JP2023037175A - Polyester film, miniature led substrate, backlight and display - Google Patents

Abstract

To provide a backlight using a fine LED which achieves high heat resistance and luminance improving effect.SOLUTION: A polyester film has a void, wherein a void ratio is 10% or more, and a ratio of at least one trans of a polyester resin constituting the surface of the film is 0.75 time or more with respect to a ratio of a gauche.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a polyester film preferably used for mini LED display applications.

Many displays using liquid crystals are used as display devices for smartphones, personal computers, televisions, and the like. These liquid crystal displays can display images by irradiating light from a surface light source called a backlight installed on the back side. As for the backlight, the mini-LED system and the micro-LED system, in which a large number of minute LEDs are arranged on a substrate, are being studied due to the recent needs for high brightness, high contrast, and a wide color gamut of the display.

On the other hand, in the conventional LED system, a reflective film is provided around the LED to prevent light from going around to the back of the LED and reduce light loss, thereby achieving high brightness. Reflective films used in backlights for such liquid crystal displays have been conventionally made of a film containing a white pigment or a film containing fine air bubbles, either alone or laminated with a metal plate or a plastic plate. has been used. In particular, a film containing fine air bubbles inside is widely used because it has a certain effect of improving brightness and making screen brightness uniform (see Patent Documents 1 and 2).

JP 2003-160682 A Japanese Patent Publication No. 8-16175

However, conventional reflective films such as those described in Patent Documents 1 and 2 may not have sufficient heat resistance to maintain fine air bubbles. Especially in the mini-LED method and micro-LED method, full-surface encapsulation, which is superior in production efficiency, may be adopted as opposed to the process of individually encapsulating micro LEDs. Not only is the film deformed and the luminance is lowered, but the deformed film places a large load on the LED, causing damage.

The present invention solves the above problems and provides a polyester film that has high brightness and is stable against dimensional changes in high-temperature environments such as the high-temperature heat history of the LED sealing process or continuous LED lighting. It is intended.

In order to solve the above problems, the present invention has the following configurations.

(1) A polyester film having at least a layer containing a polyester resin and a thermoplastic resin or inorganic particles incompatible with the polyester resin as main components, wherein the layer has a void content of 10% or more, and A polyester film, wherein the ratio of the trans isomer of the polyester resin constituting at least one surface of the polyester film is 0.75 times or more the ratio of the gauche isomer.

(2) Vb/Va is 1.2 for the void content rates Va (%) and Vb (%) of the cross section A with the film main axis direction as the cutting direction and the cross section B with the film main axis direction as the normal. (%) or more, the polyester film according to claim 1.

(3) The polyester according to claim 1 or 2, wherein the average of the coefficient of thermal expansion (CTE) at 50 ° C. to 150 ° C. in the principal axis direction of the film and in the direction perpendicular to the principal axis direction is 0 ppm / ° C. or more and 30 ppm / ° C. or less. the film.

(4) When the film is heated at 150° C. for 2 hours, the average thermal shrinkage rate in the principal axis direction of the film and in the direction perpendicular to the principal axis direction is 0.05% or more and 0.5% or less. 4. The polyester film according to any one of 3.

(5) The density of voids having a cross-sectional area of 0.1 μm ² or more and 1.0 μm ² or less in the cross section of the film is 0.25/μm ² or more and 1.25/μm ² or less. The polyester film according to any one of the above.

(6) The average number of voids per 1 μm of thickness in each thickness direction of cross section A with the film main axis direction as the cutting direction and cross section B with the film main axis direction as the normal line is 0.4 or more and 4.0 or less. The polyester film according to any one of claims 1 to 5.

(7) The polyester film according to any one of claims 1 to 6, wherein the sum of the Young's modulus in the principal axis direction and the Young's modulus in the direction perpendicular to the principal axis direction of the film is 4 GPa or more and 20 GPa or less.

(8) A mini LED substrate having the polyester film according to any one of claims 1 to 7.

(9) A backlight having the mini LED substrate according to claim 8.

(10) A display comprising the backlight of claim 9.

According to the present invention, it is possible to improve the luminance of a backlight using mini-LEDs.

FIG. 1 is a schematic cross-sectional view of an example of a backlight in which the polyester film or mini LED substrate of the present invention is preferably used, or the backlight of the present invention.

Embodiments of the present invention will be described below, but the present invention should not be construed as being limited to the embodiments including the following examples. Naturally, various changes are possible within the scope of the invention. In the present invention, "more than or equal to" means equal to or greater than the indicated numerical value. Also, "or less" means equal to or smaller than the indicated numerical value.

The polyester film of the present invention must have at least a layer containing a polyester resin and a thermoplastic resin or inorganic particles incompatible with the polyester resin as main components. Here, the term "main component" refers to a component that accounts for 50% by mass or more of the resin components that constitute the resin composition.

Preferred embodiments of the polyester resin are described below. A polyester resin is a polymer having an ester bond in its main chain, and a polyester resin having a structure in which a dicarboxylic acid and a diol are polycondensed is preferable. Examples of dicarboxylic acids include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenoxyethanedicarboxylic acid, and diphenylsulfonedicarboxylic acid. , oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid, fumaric acid and other aliphatic dicarboxylic acids; alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid; Carboxylic acid and the like can be mentioned. Moreover, each of the above dicarboxylic acids may be used as a dicarboxylic acid ester derivative. Examples of diols include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol. , 2,2-dimethyl-1,3-propanediol (neopentyl glycol) and other aliphatic dihydroxy compounds, diethylene glycol, polyethylene glycol, polypropylene glycol, polyoxyalkylene glycols such as polytetramethylene glycol, 1,4-cyclohexanediol. Alicyclic dihydroxy compounds such as methanol and spiroglycol; aromatic dihydroxy compounds such as bisphenol A and bisphenol S; Each of these may be used alone or in combination of two or more. In addition, a small amount of one or more of trimellitic acid, pyromellitic acid and their ester derivatives may be copolymerized as long as the film formability of the film is not affected.

Specific examples of polyester resins include polyethylene terephthalate (hereinafter abbreviated as PET), polyethylene-2,6-naphthalene dicarboxylate, polypropylene terephthalate, polybutylene terephthalate, poly-1,4-cyclohexylene dimethylene terephthalate, and the like. is available at a low cost and has good film-forming properties, so it can be used particularly preferably.

Of course, these polyester resins may be homopolymers or copolymers. Copolymerization components in the copolymer include aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, and diol components having 2 to 15 carbon atoms, examples of which include isophthalic acid and adipic acid. , sebacic acid, phthalic acid, sulfonic acid group-containing isophthalic acid, and ester-forming compounds thereof, ethylene glycol, 1,4-butanediol, neopentyl glycol, 1,4-cyclohexanedimethanol, spiroglycol, number average molecular weight Polyalkylene glycol having a molecular weight of 400 or more and 20,000 or less can be mentioned.

The layer of the polyester film of the present invention must contain a thermoplastic resin or inorganic particles incompatible with the polyester resin, in addition to the polyester resin. By having a layer mainly composed of a polyester resin and a thermoplastic resin or inorganic particles incompatible with the polyester resin, it not only has an excellent brightness improvement effect in backlights using minute LEDs such as the mini-LED system. , a film excellent in water resistance, durability, chemical resistance, etc. can be obtained.

It is important that the polyester film of the present invention have voids. Here, voids are minute cavities contained in an object, and any polyester film having a void content exceeding 1% in an arbitrary vertical cross section, which will be described later, is regarded as a polyester film having voids.

A thermoplastic resin that is incompatible with the polyester resin is used as a void nucleating agent that forms voids. Hereinafter, the thermoplastic resin incompatible with the polyester resin, which is used as the void nucleating agent, is also simply referred to as "void nucleating agent". A void in the present invention refers to a space existing in a layer formed by a void nucleating agent. The shape of the voids is observed in a substantially circular or substantially elliptical shape from the cross section of the film. Voids can be formed by stretching a resin obtained by mixing the above-described polyester resin and void nucleating agent in an arbitrary ratio to apply an external force to separate the polyester resin and the void nucleating agent. Specifically, there is a method of forming voids inside by melt-extruding a mixture containing a polyester resin and a void nucleating agent and then stretching in at least one direction.

It is important that the void nucleating agent is a thermoplastic resin that is incompatible with the main component resin. Herein, "incompatible" means a combination of resins whose solubility parameter (hereinafter sometimes abbreviated as "SP value") deviates by 0.5 (MPa) ^1/2 or more. The SP value was calculated based on the theory of solubility parameters proposed by Hildebrand et al. The greater the divergence of the SP values of the resins, the lower the compatibility of the resins.

The resin preferably used as the void nucleating agent is the difference Tm- Tb is preferably 10°C or higher and 130°C or lower. By setting Tm−Tb within the range of 10° C. or higher and 130° C. or lower, more preferably 20° C. or higher and 100° C. or lower, the void nucleating agent is finely dispersed during kneading, and voids are formed more reliably in the stretching process, Void disappearance in the heat treatment process can be suppressed.

Specific examples of void nucleating agents include linear or branched olefin resins such as polyethylene, polypropylene, polybutene, and polymethylpentene, cyclic olefin resins, styrene resins, poly(meth)acrylate resins, and polycarbonate resins. , polyacrylonitrile resin, polyphenylene sulfide resin, fluorine-based resin, and the like. Among them, olefin-based resins or styrene-based resins are preferred. Examples of olefin-based resins include polyethylene, polypropylene, and poly-4-methylpentene-1 (hereinafter sometimes abbreviated as "polymethylpentene" or "PMP"). , ethylene-propylene copolymers, ethylene-butene-1 copolymers and cyclic olefins, and styrene resins such as polystyrene, polymethylstyrene and polydimethylstyrene. As the void nucleating agent used in the present invention, polyolefins that easily form voids in the polyester resin and are easily compatible with film-forming properties are preferred. Specifically, polyethylene, polypropylene, polybutene, polymethylpentene, and the like are preferred. Linear or branched olefin-based resins, cyclic olefin-based resins, and the like are used. In particular, from the viewpoint of void formation and heat resistance, polymethylpentene or cyclic olefin is preferable, and cyclic olefin which has a higher glass transition temperature and can be finely dispersed in polyester is most preferable.
These may be homopolymers or copolymers, and two or more thermoplastic resins may be used in combination.

The polyester film of the present invention preferably contains inorganic particles. By containing inorganic particles, the amount of light emitted to the incident surface side due to light scattering can be increased. The average particle diameter of the inorganic particles is preferably 0.05 μm or more and 1.00 μm or less in order to obtain an effective particle scattering effect. By setting the average particle size to 0.05 μm or more, more preferably 0.10 μm or more, and even more preferably 0.15 μm or more, it is possible to obtain an effective particle scattering effect while preventing aggregation due to good dispersibility. . Also, by setting the thickness to 1.00 μm or less, more preferably 0.50 μm or less, and even more preferably 0.35 μm or less, breakage during film formation or perforation can be suppressed. In particular, when the thickness is within the range of 0.10 to 0.50 μm, the cross-sectional density of specific voids, which will be described later, and the reflection performance of the film can be effectively increased. In addition, the average particle diameter in the present invention refers to the average particle diameter measured by the dynamic light scattering method.

Materials of inorganic particles include, for example, gold, silver, copper, platinum, palladium, rhenium, vanadium, osmium, cobalt, iron, zinc, ruthenium, praseodymium, chromium, nickel, aluminum, tin, zinc, titanium, tantalum, zirconium, metals such as antimony, indium, yttrium, lanthanum, silicon,
metal oxides such as zinc oxide, titanium oxide, cesium oxide, antimony oxide, tin oxide, indium tin oxide, yttrium oxide, lanthanum oxide, zirconium oxide, aluminum oxide, magnesium oxide, silicon oxide;
Metal fluorides such as lithium fluoride, magnesium fluoride, aluminum fluoride, cryolite,
metal phosphates such as calcium phosphate,
carbonates such as calcium carbonate,
Sulfates such as barium sulfate and magnesium sulfate,
Nitrides such as silicon nitride, boron nitride, carbon nitride,
silicates such as wollastonite, sepiolite and xonotlite,
Examples include titanates such as potassium titanate and strontium titanate. Moreover, two or more kinds of these inorganic particles may be used. Among these, titanium oxide, calcium carbonate, and barium sulfate are more preferable, and titanium oxide is particularly preferably used, from the viewpoint of comprehensive effects such as refractive index, bubble formation, whiteness, and optical density.

The content of the inorganic particles is preferably 10% by mass or more and 50% by mass or less with respect to the voided layer of the polyester film. By setting the content of the inorganic particles to 10% by mass or more, more preferably 20% by mass or more, the amount of light emitted to the incident surface side due to light scattering increases, resulting in a film with excellent reflective performance. In addition, by making it 50% by mass or less, more preferably 40% by mass or less, it is possible to prevent a decrease in productivity due to a decrease in film stretchability and a decrease in workability such as perforation.

In particular, as the film thickness becomes thinner, the productivity and workability tend to decrease. It is also effective to incorporate inorganic particles into the void nucleating agent in order to improve the decrease in productivity and workability. As a method for incorporating the inorganic particles into the void nucleating agent, it is preferable to subject the inorganic particles to a surface treatment for enhancing compatibility with the thermoplastic resin. As the surface treatment agent, silicone, silane coupling agent, aluminum chelating agent, polyurea and the like are preferably used. A silane coupling agent is particularly preferred.

The surface treatment method of the inorganic particles is not particularly limited, and examples thereof include a dry method and a wet method. The dry method is a method of reacting by dropping or spraying a surface treatment agent into an inorganic material that is being stirred at high speed by a stirrer, and the wet method is to add an organic solvent such as alcohol to the inorganic material to make a slurry. In this method, a surface treatment agent is added and reacted in the state.

For example, in the case of treatment with a silane coupling agent, a method of surface-treating inorganic particles with a silane coupling agent in advance according to a conventional method and then melt-kneading them with a thermoplastic resin is preferably used. Alternatively, an integral blend method may be used in which a coupling agent is added when the inorganic particles and the thermoplastic resin are melt-kneaded without subjecting the inorganic particles to surface treatment in advance.

A structure in which the inorganic particles are contained in the void nucleating agent can be obtained by melt-kneading the masterbatch and the main component resin after preparing a masterbatch by melt-kneading the inorganic particles and the void nucleating agent.

The polyester film of the present invention preferably consists of two or more layers having different void contents. From the viewpoint of curl suppression during heating, production aptitude, and film forming properties, a layer with the highest void content (B layer) and a surface layer (A layer) are laminated in the order of A/B/A in three layers. preferable. By laminating the A layer and the B layer in the order of A/B/A, it is possible to suppress a decrease in productivity due to falling off of particles and void nucleating agents contained in the B layer, and obtain high film forming stability. Moreover, the polyester film of the present invention may have a structure of four or more layers.

The polyester film of the present invention has at least a layer containing the polyester resin described above and a thermoplastic resin or inorganic particles incompatible with the polyester resin as a main component, and the void content of the layer is 10% or more. There must be The void content rate is an index that indicates the reflective performance of each layer that makes up the film. The higher the void content rate, the higher the brightness when mounted in a backlight that uses minute LEDs such as the mini-LED system. can be improved. Preferably, the void content is 35% or more. As the void content increases, the polyester film becomes able to return more light to the incident direction, and in the backlight using minute LEDs such as the mini LED system, it is possible to obtain a film that is excellent in brightness improvement effect. can be done. On the other hand, excessive formation of voids induces connection between adjacent voids, which may reduce the brightness improvement effect in backlights using minute LEDs such as the mini-LED system. , the void content is preferably 60% or less. As a method for increasing the void content, in addition to selecting and increasing the amount of the void nucleating agent, the voids are gradually grown by adjusting the stretching ratio and stretching speed, and performing re-MD stretching and re-TD stretching, which will be described later. methods and the like.

In the polyester film of the present invention, the ratio of the ratio of the trans isomer to the ratio of the gauche isomer in the orientation state of the polyester resin constituting at least one surface of the polyester film, measured by FT-IR (ATR method) (hereinafter referred to as " It is sometimes referred to as "surface trans/gauche ratio") must be 0.75 times or more. By increasing the surface transformer/gauche ratio, it is possible to suppress film deformation and peeling from the substrate in high-temperature processing processes such as full-surface resin sealing and in high-temperature environments such as LED continuous lighting. It is easy to suppress burrs in the perforation of films such as.

Reflective films used in mini-LED displays generally have a thickness of 100 μm or less in order to prevent the reflective film from blocking the light emitted from the LEDs. In order to improve the reflectance, inorganic particles may be added in addition to the void nucleating agent. However, when forming a reflective film to which inorganic particles are added, the layer containing the inorganic particles has lower heat resistance than the reflective film to which the inorganic particles are not added, and the inorganic particles promote the oriented crystallization of the polyester resin. Therefore, tearing occurs at a lower draw ratio.

On the other hand, the polyester resin of the layer that does not contain inorganic particles has insufficient orientation because it is formed at a low stretching ratio, and in high-temperature processing processes such as full-surface resin sealing and in high-temperature environments such as LED continuous lighting. In addition, deformation of the film and peeling from the substrate may occur, and burrs may occur in the process of perforating the film, which will be described later, which may reduce workability.

In addition, when the amount of thermoplastic resin or inorganic particles added to the layer mainly composed of polyester resin, thermoplastic resin incompatible with polyester resin, or inorganic particles is reduced, the void content rate and light scattering performance due to inorganic particles decrease. As a result, the brightness improvement effect may decrease in a backlight using minute LEDs such as a mini-LED system.

On the other hand, as in the present invention, in the case of a polyester film having a surface trans/gauche ratio of 0.75 or more, the void content of the layer mainly composed of a polyester resin, a thermoplastic resin incompatible with the polyester resin, or inorganic particles is 10% or more, the highly oriented surface layer acts as an expansion-suppressing layer in a high-temperature environment, so that it is possible to obtain the effect of suppressing film deformation and peeling from the substrate.

The surface trans/gauche ratio is preferably 0.8 or greater, more preferably 0.85 or greater. When the surface trans/gauche ratio is 0.8 or more, it is possible to suppress deformation of the film even in a higher temperature environment, and when it is 0.85 or more, the surface layer is highly oriented and strengthened. Therefore, in addition to the effect of improving the heat resistance described above, it is also possible to suppress the occurrence of burrs in the drilling process. On the other hand, the surface transformer/gauche ratio is preferably 3.0 or less from the viewpoint of preventing the film from being damaged during processing such as perforation that applies a strong impact to the film.

As a method for achieving the surface trans/gauche ratio in such a range, re-stretching in MD and re-stretching in TD, which will be described later, are performed, and the draw ratio and draw speed at each drawing stage, the ratio of the total draw ratio in the MD direction and the TD direction. is controlled within a specific range, or the content and type of the void nucleating agent and inorganic particles to be added are optimized.

In the polyester film of the present invention, Vb/Va is 1 for the void content rate Va (%) and Vb (%) of each of the cross section A with the main axis direction of the film as the cutting direction and the cross section B with the main axis direction as the normal line. .2 or more is preferable. Here, the principal axis direction means that 19 vertical cross sections are prepared while rotating the cutting direction from 0 to 90 degrees with respect to the film surface in increments of 5 degrees when preparing a vertical cross section of the film, and each vertical cross section is described above. This refers to the cutting direction when the void content rate is the highest when the void content rate is measured. By increasing Vb/Va, the structure of voids becomes stronger, and heat resistance can be improved in high-temperature processing processes such as full-surface resin sealing and in high-temperature environments such as LED continuous lighting. In addition, as the Vb/Va increases, the void takes a shape elongated in the direction of the main axis, thereby improving the light diffusibility, and the backlight using minute LEDs such as the mini LED system can further improve the brightness. It is possible. Vb/Va is preferably 1.4 or more, more preferably 1.5 or more. When Vb/Va is 1.4 or more, a backlight using minute LEDs such as a mini-LED system can obtain a brightness improvement effect. A brightness improvement effect can be obtained. On the other hand, from the viewpoint of suppressing connection between adjacent voids, Vb/Va is preferably 4.0 or less.
As a method for achieving Va and Vb in such ranges, re-stretching in MD or re-stretching in TD, which will be described later, is performed, the draw ratio and draw speed of each drawing stage, and the ratio of the total draw ratio in the MD direction and the TD direction are adjusted. Examples include a method of controlling within a specific range, and a method of optimizing the content and type of the void nucleating agent and inorganic particles to be added.

The polyester film of the present invention preferably has an average coefficient of thermal expansion (CTE) of 0 ppm/° C. or more and 30 ppm/° C. or less at 50° C. to 150° C. in the principal axis direction of the film and in the direction perpendicular to the principal axis direction. The thermal expansion coefficient of 50 ° C. to 150 ° C. in the direction of the main axis and the direction perpendicular to the direction of the main axis is 0 ppm / ° C. or more and 30 ppm / ° C. or less, so that the film thermally expands in a high temperature environment. It is possible to obtain a film excellent in heat resistance, which can prevent the light source from being damaged due to the load applied to the light source. The upper limit of the coefficient of thermal expansion at 50° C. to 150° C. in the restrained main axis direction and the direction perpendicular to the main axis direction is preferably 30 ppm/° C. or less, more preferably 25 ppm/° C. or less, and particularly preferably 15 ppm/° C. or less. is. If the coefficient of thermal expansion at 50° C. to 150° C. in the direction of the main axis and in the direction perpendicular to the direction of the main axis is 25 ppm/° C. or less, blockage of the perforations can be prevented even if the perforations have a smaller diameter, and the coefficient of thermal expansion is 15 ppm/ °C or less, it is possible to obtain a film with excellent heat resistance that can prevent clogging of the perforations even in a higher temperature environment.

As a method for achieving a coefficient of thermal expansion of 50° C. to 150° C. in the main axis direction and in the direction perpendicular to the main axis direction in such a range, not only adjustment of raw materials such as selection of void nucleating agents and inorganic particles but also A method of gradually growing voids by performing re-MD stretching and re-TD stretching, which will be described later, a method of performing relaxation treatment before re-MD stretching after TD stretching, and a heat treatment temperature after re-stretching are appropriately set. A method of adjusting to a suitable range may also be mentioned.

The polyester film of the present invention preferably has an average thermal shrinkage rate of 0.05% or more and 0.5% or less when heated at 150 ° C. for 2 hours in the main axis direction of the film and in the direction perpendicular to the main axis direction. . By setting the average thermal shrinkage rate when heated at 150 ° C. for 2 hours in the main axis direction of the film and in the direction perpendicular to the main axis direction to 0.05% or more and 0.5% or less, the deformation of the film in a high temperature environment is reduced. Since it is suppressed, the heat resistance in high-temperature processing processes such as full-surface resin sealing and in high-temperature environments such as LED continuous lighting is improved. The upper limit of the average thermal shrinkage rate when heated at 150° C. for 2 hours in the principal axis direction and the direction perpendicular to the principal axis direction is preferably 0.5% or less, more preferably 0.4% or less, and particularly preferably 0.4% or less. is 0.3% or less. When the average thermal shrinkage rate when heated at 150 ° C. for 2 hours in the principal axis direction and the direction perpendicular to the principal axis direction is 0.4% or less, expansion of perforations due to deformation of the film in a high temperature environment is suppressed, It becomes possible to prevent peeling from the substrate and obtain a film with more excellent heat resistance. If it is 0.3% or less, it is possible to suppress the expansion of perforations due to deformation of the film in a high-temperature environment and prevent peeling from the substrate even for a film that has been perforated with a smaller diameter. Therefore, a film excellent in workability and heat resistance can be obtained. On the other hand, if the average thermal shrinkage rate is less than 0.05% when heated at 150° C. for 2 hours in the main axis direction and in the direction orthogonal to the main axis direction, there is a difference between the heat shrinkage of the backlight substrate. As a result, the separation from the substrate may progress. On the other hand, when it is more than 0.5%, deformation of the film and peeling from the substrate may occur in a high temperature environment. As a method for making the average of the thermal shrinkage rate when heated at 150 ° C. for 2 hours in the main axis direction and the direction perpendicular to the main axis direction within the above range, raw materials such as selection of void nucleating agents and inorganic particles are selected. A method of adjusting, a method of performing re-MD stretching and re-TD stretching described later, a method of performing relaxation treatment after TD stretching, a method of adjusting the heat treatment temperature to an appropriate range, etc. The polyester film of the present invention has a film cross section. When observed, the density of voids having a cross-sectional area of 0.1 μm ² or more and 1.0 μm ² or less is preferably 0.25/μm ² or more and 1.25/μm ^{2 or} less. Here, the void cross-sectional area is obtained by measuring the area of the white region in the binarized data when the cross-sectional analysis data is binarized using image analysis software, and the void density is within the above range. It is obtained by dividing the number of voids by the image area. If the cross-sectional area of the void is less than 0.1 μm ² , the void transmits light in the visible light region without reflecting it. The improvement effect may decrease. In addition, by setting the cross-sectional area of the voids to less than 1.0 μm ² , the film can contain many voids while maintaining the size necessary for the voids to reflect light. It is possible to obtain a film excellent in luminance improvement effect in a backlight using.
By setting the density of voids having a cross-sectional area of 0.1 μm ² or more and 1.0 μm ² or less to 0.4 voids/μm ² or more when observing the cross section of the film, the reflectance is reduced due to the multiple occurrence of light scattering. It is possible to obtain an excellent brightness improvement effect in a backlight using minute LEDs such as a mini-LED system. The lower limit of the void density is preferably 0.8 voids/μm ² or more. By making it 0.8/μm ² or more, the voids are densely filled and it is possible to reflect scattered light at various angles, so backlights using minute LEDs such as the mini LED method On the other hand, if it is less than 0.25 / μm ² , the reflection performance of the void is low, and the backlight using minute LEDs such as the mini LED method has a brightness improvement effect. may be low. Further, by setting the number of voids to 1.25/μm ² or less, it is possible to suppress a decrease in the luminance improvement effect due to miniaturization of voids.
The preferable range of the density of voids having a cross-sectional area of 0.1 μm ² or more and 1.0 μm ² or less when observing the film cross section is determined by selecting the particle size of the inorganic particles to be added as described above, or by re-MD stretching and re-stretching described later. It can be achieved by performing TD stretching.

In the polyester film of the present invention, the average number of voids per 1 μm in the thickness direction of each of the cross section A with the main axis direction of the film as the cutting direction and the cross section B with the main axis direction of the film as the normal line is 0.4 or more 4 0 or less is preferable. Here, the number of voids per 1 μm of thickness is an index that indicates the reflection performance of the film. Using the number Nw of white areas and the length D of the straight line, it is obtained by the following formula.
Number of voids per 1 μm thickness (number/μm) = Nw/D
The average number of voids per 1 μm in the thickness direction of each of the cross section A with the film main axis direction as the cutting direction and the cross section B with the film main axis direction as the normal is 0.4 or more, so that the voids can be penetrated. Since the emitted light is reflected by another void and returned to the direction of incidence, it is possible to obtain a film excellent in brightness improvement effect in a backlight using minute LEDs such as a mini-LED system.

The average number of voids per 1 μm of thickness in each thickness direction of cross section A with the film main axis direction as the cutting direction and cross section B with the film main axis direction as the normal is preferably 0.4 or more, more preferably. is 1.0 or more. When the average number of voids per 1 μm of thickness is 1.0 or more, the voids existing in the thickness direction interfere with incident light from the thickness direction, and multiple reflections and transmissions are repeated. Since more light can be reflected in the direction of incidence, it is possible to obtain a film that is excellent in brightness improvement effect in a backlight using minute LEDs such as a mini-LED system. On the other hand, if the number of voids is less than 0.4, the number of voids that reflect light is small, and the effect of improving brightness in a backlight using minute LEDs such as a mini-LED system may decrease. In addition, by making it 3.0 or less, it is possible to suppress the decrease in reflectance and the decrease in rigidity due to the connection of adjacent voids in the thickness direction, and it is difficult to break during drilling, and the mini LED method etc. It is possible to obtain a film excellent in luminance improvement effect in a backlight using.
Examples of methods for achieving such an average number of void layers include a method of adding a dispersant and selecting the particle size of the void nucleating agent, a method of performing re-MD stretching and re-TD stretching, which will be described later.

In the polyester film of the present invention, the sum of the Young's modulus in the principal axis direction of the film and the direction perpendicular to the principal axis direction is preferably 4.0 GPa or more and 20 GPa or less. By setting the sum of the Young's moduli in the principal axis direction of the film and the direction perpendicular to the principal axis direction to 5.0 MPa or more, the reduction in the rigidity of the film due to the addition of a void nucleating agent or inorganic particles is suppressed, and perforation processing etc. A film having excellent processability can be obtained. The lower limit of the sum of the Young's moduli in the principal axis direction of the film and the direction orthogonal to the principal axis direction is preferably 5.0 GPa or more, more preferably 6.0 GPa or more, and still more preferably 7.0 GPa or more. When the sum of the Young's moduli in the direction of the principal axis of the film and the direction perpendicular to the direction of the principal axis is 6.0 GPa or more, in perforation processing, the generation of burrs due to perforation can be further suppressed, and fragments remain in the film. If it is 7.0 GPa or more, it not only improves workability such as perforation, but also suppresses deformation of the film in a high-temperature environment, resulting in a film with excellent heat resistance. Obtainable. On the other hand, when the sum of the Young's moduli in the direction of the principal axis of the film and the direction perpendicular to the direction of the principal axis is less than 4.0 GPa, the generation of burrs in the perforation process is remarkable, and fragments due to perforation remain in the film. Machining defects may occur due to this, and the workability may deteriorate. In addition, when the sum of the Young's moduli in the direction of the principal axis of the film and the direction perpendicular to the direction of the principal axis is greater than 20 GPa, the flexibility of the film is lost. sometimes.
Methods for adjusting the sum of Young's moduli in the principal axis direction and the direction perpendicular to the principal axis direction of the film to the above range include a method of performing re-MD stretching and re-TD stretching, which will be described later, and a stretching ratio and stretching speed at each stretching stage. can be controlled.

In the polyester film of the present invention, the reflection, scattering, and absorption of light are excessively repeated in the film, so that the intensity of the light returned to the incident direction is reduced, and the brightness is improved when mounted in a backlight using LEDs. From the viewpoint of suppressing deterioration of the effect, the thickness of the layer containing the polyester resin and the thermoplastic resin or inorganic particles incompatible with the polyester resin as main components is preferably 10 μm or more and 188 μm or less. Moreover, from the viewpoint of improving the brightness of a backlight using minute LEDs such as a mini-LED system, the upper limit of the thickness is more preferably 100 μm or less. Moreover, when the polyester film of the present invention has a laminated structure, the ratio of the thickness of the layer to the thickness of the entire polyester film is preferably 50% or more and 90% or less.

The polyester film of the present invention utilizes its thinness, high reflective performance, whiteness, insulation, low specific gravity, high heat insulation, etc., and can be used as a reflector for displays, a reflector for lighting, wallpaper, It can be used for various purposes such as light leakage shielding labels, paper substitute printing sheets, insulating sheets for coil coating, and heat insulating spacers. and is particularly suitable for mini LED substrate applications. Here, the mini-LED is a general term for LEDs whose size is significantly miniaturized in order to meet the needs of recent displays for high brightness, high contrast, and wide color gamut. LEDs are conventional LEDs (several millimeters square, several millimeters high), mini LEDs (several millimeters square, around 100 μm high), and micro LEDs (around 100 μm or less in width and 50 μm or less in height). Although there are cases where it is classified as such, in reality, the boundary of the size that each name means is not clear. In the present application, when the term "mini-LED" is used, it means an LED with an installation height of 5 μm or more and 200 μm or less on a printed circuit board.

The polyester film of the present invention is preferably used as a reflective material for mini-LED substrates in which mini-LED chips are mounted on printed substrates. In that case, it is preferable that the film is patterned with a plurality of perforations in accordance with the size, number, and arrangement pattern of the mini LEDs. By patterning, the polyester film of the present invention can be laminated to a substrate on which mini LEDs are arranged, or mini LEDs can be arranged on a substrate in which the polyester film of the present invention is laminated. can do. As a patterning method, mechanical punching such as Thomson blade punching, pinnacle punching, or mechanical punching, or laser baking can be used.

The mini LED substrate using the polyester film of the present invention is preferably used as a backlight. A backlight is an illumination member that illuminates the display surface of a liquid crystal display from behind. When a mini-LED substrate is used, a backlight called "direct type" is generally used, in which a large number of LED light sources are arranged directly under the display surface to directly adjust the brightness of the display. An example of the configuration of the backlight is shown in FIG. 1, for example. In FIG. 1, the sealing height of the sealing material is generally designed to be higher than the installation height of the LED, but by mounting the polyester film of the present invention, the sealing height is Even if it is small, the light from the LED can be diffused over a wide range and emitted in the direction of the display surface of the liquid crystal panel, which is preferable.

<Method for producing polyester film>
Next, a specific example is given and demonstrated about the manufacturing method of the polyester film of this invention. In addition, the polyester film of the present invention is not limited to those obtained by the following manufacturing method.

At least two single-screw or twin-screw extruders, or a composite film forming apparatus having a main extruder and a sub-extruder, in which the main extruder has a resin that is used as a raw material for layer B, and the sub-extruder has a resin that is used as a raw material for layer A. to put in. Each raw material is preferably dried to a moisture content of 50 ppm or less. In this way, the raw material is supplied to each extruder, and, for example, two extruders and a mold (T die) having a linear lip are installed at the upper part of the feed block or multi-manifold to produce 3 of A / B / A. It can be a layer laminated film. The extruded unstretched sheet is cooled and solidified on a cooled casting drum to obtain an unstretched laminated film. At this time, in order to obtain a uniform film, it is preferable to apply static electricity to bring the film into close contact with the casting drum.

This unstretched film is heated by roll heating, and if necessary, by infrared heating, etc., to a glass transition temperature Tg of the unstretched film Tg or higher and Tg + 60 ° C. or lower, and stretched in the longitudinal direction (hereinafter referred to as MD). (MD stretching). MD stretching is preferably carried out using a difference in peripheral speed between two or more rolls. The MD stretching ratio is preferably 1.5 times or more and 6.0 times or less. It is 1.5 times or more, more preferably 2.0 times or more, still more preferably 2.5 times or more. The purpose of the first MD stretching is to form voids and to provide the minimum necessary orientation for improving the uniform stretchability during subsequent stretching in the width direction of the film. Therefore, when the draw ratio is greater than 6.0 times, it may not be possible to obtain a film with a sufficient draw ratio when stretching the film in the width direction described below and re-stretching in the MD direction performed after that step. be. Moreover, when the stretching ratio is less than 1.5 times, voids are not sufficiently formed, and the brightness improvement effect when mounted on an LED type backlight may decrease. The stretching speed is preferably 25%/second or more and 750%/second or less, more preferably 100%/second or more and 500%/second or less in order to enhance the luminance improvement effect when mounted on an LED type backlight. is.

After the MD stretching, the film is then stretched in a direction perpendicular to the MD (hereinafter referred to as TD) (TD stretching). As a method of TD stretching, a method using a tenter is preferable. At this time, the preheating and stretching temperature for the TD stretching is preferably Tg+10° C. or more and Tg+50° C. or less with respect to the glass transition temperature Tg of the unstretched film. The ratio of TD stretching is preferably 2.5 times or more and 6.0 times or less. The purpose of the stretching in the TD direction is to expand the voids formed by the MD stretching in the TD direction, increase the content of voids, and impart high stretchability when the film is subsequently stretched in the MD direction. To provide the minimum necessary orientation for Therefore, if the draw ratio is more than 6.0 times, it may not be possible to obtain a film with a sufficient draw ratio during re-stretching in the MD direction following this step. In addition, when the stretching ratio is less than 2 times, voids are not sufficiently expanded, and the brightness improvement effect when mounted in an LED type backlight may decrease. By setting the TD draw ratio to 2.5 times or more, more preferably 3.0 times or more, it is possible to obtain a film excellent in luminance improvement effect when mounted in an LED type backlight. Further, by setting the TD draw ratio to 6.0 times or less, more preferably 4.5 times or less, it is possible to prevent breakage during film formation. The stretching speed is preferably 5%/second or more and 150%/second or less, more preferably 10%/second or more and 100%/second or less, in order to increase the reflectance of the polyester film.

The surface trans/gauche ratio of the polyester film of the present invention can be adjusted by known methods such as adjusting the draw ratio and adjusting the heat treatment temperature after stretching. The film is prone to film tearing, and film formation at a low draw ratio is required for stable film formation. decreases. Although tearing can be suppressed by reducing the content of the void nucleating agent and inorganic particles, it is difficult to obtain a film with an excellent brightness improvement effect when mounted in an LED backlight with such a design. be.

Therefore, it is preferable to further perform MD stretching (re-MD stretching) after the aforementioned MD stretching and TD stretching. MD stretching is preferably carried out using a difference in peripheral speed between two or more rolls. By further performing MD stretching in a state in which oriented crystals are formed by MD stretching and TD stretching, stable film formation at a higher stretching ratio than film formation in a normal biaxial stretching process becomes possible, and voids or inorganic particles In addition to highly oriented layers that do not have the

And in the film of the present invention as described above, compatibility between the void content rate and the trans / gauche ratio of the surface, the void content rate in a specific range, the thermal expansion coefficient in a specific range, the thermal contraction rate in a specific range, the void in a specific range Density, a specific range of void layers per unit thickness, and a specific range of Young's modulus sums can be effectively achieved. It is presumed that this is because the void structure can be strengthened by further highly orienting the molecular chains after the voids are formed. In addition, in the laminated structure of A/B/A, it is presumed that the highly oriented A layer functions as an expansion suppressing layer.
And in the polyester film of the present invention, the void content rate in the specific range as described above Vb / Va in the specific range, the coefficient of thermal expansion in the specific range, the thermal contraction rate in the specific range, the density of voids in the specific range, the thickness in the specific range The number of voids per μm, the sum of Young's modulus in a specific range can be effectively achieved.

The draw ratio in the re-MD drawing is preferably 1.1 times or more and 3.5 times or less, more preferably 1.5 times or more and 3.0 times or less. The stretching temperature in the re-MD stretching is preferably Tg+10° C. or more and Tg+80° C. or less with respect to the glass transition temperature Tg of the unstretched film. The stretching speed is 50%/second or more to increase the surface trans/gauche ratio of the polyester film and the void content of the polyester resin, the layer mainly composed of a thermoplastic resin incompatible with the polyester resin, or inorganic particles. %/second or less, more preferably 150%/second or more and 450%/second or less.

Further, the total MD ratio obtained by multiplying the MD draw ratio and the re-MD draw ratio is 5.0 times or more and 15.0 times or less, more preferably 6.0 times or more and 13.0 times or less, so that voids are eliminated. It is possible to form and expand the film efficiently, and obtain a film excellent in luminance improvement effect in a backlight using minute LEDs such as a mini-LED system. In addition, after TD stretching, before performing MD stretching again, while heating at a temperature of +20 ° C. to +70 ° C. relative to the TD stretching temperature in the tenter, the relaxation rate in the TD direction is 1.5% or more and 5% or less. It is also preferable to perform relaxation treatment for a treatment time of 0.2 to 10 seconds. By doing so, the crystal structure before re-stretching in MD is stabilized, and the heat shrinkage in the TD direction that occurs during preheating during re-stretching in MD is suppressed. In a high-temperature environment such as lighting, it is possible not only to suppress film deformation and peeling from the substrate, but also to strengthen the structure of the voids. can be increased to

Moreover, it is also preferable to further perform TD stretching (re-TD stretching) after re-MD stretching. By repeating the biaxial stretching step alternately a plurality of times in each direction, voids can be formed more stably even in high-ratio stretching. A method using a tenter is preferable as the method of re-TD stretching. The draw ratio of the re-TD drawing is preferably 1.1 times or more and 2.5 times or less, more preferably 1.1 times or more and 2.0 times or less. By performing re-TD stretching at a draw ratio within such a range, it is possible to suppress deterioration of reflection performance due to connection of voids and breakage of the film, and to stably form a film having an excellent void content rate. as well as
The stretching temperature of the re-TD stretching is preferably Tg+20° C. or more and Tg+80° C. or less with respect to the glass transition temperature Tg of the unstretched film.

The stretching speed of the re-TD stretching is preferably 2%/second or more and 120%/second or less, more preferably 5%/second or more and 60%/second or less in order to increase the reflectance and heat resistance of the polyester film. be.

Regarding the total MD ratio Em obtained by multiplying the MD draw ratio by the re-MD draw ratio and the total TD ratio Et by multiplying the TD draw ratio by the re-TD draw ratio, the value of Em/Et obtained by dividing Em by Et is 1.1. It is preferable that it is above. When Em/Et is 1.1 or more, more preferably 1.5 or more, the reflection performance, workability and heat resistance of the film can be improved.

After re-TD-stretching, the film is subsequently heat-treated under tension or under relaxation. The heat treatment temperature is Tm-60° C. or higher and Tm-10° C. or lower, more preferably Tm-30° C. or higher and Tm-10° C. or lower with respect to the Tm of the unstretched film for a treatment time of 0.2 to 20 seconds. It is preferable to use By performing heat treatment at a temperature within such a range, the crystal structure before re-TD stretching is stabilized, and heat resistance is improved in high-temperature processing processes such as full-surface resin sealing and in high-temperature environments such as LED continuous lighting. In addition, it is possible to suppress a decrease in brightness due to relaxation of the void structure. In the case of relaxation, the relaxation rate in the TD direction is 1.5% or more and 10% or less in order to improve the heat resistance of the film while suppressing the deterioration of the luminance improvement effect due to the collapse of voids due to excessive relaxation. preferably.

Then, after uniformly slow cooling, it is cooled to room temperature and wound up on a roll.

Here, the case of stretching by the sequential biaxial stretching method was described in detail as an example, but as long as the effects of the present invention are not impaired, any of the sequential biaxial stretching method, simultaneous biaxial stretching method, and longitudinal multistage stretching method can be used. may be adopted.

In addition, in order to impart lubricity, antistatic properties, ultraviolet light absorption performance, impact resistance, etc., to the extent that the effects of the present invention are not impaired, at least one surface of the polyester film of the present invention may be coated with a known technique. may be used to apply various coating liquids, or a hard coat layer or the like may be provided. The coating may be applied during film production (in-line coating) or may be applied after film production (off-line coating).

The present invention will be described in detail below with reference to examples. Each characteristic value was measured by the following method.

(1) Void content rate Using a press heated to a temperature of 150 ° C. for both the upper mold and the lower mold, a 0.2 mm thick aluminum plate / a 0.125 mm thick polyimide film V)/sample/polyimide film same as above/aluminum plate same as above was hot-pressed for 1 minute at 3 MPa. Cut the film into a size of 5 cm × 5 cm, and use an electronic hydrometer SD-120L (manufactured by Mirage Trading Co., Ltd.) based on JIS K7112 (1980 version) to measure the specific gravity d0 of the sample before heat treatment, after heat treatment The specific gravity d1 of the sample was measured, and the void fraction φ was obtained based on the following formula. Five sheets of each film were prepared, each was measured, and the average value was taken as the specific gravity.
Void content rate (%) = (1-d0/d1) x 100
(2) A surface trans/gauche ratio film cut into a width of 5 cm and a length of 5 cm was measured by the ATR method using an infrared microscope (PerkinElmer Spectrum Spotlight 200). The intensity distribution of the orientation state on the sample surface was obtained using analysis software (Spectrum). Using the same analysis software, with respect to the distribution having a peak at 1456 cm ⁻¹ , the intensity was integrated up to twice the half width with that peak as the reference, and the integrated value was taken as the intensity of the gauche body. Intensities at 1472 cm ⁻¹ were also integrated in the same manner, and the integrated value was taken as the intensity of the trans isomer. The surface trans/gauche ratio on the measurement surface of the film was obtained by dividing the obtained intensity of the trans isomer by the intensity of the gauche isomer.

(3) Void content rate Va, Vb, Vb/Va
It was obtained by the following procedures (a1) to (a6).

(a1) Using an ion milling apparatus (Hitachi IM4000), the film was cut perpendicularly to the film surface under cooling with liquid nitrogen while suppressing deformation and damage to prepare a measurement sample. The cutting direction is rotated rightward in increments of 5° with respect to the film surface, and a vertical cross section is prepared at each rotation angle, until the total rotation angle reaches 90°. was made.

(a2) After depositing platinum-palladium on the prepared cross section, the cross section of the film is observed from directly above using a shape analysis laser microscope (VK-X1000 manufactured by Keyence Corporation, VK-X1100 for the head), and the cross-sectional shape is mapped. Measured. Magnification was set to 5,000 times, and the measurement conditions were such that the display resolution was maximized. When the film consisted of multiple layers, the thickest layer was observed.

(a3) The obtained cross-sectional analysis data was converted to 16-bit color grayscale using analysis software (attached to the apparatus) to obtain a frequency distribution related to brightness. Of the frequency distributions having multiple peaks with different luminosity, focusing on two distributions with high frequency peaks, binarization of the grayscale image is performed using the average of the mode values of each of the two distributions as the boundary value. , obtained cross-sectional analysis data consisting of black areas (cut surfaces) and white areas (voids).

(a4) For the cross-sectional analysis data created in (a3), the area Sw of the white region and the area Sb of the black region are calculated using analysis software (attached to the apparatus), and the void content rate is calculated based on the following formula. Based on V.
V=Sb/(Sw+Sb)×100 (formula).

(a5) For all 19 vertical cross sections prepared in (a1), the void content rate V is determined by the procedure of (a2) to (a4), and the cutting direction of the sample with the highest value of V is the principal axis direction of the sample. and its cross section is defined as cross section A. A cross section perpendicular to the direction of the main axis is defined as a cross section B.

(a6) The void content rate of cross section A was Va (%), the void content rate of cross section B was Vb (%), and the ratio Va/Vb was obtained.

As for the ion milling device, the shape analysis laser microscope and its head portion, and the analysis software, alternatives may be used as long as they comply with equivalent standards.

(4) Coefficient of thermal expansion (CTE)
Five specimens were collected so that the main axis direction of the film specified by the method described in (3) above and the direction perpendicular to the main axis direction were each the longitudinal direction of the strip, and the measurement was performed under the following conditions.
Thermomechanical analyzer: "TMA/SS6000" manufactured by Seiko Instruments Inc.
Strip size: width 3 mm, length 15 mm
Load condition: 29.4 mN Constant temperature condition: Temperature was raised from 30°C to 170°C at 10°C/min and held for 10 minutes. Further, the temperature was lowered from 170°C to 40°C at a rate of 10°C/min and held for 20 minutes.
CTE calculation temperature range: From 150°C to 50°C when the temperature was lowered The thermal expansion coefficient was obtained from the following formula, and the average value in each direction was calculated.
CTE = [(L ₁₅₀ −L ₅₀ )/{L ₅₀ ×(150−50)}]×10 ⁶
here,
CTE: coefficient of thermal expansion (ppm/°C)
L ₁₅₀ : lengthwise dimension of the film at 150°C L ₅₀ : lengthwise dimension of the film at 50°C.

As for the ion milling device, shape analysis laser microscope, analysis software, and thermomechanical analysis device, alternatives may be used as long as they comply with equivalent standards.

(5) Thermal shrinkage rate Five specimens were collected so that the main axis direction of the film specified by the method described in (3) above and the direction perpendicular to the main axis direction were each the longitudinal direction of the strip, and under the following conditions I made a measurement.
Dimension analyzer:
Strip size: width 10mm, length 15mm
Temperature conditions: Using an oven with the internal temperature set to 150°C, hold for 30 minutes without tension.
Using the gauge length before and after the heat treatment, the thermal shrinkage rate (150° C. thermal shrinkage rate) was obtained from the following formula, and the average value in each direction was calculated.
Thermal shrinkage rate (%) = (H0 - H) / H0) x 100
here,
H0: gauge length before heat treatment H: gauge length after heat treatment (6) Void density with a cross-sectional area of 0.1 μm ² or more and 1.0 μm ^{2 or} less of the cross section obtained in (3) (a3) above Based on the mapping data, the total number of voids (specific voids) with an area of 0.1 μm ² or more and 1.0 μm ² or less within the field of view is determined by image analysis software (attached to the device), and the number of specific voids per unit area. The number (pieces/μm ² ) was calculated. After calculating the number of specific voids per unit area for each of 19 cross sections prepared by changing the cutting direction, the average value was adopted as the void density (pieces/μm ² ).

As for the ion milling device, the shape analysis laser microscope and its head portion, and the analysis software, alternatives may be used as long as they comply with equivalent standards.

(7) Number of voids per 1 μm of thickness The cross-sectional mapping data obtained in (a3) of (3) above in each of the principal axis direction of the film specified by the method described in (3) and a direction perpendicular to the principal axis direction. When 7 straight lines parallel to the thickness direction of the film are drawn at equal intervals, dividing the film into 8 equal parts in the length direction, the number Nw of white areas existing on 5 lines excluding the two lines at both ends is calculated for each line. , the number of void layers per unit thickness (number/μm) was obtained from the following formula, and the average value thereof was calculated. After calculating the number of specific voids per unit area for each of 19 cross sections prepared by changing the cutting direction, the average value was adopted as the number of voids per unit thickness (number/μm).
Number of voids per unit thickness (number/μm) = Nw/D
here,
D: length of straight line (μm) Note that the ion milling device, the shape analysis laser microscope, its head portion, and the analysis software may be replaced as long as they comply with equivalent standards.

(8) The sum of the Young's modulus in the principal axis direction of the film and the Young's modulus in the direction perpendicular to the principal axis direction of the film specified by the method described in (3) above. Five samples each having a width of 10 mm and a length of 10 cm were cut in the longitudinal direction. The sample is set in a tensile tester (Tensilon UCT-100 manufactured by Orientec) so that the length between chucks (initial sample length) is 50 mm, and the temperature is 25 ° C. and the relative humidity is 65%. A tensile test was performed until the film broke at , and the Young's modulus was obtained from the tangent line of the rising portion of the obtained load-elongation curve. After measuring five times in each direction and calculating the average value of the Young's modulus values for five times in each direction, the average value of the Young's modulus in the principal axis direction and the Young's modulus in the direction perpendicular to the principal axis direction The sum of Young's moduli (GPa) was obtained by adding the average values.

As for the ion milling device, the shape analysis laser microscope, the analysis software, and the tensile tester, alternatives may be used as long as they comply with equivalent standards.

(9) Film thickness Cut out five cross-sections of the center part in the width direction of the film, and use a scanning electron microscope (Hitachi field emission scanning electron microscope (FE-SEM) S-4000) at 500 to 5,000 times. The thickness of the film was measured from a cross-sectional photograph taken by magnifying observation. The average value of the numerical values of 5 sheets was taken as the film thickness.

(10) Luminance Rectangular parallelepiped blue LEDs (height from substrate surface 80 μm, length 376 μm, An LED substrate on which 1152 LEDs with a width of 200 μm were arranged was used for evaluation. The substrate on which the LEDs were arranged had a surface coated with a white solder resist (SFR-6 KS F) manufactured by Sekisui Chemical Co., Ltd. in a thickness of 23 μm. Next, the polyester film was cut into the same size as the LED substrate, and a rectangular hole with a width of 500 µm and a length of 300 µm was punched by punching with a Thomson blade so that the position where the blue LED was arranged was exactly hollowed out. .

An acrylic diffuser plate was placed so that there was a gap of 5 mm from the zenith of the blue LEDs arranged on the substrate, a prism sheet was placed thereon, and left to stand for 1 hour under conditions of a temperature of 25°C and a relative humidity of 65%. . After that, an image was taken with a two-dimensional color luminance meter (CA-2000 manufactured by Konica Minolta) from a point 90 cm above the LED substrate, and the image was captured using image analysis software (CA-S20w manufactured by Konica Minolta). After that, the brightness level of the photographed image was controlled to 30,000 steps, automatically detected, converted to brightness, and used as the brightness in the absence of the reflective film.

Next, the acrylic diffuser plate and the prism sheet on the blue LEDs were once removed, and the reflective film after the perforation processing was placed over the resin substrate so that the LEDs entered the holes. After that, the acrylic diffuser plate and the prism sheet were placed again in the same manner as described above, and after standing still for 1 hour, the luminance was measured in the same manner as in the previous term, and was defined as the luminance with the reflective film. The value obtained by dividing the luminance with the reflective film by the luminance without the reflective film and multiplying the result by 100 was taken as the relative luminance. A series of measurements and calculations were performed 5 times with different films, and the average value of the 5 relative luminance values was finally taken as the relative luminance of the film. Based on the obtained relative luminance values, the luminance was determined according to the following criteria. C or more is a pass.
A: Relative brightness of 145% or more B: Relative brightness of 140% or more and less than 145% C: Relative brightness of 130% or more and less than 140% D: Relative brightness of less than 130%.

(11) Perforation processing Perforation processing (φ200 circle) is performed on the film by punching using a Thomson blade, and the shape after processing is three-dimensionally analyzed using a shape analysis laser microscope VK-X1000 manufactured by Keyence Corporation. When burrs were formed along the periphery of the hole, the height h of the burrs was measured with respect to the film surface, and workability was evaluated according to the following criteria. C or more is a pass.
A: No burrs were generated, or burrs were generated, but the height of the burrs was less than 1 μm.
B: Burrs were generated, and the height of the burrs was 1 μm or more and less than 2 μm.
C: Burrs were generated, and the height of the burrs was 2 μm or more and less than 3 μm.
D: A burr with a height of 3 μm or more was generated, or the film was torn during perforation.

(12) Heat resistance FR-4 glass epoxy material (L6504C1 manufactured by Nikkan Kogyo Co., Ltd.) was cut into 5 cm × 5 cm, and on the side where the copper foil was not laminated, a transparent acrylic adhesive with a thickness of 25 μm was cut into the same size. (OCA) (8171 manufactured by 3M) was applied. Next, a circular hole with a diameter of 200 μm (cross-sectional area of the hole: 3.14×10 ⁴ μm ² ) was formed by laser processing at the center position of the plate in which the glass epoxy material and the OCA were overlapped. Then, the film was similarly cut into a size of 5 cm×5 cm, and a circular hole with a diameter of 200 μm was formed in the center by punching using a Thomson blade. Then, the glass epoxy material and the film were adhered to each other through the OCA and laminated to obtain a laminated body having a cylindrical through-hole with a diameter of 200 μm in the central portion. Then, a thermosetting silicone resin sealing material (OE-6336A/B manufactured by Dow Corning Toray Co., Ltd.) was used to perform resin sealing in such a manner that the entire laminate including the through holes was completely covered. Next, the resulting resin-sealed laminate was heat-treated in a hot air oven heated to 150° C. for 500 hours.

The sample after treatment was subjected to three-dimensional analysis using a shape analysis laser microscope (VK-X1000 manufactured by Keyence Corporation), and the heat resistance was evaluated according to the following criteria. C or more is a pass.
A: The holes in the film were not deformed, or the change in the cross-sectional area of the holes from the time of punching was less than 1%.
B: The rate of change in the cross-sectional area of the hole from the time of punching was 1% or more and less than 3%.
C: The rate of change in cross-sectional area of the hole from the time of punching was 3% or more and less than 5%.
D: The rate of change in cross-sectional area of the hole from the time of perforation processing was 5% or more, or peeling occurred between the film and the substrate.

[raw materials used]
(1) polyester resin (a)
Terephthalic acid and ethylene glycol were polymerized by a conventional method using antimony trioxide as a catalyst to obtain polyethylene terephthalate (PET). The obtained PET had a glass transition temperature of 77° C., a melting point of 255° C., an intrinsic viscosity of 0.63 dL/g, and a terminal carboxyl group concentration of 40 eq. /t.

(2) Copolyester resin (b)
A commercially available 1,4-cyclohexanedimethanol copolymer polyester (GN001 manufactured by Eastman Chemical Co.) was used.

(3) Copolyester resin (c)
A commercially available PBT (polybutylene terephthalate)/PAG (polyalkylene glycol) block copolymer ("Hytrel 7247" manufactured by Toray DuPont) was used. The PAG in this resin mainly consists of polytetramethylene glycol.

(4) Thermoplastic resin (d) incompatible with polyester resin
A commercially available cyclic olefin resin (“TOPAS 6017” manufactured by Nippon Polyplastics Co., Ltd.) was used. (Indicated as TOPAS in the table.)
(5) Titanium dioxide master (e)
0.25 parts by mass of a silane coupling agent (“11-100Additive” manufactured by Dow Corning Toray Co., Ltd.) is added to 50 parts by mass of titanium dioxide particles (number average particle size 0.25 μm, rutile type), and After the surface treatment, 50 parts by mass of the polyester resin (a) was kneaded with a twin-screw extruder to obtain pellets of the titanium dioxide master (e).

(6) Titanium dioxide master (f)
0.25 parts by mass of a silane coupling agent ("11-100Additive" manufactured by Dow Corning Toray Co., Ltd.) is added to 50 parts by mass of titanium dioxide particles (number average particle size 0.25 μm rutile type), and the surface is After the treatment, 50 parts by mass of thermoplastic resin (d) was kneaded with a twin-screw extruder to obtain pellets of titanium dioxide master (f).

(7) Thermoplastic resin (g) incompatible with polyester resin
A commercially available polymethylpentene resin (“TPX” manufactured by Mitsui Chemicals, Inc.) was used. (Indicated as PMP in the table.)
(8) Titanium dioxide master (h)
0.25 parts by mass of a silane coupling agent ("11-100Additive" manufactured by Dow Corning Toray Co., Ltd.) is added to 50 parts by mass of titanium dioxide particles (number average particle size 0.25 μm rutile type), and the surface is After the treatment, 50 parts by mass of thermoplastic resin (g) was kneaded with a twin-screw extruder to obtain pellets of titanium dioxide master (h).

(9) Calcium carbonate master (i)
50 parts by mass of the polyester resin (a) and 50 parts by mass of the polyester resin (a) were kneaded with 50 parts by mass of calcium carbonate particles (0.6 μm in number average particle size) using a twin-screw extruder to obtain pellets of titanium dioxide master (i).

[Example 1]
Raw materials having a composition of 50 parts by mass of resin (a), 5 parts by mass of resin (b), 5 parts by mass of resin (c), 20 parts by mass of resin (e), and 20 parts by mass of resin (f) were heated at a temperature of 180° C. After vacuum-drying for 3 hours, it was supplied to the main extruder as a raw material for layer B, melt-extruded at a temperature of 280° C., and then filtered through a 30 μm cut filter. Further, 100 parts by mass of resin (a) was vacuum-dried at a temperature of 180° C. for 6 hours, supplied to a sub-extruder as a raw material for layer A, melt-extruded at a temperature of 280° C., and filtered with a 30 μm cut filter. . Subsequently, these molten polymers were combined in a T-die composite die such that the A layer was laminated on both surfaces of the B layer (A/B/A). Lamination ratio was set to 1:7:1.

Subsequently, the merged molten polymer was extruded into a sheet shape to form a molten sheet, and the molten sheet was brought into close contact with a cast drum whose surface temperature was kept at 25°C by an electrostatic application method and cooled and solidified to form an unstretched film. . The melting point Tm of the unstretched film was 250°C.

Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80° C., while irradiating from both sides with an infrared heater, 2.5 at a stretching speed of 30% / sec with a roll heated to 88 ° C. The film was MD-stretched twice and cooled by a roll group at a temperature of 30° C. to obtain a uniaxially-stretched film.

Subsequently, while holding both ends of the uniaxially stretched film with clips, it was guided to a preheating zone of 90°C in the tenter, and TD stretching was performed at a stretching temperature of 95°C and a stretching speed of 15%/second to 3.2 times. , and slowly cooled in a cooling zone of 30° C. to obtain a biaxially stretched film.

Subsequently, the biaxially stretched film was preheated by a group of rolls heated to a temperature of 90° C. and then stretched by a roll heated to 100° C. at a stretching speed of 50%/sec while being irradiated from both sides by an infrared heater. It was again MD-stretched to 0 times and cooled with a roll group at a temperature of 30°C. Next, while holding both ends of the film with clips, it is guided to a preheating zone of 100°C in the tenter, and again TD-stretched to 1.5 times at a stretching temperature of 110°C and a stretching speed of 5%/sec, and 30°C. was led to the cooling zone of , and slowly cooled. Then, the film was heat-treated at a temperature of 200° C. for 10 seconds in a heat-treatment zone in the tenter, then uniformly cooled to 30° C. and then wound on a roll to obtain a film having a thickness of 60 μm. Table 1 shows the physical properties of the polyester film (substrate) for mini LED displays.

[Example 2]
Raw materials having a composition of 50 parts by mass of resin (a), 5 parts by mass of resin (b), 5 parts by mass of resin (c), 20 parts by mass of resin (e), and 20 parts by mass of resin (f) were heated at a temperature of 180° C. After vacuum-drying for 3 hours, it was supplied to the main extruder as a raw material for layer B, melt-extruded at a temperature of 280° C., and then filtered through a 30 μm cut filter. Further, 100 parts by mass of resin (a) was vacuum-dried at a temperature of 180° C. for 6 hours, supplied to a sub-extruder as a raw material for layer A, melt-extruded at a temperature of 280° C., and filtered with a 30 μm cut filter. . Subsequently, these molten polymers were combined in a T-die composite die such that the A layer was laminated on both surfaces of the B layer (A/B/A). The lamination ratio was 1:7:1, and the discharge rate of the extruder was adjusted so that the final film thickness was 60 μm. Subsequently, the merged molten polymer was extruded into a sheet shape to form a molten sheet, and the molten sheet was brought into close contact with a cast drum whose surface temperature was kept at 25°C by an electrostatic application method and cooled and solidified to form an unstretched film. . The melting point Tm of the unstretched film was 250°C.

Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80 ° C., while irradiating from both sides with an infrared heater, 2.5 at a stretching speed of 150% / sec with a roll heated to 88 ° C. The film was MD-stretched twice and cooled by a roll group at a temperature of 30° C. to obtain a uniaxially-stretched film.

Subsequently, while holding both ends of the uniaxially stretched film with clips, it was guided to a preheating zone of 90°C in the tenter, and TD stretching was performed at a stretching temperature of 95°C and a stretching speed of 15%/second to 3.2 times. , and slowly cooled in a cooling zone of 30° C. to obtain a biaxially stretched film.

Subsequently, the biaxially stretched film was preheated with rolls heated to a temperature of 90° C., and then stretched with rolls heated to 100° C. at a stretching speed of 250%/sec while being irradiated from both sides by infrared heaters. It was again MD-stretched to 0 times and cooled with a roll group at a temperature of 30°C. Next, while holding both ends of the film with clips, it is guided to a preheating zone of 100°C in the tenter, and again TD-stretched to 1.5 times at a stretching temperature of 110°C and a stretching speed of 5%/sec, and 30°C. was led to the cooling zone of , and slowly cooled. Then, the film was heat-treated at a temperature of 200° C. for 10 seconds in a heat-treatment zone in the tenter, then uniformly cooled to 30° C. and then wound on a roll to obtain a film having a thickness of 60 μm. Table 1 shows the physical properties of the polyester film (substrate) for mini LED displays.

[Example 3]
Raw materials having a composition of 50 parts by mass of resin (a), 5 parts by mass of resin (b), 5 parts by mass of resin (c), 20 parts by mass of resin (e), and 20 parts by mass of resin (f) were heated at a temperature of 180° C. After vacuum-drying for 3 hours, it was supplied to the main extruder as a raw material for layer B, melt-extruded at a temperature of 280° C., and then filtered through a 30 μm cut filter. Further, 100 parts by mass of resin (a) was vacuum-dried at a temperature of 180° C. for 6 hours, supplied to a sub-extruder as a raw material for layer A, melt-extruded at a temperature of 280° C., and filtered with a 30 μm cut filter. . Subsequently, these molten polymers were combined in a T-die composite die such that the A layer was laminated on both surfaces of the B layer (A/B/A). The lamination ratio was 1:7:1, and the discharge rate of the extruder was adjusted so that the final film thickness was 60 μm.

Subsequently, the merged molten polymer was extruded into a sheet shape to form a molten sheet, and the molten sheet was brought into close contact with a cast drum whose surface temperature was kept at 25°C by an electrostatic application method and cooled and solidified to form an unstretched film. . The melting point Tm of the unstretched film was 250°C.

Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80 ° C., while irradiating from both sides with an infrared heater, 3.0 at a stretching speed of 150% / sec with a roll heated to 88 ° C. The film was MD-stretched twice and cooled by a roll group at a temperature of 30° C. to obtain a uniaxially-stretched film.

Subsequently, while holding both ends of the uniaxially stretched film with clips, it was guided to a preheating zone of 90°C in the tenter, and TD stretching was performed at a stretching temperature of 95°C and a stretching speed of 15%/second to 3.2 times. , and slowly cooled in a cooling zone of 30° C. to obtain a biaxially stretched film.

Subsequently, the biaxially stretched film was preheated with rolls heated to a temperature of 90° C., and then stretched with rolls heated to 100° C. at a stretching speed of 250%/sec while being irradiated from both sides by infrared heaters. It was again MD-stretched to 0 times and cooled with a roll group at a temperature of 30°C. Next, while holding both ends of the film with clips, it is guided to a preheating zone of 100°C in the tenter, and again TD-stretched to 1.5 times at a stretching temperature of 110°C and a stretching speed of 5%/sec, and 30°C. was led to the cooling zone of , and slowly cooled. Then, the film was heat-treated at a temperature of 200° C. for 10 seconds in a heat-treatment zone in the tenter, then uniformly cooled to 30° C. and then wound on a roll to obtain a film having a thickness of 60 μm. Table 1 shows the physical properties of the polyester film (substrate) for mini LED displays.

[Example 4]
Raw materials having a composition of 50 parts by mass of resin (a), 5 parts by mass of resin (b), 5 parts by mass of resin (c), 20 parts by mass of resin (e), and 20 parts by mass of resin (f) were heated at a temperature of 180° C. After vacuum-drying for 3 hours, it was supplied to the main extruder as a raw material for layer B, melt-extruded at a temperature of 280° C., and then filtered through a 30 μm cut filter. Further, 100 parts by mass of resin (a) was vacuum-dried at a temperature of 180° C. for 6 hours, supplied to a sub-extruder as a raw material for layer A, melt-extruded at a temperature of 280° C., and filtered with a 30 μm cut filter. . Subsequently, these molten polymers were combined in a T-die composite die such that the A layer was laminated on both surfaces of the B layer (A/B/A). The lamination ratio was 1:7:1, and the discharge rate of the extruder was adjusted so that the final film thickness was 60 μm.

Subsequently, the merged molten polymer was extruded into a sheet shape to form a molten sheet, and the molten sheet was brought into close contact with a cast drum whose surface temperature was kept at 25°C by an electrostatic application method and cooled and solidified to form an unstretched film. . The melting point Tm of the unstretched film was 250°C.

Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80 ° C., while irradiating from both sides with an infrared heater, 3.0 at a stretching speed of 150% / sec with a roll heated to 88 ° C. The film was MD-stretched twice and cooled by a roll group at a temperature of 30° C. to obtain a uniaxially-stretched film.

Subsequently, while holding both ends of the uniaxially stretched film with clips, it was guided to a preheating zone of 90°C in the tenter, and TD stretching was performed at a stretching temperature of 95°C and a stretching speed of 15%/second to 3.2 times. , and slowly cooled in a cooling zone of 30° C. to obtain a biaxially stretched film.

Subsequently, the biaxially stretched film was preheated by a group of rolls heated to a temperature of 80° C., and then irradiated from both sides with an infrared heater while being stretched by a roll heated to 95° C. at a stretching rate of 250%/second. It was again MD-stretched to 0 times and cooled with a roll group at a temperature of 30°C. Next, while holding both ends of the film with clips, it is guided to a preheating zone of 100°C in the tenter, and again TD-stretched to 1.5 times at a stretching temperature of 110°C and a stretching speed of 5%/sec, and 30°C. was led to the cooling zone of , and slowly cooled. Then, the film was heat-treated at a temperature of 200° C. for 10 seconds in a heat-treatment zone in the tenter, then uniformly cooled to 30° C. and then wound on a roll to obtain a film having a thickness of 60 μm. Table 1 shows the physical properties of the polyester film (substrate) for mini LED displays.
[Example 5]
The MD draw ratio was changed to 2.2 times, the TD draw ratio was changed to 3.6 times, the re-MD draw ratio was changed to 1.7 times, and the re-TD draw ratio was changed to 1.8 times, and the final film thickness was 60 μm. A polyester film shown in Table 1 was obtained in the same manner as in Example 2, except that the discharge rate of the extruder was adjusted so that
[Example 6]
The MD draw ratio was changed to 3.4 times, the TD draw ratio was changed to 2.7 times, the re-MD draw ratio was changed to 2.2 times, and the re-TD draw ratio was changed to 1.2 times, and the final film thickness was 60 μm. A polyester film shown in Table 1 was obtained in the same manner as in Example 2, except that the discharge rate of the extruder was adjusted so that
[Example 7]
The MD draw ratio was changed to 2.2 times, the TD draw ratio was changed to 2.7 times, the re-MD draw ratio was changed to 1.7 times, and the re-TD draw ratio was changed to 1.2 times, and the final film thickness was 60 μm. A polyester film shown in Table 1 was obtained in the same manner as in Example 2, except that the discharge rate of the extruder was adjusted so that

[Example 8]
As a raw material to be supplied to the B layer, a composition of 50 parts by mass of resin (a), 5 parts by mass of resin (b), 5 parts by mass of resin (c), 20 parts by mass of resin (e), and 20 parts by mass of resin (h) A polyester film shown in Table 1 was obtained in the same manner as in Example 2, except that the raw materials were used and the discharge rate of the extruder was adjusted so that the final film thickness was 60 μm.

[Example 9]
As a raw material to be supplied to layer B, a raw material having a composition of 40 parts by mass of resin (a), 30 parts by mass of resin (e), and 30 parts by mass of resin (i) was used so that the final film thickness was 60 μm. A polyester film shown in Table 1 was obtained in the same manner as in Example 2, except that the discharge rate of the extruder was adjusted.

[Example 10]
As a raw material to be supplied to layer B, a raw material having a composition of 40 parts by mass of resin (a) and 60 parts by mass of resin (e) was used, and the discharge rate of the extruder was adjusted so that the final film thickness was 60 μm. A polyester film shown in Table 1 was obtained in the same manner as in Example 2 except for the above.

[Example 11]
As a raw material to be supplied to the B layer, a composition of 40 parts by mass of resin (a), 5 parts by mass of resin (b), 5 parts by mass of resin (c), 10 parts by mass of resin (d), and 40 parts by mass of resin (e) A polyester film shown in Table 1 was obtained in the same manner as in Example 2, except that the raw materials were used and the discharge rate of the extruder was adjusted so that the final film thickness was 60 μm.

[Example 12]
After the TD stretching, the film was subjected to relaxation treatment in the TD direction at a relaxation rate of 3% at 150° C. for 5 seconds, and then subjected to MD stretching again, and the extruder discharge rate was adjusted so that the final film thickness was 60 μm. A polyester film shown in Table 1 was obtained in the same manner as in Example 2 except for the above.

[Example 13]
In the same manner as in Example 10, except that the heat treatment temperature after re-TD stretching was changed to 230 ° C. and the discharge rate of the extruder was adjusted so that the final film thickness was 60 μm, the polyester described in Table 1 got the film.

[Example 14]
A polyester film shown in Table 1 was obtained in the same manner as in Example 2, except that the discharge rate of the extruder was adjusted so that the final film thickness was 188 μm.
[Example 15]
Raw materials having a composition of 50 parts by mass of resin (a), 5 parts by mass of resin (b), 5 parts by mass of resin (c), 20 parts by mass of resin (e), and 20 parts by mass of resin (f) were heated at a temperature of 180° C. After vacuum-drying for 3 hours, it was supplied as a raw material to an extruder, melt-extruded at a temperature of 280° C., and then filtered through a 30 μm cut filter. Subsequently, these molten polymers are extruded in sheet form from a T-die die to form a molten sheet, and the molten sheet is brought into close contact with a cast drum whose surface temperature is maintained at 25° C. by an electrostatic application method and cooled and solidified. A stretched film was obtained. The melting point Tm of the unstretched film was 250°C. At this time, the discharge rate of the extruder was adjusted so that the final thickness of the film was 60 μm.

Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80 ° C., while irradiating from both sides with an infrared heater, 2.5 at a stretching speed of 150% / sec with a roll heated to 88 ° C. The film was MD-stretched twice and cooled by a roll group at a temperature of 30° C. to obtain a uniaxially-stretched film.

Subsequently, while holding both ends of the uniaxially stretched film with clips, it was guided to a preheating zone of 90°C in the tenter, and TD stretching was performed at a stretching temperature of 95°C and a stretching speed of 15%/second to 3.2 times. , and slowly cooled in a cooling zone of 30° C. to obtain a biaxially stretched film.

Subsequently, the biaxially stretched film was preheated with rolls heated to a temperature of 90° C., and then stretched with rolls heated to 100° C. at a stretching speed of 250%/sec while being irradiated from both sides by infrared heaters. It was again MD-stretched to 0 times and cooled with a roll group at a temperature of 30°C. Next, while holding both ends of the film with clips, it is guided to a preheating zone of 100°C in the tenter, and again TD-stretched to 1.5 times at a stretching temperature of 110°C and a stretching speed of 5%/sec, and 30°C. was led to the cooling zone of , and slowly cooled. Then, the film was heat-treated at a temperature of 200° C. for 10 seconds in a heat-treatment zone in the tenter, then uniformly cooled to 30° C. and wound up on a roll to obtain a polyester film shown in Table 1.

[Comparative Example 1]
An unstretched film was obtained in the same manner as in Example 1, except that the extrusion conditions were adjusted so that the thickness of the film finally obtained was 60 μm.

Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80 ° C., while irradiating from both sides with an infrared heater, 2.5 at a stretching speed of 150% / sec with a roll heated to 88 ° C. The film was MD-stretched twice and cooled by a roll group at a temperature of 30° C. to obtain a uniaxially-stretched film.

Subsequently, while holding both ends of the uniaxially stretched film with clips, it is guided to a preheating zone of 90°C in the tenter, and TD stretching is performed at a stretching temperature of 95°C and a stretching rate of 15%/second to 3.5 times. Then, the film was heat-treated at a temperature of 200° C. for 10 seconds in a heat-treatment zone in the tenter, then uniformly cooled to 30° C. and then wound on a roll to obtain a film shown in Table 1.

[Comparative Example 2]
Extrusion conditions were adjusted so that the thickness of the film finally obtained was 60 μm, and the same procedure as in Comparative Example 1 was performed except that the MD draw ratio was changed to 3.5 times and the TD draw ratio was changed to 3.7 times. , the polyester film described in Table 1 was obtained. Film breakage occurred frequently during film formation, and the stretching conditions were not suitable for stable film formation.

[Comparative Example 3]
After vacuum-drying 100 parts by mass of resin (a) at a temperature of 180° C. for 6 hours, it was supplied as a raw material to an extruder, melt-extruded at a temperature of 280° C., and then filtered through a 30 μm cut filter. Subsequently, these molten polymers are extruded in sheet form from a T-die die to form a molten sheet, and the molten sheet is brought into close contact with a cast drum whose surface temperature is maintained at 25° C. by an electrostatic application method and cooled and solidified. A stretched film was obtained.

Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80 ° C., while irradiating from both sides with an infrared heater, 3.0 at a stretching speed of 150% / sec with a roll heated to 88 ° C. The film was MD-stretched twice and cooled by a roll group at a temperature of 30° C. to obtain a uniaxially-stretched film.

Subsequently, while holding both ends of the uniaxially stretched film with clips, it was guided to a preheating zone of 90°C in the tenter, and TD stretching was performed at a stretching temperature of 95°C and a stretching speed of 15%/second to 3.2 times. , and slowly cooled in a cooling zone of 30° C. to obtain a biaxially stretched film.

Subsequently, the biaxially stretched film was preheated with rolls heated to a temperature of 90° C., and then stretched with rolls heated to 100° C. at a stretching speed of 250%/sec while being irradiated from both sides by infrared heaters. It was again MD-stretched to 0 times and cooled with a roll group at a temperature of 30°C. Next, while holding both ends of the film with clips, it is guided to a preheating zone of 100°C in the tenter, and again TD-stretched to 1.5 times at a stretching temperature of 110°C and a stretching speed of 5%/sec, and 30°C. was led to the cooling zone of , and slowly cooled. Then, the film was heat-treated at a temperature of 200° C. for 10 seconds in a heat treatment zone in the tenter, then uniformly cooled to 30° C. and then wound on a roll to obtain a polyester film having a thickness shown in Table 1.

[Comparative Example 4]
An unstretched film was obtained in the same manner as in Example 1, except that the extrusion conditions were adjusted so that the thickness of the film finally obtained was 60 μm.
Subsequently, after preheating the unstretched film with a roll group heated to a temperature of 80 ° C., while irradiating from both sides with an infrared heater, 1.9 at a stretching speed of 150% / sec with a roll heated to 88 ° C. MD stretching was performed twice. Next, a group of rolls heated to 90°C is guided to a re-MD stretching section, and while being irradiated from both sides with an infrared heater, the rolls heated to 95°C are stretched again to 2.0 times at a stretching speed of 300%/sec. to obtain a multistage uniaxially stretched film.
Subsequently, while holding both ends of the multistage uniaxially stretched film with clips, it is guided to a preheating zone of 95°C in the tenter, and TD stretched 3.8 times at a stretching temperature of 100°C and a stretching rate of 15%/sec. and then heat-treated at a temperature of 200.degree. However, tearing of the film occurred frequently, and it was not possible to collect a film that could be evaluated.

ADVANTAGE OF THE INVENTION According to this invention, the reflective film excellent in heat resistance and the brightness improvement effect which can be used suitably for the backlight using minute LED can be provided.

1: Substrate (PCB)
2: Reflective film 3: Blue LED
4: sealing material 5: wavelength conversion sheet 6: diffusion plate 7: LCD

Claims (10)

A polyester film having at least a layer containing a polyester resin and a thermoplastic resin or inorganic particles incompatible with the polyester resin as main components, wherein the layer has a void content of 10% or more, and the polyester film A polyester film in which the ratio of the trans isomer of the polyester resin constituting at least one surface of is 0.75 times or more the ratio of the gauche isomer. Vb/Va is 1.2 (%) for the void content rates Va (%) and Vb (%) of the cross section A with the film main axis direction as the cutting direction and the cross section B with the film main axis direction as the normal line. The polyester film according to claim 1, which is the above. 3. The polyester film according to claim 1, wherein the average coefficient of thermal expansion (CTE) at 50° C. to 150° C. in the principal axis direction of the film and in the direction perpendicular to the principal axis direction is 0 ppm/° C. or more and 30 ppm/° C. or less. 4. Any one of claims 1 to 3, wherein when the film is heated at 150° C. for 2 hours, the average thermal shrinkage rate in the principal axis direction of the film and in the direction perpendicular to the principal axis direction is 0.05% or more and 0.5% or less. The polyester film described in 1. 5. Any one of claims 1 to 4, wherein the density of voids having a cross-sectional area of 0.1 μm ² or more and 1.0 μm ² or less in the cross section of the film is 0.25/μm ² or more and 1.25/μm ² or less. The polyester film described. The average number of voids per 1 μm of thickness in each thickness direction of cross section A with the film main axis direction as the cutting direction and cross section B with the film main axis direction as the normal is 0.4 or more and 4.0 or less. The polyester film according to any one of claims 1-5. 7. The polyester film according to any one of claims 1 to 6, wherein the sum of the Young's modulus in the principal axis direction and the Young's modulus in the direction perpendicular to the principal axis direction of the film is 4 GPa or more and 20 GPa or less. A mini LED substrate having the polyester film according to any one of claims 1 to 7. A backlight comprising the mini LED substrate according to claim 8 . A display comprising a backlight according to claim 9.

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