Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Process of making developer composition
Reexamination Certificate
2001-04-10
2003-04-22
Dote, Janis L. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Process of making developer composition
C430S111100, C430S111350
Reexamination Certificate
active
06551754
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for coating carrier particles. It relates especially to a method for coating carrier particles to be used as carrier particles in multi-component developers for electrostatic imaging with magnetic brush development as well as cascade development.
BACKGROUND OF THE INVENTION
There are several methods for coating solid with one or more chemical substances. For instance, a fluidized bed of carrier particles can be created whereto the coating solution of a chemical substance in a solvent is then added. Subsequently, the solvent is evaporated. This is for instance disclosed in “Method of and apparatus for fluidized bed coating of electrophotographic toner carrier particles”, Anonymous, Product Licensing Index no. 100, Aug. 1, 1972, pages 69-71, XP-002147126; and in the German published patent application DE 3825954 A1; and in U.S. Pat. No. 5,340,677 where a fluidized bed coater (referred to as the SPIRACOATER™) is used. This method may give good coating results, but the amount of air required to form the fluidized bed is such that the evaporated solvent (which in most cases is an organic solvent) contaminates a large volume of air, which can not without further treatment be vented in the open atmosphere. Therefore the installations for fluidized bed coating are quite large and expensive. Moreover, in a fluidized bed the carrier particles are strongly agitated and many collisions occur which damage the coating around the carrier particles.
In another method, the carrier particles are mixed (dispersed) in a solution of the chemical compound or compounds that are to be applied on the surface of the carrier particles and the particles are then spray-dried. Again this method gives good coating results, but the installation required for spray drying is expensive. Moreover, in these prior-art coating methods, based on a fluidized bed or spray drying, frequently some of the carrier particles to be coated adhere during the coating process to the wall of the container of the fluidized bed or spray drying apparatus, and these particles are only coated from one side.
In several documents it is disclosed to bring a solution of coating compounds in a low temperature boiling solvent in contact with the carrier particles to be coated in a vessel. Usually the carrier particles in the vessel or subjected to mechanical agitation, optionally combined with or followed by an act of evaporating the solvent by heating at a constant elevated temperature. An example thereof is U.S. Pat. No. 3,507,686, which discloses a method of coating carrier particles, wherein the carrier particles in the vessel are first subjected simultaneously to pre-heating at an elevated temperature and oscillatory vibration. The vibration is terminated and the coating solution is added to the heated carrier particles and thereafter the oscillatory vibration is reinitiated. Other examples are for instance U.S. Pat. No. 5,102,769 and European patent application EP0800118 where the coating solution and the carrier particles are mixed while heating at an elevated constant temperature. A further example is disclosed in the British patent application GB2014876 where the coating solution and the carrier particles are agitated by oscillatory vibration while heating at an unspecified temperature. In some of these disclosures, fairly large amounts of solvents are used for bringing the coating chemicals on the carrier particles. This is undesirable as these large amounts of solvents have to be evaporated and, with respect to environmentally friendly manufacture, recuperated.
A method using a small amount of solvent for bringing coating chemicals to the carrier particles has been disclosed in the European patent application EP898206 (or its US counterpart U.S. Pat. No. 5,888,692). In this disclosure carrier particles are coated by forming first a loose bed of these carrier particles by mechanical means instead of by an air flow as in a fluidized bed or spray coater and then continuously adding the coating solution to that loose bed at such a rate that, at any time, said solvent is present in an amount lower than 1.25 10
−4
ml per cm
2
of surface of said carrier particles to be coated. During this whole process the carrier particles are heated to an elevated constant temperature.
When coating carrier particles for use in electrostatic imaging the coating has to be very homogenous over the surface of the particles to be coated. In electrostatographic imaging the carrier particles are mixed with toner particles (and with other ingredients) to form a developer. When the developer is to be used in magnetic brush development, the carrier particles are magnetic, when the developer is used in cascade development, the carrier particles can be coated glass beads. In any case the rubbing of the carrier particles and the toner particles induces a tribo-electric charge in the toner particles and the nature of the coating on the carrier particles determines, together with the toner ingredients, the polarity of the charge on the toner particles as well as the amount of the charge. When the coating of the surface of the carrier particles is not even and has interruptions, problems in charging the toner particles can occur. Thus in coating carrier particles it is of utmost importance to have an even, closed coating on the surface of the particles.
The prior-art methods described may provide coated carrier particles with good properties, but the homogeneity of the coating, i.e. having the same quality coating on each carrier particle, leaves still room for improvement.
Thus, it is still desirable to have a simple, inexpensive, environmentally friendly method for coating particles with a homogenous surface layer.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for coating carrier particles with chemical substances that is simple, fast and reliable and that makes it possible to apply a thin homogenous layer, without interruptions, on the surface of the particles.
It is another object of the invention to provide a method enabling the application of a thin homogeneous coating layer on carrier particles in a vessel by preventing the carrier particles from sticking to each other as well as sticking to the wall of the vessel while using limited mechanical agitation in order to prevent damaging of the coating.
It is a further object of the invention to provide a method for coating carrier particles in an environmental friendly way by using low amounts of organic solvent.
It is still a further object of the invention to provide a method for coating carrier particles wherein only a low volume of air is contaminated by organic solvents and wherein said contaminated air can be easily purified.
The objects of the invention are realized by providing a method for coating carrier particles having a volume average diameter between 20 and 200 &mgr;m, for use in electrostatic developers, comprising the acts of:
a) forming a coating mixture in a vessel at a temperature from 15 to 40 degrees C by adding a coating solution comprising B ml of at least one solvent to carrier particles having a volume of A ml in said vessel, said volume A and said volume B being selected such that 0.5≦A/B≦5; and
b) evaporating said solvent by mechanically agitating and gradually heating said coating mixture to a temperature of at most 10 degrees C. above the boiling point of the solvent with the lowest boiling point.
In a preferred embodiment of the invention, the carrier particles to be coated are brought in a stationary vessel wherein the mechanical agitation is executed with a mechanical stirrer.
DETAILED DESCRIPTION OF THE INVENTION
The method described in EP898206 or in the US-counterpart U.S. Pat. No. 5,888,692, necessitates quite rigorous control of the rate of addition of the coating solution. Although this control poses no insurmountable problem a method wherein less control of the rate of addition of the coating solution is necessary remains desirable. The inventors have
Dote Janis L.
Knobbe Martens Olson & Bear LLP
Xeikon N.V.
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