JP2017061036A - Biaxially oriented polyester film for optical application - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyester film having good optical performance, which can achieve high luminance and give a high-quality image when used as a member in an LCD, a PDP, an organic EL, a projection display or the like.SOLUTION: A biaxially oriented polyester film for optical application is provided, which is a polyester film having a layered structure of three or more layers. One of the layers other than layers constituting two surfaces of the above polyester film contains a fluorescent brightener, and the content of the fluorescent brightener in the film is 0.001 to 0.01 wt.% with respect to the whole polyester constituting the film. The optical film has a film haze of 2.0% or less.SELECTED DRAWING: None

Description

  The present invention relates to an optical polyester film, and more specifically, various optical members used for liquid crystal displays (hereinafter abbreviated as LCD), plasma displays (hereinafter abbreviated as PDP), etc. Polyester film for use in protective films, release films, etc. used in the manufacturing process of products in the field, which has excellent optical properties and can contribute to improving the quality of optical products and reducing energy consumption It is about.

  Conventionally, polyester films, especially biaxially stretched films of polyethylene terephthalate and polyethylene naphthalate, have excellent mechanical properties, heat resistance, and chemical resistance. Magnetic tape, ferromagnetic thin film tape, photographic film, packaging It is widely used as a material for films, films for electronic parts, electrical insulation films, metal laminate films, films to be attached to glass surfaces such as glass displays, and protective films for various members.

  In recent years, polyester films have been widely used for various optical films, in particular, prism films for LCD members, light diffusion sheets, reflectors, touch panels, and other base films, antireflection base films, display explosion-proof base films, and PDPs. It is used for various applications such as filter films. In these optical products, in order to obtain a bright and clear image, the base film used as an optical film has good transparency from its usage pattern and does not have defects such as foreign matter and scratches that affect the image. Is required.

  Brightness is an important characteristic in displays, and it is known that the polyester film used as a member affects the brightness. In particular, when a high-quality image is obtained, a high brightness is required. Therefore, the polyester film has been required to have a characteristic for realizing a high brightness. For this purpose, it goes without saying that the polyester film has a high degree of transparency, but in addition to this, it is necessary to have further optically advantageous properties. In particular, among LCD members, a film used for a backlight requires a material capable of increasing luminance for the purpose of reducing power consumption. Specifically, in the case where a transparent film is used among light diffusing sheets, prism sheets, and light reflecting sheets, improvement in luminance is strongly desired.

  Another important characteristic of the display is color tone, and it is known that the characteristic of the polyester film used as a member also affects the color tone. For example, when the polyester film of the base material is strong in yellow, the display image tends to have a yellowish color tone, which is not preferable. Therefore, it is calculated | required that the yellowishness (color tone y value) should be reduced with respect to the polyester film used for the member for a display.

JP 2006-176551 A

  The present invention has been made in view of the above circumstances, and the problem to be solved is that, when used as a member of an LCD, PDP, organic EL, projection display, etc., it achieves high brightness and produces a high-quality image. An object of the present invention is to provide a polyester film having good optical performance that can be provided.

  As a result of intensive studies in view of the above problems, the present inventors have found that a film having a specific configuration is suitable as an optical film, and have completed the present invention.

  That is, the gist of the present invention is that the fluorescent whitening agent is contained in any layer other than the layers constituting both surfaces of the polyester film having a laminated structure of three or more layers, and the content of the fluorescent whitening agent in the film Is a film in the range of 0.001 to 0.01% by weight with respect to all the polyesters constituting the film, and the film haze of the film is 2.0% or less. Lies in oriented polyester film.

  According to the present invention, a polyester film with good optical performance that can provide high-quality brightness and give a high-quality image when used as a member such as an LCD, PDP, organic EL, or projection display. The industrial value of the present invention is high.

  The polyester used for the polyester film in the present invention refers to a polyester obtained by polycondensation of an aromatic dicarboxylic acid and an aliphatic glycol. Examples of the aromatic dicarboxylic acid include terephthalic acid and 2,6-naphthalenedicarboxylic acid, and examples of the aliphatic glycol include ethylene glycol, diethylene glycol, and 1,4-cyclohexanedimethanol. Representative polyesters include polyethylene terephthalate (PET), polyethylene-2,6-naphthalenedicarboxylate (PEN), and the like. In this invention, it is preferable to contain 2.0-10.0 mol% of 3rd components other than a main structural component, More preferably, it is 2.5-10 mol%, Most preferably, it is 3.0-8.0. Mol%.

As a method for containing the third component, a copolymer polyester containing a predetermined amount of a copolymer component as a raw material for producing a film may be used, or a copolymer polyester containing a copolymer component larger than a predetermined amount. Further, a raw material obtained by blending a copolymer polyester or homopolyester having a small content of copolymer components may be used. Examples of the third component herein include isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, adipic acid, sebacic acid and the like as the dicarboxylic acid component, and P-oxybenzoic acid as the oxycarboxylic acid. Examples of the glycol component include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol, and the like. Among these, when diethylene glycol, triethylene glycol, or 1,4-cyclohexanedimethanol is used, the optical unevenness of the film due to the orientation and thickness unevenness of the polymer can be efficiently reduced, and the flatness of the film and This is preferable in that heat resistance and dimensional stability can be maintained at a high level. Here, as a third component contained in the polyester, diethylene glycol by-produced from ethylene glycol during polymerization is also included.

  The polyester in the present invention is a conventionally known method, for example, a method of directly obtaining a low-polymerization degree polyester by reaction of a dicarboxylic acid and a diol, or a lower alkyl ester of a dicarboxylic acid and a diol reacted with a conventionally known transesterification catalyst. Then, it can obtain by the method of performing a polymerization reaction in presence of a polymerization catalyst. As the polymerization catalyst, a known catalyst such as an antimony compound, a germanium compound, or a titanium compound may be used. Preferably, the amount of the antimony compound is zero or antimony is reduced to 100 ppm or less to reduce film dullness. .

  The polyester to be used may be chipped after melt polymerization, and further subjected to solid phase polymerization as necessary under heating and reduced pressure or in an inert gas stream such as nitrogen. The intrinsic viscosity of the obtained polyester is preferably 0.50 dl / g or more, and preferably 0.50 to 0.90 dl / g.

  You may mix | blend particle | grains in the polyester layer in the film of this invention with the main objective of providing easy slipperiness. The kind of the particle to be blended is not particularly limited as long as it is a particle capable of imparting slipperiness. Specific examples thereof include silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, and phosphoric acid. Examples thereof include particles of magnesium silicon oxide, kaolin, aluminum oxide, titanium oxide and the like. Further, the heat-resistant organic particles described in JP-B-59-5216, JP-A-59-217755 and the like may be used. Examples of other heat-resistant organic particles include thermosetting urea resins, thermosetting phenol resins, thermosetting epoxy resins, benzoguanamine resins, and the like. Furthermore, precipitated particles obtained by precipitating and finely dispersing a part of a metal compound such as a catalyst during the polyester production process can also be used.

  On the other hand, the shape of the particles to be used is not particularly limited, and any of a spherical shape, a block shape, a rod shape, a flat shape, and the like may be used. Moreover, there is no restriction | limiting in particular also about the hardness, specific gravity, a color, etc. These series of particles may be used in combination of two or more as required.

  Moreover, the average particle diameter of the particle | grains to be used has the preferable range of 0.01-5 micrometers normally. If the average particle size is less than 0.01 μm, the particles tend to aggregate and the dispersibility may be insufficient. On the other hand, if the average particle size exceeds 5 μm, the surface roughness of the film becomes too rough and transparent. It may become inferior.

  Furthermore, the particle content in the polyester film is usually in the range of 0.0003 to 1.0% by weight, preferably 0.0005 to 0.5% by weight. When the particle content is less than 0.0003 wt%, the slipperiness of the film may be insufficient. On the other hand, when the content exceeds 1.0 wt%, the transparency of the film is insufficient. There is a case.

  The method for adding particles to the polyester is not particularly limited, and a conventionally known method can be adopted. For example, it can be added at any stage for producing the polyester, but the polycondensation reaction may proceed preferably after the esterification stage or after the transesterification reaction. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a blending of dried particles and a polyester raw material using a kneading extruder. It is done by methods.

  In addition to the above-mentioned particles, conventionally known antioxidants, heat stabilizers, lubricants, antistatic agents, dyes, pigments, and the like can be added to the polyester film in the present invention as necessary. Depending on the application, an ultraviolet absorber, particularly a benzoxazinone-based ultraviolet absorber, may be contained.

  The thickness of the polyester film of the present invention is not particularly limited as long as it can be formed as a film, but is usually 9 to 300 μm, preferably 20 to 250 μm, and more preferably 25 to 200 μm.

  The film of the present invention has an essential requirement that the film haze is 2.0% or less. The film of the present invention is widely used for optical applications because of its excellent transparency. However, when the film haze exceeds 2.0%, it is unsuitable for optical use.

  The polyester film for optical use of the present invention, particularly a display, preferably has a color tone y value measured by one transmission method of 0.3175 or less. If the y value exceeds 0.3175, it may be inappropriate in that the color tone of the image becomes inferior or the luminance becomes low when used for a display because of its strong yellowishness.

In the film of the present invention, the sum of the front and back surfaces of the oligomer (ester cyclic trimer) precipitation amount on the film surface after heat treatment at 180 ° C. for 10 minutes is preferably 15 mg / m ² or less, more preferably. Is 10.0 mg / m ² or less, particularly preferably 8.0 mg / m ² or less. When the amount of oligomer precipitation on the film surface exceeds 15 mg / m ² , the oligomer may crystallize on the surface and dissolve into a functional layer provided on the film, which may cause problems such as affecting properties.

  In order to make the oligomer precipitation amount on the film surface by heat treatment within the above range, it is particularly a laminated film composed of at least three layers by coextrusion, and a polyester having a low oligomer content is used as the outermost layer constituting the surface layer, By providing the coating layer inline / offline, the oligomer deposition amount on the film surface after the heat treatment can be controlled within the above range by suppressing the oligomer deposition on the film surface.

  The film of the present invention can be formed into a laminated structure using a coextrusion method. In this case, the thickness of the outermost layer is usually 3 μm or more, preferably 5 μm or more, more preferably 10 μm or more, only on one side. And it is preferable that it is 1/4 or less of the total thickness. When the thickness is less than 3 μm, oligomers (cyclic trimers) contained in the inner layer are deposited on the film surface due to heat history during processing, etc., and contamination of the production line and an increase in the amount of foreign matter on the film surface are observed. On the other hand, if it is thicker than 1/4 of the total thickness, the amount of particles blended in the outermost layer may increase and the transparency may be impaired.

  When it manufactures as this laminated | multilayer film, it is preferable that content of the 3rd component in this invention sets the amount of 3rd component contained in the whole film with respect to polyester of the whole film as the above-mentioned range. This is because the prevention of optical unevenness, which is a problem to be improved in the present invention, relates to the transmitted light of the film and relates not only to the film surface and the inside but also to the entire characteristics.

  In the present invention, a fluorescent whitening agent is contained in the film in order to improve luminance when used as an optical member, particularly an LCD backlight unit, and to reduce yellowing. The content of the optical brightener is 0.001 to 0.01% by weight, preferably 0.003 to 0.006% by weight, based on the total polyester constituting the film. When the content of the optical brightener is less than 0.0001% by weight, the effects of improving luminance and reducing yellowing are insufficient, and the object of the present invention cannot be achieved. On the other hand, if it exceeds 0.01% by weight, the brightness of the film may decrease due to the change in the color tone of the film, and problems such as bleed out on the surface of the film and adversely affecting the use may occur. Absent.

  Furthermore, in order to prevent the bleed-out of the optical brightener, in the present invention, the film has a laminated structure of three or more layers, and the fluorescent brightener is contained in any layer other than the layer forming the surface. That is, for example, the A / B / A structure and the A / B / C structure B layer may be used. In this case, the fluorescent brightening agent can be contained in layers other than the B layer, but the fluorescent brightening agent content in the A layer and the C layer is preferably lower than that in the B layer. When such an addition method is used, the content of the optical brightener in the film needs to be in the above-described range as the content with respect to all the polyesters constituting the film.

  Examples of fluorescent brighteners that can be suitably used in the present invention include a product “Ubitec” manufactured by Ciba Specialty Chemicals, a product “OB-1” manufactured by Eastman, and the like. The method of adding the optical brightener may be added during the production process of the polyester, or may be a method in which the polyester raw material is made into a chip shape and then applied, or may be added during the melt extrusion process of the polyester. A method of adding a polyester raw material added at a high concentration by such a method as a master batch in a melt extrusion process at the time of film formation can also be adopted. In any case, the effect of the present invention will be exhibited if the obtained film contains a predetermined amount of fluorescent brightening agent.

In addition, since the film of the present invention is used for optical purposes, the number of foreign matters having a maximum diameter of 150 μm or more present in the film is preferably 0.0 pieces / m ² or less, and foreign matters having a maximum diameter of 30 μm or more. Is usually 1.5 pieces / m ² or less, preferably 1.0 pieces / m ² or less. When foreign matter with a maximum diameter of 150 μm or more is 0.0 pieces / m ² or less, or foreign matter with a maximum diameter of 30 μm or more is 1.5 pieces / m ² or less, when used as a member of LCD or PDP Defects in the image may cause the quality to deteriorate. Further, the number of scratches having a width of 10 μm or more present on the film surface is preferably 10 / m ² or less, more preferably 5 / m ² or less. If the number of scratches having a width of 10 μm is larger than 10 / m ² , the quality of the LCD or PDP image may be similarly lowered.

  In order to overcome such problems, a method for preventing foreign matters from being mixed during polyester raw material production and film production and removing foreign matters by using a high-accuracy filter has been adopted. It is preferable to take measures such as suppressing unevenness in speed at the time of contact with each roll in the process and preventing foreign matter from entering between the roll and the film.

  The thus-obtained film of the present invention is used for various optical applications. In this case, it is preferable that the optical unevenness of the film due to the orientation of the polymer and the unevenness of the thickness is reduced. The difference between the maximum and minimum retardation values in the plane of the film measured by the method is preferably 200 nm or less, more preferably 100 nm or less, and particularly preferably 50 nm or less. When this requirement is satisfied, the image can be excellent in sharpness, brightness, and uniformity when used for optical applications.

  Hereinafter, although the manufacturing method of the polyester film of this invention is demonstrated concretely, as long as the summary of this invention is satisfied, this invention is not specifically limited to the following illustrations.

  First, a dried or undried polyester chip by a known method is supplied to a melt-extrusion apparatus and heated to a temperature equal to or higher than the melting point of each polymer and melted. Next, the molten polymer is extruded from a die and rapidly cooled and solidified on a rotary cooling drum so that the temperature is equal to or lower than the glass transition temperature to obtain a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed.

  In the present invention, the sheet thus obtained is stretched biaxially to form a film. Specifically describing the stretching conditions, the unstretched sheet is preferably stretched 2.7 to 5 times at 70 to 145 ° C. in the longitudinal direction to form a longitudinal uniaxially stretched film, and then 90 to 160 in the lateral direction. It is preferable that the film is stretched 3 to 5 times at ° C and heat-treated at 210 to 240 ° C for 10 to 600 seconds. Further, at this time, a method of relaxing 0.1 to 20% in the longitudinal direction and / or the transverse direction in the maximum temperature zone of the heat treatment and / or the cooling zone at the heat treatment outlet is preferable. Further, it is possible to add re-longitudinal stretching and re-lateral stretching as necessary.

  In the present invention, as described above, two or three or more polyester melt extruders can be used to form a laminated film of two layers or three or more layers by a so-called coextrusion method. As the layer structure, an A / B structure using an A raw material and a B raw material, or an A / B / A structure, and further using a C raw material to form an A / B / C structure or other film. Can do. For example, the surface shape of the A layer can be designed using specific particles as the A raw material, and a film having an A / B or A / B / A structure can be formed using a raw material not containing particles as the B raw material. In this case, since the raw material of B layer can be selected freely, a cost advantage etc. are large. Further, even if the recycled material of the film is blended with the B layer, the surface roughness can be designed by the surface A layer, so that the cost advantage is further increased.

  In particular, since the film of the present invention is used for optical applications, a light diffusion layer, a hard coat layer, an antireflection layer, an antiglare layer, or the like is provided, or a vapor deposition layer is provided. A coating layer as an undercoat layer can be provided for the purpose of improving the coating property and adhesiveness of the material, or preventing charging in order to keep the surface clean.

  In forming such a coating layer, the method of forming a film, in particular, after stretching in the longitudinal direction and before stretching in the lateral direction can form a very thin coating layer, and the coating solution drying and curing reaction Is preferable in that it can be carried out in the film forming process.

  The coating layer is preferably composed of a combination of a crosslinking agent and various binder resins, and as the binder resin, a polymer usually selected from polyester, acrylic polymer and polyurethane is adopted from the viewpoint of adhesiveness. Is preferred. Each of the above polymers shall also include derivatives thereof. The derivative here refers to a polymer obtained by reacting a reactive compound with a copolymer or a functional group with another polymer.

  In addition, since the film of the present invention is used for optics, it is also preferable to form a functional multilayer thin film for the purpose of preventing dust reflection due to reflection of external light and static electricity, as well as electromagnetic shielding, in addition to improving adhesiveness. .

  Among the polyesters, acrylic polymers, and polyurethanes used as coating agents in the present invention, particularly preferred polymers have a glass transition temperature (Tg) of 0 ° C. or higher, more preferably 40 ° C. or higher. Polyester polyurethane is a polymer having a carboxylic acid residue, at least a part of which is made aqueous with an amine or ammonia.

  As the crosslinking agent resin, melamine-based, epoxy-based, and oxazoline-based resins are generally used, but melamine-based resins are particularly preferable from the viewpoints of coatability and durable adhesiveness. The melamine-based resin may be either a monomer, a condensate composed of a dimer or higher multimer, or a mixture thereof.

  In the present invention, it is preferable to contain inorganic particles or organic particles in the coating layer in order to further improve the slipperiness, adhesion and the like. The amount of the particles in the coating agent is usually 0.5 to 10% by weight, preferably 1 to 5% by weight. When the blending amount is less than 0.5% by weight, the blocking resistance may be insufficient. When the blending amount exceeds 10% by weight, the transparency of the film is hindered and the sharpness of the image tends to be lowered.

  Examples of the inorganic particles include silicon dioxide, alumina, zirconium oxide, kaolin, talc, calcium carbonate, titanium oxide, barium oxide, carbon black, molybdenum sulfide, and antimony oxide. Among these, silicon dioxide is easy to use because it is inexpensive and has various particle sizes. On the other hand, examples of the organic particles include polystyrene or polyacrylate polymethacrylate in which a crosslinked structure is achieved by a compound (for example, divinylbenzene) containing two or more carbon-carbon double bonds in one molecule.

  The above inorganic particles and organic particles may be surface-treated. Examples of the surface treatment agent include a surfactant, a polymer as a dispersant, a silane coupling agent, a titanium coupling agent, and the like. The content of the particles in the coating layer is preferably 10% by weight or less, more preferably 5% by weight or less as an appropriate addition amount that does not impair the transparency.

  Further, the coating layer may contain an antistatic agent, an antifoaming agent, a coating property improving agent, a thickener, an antioxidant, an ultraviolet absorber, a foaming agent, a dye, a pigment, and the like.

  As long as water is the main medium, the coating agent may contain a small amount of an organic solvent for the purpose of improving dispersion in water or improving the film-forming performance. It is necessary to use the organic solvent as long as it is soluble in water. Examples of the organic solvent include aliphatic or alicyclic alcohols such as n-butyl alcohol, n-propyl alcohol, isopropyl alcohol, ethyl alcohol, and methyl alcohol, glycols such as propylene glycol, ethylene glycol, and diethylene glycol, n-butyl cellosolve, Examples include glycol derivatives such as ethyl cellosolve, methyl cellosolve, propylene glycol monomethyl ether, ethers such as dioxane and tetrahydrofuran, esters such as ethyl acetate and amyl acetate, ketones such as methyl ethyl ketone and acetone, and amides such as N-methylpyrrolidone. Can be mentioned. These organic solvents may be used in combination of two or more as required.

  As a coating method of the coating agent, for example, a reverse roll coater, a gravure coater, a rod coater, an air doctor coater, or a coating apparatus other than these as shown in Yuji Harasaki, Tsuji Shoten, published in 1979, “Coating Method” Can be used.

  The coating layer may be formed only on one side of the polyester film or on both sides. When formed only on one side, other characteristics can be imparted by forming a coating layer different from the above-mentioned coating layer on the opposite surface as necessary. In addition, in order to improve the applicability | paintability and adhesiveness to the film of a coating agent, you may give a chemical process and an electrical discharge process to a film before application | coating. Further, in order to further improve the surface characteristics, a discharge treatment may be performed after the coating layer is formed.

  The thickness of the coating layer is usually 0.01 to 0.5 μm, preferably 0.015 to 0.3 μm as the final dry thickness. When the thickness of the coating layer is less than 0.01 μm, the effect of the present invention may not be sufficiently exhibited. When the thickness of the coating layer exceeds 0.5 μm, the films tend to stick to each other, and particularly when the coated film is re-stretched to increase the strength of the film, it adheres to the roll in the process. There is a tendency to become easy. The above problem of sticking appears particularly when the same coating layer is formed on both sides of the film.

  When such a coating film is applied to an optical application, the coating layer surface coating is problematic when an antireflection layer or the like is provided on the coating layer. The reason for the occurrence of coating leakage is not clear, but foreign matter in the film creates coarse protrusions on the surface of the film, which acts as a core and the coating agent repels and stretches to cause coating leakage or the film surface. There are cases where oligomers and dust adhering to the core serve as nuclei, and the coating agent repels and disappears from the nuclei. Therefore, it is necessary to form a film under such a condition that dust and foreign matters that can become nuclei are removed as much as possible. Such foreign substances include oligomers adhered or deposited on the film, so that reducing the amount of oligomers contained in the film has the effect of reducing the amount of coating.

  When the film of the present invention thus obtained has a coating layer, the number of coating spots (N) is preferably in the range of 0 to 20, more preferably 10 or less, particularly preferably, per 30 m2 of film. 3 or less.

  In any case, in an optical film that will become more and more severe in the future, it is necessary to make coating coating as zero as possible.

The film of the present invention exerts particularly excellent effects when used for optical purposes. As specific members thereof, a diffuser plate, a reflective plate, and a prism sheet for a backlight for a liquid crystal display , Lens sheets, brightness enhancement films, etc., and materials for so-called PDP filters for plasma displays, such as electromagnetic wave shielding, near-infrared shielding, color correction, ultraviolet shielding, antireflection, etc. It is effectively used as a base material that requires high transparency. In particular, the addition of the fluorescent whitening agent of the present invention exhibits an excellent effect when used as a backlight member for LCD, as a base material of a diffusion plate, a prism sheet, or a reflection plate. Similarly, when used as a base material for various light-emitting type display boards, such as diffusion boards and reflectors used for information boards, advertising displays, etc.,
Brightening the display and making it easier to see can have a great effect on energy saving.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded. The measuring method used in the present invention is as follows.

(1) Measurement of Intrinsic Viscosity of Polyester 1 g of polyester was accurately weighed and dissolved by adding 100 ml of a mixed solvent of phenol / tetrachloroethane = 50/50 (weight ratio), and measured at 30 ° C.

(2) Measurement of average particle size (d50: μm) 50% integrated (weight basis) in equivalent spherical distribution measured using centrifugal sedimentation type particle size distribution measuring device (SA-CP3 type manufactured by Shimadzu Corporation) The value was defined as the average particle size.

(3) Film haze Film haze was measured with an integrating sphere turbidimeter “NDH2000” manufactured by Nippon Denshoku Industries Co., Ltd. according to JIS-K-7136.

(4) Color tone transmission method y value The color tone transmission method y value of one film was measured with a Minolta spectrocolorimeter “CM-3700d” according to JIS-Z-5722.

(5) Suitability of optical member As a representative optical member, characteristics when used as a diffusion plate were evaluated. That is, an acrylic binder containing particles was applied to one side of the film to form a light diffusion layer. The obtained diffusion sheet was incorporated into a backlight unit, and the quality of the obtained planar light emission was evaluated from the following viewpoints.
Luminance level (evaluated using a luminance meter, compared with the case of using a conventional PET film, that is, the film of Comparative Example 1 in the present invention)
A: Brightness increased by 1% or more, and significant improvement was observed. B: Brightness increased by 0.5% or more, and improvement was observed. C: Brightness was not improved or decreased.

(6) Method for measuring amount of ester cyclic trimer contained in polyester raw material About 200 mg of the polyester raw material is weighed and dissolved in 2 ml of a mixed solvent of chloroform / HFIP (hexafluoro-2-isopropanol) ratio 3: 2. After dissolution, add 20 ml of chloroform, and then add 10 ml of methanol little by little. The precipitate is removed by filtration, and the precipitate is further washed with a mixed solvent having a chloroform / methanol ratio of 2: 1. The filtrate / washing solution is collected, concentrated by an evaporator, and then dried. After the dried product was dissolved in 25 ml of DMF (dimethylformamide), this solution was supplied to liquid chromatography (manufactured by Shimadzu Corporation: LC-7A) to determine the amount of ester cyclic trimer in DMF. Divided by the amount of the polyester raw material dissolved in the / HFIP mixed solvent to obtain the amount of ester cyclic trimer (weight%). The amount of ester cyclic trimer in DMF was determined from the peak area ratio between the standard sample peak area and the measured sample peak area (absolute calibration curve method).

  The standard sample was prepared by accurately weighing the pre-sorted ester cyclic trimer and dissolving it in DMF accurately weighed. The concentration of the standard sample is preferably in the range of 0.001 to 0.01 mg / ml.

  The conditions for the liquid chromatograph were as follows.

Mobile phase A: Acetonitrile Mobile phase B: 2% acetic acid aqueous solution Column: “MCI GEL ODS 1HU” manufactured by Mitsubishi Chemical Corporation
Column temperature: 40 ° C
Flow rate: 1 ml / min Detection wavelength: 254 nm

(7) Method for measuring the amount of ester cyclic trimer extracted from the surface of the laminated polyester film In advance, the untreated laminated polyester film is heated in air at 180 ° C. for 10 minutes. Thereafter, the heat-treated film is brought into close contact with the inner surface of a box having an upper portion of 10 cm in length and width of 3 cm and a height of 3 cm to form a box shape. Next, 4 ml of DMF is placed in the box prepared by the above method and left for 3 minutes, and then DMF is recovered. The recovered DMF is supplied to liquid chromatography (manufactured by Shimadzu Corporation: LC-7A) to determine the amount of ester cyclic trimer in DMF, and this value is divided by the film area in contact with DMF to obtain the film surface. The amount of ester cyclic trimer (mg / m ² ) was used. The amount of ester cyclic trimer in DMF was calculated according to the absolute calibration method described in (6) Method for measuring amount of ester cyclic trimer contained in polyester raw material.

  Examples and Comparative Examples are shown below, and the method for producing the polyester used in the Examples and Comparative Examples is as follows.

<Manufacture of polyester>
<Method for producing polyester (a)>
Using 100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol as starting materials, 0.09 parts by weight of magnesium acetate tetrahydrate as a catalyst is placed in the reactor, the reaction start temperature is set to 150 ° C., and the methanol is distilled off gradually. The reaction temperature was raised to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. After 0.04 part of ethyl acid phosphate was added to this reaction mixture, 0.03 part of antimony trioxide was added and a polycondensation reaction was carried out for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.67 due to a change in stirring power of the reaction vessel, and the polymer was discharged under nitrogen pressure. The obtained polyester (a) had an intrinsic viscosity of 0.67 and an ester cyclic trimer content of 0.97% by weight.

<Method for producing polyester (b)>
In the method for producing polyester (a), after adding 0.04 part of ethyl acid phosphate, 0.4 part of silica particles having an average particle diameter of 2.2 μm dispersed in ethylene glycol and 0.03 part of antimony trioxide are added. In addition, polyester (b) was obtained using the same method as the production method of polyester (a) except that the polycondensation reaction was stopped at the time corresponding to the intrinsic viscosity of 0.66. The obtained polyester (b) had an intrinsic viscosity of 0.67 and an ester cyclic trimer content of 0.85% by weight.

<Method for producing polyester (c)>
In the method for producing polyester (a), the starting material is 100 parts by weight of dimethyl terephthalate, 60 parts by weight of ethylene glycol and 2 parts by weight of diethylene glycol, and the method for producing polyester (a) is used except that germanium oxide is used as a polymerization catalyst. A polyester (c) was obtained using the same method as described above. In addition, the addition method of germanium oxide employ | adopted the well-known method, The addition amount was 100 ppm with respect to the raw material weight as germanium. The obtained polyester (c) had an intrinsic viscosity of 0.66 and an ester cyclic trimer content of 0.89% by weight.

<Production method of polyester (d)>
A blend of 99.5 parts by weight of polyester (c) and 0.5 parts by weight of fluorescent whitening agent “OB-1” manufactured by Eastman, and melt-kneading using a twin screw extruder, and a master containing fluorescent whitening agent Created a batch.

<Method for producing polyester (e)>
Polyester (a) is pre-crystallized at 160 ° C. in advance and then solid-phase polymerized in a nitrogen atmosphere at a temperature of 220 ° C. to obtain a polyester having an intrinsic viscosity of 0.75 and an ester cyclic trimer content of 0.46% by weight. (E) was obtained.

Example 1:
A mixed raw material obtained by mixing the above polyesters (b) and (e) at a ratio of 12% and 88%, respectively, is used as a raw material for the A layer, and polyesters (c) and (d) are respectively 99.3% and 0.7%. The raw materials mixed at a ratio of B are fed to the two bent twin-screw extruders and melted at 285 ° C., the A layer is the outermost layer (surface layer), and the B layer is the intermediate layer. Co-extruded in a layer configuration of 2 types, 3 layers (A / B / A), extruded from the die, cooled and solidified on a cooling roll set at a surface temperature of 40 ° C. using an electrostatic application adhesion method, and an unstretched sheet Obtained. Next, the film was stretched 3.3 times in the machine direction at a film temperature of 81 ° C. using the difference in peripheral speed of the roll, and then applied to a tenter after applying a coating agent having the composition shown below, and 3. The film was stretched 6 times, heat treated at 225 ° C., and then relaxed 2% in the transverse direction to obtain a laminated polyester film having a thickness of 250 μm. The thickness of each layer of the obtained film was 12/226/12 μm. The thickness of the coating layer was 0.08 μm.

(Coating agent composition: weight ratio)
I / II / III / IV = 47/20/30/3 (where I is terephthalic acid / isophthalic acid / 5-sodium sulfoisophthalic acid / ethylene glycol / diethylene glycol / triethylene glycol = 31/16/3 / 22/21 (molar ratio) polyester dispersion, II is an emulsion polymer (emulsifier: anion) of methyl methacrylate / ethyl acrylate / acrylonitrile / N-methylol methacrylamide = 45/45/5/5 (molar ratio) System surfactant), III is hexamethoxymethylmelamine (melamine crosslinking agent), and IV is an aqueous dispersion (inorganic particles) of silicon oxide having a particle size of 0.06 μm)

Example 2:
A polyester film having a thickness of 250 μm was prepared in the same manner as in Example 1 except that the mixed raw material in which the polyesters (c) and (d) were mixed at a ratio of 99.8% and 0.2%, respectively, was used as the raw material for the B layer. Obtained.

Example 3:
A polyester film having a thickness of 250 μm was prepared in the same manner as in Example 1 except that the mixed raw material in which the polyesters (c) and (d) were mixed at a ratio of 98.2% and 1.8%, respectively, was used as the raw material for the B layer. Obtained.

Example 4:
A polyester film having a thickness of 250 μm was prepared in the same manner as in Example 1 except that a mixed raw material in which polyesters (a) and (d) were mixed in proportions of 99.3% and 0.7%, respectively, was used as the raw material for the B layer. Obtained.

Example 5:
A polyester film having a thickness of 250 μm was obtained in the same manner as in Example 1 except that the thickness of each layer of the film was 6/238/6 μm.

Example 6:
A polyester film having a thickness of 250 μm was obtained in the same manner as in Example 1 except that the thickness of each layer of the film was set to 2/246/2 μm.

Example 7:
A polyester film having a thickness of 250 μm was obtained in the same manner as in Example 1 except that the mixed raw material obtained by mixing polyester (b) and (a) at a ratio of 12% and 88%, respectively, was used as the A layer.

Comparative Example 1:
A polyester film was obtained in the same manner as in Example 1 except that 100% of the polyester (c) was used as the raw material for the B layer.

Comparative Example 2:
A polyester film was obtained in the same manner as in Example 1 except that a mixed raw material in which polyesters (c) and (d) were mixed in proportions of 99.98% and 0.02%, respectively, was used as the raw material for the B layer. It was.

Comparative Example 3:
A polyester film was obtained in the same manner as in Example 1 except that a mixed raw material in which polyesters (c) and (d) were mixed in a proportion of 97.3% and 2.7%, respectively, was used as the raw material for the B layer. It was.

Comparative Example 4:
A polyester film having a thickness of 250 μm was obtained in the same manner as in Example 1 except that the mixed raw material in which polyesters (b) and (c) were mixed in proportions of 25% and 75%, respectively, was used as the A layer.

  As described above, the physical properties of the obtained film and the characteristics when the film is used as the base material of the diffusion plate are summarized in Table 1 below. It can be seen that a film satisfying the requirements of the present invention is highly suitable for this application.

  The film of the present invention can be suitably used as a base material for various optical members used in, for example, LCDs and PDPs.

Claims (1)

Any polyester other than the layers constituting both surfaces of the polyester film having a laminate structure of three or more layers contains the fluorescent brightening agent, and the content of the fluorescent brightening agent in the film is all polyester constituting the film A biaxially oriented polyester film for optical use, characterized in that the film is in the range of 0.001 to 0.01% by weight and the film haze of the film is 2.0% or less.