Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing... – Sealing of liquid crystal
Reexamination Certificate
1999-11-30
2001-02-20
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
Sealing of liquid crystal
C349S153000
Reexamination Certificate
active
06191841
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a liquid crystal display cell.
A liquid crystal display device, which is thin, light in weight, and permits suppressing power consumption, is widely used as a display device for a notebook type or sub-notebook type portable personal computer. In recent years, an enlargement in the display capacity or the display area and an improved image quality are required for the liquid crystal display device in accordance with improvement in the performance of the personal computer.
In general, a liquid crystal display device comprises a liquid crystal display cell including two insulating substrates, e.g., glass substrates arranged to face each other with spacers interposed therebetween. These two glass substrates are bonded to each other with a sealing agent, and a liquid crystal material is held between the two glass substrates. The liquid crystal display cell of the particular construction is assembled as follows.
In the first step, two glass substrates are held by suction on a pair of upper and lower stages arranged to face each other. For example, a first glass substrate having a large number of electrodes (scanning electrodes, signal electrodes, and etc.) formed on a main surface and having a peripheral region outside the display region coated with a sealing agent in the shape of a rectangular frame is held on the lower stage such that the surface of the first glass substrate on which the electrodes and the sealing agent are formed faces upward. Also, spacers, e.g. plastic beads, for ensuring a gap between the two glass substrates is arranged on the upper surface of the first glass substrate. On the other hand, a second glass substrate having an opposite electrode, a color filter, etc. provided on a main surface is held by suction on the upper stage such that the main surface of the second glass substrate faces downward.
Then, the first and second glass substrates held on the lower and upper stages, respectively, are superposed one upon the other, followed by moving these glass substrates in X-, Y- and &thgr;-directions to align the positions thereof. Further, these two glass substrates are provisionally fixed to each other with, for example, an ultraviolet light curing type adhesive to prevent these two glass substrates from being deviated from each other. The resultant structure is called hereinafter a cell assembly.
Then, the sealing agent is cured under heat while pressurizing the cell assembly to set the gap between the two glass substrates at a predetermined value, thereby sealing the two insulating substrates. Further, a liquid crystal composition is injected into the vacant cell, followed by sealing the injecting port of the liquid crystal composition with a sealing agent such as an ultraviolet light curing resin.
In recent years, a new system is employed in the step of sealing he two glass substrates by curing under heat the sealing agent included in the method of manufacturing such a liquid crystal display cell. Specifically, it was customary in the past to arrange scores of cell assemblies in a superposed fashion within a press tool and to heat the superposed cell assemblies within a hot air circulating type oven while pressurizing the cell assemblies. Alternatively, a far infrared ray heater was used for heating the superposed cell assemblies. However, employed in recent years is a one-by-one sealing method using an air pressurizing system in which the cell assemblies are heated one by one while pressurizing the cell assembly with an atmospheric pressure.
The one-by-one sealing method makes it possible to improve the accuracy of the gap between the glass substrates and to improve the image quality of the liquid crystal display device. Also, since the cell assemblies are heated one by one, the heating efficiency can be improved and the sealing agent can be cured in a short time.
However, where the temperature is rapidly elevated in the conventional one-by-one sealing method, due to the influence of the gas in the cell assembly, the sealing agent layer is broken or bubbles are generated in the sealing agent layer so as to lower the sealing strength between the two substrates. As a result, the reliability of the liquid crystal display cell is lowered.
BRIEF SUMMARY OF THE INVENTION
The present invention has been contrived in consideration of the above circumstances, and its object it to provide a method of manufacturing a liquid crystal display cell, which makes it possible to prevent the break of the sealing agent layer and the bubble generation within the sealing agent layer in the step of sealing the substrates by curing the sealing agent, thereby manufacturing a liquid crystal display cell having an improved reliability.
According to a first aspect of the present invention, there is provided a method of manufacturing a liquid crystal display cell, comprising the steps of coating a first surface of a first substrate with a sealing agent in a manner to surround an outer periphery of a display region of the first substrate; arranging a second substrate to face the first substrate a predetermined distance apart from the first surface of the first substrate; heating the sealing agent while pressurizing the first and second substrates to cure the sealing agent so as to seal the first and second substrates; and injecting a liquid crystal material into the gap between the first and second substrates, wherein the sealing agent maintains a viscosity of at least 110 poises in the-curing step of the sealing agent.
In the method of the present invention for manufacturing a liquid crystal display cell, used is a sealing agent that is expanded at a rate of 4 &mgr;m/sec or less in the curing step of the sealing agent.
In the present invention, the viscosity of the sealing agent throughout the curing step of the sealing agent is limited to 110 poises or more. The viscosity of the sealing agent is related to the break or the bubble generation within the sealing agent layer in the step of curing the sealing agent. Specifically, if the temperature of the sealing agent is rapidly elevated, the viscosity of the sealing agent is rapidly lowered. In this case, it is impossible for the entire gas within the cell expanded by the heating to be released to the outside through the injecting port of the liquid crystal layer. Naturally, the remaining gas is considered to break the surface of the sealing agent layer having a lowered viscosity so as to enter the sealing agent layer, thereby forming bubbles within the sealing agent layer. Therefore, if the sealing agent maintains a viscosity high enough to withstand sufficiently the pressure of the thermal expansion of the gas within the cell throughout the step of curing the sealing agent, it is considered possible to prevent completely the bubble generation and the like within the sealing agent layers.
As a result of various experiments, the present inventors have found that a sealing agent layer having a viscosity of at least 110 poises is capable of withstanding the thermal expansion of the gas within the cell. In other words, it has been found that the gas is incapable of permeating through the sealing agent layer having a viscosity of at least 110 poises.
A sealing agent maintaining a viscosity of at least 110 poises throughout the curing step of the sealing agent can be obtained by improving the conventional epoxy series or phenolic series resin composition.
To be more specific, the reasons for the viscosity reduction caused by the temperature elevation in the conventional sealing agent are considered to be as follows:
1) The main component of the sealing agent such as an epoxy resin has a small molecular weight and, thus, has a low viscosity at room temperature;
2) The solvent mixed in the sealing agent has a high boiling point and, thus, fails to be evaporated sufficiently in the pre-curing step performed before the sealing step, with the result that the viscosity of the sealing agent is lowered; and
3) The curing agent used is solid at room temperature and
Okano Kazuyoshi
Ootaguro Hiroshi
Kabushiki Kaisha Toshiba
Parker Kenneth
Pillsbury Madison & Sutro LLP Intellectual Property Group
Qi Mike
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