U.S. patent number 3,923,503 [Application Number 05/489,631] was granted by the patent office on 1975-12-02 for electrostatic latent image development employing steel carrier particles.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Robert J. Hagenbach.
United States Patent |
3,923,503 |
Hagenbach |
December 2, 1975 |
Electrostatic latent image development employing steel carrier
particles
Abstract
An electrostatographic developer mixture comprising
finely-divided toner particles electrostatically adhering to the
surface of highly shape-classified low carbon hypereutectoid steel
carrier particles, the steel carrier particles comprising carrier
cores having a composition comprising about 95.0 to 99.0 parts
iron, 0.1 to 2.0 parts carbon, 0.5 to 2.0 parts manganese, 0.5 to
2.0 parts silicon, 0.03 to 0.1 parts aluminum, 0.01 to 0.05 parts
phosphorus, and 0.02 to 0.05 parts sulphur, said carrier cores
having been heat treated to a tempered martensitic microstructure
with an average hardness of 40-55 Rockwell C and then overcoated
with a coating material capable of electrostatically releasing
toner particles adhered thereto to an electrostatic latent image.
Processes of employing said developer mixture in
electrostatographic imaging systems are also disclosed.
Inventors: |
Hagenbach; Robert J.
(Rochester, NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
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Family
ID: |
27004420 |
Appl.
No.: |
05/489,631 |
Filed: |
July 18, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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369036 |
Jun 11, 1973 |
3849182 |
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834862 |
Jun 19, 1969 |
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Current U.S.
Class: |
430/123.58;
430/111.35; 430/111.34; 430/121.1 |
Current CPC
Class: |
G03G
9/1075 (20130101); G03G 9/113 (20130101); G03G
9/1136 (20130101); G03G 9/1133 (20130101) |
Current International
Class: |
G03G
9/113 (20060101); G03G 9/107 (20060101); G03g
013/08 () |
Field of
Search: |
;96/1SD ;117/17.5,27,1M
;252/62.1P ;75/.5BA,124,.5R |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Metals Handbook," 1948, American Society for Metals, pp.
308-311..
|
Primary Examiner: Torchin; Norman G.
Assistant Examiner: Miller; John R.
Parent Case Text
This is a division, of application Ser. No. 369,036, filed June 11,
1973 now U.S. Pat. 3,849,182, which is a continuation-in- part of
U.S. Ser. No. 834,862 filed on June 19, 1969, now abandoned.
Claims
What is claimed is:
1. A method of forming a visible toner image comprising the steps
of forming an electrostatic latent image on a recording surface and
contacting said electrostatic latent image with a developer mixture
until at least a portion of said toner particles are attracted to
and held on said surface in conformance to said electrostatic
latent image, said developer mixture comprising about one part by
weight of finely-divided toner particles electrostatically adhering
to the surface of about 10 parts to about 200 parts by weight of
highly shape-classified low carbon hypereutectoid steel carrier
particles having an average particle size between about 50 microns
and about 1,000 microns and a specific gravity of at least about 7
grams per cubic centimeter, said steel carrier particles having
been heat treated to a tempered martensitic microstructure with an
average hardness of 40-55 Rockwell C in an air or oxygen atmosphere
at temperatures ranging between about 500.degree.F and about
1,000.degree.F to provide an oxide surface layer comprising
Fe.sub.3 O.sub.4 on said steel carrier material, said oxide surface
layer having a thickness between about 0.2 and about 5.0 microns,
said steel carrier particles comprising carrier cores having a
composition comprising about 95.0 to about 99.0 parts iron, about
0.1 to about 2.0 parts carbon, about 0.5 to about 2.0 parts
manganese, about 0.5 to about 2.0 parts silicon, about 0.03 to
about 0.1 parts aluminum, about 0.01 to about 0.05 parts
phosphorus, and about 0.02 to about 0.05 parts sulphur, said
treated carrier particles being overcoated with a resinous coating
material capable of electrostatically releasing said
electrostatically adhering toner particles to said electrostatic
latent image.
2. A method according to claim 1 wherein said carrier cores
comprise about 96.0 to about 98.0 parts iron, about 0.85 to about
1.5 parts carbon, about 0.9 to about 1.2 parts manganese, about 0.9
to about 1.2 parts silicon, about 0.035 to about 0.045 parts
aluminum, about 0.015 to about 0.05 parts phosphorus, and about
0.03 to about 0.05 parts sulphur.
3. A method according to claim 1 wherein said carrier cores
comprise about 96.9 parts iron, about 1.0 parts carbon, about 1.0
parts manganese, about 1.0 parts silicon, about 0.04 parts
aluminum, about 0.015 parts phosphorus, and about 0.035 parts
sulphur.
4. A method according to claim 1 wherein said highly
shape-classified steel carrier particles comprise at least about 95
percent by weight of round carrier particles.
5. A method according to claim 1 wherein said highly
shape-classified steel carrier particles are essentially free of
non-round carrier particles.
6. A method according to claim 1 wherein said highly
shape-classified steel carrier particles are obtained by
mechanically separating round carrier particles from
irregularly-shaped particles through controlled vibration.
7. A method according to claim 1 wherein said highly
shape-classified steel carrier particles are obtained by separating
non-round particles from round carrier particles by employing a
double-spiral separator.
8. A method according to claim 1 wherein at least about 95 percent
of said steel carrier particles have been classified as rounds.
9. A method according to claim 1 wherein said carrier coating
material comprises a styrene-methacrylate-organo-silicon
terpolymer.
Description
BACKGROUND OF THE INVENTION
This invention relates in general to an imaging system and, more
particularly, to an electrostatographic material.
Electrostatography is best exemplified by the process of xerography
as first described in U.S. Pat. No. 2,297,691 to C. F. Carlson. In
this process a photoconductor is first provided with a uniform
electrostatic charge over its surface and is then exposed to an
image of activating electromagnetic radiation which selectively
dissipates the charge in illuminated areas of the photoconductor
while the charge in the non-illuminated areas is retained thereby
forming an electrostatic latent image. This electrostatic latent
image is then developed or made visible by the desposition of
finely-divided electroscopic marking particles 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 powdered
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 fusing. Instead of forming latent images
by uniformly charging a photoconductive layer and then exposing the
layer to a light-and-shadow image, an image may be formed by
directly charging the layer or an insulating member in image
configuration. The powder image may be fixed to the imaging layer
if elimination of the powder image transfer step is desired. Other
suitable means such as solvent or overcoating treatment may be
substituted for the foregoing heat fixing steps.
Several methods are known for applying the electroscopic particles
to the electrostatic latent image to be developed. One well-known
commercial method for developing electrostatic images is the
"cascade" process disclosed by L. E. Walkup in U.S. Pat. No.
2,618,551 and E. N. Wise in U.S. Pat. No. 2,618,552. In this method
a developer material comprising relatively large carrier beads
having fine toner particles electrostatically coated thereon is
conveyed to or rolled or cascaded across the electrostatic
image-bearing surface. The composition of the carrier particles is
so chosen as to triboelectrically charge the toner particles to the
desired polarity. As the image cascades or rolles across the
image-bearing surface, the toner particles are electrostatically
deposited and secured to the charged portion of a latent image and
are not deposited on the uncharged or background portion of the
image. Most of the toner particles accidentally deposited in the
background areas are removed by the rolling carrier, due apparently
to the greater electrostatic attraction between the toner and
carrier than between the toner and the discharged background. The
carrier and excess toner are then recycled.
In most commercial processes the cascade technique is carried out
in automatic machines. In these machines small buckets on an
endless belt conveyor scoop the developer mixture comprising
relatively large carrier beads and smaller toner particles and
convey to a point above an electrostatic image-bearing surface
where the developer mixture is allowed to fall and roll by gravity
across the image-bearing surface. The carrier beads along with any
unused toner particles are then returned to the sump for recycling
through the developing system. Small quantities of toner material
are periodically added to the developer mixture to compensate for
the toner depleted during the development process. This process is
repeated for each copy produced in the machine and is ordinarily
repeated many thousands of times during the usable life of the
developer mixture. It is apparent that in this process, as well as
in other development techniques, the developer mixture is subjected
to a great deal of mechanical attrition which tends to degrade both
the toner and carrier particles. This degradation, of course,
occurs primarily as a result of shear and impact forces due to the
tumbling of the developer mixture on the image-bearing plate and
the movement of the bucket conveyor through the developer material
in the sump.
In prior art processes both coated and uncoated carrier beads were
employed with varying degrees of success. However, coated carrier
beads are subject to deterioration or degradation characterized by
the separation of portions of or the entire carrier coating from
the carrier core. The separation may be in the form of chips,
flakes, or entire layers and is primarily caused by poorly adhering
coating materials which fail upon impact and abrasive contact with
machine parts and other carrier particles. Carriers having coatings
tend to chip and otherwise separate from the carrier core and must
be frequently replaced, thereby increasing expense and consuming
time. Print deletion and poor print quality occur when carrier
particles having damaged coatings are not replaced. Fines and grit
formed by the carrier coating disintegration 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 poor print quality.
Uncoated carrier beads, on the other hand, have three main
deficiencies. First, often they lack the weight required to insure
against adherence of the granular carrier material to the charged
plate. Desirably, the specific gravity of the carrier material
should be between about 3 and about 8. Heavier carrier bead
materials cause impact damage to the surface of the image bearing
layer. Secondly, the prior art uncoated carrier materials lacked
the triboelectric qualities required of an electrostatographic
material. Problems encountered when carrier materials lack these
properties are set out in the following discussion. In the
reproduction of high contrast copies such as letters, tracings, and
the like, it is desirable to select the electroscopic powder and
carrier materials so that their mutual electrification is
relatively large; the degree of such electrification being
determined in most cases by the distance between their relative
positions in the triboelectric series. However, when otherwise
compatible electroscopic powder and carrier materials are removed
from each other in the triboelectric series by too great a
distance, the resulting images are very faint because the
attractive forces between the carrier and toner particles compete
with the attractive forces between the electrostatic latent image
and the toner particles. Although the image density described in
the immediately preceding sentence may be improved by increasing
the toner concentration in the developer mixture, undesirably high
background toner deposition as well as increased toner impaction
and agglomeration is encountered when the developer mixture is
overtoned. The initial electrostatographic plate charge may be
increased to improve the density of the deposited powdered image
but the plate charge would ordinarily have to be excessively high
in order to attract the electroscopic powder away from the carrier
particle. Excessively high electrostatographic plate charges are
not only undesirable because of the high power consumption
necessary to maintain the electrostatographic plate at high
potentials but also because a high potential causes the carrier
particles to adhere to the electrostatographic plate surface rather
than merely roll across and off the electrostatographic plate
surface. Print deletion and massive carryover of carrier particles
often occur when carrier particles adhere to reusable
electrostatographic imaging surfaces. Massive carrier carryover
problems are particularly acute when the developer is employed in
solid area coverage machines where excessive quantities of toner
particles are removed from carrier particles thereby leaving many
carrier particles substantially bare of toner particles. Further,
adherence of carrier particles to reusable electrostatographic
imaging surfaces promotes the formation of undesirable scratches on
the surfaces during image transfer and surface cleaning operations.
It is, therefore, apparent that many materials which otherwise have
suitable properties for employment as carrier particles are
unsuitable because they possess too high a triboelectric value.
Desirably, the triboelectric value for conventional
electrostatography measured in micro-coulombs per gram of toner
should be between 8 and 30.
Finally, the triboelectric value of a carrier material should not
be significantly affected by ambient humidity conditions since such
effect would destroy print quality at higher humidities and
complicate machine design and operation. Many prior art uncoated
materials were never commercially successful because of their great
humidity sensitivity.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a system
for developing electrostatic latent images which overcome the
above-noted deficiencies.
It is another object of this invention to provide a system for
developing electrostatic latent images which is relatively
insensitive to humidity conditions.
It is another object of this invention to provide a carrier
material which is relatively resistant to abrasion.
It is another object of this invention to provide a carrier
material which has relatively improved triboelectric
properties.
It is another object of this invention to provide a carrier
material which has a relatively high density.
It is still another object of this invention to provide a carrier
core material which allows for greater adhesion of carrier coating
materials.
It is yet another object of this invention to provide a coated
carrier material which is not subject to deterioration of
degradation.
It is a further object of this invention to provide a carrier
material wherein the carrier coating does not separate from the
core material during machine operation.
The foregoing objects and others are accomplished in accordance
with this invention by providing highly shape-classified low carbon
hypereutectoid steel beads which have been heat treated to a
tempered martensitic microstructure with an average hardness of
40-55 Rockwell C for use as an electrostatographic carrier core.
The highly shape-classified low carbon hypereutectoid steel bead
electrostatographic carrier cores of this invention comprise about
95.0 to 99.0 parts iron, 0.1 to 2.0 parts carbon, 0.5 to 2.0 parts
manganese, 0.5 to 2.0 parts silicon, 0.03 to 0.1 parts aluminum,
0.01 to 0.05 parts phosphorus, and 0.02 to 0.05 parts sulphur. One
of the desirable properties of this type of core material is that
the slight porosity of its surface area improves and yields greater
adhesions of polymer coatings. Another desirable property of the
electrostatographic carrier core materials of this invention is the
presence of a rough surface thereon obtained by the aforementioned
heat treatment in an air or oxygen atmosphere at temperatures
ranging between about 500.degree.F and about 1000.degree.F. The
heat treatment of the core material provides oxidized carrier cores
having a surface oxide layer comprising Fe.sub.3 0.sub.4 of a
thickness between about 0.2 and about 5.0 microns. It has been
found that thus oxidized low carbon hypereutectoid steel carrier
beads when subsequently coated with polymer materials provide
coated electrostatographic carrier beads having increased coating
to core adhesion resulting in longer machine life, less gritting
characteristics, and permitting faster machine speeds. Particularly
good adhesion of carrier coatings to the core material is obtained
with an oxidized surface carrier core composition comprising about
96.0 to 98.0 parts iron, 0.85 to 1.5 parts carbon, 0.9 to 1.2 parts
manganese, 0.9 to 1.2 parts silicon, 0.035 to 0.045 parts aluminum
0.015 to 0.05 parts phosphorus, and 0.03 to 0.05 parts sulphur.
Optimum adhesion of carrier coatings to core material is obtained
with an oxidized surface carrier core composition comprising about
96.9 parts iron, 1.0 part carbon, 1.0 part manganese, 1.0 part
silicon, 0.04 parts aluminum, 0.015 parts phosphorus, and 0.035
parts sulfur.
The polymer coated carrier beads of this invention have a minimum
density of about 7.0 grams per cubic centimeter.
The raw low carbon hypereutectoid steel beads when received as
manufactured generally are not satisfactory for use as
electrostatographic carrier cores because they usually contain at
least about 30 percent by weight of non-round materials. The raw
steel beads are generally manufactured by the so-called Rotating
Electrode Process or in an electric arc furnace well known in the
metals processing industry. In the Rotating Electrode Process, a
rapidly rotating electrode, on the order of 5,000 to 12,000 RPM, of
the metal to be spheroidized is made molten by an electric arc from
an opposing stationary tungsten electrode. Centrifugal forces throw
off the molten droplets of metal which solidify in a chosen
controlled atmosphere, for example, helium, before hitting the
walls of a cylindrical collection chamber surrounding the
electrodes. Thus, although spheroidal particles are produced, a
mixture of round and irregularly shaped particles generally result,
Non-round materials are undesirable because they contain slag,
hollow particles, chip particles, and flat particles which cause
variations in electrostatographic carrier bead density resulting in
carrier bead sticking to electrostatographic photoreceptor drum
surfaces causing print deletions, scratches on the photoreceptor
drum, and non-uniform developer triboelectric properties. In
addition, undersized particles create machine leakage problems and
oversized particles lessen the amount of developability, that is,
edge definition and background levels suffer in quality.
Consequently, the raw steel beads must be sized and classified for
usage as electrostatographic carrier cores.
In accordance with this invention, one method of obtaining highly
shape-classified steel carrier cores is by mechanically separating
round from irregularly-shaped beads through controlled vibration
such as by employing a vibrating table set at a predetermined
slope. A particularly useful vibrating table for classifying round
beads in the 100 micron size range is TAFA Model 41-5 Vibrating
Table available from TAFA Division, Humphreys Corp., Bow-Concord,
New Hampshire. For separating 250 and 450 micron round carrier core
material from non-round materials, a particularly useful method is
by employing a Krussow Double Spiral Separator available from
Cleland Manufacturing Company, Minneapolis, Minnesota. For
obtaining the highly shape-classified 250 micron and 450 micron
steel carrier cores, respectively, of this invention, it was found
that the 3 1/2 spiral width and the 5 1/2 spiral width,
respectively, of the aforementioned separator provided better than
95.0 percent rounds and the removal of all flakes and non-rounds
with no appreciable loss of round material.
It has been found that the highly shape-classified low carbon
hypereutectoid oxidized steel carrier beads of this invention when
coated with a polymer coating provide satisfactory results in an
electrostatographic device when at least about 95 percent of the
carrier beads have been classified as rounds. For even better
results, it is preferred that at least about 97 percent of the
carrier beads have been classified as rounds. Optimum results in an
electrostatographic device are obtained when at least about 98
percent of the carrier beads have been classified as rounds. As an
indication of the value of shape-classified carrier beads, the
voltage required to adhere carrier beads of various roundness
percentages was measured by rolling developer mixtures prepared
therefrom down an electrode inclined at 25 degrees. It was found
that as the percentage of round classified carrier beads increased,
so did the voltage required to adhere the carrier beads to the
electrode. For example, at about 25 percent rounds, the voltage
required was about 360; at about 50 percent round, the voltage
required was about 440; at about 75 percent rounds, the voltage
required was about 570; and at about essentially 100 percent
rounds, the voltage required was about 860. Thus, as the percentage
of round carrier beads increases so does the voltage required to
cause carrier bead sticking on an electrically charged surface such
as a photoreceptor drum. Therefore, when employing highly
shape-classified carrier beads in an electrostatographic
development system, carrier bead sticking is minimized,
photoreceptor surface scratching and damage is likewise minimized,
and print quality is greatly improved.
After classification, the carrier cores may be coated with any
suitable carrier coating material. Typical carrier coating
materials include natural resin, thermoplastic resin, or partially
cured thermosetting resin. Typical natural resins include:
caoutchouc, colophony, copal, dammar, dragon's blood, jalep,
storax, and mixtures thereof. Typical thermoplastic resins include:
the polyolefins such as polyethylene, polypropylene, chlorinated
polyethylene, and chlorosulfonated polyethylene; polyvinyls and
polyvinylidenes such as polystyrene, polymethylstyrene,
polymethylmethacrylate, polyacrylonitrile, polyvinylacetate,
polyvinylalcohol, polyvinylbutyral, polyvinylchloride,
polyvinylcarbazole, polyvinyl ethers, and polyvinyl ketones;
fluorocarbons such as polytetrafluoroethylene, polyvinylfluoride,
polyvinylidene flouride; and polychlorotrifluoroethylene;
polyamides such as polycoproloctamo and polyhexamethylene
adipimide; polyesters such as polyethylene terephthalate;
polyurethanes; polysulfides; polycarbonates; and mixtures thereof.
Typical thermosetting resins include: phenolic resins such as
phenol formaldehyde, phenol furfural and resorcinol formaldehyde;
amino resins such as urea formaldehyde and melamine formaldehyde;
polyester resins; epoxy resins; and mixtures thereof. A
styrene-methacrylate-organosilicon terpolymer carrier coating
composition such as described in U.S. Pat. No. 3,526,533 is
particularly preferred because of its excellent triboelectric
characteristics. The highly shape-classified oxidized carrier core
materials of this invention may be coated with carrier coating
material by any conventional carrier coating technique, such as,
for example, the technique described in U.S. Pat. No.
2,618,551.
After the highly shape-classified carrier beads of this invention
are coated with carrier coating material, the coated carrier beads
are screened to remove any agglomerates formed during the coating
process. The screen mesh sizes employed naturally depend upon the
carrier bead size desired. The thus-obtained carrier beads are now
ready for mixing with an appropriate toner material to provide a
developer mixture.
Any suitable pigmented or dyed electroscopic toner material may be
employed with the carriers of this invention. Typical toner
materials include: gum copal; gum sandarac; rosin; cumaromeindene
resin; asphaltum; phenol formaldehyde resins; rosin modified phenol
formaldehyde resins; methacrylic resins; polystyrene resins;
polypropylene resins; epoxy resins; polyethylene resins; polyester
resins; and mixtures thereof. The particular toner material to be
employed usually depends upon the separation of the toner particles
from the carrier beads in the triboelectric series and whether a
negatively or positively charged image is to be developed. Among
the patents describing electroscopic toner compositions are U.S.
Pat. No. 2,659,670 to Copley, U.S. Pat. No. 2,753,308 to Landrigan,
U.S. Pat. No. 3,079,342 to Insalaco, U.S. Patent Re-issue 25,136 to
Carlson, and U.S. Pat. No. 2,788,288 to Rheinfrank et al. These
toners generally have an average particle diameter between about 1
and 30 microns. A toner comprising a styrene-N-butyl methacrylate
copolymer, polyvinylbutyral and carbon black produced by the method
disclosed by M. A. Insalaco in Example I of U.S. Pat. No. 3,079,342
is preferred because of its excellent triboelectric qualities and
its deep black color.
An ultimate coated carrier particle diameter between about 50
microns to about 1,000 microns is preferred because the carrier
particles then possess sufficient density and inertia to avoid
adherence to the electrostatic images during the cascade
development process. Adherence of carrier beads to
electrostatographic drum surfaces is undesirable because of the
formation of deep scratches on the surface during the image
transfer and drum cleaning steps, particularly where cleaning is
accomplished by a web cleaner such as the web disclosed by W. P.
Graff, Jr., et al in U.S. Pat. No. 3,186,838. Also, print deletion
occurs when carrier beads adhere to electrostatographic imaging
surfaces. Generally, speaking, satisfactory results are obtained
when about 1 part toner is used with about 10 to about 200 parts by
weight of carrier.
The developer compositions of the instant invention may be employed
to develop electrostatic latent images on any suitable
electrostatic latent image-bearing surface including conventional
photoconductive surfaces. Well-known photoconductive materials
include vitreous selenium, organic or inorganic photoconductors
embedded in a non-photoconductive matrix, organic or inorganic
photoconductors embedded in a photoconductive matrix, and the like.
Representative patents in which photoconductive materials are
disclosed include U.S. Pat. No. 2,803,542 to Ullrich, U.S. Pat. No.
2,970,906 to Bixby, U.S. Pat. No. 3,121,006 to Middleton, U.S. Pat.
No. 3,121,007 to Middleton, and U.S. Pat. No. 3,151,982 to Corrsin.
The developer compositions of this invention provide improved print
quality such as image sharpness, solid area coverage, and character
fill-in when employed in electrostatographic machines.
DESCRIPTION OF PREFERRED EMBODIMENTS
The following examples further define, describe, and compare
methods of preparing developer materials of the present invention
and of utilizing them to develop electrostatic latent images. Parts
and percentages are by weight unless otherwise indicated.
EXAMPLE I
Highly shape-classified 100 micron steel carrier cores are obtained
by the following method. A TAFA Model 41-5 vibrating table
available from TAFA Division, Humphreys Corp., Bow-Concord, New
Hampshire, is placed on a firm bench or table. Using an
inclinometer, such as available from Berger Instruments, Boston,
Mass., the table slope is adjusted to provide about 2.degree.
decline from back to front and about 1.5.degree. decline from the
feed-side to the discharge-side. The vibration amplitude rheostat
is set at about 32. The table is cleaned with a camel hair brush to
remove dust which impedes the flow of carrier core. A feeder, such
as Syntron Model EB-70 Parts Feeder available from Syntron
Division, FMC Corporation, Homer City, Pa., isolated from table
vibration, is regulated to provide a steady material flow with good
dispersion across the vibrating table. The two carrier core
fractions are collected in the respective round and non-round
collection containers. In this way, 100 micron carrier core
particles are mechanically separated into round and irregularly
shaped beads as to provide highly shape-classified carrier
cores.
EXAMPLE II
Highly shape-classified 250 micron steel carrier cores are obtained
by the following method. The as-received carrier cores are poured
into a gravity separator, such as the Krussow Double Spiral
Separator available from Cleland Manufacturing Company,
Minneapolis, Minnesota, employing a 3 1/2 spiral width. The rounds
are consolidated into one container. In this way, highly
shape-classified carrier core particles being at least about 95.0
percent rounds are obtained and flaked, non-round, irregularly
shaped particles are separated from the starting material with no
appreciable loss of round material.
EXAMPLE III
A highly shape-classified 250 micron coated carrier material is
prepared as follows. The carrier core material comprises about 96.9
parts iron, 1.0 part carbon, 1.0 part manganese, 1.0 part silicon,
0.04 parts aluminum, 0.15 parts phosphorus, and 0.035 parts
sulphur. The surface of the carrier core material has an oxide
layer comprising Fe.sub.3 O.sub.4 of about 1.0 micron in thickness
obtained by heat treating the carrier cores at about 900.degree.F
in an air atmosphere for about 6 hours. The oxidized carrier core
material was exposed to a 0.05 M CuSO.sub.4 solution to ascertain
the degree of surface oxidation. Theoretically, the copper should
plate any area of the carrier core not having an oxide coating. By
the foregoing, only a few carrier beads became plated with copper.
The carrier beads were then classified to obtain at least about
95.0 percent rounds in accordance with Example II. The highly
shape-classified carrier beads were then coated with about 0.28
percent by weight, based on the weight of the beads, of the methyl
terpolymer coating composition disclosed in U.S. Pat. No. 3,526,533
and wherein the coating composition contained about 8.0 percent by
weight, based on the weight of the coating composition, of DuPont
Oil Red dye. The highly shape-classified coated carrier beads are
then screened to remove any agglomerated particles. The classified
and screened coated carrier cores are then evaluated for carrier
coating abrasion resistance according to the following procedure:
Approximately 25.0 grams of coated carrier beads are weighed to the
nearest 0.1 milligram and placed in a 250 ml stainless steel beaker
fitted with a rubber stopper. The beaker and the carrier beads are
agitated on a Red Devil Paint shaker for 15 minutes. The stopper is
removed and any particles adhering thereto are carefully washed
into the beaker with a wash bottle. 100 ml of distilled water and
two drops of Triton X-100, a nonionic wetting agent available from
Rohm & Haas, Philadelphia, Pa., are added to the beaker. The
mixture is agitated for 30 seconds at 7 amps using a Branson
Sonifier. The mixture is carefully filtered through a 5 micron
cellulose, millipore filter weighed to the nearest 0.1 milligram.
The mixture is washed with two additional 100 ml portions of
distilled water and filtered. The filter paper is dried to constant
weight in a circulating air oven at 60.degree.C. The coating weight
removed is determined by subtracting the original filter weight
from the final weight. The coating weight removed was found to be
about 17.0 percent.
A developer mixture is prepared by mixing about 100 parts of the
highly shape-classified coated carrier beads obtained with about
one part of toner material comprising styrene-n-butyl methacrylate
copolymer, polyvinylbutyral and carbon black produced by the method
disclosed in Example I of U.S. Pat. No. 3,079,342 having an average
particle size of about 10 to about 20 microns. The developer
mixture is cascaded across an imaging surface bearing a positively
charged electrostatic image. The resulting developed image is
transferred by electrostatic means to a sheet of paper whereon it
is fused by heat. The resulting image is dense and substantially
free of background toner deposits. After the development of about
13,000 images, the carrier material is examined. The material
overcoating the carrier core is found not to have flaked,
separated, or deteriorated in any manner.
EXAMPLE IV
A coated carrier material is prepared as follows. The carrier core
material comprises about 96.9 parts iron, 1.0 part carbon, 1.0 part
manganese, 1.0 part silicon, 0.04 parts aluminum, 0.15 parts
phosphorus, and 0.035 parts sulphur. The carrier core material is
not heat-treated as in Example III and has substantially no oxide
layer thereon. The carrier core material was exposed to a 0.05 M
CuSO.sub.4 solution to determine the degree of surface oxidation.
Substantially all of the carrier core material became plated with
copper indicating substantially no surface oxidation. The carrier
core material was then classified, coated, screened, and evaluated
for carrier coating abrasion resistance as in Example III. The
coating weight removed was found to be about 73.0 percent.
A developer mixture is produced by mixing about 100 parts of the
coated carrier beads with about one part of the toner material
described in Example III and employed as in Example III to develop
an imaging surface bearing a positively charged electrostatic
image. After development of several hundred images, the carrier
material is examined. The material overcoating the carrier core is
found to have flaked, separated, and deteriorated. The developed
images are found to have hollow characters, poor edge definition,
and contain an unsatisfactory level of background deposits. These
results show that an oxidized carrier substrate provides a much
stronger coating/core bond providing much improved carrier coating
adhesion with resulting improved print quality.
EXAMPLE V
The carrier material of Example III was evaluated for carrier
coating abrasion resistance as in Example III except that the
carrier cores were coated with 0.2 percent and 0.6 percent coating
weights. The object was to determine the effect of coating weight
on abrasion resistance. The results, based on the abrasion test,
indicate that carrier coating abrasion resistance is not influenced
by coating weight.
EXAMPLE VI
250 micron size carrier core material comprising about 98.9 parts
iron, 0.1 parts carbon, 0.5 parts manganese, 0.5 parts silicon,
0.03 parts aluminum, 0.01 parts phosphorus, and 0.02 parts sulphur
was oxidized in an air atmosphere under various conditions in a
Lindberg Hevi-Duty Oven to determine the effect on carrier coating
abrasion resistance. All trials were run at 500.degree.C with the
only variable being time of treatment. The as-received core
material was poured into a crucible and placed in the oven. The
power for the oven was switched on and the temperature controls
were set for 500.degree.C. When the control temperature was
reached, the treatment was begun. After the desired treatment time,
the crucible was removed from the oven and allowed to air cool
overnight. Oxidation times allowed were 1, 3, and 7 hours. Samples
of the as-received carrier core material and the carrier core
materials air-oxidized for 1, 3, and 7 hours, respectively, were
then coated with 0.28 percent by weight, based on the weight of the
carrier cores, with the coating composition of Example III. The
four sets of carrier materials were evaluated for carrier coating
abrasion resistance in accordance with the procedure set forth in
Example III. It was found that the as-received coated core sample
provided the poorest carrier coating abrasion resistance losing
about 71.0 percent total coating after 15 minutes agitation. The
core material sample air-oxidized for one hour lost about 54.0
percent total coating after 15 minutes agitation; the core material
sample air-oxidized for three hours lost about 55.0 percent total
coating after 15 minutes agitation; and the core material sample
air-oxidized for 7 hours lost about 43.0 percent total coating
after 15 minutes agitation. Thus, a significant improvement in the
coating adhesion to carrier core was observed in the air-oxidized
samples.
Although specific components and proportions have been stated in
the above description of preferred embodiments of the invention,
other typical materials as listed above where suitable may be used
with similar results. In addition, other materials may be added to
the mixture to synergize, enhance, or otherwise modify the
properties of the carrier beads. For example, a material to improve
the sphericity of the beads may be incorporated during
manufacture.
The expressions "developer" and "developing material" as employed
herein are intended to include electroscopic toner material or
combinations of toner material and carrier material.
Other modifications and ramifications of the present invention will
occur to those skilled in the art upon a reading of this
disclosure. These are intended to be included within the scope of
this invention.
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