JP2004271568A - Light diffusive film and vertical type surface light source using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a light diffusive film which is thin and has luminance unevenness reduction effect needed for a right-under type surface light sources while maintaining high luminance. <P>SOLUTION: The light diffusive film is a film of 15 to 60% in total light beam transmissivity T(%) and ≥80% in total value of diffused light reflectivity R(%) and total light beam transmissivity T(%), and the right-under type surface light source is mounted with the light diffusive film on its projection surface. <P>COPYRIGHT: (C)2004,JPO&NCIPI

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]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light diffusing film suitably used for a planar light source such as a backlight or a light box of a liquid crystal display, an illuminated signboard device, and a planar lighting device. More specifically, a method 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, that is, light preferably used to be mounted on an emission surface of a so-called direct-type surface light source The present invention relates to a diffusible film and a direct-type surface light source using the same.
[0002]
[Prior art]
In recent years, many displays using liquid crystal have been used as display devices for personal computers, televisions, mobile phones, and the like. In order to use these liquid crystal displays as an image display device, a display is generally performed by arranging a surface light source called a backlight on the back side as viewed from an observer and irradiating light from the back side.
[0003]
At this time, if there is uneven brightness in the light emitted from the backlight, the image quality of the liquid crystal display deteriorates.
[0004]
Large LCDs, such as large liquid crystal TVs, that require extremely high brightness require a direct-type backlight, which can reduce the loss of light even when using a large number of light sources and achieve high brightness. Backlights called are preferred.
[0005]
The direct-type backlight is a planar light source of a system 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 structure in which a large number of light sources such as cold cathode ray tubes are arranged at a position immediately below the light emission surface (that is, a position immediately behind the liquid crystal screen when a liquid crystal display is used).
[0006]
For this reason, the direct backlight has a problem that a large difference in luminance is likely to occur between a position directly above the light source on the screen and a position other than the position above the light source, and is easily recognized as luminance unevenness. For this reason, in general, a translucent milky plate (a so-called light diffusing plate) having a very strong light diffusing property is used for the light emitting surface to reduce the luminance unevenness as much as possible. As this light diffusion plate, a resin plate made of acrylic or polycarbonate having a thickness of several mm mixed with a light scattering substance such as organic or 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]
Further, if the uniformity is still insufficient, (1) a light reflector is deformed in accordance with the arrangement of the light source to control the light emission direction (for example, see Patent Documents 3 and 4). A light-shielding pattern printed directly or a light-shielding pattern printed on a transparent film is superimposed on a light diffusion plate to partially block light transmitted from above the light source, making the brightness of the entire screen uniform. (See, for example, Patent Document 5).
[0008]
Further, a film containing bubbles is known as a light diffusing film (see Patent Document 6).
[0009]
[Patent Document 1] JP-A-5-19311
[0010]
[Patent Document 2] JP-A-6-73296
[0011]
[Patent Document 3] JP-A-2001-318614
[0012]
[Patent Document 4] JP-A-2002-82624
[0013]
[Patent Document 5] JP-A-11-212090
[0014]
[Patent Document 6] JP-A-11-268211
[0015]
[Problems to be solved by the invention]
Large monitors, televisions, internally illuminated display panels, etc. that use direct backlights are required to be thinner, lighter, more reliable, and at the same time have higher functionality and higher productivity. .
[0016]
However, the conventional diffusion plate requires a thickness of several mm in order to obtain the required effect of reducing luminance unevenness, and thus there has been a limit in reducing the thickness and weight of the backlight.
[0017]
At the same time, the diffusion plate must be cut into the required shape, and furthermore, printing of a light-shielding pattern, moisture-proof (gas barrier) processing, antistatic processing, hard coating processing, anti-reflection processing, and processing for forming a transparent conductive film Further, by performing surface processing such as fine unevenness and fine surface shape processing of a prism shape, the function is further enhanced.However, in the case of a thick resin plate, these processing require a very large apparatus and However, since it is necessary to process each sheet one by one, there is a problem in terms of productivity, resulting in a high cost.
[0018]
In addition, since these resin plates require high uniformity and defect-free properties, large dedicated equipment may be required for handling and storing thick resin plates in backlight assembly, resulting in reduced productivity. Was.
[0019]
Furthermore, a low-cost acrylic light diffusion plate has low heat resistance and a large dimensional change due to moisture absorption, so that the light diffusion plate is likely to bend or deform due to heat and humidity changes, and as a result, a thicker diffusion There was a problem that a board was required.
[0020]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention provides a method in which a total light transmittance T (%) is 15% or more and 60% or less, and a total value of diffuse light reflectance R (%) and total light transmittance T (%). Is 80% or more.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
The present inventors have studied and considered that the above-mentioned problem can be solved if a thin film having optical characteristics that can be used in place of a thick diffusion plate can be realized.
[0022]
As a result, in the conventional light diffusion plate, it was said that the total light transmittance had to be increased in order to ensure sufficient luminance when used as a backlight. As a result of a detailed study focusing on the fact that there is no perfect correlation with the total light transmittance, if the total light transmittance is 15% or more, the total value with the diffuse light reflectance is set to 80% or more. It was found that there was no decrease in brightness.
[0023]
Further, in order to obtain a film having such a strong light diffusing property 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 preferably contains bubbles inside. The cells may be closed cells or open cells, but are preferably closed cells in that the mechanical strength of the film is easily ensured.
[0025]
The size of the bubbles contained in the film of the present invention is 0.1 μm or more and 30 μm or less, more preferably 0.5 μm or more and 10 μm or less, as indicated by the average bubble diameter. If the bubble size is less than 0.1 μm, the transmitted light tends to be colored and the brightness tends to be low. If the bubble size is larger than 30 μm, it may be visually recognized as a luminance defect. The average of the number of bubbles means an average based on the number of bubbles. ^* Is as shown in the following equation, and hereinafter, the average of the number of bubbles is also simply referred to as an average.
[0026]
A ^* = ΣN _i A _i / ΣN _i (N _i Is the parameter A = A _i The number of bubbles)
The light diffusing film of the present invention has a total light transmittance T (%) of 15% or more and 60% or less. If the total light transmittance is less than 15%, the brightness at the time of a backlight becomes insufficient, and if it exceeds 60%, it becomes difficult to eliminate the uneven brightness at the time of a backlight. A more preferable range of the total light transmittance T is 20% or more and 40% or less, but 30% or more and 60% or less in an application in which an absolute value of the luminance is prioritized over the luminance unevenness (for example, an internal illumination type illumination application). It is preferable that
[0027]
One point of the present invention is that the total value of the diffuse light reflectance R (%) and the total light transmittance T (%) is 80% or more. The total value of R and T is preferably at least 90%. In order to satisfy this property, it is preferable to include air bubbles inside the film in terms of the production method, which is easy.
[0028]
The diffuse reflectance is defined as the intensity of light reflected in a 45 ° direction when a light beam is incident on the film surface from a normal direction (this is 0 °) with respect to the film surface. Is expressed as 100%.
[0029]
Inorganic or organic particles imparting light diffusing properties can be added in addition to the bubbles, but in this case, the content 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, and preferably includes a biaxially stretched polyester film containing air bubbles or a biaxially stretched polyester film containing air bubbles. A film comprising a laminate of the above films. Further, the total thickness is preferably 1000 μm or less. More preferably, it is 500 μm or less.
[0031]
It is a matter of course that extremely high heat resistance and dimensional stability can be obtained by using a biaxially stretched polyester film as the film containing bubbles, but at the same time, the film has a thickness of 1000 μm or less due to its strong tensile modulus. However, it is also possible to obtain a film having a strong bending strength equivalent to that of a thick diffusion plate of several mm. For this reason, the same method as that of the current resin plate can be adopted up to the method of installing the direct-type backlight in the hollow casing. However, if the film thickness is 20 μm or less, unevenness due to the generation of wrinkles may occur due to aging during use unless a separate support is prepared, which is not preferable.
[0032]
When laminated with a film or the like having no light diffusing property, the thickness of the layer having light diffusing property such as having air bubbles of the light diffusing film of the present invention is selected to be optimal according to the use form. However, it is preferable that the film containing the necessary air bubbles or the like with a thickness of 75 μm or more and 350 μm or less as a single film, and satisfy the required optical characteristics is preferable, and when a thickness exceeding 350 μm is required. Is preferably used as a laminate bonded to another prepared highly transparent film from the viewpoint of high productivity.
[0033]
The number of bubbles contained in the film of the present invention is such that, when a straight line penetrating in the film thickness direction is drawn, the straight line intersects 3 to 30 bubbles (6 to 60 bubble interfaces) on average. It is preferable (more preferably, 5 to 20). If the number of bubbles is too small, the diffused 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 plane is larger than the average thickness in the thickness direction of the film, that is, the cells are flat in the direction parallel to the film plane, and are high when used as a backlight. This is preferable in that luminance can be obtained. In addition, regarding the average length in the plane parallel direction, when the longitudinal and horizontal stretch ratios of the film are different and there is anisotropy in the bubble length in the plane parallel direction, measurement is performed so that the average is obtained.
[0035]
The reason for this is not clear, but in the case of perfectly spherical bubbles that are not flat or irregular bubbles that do not have directionality, of the light flux incident from the film surface, it becomes stray light inside the film and is absorbed by the film. This is considered to be due to the fact that components that are lost due to emission from the end face of the film tend to increase. In particular, when the average flatness of the bubbles is 1.5 or more and 10 or less (more preferably 2 or more and 5 or less), it is possible to achieve both high luminance and a high level of luminance unevenness suppression. If the average flatness is less than 1.5, the luminance tends to decrease, and if it exceeds 10, luminance unevenness tends to occur.
[0036]
In addition, when observing and counting bubbles with an electron microscope, in consideration of the resolution limit, bubbles having a size of preferably less 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 for a case of a biaxially stretched polyester film containing the most preferable flat cells inside the film.
[0038]
As the polyester used in the present invention, polyethylene terephthalate or polyethylene-2,6-naphthalate is preferably used as a main component.
[0039]
As a method for producing a polyester film containing bubbles,
(I) A mixture containing a main resin component (a) and an incompatible component (b) with respect to the resin component (a) is melt-extruded and then stretched in at least one direction to form pores therein. By forming an interface,
(II) a method of forming pores by adding foamable particles and foaming inside the film by melt extrusion,
(III) a method of forming pores inside the film by injecting a gas such as carbon dioxide gas and extruding and foaming;
(IV) a method of mixing two or more components of a polymer, an organic substance, or an inorganic substance, melt-extruding, and then dissolving at least one component by solvent extraction to form pores inside the film;
(V) a method in which a resin to which hollow particles are added is melt-extruded, or a solution is applied and dried to form pores;
(VI) A method of forming a dry porous layer by coating a base film with a urethane resin for moisture permeation processing and drying.
However, in the present invention, it is preferable to use the method (I) because a preferable flat bubble can be easily generated. In addition, it is preferable to adjust the bubble flatness or to adjust the diffusivity of the reflected light by combining other methods, mainly the method (I). A method of adding to a solution, applying to the surface layer of a film, and drying.
[0040]
In the method (I), when polyester is selected as the main resin component (a), a polyolefin such as polypropylene, polyethylene, polymethylpentene, polycycloolefin, or polystyrene is preferably used as the incompatible component (b). Although it is used, it is particularly preferable to use polymethylpentene or polycycloolefin from the viewpoint of the flatness of generated bubbles.
[0041]
Furthermore, in order to control the dispersion diameter of the incompatible component (b) in the polyester and the state of bubble formation by stretching, it is preferable to add a dispersion control agent as the third component. As such dispersion controlling 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 shape from a slit-shaped die, dropped on a cooled drum, quenched and solidified, and substantially in an amorphous state. Get a sheet.
[0043]
Then, after heating the film to the stretching temperature using a roll group, a hot air oven, an infrared heater, or the like, while gripping both ends with a pair of rolls having different peripheral speeds, and a clip running on a rail that gradually increases in interval. Stretch.
[0044]
In the case of biaxial stretching, stretching is performed in both the longitudinal direction and the width direction, but the order is not limited, and simultaneous biaxial stretching may be performed. Alternatively, a method of stretching in multiple stages may be used. The shape of the bubble can also be controlled by selecting the stretching method.
[0045]
Although not particularly limited, the stretching ratio is preferably 1.1 to 8 (more preferably, 1.2 to 6, more preferably 1.5 to 5). If the value is below the lower limit of the numerical range, the heat shrinkage is too large, which is not preferable. On the other hand, if the value exceeds the upper limit of the numerical range, the stretching ratio is too high, and the stretching process becomes difficult, which is not preferable. The stretching ratio may be the same in the vertical and horizontal directions or may be 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, depending on the composition of the polyester which is the main component (a), the type of the incompatible component (b), the same amount, the type of the dispersion control agent, the same amount, the stretching temperature, the magnification, and the heat setting temperature, It is possible to control the size, number, and shape of the bubbles generated at a time.
[0048]
For example, as a method of reducing the flatness of bubbles, a low-melting polyester such as polyethylene terephthalate obtained by copolymerizing polyethylene isophthalate as a polyester, an incompatible component (b) as a resin having a high softening temperature, and a draw ratio of Operations such as lowering the temperature, increasing the stretching temperature, and increasing the heat setting temperature may be performed.
[0049]
Next, the light-diffusing film of the present invention can be obtained by subjecting the obtained biaxially stretched polyester film to a necessary surface treatment such as coating and drying a resin solution containing hollow particles with a coater, if necessary.
[0050]
When the light diffusing film is used as a laminate, the laminate is produced by laminating the obtained bubble-containing polyester film and a separately prepared transparent biaxially stretched polyester film using a dry laminator or the like.
[0051]
[Method of measuring and evaluating characteristics]
(1) Total light transmittance (T)
The total light transmittance was measured from both sides of the film using a fully automatic direct reading haze computer HGM-2DP (manufactured by Suga Test Instruments Co., Ltd.), and the lower value was taken as the total light transmittance.
[0052]
(2) Diffuse light reflectance (R)
Using a spectral colorimeter SE-2000 (manufactured by Nippon Denshoku Industries Co., Ltd.), 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 determined.
[0053]
At this time, the sample film to be measured is superimposed on a black polyester film ("Lumirror" 125X30, manufactured by Toray Industries, Inc.) having the same size as the sample, and the reflectance of the sample film is measured.
[0054]
This was performed on both the front and back surfaces of the sample film, and the higher value was taken as the diffused 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. Then, by using a scanning electron microscope Model S-2100A (Hitachi, Ltd.), the cut cross section can be observed so that particles of 1 μm or more can be observed, and the entire cross section from the film front surface to the back surface can be viewed as much as possible. Magnification observation is performed at a magnification (generally about 3000 to 10000 times) and a photograph is taken. If the entire cross section from the film front surface to the back surface does not fit in one field of view, photographs taken a plurality of times are connected and synthesized to obtain an image of the entire cross section.
[0056]
This observation and photographing are performed for any 10 points of the film.
[0057]
Next, when a straight line is drawn perpendicularly to the film surface from an arbitrary point on the film surface to the other film surface in the obtained cross-sectional image, the number of interfaces between the bubbles and the resin existing on the line. Count. At this time, the number of interfaces is counted as 1 both for the interface from the gas phase to the solid phase and for the interface from the solid phase to the gas phase. This is performed for any 10 points in one image.
[0058]
A simple average value of data of 100 points (10 points per image) × (10 images) thus obtained is calculated, and a value obtained by dividing by 2 is taken as an average number of bubbles (N) in the film thickness direction ( One bubble has two interfaces, upper and lower).
[0059]
When the average number of bubbles in the film thickness direction is different depending on the case where the film is cut in the longitudinal direction and the case where the film is cut in the width direction when further cutting the film, after calculating the average number of bubbles in the film thickness direction in each direction, it is large. The value of the other is defined as the average number of bubbles (N) in the film thickness direction.
[0060]
(4) Length of bubble in the direction parallel to the film surface (X), thickness in the film thickness direction (Y), average flatness (A)
An enlarged cross-sectional image is obtained by the same method as that used in the measurement (3) of the average number of bubbles, and 50 points are arbitrarily selected for a closed-shaped bubble portion in the cross-sectional photograph.
[0061]
For the selected bubbles, the length in the direction parallel to the film surface and the thickness in the direction perpendicular to the film surface were measured. The values were defined as the length of the bubble in the direction parallel to the film surface (X) and the thickness in the film thickness direction (Y).
[0062]
The value obtained by dividing the length (X) of the bubbles in the direction parallel to the film surface by the thickness (Y) in the film thickness direction was defined as the average flatness (A).
[0063]
(5) Average bubble diameter (φ)
An enlarged cross-sectional image is obtained by the same method as 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]
After overlaying a transparent film on the photo, tracing each outline with an oil-based pen, painting the part corresponding to the cavity, taking an image of each cavity (bubble), and taking the captured trace image The area is measured with an image analyzer, and the equivalent circle diameter is calculated.
[0065]
This was performed for the selected 50 points, and the average value was calculated to be the average bubble diameter (φ).
[0066]
(6) Surface light source luminance and luminance unevenness
The luminance was measured using a direct-type backlight having a structure shown in FIG. 1 mounted on an 18-inch liquid crystal monitor.
[0067]
As shown in FIG. 1, the measurement sample 1 is placed on the cold cathode tube 2, the cold cathode ray tube 2 is turned on for 60 minutes to stabilize the light source, and then the color luminance meter BM-7fast (stock (Cd / m 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 as follows: 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 at the center of each area obtained by dividing the backlight surface into 16 equal areas of 4 × 4, and the average value was defined as the luminance.
[0071]
In addition, luminance unevenness was evaluated based on variations in the measurement results at 16 points.
[0072]
Brightness unevenness (%) = (maximum value−minimum value) / (average value) × 100
[0073]
【Example】
Hereinafter, the present invention will be described with reference to examples, but the present invention is not necessarily limited thereto.
[0074]
[Example 1]
Raw materials having the following compositions 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 were co-extruded from a 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 mixture was quenched with a mirror cooling drum whose surface temperature was maintained at 25 ° C. to obtain a substantially amorphous composite unstretched sheet film. The thickness ratio of the composite unstretched sheet was adjusted to B / A / B (7/84/7) by adjusting the extrusion amount of each extruder.
[0076]
This sheet was stretched three times in the longitudinal direction at 85 ° C. Thereafter, the end of the sheet was gripped by 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, heat treatment was continuously performed at 90 ° C. for 20 seconds and in an atmosphere of 220 ° C. for 10 seconds to form bubbles, thereby obtaining a 125 μm-thick biaxially stretched polyester film.
[0077]
This biaxially stretched polyester film is referred to as film 1.
[0078]
The obtained film 1 is attached to a metal frame having an opening having the same shape as the emission surface of the evaluation surface light source while applying a slight tension so that wrinkles and undulations do not enter, and placed on the emission surface of the evaluation surface light source. And evaluated as a light diffusing film. Table 1 shows the evaluation results.
[0079]
[Example 2]
One side of a transparent PET film (188 μm in thickness, “Lumilar” 188U34, manufactured by Toray Industries, Inc.) was coated with a hot melt adhesive (Aronmelt PES-304) manufactured by Toa Gosei Co., Ltd. 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. on both sides of the film 1 obtained in Example 1 using a hot laminator to obtain a light diffusion film having a thickness of 505 μm.
[0081]
Since the obtained film is a strong biaxially stretched PET film laminate having a high strength and a thickness of 505 μm, it is very stiff and has a light diffusing property without requiring a supporting structure such as a metal frame. 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 in the cooling drum, and the final film thickness was set to 100 μm.
[0083]
The obtained film was evaluated as a light-diffusing film in the same manner as in Film 1. The evaluation results are shown in Table 1.
[0084]
[Example 4]
Raw materials having the following compositions 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 parts by weight
Sub-extruder B:
PET: 100 parts by weight
Barium sulfate particles having an average particle size 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 the raw materials of the main extruder A were co-extruded from the slit-shaped die so that the molten raw materials of the sub-extruder B became both inner layers. Thereafter, the mixture was quenched with a mirror cooling drum whose surface temperature was maintained at 25 ° C. to obtain a substantially amorphous composite unstretched sheet film. The thickness ratio of the composite unstretched sheet was adjusted 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 sheet end was gripped by a clip and guided 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. Further, heat treatment was continuously performed in an atmosphere at 235 ° C. for 10 seconds to obtain a biaxially stretched polyester film having a thickness of 125 μm.
[0087]
The obtained film was evaluated as a light-diffusing film in the same manner as in Film 1. The evaluation results are shown in Table 1.
[0088]
[Comparative Example 1]
Raw materials having the following compositions 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 the raw materials of the main extruder A were co-extruded from the slit-shaped die so that the molten raw materials of the sub-extruder B became both inner layers. Thereafter, the mixture was quenched with a mirror cooling drum whose surface temperature was maintained at 25 ° C. to obtain a substantially amorphous composite unstretched sheet film. The thickness ratio of the composite unstretched sheet was adjusted 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 by a clip and guided into a tenter heated to 95 ° C. for preheating, and then continuously stretched 3.5 times in the width direction in an atmosphere at 100 ° C. Further, a heat treatment was continuously performed in an atmosphere at 220 ° C. for 10 seconds to obtain a biaxially stretched polyester film having a thickness of 125 μm.
[0091]
The obtained film was evaluated as a light-diffusing film in the same manner as in 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 and high uniformity but low luminance.
[0092]
[Comparative Example 2]
A light diffusing film was obtained in the same manner as in Example 1, except that the raw materials supplied to the main extruder A had 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 in the same manner as in Film 1. The evaluation results are shown in Table 1. The direct-type surface light source using this film has high total light transmittance and high luminance, but lacks practicality due to large luminance unevenness due to the thin film.
[0094]
[Comparative Example 3]
Raw materials having the following compositions 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
Copolymerized polyester obtained by copolymerizing PET with 10.0% by weight of polyethylene glycol: 3.0 parts by weight
Sub-extruder B:
PET: 100 parts by weight
The respective raw materials were melt-extruded at 285 ° C. from extruders A and B, and co-extruded from a slit-shaped die so that the molten raw material of main extruder A became an inner layer and the molten raw material of sub-extruder B became both surface layers. Thereafter, the mixture was quenched with a mirror cooling drum whose surface temperature was maintained at 25 ° C. to obtain a substantially amorphous composite unstretched sheet film. The thickness ratio of the composite unstretched sheet was adjusted 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 sheet end was gripped by a clip and guided into a tenter heated to 85 ° C. for preheating, and then continuously stretched 3.6 times in the width direction in an atmosphere at 105 ° C. Further, heat treatment was continuously performed in an atmosphere at 220 ° C. for 10 seconds to obtain a biaxially stretched polyester film having a thickness of 100 μm.
[0097]
The obtained film was evaluated as a light-diffusing film in the same manner as in 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 had low brightness and low efficiency.
[0098]
From the above results, the light diffusion sheet of the present invention, when used as a light diffusion sheet for a direct-type surface light source, is a thin film, but has high luminance and excellent uniformity by suppressing luminance unevenness. You can see that there is.
[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 mm which is currently used in a direct-type surface light source. In addition, the thickness and weight of the surface light source can be reduced.
[0101]
Furthermore, a thin film has not only the advantages of thinning and weight reduction by itself, but also easy winding in a roll shape. 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 diffusing film that is resistant to heat and humidity can be obtained.
[0103]
The light-diffusing film of the present invention and a direct-type surface light source using the same can be used for backlights for liquid crystal displays, internally illuminated display panels, internally illuminated lighting fixtures, and the like, which require high luminance and high luminance uniformity. It can be used as a light source.
[Brief description of the drawings]
FIG. 1 is a schematic longitudinal sectional view of an apparatus for schematically 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 (6)

A light diffusing film having a total light transmittance T (%) of 15% or more and 60% or less, and a total value of diffuse light reflectance R (%) and total light transmittance T (%) of 80% or more. The light-diffusing film according to claim 1, wherein the total thickness is 1000 µm or less. 3. The light-diffusing film according to claim 1, wherein the film contains fine bubbles. The light diffusing film according to claim 3, wherein the average number of bubbles in the thickness direction of the film is 3 to 30. The light diffusing film according to claim 3, wherein the average flatness of the bubbles is 1.5 or more and 10 or less. A direct-type surface light source on which the light-diffusing film according to claim 1 is mounted.

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