JP2007156287A - Polyester film for prism sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light-scattering biaxially oriented polyester film which has antistatic property and whose productivity is excellent and which can be suitably used as the base film of a film or a sheet which easily causes the problem of deposition of dust or the like by electrostatic charges caused when peeling a protective film or the like, for example, a diffusion plate or the prism sheet of a backlight unit or the lens sheet of a projector screen. <P>SOLUTION: The light-scattering biaxially oriented polyester film contains organic particles having an average particle size of 2.0 to 40 μm by 0.10 to 10 wt.% and has surface resistivity of 5×10<SP>12</SP>Ω or lower. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a polyester film film for a prism sheet that serves as a base film for forming a prism sheet used in a liquid crystal display or the like. Specifically, the present invention relates to a polyester film for a prism sheet that is excellent in optical characteristics and can contribute to improving the quality of the prism sheet.

  As typified by the colorization, size reduction, and weight reduction of notebook personal computers in recent years, there has been a remarkable spread of size reduction, weight reduction, and colorization of portable electric devices. In particular, products using color liquid crystal displays are not limited to color notebook computers, but a wide variety of products such as portable color liquid crystal televisions, video-integrated color liquid crystal televisions, handy label printers, and portable communication devices (such as PDAs). There is.

  In such a device, a battery is used in order to be portable, and its operating time is limited. In particular, the power consumption of a liquid crystal display, especially a color liquid crystal display, greatly affects the operating time of the battery. By reducing this power consumption, the practical product value of the portable electric device described above is increased. Can do. At the same time, achieving energy savings is also meaningful in terms of the global environment.

  As a method for reducing power consumption, it is the simplest method to suppress power consumption of a backlight used in a liquid crystal display. However, in this case, since the brightness of the backlight is lowered, the brightness of the liquid crystal display is also lowered, and the display function is significantly lowered.

  In order to solve this problem, a method for improving the optical efficiency of the backlight has been proposed. For example, a backlight in which a lens sheet such as a lens sheet in which a large number of lens units in a prism array are formed on one side or a sheet in which a large number of lens units in a lenticular array are formed is provided on the output light surface side of the backlight light guide Has been proposed.

  Such a lens sheet improves the front luminance by directing light emitted from the backlight toward the front of the display by refraction. Normally, a lens sheet has a structure in which a plastic resin having excellent transparency such as a polycarbonate resin, an acrylic resin, and a polyester resin is used as a base material because of its good moldability, and a large number of lens units are formed on at least one side. ing.

  On the other hand, a polyester film used as a lens sheet, particularly a biaxially stretched film of polyethylene terephthalate or polyethylene naphthalate, has excellent mechanical properties, heat resistance, and chemical resistance.

  In order to obtain a bright and clear image in an optical product using a prism sheet, the polyester film used as a base material has good transparency from its usage pattern and is free from defects such as foreign matter and scratches that affect the image. It will be necessary. In addition to this, even when polarized light is used, it is necessary to obtain uniform images with uniform retardation due to uneven orientation of the polymer and uneven thickness.

The polyester film is usually obtained by subjecting an amorphous sheet obtained by melt-extrusion into a sheet shape and rapid solidification to stretching in the MD direction (longitudinal direction) and TD direction (transverse direction), followed by heat treatment. If the uniformity of cooling and stretching is not sufficient in these steps, the above-described retardation unevenness remains, and there is a problem that the image is deteriorated when used for optics.
Japanese Patent Laid-Open No. 11-271503

  The present invention has been made in view of the above circumstances, and the solution to the problem is that when used as a prism sheet, there is no unevenness or defect, high brightness can be achieved, and a high-quality image can be provided. The object is to provide a polyester film having good optical performance.

  As a result of intensive studies in view of the above problems, the present inventor has found that the above problems can be easily solved by a film having a specific configuration, and has completed the present invention.

  That is, the gist of the present invention is that the difference between the maximum and minimum retardation values in the MD direction of the film is 300 nm or less, and the change amount of the retardation per 1 mm in the MD direction is 10 nm or less. Lies in polyester film.

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, a third component other than the main constituent component may be contained.

  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 is usually used.

  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 as necessary under heating under reduced pressure or in an inert gas stream such as nitrogen. The intrinsic viscosity of the obtained polyester is preferably 0.40 dl / g or more, and 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 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. It may 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. 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 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, fluorescent brighteners, dyes, pigments, and the like are added to the polyester film in the present invention as necessary. be able to. Depending on the use, 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 a film haze of 0 to 3.0%, preferably 0 to 1.5%, more preferably 0 to 1.2%, and particularly preferably 0 to 1.0%. The film of the present invention is widely used for optical applications because of its excellent transparency. However, when the film haze exceeds 3.0%, it is unsuitable for optical use.

  The polyester film for optical use of the present invention, particularly for a prism sheet, 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. .5 to +1. When the b * value exceeds +3, it is often inappropriate because the color tone of the image becomes inferior or the brightness becomes low when used as a prism sheet because of its strong yellowishness. On the other hand, although there is a problem of color tone in a film lower than -5, since a b * value is not lower than -5 in a normal polyester film, a method such as using an additive is used. Then, there may be a problem in the bleed out of the additive and the 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 not heated to an unnecessarily high temperature during polymerization and film formation. It is possible to adopt methods such as reducing the melting time, reducing the blending amount of the regenerated raw material, and the like.

  The film of the present invention can be formed into a laminated structure using a co-extrusion method. In this case, the outermost layer thickness is a thickness on one side only, usually 3 μm or more and 1/4 or less of the total thickness. preferable. 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.

  On the other hand, when the present invention is carried out with a single layer, it is also preferred that the film contains as little particles as possible and that the front and back coating layers contain particles.

  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.

  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.

  The film of the present invention thus obtained is used for a prism sheet, and at that time, it is necessary that the optical unevenness of the film due to the orientation and thickness unevenness of the polymer is reduced, specifically, The difference between the maximum and minimum retardation values in the MD direction of the film measured by the method described later is 300 nm or less, and the amount of change in retardation per 1 mm MD is 10 nm or less. Preferably, the difference between the maximum value and the minimum value in the MD direction is 200 nm or less and the amount of change in retardation per MD 1 mm is 8 nm or less, more preferably the difference between the maximum value and the minimum value in the MD direction is 150 nm or less and the retardation per MD 1 mm Is 5 nm or less. When satisfying such requirements, it is possible to obtain a product having excellent image clarity, brightness and uniformity when used for prism sheet applications.

Here, retardation refers to the optical properties of the film represented by the following formula.
Re = Δn · d
(In the above formula, Δn is the birefringence of the film in the 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 speeds pass. Since the speeds are different, a phase difference occurs between the two polarized waves. This phase difference is called retardation (hereinafter referred to as “Re”).

  This Re value affects the interference color and light transmission in the visual inspection by the crossed Nicols method. As can be seen from the Michel-Levy interference color chart, when the Re value is fluctuated, the color unevenness occurs. Observed.

  In the present invention, a specific prescription satisfying that the difference between the maximum value and the minimum value of Re of the polyester film is 300 nm or less and the change amount per MD of 1 mm is 10 nm or less is, for example, that the thickness unevenness is as small as possible. It is preferable to obtain a stretched film. Further, the thinner unstretched film is less likely to cause uneven thickness and partial spherulite formation due to insufficient cooling, so that the above requirements are satisfied. Specifically, when the thickness of the unstretched film exceeds 2.5 mm, it is difficult to obtain a desirable unstretched film.

  Further, in roll stretching of sequential biaxial stretching, single-sided heating is alternately performed in the preheating step and the stretching step, so that temperature distribution in the thickness direction is likely to occur, and it is often difficult to keep Re uniform. . 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.

  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 to 6 times at 70 to 145 ° C. in the MD direction to form a longitudinal uniaxially stretched film, and then at 90 to 160 ° C. in the TD 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, since the film of the present invention is used for prism sheet applications, it is intended to improve coating properties and adhesion when forming a processed layer, or to prevent charging in order to keep the surface clean. A coating layer as a pulling layer can be provided. In forming such a coating layer, a method of performing a film manufacturing process, particularly stretching in the longitudinal direction and before stretching in the lateral direction is called in-line coating, and a very thin coating layer can be formed. This is preferable in that the drying and curing reaction of the coating solution 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.

  In addition, you may coat by an offline coat after manufacture of a film as needed. Moreover, it does not matter on one side or both sides. The coating material may be either water-based and / or solvent-based for offline 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 a prism sheet, it is also preferable to form a functional multilayer thin film in addition to improving adhesion, preventing reflection of external light and dust due to static electricity.

  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, 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 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. 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 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 preferable 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 also has the effect of reducing the amount of coating waste.

  The film of the present invention thus obtained needs to have a difference between the maximum value and the minimum value of retardation in the film MD direction of 300 nm and a change amount per MD of 1 mm of 10 nm or less. When this condition is not satisfied, color unevenness when the film is observed through polarized light is large, which causes a problem of LCD screen display using the prism sheet.

  The film of the present invention is excellent in optical properties such as transparency, low haze, and optical uniformity, and can contribute to improvement in backlight quality and energy consumption, and was used as a prism sheet film. In that case, 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 Retardation (Re) Using a cell gap inspection apparatus RETS-1100A manufactured by Otsuka Electronics Co., Ltd., 50 points were measured every 20 mm in the film MD direction. The difference between the maximum value and the minimum value of the measured values, and the maximum value among the values obtained by differentiating the obtained values (the following formula) was defined as the amount of change per 1 mm of film. The retardation was measured at 23 ° C. using an optical interference method with an aperture diameter of 5 mm.
(Derivation of differential value)
When the Re value (nm) at points x (mm) and x + Δx (mm) is f (x) and f (x + Δx), the differential value is | (f (x + Δx) −f (x)) / Δx. |

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

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

(4) Thickness Determined by a micrometer.

(5) Thickness variation of the film Measured along the longitudinal direction and lateral direction of the biaxially stretched film with an anritsu continuous film thickness measuring instrument (using an electronic micrometer). Calculated from the formula.
Thickness unevenness = (maximum thickness-minimum thickness) / average thickness x 100 (%)

(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 placed under crossed Nicols, and the influence of light interference was evaluated with a white light source according to the following criteria. That is, FIG. 1 is a schematic explanatory diagram of this visual inspection by the crossed Nicols method. Evaluation was performed according to the following classification.
○: Color unevenness due to light interference cannot be observed. Δ: Color unevenness due to light interference is present, but there is no problem in use. ×: Problems due to color unevenness due to light interference.

(8) Suitability of optical member As a representative optical member, characteristics when used as a prism sheet were evaluated. That is, an acrylic binder was applied to one side of the film to form a prism layer. The obtained prism sheet was incorporated into a backlight unit, and the quality of the obtained planar light emission was evaluated from the following viewpoints.
○: Color unevenness is hardly seen Δ: Color unevenness is slightly seen but practical level ×: Color unevenness is remarkable and affects the image on the liquid crystal display

(9) Comprehensive evaluation The sample film was comprehensively evaluated according to the following criteria.
<Criteria>
A: The difference between the maximum value and the minimum value of Re is 150 nm or less, the amount of change per 1 mm of MD is 5 nm or less, and the brightness, visual inspection evaluation, and optical member suitability are all good (very good).
○: The difference between the maximum value and the minimum value of Re is 300 nm or less, the amount of change per MD 1 mm is 10 nm or less, and the brightness, visual inspection evaluation, and optical member suitability are all good (good).
Δ: Brightness, visual inspection evaluation, or optical member suitability is Δ.
X: Brightness, visual inspection evaluation, or optical member suitability is x (defect)

  Examples / Comparative Examples are shown below, and the production method of the polyester used in this example is as follows.

<Manufacture of polyester>
(Method for producing polyester A)
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 that 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 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 0.65.

(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. Four hours later, 0.45 parts of silica particles having an average particle size of 3.0 μm dispersed in an ethylene glycol slurry were added to the reaction mixture which had been substantially transesterified, and ethyl acid phosphate 0.04 was added. Part 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.

Example 1:
Two vented twin-screw extrusions using the above-mentioned polyester A and B 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. The thickness of the obtained unstretched film was 2.3 mm. Next, the film was stretched 3.1 times in the longitudinal direction at a longitudinal stretching temperature of 83 ° C. using the difference in peripheral speed of the roll. 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 188 μm. The thickness of each layer of the obtained film was 10/168/10 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) (Crosslinking agent); d is an aqueous dispersion (inorganic particles) of silicon oxide having a particle size of 0.06 μm
It is.

Example 2:
In Example 1, a polyester film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the transverse draw ratio was 3.6 times.

Example 3:
In Example 1, a polyester film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the longitudinal draw ratio was 3.3 times.

Comparative Example 1:
A polyester film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the longitudinal stretching temperature was 79 ° C. and the lateral stretching ratio was 3.6 times in Example 1.

Comparative Example 2:
A polyester film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the longitudinal stretching ratio in Example 1 was 3.3 times, the longitudinal stretching temperature was 79 ° C., and the lateral stretching ratio was 3.6 times.

Comparative Example 3:
A polyester film having a thickness of 188 μm was obtained in the same manner as in Example 1 except that the longitudinal stretch ratio was 3.5 and the lateral stretch ratio was 4.0 in Example 1.

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

  In Table 1, the difference in Re indicates the difference between the maximum value and the minimum value in the measurement data, and the amount of change in Re indicates the amount of change per MD 1 mm.

  The film of the present invention can be suitably used as a base film for forming a prism sheet used for, for example, a liquid crystal display.

It is a schematic diagram which shows the visual inspection by the cross Nicol method. The film was sandwiched between polarizing plates (polarizer / analyzer) crossed at right angles, and inspected with a white light source.

Explanation of symbols

1: White light source 2: Analyzer 3: Film 4: Polarizer 5: Eye of person to be inspected

Claims (1)

A polyester film for a prism sheet, wherein the difference between the maximum value and the minimum value of retardation in the film MD direction is 300 nm or less, and the amount of change in retardation per mm direction in the MD direction is 10 nm or less.

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