Electrostatic fuser rolls and belts

#This invention relates to electrophotographic processes and, particularly, to hot rolls and belts used in the fusing step of such processes.

#In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor's surface, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor's surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.

#A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor's surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rollers or belts, under heat, melts and fixes the toner in the paper, subsequent to direct transfer or indirect transfer when an intermediate transfer medium is used, producing the printed image.

#FIG. 1 is a schematic diagram of a test fixture which can be used to determine fuser belt or roll dielectric breakdown voltage and time constant.

#FIG. 2 is a graph of typical results obtained using the test fixture shown in FIG. 1.

#FIG. 3 is a graph showing measured dielectric breakdown voltage versus coating thickness and carbon loading of the outer layer for fuser belts exemplified in the present application.

#The present invention relates to hot fuser rolls and fuser belts which are used in the fixing portion of the electrophotographic process. Specifically, the present invention recognizes the importance of the dielectric breakdown (or charge acceptance) value of a hot roll or a fuser belt coating in order to limit the build-up of charge on the fuser members, rather than (as the prior art does) focussing on the resistivity of the rollers or belts. This approach limits field magnitude and toner contamination associated with fuser electrostatics without requiring that each and every layer of the roller or belt be rendered resistive by the addition of conductive particles, fibers, or ionic additives. This provides much greater flexibility in the formulation of fuser hot rolls and fuser belts and, importantly, allows such rolls and belts to be formulated without particulate material in the topcoat or release layer, thereby improving the release properties of the printed pages from the fuser system.

#The present invention is based on the finding that hot rolls and belts, which exhibit electrical breakdown at 250 volts or less (as applied with a corona and measured with an electrostatic probe), eliminate a particular toner contamination problem associated with charge accumulation on the fuser belt or hot roll. The distinction between a resistive coating and a coating that exhibits dielectric breakdown is an important one, since lower loadings of electrically conductive particles or ionic conductive agents in thick fluororesin layers or thin fluororesin coatings with no conductive particles or ionic conductive agents can be used to achieve a total composite coating electrical breakdown in the range of 250 volts or less. Functionally, the release characteristics of the fluoropolymer coating are significantly improved when conductive agents are reduced in concentration or eliminated.

#The fuser hot rolls of the present invention comprise a core member, generally cylindrical in shape having laminated (coated) thereon a plurality of concentric layers which provide various functions. The core members are well known in the art and can be made from any material that conducts heat. Examples of such materials include aluminum, copper, aluminum alloys, copper alloys, steel and stainless steel. Aluminum is a preferred material because it is light in weight, heat conductive and relatively inexpensive. The core member is generally hollow, which permits a heating lamp to be placed within it thereby providing the heat energy to the fuser roll. The core is coated by two or more layers which entirely coat the outer surface of the core material. These layers provide the appropriate surface characteristics for the hot fuser roll, are sufficiently heat resistant to withstand fuser temperatures (e.g., 180° C.), and (alone and in combination with conductive materials) provide the required electrical breakdown characteristics of the roller. The total thickness of the surface layers is preferably in the range of from about 1 to about 50 μm. Examples of materials which can be used in the surface layers include fluorine-containing resins, polyimide resins, polyamidoimide resins, silicone resins, polybenzimidazol resins, polyphenylene oxide resins and polybutylene terephthalate resins. Fluorine-containing resins are preferred. Examples of fluorine-containing resins include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroakylvinyl ether co-polymer (PFA), and tetrafluoroethylene-hexafluoropropylene co-polymer (FEP). Key to the present invention is that at least one of the coated layers does not contain electrically conductive materials and that the roll itself exhibits electrical breakdown at about 250 volts or less.