Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
2000-10-02
2003-03-11
Dote, Janis L. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S110400, C430S137140, C523S333000, C523S335000
Reexamination Certificate
active
06531254
ABSTRACT:
The present invention relates to a process for making particulate compositions. Such compositions have particular utility in the field of electroreprography. Preferred aspects of the invention relates to processes for making toner compositions for use in electroreprography.
Electroreprography is any process in which an image is reproduced by means of electricity and incident radiation, usually electromagnetic radiation more usually visible light. Electroreprography comprises the technology of electrophotography which encompasses photocopying and laser printing technologies. In both these technologies a latent electrostatic image in charge is produced by exposure of a photoconductive drum to light. This can be either reflected light from an illuminated image (photocopying) or by scanning the drum with a laser usually under instruction from a computer (laser printing). Once a latent image has been produced in charge it must be developed to form a visible image on the drum which can then be transferred onto a suitable substrate so a hard copy of the image is obtained (e.g. by printing onto paper).
Suitable developers, which may be liquid or dry compositions, comprise particles of a toner which are electrostatically attracted to the latent image. Liquid developers comprise a toner dispersed in a suitable insulating liquid. Dry developers may comprise single component systems comprising a toner, or two component systems which comprise a mixture of a toner and a carrier. A toner may comprise particles of a polymeric component, a colouring agent and optionally other internal and/or external additives such as charge control agents and/or surface additives to improve the flowability of the toner particles. The polymeric component of the toner is electrically insulating to enable the toner to be electrostatically charged during the electroreprographic process and also acts to fix the toner to the printed substrate, usually by fusion of the polymer onto the substrate by heating. The colouring agent, which is usually a pigment, imparts the required colour to the toner.
During use in an electroreprographic device, friction between particles of toner with their carrier and/or with parts of the device in which the toner is used cause the toner particles to become charged with an electrostatic charge (tribocharge). The exact mechanism to produce the toner image will then vary according to the specific device used. For example in a conventional photocopier the toner composition may be formulated so that tribocharged toner particles will be opposite in sign to the latent image on the drum and toner will be attracted to the latent image on the drum to develop an image in toner on the drum which corresponds to the original document. The developed image is then transferred to a substrate such as paper (e.g. by a pressure roller and/or voltage). The transferred image is fixed to the substrate (e.g. by heat, pressure and/or suitable solvents) to produce a hard copy of the image. The image drum is then cleaned and the device is ready to produce the next copy. Thus developer compositions are used both to develop the latent image on the drum and to produce the final hard copy.
There are a number of methods for making toners. The most common method is to mix the polymer and optional other ingredients (e.g. colorant) together by kneading in a ball mill above the melting point of the resin. The optional ingredients may be added simultaneously or sequentially to the resin before or after melting the resin, but are generally added to the resin when molten. Generally, this involves mixing the molten composition for several hours at temperatures from 120° C. to 200° C., in order to uniformly distribute any optional ingredients (if present) throughout the toner resin. The resultant melt may then be cooled, extruded and then formed into particles with a mean diameter of typically below 20 &mgr;m. The particle formation is achieved by physical processes such as crushing, grinding, milling, and/or pulverising the extrudate. The fine powder of colour toner or toner-resin so obtained is either used directly, is diluted with an inert solid as carrier and/or is coated with surface additives such as silica by mixing for example in a suitable blending machine.
As well as being extremely energy intensive, such physical processes result in a wide distribution of particle sizes within the toner. This leads to significant disadvantages. A wide particle size range generates more uneven tribocharge within the toner which leads to an uneven print density in the final image. The fine dust within such toner compositions leads to fogging of the image produced and more readily contaminates the interior of the device in which the toner is used. The larger particles reduce the resolution of images developed with the toner. Methods for classifying this wide particle size (such as air classification or sieving) are wasteful as material outside the required size range is recycled which adds to the cost.
Modem electroreprographic devices require toners which avoid some or all of the preceding disadvantages and have some or all of the following properties: low temperature at which the toner image fixes onto the printed substrate; wide temperature range over which fusion of the toner occurs; low contamination of the device in which it is used; ability to generate tribocharge at a controlled level, which is stable with time and which is reasonably independent of either temperature or humidity; small particle size (preferably <7 &mgr;m) with narrow size distribution to provide good image resolution; cheap to produce in large volumes; uniform dispersion of colorant(s) and other additives [e.g. charge control agents (CCAs) and waxes]; ability to produce matt or gloss images as required; high optical density; wide colour gamut; and/or resistance to smudging and smearing in the final image. These properties are strongly influenced by the choice of toner resins. It is not feasible or cost effective to produce a toner having these parameters using the conventional extrusion and milling processes described above.
Therefore to overcome these disadvantages, methods for chemically producing toners have been developed in which the toner particles are prepared by chemical processes such as aggregation or suspension rather than abraded from much larger sized material by physical processes. Chemically produced toners made by prior art suspension methods are less satisfactory as it is difficult to control particle shape or obtain a narrow distribution of particle size using such methods. Aggregation processes are preferred as they provide a greater degree of control of the properties of resultant toner particles such as size distribution, particle shape and/or particle composition.
Certain prior art applications (for example JP 2-259770, JP 2-259771, JP 2-11968, JP 2-061650 and JP 2-093659 [Kokai] and U.S. Pat. Nos. 4,983,488, 5,066,560 and EP 0162577 all to Hitachi) disclose methods for chemical production of toners using an irreversible coagulation method for particle growth. JP 2-061650 is typical of these and describes mixing aqueous dispersions of latex and a pigment followed by a coagulation step. These Hitachi patents all describe use of coagulating agents, such as suitable salts, which reduce the stability of the colloid irreversibly.
The mechanism of the Hitachi processes is as follows. In a colloid stabilised by charged surfactants, surrounding each dispersed particle in the continuous (typically aqueous) phase there will be a so called ‘double layer’ where counter ions (of opposite charge to the net charge on the particle) will be in excess. The degree to which the counter ions are in excess will decrease with increasing distance from the dispersed particle. The thickness of this double layer will be determined by the rate at which the net charge decreases with distance from the particle which is dependent on inter alia the ionic strength of the colloid. The colloid will only be stable whilst the ionic repulsion between
Bedells Alison Dawn
Morris Daniel Patrick
Avecia Limited
Dote Janis L.
Pillsbury & Winthrop LLP
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