Note: Descriptions are shown in the official language in which they were submitted.
5~0
BACKGROUND OF THE INVENTION
This invention relates in general to electrophotography,and more particularly, to carrier materials useful in the
development of electrostatic latent images.
The formation and development of images on the surface
of photoconductive materials by electrostatic means is well known.
The basic electrostatographic process, as taught by C.F. Carlson
in U.S. Patent 2,297,691, involves placing a uniform electrostatic
charge on a photoconductive insulating layer, exposing ~he layer
to a light and shadow image to dissipate the charge on the areas
of the layer exposed to the light and developing the resulting
electrostatic latent image by depositing on the image a finely-
divided electroscopic material referred to in the art as "toner'.
The toner will normally be attracted to those areas of the layer
which retain a charge, thereby forming a toner image corresponding
to the electrostatic latent image. This powder image may then be
transferred to a support surface such as paper. The transferred
image may subsequently be permanently affixed to the support
surface as by heat. Instead of latent image formation by uniformly
charging the photoconductive layer and then exposing the layer to
a light and shadow image, one may form the latent image by
directly charging the layer in image configuration. The powder
image may be fixed to the photoconductive layer if elimination of
the powder image transfer step is desired. Other suitable
fixing means such as solvent or overcoating treatment may be
substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic
particles to the electrostatic latent image to be developed. One
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development method, as disclosed by E. N. Wise in U.S. Patent
2,618,522 is known as "cascade" development. In this method,
a developer material comprising relatively large carrier particles
having finely-divided toner particles electrostatically clinging
to the surface of the carrier particles is conveyed to and rolled
or cascaded across the electrostatic latent image-bearing surface.
The composition of the toner particles is so chosen as to have a
triboelectric polarity opposite that of carrier particles. As
the mixture cascades or rolls across the image-bearing surface,
the toner particles are electrostatically deposited and secured
to the charged portion of the latent image and are not deposited
on the uncharged or background portions of the image. Most of
the toner particles accidentally deposited in the background
are removed by the rolling carrier, due apparently, to the
greater electrostatic attraction between the toner and the carrier
than between the toner and the discharged background. The
carrier particles and unused toner particles are then recycled.
This technique is çxtremely good for the development of line
copy images. The cascade development process is the most widely
used commercial electrostatographic development technique. A
general purpose office copying machine incorporating this
technique is described in U.S. Patent 3,009,943.
Another technique for developing electrostatic latent
images is the "magnetic brush" process as disclosed, for example,
in U.S. Patent 2,874,063. In this method, a developer material
containing toner and magnetic carrier particles is carried by a
magnet. The magnetic field of the magnet causes alignment of the
magnetic carriers in a brush-like configuration. This "magnetic
brush" is engaged with an electrostatic-image bearing surface
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and the toner particles are drawn from the brush to the electro-
static image by electrostatic attraction.
In automatic reproduction equipment, it is conventional
to employ as the imaging plate, a photoconductor in a conductive
substrate in the form of a cylindrical drum or a flexible belt
which is continuously rotated through a cycle of sequential
operations including charging, exposing, developing, trans-
erring and cleaning. The developer chamber is charged with
developer mixture comprising carrier particles and enough toner
for hundreds of reproduction cycles. Generally, the freshly
charged developer mixtures contain between about 1.0 and 3.0% toner
toner based upon the weight o the developer. This initial
concentration provides sufficient toner for many reproduction
cycles without causing undesirably high background toner
deposition.
The imaging plate is usually given a uniform positive charge
by means of a corona generating device connected to a suitable
source of high potential as disclosed by L. E. Walkup in U.S.
Patent 2,777,957. The plate is then discharged in imagewise
configuration by exposure to a light image corresponding to the
original to be copies. The resultant latent image is then
brought into developing configuration with the developer mixture.
The relatively high electric fiald over the imaged areas
of the plate attracts the toner powder from the carrier
particles whereas, ideally the unimaged areas of the plate,
do not. Optimally the charged pattern on the imaged plate
corresponds to the light and dark areas of the original. However,
as explained in "Xerography and ~elated Processes", Dessauer
and Clark, The Focal Press, New York (19652, development
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fields of dark imaged areas which are very large compared with
the thickness of the photoconductive film are confined to the
edges of the images. In order to overcome this undesirable
effect, a conductive surface called a "development electrode"
is placed near the metal substrate of the imaged plate, either
with or without bias potential, to increase the electric field
above the large uniformly charged areas to aid in solid area
development and reduce background development. When such
electrodes are biased, as they usually are in commercially
available machines, a field is created between the plate and
the electrode which accurately represents the charge density
of the latent image. Ideally, such an electrode should be
held in virtual contact, since both development and background
suppression fields are increased with decreasing distance between
the electrode a~d plate. However, it is not practical to have
the development electrode in virtual contact with the plate in
development processes employing developer mixtures with solid
carrier particles.
After development the image is transferred to a copy
support surface such as paper, by electrostatically charging
the paper to cause it to attract the developed image. After
image transfer, the residual toner and carrier particles are
removed before the plate is reused in subsequent cycles. This
is generally accomplished by imparting an opposite charge to the
photoconductive surface thereby nullifying any electrostatic
attraction between the surface and the particles, then rubbing
the surface to physically remove all the remaining particles
and exposing it to light to fully discharge the surface.
It is known to employ coated and uncoated carrier
beads to prepare developer mixtures. However, most currently
available coated insulating carrier materials have an electrical
resistanc~ which is too high to produce excellent quality solid area
development. Therefore, recent efforts have been directed
toward the provision of uncoated conductive carrier particles
which have triboelectric and other physical properties rendering them
suitable for developer use. Since these carrier particles do
not require a coating to give them the proper electricals to
perform in automatic electrostatographic copy machines, they
are relatively easy to produce. Moreover, because of their
density and triboelectric properties, the carrier particles
produce prints of relatively high print density and relatively
low background development levels over wide ranges of toner
concentrations.
Though developer compositions comprising uncoated
carrier particles have been found to initially produce prints
of excellent quality, it has been observed that as toner is
depleted from the developer mixture and toner concentration
approaches low permissible levels, background development in-
creases and print quality is reduced. This problem is especially
prominent in copying machines employing magnetic brush develop-
ment techniques in which the magnetic roll has a bias potential
to make the roll perform as a development electrode. This
problem is particularly severe with uncoated metal carrier
particles. That is, when conductive carrier is employed and
conditions necessary for shorting are present, the conductive
carrier grounds out the development electrode and the resultant
shorting is manifested by heavy deposltion of background toner
due to a loss of background suppression bias potential on the
development electrode. This e~fect becomes catastrophic when
the concentration is low and the carrier is conductive.
PRIOR ART
A method of overcoming some of the aforementioned
problems is disclosed in U.S. Patent No. 3,533,835 issued to
R. J. Hagenbach et al which teaches the incorporation of finely
divided electrically conductive particulate material in at
least the surface of carrier substrates. The electrically
conductive particulate material employed therein preferably
has a volume resistivity of less than about 101 ohm-centimeters
and a maximum average particle size of less than about 15 microns.
It is also disclosed that the electrically conductive particulate
material is employed to alter the triboelectric properties of
carrier substrates. Factors affecting the quantity of conductive
particulate material to be incorporated in at least the surface
of carrier particles include: the separation in the triboelectric
series between the electroscopic marking particles and the carrier
material; the average particle size of the conductive particulate
additive; the concentration of the particulate conductive material
at the surface of the carrier particle; the average diameter of
the carrier particle; and the conductivity of the finely-divided
particulate additive. The finely-divided conductive particulate
material may be distributed only at the surface of a coated
or uncoated carrier particle or uniformly distributed throughout
an uncoated carrier particle or throughout the external coating
of a coated carrier particle.
In view of the ascending importance of carrier particles
having controlled triboelectric and conductive properties because of
the good print quality they are potentially capable of producing
and the limitations which developer mixtures containing carrier
particles presently have, it iS~an~~ ~ of an aspect of the
present invention to provide developer compositions having the
advantages, but not the disadvantages, attached to those
employing carrier particles.
It is an object of an aspect of the present invention
to provide a developer mixture containing conductive carrier
particles, especially particles fabricated from ferromagnetic
metals such as nickel, steel, or ferrites which particles are
much less dependent upon toner concentration for the produc-
10 tion of high quality prints than heretofore thought possibleand which are eminently suitable for use in a variety of
development processes.
It is an object of an aspect of this invention to
provide a developer mixture containing insulating carrier
particles which particles are much more resistant to carrier
coating degradation than heretofore known insulating carrier
particles.
It is an object of an aspect of this invention to
provide a developer mixture containing carrier particles
20 having improved triboelectric properties.
It is an object of an aspect of this invention to
provide developer materials having electrostatographic
properties superior to those of known developer materials.
Various aspects of the invention are as follows:
In an electrostatographic carrier particle
having a diameter of from between about 30 microns and
about 1000 microns, said carrier particle comprising a
core at least partially coat~d with an electrically
insulating resin, the improvement comprising that said
insulating resin is an amorphous polyamide having a
yield tenslle strength in an electrostatographic
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electrically insulating coated carrier particle having
a diameter of from between about 30 microns and about
1000 microns, said carrier particle comprising a core
having a substantially continuous coating of an
insulating resin being present in the amount of from
about 0.1 percent to about lO.0 percent by weight
based on the weight of said carrier particle, wherein
the improvement comprises that said insulating resin ;
is an amorphous polyamide having a yield tensile
strength of between about 10,700 psi at about 73F.
and about 5,130 psi at about 200 F., a tensile
modulus of between about 405,000 psi at about 73F
and about 175,000 psi at about 200F., a compressive
modulus of about 339,000 psi, and a Rockwell Hardness
at 73F. of between m-89 and m-93.
In an electrostatographic imaging process
comprising the steps of providing an electrostato-
graphic imaging member having a recording surface,
forming an electrostatic latent image on said record-
ing surface, and contacting said electrostatic latentimage with a developer mixture comprising finely-
divided toner particles electrostatically clinging to
the surface of carrier particles having a diameter of
from between about 30 microns and about 1,000 microns,
each of said carrier particles-comprising a core at
least partially coated with an electrically insulating
resin, wherein the improvement comprises that said
electrically insulating resin is an amorphous poly-
amide having a yield tensile strength in an electro-
statographic imaging process comprising the steps ofproviding an electrostatographic imaging member having
a recording surface, forming an electrostatic latent
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image on said recording surface, and contacting said
electrostatic latent image with a developer mixture
comprising finely-divided toner particles electro-
statically clinging to the surface of electrically
insulating coated carrier particles having a diameter
of from between about 30 microns and about 1,000
microns, each of said carrier particles comprising a
core having a substantially continuous coating of an
insulating resin being present in the amount of from
about 0.1 percent to about 10.0 percent by weight
based on the weight of said carrier particles, wherein
the improvement comprises that said insulating resin
is an amorphous polyamide having a yield tensile
strength of between about 10,700 psi at about 73F.
15 and about 5,130 psi at about 200 F., a tensile
modulus of between about 405,000 psi at about 73F.
and about 175,000 psi at about 200 F., a compressive
modulus of about 339,000 psi, and a Rockwell Hardness
at 73 F. of between m-89 and m-93, whereby at least a
20 portion of said finely-divided toner particles are
attracted to and deposited on said recording surface in
conformance with said electrostatic latent image.
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In a preferred embodiment where it is desired to
obtain a carrier particle having insulating properties, the
outer surface of coated or uncoated carrier beads is substantially
completely coated with the polyamides of this invention. It has
been found that such a coated carrier particle provides a
carrier particle having a useful life longer than here-to-known
coated insulating carrier materials. The coated insulating
carrier particles of this invention are characterized as
possessing mechanical toughness far superior to styrene-methacrylate-
siloxane terpolymers or al~yl methacrylate-styrene copolymers;
they obtain high degrees of coatability and coating integrity;
and possess excellent triboelectric properties even at high
relative humidity conditions.
In another preferred embodiment of this invention where
it is desired to obtain a carrier particle having conductive
properties, ferromagnetic carrier beads such as iron, steel,
ferrites, and nickel are partially coated with the polyamides.
It has been round that when such ferromagnetic carrier beads are
partially coated with the polyamides of this invention, that the
resultant carrier particles provide a carrier material having
conductive properties which enables good solid area development
when employed in an electrostatographic imaging system; the
conductive developer does not accumulate a net charge; and
the conductive developer has a rapid charging rate.
Any suitable amorphous polyamide may be employed to
completely or partially coat the carrier particles of this
invention. Typical polyamides include poly-2,2,4-trimethylhexa-
fra~/c ~a~
methylene terephthalamide available under the trad~n~c Trogamid
T from Dynamit Nobel of America, Inc., Norwood, New Jersey,
Amidel~ from Union Carbide Corporation, New York, New York;
Elvamide~ from E. I. duPont and Co., Wilmington, Delaware,condensation products of polymerized unsaturated fatty acids
with aliphatic amines available under the tradename Versamids
from General Mills, Inc., Chemical Div., Kankakee, Illinois;
poly-diacetone acrylamide; and poly-N, N-dimethylacrylamide,
including mixtures thereof. However, the preferred polyamide for
use in this invention is poly-2,2,4-trimethylhexamethylene tere-
phthalamide because its mechanical toughness is comparable to
Mylar~ polyester, J ~ polycarbonate, and polysulfone; it
possesses satisfactory and stable triboelectric properties; and
it provides a coating integrity of about 90 percent at about 0.2%
coating weight on 100 micron steel carrier cores. Further, the
amorphous polyamides of this invention have a yield tensile
strength of between about 10,700 psi at about 73~F and about
5,130 psi at about 200F; a tensile modulus of between about 405,000
psi at about 73F and about 175,000 psi at about 200F, both
values obtained according to ASTM test method D638; a compressive
modulus (modulus of elasticity) of about 339,000 psi according
to ASTM test method D695; a Rockwell Hardness at 73F of between
M-89 and M-93 according to ASTM test method D785; and a Taber
abrasion (CS 17 wheel, 1000 g. wt.) of 21 mg/1000 cycles according
to ASTM test method D1044.
As indicated, the carrier materials of the present
invention may have conductive and insulating surfaces and
may also be blended in the proper proportions to meet the
performance requirement of any given copying and duplicating
system. Conductive carrier materials are provided in accordance
with this invention by providing metals such as iron, steel,
nickel, zinc, aluminum, brass, copper, and the like, as well
as combined for~ms of such metals, with a partial coating of
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the polyamides thereon. Thus, a wide variety of particulate,
magnetically responsive materials the surface of which may be
oxidized including materials in such forms as steel and iron
particles produced by atomization of molten metal and subsequent
cooling of the droplets; particles produced by grinding, milling,
filing, turning etc; as well as particles of steel and iron
alloys having oxidizable iron on the surface thereof such as
stainless steel and iron alloys containing nickel and/or cobalt
may be employed in accordance with this invention. Insulating
carriers may be of any of the above-mentioned materials having
a surface coating comprising the polyamides herein capable of
forming a substantially continuous coating thereon. Alternatively,
the insulating carrier particles may be fabricated from an
insulating material with a surface polyamide coating thereon.
Further, conductive and insulating carrier particles made
in accordance with this invention can be admixed in such
proportions as to give a carrier composition with an overall
surface resistivity of between about 104 ohm-cm and about 1015
ohm-cm.
The polyamide coating produced upon the conductive
carrier particles of this invention may be any suitable thickness
or weight percent. However, a polyamide layer sufficiently
distributed at or near the surface to produce at least semi-
conductive electrical characteristics is preferred because the
carrier particles will then possess desirable triboelectric
features and allow their application in electrode development
systems where RC (Resista~ce-Capacitance) time constant
considerations are of importance in preventing high electrical
discharges between the development electrode and the photoreceptor.
Preferably the polyamide coating should be so distributed
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through the preparation so that the conductive carrier, where
measured in the bead aggregate, will possess a volume resistivity
between about 105 ohm-cm and 1012 ohm-cm. Carrier particles
having a partial surface polyamide coating and conductive
properties are highly desirable with respect to their use in
magnetic brush development electrostatographic copying and
duplicating devices. When employed with finely divided toner
particles to develop electrostatic latent images in a
magnetic-brush development apparatus, they have been found to
provide developed images having lower background densities and
higher resolution than prior known carrier materia~s. Although
not wishing to be bound by any theory, it is believed that the
improved results obtained are due to the conductive properties
of the carrier materials. The carrier particles may vary in
size and shape, however, the size of the carrier particles
employed will, of course, depend upon several factors, such
as the type of images ultimately developed, the machine
configuration, and so forth.
Many of the foregoing and other typical carrier
materials are described by L.E. Walkup in U.S. Patent 2,618,551;
B.B. Jacknow et al in U.S. Patent 3,526,533; and R.J. Hagenbach
et al in U.S. Patents 3,533,835 and 3,658,500. When the carrier
particles of this invention are coated with a polyamide, any
suitable electrostatographic carrier coating thickness or
weight may be employed. However, a carrier coating having a
thickness at least sufficient to form a thin continuous film
on the carrier particle is preferred when a completely coated
insulating caxrier particle is desired because the carrier
coating will then possess sufficient thickness to resist abrasion
and prevent pinholes which adversely affect the triboelectric
properties of the coated carrier particles. Generally, for
cascade and magnetic brush development, the carrier coating
may comprise from about 0.1 percent to about 10.0 percent by
weight based on the weight of the coated carrier particles.
Preferably, the carrier coating should comprise from about 0.5
percent to about 1.0 percent by weight based on the weight of
the coated carrier particles because maximum insulation, coating
durability, toner impaction resistance, and copy quality are
achieved.
- Toner materials which positively or negatively
triboelectrify the carrier materials herein can be used in the
present invention. The choice of material depends upon the
charging character of the photoconductor and whether positive
or reversal development is desired. Thus, to produce a positive
print from a positively charged selenium photoconductive member, a
carrier should be chosen which imparts a negative charge to the
toner material, whereas for reversal development of the same
photoconductor the carrier material should impart a positive
charge to the toner. In the development of other photoconductive
surfaces which accept a negative charge such as poly(vinyl
carbazole) or zinc oxide, the reverse holds true. Thus, for
positive development of such surfaces, the carrier material
should impart a positive charge to the toner material, whereas
for reversal development the carrier material should impart a
negative charge to the toner material.
The carrier particles of the present invention should
have an average particle diameter of between about 30 microns
to about 1,000 microns, preferably between about 50 to about
500 microns. When used in magnetic brush development systems,
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they must obviously be fabricated of or at least contain a
magnetic material such as iron, steel, nickel, cobalt or
ferromagnetic oxides. If development systems such as cascade
development or "shell" development systems as disclosed in
U.S. Patent 3,503,776 are employed, the carrier material need
not have ferromagnetic properties. In this case, a carrier
material such as zinc which provides excellent triboelectri-
fication of toner for development of selenium photoconductors,
is suitably employed.
Developer mixtures of the present invention are
particularly useful in reproduction systems employing a
development electrode and magnetic brush development rolls.
In practical use, magnetic rolls with a bias potential perform
the function of both the magnetic brush and the development
electrode. In a typical situation in which a selenium
photoconductor is developed by a plurality of magnetic rolls
bearing a developer mixture comprising between about 0.75 and
3~ of a pigmented polymeric toner (e.g., a 10~ carbon black
dispersion in a polymeric matrix comprising a blend of a
styrene-n-butyl methacrylate copolymer with poly(vinyl butyral),
the photoconductor bears a positive charge of about 800 volts
in dark image areas and about 100 volts in non-imaged areas.
The development electrode, i.e., the magnetic brushes, is
biased with a potential of about 200 volts. The spacing between
the development electrode and the photoconductive surface is
sufficient to prevent any shorting out between the development
electrode and the plate, but not so large as to have the
developer mixture outside of the development electric field of
the charged plate. Generally, development electrode-photoconductor
spacings of about 0.04 to 0.12 inches suffice to prevent un-
due damage to the photoconductor surface while bringing the
development electrode in sufficient proximity to the photo-
conductor surface. Using carrier beads having at least a
partially conductive surface in accordance with the present
invention, it is noted that the density of developed dark
image areas is much greater with respect to background density
than when carrier beads having an insulating surface are used
in otherwise identical conditions. It is theorized that this
is due to the formation of conductivé paths which effectively
extend the development electrode closer to the photoconductor
surace thereby effectively increasing both external development
and background suppression fields. This effect is absent
when insulating carrier beads are substituted under identical
operating conditions. Moreover, as the toner concentration
decreases from the initial level of above about 2% minimum
levels for acceptable print density to about 0.75~ of the developer
mixture, this effect is hardly diminished. Thus, the carrier
materials of the present invention are capable of providing
excellent print quality, i.e., high print density and low
background development, over wide toner concentration ranges.
Contrary to the relatively wide toner concentration
latitude available with the partially polyamide coated conductive
carrier materials of the present invention, developer mixtures
containing only uncoated conductive carrier particles, e.g.,
nickel carrier beads, produce prints of widely varied quality,
depending upon whether toner concentration is at the high or
low end of the usable toner concentration range, and whether
the photoconductor surface has any flaws through which
shorting between the development electrode and the conductive
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backing, may occur. Under ideal conditions, the photoconductor
surface is free of any imperfections; however, in practice,
photoconductor surfaces frequently are scratched through
frictional contact of developer material with machine parts.
A fresh developer mixture containing at least about 2~
toner and only completely uncoated conductive carrier beads
having an average particle diameter of about 100 microns,
produces prints of excellent quality. However, as toner
concentration decreases, the density of developed areas
begins to sharply decrease while the level o~ background
development sharply increases. It is theorized that this
effect is due to the lengthening of conductive paths and
the consequent increased chance of shorting between the
development electrode and the conductive layer of the electro~
statographic plate, since the toner, which is normally not
conductive, acts as an insulator to interrupt conductive
paths when toner concentration is sufficiently high; whereas
it does not, when toner concentration decreases. Thus, in
developer mixtures based upon completely uncoated conductive
carrier beads, toner concentration of a used developer mixture
may be sufficiently high to produce acceptable print density
under ideal conditions, but too low to prevent shorting out
between the development electrode and the grounded photoconductive
surface. However, with the carrier materials of this invention
the conductive properties of the carrier materials never
reach a conduction level as to cause shorting due to the
limited conductivity of the carrier particles.
By employing carrier particles having a partially
conductive surface and toner particles having a good triboelectric
relationship with respect to each other, the overall development
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capability of the developer mixture can be greatly increased
over that of either an insulating or completely conductive
carrier particle alone. The apparent mechanism for this
beneficial effect is that sufficient conductive paths are
formed through the partially conductive carrier-to-partially
conductive carrier contacts. If these partially conductive
paths are long enough and numerous enough, a virtual electrode
(at the potential of the development electrode) is formed in
the region between the photoconductor and the development
electrode. Since the distance from the photoconductor surface
to the development electrode is effectively decreased, the
electric fields which control, to a great extent, the
deposition of toner, are increased. Increasing the development
field increases the density of the developed image. Increasing
the background suppression field reduces the development of
background. To gain this effect the proportion of conductive
surface areas of the carrier particles to insulating surface
areas of the carrier particles should be great enough so that
conductive paths of sufficient length and number are formed
to increase the electric field. To obtain the foregoing
results, between about 10~ and about 90% of the surface area
of the carrier materials of the present invention should be of
the type having a conductive surface. The optimum proportion
of conductive to insulating carrier surface area should be
such that when the carrier particles are mixed with the minimum
amount of toner which produces prints of acceptable density,
the maximum length of uninterrupted conductive paths is less
than the spacing between the development electrode and the
photoconductor surface. In cases where the development electrode-
photoconductor spacing is in the above-noted range, it has been
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noted that the surface area of carrier particles having a
conductive surface should be about 25 percent to about 75
percent of the total carrier particles of the developer mixture.
Any suitable well known toner material may be
employed with the carrier materials of this invention. Typical
toner materials include gum copal, gum sandarac, rosin,
cumaroneindene resin, asphaltum, gilsonite, phenolformaldehyde
resins, rosin modified phenolformaldehyde resins, methacrylic
resins, polystyrene resins, polypropylene resins, epoxy resins,
polyethylene resins, polyester resins, and mixtures thereof. The
particular toner material to be employed obviously depends
upon the separation of the toner particles from the carrier
materials in the triboelectric series and should be sufficient
to cause the toner particles to electrostatically cling to
the carrier surface. Among the patents describing electroscopic
toner compositions are U.S. Patent 2,659,670 to Copley; U.S.
Patent 2,753,308 to Landrigan; U.S. Patent 3,079,342 to
Insalaco; U.S. Patent Reissue 25,136 to Carlson and U.S. Patent
2,788,288 to Rheinfrank et al. These toners generally have an
average particle diameter between about 1 and 30 microns.
Any suitable colorant such as a pigment or dye
may be employed to color the toner particles. Toner colorants
are well known and include, for example, carbon black,
nigrosine dye, aniline blue, Calco Oil Blue, chrome yellow,
ultramarine blue, Quinoline Yellow, methylene blue chloride,
Monastral Blue, Malachite Green Ozalate, lampblack, Rose
Bengal, Monastral Red, Sudan Black BM, and mixtures thereof.
The pigment or dye should be present in a quantity sufficient
to render it highly colored so that it will form a clearly
visible image on a recording member. Preferably, the pigment
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is employed in an amount from about 3 percent to about 20
percent by weight based on the total weight of the colored
toner because high quality images are obtained. If the toner
colorant employed is a dye, substantially smaller quanitities
of colorant may be used.
Any suitable conventional toner concentration may be
employed with the carrier materials of this invention. Typical
toner concentrations for development systems include about 1
part toner with about 10 to about 200 parts by weight of carrier.
The carrier materials of the instant invention may be
mixed with finely divided toner particles and employed to develop
electrostatic latent images on any suitable electrostatic latent
image-bearing surface including conventional photoconductive
surfaces. Typical inorganic photoconductor materials include:
sulfur, selenium, zinc sulfide, zinc oxide, zinc cadmium sulfide,
zinc magnesium oxide, cadmium selenide, zinc silicate, calcium
strontium sulfide, cadmium sulfide, mercuric iodide, mercuric
oxide, mercuric sulfide, indium tri-sulfide, gallium selenide
arsenic disulfide, arsenic trisulfide, arsenic triselenide,
antimony trisulfide, cadmium sulfoselenide, and mixtures thereof.
Typical organic photoconductors include: quinacridone pigments,
phthalocyanine pigments, triphenylamine, 2,4-bis(r,r'-diethylamino-
phenol)-1,3,4-oxadiazole, N-isopropylcarbazole, triphenylpyrrole,
4,5-diphenylimidazolidinone, 1,5-dicyanonaphthalene,
1,4-dicyanonaphthalene, aminophthalodinitrile, nitrophthalo-
dinitrile, 1,2,5,6-tetra-azecyclooctatetraene-(2,4,6,8),
2-mercaptobenzothiazole-2-phenyl-4-diphenylidene-oxazolone,
6-hydroxy-2,3-di(p-methoxyphenyl)-benzofuran~e, 4-dimethylamino-
benzylidene-benzhydrazide, 2-benzylidene-aminocarbazole,
polyvinyl carbazole, (2-nitrobenzylidene)-p-bromoaniline,
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2,4-diphenyl-quinazoline, 1,2,4-triazine, 1,5-diphenyl-3-
methyl-pyrazoline,2-(~'-dimethylamino phenyl)-benzoxazole,
3-amine-carbazole, and mixtures thereof. Representative patents
in which photoconductive materials are disclosed include U.S.
Patents 2,803,542 to Ullrich, U.S. Patent 3,121,007 to Middleton,
and U.S. Patent 3,151,982 to Corrsin.
The carrier materials of this invention provide
numerous advantages when employed to develop electrostatic
latent images. For example, when mixed with a toner material
the resultant developer composition is found to greatly reduce
the development electrode effect of a biased magnetic brush
and there is no opportunity for strong transient currents
occuring to damage photoreceptor surfaces or produce copy
defects. Thus, it is not possible to short circuit and draw
heavy currents between development electrodes and the photoreceptor
through the electrical path offered by these carrier materials.
Further, the conductive carrier particles herein provide
excellent solid area development; there is no net charge
accumulation resulting therefrom when employed in a developer
mixture; and the developer mixture has a rapid triboelectric
charging rate. Further still, this invention enables the
use of low cost irregular magnetic carrier core materials such
as sponge iron which is very difficult to completely coat,
yet provides a carrier material having excellent electro-
statographic properties. Therefore, developer compositions
employing these carrier materials are found to provide lower
background densities, higher image resolutions, and greatly
improved overall print qualities. Further, a wide range of
xerographic properties ma~ be derived from these carrier
materials since xerographic properties may be controlled based
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on the polyamide coated surface areas of carrier paxticles.
The following examples, other than the control
examples, further define, describe, and compare preferred
methods of preparing and utilizing the carrier materials of
the present invention in electrostatographic applications.
Parts and percentages are by weight unless otherwise indicated.
In the following examples, the relative triboelectric
values generated by contact of carrier beads with toner particles
is measured by means of a Faraday Cage. The device comprises
a brass cylinder having a diameter of one inch and a length
of one inch. A 100-mesh screen is positioned at each end
of the cylinder. The cylinder is weighed, charged with 0.5 grams
of a mixture of carrier and toner particles and connected
to ground through a capacitor and an electrometer connected
in parallel. Dry compressed air is then blown through the
brass cylinder to drive all the toner from the carrier. The
charge on the capacitor is then read on the electrometer. Next,
the chamber is reweighed to determine the weight loss. The
resulting data is used to calculate the toner concentration and the
charge in micro-coulombs per gram of toner. Since triboelectric
measurements are relative, the measurements should, for
comparative purposes, be conducted under substantially identical
conditions. Thus, a toner comprising a styrene-n-butyl
methacrylate copolymer, polyvinyl butyral, and carbon black by
the method disclosed by M.A. Insalaco in Example I of U.S.
Patent 3,079,342 is used as a contact triboelectrificiation
standard in all the examples. Obviously other suitable toners
such as those listed above may be ~ubstituted for the toner
used in the examples.
.
EXAMPLE I
A control developer mixture was prepared by mixing
about 6 grams of toner particles comprising a styrene-n-butyl
methacrylate copolymer and carbon black with about 300 grams
of carrier particles. The carrier particles comprised steel
shot having an average particle size of about 100 microns which
is commercially available from Nuclear Metals, Wes~ Concord, ~ass.
(Division of Whittaker Corporation). The developer mixture was
employed to develop electrostatic latent images in a copying
machine equipped with a magnetic brush development device. The
magnetic brush development fixture comprised 1 magnetic roll,
which transported developer to the electrostatic image area
which had been previously formed on the flat plate photocon-
ductive imaging surface of the fixture. During the test,
extreme powder cloud effect was found. Background densities
were approximately half of the solid area densities as measured
with a Welch densitometer. Image resolution was very poor.
It was concluded that this developer mixture was unsatisfactory.
EXAMPLE II
A developer mixture was prepared by mixing about 6
grams of toner particles as in Example I with about 300 grams
of carrier particles. The carrier particles were as in
Example I except that the steel shot was coated with about
0.8 percent by weight, based on the weight of the carrier
particles, of a coating composition comprising about 15
parts of styrene, 85 parts of methyl methacrylate, and 1 part
of vinyltriethoxysilane. The developer mixture was employed
to develop electrostatic latent images as in Example I. To
maintain a developed image line density of no less than 1.2, which
is considered acceptable, it was found that the toner concentration
could not be less than about 1.22 percent of the developer
mixture. At the line density of 1.2, it was found that the
solid area density of the developed iamge was only 0.17. The
background density of the developed images was acceptable. The
relative triboelectric value of the carrier material
measured by means of a Faraday Cage was about 29 micro-coulombs
per gram of toner.
EXAMPLE III
A developer mixture was prepared by mixing about 6
grams of toner particles as in Example I with about 300 grams
of carrier particles. The carrier particles were as in Example
I except that the steel shot was coated with about 0.2
percent by weight, based on the weight of the particles, of
a coating composition comprising poly-2,2,4-trimethylhexamethylene
terephthalamide (Trogamid T, available from Dynamit Nobel
of America Inc., Norwood, New Jersey). The carrier particles
were found to have a coating integrity of about 90 percent
of the surface area which is unusally high for this type of
carrier core material. The developer mixture was employed
to develop electrostatic latent images as in Example I. The
copy quality and developer life obtained was found to be
comparable to that obtained with the developer mixture of
Example II. The relative triboelectric value of this carrier
material measured by means of a Faraday Cage was about 25
micro-coulombs per gram of toner.
EXAMPLE IV
. .
A developer mixture was prepared by mixing about
6 grams of toner particles as in Example I with about 300
grams of carrier particles. The carrier particles were as
in Example I except that the steel shot was coated with about
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0.4 percent by weight, based on the weight of the particles,
of a coating composition comprising poly-2,2,4-trimethylhexamethylene
terephthalamide (Trogamid T, available from Dynamit Nobel
of America Inc., Norwood, New Jersey). The developer mixture
was employed to develop electrostatic latent images as in
Example I. The copy quality and developer life obtained was
found to be superior to that obtained with the developer
mixture of Example II. The carrier particles were found
to show a considerably slower rate of copy quality degradation
than those of Example II. In addition, these carrier
particles consistently gave less deposition o toner particles
in background areas than those of Example II. The relative
triboelectric value of this carrier material measured by
means of a Faraday Cage was about 35 micro-coulombs per gram
of toner.
EXAMPLE V
A developer mixture was prepared by mixing about 6
grams of toner particles as in Example I with about 300 grams
of carrier particles. The carrier particles were as in
Example I except that the steel shot was coated with about
0.9 percent by weight, based on the weight of the particles,
of a coating composition comprising poly-2,2,4-trimethylhexamethylene
terephthalamide (Trogamid T, available from Dynamit Nobel
of America Inc., Norwood, New Jersey). The carrier particles
were found to have a coating integrity of substantially 100
percent of the carrier surface area. The developer mixture
was employed to develop electrostatic latent images as in
Example I. The copy quality and developer life obtained was
found to be superior to that obtained with the developer
mixture of Example II. The carrier particles were found to show
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a considerably slower rate of copy quality degradation than
those of Example II. In addition, these carrier particles
consistently gaye less deposition of toner particles in back-
ground areas than those of Example II. The relative triboelectric
value of this material measured by means of a Faraday Cage
was about 43 micro-coulombs per gram of toner.
EXAMPLE ~I
A developer mixture was prepared as in Example IV
except that the toner particles comprised styrene, alkyl
methacrylate, carbon black, and a quaternary ammonium salt.
The amount of toner particles and carrier particles, the carrier
core, coating material and the amount of carrier coating were
the same as in Example IV. This developer mixture was employed
to develop electrostatic latent images as in Example I, but
in a reversal mode. The copy quality and developer life
obtained was found to be comparable to that obtained with the
developer mixture of Example IV. The relative triboelectric
of the carrier material measured by means of a Faraday Cage
was about -25 micro-coulombs per gram of toner.
Although specific components, proportions and
procedures have been stated in the above description of the
preferred embodiments of the novel carrier system, other
suitable materials, as listed above, may be used with similar
results. Further, other materials and procedures may be
employed to synergize, enhance or otherwise modify the novel
system.
other modifications and ramifications of the present
invention will appear to those skilled in the art upon the
reading of a disclosure. These are intended to be included
within the scope of this invention.
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