JPH1123814A - Optical control sheet and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture the optical control for a back light unit which converges projection light toward the front and has no glareness on the screen in the case where a liquid crystal display device is used. SOLUTION: The surface temperature of a thermoplastic resin sheet right before a plate is pressed is higher than the glass transition temperature Tg deg.C and the sheet center temperature is lower than (Tg-10) deg.C. When the sheet surface is heated and softened right before the plate is pressed, 0<Q<1000 and 10/Q<t<100/Q re satisfied, where Q is a heating quantity (kW/m<2> ) given to the sheet surface and (t) is a heating time (second); and many recessed parts of 50-10 μm are formed on the sheet surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light control sheet made of a thermoplastic resin sheet to which an embossed shape has been transferred, and a method of manufacturing the same. About.

[0002]

2. Description of the Related Art Generally, a liquid crystal display screen does not have a self-luminous property. Therefore, a light source unit called a backlight unit that shines light from the back of a liquid crystal display element has been conventionally used. Are often used on the side of a liquid crystal display device.

In a planar light source device used in the edge light system, a fluorescent tube (2) is disposed as a light source on one side surface of a transparent light guide plate (1) as shown in FIG. Is formed with a paint dot pattern (3), and a reflection plate (4) is arranged behind the light guide plate. With this dot pattern, light incident from the fluorescent tube on the side surface can be emitted from the light guide plate to the front side at a desired position on the screen.

By the way, as shown in FIG. 6, most of the light emitted from the fluorescent tube (2) and emitted from the light guide plate (1) by the dot pattern (3) is guided by a light guide pattern (5). The distribution is emitted in a direction largely deviated from the normal direction of the light plate. Therefore, a user who views the screen from the normal direction of the light guide plate obtains a small amount of light, and feels the liquid crystal display screen very dark.

Usually, in order to solve this drawback, a method of arranging a light control sheet on the front surface of the light guide plate and suppressing unnecessary light emission in the oblique direction is adopted. As a conventional light control sheet, the surface of the thermoplastic resin sheet has a roughness Ra = 100.
There is known one having fine random irregularities of about 500 μm (JP-A-62-291618). However, in the case of random irregularities having a large roughness, the amount of emitted light in the oblique direction is still large, and there is a limit to increasing the brightness of the backlight. Therefore, the applicant has previously described a sheet surface such as that shown in FIG.
Invented a sheet in which 0 μm prism-shaped projections were transferred in parallel arrangement and provided with a 50 μm coating layer on the back of the sheet, and filed an application that improved both the light-collecting property and the amount of emitted light. (JP-A-7-230001). (Hereinafter, such a sheet in which the surface of a thermoplastic resin sheet is processed into a prism shape is referred to as a “prism sheet”.)

[0006]

Incidentally, the pitch of the projections of the prism is 100 to 20 on the prism sheet.
Due to the coarseness of 0 μm, there is a problem that the light and shade of light generated by the prism can be visually observed, and the screen appears to be streaky. If the pitch of the projections is reduced to less than 100 μm, the light and shade of the light generated by the prism cannot be visually observed, and the screen does not cause the glare. However, conventionally known methods for producing a prism sheet (casting, solvent casting, profile extrusion, roll embossing while extrusion, hot pressing with a flat plate,
In the monomer casting method, the injection molding method, and the like), the influence of the surface tension of the molten resin cannot be ignored, and it has been impossible or very difficult to reduce the pitch of the projections to less than 100 μm.

The present inventor has pointed out that the conventional manufacturing method has mainly been devised such as control of the molding pressure, and has not been studied in detail of the heating temperature control closely related to the surface tension of the molten resin. By focusing attention, it has been found that there is a specific relationship between the molten resin temperature and the heating condition of the sheet surface for suppressing the influence of surface tension, and the present invention has been completed.
That is, the present invention controls the heating amount and the heating time of the sheet, so that when used in a liquid crystal display device, the backlight has a light-collecting property such that the front direction becomes sufficiently bright and does not cause glare on the screen. It is an object to provide a light control sheet for a unit.

[0008]

According to a first aspect of the present invention, there is provided a light control sheet comprising a thermoplastic resin sheet having a large number of prisms each having a substantially triangular cross section formed on the sheet surface. The light control sheet is characterized in that the pitch of the large number of projected portions is not less than 50 μm and less than 100 μm.

The material of the thermoplastic resin sheet in the first aspect of the present invention is not particularly limited. Examples of the light control sheet include polycarbonate, polymethyl methacrylate, polysulfone, polyethersulfone, polyarylate, polyetherimide, and polyetherimide. A resin material having good light transmittance, such as methylpentene and amorphous copolyester, is suitable. The pitch is measured by obtaining the distance between the vertices of the convex portions between the adjacent prisms. The pitch of the prism sheet must be 50 μm or more and less than 100 μm. If the pitch is 100 μm or more, a streak-like glare is generated on the prism sheet incorporated in the backlight, and visibility is deteriorated. If the pitch is less than 50 μm, it is necessary to more precisely control the temperature of the sheet during the transfer of the embossed shape, so that the sheet cannot be manufactured stably.

[0010] According to the second aspect of the present invention, the surface of the thermoplastic resin sheet is heated and softened, and then a plate having a reverse embossed pattern having a desired embossed shape of 50 µm or more and less than 100 µm is pressed on the sheet surface to transfer the embossed shape. When the glass transition temperature of the thermoplastic resin is Tg ° C., the sheet surface temperature immediately before pressing the plate is Tg ° C. or higher, and the sheet central temperature is (Tg−
10) The amount of heat QkW / m ² given to the sheet surface when heating and softening the sheet surface at a temperature of not more than 0 ° C. and the heating time t
A method for manufacturing a light control sheet, characterized in that heating is performed so that seconds satisfy the following relationship. 10 <Q <1000 and 10 / Q <t <100 / Q

The material of the thermoplastic resin sheet in the second aspect of the present invention is not particularly limited. However, as the light control sheet, a resin material having good light transmittance, such as polycarbonate and polymethyl methacrylate, is preferable. The method for producing the thermoplastic resin sheet includes, but is not limited to, an extrusion molding method, an injection molding method, a solvent casting method, and a monomer casting method. The sheet surface temperature immediately before pressing the plate is Tg ° C. or higher, and the sheet center temperature is (Tg−
10) It must be below ° C. Sheet surface temperature is T
If it is less than g, the sheet surface is unlikely to be plastically deformed and the emboss transferability is deteriorated. Also, the sheet center temperature is (Tg-10)
When the temperature exceeds ℃, the pressure for pressing the plate is also used for the resin flow in the surface direction, and the pressure used for the plastic deformation of the sheet surface is substantially lower than the pressure for pressing the plate. It becomes difficult to transfer irregularities. The temperature of the sheet surface and the center of the sheet in the emboss transfer is measured under each heating condition in advance using a sheet in which a thermocouple is attached to the sheet surface and the center of the sheet, and the temperature of the sheet is set within a desired temperature range. So that it can be stored.

The method of heating the sheet surface includes non-contact heating such as hot air, an infrared heater, a halogen lamp heater, and the like.
There is contact heating of a heating roll, a hot plate or the like, but there is no particular limitation. The method of heating the sheet surface is such that when the sheet surface is heated and softened, the amount of heat QkW / m ² applied to the sheet surface and the heating time t seconds are 10 <Q <1000 and 10 / Q <
The heating is performed so as to satisfy the relational expression of t <100 / Q. When Q is 10 kW / m ² or less, the heating amount is too low, and the difference between the sheet surface temperature and the sheet center temperature becomes small. Q is 10
If it is more than 00 kW / m ² , the temperature of the sheet surface will rise rapidly, and the resin on the sheet surface may be degraded or discolored by oxidation or thermal decomposition. Further, 10 / Q <t <100 / Q is a relational expression obtained by an experiment, and there is no particular unit. In this relation, when the heating time t is (10 / Q) seconds or less, the sheet surface temperature does not rise to Tg or more, and the sheet surface is hardly plastically deformed, so that the emboss transferability deteriorates. When t is (100 / Q) seconds or more, the temperature of the sheet becomes uniform, the center temperature of the sheet becomes higher than (Tg-10) ° C., and the pressure for bonding is substantially reduced, so that fine pitch irregularities cannot be transferred. .

As a molding method for transferring an embossed shape to the surface of the sheet softened by heating as described above, there are a conventionally known roll embossing method and a hot press method for a flat plate.
Any of them can be used and is not particularly limited. (Function) According to the present invention, by stably and efficiently producing a prism sheet having a convex portion pitch of less than 100 μm, the light and shade of light generated by the prism cannot be visually observed, and the light is sufficiently bright in the front direction and no glare occurs. LCD screen can be provided.

[0014]

Embodiments of the present invention will be described below. Example 1 Tg = 150 ° C., thermal conductivity λ = 1.9 × 10 ⁻⁴ kW / m
C. The polycarbonate resin at a temperature of .degree. C. was melt-extruded from a T-die and cooled on a cooling roll to prepare two types of sheets having a thickness of 0.1 mm and 0.2 mm. Two sheets of 0.1mm thickness are stacked, a K thermocouple with a wire diameter of 0.2mm is stuck between the surface and the middle, passed through a 250 ° C hot air oven and heated for 1 second to measure the sheet surface and center temperature did. At this time, the temperature difference between the surface and the center was 40 ° C. at the maximum. by the way,
The calorific value QkW / m ² given to the sheet surface was calculated by first measuring the maximum value of the temperature difference ΔT ° C. between the sheet surface and the sheet center in the heating step by attaching a thermocouple to the sheet surface and the sheet center. The thermal conductivity of the sheet λkW / m ° C;
The distance dmm between two thermocouples corresponding to half of the sheet thickness,
And ΔT ° C. were substituted into Q = λ · ΔT / d. When the amount of heat applied to the sheet surface by this method was calculated, Q = 76 kW / m ² . In addition, heating was started at 0.
After 2 seconds, the temperature of the sheet surface is 160 ° C. and the central temperature is 13 ° C.
It was 0 ° C. Next, a sheet having a thickness of 0.2 mm was
℃ hot air oven, immediately after heating for 0.2 seconds,
Pressing was performed with a cooling roll having a surface temperature of 140 ° C. and a large number of grooves formed in the circumferential direction to obtain a light control sheet having a large number of convex portions on the MD. The cross-sectional shape of the groove of the cooling roll used is as shown in FIG. 4, the depth of the groove is 25 μm, the vertex angle of the groove is 90 °, and the pitch of the groove is 50 μm. The cross-sectional shape of the obtained sheet is as shown in FIG. 1, the groove depth is 23 μm, the apex angle of the groove is 90 °, the pitch of the projections is 50 μm, and the radius of curvature of the projections is 6
μm.

Performance test As an evaluation method, luminance measurement of JCS-C7614 was performed. In FIG. 8, the light source has a thickness of 35 mm and a length of 13 mm.
5mm fluorescent tube (2), dot pattern (3) on the back
The light guide plate (1) having The light guide plate (1) is made of a polymethyl methacrylate plate having a thickness of 3 mm, a width of 205 mm, and a length of 135 mm. A reflection plate (4) was arranged on the rear side of the light guide plate (1), and a light control sheet (S) was arranged on the front side of the light guide plate (1). Further, a liquid crystal display element (8) is disposed on the front side of the light control sheet (S). The reflector is made of an opaque polyethylene terephthalate film. When the luminance in the normal direction at an applied voltage of 12 V was measured for the edge-light type backlight of each configuration described above, it was 500 cd / m ² . In addition, there was no visual problem such as glare on the screen.

COMPARATIVE EXAMPLE 1 Two sheets each having a thickness of 0.1 mm were superposed in the same manner as in Example 1, and a K thermocouple having a wire diameter of 0.2 mm was attached between the surface and the middle.
The sheet was passed through a hot air oven at 250 ° C., heated for 1 second, and the sheet surface and center temperatures were measured. The amount of heat applied to the sheet surface was Q = 76 kW / m ² as in Example 1. The sheet surface temperature 0.1 second after the start of heating is 14
The temperature was 0 ° C. and the central temperature was 100 ° C. Next, thickness 0.2
mm sheet was passed through a hot air oven at 250 ° C. and the heating time was changed to 0.1 second in the same manner as in Example 1 except that the heating time was changed to 0.1 second.
A light control sheet having a large number of ridges was obtained. The cross-sectional shape of the obtained sheet is as shown in FIG. 2, and the groove depth is 15 μm.
The vertex angle on the m-projection was 90 °, the pitch of the projections was 50 μm, and the radius of curvature of the projections was 25 μm. When the luminance was measured in the same manner as in Example 1, the luminance in the normal direction was 400
It was as dark as cd / m ² and visibility was poor.

Comparative Example 2 In the same manner as in Example 1, two sheets each having a thickness of 0.1 mm were overlapped, a K-line thermocouple having a wire diameter of 0.2 mm was attached between the surface and the middle, and passed through a hot-air oven at 250 ° C. for 1 second. The sheet was heated and the sheet surface and center temperatures were measured. The amount of heat applied to the sheet surface was Q = 76 kW / m ² as in Example 1. The sheet surface temperature 2 seconds after the start of heating is 250
° C and the central temperature was 240 ° C. Next, thickness 0.2m
m was passed through a hot-air oven at 250 ° C., and the light control sheet having many convex stripes in the MD was obtained in the same manner as in Example 1 except that the heating time was changed to 2 seconds. The cross-sectional shape of the obtained sheet was as shown in FIG. 3, the groove depth was 20 μm, the apex angle of the ridge was 90 °, the pitch of the ridges was 50 μm, and the radius of curvature of the ridges was 15 μm. When the luminance was measured in the same manner as in Example 1, the luminance in the normal direction was 450 cd.
/ M ² and the visibility was poor.

Table 1 shows the results.

[Table 1]

According to the present invention, the pitch of the projections is 100 μm.
m can be stably and efficiently manufactured. Further, according to the present invention, it is possible to provide a light control sheet for a backlight unit which has a light-collecting property such that the front direction becomes sufficiently bright when used in a liquid crystal display device and does not cause glare on a screen. it can.

[0019]

[Brief description of the drawings]

FIG. 1 is a cross-sectional view in the width direction showing a shape of a convex portion of a light control sheet obtained in Example 1.

FIG. 2 is a cross-sectional view in the width direction showing a shape of a convex portion of the light control sheet obtained in Comparative Example 1.

FIG. 3 is a cross-sectional view in the width direction showing a shape of a convex portion of the light control sheet obtained in Comparative Example 2.

FIG. 4 is a cross-sectional view showing the shape of the groove of the cooling roll for pressure bonding used for molding the light control sheet in Example 1.

FIG. 5 is a schematic diagram of an edge light type backlight unit.

FIG. 6 is a schematic diagram showing emission light characteristics of the light guide plate alone.

FIG. 7 shows an example of a prism sheet type light control sheet.

FIG. 8 is a schematic diagram showing a light source unit used for a performance test of the light control sheet.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Light guide plate 2 ... Fluorescent tube 3 ... Dot pattern 4 ... Reflector 5 ... Outgoing light pattern distribution 6 ... Convex part of a prism sheet 7 ... Groove of a prism sheet 8 ... Liquid crystal display element S ... Light control sheet

Claims (2)

[Claims] 1. A light control sheet comprising a thermoplastic resin sheet having a large number of prisms each having a substantially triangular cross section formed on a sheet surface, wherein a pitch of a large number of convex portions formed on the sheet surface is 50 μm or more and 100 μm or more. A light control sheet, wherein: 2. The surface of the thermoplastic resin sheet is heated and softened, and then the desired embossed shape of 50 μm is formed on the sheet surface.
In the method for producing a light control sheet for transferring an embossed shape by pressing a plate having an inverted concavo-convex pattern of not more than 100 μm, the glass transition temperature of the thermoplastic resin is set at Tg ° C.
When the sheet surface temperature immediately before the plate is pressed is Tg
C. or more, the sheet center temperature is (Tg-10) ° C. or less, and the heating is performed so that the heat quantity QkW / m ² given to the sheet surface when heating and softening the sheet surface and the heating time t seconds satisfy the following relationship. A method for manufacturing a light control sheet, comprising: 10 <Q <1000 and 10 / Q <t <100 / Q