Easy-adhesive polyester film, its manufacturing method and the manufacturing method of laminated polyester film

Abstract

[The technical problem to be solved] The present invention provides an easy-adhesive polyester film for use in foldable displays and exhibiting excellent adhesion to various functional layers, and a laminated polyester film having the functional layers. Technical means to solve the problem An easy-to-adhere polyester film for a foldable display has a polyester film substrate and a coating layer on at least one side of the polyester film substrate. The coating layer is formed from a composition comprising polycarbonate polyurethane resin (A), polyester resin (B), and a capped isocyanate crosslinking agent (C). When the surface of the coating layer is observed using an atomic force microscope, the proportion of the area with a deformation of more than 1 nm in the force curve of pressing with a 3 nN load is more than 0.15% relative to the entire observed surface.

Description

[Technical Field]

[0001] The present invention relates to an easy-adhesive polyester film for use in foldable displays and having excellent adhesion to various functional layers, and a laminated polyester film having the functional layers. [Previous Technology]

[0002] The front surface of touch panels, computers, televisions, LCD displays, and decorative materials uses a hard coating film formed by depositing a transparent hard coating layer. Furthermore, panel components used in displays are formed by bonding the hard coating film to a polarizing mirror or to other components, and the bonding is performed by applying an adhesive component.

[0003] As for the transparent plastic film of the substrate, transparent polyester film is usually used. In order to improve the adhesion between the polyester film of the substrate and the hard coating and adhesive, a coating layer with easy adhesion is often provided on the surface of the polyester film as such an intermediate layer.

[0004] For the aforementioned hard-coated thin film, requirements include durability, transparency, chemical resistance, scratch resistance, and stain resistance, as well as resistance to temperature, humidity, and light. Furthermore, since it is mostly used on the surface of displays and decorative materials, visual recognizability and design flexibility are also required. Therefore, in order to suppress glare and iridescent colors caused by reflected light when viewed from any angle, a multi-layered anti-reflective layer is usually provided, consisting of alternating layers of high and low refractive index layers deposited on top of 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 has been discussed depending on its composition. For example, when a thin film with a hard coating is used in a display of an LCD TV, in order to ensure high reliability for long-term use, not only is initial adhesion required immediately after deposition, but also properties such as resistance to damp heat and adhesion retention over time are required.

[0006] Similarly, the aforementioned adhesives require durability, transparency, chemical resistance, scratch resistance, and stain resistance under temperature, humidity, and light conditions. Furthermore, since they are mostly used on the surfaces of displays and decorative materials, visual recognizability and design flexibility are also required. In recent years, adhesives with various compositions have been developed, and the adhesion between the substrate and the adhesive is discussed depending on its composition. For example, when adhesive-bonded components are used in LCD TV displays, to ensure high reliability for long-term use, not only initial adhesion immediately after lamination is required, but also resistance to damp heat and moisture, and adhesion retention over time.

[0007] Furthermore, due to the impact and deformation caused by folding, there are concerns about the reduced adhesion of hard coatings, adhesives, etc. The performance is required not only to ensure adhesion in the above-mentioned free state, but also to ensure adhesion even in the harsh environment caused by folding.

[0008] In liquid crystal display devices, due to their image formation method, polarizing plates are disposed on both sides of the glass substrate forming the surface of the liquid crystal panel. Generally, polarizing plates are constructed by bonding a protective film for a polarizing mirror to both sides of a polarizing mirror made of a polyvinyl alcohol-based film and a dichroic material such as iodine, separated by a hydrophilic adhesive such as a polyvinyl alcohol-based resin. Regarding the protective film used to protect the polarizing mirror, triacetyl cellulose film has conventionally been used from the perspectives of optical properties and transparency.

[0009] However, triacetyl cellulose has insufficient durability. If a polarizing plate using triacetyl cellulose film as a protective film for a polarizing lens is used under high temperature or high humidity, the performance of the polarizing plate, such as polarization and hue, will decrease. Furthermore, in recent years, in order to meet the trend of thinner displays, there has been a demand for thinner polarizing plates, but from the viewpoint of maintaining moisture barrier properties, the thinning of triacetyl cellulose film has its limits. Therefore, some have proposed using polyester film as a protective film for polarizing lenses that has both durability and moisture barrier properties (for example, see Patent Document 1).

[0010] Triacetyl cellulose films used as protective films for polarizing lenses have undergone alkali treatment or similar processes on their surfaces and exhibit extremely high affinity for hydrophilic adhesives. Therefore, protective films made of triacetyl cellulose films have extremely high adhesion to polarizing lenses coated with hydrophilic adhesives. However, the adhesion between polyester films and hydrophilic adhesives is not sufficient, especially in the case of polyester films that have acquired orientation through stretching treatment; this tendency becomes more pronounced. Therefore, in order to improve adhesion to polarizing lenses or hydrophilic adhesives coated on polarizing lenses, it has been reported in the literature to coat the surface of polyester films with highly hydrophilic materials to form a coating layer (for example, see Patent Document 1).

[0011] In the field of conventional easy-to-adhere polyester films, there are reports of laminated polyester films formed by providing a coating layer on at least one side of the polyester film, which have excellent adhesion to surface functional layers such as hard coatings, and the coating layer includes polyester resins containing naphthalic acid components (for example, see Patent Document 2).

[0012] Furthermore, there are reports of easily bondable polyester films formed by applying a coating layer of polyurethane containing an aliphatic polycarbonate polyol with excellent flexibility and high adhesion to at least one side of a polyester film, which exhibit excellent adhesion to the optical functional layer under high temperature and humidity conditions (for example, see Patent Document 3). However, while adhesion has been confirmed in each of these methods, an easily bondable polyester film with excellent adhesion after long-term storage has not been obtained. [Prior Art Documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 2013-063610 [Patent Document 2] Japanese Patent Application Publication No. 2011-246663 [Patent Document 3] Japanese Patent Application Publication No. 2011-168053 [Summary of the Invention]

[0014] [Problem to be Solved by the Invention] The present invention was made against the backdrop of the problems of the prior art. That is, the object of the present invention is to: provide an easy-to-adhere polyester film with excellent adhesion to functional layers such as hard coatings and adhesives; provide an easy-to-adhere polyester film for use in foldable displays; and provide a laminated polyester film having the functional layer. [Method for Solving the Problem]

[0015] The present invention was completed as a result of careful exploration by the inventors in order to achieve this objective. That is, the present invention is composed of the following components.

[0016] [1] An easy-to-adhere polyester film for a foldable display has a polyester film substrate and a coating layer on at least one side of the polyester film substrate. The coating layer is formed from a composition comprising polycarbonate polyurethane resin (A), polyester resin (B), and a capping isocyanate crosslinking agent (C). When the surface of the coating layer is observed using an atomic force microscope, the proportion of the area with a deformation of 1 nm or more in the force curve under a 3 nN load is 0.15% or more relative to the entire observed surface. [2] In the easy-to-adhere polyester film for a foldable display as described in [1], the aforementioned polyester resin (B) has a naphthalene skeleton in its molecule. [3] In the easy-to-adhere polyester film for a foldable display as described in [1] or [2], the aforementioned capping isocyanate crosslinking agent (C) is a pyrazole compound. [4] An easy-to-adhere polyester film for a foldable display as described in any one of [1] to [3], wherein the aforementioned polycarbonate polyurethane resin (A) has a hexane structure in its molecule. [5] An easy-to-adhere polyester film for a foldable display as described in any one of [1] to [4], wherein the aforementioned polycarbonate polyurethane resin (A) has a hexane structure of 10% by mass or more in its molecule. [6] A laminated polyester film for a foldable display, wherein a functional layer is further provided on the coating layer of an easy-to-adhere polyester film as described in any one of [1] to [5]. [7] A method for manufacturing an easy-to-adhere polyester film for a foldable display, which is a method for manufacturing an easy-to-adhere polyester film as described in any one of [1] to [6] above, comprising a step of forming a coating layer on at least one side of a polyester film substrate, wherein the coating layer forming step includes a heating step of 180°C or higher. [8] A method for manufacturing a laminated polyester film for a foldable display, comprising the step of further coating a functional layer forming composition onto a coating layer of an easily bondable polyester film as described in [7], thereby forming a functional layer. [Effects of the Invention]

[0017] The easy-to-adhere polyester film of the present 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 adhesion reliability. Furthermore, it also possesses excellent anti-adhesion and transparency. Therefore, it is suitable for a wide range of applications, including optical applications.

Implementation Method

[0018] [Forms for Implementing the Invention] In this invention, "display" refers to the entire display device. Regarding the types of displays, there are LCDs, organic EL displays, inorganic EL displays, LEDs, FEDs, etc., but LCDs, organic ELs, and inorganic ELs with a bendable structure are preferred. In particular, organic ELs and inorganic ELs that reduce the number of layers are preferred, and organic ELs with a wide color gamut are even more preferred.

[0019] The foldable display referred to in this invention is a single, continuous display that can be folded in half or similarly when carried. Folding reduces the size, improves portability, and makes thinness possible in the folded state. The bending radius during folding refers to the radius of the inner side of the folded portion. It can be said that the smaller the bending radius, the better, but a smaller bending radius makes it easier for creases to form, which can lead to problems such as cracking of functional layers such as the hardened coating. In addition, the surface protective film described later can be located on the outer side or the inner side of the folded display. Furthermore, the foldable display can be folded into 3 folds, 4 folds, or even a rollable type, all of which are included in the scope of the foldable display referred to in this invention.

[0020] In another embodiment, the present invention can also be a laminated polyester film for a foldable display, wherein a coating layer providing easy adhesion is disposed on one side of the polyester film, and a functional layer, represented by a hard coating layer, is thereon. This is suitable for foldable displays. It can be used in any part of a foldable display as long as it is a constituent component. Hereinafter, using an organic EL display as an example, a representative structure of a foldable display and the part where the film of the present invention can be used will be described.

[0021] The essential components of a foldable organic EL display are, although they are organic EL modules, they can be further configured with a circular polarizing plate, a touch panel module, a surface protective film, a back protective film, etc., as needed.

[0022] The general structure of an organic EL module includes an electrode / electron transport layer / light-emitting layer / hole transport layer / transparent electrode.

[0023] A touch panel is preferably included in the portable terminal device. When using an organic EL display, it is preferable to place the touch panel module on top of the organic EL display or between the organic EL module and the circular polarizer. The touch panel module has a transparent substrate such as a thin film and transparent electrodes disposed thereon. A laminated polyester film, which is formed by imparting a functional layer to the easy-to-adhere polyester film of the present invention, can be used as the transparent substrate. When used as the transparent substrate of the touch panel, a refractive index adjustment layer is preferably provided.

[0024] A circular polarizer, through components inside the display, reflects external light and suppresses image quality degradation. The circular polarizer comprises a linear polarizer and a retardation plate. The linear polarizer has a protective film on at least the visual recognition side of the polarizer. A protective film may also be present on the side opposite to the visual recognition side of the polarizer, and the retardation plate can be directly deposited onto the polarizer. The retardation plate is formed by using a resin film with phase difference, such as polycarbonate or cyclic olefins, and depositing a retardation layer composed of a liquid crystal compound on the resin film. The laminated polyester film of the present invention can be used as a protective film for polarizers. In this case, the slow axis direction of the polyester film is preferably parallel or orthogonal to the absorption axis direction of the polarizer. Furthermore, a deviation of up to 10 degrees is permissible for this parallelism or orthogonality, preferably up to 5 degrees.

[0025] If an impact is applied to the display from above, there is a risk of circuit breakage in the organic EL module and touch panel module. Therefore, a surface protective film is usually provided. The laminated polyester film of the present invention is used as such a surface protective film. Surface protective films include: those installed on the outermost surface of the display and called a cover window; and those that can be applied, peeled off, and replaced by the user and called an outer layer. The laminated polyester film of the present invention can be used in either case. It is provided on the surface of the foldable display in such a way that the functional layer becomes the visual recognition side. In addition, the functional layer can also be provided on both sides.

[0026] Preferably, a protective film is also provided on the back side of the display. The laminated polyester film of the present invention can be used as the protective film on the back side.

[0027] The laminated polyester film of the present invention may be used at the folding portion of a component of a foldable display, or may be other than those described above. Among these, the laminated polyester film of the present invention is preferably used as a protective film for the cover window surface, a protective film for the back surface, a substrate film for the touch panel module, or a back protective film. More preferably, it is used as a protective film for the cover window surface or a protective film for the back surface.

[0028] Furthermore, for foldable displays, it is not necessary to use the laminated polyester film of the present invention for all of the above-mentioned applications. In foldable displays, films with functional layers, such as polyester film, polyimide film, polyamide film, polyamide-imide film, polycarbonate film, acrylic film, triacetyl cellulose film, cyclic olefin polymer film, polyphenylene sulfide film, and polymethylpentene film, can be used in combination as substrates.

[0029] Polyester Film The polyester film used as a substrate in the easy-to-adhere 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 mass or more of polyester structural units. Preferably, the polyester film contains 90% by mass or more of polyester resin, more preferably 95% by mass or more, and even more preferably 100% by mass.

[0030] The material of the polyester resin is not particularly limited, and copolymers or blends thereof formed by polycondensation of dicarboxylic acid component and diol component can be used. 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, diphenylcarboxylic acid, anthracenedicarboxylic 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, dimeric acid, sebacic acid, octanoic acid, dodecanedicarboxylic acid, etc.

[0031] Examples of diols constituting polyester resins include: ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanediethanol, 1,4-cyclohexanediethanol, decanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl) benzoxene, etc.

[0032] One or more dicarboxylic acid components and two diol components may be used respectively. In addition, other polycarboxylic acid components such as trimellitic acid and other polyol components such as trimethylolpropane may also be added appropriately.

[0033] Specifically, regarding polyester resins, examples 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, in order to control optical properties such as polarization, including other copolymer components and other polymers is also preferable. From the viewpoint of controlling the optical properties of polyester films, preferred copolymer components include diethylene glycol and copolymer components having norbornene in the side chain.

[0034] The inherent viscosity of the polyester resin (solvent: phenol / tetrachloroethane = 60:40) is, for example, 0.50 to 1.0 dl / g.

[0035] To improve the processability of polyester films, such as their sliding and winding properties, inert particles can be included in the film. Examples of inert particles include: silica, kaolin, 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 gypsum, magnesium carbonate, magnesium hydroxide, and other inorganic particles. The average particle size of the inert particles can be, for example, 200–5000 nm, and more specifically, 250–4500 nm. This average particle size is measured using the method described in the examples (average particle size based on the number of particles obtained by SEM). To maintain high transparency, the content of inert particles in the film is preferably as low as possible. Therefore, it is preferable to make a multilayer structure in which particles are present only on the surface of the film, or to make the film substantially free of particles and contain particles only in the coating layer on at least one side of the polyester film.

[0036] Furthermore, the term "substantially free of particles" means, for example, in the case of inorganic particles, that when quantitatively analyzing elements originating from particles using fluorescence X-ray analysis, the content is less than 50 ppm, preferably less than 10 ppm, and most preferably less than the detection limit. This is because, even if particles are not actively added to the substrate film, contaminants from foreign matter, raw material resins, or contaminants adhering to the production line or equipment during the film manufacturing process can still inevitably be mixed into the film through shedding.

[0037] Furthermore, when the polyester film is made into a multilayer structure, it is possible to make two types of three-layer structures in which the inner layer does not contain inert particles, but only the outermost layer (two layers) contains inert particles. In this way, both transparency and processability can be better balanced.

[0038] The polyester film serving as the substrate can be a single layer or formed by laminating two or more layers. Furthermore, various additives can be included in the film as needed, provided that the effects of the present invention are achieved. Examples of additives include: antioxidants, lightfastness agents, antigelling agents, organic wetting agents, antistatic agents, ultraviolet absorbers, and surfactants. When the film has a laminated structure, it is preferable to include additives as needed and according to the function of each layer. For example, adding an ultraviolet absorber to the inner layer to prevent light degradation of the polarizer is also a preferred approach.

[0039] Polyester film can be manufactured by conventional methods. For example, it can be obtained by melting and extruding a material containing the above-mentioned polyester resin into a film shape, and then cooling and solidifying it using a casting roller to form a film. As for 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 mechanical strength, chemical resistance and durability.

[0040] When the polyester film is a stretched film, the stretching method is not particularly limited, and longitudinal uniaxial stretching, transverse uniaxial stretching, longitudinal and transverse successive biaxial stretching, and simultaneous longitudinal and transverse biaxial stretching can be used. When stretching the polyester film, stretching can be performed before or after the easy-to-adhere coating layer described later. It is also possible to perform uniaxial stretching in the longitudinal or transverse direction before the easy-to-adhere coating layer is laminated, and to perform stretching in the other direction after the coating layer is laminated.

[0041] Coating Layer The easy-to-adhere polyester film for foldable displays of the present invention has an easy-to-adhere coating layer on at least one side of the polyester film serving as the substrate. The easy-to-adhere polyester film for foldable displays of the present invention has a polyester film substrate and a coating layer located on at least one side of the polyester film substrate. The coating layer is formed from a composition comprising polycarbonate polyurethane resin (A), polyester resin (B), and a capped isocyanate crosslinking agent (C). Furthermore, in one sample, when the surface of the coating layer is observed using an atomic force microscope, the proportion of the area with a deformation of 1 nm or more in the force curve under a 3 nN load is 0.15% or more relative to the entire observed surface. The coating layer can be formed from a composition comprising an adhesive resin and additives as needed.

[0042] The components of the coating layer will be described in detail below. The adhesive resin constituting the coating layer is a resin with good adhesion, which includes: an urethane resin having a polycarbonate structure, i.e., polycarbonate polyurethane resin, and a polyester resin. By using a coating layer composed of these components, the adhesion between the polyester film substrate and the functional layer is significantly improved.

[0043] For this reason, the following can be cited: the coating layer has good compatibility with the composition of the functional layer described later. Furthermore, the functional layer can be obtained by UV curing under UV irradiation and has a cross-linked network structure. The polycarbonate polyurethane resin and polyester resin of the coating layer interact with each other, and the resins are easily compatible and become easily entangled. Moreover, it is believed that this entangled resin, by being entangled in the network structure of the functional layer formed thereon, results in a coating layer with better adhesion than before. Furthermore, it is believed that the coating layer contains polyester resin, thus its adhesion to the polyester film as the substrate is also good, resulting in a coating layer with even better adhesion.

[0044] (Polycarbonate polyurethane resin (A)) The polycarbonate polyurethane resin is not particularly limited as long as it is an urethane resin with a polycarbonate structure in its molecule, but it is preferred that it has a hexane structure in its molecule.

[0045] The content of hexane structure in polycarbonate polyurethane resin is typically 10% by mass or more and 90% by mass or less, 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 highly flexible structures present on the film surface will increase, 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 hexane structure in polycarbonate polyurethane resin is calculated as the proportion of the mass of hexane structure relative to 100g of the total mass of polycarbonate polyurethane resin. Specifically, it can be calculated from "the molecular weight and mole % of each component constituting polycarbonate polyurethane resin, such as polycarbonate diol and diisocyanate", "the mole % of the component containing hexane structure", and "the proportion of the molecular weight occupied by the hexane structure".

[0046] The polycarbonate polyurethane resin is formed by adding a polycarbonate diol component, a diisocyanate component, and a diol component that is used as a chain extender as needed, wherein at least one component has a hexane structure.

[0047] Examples of diols used to prepare polycarbonate polyurethane resins include: ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanediethanol, 1,4-cyclohexanediethanol, 1,4-cyclohexanediethanol, decanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)benzene, etc. 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 polycarbonate polyurethane with the desired properties.

[0048] Examples of diisocyanate components used to prepare polycarbonate and polyurethane resins include: toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, xylene diisocyanate, naphthalene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, 4,4-methylene dicyclohexyl diisocyanate, 1,2-bis(isocyanomethyl)cyclohexane, and 1,4-bis(isocyanomethyl)cyclohexane. Among these, isophorone diisocyanate, 4,4-methylene dicyclohexyl diisocyanate, 1,2-bis(isocyanomethyl)cyclohexane, and 1,4-bis(isocyanomethyl)cyclohexane are particularly preferred to have a structure in which at least one hydrogen atom on the cyclohexane ring is substituted into a hydrocarbon group. Furthermore, these isocyanate components can be used alone or in combination of two or more. There is no particular limitation on the ratio when combining two or more types; it can be adjusted in a way that yields polycarbonate or polyurethane with the desired properties.

[0049] (Polyester Resin (B)) The polyester resin is formed by polycondensation of a dicarboxylic acid component and a diol component. It is not particularly limited as long as it is a copolymer polyester composed of a dicarboxylic acid component and a diol component. One or more dicarboxylic acid components and two or more diol components may be used. In this invention, the polyester resin (B) preferably has a naphthalene skeleton in its molecule; for example, it is particularly preferred that the dicarboxylic acid component has a structure containing a naphthalene ring.

[0050] 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, diphenylcarboxylic acid, anthracenedicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid. The composition includes 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, dimeric acid, sebacic acid, octanoic acid, dodecanedicarboxylic acid, and 1,1-cyclohexanediacetic acid. Among these, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, and 1,5-naphthalenedicarboxylic acid, which preferably possess the aforementioned structure containing a naphthalene ring. In a single-state sample, the polyester resin (B), in a compositional analysis using 1H-NMR, contains a composition in which a naphthalene ring is included as a dicarboxylic acid component at a proportion of 50% to 95% (preferably 60% to 90%).

[0051] In terms of diol components, examples include: ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanediethanol, 1,4-cyclohexanediethanol, 1,4-cyclohexanediethanol, decanediol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl) benzoxene, etc.

[0052] The ratio of dicarboxylic acid component to diol component is not particularly limited and can be adjusted in a way that yields a polyester with the desired properties.

[0053] The exact mechanism by which the polycarbonate, polyurethane, and polyester resins used in the coating layer exhibit the effects of the present invention is not yet clear. However, the coating layer of the present invention can maintain the rigidity of the resin while improving its flexibility, thus enabling it to perform its best function as a coating layer. In other words, the polyester film as the substrate not only exhibits excellent adhesion to the substrate such as the 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. In particular, in the present invention, the change in adhesion to the functional layer after film formation and after damp heat treatment is small, and compared with the past, it can improve properties such as resistance to damp heat and adhesion retention over time. Furthermore, in addition to the above-mentioned effects, the present invention can maintain good adhesion to hard coatings, adhesives, etc., even when subjected to forces applied due to impact or deformation caused by folding. In this way, the present invention not only ensures adhesion in the free state, but also ensures adhesion even under harsh environments caused by folding.

[0054] In this invention, it is important that the binder 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 binder 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 setting the polycarbonate polyurethane resin to 40% by mass or more, a balance between the softness and hardness of the polyurethane resin can be ensured, and adhesion over time can be ensured. 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 is also improved, thus allowing the coating layer to perform its best function.

[0055] (Crosslinking Agent) In this invention, in order to form a crosslinked structure between resins in the coating layer, the composition used to form the coating layer includes a capped isocyanate-based crosslinking agent (C). By including the crosslinking agent of this invention, the adhesion under high temperature and high humidity conditions can be further improved. Furthermore, in addition to the above-mentioned effects, with this invention, even when subjected to force due to impact or deformation caused by folding, it can still maintain good adhesion to hard coatings, adhesives, etc. In this way, this invention not only ensures adhesion in the free state, but also ensures adhesion even under harsh environments caused by folding. In this invention, in addition to the capped isocyanate-based crosslinking agent (C), examples of crosslinking agents include: urea-based, epoxy-based, melamine-based, isocyanate-based, acezoline-based, carbodiimide-based, etc., other than crosslinking agent (C). Among these, isocyanate-based crosslinking agents are preferred from the perspective of the stability of the coating over time and the improvement of adhesion under high temperature and high humidity treatment. Furthermore, catalysts can be appropriately used as needed to promote the crosslinking reaction.

[0056] From the viewpoint of adhesion, when the composition for forming the coating layer includes a binder resin and a crosslinking agent, when the total mass of the binder resin and the crosslinking agent is set to 100% by mass, the content of the binder 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 can be 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 can be maintained, and the adhesion under normal temperature, high temperature and high humidity is better. Furthermore, when the total mass of the binder 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.

[0057] When using a capped isocyanate-based crosslinking agent (C) as a crosslinking agent, it is suitable to use a capping agent in order to control the reactivity of the isocyanate. Examples of capping agents include: sulfite 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; and active compounds such as dimethyl malonate and acetone. End-capping agents include methylene compounds; thiols such as butyl mercaptan and dodecyl mercaptan; acetylaniline and acetylamine; endo-acetylamines such as ε-caprolactam and δ-pentanolide; acetylimine and maleimine; 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 for this system.

[0058] From the viewpoint of imparting water dispersibility to aqueous solvents, it is preferable to introduce hydrophilic groups in the isocyanate-based crosslinking agent. Furthermore, in terms of hydrophilicity, anionic groups such as carboxyl or sulfonic acid groups, and nonionic groups such as oxyalkyl groups, are preferred. Crosslinking agents having such hydrophilic groups can be prepared by first reacting a polyisocyanate, which forms the base of the isocyanate, with a compound having reactive groups such as hydrophilic groups and hydroxyl groups.

[0059] (Additives) In the coating layer of the present invention, known additives may be added to the coating layer without hindering 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 undesirable in the environment, etc.

[0060] To further improve the anti-blocking properties of the coating layer, adding inert particles to the coating layer is also preferable. Examples of particles contained in the coating layer include: inorganic particles and organic polymer particles. Examples of inorganic particles include: titanium dioxide, barium sulfate, calcium carbonate, calcium sulfate, silicon dioxide, 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 bentonite, zirconium oxide, tungsten oxide, lithium fluoride, calcium fluoride, etc. Examples of organic polymer particles include: styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silica-based polymer particles.

[0061] The average particle size (average particle size based on the number of particles obtained by SEM, the same below) of the inert particles in the coating layer is preferably 0.04 to 2.0 μm, more preferably 0.1 to 1.0 μm. If the average particle size of the inert particles is 0.04 μm or more, it becomes easier to form unevenness on the surface of the coating layer, thus improving the processability of the easy-to-adhere polyester film, such as its sliding and winding properties, and resulting in better processability during lamination. On the other hand, if the average particle size of the inert particles is 2.0 μm or less, it is preferable that particles are less likely to fall off. The particle concentration in the coating layer is preferably 1 to 20% by mass relative to the resin composition.

[0062] (Formation of the coating layer) The coating layer forming composition (hereinafter also referred to as "coating liquid") used to form the coating layer may further contain a surfactant for the purpose of improving the leveling properties during coating and defoaming the coating liquid. Examples of surfactants include cationic, anionic, and nonionic surfactants, preferably silica-based, acetylenol-based, or fluorine-based surfactants. These surfactants are preferably contained in the coating layer forming composition within a range that does not impair the suppression effect of iridescent color and the degree of adhesion under the 3-wavelength LED light source.

[0063] Regarding the method of coating the coating liquid onto the polyester film, either the so-called inline coating method, in which coating is performed simultaneously during the polyester film making process, or the so-called offline coating method, in which coating is performed using a separate coating machine after the polyester substrate film is made, can be applied, but the inline coating method is more efficient and better.

[0064] Regarding the coating method, any known method can be used to coat the coating liquid onto a polyethylene terephthalate (PET) film. Examples include: reverse roll coating, gravure coating, kiss coating, die coating, roller brush coating, spray coating, air knife coating, wire rod coating, tube coating, impregnation coating, curtain coating, etc. Coating can be performed using these methods alone or in combination.

[0065] In this invention, regarding the method of forming a coating layer on a polyester film, an example is a method of coating a polyester film with a coating liquid containing a solvent, particles, and resin and then drying it. Regarding the solvent, examples include water, or a mixture of water and an organic solvent; however, from an environmental perspective, water alone or a mixture 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 celelusol; amines such as triethanolamine; and amides such as N-methylpyrrolidone.

[0066] The concentration of solid components in the coating liquid varies depending on the type of binder resin and the type of solvent, but it is preferably 2% by mass or more, and 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, and more preferably 15% by mass or less.

[0067] The drying temperature after coating varies depending on the type of binder resin, the type of solvent, the presence or absence of crosslinking agent, and the concentration of solid components, but is preferably 80°C or higher. For example, the coating layer formation step may involve a heating step at 180°C or higher. The coating layer formation step is preferably 250°C or lower.

[0068] The coating amount of the coating liquid can be adjusted such that the solid content on the dried polyester film is, for example, 0.03 to 0.24 g / m², or even 0.06 to 0.18 g / m². When performing a film stretching step after coating and drying, the film can be prepared such that the solid content on the stretched polyester film is within the above range.

[0069] The stretching step can be either uniaxial stretching 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 further relaxed at 120–250°C.

[0070] The easy-to-adhere polyester film of the present invention can be manufactured by coating, drying, stretching and heat treatment as described above.

[0071] The thickness of the coating layer is 30 nm to 200 nm. As long as the coating is prepared within this range, it is easier to balance processability and adhesion, which is preferable. More preferably, it is 40 nm to 150 nm, for example, 40 nm to 130 nm, and even more preferably 40 nm to 100 nm. If the coating layer thickness is 30 nm or more, the adhesion becomes good, which is preferable. If the coating layer thickness is 200 nm or less, adhesion is less likely to occur, which is preferable. Furthermore, within the above range, there is a tendency to easily control the phase separation state of the film surface. Moreover, by having the above thickness, even in folded applications where impacts or deformations caused by folding occur, good adhesion to hard coatings, adhesives, etc., can still be maintained. In this way, the present invention, by having a coating layer thickness within the above range, not only ensures adhesion in a free state, but also ensures adhesion even under harsh environments caused by folding.

[0072] The thickness of the coating layer is determined by observing the cross-section of the cut film using a transmission electron microscope (TEM) and taking the average value of 10 randomly measured coating layer thicknesses as the coating layer thickness.

[0073] In this invention, it was discovered that the phase separation state of the film surface changes depending on the heating conditions, as do surface functional properties such as adhesion. The distribution of the low elastic modulus component of the surface, which changes due to thermal history, is numerically converted into a distribution of deformation by mapping the AFM force curve. In this invention, the control of the phase separation state of the film surface can also be performed during the drying step of the composition forming the coating layer, and during the stretching step of the film after coating and drying of the composition. For example, in this invention, to guide the control of the phase separation state of the film surface, the surface of the coating layer is observed using an atomic force microscope. A state is achieved where, in the force curve of a 3nN load pressing the surface, the area with a deformation of 1 nm or more is 0.15% or more relative to the overall observed surface. This can be achieved through the following steps. Furthermore, at the time of this application, no specific mechanism for controlling the phase separation state of the film surface using the steps described in this specification has been found, but it is speculated that, for example, by using the components and film-forming conditions described in this specification, the desired interactions will occur among the components.

[0074] The present invention provides a method for manufacturing a thin film in which the phase separation state of the thin film surface is controlled and the proportion of the area with a deformation amount of 1 nm or more is 0.15% or more relative to the overall observation surface. This method includes the following steps: For example, coating a composition forming a coating layer onto a polyester film substrate and drying it at a temperature of 30°C to 120°C, preferably at 60°C to 100°C. It is presumed that drying within this temperature range can prevent the coating layer from completely hardening, thus aiding in the control of the phase separation state in subsequent steps.

[0075] Next, preferably at a temperature of 80°C to 180°C, for example, 80°C to 170°C, the film having the coating layer is stretched (sometimes simply referred to as the stretching step). The stretching direction can be either the long side direction or the width direction. If the workability of operations such as the relaxation treatment described later is taken into consideration, stretching can also be performed in the width direction. Preferably, the stretching ratio of the film is 1.5 times to 6.0 times in the width direction, for example, 1.5 times to 5.0 times in the width direction. Furthermore, in one state, it is desirable to heat (sometimes simply referred to as the heating step) at a temperature higher than the temperature of the stretching step while keeping the length of the film fixed in the width direction. For example, the heating temperature while keeping the length of the film fixed in the width direction is 10°C to 70°C higher than the stretching temperature, specifically, heating is performed at a temperature of 90°C to 220°C, for example, 100°C to 220°C.

[0076] In the sample, 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 above-described 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 to 220°C, for example, 100°C to 220°C. Preferably, the relaxation treatment is performed within ±10°C of the above-described heating step. For example, the heating step and the relaxation treatment can be performed at the same temperature, in which case the heating step and the relaxation treatment can be performed continuously. When the heating step and the relaxation treatment are performed continuously, the highest temperature reached is performed at a temperature 10°C to 70°C higher than the stretching temperature, specifically, at a temperature of 90°C to 220°C, for example, 100°C to 220°C.

[0077] The total heating time is preferably in the range of 5 seconds to 180 seconds. When the heating step and relaxation treatment are performed at the same temperature as described above, and the process is carried out in a continuous manner, the time required for the heating step and relaxation treatment is also preferably in the range of 5 seconds to 180 seconds.

[0078] It is believed that in this invention, the heat applied to the coating film formed by coating has a significant impact on the phase separation that occurs. As the heat applied to the coating film increases, phase separation accelerates and progresses towards a rougher surface. On the other hand, as the heat applied to the coating film decreases, phase separation slows down and progresses towards a flatter surface. By appropriately adjusting the coating amount and temperature, the most suitable surface state can be constructed, resulting in the formation of a tightly adhered, easily bonded film that maintains the adhesion of this invention over time. Furthermore, even when subjected to forces due to impacts or deformation caused by folding, good adhesion to hard coatings, adhesives, etc., can still be maintained.

[0079] To confirm the distribution of deformation on the surface of the easily bondable polyester film, an atomic force microscope (hereinafter referred to as AFM) was used and measurements were performed using the PinPoint mode. A cantilever with a spring constant of approximately 1 N / m was used, and the field of view was set to 1 μm square.

[0080] Specifically, in accordance with the PinPoint mode manual, after evaluating the force slope, sensitivity, and spring constant, the measurement is carried out using the following conditions.

[0081] The measurement conditions are shown below. • Measurement apparatus: Atomic force microscope (NX10) manufactured by Park Systems. • Measurement mode: PinPoint mode. • Cantilever: CONTSCR (NanoWorld), NSC18 (Mikro Masch), etc. • Measurement environment: 23℃ / atmosphere. • Set point: 3nN. • Cantilever speed: 6μm / s. • Pixel count: 256×256

[0082] In this invention, it was found that when observing the surface of an easily bondable polyester film using the PinPoint mode of AFM, a deformation of 1 nm or more, read from the force curve obtained by applying a 3 nN pressing load, relative to the overall measured area, would have a significant impact on the adhesion when laminating the functional layer described later, resulting in higher adhesion durability. Preferably, it is 0.20% to 40.0% relative to the overall measured area, and more preferably 0.20% to 35%. For example, it can be 0.20% to 20.0%, or 0.30% to 14.0%. It was found that if it is 0.15% or more, not only is the adhesion immediately after the functional layer is applied ensured, but the adhesion of the film can also be ensured even after long periods of non-use, and a film with high adhesion stability that does not change over time can be obtained. Furthermore, if the deformation rate becomes excessively high and exceeds the scope of this invention, there is a possibility of reduced adhesion reliability. Furthermore, if the deformation rate becomes excessively high and exceeds the scope of this invention, the film's flexibility may deteriorate, making it unsuitable for foldable applications.

[0083] Laminated Polyester Film The present invention also provides a laminated polyester film formed by providing functional layers with various properties on a coating layer of an easily bondable polyester film. The term "functional layer" refers to a layer with functions such as preventing reflection, suppressing glare, suppressing iris, and suppressing damage, for example, a hard coating layer, an anti-glare layer, an anti-glare anti-reflective layer, an anti-reflective layer, a low-reflective layer, or an antistatic layer. This functional layer can be made from various types known in the art, and its type is not particularly limited.

[0084] For example, when forming a hard coating layer on the coating layer, the material used for the hard coating layer can be any known material and is not particularly limited. As such a material, resin compounds (especially curing resins) that are polymerized and / or reacted by drying, heat, chemical reaction, or irradiation with an electron beam, radiation, or ultraviolet light can be used. Examples of such curing resins include melamine-based, acrylic-based, silicone-based, and polyvinyl alcohol-based curing resins, but from the perspective of high surface hardness or achieving optical design, photocurable acrylic-based curing resins are preferred. For such acrylic-based curing resins, 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 reactive diluents, photopolymerization initiators, sensitizers, etc., into these acrylic curable resins, it is possible to obtain a coating composition for forming the aforementioned optical functional layer.

[0085] The aforementioned hard coating may have an anti-glare function that scatters external light. The anti-glare function may be achieved by forming 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 0.2% or more is also acceptable, as is 0.5% or more.

[0086] Furthermore, by setting layers with different refractive indices as functional layers and changing the light transmission characteristics, low-reflection processing (anti-reflection processing) that suppresses light reflection can be implemented. 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 especially preferably 0 to 0.5%. Of course, 0% is ideal, but 0.05% or higher is also acceptable, as is 0.1% or higher.

[0087] The applications of the laminated polyester film of the present invention mainly cover all optical films, including: base films for optical components such as prism lenses, AR (Anti-Reflection) films, hard-coated films, diffuser plates, and shatterproof films for LCDs, flat-screen TVs, and CRTs; near-infrared absorbing filters for components located on the front panel of plasma displays; and transparent conductive films such as touch panels and electroluminescent films. It is suitable for any application, but is particularly suitable for foldable displays.

[0088] Examples of acrylic resins that are cured by electron beams or ultraviolet light for forming the aforementioned functional layers include: oligomers comprising (meth)acrylates as reactive oligomers and (meth)acrylate monomers as reactive monomers (reactive diluents). Examples of (meth)acrylate oligomers include: compounds formed by reactive (meth)acrylate groups bonded to the (meth)acrylate resin backbone, polyester acrylates, epoxy acrylates, polyurethane acrylates, silicone acrylates, melamine acrylates, polyether acrylates, etc. Regarding (meth)acrylate monomers, examples 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, neopentyltetrol tri(meth)acrylate, dinepentyltetrol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and neopentyl glycol di(meth)acrylate. Third components can be appropriately added as needed, such as lower molecular weight polyester resins, polyether resins, acrylic resins, epoxy resins, urethane resins, alkyd resins, spiroacetal resins, polybutadiene resins, polythiol polyene resins, and polyols.

[0089] In the case of electron beam or ultraviolet curable acrylic resins, acetophenones, diphenyl ketones, Mihler benzoate, α-pentyl oxime ester, tetramethylthiuram monosulfide, and 9-oxosulfuron derivatives 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.

[0090] Silicon-based (siloxane-based) thermosetting resins can be manufactured by hydrolyzing and condensing one or more organosilicon compounds under acid or alkali catalysts. In particular, for low-reflection applications, the addition of one or more fluorosilane compounds for hydrolysis and condensation further improves low refractive index properties and stain resistance.

[0091] Manufacturing of the laminated polyester film The laminated polyester film of the present invention can be manufactured by providing a functional layer on a coating layer of an easy-to-adhere polyester film. Specific examples are described below, but are not limited thereto.

[0092] A functional layer forming composition (functional layer forming coating liquid) is coated onto the coating layer of the aforementioned easy-to-adhere polyester film. Examples of such functional layer forming composition include: the aforementioned electron beam or ultraviolet curable acrylic resin (including its oligomers, monomers, etc.) or silicone thermosetting resin. When the coating layer is provided on both sides of the easy-to-adhere polyester film, it can be coated onto at least one side of the coating layer. The functional layer forming coating liquid does not need to be specially diluted, but it is acceptable to dilute it with an organic solvent according to its viscosity, wettability, coating thickness, etc. After coating the film with the aforementioned functional layer forming coating liquid, the film can be dried as needed, and then cured by electron beam or ultraviolet irradiation and heating according to its curing conditions, thereby forming a functional layer.

[0093] More typically, a functional layer forming coating liquid with the above-described composition can be applied to a coating layer of an easily bondable polyester film using a wire rod or similar tool, and the solvent can be removed by drying at 60–100°C for 0.5–10 minutes. Then, the film coated with the functional layer can be irradiated with ultraviolet light at 300 mJ / cm² using, for example, a high-pressure mercury lamp, to obtain a laminated polyester film with the functional layer.

[0094] 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, the effects of the functional layer on chemical resistance, scratch resistance, and stain resistance can be effectively exerted, which is therefore preferable. On the other hand, if the thickness is 15 μm or less, the flexibility of the functional layer can be maintained, and there is no risk of cracking, which is also preferable.

[0095] The easy-to-adhere polyester film and laminated polyester film of the present invention are primarily suitable for optical applications, and therefore preferably have high transparency. The lower limit of haze is ideally 0%, and the closer to 0%, the better. The upper limit of haze is preferably 2%. If it is below 2%, the light transmittance is good, and a clear image can be obtained in the liquid crystal display device, which is therefore preferable. Haze can be measured, for example, according to the method described in the embodiments described later.

[0096] (Evaluation of Adhesion) 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. Specifically, a laminated polyester film is prepared by forming a functional layer on the coating layer of an easy-to-adhere polyester film, and its adhesion X (%) is evaluated. Alternatively, after storing the easy-to-adhere polyester film for an extended period, a functional layer is formed on its coating layer to prepare a laminated polyester film, and its adhesion Y (%) is evaluated. The long-term storage conditions were originally intended to be measured after being placed in a normal room temperature environment for several weeks to several months. However, room temperature storage requires an extremely long time. Therefore, as an alternative measurement method in this specification, accelerated conditions are used: placement in a high-temperature, high-humidity environment of 80°C and 90%RH for 24 hours, followed by placement at room temperature (10–30°C) for 12 hours. As can be seen from Table 3 of the examples, the adhesion X is typically 95% or higher, more preferably 98% or higher, and most preferably 100%. If the adhesion X is 95% or higher, it can be said that the adhesion between the coating layer and the functional layer can be sufficiently maintained. Furthermore, the adhesion Y after long-term storage is typically 95% or higher, more preferably 98% or higher, and most preferably 100%. If the adhesion Y is 95% or higher, it can be said that the adhesion between the coating layer and the functional layer can be sufficiently maintained even after long-term storage. In the easy-to-adhere polyester film of the present invention, both adhesion X and adhesion Y are high, and adhesion Y is equal to or almost unchanged relative to adhesion X.

[0097] Thus, the easy-to-adhere polyester film of the present invention has the following characteristics: high adhesion reliability to the functional layer, not only after film formation, but also after exposure to high temperature and high humidity environments (even after long-term storage). That is to say, it has the characteristics of sufficiently high adhesion X after film formation and high adhesion Y after long-term storage. In other words, the above-mentioned X (%) and Y (%) satisfy the following formula (1). XY (%) ≦ 5 … Formula (1)

[0098] The value of Equation (1) is usually less than 5%, more preferably less than 3%, and most preferably 0%. If it is less than 5%, it can be determined that there is no significant difference between the adhesion after film formation and the adhesion after wet heat treatment, and it can be determined that both are sufficiently adhered. Therefore, it can be determined that even if stored for a long time, the adhesion of the functional layer has not been reduced, and the adhesion reliability can be ensured.

[0099] The above results do not stop there. It is understood that by ensuring this level of adhesion, sufficient adhesion can still be achieved even after the bending test required for foldable display applications.

[0100] After the bending test shown below, the laminated polyester film was visually inspected to check the cracking (crack) state of the hard coating on the film surface at the bent portion, thus confirming the adhesion after the bending test. ○: No cracks (cracks) on the outer surface of the film after bending ×: Cracks (cracks) or film breakage on the outer surface of the film after bending. This adhesion is evaluated as adhesion Z (%). As can be seen from Table 3 of the examples, an easy-to-adhere polyester film with adhesion Z of ○ indicates high adhesion reliability. [Example]

[0101] Next, the present invention will be described in detail using examples, comparative examples, and reference examples, but the present invention is not limited to the following examples. Furthermore, the method for measuring the measured values ​​used in the present invention is as follows.

[0102] (1) Average particle size [Measurement method using scanning electron microscope] The average particle size of the particles present in the coating layer of the present invention can be measured by the following method. The particles are photographed using a scanning electron microscope (SEM), and the maximum diameter (distance between the two farthest points) of 300 to 500 particles is measured at a magnification of 2 to 5 mm with the size of the smallest particle. The arithmetic mean of these is taken as the average particle size.

[0103] [Dynamic Light Scattering Method] The average particle size can also be determined during the fabrication of particles and thin films using the dynamic scattering method. The sol is diluted using a dispersion medium, and the parameters of the dispersion medium are measured using a submicron particle analyzer N4 PLUS (manufactured by Beckman Coulter). The average particle size is obtained by calculation using the cumulant method. In the dynamic light scattering method, the average particle size of the particles in the sol is observed; when particles aggregate, the average particle size of these aggregated particles is observed.

[0104] (2) Refractive Index of Particles The refractive index of particles can be determined by the following method. After drying the inorganic particles at 150°C, they are pulverized using a mortar and pestle. The particles are then immersed in solvent 1 (which has a lower refractive index than the particles), and solvent 2 (which has a higher refractive index than the particles) is added in small amounts until the particles become almost transparent. The refractive index of the solution is measured using an Abbe refractometer (ATAGO Co., Ltd.). The measurement is performed at 23°C using a D-line (wavelength 589 nm). The solvents 1 and 2 mentioned above are selected to be miscible. Examples of solvents that can be used depending on the refractive index include: 1,1,1,3,3,3-hexafluoro-2-propanol, 2-propanol, chloroform, carbon tetrachloride, toluene, glycerol, etc.

[0105] (3) Haze of easy-to-adhere polyester film for optical use The haze of easy-to-adhere polyester film was measured according to JIS K 7136:2000 using a turbidimeter (Nippon Denshoku Corporation, NDH2000).

[0106] (4) Adhesion of the functional layer A hard coating is formed on the easy-to-adhere coating layer of the polyester film obtained in the example. The adhesion between the hard coating and the substrate film is determined according to the description in 8.5.1 of JIS-K5400-1990.

[0107] The coating liquid used for forming a hard coating is prepared as follows. (Preparation of coating liquid L for forming a hard coating) Isopropanol 21.00% of mass Toluene 49.00% of mass Neopentyl tetraacrylate 21.38% of mass (Niinakamura Chemicals A-DPH) Aminocarbamate acrylates 7.12% of mass (Made by Mitsubishi Chemical Corporation, UV-7600B) Photopolymerization initiator 1.50% by mass (IGM Resins BV Omnirad184)

[0108] (Preparation of coating liquid M for hard coating formation) Methyl ethyl ketone 64.40% of mass Dinepentyl tetraol hexaacrylate 27.20% of mass (Niinakamura Chemicals A-DPH) Polyethylene glycol diacrylate 3.40% by weight (Kyoei Chemical Manufacturing Co., Ltd. Light Acrylate 9EG-A) Bisphenol A diacrylate 4.00% mass (Kyoei Chemical Manufacturing Co., Ltd. Light Acrylate BP-4PA) Photopolymerization initiator 1.00% mass (IGM Resins BV Omnirad184)

[0109] (Formation of Hard Coating) The easy-to-adhere polyester film manufactured in the example was stored at 20°C and 65%RH. Twelve hours after the start of film preparation, a coating liquid (L or M) for forming the hard coating as described above was applied to the easy-to-adhere 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 ultraviolet light at 300 mJ / cm² using a high-pressure mercury lamp to obtain a hard-coated film with a hard coating thickness of 7 μm.

[0110] The specific method for measuring adhesion is as follows. Using a cutter guide with a 2mm gap, 100 grid-like cuts are made on the hard coating surface, penetrating the hard coating and reaching the substrate film. Next, cellophane adhesive tape (made by Nichiban, No. 405; 24mm wide) is applied to the grid-like cut surfaces and wiped with an eraser to ensure complete adhesion. Then, the cellophane adhesive tape is peeled vertically from the hard coating surface of the easily bondable polyester film on which the aforementioned hard coating is formed. The number of grids peeled off from the hard coating surface is visually counted, and the adhesion between the hard coating and the substrate film is calculated using the following formula. In addition, those grids that are partially peeled off are also counted as peeled grids. Adhesion (%) = {1 - (number of peeled grids / 100)} × 100

[0111] (6) Resistance to damp heat (adhesion after placement at 80°C and 90%RH) The obtained easy-to-adhere polyester film was placed in a high-temperature and high-humidity bath at 80°C and 90%RH for 24 hours, and then placed at room temperature (20°C and 65%RH) for 12 hours. Afterward, the functional layer was formed by the same method as described above, and the adhesion to the substrate film was determined.

[0112] (7) Reduced viscosity ηsp / c (unit: dl / g) 0.10g of polyester resin was dissolved in 25ml of a mixed solvent of phenol / tetrachloroethane (mass ratio 6 / 4) and measured using an Ubbelohde viscometer at 30°C.

[0113] (8) Using a transmission electron microscope for cross-sectional observation, the obtained easy-to-adhere polyester film was cut into 1mm × 10mm sections and embedded in epoxy resin. Then, a thin section parallel to the short side of the embedded sample section was prepared using an ultramicrotome. Next, the sectioned film was stained with ruthenium tetroxide, and the areas without obvious damage were observed using a transmission electron microscope (JEM2100, Nippon Electron) at an accelerating voltage of 200kV and 20,000x. From 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.

[0114] (9) The resin composition of polyester was dissolved in deuterated chloroform and analyzed by 1H-NMR using a Varian Gemini 200 nuclear magnetic resonance analyzer (NMR). The molar percentage of each component was determined by the integral ratio.

[0115] (10) Viscosity of polyester resin dispersion: Add the aqueous dispersion of polyester resin to a 140cc glass bottle, and use a viscometer with rotor No. 1 or No. 2 of model BL (TOKIMEC INC.) in a constant temperature bath at 25°C. Measure the viscosity of the aqueous dispersion of polyester resin at a rotation speed of 60 rpm for 1 minute.

[0116] (11) Adhesion evaluation in the embodiment: After film formation (X) and after wet heat treatment (Y), the procedure was performed according to the procedure described in this specification. Also, the adhesion (Z) was performed and evaluated according to the procedure described in this specification. The adhesion reliability based on the result is defined as follows. 0≦XY≦2 and Z is ○ can ensure adhesion to a high degree and is evaluated as "A", 2≦XY≦4 and Z is ○ can ensure adhesion well and is evaluated as "B", 4≦XY≦5 and Z is ○ can ensure adhesion and is evaluated as "C", 5<XY and Z is × cannot ensure adhesion and is evaluated as "D".

[0117] (12) Bending resistance of laminated polyester film: A laminated polyester film sample with a width of 20 mm × flow direction of 110 mm was prepared. Using a no-load U-shaped stretch tester (manufactured by Yuasa System Co., Ltd., DLDMLH-FS), the bending radius was set to 0.5 mm, and the sample was bent 200,000 times at a speed of 1 time / second. At this time, the sample was fixed at 10 mm from both ends of the long side, and the bending part was set to 20 mm × 90 mm. Considering the case where the laminated film is placed on the inner surface of the folded sample, the bending radius was set to 0.5 mm and a bending test was performed in a model manner. The film surface on the outside of the bending part was observed with a digital microscope (HIROX RH2000) at 800x magnification to observe whether there were any cracks (cracks) on the surface.

[0118] [Polyurethane Resin] Synthesis of polyurethane resin (PCPU-1): In a four-necked flask equipped with a stirrer, a Deutsche cooler, a nitrogen inlet tube, a silicone drying tube, and a thermometer, 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 1,4-butanediol / 2,2-dimethyl-1,3-propanediol (=75 / 25 (MoRb)) as the main raw materials (with a number average molecular weight of 1000), 7 parts by mass of dimethylolpropionic acid, and 200 parts by mass of ethyl methyl ketone as a solvent were added. The mixture was stirred at 75°C for 3 hours under nitrogen atmosphere. The infrared spectrum of the reaction solution was measured to confirm the disappearance of isocyanate groups in the reaction solution. Next, after cooling the solution to room temperature, 8.2 parts by mass of triethylamine were added to obtain a polyurethane resin (PCPU-1) solution with a solid content of 50.0% by mass. The overall hexane content of this polycarbonate polyurethane (PCPU-1) was 0.0% by mass.

[0119] Preparation of an aqueous dispersion of polyurethane resin (PCPU-1) (PCPU-1WD): In a reaction vessel equipped with a high-speed stirring homogenizer, a specified amount of water was added, the temperature was adjusted to 25°C, and the aforementioned polyurethane resin (PCPU-1) solution was slowly added while stirring and mixing at 2000 min⁻¹ to achieve aqueous dispersion. Then, ethyl methyl ketone, which was the solvent, was removed under reduced pressure. By adjusting the concentration using water, an aqueous dispersion (PCPU-1WD) of polycarbonate polyurethane resin (PCPU-1) with a solids content of 35.0% by mass was prepared.

[0120] Synthesis of polyurethane resin (PCPU-2) and preparation of aqueous dispersion (PCPU-2WD): The main raw material of polycarbonate diol was changed to 1,4-cyclohexanediol / 1,6-hexanediol (=75 / 25) (molar ratio). Otherwise, (PCPU-2) was prepared in the same manner as the synthesis of (PCPU-1), and the aqueous dispersion (PCPU-2WD) was obtained in the same manner as the preparation of (PCPU-1WD). The overall hexane content of this polycarbonate polyurethane (PCPU-2) was 11.5% by mass.

[0121] Synthesis of polyurethane resin (PCPU-3) and preparation of aqueous dispersion (PCPU-3WD): The main raw material of polycarbonate diol was changed to 1,5-pentanediol / 1,6-hexanediol (=45 / 55) (molar ratio). Otherwise, (PCPU-3) was prepared in the same manner as the synthesis of (PCPU-1) described above, and the aqueous dispersion (PCPU-3WD) was obtained in the same manner as the preparation of (PCPU-1WD). The overall hexane content of this polycarbonate polyurethane (PCPU-2) was 31.2% by mass.

[0122] Synthesis of polyurethane resin (PCPU-4) and preparation of aqueous dispersion (PCPU-4WD): 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) described above. The aqueous dispersion (PCPU-4WD) was obtained in the same manner as the preparation of (PCPU-1WD). The overall hexane content of this polycarbonate polyurethane (PCPU-4) was 80.5% by mass.

[0123] [Polyester Resin] Manufacturing of Polyester Resin (PEs-1): Polyester resin (PEs-1) was polymerized according to a known polymerization method. The composition of the obtained polymer was analyzed by 1H-NMR, and the molar percentage of each component was determined by its integral ratio. The results are recorded in Table 1. The reduced viscosity of the obtained polyester resin was 0.583 dl / g. The abbreviations recorded in Table 1 are recorded below. TPA: Terephthalic acid; IPA: Isophthalic acid; NDC: Naphthalic acid; SA: Sebacic acid; DSS: Sodium dimethyl isophthalate-5 sulfonate; EG: Ethylene glycol; HD: Hexanediol; DEG: Diethylene glycol; NPG: Neopentyl glycol.

[0124] Preparation of polyester aqueous dispersion (PEs-1WD): In a reactor equipped with a stirrer, thermometer, and reflux device, 30 parts by mass of copolymerized polyester resin (PEs-1) and 15 parts by mass of ethylene glycol n-butyl ether were added. The resin was dissolved by heating and stirring at 110°C. After the resin was completely dissolved, 55 parts by mass of water were slowly added to the polyester solution while stirring. After addition, the solution was cooled to room temperature while stirring to prepare a milky white polyester resin (PEs-1) aqueous dispersion (PEs-1WD) with a solid content of 25.1% by mass. The liquid viscosity of the obtained aqueous dispersion was 84 mPa·s.

[0125] Manufacturing of polyester resin (PEs-2): Polyester resin (PEs-2) was manufactured using a known polymerization method, similar to the polymerization of the aforementioned polyester resin (PEs-1). The composition ratio was determined in the same manner as for PEs-1, and the reduced viscosity of the resulting resin was evaluated. The results are recorded in Table 1.

[0126] Preparation of polyester aqueous dispersion (PEs-2WD): Polyester aqueous dispersion (PEs-2WD) was prepared in the same manner as the preparation of polyester aqueous dispersion (PEs-1WD). The concentration of solid components and the viscosity of liquid were evaluated in the same manner as PEs-1WD. The results are recorded in Table 2.

[0127] [Table 1] Composition ratio (by means of) 1 H-NMR) Reduced viscosity TPA IPA NDC SA GCM EG HD DEG NPG [dl / g] PEs-1 46 47 0 0 7 50 0 0 50 0.400 PEs-2 0 0 84 10 6 50 44 6 0 0.493 [Table 2] NV ηB % mPa・s PEs-1WD 25.1 47 PEs-2WD 24.9 32

[0128] [Crosslinking Agent] Preparation of an aqueous dispersion (C-1WD) of a capped isocyanate-based crosslinking agent (C-1): 125.2 parts by mass of a polyisocyanate compound with a biuret structure (manufactured by Asahi Kasei Chemicals, Durnate 24A-100, NCO concentration 23.1%) using hexamethylene diisocyanate as a raw material, 50.0 parts by mass of dipropylene glycol dimethyl ether, and 68.8 parts by mass of 3,5-dimethylpyrazole were added to a flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 70°C for 2 hours under nitrogen atmosphere. Afterwards, the infrared spectrum of the reaction solution was 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 water was added while stirring at 2000 min⁻¹. By adjusting the concentration using water, an aqueous dispersion (C-1WD) of the end-capped isocyanate crosslinking agent (C-1) with a solid content of 30.0% by weight was prepared.

[0129] [Crosslinking Agent] Preparation of an aqueous dispersion (C-1WD) of a capped isocyanate-based crosslinking agent (C-1): In a flask equipped with a stirrer, thermometer, and reflux condenser, 100 parts by weight of a polyisocyanate compound with a triisocyanate structure (Asahi Kasei Chemicals, Durnate TPA-100, NCO concentration 23.1%) prepared from 1,6-hexamethylene diisocyanate by a known method, 17.5 parts by weight of N-methylpyrrolidone, and 35.00 parts by weight of 3,5-dimethylpyrazole were added dropwise at 70°C for 1 hour under nitrogen atmosphere. Then, 12.50 parts by weight of dimethylolpropionic acid were added dropwise. The infrared spectrum of the reaction solution was measured, and after confirming the disappearance of the isocyanate group absorption, 8.72 parts by weight of N,N-dimethylethanolamine were added. After stirring for 1 hour, add an appropriate amount of water to prepare an aqueous dispersion (C-1WD) of a capped isocyanate crosslinking agent (C-1) with a solid content of 40.0% by mass.

[0130] Preparation of an aqueous dispersion (C-2WD) of a capped isocyanate-based crosslinking agent (C-2): 125.2 parts by mass of a polyisocyanate compound with a biuret structure (Asahi Kasei Chemicals, Duranat 24A-100, NCO concentration 23.1%) using hexamethylene diisocyanate as a raw material, 50.0 parts by mass of dipropylene glycol dimethyl ether, and 68.8 parts by mass of 3,5-dimethylpyrazole were added to a flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 70°C for 2 hours under nitrogen atmosphere. Afterward, the infrared spectrum of the reaction solution was measured to confirm the disappearance of isocyanate group absorption. Then, after cooling to room temperature, 6 parts by mass of polyethylene glycol (n=12) monolaurate were added, and water was added while stirring at 2000 min⁻¹. An aqueous dispersion (C-2WD) of a capped isocyanate-based crosslinking agent (C-2) with a solid content of 30.0% by mass was prepared by adjusting the concentration using water.

[0131] Preparation of an aqueous dispersion (C-3WD) of a capped isocyanate-based crosslinking agent (C-3): 64.5 parts by mass of a polyisocyanate compound (Asahi Kasei Chemicals, Durnate TPA-100, NCO concentration 23.1%) with a triisocyanate structure, using hexamethylene diisocyanate as a raw material, 50.0 parts by mass of dipropylene glycol dimethyl ether, and 20.8 parts by mass of methyl ethyl ketone oxime were added to a flask equipped with a stirrer, thermometer, and reflux condenser. The mixture was stirred at 70°C for 1 hour under nitrogen atmosphere. Then, 8.7 parts by mass of trimethylolpropionic acid were added. The infrared spectrum of the reaction solution was measured while stirring at 70°C until the absorption of the isocyanate groups disappeared. Next, after cooling to room temperature, 6.0 parts by mass of dimethylethanolamine were added, and water was added while stirring at 2000 min⁻¹. An aqueous dispersion (C-3WD) of a capped isocyanate-based crosslinking agent (C-3) with a solid content of 30.0% by mass was prepared by adjusting the concentration using water.

[0132] (Zirconium oxide particles) In a 3-liter glass container, 2283.6 g of pure water and 403.4 g of oxalic acid dihydrate were added 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., containing 39.76% by mass when converted to ZrO2) was slowly added and mixed for 30 minutes, followed by heating at 90°C for 30 minutes. Then, 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 when converted to 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 completely gelled without any unresolved gel material. The obtained sol contained 4.0% ZrO2 by mass, pH 6.8, and the average particle size obtained by dynamic light scattering was 19 nm. Furthermore, the transmittance was 88% when the sol was adjusted to a ZrO2 concentration of 2.0% by mass using pure water. Particle observation using a transmission electron microscope revealed aggregated ZrO2 primary particles around 7 nm in size. 4000 g of the zirconia sol with a ZrO2 concentration of 4.0% by mass obtained through the above hydrothermal treatment was washed and concentrated using an ultrafiltration device with pure water added slowly, yielding 953 g of zirconia sol with a ZrO2 concentration of 13.1% by mass, pH 4.9, and a transmittance of 76% at a ZrO2 concentration of 13.1% by mass. The refractive index of the obtained zirconia-based particles was 1.75.

[0133] (Zirconium oxide sol) 300g of a 13.1% by mass ZrO2 zirconia sol 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 the zirconia sol. The resulting high-concentration zirconia sol (30.5% by mass ZrO2) was then concentrated using an ultrafiltration device, yielding 129g of the zirconia sol. The obtained high-concentration zirconia sol had an average particle size of 19nm at pH 9.3, obtained by dynamic light scattering. Furthermore, the zirconia sol showed no sedimentation and remained stable for more than one month at 50°C.

[0134] [Example 1] (Preparation of coating liquid) A coating liquid with the following composition was prepared. water 42.11 parts by weight Isopropanol 30.06 parts by weight Zirconia sol 4.50 parts by weight (Zirconium oxide sol with an average particle size of 20 nm and a solid content concentration of 30% by mass) silica sol 0.87 parts by weight (Silica sol with an average particle size of 450 nm and a solid content concentration of 4% by mass) PCPU-1WD 5.62 parts by weight (Solid component concentration 35.0% by mass) PEs-1WD 11.08 parts by weight (Solid component concentration 25.1% by mass) C-1WD 5.20 parts by weight (Solid component concentration 30.0% by mass) surfactants 0.30 parts by weight (Silicon-oxygen based, solid content 10.0% by mass) high boiling point solvent 0.26 parts by weight

[0135] (Manufacturing of Easy-to-Adhere Polyester Film) As the film raw material polymer, PET resin pellets 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. Afterwards, the pellets were fed to an extruder and melt-extruded into sheets at approximately 280°C. The sheets were then rapidly cooled and solidified on a rotating cooling metal roller maintained at a surface temperature of 20°C to obtain unstretched PET sheets.

[0136] Next, the above coating solution is applied to one side of the PET film using a roller coating method and dried at 80°C, adjusting the coating weight after final stretching and drying to 0.07 g / m². Then, the film is stretched to 4.0 times its original width in a tenter frame at 150°C, and heated to 200°C while maintaining the film's width length. Further relaxation treatment in the width direction is then performed at 200°C to obtain an easy-to-adhere polyester film with a thickness of 50 μm.

[0137] The thickness of the coating layer of the easy-to-adhere polyester film is 50 nm, and the haze of the film is 1.11%.

[0138] The surface deformation of the obtained easy-to-adhere polyester film was evaluated using AFM. If the proportion of deformation greater than 1 nm was calculated, it was 0.16%.

[0139] Next, on the easy-adhesion coating layer of the easy-adhesion polyester film, a functional layer forming coating liquid (the hard coating forming coating liquids L and M of (4) above) is used to obtain a laminated polyester film with a functional layer according to the above forming method.

[0140] The adhesion of the functional layer of the obtained laminated polyester film was evaluated, and the adhesion strength X was 99%.

[0141] On the other hand, the above-mentioned easy-to-adhere polyester film was placed in a high-temperature and high-humidity bath at 80°C and 90% RH for 24 hours, and then placed at room temperature for 12 hours. After that, 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.

[0142] The adhesion of the functional layer of the laminated polyester film was evaluated, and the adhesion Y was found to be 95%.

[0143] From this result, XY = 4 (%). These results are recorded in Table 3.

[0144] Furthermore, a bending test was conducted on the easy-to-adhere polyester film with the above-mentioned functional layer to evaluate the cracks (cracks) on the surface of the film after the test. No cracks were found.

[0145] The results (adhesion Z) are recorded in Table 3. No cracks (fractures) were found, therefore it can be confirmed that adhesion reliability can be ensured. Furthermore, the evaluation results of adhesion reliability are recorded in the adhesion reliability column of Table 3.

[0146] [Example 2] The polycarbonate polyurethane resin used was changed to PCPU-2WD as described in Table 3. Otherwise, the same procedure as in Example 1 was followed to obtain an easy-to-adhere polyester film.

[0147] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0148] [Example 3] The polyester resin used was changed to PEs-2 as described in Table 3. Otherwise, the same procedure as in Example 2 was followed to obtain an easy-to-adhere polyester film.

[0149] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0150] [Examples 4-5] The temperature for heat setting was changed to the temperature listed in Table 3. Otherwise, the resin composition was the same as in Example 3, and the process was carried out in the same way as in Example 1 to obtain an easy-to-adhere polyester film.

[0151] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0152] [Examples 6-8] The coating thickness was changed to the thickness recorded in Table 3, and the heat setting temperature recorded in Table 3 was used. Otherwise, the resin composition was the same as in Example 1, and the process was carried out in the same way as in Example 3 to obtain an easy-to-adhere polyester film.

[0153] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0154] [Examples 9-12] The polycarbonate polyurethane resin used was changed to PCPU-3WD as described in Table 3, and the coating thickness and heat setting temperature were changed to the conditions described in Table 3. Otherwise, the process was carried out in the same way as in Example 3 to obtain an easy-to-adhere polyester film.

[0155] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0156] [Example 13] The polycarbonate polyurethane resin used was changed to PCPU-4WD as described in Table 3. Otherwise, the same procedure as in Example 3 was followed to obtain an easy-to-adhere polyester film.

[0157] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0158] [Examples 14-15] The crosslinking agent used was changed to that listed in Table 3, and the same procedure as in Example 3 was followed to obtain an easy-to-adhere polyester film.

[0159] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0160] [Example 16] The easy-to-adhere polyester film prepared in the same manner as in Example 3 was given the functional layer described in Table 3, and was otherwise evaluated in the same manner as in Example 1. The various evaluation results are as described in Table 3.

[0161] [Comparative Examples 1 to 4] The polycarbonate polyurethane resin, polyester resin and crosslinking agent were changed to the combination listed in Table 3, and the thickness of the coating layer was changed. Otherwise, the process was the same as in Example 1 to obtain an easy-to-adhere polyester film.

[0162] The obtained easy-to-adhere polyester film was evaluated in the same manner as in Example 1, and then the process was repeated in the same manner as in Example 1 to obtain a laminated polyester film with a functional layer. The various evaluation results are shown in Table 3.

[0163] [Table 3] Effect polycarbonate Carbamate resin Polyester resin Crosslinking agent Coating layer thickness (nm) Highest Temperature reached (°C) film Haze (%) Deformation rate (%) Functional layer types Functional layer adhesion (%) Adhesion reliability After film formation (X) After damp heat treatment (Y) XY Z Example 1 PCPU-1WD PEs-1WD C-1WD 50 200 1.11 0.16 L 99 95 4 〇 C Example 2 PCPU-2WD PEs-1WD C-1WD 50 200 0.99 0.15 L 97 95 2 〇 B Example 3 PCPU-2WD PEs-2WD C-1WD 50 200 0.78 0.19 L 99 95 4 〇 C Example 4 PCPU-2WD PEs-2WD C-1WD 50 180 0.68 0.22 L 98 95 3 〇 B Example 5 PCPU-2WD PEs-2WD C-1WD 50 230 0.63 0.17 L 100 95 5 〇 C Example 6 PCPU-2WD PEs-2WD C-1WD 100 200 0.77 0.20 L 99 97 2 〇 B Example 7 PCPU-2WD PEs-2WD C-1WD 100 180 0.86 0.36 L 100 100 0 〇 A Example 8 PCPU-2WD PEs-2WD C-1WD 100 230 0.69 0.18 L 100 99 1 〇 A Example 9 PCPU-3WD PEs-2WD C-1WD 50 200 0.75 2.50 L 100 100 0 〇 A Example 10 PCPU-3WD PEs-2WD C-1WD 50 230 0.68 11.65 L 100 99 1 〇 A Example 11 PCPU-3WD PEs-2WD C-1WD 100 200 1.11 0.38 L 100 100 0 〇 A Example 12 PCPU-3WD PEs-2WD C-1WD 100 180 1.15 0.23 L 100 100 0 〇 A Example 13 PCPU-4WD PEs-2WD C-1WD 50 200 1.32 14.60 L 98 98 0 〇 A Example 14 PCPU-2WD PEs-2WD C-2WD 50 200 1.13 0.24 L 100 100 0 〇 A Example 15 PCPU-2WD PEs-2WD C-3WD 50 200 1.01 0.15 L 100 95 5 〇 C Example 16 PCPU-2WD PEs-2WD C-1WD 50 200 0.77 0.19 M 100 100 0 〇 A Comparative Example 1 - PEs-2WD C-1WD 50 200 0.56 0.00 L 100 0 100 × D Comparative Example 2 PCPU-2WD - C-1WD 50 200 0.72 0.08 L 100 0 100 × D Comparative Example 3 PCPU-2WD PEs-2WD C-1WD 25 200 0.61 0.05 L 85 70 15 × D Comparative Example 4 PCPU-2WD PEs-2WD C-1WD 150 200 0.82 0.10 L 70 40 30 × D The composition ratio is expressed as the percentage of the mass of each solid component of the resin and crosslinking agent relative to the sum of the masses of the solid components of the resin and crosslinking agent.

[0164] The laminated polyester film of the present invention exhibits excellent adhesion between the functional layer and the polyester film (especially adhesion after long-term storage), resulting in high adhesion reliability. Furthermore, it also possesses excellent anti-blocking properties and transparency. Therefore, it is suitable for a wide range of applications, such as optical applications. On the other hand, Comparative Example 1's coating layer does not contain polycarbonate polyurethane resin (A), therefore its adhesion and adhesion reliability after damp heat treatment are significantly worse than those of the present invention. Furthermore, adhesion cannot be ensured even under harsh conditions caused by folding. Comparative Example 2's coating layer does not contain polyester resin (B), therefore its adhesion and adhesion reliability after damp heat treatment are significantly worse than those of the present invention. Furthermore, adhesion cannot be ensured even under harsh conditions caused by folding. Comparative Example 3's coating layer is thin, failing to guide the phase separation state of the film surface to the state of the present invention, and thus failing to exhibit sufficient adhesion. Furthermore, adhesion cannot be ensured even under harsh conditions caused by folding. Comparative Example 4 has a thick coating layer, which fails 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 cannot be ensured even under harsh conditions caused by folding. [Industrial Applicability]

[0165] The easy-to-adhere polyester film of the present 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 adhesion reliability. Furthermore, it also possesses excellent anti-adhesion and transparency. Therefore, it is suitable for a wide range of applications, including optical applications.

Claims

1. An easy-to-adhere polyester film for a foldable display, comprising a polyester film substrate and a coating layer on at least one side of the polyester film substrate, wherein the coating layer is formed of a composition comprising polycarbonate polyurethane resin (A), polyester resin (B), and a capped isocyanate crosslinking agent (C), wherein, when the surface of the coating layer is observed using an atomic force microscope, the proportion of the area with a deformation of 1 nm or more in the force curve under a 3 nN load is 0.15% or more relative to the overall observed surface, and the haze of the easy-to-adhere polyester film is 2% or less.

2. The easy-adhesive polyester film for foldable displays as claimed in claim 1, wherein the aforementioned polyester resin (B) has a naphthalene backbone in its molecule.

3. The easy-to-adhere polyester film for foldable displays as claimed in claim 1, wherein the aforementioned end-capping isocyanate-based crosslinking agent (C) is a pyrazole-based compound.

4. The easy-to-adhere polyester film for foldable displays as claimed in claim 1, wherein the aforementioned polycarbonate polyurethane resin (A) has a hexane structure in its molecule.

5. The easy-adhesive polyester film for foldable displays as claimed in claim 4, wherein the aforementioned polycarbonate polyurethane resin (A) has a hexane structure of more than 10% by mass in the molecule.

6. The easy-adhesive polyester film for foldable displays as claimed in claim 1, wherein the coating layer of the easy-adhesive polyester film further has a functional layer.

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 as claimed in any one of claims 1 to 6, wherein the step of forming a coating layer on at least one side of a polyester film substrate is included, wherein the coating layer forming step includes a heating step at 180°C or higher.

8. A method for manufacturing a laminated polyester film for a foldable display, comprising the step of further coating a functional layer forming composition onto a coating layer of an easy-to-adhere polyester film as claimed in claim 7, thereby forming a functional layer.