JP4300826B2 - Light diffusing film and direct surface light source using the same - Google Patents

Description

【０１００】
【発明の効果】
本発明の光拡散性フィルムは、好ましくは１０００μｍ以下の薄番手のフィルムでありながら、現在、直下型面光源で用いられている数ｍｍ厚さの光拡散板に替えて使用することができるため、面光源の薄型化、軽量化が可能である。
【０１０１】
さらに薄番手のフィルムであれば、それだけで薄型化、軽量化のメリットがあるだけでなく、ロール状に巻き取ることが容易であるため、各種の表面加工も、高生産性のロール・トゥ・ロール加工が可能であり、保管も容易である。
【０１０２】
さらにまた、耐熱性や寸法安定性の高い二軸延伸ポリエステルフィルムを用いれば熱や湿度にも強い光拡散性フィルムとすることができる。
【０１０３】
本発明の光拡散性フィルムとそれを用いた直下型面光源は、液晶ディスプレイ用バックライト、内照式表示板、内照式照明器具等、高輝度で、かつ高い輝度均一性が求められる面光源として用いることができる。
【図面の簡単な説明】
【図１】面光源の輝度を測定するための、装置構造の概略を示す装置縦断面概略図である。
【符号の説明】
１：測定サンプル
２：冷陰極線管
３：反射フィルム
４：筐体[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light diffusive film suitably used for a planar light source such as a backlight of a liquid crystal display, a light box, an electric signboard device, or a planar illumination device. More specifically, light that is preferably used for mounting on a light emitting surface of a so-called direct-type surface light source, in which a light source is arranged in a hollow housing and light emitted from the light source is emitted from one main plane of the housing. The present invention relates to a diffusive film and a direct type surface light source using the same.
[0002]
[Prior art]
In recent years, many displays using liquid crystals have been used as display devices for personal computers, televisions, mobile phones, and the like. Since these liquid crystal displays are used as an image display device, it is generally performed that a surface light source called a backlight is arranged on the back side when viewed from an observer and light is emitted from the back side.
[0003]
At this time, if there is luminance unevenness in the light emitted from the backlight, the liquid crystal display deteriorates the image quality, so that it is required to irradiate the entire screen uniformly.
[0004]
Large LCD TVs that require extremely high brightness, such as large LCD TVs, can achieve high brightness with little light loss even when a large number of light sources are used. A so-called backlight is preferred.
[0005]
The direct type backlight is a planar light source of a type in which a light source is disposed in a hollow housing and light emitted from the light source is emitted from one main plane of the housing (for example, Patent Document 1). That is, a light source such as a number of cold cathode ray tubes is arranged at a position immediately below the light emitting surface (that is, a position just behind the liquid crystal screen when a liquid crystal display is used).
[0006]
For this reason, the direct-type backlight has a problem that a large luminance difference is likely to occur between a position on the screen that is directly above the light source and a position that is not so, and is easily recognized as luminance unevenness. For this reason, in general, a translucent milky white plate (so-called light diffusing plate) having a very strong light diffusibility is used on the light emitting surface to reduce luminance unevenness as much as possible. As this light diffusing plate, a resin plate made of acrylic or polycarbonate having a thickness of several millimeters mixed with a light scattering material such as organic and inorganic fine particles, preferably silicone fine particles is used (for example, Patent Document 2). According to this, strong light diffusivity is obtained while having a total light transmittance of 60% or more.
[0007]
Furthermore, if the uniformity is still insufficient, (1) the light reflecting plate is deformed in accordance with the arrangement of the light source and the light emission direction is controlled (see, for example, Patent Documents 3 and 4), (2) the light diffusing plate Directly light-shielded pattern printed or transparent film with light-shielded pattern printed on the light diffusion plate to partially block the light transmitted from the top of the light source and make the entire screen uniform (For example, refer patent document 5) etc. are proposed.
[0008]
Moreover, the film containing a bubble is known as a light diffusable film (refer patent document 6).
[0009]
[Patent Document 1]
Japanese Patent Laid-Open No. 5-119311
[0010]
[Patent Document 2]
JP-A-6-73296
[0011]
[Patent Document 3]
JP 2001-318614 A
[0012]
[Patent Document 4]
JP 2002-82624 A
[0013]
[Patent Document 5]
Japanese Patent Laid-Open No. 11-212090
[0014]
[Patent Document 6]
JP-A-11-268211
[0015]
[Problems to be solved by the invention]
Large monitors, televisions, internally-lit display panels, etc. that use direct-type backlights are required to be thinner, lighter, more reliable, and at the same time have higher functionality and better productivity. .
[0016]
However, since the conventional diffuser plate requires a thickness of several millimeters in order to obtain the required luminance unevenness reduction effect, there is a limit to reducing the thickness and weight of the backlight.
[0017]
At the same time, it is necessary to cut the diffuser into the required shape, and furthermore, printing of light shielding patterns, moisture-proof (gas barrier) processing, antistatic processing, hard coat processing, antireflection processing, and processing to form a transparent conductive film Furthermore, by performing surface processing such as micro unevenness and prism-shaped micro surface shape processing, the functionality is further improved. However, in the case of a thick resin plate, these processes require a very large apparatus at the same time. However, since it is necessary to process each sheet one by one, there is a problem in terms of productivity, resulting in high costs.
[0018]
In addition, because these resin plates require high uniformity and defect-freeness, large special equipment may be required for handling and storing thick resin plates in backlight assembly, resulting in decreased productivity. Was.
[0019]
Furthermore, low-cost acrylic light diffusion plates have low heat resistance and large dimensional changes due to moisture absorption, so the light diffusion plates are likely to bend and deform due to heat and humidity changes, resulting in a thicker diffusion. There was a problem that a board was required.
[0020]
In order to solve the above-described problems, the present invention provides a light diffusivity having a total light transmittance T (%) of 15% or more and 60% or less. Biaxially stretched polyester the film The total value of diffused light reflectance R (%) and total light transmittance T (%) is 80% or more, the total thickness is 1000 μm or less, contains fine bubbles inside the film, The light-diffusible biaxially stretched polyester film has an average number of bubbles in the thickness direction of 3 to 30 and an average flatness of bubbles of 3.1 to 10 It is what.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention considered that the above problems could be solved if a thin count film having optical characteristics that could be used in place of a thick diffusion plate could be realized.
[0022]
As a result, it has been said that the total light transmittance has to be increased in order to ensure sufficient brightness when using a conventional light diffusion plate as a backlight. As a result of a detailed study focusing on the fact that the total light transmittance is not completely correlated, if the total light transmittance is 15% or more, the total value with the diffused light reflectance is set to 80% or more. We have determined that there is no decrease in brightness.
[0023]
Furthermore, in order to obtain a film having such strong light diffusibility and diffused light reflectance, it is preferable to contain bubbles inside the film, more preferably by controlling the shape of the bubbles. It has been found that it can be achieved.
[0024]
In the present invention, the light diffusing film may contain bubbles inside. is necessary . This bubble may be a closed cell or an open cell, but is preferably a closed cell from the viewpoint of easily securing the mechanical strength of the film.
[0025]
The size of the bubbles contained in the film of the present invention is 0.1 to 30 μm, more preferably 0.5 to 10 μm, expressed as the average bubble diameter. When the bubble size is less than 0.1 μm, the transmitted light is likely to be colored and the brightness tends to be low. On the other hand, if the bubble size is larger than 30 μm, it may be visually recognized as a luminance defect. Note that the bubble number average means an average based on the number of bubbles, for example, the bubble number average A of a certain parameter A for bubbles. ^* Is the following equation, and hereinafter, the average number of bubbles is also simply referred to as average.
[0026]
A ^* = ΣN _i A _i / ΣN _i (N _i Is the parameter A = A _i Number of bubbles
The light diffusive film of the present invention has a total light transmittance T (%) of 15% or more and 60% or less. When the total light transmittance is less than 15%, the luminance when the backlight is used is insufficient, and when it exceeds 60%, it is difficult to eliminate the luminance unevenness when the backlight is used. The more preferable range of the total light transmittance T is 20% or more and 40% or less. However, in applications in which the absolute value of luminance is given priority over luminance unevenness (for example, for internal illumination use), it is 30% to 60% or less. It is preferable that
[0027]
One point of the present invention is that the total value of the diffused light reflectance R (%) and the total light transmittance T (%) is 80% or more. The total value of R and T is preferably 90% or more. In order to satisfy this characteristic, it is preferable that air bubbles be contained inside the film because it is easy in terms of the production method.
[0028]
The diffuse reflectance refers to the intensity of light reflected in the 45 ° direction when a light beam is incident on the film surface from the normal direction to the film surface (this is 0 degree). This is expressed as 100%.
[0029]
In addition to the bubbles, inorganic or organic particles imparting light diffusibility can also be added. In that case, it is preferably 5% by weight or less, more preferably 3% by weight or less based on the total weight of the film.
[0030]
The light diffusing film of the present invention may be a single layer or a laminate. , It is a film comprising a laminate of a plurality of films including a biaxially stretched polyester film containing bubbles or a biaxially stretched polyester film containing bubbles. Also, the overall thickness should be 1000 μm or less is necessary . More preferably, it is 500 μm or less.
[0031]
It is natural that extremely high heat resistance and dimensional stability can be obtained by making the film containing air bubbles into a biaxially stretched polyester film, but at the same time, it has a thickness of 1000 μm or less due to its strong tensile elastic modulus. However, it is also possible to obtain a film having a strong bending strength equivalent to a thick diffuser plate of several mm. For this reason, it becomes possible to adopt the same method as that of the current resin plate up to the method of installing in the hollow casing of the direct type backlight. However, when the film thickness is 20 μm or less, unless a separate support is prepared, unevenness due to wrinkles may occur due to changes over time during use.
[0032]
When laminating with a non-light diffusing film, etc., the thickness of the light diffusing layer such as having a bubble of the light diffusing film of the present invention is selected in accordance with the use form. However, as a single film, it is preferable that the film contains necessary bubbles or the like with a thickness of 75 μm or more and 350 μm or less, and satisfies the required optical characteristics, and when a thickness exceeding 350 μm is required. It is preferable from the point of high productivity to use as a laminated body bonded together with the film with high transparency prepared by others.
[0033]
The number of bubbles contained in the film of the present invention intersects with 3 to 30 bubbles (6 to 60 bubble interfaces) on average when the straight line penetrating in the film thickness direction is drawn. Can is necessary (More preferably 5 to 20). If the number of bubbles is too small, the diffuse light reflectance tends to be insufficient, and if the number of bubbles is too large, the total light transmittance tends to be insufficient.
[0034]
The average length of the bubbles contained in the film of the present invention in the direction parallel to the film surface is larger than the average thickness in the film thickness direction, that is, it is high when the backlight is a flat bubble in the direction parallel to the film surface. This is preferable in that brightness is obtained. In addition, about the average length of a surface parallel direction, when the vertical and horizontal stretch rate of a film differs and there exists anisotropy of bubble length in a surface parallel direction, it measures so that it may become the average.
[0035]
The reason for this is not clear, but in the case of a perfectly spherical bubble that is not flat or an irregular bubble that does not have directionality, it becomes stray light within the film and absorbed by the film. This is considered to be due to the fact that the component that becomes loss due to emanating from the film end face tends to increase. In particular, the average flatness of the bubbles 3.1 or higher Top 10 or less (Preferably 5 or less) It is necessary It is possible to achieve both high luminance and luminance unevenness suppression at a high level. Average flatness is 3.1 If it is less than 10, the luminance tends to decrease, and if it exceeds 10, luminance unevenness tends to occur.
[0036]
When the bubbles are observed and counted with an electron microscope, in consideration of the resolution limit, bubbles smaller than 0.05 (more preferably 0.08, more preferably 0.1) μm are ignored. It is practical.
[0037]
Next, an example of the method for producing the light diffusing film of the present invention will be described in the case of a biaxially stretched polyester film containing the most preferable flat bubbles inside the film.
[0038]
As the polyester used in the present invention, polyethylene terephthalate or polyethylene-2,6-naphthalate is preferably the main component.
[0039]
As a method for producing a polyester film containing bubbles,
(I) After melt-extruding a mixture containing the main resin component (a) and the incompatible component (b) with respect to the resin component (a), it is stretched in at least one direction to form pores therein. A method of forming an interface by
(II) A method of forming pores by adding foamable particles and foaming inside the film by melt extrusion,
(III) A method of forming holes in the film by injecting a gas such as carbon dioxide gas and performing extrusion foaming,
(IV) A method of forming pores inside the film by mixing at least two components of polymer, organic substance or inorganic substance, melt-extruding, and then dissolving at least one component by solvent extraction,
(V) A method in which pores are formed by melt-extruding a resin to which hollow particles have been added or by applying and drying in a solution state;
(VI) A method of forming a dry porous layer by coating a base film with a moisture permeable urethane resin and drying it.
In the present invention, it is preferable to use the method (I) because a preferable flat bubble can be easily generated. Further, centering on the method (I), it is preferable to adjust the bubble flatness by combining other methods, or to adjust the diffusibility of reflected light. As a method that can be preferably combined, the hollow particles of (V) are resin The method of adding to a solution, apply | coating to the surface layer of a film, and drying is mentioned.
[0040]
In the method (I), when polyester is selected as the main resin component (a), polyolefin such as polypropylene, polyethylene, polymethylpentene, polycycloolefin, polystyrene, etc. are preferable as the incompatible component (b). In particular, polymethylpentene or polycycloolefin is preferably used from the viewpoint of the flatness of the generated bubbles.
[0041]
Furthermore, in order to control the dispersion diameter of the incompatible component (b) in the polyester and the bubble formation state by stretching, it is also preferable to add a dispersion control agent as the third component. As such dispersion control agents, polyethylene glycol, polyalkylene glycol, polyhydroxycarboxylic acid, copolymers thereof, and the like are known.
[0042]
The resin composition thus selected is put into an extruder, melted, extruded into a sheet form from a slit-shaped die, dropped onto a cooled drum, and rapidly cooled and solidified to be substantially in an amorphous state. Get a sheet of.
[0043]
Next, after raising the film to the stretching temperature using a roll group, hot air oven, infrared heater, etc., while gripping both ends with a pair of rolls with different peripheral speeds and clips running on rails with gradually increasing intervals Stretch.
[0044]
In the case of biaxial stretching, the stretching is performed in both the longitudinal direction and the width direction, but the order is not limited, and simultaneous biaxial stretching may be used. Or the method of extending | stretching in multiple steps may be used. The shape of the bubbles can also be controlled by selecting the stretching method.
[0045]
Although not particularly limited, the draw ratio is preferably 1.1 to 8 (more preferably 1.2 to 6, more preferably 1.5 to 5) times. When the value falls below the lower limit of the numerical range, the heat shrinkage rate is too large, which is not preferable. On the other hand, when the value exceeds the upper limit of the numerical range, the draw ratio is too high, and the drawing process becomes difficult. The draw ratios may be the same in length and breadth or different.
[0046]
The stretched film is subsequently heat-set by a hot air oven or the like set at a temperature higher than the stretching temperature and lower than the melting point of the polyester to obtain a secondary stretched polyester film containing bubbles.
[0047]
At this time, the composition of the polyester as the main component (a), the type of the incompatible component (b), the same addition amount, the type of the dispersion control agent, the same addition amount, the stretching temperature, the magnification, the temperature of the heat setting It is possible to control the size, number and shape of bubbles generated.
[0048]
For example, as a method for reducing the flatness of bubbles, a low melting point polyester such as polyethylene terephthalate copolymerized with polyethylene isophthalate is used as the polyester, and the incompatible component (b) is a resin having a high softening temperature. Operations such as lowering, increasing the stretching temperature, and increasing the heat setting temperature may be performed.
[0049]
Next, the light diffusive film of the present invention is obtained by performing necessary surface processing such as applying and drying a resin solution in which hollow particles are added to the biaxially stretched polyester film obtained as necessary, using a coater.
[0050]
Moreover, when making a light diffusable film into a laminated body, the obtained bubble containing polyester film and the transparent biaxially stretched polyester film prepared separately are bonded together using a dry laminator etc., and it manufactures.
[0051]
[Measurement and evaluation method of characteristics]
(1) Total light transmittance (T)
Using a fully automatic direct reading haze computer HGM-2DP (manufactured by Suga Test Instruments Co., Ltd.), the total light transmittance was measured from both sides of the film, and the lower value was taken as the total light transmittance.
[0052]
(2) Diffuse light reflectance (R)
Using a spectroscopic color difference meter SE-2000 type (manufactured by Nippon Denshoku Industries Co., Ltd.), the spectral reflectance in the range of 400 to 700 nm was measured at 10 nm intervals according to JIS Z-8722, and the average value was obtained.
[0053]
At this time, the sample film to be measured and a black polyester film (“Lumirror” 125X30, manufactured by Toray Industries, Inc.) having the same size as the sample are superimposed, and the reflectance on the sample film side is measured.
[0054]
This was performed on both sides of the sample film, and the higher value was taken as the diffuse light reflectance.
[0055]
(3) Average number of bubbles in the film thickness direction (N)
Using a microtome, the film is cut without crushing the cross section in the thickness direction. Next, the cut section is suitable using a scanning electron microscope S-2100A type (Hitachi, Ltd.) so that particles of 1 μm or more can be observed, and the entire section from the film surface to the back surface is within the field of view as much as possible. A magnified observation is performed at a magnification (generally about 3000 to 10,000 times), and a photograph is taken. When the entire cross section from the film surface to the back surface does not fit in one field of view, images taken in multiple times are connected and combined to obtain an image of the entire cross section.
[0056]
This observation and photography is performed on any 10 points on the film.
[0057]
Next, when a straight line is drawn perpendicularly to the film surface from one arbitrary point on the film surface to the other film surface in the obtained cross-sectional image, the number of bubbles / resin interfaces present on the line Count. At this time, the number of interfaces is counted as 1 for both the interface from the gas phase to the solid phase and the interface from the solid phase to the gas phase. This is performed for any 10 points for one image.
[0058]
The simple average value of the data of 100 points in total (10 points per image) × (10 images) obtained in this way was calculated, and the value divided by 2 was defined as the average number of bubbles (N) in the film thickness direction ( Because there are two upper and lower interfaces in one bubble).
[0059]
Further, when the average number of cells in the film thickness direction is different depending on whether the film is cut in the longitudinal direction or when cut in the width direction when cutting the film, the average number of bubbles in the film thickness direction is determined for each direction and then large. This value is taken as the average number of bubbles (N) in the film thickness direction.
[0060]
(4) Bubble length in parallel to film surface (X), film thickness direction thickness (Y), average flatness (A)
An enlarged cross-sectional image is obtained by the same method as that used in the measurement of the average number of bubbles (3), and 50 points are arbitrarily selected for the bubble portion having a closed shape in the cross-sectional photograph.
[0061]
For the selected bubbles, the length in the direction parallel to the surface of the film surface and the thickness in the direction perpendicular to the film surface were measured, and the average number of bubbles for each of the obtained length and thickness data of 50 points was measured. The value was defined as the length (X) of the bubble in the direction parallel to the film surface and the thickness (Y) in the film thickness direction.
[0062]
Moreover, the value which remove | divided the film surface parallel direction length (X) of the bubble by the film thickness direction thickness (Y) was made into average flatness (A).
[0063]
(5) Average bubble diameter (φ)
An enlarged cross-sectional image is obtained by the same method as that used in the measurement of the average number of bubbles (3), and 50 points are arbitrarily selected for the closed bubble portion in the cross-sectional photograph.
[0064]
Overlay a transparent film on the photo, trace each contour shape with an oil pen, paint the part corresponding to the cavity, copy the image of each cavity (bubble), and copy the trace image Measure the area with an image analyzer and calculate the equivalent circle diameter.
[0065]
This was performed for 50 selected points, and the average value was calculated as the average bubble diameter (φ).
[0066]
(6) Surface light source brightness and uneven brightness
Luminance was measured using a direct type backlight having the structure shown in FIG. 1 mounted on an 18-inch type liquid crystal monitor.
[0067]
As shown in FIG. 1, after placing the measurement sample 1 on the cold cathode tube 2 and lighting the cold cathode ray tube 2 for 60 minutes to stabilize the light source, the color luminance meter BM-7fast (stock) Brightness (cd / m) using Topcon ² ) Was measured.
[0068]
At this time, the distance from the upper end of the light source to the lower end of the sample film was set to 19 mm so that there was no difference depending on the thickness of the sample film.
[0069]
The luminance measurement conditions were such that the distance from the measurement surface to the luminance meter was 750 mm and the measurement viewing angle was 2 °.
[0070]
The measurement was performed on 16 points of the center point of each area obtained by dividing the backlight surface into 4 × 4 areas, and the average value was defined as the luminance.
[0071]
In addition, luminance unevenness was evaluated based on variations in measurement results at 16 points.
[0072]
Brightness unevenness (%) = (maximum value−minimum value) / (average value) × 100
[0073]
【Example】
Hereinafter, although an example is given and explained in the present invention, the present invention is not necessarily limited to these.
[0074]
[Example 1]
The raw materials having the following composition were each vacuum dried at 180 ° C. for 4 hours and supplied to a composite film forming apparatus having a main extruder A and a sub-extruder B.
[0075]
Main extruder A:
Polyethylene terephthalate (PET): 80 parts by weight
Copolyester obtained by copolymerizing 14 mol% of isophthalic acid component with PET: 20 parts by weight
Polymethylpentene: 0.7 parts by weight
Sub extruder B:
PET: 100 parts by weight
Calcium carbonate particles having an average particle size of 1.1 μm: 1.0 part by weight
The respective raw materials were melt-extruded at 280 ° C. from the extruders A and B, and co-extruded from the slit-shaped die so that the molten raw material of the main extruder A became an inner layer and the molten raw material of the sub-extruder B became both surface layers. Thereafter, the film was rapidly cooled with a mirror-cooled drum maintained at a surface temperature of 25 ° C. to obtain a substantially non-crystalline composite unstretched sheet film. The thickness composition ratio of the composite unstretched sheet was set to B / A / B (7/84/7) by adjusting the extrusion amount of each extruder.
[0076]
This sheet was stretched 3 times in the longitudinal direction at 85 ° C. Thereafter, the edge of the sheet was gripped with a clip, guided into a tenter heated to 90 ° C., preheated, and then continuously stretched 3.5 times in the width direction in an atmosphere at 100 ° C. to form a void. Further, a heat treatment was continuously performed at 90 ° C. for 20 seconds and at 220 ° C. for 10 seconds to form bubbles to obtain a biaxially stretched polyester film having a film thickness of 125 μm.
[0077]
This biaxially stretched polyester film is named film 1.
[0078]
The obtained film 1 is affixed to a metal frame having an opening having the same shape as the exit surface of the evaluation surface light source with slight tension so that no wrinkles or undulations enter, and is placed on the output surface of the evaluation surface light source. And evaluated as a light diffusing film. The evaluation results are shown in Table 1.
[0079]
[Example 2]
A hot melt adhesive (Aron Melt PES-304) manufactured by Toa Gosei Co., Ltd. was applied to one side of a transparent PET film (thickness: 188 μm, “Lumirror” 188U34, manufactured by Toray Industries, Inc.) to a dry thickness of 5 μm. Drying was performed with hot air at 150 ° C. for 2 minutes.
[0080]
This was laminated at 100 ° C. using a hot laminator on both surfaces of the film 1 obtained in Example 1 to obtain a light-diffusing film having a thickness of 505 μm.
[0081]
The resulting film is a strong biaxially stretched PET film laminate with a thickness of 505 μm, so it is very stiff, and does not require a support structure such as a metal frame and has a light diffusibility. It could be used as a film. The evaluation results are shown in Table 1.
[0082]
[Example 3]
A biaxially stretched polyester film was obtained in the same manner as in Example 1 except that the thickness of the unstretched sheet was reduced by increasing the take-up speed on the cooling drum and the final film thickness was 100 μm.
[0083]
The obtained film was evaluated as a light diffusing film by the same method as film 1. The evaluation results are shown in Table 1.
[0084]
[Example 4]
The raw materials having the following composition were each vacuum dried at 180 ° C. for 4 hours and supplied to a composite film forming apparatus having a main extruder A and a sub-extruder B.
[0085]
Main extruder A:
Polyethylene terephthalate (PET): 90 parts by weight
Copolyester obtained by copolymerizing 33 mol% of cyclohexanedimethanol component with PET: 10 parts by weight
Polymethylpentene: 1.0 part by weight
Sub extruder B:
PET: 100 parts by weight
Barium sulfate particles having an average particle diameter of 1.0 μm: 0.7 parts by weight
The respective raw materials were melt-extruded at 280 ° C. from the extruders A and B, and co-extruded from the slit-shaped die so that the molten raw material of the main extruder A became an inner layer and the molten raw material of the sub-extruder B became both surface layers. Thereafter, the film was rapidly cooled with a mirror-cooled drum maintained at a surface temperature of 25 ° C. to obtain a substantially non-crystalline composite unstretched sheet film. The thickness composition ratio of the composite unstretched sheet was set to B / A / B (10/80/10) by adjusting the extrusion amount of each extruder.
[0086]
This sheet was stretched 2.8 times in the longitudinal direction at 90 ° C. Thereafter, the end of the sheet was gripped with a clip, led into a tenter heated to 95 ° C. and preheated, and then continuously stretched 3.0 times in the width direction in an atmosphere at 100 ° C. Furthermore, the heat processing for 10 second was continuously performed in 235 degreeC atmosphere, and the 125-micrometer-thick biaxially stretched polyester film was obtained.
[0087]
The obtained film was evaluated as a light diffusing film by the same method as film 1. The evaluation results are shown in Table 1.
[0088]
[Comparative Example 1]
The raw materials having the following composition were each vacuum dried at 180 ° C. for 4 hours and supplied to a composite film forming apparatus having a main extruder A and a sub-extruder B.
[0089]
Main extruder A:
Polyethylene terephthalate (PET): 100 parts by weight
Polymethylpentene: 5.0 parts by weight
Sub extruder B:
PET: 100 parts by weight
Calcium carbonate particles having an average particle size of 1.1 μm: 0.5 parts by weight
The respective raw materials were melt-extruded at 280 ° C. from the extruders A and B, and co-extruded from the slit-shaped die so that the molten raw material of the main extruder A became an inner layer and the molten raw material of the sub-extruder B became both surface layers. Thereafter, the film was rapidly cooled with a mirror-cooled drum maintained at a surface temperature of 25 ° C. to obtain a substantially non-crystalline composite unstretched sheet film. The thickness composition ratio of the composite unstretched sheet was set to B / A / B (10/80/10) by adjusting the extrusion amount of each extruder.
[0090]
This sheet was stretched 3.0 times in the longitudinal direction at 85 ° C. Thereafter, the end of the sheet was gripped with a clip, guided into a tenter heated to 95 ° C., preheated, and then continuously stretched 3.5 times in the width direction in an atmosphere at 100 ° C. Furthermore, the heat processing for 10 second was continuously performed in 220 degreeC atmosphere, and the 125-micrometer-thick biaxially stretched polyester film was obtained.
[0091]
The obtained film was evaluated as a light diffusing film by the same method as film 1. The evaluation results are shown in Table 1. The direct type surface light source using this film had low total light transmittance of the film, high uniformity, but low brightness.
[0092]
[Comparative Example 2]
A light diffusing film was obtained by the same method as in Example 1 except that the raw material supplied to the main extruder A was changed to the following composition.
[0093]
Polyethylene terephthalate (PET): 80 parts by weight
Copolyester obtained by copolymerizing 14 mol% of isophthalic acid component with PET: 20 parts by weight
Polymethylpentene: 0.1 parts by weight
The obtained film was evaluated as a light diffusing film by the same method as film 1. The evaluation results are shown in Table 1. The direct type surface light source using this film has a high total light transmittance and high luminance, but has a large luminance unevenness due to the thin film, and lacks practicality.
[0094]
[Comparative Example 3]
The raw materials having the following composition were each vacuum dried at 180 ° C. for 4 hours and supplied to a composite film forming apparatus having a main extruder A and a sub-extruder B.
[0095]
Main extruder A:
Polyethylene terephthalate (PET): 85 parts by weight
Polymethylpentene: 12.0 parts by weight
Copolyester obtained by copolymerizing 10.0% by weight of polyethylene glycol with PET: 3.0 parts by weight
Sub extruder B:
PET: 100 parts by weight
The respective raw materials were melt-extruded at 285 ° C. from the extruders A and B, and co-extruded from the slit-shaped die so that the molten raw material of the main extruder A became an inner layer and the molten raw material of the sub-extruder B became both surface layers. Thereafter, the film was rapidly cooled with a mirror-cooled drum maintained at a surface temperature of 25 ° C. to obtain a substantially non-crystalline composite unstretched sheet film. The thickness composition ratio of the composite unstretched sheet was set to B / A / B (15/70/15) by adjusting the extrusion amount of each extruder.
[0096]
This sheet was stretched 3.5 times in the longitudinal direction at 85 ° C. Thereafter, the end of the sheet was gripped with a clip, guided into a tenter heated to 85 ° C., preheated, and then continuously stretched 3.6 times in the width direction in an atmosphere at 105 ° C. Furthermore, the heat processing for 10 second was continuously performed in 220 degreeC atmosphere, and the biaxially stretched polyester film with a film thickness of 100 micrometers was obtained.
[0097]
The obtained film was evaluated as a light diffusing film by the same method as film 1. The evaluation results are shown in Table 1. The direct type surface light source using this film had low diffused light reflectance of the film and high uniformity but low brightness and poor efficiency.
[0098]
From the above results, when the light diffusion sheet of the present invention is used as a light diffusion sheet for a direct type surface light source, it is a thin film, and has high brightness and excellent uniformity by suppressing luminance unevenness. I understand that.
[0099]
[Table 1]

[0100]
【The invention's effect】
The light diffusing film of the present invention is preferably a thin film having a thickness of 1000 μm or less, but can be used in place of a light diffusing plate having a thickness of several millimeters that is currently used in a direct type surface light source. The surface light source can be made thinner and lighter.
[0101]
In addition, a thin count film not only has the advantages of thinning and weight reduction, but also can be easily wound up into a roll. Roll processing is possible and storage is easy.
[0102]
Furthermore, if a biaxially stretched polyester film having high heat resistance and high dimensional stability is used, a light diffusive film resistant to heat and humidity can be obtained.
[0103]
The light diffusing film of the present invention and a direct surface light source using the same are surfaces requiring high brightness and high brightness uniformity, such as backlights for liquid crystal displays, internally illuminating display boards, internally illuminating lighting fixtures, etc. It can be used as a light source.
[Brief description of the drawings]
FIG. 1 is an apparatus vertical cross-sectional schematic view showing an outline of an apparatus structure for measuring the luminance of a surface light source.
[Explanation of symbols]
1: Measurement sample
2: Cold cathode ray tube
3: Reflective film
4: Housing

Claims (2)

A light diffusing biaxially stretched polyester film having a total light transmittance T (%) of 15% to 60% ,
The total value of diffused light reflectance R (%) and total light transmittance T (%) is 80% or more,
The overall thickness is 1000 μm or less,
Contains fine bubbles inside the film,
The average number of bubbles in the thickness direction of the film is 3-30,
A light-diffusing biaxially stretched polyester film having an average flatness of bubbles of 3.1 or more and 10 or less . The direct type | mold surface light source which mounted | wore the light diffusable biaxially stretched polyester film of Claim 1 .

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