TW202313785A - Composition for forming polyimide film for cover window, process for preparing the same and uses thereof - Google Patents

Abstract

Provided is a composition for forming a polyimide film for a cover window capable of satisfying a high performance required for the cover window, a process for preparing the same, and a use thereof. According to an embodiment, the composition may be usefully utilized in flexible display devices of various aspects providing a colorless and transparent film for a cover window that has excellent visibility without optical mura and deterioration in optical properties, and has excellent heat resistance and mechanical properties for use in optical applications.

Description

Composition for forming polyimide film for covering windows, preparation method of the composition and application thereof

The following invention relates to a composition for forming a polyimide film for covering windows, a method for preparing the composition and its use.

The display device includes a cover window for the display device on the display panel to protect the display panel from scratches or external impact. Recently, flexible bent (bent) or curved (curved) flexible display panels have become the focus of attention as next-generation displays, and polymer materials that can impart flexibility as cover window materials for next-generation displays are attracting attention. focus on. Among them, polyimide (PI), a polymer that is easy to synthesize and has excellent heat resistance and chemical resistance, is mainly used.

However, in order for the polyimide film to be applied to a display device, it is necessary to improve inherent yellowness index properties and impart colorless and transparent properties. Furthermore, in order for the polyimide film to be applied to a foldable display device or a flexible display device, it must be accompanied by improved mechanical properties, and thus the required properties of cover window materials for display devices are gradually becoming higher.

For this purpose, research has been continued to reduce the CTC effect by combining or changing monomers having various structures, but there is still a limitation that residual yellow color occurs and the yellowness index increases with film thickness. Furthermore, since the optical and mechanical properties have a trade-off with each other, such attempts inevitably lead to the very general result that although the optical properties of the polyimide are improved, the functionality is reduced or the mechanical properties are reduced. Performance drops.

One embodiment aims to provide a composition for forming a polyimide film for covering windows with high performance required for covering windows, and a polyimide film for covering windows prepared therefrom.

Specifically, an embodiment can provide a polyimide film for covering windows, which can achieve excellent mechanical properties and is colorless and transparent at the same time.

Another embodiment aims to provide a method for manufacturing a composition for forming a polyimide film for covering windows to achieve the above physical properties, and a method for preparing the polyimide film for covering windows.

Another embodiment aims to provide a multilayer structure including the polyimide film for covering windows.

Yet another embodiment aims to provide a cover window made of a new material that can replace tempered glass and the like, and a flexible display device including the cover window.

[化學式2]

[式1] 7,000 ≤ V _PI≤ 20,000 其中， V _PI是以該用於形成覆蓋窗用聚醯亞胺膜的組合物的總重量計，固含量為15重量%時，該用於形成覆蓋窗用聚醯亞胺膜的組合物的黏度，並且黏度是在25℃下利用布式（Brookfield）旋轉黏度計使用52Z轉子基於80%的扭矩持續2分鐘而測得的黏度（單位：cp）。 In a general scheme, a composition for forming a polyimide film for covering windows comprises: polyamic acid or polyimide comprising a structural unit derived from a dianhydride and a structural unit derived from a diamine; and a mixed solvent of an amide-based solvent and a hydrocarbon-based solvent, and satisfy the following formula 1, wherein the structural unit derived from a dianhydride includes a structural unit derived from a compound represented by the following chemical formula 1, and the structural unit derived from a diamine includes a structural unit derived from A structural unit of a compound represented by the following chemical formula 2: [chemical formula 1]

[chemical formula 2]

[Formula 1] 7,000 ≤ V _PI ≤ 20,000 Wherein, V _PI is based on the total weight of the composition for forming a polyimide film for covering windows, and when the solid content is 15% by weight, the polyimide film for forming a covering window The viscosity of the composition with the polyimide film was measured at 25° C. using a Brookfield rotational viscometer using a 52Z rotor based on 80% torque for 2 minutes (unit: cp).

。 The structural unit derived from dianhydride may further include a structural unit derived from a compound represented by the following Chemical Formula 3: [Chemical Formula 3]

.

The content of the structural unit derived from the compound represented by Chemical Formula 3 may be 10 mol% to 50 mol% based on 100 mol% of the structural unit derived from the dianhydride.

The amide-based solvent may contain dimethylacrylamide.

The hydrocarbon-based solvent may be a cyclic hydrocarbon-based solvent.

The cyclic hydrocarbon solvent can be toluene, benzene, cyclohexane or a combination of the foregoing.

The composition for forming a polyimide film for covering windows may include an amide-based solvent and a hydrocarbon-based solvent in a weight ratio of 8:2 to 5:5.

The solid content of the composition for forming a polyimide film for covering windows may be 10% by weight to 40% by weight based on the total weight of the composition for forming a polyimide film.

[化學式2]

[式1] 7,000 ≤ V _PI≤ 20,000 其中V _PI與上述式1中的定義相同。 In another general aspect, a method for preparing a composition for forming a polyimide film for covering windows includes: (A) by making a compound represented by the following Chemical Formula 1 and a compound represented by the following Chemical Formula 2 in an acyl react in an amine solvent to prepare a polyamic acid solution; and (B) adjust the viscosity by additionally adding a hydrocarbon solvent to satisfy the following formula 1: [chemical formula 1]

[chemical formula 2]

[Formula 1] 7,000 ≤ V _PI ≤ 20,000 wherein V _PI is as defined in Formula 1 above.

。 The step (A) can be reacted by further including a compound represented by the following chemical formula 3: [chemical formula 3]

.

Step (B) may include: based on the weight of the amide-based solvent described in step (A), additionally adding 25% by weight to 100% by weight of a hydrocarbon-based solvent, and stirring the resulting mixture; and additionally adding an amide-based solvent containing A mixed solvent with a hydrocarbon solvent satisfies Formula 1.

In another general aspect, a polyimide film for covering windows is obtained by curing the composition described above for forming a polyimide film for covering windows.

The polyimide film for covering windows can have a thickness of 30 micrometers to 500 micrometers, an absolute value of retardation (Rth) in the thickness direction at a wavelength of 550 nanometers can be 1,000 nanometers or less, and according to ASTM E131 The yellowness index (YI) can be 5 or less.

The polyimide film for window covering may have a modulus of 3 GPa or more according to ASTM E111, a tensile strength of 100 MPa or more, and an elongation of 10% or more.

In another general aspect, a method for preparing a polyimide film for covering a window includes coating a composition for forming a polyimide film for covering a window on a substrate, followed by heat treatment.

The heat treatment may be performed by heating the composition for forming a polyimide film for covering windows at 280°C to 350°C.

In another general aspect, the multilayer structure includes the polyimide film for cover windows as described above on one surface of the substrate.

In another general aspect, a cover window for a display device includes: the polyimide film for covering the window as described above; and a coating layer formed on the polyimide film.

The coating can be a hard coat, an antistatic layer, an anti-fingerprint layer, an anti-fouling layer, an anti-scratch layer, a low-refractive layer, an anti-reflection layer, an impact-absorbing layer, or a combination of any of the foregoing.

Hereinafter, implementation aspects of the present invention will be described in detail so that those skilled in the art to which the present invention pertains can easily implement. However, the present invention can be implemented in various forms and is not limited to the embodiments described herein. Furthermore, the embodiments are not intended to limit the scope of protection defined by the claims.

In addition, unless otherwise defined, technical terms and scientific terms used herein have the ordinary meanings understood by those skilled in the art to which the present invention belongs, and known functions and terms that unnecessarily obscure the gist of the present invention will be omitted in the following description. A description of the configuration.

Throughout this specification, unless expressly stated to the contrary, "comprising" any component will be understood as implying the further inclusion of other elements, not excluding them.

Hereinafter, when an element such as a layer, film, film, region, or plate is referred to as being "on" another element, it can be directly on the other element or it can be between them, unless otherwise defined herein. On another component while inserting another component.

Hereinafter, unless otherwise defined, the term "a combination thereof" may mean mixing or copolymerization of components.

Hereinafter, unless otherwise defined herein, "A and/or B" may refer to a solution including both A and B, or may refer to a solution selected from A and B

Unless otherwise defined herein, "polymer" includes oligomers, and includes homopolymers and copolymers. Copolymers include alternating copolymers, block copolymers, random copolymers, branched copolymers, crosslinked copolymers, or all of the foregoing.

Hereinafter, unless otherwise defined herein, "polyamic acid" may refer to a polymer comprising a structural unit having an amic acid moiety, and "polyimide" may refer to a polymer comprising a structural unit having an imide moiety. polymer.

In the following, unless otherwise defined herein, the "mura phenomenon" may be interpreted as encompassing all distortions that may be caused by light at certain angles. For example, in a display device including a polyimide film, distortions caused by light may be mentioned such as a blackout phenomenon in which the screen appears black, a hot spot phenomenon or having The rainbow phenomenon of rainbow streaks.

Hereinafter, a composition for forming a polyimide film for covering windows according to an embodiment will be described.

In order to apply polyimide films to display devices, many attempts have been made to improve optical properties and mechanical properties by combining or changing monomers having various structures. However, since the mechanical properties and the optical properties have a trade-off relationship with each other, even if the optical properties are improved, such attempts are not only insufficient, but have the very general result of a decrease in the mechanical properties. Therefore, new attempts capable of simultaneously imparting excellent mechanical properties and optical properties are required.

The composition for forming a polyimide film for covering a window according to an embodiment (hereinafter also referred to as a polyimide film forming composition for covering a window) can be provided by applying a nonpolar solvent while having an improved A polyimide film with excellent optical properties and improved mechanical properties, non-polar solvents cannot be used as polymerization solvents for polyamide acid (hereinafter also referred to as polyimide precursor) and/or polyimide, and Has no affinity for polyimides.

Specifically, the composition for forming a polyimide film for covering windows according to an embodiment can provide a yellow index having a remarkably improved yellowness index without degrading mechanical properties, and remarkably reduce distortion caused by light (distortion). Polyimide membrane. Therefore, the polyimide film prepared from the composition for forming a polyimide film for covering windows according to one embodiment can be applied to a new substrate material applicable to a foldable display device or a flexible display device or The window material is covered, and the polyimide film can have excellent visibility, thereby minimizing user's eyestrain.

The composition for forming a polyimide film for covering windows according to an embodiment may include polyamic acid and/or polyimide; a polar solvent; and a non-polar solvent. The polar solvent may be a hydrophilic solvent, may have affinity with, for example, polyamide acid and/or polyimide, and may be, for example, an amide-based solvent. In addition, the nonpolar solvent may have little affinity with polyamic acid and/or polyimide, and may be, for example, a hydrocarbon-based solvent.

A composition for forming a polyimide film for covering windows according to an embodiment includes: polyamic acid or polyimide including a structural unit derived from a dianhydride and a structural unit derived from a diamine; and A mixed solvent of an amide-based solvent and a hydrocarbon-based solvent, the structural unit derived from a dianhydride includes a structural unit derived from a compound represented by the following Chemical Formula 1, and the structural unit derived from a diamine includes a structural unit derived from a compound represented by the following Chemical Formula 2 Structural units.

[化學式2]

[式1] 7,000 ≤ V _PI≤ 20,000 其中， V _PI是以用於形成覆蓋窗用聚醯亞胺膜的組合物的總重量計，固含量為15重量%時，用於形成覆蓋窗用聚醯亞胺膜的組合物的黏度，並且黏度是在25℃下用布式（Brookfield）旋轉黏度計使用52Z轉子基於80%的扭矩持續2分鐘而測得的黏度（單位：cp）。 In addition, the composition for forming a polyimide film for covering windows according to one embodiment may satisfy Formula 1 below. While not wishing to be bound by a particular theory, a composition for forming a polyimide film for covering windows that satisfies these conditions can suppress the bulk density of the polyimide film during curing and render the film amorphous (amorphous), thereby improving optical performance. [chemical formula 1]

[chemical formula 2]

[Formula 1] 7,000 ≤ V _PI ≤ 20,000 wherein, V _PI is based on the total weight of the composition for forming a polyimide film for covering windows, and when the solid content is 15% by weight, The viscosity of the composition of the imide film, and the viscosity is a viscosity (unit: cp) measured at 25° C. with a Brookfield rotational viscometer using a 52Z rotor based on 80% torque for 2 minutes.

The composition for forming a polyimide film for covering a window according to one embodiment has the monomer combination and the mixed solvent composition as described above, and thus can provide a covering window excellent in colorless and transparent properties, optical properties, and mechanical properties Use a polyimide membrane. Specifically, the polyimide film for a cover window according to one embodiment may be colorless and transparent and have significantly low retardation, thereby effectively suppressing the mottled phenomenon and the rainbow phenomenon while achieving excellent mechanical properties.

。 The structural unit derived from dianhydride according to one embodiment may further include a structural unit derived from a compound represented by the following Chemical Formula 3: [Chemical Formula 3]

.

Based on 100 mol% of the structural units derived from dianhydride, according to an embodiment, the content of the structural units derived from the compound represented by Chemical Formula 3 may be 10 mol% to 50 mol%, specifically 20 mol% % to 40 mol%, and more specifically 20 mol% to 30 mol%. Since the content of the structural unit derived from the compound represented by Chemical Formula 3 is within the above-mentioned range, it is possible to provide a polyimide film having improved optical properties such as lower retardation in the thickness direction without lowering mechanical properties .

The composition for forming a polyimide film for covering windows according to one embodiment can effectively suppress intermolecular interactions between polymers by using a mixed solvent including an amide-based solvent and a hydrocarbon-based solvent. interactions and/or interactions between polymer and solvent. Furthermore, during curing, the intermolecular packing density is significantly reduced, so that both optical and mechanical properties can be improved.

In addition, by using a mixed solvent, the composition for forming a polyimide film for covering windows can have a reduced viscosity of the composition while having a high solid content. Therefore, the composition for forming a polyimide film for covering windows according to an embodiment may have a high solid content and a low viscosity, thereby easily forming a film by a solution process, and providing both excellent mechanical properties and Polyimide film with excellent optical properties.

Specifically, the viscosity (V _PI ) of the composition for forming a polyimide film for covering windows according to an embodiment may be 7,000 cp to 15,000 cp, or 8,000 cp to 15,000 cp, or 9,000 cp to 15,000 cp . Here, the viscosity is the viscosity when the solid content is 15% by weight based on the total weight of the composition for forming the polyimide film for covering windows. In other words, a composition for forming a polyimide film for covering windows comprising a high solid content can be more easily applied to a thin film process, and can provide a polyimide film having both more excellent optical properties and more excellent mechanical properties. Amine film. Here, the solid content may be polyamic acid and/or polyimide.

In one embodiment, the amide-based solvent refers to a compound containing an amide moiety. The amide-based solvent may be a cyclic compound or a chain compound, and is specifically a chain compound. Specifically, the chain compound may have 2 to 15 carbon atoms, and more specifically, may have 3 to 10 carbon atoms.

The amide-based solvent may include N,N-dialkylamide moieties, each dialkyl group may exist independently, may be fused to each other to form a ring, or at least one of the dialkyl groups may be Fused with another substituent in the molecule to form a ring, for example, at least one alkyl group in a dialkyl group may be fused to an alkyl group attached to the carbonyl carbon of the amide moiety to form a ring . Here, the ring may be a 4-membered ring to a 7-membered ring, for example a 5-membered ring to a 7-membered ring, for example a 5-membered ring or a 6-membered ring. The alkyl group may be, for example, a C _1-10 alkyl group, such as a C _1-8 alkyl group, such as methyl or ethyl.

More specifically, the amide-based solvent is not limited as long as it is generally used for the polymerization of polyamide acid, but may include, for example, dimethylacrylamide, diethylacrylamide, dimethylacetamide , diethylacetamide, dimethylformamide, methylpyrrolidone, ethylpyrrolidone, octylpyrrolidone or a combination of the foregoing, and specifically may include dimethylacrylamide.

In one embodiment, the hydrocarbon-based solvent may be a non-polar molecule as described above. The hydrocarbon-based solvent may be a compound composed of carbon and hydrogen. For example, the hydrocarbon-based solvent may be aromatic or aliphatic, and may be, for example, a cyclic compound or a chain compound, but specifically, may be a cyclic compound. Here, when the hydrocarbon-based solvent is a cyclic compound, the hydrocarbon-based solvent may include a single ring or polycyclic rings, and the polycyclic rings may be condensed or non-condensed rings, but specifically may be monocyclic.

The hydrocarbon-based solvent may have 3 to 15 carbon atoms, such as 6 to 15 carbon atoms, such as 6 to 12 carbon atoms. The hydrocarbon solvent may be a substituted or unsubstituted C _3-15 cycloalkane, a substituted or unsubstituted C _6-15 aromatic compound, or a combination of the foregoing. Here, the cycloalkane may include cyclobutane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, or a combination of the foregoing, and the aromatic compound may include benzene, naphthalene, or a combination of the foregoing. The hydrocarbon solvent can be unsubstituted or substituted by at least one C _1-5 alkyl cycloalkane, substituted or unsubstituted aromatic compound by at least one C _1-5 alkyl, or a combination of the foregoing, wherein the ring The alkanes and aromatic compounds are respectively as described above. The C _1-5 alkyl group may be, for example, a C _1-3 alkyl group, such as a C _1-2 alkyl group, and more specifically, may be a methyl group, but the present invention is not limited thereto. In addition, the hydrocarbon-based solvent may further include oxygen, if necessary. For example, when the hydrocarbon-based solvent contains oxygen, the hydrocarbon-based solvent may include a ketone group or a hydroxyl group, such as cyclopentanone, cresol, or a combination of the foregoing. Specifically, the hydrocarbon solvent may include but not limited to benzene, toluene, cyclohexane, cyclopentanone, cresol or a combination of the foregoing.

More specifically, the composition for forming a polyimide film for covering windows according to an embodiment may include a mixed solvent including an amide-based solvent containing dimethylacrylamide and a solvent selected from toluene, Hydrocarbon solvent for benzene and cyclohexane.

In addition, the hydrocarbon-based solvent and the mixed solvent according to one embodiment may be added after polymerization of polyamic acid or polyimide.

Therefore, the composition for forming a polyimide film for covering windows according to an embodiment may exhibit intermolecular behavior and interaction different from when a mixed solution is simply added in a polymerization step of polyamic acid. For example, when a hydrocarbon-based solvent is mixed in the polymerization step of polyamic acid, it acts as a factor that hinders polymerization, so high-molecular-weight polyamic acid may not be obtained. Meanwhile, in the composition for forming a polyimide film for covering windows according to one embodiment, after obtaining polyamic acid and/or polyimide having a sufficiently high molecular weight, a hydrocarbon-based solvent is mixed, The hydrocarbon-based solvent can be used as a catalyst to weaken the intermolecular interaction between polymers and/or the strong interaction between the polymer and the solvent, and desired optical properties can be obtained in the subsequent curing process. Here, the interaction between polyamic acid, which is a polyimide precursor, and the solvent can be adjusted to a more appropriate range by sequentially using an amide-based solvent and a hydrocarbon-based solvent. Here, adjusting may mean suppressing.

The composition for forming a polyimide film for covering windows according to an embodiment may include an amide-based solvent and a hydrocarbon-based solvent in a weight ratio of 8:2 to 5:5, specifically, a weight ratio of 7.5:2.5 to 5:5 or 7.5:2.5 to 5.5:4.5. By including the amide-based solvent and the hydrocarbon-based solvent in the above weight ratio, more excellent optical properties can be achieved, and excellent reactivity of diamine and dianhydride can be maintained. In addition, when the composition for forming a polyimide film for covering windows is cured, the composition can more properly suppress intermolecular packing density and make the film amorphous. Therefore, even at a thickness of 30 micrometers or more, it is possible to provide a polyimide film for a cover window having further improved optical properties.

The composition for forming a polyimide film for covering windows according to an embodiment may provide a film having more excellent mechanical properties and optical properties (eg, yellowness index) by including the diamine represented by Chemical Formula 2. In addition, diamines can be used in combination with one or two or more selected from the following groups, if necessary: p-phenylenediamine (PDA), m-phenylenediamine (m-PDA), 4,4'-diamine Diphenyl ether (4,4'-ODA), 3,4'-diaminodiphenyl ether (3,4'-ODA), 2,2-bis(4-[4-aminophenoxy] -phenyl)propane (BAPP), 1,4-bis(4-aminophenoxy)benzene (TPE-Q), 1,3-bis(4-aminophenoxy)benzene (TPE-R) , 4,4'-bis(4-aminophenoxy)biphenyl (BAPB), bis(4-[4-aminophenoxy]phenyl)pyridine (BAPS), bis(4-[3- Aminophenoxy]phenyl) (m-BAPS), 3,3'-dihydroxy-4,4'-diaminobiphenyl (HAB), 3,3'-dimethylbenzidine (TB ), 2,2'-dimethylbenzidine (m-TB), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB), 2,2'-bis (Trifluoromethyl)-4,4'-diaminodiphenyl ether (6FODA), 1,3-bis(3-aminophenoxy)benzene (APB), 1,4-naphthalene diamine (1 ,4-ND), 1,5-naphthalene diamine (1,5-ND), 4,4'-diaminobenzamide aniline (DABA), 6-amino-2-(4-aminobenzene base) benzoxazole and 5-amino-2-(4-aminophenyl) benzoxazole and the like, but the present invention is not limited thereto.

The composition for forming a polyimide film for covering windows according to an embodiment can provide a polyimide film having improved mechanical strength and optical properties of the film.

In addition, if necessary, the dianhydride may further include pyromelite dianhydride (PMDA), 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), 4,4'-oxodi Phthalic anhydride (ODPA), 4,4'-(4,4'-isopropyldiphenoxy)diphthalic anhydride (BPADA), 3,3',4,4'-diphenyl Diphenylene tetracarboxylic dianhydride (DSDA), p-phenylene bis(trimellitate dianhydride) (TMHQ), 2,2-bis(4-hydroxyphenyl)propane dibenzoate-3, 3',4,4'-tetracarboxylic dianhydride (ESDA), naphthalenetetracarboxylic dianhydride (NTDA) or a combination of the foregoing.

The composition for forming a polyimide film for covering windows according to one embodiment may include polyamic acid and/or Polyimide.

The weight-average molecular weight (Mw) of polyamic acid and/or polyimide is not particularly limited, but it can be above 10,000 g/mol, such as above 20,000 g/mol, such as 25,000 g/mol to 100,000 g/mol mole. Also, the glass transition temperature is not limited, but may be 100°C to 400°C, more specifically, 100°C to 380°C. Within the above range, it is preferable to provide a film having more excellent optical properties, more excellent mechanical strength, and less curl, but the present invention is not necessarily limited thereto.

Based on the total weight of the composition for forming a polyimide film for covering windows, the solid content of the composition for forming a polyimide film for covering windows according to one embodiment may satisfy 10% by weight to 40% by weight. % by weight, or a range of 10% by weight to 35% by weight or less, or 10% by weight to 20% by weight. Here, the solid content may be polyamic acid and/or polyimide.

In general, in the case of polyimides, the higher the concentration of solids content, the higher the viscosity tends to be. For example, when the above-mentioned polyamic acid and/or polyimide is dissolved alone in a traditional amide-based solvent, the viscosity of the solution may be above 20,000 cp, or above 30,000 cp. Here, the viscosity of the solution refers to the viscosity when the solid content is 15% by weight based on the total weight of the solution. When a film is produced by a solution process such as a coating process, if the fluidity of the polymer is poor due to high viscosity, it is difficult to remove air bubbles and streaks may occur during the coating process.

On the other hand, the composition for forming a polyimide film for covering windows according to one embodiment can significantly reduce the viscosity of the composition by using a mixed solvent of an amide-based solvent and a hydrocarbon-based solvent, even if the composition Contains a high solids content above 15% by weight. Therefore, the composition can be applied to a thin film coating process with high solid content and low viscosity, can improve process efficiency, and can achieve desired physical properties more effectively.

Hereinafter, a method of preparing a composition for forming a polyimide film for covering windows according to an embodiment will be described.

According to one embodiment, the method for preparing a composition for forming a polyimide film for covering windows may include: (A) by making the compound represented by Chemical Formula 1 and the compound represented by Chemical Formula 2 in an amide solvent and (B) adjusting the viscosity by additionally adding a hydrocarbon solvent to satisfy Formula 1.

The step (A) according to one embodiment may be reacted by further including the compound represented by Chemical Formula 3.

Specifically, step (A) may include: polymerizing polyamic acid by mixing diamine and dianhydride in an equivalent ratio of 1:1 to 1:1.1, wherein the diamine is dissolved in an amide-based solvent; Dianhydride is added to the solution for dissolution; and the reaction solution is stirred for 5 to 7 hours to allow them to react.

According to the step (B) of an embodiment, the above-mentioned hydrocarbon-based solvent can be additionally added and stirred, and then a mixed solvent including an amide-based solvent and a hydrocarbon-based solvent can be additionally added, thereby making it possible to form a polyimide film for covering windows. The viscosity range of the composition can satisfy formula 1.

Specifically, step (B) according to an implementation aspect may include: at room temperature (25°C), based on the weight of the amide-based solvent in step (A), additionally adding 20% by weight to 100% by weight or 20 % by weight to 50% by weight of a hydrocarbon-based solvent, and stirring for 15 to 20 hours; and after the stirring is completed, a mixed solvent including an amide-based solvent and a hydrocarbon-based solvent is added to satisfy Formula 1. While not wishing to be bound by a particular theory, a composition for forming a polyimide film for covering windows that satisfies these conditions can suppress the bulk density of the polyimide film during curing and make the film amorphous quality. Accordingly, a polyimide film having further improved yellowness index and thickness-direction retardation without lowering mechanical properties can be provided.

In addition, the composition for forming a polyimide film for covering windows according to an embodiment may exhibit intermolecular behavior and interaction different from when a mixed solution is simply added in a polymerization step of polyamic acid. For example, when a hydrocarbon-based solvent is included in the polymerization step of polyamic acid, it acts as a factor hindering polymerization, and thus high-molecular-weight polyamic acid may not be obtained.

On the other hand, after obtaining polyamic acid and/or polyimide having a sufficiently high molecular weight, a hydrocarbon system may be mixed in the composition for forming a polyimide film for covering windows according to one embodiment. Solvent, so as to obtain high molecular weight polyamic acid. In addition, hydrocarbon-based solvents can act as catalysts to weaken the intermolecular interactions between polymers and/or the strong interactions between polymers and solvents, so that desired optical properties can be obtained in the subsequent curing process. In general, optical properties such as retardation in the thickness direction (Rth) and yellowness index and mechanical properties such as modulus have a trade-off relationship with each other, and it is difficult to simultaneously improve these properties. However, according to one embodiment, these physical properties can be simultaneously improved even at thicknesses above 30 microns.

In addition, another embodiment may be a polyimide film for covering a window manufactured from a composition for forming a polyimide film for covering a window.

According to an embodiment, the polyimide film for covering the window may have a thickness of 30 microns to 500 microns, such as 40 microns to 300 microns, such as 50 microns to 200 microns. In addition, the absolute value of retardation (Rth) in the thickness direction at a wavelength of 550 nm may be 200 nm to 1000 nm, such as 300 nm to 1000 nm, such as 500 nm to 1000 nm. In addition, the yellowness index (YI) according to ASTM E131 may simultaneously satisfy the physical properties of 5 or less, or 4.5 or less, or 4 or less. In other words, the polyimide film for covering a window according to an embodiment can significantly reduce distortion due to light without deteriorating yellowness index even if the thickness is 30 μm or more.

In addition, the polyimide film for covering windows according to one embodiment may have a modulus according to ASTM E111 of 3 GPa or more, a tensile strength of 100 MPa or more, and an elongation of 10% or more. More specifically, the polyimide film for a cover window having the above-described retardation (Rth) in the thickness direction at a wavelength of 550 nm and a yellowness index can simultaneously satisfy such mechanical properties.

Specifically, the modulus of the polyimide film for covering windows according to ASTM E111 according to one embodiment may be 3 GPa or more, or 3.5 GPa or more, or 4 GPa to 6 GPa, and the tensile strength may be 100 MPa or more. , or 110 MPa or more, or 110 MPa to 200 Mpa, and the elongation may be 10% or more, or 11% or more, or 10% to 30%. In other words, the polyimide film according to one embodiment may achieve mechanical properties and durability sufficient to be applied to a cover window.

Since the polyimide film for a cover window according to the embodiment satisfies the retardation in the thickness direction, the yellowness index, and the modulus within the above-mentioned ranges, it is possible to prevent image distortion due to light, thereby providing further improved visibility ( visibility). In addition, it is possible to exhibit more uniform mechanical properties (modulus, etc.) and optical properties (retardation in the thickness direction, etc.) as a whole throughout the center portion and edge portion of the film, and further reduce film loss. In addition, the polyimide film is flexible and has excellent bending properties, and thus can be more easily restored to its original shape without deformation and/or damage even when predetermined deformation occurs repeatedly. In addition, the cover window including the polyimide film according to one embodiment may have better visibility, and may prevent the occurrence of folding marks and microcracks, and thus may impart excellent performance to a foldable display device or a flexible display device. Durability and long life.

Hereinafter, a method of manufacturing a polyimide film for covering a window according to an embodiment will be described.

In another general aspect, the method for manufacturing a polyimide film for covering a window according to an embodiment may include applying the composition for forming a polyimide film for covering a window according to an embodiment to a substrate , followed by heat treatment.

Specifically, coating can be used without limitation as long as it is generally used in this field. Non-limiting examples of coating may include knife coating, dip coating, roll coating, slot coating, lip slot coating, and slide coating (slide coating), etc., and for this, one or more times may be applied sequentially Coating of the same type or different types.

In addition, the substrate can be used without limitation as long as it is generally used in the art, and non-limiting examples of the substrate can include, but are not limited to, glass; stainless steel; or plastic films such as polyethylene terephthalate, Polyethylene naphthalate, polypropylene, polyethylene, cellulose triacetate, cellulose diacetate, polyalkyl(meth)acrylates, poly(meth)acrylate copolymers, polyvinyl chloride, polyethylene Alcohol, polycarbonate, polystyrene, cellophane, polyvinylidene chloride copolymer, polyamide, polyimide, vinyl chloride vinyl acetate copolymer, polytetrafluoroethylene and polytrifluoroethylene, etc.

In addition, according to one embodiment, the adhesive strength of the polyimide film for a cover window to a substrate such as glass may be 5 gf/in (gf/in) or more, or 10 gf/in or more, or 15 gf/in. / inch or more. The polyimide film according to one embodiment has a reduced intermolecular density, and thus does not cause screen distortion when the polyimide film is applied to a cover window of a flexible display.

In the method of manufacturing a polyimide film for covering windows according to one embodiment, the heat treatment in the heat treatment step may be performed at a temperature of 280° C. or more and 350° C. or less for 10 minutes to 60 minutes. When curing is performed at relatively lower temperatures, the film may receive less heat history and thus may tend to have a lower yellowness index. However, when the film is cured at or below the glass transition temperature (Tg), there may be a problem of reduced mechanical properties of the film or increased retardation in the thickness direction due to the orientation problem of the molecular structure. On the other hand, the polyimide film according to one embodiment can improve mechanical properties and can more isotropically align the polymerized material even if it is heat-treated at a temperature of 280°C to 350°C or 300°C to 350°C. material chain, and can further reduce the delay in the thickness direction. In other words, the polyimide film according to one embodiment may simultaneously achieve excellent yellowness index, retardation, and mechanical properties. In addition, heat treatment may be performed, for example, for 10 minutes to 50 minutes, 10 minutes to 40 minutes, 10 minutes to 30 minutes, or 10 minutes to 20 minutes, but the present invention is not necessarily limited thereto. In addition, thermal curing may be performed, for example, in a separate vacuum oven, an oven filled with an inert gas, or the like.

In addition, before the heat treatment step, a drying step may be additionally performed if necessary. The drying step may be performed at a temperature of 50°C to 150°C, 50°C to 130°C, 60°C to 100°C, or about 80°C, but the present invention is not necessarily limited thereto.

If necessary, the method of manufacturing a polyimide film for covering a window may further include coating a composition for forming a polyimide film for covering a window on a substrate, and then leaving the composition to stand at room temperature. The optical properties of the film surface can be more stably maintained by the standing step. While not wishing to be bound by a particular theory, when conventional polyimide film-forming compositions undergo this standing step prior to curing, the solvent may absorb moisture from the air, and the moisture may diffuse to in. As a result, moisture may collide with the polyamic acid and/or polyimide, causing cloudiness on the membrane surface, and clumping may occur, resulting in uneven coating. On the other hand, the composition for forming a polyimide film for covering windows according to an embodiment can achieve the following advantages: it will not be cloudy and agglomerated even if left standing in the air for a long time, and can ensure The films have enhanced optical properties. The standing step can be performed at room temperature and/or under high humidity conditions. Here, the room temperature may be below 40°C, such as below 30°C, such as below 25°C, more specifically 15°C to 25°C, particularly preferably 20°C to 25°C. Furthermore, high humidity may be, for example, a relative humidity above 50%, such as above 60%, such as above 70%, such as above 80%. The standing step may be performed for 1 minute to 3 hours, such as 10 minutes to 2 hours, such as 20 minutes to 1 hour.

In the method of manufacturing a polyimide film for covering windows according to one embodiment, the polyimide film for covering windows can be enhanced by mixing one or more kinds selected from flame retardants, Additives for additives, inorganic particles, antioxidants, UV inhibitors and plasticizers.

Hereinafter, the use of the polyimide film for covering windows according to one embodiment will be described.

In one aspect, it may be a multilayer structure including the polyimide film according to one embodiment. For example, the multilayer structure may be a cover window for a display device, the cover window including a polyimide film for the cover window; and a coating layer formed on the polyimide film. As non-limiting examples, the coating may include a hard coat, an antistatic layer, an anti-fingerprint layer, an anti-smudge layer, an anti-scratch layer, a low-refractive layer, an anti-reflection layer, an impact-absorbing layer, etc., and may include at least one or both More than one functional coating. In this case, the thickness of the coating layer may be 1 μm to 500 μm, 2 μm to 450 μm, or 2 μm to 200 μm, but the present invention is not limited thereto.

In another aspect, it may be a display device including a polyimide film for covering a window according to an embodiment.

As described above, the polyimide film according to one embodiment can have excellent optical properties and mechanical properties, in particular, can exhibit sufficiently low retardation even at various angles, and thus can be used in applications that require ensuring a wide viewing angle. various industrial fields.

For example, a display device is not particularly limited as long as the display device is a field requiring excellent optical performance, and a display panel suitable for the display device can be selected and provided. Specifically, the display device can be applied to a flexible display device, and non-limiting examples of the flexible display device can include, but are not limited to, various image display devices, such as liquid crystal display devices, electroluminescent display devices, plasma display devices, field emissive display devices, etc.

In addition, a display device including the polyimide film according to an embodiment may not only have excellent display quality, but also significantly reduce distortion due to light. In particular, the rainbow phenomenon that produces rainbow streaks can be significantly improved, and the excellent visibility can minimize user's eye fatigue. In addition, as the screen size of a display device increases, viewing of the screen from the side increases. When the polyimide film for a cover window according to one embodiment is applied to a display device, since visibility is excellent even when viewed from the side, the display device can be effectively applied to a large-sized display device.

Hereinafter, in order to describe the embodiments in detail, exemplary embodiments are described, but the present invention is not limited thereto.

In the following experiments, physical properties were measured as follows.

<Viscosity (V _PI )>

Viscosity values are measured as follows: 0.5 microliter of the composition is placed in the container, the rotor is lowered, and when the torque reaches 80%, the revolutions per minute (rpm) are adjusted for 2 minutes, after which there is no change in torque. In this case, the viscosity was measured at a temperature of 25°C using a 52Z spindle. The unit of viscosity is cp.

＜Weight average molecular weight＞

Weight average molecular weight was measured by dissolving the membrane in DMAc eluent containing 0.05 M LiCl. For gel permeation chromatography (GPC), a Waters GPC system, Waters 1515 isocratic HPLC pump, Waters 2414 refractive index detector was used. The column (column) was connected to Olexis column, Polypore column and mixed D column, using polymethyl methacrylate (PMMA STD) as a standard, and analyzed at a flow rate of 1 ml/min at 35°C.

＜Yellow Index (YI)＞

Yellowness index was measured using a spectrophotometer (Nippon Denshoku, COH-5500) according to ASTM E313 standard.

＜Delay (Rth)＞

Latency was measured using Axoscan (OPMF, Axometrics). The retardation (Rth) in the thickness direction was measured at a wavelength of 400 nm to 800 nm, and an absolute value was expressed based on a wavelength of 550 nm. Delay is in nanometers.

＜Modulus＞

The modulus was measured at 25° C. at a speed of 50 mm/min, according to ASTM E111, using UTM 3365 (Instron Corporation) to stretch a sample having a thickness of 50 μm, a length of 50 mm, and a width of 10 mm. The modulus unit is GPa.

[Example 1]

Preparation of compositions for forming polyimide films for covering windows

Put 180 grams of N,N-dimethylacrylamide (DMPA) into a reactor with nitrogen flow flowing through it, and dissolve 22.68 grams of 2,2'-bis Trifluoromethylbenzidine (TFMB). Add 16.7 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 6.5 g of 9,9-bis(3,4-dicarboxyphenyl)fluorene to it at room temperature dianhydride (BPAF), and stirred for 6 hours to prepare a polyamic acid solution. Then, 77 g of toluene was added thereto at 25° C., and the mixture was stirred for 18 hours. Thereafter, by adding a mixed solvent of DMPA: toluene = 70% by weight: 30% by weight so that the solid content is 15% by weight based on the total weight of the composition, a polymer for forming a covering window with a viscosity of 12,000 cp was prepared. Composition of imide membranes.

Manufacture of polyimide film for covering windows

The above-obtained composition for forming a polyimide film for covering windows was coated onto one surface of a glass substrate (1.0T) with a Meyer bar, and the composition was dried at 80° C. under a nitrogen stream After 30 minutes, the dried composition was heated and cured at 300° C. for 15 minutes, and then the resulting film was peeled off from the glass substrate to obtain the polyimide film for window covering of Example 1 having a thickness of 50 μm. The physical properties are shown in Table 2 below.

[Example 2 to Example 4]

Compositions for forming polyimide films for covering windows of Examples 2 to 4 were prepared in the same manner as in Example 1 except for using the monomers at the molar ratio shown in Table 1 below. In addition, polyimide films for covering windows of Examples 2 to 4 each having a thickness of 50 μm were prepared in the same manner as in Example 1. The physical properties are shown in Table 2 below.

[Example 5 and Example 6]

Prepared in the same manner as in Example 1, except adjusting the toluene content (T content) relative to the total weight of DMPA and toluene in the composition for forming a polyimide film for covering windows as shown in Table 1 below. Compositions of Examples 5 and 6 for forming polyimide films for covering windows. In addition, polyimide films for covering windows of Examples 5 to 6 each having a thickness of 50 μm were prepared in the same manner as in Example 1. The physical properties are shown in Table 2 below.

[Comparative example 1]

Preparation of compositions for forming polyimide films for covering windows

Put 211 grams of N,N-dimethylacrylamide (DMPA) into a reactor with nitrogen flow flowing through it, and dissolve 22.68 grams of 2,2'-bis Trifluoromethylbenzidine (TFMB). Add 16.7 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 6.5 g of 9,9-bis(3,4-dicarboxyphenyl)fluorene to it at room temperature dianhydride (BPAF), and stirred for 24 hours to prepare a polyamic acid solution. Then, DMPA was added at 25° C. so that the solid content was 15% by weight based on the total weight of the composition to prepare a composition for forming a polyimide film for covering windows having a viscosity of 23,000 cp.

Manufacture of polyimide film for covering windows

The above-obtained composition for forming a polyimide film for covering windows was coated onto one surface of a glass substrate (1.0 T) with a Meyer rod, and the composition was dried at 80° C. for 30 minutes under a stream of nitrogen, and the The dried composition was heated and cured at 300° C. for 15 minutes, and then the resulting film was peeled off from the glass substrate to obtain a polyimide film for window covering of Comparative Example 1 having a thickness of 50 μm. The physical properties are shown in Table 2 below.

[Comparative Example 2 and Comparative Example 3]

Prepared in the same manner as in Example 1, except adjusting the toluene content (T content) relative to the total weight of DMPA and toluene in the composition for forming a polyimide film for covering windows as shown in Table 1 below. Compositions of Comparative Example 2 and Comparative Example 3 for forming polyimide films for covering windows. In addition, polyimide films for covering windows of Comparative Example 2 to Comparative Example 3 each having a thickness of 50 μm were prepared in the same manner as in Example 1. The physical properties are shown in Table 2 below.

[Table 1] diamine Dianhydride Toluene (T) content (weight%) TFMB BPDA BPAF Example 1 0.99 80 20 30 Example 2 0.99 75 25 30 Example 3 0.99 70 30 30 Example 4 0.99 50 50 30 Example 5 0.99 80 20 25 Example 6 0.99 80 20 45 Comparative example 1 0.99 80 20 0 Comparative example 2 0.99 80 20 10 Comparative example 3 0.99 80 20 60

[Table 2] Example comparative example 1 2 3 4 5 6 1 2 3 Viscosity (cp) 12,000 13,000 15,000 14,000 13,000 9,500 23,000 21,000 unavailable Thickness (microns) 50 50 50 50 50 50 50 50 YI 3.1 3.1 3.1 3.4 3.2 3.0 3.7 3.6 Latency (at 550nm) 980 820 730 510 550 480 1110 1050 Modulus (GPa) 4.1 4.1 4.1 4.1 4.1 4.0 4.2 4.2

From Table 2, it can be confirmed that the compositions for forming a polyimide film for covering windows according to Examples 1 to 6, including a mixed solvent of an amide-based solvent and a hydrocarbon-based solvent, can have 7,000 cp to 20,000 cp viscosity, and can form a film thick enough to be used as a cover film for a flexible display. Furthermore, all polyimide films fabricated using this composition have excellent optical and mechanical properties.

On the other hand, compared with the polyimide films of Example 1 and Example 6 having the same monomer molar ratio and heat treatment temperature, the composition not including a mixed solvent containing an amide-based solvent and a hydrocarbon-based solvent The prepared polyimide film of Comparative Example 1 exhibited remarkable deterioration in yellowness index and retardation in the thickness direction.

Furthermore, the composition of Comparative Example 3 had a high initial polymeric solids content and the solution viscosity became uncontrollably high, which made it impossible to measure viscosity and polymerization, and the composition of Comparative Example 2 had a high viscosity and because the air bubbles were not removed , which is unfavorable in the process, and the surface of the coating thus prepared is not uniform. Therefore, it was confirmed that after curing, the surface of the coating was somewhat rough, evaluated as defective, and unsuitable for producing polyimide films. In addition, it was confirmed that the surface of the polyimide film-forming composition for covering windows of Comparative Example 2 was rough after coating, and thus the retardation value was significantly increased.

The composition for forming a polyimide film for covering windows according to an embodiment can inhibit the interaction between polyamic acid and mixed solvents, thereby significantly reducing the packing density between molecules during curing. Therefore, it is possible to provide a polyimide film capable of realizing colorless and transparent optical properties even within a thickness range having mechanical strength similar to that of tempered glass.

In addition, the composition for forming a polyimide film for covering windows according to an embodiment can significantly reduce the viscosity of the composition by using a mixed solvent of an amide-based solvent and a hydrocarbon-based solvent even if it contains high solid content. Accordingly, the composition for forming a polyimide film for covering windows can be applied to a thin film coating process with a high solid content and low viscosity, and desired physical properties can be effectively achieved.

Specifically, the polyimide film for a cover window according to one embodiment can ensure that the yellowness index is significantly suppressed, and has significantly low retardation in the thickness direction in the visible light region. Therefore, when the polyimide film for a cover window is used as a cover window of a display panel, the mottle phenomenon and the rainbow phenomenon which are problems of visibility are effectively suppressed, thereby increasing the display including the polyimide film for a cover window. panel reliability.

In addition, the polyimide film for covering windows according to one embodiment has excellent high-strength properties and bending properties, and can effectively prevent cracks or cracks caused by bending, and thus can be applied to various industrial fields, such as Flexible displays.

This application claims priority to Korean Patent Application No. 10-2021-0124489 filed with the Korean Intellectual Property Office on Sep. 17, 2021, the entire disclosure of which is incorporated herein by reference in its entirety.

In the above, although the embodiments have been described by way of limited examples, these limited embodiments are only provided to help a more comprehensive understanding of the present invention. Therefore, the present invention is not limited to these examples. Those skilled in the art to which the present invention pertains can make various modifications and changes based on the present description.

Therefore, the spirit of the present invention should not be limited to the above-mentioned embodiments, but the claimed scope and all modifications equal or equivalent to the claimed scope are intended to fall within the scope and spirit of the present invention.

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Claims (19)

A composition for forming a polyimide film for covering windows, comprising: polyamic acid or polyimide comprising a structural unit derived from a dianhydride and a structural unit derived from a diamine; and an amide-based A mixed solvent of a solvent and a hydrocarbon-based solvent, wherein the composition satisfies the following formula 1, and the structural unit derived from dianhydride includes a structural unit derived from a compound represented by the following chemical formula 1, the structural unit derived from diamine includes a structural unit derived from A structural unit of a compound represented by the following chemical formula 2: [chemical formula 1]

[chemical formula 2]

[Formula 1] 7,000 ≤ V _PI ≤ 20,000 Wherein, V _PI is based on the total weight of the composition for forming a polyimide film for covering windows, and when the solid content is 15% by weight, the polyimide film for forming a covering window The viscosity of the composition with the polyimide film, and the viscosity is the viscosity measured at 25° C. using a Brookfield rotational viscometer using a 52Z rotor based on 80% torque for 2 minutes, and the unit is cp. The composition according to claim 1, wherein the structural unit derived from dianhydride further comprises a structural unit derived from a compound represented by the following chemical formula 3: [chemical formula 3]

. The composition as claimed in claim 2, wherein based on 100 mol% of the structural unit derived from dianhydride, the content of the structural unit derived from the compound represented by Chemical Formula 3 is 10 mol% to 50 mol%. . The composition as claimed in item 1, wherein the amide-based solvent comprises dimethylacrylamide. The composition according to claim 1, wherein the hydrocarbon solvent is a cyclic hydrocarbon solvent. The composition as claimed in item 5, wherein the cyclic hydrocarbon solvent is toluene, benzene, cyclohexane or a combination of the foregoing. The composition according to claim 1, wherein the composition for forming a polyimide film for covering windows comprises the amide-based solvent and the hydrocarbon-based solvent in a weight ratio of 8:2 to 5:5. The composition as claimed in item 1, wherein the total weight of the composition for forming a polyimide film, the solid content of the composition for forming a polyimide film for covering windows is 10% by weight % to 40% by weight. A method for preparing a composition for forming a polyimide film for covering windows, the method comprising: (A) by making a compound represented by the following Chemical Formula 1 and a compound represented by the following Chemical Formula 2 in an amide system react in a solvent to prepare a polyamic acid solution; and (B) adjust the viscosity by additionally adding a hydrocarbon solvent to satisfy the following formula 1: [chemical formula 1]

[chemical formula 2]

[Formula 1] 7,000 ≤ V _PI ≤ 20,000 where V _PI is the same as defined in Formula 1 of Claim 1. The method according to claim 9, wherein the step (A) is reacted by further including a compound represented by the following chemical formula 3: [chemical formula 3]

. The method as described in claim item 9, wherein the step (B) includes: Based on the weight of the amide-based solvent in step (A), additionally adding 25% by weight to 100% by weight of a hydrocarbon-based solvent, and stirring the resulting mixture; and A mixed solvent containing the amide-based solvent and the hydrocarbon-based solvent is additionally added to satisfy Formula 1. A polyimide film for covering windows, which is obtained by curing the composition for forming a polyimide film for covering windows as described in any one of claims 1 to 8. The polyimide film for cover window as described in claim 12, wherein the thickness of the polyimide film for cover window is 30 micrometers to 500 micrometers, and the retardation (Rth) in the thickness direction at a wavelength of 550 nanometers The absolute value of 1,000 nm or less, and the yellowness index (YI) according to ASTM E131 is 5 or less. The polyimide film for covering windows according to claim 13, wherein the polyimide film for covering windows has a modulus (modulus) of 3 GPa or more and a tensile strength (tensile strength) of 100 according to ASTM E111 MPa or more and elongation (elongation) of 10% or more. A method for preparing a polyimide film for covering a window, the method comprising: coating the composition for forming a polyimide film for covering a window as described in any one of claims 1 to 8 on a substrate (substrate), and then heat treatment. The method according to claim 15, wherein the heat treatment is performed by heating the composition for forming a polyimide film for covering windows at 280° C. to 350° C. A multilayer structure comprising: the polyimide film for covering windows according to any one of claims 12 to 14 on one surface of a substrate. A cover window for a display device, comprising: The polyimide film for covering windows as described in any one of claims 12 to 14; and A coating formed on the polyimide film. The cover window for a display device as claimed in claim 18, wherein the coating is a hard coat layer, an antistatic layer, an anti-fingerprint layer, an anti-fouling layer, an anti-scratch layer, a low-refractive layer, an anti-reflection layer, an impact-absorbing layer layer or a combination of the foregoing.