Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-01-11
2004-09-21
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S189000, C349S086000, C349S156000
Reexamination Certificate
active
06795138
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
This invention relates to liquid crystal displays comprising cells of well-defined shape, size and aspect ratio, which are filled with liquid crystals, preferably with a guest dye, and novel processes for their manufacture.
b) Background
A polymer dispersed liquid crystal (PDLC) display usually comprises two transparent plates with electrodes placed opposing each other, separated by using spacers. A thin film of PDLC is enclosed between the two plates. The PDLC film may be up to 200 microns thick, but usually having a thickness of between 2 microns and 50 microns. The cell is hermetically sealed in order to eliminate oxygen and moisture, both of which may chemically attack the liquid crystals. A thorough review of the PDLC technologies can be found in the book “Liquid Crystal Dispersions” by P. S. Drzaic (1995).
A PDLC typically consists of micron-size droplets of a low-molecular-weight nematic liquid crystal dispersed in a polymer binder. The nematic droplets strongly scatter light and the material has a white opaque or translucent appearance (“off state”). When a voltage difference is imposed between the two electrodes (“on state”), the electric field aligns the droplets such that the ordinary refractive index of the liquid crystal nearly matches that of the isotropic polymer matrix, substantially reducing the scattering power of the droplets, and thus allowing light to transmit through. In the on state, the cell thus appears clear or transparent, in the off state it appears opaque.
In a guest-host PDLC display, a dye, particularly a pleochroic or dichroic dye, is added as a guest to the liquid crystal to produce a high color contrast display. For example, because the dye molecules have a property to orientate parallel to the liquid crystal molecules, if a dichroic dye having a bar-shaped structure is added to the liquid crystal, the direction of the dye molecules also changes if the molecular direction of the liquid crystal is changed by applying an electric field on the opposing electrodes. Because this dye is made colored or not depending on the orientation direction, it is possible to switch between a colored state (“off state”) and a colorless state (“on state”) by applying a voltage on the two electrodes. The use of dichroic or pleochroic dyes in guest-host PDLC displays to improve the contrast ratio is well known in the art.
A PDLC display may be transmissive and/or reflective. A transmissive PDLC display has an internal illumination source. Imposing a voltage on the two electrodes allows light to pass through the cell. A typical example of a transmissive PDLC display is a PDLC overhead projector. Reflective PDLC displays typically contain a reflective black or colored filter which becomes visible in the transparent state. Reflective PDLC displays may be found in PDA (personal digital assistant) devices. Transmissive and reflective PDLC displays are particularly attractive because polarizers are eliminated. Polarizers substantially reduce light and decrease brightness of both direct view and projection displays. The absence of polarizers also gives a better viewing angle.
The PDLC displays prepared by prior art processes have many shortcomings. For example, the polymer dispersed liquid crystals typically have droplets of very broad particle size distribution, which results in significant hysteresis, higher operation voltage, poor contrast ratio, undesirable red bleedthrough, and low level of multiplexing. However, the hysteresis of PDLC films must be low to show reproducible gray scales, and low voltage operation and high contrast ratio of the device is essential for most PDA applications. Monodispersed liquid crystal particles in the micron size range have been taught in U.S. Pat. No. 5,835,174, (Clikeman, et al.) U.S. Pat. No. 5,976,405 (Clikeman, et al.), and U.S. Pat. No. 6,037,058 (Clikeman, et al.) to reduce the hysteresis and operation voltage, and improve the level of multiplexity. The contrast ratio of PDLC device prepared from the monodispersed particles remains low for most applications. To improve the contrast ratio without trade off in the thickness of the PDLC film and operation voltage, guest dyes preferably, pleochroic dyes or dichroic dyes are needed. However, the prior art processes do not allow for the precise enclosure of a high concentration of guest dyes in the liquid crystal phase during the manufacturing process, such that only a low concentration of dyes may be encapsulated in the monodispersed polymer particles. Some guest dyes may be left outside of the particles, thereby resulting in an increase in Dmin and a lower contrast ratio.
It is highly desirable to create monodispered liquid crystal domains, which would alleviate the requirement of high operation voltage, allow high contrast ratio and high level of multiplexing, and reduce hysteresis.
SUMMARY OF THE INVENTION
The first aspect of the present invention is directed to a liquid crystal (LC) display comprising cells of substantially uniform shape, size and aspect ratio. The cells are filled with LC preferably with guest dye(s).
Another aspect of the invention relates to a novel process for the manufacture of such a LC display.
A further aspect of the invention relates to the preparation of cells of substantially uniform shape, size and aspect ratio. The cells enclose LC preferably with guest dye(s) and are formed from microcups prepared according to the present invention. Briefly, the process for the preparation of the microcups involves embossing a thermoplastic or thermoset precursor layer coated on a conductor film with a pre-patterned male mold, followed by releasing the mold before, during or after the thermoplastic or thermoset precursor layer is hardened by radiation, cooling, solvent evaporation, or other means. Alternatively, the microcups may be formed from imagewise exposure of the conductor film coated with a radiation curable layer followed by removing the unexposed areas after the exposed areas have become hardened.
Solvent-resistant, thermomechanically stable microcups having substantially monodispersed size and shape can be prepared by either one of the aforesaid methods. The size of microcups for most display applications is in the range of submicrons to 10 microns, more preferably 0.5 microns to 5 microns. The shape may be any shape, although a shape allowing a higher total area of interface between liquid crystal and the cups is preferred. The microcups are then filled with LC preferably with guest dye(s), and sealed.
Yet another aspect of the present invention relates to the sealing of the microcups filled with the LC preferably with guest dye(s). Sealing can be accomplished by a variety of ways. Preferably, it is accomplished by dispersing into the LC phase before the filling step, a sealant composition containing a thermoplastic or thermoset precursor. The sealant composition is immiscible with the LC and has a specific gravity lower than that of the LC. After filling, the thermoplastic or thermoset precursor phase separates and forms a supernatant layer at the top of the LC. The sealing of the microcups is then conveniently accomplished by hardening the precursor layer by solvent evaporation, interfacial reaction, moisture, heat, or radiation. UV radiation is the preferred method to seal the microcups, although a combination of two or more curing mechanisms as described above may be used to increase the throughput of sealing. Alternatively, the sealing can be accomplished by overcoating the LC with a sealant composition containing the thermoplastic or thermoset precursor. The solvent used in the sealant composition is critical. Preferably, it is immiscible with the LC and has a specific gravity tower than that of the LC. It is also important to control the surface tension and viscosity of the sealant composition to ensure a good coating uniformity. The sealing is then accomplished by hardening the sealant composition by solvent evaporation, interfacial reaction, moisture, heat, radiation, or a combination of cu
Li Weijin
Li Ying-Syi
Liang Rong-Chang
Phan Xuan
Tseng Scott C-J
Howrey Simon Arnold & White , LLP
Kung Viola T.
Parker Kenneth
SiPix Imaging Inc.
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