JP2007191572A - Polyester film for diffusion plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high quality optical diffusion plate which can give a high quality image without unevenness of retardation and defects such as foreign matter and scratches when used as a liquid crystal display (LCD) screen. Provide a polyester film.
A retardation value is in a range of 2000 to 4500 nm, an average value of a * values in crossed Nicols is 2.0 or less, and a maximum value thereof is 4.0 or less. Polyester film for diffusion plate.
[Selection figure] None

Description

  The present invention relates to a polyester film for a diffusion plate, and more particularly to a polyester film for a diffusion plate having excellent optical quality.

  Polyester film is excellent in heat resistance, water resistance, chemical resistance, mechanical strength, dimensional stability, etc., and has been used for various industrial applications, and its applications are expanding and diversifying. With such diversification, the required characteristics have become increasingly severe, but the current situation is that they have not been fully satisfied.

  For example, a polyester film is used as a base substrate of a diffusion plate for a liquid crystal display (LCD). In order to obtain bright and clear images in optical products using these polyester films, the base film used as an optical film has good transparency and foreign matter that affects the image because of its usage. There must be no defects such as scratches. In addition to this, even when polarized light is used, in order to obtain an image having no unevenness, it is necessary that there is no retardation unevenness due to uneven orientation of the polymer or uneven thickness. In addition, there is a problem that the color derived from the retardation of the film itself is emphasized due to the polarization state of the light transmitted through the base film, resulting in poor color development of the optical product.

Conventionally, a polyester film used for a diffusion plate is usually obtained by subjecting an amorphous sheet obtained by melt-extrusion into a sheet shape and rapid solidification to stretching in the machine direction and the transverse direction, followed by heat treatment. If the uniformity of cooling and stretching is not sufficient in these steps, there is a problem that the image in the optical product is deteriorated due to the color derived from the above-described retardation.
JP 2001-337201 A

  The present invention has been made in view of the above circumstances, and a solution to the problem is to provide a polyester film for a diffusion plate that is free from unevenness and defects when used as an LCD screen and can provide a high-quality image. There is to do.

  As a result of intensive studies in view of the above problems, the present inventor has found that a film having a specific configuration can provide a high-quality image, and has completed the present invention.

  That is, the gist of the present invention is that the retardation value is in the range of 2000 to 4500 nm, the average value of a * values in crossed Nicols is 2.0 or less, and the maximum value is 4.0 or less. It exists in the polyester film for diffusion plates characterized by these.

Hereinafter, the present invention will be described in 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, a third component other than the main constituent component can be contained in a range of 10 mol% or less. 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. When these third components are used, optical unevenness of the film due to polymer orientation and thickness unevenness can be reduced, and the flatness, heat resistance and dimensional stability of the film can be maintained at a high level. preferable.

  Moreover, it is preferable that the polyester film of this invention contains both a titanium compound and a phosphorus compound. The content of titanium element in at least one layer of the film of the present invention is preferably 20 ppm or less, more preferably 10 ppm or less, and the lower limit is usually 1 ppm, preferably 2 ppm. When there is too much content of a titanium compound, an oligomer byproduces in the process of melt-extruding polyester, and it may be impossible to obtain a film having low transparency and high transparency. Further, when no titanium element is contained, the productivity at the time of producing the polyester raw material tends to be inferior, and the polyester raw material reaching the target degree of polymerization may not be obtained. On the other hand, the amount of phosphorus element is preferably 1 ppm or more, preferably 5 ppm or more, and the upper limit is 300 ppm, preferably 200 ppm, more preferably 100 ppm. By containing a specific amount of the above-described titanium compound and containing a phosphorus compound, it is possible to exert a remarkable effect on the reduction of the contained oligomer. When there is too much content of a phosphorus compound, gelatinization will occur and it may become a foreign material and cause the quality of a film to fall. In the present invention, when the titanium compound and the phosphorus compound are contained in the above-described range, the by-product of the oligomer can be prevented, and the effect of the present invention is highly obtained.

  Further, the layer containing the titanium compound and the phosphorus compound preferably contains no antimony element, usually 100 ppm or less, preferably 60 ppm or less, and most preferably substantially free, ie 10 ppm or less. If the amount of the antimony element is too large, it may be reduced by the phosphorus compound during melt extrusion and aggregate to cause foreign matters, or the film may become dark and the transparency may be impaired.

  The film of the present invention satisfies the above problems to a high degree, and exhibits a particularly excellent effect for this application. That is, by using a titanium compound as a catalyst, darkening as in the case of using antimony can be prevented, and a film having excellent transparency can be obtained, and by adding a phosphorus compound at the same time, the color tone of the film can be improved. Demonstrate the effect of not worsening. Specifically, the b value of the film increases. That is, when the film is used in this application when the film becomes more yellowish, the brightness may decrease and the image may deteriorate, whereas the film of the present invention exhibits excellent characteristics in terms of transparency and color tone. To do.

  In the present invention, for example, by mixing a polyester produced using titanium as a catalyst and a polyester containing a predetermined amount of a phosphorus compound, a film having the above-described amounts of titanium and phosphorus can be obtained. , Can exhibit its excellent effect. That is, when a polyester is produced by polymerization using titanium as a catalyst, adding a phosphorus compound in the polymerization process slows down the polymerization rate, leading to a decrease in productivity and a longer polymerization time. May become a problem. On the other hand, when the polyester of the titanium catalyst is subjected to a melt extrusion process in which a film is produced in the absence of a phosphorus compound, there is a problem that yellowness increases due to thermal deterioration. In the present invention, in order to prevent the occurrence of such a problem, a method in which a polyester containing a predetermined amount of a phosphorus compound is prepared as a master batch and mixed with a polyester using a titanium catalyst is preferable. Examples of the method for producing a master batch of a phosphorus compound include a method of polymerizing with a germanium catalyst, a method of polymerizing with a minimum amount of an antimony catalyst, and a method of adding in a step of melt extrusion to a polyester with a titanium catalyst. It is particularly preferable to employ a catalyst.

  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.

  The polyester used in the present invention may be chipped after melt polymerization, and further subjected to solid layer polymerization as necessary under heating and reduced pressure or in an inert gas stream such as nitrogen. The intrinsic viscosity (IV) of the polyester used in the present invention is usually 0.52 to 0.75, preferably 0.55 to 0.70, and more preferably 0.58 to 0.67. When the IV value is less than 0.52, heat resistance, mechanical strength, and the like, which are excellent characteristics of the polyester film when used as a film, tend to be inferior. On the other hand, when the IV value exceeds 0.75, the load may be excessively increased in the extrusion process during the production of the polyester film, and the productivity may be lowered.

  You may mix | blend particle | grains in the polyester layer in this invention with the main objective of providing lubricity. The type of particles to be blended is not particularly limited as long as it is a particle capable of imparting slipperiness, and specific examples include, for example, silica, calcium carbonate, magnesium carbonate, barium carbonate, calcium sulfate, calcium phosphate, kaolin, Examples of the particles include 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 if necessary.

  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. May be inferior.

  Further, 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% by weight, the slipperiness of the film may be insufficient. On the other hand, when the content exceeds 1.0% by weight, the transparency of the film is poor. It may be enough.

  The method of 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 method of blending dried particles and a polyester raw material using a kneading extruder. It is performed by the method of blending.

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

  The polyester film as used in the present invention is a film extruded by a single layer extrusion method or a coextrusion method in which all layers are single or co-melt extruded from a die, and then biaxially stretched in the vertical and horizontal directions, and then heated as necessary. Refers to fixed objects. The film 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 500 μm, preferably 20 to 350 μm, and more preferably 50 to 250 μm.

  The film of the present invention has a total light transmittance of usually 91.0% or more, preferably 91.5% or more, more preferably 92.0% or more. The film of the present invention needs to have excellent light transmittance. If the total light transmittance is less than 91%, it may be inappropriate for this application.

  The film haze of the film of the present invention is usually 0 to 5.0%, preferably 0 to 3.5%, and more preferably 0 to 3.0%. The film of the present invention needs to have excellent transparency, and if the film haze exceeds 5.0%, it may be inappropriate for this application.

  The film of the present invention is required to have a retardation value of 2000 to 4500 nm, preferably 2500 to 4500 nm, and more preferably 2800 to 4500 nm. Further, it is necessary that the average value of the a * values is 2.0 or less and the maximum value is 4.0 or less. When the retardation value is smaller than 2000 nm, even if the retardation value slightly fluctuates, it is observed as uneven color on the LCD screen. In addition, when the retardation value exceeds 4500 nm, when observed through a polarizing plate, a strong red color is developed, and the color tone of the liquid crystal screen is remarkably deteriorated. It becomes impossible to assemble the backlight.

  In addition, when the average value of a * values under crossed Nicols is larger than 2.0, red color is observed on one side when observed with a polarizing plate interposed therebetween, and the average value of a * values under crossed Nicols is When the ratio is larger than 4.0, a partial red color may be observed, and it is difficult to form a diffusion plate with high optical quality.

  The film in the present invention is often difficult to obtain with the full width of the tenter due to the film production method, but the lateral range that satisfies the above requirements only needs to be satisfied with the product width of the film roll. Specifically, it may be satisfied in the range of 500 mm or more, preferably in the range of 600 mm or more.

Here, retardation refers to the optical properties of the film represented by the following formula.
Retardation value = Δn · d
In the above formula, Δn is the birefringence of the film with visible light (wavelength λ = 589 nm), and d is the thickness (nm) of the film.

  That is, when linearly polarized light is incident on a birefringent body such as a stretched polyester film, generally only two polarized waves having vibration directions orthogonal to each other and having different velocities pass. Since the speeds are different, a phase difference occurs between the two polarized waves. This phase difference is called retardation.

  This retardation value affects the interference color and light transmission in the visual inspection by the crossed Nicols method, and a color corresponding to the retardation value is observed as can be understood from the interference color chart of Michel-Levy. .

  Since the light transmitted through the diffuser base film has some polarization components due to the light source, reflector, and light guide plate of the backlight unit, it is necessary to design the film in consideration of the retardation value. .

  In the present invention, as a specific prescription in which the retardation value of the polyester film is within the above range, the film may be formed at a stretch ratio corresponding to the thickness of the film. This requirement may not be met with a film whose stretch ratio is biased in either the vertical or horizontal direction.

  As a specific prescription in which the average value of a * values is 2.0 or less and the maximum value is 4.0 or less, an unstretched film with as little thickness unevenness as possible may be obtained. Specifically, unstretched The thickness unevenness of the film is preferably 1.5% or less, more preferably 1.0% or less.

  Moreover, in roll stretching of sequential biaxial stretching, single-sided heating is alternately performed in the preheating step and the stretching step, so that a temperature distribution in the thickness direction is likely to occur, and it is often difficult to keep the retardation uniform. is there. In particular, as the film thickness increases, the temperature difference between the front and back of the film tends to increase further. From this point of view, it is desirable to perform sufficient preheating so as to reduce the temperature difference between the front and back sides and to optimize the stretching conditions in order to satisfy the above requirements in the present invention.

  Further, the higher the draw ratio is, the better the thickness flake becomes. Therefore, since the color in crossed Nicol in a certain area approaches uniformly, the maximum value of the a * value tends to be small.

  The polyester film of the present invention has a color tone b * value measured by a single transmission method of preferably −5 to +3, more preferably −4 to +2, and particularly preferably −3.5 to +1. It is. When the b * value exceeds +3, the yellow color becomes strong, and when used as a display, the color tone of the image may become inferior or the luminance may become low. is there. On the other hand, although there is a problem of color tone in a film smaller than −5, in a normal polyester film, the b * value is never lower than −5, and a method using an additive or the like is used. This method may cause problems due to bleeding out of the additive and reliability during long-term use. In order to obtain a film having such a color tone, the catalyst and auxiliary agent for producing the raw material polyester are selected, the amount of the catalyst is reduced as much as possible, and the polyester is heated to an unnecessarily high temperature during polymerization and film formation. Methods such as preventing the melting time from becoming long, and reducing the blending amount of the regenerated raw material can be employed.

  The thickness unevenness of the film of the present invention is preferably 5% or less of the average thickness. If the thickness unevenness exceeds 5%, the display quality may deteriorate due to retardation flare, and blistering and sagging may occur after processing, resulting in poor flatness and the possibility of being unable to be used as a diffusion plate.

  The film may be subjected to a discharge treatment or a subbing coating layer for the purpose of winding up the film during the production of the present invention, for easy adhesion of the coating agent for the diffusion plate, or for imparting antistatic properties.

  Although the manufacturing method of the polyester film for platemaking of this invention is demonstrated concretely below, as long as the structural requirements of this invention are satisfied, it 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 to 6 times in the longitudinal direction at 70 to 145 ° C. to form a longitudinal uniaxially stretched film, and then in the transverse direction at 90 to 160 ° C. 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, 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 a layer structure, an A / B structure using an A raw material and a B raw material, or an A / B / A structure, and a C raw material may be used to form an A / B / C structure or other film. it can. 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. 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.

  When processing a single layer or laminated film of the invention into a diffusion plate, it is applied as a subbing to improve necessary properties according to the required properties, such as easy printing adhesion, antistatic properties, weather resistance and surface hardness. A layer can be provided. In forming such a coating layer, a method called in-line coating, which is performed in the process of producing a film, particularly after stretching in the longitudinal direction and before stretching in the lateral direction, can form a very thin coating layer, This is preferable in that the drying and curing reaction can be performed 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.

  If necessary, the film may be coated by a method called offline coating which is coated after the production of the film. Moreover, single side | surface and both surfaces do not ask | require. The coating material may be either water-based and / or solvent-based for off-line coating, but is preferably water-based or water-dispersed for in-line coating.

  According to the present invention, when used as an LCD screen, there can be provided a polyester film for a diffusion plate that is free of unevenness and defects and can provide a high-quality image, and the industrial value of the present invention is high. .

  EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist. In addition, the measurement and determination of various characteristics in the present invention were performed by the following methods.

(1) Measurement of retardation (Re) Using a cell gap inspection apparatus RETS-1100A manufactured by Otsuka Electronics Co., Ltd., a total of 25 points of 5 points in the vertical direction and 5 points in the horizontal direction were measured at intervals of 50 mm. The average value of the measured values was used as the retardation value. The retardation was measured at 23 ° C. using an optical interference method with an aperture diameter of 5 mm.

(2) Measurement of color a * value in crossed Nicols Using a cell gap inspection apparatus RETS-1100A manufactured by Otsuka Electronics Co., Ltd., a total of 25 points of 5 points in the vertical direction and 5 points in the horizontal direction are measured at intervals of 50 mm, and 25 points The average value and the maximum value were calculated. The inclination of the polarizer and the analyzer was 90 degrees (crossed Nicols), and the inclination of the optical axis of the sample and the polarizer was 45 degrees. The aperture diameter was 5 mm, and the test was performed at 23 ° C. The light source was a D65, 10 degree field of view.

(3) Film haze, total light transmittance According to JIS-K7105, the turbidity and total light transmittance of the film were measured with the Nippon Denshoku Industries Co., Ltd. divisional turbidimeter NDH-1001DP.

(4) b * value Using a color analyzer TC-1800MKII type manufactured by Tokyo Denshoku Co., Ltd., color tone was measured according to the method of JISZ-8722.

(5) Thickness unevenness of film Measured along the longitudinal and lateral directions of unstretched film and biaxially stretched film with an anritsu continuous film thickness meter (using an electronic micrometer) It was calculated from the following formula.
Thickness unevenness = (maximum film thickness−minimum film thickness) ÷ average film thickness × 100

(5) Film thickness Determined by a micrometer.

(6) Brightness A film was placed on a flat illuminator manufactured by Dentsu Sangyo Co., Ltd., and the method of light transmission was visually inspected to determine the following.
○: Even when a film is placed, the transmitted light maintains almost the same brightness as the flat illuminator. △: When the film is placed, the light can be seen as slightly whitish and cloudy. ×: When the film is placed. , Something that makes the light cloudy

(7) Visual inspection The film was arrange | positioned under crossed Nicols, and the influence of light interference was evaluated with the following references | standards with the white light source. Evaluation was performed according to the following classification.
○: Uneven color due to light interference and / or red coloration cannot be observed Δ: Uneven color due to light interference and / or red coloration is present but there is no problem in use ×: Due to color unevenness and / or red coloration due to light interference Malfunction

(8) Suitability of optical member Properties 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 resulting planar light emission unevenness was evaluated from the following viewpoints.
Unevenness of brightness A: Unevenness is hardly seen B: Unevenness is seen slightly, but practical level C: Unevenness is noticeable and affects the image on the liquid crystal display

(9) Comprehensive evaluation The sample film was comprehensively evaluated according to the following criteria.
<Criteria>
○: Retardation value is 2000 nm or more and 4500 nm or less, average value of a * value in crossed Nicols is 2 or less, and maximum value of a * in crossed Nicols is 4 or less, brightness, visual inspection evaluation, optical All material suitability is ○ (good)
Δ: Brightness, visual inspection evaluation, or optical member suitability is Δ ×: Brightness, visual inspection evaluation, or optical member suitability is x (defect)

The raw materials used in the following examples were prepared by the following method.
(Method for producing polyester A)
Using 100 parts by weight of dimethyl terephthalate, 60 parts by weight of ethylene glycol and 2 parts by weight of diethylene glycol as starting materials, tetrabutoxytitanate as a catalyst is taken into the reactor, the reaction start temperature is 150 ° C., and the reaction temperature gradually increases with the distillation of methanol. Was raised to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially completed, and then a polycondensation reaction was performed 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 0.63 due to a change in stirring power of the reaction tank, and the polymer was discharged under nitrogen pressure. The obtained polyester (A) had an intrinsic viscosity of 0.63 and a diethylene glycol content of 4.0 mol%.

(Method for producing polyester B)
100 parts by weight of dimethyl terephthalate and 60 parts of ethylene glycol are used as starting materials, 0.09 parts by weight of magnesium acetate tetrahydrate as a catalyst is taken in the reactor, the reaction start temperature is 150 ° C., and the methanol is distilled off gradually. The reaction temperature was raised to 230 ° C. after 3 hours. After 4 hours, this reaction mixture which had substantially completed the transesterification reaction, 0.04 part of ethyl acid phosphate and 0.04 part of antimony trioxide were 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 4 hours from the start of the reaction, the reaction was stopped and the polymer was discharged under nitrogen pressure. The intrinsic viscosity of the obtained polyester A was 0.65.

(Method for producing polyester C)
100 parts by weight of dimethyl terephthalate and 60 parts of ethylene glycol are used as starting materials, 0.09 parts by weight of magnesium acetate tetrahydrate as a catalyst is taken in the reactor, the reaction start temperature is 150 ° C., and the methanol is distilled off gradually. The reaction temperature was raised to 230 ° C. after 3 hours. After 4 hours, 0.45 parts of silica particles having an average particle diameter of 3.0 μm dispersed in an ethylene glycol slurry were added to the reaction mixture which had been substantially transesterified, and 0.04 part of ethyl acid phosphate was added. Then, 0.04 part of antimony trioxide was added, and a polycondensation reaction was performed 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 4 hours from the start of the reaction, the reaction was stopped and the polymer was discharged under nitrogen pressure. The intrinsic viscosity of the obtained polyester A was 0.63.

Example 1:
Two bent-type twin-screw extrusions using the above-mentioned polyester A and C mixed at a ratio of 88 parts by weight and 12 parts by weight, respectively, as the raw material for the A layer and 100 parts by weight of the polyester A as the raw material for the B layer Each was supplied to a machine, melted at 285 ° C., and coextruded in a layer configuration of two types and three layers (A / B / A) with the A layer as the outermost layer (surface layer) and the B layer as the intermediate layer. The film was cooled and solidified on a casting drum cooled to 0 ° C. to obtain an unstretched film. Subsequently, using the difference in peripheral speed of the roll, the film was stretched 3.4 times in the longitudinal direction at a longitudinal stretching temperature of 83 ° C. The longitudinal stretching temperature was measured using an infrared radiation thermometer. Then, after applying the coating agent shown below, the film was guided to a tenter, stretched 4.0 times in the transverse direction at 120 ° C, heat-treated at 225 ° C, and then subjected to a 2% relaxation treatment in the transverse direction, followed by film Was rolled up to obtain a polyester film having a thickness of 100 μm. The thickness of each layer of the obtained film was 10/80/10 μm. The thickness of the coating layer was 0.08 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, a polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the film was stretched 3.6 times in the longitudinal direction.

Example 3:
A polyester film was obtained in the same manner as in Example 1 except that the film was stretched 3.8 times in the lateral direction in Example 1 and the film thickness was 190 μm. The thickness of each layer of the obtained film was 10/170/10 μm.

Comparative Example 1:
A polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that polyester B was used instead of polyester A in Example 1.

Comparative Example 2:
When an unstretched film was formed, a polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 with a melting temperature of 283 ° C. Since the uneven thickness of the unstretched film was worse than that of Example 1, color unevenness by visual inspection was observed.

Comparative Example 3:
A polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the film was stretched 3.3 times in the longitudinal direction in Example 1. Since the obtained film had a large retardation value, a red color on one side was observed by visual inspection.

Comparative Example 4:
A polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1, except that the film was stretched 3.2 times in the vertical direction and 3.6 times in the horizontal direction. The obtained film had a large maximum value of a *, and local red coloration was observed by visual inspection.

Comparative Example 5:
In Example 1, the film was stretched in the same manner as in Example 1 except that the film was stretched 3.5 times in the vertical direction and 3.7 times in the horizontal direction. The obtained film had a low retardation value, and was inferior to unevenness in visual inspection and unevenness in luminance in optical member suitability evaluation.
The obtained results are summarized in Table 1 below.

  The film of the present invention can be suitably used as a polyester film for a diffusion plate, for example.

Claims (1)

Retardation polyester having a retardation value in the range of 2000 to 4500 nm, an average value of a * values in crossed Nicols of 2.0 or less, and a maximum value of 4.0 or less the film.