TWI770121B - Polyester film and method for producing the same, hard coating film, foldable display and portable terminal - Google Patents

Abstract

The present invention provides a foldable display that is excellent in mass productivity, and that the image displayed on the folded portion after repeated bending is not likely to be disturbed; and a portable terminal equipped with such a foldable display; Polyester film for surface protection film and hard coat film for surface protection film.

The polyester film for a surface protection film of a foldable display is a polyester film with a thickness of 10-75 μm, characterized in that the refractive index of at least one of the longitudinal direction and the width direction is 1.57-1.64. Moreover, the hard-coat film of the polyester film for surface protection films of this foldable display, a foldable display, and a portable terminal device are provided.

Description

Polyester film and method for producing the same, hard coating film, foldable display and portable terminal

The present invention relates to a polyester film for a surface protection film of a folding display, a hard coat film for a surface protection film of a folding display, a folding display, and a portable terminal device, and is difficult to cause the surface Folding type displays and portable terminals with disturbed images caused by deformation of the film, and polyester films and hard coat films for surface protection films of the folding displays.

With the development of thin films for portable terminal equipment, portable terminal equipment represented by smartphones has become widely popular. While various functions are required for the portable terminal device, convenience is also required. Therefore, the popular portable terminal devices can be easily operated with one hand, even if they are stored in the pockets of clothes, etc., so they must be made into a small screen size of about 6 inches.

On the other hand, a tablet terminal with a screen size of 7 inches to 10 inches is expected to have high functionality not only for video content and music, but also for commercial use, drawing use, and reading. However, it cannot be operated with one hand, the portability is not good, and there are problems in terms of convenience.

In order to achieve this, there has been proposed a method of combining a plurality of displays for miniaturization such as Patent Document 1 (refer to Patent Document 1). However, since the part of the bezel is left, the image is cut off, so the visibility is not visible. Decreasing became a problem and not widespread.

Then, in recent years, a portable terminal integrating a flexible display and a foldable display has been proposed. According to this method, the video is not interrupted, and it can be conveniently carried as a portable terminal equipped with a large-screen display.

Here, with regard to the conventional displays and portable terminals that do not have a foldable structure, the surface of the display can be protected by inflexible materials such as glass, but in the case of a foldable display, it is When a part is folded to form a full-surface display, it is necessary to use a hard coat film or the like which is flexible and can protect the surface. However, in a foldable display, since a specific point corresponding to the folded portion is repeatedly folded, there are problems such as deformation of the film over time and distortion of the image displayed on the display.

Therefore, a method of locally changing the film thickness has been proposed (see Patent Document 2), but there is a problem that mass productivity is not good.

prior art literature Patent Literature

Patent Document 1 Japanese Patent Application Laid-Open No. 2010-228391

Patent Document 2 Japanese Patent Application Laid-Open No. 2016-155124

The present invention is intended to solve the problems of the surface protection member of the conventional display as described above, in order to provide excellent mass productivity, and there is no fear of causing disturbance in the image displayed on the folded portion after repeated bending. A foldable display and a portable terminal equipped with such a foldable display are provided, and a polyester film for a surface protection film and a hard coat film for a surface protection film of the foldable display are provided.

That is, the present invention has the following configuration.

1. A polyester film for a surface protection film of a foldable display, which is a polyester film with a thickness of 10 to 75 μm, characterized in that the refractive index of at least one of the longitudinal direction and the width direction is 1.57 to 1.64.

2. The polyester film for the surface protection film of the foldable display according to the above 1., wherein the refractive index in the bending direction is 1.57~1.64 (the bending direction here refers to the direction perpendicular to the folded part when the polyester film is folded). direction).

3. The polyester film for a surface protection film of a foldable display according to the above 2., wherein the bending direction corresponds to the longitudinal direction of the polyester film.

4. A hard coat film for a surface protection film of a foldable display, characterized in that at least one side of the polyester film for a surface protection film of a foldable display as any one of 1. to 3. above has a thickness of 1~40μm hard coating.

5. The hard coat film for a surface protection film of a folding display according to the above 4., wherein the pencil hardness of the hard coat layer measured with a load of 750 g according to JIS K5600-5-4:1999 is H or more.

6. A foldable display, which is a foldable display that uses a hard coat film for the surface protective film of the foldable display as described in the above 4. or 5. as a surface protective film in such a way that a hard coat layer is located on the surface. The display is characterized in that the bending radius when folded is 5 mm or less.

7. The foldable display according to the above 6., wherein a continuous single hard-coat film is provided along the folded portion of the foldable display.

8. A portable terminal device having the foldable display according to 6. or 7. above.

9. A method for producing a polyester film for a surface protection film of a foldable display, which is a method for producing a polyester film with a thickness of 10 to 75 μm, characterized by comprising: forming an unstretched polyester sheet in the longitudinal direction and the width At least one of the directions is extended to a stretching ratio of 1.0 to 3.0 times.

10. The manufacturing method of the polyester film for the surface protection film of the foldable display according to the above 9., wherein the extension ratio in the bending direction is 1.0 to 3.0 times (the bending direction here refers to the difference between folding the polyester film and folding the polyester film) in the orthogonal direction).

11. The method for producing a polyester film for a surface protection film of a foldable display according to 10. above, wherein the bending direction corresponds to the longitudinal direction of the unstretched polyester sheet.

The foldable display using the polyester film and the hard coat film for the surface protection film of the foldable display of the present invention maintains mass productivity, and the polyester film and the hard coat film do not cause deformation after repeated folding. Therefore, there will be no image disturbance in the folded part of the display. A portable terminal equipped with such a foldable display can provide beautiful images, rich functions, and excellent convenience such as portability.

1‧‧‧Folding Display

11‧‧‧Bending radius

2‧‧‧Polyester film for surface protection film of foldable display

21‧‧‧Folding part

22‧‧‧Bending direction (direction perpendicular to the folded part)

FIG. 1 is a schematic diagram showing the measurement of the bending radius when the folding display of the present invention is folded.

FIG. 2 is a schematic diagram showing the bending direction of the polyester film for the surface protection film of the foldable display of the present invention.

(monitor)

The so-called display in the present invention refers to all display devices, and the types of displays include LCD, organic EL display, inorganic EL display, LED, FED, etc., preferably LCD, organic EL, inorganic EL having a bendable structure . In particular, organic ELs and inorganic ELs that can reduce the layer structure are particularly preferred, and organic ELs with a wide color gamut are more preferred.

(foldable display)

A preferred foldable display is a continuous piece of display that can be folded into two when being carried, halving the size, and is a structure that improves portability. Also, at the same time, thin and light-weight ones are preferable. Therefore, the bending radius of the foldable display is preferably 5 mm or less, more preferably 3 mm or less. If the bending radius is 5 mm or less, it is possible to reduce the thickness of the folded state. It can be said that the smaller the bending radius, the better, but may be 0.1 mm or more, or 0.5 mm or more. Even if it is 1 mm or more, the practicability is very good compared to a conventional display that does not have a folding structure. The so-called bending radius when folded means the inner radius of the folded portion when folded, where the reference numeral 11 in the schematic diagram of FIG. 1 is measured. In addition, the surface protection film described later may be located on the outer side of the folding display, or may be located on the inner side.

(Organic EL)

The general structure of an organic EL display includes: an organic EL layer composed of an electrode/electron transport layer/light-emitting layer/hole transport layer/transparent electrode, a retardation plate for improving image quality, and a polarizing plate.

(Portable Terminal with Touch Panel)

In the case where an organic EL display is used in a portable terminal device with a touch panel, a touch panel module is arranged on the upper part of the organic EL display or between the organic EL layer and the retardation plate. At this time, when an impact is applied from the top, there is a possibility that the circuit of the organic EL and the touch panel may be disconnected, so a surface protection film is necessary. A hard coat layer is laminated on at least the surface side of the display.

(Surface Protection Film for Folding Displays)

As a surface protection film, if it is a polyimide film, polyester film, polycarbonate film, acrylic film, triacetin-based cellulose film, cycloolefin polymer film, etc., high light penetration, haze value (haze) Thin films can be used, but among them, polyimide films and polyester films having high impact resistance and sufficient pencil hardness are preferred, and polyester films that can be produced inexpensively are particularly preferred.

In the present invention, the polyester film may be a single-layered film including one or more polyester resins, and when two or more types of polyesters are used, it may be a multilayered film or a super-multilayered repetitive structure. Laminated films.

The polyester resin used for the polyester film includes, for example, polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, or the like It is a polyester film of a copolymer whose main component is the resin. Among them, a stretched polyethylene terephthalate film is particularly preferred from the viewpoint of mechanical properties, heat resistance, transparency, and price.

When a polyester copolymer is used for a polyester film, the dicarboxylic acid component of the polyester includes, for example, aliphatic dicarboxylic acids such as adipic acid and sebacic acid; terephthalic acid and isophthalic acid. , Aromatic dicarboxylic acids such as phthalic acid and 2,6-naphthalenedicarboxylic acid; polyfunctional carboxylic acids such as 1,2,4-benzenetricarboxylic acid and 1,2,4,5-benzenetetracarboxylic acid. Moreover, examples of glycol components include fatty acid glycols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, and neopentyl glycol; and aromatic glycols such as p-xylene glycol. ; Alicyclic glycols such as 1,4-cyclohexanedimethanol; polyethylene glycols with an average molecular weight of 150~20,000. The mass ratio of the copolymerized component of the preferred copolymer is less than 20% by mass. When it is less than 20 mass %, film strength, transparency, and heat resistance can be maintained, which is preferable.

In addition, in the production of polyester film, the limiting viscosity of at least one or more resin particles is preferably in the range of 0.50 to 1.0 dl/g. When the limiting viscosity is 0.50 dl/g or more, the impact resistance of the obtained film will be improved, and it is difficult to break the internal circuit of the display due to external impact, so it is preferable. On the other hand, when the limiting viscosity is 1.00 dl/g or less, the rise of the filtration pressure of the molten fluid does not become too large, and it is easy to stably operate the film production, which is preferable.

The thickness of the polyester film is preferably 10 to 75 μm, more preferably 25 to 75 μm. When the thickness is 10 μm or more, the pencil hardness improvement effect is seen, and when the thickness is 75 μm or less, in addition to being advantageous in weight reduction, flexibility, workability, and handling properties are excellent.

The surface of the polyester film of the present invention may be smooth or uneven, but from the viewpoint that it can be used for surface covering of displays, it is unfavorable that the optical properties due to the unevenness are lowered. The fog value is preferably 3% or less, more preferably 2% or less, and most preferably 1% or less. If the fog value is less than 3%, the visibility of the image can be improved. The lower limit of the haze value is preferably as small as possible, but may be 0.1% or more, or 0.3% or more.

As described above, for the purpose of reducing the haze value, it is preferable that the unevenness of the film surface is not large, but from the viewpoint of handleability, in order to impart moderate smoothness, as a method of forming unevenness, it is possible to blend The filler is formed in the polyester resin layer of the surface layer, or by coating a coating layer containing the filler in the film forming process.

As a method of blending the particles with the base film, a known method can be adopted. For example, it can be added at any stage of polyester production, but it is preferably in the form of a slurry dispersed in ethylene glycol or the like at the stage of esterification or after the end of the transesterification reaction and before the start of the polycondensation reaction. added while continuing the polycondensation reaction. In addition, according to the method of mixing the slurry of particles dispersed in ethylene glycol or water with polyester raw materials using a kneading extruder with ventilation holes; or using a kneading extruder to extrude the dried particles It is carried out by the method of mixing with a polyester raw material, etc.

Among them, it is preferable that: after the aggregated inorganic particles are homogeneously dispersed in the monomer liquid which is a part of the polyester raw material, the filtered ones are added before the esterification reaction, during the esterification reaction or after the esterification reaction. method in the remainder of the polyester feedstock. According to this method, since the monomer liquid has a low viscosity, the homogeneous dispersion of the particles and the high-precision filtration of the slurry can be easily performed, and at the same time, when added to the remaining part of the raw material, the dispersibility of the particles is good, and new particles are not easily generated. Aggregate. From this viewpoint, it is particularly preferable to add the remainder of the raw material to the low temperature state before the esterification reaction.

In addition, after obtaining polyester containing particles in advance, the number of protrusions on the film surface can be further reduced by a method such as kneading and extruding the particles and particles not containing particles (master batch replenishment method).

In addition, the polyester film may contain various additives within the range in which the total light transmittance can be maintained in a preferable range. As an additive, an antistatic agent, a UV absorber, and a stabilizer are mentioned, for example.

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

On the surface of the polyester film of the present invention, a treatment for improving the adhesiveness with a resin for forming a hard coat layer or the like can be performed.

As a method of surface treatment, for example, sandblasting, solvent treatment, etc., roughening treatment, corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone-ultraviolet irradiation treatment, flame treatment, chromic acid treatment can be mentioned. , oxidation treatment such as hot air treatment, etc., can be used without particular limitation.

Moreover, adhesiveness can also be improved by the adhesion|attachment lift layer, such as an easy-bonding layer. As the easily bonding layer, acrylic resins, polyester resins, polyurethane resins, polyether resins, etc. can be used without particular limitation, and a general coating method, preferably so-called in-mold coating (in-mold coating), can be used. line coating) method to form.

The above-mentioned polyester film can be produced, for example, through the following steps: after homogeneously dispersing inorganic particles in a monomer liquid belonging to a part of the polyester raw material and filtering it, adding it to the remaining part of the polyester raw material, and carrying out the polyester treatment. The polymerization step of polymerization: The polyester is melted and extruded into a sheet shape through a filter, and after cooling, it is stretched to form a film forming step of a base film.

Next, regarding the manufacturing method of a biaxially stretched polyester film, the example which used polyethylene terephthalate (it describes as PET) pellets as a raw material of a base film is demonstrated in detail, but it is not limited to this. In addition, a single-layer structure, a multilayer structure, etc. may be sufficient, and the number of layers is not limited.

After mixing and drying the PET pellets at a predetermined ratio, the pellets are supplied to a known extruder for fusion lamination, extruded from a slit-shaped die into a sheet shape, and cooled and solidified on a casting drum to form an unstretched film. In the case of a single layer, one extruder may be used, but in the case of making a film composed of multiple layers, two or more extruders may be used, and two or more layers of manifolds or merging blocks (eg, with squares) may be used. The junction block of the junction part), a plurality of film layers constituting each outermost surface layer are laminated, two or more sheets are extruded from a nozzle, and cooled by a casting drum to form an unstretched film.

At this time, it is preferable to perform high-precision filtration in order to remove foreign matter contained in the resin while the molten resin is maintained at an arbitrary position of about 280° C. during melt extrusion. The filter material used in the high-precision filtration of molten resin is not particularly limited, but is preferred from the viewpoint of excellent performance in removing aggregates and high-melting point organics mainly composed of Si, Ti, Sb, Ge, and Cu. The filter material of stainless steel sintered body.

Furthermore, the filter particle size (initial filtration efficiency of 95%) of the filter material is preferably 20 μm or less, and particularly preferably 15 μm or less. When the filtration particle size of the filter material (initial filtration efficiency 95%) exceeds 20μm, foreign matter larger than 20μm cannot be sufficiently removed. High-precision filtration of molten resin by using a filter material with a filter particle size (initial filtration efficiency of 95%) of 20 μm or less, although productivity may be reduced, protrusions caused by coarse particles can be obtained. For less film, it is better.

The refractive index in at least one of the longitudinal direction (mechanical flow direction) and the width direction of the polyester film is preferably 1.57 to 1.64. Then, the refractive index in the bending direction of the polyester film is preferably 1.57 to 1.64. Here, the so-called bending direction means the difference between the folded part (reference numeral 22) and the folded part (reference numeral 22), which is assumed in the application of the surface protection film of the folding display, as shown by the reference numeral 22 on the polyester film (reference numeral 2) of FIG. 2 . 21) Orthogonal directions. When the refractive index in at least one of the longitudinal direction and the width direction is 1.57 to 1.64, deformation during repeated folding is small, and there is no risk of lowering the image quality of the foldable display, which is preferable. The refractive index is preferably 1.575~1.634. Of course, this direction is preferably the above-mentioned bending direction. The refractive index of the polyester film can be effectively adjusted by adjusting the stretching ratio. Preferably, the stretching ratio in at least one of the longitudinal direction (mechanical flow direction) and the width direction of the unstretched polyester sheet is 1.0 to 3.0 times, more preferably 1.0 to 2.5 times. Then, the extending direction is preferably the above-mentioned bending direction. The stretching temperature is preferably 80 to 140°C, more preferably 90 to 130°C. In addition, the heating method at the time of extending|stretching can employ|adopt conventionally well-known means, such as a hot-air heating method, a drum heating method, and an infrared heating method. By setting the stretching temperature to 80 to 140° C., it is possible to prevent uneven thickness caused by stretching at the above stretching ratio.

The refractive index of the direction orthogonal to the bending direction (the direction of the folded portion) is preferably larger than the bending direction based on the mechanical properties of the film. It is better to extend the magnification larger than the bending direction. Stable productivity can be obtained by making a draw ratio into 2.5 times or more, and favorable impact resistance can be obtained by making a draw ratio into 5 times or less.

In addition, the bending direction of the polyester film should preferably correspond to the longitudinal direction (mechanical flow direction). In this way, stable productivity can be obtained. That is, it is possible to obtain a polyester film in which the unstretched polyester sheet is stretched at a stretching ratio of 1.0 to 3.0 times, preferably 1.0 to 2.5 times, in the longitudinal direction. Moreover, in the width direction, it can be said that it is a preferable aspect to perform stretching at a stretching ratio of 2.5 to 5.0 times.

Specifically, for example, after PET pellets are sufficiently vacuum-dried, they are supplied to an extruder, melted and extruded into a sheet shape at about 280° C., and cooled and solidified to form an unstretched PET sheet. Using a roller heated to 80 to 120° C., the obtained unstretched sheet is stretched 1.0 to 3.0 times in the longitudinal direction to obtain a uniaxially oriented PET film. Furthermore, hold the end of the film with a clip, and guide it to a hot air area that has been heated to 80~180°C. After drying, it extends 2.5~5.0 times in the width direction. Next, it can be led to a heat treatment area of 160 to 240°C for 1 to 60 seconds of heat treatment. In this heat treatment step, a relaxation treatment of 1-12% may be applied in the width direction or the longitudinal direction as required.

The limit viscosity of polyester film is preferably in the range of 0.50~1.0dl/g. When the limiting viscosity is more than 0.50dl/g, the impact resistance is improved, and it is not easy to break the internal circuit of the display caused by external impact, so it is good. On the other hand, when the limiting viscosity is 1.0 dl/g or less, the increase in the filtration pressure of the molten fluid does not become too large, and the film production is stable, which is preferable.

(Easy bonding layer)

The easy-bond layer is obtained by coating the coating liquid on one or both sides of an unstretched or longitudinally uniaxially stretched film, drying at 100-150° C., and then extending in the lateral direction. The coating amount of the final easy-bonding layer is preferably managed at 0.05~0.20g/m ² . When the coating amount is less than 0.05 g/m ² , the adhesiveness may be insufficient. On the other hand, when the coating amount exceeds 0.20 g/m ² , the blocking resistance may be lowered.

唑啉樹脂、環氧樹脂等。也可分別混合兩種以上來使用。該等係依模內塗布之性質，藉由水系塗布液進行塗布為佳，該樹脂或交聯劑係較佳係為水溶性或水分散性之樹脂和化合物。 As the resin used for the easily bonding layer, for example, polyester resin, polyurethane resin, polyester polyurethane resin, polycarbonate polyurethane can be used without particular limitation. resin, acrylic resin, etc. As the crosslinking agent of the easily bonding layer, melamine, isocyanate compound,

oxazoline resin, epoxy resin, etc. It is also possible to mix and use two or more of them, respectively. These are based on the properties of in-mold coating, preferably by a water-based coating solution, and the resin or cross-linking agent is preferably a water-soluble or water-dispersible resin and compound.

系、矽酮系等之有機聚合物系粒子等。該等係可單獨添加至易接著層，亦可組合兩種以上而添加。 In order to impart slipperiness to the easy-bonding layer, it is preferable to add particles. The average particle size of the fine particles is preferably 2 μm or less. When the average particle diameter of the particles exceeds 2 μm, the particles tend to fall off from the easily bonding layer. Examples of particles contained in the easily bonding layer include titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, Inorganic particles of tungsten oxide, lithium fluoride, calcium fluoride, etc.; and styrene-based, acrylic-based, melamine-based, benzoguanidine

Organic polymer-based particles, such as silicone-based and silicone-based particles, etc. These systems may be added individually to the easily bonding layer, or may be added in combination of two or more.

In addition, as a method of applying the coating liquid, the same known method as the above-mentioned coating layer can be adopted. For example, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll flash coating method, a spray coating method, an air knife coating method, a linear bar coating method, a tubular doctor blade method can be mentioned. methods, etc., these methods can be carried out individually or in combination.

(hard coating)

The polyester film on the surface of the foldable display to protect the display preferably has a hard coating on at least one surface thereof. The hard coat layer is preferably used in the display so that it is located on the display surface side on the polyester film. As the resin for forming the hard coat layer, an acrylic type, a siloxane type, an inorganic blend type, a urethane acrylate type, a polyester acrylate type, an epoxy type, etc. can be used without particular limitation. In addition, two or more kinds of materials may be mixed and used, and particles such as inorganic fillers and organic fillers may be added.

(film thickness of hard coat)

The thickness of the hard coat layer is preferably 1 to 40 μm. When thicker than 1 micrometer, it hardens|cures sufficiently, and favorable pencil hardness can be obtained. Moreover, by setting the thickness to be 40 μm or less, the warpage due to the curing shrinkage of the hard coating can be suppressed, and the handling properties of the film can be improved. More preferably, it is 2 to 20 μm, and even more preferably, it is 3 to 15 μm.

(coating method)

As the coating method of the hard coat layer, linear bar coating, gravure coating, die coating, knife coating, etc. can be used without particular limitation, and can be appropriately selected according to the viscosity and film thickness.

(hardening condition)

As a hardening method of the hard coat layer, a hardening method using energy rays such as ultraviolet rays and electron beams, heat curing methods, and the like can be used. In order to reduce damage to the film, a curing method using ultraviolet rays, electron beams, or the like is preferable.

(Pencil Hardness)

The pencil hardness of the hard coat layer is preferably B or higher, more preferably H or higher, and particularly preferably 2H or higher. If it is a pencil hardness of B or more, it will not be damaged easily, and will not reduce visibility. Generally speaking, the higher the pencil hardness of the hard coat layer, the better, but it may be 10H or less, or 8H or less, and even if it is 6H or less, it can be used without problems in practice.

(Characteristics of hard coating)

The hard coat layer of the present invention can be used for the purpose of improving the pencil hardness of the surface as described above to protect the display, and the one with a high transmittance is preferred. The penetration rate of the hard coating film is preferably 87% or more, more preferably 88% or more. When the penetration rate is 87% or more, sufficient visibility can be obtained. The total light transmittance of the hard coat film is generally as high as possible, but it can be less than 99% or less than 97%. In addition, the haze value of the hard coating film is generally as low as possible, preferably 3% or less. More preferably, the haze value of the hard coating film is 2% or less, and most preferably 1% or less. If the haze value is 3% or less, the visibility of the image can be improved. Generally speaking, the lower the haze value, the better, and it may be 0.1% or more, or 0.3% or more.

Other functions can be further added to the hard coat layer. For example, an anti-glare layer having 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 and other additional organic hard coat layers can also be suitably used in the present invention.

[Example]

Next, the effects of the present invention will be described using Examples and Comparative Examples. First, the evaluation method of the characteristic value implemented in the present invention is described below.

(1) Ultimate viscosity

After pulverizing and drying the film or polyester resin, it is dissolved in a mixed solvent of phenol/tetrachloroethane=60/40 (mass ratio). After the solution was subjected to centrifugal separation to remove inorganic particles, the flow time of the solution with a concentration of 0.4 (g/dl) at 30°C and the flow time of the solvent alone were measured using an Ubbelohde viscometer. The time scale, using Huggins' formula, and assuming a Huggins constant of 0.38, calculates the limiting viscosity.

(2) Bending resistance

Prepare a hard coat film sample with a size of 50 mm in the width direction x 100 mm in the flow direction. Using an unburdened U-shaped expansion tester (DLDMLH-FS, manufactured by Yuasa Systems Machinery Co., Ltd.), the bending radius was set to 2 mm, and the bending was performed 200,000 times at a speed of 1 time/second. At this time, the sample was fixed at a position of 10 mm at both ends of the long side, and the bent portion was made 50 mm×80 mm. After the bending process, the sample was placed on a flat surface with the inside of the bending as the bottom, and a visual inspection was performed.

⊚: The deformation of the sample could not be confirmed.

○: The sample is deformed, and when placed horizontally, the maximum floating height is less than 5 mm.

×: There is a crease on the sample, and when it is placed horizontally, the maximum floating height is 5 mm or more.

(3) Refractive index

According to JIS K 7142-2008 "Method for Measuring the Refractive Index of Plastics (Method A)", the refractive index in the longitudinal direction and the width in the width direction were obtained using an Abbe refractometer (NAR-4T, manufactured by ATAGO, Inc., measuring wavelength 589 nm). the refractive index.

(4) Pencil hardness

Using the pencil hardness of the hard coat film as a sample, it was measured according to JIS K 5600-5-4: 1999 under a load of 750 g and a speed of 1.0 mm/s.

(5) Total light transmittance, haze value

The hard coat film was used as a sample, and the measurement was performed using a haze meter (manufactured by Nippon Denshoku Kogyo Co., Ltd., NDH5000).

(Preparation of polyethylene terephthalate particles (a))

As the esterification reaction device, a continuous esterification reaction device composed of a three-stage complete mixing tank having a stirring device, a fractionator, a raw material addition port, and a product extraction port was used, TPA was set to 2 tons/hour, and EG was set to With respect to 2 mol of 1 mol of TPA, antimony trioxide was set to an amount of 160 ppm of Sb atom with respect to the produced PET, and these slurries were continuously supplied to the first esterification reaction tank of the esterification reaction apparatus, and then The reaction was carried out at 255°C under normal pressure with an average residence time of 4 hours. Next, the reaction product of the first esterification reaction tank was continuously taken out of the system, and supplied to the second esterification reaction tank, and in the second esterification reaction tank, the EG distilled off from the first esterification reaction tank was The EG solution containing magnesium acetate in an amount of 65 ppm of Mg atoms relative to the produced PET and 20 ppm of P atoms relative to the produced PET was added after supplying 8 mass % with respect to the produced polymer (produced PET). The amount of EG solution containing TMPA was reacted at 260°C under normal pressure with an average residence time of 1.5 hours. Next, the reaction product of 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 was added in an amount of 20 ppm of P atoms with respect to the produced PET, The reaction was carried out at 260°C under normal pressure with an average residence time of 0.5 hours. The esterification reaction product generated in the third esterification reaction tank is continuously supplied to the continuous polycondensation reaction device of 3 stages, and the polycondensation is carried out. 90% was removed) and filtered to obtain polyethylene terephthalate particles (a) with an intrinsic viscosity of 0.62 dl/g.

(Preparation of polyethylene terephthalate particles (b))

In the production step of the polyethylene terephthalate particles (a), except for adjusting the residence time of the third esterification reaction, the limiting viscosity was adjusted to 0.580 dl/g in the same manner to obtain polyethylene terephthalate. Glycol particles (b).

(Preparation of polyethylene terephthalate particles (c))

Using a rotary vacuum polymerization device, under the reduced pressure of 0.5 mmHg, the polyethylene terephthalate particles (a) were solid-phase polymerized at 220 ° C under a reduced pressure of 0.5 mmHg. Ethylene phthalate particles (c).

(Polymerization of Urethane Resin)

72.96 parts by mass of 1,3-bis(isocyanatomethyl)cyclohexane, dimethylol group were added to a 4-neck flask equipped with a stirrer, a Day's condenser, a nitrogen introduction tube, a silica gel drying tube, and a thermometer. 12.60 parts by mass of propionic acid, 11.74 parts by mass of neopentyl glycol, 112.70 parts by mass of polycarbonate diol with a number average molecular weight of 2000, and 85.00 parts by mass of propionitrile as a solvent, and 5.00 parts by mass of N-methylpyrrolidone, in Under a nitrogen atmosphere, the mixture was stirred at 75°C for 3 hours, and it was confirmed that the reaction liquid reached a predetermined amine equivalent. Next, after cooling this reaction liquid to 40 degreeC, 9.03 mass parts of triethylamines were added, and the polyurethane prepolymer D solution was obtained. Next, to a reaction vessel equipped with a homogenizer capable of high-speed stirring, 450 g of water was added, the temperature was adjusted to 25° C., and the mixture was stirred and mixed for 2000 min ⁻¹ , and the isocyanate group-terminated prepolymer was added to conduct water dispersion. Then, propionitrile and a part of water were removed under reduced pressure, and the water-soluble polyurethane resin (A) of 35 mass % of solid content was prepared.

(polymerization of water-soluble carbodiimide)

200 parts by mass of isophorone diisocyanate and 3-methyl-1-phenyl-2 as a carbodiimide catalyst were added to a flask equipped with a thermometer, a nitrogen introduction tube, a reflux cooler, a lower funnel, and a stirrer. 4 parts by mass of -phosphorus-1-oxide was stirred at 180° C. for 10 hours in a nitrogen atmosphere to obtain isocyanate-terminated isophorone carbodiimide (degree of polymerization=5). Next, 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 layer)

The following coating agents were mixed to prepare a coating liquid.

(Silica gel with an average particle size of 40nm, solid content concentration of 40% by mass)

Surfactant 0.05 parts by mass

(Silicone type, solid content concentration 100% by mass)

After the polyethylene terephthalate master batch pellets (a) were dried under reduced pressure (3 Torr) at 180°C for 8 hours, the polyethylene terephthalate pellets (a) were separated into the extruder. It was supplied to an extruder and melted at 285°C. This polymer is filtered with a filter material of stainless steel sintered body (nominal filtration accuracy of 10μm particles is separated by 95%). After being extruded into a sheet from a nozzle, electrostatically applied casting method is used to make it come into contact with a casting with a surface temperature of 30°C. Roller, cooling and solidification to make unstretched film. This unstretched film was not stretched in the longitudinal direction (stretched at 85°C), the coating liquid was applied to one side of a PET film by a roll coating method, and then dried at 80°C for 20 seconds. In addition, the coating amount after drying at the final (after biaxial stretching) was adjusted to be 0.06 g/m ² . Then, this film was stretched 4.0 times in the width direction at 95° C. using a tenter, and heat-treated at 220° C. for 5 seconds to obtain a polyethylene terephthalate film of No. 1 in Table 1. Except for some adjustment of the conditions, the polyethylene terephthalate master batch pellets (b) to (c) were supplied according to almost the same procedure as above, to obtain the polyethylene terephthalate Nos. 2 to 9 in Table 1. Ethylene formate film.

(Preparation of hard coat coating solution a)

To 100 parts by weight of a hard coating agent containing an acrylate-based inorganic nanofiller (manufactured by Nippon Kasei Co., Ltd., ELCOM (registered trademark) LP-1001, solid content concentration 61%), a leveling agent (manufactured by BYK Japan Co., Ltd., BYK307, solid content concentration 100%) 0.1 weight part, was diluted with MEK, and prepared the hard coating coating liquid a of solid content concentration 40 mass %.

(Preparation of Hard Coating Liquid b)

95 parts by weight of a urethane acrylate-based hard coat agent (manufactured by Arakawa Chemical Industry Co., Ltd., beamset (registered trademark) 577, solid content concentration 100%), a photopolymerization initiator (manufactured by BASF Japan, Irgacure (registered trademark) Trademark) 184, solid content concentration 100%) 5 weight parts, leveling agent (made by BYK Japan company, BYK307, solid content concentration 100%) 0.1 weight part are mixed, utilize the solvent of toluene/MEK=1/1 to carry out dilution, Coating liquid b with a concentration of 40% was prepared.

(Example 1)

Using a linear bar, the hard coat coating solution a was applied on one side of the polyethylene terephthalate film No. 3 so that the film thickness after drying would be 5.0 μm, and after drying at 80° C. for 1 minute, Ultraviolet rays (high pressure mercury lamp, cumulative light intensity of 200 mJ/cm ² ) were irradiated to obtain a hard coat film.

(Examples 2 to 8, Comparative Examples 1 to 2)

By the same method as in Example 1, hard coat films under the conditions shown in Table 2 were prepared.

A 25μm thick adhesive layer is interposed, and the formed film is attached to the organic EL module to form a smartphone-type foldable display that can be folded in half at the center of the whole with a radius of 3mm corresponding to the bending radius shown in Figure 1. The hard coat film is disposed on the surface of a continuous piece of display that is connected to the folded portion, and the hard coat is disposed on the surface of the display. Those using the hard coat films of the respective embodiments are used as smart phones that can be folded in half at the center portion and carried, and satisfy the operation and visibility. On the other hand, the foldable displays using the hard-coat films of the comparative examples are not preferable as the frequency of use increases, and it is felt that the folded portion of the display produces image skew.

Industrial Availability

According to the foldable display using the polyester film and the hard coat film for the surface protection film of the foldable display of the present invention, the mass productivity can be maintained without causing the polyester film and the hard coat film on the surface of the foldable display. Deformation after repeated folding, so there will be no image disorder in the folded part of the display. A portable terminal equipped with a foldable display using the polyester film and the hard coat film of the present invention as a surface protective film can provide beautiful images, and is rich in functions and excellent in convenience such as portability.

1‧‧‧Folding Display

11‧‧‧Bending radius

Claims (11)

A polyester film for a surface protection film of a foldable display is a polyester film with a thickness of 10 to 75 μm, characterized in that at least one of the lengthwise direction and the widthwise direction measured at a wavelength of 589 nm according to JIS K 7142-2008 The refractive index in one direction is 1.57 to 1.64. The polyester film for a surface protective film of a foldable display according to claim 1, wherein the refractive index in the bending direction measured at a wavelength of 589 nm according to JIS K 7142-2008 is 1.57~1.64, and the bending direction here refers to The direction orthogonal to the folded portion when folding the polyester film. The polyester film for a surface protection film of a foldable display according to claim 2, wherein the bending direction corresponds to the longitudinal direction of the polyester film. A hard coating film for a surface protection film of a foldable display, characterized in that at least one side of the polyester film for a surface protection film of a foldable display according to any one of claims 1 to 3 has a thickness of 1~3 40μm hard coat. The hard coat film for a surface protective film of a foldable display according to claim 4, wherein the pencil hardness of the hard coat measured with a load of 750 g according to JIS K5600-5-4:1999 is H or more. A foldable display, which is a foldable display configured by using a hard coat film for the surface protection film of the foldable display as claimed in claim 4 or 5 as a surface protection film in such a way that a hard coating is positioned on the surface, which The characteristic is that the bending radius when folded is 5mm or less. The foldable display of claim 6, wherein a continuous single hard-coat film is provided along the folded portion of the foldable display. A portable terminal device having the foldable display as claimed in claim 6 or 7. A method for producing a polyester film for a surface protection film of a foldable display as claimed in claim 1, which is a method for producing a polyester film with a thickness of 10 to 75 μm, characterized by comprising: placing an unstretched polyester sheet on the long side In at least one of the direction and the width direction, a step of extending to a stretching ratio of 1.0 to 3.0 times is performed. As claimed in claim 9, the method for producing a polyester film for a surface protection film of a foldable display, wherein the extension ratio in the bending direction is 1.0 to 3.0 times, and the bending direction here refers to the distance between the folded part and the folded part when the polyester film is folded. direction of handing. The method for producing a polyester film for a surface protection film of a foldable display according to claim 10, wherein the bending direction corresponds to the longitudinal direction of the unstretched polyester sheet.

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