JP6624184B2 - White polyester film for liquid crystal display, method for producing the same, and backlight for liquid crystal display - Google Patents

Description

  The present invention relates to a white polyester film suitable for use in a liquid crystal display. The present invention relates to a white polyester film which can be suitably used for a backlight device for image display and a reflection sheet of a lamp reflector, and particularly suitable for use as a reflection plate of a backlight device for image display.

In a flat-panel image display system used for liquid crystal displays, etc., white polyester film has uniform and high brightness for applications such as reflectors and reflectors for surface light source devices, back reflectors for lighting signs, and back reflectors for solar cells. It is widely used because of its properties such as thermal dimensional stability and low cost.

  In a liquid crystal display device for a liquid crystal television or a personal computer, the heat generation amount in the device increases due to the miniaturization and high performance, and the heat generation amount of a light source (for example, an LED) used for the backlight of the liquid crystal display. When the backlight was used for a long time, the phenomenon that the white film was deformed in a wavy manner and the screen became uneven was clarified. In particular, recently, the size of a liquid crystal display has been increased, and a waving phenomenon and screen unevenness have become serious problems.

  As a method of expressing high brightness, a polyester film contains a large number of inorganic particles such as barium sulfate, and uses light reflection at the interface between the polyester resin and the particles and at the cavity interface of the fine cavities generated with the particles as nuclei. (See Patent Document 1), a method using light reflection at a cavity interface of fine cavities generated by using an incompatible resin as a nucleus by mixing a polyester and an incompatible resin (Patent Document 2). Inorganic gas contained in a polyester film, such as a method of impregnating a polyester film with an inert gas in a pressure vessel to utilize light reflection at an interface of a cavity formed inside (see Patent Document 3). A method utilizing the difference in the refractive index between the particles and the polyester resin and the difference in the refractive index between the fine cavities and the polyester resin is widely used.

  As a study to reduce the initial screen unevenness in a large TV, a method of reducing the variation in the light transmittance in the longitudinal direction and the width direction of the film (see Patent Document 4), and reducing the glossiness by containing inorganic particles on the surface. A method is being studied (see Patent Document 5).

  However, in these methods, apparent specific gravity tends to be light, cushioning property and insulating property are high, and the cost per unit area is advantageous, but the thermal dimensional stability is often inferior. When a large-sized TV using an LED backlight which is likely to be heated to a high temperature is used for a long time, there is a problem that a film is wavy and a screen becomes uneven, so that there is a limit in thinning.

JP 2003-160682 A Japanese Patent Publication No. 8-16175 JP 2001-166295 A JP 2008-257229 A JP 2011-209499 A

According to the present invention, a film waving phenomenon and screen unevenness caused by use inside a liquid crystal display, particularly inside a liquid crystal display using an LED backlight are improved. This aims to maintain high reflection performance during long-term use.

The white polyester film of the present invention that solves the above problems has the following configuration. That is,
(A)
What is claimed is: 1. A white polyester film having at least two layers: a layer A containing substantially no voids and a layer B containing voids, wherein the white polyester film satisfies the following requirements (1) to (4): White polyester film for display,
(1) The layer B contains polyester containing an incompatible resin and has bubbles,
(2) The B layer has a basic configuration of polyethylene terephthalate, contains at least one or more copolymerized polyester, and the copolymerized polyester is at least 1.0% by mass and at most 20.0% by mass based on the mass of the B layer. ,
(3) the apparent density of the film is 0.5 g / cm ³ or more and 1.1 g / cm ³ or less;
(4) The heat shrinkage measured by thermomechanical analysis at 40 ° C. to 100 ° C. in the longitudinal direction and the width direction of the film is −0.5% or more and 0.0% or less;
(B)
The white polyester film for a liquid crystal display according to (A), wherein the content of the immiscible resin in the polyester of the polyester B layer is 5% by mass or more and 25% by mass or less based on the mass of the layer B.
(C)
The white polyester film for a liquid crystal display according to (A) or (B), wherein at least one surface has a relative reflectance at a wavelength of 560 nm of 97.0% or more,
(D)
The white polyester film for a liquid crystal display according to any one of (A) to (C), wherein the total thickness of the white polyester film is 50 μm or more and 500 μm or less,
(E)
The polyester constituting the B layer is basically composed of polyethylene terephthalate, a copolymer of polyethylene terephthalate and isophthalic acid, a copolymer of polyethylene terephthalate and cyclohexane dimethanol, and a copolymer of polybutylene terephthalate and polytetramethylene terephthalate A white polyester film for a liquid crystal display according to any one of (A) to (D), comprising at least two or more copolymerized polyesters selected from the group consisting of:
(F)
(A) to (E) are obtained by biaxially stretching a film containing a polyester and a component incompatible with the polyester to generate cavities in the film, and then performing a heat treatment at 100 to 200 ° C. for 5 to 50 seconds. A white polyester film for a liquid crystal display according to any one of the above,
(G)
(A) A backlight for a liquid crystal display, wherein the white polyester film for a liquid crystal display according to any one of (A) to (F) is used as a light reflecting material.
(H)
The backlight for a liquid crystal display according to (G), wherein the light source of the backlight is an LED system.
It is.

ADVANTAGE OF THE INVENTION According to this invention, the ripple phenomenon of a film and screen unevenness which arise by using inside a liquid crystal display, especially the liquid crystal display using LED backlight can be improved. Thereby, high reflective performance can be maintained during long-term use.

FIG. 2 is a diagram illustrating a relationship between temperature and heat shrinkage by thermomechanical analysis in the present invention (Example 1).

  The white polyester film of the present invention is a white polyester film having at least two layers, that is, a layer A made of polyester and having substantially no voids and a layer B containing voids.

[Layer A]
In the present invention, the layer A contains substantially no bubbles. The phrase "substantially free of bubbles" means that the layer has a porosity of less than 10%. The thickness of the A layer and the B layer is determined by observing a cross section of the layer with an electron microscope and substantially including bubbles from the surface. The thickness of the layer containing substantially no air bubbles is defined as the thickness of the layer A, and the thickness of the layer containing the air bubbles (cavities) is defined as the thickness of the layer B.

  The A layer preferably contains inorganic particles in the polyester, and has a role of scattering light. Further, it has a role of preventing light leakage to the back surface and a role of a support layer for stabilizing film formation.

  The light scattering property of the A layer can be adjusted mainly by controlling the surface roughness. As another method, for example, a method of adding particles having a different refractive index to a polyester resin can be mentioned.

  Here, the type of the inorganic fine particles contained in the layer A is not particularly limited, but preferably has a Mohs hardness of 3.0 or more. For example, calcium carbonate, titanium dioxide, zinc oxide, silicon dioxide, zinc sulfide , Barium sulfate, alumina, talc and the like. These inorganic particles can be used alone or in combination depending on the necessity of imparting a surface function such as glossiness adjustment, whiteness adjustment, and light resistance.

  In the present invention, the resin constituting the layer A and the layer B is polyester. Particularly, polyethylene terephthalate and polyethylene naphthalate are preferred.

  Further, various known additives such as an antioxidant and an antistatic agent may be added to the polyester.

[Layer B]
The B layer is whitened by containing fine bubbles inside the film. The formation of fine bubbles can be achieved by finely dispersing a polymer (incompatible resin) incompatible with polyester in a film base material, for example, polyester, and stretching (for example, biaxial stretching).

[Incompatible resin]
In the present invention, the layer B needs to contain a resin incompatible with the polyester. The resin incompatible with the polyester (hereinafter sometimes abbreviated as an incompatible resin) may be a homopolymer or a copolymer, such as polyethylene, polypropylene, polybutene, and polymethylpentene. Polyolefin resins, cyclic polyolefin resins, polystyrene resins, polyacrylate resins, polycarbonate resins, polyacrylonitrile resins, polyphenylene sulfide resins, fluorine resins, and the like are preferably used. These may be used in combination of two or more. Particularly, a resin having a large difference in critical surface tension from polyester and hardly deformed by heat treatment after stretching is preferable, and specifically, a polyolefin resin is preferable. Examples of the polyolefin-based resin include polyolefin resins such as polyethylene, polypropylene, polybutene, and polymethylpentene; cyclic polyolefin resins; and copolymers thereof. Among these, a copolymer of ethylene and a bicycloalkene, which is a cyclic olefin copolymer, is particularly preferred.
The glass transition temperature of the immiscible resin is preferably from 180 ° C to 220 ° C, more preferably from 190 ° C to 220 ° C. Regarding the glass transition temperature, in a region where the glass transition temperature is lower than 180 ° C., voids generated during stretching may be deformed and crushed in the heat treatment step in the film production step. In particular, in the case of a void having a small diameter and being finely dispersed, a small deformation causes void disappearance, which may affect a decrease in reflectance of the white polyester film and, consequently, a decrease in luminance. In addition, in a region higher than 220 ° C., when the resin constituting the layer B is melt-kneaded, the incompatible resin may not be sufficiently melted and fine dispersion may not be promoted.

  The glass transition point is the temperature at which glass transition occurs in an amorphous solid material, and is abbreviated as Tg. In the present invention, the Tg is measured by reading the intersection of the DSC curve with the tangent to the base line and the tangent to the steep drop position of the endothermic region due to the glass transition.

  The method of controlling Tg can be arbitrarily changed by controlling the copolymerization ratio of a linear olefin portion (ethylene portion) and a cycloolefin portion (methyl-norbornene portion). , By increasing the proportion of cycloolefin. The temperature can be increased to 185 ° C. or higher by further increasing the ratio of the linear olefin portion: cycloolefin portion = 3: 7.

  When the Tg is in the above range, the voids are less likely to disappear during the heat treatment, and the rigidity of the cycloolefin, which is a nucleus when the voids are developed during stretching, is high, and the void formation rate is significantly increased. is there. Since the voids can be laminated in a fine multiple layer, it is effective for improving the reflectance and, consequently, the luminance.

  By containing the incompatible resin, a cavity having the incompatible resin as a nucleus is formed at the time of stretching, and light reflection occurs at the cavity interface.

In the present invention, the preferable content of the incompatible resin contained in the layer B is 5% by mass or more and 25% by mass or less. The specific gravity decreases because the number of cavities increases by increasing the content of the incompatible resin. It is preferable for the apparent specific gravity to be 0.5 g / cm ³ or more and 1.1 g / cm ³ or less.

  In the present invention, the immiscible resin to be contained in the layer B is such that the number average particle diameter is dispersed in the matrix composed of the polyester resin at 0.4 μm or more and 3.0 μm or less. From the viewpoint of obtaining film strength, the thickness is more preferably in the range of 0.5 μm to 1.5 μm. The number average particle size as used herein means that a cross section of the film in the width direction (TD) is cut out, and the B layer portion of the cross section is observed using a scanning electron microscope (FE-SEM) S-2100A manufactured by Hitachi, Ltd. It is the average value of the diameter when the area of 100 particles is calculated and converted into a perfect circle.

  In the present invention, the B layer may contain inorganic particles, such as calcium carbonate, titanium dioxide, zinc oxide, zirconium oxide, zinc sulfide, basic lead carbonate (white lead), and barium sulfate. Among these, calcium carbonate, barium sulfate, titanium dioxide, and the like, which have low absorption in the visible light region of 400 to 700 nm, are preferable from the viewpoint of reflection characteristics, concealing properties, manufacturing costs, and the like. In the present invention, barium sulfate and titanium dioxide are most preferable from the viewpoints of winding property of the film, long-term film forming stability, and improvement of reflection characteristics. As the particle diameter of the inorganic particles, it is preferable to use those having a number average particle diameter of 0.1 μm or more and 3.0 μm or less in order to achieve excellent reflection and hiding properties.

[Copolymerized polyester]
In the present invention, it is preferable to use a copolymerized polyester for the layer B. Even if a composition containing a high concentration of an inorganic substance in the B layer, a film can be stably formed, and has a role as a dispersant for the incompatible resin in the B layer.

  In the copolymerized polyester, the main dicarboxylic acid component is terephthalic acid, the main glycol component is ethylene glycol, and the copolymerization component is an aromatic carboxylic acid or an aliphatic carboxylic acid such as isophthalic acid or naphthalenedicarboxylic acid, and tetracarboxylic acid. It is a polyester containing at least one selected from the group consisting of aliphatic diols such as methylene glycol, cyclohexanedimethanol, polyethylene glycol, and polytetramethylene glycol. The copolymerized polyester used has a basic composition of polyethylene terephthalate, a copolymer of polyethylene terephthalate and isophthalic acid, a copolymer of polyethylene terephthalate and cyclohexane dimethanol, and a copolymer of polybutylene terephthalate and polytetramethylene terephthalate. It is preferable to contain at least two or more copolyesters selected from coalescing.

  The content of the copolyester contained in the B layer is from 1% by mass to 20% by mass, preferably from 2% by mass to 15% by mass, based on the mass of the B layer. When the content is less than 1% by mass, the change in shrinkage stress with respect to temperature change is small and the thermal dimensional stability is excellent, but the dispersibility of the immiscible resin is reduced, the reflection performance is reduced, and the inorganic particles are contained. In some cases, the film may not be formed. Further, when a large amount of the copolyester is contained, the heat shrinkage tends to increase. Therefore, if it exceeds 20% by mass, the change in shrinkage stress with respect to the temperature change is large, and the thermal dimensional stability is poor. In this case, since the heat shrinkage measured by thermomechanical analysis at 40 ° C. to 100 ° C. does not satisfy −0.5% or more and 0.0% or less, the film undulates inside the backlight, The screen becomes uneven. Moreover, film formation may become difficult.

  The proportion of the copolymer component in the copolymerized polyester is preferably 1 to 30 mol%, more preferably 3 to 25 mol%, based on all dicarboxylic acid components or all diol components. If the amount is less than 1 mol%, the dispersibility of the incompatible resin is reduced and the reflection performance is reduced, and the stretching stress when inorganic particles are contained is increased, so that the film may not be formed. On the other hand, if it exceeds 30 mol%, thermal dimensional stability may be lacked, or film formation may be difficult.

  The copolymerized polyester used in the present invention preferably has a melting point of 170 ° C. or more and 230 ° C. or less. More preferably, the temperature is in the range of 180 ° C to 220 ° C. When the melting point of the thermoplastic polyester elastomer is less than 170 ° C., uniform dispersion may not be easily obtained, and when used as a reflector, the brightness may be reduced. If the temperature is higher than 230 ° C., the dispersing effect may not be recognized, which is not preferable.

[Specific gravity (apparent density)]
In the present invention, the specific gravity of the white polyester film is 0.5 (g / cm ³ ) or more and 1.1 (g / cm ³ ) or less, preferably 0.55 (g / cm ³ ) or more and 1.05 (g / cm ³ ). ³ ) or less, more preferably 0.6 (g / cm ³ ) or more and 1.0 (g / cm ³ ) or less, for obtaining higher reflectance. Further, when the specific gravity is less than 0.5 (g / cm ³ ), the film forming stability is poor. Further, since a cavity is formed around the incompatible component, the heat shrinkage tends to be larger than that of a conventional polyester film. On the other hand, when it exceeds 1.1 (g / cm ³ ), generation of fine bubbles may be insufficient.

[Total thickness]
In the present invention, the total thickness of the white polyester film is preferably from 50 μm to 500 μm, more preferably from 150 μm to 350 μm. It is preferable that the total thickness of the white polyester film is 50 μm or more from the viewpoint of reflectance, and it is also preferable from the viewpoint of luminance unevenness that the thicker the thickness is, the higher the rigidity is and the less bending occurs in the housing. The upper limit is not particularly limited, but is preferably 500 μm or less, more preferably 350 μm or less, from the viewpoint of reflectance, workability, and cost. If the thickness exceeds 500 μm, even if the thickness is more than 500 μm, an increase in the reflectance cannot be expected, and the workability (handling property) is deteriorated due to an increase in mass when working as a single wafer for incorporation into a backlight.

[Reflectance]
It is preferable that the relative reflectance at a wavelength of 560 nm of at least one surface of the white polyester film of the present invention is 97.0% or more for use as a reflector. The relative reflectance is more preferably 98.5% or more, further preferably 99.0% or more, and most preferably 99.5% or more. In the present invention, the relative reflectance refers to a spectrophotometer (U-3310) manufactured by Hitachi High-Technologies Corporation with an integrating sphere attached, and the reflectance is set to 100% using a standard white plate (aluminum oxide) attached to the spectrophotometer as a reference. Is the reflectance when

[Thermo-mechanical analysis (TMA)]
In the present invention, it is necessary that the thermal shrinkage measured by thermomechanical analysis from 40 ° C. to 100 ° C. in the longitudinal direction and the width direction of the film is −0.5% or more and 0.0% or less. .

The thermomechanical analysis (TMA) in the present invention refers to a value measured under the following conditions using a thermomechanical analyzer (TMA / SS6000) manufactured by Seiko Instruments Inc. For each data, at least one data per 1 ° C. is obtained, and the heat shrinkage at each temperature is calculated by using the following equation. Thermomechanical analysis is a method of measuring the deformation of a material as a function of temperature by applying a non-oscillating load while changing the temperature of the material according to a regulated program, and shows the change in shrinkage stress with temperature change. Things. The measurement conditions are as follows.
Sample size: width 4mm, length 20mm
Measurement temperature range: 25 ° C to 160 ° C
Heating rate: 10 ° C / min Measurement load: 19.6 mN
Measurement room environment: temperature 23 ° C, relative humidity 65%, heat shrinkage in air (T ° C) = (L (25 ° C)-L (T ° C)) / L (25 ° C) x 100
L (T ° C): sample length at T ° C, heat shrinkage (T ° C): heat shrinkage at T ° C (%).

  In recent years, as a light source of a liquid crystal display, a method that is advantageous for thinning by arranging an LED light source having a small power consumption and capable of high output on a side surface has been adopted. The temperature reaches around ℃, and even at a distance, it reaches around 60 ℃. As described above, since there is a temperature gradient inside the backlight, if the temperature deviates from the above range, the film will undulate inside the backlight when used as a reflector for backlight, resulting in screen unevenness. By setting the heat shrinkage as measured by TMA within the above range, it is possible to suppress the waving of the film and to suppress the unevenness of the screen. The value of the heat shrinkage is from -0.5% to 0.0%, more preferably from -0.4% to 0.0%.

  Furthermore, when the reflector is gripped at the end even when incorporated in a medium-sized or small-sized edge light type display, a reflector having good thermal dimensional stability is required. It is also suitable in the case of

[Production method]
Next, a method for producing the white polyester film of the present invention will be described, but the present invention is not limited to this example. A cyclic olefin as an incompatible resin; a copolymer of 1,4-cyclohexanedimethanol and polyethylene terephthalate as a dispersant (copolyester); a copolymer of polybutylene terephthalate and polytetramethylene glycol; The mixture is sufficiently mixed and dried, and is supplied to an extruder B heated to a temperature of 270 to 300 ° C. Polyethylene terephthalate containing inorganic particles of silicon dioxide is supplied to the extruder A by a conventional method, and the extruder A layer / B layer / A layer is formed so that the polymer of the extruder A layer becomes both surface layers in the T die three-layer die. A three-layer configuration was obtained.

  The melted sheet is tightly cooled and solidified by electrostatic force on a drum cooled to a drum surface temperature of 10 to 60 ° C., and the unstretched film is guided to a group of rolls heated to 80 to 120 ° C. in a longitudinal direction. It is stretched 2.8 to 4.0 times.

  More preferably, it is 3.1 to 3.4 times. When the ratio is less than 3.1 times, the reflection characteristics are poor or the thickness unevenness of the film is deteriorated, so that a good film cannot be obtained. When the ratio exceeds 3.4 times, breakage is apt to occur during film formation, and heat in the longitudinal direction is also increased. It is not preferable because the shrinkage rate increases. The longitudinally stretched film is then sequentially subjected to stretching in a direction perpendicular to the longitudinal direction (hereinafter, referred to as a transverse direction), heat fixing, and heat relaxation to obtain a biaxially oriented film. Perform while running. The process of transverse stretching starts from a temperature higher than the glass transition temperature (Tg) of the polyester. Then, the temperature is raised to a temperature (5 to 70) ° C. higher than Tg. The temperature rise in the transverse stretching process may be continuous or stepwise (sequential), but it is usually preferable to raise the temperature sequentially. For example, the transverse stretching zone of the tenter is divided into a plurality of sections along the film running direction, and the temperature is increased by flowing a heating medium at a predetermined temperature in each zone. The transverse stretching ratio is preferably 2.5 to 4.5 times, and more preferably 2.8 to 3.9 times, although it depends on the required characteristics of the application. When the ratio is less than 2.5 times, unevenness of the thickness of the film becomes worse and a good film cannot be obtained. When the ratio exceeds 4.5 times, breakage is liable to occur during film formation.

  By stretching the obtained unstretched film in at least one direction in this manner, voids can be developed with a resin incompatible with the polyester or inorganic particles capable of forming bubbles as nuclei. Here, the case of stretching by the sequential biaxial stretching method has been described in detail, but the white polyester film of the present invention may be stretched by any of the sequential biaxial stretching method and the simultaneous biaxial stretching method.

  In order to complete the crystal orientation of the obtained biaxially stretched film and to impart flatness and thermal dimensional stability, heat fixation is subsequently performed in a tenter at a temperature of 180 to 220 ° C. for 1 to 30 seconds, After uniform cooling, cool to room temperature and take up. Thereafter, a heat treatment is applied to the obtained film. During the heat treatment step, a relaxation treatment of 0.5 to 10% may be performed in the horizontal or vertical direction as necessary. The heat setting and heat treatment steps are important steps for obtaining the white polyester film for a liquid crystal display of the present invention, and will be described in detail below.

[Heat fixation]
The white polyester film for a liquid crystal display of the present invention has cavities formed around the incompatible components, whereby the apparent density is lower than that of the conventional polyester film. This void-containing polyester film tends to have a higher heat shrinkage than a conventional polyester film. In order to reduce the heat shrinkage, it is effective to increase the temperature of the heat setting after the transverse stretching. Therefore, it is preferable to perform the heat setting at 180 to 220 ° C. More preferably, it is 190-210 degreeC. When the heat setting treatment is performed at a temperature lower than 180 ° C., the crystallization in the heat setting zone is insufficiently relaxed, and a uniform low heat shrinkage in the vertical direction may not be obtained. On the other hand, when performed at a temperature higher than 220 ° C., the polyester and the incompatible resin are softened, and the cavities are crushed or disappear, so that the apparent specific gravity increases or the original purpose cannot be fulfilled. I will. Since the polyester film has low air permeability, the cavity may not be sufficiently filled with air immediately after the cavity is generated by the transverse stretching (that is, close to a vacuum state). In this state, when the polyester and the immiscible resin are softened by heat setting at a high temperature, the cavity is crushed because the cavity is in a vacuum state. If the cavity is crushed, the reflectance will decrease.

  Regarding the heat treatment step, a method performed during the production of the biaxially stretched polyester film (heat treatment A: in-line treatment) is preferred from the viewpoint of production cost, but a method in which the film once formed is passed through an oven again to perform relaxation treatment ( Heat treatment B: off-line treatment).

[Heat treatment A: In-line treatment]
At the exit of the tenter, a cutting blade is installed near the edge of the film to be released from the clip, the film edge is cut off, and the film is once cooled to room temperature. Next, by reducing the film take-up speed in an in-line oven separate from the transverse stretching and heat setting, the entire width of the film detached from the clip is heat-treated and relaxed in the longitudinal direction. As means for relaxing, the speed of the roll group on the winding side is adjusted. In particular, the heat treatment is particularly preferable in order to set the heat shrinkage as measured by TMA to −0.5% or more and 0.0% or less.

  As the rate of relaxation, the speed of the roll group is reduced with respect to the film line speed of the tenter, preferably 0.1 to 1.5%, more preferably 0.2 to 1.2%, and particularly preferably 0.3 to A speed reduction of ~ 1.0% is implemented to relax the film and adjust the heat shrinkage in the machine direction.

[Heat treatment B: Off-line treatment]
In order to fill the cavity with air, it is preferable that the biaxially stretched film is kept in air under a pressure condition of atmospheric pressure (90000 Pa) or more for a certain period of time. The temperature of the stationary environment is preferably a temperature not higher than the glass transition point (Tg) of the polyester, specifically, not higher than 70 ° C, more preferably not higher than 55 ° C, and further preferably not higher than 40 ° C. . When the temperature exceeds 70 ° C., the polyester and / or the incompatible resin may be softened, and the effect of the subsequent heat treatment is weakened, which is not preferable in terms of cost. The lower limit of the temperature is not particularly limited, but is preferably −5 ° C. or higher from the viewpoint of cost. Also, the standing time is preferably 24 hours or more in order to sufficiently fill the cavity with air.

  After leaving the film stationary as described above and filling the cavity with air, heat treatment is performed in an oven or the like. By this heat treatment, not only the heat shrinkage rate can be reduced, but also the air filled in the cavity expands and the apparent density can be reduced by expanding the cavity. In particular, heat treatment is particularly preferable in order to set the heat shrinkage measured by TMA to be -0.5% or more and 0.0% or less.

  The temperature of the heat treatments A and B is preferably equal to or lower than the melting point (Tm) of the polyester, specifically, preferably 100 to 200 ° C, more preferably 120 to 180 ° C. If the temperature of the heat treatment is lower than 100 ° C., the heat shrinkage does not become sufficiently small, and if it is higher than 200 ° C., the apparent density does not sufficiently decrease and the flatness of the film deteriorates. The heat treatment time is preferably from 5 to 50 seconds, more preferably from 10 to 40 seconds. If the heat treatment time is shorter than 5 seconds, the heat shrinkage does not become sufficiently small, and if it is longer than 50 seconds, the specific gravity does not sufficiently decrease, which is not preferable. By such a heat treatment, it is possible to achieve both high reflection performance and thermal dimensional stability, which cannot be achieved by the conventional method.

  The white polyester film of the present invention obtained in this way can improve the brightness of the liquid crystal backlight, does not damage the reflector even when used for a long time, and has a small decrease in reflectance. Can be conveniently used as a reflector of a surface light source of an edge light and a direct type light, and a reflector. The obtained white polyester film for reflection of a liquid crystal display of the present invention can maintain high luminance particularly when used as a reflection plate of a sidelight type and a direct type light type liquid crystal display using LEDs.

[Measurement of physical properties and evaluation method of effect]
The method for evaluating physical properties and the method for evaluating effects of the present invention are as follows.

(1) Film thickness / layer thickness, porosity The film thickness was measured according to JIS C2151-2006.
Using a microtome, the film was cut in the width direction (TD) without being crushed in the thickness direction to obtain a section sample.
A cross section of the section sample was imaged at a magnification of 3000 times using a scanning electron microscope (FE-SEM) Model S-2100A manufactured by Hitachi, Ltd., and the thickness of the laminated layer was measured from the image to calculate the thickness and thickness ratio of each layer. At the same time, voids (bubbles) were observed, and the area of the portion observed as bubbles was divided by the area of the observed layer and multiplied by 100 to obtain the porosity.

(2) Thermomechanical analysis (TMA)
The measurement was performed under the following conditions using a thermomechanical analyzer (TMA / SS6000) manufactured by Seiko Instruments Inc. For each data, at least one data was obtained per 1 ° C., and the heat shrinkage at each temperature was calculated using the following equation.
Sample size: width 4mm, length 20mm
Measurement temperature range: 25 ° C to 160 ° C
Heating rate: 10 ° C / min Measurement load: 19.6 mN
Measurement room environment: temperature 23 ° C, relative humidity 65%, heat shrinkage in air (T ° C) = (L (25 ° C)-L (T ° C)) / L (25 ° C) x 100
L (T ° C): sample length at T ° C, heat shrinkage (T ° C): heat shrinkage at T ° C (%).

(3) Apparent density The film was cut into a 100 x 100 mm square, and the thickness at 10 points was measured with a 10 mm diameter measuring element (No. 7002) attached to a dial gauge (No. 2109-10 manufactured by Mitoyo Seisakusho). Then, the average value d (μm) of the thickness is calculated. Further, the film is weighed by a direct reading balance, and the weight w (g) is read to a unit of 10 ⁻⁴ g. Let the value calculated by the following formula be the apparent density.
Apparent density = w / d × 100 (g / cm ³ ).

(4) Shape of end of white film The reflection film laminated in the backlight of a 32-inch liquid crystal TV LHD32K15JP manufactured by Hisense Japan Co., Ltd. was changed to a predetermined film sample, and the temperature was 60 ° C. and the relative humidity was 80% for 4 hours. Lighted. Thereafter, the backlight unit was removed from the liquid crystal TV, and the white film of the reflector was placed on a flat surface. At this time, the lifting of the film edge was measured.
The following evaluation results were obtained.
A: No deformation (0 mm or more and less than 1 mm)
B: There is a slight ripple. (1mm or more and less than 4mm)
C: There is a large wave. (4mm or more)
The above A and B were regarded as pass.

(5) Brightness unevenness (screen unevenness) and brightness The reflection film laminated in a new 32 inch liquid crystal TV LHD32K15JP backlight manufactured by Hisense Japan Co., Ltd. was changed to the reflector taken out in the above (4) and lit. After waiting for one hour in this state and stabilizing the light source, the liquid crystal screen was photographed with a CCD camera (DXC-390 manufactured by SONY), and an image was captured by an eye scale manufactured by Image Analyzer I-System. After that, the brightness level of the photographed image was controlled to 30,000 steps, automatically detected, and converted into brightness.
Luminance unevenness (%) = (maximum luminance−minimum luminance) / average luminance × 100
S: Excellent (less than 2%)
A: Good (2% or more and less than 5%)
B: Inferior (5% or more and less than 10%)
C: very poor (more than 10%)
The above S and A were regarded as acceptable.
As a luminance evaluation, Lumirror (registered trademark) # 250E6SL manufactured by Toray Industries, Inc. was used as a reference sample (100%), and the evaluation results were as follows.
S: Excellent (103% or more)
A: Good (102% or more and less than 103%)
B: Inferior (101% or more and less than 102%)
C: very poor (less than 101%)
The above S and A were regarded as acceptable.

(6) Film-forming stability Whether a film could be formed stably was evaluated according to the following criteria.
A: A film can be stably formed for 24 hours or more.
B: A film can be stably formed for 12 hours or more and less than 24 hours.
C: Breakage occurred within 12 hours, and stable film formation was not possible.
The above A and B were regarded as pass.

(7) Relative reflectance The integrating sphere was attached to a spectrophotometer (U-3310) manufactured by Hitachi High-Technologies Corporation, and the reflectance was 560 nm when the standard white plate (aluminum oxide) attached to the spectrophotometer was 100%. Was measured for relative reflectance.
97% or more was regarded as a pass.

(8) Measurement of Resin Content in Film After weighing the film, it is dissolved in a mixed solvent of hexafluoroisopropanol (HFIP) / chloroform (mass ratio 50/50). When there is an insoluble component, the insoluble component is separated by centrifugation, then the mass is measured, and the structure and mass fraction of the component are measured by elemental analysis, FT-IR, and NMR. If the supernatant component is similarly analyzed, the mass fraction and the structure of the polyester component and other components are specified. After evaporating the solvent from the supernatant component and dissolving in a mixed solvent of HFIP / deuterated chloroform (mass ratio 50/50), the NMR spectrum of 1H nucleus is measured.

  The peak area intensity of absorption specific to each component is determined from the obtained spectrum, and the molar ratio of the blend is calculated from the ratio and the number of protons. Further, the mass ratio is calculated from the formula weight corresponding to the unit unit of the polymer. Thus, the mass fraction and the structure of each component were specified.

(9) The shape of the central part of the white film The reflection film laminated in the backlight of the 32-inch liquid crystal TV LHD32K15JP manufactured by Hisense Japan Co., Ltd. was changed to a predetermined film sample, and the temperature was 60 ° C. and the relative humidity was 80% for 4 hours. Two samples were prepared for lighting.
Thereafter, the backlight unit was removed from the liquid crystal TV, the reflective film was taken out, and one sample was cut out in the long side direction at an interval of 3 cm for the entire width. The other sample was cut out from the entire width at 3 cm intervals in the short side direction. The cut sample was placed on a flat plate, the maximum value of the rising height of the reflection film edge from the flat plate was completely measured, and the maximum value in the cut sample was taken as the result of that level.
When placed on a flat plate, the top surface was not specified, and both surfaces were confirmed.
The following evaluation results were obtained.
A: No deformation (0 mm or more and less than 1 mm)
B: There is a slight ripple. (1mm or more and less than 4mm)
C: There is a large wave. (4mm or more)
The above A and B were regarded as pass.

  The present invention will be described based on examples.

[Example 1]
Polyethylene terephthalate having a L value of 62.8, a b value of 0.5, and a haze of 0.2% having a color tone (measured by a stimulus value direct reading method) of polyethylene terephthalate after polymerization of 94% by mass of polyethylene terephthalate Parts, 0.5 part by mass of a (PBT / PTMG) copolymer of polybutylene terephthalate and polytetramethylene glycol (trade name: Hytrel manufactured by Toray DuPont), and 33 mol% of 1,4-cyclohexanedimethanol based on the diol component 0.5 parts by mass of copolymerized polyethylene terephthalate (33 mol% CHDM copolymerized PET) and 5 parts by mass of a cycloolefin copolymer having a glass transition temperature of 210 ° C. (trade name: TOPAS manufactured by Polyplastics Co., Ltd.) After mixing and drying at 180 ° C for 3 hours Was supplied to the extruder B heated to 270 to 300 ° C. (B layer).

  On the other hand, 63 parts by mass of polyethylene terephthalate, 17 parts by mass of a silicon dioxide particle having a number average particle size of 3.5 μm and 17 parts by mass of a polyethylene terephthalate master (containing 6 mass% of silicon dioxide based on the total amount of the master chip), and 18 mol% of isophthalic acid in polyethylene terephthalate were used. After 20 parts by mass of the polymerized product (PET / I) was vacuum-dried at 180 ° C. for 3 hours, it was supplied to an extruder A heated to 280 ° C. (A layer), and these polymers were converted into A layer / B layer / The sheets were laminated through a laminating apparatus so as to form the A layer, and formed into a sheet shape from a T die. Further, the unstretched film cooled and solidified by a cooling drum having a surface temperature of 25 ° C. was led to a group of rolls heated to 85 to 98 ° C., longitudinally stretched 3.4 times in the longitudinal direction, and cooled by a group of rolls of 21 ° C. . Subsequently, the longitudinally stretched film was guided to a tenter while holding both ends of the film with clips, and was transversely stretched 3.6 times in a direction perpendicular to the longitudinal direction in an atmosphere heated to 120 ° C. Thereafter, heat setting at 190 ° C. was performed in a tenter, followed by a 6% relaxation treatment in the width direction at the same temperature, and then, after uniform cooling, cooling to room temperature to obtain a biaxially stretched laminated film. . Then, after performing a stationary treatment at 25 ° C. for 24 hours, heat treatment was performed in an oven at 200 ° C. for 5 seconds. Table 1 shows the physical properties of the substrate for light reflection.

[Examples 2 to 20]
A white polyester film was obtained in the same manner as in Example 1, except that the raw material compositions, the film forming conditions, and the heat treatment conditions of the A layer and the B layer were changed as described in Tables 1 and 2. In each of the examples, the film shape, luminance unevenness and luminance were good.

[Comparative Examples 1-2]
A 225 μm-thick film was obtained in the same manner as in Example 1, except that the raw material compositions of the A layer and the B layer and the film forming conditions were changed as described in Table 3. Since the content of the copolymerization component in the B layer was large and the heat treatment was not performed, the film shape and luminance unevenness were insufficient.

[Comparative Examples 3 to 6]
A white polyester film was obtained in the same manner as in Example 1 except that the raw material compositions of the A layer and the B layer and the film forming conditions were changed as described in Table 3. Although the content of the copolymerization component in the layer B was small, the heat treatment was not performed, so that the film shape, luminance unevenness, and film formation stability were insufficient.

[Comparative Example 7]
A white polyester film was obtained in the same manner as in Example 1 except that the raw material compositions of the A layer and the B layer and the film forming conditions were changed as described in Table 3. Since the content of the copolymerization component in the layer B was large, heat treatment was performed, but the shape and luminance unevenness of the film were insufficient.

Here, the abbreviations in Tables 1 to 3 represent the following contents. That is,
PET: polyethylene terephthalate,
PET / I: polyethylene terephthalate copolymerized with 18 mol% of isophthalic acid,
PET / CHDM: polyethylene-1,4-cyclohexylene dimethylene terephthalate (polyethylene terephthalate copolymer obtained by copolymerizing 33% by mole of 1,4-cyclohexanedimethanol with respect to ethylene glycol),
PBT / PTMG: polyester ether elastomer mabutylene / poly (alkylene ether) phthalate (a copolymer in which alkylene glycol is 30 mol% based on butylene terephthalate) (trade name: Hytrel manufactured by Toray DuPont),
It is.
PET / I / PEG: copolymer obtained by copolymerizing 10 mol% of polyethylene terephthalate isophthalic acid and 5 mol% of polyethylene glycol (trade name: T794M manufactured by Toray)

The white polyester film for a liquid crystal display of the present invention can be suitably used for a backlight.

Claims (8)

What is claimed is: 1. A white polyester film having at least two layers, an A layer having substantially no voids and a B layer having voids, wherein the white polyester film satisfies the following requirements (1) to (4): White polyester film for light type liquid crystal displays.
(1) The layer B contains polyester containing an incompatible resin and has bubbles,
(2) The B layer has a basic configuration of polyethylene terephthalate, contains at least one or more copolymerized polyester, and the copolymerized polyester is at least 1.0% by mass and at most 20.0% by mass based on the mass of the B layer. ,
(3) the apparent density of the film is 0.5 g / cm ³ or more and 1.1 g / cm ³ or less;
(4) The thermal shrinkage measured by thermomechanical analysis from 40 ° C. to 100 ° C. in the longitudinal direction and the width direction of the film is −0.5% or more and 0.0% or less. The white polyester film for a sidelight type liquid crystal display according to claim 1, wherein the content of the incompatible resin in the polyester of the polyester B layer is 5% by mass or more and 25% by mass or less based on the mass of the B layer. The white polyester film for a sidelight type liquid crystal display according to claim 1, wherein the relative reflectance at a wavelength of 560 nm of at least one surface is 97.0% or more. The white polyester film for a sidelight type liquid crystal display according to claim 1, wherein the total thickness of the white polyester film is 50 µm or more and 500 µm or less. The polyester constituting the B layer is basically composed of polyethylene terephthalate, a copolymer of polyethylene terephthalate and isophthalic acid, a copolymer of polyethylene terephthalate and cyclohexane dimethanol, and a copolymer of polybutylene terephthalate and polytetramethylene terephthalate selected from, a sidelight type white polyester film arm for a liquid crystal display according to claim 1, characterized in that it contains at least two kinds of copolymerized polyester. The film according to claim 1, which is obtained by biaxially stretching a film containing a polyester and a component incompatible with the polyester, generating a cavity in the film, and then heat-treating the film at 100 to 200 ° C for 5 to 50 seconds. A method for producing a white polyester film for a sidelight type liquid crystal display. A backlight for a sidelight type liquid crystal display, wherein the white polyester film for a sidelight type liquid crystal display according to claim 1 is used as a light reflecting material.   The backlight for a sidelight type liquid crystal display according to claim 7, wherein the light source of the backlight is an LED system.

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