Note: Descriptions are shown in the official language in which they were submitted.
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BACKGRO~ND OF T~ INVENTION
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This invention relates in general to electrophotography
and more particularly, to a process for preparing carrier materials
useful in the magnetic~brush type 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 the 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 subse~uently 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 clectrostatic 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 extremely good for the development of line
copy images. The cascade development process is the most widely
used commercial electrostatographic development technique. A
yeneral purpose office copying machine incorporating this
technique is described in U. S. Patent 3,099,943.
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 ~o the electro-
static image by electrostatic attraction.
In magnetic-brush development of electrostatic images,
the developer is commonly a triboelectric mixture of finely-
divided toner powder comprised of dyed or pigmented thermoplastic
resin with coarser carrier particles of a soft magnetic material
such as "ground chemical iron" (iron filings), reduced iron oxide
particles or the like. The conductivity of the ferromagnetic
carrier particles which form the "bristles" of a magnetic brush
provides the effect of a development electrode having a very close
spacing to the surface of the electrophotographic element being
developed. By virtue of this development electrode effect it is
to some extent possible to develop part of the tones in pictures
and solid blacks as well as line copy. This brush developing
sometimes makes this mode of developing advantageous where it is
desired to copy materials other than simple line copy. One
difficulty with such counter-electrode development is that the
exposure latitude obtainable is limited. Consequently, for
certain applications, it is desirable to suppress the counter-
electrode effect in order to obtain improved exposure latitude.
One method of suppressing the counter-electrode effect is to use
a carrier material which has a high electrical resistance.
It is well-known to provide a carrier material which has
a high electric resistance. A conventional method of obtaining a
carrier material which has a high electrical resistance is to
apply a coating of insulating material to a carrier substrate.
However, coated carrier beads are subject to deterioration when
the coating separates from the carrier core as the carrier particles
are repeatedly impacted and abraded against machine parts and
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other carrier particles. The separated coatings form chips and
flakes which cause print deletion and poor print quality; and
fines and grit which tend to drift and form unwanted deposits
on critical machine parts. In addition, the triboelectric
properties of the carrier material vary with deterioration of
the coating, resulting in inefficient carrier performance.
Further, most currently available insulated ferro-
magnetic carrier particles having an electrical resistance which
is too high to produce good quality solid area development. When
not insulated, they are too conductive to entirely suppress the
counter-electrode effect. One reason for the lack of uniformity
in the electrical properties of uncoated carrier particles is that
the particles carry surface dirt, such as grease, oil, and other
contaminants. It has been suggested in the prior art that typical
conductive carrier particles can be treated with methanol, iso-
propanol, and other alcohols to rid the carrier of grease, oil,
etc.
However, the prior art has not recognized the hetero-
geneous nature of the surface of the uncoated carriers used in
this art. In addition, no recognition has been given to the
resulting complications in magnetic-brush processing which arise
from using some of the presently available materials. The extraneous
surface dirt found on the available carrier particles is only part
of the problem. In addition to the dirt, the carrier particles
invariably carry a non-uniform distribution of oxide. When
materials of this type are used in conventional magnetic brushes,
the carrier is in continuous motion and the constant friction of the
particles against one another and against the various mechanical
parts gradually remove bits of this non-uniform surface material.
The attrition will, of course, vary from point to point on a
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carrier particle depending on the nature of the deposit, its
thickness, friability, adhesion to the underlying structure, and
the like. Thus, with the available carrier materials, a progressive
change occurs in the character of the carrièr surface during use.
Correspondingly, the nature of the admixed toner also changes as it
becomes contaminated with the fine particles which have separated
from the surface of the carrier.
Accordingly, there is a need for electrostatographic
carriers which will present a more nearly homogeneous surface and
which will have stable triboelectric and other properties even
during continued use. Furthermore, there is a need for carrier
particles which will enhance the counter-electrode effect and thus
improve the solid-area type development obtainable in magnetic
brush processing. Also, there is a need for carrier particles
which can repress the counter-electrode effect so that fringing
development will be induced with its accompanying improvement in
exposure latitude.
It is, therefore, an object of an aspect of this
invention to provide a method for preparing electrostatographic
carrier materials which overcomes the above noted deficiencies.
It is an object of an aspect of this invention to
provide a simple economic process for preparing magnetically
responsive carrier particles having a homogeneous surface.
It is an object of an aspect of this invention to
provide a process for preparing magnetically responsive carrier
particles which have semi-conductive properties.
An object of an aspect of this invention is to provide
new developer compositions for use in magnetic brush development.
It is an object of an aspect of this invention to
provide developer materials having physical and electrical
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properties superior to those of known developer material.
In accordance with one aspect of this invention
there is provided a magnetically-responsive, semi-
conductive electrostatographic carrier particle having an
average particle diameter of from between about 30 microns
and about 1,000 microns, said carrier particle having
been prepared by firing metallic, magnetic particles in
an atmosphere having an oxygen content of between about 1
percent and about 18 percent by weight at a temperature
of between about 900F and about 1600F for between about
10 minutes and up to about 2 hours whereby said particles
are provided with an oxide coating having a thickness of
between about 0.25 micron and.about 5 microns.
In accordance with another aspect of this inven-
15 tion there is provided an electrostatographic developer
mixture comprising finely-divided toner particles electro-
statically, clinging to the surface of a carrier, said
carrier comprising magnetically-responsive, semi-
conductive electrostatographic carrier particles ha~ing
an average partiale diameter of from between ahout 30
microns and about 1,000 microns, said carrier particles
having been prepared by firing metallic, magnetic
parti.cles in an atmosphere having an oxygen content of . .
between about 1 percent and about 18 percent by weight
25 at a temperature of between about 900F and about
1600F for between about 10 minutes and up to about 2 . -
hours whereby said carrier particles are.provided with
an oxide coating having a thickness of between about
0.25 micron and about 5 microns.
In accordance with another aspect of this inven-
tion there is provided an electrostatographic imaging
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process comprising the steps of providing an electro- . .
statographic imaging member having a recording surface,
forming an electrostatic latent image on said recording
surface, and contacting said electrostatic latent image
with a developer mixture comprising finely-divided
toner particles electrostatically clinging to the
surface of a carrier, said carrier comprising magnetic-
ally-responsive, semi-conductive electrostatographic
carrier particles having an average particle diameter of
from~between about 30 microns and about 1,000 microns,
said carrier particles having been prepared by firing
metallic, magnetic particles in an atmosphere having an
oxygen content of between about 1 percent and about
18 percent by weight at a temperature of between about
900F and about 1600F for between about 10 minutes and
up to about 2 hours whereby said carrier particles are
provided with an oxide coating having a thickness of
between about 0.25 micron and about 5 microns, whereby
at least a portion of said finely-divided toner
particles are attracted to and deposited on said record-
ing surface in conformance with said electrostatic
latent image.
In accordance with another aspect of this
invention there is provided a process for preparing
magnetically-responsive, semi-conductive electrostato-
graphic carrier particles having an average particle
diameter of from between about 30 microns and about
1,000 microns, said process comprising firing metallic,
magnetic particles in an atmosphere having an oxygen
content of betwee~ about 1 percent and about 18 percent
by weight at a temperature of between about 900F
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and about 16000F for between about 10 minutes and up to
about 2 hours whereby said carrier particles are provided
with an oxide coating having a thickness of between about
0.25 micron and about 5 microns.
By way of added explanation, in accordance with
an aspect of this invention steel shot carrier particles
are fired in an oxygen deficient atmosphere as to provide
the carrier particles with an oxide coating. That is,
steel carrier particles are fired in a controlled atmos-
phere under preselected conditions of time and temperature
~as to generate a suitable oxide coating on the particles.
More particularly, commercially available steel shot is
placed in a furnace wherein the atmosphere has an oxygen
content of less than about 18 percent oxygen and the
steel shot is fired at a temperature of between about 900F
and about 1600F for a period of time of between about 10
~inutes and up to about 2 hours. Steel shot carrier par-
ticles fired in accordance with this invention are found to
possess a surface oxide layer which is highly desirable
with respect to their use in magnetic brush development
electrostatographic copying and duplicating devices.
More specifically, steel shot carrier particles prepared in
accordance with this invention when employed with finely-
divided toner particles to develop electrostatic latent
images in a magnetic-brush development apparatus have been
found to provide developed images having lower background
densities and higher resolution than prior known carrier
materials. Although not wishing to be bound by any theory,
it is believed that the improved results obtained are due
to the semi-conductive properties of the carrier materials
prepared by the process of this invention.
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The carrier materials which are suitable for treatment
in accordance with this invention include ferromagnetic materials
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such as steel and iron powder in various forms. Thus, a wide
variety of particulate, magnetically responsive materials the
surface of which can 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 treated in accordance with this
invention. The treated ferromagnetic carrier particles may vary
in size and shape with useful results being obtained with average
particle sizes of from about 30 microns to about 1,000 microns.
Particularly useful results are obtained with average particle
sizes from about 50 microns to about 500 microns. 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.
In accordance with this invention, semi-conductive
magnetically-responsive carrier particles having satisfactory
electrostatoyraphic properties may be obtained by firing ferro-
magnetic particles in an atmosphere containing between about 1
percent and about 18 percent by weight of oxygen at a temperature
of between about 900F and about 1600F for between about 10
minutes and about 2 hours. However, it is preferred that the
carrier particles of this invention be obtained by firing ferro-
magnetic particles in an atmosphere containing between about 1
percent and about 12 percent by weight of oxygen at a temperature
of between about 900F and about 1500F for between about 30
minutes and about 60 minutes to control oxide species and quality.
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optimum results are obtained when the carrier particles of this
invention are fired in an atmosphere containing between about 4
percent and about 10 percent by weight of oxygen at a temperature
of between about 965F and about 1470F for between about 30
minutes and about 60 minutes.
Any suitable type of furnace may be employed to fire
the carrier particles of this invention. Typical furnaces include
a static furnace, a rotary kiln, a tunnel kiln, or an agitated
bed furnace. The static furnace type will generally provide for
long residence times, while the rotary kiln and agitated bed type
of furnace generally provide more uniform product reaction, con-
sistent residence time, and high capacity throughout. During the
treatment of the carrier particles as described, it is highly
desirable to agitate the particles at least occasionally, so as
to completely expose the surfaces of the particles for more uniform
treatment thereof. In any event, a furnace providing a controlled
firing temperature and controlled atmosphere profile provides a
more controlled method of preparing the carrier particles.
The oxide coating produced upon the carrier particles
of this invention may be any suitable thickness or weight percent.
However, an oxide sufficiently distributed at or near the surface
to produce semi-conductive electrical characteristics is preferred
because the carrier will then possess desirable triboelectric
features and allow their application in electrode development sys-
tems where RC (Resistance-Capacitance) time constant considerations
are of importance in preventing high electrical discharges between
the development electrode and the photoreceptor. Preferably the
oxide should be so distributed through the preparation so that the
carrier, where measured in the bead aggregate, should possess a
volume resistivity between about 105 ohm-cm and 1012 ohm-cm. An
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oxide coating having a thickness of between about 0.25 micron
and about 5 microns has been found to provide satisfactory
results.
Any suitable well known toner material may be employed
with the oxide coated carriers 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 oxide coated carrier in the tribo-
electric 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 plgment 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 ~il Blue, chrome yellow, ultramarine blue,
Quinoline Yellow, methylene blue chloride, Monastral Blue,
Malachite Green Gzalate, 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.
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Preferably, the pigment 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
quantities of colorant may be used.
Any suitable conventional toner concentration may be
employed with the oxide coated carriers 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 sur-
faces. 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, txiphenylamine, 2,4-bis(4,4'-diethylamino-
phenol)-1,3,4-oxadiazol, N-isopropylcarbazole, triphenylpyrrole,
4,5-diphenylimidazolidinone, 4,5-diphenylimidazolidinethione,
4,5-bis-(4'amino-phenyl)-imidazolidinone, 1,5-dicyanonaphthalene,
1,4-dicyanonaphthalene, aminophthalocinitrile, nitrophthalo-
dinitrile, 1,2,5,6-tetra-azacyclooctatetraene-(2,4,6,8),
2-mercaptobenzothiazole-2-phenyl-4-diphenylidene-oxazolone,
6-hydroxy-2,3-di(p-methoxyphenyl)-benzofurane, 4-dimethylamino-
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benzylidene-benzhydrazide, 3-benzylidene-aminocarbazole,
polyvinyl carbazole, (2-nitrobenzylidene)-p-bromoaniline,
2,4-diphenyl-quinazoline, 1,2,4-triazine, 1,5-diphenyl-3-
methyl-pyrazoline, 2-(4'-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 3,121,0Q7 to Middleton,
and U. S. Patent 3,151,982 to Corrsin.
The oxide coated carrier materials produced by the
process of this invention provide numerous advantages when employed
to develop electrostatic latent images. For example, when mixed
with an appropriate toner material, the resultant developer
composition is found to greatly reduce the development electrode
effect of a magnetic brush. In addition, such a developer
composition induces fringing development with an accompanying
increase in exposure latitude. Further, such developer compositions
are found to provide lower background densities, higher image
resolutions, and greatly improved overall print qualities.
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 about one inch and a length
of about one inch. ~ 100-mesh screen is positioned at each end
of the cylinder. The cylinder is weighed, charged with about 0.5
gram 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 electrometex. Next, the
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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 the 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
and carbon black as disclosed by M. A. Insalaco in U. S. Patent
3,079,342 is used as a contact triboelectrification standard.
Obviously, other suitable toners such as those listed above may
be substituted for the toner used in the examples.
The following examples, other than the control examples,
further define, describe, and compare preferred methods of pre-
paring and utilizing the oxide coated carriers of the present
invention in electrostatographic applications. Parts and percen-
tages are by weight unless otherwise indicated.
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, West Concord, Mass.
(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. Said
magnetic brush development fixture comprised 5 magnetic rolls,
in series, which transported developer to the electrostatic image
area which had been previously formed on the flat plate photo-
conductive imaging surface of the fixture. During the test,
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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 they were treated as follows: the carrier particles
were placed in a muffle furnace and heated to about 800C for about
one hour in an atmosphere containing about 5 percent of oxygen.
After this treatment, the carrier particles were allowed to cool
to ambient temperature and examined. The treated carrier particles
were found to be gray-black in color and to have an oxide coating
of between about 0.25 micron and about 5 microns in thickness.
The carrier particles were mixed with toner particles and employed
to develop electrostatic latent images as in Example I. The
oxidized carrier developer composition was found to exhibit much
better qualities than that of Example I. That is, this developer
composition provided images having good resolution with very
good print quality. Background densities were very low, that is,
on the order of about 0.002 at very high (1.3 - 1.4) solid area
densities. A slight powder cloud effect was found at a solid area
density of 1.35. The electrical characteristics of the carrier
material were found to be within the desired semiconductive
range and determined to be acceptably stable over a wide range
of environmental humidity conditions as shown in Figure 1. Thus,
reasonable assurance is provided for triboelectric charging
capability and favorable RC development electrode field relaxation
characteristics over a practical environmental humidity range.
It was concluded that this developer mixture was satisfactory.
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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 they were treated as follows: the carrier particles
were placed in a high temperature rotary furnace and heated to
about 500C for about 30 minutes in an atmosphere containing
about 10 percent of oxygen. After this treatment, the carrier
particles were allowed to cool to ambient temperature and examined.
The treated carrier particles were found to be gray-black in color
and to have an oxide coating of between about 0.25 micron and
about 1 micron in thickness. The carrier particles were mixed
with toner particles and employed to develop electrostatic
latent images as in Example I. The oxidized carrier developer
composition was found to exhibit much better qualities than that
of Example I. That is, this developer composition provided images
having good resolution with very good print quality. Background
densities were very low at very high (1.3 - 1.4) solid area
densities. The electrical characteristics of the carrier material
were found to be within the desired semi-conductive range and
determined to be acceptably stable over a wide range of
environmental humidity conditions as shown in Figure 2. It was
concluded that this developer mixture was satisfactory.
Other modifications of the present invention will occur
to those skilled in the art upon a reading of the present dis-
closure. These are intended to be included within the scope
of this invention.
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