Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1998-12-31
2002-04-09
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C523S502000, C523S503000, C523S504000, C523S505000, C523S507000, C523S523000, C523S526000, C524S457000
Reexamination Certificate
active
06369136
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of electrophotography. More particularly, it relates to polymeric compositions for use as binders in electrophotographic toners and developers, and to the process by which these polymeric compositions are prepared.
BACKGROUND OF THE INVENTION
In electrophotography, an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element and is then developed into a toner image by contacting the image with an electrographic developer. If desired, the latent image can be transferred to another surface before development. The toner image is eventually transferred to a receiver, to which it is fused, typically by heat and pressure. Electrophotographic toners must meet many system requirements. For example, for high process speeds the toner must meet certain demands; toners must have the ability to fuse at low fusing temperatures and also embody high melt strength. The combination of these two properties results in improved offset, large fusing latitudes, and extended fuser roller life.
Toners contain a binder and other optional additives, such as colorants and charge controlling agents. Binders are generally polymeric compositions selected so as to provide a balance between various conflicting constraints. For example, the chemical nature of the polymer composition should allow appropriate charging polarity and charge level for the toner incorporating the binder and yet exhibit sufficient compatibility with the receiving substrate to enhance the degree of adhesion. The problem often encountered is that binder compositions that are desirable for promoting adhesion to receiver substrates yield wrong charge sign toner particles or insufficient charge level, and vice versa. There are many polar monomers that can be used as comononers in polymer synthesis which possess specific affinity for the receiver substrate during the fusing. However, due to their polar nature these comononers also have a significant impact on the tribocharging behavior of the resulting toner particles.
Further, the polymer architecture needs to be optimized so as not to yield a very high melt viscosity of the toner incorporating the binder which can cause problems in grindability, and in fusing a toner image to a receiver. Enhanced grindability of a toner binder implies that the toner can be pulverized at a higher rate, which will lower the toner manufacturing cost. The fusing off-set latitude refers to the range of temperatures between the lowest temperature where the toner does not show cold offset, (or the lowest temperature at which toner fuses to the receiver substrate) and the highest temperature where the toner does not exhibit hot offset, (or highest temperature at which toner fuses to receiver without offsetting to the fuser roller).
In general, the grindability of a toner binder can be improved by lowering its molecular weight. However, the lowering of the molecular weight also impairs the rheological behavior of the toner binder. More specifically, the melt viscosity of the binder polymer is lowered and this results in reduced melt strength and a greater propensity for the toner to show fusing offset. Increases in molecular weight increase the melt strength, but the toner binder becomes both harder to grind and fuse. It is because of such competing requirements that toner binder properties are often compromised.
As materials suitable for providing adequate fusing to receiver substrate, polyesters resins have attracted significant attention. The use of low molecular weight polyester binders is described in U.S. Pat. Nos. 3,590,000 and 3,681,106. Toners prepared from low molecular weight polyester have limited fusing latitude since they adhere well to the receiver, but also adhere to the fuser surface.
In order to control the grindability of the polyester binders and also to control the melt rheological behavior of the polymeric binder, it is often possible to incorporate some branching or cross-linking to the polymer architecture. Examples of such binders are described in U.S. Pat. Nos. 5,135,833 and 5,489,498. Toners prepared with such cross-linked polyester resins exhibit reasonable fusing latitude, as their propensity for hot offset to fuser roller surface is reduced due to high melt elasticity of the toner binder. In addition, such resins can be readily pulverized.
However, polyester resins suffer from several drawbacks. First, the cost of the polyester resins is much higher than vinyl based toner binders. The cost of cross-linked polyesters is higher still. This difference is magnified further in high volume copiers/printers due to the large amounts of toner consumed as well as the need for higher fusing latitude required in high speed machines, which necessitates the use of cross-linked polyesters. Second, the specific gravity of the polyesters is higher than vinyl based polymers. Thus, based on volume, the cost of the toner goes up even further. Third, the polyesters form much more cohesive powders than vinyl polymers. As a result of higher cohesiveness, the powder flow characteristics of polyester resin based toners are fairly poor. In order to address the powder flow issue, it is often necessary to incorporate small amounts of inorganic oxide powders such as silica or titanium to the toner surface. Incorporating such surface treatment on the toner particles further increases toner cost.
Most vinyl polymer based toners are more cost effective due to lower binder cost. Moreover, the toners based on vinyl polymers do not suffer from powder flow concerns, as the vinyl polymers based binder polymers are tough and have higher Young's modulus than polyesters. However, the vinyl resin binders suffer from shortcomings associated with poor adhesion to receiver substrate. In addition, high molecular weight vinyl resin binders exhibit poor pulverizing behavior due to their toughness.
Often, the problems associated with pulverizing high molecular weight tough resin binders can be addressed by mixing a low molecular weight fraction with a high molecular weight fraction. This is taught by U.S. Pat. Nos. 4,973,538, to Suzuki et al; 4,486,524 to Fujisaki et al; 4,499,168 to Mitsuhashi; and 5,135,833 to Matsunaga et al. By controlling the ratio of the two molecular weight fractions, the rheological and grindability of the toner binder can be somewhat controlled. However, this approach does not provide satisfactory results if toners with higher melt elasticity are desired or if the molecular weight difference between the two distributions is very large. Further, the addition of low molecular weight fraction has a negative impact on the fusing latitude as the melt elasticity of the toner is lowered. Finally, the most severe shortcoming of low molecular weight fraction is that adhesion to the receiver substrate remains mostly unaffected because the surface energy remains the same.
Another problem with the above approach concerns the mixing of two polymers of different molecular weights and chemical compositions. Generally polyesters and styrenics are incompatible and exhibit macro-phase separation when melt blended with typical toner addenda such as colorants, charge control agents, and release addenda. Often, the resulting domains due to phase separation are larger than the desired toner particle size. This results in problems such as unacceptable developer stability and/or charging properties, poor developer flow properties, and toner batch to batch variations.
The use of high volume (high process speeds) electrophotographic copiers and or the need for full process color electrophotographic prints has increased the need for toners which are capable of being fused at lower temperatures. The demand for color engines has placed a premium on fusing quality as measured by image gloss and color clarity. Polyester as well as epoxy resins represent polymer classes that have the desired adhesive, melt-flow, and rheological properties that are suite
Sorriero Louis J.
Tyagi Dinesh
Eastman Kodak Company
Egwing Kelechi
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