Radiation imagery chemistry: process – composition – or product th – Electric or magnetic imagery – e.g. – xerography,... – Radiation-sensitive composition or product
Reexamination Certificate
2000-02-16
2002-01-29
Goodrow, John (Department: 1753)
Radiation imagery chemistry: process, composition, or product th
Electric or magnetic imagery, e.g., xerography,...
Radiation-sensitive composition or product
C430S066000, C430S132000
Reexamination Certificate
active
06342324
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a release layer, and a composition for forming the same that is particularly well suited to a photoconductor element in a liquid electrophotographic system. More specifically, this invention relates to a release coating for the photoconductor element.
BACKGROUND OF THE INVENTION
Electrophotography forms the technical basis for various well known imaging processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoconductor element, imagewise exposing the photoconductor element to activating electromagnetic radiation, also referred to herein as “light,” thereby dissipating the charge in the exposed areas, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoconductor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material.
The structure of a photoconductor element may be a continuous belt, which is supported and circulated by rollers, or a rotatable drum. The photoconductive layer is generally affixed to an electroconductive support. The surface of the photoconductor is either negatively or positively charged such that when activating electromagnetic radiation strikes the photoconductive layer, charge is conducted through the photoconductor in that region to neutralize or reduce the surface potential in the illuminated region. An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and extend the service life of the photoconductive layer. Other layers, such as adhesive or priming layers or charge injection blocking layers, are also used in some photoconductor elements.
Typically, a positively charged toner is attracted to those areas of the photoconductor element which retain a negative charge after the imagewise exposure, thereby forming a toner image that corresponds to the electrostatic latent image. The toner need not be positively charged. Some toners are attracted to the areas of the photoconductor element where the charge has been dissipated. The toner may be either a powdered material comprising a blend of polymer and colored particulates, typically carbon, or a liquid material of finely divided solids dispersed in an insulating liquid. Liquid toners are often preferable because they are capable of giving higher resolution images.
The toner image may be transferred to the substrate or an intermediate carrier by means of heat, pressure, a combination of heat and pressure, or electrostatic assist. A common problem that arises at this stage of electrophotographic imaging is poor transfer from the photoconductor to the receptor. Poor transfer may be manifested by low transfer efficiency and low image resolution. Low transfer efficiency results in images that are light and/or speckled. Low image resolution results in images that are fuzzy. These transfer problems may be alleviated by the use of a release coating.
The release layer is applied over the photoconductive layer or over the barrier layer if a barrier layer is being used. The release layer preferably adheres well to the photoconductive, a barrier layer, or a tie layer. Moreover, the release layer must not significantly interfere with the charge dissipation characteristics of the photoconductor construction.
Typical release coatings known in the electrophotographic arts include silicone polymers such as those disclosed in U.S. Pat. Nos. 4,600,673 and 5,733,698. Conventional siloxane release materials tend to swell significantly in the hydrocarbon solvents that are used as carrier liquids in electrophotography. Swollen polymers generally have reduced strength and are more easily abraded or easily delaminate when applied in an electrophotograhic system.
U.S. Pat. No. 5,652,078 describes another type of release layer that includes a cross-linked siloxane polymer with a bimodal distribution of chain lengths between crosslinks, wherein the siloxane polymer is the reaction product of the components comprising:
A) 35 to 80 parts by weight of a siloxane polymer with a high content of functional groups capable of crosslinking having the formula:
where each R
1
and R
3
independently is an alkyl group, aryl group, or alkenyl group, R
2
is, independently for each group —SiR
2
R
3
O— and each group —SiR
1
R
1
R
2
, either an alkyl group, an aryl group, or a functional group capable of cross-linking and at least 3% of R
2
are functional groups capable of crosslinking, and x is an integer greater than 0; and
B) greater than 0 and less than or equal to 50 parts by weight of a siloxane polymer with a low content of functional groups capable of crosslinking having the formula
where each R
4
and R
6
independently is an alkyl group, aryl group, or alkenyl group, R
5
is, independently for each group —SiR
5
R
6
O— and each group —Si(R
4
)
2
R
5
, either an alkyl group, an aryl group or a functional group capable of cross-linking and no more than 2.5% of R
5
are functional groups capable of cross-linking, and y is an integer of at least 50. Optionally, the siloxane polymer can include a cross-linking agent, preferably in an amount from 5 to 30 parts by weight.
SUMMARY OF THE INVENTION
What is yet needed is a photoconductor that is capable of withstanding more imaging cycles per photoconductor construction and thus, a more durable release layer is desired. Specifically, the release layer should be mechanically durable to withstand abrasion of the various rollers and scrapers which contact the photoconductor element. The release layer must also be resistant to toner carrier liquids.
One aspect of the present invention provides a photoconductor construction comprising a photoconductor layer, and an electroconductive substrate, and a release layer which displays good release properties, as well as good durability, low peel force, preferably less than about 13 g/2.54 cm, and resistance to toner carrier liquids.
Solvent resistance may be improved by adding fillers to or by cross-linking the polymer. However, highly cross-linked or filled systems tend to have increased surface energy that causes a decreased release performance. The present invention provides a release layer that has increased solvent resistance with minimal sacrifice of release properties.
In one embodiment, a release composition includes a siloxane polymer with a bimodal distribution of chain lengths between crosslinks that is, preferably, the reaction product of a polymer with high functionality, a polymer with low functionality, and a cross-linking agent. However, this polymer could alternatively be the cross-linked product of a single polymer provided the functional groups were spaced appropriately to provide a bimodal distribution of chain lengths between crosslinks. Such a polymer can be synthesized using anionic polymerization methods as are known to those skilled in the art.
As used herein, “functionality” and “functional groups” is an indication of reactive groups. A polymer with high functionality has more reactive groups than a polymer with low functionality. Some reactive groups would include those groups that undergo free radical reactions, condensation reactions, hydrosilation addition reactions, hydrosilane/silanol reactions, or photoinitiated reactions.
As used herein, “chain length between crosslinks” indicates how many monomeric units are in the backbone of the polymer between monomeric units from which branching or cross-linking has occurred. The bimodal distribution of such chain lengths indicates that there are a high number of relatively short chains between crosslinks and a high number of relatively long chains between crosslinks, but only a small number of chains having an intermediate length between crosslinks.
The crosslinking of the siloxanes can be undertaken by any of a variety of methods including free radical reactions, condensation reactions, hydrosilylation addition reactions, hydrosilane/silanol reactions, and photoi
Ali M. Zaki
Li Minyu
Morrison Eric D.
Simpson Charles W.
Stulc Leonard J.
Buharin Amelia A.
Goodrow John
Imation Corp.
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