Flexible liquid crystal display device with improved mechanical ability

[0002] The present invention relates to a liquid crystal display device comprising a substrate made of a flexible material, preferably a plastic foil substrate, and a liquid crystal cell containing a bistable ferroelectric liquid crystal arranged between covering electrodes.

[0003] This type of display device has already been described in German Published Patent Application DE 196 49 761.2. Ferroelectric liquid crystals are known for their rapid and bistable switching properties. However displays made using these crystals are usually impact sensitive and react by loosing written information as soon as a material flow occurs in the interior of the crystal layer. In the known foil displays with ferroelectric liquid crystals this problem is minimized by using special thin foil substrates and by using a liquid crystal with a so-called “bookshelf” texture.

[0004] It is an object of the present invention to provide a ferroelectric liquid crystal display device with a flexible substrate, which has a higher mechanical stability than currently known displays.

[0005] This object and others which will be made more apparent hereinafter are attained in a liquid crystal display device comprising a liquid crystal cell containing a bistable ferroelectric liquid crystal arranged between covering electrodes and a substrate made from flexible material, preferably a plastic foil substrate.

[0006] According to the invention the optical thickness of the liquid crystal cell is a fourth of the wavelength of light and spacers are arranged between the covering electrodes. Preferably the spacers are at a distance of from 40 to 60 μm from each other.

[0007] The shear force that acts on the crystal layer of this display device depends strongly on its thickness. By using a liquid crystal cell according to the invention having an optical thickness that corresponds to a quarter of the wavelength of light, the impact sensitivity is thus clearly reduced. The spacers also increase the mechanical stability of the display device by themselves, so that the display device resists compression, bending and torsion. A diffuser or a reflector can be arranged on the rear side of the display device, which insures that light passes through the quarter wave cell twice. Because of that the liquid crystal cell according to the invention has a thickness, which amounts to about 0.8 μm, which is about half that for cells of the currently known liquid crystal display devices. The spacers arranged between the covering electrodes of the liquid crystal cells can have a thickness of about 0.8 μm corresponding to that of the cells and a diameter of about 15 μm and preferably have plane surfaces on each end or side. These spacers may be made by photolithography or printing techniques. The spacers can however also be provided with an adhesive layer and can be distributed on the covering electrodes. Orienting layers for the liquid crystal comprising SiO_xor Ta₂O₅layers can be deposited by evaporation or spraying on the cover layers, obliquely or diagonally to the substrate surface. This type of orienting layer exerts a great anchoring influence on the molecules of the liquid crystal material. An additional increase in the mechanical stability may be obtained by producing a low pressure in the liquid crystal cell. The mechanical properties of a display device according to the invention having about 3 cm²surface area include typically a compressive load over 120 N/cm², a bending radius of less than 1.5 cm and a switching potential of under 5 V at temperatures of 0° C. to 60° C.

[0008] The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiment, with reference to the accompanying sole FIGURE that is a cross-sectional view of a preferred embodiment of the display device according to the invention.

[0009] The sole FIGURE shows a cross-sectional view of one embodiment of a display device 10 according to the invention. The display device includes a flexible substrate 11 made from a flexible plastic material and a liquid crystal cell 15 containing a bistable ferroelectric liquid crystal 12. In the liquid crystal cell 15 the liquid crystal 12 is arranged between two electrodes 13 and two orienting layers 14. The spacers 16 distributed between the two orienting layers 14 as shown in the figure determine the thickness of the liquid crystal cell. The orienting layers 14 are made from obliquely or diagonally deposited SiO_xlayers deposited by vacuum vaporization or spraying. A low pressure can also be provided in the liquid crystal cell 15 by an adhesive border 17 arranged laterally around the display device 10 that is admixed with unshown spacer elements, for example gold-coated polymeric spheres. These polymeric spheres can also act at the same time to provide a contact for the opposing plate. A reservoir for the liquid crystal, which can also contain vacuoles, arises along the relatively thick adhesive border 17. A diffuser or mirror 18 is also arranged on the rear side or the display device 10, which causes light to pass twice through the liquid crystal cell 15. In this manner the optical thickness of the liquid crystal cell can be reduced to a fourth of the wavelength of the light. A polarizer 19 is provided on the visible side of the display device 10.

[0010] The term “wavelength of light” in the specification and appended claims means the average wavelength of visible light passing through the liquid crystal.

[0011] The term “low pressure” means a pressure below normal atmospheric pressure.

[0012] The disclosure in German Patent Application 198 22 150.9 of May 16, 1998 is incorporated here by reference. This German Patent Application describes the invention described hereinabove and claimed in the claims appended hereinbelow and provides the basis for a claim of priority for the instant invention under 35 U.S.C. 119.

[0013] While the invention has been illustrated and described as embodied in a liquid crystal display device, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention.

[0014] Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

[0015] What is claimed is new and is set forth in the following appended claims.