FR2495805A1 - Moulded plastics casings for liquid crystal display units - to simplify construction and assembly - Google Patents

Abstract

The casing elements for an electrically activated liquid crystal visual display panel, are made from a polymeric moulding material, specifically polymethylmethacrylate, polyethylene terephthalate or polycarbonate, and carry on part of their internal faces conductive coatings for selective activation of the enclosed crystals. The moulded elements are used instead of conventional glass plates, to reduce or eliminate ancillary machining or assembly involving separate spacers. Applicable to systems using either helical molecular crystals or phase variable 'cholesteric' crystals, linked by transparent conductive paths of suitably etched deposits of Sn or In oxide.

Description

ELECTRO..OPTOUE CRYSTAL DISPLAY DEVICE
LIQUID AND ITS MANUFACTURING METHOD
The present invention relates to an electrooptical liquid crystal display device. Usually this device comprises two series of electrodes arranged on the internal face of two glass slides placed opposite, at a short distance from each other. The liquid crystal is placed between these two plates. If the thin layer of liquid crystal thus created is subjected to an electric field by connecting the two opposite electrodes to a voltage source, the electric field induces within the liquid crystal disturbances observable by transmission or reflection of rays light produced by an auxiliary source.

 There are two approaches to induce these disturbances.

 According to the first approach, the electric field modifies the orientation of the liquid crystal causing a variation in birefringence and transmission of a light ray passing through the layer of liquid crystal. According to this same approach, if polarized rays are involved, the electric field modifies the polarization of the transmitted or reflected rays. In this case, helical or "twisted" nematic liquid crystals are used.

 According to the second approach, the electric field causes a glectro-hydrodynamic effect, one consequence of which can be a loss of the transparency characteristics of the liquid crystal. When we illuminate the liquid crystal, it diffuses the light (dynamic diffusion). According to this same approach, it is also possible to induce a phase change in liquid crystals of the cholesteric type.

 There are many types of electro-optical liquid crystal devices operating according to either of these approaches. By way of non-limiting example, mention may be made of: digital display cells of the "seven segment" type, cells with matrix display of the "5 x 7" type and more generally devices in the form of flat access panels matrix. Such panels can be used to render television images.

 Furthermore, according to the first approach, in most applications, it is essential to add crossed polarizing elements to the device.

In the known art, the methods of manufacturing these devices usually include the following steps:
- depositing on the glass slides a thin metallic layer by sputtering or evaporation. The thickness must be small enough so that the metal layer is transparent to light rays.

It can be a deposit of tin oxide - indium oxide;
the electrodes are then produced by etching the deposited layer according to a pre-established configuration;
- The internal faces of the glass slides are then treated so that the liquid crystal which will be brought into contact with these faces is oriented in a preferred direction. To do this, a surfactant is deposited, for example by evaporation of silica under grazing incidence. We can still rub these surfaces in two orthogonal directions;
- The device is then formed by placing shims between the two blades and sealing everything by gluing;
- a filling hole is then made and the volume delimited by the blades and shims is then filled with liquid crystal and the hole is plugged.

The Frocédésde realization of known art, which have just been recalled in their main line, have several drawbacks:
the precise positioning of the glass slides carrying the electrodes engraved with one relative to the other is difficult to achieve;
- it is necessary in all cases to use shims; If these are etched in the glass, the operation is long and costly;
- drilling the filling holes is also difficult to achieve;
- it is finally the same for the final collage.

 To overcome these drawbacks, the invention provides electro-optical liquid crystal devices which do not require the use of a glass slide. According to the invention, these glass slides are replaced by two moldable polymer frames, at least one of which is transparent, which makes it easier and cheaper to produce.

 Other advantages arise from the fact that a moldable polymer is used: the positioning can be facilitated by emboiling; the shims can be omitted, because these can also be made by molding; the molding also makes it possible to print a network on the internal face of the polymer armatures so as to orient the liquid crystal, and finally the polarizing elements can be deposited directly on the other face of the armatures.

 The invention therefore relates to a liquid crystal display device in which the liquid crystal is inserted between first and second plates comprising electrodes of electrically conductive material arranged on the faces of the plates in contact with the liquid crystal; device mainly characterized in that the reinforcements are made of polymer material.

The invention will be better understood and other advantages will appear on reading the description below and on examining the appended figures among which:
- Figure 1 shows schematically, in section, a display device of the known art;
- Figure 2 illustrates the operation of a display device in which the direction of polarization of the transmitted light rotates under the effect of an electric field;
- Figure 3 shows a first embodiment of a display device according to the invention;
- Figure 6 illustrates the application of the invention to the production of a liquid crystal display cell of the "seven segment" type.

FIG. 1 schematically represents a display device 1 produced according to known art. It consists of a stack comprising a first glass slide 10 covered on its internal face with a metal layer 11 and a second glass slide 12 also covered on its internal face with a metal layer 13. When reading information displayed by the device is done by light transmission, the two layers 11 and 13 must have a thickness sufficiently small to be transparent to light rays. In the case of reading by reflection, at least one of the metal layers, for example 11, is transparent, the other having a thickness sufficient to be reflective. The two glass blades 10 and 12 are maintained in parallel relation by means of wedges or spacers 15 or 14. These wedges do not need to be made of glass, they can for example be made of plastic material. The volume defined by this set of elements is then filled with a liquid crystal 16. Many liquid crystals can be used and we will find nonlimiting examples in the article by JC DUBOIS: "NEW MATERIALS
MESOMORPHS AND APPLICATIONS ", published in the Annales de Physique, 1978, volume 3, page 131. As has been pointed out, many types of device are known. Some examples of these devices are described in the article by M. HARENG and S. LE BERRE: "STUDY ON THE
APPLICATIONS FOR THE VISUALIZATION OF LIQUID CRYSTALS,
TRENDS, "published in the Annales de Physique, 1978, volume 3, pages 317 to 324.

 The methods for producing these devices have also been mentioned previously, as well as the drawbacks due to the use of glass plates.

 To fix the ideas, without this being limiting, it will be considered in what follows, the case of a liquid crystal display cell allowing the visualization of information by variation of the direction of polarization of a light ray passing through the cell, that is to say according to one of the variants of the first approach.

 Figure 2 schematically illustrates the operation of such a cell. It consists of the actual electro-optical cell 1 described in relation to FIG. 1, comprising two glass plates 10 and 12 comprising on their internal face electrodes represented in FIG. 2 by uniform metallization layers 11 and 13. It is understood that these electrodes have a particular configuration according to the intended use, a configuration of which will be described in more detail with reference to FIG. 6. The device is completed by a first plate 2 playing the role of a direction polarizer polarization P2 and a second blade 3 playing the role of an analyzer having a polarization direction P3, the two directions P2 and P3 being orthogonal to each other. The shims have not been shown in the figure and, in general, the slides 2 and 3 are glued to the upper and lower faces of the cell. The space delimited by the two glass plates 10 and 12 is filled with a liquid crystal, which in the example described is of the nematic helical type with high positive dielectric anisotropy. The internal faces of the plates 10 and 12 are treated so that the liquid crystal molecules 16 (16 ') are, in the absence of an electric field, oriented in the vicinity of the two internal faces, respectively in a direction P'2 parallel to the direction of polarization of the polarizing blade 2: P2, and P'3 parallel to the direction of polarization P3 of the analyzer blade 3. We can obtain such a configuration, either by rubbing the internal walls of the blades 10 and 12 according to orthogonal directions between them, either by subjecting these walls to an evaporation of silicon oxide under grazing incidence, according to these same directions. The molecules of the liquid crystal will take all identical directions of orientation for the same XY plane of the Z axis. An incident ray Ri produced by a non-polarized light source 4, comes out polarized from the plate 2 in the polarization direction P2 and sees its direction of polarization rotate by 2 to be parallel to the axis of polarization P3 at the output of cell 1 in the absence of an electric field within the liquid crystal. If we connect the two electrodes 11 and 13 to an electric voltage generator, an electric field is created orthogonal to the plane of these electrodes. The liquid crystal molecules 16 ′ will tilt and take, if this voltage is sufficient, an orientation parallel to the axis Z, as shown on the right-hand side of FIG. 2. If we consider an incident ray R ′. produced by the light energy source 4, it comes out polarized in a direction parallel to the direction of polarization P2 of the polarizing plate 2. This ray keeps its direction of polarization unchanged at the crossing of the cell 1 and comes out with the same direction of polarization, direction orthogonal to the direction of polarization P3 of the analyzer blade 3. It follows that no light energy emerges from the blade 3 and an observer 5 sees no light manifestation unlike the case previous.

 An example of a cell of this type produced according to the invention will now be described in relation to FIG. 3. According to the most important characteristic of the invention, the glass slides are replaced by two polymer frames, and in the context of the embodiment described with regard to Figure 3, by two half-shells 30 and 32 which can fit one into the other.

 To do this, use is made of positioning indexes produced in the form of bosses and cavities with complementary profiles, respectively 302 and 322. The figure shows the two electrodes 31 and 33 placed opposite and between which will be subsequently arranged the liquid crystal. In general, one of the electrodes, 33 in FIG. 3, is a multiple electrode produced according to a determined configuration depending on the application.

 An example of configuration will be described in the following in connection with FIG. 6. The other electrode is an integral electrode covering the entire region covered by the electrode 33. The different parts of the electrode 33 and the electrode 31 must be connected to electronic circuits external to the device. To do this, the demicoques 30 and 32 are extended by plates 320 and 301 on which extend the different parts of the electrode 33, represented in FIG. 3 by two tongues 330 and 331, as well as the electrode 31 represented by a tab 310. These tabs are then connected using connection wires C1 to C3 to external circuits not shown in the figure.

These connection wires can be soldered or glued using a conductive glue.

 The two half-shells 30 and 32 can each be obtained in a single operation and preferably by molding. In addition to the positioning indexes 302 and 322 previously described, the two half-shells are provided with cavities and tongues with complementary profile 300, 303 and 321. The exact shape of these tongues as well as that of the housing formed by the two half-shells 30 and 32 depend on the intended use and are only given in FIG. 3 as an illustration of the invention. In a variant not described, only one of the half-shells can comprise a cavity intended to receive the liquid crystal, the other being substantially in the form of a flat blade. It should also be noted that the scale cannot be observed in the drawing, the space between two electrodes being of the order of ten microns.

 The metallization making it possible to produce the electrodes 31 and 33 is carried out by any suitable method and does not differ in any way from the known art. In the case of the example considered, the two electrodes must between transparent and can be made of tin oxide - indium oxide deposited by sputtering for example, in an apparatus comprising a magnetron and a tin-indium target ( in the respective proportions from 1 to 9) under a pressure of the order of 10 5 Pascal. The surface temperature of the substrate is around 1500C. This temperature is compatible with the use of polymer as the material constituting the two half-shells.

 In the example considered, the two half-shells must be transparent, it follows that the polymers used must be also transparent polymers. When the reading is carried out by reflection, only one of the half-shells is necessarily transparent.

 The polymers which can be used in the context of the invention are, by way of nonlimiting examples: polymethyl methacrylate, polyethylene terephthalate, or polycarbonate.

 According to another important characteristic of the invention, the internal face of the half-shells is covered before metallization, with a protective layer. The purpose of this layer is to avoid attack of the polymer by the liquid crystal, an attack which makes the polymer diffusing and prohibits its use as a light-transmitting element. In addition, the liquid crystal loses its properties in the vicinity of the attacked regions.

 In the known art, only the shims have been made of plastic material because they do not have to be transparent and are located outside the useful field of display (periphery of the device). The nature of this protective layer as well as the deposition method will be described in more detail in the following.

 Two other advantages brought by the invention which uses polymeric materials for the realization of the case of the device are the following one can engrave directly by molding two networks of orthogonal direction between it allowing the orientation of the molecules of the liquid crystal to obtain the described configuration in connection with FIG. 2, when the liquid crystal is of the nematic helical type and it is possible to directly deposit a suitable material on the external faces of demicoques 30 and 32, the polarizer and analyzer 2 and 3 described in relation with Figure 2. This deposition can be carried out by coating with a quinolinium iodide solution. This deposition is more difficult to achieve on the glass slides of devices of the known art.

 The half-shells are kept fitted by heat-sealing or gluing. The filling with liquid crystal can be carried out according to the methods of the known art.

 Other arrangements can also be retained allowing the simultaneous positioning and nesting of the two half-shells 30 and 32.

The faces facing the walls can be provided with half-shells with grooves or with complementary profiles as illustrated in FIG. 4. The two half-shells 40 and 42 are molded so as to fit the one over the other using complementary profiles 46 and 47. In this case, there is also locking. We find the metallization layers 42 and 43 as well as the protective layers 44 and 45 of the polymer materials.

 The invention remains compatible with the previously known techniques consisting in introducing the shims between the two frames. Figure 5 illustrates such a configuration. The two half-shells 50 and 52 are reduced to two plastic blades covered on their internal face with two protective layers respectively 54 and 55 as well as metallizations constituting electrodes 51 and 53. The spacing between these two blades is kept constant using a wedge 56. It is also possible to use polymer microbeads which are mixed with the liquid crystal. This well known technique is particularly advantageous in the case of large panels, for example flat liquid crystal television screens and avoids bending of the two blades facing each other.

The arrangement consisting in protecting the polymer layer using a thin layer of material as well as the methods of depositing this material will now be described in more detail. Obtaining a protective layer can be obtained according to two main variants:
- by a deposit of polymer in the gas phase,
- Or by depositing a polymer in solution followed by evaporation of the solvent and subsequent cooking.

 In all cases it is a question of obtaining a thin layer of insoluble polymer, the thickness of this layer being of the order of a few micrometers and preferably comprised in the range of 0.5 to 10 μm.

According to the first variant, the deposition of polymer in the gas phase can be carried out in various ways, by direct deposition of polymeric material, such as poly P - xylidene or by polymerization of a monomer using an initiated light discharge. by a high frequency current. An apparatus and a method for carrying out this polymerization is described in French patent application No. 73 37 360 filed on 19
October 1973 and published under the number 2 248 529, and more particularly in connection with FIG. 1 of this patent application. According to the teaching of this patent application, a layer of polymer with oriented molecules is formed on one of the faces of a blade placed on one of the discharge electrodes of an apparatus operating at high frequency, and containing in a chamber a monomer vapor at very low pressure (10 2 to 10 4 Pascal). In the context of the invention, the monomer can be perfluorobutene - 2 for example.

 According to the second variant, the polymer deposition is carried out from a solution. The product used is then polyimide or polyimidamide.

Such products are commercially available under different brand names. The solution is coated on the internal faces of the polymer reinforcement. These reinforcements are then heated to evaporate the solvent and obtain crosslinking of the deposited layer.

 The teachings of the invention can be applied to all types of electro-optical display devices comprising two series of opposite electrodes between which the liquid crystal has been inserted and whose armatures are made in the known art using two flat glass slides. By way of example, FIG. 6 schematically illustrates an individual, digital cell of the "seven segment" type, and more particularly the configuration of its electrodes. On the internal face of the half-shell 62 was deposited a multiple electrode under the general reference 63, electrode comprising "seven segments" allowing the display of the numbers 0 to 9 as well as additional elements in the form of a point and d 'a dash.These electrodes are extended by nine tabs under the general reference 630, tabs on which can be welded external connections not shown. On the internal face of the half-shell 60 was deposited a single electrode 61 whose configuration allows the covering of the multiple electrode 63. This electrode is also extended by a tongue 610 on which an external connection can be welded.

 Finally, the recording of information in certain types of liquid crystal display device is effected by thermoelectric effect. These devices also include two parallel frames between which the liquid crystal is placed. In addition, networks of resistive wires are arranged on the internal faces of these two armatures, for example in the form of two orthogonal networks between them of parallel lines, the assembly forming a device with matrix access. If two othogonal lines are crossed by a current, in the region near the point of intersection of two orthogonal lines, the temperature of the liquid crystal becomes higher than the temperature of erasure. Information previously recorded in this region is thus deleted. In these devices, it is particularly important to choose the coefficient of thermal diffusivity of the reinforcements supporting the resistive lines in predetermined relationship with the coefficient of thermal diffusivity of the liquid crystal included between these two. frames. In general, this coefficient should be lower than that of glass. An example of such a device is described in French patent application No. 79 19 426 filed on July 27, 1979. There are polymers with a coefficient of tin diffusivity much lower than that of glass. It is therefore an additional advantage of the invention which allows the use of polymers for reinforcements. The glass has a diffusive eoefficient of the order of 5.5. 10 3 cm2îs and the commonly used liquid crystals, a coefficient of the order of 2.10-3 cm20s. A polymer material such as polyethylene glycol polyterephthalate has a coefficient of friction equal to 3.10-4 cm2is and can be used for this type of application.

Claims (16)

 1. Electro-optical liquid crystal display device in which the liquid crystal is inserted between first t30) and second (32) armatures comprising electrodes (31, 33) of electrically conductive material arranged on the faces of the armatures in contact with the liquid crystal; device characterized in that the reinforcements are made of polymer material.  2. Device according to claim 1, characterized in that the polymer materials are chosen from the following: polymethyl methacrylate, polyethylene terephthalate glycol or polycarbonate.  3. Device according to any one of Claims 1 to 2, characterized in that the internal faces of the reinforcements are covered with a protective layer of highly crosslinked polymer material (34, 35) and in that the thickness of this layer is in the range 0.5 to 10 micrometers.  4. Device according to any one of claims 1 to 3, characterized in that the reinforcements are half-shells (30, 32) obtained by molding each being in the form of a polymer plate comprising a delimited parallelepiped cavity by side walls and in that these two half-shells are fitted one on the other so that the cavities are positioned in a pre-established relation to each other to define an enclosure intended to contain the liquid crystal.  5. Device according to claim 4, characterized in that the surfaces of the side walls of the half-shells in contact comprise a set of bosses (302) and recesses (322) with complementary profiles allowing the positioning in pre-established relation of two cavities.  6. Device according to claim 4, characterized in that the side walls of the two half-shells (40, 42) comprise locking means (46, 47) consisting of bosses and recesses with complementary profile intended to receive said bosses when the two half-shells are fitted.  7. Device according to any one of claims 1 to 3, characterized in that the two reinforcements each consist of a parallelepiped blade of polymeric material (50, 52) and are maintained in parallel relation one with respect to the 'other by shims (56).  8. Device according to claim 7, characterized in that the shims consist of microbeads.  9. Device according to any one of claims 1 to 8, characterized in that the internal faces of the reinforcements each comprise a relief network molded in the polymer material intended to induce a direction (P'2, P'3) of preferred orientation of a liquid crystal molecule and in that the directions induced by the first and second reinforcements are orthogonal to one another.  10. Device according to any one of claims 1 to 8, characterized in that the external faces of the armatures are covered with a layer of material having properties of polarization of light in a preferred direction and in that the directions of polarization of the layers (P2, P3) covering the first and second reinforcements are orthogonal to each other.  11. Device according to claim 10, characterized in that the material having light polarization properties is quinolinium iodide.  12. A method of manufacturing an electro-optical display device according to any one of claims 1 to 11, characterized in that it comprises at least one step consisting in molding two half-shells (30, 32) of material polymer according to a predetermined profile, to form the reinforcements (30, 32).  13. The method of claim 12, characterized in that it comprises a step of depositing a polymer material in the gas phase on the internal face of the two half-shells to form a protective layer (34, 35) and in that that this polymeric material is poly p - xylididene.  14. Method according to claim 12, characterized in that it comprises an additional step of depositing a monomeric material in the gas phase on the internal face of the two half-shells to form a protective layer (34, 35) followed by polymerization of this monomeric material using a light discharge and in that this monomeric material is perfluorobutene-2.  15. The method of claim 12, characterized in that it comprises an additional phase of despite a polymeric material in solution on the internal face of the two half-shells to form a protective layer (34, 35) followed by evaporation of the solvent, and in that the polymeric material is a polyimide or a polyimidamide.  16. The method of claim 12, characterized in that it comprises a step consisting in bonding or heat sealing the two half-shells together.