JP2007293316A - Light diffusion film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light diffusion film having excellent light transmittance and light diffusing property while maintaining excellent heat resistance, mechanical strength and thickness accuracy intrinsic to a biaxially stretched film, and suppressing generation of curling after a heat treatment. <P>SOLUTION: The light diffusion film is composed of a biaxially stretched multilayer film having a support layer made of a crystalline polyester and a light diffusion layer laminated on at least one surface of the support layer. by co-extrusion. The light diffusion layer contains a crystalline polyester by 60 to 98 parts by mass and a light diffusing additive incompatible with the polyester by 2 to 40 parts by mass. The light diffusion film has a plane orientation degree (ΔP) of 0.080 to 0.160, a total light transmittance of not less than 85%, and a haze of not less than 30%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a light diffusing film used for a backlight unit of a liquid crystal display, an illumination device, and the like. More specifically, the present invention relates to a light diffusing film having excellent heat resistance, mechanical strength and thickness accuracy inherent to a biaxially stretched film, and having excellent light transmittance and light diffusibility.

  In recent years, the technological progress of liquid crystal displays has been remarkable and widely used as display devices for personal computers, televisions, mobile phones and the like. Since these liquid crystal displays do not have a light emitting function by themselves, a liquid crystal display unit can be displayed by installing a backlight unit on the back surface thereof.

  There are various types of backlight units, but they are roughly classified into two types. In general, the most common method is an internal illumination method or a direct type, in which the light source is inside the illumination surface. In this method, a large number of light sources such as cold cathode ray tubes can be arranged directly under the illumination surface. Therefore, compared to the edge light method described later, extremely high luminance is obtained and light loss is small. is doing. Therefore, a direct type is often used for a large-sized liquid crystal display such as a large-sized liquid crystal TV that requires high luminance.

  However, the direct type has a problem in that a large luminance difference is easily generated between a position on the screen that is directly above the light source and a position that is not, and is easily recognized as luminance unevenness. For this reason, a light diffusing plate made of acrylic or polycarbonate having a thickness of several millimeters mixed with a light scattering material such as organic and inorganic fine particles on the light emitting surface, and, if necessary, the surface of the biaxially stretched polyester film A light diffusing film subjected to a light diffusing process is installed to reduce luminance unevenness.

  The other method is called an edge light type. The light source is arranged outside the illumination surface, and a fluorescent lamp (mostly a cold lamp is provided on one or two sides of the light guide plate made of a transparent acrylic resin plate or the like as the illumination surface. For example, a substantially linear light-emitting body such as a cathode discharge tube) is closely attached, and a lamp cover made of a reflector is provided to introduce light into the light guide plate. This method has the characteristics that it consumes less power and can be made smaller and thinner than the direct backlight unit described above. Therefore, when a thin display such as a notebook personal computer is required to be particularly thin and light, an edge light type backlight unit is widely used.

  The necessary functions required for the light guide plate of the edge light type backlight unit are a function of sending light incident from the end portion forward and a function of emitting the sent light to the liquid crystal display element side. The former function depends on the material used and the interface reflection characteristics, and the latter function depends on the shape of the light guide plate surface that avoids the total reflection condition. Regarding the shape of the surface of the light guide plate that avoids this total reflection condition, a method of forming a white diffusing material on the surface of the light guide plate and a method of forming a Fresnel shape of lenticular or prism on the surface of the light guide plate are known.

  However, the light emitted from the light guide plate in which these shapes are formed has a non-uniform light distribution according to the shape. Therefore, in order to obtain a high-quality image, an attempt is made to install a light diffusive film on the light guide plate, diffuse and scatter light passing through the light diffusion layer, and make the luminance of the light exit surface uniform.

  In order to further improve the front luminance of these backlight units, a light collecting sheet may be used so as to collect light emitted through the light diffusing film in the front direction as much as possible. This condensing sheet is a transparent sheet with many prisms, waves, pyramids, etc. arranged on the surface, refracting the emitted light that has passed through the light diffusing film and collecting it on the front, The brightness is improved. Such a condensing sheet is used by superimposing one or two sheets on the surface side of the light diffusing film.

  Furthermore, in order to diffuse and scatter the unevenness of brightness and the defects of the light collecting sheet caused by the light collecting sheet, the light diffusing film may be provided on the surface side of the light collecting sheet. is there.

  Each member (light diffusing plate, light guide plate, light diffusing film, light condensing sheet, etc.) constituting the backlight unit transmits light in order to suppress light loss and improve light utilization efficiency. A material with a high rate is required.

  On the other hand, these members generally have a structure in which a functional layer is laminated on a base film, and these members are bonded together with an adhesive to make a composite. By reducing the number of supports of these members, the number of times light is reflected at the interface between members having different refractive indexes can be reduced. Therefore, reducing the number of members is also effective for increasing the light utilization efficiency. On the other hand, attempts to impart other functions (for example, light diffusibility) to a single base film itself are also being studied.

As the light diffusing film used for the backlight unit, for example, a film obtained by forming a transparent thermoplastic resin into a sheet shape and then physically processing the surface, or a biaxially stretched polyester film A film obtained by coating a light diffusion layer made of a transparent resin containing fine particles on the surface is disclosed (see, for example, Patent Documents 1 and 2).
JP-A-4-275501 JP-A-6-59108

  In particular, a biaxially stretched polyester film obtained by coating a light diffusing layer made of a transparent resin containing fine particles has a high light transmittance and excellent light diffusibility. It is widely used because it has excellent heat resistance, mechanical strength, and excellent thickness uniformity, which are the characteristics of an axially stretched polyester film.

  However, in this method, there is a problem that curling tends to occur when the light diffusing film is heated due to a difference in linear expansion coefficient between the base film and the light diffusing layer. This problem is becoming an important problem particularly in a liquid crystal display employing a direct type backlight unit that requires a large size and extremely high luminance, such as a large liquid crystal TV in recent years. This is because the larger the area of the light diffusing film, the larger the area of the interface between the layers having different linear expansion coefficients, and the curling becomes conspicuous when the light diffusing film is heated. Furthermore, the higher the brightness of the display, the greater the power consumption of the light source, that is, the amount of heat generated by the backlight unit. Further, in this method, the light diffusion layer is formed by post-processing, which is disadvantageous with respect to the market demand for cost reduction.

  On the other hand, for the purpose of cost reduction such as reduction of the number of backlight unit parts by integration with other optical functional films such as light diffusing film and condensing sheet and simplification of manufacturing process, biaxial Many attempts have been made to impart light diffusibility to the stretched polyester film itself. And the approach to give the light diffusibility to the biaxially stretched polyester film itself having excellent heat resistance, mechanical strength, and excellent thickness uniformity leads to the solution of the problem of heating curl. . Therefore, its industrial value is very large.

However, any of the previously proposed attempts to impart light diffusibility to the biaxially stretched polyester film itself impairs any of the inherent characteristics of the biaxially stretched polyester film, Properties that the light diffusive film should have, such as transmittance and light diffusibility, are impaired, and it has not been put into practical use. For example, a biaxially stretched polyester film comprising a composite film of two or more layers and at least one layer containing fine bubbles therein is disclosed (for example, see Patent Document 3).
JP-A-11-268211

  This method has the characteristics inherent to the biaxially stretched polyester film, such as excellent heat resistance, mechanical strength, and excellent thickness uniformity. However, since the light diffusibility is imparted by bubbles existing inside the layer, there is a problem that the light transmittance is low. Because the bubbles generated in the biaxial stretching process of the film have a flat plate shape parallel to the film surface, when this is used in a backlight unit as a light diffusive film This is because the light emitted from the illumination surface is scattered back and the light transmittance is lost. In fact, the total light transmittance shown in the examples is only 83% even at the highest.

In addition, a multilayer type biaxially stretched polyester film using a low melting point polyester resin having a melting point of 200 ° C. or less as a resin constituting the light diffusion layer made of polyethylene terephthalate is disclosed (for example, (See Patent Document 4).
JP 2001-272508 A

  In the above method, consideration is given to suppression of voids that develop around the light diffusing agent and inhibit transparency. Therefore, the balance between light transmittance and light diffusivity is the same as that of conventional light diffusive films obtained by coating the surface of a biaxially stretched polyester film with a light diffusing layer made of a transparent resin containing fine particles. Comparable.

  However, the light diffusing film obtained by the method described in Patent Document 4 has a large melting point difference between the polyester resin constituting the base material layer and the polyester resin constituting the light diffusing layer. As a result, since the obtained biaxially stretched film has different linear expansion coefficients between the light diffusion layer and the base film, the biaxially stretched film itself tends to curl during heat treatment. For this reason, curling may occur due to heat treatment in the post-processing process, or curling may occur depending on the usage environment (temperature) of the liquid crystal display, and the luminance of the light exit surface of the backlight unit may be non-uniform.

In addition, a light diffusing layer in which a copolymer polyester or amorphous polyester having a melting point of 210 ° C. or less is used as a constituent resin and a light diffusing additive composed of incompatible particles or a thermoplastic resin is blended with the constituent resin is used as an intermediate. As a layer, a film is disclosed in which a crystalline polyester resin layer that forms a smooth surface made of polyethylene terephthalate is laminated on both surfaces (see, for example, Patent Documents 5 to 11).
JP 2001-324605 A JP 2002-162508 A JP 2002-182013 A JP 2002-196113 A JP 2002-372606 A JP 2004-219438 A JP 2004-354558 A

  The method described above is different from Patent Document 4 in that the structure of the film is the target structure. Therefore, the curling due to the asymmetric structure that occurs in Patent Document 4 is improved. However, since a resin having a significantly different melting point and crystallinity is used as a constituent component, there is a factor in increasing the occurrence of curling due to fluctuations in manufacturing conditions and the like. Moreover, since the above method comprises a layer composed of a low-melting point copolymer polyester resin or an amorphous polyester resin, 80% or more of the total thickness of the laminated film is a characteristic characteristic of the crystalline biaxially stretched polyester film, heat resistance, Excellent properties such as mechanical strength and thickness uniformity are impaired. As a result, the original purpose of the light diffusing film is achieved, which causes significant dimensional changes and poor flatness in high temperature processing and use in high temperature environments, and makes the brightness of the light exit surface of the backlight unit uniform. Can not.

Moreover, the biaxially-stretched polyethylene terephthalate film which mix | blended the spherical or convex lens-shaped particle | grains of a specific particle diameter is disclosed (for example, refer patent document 12).
Japanese Patent Laid-Open No. 2002-37898

  In the above document, Example 1 discloses a film having a total light transmittance of 88% and a diffuse transmittance of 68% while using polyethylene terephthalate as a raw material for polyester. In Example 5, a film having a total light transmittance of 85% and a diffuse transmittance of 63% is disclosed. These light transmittances are excellent characteristic values comparable to those obtained by coating a light diffusion layer made of a transparent resin containing fine particles on the surface of the biaxially stretched polyester film.

  However, the basic properties such as heat resistance, mechanical strength, and thickness accuracy of these films are not disclosed, and heat resistance, mechanical strength, and high thickness accuracy, which are inherent characteristics of biaxially stretched polyethylene terephthalate films, are not disclosed. The probability of obtaining is not recognized at all. This is because these films are obtained by stretching an unstretched film having a thickness of 200 μm by 3.0 times in the vertical, horizontal, and both directions, that is, by an area magnification of 9.0 times. Is 50 μm, and the actual area stretch ratio calculated from the thickness ratio before and after stretching is only 4.0 times.

  In other words, the set draw ratio and the actual draw ratio deviate significantly due to the effects of width shrinkage that occurs during longitudinal stretching, the distribution of the draw ratio in the width direction that occurs during transverse stretching, and dimensional changes during heat treatment. It is thought that. And, when the actual area draw ratio is about 4 times, even if excellent light transmittance is obtained, the heat resistance, mechanical strength and high thickness accuracy which are the original characteristics of the biaxially stretched film are achieved. That is impossible.

  From the above situation, the method of imparting light diffusibility to the biaxially stretched film itself maintains the heat resistance, mechanical strength, and high thickness accuracy that are the characteristics of the biaxially stretched film, while maintaining light transmittance and light diffusion. From the standpoint of comprehensive quality that makes the properties compatible, the method does not reach the method of post-processing the light diffusion layer on the transparent base film. Therefore, this method has not been put to practical use.

The objects of the present invention are the following two points.
(1) To provide a light diffusive film having excellent heat resistance, mechanical strength, thickness accuracy and the like inherent in a biaxially stretched polyester film, and having excellent total light transmittance and light diffusibility. In addition, the reduction of the number of backlight unit parts, the simplification of the manufacturing process, and the cost reduction by integrating the light diffusive film and the other optical functional film.
(2) To provide a light diffusing film in which the occurrence of curling after heat treatment caused by laminating resin layers having different linear expansion coefficients is suppressed. In addition, the brightness of the light exit surface should be made uniform in a liquid crystal display that employs a direct type backlight unit that is large and requires extremely high brightness.

  The present invention is a light diffusing film comprising a biaxially stretched laminated film having a support layer made of crystalline polyester and a light diffusion layer laminated on at least one side of the support layer by coextrusion, Contains 60 to 98 parts by mass of crystalline polyester and 2 to 40 parts by mass of a light diffusible additive incompatible with the polyester, and the light diffusive film has a degree of plane orientation (ΔP) of 0.080 to 0.001. The light diffusive film has a total light transmittance of 85% or more and a haze of 30% or more.

  Since the light diffusing film of the present invention is a biaxially stretched film made of polyester with high crystallinity, the biaxially stretched polyester film has excellent heat resistance, mechanical strength, thickness accuracy, and the like inherent to the biaxially stretched polyester film. In addition, the light diffusive film of the present invention adopts a specific layer structure, and controls the degree of plane orientation within a specific range, so that both the light transmittance and the light diffusivity are compatible at a high level. Yes. Furthermore, since the light diffusing film of the present invention has a high crystallinity and a multilayer structure including the same kind of polyester as a constituent component, the curl after the heat treatment caused by the lamination of resin layers having different linear expansion coefficients. Can be suppressed.

  The light diffusing film of the present invention is a light diffusing film comprising a biaxially stretched laminated film having a support layer made of crystalline polyester and a light diffusing layer laminated on at least one side of the support layer by a coextrusion method. The light diffusing layer contains 60 to 98 parts by mass of crystalline polyester and 2 to 40 parts by mass of a light diffusing additive incompatible with the polyester, and the light diffusing film has a plane orientation degree (ΔP) of 0. 0.080 to 0.160, the total light transmittance is 85% or more, and the haze is 30% or more.

(material)
The crystalline polyester used in the present invention is arbitrary without any limitation as long as it is a polyester in which a clear crystal melting heat peak (melting point) is observed by measurement using a differential scanning calorimeter. However, in order to achieve the inherent excellent heat resistance, mechanical strength, thickness accuracy, etc. of the biaxially stretched polyester film, the melting point of the crystalline polyester used as a raw material is preferably 250 ° C. or higher, and more preferably 253 ° C. or higher. It is preferable that

  A preferred upper limit of the melting point is 300 ° C. Further, the heat of crystal fusion is preferably 15 mJ / mg or more, more preferably 20 mJ / mg or more, and most preferably 30 mJ / mg or more. A preferable upper limit of the heat of crystal fusion is 50 mJ / mg. The most preferable crystalline polyester having such characteristics includes polyethylene terephthalate or polyethylene naphthalate homopolymer, and polyethylene terephthalate is most excellent in terms of cost performance.

  Moreover, said crystalline polyester may also contain a copolymerization component as needed. It is possible to control the degree of plane orientation of the biaxially stretched film by introducing a slight amount of a copolymer component into the polyester.

  Here, the components copolymerizable with the crystalline polyester include aromatic dicarboxylic acids such as isophthalic acid and naphthalenedicarboxylic acid or esters thereof, diethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl. Examples thereof include polyesters produced by polycondensation with glycols such as glycols. In addition to the direct weight method in which an aromatic dicarboxylic acid and a glycol are directly reacted, these polyesters can be transesterified by an alkyl ester of an aromatic dicarboxylic acid and a glycol and then subjected to a polycondensation, or an aromatic method. It can be produced by a method such as polycondensation of diglycol ester of dicarboxylic acid.

  However, when these copolymerization components are introduced excessively, the melting point of the polyester is lowered, and the original excellent characteristics of the biaxially stretched polyester film cannot be obtained. A preferable upper limit of the content of the copolymer component is 5 mol%, and a more preferable upper limit is 3 mol%. Maintaining the melting point of the polyester at 250 ° C. or higher by setting the content of the copolymer component to 5 mol% or less, and achieving the excellent heat resistance, mechanical strength, thickness accuracy, etc. inherent to the biaxially stretched polyester film Can do.

  Moreover, it is preferable that said polyester does not contain substantially particles other than the light diffusible additive mentioned later. For example, in the case of inorganic particles, the above-mentioned “substantially free of particles” is 50 ppm or less, preferably 10 ppm or less, and most preferably the detection limit or less when inorganic elements are quantified by fluorescent X-ray analysis. Means content. Thus, by using a clean polyester raw material without impurities, it is possible to suppress the occurrence of optical defects in the light diffusing film for liquid crystal displays.

(Light diffusing additive)
The light diffusing additive in the present invention is not particularly limited as long as it is a material incompatible with the polyester, but it is preferable to use the following materials.

(1) Thermoplastic resin incompatible with polyester As the light diffusing additive used in the present invention, a thermoplastic resin incompatible with polyester is most preferable. That is, by utilizing the incompatibility between the crystalline polyester and the thermoplastic resin, a thermoplastic resin that is incompatible with the polyester in the matrix made of the crystalline polyester in the production process (melting / extrusion process) of the biaxially stretched film. This is a technology in which domains made of resin are dispersed and used as a light diffusing substance.

  By using this technique, foreign matter can be filtered with a high-accuracy filter in the film melting / extrusion step, and the cleanliness required for a light diffusing film for a liquid crystal display can be achieved.

  On the other hand, when using non-melting polymer particles and inorganic particles, which will be described later, as a light diffusing additive, if the pore size of the filter is made fine in order to remove foreign substances, these particles are captured by the filter, and light diffusing properties are obtained. Not only decreases, but also makes it difficult to produce industrially due to filter clogging. On the other hand, when the pore size of the filter is increased in order to avoid clogging of the filter, foreign matter that becomes an optical defect of the liquid crystal display increases.

Examples of the thermoplastic resin incompatible with the polyester that can be used as the light diffusing additive of the present invention include the following materials.
(A) Polyolefin resins such as polyethylene, polypropylene, polymethylpentene, and cyclic olefins (b) Polycarbonate resins (c) Polystyrene resins such as atactic polystyrene, syndiotactic polystyrene, isotactic polystyrene (d) Polyamide resins (e) Polyether resin (f) Polyester amide resin (g) Polyphenylene sulfide resin (h) Polyphenylene ether resin (i) Polyether ester resin (j) Polyvinyl chloride resin (k) Acrylic resin represented by polymethacrylic ester (L) a copolymer comprising any one of (a) to (k) as a main component, or a mixture of these resins

  Among these, it is particularly preferable to use an amorphous transparent polymer in order to produce a light diffusing film having a high light transmittance. On the other hand, when a crystalline polymer is used as a light diffusing additive, the crystalline polymer becomes cloudy, the internal haze of the film increases, and the light transmittance may decrease.

  Examples of the amorphous transparent polymer that can be used in the present invention include polystyrene resins, styrene resins such as acrylonitrile / styrene copolymers, methyl methacrylate / styrene copolymers, cyclic olefin resins, and methacrylic resins. Examples thereof include acrylic resins and polycarbonate resins. Of these amorphous transparent polymers, polystyrene resins or styrene copolymers are particularly preferable.

  The melt viscosity ηs of the polystyrene resin is preferably 1000 poise or more, more preferably 3000 poise from the viewpoint of making the distribution of the polystyrene resin phase in the dispersed particles uniform and stabilizing the phase structure of the dispersed particles. On the other hand, the melt viscosity ηs of the polystyrene resin is preferably 10000 poise or less, more preferably 7000 poise, from the viewpoint of improving dispersibility in the extruder and stabilizing the size of the dispersed particles.

(2) Non-melting polymer particles Non-melting polymer particles that can be used as the light diffusing additive of the present invention are 30 ° C. to 350 ° C. using a melting point measuring device (manufactured by Stanford Research Systems, Inc., MPA100 type). The composition is not limited as long as the particles do not undergo flow deformation due to melting when the temperature is raised to 10 ° C./min. Examples thereof include acrylic resins, polystyrene resins, polyolefin resins, polyester resins, polyamide resins, polyimide resins, fluorine resins, urea resins, melamine resins, and organic silicone resins. The shape of the particles is preferably spherical, particularly preferably spherical. The particles may or may not have pores. Furthermore, you may use both together.

  When the non-melting polymer particles are made of a polymer having a melting point of 350 ° C. or higher, non-cross-linked polymer particles may be used, but from the viewpoint of heat resistance, it is necessary to use cross-linked polymer particles made of a polymer having a cross-linked structure. preferable.

  The average particle size of the non-melting polymer particles is preferably 0.5 to 50 μm. The lower limit of the average particle size of the non-melting polymer particles is more preferably 1.0 μm, and particularly preferably 2.0 μm. When the average particle size of the non-melting polymer particles is less than 0.5 μm, it is difficult to obtain a good light diffusion effect.

  On the other hand, the upper limit of the average particle size of the non-melting polymer particles is more preferably 30 μm, and particularly preferably 20 μm. When the average particle diameter of the non-melting polymer particles exceeds 50 μm, the film strength and the total light transmittance are likely to decrease. The non-melting polymer particles are preferably particles having a sharp particle size distribution as much as possible.

  One type of non-melting polymer particles may be used, or two or more types may be used. Use of a plurality of non-melting polymer particles having a sharp particle size distribution (meaning that the particle size of the particles is uniform) and different average particle sizes is mixed with coarse particles, which is a film defect. Is a preferred embodiment.

In addition, the measurement of the average particle diameter of said particle | grain is performed with the following method.
Take a photograph of the particles with a scanning electron microscope (SEM), measure the maximum diameter of 300-500 particles at a magnification such that the size of one smallest particle is 2-5 mm, and calculate the average value. Average particle diameter. Moreover, when the particle | grains contained in a film are individual, the largest diameter of each particle | grain is measured and let the average value be an average particle diameter.

(3) Inorganic particles Examples of inorganic particles that can be used as the light diffusing additive include silica, calcium carbonate, barium sulfate, calcium sulfate, alumina, kaolinite, and talc.

  The average particle diameter of the inorganic particles is preferably 0.1 μm, more preferably 0.5 μm, particularly preferably 1 μm, from the viewpoint of obtaining a good light diffusion effect. On the other hand, the average particle diameter of the inorganic particles is preferably 50 μm, more preferably 30 μm, and particularly preferably 20 μm from the viewpoint of suppressing a decrease in film strength.

  The particle size distribution of the inorganic particles is preferably as sharp as possible. When it is necessary to widen the particle size distribution, it is preferable to use particles having a sharp particle size distribution and a different average particle size in order to suppress the incorporation of coarse particles, which is a drawback of the film.

In addition, the measurement of the average particle diameter of particle | grains is performed with the following method.
Take a photograph of the particles with a scanning electron microscope (SEM), measure the maximum diameter of 300-500 particles at a magnification such that the size of one smallest particle is 2-5 mm, and calculate the average value. Average particle diameter. Moreover, the maximum diameter of the particle | grains contained in a film is measured, and let the average value be an average particle diameter.

  The shape of the inorganic particles is not limited, but is preferably substantially spherical or true spherical. The particles may be non-porous or porous. Furthermore, you may use both together.

  As the light diffusing additive, one of the above three types may be used, or two or more types may be used in combination.

(4) Mixing ratio of light diffusing additive The light diffusing layer in the light diffusing film of the present invention comprises 60 to 98 parts by mass of crystalline polyester and 2 to 40 light diffusing additive incompatible with the polyester. It consists of a composition containing parts by mass.

  When the mixing ratio of the light diffusing additive is less than 2 parts by mass, the light diffusing performance is insufficient. On the other hand, when the mixing ratio of the light diffusing additive exceeds 40 parts by mass, the number and size of voids generated around the light diffusing additive increases to increase the internal haze, and the total light transmittance. Tends to decrease. Moreover, since the composition of the light diffusion layer and the support layer is greatly different, the film tends to curl easily. Furthermore, since the light diffusible additive easily falls off during biaxial stretching of the film and the fallout causes foreign substances, this is not preferable.

  The mixing ratio of the light diffusing additive varies depending on the material used, and is adjusted within the above range so that the total light transmittance is 85% or more and the haze is 30% or more. The adjustment of the mixing ratio is a design matter that can be adjusted by conducting a preliminary experiment, and does not require excessive trial and error.

(Layer structure)
The light diffusing film of the present invention is a composition comprising the crystalline polyester and the light diffusing additive incompatible with the crystalline polyester on at least one side of the support layer (A) made of the crystalline polyester. It is important that the light diffusing layer (B) made of a multilayer structure is formed by coextrusion. By adopting such a multilayer structure, a light diffusive film having a high haze and a high total light transmittance can be obtained. That is, the light diffusion (internal haze) inside the film is suppressed to achieve a high total light transmittance, and the light diffusion effect (surface haze) generated by the irregularities on the surface of the light diffusion layer (B) is effectively utilized. High haze can be achieved.

  The layer structure of the light diffusing film of the present invention may be a two-layer structure as described above, or may be a multilayer structure of three or more layers as necessary.

  When the light diffusing film of the present invention is used as the light diffusing sheet on the lower surface of the prism sheet (light collecting sheet), the light diffusing layer (B) is coextruded and laminated on both surfaces of the support layer (A). Since the light diffusing effect caused by surface irregularities can be obtained on both sides of the film, a light diffusive film having higher haze can be obtained while maintaining high light transmittance.

Further, when the light diffusive film of the present invention is used as a light diffusive sheet on the upper surface of the prism sheet (light collecting sheet), the surface facing the prism sheet (light collecting sheet) needs to be smooth, What is necessary is just to set it as the structure which provided the light-diffusion layer (B) in the single side | surface of the support layer (A).
In addition, in order to perform prism processing on the light diffusive film of the present invention and to integrate the light diffusive film and the prism sheet (light collecting sheet), the light diffusing layer (B) is used as the support layer (A In the preferred embodiment, it is possible to effectively utilize the characteristics of the light diffusive film of the present invention by subjecting the surface of the support layer (A), which is a smooth surface of the structure, to prism processing. is there.

  Further, layers having functions other than the support layer (A) and the light diffusion layer (B) may be coextruded and laminated. In particular, in order to prevent a light diffusing additive contained in the light diffusing layer (B) from bleeding out or falling off the film surface and to produce a light diffusing film having no defects, the light diffusing layer (B It is effective to co-extrusion the protective layer (C) on the surface of In this case, the light diffusing additive is not blended in the protective layer (C) or even when blended, the blending ratio needs to be less than the blending ratio to the light diffusing layer (B).

  As the main raw material of the protective layer (C), the above-mentioned crystalline polyester may be used, a low crystalline polyester having a melting point of less than 250 ° C., or an amorphous property in which no clear crystal melting heat peak (melting point) is observed. The polyester may be used.

  In the present invention, the thickness of the light diffusion layer (B) is preferably 3 μm, and more preferably 50 μm, in order to make the laminated structure of the film uniform and make the light diffusibility uniform. On the other hand, the upper limit of the thickness of the light diffusion layer (B) is preferably 70 μm, more preferably 50 μm, in order to increase the surface haze and suppress the decrease in the total light transmittance due to the increase in the internal haze. To do.

  Moreover, it is preferable that the ratio with respect to the film whole thickness of the said light-diffusion layer is 3 to 50%, More preferably, it is 4 to 40%, Most preferably, it is 5 to 30%. When the ratio of the light diffusing layer to the total film thickness is less than 3%, the laminated structure of the film becomes nonuniform and the light diffusibility becomes nonuniform.

  On the other hand, when the ratio of the light diffusion layer (B) to the total film thickness exceeds 50%, curling due to a difference in thermal or mechanical properties between the support layer (A) and the light diffusion layer (B) occurs. Since it becomes easy to generate | occur | produce, it is not preferable. Furthermore, an unnecessary increase in the ratio of the light diffusing layer to the total film thickness is undesirable from the viewpoint of reducing the smoothness of the surface of the support layer (A).

  Moreover, when providing a protective layer (C), it is preferable to make thickness of a protective layer (C) thinner than the thickness of the said light-diffusion layer. When a low crystalline polyester having a melting point of less than 250 ° C. or an amorphous polyester is used as a raw material for the protective layer (C), the thickness of the protective layer (C) is preferably less than 20 μm. Preferably it is less than 10 μm.

(Characteristics of light diffusing film)
It is important that the light diffusive film of the present invention has a degree of plane orientation (ΔP) of 0.080 to 0.160, a total light transmittance of 85% or more, and a haze of 30% or more.

  The lower limit of the degree of plane orientation (ΔP) is more preferably 0.100, and still more preferably 0.110. On the other hand, the upper limit of the degree of plane orientation (ΔP) is more preferably 0.150, and still more preferably 0.140. When the plane orientation degree (ΔP) exceeds 0.160, the unevenness on the surface of the light diffusion layer (B) is lowered (flattened), and the light diffusion effect (surface haze) caused by the surface unevenness is remarkably reduced. Also, when the degree of plane orientation (ΔP) exceeds 0.160, the number and size of voids generated around the light diffusible additive increase depending on the type of the light diffusible additive used, Haze increases. Therefore, the total light transmittance tends to decrease. In any case, when the degree of plane orientation (ΔP) exceeds 0.160, the balance between the total light transmittance and the light diffusibility cannot be achieved, which is not preferable.

  In order to reduce the degree of plane orientation of the film, (1) a method of reducing the stretching stress at the time of biaxial stretching, or (2) a method of relaxing the stress remaining on the film after biaxial stretching. As the former method (1), for example, (a) a method of decreasing the stretching ratio, (b) a method of increasing the stretching temperature, (c) a method of decreasing the stretching speed, (d) And a method in which a polymerized polyester is used in combination. Moreover, as the latter method (2), a method of increasing the heat setting temperature higher than usual is exemplified. These methods are used alone or in combination to adjust the plane orientation degree of the film.

  On the other hand, when the plane orientation degree is less than 0.080, the characteristics as a biaxially stretched film are lost, and the mechanical strength is remarkably lowered. Moreover, the film thickness uniformity also deteriorates.

  Therefore, in the present invention, in view of the balance between the total light transmittance and light diffusibility, and the heat resistance, mechanical strength, and thickness accuracy of the biaxially stretched polyester film, in particular, stretching in the machine direction and transverse direction. A method of controlling the degree of plane orientation is preferable by completing all stretching from the start to the end of stretching at a slow stretching speed of less than 80% / second.

  The preferable lower limit of the total light transmittance in the light diffusing film of the present invention is 86%, the more preferable lower limit is 88%, the still more preferable lower limit is 89%, and the most preferable lower limit is 90%. On the other hand, the preferable upper limit of the total light transmittance is 98%.

  The preferable lower limit of haze in the light diffusing film of the present invention is 40%, the more preferable lower limit is 50%, and the most preferable lower limit is 60%. On the other hand, the preferable upper limit of haze is 100%.

  Furthermore, in the present invention, the surface haze, which is the difference between the above haze and the internal haze obtained by the following method, is preferably 20% or more. The surface haze is more preferably 30% or more, and further preferably 40% or more. A preferable upper limit of the surface haze is 100%.

[Evaluation of internal haze]
The value obtained by subtracting the haze (one haze) measured by the above-described normal method from the haze (two hazes) measured by overlapping the two sheets with a cedder oil is defined as the internal haze.

  The surface haze is imparted by surface irregularities on the surface of the light diffusion layer, and is imparted by light scattering generated when light is emitted from the film surface or incident on the film surface. Therefore, the surface haze and the total light transmittance are basically irrelevant, and the increase in the surface haze can increase the haze while suppressing the decrease in the total light transmittance.

  On the other hand, the internal haze is imparted by light scattering inside the film and causes backscattering of incident light, so that the total light transmittance is lowered. Therefore, in order to produce a light diffusive film having a high haze and a high total light transmittance, it is an effective means to increase the surface haze and reduce the internal haze as much as possible.

  The internal haze in the present invention is preferably 12% or less, more preferably 10% or less, and most preferably 5% or less. Moreover, the minimum with preferable internal haze is 0.1%, and a more preferable minimum is 1%. In order to reduce the internal haze to 0.1% or less, the light diffusibility additive can only be blended in the vicinity of the film surface layer, so that the light diffusibility may be nonuniform.

Next, the heat resistance, mechanical strength, and thickness accuracy of the light diffusing film of the present invention will be described. It should be noted that the characteristics described below explain the effects obtained as a result of the present invention and do not limit the present invention.
The light diffusive film of the present invention causes significant dimensional change and deterioration of flatness in processing at high temperatures and use in a high temperature environment, and makes the brightness of the light exit surface of the backlight unit uniform. In order to achieve the original purpose of the film, the dimensional change rate at 150 ° C. is preferably 3% or less. More preferably, it is 2% or less, more preferably 1.0% or less, and most preferably 0.5% or less.

  In addition, the light diffusing film of the present invention has sufficient mechanical strength of the biaxially stretched film, and its tensile strength in the machine direction and in order to suppress problems such as cracking, tearing, and folding in the film processing step. It is preferable that it is 100 MPa or more in both lateral directions. The lower limit of the tensile strength is preferably 110 MPa, more preferably 140 MPa, and particularly preferably 150 MPa.

  In addition, the light diffusive film of the present invention prevents deterioration of flatness due to wrinkles and bumps when winding the film on a roll, and makes the luminance of the light exit surface in the backlight unit uniform by the following method. The measured thickness variation is preferably 10% or less. The thickness unevenness is more preferably 5.0% or less, and further preferably 4.0% or less. Although it is desirable that the thickness unevenness is small, it is technically difficult to make the thickness unevenness 0.1% or less, and since there is no significant difference in practical quality, the lower limit of the thickness unevenness is 0. It may be 1%.

[Evaluation of thick spots]
A tape-like sample (length 1 m) continuous in the longitudinal stretching direction is collected, and the thickness at 100 points is measured at a pitch of 1 cm using an electronic micrometer manufactured by Seiko EM Co., Ltd. and Millitron 1240. From the measured values, the maximum value (dmax), the minimum value (dmin), and the average value (d) of the thickness were obtained, and the thickness unevenness (%) was calculated by the following formula. In addition, the measurement was performed 3 times and the average value was calculated | required.
Thickness unevenness (%) = ((dmax−dmin) / d) × 100

  The light diffusing film of the present invention preferably has a curl value of 20 mm or less after heat treatment at 100 ° C. for 30 minutes under no load for the following reasons (a) and (b). More preferably, it is 10 mm or less, further 5 mm or less, and most preferably 2 mm or less.

(A) For example, it is possible to prevent deterioration in handling properties during work under no tension when incorporated into a final product as a light diffusing film.
(B) Further, in processing at a high temperature and use in a high temperature environment, the occurrence of distortion of the light diffusing film is suppressed, and the luminance of the light emitting surface in the backlight unit is made uniform.

  Regarding curling suppression, in the present invention, both the support layer (A) and the light diffusion layer (B) are suppressed by using a crystalline polyester, but it is preferable to use the same type of polyester for both layers. .

  In addition, the curl caused by the structural difference between the front and back of the film applied in each process, such as preheating, stretching, cooling, and winding, is controlled, including the crystallinity in the film thickness direction due to the cooling rate difference between the front and back cooling during extrusion. Therefore, it is preferable to apply a method of actively generating a structural difference between the front and back of the film and complementing the necessary structural difference to bring the curl value close to zero.

Specifically, for the longitudinal curl immediately after film formation, the stretching temperature of the film back and front during longitudinal stretching may be controlled. The surface stretched at a lower temperature enhances the molecular orientation of the polyester than the opposite surface. Therefore, the linear expansion coefficient of the film decreases. By utilizing this behavior, the linear expansion coefficient on the front and back of the film can be controlled, and the curl in the vertical direction can be controlled.
Similarly, the curling in the lateral direction can be controlled by controlling the stretching temperature during transverse stretching separately for the front and back sides.

(Manufacture of biaxially stretched film)
In the present invention, the method for imparting the above properties to the light diffusing film is not limited, but the following method is a preferred embodiment.

  In the method for producing a light diffusing film of the present invention, in the method for producing a film at a draw ratio, which is carried out in the production of a normal polyester film in which a film having high thickness accuracy is obtained using the crystalline polyester as a constituent component. The greatest feature is to obtain a light diffusing film having the above characteristics by suppressing formation of voids generated around the light diffusing additive during stretching and forming sufficient irregularities on the surface of the light diffusing layer. In order to achieve the object, it is important to perform biaxial stretching of the film at specific stretching conditions, particularly at a slow stretching speed in both the machine direction and the transverse direction.

  Hereinafter, a preferred example of the method for producing a light diffusing film of the present invention will be described in detail with respect to a representative example using polyethylene terephthalate (hereinafter referred to as PET) pellets as crystalline polyester which is a raw material of the light diffusing layer (B). However, of course, it is not limited to this.

  Usually, the above pellets are transported by air using a predetermined pipe. In this case, it is preferable to use air that has been purified using a HEPA filter in order to prevent dust contamination. The HEPA filter used at this time is preferably a filter having a performance of cutting 95% or more of dust having a nominal filtration accuracy of 0.5 μm or more.

  First, as a film material, polyester and a thermoplastic resin that is incompatible with polyester are dried by vacuum drying or hot air drying so that the moisture content is less than 100 ppm. Next, each raw material is weighed and mixed, supplied to an extruder, and melt extruded into a sheet. Furthermore, the sheet in the molten state is brought into close contact with a metal rotating roll (chill roll) controlled at a surface temperature of 10 to 50 ° C. using an electrostatic application method, and further cooled and solidified by blowing cold air from the opposite surface. An unstretched PET sheet is obtained.

  At this time, it is possible to control the resin temperature up to the melting part, kneading part, polymer tube, gear pump, and filter of the extruder to 280 to 290 ° C., and the polymer temperature to the subsequent polymer tube and die to 270 to 295 ° C., This is preferable in order to suppress the generation of foreign matters such as deteriorated products.

  In addition, high-precision filtration is performed at any place where the molten resin is maintained at about 280 ° C. in order to remove foreign substances contained in the resin. The filter medium used for high-precision filtration of the molten resin is not particularly limited, but in the case of a stainless steel sintered filter medium, the removal performance of aggregates and high melting point organic substances mainly composed of Si, Ti, Sb, Ge, Cu Excellent and suitable. When performing the high-precision filtration, when the temperature of the molten resin is lower than 280 ° C., the filtration pressure increases, so that it is necessary to take measures such as reducing the discharge amount of the raw material resin, and the productivity is lowered.

  Further, the filter particle size (initial filtration efficiency 95%) of the filter medium is preferably 20 μm or less, particularly preferably 15 μm or less. When the filter particle size of the filter medium (initial filtration efficiency 95%) exceeds 20 μm, foreign matters having a size of 20 μm or more cannot be sufficiently removed. Productivity may be reduced by performing high-precision filtration of molten resin using a filter medium having a filter particle size (initial filtration efficiency of 95%) of 20 μm or less. However, a film with less optical defects due to coarse particles may be used. It is an important process in obtaining. In the present invention, the above-described high-accuracy filtration is possible by using a thermoplastic resin that is incompatible with polyester as the light diffusibility-expressing substance.

  In order to coextrude and laminate the light diffusion layer (B) and the support layer (A), the raw materials of each layer are extruded using two or more extruders, and a multi-layer feed block (for example, a confluence having a rectangular confluence) The two layers are joined together using a block), extruded into a sheet from a slit die, and cooled and solidified on a casting roll to form an unstretched film. Alternatively, a multi-manifold die may be used instead of the multilayer feed block.

Moreover, in the light diffusable film of this invention, it is preferable to have a coating layer on at least one surface, and also you may have a coating layer on both surfaces. A preferable coating amount is in the range of 0.005 to 0.20 g / m ² . By providing the coating layer on the film surface, generation of reflected light on the film surface can be suppressed, and the total light transmittance can be further increased. Further, when prism processing or hard coat processing is performed on the surface opposite to the light diffusion layer, easy adhesion can be imparted.

  In this case, after an application layer is provided on the unstretched film obtained by the above method, simultaneous biaxial stretching is performed. Moreover, when performing by a sequential extending | stretching method, after providing an easily bonding layer in the film uniaxially stretched to the vertical or horizontal direction, it extends | stretches to an orthogonal direction and performs biaxial stretching.

  The method for applying the coating layer layer forming coating solution to an unstretched film or a uniaxially stretched film can be selected from any known methods, such as reverse roll coating, gravure coating, kiss coating, and die coating. Coating method, roll brushing method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, impregnation coating method, curtain coating method, etc., and these methods can be applied alone or in combination. .

  The resin constituting the coating layer is selected from the group consisting of a copolyester resin, a polyurethane resin, and an acrylic resin from the viewpoint of ensuring better adhesion with other members in light diffusive film applications. It is preferable that the main component is one or more selected from the above. These resins are also recommended from the viewpoint of suppressing the generation of reflected light on the film surface. In addition, the said "main component" in an easily bonding layer means that at least 1 type of resin enumerated above is 50 mass% or more in 100 mass% of resin which comprises this layer.

  In order to increase the transparency of the film, if the support layer (A) does not contain particles, or if it contains only a small amount to the extent that the transparency is not hindered, the slipperiness of the film becomes insufficient and handling properties are improved. It may get worse. Therefore, you may add the particle | grains for the purpose of providing slipperiness to said application layer. For these particles, it is important to use particles having an extremely small average particle diameter equal to or smaller than the wavelength of visible light in order to ensure transparency.

  Examples of the particles include inorganic particles such as calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, and molybdenum sulfide; crosslinked polymer particles; calcium oxalate And organic particles. Silica is particularly preferable when the coating layer is formed mainly of the copolymer polyester resin. Since silica has a relatively close refractive index to that of polyester, it is most preferable in that it can secure a light diffusive film having more excellent transparency.

  The particles contained in the coating layer preferably have an average particle size (SEM observation particle size) of 0.005 to 1.0 μm from the viewpoint of ensuring transparency, handling properties, and scratch resistance of the light diffusing film. . The upper limit of the average particle size of the particles is more preferably 0.5 μm, particularly preferably 0.2 μm, from the viewpoint of transparency. Further, the lower limit of the average particle diameter of the particles is more preferably 0.03 μm from the viewpoints of handling properties and scratch resistance.

In addition, the measurement of the average particle diameter of said particle | grain is performed with the following method.
Take a photograph of the particles with a scanning electron microscope (SEM), measure the maximum diameter of 300-500 particles at a magnification such that the size of one smallest particle is 2-5 mm, and calculate the average value. Average particle diameter. Moreover, when calculating | requiring the average particle diameter of the particle | grains contained in a coating layer, using a transmission electron microscope (TEM), the cross section of a coating film with the magnification | multiplying_factor which makes the size of one smallest particle | grain become 2-5 mm The maximum diameter of particles existing in the cross section of the coating layer is obtained. The average particle diameter of the particles composed of aggregates is obtained by photographing 300 to 500 cross sections of the coating layer of the coating film using an optical microscope at a magnification of 200 times and measuring the maximum diameter.

  The content of the particles in the coating layer is 0.1 to 60% by mass relative to the total amount of the constituent components of the coating layer, ensuring transparency, adhesion, handling properties, and scratch resistance of the optical laminated film. It is preferable from the point. The upper limit of the content of particles is more preferably 50% by mass, particularly preferably 40% by mass, from the viewpoints of transparency and adhesion. Further, the lower limit of the content of the particles is more preferably 1% by mass from the viewpoints of handling properties and scratch resistance.

  Two or more kinds of the particles may be used in combination, and the same kind of particles having different particle sizes may be blended, but in any case, the average particle size of the whole particles and the total content are within the above range. Is preferably satisfied.

  Next, the unstretched film obtained by the above method is simultaneously biaxially stretched or sequentially biaxially stretched, and then heat-treated.

  It is important that the above biaxial stretching is performed at a stretching ratio of 2.8 times or more in both the vertical, horizontal and both directions. In addition, the draw ratio defined by this invention is the actual draw ratio by which the film was actually extended | stretched. This stretch ratio can be grasped by writing a mass change rate per unit area before and after each stretching step and a lattice-shaped magnification marker on an unstretched film. If the draw ratio in either the machine direction or the transverse direction is less than 2.8 times, the heat resistance and mechanical strength inherent to the biaxially stretched film cannot be obtained. In addition, the film thickness uniformity is significantly deteriorated. The lower limit of the preferred draw ratio in the present invention is 2.9 times, the more preferred lower limit is 3.0 times, and the most preferred lower limit is 3.1 times. The preferable upper limit of the draw ratio is 6.0 times.

  In the biaxial stretching in the present invention, it is particularly important to perform stretching in both the longitudinal and transverse directions at a stretching speed of less than 80% / second, more preferably at a stretching speed of 50% / second or less. The stretching speed in the present invention represents the deformation rate of the film per unit time based on the dimensions of the unstretched film, and the stretching speed in the machine direction and the transverse direction (unit:% / second) is Each is defined by the following formula.

Longitudinal stretching speed (% / second) = Acceleration during film running (m / second / second)
÷ Speed of unstretched film (m / sec) × 100

Stretching speed in the transverse direction (% / second) = width change rate per second (m / second)
÷ Unstretched film width (m) x 100

  Then, all stretching in the machine direction and transverse direction from the start of stretching to the end of stretching is completed at a stretching speed of less than 80% / second. For the first time, this makes it possible to industrially stably produce a product having a plane orientation of 0.160 or less while using crystalline polyester as a matrix polymer. As a result, a light diffusive biaxially stretched film having high total light transmittance, excellent light diffusibility, combined with the original heat resistance and mechanical strength of the biaxially stretched polyester film can be produced with excellent thickness accuracy. It becomes possible.

  On the other hand, the lower limit of the stretching speed is not limited, but if the stretching speed is made slower than necessary, the productivity of the film is lowered or excessive equipment investment is required in the production of the film on an industrial scale. Therefore, in the present invention, the maximum stretching speed between the start of stretching and the end of stretching is preferably 5% / second or more, and more preferably 10% / second or more.

  In a sequential biaxial stretching method that is generally performed, a roll-type stretching machine is used for stretching in the machine direction. However, roll-type stretching has a very high stretching speed, and it is difficult to obtain the effects of the present invention.

  As a biaxial stretching machine that can control the stretching speed in the machine direction and the transverse direction as described above, it guides to the tenter while holding both ends of the film with clips, and controls the width between clips and the transport speed of clips. Thus, a tenter type simultaneous biaxial stretching machine equipped with a mechanism capable of continuous stretching in both the longitudinal and lateral directions is suitable. As long as the equipment has this function, the clip transport mechanism is optional and is not particularly limited, but a conventionally known apparatus such as a pantograph system, a linear motor system, or a screw system can be adopted.

  In the biaxial stretching of the film, the detailed conditions such as stretching temperature, heat treatment temperature, and time are the characteristics of the matrix polymer and the characteristics required for the film, for example, optical characteristics such as refractive index, mechanical characteristics, dimensional change rate. It is possible to select appropriately according to the thermal characteristics such as, the desired crystallinity, etc., and there is no particular limitation.

  However, when PET is used as the matrix polymer and a simultaneous biaxial stretching machine is used for biaxial stretching of the film, the preferred stretching temperature is 95 ° C to 110 ° C. When the stretching temperature (maximum temperature) exceeds 110 ° C., it becomes difficult to control the plane orientation degree of the film to 0.080 or more. Furthermore, the uniformity such as the thickness accuracy of the film also decreases. On the other hand, when the stretching temperature (maximum temperature) is less than 95 ° C., it is difficult to uniformly control the plane orientation degree of the film to 0.160 or less.

  The heat treatment temperature of the film is preferably in the range of 200 ° C. to 250 ° C., and the heat treatment time is preferably in the range of 10 seconds to 100 seconds. Moreover, you may perform the relaxation process of the vertical direction and / or a horizontal direction simultaneously with heat processing or after heat processing.

  Next, the effect of this invention is demonstrated using an Example and a comparative example. First, the evaluation method of the characteristic values used in the present invention is shown below.

[Evaluation methods]
(1) Intrinsic viscosity of polyester resin In accordance with JIS K 7367-5, a mixed solvent of phenol (60% by mass) and 1,1,2,2-tetrachloroethane (40% by mass) is used as a solvent at 30 ° C. It was measured.

(2) Calorie melting calorie and melting point It is determined using a differential scanning calorimeter (DSII Nanotechnology, DSC 6220). In a nitrogen atmosphere, the sample is heated and melted at 300 ° C. for 5 minutes and then rapidly cooled with liquid nitrogen. Next, 10 mg of the sample is heated at a rate of 20 ° C./min. From the obtained DSC curve, the heat of fusion was determined from the area of the endothermic peak accompanying the melting of the crystal, and this was divided by the mass of the sample to calculate the heat of crystal melting. The peak of the endothermic peak was defined as the melting point.

(3) Melt viscosity The melt viscosity at a resin temperature of 285 ° C. and a shear rate of 100 / sec was measured using a flow tester (manufactured by Shimadzu Corporation, CFT-500). Note that measurement of melt viscosity at a shear rate of 100 / sec is difficult to perform with the shear rate fixed at 100 / sec. Therefore, using an appropriate load, any shear rate of less than 100 / sec and The melt viscosity was measured at an arbitrary shear rate greater than the rate, the melt viscosity was plotted on the vertical axis, and the shear rate was plotted on the horizontal axis, and plotted on a log-log graph. The above two points were connected by a straight line, and the melt viscosity (unit: poise) at a shear rate of 100 / sec was determined by interpolation.

(4) Thickness unevenness of film A tape-like sample (length 1 m) continuous in the longitudinal stretching direction was collected, and a thickness of 100 points was obtained at a pitch of 1 cm using an electronic micrometer manufactured by Seiko EM Co., Ltd., Millitron 1240. taking measurement. From the measured values, the maximum value (dmax), the minimum value (dmin), and the average value (d) of the thickness were obtained, and the thickness unevenness (%) was calculated by the following formula. In addition, the measurement was performed 3 times and the average value was calculated | required.
Thickness unevenness (%) = ((dmax−dmin) / d) × 100

(5) Haze, total light transmittance Measured according to JIS K 7105 “Testing method for optical properties of plastic” haze (cloudiness value). NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd. was used for the measuring instrument.
In the case of a film in which the light diffusion layer (B) is laminated only on one side, the measurement is performed by arranging the support layer (A) side on the incident light side and the light diffusion layer (B) side on the outgoing light side. did.

(6) Internal haze, surface haze [Internal haze evaluation method]
The value obtained by subtracting the haze (one haze) measured by a usual method from the haze (two hazes) measured by overlapping with two layers of film through cedar oil was defined as the internal haze.
Moreover, the value which deducted the internal haze calculated | required by the said method from the haze (one sheet haze) measured by the normal method was made into surface haze.
In the case of a film having a non-target structure on the front and back, such as a film in which the light diffusion layer (B) is laminated only on one side, the light diffusion layer (B) surface and the support layer (A) surface of each film are separated from each other. It is necessary to measure it over the oil. Also in this case, the measurement was performed with the support layer (A) surface disposed on the incident light side and the light diffusion layer (B) surface disposed on the outgoing light side.

(7) Tensile strength Measured according to JIS C 2318-1997 5.3.3 (tensile strength and elongation).

(8) Dimensional change rate It measured based on JIS C 2318-1997 5.3.4 (dimensional change).

(9) Degree of plane orientation (ΔP)
In accordance with JIS K 7142-1996 5.1 (Method A), the refractive index in the film longitudinal direction (nx), the refractive index in the width direction (ny), and the refractive index in the thickness direction (nz) using an Abbe refractometer with the sodium D line as the light source. ) And the degree of plane orientation (ΔP) was calculated by the following formula.
ΔP = {(nx + ny) −2nz} / 2

(10) Curl value The film is cut into a sheet of 100 mm in the longitudinal direction and 100 mm in the width direction, heat-treated at 100 ° C. for 30 minutes under no load, and then a horizontal glass plate with the convex portion of the film facing down Leave on top. Next, a vertical distance between the glass plate and the lower end of the four corners of the rising film is measured with a ruler. The maximum value of these four measured values was taken as the curl value. Three samples were prepared and repeatedly measured, and the average value was taken as the curl value. When the curl was 1 mm or less, the measurement was performed with an accuracy of 0.5 mm, and when the curl exceeded 1 mm, the measurement was performed with an accuracy of 1 mm.

Example 1
(1) Production of PET resin (M1) The temperature of the esterification reaction can was increased, and when it reached 200 ° C, a slurry consisting of 86.4 parts by mass of terephthalic acid and 64.4 parts by mass of ethylene glycol was charged. While stirring, 0.017 parts by mass of antimony trioxide and 0.16 parts by mass of triethylamine were added as catalysts. Next, the pressure was increased and the pressure esterification reaction was performed under the conditions of a gauge pressure of 3.5 kgf / cm ² and 240 ° C. Thereafter, the inside of the esterification reaction vessel was returned to normal pressure, and 0.071 part by mass of magnesium acetate tetrahydrate and then 0.014 part by mass of trimethyl phosphate were added. Further, the temperature was raised to 260 ° C. over 15 minutes, and 0.012 parts by mass of trimethyl phosphate and then 0.0036 parts by mass of sodium acetate were added. After 15 minutes, the obtained esterification reaction product was transferred to a polycondensation reaction can, gradually heated from 260 ° C. to 280 ° C. under reduced pressure, and subjected to a polycondensation reaction at 285 ° C.

  After completion of the polycondensation reaction, it is filtered through a NASRON filter with a 95% cut diameter of 5 μm, extruded into a strand from a nozzle, and cooled and solidified using cooling water that has been filtered (pore diameter: 1 μm or less) in advance. And cut into pellets. The obtained PET resin (M1) has a heat of crystal fusion of 35 mJ / mg, a melting point of 256 ° C., an intrinsic viscosity of 0.616 dl / g, an Sb content of 144 ppm, an Mg content of 58 ppm, a P content of 40 ppm, and a color. The L value was 56.2, the color b value was 1.6, and inert particles and internally precipitated particles were substantially not contained.

(2) Manufacture of polystyrene masterbatch (M2) The thing which pellet-mixed 30 mass parts of polystyrene resins (PS) (G797N by Nippon Polyst Co., Ltd.) whose melt viscosity is 3900 poise, and 70 mass parts of said PET (M1), A polystyrene masterbatch (M2) was prepared by supplying to a bent twin screw extruder, kneading and melt extrusion, and cooling and cutting the obtained strand.

(3) Preparation of coating solution (M3) 95 parts by mass of dimethyl terephthalate, 95 parts by mass of dimethyl isophthalate, 35 parts by mass of ethylene glycol, 145 parts by mass of neopentyl glycol, 0.1 part by mass of zinc acetate and 0.1% of antimony trioxide A mass part was charged in a reaction vessel, and a transesterification reaction was performed at 180 ° C. over 3 hours. Next, 6.0 parts by mass of 5-sodium sulfoisophthalic acid was added, and esterification was performed at 240 ° C. over 1 hour, and then at 250 ° C. under reduced pressure (10 to 0.2 mmHg) over 2 hours. A polycondensation reaction was performed to obtain a copolymerized polyester resin having a number average molecular weight of 19,500 and a softening point of 60 ° C.

  7.5 parts by mass of a 30% by mass aqueous dispersion of the obtained copolyester resin and a 20% by mass aqueous solution of a self-crosslinking polyurethane resin containing an isocyanate group blocked with sodium bisulfite (manufactured by Daiichi Kogyo Seiyaku) , Elastron H-3) 11.3 parts by mass, Elastron catalyst (Daiichi Kogyo Seiyaku, Cat 64) 0.3 parts by mass, water 39.8 parts by mass and isopropyl alcohol 37.4 parts by mass, respectively. Mixed.

  Furthermore, colloidal silica (manufactured by Nissan Chemical Industries, Snowtex OL; average particle) as 0.6 parts by mass of 10% by mass aqueous solution of a fluorine-based nonionic surfactant (manufactured by Dainippon Ink and Chemicals, MegaFuck F142D) 2.3 mass parts of a 20 mass% aqueous dispersion having a diameter of 40 nm), and 3.5 mass% aqueous dispersion of dry process silica (Nippon Aerosil, Aerosil OX50; average particle diameter 200 nm, average primary particle diameter 40 nm) as particles B 0.5 parts by mass of the liquid was added. Next, the pH of the coating solution was adjusted to 6.2 with a 5% by weight aqueous sodium bicarbonate solution, and the solution was finely filtered with a felt type polypropylene filter having a filtration particle size (initial filtration efficiency: 95%) of 10 μm. Adjusted.

(4) Production of light diffusing film As raw materials for the light diffusing layer (B), 67 parts by mass of PET (M1) and 33 parts by mass of polystyrene masterbatch (M2) were each dried under reduced pressure at 135 ° C. for 6 hours (1 Torr ), And mixed and supplied to the extruder 2. Further, PET (M1) as a raw material of the support layer (A) was dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours and then supplied to the extruder 1. The raw material supplied to the extruder 2 and the extruder 1 has a resin temperature of 280 ° C. until the melting part, kneading part, polymer tube, gear pump, and filter of the extruder, and 275 ° C. in the subsequent polymer tube. It laminated | stacked using the layer confluence | merging block, and melt-extruded in sheet form from the nozzle | cap | die. In addition, the thickness ratio of the (A) layer and the (B) layer was controlled using the gear pump of each layer so that it might become 80:20. In addition, a stainless steel sintered filter material (filtration accuracy: 95% cut of 10 μm particles) was used for each of the filters. The temperature of the die was controlled so that the temperature of the extruded resin was 275 ° C.

  Then, the extruded resin was cast on a cooling drum having a surface temperature of 30 ° C. and adhered to the surface of the cooling drum using an electrostatic application method to be cooled and solidified, thereby producing an unstretched film having a thickness of about 1.2 mm. . At this time, the layer surface (B) was a surface in contact with the cooling drum.

Subsequently, the easily bonding layer was apply | coated to the single side | surface (A) of the obtained unstretched film. As the coating solution, a solution obtained by subjecting the coating solution (M3) to microfiltration with a felt type polypropylene filter medium having a filtration particle size of 5 μm (initial filtration efficiency of 95%) was used. Moreover, the reverse roll method was employ | adopted for the application | coating method, and it apply | coated so that the wet application amount might be about 20 g / m < ² >. Thereafter, in a drying furnace divided into two zones, the coated surface was dried at a first zone temperature of 100 ° C., a wind speed of 20 m / second, for 10 seconds, at a second zone temperature of 70 ° C., a wind speed of 20 m / second, for 10 seconds.

  Next, both ends of the unstretched film having the coating layer were held by clips and guided to a simultaneous biaxial stretching machine, and a biaxially stretched film was prepared under the following conditions.

  After preheating for 35 seconds with hot air of 105 ° C., simultaneous biaxial stretching was performed at 105 ° C. at a draw ratio of 3.2 times in the longitudinal direction and 3.7 times in the transverse direction. At this time, the stretching ratios in the vertical and horizontal directions were set as shown in FIG. The stretching speed in the longitudinal and transverse directions in this stretching process is as shown in FIG. 2, the maximum stretching speed in the longitudinal direction is 20.3% / second, and the maximum stretching temperature in the transverse direction is 23.5% / second. Was controlled as follows. Next, heat treatment was performed at 230 ° C. for 17.5 seconds with a constant tenter width and a constant clip interval. Further, in the process of cooling to 60 ° C. over 15 seconds, 3% relaxation treatment was performed in the vertical and horizontal directions.

  Next, the clip that had gripped both ends of the film was released, and both ends of the film were trimmed and wound into a roll to produce a biaxially stretched film having a thickness of about 110 μm. In addition, when an actual stretch ratio was measured with a lattice-shaped magnification marker written on the unstretched film, it was confirmed that the stretch ratio was as set above.

(5) Characteristics of Light Diffusing Film Table 1 shows the characteristics of the light diffusing film obtained in Example 1. As can be seen from Table 1, the light diffusive film obtained by the method of the present invention has excellent heat resistance and mechanical strength inherent to the biaxially stretched film, and has excellent light transmittance and light diffusibility. It can be seen that

Comparative Example 1
The unstretched film obtained by the same method as Example 1 was biaxially stretched by a conventionally known method.
First, after preheating the film with a roll group heated to 75 ° C., the film was heated to 96 ° C. using a non-contact infrared heater, and longitudinally stretched 3.4 times between rolls having different peripheral speeds. . At this time, the distance between the contact points of the film was 200 mm, and the peripheral speed of the low-speed roll was 12 m / min. If the film speed between rolls is represented by an intermediate value between the low-speed roll peripheral speed and the high-speed roll peripheral speed, the film speed between rolls is 26.4 m / min, and the passing time between rolls is about 0.45 seconds. . Therefore, it is 3.4 times, that is, 240% stretching is performed in 0.45 seconds, and the stretching speed is about 530% / second.

Next, both ends of the above-mentioned longitudinally stretched film were gripped with clips, and transversely stretched. The transverse stretching temperature was 135 ° C., the transverse stretching ratio was 3.7 times, and the transverse stretching speed was constant at 25% / second. Next, heat treatment was performed at 230 ° C. for 15 seconds, and 2.5% relaxation treatment was performed in the width direction in the process of cooling to 60 ° C.
Next, the clip that had gripped both ends of the film was released, and both ends of the film were trimmed and wound into a roll to produce a biaxially stretched film. The properties of the light diffusing film obtained in Comparative Example 1 are shown in Table 2.

  The light diffusive film obtained in Comparative Example 1 had high haze and good light diffusibility, but had low total light transmittance and low quality. Moreover, the dimensional change rate was also inferior to the light diffusable film obtained in Example 1. The light diffusing film obtained in this comparative example had a high degree of plane orientation, and voids were formed around the light diffusing additive. Since the internal haze is higher than that in Example 1, the reason why the total light transmittance was low is presumed to be due to the influence of this void.

Comparative Example 2
In the method of Example 1, the raw material of the light diffusion layer (B) was changed to a mixture of 95 parts by mass of PET (M1) and 5 parts by mass of polystyrene masterbatch (M2). Otherwise, a biaxially stretched film was produced in the same manner as in Example 1. The properties of the light diffusing film obtained in Comparative Example 2 are shown in Table 2.

  The light diffusive film obtained in this Comparative Example 2 had a low quality because the haze was insufficient and the light diffusibility required for the light diffusible film and the total light transmittance were not balanced.

Comparative Example 3
In the method of Comparative Example 1, the intrinsic viscosity is 100 mol% of the terephthalic acid unit as the raw material of the light diffusion layer (B), 70 mol% of the ethylene glycol unit and 30 mol% of the neopentylglycol unit as the diol component. It changed into the mixture of 50 mass parts of 0.69 dl / g non-crystalline copolymer polyester, and 50 mass parts of polystyrene masterbatch (M2). The copolyester was dried under reduced pressure (1 Torr) at 60 ° C. for 72 hours and then subjected to mixing with a polystyrene masterbatch (M2).
A biaxially stretched film was produced in the same manner as in Comparative Example 1 except for the above. The properties of the light diffusing film obtained in Comparative Example 3 are shown in Table 2.

  The light diffusive film obtained in Comparative Example 3 uses a non-crystalline copolymer polyester as a raw material polyester for the light diffusion layer (B), and thus the heating curl is increased. Further, the haze was low and the quality was low.

Comparative Example 4
In the method of Example 1, when biaxial stretching was performed using a simultaneous biaxial stretching tenter, the preheating temperature was changed to 110 ° C. and the stretching temperature was changed to 115 ° C. Otherwise, a biaxially stretched film was prepared in the same manner as in Example 1. The properties of the light diffusing film obtained in Comparative Example 4 are shown in Table 2.
The film obtained in Comparative Example 4 had a plane orientation degree (ΔP) of less than 0.080, the tensile strength was remarkably reduced, and the film was low quality. Moreover, the thickness spot was also getting worse.

Comparative Example 5
In the method of Example 1, when biaxial stretching was performed using a simultaneous biaxial stretching tenter, both the preheating temperature and the stretching temperature were changed to 92 ° C. Otherwise, a biaxially stretched film was prepared in the same manner as in Example 1. Table 2 shows the characteristics of the light diffusing film obtained in Comparative Example 5.
The film obtained in Comparative Example 5 had a degree of plane orientation (ΔP) exceeding 0.160, and haze decreased. Also, the internal haze increased and the total light transmittance also decreased.

Example 2
In Example 1, the coating layer was provided on both surfaces of the unstretched film. The same coating solution as in Example 1 was used. The coating and drying method on the (A) side was performed by the method described in Example 1. However, the wire bar method was adopted as the coating method on the (B) side, and the wet coating amount was applied so as to be about 20 g / m ² . Other production conditions were the same as in Example 1 to prepare a biaxially stretched film. Table 1 shows the characteristics of the light diffusing film obtained in Example 2.
It can be seen that the film obtained in Example 2 has further improved total light transmittance as compared with Example 1, and has excellent characteristics as a light diffusive film.

Example 3
As raw materials for the light diffusion layer (B), 50 parts by mass of PET (M1) and 50 parts by mass of polystyrene masterbatch (M2) were each dried at 135 ° C. under reduced pressure (1 Torr) for 6 hours, and then mixed and extruded. 2 was supplied. Further, PET (M1) as a raw material of the support layer (A) was dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours and then supplied to the extruder 1. The raw material supplied to the extruder 2 and the extruder 1 has a resin temperature of 280 ° C. until the melting part, kneading part, polymer tube, gear pump, and filter of the extruder, and 275 ° C. in the subsequent polymer tube. It laminated | stacked using the layer confluence | merging block, and it melt-extruded in the sheet form from the nozzle | cap | die. In addition, the thickness ratio of the (A) layer and the (B) layer was controlled using the gear pump of each layer so that it might become 80:20. In addition, a stainless sintered body filter material (nominal filtration accuracy: 95% cut of 10 μm particles) was used for each of the filters. The temperature of the die was controlled so that the temperature of the extruded resin was 275 ° C.

  Then, the extruded resin was cast on a cooling drum having a surface temperature of 40 ° C., and adhered to the surface of the cooling drum using an electrostatic application method to be cooled and solidified to prepare an unstretched film having a thickness of about 2.0 mm. . At this time, the layer surface (A) was a surface in contact with the cooling drum. Further, a multi-duct in which cooling air blowing nozzles and suction nozzles were alternately arranged continuously was installed along the outer periphery of the cooling drum, and the layer surface (B) was air-cooled from a position about 30 mm away from the cooling drum.

  Subsequently, the coating layer was apply | coated to the single side | surface (A) of the obtained unstretched film by the method similar to Example 1. FIG.

  Next, both ends of the unstretched film having the coating layer were gripped with clips and led to a simultaneous biaxial stretching machine to prepare a biaxially stretched film. The biaxial stretching conditions were the same as in Example 1 except that the preheating hot air temperature was corrected to 110 ° C. In this example as well, when the actual draw ratio was measured with a grid-like magnification marker written on the unstretched film, it was confirmed that the draw ratio was as set.

  Table 1 shows the characteristics of the light diffusing film obtained in Example 3. As can be seen from Table 1, the light diffusive film obtained by the method of the present invention has excellent heat resistance and mechanical strength inherent to the biaxially stretched film, and has excellent light transmittance and light diffusibility. It can be seen that

Example 4
In the production of the unstretched film of Example 1, the raw material of the light diffusion layer (B) was 97 parts by mass of PET (M1) and a cyclic olefin copolymer having a glass transition temperature of 160 ° C. (Topas Advanced Polymers, TOPAS 6015) 3 The mixture was changed to parts by weight. Moreover, the thickness ratio of A layer and B layer was changed so that it might be set to 90:10, and the unstretched film about 1.2 mm thick was created.
The obtained unstretched film was preheated with hot air at 100 ° C. for 40 seconds and then simultaneously biaxially stretched 3.5 times in the longitudinal and lateral directions at a constant stretching speed of 50% / second. Next, heat treatment was performed for 10 seconds with hot air at 220 ° C., and cooled to room temperature to prepare a biaxially stretched film.
Table 1 shows the characteristics of the light diffusing film obtained in Example 4. The film obtained in Example 4 had excellent characteristics as in Example 1.

  The light diffusing film of the present invention can be used as a light diffusing film for a liquid crystal display, particularly for a large liquid crystal display employing a direct type backlight unit. Moreover, by performing prism processing on one surface, the light diffusing film and the light collecting sheet can be integrated, and the number of backlight unit parts can be reduced, the manufacturing process can be simplified, and the cost can be reduced.

It is explanatory drawing which shows the relationship between the process in the extending machine of the film at the time of film manufacture of Example 1, and a longitudinal draw ratio or a transverse draw ratio. It is explanatory drawing which shows the relationship between the process in the extending machine of the film at the time of film manufacture of Example 1, and a longitudinal stretch speed or a horizontal stretch speed.

Explanation of symbols

1: Transverse stretching 2: Longitudinal stretching 10: Preheating zone 11: Stretching zone (film transit time: 18 seconds)
12: Heat treatment zone

Claims (6)

A light diffusing film comprising a biaxially stretched laminated film having a support layer made of crystalline polyester and a light diffusing layer laminated on at least one side of the support layer by coextrusion,
The light diffusing layer contains 60 to 98 parts by mass of crystalline polyester and 2 to 40 parts by mass of a light diffusing additive incompatible with the polyester,
The light diffusing film has a plane orientation degree (ΔP) of 0.080 to 0.160, a total light transmittance of 85% or more, and a haze of 30% or more.   The light diffusing film according to claim 1, wherein the light diffusing additive is a thermoplastic resin incompatible with polyester.   The thermoplastic resin incompatible with polyester is an amorphous transparent polymer selected from any of polystyrene resin, styrene copolymer resin, cyclic olefin resin, acrylic resin, and acrylic resin. The light diffusable film according to claim 2.   4. The light diffusing film according to claim 3, wherein the amorphous transparent polymer is a polystyrene resin or a styrene copolymer resin having a melt viscosity of 1000 to 10,000 poise.   It has a coating layer mainly comprising at least one copolymer polyester resin, polyurethane resin, or acrylic resin provided on the surface of the light diffusion layer before completion of stretching and orientation of the film. The light diffusing film according to claim 1.   The light diffusing film according to claim 1, further comprising a coating layer mainly comprising at least one of a copolyester resin, a polyurethane resin, or an acrylic resin on a surface opposite to the light diffusing layer. A light diffusive film for a light-collecting sheet substrate.

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