JP2022031193A - Polyester film - Google Patents

Abstract

[Problem] To provide a polyester film with excellent bending resistance and flatness. [Solution] A polyester film having a Young's modulus of 2.8 GPa or more in the direction (a direction) in which the Young's modulus is greatest, and a stretch absorption parameter in both the a direction and the direction perpendicular to the a direction (b direction) of 0.7 to 1.5. [Selected Figures] None

Description

The present invention relates to a polyester film.

In recent years, an image display device (hereinafter referred to as "organic electroluminescence display device") using a self-luminous body called an organic light emitting diode has been put into practical use. Compared to conventional liquid crystal displays, this organic electroluminescence display device uses a self-luminous body, so it is not only superior in terms of visibility and response speed, but also an auxiliary lighting device such as a backlight. Therefore, it is possible to make the display device thinner and more flexible. For this reason, the development of flexible displays that can be folded and wound up is accelerating, and bending resistance is also required for cover films that prevent scratches on the surface of display devices.

For example, as a method for improving scratch resistance for an image display device, a hard coat film in which a hard coat layer is laminated on both sides of a base material containing a polyester film has been proposed (Patent Document 1). Further, for a flexible display, an antireflection film in which an antireflection layer is provided on at least one surface of a transparent resin film having flexibility including a polyester film has been proposed (Patent Document 2).

Japanese Unexamined Patent Publication No. 2015-61750 Japanese Unexamined Patent Publication No. 2016-75869

However, the films used for the image display devices described in Patent Document 1 and Patent Document 2 are characterized by a hard coat layer and an antireflection layer, and the film as a base material does not consider bending resistance. , It was difficult to apply it to flexible displays. Further, when the film is made flexible in order to improve the bending resistance of the film, the flatness when a curable resin layer such as a hard coat layer is applied to the film as a scratch resistant layer deteriorates. It turned out that a problem would occur.

Therefore, an object of the present invention is to solve the above-mentioned problems, and to provide a polyester film having both bending resistance and flatness.

The polyester film of the present invention adopts the following means in order to solve such a problem.
(1) The Young's modulus in the direction in which the Young's modulus is maximum (a direction) is 2.8 GPa or more, and the elongation absorption parameters in both the a direction and the direction orthogonal to the a direction (b direction) are 0.7. Must be 1.5 or less.
(2) The heat shrinkage start temperature obtained from the heat shrinkage rate curve is 65 ° C. or higher and 120 ° C. or lower in both the a direction and the b direction.
(3) The bending hysteresis 2HB obtained by the following measurement method is 0.04 gf · cm / cm or more and 0.10 gf · cm / cm or less in both the a direction and the b direction.
[Measurement method of bending hysteresis 2HB]
Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., bending hysteresis 2HB measurement is performed in each of the a direction and the b direction under the following conditions. The bending hysteresis 2HB is a value obtained in the 5th cycle (5 repetitions) with a curvature of −2.5 to +2.5 cm ^-1 as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2x5
Curvature: -2.5 to +2.5 cm ^-1
Deformation speed: 0.50 cm ^-1 / sec
Sample size: a direction x b direction = 20 cm x 20 cm
(4) The flexural rigidity obtained by the following measurement method shall be 0.06 gf · cm ² / cm or more and 0.12 gf · cm ² / cm or less in both the a direction and the b direction.
[Measurement method of flexural rigidity]
Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., the bending hardness is measured in the a direction and the b direction under the following conditions. The flexural rigidity is a value obtained in the 5th cycle (5 repetitions) with a curvature of −2.5 to +2.5 cm ^-1 as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2x5
Curvature: -2.5 to +2.5 cm ^-1
Deformation speed: 0.50 cm ^-1 / sec
Sample size: a direction x b direction = 20 cm x 20 cm
(5) The weight average molecular weight Mw is 20,000 or more and 50,000 or less.
(6) The number of bending breaks in the a direction is 200,000 or more and 1 million or less.
(7) The film thickness is 5 μm or more and 30 μm or less.
(8) The coefficient of static friction of at least one surface is 0.2 or more and 0.5 or less.
(9) The surface resistivity value of at least one surface shall be 1 × 10 ¹² Ω / □ or less.
(10) Used for flexible displays.

According to the present invention, by controlling the Young's modulus and the elongation absorption parameter in a specific range, it is possible to provide a polyester film having both bending resistance and flatness. Such a polyester film can be particularly preferably used as a cover film for a flexible display such as an organic electroluminescence display device.

It is a schematic diagram which shows the winding resistance evaluation.

Hereinafter, the polyester film according to the present invention will be described in detail together with embodiments.

A preferred embodiment of the polyester film of the present invention is a Young's modulus of 2.8 GPa or more in the direction in which the Young's modulus is maximum (direction a). Young's modulus indicates the rigidity of the film, and is an index for maintaining the flatness when a curable resin such as a hard coat is applied to the film. When the Young's modulus in the a direction is 2.8 GPa or more, it is possible to maintain the flatness when a curable resin such as a hard coat is applied. If the Young's modulus in the a direction is less than 2.8 GPa, the flatness when hard-courted may be inferior. From the viewpoint of bending resistance, the Young's modulus in the a direction is preferably less than 5.5 GPa. Further, from the viewpoint of flatness, it is preferable that the Young's modulus in the b direction, which is an orthogonal direction in the a direction, is 2.8 GPa or more. Further, from the viewpoint of flatness, the Young's modulus in the b direction is preferably 3.5 GPa or more, and most preferably 3.8 GPa or more. The upper limit is not particularly limited, but the Young's modulus in the b direction is preferably 10.0 GPa or less. In order to make the Young's modulus in the a direction 2.8 GPa or more, it is preferable to use a resin mainly composed of polyethylene terephthalate, polyethylene naphthalate, or polycyclohexylene methylene terephthalate as the polyester. Here, the main component means that each polyester constituent unit is 75 mol% or more, and it is most preferably homopolyester from the viewpoint of increasing Young's modulus. The film forming conditions can be controlled by using the above resin and stretching it 3.5 times or more. Since the Young's modulus increases as the film is oriented in the stretching direction, the direction in which the film is stretched most is the a direction. For example, if the stretching ratio in a certain direction (uniaxial direction) and the direction orthogonal to it are 3.5 times or more and the area magnification is 12.25 times or more, the Young's modulus is 2.8 GPa or more in both the a direction and the b direction. Can be. Since the Young's modulus is improved by increasing the draw ratio, it may be appropriately adjusted according to the required function. However, if the draw ratio is increased too much, it may break during film formation, so the draw ratio is set. It is preferably 4.0 times or less, and the area magnification is preferably 16.0 times or less. When the polyester film of the present invention is used for a take-up type flexible display, if the polyester film of the present invention is used so that the a direction is the take-up direction, long-term durability is improved by taking advantage of the excellent bending resistance during repeated winding. It is preferable because it becomes good.

The polyester film of the present invention is preferably a biaxially oriented polyester film. The biaxially oriented polyester film may be stretched in the biaxial direction by a method described later.

A preferred embodiment of the polyester film of the present invention is that the elongation and absorption parameters in the a direction and the direction orthogonal to the a direction (b direction) are both 0.7 or more and 1.5 or less. Here, the stretch absorption parameter refers to a numerical value obtained by the evaluation method (9) stretch absorption parameter described in the examples, and is an index showing how easily the direction orthogonal to the stretch direction shrinks when the film is stretched. It was conceived and found from the r value obtained from the composition strain ratio test method of the thin plate metal material described in JIS standard Z254: 2008. In a flexible display, repeated winding and bending loads such as bending and winding are assumed, and how to absorb these loads is important. The present inventors focused on the elongation absorption parameter as an index for absorbing the load such as repeated bending and winding, and by controlling the extension absorption parameter within the range of the present invention, the bending resistance such as repeated winding and the like is prevented. I found it to be excellent in sex. The larger the value of the elongation absorption parameter, the more preferable it is, and it is preferable that the extension absorption parameter is 0.8 or more. Further, it is preferably 1.0 or more, and more preferably 1.2 or more. If the elongation absorption parameter is less than 0.7, the bending resistance may be poor. In addition, it is difficult for the stretch absorption parameter to exceed 1.5 when a polyester film is used.

In order to set the elongation absorption parameter in the a direction and the direction orthogonal to the a direction (b direction) to 0.7 or more and 1.5 or less, the film bulk structure is densely formed and external force absorption such as bending due to molecular chain entanglement is performed. It is presumed that it is important to enhance the effect, and based on this policy, film forming conditions ((1) step stretching, (2) cooling to 50 ° C or lower after film biaxial stretching), surface orientation coefficient, film It can be controlled by the intrinsic viscosity of the film and the minute heat absorption temperature peak Tmeta. This will be described in detail below.

First, as a biaxial stretching condition of the film, it is important to carry out step-stretching (for example, stretching in three or more steps) in the stretching direction of each axis in order to form a dense molecular structure. For example, in the case where the stretching of the first axis is carried out in three stages, when the stretching ratios of the three stages are set to MD1, MD2, and MD3 in order, setting the stretching ratio as follows is to obtain a precise bulk structure. Is important to.
MD1: 1.03 times or more and 1.8 times or less.
MD2: 1.05 times or more and 1.12 times or less.
MD3: 3.0 times or more, and MD3 occupying the total draw ratio (MD1 × MD2 × MD3) of the first axis is 80% or more.
Total draw ratio of the first axis: 3.5 times or more.

Further, in the case where the second axis stretching is carried out in three stages, when the stretching ratios in the three stages are set to TD1, TD2, and TD3 in order, setting the stretching ratio as follows takes a precise bulk structure. Is important for.
TD 1: 1.5 times or more and 2.2 times or less.
TD2: 1.3 times or more and 1.8 times or less.
TD3: 1.2 times or more and 1.5 times or less.
The total draw ratio of the second axis (TD1 × TD2 × TD3): 3.5 times or more.

Further, the elongation absorption parameter tends to increase in the range showing a positive value when the plane orientation coefficient is increased, and the plane orientation coefficient is preferably 0.164 or more. Increasing the plane orientation coefficient is advantageous in that the elongation absorption parameter is increased, but it may deteriorate the dimensional stability. Therefore, it may be appropriately adjusted according to the required thermal characteristics. However, when the minute endothermic temperature peak Tmeta is observed in the range exceeding 200 ° C., it is difficult to set the extension endothermic parameter to 0.7 or more and 1.5 or less even if the plane orientation coefficient is controlled. Further, the plane orientation coefficient can be controlled by the draw ratio, and can be increased by increasing the draw ratio. In order to set the elongation absorption parameter in the a direction and the direction orthogonal to the a direction (b direction) to 0.8 or more, it is important to set the plane orientation coefficient to 0.166 or more. There is no particular upper limit, but if the plane orientation coefficient exceeds 0.17, tearing may occur frequently when stretched, resulting in inferior productivity.

In the present invention, it is important to have a heat treatment step after the film is biaxially stretched and then cooled to 50 ° C. or lower. In the conventional polyester film manufacturing method, after sequential biaxial stretching and simultaneous biaxial stretching in terms of productivity and thermal efficiency, heat treatment is performed as it is in the heat treatment step without cooling to 50 ° C. or lower. In order to form it densely and increase the entanglement of molecular chains, it is important to cool it to 50 ° C. or lower once after biaxial stretching. Although the detailed mechanism is unknown, the film bulk structure becomes denser by performing heat treatment after stabilizing the crystal structure formed by biaxial stretching at 50 ° C or lower, and as a result, the elongation absorption parameter is 0.7 or more. It is presumed that it is easy to control to 5 or less. Therefore, if the heat treatment is performed without cooling to 50 ° C. or lower after biaxial stretching, the elongation absorption parameter may not be controlled to 0.7 or more and 1.5 or less. It is more preferable to have a heat treatment step after undergoing a step of biaxially stretching and then cooling to 35 ° C. or lower.

In the present invention, the elongation absorption parameter tends to increase as the intrinsic viscosity of the film increases. The elongation absorption parameter can be set in a suitable range by controlling it in combination with other requirements, but the intrinsic viscosity of the film is preferably 0.66 dl / g or more and 1.15 dl / g or less. If the intrinsic viscosity is increased, the filtration pressure tends to increase in the extrusion process for forming the film, which may reduce mass productivity. The intrinsic viscosity of the film can be adjusted by the intrinsic viscosity of the resin used. Although there is no particular upper limit, the filter pressure becomes high in the extrusion process for forming the film, and it becomes necessary to reduce the discharge amount, which may reduce mass productivity. The intrinsic viscosity of the film can be adjusted by the intrinsic viscosity of the resin used.

In the present invention, as described above, it is important to set the micro endothermic temperature peak Tmeta to less than 200 ° C. or not to have it in order to set the extended endothermic parameter to 0.7 or more and 1.5 or less. In order to set the elongation absorption parameter to 0.8 or more while setting the intrinsic viscosity and the film forming and stretching conditions within the preferable ranges of the present invention, it is important to set Tmeta to less than 195 ° C. To make it 0 or more, it is important to keep Tmeta below 185 ° C, and to make the elongation absorption parameter 1.2 or more, it is important to keep Tmeta below 175 ° C. In order to set the micro endothermic temperature peak Tmeta to less than 200 ° C., it can be controlled by setting the maximum heat history temperature (usually indicating the heat treatment temperature) in the film manufacturing process to 200 ° C. or lower.

From the viewpoint of heat resistance to drying, the polyester film of the present invention preferably has a heat shrinkage start temperature of 65 ° C. or higher and 120 ° C. or lower in both the a direction and the b direction. In order to control this range, the Young's modulus in the a direction is adjusted to be 2.8 GPa or more, and then heat treatment is performed so that Tmeta is 180 ° C. or more. Further, more preferably, the heat shrinkage start temperature is 70 ° C. or higher and 100 ° C. or lower in both the a direction and the b direction. In order to control this range, the stretching temperature ED1 of the first axis during biaxial stretching is set to be 10 ° C. or higher higher than the glass transition temperature Tg of the resin, and the stretching temperature ED2 of the second axis is the following formula. It can be controlled by heat-treating so that the temperature of Tmeta is 180 ° C. or higher.
｜ ED1-ED2 ｜ ≦ 5 ℃
In the case of stepwise stretching, ED1 and ED2 refer to the temperatures of TD1 and TD2 in each stretching step.

The polyester film of the present invention preferably has a bending hysteresis 2HB in both the a direction and the b direction of 0.04 gf · cm / cm or more and 0.10 gf · cm / cm or less. When the bending hysteresis 2HB is in the above range, the dynamic bending resistance is excellent. Here, the bending hysteresis 2HB is a value obtained by a measuring method described later, and is an index showing bending repulsion. If the bending hysteresis 2HB exceeds 0.10 gf · cm / cm, the dynamic flexibility deteriorates, and if it is less than 0.04 gf / cm / cm, the flatness when the curable resin layer is provided may be inferior. ..

The stress applied to the film when the dynamic bending test is performed is compression on the inside of the film bending and tension on the outside of the bending. To obtain resistance to bending, the resistance to stress applied in either the film surface direction or the thickness direction is required. It is important to have. When the bending hysteresis 2HB is controlled to 0.10 gf · cm / cm or less, for example, when a polyester film is used and the plane orientation is increased, resistance to the plane direction, that is, resistance to pulling on the outside of bending in dynamic bending can be obtained. However, since the refractive index in the thickness direction is lowered, resistance to compression inside the bending cannot be obtained, and the bending hysteresis 2HB may not be 0.10 gf · cm / cm or less. Therefore, in order to set the bending hysteresis 2HB to 0.10 gf · cm / cm or less, it is preferable to set the refractive index in the thickness direction to 1.46 or more. Further, from the viewpoint of obtaining bending resistance in the plane direction, the plane orientation coefficient obtained by subtracting the thickness direction refractive index from the average refractive index in the a direction and the b direction, which indicates the degree of orientation in the plane direction, is set to 0.166 or more and 0.17 or less. It is preferable to control.

The refractive index and the surface orientation coefficient in the plane direction and the thickness direction of the film can be adjusted by the stretching temperature under the film forming conditions, the area stretching ratio, the heat treatment temperature after stretching, and the crystallinity of the resin. In order to make the thickness direction refractive index 1.46 or more in the manufacturing method that employs sequential biaxial stretching, the stretching temperature in the longitudinal direction is set to the glass transition temperature or higher and the glass transition temperature + 20 ° C. or lower in the longitudinal direction, and the stretching temperature in the width direction is set. Examples thereof include a method in which the glass transition temperature of the resin is + 20 ° C. or higher and the glass transition temperature is + 60 ° C. or lower, and the area stretching ratio is 10 times or more and 16 times or less. Further, the higher the crystallinity, the smaller the refractive index in the thickness direction after stretching tends to be, and the crystallinity of the resin can be controlled, for example, by introducing a copolymerization component. Taking the case of using polyethylene terephthalate among polyesters as an example, the crystallinity can be lowered by copolymerizing isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedimethanol, spiroglycol and the like.

From the viewpoint of maintaining flatness when a curable resin layer such as a hard coat layer is applied, the polyester film of the present invention has a bending hardness of 0.06 gf. It is preferably cm ² / cm or more and 0.12 gf · cm ² / cm or less. More preferably, the bending hardness in both the a direction and the b direction is 0.08 gf · cm ² / cm or more and 0.12 gf · cm ² / cm or less. The flexural rigidity tends to increase as the thickness of the film increases and the orientation of the film increases. The flexural rigidity can be controlled within the above range by adjusting the thickness and the orientation of the film according to other necessary properties.

The polyester film of the present invention preferably has a film thickness of 5 μm or more and 30 μm or less, preferably 9 μm or more and 28 μm or less, from the viewpoint of handleability, scratch resistance, bending resistance, and flatness when a curable resin is applied. It is more preferable, and most preferably 12 μm or more and 26 μm or less. Further, when the thickness exceeds 30 μm, the bending hysteresis 2HB tends to be very high regardless of the material, and therefore it is preferably 30 μm or less.

The polyester film of the present invention contains polyester as a main constituent. Glycols or derivatives thereof that give polyester include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, and 1, , An aliphatic dihydroxy compound such as 6-hexanediol and neopentyl glycol, a polyoxyalkylene glycol such as diethylene glycol, polyethylene glycol, polypropylene glycol and polytetramethylene glycol, and an alicyclic group such as 1,4-cyclohexanedimethanol and spiroglycol. Examples thereof include aromatic dihydroxy compounds such as dihydroxy compounds, bisphenol A and bisphenol S, and derivatives thereof.

Further, as the dicarboxylic acid or a derivative thereof that gives the polyester used in the present invention, terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonicdicarboxylic acid, diphenoxyetanedicarboxylic acid, 5 -Aromatic dicarboxylic acids such as sodium sulfonedicarboxylic acid, aliphatic dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid and fumaric acid, and alicyclic such as 1,4-cyclohexanedicarboxylic acid. Examples thereof include oxycarboxylic acids such as group dicarboxylic acids and paraoxybenzoic acids, and derivatives thereof. Examples of the derivative of the dicarboxylic acid include dimethyl terephthalate, diethyl terephthalate, 2-hydroxyethylmethyl ester terephthalic acid, dimethyl 2,6-naphthalenedicarboxylate, dimethyl isophthalate, dimethyl adipate, diethyl maleate, and dimethyl dimerate. The esterified product can be mentioned.

As for the composition of the polyester in the present invention, 80 mol% or more of the glycol unit is preferably a structural unit derived from ethylene glycol, more preferably 85 mol% or more, and most preferably 90 mol% or more. Further, it is preferable that 80 mol% or more of the dicarboxylic acid unit is a structural unit derived from terephthalic acid, more preferably 85 mol% or more, and most preferably 90 mol% or more. Further, instead of a single resin, another polyester resin may be mixed. For example, mixing polybutylene terephthalate can reduce Young's modulus, and adding polyethylene naphthalate tends to increase Young's modulus. As appropriate, it can be mixed according to the required characteristics.

The polyester film of the present invention preferably has a weight average molecular weight Mw of 20,000 or more and 50,000 or less, and more preferably 20,000 or more and 27,500 or less. When the weight average molecular weight is in the range, as described above, the increase in the entanglement of the molecular chains makes it easier to control the elongation absorption parameter within the range of the present application as compared with the film obtained under the same film forming conditions. The method for setting the weight average molecular weight Mw to 20,000 or more and 50,000 or less is not particularly limited. For example, the higher the intrinsic viscosity of the polyester used as a raw material, the higher the weight average molecular weight Mw tends to be, and when the melt extrusion temperature is higher than the melting point of the resin, the weight average molecular weight Mw tends to be lower. When the residence time from the mouthpiece to bleeding is lengthened, the weight average molecular weight Mw tends to decrease. Therefore, the preferable weight average molecular weight Mw can be obtained by controlling each condition. In addition, after using a resin having an intrinsic viscosity of 0.70 or more for polyester, the extrusion temperature is controlled to the melting point of the resin + 40 ° C. or less, and the residence time until the polyester is charged into the extruder and bleeds from the mouthpiece is 10. A method with a minute or less is mentioned as a preferable method. From the viewpoint of increasing the weight average molecular weight Mw, it is more preferable to use a resin having an intrinsic viscosity of 0.80 or more.

The polyester film of the present invention preferably has 200,000 or more and 1 million or less bending breaks in the a direction. When the number of bending resistance fractures is within this range, the bending resistance is repeatedly improved. Therefore, a take-up type flexible display used so that the a direction is the take-up direction is preferable because it has good long-term durability by taking advantage of its excellent bending resistance during repeated winding. It can be controlled by setting the plane orientation coefficient of the film to 0.165 or more and 0.170 or less.

The polyester film of the present invention preferably has a static friction coefficient of at least one surface of 0.2 or more and 0.5 or less from the viewpoint of preventing scratches when winding is assumed, such as in a take-up display. The coefficient of static friction can be controlled by including particles or the like in the film. As the particles, for example, it is preferable that at least one surface layer has an inorganic particle having an average particle size of 0.005 μm or more and 10 μm or less, and / or a layer containing 0.01% by mass or more of organic particles. From the viewpoint of display transparency, the particle content is preferably 3.0% by mass or less. As the inorganic particles, for example, wet and dry silica, colloidal silica, aluminum silicate, titanium oxide, calcium carbonate, calcium phosphate, barium sulfate, aluminum oxide, mica, kaolin, clay and the like can be used. Further, as the organic particles, particles containing styrene, silicone, acrylic acids, methacrylic acids, polyesters, divinyl compounds and the like as constituents can be used. Among them, it is preferable to use wet and dry silica, inorganic particles such as alumina and calcium carbonate, and particles containing styrene, silicone, acrylic acid, methacrylic acid, polyester, divinylbenzene and the like as constituents. Further, two or more kinds of these inorganic particles and organic particles may be used in combination. If the coefficient of static friction is less than 0.2, the film becomes slippery and may be inferior in handleability. On the other hand, if the coefficient of static friction exceeds 0.5, the surface may be easily scratched at the time of winding assuming a rollable display or the like.

The polyester film of the present invention preferably has a surface resistivity value of at least one surface of 1 × 10 ¹² Ω / □ or less. Particularly preferably, it is 0.01 × 10 ¹² (Ω / □) or more and 1 × 10 ¹² (Ω / □) or less. When the surface resistivity is within this range, it is possible to prevent the adhesion of dust, the adhesion between products due to charging, and the prevention of electrostatic damage such as the destruction of precise electronic circuit material elements due to electrostatic discharge. When the surface resistivity exceeds 1 × 10 ¹² (Ω / □), static electricity is likely to be generated in the film, and foreign matter is likely to adhere to the surface of the film of the present invention in the process of laminating another layer, which is a drawback. There is. If it is less than 0.01 × 10 ¹² (Ω / □), the film may conduct, which may not be preferable. In order to control the surface resistivity within this range, it can be controlled by corona-treating the surface and laminating an easy-adhesive resin layer having a surface free energy of 38 mN / m or more on at least one surface.

A preferred embodiment of the polyester film of the present invention is a laminated sheet having a layer containing a curable resin on at least one side of the polyester film. By forming a laminated sheet having a layer containing a curable resin on at least one side of the polyester film of the present invention, it is possible to enhance the effect of suppressing damage to impact from the curable resin layer side, and thus organic electroluminescence. It can be suitably used for a luminous display device application.

Here, the curable resin refers to a resin that forms a crosslinked structure and is cured by irradiating it with heat or light. The curable resin is not particularly limited, but is preferably a thermosetting resin or an ultraviolet curable resin, and specifically, an organic silicone-based, a polyol-based, a melamine-based, an epoxy-based, a polyfunctional acrylate-based, or a urethane-based resin. Examples thereof include isocyanate-based resins, organic-inorganic hybrid-based resins which are composite materials of organic materials and inorganic materials, and silicone-based resins having a curable functional group. More preferably, it is an epoxy-based, polyfunctional acrylate-based, organic-inorganic hybrid-based, or silsesquioxane-based resin. More preferably, it is a polyfunctional acrylate-based, organic-inorganic hybrid-based, or silsesquioxane-based resin.

As the polyfunctional acrylate-based and silsesquioxane-based resins used as the curable resin, polyfunctional acrylate monomers, oligomers, urethane acrylate oligomers, alkoxysilanes, alkoxysilane hydrolysates, alkoxysilane oligomers, and the like are preferable. Examples of the polyfunctional acrylate monomer include a polyfunctional acrylate having two or more (meth) acryloyloxy groups in one molecule and a modified polymer thereof, and specific examples thereof include pentaerythritol tri (meth) acrylate and pentaerythritol. Tetra (meth) acrylate, dipentaerythritol tri (meth) acrylate, dipentaerythritol tetra (meth) acrylate, dipentaerythritol penta (meth) acrylate, dipentaerythritol hexa (meth) acrylate, trimethyrole propanetri (meth) acrylate , Penta-eristol triacrylate hexanemethylene diisocyanate urethane polymer and the like can be used. These monomers may be used alone or in admixture of two or more.

Further, in the present invention, it is preferable that the layer containing the curable resin contains one or more kinds of particles. Here, the particles may be either inorganic particles or organic particles, but it is preferable to contain the inorganic particles in order to improve the surface hardness. The inorganic particles are not particularly limited, and examples thereof include metal and metalloid oxides, siliconized products, nitrides, boronized products, chlorides, and carbonates. Specifically, silica (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), zinc oxide (ZnO), zirconium oxide (ZrO ₂ ), titanium oxide (TIO ₂ ), antimony oxide (Sb ₂ O ₃ ) and indium. At least one particle selected from the group consisting of tin oxide (In ₂ O ₃ + SnO ₂ ) is preferred. When introducing particles for the purpose of improving the surface hardness, the particle diameter is preferably 1 nm or more and 300 nm or less. The particle size is more preferably 50 nm or more and 200 nm or less, and further preferably 100 nm or more and 150 nm or less in terms of achieving both surface hardness and bending resistance at a higher level. The particle size here means a number average particle size, and means the particle size observed in the cross section of the film. If the shape is not a perfect circle, the value converted into a perfect circle with the same area is used as the particle diameter. Here, the number average particle size Dn can be obtained by the following steps (1) to (4).
(1) First, a microtome is used to cut a cross section of the film without crushing it in the thickness direction, and a scanning electron microscope is used to obtain a magnified observation image. At this time, cutting is performed so as to be parallel to the film a direction.
(2) Next, for each particle observed in the cross section in the image, the cross-sectional area S is obtained, and the particle size d is obtained by the following formula.
d = 2 × (S / π) ^1/2
(3) Using the obtained particle diameter d and the number n of resin particles, Dn is obtained by the following equation.
Dn = Σd / n
However, Σd is the total particle size of the particles in the observation surface, and n is the total number of particles in the observation surface.
(4) The above (1) to (3) are carried out at five different locations, and the average value is taken as the number average particle size of the particles. The above evaluation is carried out in a region of 2500 μm ² or more per observation point.

Further, the content ratio of the curable resin and the particles is preferably particles / resin = 20/80 to 80/20 in terms of mass ratio. If the particle / resin is less than 20/80, the surface hardness may be insufficient, and if it exceeds 80/20, the bending resistance may be lowered. The mass ratio of particles / resin is more preferably 30/70 to 70/30, and even more preferably 40/60 to 60/40, because both surface hardness and bending resistance are compatible at a higher level. ..

Next, an example of a specific manufacturing method of the polyester film of the present invention will be described. Here, polyethylene terephthalate is used as an example of the resin constituting the film, but the present invention is not limited to such an example.

First, as a resin used for a film, a polyethylene terephthalate resin having an intrinsic viscosity of 0.85 gl / d is dried and pre-crystallized, and then supplied to a single-screw extruder for melt extrusion. At this time, it is preferable to control the resin temperature to 265 to 290 ° C. Next, the foreign matter is removed and the extrusion amount is proportionalized through the filter and the gear pump, and the foreign matter is discharged from the T-die onto the cooling drum in the form of a sheet. At that time, the electrostatic application method in which the cooling drum and the resin are brought into close contact with each other by electrostatic force using an electrode to which a high voltage is applied, the casting method in which a water film is provided between the casting drum and the extruded polymer sheet, and the casting drum temperature are set to polyester resin. The sheet-like polymer is brought into close contact with the casting drum, cooled and solidified, and unstretched by a method of adhering the extruded polymer from the glass transition point to (glass transition point -20 ° C) or a method of combining a plurality of these methods. Get the film. Among these casting methods, when polyester is used, the method of electrostatically applying is preferably used from the viewpoint of productivity and flatness.

The unstretched film obtained in the casting step is stretched in the longitudinal direction and then stretched in the width direction, or stretched in the width direction and then stretched in the longitudinal direction by a sequential biaxial stretching method, or in the longitudinal direction of the film. It can be obtained by stretching by a simultaneous biaxial stretching method in which the width direction is stretched almost simultaneously. As a biaxial stretching method for such a film, for example, stretching in multiple stages in the stretching direction of each axis is important for forming a dense molecular structure. For example, in the case where the stretching of the first axis is carried out in three stages, when the stretching ratios of the three stages are set to MD1, MD2, and MD3 in order, it is necessary to set the magnifications as follows in order to obtain a precise bulk structure. is important.
MD1: 1.03 times or more and 1.8 times or less.
MD2: 1.05 times or more and 1.12 times or less.
MD3: 3.0 times or more, and MD3 occupying the total draw ratio (MD1 × MD2 × MD3) of the first axis is 80% or more.
Total draw ratio of the first axis: 3.5 times or more.

Further, in the case where the second axis stretching is carried out in three stages, when the stretching ratios in the three stages are set to TD1, TD2, and TD3 in order, it is necessary to set the magnifications as follows to obtain a precise bulk structure. Is important for.
TD 1: 1.5 times or more and 2.2 times or less.
TD2: 1.3 times or more and 1.8 times or less.
TD3: 1.2 times or more and 1.5 times or less.
The total draw ratio of the second axis (TD1 × TD2 × TD3): 3.5 times or more.

In the present invention, from the viewpoint of setting the Young's modulus to 2.8 GPa or more and the viewpoint of thickness unevenness, it is preferable to stretch 3.5 times or more in each direction, and the plane orientation coefficient is 0.165 or more and 0.17 or less. In some cases, the area stretch ratio is preferably 12.25 times or more and 16 times or less. Further, the stretching temperature is preferably set to such an extent that stretching unevenness does not occur, and from the viewpoint of further setting the thickness direction refractive index to 1.46 or more, for example, sequentially biaxially stretching in the longitudinal direction and then stretching in the width direction. When the stretching method is adopted, it is preferable that the preheating temperature in the longitudinal direction is the glass transition temperature of the resin -20 ° C or higher and the glass transition temperature + 0 ° C or lower, and the stretching temperature is the glass transition temperature of the resin or higher and the glass transition temperature + 30 ° C or lower. It is preferable that the preheating temperature in the width direction is the glass transition temperature of the resin of −10 ° C. or higher and the glass transition temperature of + 20 ° C. or lower, and the stretching temperature is the glass transition temperature of the resin or higher and the glass transition temperature of + 60 ° C. or lower.

The polyester film of the present invention is preferably subjected to heat treatment of the film after biaxial stretching. The heat treatment can be performed by any conventionally known method such as in an oven or on a heated roll. However, as described above, in the present invention, in order to form the film bulk structure densely and increase the molecular chain entanglement, it is important to cool the film to 50 ° C. or lower once after biaxial stretching before the heat treatment. .. After biaxial stretching and cooling to 50 ° C. or lower, heat treatment may be performed on the premise that the micro endothermic temperature peak Tmeta is less than 200 ° C. as described above. Dimensional stability can be improved by heat treatment. In order to set the endothermic temperature peak Tmeta to less than 200 ° C., the heat treatment temperature may be set to 200 ° C. or lower. The heat treatment may be divided into a plurality of zones and the temperature may be raised or lowered stepwise, or a method of finely stretching the temperature in the width direction to about 1.01 to 1.2 times in the heat treatment step can also be used. The heat treatment time can be arbitrary as long as the characteristics are not deteriorated, and is preferably performed for 10 to 60 seconds, more preferably 15 to 30 seconds. Further, the heat treatment may be performed by relaxing the film in the longitudinal direction and / or the width direction.

Further, in the polyester film of the present invention, when a layer containing a curable resin is laminated, the surface is corona-treated from the viewpoint of adhesion to the layer, or at least one surface thereof has a thickness of 10 nm or more and 500 nm or less. It is preferable to laminate an easily adhesive resin layer having a surface free energy of 38 mN / m or more. As a method for forming the easy-adhesive resin layer, the film surface is coated with the easy-adhesive resin (composite melt extrusion method, hot melt coat method, solvent other than water, in-line or offline coat from water-soluble and / or water-dispersible resin). (Method, etc.), and the surface lamination method of the same composition or its blended product can be mentioned. Among them, an in-line coating method in which a film coating agent is applied to one surface of a film before alignment crystallization is completed, stretched in at least one direction, and heat-treated to complete alignment crystallization is used for uniform film formation. Industrially preferable. Further, when the easy-adhesion resin layer is provided by coating, the resin to which the easy-adhesion resin layer is applied is not particularly limited, but for example, an acrylic resin, a urethane resin, a polyester resin, an olefin resin, etc. Fluorine-based resin, vinyl-based resin, chlorine-based resin, styrene-based resin, various graft-based resins, epoxy-based resin, silicone-based resin and the like can be used, and a mixture of these resins can also be used. From the viewpoint of adhesion, it is preferable to use a polyester resin, an acrylic resin, or a urethane resin. When a polyester resin is used as a water-based coating liquid, a water-soluble or water-dispersible polyester resin is used. For such water-soluble or water-dispersible, a compound containing a sulfonic acid base or a carboxylic acid is used. It is preferable to copolymerize a compound containing an acid base. When the acrylic resin is used as an aqueous coating liquid, it must be in a state of being dissolved or dispersed in water, and examples of the emulsifier include surfactants (for example, polyether compounds), but the emulsifiers are not limited. No.) may be used.

Further, in the easy-adhesive resin layer used in the present invention, various cross-linking agents can be used in combination with the resin in order to further improve the adhesiveness. As the cross-linking agent resin, a melamine-based resin, an epoxy-based resin, or an oxazoline-based resin is generally used. Examples of the particles contained in the resin layer of the present invention include inorganic particles and organic particles, but inorganic particles are more preferable because they improve slipperiness and blocking resistance. As the inorganic particles, silica, alumina, kaolin, talc, mica, calcium carbonate, titanium and the like can be used.

Since the polyester film of the present invention is excellent in bending resistance and flatness after coating with a curable resin layer, it can be particularly preferably used as a cover film for an organic electroluminescence display device. By applying it as a cover film for an organic electroluminescence display device, it is possible to prevent scratches on the surface of the display device without impairing the flexibility of the display device. Further, it is also preferable to use it not only as an optical film but also as a work material film such as various cover films utilizing the characteristics of the present invention and packaging applications.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to these Examples. The characteristics were measured by the following methods.

(1) Film thickness When measuring the total thickness of the film, the thickness of the sample cut out from the film was measured at five arbitrary locations using a dial gauge, and the average value was calculated.

(2) Composition of resin constituting the film The film is dissolved in hexafluoroisopropanol (HFIP), and the content of each monomer residue and by-product diethylene glycol is quantified using ¹ H-NMR and ¹³ C-NMR.

(3) Glass transition temperature and melting point Tm, Tmeta of the resin constituting the polyester film
Using the differential scanning calorimetry robot DSC-RDC6220 manufactured by Seiko Instruments Co., Ltd. and the thermal analysis rheology system software "Muse" manufactured by SI Nanotechnology Co., Ltd. for data analysis, the glass transition temperature [℃] The melting point Tm [° C.] and the minute heat absorption peak temperature Tmeta [° C.] were measured and analyzed in accordance with JIS K7121 (1987).
Specifically, the temperature of the apex of the exothermic peak obtained from the DSC curve when the temperature of 5 mg of the sample was raised from 25 ° C. to 300 ° C. at 20 ° C./min was Tcc, and the temperature of the endothermic peak apex obtained from the DSC curve. Was defined as the melting point Tm, and the minute endothermic peak observed at the midpoint between Tm and Tcc was defined as Tmeta. The glass transition temperature is the straight line at the same distance in the vertical direction from the extended straight line of each baseline and the curve of the stepwise change part of the glass transition in the stepwise change part of the glass transition in the differential scanning calorimetry chart. I asked for it from the point of intersection.

(4) Intrinsic Viscosity 0.1 g of the sample was weighed with an accuracy of 0.001 g or less, and dissolved by heating at 100 ° C. for 30 minutes using 10 mL of o-chlorophenol. The solution was cooled to room temperature, 8 mL of the solution was charged into an Ostwald viscometer installed in a water tank at 25 ° C., and the number of seconds passed through the marked line was measured (A seconds). In addition, the number of seconds passed through the marked line was measured with an Ostwald viscometer installed in a water tank at 25 ° C. using only 8 mL of o-chlorophenol (B seconds). The intrinsic viscosity was calculated by the following formula.

IV = -1 + [1 + 4 × K × {(A / B) -1}] ^0.5 / (2 × K × C)
Here, K is 0.343 and C is the concentration of the sample solution (g / 100 mL).

(5) Surface orientation coefficient fn of polyester film
Refractive index in the a direction, b direction and thickness direction of the film using a sodium D line (wavelength 589 nm) as a light source, methylene iodide as a mounting solution, and an Abbe refractometer 4T (manufactured by Atago Co., Ltd.) at 25 ° C. (NMD, nTD, nZD, respectively) were measured according to the JIS K7142 (2014) A method. The refractive index of the test piece used was 1.74. From the obtained refractive index, the plane orientation coefficient (fn) of the B layer was calculated by the following formula (1).
fn = (nMD + nTD) /2-nZD ... (1)
(6) Weight average molecular weight Mw
To prepare the sample solution, 5 mL of hexafluoroisopropanol added with sodium trifluoroacetate was added to 3 mg of the sample, and the mixture was gently stirred at 40 ° C. for 3 hours. After that, filtration was performed using a 0.5 μm filter. Next, the weight average molecular weight was measured under the following conditions using a gel permeation chromatograph GPC (detector: differential refractive index detector RI Tosoh RI-8020) using hexafluoroisopropanol added with sodium trifluoroacetate as a solvent. did.
Column: shodexHFIP-LG 1 piece (φ8.0 mm x 5 cm, manufactured by Showa Denko)
shodexHFIP-806M 2 pieces (φ8.0mm x 30cm, manufactured by Showa Denko)
Flow rate: 0.5 mL / min
Column temperature: 40 ° C
Injection volume: 0.2 mL
Molecular weight calibration: Showa Denko monodisperse polymethylmethacrylate (PMMA) (standard sample).

(7) Thickness of Curable Resin Layer and Easy Adhesive Resin Layer The thickness of the curable resin layer on the film was measured by observing the cross section using a transmission electron microscope (TEM). The thickness of the curable resin layer was read from an image taken by TEM at a magnification of 100,000 times. The thicknesses of the curable resin layer and the easy-adhesive resin layer at 10 points in total were measured, and the average value was used. The observation magnification may be other than 100,000 times as long as the thickness can be measured.

(8) Young's modulus A strip-shaped sample having a length of 150 mm and a width of 10 mm is rotated clockwise by 10 °, and the Young's modulus is measured for 18 samples from 0 ° to 170 °, and the direction in which the Young's modulus is highest. Was the a direction, and the direction orthogonal to it was the b direction. Young's modulus was measured using an Instron type tensile tester according to the method specified in JIS Z1702 (1994). The measurement was carried out under the following conditions, and the measurement was performed for each of 10 samples, and the average value was obtained.
Measuring device: Orientec Co., Ltd. film strength and elongation automatic measuring device "Tensilon" (registered trademark) AMF / RTA-100
Sample size: width 10 mm x trial length 50 mm
Tensile speed: 300 mm / min Measurement environment: Temperature 23 ° C, humidity 65% RH.

(9) Stretch absorption parameter A strip-shaped sample with a length of 150 mm and a width of 10 mm is cut out and used so that the film a and b directions are in the length direction, and shrinkage in the width direction at an elongation of 60% in each direction. By measuring the rate with a universal tester, the elongation absorption parameter was calculated by the following formula. The tensile test was measured using an instron type according to the method specified in JIS Z1702 (1994). The measurement was performed under the following conditions, and the test was stopped after pulling each sample to an elongation of 60% with 10 samples, and the initial length was L ₀ , the length at an elongation of 60% was L ₆₀ , and the initial in the width direction. When the value was W ₀ and the width and length at an elongation of 60% were W ₆₀ , the elongation absorption parameter was calculated from the following formula. For L ₀ and L ₆₀ , select the position where the maximum value is obtained from the measurement sample, and for W ₀ and W ₆₀ , use the values extracted from the position where the minimum value is obtained. If necessary, it can be measured using a universal projector.
Universal testing machine: Orientec Co., Ltd. film strength and elongation automatic measuring device "Tensilon" (registered trademark) AMF / RTA-100
Sample size: width 10 mm x trial length 50 mm
Tensile speed: 300 mm / min Measurement environment: Temperature 23 ° C, humidity 65% RH.
Elongation absorption parameter =-(ln (W ₆₀ / W ₀ ) / (ln (L ₆₀ / L ₀ )))
* Ln: Natural logarithm (10) Bending hysteresis 2HB
Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., measurements were performed in the a direction and the b direction under the following conditions to obtain bending hysteresis 2HB. For the bending hysteresis 2HB, the data obtained in the 5th cycle (5 repetitions) was adopted with the curvature −2.5 to +2.5 cm ^-1 as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2x5
Curvature: -2.5 to +2.5 cm ^-1
Deformation speed: 0.50 cm ^-1 / sec
Sample size: a direction x b direction = 20 cm x 20 cm
(11) Flexural rigidity Using a multipurpose bending tester (KES-FB2) manufactured by Katou Tech Co., Ltd., measurements were performed in the a and b directions under the following conditions to obtain the flexural rigidity. For the flexural rigidity, the data obtained in the 5th cycle (5 repetitions) was adopted with the curvature −2.5 to +2.5 cm ^-1 as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2x5
Curvature: -2.5 to +2.5 cm ^-1
Deformation speed: 0.50 cm ^-1 / sec
Sample size: a direction x b direction = 20 cm x 20 cm
(12) Static friction coefficient Using a slip tester manufactured by Toyo Seiki Co., Ltd., the initial rising resistance value when both sides of the film are overlapped and rubbed is measured according to JIS-K7125 (1999), and the static friction coefficient is measured. It was set to μs. The sample was a rectangle having a width of 80 mm and a length of 200 mm, and 3 sets (6 sheets) were cut out from the roll so that the length direction of the rectangle was the a direction. The measurement was performed three times, and the average value was calculated.

(13) Scratch resistance during winding The polyester film of the present invention was subjected to a rubbing test under the following conditions using a rubbing tester, and used as an index of scratch resistance. The same polyester film shall be used for the sample and the film used for the scraping material.
Evaluation environmental conditions: 25 ° C, 60% RH
Rubbing material: Wrap around the rubbing tip (1 cm x 1 cm) of a tester that comes into contact with the polyester film sample of the present invention, and fix the band.
Travel distance (one way): 13 cm,
Rubbing speed: 13 cm / sec,
Load: 100 g / cm ² ,
Tip contact area: 1 cm x 1 cm, number of rubs: 5 reciprocations.
Oil-based black ink was applied to the back side of the rubbed sample, and scratches on the rubbed portion were visually observed with reflected light and evaluated according to the following criteria. The evaluation repeated the above test three times.
○: No scratches can be seen.
Δ: Slight scratches are visible.
×: There are scratches that can be seen at a glance.

(14) Surface resistivity The film is allowed to stand at a temperature of 23 ° C. and a relative humidity of 65% for 24 hours to control the humidity, and then applied voltage using a digital ultrahigh resistivity / micro ammeter (R8340A manufactured by Advantest) under the same conditions. The surface resistivity is measured at 100 V. Measure 5 times, and use the average value as the surface resistivity (Ω / □).

(15) Number of bending fractures A sample cut into a size of 110 mm in length (measurement direction) and 15 mm in width according to JIS P8115 (2001) using a MIT folding resistance tester (Tester No. 702 manufactured by Maze). A bending test was performed with a load of 1000 g, a bending angle of 135 ° to the left and right (R: + 135 °, L: -135 °), a bending speed of 175 times / minute, and a chuck tip R: 0.38 mm. The number of times was defined as the number of bending breaks. The test was carried out three times, and the average value was adopted.

(16) Dynamic bending resistance The polyester film of the present invention was cut into a sample having a width of 108 mm and a length of 112 mm using a U-shaped expansion / contraction tester (DLDMLLH-FS manufactured by Yuasa System Equipment), and the tilt clamp was in a horizontal state. Then, attach it to the end of the tilt clamp so that the stroke direction is the sample length direction, bend it 10,000 times at a test speed of 60 r / min, a test stroke of 60 mm, and an inter-plane distance of 3 mm, and use a fluorescent lamp for the sample after the test. The following judgment was made based on the reflected light and the appearance.
A: There was no change in appearance, and no distortion of reflected light was observed.
B: There was no change in appearance, but distortion of reflected light was observed.
C: Bending lines were clearly observed on the appearance.

(17) Winding resistance by mandrel test A film cut out in a direction x b direction = 15 mm x 30 mm was humidity-controlled for 24 hours under 23 ° C. 60% RH conditions, and the center of the pedestal of the mandrel tester (manufactured by Ueshima Seisakusho). Install as shown in Fig. 1 so that the center of the film and the center of the film coincide with each other. A test rod with a diameter of 1.0 mm is installed, the film is wound 180 °, and the film is allowed to stand for 24 hours. After that, the film was taken out, immediately placed on a flat floor, and the height raised from the ground was read. The film cut out in the b direction × a direction = 15 mm × 30 mm is also measured in the same manner, and the raised height is read. The film cut out in the a direction x b direction = 15 mm x 30 mm and the film cut out in the b direction x a direction = 15 mm x 30 mm were measured 5 times each, and the winding resistance was measured from the average value of the raised height 10 times in total. The takeability was determined as follows.

Less than 0.5 mm: ◎
0.5 mm or more and less than 0.8 mm: ○
0.8 mm or more and less than 1.0 mm: □
1.0 mm or more and less than 1.2 mm: △
1.2 mm or more: ×
(18) Flatness after coating the curable resin The curable resin Q described below (formulation of the curable resin Q) is applied using a slot die coater by controlling the flow rate so that the thickness after drying becomes 5 μm. Then, it was dried at 100 ° C. for 1 minute to remove the solvent. Next, the film coated with the curable resin was irradiated with ultraviolet rays of 300 mJ / cm ² using a high-pressure mercury lamp to obtain a curable resin laminated sheet of the polyester film. The flatness of the obtained laminated sheet after application of the curable resin layer was determined as follows for (a) wrinkles and (b) curls.
(A) No wrinkles and good flatness of the entire sheet: ◎
There is slight distortion of the film, but there is no problem in practical use: ○
Obvious film distortion is occasionally seen, but there is no problem in practical use: △
The entire film swells and cannot be used: ×
(B) When the curl film was cut out to A4 size and placed on a flat surface, it was determined as follows based on the height at which it was lifted from the ground.
No film lift: ◎
The lift of the film is less than 10 mm above the ground: ○
The lift of the film is 10 mm or more and less than 15 mm from the ground: Δ
The lift of the film exceeds 15 mm from the ground: ×.

(Preparation of curable resin Q)
Silica particles with a particle size of 100 nm (organo silica sol manufactured by Nissan Chemical Industry Co., Ltd.) and polyfunctional acrylate (“KAYARAD” (registered trademark) PET30 manufactured by Nippon Kayaku Co., Ltd.) are mixed at a mass ratio of 50:50 and toluene / A curable resin was prepared by diluting with a MEK mixed solvent (mass ratio 50:50).

(19) Antistatic effect The curable resin laminated sheet of each polyester film obtained by the method described in (18) is cut into □ 20 cm (400 cm2), and then an X35S black film manufactured by Toray Industries, Inc. is laid on the base. The sample cut out was placed in a fluorescent lamp environment of 500 to 1,000 looks, and the major axis of the defect was visually measured with a JIS P8208 / P8145 general dot cage, and the number of defects of 0.1 mm or more was counted. A street judgment was made.
◯: 10 or more / 400 cm ² .
Δ: Less than 10 pieces / 400 cm ² .

(20) Thermal shrinkage start temperature obtained from the thermal shrinkage rate curve Using a thermal mechanical measuring device TMA / SS6100 (manufactured by Seiko Instruments), a load of 3 g is applied to a sample having a sample width of 4 mm and a sample length (distance between chucks) of 20 mm. Loaded. The temperature was raised from room temperature to 170 ° C. at a heating rate of 5 ° C./min and held for 10 minutes after reaching 170 ° C. to obtain a heat shrinkage curve when the temperature was raised from room temperature to 170 ° C. Then, the obtained heat shrinkage curve was differentiated with respect to the measurement time to obtain DTMA. The temperature at which DTMA became 0 was defined as the heat shrinkage start temperature.

(21) Dry heat resistance Screen printing was performed on the surface of the film. Printing was performed using an ink U-PET (517) manufactured by Mino Group Co., Ltd. and a screen SX270T under the conditions of a squeegee speed of 300 mm / sec and a squeegee angle of 45 °, and then dried in a hot air oven under 80 ° C. conditions for 5 minutes. And obtained an evaluation film. The appearance of the obtained evaluation film was evaluated according to the following criteria.
A: No wrinkles were confirmed even after drying, and the appearance was good.
B: Some wrinkles were confirmed after drying, but the appearance was good.
C: Wrinkles were confirmed after drying, but it was at a level where there was no practical problem as a printed film.

(Manufacturing of polyester)
The polyester resin used for film formation was prepared as follows.

(Polyester A)
A polyethylene terephthalate resin (intrinsic viscosity 0.75) having 100 mol% of terephthalic acid component as a dicarboxylic acid component and 100 mol% of ethylene glycol component as a glycol component.

(Polyester B)
A polyethylene terephthalate resin (intrinsic viscosity 0.70) having 100 mol% of terephthalic acid component as a dicarboxylic acid component and 100 mol% of ethylene glycol component as a glycol component.

(Polyester C)
A polyethylene terephthalate resin (intrinsic viscosity 0.80) having 100 mol% of terephthalic acid component as a dicarboxylic acid component and 100 mol% of ethylene glycol component as a glycol component.

(Polyester D)
A polyethylene terephthalate resin (intrinsic viscosity 0.85) having a terephthalic acid component of 100 mol% as a dicarboxylic acid component and an ethylene glycol component of 100 mol% as a glycol component.

(Polyester E)
A polybutylene terephthalate resin (intrinsic viscosity 1.2) containing 100 mol% of a terephthalic acid component as a dicarboxylic acid component and 100 mol% of a 1,4-butanediol component as a glycol component.

(Particle Master A)
Polyethylene terephthalate particle master (intrinsic viscosity 0.65) containing calcium carbonate particles having an average particle diameter of 1.2 μm in polyester A at a particle concentration of 1% by mass.

(Particle Master B)
Polyethylene terephthalate particle master (intrinsic viscosity 1.1) containing calcium carbonate particles having an average particle diameter of 1.2 μm in polyester E at a particle concentration of 1% by mass.

(Preparation of easy-adhesive resin P)
The easy-adhesive resin layer to be laminated on the surface of the film was prepared as follows.

Resin solution (a): Water-soluble polyester resin composed of terephthalic acid (88 mol%) as an acid component, 5-sodium sulfoisophthalic acid (12 mol%), ethylene glycol (100 mol%) as a diol component, and a diol component. 70 parts by mass of the resin coating solution, terephthalic acid (50 mol%), isophthalic acid (49 mol%), 5-sodium sulfoisophthalic acid (1 mol%) and diol component ethylene glycol (55 mol%), neo A solution obtained by mixing 30 parts by mass of an aqueous dispersion of a polyester resin composed of an acidic component of pentylglycol (44 mol%) and polyethylene glycol (molecular weight: 4000) (1 mol%) and a diol component.
Cross-linking agent (b): Methylol-based melamine cross-linking agent Cross-linking agent (c): Oxazoline group-containing cross-linking agent Particles (d): Aqueous dispersion particles of collodyl silica particles having a particle diameter of 150 nm: Corodies having a particle diameter of 300 nm Aqueous dispersion of lusilica particles Fluorine-based surfactant (f): "Megafuck" (registered trademark) F-444 manufactured by DIC Co., Ltd.
By mass ratio of these, (a) / (b) / (c) / (d) / (e) / (f) = 47 parts by mass / 19 parts by mass / 20 parts by mass / 4.9 parts by mass / 0 The mixture was mixed in an amount of 7. parts by mass / 0.1 part by mass.

The refractive index of the easy-adhesive resin P after drying was 1.57.

(Preparation of curable resin Q)
Silica particles with a particle size of 100 nm (organo silica sol manufactured by Nissan Chemical Industry Co., Ltd.) and polyfunctional acrylate (“KAYARAD” (registered trademark) PET30 manufactured by Nippon Kayaku Co., Ltd.) are mixed at a mass ratio of 50:50 and toluene / A curable resin was prepared by diluting with a MEK mixed solvent (mass ratio 50:50).

(Examples 1 to 8, 10 to 19, comparative examples 1 to 6)
As shown in Table 1, the resin type and the particle master type are mixed at the contents shown in Table 1 and charged into the extruder, then melted at the extruder temperature shown in Table 1 and heated to 25 ° C. from the T die. It was discharged in the form of a sheet on the cooling drum controlled to. At that time, an unstretched sheet was obtained by electrostatically applying static electricity using a wire-shaped electrode having a diameter of 0.1 mm and bringing it into close contact with a cooling drum. Then, it was rapidly cooled with a cooling roll whose temperature was controlled to 20 ° C., subsequently stretched with each roll in the MD direction at the stretching temperature and stretching ratio shown in Table 2, and then once cooled (intermediate cooling). Next, both sides of this uniaxially stretched film were subjected to corona discharge treatment to set the wetting tension of the film to 55 mN / m, and the easy-adhesive resin P was applied to both sides of the film, and then the first oven tenter was used to show in Table 2. The film was stretched in the TD direction at the stretching temperature and the stretching ratio of the above, and cooled to the temperature shown in Table 2 as intermediate cooling. Then, in the tenter of the second oven, the film was heat-treated at the heat treatment temperature shown in Table 2 and relaxed in the width direction to obtain a film having the film thickness shown in Table 1. The physical characteristics of the obtained film are as shown in Tables 3 and 4, and the film was in the a direction in the MD direction and the b direction in the TD direction. In the examples, the bending resistance and the flatness after applying the curable resin were excellent.

(Example 9)
A polyester film was obtained in the same manner as in Example 5 except that the easy-adhesive resin P was not applied. As in the other examples, it was excellent in bending resistance and flatness after coating with a curable resin.

Since the polyester film of the present invention has Young's modulus and elongation absorption parameters within a specific range, it can be particularly preferably used as a cover film for, for example, a flexible image display device.

Claims (10)

The Young's modulus in the direction in which the Young's modulus is maximum (a direction) is 2.8 GPa or more, and the elongation absorption parameters in the a direction and the direction orthogonal to the a direction (b direction) are both 0.7 or more. A polyester film of 5 or less. The polyester film according to claim 1, wherein the heat shrinkage start temperature obtained from the heat shrinkage curve is 65 ° C. or higher and 120 ° C. or lower in both the longitudinal direction and the width direction. The polyester film according to claim 1 or 2, wherein the bending hysteresis 2HB obtained by the following measuring method is 0.04 gf · cm / cm or more and 0.10 gf · cm / cm or less in both the a direction and the b direction.
[Measurement method of bending hysteresis 2HB]
Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., bending hysteresis 2HB measurement is performed in each of the a direction and the b direction under the following conditions. The bending hysteresis 2HB is a value obtained in the 5th cycle (5 repetitions) with a curvature of −2.5 to +2.5 cm ^-1 as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2x5
Curvature: -2.5 to +2.5 cm ^-1
Deformation speed: 0.50 cm ^-1 / sec
Sample size: a direction x b direction = 20 cm x 20 cm The polyester according to any one of claims 1 to 3, wherein the bending hardness obtained by the following measuring method is 0.06 gf · cm ² / cm or more and 0.12 gf · cm ² / cm or less in both the a direction and the b direction. the film.
[Measurement method of flexural rigidity]
Using a multipurpose bending tester (KES-FB2) manufactured by Kato Tech Co., Ltd., the bending hardness is measured in the a direction and the b direction under the following conditions. The flexural rigidity is a value obtained in the 5th cycle (5 repetitions) with a curvature of −2.5 to +2.5 cm ^-1 as one cycle.
Number of repetitions: 5 times (5th cycle of recruitment data)
SENS: 2x5
Curvature: -2.5 to +2.5 cm ^-1
Deformation speed: 0.50 cm ^-1 / sec
Sample size: a direction x b direction = 20 cm x 20 cm The polyester film according to any one of claims 1 to 4, wherein the weight average molecular weight Mw is 20,000 or more and 50,000 or less. The polyester film according to any one of claims 1 to 5, wherein the number of bending breaks in the a direction is 200,000 times or more and 1 million times or less. The polyester film according to any one of claims 1 to 6, wherein the film thickness is 5 μm or more and 30 μm or less. The polyester film according to any one of claims 1 to 7, wherein the static friction coefficient of at least one surface is 0.2 or more and 0.5 or less. The polyester film according to any one of claims 1 to 8, wherein the surface resistivity value of at least one surface is 1 × 10 ¹² Ω / □ or less. The polyester film according to any one of claims 1 to 9, which is used for a flexible display.

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