JPH0427902A - Optical sheet and liquid crystal display device using same - Google Patents

Abstract

PURPOSE:To reduce coloring of display and to improve the contrast by using an optical sheet made of a synthetic resin having a unique refraction characteristic to reduce the difference between the picture on a liquid crystal display device (LCD) viewed from the front and that viewed slantwise. CONSTITUTION:Boundary faces between the surfaces of a material 2 kept at a temperature higher than the glass transition temperature and lower than the melting point and inside faces of compression dies 1 are made lubricative, and the material 2 is compressed by compression dies 1 and is multiaxially orientated by plug flow and is cooled as it is to obtain a multiaxially orientated sheet 3. In this case, various elements are controlled to satisfy inequalities where (d), bmax, nmin, and n2 are the thickness, a maximum refractive index in each direction on a plane, a minimum refractive index, and the refractive index in the perpendicular direction of this sheet 3 respectively. When this optical sheet is used in the LCD, it eliminates the phase difference between the picture on the LCD viewed from the front and that viewed slantwise.

Description

Detailed Description of the Invention [Industrial Application Decomposition] The present invention relates to an optical sheet made of synthetic resin having unique refractive properties and a liquid crystal display device (hereinafter referred to as rLCD) using the optical sheet.
(abbreviated as J).

[Prior Art] Conventionally, as an optical sheet made of synthetic resin, for example, super twisted nematic (STN: 5upper
Twisted Nematic) L using liquid crystal
Retardation compensation sheets for CDs are known.

To further explain the above retardation compensation sheet, STN
In order to achieve black and white display on a liquid crystal LCD, S
It is necessary to remove coloration due to birefringence of TN liquid crystal. As a method for this purpose, a two-layer cell method has been put into practice in which optical compensation liquid crystal cells are further stacked to erase the coloring of the liquid crystal, but the optical compensation cells are expensive, and not only do they make the device thick and heavy, but
Since the transmittance of light rays decreases, it has disadvantages such as requiring a stronger light source.

As a way to solve these drawbacks of the two-layer cell system,
There is a method of replacing it with an optical sheet made of synthetic resin that has optical characteristics similar to those of a liquid crystal cell for optical compensation. This optical sheet is a retardation compensation sheet, which has birefringence characteristics and must be highly transparent.

The uniformity of this retardation compensation sheet is important because the color unevenness of the display is determined by the thickness of the sheet and the uniformity of the birefringence properties.

The above-mentioned retardation compensating sheet is manufactured by stretching a base sheet after heating or while heating.

That is, a difference in the birefringence of light (Δn) is created by creating a difference in the stretching ratio in the vertical and horizontal directions, and the value obtained by multiplying this Δn by the thickness d of the sheet, that is, the retardation R-Δn-d
Compensation is done by aligning the same with the liquid crystal cell.

[Problems to be Solved by the Invention] By the way, the optical sheet used as the conventional retardation compensation sheet described above does not compensate for the retardation when viewed perpendicularly to the liquid crystal cell and the retardation compensation sheet. However, the retardation generated in the liquid crystal differs when viewed perpendicularly to the liquid crystal cell and the retardation compensation sheet and when viewed obliquely. In other words, it is preferable that the retardation compensation be performed in the same way whether viewed perpendicularly or obliquely to the liquid crystal cell and the retardation compensation sheet, but currently there is a problem that causes a difference in this. be.

The object of the present invention is to provide an optical sheet with characteristics that can solve the above-mentioned problems, and to make an LCD easier to see.

[Means and effects for solving the problem] To explain the means claimed in the present invention to solve the above problem, in the invention of claim 1, transparent uniform multiaxial stretching of synthetic resin A sheet, where the thickness of the sheet is d1 the maximum refractive index in each direction on the plane of the sheet (X-Y plane) is nlllllX%, the minimum refractive index is n5aln, and the thickness direction of the sheet (Z direction) is If the refractive index is n2, then Cnma
x-nmin) ・d<20nmnwa
A measure is taken to provide an optical sheet that satisfies a* + n win I n, -l d<50rv. Preferably, n wax + n m1n n, >□, and further 1nx - 2 wax + n + mln l value is (n
An optical sheet having a value of 10 times or more, more preferably 15 times or more, is used.

Further, the invention as claimed in claim 4 requires means for providing an LCD including the above-mentioned optical sheet.

The synthetic resin used in the present invention is a synthetic resin with excellent transparency, and examples of non-crystalline resins include methacrylic resin, polystyrene, and styrene-acrylonitrile copolymer (rA
sJ), polycarbonate, etc. can be used. As the polycarbonate, aromatic polycarbonate synthesized from bisphenol A is used. Further, as the crystalline resin, for example, polyester such as polyethylene terephthalate, polyolefin such as polypropylene, etc. can be used, and polypropylene is particularly suitable.

Among the above synthetic resins, (nmax ns+In) ・d <20nmI of the present invention
As a synthetic resin that can satisfactorily form a uniform multiaxially stretched sheet satisfying nt "ax""' and d>50 nm, it is preferable to use a synthetic resin whose refractive index changes moderately depending on the stretching ratio. However, if a synthetic resin whose refractive index does not easily change is used, it is difficult to increase the may + n m1n n □I-d value, and if a synthetic resin whose refractive index easily changes by stretching is used, (n wax −n mln ) ”It is difficult to reduce the value. From this, the rate of change (I n, -ny
l/stretching ratio) is 5 X 10-' to 5 X 10-'
It is preferable to use a synthetic resin that can be stretched 3 times as much as
It can be calculated by measuring n, -ny l) and dividing that value by d. This value is based on the type of synthetic resin, 1
It varies depending on the stretching conditions such as the degree of axial stretching.

Generally, there is a range of stretching temperature that is most suitable for each synthetic resin, and stretching is carried out within that range. For the methyl methacrylate (hereinafter referred to as rMMAJ) resin described below, 120
The temperature range is preferably 170°C, more preferably 130°C or more.
The temperature range is 160°C.

From the above viewpoint, transparency, hardness, rigidity, etc., in the present invention, M
MA-based resins are preferred, particularly those containing MMA of 50% by weight or more, and more preferably 50 to 95% by weight.

Examples of the above-mentioned MMA-based resin include polymethyl methacrylate (hereinafter referred to as rPMMAJ), a copolymer of MMA and alkyl acrylate (hereinafter referred to as rCo (MMA-A
A) J), MMA and styrene copolymer, MM
A-Maleic anhydride-styrenic ternary copolymer, MM
Examples include A-acrylamide copolymers and blends of these polymers.

Examples of the above blends include PMMA-AS, Co
(MMA-AA) -AS system, CO (MMA-AA)
-Polyvinylidene fluoride etc. can be mentioned.

In addition, MMA resins such as PMMA and Co (MMA-AA) have relatively small photoelastic coefficients, and polycarbonate,
Since polystyrene, AS, etc. have a relatively large photoelastic coefficient, a synthetic resin between these two is most preferable. for example,
PMMA-AS blend, Co (MMA-AA)-
AS blend, Co (MMA-3t), MMA-S
A t-acrylonitrile terpolymer or the like can be used satisfactorily. In this case, it is preferable that the MMA component is contained in an amount of 50 to 95% by weight, particularly 60 to 90% by weight.

The thickness of the optical sheet of the present invention is preferably 1.0 mm or less, more preferably 10 μm to 1.0 mm.

Most preferably it is 50 μm to 200 μm. Further, in the present invention, in addition to the planar optical sheet, a curved optical sheet may be used as necessary.

The uniformity of multiaxial alignment in the present invention can be judged by the rainbow pattern, brightness, etc. when the sheet to be measured is sandwiched between two polarizing plates and viewed through light, but quantitatively it can be measured by a polarized fluorometer. , birefringent glue tarp John (nmax -n+
This can be done by measuring ++In)'d.

In particular, the smaller the retardation, the more uniform the multiaxial stretching is. In the present invention, a small retardation is one of the requirements, so in the present invention, the uniformity of the multiaxial stretching is It is convenient to judge by retardation.

In addition, a method for measuring molecular orientation using a polarized fluorometer is described in J.
, Poly, Sci., c-15,237 (19
86) and Jasco Report, 6.1[i6
(196!l), etc., and is a method of determining the state of fluorescent molecule orientation from the angular distribution of the polarization characteristics of fluorescence emitted by fluorescent molecules fixed in a sheet. The feature of this method is that it utilizes the double optical anisotropy exhibited by fluorescent molecules when absorbing light and when emitting fluorescence, which can only be described in an averaged form as the so-called degree of orientation. The molecular orientation determined by the method is directly measured as an orientation pattern indicating the state of orientation by determining the orientation type.

The optical sheet of the present invention has a thickness of d and a maximum refractive index of 0□8 in each longitudinal direction on the plane of the sheet (X-Y plane).
, the minimum refractive index is nll1in, the thickness direction of the sheet (
If the refractive index in the
This is an optical sheet that satisfies the following: m --= (A) max 10n m1n n, -1.d>50nm= (B). (n5ax of the above formula (A)
The smaller nm1n)'d is, the more homogeneously-multiaxially oriented it is. In the present invention, this (n
The smaller the d value is, the more preferable it is;
Preferably it is 15 rv or less, more preferably 10 nm or less.

Above (B) formula no i n , - 1 lot 2 construction ridge 2 1 d
is a coefficient that indicates the difference in birefringence when light enters the plane of the optical sheet from a direction perpendicular to that when light enters from an oblique direction. The larger this value is, the greater the difference in birefringence that occurs. , preferably 70 nm or more, more preferably 80 to 800 nm.

Particularly preferred is 100 to 500 nm. Further, the sheet of the present invention preferably has a value of (nw
*ax nm1n) value is preferably 10 times or more, more preferably 20 times or more.

The above ng+mx + nm1n and n2 can be measured using various methods. The measurement temperature is room temperature (23° C.), the measurement wavelength is visible light, and the measurement is performed in the range of 400 to 700 r++n. Measuring instruments can be selected appropriately, but for example
Oak's newly manufactured micro polarization spectrophotometer can be used successfully.

When the optical sheet of the present invention is used in an LCD, it can be used to match the phase difference when the LCD is viewed from the front and when viewed from an oblique direction.

That is, when the LCD is viewed from the front, Δn-d is small;
Used when the phase difference becomes large when viewed from an angle. In order to reduce the phase difference when the LCD is viewed diagonally, ln, --shi2-nigo and Lrib-1・d (large one 7 of a) are used.
- Use

, −+nax + nm A sheet with a positive value of m1n−·d and a sheet with a negative value are used as necessary. Sheets made of synthetic resins such as PMMA, AS, polystyrene, etc. that are homogeneously stretched have positive values, and sheets made of polypropylene, polycarbonate, polyvinyl alcohol, etc. that are homogeneously stretched are particularly preferred for the present invention. The above value becomes a negative value.

Next, an example of a method for forming a uniform multiaxially oriented sheet, which is the optical sheet of the present invention, can be formed by, for example, a method in which a synthetic resin base is compressed in a compression die to form a sheet.

The compression molding method shown in FIG. 1 is a method suitable for obtaining a thick multiaxially oriented sheet 3, in which a base material 2 whose temperature is higher than the melting point of the glass 1 and lower than the melting point of the glass 1 is pressed between the surface of the base material 2 and the compression die 1. This is a method in which the interface on the inner surface of the sheet is lubricated, compressed with a compression die 1, the substrate 2 is caused to plug flow to be multiaxially oriented, and then cooled as it is to form a multiaxially oriented sheet 3.

Further, the method shown in FIG. 2 is suitable for obtaining a thin multiaxially oriented sheet 3. That is, FIG. 2(a)~
As shown in (d), 1) two or more layers of the substrate 2 are placed one on top of the other in a non-adhesive state in the compression die 1, and 2) the interface between the inner surface of the compression die 1 and the surface of the substrate 2 is kept in a lubricated state. , 3) Orient the substrate 2 by compressing it at a temperature equal to or higher than the glass transition temperature of the substrate 2, 4) After cooling, take it out from the compression die 1 and press the multiaxially oriented sheet 3, which has been oriented and molded from each substrate 2, into a moving part with respect to each other. This is a compression molding method in which two or more multiaxially oriented sheets 3 are obtained.

In the compression molding method shown in FIGS. 1 and 2, in order to make the interface between the inner surface of the compression die 1 and the surface of the substrate 2 lubricated, a lubricant is applied to the inner surface of the compression die 1 or
This can be achieved by interposing a sheet containing a lubricant at the interface between the inner surface and the substrate 2, or by interposing a sheet containing a lubricant, or both.

In order to compress and orient the substrate 2 at a temperature higher than the glass transition temperature of the substrate 2, the stacked substrates 2 may be uniformly plug-flowed in the compression die 1 by the compression force. Compression die 1 inner surface and substrate 2
By keeping the surface interface well lubricated, uniform plug flow molding can be achieved.

In particular, in order to obtain a multiaxially oriented sheet 3 with high uniformity, it is preferable that the substrate 2 be formed into a circular plate shape and compressed with a circular compression die 1. Further, the stretching ratio depends on the thickness of the substrate 2, but can be selected from a range of 1.5 to 50 times in terms of area ratio, if necessary.

The non-adhesive state described here refers to a state in which the multiaxially oriented sheets 3 formed from the respective substrates 2 can be easily peeled off from each other after two or more layers of substrates 2 are piled up and compression-molded.

The non-adhesive state in the compression molding method shown in FIG. 2 can be obtained by placing a release sheet 4 of a non-adhesive film or sheet on the interface of each substrate 2. In this case, during molding, the viscosity of the release sheet 4 is close to the viscosity of the substrate 2, preferably 1/30 to 30 times. When the difference in viscosity between the substrate 2 and the release sheet 4 is small, when the substrate 2 is stretched by compression, the release sheet 4 is also stably stretched together with it, and a uniform multiaxially oriented sheet 3 can be obtained. In particular, it is preferable that the release sheet 4 has a viscosity of 1 to 20 times the viscosity of the substrate 2 during molding and has a smooth surface. In this case, the surface of the release sheet 4 is transferred to the surface of the multiaxially oriented sheet 3, and the multiaxially oriented sheet 3 with a smooth surface is obtained.

The non-adhesive state between the substrates 2 can be achieved by applying a lubricant or a release agent to the interface between each substrate 2, as shown in FIG. 3(a), without using the above-mentioned release sheet 4. As shown in (b), it can also be obtained by overlapping the non-adhesive substrates 2 and 2° so that they are adjacent to each other.

Further, as shown in FIG. 3(C), it is preferable to vacuum-package the entire stacked base material 2 using a non-adhesive resin film or sheet as a sealing sheet 5 and then perform the molding. In this way, the superimposed base material 2
The multiaxially oriented sheet 3 obtained by leaving air at the interface between
It is possible to prevent the surface condition from deteriorating.

This optical sheet is used, for example, in a 5TN-LCD.

A general STN liquid crystal-LCD includes a light source unit 6, a polarizing film 7.5 and a TNLCD unit 8, as shown in FIG.
and a polarizing film 9.

In the 5TN-LCD described above, since the STN liquid crystal has a different refractive index depending on the wavelength of light, linearly polarized light becomes elliptically polarized light in the STN liquid crystal. Fix this elliptically polarized light! Conventionally, a retardation compensation sheet 10 has been used to make the light S-polarized.

However, it is difficult to completely restore the phase difference using only the retardation compensating sheet 1o, and for this reason, the present optical sheet 11 is used. That is, in the STN liquid crystal and the retardation compensation sheet 10, the LC
Because the discordance when viewing the image of D from an angle is different,
By using the present optical sheet 11, even when viewed obliquely, the polarized light can be made closer to linearly polarized light.

In this case, the optical sheet 11 and the retardation compensating sheet 10 may be placed in either direction. Further, the optical sheet 11 can be placed between the polarizing film 7 on the light source unit 6 side and the 5TN-LCD unit 8, if necessary.

This optical sheet 11 is generally used in combination with a normal retardation compensating sheet 10 as shown in FIG. As a normal phase difference compensating sheet 10, Δ
There are λ plates with n-d in the range of visible light (400 to 700 nIn), 1/2 λ plates that are 1/2 of that, and 1/4 λ plates that are 1/4, and one with an appropriate nm is selected.

In addition, in FIG. 4, 12 is a light guide plate having a reflective layer 13;
14 is a diffusion sheet, 15 is a glass plate, 16 is an STN liquid crystal, 17 is a front plate, and 18 is a light source.

The relationship between the refractive index in each direction of the present optical sheet 11 will be explained.

As shown in FIG. 5, the refractive index of the optical sheet 11 in the X direction and in the direction perpendicular to the X direction (Y direction) is n.
The maximum and minimum refractive index in each direction of the X + y and x-y planes are n□ The refractive index in the Y° direction and the Z direction is nXZn
z°+n)"+nl, and the thickness of the optical sheet 11 is d (mm).

In the optical sheet 11 of the present invention, nxgony (I nmax nm1n I d<20 nm)
++ (1) n X −ny
・・・・・・(2)n z 1n
0.000. (3) n x < n x'
<n! ' or n, > n, '> i!
+...There is the relationship shown in (4). That is,
Since the optical sheet 11 of the present invention is uniformly multiaxially stretched, the formulas (1) to (3) hold true. As mentioned above, the present invention is characterized by
The smaller n + m1nd is, the more preferable it is, preferably 15 nm or less, and more preferably 10 nm or less. Further, it is at least flffi shown in the above formula (5), more preferably from 80 to 800 nm.

This optical sheet 11 must be a normal retardation compensation sheet 10.
It does not have to be used in conjunction with
As shown in FIG. 6, the viewing angle can also be adjusted by arranging only the present optical sheet 11 without using the usual retardation compensating sheet 10. In this case, the present optical sheet 11
If necessary, attach the polarizing film 7 on the light source unit 6 side.
It can also be placed between the 5TN-LCD unit 8 and the 5TN-LCD unit 8.

[Example] Example I AS (styrene/acrylonitrile weight ratio = 80/20
) were dissolved and mixed at a weight ratio of MMA/AS = 85/15, and this was polymerized by cell cast polymerization method to obtain PM
A 1 mm thick sheet with a weight average molecular weight of MA of about 1.8 million and a smooth surface of PMMA/AS=85/15 (weight ratio) was obtained.

Using this sheet as a base material, it was uniformly stretched to an area ratio of 5 times by the compression molding method shown in FIG. 2 to obtain a sheet having a thickness of 0.2111 m. The refractive index of the sheet was as follows. In addition, the following n maX + n 1ain + nX is,
■Measurement was performed at 23°C using a micropolarization spectrophotometer 120AFTJ manufactured by Oak Seisakusho.

nmax = 1.48939 nl, = 1.48931 nX = 1.49140 (nmax sin) ・d
= 16nm n TMF- The above sheet also had a high total light transmittance (92%) and was excellent as an optical sheet.

Example 2 Stack 10 0.1mm thick polypropylene sheets (non-oriented transparent sheets)! ■ A uniformly multiaxially stretched sheet with a thickness of 0.2mm, which was made into a 0.2mm thick piece by uniformly polyaxially stretching the 1mmmm-t as a base material by the compression molding method shown in Figure 2 to an area ratio of 5 times. I got it. The refractive index of the sheet was measured in the same manner as in Example 1 and was as follows.

n,,X=1,49079n+m
ln=1. 49075 n, =1. 48846 (nmax rl+++In) ・d=8
．． 6nm (n, -”” ””’) ・d=456r
+m The above sheet was excellent as an optical sheet.

[Effects of the Invention] The present invention is as explained above, and by using the optical sheet of the present invention in, for example, an LCD using STN liquid crystal, the LCD can be viewed from the front and from an angle. The difference between cases is reduced, coloring of the display is reduced, contrast ratio is improved, and the LCD becomes easier to see.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(b) are explanatory diagrams showing an example of a method for manufacturing a uniform multiaxially stretched sheet that is the optical sheet of the present invention, and FIG.
Figures (a) to (d) are explanatory diagrams showing other examples, and Figure 3 (
a) to (c) are explanatory diagrams of the overlapping state of the substrates, and the fourth
The figure is an explanatory diagram of an LCD using the present optical sheet, Figure 5 is an explanatory diagram of the relationship between the refractive index in each direction of the present optical sheet, and Figure 6 is an explanation showing another example of an LCD using the present optical sheet. It is a diagram. 1: Compression die, 2: Base material, 3: Multiaxially oriented sheet, 4: Release sheet, 5: Sealing sheet, 6: Light source unit, 7: Polarizing film, 8: LCD unit, 9: Polarizing film, 10: Retardation Compensation sheet, 11: Main optical sheet, 1
2: Light guide plate, 13: Reflective layer, 14: Diffusion sheet, 15 Glass plate, 16: STN liquid crystal, 17: Front plate, 18
:light source.

Claims (4)

[Claims] (1) A transparent uniform multiaxially stretched sheet of synthetic resin,
The thickness of the sheet is d, the maximum refractive index in each direction on the plane of the sheet (X-Y plane) is n_m_a_x, the minimum refractive index is n_m_i_n, the thickness direction of the sheet (Z-axis direction)
When the refractive index of is n_z, (n_m_a_x−n_m_i_n)・d<20nm |
n_x−(n_m_a_x+n_m_i_n)/2|・
An optical sheet characterized by satisfying d>50 nm. (2) n_z>(n_m_a_x+n_m_i_n)/
2. The optical sheet according to claim 1, wherein: (3) |n_z−(n_m_a_x+n_m_i_n)
/2 | value is (n_m_a_x - n_m_i_n) value 1
Claim 1 or 2 characterized in that it is 0 times or more
Section optical sheet. (4) A liquid crystal display device comprising the optical sheet according to any one of claims 1 to 3.