Flexible window film and display device including same

Abstract

Disclosed herein are a flexible window film and a display device including the same. The flexible window film includes a base layer, a hard coating layer on an upper surface of the base layer, a functional coating layer stacked on a lower surface of the base layer, and a buffer layer formed between the base layer and the functional coating layer, wherein the functional coating layer has a modulus of about 50 MPa to about 300 MPa at-20 DEG C.

Description

[0001] Citations of Related Applications

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2023-0159230 filed in the Korean Intellectual Property Office on November 16, 2023, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to a flexible window film, and to a display device including the flexible window film. Background Art

[0004] There is a growing interest in flexible display devices. In response to this interest, window films mounted on flexible display devices are generally flexible. Because such window films can be configured to protect the internal optical devices of the display device, it may be desirable for the window films to have a hard coating layer. For example, the window film includes a base layer and a hard coating layer formed on the base layer.

[0005] When included in a flexible display device, the window film is folded and unfolded back to its original state hundreds of thousands of times or more in the direction of the base layer or hard coating. For example, a polyimide film can constitute the base layer. It can be expected that the window film has good bending reliability under low temperature conditions and high temperature and high humidity conditions, while having good impact resistance.

[0006] The technical background of the present disclosure is disclosed in Japanese Patent Laid-Open No. 2008-037101 and the like. Summary of the invention

[0007] An exemplary aspect of the present disclosure provides a flexible window film without rainbow mura.

[0008] Another exemplary aspect of the present disclosure provides a flexible window film having good bending reliability under low temperature conditions as well as high temperature and high humidity conditions.

[0009] According to an exemplary aspect of the present disclosure, a flexible window film includes a base layer, a hard coating layer on an upper surface of the base layer (e.g., stacked on an upper surface of the base layer), a functional coating layer on a lower surface of the base layer (e.g., stacked on a lower surface of the base layer), and a buffer layer formed between the base layer and the functional coating layer, wherein the functional coating layer has a modulus of 50 MPa to 300 MPa at -20°C.

[0010] According to another aspect of the present disclosure, an optical display device includes the above-mentioned flexible window film.

[0011] Exemplary embodiments of the present disclosure provide a flexible window film without rainbow-like unevenness.

[0012] Exemplary embodiments of the present disclosure provide a flexible window film having good bending reliability under low temperature conditions and under high temperature and high humidity conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a cross-sectional view of a window film according to an exemplary embodiment. DETAILED DESCRIPTION

[0014] The exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings to facilitate the practice of those skilled in the art to which the present disclosure belongs. It should be understood that the present disclosure can be embodied in different ways and is not limited to the following exemplary embodiments.

[0015] In the accompanying drawings, for the sake of clarity, parts not related to the description will be omitted. Throughout the specification, the same components will be represented by the same reference numerals. The lengths, sizes, etc. of the components in the accompanying drawings are for the purpose of illustrating the present disclosure, and the present disclosure is not limited thereto.

[0016] The terms used herein are for the purpose of describing exemplary embodiments and are not intended to limit the present disclosure.As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0017] As used herein, "combinations thereof" refers to mixtures, stacks, composites, copolymers, alloys, blends, reaction products, etc., of the components.

[0018] Furthermore, it should be understood that the terms “includes,” “comprises,” “including” and / or “comprising” when used in this specification refer to the presence of stated features, quantities, steps, components or combinations thereof, but do not exclude the presence or addition of one or more other features, quantities, steps, components or combinations thereof.

[0019] In this document, spatially relative terms such as "upper" and "lower" are defined with reference to the accompanying drawings. Thus, it will be understood that the term "upper surface" can be used interchangeably with the term "lower surface", and when an element such as a layer or film is referred to as being placed "on" another element, the element can be placed directly on the other element, or there can be intervening elements. On the other hand, when an element is referred to as being placed "directly" "on" another element, there are no intervening elements therebetween.

[0020] As used herein, the term "(meth)acryloyl" refers to acryloyl and / or methacryloyl.

[0021] As used herein, to express a specific numerical range, the expression “X to Y” means “greater than or equal to X and less than or equal to Y (X≤ and ≤Y)”.

[0022] When the term "about" is used in conjunction with a numerical value in this specification, it is intended that the associated numerical value includes a tolerance of ±10% around the numerical value. When a range is specified, the range includes all values ​​therebetween, such as increments of 0.1%.

[0023] According to one aspect of the present disclosure, a flexible window film includes a base layer, a hard coating layer on an upper surface of the base layer (e.g., stacked on the upper surface of the base layer), a functional coating layer on a lower surface of the base layer (e.g., stacked on the lower surface of the base layer), and a buffer layer formed between the base layer and the functional coating layer, wherein the functional coating layer has a modulus of 50 MPa to 300 MPa at -20° C. By adjusting the modulus of the functional coating layer to the above range in combination with forming a buffer layer between the base layer and the functional coating layer, the flexible window film can have improved bending reliability under low temperature conditions and under high temperature and high humidity conditions.

[0024] Herein, the "modulus" of the base layer refers to the storage modulus (E') of the base layer measured using a dynamic mechanical analyzer (DMA) (Q800 model, TAinstruments Inc.). For example, the storage modulus is obtained using dynamic mechanical analysis in a tensile mode at a frequency of 1 Hz and a heating rate of 3°C / min from -40°C to 70°C after clamping both ends of the sample with a space of about 7 mm therebetween.

[0025] Herein, the "modulus" of the functional coating refers to the storage modulus (E') of the functional coating measured using a dynamic mechanical analyzer (Q800 model, TAinstruments Inc.). For example, the storage modulus is obtained using dynamic mechanical analysis in a tensile mode at a frequency of 1 Hz and a heating rate of 3°C / min from -40°C to 70°C after clamping both ends of the sample with a space of about 7 mm therebetween.

[0026] The base layer supports the window film to enhance the mechanical strength of the window film.

[0027] The base layer may be formed of or include an optically transparent flexible resin. According to an exemplary embodiment, the resin may include at least one of a polyamide-imide resin and a polyimide resin. Each or at least one of the polyamide-imide resin and the polyimide resin may be prepared by a typical method known in the art. The polyamide-imide resin may include a block copolymer of poly(amide-imide), but is not limited thereto. For example, the base layer may be or include a polyimide (PI) resin film.

[0028] According to an exemplary embodiment, the base layer may be or include a soluble polyimide resin film. The soluble polyimide resin film is partially dissolved in a solvent or the like to help form a buffer layer described below at the boundary of the resin film. According to an exemplary embodiment, the solvent may include at least one of methyl ethyl ketone, methyl isobutyl ketone, propylene glycol monomethyl ether, N,N-dimethylacetamide, and methyl pyrrolidone.

[0029] The soluble polyimide resin film may be prepared by a typical method known in the art.

[0030] The base layer may have a modulus of about 3 GPa to about 10 GPa (e.g., 3, 4, 5, 6, 7, 8, 9, 10 GPa, e.g., 6 GPa to 10 GPa, e.g., 6 GPa to 8 GPa) at 25° C. Within these ranges, when the window film is folded in the direction of the base layer and / or in the direction of the hard coating layer and unfolded to its original state, the base layer may reduce or prevent cracking of the window film while ensuring good bending reliability of the window film.

[0031] The modulus of the base layer may be adjusted to the above range by adjusting the weight average molecular weight of the resin in the resin film forming the base layer, but is not limited thereto.

[0032] According to an exemplary embodiment, the base layer may have a modulus gradient relative to the functional coating to help improve the bending reliability and impact resistance of the window film under low temperature conditions and under high temperature and high humidity conditions. For example, the ratio of the modulus of the functional coating at about 25°C to the modulus of the base layer at 25°C (modulus of the functional coating at 25°C: modulus of the base layer at 25°C) may be in the range of about 1:50 to about 1:2,000 (e.g., 1:50 to 1:1,000). In this article, the modulus of each of the base layer and the functional coating is a value in MPa.

[0033] The base layer may have a thickness of about 10 μm to about 200 μm, such as 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150 μm, 20 to 150 μm, or 30 to 100 μm. Within these ranges, the base layer may be included in the window film.

[0034] The base layer may be or include a single layer film. However, the present disclosure is not limited thereto, and the base layer may be or include a stack of two or more identical or different resin films attached to each other by, for example, an adhesive layer, a bonding layer, or an adhesive bonding layer.

[0035] The base layer may further include a primer layer or coating providing additional functionality on one or both surfaces thereof. According to an exemplary embodiment, the base layer may be or include a single layer film formed of or including any of the above resins without a primer layer or an adhesion enhancing layer on one or both surfaces thereof.

[0036] The buffer layer is formed on the lower surface of the base layer. According to an exemplary embodiment, the buffer layer may be directly formed on the base layer. Herein, the expression "directly formed" refers to the fact that no adhesive layer or bonding layer is interposed between the base layer and the buffer layer.

[0037] According to an exemplary embodiment, the buffer layer is an intermixing layer of a base layer and a functional coating. In this article, "intermixing layer" means that the buffer layer is a layer in which the main component of the base layer is mixed with the main component of the functional coating. The buffer layer can promote the reduction or elimination of rainbow-like unevenness. For example, compared with forming a buffer layer between the base layer and the hard coating, forming a buffer layer between the base layer and the functional coating can effectively reduce or eliminate rainbow-like unevenness.

[0038] The buffer layer is formed to a predetermined or desired thickness at the interface between the base layer and the functional coating. The buffer layer can mitigate the refractive index difference between the base layer and the functional coating in the thickness direction of the window film. Therefore, when included in a display device, the window film can ensure good screen quality by reducing or eliminating rainbow-like unevenness. The refractive index difference between the base layer and the functional coating can be in the range of about 0 to about 0.7 (e.g., 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.1 to 0.4) (the base layer has a higher refractive index than the functional coating). Within these ranges, the buffer layer can be highly effective in reducing or eliminating rainbow-like unevenness.

[0039] In combination with a functional coating having a modulus within the range described below at -20°C, the buffer layer can contribute to improving the bending reliability of the flexible window film under low temperature conditions and under high temperature and high humidity conditions.

[0040] According to one exemplary embodiment, in the buffer layer, the content of the main component of the base layer (eg, polyimide resin) may gradually decrease from an interface between the buffer layer and the base layer to an interface between the buffer layer and the functional coating layer.

[0041] According to an exemplary embodiment, in the buffer layer, the content of the main component of the functional coating (for example, at least one of a polyurethane (urethane) resin and a polyurethane (meth) acrylic resin) can be gradually reduced from the interface between the buffer layer and the functional coating to the interface between the buffer layer and the base layer.

[0042] In one exemplary embodiment, the buffer layer may refer to a layer in which a main component of the base layer (eg, polyimide resin) is mixed with a main component of the functional coating layer (eg, at least one of polyurethane resin and polyurethane (meth) acrylic resin).

[0043] According to one exemplary embodiment, when the functional coating composition is applied to the lower surface of the base layer, the buffer layer may be formed by partially dissolving the base layer with a solvent included in the functional coating composition described below.

[0044] The presence of the buffer layer in the flexible window film may be confirmed by examining a cross section of the window film using a scanning electron microscope (SEM) or by scanning a cross section of the window film in its thickness direction by surface infrared spectroscopy.

[0045] The buffer layer may have a thickness of about 0.1 μm to about 5 μm, such as 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3 μm, 0.3 μm to 3 μm. Within these ranges, the buffer layer can be included in the flexible window film.

[0046] The functional coating is formed on the lower surface of the buffer layer. According to an exemplary embodiment, the functional coating is directly formed on the buffer layer. Herein, the expression "directly formed" means that there is no adhesive layer or bonding layer interposed between the functional coating and the buffer layer.

[0047] The functional coating has a modulus of about 50 MPa to about 300 MPa at -20°C. When the modulus of the functional coating at -20°C is less than about 50 MPa, the flexible window film may have poor bending reliability under high temperature and high humidity conditions. When the modulus of the functional coating at -20°C exceeds about 300 MPa, the flexible window film may have poor bending reliability under low temperature conditions. In an exemplary embodiment, the functional coating may have the following modulus at -20°C: 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300 MPa, 50 MPa to 200 MPa.

[0048] According to an exemplary embodiment, the functional coating may have a modulus of about 5 MPa to about 200 MPa (eg, 5 MPa to 150 MPa, eg, 10 MPa to 100 MPa) at 25° C. Within these ranges, the functional coating can contribute to achieving the desired effects of the flexible window film according to the present disclosure.

[0049] According to an exemplary embodiment, the functional coating can have a modulus of about 1 MPa to about 150 MPa (e.g., 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150 MPa, 1 MPa to 100 MPa, or such as 1 MPa to 50 MPa) at 60° C. Within these ranges, the functional coating can contribute to achieving the desired effects of the flexible window film according to the present disclosure.

[0050] According to an exemplary embodiment, the ratio of the modulus of the functional coating at 60°C to the modulus of the functional coating at -20°C (modulus of the functional coating at 60°C:modulus of the functional coating at -20°C) may be in the range of about 1:2 to about 1:10 (e.g., 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:4 to 1:8). Within these ranges, the flexible window film can provide stable bending reliability in a wide low temperature range and a wide high temperature range and a high humidity range.

[0051] Although not particularly limited, the functional coating may be or include an impact-resistant coating. When the impact-resistant coating is the functional coating, the flexible window film can be arranged at the outermost side of the optical display device.

[0052] According to an exemplary embodiment, the functional coating may include a cured product of a functional coating composition containing an elastomeric resin. For example, the elastomeric resin may include at least one of a polyurethane resin and a polyurethane (meth) acrylic resin. Each or at least one of the polyurethane resin and the polyurethane (meth) acrylic resin may be prepared by methods known in the art, or may be or include a commercially available product. For example, the elastomeric resin is or includes a polyurethane (meth) acrylic resin. Using a polyurethane (meth) acrylic resin as an elastomeric resin can contribute to achieving the desired effects described herein. For example, a polyurethane (meth) acrylic resin may be or include a polyurethane (meth) acrylate resin.

[0053] In an exemplary embodiment, the elastomer resin is a polyurethane (meth) acrylic resin and, before curing, may have a weight average molecular weight of about 100 g / mol to about 500 g / mol (e.g., 100, 150, 200, 250, 300, 350, 400, 450, 500 g / mol, 150 g / mol to 400 g / mol). In this article, the weight average molecular weight can be obtained based on polystyrene conversion by gel permeation chromatography.

[0054] In an exemplary embodiment, polyurethane (meth) acrylic resin can be prepared by reacting a difunctional or higher functional polyol with a difunctional or higher functional isocyanate to prepare a polyurethane prepolymer, and then reacting the prepared polyurethane prepolymer with a (meth) acrylate containing an alkyl group having a hydroxyl group. The polyol can include at least one of an aromatic polyol, an aliphatic polyol, and an alicyclic polyol. For example, the polyol is a polyurethane compound, which is formed by at least one of an aliphatic polyol and an alicyclic polyol or includes at least one of an aliphatic polyol and an alicyclic polyol. The polyol can include at least one of polyester diols, polycarbonate diols, polyolefin diols, polyether diols, polythioether diols, polysiloxane diols, polyacetal diols, and polyesteramide diols, but is not limited thereto. The polyfunctional isocyanate can include at least one of any aliphatic, alicyclic, and aromatic isocyanates. The (meth)acrylate containing an alkyl group having a hydroxyl group is a (meth)acrylate containing a C1 to C10 alkyl group having at least one hydroxyl group at an ester site thereof, and may include 2-hydroxyethyl (meth)acrylate and the like.

[0055] In the preparation of the polyurethane prepolymer, a chain extender may also be used. The chain extender may include at least one of a diol (such as an aliphatic diol), an amino alcohol, a diamine, a hydrazine, a hydrazide, and a mixture thereof. In addition, in the preparation of the polyurethane prepolymer, a tin compound (such as at least one of a tin salt of a carboxylic acid) or an amine (such as dimethylcyclohexylamine or triethylenediamine) may also be included as a catalyst to promote the formation of polyurethane bonds. In addition, in the preparation of the polyurethane prepolymer, other typical additives may also be used, such as at least one of a surfactant, a flame retardant, a filler, and a pigment.

[0056] The elastomeric resin is a polyurethane (meth) acrylic resin, and may be or include an elastomeric product manufactured by DuPont.

[0057] The modulus of the functional coating at -20°C, 25°C and 60°C can be adjusted to the above range by adjusting the weight average molecular weight of the elastomeric resin, the content of the elastomeric resin in the functional coating composition, the curing rate of the functional coating composition (for example, the photocuring rate or thermal curing rate of the composition), the content of the photoinitiator in the functional coating composition, etc.

[0058] In an exemplary embodiment, the photoinitiator may be or include a free radical photoinitiator. The photoinitiator may be or include any suitable photoinitiator that can cure the elastomer resin by light radiation or the like in a curing process, but is not limited thereto. For example, the photoinitiator may be or include at least one of a benzoin photoinitiator, an acetophenone photoinitiator, a hydroxyketone photoinitiator, an aminoketone photoinitiator, and a phosphine oxide photoinitiator. For example, the photoinitiator may include at least one of benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin n-butyl ether, benzoin isobutyl ether, acetophenone, dimethylaminoacetophenone, 2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy-2-phenylacetophenone, 2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinyl-propan-1-one, 4-(2-hydroxyethoxy)phenyl-2-(hydroxy-2-propyl)ketone, dibenzophenone , p-phenylbenzophenone, 4,4-diethylaminobenzophenone, dichlorobenzophenone, 2-methylanthraquinone, 2-ethylanthraquinone, 2-tert-butylanthraquinone, 2-aminoanthraquinone, 2-methylthioxanthone, 2-ethylthioxanthone, 2-chlorothioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, benzyl dimethyl ketal, acetophenone dimethyl ketal, p-dimethylaminobenzoate, oligo[2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone], 2,4,6-trimethylbenzoyl-diphenyl-phosphine oxide, 1-hydroxy-cyclohexyl-phenyl ketone, etc. For example, the photoinitiator is or includes a hydroxyketone photoinitiator.

[0059] According to an exemplary embodiment, the functional coating may be formed from or include a functional coating composition, which comprises, by solid content, about 100 parts by weight of a base resin, the base resin comprising at least one of a polyurethane resin and a polyurethane (meth) acrylic resin; and about 0.5 parts by weight to about 1.5 parts by weight (e.g., 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 parts by weight, 0.5 parts by weight to 1 part by weight) of a photoinitiator, wherein the functional coating composition further comprises a solvent. The solvent will be described in further detail below. Within this content range of the photoinitiator, the functional coating can easily meet the above modulus parameters.

[0060] The functional coating composition may further comprise typical additives such as or including at least one of a UV absorber, a leveling agent, a thermal stabilizer, and the like.

[0061] According to an exemplary embodiment, the functional coating composition includes a solvent configured to dissolve the base layer to form a buffer layer. The solvent is configured to dissolve a portion of the base layer to form a buffer layer between the base layer and the functional coating. Depending on the type of the main component of the base layer, the solvent can be appropriately selected. According to an exemplary embodiment, when the base layer is a polyimide resin, the solvent can include at least one of methyl isobutyl ketone, propylene glycol monomethyl ether, N,N-dimethylacetamide and methyl pyrrolidone.

[0062] A hard coat is formed on the base layer. According to an exemplary embodiment, the hard coat is directly formed on the base layer. In this article, the expression "directly formed" refers to that there is no adhesive layer, bonding layer or adhesive bonding layer inserted between the hard coat and the base layer. As will be described below, the hard coat can be formed by directly applying the hard coat composition to a surface of the base layer and then curing.

[0063] In one exemplary embodiment, a buffer layer as described above is not formed between the hard coating layer and the base layer.

[0064] The hard coating layer may have a thickness of about 1 μm to about 50 μm (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50 μm, 3 μm to 30 μm, more e.g., 4 μm to 20 μm). Within these ranges, the hard coating layer may be included in the flexible window film.

[0065] The hard coating layer may be formed from or include a hard coating layer composition containing at least one (meth) acrylic resin. The hard coating layer composition may also include at least one of a cross-linking agent and an initiator. For example, the hard coating layer composition may be composed of or include: a (meth) acrylic resin and an initiator, but without a cross-linking agent.

[0066] In one exemplary embodiment, the hard coating layer may consist of or include a (meth)acrylic resin and an initiator without a cross-linking agent.

[0067] The (meth)acrylic resin may include a (meth)acrylic resin obtained by polymerizing a single (meth)acrylic monomer or by polymerizing a (meth)acrylic monomer and a comonomer, the comonomer being copolymerizable with the (meth)acrylic monomer. The (meth)acrylic monomer may include a typical monomer known in the art. The comonomer may include a typical monomer known in the art that is copolymerizable with the (meth)acrylic monomer.

[0068] In an exemplary embodiment, the (meth) acrylic resin may include a resin having a (meth) acrylate group at its terminal. For example, the (meth) acrylic resin may include a dendritic aliphatic compound having a (meth) acrylate group at its terminal. Through the dendritic structure, the aliphatic compound may have a plurality of (meth) acrylate groups at its molecular terminal and thus may have high reactivity.

[0069] (Meth) acrylic resin can include one or more (meth) acrylic resins. These (meth) acrylic resins can be included individually or in combination. Specifically, (meth) acrylic resin can include at least one of dendrimer type (dendrimer type) (meth) acrylic resin and hyperbranched (meth) acrylic resin. In this article, dendrimer type (meth) acrylic resin refers to a (meth) acrylic resin branched with high regularity, and hyperbranched (meth) acrylic resin refers to a (meth) acrylic resin branched with relatively low regularity compared to dendrimer type (meth) acrylic resin. Due to the low viscosity of hyperbranched (meth) acrylic resin compared to linear (meth) acrylic resin, hyperbranched (meth) acrylic resin can be highly soluble in solvent.

[0070] The dendrimer (meth) acrylic resin may be or include a multi-branched (dendrimer) polyester (meth) acrylate having a (meth) acrylate group at its terminal, but is not limited thereto. The hyperbranched (meth) acrylic resin may be or include a multi-branched (dendritic) poly (meth) acrylate having a (meth) acrylate group at its terminal, but is not limited thereto. In an exemplary embodiment, the hyperbranched (meth) acrylic resin may be or include a multi-branched (dipentaerythritol hexaacrylate-linked) poly (meth) acrylate having dipentaerythritol as a core and a (meth) acrylate group at its terminal, but is not limited thereto.

[0071] The (meth)acrylate resin may have a weight average molecular weight of about 5,000 g / mol to about 30,000 g / mol (eg, 10,000 g / mol to 25,000 g / mol). Within these ranges, the window film can have good hardness and scratch resistance.

[0072] The initiator is or includes a free radical photoinitiator and may include a typical photoinitiator known in the art. For example, the initiator may include at least one of the following: a hydroxyketone photoinitiator, a phosphine oxide photoinitiator, a benzoin photoinitiator, or an aminoketone photoinitiator. The initiator may be present in an amount of about 1 part by weight to about 10 parts by weight (e.g., 1 part by weight to 5 parts by weight) relative to 100 parts by weight of the (meth) acrylic resin. Within these ranges, the initiator can ensure sufficient curing of the hard coating composition without deteriorating the optical or physical properties of the window film.

[0073] In another exemplary embodiment, the hard coating layer may include a (meth)acrylic resin, a cross-linking agent, and an initiator.

[0074] The crosslinking agent forms a crosslinking structure by reacting with a (meth)acrylic resin, and may include a (meth)acrylate compound having at least one (meth)acrylate group, such as at least two (meth)acrylate groups, and for example, 2 to 20 (meth)acrylate groups.

[0075] In one exemplary embodiment, the crosslinking agent may include multifunctional (meth)acrylate. The multifunctional (meth)acrylate may further improve the hardness and flexibility of the hard coating layer.

[0076] The crosslinking agent may be present in an amount of about 5 parts by weight to about 150 parts by weight (eg, 5 parts by weight to 100 parts by weight) relative to 100 parts by weight of the (meth)acrylic resin. Within these ranges, the crosslinking agent can improve the hardness and bending reliability of the hard coating layer.

[0077] The hard coating composition may also include additives. The additives may provide additional functions for the window film. The additives may include any additives that are usually added to the window film. For example, the additives may include at least one of the following: leveling agent, UV absorber, reaction inhibitor, adhesion enhancer, thixotropy imparting agent, conductivity imparting agent, color regulator, stabilizer, antistatic agent and antioxidant. Relative to 100 parts by weight of (meth) acrylic resin, the additive may be present in an amount of about 0.01 parts by weight to about 5 parts by weight (e.g., 0.1 parts by weight to 3 parts by weight). Within these ranges, the additives can provide the desired effect while ensuring good hardness and flexibility of the window film.

[0078] The hard coating composition may further include a solvent to ensure coatability, operability or processability. The solvent may include at least one of methyl ethyl ketone, methyl isobutyl ketone, propylene glycol monomethyl ether and N,N-dimethylacetamide, but is not limited thereto. The solvent may be present in the hard coating composition in a residual amount.

[0079] The hard coating layer can be formed by applying the hard coating composition to a surface of the base layer and then curing. There is no particular limitation on the method for applying the hard coating composition to the base layer. For example, the hard coating layer can be applied to the base layer by rod coating, spin coating, dip coating, roller coating, flow coating, die coating, etc. The curing of the hard coating composition can include at least one of photocuring and thermal curing. Photocuring can include curing at about 10mJ / cm 2 About 1,000mJ / cm 2 The hard coating composition is irradiated with light having a wavelength of about 400 nm or less at an energy fluence of about 100 nm. Thermal curing may include applying the hard coating composition to the base layer to a predetermined or desired thickness, followed by drying at a temperature in the range of about 80° C. to about 150° C. for about 5 to about 30 minutes.

[0080] The window film may further include an anti-fingerprint layer formed on the upper surface of the hard coating layer. When the window film is arranged on the outermost side of the display device, the anti-fingerprint layer can hinder or prevent the screen of the display device from being soiled by the finger touching the screen. The anti-fingerprint layer may be or include a fluorine anti-fingerprint layer, but is not limited thereto. The anti-fingerprint layer may have a thickness of about 0.1 μm to about 1 μm.

[0081] The window film can be optically transparent to be included in a transparent display device. The window film can have a light transmittance of about 80% or more (e.g., 85% to 100%) and a haze of about 1% or less (e.g., 0% to 1%) as measured in the visible spectrum (e.g., at a wavelength of about 400 nm to about 800 nm). Within this range, the window film can be included as a window film for a display device.

[0082] In evaluating the flexibility of the window film after repeating a cycle in which the window film is bent to a curvature radius of 1 mm under high temperature and high humidity conditions (e.g., at 60° C. and 95% relative humidity (RH)) and / or under low temperature conditions (e.g., at -40° C.), the minimum number of cycles at which cracks occur on the hard coating layer or the base layer may be about 100,000 or more. Therefore, due to its good bending reliability under high temperature and high humidity conditions and / or low temperature conditions, the window film can be included in a flexible display device.

[0083] The window film may have a thickness of about 30 μm to about 200 μm (eg, 30 μm to 80 μm). Within these ranges, the window film may be a flexible window film for a display device.

[0084] In the following, reference will be made to Figure 1 A window film according to an exemplary embodiment of the present disclosure is described.

[0085] Reference Figure 1 The window film includes a base layer 110 , a hard coating layer 130 formed on one surface of the base layer 110 , a functional coating layer 120 formed on the other surface of the base layer 110 , and a buffer layer 140 formed at an interface between the base layer 110 and the functional coating layer 120 .

[0086] exist Figure 1 , each of the interfaces between the base layer 110 and the buffer layer 140 and the interfaces between the buffer layer 140 and the functional coating layer 120 is represented by a dotted line. This indicates that the base layer 110 and the buffer layer 140 are not divided from each other by a plane, and the buffer layer 140 and the functional coating layer 120 are not divided from each other by a plane.

[0087] Hereinafter, a method of manufacturing a window film will be described.

[0088] The window film may be manufactured by applying a hard coating composition onto an upper surface of a base layer, followed by curing to form a hard coating layer on the base layer, and applying a functional coating composition onto a lower surface of the base layer, followed by curing to simultaneously form a buffer layer and a functional coating layer.

[0089] Each composition may be coated by typical methods known in the art. For example, each composition may be coated by spraying, die coating, or spin coating, but is not limited thereto. Curing may include at least one of photocuring and thermal curing. The conditions of photocuring or thermal curing may be adjusted according to the thickness of each layer, the type of material forming each layer, etc. In an exemplary embodiment, each composition may be cured in combination with drying, etc. to reduce the surface roughness of each layer or shorten the time required to complete the curing.

[0090] Hereinafter, a display device according to the present disclosure will be described.

[0091] The display device according to the present disclosure includes the flexible window film according to the present disclosure as described above. The display device may be or include a flexible display device, or may be or include a non-flexible display device. For example, the display device may be or include a light-emitting diode display (including an organic light-emitting diode display device, etc.), a liquid crystal display, etc., but is not limited thereto.

[0092] Next, the present disclosure will be described in more detail with reference to some embodiments. It should be understood that these embodiments are provided for illustration only and should not be interpreted as limiting the present disclosure in any way.

[0093] Preparation Example: Preparation of Hard Coating Composition

[0094] A hard coating composition was prepared by mixing 67 parts by weight of a dendrimer type acrylic resin (Sirius 501, Osaka Organic Chemical Industry Ltd.) with 1 part by weight of a photoinitiator (Irgacure-184, BASF Co., Ltd.), 31.5 parts by weight of methyl ethyl ketone, and 0.5 parts by weight of a leveling agent (BYK-350).

[0095] Example 1

[0096] An impact-resistant layer composition including 100 parts by weight of a polyurethane resin (Elastomer E24, DuPont Inc.), 0.5 parts by weight of a photoinitiator (1-hydroxy-cyclohexyl-phenyl ketone, Irgacure-184, BASF Co., Ltd.), and 15 parts by weight of N,N-dimethylacetamide as a solvent was prepared.

[0097] A polyimide film (thickness: 50 μm, modulus: 6.5 GPa, Kolon Industries Inc.) was used as the base layer. The modulus of the base layer was measured by the above-mentioned method.

[0098] The hard coating composition prepared in the preparation example was applied to the upper surface of the base layer, followed by drying at 100° C. for 10 minutes, thereby forming a hard coating layer having a thickness of 5 μm. The prepared impact-resistant layer composition was applied to the lower surface of the base layer to a predetermined or desired thickness, followed by drying at 80° C. for 30 minutes, and then heated at 900 mJ / cm 2 The composition was irradiated with UV light at an energy fluence of 1000 nm to form a buffer layer and an impact-resistant layer having a thickness of 40 μm, thereby manufacturing a window film. A buffer layer composed of a mixture of a polyimide resin and a polyurethane resin was formed between the base layer and the impact-resistant layer. The base layer, the buffer layer, and the impact-resistant layer were formed integrally with each other.

[0099] Example 2

[0100] The window film was manufactured in the same manner as in Example 1, except that the thickness of the impact-resistant layer was changed to 20 μm. The window film had a buffer layer composed of a mixture of polyimide resin and polyurethane resin between the base layer and the impact-resistant layer. The base layer, the buffer layer, and the impact-resistant layer were formed integrally with each other.

[0101] Example 3

[0102] An impact-resistant layer composition comprising 100 parts by weight of a polyurethane resin (Elastomer E24, DuPont Inc.) and 1 part by weight of a photoinitiator (Irgacure-184, BASF Co., Ltd.) was prepared. A window film was manufactured in the same manner as in Example 1, except that the prepared impact-resistant layer composition was used. The window film had a buffer layer composed of a mixture of a polyimide resin and a polyurethane resin between a base layer and an impact-resistant layer. The base layer, the buffer layer, and the impact-resistant layer were formed integrally with each other.

[0103] Comparative Example 1

[0104] An impact-resistant layer composition comprising 100 parts by weight of a polyurethane resin (Elastomer E24, DuPont Inc.) and 2 parts by weight of a photoinitiator (Irgacure-184, BASF Co., Ltd.) was prepared. A window film was manufactured in the same manner as in Example 1, except that the prepared impact-resistant layer composition was used. The window film had a buffer layer composed of a mixture of a polyimide resin and a polyurethane resin between a base layer and an impact-resistant layer. The base layer, the buffer layer, and the impact-resistant layer were formed integrally with each other.

[0105] Comparative Example 2

[0106] An impact-resistant layer composition comprising 100 parts by weight of a polyurethane resin (Elastomer E24, DuPont Inc.) and 0.2 parts by weight of a photoinitiator (Irgacure-184, BASF Co., Ltd.) was prepared. A window film was manufactured in the same manner as in Example 1, except that the prepared impact-resistant layer composition was used. The window film had a buffer layer composed of a mixture of a polyimide resin and a polyurethane resin between the base layer and the impact-resistant layer.

[0107] Comparative Example 3

[0108] An impact-resistant layer composition comprising 100 parts by weight of a polyurethane resin (Elastomer E24, DuPont Inc.) and 0.5 parts by weight of a photoinitiator (Irgacure-184, BASF Co., Ltd.) was prepared. The impact-resistant layer composition did not contain dimethylacetamide as a solvent. A window film was manufactured in the same manner as in Example 1, except that the prepared impact-resistant layer composition was used.

[0109] The window film does not have a buffer layer composed of a mixture of a polyimide resin and a polyurethane resin between the base layer and the impact-resistant layer.

[0110] Comparative Example 4

[0111] An impact-resistant layer composition was prepared including 100 parts by weight of a polyurethane resin (Elastomer E24, DuPont Inc.) and 0.5 parts by weight of a photoinitiator (Irgacure-184, BASF Co., Ltd.) The impact-resistant layer composition did not contain dimethylacetamide as a solvent.

[0112] A hard coating composition was prepared by mixing 67 parts by weight of a dendrimer type acrylic resin (Sirius 501, Osaka Organic Chemical Industry Ltd.) with 1 part by weight of a photoinitiator (Irgacure-184, BASF Co., Ltd.), 31.5 parts by weight of N,N-dimethylacetamide, and 0.5 parts by weight of a leveling agent (BYK-350).

[0113] A window film was manufactured in the same manner as in Example 1, except that the prepared impact-resistant layer composition and the prepared hard coating composition were used. The window film had a buffer layer between the base layer and the hard coating layer instead of between the base layer and the impact-resistant layer.

[0114] The following properties were evaluated for each of the window films manufactured in Examples 1 to 3 and Comparative Examples 1 to 4. The results are shown in Table 1.

[0115] Performance Evaluation

[0116] (1) Modulus of impact-resistant layer (unit: MPa): The modulus was measured using a dynamic mechanical analyzer (DMA) (Q800 model, TAinstruments Inc.). For example, after clamping both ends of the sample with a space of about 7 mm between them, the storage modulus of the impact-resistant layer was measured using dynamic mechanical analysis in a tensile mode at a frequency of 1 Hz and a heating rate of 3°C / min from -40°C to 70°C. The storage modulus was measured twice to ensure the reliability of the results. Here, a 20 μm thick impact-resistant layer sample was used.

[0117] (2) Impact resistance (unit: cm, measured by pen drop test): An adhesive layer was formed on the lower surface (i.e., the surface of the functional coating) of each window film manufactured in Examples 1 to 3 and Comparative Examples 1 to 4, and then a polyethylene terephthalate (PET) film (thickness: 125 μm) was stacked on the other surface of the window film to prepare a sample. A BIC ballpoint pen was freely dropped onto the sample from different heights above the hard coating layer of the sample to determine the minimum height of the PET film (thickness: 125 μm) ruptured by the drop of the ballpoint pen. The rupture was observed using an optical microscope. The larger the minimum height, the better the resistance to pen drop impact.

[0118] (3) Light transmittance and haze (unit: %): The haze and light transmittance of each window film manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 were measured using a haze meter (NDH2000, Nippon Denshoku Industries Co., Ltd.) and a spectrophotometer (CM-3600A, Konica Minolta Inc.), respectively.

[0119] (4) Rainbow unevenness: The adhesive and each of the window films manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 were sequentially attached to the upper surface of a black sheet (the hard coating layer of the window film faced upward), and then interference unevenness and rainbow unevenness were evaluated by naked eyes at an angle of 45° under a three-wavelength light source. The rainbow unevenness of the window film was evaluated according to the following criteria:

[0120] ◎: Rainbow-like unevenness does not exist at all.

[0121] ○: Rainbow-like unevenness exists slightly, but is not visible to the naked eye.

[0122] Δ: Rainbow-like unevenness appears slightly, but it is not easily visible to the naked eye.

[0123] ×: Rainbow-like unevenness is easily visible to the naked eye.

[0124] ××: Rainbow-like unevenness is very easy to see with the naked eye.

[0125] ◎ to Δ indicate that the corresponding window film can be effectively included in the display device, and × and ×× indicate that the corresponding window film cannot be included in the display device.

[0126] (5) Bending reliability (unit: number of cycles): The bending reliability of each window film manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 under repeated bending to a curvature radius of 1 mm was evaluated under high temperature and high humidity conditions (60° C. and 95% RH) and low temperature conditions (−40° C.). The window film was cut into a rectangle having a size of 2.5 cm×15 cm (width×length) to prepare a sample. While repeating a cycle in which the sample was bent so that the hard coating layer came into contact with a fixture having a curvature radius of 1 mm and then straightened back to its original state, the minimum number of cycles at which cracks began to appear on the window film was determined. In the evaluation of bending reliability, when the minimum number of cycles was 100,000 or more, the corresponding window film can be considered to have good bending reliability.

[0127] Table 1

[0128]

[0129]

[0130] *Forms a buffer layer between the base layer and the hard coat layer.

[0131] As can be seen from Table 1, the window film according to the present disclosure is remarkably effective in reducing or eliminating rainbow-like non-uniformity while providing good bending reliability under low temperature conditions as well as high temperature and high humidity conditions.

[0132] It should be understood that various modifications, changes, variations, and equivalent embodiments may be made by those skilled in the art without departing from the spirit and scope of the present disclosure.

Claims

1. A flexible window film, comprising: Base layer; a hard coating layer on the upper surface of the base layer; a functional coating on a lower surface of the base layer; as well as a buffer layer between the base layer and the functional coating, Wherein, the functional coating has a modulus of 50 MPa to 300 MPa at -20°C.

2. The flexible window film according to claim 1, wherein: The functional coating has a modulus of 5 MPa to 200 MPa at 25°C.

3. The flexible window film according to claim 1, wherein: The functional coating has a modulus of 1 MPa to 150 MPa at 60°C.

4. The flexible window film according to claim 1, wherein: The ratio of the modulus of the functional coating at 60°C to the modulus of the functional coating at -20°C is in the range of 1:2 to 1:

10.

5. The flexible window film according to claim 1, wherein: The functional coating includes an impact-resistant coating.

6. The flexible window film according to claim 1, wherein: The functional coating includes a cured product of a composition containing an elastomeric resin.

7. The flexible window film according to claim 6, wherein: The elastomer resin includes at least one of a polyurethane resin and a polyurethane (meth) acrylic resin.

8. The flexible window film according to claim 6, wherein: The functional coating includes a cured product of a composition including: about 100 parts by weight of a base resin including at least one of a polyurethane resin and a polyurethane (meth) acrylic resin; and 0.5 to 1.5 parts by weight of a photoinitiator.

9. The flexible window film according to claim 1, wherein: The buffer layer includes a mixed layer of the base layer and the functional coating layer.

10. The flexible window film according to claim 1, wherein: The content of at least one of the polyurethane resin and the polyurethane (meth) acrylic resin in the buffer layer gradually decreases from an interface between the buffer layer and the functional coating layer to an interface between the buffer layer and the base layer.

11. The flexible window film according to claim 1, wherein: The base layer has a modulus of 3 GPa to 10 GPa at 25°C.

12. The flexible window film according to claim 1, wherein: The base layer includes a soluble polyimide resin film.

13. The flexible window film according to claim 12, wherein: The soluble polyimide resin film includes a polyimide resin soluble in a solvent including at least one of methyl ethyl ketone, methyl isobutyl ketone, propylene glycol monomethyl ether, N,N-dimethylacetamide, and methyl pyrrolidone.

14. The flexible window film according to claim 1, wherein: A modulus ratio of the functional coating at 25°C to the base layer at 25°C is in the range of 1:50 to 1:2000.

15. The flexible window film according to claim 1, wherein: The hard coating layer includes a (meth)acrylic coating layer.

16. The flexible window film according to claim 1, further comprising: An anti-fingerprint layer is on an upper surface of the hard coating layer.

17. A display device comprising the flexible window film according to any one of claims 1 to 16.

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