JP2008256756A - Polyester film for optical use - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film achieving high luminance without unevenness or any defect, and providing a high-quality image, when used as a prism sheet member to be used in a liquid crystal display. <P>SOLUTION: The biaxially oriented polyester film for the prism sheet for the liquid crystal display is composed of a polyester containing 0.01-1.00 pts.wt. fluorescent brightener, and has total transmittance of 91.0% or more. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to an optical polyester film, and more specifically, a polyester film for a prism sheet substrate used in a liquid crystal display (hereinafter abbreviated as LCD), which has excellent optical characteristics and quality of optical products. The present invention relates to an optical polyester film that can contribute to improvement and has excellent workability during production.

  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.

  Polyester films have been used in recent years, especially for various optical films. Base films for LCD components such as prism sheets, light diffusion sheets, reflectors, and touch panels, antireflection base films, display explosion-proof base films, and plasma It is used for various applications such as a display panel (hereinafter abbreviated as PDP) filter film. 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, and there are no defects such as foreign matter and scratches that affect the image. Necessary.

  The polyester film is usually obtained by subjecting an amorphous sheet obtained by melt extrusion to a sheet shape and rapid cooling and solidification to the longitudinal direction and the transverse direction, followed by heat treatment. If the uniformity of cooling and stretching is not sufficient in these steps, uneven retardation remains, and there is a problem that the image is deteriorated when used for optics.

Brightness and image contrast are listed as important characteristics in displays, and it is known that the polyester film used as a member affects the brightness and contrast. In particular, when obtaining a high-quality image, the above characteristics are required. Therefore, the polyester film is required to have a characteristic for realizing a high-quality image. For this purpose, it goes without saying that the polyester film has a high degree of transparency, but in addition, it is necessary to have optically advantageous characteristics such as film color tone.
Japanese Patent Publication No.59-5216 JP 59-217755 A

  The present invention has been made in view of the above-described circumstances, and the problem to be solved is that when used as a member of a prism sheet of an LCD, there is no unevenness or a defect, high brightness is achieved, and a high-quality image is provided. It is in providing the polyester film which can be.

  That is, the gist of the present invention is a film comprising a polyester containing 0.01 to 1.00 parts by weight of a fluorescent brightening agent, wherein the total light transmittance is 91.0% or more. It exists in the biaxially oriented polyester film for prism sheets of displays.

Hereinafter, the present invention will be described in more detail.
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 the present invention, the third component other than the main constituent is usually contained in an amount of 2.0 to 10.0 mol%, preferably 2.5 to 10.0 mol%, more preferably 3.0 to 8.0 mol. %.

  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, optical unevenness of the film due to polymer orientation and thickness unevenness 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.

  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.

  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 antimony compound is reduced to zero or 100 ppm or less as antimony to reduce film dullness. preferable.

  In addition, the polyester used in the present invention may be obtained by forming a chip after melt polymerization, and further subjecting it to solid-phase polymerization under heating or reduced pressure or in an inert gas stream such as nitrogen as necessary. The intrinsic viscosity of the obtained polyester is preferably 0.40 dl / g or more, and more preferably 0.40 to 0.90 dl / g.

  You may mix | blend particle | grains in the polyester layer in this invention with the main objective of providing lubricity. 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 of the particles include magnesium, silicon oxide, kaolin, aluminum oxide, and titanium oxide. 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.

  Further, the average particle size of the particles used is usually preferably 0.01 to 5 μm. 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 will become inferior.

  Furthermore, the particle content in the polyester is usually in the range of 0.0003 to 1.0% by weight, preferably 0.0005 to 0.5% by weight, based on the total polyester constituting the film. 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 are cases.

  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 preferably at the stage of esterification or after completion of the transesterification reaction and before the polycondensation 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 the present invention, a fluorescent whitening agent is included in the film in order to improve the luminance when used as an optical member, particularly an LCD backlight unit. The content of the optical brightener is 0.01 to 1.00% by weight, preferably 0.01 to 0.50% by weight, more preferably 0.02 to 0.30% by weight. When the content of the optical brightener is less than 0.01% by weight, the effect of improving the luminance is insufficient and the object of the present invention cannot be achieved. On the other hand, if the content exceeds 1.0% by weight, the effect of improving the brightness will be saturated, the brightness may be lowered by changing the color tone of the film, or the film may bleed out to the surface of the film. Since problems such as adverse effects during use may occur, it is not preferable.

  Furthermore, in order to prevent the bleed-out of the fluorescent whitening agent, in the present invention, it is desirable that the film has a laminated structure of three or more layers and the fluorescent whitening agent is contained in layers 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 is exhibited if the obtained film contains a predetermined amount of fluorescent brightening agent.

  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 this invention is 9-500 micrometers normally, Preferably it is 20-300 micrometers, More preferably, it is the range of 20-250 micrometers.

  The film of the present invention has a total light transmittance of 91.0% or more, preferably 91.5% or more, more preferably 92.0% or more. The film of the present invention is widely used in optical applications because of its excellent light transmittance. However, when the total light transmittance is less than 91.0%, it is unsuitable for optical use.

  The polyester film for optical use of the present invention, particularly for display, has a color tone b * value measured by a single transmission method of preferably -5 to +3, more preferably -4 to +2, particularly preferably -3. The range is 5 to +1. When the b * value exceeds +3, it is often inappropriate in terms of strong yellowness and low brightness. On the other hand, when the film is lower than -5, the bluish color is strong and the color tone of the image may be inferior when used for a display.

In the film of the present invention, the sum of the front and back surfaces of the oligomer (cyclic trimer) precipitation amount on the film surface after heat treatment at 180 ° C. for 10 minutes is preferably 15.0 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 deposition on the film surface exceeds 15.0 mg / m ² , the oligomer crystallizes on the surface to lower the transparency of the film, or it dissolves in the functional layer provided on the film and affects the properties. It may cause problems such as.

  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 in-line or off-line, the oligomer precipitation amount on the film surface after the heat treatment can be controlled within the above range by suppressing the oligomer precipitation 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 outermost layer thickness is a thickness on one side only, preferably 3 μm or more, more preferably 5 μm or more, and the total thickness. Is preferably 1/4 or less, more preferably 1/5 or less, and particularly preferably 1/10 or less. 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, content of the 3rd component in this invention needs to make the amount of 3rd component contained in the whole film with respect to polyester of the whole film into 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.

  On the other hand, when carrying out by a single layer, it is also preferable to make a film not contain particle | grains as much as possible, and to make a coating layer of front and back contain particle | grains. Moreover, when adding additives, such as the said ultraviolet absorber and dye, it is preferable to mix | blend with the intermediate | middle layer of a laminated | multilayer film.

  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 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 to 6 times at 70 to 145 ° C. in the longitudinal direction to form a longitudinal uniaxially stretched film, and then at 90 to 160 ° C. in the lateral direction. It is preferable to perform 2-6 times stretching and to heat-process at 150-240 degreeC for 1 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, the film of the present invention can be provided with a coating layer as an undercoat layer for the purpose of improving the coating property and adhesiveness when forming the prism layer and preventing static electricity for preventing dust adhesion. 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 a combination of a crosslinking agent and various binder resins, and as the binder resin, a polymer selected from polyester, acrylic polymer and polyurethane is usually employed from the viewpoint of adhesiveness. 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, you may provide an application layer by offline coating after manufacture of a film as needed. Moreover, the surface which provides an application layer does not ask | require one side and both surfaces. The coating material may be either water-based or solvent-based for off-line coating, but is preferably water-based or water-dispersed for in-line coating.

  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 adhesion due to static electricity in addition to improving adhesiveness.

  Among the polyesters, acrylic polymers, and polyurethanes that are 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. It is a polyurethane having a carboxylic acid residue, at least a part of which is made water-based using 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, stickiness 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 oxide 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 containing two or more carbon-carbon double bonds in one molecule (for example, divinylbenzene).

  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. 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, 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, 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 it is formed only on one side, an application layer different from the above-mentioned application layer can be formed on the opposite side as necessary to give other characteristics. 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. Moreover, 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 coated film is applied to an optical application, coating missing on the surface of the coating layer is a problem when a prism 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 film surface, which acts as a core and the coating agent repels, and it is stretched to generate 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 has a coating layer, the number of coating spots having a major axis of 3 mm or more is preferably 50 or less, more preferably 30 or less, and particularly preferably 10 or less, per 10 m ^{2 of} film. 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 its excellent effect particularly when used for optics, but its specific member is effective as a base material that requires high transparency of prism sheets for liquid crystal displays. Used for.

  The film of the present invention is excellent in optical properties such as transparency, low haze, and optical uniformity, and can contribute to improving the quality of optical products and reducing energy consumption, and was used as a prism sheet member. In some cases, the advanced characteristics are exhibited, and the industrial value is extremely high.

  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 third component (copolymerization component) content About a solution in which a resin sample was dissolved in a deuterated chloroform / hexafluoroisopropanol (weight ratio 7/3) mixed solvent to a concentration of 3% by weight, Using a nuclear magnetic resonance apparatus (“JNM-EX270 type” manufactured by JEOL Ltd.), 1 H-NMR was measured to assign each peak, and the content of the copolymer component was calculated from the integrated value of the peak.

(3) Measurement of average particle size (d ₅₀ : μm) Integration (weight basis) 50% in equivalent spherical distribution measured using centrifugal sedimentation type particle size distribution measuring device (SA-CP3 type manufactured by Shimadzu Corporation) Was the average particle size.

(4) Total light transmittance Total light transmittance was measured with an integrating sphere turbidimeter “NDH-300A” manufactured by Nippon Denshoku Industries Co., Ltd. according to JIS-K-7105.

(5) Suitability of optical member The characteristics when used as a prism sheet 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 and compared with the case of using the film of Comparative Example 3)
A: Brightness increased and significant improvement was observed B: No increase in brightness C: Brightness decreased

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 adding 0.04 part of ethyl acid phosphate to this reaction mixture, 0.01 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 0.68 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 0.68 and a diethylene glycol content of 1.0 mol%.

<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 2.1 μm dispersed in ethylene glycol and 0.01 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 0.66 and a diethylene glycol content of 1.0 mol%.

<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 0.68, and the diethylene glycol content in the polymer was 4.0 mol%.

<Method for producing polyester (D)>
The starting material is 100 parts by weight of dimethyl terephthalate, 54 parts by weight of ethylene glycol and 25 parts by weight of 1,4-cyclohexanedimethanol, 0.011 part by weight of tetrabutoxy titanate is used as a catalyst, and the reaction start temperature is 150 ° C. The reaction temperature was gradually increased with the distillation of methanol, and the temperature was raised to 230 ° C. after 3 hours, and the reaction was continued for another hour. Thereafter, the temperature of the reaction product was gradually raised from 230 ° C. and the pressure was gradually reduced from the normal pressure, and finally the temperature was 280 ° C. and the pressure was 0.3 mmHg. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.70 due to a change in stirring power of the reaction vessel, and the polymer was discharged under nitrogen pressure. The intrinsic viscosity of the obtained polyester (D) was 0.70, the content of 1,4-cyclohexanedimethanol was 33 mol%, and the content of diethylene glycol was 1.0 mol%.

<Method for producing polyester (E)>
99.5 parts by weight of polyester (A) and 0.5 parts by weight of brightening agent “OB-1” manufactured by Eastman Co., Ltd. are melt-kneaded using a twin screw extruder, and a master containing fluorescent brightening agent Created a batch.

<Method for producing polyester (F)>
A blend of 99.5 parts by weight of polyester (C) and 0.5 parts by weight of brightening agent “OB-1” manufactured by Eastman, and melt-kneading using a twin screw extruder, and a master containing fluorescent brightening agent Created a batch.

Example 1:
A mixed raw material obtained by mixing the polyesters (B) and (C) at a ratio of 12% and 88%, respectively, is used as the raw material for the A layer, and the polyesters (C) and (E) are 50% and 50% of the raw material is B. As raw materials for the layers, each was supplied to two bent twin-screw extruders and melted at 280 ° C., respectively, and two types and three layers with the A layer as the outermost layer (surface layer) and the B layer as the intermediate layer (A / B / A) was coextruded with a layer structure, extruded from a die, and cooled and solidified on a cooling roll set at a surface temperature of 40 ° C. using an electrostatic application adhesion method to obtain an unstretched sheet. Next, the film was stretched 3.4 times in the machine direction at a film temperature of 85 ° C. by utilizing the difference in peripheral speed of the roll, and after applying a coating agent having the composition shown below, it was led to a tenter, and 3. The film was stretched 8 times, heat treated at 230 ° C., and then relaxed 2% in the transverse direction to obtain a laminated polyester film having a thickness of 100 μm. The thickness of each layer of the obtained film was 5/90/5 μm. The thickness of the coating layer was 0.10 μm.

(Coating agent composition: weight ratio)
a / b / c / d = 47/20/30/3
Here, a is a polyester dispersion of terephthalic acid / isophthalic acid / 5-sodium sulfoisophthalic acid / ethylene glycol / diethylene glycol / triethylene glycol = 31/16/3/22/21 (molar ratio); b is Methyl methacrylate / ethyl acrylate / acrylonitrile / N-methylol methacrylamide = 45/45/5/5 (molar ratio) emulsion polymer (emulsifier: anionic surfactant); c is hexamethoxymethylmelamine (melamine type) D) is an aqueous dispersion (inorganic particles) of silicon oxide having a particle size of 0.06 μm.

Example 2:
In Example 1, the raw material of the B layer is a raw material in which the polyesters (C), (D), and (F) are mixed at a ratio of 45%, 5%, and 50%, respectively, and the A layer is the polyester (B), (C ) And (D) were used in the same manner as in Example 1 except that 12%, 83%, and 5% were mixed to obtain a polyester film having a thickness of 100 μm.
The thickness of the coating layer was 0.11 μm.

Comparative Example 1:
Example 1 except that the mixed raw materials obtained by mixing polyester (B) and (C) at a ratio of 12% and 88%, respectively, were used as the raw material for the A layer, and the raw material of 100% polyester (C) was used as the raw material for the B layer. In the same manner as above, a polyester film having a thickness of 100 μm was obtained.
The thickness of the coating layer was 0.11 μm.

Comparative Example 2:
In Example 1, a polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that a raw material of 100% polyester (F) was used as the raw material for the B layer. Since the obtained film contained too much fluorescent brightening agent, the color and haze values of the film slightly increased, and the effect of improving the luminance was no longer obtained. The thickness of the coating layer was 0.13 μm.

Comparative Example 3:
In Example 2, the raw material of the B layer is a raw material in which the polyesters (C), (D), and (F) are mixed in proportions of 20%, 30%, and 50%, respectively, and the A layer is the polyester (B), (C ), (D) A polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the raw materials were mixed in proportions of 12%, 53% and 35%, respectively. The thickness of the coating layer was 0.12 μm. The resulting film has too much content of the third component, so when used for optics, its properties deteriorate, heat resistance is insufficient, and the flatness of the film during actual use is insufficient. It was seen.

Comparative Example 4:
In Example 1, the raw material of the A layer is a raw material in which the polyesters (A) and (B) are mixed at a ratio of 88% and 12%, respectively, and the B layer is 50% and 50% of the polyester (B) and (E), respectively. A polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the raw material was mixed at a ratio of%. The thickness of the coating layer was 0.11 μm.

  The physical property values and optical member suitability of the obtained film are summarized in Tables 1 and 2 below. It can be seen that a film satisfying the requirements of the present invention has high suitability for optical use.

Abbreviations in Table 2 are as follows.
DEG: Diethylene glycol 1,4-CHDM: 1,4-cyclohexanedimethanol The contents of the third component in Example 2 and Comparative Example 3 also include DEG.

  The film of the present invention can be suitably used as a member of a prism sheet used in a liquid crystal display.

Claims (2)

Biaxial orientation for prism sheet of liquid crystal display, characterized in that it is a film comprising polyester containing 0.01 to 1.00 parts by weight of a fluorescent brightening agent, and has a total light transmittance of 91.0% or more Polyester film. The main component of the polyester is polyethylene terephthalate, and the third component is one or more selected from diethylene glycol, triethylene glycol, and 1,4-cyclohexanedimethanol, and is 2.0 to 10.0%. The biaxially oriented polyester film for a prism sheet according to claim 1, wherein the biaxially oriented polyester film is contained in a range.