JP4715511B2 - Light diffusion film - Google Patents

Description

The present invention is a backlight unit of a liquid crystal display, about the light-diffusing film used in a lighting device or the like.

  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 diffusing layer is formed by post-processing, which is disadvantageous in terms of cost reduction requirements. 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 a biaxially stretched film itself that is widely used as a base film of a light diffusive film.

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 layer formed by melt mixing, and a layer comprising a crystalline polyester obtained by co-extrusion, biaxially oriented laminated film (e.g., see Patent Documents 10 and 11), etc. 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

  However, in the attempts (4) to (8) for imparting light diffusibility to the biaxially stretched film itself, the light diffusing layer is post-processed on the transparent substrate film in terms of both heat resistance and light transmittance. It has not reached the methods (1) and (2), and has not been put into practical use.

  Because, in the method of imparting light diffusibility to the biaxially stretched film itself, it is necessary to suppress the generation of voids (bubbles) in the biaxial stretching process of the film in order to achieve both light diffusion performance and light transmittance. is there. However, when a crystalline polyester resin is used as the matrix polymer (4), although excellent heat resistance is obtained, voids frequently occur at the interface between the matrix polymer and the incompatible resin or particles. 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 a crystalline 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 characteristic of a crystalline biaxially stretched polyester film cannot be obtained, and a significant dimensional change or deterioration of flatness occurs during processing at a high temperature or use in a high temperature environment. The original purpose of the light diffusing film, which is to make the brightness of 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.

  The object of the present invention is to produce a light diffusive film having excellent heat resistance and mechanical strength inherent in a biaxially stretched film, and having excellent light transmittance and light diffusivity, and light obtained therefrom. It is to provide a diffusive film.

1st invention of this invention has a light-diffusion layer obtained by biaxially stretching the unstretched sheet which consists of 50-99 mass parts of crystalline polyester and 1-50 mass parts of non-meltable polymer particles. It is a light diffusing film , the melting point of the crystalline polyester is 230 ° C. or higher, the total light transmittance is 85% or higher, the haze is 50% or higher, and the dimensional change rate at 150 ° C. is 3% in both the vertical and horizontal directions. Hereinafter, the tensile strength is 100 MPa or more in both the longitudinal direction and the transverse direction, and the biaxial stretching is performed at a stretching ratio of 2.5 times or more and a stretching speed of less than 100 % / second in the longitudinal direction and the transverse direction, respectively. a light diffusion film, which comprises carrying out.

The second invention is a biaxially stretched above, a light-diffusing film according to the first invention which is characterized in that by using a simultaneous biaxial stretching machine.

Furthermore, the third invention is the light diffusing film according to the first or second invention , wherein the crystalline polyester is polyethylene terephthalate.

Since the light diffusing film of the present invention uses crystalline polyester as the matrix polymer of the light diffusing layer, it has excellent heat resistance and mechanical strength inherent to the biaxially stretched film. Further, since the biaxial stretching of the film is performed at a stretching ratio of 2.5 times or more in the machine direction and the transverse direction, and the stretching in both directions is performed at a stretching speed of less than 100% / second, it is accompanied by stretching. Generation of voids is suppressed, and both excellent light transmittance and light diffusibility can be achieved.

The light diffusing film of the present invention has a light diffusing layer obtained by biaxially stretching an unstretched sheet made of a mixture containing 50 to 99 parts by weight of crystalline polyester and 1 to 50 parts by weight of non-meltable polymer particles. It is a diffusible film , the melting point of the crystalline polyester is 230 ° C. or higher, the total light transmittance is 85% or higher, the haze is 50% or higher, and the dimensional change rate at 150 ° C. is 3% or lower in both the vertical and horizontal directions. The tensile strength is 100 MPa or more in both the longitudinal direction and the transverse direction, the biaxial stretching is performed at a stretching ratio of 2.5 times or more in the longitudinal direction and the transverse direction, and stretching in both directions is 100 % / second. It is characterized by being carried out at a drawing speed of less than.

  First, the raw material used when manufacturing the light diffusable film of this invention is demonstrated in detail about the manufacturing conditions and manufacturing method of a film then.

(material)
(1) Crystalline polyester The crystalline polyester that can be used as a raw material of the light diffusion layer in the present invention means a polyester having a heat of crystal melting of 10 mJ / mg or more. When the heat of crystal fusion is less than 10 mJ / mg, the heat resistance of the biaxially stretched film decreases, and curling may occur due to heat treatment in the post-processing step or the use environment (temperature) of the liquid crystal display. It may be insufficient. In either case, it is difficult to achieve both the excellent heat resistance inherent in the biaxially stretched film and the mechanical strength. A more preferred lower limit of the heat of crystal fusion is 15 mJ / mg, a more preferred lower limit is 20 mJ / mg, and a most preferred lower limit is 30 mJ / mg.

The melting point of the crystalline polyester of the present invention, 230 ° C. or higher, most preferably 240 ° C. or higher.

  Here, the polyester means 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, etc. 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. Among these polyesters, 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.

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

  When the non-melting polymer particles are made of a polymer having a melting point of 350 ° C. or higher, non-crosslinked polymer particles may be used, but from the viewpoint of heat resistance, crosslinked polymer particles made of a polymer having a crosslinked structure are used. It is preferable.

  The average particle diameter of the non-melting polymer particles is preferably 0.1 to 50 μm. The lower limit of the average particle size of the non-melting polymer particles is more preferably 0.5 μm, and particularly preferably 0.5 μm. When the average particle diameter of the non-melting polymer particles is less than 0.1 μm, it is difficult to obtain a good light diffusion effect.

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

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

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

  The refractive index of the non-melting polymer particles is preferably 1.35 to 1.85. The lower limit of the refractive index of the non-melting polymer particles is more preferably 1.40, and the upper limit is more preferably 1.80. If the above range is exceeded, the light transmittance and light diffusivity cannot be balanced, which is not preferable.

  The mixing ratio of the non-melting polymer particles is preferably 1 to 50 parts by mass with respect to 100 parts by mass of the crystalline polyester. The lower limit of the mixing ratio is more preferably 4 parts by mass, and particularly preferably 8 parts by mass. When the mixing ratio is less than 1 part by mass, the light diffusion performance is insufficient. On the other hand, the upper limit of the mixing ratio is preferably 40 parts by mass, particularly preferably 30 parts by mass. When the mixing ratio exceeds 50 parts by mass, the non-melting polymer particles are likely to fall off during biaxial stretching of the film, and this is not preferable.

  In the present invention, in addition to the non-melting polymer particles, a crystalline polyester and an incompatible thermoplastic resin may be used in combination. The thermoplastic resin incompatible with the crystalline polyester exists as a particulate dispersion inside the crystalline polyester of the matrix.

  Examples of thermoplastic resins that are incompatible with crystalline polyester include polyethylene, polypropylene, polymethylpentene, polyolefins such as various cyclic olefin polymers, polycarbonate, atactic polystyrene, syndiotactic polystyrene, and isotactic polystyrene. , Polyamides, polyethers, polyester amides, polyphenylene sulfide, polyphenylene ethers, polyether esters, polyvinyl chloride, polymethacrylic esters, and other acrylic resins, and copolymers containing these as main components, or mixtures of these resins, etc. Is mentioned.

  When an incompatible thermoplastic resin is used in combination with the crystalline polyester, the mixing ratio is such that the total amount of the non-melting polymer particles and the thermoplastic resin incompatible with the crystalline polyester is 100 parts by mass of the crystalline polyester. It is preferable that it is 1-50 mass parts with respect to. The lower limit is more preferably 4 parts by mass, particularly preferably 8 parts by mass. On the other hand, the upper limit is more preferably 40 parts by mass, and particularly preferably 30 parts by mass.

  In the present invention, inorganic particles may be used in combination as necessary. Examples of the inorganic particles include silica, calcium carbonate, barium sulfate, calcium sulfate, alumina, kaolinite, talc and the like. The refractive index of the inorganic particles is preferably 1.35 to 1.85, and the average particle size is preferably 0.1 to 50 μm.

  The mixing ratio of the inorganic particles is preferably such that the total amount of the non-melting polymer particles and the inorganic particles is 1 to 50 parts by mass with respect to 100 parts by mass of the crystalline polyester. The lower limit is more preferably 4 parts by mass, particularly preferably 8 parts by mass. On the other hand, the upper limit is more preferably 40 parts by mass, and particularly preferably 30 parts by mass.

  In the present invention, the above-mentioned crystalline polyester and non-melting polymer particles, incompatible thermoplastic resin and inorganic particles may be mixed in the production process of crystalline polyester, or crystalline polyester and non-melting may be performed. Polymer particles, incompatible thermoplastic resins, inorganic particles, and the like may be prepared by melt-kneading.

  In the present invention, the crystalline polyester and the non-melting polymer particles, the incompatible thermoplastic resin or the inorganic particles may be mixed in the production process of the crystalline polyester, or in the film production stage, the crystalline polyester may be mixed. And non-melting polymer particles, incompatible thermoplastic resin or inorganic particles may be prepared by melt-kneading.

(Manufacture of biaxially stretched film)
The method for producing a light diffusing film of the present invention is characterized in that biaxial stretching of a film is carried out at specific stretching conditions, particularly at slow stretching speeds in both the longitudinal and transverse directions.

  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, (1) crystalline polyester containing 1 to 50 parts by mass of non-melting polymer particles, or (2) crystals containing non-melting polymer particles at a high concentration (10 to 50 parts by mass) A crystalline polyester that does not contain non-meltable polymer particles is prepared. Each polyester of (1) or (2) is 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. It is preferable in order to suppress the generation of foreign matters such as deteriorated products.

  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 molten polyester is not particularly limited, but in the case of a stainless sintered filter medium, removal of aggregates and high-melting-point organic substances mainly composed of Si, Ti, Sb, Ge, and Cu. Excellent performance and suitable. When high-precision filtration is performed at a temperature of the molten resin lower than 280 ° C., the filtration pressure increases. Therefore, if the discharge amount of the raw material polyester is lowered, the productivity is not preferable.

  The layer structure of the light diffusing film of the present invention may be a single layer or a multilayer structure as long as it has the light diffusing layer (A). In order to carry out post-processing and to have other optical functionalities, for example, a function as a prism sheet, a multilayer structure is preferable. In that case, what is necessary is just to laminate | stack the polyester layer (B) which does not contain a non-meltable polymer particle substantially on the single side | surface or both surfaces of a light-diffusion layer (A) using a coextrusion method.

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

  About the lamination | stacking ratio in this case, 3 to 50% is preferable and, as for the ratio with respect to the total thickness of a light-diffusion layer (A), 10 to 30% is more preferable. When the ratio of the light diffusion layer (A) is smaller than 3%, only non-uniform light diffusion performance can be obtained. On the other hand, when the ratio with respect to the total thickness of the light diffusion layer (A) exceeds 50%, the surface smoothness of the polyester layer (B) decreases, and post-processing such as prism sheet processing on the surface of the polyester layer (B) is difficult. It becomes.

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 may be provided on at least the surface of the polyester layer (B). preferable.

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

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

  The resin constituting the 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 addition, you may contain particle | grains in an easily bonding layer.

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

  It is important that the biaxial stretching is performed at a stretching ratio of 2.5 times or more in the longitudinal, lateral and both directions. If the draw ratio in either the machine direction or the transverse direction is less than 2.5, the excellent heat resistance and mechanical strength inherent to the biaxially stretched film cannot be obtained, and the thickness uniformity of the film 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 preferable upper limit of the draw ratio is 6.0 times.

In the biaxial stretching in the present invention, it is particularly important that both the longitudinal and lateral directions are stretched at a stretching speed of less than 100 % / second using a biaxial stretching machine. 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

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

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

Then, all stretching in the machine direction and transverse direction from the start of stretching to the end of stretching is completed at a stretching speed of less than 100 % / second. This stretching speed condition is the most important requirement of the present invention, and for the first time, while using crystalline polyester as a matrix polymer, the generation of stretched voids is suppressed, excellent light transmittance, and biaxially stretched polyester. It becomes possible to obtain a light diffusive biaxially stretched film that has both the original heat resistance and mechanical strength of the film.

On the other hand, if the stretching speed exceeds 100 % / second in either the vertical or horizontal direction, it is difficult to suppress voids generated during stretching, resulting in poor light transmittance.

  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.

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

  The biaxial stretching machine that can control the stretching speed in the machine direction and the transverse direction as described above is guided to the tenter while holding both ends of the film with clips, and the width between the clips and the transport speed of the clips are controlled. Thus, a tenter type biaxial stretching machine having a mechanism capable of continuous stretching in both the vertical and horizontal directions is suitable. As long as the equipment has the function, the clip transport mechanism is arbitrary and is not particularly limited, but a linear motor system, a pantograph system, or a screw system can be adopted.

  In the tenter-type biaxial stretching, the stretching method may be a longitudinal / lateral or lateral / longitudinal sequential biaxial stretching method, or a so-called simultaneous biaxial stretching method in which stretching is performed simultaneously in the longitudinal direction and the transverse direction. Further, stretching may be performed in multiple stages in the longitudinal direction or the transverse direction. However, the simultaneous biaxial stretching method has the advantage that it can be performed with relatively compact equipment because biaxial stretching can be performed simultaneously.

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

(Characteristics of light diffusing film)
The characteristics of the light diffusing film obtained by the above-described method are as follows: total light transmittance is 85% or more, haze is 50% or more, dimensional change rate at 150 ° C. is 3% or less in both length and width, tensile strength is length, Both sides are characterized by 100 MPa or more.

  The preferable lower limit of the total light transmittance in the light diffusing film of the present invention is 88%, and the particularly preferable lower limit is 89%.

  Moreover, the preferable minimum of haze in the light diffusable film of this invention is 60%, and a preferable upper limit is 100%.

  Further, the preferable upper limit of the dimensional change rate at 150 ° C. is 2%, the more preferable upper limit is 1.0%, the still more preferable upper limit is 0.5%, and the most preferable upper limit is 0.3%.

  The preferred lower limit of tensile strength is 110 MPa, the more preferred lower limit is 140 MPa, and the particularly preferred lower limit is 150 MPa.

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

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

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

(3) Melt viscosity The melt viscosity at a resin temperature of 285 ° C. and a shear rate of 100 / sec was measured using a flow tester (manufactured by Shimadzu Corporation, CFT-500). Note that measurement of melt viscosity at a shear rate of 100 / sec is difficult to perform with the shear rate fixed at 100 / sec. Therefore, using an appropriate load, any shear rate of less than 100 / sec and The melt viscosity was measured at an arbitrary shear rate 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) Tensile strength Measured according to JIS C 2318-1997 5.3.3 (tensile strength and elongation).

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

(8) Curl The sample after measuring the dimensional change rate was visually evaluated.

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 kg / 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 heat of crystal fusion of 35 mJ / mg, 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 The L value was 56.2, the color b value was 1.6, and inert particles and internally precipitated particles were substantially not contained.

(2) Production of cross-linked polystyrene particle-containing PET (M2) 30 parts by mass of an approximately monodisperse and spherical cross-linked polystyrene particle having an average particle diameter of 12 μm and 70 parts by mass of the above-described PET (M1) are mixed to form a bent biaxial The resulting strand was supplied to an extruder, kneaded and melt-extruded, and the obtained strand was cooled and cut to obtain pellets of crosslinked polystyrene particle-containing PET (M2).

(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.

(4) Production of Light Diffusing Film As raw materials for the light diffusing layer (A), 72 parts by mass of PET (M1) and 28 parts by mass of PET (M2) containing crosslinked polystyrene particles are each at 135 ° C. for 6 hours. After drying under reduced pressure (1 Torr), they were mixed and supplied to the extruder 2. Further, PET (M1) as a raw material of the B 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 is 280 ° C. in the melting part, kneading part, polymer pipe and gear pump of the extruder, and 275 ° C. in the subsequent polymer pipe. It laminated | stacked using the block and melt-extruded in the sheet form from the nozzle | cap | die.

  In addition, the thickness ratio of A layer and B layer was controlled using the gear pump of each layer so that it might become 25:75. 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 cooling drum surface using an electrostatic application method, and solidified by cooling to prepare an unstretched film having a thickness of 1.3 mm. 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 application | coating method, and it apply | coated so that the wet application quantity might be 20 g / m < ² >. Thereafter, in a drying furnace divided into two zones, the coated surface was dried at a first zone temperature of 100 ° C. and a wind speed of 20 m / second for 10 seconds, a second zone temperature of 70 ° C. and a wind speed of 20 m / second for 10 seconds.

  Next, the unstretched film having the coating layer was preheated with hot air at 105 ° C. for 40 seconds using a pantograph simultaneous biaxial stretching machine, and then 20 seconds, and 3. Biaxial stretching was performed 7 times at a time. At this time, the draw ratios in the vertical and horizontal directions were as shown in FIG. 1, and the drawing speed was as shown in FIG.

  Next, with the film dimensions fixed, heat treatment is performed at 230 ° C. for 30 seconds, and in the process of cooling to room temperature, 3% relaxation treatment is performed in the vertical and horizontal directions to form a biaxially stretched film having a thickness of 100 μm. Manufactured. The properties of the obtained film are shown in Table 1.

  The light diffusive film obtained in Example 1 has excellent heat resistance and mechanical strength inherent to the biaxially stretched film, and has excellent light transmittance and light diffusibility. It was high quality.

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

  Next, both ends of the above-mentioned longitudinally stretched film were gripped with clips, and transversely stretched. The transverse stretching temperature was 135 ° C., the transverse stretching ratio was 3.7 times, and the transverse stretching speed was constant at 25% / second. Next, heat treatment was performed at 230 ° C. for 15 seconds, and 2.5% relaxation treatment was performed in the width direction in the process of cooling to 60 ° C.

  Next, the clip that had gripped both ends of the film was released, and both ends of the film were trimmed and wound into a roll to produce a biaxially stretched film. The properties of the obtained film are shown in Table 1.

  The light diffusive film obtained in Comparative Example 1 had high haze and good light diffusibility, but low light transmittance, and the light diffusivity and light transmittance were not well balanced and had low quality. . Moreover, the dimensional change rate was also inferior to the light diffusable film obtained in Example 1.

Comparative Example 2
The same as Comparative Example 1 except that 93.3 parts by mass of PET (M1) and 6.7 parts by mass of crosslinked polystyrene particle-containing PET (M2) are used as raw materials for the light diffusing layer (A). A biaxially stretched film was produced by this method. The properties of the obtained film are shown in Table 1.

  The light diffusive film obtained in this Comparative Example 2 has high light transmittance, but has a low haze value and insufficient light diffusivity, and the light diffusivity and light transmittance are not well balanced and low quality. Met. Moreover, the dimensional change rate was also inferior to the light diffusable film obtained in Example 1.

Example 2
In the method of Example 1, a light-diffusing film was obtained in the same manner as in Example 1 except that instead of the crosslinked polystyrene particles, average monodisperse spherical crosslinked acrylic particles having an average particle size of 8 μm were used. . The properties of the obtained film are shown in Table 1.

  Similar to the light diffusive film obtained in Example 1, the light diffusible film obtained in Example 2 had a high quality and balanced properties.

Light-diffusing film of the present invention, a biaxially stretched film having an original excellent heat resistance and mechanical strength, and excellent because it is both a light transmittance and light diffusibility, other optical functional It can be integrated with a 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.

It is explanatory drawing which shows the relationship between the elapsed time from the extending | stretching start at the time of film manufacture of Example 1, and a draw ratio. It is explanatory drawing which shows the relationship between the elapsed time from the extending | stretching start at the time of film manufacture of Example 1, and a stretching speed.

Claims (3)

It is a light diffusing film having a light diffusion layer obtained by biaxially stretching an unstretched sheet composed of a mixture containing 50 to 99 parts by weight of crystalline polyester and 1 to 50 parts by weight of non-meltable polymer particles.
The melting point of the crystalline polyester is 230 ° C. or higher,
A total light transmittance of 85% or more, a haze of 50% or more, 3% dimensional change rate of both the longitudinal and transverse directions at 0.99 ° C. or less state, and are the tensile strength of the longitudinal direction and both lateral 100MPa or more,
A light diffusing film, wherein the biaxial stretching is performed in a longitudinal direction and a lateral direction at a stretching ratio of 2.5 times or more and a stretching speed of less than 100% / second . The light diffusive film according to claim 1, wherein the biaxial stretching is performed using a simultaneous biaxial stretching machine . The light diffusing film according to claim 1, wherein the crystalline polyester is polyethylene terephthalate.

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