Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-12-21
2004-01-06
Dudek, James (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
Reexamination Certificate
active
06674503
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display element including a spacer which is sandwiched between a pair of substrates made of plastic and the like, and a manufacturing method thereof.
BACKGROUND OF THE INVENTION
Conventionally, as a pair of substrates for composing a liquid crystal display element by holding a liquid crystal, substrates made of plastic and the like have been used because of superior impact resistance. However, a substrate made of plastic and the like has a problem of an inferior gas barrier, i.e., gas is easily transmitted through the substrate, thereby arising such a problem that the gas transmitted through the substrate dissolves in a liquid crystal layer so as to allow bubbles to be generated more easily.
More specific explanation is provided as follows. When composing a liquid crystal display element, a polarizer, etc. are attached to a substrate. Since the substrate and the polarizer generally have different elongation ratio, due to environmental change (change in a temperature, humidity, etc.) is deformed the substrate to a bimetallic form. A cell gap between the pair of substrates is changed due to the deformation of the substrate, thereby changing pressure between the pair of substrates sandwiching the liquid crystal. In addition, the change in the pressure causes the gas that dissolved in the liquid crystal layer to be vaporized, and the bubbles are generated.
On the other hand, a technique disclosed in Japanese Unexamined Patent Publication No. 281941/1994 (Tokukaihei 6-281941 published on Oct. 7, 1994), etc., is such that, by increasing the number of spacer sandwiched by the pair of substrates, deformation of the substrate particularly under a high temperature and high humidity is reduced, thereby attempting to suppress generation of the bubbles in the case of using a substrate of an inferior gas barrier.
Further, the foregoing publication also discloses a technique of adopting a spacer of an adhesive type, suppressing movement of the spacer by adhering the spacer to the substrate in a manufacturing process, and preventing the deformation of the substrate.
Note that, such a liquid crystal display element is commonly formed by injecting a liquid crystal between a pair of substrates, surroundings of which are sealed with a sealing agent by a vacuum impregnation method, except a liquid crystal injection slit (liquid crystal impregnation slit), and sealing the liquid crystal injection slit with end seal (sealing agent) made of UV setting resin, etc. Here, for example, a technique disclosed in Japanese Unexamined Patent Publication No. 142506/1993 (Tokukaihei 5-142506 published on Jun. 11, 1993), etc., is such that, after injecting the liquid crystal, a panel which is made up of a pair of substrates is pressed so as to squeeze out the liquid crystal, then, the liquid crystal injection slit is sealed, thereby uniforming the cell gap between the pair of substrates.
However, in the case of using the spacer of the adhesive type disclosed in the publication No. 6-281941, as the spacer of the adhesive type is adopted a spacer, a surface of which is coated with a thermoplastic adhesive. In that case, the unit cost of the spacer becomes high, thereby arising a problem of increase in the cost of a liquid crystal display element.
Further, in that case, adhesion of the spacer to the substrate is carried out simultaneously with the heat treatment for setting a seal material in the manufacturing process, but when using a UV setting resin as the seal material, a heat treatment process should separately be provided, thereby arising problems of complicated processes and increase in cost.
Further, in that case, since the thermoplastic adhesive exists inside the liquid crystal, the thermoplastic adhesive dissolves in the liquid crystal when performing the heat treatment, thereby possibly resulting in exacerbation of the performance of the liquid crystal.
Further, in the technique disclosed in the publication No. 6-281941, an effect is produced with respect to the generation of bubbles due to the deformation of the substrate under a high temperature and high humidity. However, when the number of the spacers is excessively large, a relative quantity of the liquid crystal inside the cell is reduced. In such case where the relative quantity of the liquid crystal is small, and particularly, in case where the liquid crystal is kept under a condition of a low temperature and the liquid crystal itself shrinks largely, there arises such a problem that a mechanical impact from outside allows bubbles to be readily generated in a vacuum region (a space where the liquid crystal is eliminated and substantially no gases are included).
Furthermore, when the number of the spacers is excessively large, a problem of reduction in contrast of the liquid crystal display element arises.
On the other hand, when the number of the spacers is excessively small, the relative quantity of the liquid crystal inside the cell is increased, and the degree of generation of bubbles due to the mechanical impact (characteristic of resistance to a mechanical impact bubbles) is increased. However, the small number of the spacers results in a reduction in a support between the substrates. Consequently, there arises problems such as readily deformation of a substrate, deterioration of resistance to pressure, and nonuniformity in cell gap.
Particularly, when using a substrate which is made of a material such as plastic having smaller stiffness than that of glass and the like, it is necessary to attain stiffness of the liquid crystal display element by increasing a spacer density to a certain extent.
Meanwhile, in the case of an STN (Super-Twisted Nematic) type liquid crystal cell which requires highly accurate adjustment of cell gap, it is crucial to maintain the cell gap uniformly. Specifically, in the STN type liquid crystal cell, a product &Dgr;nd (retardation) of a cell gap d and birefringence &Dgr;n of a liquid crystal injected inside the cell determines background color gradations, and therefore a partial variation in the cell gap d results in color shading.
In order to uniform the cell gap d, the application of pressure to a panel prior to sealing of the liquid crystal injection slit as disclosed in the publication No. 5-142506 is effective, but when the pressure here is high, the liquid crystal is discharged excessively. Therefore, when the panel after sealing is left to stand in an environment of a low temperature, a liquid crystal inside the panel shrinks so as to form bubbles in the vacuum region inside the panel.
Further, in the case of a panel using a plastic substrate in particular, when the spacer density (spacer scattering density) is low, as shown in
FIG. 7
, a substrate is bent in accordance with presence or absence of the spacer.
FIG. 7
is a cross sectional view showing a state of a liquid crystal display element
10
including spacers
4
whose density is low. In the liquid crystal display element
10
, a display side substrate
1
a
and a rear side substrate
1
b
are bent inward at a portion where an interval between the spacers
4
is wide, and thereby a cell of this portion becomes small. This is recognized as color shading in the case of the STN type liquid crystal cell. Furthermore, the spacers
4
can uniformly support the display side substrate
1
a
and the rear side substrate
1
b
when the spacers
4
are completely spaced equidistantly, but it is difficult to attain such complete equidistant spacing by a common spacer scattering technique. Consequently, since the support by the spacers
4
varies depending on a location, the cell gap fails to be uniform, thereby occasionally emphasizing the color shading.
On the other hand, when the spacer density is high, bending of the display side substrate
1
a
and the rear side substrate
1
b
is suppressed, and moreover, the addition of pressure allows the spacers
4
to support the substrates uniformly, and thus it is relatively easy to uniform the cell gap. However, since a quantity of a liquid cry
Inou Ippei
Niiya Hirotaka
Conlin, Esq. David C.
Dudek James
Edwards & Angell LLP
Roos, Esq. Richard J.
Sharp Kabushiki Kaisha
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