Measuring and testing – Testing of material
Reexamination Certificate
2003-05-30
2004-09-28
Raevis, Robert (Department: 2856)
Measuring and testing
Testing of material
C073S15000R
Reexamination Certificate
active
06796197
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of liquid electrophotography, and specifically to a method and apparatus for screening liquid toners and receptors for use in electrophotographic printing devices.
BACKGROUND OF THE ART
In electrophotographic and electrostatic and imaging processes (collectively electrographic processes), an electrostatic image is formed on the surface of a photoreceptive element or dielectric element, respectively. The photoreceptive element or dielectric element may be an intermediate transfer sheet, drum or belt or the substrate for the final toned image itself, as described by Schmidt, S. P. and Larson, J. R. in
Handbook of Imaging Materials
, Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp 227-252, and U.S. Pat. Nos. 4,728,983; 4,321,404; and 4,268,598.
In electrostatic printing, a latent image is typically formed by (1) placing a charge image onto a dielectric element (typically the receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a latent charge image. This latent charge image is developed or toned by (2) applying toner to the charge image, and (3) fixing the toned image. An example of this type of process is described in U.S. Pat. No. 5,262,259.
In electrophotographic printing, also referred to as xerography, electrophotographic technology is used to produce images on a final image receptor, such as paper, film, drums, or the like. Electrophotographic technology is incorporated into a wide range of equipment including photocopiers, laser printers, facsimile machines, and the like.
Electrophotography typically involves the use of a reusable, light sensitive, temporary charge accepting, temporary image receptor, known as a photoreceptor. The photoreceptor is used in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce a visible toned image on a receptor, including charging of the photoreceptor, exposure to dissipate the charge in an imagewise manner and form a latent charge image, toner development of the latent charge image, transfer of the toned image, fusing of the transferred toned image, cleaning of the photoreceptor, and erasure of residual charge on the photoreceptor.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona device or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent charge image by selectively discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent charge image on the photoreceptor, typically using a developer that is electrically-biased to a potential opposite in polarity to the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent charge image via electrostatic forces, forming a temporary toned image on the photoreceptor.
In the transfer step, the temporary toned image is transferred from the photoreceptor to the desired final image receptor. An intermediate transfer element is sometimes used to effect transfer of the toned image (usually to accomplish a desired order of color planes in the image) from the photoreceptor with subsequent transfer of the toned image to a final image receptor. In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor to form a final and permanent image. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent (or gives the toner a liquid appearance), e.g., typically less than 10 weight percent solvent and preferably less then 8% or less then 5% by total weight of toner (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and the dry toner particles are capable of carrying a triboelectric charge. This relative proportion of liquid carrier is a physical characteristic that distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically a nonconductive dispersant liquid, the lack of charge carrying capability being necessary to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated or stabilized (dispersed and suspended) to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent (by total weight of the toner) of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but the toner particles do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron size, liquid toners are capable of producing very high-resolution toned images.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process, supporting the visual enhancement additive during toning and fusing the visual enhancement additive during formation of the permanent image. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g., glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g., temperature, pressure and fuser configuration) impact the durability (e.g., blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
Polymeric binder materials suitable for use in liquid toner particles typically exhibit glass transition temperatures of from about −24° C. to 55° C., which is lower than the range of glass transition temperatures (50-100° C.) typical for polymeric binders used in dry toner particles. In particular, some liquid toners are known to incorporate polymeric binders exhibiting glass transition temperatures (T
g
) below room temperature (25° C.) to rapidly self fix, e.g., by film formation, in the liquid electrophotographic imaging process; see e.g., U.S. Pat. No. 6,255,363. However, such liquid toners arc also known to exhibit inferior image durability (e.g., poor blocking properties and erasure resistance) resulting from the low T
g
after fusing the toned image to a final image receptor.
In other printing processes using liquid toners, self-fixing is not required. In such a system, the image developed on the photoconductive surface is transferred to an intermediate transfer belt (“ITB”) or intermediate transfer membe
Chou Hsin Hsin
Edwards William D.
Kellie Truman Frank
Teschendorf Brian P.
Mark A. Litman & Associates P.A.
Raevis Robert
Samsung Electronics Co,. Ltd.
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