KR102575489B1 - Polyester protective film for flexible display device - Google Patents

Abstract

The protective film according to one embodiment implements a very low reflectance through a multi-layer structure with an adjusted refractive index, and does not cause whitening or cracking even when frequently bent or folded. Accordingly, the protective film may exhibit excellent optical and mechanical properties when applied to a cover of a flexible display device, particularly a foldable display device.

Description

Polyester protective film for flexible display device {POLYESTER PROTECTIVE FILM FOR FLEXIBLE DISPLAY DEVICE}

Embodiments relate to a polyester protective film for a flexible display device. More specifically, embodiments relate to a protective film having a controlled modulus at room temperature, a laminate including the protective film, and a flexible display device.

Display technology is constantly evolving due to demand according to the development of IT devices, and technologies such as curved displays and bended displays have already been commercialized. Recently, in the field of mobile devices requiring both large screen and portability, a flexible display device that can be flexibly bent or folded according to an external force is preferred. In particular, a great advantage of a foldable display device is that it can be folded and made small to increase portability when not in use, and can be widened to realize a large screen when in use.

Referring to FIGS. 4A and 4B , such foldable display devices include an in-folding type (1) in which the screen is positioned inside the folding direction, and an out-folding type (1) in which the screen is positioned outside the folding direction. It is being developed as an out-folding type (2). As the transparent substrate 200 applied to the cover window of these foldable display devices, for example, a polyimide-based film for an in-folding type and an ultra-thin glass (UTG) for an out-folding type are mainly used. In addition, the protective film 100 is applied to the surface of the transparent substrate 200 to mitigate impact, prevent scattering, prevent scratches, etc., and recently, there is an attempt to manufacture a protective film for a flexible display device using a polyester resin. .

In addition, recently, various surface treatment technologies have been developed to reduce the reflectance of films applied to the cover of the display device as much as possible in order to prevent deterioration of visibility due to reflection of external light on the front surface of the display device.

Korean Patent Publication No. 2014-0036536

A characteristic that is as important as flexibility in materials applied to flexible display devices is that the original characteristics do not deteriorate even when frequently bent or folded. When a general material is completely folded and then unfolded, marks are formed and it is almost impossible to return to its original shape. Therefore, development of a material applied to a flexible display device must be accompanied by research to overcome these limitations.

Specifically, with reference to FIG. 4A, in the case of the infolding type (1), by deformation due to the stress applied to the point (a) that is folded small inward, and also with reference to FIG. 4B, of the outfolding type (2) When the protective film 100 or the like is whitened or cracked due to deformation due to the stress generated at the point (b) where it is greatly folded outward, the properties are likely to be deteriorated. These whitening and cracking can generally be resolved when the modulus of the protective film is low at room temperature, but polyester resins such as polyethylene terephthalate (PET) generally have high modulus at room temperature, so whitening and cracking when applied to flexible display devices There is a problem with this that easily arises.

Meanwhile, in order to lower the reflectance of the protective film used in the display device, a technique of forming various functional layers on the surface of the protective film is known. However, the functional layers newly formed in this way cause optical interference with the existing film layer, which makes it difficult to adjust the reflectance or deteriorates the optical properties. In addition, since a polyester film such as polyethylene terephthalate has a very high birefringence, it causes distortion in the polarization state between the polarizer and the liquid crystal, and accordingly, there is a problem in that visibility is remarkably reduced due to occurrence of rainbow spots and the like.

As a result of research by the present inventors to solve these problems, it was found that optical and mechanical properties can be improved by designing a multilayer structure in which refractive indices are combined and adjusting the modulus of the film.

Accordingly, embodiments are intended to provide a protective film that does not generate whitening and cracks even when frequently bent or folded while implementing a very low reflectance through a multilayer structure with a controlled refractive index, a laminate and a display device including the protective film.

A method for manufacturing a protective film according to an embodiment includes extruding a composition containing a polyester resin to obtain an unstretched sheet; preheating the unstretched sheet at 70° C. to 90° C. and then stretching the sheet at a stretching ratio in the longitudinal direction of 2.0 to 5.0 and a stretching ratio in the width direction of 2.0 to 5.0; preparing a base layer by heat-setting the stretched sheet at 150° C. to 250° C.; and forming a primer layer on the base layer.

A protective film according to another embodiment includes a base layer containing a polyester resin, and a primer layer disposed on the base layer, has a reflectance of 5% or less for light having a wavelength of 550 nm, and the base layer is whitened. The strain until occurrence is 10% or more, the base layer has a moisture permeability of 10 g/m ² day to 100 g/m ² ·day, and the following formula (1) is satisfied.

0.7 ≤ n2/n1 ≤ 1.0 (1)

In Equation (1), n1 and n2 mean the refractive indices of the base layer and the primer layer, respectively.

A laminate according to another embodiment includes a transparent cover for a flexible display device; and a protective film disposed on the transparent cover and including a substrate layer comprising a polyester resin, and a primer layer disposed on the substrate layer, wherein the protective film has a reflectance of 550 nm for light having a wavelength of 5 % or less, the strain until whitening of the transparent cover and the base layer is 10% or more, and the moisture permeability of the base layer is 10 g/m ² day to 100 g/m ² day, The above expression (1) is satisfied.

The protective film according to the embodiment has a very low reflectance due to a combination of refractive indices between constituent layers, so that visibility is not impaired by external light.

In addition, since the modulus of the protective film according to the embodiment is controlled at room temperature, whitening and cracking can be remarkably suppressed even when frequently bent or folded.

Due to these characteristics, the protective film can exhibit excellent optical and mechanical properties when applied to a cover of a flexible display device, particularly a foldable display device.

1 is a cross-sectional view of a protective film having a base layer and a primer layer.
2 is a cross-sectional view of a protective film having a base layer, a primer layer, a hard coating layer, a first refractive layer, and a second refractive layer.
3 is a cross-sectional view of a laminate of a transparent cover and a protective film for a flexible display device.
4A and 4B are cross-sectional views of an in-folding type and an out-folding type flexible display device, respectively.

In the case where each film, panel, layer, etc. described in the following embodiments is described as being formed "on" or "under" each film, panel, or layer, the "upper ( "on" and "under" include both directly or indirectly formed through other components.

In addition, the size of each component in the drawings may be exaggerated for description, and does not mean a size that is actually applied.

In addition, it should be understood that all numerical ranges representing physical property values, dimensions, etc. of components described in this specification are modified by the term "about" in all cases unless otherwise specified.

[Protection Film]

A protective film according to another embodiment includes a base layer containing a polyester resin, and a primer layer disposed on the base layer, has a reflectance of 5% or less for light having a wavelength of 550 nm, and the base layer is whitened. The strain until the occurrence is 10% or more, the water vapor transmission rate of the base layer is 10 g/m ² day to 100 g/m ² ·day, and the following formula (1) is satisfied.

0.7 ≤ n2/n1 ≤ 1.0 (1)

In Equation (1), n1 and n2 mean the refractive indices of the base layer and the primer layer, respectively.

1 is a cross-sectional view of the protective film 100 according to the embodiment, showing the protective film including a base layer 110 and a primer layer 120.

The protective film may further include one or more additional coating layers other than the primer layer 120 on the base layer 110 .

For example, the protective film may further include a functional coating layer for improving hardness, preventing static electricity, preventing scattering, adjusting the refractive index, and protecting the surface.

Referring to FIG. 2 , the protective film 100 may further include a hard coating layer 130, a first refractive layer 140, and a second refractive layer 150 disposed on the primer layer 120. there is.

In addition, the protective film 100 may further include an anti-static or anti-sparkling coating layer disposed on the primer layer 120 .

Refractive index and thickness per layer

The protective film according to the embodiment satisfies the following formula (1):

0.7 ≤ n2/n1 ≤ 1.0 (1)

In Equation (1), n1 and n2 mean the refractive indices of the base layer and the primer layer, respectively.

Specifically, the n2/n1 ratio may be 0.8 to 1.0, 0.9 to 1.0, 0.7 to 0.9, 0.7 to 0.8, or 0.8 to 0.9. Alternatively, the n2/n1 ratio may be 0.7 or more and less than 1.0, 0.8 or more and less than 1.0, or 0.9 to less than 1.0.

According to another embodiment, the protective film may further satisfy the following formula (2).

0 ≤ n1-n2 ≤ 0.10 (2)

In Equation (2), n1 and n2 mean the refractive indices of the base layer and the primer layer, respectively.

According to another embodiment, the protective film may further include a hard coating layer disposed on the primer layer, and may further satisfy the following formulas (3) to (5):

n3 < n2 < n1 (3)

0.05 ≤ n1-n3 ≤ 0.20 (4)

0 < n2-n3 ≤0.13 (5)

In Equations (3) to (5), n1, n2, and n3 mean refractive indices of the base layer, the primer layer, and the hard coating layer, respectively.

The refractive index (n1) of the base layer may be 1.61 to 1.69. Alternatively, the refractive index (n1) of the base layer may be 1.63 to 1.68, 1.63 to 1.67, or 1.63 to 1.66.

The refractive index (n2) of the primer layer may be 1.50 to 1.68. Alternatively, the refractive index (n2) of the primer layer may be 1.54 to 1.63, 1.54 to 1.58, 1.54 to 1.62, 1.55 to 1.61, or 1.58 to 1.63.

The refractive index (n3) of the hard coating layer may be 1.40 to 1.67. Alternatively, the refractive index (n3) of the hard coating layer may be 1.45 to 1.60, or 1.50 to 1.53.

The base layer may have a thickness of 10 μm to 100 μm. Alternatively, the base layer may have a thickness of 10 μm to 80 μm, 20 μm to 60 μm, or 40 μm to 60 μm.

The primer layer may have a thickness of 10 nm to 200 nm. Alternatively, the thickness of the primer layer may be 50 nm to 120 nm, 80 nm to 95 nm, 80 nm to 90 nm, or 80 nm to 85 nm.

The hard coating layer may have a thickness of 0.5 μm to 100 μm. Alternatively, the hard coating layer may have a thickness of 1 μm to 10 μm, 1 μm to 8 μm, 1 μm to 5 μm, or 1.5 μm to 3.5 μm.

According to a specific example, the base layer has a refractive index (n1) of 1.61 to 1.69 and a thickness of 20 μm to 60 μm, and the primer layer has a refractive index (n2) of 1.54 to 1.63 and a thickness of 20 nm to 120 nm, , The hard coating layer may have a refractive index (n3) of 1.50 to 1.53 and a thickness of 1 μm to 5 μm.

When it is within the above preferred range, it may be more advantageous to suppress generation of rainbow stains and improve visibility when applied to optical components for display devices.

According to another embodiment, the protective film further includes a first refractive layer disposed on the hard coating layer and a second refractive layer disposed on the first refractive layer, and the following formulas (6) and (7) ) is more satisfactory.

n5 < n3 < n2 < n1 < n4 (6)

0.25 ≤ n4-n5 ≤ 0.60 (7)

In Equations (6) and (7), n1, n2, n3, n4, and n5 denote refractive indices of the base layer, the primer layer, the hard coating layer, the first refractive layer, and the second refractive layer, respectively.

The refractive index n4 of the first refractive layer may be 1.60 to 1.80, 1.60 to 1.75, 1.70 to 1.80, or 1.70 to 1.75.

In addition, the refractive index (n5) of the second refractive layer may be 1.10 to 1.40 or 1.25 to 1.35.

In addition, the refractive index difference (n1-n3) between the base layer and the hard coating layer may be 0.05 to 0.20, 0.09 to 0.16, 0.05 to 0.15, or 0.10 to 0.20.

Also, the refractive index difference (n4-n5) between the first refractive layer and the second refractive layer may be 0.30 to 0.60, 0.40 to 0.50, 0.30 to 0.50, or 0.40 to 0.60.

The thickness of the first refractive layer is 5 nm to 800 nm, 60 nm to 800 nm, 30 nm to 500 nm, 50 nm to 300 nm, 50 nm to 200 nm, 5 nm to 150 nm, 10 nm to 130 nm, or 20 nm to 100 nm.

The second refractive layer may have a thickness of 10 nm to 1 μm, 80 nm to 120 nm, 80 nm to 100 nm, 90 nm to 110 nm, or 100 nm to 120 nm.

According to a more specific example, the base layer has a refractive index (n1) of 1.61 to 1.69 and a thickness of 20 μm to 60 μm, and the primer layer has a refractive index (n2) of 1.54 to 1.63 and a thickness of 20 nm to 120 nm The hard coating layer has a refractive index (n3) of 1.50 to 1.53 and a thickness of 1 μm to 5 μm, and the first refractive layer has a refractive index (n4) of 1.60 to 1.75 and a thickness of 60 nm to 800 nm, The second refractive layer may have a refractive index (n5) of 1.25 to 1.35 and a thickness of 80 nm to 120 nm.

When it is within the above preferred range, when applied to an optical component for a display device, etc., reflection may be reduced and other optical properties may be improved.

Physical properties of base layer

The base layer may have a strain of 10% or more, 12% or more, 15% or more, or 20% or more until whitening occurs. When within the above range, whitening does not occur even when the film is deformed due to frequent folding, so it is advantageous to apply it to a flexible display device. The strain refers to the ratio of the changed dimension to the original dimension of the film. In this case, the changed dimension may be an increased dimension or a decreased dimension.

The number of repeated folding at an angle of 135 ° until breakage of the base layer may be 100 times, 1000 times, 10,000 times, 50,000 times, 100,000 times, 150,000 times, or 200,000 times or more. When it is within the above preferred range, it is advantageous to apply the flexible display device because breakage does not occur even during frequent folding.

In addition, the base layer may have an elongation retention of 80% or more, 85% or more, or 90% or more when treated at 1.2 atm and 120° C. for 72 hours.

In addition, the base layer has a moisture permeability of 10 g/m ² day to 100 g/m ² day, 10 g/m ² day to 50 g/m ² day, or 10 g/m ² day to 30 g/m 2 day. It may be m ² ·day.

In addition, the base layer may have a transmittance of 10% or less, 5% or less, or 3% or less at a wavelength of 380 nm.

In addition, the base layer may have a surface hardness of 5B or more, 1H or more, 2H or more, or 3H or more, which may be the surface hardness of a hard coating process having a thickness of 2 μm to 4 μm.

The base layer may have a ratio (s2/s1) of a thermal contraction rate (s2) in the second direction to a thermal contraction rate (s1) in the first direction under conditions of 85° C. and 24 hours. Specifically, the thermal shrinkage ratio (s2/s1) may be 1.4 to 9, 1.5 to 8, or 1.6 to 7.

In addition, the s1 may be 1% or less, 0.8% or less, 0.6% or less, 0.4% or less, or 0.2% or less. For example, s1 may be 0% to 1.0%, 0% to 0.8%, 0% to 0.6%, 0% to 0.4%, or 0% to 0.2%.

In addition, the s2 may be 3% or less, 2% or less, 1.5% or less, 1.2% or less, 1% or less, 0.8% or less, or 0.7% or less. For example, s2 may be 0.2% to 3%, 0.3% to 2%, 0.4% to 1%, or 0.5% to 0.8%.

Since the base layer has the above-described heat shrinkage characteristics, lifting due to shrinkage may not occur under high temperature conditions. Accordingly, it is possible to prevent wavy patterns, glitter, delamination between layers, cracks, and the like in advance.

Modulus of the substrate layer

The base layer has a low level of modulus (m1, m2) in the first and second directions perpendicular to each other in the plane at 25°C.

In particular, the base layer has a small difference (|m1-m2|) between m1 and m2 of the film at 25°C. That is, the m2/m1 ratio may be close to 1 at 25°C.

Accordingly, the protective film is advantageously applied to a flexible display device because whitening does not occur even when frequently bent or folded.

Both m1 and m2 may be 5 GPa or less, 4.5 GPa or less, 4 GPa or less, 3.5 GPa or less, or 3 GPa or less at 25°C. Specifically, m1 and m2 may be 2 GPa to 5 GPa, 2.5 GPa to 5 GPa, 2 GPa to 4.5 GPa, 2 GPa to 4.5 GPa, or 2 GPa to 4 GPa at 25°C.

In addition, the difference (|m1-m2|) between the m1 and the m2 may be 1.0 GPa or less at 25°C. Specifically, the difference between m1 and m2 (|m1-m2|) may be 0.7 GPa or less, 0.5 GPa or less, 0.3 GPa or less, 0.2 GPa or less, or 0.1 GPa or less at 25°C.

Also, the m2/m1 ratio may be 0.7 to 1.3, 0.8 to 1.2, or 0.9 to 1.1 at 25°C.

As an example, the first direction may be the width direction (TD) of the film, and the second direction may be the length direction (MD) of the film.

In this case, the m2/m1 ratio may be 0.7 to 1.0, 0.8 to 1.0, 0.9 to 1.0, 0.7 to 0.9, 0.7 to 0.8, or 0.8 to 0.9 at 25°C.

Alternatively, the m2/m1 ratio may be 1.0 to 1.3, 1.0 to 1.2, 1.0 to 1.1, 1.1 to 1.3, 1.2 to 1.3, or 1.1 to 1.2 at 25°C.

Due to these characteristics, the protective film can be applied to a cover of a flexible display device, in particular, a cover of a foldable display device. Characteristics may not be degraded even by deformation occurring at a folding point.

In addition, m1 and m2 may both be 3.5 GPa or less, 3.0 GPa or less, 2.5 GPa or less, 2.0 GPa or less, or 1.5 GPa or less at 85°C.

In addition, m1 may have a difference of 1 GPa to 4 GPa, a difference of 2 GPa to 4 GPa, or a difference of 1 GPa to 3 GPa at 25 ° C and 85 ° C.

In addition, m2 may have a difference of 1 GPa to 4 GPa, a difference of 2 GPa to 4 GPa, or a difference of 1 GPa to 3 GPa at 25 ° C and 85 ° C.

As a specific example, the base layer, when defining a first direction and a second direction perpendicular to each other in the plane, the ratio (m2) of the modulus (m2) in the second direction to the modulus (m1) in the first direction /m1) may be 0.7 to 1.3 at 25 °C. In this case, both m1 and m2 may be 4.5 GPa or less at 25°C. In addition, at this time, both the m1 and the m2 may be 2.5 GPa or less at 85°C.

phase difference of base layer

The substrate layer may have an in-plane retardation (Ro) of 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, or 200 nm or less. When within the above preferred range, occurrence of rainbow stains can be reduced to a minimum.

Meanwhile, the lower limit of the in-plane retardation of the base layer may be 0 nm, or the lower limit of the in-plane retardation (Ro) may be 10 nm or more, 30 nm or more, or 50 nm or more to balance optical properties and mechanical properties. there is.

In addition, the base layer may have a thickness direction retardation (Rth) of 4,000 nm or more, 5,000 nm or more, or 5,500 nm or more.

The retardation in the thickness direction may be a measured value based on a thickness of 40 μm to 50 μm. When within the preferred range, the degree of molecular orientation is large, and crystallization is promoted, which is preferable in terms of mechanical properties. In addition, since the ratio (Rth/Ro) of the thickness direction retardation Rth to the in-plane retardation Ro increases as the thickness direction retardation Rth increases, rainbow spots can be effectively suppressed.

On the other hand, in consideration of the thickness limit and cost for removing rainbow stains from the base layer, the upper limit value of the thickness direction retardation (Rth) may be 16,000 nm or less, 15,000 nm or less, or 14,000 nm or less.

Here, the in-plane retardation (Ro) is the product of the anisotropy of the refractive index of two orthogonal axes in the plane of the film (Δnxy = |nx - ny|) and the film thickness (d) (Δnxy Х d) A parameter defined by , which is a measure of optical isotropy or anisotropy.

In addition, the thickness direction retardation (Rth) refers to the two birefringences of △nxz (=|nx - nz|) and △nyz (=|ny - nz|) when viewed from the cross section in the film thickness direction, respectively. It is a parameter defined as the average of phase differences obtained by multiplying the thickness (d).

In addition, the base layer may have a ratio of thickness direction retardation (Rth) to in-plane retardation (Ro) (Rth/Ro) of 10 or more, 15 or more, or 20 or more. The smaller the in-plane retardation (Ro) and the larger the thickness-direction retardation (Rth), the more advantageous it is to prevent rainbow stains.

base layer composition

The base layer includes a polyester resin.

The polyester resin may be a homopolymer resin or a copolymer resin obtained by polycondensation of a dicarboxylic acid and a diol. In addition, the polyester resin may be a blend resin in which the homopolymer resin or the copolymer resin is mixed.

Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, orthophthalic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,5- Naphthalenedicarboxylic acid, diphenylcarboxylic acid, diphenoxyethanedicarboxylic acid, diphenylsulfonecarboxylic acid, anthracene dicarboxylic acid, 1,3-cyclopentanedicarboxylic acid, 1,3-cyclo Hexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, hexahydroterephthalic acid, hexahydroisophthalic acid, malonic acid, dimethylmalonic acid, succinic acid, 3,3-diethylsuccinic acid, glutaric acid, 2 ,2-dimethylglutaric acid, adipic acid, 2-methyladipic acid, trimethyladipic acid, pimelic acid, azelaic acid, sebacic acid, suberic acid, dodecadicarboxylic acid, and the like.

In addition, examples of the diol include ethylene glycol, propylene glycol, hexamethylene glycol, neopentyl glycol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, decamethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-bis(4-hydroxyphenyl)propane, and bis(4-hydroxyphenyl)sulfone.

Preferably, the polyester resin may be an aromatic polyester resin having excellent crystallinity, and for example, polyethylene terephthalate (PET) resin may be used as a main component.

As an example, the base layer may include about 85% by weight or more of a polyester resin, specifically, a PET resin, and more specifically, 90% by weight or more, 95% by weight or more, or 99% by weight or more. As another example, the base layer may further include other polyester resins in addition to the PET resin. Specifically, the base layer may further include about 15% by weight or less of a polyethylene naphthalate (PEN) resin. More specifically, the base layer may further include about 0.1 wt % to 10 wt %, or about 0.1 wt % to 5 wt % of the PEN resin.

The substrate layer is preferably a stretched film in view of its high crystallinity and excellent mechanical properties. Specifically, the base layer may be a biaxially stretched polyester film, for example, a film stretched at a stretch ratio of 2.0 to 5.0 in the machine direction (MD) and the width direction (TD), respectively.

In addition, since the base layer is made of polyester as a main component, crystallinity may be increased and mechanical properties such as tensile strength may be improved during the manufacturing process through heating and stretching.

primer layer

The primer layer serves to improve adhesion between the base layer and other functional coating layers.

The primer layer may be formed from a coating composition containing a thermosetting resin. For example, the coating composition may include a polyester-based resin, a polyurethane-based resin, or a mixture thereof.

The primer layer may be formed using a composition obtained by mixing and dispersing a raw material resin, a photopolymerization initiator, and other additives in a solvent, if necessary. Water may be preferably used as the solvent, and accordingly, the composition for the primer coating layer may be prepared in the form of an aqueous solution such as an aqueous solution, an aqueous dispersion, or an emulsion. Also, some organic solvents may be used.

For example, the composition for the primer layer preferably has a solid content in the composition of 3% to 20% by weight, or 4% to 10% by weight.

hard coating layer

The hard coating layer serves to improve the hardness of the surface of the protective film.

As an example, the one or more coating layers may include a hard coating layer having a thickness of 2 μm to 4 μm, and the protective film may have a surface hardness of 2H or more.

The hard coating layer may include a photocurable resin. Examples of the photocurable resin include compounds having one or two or more unsaturated bonds such as acrylate-based compounds.

In addition, the composition for the hard coating layer may include a thermosetting resin. Examples of the thermosetting resin include phenol resins, urea resins, diallylphthalate resins, melamine resins, guanamine resins, unsaturated polyester resins, polyurethane resins, epoxy resins, amino alkyd resins, melamine-urea cocondensation resins, silicon resins, A polysiloxane resin etc. are mentioned.

The first refractive layer and the second refractive layer

The first refractive layer and the second refractive layer serve to lower the reflectance by combining the refractive index with the lower coating layer.

The first refractive layer may include a resin containing metal oxide fine particles having a refractive index of 1.70 to 2.80.

The metal oxide fine particles are, for example, titanium oxide (TiO ₂ ), zirconium oxide (ZrO ₂ ), cerium oxide (CeO ₂ ), tin oxide (SnO ₂ ), antimony oxide (Sb ₂ O ₅ ), zinc antimonate ( ZnSb ₂ O ₆ ), antimony-tin oxide (ATO), indium-tin oxide (ITO), phosphorus-tin oxide (PTO), aluminum-zinc oxide (AZO), gallium-zinc oxide (GZO), and combinations thereof. It may be one selected from the group consisting of

The second refractive layer is, for example, i) a resin containing low refractive index inorganic particles such as silica or magnesium fluoride, ii) a fluorine-based resin that is a low refractive index resin, iii) containing low refractive index inorganic particles such as silica or magnesium fluoride. It may include any one of a fluorine-based resin, iv) a low refractive index inorganic material such as silica or magnesium fluoride. The silica is preferably hollow silica fine particles.

The fluorine-based resin may be a polymerizable compound or a polymer containing a fluorine atom in at least a molecule thereof. Although it does not specifically limit as a polymeric compound at this time, For example, what has hardening reactive groups, such as a photocurable functional group and a thermosetting polar group, is preferable.

As the polymerizable compound having both the photocurable functional group and the thermosetting polar group, partially and fully fluorinated alkyl, alkenyl, and aryl esters of acrylic or methacrylic acid, fully or partially fluorinated vinyl ethers, and fully or partially fluorinated vinyl esters , fully or partially fluorinated vinyl ketones, and the like can be exemplified.

As a specific example, the base layer includes a polyethylene terephthalate-based resin, the primer layer includes a thermosetting polyurethane-based resin, the hard coating layer includes a photocurable acrylate-based resin, and the first refractive layer includes an oxidized resin. Titanium (TiO ₂ ) or zirconium oxide (ZrO ₂ ) may be included, and the second refraction layer may include silica fine particles.

Characteristics of protective film

The protective film according to the embodiment has a very low reflectance for visible light due to a combination of refractive indices between constituent layers, so that visibility is not impaired by external light.

That is, the protective film may have a reflectance of 5% or less, 4% or less, 3% or less, or 2% or less, more specifically, 0% to 5%, 3% to 5%, or 0% for light having a wavelength of 550 nm. It may have a reflectance of 3% to 3%. Accordingly, since the protective film hardly reflects external light, visibility may be improved.

[Method of manufacturing protective film]

A method of manufacturing a protective film according to an embodiment includes extruding a composition containing a polyester resin to obtain an unstretched sheet; preheating the unstretched sheet at 70° C. to 90° C. and then stretching the sheet at a stretching ratio in the longitudinal direction of 2.0 to 5.0 and a stretching ratio in the width direction of 2.0 to 5.0; preparing a base layer by heat-setting the stretched sheet at 150° C. to 250° C.; and forming a primer layer on the base layer.

Specifically, the protective film is obtained by extruding a composition containing a polyester resin to obtain an unstretched film; stretching the unstretched film in the longitudinal direction and the width direction; preparing a base layer by heat-setting the stretched film; and forming one or more coating layers on the base layer.

More specifically, the protective film is obtained by extruding a polyester resin to obtain an unstretched sheet; preheating the unstretched sheet at 70° C. to 90° C. and then stretching the sheet at a stretching ratio in the longitudinal direction of 2.0 to 5.0 and a stretching ratio in the width direction of 2.0 to 5.0; and preparing a base layer by heat-setting the stretched sheet at 150° C. to 250° C.; and sequentially forming a primer layer, a hard coating layer, a first refractive layer, and a second refractive layer on the base layer.

According to the embodiment, a step of sequentially forming a hard coating layer, a first refractive layer, and a second refractive layer on the primer layer may be included.

In addition, the protective film may additionally form an anti-static or anti-sparkling coating layer disposed on the primer layer as another functional coating layer.

[Laminate]

A laminate according to another embodiment includes a transparent cover for a flexible display device; and a protective film disposed on the transparent cover and including a substrate layer comprising a polyester resin, and a primer layer disposed on the substrate layer, wherein the protective film has a reflectance of 550 nm for light having a wavelength of 5 % or less, the strain until whitening of the transparent cover and the base layer is 10% or more, and the moisture permeability of the base layer is 10 g/m ² day to 100 g/m ² day, The above expression (1) is satisfied.

3 is a cross-sectional view of a laminate of a transparent cover and a protective film for a flexible display device.

Referring to FIG. 3 , a laminate 10 according to an embodiment includes a transparent cover 200 for a flexible display device; and a protective film 100 including a substrate layer disposed on the transparent cover 200 and including a polyester resin, and a primer layer disposed on the substrate layer.

In the laminate, the protective film has substantially the same configuration and characteristics as the protective film according to the embodiment described above.

transparent cover

The transparent cover may be a cover window of a flexible display device.

The transparent cover may be a polymer film or a glass substrate. Specifically, the transparent cover may be a polyimide-based film or ultra-thin glass (UTG).

As an example, the transparent cover may include a polyimide-based resin.

The transparent cover may have a surface hardness of HB or more and light transmittance of 80% or more at a wavelength of 550 nm. In addition, the transparent cover may have a yellowness of 5 or less and a haze of 2% or less based on a thickness of 50 μm.

The transparent cover may have a strain of 10% or more, 12% or more, 15% or more, or 20% or more until whitening occurs. When within the above range, whitening does not occur even when deformation occurs due to frequent folding, and thus, application to a flexible display device is advantageous. The strain refers to the ratio of the changed dimension to the original dimension of the film. In this case, the changed dimension may be an increased dimension or a decreased dimension. In particular, both the protective film and the transparent cover may have strains of 10% or more until whitening occurs.

The transparent cover may be repeatedly folded at an angle of 135° until breakage of 100 times, 1000 times, 10,000 times, 50,000 times, 100,000 times, 150,000 times or more, or 200,000 times or more. When within the above range, breakage does not occur even during frequent folding, so it is advantageous to apply it to a flexible display device.

The laminate may satisfy the following equations (11) to (13) at 25° C. when defining first and second directions perpendicular to each other in-plane.

0.7 ≤ m2/m1 ≤ 1.3 (11)

0.5 ≤ m1/m3 ≤ 0.9 (12)

0.4 ≤ m2/m4 ≤ 0.9 (13)

In Equations (11) to (13), m1 and m2 are the modulus of the base layer in the first and second directions, respectively, and m3 and m4 are the transparent properties in the first and second directions, respectively. is the modulus of the cover.

In addition, the transparent cover has low in-plane moduli (m3, m4) in first and second directions perpendicular to each other at 25°C.

In particular, the transparent cover has a small difference (|m3-m4|) between the m3 and the m4 at 25°C. That is, the m4/m3 ratio may be close to 1 at 25°C.

Accordingly, the transparent cover is advantageously applied to a flexible display device because whitening and cracking do not occur even when frequently bent or folded.

Both m3 and m4 may be 10 GPa or less, 9.5 GPa or less, 9 GPa or less, 8.5 GPa or less, or 8 GPa or less at 25°C. Specifically, m3 and m4 may be 5 GPa to 10 GPa, 5 GPa to 9.5 GPa, 5 GPa to 8.5 GPa, or 5 GPa to 8 GPa at 25°C.

In addition, the difference (|m3-m4|) between m3 and m4 may be 2.0 GPa or less at 25°C. Specifically, the difference between m3 and m4 (|m3-m4|) may be 1.5 GPa or less, 1.0 GPa or less, 0.5 GPa or less, 0.2 GPa or less, or 0.1 GPa or less at 25°C.

In addition, the m4/m3 ratio may be 0.7 to 1.3, 0.8 to 1.2, or 0.9 to 1.1 at 25°C.

As an example, the first direction may be the width direction (TD) of the film, and the second direction may be the length direction (MD) of the film.

In this case, the m4/m3 ratio may be 0.7 to 1.0, 0.8 to 1.0, 0.9 to 1.0, 0.7 to 0.9, 0.7 to 0.8, or 0.8 to 0.9 at 25°C.

Alternatively, the m4/m3 ratio may be 1.0 to 1.3, 1.0 to 1.2, 1.0 to 1.1, 1.1 to 1.3, 1.2 to 1.3, or 1.1 to 1.2 at 25°C.

In addition, the modulus (m1, m2) of the protective film in the first and second directions and the modulus (m3, m4) of the transparent cover in the first and second directions are calculated according to the above formulas (12) and (13) satisfies

Specifically, the m1/m3 ratio may be 0.5 to 0.9, 0.5 to 0.8, 0.5 to 0.7, 0.6 to 0.9, 0.7 to 0.9, or 0.6 to 0.8.

In addition, the m2/m4 ratio may be 0.4 to 0.9, 0.4 to 0.8, 0.4 to 0.7, 0.5 to 0.9, 0.6 to 0.9, or 0.5 to 0.8.

Also, m1 and m2 may be 2.5 Gpa to 5 Gpa at 25°C, and m3 and m4 may be 5 Gpa to 10 Gpa at 25°C.

Due to these characteristics, the laminate can be applied to a cover of a flexible display device, in particular, a cover of a foldable display device. Characteristics may not be degraded even by deformation occurring at a folding point.

additional functional layer

The laminate 20 may further include one or more functional coating layers formed between the protective film 100 and the transparent cover 200 .

For example, the at least one functional coating layer may be a layer for improving adhesion between the protective film and the transparent cover.

[Display device]

A display device according to an embodiment includes a display panel; and a protective film according to the embodiment disposed on one surface of the display panel.

A display device according to another embodiment includes a display panel; and the laminate according to the embodiment disposed on one surface of the display panel.

The display device may be a flexible display device, particularly a foldable display device.

Specifically, the foldable display device may be an in-folding type or an out-folding type according to a folding direction.

In the display device, the protective film or the laminate has substantially the same configuration and characteristics as the protective film or the laminate according to the embodiment described above.

Specifically, since the difference in modulus of the protective film or the laminate in two directions perpendicular to the plane is small, the whitening phenomenon can be remarkably suppressed even with frequent folding. In addition, since the protective film or the laminate has excellent mechanical properties, thermal properties, and durability, it can serve as a protective film to mitigate impact, prevent scattering, and prevent scratches.

[Example]

It will be described in more detail through the following examples, but is not limited to these ranges.

Preparation and evaluation of polyester films (Examples A1 to A4 and Comparative Examples A1 and A2)

A polyethylene terephthalate (PET) resin (SKC Co.) was extruded from an extruder and cast from a casting roll to prepare an unstretched sheet. The unstretched sheet was stretched in the machine direction (MD) and the width direction (TD). Thereafter, the stretched sheet was heat-set, annealed and cooled to prepare a polyester film. At this time, relaxation was performed in two steps while lowering the temperature in the annealing step.

Table 1 summarizes specific process conditions.

division Raw material thickness
(μm) Preheat
Temperature (℃) MD
stretch ratio TD
stretch ratio heat fixation
Temperature (℃) Example A1 PET 40 78 3.3 3.5 180 Example A2 PET 40 78 3.1 3.4 230 Example A3 PET 50 78 3.1 3.4 230 Example A4 PET 30 78 3.1 3.4 230 Comparative Example A1 PET 50 78 3.2 4.2 230 Comparative Example A2 PET 80 78 1.2 4.3 210

The prepared polyester film was tested as follows and the results are shown in Table 2.

(1) Modulus measurement

The modulus (Young's modulus) was measured at 25 ° C and 85 ° C in each of the MD and TD directions of the film sample, and the results are shown in Table 2 below. Specifically, the film sample was cut to a width of 1.5 cm and mounted on a universal testing machine (UTM, 5566A, Instron Co.), and the modulus was calculated by measuring the resulting stress while increasing the length.

(2) folding evaluation (MIT folding test)

An MIT folding test according to ASTM D 2176 and TAPPI T 511 was performed on the film samples by a folding endurance tester (MIT-DA, Toyoseiki). Specifically, the MIT folding test was performed under conditions of a folding angle of 135°, a sample size of 10 mm in width, a load of 500 gf, a radius of curvature of 0.38R, and a folding speed of 175 times/min. The results were classified according to the criteria below and are shown in Table 2 below.

- O: 100,000 or more repetitions of folding until cracks or fractures occur.

- X: The number of repetitions of folding until cracks or breaks are less than 100,000 times.

(3) Whitening evaluation (Crack strain test)

For the film samples, the minimum elongation until whitening occurs was measured using a universal testing machine (UTM, 5566A, Instron) at increments of 1%, and the results were classified according to the following criteria and are shown in Table 2 below.

- O: Whitening does not occur even at an elongation of 10% or more.

- X: Whitening occurs when the elongation is less than 10%.

division MD modulus (GPa)
/TD modulus (GPa)
(measured at 25°C) MD modulus (GPa)
/TD modulus (GPa)
(Measured at 85℃) folding
evaluation all sorts of flowers
evaluation Example A1 3.7/3.8 1.57 / 1.9 O O Example A2 3.7/4.0 1.5 / 1.62 O O Example A3 3.8/3.9 1.95 / 2.1 O O Example A4 4.0 / 4.1 1.62 / 1.675 O O Comparative Example A1 3.8/4.6 2.3 / 2.87 X X Comparative Example A2 2.2 / 5.8 1.5 / 3.2 X X

As shown in Table 2, the polyester films of Examples A1 to A4 having MD modulus and TD modulus adjusted to the preferred range passed both folding evaluation and whitening evaluation, and were suitable as protective films for flexible display covers.

On the other hand, the polyester films of Comparative Examples A1 and A2 whose MD modulus and TD modulus were outside the preferred ranges did not pass the folding evaluation and whitening evaluation, and thus were unsuitable for use as protective films for flexible display covers.

Preparation and evaluation of protective films (Examples B1 to B6 and Comparative Examples B1 and B2)

A polyester film was prepared in the same manner as in Example A1 above. In the process of manufacturing the polyester film, a primer layer was formed on the polyester film after stretching in the machine direction (MD) and before stretching in the width direction (TD). At this time, the thermosetting polyurethane-based resin composition was applied on the polyester film using a Mayer bar and dried to form a primer layer.

The prepared protective film was tested as follows.

(1) Refractive index and in-plane retardation

For the film samples, the refractive index ((nx + ny)/2), in-plane retardation (Ro), thickness direction retardation (Rth), and in-plane retardation (|ΔRo|/|Δx|) within the effective width were measured.

First, the orientation axis direction of the film sample was obtained using two polarizing plates, and the orientation axis direction was cut into a 4 cm × 2 cm rectangle so that the orientation axis direction was orthogonal. In-plane retardation (Ro) and thickness direction retardation (Rth) were measured at the center of the width using a retardation measuring instrument (Axoscan, manufactured by Axometrics, measuring wavelength: 550 nm). In addition, the refractive index of the film sample, which is the basic data of the retardation meter, was measured by an Abbe refractometer (manufactured by Atago, NAR-4T, measuring wavelength 589.3 nm), and the thickness d (μm) of the film was measured by an electric micrometer (Fine Ryup). manufacture, Militron 1245D).

(2) Total light transmittance

Total light transmittance and haze were measured using a haze meter (Nippon Denshoku Kogyotk Co., Ltd., NDH 5000W).

(3) interference stains

A black tape was attached to the back of the film sample, and it was visually observed whether or not interference stains were generated.

- None: No rainbow stains or colors were observed in any direction.

- Amblyopia: Light interference stains are observed in some directions.

- Kang Si-in: interference stains are clearly observed.

The results are shown in Table 3 below.

division base layer
In-plane phase difference
(nm) primer layer
thickness (nm) electric light
transmittance
(%) Primer layer/
base layer
refractive index ratio Interference
stain Example B1 350 80 92.5 0.89 radish Example B2 268 40 92.8 0.95 radish Example B3 289 85 93.8 0.92 radish Example B4 148 100 92.1 0.98 radish Example B5 541 28 93.0 0.97 radish Example B6 489 48 93.7 0.89 radish Comparative Example B1 156 54 91.2 0.74 Poet Kang Comparative Example B2 652 26 92.4 1.0 amblyopia

As shown in Table 3, the protective film according to Example had excellent transmittance and no interference stains were observed, whereas the protective film according to Comparative Example showed interference up-look.

Preparation and evaluation of protective film (Example C1)

Step (1) Coating of the primer layer

A primer layer was coated on the polyester base layer in the same manner as before.

Step (2) Formation of hard coating layer

A hard coating layer composition was prepared by dissolving pentaerythritol triacrylate (PETA) in a methyl isobutyl ketone (MIBK) solvent at a concentration of 30% by weight and adding 5% by weight of a photopolymerization initiator (Irg184, BASF) based on the solid content. did The hard coat layer composition was coated on the primer layer with a bar coater so that the film thickness after drying was 5 μm to form a coating film. The formed coating film was dried at 80° C. for 1 minute to remove the solvent and fixed by irradiation of about 300 mJ/cm 2 of ultraviolet rays to form a hard coating layer.

Step (3) Formation of the first refractive layer

On the hard coating layer, a first refractive layer was formed using a composition including a multifunctional acrylate resin and titanium oxide (TiO ₂ ). After coating the composition on the hard coating layer, it was dried at 80° C. for 1 minute, and ultraviolet rays of about 650 mJ/cm 2 were irradiated under nitrogen purging conditions to form a first refractive layer. At this time, the composition contained about 40% by weight of solids and had a viscosity of about 6.5 mPa·s at 25°C.

Step (4) Formation of the second refractive layer

On the first refractive layer, a second refractive layer was formed using a resin containing fine silica particles.

The refractive index and thickness of each layer of the above-prepared protective film (Example C1) are summarized in Table 4 below.

floor name refractive index thickness base layer 1.66 40 μm primer layer 1.60 85 nm hard coating layer 1.52 2.5 μm first refractive layer 1.75 60 nm second refractive layer 1.29 100 nm

The prepared protective film was tested as follows.

(1) Reflectance measurement

Reflectance was measured for the protective film prepared in Example C1.

After attaching a black adhesive tape (Nitto Co.) on glass having little light absorption and attaching an optical multilayer film sample thereon, the reflectance was measured using a spectrophotometer (U-4100, Hitachi Co.).

As a result, the reflectance for light having a wavelength of 550 nm was measured to be 5% or less.

Preparation and Evaluation of Laminates (Example D1)

A primer layer was formed on the polyester films of Examples A1 to A4 and Comparative Examples A1 and A2 in the same manner as before to obtain respective protective films.

The obtained protective film was laminated with a polyimide film (TPI, SKC Co.) through an optically transparent adhesive (OCA 8146-x, 3M Co.) to prepare laminates, respectively. At this time, the modulus of the polyimide film was about 6.5 GPa in the MD and TD directions, respectively, at about 25°C.

In addition, folding evaluation and whitening evaluation were performed on the laminates prepared as described above, and as a result, the laminates including the polyester films of Examples A1 to A4 passed the folding evaluation and whitening evaluation, but Comparative Example A1 and the polyester film of A2 did not pass the folding evaluation and whitening evaluation.

1: In-folding type flexible display device,
2: out-folding type flexible display device,
10: laminate, 100: protective film,
110: base layer, 120: primer layer,
130: hard coating layer, 140: first refractive layer,
150: second refraction layer, 200: transparent cover,
300: flexible display device body,
a, b: folding points.

Claims (10)

delete delete A base layer comprising a polyester resin, and
Including a primer layer disposed on the substrate layer,
The reflectance for light with a wavelength of 550 nm is 5% or less,
The strain until whitening of the base layer occurs is 10% or more,
When the base layer defines a first direction and a second direction perpendicular to each other in plane, the difference between the modulus (m1) for the first direction and the modulus (m2) for the second direction (|m1-m2|) is less than or equal to 0.3 GPa at 25°C;
Both the modulus (m1) and the modulus (m2) at 25 ° C are 4 GPa or less,
Both the modulus (m1) and the modulus (m2) at 85 ° C are 2.5 GPa or less,
The number of repetitions of folding at an angle of 135 ° until the base layer is broken is 100,000 or more,
A protective film for a foldable display that satisfies the following formula (1):
0.7 ≤ n2/n1 ≤ 1.0 (1)
In Equation (1), n1 and n2 mean the refractive indices of the base layer and the primer layer, respectively. According to claim 3,
When the base layer defines a first direction and a second direction perpendicular to each other in-plane, the ratio (m2/m1) of the modulus (m1) in the first direction to the modulus (m2) in the second direction is 25 0.7 to 1.3 at ° C,
The ratio (s2 / s1) of the thermal contraction rate (s2) in the second direction perpendicular to the first direction compared to the thermal contraction rate (s1) in the first direction under the condition of 85 ° C. and 24 hours of the base layer is 1 to 10, Protective film for foldable displays. According to claim 3,
The protective film for a foldable display, wherein the base layer has an in-plane retardation (Ro) of 600 nm or less and a thickness direction retardation (Rth) of 4,000 nm or more. According to claim 3,
A protective film for a foldable display, wherein the protective film further satisfies the following formula (2):
0 ≤ n1-n2 ≤ 0.10 (2)
In Equation (2), n1 and n2 mean the refractive indices of the base layer and the primer layer, respectively. According to claim 3,
the protective film
Further comprising a hard coating layer disposed on the primer layer,
A protective film for a foldable display that further satisfies the following formulas (3) to (5):
n3 < n2 < n1 (3)
0.05 ≤ n1-n3 ≤ 0.20 (4)
0 < n2-n3 ≤ 0.13 (5)
In Equations (3) to (5), n1, n2, and n3 mean refractive indices of the base layer, the primer layer, and the hard coating layer, respectively. According to claim 7,
the protective film
A first refractive layer disposed on the hard coating layer, and
Further comprising a second refractive layer disposed on the first refractive layer,
A protective film for a foldable display that further satisfies the following formulas (6) and (7):
n5 < n3 < n2 < n1 < n4 (6)
0.25 ≤ n4-n5 ≤ 0.60 (7)
In Equations (6) and (7), n1, n2, n3, n4, and n5 denote refractive indices of the base layer, the primer layer, the hard coating layer, the first refractive layer, and the second refractive layer, respectively. a transparent cover for a foldable display device; and
A protective film comprising a base layer disposed on the transparent cover and containing a polyester resin, and a primer layer disposed on the base layer,
The reflectance of the protective film for light having a wavelength of 550 nm is 5% or less,
The transparent cover and the base layer have a strain of 10% or more until whitening occurs,
When the base layer defines a first direction and a second direction perpendicular to each other in plane, the difference between the modulus (m1) for the first direction and the modulus (m2) for the second direction (|m1-m2|) is less than or equal to 0.3 GPa at 25°C;
Both the modulus (m1) and the modulus (m2) at 25 ° C are 4 GPa or less,
Both the modulus (m1) and the modulus (m2) at 85 ° C are 2.5 GPa or less,
The number of repetitions of folding at an angle of 135 ° until the base layer is broken is 100,000 or more,
A laminate that satisfies the following formula (1):
0.7 ≤ n2/n1 ≤ 1.0 (1)
In Equation (1), n1 and n2 mean the refractive indices of the base layer and the primer layer, respectively. delete

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