Heterogeneous display elements and methods for their...

Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Utilizing plasma – electric – electromagnetic – particulate – or...

Reexamination Certificate

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Details

C264S001700, C264S437000, C264S438000, C264S004100

Reexamination Certificate

active

06241921

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to microparticles and their production, functional fluids containing microparticles, and microparticle-based display and information-bearing elements.
BACKGROUND OF THE INVENTION
Colors are imparted to various media by dissolving and/or dispersing organic dyes in them, or by suspending insoluble inorganic pigments in the media during their manufacture. Colored media may serve as the material out of which an end product is fabricated (e.g., in the case of plastic articles), or may be applied to a substrate in the form of an ink, paint or toner. Electrophoretic toners, for example, consist of pigments dispersed in an oil-based, low-viscosity carrier. The pigments are given electrical charges so they can be attracted by electrical charges of opposite polarity on a substrate. During an imaging procedure, the pigment particles migrate, driven by electrostatic attraction, towards the substrate surface in response to an imagewise electrical charge pattern applied thereto.
Pigments may also be retained within a permanent liquid carrier to form displays. An electrophoretic display utilizes charged particles of one color suspended in a dielectric liquid medium of a different color. The suspension is housed in a cell located between (or partly defined by) a pair of oppositely disposed electrodes, one of which is optically transmissive. When the electrodes are operated to apply a DC or pulsed field across the medium, the particles migrate toward the electrode of opposite sign. The result is a visually observable color change. In particular, when a sufficient number of the particles reach the optically transmissive electrode, their color dominates the display; if the particles are drawn to the other electrode, however, they are obscured by the color of the liquid medium, which dominates instead.
As a rule, pigments consist of poorly dispersed, agglomerated discrete particles whose approximate coloring effects are derived from the widely varying shape and size distributions throughout the carrier material. Different colors and hues are obtained by mixing quantities of different color pigments. It has been found, however, that the purity and predictability of color relates quite specifically to the particular size, shape, and morphology of the pigment particles. For demanding colorant applications, it is desirable to utilize pigment particles exhibiting a narrow size distribution (i.e., which are monodispersed), identical shape (frequently, but not always spherical), identical bulk properties, and identical surface electrical properties (directly or through additives). Heretofore, such uniform synthetic pigment particles have been difficult to manufacture in a controllable, inexpensive fashion.
At the same time, while pigment particles have been utilized in electrophoretic displays, many common forms of electronic display require larger and/or specialized elements that are expensive to fabricate. It would be beneficial to utilize the versatility of pigments in connection with the many types of display and information-bearing devices that currently require specialized components.
DESCRIPTION OF THE INVENTION
Brief Summary of the Invention
In a first aspect, the present invention comprises methods of fabricating pigment particles of uniform morphology and of a narrow size range. The particles may be spherical or anisotropic (e.g., disk-like), and typically have sizes that can range from several nanometers to 100 &mgr;m. The particles may be provided with a polymer coating to confer desired charge, photoresponse or density characteristics. By matching the density of the particles to that of the surrounding medium, it is possible to fabricate particle-based displays that avoid problems of agglomeration. In addition, by conferring different densities to different classes of particle, it is possible to separate the particles into distinct regions by centrifugation.
A polymer-coated particle may be formed from a conventional inorganic or organic pigment as follows. First, the polymer material is dissolved in a solvent to form a solution, which is combined with a homogeneous suspension of pigment particles. The dissolved polymer is then caused to adsorb onto the surfaces of the pigment particles, and the adsorbed polymer is precipitated to form coatings on the particles. The thickness of a coating may be controlled so as to form only a thin shell (e.g., for purposes of conferring a surface charge) or a thick covering. In the latter case, the volume of the polymer coating may be substantially greater than that of the surrounded core, thereby dominating the density characteristics of the composite particle; in addition, the shape of a thick outer coating can be controlled so as to establish the overall shape and dimension of the final particle. For example, if precipitation occurs under centrifugation, the particles will exhibit a disk-like (rather than spherical) shape.
In a second aspect, the invention provides for manufacture of optically heterogeneous display elements using particles as described above. These particles may be microencapsulated prior to formation of the display element, so that the element is formed internally within the container in which it is permanently housed. In accordance with this aspect of the invention, two types of particles, generally in equal proportions, are encapsulated in microcontainers. The two particles are optically distinct (e.g., of different colors or shades, different refractive indices, different fluorescent properties, and/or different phosphorescent properties) and also respond differentially to an external stimulus (e.g., an electric, magnetic, or gravitational field). The external stimulus is applied to the microcontainers, thereby separating the particles into distinct regions within the microcontainers, and the particles, thus separated, are fused into a single encapsulated element. Fusing may be accomplished by exposure to energy in the form of, for example, heat, actinic radiation, ultrasound or radio-frequency (RF) radiation. If the differential response persists within the fused element, it can be used to orient the element within the microcontainer.
In a variation to this approach, optical contrast is provided not by complementary halves of the display element, but instead by a carrier medium that contrasts with a unitary display element. The element may be a hemisphere contained within a spherical microcapsule and surrounded by a carrier medium (typically, although not necessarily, liquid), the element and the carrier contrasting in terms of at least one optical property of interest. For example, the hemisphere and the carrier medium may be differently colored, so that if the hemisphere is adjacent the portion of the microcapsule that is viewed, its color dominates the display; if the element is drawn to the other side of the display, however, it is obscured by the color of the carrier medium, which dominates instead.
More generally, the display element is formed by encapsulating a suspension of particles within an optically transmissive container having an interior surface contour. The particles are responsive to an external stimulus and are suspended within a carrier medium that contrasts visually with the particles. The external stimulus is applied to the encapsulated particles to aggregate the particles, which are joined into a single encapsulated element having a surface conforming to the interior surface contour of part of the container. In this way, the element is visible through a portion of the container and obscured by the carrier medium through another portion of the container, so that what the viewer observes depends on the relative position of the element.
In a third aspect, the invention provides for manufacture of light-valve elements based on fused particles. A suspension of particles opaque to light is encapsulated within an optically transmissive container. The particles are caused to form a disk, following which they are joined together into a single encapsulated element.
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