by weight to lower than 100% by weight, or 80% by weight to 95% by weight, but it is not limited to these. As another example, the solvent contains isopropanol. Specifically, depending on the total weight of the solvent, the isopropanol content can be less than 50% by weight, 40% by weight or less, 30% by weight or less, 25% by weight or less, less than 25% by weight, 20% by weight or less, or 15% by weight or less, and more than 0% by weight, 1% by weight or more, 5% by weight or more, 10% by weight or more, 15% by weight or more, or 20% by weight or more. Specifically, depending on the total weight of the solvent, the isopropanol content can be from 0% by weight to less than 25% by weight, from more than 0% by weight to less than 25% by weight, or from 5% by weight to 20% by weight, but it is not limited to these. As a specific example, depending on the total weight of the solvent, the solvent may contain more than 75% by weight of water and less than 25% by weight of isopropanol. In addition, the primer composition may further include an additive, such as a photoinitiator. The surface energy of the primer layer composition can be between 25 and 40 dynes or between 30 and 40 dynes, but it is not limited to this. The method described above or other known methods can be used as a method for measuring this surface energy. The primer coating composition can be applied onto a substrate by bar coating, knife coating, roll coating, blade coating, die coating, micro gravure coating, comma coating, slot die coating, lip coating, solution casting, or similar methods, and then dried and cured to form a primer layer. The solvent contained in the primer coating composition can be removed via the drying step. The drying temperature can be 70°C or higher, 90°C or higher, or 110°C or higher, for example, 70°C to 200°C or 90°C to 150°C. The drying time can be, for example, 1 minute to 20 minutes, specifically 1 minute to 10 minutes or 3 minutes to 7 minutes, but it is not limited thereto. The primer layer can be cured by light and / or heat. As an example, the primer layer can be UV cured, and the light dose during UV curing can be 100 mJ or higher, 200 mJ or higher, or 300 mJ or higher, for example, 100 mJ to 1,000 mJ or 300 mJ to 700 mJ. Furthermore, the curing can be partial or complete. The primer layer may have a thickness ranging from 0.03 μm to 1.0 μm. For example, the thickness of the primer layer may be 0.03 μm or higher, 0.05 μm or higher, 0.07 μm or higher, 0.1 μm or higher, 0.2 μm or higher, 0.3 μm or higher, 0.4 μm or higher, or 0.5 μm or higher, and 1.0 μm or lower, 0.9 μm or lower, 0.8 μm or lower, 0.7 μm or lower, 0.6 μm or lower, 0.5 μm or lower, 0.4 μm or lower, 0.3 μm or lower, 0.2 μm or lower, or 0.1 μm or lower. As a specific example, the thickness of the primer layer may be from 0.03 μm to 0.1 μm or from 0.1 μm to 0.5 μm. basal layer The substrate layer serves as a base film for the primer layer, while also imparting mechanical properties to the multilayer film. According to one specific example, the substrate layer comprises a polyamide polymer. Specifically, the substrate layer can be a transparent polyamide film. The polyamide polymer comprises a polyamide repeating unit. The polyamide repeating unit can be formed by polymerizing a diamine compound and a dicarbonyl compound. That is, the polyamide polymer can be prepared by reacting a reactant comprising a diamine compound and a dicarbonyl compound simultaneously or sequentially. In some specific examples, the polyamide polymer may further comprise a amide-type repeating unit. Specifically, the polyamide polymer can be a copolymer comprising both an amide-type repeating unit and an amide-type repeating unit, i.e., a polyamide-amide polymer. The amide-type repeating unit can be formed by polymerizing a diamine compound and a dianhydride compound. That is, the polyamide-amide polymer can be prepared by reacting a reactant comprising a diamine compound, a dianhydride compound, and a dicarbonyl compound simultaneously or sequentially. The diamine compound can form a amide bond with the dianhydride compound and an amide bond with the dicarbonyl compound, thereby forming a polymer. The diamine compound is not particularly limited, but it can be, for example, an aromatic diamine compound containing an aromatic structure. For example, the diamine compound can be a compound represented by the following chemical formula 1: [Chemical Formula 1] In chemical formula 1, the E series is selected from: monosubstituted or unsubstituted divalent C. 6-C 30 Aliphatic cyclic groups, monosubstituted or unsubstituted divalent carbon 4-C 30 Aliphatic cyclic groups, monosubstituted or unsubstituted divalent carbon 6-C 30 Aromatic cyclic groups, monosubstituted or unsubstituted divalent carbon 4-C 30 heteroaromatic cyclic groups, monosubstituted or unsubstituted C 1-C 30 alkyl groups, monosubstituted or unsubstituted C 2-C 30 Enyl group, monosubstituted or unsubstituted C 2-C 30 Alkyne groups, -C(=O)-, -CH(OH)-, -S(=O) 2-, -Si(CH) 3)2 -、-C(CH 3) 2- and -C(CF) 3) 2-. The e series consists of integers selected from 1 to 5. When the e series is 2 or higher, these E series can be the same or different from each other. In chemical formula 1, (E) e It may be selected from groups represented by the following chemical formulas 1-1a to 1-14a, but it is not limited thereto. Specifically, (E)e in chemical formula 1 can be a group selected from those represented by chemical formulas 1-1b to 1-13b below, but it is not limited to them. More specifically, (E)e in chemical formula 1 can be a group represented by chemical formulas 1-6b above or a group represented by chemical formulas 1-9b above. In one specific example, the diamine compound may comprise either a compound having a fluorinated substituent or a compound having an ether group (-O-). The diamine compound can be composed of a compound having a fluorinated substituent. In such a case, the fluorinated substituent can be a fluorinated hydrocarbon group, and more specifically, a trifluoromethyl group. However, it is not limited to this. In certain specific examples, a diamine compound may be used as the diamine compound. That is, the diamine compound may consist of a single component. For example, the diamine compound may contain 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl [2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, TFDB] represented by the following chemical formula, but it is not limited thereto. Because the dianhydride compound has a low birefringence value, it can help enhance the optical properties (such as transmittance) of the film containing a amide-type repeating unit. The dianhydride compound is not particularly limited, but it can be, for example, an aromatic dianhydride compound containing an aromatic structure. Specifically, the aromatic dianhydride compound can be a compound represented by the following chemical formula 2. [Chemical Formula 2] In chemical formula 2, G can be a group selected from the following: a monosubstituted or unsubstituted tetravalent C. 6-C 30 Aliphatic cyclic group, monosubstituted or unsubstituted tetravalent carbon 4-C 30 Heteroaliphatic cyclic groups, monosubstituted or unsubstituted tetravalent carbon 6-C 30 Aromatic cyclic groups or monosubstituted or unsubstituted tetravalent carbon 4-C 30 A heteroaromatic cyclic group, wherein the aliphatic cyclic group, the heteroaliphatic cyclic group, the aromatic cyclic group, or the heteroaromatic cyclic group may exist alone, may be fused together to form a fused ring, or may be bonded to a C selected from a substituted or unsubstituted C. 1-C 30 alkyl groups, monosubstituted or unsubstituted C 2-C 30Enyl group, monosubstituted or unsubstituted C 2-C 30 Alkyne groups, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) 2-, -Si(CH) 3) 2-、-C(CH 3) 2- and -C(CF) 3) 2-bonding groups. In the above chemical formula 2, G can be selected from the groups represented by the following chemical formulas 2-1a to 2-9a, but it is not limited thereto. For example, G in chemical formula 2 can be a group represented by chemical formula 2-2a, chemical formula 2-8a, or chemical formula 2-9a. In one specific example, the dianhydride compound may comprise a compound having a fluorine-containing substituent, a compound having a biphenyl group, or a compound having a ketone group. The fluorinated substituent can be a fluorinated hydrocarbon group, and more specifically, a trifluoromethyl group. However, it is not limited to this. In another specific example, the dianhydride compound may consist of a single component or a mixture of one or two components. For example, the dianhydride compound may contain at least one selected from the group consisting of 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride [2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, 6-FDA] and 3,3',4,4'-biphenyltetracarboxylic dianhydride [3,3',4,4'-biphenyltetracarboxylic dianhydride, BPDA] having the following structures, but is not limited thereto. The diamine compound and the dianhydride compound can be polymerized to form a polyamide. Subsequently, the polyamide can be converted into a amide-type repeating unit via a dehydration reaction. This amide-type repeating unit can be represented by the following chemical formula A. [Chemical Formula A] In chemical formula A, E, G, and the e series are as described above. For example, the amide-type repeating unit may contain a repeating unit represented by the following chemical formula A-1, but it is not limited thereto. [Chemical Formula A-1] In chemical formula A-1, n is an integer from 1 to 400. The dicarbonyl compound is not particularly limited, but it can be, for example, a compound represented by the following chemical formula 3. [Chemical Formula 3] In chemical formula 3, J is selected from monosubstituted or unsubstituted divalent C. 6-C 30 Aliphatic cyclic groups, monosubstituted or unsubstituted divalent carbon 4-C 30 Aliphatic cyclic groups, monosubstituted or unsubstituted divalent carbon 6-C 30 Aromatic cyclic groups, monosubstituted or unsubstituted divalent carbon 4-C 30 heteroaromatic cyclic groups, monosubstituted or unsubstituted C 1-C 30 alkyl groups, monosubstituted or unsubstituted C 2-C 30 Enyl group, monosubstituted or unsubstituted C 2-C 30 Alkyne groups, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) 2-, -Si(CH) 3) 2-、-C(CH 3) 2- and -C(CF) 3) 2-. j is an integer selected from 1 to 5. When j is 2 or higher, these j can be the same or different from each other. X is a halogen atom. Specifically, X can be F, Cl, Br, I, or similar substances. More specifically, X can be Cl, but it is not limited to this. In the above chemical formula 3, (J)j can be selected from the groups represented by the following chemical formulas 3-1a to 3-14a, but it is not limited thereto. Specifically, (J)j in the above chemical formula 3 may be selected from the groups represented by the following chemical formulas 3-1b to 3-8b, but it is not limited thereto. More specifically, (J)j in chemical formula 3 can be a group represented by chemical formula 3-1b, chemical formula 3-2b, chemical formula 3-3b, or chemical formula 3-8b. In one specific example, a dicarbonyl compound may be used alone, or a mixture of at least two different dicarbonyl compounds may be used as the dicarbonyl compound. If two or more dicarbonyl compounds are used, at least two dicarbonyl compounds whose (J)j in the above chemical formula 3 are selected from those groups represented by the above chemical formulas 3-1b to 3-8b may be used as the dicarbonyl compound. In another specific example, the dicarbonyl compound can be an aromatic dicarbonyl compound containing an aromatic structure. For example, the dicarbonyl compound may comprise a first dicarbonyl compound and / or a second dicarbonyl compound different from the first dicarbonyl compound. The first dicarbonyl compound and the second dicarbonyl compound can each be an aromatic dicarbonyl compound. The first dicarbonyl compound and the second dicarbonyl compound can be different aromatic dicarbonyl compounds, but they are not limited to this. If the first dicarbonyl compound and the second dicarbonyl compound are each an aromatic dicarbonyl compound containing a benzene ring, they can contribute to the improvement of the mechanical properties (such as surface hardness and tensile strength) of a film formed from the polyamide resin. The dicarbonyl compound may contain terephthaloyl chloride (TPC), isophthaloyl chloride (IPC), and 1,1'-biphenyl-4,4'-dicarbonyl dichloride (BPDC), or a combination thereof, represented by the following chemical formulas. However, it is not limited to these. For example, the first dicarbonyl compound may contain BPDC, and the second dicarbonyl compound may contain TPC, but they are not limited thereto. Specifically, if BPDC is used as the first dicarbonyl compound and TPC is used as the second dicarbonyl compound in a suitable combination, a film containing the polyamide-type resin thus generated can have high oxidation resistance. Optionally, the first dicarbonyl compound may contain IPC (isophthalic acid chloride), and the second dicarbonyl compound may contain TPC, but they are not limited thereto. Specifically, if IPC is used as the first dicarbonyl compound and TPC is used as the second dicarbonyl compound in a suitable combination, a film containing the polyamide-type resin thus generated can have high oxidation resistance, along with reduced manufacturing costs. The diamine compound and the dicarbonyl compound can be polymerized to form a repeating unit represented by the following chemical formula B. [Chemical Formula B] In chemical formula B, E, J, e, and j are as described above. For example, the diamine compound and the dicarbonyl compound can be polymerized to form amide repeating units represented by the following chemical formulas B-1 and B-2. Optionally, the diamine compound and the dicarbonyl compound can be polymerized to form amide repeating units represented by the following chemical formulas B-2 and B-3. [Chemical Formula B-1] In chemical formula B-1, x is an integer from 1 to 400. [Chemical formula B-2] In chemical formula B-2, y is an integer from 1 to 400. [Chemical formula B-3] In chemical formula B-3, y is an integer from 1 to 400. According to a specific example, the polyamide polymer may comprise a repeating unit represented by chemical formula A below and a repeating unit represented by chemical formula B below. [Chemical Formula A] In chemical formula A, the E, G, and e series are as described above. [Chemical formula B] In chemical formula B, E, J, e, and j are as described above. The polyamide polymer may comprise an imino repeating unit and an amino repeating unit in a molar ratio of 0:100 to 75:25. For example, the molar ratio of the imino repeating unit to the amino repeating unit in the polyamide polymer may be 2:98 to 70:30, 0:100 to 60:40, 2:98 to 60:40, 5:95 to 60:40, 0:100 to 55:45, 5:95 to 55:45, 0:100 to 50:50, or 5:95 to 50:50, but it is not limited thereto. Specifically, in this polyamide polymer, the molar ratio of repeating units represented by chemical formula A to repeating units represented by chemical formula B can be from 2:98 to 75:25. Specifically, the molar ratio of repeating units represented by chemical formula A to repeating units represented by chemical formula B can be from 2:98 to 70:30, 2:98 to 60:40, 5:95 to 60:40, 5:95 to 55:45, or 5:95 to 50:50, but it is not limited to these. The base layer may further include one or more of the following as additives: fillers, pigments and UV absorbers. A method for preparing a polyamide-based film according to a specific example includes: preparing a polyamide-based polymer solution (S100); casting the solution and then drying it to prepare a colloidal sheet (S200); and heat-treating the colloidal sheet (S300). First, a polyamide-based polymer is prepared by solvent coating (S100). The polyamide-based polymer solution can be prepared by polymerizing a diamine compound, a dicarbonyl compound, and optionally a dianhydride compound in an organic solvent. Specifically, the polyamine-based polymer solution can be prepared by simultaneously or sequentially mixing a diamine compound, a dicarbonyl compound, and optionally a dianhydride compound in an organic solvent within a reactor, and then reacting the mixture. As an example, the polymer solution can be prepared by simultaneously mixing a diamine compound and a dicarbonyl compound in an organic solvent and reacting them. As another example, the polymer solution can be prepared by simultaneously mixing a diamine compound, a dianhydride compound, and a dicarbonyl compound in an organic solvent and reacting them. As another example, the steps for preparing the polymer solution may include: first mixing the diamine compound and the dianhydride compound in a solvent and reacting them to generate a polyacrylic acid (PAA) solution; and then mixing the polyacrylic acid (PAA) solution and the dicarbonyl compound and reacting them to form a amide bond and a amide-imine bond. The polyacrylic acid solution is a solution containing a polyacrylic acid. Alternatively, the steps for preparing the polymer solution may include: first mixing the diamine compound and the dianhydride compound in a solvent and reacting them to generate a polyamide solution; subjecting the polyamide solution to dehydration to generate a polyimide (PI) solution; and then mixing the polyimide (PI) solution and the dicarbonyl compound and reacting them to further form a amide bond. The polyimide solution is a solution containing a polymer having a amide repeating unit. As another example, the steps for preparing the polymer solution may include: first mixing the diamine compound and the dicarbonyl compound in a solvent to generate a polyamide (PA) solution; and then mixing the polyamide (PA) solution and the dianhydride compound to further form a amide bond. The polyamide solution is a solution containing a polymer having a amide repeating unit. The details of the diamine compound, the dianhydride compound, and the dicarbonyl compound are as described above. The solid content in the polymer solution may be from 10% to 30% by weight. Alternatively, the solid content in the polymer solution may be from 15% to 25% by weight, but it is not limited to this. Next, the polymer solution is cast to prepare a colloidal sheet (S200). For example, the polymer solution can be extruded, coated, and / or dried over a support to form a colloidal sheet. Furthermore, the casting thickness of the polymer solution can be from 200 μm to 700 μm. Because the polymer solution is cast to a thickness within the above range, the final film formed after drying and heat treatment can have a suitable and uniform thickness. The polymer solution can have a viscosity of 80,000 cps to 500,000 cps at room temperature. Because the viscosity meets the above range, the polymer solution can be cast to a defect-free, uniform thickness, and a polyamide-based film with a substantially uniform thickness can be formed during drying without local / partial thickness variations. The polymer solution is cast and then dried at a temperature of 60°C to 150°C, 70°C to 150°C, or 80°C to 150°C for 5 to 60 minutes to prepare a colloidal sheet. Specifically, the polymer solution is dried at a temperature of 70°C to 140°C for 15 to 40 minutes to prepare a colloidal sheet. The solvent in the polymer solution can be partially or completely evaporated during the drying process to prepare the colloidal sheet. The dried colloidal sheet is then heat-treated to form a polyamide-based thin film (S300). The step of heat-treating the colloidal sheet includes heat treatment via at least one heater. Furthermore, the step of heat-treating the colloidal sheet may further include heat treatment using hot air. Heat treatment using hot air can be carried out at a temperature range of 60°C to 500°C for 5 to 200 minutes. Specifically, the heat treatment of the colloidal sheet can be carried out at a temperature rise rate of 2°C / min to 80°C / min at a temperature range of 80°C to 350°C for 10 to 150 minutes. In such a case, the initial temperature of the heat treatment of the colloidal sheet can be 60°C or higher. Specifically, the initial temperature of the heat treatment of the colloidal sheet can be 80°C to 180°C. Furthermore, the maximum temperature in this heat treatment can be 200°C to 500°C. The at least one heater may include an IR heater. However, the type of the at least one heater is not limited to the above-described example and can be varied in many ways. The heat treatment by the at least one heater can be carried out in a temperature range of 300°C or higher. Specifically, the heat treatment by the at least one heater can be carried out in a temperature range of 300°C to 500°C for 1 minute to 30 minutes. The substrate layer may have a thickness of 20 μm or higher, 30 μm or higher, 40 μm or higher, 50 μm or higher, or 100 μm or higher, and 500 μm or lower, 400 μm or lower, 300 μm or lower, or 200 μm or lower. As a specific example, the thickness of the substrate layer may be from 20 μm to 500 μm, more specifically from 40 μm to 200 μm or from 50 μm to 200 μm. Functional layer The multilayer film may further include a functional layer. The functional layer may be formed on the primer layer, and thus the primer layer may be inserted between the functional layer and the substrate layer. The functional layer may be a rigid coating, an optical adhesive layer, a printed coating layer, an anti-fingerprint layer, an antistatic layer, an anti-glare layer, an anti-reflective layer, or the like, but it is not particularly limited to these. For example, the functional layer may be selected from at least one of the following groups: a rigid coating layer, an optical adhesive layer, and a printed coating layer. According to one specific example, the multilayer film may further include a rigid coating layer formed on the primer layer. The rigid coating layer may enhance the mechanical and / or optical properties of the multilayer film. The rigid coating may comprise at least one of an organic component, an inorganic component, and an organic-inorganic composite component as a hard coating agent. As an example, the rigid coating may comprise an organic resin. Specifically, the organic resin may be a curable resin. Thus, the rigid coating may be a curable coating. Specifically, the rigid coating may comprise at least one selected from the group consisting of: a urethane acrylate-based compound, an acrylate-based compound, and an epoxy acrylate-based compound. More specifically, the rigid coating may comprise a urethane acrylate-based compound and an acrylate-based compound. The content of the organic resin can be from 30% to 100% by weight, depending on the total weight of the rigid coating. Specifically, the content of the organic resin can be from 40% to 90% by weight or from 50% to 80% by weight, depending on the total weight of the rigid coating. The rigid coating may optionally further comprise a filler. The filler may be, for example, inorganic particles. Exemplary examples of the filler include silicon oxide, barium sulfate, zinc oxide, and aluminum oxide. The filler may have a particle size of 1 nm to 1,000 nm. Depending on the total weight of the functional layer, the filler content may be 25% by weight or higher, 30% by weight or higher, or 35% by weight or higher, and 50% by weight or lower, 45% by weight or lower, or 40% by weight or lower. The rigid coating may further contain a photoinitiator. The photoinitiator may be used alone or in combination of two or more different forms. Furthermore, the rigid coating may further contain additives such as antifouling agents, antistatic agents, surfactants, UV absorbers, UV stabilizers, anti-yellowing agents, leveling agents, and dyes to improve color values. The content of these additives can be varied within a range that does not impair the physical properties of the rigid coating. For example, it can be from 0.01% to 10% by weight, depending on the total weight of the rigid coating, but it is not limited to this. The rigid coating may have a thickness of 2 μm or higher, 3 μm or higher, 5 μm or higher, or 10 μm or higher, and 50 μm or lower, 30 μm or lower, 20 μm or lower, or 10 μm or lower. For example, the thickness of the rigid coating may be from 2 µm to 20 µm. Specifically, the thickness of the rigid coating may be from 5 µm to 20 µm. According to another specific example, the multilayer film may further include a printed coating layer formed on the primer layer. The printed coating may contain a binder resin, for example, an ester-based compound, a carbamate-based compound, an epoxy-based compound, or a mixture thereof. Specifically, the ester-type compound may comprise at least one selected from the group consisting of: a phthalic acid resin, an isophthalic acid resin, and a terephthalic acid resin; the carbamate-type compound may comprise a resin in which an isocyanate compound such as diphenylmethane diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate is polymerized with a polyol; and the epoxy compound may comprise at least one selected from the group consisting of: a bisphenol A-type epoxy resin, a brominated bisphenol A-type epoxy resin, and a phenolic resin. A resin and a glycidyl amine-based resin. The printed coating may further comprise a pigment or a dye. The pigment may be: an inorganic pigment such as carbon black, barium sulfate, calcium carbonate, titanium dioxide, yellow iron oxide, black iron, chrome yellow, chrome vermilion, cadmium yellow, cadmium red, or royal blue; an organic pigment such as insoluble azo, soluble azo, phthalocyanines, quinacridones, or polyazos; or a mixture thereof. The printed coating layer can be formed by applying a printing coating layer composition onto the primer layer, and the coating can be performed using a silk screen or a thermal transfer. As an example, the printed coating composition can be a thermosetting resin composition. The thermosetting resin composition may include a binder resin, a pigment, and a solvent. Specifically, based on the total weight of the thermosetting resin composition, it may contain 10 to 50% by weight of the binder resin, 1 to 20% by weight of the pigment, and 40 to 80% by weight of the solvent. The solvent can be ketone-based or alcohol-based. Specifically, the solvent can be: ketone-based, such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, 2-heptanone, and 3-heptanone; alcohol-based, such as ethanol, propanol, butanol, hexanol, cyclohexanol, ethylene glycol, diethylene glycol, and glycerol; or a mixture thereof. According to one specific example, the thermosetting resin composition may further include an additive. Specifically, the additive may be selected from at least one of the group consisting of: a leveling agent, a dispersant, and a thickener. According to one specific example, the thermosetting resin composition may contain the additive in an amount of 1 to 10% by weight, depending on the total weight of the thermosetting resin composition. The printed coating may have a thickness ranging from 1 µm to 50 µm. For example, the thickness of the printed coating may be 3 to 50 µm, 3 to 20 µm, 10 to 50 µm, 15 to 40 µm, 20 to 50 µm, 15 to 20 µm, 10 to 15 µm, 2 to 10 µm, 5 to 10 µm, or 2 to 7 µm. According to another specific example, the multilayer film may further include an optical adhesive layer formed on the primer layer. The optical adhesive layer may contain an acrylic resin, an epoxy resin, a urethane resin, or the like, but is not particularly limited thereto. As an example, the optical adhesive layer may comprise an acrylic resin, and it may be formed by reacting one, two, or more acrylic compounds. The acrylic compound may be, for example, selected from at least one of the following: 2-ethylhexyl acrylate, 4-hydroxybutyl acrylate, methyl acrylate, acrylic acid, and the like. The reaction of the acrylic compound may be carried out in a solvent, and examples of such solvents include ethyl acetate, butyl acetate, toluene, or a mixture thereof. The optical adhesive layer may further contain a photoinitiator. For example, 2,2'-azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO), or similar substances may be used as the photoinitiator. The optical adhesive layer may further comprise a crosslinking agent. For example, a glycidyl-based crosslinking agent, an isocyanate-based crosslinking agent, an epoxy-based crosslinking agent, or a mixture thereof may be used as the crosslinking agent. The content of the crosslinking agent may be 0.2 to 0.6 parts by weight, depending on 100 parts by weight of the resin used to prepare the optical adhesive layer. The optical adhesive layer may further contain organic or inorganic particles. For example, the organic particles may include at least one of polybutyl acrylate (PBA) and polymethyl methacrylate (PMMA). The content of the organic or inorganic particles may be 1 to 10 parts by weight, depending on 100 parts by weight of the resin used to prepare the optical adhesive layer. As an example, the optical adhesive layer can be formed by coating the optical adhesive layer composition onto the primer layer of the multilayer film. As another example, the optical adhesive layer is pre-prepared as a sheet that can be attached to the primer layer of the multilayer film. The optical adhesive layer may have a thickness of 30 μm or higher, 40 μm or higher, or 50 μm or higher, or 200 μm or lower, 150 μm or lower, or 100 μm or lower. Efficacy and Uses Because, according to a specific example, the multilayer film has an excellent adhesion strength to a rigid coating layer and an OCA layer on a polyamide base layer, consisting of a primer layer composed of a specific composition containing a polyester-based resin, and also has excellent optical and mechanical properties. Therefore, in order to achieve the features required for a cover of a display device (such as a flexible display device), the multilayer film, according to a specific example, can be provided with various functional layers, such as a rigid coating layer, an anti-fingerprint layer, an antistatic layer, an anti-glare layer, an anti-reflective layer, and an optical adhesive layer. According to one specific example, the multilayer film can be used in a display device. For example, according to one specific example, the multilayer film can be applied as a cover window of one of the display devices. The display device can be a flexible display device. As an example, it can be a foldable display device. According to another specific example, the display device includes: a display unit; and a cover window disposed on the viewing side of the display unit, wherein the multilayer film described above is applied as the cover window. An adhesive layer may be formed between the cover window and the display panel. For example, the adhesive layer may contain an optically transparent adhesive. The display panel (20) can be a liquid crystal display (LCD) panel. Alternatively, the display panel (20) can be a light-emitting diode (LED) display panel. Modes used in this invention The specific examples described below are provided to aid understanding, but the scope of implementation is not limited thereto. Preparation Example 1A : Substrate thin film A 1-liter glass reactor equipped with a temperature-controlled double-jacket was filled with dimethylacetamide (DMAc) under a nitrogen atmosphere at 20°C. Next, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) was slowly added to the DMAc to dissolve the TFDB. Then, 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6-FDA), a dianhydride compound, was slowly added, and the mixture was stirred for 1 hour. Next, terephthalic acid chloride (TPC) and isophthalic acid chloride (IPC), dicarbonyl compounds, were added, followed by stirring for another 1 hour, thereby preparing a polymer solution. The types and molar ratios of the monomers used to prepare this polymer solution were adjusted as shown in Table 1 below. The resulting polymer solution was coated onto a glass plate and then dried with hot air at 80°C for 30 minutes. It was then peeled off from the glass plate, fixed to a pin frame, and heat-treated at a temperature rise rate of 2°C / min within a temperature range of 80°C to 350°C to obtain a polyamide film with a thickness of 50 µm. Preparation Example 1B To 1D : Substrate thin film Except for changing the type of monomer and molar ratio used to prepare the polymer solution as shown in Table 1 below, the operating procedure for preparing Example 1A was repeated to prepare the films for Examples 1B to 1D. [Table 1] Preparation Example 2 Optical adhesive (OCA) 100 parts by weight of a mixture of 2-ethylhexyl acrylate and 4-hydroxybutyl acrylate (80:20, w / w), 4 parts by weight of organic particles [a core-shell structure with a polybutyl acrylate (PBA) core and a polymethyl methacrylate (PMMA) shell, a particle size of 130 nm and a refractive index of 1.48], and 0.005 parts by weight of a photoinitiator (Irgacure™ 651) were mixed in a glass container. Dissolved oxygen in the glass container was replaced with nitrogen, and the polymerization reaction was carried out for several minutes using ultraviolet radiation from a low-pressure lamp (BL Lamp, Sankyo). As a result, a (meth)acrylic-based copolymer containing organic particles, having a viscosity of approximately 1,000 cps and a refractive index of 1.47 to 1.48, was obtained. Here, 0.35 parts by weight of a photoinitiator (Irgacure™ 184) was added to prepare an adhesive composition. This adhesive composition was coated onto a polyester-based film (release film, PET, 50 µm thick) to form an adhesive film with a thickness of 100 µm. The upper surface of this adhesive film was covered with a release film with a thickness of 75 μm, and both sides were then irradiated with a low-pressure lamp (BL Lamp, Sankyo) for approximately 6 minutes to obtain an optically conductive film (OCA). Preparation Example 3 Hard coating composition 54.32 parts by weight of a urethane acrylate oligomer (PU2050, Miwon Specialty Chemical), 23.28 parts by weight of a multifunctional acrylate monomer (M300, Miwon Specialty Chemical), 19.4 parts by weight of a silica sol (MA-ST, Nissan Chemical) (in which fine silica particles having an average particle size of 10 to 15 nm are dispersed in methanol at 30% by weight), and 3 parts by weight of a photoinitiator (I-184, BASF) are mixed to prepare a rigid coating composition. Preparation Example 4 : Composition of printed coating layer A colored pigment (carbon black) of 50% by weight, a polymer dispersant (BYK-P104S) of 2% by weight, an acrylic resin of 35% by weight (depending on solids content), and ethyl acetate of 13% by weight were compounded and premixed for 30 minutes, and then milled (6 to 8 times) using a Dyno™ Mill to achieve a particle size distribution of 10 μm or less. A mill base of 20% by weight, ethyl acetate of 60% by weight, an acrylic resin of 18.6% by weight (depending on solids content), and a silane-based compound [γ-glycidoxypropyl trimethoxysilane, trade name Silquest A-187] of 1.4% by weight were mixed and stirred to prepare a printing coating composition. Example 1 Preparation steps of a multilayer thin film (1): Preparation of a primer layer composition A polyester-based resin (PLASCOAT 446, GOO Chemical, Japan) and a polyurethane-based resin (28% solids by weight, H-15, DKS Co. Ltd., Japan) were mixed in a weight ratio of 7:3 (weight ratio of solids), while a mixture of water (H... A mixed solvent of 2O and isopropanol (IPA) in a weight ratio of 85:15 was used to compound the primer, and the mixture was stirred at room temperature for 30 minutes to prepare a primer composition. Step (2): Formation of a primer layer The primer composition obtained in the above steps was applied to one side of the substrate obtained in Example 1 using a bar coater and then dried to form a primer layer. The coating amount was controlled by the size of the bar, and the thickness of the primer layer after drying was measured using an optical thickness meter (Film Metrics F-20), as shown in Table 2 below. As a result, a multilayer film in which a primer layer is formed on a substrate layer is obtained. Example 2 Up to 12 Except for the film used in the substrate, the composition and content of the primer layer, and the thickness of the primer layer being changed as shown in Table 2 below, the same operating procedure as in Example 1 was repeated to prepare a multilayer film. Comparative Example 1 Up to 3 Except that no polyester-based resin was added during the preparation of the primer composition as shown in Table 2 below, a polyurethane-based resin (28% by weight solids, H-15, DKS Co. Ltd., Japan) and an acrylic-based resin (55% by weight solids, PRIMAL AC-26, Dow Chemical) were blended in a weight ratio (by weight solids), and the composition of the solvent in the primer composition and the thickness of the primer layer were changed, the same operating procedure as in Example 1 was repeated to prepare a multilayer film. Comparative Example 4 Except for the polyimide (PI) film used in Example 1D as shown in Table 2 below, which was used as the component of the substrate layer and the primer layer, the same operating procedure as in Example 1 was repeated to prepare a multilayer film. Comparative Example 5 The polyamide-imide film obtained in Example 1A was used as a base layer because it did not form a primer layer. [Table 2] Experimental Demonstration Example 1: Between The primer layer and The base layer Between Adhesion strength ( Hundred-grip knife test ) The adhesion strength between the primer layer and the substrate layer of a thin film sample was evaluated using a cross-cut adhesion test. According to ASTM D3359 (Method B), the surface of the primer layer was cut at regular intervals to form a grid shape. A Nitto Tape 50B was applied and then peeled off, and the degree to which the grid units detached from the surface was evaluated. It was graded from 0B to 5B according to the following criteria, with 5B being evaluated as the best (see Figure 4). - 5B: The cut surface is clean, and the grid squares are not peeled off [0% of the grid area]. - 4B: Small patches of coating are peeled off at intersections (less than 5% of the grid area). - 3B: Small patches of coating are peeled off along the edges and at cut-off intersections (5-15% of the grid area). - 2B: The coating is peeled off at the cut edge and part of the rectangle (15-35% of the grid area). - 1B: The coating is extensively peeled off along the edge of the cut surface, and the squares are peeled off (35-65% of the grid area). - 0B: The coating is peeled off, and the degree of detachment of the squares is more severe (more than 65% of the grid area). Experimental Demonstration Example 2 The adhesive strength between the rigid coating and the substrate. Hundred-grid knife test) A rigid coating is formed on the primer layer of a thin film sample, and the adhesion strength between the rigid coating and the substrate is evaluated by a cross-cut test. First, the rigid coating composition obtained in Example 3 was coated onto the primer layer of a thin film sample (or directly onto the substrate layer if no primer layer was present) using a stencil coating method. The solvent was dried at 80°C for approximately 1 minute, and then cured by irradiation with ultraviolet light using a high-pressure mercury lamp at a light dose of 1,000 mJ / cm². As a result, a thin film with a rigid coating layer of 5 µm thickness located on the substrate layer and the primer layer was obtained. Subsequently, a cross-cut test was performed on the surface of the hard coating according to ASTM D 3359 (Method B) standard, as in Test Example 1. Experimental Demonstration Example 3 The adhesion strength between the printed coating layer and the substrate layer ( Hundred-grid knife test) A printed coating is formed on the primer layer of a thin film sample, and the adhesion strength between the printed coating and the substrate is evaluated by a cross-cut test. First, the printed coating composition obtained in Example 4 was coated onto the primer layer of a thin film sample (or directly onto the substrate layer if no primer layer is present) using a silk screen coating method. It was then subjected to solvent drying and curing at 80°C for approximately 30 minutes. As a result, a thin film was obtained containing a printed coating layer with a thickness of 3.0 µm located on the substrate layer and the primer layer. Subsequently, a cross-cut test was performed on the surface of the printed coating in accordance with ASTM D 3359 (Method B) standard as in Test Example 1. Experimental Demonstration Example 4 The adhesion strength between the optical adhesive layer and the substrate layer (180°) Peel strength) An optical adhesive (OCA) layer is formed on the primer layer of a thin film sample, and the adhesive strength between the OCA layer and the substrate layer is evaluated by a 180° peel strength test. First, the OCA sheet obtained in Example 2 was adhered to the primer layer of a film sample (or directly coated onto the substrate layer if no primer layer is present) by reciprocating a rubber-coated roller once under a load of 2 kg and at a speed of 5 mm / s. As a result, a film having an OCA layer on the substrate layer and the primer was obtained. The 180° peel strength was measured on the OCA layer using a universal testing machine (UTM) at a rate of 300 mm / min according to ASTM D903 (see Figure 5). Experimental Demonstration Example 5 Surface energy The surface energy is obtained by measuring the contact angle of a test solution (deionized water, diiodomethane) on the surface of the primer layer (base layer, if no primer layer is present) of a thin film sample using a KRUSS MSA [mobile surface analyzer] with the non-drip method (see Figure 6). First, deionized water with a known surface tension is dripped, and the contact angle is obtained. This procedure is repeated five times to obtain the average of one of the five obtained contact angle values. Similarly, diiodomethane with a known surface tension is dripped, and the contact angle is obtained. This procedure is repeated five times to obtain the average of one of the five obtained contact angle values. Then, the average of the contact angle values ​​of the deionized water and diiodomethane is used to calculate the surface energy according to Young's Equation. Here, Θ represents the contact angle of a liquid with respect to a thin film, and σ... l The system represents the surface tension of the liquid, σ. s It is the surface energy of the thin film, and σ sl It is the interfacial tension between the film and the liquid. The interfacial tension between the film and the liquid can be obtained using the known Owens-Wendt-Rabel-Kaelble method. For more specific methods, see known literature (e.g., Supplementary Information - Surface energy and Wettability of van der Waals structures, Nanoscale, Issue 10, 2016). Experimental Demonstration Example 6 Measurement of transmittance and haze Total transmittance and haze were measured according to JIS K 7136 standard using a haze meter NDH-5000W manufactured by Nippon Denshoku Kogyo. Experimental Demonstration Example 7 Measurement of the yellow index The yellow index (YI) was measured using a spectrometer (UltraScan PRO, Hunter Associates Laboratory) under conditions of d65 and 10° according to the ASTM-E313 standard. The results of these experimental examples are shown in the table below. [Table 3] As can be seen from the table above, in which a polyamide-based film (PAI or PA) is adopted as a base layer and the primer layer contains a polyester-based resin, all of these multilayer films have high surface energy and are excellent in the cross-cut test of the rigid coating layer and the printed coating layer, resulting in high peel strength of the adhesive layer. Conversely, the multilayer films of Comparative Examples 1 to 3, in which the primer layer does not contain a polyester-based resin, have low surface energy and perform poorly in the cross-cut adhesion test of the rigid coating, resulting in low peel strength of the adhesive layer. Furthermore, the multilayer film of Comparative Example 4, in which a polyimide (PI) is incorporated, and the multilayer film of Comparative Example 5, in which no primer layer is formed, perform poorly in the cross-cut adhesion test of the rigid coating, resulting in low peel strength of the adhesive layer. 3: Test solution 10a: Sample (substrate layer) 10b: Sample (functional layer) 10c: Sample (surface) 11: Multilayer film according to one specific example 12: Multilayer film according to another specific example 13: Multilayer film according to yet another specific example 21: Calibration plate 22a: Upper clamp 22b: Lower clamp 100: Substrate layer 200: Primer layer 210: First primer layer 220: Second primer layer 300: Functional layer 310: First functional layer 320: Second functional layer Figure 1 shows a cross-sectional view of one of the multilayer films according to a specific example. Figure 2 shows a cross-sectional view of one of the multilayer films according to another specific example. Figure 3 shows a cross-sectional view of one of the multilayer films according to another specific example. Figure 4 shows an evaluation standard for the cross-cut test according to ASTM D3359 Method B. Figure 5 illustrates a 180° peel strength test method according to the ASTM D903 standard. Figure 6 illustrates a sessile drop method for calculating surface energy. 11: Multilayer thin films according to a specific example 100: Basal layer 200: Primer layer

Claims

1. A multilayer film comprising: a base layer comprising a polyamide polymer, and a primer layer formed on the base layer, wherein the polyamide polymer comprises an imide repeating unit and an amide repeating unit in a molar ratio of 2:98 to 70:30, wherein the primer layer comprises a polyester resin comprising 60% to 95% by weight of the total weight of the primer layer, wherein the multilayer film further comprises a functional layer interposed between the base layer and the functional layer, the functional layer being selected from at least one of the group consisting of: a rigid coating layer, an optical adhesive layer, and a printed coating layer, wherein the surface of the rigid coating layer or the printed coating layer has a cross-cut adhesion test result of 5B or higher according to ASTM D3359, and the optical adhesive layer has a surface area of ​​0.8 relative to the base layer according to ASTM D903. A peel strength of 180° or higher of kgf / inch, wherein the primer layer has a thickness ranging from 0.03 μm to 1.0 μm; as well as The multilayer thin film is prepared by a method comprising: preparing the substrate layer; And a primer layer composition is applied on the substrate and dried thereon, wherein the primer layer composition comprises a polymer resin and a solvent, and the polymer resin comprises a polyester-based resin comprising 60% to 95% by weight of the total solids content of the primer layer composition, wherein the solvent comprises more than 75% by weight of water and less than 25% by weight of isopropanol based on the total weight of the solvent.

2. The multilayer film of claim 1, wherein the primer layer further comprises a polyurethane-based resin comprising 5% to 40% by weight of the total weight of the primer layer.

3. The multilayer film of claim 1, wherein the surface energy of the primer layer is 30 dynes to 70 dynes.

4. The multilayer film of claim 1, having a haze of 3% or less, a transmittance of 85% or more, and a yellow index (YI) of 5 or less.

5. The multilayer film as requested in item 1, wherein the surface of the primer layer is rated 4B or higher according to the cross-cut adhesion test result of ASTM D3359.

6. A method for preparing a multilayer thin film as claimed in claim 1, comprising: Prepare a substrate layer containing a polyamide-based polymer; And to coat a primer composition on the substrate and dry it, wherein the primer composition comprises a polymer resin and a solvent, and the polymer resin comprises a polyester-based resin comprising 60% to 95% by weight of the total solids content of the primer composition, wherein the solvent comprises more than 75% by weight of water and less than 25% by weight of isopropanol based on the total weight of the solvent.

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