WO2010016542A1

WO2010016542A1 - Polyester film with light-diffusing surface

Info

Publication number: WO2010016542A1
Application number: PCT/JP2009/063935
Authority: WO
Inventors: 芳紀斎宮; 史朗濱本; 俊一朗馬場; 睦夫西
Original assignee: 東洋紡績株式会社
Priority date: 2008-08-06
Filing date: 2009-08-06
Publication date: 2010-02-11
Also published as: JP2010036486A; JP4370538B1

Abstract

A polyester film having a light-diffusing surface is provided which is excellent in heat resistance, mechanical strength, thickness precision, etc. The film reconciles total light transmittance with light-diffusing properties. The film is inhibited from suffering the thermal curling attributable to a bimetal structure. The film brings about satisfactory brightness.

Description

Surface light diffusing polyester film

The present invention relates to a light diffusive film used for a backlight unit of a liquid crystal display having a large screen and a high luminance, a lighting device, and the like. More specifically, the present invention relates to a surface light diffusible polyester film that achieves both light diffusibility and light transmittance and is less likely to curl due to temperature changes.

In recent years, technological advances in liquid crystal displays have been remarkable, and they are widely used as display devices for personal computers, televisions, mobile phones and the like. In particular, in recent years, high definition has been advanced for various uses of liquid crystal displays. Especially for television applications, with the widespread use of high-definition broadcasting, horizontal 1920 × vertical 1080 dots, which has traditionally been mainly used for large-screen liquid crystal televisions. The so-called full HD display liquid crystal panel has been adopted for a relatively small screen size LCD TV, and the demand for higher definition is increasing. 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 divided into two types. In general, the most common method is a method called a direct type, in which the light source is inside the illumination surface. In this system, a large number of light sources such as cold-cathode ray tubes can be arranged directly under the illumination surface, so that extremely high luminance is obtained and light loss is small. For this reason, it is often used for large liquid crystal displays such as large liquid crystal TVs that require high brightness.

The other method is called an edge light type. The light source is arranged outside the illumination surface, and a fluorescent lamp (many of them are placed on one or two sides of a light guide plate made of a transparent acrylic resin plate or the like arranged on the illumination surface. Is a system in which a substantially linear light-emitting body such as a cold cathode discharge tube is closely attached and a lamp cover made of a reflector is provided to introduce light into the light guide plate. This method has the characteristics that power consumption is small, and that it can be reduced in size and thickness. For this reason, it is widely used in applications that require a reduction in thickness and weight, such as a small display such as a notebook personal computer.

The functions required of the light guide plate of the edge light type backlight unit are a function of transmitting light incident from the end portion forward and a function of emitting the transmitted light to the liquid crystal display element side. The former function depends on the material used and the interface reflection characteristics. The latter function is determined according to the shape of the light guide plate surface that avoids the total reflection condition. As a method of forming the shape of the surface of the light guide plate, a method of applying a white diffusing material to the surface of the light guide plate and a method of providing a Fresnel shape of lenticular or prism to the surface of the light guide plate are known. However, the light emitted from the light guide plate having these surface shapes exhibits a non-uniform distribution due to the shape. Therefore, in order to obtain a high-quality image, a light diffusive film is installed on the light guide plate, and the light emitted from the light guide plate is diffused and scattered, and the brightness of the illumination surface is made uniform.

In order to further improve the front luminance of these backlight units, a prism sheet or a sheet having a light collecting function called a lens sheet is used so as to collect the light transmitted through the light diffusing film in the front direction as much as possible. May be. The surface of the sheet has a large number of minute irregularities such as a prism shape, a wave shape, and a pyramid shape, and the emitted light that has passed through the light diffusing film is refracted and collected in the front to improve the luminance of the illumination surface. Such a prism sheet is used by being arranged one or two on the surface side of the light diffusing film.

Furthermore, in order to make the luminance unevenness and the defect of the prism sheet caused by the arrangement of the prism sheet inconspicuous (improve concealment), a light diffusing film may be arranged on the surface side of the prism sheet. .

The light diffusing film used in the backlight unit as described above is obtained by coating a light diffusing layer made of a transparent resin containing fine particles on the surface of a biaxially stretched polyester film (for example, Patent Document 1). 2) is the mainstream.

However, in this method, since it is necessary to provide a light diffusion layer by coating on one side of the base film, the light diffusion film has a bimetallic structure due to the difference in the linear expansion coefficient between the light diffusion layer and the base film. There is a problem that curling is likely to occur due to heating. This problem is becoming an important problem particularly in a liquid crystal display employing a direct type backlight unit that requires a large size and extremely high luminance, such as a large liquid crystal TV in recent years. This is because the larger the area of the light diffusing film, the more the curling becomes more prominent, and the higher the brightness of the display, the greater the power consumption of the light source, that is, the amount of heat generated by the backlight unit. is there.

¡To solve this problem, it is necessary to eliminate bimetal. Generally, a hard coat layer (non-light diffusible layer) is formed on the surface of the light diffusion layer of the base film with a thickness of several μm to several tens of μm, and linear expansion stress is formed on both sides of the light diffusion layer. The measures to balance are taken.

However, the thickness of the hard coat layer is originally unnecessary, which causes an increase in thickness unnecessary for the light diffusing film and an increase in manufacturing cost. Furthermore, there is a limit to measures for balancing the linear expansion stresses on the front and back sides, and the above-described large screen and high brightness display can provide only an insufficient effect.

In recent years, many studies have been made to integrate a light diffusive film with another optical functional film in order to reduce the number of backlight unit parts, simplify the manufacturing process, and reduce the cost.

For example, a prism row is formed on the first surface side of a plate-shaped translucent base material having two main surfaces, the first surface and the second surface, and a large number of light transmitting surfaces are formed on the second surface side of the base material. There is disclosed a prism sheet (see Patent Document 3), in which a light diffusion layer containing a light bead is formed.

Further, at least two layers of a light diffusing layer composed of a thermoplastic resin layer kneaded with a light diffusing agent and a prism shape forming layer having a prism shape formed on the surface of a thermoplastic resin layer not kneaded with the light diffusing agent are laminated. A lens sheet for a liquid crystal display device (see Patent Document 4) is disclosed.

Furthermore, a light-scattering biaxially stretched polyester film for a prism sheet is disclosed (see Patent Document 5) in which light diffusibility is imparted by a light-scattering agent added inside the film and voids generated around the light-scattering agent.

However, the method disclosed in Patent Document 3 has a problem in that the translucent bead having a lens action is placed on the light incident surface side, so that a so-called reverse diffusion state occurs and the front luminance is greatly reduced. For this reason, this method cannot provide sufficient luminance and light diffusibility.

On the other hand, in the methods disclosed in Patent Document 4 and Patent Document 5, since light diffusibility is imparted by the light scattering material inside the substrate, some incident light undergoes backscattering, and the light transmittance is reduced. There is a problem of doing.

In recent years, many approaches have been made to impart light diffusivity to a biaxially stretched polyester film itself having excellent heat resistance, mechanical strength, and thickness uniformity. Providing light diffusibility to a polyester film consisting essentially of a single material opens the way for solving the above-mentioned heating curl problem and integrating the functions of the diffusion sheet and prism sheet. The value is very great.

However, any of the previously proposed proposals for imparting light diffusibility to the biaxially stretched polyester film itself is one of the inherent characteristics (heat resistance, mechanical strength, etc.) of the biaxially stretched polyester film. Or the characteristics that the light diffusive film should have such as light transmittance and light diffusibility are impaired, and it has not been put into practical use.

For example, the film disclosed in Patent Document 5 is presumed to have characteristics inherent to a biaxially stretched polyester film, such as excellent heat resistance, mechanical strength, and excellent thickness uniformity. However, since the light diffusibility is imparted by bubbles existing inside the layer, there is a problem that the light transmittance is low. Bubbles (voids) generated in the biaxial stretching process of the film have a flat plate shape parallel to the film surface. Therefore, when used as a light diffusive film in a backlight unit, most of the light emitted from the illumination surface is backscattered, and the light transmittance is impaired. In fact, the total light transmittance shown in the examples is only 85.3% at the highest.

Further, as a constituent polyester resin of a light diffusion layer containing fine particles, an internal light diffusion film using an amorphous polyester obtained by copolymerizing 25 mol% of an isophthalic acid component with polyethylene terephthalate (PET) is laminated on at least one surface thereof. A laminated light diffusing film made of PET film (see Patent Document 6) is disclosed.

In the above method, since the elimination of voids is considered, the light transmittance is improved. However, even in this method, the light diffusibility is imparted by light scattering inside the film, and a decrease in light transmittance accompanying backscattering of incident light is inevitable.

In the film of Patent Document 6, the crystallinity of the constituent resin (PET homopolymer) of the base material layer and the constituent resin (amorphous polyester) of the light diffusion layer is remarkably different. As a result, the obtained biaxially stretched film itself has a bimetallic structure, and the biaxially stretched film itself is easily curled by heating. For this reason, curling may occur depending on the heat treatment in the post-processing step and the use environment (temperature) of the liquid crystal display.

In addition, a light diffusing layer in which a melting point is 210 ° C. or less or amorphous polyester is used as a constituent resin, and a light diffusing additive composed of incompatible particles or thermoplastic resin is blended with the constituent resin is used as an intermediate layer. A film in which a crystalline polyester resin layer is laminated on both sides is disclosed (see Patent Documents 7 to 13).

In these methods, since the film structure is the front and back object, the occurrence of curl due to the asymmetric structure is improved to some extent. However, there is no difference in crystallinity between the light diffusing intermediate layer and the surface layer, and the flatness during temperature changes is significantly deteriorated due to slight layer thickness fluctuations and physical properties fluctuations on both sides. Is inherent.

In these methods, most of the film is made of amorphous or extremely poorly crystalline polyester, so that the excellent heat resistance, mechanical strength and thickness uniformity inherent in the biaxially stretched film can be obtained. I can't.

Also, a biaxially stretched polyethylene terephthalate film containing spherical or convex lens-shaped particles having a specific particle diameter is disclosed (see Patent Document 14).

Patent Document 14 discloses a film having a total light transmittance of 88% and a diffuse transmittance of 68% while using polyethylene terephthalate as a raw material for polyester. Furthermore, a film having a total light transmittance of 85% and a diffuse transmittance of 63% is disclosed. However, the basic properties such as heat resistance, mechanical strength, and thickness accuracy of these films are not disclosed, and heat resistance, mechanical strength, and high thickness accuracy, which are inherent characteristics of biaxially stretched polyethylene terephthalate films, are not disclosed. The probability of obtaining is not recognized at all.

This is because these films are obtained by stretching an unstretched film having a thickness of 200 μm by 3.0 times in the vertical, horizontal, and both directions, that is, by an area magnification of 9.0 times. Is 50 μm, and the actual area stretch ratio calculated from the thickness ratio before and after stretching is only 4.0 times. In other words, due to the influence of width shrinkage that occurs during longitudinal stretching, stretching ratio distribution that occurs during transverse stretching, and dimensional changes during heat treatment, the setting ratio of the stretching equipment and the actual stretching ratio are significantly different. Conceivable. And, when the actual area draw ratio is about 4 times, even if excellent light transmittance is obtained, the heat resistance, mechanical strength and high thickness accuracy which are the original characteristics of the biaxially stretched film are achieved. That is impossible.

As described above, the problem that the bimetallic film base material is likely to be curled as the backlight unit for liquid crystal display is increased in size and output is increasing, and the offline coating method is used to solve the above problem. However, it is desirable to use the actual stretched film itself (Patent Documents 1 and 2). However, in the method of imparting light diffusibility to the biaxially stretched film itself, generation of voids due to the light diffusing particles is unavoidable, and there is a problem that the total light transmittance is reduced (Patent Documents 3, 4, and 5). . As a method for avoiding the generation of voids, there has been a problem that the problem of curling cannot be solved by changing the resin properties and stretching conditions that have been made conventionally (Patent Document 6) or the mechanical strength of the film is reduced (patent) References 7-14). That is, since the mechanical properties and optical properties of the biaxially stretched film are in a trade-off relationship, no film satisfying any of the properties has been obtained. For this reason, in terms of overall quality, the method has not been put into practical use, in comparison with the conventional method of applying a light diffusion layer to a transparent substrate film by post-processing.

In view of the problems as described above, the biaxially stretched polyester film inherently has excellent heat resistance, mechanical strength, thickness accuracy, etc. by using a light diffusion layer mainly made of crystalline polyester, and mainly diffuses light by surface haze. In order to provide a surface light diffusible polyester film in which the total light transmittance and the light diffusibility are both achieved by imparting the property, and the generation of the heating curl derived from the bimetal structure is suppressed, the inventors of the present application have first proposed. The invention of the prior application (I) (Japanese Patent Application No. 2007-316712) was conducted.

The invention (I) of the prior application is a surface light diffusible polyester that suppresses the occurrence of curling by heating, has the original excellent mechanical properties of the biaxially stretched polyester film, and further achieves both total light transmittance and light diffusibility. A film is provided. In order to achieve both of the above characteristics, the inventors of the present application have made extensive studies by paying particular attention to the plane orientation coefficient of the film and the relationship between the internal haze and the surface haze. As a result, the inventors of the present application have found that the above-mentioned contradictory characteristics can be achieved by taking the measures described in [1] to [7], which will be described later, and have arrived at the prior invention (I).

JP-A-6-59108 Japanese Patent No. 3698978 specification JP-A-9-281310 Japanese Patent No. 3732253 JP 2005-181648 A JP 2001-272508 A Japanese Patent Laid-Open No. 2001-324606 JP 2002-162508 A JP 2002-182013 A JP 2002-196113 A JP 2002-372606 A JP 2004-219438 A JP 2004-354558 A Japanese Patent Laid-Open No. 2002-37898

In the surface light diffusible polyester film of the invention (I) of the prior application, since the support layer and the light diffusing layer both have a multilayer structure mainly composed of crystalline polyester, generation of heating curl derived from the bimetallic structure is suppressed. In addition, the biaxially stretched polyester film had excellent heat resistance, mechanical strength and thickness accuracy inherent to the biaxially stretched polyester film.

The surface light diffusing polyester film of the invention (I) of the prior application uses crystalline polyester containing a copolymer component as the main raw material of the light diffusing layer, and the plane orientation coefficient of the entire film is controlled within a specific range. The voids are not substantially generated around the incompatible additive added in the light diffusion layer, and the surface of the light diffusion layer has an uneven structure. Therefore, it has both excellent surface light diffusibility and high light transmittance.

When the surface light diffusive film is used in combination with a lens sheet, a prism sheet, or a lens layer, not only the light diffusibility of the light diffusive film alone and the light transmittance are required, but the light diffusive film and the lens sheet or The front luminance exhibited when combined with the prism sheet is required. As a result of studying various forms of use of the surface light diffusible polyester film of the invention (I) of the prior application, it becomes clear that the front luminance may be lowered particularly in an environment-friendly low power consumption type liquid crystal display. It was. In the low power consumption type, the amount of irradiation required for the backlight is minimized. Therefore, when the surface light diffusing polyester film is used in combination with a lens sheet or a prism sheet, it is necessary to exhibit excellent front luminance even if it is a low power consumption type.

The present invention makes it possible to utilize the excellent characteristics of the surface light diffusible polyester film of the invention (I) of the prior application in a wide variety of usage forms, particularly when used in combination with a lens sheet, a prism sheet, or a lens layer. An object of the present invention is to provide a surface light diffusing polyester film having excellent luminance characteristics even if it is a type.

The surface light diffusing polyester film of the present invention capable of achieving the above-mentioned object has the following configuration.

That is, among the present inventions, the structure of the first invention is a light diffusing polyester film made of a biaxially oriented polyester film, and is characterized by satisfying the following requirements (1) to (6).
(1) A support layer comprising a crystalline homopolyester or a crystalline polyester containing a copolymer component, and a copolymer component having a melting point of 235 to 255 ° C. laminated on at least one surface of the support layer by a co-extrusion method. A light diffusing layer comprising a blended composition of 50 to 99 parts by mass of the crystalline polyester contained and 1 to 50 parts by mass of an additive incompatible with the polyester.
(2) The plane orientation coefficient ΔP of the film defined by the following formula is 0.08 to 0.16.
ΔP = (nx + ny) / 2−nz
Here, nx, ny, and nz represent the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction, respectively.
(3) The surface haze is 15% or more.
(4) The internal haze is less than the surface haze.
(5) The rate of dimensional change at 150 ° C. is 3% or less in both the vertical and horizontal directions, and the tensile strength is 100 MPa or higher in both the vertical and horizontal directions.
(6) The average inclination gradient (Δa) on the surface of the light diffusion layer is 0.03 or more.
The structure of the second invention is characterized in that, in the above invention, the total light transmittance is 86% or more, and the image definition at a comb width of 2 mm is 50% or less.
According to a third aspect of the present invention, the main component is at least one or more of a copolyester resin, a polyurethane resin, or an acrylic resin provided on the surface of the light diffusion layer in the invention before completion of stretching and orientation of the film. It is characterized by having a coating layer.
According to a fourth aspect of the present invention, in the above invention, at least one or more of a copolyester resin, a polyurethane resin, or an acrylic resin is provided on both the light diffusing layer side and the support layer side of the light diffusing polyester film. It has the coating layer which has as a main component.
According to a fifth aspect of the invention, the surface light diffusing polyester film is for a prism sheet, and at least one of a copolyester resin, a polyurethane resin, or an acrylic resin is mainly formed on the surface opposite to the light diffusing layer. It has a coating layer as a component.

The surface light diffusing polyester film of the present invention has curling caused by heating achieved in the invention (I) of the prior application, has the original excellent mechanical properties of the biaxially stretched polyester film, and further has a total light transmittance. In addition to the effect of balancing light diffusivity, by controlling the gradient of the surface irregularities of the light diffusing layer, it is also possible to achieve high brightness when combined with lens sheets, prism sheets, and lens layers. ing.

The present invention suppresses the occurrence of curling due to heating, has the original excellent mechanical properties of the biaxially stretched polyester film, and further achieves both total light transmittance and light diffusibility, and also has excellent front luminance. The surface light diffusible polyester film which also realizes the effect of exhibiting the above is provided. In order to solve the above-mentioned problems, the inventors of the present application pay particular attention to the surface unevenness structure of the light diffusion layer in addition to the relationship between the plane orientation coefficient of the film and the relationship between the internal haze and the surface haze. went.

The light emitted from the light emitter (cathode tube or light guide plate) passes through the light diffusing film and the lens sheet. In this case, the light rays diffused by the light diffusing film are emitted in the front direction after the focusing angle is adjusted mainly in the prism type lens provided on the lens sheet. For this reason, it is necessary for the light diffusing film to have an optical design that can provide a predetermined front luminance even when combined with a lens sheet in addition to the diffusibility.

When light is diffused at a wide angle by the light diffusing film, the ratio of the amount of light that is captured and collected by the lens sheet, prism sheet, and lens layer decreases, so that the brightness of the transmitted light emitted to the front surface decreases. On the other hand, when light is diffused at a narrow angle by the light diffusing film, the ratio of the amount of light captured and condensed by the lens sheet or lens layer is increased, but the diffused component is reduced. For this reason, the light diffusibility of the transmitted light is lowered, and the concealability by the diffusion film and the uniformity of the luminance on the entire irradiated surface are lowered. Therefore, it is necessary to attain a high level of compatibility between the brightness when combining lens sheets or lens layers and the diffusibility of the light diffusing film alone.

As a result of earnestly examining measures to obtain a film having an optimal optical design that matches the light collection angle of the lens sheet, the inventor of the present application is able to obtain the front luminance in the fine gradient gradient of the uneven structure on the surface of the light diffusion layer. The significance of optical design was discovered and the present invention was achieved. That is, the present inventor has designed an optical design that provides excellent front luminance when the diffusion distribution angle of light diffusion is combined with the lens sheet by controlling the average gradient (Δa) of the surface of the light diffusion layer. This led to the realization of a light diffusive film.

The surface of the light diffusion layer has an uneven surface structure due to the light diffusing additive. When the surface unevenness profile on the surface of the light diffusion layer is observed using a micromap, a mountain-shaped fine unevenness profile is observed. Reflection of light occurs on the inclined surface formed by this mountain-shaped unevenness, but if this inclination gradient is more than a predetermined value, the light is efficiently captured and condensed by combining with the lens sheet, and the brightness is improved. is there.

Here, the average inclination gradient (Δa) is obtained from the surface unevenness profile observed on the micromap. The height (y) of the surface unevenness profile was measured at every predetermined pitch (x), and the difference in height (y _n −y _{n + 1} ) at two consecutive measurement points was divided by the measurement pitch interval (x). This was measured as a slope, measured over a predetermined length in two directions perpendicular to the longitudinal direction (the longitudinal direction of the film) and the lateral direction (the width direction of the film), and the average was determined as the average slope gradient (Δa). . That is, the average gradient (Δa) expresses an average gradient (gradient) due to the uneven structure formed on the surface of the light diffusion layer. In the present application, the average gradient (Δa) is a factor that governs the coexistence of the light diffusion caused by the uneven structure on the surface of the light diffusion layer and the luminance exhibited when combined with the lens sheet.

In the surface light diffusing polyester film of the present invention, it is important that the average slope gradient (Δa) of the light diffusing layer surface is 0.03 or more. When the Δa is 0.03 or more, not only the light diffusibility necessary for concealing properties such as a cathode ray tube but also a sufficient luminance when combined with the lens film can be achieved even at a low irradiation amount. . The lower limit of Δa is preferably 0.04 or more, and more preferably 0.05 or more. The upper limit of Δa is preferably 0.10 or less, more preferably 0.09 or less, and even more preferably 0.08 or less. When Δa exceeds 0.10, depending on the lens sheet used, back reflection due to in-plane reflection may occur due to optical design, and front luminance may not be improved.

In order to achieve the above structural requirements, the present inventors have found that such characteristics can be realized by taking the means described in [1] to [8] below, and have reached the present invention. First, the characteristics of these achievement means will be described. In order to realize the above characteristics, only one of the following means [1] to [8] does not contribute effectively, but the means [1] to [8] are combined. It is considered that the above characteristics can be realized for the first time by using it.
[1] Control of resin melting point of light diffusion layer [2] Control of difference in melting point between support layer and light diffusion layer [3] Control of laminated structure of light diffusion layer [4] Control of thickness of light diffusion layer [5] Light Control of intrinsic viscosity of diffusion layer constituent resin [6] Control of difference in melt viscosity between base polymer and incompatible resin [7] Control of stretching temperature [8] Mutual control of melting point and heat treatment temperature condition of light diffusion layer

[1] Control of resin melting point of light diffusing layer (B) The surface light diffusing polyester film of the present invention has a support layer (A) made of a crystalline homopolyester or a crystalline polyester containing a copolymer component. A light diffusion layer (B) comprising a blended composition of a crystalline polyester containing a polymerization component and the incompatible additive. Here, crystalline polyester / crystalline homopolyester refers to polyester / homopolyester having a melting point. The melting point is the endothermic peak temperature at the time of melting detected at the time of primary temperature rise in so-called differential scanning calorimetry (DSC). Any polyester / homopolyester in which a clear crystal melting heat peak is observed as a melting point when measured using a differential scanning calorimeter is included in the crystalline polyester / crystalline homopolyester.

From the viewpoint of heat resistance, mechanical strength and thickness accuracy of the film, the higher the melting point of the resin, the better. However, when the melting point of the resin is high, the stretching stress generated during stretching increases. Therefore, if there are incompatible particles in the resin, voids (cavities) are easily generated, and the total light transmittance is reduced. The ease with which voids are generated is influenced by the stretching conditions as described later, but is strongly related to the plane orientation coefficient of the produced film. The plane orientation coefficient indicates the orientation state of the polymer chain formed in the stretched film. The higher the orientation state, the stronger the mechanical strength, but the more voids are generated in the film. Therefore, it is desirable to control the melting point of the resin constituting the light diffusion layer (B) within a certain range in order to reduce the degree of plane orientation of the film and suppress the generation of voids. The lower limit of the melting point of the crystalline polyester containing the copolymer component constituting the light diffusion layer (B) is preferably 235 ° C, more preferably 240 ° C. When the melting point is 235 ° C. or higher, an orientation coefficient that can exhibit desirable heat resistance, mechanical strength, and thickness accuracy can be obtained. Further, the upper limit of the melting point of the crystalline polyester containing the copolymer component constituting the light diffusion layer (B) is preferably 255 ° C. If melting | fusing point is 255 degrees C or less, since generation | occurrence | production of the void in a light-diffusion layer (B) is suppressed, it is preferable.

[2] Control of Difference in Melting Point The surface light diffusing polyester film of the present invention has a support layer (A) made of crystalline homopolyester or crystalline polyester containing a copolymer component. In order to obtain predetermined heat resistance, mechanical strength and thickness accuracy as a film, it is preferable that the crystalline polyester / crystalline homopolyester constituting the support layer (A) has a higher melting point. However, when the melting point difference between the resins constituting the two layers of the support layer (A) and the light diffusion layer (B) is large, curling due to the bimetal structure is likely to occur. Therefore, the melting point difference between the crystalline polyester / crystalline homopolyester constituting the support layer (A) and the crystalline polyester constituting the light diffusion layer (B) is preferably within 25 ° C., and within 20 ° C. More preferably, it is more preferably within 10 ° C, particularly preferably within 5 ° C. If the melting point difference is within 25 ° C., the occurrence of curling due to the bimetallic structure can be suppressed within the practical range. Since the melting point of the resin constituting the light diffusion layer (B) is preferably within the above range, the upper limit of the melting point of the crystalline polyester / crystalline homopolyester constituting the support layer (A) is preferably 270 ° C.

The melting point of the crystalline polyester constituting the support layer (A) and the light diffusion layer (B) can be controlled by introducing a copolymer component. In particular, in the present invention, it is desirable to introduce a predetermined amount of a copolymer component into the crystalline polyester constituting the light diffusion layer (B). By introducing the copolymer component into the polyester, the plane orientation coefficient of the biaxially stretched film can be controlled, and the light transmittance and the light diffusivity can be highly compatible. However, if the copolymer component is excessively introduced, the melting point of the polyester is lowered, and the original excellent characteristics of the biaxially stretched polyester film cannot be obtained. The introduction amount of the copolymer component is preferably 3 mol% or more, more preferably 5 mol% or more, and particularly preferably 8 mol% or more with respect to the entire aromatic dicarboxylic component or the entire glycol component. When the content of the copolymer component is larger than 3 mol%, it is preferable because generation of voids is suppressed and the light transmittance and the light diffusibility are highly compatible. On the other hand, the upper limit of the introduction amount of the copolymer component is preferably 20 mol% or less, more preferably 18 mol% or less, and particularly preferably 15 mol% or less with respect to the above components. When the content of the copolymer component is 20 mol% or less, it is preferable because the melting point is such that the mechanical properties of the biaxially stretched polyester film are within the practical range. The composition of the copolymer component that can be used in the present invention will be described later.

[3] Control of laminated structure of light diffusing layer (B) The surface light diffusing polyester film of the present invention is formed on at least one surface of the support layer (A) comprising the crystalline homopolyester or the crystalline polyester containing a copolymer component. It is important that the light diffusion layer (B) comprising a blended composition of a crystalline polyester containing the copolymer component and an additive incompatible with the polyester has a multilayer structure laminated by a coextrusion method. .

The diffusion of light in the light diffusion layer (B) is divided into scattering caused by the surface structure of the film and scattering caused by the internal structure of the film. The scattering can be evaluated as surface haze, and the post-scattering can be evaluated as internal haze. Light scattering by internal structures such as voids is accompanied by backscattering, so that a high total light transmittance cannot be obtained. On the other hand, the light scattering by the surface structure can obtain high light diffusibility without greatly reducing the total light transmittance. However, in order to achieve an effective surface haze in the light diffusion layer (B), it is difficult to avoid the occurrence of curling due to the bimetallic structure. In the present invention, by taking the means disclosed in (1) to (7), it is possible to provide a film having a high surface haze while suppressing the occurrence of heating curl. That is, the surface light diffusible polyester film of the present invention employs the multilayer structure described above, thereby imparting light diffusibility by the concavo-convex structure on the surface of the light diffusion layer (B) caused by the incompatible additive, and the film It is possible to achieve high total light transmittance by suppressing light scattering (internal haze) in the inside. Thereby, both high light transmittance and light diffusibility can be achieved.

When the surface light diffusing polyester film of the present invention is used as a prism sheet, a film obtained by laminating the light diffusing layer (B) on one side of the support layer (A) is used as a base material, and the surface opposite to the light diffusing layer (B) is used. It can use suitably by providing a prism structure. The layer structure of the surface light diffusible polyester film of the present invention may be a two-layer structure as described above, and may be a multilayer structure of three or more layers as necessary if the effects of the present invention can be obtained. When a film having a flat surface (not having a concavo-convex structure) is stacked on a flat transparent member, Newton rings are generated, and visibility may be lowered. Therefore, when the film of the present invention is used alone as a light diffusing sheet, the light diffusing layer (B) is formed on both sides of the support layer (A) in order to prevent Newton rings from being superimposed on the light guide plate or the prism sheet. ) Is preferably laminated. The composition of the incompatible additive that can be used in the present invention will be described later.

[4] Control of thickness of light diffusing layer (B) The surface light diffusing polyester film of the present invention has a support layer (A) and a light diffusing layer (B), in order to obtain the surface light diffusing polyester film of the present invention. The thickness of the light diffusion layer (B) is important. The surface haze of the light diffusion layer (B) tends to increase as the surface irregularity increases. Therefore, it is desirable that the particle size of the additive in the light diffusion layer (B) is large. In order to obtain a particle size effective for surface haze, the lower limit of the thickness of the light diffusion layer (B) is preferably 3 μm, more preferably 4 μm, and particularly preferably 5 μm.

On the other hand, when the thickness of the light diffusing layer (B) exceeds the particle size of the incompatible additive to a considerable extent, it becomes difficult to form a surface uneven structure effectively. Therefore, when the thickness of the light diffusion layer (B) is increased, the formation of surface irregularities is reduced and the surface haze is reduced. Further, according to the thickness of the light diffusion layer (B), the internal haze due to the internal structure of the light diffusion layer (B) increases, and the total light transmittance decreases. In order to achieve both high total light transmittance and light diffusibility, it is desirable to control the thickness of the light diffusion layer (B) within a predetermined range. Therefore, the upper limit of the thickness of the light diffusion layer (B) is preferably 50 μm, more preferably 30 μm, and particularly preferably 20 μm.

Also, when the ratio of the light diffusion layer (B) to the total film thickness (A + B) is increased, curling due to the bimetallic structure is likely to occur. Furthermore, since the ratio of the light diffusion layer (B) having a relatively low melting point as compared with the support layer (A) is increased, thickness unevenness tends to occur as a whole film, and the surface smoothness is impaired. Moreover, since a light-diffusion layer (B) contains many copolymerization components, an orientation coefficient falls as the whole film, and a mechanical characteristic falls. On the other hand, if the ratio of the light diffusion layer (B) to the total film thickness is small, the additive in the light diffusion layer (B) may bleed out on the surface of the film or may fall off. Therefore, the ratio of the light diffusion layer (B) to the total film thickness is desirably controlled within a predetermined range, and preferably in the range of 2 to 50%. The lower limit of the ratio of the light diffusion layer (B) to the total film thickness is preferably 2%, more preferably 3%, and particularly preferably 4%. On the other hand, the upper limit of the ratio of the light diffusion layer (B) to the total film thickness is preferably 50%, more preferably 35%, and particularly preferably 20%.

[5] Control of intrinsic viscosity of light diffusion layer (B) constituting resin The present invention is characterized in that the light diffusion layer (B) is applied by a coextrusion method. Since the surface light diffusible polyester film of the present invention is intended for optical applications, it is preferable that there are few optical defects due to foreign matters, and when a resin is supplied by a coextrusion method, it is desirable to provide a foreign matter removing filter in the melt line. In order to allow the resin to pass through the foreign matter removal filter, a certain extrusion pressure is required. However, when the intrinsic viscosity of the resin is low, the ejection stability at the time of extrusion of the molten resin is lowered, so that stable film formation becomes difficult. Moreover, when the intrinsic viscosity of resin is low, the plane orientation coefficient of the obtained light-diffusion layer (B) becomes low, and the mechanical strength of a film falls. Therefore, it was considered that the intrinsic viscosity of the crystalline polyester containing the copolymer component constituting the light diffusion layer (B) is preferably higher. However, the present inventors have found the surprising relationship described below between the intrinsic viscosity of the polyester and the surface haze.

When the intrinsic viscosity of the crystalline polyester is increased, the shearing force during melting and stirring is increased. Therefore, when the crystalline polyester and an incompatible additive are stirred and mixed in an extruder, the higher the intrinsic viscosity of the crystalline polyester, the higher the shearing force in melting and stirring, and the higher the dispersibility of the additive. Rise. This is thought to be due to the additive becoming finer due to the shearing force of the solvent. As a result, the particle size of the additive becomes small, and an effective dispersion diameter cannot be obtained to such an extent that a good uneven structure is imparted to the surface of the light diffusion layer (B), and the surface haze is lowered. Therefore, in order to achieve both the mechanical strength of the light diffusion layer (B) and good light characteristics, the intrinsic viscosity of the crystalline polyester containing the copolymer component constituting the light diffusion layer can be controlled within a predetermined range. It turned out to be preferable.

The lower limit of the intrinsic viscosity of the crystalline polyester is preferably 0.50 dl / g, more preferably 0.52 dl / g. When the intrinsic viscosity is less than 0.50 dl / g, when a foreign matter removing filter is provided in the melt line, the discharge stability during extrusion of the molten resin tends to be lowered. The upper limit of the intrinsic viscosity of the crystalline polyester is preferably 0.61 dl / g, and more preferably 0.59 dl / g. When the intrinsic viscosity exceeds 0.61 dl / g, the dispersion diameter in the polyester of the additive becomes small, and the light diffusibility tends to decrease.

[6] Control of the difference in melt viscosity between the base polymer and the incompatible resin The inventor of the present invention describes the difference in melt viscosity between the crystalline polyester constituting the light diffusion layer (B) and the incompatible additive, It has been found that it has the following relationship with the surface haze. In the present invention, surface irregularities are formed by the incompatible additive in the light diffusion layer (B), and a predetermined surface haze is obtained. The crystalline polyester containing the copolymer component constituting the light diffusion layer (B) and the incompatible additive are stirred and mixed in the extruder. As an aspect of the incompatible additive, a thermoplastic resin is preferable. However, when the melt viscosity of the crystalline polyester and the melt viscosity of the additive are approximately the same, the two components are easily dispersed, and the additive is a fine additive. Granulate. When the dispersion diameter of the additive is reduced, a good uneven structure cannot be obtained on the surface of the light diffusion layer (B), and the surface haze is lowered. Therefore, in the present invention, it is preferable that the difference in melt viscosity between the crystalline polyester containing the copolymerization component constituting the light diffusion layer (B) and the incompatible additive is large. The difference in melt viscosity is preferably 35 Pa · s or more, and more preferably 40 Pa · s or more. When the difference in melt viscosity is 35 Pa · s or more, the additive in the polyester as an additive has a good dispersion diameter, and good light diffusibility is obtained.

[7] Control of stretching temperature The mechanical properties and optical properties of the film can also be controlled by the film forming conditions. When the stretching temperature of the film is raised, the stretching stress is lowered, so the orientation coefficient is lowered and the generation of voids is suppressed. In addition, since surface irregularities due to incompatible additives are easily formed, it is desirable to stretch at a high temperature from the viewpoint of achieving both total light transmittance and light diffusibility. However, when the stretching temperature is increased, the thickness variation of the film increases, resulting in thickness unevenness and the like, and it is difficult to obtain the original mechanical characteristics of the film. In the surface light diffusible polyester film of the present invention, in order to achieve both excellent mechanical properties and total light transmittance and light diffusibility, film forming conditions according to the resin properties and required properties, particularly the temperature during stretching. It is desirable to control appropriately. When the surface light diffusible polyester film of the present invention is produced by stretching a polyester resin, the temperature during the transverse stretching is desirably in the temperature range of 120 ° C to 160 ° C.

[8] Mutual Control of Melting Point of Light Diffusing Layer and Heat Treatment Temperature Conditions The surface light diffusing polyester film of the invention (I) of the prior application was achieved by relating the above means [1] to [7]. However, in order to achieve excellent luminance characteristics in the present invention, it is important that the average gradient (Δa) is 0.03 or more in the uneven structure on the surface of the light diffusion layer (B). For this purpose, it is desirable to provide a concavo-convex structure on the surface of the light diffusion layer (B) by means such as the above [5], [6]. In particular, the stretching step contributes to effective formation of unevenness. This is considered to be due to the fact that the incompatible additive is pushed outward by the stretching stress generated inside the film, and an effective uneven structure is formed.

However, even if the concavo-convex structure is provided at the initial stage of the film forming process, the concavo-convex structure having an average inclination gradient (Δa) to the extent that the concavo-convex structure is flattened in the subsequent film forming process and a predetermined luminance is generated when combined with the lens sheet. Is not retained. For example, in the prior invention (I), in the heat treatment step, heat treatment was performed at a high temperature of 235 to 250 ° C. in order to reduce voids inside the film. In this case, the crystalline polyester containing the copolymer component constituting the light diffusion layer (B) was softened by a high-temperature heat treatment, and the gradient formed by the concavo-convex structure was flattened.

Therefore, as a method for achieving the average gradient (Δa) of the present invention, it is desirable to increase the difference between the melting point of the resin constituting the light diffusion layer (B) and the heat treatment temperature. When the difference between the melting point of the resin constituting the light diffusion layer and the heat treatment temperature is reduced, the light diffusion layer is softened in the heat treatment step, and as a result, a surface uneven structure having an average gradient (Δa) excellent in luminance is formed. Disappear. However, when the difference between the melting point of the resin constituting the light diffusion layer (B) and the heat treatment temperature is increased, the heat treatment temperature is lowered, so that the thermal contraction rate of the film is deteriorated. Further, when the melting point of the resin constituting the light diffusion layer is increased, voids generated around the incompatible resin contained in the light diffusion layer are not lost even by heat treatment and remain. A film in which voids are generated is not preferable because the total light transmittance is reduced due to an increase in internal haze. Therefore, the difference between the melting point of the light diffusion layer (B) and the heat treatment temperature is preferably controlled within a range of 9 ° C. or more and 25 ° C. or less, more preferably 11 ° C. or more and 23 ° C. or less, More preferably, it is 13 ° C. or more and 21 ° C. or less.

In order to achieve the requirement (1) described in claim 1, it is possible to achieve it by executing the condition control of the means [1] to [3].
In order to achieve the requirement (2) described in claim 1, it is possible to achieve it by executing the condition control of the means [4] to [7].
In order to achieve the requirement (3) described in claim 1, it is possible to achieve it by executing the condition control of the means [3] to [7].
In order to achieve the requirement (4) described in claim 1, it is possible to achieve it by executing the condition control of the means [3] to [7].
In order to achieve the requirement (5) described in claim 1, it is possible to achieve the condition control of the means [1] to [4] and [7].
In order to achieve the requirement (6) described in claim 1, it is possible to achieve the condition control of the means [1], [5] to [8].
In the present invention, it is considered that the above means [1] to [8] are related to each other to obtain a predetermined effect. However, it can be achieved by a method different from the method described above as long as it does not depart from the spirit of the present invention. Specifically, the following means can be mentioned.

In [2] above, a method for suppressing the occurrence of curling due to the bimetal structure was shown. In the above description, the light transmittance and the light diffusivity are both highly compatible, and if the difference between the linear expansion coefficients of the light diffusion layer (B) and the support layer (A) is reduced, Although the technical idea that a surface light diffusible polyester film can be obtained is disclosed, those skilled in the art can easily implement such a technical idea by a method different from the method described above, The surface light diffusing polyester film of the present invention can be obtained by different methods.

That is, even when the difference in melting point between the crystalline polyester / crystalline homopolyester constituting the support layer (A) and the crystalline polyester constituting the light diffusion layer (B) is greater than 25 ° C., By providing a difference in the stretching temperature of each surface of the support layer (A) and the light diffusion layer (B), the support layer (A) surface side and the light diffusion layer (B) surface side are aligned by stretching. By providing the difference and controlling the difference between the linear expansion coefficients on both sides of the film, it is possible to obtain a surface light diffusible polyester film that suppresses the occurrence of curling due to the bimetallic structure.

In [5] above, a method for controlling the surface haze by the surface irregularities formed by the dispersed additive was shown. In the above description, the technical idea of how to control the dispersion diameter of the additive is disclosed. However, those skilled in the art will consider such a technical idea as a method different from the method described above. Can be implemented more easily.

That is, even when the intrinsic viscosity of the crystalline polyester containing the copolymerization component constituting the light diffusion layer (B) exceeds 0.61 dl / g, from the polymer tube after the kneading part in the extruder to the die exit. By controlling the retention time of the additive, it is possible to secure a time for the finely divided additive to agglomerate, and to obtain a surface haze due to surface irregularities formed by controlling the dispersion diameter of the additive. Moreover, the dispersion diameter of the additive can be controlled by controlling the shear force at the time of discharging the molten resin by controlling the slit interval of the T die. Further, the dispersion diameter of the additive can be controlled by adding a flocculant having an effect of agglomerating the finely divided additive to the dissolved resin once dispersed in the polymer tube after kneading. For example, when a polystyrene resin is used as an additive, by adding an acrylic-styrene copolymer or the like as an aggregating agent, aggregation of the styrene resin is promoted and a dispersion diameter effective for light diffusion can be obtained. Such an acrylic-styrene copolymer can also be obtained by copolymerizing 1 mol of glycidyl methacrylate and 2 mol of styrene monomer.

In the description of [7] above, a method of controlling the stretching stress and suppressing the generation of voids by increasing the stretching temperature of the film was shown. In the above description, the technical idea of how to reduce the stretching stress is shown. However, those skilled in the art can easily perform the technical idea by a method different from the method described above. Can be implemented. That is, even when the film stretching temperature is low, by using a simultaneous biaxial stretching machine, the stretching speed can be lowered to control the stretching stress and suppress the generation of voids.

In the description of [8] above, the method of controlling the average gradient (Δa) of the surface of the light diffusion layer (B) by controlling the difference between the melting point of the light diffusion layer and the heat treatment temperature is shown. In the above description, the technical idea of how to maintain the uneven structure on the surface of the light diffusion layer (B) in the heat treatment step is shown. However, those skilled in the art described the technical idea above. Even a method different from the method can be implemented. In other words, (i) using a material excellent in heat resistance strength as a light diffusing additive, or (ii) providing a temperature difference between the front and back in the heat treatment step, lowering the processing temperature of the light diffusion layer (B) surface, The average gradient (Δa) on the surface of the light diffusion layer (B) can also be controlled by increasing the processing temperature of the surface of the support layer (A).

Furthermore, the structure for obtaining the surface light diffusable polyester film of the present invention and the characteristics will be described in detail below.
(material)
The crystalline homopolyester used as a film raw material in the present invention includes aromatic dicarboxylic acids or esters thereof such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, ethylene glycol, diethylene glycol, 1,3-propanediol, 1,4-butane. Polyester produced by polycondensation with glycols such as diol and neopentyl 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.

Representative examples of the polyester include polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate. The polyester may be a homopolymer or may be a copolymer of the third component within a range that does not substantially impair the crystallinity thereof. 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.

The crystalline polyester containing a copolymerization component that can be used in the present invention is a polyester in which a third component (copolymerization component) is introduced into the main chain using the above crystalline homopolyester as a basic skeleton. The structure, molecular weight, and composition are not limited and are arbitrary.

Further, the surface light diffusing polyester film of the present invention comprises a copolyester composed of an aromatic dicarboxylic acid component and a glycol component containing ethylene glycol and at least one of a branched aliphatic glycol or alicyclic glycol. It is preferable to use it for some or all of the raw materials.

Examples of branched aliphatic glycols include neopentyl glycol, 1,2-propanediol, and 1,2-butanediol. Examples of the alicyclic glycol include 1,4-cyclohexanedimethanol and tricyclodecane dimethylol.

Of these, neopentyl glycol and 1,4-cyclohexanedimethanol are particularly preferable. Furthermore, in the present invention, it is a more preferred embodiment that 1,3-propanediol or 1,4-butanediol is used as a copolymerization component in addition to the glycol component. Introducing and using these glycols as copolymerization components in the above-mentioned range is suitable for imparting the above-mentioned characteristics, and further reduces voids in the light diffusion layer, and reduces light transmittance and light. It is also preferable from the viewpoint of achieving both high diffusibility.

Furthermore, if necessary, one or more dicarboxylic acid components and / or glycol components as described below may be used in combination with the polyester as a copolymerization component.

Other dicarboxylic acid components that can be used in combination with terephthalic acid or its ester-forming derivatives include (1) isophthalic acid, 2,6-naphthalenedicarboxylic acid, diphenyl-4,4'-dicarboxylic acid, diphenoxyethanedicarboxylic acid Aromatic dicarboxylic acids such as acid, diphenylsulfone dicarboxylic acid, 5-sodium sulfoisophthalic acid, phthalic acid or their ester-forming derivatives, (2) oxalic acid, succinic acid, adipic acid, sebacic acid, dimer acid, maleic acid Aliphatic dicarboxylic acids such as fumaric acid and glutaric acid or ester-forming derivatives thereof, (3) alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid or ester-forming derivatives thereof, (4) p-oxybenzoic acid, Oxycarboxylic acids such as oxycaproic acid or their Le forming derivatives, and the like.

On the other hand, other glycol components that can be used in combination with ethylene glycol and branched aliphatic glycol and / or alicyclic glycol include aliphatic glycols such as pentanediol and hexanediol, bisphenol A, bisphenol S, and the like. Aromatic glycols and their ethylene oxide adducts, diethylene glycol, triethylene glycol, dimer diol and the like can be mentioned.

Furthermore, if necessary, the polyester may be further copolymerized with a polyfunctional compound such as trimellitic acid, trimesic acid, or trimethylolpropane.

Examples of the catalyst used for producing the polyester include alkaline earth metal compounds, manganese compounds, cobalt compounds, aluminum compounds, antimony compounds, titanium compounds, titanium / silicon composite oxides, and germanium compounds. Of these, titanium compounds, antimony compounds, germanium compounds, and aluminum compounds are preferred from the viewpoint of catalytic activity.

When producing the polyester, it is preferable to add a phosphorus compound as a heat stabilizer. As said phosphorus compound, phosphoric acid, phosphorous acid, etc. are preferable, for example.

In the surface light diffusing polyester film of the present invention, the copolymerized polyester may be used as a raw material for the film as it is, or a copolymerized polyester having a large amount of copolymerized components is blended with a homopolyester (for example, polyethylene terephthalate) to copolymerize. You may adjust the amount.

In particular, by producing a film using the latter blending method, a co-polymer having a high melting point (heat resistance) while achieving both the light diffusibility and the total light transmittance equivalent to the case of using only the copolyester. A crystalline polyester containing a polymerization component can be prepared.

Alternatively, a method may be employed in which two different kinds of crystalline polyesters are melt-mixed and a third component (copolymerization component) is introduced into the main chain by utilizing a transesterification reaction between them. In particular, the copolymer polyester, polyethylene terephthalate, and at least one homopolyester other than polyethylene terephthalate (for example, polytetramethylene terephthalate or polybutylene terephthalate) are blended and used as a raw material for the surface light diffusing polyester film of the present invention. The use is further preferable from the viewpoint of reducing voids.

In addition, it is preferable that the polyester constituting the support layer (A) does not substantially contain particles. Moreover, it is preferable that the crystalline copolyester constituting the light diffusion layer does not substantially contain particles other than the additive described later. The above-mentioned “substantially contain no particles” means, for example, in the case of inorganic particles, a content of 50 ppm or less, preferably 10 ppm or less, particularly preferably a detection limit or less when inorganic elements are quantified by fluorescent X-ray analysis. Means quantity. By using a clean polyester raw material free from impurities, the occurrence of optical defects in the liquid crystal display can be suppressed.

(Additive <surface irregularity imparting agent>)
The additive in the present invention is added for the purpose of imparting unevenness to the surface of the light diffusion layer and exhibiting surface light diffusion performance. Light incident on the light diffusing layer (emitted from the light diffusing layer) is refracted and diffused in a random direction by the unevenness imparted to the film surface, and surface light diffusibility is exhibited. The additive is not particularly limited as long as it is a material incompatible with the polyester, but it is preferable to use the following materials.

(Thermoplastic resin incompatible with polyester)
The most excellent additive that can be used in the present invention is a thermoplastic resin that is incompatible with the polyester. That is, by utilizing the incompatibility between polyester and thermoplastic resin, in the production process (melting / extrusion process) of the biaxially stretched film, the matrix made of polyester is made of a thermoplastic resin that is incompatible with the polyester. This is a technology that forms domains in a dispersed manner and uses them as surface irregularity forming agents. By using this technique, foreign matter can be filtered with a high-accuracy filter in the film melting / extrusion step, and the cleanliness required for a liquid crystal display film can be achieved.

On the other hand, when using non-melting polymer particles and inorganic particles, which will be described later, as an additive, there is a limit to the fineness of filter openings that can be used in the film manufacturing process, and foreign substances are removed with high accuracy. It becomes difficult. Furthermore, when polymer particles or inorganic particles are used, voids are likely to be generated at the interface between the particles and the polyester, and it is difficult to achieve a high balance between light diffusibility and total light transmittance.

Examples of thermoplastic resins that are incompatible with polyester that can be used as the additive include the following materials. That is, polyolefins such as polyethylene, polypropylene, polymethylpentene, various cyclic olefin polymers, polycarbonate, polystyrene such as atactic polystyrene, syndiotactic polystyrene, isotactic polystyrene, polyamide, polyether, polyesteramide, polyphenylene sulfide, polyphenylene Examples thereof include acrylic resins such as ether, polyether ester, polyvinyl chloride, and polymethacrylic acid ester, copolymers having these as main components, or mixtures of these resins.

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

Examples of the amorphous transparent polymer that can be used in the present invention include the following. That is, polystyrene (PS resin), acrylonitrile / styrene copolymer (AS resin), methyl methacrylate / styrene copolymer (MS resin), cyclic olefin polymer, methacrylic resin, PMMA, and the like are exemplified.

Among these, it is more preferable from the viewpoint of void reduction to select an amorphous transparent polymer having a polymer surface tension close to that of a matrix made of polyester. As such an amorphous transparent polymer having a surface tension close to that of polyester, polystyrene (PS resin), PMMA and the like are particularly preferable.

(Non-melting polymer particles)
The non-melting polymer particles that can be used as the additive of the present invention are obtained when the temperature is raised from 30 ° C. to 350 ° C. at 10 ° C./min using a melting point measuring device (manufactured by Stanford Research Systems, MPA100 type). The composition of the particles is not limited as long as the particles do not undergo flow deformation due to melting. 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 or elliptical. The particles may or may not have pores. Furthermore, you may use both together.

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

The average particle size 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 5 μm. In order to exhibit a good light diffusion effect, the average particle size of the non-melting polymer particles is preferably 0.1 μm or more.

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

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

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

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

The average particle size of the inorganic particles is usually preferably 0.1 to 50 μm. 0.5-30 μm is more preferable, and 1-20 μm is even more preferable. If the average particle size is less than 0.1 μm, a good light diffusion effect cannot be obtained. On the contrary, when it exceeds 50 μm, it is not preferable because it leads to a decrease in film strength and the like. The particle size distribution of the inorganic particles is preferably as sharp as possible. When it becomes necessary to widen the particle size distribution, it is preferable to mix a plurality of particles having a sharp particle size distribution. By the correspondence, mixing of particles having a large particle diameter, which is a defect of the film, can be suppressed.

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

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

The additive used in the present invention may be one of the above three types, or two or more types may be used in combination.

(Mixing ratio of additives)
The light diffusing layer in the surface light diffusing polyester film of the present invention comprises a blended composition of 50 to 99 parts by mass of the crystalline polyester containing the copolymer component and 1 to 50 parts by mass of an additive incompatible with the polyester. . A preferred blending ratio of both is a blend of 75 to 98 parts by weight of polyester and 2 to 25 parts by weight of additive, and more preferably a blend of 80 to 97 parts by weight of polyester and 3 to 20 parts by weight of additive.

When the mixing ratio of the additive is less than 1 part by mass, the film surface unevenness forming ability by the additive is insufficient, and sufficient surface light diffusion performance cannot be obtained. On the other hand, when the mixing ratio of the additive exceeds 50 parts by mass, light scattering at the additive / polyester interface increases, and the stretching stress of the polyester increases to easily generate voids around the additive. As a result, the internal haze of the light diffusion layer increases, and the total light transmittance tends to decrease. Furthermore, the additive easily falls off during biaxial stretching of the film, and the fallout can cause foreign matter.

[Characteristics of light diffusing polyester film]
(Plane orientation coefficient)
It is important that the surface light diffusing polyester film of the present invention has a plane orientation coefficient (ΔP) of 0.08 to 0.16. The lower limit of the plane orientation coefficient (ΔP) is more preferably 0.09, and particularly preferably 0.10. On the other hand, the upper limit of the plane orientation coefficient (ΔP) is more preferably 0.15, and particularly preferably 0.14.

It is desirable that the surface orientation coefficient (ΔP) is 0.16 or less because the unevenness on the surface of the light diffusion layer (B) is effectively formed and the light diffusion effect (surface haze) generated by the surface unevenness is exhibited.

Also, when the plane orientation coefficient (ΔP) exceeds 0.16, depending on the type of additive used, the number and size of voids generated around the additive tend to increase. Therefore, internal scattering (internal haze) increases, and the total light transmittance tends to decrease. In any case, when the plane orientation coefficient (ΔP) is 0.16 or less, both the total light transmittance and the light diffusibility can be achieved.

On the other hand, when the plane orientation coefficient is 0.08 or more, the characteristics as a biaxially stretched film are exhibited, heat resistance, mechanical strength, thickness uniformity and the like are good, and the occurrence of heating curl is suppressed.

The method for controlling the plane orientation coefficient within the above range is arbitrary, but can be controlled, for example, by adjusting the ratio of the copolymer component to the crystalline polyester containing the copolymer component. If the ratio of the copolymerization component in the light diffusion layer (B) or the support layer (A) is increased, the plane orientation coefficient is decreased, and if the ratio of the copolymerization component is decreased, the plane orientation coefficient is increased. be able to. The ratio of the preferable copolymerization component is as described above.

Further, the glass transition point of the crystalline polyester containing the copolymer component may be controlled by polymer blend or copolymerization. If the glass transition point is lowered, the orientation in the biaxial stretching step described later is lowered, and the plane orientation coefficient can be lowered. Moreover, the same effect is acquired even if the intrinsic viscosity of the raw material polyester used for a light-diffusion layer is reduced. A preferred intrinsic viscosity is as described above.

Further, the plane orientation coefficient can be controlled to some extent by adjusting the biaxial stretching conditions described later. In order to reduce the plane orientation coefficient, the stretching temperature for longitudinal stretching or transverse stretching may be set high, the stretching ratio may be set low, or the heat treatment temperature may be set high. Preferred biaxial stretching conditions will be described later.

(Optical characteristics)
Next, the present invention is characterized in that the surface haze is 15% or more and the internal haze is less than the surface haze. The surface haze is a characteristic derived from surface irregularities of the light diffusion layer. Therefore, when light is emitted from the film surface, or when light is incident on the film surface, the surface haze is increased due to the light being refracted by the surface irregularities of the light diffusion layer. Therefore, surface haze and total light transmittance are basically irrelevant. Therefore, by increasing the surface haze, the light diffusibility can be enhanced in a state where the decrease in the total light transmittance is suppressed.

On the other hand, the internal haze is a characteristic derived from light scattering inside the film. Therefore, the total light transmittance is reduced due to the influence of backscattering of incident light. Therefore, in order to produce a light diffusible polyester film having excellent light diffusibility and high total light transmittance, it is effective means to increase the surface haze and reduce the internal haze as much as possible.

The surface haze of the surface light diffusible polyester film of the present invention is 15% or more, and the preferred lower limit is 20%. If the surface haze is 15% or more, an effective diffusing effect is exerted on the printed pattern of the light guide plate and the lamp image of the cold cathode tube, and light diffusing performance effective as a light diffusing film is obtained.

On the other hand, the preferable upper limit of the surface haze is 60%, the more preferable upper limit is 70%, and the more preferable upper limit is 80%. If the surface haze is 60% or less, the internal haze is suppressed and the total light transmittance tends to increase.

Also, it is important that the internal haze is less than the surface haze. The upper limit of the internal haze is preferably 40%, more preferably 30%, still more preferably 20%, and particularly preferably 10%.

If the internal haze is the same as or exceeds the surface haze, the internal haze is responsible for the light diffusing function of the film, causing light scattering (with backscattering) inside the film and total light transmission. The rate is greatly reduced. On the other hand, the lower limit of the internal haze is preferably 1%. In a film having an internal haze of less than 1%, sufficient surface haze tends not to be obtained.

The surface light diffusing polyester film of the present invention preferably has a total light transmittance of 86% or more. A more preferable lower limit of light transmittance is 87%, and a more preferable lower limit is 88%.

Further, the light diffusion performance of the light diffusing film can be quantitatively evaluated by, for example, the image definition. Image sharpness is an index indicating the sharpness when a light source such as a fluorescent lamp is viewed through a film, and is a normal method for measuring in accordance with JIS K 7105 “Plastic Optical Properties Test Method” image sharpness. This is the image definition evaluated in. The smaller the image definition, the better the concealing property and the better the light diffusion performance.

In the surface light diffusible polyester film of the present invention, it is possible to obtain an image definition of 50% or less in a transmission method with an optical comb width of 2 mm. A more preferable upper limit of image definition is 40%, and a further preferable upper limit is 20%. The smaller the image definition is, the better. However, if the image definition is reduced more than necessary, the internal haze increases and the total light transmittance decreases. In the present invention, the lower limit of the image definition is preferably 1%, more preferably 3%.

The light diffusing performance of the light diffusing film can be further quantitatively evaluated by, for example, transmitted light intensity using a goniophotometer GP-200 manufactured by Murakami Color Research Laboratory. Of the transmitted light intensity, the value of the light receiving angle 0 degree is I (0), the value of the light receiving angle N degree is I (N), and the transmitted light intensity ratio obtained by the following formula is S (N). For example, when the value of S (1) when N = 1 degree is large, the transmitted light diffused around the transmitted light at 0 degree increases, so that the sharpness of the image seen through the film can be reduced. Good concealability can be obtained. In the surface light diffusing polyester film of the present invention, S (1) can obtain a value of 75% or more. The larger the transmitted light intensity ratio, the better the concealing property and the better the light diffusing property. If the value of S (1) is less than 75%, the light diffusibility is lowered and good concealability cannot be obtained.
S (N) = I (N) / I (0) × 100

The larger the transmitted light intensity ratio, the better the light diffusibility. However, if the transmitted light intensity ratio is increased more than necessary, I (0) often decreases, and as a result, the front luminance in the backlight unit is It will decline. In the surface light diffusing polyester film of the present invention, the upper limit value of S (1) is preferably 99%, more preferably 95%, still more preferably 85%.

In the present invention, the luminance is the luminance derived from the ratio of the parallel light transmittance and the total light transmittance when the light diffusing film and the lens sheet are superimposed and irradiated with light, with the ratio of the lens sheet alone being 100. Evaluated as a ratio. If the said brightness | luminance ratio is 101% or more, high brightness | luminance can be ensured compared with the case where there is no light diffusable film. In the present invention, the luminance ratio is preferably high, specifically 103% or more, and more preferably 105% or more. When the luminance ratio is 103% or higher, high luminance can be ensured even when the amount of light is low.

(Mechanical characteristics)
In the present invention, since crystalline polyester is used as the raw material for the film, the inherently excellent heat resistance, mechanical strength, and excellent thickness accuracy of the biaxially stretched film can be obtained.

Regarding the heat resistance, the dimensional change rate at 150 ° C. is preferably 3% or less in both the horizontal direction and the vertical direction, the more preferable upper limit is 2.5%, and the more preferable upper limit is 2%. A preferred upper limit is 1.5%, and a more particularly preferred upper limit is 1%. On the other hand, a smaller dimensional change rate in the horizontal and vertical directions at 150 ° C. is desirable, but 0% is considered the lower limit. When the dimensional change rate is 3% or less, the dimensional change and flatness are not deteriorated in processing at a high temperature and use in a high temperature environment, and good flatness is maintained. As a result, it is possible to achieve the original purpose of the light diffusing film, which is to uniform the luminance of the light emitting surface in the backlight unit. In the present invention, the longitudinal direction refers to the film flow direction (winding direction) during film formation, and the lateral direction refers to a direction perpendicular thereto.

Further, the lower limit of the tensile strength of the film is preferably 100 MPa, more preferably 130 MPa, and particularly preferably 160 MPa. When the tensile strength is 100 MPa or more, the mechanical strength of the biaxially stretched film is exhibited, and problems such as cracks, tears, breaks, and tears are less likely to occur in the film processing step.

The surface light diffusible polyester film of the present invention preferably has a thickness unevenness of 5.0% or less.

When the thickness unevenness of the film is 5.0% or less, when the film is rolled up, wrinkles and bumps are hardly generated, and flatness is maintained. As a result, the luminance of the light emitting surface in the backlight unit becomes uniform, and the original purpose of the light diffusing film can be achieved.

The surface light diffusing polyester film of the present invention preferably has a curl value of 5 mm or less after heat treatment at 100 ° C. for 30 minutes under no load.

When the curl value is 5 mm or less, for example, the handling property when working under no tension when incorporated into a final product as a light diffusing film is improved. Further, even when processing at a high temperature or use in a high temperature environment, the original purpose of the light diffusing film can be achieved, in which the distortion of the film is suppressed and the luminance of the light exit surface of the backlight unit is made uniform.

As described above, curling can be controlled by controlling the difference in melting point between the support layer (A) and the light diffusion layer (B). In order to control the curl due to the structural difference between the front and back of the film applied in each process such as preheating, stretching, cooling, winding, etc., including the crystallinity in the thickness direction, positively generate structural differences between the front and back of the film It is preferable to apply a method of making the curl value close to zero by complementing the necessary structural difference.

Specifically, in the stretching process such as longitudinal stretching and lateral stretching, and the heat treatment process, the degree of orientation of the film front and back is independently controlled by setting the temperature or heat quantity of the film front and back to different values. By adopting conditions that achieve both physical properties, zero curl film formation is realized.

Also, as a basic requirement for stable production of curls in a low state over the entire width, it is also important to use a stretch recipe with less thickness unevenness.

More specifically, for the longitudinal curl immediately after film formation, the structural difference between the front and back of the film during longitudinal stretching is controlled, and the lateral curl is controlled by controlling the structural difference between the front and back of the film during lateral stretching and heat setting. It is preferable to create an internal strain in the opposite direction, to make it compatible with the internal strain due to the structural difference between the front and back of the film, and to suppress curling.

The thickness of the surface light diffusing polyester film of the present invention is arbitrary and is not particularly limited, but is preferably in the range of 25 to 500 μm, more preferably in the range of 75 to 350 μm.

(Manufacture of biaxially stretched film)
In the present invention, as a method for satisfying the above characteristics, for example, the following production method is preferably used.

Hereinafter, a polyethylene terephthalate copolymer (hereinafter simply abbreviated as polyester) is used as a crystalline polyester containing a copolymer component that is a raw material of the light diffusion layer (B) for a preferable method for producing a surface light diffusible polyester film of the present invention. A typical example using the pellets of (sometimes) will be described in detail.

First, as a film raw material, polyester and a thermoplastic resin incompatible with the polyester are dried by vacuum drying or hot air drying so that the moisture content is less than 100 ppm. Next, each raw material is weighed and mixed, supplied to an extruder, and melt extruded into a sheet. Further, the molten sheet is brought into close contact with a metal rotating roll (chill roll) controlled at a surface temperature of 10 to 50 ° C. by using an electrostatic application method to obtain an unstretched polyester sheet. In the present invention, among the raw materials, the incompatible additive is used as a pre-kneading master pellet in which all or part of the base polymer and the incompatible additive are previously melt-mixed using an extruder. is important.

At this time, the resin temperature up to the melting section, kneading section, polymer tube, gear pump, and filter of the extruder is controlled to 220 to 290 ° C., and the resin temperature up to the subsequent polymer tube and die is controlled to 210 to 295 ° C., This is preferable in order to suppress the generation of foreign matters such as deteriorated products.

Also, high-precision filtration is performed at any place where the molten resin is maintained at a constant temperature of 275 ° C. in order to remove foreign substances contained in the resin. As a filter medium used for high-precision filtration of molten resin, the filter medium of stainless steel sintered body is capable of removing aggregates mainly composed of Si, Ti, Sb, Ge, Cu and high melting point organic substances in the resin. Excellent and suitable. When performing high-precision filtration, if the temperature of the molten resin is lower than 275 ° C., the filtration pressure rises, so an operation such as reducing the discharge amount of the raw material resin is performed.

Furthermore, the filter particle size (initial filtration efficiency 95%) of the filter medium is preferably 20 μm or less, particularly preferably 15 μm or less. When the filter particle size (initial filtration efficiency 95%) of the filter medium exceeds 20 μm, it becomes difficult to sufficiently remove foreign matters having a size of 20 μm or more. Productivity may be reduced by performing high-precision filtration of molten resin using a filter medium having a filter particle size (initial filtration efficiency of 95%) of 20 μm or less. However, a film with less optical defects due to coarse particles may be used. It is an important process to obtain. In the present invention, high-accuracy filtration as described above can be performed by using an incompatible thermoplastic resin for the crystalline copolyester as an additive.

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

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

In this case, after an application layer is provided on the unstretched film obtained by the above method, biaxial stretching is performed. The simultaneous biaxial stretching method or the sequential biaxial stretching method may be used. However, when the sequential stretching method is performed, an easy-adhesion layer is provided on a film uniaxially stretched in the longitudinal or transverse direction, and then stretched in the orthogonal direction and biaxially stretched. I do.

The method for applying the coating layer forming coating solution to an unstretched film or a uniaxially stretched film can be selected from known arbitrary methods, such as reverse roll coating, gravure coating, kiss coating, and die coater. Method, roll brush method, spray coating method, air knife coating method, wire bar coating method, pipe doctor method, impregnation coating method, curtain coating method, and the like. These methods are applied alone or in combination.

The resin constituting the coating layer is a copolyester resin, polyurethane resin, or acrylic resin from the viewpoint of securing better adhesion to other optical functional layers in prism sheet applications and light diffusing film applications. It is preferable that at least one of these is a main component. These resins are also recommended from the viewpoint of suppressing the generation of reflected light on the surface of the light diffusion layer. In the resin constituting the coating layer, the “main component” means that at least one of the resins is contained in an amount of 50% by mass or more with respect to 100% by mass of the resin constituting the coating layer. Means.

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

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

The particles contained in the coating layer have an average particle diameter (average maximum diameter of number-based particles observed by SEM) of 0.005 to 1.0 μm, so that the transparency, handling properties, and scratch resistance of the film It is preferable from the viewpoint of securing. The upper limit of the average particle size of the particles is more preferably 0.5 μm, particularly preferably 0.2 μm, from the viewpoint of transparency. Further, the lower limit of the average particle diameter of the particles is more preferably 0.01 μm, particularly preferably 0.03 μm from the viewpoints of handling properties and scratch resistance.

In addition, the measurement of the average particle diameter of said particle | grain is performed with the following method.
Take a photograph of the particles with a scanning electron microscope (SEM), measure the maximum diameter of 300-500 particles at a magnification such that the size of one smallest particle is 2-5 mm, and calculate the average value. Average particle diameter. When determining the average particle size of the particles contained in the coating layer, the cross section of the coating film is measured using a transmission electron microscope (TEM) at a magnification such that the size of one smallest particle is 2 to 5 mm. The maximum diameter of particles existing in the cross section of the coating layer is obtained. The average particle size of the aggregated particles is measured by taking 300 to 500 cross sections of the coated layer of the coated film using an optical microscope at a magnification of 200 times and measuring the maximum diameter.

The content of the particles in the coating layer is 0.1 to 60% by mass with respect to the composition constituting the coating layer. It is preferable from the viewpoint of ensuring. The upper limit of the content of the particles is more preferably 50% by mass from the viewpoint of transparency and adhesion, and particularly preferably 40% by mass. Further, the lower limit of the content of the particles is more preferably 1% by mass, particularly preferably 0.5% by mass from the viewpoints of handling properties and scratch resistance.

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

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

It is important to perform the above biaxial stretching at a stretching ratio of 2.8 times or more in both the vertical, horizontal and both directions. In addition, the draw ratio defined by this invention is the actual draw ratio by which the film was actually extended | stretched. This stretch ratio can be grasped by writing a mass change rate per unit area before and after each stretching step and a lattice-shaped magnification marker on an unstretched film.

When the draw ratio in either the machine direction or the transverse direction is less than 2.8 times, the thickness unevenness of the resulting film is lowered, and the original excellent heat resistance and mechanical strength of the biaxially stretched film cannot be obtained. . In addition, the film thickness uniformity is significantly deteriorated. The lower limit of the preferred draw ratio in the present invention is 3.0 times, and the more preferred lower limit is 3.2 times. Moreover, the preferable upper limit of a draw ratio is 5 times. The suitable stretching temperature conditions are as described above.

Next, the present invention will be specifically described using examples and comparative examples. First, the evaluation method of the characteristic values used in the present invention is shown below.

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

(2) Calorie melting calorie, melting point and glass transition temperature It is determined using a DSC 6220 type differential scanning calorimeter manufactured by SII Nanotechnology. In a nitrogen atmosphere, the resin sample was heated and melted at 300 ° C. for 5 minutes, then rapidly cooled with liquid nitrogen, and 10 mg of the pulverized resin sample was heated at a rate of 20 ° C./min, and differential thermal analysis was performed. The amount of heat of crystal fusion is obtained by integrating the DSC curve surrounding the melting peak temperature (Tpm), extrapolation melting start temperature (Tim) and extrapolation melting end temperature (Tem) as defined in JIS-K7121-1987, Section 9.1. And asked. The melting peak temperature (Tpm) was taken as the melting point. Further, the glass transition temperature (Tg) was determined based on JIS-K7121-1987, Section 9/3.

(3) Melt viscosity The viscosity of the resin sample is in accordance with JIS K 7199 “Plastic-capillary rheometer and slit flow rheometer test method for plastic flow”, method A (capillary die) in 5.1.3. It was measured. Using a capillary die with a diameter of 1 mm and L / D = 10 in a Capillograph 1B manufactured by Toyo Seiki Co., Ltd., a dried resin sample was filled into a cylinder maintained at 270 ° C., melted for about 1 minute, and then a shear rate of 608.0 sec. ^The melt viscosity was measured under ^-1 . When a plurality of resins are used as the base polymer, the melt viscosity of the base polymer is measured by the same method as described above after thoroughly mixing a plurality of resin samples and filling the cylinder. did.

(4) Thickness unevenness of film A continuous tape-shaped sample having a length of 3 m in the transverse stretching direction and a length of 5 cm in the longitudinal stretching direction is taken up, and the thickness of the film is measured with a continuous film thickness measuring machine (manufactured by Anritsu Corporation). Record on the recorder. From the chart, the maximum value (dmax), minimum value (dmin), and average value (d) of the thickness were obtained, and the thickness unevenness (%) was calculated by the following formula. In addition, when the length of a horizontal extending direction is less than 3 m, it joins and performs. The connected part is deleted from the data analysis.
Thickness unevenness (%) = ((dmax−dmin) / d) × 100

The measurement was performed 3 times, the average value was calculated | required, and the following reference | standard evaluated.
○: Thickness unevenness is 5% or less ×: Thickness unevenness exceeds 5%

(5) Haze and total light transmittance The haze (cloudiness value) and total light transmittance of the film test piece were measured according to JIS K 7105 "Testing method for optical properties of plastics". The film test piece was placed with the longitudinal direction of the film in the vertical direction and the light diffusion layer (B) faced toward the light source, and measured using a NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd.

(6) Internal haze, all hazes, surface haze A highly transparent polyethylene terephthalate film having a haze of less than 1.0% (coating amount: 20 ± 10 g / m ² per side) is applied to both sides of the film test piece. For example, a sample sandwiched between two sheets (Toyobo Co., Ltd., A4300, thickness: 100 μm) was used as a sample for measuring internal haze. In addition, a blank sample was obtained by superposing the two highly transparent polyethylene terephthalate films with cedar oil.

Next, the haze of the internal haze measurement sample and the blank sample was measured by the method described in (5). Then, the haze value of the blank sample was subtracted from the haze value of the sample for measuring internal haze to determine the internal haze. Moreover, the haze in the film test piece single piece measured by the method of (5) was made into the total haze, the internal haze value was subtracted from the total haze value, and the surface haze was calculated | required.

(7) Image Sharpness Measured by the transmission method according to JIS K 7105 “Testing method for optical properties of plastic” image sharpness. The film test piece was measured with the film longitudinal direction as the vertical direction and the surface of the light diffusion layer (B) facing the light source. As a measuring instrument, an ICM-1T image clarity measuring instrument manufactured by Suga Test Instruments Co., Ltd. was used.

(8) Light diffusivity The light diffusivity was measured using a Goniometer Photometer GP-200 manufactured by Murakami Color Research Laboratory. A halogen lamp (12V, 50W) was used as the light source, and the light emitted from the light source was extracted as horizontal parallel light through a condenser lens, pinhole, and collimator, and then dimmed with an ND filter having a transmittance of 1%. . The light source beam stop was 10.5 mm, and the light receiving stop of the light receiver was 9.1 mm. The film test piece was set in the sample holder so that the surface of the light diffusing layer of the sample film was the light source side, the film main surface was perpendicular to the light source luminous flux, and the vertical direction of the film was up and down. The light incident on the sample film is transmitted to the opposite side of the film, reaches the light receiver, and the intensity is measured. The direction in which the light source light beam is coaxially extended is 0 degree, and the light receiver is rotated horizontally around the intersection of the optical axis of the light source light beam and the incident surface of the film, and from −80 degrees to +80 in 0.1 degree steps. The transmitted light intensity was measured in the range of degrees.
Transmitted light intensity ratio S (N) obtained by the following formula when the transmitted light intensity measured by the above method is I (0) and the transmitted light intensity of angle ± N degrees is I (N). [%] Was used as an index of light diffusivity. In the present invention, S (1) is used as a value that shows a correlation with the sharpness of the image observed through the light diffusing film.
S (1) was evaluated as 75% or more as ◯, and S (1) as less than 75% as x.
S (N) = I (N) / I (0) × 100

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

(10) Dimensional change rate Measured according to JIS C 2318-1997 5.3.4 (dimensional change).

(11) Plane orientation coefficient (ΔP)
According to JIS K 7142-1996 5.1 (Method A), the refractive index in the film longitudinal direction (nx), the refractive index in the width direction (ny), and the refractive index in the thickness direction (nz) using an Abbe refractometer with the sodium D line as the light source ) And the plane orientation coefficient (ΔP) was calculated by the following formula.
ΔP = (nx + ny) / 2−nz

(12) Curl value The film is cut into a sheet of 100 mm in the longitudinal direction and 50 mm in the width direction, heat-treated at 100 ° C. for 30 minutes under no load, and then a horizontal glass plate with the convex portion of the film facing down. The vertical distance between the glass plate and the lower end of the four corners of the rising film was measured using a ruler in units of a minimum scale of 0.5 mm, and the average value of the measured values at these four locations was determined. The same measurement was performed on the three film specimens, and the average value was taken as the curl value and evaluated according to the following criteria.
○: Curl value is 5 mm or less ×: Curl value is 5 mm or more

(13) Average slope (Δa)
With the light diffusion layer (B) of the film facing upward, the surface unevenness profile of the surface of the light diffusion layer (B) is measured using a three-dimensional shape measuring device (manufactured by Ryoka System Co., Ltd., Micromap TYPE 550, objective lens 10 times) did. From the measured profile, a cross-sectional profile was cut out along two axes perpendicular to the longitudinal direction (longitudinal direction) and lateral direction (width direction) of the film. The height (y) is measured continuously at a measurement length of 1.0 mm and an interval of 2.5 μm in each direction, and the respective heights y ₁ , y ₂ , y ₃ , and the pitch at intervals of 2.5 (μm) ,, _{y n} a (μm) was output to Excel file. The wave function of the analysis software (SX-Viewer, manufactured by Ryoka System Co., Ltd.) was used to output the protrusion height to an Excel file. Further, Δa was derived by calculating the following equation. For Δa, an average slope gradient was derived for each of the vertical direction and the horizontal direction, and values obtained by averaging the values in the vertical direction and the horizontal direction were used.
Δa = [(y ₁ −0) /2.5+ (y ₂ −y ₁ ) /2.5+··+ (y _n −y _n−1 ) /2.5] / n

(14) Luminance ratio As a lens sheet to be combined with the luminance evaluation of the obtained surface light diffusive film, a lens sheet mounted on a liquid crystal television manufactured by Sharp Corporation (Aquos LC-37GS10, manufactured in 2007) was used. Two films were overlapped so as to overlap the support layer (A) surface of the cut light diffusing film piece and the lens back surface of the lens sheet. The light diffusive film piece was placed on the turbidimeter with the light diffusion layer (B) surface facing the light source so that the vertical direction of the light diffusive film piece (longitudinal direction of film formation) was the vertical direction. The NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd. was used as the turbidimeter. The measurement method was carried out in accordance with JIS K 7105 “Testing method for optical properties of plastics”. Deriving a value obtained by dividing the parallel light transmittance obtained by measurement by the total light transmittance, and a luminance ratio (%) to a value obtained by dividing the parallel light transmittance obtained by measuring the lens sheet alone by the total light transmittance ) Was derived.

Example 1
(1) Production of crystalline homopolyester resin (M1) When the temperature of the esterification reaction can reached 200 ° C, from terephthalic acid (86.4 parts by mass) and ethylene glycol (64.4 parts by mass) The resulting slurry was added and antimony trioxide (0.017 parts by mass) and triethylamine (0.16 parts by mass) were added as catalysts while stirring. Next, the pressure was increased and the pressure esterification reaction was performed under the conditions of a gauge pressure of 3.5 kgf / cm ² and 240 ° C. Thereafter, the inside of the esterification reaction vessel was returned to normal pressure, and magnesium acetate tetrahydrate (0.071 parts by mass) and then phosphoric acid (0.014 parts by mass) were added. Further, the temperature was raised to 260 ° C. over 15 minutes, and trimethyl phosphate (0.012 parts by mass) and then sodium acetate (0.0036 parts by mass) 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. until a predetermined intrinsic viscosity was reached. went.

After completion of the polycondensation reaction, filtered with a NASRON filter with a filtration particle size of 5 μm (initial filtration efficiency: 95%), extruded into a strand from a nozzle, and preliminarily filtered (pore size: 1 μm or less) It was cooled and solidified, and cut into pellets. The obtained crystalline homopolyester resin (M1) has a crystal melting heat of 35 mJ / mg, a melting point of 256 ° C., an intrinsic viscosity of 0.56 dl / g, a melt viscosity of 91 Pa · s, an Sb content of 144 ppm, and an Mg content. The amount was 58 ppm, the P content was 40 ppm, the color L value was 56.2, and the color b value was 1.6. Further, inert particles and internally precipitated particles were not substantially contained.

(2) Production of copolymer polyester resin (M2) Intrinsic viscosity comprising 100 mol% terephthalic acid unit as the aromatic dicarboxylic acid component, 70 mol% ethylene glycol unit and 30 mol% neopentylglycol unit as the diol component Of 0.59 dl / g and a melt viscosity of 121 Pa · s were produced in accordance with the production method of (M1).

(3) Polystyrene (M3)
A polystyrene resin (PS) having a melt viscosity of 147 Pa · s was used.

(4) Preparation of coating solution (M4) Dimethyl terephthalate (95 parts by mass), dimethyl isophthalate (95 parts by mass), ethylene glycol (35 parts by mass), neopentyl glycol (145 parts by mass), zinc acetate (0.1 Parts by mass) and antimony trioxide (0.1 parts by mass) were charged into a reaction vessel, and a transesterification reaction was performed at 180 ° C. over 3 hours. Next, 5-sodium sulfoisophthalic acid (6.0 parts by mass) was added and the esterification reaction was performed at 240 ° C. over 1 hour, and then at 250 ° C. under reduced pressure (10 to 0.2 mmHg) for 2 hours. The polycondensation reaction was carried out to obtain a copolyester resin having a number average molecular weight of 19,500.

7.5 parts by mass of a 30% by mass aqueous dispersion of the obtained copolyester resin and 11.3 parts by mass of a 20% by mass aqueous solution of a self-crosslinking polyurethane resin containing an isocyanate group blocked with sodium bisulfite. The organic tin catalyst was mixed in an amount of 0.3 parts by weight, 39.8 parts by weight of water and 37.4 parts by weight of isopropyl alcohol.

Furthermore, 0.6 parts by mass of a 10% by mass aqueous solution of a fluorine-based nonionic surfactant, 2.3 parts by mass of a 20% by mass aqueous dispersion of colloidal silica (average particle size 40 nm) as particles A, and dry as particles B 0.5 parts by mass of a 3.5% by mass aqueous dispersion of method silica (average particle size 200 nm, average primary particle size 40 nm) was added. Next, the pH of the coating solution is adjusted to 6.2 with a 5% by weight aqueous sodium bicarbonate solution, and the solution is precisely filtered with a felt type polypropylene filter having a filtration particle size (initial filtration efficiency: 95%) of 10 μm. Was prepared.

(5) Production of surface light diffusing polyester film As raw materials for the light diffusing layer (B), 74 parts by mass of crystalline homopolyester (M1) dried at 135 ° C. for 6 hours under reduced pressure (1 Torr), and dried under reduced pressure at 70 ° C. for 12 hours. 23 parts by mass of (1 Torr) copolymerized polyester (M2) and 3 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2. Further, a crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours as a raw material for the support layer (A) was supplied to the extruder 1.

Supplied from Extruder 2 and Extruder 1 with a set temperature of 275 ° C. up to the melting section, kneading section, polymer pipe, gear pump and filter of each extruder and 270 ° C. set temperature of the polymer pipe after the filter. Each raw material was laminated using a two-layer merging block and melt-extruded into a sheet form from the die.

In addition, the thickness ratio of the (A) layer and the (B) layer was controlled using a gear pump of each layer so as to be 89:11. In addition, a stainless sintered body filter material (nominal filtration accuracy: 95% cut of 10 μm particles) was used for each of the filters. The temperature of the die was controlled so that the temperature of the extruded resin was 275 ° C.

The extruded resin was brought into close contact with a cooling drum having a surface temperature of 30 ° C. using an electrostatic application method and solidified by cooling to produce an unstretched film. At this time, the layer surface (A) was a surface in contact with the cooling drum. The take-up speed of the unstretched film by the cooling drum was 12 m / min.

The obtained unstretched film was heated to 79 ° C. using a preheating roll, and stretched 3.4 times in the flow direction between rolls having different peripheral speeds. At this time, the temperature of the film was monitored with an infrared radiation thermometer, and the heater temperature was controlled so that the maximum temperature of the film was 100 ° C.

After completion of the longitudinal stretching, the obtained uniaxially stretched film was cooled to 50 ° C., and then the coating liquid (M4) was applied to both surfaces of the film. The coating amount of the solution was controlled so as to be about 15 g / m ^{2 on} both sides. Thereafter, the coated surface was dried in a drying furnace.

The both ends of the uniaxially stretched film having the coating layer are gripped by clips, guided to a tenter, preheated to 120 ° C, stretched 2.5 times in the width direction at 135 ° C, and then 1.6 times in the width direction at 140 ° C. The film was stretched twice, further heat treated at 231 ° C. for 10 seconds, and subjected to a relaxation treatment of 3.3% in the width direction in the process of cooling to 60 ° C., thereby producing a surface light diffusible polyester film having a total thickness of 188 μm.

In addition, in order to measure the melting point and intrinsic viscosity of the polyester of each layer, the discharge of the (B) layer was temporarily stopped, and an unstretched film of the (A) layer alone was collected. Similarly, the discharge of the (A) layer was temporarily stopped, and an unstretched film of the (B) layer alone was collected.

(6) Properties of Film The properties of the film obtained in Example 1 are shown in Table 1. As can be seen from Table 1, the surface light diffusible polyester film obtained in the present invention has excellent heat resistance, mechanical strength and thickness accuracy inherent to the biaxially stretched film. Also, the internal haze is small and the light transmittance is high. Further, it can be seen that most of the total haze is imparted by the surface haze, and the light diffusibility is also excellent. In addition, high brightness was obtained when combined with a lens sheet.

Example 2
After stretching in the width direction, a surface light diffusible polyester film of Example 2 was produced in the same manner as shown in Example 1 except that heat treatment was performed at 222 ° C. for 10 seconds.

The characteristics of the film obtained in Example 2 are shown in Table 1. From Table 1, it can be seen that Example 2 has excellent characteristics as in Example 1.

Example 3
As raw materials for the light diffusion layer (B), 51 parts by mass of crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and copolymer polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours. 46 parts by mass and 3 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2, and the drawing speed of the unstretched film by the cooling drum was adjusted so that the film thickness after stretching was 100 μm. The thickness ratio between the (A) layer and the (B) layer was controlled to be 80:20, the coating liquid (M4) was applied only to the (A) layer, and the width direction was 2.4 at 135 ° C. After stretching twice, stretched 1.6 times in the width direction at 140 ° C, further heat treated at 223 ° C for 17 seconds, and cooled to 60 ° C, except that 1.0% relaxation treatment was performed in the width direction. In Example 1 To produce a surface light-diffusing polyester film of Example 3 having a thickness of 100μm in the same manner as the the.

The properties of the film obtained in Example 3 are shown in Table 1. From Table 1, it can be seen that Example 3 has excellent characteristics as in Example 1.

Example 4
As raw materials of the light diffusion layer (B), 69 parts by mass of crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and copolymer polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours. 21 parts by mass and 10 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2, heat treated at 232 ° C. for 17 seconds, adjusted to a thickness of 100 μm, layer (A) and ( B) A surface light diffusing polyester film of Example 4 was prepared in the same manner as shown in Example 3 except that the thickness ratio with the layer was 70:30.

The properties of the film obtained in Example 4 are shown in Table 1. From Table 1, it can be seen that Example 4 has excellent characteristics as in Example 1.

Example 5
As raw materials for the light diffusion layer (B), 55 parts by mass of crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and copolymer polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours. 38 parts by weight and 7 parts by weight of polystyrene (M3) were mixed and supplied to the extruder 2, and the same method as shown in Example 1 was performed except that the heat treatment was performed at 224 ° C. for 10 seconds. The surface light diffusing polyester film of Example 5 was produced.

The properties of the film obtained in Example 5 are shown in Table 1. From Table 1, it can be seen that Example 5 has excellent characteristics as in Example 1.

Comparative Example 1
As raw materials for the light diffusion layer (B), 58 parts by mass of crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and copolymer polyester (M2) dried under reduced pressure (1 Torr) for 12 hours at 70 ° C. 32 parts by weight and 10 parts by weight of polystyrene (M3) were mixed and supplied to the extruder 2. Heat treatment at 234 ° C. for 17 seconds and cooling to 60 ° C. 3.3% relaxation treatment in the width direction A surface light diffusible polyester film of Comparative Example 1 was prepared in the same manner as shown in Example 1 except that the above was performed.

The characteristics of the film obtained in Comparative Example 1 are shown in Table 1.

Comparative Example 2
A surface light diffusable polyester film of Comparative Example 2 was produced in the same manner as shown in Example 3 except that the film was heat treated at 233 ° C. after stretching in the width direction.

Table 1 shows the characteristics of the film obtained in Comparative Example 2.

Comparative example 3

As raw materials for the light diffusion layer (B), 63 parts by mass of crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and copolymer polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours. 34 parts by weight and 3 parts by weight of polystyrene (M3) were mixed and supplied to the extruder 2, the longitudinal draw ratio was 3.3 times, heat-treated at 240 ° C. for 17 seconds after transverse stretching, A surface light diffusive film of Comparative Example 3 was produced in the same manner as shown in Example 1 except that 1.3% relaxation treatment was performed in the width direction in the course of cooling to 60 ° C.

The surface light diffusing polyester film of the present invention can be used as a light diffusing film used in a backlight unit of a liquid crystal display, a lighting device, or the like. Moreover, it can use as a base film for prism sheets. Therefore, it is important to contribute to the industry.

Claims

A light diffusing polyester film comprising a biaxially oriented polyester film, which satisfies the following requirements (1) to (6):
(1) A support layer comprising a crystalline homopolyester or a crystalline polyester containing a copolymer component, and a copolymer component having a melting point of 235 to 255 ° C. laminated on at least one surface of the support layer by a co-extrusion method. And (2) a film surface defined by the following formula: having a light diffusing layer comprising a blended composition of 50 to 99 parts by mass of a crystalline polyester and 1 to 50 parts by mass of an additive incompatible with the polyester The orientation coefficient ΔP is 0.08 to 0.16 ΔP = (nx + ny) / 2 −nz
Here, nx, ny, and nz represent the refractive index in the longitudinal direction, the refractive index in the width direction, and the refractive index in the thickness direction, respectively.
(3) The surface haze is 15% or more. (4) The internal haze is less than the surface haze. (5) The dimensional change rate at 150 ° C. is 3% or less in both the longitudinal direction and the transverse direction, and the tensile strength is the longitudinal direction. (6) The average slope gradient (Δa) of the light diffusion layer surface is 0.03 or more.
2. The surface light diffusing polyester film according to claim 1, wherein the total light transmittance is 86% or more and the image sharpness at a comb width of 2 mm is 50% or less.
A coating layer mainly comprising at least one of a copolyester resin, a polyurethane resin, or an acrylic resin provided before completion of stretching and orientation of the film is provided on the surface of the light diffusion layer. The surface light diffusable polyester film according to claim 1.
The light diffusing polyester film has a coating layer mainly composed of at least one of a copolyester resin, a polyurethane resin, or an acrylic resin on both the light diffusing layer side and the support layer side. The surface light diffusable polyester film according to claim 1.
It has the coating layer which has at least 1 sort (s) of a copolyester resin, a polyurethane-type resin, or an acrylic resin as a main component on the opposite surface to the light-diffusion layer of the surface light diffusable polyester film of Claim 1. Surface light diffusing polyester film for prism sheet.