WO2021177648A1 - Flexible window film and display device comprising same - Google Patents

Abstract

Provided are a flexible window film and a display device comprising same, the flexible window film comprising: a substrate layer; a first intermixing layer and a hard coating layer which are sequentially formed on the upper surface of the substrate layer; and a second intermixing layer and a back coating layer which are sequentially formed on the lower surface of the substrate layer, wherein the thickness ratio of formula 1, of the first intermixing layer and the second intermixing layer, is approximately 0.5-100.

Description

Flexible window film and display device including same

The present invention relates to a flexible window film and a display device including the same. More particularly, the present invention relates to a flexible window film having excellent appearance and excellent bending reliability because rainbow and/or mura are not visually recognized, and a display device including the same.

Recently, interest in flexible display devices is increasing. In response to this interest, a window film mounted on a flexible display device is also required to have flexibility. Since the window film serves to protect the internal optical element of the display device, it is inevitably provided with a hard coating layer. In one example, the window film includes a base layer and a hard coating layer formed on the base layer. Since the window film is located at the outermost part of the display device, it must have excellent appearance without rainbow and/or mura.

For the reliability of bending of the window film, a polyimide-based resin film may be used as the base layer. When the hard coating layer is formed on the polyimide-based resin film with the composition for the hard coating layer, mura and/or rainbow may occur due to a difference in refractive index between the polyimide-based resin film and the hard coating layer, resulting in poor appearance.

As a method of resolving the appearance defect, a method of alleviating the refractive index difference between the hard coating layer and the base layer may be considered. However, the difference in refractive index between the (meth)acrylic hard coating layer and the polyimide-based substrate layer is so large that there is a limit to alleviating the refractive index difference, and when the refractive index of the hard coating layer is increased, reflection by external light may occur.

Background art of the present invention is disclosed in Japanese Patent Application Laid-Open No. 2008-037101 and the like.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a flexible window film having an excellent appearance because rainbow and/or mura are not visually recognized.

Another object of the present invention is to provide a flexible window film having excellent bending reliability.

Another object of the present invention is to provide a flexible window film having excellent optical transparency.

One aspect of the present invention is a flexible window film.

1. The flexible window film includes a base layer; a first intermixing layer and a hard coating layer sequentially formed on the upper surface of the base layer; and a second intermixing layer and a back coating layer sequentially formed on the lower surface of the base layer, wherein the thickness ratio of the first intermixing layer and the second intermixing layer in Equation 1 is about 0.5 to 100:

[Equation 1]

Thickness ratio = (thickness of first intermixing layer)/(thickness of second intermixing layer).

In 2.1, the first intermixing layer is continuously formed on the base layer and the hard coating layer without an interface, respectively, and the second intermixing layer is continuously formed on the base layer and the back coating layer without an interface, respectively. .

In 3.1-2, the first intermixing layer and the second intermixing layer may have different refractive indices.

In 4.1-3, the first intermixing layer may include both the resin contained in the hard coating layer and the resin contained in the base layer.

In 5.1-4, the second intermixing layer may include both the resin contained in the base layer and the resin contained in the back coating layer.

In 6.1-5, the hard coating layer may have a lower refractive index than the base layer.

In 7.6, the hard coating layer may have a refractive index of about 1.30 to about 1.60, and the base layer may have a refractive index of about 1.40 to about 1.70.

In 8.1-7, the back coating layer may have a lower refractive index than the base layer.

In 9.8, the back coating layer may have a refractive index of about 1.30 to about 1.60, and the base layer may have a refractive index of about 1.40 to about 1.70.

In 10.1-9, the base layer may include a film formed of at least one of a polyimide-based resin and a polyamide-imide-based resin.

In 11.1-10, the hard coating layer may be a (meth)acrylic coating layer.

In 12.1-11, the back coating layer may be a polyurethane-polyimide-based coating layer.

In 13.1-12, the first intermixing layer and the second intermixing layer may be a solvent erosion layer or a solvent dissolving layer for the base layer, respectively.

In 14.1-13, at least one of the base layer, the first intermixing layer, the hard coating layer, the second intermixing layer, and the back coating layer may further include a dye having a maximum absorption wavelength of about 550 nm to about 650 nm can

The display device of the present invention includes the flexible window film of the present invention.

The present invention provides a flexible window film having an excellent appearance because rainbow and/or mura are not visually recognized.

The present invention provides a flexible window film having excellent bending reliability.

The present invention provides a flexible window film having excellent optical transparency.

1 is a cross-sectional view of a flexible window film according to an embodiment of the present invention.

With reference to the accompanying drawings, the present invention will be described in detail so as to be easily practiced by those skilled in the art to which the present invention pertains by way of example. The present invention may be embodied in many different forms and is not limited to the embodiments described herein.

In order to clearly explain the present invention in the drawings, parts irrelevant to the description are omitted. In the drawings, the length and/or thickness of each component is shown to explain the present invention, and the present invention is not limited to the length and/or thickness described in the drawings.

As used herein, "upper" and "lower" are defined based on the drawings, and "upper" may be changed to "lower" and "lower" to "upper" depending on the viewpoint, and "on" or What is referred to as “on” may include cases where other structures are interposed in the middle as well as immediately above. On the other hand, "directly on" or "directly on" means that other structures are not interposed therebetween.

As used herein, “(meth)acryl” may mean acryl and/or methacrylic.

In the present specification, when describing a numerical range, “X to Y” means “X or more and Y or less (X≤ and ≤Y)”.

In the flexible window film (hereinafter also referred to as 'window film') in which a back coating layer, a base layer, and a hard coating layer are sequentially stacked, a first intermixing layer is formed between the base layer and the hard coating layer, A second intermixing layer was formed between the base layer and the back coating layer, and a thickness ratio between the thickness of the second intermixing layer and the thickness of the first intermixing layer was controlled. Through this, the window film secures the bending reliability described in detail below in the window film including the base layer described in detail below, the hard coat layer described in detail below, and the back coating layer described in detail below, and the rainbow and/or mura are not visible at all or the rainbow and/or mura was not easily recognized with the naked eye. Preferably, the rainbow and/or mura may not be visible at all.

The first intermixing layer is a layer continuously formed between the base layer and the hard coating layer. The second intermixing layer is a layer continuously formed between the base layer and the back coating layer.

The window film has a minimum number of cracks in the hard coating layer, the base layer and/or the back coating layer when the flexural reliability is evaluated with a radius of curvature of about 1 mm at about 60° C. and about 95% relative humidity and/or about -40° C. After 200,000 cycles or more, bending reliability is excellent in both high temperature, high humidity and/or low temperature conditions, so that it can be sufficiently used in a flexible display device.

Since the window film is optically transparent, it can be used in a transparent display device. The window film may have a light transmittance of about 80% or more, specifically, about 85% to about 100%, and a haze of about 1% or less, specifically about 0% to 1% in the visible light region, specifically, a wavelength of about 400 nm to about 800 nm. have. It can be used as a window film in the above range.

The window film may have a pencil hardness of about 3H or more, for example, about 3H to about 9H, and a radius of curvature of about 0mm to about 2mm, for example, about 0.1mm to about 2mm. In the above range, it can be used as a window film having good hardness and flexibility.

When the window film is evaluated for flexibility with a radius of curvature of about 1 mm at about 60° C. and about 95% relative humidity and/or about -40° C., the minimum number of cracks in the hard coating layer, the substrate layer and/or the back coating layer is about 20 It can be fully used in a flexible display device because it has excellent bending reliability under both conditions of high temperature, high humidity and/or low temperature.

The window film may have a thickness of about 30 μm to 200 μm, for example, about 30 μm to about 100 μm. It can be used as a flexible window film in the above range.

Hereinafter, a window film according to an embodiment of the present invention will be described with reference to FIG. 1 .

Referring to FIG. 1 , the window film includes a base layer 110 , a hard coating layer 120 , a first intermixing layer 130 , a back coating layer 140 , and a second intermixing layer 150 . A second intermixing layer 150 , a base layer 110 , a first intermixing layer 130 , and a hard coating layer 120 are sequentially formed on the upper surface of the back coating layer 140 .

base layer

The base layer 110 may support the window film to increase the mechanical strength of the window film.

The base layer 110 may be formed of an optically transparent and flexible resin. The resin may include at least one of a polyimide-based resin and a polyamideimide-based resin. Each of the polyimide-based resin and the polyamideimide-based resin may be prepared by a conventional method known to those skilled in the art.

The base layer 110 has a higher refractive index than the hard coating layer 120 . In this situation, the window film may prevent the rainbow and/or mura from being recognized at all or the rainbow and/or mura from being easily recognized with the naked eye by satisfying the above-described thickness ratio. Specifically, the base layer 110 may have a refractive index of 1.40 to 1.70, more specifically 1.50 to 1.70.

The base layer 110 may have a thickness of about 10 μm to about 200 μm, specifically about 20 μm to about 150 μm, and more specifically about 30 μm to about 100 μm. It can be used for the window film in the above range.

The base layer 110 does not include a resin forming the hard coat layer 120 and a resin forming the back coat layer 140 . Through this, the base layer 110 may be distinguished from the first intermixing layer 130 . Through this, the base layer 110 may be distinguished from the second intermixing layer 150 .

As shown in FIG. 1 , the base layer 110 may be a single-layer film. However, a case in which the base layer is a laminate in which two or more resin films of the same or different types are laminated on each other by an adhesive layer, an adhesive layer, or an adhesive layer may be included in the scope of the present invention.

Although not shown in FIG. 1 , a primer layer or a functional coating layer for providing an additional function may be additionally formed on one or both surfaces of the base layer 110 . Preferably, the base layer 110 may be a single-layer film formed of the above-described resin without a primer layer or an easily adhesive layer on one or both sides.

hard coating layer

The hard coating layer 120 is directly formed on the upper surface of the first intermixing layer 130 . The "directly formed" means that the hard coating layer 120 is formed directly on the first intermixing layer 130 without an adhesive layer, an adhesive layer, or an adhesive layer.

The hard coating layer 120 may have a thickness of about 0.1 μm to about 20 μm, specifically about 0.1 μm to about 10 μm, more specifically about 1 μm to about 10 μm, and most specifically about 5 μm to about 10 μm. have. Within the above range, it may be used for a flexible window film.

The hard coating layer 120 has a lower refractive index compared to the base layer 110, and the difference in refractive index between the refractive index of the base layer and the refractive index of the hard coating layer may be from about 0.01 to about 0.3, specifically from about 0.1 to about 0.2. Even in the refractive index magnitude and refractive index difference, the window film has a thickness ratio of about 0.5 to about 100 (for example, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100), so that there is no mura and/or rainbow and the appearance is excellent.

In one embodiment, the hard coating layer 120 has a refractive index of about 1.30 to about 1.60 (eg 1.30, 1.40, 1.50 or 1.60), specifically about 1.40 to about 1.60 (eg, 1.40, 1.50, or 1.60) this can be In the above range, when the thickness ratio of Equation 1 is about 0.5 to 100, there is no mura and/or rainbow, and thus the appearance may be excellent.

The hard coating layer 120 may be formed of a composition for a hard coating layer including at least one of (meth)acrylic resins and a solvent. The composition for the hard coating layer may further include at least one of a crosslinking agent and an initiator. Preferably, the composition for the hard coating layer may include a (meth)acrylic resin, an initiator, and a solvent without a crosslinking agent. The composition for the hard coating layer essentially contains a solvent.

In one embodiment, the composition for the hard coating layer may include a (meth)acrylic resin, an initiator, and a solvent without a crosslinking agent.

The (meth)acrylic resin is a (meth)acrylic monomer alone; Alternatively, it may include a (meth)acrylic resin formed by polymerization between a (meth)acrylic monomer and a (meth)acrylic monomer and a copolymerizable comonomer. The (meth)acrylic monomer may include a conventional monomer known to those skilled in the art. The comonomer may include a common monomer copolymerizable with a (meth)acrylic monomer, known to those skilled in the art.

In one embodiment, the (meth)acrylic resin may include a resin having a (meth)acrylate group at the terminal. Preferably, the (meth)acrylic resin may include a dendritic aliphatic compound having a (meth)acrylate group at the terminal. By including the resin, it can have many (meth)acrylate groups at the molecular terminals, and thus the reactivity can be increased.

(meth)acrylic resin may be used individually by 1 type, and may be used in combination of 2 or more type. Among (meth)acrylic resins, one or more of dendrimer-type resins and hyperbranched-type resins can be included. The dendrimer-type resin is a highly branched resin with a high regularity, and the hyper-branched resin is a branched resin with a relatively low regularity than the dendrimer-type resin. The hyper-branched resin may have excellent solubility in solvents due to low viscosity compared to linear resins.

As the dendrimer-type resin, a multi-branched (dendrimer-type) polyester (meth)acrylate having a (meth)acrylate group at the terminal may be used, but is not limited thereto. As the hyper-branched resin, a multi-branched (dendrimer-type) poly(meth)acrylate having a (meth)acrylate group at the terminal may be used, but is not limited thereto. In one embodiment, a multi-branched (dipentaerythritol hexaacrylate-linked) poly(meth)acrylate having dipentaerythritol as a core and a (meth)acrylate group at the terminal may be used, but is not limited thereto. .

The (meth)acrylic resin may have a weight average molecular weight of about 5,000 to about 30,000, for example, about 10,000 to about 25,000. In the above range, the hardness and scratch resistance of the window film may be excellent.

The initiator may comprise conventional photoinitiators known to those skilled in the art as photo-radical initiators. For example, a hydroxyketone-based, phosphine oxide-based, benzoin-based or aminoketone-based photoradical initiator may be used. The initiator may be included in an amount of about 1 part by weight to about 10 parts by weight, preferably about 1 part by weight to about 5 parts by weight based on about 100 parts by weight of the (meth)acrylic resin. Within the above range, there may be an effect of not reducing optical properties or physical properties while curing is performed properly.

The solvent is included in the composition for the hard coat layer for the ease of coating the composition for the hard coat layer and the formation of the first intermixing layer to be described in detail below. The solvent may facilitate the formation of the first intermixing layer by selecting and using a type capable of dissolving the base layer.

In one embodiment, when the base layer is a film including at least one of polyimide-based resin and polyamideimide-based resin, the solvent is methyl ethyl ketone, methyl isobutyl ketone, propylene glycol monomethyl ether, N,N- It may include one or more of dimethylacetamide.

In another embodiment, the composition for the hard coating layer may include a (meth)acrylic resin, a crosslinking agent, an initiator, and a solvent.

The crosslinking agent crosslinks with the (meth)acrylic resin, and may include a (meth)acrylate compound having one or more (meth)acrylate groups, preferably two or more, more preferably 2 to 20. have.

In one embodiment, the crosslinking agent may include a polyfunctional (meth)acrylate. The polyfunctional (meth)acrylate may be included in the composition for the hard coat layer to further improve the hardness and flexibility of the hard coat layer.

The crosslinking agent may be included in an amount of about 5 parts by weight to about 150 parts by weight, preferably about 5 parts by weight to about 100 parts by weight, based on about 100 parts by weight of the (meth)acrylic resin. Within the above range, there may be an effect of improving high hardness and bending flexibility.

The hard coating layer 120 is formed simultaneously with the first intermixing layer 130 . This will be described in detail below.

Although not shown in FIG. 1 , an additional functional layer, for example, an anti-fingerprint layer, an anti-reflection layer, and an anti-glare layer, may be additionally formed on the upper surface of the hard coating layer 120 .

first intermixing layer

The first intermixing layer 130 is formed between the base layer 110 and the hard coating layer 120 . The first intermixing layer 130 is directly formed on the upper surface of the base layer 110 and the lower surface of the hard coating layer 120 , respectively.

The first intermixing layer 130 refers to a layer in which a resin forming the base layer 110 and a resin forming the hard coating layer 120 are mixed. As shown in FIG. 1 , the first intermixing layer 130 is continuously formed without an interface compared to the base layer 110 and the hard coating layer 120 .

The first intermixing layer 130 may include a resin contained in the hard coating layer 120, for example, a (meth)acrylic resin, and a resin contained in the base layer 110, for example, a polyimide-based resin or a polyamide-imide-based resin. Includes all one or more species. The first intermixing layer 130 further includes a resin contained in the base layer 110 compared to the hard coating layer 120 . Through this, the first intermixing layer 130 may be distinguished from the hard coating layer 120 .

To this end, the first intermixing layer 130 is formed simultaneously with the hard coating layer 120 .

That is, since the composition for the hard coat layer contains a solvent for dissolving the base layer, when the composition for the hard coat layer is coated on the base layer 110, a part of the base layer 110 is dissolved or eroded and the hard coat layer By curing the composition, the hard coating layer 120 and the first intermixing layer 130 may be simultaneously formed. Accordingly, the first intermixing layer 130 may be a solvent erosion layer or a solvent dissolving layer for the base layer 110 .

Specifically, it may be formed by applying the hard coating composition to a predetermined thickness on the upper surface of the base layer 110 and curing it. A method of coating the composition for the hard coating layer on the upper surface of the base layer 110 is not particularly limited. For example, it may be bar coating, spin coating, dip coating, roll coating, flow coating, die coating, and the like. Curing may include at least one of photocuring and thermal curing. Photocuring may involve irradiation with light intensity of about 10 mJ / cm ² to about 1,000mJ / cm ² at a wavelength of about 400nm or less. Thermal curing may include applying the composition for the hard coating layer to a predetermined thickness and drying the composition at about 80° C. to about 150° C. for about 5 minutes to about 30 minutes.

The first intermixing layer 130 satisfies the thickness ratio of Equation 1 described in detail below. To this end, when the hard coating layer 120 and the first intermixing layer 130 are formed, the solvent in the composition for the hard coating layer may be included in an amount of about 20 wt% to about 70 wt%, preferably 30 wt% to 50 wt% have.

The first intermixing layer 130 may have a thickness of about 200 nm to about 2500 nm, specifically about 500 nm to about 2000 nm, and more specifically about 800 nm to about 1500 nm. In the above range, it may be easy to satisfy the thickness ratio of Equation 1.

back coating layer

The back coating layer 140 is formed on the lower surface of the base layer 110 via the second intermixing layer 150 . A window film in which only the second intermixing layer 150 satisfying the thickness ratio of Equation 1 is formed on the lower surface of the base layer 110 without the back coating layer 140 may cause severe rainbow and/or mura.

The back coating layer 140 is thinner than the hard coating layer 120 , and for example, may have a thickness of about 0.1 μm to about 5 μm, specifically, about 0.1 μm to about 2.5 μm. In the above range, there is no rainbow and/or mura, so that the appearance defect is improved, there is no crack generation during bending, and the haze of the film may not rise.

The back coating layer 140 may have a lower refractive index than the base layer 110 . In one embodiment, the difference between the refractive index of the base layer 110 and the refractive index of the back coating layer 140 may be 0.01 to 0.2, specifically 0.01 to 0.15. When the thickness ratio of Equation 1 is satisfied in the above range, it may help to prevent the rainbow and mura from being recognized.

In one embodiment, the window film comprises a first intermixing layer between the hard coating layer and the base layer and a second intermixing layer between the base layer and the back coating layer when the refractive index increases in the order of the hard coating layer, the back coating layer and the base layer, respectively. By including and satisfying the thickness ratio of Equation 1, the effect of preventing the rainbow and mura from being recognized may be higher.

The back coating layer 140 may have a refractive index of about 1.30 to about 1.60 (eg, 1.30, 1.40, 1.50, or 1.60), specifically, about 1.40 to about 1.60.

The back coating layer 140 may be formed of a composition for a back coating layer including a polyurethane-polyimide-based polymer and a solvent. The composition for the back coating layer may further include at least one of a crosslinking agent and an initiator. The composition for the back coating layer may include a polyurethane-polyimide-based polymer and a solvent without a crosslinking agent. The composition for the back coating layer essentially contains a solvent. The polyurethane-polyimide-based polymer may be easy to implement the effects of the present invention.

The polyurethane-polyimide-based polymer is a polymer including a urethane structural unit and an imide structural unit, and these may include random, block, or alternate copolymers. The urethane structural unit may be represented by the following Chemical Formula 1, and the imide structural unit may be represented by the following Chemical Formula 2: In the polyurethane-polyimide-based polymer, one or more or two or more different structural units of Chemical Formula 1 may be included, and one or more or two or more different structural units of Formula 2 may be included:

[Formula 1]

*-(-Y-NH-C(=O)-O-Z-)-*

(In Formula 1, * is a connection site,

Y and Z are each independently a substituted or unsubstituted divalent C1 to C30 aliphatic organic group, a substituted or unsubstituted divalent C2 to C30 alicyclic organic group, a substituted or unsubstituted C6 to C30 aromatic organic group, a substituted or an unsubstituted C4 to C30 heteroaromatic organic group, or a combination thereof,

The alicyclic organic group, the aromatic organic group, the heteroaromatic organic group, or a combination thereof is a monocyclic ring, a condensed ring in which two or more rings are fused, or two or more rings selected from the monocyclic and condensed rings are Single bond, fluorenylene group, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) ₂ -, -Si(CH ₃ ) ₂ -, - (CH ₂ ) _p -, -(CF ₂ ) _q -, -C(C _n H _2n+1 ) ₂ -, -C(C _n F _2n+1 ) ₂ -, -(CH ₂ ) _p -C( C _n H _2n+1 ) ₂ -(CH ₂ ) _q -, -(CH ₂ ) _p -C(C _n F _2n+1 ) ₂ -(CH ₂ ) _q - (where n, p, q are each an integer) and 1≤n≤10, 1≤p≤10, and 1≤q≤10), -C(CF ₃ )(C ₆ H ₅ )-, or -C(=O)NH-.

[Formula 2]

(In Formula 2, * is a connection site,

D is a substituted or unsubstituted tetravalent C4 to C30 alicyclic organic group, a substituted or unsubstituted tetravalent C6 to C30 aromatic organic group, a substituted or unsubstituted tetravalent C4 to C30 heteroaromatic organic group, or these is a combination of

The alicyclic organic group, the aromatic organic group, the heteroaromatic organic group, or a combination thereof is a monocyclic ring, a condensed ring in which two or more rings are fused, or two or more rings selected from the monocyclic and condensed rings are Single bond, fluorenylene group, -O-, -S-, -C(=O)-, -CH(OH)-, -S(=O) ₂ -, -Si(CH ₃ ) ₂ -, - (CH ₂ )p-, -(CF ₂ )q-, -C(C _n H2 _n+1 ) ₂ -, -C(C _n F _2n+1 ) ₂ -, -(CH ₂ ) _p -C( C _n H2 _n+1 )2-(CH ₂ ) _q -, -(CH ₂ )pC(C _n F _2n+1 ) ₂ -(CH ₂ ) _q - (wherein n, p, q are each an integer, 1≤n≤10, 1≤p≤10, and 1≤q≤10), -C(CF ₃ )(C ₆ H ₅ )-, or -C(=O)NH-.

In the present specification, "substituted" in "substituted or unsubstituted" refers to a C1 to C10 alkyl group, a hydroxyl group, a C3 to C10 cycloalkyl group, a C6 to C20 aryl group, a C7 to C20 arylalkyl group, and an amino group among the corresponding functional groups. or halogen-substituted.

The polyurethane-polyimide-based polymer may be prepared by a conventional method known to those skilled in the art.

The solvent is included in the composition for the back coating layer to facilitate coating of the composition for the back coating layer and to form the second intermixing layer to be described in detail below. The solvent may facilitate the formation of the second intermixing layer by selecting and using a type capable of dissolving the base layer. In one embodiment, when the base layer is a film including at least one of polyimide-based resin and polyamideimide-based resin, the solvent is methyl ethyl ketone, methyl isobutyl ketone, propylene glycol monomethyl ether, N,N- It may include one or more of dimethylacetamide.

The back coating layer 140 is formed simultaneously with the second intermixing layer 150 . This will be described in detail below.

2nd intermixing layer

The second intermixing layer 150 is formed between the base layer 110 and the back coating layer 140 . The second intermixing layer 150 is directly formed on the lower surface of the base layer 110 .

The second intermixing layer 150 refers to a layer in which a resin forming the base layer 110 and a resin forming the back coating layer 140 are mixed. As shown in FIG. 1 , the second intermixing layer 150 is continuously formed without an interface compared to the base layer 110 and the back coating layer 140 .

The second intermixing layer 150 may include a resin contained in the back coating layer 140, for example, a polyurethane-polyimide-based polymer, and a resin contained in the base layer 110, for example, a polyimide-based resin or a polyamide-imide-based polymer. All of at least one kind of resin is included. The second intermixing layer 150 further includes a resin contained in the base layer 110 compared to the back coating layer 120 . Through this, the second intermixing layer 150 may be distinguished from the back coating layer 120 .

To this end, the second intermixing layer 150 is formed simultaneously with the back coating layer 140 . That is, since the composition for the back coating layer contains a solvent for dissolving the base layer, when the composition for the back coating layer is coated on the base layer 110 , a part of the base layer 110 is dissolved or eroded, and the back coating layer is used for As the composition is cured, the back coating layer 140 and the second intermixing layer 150 may be simultaneously formed. Accordingly, the second intermixing layer 150 may be a solvent erosion layer or a solvent dissolving layer for the base layer 110 .

Specifically, it may be formed by applying the composition for a back coating layer to a predetermined thickness on the lower surface of the base layer 110 and curing the composition. A method of coating the composition for the back coating layer on the lower surface of the base layer 110 is not particularly limited. For example, it may be bar coating, spin coating, dip coating, roll coating, flow coating, die coating, and the like. Curing may include at least one of photocuring and thermal curing. Photocuring may involve irradiation with light intensity of approximately 10mJ / cm ² to about 1,000mJ / cm ² at a wavelength of about 400nm or less. Thermal curing may include drying at about 80° C. to about 150° C. for about 5 minutes to about 30 minutes.

The second intermixing layer 150 satisfies the thickness ratio of Equation 1 described in detail below. To this end, when the back coating layer 140 and the second intermixing layer 150 are formed, the solvent in the composition for the back coating layer is included in an amount of about 70 wt% to about 95 wt%, specifically about 75 wt% to about 95 wt% The thickness of the composition for the back coating layer may be about 1 to about 1.5 times the total thickness of the back coating layer and the second intermixing layer.

The second intermixing layer 150 may have a thickness of about 20 nm to about 500 nm, specifically about 100 nm to about 500 nm, and specifically about 100 nm to about 450 nm. In the above range, it may be easy to satisfy the thickness ratio of Equation 1.

The thickness ratio of the second intermixing layer 150 to the first intermixing layer 140 in Equation 1 below is about 0.5 to about 100. In the above range, in the window film of the present invention, mura and/or rainbow are not recognized at all, or rainbow and/or mura are not easily recognized with the naked eye, so that the appearance may be excellent. Preferably, the thickness ratio of Equation 1 may be from about 1 to about 50, more specifically, from about 2 to about 5.

[Equation 1]

Thickness ratio = (thickness of the first intermixing layer)/(thickness of the second intermixing layer)

The second intermixing layer 150 may have a refractive index different from that of the first intermixing layer 130 . In one embodiment, the second intermixing layer 150 may have a higher refractive index than the first intermixing layer 130 .

Although not shown in FIG. 1 , at least one of the base layer 110 , the first intermixing layer 130 , the hard coating layer 120 , the second intermixing layer 150 , and the back coating layer 140 has a maximum absorption wavelength about 550 nm to about 650 nm (e.g., about 550 nm, about 560 nm, about 570 nm, about 580 nm, about 590 nm, about 600 nm, about 610 nm, about 620 nm, about 630 nm, about 640 nm , or about 650 nm). In the maximum absorption wavelength range, when the substrate layer is a film formed of at least one of a polyimide-based resin and a polyamide-imide-based resin, yellowness may be lowered to suppress yellow visibility. Commercially available products may be used for these dyes, and specifically blue-B (Kyungin Corporation, maximum absorption wavelength: about 610 nm) may be used, but is not limited thereto. The "maximum absorption wavelength" means a wavelength at which the maximum absorbance appears when the absorbance is measured for a solution having a concentration of about 10 ppm in methyl ethyl ketone.

Hereinafter, the display device of the present invention will be described.

The display device of the present invention includes the above-described flexible window film of the present invention. The display device may be a flexible display device or a non-flexible display device. For example, the display device may be a light emitting display device including an organic light emitting display device, a liquid crystal display device, or the like, but is not limited thereto.

Hereinafter, the present invention will be described in more detail through examples, but these examples are for illustrative purposes only and should not be construed as limiting the present invention.

Example 1

(1) Preparation of composition for hard coating layer

A composition for a hard coating layer was prepared by mixing 67 parts by weight of a dendrimer-type acrylic resin (Osaka organic chemical), 1 part by weight of a photoinitiator (Irgacure-184, BASF), and propylene glycol monomethyl ether (PGME). In the composition for the hard coating layer, the solvent PGME is included in an amount of 40% by weight.

(2) Preparation of composition for back coating layer

After adding 1,4-butanediol and isophorone diisocyanate, the reaction was stirred at 70° C. for 4 hours. In addition, dimethylacetamide, 2,2'-bis(trifluoromethyl)benzidine (2,2'-bis(trifluoromethyl)benzidine), 3,3',4,4'-biphenyltetracarboxy dianhydride ( 3,3',4,4'-biphenyltetracarboxylic dianhydride) and 2,2-bis(3,4-dicarboxyphenyl hexafluoropropane dianhydride) was added and stirred for 24 hours, followed by addition of pyridine and acetic anhydride and further stirring for 24 hours to obtain a final solution containing a polyurethane-polyimide-based polymer.

A composition for a back coating layer was prepared by mixing 10 parts by weight of a polyurethane-polyimide-based polymer and 90 parts by weight of propylene glycol monomethyl ether (PGME).

(3) Manufacture of window film

A polyimide film (thickness: 50 µm, Kolon Corporation, refractive index: 1.65) was used as the base layer. The composition for the hard coating layer prepared above was applied to the upper surface of the polyimide film to a thickness of 7 μm, dried at 80° C. for 2 minutes, ^{and irradiated with UV of 500 mJ/cm 2} for first intermixing on the upper surface of the polyimide film. A layer and a hard coating layer (refractive index: 1.48) were formed.

The prepared composition for a back coating layer was applied to the lower surface of the polyimide film to a thickness of 1.5 μm, dried at 100° C. for 2 minutes, and a second intermixing layer and a back coating layer (refractive index: 1.58) on the lower surface of the polyimide film. By forming a window film was prepared.

Examples 2 to 4 and Examples 6 to 7

A window film was manufactured in the same manner as in Example 1, except that in Example 1, each layer was prepared and the thickness of each layer was adjusted as shown in Table 1 below.

Example 5

A window film was prepared in the same manner as in Example 1, except that in Example 1, 1 part by weight of Blue-B (Kyungin Corporation) dye was additionally included in the composition for the back coating layer.

Comparative Example 1

A window film was prepared in the same manner as in Example 1, except that in Example 1, the solvent content in the composition for the back coating layer was changed to 50% by weight as shown in Table 1 below. Since the content of the solvent in the composition for the back coating layer was low, in Comparative Example 1, the second intermixing layer was not formed at all.

Comparative Example 2

A window film was prepared in the same manner as in Example 1, except that in Example 1, the solvent content in the composition for the hard coating layer was changed to 10% by weight as shown in Table 1 below. The first intermixing layer was not formed at all in Comparative Example 2 because the solvent content in the composition for the hard coating layer was low.

Comparative Example 3 and Comparative Example 4

A window film was prepared in the same manner as in Example 1, except that the thickness of each layer was adjusted as shown in Table 1 below by changing the solvent content in the composition for the back coating layer and the composition for the hard coating layer in Example 1, and the thickness of each application. prepared.

The following physical properties were evaluated for the window films prepared in Examples and Comparative Examples, and the results are shown in Table 1 below.

Properties

(1) Light transmittance and haze (unit: %): For the window film, haze and light transmittance were measured using Hazemeter (NDH2000, Nippon Denshoku) and light transmittance CM-3600A (Konica Minolta).

(2) Rainbow and Mura: After sequentially laminating an adhesive and a window film on the upper surface of the black sheet (at this time, the hard coating layer is on top), the interference Mura and Rainbow Mura were visually evaluated at an angle of 45° under three wavelengths of light. . Rainbow was evaluated based on the following criteria. In the case of ◎ to △, it can be practically used in a display device. In the case of x to x x, it cannot be used for the display device.

◎: When rainbow and mura are not visually recognized at all

○: When there is a little rainbow and mura, but it is not visually recognized

△: When there is a little rainbow and mura, but it is not easily recognized with the naked eye

x: When rainbow and mura are easily recognized with the naked eye

x x: When rainbow and mura are visually recognized very easily

(3) Scratch resistance (Scuff test): Using a scuff tester, place a tip (diameter: 11 mm) equipped with steel wool (ribenon #0000) on the window coating layer of the window film, then load 1.5 kg, speed It moves on the surface of the window coating layer under the conditions of 60 mm/sec and a moving distance of 40 mm. Record the number of scratches that occurred after repeating this 10 times. The lower the number, the better the scratch resistance. If the number is less than 1, it can be used as a window film, and thus it was evaluated as 'pass'.

(4) Bending reliability: The window film was evaluated for operational bending reliability at a curvature radius of 1 mm, high temperature and high humidity at 60°C and 95% relative humidity, and low temperature at -40°C. A specimen was prepared by cutting the window film into a rectangle having a length of 2.5 cm and a width of 15 cm. The minimum number of times that cracks start to occur in the window film was evaluated when the prepared specimen was wound on a jig with a radius of curvature of 1 mm so that the hard coating layer was in contact, and then rolled and unrolled. A case in which the minimum number of cracks occurring in the hard coating layer, the base layer and/or the back coating layer was 200,000 or more was evaluated as 'passed'.

	Example							comparative example
	One	2	3	4	5	6	7	One	2	3	4
Solvent content (% by weight) in the hard coating layer composition	40	40	30	50	40	70	20	40	10	15	80
Solvent content in the back coating layer composition (wt%)	90	80	95	80	90	70	95	50	90	95	70
film structure	Figure 1	Figure 1	Figure 1	Figure 1	Figure 1	Figure 1	Figure 1	-	-	Figure 1	Figure 1
Hard coating layer thickness (㎛)	6.98	6.71	6.85	6.70	6.98	6.99	6.91	7.08	7.0	7.01	6.85
First intermixing layer thickness (nm)	1130	1050	870	1250	1130	2190	244	1270	0	150	2890
Second intermixing layer thickness (nm)	377	236	420	251	377	23	452	0	350	445	25
Back coating layer thickness (㎛)	1.51	1.01	1.66	1.22	1.51	1.21	1.18	1.5	1.48	1.2	1.25
Equation 1	3.00	4.45	2.07	4.98	3.00	95.2	0.54	-	-	0.34	115.6
light transmittance	89.1	90.0	89.0	89.9	89.1	89.9	89.8	90.2	89.2	89.8	89.7
haze	0.8	0.5	0.8	0.6	0.8	0.8	0.7	0.5	0.9	0.6	0.7
Rainbow and Mura	◎	○	◎	○	◎	△	△	xx	xx	x	x
scratch resistance	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass
Bend Reliability	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass	Pass

As shown in Table 1, the window film of the present invention has excellent optical properties due to high light transmittance and low haze, and has no or weak rainbow and mura visibility, and has excellent bending reliability.

On the other hand, in the window film of the present invention, Comparative Example 1 without the second intermixing layer, Comparative Example 2 without the first intermixing layer, and Comparative Example 3 and Comparative Examples that do not satisfy the thickness ratio of Equation 1 of the present invention 4, rainbow and mura were recognized and could not be used for display purposes.

Simple modifications or changes of the present invention can be easily carried out by those of ordinary skill in the art, and all such modifications or changes can be considered to be included in the scope of the present invention.

Claims (15)

1. base layer; a first intermixing layer and a hard coating layer sequentially formed on the upper surface of the base layer; and a second intermixing layer and a back coating layer sequentially formed on the lower surface of the base layer,

   The flexible window film, wherein the first intermixing layer and the second intermixing layer have a thickness ratio of about 0.5 to about 100 in Equation 1 below:

   [Equation 1]

   Thickness ratio = (thickness of the first intermixing layer)/(thickness of the second intermixing layer).
2. According to claim 1, wherein the first intermixing layer is continuously formed without an interface on the base layer and the hard coating layer, respectively,

   wherein the second intermixing layer is continuously formed on the base layer and the back coating layer without an interface, respectively.
3. The flexible window film of claim 1 , wherein the first intermixing layer and the second intermixing layer have different refractive indices.
4. The flexible window film of claim 1 , wherein the first intermixing layer includes both a resin contained in the hard coating layer and a resin contained in the base layer.
5. The flexible window film of claim 1 , wherein the second intermixing layer includes both a resin contained in the base layer and a resin contained in the back coating layer.
6. The flexible window film of claim 1 , wherein the hard coating layer has a lower refractive index than the base layer.
7. The method of claim 6, wherein the hard coating layer has a refractive index of about 1.30 to about 1.60,

   The base layer has a refractive index of about 1.40 to about 1.70, the flexible window film.
8. The flexible window film of claim 1 , wherein the back coating layer has a lower refractive index than the base layer.
9. 9. The method of claim 8, wherein the back coating layer has a refractive index of about 1.30 to about 1.60;

   The base layer has a refractive index of about 1.40 to about 1.70, the flexible window film.
10. The flexible window film of claim 1 , wherein the base layer comprises a film formed of at least one of a polyimide-based resin and a polyamideimide-based resin.
11. The flexible window film of claim 1, wherein the hard coating layer is a (meth)acrylic coating layer.
12. The flexible window film of claim 1 , wherein the back coating layer is a polyurethane-polyimide-based coating layer.
13. The flexible window film of claim 1, wherein the first intermixing layer and the second intermixing layer are a solvent erosion layer or a solvent dissolution layer for the base layer, respectively.
14. According to claim 1, wherein at least one of the base layer, the first intermixing layer, the hard coating layer, the second intermixing layer, and the back coating layer further comprises a dye having a maximum absorption wavelength of about 550 nm to about 650 nm That is, a flexible window film.
15. The display device comprising the flexible window film of any one of claims 1 to 14.

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