METHOD OF MANUFACTURING PLASTIC PARTICLES FOR A PARTICLE DISPLAY

Abstract: A particle display comprises a plurality of particles which in an electrical field rotate to expose either their white or black surface depending upon the polarity of the field. The particle display can be used for a flat panel ambient-illuminated numeric, alpha-numeric and other forms of visual displays.

Description: This invention relates to particle displays and more particularly to the method of manufacturing particles used in the display. Philips Electronics of The Netherlands developed a display using small magnetized spheres coated half white and half black. In a magnetic or electrostatic field, these particles rotate to expose a white or black surface depending on the polarity of the field. The display can be used for flat panel ambient-illuminated numeric, alpha-numeric and other forms of visual displays, such as television screens. The magnetized spheres which make up the display have been made by Phillips Electronics by compounding polyethylene with magnetite. The polymer composition is then processed to produce fine particles by centrifugal spraying of the polymer melt. The magnetite particles in the particles of polyethylene are magnetized. The particles are then aligned in a magnetic field to produce a single layer of particles. With the particles held in the single layer, aluminum is vacuum deposited on the upper half of the surface of the spherical particles to coat each particle with a white coating. The remaining uncoated portion of the sphere, as compounded with magnetite, has a dark or black appearance. This method of production of the spheres is very tedious and difficult to adopt for mass production. In US-A-3982334, there is disclosed a device adapted to provide a visual display under the influence of an exterior magnetic field, which comprises a plurality of rotatable, multi-coloured, non-magnetically interactive, permanently magnetized micromagnets disposed in a cavity within a body having a transparent surface and a plurality of relatively small compartments within the cavity. The device can be a sheet containing such micromagnets between the thin cavity of the front and rear surfaces of the sheet wherein the relatively even dispersion of the micromagnets in the sheet is maintained by such compartments whose barriers minimize displacement so that physical pressure exerted on the sheet, such as by a twist or flex, does not substantially alter the even dispersion of the micromagnets. In US-A-4143103, there is described a method of making a display characterized by a plurality of particles which have an electrical anisotropy due to hemispherical surface coatings of different Zeta potential and their distribution in a volume of a dielectric liquid and which also have an optical anisotropy due to the hemispherical surface coatings having different optical characteristics. The particles are mixed with a light transparent liquid which is subsequently cured to form an elastomeric or rigid slab. Following curing of the liquid, the slab is emersed in a plasticizer (dielectric liquid) which is absorbed by the slab and which causes the slab to expand slightly. Expansion of the slab around the particles provides a plasticizer-filled cavity around each particle, which cavities allow the particles to rotate to provide a display in accordance with their optical anisotropy but does not allow substantial translation of the particles. According to the present invention, there is provided a method of making spherical plastic particles composed of first and second portions each of a distinct colour, the first portion including a material which imparts thereto a predetermined electrical property which is different from the electrical properties of the second portion, said first and second portions being substantially hemispherical portions of the two distinct colours, characterised in that the method of making the spherical plastic particles comprises the steps of: (a) adding to a heated liquid bath non-spherical particles consisting of (i) a first layer of plastic having a first colour, said first layer including a material which imparts a predetermined electrical property thereto, and (ii) a second layer of plastic having a second colour, said second layer including a material which imparts thereto an electrical property different from that of said first layer, the non-spherical particles having been formed by extruding said first and second layers of plastic in the shape of an elongate rod having said first and second layers adjoining one another centrally of and along said rod, said rod having been cut into predetermined lengths wherein the resultant aspect ratio of fibre length to diameter is in the range of 0.5 up to 5.0; (b) establishing in said bath a temperature which is sufficient to cause melting of said first and second layers of plastic, an anti-agglomerating agent being present in said bath to preclude the melting non-spherical particles in said bath from adhering to one another; (c) allowing the melting non-spherical particles to form spherical plastic particles in said bath; (d) solidifying said spherical plastic particles in said bath; and (e) removing said spherical plastic particles from said bath. Figure 1 of the drawings shows, in magnified form, spherical plastic particles (hereinafter referred to as spheres), made in accordance with this invention for use in magnetic particle displays and the like. The spheres, as prepared in accordance with this invention, may be incorporated into flat display panels in accordance with standard procedure. Such procedure involves encapsulating each sphere in one manner or another to form individual cells. The individual cells are then incorporated into a display. There are various examples of such techniques, such as disclosed in the above-mentioned US-A-4143103. In that patent, the difference in surface colours for the two hemispheres of the spherical particle provides the necessary difference in electrical properties. It is disclosed in that patent that, when particles are dispersed in a dielectric liquid, the particles acquire an electrical charge related to the Zeta potential of their surface coating. Due to the optical anisotropy of the particles by virtue of the different surface coatings, then the particles present different Zeta potentials with the result that the particles have an electrical anisotropy in addition to their optical anisotropy. Under the action of an addressing electric field, such as provided by conventional matrix addressing, selected ones of the electrically anisotropic particles are made to rotate or otherwise shift their orientation within their cavities to provide a display by the selective adsorption and reflection of ambient light. As previously noted, it is possible to provide the desired electrical property in each particle by incorporating a magnetic material therein. Hence, it is understood that by the use of the term electrical property, it is intended to cover not only electrical charges on the particle, but also any other form of electrical effect including a magnetic field. The invention is applicable to forming spheres from any form of suitable plastic which can provide for the compatible marrying of one colored material to a different colored material, where one of the materials optionally contains magnetite. There are available many suitable thermoplastic polymers which fall into this category such as polyolefins of polyethelene and polypropylene or, polyamides and acrylics. The selected polymer should have a relatively low viscosity in the molten state so as to assume the shape of a sphere within a relatively short time frame when the particles are immersed in a heated bath. The prepared particles, according to an aspect of the invention, consist of one layer of polymer of one color and a second layer of polymer of another color. One of the layers may contain magnetite. The particles are charged into a tank containing a bath medium which is of a sufficiently high temperature to cause a melting of the particles. To prevent agglomeration of melted particles when in the bath, an additive which prevents particle agglomeration is either present in the bath or coated onto the particles before the particles are introduced to the bath. Stirring of the bath may be provided to circulate the particles. As the particles melt, the anti-agglomeration material adheres to the melted spherical surfaces of the particles and prevents adjacent coated particles from agglomerating to form larger particles. Suitable anti-agglomeration powders include clays, such as kaolin clays, which may be coated on the dry particles before introduction to the bath or the kaolin clay powder may be introduced to the bath before the particles of polymer are charged into the bath. Other suitable anti-agglomeration additives include surfactants which perform the same function in preventing agglomeration of the particles when they are present in the bath. Once the particles have formed spheres, the bath is cooled to room temperature to solidify the particles in their spherical form. The particles are then filtered from the bath or in some other manner removed from the bath for further treatment and set up for use in displays. According to one aspect of the invention, the particles may be prepared by coextruding a fibre consisting of a semi-circular layer of polyethylene pigmented white and a black layer of polyethylene containing magnetite. The resultant fiber is chopped into fine particles to provide a layer containing the white pigment married to a layer containing the magnetite. The fibre particles have an aspect ratio; i.e. fiber length to diameter, in the range of 0.5 up to 5.0. The extruded diameters may be in the range of 10 microns up to 10 millimeters depending upon the desired size of the sphere to be formed from the particle. The particles may then be mixed with clay or other anti-agglomeration material. The coated polyethylene particles are charged into a stirred tank reactor containing oil or other liquid preheated to approximately 120°C. The particles are mixed for approximately ten minutes in the bath, during which time the particles form spheres without agglomerating with other adjacent spheres. The bath is then cooled to room temperature to solidify the particles. The bath medium is discharged and the particles are filtered from the medium. It is appreciated that the plastic for each layer may be different. In the coextrusion of the different plastics, it may not be necessary to use an adhesive to bond the layers together, because the layers may still fuse. However in situations where the layers do not form a proper bond at their interface, a suitable adhesive may be employed in the coextrusion process. Figure 1 shows the resulting spheres 10 and 12. Each sphere is essentially round and has a distinct white hemispherical portion 14 compared to a distinct black hemispherical portion 16. The black hemispherical portion contains the magnetite. Each particle has a slightly warped interface 18 formed during the melting of the non-spherical particle. It is appreciated that for fibers when the aspect ratio is approximately 1.0, the resultant spheres have an essentially planar interface. The process, according to this invention, provides in a rapid efficient manner spherical particles having a hemispherical portion of different electrical properties for use in various types of particle displays.

Claims: A method of making spherical plastic particles (10) composed of first (14) and second (16) portions each of a distinct colour, the first portion (14) including a material which imparts thereto a predetermined electrical property which is different from the electrical properties of the second portion (16), said first and second portions (14, 16) being substantially hemispherical portions of the two distinct colours, characterised in that the method of making the spherical plastic particles (10) comprises the steps of: (a) adding to a heated liquid bath non-spherical particles consisting of (i) a first layer of plastic having a first colour, said first layer including a material which imparts a predetermined electrical property thereto, and (ii) a second layer of plastic having a second colour, said second layer including a material which imparts thereto an electrical property different from that of said first layer, the non-spherical particles having been formed by extruding said first and second layers of plastic in the shape of an elongate rod having said first and second layers adjoining one another centrally of and along said rod, said rod having been cut into predetermined lengths wherein the resultant aspect ratio of fibre length to diameter is in the range of 0.5 up to 5.0; (b) establishing in said bath a temperature which is sufficient to cause melting of said first and second layers of plastic, an anti-agglomerating agent being present in said bath to preclude the melting non-spherical particles in said bath from adhering to one another; (c) allowing the melting non-spherical particles to form spherical plastic particles (10) in said bath; (d) solidifying said spherical plastic particles (10) in said bath; and (e) removing said spherical plastic particles (10) from said bath. A method according to claim 1, wherein said bath is an oil bath heated to a sufficient temperature to cause formation of said spherical plastic particles (10), said oil bath being cooled to solidify said formed spherical plastic particles (10). A method according to claim 1 or 2, wherein said first layer has said electrical property imparted therein by incorporating into said first layer a magnetizable material. A method according to claim 3, wherein said magnetizable material is magnetite, said magnetite pigmenting said first layer black, and wherein said second layer is pigmented white with titanium dioxide. A method according to any one of the preceding claims, wherein said anti-agglomerating agent is selected from clays and surfactants. A method according to any one of the preceding claims, wherein said plastics are selected form thermoplastic polymers, polyamides and acrylics.

Inventors: LEIDNER, JACOB
Applicants: ORTECH CORPORATION
Application year: 1987
Main classification: G09F9
IPC codes: B29B9, B29K77, H01F1, G02B26, B29C67, C08J3, G09F9, B29K33