Easily bondable polyester film

Abstract

This invention provides an easily adhesive polyester film for use in foldable displays, exhibiting excellent adhesion to various functional layers; and a laminated polyester film having the functional layers. The easily adhesive polyester film for foldable displays has a polyester film substrate and a coating layer on at least one side of the substrate. This coating layer is formed from a composition comprising a polycarbonate polyurethane resin (A), a polyester resin (B), and a capped isocyanate crosslinking agent (C). In a force curve observed using an atomic force microscope on the surface of the coating layer under a load of 3 nN, the proportion of the area with a deformation of 1 nm or more is 0.15% or more relative to the overall observed surface.

Description

Technical Field

[0001] This invention relates to an easily adhesive polyester film, which is a polyester film used in foldable displays and has excellent adhesion to various functional layers; and a laminated polyester film having the functional layers. Background Technology

[0002] Hard-coated films with a transparent hard coating are used on the front surfaces of touch panels, displays for computers, televisions, liquid crystal displays, and decorative materials. Furthermore, panel components used in displays are formed by bonding the hard-coated film to a polarizer or to other components, but in this bonding process, an adhesive component is applied before bonding.

[0003] Transparent plastic films used as substrates are typically transparent polyester films. To improve the adhesion between the polyester film substrate and the hard coating and adhesive, an easily adhesive coating layer is usually applied to the surface of the polyester film as an intermediate layer.

[0004] The aforementioned hard-coated films require durability against temperature, humidity, and light, as well as transparency, chemical resistance, scratch resistance, and stain resistance. Furthermore, since they are often used on the surfaces of displays and decorative materials, visual legibility and design flexibility are also required. Therefore, to suppress glare and iridescent spots caused by reflected light when viewed from any angle, a multi-layered anti-reflective layer, consisting of stacked high-refractive-index and low-refractive-index layers, is typically applied over the hard coating.

[0005] In recent years, hard coatings with various compositions have been developed, and the adhesion between the substrate and the hard coating is discussed based on their composition. For example, when using a film with a hard coating laminated on it in a liquid crystal television display, in order to ensure high reliability for long-term use, not only the initial adhesion immediately after lamination is required, but also properties such as resistance to damp heat and retention of adhesion over time are also required.

[0006] For the aforementioned adhesives, similar requirements exist regarding durability against temperature, humidity, and light, transparency, chemical resistance, scratch resistance, and stain resistance. Furthermore, since they are frequently used on the surfaces of displays and decorative materials, visual legibility and design flexibility are also essential. In recent years, adhesives with various compositions have been developed, and the adhesion between the substrate and the adhesive is discussed based on their composition. For example, when components bonded using adhesives are used in the displays of LCD televisions, to ensure high reliability for long-term use, not only initial adhesion after lamination is required, but also resistance to damp heat and moisture, and retention of adhesion over time.

[0007] Furthermore, due to the impact and deformation caused by folding, there are concerns about a reduction in the adhesion of hard coatings, adhesives, etc. Therefore, it is required that the adhesion be ensured not only in the aforementioned free state, but also in the harsh environment caused by folding.

[0008] In liquid crystal display devices, polarizing plates are disposed on both sides of the glass substrate on which the surface of the liquid crystal panel is formed, depending on the image formation method. The polarizing plates typically have a structure in which a protective polarizing film is bonded to both sides of a polarizer made of a polyvinyl alcohol-based film and a dichroic material such as iodine via a hydrophilic adhesive such as a polyvinyl alcohol-based resin. From the viewpoint of optical properties and transparency, triacetyl cellulose film has traditionally been used as the protective film for protecting the polarizer.

[0009] However, the durability of triacetylcellulose is not sufficient. When using a polarizing plate with a triacetylcellulose film as a protective film for polarizers under high temperature or high humidity conditions, the polarization degree, hue, and other performance characteristics of the polarizer may sometimes decrease. Furthermore, in recent years, to meet the demand for thinner displays, there is a requirement for thinner polarizing plates; however, from the viewpoint of maintaining moisture barrier properties, there are limits to the thinning of triacetylcellulose films. Therefore, a solution using polyester film has been proposed as a protective film for polarizers that possesses both durability and moisture barrier properties (for example, see Patent Document 1).

[0010] Triacetyl cellulose film used as a protective film for polarizers undergoes alkali treatment on its surface, resulting in extremely high affinity for hydrophilic adhesives. Therefore, the protective film composed of triacetyl cellulose film exhibits extremely high adhesion to polarizers coated with hydrophilic adhesives. However, the adhesion between polyester film and hydrophilic adhesives is not sufficient, especially in the case of polyester films that have been oriented through stretching treatment, where this tendency becomes more pronounced. Therefore, to improve adhesion to polarizers or hydrophilic adhesives coated on polarizers, it has been reported to form a covering layer by coating the surface of the polyester film with a highly hydrophilic material (see, for example, Patent Document 1).

[0011] In the field of readily adhesive polyester films, it has been reported that laminated polyester films, in which a coating layer containing a polyester resin containing a naphthalene dicarboxylic acid component is provided on at least one side of the polyester film, exhibit excellent adhesion to surface functional layers such as hard coatings (for example, see Patent Document 2).

[0012] In addition, it has been reported that an easy-to-adhere polyester film with a coating layer provided on at least one side of the polyester film exhibits excellent adhesion to an optical functional layer under high temperature and high humidity, wherein the coating layer contains a polyurethane resin with aliphatic polycarbonate polyol as a component, which has excellent flexibility and high adhesion (for example, see Patent Document 3).

[0013] However, although the adhesion was confirmed, an easy-to-adhere polyester film with excellent adhesion after long-term storage was not obtained.

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent Application Publication No. 2013-063610

[0017] Patent Document 2: Japanese Patent Application Publication No. 2011-246663

[0018] Patent Document 3: Japanese Patent Application Publication No. 2011-168053 Summary of the Invention

[0019] The problem the invention aims to solve

[0020] This invention was made against the backdrop of the aforementioned problems in the prior art. Specifically, the object of this invention is to provide an easily bondable polyester film with excellent adhesion to functional layers such as hard coatings and adhesives, and to provide an easily bondable polyester film for use in foldable displays. Furthermore, a laminated polyester film having this functional layer is provided.

[0021] Solution for solving the problem

[0022] To achieve the above objectives, the inventors conducted in-depth research, resulting in the completion of this invention. Specifically, this invention comprises the following technical solutions. [1]

[0024] An easily adhesive polyester film for foldable displays, comprising a polyester film substrate having a coating layer on at least one side of the substrate.

[0025] The coating layer is formed from a composition comprising polycarbonate polyurethane resin (A), polyester resin (B), and a capped isocyanate crosslinking agent (C).

[0026] In the force curve observed using an atomic force microscope on the surface of the coating layer and pressed under a load of 3nN, the proportion of the area with a deformation of more than 1nm is more than 0.15% of the total observed surface. [2]

[0028] According to [1], the easy-to-adhere polyester film for foldable displays, wherein the polyester resin (B) has a naphthalene skeleton in its molecule. [3]

[0030] According to [1] or [2], the easy-to-adhere polyester film for foldable displays, wherein the end-capping agent of the end-capping isocyanate-based crosslinking agent (C) is a pyrazole compound. [4]

[0032] The easy-to-adhere polyester film for foldable displays according to any one of [1] to [3], wherein the polycarbonate polyurethane resin (A) has a hexane structure in its molecule. [5]

[0034] The easy-to-adhere polyester film for foldable displays according to any one of [1] to [4], wherein the polycarbonate polyurethane resin (A) has a hexane structure of more than 10% by mass in the molecule. [6]

[0036] A laminated polyester film for a foldable display, wherein a functional layer is further provided on the coating layer of the foldable easy-to-adhere polyester film for a foldable display as described in any one of [1] to [5]. [7]

[0038] A method for manufacturing an easy-to-adhere polyester film for a foldable display, which is the method for manufacturing an easy-to-adhere polyester film for a foldable display as described in any one of [1] to [5], comprising: a step of forming a coating layer on at least one side of a polyester film substrate,

[0039] The coating layer formation process includes a heating process at 180°C or higher. [8]

[0041] The method for manufacturing a laminated polyester film for a foldable display according to [6] includes: further coating a functional layer forming composition onto a coating layer of an easily adhesive polyester film obtained by the manufacturing method described in [7] to form a functional layer.

[0042] The effects of the invention

[0043] The easily bondable polyester film of this invention is suitable for use as a film for foldable displays. It exhibits excellent adhesion between the functional layer and the polyester film (especially after long-term storage), resulting in high bonding reliability. Furthermore, it also possesses excellent anti-blocking properties and transparency. Therefore, it can be widely used in optical applications and the like. Detailed Implementation

[0044] The term "display" in this invention refers to a display device in general. Types of displays include LCDs, organic EL displays, inorganic EL displays, LEDs, and FEDs, with LCDs, organic EL displays, and inorganic EL displays having a bendable structure being preferred. From the perspective of reducing the number of layers required for the display, organic EL displays and inorganic EL displays are preferred, and from the perspective of a wide color gamut, organic EL displays are further preferred.

[0045] The foldable display mentioned in this invention refers to a display that can be folded in half or similarly when carried. Folding reduces size, improving portability and achieving a thinner profile in the folded state. The bending radius during folding refers to the radius of the inner side of the folded portion. A smaller bending radius is generally better, but it also increases the likelihood of creases and cracking of the layered hard coating and other functional layers. It should be noted that the surface protective film described later can be located on either the outer or inner side of the folded portion of the foldable display.

[0046] In addition, the foldable display can be two-fold, three-fold, four-fold, or it can be called a rollable type, all of which are included in the foldable display described in this invention.

[0047] Alternatively, the present invention can be a laminated polyester film for foldable displays, wherein a coating layer providing easy adhesion is provided on one side of the polyester film, and a functional layer, represented by a hard coating layer, is present thereon. As an example, it is applied to foldable displays. The film of the present invention can be used in any part of a foldable display as long as it is a component. Hereinafter, taking an organic EL display as an example, a representative configuration of a foldable display and the parts of a foldable display in which the film of the present invention can be used will be described.

[0048] In a foldable organic EL display, an organic EL module is included as an essential component, and further, as needed, a circular polarizing plate, a touch panel module, a surface protective film, a back protective film, etc.

[0049] The general structure of an organic EL module includes electrodes, an electron transport layer, a light-emitting layer, a hole transport layer, and a transparent electrode.

[0050] Portable terminal devices preferably have a touch panel. In the case of an organic EL display, the touch panel module is preferably disposed on the upper part of the organic EL display, or between the organic EL module and the circular polarizer. The touch panel module may have a transparent substrate such as a thin film and transparent electrodes disposed thereon. A laminated polyester film to which the easy-to-adhere polyester film of the present invention has been given a functional layer can be used as the transparent substrate. When a laminated polyester film is used as the transparent substrate of the touch panel, it is preferable to provide a refractive index adjustment layer on the film.

[0051] Circular polarizers can suppress image quality degradation caused by reflection of external light from internal components of the display. A circular polarizer can have both a linear polarizer and a retardation plate. A linear polarizer can have a protective film on at least the visible side of the polarizer. A linear polarizer can also have a protective film on the side of the polarizer opposite to the visible side, or a retardation plate can be directly laminated onto the polarizer. As a retardation plate, a resin film with a phase retardation, such as polycarbonate or cyclic olefins, can be used; or an article having a retardation layer formed of a liquid crystal compound disposed on the resin film. The laminated polyester film of the present invention can be used as a protective film for the polarizer. In these cases, it is preferable that the slow axis direction of the polyester film is parallel or orthogonal to the absorption axis direction of the polarizer. Furthermore, a deviation of 10 degrees, preferably 5 degrees, relative to this parallelism or orthogonality is acceptable.

[0052] If an impact is applied to the display from above, the circuitry of the OLED module and touch panel module may break. Therefore, in most cases, a surface protective film is provided for the display. The laminated polyester film of the present invention can be used as this surface protective film. The surface protective film can be either a film called a cover window assembled to the outermost surface of the display, or a film called an outer layer that can be applied, peeled off, and replaced by the user. The laminated polyester film of the present invention can be used on the surface of a foldable display such that the side with the functional layer is the visible side of the display. It should be noted that the functional layer can be provided on both sides of the laminated polyester film.

[0053] It is also preferable to provide a protective film on the back side of the display. The laminated polyester film of the present invention can be used as the protective film on this back side.

[0054] The laminated polyester film of the present invention can be any laminated polyester film other than those described above, as long as it is used for the portion of the components of a foldable display that is folded.

[0055] Among these, the laminated polyester film of the present invention is preferably used to cover the window surface protective film, the back surface protective film, the substrate film of the touch panel module, and the back protective film. Furthermore, it is preferably used to cover the window surface protective film and the back surface protective film.

[0056] Furthermore, as a foldable display, it is not necessary to use the laminated polyester film of the present invention in all of the above applications. In foldable displays, films having functional layers based on polyester films, polyimide films, polyamide films, polyamide-imide films, polycarbonate films, acrylic films, triacetyl cellulose films, cyclic olefin polymer films, polyphenylene sulfide films, polymethylpentene films, etc., can be used as appropriate, depending on adaptability.

[0057] Polyester film

[0058] The polyester film used as the substrate in the easily adhesive polyester film of the present invention is a film mainly composed of polyester resin. Here, "film mainly composed of polyester resin" means a film formed from a resin composition containing 50% by mass or more of polyester resin. When blended with other polymers (e.g., polycarbonate resin, polyimide resin, etc.), it means containing 50% by mass or more of polyester resin; when copolymerized with other monomers, it means containing 50% by mole or more of polyester structural units. Preferably, the polyester film contains 90% by mass or more, more preferably 95% by mass or more, and even more preferably 100% by mass of polyester resin.

[0059] There are no particular limitations on the materials of polyester resins; copolymers formed by the condensation of dicarboxylic acid components and diol components, or mixtures thereof, can be used.

[0060] Examples of dicarboxylic acid components that constitute polyester resins include terephthalic acid, isophthalic acid, phthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracene dicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyl adipic acid, trimethyl adipic acid, pimelic acid, azelaic acid, dimer acid, sebacic acid, octanoic acid, and dodecanedicarboxylic acid.

[0061] Examples of diols that constitute polyester resins include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanediethanol, 1,4-cyclohexanediethanol, decamethyldiol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.

[0062] One or more dicarboxylic acid components and two diol components can be used. In addition, other polycarboxylic acid components such as trimellitic acid and other polyol components such as trimethylolpropane can also be added appropriately.

[0063] Specifically, examples of polyester resins include polyethylene terephthalate, polyethylene terephthalate, polyethylene terephthalate, and polyethylene naphthalate. Among these, polyethylene terephthalate is preferred from the perspective of balancing physical properties and cost. Furthermore, including other copolymer components and other polymers is also a preferred approach to control optical properties such as polarization. From the viewpoint of controlling the optical properties of the polyester film, preferred copolymer components include diethylene glycol and copolymer components with norbornene side chains.

[0064] The intrinsic viscosity of the polyester resin (solvent: phenol / tetrachloroethane = 60:40) is, for example, 0.50~1.0 dl / g.

[0065] To improve the handling properties of polyester films, such as their slip properties and winding properties, the films can contain inactive particles. Examples of inactive particles include silica, kaolinite, talc, light calcium carbonate, heavy calcium carbonate, zeolite, alumina, barium sulfate, carbon black, zinc oxide, zinc sulfate, zinc carbonate, titanium dioxide, satin white, aluminum silicate, diatomaceous earth, calcium silicate, aluminum hydroxide, hydrated halloysite, magnesium carbonate, and magnesium hydroxide. The average particle size of these inactive particles can be, for example, 200–5000 nm, or even 250–4500 nm. This average particle size is determined using the method described in the examples (average particle size based on the number of particles measured by SEM).

[0066] To maintain high transparency, it is preferable that the content of inactive particles in the film is as low as possible. Therefore, it is preferable to form a multilayer structure in which particles are present only on the surface of the film, or to have no particles in the film at all, with particles present only in at least one side of the coating layer laminated on the polyester film.

[0067] It should be noted that "substantially free of particles" means, for example, in the case of inorganic particles, that when quantitative analysis of elements derived from particles is performed by fluorescence X-ray analysis, the content is 50 ppm or less, preferably 10 ppm or less, and most preferably below the detection limit. This is because, even without actively adding particles to the substrate film, contaminants from foreign matter, raw material resins, or dirt adhering to the production line or equipment during the film manufacturing process can inevitably be mixed into the film.

[0068] Furthermore, when the polyester film is made into a multilayer structure, two three-layer structures can be produced, in which the inner layer substantially does not contain inactive particles, and only the outermost layer (second layer) contains inactive particles. This allows for a balance between transparency and processability, and is therefore preferred.

[0069] The polyester film used as the substrate can be a single layer or composed of two or more layers laminated together. Furthermore, various additives can be included in the film as needed, provided they are within the scope of achieving the effects of this invention. Examples of additives include antioxidants, lightfastness agents, anti-gelling agents, organic wetting agents, antistatic agents, ultraviolet absorbers, and surfactants. When the film has a laminated structure, it is preferable to include additives according to the function of each layer. For example, adding ultraviolet absorbers to the inner layers to prevent light degradation of the polarizer is also a preferred approach.

[0070] Polyester film can be manufactured using conventional methods. For example, it can be obtained by melting and extruding a material containing the aforementioned polyester resin into a film shape and then cooling and solidifying it using a casting drum. As the polyester film of the present invention, either unstretched film or stretched film can be used, but stretched film is preferred from the perspective of durability, such as mechanical strength and chemical resistance.

[0071] When the polyester film is a stretched film, the stretching method is not particularly limited, and can include longitudinal uniaxial stretching, transverse uniaxial stretching, longitudinal and transverse successive biaxial stretching, and simultaneous longitudinal and transverse biaxial stretching. When stretching a polyester film, stretching can be performed before or after the lamination of the easily bondable coating layer (described later). Alternatively, uniaxial stretching can be performed longitudinally or transversely before laminating the easily bondable coating layer, and then stretched in the other direction after laminating the coating layer.

[0072] Coating layer

[0073] The easy-to-adhesive polyester film for foldable displays of the present invention has an easy-to-adhesive coating layer laminated on at least one side of the polyester film, which serves as the substrate.

[0074] The easy-to-adhere polyester film for foldable displays of the present invention has a polyester film substrate and a coating layer on at least one side of the substrate, said coating layer being formed of a composition comprising a polycarbonate polyurethane resin (A), a polyester resin (B), and a capped isocyanate crosslinking agent (C).

[0075] Furthermore, in one embodiment, in a force curve obtained by observing the surface of the coating layer under an atomic force microscope and pressing it with a load of 3 nN, the proportion of the area with a deformation of 1 nm or more is 0.15% or more relative to the overall observed surface. The coating layer can be formed from a composition comprising a binder resin and additives as needed.

[0076] The components of the coating layer are described in detail below. The adhesive resin constituting the coating layer is a resin with good adhesion, including polycarbonate polyurethane resin (A), which is a urethane resin with a polycarbonate structure, and polyester resin (B). By using a coating layer composed of these, the adhesion between the polyester film substrate and the functional layer is significantly improved.

[0077] The reason for this is the good compatibility between the coating layer and the functional layer described later. Furthermore, the functional layer, obtained by UV curing via UV irradiation, has a cross-linked mesh structure. The polycarbonate polyurethane resin and polyester resin in the coating layer interact with each other, easily fusing and entangled. Moreover, it is believed that by entangled these intertwined resins with the mesh structure of the functional layer formed thereon, a coating layer with superior adhesion compared to previous methods is achieved. Furthermore, since the coating layer contains polyester resin, its adhesion to the polyester film as the substrate is also good, resulting in a coating layer with even superior adhesion.

[0078] (Polycarbonate polyurethane resin (A))

[0079] There are no particular limitations on polycarbonate polyurethane resins as long as they are urethane resins with a polycarbonate structure in their molecules. They may not have a hexane structure in their molecules, but it is preferred to have one.

[0080] When the polycarbonate polyurethane resin has a hexane structure, the content of the hexane structure is typically 10% by mass or more, 10% by mass or more and 90% by mass or less, preferably 11% by mass or more and 85% by mass or less, more preferably 15% by mass or more and 80% by mass or less, and even more preferably 20% by mass or more and 70% by mass or less. If the content is 10% by mass or more and 90% by mass or less, the proportion of the highly flexible structure present on the film surface increases, thus maintaining the surface flexibility of the coating layer, and making it easier to achieve good adhesion under normal temperature and constant temperature and humidity conditions. The content of the hexane structure in the polycarbonate polyurethane resin is obtained by calculating the ratio of the mass of the hexane structure to 100g of the total mass of the polycarbonate polyurethane resin. Specifically, it can be calculated from "the molecular weight and molar percentage of each component constituting the polycarbonate polyurethane resin, such as the polycarbonate diol component and the diisocyanate component", "the molar percentage of the component having the hexane structure", and "the proportion of the molecular weight of the hexane structure".

[0081] Polycarbonate polyurethane resin is a resin obtained by adding polycarbonate diol, diisocyanate, and diol as a chain extender as needed, wherein at least one component has a hexane structure.

[0082] Examples of diols required for preparing polycarbonate polyurethane resins include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanediethanol, 1,4-cyclohexanediethanol, 1,4-cyclohexanediethanol, decamethyl glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone. Among these, 1,2-cyclohexanediethanol, 1,4-cyclohexanediethanol, 1,4-cyclohexanediethanol, and 1,6-hexanediol, which have the aforementioned hexane structure, are particularly preferred. Furthermore, these diols can be used alone or in combination of two or more. The ratio when combining two or more is not particularly limited and can be adjusted to obtain a polycarbonate polyurethane resin possessing the necessary properties.

[0083] Examples of diisocyanate components required for preparing polycarbonate polyurethane resins include toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, phenylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, 4,4-methylene dicyclohexyl diisocyanate, 1,2-bis(isocyanate methyl)cyclohexane, and 1,4-bis(isocyanate methyl)cyclohexane. Among these, isophorone diisocyanate, 4,4-methylene dicyclohexyl diisocyanate, 1,2-bis(isocyanate methyl)cyclohexane, and 1,4-bis(isocyanate methyl)cyclohexane are particularly preferred, as they have a structure in which at least one hydrogen atom on the cyclohexane ring is substituted with a hydrocarbon group. Furthermore, these isocyanate components can be used alone or in combination of two or more. The ratio when combining two or more is not particularly limited and can be adjusted to obtain a polycarbonate polyurethane resin with the desired properties.

[0084] (Polyester resin (B))

[0085] There are no particular limitations on the polyester resin as long as it is a copolyester composed of dicarboxylic acid and diol components, and is formed by the condensation polymerization of dicarboxylic acid and diol components. One or more dicarboxylic acid and diol components can be used respectively. In this invention, the polyester resin (B) preferably has a naphthalene skeleton in its molecule, for example, it is particularly preferred to have a structure containing a naphthalene ring in the dicarboxylic acid component.

[0086] Examples of dicarboxylic acid components include terephthalic acid, isophthalic acid, phthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracene dicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, cyclohexylmethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, cyclohexylmethylsuccinic acid, glutaric acid, 2,2-dimethylglutaric acid, adipic acid, 2-methyl adipic acid, trimethyl adipic acid, pimelic acid, azelaic acid, dimer acid, sebacic acid, octanoic acid, dodecanedicarboxylic acid, and 1,1-cyclohexanediacetic acid. Among them, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, and 1,5-naphthalenedicarboxylic acid have the above-mentioned structure containing a naphthalene ring, and are particularly preferred.

[0087] In one embodiment, polyester resin (B) is used 1 In the H-NMR composition ratio analysis, the dicarboxylic acid component includes a component containing a naphthalene ring in a proportion that is preferably 50 mol% or more and 95 mol% or less, more preferably 60 mol% or more and 90 mol% or less relative to the total amount of dicarboxylic acid components.

[0088] Examples of diols include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanediethanol, 1,4-cyclohexanediethanol, 1,4-cyclohexanediethanol, decamethyl glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.

[0089] The ratio of the dicarboxylic acid component to the diol component is not particularly limited and can be adjusted to obtain a polyester with the desired properties.

[0090] The exact mechanism by which the polycarbonate polyurethane resin and polyester resin used in the coating layer exhibit the effects of the present invention is not yet clear, but the coating layer of the present invention can improve the flexibility of the resin while maintaining its rigidity, thus performing its function optimally as a coating layer. That is, it not only has excellent adhesion to the substrate, such as polyester film and functional layer, but also significantly improves adhesion over time. As a result, a film that maintains its quality for a long time can be obtained.

[0091] In particular, in this invention, the change in the adhesion to the functional layer is small after film formation and after damp heat treatment. Compared with the past, it can improve the resistance to damp heat and the retention of adhesion over time.

[0092] Furthermore, based on the aforementioned effects, according to the present invention, even when subjected to forces due to impacts or deformation caused by folding, good adhesion to hard coatings, adhesives, etc., can be maintained. Thus, the present invention ensures adhesion not only in the free state but also in harsh environments caused by folding.

[0093] In this invention, it is important that the adhesive resin comprises polycarbonate polyurethane resin and polyester resin. The ratio of the two is not particularly limited as long as it ensures the physical properties of the resulting coating layer. When the total amount of the adhesive resin is set to 100% by mass, the polycarbonate polyurethane resin is generally preferably 40% by mass or more and 85% by mass or less, more preferably 45% by mass or more and 80% by mass or less, and even more preferably 50% by mass or more and 75% by mass or less. By making the polycarbonate polyurethane resin 40% by mass or more, a balance between the softness and hardness of the urethane resin can be ensured, thus ensuring adhesion over time. Furthermore, by setting it to 85% by mass or less, a balance between the hardness and softness of the coating layer can be maintained, and softness can also be improved, thus allowing the coating layer to function optimally.

[0094] (Cross-linking agent)

[0095] In this invention, in order to form a cross-linked structure between the resins in the coating layer, the composition used in forming the coating layer includes a capped isocyanate-based cross-linking agent (C). By containing the cross-linking agent of this invention, the adhesion under high temperature and high humidity conditions can be further improved. Furthermore, based on the above effects, according to this invention, even when subjected to forces applied due to impacts or deformation caused by folding, good adhesion to hard coatings, adhesives, etc., can be maintained. Thus, the present invention ensures adhesion not only in the free state but also in harsh environments caused by folding.

[0096] In this invention, in addition to the isocyanate-based crosslinking agent (C), other crosslinking agents include urea-based, epoxy-based, melamine-based, isocyanate-based, oxazoline-based, and carbodiimide-based agents, among others. From the viewpoint of improving the stability of the coating solution over time and enhancing adhesion under high temperature and humidity conditions, isocyanate-based crosslinking agents are preferred. Furthermore, catalysts may be used as needed to promote the crosslinking reaction.

[0097] When the composition for forming the coating layer includes an adhesive resin and a crosslinking agent, and the total mass of the adhesive resin and the crosslinking agent is set to 100% by mass, from the viewpoint of adhesion, the content of the adhesive resin is preferably 50-95% by mass, more preferably 55-90% by mass, further preferably 60-90% by mass, and most preferably 80-90% by mass. If it is 95% by mass or less, the strength of the coating film is maintained, and the adhesion under high temperature and high humidity is good. If it is 50% by mass or more, the flexibility of the coating layer is maintained, and the adhesion under room temperature, high temperature and high humidity is maintained, which is preferred. In addition, when the total mass of the adhesive resin and the crosslinking agent is set to 100% by mass, the content of the crosslinking agent is preferably 5-50% by mass, more preferably 10-45% by mass, further preferably 10-40% by mass, and most preferably 10-20% by mass.

[0098] When using a capped isocyanate-based crosslinking agent (C) as the crosslinking agent, a capping agent is preferred to control the reactivity of the isocyanate. Examples of capping agents include: bisulfite compounds such as sodium bisulfite; pyrazole compounds such as 3,5-dimethylpyrazole, 3-methylpyrazole, 4-bromo-3,5-dimethylpyrazole, and 4-nitro-3,5-dimethylpyrazole; phenols such as phenol and cresol; aliphatic alcohols such as methanol and ethanol; active methylene compounds such as dimethyl malonate and acetylacetone; thiols such as butyl mercaptan and dodecyl mercaptan; amides such as acetanilide and acetamide; lactams such as ε-caprolactam and δ-valeramide; imides such as succinimide and maleimide; oximes such as acetaldehyde oxime, acetone oxime, and methyl ethyl ketone oxime; and amines such as diphenylaniline, aniline, and ethyleneimine. From a reactivity point of view, pyrazole compounds with a pyrazole skeleton are suitable as end-capping agents in this system.

[0099] From the viewpoint of imparting water dispersibility in aqueous solvents, it is preferable to introduce hydrophilic groups into the end-capped isocyanate-based crosslinking agent. Furthermore, as hydrophilic groups, anionic groups such as carboxyl or sulfonic acid groups, and nonionic groups such as oxyalkyl groups are preferred. These crosslinking agents with hydrophilic groups can be prepared by reacting a polyisocyanate, which is the base of the end-capped isocyanate, with a compound having hydrophilic groups and reactive groups such as hydroxyl groups.

[0100] (additive)

[0101] In the coating layer of the present invention, known additives may be added without impairing the effects of the present invention, such as surfactants, antioxidants, heat stabilizers, weather stabilizers, ultraviolet absorbers, organic slip agents, pigments, dyes, organic or inorganic particles, antistatic agents, nucleating agents, etc. However, it is preferable not to use substances that are environmentally unfriendly.

[0102] To further improve the anti-blocking properties of the coating layer, adding inactive particles to the coating layer is also a preferred method. Examples of particles contained in the coating layer include inorganic particles and organic polymer-based particles. Examples of inorganic particles include titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, or mixtures thereof. Furthermore, they can be used in combination with other common inorganic particles, such as calcium phosphate, mica, lithium montmorillonite, zirconium oxide, tungsten oxide, lithium fluoride, and calcium fluoride. Examples of organic polymer-based particles include styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and organosilicon-based polymer-based particles.

[0103] The average particle size (average particle size based on the number of SEM particles, the same below) of the inactive particles in the coating layer is preferably 0.01 to 20 μm, more preferably 0.04 to 2.0 μm, and even more preferably 0.1 to 1.0 μm. When the average particle size of the inactive particles is 0.04 μm or more, it is easy to form unevenness on the surface of the coating layer, thus improving the operability of the easily bondable polyester film, such as its sliding and winding properties, and improving the processability during lamination, which is therefore preferred. On the other hand, when the average particle size of the inactive particles is 2.0 μm or less, particle shedding is less likely to occur, which is also preferred. The particle concentration in the coating layer is preferably 1 to 20% by mass relative to the resin composition.

[0104] (Formation of the coating layer)

[0105] In the coating layer forming composition (hereinafter also referred to as "coating liquid") used to form the coating layer, a surfactant may be further contained to improve the leveling properties during coating and to defoam the coating liquid. Examples of surfactants include cationic, anionic, and nonionic surfactants, with silicone-based, acetylenol-based, or fluorinated surfactants being preferred. These surfactants are preferably contained in the coating layer forming composition to the extent that they do not impair the suppression effect of iris spots under the three-wavelength LED light source or the degree of adhesion.

[0106] As a method for coating a coating liquid onto a polyester film, it can be applied to either the so-called online coating method, in which the coating is performed simultaneously during the polyester film making process, or the so-called offline coating method, in which the polyester substrate film is made and then coated separately using a coating machine. However, the online coating method is more efficient.

[0107] As a coating method, any known method can be used to coat the coating solution onto a polyethylene terephthalate (PET) film. Examples include reverse roller coating, gravure coating, kiss coating, die coating, roller brush coating, spray coating, air knife coating, wire rod coating, tube doctor blade coating, dip coating, and curtain coating. These methods can be used individually or in combination.

[0108] In this invention, a method for forming a coating layer on a polyester film can be described as applying a coating liquid containing a solvent, particles, and resin onto the polyester film and then drying it. As a solvent, water or a mixture of water and an organic solvent can be cited, but from an environmental perspective, water alone or a solvent composed of water and a water-soluble organic solvent is preferred. Examples of water-soluble organic solvents include alcohols such as isopropanol and ethanol; ketones such as methyl ethyl ketone; ethers such as butyl cellosolve; amines such as triethanolamine; and amides such as N-methylpyrrolidone.

[0109] The concentration of solid components in the coating liquid also depends on the type of binder resin, the type of solvent, etc., and is preferably 2% by mass or more, more preferably 4% by mass or more, relative to the total mass of the coating liquid. The concentration of solid components in the coating liquid is preferably 35% by mass or less, more preferably 15% by mass or less.

[0110] The drying temperature after coating depends on the type of adhesive resin, the type of solvent, the presence or absence of a crosslinking agent, and the concentration of solid components, but is preferably 80°C or higher. For example, the coating layer formation process may involve a heating step at 180°C or higher. The coating layer formation process is preferably at 250°C or lower.

[0111] The coating amount of the coating solution can be adjusted to achieve a solid content on the dried polyester film, for example, 0.03~0.24 g / m². 2 Further, the concentration was 0.06~0.18 g / m³. 2 When a film stretching process is performed after coating and drying, the film can be prepared in such a way that the amount of solids on the stretched polyester film reaches the range described above.

[0112] The stretching process can be either uniaxial or biaxial stretching. The stretched polyester film can be heat-treated (e.g., 70~250°C, preferably 80~245°C) while fixed in a tenter frame, and then further relaxed at 120~250°C.

[0113] As described above, the easy-to-adhere polyester film of the present invention is manufactured through coating, drying, stretching and heat treatment processes.

[0114] The thickness of the coating layer can be 30 nm or more and 200 nm or less. If it is prepared within this range, it is easy to balance processability and adhesion, and is therefore preferred. More preferably, it is 40 nm or more and 150 nm or less, for example, 40 nm or more and 130 nm or less, and even more preferably 40 nm or more and 100 nm or less. When the thickness of the coating layer is 30 nm or more, the adhesion becomes good, and this is preferred. When the thickness of the coating layer is 200 nm or less, adhesion is less likely to occur, which is also preferred. Furthermore, within the above ranges, there is a tendency to easily control the phase separation state of the film surface.

[0115] Furthermore, by having the aforementioned thickness, even in foldable applications, the adhesion to the hard coating, adhesive, etc., can be well maintained despite forces exerted due to impacts or deformation caused by folding. Thus, by keeping the coating thickness within the aforementioned range, the present invention ensures adhesion not only in the free state but also in harsh environments caused by folding.

[0116] Regarding the thickness of the coating layer, the cross-section of the cut film was observed using a transmission electron microscope (TEM), and the average value obtained by randomly measuring the thickness of the coating layer at 10 points was taken as the thickness of the coating layer.

[0117] In this invention, it was discovered that the phase separation state of the thin film surface, which is related to surface functional properties such as adhesion, changes according to heating conditions. The distribution state of the low elastic modulus component of the surface, which changes according to the thermal history, is numerically represented as the distribution of deformation by AFM force curve mapping.

[0118] In this invention, the control of the phase separation state on the film surface can be performed in the drying process of the composition forming the coating layer. If a film stretching process is performed after the coating and drying of the composition, the phase separation state can also be controlled in the stretching process.

[0119] For example, in this invention, by controlling the phase separation state of the thin film surface, in order to derive a state in which the area with a deformation of more than 1 nm is greater than 0.15% of the overall observed surface in the force curve observed by an atomic force microscope and pressed with a load of 3 nN, the following steps can be used.

[0120] It should be noted that, at the time of this application, no specific mechanism was found that could control the phase separation state of the film surface through the process described in this specification, but it is speculated that, for example, by using the components and film-forming conditions described in this specification, the desired interaction can be generated among the components.

[0121] In this invention, a method for manufacturing a thin film in which the phase separation state of the thin film surface is controlled and the ratio of the area with a deformation amount of 1 nm or more to the overall observation surface is 0.15% or more includes the following steps.

[0122] For example, the composition forming the coating layer is coated onto a polyester film substrate and dried at a temperature of 30°C or higher and 120°C or lower, preferably at 60°C or higher and 100°C or lower.

[0123] It is speculated that drying within this temperature range can prevent the coating from completely curing, which helps control the phase separation state in the next process and subsequent processes.

[0124] Next, the film with the coating layer is preferably stretched (sometimes simply referred to as the stretching process) at a temperature of 80°C or higher and 180°C or lower, for example, 80°C or higher and 170°C. The stretching direction can be either the length direction or the width direction. If workability such as the relaxation treatment described later is taken into consideration, stretching can also be performed along the width direction.

[0125] The preferred film has a stretch ratio of 1.5 times or more and 6.0 times or less in the width direction, for example, 1.5 times or more and 5.0 times or less in the width direction.

[0126] Furthermore, in one approach, it is preferable to heat the film at a temperature higher than the temperature of the stretching process (sometimes simply referred to as the heating process) while the film's width length is fixed. For example, the heating temperature while the film's width length is fixed is performed at a temperature 10°C to 70°C higher than the stretching temperature, specifically at a temperature of 90°C or higher and 220°C or lower, for example, 100°C or higher and 230°C or lower.

[0127] In one embodiment, the film having the coating layer is preferably subjected to a relaxation treatment in the width direction (sometimes simply referred to as relaxation treatment) after the aforementioned heating step. The relaxation treatment is preferably performed at a temperature higher than the temperature of the stretching step. For example, it is performed at a temperature 10°C to 70°C higher than the stretching temperature, specifically at a temperature of 90°C or higher and 220°C or lower, for example, 100°C or higher and 220°C or lower. Preferably, the relaxation treatment is performed within ±10°C of the aforementioned heating step.

[0128] For example, the heating process and relaxation process can be performed at the same temperature, and in this case, the heating process and relaxation process can be performed continuously. When the heating process and relaxation process are performed continuously, the maximum temperature reached is 10°C to 70°C higher than the stretching temperature, specifically, heating is performed at a temperature of 90°C or higher and 220°C or, for example, 100°C or higher and 230°C or lower.

[0129] The total heating time is preferably within the range of 5 to 180 seconds. As described above, when the heating process and relaxation process are performed at the same temperature in a continuous process, the time required for the heating process and relaxation process is also preferably within the range of 5 to 180 seconds.

[0130] In this invention, it is believed that the heat applied to the coating film has a significant impact on the phase separation formed. As the heat applied to the coating film increases, phase separation accelerates, and the surface tends to become rougher. On the other hand, as the heat applied to the coating film decreases, phase separation slows down, and the surface tends to become smoother.

[0131] By adjusting the amount and temperature of the coating liquid within the conditions described in this specification, an optimal surface condition can be constructed, resulting in the formation of the easily adhesive film of the present invention that can maintain a tight seal over time.

[0132] Furthermore, even when subjected to forces due to impacts or deformation caused by folding, it can maintain good adhesion to hard coatings, adhesives, etc.

[0133] To confirm the distribution of deformation on the surface of easily bondable polyester films, measurements can be performed using atomic force microscopy (AFM) in PinPoint mode. A cantilever with a spring constant of approximately 1 N / m is used, and the field of view can be set to 1 μm square.

[0134] Specifically, following the PinPoint model manual, after evaluating the force slope, sensitivity, and spring constant, the measurements are performed under the following conditions.

[0135] The measurement conditions are as follows.

[0136] • Measurement apparatus: Atomic force microscope (NX10) manufactured by Park Systems.

[0137] • Measurement mode: PinPoint mode

[0138] • Cantilever: CONTSCR (NanoWorld), NSC18 (Mikro Masch), etc.

[0139] • Measurement atmosphere: 23℃ • Atmosphere

[0140] • Set point: 3nN

[0141] • Cantilever speed: 6μm / s

[0142] • Pixel count: 256×256

[0143] In this invention, it was found that when observing the surface of an easily bondable polyester film in the PinPoint mode of AFM, the deformation of more than 1 nm obtained from the force curve obtained by the 3nN indentation load is more than 0.15% of the overall measured area. This has a significant impact on the adhesion when the functional layers described later are laminated, resulting in higher adhesion durability.

[0144] Preferably, the percentage relative to the total measured area is 0.20% or more and 40.0% or less, more preferably 0.20% or more and 35% or less. For example, it can be 0.20% or more and 20.0% or less, or 0.30% or more and 14.0% or less.

[0145] If the value is 0.15% or higher, it can be seen that not only is the adhesion immediately after the functional layer is applied, but also the adhesion of the film after long-term non-use can be ensured, resulting in a film with high adhesion stability that does not change over time. However, if the deformation rate becomes too high and exceeds the scope of this invention, the adhesion reliability may decrease. Furthermore, when the deformation rate is too high and exceeds the scope of this invention, the flexibility of the film deteriorates, potentially making it unsuitable for foldable applications.

[0146] laminated polyester film

[0147] This invention also provides a laminated polyester film in which a functional layer having various properties is disposed on a coating layer of an easily adhesive polyester film. The functional layer refers to a layer with functions such as preventing reflection, suppressing glare, suppressing iris, and suppressing scratches, for example, a hard coating layer, an anti-glare layer, an anti-glare and anti-reflective layer, an anti-reflective layer, a low-reflection layer, and an antistatic layer. Various functional layers known in the art can be used for this purpose, and there is no particular limitation on their type. For example, the laminated polyester film can be manufactured by further coating a functional layer forming composition onto the coating layer and forming the functional layer.

[0148] For example, when forming a hard coating layer on the coating layer, the material for the hard coating layer can be a known material without particular limitation. As such a material, a resin compound (especially a curable resin) that undergoes polymerization and / or reaction by drying, heat, chemical reaction, or irradiation with an electron beam, radiation, or ultraviolet light can be used. Examples of such curable resins include melamine-based, acrylic-based, silicone-based, and polyvinyl alcohol-based curable resins. From the perspective of obtaining high surface hardness or optical design, a photocurable acrylic-based curable resin is preferred. As such an acrylic-based curable resin, polyfunctional (meth)acrylate monomers and acrylate oligomers can be used. Examples of acrylate oligomers include polyester acrylates, epoxy acrylates, urethane acrylates, polyether acrylates, polybutadiene acrylates, and silicone acrylates. By mixing a reaction diluent, a photopolymerization initiator, a sensitizer, etc., into these acrylic-based curable resins, a coating composition for forming the aforementioned optical functional layer can be obtained.

[0149] The aforementioned hard coating can possess an anti-glare function that scatters external light. This anti-glare function is achieved by creating an uneven surface on the hard coating. Ideally, the haze of the film is 0-50%, more preferably 0-40%, and particularly preferably 0-30%. Of course, 0% is ideal, but it can be 0.2% or more, or even 0.5% or more.

[0150] Furthermore, by setting layers with different refractive indices as functional layers, low-reflection processing (anti-reflection processing) can be implemented by changing the light transmission characteristics, thereby suppressing light reflection. Ideally, the refractive index of functional layers such as hard coatings should be adjusted to a reflectance of 0 to 1.0%, more preferably 0 to 0.8%, and particularly preferably 0 to 0.5%. Of course, 0% is ideal, but it can be 0.05% or more, or 0.1% or more.

[0151] The laminated polyester film of this invention is primarily used in the broader field of optical films, including base films for optical components such as LCDs, flat panel TVs, and CRTs, such as prism lenses, AR (anti-reflective) films, hard-coated films, diffuser plates, and shatterproof films; near-infrared absorbing filters for front panel components of plasma displays; and transparent conductive films such as touch panels and electroluminescent films. It is suitable for any application, but particularly well-suited for foldable displays.

[0152] Examples of acrylic resins that are cured by electron beam or ultraviolet light to form the above-mentioned functional layers include compositions comprising oligomers of (meth)acrylates as reactive oligomers and (meth)acrylate-based monomers as reactive monomers (reactive diluents).

[0153] Examples of oligomers of (meth)acrylates include compounds with reactive (meth)acryloyl groups bonded to the backbone of (meth)acrylate resins, polyester acrylates, epoxy acrylates, polyurethane acrylates, silicone acrylates, melamine acrylates, polyether acrylates, etc.

[0154] Examples of (meth)acrylate monomers include monofunctional monomers such as ethyl (meth)acrylate and ethylhexyl (meth)acrylate; and polyfunctional monomers such as trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentyl glycol di(meth)acrylate.

[0155] As needed, a third component may be added, such as polyester resin, polyether resin, acrylic resin, epoxy resin, urethane resin, alkyd resin, spiroacetal resin, polybutadiene resin, polythiol polyene resin, polyol, etc., with a low molecular weight.

[0156] In the case of electron beam or ultraviolet-curable acrylic resins, acetophenones, benzophenones, Michaelis-Menten benzoylbenzoate, α-pentyl oxime ester, tetramethylthiuram monosulfide, and thioxanones can be used as photopolymerization initiators in the aforementioned resins. Furthermore, the photopolymerization initiator can be mixed with photosensitizers such as n-butylamine, triethylamine, and tri-n-butylphosphine.

[0157] Organosilicon (siloxane) thermosetting resins can be manufactured by hydrolyzing and condensing two or more organosilicon compounds, either alone or in combination, under acid or alkaline catalysts. Particularly in low-reflection applications, mixing one or more fluorosilane compounds and subjecting them to hydrolysis and condensation is preferred for improving low refractive index properties and stain resistance.

[0158] Manufacturing of laminated polyester films

[0159] The laminated polyester film of the present invention can be manufactured by providing a functional layer on a coating layer of an easily adhesive polyester film. Specific methods will be described below, but are not limited thereto.

[0160] A functional layer forming composition (coating liquid for forming a functional layer) is applied to the coating layer of the aforementioned easy-to-adhere polyester film. Examples of this functional layer forming composition include, for instance, the aforementioned electron beam or ultraviolet-curable acrylic resin (including its oligomers, monomers, etc.) or siloxane-based thermosetting resin. When the coating layer is applied to both sides of the easy-to-adhere polyester film, it can be applied to at least one coating layer. The functional layer forming coating liquid does not require special dilution, but dilution with an organic solvent is acceptable depending on the requirements of its viscosity, wettability, and film thickness. After applying the functional layer forming coating liquid to the film, the coating film is dried as needed, and then cured by electron beam or ultraviolet irradiation and heating according to its curing conditions, thereby forming a functional layer.

[0161] More typically, a coating liquid for forming a functional layer, as described above, can be used on a coating layer of an easily adhesive polyester film. The coating is applied using a wire rod or similar tool, and then dried, for example, at 60–100°C for 0.5–10 minutes to remove the solvent. Next, the film coated with the functional layer is irradiated with, for example, a high-pressure mercury lamp at 300 mJ / cm². 2 Ultraviolet light can be used to obtain laminated polyester films with functional layers.

[0162] In this invention, the thickness of the functional layer is preferably 1 to 15 μm. If the thickness of the functional layer is 1 μm or more, its effects on chemical resistance, scratch resistance, and stain resistance can be effectively exerted, and therefore it is preferred. On the other hand, if the thickness is 15 μm or less, the flexibility of the functional layer is maintained, and there is no concern about cracking, etc., and therefore it is preferred.

[0163] The easily bondable polyester film and laminated polyester film of the present invention are primarily suitable for optical applications, and therefore high transparency is preferred. Ideally, the lower limit of haze is 0%, and the closer to 0%, the better. The upper limit of haze is preferably 2%. If it is below 2%, light transmittance is good, and a clear image can be obtained in the liquid crystal display device, which is therefore preferred. Haze can be measured, for example, according to the method described in the embodiments described later.

[0164] (Evaluation of fit)

[0165] The adhesion between the polyester film and the functional layer in a laminated polyester film can be evaluated using the methods described in the examples.

[0166] Specifically, a laminated polyester film was prepared by forming a functional layer on the coating layer of an easily bondable polyester film, and its adhesion X (%) was evaluated. Furthermore, after long-term storage of the easily bondable polyester film, a laminated polyester film was prepared by forming a functional layer on its coating layer, and its adhesion Y (%) was evaluated.

[0167] Ideally, long-term storage should be performed by placing the sample at room temperature for several weeks to several months. However, this requires an extremely long time. Therefore, as an alternative method in this manual, the sample is placed at a high temperature and high humidity of 80°C and 90%RH for 24 hours, followed by 12 hours at room temperature (10~30°C).

[0168] As can be seen from Table 3 of the embodiments, the adhesion X is typically above 95%, further above 98%, and particularly 100%. When the adhesion X is above 95%, it can be said that the adhesion between the coating layer and the functional layer is sufficiently maintained.

[0169] In addition, the adhesion Y after long-term storage is usually above 95%, further above 98%, and especially 100%. When the adhesion Y is above 95%, it can be said that the adhesion between the coating layer and the functional layer is fully maintained even after long-term storage.

[0170] In the easily bondable polyester film of the present invention, both adhesion X and adhesion Y are high, and adhesion Y is equal to or almost unchanged compared with adhesion X.

[0171] Thus, the easily bondable polyester film of the present invention has the characteristics of high adhesion reliability with the functional layer, and high adhesion not only after film formation but also after exposure to high temperature and high humidity environments (after long-term storage). That is, it has the characteristics of high adhesion X after film formation and high adhesion Y after long-term storage. That is, the above-mentioned X (%) and Y (%) satisfy the following formula (1).

[0172] XY (%) ≤ 5… Equation (1)

[0173] The value of formula (1) is usually less than 5%, further less than 3%, and particularly preferably 0%. If it is less than 5%, it can be determined that there is no significant difference between the tightness after film formation and the tightness after wet heat treatment, and it can be determined that both are sufficiently tight. Therefore, even if stored for a long time, the tightness of the functional layer will not decrease, and it can be determined that the tightness reliability can be ensured.

[0174] The results are not limited to this; it was also found that by ensuring this level of tightness, a sufficient seal can be achieved even after bending tests required for foldable display applications.

[0175] Perform the following visual tests: confirm the laminated polyester film after the bending test shown below, confirm the film surface at the bending point, confirm the cracking (crack) state of the hard coating, and confirm the adhesion after the bending test.

[0176] 〇: The outer surface of the film is not cracked.

[0177] ×: The film surface on the outer side of the bend has cracks or breaks.

[0178] The tightness is rated as tightness Z.

[0179] As can be seen from Table 3 of the examples, the film with an adhesion Z of 0 has high adhesion reliability.

[0180] Example

[0181] Next, the present invention will be described in detail using examples and comparative examples, but the present invention is of course not limited to the following examples. Furthermore, the method for measuring the measured values ​​used in the present invention is described below.

[0182] (1) Average particle size

[0183] [Method using scanning electron microscopy]

[0184] The average particle size of the particles present in the coating layer of this invention can be determined by the following method: Using a scanning electron microscope (SEM), the particles are photographed, and the maximum diameter (distance between the two furthest points) of 300 to 500 particles is measured at a magnification of 2 to 5 mm for the smallest particle size. The arithmetic mean of these particles is taken as the average particle size.

[0185] Dynamic light scattering method

[0186] The average particle size can also be determined during the manufacturing of particles or films using dynamic light scattering. The sol is diluted with a dispersion medium, and the parameters of the dispersion medium are measured using a submicron particle analyzer N4 PLUS (Beckman Coulter). The average particle size is calculated using the cumulative moment method. The average particle size in the sol is then observed using dynamic light scattering; if particle aggregation is present, the average particle size of these aggregated particles is also observed.

[0187] (2) Refractive index of the particles

[0188] The refractive index of the particles can be determined by the following method. Inorganic particles are dried at 150°C, pulverized in a mortar, and then immersed in solvent 1 (a solvent with a lower refractive index than the particles). Solvent 2 (a substance with a higher refractive index than the particles) is then added in small, incremental amounts until the particles become approximately transparent. The refractive index of the liquid is measured using an Abbe refractometer (Abbe refractometer manufactured by Atago Co., Ltd.). The measurement is performed at 23°C under D-rays (wavelength 589 nm). Solvent 1 and solvent 2 are selected to be miscible solvents; examples, based on the refractive index, include 1,1,1,3,3,3-hexafluoro-2-propanol, 2-propanol, chloroform, carbon tetrachloride, toluene, and glycerol.

[0189] (3) Haze of easy-to-adhere polyester film for optical applications

[0190] The haze of the easy-to-adhere polyester film was measured using a turbidimeter (Nippon Denshoku, NDH2000) according to JIS K 7136:2000.

[0191] (4) The tightness of the functional layer

[0192] A hard coating is formed on the easy-to-adhere coating layer of the polyester film. The adhesion between the hard coating and the substrate film is determined according to 8.5.1 of JIS-K5400-1990.

[0193] The coating solution used to form the hard coating is prepared as follows.

[0194] (Preparation of coating liquid L for hard coating formation)

[0195] Isopropanol 21.00% by mass

[0196] Toluene 49.00% by mass

[0197] Pentaerythritol triacrylate 21.38% by mass

[0198] (A-DPH produced by Shin-Nakamura Chemical)

[0199] 7.12% by mass of urethane acrylate

[0200] (Mitsubishi Chemical Corporation UV-7600B)

[0201] Photopolymerization initiator 1.50% by mass

[0202] (Omnirad184 made by IGM Resins BV)

[0203] (Preparation of coating liquid M for hard coating formation)

[0204] Methyl ethyl ketone 64.40% by mass

[0205] Dipentaerythritol hexaacrylate 27.20% by mass

[0206] (A-DPH produced by Shin-Nakamura Chemical)

[0207] 3.40% by mass of polyethylene glycol diacrylate

[0208] (Kyoei Chemical Manufacturing Co., Ltd. Light Acrylate 9EG-A)

[0209] Bisphenol A diacrylate 4.00% by mass

[0210] (Kyoei Chemical Manufacturing Co., Ltd. Light Acrylate BP-4PA)

[0211] Photopolymerization initiator 1.00% by mass

[0212] (Omnirad184 made by IGM Resins BV)

[0213] (Formation of hard coating)

[0214] The easily bondable polyester film was stored at 20°C and 65% RH. Twelve hours after film preparation, a hard coating forming solution (L or M) was applied to the easily bondable coating layer using a #14 wire rod. The film was then dried at 70°C for 1 minute to remove the solvent. Next, the film coated with the hard coating was irradiated with a high-pressure mercury lamp at 300 mJ / cm². 2 Ultraviolet light was used to obtain a hard coating film with a hard coating thickness of 7 μm.

[0215] The specific method for measuring adhesion is as follows. Using a cutting guide with a 2mm gap, 100 squares are scratched on the hard coating surface, penetrating the hard coating and reaching the substrate film. Next, cellophane tape (manufactured by NICHIBAN Co., Ltd., No. 405; 24mm width) is applied to the square-shaped cut surfaces and rubbed with an eraser to ensure complete adhesion. Then, the cellophane tape is peeled vertically from the hard coating surface of the easy-to-adhere polyester film with the hard coating formed thereon. The number of squares peeled off from the hard coating surface is visually counted, and the adhesion between the hard coating and the substrate film is determined by the following formula. It should be noted that squares that are partially peeled off are also counted as peeled squares.

[0216] Adhesion (%) = {1 - (number of peeled squares / 100)} × 100

[0217] (6) Resistance to damp heat (sealing performance after placement at 80℃ and 90%RH)

[0218] An easily bondable polyester film was placed in a high-temperature, high-humidity bath at 80°C and 90%RH for 24 hours, followed by a humid heat treatment at room temperature (20°C and 65%RH) for 12 hours. Then, a functional layer was formed using the same method as described above, and the adhesion to the substrate film was determined.

[0219] (7) Specific viscosity ηsp / c (unit: dl / g)

[0220] 0.10 g of polyester resin was dissolved in 25 ml of a mixed solvent of phenol / tetrachloroethane (mass ratio 6 / 4), and the viscosity was measured using an Ubbelohde viscometer at 30 °C.

[0221] (8) Observation using cross-section of a transmission electron microscope

[0222] After cutting the easily-adhesive polyester film into 1 mm × 10 mm pieces and embedding in epoxy resin, an ultramicrotome was used to prepare thin cross-sections parallel to the short side of the embedded sample sheet. Next, the sliced film was stained with ruthenium tetroxide, and portions without significant damage were observed using a transmission electron microscope (JEM2100 manufactured by JEOL Ltd.) at an accelerating voltage of 200 kV and a magnification of 20000×. Based on the observed image of the coating layer, the thickness of the coating layer was measured at 10 points at each level, and the average value was taken as the thickness of the coating layer.

[0223] (9) Resin composition of polyester

[0224] Polyester resin is dissolved in deuterated chloroform, and a Varian nuclear magnetic resonance analyzer (NMR) GEMINI-200 is used to conduct 1 H-NMR analysis, and the mole percentage ratio of each component is determined from the integral ratio.

[0225] (10) Viscosity of polyester resin dispersion

[0226] An aqueous polyester resin dispersion is added into a 140 cc glass bottle, and a No. 1 or No. 2 rotor of a model BL viscometer (TOKIMEC INC.) is used in a constant temperature bath at 25°C. The viscosity of the aqueous polyester resin dispersion is measured at a rotation speed of 60 rpm for 1 minute.

[0227] (11) Adhesion evaluation

[0228] Adhesion after film formation (X) and adhesion after humid heat treatment (Y) shown in Table 3 were carried out according to the procedures described in this specification.

[0229] In addition, adhesion (Z) was also carried out and evaluated according to the procedures described in this specification.

[0230] The adhesion reliability based on the results was evaluated according to the following criteria.

[0231] A sample satisfying 0 ≤ X-Y < 2 and Z being ○ is a sample that can very sufficiently ensure adhesion, rated as "A";

[0232] A sample satisfying 2 ≤ X-Y < 4 and Z being ○ is a sample that can well ensure adhesion, rated as "B";

[0233] A sample satisfying 4 ≤ X-Y ≤ 5 and Z being ○ is a sample that can ensure adhesion, rated as "C";

[0234] A sample satisfying 5 < X-Y and Z being × is a sample that cannot ensure adhesion, rated as "D".

[0235] (12) Bending resistance of laminated polyester film

[0236] Prepare a laminated polyester film sample measuring 20 mm in width and 110 mm in flow direction. Use a no-load U-shaped stretch tester (manufactured by YUASA SYSTEM Co., Ltd., DLDMLH-FS) with a bending radius of 0.5 mm, bending 200,000 times at a rate of 1 cycle / second. At this point, fix the sample 10 mm from both ends of the long side, with the bent portion measuring 20 mm × 90 mm. Imagine the laminated film is placed on the inner surface of a folded display; set the bending radius to 0.5 mm. Observe the outer surface of the bent portion of the sample film using a digital microscope (HIROX, RH2000) at 800x magnification, checking for any cracks or fissures.

[0237] [Polycarbonate polyurethane resin]

[0238] Synthesis of polycarbonate polyurethane resin (PCPU-1):

[0239] 29.4 parts by mass of dicyclohexylmethane-4,4'-diisocyanate, 63.6 parts by mass of polycarbonate diol with a melting point of 33°C and using 1,4-butanediol / 2,2-dimethyl-1,3-propanediol (molar ratio = 75 / 25) as the main raw materials (number average molecular weight 1000), 7 parts by mass of dimethylolpropionic acid, and 200 parts by mass of methyl ethyl ketone as solvent were added to a four-necked flask equipped with a stirrer, a serpentine condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer. The mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere. The infrared spectrum of the reaction solution was measured to confirm the disappearance of isocyanate groups in the reaction solution. Then, after cooling the solution to room temperature, 8.2 parts by mass of triethylamine were added, thereby obtaining a polycarbonate polyurethane resin (PCPU-1) solution with a solid content of 50.0% by mass.

[0240] The overall hexane content of this polycarbonate polyurethane resin (PCPU-1) is 0.0% by mass.

[0241] Preparation of an aqueous dispersion of polycarbonate polyurethane resin (PCPU-1) (PCPU-1WD):

[0242] Add the specified amount of water to a reaction vessel equipped with a high-speed homogenizer, adjust the temperature to 25°C, and stir at a speed of 2000 min... -1 The polycarbonate polyurethane resin (PCPU-1) solution was slowly added while stirring and mixing to achieve aqueous dispersion. Then, methyl ethyl ketone, used as a solvent, was removed under reduced pressure. The concentration was adjusted by water to prepare an aqueous dispersion (PCPU-1WD) of polycarbonate polyurethane resin (PCPU-1) with a solid content of 35.0% by mass.

[0243] Synthesis of polycarbonate polyurethane resin (PCPU-2) and preparation of its aqueous dispersion (PCPU-2WD): The main raw material of polycarbonate diol was changed to 1,4-cyclohexanediol / 1,6-hexanediol (=75 / 25) (molar ratio). Except for this, PCPU-2 was prepared in the same manner as PCPU-1, and the aqueous dispersion (PCPU-2WD) was obtained in the same manner as PCPU-1WD. The overall hexane content of this polycarbonate polyurethane resin (PCPU-2) was 11.5% by mass.

[0244] Synthesis of polycarbonate polyurethane resin (PCPU-3) and preparation of its aqueous dispersion (PCPU-3WD): The main raw material of polycarbonate diol was changed to 1,5-pentanediol / 1,6-hexanediol (=45 / 55) (molar ratio). Except for this, (PCPU-3) was prepared in the same manner as (PCPU-1), and the aqueous dispersion (PCPU-3WD) was obtained in the same manner as (PCPU-1WD). The overall hexane content of this polycarbonate polyurethane resin (PCPU-3) was 31.2% by mass.

[0245] Synthesis of polycarbonate polyurethane resin (PCPU-4) and preparation of its aqueous dispersion (PCPU-4WD):

[0246] The main raw material of polycarbonate diol was changed to 1,6-hexanediol. Otherwise, PCPU-4 was prepared in the same manner as the synthesis of PCPU-1, and an aqueous dispersion (PCPU-4WD) was obtained in the same manner as the preparation of PCPU-1WD. The overall hexane structure ratio (content) of this polycarbonate polyurethane resin (PCPU-4) was 80.5% by mass.

[0247] [Polyester resin]

[0248] Manufacturing of polyester resin (PEs-1):

[0249] Polyester resin (PEs-1) was polymerized according to a known polymerization method. The composition of the resulting polymer was then analyzed. 1 ¹H-NMR analysis was performed, and the molar percentage of each component was determined by its integral ratio. The results are recorded in Table 1. The specific viscosity of the obtained polyester resin was 0.583 dl / g.

[0250] The following describes the abbreviations recorded in Table 1.

[0251] TPA: Terephthalic acid

[0252] IPA: Isophthalic acid

[0253] NDC: Naphthalene dicarboxylic acid

[0254] SA: Sebacic acid

[0255] DSS: Sodium dimethyl isophthalate-5-sulfonate

[0256] EG: Ethylene glycol

[0257] HD: Hexanediol

[0258] DEG: Diethylene glycol

[0259] NPG: Neopentyl Glycol

[0260] Preparation of polyester aqueous dispersion (PEs-1WD):

[0261] In a reactor equipped with a stirrer, thermometer, and reflux device, 30 parts by weight of copolyester resin (PEs-1) and 15 parts by weight of ethylene glycol-n-butyl ether were added. The mixture was heated and stirred at 110°C to dissolve the resin. After the resin was completely dissolved, 55 parts by weight of water were slowly added to the polyester solution while stirring. After the water was added, the liquid was cooled to room temperature while stirring to prepare a milky white aqueous dispersion of polyester resin (PEs-1) (PEs-1WD) with a solid content of 25.1% by weight. The viscosity of the obtained aqueous dispersion was 84 mPa·s.

[0262] Manufacturing of polyester resin (PEs-2):

[0263] Similar to the polymerization of the polyester resin (PEs-1) described above, polyester resin (PEs-2) was produced using a known polymerization method. The composition ratio was determined in the same manner as for PEs-1, and the specific viscosity of the resulting resin was evaluated. The results are recorded in Table 1.

[0264] Preparation of polyester aqueous dispersion (PEs-2WD):

[0265] Polyester aqueous dispersion (PEs-2WD) was prepared by replacing PEs-1 with PEs-2 in the same manner as the preparation of the polyester aqueous dispersion (PEs-1WD) described above. The solid component concentration (mass%) and liquid viscosity (mPa·s) were evaluated in the same manner as PEs-1WD. The results are recorded in Table 2.

[0266] [Table 1]

[0267]

[0268] [Table 2]

[0269]

[0270] [Cross-linking agent]

[0271] Preparation of an aqueous dispersion (C-1WD) of a capped isocyanate-based crosslinking agent (C-1):

[0272] In a flask equipped with a stirrer, thermometer, and reflux condenser, 125.2 parts by weight of a biuret-structured polyisocyanate compound (Asahi Kasei Chemicals, DURANATE 24A-100, NCO concentration 23.1%) based on hexamethylene diisocyanate, 50.0 parts by weight of dipropylene glycol dimethyl ether, and 68.8 parts by weight of 3,5-dimethylpyrazole were added. The mixture was stirred continuously at 70°C under a nitrogen atmosphere for 2 hours. The infrared spectrum of the reaction solution was then measured to confirm the disappearance of the isocyanate group absorption. Next, after cooling to room temperature, 6 parts by weight of polyethylene glycol (n=12) monolaurate were added, and the mixture was stirred at 2000 min... -1 Water was added while stirring and mixing. The concentration was adjusted by water to prepare an aqueous dispersion (C-1WD) of 30.0% by mass of the end-capped isocyanate crosslinking agent (C-1).

[0273] Preparation of an aqueous dispersion (C-2WD) of a capped isocyanate-based crosslinking agent (C-2):

[0274] In a flask equipped with a stirrer, thermometer, and reflux condenser, 125.2 parts by weight of a biuret-structured polyisocyanate compound (Asahi Kasei Chemicals, DURANATE 24A-100, NCO concentration 23.1%) based on hexamethylene diisocyanate, 50.0 parts by weight of dipropylene glycol dimethyl ether, and 68.8 parts by weight of 3,5-dimethylpyrazole were added. The mixture was stirred continuously at 70°C under a nitrogen atmosphere for 2 hours. The infrared spectrum of the reaction solution was then measured to confirm the disappearance of the isocyanate group absorption. Next, after cooling to room temperature, 6 parts by weight of polyethylene glycol (n=12) monolaurate were added, and the mixture was stirred at 2000 min... -1 Water was added while stirring and mixing. The concentration was adjusted by water to prepare an aqueous dispersion (C-2WD) of 30.0% by mass of the end-capped isocyanate crosslinking agent (C-2).

[0275] Preparation of an aqueous dispersion (C-3WD) of a capped isocyanate-based crosslinking agent (C-3):

[0276] In a flask equipped with a stirrer, thermometer, and reflux condenser, 64.5 parts by weight of a polyisocyanate compound with an isocyanurate structure (Asahi Kasei Chemicals, DURANATE TPA-100, NCO concentration 23.1%) based on hexamethylene diisocyanate, 50.0 parts by weight of dipropylene glycol dimethyl ether, and 20.8 parts by weight of methyl ethyl ketone oxime were added. The mixture was stirred at 70°C under a nitrogen atmosphere for 1 hour. Then, 8.7 parts by weight of dimethylolpropionic acid were added. The infrared spectrum of the reaction solution was measured while stirring directly at 70°C, and the reaction continued until the absorption of the isocyanate groups disappeared. Next, after cooling to room temperature, 6.0 parts by weight of dimethylethanolamine were added, and the mixture was stirred at a speed of 2000 min... -1 Water was added while stirring and mixing. The concentration was adjusted by water to prepare an aqueous dispersion (C-3WD) of 30.0% by mass of the end-capped isocyanate crosslinking agent (C-3).

[0277] (Zirconium oxide particles)

[0278] 2283.6 g of pure water and 403.4 g of oxalic acid dihydrate were added to a 3-liter glass container and heated to 40°C to prepare a 10.72% by mass oxalic acid aqueous solution. While stirring the aqueous solution, 495.8 g of zirconium oxycarbonate powder (ZrOCO3, manufactured by AMR International Corp., equivalent to ZrO2, containing 39.76% by mass) was slowly added. After mixing for 30 minutes, the mixture was heated at 90°C for 30 minutes. Next, 1747.2 g of a 25.0% by mass tetramethylammonium hydroxide aqueous solution (manufactured by Tama Chemical Industry Co., Ltd.) was slowly added over 1 hour. At this point, the mixture was in slurry form, containing 4.0% by mass of ZrO2. The slurry was transferred to a stainless steel autoclave and subjected to hydrothermal treatment at 145°C for 5 hours. The product after hydrothermal treatment was free of undissolved matter and completely gelled. The obtained sol contained 4.0% by mass ZrO2, had a pH of 6.8, and an average particle size of 19 nm based on dynamic light scattering. Furthermore, the transmittance was measured to be 88% when the sol was adjusted to a ZrO2 concentration of 2.0% by mass with pure water. Particle observation using a transmission electron microscope revealed aggregated ZrO2 particles of approximately 7 nm in size. Using an ultrafiltration device, 4000 g of the 4.0% by mass ZrO2 sol obtained through hydrothermal treatment was washed and concentrated with the slow addition of pure water, yielding 953 g of a 13.1% by mass ZrO2 sol with a transmittance of 76% at a pH of 4.9. The refractive index of the obtained zirconia-based microparticles was 1.75.

[0279] (Zirconium oxide sol)

[0280] 300g of a zirconia sol with a ZrO2 concentration of 13.1% by mass, obtained through the above washing and concentration process, was further concentrated by adding 3.93g of a 20% by mass citric acid aqueous solution and 11.0g of a 25% by mass tetramethylammonium hydroxide aqueous solution to 300g of zirconia sol using an ultrafiltration device. This yielded 129g of a high-concentration zirconia sol with a ZrO2 concentration of 30.5% by mass. The resulting high-concentration zirconia sol had a pH of 9.3 and an average particle size of 19nm based on dynamic light scattering. Furthermore, this zirconia sol exhibited no precipitate and remained stable for more than one month at 50°C.

[0281] [Example 1]

[0282] (Preparation of coating solution)

[0283] Prepare a coating solution with the following composition.

[0284] 42.11 parts by weight of water

[0285] 30.06 parts by weight of isopropanol

[0286] 4.50 parts by weight of zirconium oxide sol

[0287] (Zirconium oxide sol with an average particle size of 20 nm and a solid content concentration of 30% by mass)

[0288] 0.87 parts by weight of silica sol

[0289] (Silica sol with an average particle size of 450 nm and a solid content concentration of 4% by mass)

[0290] PCPU-1WD 5.62 parts by weight

[0291] (Solid component concentration 35.0% by mass)

[0292] PEs-1WD 11.08 parts by weight

[0293] (Solid component concentration 25.1% by mass)

[0294] C-1WD 5.20 parts by weight

[0295] (Solid component concentration 30.0% by mass)

[0296] 0.30 parts by weight of surfactant

[0297] (Organosilicon-based, solid content concentration 10.0% by mass)

[0298] 0.26 parts by weight of high-boiling-point solvent

[0299] (Manufacturing of easy-to-adhere polyester film)

[0300] As a polymer raw material for films, PET resin granules with an intrinsic viscosity (solvent: phenol / tetrachloroethane = 60 / 40) of 0.62 dl / g and substantially free of particles were dried at 135°C for 6 hours under reduced pressure of 133 Pa. The dried granules were then fed to an extruder and melt-extruded into sheets at approximately 280°C. These sheets were then rapidly cooled and solidified on rotating cooling metal rollers with a surface temperature maintained at 20°C to obtain unstretched PET sheets.

[0301] Next, the coating solution was applied to one side of the PET film using a roller coating method and then dried at 80°C, resulting in a final coating weight of 0.07 g / m² after stretching and drying. 2 The film was then adjusted in the following manner. Next, it was stretched to 4.0 times its original length in the width direction using a tenter frame at 150°C. With the length in the width direction of the film fixed, it was heated to 200°C and then further relaxed in the width direction at 200°C to obtain an easy-to-adhere polyester film with a thickness of 50μm.

[0302] The thickness of the adhesive coating layer of the obtained adhesive polyester film is 50 nm, and the film haze is 1.11%.

[0303] The deformation was evaluated using AFM (Aspect-Factor Method) by applying a 3 nN load to the surface of the obtained easily bondable polyester film. The proportion of films deformed by more than 1 nm was calculated to be 0.16%.

[0304] Next, on the easy-to-adhere coating layer of the easy-to-adhere polyester film, a functional layer forming coating liquid (the hard coating forming coating liquids L and M of (4) above) is used to form a laminated polyester film with a functional layer according to the above forming method.

[0305] The adhesion of the functional layers of the obtained laminated polyester film was evaluated, and the adhesion force X was 99%.

[0306] On the other hand, the aforementioned easy-to-adhere polyester film was placed in a high-temperature and high-humidity bath at 80°C and 90% RH for 24 hours, followed by 12 hours at room temperature. Then, a functional layer was formed on the easy-to-adhere coating layer of the treated easy-to-adhere polyester film using a functional layer forming coating liquid (as above) to obtain a laminated polyester film.

[0307] The adhesion of the functional layers of the laminated polyester film was evaluated, and the adhesion force Y was found to be 95%.

[0308] Based on this result, XY = 4 (%). These results are recorded in Table 3.

[0309] Furthermore, a bending test was conducted on an easy-to-adhere polyester film with the aforementioned functional layer to evaluate the cracks on the film surface after the test. No cracks were confirmed.

[0310] The results (sealing reliability Z) are recorded in Table 3. Since no cracks were detected, it can be confirmed that the sealing reliability can be ensured. Therefore, the evaluation results of the sealing reliability are recorded in the sealing reliability column of Table 3.

[0311] [Example 2]

[0312] Except for changing the polycarbonate polyurethane resin aqueous dispersion used to PCPU-2WD as described in Table 3, an easy-to-adhere polyester film was obtained in the same manner as in Example 1.

[0313] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0314] [Example 3]

[0315] Except for changing the polyester resin aqueous dispersion used to PEs-2WD as described in Table 3, an easy-to-adhere polyester film was obtained in the same manner as in Example 2.

[0316] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0317] [Examples 4-5]

[0318] Except for changing the heat-fixing temperature to the temperature recorded in Table 3 and setting the same resin composition as in Example 3, an easy-to-adhere polyester film was obtained in the same manner as in Example 1.

[0319] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0320] [Examples 6-8]

[0321] Except that the coating thickness was changed to the thickness recorded in Table 3 and the heat setting temperature recorded in Table 3 was used, an easy-to-adhere polyester film was obtained in the same manner as in Example 3.

[0322] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0323] [Examples 9-12]

[0324] Except for changing the polycarbonate polyurethane resin aqueous dispersion used to PCPU-3WD as described in Table 3, and changing the coating thickness and heat setting temperature to the conditions described in Table 3, an easy-to-adhere polyester film was obtained in the same manner as in Example 3.

[0325] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0326] [Example 13]

[0327] Except for changing the polycarbonate polyurethane resin aqueous dispersion used to PCPU-4WD as described in Table 3, an easy-to-adhere polyester film was obtained in the same manner as in Example 3.

[0328] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0329] [Examples 14-15]

[0330] Except for changing the aqueous dispersion of the crosslinking agent used to the substance described in Table 3, an easy-to-adhere polyester film was obtained in the same manner as in Example 3.

[0331] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0332] [Example 16]

[0333] Except for imparting the functional layers described in Table 3 to the easily bondable polyester film prepared in the same manner as in Example 3, the laminated polyester film was prepared and evaluated in the same manner as in Example 1. The various evaluation results are described in Table 3.

[0334] [Comparative Examples 1-4]

[0335] Except for changing the aqueous dispersion of polycarbonate polyurethane resin, the aqueous dispersion of polyester resin, and the thickness of the coating layer to those described in Table 3, an easy-to-adhere polyester film was obtained in the same manner as in Example 1.

[0336] The resulting easily bondable polyester film was evaluated in the same manner as in Example 1, and then the process was repeated as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0337] [Table 3]

[0338]

[0339] The functional layer of the laminated polyester film of this invention exhibits excellent adhesion to the polyester film (especially after long-term storage), resulting in high adhesion reliability. Furthermore, it also demonstrates excellent anti-adhesion and transparency. Therefore, it can be widely used in optical applications and the like.

[0340] On the other hand, in Comparative Example 1, the coating layer did not contain polycarbonate polyurethane resin (A), resulting in significantly worse adhesion and adhesion reliability after damp heat treatment compared to the present invention. Furthermore, adhesion could not be ensured even under harsh conditions caused by folding.

[0341] In Comparative Example 2, the coating layer did not contain polyester resin (B), resulting in significantly worse adhesion and adhesion reliability after damp heat treatment compared to the present invention. Furthermore, adhesion could not be guaranteed even under harsh conditions caused by folding.

[0342] In Comparative Example 3, the coating layer was too thin to guide the phase separation state of the film surface to the state of the present invention, thus failing to exhibit sufficient adhesion. Furthermore, adhesion could not be ensured even under harsh conditions caused by folding.

[0343] In Comparative Example 4, the coating layer was too thick to guide the phase separation state of the film surface to the state of the present invention, thus failing to exhibit sufficient adhesion. Furthermore, adhesion could not be ensured even under harsh conditions caused by folding.

[0344] Industrial availability

[0345] The easily bondable polyester film of this invention is suitable for use in foldable displays. It exhibits excellent adhesion between the functional layer and the polyester film (especially after long-term storage), resulting in high bonding reliability. Furthermore, it also demonstrates excellent anti-blocking properties and transparency. Therefore, it can be widely used in optical applications and the like.

Claims

1. An easily adhesive polyester film for a foldable display, comprising a polyester film substrate and a coating layer on at least one side of the substrate. The coating layer is formed from a composition comprising polycarbonate polyurethane resin (A), polyester resin (B), and a capped isocyanate crosslinking agent (C). In the force curve observed using an atomic force microscope on the surface of the coating layer and pressed under a load of 3nN, the proportion of the area with a deformation of more than 1nm is more than 0.15% of the total observed surface.

2. The easily adhesive polyester film for foldable displays according to claim 1, wherein, The polyester resin (B) has a naphthalene skeleton in its molecule.

3. The easily bondable polyester film for foldable displays according to claim 1, wherein, The isocyanate-based crosslinking agent (C) is a pyrazole compound.

4. The easily adhesive polyester film for foldable displays according to claim 1, wherein, The polycarbonate polyurethane resin (A) has a hexane structure in its molecule.

5. The easily adhesive polyester film for foldable displays according to claim 4, wherein, The polycarbonate polyurethane resin (A) has a hexane structure of more than 10% by mass in its molecule.

6. A laminated polyester film for a foldable display, further having a functional layer on the coated layer of the easy-to-adhere polyester film for a foldable display as claimed in claim 1.

7. A method for manufacturing an easy-to-adhere polyester film for a foldable display, comprising the method for manufacturing an easy-to-adhere polyester film for a foldable display according to any one of claims 1 to 5, and comprising: The process of forming a coating layer on at least one side of a polyester film substrate. The coating layer formation process includes a heating process at 180°C or higher.

8. A method for manufacturing a laminated polyester film for a foldable display according to claim 6, comprising: A functional layer is formed by further coating the coating layer of the easily adhesive polyester film obtained by the manufacturing method of claim 7 with a composition for forming a functional layer.

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