JP2010249929A - Light diffusion film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light diffusion film which is suitable for a base film of a laminated body. <P>SOLUTION: This light diffusion film satisfies requirements (1) to (4). (1) A light diffusion layer containing a light-diffusing component is disposed, on at least one surface of a biaxial stretched polyethylene terephthalate resin film. (2) The light diffusion layer does not substantially contain voids. (3) The light diffusion layer is formed of at least light transmissive resin and fine particles as the light diffusing component. (4) The average of the heights of the cambers at four corners is 0.5 mm or more and 5.0 mm or less, when a sample obtained by sampling the film by 300 mm in the longitudinal direction of the film production and by 210 mm in the width direction rectangular to the longitudinal direction is heat-treated at 150°C for 30 minutes. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a light diffusing film. Specifically, the present invention relates to a light diffusing film suitable for a base film of a laminate having excellent planarity.

  A light diffusing film using a biaxially stretched polyethylene terephthalate resin film as a base film is used as a backlight film for liquid crystal displays because of its excellent light transmittance, light diffusibility, and chemical resistance. In particular, a light diffusing film excellent in light transmittance, light diffusibility, and mechanical strength while having a smooth surface has been proposed (Patent Documents 1 and 2). By providing various functional layers made of the curing shrinkable resin composition on the smooth surface of such a light diffusing film, it becomes possible to have a combined function.

  The light diffusive film for use as described above is required to have good flatness (flatness when placed on a flat table) as compared with a film for normal packaging use or the like. In particular, with the recent high performance of products, the demand for flatness is high, and the disturbance of flatness becomes a quality defect.

  On the other hand, with respect to suppressing curling of a film cut out from a roll-shaped film and curling after heating for the purpose of obtaining a void-containing polyester-based film having good flatness, as in Patent Documents 3 and 4, a longitudinal stretching step Describes a method of changing the output of the infrared rays on the front and back sides and performing a temperature difference between the film in the transverse stretching step and the heat setting step.

  Furthermore, for the purpose of obtaining a film with good flatness, regarding the curling of the film cut out from the film under load, as in Patent Document 5, the temperature difference between the front and back of the film is set to 10 ° C. or less during longitudinal stretching. It is described.

JP 2001-272508 A JP 2001-324606 A JP 2001-342273 A JP 2001-342274 A Japanese Patent Laid-Open No. 10-258458

  A laminate produced using a light diffusing film as a base film is also required to have good flatness. In order to obtain a laminate having good flatness, a relatively thick base film or a base film having excellent flatness is used as described above. Further, even when the laminate itself has a warp, when producing an assembly using the laminate as a member, the planarity of the laminate is adjusted by devising an assembly method. Therefore, even a conventional base film can be used without any problem.

  However, it was found that when the curing shrinkable resin composition was laminated, a minute warp was generated on the side of the curable resin layer due to curing shrinkage accompanying the curing of the resin. For this reason, even in a laminate that has been considered to have good flatness in the past, in applications that require a larger area, it has been considered that a material with a small degree of warping is useful for improving precision.

  The present invention relates to a light diffusing film. Specifically, the present invention relates to a light diffusing film suitable for a base film of a laminate having excellent planarity. For example, when a curing shrinkable resin composition is laminated on a light diffusing film, even if curing shrinkage of the curing shrinkable resin composition occurs, the base film has shrinkage on the opposite side, The present invention relates to a light diffusing film that can be suitably used as a base film that has a balanced warpage and can have excellent flatness as a whole laminate.

Among the present inventions, the first invention is a light diffusing film that satisfies the following requirements (1) to (4).
(1) It has a light diffusion layer having a light diffusion component inside on at least one surface of a biaxially stretched polyethylene terephthalate resin film. (2) The light diffusion layer is substantially free of voids (3 ) The light diffusing layer is composed of at least a light transmissive resin and fine particles as a light diffusing component. (4) A sample of 300 mm is taken in the longitudinal direction of the film and 210 mm in the width direction perpendicular thereto. The sample was placed on a mount with one side facing up, and heat-treated at 150 ° C. for 30 minutes in a heating oven. Then, the sample was removed from the heating oven together with the mount, and the sample was allowed to stand at room temperature for 30 minutes. When measuring the height of the sled (height in the vertical direction from the horizontal plane) with a JIS metal ruler (0.5 mm scale), the average height of the sleds at the four corners is 0.5 mm or more and 5.0 mm or less. (If the height of the warp of the sample after standing at room temperature after heating is 0 mm, or if the cross section of the sample is M-shaped, the upper and lower surfaces of the sample are reversed. Measure the height of the sled.)
2nd invention is the said light diffusable film whose thickness of the said light diffusable film is 100 micrometers or more and 400 micrometers or less.
3rd invention is the said light diffusable film characterized by the total light transmittance of a light diffusable resin film being 80% or more, and haze being 60% or more.
4th invention has a coating layer in at least one surface of the said light diffusable film, and provided the prism row | line | column which consists of an ultraviolet curable type or an electron beam curable acrylic resin on the surface of the said coating layer, or a light-diffusion layer. It is the laminated body characterized by becoming.

  The light diffusing film of the present invention has good flatness as a single film or a laminate. Therefore, as a preferred embodiment, even when a film of the present invention is used as a base film and a curing shrinkable resin composition such as a hard coat layer or a lens layer is laminated, the planarity of the whole laminate is good. Further, as a preferred embodiment, even when materials having different shrinkage properties or shrinkage properties are laminated or bonded, the planarity of the entire laminate is good.

  The polyethylene terephthalate film, which is a base film of the light diffusing film of the present invention, contains ethylene glycol and terephthalic acid as main components. Other dicarboxylic acid components and glycol components may be copolymerized as long as the object of the present invention is not impaired. Examples of the other dicarboxylic acid components include isophthalic acid, p-β-oxyethoxybenzoic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-dicarboxybenzophenone, bis- (4-carboxyphenylethane), adipine Examples include acid, sebacic acid, 5-sodium sulfoisophthalic acid, cyclohexane-1,4-dicarboxylic acid, and the like. Examples of the other glycol component include propylene glycol, butanediol, neopentyl glycol, diethylene glycol, bisphenol A and other ethylene oxide adducts, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and the like. In addition, oxycarboxylic acid components such as p-oxybenzoic acid can also be used.

  The light diffusing film of the present invention has a structure in which a light diffusing layer having a light diffusing component is laminated on at least one surface of a polyethylene terephthalate film.

  The light diffusion layer of the present invention comprises at least a light transmissive resin and a light diffusion component.

  Examples of the light transmissive resin preferably used in the present invention include polyesters such as polyethylene terephthalate, polyethylene-2, 6-naphthalate, polypropylene terephthalate, and polybutylene terephthalate, polyolefins such as polycarbonate, polystyrene, and polypropylene, polyamides, polyethers, Polyester amide, polyether ester, polyvinyl chloride, polymethacrylic acid ester, a copolymer containing these as main components, a mixture of these resins, or the like can be suitably used.

  The light-transmitting property of the light-transmitting resin here means that it is transparent with respect to light rays in the visible light region. Specifically, it refers to a resin having a total light transmittance of 80% or more in a film having a thickness of 450 μm made of only those resins. Here, the total light transmittance can be measured using a haze meter.

  In addition, the refractive index of the resin is generally preferably about 1.3 to 1.7.

  The light diffusing component used in the present invention may be anything that diffuses incident light, and various fine particles are preferably used.

Examples of the fine particles used in the present invention include inorganic fine particles such as glass, silica, barium sulfate, titanium oxide, magnesium sulfate, magnesium carbonate, and calcium carbonate, or acrylic resin, organic silicone resin, polystyrene, polyolefin, polyester, and urea resin. And organic fine particles such as formaldehyde condensate and fluororesin.

In the biaxially stretched film manufacturing process (melting / extrusion process), in the matrix made of light-transmitting resin, by dispersing and forming domains made of incompatible thermoplastic resin in the light-transmitting resin, The fine particles may be formed. Examples of the thermoplastic resin incompatible with the light transmissive resin include polyolefins such as polyethylene, polypropylene, polymethylpentene, and various cyclic olefin polymers, polycarbonate, atactic polystyrene, syndiotactic polystyrene, isotactic polystyrene, and the like. Acrylic resins such as polystyrene, polyamide, polyether, polyesteramide, polyphenylene sulfide, polyphenylene ether, polyether ester, polyvinyl chloride, polymethacrylic acid ester, and copolymers containing these as main components, or of these resins A mixture etc. can be mentioned.

  The shape of these fine particles is preferably substantially spherical or true spherical. The substantially spherical shape here may be a shape having at least a spherical portion on the surface of the fine particles. Furthermore, an aggregate of these fine particles may be present. When the fine particles are substantially spherical or spherical, a uniform diffusion effect without directionality can be obtained. In this respect, the fine particles are more preferably spherical. These average particle diameters are usually 0.1 to 50 μm, more preferably 0.1 to 30 μm, and most preferably 0.1 to 20 μm. By making the particle diameter of the fine particles larger than 0.1 μm, the total light transmittance of the film is increased, and by making the particle diameter smaller than 50 μm, a good light diffusion effect can be obtained without reducing the film strength.

  Further, it is important that the refractive index of the fine particles is different from the refractive index of the light-transmitting resin. If the refractive indexes of the fine particles and the light-transmitting resin are the same, the refractive scattering phenomenon does not occur at the interface between the resin and the fine particles, and as a result, a desired light diffusion effect cannot be obtained. Furthermore, in order to obtain substantially effective light diffusibility, it is preferable that the difference in refractive index between the light-transmitting resin and the fine particles is 0.012 or more. If the refractive index difference is less than 0.012, the light diffusion effect is small, and in order to obtain a good diffusion effect, it is necessary to add a large amount of fine particles or increase the film thickness, and the mechanical strength becomes weak or desired. It is not preferable because the influence is too thick.

  The light-transmitting resin used as the raw material of the light diffusing layer of the present invention and the fine particles as the light diffusing component are supplied to pellets prepared by being melt-kneaded and blended uniformly in advance or directly kneaded extruder. Melt knead.

  Moreover, the ratio of the fine particles blended in the light diffusion layer is appropriately selected depending on the desired degree of light diffusion. Preferably, the volume fraction is 1 to 75%, more preferably 1 to 50%.

  The total light transmittance of the light diffusing film is 80% or more, preferably 83% or more, more preferably 86% or more, further preferably 89% or more, and the haze is 60% or more, preferably 70% or more. More preferably, it is 80% or more, More preferably, it is 90% or more.

  Moreover, it is preferable that a light-diffusion layer does not contain a void substantially. A void here refers to the fine bubble produced | generated inside the film. This void can be confirmed by using a scanning electron microscope, a transmission electron microscope, or the like for the film cross section.

  The light diffusing film of the present invention is constituted by laminating a light diffusing layer on a biaxially stretched polyethylene terephthalate film that is a base film. A film consisting of a single film consisting only of the light diffusing layer can be obtained as a film having a high total light transmittance and a high haze, but is not preferred because it is inferior in the mechanical strength of the film such as bending rigidity. If the bending rigidity is weak, a handling problem such as a crease tends to occur during handling after film formation is not preferable. Moreover, the light diffusion component contained inside inhibits the flatness of the surface. Therefore, the light diffusing film of the present invention is formed by laminating a light diffusing layer and a biaxially stretched polyethylene terephthalate film excellent in mechanical strength and surface flatness. Here, the lamination method will be described. For example, a raw material for the light diffusion layer and a raw material for the biaxially stretched polyethylene terephthalate film are melted by separate extruders, and a method such as coextrusion molding through a T-die or the like. After forming into a film, a desired light diffusion layer is obtained through a stretching process, a heat setting process, a heat treatment process, and the like as necessary. Here, when a stretching process is required, voids may be generated in the film after stretching. When a large amount of voids are generated in the film, the total light transmittance may be lowered. Therefore, it may be necessary to eliminate the voids through processes such as heat setting and heat treatment.

  As a polymerization method of polyethylene terephthalate (hereinafter simply referred to as PET) used as a base material, a direct polymerization method in which terephthalic acid and ethylene glycol and, if necessary, other dicarboxylic acid component and diol component are directly reacted, and terephthalic acid Any production method such as a transesterification method in which a dimethyl ester (including methyl esters of other dicarboxylic acids as required) and ethylene glycol (including other diol components as necessary) are transesterified is used. obtain.

  When the film of the present invention is formed of PET, the intrinsic viscosity (IV) of the raw material PET is preferably 0.45 to 0.70 dl / g, and more preferably 0.55 to 0.65 dl / g. When the intrinsic viscosity of the PET raw material is 0.45 or less, the degree of polymerization of the PET after being recovered and passed through the extruder again becomes too low, and the stretchability of the film deteriorates or the tear resistance decreases. Is not preferable. On the other hand, if the intrinsic viscosity exceeds 0.70 dl / g, the filtration pressure becomes excessively high and high-precision filtration becomes difficult, which is not preferable. In addition, IV of the resin raw material is calculated | required with the following methods, for example, and is represented as [(eta)].

[Intrinsic viscosity (IV)]
After the PET ground sample is dried, it is dissolved in a mixed solvent of phenol / tetrachloroethane = 60/40 (weight ratio), and a solution having a concentration of 0.4 (g / dl) is obtained at 30 ° C. using an Ostwald viscometer. The flow time and the flow time of the solvent alone are measured, and the calculation is performed from the time ratio on the assumption that the constant of Huggins is 0.38 using the Huggins formula.

  Moreover, when forming the film of this invention by PET, the range of 3-30 eq / t is preferable and, as for the acid value (AV) of PET raw material, it is more preferable in it being 5-25 eq / t. An acid value of 3 eq / t or less is not preferable because the polymerization rate becomes slow and the production efficiency is lowered. On the other hand, an acid value of 30 eq / t or more is not preferable because hydrolysis tends to proceed and the degree of polymerization tends to decrease. The acid value of the resin raw material is determined by the following method, for example.

[Acid value]
After pulverizing the raw material, it is dissolved in benzyl alcohol, and after adding chloroform, neutralization titration with a sodium hydroxide solution is performed to calculate the equivalent of sodium hydroxide per 1 ton of PET.

  When a rectangular film sample of 300 mm in the longitudinal direction (longitudinal direction) of the film formation and 210 mm in the width direction perpendicular thereto is cut out from the light diffusing film of the present invention, heat treatment is performed at 150 ° C. for 30 minutes under the following measurement conditions. It is necessary that the average height of the four corners be 0.5 mm or more and 5.0 mm or less.

The warpage after heating is measured by the following method.
(1) A rectangular film sample of 300 mm × width direction 210 mm is cut out in the longitudinal direction (also referred to as longitudinal direction or machine direction) of film formation.
(2) The sample is placed on a flat mount with one side (for example, x-plane) facing up, and placed on the shelf of a heating oven. Here, the mount is also referred to as cardboard or paperboard, and a thickness of about 1 mm is suitable.
(3) Adjust the heating oven to 150 ° C. and heat-treat for 30 minutes.
(4) After the heat treatment, the sample is taken out together with the mount and left at room temperature for 30 minutes. The room temperature conditions here are preferably controlled at a temperature of 23 ± 2 ° C. and a humidity of 65 ± 5%.
(5) The sample left for 30 minutes is placed on a horizontal glass plate (preferably having a thickness of about 5 mm), and the heights of the four corners of the sample (the height in the vertical direction from the horizontal plane) are set to a JIS scale (0 .5mm scale) and visually measure to 1/10 of the minimum scale. The height of the four corners is measured for all samples, and the average of the heights of the four corners is obtained.
(6) In addition, when the height of the warp of the sample after standing at room temperature after heating is 0 mm, or when the cross section of the sample (one side of the rectangle) is M-shaped, The height of the warp is measured with the upper and lower surfaces of the sample opposite (with the x-plane facing down). That is, the warp of the present application measures the heights of the four corners with the concave surface when the warp occurs in the film after heating.

  The average of the heights of the four corners by heat treatment at 150 ° C. for 30 minutes measured under the above conditions of the light diffusing film of the present invention is more preferably 0.6 m or more, and further preferably 0.7 mm or more. When the average height of warps at the four corners by heat treatment at 150 ° C. for 30 minutes measured under the above conditions is less than 0.5 mm, when the curing shrinkable resin is laminated, it cannot resist the warping due to the curing shrinkage of the resin. And the laminated body tends to warp to the cured resin composition layer side as a whole. Further, the average of the heights of the four corners by heat treatment at 150 ° C. for 30 minutes measured under the above conditions is more preferably 4.0 mm or less, and even more preferably 3.5 mm or less. When the average height of the four corner warps measured by heat treatment at 150 ° C. for 30 minutes measured under the above conditions exceeds 5.0 mm, when a curing shrinkable resin is laminated, the warp stronger than the warp due to the curing shrinkage of the resin. This is not preferable because the laminated body tends to warp toward the base film as a whole.

  When the average of the heights of the four corners by heat treatment at 150 ° C. for 30 minutes measured under the above conditions is 0.5 mm or more and 5.0 mm or less, the curing shrinkable resin composition is laminated on one side and cured with curing. Even if contraction occurs, the planarity of the entire laminate is maintained. For example, in the case of a rectangular laminate having a longitudinal direction (longitudinal direction) of 300 mm and a width direction of 210 mm, the average flatness of the four corners is 0, although the allowable range of planarity as a whole laminate depends on the application. 0.5 mm or less is preferable.

  The light diffusing film of the present invention desirably has good flatness in the state before the heat treatment. Therefore, when a rectangular film sample of 300 mm in the longitudinal direction and 210 mm in the width direction perpendicular thereto is cut out, when the warp heights at the four corners are measured without performing heat treatment, the maximum value of the warp height is It is less than the film thickness, and the average value of the height of the four corners is preferably 20% or less of the film thickness.

  When the heat treatment is not performed, the maximum value of the height of the warp is preferably not more than the film thickness, more preferably not more than 90% of the film thickness, still more preferably not more than 80%, and 50 % Or less is particularly preferable. Moreover, it is preferable that the average of the height of the warp when not heat-processing is 20% or less. When the maximum warp height when the heat treatment is not performed is less than the film thickness, or the average value is 20% or less, there is little distortion in flatness during film processing such as application of curable resin, and processing accuracy Is preferable from the viewpoint of yield.

  The thickness of the film constituting the light diffusing film of the present invention is not particularly limited. However, the base film of the laminate is preferably 100 μm or more and 400 μm or less. Further, the thickness of the film is more preferably 110 μm or more, and further preferably 120 μm or more. If the thickness of the film is 100 μm or more, it is preferable because the film can be handled easily. Further, the thickness of the film is preferably 400 μm or less, more preferably 300 μm or less, and further preferably 250 μm or less. If the thickness of the film is 400 μm or less, it is preferable because cutting processing becomes easy.

  Moreover, in order to give functionality other than light diffusibility to a film, it is good also as a polyethylene terephthalate type | system | group light diffusive film which has a multilayered structure of three or more layers. When the surface on which the slippery layer or the easy-adhesion layer is applied is the A layer, the light diffusion layer is the B layer, and the other surface is the C layer, the layer structure in the film thickness direction is A / B, A / B / A , A / B / C, A / C / B, A / C / B / C / A, etc. are conceivable. The layers A to C may be made of the same material or different materials.

  There is no problem even if various coatings are applied to one side or both sides of the light diffusing film of the present invention at the time of film formation for the purpose of improving the adhesiveness during the post-processing step or for improving the slipperiness.

  In order to improve the adhesiveness to the curable resin, it is preferable to have a coating layer mainly composed of at least one of a polyester resin, a polyurethane resin or a polyacrylic resin on at least one side of the film of the present invention. Here, the “main component” refers to a component that is 50% by mass or more of the solid components constituting the coating layer. The coating solution used for forming the coating layer of the present invention is preferably an aqueous coating solution containing at least one of water-soluble or water-dispersible copolymerized polyester resin, acrylic resin and polyurethane resin. Examples of these coating liquids include water-soluble or water-dispersible co-polymers disclosed in Japanese Patent No. 3567927, Japanese Patent No. 3589232, Japanese Patent No. 3589233, Japanese Patent No. 3900191, and Japanese Patent No. 4150982. Examples thereof include a polymerized polyester resin solution, an acrylic resin solution, and a polyurethane resin solution.

The coating layer can be obtained by applying the coating liquid on one or both sides of a uniaxially stretched film in the longitudinal direction, drying at 100 to 150 ° C., and further stretching in the transverse direction. The final coating amount of the coating layer is preferably controlled to 0.05 to 0.20 g / m ² . If the coating amount is less than 0.05 g / m ² , adhesion with the resulting curable resin may be insufficient. On the other hand, when the coating amount exceeds 0.20 g / m ² , blocking resistance may be lowered. When coating layers are provided on both sides of the polyester film, the coating amounts of the coating layers on both sides may be the same or different, and can be independently set within the above range.

  It is preferable to add particles to the coating layer in order to provide easy slipping. It is preferable to use particles having an average particle size of 2 μm or less. When the average particle diameter of the particles exceeds 2 μm, the particles easily fall off from the coating layer. Examples of the particles to be contained in the coating layer include the same particles as those described above.

  Moreover, a well-known method can be used as a method of apply | coating a coating liquid. For example, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, etc. can be mentioned. Or it can carry out in combination.

  In addition, the light diffusing film constituting the film of the present invention may be subjected to corona treatment, coating treatment, flame treatment or the like in order to improve the adhesion of the film surface.

  Moreover, in the polyethylene terephthalate-type resin film which comprises the film of this invention, microparticles | fine-particles can be added as needed. Examples of the fine particles added at that time include known inorganic fine particles and organic fine particles. Furthermore, in the resin forming the film, various additives as necessary, for example, waxes, antioxidants, antistatic agents, crystal nucleating agents, viscosity reducing agents, heat stabilizers, coloring pigments, An anti-coloring agent, an ultraviolet absorber and the like can be added. It is preferable to add fine particles to the polyethylene terephthalate resin in the present invention to improve the workability (slidability) of the polyethylene terephthalate resin film. Any fine particles can be selected. Examples of inorganic fine particles include silica, alumina, titanium dioxide, calcium carbonate, kaolin, and barium sulfate. Examples of the organic fine particles include acrylic resin particles, melamine resin particles, silicone resin particles, and crosslinked polystyrene particles. The average particle diameter of the fine particles can be appropriately selected as necessary within a range of 0.05 to 2.0 μm.

In addition, the measurement of the average particle diameter of said particle | grain is performed with the following method.
Take a picture 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.

  As a method of blending the above-mentioned particles into the polyethylene terephthalate resin film, for example, it can be added at any stage of producing the polyethylene terephthalate resin, but preferably after the esterification stage or after completion of the transesterification reaction. It may be added as a slurry dispersed in ethylene glycol or the like at the stage before the start of the condensation reaction to proceed the polycondensation reaction. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyethylene terephthalate resin raw material, or a dried particle and a polyethylene terephthalate system using a kneading extruder It can be performed by a method of blending with a resin raw material.

The technical idea for the warp of the present invention is as follows.
(1) The curing shrinkable resin composition forms a crosslinked structure by curing, and the volume decreases by about 10% before and after curing. Therefore, when a curable resin composition is laminated on one side of the base film, warping that causes the curable resin composition side to become a concave portion as a whole of the laminate occurs due to curing shrinkage accompanying the curing of the resin.
(2) In order to prevent the warpage of (1), it was considered that if the warp on the opposite side occurred in the base film so as to resist the curing shrinkage of the curable resin, the warpage could be offset as the whole laminate. For this reason, it is necessary for a base film alone without a curable resin composition layer to have a warp on the side opposite to the laminated surface.
(3) The present inventors have obtained the idea of the present invention by confirming that when one side of the base film is subjected to heat treatment for a short time at a high temperature, the heat treatment surface contracts and a slight warp is generated. From this, it was considered that the heat shrinkage rate of the front and back of the film should be different in order to generate warp in the base film. When there is a difference in heat shrinkage between the front and back of the film, the heat generated during the curing process of the curable resin generates a warp such that the surface having a large shrinkage becomes a recess. However, the warp developed by heating was slight. However, even when the present inventors applied a curable shrinkable resin composition having a large shrinkage ratio of about 10%, the entire laminate was resisted by the slight shrinkage of the base film against the curable shrinkage. As a result, they found a surprising effect that the flatness was maintained.
(4) In addition, from the viewpoint of workability, this base film has no warp and is substantially flat when not subjected to heat treatment, and must be usable in the same manner as a conventional base film. That is, the film of the present invention is a film having an unprecedented characteristic that it has a potential warp that becomes apparent by heating, even though it is flat before the heat treatment.

[Production Method of Film of the Present Invention]
<Problems of conventional stretching methods>
Until now, as a method for controlling the flatness of a film, methods described in Patent Document 3 (Japanese Patent Laid-Open No. 2001-342273) and Patent Document 4 (Japanese Patent Laid-Open No. 2001-342274) have been disclosed. In Patent Document 3 and Patent Document 4, in the case of a void-containing polyester-based film, “(1) reducing the volume fraction of the cavity and reducing the size of each cavity as a means for suppressing curling due to the structural difference between the front and back of the film. By suppressing the occurrence of curling by resisting internal strain, (2) a method of providing a distribution in the cavities in the film thickness direction, and (3) a crystal in the film thickness direction due to a cooling difference during extrusion. In order to control the curl caused by the structural difference between the front and back of the film that is applied in each process starting from the difference in the degree of conversion, the structural difference between the front and back of the film is positively generated and complemented with the necessary structural difference. "A method for bringing the value closer to zero" is described.

  Furthermore, “With a void-containing polyester film containing a large number of voids derived from a thermoplastic resin incompatible with the polyester resin inside the film, it is very difficult to obtain a film with little curl by the conventional technology.” "In ordinary transparent polyester films, the occurrence of curling curls is very slow and is very unlikely to be a problem."

  However, while Patent Documents 3 and 4 focus on the point of eliminating curling, the present invention aims to positively impart a potential warp to the film. Furthermore, while Patent Documents 3 and 4 presuppose a void-containing film, the present invention provides a potential warp in a transparent polyethylene terephthalate film that does not contain voids.

  The thermal conductivity is different between the void-containing film and the polyethylene terephthalate-based resin film not including the void (the void-containing film has a small thermal conductivity, but the film without the void has a high thermal conductivity). Therefore, in the case of a polyethylene terephthalate-based resin film that does not contain cavities, it is difficult for the methods of Patent Documents 3 and 4 to provide a temperature difference between the front and the back that is necessary for the expression of a potential warp during the manufacturing process.

  Furthermore, the temperature difference between the front and back of the film in the longitudinal stretching step is 10 ° C. or less by the method described in Patent Document 5 (JP-A-10-258458) for the purpose of preventing curling when the film is loaded. A method of setting to is disclosed.

  Patent Document 3 aims to obtain a film that does not cause curling, whereas in the present invention, a film having a potential warp that can be balanced even if the film shrinks due to curing of the curable resin. The purpose is to obtain. Therefore, in order to make the thermal shrinkage due to heating different between the front and the back, as in Patent Document 5, the one-stage stretching is insufficient. In order to obtain the film of the present invention, the reason is not clear, but multistage stretching was necessary as described later.

  In order to solve the problems of the above-described conventional stretching methods, the present inventors obtain a film having a potential warp that becomes apparent by heating even though it is flat before the heat treatment. We studied earnestly to see if it could be done. As a result, unlike the conventional stretching method, it has been found that a film in which warpage is manifested by heating can be obtained by positively providing a molecular orientation difference between the front and back surfaces in the film production process, and the present invention has been completed. It was. More specifically, by correlating the achievement means described in the following (1) to (3), the potential to be manifested by heating despite being flat before the heat treatment is shown. A film having an unprecedented characteristic of having a warp was obtained. Furthermore, by taking the means of the following (4), a light diffusing layer substantially free of voids can be obtained, and a film excellent in light transmittance and light diffusibility while having a smooth surface is obtained. As a result, the present invention has been devised.

<Characteristics of the film production method of the present invention>
(1) Temperature difference between front and back of unstretched sheet In the production of the film of the present invention, a molten resin is first extruded from a die, and rapidly cooled and solidified by winding on a cooled casting drum to obtain an unstretched sheet. At this time, the thickness of the unstretched sheet is, for example, about 1000 μm or more in a thick film of 100 μm or more. Since the unstretched sheet is cooled from the sheet surface, the cooling efficiency is different between the surface in contact with the casting drum (hereinafter referred to as the front surface (F)) and the opposite surface (hereinafter referred to as the back surface (B)). A temperature difference occurs between the front and back of the unstretched sheet.

  Although depending on the thickness of the unstretched sheet, the expression of the curl of the film varies depending on the temperature difference between the front and back surfaces of the unstretched sheet. When the temperature difference between the front and back surfaces becomes large, the sheet itself curls, and the central portion of the sheet does not adhere to the roll surface, and contact on the roll becomes insufficient. Thereby, it contacts with the unintended part in a process, and a crack will generate | occur | produce in a film. For this reason, usually, the temperature difference between the front and back sides of the sheet is not excessively increased. However, in the present invention, in order to develop warp by heat treatment, a difference in crystallinity is provided on the front and back sides of the sheet, thereby causing a difference in stretch orientation in the stretching process, thereby achieving the development of warp after heating. It is. In order to provide a difference in the crystallinity between the front and back surfaces, it was estimated that there was an appropriate range, even if the temperature difference between the front and back surfaces was low or high.

  In order to satisfy the above characteristics, the inventor of the present application has intensively studied the temperature difference between the front and back sides of the unstretched sheet and the optimization of the temperature of the entire sheet. As a result, when the surface temperature of the surface (F) of the unstretched sheet is F and the surface temperature of the back surface (B) is B when the second cooling roll (separating roll) following the casting drum is separated, it is unstretched. It has been found that the surface temperature difference (FB) between the front and back of the sheet is preferably 0 ° C. or higher and 33 ° C. or lower.

  Specifically, the surface temperature difference between the front and back of the sheet at the outlet of the second cooling roll is more preferably 5 ° C. or higher, further preferably 8 ° C. or higher, and particularly preferably 10 ° C. or higher. The surface temperature difference between the front and back of the sheet is more preferably 30 ° C. or less, further preferably 28 ° C. or less, and particularly preferably 25 ° C. or less.

  As a method of controlling the surface temperature difference (FB) between the front and back of the unstretched sheet within the above range, it is desirable to appropriately control the cooling time and the temperature of the cooling roll. The front surface (F) in direct contact with the cooled casting drum is cooled earlier than the back surface (B). Therefore, when the thickness is increased to 1,000 μm or more, the surface temperature difference becomes large in the surface temperature difference between the front and back surfaces of the sheet while being cooled by the casting drum. Thereafter, when there is a second cooling roll following the casting drum, the back surface (B) is cooled by the second cooling roll, and the surface temperature difference between the front and back of the sheet is reduced. In such a case, for example, the back surface is cooled by using cooling air, or the surface temperature difference between the front and back surfaces of the sheet is controlled by cooling the back surface with the second cooling roll early by reducing the casting drum diameter. I can do it. Further, since the time required for cooling depends on the thickness of the sheet, the speed of the cooling roll, and the like, it is preferable to appropriately adjust the temperature of the cooling air, the cooling range, the temperature of the second cooling roll, and the like.

(2) Temperature difference between front and back in longitudinal stretching In order to obtain the film of the present invention, it is desirable to provide a temperature difference between the front and back of the film in the longitudinal stretching step and change the degree of molecular orientation on the front and back of the film. If a temperature difference between the front and back of the film is provided in the longitudinal stretching step, orientation distortion remains on the side having a lower surface temperature than on the side having a higher surface temperature, and a potential warp that is manifested by heat treatment tends to occur. During the longitudinal stretching in the production of the film of the present invention, in order to provide a temperature difference between the front and back, roll temperature setting, non-contact infrared irradiation, heating with high-speed heating air, other heating or cooling means may be used. Is possible.

  Furthermore, in order to obtain the film of the present invention, it is desirable to perform longitudinal stretching with a temperature difference between the front and back in multiple stages, at least two stages or more. In order to provide a potential warp that is manifested by heating, which is the purpose of the present application, even if stretching is performed in one step, the effect due to the temperature difference is small, and further stretching with a temperature difference is performed in a state where the stretching orientation has advanced. It is desirable. That is, by repeating the stretching operation with a temperature difference between the front and back at least twice, the state where the orientation of the front and back is different by the first stage treatment is further antagonized by curing and shrinking by further stretching the temperature difference. It is thought that sufficient orientation strain cannot be obtained. In particular, it is more preferable in terms of effectively providing orientation strain that the film is stretched in one step with a temperature difference between the front and back sides and then once cooled and then subjected to longitudinal stretching with a difference in temperature between the front and back sides.

  When performing longitudinal stretching in two or more stages, it is desirable to set the stretching ratio and the temperature difference between the front and back sides according to the thickness of the film. Specifically, when a film having a product thickness of 100 to 400 μm is formed between the rolls having a difference in peripheral speed by an infrared heater and a film having a product thickness of 100 to 400 μm is formed, 2.0 times or more in the longitudinal direction (longitudinal direction). After stretching to a magnification of 2 times or less, the film after the longitudinal stretching is passed between nip rolls whose surface temperature is cooled, and a magnification of 1.03 times to 1.5 times is obtained in the longitudinal direction. Thus, it is preferable to perform two or more steps of stretching. Regarding the temperature difference between the front and back, for example, when creating a film having a product thickness of 125 μm, for the first stage, the average of the front and back temperatures is 70 ° C. or more and 115 ° C. or less, and the temperature difference between the front and back is 0.3 ° C. It is preferable to adjust so that it may become 3 degrees C or less above, and about the 2nd step | paragraph, it is preferable to adjust the temperature difference of front and back to 2 degrees C or more and 5 degrees C or less. If the average temperature of the front and back surfaces is less than 70 ° C., stretching is difficult and thickness spots are likely to occur, and if it exceeds 115 ° C., it becomes difficult to obtain the effect of providing a temperature difference between the front and back surfaces. Moreover, when the temperature difference between the front and back exceeds 5 ° C., curling occurs in the sheet itself after longitudinal stretching, and it does not follow the roll, which causes scratches. (In addition, the temperature of the film front and back in the longitudinal stretching step refers to two other than the center obtained by dividing the sheet into three in the thickness direction. Specifically, it can be obtained by heat transfer calculation.)

  In the stretching step, when a temperature difference is provided on the front and back of the film to provide orientation strain, a higher stretching deformation rate is suitable. Therefore, in providing the front and back orientation strains, the longitudinal stretching process is more suitable than the lateral stretching process as described above. However, in the transverse stretching process, it is possible to provide an orientation strain on the front and back of the film by providing a temperature difference in the vertical direction and performing multi-stage stretching.

(3) Temperature difference between upper and lower heat setting temperature In the present invention, in the heat setting step of heat setting the film after biaxial stretching, the temperature of the front and back of the film is a temperature of 0.1 ° C. or more and 0.5 ° C. or less. It is preferable to provide a difference. This is to substantially change the shrinkage ratio between the front and back surfaces by providing a difference in the degree of heat treatment between the front and back surfaces. In order to provide a temperature difference between the front and back sides in the heat setting step, for example, it is possible to change the temperature up and down through the film of the heat setting device or / and to provide a wind speed difference. In order to provide the above temperature difference between the front and back of the film, the temperature difference between the top and bottom of the heat setting device is preferably 3 ° C. or higher and 30 ° C. or lower. If the temperature is less than 3 ° C., the difference in wind speed between the top and bottom exceeds 5 m / sec to give the temperature difference of the film, and the distortion force acts on the film, which makes it difficult to control the heat shrinkage rate and causes unevenness in flatness. May occur or the thickness may change. On the other hand, if the temperature is higher than 30 ° C., the air balance is liable to be lost due to the difference in density between the air above and below the film. It may be difficult to actually measure the front and back temperatures in the heat setting process. Therefore, it is possible to estimate the front and back temperature by simulation.

(4) Adjustment of heat setting temperature The light diffusive film of the present invention is formed by laminating a light diffusion layer on at least one surface of a polyethylene terephthalate film as described above, and the melting point of the polyethylene terephthalate film on the surface layer side is Tm1. Including a step of heat-setting at a temperature Ts that satisfies Tm1>Ts> Tm2, where Tg1 is the glass transition temperature and Tm2 is the melting point of the thermoplastic resin having the highest melting point among the thermoplastic resins used in the light diffusion layer. It is characterized by. By heat-fixing at a temperature lower than Tm1, work problems such as adhesion during the treatment process do not occur, and by heat-treating at a temperature higher than Tm2, defects such as internal unevenness of the stack are eliminated and the surface Flattening is possible.

  Although effective surface heat treatment is possible with Ts satisfying Tm1> Ts, it is preferable to perform treatment at a temperature Ts satisfying Tm1> Ts + 10 (° C.), more preferably Tm1> Ts + 20 (° C.). In order to effectively eliminate internal defects, Ts> Tm2 + 10 (° C.) is preferable, and Ts> Tm2 + 20 (° C.) is more preferable.

Moreover, although the ratio for which a light-diffusion layer occupies among the light diffusable films of this invention is not specifically limited, Usually, it selects in the range of 1-50 micrometers, Preferably it is 1-30 micrometers.

  In the present invention, the above achievement means (1) to (3) are appropriately selected in order to obtain a film having a potential warp that is manifested by heating even though it is flat before the heat treatment, Or it is desirable to combine. Although it is difficult to directly evaluate the heat shrinkage rate of the front and back surfaces of the present invention, it is considered that the idea of delicately controlling the physical property called the heat shrinkage rate of the front and back surfaces has been achieved by the above-mentioned achieving means.

  In addition to the method described in detail above, it is also possible to use other methods such as passing a heating roll that performs single-side heat treatment during film formation, or using infrared heating, hot-air heating, etc. on the opposite side of the single-side cooling. Furthermore, the film after biaxial stretching is heat-treated by changing the temperature of the front and back films offline to change the heat shrinkage rate of the front and back, so that the desired heat shrinkage rate can be obtained after heating. It is also possible to make a difference.

  In addition, by adopting the above method (4), it is possible to improve the “void when the cross section of the light diffusion layer is observed”. In the above description, a method of processing at the heat fixing temperature Ts is shown. The above description discloses the technical idea of how the film of the present invention can be obtained by processing at a temperature Ts satisfying Tm1> Ts> Tm2 at the production level. Such a technical idea can be easily carried out by a method different from the method described above, and the film of the present invention can be obtained by a different method.

  The achievement means in the present invention is as described above, but the production conditions and production steps other than those described above can take the modes described later.

  When the raw resin is melt-extruded, the polyethylene terephthalate resin raw material is preferably dried using a dryer such as a hopper dryer or a paddle dryer, or a vacuum dryer. After the polyethylene terephthalate-based resin material is dried in such a manner, it is melted at a temperature of 200 to 300 ° C. and extruded into a film using an extruder. For this extrusion, any existing method such as a T-die method or a tubular method can be employed.

  In the film of the present invention, when the above-described longitudinal-lateral stretching is performed, the longitudinal stretching ratio is preferably adjusted to 2.5 times or more and 4.5 times or less. If the longitudinal stretching ratio is larger than 4.5 times, it is easy to break in the next lateral stretching step. On the other hand, if the longitudinal draw ratio is less than 2.5 times, the draw tension is extremely lowered, and as a result, the film thickness tends to be deteriorated.

  Moreover, in this invention, a heat setting process is performed following a horizontal extending process. The temperature in the heat setting treatment step is preferably 180 ° C. or higher and 240 ° C. or lower. If the temperature of the heat setting treatment is less than 180 ° C., the absolute value of the heat shrinkage rate is increased, which is not preferable. On the other hand, if the temperature of the heat setting process exceeds 240 ° C., the frequency of fracture increases, which is not preferable. A suitable heat setting method will be described later.

  It is effective to control the heat shrinkage rate, particularly the heat shrinkage rate in the width direction, by narrowing the guide rail of the gripping tool by the heat setting process. The temperature for the relaxation treatment can be selected in the range from the heat setting treatment temperature to the glass transition temperature Tg of the polyethylene terephthalate resin film, and is preferably (heat setting treatment temperature) −10 ° C. to Tg + 10 ° C. The width relaxation rate is preferably 1 to 6%. If it is less than 1%, the effect is small, and if it exceeds 6%, the flatness of the film is deteriorated.

  The film of the present invention produced by the above-described method is excellent in transparency and thickness unevenness (particularly thickness unevenness in the longitudinal direction), and in particular, a cured resin for a base film on which a curing shrinkable resin composition is applied. Can be suitably used to maintain the flatness after curing shrinkage.

  In particular, when a plurality of optical functions are imparted to a single film, a prism body comprising a cured shrinkable resin composition or a light diffusion layer provided on at least one surface of the light diffusive film As described above, when a plurality of optical functions are imparted to one film, the light diffusing film of the present invention can be suitably used. Examples of the curing shrinkable resin composition used in this case include ultraviolet curable or electron beam curable acrylic resins.

  The ultraviolet curable or electron beam curable acrylic resin is a resin having an acrylate functional group, and is preferably a polyester acrylate or a urethane acrylate. The polyester acrylate is composed of a polyester polyol oligomer acrylate or methacrylate (hereinafter, acrylate and / or methacrylate may be referred to as (meth) acrylate), or a mixture thereof. Moreover, urethane (meth) acrylate is comprised from what formed the oligomer which consists of a polyol compound and a diisocyanate compound into (meth) acrylate.

The ultraviolet curable or electron beam curable acrylic resin is cured by irradiation with ultraviolet rays or electron beams. Case of irradiation with ultraviolet rays, ultra-high pressure mercury lamp, high pressure mercury lamp, low pressure mercury lamp, carbon arc, a metal halide lamp, 100 to 400 nm, preferably, at a wavelength of 200~400nm region, the ultraviolet energy 100~3000mJ / ^{m 2} Irradiate. In the case of irradiating an electron beam, a scanning or curtain type electron beam accelerator is used to irradiate an electron beam having an acceleration voltage of 1000 keV or less, preferably 100 to 300 keV and having a wavelength region of 100 nm or less. At this time, a latent warp of the film of the present invention is expressed by heat generated by irradiation with ultraviolet rays or electron beams, and a laminate having excellent flatness as a whole is formed.

  The thickness of the prism row or the light diffusing layer may be determined in accordance with the application within the range of 0.1 to 30 μm. More preferably, it is 1-15 micrometers.

  In the case where the light diffusing film of the present invention is provided with a prism array made of a curing shrinkable resin composition or a light diffusing layer, at least one of a polyester resin, a urethane resin, and an acrylic resin is used in advance in order to improve adhesion. It is also a desirable embodiment of the present invention to provide a coating layer containing. By providing such a coating layer, it is easy to provide easy adhesion and to provide another functional layer.

  The laminated body which uses the light-diffusion film of this invention as a base film has favorable planarity. Here, the planarity of the laminate can be evaluated as follows. A rectangular sample of 300 mm in the longitudinal direction and 210 mm in the width direction perpendicular thereto is cut out from the laminate, and the sample is allowed to stand for 30 minutes or more in a room controlled at a temperature of 23 ± 2 ° C. and a humidity of 65 ± 5%. Then, the height of warping of the four corners of the film is measured in the vertical direction with reference to the stationary surface. Under the present circumstances, it is preferable that the height of the curvature of four corners is 0.5 mm or less.

  Hereinafter, the present invention will be described in detail by way of examples. However, the present invention is not limited to the embodiments of the examples, and can be appropriately changed without departing from the spirit of the present invention. . In addition, the evaluation method of a film characteristic is as follows.

(1) Flatness of film 1
Fifty samples of 300 mm in the longitudinal direction and 210 mm in the width direction are collected from the film. In a room controlled at a temperature of 23 ± 2 ° C. and a humidity of 65 ± 5% with the surface (surface (F)) that first contacted the cooling roll in the unstretched sheet process facing upward, a horizontal glass plate ( The height of the four corners of the film sample (height in the vertical direction from the horizontal plane) is measured with a JIS metal ruler (0.5 mm scale). If the height of the four corners is “0” or the cross section appears to be M-shaped, measure the warp with the opposite side facing up. The average value of the heights of the four corners measured in all the samples and the maximum value of the heights of the four corners measured in all the samples are displayed.

(2) Five samples of 300 mm in the longitudinal direction and 210 mm in the width direction are collected from the warped film after the film is heated. This sample was placed on a mounting board (thickness 1 mm) on a shelf board of a heating oven adjusted to 150 ° C. with the surface (table) that first contacted the cooling roll in the unstretched sheet process being placed on for 30 minutes. Heat treatment. Thereafter, the film sample together with the mount placed on the shelf board is taken out of the heating oven and left in a room controlled at a temperature of 23 ± 2 ° C. and a humidity of 65 ± 5% for 30 minutes. After standing for 30 minutes, the film sample is transferred to a horizontal glass plate (thickness 5 mm), and the heights of the four corners of the film (the height in the vertical direction from the horizontal surface) are measured with a JIS metal scale (0.5 mm scale). . If the height of the four corners is “0” or the cross section appears to be M-shaped, measure the warp with the opposite side facing up. The average height of the four corners measured in all samples is displayed.

(3) Flatness 2 of curable resin laminated film (evaluation by applying a curing shrinkable resin composition to the film)
Toagosei Co., Ltd., M-315 40 parts by mass, Mitsubishi Rayon Co., Ltd., nonabutylene glycol dimethacrylate (PBOM) 40 parts by mass, Shin-Nakamura Chemical Co., Ltd. urethane acrylate (U-2PHA) A resin composition was prepared using 20 parts by mass, Ciba Specialty Chemicals Co., Ltd., and 1.2 parts by mass of Irgacure 184. Using a film applicator, the resin composition has a thickness after curing on the surface of the film that becomes convex with a warp by heating (the surface in contact with the mount in the measurement of the warp after the heating of the film (2)). It applied so that it might become 2 mm. Using the high-pressure mercury lamp having a lamp emission length of 50 cm and 160 W / cm as a light source, the resin composition was irradiated with ultraviolet rays having an irradiation amount of 1 J / cm ² (measuring instrument: manufactured by Oak Manufacturing Co., Ltd., UV-350). Was cured. When the curing shrinkage was measured by the following measurement method, the resin composition exhibited a curing shrinkage of about 8.0%. Five samples cut from the curable resin laminated film thus obtained into a size of 300 mm in the longitudinal direction and 210 mm in the width direction were collected. The resin composition surface is placed on a horizontal glass plate (thickness 5 mm), and the heights of the four corners (vertical distance from the horizontal plane) are measured with a JIS metal ruler (0.5 mm scale). The average height of the four corners measured in all samples is displayed.

The weight a in the air and the weight b in the water of the cured product are measured, the solid specific gravity ds is obtained from the formula (i), and then the liquid specific gravity d _l of the curable resin composition before curing using a specific gravity bottle. And the cure shrinkage s (%) was determined from the formula (ii) from the solid specific gravity d _s value.
d _s = a / (ab) (i)
s = (1−d ₁ / d _s ) × 100 (ii)

(4) Film thickness The thickness of the film after film formation is a direction perpendicular to the longitudinal direction of the film sample cut out to 300 mm in the longitudinal direction and 210 mm in a direction perpendicular to the longitudinal direction using an electronic micrometer MILLITRON (Seiko Precision Machinery Sales). 10 times at a position of about 20 mm, and the average value is obtained.

  In addition, Table 1 shows film forming conditions of the films in Examples and Comparative Examples.

[Example 1]
In a main extruder, true spherical silica particles (refractive index 1.46) having an average particle diameter of 4.5 μm are added to a polyester resin (refractive index 1.61) obtained by copolymerizing 23 mol% of an isophthalic acid component with polyethylene terephthalate (PET). Pellets blended to a volume ratio of 10% were fed and melted at a temperature of 280 ° C. in an extruder. Separately, PET having a refractive index of 1.61 that does not contain fine particles is supplied to the sub-extruder and melted at a temperature of 285 ° C. in the extruder, and the molten resin is filtered into a stainless steel sintered body (nominal filtration accuracy) in the extruder. : Particles of 10 μm or more were cut by 90%). Next, the resin sheet was extruded from a T-shaped die, wound around a casting drum having a surface temperature of 22 ° C. using an electrostatic application casting method, and cooled and solidified to obtain an unstretched sheet of 1710 μm. Furthermore, the surface temperature difference (F−B) between the front and back surfaces of the unstretched sheet separated from the second cooling roll (detaching roll) at 22 ° C. was as shown in Table 1. At this time, extrusion was performed so that the PET layer containing particles contacted the cast drum.

  The obtained unstretched sheet is heated with a group of heated rolls, and then heated between the first nip roll and the second nip roll arranged before and after, and an infrared heater provided between the nip rolls (first While being heated by a single infrared heater, the film was stretched 2.77 times in the longitudinal direction (longitudinal direction) (first-stage longitudinal stretching). At this time, in the first infrared heater, assuming that the infrared output on the front side was 100%, the infrared output on the back side was 90%. Here, the rear second nip roll was cooled.

  Thereafter, while the film after the longitudinal stretching is heated by the infrared heater (second infrared heater) provided between the second nip roll and the third nip roll disposed immediately thereafter, the longitudinal direction ( The film was stretched 1.17 times in the longitudinal direction (second-stage longitudinal stretching). Further, the film was stretched 1.08 times in the longitudinal direction (longitudinal direction) while being heated by an infrared heater (third infrared heater) provided between the nip rolls between the third nip roll and the fourth nip roll disposed immediately thereafter. (Third-stage longitudinal stretching). In the second and third infrared heaters, assuming that the infrared output on the front side is 100%, the infrared output on the back side is 95%. Note that the relationship between the output of the infrared heater and the surface temperature is measured in advance with a model machine, and according to the above settings, the temperature difference on the film surface is front and back, the first stage is 2 ° C., the second stage is 3 The temperature was adjusted to 3 ° C. in the third stage.

  As described above, the unstretched film was stretched in the longitudinal direction in three stages, then led to a tenter, and subjected to a 4-fold transverse stretching at 135 ° C. Thereafter, a heat setting treatment was performed at 233 ° C., and a transverse relaxation treatment of 2.2% was performed at 225 ° C. A light diffusing film was produced by winding a biaxially stretched film of 125 μm thickness and 3,300 mm width over a length of about 3,000 m by cutting and removing both edges. . And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

  When the cross section of the obtained light diffusible film was cut out and the cross section was observed using the transmission electron microscope HU-12 type (Hitachi Ltd.), the void was not observed but disappeared.

  About the obtained light diffusable film, the total light transmittance and haze were measured using fully automatic direct reading haze computer HGM-2DP (made by Suga Test Instruments Co., Ltd.). The total light transmittance was 89% and haze was 87%.

  Moreover, in order to actually visually evaluate the diffusion performance of the obtained light diffusive film, the following experiment was conducted. That is, a visible light laser (He-Ne laser, wavelength 632.8 nm) is irradiated onto the obtained light diffusive film, and the transmitted light is projected onto a screen to observe how much the incident light is spread. And evaluated. A case where no intensity distribution was observed and a uniform and wide diffusion range was marked with ◯, and a case where a remarkable intensity distribution or a narrow diffusion range or a darkness with little transmitted light amount was marked as x. It was found that the laser beam spreads through the film and shows good diffusion performance. Moreover, the surface flatness was good and the bending rigidity was also excellent. The evaluation results are shown in Table 2.

[Example 2]
In a main extruder, a spherical resin having a spherical average particle size of 4.5 μm (refractive index of 1.1) is added to a polyester resin (refractive index of 1.61) obtained by copolymerizing 23 mol% of an isophthalic acid component with polyethylene terephthalate (PET). 46) is supplied at a volume ratio of 10%, and the sub-extruder is supplied with PET having a refractive index of 1.61 that does not contain fine particles. The surface of the containing PET was melt co-extruded with particle-free PET, and cooled on a mirror-casting drum by an electrostatic application method to prepare a three-layer laminated sheet. This three-layer sheet has a structure in which both surfaces of particle-containing PET are coated with non-particle PET.

  The winding speed around the casting drum was changed from that in Example 1, and when it was wound around the casting drum, it was cooled and solidified using 19 ° C. cooling air by air, and the thickness of the unstretched sheet was 3150 μm. The surface temperature difference (FB) of the unstretched sheet separated from the second cooling roll (detaching roll) at 30 ° C. was as shown in Table 1. The obtained unstretched sheet was stretched longitudinally as shown in Table 1, and further stretched in the same manner as in Example 1. In Table 1, the infrared output on the back side is shown as an output ratio (%) when the output on the front side is 100. Thereafter, a light diffusive film having a thickness of 250 μm was manufactured by performing a transverse relaxation treatment of 1.7% at 225 ° C.

  When the cross section of the obtained film was observed in the same manner as in Example 1, no voids were observed inside the film.

[Example 3]
This was carried out in the same manner as in Example 1 except that the undrawn sheet was adjusted to take up speed, the thickness of the unstretched sheet was changed to 2440 μm, and longitudinally stretched as shown in Table 1. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

[Example 4]
Crosslinked acrylic resin fine particles (refractive index 1.47) 20 having a spherical shape and an average particle diameter of about 8 μm are added to 100 parts by mass of a polyester resin (refractive index 1.61) obtained by copolymerizing 23 mol% of an isophthalic acid component with polyethylene terephthalate (PET). The same procedure as in Example 1 was carried out except that the pellets mixed with parts by mass were adjusted for the take-up speed of the unstretched sheet to change the thickness of the unstretched sheet to 3150 μm and longitudinally stretched as shown in Table 1. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

  When the cross section of the obtained film was observed in the same manner as in Example 1, no voids were observed inside the film.

[Example 5]
A biaxially stretched film was obtained in the same manner as in Example 2 except that the unstretched film obtained in the same manner as in Example 2 was changed as shown in Table 1. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The warp after heating was opposite to that in Example 2. The evaluation results are shown in Table 2.

[Example 6]
The unstretched film obtained in the same manner as in Example 1 was heated only on the surface by an infrared heater provided immediately before the first nip roll, and a temperature difference between the front and back of the film as shown in Table 1 was provided. Thereafter, a light diffusing film was obtained in the same manner as in Example 1 except that the film was longitudinally stretched. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

[Example 7]
The unstretched film obtained in the same manner as in Example 2 was heated only at the surface with high-speed heated air provided immediately before the first nip roll, and a temperature difference between the front and back of the film as shown in Table 1 was provided. Thereafter, a light diffusing film was obtained in the same manner as in Example 2 except that the film was stretched longitudinally. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

[Example 8]
The unstretched film obtained in the same manner as in Example 5 was cooled only on the back surface by high-speed cooling air provided immediately before the first nip roll, and a temperature difference between the front and back of the film as shown in Table 1 was provided. Thereafter, a light diffusing film was obtained in the same manner as in Example 5 except that the stretching conditions were changed as shown in Table 1. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The warp after heating was opposite to that in Example 2. The evaluation results are shown in Table 2.

[Comparative Example 1]
Example obtained after obtaining an unstretched sheet in the same manner as in Example 1 and then adjusting the output of the infrared heaters in the first and second stages of the longitudinal stretching so that there was no difference in output between the front and back sides. In the same manner as in Example 1, a light diffusing film was obtained. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

[Comparative Example 2]
After obtaining an unstretched sheet in the same manner as in Example 3, longitudinal stretching was performed as shown in Table 1. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

[Comparative Example 3]
After obtaining an unstretched sheet in the same manner as in Example 5, longitudinal stretching was performed as shown in Table 1. And the characteristic of the obtained film was evaluated by each measuring method mentioned above.

[Comparative Example 4]
After obtaining an unstretched sheet in the same manner as in Example 1, longitudinal stretching was performed as shown in Table 1. And the characteristic of the obtained film was evaluated by each measuring method mentioned above.

[Comparative Example 5]
An unstretched sheet obtained in the same manner as in Example 1 was heated only on one side by an infrared heater provided immediately after the first nip roll. Thereafter, a biaxially stretched film was obtained in the same manner as in Example 1 except that only the first stage of Example 1 was used for longitudinal stretching in one stage. And the characteristic of the obtained film was evaluated by each measuring method mentioned above. The evaluation results are shown in Table 2.

[Comparative Example 6]
The unstretched sheet obtained in the same manner as in Example 2 was subjected to longitudinal stretching as shown in Table 1 to obtain a light diffusing film. And the characteristic of the obtained film was evaluated by each measuring method mentioned above.

  When the average value of the flatness 1 is 20% or less of the thickness of the film, the maximum value is not more than the thickness of the film, and the average value of the flatness 2 is 0.5 mm or less, it is determined to be acceptable and is set as “O”. , Even one that failed was judged as “x”.

  From Table 2, all of the films of the examples are excellent in light transmittance and light diffusibility, have good flatness, and are coated with a curing shrinkable resin composition. The flatness of the entire laminate is very good due to the difference in heat shrinkage between the front and back surfaces.

  The light diffusing film of the present invention has excellent flatness and is suitable as a base film for a laminate. For example, it is suitable as a base film for laminates such as various optical films such as a lens film, a diffusion film, a hard coat film, and an NIR film, a touch panel, and ITO. Further, it is also suitable as a base film for use in building materials for applying and laminating curable coatings, for recording materials using curable resin ink, and for bonding members using two or more films bonded together.

Claims (4)

A light diffusing film satisfying the following requirements (1) to (4).
(1) It has a light diffusion layer having a light diffusion component inside on at least one surface of a biaxially stretched polyethylene terephthalate resin film. (2) The light diffusion layer is substantially free of voids (3 ) The light diffusing layer is composed of at least a light transmissive resin and fine particles as a light diffusing component. (4) A sample of 300 mm is taken in the longitudinal direction of the film and 210 mm in the width direction perpendicular thereto. The sample was placed on a mount with one side facing up, and heat-treated at 150 ° C. for 30 minutes in a heating oven. Then, the sample was removed from the heating oven together with the mount, and the sample was allowed to stand at room temperature for 30 minutes. When measuring the height of the sled (height in the vertical direction from the horizontal plane) with a JIS metal ruler (0.5 mm scale), the average height of the sleds at the four corners is 0.5 mm or more and 5.0 mm or less. (If the height of the warp of the sample after standing at room temperature after heating is 0 mm, or if the cross section of the sample is M-shaped, the upper and lower surfaces of the sample are reversed. Measure the height of the sled.)   The light diffusable film according to claim 1, wherein the light diffusible film has a thickness of 100 μm or more and 400 μm or less.   3. The light diffusing film according to claim 1, wherein the light diffusing resin film has a total light transmittance of 80% or more and a haze of 60% or more.   A prism array comprising a coating layer on at least one side of the light diffusing film according to any one of claims 1 to 3, and a surface of the coating layer comprising an ultraviolet curable or electron beam curable acrylic resin, or a light diffusion A laminate comprising a layer.