Biaxially stretched polyester film for light diffusion film, and display device provided with same

Abstract

The purpose of the present invention is to provide a biaxially stretched polyester film for a diffusion film, which has high breaking strength and has a reduced amount of foreign matter.　The present invention relates to a biaxially stretched polyester film for a light diffusion film, the biaxially stretched polyester film being formed of a polyester resin that contains an antimony compound, an alkaline earth metal compound, and a phosphorus compound, wherein the content of the element antimony in the biaxially stretched polyester film is 125 ppm or less, and the limiting viscosity of the biaxially stretched polyester film is 0.55 dl / g or more.

Description

Biaxially stretched polyester film for light diffusion film and display device including the same

[0001] The present invention relates to a biaxially stretched polyester film for use as a light diffusion film, and a display device including the same.

[0002] A liquid crystal display has a backlight unit on the rear side of the liquid crystal cell.

[0003] A light diffusion film is installed in the backlight unit, and this film diffuses and scatters light to make the brightness of the illuminated surface uniform.

[0004] Polyester films are used as light-diffusing films. In recent years, with the demand for higher precision, higher speed, and a more luxurious appearance in display devices, various problems have been identified, such as poor appearance of the film surface and difficulty in viewing images due to light scattering when the film is used as the surface layer of a liquid crystal display screen. One of these problems is thought to be caused by the presence of polycondensation catalysts such as antimony compounds and various additives used in producing polyester resins by polymerizing monomers, which form foreign matter in the produced polyester resin. Furthermore, since the presence of colored foreign matter in a film tends to block light that passes through the light-diffusing film, a film with minimal colored foreign matter is desirable.

[0005] Furthermore, even if a minute foreign particle passes through the filter material during the extrusion process of the molten resin, crystallization progresses around the foreign particle during the cooling process of the sheet-shaped molten material, which causes non-uniformity in the stretching process, resulting in minute differences in thickness and forming a lens-like state. Here, light is refracted or scattered as if there were a lens, and when observed with the naked eye, the foreign particle appears larger than it actually is, resulting in problems such as uneven brightness.

[0006] As a measure against such foreign matter, a method has been reported in which specific silica particles and an antioxidant are simultaneously incorporated into a biaxially oriented polyester film (Patent Document 1).

[0007] Japanese Patent Application Laid-Open No. 2004-263133

[0008] The present inventors have found that, in order to reduce the amount of foreign matter contained in a polyester film for a light diffusion film, they reduced the amount of antimony compound used during polycondensation, and although the amount of foreign matter in the polyester film for a light diffusion film was reduced, the intrinsic viscosity was low, the acid value was high, and the breaking strength was low.

[0009] Based on this finding, further research was conducted and it was found that a reduction in the amount of foreign matter and a high breaking strength can be achieved by reducing the amount of antimony compound used and lengthening the polymerization time or by carrying out solid-state polymerization, and this finding led to the completion of the present invention.

[0010] The present invention typically includes the following aspects: Item 1. A biaxially stretched polyester film for use as a light diffusion film, containing an antimony compound, an alkaline earth metal compound, a phosphorus compound, and a polyester resin, wherein the content of antimony element in the biaxially stretched polyester film is 125 ppm or less, and the intrinsic viscosity of the biaxially stretched polyester film is 0.55 dl / g or more. Item 2. The number of foreign matter particles with a major axis of 1 μm or more contained in the biaxially stretched polyester film is 20 particles / mm ２ Item 3. The biaxially stretched polyester film for a light diffusion film according to Item 1, wherein the number of particles filtered out on a membrane filter by the following filtration method is less than 1 mm. ２Item 1 or 2. The biaxially stretched polyester film for a light diffusion film according to Item 1 or 2, wherein the number of particles per kg of the biaxially stretched polyester film is 500 or less. Filtration method: 10 g of the biaxially stretched polyester film is dissolved in 80 ml of a p-chlorophenol / tetrachloroethane mixed solution to prepare a solution, and the solution is filtered under reduced pressure through a membrane filter with an average pore size of 0.5 μm. The p-chlorophenol / tetrachloroethane mixed solution consists solely of p-chlorophenol and tetrachloroethane in a mass ratio of 3 parts p-chlorophenol to 1 part tetrachloroethane, and the membrane filter is made of polytetrafluoroethylene, has a circular shape with a diameter of 47 mm, and is 90 μm thick. Item 4. The biaxially stretched polyester film for a light diffusion film according to any one of Items 1 to 3, wherein the antimony element content of the particles filtered on the membrane filter by the filtration method according to Item 3 is 10 mg or less per kg of the biaxially stretched polyester film. Item 5. The biaxially stretched polyester film for a light diffusion film according to any one of Items 1 to 4, wherein the ratio (A / B) of the antimony element content (A) of the particles filtered on the membrane filter by the filtration method according to Item 3 to the antimony element content (B) of the biaxially stretched polyester film is 0.75 or less. Item 6. When light from an LED light source is transmitted from the back of the film, a measurement is made of an area of ​​30 m at a position where the distance (WD) between the film and the lens is 340 mm. ２ When the film is photographed using a CCD camera with a resolution of 10 μm, the number of colored foreign particles detected is 30 / m ２or less, wherein the colored foreign matter is a defect detected by a CCD camera and having a minimum pixel value of 20 or more in a grayscale image (an image expressed in 256 levels, with a pixel value of 0 being black and a pixel value of 255 being white). Item 7. The biaxially stretched polyester film for a light diffusion film according to any one of Items 1 to 6, wherein the proportion of particles containing elemental antimony among the particles filtered out on the membrane filter by the filtration method according to Item 3 is 30% or less. Item 8. The biaxially stretched polyester film for a light diffusion film according to any one of Items 1 to 7, wherein the content of alkaline earth metal elements in the biaxially stretched polyester film is 5 to 160 ppm and the content of elemental phosphorus in the biaxially stretched polyester film is 1 to 50 ppm. Item 9. The biaxially stretched polyester film for use in a light diffusion film according to any one of Items 1 to 8, wherein the biaxially stretched polyester film has an intrinsic viscosity of 0.55 to 0.65 dl / g. Item 10. The biaxially stretched polyester film for use in a light diffusion film according to any one of Items 1 to 9, wherein the biaxially stretched polyester film has an acid value of 40 to 50 eq / ton. Item 11. The biaxially stretched polyester film for use in a light diffusion film according to any one of Items 1 to 10, wherein the biaxially stretched polyester film has a heat shrinkage rate of 1.4% or less in the longitudinal direction under heat conditions of 150°C for 30 minutes. Item 12. The biaxially stretched polyester film for use in a light diffusion film according to any one of Items 1 to 11, wherein the biaxially stretched polyester film has an easy-adhesion layer on at least one surface thereof. Item 13. The biaxially stretched polyester film for a light diffusion film according to Item 12, wherein the easy-adhesion layer contains a polyester resin, a urethane resin having a branched structure, and a crosslinking agent, and the mass of the polyester resin is 40 to 70% of the total mass of the solid contents of the polyester resin, the urethane resin, and the crosslinking agent.Item 14. The biaxially stretched polyester film for use in a light diffusion film according to Item 12 or 13, wherein, when the biaxially stretched polyester film has an easy-adhesion layer on only one surface thereof, a hard coat layer is also present on the surface of the easy-adhesion layer, and when the biaxially stretched polyester film has easy-adhesion layers on both surfaces thereof, a hard coat layer is also present on the surface of one or both of the easy-adhesion layers. Item 15. A display device comprising the biaxially stretched polyester film for use in a light diffusion film according to any one of Items 1 to 14, a light source, and a liquid crystal cell.

[0011] According to the present invention, a biaxially oriented polyester film for a light diffusion film having a low amount of foreign matter and high breaking strength can be provided.

[0012] As used herein, the phrase "comprising" is intended to encompass the phrases "consisting essentially of" and "consisting of."

[0013] In the numerical ranges described in stages in this specification, the upper or lower limit of a numerical range in a certain stage can be arbitrarily combined with the upper or lower limit of a numerical range in the same stage or in another stage. In addition, in the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example.

[0014] In this specification, with regard to a numerical range, the symbol "to" means equal to or greater than the leftmost value and equal to or less than the rightmost value. For example, "0.5 to 10% by weight" and "0.5% by mass to 10% by weight" both mean "0.5% by weight or greater and 10% by weight or less." Furthermore, with regard to a numerical range, "equal to or greater than" means "the same as or greater than," and "equal to or less than" means "the same as or less than."

[0015] The biaxially stretched polyester film for a light diffusion film of the present invention is a biaxially stretched film containing an antimony compound, an alkaline earth metal compound, a phosphorus compound, and a polyester resin, characterized in that the antimony content of the biaxially stretched polyester film is 125 ppm or less and the intrinsic viscosity of the biaxially stretched polyester film is 0.55 or more. The polyester film of the present invention can be used in a display device. The display device includes the polyester film of the present invention, a light source, and a liquid crystal cell. The light source is, for example, an LED. The polyester film of the present invention can be provided between the light source and the liquid crystal cell.

[0016] The antimony compound contained in the polyester film of the present invention may be derived from a polymerization catalyst described later, and may be used alone or in combination of two or more. The antimony compound will be described in detail later.

[0017] The alkaline earth metal compound contained in the polyester film of the present invention may be derived from the material used in the polymerization described below, and may be used alone or in combination of two or more. The alkaline earth metal compound will be described in detail later.

[0018] The phosphorus compound contained in the polyester film of the present invention may be derived from a material used in the polymerization described below, and may be used alone or in combination of two or more. The phosphorus compound will be described in detail later.

[0019] The number of foreign particles with a major axis of 1 μm or more contained in the polyester film of the present invention is 20 / mm ２ Foreign matter with a major axis of 1 μm or more can cause uneven brightness. Foreign matter with a major axis of 1 μm or more can be catalyst residue or dust mixed in the film. The number of foreign matter with a major axis of 1 μm or more is 15 / mm ２ Preferably, 10 pieces / mm or less ２ More preferably, 6 or less pieces / mm ２The following is even more preferable. In the present invention, the amount of such foreign matter can be reduced by reducing the amount of antimony compound used in the polyester film production process. The number of foreign matter particles with a major axis of 1 μm or more can be determined by observing the film with a confocal microscope. Specifically, it can be determined by the method described in the examples.

[0020] The antimony content of the polyester film of the present invention may be 125 ppm or less, based on the total mass of the film. The antimony content may be 120 ppm or less, preferably 110 ppm or less, and more preferably 90 ppm or less. The antimony content may be 30 ppm or more. The antimony content is preferably 50 ppm or more, more preferably 70 ppm or more. The antimony content may be 30 to 125 ppm, 30 to 120 ppm, 30 to 110 ppm, 30 to 90 ppm, 50 to 125 ppm, 50 to 120 ppm, 50 to 110 ppm, 50 to 90 ppm, 70 to 125 ppm, 70 to 120 ppm, 70 to 110 ppm, 70 to 90 ppm, etc. By ensuring that the antimony element content in the film is within this range, the amount of foreign matter in the film can be reduced while ensuring the intrinsic viscosity of the film is at or above the minimum required level. That is, the amount of foreign matter in the film can be reduced while ensuring the breaking strength required for light diffusion film applications. The antimony element content in the film can be determined using an ICP optical emission spectrometer. Specifically, it can be determined by the method described in the examples.

[0021] The alkaline earth metal element content of the polyester film of the present invention may be 160 ppm or less, 100 ppm or less, 80 ppm or less, 70 ppm or less, 60 ppm or less, etc., relative to the total mass of the film. The alkaline earth metal element content may be 5 to 160 ppm, 5 to 100 ppm, 5 to 80 ppm, 5 to 70 ppm, 5 to 60 ppm, etc. The alkaline earth metal element may form foreign matter in the film. The alkaline earth metal element content in the film can be determined using an ICP optical emission analyzer. Specifically, it can be determined by the method described in the examples.

[0022] The phosphorus content of the polyester film of the present invention may be 50 ppm or less, 40 ppm or less, 30 ppm or less, etc., based on the total mass of the film. The phosphorus content may be 1 to 50 ppm, 1 to 40 ppm, 1 to 30 ppm, etc. The phosphorus content in the film can be determined using an ICP emission spectrometer. Specifically, it can be determined by the method described in the examples.

[0023] The filtration method of the present invention will now be described. A solution prepared by dissolving 10 g of the polyester film of the present invention in 80 ml of a p-chlorophenol / tetrachloroethane mixed solution is filtered under reduced pressure through a membrane filter, and the filtered filter is dried to obtain particles on the dried filter. Here, the p-chlorophenol / tetrachloroethane mixed solution consists solely of p-chlorophenol and tetrachloroethane, with a mass ratio of 3:1 parachlorophenol:tetrachloroethane. The membrane filter may have an average pore size of 0.5 μm, be made of polytetrafluoroethylene, and be circular with a diameter of 47 mm and a thickness of 90 μm. The membrane filter may be, for example, T050A047A manufactured by ADVANTEC Corporation.

[0024] The number of particles obtained on the dry filter is ２ The number of particles on the dry filter may be 500 or less, 400 or less, etc. per 1 mm of the membrane filter. Hereinafter, the solution will be simply referred to as the "solution" and the drying filter will be simply referred to as the "drying filter." These particles may cause uneven brightness in the film. The number of particles on the dry filter is calculated based on the number of particles per 1 mm of the membrane filter. ２Preferably, the number of particles is 350 or less, more preferably 300 or less, and even more preferably 200 or less per particle. The number of particles may be 1 to 500, 1 to 400, 1 to 350, 1 to 300, 1 to 200, 10 to 500, 10 to 400, 10 to 350, 10 to 300, 10 to 200, 50 to 500, 50 to 400, 50 to 350, 50 to 300, 50 to 200, 100 to 500, 100 to 400, 100 to 350, 100 to 300, 100 to 200, etc. In the present invention, the number of such particles can be reduced by reducing the amount of antimony compound used in the polyester film production process. The number of particles on the dry filter can be determined by observing the dry filter using a scanning electron microscope (SEM) at a magnification of 1,000 times. Specifically, it can be determined by the method described in the Examples.

[0025] The antimony element content of the particles collected by filtration on the dry filter may be 10 mg or less per 1 kg of polyester film. The antimony element content is preferably 6 mg or less, more preferably 5 mg or less, and even more preferably 4 mg or less per 1 kg of polyester film. The antimony element content of the particles may be 0.1 to 10 mg, 0.1 to 6 mg, 0.1 to 5 mg, 0.1 to 4 mg, etc. per 1 kg of polyester film. The antimony element content of the particles collected by filtration on the dry filter can be determined by measuring the particles on the dry filter by X-ray fluorescence (XRF) to determine the amount of Sb element per 10 g of polyester film, and converting this to ppm or the amount (mg) per 1 kg of polyester film. Specifically, it can be determined by the method described in the Examples.

[0026] The proportion of particles containing elemental antimony among the particles filtered out on the dry filter may be 30% or less. The presence of such particles in the film may cause uneven brightness. The proportion of particles containing elemental antimony among the particles on the dry filter is preferably 25% or less, more preferably 22% or less, and even more preferably 20% or less. This proportion may be 1% to 30%, 1% to 25%, 1% to 22%, 1% to 20%, 5% to 30%, 5% to 25%, 5% to 22%, 5% to 20%, etc. The proportion of particles containing elemental antimony can be determined by performing elemental analysis of the particles on the dry filter using a scanning electron microscope (SEM) at 1,000x magnification, counting the number of particles in which elemental antimony is detected (Sb-containing particles), and dividing the number of particles in which elemental antimony is detected by the total number of particles on the dry filter to calculate the proportion (%) of the number of Sb-containing particles. Specifically, it can be identified by the method described in the Examples.

[0027] The intrinsic viscosity of the polyester film of the present invention may be 0.55 dl / g or more. The intrinsic viscosity can be controlled by adjusting the polymerization conditions (polymerization time, addition of solid-state polymerization, etc.). The intrinsic viscosity may be 0.56 dl / g or more, 0.57 dl / g or more, 0.58 dl / g or more, 0.55 to 0.65 dl / g, 0.56 to 0.65 dl / g, 0.57 to 0.65 dl / g, 0.58 to 0.65 dl / g, 0.55 to 0.62 dl / g, 0.56 to 0.62 dl / g, 0.57 to 0.62 dl / g, or 0.58 to 0.62 dl / g, etc. An intrinsic viscosity within the above range is advantageous in that the film has high breaking strength and low heat shrinkage. The intrinsic viscosity can be determined in accordance with JIS K 7367-5 by measuring the polyester film in a mixed solvent of phenol (6 parts by weight) and 1,1,2,2-tetrachloroethane (4 parts by weight) at a temperature of 30° C. Specifically, the intrinsic viscosity can be determined by the method described in the examples.

[0028] The acid value of the polyester film of the present invention may be 40 to 50 eq / ton. The acid value can be controlled by selecting the raw materials and reaction conditions. The acid value is preferably 41 to 50 eq / ton, more preferably 42 to 50 eq / ton. An acid value within the above range is advantageous in that the adhesion between the polyester film and the easy-adhesion layer is improved. The acid value can be determined by the method described in the examples.

[0029] The heat shrinkage rate in the longitudinal direction of the polyester film of the present invention can be 1.4% or less under heat conditions of 150°C for 30 minutes. The heat shrinkage rate in the longitudinal direction can be controlled by adjusting the intrinsic viscosity of the polyester film. The heat shrinkage rate in the longitudinal direction is preferably 1.0% or less, more preferably 0.9% or less. When the heat shrinkage rate in the longitudinal direction is within the above range, the film has excellent dimensional stability at high temperatures. Therefore, dimensional change is small during processing at high temperatures and use in high-temperature environments, which contributes to uniform brightness of the light exit surface of a backlight unit. The heat shrinkage rate in the longitudinal direction can be determined by heat-treating a film with a width of 10 mm and a longitudinal direction of 220 mm in a 150°C hot air oven for 30 minutes and comparing the longitudinal dimensions before and after heat treatment. Specifically, it can be determined by the method described in the examples.

[0030] The longitudinal breaking strength of the polyester film of the present invention may be 170 MPa or more, 180 MPa or more, or 190 MPa or more. The breaking strength is preferably 170 to 300 MPa, more preferably 180 to 280 MPa, and even more preferably 190 to 260 MPa. A longitudinal breaking strength within the above range is advantageous in that defects such as cracking, tearing, folds, and rips are less likely to occur during the processing and use of the film. The longitudinal breaking strength can be determined by measurement in accordance with JIS K 7127. Specifically, it can be determined by the method described in the examples.

[0031] The haze of the polyester film of the present invention may be 1.0% or less, preferably 0.9% or less. The haze can be determined by measuring with a haze meter. The haze meter may be an NDH2000 manufactured by Nippon Denshoku Corporation. The haze can be specifically determined by the method described in the examples.

[0032] The number of locations where brightness unevenness is observed in the polyester film of the present invention may be 5 or less per A4 size sample film, and preferably 0. If brightness unevenness is large, the film yield will deteriorate. Brightness unevenness can be identified by observation with crossed Nicols. Specifically, it can be identified by the method described in the examples.

[0033] The thickness of the light-diffusing polyester film of the present invention is not particularly limited and may be any thickness, but is preferably from 25 to 500 μm, and more preferably from 75 to 350 μm.

[0034] In the polyester film of the present invention, the ratio (A / B) of the antimony element content (A) of particles filtered on a membrane filter by the filtration method to the antimony element content (B) of the polyester film can be 0.75 or less. When the ratio (A / B) is 0.75 or less, antimony foreign matter with a major diameter greater than or equal to the filter diameter, i.e., coarse antimony foreign matter, is reduced, thereby suppressing brightness unevenness and brightness reduction. The antimony foreign matter may be catalyst residue mixed into the film. The ratio (A / B) is preferably 0.50 or less, more preferably 0.45 or less. In the present invention, the amount of the ratio (A / B) can be reduced by reducing the amount of antimony compound used in the polyester film manufacturing process. On the other hand, the ratio (A / B) is preferably 0.01 or more. When the ratio (A / B) is 0.01 or more, the film does not contain very fine antimony foreign matter, thereby reducing the manufacturing costs required for micronization technology. The ratio (A / B) can be between 0.01 and 0.75, between 0.01 and 0.50, between 0.01 and 0.45, between 0.05 and 0.75, between 0.05 and 0.50, between 0.05 and 0.45, between 0.1 and 0.75, between 0.1 and 0.50, between 0.1 and 0.45, and the like.

[0035] The antimony element content (A) of the particles filtered on the membrane filter (dry filter) can be determined by X-ray fluorescence measurement (XRF) as described above in the description and examples. The antimony element content (B) of the polyester film can be determined by ICP emission spectrometry as described above in the description and examples. Specifically, it can be determined by the method described in the examples.

[0036] The colored foreign matter is explained. When light from an LED light source is transmitted from the back side of the polyester film, the distance between the film and the lens (WD) is 300 to 500 mm (for example, 340 mm), and the area of ​​at least 20 m ２ or more (for example, 30 m ２ ) and a CCD camera with a resolution of 5 to 20 μm (for example, 10 μm). Defects are detected in the captured image. In the grayscale image (an image expressed in 256 gradations, with pixel value 0 being black and pixel value 255 being white), defects with a minimum pixel value of 20 or more are defined as colored foreign matter. The number of colored foreign matter contained in the film is 30 / m ２ It is preferable that the minimum pixel value is 20 or less. Colored foreign matter having a pixel value of 20 or more can cause a decrease in brightness. If the resolution of the CCD camera is greater than 20 μm, colored foreign matter will go undetected, while if the resolution of the CCD camera is less than 5 μm, it is thought that colored foreign matter with a small major axis that does not cause a decrease in brightness can be detected. The resolution of the CCD camera is preferably 5 to 15 μm. The number of colored foreign matter contained in the film is 10 particles / m ２ More preferably, 5 or less per m ２ The following is even more preferable: The number of colored foreign matters contained in the film is 0.1 to 30 per m ２ , 0.1-10 pieces / m ２ , 0.1-5 pieces / m ２ , 0.5-30 pieces / m ２ , 0.5-10 pieces / m ２ , 0.5-5 pieces / m ２ , 1-30 pieces / m ２ , 1-10 pieces / m ２ , 1-5 pieces / m ２ In the present invention, the amount of colored foreign matter can be reduced by reducing the amount of antimony compound used in the polyester film production process.

[0037] The polyester resin used as the raw material for the polyester film of the present invention can be produced by a direct polymerization method in which a dicarboxylic acid and a glycol are directly reacted, or by a transesterification method in which an alkyl ester of a dicarboxylic acid and a glycol are transesterified, followed by polycondensation. For example, polyethylene terephthalate can be produced by esterifying or transesterifying terephthalic acid or dimethyl terephthalate with ethylene glycol to produce an oligomer mixture such as bis(2-hydroxyethyl) terephthalate, and then melt-polymerizing this mixture at high temperature under vacuum using a catalyst. In addition, the molten polymer can be solid-phase polymerized at a temperature below the melting point. Solid-phase polymerization after melt polymerization can increase the intrinsic viscosity and reduce the acid value while suppressing the generation of foreign matter with a major axis of 1 μm or more, colored foreign matter, and antimony-based foreign matter (particles containing antimony element). The polyester resin content in the polyester film of the present invention can be 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, 99% by mass or more, 99.9% by mass or more, etc. The polyester film of the present invention may contain a resin other than a polyester resin.

[0038] The dicarboxylic acids may be used singly or in combination of two or more. The dicarboxylic acids may be aromatic dicarboxylic acids, aliphatic dicarboxylic acids, etc., with aromatic dicarboxylic acids being preferred.

[0039] The diols may be used singly or in combination of two or more. The diols may be aliphatic glycols, aromatic glycols, etc., with aliphatic glycols being preferred.

[0040] In the present invention, the polyester resin is preferably a polyester resin whose main dicarboxylic acid component is terephthalic acid or a polyester resin whose main diol component is ethylene glycol, and more preferably a polyester resin whose main dicarboxylic acid component is terephthalic acid and whose main diol component is ethylene glycol (polyethylene terephthalate).

[0041] Here, the term "main dicarboxylic acid component" means that, when the total dicarboxylic acid components in the polyester resin is taken as 100 mol%, the dicarboxylic acid or its ester component accounts for 70 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more, and even more preferably 100 mol%.

[0042] Furthermore, the term "main diol component" can mean that, when the total amount of all diol components in the polyester resin is 100 mol%, the diol component accounts for 80 mol% or more, preferably 90 mol% or more, more preferably 95 mol% or more, and even more preferably 100 mol%.

[0043] A polymerization catalyst may be used in the melt polymerization. The polymerization catalyst may be used alone or in combination of two or more. An antimony compound is preferred as the polymerization catalyst. The antimony compound may be used alone or in combination of two or more. The antimony compound may be an antimony salt of an aliphatic carboxylic acid. The antimony salt of an aliphatic carboxylic acid may be antimony trioxide, antimony pentoxide, or antimony acetate, with antimony trioxide being preferred in terms of polycondensation reactivity, the color tone of the resulting polymer, and inexpensive availability. Examples of catalysts other than antimony compounds include alkaline earth metal compounds, manganese compounds, cobalt compounds, aluminum compounds, titanium compounds, titanium / silicon composite oxides, and germanium compounds. Catalysts other than antimony compounds may be used to the extent that they do not cause problems with the properties of the polyester film.

[0044] When producing a polyester resin, it is preferable to add an alkaline earth metal. The alkaline earth metal compound can be used alone or in combination of two or more. The alkaline earth metal compound is preferably a magnesium compound. The magnesium compound may be an inorganic acid salt selected from saturated aliphatic carboxylates, unsaturated aliphatic carboxylates, aromatic carboxylates, halogen-containing carboxylates, hydroxycarboxylates, sulfates, nitric acid, phosphoric acid, phosphonic acid, hydrogen phosphate, hydrogen sulfide, sulfurous acid, thiosulfuric acid, hydrochloric acid, hydrobromic acid, chloric acid, and bromic acid, an organic sulfonate, an organic sulfate, a chelate compound, or an oxide of magnesium metal. From the viewpoints of ease of handling and availability, saturated aliphatic carboxylates of magnesium metal are preferred, and magnesium acetate is more preferred.

[0045] When producing a polyester resin, it is preferable to add a phosphorus compound as a heat stabilizer. The phosphorus compounds may be used alone or in combination of two or more. Examples of phosphorus compounds include phosphoric acid, phosphorous acid, phosphonic acid, and derivatives thereof. Specific examples include phosphoric acid, trimethyl phosphate, tributyl phosphate, triphenyl phosphate, monomethyl phosphate, dimethyl phosphate, monobutyl phosphate, dibutyl phosphate, phosphorous acid, trimethyl phosphite, tributyl phosphite, methylphosphonic acid, dimethyl methylphosphonate, diethyl ethylphosphonate, dimethyl ethylphosphonate, dimethyl phenylphosphonate, diethyl phenylphosphonate, and diphenyl phenylphosphonate. Among these, trimethyl phosphate, diethyl ethylphosphonate, and / or phosphoric acid are preferred, and trimethyl phosphate is more preferred.

[0046] When producing the polyester resin, various compounds and additives may be added within the range that does not cause problems in the properties of the polyester film obtained from the polyester resin.

[0047] An example of the production of a polyester resin as a raw material for the polyester film of the present invention is as follows. The polyester resin is polyethylene terephthalate. The present invention may be a method for producing a biaxially stretched polyethylene terephthalate film for use as a light diffusion film, comprising the following steps:

[0048] [Step 1] A step of melt-polymerizing a polyester resin using terephthalic acid as the dicarboxylic acid component, ethylene glycol as the diol component, and an antimony compound as the main polymerization catalyst to obtain a polyester resin. [Step 2] Optionally, a step of solid-state polymerizing the polyester resin obtained in Step 1 at 197 to 225°C for 5 to 10 hours to obtain a polyester resin. [Step 3] A step of stretching the polyester resin obtained in Step 1 or 2 to obtain a biaxially stretched polyethylene terephthalate film for a light diffusion film.

[0049] The melt polymerization may be a batch polymerization method or a continuous polymerization method. In either method, the esterification reaction or transesterification reaction may be carried out in one stage, but is preferably carried out in multiple stages. In the melt polymerization reaction, the number and size of reactors and the production conditions for each step can be selected as appropriate without any limitations. The melt polymerization reaction may be carried out in one stage or in multiple stages, preferably in two to five stages, more preferably in three to four stages, and even more preferably in three stages. The melt polymerization reaction is preferably carried out in a continuous reactor. A continuous reactor is a method in which a reaction vessel for the esterification reaction or transesterification reaction and a melt polymerization reaction vessel are connected by piping, and raw materials are continuously introduced into each reaction vessel without emptying, transferred to the melt polymerization reaction vessel via the piping, and the resin is withdrawn from the melt polymerization reaction vessel.

[0050] The steps of the continuous polymerization method are as follows.

[0051] 1) Slurry Preparation Step A dicarboxylic acid component and a diol component are introduced into a slurry preparation tank to prepare a slurry. The content ratio of these components in the slurry is not particularly limited as long as the slurry has sufficient fluidity to be transported to an esterification reaction tank. From an economical viewpoint, it is preferable to reuse the diol component recovered in the polycondensation step as a slurry raw material. In the present invention, recycled raw materials such as dicarboxylic acid components and diol components obtained by a chemical decomposition and recovery method may also be used.

[0052] 2) Esterification Reaction Step: The slurry obtained above is introduced into two or more esterification reaction vessels connected in series and subjected to an esterification reaction to obtain an oligomer compound in which a diol is condensed with both terminal carboxyl groups of a dicarboxylic acid component. The esterification reaction is preferably carried out while removing water produced by the reaction from the system using a distillation column. The number and size of the reaction vessels in the esterification reaction step can be selected as appropriate without any limitations. The production conditions for each step can be selected as appropriate depending on the type and amount of the polycondensation catalyst and additives for improving electrostatic adhesion, the number and size of the reaction vessels, etc. For example, in a system having three esterification reaction vessels, the temperature of the first esterification reaction vessel can be 240 to 270°C, the pressure can be 100 to 160 kPa absolute, and the average residence time can be 2 to 5 hours. When solid-state polymerization is not used, the average residence time is preferably 3.5 to 5 hours, more preferably 3.9 to 5 hours. The temperature of the second and third esterification reaction vessels may be 250 to 280°C, the pressure may be 0 to 100 kPa absolute, and the average residence time may be 0.1 to 2.5 hours. When solid-state polymerization is not used, the average residence time of the second esterification reaction is preferably 1.4 to 2.5 hours, more preferably 1.5 to 2.5 hours, and the average residence time of the third esterification reaction is preferably 1.0 to 2.5 hours, more preferably 1.1 to 2.5 hours. It is desirable that the final esterification reaction rate reaches 60% or more, preferably 70% or more. Furthermore, the esterification reaction vessel may be a multi-stage vessel provided with a weir or the like inside.

[0053] In the esterification step, it is preferable to additionally supply the diol component after the second esterification reaction vessel. Supplying the entire amount of the diol component in the slurry preparation can cause problems such as fluctuations in the diol component composition in the polyester resin and a decrease in the esterification reaction rate during long-term continuous production. By additionally supplying the diol component after the second esterification reaction vessel, these problems can be suppressed.

[0054] In the esterification step, a phosphorus compound, an alkali metal compound, an alkaline earth metal compound, etc. may be added. The timing of addition may be any time from before the esterification reaction to the start of the polycondensation reaction, but in the continuous polymerization method, it is preferable to add them in the third esterification reaction tank or later.

[0055] When producing a polyester resin by a batch polymerization method or a continuous polymerization method, the antimony compound may be added as a powder, an ethylene glycol slurry, an ethylene glycol solution, etc., but it is preferably added as an ethylene glycol solution. The addition time may be either before the esterification reaction and the ester exchange reaction, or between the end of the ester exchange reaction and the esterification reaction and the start of the polycondensation reaction.

[0056] 3) Polycondensation Reaction Step The oligomer compound that has undergone the esterification reaction is subsequently transferred to a polycondensation reaction tank and subjected to the polycondensation reaction. The number and size of the reaction tanks in the polycondensation reaction step are not limited and can be selected appropriately. Furthermore, the production conditions for each step can be selected appropriately depending on the type and amount of the polycondensation catalyst and additives, the number and size of the reaction tanks, and the like. For example, when three polycondensation reaction tanks are used, the temperature of the first polycondensation reaction tank can be 260 to 290°C, the pressure can be 2 to 8 kPa, and the average residence time can be 0.1 to 1 hour. When solid-state polymerization is not used, the average residence time of the first polycondensation reaction is preferably 0.8 to 1 hour, more preferably 0.9 to 1 hour. The temperature of the second polycondensation reaction tank can be 270 to 290°C, the pressure can be 0.5 to 1.5 kPa, and the average residence time can be 0.1 to 2 hours. When solid-state polymerization is not used, the average residence time for the second polycondensation is preferably 1.0 to 2 hours, more preferably 1.1 to 2 hours. The temperature of the third polycondensation reaction vessel may be 270 to 290°C, the pressure may be 0.01 to 0.5 kPa, and the average residence time may be 0.1 to 2 hours. When solid-state polymerization is not used, the average residence time for the third polycondensation is preferably 1.0 to 2 hours, more preferably 1.1 to 2 hours. It is preferable that the degree of increase in intrinsic viscosity achieved in each of these polycondensation reaction steps be smoothly distributed. Since the diol component is distilled in the polycondensation reaction step, it is preferable to recover, purify, and reuse it. This recovery and purification can be carried out in a distillation column, as in the esterification reaction step.

[0057] The intrinsic viscosity of the polyester resin produced by melt polymerization is preferably 0.52 to 0.59 dL / g, more preferably 0.52 to 0.55 dL / g. When the intrinsic viscosity of the polyester resin is within the above range, the effect of suppressing foreign matter and the color tone are good.

[0058] It is preferable to further subject the polyester resin obtained by melt polymerization to solid-state polymerization, since this makes it possible to increase the intrinsic viscosity and reduce the acid value while suppressing the generation of foreign matter having a major axis of 1 μm or more, the generation of colored foreign matter, and the generation of antimony-based foreign matter (particles containing antimony element).

[0059] The solid state polymerization can be carried out on a polyester resin in the form of granules, which means chips, pellets, flakes, or powder, preferably pellets.

[0060] Solid-state polymerization can be carried out by heating the granular polyester resin at a temperature below the melting point of the polyester resin under an inert gas flow or under reduced pressure. The solid-state polymerization is preferably carried out under reduced pressure. The solid-state polymerization process may be carried out in one stage or in multiple stages. The granular polyester resin to be supplied to the solid-state polymerization process is preferably first heated to a temperature lower than the temperature used for solid-state polymerization to crystallize it, and then supplied to the solid-state polymerization process. The crystallization process is preferably carried out by heating the granular polyester at a temperature of 70 to 90°C for 3 to 5 hours to dry it, and then heating it at a temperature of typically 120 to 200°C, preferably 130 to 150°C, for 1 to 4 hours.

[0061] 4) Stretching Step: The polyester resin is stretched to form a film. The film formation method is not particularly limited, and known methods can be used. For example, a polyester resin film can be obtained by melting the polyester resin at a temperature above its melting point, extruding it to form a polyester resin sheet, and then stretching the resulting polyester resin sheet. For example, a polyester resin film can be obtained by melt-extruding the polyester resin into a film at 250 to 320°C, solidifying it, and forming an amorphous sheet. The sheet is then biaxially stretched lengthwise and widthwise, either sequentially or simultaneously, at 70 to 140°C, followed by heat treatment at 160 to 240°C. The stretching ratios are typically selected from the range of 1.1 to 10 times in both the lengthwise (longitudinal) and widthwise (transverse) directions. The stretching ratio in the lengthwise direction is preferably 2.5 to 5.0 times, more preferably 2.8 to 5.0 times, and even more preferably 3.0 to 5.0 times. The stretching ratio in the widthwise direction is preferably 2.5 to 5.0 times, more preferably 2.8 to 5.0 times, and even more preferably 3.0 to 5.0 times. A stretching ratio within the above range is advantageous in that thickness unevenness of the obtained film is suppressed and that the film has excellent heat resistance and mechanical strength. Note that the stretching ratio defined in the present invention refers to the actual stretching ratio at which the film is actually stretched. This stretching ratio can be determined by measuring the mass change rate per unit area before and after each stretching step or by marking a grid-shaped stretching ratio marker on the unstretched film.

[0062] (Adhesion-Enhancing Layer) The polyester film of the present invention may have an adhesion-enhancing layer on one or both surfaces thereof. By providing an adhesion-enhancing layer, the adhesion between the film and, for example, a hard coat layer is improved. The coating amount of the adhesion-enhancing layer is 0.005 to 0.20 g / m ２ It is preferable to control the coating amount to 0.005 g / m ２ On the other hand, when the coating amount is 0.20 g / m or more, the adhesiveness increases, which is preferable. ２ If it is less than this, blocking resistance can be obtained, which is preferable.

[0063] The adhesive layer may contain a resin and a crosslinking agent. Examples of resins that can be used include, but are not limited to, polyester resins, urethane resins, and acrylic resins. These resins may be used singly or in combination. The adhesive layer may preferably contain polyester resins and urethane resins, and more preferably polyester resins and urethane resins having a branched structure.

[0064] The adhesive layer may contain a crosslinking agent. Examples of the crosslinking agent include melamine compounds, isocyanate compounds, oxazoline compounds, epoxy compounds, carbodiimide compounds, and polymers thereof. These crosslinking agents may be used alone or in combination of two or more. Block polyisocyanate is preferred as the crosslinking agent.

[0065] The easy-adhesion layer may contain fine particles. The inclusion of particles imparts slipperiness to the film. The average particle size of the fine particles is preferably 2 μm or less. If the average particle size of the fine particles exceeds 2 μm, the particles tend to fall off from the easy-adhesion layer. Examples of fine particles to be contained in the easy-adhesion layer include inorganic particles such as calcium carbonate and silica, and organic polymer particles such as acrylic particles, with silica being preferred. The fine particles may be used alone or in combination of two or more types.

[0066] (Hard Coat Layer) When the polyester film of the present invention has an easy-adhesion layer on only one surface thereof, it may have a hard coat layer on the surface of the easy-adhesion layer. When the polyester film of the present invention has easy-adhesion layers on both surfaces thereof, it may have a hard coat layer on the surface of either or both of the easy-adhesion layers.

[0067] The resin contained in the hard coat layer may be an acrylic resin, a siloxane resin, an inorganic hybrid resin, a urethane acrylate resin, a polyester acrylate resin, an epoxy resin, or the like, without any particular limitation. The resin contained in the hard coat layer may be used alone or in combination of two or more. The hard coat layer may or may not contain particles such as an inorganic filler or an organic filler.

[0068] The thickness of the hard coat layer is preferably 1 to 50 μm. A thickness of 1 μm or more is preferable because it cures sufficiently and the pencil hardness is high. Furthermore, by making the thickness 50 μm or less, curling due to cure shrinkage of the hard coat can be suppressed, and the handling properties of the film can be improved.

[0069] The hard coat layer can be cured by energy rays such as ultraviolet rays and electron beams, or by heat, and curing methods using ultraviolet rays or electron beams are preferred in order to reduce damage to the film.

[0070] (Display Device) The polyester film of the present invention can be used as a light diffusion film conventionally used in display devices such as liquid crystal displays. Display devices include a liquid crystal cell and a backlight unit, and a light source and a light diffusion film are installed in the backlight unit. The light diffusion film diffuses and scatters light emitted from the light source, thereby making the brightness of the illuminated surface uniform. In addition, a lens sheet may be used to collect light transmitted through the light diffusion film in the forward direction as much as possible. In order to make brightness unevenness and defects in the lens sheet caused by the arrangement of the lens sheet less noticeable (to improve concealment), a light diffusion film may also be arranged on the front side of the lens sheet.

[0071] Hereinafter, one embodiment of the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto. Methods for determining various physical properties in the examples will be described below.

[0072] (Quantitative Determination of Sb, Mg, and P Elements in Polyester Film (ICP Atomic Emission Analysis)) Nitric acid and ultrapure water were added to 0.5 g of polyester film, and the mixture was treated using a microwave decomposition apparatus (UltraWAVE manufactured by Milestone General Co., Ltd.) to prepare a measurement sample. Then, the contents (ppm) of Sb, Mg, and P relative to the mass of the polyester film were determined using an ICP optical emission analyzer (5900 SVDV ICP OES manufactured by Agilent).

[0073] (Number of foreign particles with a major axis of 1 μm or more by film observation) The polyester film was observed using a confocal microscope (OPTELICS HYBRID C3, manufactured by Lasertec Corporation). The observation conditions were as follows: a 20x objective lens was used, and a range of 700 μm × 750 μm was defined as one visual field, with four visual fields (total of 2.1 mm ２ When measuring the number of foreign particles, the focus was adjusted from the surface to the bottom of the film, and the number of foreign particles with a major axis of 1.0 μm or more that came into focus during the above-mentioned operation was counted. The number of foreign particles with a major axis of 1.0 μm or more that were counted was calculated based on the above-mentioned observation area (2.1 mm ２ ) and divide by 1 mm of film ２ The number per unit was calculated.

[0074] (Polyester film solution) 10 g of polyester film was washed with water and dried, and then dissolved in a p-chlorophenol / tetrachloroethane mixed solution (a solution consisting only of parachlorophenol and tetrachloroethane in a mass ratio of 3:1). Thus, a polyester film solution was obtained.

[0075] (Filtration of Polyester Film Dissolution) The polyester film solution was filtered through a membrane filter (PTFE membrane filter manufactured by Advantec, product number: T050A430A), and the filtered filter was dried to obtain a dried filter. The membrane filter was made of polytetrafluoroethylene and had an average pore size of 0.5 μm and a circular shape with a diameter of 47 mm (147.6 mm ２ ) and its thickness was 90 μm.

[0076] (Number of Total Particles Collected by Filtration of Dissolution Solution) The dried filter was observed at a magnification of 1,000 times using a scanning electron microscope (SEM), and the number of particles was counted.

[0077] (Number of Sb-containing particles collected by filtration of solution and their proportion to the total number of particles) Elemental analysis was performed on the particles on the dry filter using a scanning electron microscope (SEM) at a magnification of 1,000 times, and the number of particles in which Sb element was detected (Sb-containing particles) was counted. The number of particles in which Sb element was detected was divided by the number of all particles to calculate the proportion (%) of the number of Sb-containing particles.

[0078] (Amount of Sb element in particles collected by filtration of solution (X-ray fluorescence analysis (XRF))) The particles on the dry filter were measured by X-ray fluorescence to determine the amount of Sb element per 10 g of polyester, which was converted into the amount (mg) per kg of polyester.

[0079] (Foreign matter evaluation) Using a film roll with a width of 300 mm, light from an LED light source was transmitted from the back side of the film, and a CCD camera was set at a position where the distance (WD) between the film and the lens was 340 mm. ２ The number of colored foreign particles was counted when photographing the area using a CCD camera with a resolution of 10 μm. Here, colored foreign particles are defined as defects detected by the CCD camera with a minimum pixel value of 20 or more in a 256-level grayscale image, with pixel value 0 being black and pixel value 255 being white. The number of detected colored foreign particles was converted into the number per unit area.

[0080] (Intrinsic Viscosity of Polyester Film) The intrinsic viscosity (dl / g) was measured in accordance with JIS K 7367-5. In the measurement, the polyester film was immersed in a mixed solvent of phenol (6 parts by weight) and 1,1,2,2-tetrachloroethane (4 parts by weight) at a temperature of 30°C.

[0081] (Acid Value of Polyester Film) A. Sample Preparation: Polyester film was crushed and vacuum-dried at 70°C for 24 hours. The resulting sample was weighed to within 0.20±0.0005 g using a balance. The resulting mass was designated W (g). 10 ml of benzyl alcohol and a weighed sample were added to a test tube, which was then immersed in a benzyl alcohol bath heated to 205°C. The sample was dissolved while stirring with a glass rod. Samples obtained after dissolution times of 3, 5, and 7 minutes were designated A, B, and C, respectively. Next, new test tubes were prepared, and only benzyl alcohol was added. The same procedure was repeated. Samples obtained after dissolution times of 3, 5, and 7 minutes were designated A, B, and C, respectively. B. Titration: Titration was performed using a 0.04 mol / L potassium hydroxide solution (ethanol solution) with a known titration factor. Phenol red was used as the indicator, and the change from yellow-green to pale pink was used as the endpoint, determining the titration volume (ml) of potassium hydroxide solution. The titer amounts for samples A, B, and C were designated XA, XB, and XC (ml). The titer amounts for samples a, b, and c were designated Xa, Xb, and Xc (ml). C. Calculation of acid value Using the titer amounts XA, XB, and XC for each dissolution time, the titer amount V (ml) at dissolution time 0 minutes was calculated by the least squares method. Similarly, the titer amount V0 (ml) was calculated using Xa, Xb, and Xc. Next, the carboxyl terminal concentration was calculated according to the following formula: Acid value (eq / ton) = [(V - V0) x 0.04 x NF x 1000] / W NF: Factor of 0.04 mol / l potassium hydroxide solution W: Sample mass (g)

[0082] (Longitudinal Heat Shrinkage of Polyester Film) A polyester film was sampled to a size of 10 mm in the transverse direction and 220 mm in the longitudinal direction. Reference lines were marked on the sample at 200 mm intervals in the longitudinal direction, and the interval between the reference lines (L0) was measured. The sample was then sandwiched between sheets of paper and placed in a hot air oven controlled at a temperature of 150°C. After 30 minutes of treatment, the sample was removed and the interval between the reference lines (L) was measured. The heat shrinkage was calculated using the following formula: Heat shrinkage (%) = {(L0 - L) / L0} x 100

[0083] (Breaking Strength of Polyester Film) Breaking strength is the stress required to break a film. Specifically, a tensile force is gradually applied to a film, and the force at which the film breaks is determined. This force is converted into a stress per unit area (unit: MPa). Breaking strength was measured in accordance with JIS K 7127, specifically by the following method. That is, a film test piece having a width of 12.7 mm and a length of 200 mm was sampled, and the film test piece was set in a tensile tester (e.g., AG-X manufactured by Shimadzu Corporation). The film test piece was elongated at a chuck distance of 100 mm and a take-up speed of 100 mm / min under an environment of a temperature of 23°C and a humidity of 65% RH. The breaking strength was calculated from the measured values ​​of the elongation at break of the film test piece and the load required for breakage.

[0084] (Haze Measurement) The haze of the polyester film was measured at three randomly selected locations using a haze meter (NDH2000, manufactured by Nippon Denshoku Industries Co., Ltd.), and the average value was taken as the haze.

[0085] (Yield Evaluation Based on Brightness Unevenness) Using a commercially available polarized film sheet and a light box (Fujicolor Light Box NEW5000 Inverter), 10 film samples cut to A4 size were observed in a crossed Nicol position. Here, crossed Nicol observation generally refers to observing a sample sandwiched between two polarizers (Nicols), which generally become dark when two polarizers (Nicols) are superimposed in directions perpendicular to each other. The number of areas where brightness unevenness was observed was counted, and the number per A4 sheet was calculated. This was used as an index of yield based on brightness unevenness based on the following criteria. While a yield rating of A is naturally ideal, films with a yield rating of B or higher can be suitably used for light diffusion applications. Yield rating A: 0 locations where uneven brightness is observed Yield rating B: 1 to 5 locations where uneven brightness is observed Yield rating C: 6 to 10 locations where uneven brightness is observed Yield rating D: 11 or more locations where uneven brightness is observed

[0086] (Optical suitability for display applications) The number of foreign particles with a major diameter of 1 μm or more is 10 / mm ２Hereinafter, polyester films that satisfy all the conditions of a haze of 1.0% or less and a yield rating of B or higher based on brightness unevenness are judged to have passed the evaluation of optical suitability for display applications and are marked with "Good" in Table 3. On the other hand, those that do not satisfy these conditions are judged to have failed and are marked with "Poor" in Table 3.

[0087] (Strength Suitability for Display Applications) Polyester films that satisfied the condition of a longitudinal breaking strength of 180 MPa or more were judged to have passed the evaluation of optical suitability for display applications and were marked with "Good" in Table 3. On the other hand, those that did not satisfy this condition were judged to have failed and were marked with "Poor" in Table 3.

[0088] (Overall Evaluation for Display Use) When both the optical composition and strength compatibility were evaluated as "pass," the overall evaluation was "pass" and marked with "◯" in Table 3. On the other hand, when either or both of the optical composition and strength compatibility were evaluated as "fail," the overall evaluation was "fail" and marked with "x" in Table 3.

[0089] Example 1 <Production of Polyester Resin (Melt Polymerization)> (Slurry Preparation) A slurry was prepared by continuously feeding terephthalic acid and ethylene glycol in a ratio of 46.4 parts by mass of ethylene glycol to 100 parts by mass of terephthalic acid into a slurry preparation tank while stirring under a nitrogen flow.

[0090] (Esterification Reaction) A continuous esterification reactor consisting of a three-stage complete mixing vessel equipped with a stirrer, a distillation column, a raw material inlet, and a product outlet was used as the esterification reactor. An ethylene glycol solution of antimony trioxide (antimony trioxide concentration: 12 g / L) was supplied to the first esterification reactor together with the prepared slurry, and the esterification reaction was carried out at an absolute pressure of 126 kPa, a temperature of 258°C, and an average residence time of 3.3 hours. The reaction liquid was withdrawn from the first esterification reactor so that the liquid level remained constant, and then introduced into the second esterification reactor. Ethylene glycol was introduced into the second esterification reactor through another inlet at an average rate of 230 kg / hour, and the esterification reaction was carried out at atmospheric pressure, a temperature of 261°C, and an average residence time of 1.3 hours. The reaction liquid was withdrawn from the second esterification reactor so that the liquid level remained constant, and then introduced into the third esterification reactor. In the third esterification reaction tank, an ethylene glycol solution containing magnesium acetate, an ethylene glycol solution containing sodium acetate, and an ethylene glycol solution containing trimethyl phosphate were each fed in equal amounts through separate inlets, and an esterification reaction was carried out at a temperature of 260°C under atmospheric pressure for an average residence time of 0.9 hours.

[0091] (Polycondensation Reaction) The reaction solution was removed from the third esterification reaction tank so that the liquid level remained constant, and then introduced into the first polycondensation reaction tank of a three-stage continuous polycondensation reaction apparatus, where the first polycondensation reaction was carried out at a pressure of 5.6 kPa, a temperature of 278°C, and an average residence time of 0.7 hours. The reaction solution was removed from the first polycondensation reaction tank so that the liquid level remained constant, and then introduced into the second polycondensation reaction tank. The second polycondensation reaction was carried out at a pressure of 0.75 kPa, a temperature of 282°C, and an average residence time of 0.9 hours. The reaction solution was removed from the second polycondensation product so that the liquid level remained constant, and then introduced into the third polycondensation reaction tank. The degree of vacuum (pressure) was adjusted so that the intrinsic viscosity of the reaction product was 0.53 dl / g at a temperature of 282°C and an average residence time of 0.9 hours. The pressure was in the range of 0.08 to 0.15 kPa. The polyester resin obtained through the above process was extruded into a strand shape, cooled in water, and then cut into pellets.

[0092] <Production of Polyester Resin (Solid-State Polymerization)> The polyester resin obtained by melt polymerization was charged into a solid-state polymerization apparatus. After drying at 90°C for 3.5 hours, it was crystallized at 130°C for 4.5 hours. The temperature was then gradually increased from 197°C to 220°C, and solid-state polymerization was carried out at a pressure of 40 Pa for 7 hours to obtain a polyester resin with an intrinsic viscosity of 0.617 dl / g.

[0093] (Polymerization of Urethane Resin) 38 parts by mass of 4,4-dicyclohexylmethane diisocyanate, 9 parts by mass of dimethylolpropanoic acid, 53 parts by mass of polyhexamethylene carbonate diol having a number average molecular weight of 1000, and 84.00 parts by mass of acetone as a solvent were added to a four-neck flask equipped with a stirrer, a Dimroth condenser, a nitrogen inlet tube, a silica gel drying tube, and a thermometer. The mixture was stirred at 75°C for 3 hours under a nitrogen atmosphere, and it was confirmed that the reaction solution had reached the predetermined amine equivalent. Next, the reaction solution was cooled to 40°C, and then 5.17 parts by mass of triethylamine was added to obtain a polyurethane prepolymer solution. Next, 450 g of water was added to a reaction vessel equipped with a homodisperser capable of high-speed stirring, and the temperature was adjusted to 25°C. While stirring and mixing for 2000 min, the polyurethane prepolymer solution was added and dispersed in water. Thereafter, the acetone and a portion of the water were removed under reduced pressure to prepare a water-dispersible urethane resin solution (A) with a solids content of 35%.

[0094] (Polymerization of Blocked Isocyanate Crosslinking Agent) In a flask equipped with a stirrer, a thermometer, and a reflux condenser, 66.04 parts by mass of a polyisocyanate compound having an isocyanurate structure derived from hexamethylene diisocyanate (Duranate TPA, manufactured by Asahi Kasei Chemicals Corporation) and 25.19 parts by mass of 3,5-dimethylpyrazole (dissociation temperature: 120°C, boiling point: 218°C) were added dropwise to 17.50 parts by mass of N-methylpyrrolidone, and the mixture was maintained at 70°C for 1 hour under a nitrogen atmosphere. Thereafter, 5.27 parts by mass of dimethylolpropanoic acid were added dropwise. The infrared spectrum of the reaction solution was measured, and after confirming that the absorption of the isocyanate group had disappeared, 5.59 parts by mass of N,N-dimethylethanolamine and 132.5 parts by mass of water were added to obtain a blocked polyisocyanate aqueous dispersion (B) (crosslinking agent (B)) with a solids content of 40% by mass. The blocked isocyanate crosslinking agent has four functional groups.

[0095] (Polymerization of polyester resin for coating liquid) A stainless steel autoclave equipped with a stirrer, thermometer, and partial reflux condenser was charged with 194.2 parts by mass of dimethyl terephthalate, 184.5 parts by mass of dimethyl isophthalate, 14.8 parts by mass of dimethyl-5-sodium sulfoisophthalate, 233.5 parts by mass of diethylene glycol, 136.6 parts by mass of ethylene glycol, and 0.2 parts by mass of tetra-n-butyl titanate, and a transesterification reaction was carried out at a temperature of 160 ° C. to 220 ° C. for 4 hours. The temperature was then raised to 255 ° C., and the reaction system was gradually reduced in pressure, followed by reaction for 1 hour and 30 minutes under a reduced pressure of 30 Pa, to obtain polyester resin (C). The obtained polyester resin (C) was pale yellow and transparent.

[0096] (Preparation of Polyester Water Dispersion) 15 parts by mass of polyester resin (C) and 15 parts by mass of ethylene glycol n-butyl ether were placed in a reactor equipped with a stirrer, a thermometer, and a reflux device, and the mixture was heated to 110°C and stirred to dissolve the resin. After the resin was completely dissolved, 70 parts by mass of water was gradually added to the polyester solution with stirring. After the addition, the liquid was cooled to room temperature with stirring to prepare a milky white polyester water dispersion (Cw) with a solids content of 15% by mass.

[0097] (Preparation of coating liquid for forming easy-adhesion layer) The following coating agents were mixed with a mixed solvent of water and isopropanol to prepare a coating liquid having a solids mass ratio of urethane resin solution (A) / crosslinking agent (B) / polyester aqueous dispersion (Cw) of 25 / 10 / 68. Urethane resin solution (A) 3.65 parts by mass Crosslinking agent (B) 1.22 parts by mass Polyester aqueous dispersion (Cw) 22.27 parts by mass Particles 0.47 parts by mass (dry-process silica having an average particle size of 200 nm, solids concentration 3.5%) Particles 1.85 parts by mass (silica sol having an average particle size of 40 to 50 nm, solids concentration 30% by mass) Surfactant 0.30 parts by mass (silicone-based, solids concentration 10% by mass)

[0098] (Production of Biaxially Stretched Polyester Film for Light Diffusion Film) Polyester pellets as the film raw material polymer were dried at 135°C for 6 hours under a reduced pressure of 133 Pa. The pellets were then fed into an extruder and melt-extruded into a sheet at approximately 280°C. The extruded pellets were then rapidly cooled and solidified on a rotating cooling metal roll maintained at a surface temperature of 20°C to obtain an unstretched PET sheet. This unstretched PET sheet was heated to 100°C using a group of heated rolls and an infrared heater, and then stretched 3.5 times in the longitudinal direction using a group of rolls with different peripheral speeds to obtain a uniaxially stretched PET film. The coating solution, which had been allowed to stand at room temperature for 5 hours or more, was then applied to one side of a PET film by roll coating and then dried at 80°C for 20 seconds. The final (biaxially stretched) coating weight after drying was adjusted to 0.15 g / m (coating layer thickness after drying: 150 nm). Subsequently, the film was stretched 4.0 times in the width direction at 120°C in a tenter, and while the length of the film in the width direction was fixed, it was heated at 230°C for 5 seconds, and further subjected to a 3% width direction relaxation treatment at 100°C for 10 seconds to obtain a 75 μm biaxially stretched polyester film for light diffusion film.

[0099] Examples 2 to 5 and Comparative Examples 1 to 6 In producing polyester pellets, biaxially stretched polyester films for light diffusion films were obtained in the same manner as in Example 1, except that the amount of antimony trioxide solution supplied was set to the Sb content (ppm) shown in Table 2, the esterification reaction time and polycondensation reaction time were set to the times (hours) shown in Table 1, and the presence or absence of solid-state polymerization was set as shown in Table 1. In Table 1, in examples where the column for solid-state polymerization is marked with 0, solid-state polymerization was not performed.

[0100] Tables 1 to 4 show the production conditions of the polyester resin, various analyses of the biaxially stretched polyester film for use as a light diffusion film, and the measurement results.

[0101]

[0102]

[0103]

[0104]

[0105] The films of Comparative Examples 1, 2, and 4 to 6 contained a large number of foreign particles with a major axis of 1 μm or more, resulting in inferior optical properties as a light diffusion film. The film of Comparative Example 3, which used a lower amount of antimony compound than Comparative Example 1, contained fewer foreign particles with a major axis of 1 μm or more, but had a low intrinsic viscosity, a high acid value, and low longitudinal breaking strength, resulting in problems with strength suitability for display applications. Comparative Example 3 shows that although the amount of foreign particles can be reduced by reducing the amount of antimony compound used, other properties are inferior to those of the film of Comparative Example 1, making it insufficient as a light diffusion film. Despite using a lower amount of antimony compound than Comparative Example 1, the film of Example 1, by using solid-state polymerization in combination, exhibited breaking strength comparable to that of Comparative Example 1 and other optical properties superior to those of Comparative Example 1, making it suitable for use as a light diffusion film. Although the amount of antimony compound used in the films of Examples 2 and 3 was smaller than that in Comparative Example 1, the films of Examples 2 and 3 exhibited breaking strengths comparable to that of Comparative Example 1 by setting the esterification reaction time and polycondensation reaction time longer than those in Comparative Example 1, and were superior to Comparative Example 1 in terms of the evaluation of brightness unevenness yield, and were therefore suitable for use as light diffusion films.

[0106] The films of Comparative Examples 4 and 5 have a large ratio of XRF-Sb amount (antimony element content of particles filtered on the membrane filter after filtration using the above method) to ICP-Sb amount (antimony element content of the polyester film), which may result in reduced brightness. Furthermore, the films of Comparative Examples 4 and 6 contain a large number of colored foreign matter, which may result in reduced brightness. The film of Comparative Example 3 has a low intrinsic viscosity, resulting in low longitudinal breaking strength, which may pose a problem in terms of strength suitability for display applications. Despite using a smaller amount of antimony compound than Comparative Example 4, the films of Examples 4 and 5, by incorporating solid-state polymerization, exhibit breaking strength comparable to that of Comparative Example 4, and are therefore superior in terms of strength suitability for display applications. Furthermore, the films of Examples 3 and 4 have a small ratio of XRF-Sb amount to ICP-Sb amount and a small number of colored foreign matter, making them suitable for use as light diffusion films. Although the film of Example 3 uses a smaller amount of antimony compound than the film of Comparative Example 4, by extending the polymerization time, it exhibits a higher breaking strength than the film of Comparative Example 4, and is superior in terms of strength suitability for display applications. In addition, since the ratio of XRF-Sb amount / ICP-Sb amount is small and the number of colored foreign matter is also small, it is suitable for use as a light diffusion film.

Claims

1. A biaxially oriented polyester film for use as a light diffusion film, comprising an antimony compound, an alkaline earth metal compound, a phosphorus compound, and a polyester resin, wherein the antimony element content of the biaxially oriented polyester film is 125 ppm or less, and the intrinsic viscosity of the biaxially oriented polyester film is 0.55 dl / g or more.

2. The number of foreign particles with a major diameter of 1 μm or more contained in the biaxially oriented polyester film is 20 / mm ２ The biaxially stretched polyester film for a light diffusion film according to claim 1, wherein:

3. The number of particles filtered on the membrane filter by the following filtration method is ２ The biaxially stretched polyester film for a light diffusion film according to claim 1, wherein the number of particles per particle is 500 or less. Filtration method: 10 g of the biaxially stretched polyester film is dissolved in 80 ml of a p-chlorophenol / tetrachloroethane mixed solution to prepare a solution, and the solution is filtered under reduced pressure through a membrane filter with an average pore size of 0.5 μm. The p-chlorophenol / tetrachloroethane mixed solution consists solely of p-chlorophenol and tetrachloroethane in a mass ratio of 3 parts p-chlorophenol to 1 part tetrachloroethane, and the membrane filter is made of polytetrafluoroethylene, has a diameter of 47 mm, is circular, and is 90 μm thick.

4. The biaxially oriented polyester film for light diffusion film according to claim 1, wherein the antimony element content of the particles filtered on the membrane filter by the filtration method according to claim 3 is 10 mg or less per kg of the biaxially oriented polyester film.

5. A biaxially oriented polyester film for use as a light diffusion film according to claim 1, wherein the ratio (A / B) of the antimony element content (A) of particles filtered on a membrane filter by the filtration method according to claim 3 to the antimony element content (B) of the biaxially oriented polyester film is 0.75 or less.

6. When the LED light source is transmitted from the back of the film, the distance between the film and the lens (WD) is 340 mm, and the area is 30 m ２ When the film is photographed using a CCD camera with a resolution of 10 μm, the number of colored foreign particles detected is 30 / m ２ 2. The biaxially stretched polyester film for a light diffusion film according to claim 1, wherein the colored foreign matter is a defect detected by a CCD camera, the minimum pixel value of which in a grayscale image (an image expressed in 256 gradations with a pixel value of 0 being black and a pixel value of 255 being white) is 20 or more.

7. A biaxially stretched polyester film for use as a light diffusion film according to claim 1, in which the proportion of particles containing antimony element among the particles filtered on the membrane filter by the filtration method according to claim 3 is 30% or less.

8. The biaxially oriented polyester film for use as a light diffusion film according to claim 1, wherein the content of alkaline earth metal elements in the biaxially oriented polyester film is 5 to 160 ppm and the content of phosphorus elements is 1 to 50 ppm.

9. The biaxially oriented polyester film for use as a light diffusion film according to claim 1, wherein the intrinsic viscosity of the biaxially oriented polyester film is 0.55 to 0.65 dl / g.

10. The biaxially oriented polyester film for use as a light diffusion film according to claim 1, wherein the acid value of the biaxially oriented polyester film is 40 to 50 eq / ton.

11. The biaxially oriented polyester film for use as a light diffusion film according to claim 1, wherein the heat shrinkage rate of the biaxially oriented polyester film in the longitudinal direction under heat conditions of 150°C for 30 minutes is 1.4% or less.

12. The biaxially oriented polyester film for use as a light diffusion film according to claim 1, which has an easy-adhesion layer on at least one surface of the biaxially oriented polyester film.

13. A biaxially stretched polyester film for a light diffusion film according to claim 12, wherein the easy-adhesion layer contains a polyester resin, a urethane resin having a branched structure, and a crosslinking agent, and the mass of the polyester resin is 40 to 70% of the total mass of the solid contents of the polyester resin, the urethane resin, and the crosslinking agent.

14. A biaxially oriented polyester film for a light diffusion film according to claim 12, wherein, when the biaxially oriented polyester film has an easy-adhesion layer on only one surface, it has a hard coat layer on the surface of the easy-adhesion layer, and when the biaxially oriented polyester film has easy-adhesion layers on both surfaces, it has a hard coat layer on the surface of either or both of the easy-adhesion layers.

15. A display device comprising the biaxially stretched polyester film for use as a light diffusion film according to any one of claims 1 to 14, a light source, and a liquid crystal cell.

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