JP2009008785A - Light diffusing film and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light diffusing film having excellent light transmittance and light diffusivity while maintaining excellent heat resistance of a stretched film. <P>SOLUTION: The light diffusing film is obtained by stretching an unstretched sheet having a support layer composed of crystalline polyester and a light diffusing layer laminated on at least one side of the support layer by co-extrusion method at least in one axial direction. The light diffusing layer comprises 60-98 pts.mass of crystalline polyester and 2-50 pts.mass of a light diffusing additive non-compatible to the polyester, and the light diffusing film has refractive index difference (Δn) between longitudinal refractive index and lateral refractive index of 0.040-0.120, face orientation (ΔP) of 0.040-0.160, full light transmittance of 85% or more, haze of 40% or more, and dimensional change rate at 150°C of 3.0% or less in both longitudinal and lateral directions. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a light diffusing film used in a backlight unit of a liquid crystal display, an illumination device, and the like, and a light diffusing film obtained therefrom.

  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. 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. And edge light type backlight units are widely used when a small display such as a notebook personal computer is required to be thin and light.

  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.

  Furthermore, in order to improve the front luminance of the backlight unit, a condensing sheet is used so as to collect the light transmitted through the light diffusing film and emitted 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 light condensing sheet is disposed and used in one or two layers on the surface side of the light diffusing film.

  In addition, in order to increase the brightness of the display screen and reduce the power consumption, each member (light guide plate, light diffusing film, condensing sheet, etc.) through which the light of the backlight unit transmits is made of a material having a high light transmittance. Ingenuity has been devised to reduce light loss and improve light utilization efficiency.

  Examples of the light diffusive film used in the backlight unit as described above include (1) a film obtained by forming a transparent thermoplastic resin into a sheet shape and then physically processing the surface ( For example, see Patent Document 1), (2) obtained by coating a light diffusing layer made of a transparent resin containing fine particles on a transparent substrate film such as polyester resin (see, for example, Patent Document 2) ), (3) A product obtained by melt-mixing beads in a transparent resin and extruding the beads (for example, see Patent Document 3) is generally used.

JP-A-4-275501 JP-A-6-59108 JP-A-6-123802

  The methods disclosed in the above (1) and (2) are widely adopted because a film having a balanced light transmittance and light diffusibility can be obtained. However, the light diffusion layer is formed by post-processing (off-line), which is disadvantageous with respect to the cost reduction requirement. On the other hand, the film and sheet obtained by the method disclosed in the above (3) have a problem that heat resistance and solvent resistance are inferior.

  In addition to the above general light diffusive film, the backlight unit can be reduced in size by integrating the light diffusive film and other optical functional film, and the backlight unit configuration and manufacturing process can be simplified. For the purpose of cost reduction, many attempts have been made to impart light diffusibility to the base film itself of the optical functional film by in-line processing.

  For example, (4) a polyester resin and a resin incompatible with the resin are melt-mixed and biaxially stretched, and a film containing voids inside (for example, see Patent Document 4), (5) low A film substantially free of voids obtained by mixing spherical copolymer particles with a crystalline copolymer polyester resin and biaxially stretching (for example, see Patent Documents 5 and 6), (6) Low crystalline copolymer A film substantially free of voids obtained by melt-mixing a polyester resin and a resin incompatible with the resin and biaxially stretching (see, for example, Patent Documents 7 and 8), (7) Low crystalline copolymer polyester A film containing voids inside (for example, see Patent Document 9), melted and mixed with a resin and a resin incompatible with the resin and biaxially stretched, (8) Amorphous copolymer polyester Resin and tree incompatible with resin A biaxially stretched laminated film (for example, see Patent Documents 10 and 11), (9) crystalline polyester resin or amorphous A film containing a copolyester resin and a resin incompatible with the resin are melt-mixed and stretched to contain anisotropic bubbles (for example, see Patent Documents 12 and 13). It is disclosed.

JP-A-11-268211 JP 2001-272508 A JP 2001-324606 A JP 2002-162508 A JP 2002-182013 A JP 2002-196113 A JP 2002-372606 A JP 2004-354558 A JP 2003-344613 A JP 2004-12681 A

  However, in the attempts (4) to (8) for imparting light diffusibility to the base film itself, a method of post-processing the light diffusion layer on the transparent base film in terms of both heat resistance and light transmittance. It is less than (1) and (2) and has not been put into practical use.

  This is because, in the method of imparting light diffusibility to the base film itself, in order to maintain the high light transmittance inherent in the base film, it is necessary to suppress the generation of voids (bubbles) in the film stretching process. . However, when a crystalline polyester resin is used as the matrix polymer (4), although excellent heat resistance is obtained by the crystalline polyester resin, voids are formed at the interface between the matrix polymer and the incompatible resin or particles. It will occur frequently. Since the voids generated in this way have a flat plate shape parallel to the film surface, the light emitted from the light guide plate when used in a backlight unit as a light diffusing film. Will be scattered back and light transmittance will be impaired.

  On the other hand, when an amorphous polyester resin is used as the matrix polymer (5) to (7), generation of voids is suppressed depending on the degree of amorphousness, and excellent light transmittance is obtained. However, the heat resistance and solvent resistance that are characteristic of crystalline polyester films cannot be obtained, resulting in significant deterioration in dimensional change when processed at high temperatures or in high temperature environments, and the brightness of the light exit surface of the backlight unit. The original purpose of the light diffusing film, that is, making the film uniform cannot be achieved.

  In the biaxially stretched laminated film (8) obtained by co-extrusion of a non-crystalline copolymer polyester resin, a layer obtained by melt-mixing a resin incompatible with the resin, and a layer made of crystalline polyester. Although a certain heat resistance improvement effect can be obtained, an essential heat resistance improvement effect cannot be obtained. This is because when layers having different crystallinity are laminated, a so-called bimetallic structure is formed in which films having different linear expansion rates and heat shrinkage rates are laminated. 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 will be uneven. It is. 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 more the curling becomes more prominent, and the higher the brightness of the display, the greater the power consumption of the light source, that is, the amount of heat generated. In order to solve this problem, it is necessary to eliminate the bimetal structure, and it is desirable that the resin composition constituting the support layer and the light diffusion layer be the same.

A film (9) containing a crystalline polyester resin or an amorphous copolyester resin and a resin incompatible with the resin and stretched and stretched, and containing anisotropic voids (voids) In this case, although a certain light diffusibility and brightness improvement effect can be obtained, the light transmittance is reduced by bubbles, and when used as a light diffusive film in a backlight unit, back scattering is caused by bubbles. Will occur, resulting in a substantial decrease in luminance.

  Further, in the biaxially stretched films of (8) and (9) obtained by using crystalline polyester for the support layer and stretched at approximately equal magnification and high magnification in the biaxial direction, the polyester is stretched in the longitudinal and lateral biaxial directions. Since the polymer is highly oriented, it is considered that the molecular orientation is formed in the form of a network in the vertical and horizontal directions, which impedes the transmission of light and causes a decrease in luminance.

  As described above, it is preferable to impart light diffusibility to the base film by in-line from the viewpoint of simplification of the process and cost reduction. However, when diffusibility is imparted by an incompatible resin or the like, the light transmittance decreases due to the generation of voids, and the original function of the base film cannot be performed. Moreover, if an amorphous polyester resin is used to suppress the generation of voids, the heat resistance and solvent resistance that the base film should originally have cannot be obtained. Furthermore, a light diffusing biaxially stretched polyester film produced using a crystalline polyester resin for the support layer and an amorphous polyester resin for the light diffusing layer causes curling due to the bimetallic structure. In addition, by performing biaxial stretching, there is a risk of luminance reduction due to molecular orientation.

  The object of the present invention is to solve the above-mentioned problems in the prior art, and has the heat resistance and solvent resistance inherent in the stretched film, and light having excellent light transmittance and light diffusibility. It is providing the manufacturing method of a diffusable film and a light diffusable film.

  The light diffusing film of the first invention in the present invention capable of solving the above problems is a support layer made of crystalline polyester, and a light diffusion layer laminated on at least one side of the support layer by a coextrusion method. A light diffusive film obtained by stretching an unstretched sheet having at least uniaxial direction, wherein the light diffusing layer comprises 60 to 98 parts by mass of crystalline polyester and light diffusibility incompatible with the polyester. The light diffusing film contains 2 to 40 parts by mass of the additive, and the refractive index difference (Δn) between the refractive index in the longitudinal direction and the refractive index in the width direction is 0.040 to 0.120, and the degree of plane orientation ( ΔP) is 0.040 to 0.160, the total light transmittance is 85% or more, the haze is 40% or more, and the dimensional change rate at 150 ° C. is 3.0% or less in both the longitudinal direction and the width direction. It is characterized by A light diffusion film that.

  The second invention is the light diffusing film according to claim 1, wherein the crystalline polyester is made of polyethylene terephthalate or a polyethylene terephthalate copolymer.

  Furthermore, a third invention is a method for producing the light diffusing film of the first or second invention, wherein the draw ratio is 2.5 times or more and the draw speed is less than 80% / second. It is a method for producing a light diffusing film characterized by stretching only in a uniaxial direction.

  Since the light diffusing film of the present invention uses crystalline polyester or polyethylene terephthalate copolymer as the matrix polymer of the light diffusing layer, it has excellent heat resistance and solvent resistance inherent to the stretched film. In addition, since the film is stretched at a stretching speed of less than 83% / second in at least uniaxial direction, generation of voids accompanying stretching is suppressed, and both excellent light transmittance and light diffusibility can be achieved. it can.

(Characteristics of light diffusing film)
The light diffusing film of the present invention is obtained by stretching an unstretched sheet having a support layer made of crystalline polyester and a light diffusion layer laminated on at least one side of the support layer by a coextrusion method at least in a uniaxial direction. The obtained light diffusing film, wherein 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 refractive index difference (Δn) between the refractive index in the longitudinal direction and the refractive index in the width direction is 0.04 to 0.12, the degree of plane orientation (ΔP) is 0.040 to 0.160, The transmittance is 85% or more, the haze is 40% or more, and the dimensional change rate at 150 ° C. is 3.0% or less in both the longitudinal direction and the width direction.

(Refractive index difference Δn)
It is important that the refractive index difference (Δn) between the refractive index in the longitudinal direction and the refractive index in the width direction in the light diffusing film of the present invention is 0.040 to 0.120. The lower limit of the refractive index difference (Δn) is preferably 0.060, more preferably 0.070. The upper limit of the refractive index difference (Δn) is preferably 0.110, and more preferably 0.100. When the difference in refractive index is less than 0.040, the molecules are oriented to the same extent in both the vertical and horizontal directions, and the molecules oriented in a network form weaken the light transmission in the direction perpendicular to the film surface. This is not preferable. On the other hand, if the refractive index exceeds 0.120, molecules are extremely strongly aligned in one direction, and tearing tends to occur along the alignment direction, which is not preferable from the viewpoint of mechanical strength.

Here, the longitudinal direction refers to a direction parallel to the film winding direction, and the width direction refers to a direction perpendicular to the film winding direction. When the refractive index in the longitudinal direction is Nx and the refractive index in the width direction is Ny, the absolute value of these differences is the refractive index difference Δn as shown by the following equation.
Δn = | Nx−Ny |

(Plane orientation degree ΔP)
It is important that the degree of plane orientation (ΔP) in the light diffusing film of the present invention is 0.040 to 0.160. The lower limit of the degree of plane orientation (ΔP) is preferably 0.050, more preferably 0.060. Moreover, the upper limit of the refractive index difference (Δn) is preferably 0.140, and more preferably 0.120. If the degree of plane orientation is less than 0.040, the excellent heat resistance and solvent resistance of the stretched polyester film cannot be obtained. When the plane orientation degree exceeds 0.160, the unevenness on the surface of the light diffusion layer is lowered (flattened), and the light diffusion effect caused by the surface unevenness is remarkably reduced. On the other hand, when the degree of plane orientation exceeds 0.160, the number and size of voids generated around the light diffusing additive increases, and thus the light transmittance tends to decrease.

(Total light transmittance)
The total light transmittance in the light diffusing film of the present invention is 85% or more, more preferably 86% or more, still more preferably 87% or more, and particularly preferably 88% or more. On the other hand, the preferable upper limit of the total light transmittance is 100%. A total light transmittance of 85% or more is preferable from the viewpoint of luminance required for optical applications.

(Haze)
The haze in the light diffusing film of the present invention is 40% or more, preferably 50% or more. On the other hand, the preferable upper limit of haze is 100%. When the haze is 40% or more, preferable light diffusibility is obtained such that the luminance of the light irradiation surface of the backlight becomes uniform.

(Thermal dimensional stability)
The dimensional change rate at 150 ° C. of the light diffusing film of the present invention is 3.0% or less in the longitudinal direction and the width direction, more preferably 2.0% or less, and further preferably 1.5% or less. is there. If the dimensional change rate at 150 ° C. is 3.0% or less, the flatness is maintained even during high-temperature processing or use in a high-temperature environment. For this reason, it can be applied to a high output backlight unit.

(material)
(1) Crystalline polyester The crystalline polyester that can be used as the raw material of the light diffusion layer in the present invention is a polyester in which a clear crystal melting heat peak (melting point) can be observed by measurement using a differential scanning calorimeter. If it is, it is arbitrary without any limitation. However, in order to achieve the original heat resistance and mechanical strength of the polyester film, the melting point of the crystalline polyester used as a raw material is preferably 200 ° C. or higher, more preferably 220 ° C. or higher, more preferably 230 ° C. or higher. Particularly preferably, it is 240 ° C. or higher, and most preferably 250 ° C. or higher.

  Here, the polyester is an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid or an ester thereof and ethylene glycol, diethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, or the like. Polyester produced by polycondensation with glycol. 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.

  Typical examples of the polyester include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate. The polyester may be a homopolymer or a copolymer of a third component. As the polyester-based resin copolymer, a polyester having an ethylene terephthalate unit or an ethylene-2,6-naphthalate unit of 70 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more is preferable. Among these, as the crystalline polyester used in the present invention, polyethylene terephthalate or a polyethylene terephthalate copolymer can be suitably used from the viewpoint of heat resistance, transparency, and solvent resistance. In the light diffusing film of the present invention, the same crystalline polyester is preferably used for both the support layer and the light diffusing layer from the viewpoint of curling prevention at high temperatures.

  Moreover, it is preferable that the polyester does not substantially contain particles used for the purpose of imparting slipperiness in order to increase the total light transmittance. The term “substantially free of particles” means, for example, in the case of inorganic particles, when inorganic elements are quantified by fluorescent X-ray analysis, they are 50 ppm or less, preferably 10 ppm or less, most preferably the detection limit or less. Means content. This means that even if particles are not actively added to the polyester, contaminants derived from foreign substances, raw material resin, or dirt attached to the line or device in the film manufacturing process will be peeled off and mixed into the film. This is because there are cases.

(2) Light diffusing additive The light diffusing additive used in the present invention is not limited as long as it is an incompatible material with the polyester, but the following materials should be used. Is preferred. In addition, one of the following types of light diffusing additives may be used, or two or more types may be used in combination.

(2-1) 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, using the incompatibility between the crystalline polyester and the thermoplastic resin, the stretched film is made of a thermoplastic resin that is incompatible with the polyester in the matrix made of the crystalline polyester in the production process (melting and extrusion process). Disperse the domain and use it as a light diffusing substance.

  Examples of the thermoplastic resin incompatible with polyester that can be used in the present invention include polyolefins such as polyethylene, polypropylene, polymethylpentene, and various cyclic olefin polymers, polycarbonate, atactic polystyrene, syndiotactic polystyrene, Polystyrene such as tactic polystyrene, polyamide, polyether, polyester amide, polyphenylene sulfide, polyphenylene ether, polyether ester, polyvinyl chloride, copolymer such as polymethacrylic acid ester, and copolymers having these as main components, or Although the mixture of these resin etc. are mentioned, it does not specifically limit to these.

Among these, it is particularly preferable to use an amorphous transparent polymer in order to produce a light diffusing film having high light transmittance. In particular, in the case of a polystyrene resin, the melt viscosity is preferably 1000 poise or more, more preferably 3000 poise, from the viewpoint that it is uniformly dispersed in the crystalline polyester resin and the dispersed particle size is stabilized.

(2-2) Non-melting polymer particles The non-melting polymer particles that can be used as the light diffusing additive of the present invention are obtained by using a melting point measuring device (for example, MPA100 type manufactured by Stanford Research System). The composition is not particularly limited as long as it does not cause flow deformation due to melting when it is heated from 30 ° C. to 350 ° C. at a rate of 10 ° C./min. Examples thereof include acrylic resins, polystyrene resins, polyolefin resins, polyester resins, polyamide resins, polyimide resins, fluorine resins, melamine resins, urea resins, and organic silicone resins. The average particle size of the non-melting polymer particles is preferably 0.5 to 50 μm. One type of non-soluble polymer particles may be used, or two or more types may be used.

(2-3) 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 system of the inorganic particles is preferably 0.5 to 50 μm. One type of non-soluble polymer particles may be used, or two or more types may be used.

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) and at a magnification such that the size of one smallest particle is 2-5 mm, the maximum diameter of 300-500 particles (between the two most distant points) Distance) is measured, and the average value is taken as the average particle diameter.

(3) 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. The mixing ratio of the light diffusing additive is more preferably 3 to 20 parts by mass, and further preferably 5 to 15 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, which is not preferable. In addition, 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, and the light transmittance decreases. In addition, since the compositions of the light diffusion layer and the support layer are greatly different, the film tends to curl easily.

(Layer structure)
The light diffusing film of the present invention comprises a mixed composition of the crystalline polyester and the light diffusing additive incompatible with the crystalline polyester on at least one side of the support layer (A layer) made of the crystalline polyester. It is important that the light diffusing layer (B layer) made of a product has a multilayer structure laminated by a coextrusion method. By adopting such a multilayer structure, a light diffusing film having high haze and high total light transmittance can be obtained. That is, it is possible to achieve high haze by suppressing the light diffusibility inside the film to achieve a high total light transmittance and effectively utilizing the light diffusion effect caused by the irregularities on the surface of the light diffusion layer.

  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 diffusive film of the present invention is used as a light diffusive sheet on the lower surface of the prism sheet, surface unevenness is obtained by coextrusion and lamination of the light diffusion layer (B layer) on both sides of the support layer (A layer). Since the light diffusing effect produced in step 1 can be obtained on both sides of the film, a light diffusive film having higher haze can be obtained while maintaining high light transmittance.

  When the light diffusing film of the present invention is used as a light diffusing sheet on the upper surface of the prism sheet, the surface facing the prism sheet is required to have smoothness, and therefore the light diffusing layer (B layer) is used as a support layer ( What is necessary is just to set it as the structure provided in the single side | surface of (A layer).

  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, the light diffusion layer (B layer) is provided on one side of the support layer (A layer). As a configuration, it is a preferred embodiment that the surface of the support layer (A layer), which is a smooth surface of the configuration, is subjected to prism processing so that the characteristics of the light diffusing film of the present invention can be effectively utilized. In this case, it is preferable to provide an easy adhesion layer on the smooth surface of the support layer (A layer) in order to improve the adhesion to the acrylic resin constituting the prism.

  The thickness of the entire light diffusing film of the present invention is preferably 50 to 500 μm, more preferably 70 to 400 μm, and further preferably 100 to 250 μm from the viewpoint of strength. Moreover, it is preferable that the ratio with respect to the film whole thickness of the said light-diffusion layer (B layer) is 3 to 50%, It is more preferable that it is 4 to 40%, It is especially preferable that it is 5 to 30%. When the ratio of the light diffusion layer (B layer) to the whole film is less than 3%, the laminated structure of the film becomes non-uniform and the light diffusibility becomes non-uniform. On the other hand, when the ratio of the light diffusion layer (B) to the total film thickness exceeds 50%, the total light transmittance is lowered, which is not preferable.

(Method for producing light diffusing film)
In order to produce the light diffusive film of the present invention, in order to achieve both high total light transmittance and high light diffusibility, the degree of plane orientation (ΔP) is lowered to suppress the generation of voids generated inside the film. This is very important. Furthermore, it is also important to improve the luminance by improving the network molecular orientation formed by the vertical and horizontal molecular orientation while maintaining the original physical strength of the stretched film. The inventor of the present application has found that it is important to highly control the ratio of the stretching speed, the stretching ratio, and the stretching ratio in two directions perpendicular to the light diffusive film having such characteristics. Invented. Each content will be described below. In addition, an achievement means is not limited only to this, Even if it obtains the light diffusable film which has the characteristic of this invention using another means, it is in the scope of the present invention.

(1) Stretching speed It is particularly important that the stretching speed in the production of the light diffusing film of the present invention is performed at a stretching speed of less than 80% / second in both the longitudinal and transverse directions. 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 (% / sec) = Acceleration during film running (m / sec / sec) ÷ Unstretched film speed (m / sec) × 100

  Stretching speed in the transverse direction (% / second) = width change rate per second (m / second) ÷ width of unstretched film (m) × 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. This stretching speed condition is the most important requirement of the present invention. By making the stretching speed less than 80% / second, while using crystalline polyester as a matrix polymer, the generation of stretched voids is suppressed, and excellent light transmittance, inherent heat resistance and solvent resistance of stretched polyester film It is possible to stably obtain a light diffusable stretched film satisfying both of the above.

  On the other hand, when the stretching speed exceeds 80% / second in either the vertical or horizontal direction, the degree of surface orientation of the light diffusing film increases, and it becomes difficult to suppress voids generated during stretching. The transmittance is poor. The upper limit of the preferred stretching speed in the present invention is 50% / second, and the more preferred upper limit is 25% / second.

  On the other hand, the lower limit of the stretching speed is not limited, but if the stretching speed is slowed more than necessary, it is not preferable because the production of the film on an industrial scale decreases the productivity of the film or requires excessive capital investment. . 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.

(2) Stretch ratio It is important that the stretch in the production of the light diffusive film of the present invention is performed at a stretch ratio of 2.5 times or more in the direction in which either the longitudinal direction or the width direction has a large stretch ratio. If the draw ratio is less than 2.5 times in either the longitudinal direction or the transverse direction, the original excellent heat resistance and solvent resistance of the stretched film cannot be obtained, and the film thickness uniformity is insufficient. Become. The lower limit of the preferred draw ratio in the present invention is 2.8 times, and the more preferred lower limit is 3.0 times. The upper limit of the draw ratio is preferably 6.0 times, more preferably 5.0 times, and particularly preferably 4.5 times. When the draw ratio exceeds 6.0 times, the degree of plane orientation (ΔP) increases and it becomes difficult to suppress voids generated during drawing, resulting in poor light transmittance.

(3) Stretch ratio ratio in the orthogonal biaxial direction The ratio of the stretch ratio in the orthogonal biaxial direction (longitudinal direction and width direction) in the production of the light diffusive film of the present invention is set to 1 where the stretch ratio is smaller. Sometimes, it is important that the other draw ratio is 1.5 to 6 times. If the ratio of the draw ratios in the orthogonal biaxial direction is 1.5 or more, the linear polymer of the polyester is oriented only in one direction by stretching, so that the obstacle of light transmission is reduced. High brightness tends to be obtained. When the ratio of the draw ratios in the perpendicular biaxial direction exceeds 6.0, the film tends to tear in the direction in which the draw ratio is large, and the mechanical strength cannot be obtained. Specifically, the stretching ratio in the direction in which the stretching ratio in either the longitudinal direction or the width direction is small is preferably 1 to 2.0 times, and more preferably 1 to 1.5 times. In the present invention, it is the most preferred embodiment that the stretching is performed only in one of the longitudinal direction and the lateral direction.

  In a generally used stretching method, 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. Therefore, as a stretching machine capable of suitably controlling the stretching method as described above, the film is continuously stretched in the transverse direction by guiding it to the tenter while gripping both ends of the film and controlling the transport speed of the clip. A tenter type stretching machine having a possible mechanism is suitable. As long as the equipment has the function, the clip transport mechanism is arbitrary and is not particularly limited.

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

  In order to transfer the pellets, it is usually carried out by air using a predetermined pipe. In this case, it is preferable to use purified air 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 raw material, polyester and a thermoplastic resin incompatible with the polyester are each 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 melted sheet is brought into close contact with a rotating metal roll (casting roll) using an electrostatic application method and cooled and solidified to obtain an unstretched PET sheet.

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

  Further, high-precision filtration is performed at any place where the molten resin is kept at 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. By performing high-precision filtration of the molten resin using a filter medium having a filter particle size (initial filtration efficiency of 95%) of 20 μm or less, productivity may be reduced, but a film with few protrusions due to coarse particles is obtained. This is an important process. In the present invention, when a thermoplastic resin that is incompatible with polyester is used as the light diffusive substance, high-precision filtration as described above is possible. Here, the initial filtration efficiency is a numerical value measured by ANSI / B93.36-1973.

  If foreign substances present in the raw material polymer are present inside the film, the orientation of the polyester molecules is disturbed around the foreign substances in the stretching process during film formation, and optical distortion occurs. Because of this optical distortion, it is recognized as a defect that is considerably larger than the actual size of the foreign material, and the quality is significantly impaired. For example, a foreign substance having a size of 20 μm may be optically recognized as a size of 50 μm or more, and may be recognized as an optical defect having a size of 100 μm or more.

  In order to obtain a highly transparent film, it is desirable not to contain inorganic particles for imparting slipperiness in the base film, or to contain only a small amount so as not to impair the transparency. As the transparency of the film increases, optical defects due to minute foreign substances tend to become clearer. Further, as the film becomes thicker, the content of foreign matters per unit area of the film tends to be larger than that of a thin film, and this problem is further increased.

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

When the light diffusing film of the present invention has a multilayer structure, an easy adhesion layer having a coating amount of 0.005 to 0.20 g / m ^{2 on} the surface of the support layer (A layer) as described above. Is preferably provided.

  The method for applying the easy-adhesion layer-forming coating solution to the unstretched film can be selected from any known method, for example, reverse roll coating method, gravure coating method, kiss coating method, die coater method, roll Examples thereof include a brush method, a spray coating method, an air knife coating method, a wire bar coating method, a pipe doctor method, an impregnation coating method, a curtain coating method, and the like, and these methods are applied alone or in combination.

  The resin constituting the easy-adhesion layer is 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 the light diffusive film application. It is preferable that the main component is one or more selected. 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 layer) does not contain particles, or if it contains only a small amount so as not to impair the transparency, the slipperiness of the film becomes insufficient and handling properties are improved. Gets worse. Therefore, it is preferable to add particles for the purpose of imparting slipperiness to the easy adhesion 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 calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, and the like; crosslinked polymer particles; calcium oxalate And organic particles. Silica is particularly preferable when the easy-adhesion layer is formed mainly of the above-mentioned copolymerized 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 average particle size (SEM) of the particles contained in the easy-adhesion layer is preferably 0.005 to 1.0 μm from the viewpoint of ensuring the 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, particularly preferably 0.01 μm, from the viewpoint of handling properties and scratch resistance.

  The content of the particles of the easy-adhesion layer is 0.1 to 60% by mass with respect to the total amount of the constituent components of the easy-adhesion layer. It is preferable from the viewpoint of securing. The upper limit of the content of the particles is more preferably 50% by mass from the viewpoint of transparency and adhesion, and particularly preferably 40% by mass. Further, the lower limit of the content of the particles is more preferably 1% by mass, particularly preferably 0.5% 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 uniaxially stretched or sequentially biaxially stretched according to the stretching method, and then heat-treated.

  In the stretching of the film, the detailed conditions such as the stretching temperature, heat treatment temperature, and time are the characteristics of the matrix polymer and the characteristics required for the film, such as optical characteristics such as refractive index, mechanical characteristics, dimensional change rate, etc. It can be appropriately selected according to thermal characteristics, desired crystallinity, etc., and is not particularly limited. When PET is used as a matrix polymer, a preferable stretching temperature is 80 ° C to 110 ° C, a preferable heat treatment temperature is 180 to 250 ° C, and a preferable heat treatment time is 10 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) Melting point The melting point is determined using a DSC 6220 type differential scanning calorimeter manufactured by SII Nano Technology. In a nitrogen atmosphere, the resin sample was heated and melted at 300 ° C. for 5 minutes, then rapidly cooled with liquid nitrogen, and 10 mg of the pulverized resin sample was heated at a rate of 20 ° C./min, and differential thermal analysis was performed. The peak temperature of the endothermic peak defined in JIS K 7121, 1987, 9.1 is 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 higher 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 two points were connected with a straight line, and the melt viscosity (unit: poise) at a shear rate of 100 / sec was determined by interpolation.

(4) Film thickness Measured in accordance with JIS K 7130 “Plastic-Film and Sheet-Thickness Measuring Method” by mechanical scanning (Method A). The measuring instrument used was an electronic micrometer (manufactured by Marl, Millitron 1240).

(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.

(6) Refractive index In accordance with JIS K 7142 “Plastic Refractive Index Measuring Method”, a polarizing plate is attached to the eyepiece using sodium D line (wavelength 589 nm) as a light source and an Abbe refractometer 4T manufactured by Atago Co., Ltd. The direction of attachment, the direction of the polarizing plate, and the direction of the film were adjusted, and the refractive index (Nz) in the thickness direction, the refractive index (Ny) in the width direction, and the refractive index (Nx) in the longitudinal direction were measured. Jodomethane was used as an intermediate solution. Measurement of establishment of chain extender in each direction was carried out on each side of the film with n = 3 for each sample, and the average value was taken as the refractive index in each direction. Here, the width direction means a direction perpendicular to the roll unwinding direction and a direction parallel to the roll unwinding direction. The degree of plane orientation (ΔP) and the difference in refractive index (Δn) in the plane direction were determined by the following formula.
ΔP = ((Nx + Ny) / 2) −Nz
Δn = | Nx−Ny |

(7) Void amount A 20 mm × 20 mm film piece is prepared and fixed to the preparation glass with a tape. An objective lens (Nikon Corporation, oil immersion, numerical aperture 1.25, 100 times) is attached to an optical microscope (Nikon reflective bright / dark field microscope OPTIPHOT), and the prepared glass on which the film piece is placed is fixed in place. . A drop of silicone oil (about 30μ ■) is dropped on the film piece to bring the objective lens into contact with the oil. The reflected light is used to observe voids (bright parts) in the diffusion layer, and a photograph is taken to determine the area ratio (%) of voids (bright parts) in the entire photographic field. This operation is repeated for 10 film pieces, and the average value is defined as the void amount (%) of the film.

(8) Dimensional change rate It measured based on JIS C 2318-1997 5.3.4 (dimensional change). In the direction to be measured, the film is cut to a width of 10 mm and a length of 250 mm, marked at intervals of 200 mm, and the distance (A) between the marks is measured under a constant tension of 5 gf. Next, the film is placed in an oven in an atmosphere of 150 ° C., subjected to heat treatment at 150 ± 3 ° C. for 30 minutes under no load, and then the mark interval (B) is measured under a constant tension of 5 gf. The thermal shrinkage rate was calculated from the following formula.
Thermal contraction rate (%) = (A−B) / A × 100

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. Furthermore, the temperature was raised to 260 ° C. over 15 minutes, and 0.012 part by mass of trimethyl phosphate and then 0.0036 part 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 melting point of 257 ° 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, a color L value of 56.2, and a color. The b value was 1.6, and inert particles and internally precipitated particles were not substantially contained.

(2) Production of copolymer polyester resin (M2) As aromatic dicarboxylic acid component, terephthalic acid unit 100 mol%, diol component as ethylene glycol unit 70 mol% and neopentyl glycol unit 30 mol% as constituent components A copolyester resin (M2) having a viscosity of 0.69 dl / g was produced.

(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% by mass of a 30% by mass aqueous dispersion of the obtained copolyester resin (A) and a 20% by mass aqueous solution of a self-crosslinking polyurethane resin (B) containing an isocyanate group blocked with sodium bisulfite. 11.3 parts by mass (Daiichi Kogyo Seiyaku Co., Ltd., Elastolon H-3), 0.3 parts by mass of Elastolone Catalyst (Daiichi Kogyo Seiyaku, Cat 64), 39.8 parts by mass of water and 37 isopropyl alcohol 4 parts by mass were 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.

(3) Manufacture of light diffusing film As a raw material for the light diffusing layer (B layer), 90 parts by mass of PET (M1) and 10 parts by mass of PS (G797N manufactured by Nippon Polyste) having a melt viscosity of 3900 poise were mixed. After drying at 135 ° C. under reduced pressure (1 Torr) for 6 hours, the mixture was supplied to the extruder 2. Further, PET (M1) as a raw material of the support layer (A layer) 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 B layer and A layer was controlled using the gear pump of each layer so that it might become 10:90. In addition, a stainless steel sintered filter medium (nominal filtration accuracy: 95 μm 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 brought into close contact with 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 750 μm. At this time, the layer B surface was a surface in contact with the cooling drum.

Subsequently, the easily bonding layer was apply | coated to the single side | surface (B layer surface) 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 coating method, and it apply | coated so that the coating amount after drying might be 0.1 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, the unstretched film having the coating layer was gripped with a clip and subjected to lateral stretching. The transverse stretching temperature was 100 ° C., the transverse stretching ratio was 3.6 times, and the transverse stretching speed was constant at 25% / second. Next, heat treatment was performed at 200 ° C. for 15 seconds to obtain a light diffusing film.

(4) 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 inherent to the stretched film, and has excellent light transmittance and light diffusibility. I understand. Further, even when used as a light diffusing film in a known backlight unit, the luminance was not lowered, and the properties excellent as a light diffusing film were exhibited.

Example 2
A light diffusing film was produced in the same manner as in Example 1 except that the transverse stretching temperature was 95 ° C., the transverse stretching ratio was 4.0 times, and the heat treatment temperature was 230 ° C.

Example 3
Other than mixing and using 60 parts by mass of PET (M1), 30 parts by mass of a copolyester resin (M2), and 10 parts by mass of PS (G797N manufactured by Nippon Polyste) as a raw material for the light diffusion layer (B layer) Produced a light diffusing film in the same manner as in Example 1. In addition, melting | fusing point of the said mixed crystalline polyester resin which comprises a light-diffusion layer was 257 degreeC.

Comparative Example 1
An unstretched film having a coating layer obtained by the same method as in 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 longitudinal stretching speed was 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 in the process of cooling to 60 ° C., 3% relaxation treatment was performed in the width direction to produce a light diffusing film.

  The film obtained in Comparative Example 1 has a high haze and good light diffusivity, but has a low light transmittance, and the light diffusivity and light transmittance required for the light diffusible film can be balanced. The quality was low.

Comparative Example 2
An unstretched film having a coating layer obtained in exactly the same manner as in Example 1 was first preheated with a roll group heated to 75 ° C., and then the film was heated to 96 ° C. using a non-contact infrared heater. Thus, longitudinal stretching was performed 3.4 times between rolls having different peripheral speeds. At this time, the longitudinal stretching speed was 530% / second. Next, heat treatment was performed at 200 ° C. for 15 seconds to obtain a light diffusing film.

  The film obtained in Comparative Example 2 had high haze and good light diffusibility, but had low light transmittance and low quality.

  The light diffusive film obtained by the production method of the present invention has excellent heat resistance inherent in the stretched film, and has both excellent light transmittance and light diffusibility, so other optical functionalities It can be integrated with the film, and can be used for reducing the size of the backlight unit, simplifying the structure and manufacturing process of the backlight unit, reducing costs, and the like.

Claims (3)

  A light diffusing film obtained by stretching at least uniaxially a non-stretched sheet 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 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 refractive index in the longitudinal direction and a width direction. The refractive index difference (Δn) with respect to the refractive index is 0.040 to 0.120, the degree of plane orientation (ΔP) is 0.040 to 0.160, and the total light transmittance is 85% or more, A light diffusing film characterized by having a haze of 40% or more and a dimensional change rate at 150 ° C. of 3.0% or less in both the longitudinal direction and the width direction.   2. The light diffusing film according to claim 1, wherein the crystalline polyester is made of polyethylene terephthalate or a polyethylene terephthalate copolymer. A method for producing the light diffusive film according to claim 1, wherein the film is stretched only in a uniaxial direction at a stretching ratio of 2.5 times or more and at a stretching speed of less than 80% / second. A method for producing a light diffusing film characterized by the above.