JPH07174911A - Prism sheet - Google Patents

Abstract

PURPOSE:To obtain a prism sheet having improved front luminance and ensuring satisfactory handleableness and productivity by forming an array of prisms on one side of a transparent substrate from a UV-curing resin compsn. having a higher refractive index than the substrate. CONSTITUTION:A UV-curing resin compsn. having a higher refractive index than a transparent substrate 13 is injected into a prism mold, the substrate 13 is superposed and the compsn. is polymerized by irradiation with UV from a UV light source through the substrate 13 to form an array of prisms 14 each having 80 to 150 deg. vertical angle on one side of the substrate 13. This array is released from the mold and the objective prism sheet is obtd. The UV-curing resin compsn. contains a multifunctional (meth)acrylic compd. such as bis(methacryloylthiophenyl) sulfide. The substrate 13 may be a flexible glass sheet transmitting UV, but a film of a transparent synthetic resin such as acrylic resin or polycarbonate resin is preferably used in general.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for improving the front brightness of a backlight unit used in a liquid crystal display device or the like by optical means.

[0002]

2. Description of the Related Art In recent years, power consumption is large in a portable notebook personal computer equipped with a color liquid crystal display device, a portable liquid crystal TV using a color liquid crystal panel, a video integrated liquid crystal TV, or the like which is driven by a battery. The liquid crystal display device is an obstacle to extending the battery drive time. Above all, the proportion of power consumption of the backlight used for this is large, and keeping it low is an important target for extending the battery drive time and increasing the practical value of the above products.

At this time, if the brightness of the backlight is reduced in order to suppress power consumption, the display is difficult to see, which is not preferable. Therefore, in order to suppress the power consumption without sacrificing the brightness, it is desired to improve the optical efficiency of the backlight, but as a means to realize this,
As shown in FIG. 1, the method of placing the prism sheet 1 having the prism array 2 on one surface on the light emitting surface 4 of the backlight 3 is as follows.
It is currently in practical use.

This prism sheet is formed by press-molding a prism array on one surface of a thermoplastic transparent plastic plate, or a prism array is formed on one surface of a transparent sheet by an ultraviolet curable resin composition.
Thermoplastic transparent plastic is PMMA (refractive index: n = 1.
49), PC (n = 1.59), and other UV-curable resin compositions use acrylate resins with a refractive index of about 1.49 to 1.55.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have completed the present invention as a result of repeated studies aiming at clarifying the form of the prism sheet having a better effect of improving the brightness.

[0006]

[Means for Solving the Problems] First, as a result of analyzing the mechanism of the brightness enhancement by the prism sheet, it was found that the following two actions were working.

The first action is due to the refraction action of light rays by the prism sheet. Light rays 8 and 8'in FIG. 2 indicate light rays that enter the prism sheet from the light emitting surface 4 and exit in the front direction. As can be seen from FIG. 2, since these light rays are refracted by the incident surface 10 and the prism inclined surfaces 11 and 11 ′, the incident angle when entering the prism sheet is as follows.
It becomes α of (1), which is different from the ray component that was originally directed in the front direction.

[0008]

[Equation 1]

In the formula (1), n is the refractive index of the prism. Generally, the directivity of the edge light type backlight used in a thin display has a property that the light density increases as the emission angle increases. By utilizing the light ray component of the angle α, the front brightness is increased.

The effect of improving the front luminance due to this effect is largely influenced by the directivity of the backlight used, and this effect can hardly be expected in a direct type backlight as shown in FIG.

The second function of the prism sheet is to increase the brightness of the light emitting surface of the backlight by the totally reflected light from the prism surface. Light rays 9 in FIG. 2 enter the prism sheet from the light emitting surface 4, are totally reflected by the prism surface, and return to the light emitting surface again. At this time, the light ray 9 must make an incident angle with the normal to both slopes of the prism and at a critical angle θc (= sin ⁻¹ (1 / n): n is the refractive index of the prism) or more.
Therefore, the size of such totally reflected light depends on the apex angle and refractive index of the prism, the directivity of the backlight, etc., but a typical prism sheet with a prism apex angle of 90 ° to 110 ° and a refractive index of 1.49 to 1.59. In the case of omnidirectional backlight,
It is estimated that about 30 to 40% of the total incident light is totally reflected and returns to the light emitting surface 4. A diffuser sheet that diffuses and reflects normal light is used for the light emitting surface 4. By diffusing and reflecting the totally reflected light 9, the brightness of the light emitting surface 4 is increased, and as a result, the front brightness is increased. The increase in the front luminance due to this action is recognized in both the edge light type and the direct type. For example, a PMMA prism sheet with a prism apex angle of 90 ° is used for the direct type backlight as shown in FIG. The front brightness increase was measured.

In practice, these two effects combine
These effects depend on the prism apex angle and the refractive index of the prism, since a front brightness increase of ~ 50% or more is achieved.

First, the smaller the prism apex angle, the larger the angle α, so the first effect can be expected to increase. On the other hand, however, the total reflected light beam 9 decreases and the second effect decreases. Therefore, there is an optimum value for the prism apex angle, and a value of 90 ° to 110 ° is most often adopted.

On the other hand, the higher the refractive index of the prism, the greater the angle α, and the greater the first effect, the critical angle θ
The increase of the total reflected light beam 9 due to the decrease of c increases the second action, and good results can be expected through both actions.

As described above, it has been found that it is preferable to use a material having a high refractive index for the material of the prism in order to increase the front luminance, but on the other hand, a material having a high refractive index has a low light transmittance. It also has the disadvantage of being easy. One of the causes is the principle that the Fresnel reflection on the surface increases as the refractive index increases, but the other cause that occurs especially in the case of transparent plastic is a molecule containing a double bond with a high refractive index. The structure is likely to cause coloring and unevenness, and the light transmittance may be reduced by absorption or scattering. For this reason, a prism sheet using only high refractive index plastic does not always give good results.

That is, it is preferable to use a material having a refractive index as high as possible in order to enhance the front luminance improving effect of the prism sheet, but conversely, in order to suppress the causes of loss such as surface reflection, absorption and scattering, PMMA and the like ( There is a contradiction that materials with a relatively low refractive index (n = 1.492) are preferred.

The prism sheet of the present invention uses a plastic having a relatively low refractive index and a high transparency as a base and a plastic having a relatively high refractive index as a material of the prism so that the prism sheet has a two-layer structure and thus the above-mentioned contradiction occurs. In addition, by making the prism layer an ultraviolet-curable resin composition, the refractive index can be easily adjusted and the workability is also improved.

That is, by using a sheet of PMMA, PC, etc., having a thickness of several hundreds of μm, which is excellent in transparency, as the base film, excellent handling and transparency of the product are ensured, and prism rows are formed by ultraviolet curing. As a result, excellent workability is realized. That is, since the prism layer of the ultraviolet curable resin composition is as thin as several tens of μm, even if the transparency is somewhat inferior, the light intensity is not attenuated here, and the physical strength of the prism sheet is several hundreds of μm. The layer is determined, and UV curing also improves the productivity.

The UV-curable resin composition used in the present invention includes bis (methacryloylthiophenyl) sulphoid, 2,4-dibromophenyl (meth) acrylate, 2,3,5-tribromophenyl (meth) acrylate, 2,2-bis (4- (meth) acryloyloxyphenyl) -propane, 2,2-bis (4- (meth) acryloyloxyethoxyphenyl) -propane, 2,
2-bis (4- (meth) acryloyloxydiethoxyphenyl) -propane, 2,2-bis (4- (meth) acryloyloxypentaethoxyphenyl) -propane,
2,2-bis (4- (meth) acryloyloxyethoxy-3,5-dibromophenyl) -propane, 2,2-
Bis (4- (meth) acryloyloxydiethoxy-
3,5-dibromophenyl) -propane, 2,2-bis (4- (meth) acryloyloxypentaethoxy-
3,5-Dibromophenyl) -propane, 2,2-bis (4- (meth) acryloyloxyethoxy-3,5-
Dimethylphenyl) -propane, 2,2-bis (4-
(Meth) acryloyloxyethoxy-3-phenylphenyl) -propane, bis (4- (meth) acryloyloxyphenyl) -sulfone, bis (4- (meth) acryloyloxyethoxyphenyl) -sulfone, bis (4- (meth) ) Acryloyloxydiethoxyphenyl) -sulfone, bis (4- (meth) acryloyloxypentaethoxyphenyl) -sulfone, bis (4
-(Meth) acryloyloxyethoxy-3-phenylphenyl) -sulfone, bis (4- (meth) acryloyloxyethoxy-3,5-dimethylphenyl) -sulfone, bis (4- (meth) acryloyloxyphenyl) -sulfide , Bis (4- (meth) acryloyloxyethoxyphenyl) -sulfide, bis (4-
(Meth) acryloyloxypentaethoxyphenyl)
-Sulfide, bis (4- (meth) acryloyloxyethoxy-3-phenylphenyl) -sulfide, bis (4- (meth) acryloyloxyethoxy-3,5-
Examples thereof include polyfunctional (meth) acrylic compounds such as dimethylphenyl) -sulfide, di ((meth) acryloyloxyethoxy) -phosphate and tri ((meth) acryloyloxyethoxy) -phosphate. These are used in one type or in a mixed system of two or more types, but the refractive index after mixing needs to be higher than the refractive index of the base material.

The details of the present invention will be described below with reference to the drawings. The prism sheet according to the present invention comprises a transparent base material 13 and an ultraviolet curable resin 14 as shown in FIG. The material of the transparent substrate 13 may be a flexible glass plate that allows passage of ultraviolet rays, but is generally an acrylic resin, a polycarbonate resin,
A transparent synthetic resin film such as a vinyl chloride resin or a polymethacrylimide resin is desirable. Among them, PMMA having a relatively low refractive index and extremely high transparency is considered to be the most suitable resin at present. In a preferred embodiment, the refractive index of the transparent substrate can be 1.50 or less, and the refractive index of the prism can be 1.55 or more. In still another aspect, it is preferable that the difference in refractive index between the transparent substrate and the ultraviolet curable resin composition is 0.2 or more.

Next, as shown in FIG. 5, an ultraviolet curable resin composition 17 for forming a prism shape is prepared, poured into the prism mold 16, and then the transparent base material 13 is superposed on the transparent base material 13. Ultraviolet rays are radiated from the ultraviolet ray source 18 through the polymer to be polymerized, and then peeled as shown in FIG. 6 to obtain the prism sheet 15.

The resin composition is composed of an ultraviolet curable resin, a catalyst that generates radicals upon irradiation with ultraviolet rays, and the like. Examples of the ultraviolet curable resin used at this time include known resins such as ester (meth) acrylate-based, epoxy (meth) acrylate-based, urethane (meth) acrylate-based resins, and at least a polymerizable unsaturated double resin in the molecule. A compound having a bond can be used. Examples of the polymerizable unsaturated double bond include (meth) acryloyl group.

Further, in order to control the refractive index of the above-mentioned ultraviolet curable resin composition, styrene, vinyltoluene, chlorostyrene, dichlorostyrene, bromostyrene, dibromostyrene, divinylbenzene, 1-vinylnaphthalene, 2-vinylnaphthalene. , Vinyl compounds such as N-vinylpyrrolidone, (meth) acrylates such as phenyl (meth) acrylate, benzyl (meth) acrylate, biphenyl (meth) acrylate, allyl such as diallyl phthalate, dimethallyl phthalate and diallyl biphenylate Compounds, (meth) acrylic acid and barium,
A metal salt of lead, antimony, titanium, tin, zinc or the like can be added to the above-mentioned ultraviolet curable resin liquid as long as the curing is not hindered. These are used alone or in a mixture of two or more.

As a catalyst which is mixed with the above ultraviolet curable resin composition and generates radicals upon irradiation with ultraviolet rays, 2
-Hydroxy-2-methyl-1-phenylpropane-1
-One, hydroxycyclohexyl phenyl ketone, methylphenyl glyoxylate, 2,4,6-trimethylbenzoyldiphenylphosphine oxide, benzyl dimethyl ketal and the like can be used.

If necessary, additives such as an antioxidant, an ultraviolet absorber, an anti-yellowing agent, a bluing agent, a pigment and a diffusing agent are added to the ultraviolet curable resin liquid of the present invention so as to prevent the curing. You may mix in the range.

It is desirable that the dope in which the ultraviolet curable resin, the polymer for adjusting the refractive index, the catalyst and the like are mixed is defoamed in advance and that the dust in the liquid is filtered by a filter.

The prism mold 16 having the prism shape shown in FIG. 7 used for forming the prism sheet shape includes metal molds such as aluminum, brass, steel, silicon resin, urethane resin, epoxy resin and ABS. It is possible to use a mold made of a resin, a fluorine resin, a synthetic resin such as polymethylpentene resin, and / or a mold made by plating the above materials or mixing various metal powders. It is desirable to use a metal mold in terms of strength.

As the ultraviolet light source 18, an ordinary ultraviolet light source such as a fluorescent lamp, a chemical lamp, a high pressure mercury lamp, a low pressure mercury lamp, a metal halide lamp or the like can be used.

[0029]

【Example】

Example An ultraviolet curable resin liquid having the following composition was applied to a roughly brass A4 size plate as shown in FIG.
m and a prism shape with an apex angle of 95 ° were engraved into the prism mold of Example 1, and then PMMA sheets 13 having a thickness of 250 μm and having substantially the same size were overlaid, and then the PMMA
80w / c installed at 300mm above the sheet
The resin was cured by irradiating it with ultraviolet rays for 30 seconds with an ultraviolet lamp of 6.4 kw manufactured by Western Quartz Co., Ltd. with an irradiation intensity of m, and then peeled to obtain a prism sheet.

* UV-curable resin composition bis (methacryloylthiophenyl) sulfoid, 2,4-dibromophenyl (meth) acrylate: 60% by weight trimethylolpropane triacrylate: 40% by weight manufactured by Merck ( Photocuring catalyst) "Darocur 1173" ・ ・ ・ 1.5% by weight (based on the sum of the above 4 resin liquids)

The refractive index of the resin composition after polymerization is n
= 1.63.

Next, as shown in FIG. 1, the prism sheet is set on the backlight unit 3 including the cold cathode tube 5 made by Stanley, the acrylic light guide 7 made by Mitsubishi Rayon, and the diffusion film 4 made by Toray. The brightness was measured with a BM7 type brightness meter manufactured by Topcon Corporation from 1 m directly above the backlight unit.

Comparative Example 1 An ultraviolet-curing resin liquid having the composition of Example 1 was poured into a silicone rubber packing around the prism mold 16 of Example 1, and then glass was formed so as to prevent bubbles from entering. The plate was covered and fixed, ultraviolet rays were similarly irradiated to cure the resin, and then the resin portion was peeled from the glass to obtain a prism sheet. Luminance was similarly measured using this. The prism sheet obtained had no strength and was difficult to handle.

Comparative Example 2 A 250 mm thick PMMA film manufactured by Mitsubishi Rayon Co., which is slightly smaller than the engraved portion of the prism type 16 of Example 1, was superposed on the engraved portion, and then a 3 mm polished stainless steel plate was used. After stacking, stack 1 above
While heating at 80 ° C., a load of 50 t was evenly applied, and after standing for 3 hours, it was peeled after waiting for cooling, to prepare a prism sheet. Luminance was similarly measured using this. The refractive index of PMMA film manufactured by Mitsubishi Rayon Co., Ltd.
It was n = 1.492.

Table 1 shows the measurement results of the above Examples and Comparative Examples 1 and 2. Here, the luminance when there is no prism sheet is 1, and the luminance when each prism sheet is placed is represented by a ratio. It can be seen that the examples of the present invention have a better brightness improving effect than any of the comparative examples.

[0036]

[Table 1]

The brightness ratios shown in Table 1 vary in absolute value when the backlight used is different, but within the range examined by the present inventors, the brightness ratio is different from that of the comparative example. The magnitude relationship of the examples did not reverse.

[0038]

As described above, according to the present invention, by forming a prism array on one surface of a transparent substrate with an ultraviolet curable resin composition having a refractive index higher than that of the transparent substrate, the effect of improving front luminance can be obtained. It was possible to realize a prism sheet that is high and has good handleability and productivity.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a prism sheet and its usage pattern.

FIG. 2 is a functional explanatory diagram of a prism sheet.

FIG. 3 is a diagram showing different usage forms of the prism sheet.

FIG. 4 is a cross-sectional view of the prism sheet of claim 2.

FIG. 5 is a diagram illustrating a method for producing the prism sheet according to claim 2;

FIG. 6 is a diagram illustrating a method of producing the prism sheet according to claim 2;

FIG. 7 is a perspective view of an example of a prism type for a prism sheet.

[Explanation of symbols]

 1 ... Prism sheet 2 ... Prism row 3 ... Backlight 4 ... Diffusion film 5 ... Cold cathode tube 6 ... Reflective film 7 ... Light guide 8,8 '...・ Light rays coming out in front 9 ・ ・ ・ Front reflected light 10 ・ ・ ・ Incident surface of prism sheet 11, 11 '・ ・ ・ Slope of prism 12 ・ ・ ・ Reflector 13 ・ ・ ・ Base film 14 ・ ・ ・ By UV curable resin Prism 15 ・ ・ ・ Prism sheet 16 ・ ・ ・ Prism type 17 ・ ・ ・ UV curable resin liquid 18 ・ ・ ・ UV light source

Claims (1)

[Claims] 1. A prism sheet in which a prism row having an apex angle of 80 ° to 150 ° is formed on one surface of a transparent substrate by an ultraviolet curable resin composition having a refractive index higher than that of the transparent substrate.