TW201842006A - Polyester film as surface protective film for foldable display and application thereof - Google Patents

Abstract

Provides a foldable display that is excellent in mass productivity and does not cause disturbance of the image displayed in the folded portion after repeated bending, and a mobile terminal equipped with such a foldable display. Hard coating film for ester film and surface protection film.

A polyester film for a surface protection film of a folding display, which is a polyester film with a thickness of 10 to 75 μm, in which the 0.2% guaranteed strength point strain of at least one of the long side direction and the width direction is 2.6 to 5.0% . A hard coating film using a polyester film for a surface protection film of a folding display, a folding display, and a mobile terminal.

Description

   Polyester film for surface protection film of folding display and use thereof   

The present invention relates to a polyester film for a surface protection film of a foldable display, a hard coating film for a surface protection film of a foldable display, a foldable display, and a mobile terminal, and relates to a film that is unlikely to occur on the surface even after repeated folding Folding displays and mobile terminals with distorted images and polyester films and hard coating films for surface protection films of the aforementioned folding displays.

Thin films for mobile terminals are being promoted, and mobile terminals such as smart phones are widely used. Various functions are required for mobile terminal systems, and convenience is also required. Therefore, a popular mobile terminal can be easily operated with one hand, and it is premised on being able to be stored in a pocket of clothes, etc., so it needs to be a small screen size of about 6 inches.

On the other hand, in the case of tablet terminals with a screen size of 7 inches to 10 inches, not only image information or music, but also commercial use, drawing use, and reading should be envisaged. It is highly functional. However, it cannot be operated with one hand, the portability is poor, and there are problems in terms of convenience.

In order to meet these requirements, some people have proposed a method of reducing weight by connecting a plurality of displays together (Patent Document 1). However, because there are frames remaining, the image is cut off, which causes the problem of reduced visibility and cannot be popularized. .

Therefore, in recent years, some people have proposed mobile terminals incorporating flexible displays and folding displays. With this method, the image is not cut off, and it can be conveniently carried in the form of a mobile terminal equipped with a large-screen display.

Here, as for a display or a mobile terminal without a current folding structure, although the surface of the display can be protected with a material that does not have flexibility such as glass, the folding type display is made by folding one side. In the case of a display, it is necessary to use a hard coating film or the like which has flexibility and can protect the surface. However, in the foldable display, since a certain folded portion is repeatedly bent, the film in the place is deformed with the passage of time, and there is a problem that the film is distorted to the image displayed on the display.

Therefore, a method of partially changing the film thickness has also been proposed (see Patent Document 2), but there is a problem of lack of mass productivity.

    [Prior technical literature]         [Patent Literature]    

[Patent Document 1] Japanese Patent Laid-Open No. 2010-228391

[Patent Document 2] Japanese Patent Laid-Open No. 2016-155124

The present invention is an invention which aims to solve the problems of the surface protection member of the conventional display as described above, and is intended to provide excellent mass productivity and to avoid disturbance of the image displayed on the folded portion after repeated bending. The invention of a foldable display and a mobile terminal equipped with such a foldable display provides a polyester film for a surface protective film and a hard coating film for a surface protective film of a foldable display.

That is, this invention is comprised by the following structures.

1. A polyester film for a surface protection film of a foldable display, which is a polyester film having a thickness of 10 to 75 μm, which is characterized by a 0.2% guaranteed strength point strain of at least one of the longitudinal direction and the width direction: 2.6 ~ 5.0%.

2. The polyester film for the surface protection film of the folding display according to item 1 above, characterized in that the 0.2% guaranteed strength point strain in the bending direction is 2.6 to 5.0%.

(Here, the bending direction refers to a direction orthogonal to the folded portion when the polyester film is folded.)

3. The polyester film for the surface protection film of the folding display according to item 1 or 2 above, characterized in that the limiting viscosity of the film is 0.60 to 1.0 dl / g.

4. A hard coating film for a surface protection film of a folding display, characterized in that at least one side of the polyester film for a surface protection film of a folding display according to any one of items 1 to 3 above, has Hard coating with a thickness of 1 ~ 50μm.

5. The hard coating film for a surface protection film of the folding display according to the item 4 above, characterized in that the hardness of the hard-coat pencil pencil measured according to JIS K5600-5-4: 1999 under a load of 750 g is H or more.

6. A foldable display, which is a foldable display in which the surface protective film of the foldable display according to item 4 or 5 above is arranged so that the hard coating layer is on the surface, and the hard coating film is used as the surface protective film. , Characterized in that the bending radius during folding is 5 mm or less.

7. The folding display according to item 6 above, characterized in that it is provided with a single hard coating film that is continuous through the folded portion of the folding display.

8. A mobile terminal having a foldable display as described in the above item 6 or item 7.

The folding type display using the polyester film or hard coating film for the surface protection film of the folding type display of the present invention, while maintaining mass productivity, the polyester film or hard coating film will not be repeatedly folded. It is distorted, so the image of the folded part of the display is not disturbed. The mobile terminal equipped with the foldable display as described above is a mobile terminal that provides beautiful images, rich functionality, and convenience such as portability.

1‧‧‧ folding display

11‧‧‧ bending radius

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

21‧‧‧Folding Department

22‧‧‧ Bending direction (direction orthogonal to the folded portion)

O‧‧‧ origin

A‧‧‧ the farthest point from O where Hooke's law is established

P‧‧‧ Draw a line parallel to the OA line from Q, its point crossing the stress-strain curve

Q‧‧‧ The point of 0MPa stress and 0.2% elongation (strain)

H‧‧‧ Draw a line parallel to the vertical axis from P, the point that intersects the horizontal axis (0.2% guaranteed strength point strain (%))

Stress value of σ _0.2 ‧‧‧P (MPa)

FIG. 1 is a schematic diagram showing a measurement place of a bending radius at the time of folding in the present invention.

FIG. 2 is a schematic view showing a bending direction of the polyester film for a surface protection film of a folding display according to the present invention.

FIG. 3 is a schematic diagram for explaining the 0.2% guaranteed strength point strain in the present invention.

    [Form of Implementing Invention]    

(Monitor)

The display referred to in the present invention refers to all display devices. As the type of display, there are LCD, organic EL display, inorganic EL display, LED, FED, etc., but it is preferably an LCD that can be bent or an organic EL. , Inorganic EL. In particular, organic ELs and inorganic ELs which can reduce the layer structure are particularly preferable, and organic ELs having a wide color gamut are particularly preferable.

(Foldable display)

The foldable display preferably has a structure in which a continuous one-piece display is halved in size by being folded in half when being carried, and portability is improved. In addition, it is expected to be thinner and lighter. Therefore, the bending radius of the foldable display is preferably 5 mm or less, and even more preferably 3 mm or less. When the bending radius is 5 mm or less, the thickness can be reduced in the folded state. Although the smaller the bending radius, the better, but it can be 0.1 mm or more and 0.5 mm or more. Even if it is 1 mm or more, compared with the conventional display which does not have a folding structure, practicability is still very good. The bending radius at the time of folding refers to the bending radius at the place where the symbol 11 in the schematic diagram of FIG. 1 is measured, and the inside radius of the folded portion at the time of folding. The surface protection film described later may be located outside the fold of the foldable display, or may be located inside.

(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.

(Mobile terminal with touch panel)

When an organic EL display is used in a mobile terminal with a touch panel, the touch panel module is arranged on the upper portion of the organic EL display or between the organic EL layer and the retardation plate. At this time, if an impact is applied from above, the circuit of the organic EL and the touch panel may be disconnected. Therefore, a surface protection film is required. As for the film disposed on the front surface of the display as the surface protection film, it is preferable that A film having a hard coat layer laminated on at least the surface side.

(Surface Protective Film for Folding Display)

As the surface protective film, if it is a film having high light transmittance and low haze, such as a polyimide film, a polyester film, a polycarbonate film, an acrylic film, a cellulose triacetate film, a cycloolefin polymer film, etc., Among them, a polyimide film and a polyester film having high impact resistance and sufficient pencil hardness are preferred, and a polyester film that can be produced inexpensively is particularly preferred.

In the present invention, the polyester film may be a single-layer film including one or more polyester resins, and when two or more polyesters are used, it may be a multilayer structure film or a repeating structure film. Super multilayer laminated film.

Examples of the polyester resin include polyethylene terephthalate, polybutylene terephthalate, polyethylene 2,6-naphthalene dicarboxylate, or a main component of these resins. Polyester film composed of copolymers of ingredients. Among them, stretched polyethylene terephthalate film is particularly preferred from the aspects of mechanical properties, heat resistance, transparency, and price.

When a polyester copolymer is used for a polyester film, examples of the dicarboxylic acid component of the polyester include aliphatic dicarboxylic acids such as adipic acid and sebacic acid; terephthalic acid, Aromatic dicarboxylic acids such as isophthalic acid, phthalic acid and 2,6-naphthalenedicarboxylic acid; polyfunctional carboxylic acids such as trimellitic acid and pyromellitic acid. Examples of the diol component include fatty acid diols such as ethylene glycol, diethylene glycol, 1,4-butanediol, propylene glycol, neopentyl glycol, and aromatics such as p-xylylene glycol. Alicyclic diols such as diols, 1,4-cyclohexanedimethanol, and polyethylene glycols 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% by mass. When it is less than 20% by mass, it is preferable to maintain the film strength, transparency, and heat resistance.

In addition, in the production of the polyester film, the limiting viscosity of at least one or more resin pellets is preferably in the range of 0.60 to 1.0 dl / g. When the limiting viscosity is 0.60 dl / g or more, the impact resistance of the obtained film is improved, and disconnection of the internal circuit due to external impact is less likely to occur, which is preferable. In addition, it also helps to reduce the deformation in the case of repeated bending, so it is preferable. On the other hand, if the limiting viscosity is 1.00 dl / g or less, the increase in the filtration pressure of the molten fluid does not become excessively large, and it is easy to handle the production of the film stably, which is preferable.

Regardless of whether the film has a single-layer structure or a laminated structure, the limiting viscosity of the film is preferably 0.60 dl / g or more. More preferably, it is 0.62 dl / g or more. More preferably, it is 0.68 dl / g or more. If it is 0.60 dl / g or more, fatigue resistance can be provided and the effect of bending resistance can be fully acquired. On the other hand, a film having an intrinsic viscosity of 1.00 dl / g or less can be produced with good operability, and is therefore preferred.

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

The surface of the polyester film of the present invention may be smooth or may have unevenness. However, from the use for a display surface cover, the reduction in optical characteristics due to unevenness is not good. The haze is preferably 3% or less, even more preferably 2% or less, and most preferably 1% or less. If the haze is 3% or less, the visibility of the image can be improved. Although the lower limit of the haze is better, it may be 0.1% or more and 0.3% or more.

As mentioned above, for the purpose of reducing the haze, it is preferable that the unevenness on the surface of the film is not too large, but from the viewpoint of handleability, in order to give a considerable degree of slip, a filler can be blended in the surface layer A polyester resin layer, or a coating layer with a filler applied during the film formation, is formed as a method for forming unevenness.

As a method for blending particles into a base film, a known method can be adopted. For example, it can be added at any stage of polyester production, and it is preferably a slurry prepared by dispersing it in ethylene glycol or the like after the esterification stage or the end of the transesterification reaction and before the start of the polycondensation reaction. Add it in the form of polycondensation. In addition, it can be performed by a method such as a method of mixing a slurry of particles dispersed in ethylene glycol or water with a polyester raw material using a kneading extruder with a vent, or using kneading extrusion Machine, a method of blending dry particles and polyester raw materials.

Among them, it is preferred that the aggregated inorganic particles are homogeneously dispersed in a monomer liquid that becomes a part of the polyester raw material and then filtered, and the polyester raw material is added before, during, or after the esterification reaction. Method of the remainder. When this method is used, the monomer liquid has a low viscosity, so it can easily perform homogeneous particle dispersion or high-precision filtration of the slurry. At the same time, when it is added to the remaining part of the raw material, the particle dispersibility is good and it is not easy to produce new particles. Condensate. From such a viewpoint, it is particularly preferable that the remaining portion of the raw material is added to the low-temperature state before the esterification reaction.

In addition, the number of protrusions on the surface of the film can be further reduced by a method (masterbatch method) such as kneading and extruding the pellets and pellets that do not contain the polyester containing particles in advance. .

In addition, the polyester film may contain various additives within a range that maintains a preferable range of the total light transmittance. Examples of the additive include an antistatic agent, a UV absorber, and a stabilizer.

The total light transmittance of the polyester film is preferably 85% or more, and even more preferably 87% or more. With a transmittance of 85% or more, sufficient visibility can be ensured. Although it can be said that the higher the total light transmittance of the polyester film, the better, but it may be 99% or less and 97% or less.

The surface of the polyester film of the present invention can be subjected to a treatment for improving adhesion to a resin forming a hard coat layer or the like.

Examples of the method based on the surface treatment include: roughening treatment using sandblasting treatment, solvent treatment, and the like; corona discharge treatment, electron beam irradiation treatment, plasma treatment, ozone and ultraviolet irradiation treatment, flame treatment, and chromium Oxidation treatments such as acid treatment and hot air treatment can be used without particular limitation.

In addition, adhesion can also be improved by an adhesion promotion layer such as an easy adhesion layer. As the easy-adhesion layer, acrylic resin, polyester resin, polyurethane resin, polyether resin, and the like can be used without particular limitation, and can be applied by a general coating method, preferably by a so-called in-line method. In-line coat formulation to form.

The above-mentioned polyester film can be produced through, for example, a polymerization step and a film formation step; this polymerization step is to homogeneously disperse the inorganic particles in a monomer liquid that becomes a part of the polyester raw material, filter the mixture, and add the polyester The remaining portion of the raw material is used to polymerize the polyester; the film forming step is to melt extrude the polyester through a filter into a sheet shape, cool it, and stretch it to form a base film.

Next, the manufacturing method of a biaxially stretched polyester film will be described in detail using an example in which pellets of polyethylene terephthalate (hereinafter referred to as PET) are used as a raw material of a base film, but not Limited to these. In addition, the number of layers such as a single-layer structure and a multilayer structure is not limited.

The pellets of PET are mixed and dried at a predetermined ratio, and then supplied to a known extruder for melt-lamination, extruded into a sheet shape from a slit die, and cooled and solidified on a casting roll to form an unstretched material. film. In the case of a single layer, it can be one extruder, and in the case of manufacturing a film composed of multiple layers, two or more extruders, two or more layers of manifolds, or a confluence block (for example, having an angle type) can be used. The confluence block of the confluence part) is formed by laminating a plurality of film layers constituting each outermost layer, extruding two or more sheets from an extrusion nozzle, and cooling with a casting roll to form an unstretched film.

In this case, it is preferable to perform a high-precision filtration to remove a foreign substance contained in the resin while the molten resin is maintained at about 280 ° C. at the time of melt extrusion. The filter material used for high-precision filtration of the molten resin is not particularly limited, and the filter material of the stainless steel sintered body is preferably agglomerates and high-melting-point organic compounds having Si, Ti, Sb, Ge, and Cu as main components, and is therefore excellent in removal performance.

The filter particle size (initial filtration efficiency 95%) of the filter medium is preferably 20 μm or less, and particularly preferably 15 μm or less. If the filter particle size (initial filtration efficiency 95%) of the filter medium exceeds 20 μm, foreign matter having a size of 20 μm or more cannot be sufficiently removed. Although a filter material with a filter particle size (initial filtration efficiency of 95%) of 20 μm or less is used for high-precision filtration of molten resin, productivity may be reduced, but in terms of obtaining a film with fewer protrusions due to coarse particles, Is better.

In addition, the 0.2% guaranteed strength point strain of at least one of the longitudinal direction (mechanical flow direction) and the width direction of the polyester film is preferably 2.6 to 5.0%, and even more preferably 3.0 to 5.0%. The 0.2% guaranteed strength point strain refers to the value of the drop point whether it appears or does not appear in the stress-strain curve, and can be used as an indicator of the elastic region. FIG. 3 is a schematic diagram showing a stress-strain curve for supplying and demanding a 0.2% guaranteed strength point strain. FIG. 3 is an example of a case where no fall point occurs. The 0.2% guaranteed strength point strain is determined by a normal tensile test. In the stress-strain curve, from a point Q with a stress of 0 MPa and a strain of 0.2%, draw a line parallel to the OA where Hooke's law is established, and set the point where it intersects the stress-strain curve as the guaranteed strength point P, from there The value of the strain at the point of intersection H with the strain axis when the parallel line of the vertical axis is lowered is 0.2% of the guaranteed strength point strain. Operating in the manner described above, 0.2% of the guaranteed strength point strain can be determined from the stress-strain curve. When the stress-strain curve is obtained by a tensile tester (manufactured by Shimadzu, AUTOGRAPH AG-X Plus 1kN), if When using Auto Graph Software TRAPEZIUM X manufactured by Shimadzu Corporation, you can directly obtain the 0.2% guaranteed strength point strain calculation result by setting the strain to 0.2%.

In the case where the 0.2% guaranteed strength point strain is 2.6% or more, it is not easy to be deformed due to the strain generated during folding, so it is preferable. When it is 5.0% or less, it is preferable to obtain good impact resistance. Moreover, the 0.2% guaranteed strength point strain of the polyester film in the bending direction is preferably 2.6 to 5.0%, and even more preferably 3.0 to 5.0%. Here, the bending direction, as shown by the symbol 22 on the polyester film (symbol 2) of FIG. 2, refers to a direction orthogonal to the folded portion (symbol 21) which is supposed to be used for the surface protective film of the folding display. direction. The bending direction is not limited to either the longitudinal direction or the width direction of the film.

0.2% guaranteed strength point strain can be adjusted within the above range by adjusting the stretching ratio, stretching temperature, heat setting temperature, polyester raw materials, etc.

The 0.2% guaranteed strength point strain of the polyester film can be effectively adjusted by adjusting the draw ratio. The stretching ratio of at least one of the longitudinal direction (mechanical flow direction) and the width direction of the unstretched polyester sheet is preferably 1.0 to 3.4 times, and even more preferably 1.0 to 3.0 times. By setting the stretch magnification to 1.0 to 3.4 times, the 0.2% guaranteed strength point strain can be adjusted within the above-mentioned range, and the deformation upon repeated folding is small. The stretching direction is preferably the aforementioned bending direction. The stretching temperature is preferably 80 to 130 ° C, and even more preferably 90 to 130 ° C. Moreover, the heating method at the time of a stretching can employ conventionally well-known methods, such as a hot air heating method, a roll heating method, and an infrared heating method. By setting the stretching temperature to 80 to 130 ° C., it is possible to prevent thickness unevenness caused by stretching at the above-mentioned stretching ratio.

From the mechanical properties of the film, it is preferable that the stretching ratio in a direction orthogonal to the bending direction at the time of folding (the direction of the folded portion) is larger than the bending direction, and the stretching ratio in a direction orthogonal to the bending direction can be Examples are 2.5 to 5.0 times. By setting the stretching ratio to 2.5 times or more, stable productivity can be obtained, and by setting the stretching ratio to 5.0 times or less, good impact resistance can be obtained.

In addition, the degree of crystallization of the polyester film is preferably 35 to 54%, and even more preferably 43 to 54%. By increasing the degree of crystallization, 0.2% guaranteed strength point strain and impact resistance can be improved. In order to effectively increase the degree of crystallization, it is preferable to apply a heat treatment at 180 to 250 ° C after biaxial stretching. If the temperature is 180 ° C or higher, the degree of crystallization can be effectively improved. If the temperature is 250 ° C or lower, the surface of the polyester film does not have to be melted, and the appearance can be maintained well, which is preferable.

Specifically, for example, the pellets of PET are sufficiently vacuum-dried, then supplied to an extruder, melt-extruded into a sheet shape at about 280 ° C, and cooled and solidified to form an unstretched PET sheet. The obtained unstretched sheet was stretched 1.0 to 3.4 times in the longitudinal direction with a roller heated at 80 to 130 ° C to obtain a uniaxially oriented PET film. In addition, the ends of the film were clamped by a clamp, guided to a hot air zone heated to 80 to 180 ° C, and dried, and then stretched in the width direction by 2.5 to 5.0 times. Then, it is guided to a heat treatment zone of 180 to 250 ° C., and heat treatment is performed for 1 to 60 seconds to complete the crystal alignment. In this heat treatment step, a 1 to 12% relaxation treatment may be applied in the width direction or the long side direction as required.

(Hard coating)

It is preferable that the polyester film which protects a display on the surface of a foldable display has a hard-coat layer on the surface. The hard coat layer is preferably used in a display in such a manner that it is positioned on the display surface side of the polyester film. As the resin for forming the hard coat layer, acrylic, siloxane, inorganic mixture, urethane acrylate, polyester acrylate, epoxy and the like can be used without particular limitation. In addition, two or more materials may be mixed and used, or particles such as an inorganic filler or an organic filler may be added.

(Film thickness)

The film thickness of the hard coat layer is preferably 1 to 50 μm. More preferably, it is 1 to 40 μm. If it is thicker than 1 μm, it can be sufficiently hardened to obtain good pencil hardness. In addition, by setting the thickness to 50 μm or less, it is possible to suppress curl caused by the hardening shrinkage of the hard coating, and to improve the handleability of the film. It is more preferably 2 to 25 μm, particularly preferably 2 to 20 μm, and most preferably 3 to 15 μm.

(Coating method)

As the coating method of the hard coat layer, a Mayer bar, a gravure coating, a die coater, a blade coater, or the like can be used without particular limitation, and can be appropriately selected depending on the viscosity and film thickness.

(Hardening conditions)

As a hardening method of the hard coating layer, energy rays such as ultraviolet rays or electron rays, or heat curing methods can be used. However, in order to reduce damage to the film, a hardening method utilizing ultraviolet rays or electron rays is preferred.

(Pencil hardness)

The pencil hardness of the hard coat layer is preferably B or more, even more preferably H or more, and particularly preferably 2H or more. With a pencil hardness of B or higher, it is not easy to be injured and the visibility is not reduced. In general, a hard-coated pencil having a high hardness is preferred, but it may be 10H or less and 8H or less. In practice, it can be used without problems even if it is 6H or less.

(Kind of hard coating)

The hard coat layer in the present invention may be one having added other functions if it can be used for the purpose of protecting the display in order to increase the pencil hardness of the surface as described above. For example, functional hard coatings such as the anti-glare layer, anti-glare anti-reflection layer, anti-reflection layer, low-reflection layer, and antistatic layer with a certain pencil hardness, as described above, can also be preferably applied. In the present invention.

    [Example]    

Next, the effect of this invention is demonstrated using an Example and a comparative example. First, an evaluation method of characteristic values used in the present invention is shown below.

(1) 0.2% guaranteed strength point strain

In a measurement direction of 140 mm, the film was cut into short strips with a width of 10 mm as a sample, and a tensile tester (AUTOGRAPH AG-X Plus 1kN manufactured by Shimadzu Corporation) was used to perform a tensile test at a tensile speed of 100 mm / min. A stress-strain curve is obtained. Then, using Auto Graph Software TRAPEZIUM X manufactured by Shimadzu Corporation, the strain was set at 0.2%, and the 0.2% guaranteed strength point strain was calculated.

(2) Limit viscosity

After the film or polyester resin is pulverized and dried, it is dissolved in a mixed solvent of phenol / tetrachloroethane = 60/40 (mass ratio). After centrifuging the solution to remove the inorganic particles, the flow time of the solution with a concentration of 0.4 (g / dl) and the flow time of only the solvent were measured at 30 ° C using a Ubbelohde viscometer. From these time ratios, Using Huggins' formula, suppose the constant of Huggins is 0.38 to calculate the limiting viscosity. In the case of a laminated film, according to the thickness of the laminated layer, the corresponding polyester layer of the film is cut off, so as to evaluate the limiting viscosity of each layer monomer.

(3) Degree of crystallization

The density of the thin film sample was measured at three places. The average value is taken as the degree of crystallinity. The resin component is a completely amorphous and completely crystalline mixture, and its density is as described later. Based on the assumption that the density of the sample is a value obtained by dividing the sum of the masses of the components constituting the sample by the sum of the volumes of the respective components, each resin is estimated. Degree of crystallization (weight ratio). The density of the sample was measured according to the method (density gradient tube method) according to JISK-7112-1980. In addition, the individual density of each component uses the following value (unit: g / cm3)

Polyethylene terephthalate resin: completely amorphous 1.34, completely crystalline 1.46

Polyethylene naphthalate resin: completely amorphous 1.32, completely crystalline 1.41

(4) Bending resistance

The polyester film was cut into a size of 200 mm (bending direction) × 50 mm (orthogonal direction) to prepare a sample for measurement. Spacers of various thicknesses were arranged at the ends of two glass plates having a thickness of 5 mm to make space, and the film was held for 10 seconds. Immediately thereafter, the film was made to reflect the light of a fluorescent lamp, and the folded portion was observed, and the interval at which no folding mark was caused was recorded.

(Circle): The space | interval which does not cause a folding mark is less than 6.5 mm.

(Triangle | delta): The space | interval which does not cause a folding mark is 6.5 mm or more and less than 7.0 mm.

×: The interval at which no folding marks were caused was 7.0 mm or more.

(5) Repeated bending resistance

A sample having a size of 50 mm in the width direction and 100 mm in the flow direction was prepared. An unloaded U-shaped telescopic tester (DLDMLH-FS, manufactured by Yuasa System Equipment Co., Ltd.) was used, and a bending radius of 3 mm was set, and it was bent 50,000 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 portion to be bent was 50 mm × 80 mm. After the bending process is finished, the inside of the bending of the sample is faced down and placed on a flat surface for visual inspection.

○: The sample is not deformed, or even if it is deformed, the maximum bulging height is less than 3 mm when placed horizontally.

△: The sample is deformed. When placed horizontally, the maximum bulging height is 3 mm or more and less than 5 mm.

×: When the sample is creased or placed horizontally, the maximum bulging height is 5 mm or more.

(6) Pencil hardness

According to JIS K 5600-5-4: 1999, the hard-coated polyester film was measured at a load of 750 g and a speed of 0.5 mm / s.

(Coating liquid 1 for forming a hard coat layer)

To 100 parts by weight of a hard coating material (manufactured by JSR, Opstar (registered trademark) Z7503, concentration 75%), 0.1 part by weight of a leveling agent (by BYK-Chemie Japan, BYK307, concentration 100%) was added, and methyl ethyl Alkone is diluted to prepare a hard coat coating liquid 1 having a solid content concentration of 40% by weight.

(Coating liquid 2 for forming a hard coat layer)

95 parts by weight of a mixed urethane acrylate-based hard coating agent (manufactured by Arakawa Chemical Industries, BEAMSET (registered trademark) 577, solid content concentration 100%), photopolymerization initiator (manufactured by BASF Japan, Irgacure (registered trademark) 184, solid content concentration 100%) 5 parts by weight, leveling agent (by BYK-Chemie Japan, BYK307, solid content concentration 100%) 0.1 part by weight, diluted with a solvent of toluene / MEK = 1/1, and prepared Coating liquid 2 with a concentration of 40%.

(Preparation of polyethylene terephthalate pellet A)

A continuous esterification reaction device consisting of a stirring device, a partial condenser, and a three-stage complete mixing tank with a raw material input port and a product take-out port was used as the esterification reaction device. TPA was set to 2 tons / hr. EG was set to 2 mol relative to 1 mol TPA, and antimony trioxide was set to an amount of 160 ppm of Sb atoms relative to the PET produced, and these slurry were continuously supplied to the esterification reaction device. In the first esterification reaction tank, the reaction was performed 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 the mass was 8 masses with respect to the polymer (the PET produced). % Of EG distilled from the first esterification reaction tank was supplied to the second esterification reaction tank, and an EG solution containing magnesium acetate in an amount of 65 ppm relative to the produced PET was further added to the EG solution. And an EG solution containing TMPA in an amount of 20 ppm relative to the produced PET with P atoms, and the reaction was performed 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 the amount of P atoms was further added to 20 ppm relative to the produced PET. The EG solution of TMPA was allowed to react 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 was continuously supplied to a three-stage continuous polycondensation reaction device, polycondensation was performed, and then a filter material of a stainless steel sintered body (with a nominal filtration accuracy of 5 μm and 90% retention of particles) was used. Filtration was performed to obtain polyethylene terephthalate pellet A having an limiting viscosity of 0.62 dl / g.

(Preparation of polyethylene terephthalate pellet B)

Using a rotary vacuum polymerization device, under a reduced pressure of 0.5 mmHg and at 220 ° C., the polyethylene terephthalate pellets A were subjected to solid-phase polymerization at different times to form a polymerization pair with an ultimate viscosity of 0.72 dL / g. Ethylene phthalate pellet B.

(Polyethylene 2,6-naphthalene dicarboxylate pellet C)

Using 100 parts of dimethyl 2,6-naphthalene dicarboxylate, 60 parts of ethylene glycol, and 0.03 part of manganese acetate tetrahydrate as a transesterification catalyst, the transesterification reaction was performed according to a usual method, and then triethyl acetate was added. 0.042 parts of phosphonium phosphonium ester substantially ended the transesterification reaction. Next, 0.024 parts of antimony trioxide was added, and then a polymerization reaction was performed by a conventional method under a high temperature and a high vacuum to obtain polyethylene 2,6-naphthalene dicarboxylate (PEN) with an inherent viscosity of 0.60 dl / g. After that, pre-drying was performed at 150 to 160 ° C for 3 hours, and then solid-phase polymerization was performed at 210 ° C, 13 kPa, and nitrogen to obtain polyethylene 2,6-naphthalene dicarboxylate pellets having an inherent viscosity of 0.72 dl / g.料 C。 Material C.

(Example 1)

The polyethylene terephthalate master batch B was dried under reduced pressure (3 Torr) at 150 ° C for 8 hours, and then the pellets B of polyethylene terephthalate were supplied to the extruder. It was melted at 285 ° C. This polymer was filtered with a stainless steel sintered body filter material (with a nominal filtration accuracy of 10 μm and 95% of the particles were retained). After extruding from the extrusion nozzle into a thin sheet, the casting drum was brought into contact with a surface temperature of 30 ° C by an electrostatic casting method. Then, it is cooled and solidified to produce an unstretched film. This unstretched film was uniformly heated to 75 ° C using a heating roll, and heated to 100 ° C using a non-contact heater to perform roll stretching (vertical stretching) at 3.4 times. The obtained uniaxially stretched film was guided to a tenter, heated to 140 ° C. and stretched to 4.0 times in a transverse direction, and was fixed to a width and subjected to a heat treatment at 240 ° C. for 5 seconds, and further brought to a width of 210 ° C. 4% relaxation was performed in the direction to obtain a polyethylene terephthalate film having a thickness of 50 μm.

(Examples 2 to 6)

A polyester film was obtained in the same manner as in Example 1 except that the stretching ratio and heat treatment temperature described in Table 1 were changed.

(Examples 7 and 8)

A polyester film was obtained in the same manner as in Example 1 except that the thickness described in Table 1 was changed.

(Example 9)

A polyester film was obtained in the same manner as in Example 1 except that the polyethylene 2,6-naphthalene dicarboxylate pellet C was used as shown in Table 1 and the temperature was adjusted.

(Example 10)

A polyester film was obtained in the same manner as in Example 1 except that the polyethylene terephthalate master batch A was changed.

(Comparative example 1)

A polyester film was obtained in the same manner as in Example 1 except that the longitudinal stretching ratio was changed to 3.5 times.

(Comparative Examples 2, 3)

A polyester film was obtained in the same manner as in Example 1 except that the heat-fixing temperature in Table 1 was changed.

On one side of the polyester films obtained in Examples 1 to 10 and Comparative Examples 1 to 3 above, a coating liquid 1 for forming a hard coat layer was applied using a Mel bar so that the film thickness after drying became 5.0 μm. After drying at 80 ° C. for 1 minute, ultraviolet rays (high-pressure mercury lamp, cumulative light amount of 200 mJ / cm ² ) were irradiated to obtain a hard coating film. In Example 11, except that the polyester film obtained in Example 1 was applied so that the film thickness after drying became 10.0 μm, the same procedure as in Example 1 was performed to obtain a hard coat film. In Example 12, a hard coating film was obtained in the same manner as in Example 1 except that the polyester film obtained in Example 1 was coated with a coating solution 2 for forming a hard coat layer.

These hard coating films were laminated to an organic EL module through an adhesive layer having a thickness of 25 μm, and a folding type display of a smartphone type capable of being folded in half at the entire central portion with a bending radius equivalent to 3 mm in FIG. 1 was formed. . The hard coating film is disposed on the surface of a continuous display through the folded portion, and the hard coating is disposed on the surface of the display. Those who use the hard-coating film of each embodiment are satisfied with the operation and visibility of a smartphone that can be folded and folded in half at the center. On the other hand, the folding type display using the hard coating film of each comparative example is not a good folding type display, as the frequency of use is increased, and image distortion is gradually generated in the folded portion of the display.

    [Industrial availability]    

According to the foldable display using the polyester film for the surface protective film or the hard coating film of the foldable display of the present invention, the polyester film on the surface of the foldable display does not occur while maintaining mass productivity. Or the hard coating film is deformed after being repeatedly folded, so that the image of the folded portion of the display is not disturbed. A mobile terminal equipped with a foldable display using the polyester film or the hard coating film of the present invention as a surface protection film is a mobile terminal that provides beautiful images, rich functionality, and convenience such as portability.

Claims (8)

    A polyester film for a surface protection film of a folding display, which is a polyester film with a thickness of 10 to 75 μm, in which the 0.2% guaranteed strength point strain of at least one of the long side direction and the width direction is 2.6 to 5.0% .         For example, the polyester film for the surface protection film of the foldable display of claim 1, in which the 0.2% guaranteed strength point strain in the bending direction is 2.6 to 5.0%. Here, the bending direction refers to the folded portion when the polyester film is folded Orthogonal direction.         For example, the polyester film for the surface protection film of the foldable display of claim 1 or 2, wherein the limiting viscosity of the film is 0.60 ~ 1.0dl / g.         A hard coating film for a surface protection film of a foldable display, wherein 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 to 50 μm. coating.         The hard coating film for a surface protective film of a folding display as claimed in claim 4, wherein the hardness of the hard coating pencil measured according to JIS K5600-5-4: 1999 at a load of 750 g is H or more.         A foldable display, which is a foldable display in which the surface protective film of the foldable display such as the item 4 or 5 is arranged with the hard coating layer on the surface, and the hard coating film is used as the surface protective film. The bending radius is 5mm or less.         The folding type display according to claim 6, wherein a single hard coating film is provided which is continuous through the folded portion of the folding type display.         A mobile terminal having a foldable display as claimed in claim 6 or 7.