Compliant transfer member having multiple parallel electrodes and method of using

A transfer member includes separately addressable electrodes separated from the surface of the member by a compliant layer. Preferably, the member is an intermediate transfer member having a thin, hard outer layer usable to receive toner images from an image member and to transfer them to a receiving sheet.

This invention relates to the formation of toner images and, more 
specifically, to an transfer member particularly usable in the formation 
of toner images and a method of forming toner images using the transfer 
member. 
Non-compliant intermediate transfer members have been used commercially in 
electrophotographic equipment to transfer toner images from an imaging 
member to a receiver. They have been used both in single color (black and 
white) copiers and in color copiers and printers. 
U.S. Pat. No. 5,084,735, granted to Rimai et al, and U.S. Pat. No. 
5,370,961 to Zaretsky et al, suggest that using an intermediate transfer 
member having a thick compliant layer with a very thin, hard overcoat 
greatly improves the transfer efficiency of small toner particles compared 
to non-compliant intermediate transfer members. The above-mentioned 
Zaretsky et al patent and Zaretsky U.S. Pat. No. 5,187,526 also point out 
that best results are obtained if the intermediate transfer member is 
semi-conducting to optimize the electrostatic force which enables the 
transfer of toner. 
Although compliant intermediates exhibit significant improvements compared 
to non-compliant intermediates, difficulty still exists due to limitations 
imposed by air breakdown (ionization) in the vicinity of the transfer nip, 
both to the intermediate transfer member and away from it to the final 
receiving sheet. Air breakdown degrades the transfer efficiency and image 
quality of toner images, especially multicolor images by altering the 
quantity of charge on the toner particles. In practice, these difficulties 
are amplified because the compliant intermediates are typically composed 
of materials that are sensitive to fluctuations in temperature and 
relative humidity. 
U.S. Pat. Nos. 5,276,490 to Bartholmae et al, 5,303,013 to Koike et al, and 
U.S. Pat. No. 5,459,560 to Bartholmae, granted Oct. 17, 1995, disclose the 
use of transfer rollers containing multiple parallel electrodes to aid 
paper handling and also to control the application of an electrical bias 
during the transfer of toner images. 
SUMMARY OF THE INVENTION 
We have found that we can substantially improve transfer over the above 
systems by combining their benefits in an intermediate transfer member 
that includes a compliant layer, a thin, hard layer on the compliant layer 
having a surface away from the compliant layer for receiving a toner image 
and a set of separately addressable electrodes positioned separated from 
the thin, hard layer by at least a portion of the compliant layer. 
It is also an aspect of the invention to use this intermediate transfer 
member as an intermediate in forming images, especially multicolor images. 
According to a preferred embodiment, the compliant layer has a thickness 
measured from the addressable electrodes to the thin, hard layer of at 
least 0.5 millimeters. The compliant layer has a Young's modulus less than 
10.sup.7 Pascals and the thin, hard layer has a Young's modulus of at 
least 10.sup.8 Pascals. Preferably, the Young's modulus of the compliant 
layer is between 1.times.10.sup.6 Pascals and 5.times.10.sup.6 Pascals. 
The thin, hard layer has a thickness less than 50 microns, preferably less 
than 15 microns and a resistivity greater than 10.sup.5 ohm-cm. 
Preferably, the compliant layer has a resistivity divided by the layer's 
thickness of between 10.sup.5 ohm and 10.sup.14 ohm with an especially 
preferred range of between 10.sup.7 ohm and 10.sup.10 ohm.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, an image forming apparatus includes an intermediate 
transfer member 1 and an image member 2. In general operation, toner 
images are formed on image member 2 and transferred electrostatically to 
intermediate transfer member 1. Formation of the toner images on image 
member 2 is preferably done electrophotographically, although other 
processes for forming images are known and could be used. 
Electrophotographically, the image member 2 is photoconductive and is 
charged at a charging station 5, imagewise exposed at an exposure station, 
for example, a laser exposure station 7, to form an electrostatic image on 
the surface of image member 2. The electrostatic image is toned by 
application of toner from one of toning stations 9, 10, 11 or 12 to form a 
toner image. Each of toning stations 9, 10, 11 and 12 contain a different 
color toner so that the color of the toner image can be chosen from one of 
four colors. 
The toner image is transferred from image member 2 to the outside surface 
of intermediate transfer member 1 electrostatically in a nip 15 by a 
process which will be discussed in more detail below. The toner image is 
then or ultimately transferred electrostatically to a receiving sheet fed 
from a receiving sheet supply 20 to a nip 25 formed between intermediate 
transfer member 1 and a transfer backing roller 22. The receiving sheet 
with the toner image is transported to a fuser 30 where it is fixed and 
ultimately deposited in an output tray 32. 
The process just described provides single color images on the receiving 
sheet. The invention can be used to form single color images, but is 
particularly advantageous in forming multicolor images. To accomplish 
this, a series of electrostatic images are formed on image member 2, each 
of which are toned by a different color toner from stations 9, 10, 11 and 
12 to form a series of different color toner images on image member 2. The 
different color toner images are transferred sequentially in registration 
to the surface of transfer member 1 where they form a multicolor toner 
image. The general process described above is conventional and well known 
in the art. 
High efficiency transfer of extremely small toner particles desired for 
multicolor images is one of the most challenging aspects of providing 
multicolor images electrophotographically. FIGS. 2 and 3 show perspective 
and cross-section views of transfer member 1 which substantially improves 
the efficiency and quality of transfer in both nips 15 and 25. Referring 
to FIGS. 2 and 3, intermediate transfer member 1 includes a compliant 
layer 35 having a thin, hard overcoat 37. Separately addressable 
electrodes 40 are substantially separated from the thin, hard overcoat 
layer by at least a portion of the compliant layer 35. Although the 
electrodes 40 could be positioned in the middle of layer 35, they are 
preferably on the bottom edge of layer 35 on or supported by an insulating 
layer 42. The intermediate transfer member can be a web, belt or roller, 
depending on the geometry of the apparatus. Thus, insulating layer 32 can 
be supported by an aluminum roller or polyester web or belt support or the 
like, well known in the art. 
The compliant layer 35 separates the electrodes 40 from the thin, hard 
overcoat preferably by at least 0.5 millimeters. In some applications, 
thicknesses greater than 1 millimeter are preferred. The compliant layer 
35 further has a Young's modulus less than 10.sup.7 Pascals, preferably 
between 1.times.10.sup.6 Pascals and 5.times.10.sup.6 Pascals. Its 
resistivity divided by its thickness is preferably between 10.sup.5 ohm 
and 10.sup.14 ohm with a preferred range between 10.sup.7 ohm and 
10.sup.10 ohm. A conventional polyurethane used for transfer drams per se 
having a small mount of an antistat material can easily provide these 
characteristics, as can other elastomeric materials. The thin, hard 
overcoat layer 37 should have a thickness less than 50 microns, preferably 
less than 15 microns. It should have a Young's modulus greater than 
10.sup.8 Pascals and a resistivity greater than 10.sup.5 ohm-cm. Harder 
polyurethanes, sol-gels, ceramers and fluorinated copolymers are all 
materials that can be used for overcoats and can be made to provide these 
characteristics with an appropriate amount of an antistat added to the 
formulation. 
The electrodes are positioned generally parallel to each other and in a 
cross-track (across the in-track) direction of the transfer member. They 
can be composed of any suitably conductive material such as copper, nickel 
or carbon. The electrodes are used to apply an electric field in the 
transfer nip so that the toner particles transfer from the imaging member 
to the intermediate transfer member and, subsequently, from the 
intermediate transfer member to a receiver such as paper. The electrodes 
are selectively electrically biased so that a large electric field exists 
at least in part of the transfer nip and a small electric field exists at 
least in a part of the region just prior to the transfer nip. For example, 
in a system in which the image member is grounded (conventional), in the 
region just prior to the transfer nip the electrodes are connected to a 
ground potential preferably at least 1 millimeter prior to the beginning 
of the nip (actual contact). In the transfer nip the electrodes, starting 
from 1 millimeter into the nip and extending to the nip exit or beyond, 
are set to a full transfer potential at least 200 volts different from the 
bias applied to the image member and as an example 500 volts. Other 
variations and sophistications in field control can be worked out by a 
person skilled in the art and will vary substantially according to the 
parameters of the system, especially the actual width of the nip. The 
primary advantage of the invention is to provide a strong transfer field 
in the nip where the toner is actually contacting the surface to which it 
is to be transferred while largely eliminating pre-nip ionization. Pre-nip 
ionization traditionally has caused imaging problems in all transfer 
systems, but it is especially troublesome when small toners with varying 
stack heights are to be transferred using a transfer member subject to 
conductivity variations from humidity and temperature changes. 
The electrode structure has a wavelength structure .lambda. (essentially 
the pitch of the electrodes) which should satisfy the following 
conditions: .lambda..ltoreq.the thickness of the compliant layer divided 
by x and .lambda..ltoreq.the width of the transfer nip divided by x, where 
x is 3 but preferably where x is 5. For example, using a transfer nip 
having a width of 0.5 millimeters and a compliant layer having a thickness 
of 1 millimeter, the .lambda. of the electrodes is preferably not greater 
than 0.33 millimeters and is much preferably not greater than 0.2 
millimeters. 
The biases applied to the electrodes are controlled by a multiple bias 
source 50 which are connected to bias applying structures 52 and 54 for 
nips 15 and 25, respectively. The application of variable biases to 
cross-track oriented electrodes has been disclosed in U.S. Pat. No. 
5,276,490, granted to Bartholmae et al Jan. 4, 1994, U.S. Pat. No. 
5,459,560 to Bartholmae Oct. 17, 1995, and U.S. Pat. No. 5,303,013, 
granted to Koike et al Apr. 12, 1994, referred to above, which patents are 
hereby incorporated by reference herein. Typically these biases can be 
applied by a set of brushes or rollers which contact extensions of the 
electrodes extending beyond the end of compliant layer 35 and which can be 
separately biased in the pre-nip, in-nip and post-nip regions to great 
advantage in transfer field control A similar set of brushes or rollers 
may be desirable at other stations (for example, an articulatable 
conductor brush cleaning station (not shown)) to ground or bias the 
electrodes. 
It is believed that the excellent results obtainable with this structure 
are due to the fact that control of the field can be maintained though the 
electrodes which are separated from the nip by the compliant layer. This 
allows the compliant layer to conform to the surface of the image member 2 
and the paper or other receiving sheet at nip 25 without interference from 
the electrodes, thereby assuring excellent contact between the toner and 
the surface to be transferred to. 
The preferred thickness of compliant layer 35 depends on the pressure in 
the nip. With greater pressure, thinner compliant layers, for example, 1 
millimeter thick layers, can be used. For lower pressures, thicknesses of 
5 millimeters or more may be preferred. 
Further, the hard layer 37 provides desired release characteristics in both 
accepting the toner from the image member 2 in nip 15 and, more 
importantly, in releasing it to the receiving sheet in nip 25. Its 
thinness allows the compliance of layer 35 to be effective. 
Although the results are not nearly as dramatic, the transfer member 1 
could also be used as a backing roller to a receiving sheet for direct 
transfer of a toner image to the receiving sheet (attached to member 1), 
for example, in nip 15. In this instance, the compliance of layer 35 is 
still useful as is the separate addressability of the electrodes. The hard 
layer 37 would not be necessary in this embodiment. 
The invention has been described in detail with particular reference to a 
preferred embodiment thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention as described hereinabove and as defined in the appended claims.