Composite to enable contact electrostatic voltage sensing

A contact for use in contacting a moving photoreceptor surface, is formed from a structure (preferably a pultrusion) including a plurality of continuous strand fibers of high electrical resistance, and a thermally stable insulating component between the resistive fibers. The resistive fibers are configured to form a brush extending from the insulating component for contact with the photoreceptor surface. The resistance of the fibers is sufficiently high to reduce leakage of surface charges away from the photoreceptor and to provide a high resistance between adjacent fibers. The insulating component, which serves to interface the carbon filaments from each other and from a host polymer may comprise an organic compound, such as a polyimide composition, or may comprise an inorganic compound such as Al.sub.2 O.sub.3 or water glass, and is thermally stable below about 1000.degree. C. The host polymer, on the other hand volatilizes rapidly and cleanly upon direct exposure to laser energy.

This invention relates to improvements in electrical contacts in general, 
and more particularly to improvements in brush type electrical contacts, 
and still more particularly to improvements in brush type surface contacts 
for contacting a surface on which an electrostatic charge is carried 
without substantially redistributing the electrostatic change. The 
invention also relates to improvements in electrostatic voltmeters, and to 
probes and photoreceptive surface contacts for use with such electrostatic 
voltmeters. 
The invention has wide applications; however, as will become apparent, a 
preferred embodiment of the invention is particularly suitable for 
applications in electrostatographic reproducing machines. In a typical 
electrostatographic reproducing machine, a photoconductive insulating 
surface, often in the form of a moving belt, is uniformly charged and 
exposed to a light image from an original document. The light image causes 
the exposed or background areas to become discharged, and creates an 
electrostatic latent image on the surface corresponding to the image 
contained within the original document. Alternatively, a light beam such 
as a laser beam may be modulated and used to selectively discharge 
portions of the photoconductive surface to record the desired information 
thereon. The electrostatic latent image is made visible by developing the 
image with a developer powder, referred to in the art as toner, which may 
be subsequently transferred to a support surface such as paper to which it 
may be permanently affixed by the application of heat and/or pressure. 
To insure optimum machine performance, adjustment or tuning of the various 
machine processing components, such as adjustment of the power input to a 
corona generating device, or adjustment of the voltage bias to magnetic 
brush developing apparatii, or the like, are made. These adjustments may 
be performed through the use of an electrostatic voltage measuring device, 
sometimes termed an electrometer, which measures the voltage on a 
photoreceptive surface within the machine, such as the surface of a belt 
on which the electrostatic charge representing the image to be reproduced 
is carried. 
One of the main problems which now exists is the lack of an adequate 
contact by which the electrostatic charges on the photoreceptive surface 
can be established, for use, for instance, by the electrostatic voltmeter. 
If for example, a brush contact were to be used, ideally the net sum of 
all of the charges contacted by the brush would be integrated and 
transmitted by a common connection directly to the input of the 
electrostatic voltmeter. However, since all of the fibers in the typical 
brush are generally connected together, not only at the connection point 
but also at various other points along the lengths of the fibers, 
electrostatic charges can freely move from fiber to fiber and up and down 
the individual fibers at random, essentially rearranging the charge 
distribution on the photoreceptive surface. The undesired result of this 
"cross talk" mechanism of the brush and of the resulting charge 
redistribution is electrostatic image smearing. Smearing of the 
electrostatic image produces unacceptable variations in image density on 
the output copies which is a serious problem. 
Although it is desired that the electrostatic voltmeter contact be 
continuously in contact with the photoreceptor, it can be seen that this 
goal is not acceptably achievable at the present time thorugh the use of 
such contacts in which the "cross talk" mechanisms exists. 
It has been proposed to use a lift-off device to remove the contacting 
element of the electrostatic voltmeter from the photoreceptor at critical 
times in the electrostatic reproduction cycle, but this 
"removable-contact" approach is not very attractive, since it adds 
unnecessary cost and complexity to the machinery. 
PRIOR ART 
U.S. Pat. No. 4,358,699 to Wilsdorf describes an electrical fiber brush and 
method of making it. The fiber brush consists of a brush body of a matrix 
material, at least one fibrous part formed in the brush by removing most 
or all of the matrix material, at least one working surface which will 
make an electrical connection with some object(s) to which electrical 
connection shall be made, and at least one set of electrically conductive 
fiber wires which form a part of the working surface as well as of the 
fibrous part. The electrical properties of the brush are controlled by the 
fiber wires. By making extremely large numbers of fiber wires of very 
small diameters to contact the object at the working surface of the brush, 
quantum-mechanical tunneling is expected to become the predominant 
mechanism of current conduction, yielding extremely good brush 
performance, while at the same time brush wear is forecast to be very low. 
U.S. Pat. No. 4,761,709 to Ewing et al. describes a contact brush charging 
device having a plurality of resiliently flexible thin fibers having a 
resistivity of from about 10.sup.2 ohms-cm to about 10.sup.6 ohm-cm and 
being substantially resistivity stable to changes in relative humidity and 
temperature. Preferably the fibers comprise partially carbonized 
polyacrylonitrile fibers. 
U.S. Pat. No. 4,336,565 to Murray et al., assigned to the assignee of the 
present application, describes a charge process with a carbon fiber brush 
electrode. The process imposes an electric charge on an electrically 
insulating surface of a moving web by a brush electrode which contacts the 
surface. The brush is made up of extremely soft and flexible filaments 
comprising carbon mounted on a metallic brace serving as an electrical 
contact to supply the brush with DC potential. 
U.S. Pat. No. 4,455,078 to Mukai et al. describes a charging device having 
a conductive particle impregnated strand lined contact member. The 
charging device comprises a contact piled cloth which is formed of pliable 
material and having an electrical resistance chosen to be 1.times.10.sup.8 
ohm-cm, and contacts with a photosensitive layer of a photosensitive drum. 
The contact piled cloth is provided with a multitude of raised furs formed 
of artificial fibers with conductive particles dispersed therein. 
U.S. Pat. No. 4,741,873 to Fischer et al. describes a method for forming 
rigid composite preforms, providing a way for rigidizing the preform prior 
to resin or matrix introduction. The preforms are fabricated from 
reinforcement strands served with a thermoplastic thread. The 
reinforcement strands may be glass fibers or carbon fibers and the 
thermoplastic threads may be polyamides or other similar compounds. 
U.S. Pat. No. 4,149,119 to Buchheit, assigned to the assignee of the 
present invention, describes an electrostatic voltmeter, or electrometer, 
that includes a probe sensor element for measuring charge on a test 
surface. The electrometer includes means to automatically neutralize 
changes in spacing between the electrometer probe and the surface being 
measured. The probe receives both AC and DC signals and is compensated for 
drift in the signals. 
U.S. Pat. No. 4,801,967 to Snelling, assigned to the assignee of the 
present invention, describes a voltage sensor for sensing voltage on a 
photoreceptor surface. A wire electrode is placed parallel and across from 
a photoreceptor surface. Voltage is measured from the electrode and used 
in charging the photoreceptor to a uniform voltage level. 
U.S. Pat. No. 4,106,869 to Buchheit, assigned to the assignee of the 
present invention, describes an electrostatic voltmeter with drift 
compensation. 
U.S. Pat. No. 4,569,583 to Robson et al, describes an electrostatic charge 
probe for an electrophotographic apparatus. 
SUMMARY OF THE INVENTION 
In light of the above, therefore, it is an object of the invention to 
provide an improved electrical contact of the type described for use in 
electrostatic voltage sensing. 
It is still another object of the invention to provide an electrical 
contact of the type described which can be used to contact electrostatic 
charge on a moving photoreceptive surface without substantially 
redistributing the charge. 
It is yet another object of the invention to provide an electrical contact 
of the type described which can be used to sense electrostatic charge on a 
photoreceptor of an electrostatographic reproducing machine without 
electrostatic image smearing. 
It is still yet another object of the invention to provide a electrical 
contact of the type described in which the "cross talk" mechanisms between 
the fibers of the device are reduced or eliminated. 
It is yet another object of the invention to provide an electrical contact 
of the type described that can be allowed to remain continuously in 
contact with the photoreceptor, without substantially affecting the 
quality of the final copy image or of the photoreceptive surface. 
It is yet another object of the invention to provide an improved electrical 
contact formed of a pultrusion composition, and a method of making same. 
These and other objects, features, and advantages of the invention will be 
apparent to those skilled in the art from the following detailed 
description of the invention, when read in conjunction with the 
accompanying drawings and appended claims. 
In accordance with a broad aspect of the invention, a contact is presented 
for such use as contacting a moving photoreceptor surface, and is formed 
of a structure including a plurality of continuous strand fibers of high 
electrical resistance, and a thermally stable insulating component between 
the resistive fibers. The resistive fibers are configured to form a brush 
extending from the insulating component for contact with the photoreceptor 
surface. The resistance of the fibers is sufficiently high to reduce 
leakage of surface charges away from the photoreceptor and to provide a 
high resistance between adjacent fibers. The insulating component, which 
serves to insulatively interface the carbon filaments from each other and 
from the host polymer may comprise an organic compound, such as a 
polyimide composition, or may comprise an inorganic compound such as 
Al.sub.2 O.sub.3, and is thermally stable below about 1000.degree. C. The 
host polymer, on the other hand, thermally decomposes and volatilizes 
rapidly and cleanly upon direct exposure to the heat delivered by a laser. 
In a preferred embodiment, the structure is a pultrusion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, an improved electrical contact 
device is provided that is of improved reliability, is of low cost and is 
easily manufacturable. The invention will be described with reference to a 
manufacturing process known generally as a pultrusion process, with the 
fibrillation of at least one end of the pultrusion. A structure formed by 
such a pultrusion process is referred to herein as a "pultrusion." 
However, structures other than a pultrusion are possible as long as the 
fibers are insulated from each other. For example, as will become more 
apparent from the description herein, the structure for holding the fibers 
may be heat shrunk tubing or any other mechanism for securing the fibers 
11 together. Thus, while the invention is described with reference to a 
pultrusion, other structures are possible. 
With reference now to FIG. 1, the pultrusion composition 10 in accordance 
with a preferred embodiment of the invention comprises continuous fibers 
or strands 11 of resistive carbon fiber filler within a host polymer 12, 
and within a thermally stable, insulating compound 13 that forms a 
continuous interfacial layer between the individual fibers 11 and the host 
polymer 12. The pultrusion composition 10 is shown in contact with a 
photosensitive surface 16 on one side, and a connection plate 17 on the 
other side to which electrical connection is established. Such carbon 
fiber pultrusions are a subcategory of high performance conductive 
composite plastics, and comprise one or more types of continuous, 
conductive reinforcing filaments in a binder polymer. They provide a 
convenient way to handle, process and use fine diameter, carbon fibers 
without the problems typically encountered with free conductive fibers. 
The pultrusion process generally consists of pulling continuous lengths of 
fibers first through a resin bath or impregnator, then into a performing 
fixture where the resulting section is at least partially shaped and 
excess resin and/or air removed. The section is then pulled into heated 
dies where it is continuously cured. For a detailed discussion of 
pultrusion technology, reference is directed to "Handbook of Pultrusion 
Technology" by Raymond W. Meyer, first published in 1985 by Chapman and 
Hall, New York. 
More specifically, in the practice of the invention, conductive carbon 
fibers are submersed in a liquid polymer bath and drawn through a die 
opening of suitable shape at high temperature to produce a solid piece 
having dimensions and shapes of that imparted by the die. The solid piece 
can then be cut, shaped, and/or machined. As a result, a structure can be 
achieved that has thousands of fine diameter, conductive fiber elements 
contained within the polymer matrix, the ends of the fiber elements being 
exposed to provide a very large number of individual electrical contacts. 
The very large redundancy and availability of electrical contacts enables 
a substantial improvement in the reliability of these devices. 
Since the plurality of small diameter conductive fibers are pulled through 
the polymer bath and heated die in continuous lengths, the shaped member 
can be formed with the fibers being continuous and generally collimated 
from one end of the member to the other. Accordingly, the pultruded 
composite may be formed in a continuous length during the pultrusion 
process, then cut to any suitable dimension, with a very large number of 
filamentary electrical contacts provided at each end. Such pultruded 
composite members may have either one or both of its ends subsequently 
fibrillated. 
Any suitable fiber having a high resistance may be used in the practice of 
the invention. However, carbon fibers are particularly well suited as 
preferred fiber because they are small in size and supple, non-abrasive, 
chemically and environmentally inert, possess high strength and 
resilience, can be tailored to a wide range of resistivities, and exhibit 
a negative coefficient of thermal resistivity. The conductive fibers can 
be formed to have a DC volume resistivity of from about 1.times.10.sup.-3 
.OMEGA..cm to about 1.times.10.sup.10 .OMEGA..cm and preferably from about 
5.times.10.sup.8 .OMEGA..cm to about 5.times.10.sup.9 .OMEGA..cm. 
In addition, the individual conductive fibers 11 can be made circular in 
cross section with a diameter generally in the order of from about 4 
micrometers to about 50 micrometers and preferably from about 7 
micrometers to 10 micrometers. This provides a very high degree of fibrous 
contact redundancy in a small cross-sectional area. Thus, as contact 
materials, the fibers provide a multiple redundancy of contact points, for 
example, in the range between about 0.05.times.10.sup.5 and 
5.times.10.sup.5 contacts/cm.sup.2, preferably about 0.0558.times.10.sup.5 
contacts/cm.sup.2. This is believed to enable extraordinarily high contact 
reliability. Moreover, for instance, in electrostatic reproducing 
machines, imbedding such fibers within a polymeric binders such as within 
a pultrusion is also likely to minimize harmful contamination effects from 
long lengths of broken fibers transported randomly within the machines. 
The fibers 11 are typically flexible and compatible with the polymer 
systems within which they are imbedded. Typical fibers may include carbon, 
carbon/graphite, metalized or metal coated carbon fibers, carbon coated 
glass, surface carbonized polymeric or glass fibers, and metal coated 
glass fibers. A particularly preferred class of fibers that may be used 
are those fibers that are obtained from controlled heat treatment 
processing to yield complete or partial carbonization of polyacrylonitrile 
(PAN) precursor fibers. It has been found for such fibers that by 
carefully controlling the temperature of carbonization within certain 
limits that precise electrical resistivities for the carbonized carbon 
fibers may be obtained. The carbon fibers from polyacrylonitrile (PAN) 
precursor fibers are commercially produced by the Stackpole Company, 
Celion Carbon Fibers, Inc., division of BASF and others in yarn bundles of 
1,000 to 160,000 filaments. The yarn bundles are generally carbonized in a 
two-stage process. The first stage involves stabilizing the PAN fibers at 
temperatures on the order of 300.degree. C. in an oxygen atmosphere to 
produce preoxstabilized PAN fibers. The second stage involves 
carbonization of the fibers at elevated temperatures in an inert 
atmosphere, such as an atmosphere containing nitrogen. The DC electrical 
resistivity of the resulting fibers is controlled by the selection of the 
time and temperature of carbonization. For example, carbon fibers having 
an electrical resistivity of from about 10.sup.6 .OMEGA..cm to 10.sup.10 
.OMEGA..cm are obtained if the carbonization temperature is controlled in 
the range of from about 500.degree. C. to about 1000.degree. C. For 
further reference to the processes that may be employed in making these 
carbonized fibers attention is directed to U.S. Pat. No. 4,761,709 to 
Ewing et al and the literature sources cited therein at column 8. 
The carbon fibers 11 are enclosed in any suitable polymer matrix 12. (If 
the structure is not a pultrusion, then the fibers can be secured by heat 
shrunk tubing instead of the host polymer 12.) The polymer matrix should 
be of a resin binder material that will volatilize rapidly and cleanly 
upon direct exposure to the laser beam during laser processing below 
described. While thermosetting polymers such as vinyl ester, polyester, 
and epoxies are in general use for pultrusions and are acceptable for use 
in this invention, thermal plastic polymers such as low molecular weight 
polyethylene, polypropylene, polystyrene, polyvinylchloride, nylon, 
polyphenylene sulfide, and polyurethane may be particularly advantageously 
employed. 
Between the carbon fibers and the host polymer is an interfacial compound 
13. The interfacial compound 13 has a high insulating resistivity as well 
as a high thermal stability, for instance, having the ability to withstand 
temperatures of 200.degree. C. to beyond 500.degree. C. which can be 
created by a laser during the fibrillation process. The high thermal 
stability is necessary to enable laser or other thermal processing 
operations to be performed without volatilizing the interfacial compound 
or decomposing it to a conductive moiety. The interfacial compound 13 also 
has good compatibility with the host polymer 12 to enhance 
polymer-to-fiber adhesion where this may be required. Accordingly, the 
interfacial compound 13 can be an organic compound, such as polyimide, 
poly ether ether ketone (PEEK), polytetrafluoroethylene (teflon), 
polybenzimidolimide (PBl), Kevlar, hydrofluoro elastomers (e.g. Viton GF), 
or other high temperature, film forming- organic polymeric compounds. 
Alternatelly, insulative, inorganic compounds, such as, for example, 
aluminum oxide (Al.sub.2 O.sub.3), synthetic diamond, water glass (e.g. 
potassium silicate or lithium silicate), or any other insulative, 
film-forming compound may be suitably adapted for application in this 
invention. 
Either organic or inorganic type interfacial compounds can be applied 
directly to the fiber by solution, immersion or other solution coating 
techniques, by thermal, coating techniques from the melted compound(s), or 
from any other suitable coating technique. 
A laser (not shown) can be used to cut individual components for use as 
electrical contacts or specifically the charge pick-up contact in 
accordance with the present invention. For example, a focused CO.sub.2 
laser can be used to cut the pultrusion and simultaneously volatilize the 
binder resin in a controlled manner a sufficient distance back from the 
cut to produce in one step the distributed filament contact illustrated in 
FIG. 1. The length of exposed carbon fiber can be controlled by the laser 
power and cut rate. Various tip shapes can be achieved by changing the 
laser incidence angle. Thus, a suitable protrusion can be cut by laser 
techniques to form a contact 10 of desired length from the longer 
pultrusion length, and both severed ends can be fibrillated to provide a 
high redundancy fiber contact member downstream to contact the 
photosensitive surface 16 and a high redundancy fiber contact upstream to 
contact the contact plate 17. Any suitable laser can be used which has 
suitable light energy to be absorbed by the matrix of the host polymer 12, 
so that the host polymer 12 will be volatilized. Specific lasers which may 
be used include a carbon dioxide laser, a carbon monoxide laser, or the 
YAG laser. The carbon dioxide laser is particularly suited for this 
application, owing to the fact that it is highly reliable, highly 
compatible for efficient light to heat conversion by polymer matrix 
absorption, and is most economical in manufacturing environments. 
As mentioned, a preferred embodiment of the invention provides a high 
resistance electrode element. This element enables a contacting 
electrostatic voltmeter to be useful to continuously measure the 
electrostatic charge on the photoreceptive surface 16. That is, a contact 
member 10, formed as described above in accordance with a preferred 
embodiment of the invention can be used to contact the moving 
photoreceptive surface 16 for continuously measuring the voltage or 
electrostatic charge thereon. 
For a complete understanding of the benefits afforded by the contact of the 
invention, the mechanism for electrostatic image smear by a contacting 
element will now be described. As shown in FIG. 2, an electrostatically 
charged photoreceptor surface 16 can be viewed as a distributed capacitor 
presenting a plurality of parallel configured capacitors 20, each charged 
to a charge value determined by the degree to which the light image to 
which it has been exposed has caused the particular capacitor areas to 
become discharged via photodischarge of the previously uniformly charged 
photoconductor. A brush-like contacting member, for example, of a tip 
fibrillated pultrusion 25, shown in FIG. 3 may be made up of thousands of 
7 to 10 micron diameter fibers 24. Each of the fibers 24 is continuous in 
length and has a finite, relatively high, resistance. At any instant 
during movement of the photoreceptor surface 16 under the brush 25, as 
indicated by the arrow 27, the tip of every fiber 24 contacts a different 
area of the photoreceptor surface 16, in effect contacting a different 
capacitor. In the absence of a suitable interfacial layer 13 of FIG. 1, 
all of the fibers 24 in the pultrusion 25 of FIG. 3 are connected 
together, not only at the common connection 26, but also at various other 
points along the lengths of the fibers via the fiber to fiber resistance 
30. Here, charges can freely move from fiber to fiber and up and down the 
fibers at random. It is this charge transfer or "cross talk" mechanism via 
electrical contact between fibers that is believed to be the cause for 
electrostatic image smearing. 
Under ideal operation, the net sum of all of the charges contacted by the 
brush 25 is integrated and transmitted by a common connection 26 directly 
to the input of an electrostatic voltmeter 28, as shown in FIG. 4. Since 
if according to a preferred embodiment of the invention, each fiber is 
isolated from all of the other fibers along their entire lengths by the 
interfacial layer 13, as illustrated in an equivalent electrical circuit 
in FIG. 5, electrostatic image smear can be reduced or eliminated. Thus, 
the charge moved by any individual fiber tip 11 must travel along that 
same fiber to the common connection 17 and back down to the surface 16 of 
the photoreceptor in order to redistribute any of the charge in the 
original pattern on the photoreceptor surface. As the photoreceptor 
surface 16 moves under the brush, in the direction shown by the arrow 27 
each fiber 11 contacts a specific region on the photoreceptor for a 
finite, but very short length of time. If the resistance of the individual 
fibers 11 is properly selected (for any given photoreceptor capacitance 
and velocity), then the time constant of the charge traveling from the 
photoreceptor surface 16 through a single fiber 11 and back down via 
another adjacent fiber 15 in FIG. 1 to the photoreceptor surface 16 is 
sufficiently long so that no significant amount of charge can be 
transferred while the fiber 11 is over any one point. Consequently no 
image smear occurs. 
Although the invention has been described and illustrated with a certain 
degree of particularity, it is understood that the present disclosure has 
been made only by way of example, and that numerous changes in the 
combination and arrangement of parts or materials can be resorted to by 
those skilled in the art without departing from the spirit and scope of 
the invention as hereinafter claimed.