Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Post imaging process – finishing – or perfecting composition...
Reexamination Certificate
1999-11-30
2001-03-06
Goodrow, John (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Post imaging process, finishing, or perfecting composition...
C430S111400
Reexamination Certificate
active
06197466
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrophotographic imaging and in particular to a formulation and method for making electrophotographic toner materials surface treated with metal oxides.
BACKGROUND OF THE INVENTION
Digital electrophotographic printing products are being developed for printing high quality text and half tone images. Thus, there is a need to formulate electrophotographic toners and developers that have improved image quality. Surface treatment of toners with fine metal oxide powders, such as fumed silicon dioxide or titanium dioxide, results in toner and developer formulations that have improved powder flow properties and reproduce text and half tone dots more uniformly without character voids. See, for example, Schinichi Sata, et al. Study On The Surface Properties Of Polyester Color Toner, IS&T NIP13, 149-152 (1997). The improved powder fluidity of the toner or developer can, however, create unwanted print density in white image areas.
The triboelectric charging level of electrophotographic developers is known to change as prints are made. See, Nash, R. and Muller, R. N. “
The effect of Toner and Carrier Composition on the Relationship between Toner Charge to Mass Ratio and Toner Concentration
,” IS&T NIP 13, 112-120, (1997). This instability in charging level is one of the factors that require active process control systems in electrophotographic printers in order to maintain consistent image density from print to print. Toners with a low triboelectric charge level produce prints with high reflection optical density; toners with a high triboelectric charge level produce prints with a low reflection optical density. A toner with a constant triboelectric charge level would consistently produce prints with the same reflection optical density.
What is needed in the art are toners with more stable triboelectric charge levels which decrease the incidence of dusting (defined below).
SUMMARY OF THE INVENTION
The present invention describes toner particles that are surface-treated with metal oxides thereby making toners with more stable triboelectric charge. These toners that form less low-charge toner dust and image background, and produce images with fewer image voids. Formulations for surface treated toners have been described in U.S. Pat. Nos. 5,272,040; 4,513,074; 4,623,605; and 4,933,251, but there is no teaching that a process of applying the surface treatment could cause embedment of metal oxide particles below the surface of the toner particles and affect the performance of the toner. Toners made by the process described herein have lower levels of voids in printed characters and a lower background level in the non image areas of the print. “Character voids” are image defects where a complete letter character is not formed, there are areas where toner has not been deposited resulting in white spots in the character. “Background” is a image defect where toner is deposited in the white portion of a print, causing the print to look less sharp and white print areas to look slightly gray.
The present invention also discloses that the atomic percent of elemental metal in the metal oxide on the toner particle surface: the total weight percent of metal oxide in the toner formulation (herein referred to as “bulk metal oxide”) affects the triboelectric properties and imaging characteristics of the toner. The present invention also discloses that within this preferred ratio range, toner fluidity and image quality are improved. The examples of the present invention demonstrate that there is a preferred concentration range for metal oxide on the surface of the toner particles and that toners falling within the preferred range provide the best image quality. The concentration of metal oxide on the surface of the toner particle is controlled by the process used in mixing and blending the toner particles with the fine metal oxide powder.
Hence, the present invention provide an electrophotographic toner composition comprising toner particles admixed with metal oxide, wherein the metal oxide is selected from titanium dioxide and silicon dioxide; the metal oxide is 0.1 to 5.0 weight percent of the toner composition; and the ratio of titanium dioxide on the surface of the toner particles: total titanium dioxide in the toner composition is in the range of 1.0-3.0:1.0 and the ratio of silicon dioxide on the surface of the toner particles: total silicon dioxide in the toner composition is in the range of 10.0-25.0:1.0.
The present invention provides toners that produce images having a low level of character voids and reduced background levels in the white image areas. Further, replenishment toners create lower levels of airborne toner particles when mixed with developers, resulting in cleaner printer operation.
DETAILED DESCRIPTION OF THE INVENTION
“Dusting characteristics” as used herein, refers to the amounts of uncharged or low charged particles that are produced when fresh replenishment toner is mixed in with aged developer. Developers in a two component electrophotographic developer system are a mixture of electrostatically charged carrier particles and oppositely charged toner particles. Developers that result in very low dust levels are desirable. Toner dust results from uncharged or low charge toner particles. This dust can be deposited in the non-image area of a paper print resulting in unwanted background. In a printer, replenishment toner is added to the developer station to replace toner that is removed in the process of printing copies. This added fresh toner is uncharged and gains a triboelectric charge by mixing with the developer. During this mixing process uncharged or low charged particles can become airborne and result in background on prints or dust contamination within the printer. A “dusting test” is described herein below to evaluate the potential for a replenishment toner to form background or dust.
“Low charge characteristics ” as used herein refers to the ratio of charge to mass of the toner in a developer. Low charged toners are easier to transport through the electrostatographic process, for example from the developer station to the photoconductor, from the photoconductor onto paper, etc. Low charge is particularly important in multi-layer transfer processes in color printers, in order to minimize the voltage above already transferred layers as this maximizes the ability to transfer subsequent layers of toner. However, typically low charge toners also result in significant dust owing to the low charge. Toner dust is uncharged or low-charged toner particles that are produced when fresh replenishment toner is mixed in with aged developer. Developers that result in very low dust levels are desirable. Typically toners that exhibit high charge to mass ratios exhibit low levels of dust, and vice-versa. Toners that exhibit low charge to mass ratios and low dust characteristics are thus desirable.
“Bulk metal oxide” as used herein refers to the amount of silicon dioxide and/or titanium dioxide in the toner formulation, typically 0.1 to 5.0 weight percent, preferably 0.1 to 2.0 and most preferably to 0.15 to 0.35.
TABLE 1
Toner Formulation
Parts by
Component
weight
Supplier
styrene acrylic copolymer
100
Eastman Kodak
CAS # 60806-47-5
Regal 300 Carbon Black
7
Cabot Corporation
CAS # 1333-86-4
T77 Charge Control Agent
1.5
Hodagaya
Organo iron chelate
CAS # 115706-73-5
The components were powder blended, melt compounded, ground in an air jet mill, and classified by particle size to remove fine particles (particles less than 5 microns ion diameter). The resulting toner had a median volume diameter particle size of 11.5 microns.
Surface Treatment of Toner to Form Concentrate
Toner can be surface treated by powder blending non surface treated toner and a metal oxide concentrate consisting of about 10 weight % metal oxide and 90 weight % toner in a high-energy Henschel mixer. Concentrates were made from: 1800 gm toner and 200 gm silicon dioxide or titanium dioxide, and mixed in a 10 liter Henschel mixer with a 6 element, 20 cm diame
Fields Robert D.
Srinivasan Satyanarayan A.
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