Process for producing liquid crystal panel

Abstract: A large-sized liquid crystal panel uniformly filled with a liquid crystal material may be produced through a process including the steps of: fixing a pair of substrates with a sealing member disposed therebetween to form a blank panel having an injection port, reducing a pressure within the panel, applying a liquid crystal material onto the injection port, heating the liquid crystal material to its isotropic temperature, injecting the liquid crystal material into the panel under a pressure, and filling the panel with the liquid crystal material under heating and pressurization from the injection port toward the panel inside. The pressure within the panel before the injection is reduced at a rate not exceeding 25 torr/min. The pressure for injection is increased at varying rates, e.g., with an intermediate constant pressure period. In the filling step, a portion of the liquid crystal material near the injection port is held in its isotropic phase and the liquid crystal material except for the portion is transformed at least once into a smectic phase.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to a process for producing a liquid crystal panel characterized by a step of injecting a liquid crystal into a cell gap or panel gap defined between a pair of opposing substrates and a filling step after the injection step. In a conventional process for preparing a liquid crystal panel, a pair of substrates are applied to each other with adhesive particles and a sealing material disposed therebetween to form a blank panel, a liquid crystal material and the panel are held in a reduced-pressure chamber, the interior of the chamber is reduced in pressure and heated, the liquid crystal material is applied onto an injection port of the panel, and the chamber is restored to the atmospheric pressure and further pressurized to inject the liquid crystal material at a lowered viscosity into the panel. When the panel is restored to room temperature after the liquid crystal injection, a void can occur within a panel, particularly within a large-sized panel, due to shrinkage of the liquid crystal material. For this reason, it has been conventionally practiced to further heat the liquid crystal and pressurization of the panel inside from the injection port so as to uniformly increase the filling or packing density of the liquid crystal and remove the void. Hitherto, in the injection step, it has been principally considered to minimize the time for injecting and filling the liquid crystal material, and there has not been paid any attention to appropriate control of the rate of reducing the pressure within the vessel, control of the rate of liquid crystal material progressing within the panel during the injection step and consideration of adverse effects by placing the liquid crystal material at an elevated temperature condition throughout the entire process for a long period. EP-A-0 347 913 discloses a liquid crystal device including a pair of substrates disposed spaced apart from each other, a liquid crystal disposed between the substrates and a plurality of adhesive members bondinq the substrates which are dispersed along the inner surfaces of the substrates at a rate of 100 members/mm 2 or less with an average adhesion area per member of 1.25 x 10 -3 mm 2 or less. EP-A-0 343 902 discloses a display device comprising first and second substrates located in a substantially parallel relative arrangement to define an active display region therebetween when filled with liquid crystal material. The cell includes means to effect filling of the active display region with liquid crystal material from at least a part of an edge of an active display region which increases the cross-sectional area into the active display region through which liquid crystal material can flow. The preamble of claim 1 is based on this document. SUMMARY OF THE INVENTION Accordingly, a principal object of the present invention is to provide a process for producing a liquid crystal panel with improved liquid crystal injection and liquid crystal filling steps. According to the present invention, there is provided a process for producing a liquid crystal panel according to claim 1. Preferred embodiments are defined in the sub-claims. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing a generalized pressure-increasing profile used in a liquid crystal injection step according to an embodiment of the invention. Figure 2 is a graph showing a generalized pressure-increasing profile used in a liquid crystal injection step according to a comparative example. Figures 3 and 4 are graphs showing specific pressure-increasing profiles adopted in Examples 2 and 3, respectively, described hereinafter. Figure 5 is a schematic illustration of an intermediate state in a step of injecting a liquid crystal material into a panel according to the invention. Figure 6 is a schematic illustration of a state of a liquid crystal cell in a liquid crystal filling step after liquid crystal injection according to the invention. Figure 7 is an illustration of a liquid crystal panel including defects formed by a conventional liquid crystal injection process. Figure 8 is a graph showing a pressure profile adopted in a conventional liquid crystal injection step. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, a liquid crystal panel is produced as defined in claim 1 wherein the pressure is held once at a constant pressure before it reaches a final pressure as shown in Figure 1, more specifically with a smaller pressure-increasing rate in an intermediate stage, thereby preventing the occurrence of an injection failure, such as a poor injection region or streak defects 135 between the injected liquid crystal portions 132, 133 and 134 during injection through an injection port 131 as shown in Figure 8. Herein, the constant pressure may preferably be held for a period of 1 - 5 hours, more preferably 2 - 5 hours. If the holding period is less than 1 hour, an irregularity in alignment state can occur under severe drive conditions of the resultant liquid crystal panel. If the holding period exceeds 5 hours, streak defects can occur. Except for the constant pressure-holding period, the pressure may preferably be increased at a rate of at least 135 torr/hr., more preferably at least 200 torr/hr. If the pressure-increasing rate is below 135 torr/hr, the streak defects are liable to occur under severe drive conditions of the resultant liquid crystal panel. By adopting the above-mentioned temperature-increasing and pressure-increasing step, the liquid crystal material is controlled to progress at a rate v(t) at time t from the start of the liquid crystal injection so as to satisfy a range of 0.5v a ≦ v(t) ≦ 1.5v a , wherein v a denotes an average progress speed which may preferably be controlled within the range of 0.005 - 0.5 cm/min. The above-mentioned average progress speed v a is determined by If the average progress speed is below 0.005 cm/min., the streak defects are liable to occur because the injection speed is too small. In excess of 0.5 cm/min., alignment irregularity is liable to occur even if v(t) is controlled to satisfy the above range. Further, if v(t) is outside the above range, the liquid crystal progress speed during the injection fluctuates too much, so that it is difficult to obtain a homogeneous alignment state over the entire panel. As a result, a homogeneously good alignment state can be obtained even in a large-sized panel. Accordingly, the liquid crystal material within the panel is held in its smectic phase to cause a high density and suck the liquid crystal material in the isotropic state showing a low viscosity in the vicinity of the injection port into the inner port of the panel, whereby the period of the filling step, particularly the period of retaining a high temperature and high pressure state, can be shortened than before. Further, the influence on alignment control films formed on the inner faces of the panel can be minimized, thus minimizing the change in pretilt angle, whereby liquid crystal panels can be produced with stable qualities. The liquid crystal used in the present invention may preferably be a smectic liquid crystal, more preferably a ferroelectric smectic liquid crystal. Hereinbelow, the present invention will be described more specifically based on Examples. Example 1 A liquid crystal panel of almost A4-size (having a length L of 30 cm and a width W of 25 cm of an inner space to be filled with a liquid crystal) was prepared according to the second embodiment of the present invention. First of all, two glass substrates were applied to each other with a sealing member disposed therebetween to form a blank panel. The inner space of the panel was reduced in pressure from the atmospheric pressure to 10 -6 torr. Then, the panel was restored to a level of pressure not affecting a liquid crystal material, and a phenylpyrimidine-based ferroelectric liquid crystal composition in its smectic phase was applied onto the injection port of the panel. Then, the panel was restored to the atmospheric pressure and then again reduced in pressure to 1 torr. In this instance, the panel was warmed to convert the liquid crystal material into the isotropic phase and, after confirming that the liquid crystal material was in the isotropic phase, the pressure was started to be increased. The pressure was increased according to a pressure-increasing profile as shown in Figure 1. More specifically, the pressure was increased at a rate of 190 torr/hr while an intermediate pressure was retained for 2 hours and a final pressure of 2000 torr was retained for 6 hours. The liquid crystal panel produced through injection in the above-described manner was free from streak defects as injection traces or regions of injection failure and showed no defects during drive at room temperature. The average liquid crystal progress speed v a during the above injection step was 0.020 cm/min., and the progress speed v(t) was controlled within the range of 0.018 cm/min. ≦ v(t) ≦ 0.022 cm/min. as shown in Figure 7. Example 2 A blank panel identical to the one in Example 1 was provided. The inner space of the panel was reduced in pressure from the atmospheric pressure to 10 -6 torr. Then, the panel was restored to a level of pressure not affecting a liquid crystal material, and a phenylpyrimidine-based ferroelectric liquid crystal composition in its smectic phase was applied onto the injection port of the panel. Then, the panel was restored to the atmospheric pressure and then again reduced in pressure to 100 torr (about 20 min.). In this instance, the panel was warmed to convert the liquid crystal material into the isotropic phase and, after keeping this state for 100 min, the pressure was started to be increased. The pressure was increased according to a pressure-increasing profile as shown in Figure 3. More specifically, the pressure was initially increased to 1400 torr at a rate of 217 torr/hr. within 6 hours, the pressure was retained for 2.5 hours and then increased to 1670 torr at the same rate within 1.5 hours, the pressure was retained for 2.5 hours and then increased to 1950 torr at the same rate, and the pressure of 1950 torr was retained for 6 hours and 15 min. The pressure was then restored to the atmospheric pressure within 1.5 hours to complete the liquid crystal injection. During the injection step, the average liquid crystal progress speed v a was 0.020 cm/min. and the liquid crystal progress speed with time t(v) was within the range of 0.018 - 0.022 cm/min. As a result of controlling the liquid crystal progress speed in the above-described manner in the liquid crystal injection step, the resultant liquid crystal panel showed a good alignment state over the entire A4-size area. Further, the thus-obtained liquid crystal panel was free from streak defects as injection traces or regions of injection failure and showed no defects during drive over a wide temperature range. Example 3 A blank panel identical to the one in Example 1 was provided. The inner space of the panel was reduced in pressure from the atmospheric pressure to 10 -6 torr. Then, the panel was restored to a level of pressure not affecting a liquid crystal material, and a phenylpyrimidine-based ferroelectric liquid crystal composition in its smectic phase was applied onto the injection port of the panel. Then, the panel was restored to the atmospheric pressure and then again reduced in pressure to 100 torr (about 20 min.). In this instance, the panel was warmed to convert the liquid crystal material into the isotropic phase and, after keeping this state for 100 min, the pressure was started to be increased. The pressure was increased according to a pressure-increasing profile as shown in Figure 4. More specifically, the pressure was initially increased to 1400 torr at a rate of 217 torr/hr. within 6 hours, the pressure was retained for 2.5 hours and then increased to 1650 torr at a rate of 167 torr/hr. within 1.5 hours, the pressure was retained for 2.5 hours and then increased to 1950 torr at a rate of 200 torr/hr., and the pressure of 1950 torr was retained for 6 hours. The pressure was then restored to the atmospheric pressure within 1.5 hours to complete the liquid crystal injection. The thus-obtained liquid crystal panel showed no defects during drive over a wide temperature range. Comparative Example 1 A blank panel identical to the one in Example 1 was provided. Similarly as in Example 1, the inner space of the panel was reduced in pressure from the atmospheric pressure to 10 -6 torr. Then, the panel was restored to a level of pressure not affecting a liquid crystal material, and a phenylpyrimidine-based ferroelectric liquid crystal composition in its smectic phase was applied onto the injection port of the panel. Then, the panel was restored to the atmospheric pressure and then again reduced in pressure to 1 torr. In this instance, the panel was warmed to convert the liquid crystal material into the isotropic phase and, after confirming that the liquid crystal material was in the isotropic phase, the pressure was started to be increased. The pressure was increased according to a pressure-increasing profile as shown in Figure 14. More specifically, the pressure was increased linearly to a final pressure of 2000 torr at rate of 120 torr/hour without a constant pressure-holding period. As a result, the liquid crystal panel obtained through the above-described injection step showed streak defects 135 at boundaries between regions 132, 133 and 134 where the liquid crystal progress speed was believed to vary as shown in Figure 7. The streak defects did not disappear during drive of the liquid crystal panel. As described above, according to the present invention, the following advantages may be obtained through improvements in the liquid crystal injection and/or filling steps. (1) The pressure applied to the panel from the inside toward the outside thereof can be minimized during the evacuation step to prevent the occurrence of gap irregularity and/or panel breakage. (2) By appropriately controlling the pressurization conditions during the liquid crystal injection step, the liquid crystal progress speed within the panel may be controlled within an appropriate range, whereby a homogeneous alignment state can be accomplished over the entire panel area, and the injection failure, such as the occurrence of a region of injection failure and/or injection defects like streak defects, can be prevented. Further, the time required for injection can be shortened compared with the case where the pressure is linearly increased. (3) By adopting an appropriate temperature control program in the liquid crystal filling step after the liquid crystal injection, the removal of void within the panel can be ensured to provide an increased yield. Further, the period of high temperature and high pressure can be shortened than before, thus providing a liquid crystal panel with stable quality and performances, through, e.g., prevention of a change in pre-tilt angle of liquid crystal molecules.

Claims: A process for producing a liquid crystal panel, comprising the steps of: fixing a pair of substrates with a sealing member disposed therebetween to form a blank panel having a prescribed gap between the substrates havinq an injection port, reducing the pressure within the panel before applying a liquid crystal material onto the injection port, and heating at least a part of the liquid crystal material to its isotropic temperature before injecting it into the panel, injecting the liquid crystal material through the injection port into the panel by applying a time-varying pressure to the liquid crystal material, characterized in that the pressure is applied so that it includes a constant pressure-holding period so that the liquid crystal material progresses within the panel at a speed v(t), and in that the speed v(t) is controlled so as to satisfy 0.5v a ≤ v(t) ≤ 1.5 v a , wherein v a denotes an average speed within a range of 0.005 - 0.5 cm/min. determined by an equation wherein T denotes a total period of the injection step. A process for producing a liquid crystal panel according to claim 1 wherein after reducing the pressure within the panel, and heating the liquid crystal material to its isotropic temperature, the liquid crystal material is injected into the panel under a pressured which increases at varying rates up to a maximum pressure. A process according to claim 1, wherein the prescribed period is 1 - 5 hours. A process according to any of claims 1 or 3, wherein the pressure for injection is increased at a rate of at least 135 torr/hour except for a period of constant pressure. A process according to any of the preceding claims, wherein the liquid crystal material comprises a ferroelectric liquid crystal material.

Inventors: ENOMOTO TAKASHI, ISHIWATA KAZUYA, MITSUI MUTSUO, MURATA TATSUO, NAKAI NORIYUKI, NISHIDA NAOYA, SHIMAMUNE KASAYUKI, SUZUKI MASAAKI, WATANABE YASUYUKI, ENOMOTO, TAKASHI, C/O CANON K.K., ISHIWATA, KAZUYA, C/O CANON K.K., MITSUI, MUTSUO, C/O CANON K.K., MURATA, TATSUO, C/O CANON K.K., NAKAI, NORIYUKI, C/O CANON K.K., NISHIDA, NAOYA, C/O CANON K.K., SHIMAMUNE, KASAYUKI, C/O CANON K.K., SUZUKI, MASAAKI, C/O CANON K.K., WATANABE, YASUYUKI, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: G02F1
IPC codes: G02F1