JP2014200958A - White polyester film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a white polyester film having high brightness and high durability and preventing brightness unevenness caused by adhesion unevenness with another member at a low cost.SOLUTION: A polyester film 1 is composed of at least two layers of a polyester layer (A) and a polyester layer (B). The polyester layer (B) is a whitened layer. The polyester layer (A) is a layer for forming at least one surface layer, has an internal haze of 2.0% or less, and is a layer imparted with a regular uneven shape at least in one direction, wherein the average reflectance on the surface of the layer is 97% or more. Fine air bubbles and/or white inorganic particles are incorporated into the polyester layer (B) to whiten the polyester layer (B).

Description

  The present invention relates to a white polyester film, and more particularly to a film having a surface provided with an uneven shape with respect to a white polyester film containing fine bubbles.

A film that reflects light (hereinafter referred to as a light reflecting film) is widely used for various light sources, for example, backlights of liquid crystal display devices, street advertisements, indoor lighting, and the like. With respect to these light sources, it is known that the light reflecting film has a merit of easily improving luminance and illuminance by having a role of preventing escape of the illumination light. Since this reflector is required to have a thin film and high light reflectivity, for example, a backlight of a liquid crystal display device contains fine bubbles inside, as shown in Patent Document 1, and has a bubble. A film that is whitened by reflecting light at the interface is mainly used.
In order to contain these fine bubbles, there is a method in which a high melting point incompatible polymer or inorganic particles are finely dispersed in a film base material such as polyester and then stretched (for example, biaxially stretched). . During stretching, voids (bubbles) are formed around the incompatible polymer or inorganic particles, and this exerts a light scattering action, resulting in whitening. In recent years, display devices such as liquid crystal displays are required to have unprecedented advanced display capability. In particular, backlights are required not only to have high brightness but also to be durable under various conditions. Yes.

In the prior art, there is a method of adding a large amount of inorganic particles and void nucleating agent in order to increase brightness and improve durability. For example, ultraviolet light emitted from a light source of illumination light by the method of Patent Document 2 However, the addition of a large amount of inorganic particles and void nucleating agent may induce film breakage in the biaxial stretching process during film formation, which may result in poor productivity.
In addition, there is a method of achieving high brightness and improved durability by controlling the surface shape by coating, sandblasting, embossing and the like by an additional process. These are difficult to control the surface shape, and for example, there is a reflective film in which spherical particles are coated on a white film as in Patent Document 3, but the cost, the damage of the member due to rubbing with other members, There are problems such as durability against shaving and uneven brightness due to uneven contact at contact points. Therefore, the development of a film that satisfies the demands for both cost reduction and high performance is becoming a particularly important issue in recent years.

Japanese Patent No. 2099443 Japanese Patent No. 5045851 International Publication No. 2011/105294

  It is an object of the present invention to prevent scratches due to rubbing with other members in the backlight, have durability against shaving, prevent uneven brightness due to uneven contact with other members, and reduce costs. To do. In order to solve the problem, an object of the present invention is to provide a white polyester film for a reflective film having a regular uneven shape on the surface.

In order to achieve this object, the present invention provides the following white polyester film.
(1) A polyester film comprising two or more polyester layers including a polyester layer (A) and a polyester layer (B), wherein the polyester layer (B) is whitened, and the polyester layer (A) Is a layer forming at least one surface layer, the internal haze is 2.0% or less, and one of the polyester layers (A) forming the surface layer is regular in at least one direction in the film plane. A white polyester film having a relative average reflectance of 97% or more on the surface of the polyester layer (A).
(2) The white polyester film in which the whitening is performed by containing fine bubbles and / or white inorganic particles in the polyester layer (B).
(3) The white polyester film has a two-layer structure of polyester layer (A) / polyester layer (B), or a white layer having a three-layer structure of polyester layer (A) / polyester layer (B) / polyester layer (A). Polyester film.
(4) A white polyester film produced by causing the fine bubbles to contain a resin and / or inorganic particles incompatible with polyester and biaxially stretching.
(5) A white polyester film having a triangular or semi-elliptical shape in a cross section in which the uneven shape is formed by the one direction and a direction perpendicular to the film surface.
(6) The white polyester film whose length between the adjacent top parts of the said cross-sectional shape in the said direction is 2.0 micrometers or more and 30 micrometers or less.
(7) The white polyester film whose height between the adjacent top part and bottom part of the said cross-sectional shape in the said direction is 2.0 micrometers or more and 20 micrometers or less.
(8) The white polyester film in which the uneven shape has a wave shape in the one direction, the waveform has a periodicity in which the position variation is uniform, and the magnitude (variation width) of the position variation is 30 μm or less.

  The present invention prevents scratches due to rubbing with other members in the backlight, has durability against shaving, prevents luminance unevenness due to uneven contact with other members, and can reduce costs. It is.

It is the figure which showed the general | schematic cross section of the liquid crystal screen incorporating the white polyester film of this invention, and the outline of the luminance measuring method. It is a schematic sectional drawing which shows the liquid crystal screen of the sidelight system incorporating the white polyester film of this invention.

The polyester film of the present invention has a laminated structure of two or more layers. In the present invention, from the viewpoint of the performance as a light reflecting film and the production of the surface uneven shape, it is a laminated structure using at least the polyester layers (A) and (B), and the polyester layer (A) has a low haze. The polyester layer is a layer in which the polyester layer (B) is whitened and has a structure in which the polyester layer (A) is laminated on at least one surface layer portion.

  In the present invention, as long as it has the above structure, it may be a laminate of various polyester layers or may be composed of a large number of layers. For example, the polyester layer (A) / polyester layer (B) may be of a two-layer configuration, a polyester layer (A) / polyester layer (B) / polyester layer (A), or a polyester layer. A three-layer structure of (A) / polyester layer (B) / polyester layer (C) may be used. Furthermore, the structure of 4 types or more or 4 layers or more may be sufficient. By adopting a multilayer structure, in the laminated polyester film, the characteristics of each layer are expressed, and various characteristics can be controlled.

  Considering the ease and effect on film formation, a two-layer structure or a three-layer structure comprising a polyester layer (A) / polyester layer (B) / polyester layer (A) is preferable. That is, it is preferable that the core layer portion is the polyester layer (B) and the surface layer portion on one side or both sides is the polyester layer (A).

  The polyester constituting the present invention is a polymer obtained by condensation polymerization from a diol and a dicarboxylic acid. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, adipic acid, sebacic acid, and the like. The diol is represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol and the like. Specifically, for example, polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexylene dimethylene terephthalate, polyethylene-2,6-naphthalenedicarboxylate, etc. It is done. In the case of the present invention, polyethylene terephthalate (hereinafter sometimes abbreviated as PET), polybutylene terephthalate (hereinafter sometimes abbreviated as PBT), and polyethylene naphthalate (hereinafter sometimes abbreviated as PEN) are particularly preferable. .

When polyethylene terephthalate is used as a basic component, from the viewpoint of film formation stability, it is preferably 1 mol% or more and 15 mol% or less, more preferably 3 mol% or more and 14 mol% or less, and most preferably 5 mol per total dicarboxylic acid component. It is preferable to use a copolymerized polyester containing a copolymer component in an amount of not less than 13% and not more than 13 mol%. If it is less than 1 mol%, film formation may not be possible. Even if it exceeds 15 mol%, film formation may not be possible. Examples of the copolymer component include dicarboxylic acid components such as isophthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, adipic acid, sebacic acid, phthalic acid, and 5-sodium sulfoisophthalic acid. Can be mentioned. Examples of the diol include diethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, 1,6-hexanediol, neopentyl glycol, and polyalkylene glycol. In particular, isophthalic acid and 2,6-naphthalenedicarboxylic acid are preferably used in order to obtain good film forming properties.
From the viewpoint of film formation stability, it is also preferable to include a polyester such as polybutylene terephthalate in the polyethylene terephthalate film. Polybutylene terephthalate and polyethylene naphthalate are particularly preferred from the viewpoint of easy availability and performance development of the white polyester film.

  In addition, various known additives such as an antioxidant and an antistatic agent may be added to the polyester. As the polyester used in the present invention, polyethylene terephthalate is particularly preferable. Polyethylene terephthalate film is excellent in water resistance, durability, chemical resistance and the like.

  The white polyester film of the present invention has a relative average reflectance of 97% or more on the surface of the polyester layer (A) provided with regular irregularities on the surface layer. If it is less than 97%, the luminance as a backlight may be insufficient when applied to a liquid crystal display. In the present invention, the average reflectance means that a spectrophotometer (U-3310) manufactured by Hitachi High-Technologies is attached with a 60 mmφ integrating sphere, and a standard white plate of aluminum oxide (part No. 210-0740 manufactured by Hitachi High-Technologies) is 100%. The reflectance is measured over 400 to 700 nm, and the reflectance is read at an interval of 5 nm from the obtained chart, and is an arithmetic average value.

  In order to make the average reflectance 97% or more, it is important that the film is whitened by containing bubbles, and this can improve the average reflectance because it exhibits a light scattering action. . Preferably, the relative average reflectance is 98% or more, more preferably 100% or more. There is no particular upper limit for the relative average reflectance, but in order to increase the relative average reflectance, for example, it is necessary to increase the amount of inorganic particles added, and in this case, the film forming property may become unstable. % Or less is preferable.

Formation of fine bubbles preferably used for whitening of the polyester layer (B) is achieved by finely dispersing a polymer incompatible with the high melting point polyester in a film base material, for example, polyester, and stretching it. Is done. At the time of stretching, bubbles are formed around the incompatible polymer particles, which exert a scattering action on the light, so that it becomes white and high reflectance can be obtained.
The size of the fine bubbles is determined by the size of the incompatible polymer particles and the draw ratio at the time of drawing. The size of the incompatible polymer particles is 8 μm in number average particle size when the particle size is measured. Or less, more preferably 6 μm or less. When the number average particle diameter is larger than 8 μm, it is difficult to obtain high reflectivity, and film breakage is likely to occur during production.

  As a stretching method applicable to the polyester film in order to contain a large number of bubbles, uniaxial stretching and biaxial stretching can be selected. However, in the present invention, when uniaxial stretching is selected, the formation of fine bubbles becomes insufficient. Since it is difficult to obtain the high reflectance required in the present invention, the biaxial stretching method is preferable. As long as it is a biaxial stretching method, sequential biaxial stretching or simultaneous biaxial stretching may be used. The biaxial stretching ratio is preferably 2.5 to 4.5 times for each of longitudinal stretching and lateral stretching, and the area ratio (longitudinal stretching ratio × lateral stretching ratio) is 9 to 16 times. Is preferred. If the area magnification is less than 9 times, the whiteness of the resulting film becomes poor. Conversely, if it exceeds 16 times, the film tends to be broken during stretching and the film forming property tends to be poor.

  When the average particle diameter and the area magnification are in the above ranges, the white polyester film in the present invention can contain a large number of fine bubbles, and a high reflectance suitable as a liquid crystal display reflector can be obtained.

  As the incompatible polymer, a linear, branched or cyclic polyolefin can be preferably used. Examples of linear and branched polyolefins include poly-3-methylbutene-1, poly-4-methylpentene-1, polyvinyl tert-butane, 1,4-trans-poly-2,3-dimethyl. Butadiene, polyvinylcyclohexane, polystyrene, polymethylstyrene, polydimethylstyrene, polyfluorostyrene, poly-2-methyl-4-fluorostyrene, polyvinyl-t-butyl ether, cellulose triacetate, cellulose tripropionate, polyvinyl fluoride, Examples thereof include a polymer having a melting point of 200 ° C. or higher selected from polychlorotrifluoroethylene. Among them, poly-4-methylpentene is preferable for the polyester base material.

  Cyclic polyolefins include bicyclo [2,2,1] hept-2-ene, 6-methylbicyclo [2,2,1] hept-2-ene, and 5,6-dimethylbicyclo [2,2,1]. Hept-2-ene, 1-methylbicyclo [2,2,1] hept-2-ene, 6-ethylbicyclo [2,2,1] hept-2-ene, 6-n-butylbicyclo [2,2 , 1] hept-2-ene, 6-i-butylbicyclo [2,2,1] hept-2-ene, 7-methylbicyclo [2,2,1] hept-2-ene, tricyclo [4,3 , 0,12.5] -3-decene, 2-methyl-tricyclo [4,3,0,12.5] -3-decene, 5-methyl-tricyclo [4,3,0,12.5]- 3-decene, tricyclo [4,4,0,12.5] -3-decene, 10-methyl-to Examples include polymers obtained by copolymerizing licyclo [4,4,0,12.5] -3-decene. In addition, those obtained by copolymerizing linear and branched olefins with cyclic polyolefins can also be preferably used. Examples of the linear and branched olefins in this case include ethylene, propylene, Examples include 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and the like. Among these, a copolymer of bicyclo [2,2,1] hept-2-ene and ethylene is preferable.

  The addition amount of the incompatible thermoplastic resin is preferably 5% by mass or more and 30% by mass or less when the total weight of the entire polyester film is 100% by mass. If the content is less than 5% by mass, the formation of fine bubbles is reduced, making it difficult to obtain high reflectivity. If the content is more than 30% by mass, mechanical properties such as the strength of the film itself may be too low.

  The incompatible thermoplastic resin is more preferably dispersed uniformly in the film. By uniformly dispersing, fine bubbles are uniformly formed inside the film, and a high reflectance can be obtained uniformly. In order to uniformly disperse the incompatible thermoplastic resin, it is effective to add a low specific gravity agent as a dispersion aid. The low specific gravity agent is a compound having an effect of reducing the specific gravity, and the effect is recognized in a specific compound. For example, for polyester, polyalkylene glycol such as polyethylene glycol, methoxypolyethylene glycol, polytetramethylene glycol, polypropylene glycol, ethylene / propylene oxide copolymer, sodium dodecylbenzenesulfonate, alkylsulfonate It is represented by sodium salt, glycerin monostearate, tetrabutylphosphonium paraaminobenzenesulfonate and the like. In the case of the present invention, polyalkylene glycol, particularly polyethylene glycol is particularly preferable. A copolymer of polybutylene terephthalate and polytetramethylene glycol is also preferably used to improve the dispersibility of the incompatible thermoplastic resin. The addition amount is preferably 3% by mass or more and 30% by mass or less, particularly preferably 5% by mass or more and 25% by mass or less, based on the total weight of the polyester (B) layer. Especially preferably, it is 10 mass% or more and 20 mass% or less. If the addition amount of the low specific gravity agent is too small, the effect of the addition is diminished, and if it is too much, the original properties of the film base material may be impaired. Such a low specific gravity agent can be added to the film base polymer in advance and adjusted as a master polymer (master chip).

By making the white polyester film contain fine bubbles using an incompatible thermoplastic resin, the apparent specific gravity of the polyester film becomes lower than that of a normal polyester film. If a lower specific gravity agent is further added, the specific gravity is further lowered. That is, a white and light film can be obtained. In order to reduce the weight of the white polyester film while maintaining the mechanical properties as a substrate for a liquid crystal display reflector, the specific gravity is preferably 0.3 or more and 1.3 or less. If the specific gravity is less than 0.3, there are too many fine bubbles, which may cause the mechanical properties such as the strength of the film itself to be too low, and the film-forming stability is significantly reduced. When the specific gravity is 1.3 or more, the formation of fine bubbles is reduced and it becomes difficult to obtain a high reflectance, and when used as a liquid crystal display reflector, the relative average reflectance and luminance are inferior, which is not preferable.
Moreover, the specific gravity of the white polyester film for a liquid crystal display reflector of the present invention is 0.3 or more and 1.3 or less, preferably 0.4 or more and 1.1 or less, when an incompatible thermoplastic resin is used. Preferably it is 0.5 or more and 0.9 or less in order to obtain a higher reflectance.
In the present invention, the above-described incompatible thermoplastic resin can be suitably used to form fine bubbles, but the use of inorganic particles to form fine bubbles is also preferably exemplified. In this case, preferably, the inorganic particles and the incompatible resin are mixed so as not to cancel each other out of their performance.
When using inorganic particles, to obtain high optical properties and stable film-forming properties, the types of particles are titanium dioxide, barium sulfate, calcium sulfate, magnesium sulfate, aluminum oxide, zinc oxide, magnesium oxide, calcium carbonate, carbonic acid. Barium, silica and the like are preferable. Further, they can be used alone or in combination of two or more. Among them, barium sulfate particles and titanium dioxide particles are particularly preferable because film formation stability can be obtained with high optical characteristics.

  In order to form fine bubbles, when only inorganic particles are used, it is preferable to contain 10% by mass or more and 50% by mass or less of inorganic particles with respect to the total weight of the polyester film. More preferably, it is 15 to 50 mass%, More preferably, it is 20 to 45 mass%.

  If the content is less than the lower limit of this range, fine bubbles due to inorganic particles may be insufficient and sufficient reflection performance may not be obtained. If the amount is larger than the upper limit of this range, the film-forming stability may be significantly lowered.

By including fine bubbles in the white polyester film using such inorganic particles, the apparent specific gravity of the polyester film becomes lower than that of a normal polyester film. When using only inorganic particles such as barium sulfate and containing a large number of fine bubbles, the apparent specific gravity is 1.0 or more in order to satisfy the optical characteristics while maintaining the mechanical characteristics as a substrate for a liquid crystal display reflector. It is preferably 1.3 or less, more preferably 1.1 or more and 1.3 or less, in order to obtain a higher reflectance. When the apparent specific gravity is within the range of the present invention, a large number of fine bubbles can be present while maintaining the film strength, and a high reflectance can be obtained. That is, when it is used as a liquid crystal display reflector, it exhibits remarkably excellent reflection performance in terms of screen brightness.
When inorganic particles are used to contain a large number of fine bubbles, the size of the bubbles is determined by the size of the inorganic particles and the draw ratio during biaxial stretching, but the size of the inorganic particles is determined by measuring the particle size. In addition, the average particle size is preferably 7 μm or less, more preferably 5 μm or less. When the average particle diameter is larger than 7 μm, it is difficult to obtain a high reflectance, and film breakage is likely to occur during production.

In order to form fine bubbles, it is also preferable to mix inorganic particles and incompatible resin. In that case, the total amount of inorganic particles and incompatible resin is added to the polyester layer (B). It is preferable to be 5% by mass or more and 55% by mass or less based on the total weight. If the amount is less than 5% by mass, the formation of fine bubbles is reduced and it becomes difficult to obtain a high reflectance. If the amount is more than 55% by mass, the film formation stability of the film itself may be significantly reduced. Also when the inorganic particles and the incompatible resin are mixed, the addition amount of the inorganic particles is 10% by mass to 50% by mass with respect to the total weight of the polyester layer (B), and the addition amount of the incompatible resin is 5%. It is preferable to adjust so that it may become mass% or more and 30 mass% or less.
As a stretching method that can be applied to the polyester film in order to contain a large number of fine bubbles, uniaxial stretching and biaxial stretching can be selected. However, in the present invention, when uniaxial stretching is selected, formation of fine bubbles is insufficient. Thus, since it is difficult to obtain the high reflectance required in the present invention, the biaxial stretching method is preferable. As long as it is a biaxial stretching method, sequential biaxial stretching or simultaneous biaxial stretching may be used.

  As described above, if the content of the inorganic particles is sufficient, a large number of fine bubbles are contained, and the reflective performance sufficient for application as a liquid crystal display reflector is exhibited. However, the inorganic particles have a particle size of 0. If particles having a thickness of .05 μm to 0.5 μm are selected, the performance as a light reflecting film can be improved without forming bubbles. In addition, about a particle diameter, More preferably, they are 0.1 micrometer-0.4 micrometer, More preferably, they are 0.2 micrometer-0.4 micrometer. In this case, as the kind of particles, any of the above-mentioned particles that can generate fine bubbles can be used. Among them, it is preferable to select titanium dioxide particles and barium sulfate particles.

  The reflector film has an uneven shape on the surface, so that it contacts only the top of the uneven shape with a small area when in contact with other members. Due to the above-described effects, it is possible to prevent the reflector film and other members from being scraped or damaged when, for example, a product is rubbed with another member that is in contact with the product or due to heat from the light source. This is directly connected to the improvement in durability as a reflector film, and as a result, the effect of improving the durability as a backlight in a liquid crystal display device can be obtained.

  At this time, it is one of preferred modes to select a material that does not damage the other member, that is, has a certain degree of flexibility, at the top of the unevenness that is a contact point with the other member.

On the other hand, if a material that is too flexible for the top of the uneven shape is selected, there is a concern that the top may be crushed by contact with other members, resulting in uneven brightness of the backlight and spot-like contamination due to the crushed. It is preferable to select a material having a thickness.
In selecting the optimum polymer for such conditions, in the present invention, it is possible to apply the polyester selected so far to obtain good film forming properties and good reflection characteristics as a material for forming an uneven shape. It can be illustrated as a preferred embodiment.
In the present invention, as described above, the process of forming a uniform unevenness on the surface is preferable in view of producing a surface that is not scraped or scratched in contact with other members and the subsequent behavior. It is preferable to select an imprint process by making the performance improvement as a reflective film according to the shape compatible, and further considering the cost for realization. If it is imprint processing, the kind is not ask | required, but when considering that it can respond to various shapes at low cost, for example, it has the shape of fine unevenness as described in JP, 2010-105314, A It is preferable to select a thermal imprint process in which the mold is heated and intermittent press thermoforming is performed.

At this time, in the polyester film which performs imprint processing, it is preferable to have a low haze surface, and it is preferable to perform imprint processing on the surface. In this case, in the polyester layer (A) on the surface on which imprinting is performed, the internal haze needs to be 2.0% or less, preferably 1.5% or less, more preferably 1.0%. % Or less. In addition, in this invention, the internal haze of a polyester layer (A) isolate | separates a polyester layer (A) and another layer from a white polyester film, and the measurement of the internal haze mentioned later is carried out with respect to a polyester layer (A). Indicates the value that was performed. When the internal haze measured in this way exceeds 2.0%, there is a possibility that sufficient reflectivity cannot be obtained because it is affected by light scattering inside the polyester layer (A).
The uneven shape in the present invention is preferably a stripe shape in one plane direction, and the uneven shape forming the stripe is preferably a cross-sectional shape in which small triangles or semi-elliptical shapes are connected. When the uneven shape takes the shape, the effect of the uneven shape is not only the improvement of the durability against the contact and rubbing behavior with other members, but also the improvement of the reflectivity as a reflection film, that is, the reflection film is used as a liquid crystal display device. When incorporated in the backlight, the effect of improving the backlight brightness is exhibited. The irregular shape is not limited in its form, but having regularity and periodicity is essential for maintaining a high reflectivity, and has a linearity or periodicity with a constant peak-valley height. It is a preferable aspect to take a waveform having the same.
In the present invention, hereinafter, a direction having irregularity regularity is an x axis, a direction perpendicular to the x axis in the film plane is a y axis, and a film thickness direction is a z axis.
The stripe interval is preferably 2.0 μm or more and 30 μm or less when the length in the x-axis direction of the uneven top portion-bottom portion-top portion is defined as the stripe interval. When the distance is less than 2.0 μm or 30 μm or more, the durability improvement effect in the produced uneven shape and the reflectance improvement effect by optical path control may be insufficient. In addition, if an attempt is made to obtain unevenness having a stripe interval of 30 μm or more, the number of unevenness may be reduced, and durability in contact with other members may be inferior.
When the stripe takes a waveform, it is preferable that the position variation in the x-axis direction is a repetition of the same wave. In the case of a concave / convex shape with random position variations, the optical path control by the concave / convex shape becomes insufficient, and when the white polyester film is incorporated in a backlight for a liquid crystal display device, it may cause uneven brightness. The period of the waveform in the x-axis direction is not particularly specified, but is preferably 30 μm or more in consideration of the stability of the process for producing the uneven shape. When the thickness is less than 30 μm, the deterioration of the mold that is the basis of the uneven shape becomes severe, and it becomes difficult to maintain the uneven shape in mass production, and thus to maintain the performance of the white polyester film.
When the stripe has a waveform, the position variation in the x-axis direction is preferably 20 μm or less. When the fluctuation is larger than 20 μm, the unevenness contributing to the optical path control may be reduced. In this case, since the effect of preventing uneven adhesion is reduced, the luminance is reduced when the white polyester film is incorporated in a backlight for a liquid crystal display device. It causes unevenness.
In the present invention, the uneven shape is preferably a shape in which small triangles or semi-elliptical shapes are connected when the cross section is cut along a plane perpendicular to the stripe direction. In this shape, the obtained triangle or semi-ellipse may be a single shape repeated or a mixture of a plurality of shapes, and the sizes of the shapes may be different from each other. However, in the shape, it is preferable that the interval between adjacent apexes is 2.0 μm or more and 30 μm or less. If the distance between the tops is less than 2.0 μm, when imprinting the uneven shape, there is a high possibility that the shape of the top or bottom will be deformed. In that case, the effect of improving the reflectivity and durability by the uneven shape is insufficient. It becomes. On the other hand, if it is larger than 30 μm, the durability improvement effect in the produced uneven shape and the reflectance improvement effect by optical path control may be insufficient. In addition, when trying to obtain unevenness with a gap between the tops of more than 30 μm, the number of unevenness is reduced, and durability in contact with other members may be inferior.
Furthermore, the uneven shape in the present invention requires that the difference in the z-axis direction between the adjacent top and bottom is 2.0 μm or more and 20 μm or less. If this difference is less than 2.0 μm, the effect of preventing uneven adhesion in the produced uneven shape is reduced, and this causes uneven brightness when the white polyester film is incorporated in a backlight for a liquid crystal display device. In addition, if it is larger than 20 μm, the effect of improving the reflectance may be insufficient, and when trying to obtain unevenness, the ratio of the polyester layer (A) in the polyester film becomes high, thereby forming fine bubbles. Since the ratio of the polyester layer (B) is small, the reflectance may be inferior.

[Measurement of physical properties and evaluation method of effects]
(1) Film thickness (total thickness of each layer)
The film sample is measured with a calibrated digital micrometer (M-30, manufactured by Sony Precision Technology), and the thickness at 10 points is measured, and the average value is taken as the film thickness.

(2) Thickness of each layer The film is sampled to 5 mm × 10 mm, and a cross section in the direction parallel to the film width direction (hereinafter sometimes referred to as “lateral direction”) is cut out using the microtome. Using a transmission electron microscope type HU-12 (manufactured by Hitachi, Ltd.), the cross section of the cut polyester layer (A) and the polyester layer (B) was observed and laminated from a cross-sectional photograph magnified 250 times. Calculate the thickness.
In this example and the comparative example, the total thickness is a thickness converted from the length from the top of the uneven shape to the outermost surface of the opposite surface, and is the thickness of the polyester layer (A) having the uneven shape. Is the top of the unevenness of the polyester layer (A) having an uneven shape from the boundary between the polyester layer (A) having an uneven shape and a polyester layer (for example, polyester layer (B)) different from the polyester layer (A) in contact with the polyester layer (A) By taking the length up to and converting, the thickness of the layer is obtained.
In addition, when each uneven shape is not a continuum of the same shape, 5 points are obtained for the tops of the uneven shape, the thickness measurement is repeated at each of the tops, and the average value is obtained to obtain the total thickness and the uneven shape. The thickness of the polyester layer (A) is measured.

(3) Measurement of uneven shape (3) -1: Interval between adjacent apexes in the uneven shape of the cross section perpendicular to the stripe direction Cut out the cross section in the same manner as in (2) and cut so that the uneven shape is on the upper side of the photograph Except for setting the prepared sample, the cross section is observed in the same manner as in (2) to obtain a cross-sectional photograph.
From the obtained photograph, a triangular or semi-elliptical vertex is marked, and the length between adjacent markings is measured and converted.
(3) -2: Height between top and bottom in an uneven shape with a cross section perpendicular to the stripe direction. A cross-sectional photograph is obtained as in (3) -1. From the obtained photograph, the vertex and bottom of a triangle or semi-ellipse are marked, and the adjacent vertices and the adjacent bottoms are connected by lines. The line and the length of the line are measured and converted.
(3) -3: Stripe shape The irregular surface is magnified about 10 times with an arbitrary stereomicroscope or loupe, and it is determined whether it is a straight line or a waveform.

  If it is a waveform, the film was sampled to 5 mm × 10 mm, and the surface of the cut sample was observed using a transmission electron microscope HU-12 (manufactured by Hitachi, Ltd.) and enlarged 250 times. Measure and convert the wavelength and height from

(4) In a haze meter HZ-2 manufactured by Internal Haze Suga Test Instruments, a transparent plastic cell having a size of 10 mm × 50 mm × 50 mm is filled with liquid paraffin, and standard alignment is performed. Thereafter, the measurement film cut to 50 mm × 50 mm is immersed so that the liquid paraffin is covered, the cell thus prepared is set in the haze meter, and the haze value measured based on JIS Z8741 is read, Let it be haze.
In the present invention, when measuring the internal haze of the polyester (A) layer, other layers are scraped off with a cutter knife, or the polyester (A) layer is peeled off from the polyester layer in contact with the polyester (A ) Only the layer is taken out, and the internal haze of the polyester (A) layer is obtained by carrying out the above measurement on the polyester (A) thus taken out.

(5) Relative average reflectivity A 60 mmφ integrating sphere was attached to a spectrophotometer (U-3310) manufactured by Hitachi High-Technologies, and an aluminum oxide standard white plate (manufactured by Hitachi High-Technologies, part No. 210-0740) was defined as 100%. The hourly reflectance is measured over 400-700 nm. The reflectance is read from the obtained chart at intervals of 5 nm, the arithmetic average value is calculated, and the relative average reflectance is obtained.

(6) Specific gravity The film is accurately sampled to 100 mm × 100 mm, and accurately weighed to the nearest 0.1 mg with an electronic balance (AC100 manufactured by Mettler). After taking out the weighed sample, the total thickness of each layer of the film is measured using a constant pressure thickness measuring instrument, and the specific gravity is calculated by applying to the following formula.
Specific gravity = (Weighed value (g)) / (Total thickness (μm) of each layer) × 100.

(7) Relative luminance and luminance unevenness As shown in FIG. 1, the reflective plate attached in the backlight of 181BLM07 (manufactured by NEC Corporation) is changed to a predetermined sample of white polyester film 1 for light reflection and turned on. I let you. Here, the backlight is formed by stacking elements of the light reflecting white polyester film 1, the cold cathode tube 2, the milky white plate 3, the diffusion plate 4, the prism sheet 5, and the polarizing prism sheet 6 in order from the bottom to the top in FIG. Composed. After waiting for 1 hour in the lighting state to stabilize the light source, the liquid crystal screen was photographed with a CCD camera 7 (DXC-390 made by SONY), and an image was captured by an eye scale 8 made by an image analyzer Eye System. Thereafter, the luminance level of the photographed image was controlled to 30,000 steps and automatically detected, and converted into luminance to obtain a value (unit: cd).

  In the present invention, the luminance was evaluated as relative luminance (unit:%) when # 250E6SL manufactured by Toray Industries, Inc. was used as a reference sample and the value was taken as 100%.

  Further, luminance unevenness (%) was obtained by the following equation.

Unevenness of luminance (%) = (maximum relative luminance value−minimum relative luminance value) / average relative luminance value × 100
In the present invention, the luminance unevenness was evaluated as relative luminance unevenness (unit:%) when # 250E6SL manufactured by Toray Industries, Inc. was used as a reference sample and the value was taken as 100%.

(8) Light guide plate scratches (side light method)
The reflective plate attached in the backlight of Sony Corporation VAIO (VGN-S52B / S) as shown in FIG. 2 is changed to a sample of a predetermined white polyester film 10 for light reflection. After making contact, the white polyester film for light reflection was removed, and the surface of the light guide plate was observed. Here, the backlight is configured by stacking elements of a cold cathode tube 9, a light reflecting white polyester film 10, a light guide plate 11, and a prism sheet 12 in order from the bottom to the top in FIG.

  The presence or absence of scratches was visually confirmed, and the light guide plate scratches were graded according to the following criteria.

A: No scratches B: Slight scratches but practical level F: Scratches practical level

  The present invention will be described based on examples. The following examples are examples of embodiments of the present invention, and the present invention is not limited thereto.

[Examples 1 to 23]
As the raw material polymer and inorganic particles of the polyester layer (A) and the polyester layer (B), the raw materials shown below were mixed at a blending ratio shown in Table 1. The raw material of the polyester layer (A) was vacuum dried at 180 ° C. for 3 hours, and then the extrusion temperature was set to 320 ° C. and supplied to the extruder A heated to 270 to 300 ° C. In parallel, after mixing the raw materials of the polyester layer (B) shown in Table 1 and vacuum drying at 180 ° C. for 3 hours, the extrusion temperature was set to 320 ° C. and the extruder B heated to 270 to 300 ° C. Supplied to.

(1) Base resin / polyethylene terephthalate chip (PET) (Example: F20S manufactured by Toray Industries, Inc.)
・ Polyethylene naphthalate chip (PEN) (Example: “Teonex” manufactured by Teijin Chemicals Ltd.)
Polyester copolymer (abbreviated as PET-I) having polyethylene terephthalate as the main skeleton and copolymerized with isophthalic acid (eg, F51M manufactured by Toray Industries, Inc.)
-Polyester copolymer (abbreviated as PET-N) (manufactured as a prototype) having polyethylene terephthalate as the main skeleton and copolymerized with naphthalenedicarboxylic acid.

(2) Low specific gravity agent Polyester copolymer (abbreviated as PBT-PTMG) in which polybutylene terephthalate and polyalkylene glycol are copolymerized (Example: “Hytrel” manufactured by Toray DuPont Co., Ltd.)
-Polyester copolymer (abbreviated as PET-I-PEG) having polyethylene terephthalate as the main skeleton and copolymerized with isophthalic acid and polyethylene glycol (Example: T794M manufactured by Toray Industries, Inc.)
(3) Incompatible polymer, polymethylpentene (PMP) (Example: “TPX” manufactured by Mitsui Chemicals, Inc.)
A copolymer of cyclic olefin and ethylene (abbreviated as COC) having a glass transition temperature of 190 ° C. (eg, “TOPAS” manufactured by Polyplastics Co., Ltd.)
(4) Inorganic particles and barium sulfate (purchased as a mixed chip with polyethylene terephthalate)
・ Titanium dioxide (purchased as a mixed chip with polyethylene terephthalate)
・ Silica (purchased as a mixed chip with polyethylene terephthalate).

The raw material polymers of the polyester layers (A) and (B) were laminated through a laminating apparatus so that the layer configuration and the layer thickness were as shown in Tables 1 and 2, and formed into a sheet shape by a T-die. Further, the unstretched sheet obtained by cooling and solidifying the sheet with a cooling drum having a surface temperature of 25 ° C. is led to a group of seven rolls adjusted to a heating temperature of 80 to 98 ° C., and longitudinally increased by 2.5 to 4.5 times. It extended | stretched and cooled with the roll group of 25 degreeC. The film was stretched by 2.5 to 4.5 times in a direction perpendicular to the longitudinal direction in an atmosphere heated to 130 ° C. while being guided by a clip while holding both ends of the film with clips. Thereafter, heat setting was carried out at 190 ° C. in a tenter, and after gradually cooling in the width direction, the film was cooled to room temperature and wound up to obtain a white film.
In the thermal imprint processing apparatus which set the flat metal plate (made of copper) and the imprint mold (copper) which can obtain the uneven shape shown in Tables 1 and 2 to the white film thus obtained, on the opposite surface side Then, after the imprint mold is heated to 110 ° C., the imprint mold is sandwiched between the metal plate and the film is pressed. The pressing pressure was 7 MPa on the mold surface, and the pressing time was 30 seconds. Thereafter, the temperature control plate is cooled both up and down while pressing is continued. When the temperature reaches 60 ° C., the cooling is stopped and the press pressure is released. The imprint mold was raised to the upper limit, the release device was driven, the film was released, and the film shown in Tables 1 and 2 was obtained by winding it with a winding device. The performance of the obtained film as a white polyester film is as shown in Tables 3 and 4, and all obtained films having high film forming properties and high relative average reflectance.

[Comparative Examples 1 to 7]
Using the raw materials listed in Table 5, a film was formed in the same manner and conditions as in the examples, and an imprint mold capable of obtaining the uneven shapes in Table 5 was prepared. Went. Although the performance of the obtained film is shown in Table 6, all were inferior in reflectance and resulted in inferior performance as a white polyester film for light reflection.

[Comparative Example 8]
Using the same raw material as Example 1, it formed into a film by the method and conditions similar to an Example, and obtained the polyester film. The embossing roll, which is striped in the width direction and has a continuous triangular groove with a distance of 10 μm between adjacent tops and a height of 5 μm between top and bottom, is heated to 160 ° C., and the polyester film is subjected to a pressure of 1 MPa. It passed at a speed of 10 m / min. The irregularities of the film thus obtained were as shown in Table 5, and the performance of the film was as shown in Table 6. Since the formation of triangles on the film surface was insufficient, the effect of the shape could not be exhibited, resulting in a film with poor reflectivity.

[Comparative Example 9]
Using the same raw material as Example 1, it formed into a film by the method and conditions similar to an Example, and obtained the polyester film. A white polyester film was obtained by applying 10 μm acrylic particles by gravure roll method to the polyester film using an acrylic binder paint. Although the characteristics as a film are shown in Table 6, the reflectance was inferior, and the performance as a white polyester film for light reflection was inferior.

The present invention relates to a white polyester film, and more particularly to a white polyester film used for the purpose of reflecting light having an uneven surface.

DESCRIPTION OF SYMBOLS 1 White polyester film for light reflection 2 Cold cathode tube 3 Milky white plate 4 Diffusion plate 5 Prism sheet 6 Polarizing prism sheet 7 CCD camera 8 Eye scale 9 made from an image analyzer Eye system Cold cathode tube 9
10 White polyester film for light reflection 11 Light guide plate 12 Prism sheet 13 CCD camera

Claims (8)

A polyester film comprising two or more polyester layers including a polyester layer (A) and a polyester layer (B), wherein the polyester layer (B) is whitened, and at least one of the polyester layers (A) The inner haze is 2.0% or less, and one of the polyester layers (A) forming the surface layer is given a regular uneven shape in at least one direction in the film plane. And a white polyester film having a relative average reflectance of 97% or more on the surface of the polyester layer (A). The white polyester film according to claim 1, wherein the whitening is performed by containing fine bubbles and / or white inorganic particles in the polyester layer (B). The said white polyester film consists of 2 layer structure of a polyester layer (A) / polyester layer (B), or 3 layer structure of a polyester layer (A) / polyester layer (B) / polyester layer (A). 2. The white polyester film according to 2. 4. The white polyester film according to claim 2, wherein the fine bubbles are produced by adding a resin and / or inorganic particles incompatible with polyester and biaxially stretching. 5. The white polyester film according to claim 1, wherein the uneven shape has a triangular or semi-elliptical shape in a cross section formed by the one direction and a direction perpendicular to the film surface. The white polyester film according to claim 5, wherein a length between adjacent top portions of the cross-sectional shape in the direction is 2.0 μm or more and 30 μm or less. The white polyester film according to claim 6, wherein a height between adjacent top and bottom portions of the cross-sectional shape in the direction is 2.0 μm or more and 20 μm or less. 2. The white polyester according to claim 1, wherein the uneven shape is a waveform in the one direction, the waveform has a periodicity in which the position variation is uniform, and the size of the position variation (variation width) is 30 μm or less. the film.

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