TWI854963B - Polyester film for surface protection film of flexible display - Google Patents

Abstract

In the case of laminated hard coating, a polyester film with high pencil hardness is provided at a low cost.

A polyester film for a surface protective film of a flexible display, which has a limiting viscosity of 0.60-1.0 dL/g, a crystallization degree of 48% or more as measured by a density method, and a crystallization degree of 1.20 or more on the surface of the polyester film as measured by an ATR method. A hard coating film and a display using the polyester film.

Description

Polyester film for surface protection film of flexible display

The present invention relates to a polyester film, and more particularly to a polyester film which can obtain a high pencil hardness when a hard coating layer is laminated on the polyester film.

Mobile terminals such as displays and smartphones are moving towards thin-film weight reduction. Currently, since they all use hard frames, tempered glass is generally used to protect the display. On the other hand, in recent years, there has been a preference for displays that emphasize design. For example, there has been a preference for displays with special shapes such as round shapes. In particular, in recent years, there has been a demand for curved displays for the purpose of setting displays on curved surfaces.

However, since curved displays cannot use glass materials, acrylic sheets have been used. However, thin acrylic sheets are easily scratched and have poor impact resistance. In addition, although polycarbonate is used, it is expensive and has poor pencil hardness. On the other hand, polyester film is cheap and has excellent impact resistance, but it has insufficient pencil hardness.

In order to meet these requirements, a material having high hardness and flexibility is proposed as a hard coating material in Patent Document 1. However, the material is expensive, so there is a problem of versatility, and there is also a problem of low surface abrasion resistance.

In addition, as in Patent Document 2, a transparent polyimide material capable of achieving high pencil hardness without relying on a hard coating material has been proposed. However, this material is very expensive and has problems with mass production.

Prior Art Literature Patent Literature

Patent Document 1 Japanese Patent Application Publication No. 2017-008148

Patent Document 2 Japanese Patent Application Publication No. 2016-222797

The present invention is intended to solve the above-mentioned transparent film problem and to provide a polyester film having a high pencil hardness at a low cost when a hard coating is laminated.

That is, the present invention is constituted by the following structures.

1. A polyester film for a surface protective film of a flexible display, wherein the polyester film has an ultimate viscosity of 0.60-1.0 dL/g, a crystallization degree of 48% or more as measured by a density method, and a crystallization degree of 1.20 or more as measured by an ATR method.

2. The polyester film used as the surface protection film of the flexible display as described in item 1 above has a thickness of 25 to 188 μm.

3. A polyester film for use as a surface protective film for a flexible display as described in item 1 or 2 above, wherein the polyester film is a biaxially stretched polyester film, and an easy-to-bond layer is provided on at least one side of the biaxially stretched polyester film.

4. A hard coating film for use as a surface protective film for a flexible display, which has a hard coating layer on at least one side of the polyester film as described in any one of items 1 to 3 above.

5. A display using the hard coating film as described in item 4 above as a surface protection film.

The polyester film of the present invention has a higher surface hardness than ordinary polyester films when a hard coating layer is laminated. By using the hard coating film as a surface protection film of a display, a display having excellent properties such as scratch resistance, impact resistance, and deformation resistance on the surface of the display and excellent design can be provided. The above-mentioned display can provide beautiful images.

The form used to implement the invention (Display)

The display body referred to in the present invention generally refers to a display device. The types of displays include LCD, organic EL display, inorganic EL display, LED, FED, etc., but preferably, LCD, organic EL, or inorganic EL with a bendable structure is used. In particular, organic EL with a reduced layer structure and a wide color gamut is more preferred.

The display body can be used in any form, including large screens, monitors, digital electronic signage, curved mobile terminals, personal computers, etc. In particular, it is preferably used for a display with a touch panel having a touch panel function for direct touch operation, a digital electronic signage, a curved mobile terminal, a personal computer, etc. Furthermore, the display body is particularly preferably a flexible display as follows.

(Flexible Display)

A flexible display is a structure that can be bent and formed into a continuous sheet of display according to the application, and it is ideal to be thin and light at the same time. In particular, in addition to foldable displays, rollable displays, and the like, there are also those that can be used for paper-like displays.

(Polyester film)

Transparent films include polyimide films, polyester films, polycarbonate films, acrylic films, cellulose triacetate films, cycloolefin polymer films and other films with high light transmittance and low haze. Among them, polyester films with high impact resistance and moderate pencil hardness are preferred, and polyethylene terephthalate films that can be manufactured inexpensively are particularly preferred.

In the present invention, the polyester film may be a single-layer film containing one or more polyester resins, and when two or more polyesters are used, it may be a multi-layer structure film or a super multi-layer laminate film with a repeated structure.

Examples of the polyester resin include polyethylene terephthalate, polybutylene terephthalate, polyethylene 2,6-naphthalate, or a polyester film containing a copolymer having the constituent components of these resins as the main component. Among them, biaxially stretched polyester films are preferred, and biaxially stretched polyethylene terephthalate films are particularly preferred, from the perspectives of mechanical properties, heat resistance, transparency, price, etc.

When the polyester copolymer is used for the base film, the dicarboxylic acid component of the polyester includes, for example, aliphatic dicarboxylic acids such as adipic acid and sebacic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalene dicarboxylic acid; and polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. In addition, as the diol component, for example, fatty acid diols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; aromatic diols such as terephthalyl alcohol; alicyclic diols such as 1,4-cyclohexanedimethanol; and polyethylene glycol having an average molecular weight of 150 to 20,000. The mass ratio of the copolymerization component of the preferred copolymer is less than 20 mass %. When the content is less than 20% by mass, the film strength, transparency and heat resistance can be maintained.

In the production of polyester film, the limiting viscosity of at least one resin pellet is preferably in the range of 0.60 to 1.0 dl/g. If the limiting viscosity is 0.60 dl/g or more, the pencil hardness after hard coating is improved, which is preferred. It is more preferably 0.65 dl/g or more, and particularly preferably 0.70 dl/g or more. On the other hand, if the limiting viscosity is 1.00 dl/g or less, the filter pressure of the molten fluid does not increase too much, and the production of the film can be easily and stably operated, which is preferred.

Regardless of whether the film is a single-layer structure or a laminate structure, the limiting viscosity of the film is preferably 0.60 dl/g or more. As the molecular weight of the polyester resin increases, the elastic behavior of the film is improved, and deformation relative to the pencil load during pencil hardness measurement can be effectively suppressed, which is preferred. More preferably, it is 0.65 dl/g or more, and particularly preferably, it is 0.70 dl/g or more. On the other hand, as the limiting viscosity increases, the pencil hardness also increases, but there is a concern about reduced productivity. Therefore, it is preferred that the limiting viscosity is 1.00 dl/g or less because it can be manufactured with good workability.

The thickness of the polyester film is preferably 25 to 188 μm, more preferably 25 to 125 μm or more. If the thickness is 25 μm or more, the handling and impact resistance are improved and better, and if the thickness is 188 μm or less, in addition to being beneficial to weight reduction, the flexibility and processability are excellent and better.

The surface of the polyester film of the present invention may be smooth or have concavoconvexity, but from the perspective of the use of the protective film for the display surface, the reduction of optical properties due to the concavoconvexity is not good. As for the haze, it is preferably less than 3%, more preferably less than 2%, and most preferably less than 1%. If the haze is less than 3%, the visibility of the image can be improved. Although the lower limit of the haze is as small as possible, it can be more than 0.1% and can also be more than 0.3%.

As mentioned above, in order to reduce the haze, it is better that the surface roughness of the film is not too large. From the perspective of handling, in order to provide a certain degree of slippage, as a method of forming roughness, fillers can be mixed into the polyester resin layer on the surface, or a coating layer containing fillers can be applied during film formation.

As a method for blending particles into the base film, a known method can be adopted. For example, the particles can be added at any stage of the polyester production, preferably at the stage of esterification, or after the end of the transesterification reaction and before the start of the polycondensation reaction, in the form of a slurry dispersed in ethylene glycol or the like, and the polycondensation reaction can be carried out. In addition, the particles can be blended into a polyester raw material by using a mixing and kneading extruder with a vent, or by using a mixing and kneading extruder to blend dried particles with the polyester raw material.

Among them, the preferred method is to disperse the aggregated inorganic particles homogeneously in the monomer liquid that is a part of the polyester raw material, filter it, and then add it to the remaining polyester raw material before, during or after the esterification reaction. If this method is used, the monomer liquid has a low viscosity, so it is easy to homogeneously disperse the particles or filter the slurry with high precision. At the same time, when adding it to the remaining part of the raw material, the dispersibility of the particles is good and it is difficult to generate new aggregates. From this point of view, it is particularly preferred to add it to the remaining part of the raw material in a low temperature state before the esterification reaction.

Furthermore, the number of protrusions on the film surface can be further reduced by a method (masterbatch method) in which a polyester containing particles is obtained and then pellets thereof are kneaded and extruded with pellets containing no particles.

In addition, the polyester film may contain various additives within a range in which the total light transmittance is maintained within a preferred range. Examples of additives include antistatic agents, UV absorbers, and stabilizers.

The total light transmittance of the polyester film is preferably 85% or more, more preferably 87% or more. If the transmittance is 85% or more, visibility can be fully ensured. Although the total light transmittance of the polyester film is as high as possible, it can be 99% or less, or 97% or less.

The surface of the polyester film of the present invention can be treated to improve the adhesion with a resin forming a hard coat layer or the like.

As the method based on surface treatment, for example, roughening treatment based on sandblasting, solvent treatment, etc., or oxidation treatment based on corrugation treatment such as corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone, ultraviolet irradiation treatment, flame treatment, chromic acid treatment, hot air treatment, etc. can be cited, and they can be used without particular limitation.

In addition, the adhesion can be improved by using an adhesion-enhancing layer such as an easy-adhesion layer. As the easy-adhesion layer, acrylic resin, polyester resin, polyurethane resin, polyether resin, etc. can be used without particular limitation, and can be formed by a general coating method, preferably a so-called in-line coating method.

The above-mentioned polyester film, for example, can be manufactured through a polymerization step and a film forming step. The polymerization step is to uniformly disperse the inorganic particles in a monomer solution that is a part of the polyester raw material and filter it, and then add it to the remaining part of the polyester raw material to polymerize the polyester. The film forming step is to melt-extrude the polyester into a thin sheet through a filter, cool it, and then stretch it to form a substrate film.

Next, the method for producing a biaxially stretched polyester film is described in detail using polyethylene terephthalate (hereinafter referred to as PET) pellets as a raw material for the base film, but the present invention is not limited thereto. In addition, the number of layers such as a single-layer structure and a multi-layer structure is not limited.

After PET pellets are mixed and dried at a predetermined ratio, they are supplied to a known melt-laminated extruder, extruded from a slit-shaped die into a sheet, and cooled and solidified on a casting roll to form an unstretched film. In the case of a single layer, one extruder may be used, and in the case of manufacturing a film having a multi-layer structure, two or more extruders, two or more manifolds or confluence blocks (for example, a confluence block having a square confluence portion) may be used to laminate a plurality of film layers constituting each outermost layer, extrude two or more sheets from an extrusion nozzle, and cool them on a casting roll to form an unstretched film.

In this case, it is preferred to perform high-precision filtration at any place where the molten resin is kept at about 280°C during melt extrusion in order to remove foreign matter contained in the resin. The filter material used for high-precision filtration of the molten resin is not particularly limited, but a filter material made of sintered stainless steel is preferred because it has excellent removal performance for agglomerates and high-melting-point organics whose main components are Si, Ti, Sb, Ge, and Cu.

In addition, the filter particle size (initial filtration efficiency 95%) of the filter is preferably 20μm or less, and particularly preferably 15μm or less. If the filter particle size (initial filtration efficiency 95%) of the filter exceeds 20μm, foreign matter larger than 20μm cannot be sufficiently removed. Although productivity may be reduced by using a filter with a filter particle size (initial filtration efficiency 95%) of 20μm or less for high-precision filtration of molten resin, it is preferable in terms of obtaining a film with fewer protrusions caused by coarse particles.

Specifically, for example, after the PET pellets are fully vacuum dried, they are supplied to an extruder, melt-extruded into a thin sheet at about 280°C, and cooled and solidified to form an unstretched PET sheet. The unstretched sheet obtained is stretched 2.5 to 5.0 times in the longitudinal direction using a roller heated to 80 to 120°C to obtain a uniaxially oriented PET film. In addition, the end of the film is clamped with a clamp and guided to a hot air zone heated to 80 to 180°C. After drying, it is stretched 2.5 to 5.0 times in the width direction. Then, it is guided to a heat treatment zone at 160 to 250°C and heat treated for 1 to 60 seconds to complete the crystal orientation. In this heat treatment step, a relaxation treatment of 1 to 12% can be applied in the width direction or the longitudinal direction as needed.

(Degree of crystallization)

The degree of crystallization is mainly determined by the amount of heat applied to the film in the heat treatment zone. The amount of heat depends on various conditions such as film-making equipment conditions, film-making speed, and film thickness. Therefore, the degree of crystallization is not determined only by the heat treatment temperature. As a heat treatment temperature, it can be said that it is preferably 160~250℃, and more preferably 200~240℃. By setting it to 160℃ or more, crystallization can be promoted. By heat treating at 250℃ or less, it is better to suppress the decrease in crystallization caused by the remelting of polyester.

In order to ensure sufficient pencil hardness after hard coating, the crystallization degree of the film by the density method is preferably 48% or more, and the crystallization degree of the film surface by the ATR method is preferably 1.20 or more.

The crystallization degree of the polyester film by the density method is preferably 48% or more, more preferably 50% or more, and even more preferably 52% or more. The higher the crystallization degree, the better, but the upper limit of the film that can be produced is about 65%.

The crystallization degree of the thin film surface by the ATR method is preferably 1.20 or more, more preferably 1.25 or more, and even more preferably 1.27 or more. The higher the crystallization degree of the thin film surface by the ATR method, the better, but the upper limit of the thin film that can be produced is about 3.0.

(Easy to bond layer)

The easy-adhesive layer can be obtained by applying the coating liquid on one or both sides of an unstretched or longitudinally uniaxially stretched film, drying at 100-150°C, and further stretching in one direction or two directions. The final coating amount of the easy-adhesive layer is preferably managed to be 0.05-0.20 g/m ^2. If the coating amount is 0.05 g/m ² or more, the adhesion is preferably satisfied. On the other hand, if the coating amount is 0.20 g/m ² or less, the anti-sticking property is preferably obtained.

唑啉系、環氧系等交聯劑。也能夠混合2種以上來使用。它們在連線式塗布的性質方面，較佳為藉由水系塗布液予以塗敷，前述的樹脂、交聯劑較佳為水溶性或水分散性的樹脂、化合物。 As the resin used for the easy bonding layer, for example, polyester resins, polyurethane resins, polyester polyurethane resins, polycarbonate polyurethane resins, acrylic resins, etc. can be used without particular limitation. As the crosslinking agent for the easy bonding layer, melamine resins, isocyanate resins,

Azoline-based, epoxy-based crosslinking agent and the like can also be used in combination of two or more. In terms of the properties of the in-line coating, it is preferred that they be applied by means of a water-based coating liquid, and the aforementioned resin and crosslinking agent are preferably water-soluble or water-dispersible resins and compounds.

In order to impart lubricity to the easy-adhesion layer, it is preferred to add particles. The average particle size of the microparticles is preferably 2 μm or less. If the average particle size of the particles exceeds 2 μm, the particles become easy to fall off from the easy-adhesion layer. As particles contained in the easy-adhesion layer, for example, there can be cited: inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alum, talc, kaolin, clay, calcium phosphate, mica, lithium montmorillonite, zirconium oxide, tungsten oxide, lithium fluoride, and calcium fluoride, or organic polymer particles such as styrene-based, acrylic-based, melamine-based, benzoguanamine-based, and silicone-based particles. They can be added to the easy-adhesion layer alone, or two or more can be added in combination.

In addition, as a method for applying the coating liquid, a known method can be used in the same manner as the above-mentioned coating layer. For example, there can be cited: reverse roller coating method, gravure coating method, kiss coating method, roller brush method, spray coating method, air knife coating method, winding rod coating method, pipe doctor method, etc. These methods can be performed alone or in combination.

(Hard coating)

As the resin forming the hard coating layer, (meth)acrylate, silicone, inorganic mixture, urethane acrylate, polyester acrylate, epoxy, etc. can be used without particular limitation. In addition, two or more materials can be mixed and used, or particles of inorganic fillers, organic fillers, etc. can be added. Among them, (meth)acrylate and acrylate resins are preferred, and as an essential component, resins having three or more reactive groups in the molecule are preferred.

(Resin with 3 or more reactive groups)

Examples of the resin having three or more reactive groups include tris(2-acryloxyethyl) isocyanurate, tris(3-acryloxypropyl) isocyanurate, tris(2-methacryloxyethyl) isocyanurate, tris(3-methacryloxypropyl) isocyanurate, tris((meth)acryloxyalkyl) isocyanurate, and the like; tris(hydroxymethyl)propane tri(meth)acrylate, diquaternary isocyanurate, and the like. (Meth)acrylates such as pentaerythritol tri(meth)acrylate, propionic acid-modified dipentaerythritol tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propylene oxide-modified trihydroxymethylpropane tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, propionic acid-modified dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and caprolactone-modified dipentaerythritol hexa(meth)acrylate. These may be used alone or in combination of two or more.

In addition, commercially available hard coating agents can be used. For example, BEAMSET (registered trademark) series manufactured by Arakawa Chemical Industries, Ltd., Aica-Aitron (registered trademark) series manufactured by Aica Industries, Ltd., Liodurad (registered trademark) series manufactured by Toyo Ink Co., Ltd., etc. can be used without particular limitation, and can also be used in a mixture with other (meth)acrylate compounds.

(Photopolymerization initiator)

、1-羥基環己基苯基酮、苄基二苯基硫醚、一硫化四甲基秋蘭姆(tetramethylthiuram monosulfide)、偶氮雙異丁腈、二苯乙二酮(benzil)、聯苄(dibenzil)、雙乙醯、β-氯蒽醌、(2,4,6-三甲基苄基二苯基)氧化膦、2-苯并噻唑-N,N-二乙基二硫胺甲酸酯等。較佳為可作成表面硬化性優異的2-羥基-1-{4-[4-(2-羥基-2-甲基-丙醯基)-苄基]-苯基}-2-甲基丙烷-1-酮、1-羥基-環己基-苯基-酮、2-羥基-2-甲基-1-苯基-丙烷-1-酮、2-甲基-1-[4-(甲硫基)苯基]-2-嗎啉基丙烷-1-酮，其中特佳為2-羥基-2-甲基-1-苯基-丙烷-1-酮、2-甲基-1-[4-(甲硫基)苯基]-2-嗎啉基丙烷-1-酮。它們可以單獨使用，也可以組合2種以上來使用。 When the hard coating of the present invention is cured by ultraviolet rays, it is necessary to add a photopolymerization initiator. Specifically, the photopolymerization initiator includes benzophenone, acetophenone, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, benzoin benzoic acid, benzoin benzoic acid methyl ester, benzoin dimethyl ketal, 2,4-diethylthiosulfate, benzoin methyl ether ...

, 1-hydroxycyclohexyl phenyl ketone, benzyl diphenyl sulfide, tetramethylthiuram monosulfide, azobisisobutyronitrile, benzil, dibenzil, diacetyl, β-chloroanthraquinone, (2,4,6-trimethylbenzyldiphenyl)phosphine oxide, 2-benzothiazole-N,N-diethyldithiamine formate, etc. Preferred are 2-hydroxy-1-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]-phenyl}-2-methylpropane-1-one, 1-hydroxy-cyclohexyl-phenyl-one, 2-hydroxy-2-methyl-1-phenyl-propane-1-one, and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinylpropane-1-one, which can provide excellent surface hardening properties. Among them, 2-hydroxy-2-methyl-1-phenyl-propane-1-one and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinylpropane-1-one are particularly preferred. These may be used alone or in combination of two or more.

The amount of photopolymerization initiator added is not particularly limited. For example, it is preferably used in an amount of about 1 to 10% by mass relative to the resin used. By adding 1% by mass or more, sufficient curability can be obtained, and by setting it to 10% by mass or less, a high content ratio of the resin having three or more reactive groups can be made, and a hard coating film with high hardness due to a high crosslinking density can be produced.

(Film thickness)

The thickness of the hard coating layer is preferably 1 to 40 μm. If it is thicker than 1 μm, it can be fully cured and obtain good pencil hardness. In addition, by setting the thickness to 40 μm or less, the curling caused by the curing shrinkage of the hard coating can be suppressed, and the handling property of the film can be improved.

(Application method)

As the coating method of the hard coating layer, Mayer bar, gravure coating, die coating machine, knife coating machine, etc. can be used without particular limitation, and can be appropriately selected according to the viscosity and film thickness. The drying temperature is preferably 60~100℃, more preferably 60~90℃. If it is 60℃ or higher, it can be fully dried in a short time, and if it is 100℃ or lower, it is better because deformation of the polyester film does not occur.

(Hardening conditions)

As a method for curing the hard coating layer, a curing method using energy rays such as ultraviolet rays and electron rays, or heat can be used. However, in order to reduce damage to the film, ultraviolet rays, electron rays, etc. are preferred.

(Ultraviolet)

As ultraviolet lamps, there are high-pressure mercury lamps, electrodeless lamps, etc., which can be appropriately selected and used. As for the cumulative light amount of ultraviolet rays, it is preferably 50~500mJ/ ^cm2 , and more preferably 100~200mJ/ ^cm2 . If the cumulative light amount is 50mJ/ ^cm2 or more, sufficient pencil hardness can be obtained. If the cumulative light amount is 500mJ/ ^cm2 or less, the processing speed can be increased, and the economy is improved, which is better. It is also possible to improve the hardening property by irradiating ultraviolet rays in an inert gas such as nitrogen, argon, and carbon dioxide, so it can be appropriately selected.

(Electronic wire)

Electron beam irradiation devices include area type and scanning type, and can be selected appropriately. The cumulative irradiation dose of electron beams is preferably 25~500kGy, and more preferably 50~300kGy. If the cumulative irradiation dose is 25kGy or more, sufficient pencil hardness can be obtained. If it is less than 500kGy, the processing speed can be increased, and the economy is improved. It is better to irradiate in an inert gas such as nitrogen and argon with an oxygen concentration of less than 500ppm. By setting the oxygen concentration to less than 500ppm, ozone generation can be suppressed and safety during operation can be improved.

(Pencil hardness)

The pencil hardness of the hard coating is preferably 2H or more, more preferably 3H or more, and particularly preferably 4H or more. If the pencil hardness is 2H or more, it is not easy to deform and the visibility will not be reduced. Generally speaking, the pencil hardness of the hard coating is preferably high, but it can be 10H or less, or 8H or less, and even 6H or less can be used without problems in practical use.

(Types of hard coating)

The hard coating layer in the present invention may be one with other functions if it can be used to improve the pencil hardness of the surface as described above and protect the display. For example, a hard coating layer with additional functions such as an anti-glare layer with a certain pencil hardness as described above, an anti-glare anti-reflection layer, an anti-reflection layer, a low-reflection layer, and an anti-static layer may also be preferably used in the present invention.

[Implementation example]

Next, the effects of the present invention are described using examples and comparative examples. First, the evaluation method of the characteristic values used in the present invention is shown below.

(1) Ultimate viscosity

Film or polyester resin is pulverized and dried, and then dissolved in a mixed solvent of phenol/tetrachloroethane = 60/40 (mass ratio). After the solution is centrifuged to remove inorganic particles, the flow time of a solution with a concentration of 0.4 (g/dl) and the flow time of the solvent alone are measured at 30°C using an Oodel viscometer. From these time ratios, the limiting viscosity is calculated using the Huggins formula, assuming that the Huggins constant is 0.38. In the case of a laminated film, the corresponding polyester layer of the film is cut according to the laminate thickness to evaluate the limiting viscosity of each layer monomer.

(2) Crystallization degree based on density method

The density of the film sample was measured at three points, and the average value was taken as the degree of crystallization. The resin component is a mixture of completely amorphous and completely crystalline, and its density is described below. Based on the assumption that the density of the sample is the sum of the masses of the components constituting the sample divided by the sum of the volumes of the components, the degree of crystallization (weight ratio) of each resin was estimated. The density of the sample was measured according to the method (density gradient tube method) based on JISK-7112-1980. The calculation formula for the degree of crystallization is as follows, and the following values (unit: g/cm ³ ) are used for the individual density of each component.

Degree of crystallization (%) = {dc (d-da) / d (dc-da)} × 100

Here, dc: density of the completely crystalline part, da: density of the completely amorphous part, d: density of the sample

Polyethylene terephthalate resin: Density of the completely amorphous part (da) = 1.34, density of the completely crystalline part (dc) = 1.46

Polyethylene naphthalate resin: Density of the completely amorphous part (da) = 1.32, density of the completely crystalline part (dc) = 1.41

(3) Surface crystallization degree (ATR method)

The degree of crystallization was calculated from the intensity ratio of the absorption appearing near 1340 cm ^-1 and the absorption appearing near 1410 cm ^-1 (1340 cm ^-1 /1410 cm ^-1 ) on one side of the stretched film (the non-corona treated side when either side of the film was corona treated) under the following conditions. Here, 1340 cm ^-1 is the absorption caused by the bending vibration of CH ₂ (trans structure) of ethylene glycol, and 1410 cm ^-1 is the absorption that has nothing to do with crystallization or orientation.

(Measurement equipment and conditions)

FT-IR equipment: "FTS-60A/896" manufactured by Bio RadDIGILAB

1st reflection ATR accessory: SPECAC's "golden gate MKII"

Internal reflective element: Diamond

Angle of incidence: 45°

Resolution: 4cm ^-1

Total times: 128 times

(4) Pencil hardness

According to JIS K 5600-5-4:1999, the test was performed at a load of 750 g and a speed of 1.0 mm/s, and the presence or absence of deformation was visually confirmed. The test without deformation was rated as OK. The test was performed 5 times, and the pencil hardness with an OK rating of 4 or more was set as the measured value.

(Polymerization of urethane resin)

Into a four-necked flask equipped with a stirrer, a Dimroth condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer, 72.96 parts by mass of 1,3-bis(isocyanatemethyl)cyclohexane, 12.60 parts by mass of dihydroxymethylpropionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of a polycarbonate diol having a number average molecular weight of 2000, and 85.00 parts by mass of acetonitrile and 5.00 parts by mass of N-methylpyrrolidone as a solvent were added, and the mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere to confirm that the reaction solution reached a predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and 9.03 parts by mass of triethylamine was added to obtain a polyurethane prepolymer D solution. Next, 450 g of water was added to a reaction container equipped with a homodispersor capable of high-speed stirring, and while the mixture was adjusted to 25°C and stirred and mixed, the isocyanate group-terminated prepolymer was added and dispersed in water. Thereafter, acetonitrile and part of the water were removed under reduced pressure to prepare a water-soluble polyurethane resin (A) having a solid content of 35% by mass.

(Polymerization of water-soluble carbodiimide compounds)

Into a flask equipped with a thermometer, a nitrogen inlet tube, a reflux cooler, a dropping funnel, and a stirrer, 200 parts by weight of isophorone diisocyanate and 4 parts by weight of 3-methyl-1-phenyl-2-phospholene-1-oxide as a carbodiimidization catalyst were added, and stirred at 180°C for 10 hours in a nitrogen atmosphere to obtain isocyanate-terminated isophorone carbodiimide (polymerization degree = 5). Then, 111.2 g of the obtained carbodiimide and 80 g of polyethylene glycol monomethyl ether (molecular weight 400) were reacted at 100°C for 24 hours. Water was slowly added thereto at 50° C. to obtain a yellow transparent water-soluble carbodiimide compound (B) having a solid content of 40% by mass.

(Preparation of coating liquid for easy bonding)

Mix the following coating agents to prepare a coating solution.

(Preparation of Hard Coat Coating Liquid 10)

95 parts by weight of pentaerythritol triacrylate (A-TMM-3 manufactured by Shin-Nakamura Chemical Industry Co., Ltd., solid content concentration 100% by mass), 5 parts by weight of a photopolymerization initiator (Irgacure (registered trademark) 907 manufactured by BASF Japan, solid content concentration 100%), and 0.1 parts by weight of a leveling agent (BYK-Chemie Japan Co., Ltd., BYK307, solid content concentration 100%) were mixed and diluted with a solvent of toluene/MEK = 1/1 to prepare a coating liquid 10 with a concentration of 40%.

(Preparation of hard coat coating liquid 11)

50 parts by weight of urethane acrylate (UA-306H manufactured by Kyoeisha Chemical Co., Ltd., solid content concentration 100%), 45 parts by weight of pentaerythritol triacrylate (A-TMM-3 manufactured by Shin-Nakamura Chemical Co., Ltd., solid content concentration 100% by mass%), 5 parts by weight of photopolymerization initiator (Irgacure (registered trademark) 907 manufactured by BASF Japan, solid content concentration 100%), and 0.1 parts by weight of leveling agent (BYK-Chemie Japan, BYK307, solid content concentration 100%) were mixed and diluted with a solvent of toluene/MEK = 1/1 to prepare a coating liquid 11 with a concentration of 40%.

(Preparation of polyethylene terephthalate pellets (a))

As the esterification reaction apparatus, a continuous esterification reaction apparatus consisting of a stirring device, a separator, and a three-stage complete mixing tank with a raw material input port and a product output port was used. TPA was set to 2 tons/hr, EG was set to 2 mol relative to 1 mol of TPA, and antimony trioxide was set to an amount of Sb atoms of 160 ppm relative to the generated PET. These slurries were continuously supplied to the first esterification reaction tank of the esterification reaction apparatus, and reacted at 255°C under normal pressure with an average residence time of 4 hours. Next, the reaction product in the first esterification reaction tank was continuously taken out of the system and supplied to the second esterification reaction tank, and 8 mass % of EG distilled out from the first esterification reaction tank was supplied to the second esterification reaction tank relative to the produced polymer (produced PET), and further added: an EG solution containing magnesium acetate in an amount of 65 ppm of Mg atoms relative to the produced PET; and an EG solution containing TMPA in an amount of 20 ppm of P atoms relative to the produced PET, and reacted at 260° C. under normal pressure with an average residence time of 1.5 hours. Next, the reaction product in the second esterification reaction tank was continuously taken out of the system and supplied to the third esterification reaction tank, and an EG solution containing TMPA in an amount of 20 ppm of P atoms relative to the generated PET was further added, and the reaction was carried out at 260° C. under normal pressure with an average retention time of 0.5 hours. The esterification reaction product produced in the third esterification reaction tank was continuously supplied to a three-stage continuous polycondensation reaction device for polycondensation, and further filtered with a stainless steel sintered body filter (nominal filtration accuracy 5 μm particles 90% retention) to obtain polyethylene terephthalate pellets (a) with an ultimate viscosity of 0.58 dl/g.

(Preparation of polyethylene terephthalate pellets (b))

In the production step of polyethylene terephthalate pellets (a), except for adjusting the residence time of the third esterification reaction, the limiting viscosity was adjusted to 0.62 dl/g in the same manner to obtain polyethylene terephthalate pellets (b).

(Preparation of polyethylene terephthalate pellets (c))

The polyethylene terephthalate pellets (a) were solid-phase polymerized at 220° C. under a reduced pressure of 0.5 mmHg using a rotary vacuum polymerization apparatus to produce polyethylene terephthalate pellets (c) having an ultimate viscosity of 0.67 dL/g.

(Preparation of polyethylene terephthalate pellets (d))

The polyethylene terephthalate pellets (a) were solid-phase polymerized using a rotary vacuum polymerization apparatus at 220°C under a reduced pressure of 0.5 mmHg, with the time being changed compared to the above (c) to produce polyethylene terephthalate pellets (d) having an ultimate viscosity of 0.75 dL/g.

(Preparation of polyethylene terephthalate pellets (e))

The polyethylene terephthalate pellets (a) were solid-phase polymerized using a rotary vacuum polymerization apparatus at a reduced pressure of 0.5 mmHg and at 220°C, with the time being changed compared to the above (c) and (d), to produce polyethylene terephthalate pellets (e) having an ultimate viscosity of 0.83 dL/g.

(Examples 1 to 5, Comparative Examples 1 to 3)

After the above-mentioned polyethylene terephthalate pellets (c) were dried under reduced pressure (3 Torr) at 180°C for 8 hours, the polyethylene terephthalate pellets ( _c ) were supplied to an extruder and melted at 285°C. The polymer was filtered with a filter material of a sintered stainless steel body (nominal filter accuracy 10μm particles 95% retention), extruded from an extruder nozzle into a sheet, and then brought into contact with a casting drum with a surface temperature of 30°C by electrostatic casting to cool and solidify to produce an unstretched film. The unstretched film was stretched 3.4 times in the longitudinal direction at 85°C. The aforementioned easy-bonding layer coating liquid was applied to one side of the PET film by a roll coating method, and then dried at 80°C for 20 seconds. The coating weight after drying (after biaxial stretching) was adjusted to be 0.06 g/ ^m2 . The uniaxially stretched film was stretched 4.2 times in the width direction at 95°C using a tenter, and heat-treated at 235°C for 5 seconds to obtain the polyethylene terephthalate film of Example 1 in Table 1.

In addition, polyethylene terephthalate pellets (b) to (e) were used to obtain polyethylene terephthalate films of Examples 2 to 5 and Comparative Examples 1 to 3 in Table 1 by following almost the same steps as Example 1 except for changing the heat treatment temperature and heat treatment time.

(Implementation Example 1-1)

The hard coating liquid 10 was applied to the easy-adhesion layer of the polyethylene terephthalate film obtained in Example 1 using a Mel bar to a film thickness of 5.0 μm after drying, dried at 80°C for 1 minute, and then irradiated with ultraviolet rays (accumulated light dose 200 mJ/cm ² ) to obtain a hard coating film.

(Implementation Examples 1-1~5-1, 1-2, 2-2 and Comparative Examples 1-1~3-1)

Using the conditions listed in Table 2, a hard coating film was prepared in the same manner as in Example 1-1.

The hard coating film is bonded to the organic EL module through a 25μm thick adhesive layer to produce a flexible display. The hard coating film can protect the organic EL module from external impact and achieve good visibility.

Industrial availability

According to the present invention, it is possible to provide a polyester film whose surface hardness increases when a hard coating is laminated, and by using the hard coating film of the present invention in which a hard coating is laminated on the surface of a polyester film as a surface protective film for a display such as a flexible display, it is possible to provide a display having excellent properties such as scratch resistance, impact resistance, and deformation resistance on the surface of the display and excellent design properties.

Claims (4)

A polyester film for a surface protective film of a flexible display, which is a polyester film with a thickness of 25-188 μm and a limiting viscosity of 0.60-1.0 dL/g, a total light transmittance of 85% or more, a haze of 3% or less, a crystallization degree of 48% or more and 65% or less by density method, and a crystallization degree of 1.20 or more and 3.0 or less on the surface of the polyester film by total reflection infrared absorption measurement (FT-IR ATR measurement), and the polyester is polyethylene terephthalate. A polyester film for a surface protection film of a flexible display as claimed in claim 1, wherein at least one side of the polyester film has an easy-to-bond layer, and the easy-to-bond layer comprises a polyurethane resin. A hard coating film for a surface protection film of a flexible display, which has a hard coating layer on at least one side of the polyester film as claimed in claim 1 or 2. A display body using a hard coating film as claimed in claim 3 as a surface protection film.