Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ELECTRICALLY CONDUCTIVE DOCUMENT TRAIISPORT BELT
Back round of the Invention
This invention relates to a self-grounding
document transport system belt and more particularly to
a document transport belt used in such a system having
a light reflective outer layer and an electrically
conductive inner layer.
Background Art
In devices which utilize rubber or plastic belts
in conjunction with mechanical drive means for the
movement of paper and documents from one position to
another, the movement of the transport belt relative to
other parts of the apparatus builds a static electric
charge on the rubber or plastic belt. The static
charge makes the separation of the paper or document
from the belt difficult. Such static charge build-up
limits the speed at which such transport devices can be
operated and thus limits the speed of operation of the
device. In devices such as electrostatic or
photosensitive copiers, such limitations on speed of
document or paper feeding is of primary importance to
the performance and desirability of such copy machines.
In photosensitive or electrostatic copying devices
the document transport belt should be highly light
reflective in order to present an acceptable backdrop
for the document at the image making station. The
light reflective character of the transport belt
surface must be maintained over time despite the
exposure of such transport belts to mechanical stress
and degradation, oxidative degradation, ozone and
ultraviolet Light exposure over long periods of
operation.
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It is Nemo in the art that certain rubber
polymers can be formed into a document transport belt
which exhibits acceptable initial properties such as
the coefficient of friction and degree of light
reflectivity. Synthetic polyisoprene belts with
special low sulfur cure systems are known to give an
acceptable initial coefficient of friction. Such belts
exhibit higher resistance to degradation due to
mechanical stress however, such synthetic
polyisoprene belts cannot be effectively protected
against ozone degradation without sacrificing the
desirable light reflective white surface. Ozone
degradation causes fine cracks to develop in the
surface. these cracks degrade the mechanical
properties of the belt and pick up dirt and
discoloration on the surface of the document feed belt.
A rubber document transport belt is known which
incorporates ethylene, propylene, dine rubber (EPDM)
polymers compounded with fillers and whitening agents
I such as titanium dioxide. These EPDM transport belts
exhibit excellent resistance to environmental attack
from oxygen, and ozone as well as mechanical
degradation of the belt properties. The deficiency of
the EPDM belts known in the art in high speed
electrostatic or photosensitive copiers is that such
belts are not electrically conductive due to the
inherent insulating properties of the EPDM polymer. As
a result excessive static build up develops during
prolonged high speed copying operations. One known
approach to the static charge build up problem has been
to utilize a two layer construction where the outer
layer is composed of a flexible light reflective
material such as white polychloroprene rubber and a
second inner layer formed from a flexible electrically
conductive material such as graphite impregnated
polychloroprene rubber, Po:Lychloroprene has moderate
resistance to environmental attack but it does degrade
on exposure to ultraviolet -light, heat and oxygen. The
degradation causes the surface to turn brown, thus
reducing the light reflectivity of the surface, In
addition ozone attack causes tiny cracks to appear in
the surface. These cracks would collect dirt and
further decrease the light reflectivity. The use of
graphite as a conductive material in the inner layer
produces an inner layer which exhibits poor abrasion
resistance and inadequate physical properties such as
ultimate tensile strength and elongation. In addition,
the inner static conducting layer utilizing graphite
with its poor abrasion characteristics would have a
tendency to be worn and abraded away by the mechanical
drive means and roller assemblies supporting such belt
in the document drive apparatus. This abrasion problem
would become more pronounced as the polymer physically
degraded upon exposure to the oxygen, heat, ozone, US
light during prolonged copier operation. The abraded
particles would accumulate inside of the copier
apparatus causing discoloration of the upper belt
surface, masking of the imaging apparatus and
accumulation in mechanical drive portions of the
copying device. The document transport belts as known
in the art were deficient either in their ability to
maintain uniform physical properties and adequate light
reflectivity or in their ability to dissipate static
electrical charge build up around the belt.
Brief Description of the Invention
This invention is directed to an improved document
transport belt of composite construction which is
electrically conductive and has a light reflective
surface for use in a document copying machine. In
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particular this invention is a belt having means for
dissipating static electricity comprising an outer
layer of ethylene propylene rubber or ethylene
propylene dine rubber containing tight reflective
pigments and an electrically conductive, high tensile
strength inner layer of ethylene propylene rubber or
ethylene propylene dine rubber containing conductive
carbon black.
More Detailed Description
This invention provides a belt having means for
dissipating static electrical comprising: an outer
layer of ethylene propylene rubber or ethylene
propylene dine rubber, said outer layer containing
light reflective pigments and an electrically
conductive, high tensile strength inner layer of
ethylene propylene rubber or ethylene propylene dine
rubber, said inner layer containing conductive carbon
black.
This invention also provides a self-grounding belt
conveying system comprising: an endless flexible belt
comprising an outer layer of ethylene propylene rubber
or ethylene propylene dine rubbers said layer
containing light reflective pigments and an
electrically conductive inner layer of ethylene
propylene rubber or ethylene propylene dine rubber,
said inner layer containing conductive carbon black;
means for supporting and guiding said endless belt;
means for driving said endless belt.
In order to achieve the dual objectives of
providing a paper transport system which is capable of
dissipating static electrical charge build up during
high speed operation and also presenting a highly
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reflective surface to the image making portion of a
copy producing device, this invention employs a
specific composite structure for the belt which
actually contacts and conveys the paper or document
through the desired operation. This document transport
belt is a part of a larger system designed to convey
documents or papers from one point to another within
the copying device. The apparatus may contain Means
for support and means for driving the document
transport belt as well as other means to adjust the
tension and inclination of the document transport belt.
The document transport belt contains two distinct
layers which together form the composite belt.
Referring to the Figure the outer layer 1 is the
portion of the belt which actually contacts and conveys
the document or paper. The inner layer 2 which engages
the drive and support means of the document transport
apparatus, (not shown). The outer layer of the
improved document transport belt is made from high
molecular weight, synthetic rubber polymers derived
from ethylene and propylene. The copolymer rubbers
(ERR) are polymerized from ethylene and propylene
monomers and are completely saturated polymer
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backbones. The terpolymers (EPDM) are polymerized from
ethylene; propylene, and a small percentage of a dine
which provides unsaturation in side chains pendant from
the saturated backbone. The common dine monomer
component may be 1,4-hexadiene, ethylidene norbornene,
or dicyclopentadiene. The compounding of EPDM to
achieve desired physical properties in belts such as a
stable coefficient of friction and particular tensile,
elongation and elastic modulus levels is well known in
the art as described in So Patent 4,314,006. The
compounding of ERR is in all respects similar to EPDM
except that ERR rubbers are not curable using sulfur or
sulfur donor curing systems. The preferred cure system
to be employed is one in which free radical
cross linking is carried out using commonly available
peroxide curing materials such as ditertiary bottle
peroxide, dicumyl peroxide, bouncily peroxide, etc. Such
additional secondary curing agents as zinc
2-mercaptotlylimidazole, red lead, triallyl sonority,
triallyl isocyanurate, difunctional acrylates and
trifunctional acrylates may be utilized to achieve
specific desired cured physical properties.
Plasticizers may be used in the compounding of the
composition for the outer layer. Petroleum based
process oils are commonly used for this purpose. The
oils may be primary paraffinic in character although
naphthenic oils may be utilized in order to achieve
desired physical properties. Such plasticizers may be
added to the polymer at from 5 to 50 parts by weight of
plasticizer based on a 100 parts by weight of EPDM or
ERR. Fillers and reinforcing materials may be added to
enhance the tensile strength, elongation and modulus
values of the finished rubber composition. Preferred
reinforcers are the hydrated silicas and aluminum
oxides. For the outer layer of said belt carbon black
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is not an acceptable rein-forcing agent since it is
desired that this layer be highly light reflective.
Whiteners must be added to the outer layer in order to
achieve the desired level of light reflectance. The
preferred whitener is titanium dioxide which may be
added at levels of from 10 to ill parts by weight based
on 100 parts by weight of rubber polymer. Cure
activators may be added to the rubber composition in
order to enhance the rate of cure. Zinc oxide is a
preferred activator and may be incorporated at levels
of from 1 to 10 parts by weight.
The base polymer of the electrically conductive
inner layer is EPDM or ERR rubber and may contain
plasticizers, fillers, activators, and other
ingredients. The polymer composition and curing system
of the inner and outer layers may be the same or
different. However, it is most preferred that the
curing system and the EPDM or ERR polymer be the same
as those employed in the outer layer. Such
similarities in the base polymer and the cure system
are desirable since the outer and inner layers must be
securely bonded together and similar polymer and cure
systems will assure secure bonding between the two
layers after curing.
An essential component of the inner layer is
conductive carbon black. Among the various types of
carbon blacks available, acetylene blacks and selected
grades of furnace blacks produced from oil feed stocks
are the types which are recognized by practitioners in
rubber compounding as conductive carbon blacks. The
degree of electrical conductivity of a carbon black
loaded rubber depends on a number of factors including
the number of conductive paths provided by the black
and the resistance of the carbon black particles. The
chain structure and the level of combined oxygen
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present at the surface of the carbon black particles
are factors that affect tune conductivity of a
particular type of carbon black. High chain structure,
low oxygen carbon blacks are generally efficient
conductors. A commonly used method of classifying the
conductive character of a cured rubber composition is
to measure the electrical resistivity (ohms/cm) of the
rubber composition. For the purposes of this
invention, a carbon black is considered conductive if
it exhibits electrical resistivity of less than 106
ohms per centimeter when incorporated in the rubber at
the desired level with all other compound ingredients.
Currently available carbon blacks which exhibit such
resistivity include Shawinigan acetylene black
available from Shawinigan Carbide Corporation,
conductive furnace blacks available from Cabot
Corporation, ketjen black available from Norway Chemical
Corporation. The most preferred carbon blacks are the
Cabot XC conductive furnace blacks. The carbon black
may be added at levels of from about 30 to about 120
parts by weight per 100 parts by weight of the rubber
polymer.
It is desirable that the outer layer have a
minimum tensile strength of 1000 psi (6.9 Ma) and
minimum ultimate elongation of 200%. The inner layer
is subjected to the mechanical stress of the drive
means and must have higher physical properties than the
outer layer. It is desirable that the inner layer have
a minimum tensile strength of 1800 psi ~12.4 Ma and
minimum ultimate elongation of 150%. Such properties
being determined in accordance with ASTM D412.
Graphite differs from conductive carbon black in
several important aspects. The density of conductive
carbon black is lower. This is attributable to the
three dimensional ordered crystalline structure of
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crystalline structure of carbon black. The completely
uniform structure of graphite imparts minimal
reinforcement and abrasion resistance when compounded
into rubber polymers. The tensile strength is roughly
30% less than a conductive furnace type carbon black.
These deficiencies in physical properties make graphite
an unacceptable conductive material in the inner layer
of the belt of this invention. Use of graphite in the
inner layer would lead to abrading and fretting of the
surface by the drive and support means of the document
transport device. The abraded black particles would
accumulate on the white belt surface, imaging window
and in movable mechanical parts causing poor copy
quality and premature wear of mechanical parts.
The various ingredients may be incorporated into
the EPDM or or EN polymer in any conventional manner
including Danbury or mill mixing. The mixing should be
carried out in such a manner that all ingredients of
the composition are uniformly dispersed in the base
I polymer.
The document handling composite belt may be
produced by forming uncured sheets of desired thickness
from each of the different compositions: One sheet
from the outer layer composition which includes a
whitening agent and one sheet from the inner layer
composition which includes conductive carbon black.
Such sheets may be produced by calendaring, extrusion,
casting from solvent or any other suitable means. The
sheets of the outer layer composition and the inner
layer composition are plied together in intimate
contact at the interface of the plies. The plied sheet
may then be applied to a mandrel of the appropriate
diameter and spliced to form a smooth surface. Heat
and pressure are then applied in order to initiate
curing of the base polymer in both compositions. The
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curing of the base polymer -in both compositions. The
curing means may be any known conventional method
including autoclave, compression molding, liquid curing
by high temperature molten metals of salts, radiation
curing, etc.
A Preferred Embodiment
A document transport belt was made using an EPDM
terpolymer composed of ethylene propylene and
ethyldiene norbornene, specifically VistalonT~2504
available from Exxon Chemical Corporation. The outer
reflective layer and inner electrically conductive
layer compositions were peroxide cured. The outer
reflective layer contained US parts titanium dioxide
and 40 parts hydrated silica. The electrically
conductive inner layer contained no fillers other than
75 parts by weight based on 100 parts by weight of EPDM
of conductive carbon black, specifically VulcanTYXC-72
type furnace black commercially available through Cabot
Corporation. The physical properties of the outer and
inner layers are summarized in Table I.
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TABLE I
PHYSICAL PROPERTIES*
Electrically Light
Conductive Reflective
Inner Layer Outer Layer
Tensile Strength,
psi (Ma) 2610 (18.0) 1250 (8.6)
Ultimate Elonga-
lion, % 220 310
10 Shore A Dormitory 79 72
100% Modulus,
psi (Ma) 1770 (12.1) 435 (3.0)
Tear, Die C
lb/in-(kN/m)200 (35.2) 177 (29.8)
*Test sheets (1.5 mm) press cured for 30 minutes
@ 166C.
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The belt was produced in the following manner: 1)
The compositions as described above were Danbury mixed
to assure adequate dispersion of all ingredients; 2)
Each of the two compositions were calendared to the
appropriate gauge; 3) Laying up on a hollow rigid
mandrel a first ply of the electrically conductive
layer having a thickness of .025 inches, (.6 mm) then
laying up a second ply of the reflective layer having a
thickness of .015 inches (.4 millimeters. The
construction was inspected carefully to assure that the
two plies were in intimate contact with each other and
no air spaces existed between the plies. The uncured
belt construction was then cured under high pressure
steam conditions and subsequently cut to the desired
width to form an endless belt for use as a document
transport belt in a copying apparatus. The electrical
resistivity as expressed in ohms-cm was determined for
each of the layers of the fully cured belt. The light
reflective outer layer had a volume resistivity of 2.7
x 1014Ohm-cm and a surface resistivity of 3.5 x
1013Ohm-cm measured in accordance with ASTM D~57. The
electrically conductive inner layer was determined to
have a volume resistivity of 2800 ohm-cm measured in
accordance with British Standard (1953) BY 2044-2050,
potentiometric method number 2.
Industrial Applicability
The use of this invention in modern high speed
photocopying and electrostatic copying equipment allows
the realization of several desirable characteristics in
the document transportation devices. First the
dissipation of undesirable static charge build up
during high speed operations will be eliminated by the
use of this invention. Paper may be removed easily and
quickly from the transportation belt due to the absence
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of static electrical build up. The belt of this
invention embodies the further desirable
characteristics of the EPDM base polymer including
outstanding resistance to ultraviolet radiation, ozone,
and high temperature oxidation which are all present in
the working environment of the belt. The physical
properties of the belt are maintained over prolonged
periods of operation due to its ability to resist the
degrading effects of these and the resulting
degradation of such properties. The desirable light
reflective characteristics of the outer layer of the
belt can be maintained over long periods of operation
which maintains outstanding copy quality over the
extended service life of this belt construction.
