HK1006594A1

HK1006594A1 - Liquid crystal cell with bubble traps

Info

Publication number: HK1006594A1
Application number: HK98105821A
Authority: HK
Inventors: Grupp Joachim
Original assignee: Asulab S.A.
Priority date: 1988-09-07
Filing date: 1998-06-20
Publication date: 1999-03-05
Also published as: DE68911070D1; EP0358142B1; DE68911070T2; EP0358142A1

Description

The invention relates to a liquid crystal cell with bubble traps, designed to prevent the formation of bubbles in the active surface of the cell during large variations in the liquid crystal volume, due for example to temperature variations, particularly for liquid crystal cells of the high torque nematic type commonly referred to as super twisted nematic, dichroic, ferroelectric or analogous.

Liquid crystal cells are usually made of a film of liquid crystal interspersed between two glass plates, each covered on its face with electrodes, and connected by a sealing frame extending along the perimeter of the plates.

Such cells present several problems both in manufacture and use.

The first problem is the filling of the cell with liquid crystal, which must be done in such a way as to avoid any pollution, dust, bubbles, etc.

The method of filling the cell is usually done by dipping the cell in liquid crystal, the cell having been placed under a high vacuum, on the side of a clearance opening as part of the sealing process. Unfortunately, it is found that this filling technique causes the formation of gas bubbles, which is a visible defect in the cell, especially for large cells.

To remedy this, JP 61-35431 contains a liquid crystal display cell of general rectangular shape with an additional inner frame defining with the sealing frame a secondary chamber, in addition to the main chamber defining the active surface of the cell. These two chambers communicate by a single passage opening opposite the filling opening. The secondary chamber defines a passive surface and is intended to trap gas bubbles appearing during filling in order to remove them from the active surface.

However, when liquid crystal is introduced into the cell through the filling opening, the flow law causes the cell to have a convex filling profile so that, when the liquid crystal reaches the passage opening, there remains in each corner of the main chamber on either side of the opening a gas residue which produces a visible defect on the surface of the cell and which can easily move through the main chamber.

The second problem is at the level of the cell sealing frame: the sealing frame defining the gap between the two blades must be of a constant thickness to maintain a correct parallelism between the two blades in order to obtain a flawless working surface.

It should be noted that a variation in blade spacing of about 0,1 μm causes a defect visible to the naked eye on the cell surface.

To overcome this problem, ball-shaped or fibre-shaped gaps of a certain diameter are usually used, particularly for large cells, the latter being set up before the two blades are assembled.

An immediate solution would be to play on the number of gaps per unit area, so as to allow the volume of the cell to adapt significantly to the change in crystal volume and thus decrease or avoid bubble formation; however, if the cell is subjected to vibration or external pressures, the result is variations in the gap between the two blades which are detrimental to the quality of the cell's display.

A cell described in JP 62-257129 attempts to provide a starting point for solving the problems mentioned above.

This solution consists of a liquid crystal cell with two adjacent obstructions arranged immediately in front of the filling opening and forming a space communicating with the main chamber of the cell. A gas bubble is intentionally trapped at the filling of the cell, this bubble having a volume that can vary to compensate for the volume changes of the crystal contained in the cell, when subjected to very low temperatures. However, the trapping of the bubble is not totally guaranteed, and it is very likely that at a significant change in its volume, the bubble will leave the space and circulate in the main chamber of the cell. The bubble can then no longer be controlled.

Finally, JP-54-107752 describes a liquid crystal cell with a bubble trap consisting of an additional rectangular inner frame which defines with the sealing frame a secondary chamber, the main chamber defining the active surface of the cell.

The cell described in this document does not include a filling hole and is filled by depositing a specified volume of liquid crystal in the form of a drop on one blade and then returning the other blade and sealing it on the first.

Once again, this filling process causes bubbles to form in the two opposite corners of the interior frame without access openings to the secondary chamber.

On the other hand, bubbles which may be trapped in the secondary chamber circulate freely there, so that the risk of bubbles leaving this chamber during large changes in the volume of the crystal is great.

The main purpose of the invention is therefore to remedy the disadvantages of the above-mentioned prior art by providing a liquid crystal cell with a constant optical quality over a wide range of external temperatures and pressures, and which, among other things, can be easily used in structures subjected to high vibrations.

To this end, the present invention concerns a liquid crystal cell comprising at least one transparent front blade, one rear blade, and a sealing frame interposed between the two blades, the assembly thus formed defining a watertight volume in which is enclosed a film of liquid crystal whose optical characteristics are liable to be altered when subjected to certain phenomena, that volume being divided by at least one partition wall into a main chamber defining an active surface and a secondary chamber defining a passive surface extending over the entire perimeter of the cell, the said secondary chamber being partially filled by liquid crystal and a gas chamber, the main chamber communicating a plurality of openings of liquid crystal, which are separated by passing.

In accordance with the invention, the secondary chamber is divided into a plurality of compartments, each of which communicates with the main chamber through at least one of these openings.

Thus, thanks to this plurality of compartments, each connected to the main chamber by at least one passageway, the bubbles are permanently trapped in the bottom of the compartments so that a random circulation of these into the secondary chamber is greatly reduced and the risk of the bubbles trapped in the secondary chamber passing into the main chamber is practically eliminated.

In addition, the lengths of the access paths of the liquid crystal contained in the main chamber to the secondary chamber are considerably reduced so that the circulation of the liquid crystal is facilitated. This effectively prevents the formation of bubbles in the main chamber both when the cell is filled and when the liquid crystal volume varies.

According to an advantageous embodiment of the invention, the compartments each have a roughly equal volume.

Other features and advantages of the invention will be apparent from the following description of non-limiting embodiments in conjunction with the attached drawings, including: Figures 1 and 2 each show a schematic top view of a liquid crystal cell of the invention applied to a rear-view mirror with adjustable coefficient of reflection for a motor vehicle;Figure 3 shows a schematic top view of a liquid crystal cell in cross section according to lines III to III of Figure 1; andFigures 4 and 5 each show a schematic top view of a liquid crystal cell of the invention applied to a matrix display for a dashboard of a motor vehicle.

The detailed description of a liquid crystal cell according to the following invention will be made in the context of an application to a rear-view mirror for a motor vehicle and an indicator for a dashboard for a motor vehicle.

The invention is not, of course, limited to the applications and embodiments which will be described or envisaged. e.g. the cells of the invention can be used in a conventional way as a display or without electrodes with a liquid crystal of the cholesterol type as a thermometer.

Reference to Figures 1 to 3 shows a rear-view mirror for motor vehicles designated by general reference 1 comprising a liquid crystal cell 2 according to the invention.

Cell 2 is mounted in a frame 3 and the latter is fixed by gluing to the rear-view mirror 1 box 4.

As can be seen in particular from Figure 3, cell 2 is known to comprise a transparent front blade 5 which has undergone anti-reflective treatment and a rear blade 6 which are joined together by a sealing frame 7 defining a watertight volume in which a mixture 8 composed of a nematic liquid crystal and a dichroic dye is trapped.

The front 5 blade has a transparent 9 electrode on its inner face extending over the entire blade, e.g. a indium oxide/tin mixture, as well as an alignment layer (not shown).

In addition, the rear blade 6 carries a counter electrode 10 and a reflective layer which may constitute the electrode, and an alignment layer (not shown) over its entire inner face.

Finally, the cell has a filling opening 11a closed by a glue joint 12 (Fig. 2) after liquid crystal is introduced into the cell. An electrical control box 13 powered by a battery, for example, has an input connected to a photo-sensitive sensor 14 which gives a control signal representative of the intensity of light received by the sensor to two outputs connected to electrode 9 and counter-electrode 10 respectively to control the degree of absorption of the cell 2.

It should be noted that the drawing does not reflect the exact thickness of the assembly thus formed, this thickness being greatly exaggerated for clarity.

According to the invention, the said watertight volume is divided into a main chamber 15 completely filled with liquid crystal and a secondary chamber 16 partially filled with liquid crystal and a gas in the form of bubbles 17, e.g. nitrogen. The main chamber 15 defines an active surface of the cell, i.e. the surface presented to the user's view, which must be completely free of visible defects such as bubbles, colour variation, etc., while the secondary chamber defines a passive surface which is not normally visible when the cell is mounted on a support.

The secondary chamber 16 is defined on the one hand by the sealing frame 7 and on the other by a partition wall forming an inner frame 18 extending along the perimeter of the sealing frame 7 at a short distance from it. This secondary chamber forms a channel the width of which is chosen so that the gas bubbles 17 are trapped by capillarity when filled. For example, for a 150 x 100 mm cell this distance is about 3 mm. The surface of the main chamber 15 is therefore bounded by the inner periphery of the frame 18.

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It is also shown in Figures 1 and 2 that the secondary chamber 16 is divided into a plurality of compartments 19 according to the invention, each of which is connected to the main chamber by at least one passage opening 20 for the circulation of liquid crystal.

The compartmentation of the secondary chamber 16 and the position of the various passage openings 20 shall be determined according to the geometry of the sealing frame 7 so that, when filled, preferably at least one bubble 17 is trapped in each compartment 19.

In the example, passageways 20 lead into the middle part of compartments 19 so that a bubble 17 is trapped in the extreme parts of each compartment and, given the rectangular shape, a passageway is provided in each corner of the inner frame 18.

The amount of gas and liquid crystal contained in each compartment is determined so that for any maximum volume change the liquid crystal can undergo, there is enough liquid crystal left in the compartments for the bubbles to remain trapped.

For example, for a 150 x 100 mm cell that can be subjected to temperatures in the range of -40 °C to +85 °C, the ratio of bubble volume to the volume of liquid crystal in the compartments may vary in the range of 1/100 to 1/1000, depending on the cell configuration.

Furthermore, it can be seen in Figure 2 that compartment 21 comprising the filling opening 11b communicates with the main chamber 15 by means of two passage openings 20a and 20b arranged on either side of the filling opening so that, when filled, liquid crystal 8 flows simultaneously from two separate points.

Reference now to Figures 4 and 5 shows two variants of another method of making a cell according to the invention in which the same elements as those described above have been designated by the same numerical references.

Cell 2 shown in Figures 4 and 5 is for example a matrix display cell in STN mode intended to be mounted in a vehicle dashboard.

Cell 2 includes, here in a central part, an area 23 not containing liquid crystal. In this area a central hole is reserved for the passage of a needle 24 which can be driven by a stepper motor and gear placed behind the cell, the needle moving along a graduation produced by an appropriate command of the liquid crystal cell indicating e.g. speed.

In this embodiment, the cell comprises an additional secondary chamber 25 arranged along the perimeter of zone 23 This chamber 25 is of an annular shape and is divided into three compartments 26, each of which is communicated to the main chamber by a passage opening 27; each compartment is partially filled with liquid crystal and at least one gas bubble.

It is shown in the embodiment in Figure 4 that the communication openings 20 with the main chamber are arranged at least in each corner of the cell and, furthermore, that the filling opening 11a leads directly into the secondary chamber.

The filling behaviour of the cell (Figure 2) according to the invention is as follows: The liquid crystal enters the chamber in front of the filling hole by trapping a gas bubble at each end of the chamber, and then the liquid crystal enters the main chamber through the two passage openings provided for this purpose. The two openings are point sources from which the crystal gradually flows into the main chamber. The flow of the liquid crystal occurs along two convex profiles, gradually sealing to form only one. The gas in the main chamber is refilled, and fills the different compartments of the secondary chamber before the liquid crystal through the opening openings so that each passage is joined as gas trapped in the compartment.

The passage openings according to the invention, especially those in each corner of the interior frame, prevent the formation of gas bubbles at these critical points, so that the main chamber is completely filled with liquid crystal, resulting in a flawless working surface.

In addition, a cell of the invention can be used in a wide temperature range, as if a sharp drop in temperature occurs, the liquid crystal shrinks while the volume of the main and secondary chambers remains substantially unchanged. However, the main chamber communicates with the secondary chamber through a plurality of passage openings, which compensates for the change in volume of the liquid crystal contained in the main chamber by the liquid crystal contained in the secondary chamber, while the volume of the bubbles contained in the secondary chamber increases.

The multiplicity of passage openings decreases the length of the path that the liquid crystal in the secondary chamber must travel to compensate for the volume change, resulting in safe and rapid compensation so that bubble formation in the main chamber is made impossible.

When the temperature rises, the opposite phenomenon occurs, the liquid crystal expands, and the volume change is then compensated by the compression of the gas bubbles trapped in the secondary chamber.

Claims

Liquid crystal cell comprising at least one transparent front plate (5), a back plate (6) and a sealing frame (7) interposed between the two plates, the assembly thus formed defining a tight volume in which is contained a liquid crystal film (8) at least one optical characteristic of which is capable of being modified when it is subjected to specific phenomena, said volume being divided by at least one separating wall (18) into a main chamber (15) defining an active surface and a secondary chamber (16) defining an passive surface extending over the full extent of a circumference of the cell, said secondary chamber (16) being partially filled with the liquid crystal (8) and with a gas (17), the main chamber (15) communicating with the secondary chamber (16) by a plurality of openings (20, 20a, 20b, 27) provided in the separating wall (18), characterized in that the secondary chamber (16) is divided into a plurality of compartments (19, 21, 26), each of which is in communication with the main chamber (15) by the intermediary of at least one of said openings.
Cell according to claim 1, characterized in that the compartments (19, 26) have each a substantially equal volume.
Cell according to any one of claims 1 or 2, characterized in that the secondary chamber (16) is disposed at the outer periphery of the main chamber (15) of the cell (2).
Cell according to any one of claims 1 to 3, characterized in that the secondary chamber (16) is defined by the sealing frame (8), on the one hand, and by said separating wall forming an inner frame (18) extending around the main chamber (15), on the other hand.
Cell according to claim 4, characterized in that the inner frame (18) presents a polygonal configuration, characterized in that the passage openings (20) are at least provided in the corners of the inner frame (18).
Cell according to any one of claims 1 to 5, characterized in that the openings (20, 27) open substantially into the median part of the compartments (19, 21, 26).
Cell according to any one of the preceding claims, characterized in that the passage openings (20) are regularly spaced around the main chamber (15).
Cell according to claim 7, comprising a filling opening (11a) traversing the sealing frame (7), characterized in that the filling opening (11a) communicates directly with the main chamber (15).
Cell according to any one of claims 1 to 5, comprising a filling opening (11b) traversing the sealing frame (7), characterized in that the filling opening (11b) communicates with a compartment (21) comprising two passage openings (20a, 20b) offset in relation to the filling opening (11b) and disposed in the inner frame (18).
Cell according to any one of the preceding claims comprising a zone (23) not containing any liquid crystal, characterized in that it comprises a secondary chamber divided into a plurality of compartments (25) disposed along the circumference of the said zone.
Cell according to any one of the preceding claims, characterized in that the plates (5, 6) are provided on their opposite faces with an electrode (9, 10) joined to an electrical control device (13).
Automobile rear view mirror, characterized in that it comprises a liquid crystal cell according to claim 11, the back plate (6) of this cell (2) comprising a reflective coating and in that it comprises at least one element (14) capable of delivering a signal representing the intensity of the light reaching it, the control device (13) acting on the transparency of the cell (2) in response to said signal.
Matrix display device of the STN type, characterized in that it comprises a cell (2) according to claim 11.