JP7565714B2 - Cover film for flexible displays - Google Patents

Description

The present invention relates to a cover film for flexible displays.

A flexible display is a display that uses a flexible substrate and can be bent, folded or rolled without damage. Such flexible displays use plastic materials or plastic films as the substrate, so they are light and thin, and have the advantage of not breaking even when subjected to impact. As a result, they are being considered for use as displays for mobile devices, and because they can be bent or otherwise deformed, they are a promising industry that is expected to see explosive demand if they are spread to the fields of everyday items and automobiles in the future.

Currently known flexible displays use glass or high-elasticity resin as the cover window, but none of these cover windows are able to achieve high pencil hardness while also providing flexibility, scratch resistance, and solvent resistance.

For flexible displays to be commercialized in various fields such as portable display devices, they must not only not break when subjected to external pressure or force, but also be flexible enough to be bent and folded. At the same time, they must be scratch-resistant, solvent-resistant, and have high pencil hardness. Having high pencil hardness means that they are less likely to be damaged by scratches.

To achieve these characteristics, cover films for flexible displays must be able to protect products from damage caused by external mechanical, physical, and chemical influences, and a wide range of research and development efforts are being conducted.

JP 2016-2764 A

The present invention was made in consideration of the above circumstances, and aims to achieve high pencil hardness while providing flexibility, scratch resistance, and solvent resistance.

In the past, many attempts were made to increase the elastic modulus of a film in order to improve scratch resistance. However, when the elastic modulus of a film is increased, there is an upper limit to the elastic modulus of the film itself in terms of balancing flexibility with this, and once the resin is destroyed, it does not recover its shape, leaving scratches, and as a result, it has not been possible to sufficiently improve scratch resistance while maintaining flexibility. The inventors came up with a different idea from these conventional attempts to increase the elastic modulus of a film, and came up with the idea of achieving both high flexibility, sufficient scratch resistance, and high pencil hardness by increasing the deformation recovery rate of the film.

Then, when a polycarbonate diol that is liquid at 25°C was used as the polyol component for the surface layer and a specific slip agent was added to improve the flexibility of the soft segment and impart slipperiness, the resin surface was not left scratched despite the soft resin being formed due to the synergistic effect of improving the slipperiness of the surface and the stress dispersion effect brought about by the flexibility, and as a result, a resin molded product was obtained that was flexible, scratch-resistant, and solvent-resistant at the same time. Furthermore, it was discovered that by achieving both hardness and flexibility in the lower layer, a surprisingly high pencil hardness was achieved, and the present invention was completed. It was also discovered that a higher pencil hardness could be obtained by making the lower layer as thick as or thicker than the surface layer.

In the present invention, the surface layer refers to a layer located on the outermost surface of a flexible display, and the lower layer refers to a layer located closer to the display device than the surface layer. The cover film for flexible displays of the present invention may have other layers as long as it has the surface layer and the lower layer, and each of the surface layer and the lower layer may be composed of multiple layers.

The cover film for flexible displays of the present invention has a surface layer (X) having a thickness of 1 μm or more and 200 μm or less, and a lower layer (Y) having a thickness of 20 μm or more and 1000 μm or less that contacts the surface layer (X), and the surface layer (X) is a cured product formed from a resin composition containing a main agent (i), a curing agent (ii), and a lubricant (iii), the main agent (i) contains a polymer (A), the polymer (A) is a polyol (a) or a reaction product of polyol (a) and polyisocyanate (b) and has a hydroxyl group, the polyol (a) contains a polycarbonate polyol (a1) that is liquid at 25° C., the curing agent (ii) contains a polyisocyanate compound (d), and the lubricant (iii) contains a compound (B) represented by the following formula (1),

［式（１）中、Ｒ^１、Ｒ^２は、それぞれ独立に、メチル基又は水酸基を表す。ｎは、１～３，０００の整数を表す。］
前記表層に接する下層（Ｙ）が、二液硬化型ウレタン樹脂組成物の硬化物を含むものであって、以下の伸長試験法により測定した１００％伸長時の引張応力が３ＭＰａ以上、伸び率が１０％以上、破断強度が２０ＭＰａ以上であることを特徴とする。
［伸長試験法］
厚さ３０μｍ、幅５ｍｍ、長さ５０ｍｍに成形し、温度２３℃の環境下、引張試験機を用いて、初期のチャック間距離４０ｍｍ、引張速度１０ｍｍ／秒の条件で伸長し、１００％伸長した際の応力を１００％伸長時の引張応力、破断時の伸度を伸び率、破断時の応力を破断強度とする。

[In formula (1), R ¹ and R ² each independently represent a methyl group or a hydroxyl group, and n represents an integer of 1 to 3,000.]
The lower layer (Y) in contact with the surface layer contains a cured product of a two-component curing urethane resin composition, and is characterized in that the lower layer (Y) has a tensile stress of 3 MPa or more at 100% elongation, an elongation rate of 10% or more, and a breaking strength of 20 MPa or more, as measured by the following elongation test method.
[Extension test method]
The specimen was molded to a thickness of 30 μm, width of 5 mm, and length of 50 mm, and elongated using a tensile tester in an environment of 23° C. under conditions of an initial chuck distance of 40 mm and a tensile speed of 10 mm/sec. The stress at 100% elongation was defined as the tensile stress at 100% elongation, the elongation at break was defined as the elongation rate, and the stress at break was defined as the breaking strength.

The cover film for flexible displays of the present invention is capable of achieving both flexibility, scratch resistance, and solvent resistance, and also high pencil hardness.

The cover film for flexible displays of the present invention has at least a surface layer (X) and a lower layer (Y) in contact with the surface layer (X).

The surface layer (X) is a cured product formed from a resin composition containing a base agent (i), a curing agent (ii), and a lubricant (iii). In the present invention, the base agent (i) and the curing agent (ii) may each contain components other than those directly involved in the formation of the cured product (e.g., polymers, compounds having a curing action, etc.), and may be a composition of the base agent (i) and a composition of the curing agent (ii).

The main component (i) contains a polymer (A).

The polymer (A) is a polyol (a) or a reaction product of polyol (a) and polyisocyanate (b) and has a hydroxyl group.

The polyol (a) includes polycarbonate polyol (a1) that is liquid at 25°C (hereinafter, may be referred to as "liquid polycarbonate polyol (a1)").

The liquid polycarbonate polyol (a1) may be, for example, an esterification reaction product of a carbonate ester and a polyhydric alcohol, or a reaction product of a polyhydric alcohol and phosgene, which is liquid at 25°C.

The carbonate ester may be one or more of different types, and examples of such carbonate esters include aliphatic carbonates, alicyclic carbonates (hereinafter, those containing an alicyclic structure may be referred to as "alicyclic"), and aromatic carbonates (hereinafter, those containing an aromatic structure may be collectively referred to as "aromatic"). Examples of the aliphatic carbonate include saturated aliphatic carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, di-n-butyl carbonate, diisobutyl carbonate, ethyl-n-butyl carbonate, and ethyl isobutyl carbonate; and unsaturated aliphatic carbonates such as ethylene carbonate, trimethylene carbonate, tetramethylene carbonate, 1,2-propylene carbonate, 1,2-butylene carbonate, 1,3-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 1,3-pentylene carbonate, 1,4-pentylene carbonate, 1,5-pentylene carbonate, 2,3-pentylene carbonate, and 2,4-pentylene carbonate. Examples of aromatic carbonates include diphenyl carbonate and dibenzyl carbonate.

As the polyhydric alcohol, one or more types can be used, and examples thereof include linear or branched diols such as ethylene glycol, propylene glycol, butanediol, pentanediol, 3-methyl-1,5-pentanediol, hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, polyethylene glycol, and polypropylene glycol; alicyclic diols such as 1,4-cyclohexanedimethanol and hydrogenated bisphenol A; and trifunctional or higher polyols such as trimethylolmethane, trimethylolpropane, ditrimethylolpropane, pentaerythritol, and dipentaerythritol.

The liquid polycarbonate polyol may be an amorphous or crystalline polycarbonate polyol as long as it is liquid at 25°C, and is preferably an amorphous polycarbonate polyol. The number of hydroxyl groups contained in the liquid polycarbonate polyol is preferably 2.

The viscosity of the liquid polycarbonate polyol at room temperature (25°C) is preferably 10,000 mPa·s or less, more preferably 7,000 mPa·s or less, and even more preferably 5,000 mPa·s or less, for example 100 mPa·s or more, preferably 500 mPa·s or more.

The number average molecular weight of the liquid polycarbonate polyol is preferably more than 500, more preferably 600 or more, and even more preferably 700 or more, and is preferably less than 2,000, more preferably 1,800 or less, and even more preferably 1,500 or less.

In this specification, the number average molecular weight and weight average molecular weight are conversion values obtained by gel permeation chromatography (GPC) using polystyrene as a standard sample.

The glass transition temperature of the liquid polycarbonate polyol is preferably -100°C or higher, more preferably -90°C or higher, even more preferably -80°C or higher, particularly preferably -75°C or higher, and is preferably -5°C or lower, more preferably -15°C or lower, even more preferably -25°C or lower, particularly preferably -35°C or lower.

The content of the liquid polycarbonate polyol in the polyol (a) is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

The polyol (a) may contain other polyols (a2) in addition to the polycarbonate polyol (a1) that is liquid at 25°C. Examples of the other polyols (a2) include polyester polyols, polyether polyols, polycarbonate polyols other than the liquid polycarbonate polyol (a1), and low molecular weight polyols.

The polyester polyol may be one or more of these. Examples of the polyester polyol include polyester polyols obtained by reacting a low molecular weight polyol with a polycarboxylic acid; polyester polyols obtained by ring-opening polymerization of a cyclic ester compound such as ε-caprolactone; and polyester polyols obtained by copolymerizing these.

The low molecular weight polyol used in the production of the polyester polyol may be one or more, for example, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propanediol, 1,3-propanediol, dipropylene glycol, tripropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, neopentyl glycol, 1,5-hexanediol, 1,6-hexanediol, 2,5-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol ... Examples of such polyols include aliphatic polyols with a molecular weight of 50 to 300, such as hexanediol, 1,11-undecanediol, 1,12-dodecanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2-ethyl-1,3-hexanediol, 2-methyl-1,8-octanediol, glycerin, trimethylolpropane, ditrimethylolpropane, trimethylolpropane, and pentaerythritol; polyols with an alicyclic structure, such as cyclohexanedimethanol and hydrogenated bisphenol A; and polyols with an aromatic structure, such as bisphenol A and bisphenol F.

The polycarboxylic acid may be one or more of these, and examples thereof include aliphatic polycarboxylic acids such as succinic acid, adipic acid, sebacic acid, and dodecanedicarboxylic acid; alicyclic polycarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and cyclohexanetricarboxylic acid; aromatic polycarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and naphthalenedicarboxylic acid; and anhydrides or esters thereof.

As the polyether polyol, one or more kinds can be used, and examples thereof include compounds obtained by addition polymerization of alkylene oxide using one or more compounds having two or more active hydrogen atoms as an initiator. Examples of the compound having two or more active hydrogen atoms include propylene glycol, trimethylene glycol, 1,3-butanediol, 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, glycerin, diglycerin, trimethylolethane, trimethylolpropane, water, hexanetriol, and the like. Examples of the alkylene oxide include propylene oxide, butylene oxide, styrene oxide, epichlorohydrin, tetrahydrofuran, and the like.

Examples of polycarbonate polyols other than the liquid polycarbonate polyol (a1) include polycarbonate polyols that are solid at 25°C.

The number average molecular weight of the polyester polyol, the polyether polyol, and the polycarbonate polyol other than the polycarbonate polyol (a1) is preferably more than 300, more preferably 500 or more, and even more preferably 700 or more, and is preferably 10,000 or less, more preferably 5,000 or less, and even more preferably 4,000 or less.

The low molecular weight polyol may be one or more of these, for example, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propanediol, 1,3-propanediol, dipropylene glycol, tripropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, neopentyl glycol, 1,5-hexanediol, 1,6-hexanediol, 2,5-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11 aliphatic polyols with a molecular weight of 50 to 300, such as 1,2-undecanediol, 1,12-dodecanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2-ethyl-1,3-hexanediol, 2-methyl-1,8-octanediol, glycerin, trimethylolpropane, ditrimethylolpropane, trimethylolpropane, and pentaerythritol; polyols with an alicyclic structure, such as cyclohexanedimethanol and hydrogenated bisphenol A; and polyols with an aromatic structure, such as bisphenol A and bisphenol F.

The content of the other polyol in the polyol (a) is, for example, 30% by mass or less, 20% by mass or less, or 10% by mass or less, with the lower limit being 0% by mass.

The polyisocyanate (b) may be one or more of different types, including aliphatic polyisocyanates, alicyclic polyisocyanates, and aromatic polyisocyanates, and it is preferable to use an aliphatic or alicyclic polyisocyanate. The aliphatic or alicyclic polyisocyanate may be one or more of them. For example, tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, dodecamethylene diisocyanate, trimethylhexamethylene diisocyanate, 1,3-cyclopentylene diisocyanate, 1,3-cyclohexylene diisocyanate, 1,4-cyclohexylene diisocyanate, 1,3-di(isocyanatemethyl)cyclohexane, 1,4-di(isocyanatemethyl)cyclohexane, lysine diisocyanate, isophorone diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 2,4'-dicyclohexylmethane diisocyanate, 2,2'-dicyclohexylmethane diisocyanate, 3,3'-dimethyl-4,4'-dicyclohexylmethane diisocyanate, and the like may be used. In addition, allophanate-modified or isocyanurate-modified aliphatic or alicyclic polyisocyanates can be used. Among these, allophanate-modified or isocyanurate-modified aliphatic or alicyclic polyisocyanates are preferred, isocyanurate-modified aliphatic or alicyclic polyisocyanates are more preferred, and isocyanurate-modified aliphatic polyisocyanates are even more preferred.

The molar ratio (NCO/OH) of the isocyanate groups contained in the polyisocyanate (b) to the hydroxyl groups contained in the polyol (a) is preferably 0.8 or more, more preferably 0.9 or more, even more preferably 1.0 or more, and is preferably 1.3 or less, more preferably 1.2 or less, even more preferably 1.1 or less.

The polymer (A) may be the polyol (a) as is, or may be produced by reacting the polyol (a) with the polyisocyanate (b).

When reacting the polyol (a) and the polyisocyanate (b), a urethanization catalyst may be present, if necessary. As the urethanization catalyst, one or more types may be used, and examples thereof include nitrogen-containing compounds such as triethylamine, tributylamine, benzyldibutylamine, triethylenediamine, and N-methylmorpholine; organic metal compounds such as titanium tetrabutoxide, dibutyltin oxide, dibutyltin dilaurate, tin 2-ethylcaproate, zinc naphthenate, cobalt naphthenate, zinc 2-ethylcaproate, molybdenum glycolate, potassium acetate, zinc stearate, tin octoate, dibutyltin dilaurate, and dioctyltin dineodecane; and inorganic compounds such as iron chloride and zinc chloride.

During the reaction, a reaction solvent may be present. One or more reaction solvents may be used, and examples of such reaction solvents include aromatic hydrocarbon solvents such as toluene and xylene; ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone and acetylacetone; ether solvents such as tetrahydrofuran and dioxane; ester solvents such as ethyl acetate and butyl acetate; nitrile solvents such as acetonitrile; and amide solvents such as dimethylformamide and N-methylpyrrolidone.

The reaction can be carried out at a temperature of, for example, 50 to 90°C for a reaction time of, for example, 2 to 24 hours. The reaction pressure can be normal pressure, elevated pressure, or reduced pressure. The reaction atmosphere can be an inert gas atmosphere such as nitrogen or argon, or can be a dry air atmosphere, sealed conditions, or other conditions that do not allow moisture to be mixed in.

The hydroxyl equivalent of the polymer (A) is preferably 400 g/eq. or more, more preferably 450 g/eq. or more, and is preferably 4000 g/eq. or less, more preferably 3000 g/eq. or less.

The curing agent (ii) includes a polyisocyanate compound (d). The polyisocyanate compound (d) has two or more isocyanate groups. One or more types of polyisocyanate compounds (d) can be used. For example, diisocyanate compounds having two isocyanate groups include diisocyanates having an alicyclic structure such as cyclohexane diisocyanate, dicyclohexylmethane diisocyanate, and isophorone diisocyanate; aliphatic diisocyanates such as hexamethylene diisocyanate, lysine diisocyanate, xylylene diisocyanate, and tetramethylxylylene diisocyanate; and polyisocyanate compounds having three or more isocyanate groups include isocyanurates of diisocyanates and trimethylolpropane adducts of diisocyanates, with isocyanurates of diisocyanates being preferred. Examples of the diisocyanate include aliphatic diisocyanates such as hexamethylene diisocyanate, and alicyclic diisocyanates such as isophorone diisocyanate.

In particular, the polyisocyanate compound (d) preferably contains a polyisocyanate compound having three or more isocyanate groups. The content of the polyisocyanate compound having three or more isocyanate groups in the polyisocyanate compound (d) is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

The content of the curing agent (ii) in the resin composition (in the components forming the resin composition) is less than 50% by mass, preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 20% by mass or less, with the lower limit being, for example, 0.1% by mass.

The molar ratio (NCO/OH) of the hydroxyl groups of the polymer (A) contained in the base agent (i) to the isocyanate groups of the polyisocyanate compound (d) contained in the curing agent (ii) is preferably 0.8 or more, more preferably 0.9 or more, and is preferably 1.5 or less, more preferably 1.2 or less.

The lubricant (iii) contains a compound (B) represented by the following formula (1):

［式（１）中、Ｒ^１、Ｒ^２は、それぞれ独立に、メチル基又は水酸基を表す。ｎは、１～３，０００の整数を表す。］

[In formula (1), R ¹ and R ² each independently represent a methyl group or a hydroxyl group, and n represents an integer of 1 to 3,000.]

The lubricant (iii) may be mixed with the base agent (i) and the curing agent (ii) in any order as long as it is contained in the resin composition.

In formula (1), R ¹ and R ² are preferably methyl groups from the viewpoint of scratch resistance, and n is preferably 1 to 3,000, more preferably 5 to 2,500, and even more preferably 6 to 2,200.

The molecular weight of the compound (B) is, for example, 500 or more, more preferably 1,000 or more, even more preferably 5,000 or more, and is preferably 150,000 or less, more preferably 120,000 or less, even more preferably 110,000 or less.

The molecular weight of the compound (B) can be calculated based on the kinetic viscosity of the compound (B) at 25°C using the A. J. Barry formula.

The compound (B) may be mixed directly with the base agent (i) and the curing agent (ii), or may be dispersed in a dispersion medium such as water before mixing.

The content of the compound (B) is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and even more preferably 0.5 parts by mass or more, relative to 100 parts by mass of the total of the main agent (i) and the curing agent (ii), and is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, and even more preferably 3 parts by mass or less.

The resin composition may contain an organic solvent (C) in the main agent (i) or the curing agent (ii) (preferably in the main agent (i)). As the organic solvent (C), one or more types may be used, and examples thereof include aromatic hydrocarbon solvents such as toluene and xylene; ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone and acetylacetone; ether solvents such as tetrahydrofuran and dioxane; ester solvents such as ethyl acetate and butyl acetate; nitrile solvents such as acetonitrile; and amide solvents such as dimethylformamide and N-methylpyrrolidone.

The content of the organic solvent (C) in the resin composition is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, and is preferably 80% by mass or less, more preferably 70% by mass or less, even more preferably 65% by mass or less.

The resin molded body of the surface layer (X) may contain other resins in addition to the polymer (A). Examples of such other resins include acrylic resins and silicone resins.

The resin composition may contain other additives (D) in addition to the main agent (i), the curing agent (ii), and the lubricant (iii). Examples of the other additives (D) include tackifiers, leveling agents, catalysts, plasticizers, stabilizers, fillers, pigments, dyes, and flame retardants, and these may be premixed with part or all of the main agent (i), the curing agent (iii), and the lubricant (iii).

When the other additive (D) is included, the content thereof is, for example, 30% by mass or less, 10% by mass or less, or 5% by mass or less of the total amount of the resin composition, with the lower limit being 0% by mass.

The resin molding of the surface layer (X) has a 100% modulus of 5 MPa or less, measured by the following method. The 100% modulus is preferably 4.5 MPa or less, more preferably 4.0 MPa or less, and even more preferably 2.0 MPa or more, for example, 2.5 MPa or more, 3.0 MPa or more.

[Method for measuring 100% modulus]
The sheet is molded to a thickness of 30 μm, width of 5 mm, and length of 50 mm, and is stretched using a tensile tester in an environment of 23° C. under conditions of an initial chuck distance of 40 mm and a tensile speed of 10 mm/sec. The stress at 100% stretching is defined as the 100% modulus.

The resin molding of the surface layer (X) has a hysteresis loss at 100% elongation recovery of 50% or less, measured by the following method. The hysteresis loss is preferably 10% or less, more preferably 5% or less, and even more preferably 1% or less, with the lower limit being preferably 0%.

[Method for measuring hysteresis loss at 100% elongation and recovery]
The specimen is molded to a thickness of 30 μm, width of 5 mm, and length of 50 mm, and is stretched using a tensile tester under conditions of an initial chuck distance of 40 mm and an extension speed of 10 mm/sec in an environment at a temperature of 23° C. Immediately after 100% extension, the specimen is stretched under conditions of an inter-chuck distance of 40 mm and an extension/contraction speed of 10 mm/sec. The hysteresis loss is calculated as the percentage of heat loss calculated from the difference between the amount of heat given when the specimen is stretched and the amount of heat released by the specimen when it is pulled back, based on the integral value of a graph of the applied load and the amount of deformation.

The resin molded body of the surface layer (X) has a kinetic friction force of 2 kN or less, measured by the following method. The kinetic friction force is preferably 1 kN or less, more preferably 0.5 kN or less, and even more preferably 0.1 kN or less. For example, it is acceptable for the kinetic friction force to be 0.1 kN or more, or even 0.5 kN or more.

[Method of measuring friction force]
The friction force measured by molding the sample to a thickness of 30 μm, width of 5 mm and length of 50 mm under conditions of a temperature of 23° C., a load of 200 gf and a test speed of 120 cm/min using a stainless steel ball indenter having a diameter of 10 mm is defined as the kinetic friction force.

The surface layer (X) of the cover film for flexible displays of the present invention can be obtained by mixing the main agent (i), the curing agent (ii), and the slippage imparting agent (iii), forming a coating film by various coating methods, and curing the coating. The curing temperature can be, for example, 40 to 250°C, and the curing time can be, for example, about 1 to 1,000 seconds. A drying step can be carried out before the curing, and a secondary curing step can be carried out after the curing. The secondary curing temperature can be, for example, 40 to 100°C, and the secondary curing time can be, for example, 1 to 24 hours.

The thickness of the surface layer (X) is 1 μm or more, preferably 10 μm or more, more preferably 30 μm or more, and is 200 μm or less, preferably 100 μm or less, more preferably 50 μm or less.

In the surface layer (X), the concentration of the lubricant (iii) may be uniform or may have a distribution in the thickness direction. The distribution may be continuous or stepwise, and if it is stepwise, the surface layer may have two or more layers.

Examples of the molding method include a coating method and other molding methods. Examples of the coating method include a dip coating method, a spin coating method, a flow coating method, a spray coating method, a roll coating method, a gravure roll coating method, an air doctor coating method, a blade coating method, a wire doctor coating method, a knife coating method, a reverse coating method, a transfer roll coating method, a microgravure coating method, a kiss coating method, a cast coating method, a slot orifice coating method, a calendar coating method, a die coating method, and the like.
As other molding methods, methods such as a mold molding method and a laminate molding method can be appropriately adopted.
The surface layer (X) and the lower layer (Y) may be molded one on top of the other, or may be molded separately and then bonded together. In addition, any process may be used as long as the layers are disposed in a position that covers the surface of the display. For example, the layers may be molded by directly coating the display, or a molded product may be bonded to a display panel.

The lower layer (Y) is a resin molded body having a tensile stress of 3 MPa or more at 100% elongation, an elongation rate of 10% or more, and a breaking strength of 20 MPa or more, as measured by the following elongation test method, and a thickness of 250 μm or more.

The thickness of the lower layer (Y) is 20 μm or more, preferably 300 μm or more, more preferably 350 μm or more, and is 1000 μm or less, preferably 700 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less.

The ratio (Y/X) of the thickness of the lower layer (Y) to the thickness of the surface layer (X) is 1 or more, preferably 5 or more, more preferably 10 or more, and preferably 50 or less, more preferably 40 or less, and even more preferably 30 or less.

[Extension test method]
The specimen was molded to a thickness of 30 μm, width of 5 mm, and length of 50 mm, and elongated using a tensile tester in an environment of 23° C. under conditions of an initial chuck distance of 40 mm and a tensile speed of 10 mm/sec. The stress at 100% elongation was defined as the tensile stress at 100% elongation, the elongation at break was defined as the elongation rate, and the stress at break was defined as the breaking strength.

The tensile stress at 100% elongation is preferably 20 MPa or less, more preferably 15 MPa or less, and is 3.0 MPa or more, preferably 3.5 MPa or more, more preferably 4.0 MPa or more.

The elongation is 10% or more, preferably 50% or more, more preferably 100% or more, and may be, for example, 700% or less, 500% or less.

The breaking strength is 20 MPa or more, preferably 25 MPa or more, more preferably 30 MPa, and may be, for example, 100 MPa or less, 70 MPa or less.

The resin molding of the lower layer (Y) preferably has a hysteresis loss at 100% elongation recovery of 90% or less, as measured by the following method. The hysteresis loss is more preferably 70% or less, and preferably 25% or more. The hysteresis loss can be measured by the same method as for the resin molding of the surface layer (X).

The tensile stress, elongation, breaking strength and hysteresis loss at 100% elongation can be achieved, for example, by adjusting the ratio of rigid and flexible parts in the molecular chain of the resin contained in the resin molded body, the degree of rigidity, flexibility, degree of crosslinking, etc. For example, from the viewpoint of reducing the tensile stress at 100% elongation and increasing the elongation, it is preferable that the resin has a flexible part, and when the flexible part has a glass transition temperature, the glass transition temperature is, for example, 20°C or less, preferably 0°C or less, -5°C or less, -15°C or less, -25°C or less, -35°C or less. From the viewpoint of reducing the hysteresis loss and increasing the breaking strength, it is preferable that the resin has a rigid part. The rigid portion is a portion that crosslinks a portion that is physically or chemically flexible. Examples of physical crosslinks include crosslinks by functional groups that can form hydrogen bonds (hydroxy groups, carboxy groups, ester bond groups, urethane bond groups, urea bond groups, etc.), and examples of chemical crosslinks include crosslinks by crosslinking agents (multifunctional monomers, vulcanizing agents, etc.). While physical crosslinks can be formed reversibly, chemical crosslinks tend to be irreversibly broken when the deformation amount increases, resulting in high hysteresis loss. Therefore, it is more preferable for the rigid portion to have a higher proportion of crosslinks by physical crosslinks than by chemical crosslinks as a method for reducing hysteresis loss.

The resin molded body of the lower layer (Y) preferably contains at least one resin selected from urethane resin, acrylic resin, and silicone resin, and preferably contains urethane resin.

The urethane resin is preferably a cured product of a two-component curing type urethane resin composition containing a main agent (i') and a curing agent (ii'), and the main agent (i') preferably contains a polymer (A') having a hydroxyl group. In the present invention, the main agent (i') and the curing agent (ii') may each contain components other than those directly involved in the formation of the cured product (e.g., polymers, compounds having a curing action, etc.), and may be a composition of the main agent (i') and a composition of the curing agent (ii').

The hydroxyl group-containing polymer (A') is preferably a polyol (a') or a reaction product of polyol (a') and polyisocyanate (b').

The polyol (a') may be one or more of different types, and examples of such polyols include polymer polyols such as polyester polyols, polyether polyols, and polycarbonate polyols; low molecular weight polyols, and the like.

The polyester polyols, polyether polyols, polycarbonate polyols, and low molecular weight polyols may be the same compounds as the polyether polyols, polycarbonate polyols, and low molecular weight polyols exemplified as polyol (a) above.

Among them, the polyol (a') preferably contains a polyether polyol and a low molecular weight polyol. The content of the low molecular weight polyol in the polyol (a') is preferably 1 part by mass or more, more preferably 3 parts by mass or more, and preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the polyether polyol.

The low molecular weight polyol preferably contains a diol having two hydroxyl groups and a polyol having three or more hydroxyl groups. The content of the polyol having three or more hydroxyl groups in the low molecular weight polyol is, for example, 20% by mass or more, preferably 30% by mass or more, and, for example, 80% by mass or less, preferably 70% by mass or less.

As the polyisocyanate (b'), the same compounds as those exemplified as the polyisocyanate (b) can be used.

The molar ratio (NCO/OH) of the isocyanate groups contained in the polyisocyanate (b') to the hydroxyl groups contained in the polyol (a') is preferably 0.8 or more, more preferably 0.9 or more, even more preferably 1.0 or more, and is preferably 1.3 or less, more preferably 1.2 or less, even more preferably 1.1 or less.

The polymer (A) may be the polyol (a') as it is, or may be produced by reacting the polyol (a') with the polyisocyanate (b'). The reaction method of the polyol (a') with the polyisocyanate (b') may be the same as that described for the reaction method of the polyol (a) with the polyisocyanate (b), and a urethane catalyst, a reaction solvent, etc. may also be used.

The reaction can be carried out at a temperature of, for example, 50 to 90°C for a reaction time of, for example, 2 to 24 hours. The reaction pressure can be normal pressure, elevated pressure, or reduced pressure. The reaction atmosphere can be an inert gas atmosphere such as nitrogen or argon, or can be a dry air atmosphere, sealed conditions, or other conditions that do not allow moisture to be mixed in.

The hydroxyl equivalent of the polymer (A') is preferably 400 g/eq. or more, more preferably 450 g/eq. or more, and is preferably 4000 g/eq. or less, more preferably 3000 g/eq. or less.

The curing agent (ii') preferably contains a polyisocyanate compound (d'). As the polyisocyanate compound (d'), a compound similar to the compound described as the polyisocyanate compound (d) can be used.

In particular, the polyisocyanate compound (d') preferably contains a polyisocyanate compound having three or more isocyanate groups. The content of the polyisocyanate compound having three or more isocyanate groups in the polyisocyanate compound (d) is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

The content of the curing agent (ii') in the total of the main agent (i') and the curing agent (ii') is less than 50% by mass, preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 20% by mass or less, and the lower limit is, for example, 0.1% by mass.

The molar ratio (NCO/OH) of the hydroxyl groups of the polymer (A') contained in the base agent (i') to the isocyanate groups of the polyisocyanate compound (d') contained in the curing agent (ii') is preferably 0.8 or more, more preferably 0.9 or more, and is preferably 1.5 or less, more preferably 1.2 or less.

The content of the urethane resin in the resin contained in the resin molded body constituting the lower layer (Y) is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more, with the upper limit being 100% by mass.

The resin composition that may form the lower layer (Y) may contain the organic solvent (C). As the organic solvent (C), one or more types may be used, and examples thereof include aromatic hydrocarbon solvents such as toluene and xylene; ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone and acetylacetone; ether solvents such as tetrahydrofuran and dioxane; ester solvents such as ethyl acetate and butyl acetate; nitrile solvents such as acetonitrile; and amide solvents such as dimethylformamide and N-methylpyrrolidone.

The resin composition that may form the lower layer (Y) may contain other additives (D). Examples of the other additives (D) include tackifiers, leveling agents, catalysts, plasticizers, stabilizers, fillers, pigments, dyes, and flame retardants.

When the other additive (D) is contained, the content thereof is, for example, 30% by mass or less, 10% by mass or less, or 5% by mass or less of the total amount of the resin composition, with the lower limit being 0% by mass.

The method for forming the lower layer (Y) can be the same as the method for forming the surface layer (X).

As a method for producing a multilayer film, in addition to coating a surface layer (X) on a lower layer (Y), a lower layer (Y) may be coated on a surface layer (X) coated on a release substrate. Also, a film of a lower layer (Y) or a surface layer (X) previously produced may be laminated on a surface layer (X) or a lower layer (Y) coated on a release substrate. Corona treatment or the like may be performed to improve adhesion between the layers.

The film of the present invention is useful as a cover film for flexible displays. In the present invention, the cover film constitutes a part of the display, and does not include a protective film that is separately laminated on the surface of the display.

The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to the following examples, and it is of course possible to carry out the invention with appropriate modifications within the scope of the above and below-mentioned aims, and all such modifications are included in the technical scope of the present invention.

Preparation Example 1
In a nitrogen-substituted vessel equipped with a thermometer, a nitrogen gas inlet tube, and a stirrer, 100 parts by mass of methyl ethyl ketone (MEK) and 100 parts by mass of polycarbonate diol (DURANOL T-5652, manufactured by Asahi Kasei Corporation) were added and stirred. Then, 8 parts by mass of an isocyanurate of hexamethylene diisocyanate (Burnoc DN-902S, manufactured by DIC Corporation) and 0.01 parts by mass of dioctyltin versatate were added. After raising the internal temperature to 60°C, the mixture was stirred for 3 hours while maintaining the temperature, to obtain a surface layer urethane resin/main agent (i-1) containing a prepolymer (A-1) having a hydroxyl group at the molecular end with an equivalent weight of 1954 hydroxyl groups.

[Preparation Examples 2 to 7]
Except for changing the type and amount of polyol and the type and amount of polyisocyanate as shown in Table 1, the same procedure as in Preparation Example 1 was repeated to obtain main components (i-2) to (i-4) of urethane resins for surface layers containing prepolymers (A-2) to (A-4) having equivalent weights of hydroxyl groups shown in Table 1.

Example 1
100 parts by mass of polytetramethylene ether glycol, 3 parts by mass of butanediol, and 4 parts by mass of trimethylolpropane were mixed and stirred, and then 61 parts by mass of hydrogenated MDI ("Desmodur W" manufactured by Evonik Corporation) was mixed and stirred, and the mixture was applied to a release PET film using a knife coater so that the coating thickness after curing would be 300 μm. The mixture was then dried at 140° C. for 5 minutes using a hot air dryer, thereby obtaining a film made of a lower layer urethane resin (y-1). Further, on this film, 100 parts by mass of the surface layer urethane resin/main agent (i-1) obtained in the preparation example, 5.1 parts by mass of a curing agent (isocyanurate of hexamethylene diisocyanate (Burnoc DN-902S) were mixed so that the NCO/OH ratio was 1.10, and further 0.5 parts by mass of a slip imparting agent Wacker AK-10000 (dimethylpolysiloxane oil) and 3 parts by mass of a leveling agent (Silclean 3700, manufactured by BYK-Chemie Japan) were mixed. Next, the mixture was applied to a film made of the lower layer urethane resin (y-1) using a bar coater so that the coating thickness after drying would be 50 μm, and then dried using a hot air dryer at 70 ° C. for 2 minutes and then at 110 ° C. for 2 minutes to obtain a multilayer film (1) made of the surface layer urethane resin (x-1) and the lower layer urethane resin (y-1).

[Examples 2 to 8, Comparative Examples 1 to 5]
Except for changing the urethane resin for the surface layer as shown in Table 1, the urethane resin for the lower layer as shown in Table 2, and the multilayer structure as shown in Table 3 or 4, the same procedure as in Example 1 was repeated to obtain multilayer films (2) to (8) and comparative films (1) to (7).

In Table 1, PTMG2000 represents polyoxytetramethylene glycol (PTMG2000, molecular weight 2,000, manufactured by Mitsubishi Chemical Corporation), G3452 represents polycarbonate polyol (G3452, number average molecular weight 2,000, manufactured by Asahi Kasei Corporation), T5652 represents polycarbonate polyol (T5652, number average molecular weight 2,000, manufactured by Asahi Kasei Corporation), UM-90 represents polycarbonate polyol (UM-90, number average molecular weight 900, manufactured by Ube Industries, Ltd.), DN-902S represents an isocyanurate of hexamethylene diisocyanate (Burnoc DN-902S, manufactured by DIC Corporation), and GCA-11 represents an isocyanurate of hexamethylene diisocyanate (Pandex GCA-11, manufactured by DIC Corporation). AK1000, AK10000, and AK100000 represent dimethylpolysiloxane (Wacker AK1000, AK10000, AK100000, Wacker Asahi Kasei Silicone Co., Ltd.), and KF53 represents phenylmethylpolysiloxane (KF-53, manufactured by Shin-Etsu Chemical Co., Ltd.).

[Methods for evaluating tensile stress, elongation rate, and breaking strength at 100% elongation]
The surface layer urethane resin/main agent (i) and the curing agent (an isocyanurate of hexamethylene diisocyanate (Burnoc DN-902S) were mixed to an NCO/OH ratio of 1.10, and the mixture was applied to a PET film using a knife coater so that the solid film thickness was 30 μm. The mixture was then dried at 70° C. for 2 minutes and then at 110° C. for 2 minutes to prepare a surface layer polyurethane resin film. Next, the polyurethane resin film was cut into a rectangular specimen having a width of 5 mm and a length of 50 mm, and a tensile test was carried out using a tensile tester at a chuck distance of 40 mm, a tensile speed of 10 mm/sec, and a temperature of 23° C., and the specimen was stretched to 100%. The stress and elongation at break were measured. Similarly, the uncured resin of the lower layer urethane resin (y) was applied to a PET film by a knife coater so that the solid film thickness was 30 microns, and dried at 140 ° C for 5 minutes to prepare a lower layer polyurethane resin film. Next, the polyurethane resin film was cut into a strip of 5 mm wide and 50 mm long, and a tensile test was performed using a tensile tester under an environment of chuck distance 40 mm, tensile speed 10 mm / sec, and temperature 23 ° C., and the stress at 100% elongation (abbreviated as "100% M" in the table), elongation at break, and breaking strength were measured.

[Method for measuring hysteresis loss at 100% elongation and recovery]
The surface layer urethane resin/main agent (i) was molded to a thickness of 30 μm, width of 5 mm, and length of 50 mm in the same manner as above, and elongated using a tensile tester under conditions of an initial chuck distance of 40 mm and an elongation speed of 10 mm/sec in an environment of 23° C., and immediately after 100% elongation, it was stretched under conditions of an inter-chuck distance of 40 mm and an extension/contraction speed of 10 mm/sec. At that time, the hysteresis loss was calculated as the rate of heat loss calculated from the difference between the amount of heat given when the test piece was pulled and the amount of heat released by the test piece when it was pulled back, from the integral value of the graph of the applied load and the amount of deformation.

The resulting multilayer film and comparative film were evaluated using the following methods.

[Evaluation method of pencil hardness]
The test was carried out in accordance with JIS K5600.
○: 7H or more △: 2H or more but less than 7H ×: Less than 2H

[Method for evaluating steel wool scratch resistance]
The evaluation was carried out using a RUBBING TESTER (manufactured by Taihei Rika Kogyo Co., Ltd.). Specifically, steel wool (manufactured by Nippon Steel Wool Co., Ltd., product name BONSTAR, product number No. 0000) was used as an abrasive, and the coating surface was rubbed back and forth 500 times with the abrasive under a load of 500 g. The state of the coating surface after rubbing was visually evaluated according to the following evaluation criteria.
A: No scratches were observed on the surface of the film.
Δ: Less than three thin streaks were observed on the coating surface.
×: Three or more dark streaks were observed on the surface of the film.

[Method for evaluating solvent resistance]
A drop of MEK was dropped onto the polyurethane resin film obtained in the same manner as above using a dropper, and the film was wiped off after 10 seconds. The change in appearance was checked. When the dropped portion swelled and was clearly visible to the naked eye, it was rated as ×, and when it was not visible, it was rated as ◯.

Examples 1 to 8 are examples of the present invention, and were able to achieve both scratch resistance, solvent resistance, and high pencil hardness. Comparative Example 1 had a thin surface layer, and scratch resistance and pencil hardness were insufficient. Comparative Example 2 had a thin lower layer, and scratch resistance and pencil hardness were insufficient. Comparative Examples 3 and 4 were examples in which polycarbonate polyol, which is liquid at 25°C, was not used as the polymer, and solvent resistance was insufficient. Comparative Example 5 was an example in which the slipping agent (iii) was not included, and scratch resistance and pencil hardness were insufficient. Comparative Examples 6 and 7 had low breaking strength and tensile stress at 100% elongation in the lower layer, and scratch resistance and pencil hardness were insufficient.

Claims (7)

A surface layer (X) having a thickness of 1 μm or more and 200 μm or less, and a lower layer (Y) having a thickness of 20 μm or more and 1000 μm or less and in contact with the surface layer (X),
the surface layer (X) is a cured product formed from a resin composition containing a base agent (i), a curing agent (ii), and a lubricant (iii);
the base agent (i) contains a polymer (A), the polymer (A) is a polyol (a) or a reaction product of the polyol (a) and a polyisocyanate (b) and has a hydroxyl group, the polyol (a) contains a polycarbonate polyol (a1) which is liquid at 25°C, the curing agent (ii) contains a polyisocyanate compound (d), and the lubricant (iii) contains a compound (B) represented by the following formula (1),

[In formula (1), R ¹ and R ² each independently represent a methyl group or a hydroxyl group, and n represents an integer of 1 to 3,000.]
the lower layer (Y) in contact with the surface layer contains a cured product of a two-component curing type urethane resin composition containing a main agent (i') and a curing agent (ii') ,
The main component (i') contains a polymer (A') having a hydroxyl group,
the polymer (A') is a reaction product of a polyol (a') and a polyisocyanate (b');
The polyol (a') includes one or more selected from the group consisting of polyester polyols, polyether polyols, and polycarbonate polyols,
The curing agent (ii') contains a polyisocyanate compound (d'),
the content of the two-component curing urethane resin composition is 50 mass% or more in the resin contained in the resin molded body constituting the lower layer (Y),
A cover film for flexible displays, in which the lower layer (Y) in contact with the surface layer has a tensile stress at 100% elongation of 3 MPa or more and 20 MPa or less , an elongation percentage of 10% or more and 700% or less , and a breaking strength of 20 MPa or more and 100 MPa or less , as measured by the following elongation test method.
[Extension test method]
The specimen was molded to a thickness of 30 μm, width of 5 mm, and length of 50 mm, and elongated using a tensile tester in an environment of 23° C. under conditions of an initial chuck distance of 40 mm and a tensile speed of 10 mm/sec. The stress at 100% elongation was defined as the tensile stress at 100% elongation, the elongation at break was defined as the elongation rate, and the stress at break was defined as the breaking strength. The cover film for flexible displays according to claim 1, wherein the concentration of the slip imparting agent (iii) has a distribution in the thickness direction. The surface layer (X) is composed of two or more layers ,
Each of the two or more layers contains the base agent (i), the curing agent (ii), and the lubricant (iii),
The cover film for flexible displays according to claim 2 , wherein the concentration of the slipping agent (iii) in the surface layer (X) is graded in the thickness direction . The cover film for flexible displays according to any one of claims 1 to 3 , wherein the polyol (a') further comprises one or two selected from the group consisting of 1,4-butanediol and trimethylolpropane . The cover film for a flexible display according to claim 1 , wherein the polyisocyanate (b) contains an isocyanate compound having three or more isocyanate groups . The cover film for a flexible display according to claim 1 , wherein the polyisocyanate compound (d) contains an isocyanate compound having three or more isocyanate groups . The cover film for flexible displays according to any one of claims 1 to 6, wherein the thickness of the lower layer (Y) is equal to or greater than the thickness of the surface layer (X).