Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Process of making radiation-sensitive composition
Reexamination Certificate
1999-09-30
2002-10-01
Wu, David W. (Department: 1713)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Process of making radiation-sensitive composition
C430S109500, C430S136000, C430S137170, C430S905000, C430S914000, C524S458000, C523S201000, C526S201000
Reexamination Certificate
active
06458501
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to a surfactant-free emulsion polymerization process and to a method for preparing emulsion aggregation toners wherein the latex is formed by surfactant-free emulsion polymerization. The aforementioned toners are especially useful for imaging processes, especially xerographic processes, which usually require high toner transfer efficiency, such as those having a compact machine design without a cleaner or those that are designed to provide high quality colored images with excellent image resolution and signal-to-noise ratio, and image uniformity.
2. Description of Related Art
It is known in the art to form toners by aggregating a colorant with a latex polymer formed by emulsion polymerization. For example, U.S. Pat. No. 5,853,943, which is herein incorporated by reference, is directed to a process for preparing a latex by first forming a seed polymer. In particular, the '943 patent describes a process comprising:
(i) providing a reactor with an initial liquid composition comprising water and disulfonate surfactant or surfactants;
(ii) conducting a pre-reaction monomer emulsification which comprises emulsification of the polymerization reagents of monomers, chain transfer agent, the composition of step (i), and optional, but preferably, an initiator, wherein the emulsification is accomplished at a low temperature of, for example, from about 5° C. to about 40° C.;
(iii) preparing a seed particle latex by aqueous emulsion polymerization of a mixture comprised of (a) part of the monomer emulsion, from about 0.5 to about 50 percent by weight, and preferably from about 3 to about 25 percent by weight, of the monomer emulsion prepared in (ii), and (b) a free radical initiator, from about 0.5 to about 100 percent by weight, and preferably from about 3 to about 100 percent by weight, of the total initiator used to prepare the latex polymer at a temperature of from about 35° C. to about 125° C., wherein the reaction of the free radical initiator and monomer produces the seed latex comprised of latex resin wherein the particles are stabilized by surfactants;
(iv) heating and feed adding to the formed seed particles the remaining monomer emulsion, from about 50 to about 99.5 percent by weight, and preferably from about 75 to about 97 percent by weight, of the monomer emulsion prepared in (ii), and optionally a free radical initiator, from about 0 to about 99.5 percent by weight, and preferably from about 0 to about 97 percent by weight, of the total initiator used to prepare the latex polymer at a temperature from about 35° C. to about 125° C.; and
(v) retaining the above contents in the reactor at a temperature of from about 35° C. to about 125° C. for an effective time period to form the latex polymer, for example from about 0.5 to about 8 hours, and preferably from about 1.5 to about 6 hours, followed by cooling.
In known emulsion polymerization processes, a surfactant (that is, an emulsifier) is used to stabilize the emulsion during emulsion polymerization. The presence of a good surfactant is important for stabilizing the emulsion polymerization process. However, the same surfactants that contribute advantage in the emulsion polymerization step can be detrimental to the quality or processing of the final toners. In particular, the presence of a surfactant can contribute to problems such as filter blinding, over-dispersed particles, persistent emulsion and/or undesirable final toner characteristics, such as sensitivity to relative humidity, low tribo charge, dielectric loss, aging and poor toner flow.
Current emulsion aggregation processes have a disadvantage in that tribo charge depends on environmental changes to a large extent. Tribo charge declination is observed especially in an environment of high temperature and high humidity. This suggests that the tribo charge of the emulsion aggregation toner particles at high relative humidity can be controlled by avoiding the presence of surfactants on the particle surface. Another disadvantage is that the adhesive properties between the toner particles and the substrate is poor at high relative humidity owing to the presence of surfactants on the particles.
As a result, surfactants used in emulsion aggregation emulsion polymerization processes should be removed from the particle by washing to obtain useful tribo electric properties. However, surfactants for emulsion polymerization are known to form hydrogen-bonded complexes with carboxylic acids and are thus difficult to remove from the surface of acrylic acid-containing particles in particular. In addition, often the removal of these surfactants from the emulsion aggregation particles is very tedious and resource consuming, since surfactant removal is an equilibrium process and requires acceleration in order to be cost-effective.
Processes that eliminate or at least minimize the need for surfactants are therefore desired.
SUMMARY OF THE INVENTION
The present invention is directed to a method for preparing latex polymers by an emulsion polymerization process that does not require the addition of surfactants. The process provides for emulsion aggregation toners with good tribo charge stability, especially in an environment of high temperature and high humidity. In addition, because surfactants do not remain with the latex particles, the toner tribo charge is not as influenced by environmental changes. Furthermore, the process of the present invention can provide for a surfactant-free emulsion with high solids loading, such as greater than 40 wt %.
The process of the present invention comprises preparing an emulsion of monomers in water without the use of surfactants. In particular, neither ionic nor nonionic surfactants are used. To form the emulsion, a monomer is generally added to water and agitated to form an emulsion.
After the emulsion has been formed, a free radical initiator is then mixed with at least a portion of the emulsion to initiate seed polymerization. In this process, the initiator is a free radical initiator that attaches to the seed polymer to form ionic, hydrophilic end groups on the polymer.
After forming seed particles, additional monomer is then added to the composition to complete polymerization thus forming a latex polymer. The monomer added after seed polymerization may or may not be in the form of a monomer emulsion. During this process, additional initiator may also be added. If added, this initiator is preferably a free radical initiator. It can, but need not, be a free radical initiator that attaches to the polymer to form ionic, hydrophilic end groups on the polymer.
After forming the latex polymer, the latex may then be aggregated with a colorant, preferably in the form of a colorant dispersion, to form aggregate particles that are then coalesced or fused to form toner particles.
In forming the latex, it is not necessary to add any surfactant to the composition. However, surfactant may be present in or added to the colorant dispersion. As such, even if no surfactant is used in forming the latex polymer, surfactant may be present in the toner formed. However, the emulsion aggregation approach in which no surfactant is used in forming the latex polymer provides for toner with less surfactant. In particular, using the surfactant-free latexes in emulsion aggregation toner will generally enable at least 85% surfactant reduction since the bulk of the surfactant in typical toners comes from the latex rather than from the colorant dispersion. Such emulsion aggregation toner particles require considerably less washing, if any, to achieve maximum tribo levels than is needed with the surfactant-containing latexes.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
One or more monomers may be used to form a latex polymer in the present invention. Any suitable monomers may be used. Monomers particularly useful in the surfactant-free process of the present invention include, but are not limited to, acrylic and methacrylic esters, styrene, vinyl esters of aliphatic acids, ethylenically unsaturated carb
Chen Allan K.
Cheng Chieh-Min
Kmiecik-Lawrynowicz Grazyna E.
Egwim Kelechi C.
Palazzo Euguene
Xerox Corporation
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