JP5696356B2 - Light diffusing polyester film - Google Patents

Description

  The present invention relates to a light diffusing film used for a backlight unit of a liquid crystal display, an illumination device, and the like. More specifically, the present invention relates to a light diffusible polyester film having both light diffusibility and light transmittance, no curling even at high temperature treatment, and good brightness.

  In recent years, the technological progress of liquid crystal displays has been remarkable and widely used as display devices for personal computers, televisions, mobile phones and the like. 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.

  The backlight unit includes a direct type in which a light source is installed inside the illumination surface, and an edge light type in which light is introduced from the outside of the illumination surface through a light guide plate. These backlight units are further provided with a light diffusing film, and are devised to diffuse and scatter light to make the luminance of the illumination surface uniform. Moreover, in order to improve the front luminance, a sheet having a light collecting function called a lens sheet may be used so that light transmitted through the light diffusing film is collected in the front direction as much as possible. 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 lens sheet is disposed and used on the surface side of the light diffusing film in one or two layers. In addition, in order to make the luminance unevenness and the defect of the lens sheet caused by the arrangement of the lens sheet inconspicuous (improve concealment), a light diffusing film may be arranged on the surface side of the lens 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 base film (for example, Patent Documents 1 and 2). Are used).

  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. 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.

  On the other hand, in recent years, many studies have been made to integrate a light diffusing film with another optical functional film for the purpose of reducing the number of backlight unit parts, simplifying the manufacturing process, and reducing the cost (patents). References 3, 4). However, since the light diffusibility is imparted by the light scattering material inside the substrate, there is a problem that a part of the incident light causes back scattering and the light transmittance is lowered.

  In recent years, an approach has been made to impart light diffusibility to a biaxially stretched polyester film itself having excellent heat resistance, mechanical strength, and thickness uniformity (Patent Document 5). However, characteristics that the biaxially stretched polyester film inherently have (heat resistance, mechanical strength, etc.) are impaired, or the light diffusive film should have a light diffusive film property such as light transmittance and light diffusibility. Is detrimental.

  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. Moreover, the film which laminated | stacked the crystalline polyester resin layer on the both surfaces is disclosed (refer patent documents 6-13).

  However, there is still a large difference in crystallinity between the light diffusing intermediate layer and the surface layer, and the flatness at the time of temperature change is significantly deteriorated due to slight variations in layer thickness and physical properties on the front and back. The inherent problem is that the mechanical strength decreases.

  In view of the problems as described above, the biaxially stretched polyester film inherently has excellent heat resistance and mechanical strength by using a light diffusion layer mainly composed of crystalline polyester, and mainly imparts light diffusibility by surface haze. Thus, a light diffusible polyester film having both total light transmittance and light diffusibility has been proposed (Patent Documents 14 and 15).

JP-A-6-59108 Japanese Patent No. 3698978 specification JP-A-9-281310 Japanese Patent No. 3732253 JP 2005-181648 A JP 2001-324606 A JP 2002-162508 A JP 2002-182013 A JP 2002-196113 A JP 2002-372606 A JP 2004-219438 A JP 2004-354558 A JP 2004-354558 A JP 2009-48156 A JP 2009-139684 A

  The light diffusible polyester films disclosed in Patent Documents 14 and 15 have excellent mechanical strength, light diffusibility, and high light transmittance.

  When the light diffusive film is used in combination with a lens sheet, a prism sheet, or a lens layer, not only the light diffusibility and light transmittance of the light diffusive film alone are required, but the light diffusible film and the lens sheet or The front luminance exhibited when combined with the prism sheet is required. Therefore, when applied to various usage forms of the light diffusing polyester film, the front luminance may be lowered particularly in an environment-friendly low power consumption type liquid crystal display. In the low power consumption type, the amount of irradiation necessary for the backlight is minimized to save energy. 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.

  Further, the light diffusing polyester film has a smooth surface opposite to the light diffusing layer surface, and it is possible to prepare a lens sheet by suitably providing a lens layer. When a lens layer is provided on a light-diffusing polyester film, a roll-type mold member that normally transfers the lens shape is used, and a light-diffusing polyester film in which a lens resin such as an ultraviolet curable resin is laminated is passed through while being active. The lens layer is formed by applying energy.

  However, with the improvement in productivity, the speed at which the base film passes through the roll-type mold member is increasing. Therefore, a film that can cope with processing conditions at higher temperatures (for example, 100 ° C. or more) is being demanded by improving the productivity of lens processing.

  Accordingly, an object of the present invention is to provide a light diffusible polyester film having excellent mechanical strength, light diffusibility, and high light transmittance, and excellent in high temperature processability.

  As a result of diligent studies to solve the above problems, the present inventor solves the above problems by a light diffusible polyester film having a three-layer structure, each of which has a different action and structure, laminated by a coextrusion method. This has been found and the present invention has been achieved. The light diffusable polyester film of the present invention capable of achieving the above object has the following constitution.

That is, among the present invention, the first invention is a light diffusing polyester film made of a biaxially oriented polyester film, having a light diffusing layer (B) on one side of the intermediate layer (A), and on the opposite side. It consists of a three-layer structure laminated by a coextrusion method having a smooth layer (C), and the intermediate layer (A) is made of a crystalline homopolyester or a crystalline polyester containing a copolymer component, and the light diffusion layer (B) Is composed of 50 to 99 parts by mass of a crystalline polyester containing a copolymer component having a melting point of 225 to 255 ° C. and 1 to 50 parts by mass of an additive incompatible with the polyester, and the smooth layer (C) has a melting point of It is a light diffusible polyester film which consists of crystalline polyester containing the copolymerization component which is 225-255 degreeC, and whose average inclination gradient ((DELTA) a) of a light-diffusion layer (B) surface is 0.03 or more.
Of the present invention, the second invention is the light diffusing polyester film having a surface haze of 15% or more and an internal haze of less than the surface haze.
Among the present inventions, the third invention is such that the dimensional change rate at 150 ° C. is 3% or less in both the longitudinal direction and the transverse direction, the tensile strength is 100 MPa or more in both the longitudinal direction and the transverse direction, and the tensile elongation is 100% or more. It is a light diffusing polyester film.
Among the present inventions, the fourth invention has an average height of curls at four corners of a rectangular film sample of 300 mm in the longitudinal direction and 210 mm in the width direction after heat treatment at 150 ° C. for 30 minutes in a heating oven. It is the said light diffusable polyester film which is 0 mm or less.
Among the present inventions, the fifth invention is the light diffusible polyester film having a total light transmittance of 86% or more and an image sharpness of 40% or less at a comb width of 2 mm.
Among the present inventions, the sixth invention is characterized in that at least one or more copolymer polyester resin, polyurethane resin, or acrylic resin provided on the surface of the light diffusion layer (B) before completion of stretching and orientation of the film. This light diffusible polyester film has a coating layer containing as the main component.
Of the present invention, a seventh invention is characterized in that at least a copolymer polyester resin, a polyurethane resin, or an acrylic resin is provided on both surfaces of the light diffusion layer (B) and the smooth layer (C) of the light diffusible polyester film. It is the said light diffusable polyester film which has a coating layer which has 1 or more types as a main component.
Of the present invention, the eighth invention is an application comprising at least one copolymer polyester resin, polyurethane resin, or acrylic resin as a main component on the surface of the smooth layer (C) of the light diffusing polyester film. It is the light diffusable polyester film for lens sheets which has a layer.

  The light diffusing polyester film of the present invention has excellent mechanical properties inherent to a biaxially stretched polyester film, and in addition to the effect of achieving both total light transmittance and light diffusibility, There is no curling due to heating, and furthermore, by controlling the inclination gradient of the surface unevenness of the light diffusion layer, there is an effect that high luminance can be obtained when combined with a lens sheet, a prism sheet, or a lens layer.

  The light diffusing polyester film of the present invention has a three-layer structure laminated by a coextrusion method having a light diffusing layer (B) on one side of the intermediate layer (A) and a smooth layer (C) on the opposite side. The intermediate layer (A) is made of a crystalline homopolyester or a crystalline polyester containing a copolymer component, and the light diffusion layer (B) is a crystalline polyester 50 containing a copolymer component having a melting point of 225 to 255 ° C. ~ 99 parts by mass and 1 to 50 parts by mass of an additive incompatible with the polyester, and the smooth layer (C) is made of a crystalline polyester containing a copolymer component having a melting point of 225 to 255 ° C, and is light diffusing. The average gradient (Δa) on the surface of the layer (B) is 0.03 or more. The technical meaning of adopting such a special layer structure will be described in detail below.

(1) Light diffusion layer (B)
The light diffusing polyester film of the present invention has a light diffusing layer (B) comprising a crystalline polyester containing a copolymer component and an additive incompatible with the polyester. Here, the crystalline polyester means a polyester 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). If it is a polyester in which a clear crystal melting heat peak is observed as a melting point when measured using a differential scanning calorimeter, it is included in a crystalline polyester. Thus, since the polyester which comprises the light-diffusion layer (B) of this invention has a crystal structure, the mechanical characteristics peculiar to polyester films, such as heat resistance, mechanical strength, and thickness precision, are suitably hold | maintained.

  From the viewpoint of heat resistance, mechanical strength, and thickness accuracy of the film, it is advantageous that the crystal structure is large. Therefore, the higher the melting point of the polyester resin, the better. However, when the melting point of the polyester resin is high, the stretching stress generated during stretching increases, so if there is an incompatible additive in the resin, voids (cavities) are likely to occur, and the total light transmittance is reduced. To do. Therefore, in order to suppress the generation of voids while maintaining the mechanical properties as polyester, it is desirable to control the melting point of the resin constituting the light diffusion layer (B) within a certain range. The lower limit of the melting point of the crystalline polyester containing the copolymer component constituting the light diffusion layer (B) is preferably 225 ° C, more preferably 230 ° C, and even more preferably 235 ° C. If melting | fusing point is 225 degreeC or more, desirable heat resistance, mechanical strength, and thickness precision can be exhibited suitably. Moreover, 255 degreeC is preferable, as for the upper limit of melting | fusing point of crystalline polyester containing the copolymerization component which comprises a light-diffusion layer (B), 250 degreeC is more preferable, and 245 degreeC is still more preferable. If melting | fusing point is 255 degrees C or less, generation | occurrence | production of the void in a light-diffusion layer (B) can be suppressed suitably.

  The melting point of the crystalline polyester constituting the light diffusion layer (B) can be controlled by introducing a copolymer component. By introducing the copolymer component into the polyester, it is possible to suppress the generation of cavities and achieve both high light transmittance and light diffusibility. 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.

  The light diffusing layer of the present invention contains an additive that is incompatible with polyester and exhibits suitable light diffusibility. The suitable aspect of the light-diffusion layer of this invention has the uneven | corrugated shape by an incompatible additive on the light-diffusion layer surface. 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. Therefore, it is desirable that the light diffusing film of the present invention has an internal haze less than the surface haze.

  Light diffusion in the light diffusion layer (B) is divided into scattering due to the surface structure of the film and scattering due to 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. Note that materials incompatible with the polyester that can be used in the present invention will be described later.

  The light diffusing layer in the 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 preferable 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.

  And when the mixing ratio of the said additive is less than 1 mass part, the uneven | corrugated formation capability of the film surface by an additive is insufficient, and sufficient surface light-diffusion performance is not 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.

  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 or more, more preferably 4 μm, and particularly preferably 5 μm.

  On the other hand, if the thickness of the light diffusing layer (B) is considerably larger than the particle size of the incompatible additive, it is difficult to effectively form the surface uneven structure. 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.

(2) Smooth layer (C)
It is suitable for lens processing to impart appropriate flexibility to the surface on which the lens of the film is laminated. That is, when the hardness of the lens resin is increased or the lens shape is complicated, a strong stress may be generated on the surface of the base film. Moreover, when the productivity in lens processing is improved and the base film passes through the roll-type mold member at a high speed, the base film may hardly follow the roll-type mold member. Therefore, imparting flexibility to the smooth layer (C) that imparts the lens layer is preferable for promoting stress relaxation on the film surface and improving the adhesion of the lens layer.

  The tensile elastic modulus of the smooth layer (C) of the light diffusing polyester film of the present invention is preferably less than 4.0 MPa, more preferably 3.8 MPa or less, and even more preferably 3.7 MPa or less. preferable. When the tensile elastic modulus of the smooth layer (C) is not more than the above upper limit, flexibility suitable for lens processing is exhibited. On the other hand, the tensile elastic modulus of the smooth layer (C) is preferably 2.0 MPa or more from the viewpoint of maintaining the mechanical strength of the film surface.

  In order to control the tensile elastic modulus of the smooth layer (C) within the above range, it is desirable to control the melting point of the resin constituting the smooth layer (C). Therefore, it is preferable that the light diffusable polyester film of this invention has a smooth layer (C) which consists of crystalline polyester containing the copolymerization component whose melting | fusing point is 225-255 degreeC. From the viewpoint of the mechanical properties of the film as described above, the higher the melting point of the polyester resin, the better. However, in order to improve the flexibility of the film surface to which the lens layer is applied, it is desirable to control the melting point of the resin constituting the smooth layer (C) within a certain range. 225 degreeC is preferable, as for the minimum of melting | fusing point of crystalline polyester containing the copolymerization component which comprises a smooth layer (C), 230 degreeC is more preferable, 235 degreeC is further more preferable, and 240 degreeC is further more preferable. If melting | fusing point is 225 degreeC or more, the heat resistance, mechanical strength, and thickness accuracy which are desirable as a base film can be exhibited suitably. Moreover, 255 degreeC is preferable and, as for the upper limit of melting | fusing point of crystalline polyester containing the copolymerization component which comprises a smooth layer (C), 250 degreeC is more preferable. If melting | fusing point is 255 degrees C or less, a smooth layer (C) will exhibit the softness | flexibility suitable for provision of a lens layer, and there can show suitable lens adhesiveness.

  The melting point of the crystalline polyester constituting the smooth layer (C) can be controlled by introducing a copolymer component. 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 the flexibility of the film surface is improved during lens application and lens processing is facilitated. 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.

  The surface of the smooth layer (C) is preferably a smooth surface so that processing such as lens processing or hard coat processing can be easily performed. Specifically, the three-dimensional surface roughness (SRa) of the smooth layer (C) is preferably 0.02 μm or less, and more preferably 0.01 μm or less.

  Furthermore, since the light diffusion film of the present invention has a resin structure with a low melting point and a relatively low refractive index in both outermost layers, the refractive index changes stepwise until reaching the central layer. For this reason, total reflection is unlikely to occur when light enters and exits, and this is preferable in view of high luminance in combination with the surface irregularities described later.

(3) Intermediate layer (A)
The light-diffusing polyester film of the present invention has a three-layer structure as described above, and the two layers of the surface layer are each made of a crystalline polyester containing a copolymer component. Further, the light diffusing polyester film of the present invention has an intermediate layer (A) made of crystalline homopolyester or crystalline polyester containing a copolymer component. The intermediate layer (A) mainly has an action of imparting mechanical strength of the film. That is, by providing the lower layer as a base film with the intermediate layer (A), the role of the smooth layer (C) can be shared, and both flexibility and lower back can be achieved.

  The crystalline homopolyester / crystalline polyester constituting the intermediate layer (A) preferably has a high melting point from the viewpoint of mechanical properties. Specifically, the lower limit of the melting point is preferably 250 ° C. or higher, and more preferably 255 ° C. or higher. When the melting point is 250 ° C. or higher, mechanical properties suitable as a support layer can be obtained. The upper limit of the melting point is considered to be about 260 ° C. due to the properties of polyester.

(4) Layer structure When a rise in heating temperature accompanying the productivity improvement of lens processing or a high-temperature heat treatment (for example, 100 ° C. or higher) in post-processing is assumed, curling occurs in the light diffusing polyester film, and the optical design There may be a problem in terms of workability. Accordingly, in order to suppress curling due to heating, the present inventors focused on the thermal stress of each layer constituting the light diffusing polyester film, and laminated structure of the light diffusing polyester film, the linear expansion coefficient and the elastic modulus of each layer. Considered to adjust. As a result of investigation, each layer, particularly the outermost layer, the light diffusion layer (B) and the smooth layer, are controlled by controlling the melting point and the layer thickness of the intermediate layer (A), the light diffusion layer (B), and the smooth layer (C). By causing the linear expansion coefficient and the elastic modulus in (C) to correspond to each other, curling can be suitably suppressed not only in the processing characteristics of the laminated body but also in the case where high temperature processing is performed.

  That is, the curl amount of the laminate having a bimetallic structure is determined by the heating temperature, the elastic modulus of each layer, and the linear expansion coefficient. The above-mentioned physical property values act as a force that contracts or expands (thermal stress). When the thermal stress is uneven in the thickness direction, the thermal stress is deformed from the planar shape and is a force that takes a stable structure by warping. work. Therefore, curling can be controlled by controlling the thermal stress in the thickness direction within an appropriate range. Therefore, the present inventors tried to optimize the thermal stress of each layer obtained by heating. That is, the laminated body was divided into two in the thickness direction, and the laminated structure was designed so that the upper half thermal stress and the lower half thermal stress were constant at an assumed temperature.

ここで、σは熱応力，ｎは積層フィルムの分割数，ｌ_ｉは分割した層の中心と積層フィルムの中心線との距離，σ_ｉは分割した層が有する熱応力，Ｅ_ｉは分割した層の弾性率，Ｔ_１は加熱前の温度，Ｔ_２は加熱温度，α_ｉは分割した層をＴ_１からＴ_２まで温度変化したときの線膨張係数である。 The thermal stress can be derived from the equations (1) and (2) based on linear elasticity theory in composite material mechanics.

Here, σ is the thermal stress, n is the number of divisions of the laminated film, l _i is the distance between the center of the divided layer and the center line of the laminated film, σ _i is the thermal stress of the divided layer, and E _i is divided The elastic modulus of the layer, T ₁ is the temperature before heating, T ₂ is the heating temperature, and α _i is the linear expansion coefficient when the temperature of the divided layer is changed from T ₁ to T ₂ .

  Further, in the case of a three-layer structure, it is necessary to consider not only the thermal stress but also the distance between the center line and each layer and the degree of stress generation at that distance point. That is, by converting the force into a moment, it becomes possible to cancel the difference due to the layered configuration. Therefore, as a method for calculating the moment, it is desirable to further subdivide the laminated structure so that the upper half of the moment total value corresponds to the lower half of the moment total value. As the subdivision unit, the minimum unit of the thickness of each layer can be adopted. For example, in the case of a 100 μm film in which three layers of 10 μm, 50 μm and 40 μm are laminated, the minimum unit of the three layers is 10 μm, so it is divided into 10 μm × 10 layers and multiplied by the thermal stress and the distance to the center line. By adding together, the moment difference between the upper half and the lower half can be obtained. The difference in moment between the upper half and the lower half thus obtained is the thermal stress. In order to suppress the curling of the light diffusing polyester film of the present invention during high temperature heating, it is preferable to reduce the thermal stress. Specifically, the thermal stress at 150 ° C. is preferably 0.7 N / m or less, more preferably 0.5 N / m or less, still more preferably 0.2 N / m or less, and particularly preferably 0 N / m. If the thermal stress at 150 ° C. is within the above range, curling by heat treatment at 150 ° C. for 30 minutes can be suitably 3.0 mm or less.

In order to reduce the moment difference and control the curl as described above, the linear expansion coefficient and tensile elastic modulus between the light diffusion layer (B) and the smooth layer (C) which are the outermost layers should be adjusted to appropriate conditions. Is preferred. Specifically, the difference in coefficient of linear expansion between the light diffusion layer (B) and the smooth layer (C) is preferably 1.2 × 10 ⁻⁵ / ° C. or less, and 1.0 × 10 ⁻⁵ / ° C. or less. More preferred is 0.5 × 10 ⁻⁵ / ° C. or less. The difference in tensile elastic modulus between the light diffusion layer (B) and the smooth layer (C) is preferably 1.0 MPa or less, and more preferably 0.5 MPa or less. The linear expansion coefficient and the tensile elastic modulus are correlated with the melting point as long as the polyester resin is used. Therefore, by controlling the melting point of each layer within the aforementioned range, curl control during heating can be suitably performed.

  That is, in order to highly control the occurrence of curling in the high temperature treatment, it is desirable to highly control the melting point of the light diffusion layer (B) and the melting point of the smooth layer (C). As described above, since a suitable melting point range is set for the light diffusion layer (B) in terms of optical characteristics, the smoothing layer (C) has a melting point corresponding to the melting point of the light diffusion layer (B). It is desirable to be made of a crystalline polyester containing a copolymer component at 225 to 255 ° C. When the melting point of the smooth layer (C) exceeds the upper limit, curling tends to occur on the smooth layer (C) side, and when the melting point of the smooth layer (C) is less than the lower limit, the light diffusion layer (B) side Curling tends to occur. The lower limit of the melting point of the crystalline polyester containing the copolymer component constituting the smooth layer (C) is more preferably 230 ° C, further preferably 235 ° C, and further more preferably 240 ° C. The upper limit of the melting point of the light diffusion layer is more preferably 250 ° C. In order to control the melting point of the crystalline polyester constituting the smooth layer (C), it can be controlled by the addition amount of the copolymerization component as described above. Further, the difference between the melting point of the light diffusion layer (B) and the melting point of the smooth layer (C) is preferably 10 ° C. or less, more preferably 8 ° C. or less, and further preferably 5 ° C. or less. When the melting point difference is 10 ° C. or less, the occurrence of curling during heating can be more suitably suppressed.

  Furthermore, in order to more suitably control curling due to heating, it is also preferable to control the thickness of each layer. When 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. On the other hand, when the ratio of the light diffusion layer (B) to the entire film thickness is increased, the total light transmittance may be lowered. 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%.

  On the other hand, with respect to the smooth layer (C), it is preferable to control the thickness of the smooth layer (C) corresponding to the light diffusion layer (B) in terms of controlling curling due to heating. Specifically, the thickness of the smooth layer (C) is preferably in the range of 5 to 50% of the total thickness, more preferably 10 to 40%, and further preferably 15 to 30%. Is preferred. When the thickness of the smooth layer (C) is within the above range, curling can be reduced more preferably.

  The light diffusing polyester film of the present invention preferably has a curl of 3.0 mm or less by heat treatment at 150 ° C. for 30 minutes. The curl is more preferably 2 mm or less, and further preferably 1.0 mm or less. When the curl measured under the above conditions is 3 mm or more, curl occurs in the film during post-processing at a high temperature, and therefore it may not be possible to maintain a high optical design when applying the lens layer.

(5) Uneven surface shape 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 captured and condensed by the lens sheet, prism sheet, and lens layer is reduced, so that the luminance of transmitted light emitted to the front surface is lowered. 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 preferable to achieve a high degree of compatibility between the luminance when the lens sheet or the lens layer is combined and the diffusibility of the light diffusion film alone.

  As a result of diligent research on a method for obtaining a film having an optimal optical design that matches the light collection angle of the lens sheet, the inventor of the present application obtains the front luminance on the fine slope gradient of the uneven structure on the surface of the light diffusion layer. The significance in optical design was found. That is, the present inventor controls the average inclination gradient (Δa) on the surface of the light diffusing layer so as to achieve an excellent front luminance when the diffusion distribution angle of the light diffusion is combined with the lens sheet. Has led to the realization of a light diffusive film with

  As described above, the surface of the light diffusion layer (B) has an uneven surface structure resulting from an incompatible 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 gradient (Δa) is obtained from the surface unevenness profile observed on the micromap. The height (y) of the surface unevenness profile was measured for each 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). The product was defined as a slope, and the longitudinal direction (the longitudinal direction of the film) was measured over a predetermined length in two directions orthogonal to the transverse direction (the width direction of the film), and the average was obtained 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 inclination 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 inclination gradient (Δa) on the surface of the light diffusing layer 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 sufficient brightness when combined with the lens film even at a low irradiation amount. it can. 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. If Δa exceeds 0.10, depending on the lens sheet used, back reflection due to in-plane reflection may occur due to optical design, and the front luminance may not be improved.

  The composition having an incompatible additive constituting the light diffusion layer (B) contributes to effective formation of unevenness in the stretching step. This is considered to be due to the incompatible additive being pushed out by the stretching stress generated inside the film in the stretching process to form an effective uneven structure.

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

  Therefore, as a method of 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 (B) and the heat treatment temperature is reduced, the light diffusion layer is softened in the heat treatment step, and as a result, surface irregularities having an average gradient (Δa) excellent in luminance. No structure is formed. 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 (B) is increased, voids generated around the incompatible resin contained in the light diffusion layer (B) 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.

Furthermore, the structure for obtaining the 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 is an aromatic dicarboxylic acid or ester 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.

  Typical 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.

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

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

  Among these, neopentyl glycol and 1,4-cyclohexanedimethanol are particularly preferable. Furthermore, in the present invention, it is a more preferable 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 esthetics 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 in 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 manufacturing the said 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 light diffusing polyester film of the present invention, the copolymer polyester may be used as a film raw material as it is, or a copolymer polyester having a large amount of copolymer components is blended with a homopolyester (for example, polyethylene terephthalate) to produce a copolymer. 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 types of crystalline polyesters are melt-mixed and a third component (copolymerization component) is introduced into the main chain by utilizing both ester exchange reactions. 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 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 which comprises the said intermediate | middle layer (A) does not contain particle | grains substantially. Moreover, it is preferable that the crystalline copolyester constituting the light diffusion layer (B) 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.

  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. Further, when crystalline polyester is used as the structure of the light diffusion layer (B), if the intrinsic viscosity of the crystalline polyester is increased, the shearing force during melting and stirring is increased. (B) An effective dispersion diameter may not be obtained to such an extent that a good uneven structure is imparted to the surface. Therefore, the upper limit of the intrinsic viscosity of the crystalline polyester is preferably 0.61 dl / g, 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.

(Additive <Surface unevenness 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. The additive is not particularly limited as long as it is a material incompatible with polyester, and the following materials are preferably used.

(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 the thermoplastic resin incompatible with the 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 these, 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 further 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.

  When the present inventors have the same melt viscosity difference between the crystalline polyester constituting the light diffusion layer (B) and the incompatible thermoplastic resin, the two components are easily dispersed, and the thermoplastic resin is There is a case where the fine particles are formed and 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 thermoplastic resin 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 more easily exhibits light diffusibility.

(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 diameter of the non-melting polymer particles is preferably 0.1 to 50 μm. The lower limit of the average particle size of the non-melting polymer particles is more preferably 0.5 μm, and particularly preferably 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 size of the non-melting polymer particles is more preferably 30 μm, and particularly preferably 20 μm. When the average particle diameter of the non-melting polymer particles exceeds 50 μm, the film strength and the total light transmittance are likely to decrease. The non-melting polymer particles are preferably particles having a sharp particle size distribution as much as possible.

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

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

(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.

  As for the average particle diameter of the said inorganic particle, 0.1-50 micrometers is preferable normally. 0.5-30 micrometers is more preferable and 1-20 micrometers is still 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. It is preferable that the particle size distribution of the inorganic particles is as sharp as possible. When it is 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 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 for this invention may use 1 type in said 3 types, and may use 2 or more types together.

[Characteristics of light diffusing polyester film]
(Plane orientation coefficient)
The light diffusing polyester film of the present invention preferably 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.

  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 may increase. For this reason, internal scattering (internal haze) increases, and the total light transmittance may decrease.

  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, etc. are good, and the occurrence of heating curl is suppressed.

  Although the method for controlling the plane orientation coefficient within the above range is arbitrary, it can be controlled, for example, by adjusting the ratio of the copolymer component in the crystalline polyester containing the copolymer component. If the ratio of the copolymerization component in the light diffusion layer (B) or the smoothing layer (C) 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.

(Optical characteristics)
Next, in the present invention, it is preferable 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 light diffusing polyester film of the present invention is preferably 15% or more, and a more preferable lower limit is 20%. If the surface haze is 15% or more, an effective diffusion effect is exerted on the printed pattern of the light guide plate and the lamp image of the cold cathode tube, and light diffusion performance effective as a light diffusing film is easily 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 80% or less, the internal haze may be suppressed and the total light transmittance may be increased.

  The internal haze is preferably 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 light diffusing polyester film of the present invention desirably 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. The 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 of measuring in accordance with JIS K 7105 “Testing method for optical properties of plastic” 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 light diffusing 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%. Note that the smaller the image definition, the better. However, if the image definition is to be 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%.

  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.

  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.

  Furthermore, the lower limit of the tensile elongation of the film is preferably 100%, more preferably 120%, and particularly preferably 140%. When the tensile elongation is 100% or more, flexibility is imparted to the film, and problems such as cracking, tearing, breaking, and tearing are less likely to occur in the winding and cutting process even after lens coating.

  Moreover, it is preferable that the thickness diffuseness of the light diffusable polyester film of this invention is 5.0% or less. When the thickness unevenness of the film is 5.0% or less, when the film is wound on a roll, it is difficult to cause wrinkles and bumps, and flatness is maintained. As a result, the luminance of the light exit surface of the backlight unit becomes uniform, and the original purpose of the light diffusing film can be achieved.

  Moreover, the thickness of the light diffusable polyester film of this invention is arbitrary, Although it does not restrict | limit, It is preferable that it is the range of 25-500 micrometers, More preferably, it is the range of 75-350 micrometers.

(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.

  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. Further, 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 the total light ray transmittance and the 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 light diffusible polyester film of the present invention, in order to achieve both excellent mechanical properties and total light transmittance and light diffusibility, the film forming conditions according to the resin properties and required properties, particularly the temperature during stretching. It is desirable to appropriately control the temperature of this. When the light diffusing polyester film of the present invention is produced by stretching a polyester resin, the temperature during transverse stretching is preferably in the temperature range of 120 ° C to 160 ° C.

  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). 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 each dried by vacuum drying or hot air drying so that the moisture content is less than 100 ppm. Next, each raw material is weighed and mixed, supplied to an extruder, and melt extruded into a sheet. 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. using an electrostatic application method to obtain an unstretched PET 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, it is possible to control the resin temperature up to the melting part, kneading part, polymer pipe, gear pump, filter of the extruder to 220 to 290 ° C., and the polymer temperature to the subsequent polymer pipe and die to 210 to 295 ° C. This is preferable in order to suppress the generation of foreign matters such as deteriorated products.

  In addition, high-precision filtration is performed at any place where the molten resin is maintained at 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.

  Further, the filter particle size (initial filtration efficiency 95%) of the filter medium is preferably 20 μm or less, particularly preferably 15 μm or less. When the filter particle size (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 co-extrusion and laminate the light diffusion layer (B), the intermediate layer (A), and the smooth layer (C), the raw materials of each layer are extruded using two or more extruders, and a multi-layer feed block (for example, square The layers are joined using a joining block having a joining part), extruded from a slit-like die into a sheet, and cooled and solidified on a casting roll to produce an unstretched film. Alternatively, a multi-manifold die may be used instead of the multilayer feed block.

Moreover, in the 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 (B), generation of reflected light on the film surface can be suppressed and the total light transmittance can be further increased. Further, when a coating layer is provided on the smooth layer (C) and lens sheet processing or hard coating processing is performed on the surface of the coating layer, 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 ensuring better adhesion to other optical functional layers in lens 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 particles are not contained in the intermediate layer (A) or are contained in such a small amount 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 inorganic particles such as calcium carbonate, calcium phosphate, silica, kaolin, talc, titanium dioxide, alumina, barium sulfate, calcium fluoride, lithium fluoride, zeolite, and molybdenum sulfide; crosslinked polymer particles; calcium oxalate And organic particles. Silica is particularly preferable when the coating layer is formed mainly of the copolymer polyester resin. Since silica has a relatively close refractive index to that of polyester, it is most preferable in that 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, which means transparency of the film, handling properties, and scratch resistance. 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. Moreover, when calculating | requiring the average particle diameter of the particle | grains contained in a coating layer, using a transmission electron microscope (TEM), the cross section of a coating film with the magnification | multiplying_factor which makes the size of one smallest particle | grain become 2-5 mm The maximum diameter of particles existing in the cross section of the coating layer is obtained. The average particle diameter of the particles composed of aggregates is obtained by photographing 300 to 500 cross sections of the coating layer of the coating film using an optical microscope at a magnification of 200 times and measuring the maximum diameter.

  The content of the particles in the coating layer is 0.1 to 60% by mass with respect to the composition constituting the coating layer. The transparency, adhesion, handling properties, and scratch resistance of the laminated optical film are as follows. It is preferable from the viewpoint of ensuring. The upper limit of the content of particles is more preferably 50% by mass, particularly preferably 40% by mass, from the viewpoints of transparency and adhesion. Further, the lower limit of the content of the particles is more preferably 1% by mass, 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 that the above biaxial stretching is performed at a stretching ratio of 2.8 times or more in both the vertical, horizontal and both directions. In addition, the draw ratio defined by this invention is the actual draw ratio by which the film was actually extended | stretched. This stretch ratio can be grasped by writing a mass change rate per unit area before and after each stretching step and a lattice-shaped magnification marker on an unstretched film.

  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.

  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. Further, 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 the total light ray transmittance and the light diffusibility. However, when the stretching temperature is increased, the thickness variation of the film increases, resulting in thickness spots and the like, and it may be difficult to obtain the original mechanical characteristics of the film. In the 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 resin properties and required properties, particularly the temperature during stretching. It is desirable to appropriately control the temperature of this. 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.

  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 method]
(1) Intrinsic viscosity Based on JIS K7367-5, it measured at 30 degreeC, using the mixed solvent of phenol (60 mass%) and 1,1,2,2-tetrachloroethane (40 mass%) as a solvent.

(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 that surrounds the melting peak temperature (Tpm), extrapolation melting start temperature (Tim) and extrapolation melting end temperature (Tem) defined in JIS-K7121-1987, item 9/1. And asked. The melting peak temperature (Tpm) was taken as the melting point. Furthermore, based on JIS-K7121-1987, 9.3, the glass transition temperature (Tg) was calculated | required.

(3) Melt viscosity The viscosity of the resin sample is in accordance with JIS K 7199 “Plastic-capillary rheometer and slit die rheometer plastic flow characteristic test method”, 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), the minimum value (dmin), and the average value (d) of the thickness were obtained, and the thickness unevenness (%) was calculated by the following formula. In addition, 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) Smooth layer (C) surface three-dimensional surface roughness (SRa)
Using the stylus type three-dimensional roughness meter (SE-3AK, manufactured by Kosaka Laboratory Ltd.), the surface of the smooth layer (C) is placed in the longitudinal direction of the film under the conditions of a needle radius of 2 μm and a load of 30 mg. Measured at a cut-off value of 0.25 mm, a measurement length of 1 mm, a needle feed rate of 0.1 mm / second, and divided into 500 points at a 2 μm pitch, and the height of each point was measured with a three-dimensional roughness analyzer (SPA- 11). The same operation was performed 150 times continuously at intervals of 2 μm in the width direction of the film, that is, over 0.3 mm in the width direction of the film, and the data was taken into the analyzer. Next, the center plane average roughness (SRa) was determined using an analyzer.

(6) 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 installed with the film longitudinal direction in the vertical direction and the light diffusion layer (B) side facing the light source, and the measurement was performed using a NDH-300A type turbidimeter manufactured by Nippon Denshoku Industries Co., Ltd.

(7) Internal haze, total haze, surface haze Highly transparent polyethylene terephthalate film with a haze of less than 1.0% by applying cedar oil to both sides of the film test piece (coating amount: 20 ± 10 g / m ² per side) 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.

  Subsequently, the internal haze measurement sample and the haze of the blank sample were measured by the method described in (6). 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. Further, the haze of the film test piece alone measured by the method described in (6) was defined as the total haze, and the internal haze value was subtracted from the total haze value to determine the surface haze.

(8) Image Sharpness Measured by the transmission method in accordance with 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.

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

(10) Linear expansion coefficient A film sample of each layer alone was collected. The linear expansion coefficient of each layer was measured in a measurement temperature range of 25 to 200 ° C. using a thermomechanical analyzer (TMA SS-6100) manufactured by Seiko in accordance with JIS K 7197.

(11) Tensile modulus A film sample of each layer alone was collected. The tensile elastic modulus of each layer was measured according to JIS K 7161 using a Toyo Baldwin strong elongation analyzer (TMI RTM-100).

ここで、σは熱応力，ｎは積層フィルムの分割数，ｌ_ｉは分割した層の中心と積層フィルムの中心線との距離，σ_ｉは分割した層が有する熱応力，Ｅ_ｉは分割した層の弾性率，Ｔ_１は加熱前の温度，Ｔ_２は加熱温度，α_ｉは分割した層をＴ_１からＴ_２まで温度変化したときの線膨張係数である。計算においては、下記の数値を用いた。まず、Ｔ_１とＴ_２については、カール測定方法と対応させるため、Ｔ_１を２５℃，Ｔ_２を１５０℃とした。さらに、Ｅ_ｉとα_ｉは、（９），（１０）で求めた値を使用した。ｎは、３層を細分化したときの最小単位数をもとに決定した。なお、ｌ_ｉは中心線より上をプラス，下をマイナスとした。 (12) Thermal stress Derived using the following equations (1) and (2).

Here, σ is the thermal stress, n is the number of divisions of the laminated film, l _i is the distance between the center of the divided layer and the center line of the laminated film, σ _i is the thermal stress of the divided layer, and E _i is divided The elastic modulus of the layer, T ₁ is the temperature before heating, T ₂ is the heating temperature, and α _i is the linear expansion coefficient when the temperature of the divided layer is changed from T ₁ to T ₂ . In the calculation, the following numerical values were used. First, for T ₁ and T ₂ , T ₁ was set to 25 ° C. and T ₂ was set to 150 ° C. in order to correspond to the curl measurement method. Further, E _i and α _i used the values obtained in (9) and (10). n was determined based on the minimum number of units when the three layers were subdivided. Note that l _i is positive above the center line and negative below.

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

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

(15) Curl value Curling during high temperature heating is measured by the following methods (a) to (f).
(A) A rectangular film sample having a length of 300 mm in the film formation and 210 mm in the width direction is cut out.
(B) The sample is placed on a flat mount with the light diffusion layer (B) side up, and is placed on a shelf plate of a heating oven adjusted to 150 ° C., and heat-treated for 30 minutes.
(C) After the heat treatment, the sample is taken out together with the mount and allowed to stand at room temperature for 30 minutes. Here, the room temperature condition is desirably a condition controlled at a temperature of 23 ± 2 ° C. and a humidity of 65 ± 5%.
(D) The sample left for 30 minutes is placed on a horizontal glass plate with the light diffusion layer (B) facing upward, and the height of curls at the four corners of the sample (height in the vertical direction from the horizontal plane) (0.5 mm scale) and visually measure to 1/10 of the minimum scale. The heights of the curls at the four corners of all the samples are measured, and the average of the heights of the curls at the four corners is obtained to obtain the high temperature curl of the sample.
(E) On the other hand, when the curl height of the sample after standing at room temperature after heating is 0 mm, or when the cross section of the sample (one side of the rectangle) is M-shaped, The height of curls at the four corners is measured with the upper and lower surfaces of the sample opposite (with the light diffusion layer (B) facing down), and the average of the heights of the curls at the four corners is obtained and used as the high-temperature curl of this sample.
(F) The value of the high-temperature curl measured as described above is given a sign to clearly indicate the direction of the curl. That is, if the curl has a concave portion on the light diffusion layer (B) side, the value is given a + sign. On the other hand, when the curl has a concave portion on the intermediate layer (A) side, the value is given a minus sign.

(16) Lens adhesion After an ultraviolet curable resin composition was injected and spread on a transfer roll (diameter 300 mm) having a lens shape with a prism apex angle of 65 ° and a pitch of 50 μm, and coated with a film at a speed of 2 m / min, A high-pressure mercury lamp is irradiated from the film side so that ultraviolet light having a wavelength of 200 to 600 nm is 0.8 J / cm ^2, and the smooth layer (C) surface of the light diffusing polyester film (Comparative Example 2 is B layer, Comparative Example) A lens mold was formed on layer 3). The case where the lens array remained observed in the lens mold was indicated as x, and the case where the lens array was not observed was indicated as ◯.

(17) Average slope (△ a)
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. A cross-sectional profile was cut out from the measured profile along two orthogonal axes in 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 intervals of 2.5 (μm) are measured. ,, _{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, average slopes in the vertical and horizontal directions were derived and averaged in the vertical and horizontal directions.
Δa = [(y ₁ −0) /2.5+ (y ₂ −y ₁ ) /2.5 +. + (Y _n −y _n−1 ) /2.5] / n

(18) Luminance ratio As a lens sheet combined with the luminance evaluation of the obtained surface light diffusion 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 intermediate 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. Furthermore, it heated up to 260 degreeC over 15 minutes, and phosphoric acid (0.012 mass part) and then sodium acetate (0.0036 mass part) 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) had a heat of crystal fusion of 35 mJ / mg, a melting point of 256 ° C., an intrinsic viscosity of 0.56 dl / g, and a melt viscosity of 91 Pa · s. 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 liquid (M4) Transesterification reaction and polycondensation reaction were carried out by a conventional method to obtain 46 mol% terephthalic acid, 46 mol% isophthalic acid and 5 mol as dicarboxylic acid components (relative to the total dicarboxylic acid components). -Preparation of water-dispersible sulfonic acid metal base-containing copolymer polyester resin having a composition of 8 mol% sodium sulfonatoisophthalate, 50 mol% ethylene glycol and 50 mol% neopentyl glycol (relative to the total glycol component) as the glycol component did. Next, 51.4 parts by mass of water, 38 parts by mass of isopropyl alcohol, 5 parts by mass of n-butyl cellosolve, 0.06 parts by mass of nonionic surfactant were mixed and then heated and stirred. After adding 5 parts by mass of a water-dispersible sulfonic acid metal base-containing copolymer polyester resin and continuing to stir until the resin is no longer agglomerated, the resin water dispersion is cooled to room temperature to obtain a solid content concentration of 5.0% by mass. A uniform water-dispersible copolymerized polyester resin liquid was obtained.

  Furthermore, after dispersing 3 parts by mass of aggregated silica particles (Silicia 310, manufactured by Fuji Silysia Co., Ltd.) in 50 parts by mass of water, 99.46 parts by mass of the water-dispersible copolyester resin solution was mixed with 99.46 parts by mass of Silicia 310. 0.54 parts by mass of the aqueous dispersion was added, and 20 parts by mass of water was added with stirring to obtain a coating liquid (M4).

(5) Production of 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 intermediate layer (A) was supplied to the extruder 1. Further, 74 parts by mass of crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours as a raw material for the smooth layer (C), and copolymer polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours 23 parts by mass were supplied to the extruder 3.

  Extruder 2, Extruder 3, and Extruder are set to a melt temperature, a kneading section, a polymer tube, a gear pump, a filter at a set temperature of 275 ° C., and a polymer tube after the filter at a set temperature of 270 ° C. Each raw material supplied from 1 was laminated using a three-layer merging block, and melt-extruded into a sheet form from a die.

  In addition, the thickness ratio of (B) layer, (A) layer, and (C) layer was controlled using the gear pump of each layer so that it might become 11: 78: 1. 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 prepare an unstretched film. At this time, the (C) layer surface 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 amount of solution applied was controlled so that the final film thickness was 0.08 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 light-diffusing polyester film having a total thickness of 188 μm.

  In order to measure the melting point and intrinsic viscosity of the polyester of each layer, the discharge of the (B) layer and (C) layer was temporarily stopped, and an unstretched film of the (A) layer alone was collected. Similarly, unstretched films of the (B) layer and (C) layer alone were 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 light diffusing polyester film obtained in the present invention has the 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, almost no curling was observed even during high-temperature treatment, and high brightness was obtained when combined with a lens sheet.

Example 2
As raw materials for the smooth layer (C), 87 parts by mass of crystalline homopolyester (M1) dried under reduced pressure (1 Torr) at 135 ° C. for 6 hours, and copolymer polyester (M2) 13 dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours. The mass part was mixed and supplied to the extruder 3, the thickness ratio of the (B) layer, the (A) layer, and the (C) layer was controlled to be 11:67:22, in the width direction. After stretching, a light diffusing polyester film of Example 2 was prepared in the same manner as shown in Example 1 except that the film was heat treated 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. As a raw material for the smooth layer (C), a crystalline homopolyester (1 Torr) dried at 135 ° C. under reduced pressure for 6 hours (1 Torr) M1) 53 parts by mass and 47 parts by mass of copolyester (M2) dried at 70 ° C. for 12 hours under reduced pressure (1 Torr) were mixed and supplied to the extruder 3 so that the film thickness after stretching was 100 μm. In addition, the take-up speed of the unstretched film by the cooling drum was adjusted, and the thickness ratio of the (B) layer, the (A) layer, and the (C) layer was controlled to be 22:56:22. The coating solution (M4) was applied only to the layer (C), and was stretched 2.4 times in the width direction at 135 ° C., then 1.6 times in the width direction at 140 ° C., and further 17 at 223 ° C. The light diffusion of Example 3 having a thickness of 100 μm was performed in the same manner as shown in Example 1 except that 1.0% relaxation treatment was performed in the width direction in the process of heat treatment for 2 seconds and cooling to 60 ° C. A conductive polyester film was prepared.

  The characteristics of the film obtained in Example 3 are shown in Table 1. From Table 1, it can be seen that the present 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. As a raw material for the smooth layer (C), a crystalline homopolyester (1 Torr) dried at 135 ° C. for 6 hours under reduced pressure (1 Torr) M1) 67 parts by mass and 33 parts by mass of copolyester (M2) dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours were supplied to the extruder 3, heat treated at 232 ° C. for 17 seconds, thickness The same method as shown in Example 3 except that the thickness was adjusted to 100 μm and the thickness ratio of the (B) layer, the (A) layer, and the (C) layer was 11: 84: 7. It created a light-diffusing polyester film of Example 4 Te.

  The characteristics 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 mass and 7 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2. As a raw material for the smooth layer (C), a crystalline homopolyester (1 Torr) dried at 135 ° C. for 6 hours under reduced pressure (1 Torr) M1) 60 parts by weight and 40 parts by weight of copolymerized polyester (M2) dried under reduced pressure (1 Torr) at 70 ° C. for 12 hours were mixed and supplied to the extruder 3 except that heat treatment was performed at 224 ° C. for 10 seconds. The light diffusing polyester film of Example 5 was prepared by the same method as shown in Example 1.

  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), 74 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. 23 parts by mass and 3 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2. As a raw material for the smooth layer (C), a crystalline homopolyester (1 Torr) dried at 135 ° C. for 6 hours under reduced pressure (1 Torr) M1) 27 parts by mass and 73 parts by mass of copolymerized polyester (M2) dried under reduced pressure at 70 ° C. for 12 hours (1 Torr) were mixed and supplied to the extruder 3, and the layers (B) and (A) (C) It was shown in Example 1 except that the thickness ratio with the layer was 11:66:33 and that a relaxation treatment of 3.3% was performed in the width direction in the process of cooling to 60 ° C. Comparative example using the same method as It was created of the light-diffusing polyester film.

Comparative Example 2
As raw materials for the light diffusion layer (B), 74 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. 23 parts by mass and 10 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2, a light diffusion layer (B) was provided instead of the smooth layer (C), and the layer (B) ( A light diffusion film of Comparative Example 2 was prepared in the same manner as shown in Example 1 except that the thickness ratio of the A) layer and the (B) layer was 11:78:11.

Comparative Example 3
As raw materials for the light diffusion layer (B), 74 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. 23 parts by mass and 3 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2, the smooth layer (C) was not provided, and the thickness ratio of the (B) layer and the (A) layer was 11 A light diffusion film of Comparative Example 3 was prepared in the same manner as shown in Example 1 except that the pair was 89.

Comparative Example 4
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 mass and 3 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2, and a crystalline homopolyester (1 Torr) dried at 135 ° C. under reduced pressure (1 Torr) for 6 hours as a raw material of the sliding layer (C) ( M1) 60 parts by mass and 40 parts by mass of copolymerized polyester (M2) dried under reduced pressure (1 Torr) for 12 hours at 70 ° C. were mixed and supplied to the extruder 3, and the draw ratio in the longitudinal direction was 3.3 times. The same method as shown in Example 1 except that after the transverse stretching, heat treatment was performed at 240 ° C. for 17 seconds, and a relaxation treatment of 1.3% was performed in the width direction in the process of cooling to 60 ° C. Compare with 4 created the light diffusion film.

Reference 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 mass and 10 parts by mass of polystyrene (M3) were mixed and supplied to the extruder 2, the smooth layer (C) was not provided, and the thickness ratio of the (B) layer to the (A) layer was 11 The method was the same as that shown in Example 1, except that a heat treatment was performed at 234 ° C. for 17 seconds and 3.3% relaxation treatment was performed in the width direction in the process of cooling to 60 ° C. Thus, a light diffusing film of Reference Comparative Example 1 was prepared.

Reference Comparative Example 2
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. After the smooth layer (C) was not provided, the thickness ratio of the (B) layer and the (A) layer was controlled to be 20 to 80, and the film was stretched 2.4 times in the width direction at 135 ° C. Example 1 except that the film was stretched 1.6 times in the width direction at 140 ° C., further heat treated at 233 ° C. for 17 seconds, and subjected to 1.0% relaxation treatment in the width direction in the process of cooling to 60 ° C. Same as shown in It created a light-diffusing film of Comparative Reference Example 2 having a thickness of 100μm by the method.

  The 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 and the like. Moreover, it can use as a base film for prism sheets. Therefore, it is important to contribute to the industry.

Claims (8)

A light diffusing polyester film comprising a biaxially oriented polyester film,
The intermediate layer (A) has a three-layer structure laminated by a coextrusion method having a light diffusion layer (B) on one side and a smooth layer (C) on the opposite side,
The intermediate layer (A) is made of a crystalline homopolyester or a crystalline polyester containing a copolymer component,
The light diffusion layer (B) is composed of 50 to 99 parts by mass of a crystalline polyester containing a copolymer component having a melting point of 225 to 255 ° C. and 1 to 50 parts by mass of an additive incompatible with the polyester.
The smooth layer (C) is made of a crystalline polyester containing a copolymer component having a melting point of 225 to 255 ° C.
Average inclination gradient of the light diffusing layer (B) surface (△ a) is 0.03 or more state, and are 0.10 or less,
The thickness of the smooth layer (C) is 5 to 50% of the total thickness.
Light diffusing polyester film. The light diffusible polyester film according to claim 1, wherein the surface haze is 15% or more and the internal haze is less than the surface haze. The light-diffusing polyester according to claim 1 or 2, wherein the dimensional change rate at 150 ° C is 3% or less in both the longitudinal and transverse directions, the tensile strength is 100 MPa or more in both the longitudinal and transverse directions, and the tensile elongation is 100% or more. the film. The average height of curls at the four corners after heat treatment at 150 ° C for 30 minutes in a heating oven with respect to a rectangular film sample of 300 mm in the longitudinal direction and 210 mm in the width direction is 3.0 mm or less. A light diffusing polyester film according to claim 1. The light-diffusing polyester film according to any one of claims 1 to 4, wherein the total light transmittance is 86% or more and the image definition at a comb width of 2 mm is 40% or less. The surface of the said light-diffusion layer (B) has a coating layer which has as a main component at least 1 or more types of the copolyester resin, polyurethane-type resin, or acrylic resin provided before completion | stretching and orientation of a film. The light diffusable polyester film in any one of 1-5. On the surface of both the light diffusing layer (B) and the smooth layer (C) of the light diffusing polyester film, a coating layer containing at least one copolymer polyester resin, polyurethane resin, or acrylic resin as a main component is provided. The light diffusable polyester film according to claim 1. The coating layer which has as a main component at least 1 or more types of a copolyester resin, a polyurethane-type resin, or an acrylic resin on the surface of the smooth layer (C) of the light diffusable polyester film in any one of Claims 1-5. A light diffusing polyester film for a lens sheet.