Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Process of making developer composition
Reexamination Certificate
2002-04-23
2003-12-16
Chapman, Mark A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Process of making developer composition
C516S077000
Reexamination Certificate
active
06664018
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to an in-situ method of forming an aqueous zinc stearate dispersion. The aqueous dispersion of zinc stearate finds particular utility as an external additive for toner, and more specifically as an external additive to a toner particle used as a cleaning additive in a developer composition.
2. Description of Related Art
Zinc stearate (ZnSt) is a known additive in the art of toners. It is typically included as an external additive upon the surface of toner particles to impart desired characteristics to the toner. For example, zinc stearate may be present as an additive in the external additive package to provide lubricating properties. In toner, zinc stearate provides triboelectric enhancement due to its lubricating nature. In addition, zinc stearate enables higher toner charge and charge stability by increasing the number of contacts between toner and carrier particles.
Where the toner particles are formed via aqueous based processes, for example such as by the known emulsion aggregation technique, it is then necessary to incorporate the zinc stearate external additive into the toner particles in an aqueous dispersion form so as to ensure compatibility with the toner particles. Use of the aqueous dispersion of zinc stearate particles also ensures that use of harsh organic solvents that might degrade the toner particles and properties thereof can be avoided.
However, it is difficult to obtain stable aqueous dispersions of zinc stearate particles. For example, if a commercially available zinc stearate particle is simply mixed with surfactant and water, although a dispersion may result for some amount of time, it will settle quickly and thus is not stable. Commercially available zinc stearate particles have particle sizes of between 20 microns and 50 microns that makes it difficult to form a suspension or stable dispersion with surfactants and water. This makes it practically impossible to use such form of zinc stearate because the dispersion has no shelf life, and if used to coat toner particles after settling, it will create unacceptable inconsistencies in the coated toner obtained. Furthermore, it is not feasible to coat a 3 to 4 microns size non-pigmented resin particle with a dispersion consisting of primary particles in the range of 20 to 50 microns size such as zinc stearate.
One method in use to obtain stable zinc stearate particle dispersions is a homogenization method. See, for example, U.S. Pat. No. 6,340,549 describing the use of a homogenizer in making a dispersion of a toner additive that may be zinc stearate (see column 7, lines 38-50 and column 8, lines 1-18. In this method, a stable zinc stearate dispersion is prepared in a sealed reactor by melting zinc stearate in water containing a surfactant, for example at about 140° C., and then homogenizing the solution at a pressure in excess of 4,000 psi using a high pressure piston homogenizer. The high-temperature, high-shear homogenization is required to breakdown the zinc stearate particles from a micron size to the necessary nanometer size so that the dispersion becomes stable.
However, there are major challenges associated with this homogenization process. First, the commercial high-pressure homogenizer (e.g., a Gaulin homogenizer) together with the high-pressure reactor system is very expensive. Second, the high temperature (>140° C.) and high pressure (>4,000 psi) process requirements, coupled with the inherently abrasive nature of the zinc stearate, wears down the equipment very quickly, causing unacceptable downtime and costly mechanical parts repair (e.g., of the compression chamber, plunger, valve seats, etc). Third, in the process, the molten nanometer sized particles must be quenched to a temperature below about 50° C. before discharge in order to prevent re-aggregation of the particles due to crystallization. Fourth, the process time to make the zinc stearate dispersion is very lengthy, for example on the order of 6 to 8 hours. In addition, additional time is required to frequently clean and maintain the equipment as mentioned above.
U.S. Pat. No. 6,162,836 describes a process for preparing an aqueous dispersion of a higher fatty acid zinc salt which comprises adding a molten higher fatty acid to an aqueous dispersion of zinc oxide and reacting the higher fatty acid with the zinc oxide in the presence of a surfactant or a water-soluble polyvinyl alcohol with stirring while the aqueous dispersion is maintained at a temperature that is higher than the melting point of the higher fatty acid used. This patent thus reacts a zinc oxide with a higher fatty acid, including stearic acid, to obtain a zinc salt dispersion, including zinc stearate. However, the dispersion is obtained through the use of an acid and with a required use of high temperature (above the melting point of the higher fatty acid). The use of salts (e.g., stearates) instead of a higher fatty acid is not taught or suggested, nor is the use of lower process temperatures. Further, the examples indicate that the process achieves large zinc stearate particles, for example on the order of larger than 1 &mgr;m.
U.S. Pat. No. 4,410,446 describes stable, fluid zinc-containing dispersions and the preparations thereof by the high temperature decomposition of zinc acetate to ZnO in a dispersant-containing fluid, the dispersant being stable at the temperature of decomposition.
U.S. Pat. No. 5,998,523 discloses a composition of an essentially solid phase state having as components a liquid hydrophilic organic polymer, an aqueous salt solution containing at least one metallic or metalloid element, and a coagulating agent. The composition on calcination provides a metal-containing powder having an average particle size of 1 &mgr;m or less. Such metal-containing powders are described to be of value in the preparation of industrial catalysts, ceramics, electronic components, or as fillers in plastics, paints or cosmetics.
U.S. Pat. No. 5,939,079 describes a process for the dispersion of at least one pulverulent organic filler and/or of at least one pulverulent inorganic filler in a vehicle composed of at least one fatty phase, by mixing the filler or fillers in the vehicle by high-pressure homogenization in one or a number of passes, and describes cosmetic or dermatological compositions capable of being obtained by the process. The use of the compositions obtained as the basis for care, make-up and/or hygiene products is indicated, the compositions providing protection against the effects of ultraviolet radiation.
It is an object of the invention to develop an improved method of making a stable zinc stearate particle aqueous dispersion.
SUMMARY OF THE INVENTION
These and other objects are achieved by the present invention, which in embodiments relates to a method of forming an aqueous dispersion of particles of zinc stearate, the method comprising forming a mixture of at least one surfactant and water, heating the mixture to a temperature of from about 40° C. to less than a melting temperature of zinc stearate, adding a metal-containing stearate to the mixture under agitation, adding to the mixture a solution comprised of a zinc salt in water, whereby upon inclusion of both the metal-containing stearate and the zinc salt in the mixture, the zinc salt reacts with the metal-containing stearate to form the particles of zinc stearate, continuing the heating and agitation following completion of the latter of the adding of the solution of zinc salt or the adding of the metal-containing stearate to continue the reaction, and cooling to obtain the aqueous dispersion comprised of particles of zinc stearate.
The method of the invention achieves an aqueous dispersion of zinc stearate that is stable, and is preferably comprised of zinc stearate particles having a size of less than 1 micron, preferably less than about 900 nm.
In embodiments, the invention also relates to a method of forming toner particles coated with zinc stearate particles, the method comprising forming a mixture of at least one
Chen Allan K.
Damato Michael
Gerroir Paul J.
Jiang Lu
Ng Tie Hwee
Chapman Mark A.
Palazzo Eugene O.
Xerox Corporation
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