HIGH-QUALITY COLOR PRINTER

Technical Field

The invention relates to a liquid electrostatic dyestuff or toner composition useful in electrostatic transferring printers and capable of producing multicolor images upon a hydrophobic polymer surface and to an electrostatic printing method using the aforementioned liquid electrostatic dyestuff.

Background Art

The rapid increase of computers and their application to both image processing and image generation has greatly increased the demand for color printers that can make hard copy of the color images generated and shown on CRT monitors. Historically, conventional photographic films and instant print photographic films have been used to directly photograph the CRT images. The cost of film and/or print paper to produce 8 x 10 inch size prints limits the use of these materials to

specialized applications of low volume where cost is not an important criterion.

Presently, the marketplace is seeing many new printers based on thermal transfer (wax-based transfer inks) and ink jet printers. The printers produce low cost prints with a limited range of colors and no gray scale or a limited gray scale. When gray scale is achieved it is achieved by a sacrifice in image resolution. However, this performance is more than satisfactory for many applications, such as bar chart graphs and simplified poster-like pictures. There is a large market for these applications and the sales introductions of new model thermal and ink jet printers is booming.

The rapid growth of computers and their associated memory capacity and processing capability has reached the point where widespread use of computers to process high quality photographic color images is already beginning. This will intensify the need and the search for a rapid and low cost means to print high quality photographic images in color following the electronic generation and/or processing of such images. Electrographic color copiers have been under development for many years but are only on the market in limited numbers. The high gamma of most photconductor materials combined with the normal optical imaging and associated electrostatic toners (both dry and wet) have resulted in color copies that are not of photographic quality. Generally, such color copiers have used pigmented toners or dye toners with binders which serve to bind the pigments

and/or dyes to paper. This reduces the brilliance of color on the prints.

Disclosure of Invention

The present invention is directed to solving one or more of the problems as set forth above.

In accordance with the present invention a liquid electrostatic dyestuff is set forth consisting essentially of (1) an inert hydrophobic solvent; (2) dye particles dispersed in said solvent, the particles being substantially insoluble in the solvent but being selected to be soluble in a hydrophobic polymer surface at temperatures above a selected temperature and (3) a charge control agent which preferentially partitions onto the dye particles. The dyestuff is substantially binder-free.

In accordance with another embodiment of the present invention a method of electrostatic printing is set forth which comprises contacting an imaged liquid electrostatic dyestuff as set forth above with a polymeric surface of a sheet. The polymeric surface is heated to a dissolving temperature above the selected temperature, the dissolving temperature being such that the dye particles dissolve into said polymeric surface at the dissolving temperature.

When operating in accordance with the present invention, and in particular when utilizing the novel liquid electrostatic dyestuff thereof, one can attain sharp color images on hydrophobic surfaces. And, this can be accomplished utilizing conventional technology for imaging purposes. An

excellent gray scale is attainable without loss of any other qualities.

Best Mode For Carrying Out Invention

The color printer of the present invention makes use of the electrostatic copying process and can use any of the basic electrostatic processing techniques. The three major types of electrostatic processing techniques, as is well known, are: (1) a photoconductor coated sheet is successively charged, imaged and toned with, generally, three or four toners, for example yellow, cyan, magenta and black. This sheet then becomes the output copy; (2) a photoconductor drum, belt, or other configuration is successively charged, imaged, and the resulting charge image transferred to a dielectric receiving sheet which is again successively toned with the colored toners; and (3) a photoconductor drum, belt, or other configuration is successfully charged, imaged and then toned, with each of the toner images being transferred to a final receiving sheet.

In accordance with the present invention certain modifications are made to the basic electrostatic processing steps set forth above. The most basic change is that the toner is different from previous color toners. In accordance with the present invention the toner is binder-free and consists essentially of small physical particles of a dispersed dye. The dye is dispersed in a hydrophobic solvent which may be, for example, a simple hydrocarbon solvent such as petroleum ether, benzene, or one of the Isopars (Trademark of Exxon Corporation) . The chemical makeup of the solvent is unimportant so long as it does not deleteriously

effect the dye, the equipment or the sheet and so long as it will not dissolve the dye sufficiently to lead to coloring of the solvent and resulting coloring of the sheet where contacted by the solvent, but not by the dye particles.

The dye particles themselves must be selected so as to be substantially insoluble in the solvent at the temperature that they are at when dispersed in the solvent (generally below about 40 ^β C) but at the same time to be soluble in a hydrophobic polymer surface when the polymer surface is sufficiently softened, as by being heated above a softening temperature. This temperature is generally in the neighborhood of the glass transition temperature of the hydrophobic polymer surface and will prefereably be at or slightly above the glass transition temperature, but can also be somewhat below that temperature. Thus, in a more general sense, the dye particles must be such that they will be soluble in the hydrophobic polymer surface at a dissolving temperature which is above the temperature of application of the toner to the hydrophobic polymer surface and which is above said selected temperature. The second requirement of the method of the present invention is that the surface of the output copy be a hydrophobic polymer surface. A number of hydrophobic polymers may form this surface. For example, the surface may consist of polyamideε, cellulose esters or polyesters. The chemical nature of the polymer is relatively unimportant so long as the polymer is hydrophobic, will dissolve the dye particles at the dissolving temperature and will not deleteriously effect the

dye. Preferably, the hydrophobic polymer surface will be a polyester surface.

For ease of operation it is generally preferred that the polymer be selected so as to be such that the dye particles will dissolve therein at a dissolving temperature in the range from about 150 ^β C to about 250°C.

It is preferred that the particles have an average particle size whch falls in a range from about 0.1 micron to about 10 microns. More preferably, the particle size will fall within a range from about 0.1 micron to about 1 micron.

The toner must include a charge control agent which preferentially partitions away from the hydrophobic solvent and into the dye particles. The - charge control agent may be, for example, lethicin, a metallic sulfonate, a. soap or a detergent. Examples of useable charge control agents include Aerosol OT (Trademark of American Cyanamid Co.) , Aerosol TR 70 (Trademark of American Cyanamid Co.), salts of long chain alkyl napthalene sulfonates, calcium and barium salts of long chain dialkyl sulfosuccinates, metal salts of fatty acids and the like. It is particularly important that the dye particles not be soluble in the inert hydrophobic solvent at the temperature at which they are dispersed in the solvent. This is important to ensure that the resulting colors appear clean and distinct. If the dye particles did dissolve somewhat in the solvent, then when the dye particles were contacted with the hydrophobic polymer surface and the hydrophobic polymer surface was heated the color would appear not only at the spots where the

particles were located, but also to a lesser extent in any area which was covered by the inert hydrophobic solvent. Thus, when the requirement is set forth that the dye particles be substantially insoluble in the solvent what is meant is that the dye particles be sufficiently insoluble in the solvent (at the relevant temperature) so that the color is not noticeably smeared due to dissolved dye in the solvent. In order to get the dye to dissolve into the hydrophobic polymer surface it is necessary to heat the polymeric surface, with the imaged dyestuff in contact therewith, to a dissolving temperature whereat the hydrophobic polymer surface is sufficiently softened so that the dye is soluble therein. Generally, this temperature will be about the glass transition temperature as mentioned above. Generally, the dissolving of the dye into the hydrophobic polymer surface will take from about 20 seconds to about 2 minutes, more usually from about 30 seconds to about 90 seconds. This time can, however, be greatly reduced if the surface is melted. Thus, it is conceivable to use dissolving times of the order of a few milliseconds. As a general rule it is preferred that the dissolving time be as short as possible so as to allow for the shortest printing time possible.

In some of the embodiments of the present invention it is necessary that the dye particles be of a sublimable dye, generally one wherein the dye particles sublime at a temperature above about 120 ^β C. This is necessary wherein the printed upon copy with the hydrophobic polymer surface is one to which the dye must be transferred during the heating

process. This takes place in the situation wherein a photoconductor drum, belt, or other configuration is successively charged, imaged and then _, toned with each of the toner images then being transferred to the final receiving sheet.

During the heating process sufficient pressure is generally applied to insure surface contact. This assures that good printing of the dye onto the hydrophobic polymer surface can occur. Generally, the pressure will be nominal.

When the liquid electrostatic dyestuff is spoken of as consisting essentially of an inert hydrophobic solvent, the aforementioned dye particles, and the aforementioned charge control agent, this language is not meant to exclude other components which may be present. If an additional component or components are present such additional component or components must be electrostatically compatible and practically compatible with the other components. It is a part of the present invention that a binder is not present in the liquid electrostatic dyestuff of the present invention since this interferes with the dissolving of the dyestuff into the hydrophobic polymer surface, the sublimation of the dyestuff, its diffusion into the hydrophobic polymer surface, and/or the like.

When a hydrophobic polymer surface is spoken of herein it should be understood that this includes not only sheets which are completely made of hydrophobic polymer material, but also sheets which are made of other materials, hydrophobic or hydrophilic, and which are coated with the required hydrophobic polymer surface. For example, thin layers of a hydrophobic polymer upon ordinary paper

will provide an appropriate hydrophobic polymer surface. This allows the production of opaque copies. Opaque copies can also be obtained when the sheet is made of hydrophobic polymer material having appropriate opaque, e.g., white, fillers. When the hydrophobic polymer surface is part of an overall hydrophobic polymer sheet which does not include opaque fillers, one can obtain clear copies, for example transparencies. One particular advantage of the present invention is that the exposure of the charged photoconductor can be made by laser or other array of small modulated light sources to create an image from an electronic source such as a computer memory. In such an instance the computer may have been used to process the image, data to correct for the gamma characteristics of the electrostatic photoconductor in use in order to match the exposure of the photoconductor to achieve the gray scale range of the image being printed. The photoconductor is charged and is imagewise discharged by a directed light beam with the image being created from the computer which controls the light beam.

Gray scale modulation can be achieved by the normal reduction of the photoconductor charge voltage, by controlled exposure of the charge image or by the creation of half tone-like dots of variable size at a fixed voltage level. A combination of these two processes can also be incorporated.

The heating of the hydrophobic polymer surface to the dissolving temperature may take place following the imaging of each color or following imaging of all of a series of colors if successive

toning is accomplished without a heating step following each toning step. The resulting color image will generally consist of well mixed pure dyes dissolved into a thin clear layer of the hydrophobic polymer. As previously mentioned the hydrophobic polymer can be coated over a white reflecting sublayer. The color quality achieved is equal to that or exceeds that of photographic color prints and quality color printers. Generally, the thickness of the hydrophobic polymer surface on the white reflecting sublayer can be of any value. In some applications the white or other desired color may be incorporated in the polymer surface. For example, white pigmented polyester may be employed.

Industrial Applicability

The liquid electrostatic dyestuff and the method of the present invention are such as to produce very high quality color prints onto hydrophobic polymer surface. They can be used to produce very high quality color reproductions of, for example, displays on CRT monitors.

Other aspects, objectives, and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.