Note: Descriptions are shown in the official language in which they were submitted.
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13AC~iGROUND OF TIIL INVF,ilTIOII
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q'his invention relates in general to ~lectrostatography,
and, in particularl to a proccss for the production of improved
ferrite electrostatographic carrier materials and their use.
The formation and developmen-t of imaqes on the surface
of photoconductor materials by electrostatic means is well known.
The basic electrostatographic imaging process, as taught by
C. F. Carlson in U. S. Patent No. 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 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 lay~r which retain a charge, thereby forming a toner
image corresponding to the electrostatic latent imageO This
powder image may then be trans~erred to a support surface such as
paper. The ~ransferred 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
~onfiguration. The powder image may be fixed to the photocon-
ductive 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 steps~
Several methods are known for applying the electro
scopic particles to the electrostatic latent image to be developed.
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One development method, as disclosed by E. ~. Wise in U. S.
Patent No. 2,61B,552, is known as "cascade" development. In
this method, a developer material comprising relatively large
carrie particles having finely-divided toner particles electro-
~tatically coated thereon is conveyed to and rolled or cascaded
across the electrostatic latent image bearing surface. The
composition of the carrier particles is so selected as to
tri~oelectrically charge the toner particles to the desired
polarity. As the mixture cascades or rolls across the image
bearing surface, the toner particles are electrostatically
deposlted 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 accidentially
deposited in the background are removed hy the rolling carrier,
duP apparently, to the greater electrostatic attraction between
the toner and the carrier than between the toner and the
discharged background. The carrier and excess toner are then
recycled. This technique is extremely good for the development
of line copy images.
Rnother method of developing electrostatic latent
images is the "magnetic brush" developmen~ process as disclosed
or examplet in U. $. Patent No. 2,874,063. In this method, a
developer material containing toner and magnetic carrier
particles are carried by a magnet. The magnetic field of the
magnet causes alignment of the magnetic carrier into a brushlike
configuration. This "magnetic brush" is engaged with the electro-
static image-bearing surface and the toner particles are drawn
from the brush to the latent image by electrostatic attraction.
Thus, a developer mixture may be provided comprising a toner
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material a~d a carrier matcrial which consists of particles
WhiCIl arc magnetically attractable. Consequently, iron and
magnetic ferri~c materi.als have.been employed as the carrier
material in the electrostato~raphic arts.
Generally, in cascade or magnetic brush development
typical carrier core materials include sodium chloride, ammonium
chloride, aluminum potassium chloride, Rochelle salt, sodium
nitrate, potassium chlorate, granular zircon, granular silicon
methyl methacrylate, glass, silicon dioxide, flintshot, iron,
steel, ferrite, nickel, carborundum and mixtures thereof. Many
~f the foregoing and other typical carriers are described by
L. E. Walkup in U. S. Patent No. 2,618,551; L. E. Wallcup et al.
in U. S, Patent No. 2,638,416 and E. N. Wise in U. S. Patent No.
2,618,552. Generally, an average carrier particle.diameter
between about 30 microns to about 1,000 microns is preferred for
electrostatographic use because the carrier particle then possesses
sufficient density and inertia to avoid adherence to the
electrostatic latent images during the cascade development
process. In magnetic brush development, the ferrite carrier
materials are generally homogenous, rounded or irregularly shaped
particles having nominal particle sizes less than about 300
microns and more preferably between about 50 and 150 microns, the
latter size range providing optimum image quality during
extended useO
Ferrite materials ar~ gaining ever increasing importance
in the electronics industry and in the electrostatographic arts.
Their use as low conductivity magnetic core materials and as
carrier materials for photoconductive insulating materials is well
known. Briefly, ferrites may be described in general as compounds
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of magnetic oxidcs containing iron as ~ major metallic component.
Thus, compounds o ferric oxide, Fe203, formed with basic metallic
oxides having the gener~l formula MEIeO2 or MFe20~ where M
represents a mono or divalent metal and the iron is in the
oxidation state of +3 are ferrites. Ferrites are also referred
to as ferrospinels since they have the same crystal structure
of the mineral spinel MgA120~. }Iowever, not all ferrites are
magnetic such as, for example, ZnFe204 and CdFe20~. This lack
of magnetic property is due to the configuration of the ferrite
lattice structure, Further, some ferrites, such as magnetobarite,
BaFel2019, which exhibit permanent magnetic properties are
referred to as "hard" ferrites. A "hard" ferrite is difficult
to magnetize and demagnetize and thus is the type of ferrite that
is desirable in a permanent magnet. A "soft" ferrîte has the
opposite property; it is easily magnetized and demagnetized.
The "soter" the ferrite material is, the better it is suited to
various electrical devices in which magnetization must be
reversed very often per unit of time. If one plots the
characteristics of a "hard" ferrite and a 'isoft" ferrite on a
graph in which the imposed magnetic field forms the horizontal
axis and the total magnetization forms the vertical axis, one
obtains a characteristic curve resembling a thick S known as
a hysteresis loop. A "hard" ferrite has a wide hysteresis loop
and a "soft" ferrite has a thin one. Since each traversal of
a loop represents energy lost, a narrow loop is desirable in
devices in which magnetization must be reversed fre~uently.
The ferrite materials of main interest in the electro-
statographic arts are the soft erritcs. The soft ferrites may
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further be cllaractcrizcd as being magnetic, polycrystalline,
highly resistive ceramic ma~erials exemplified by intimate
mixtures of nlck~l, manganese, magnesium, zinc, iron or other
suit~ble metal oxides with iron oxide. Upon firing or sintering,
the oxide mixtures assume a particular lattice structure which
governs the m~gnetic and electrical properties of the resulting
ferrite.
In the past, ferrite materials have generally been
prepared by dry and wet methods. The dry method involves the
intimate mixing of pure oxides or carbonates of the desired
metallic constituents and causing the mixture to react at
elevated temperatures to form the desired structure. This method
requires extensive ball-milllng of the oxides or carbonates,
usually dispersed in a liquid, until an efficient degree of
mixing is obtained. The mixture is usually then dried,
gxanulated, pre-sintered to form the desired structure, reground
to attain a suitable particle size distribution, pressed or
compacted with a binder material, and finally sintered or
refired at temperatures above the pre-sintering temperature.
The wet method generally involves the formation of an intimate
mixture of the desired components by co-precipitation from
solution. Usually, the components are dissolved as nitrates
and co-precipitated as hydroxides, carbonates or oxalates. The
product, after filtration and washingl is then prefired, reground,
sized, compacted with a binder, and inally sintered or refired
at temperatures above the pre-sintering temperature.
, Several methods of preparing a manganese-zinc-ferrite
are disclosed. For example, in U. S. Patent No~ 3,567,641, an
oxide mixture is prepared, the mixture i5 pre~sintered at about
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700-900C. for about an hour, the pre-sintered mixture is we~
ground with CaO, thc material is presscd to shape and sintered
at l,100-1,300C. for 1 to ~ hours in a low oxygen atmosphere,
and then cooled in a substantially pure neutral atmosphere such
as nitro~cn. In U. S. Patent No. 3,56S,806 a ferrite material
is produced by providing a mixture of the oxides, forming ferrite
blanks from the oxide mixture, sintering the ferrite blanks at
1,200-1,300C. for about 4 to 20 hours, and during the last
half of the sïntering period the sintering occurs in an inert
gas atmosphere containing less than 0.2 percent by volume of
oxygen, and then cooling the sintered ferrite blanks to a
temperature of about 300C. in the same inert atmosphere.
In U. S. Patent No. 3,839,029, Berg et al. teach a spray-drying
process wherein a slurry of metal oxides is prepared in a
liquid, the slurry is spray-dried to form spherical beads, and
the beads are sintered to form ferrite beads. When employing
a rotary kiln during the sintering step, a flow-promoting
ingredient selected from aluminum oxide and zirconium oxide is
mixed with the spray-dried beads to minimize bead-to-bead
agglomeration and adherence of the beads to the furnace walls.
However, all of the aforementloned processes suffer
from various disadvantages. More particularly, it has been
found that thus-prepared ferrite materials when employed in an
electrostatographic system for the development of electrostatic
latent imayes are too sensitive to relative humidity changes to
be acceptable for use in high speed electrostatographic devices
employing magnetic brush development. One of the main reasons
~or poor performance of the ferrite materials at high humidity
in the electrostatographic device was ound to be the presence
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of certain species on the surface of the ferrite particles
which changcd surface conductivity and dielectric loss, and
caused variations in charge relaxation of a dcvclop~r mixture.
The exact mechanism of this phenomenon and the identification
of all contributing surface species is not now fully kno~n.
However, it has been found that surface sodium, perhaps combined
with sulfate, lS a major contributory contaminant. Surface ~inc
oxide has been found to be another major contaminant. Other
contaminants may be calcium and potassium. However! sodium
and zinc oxide are found to be present in by far the greatest
amoun.s on the surface of ferrite materials prepared by prior
art processes It was found in machine testing that maximum
acceptable levels of these surface species is about 20 parts
per million for sodium and about 5, obo parts per million for
zinc. The excesc zinc oxide is usually due to the non-stoichiometry
of the ferrite formulation. The major source of sodium contaminant
was found to be due to its presence in the materials composition
used and espccially the deflocculant used in dispersing metal
oxide slurries in the initial process steps.
Thus, previously known ferrite preparatio~ processes
and the resultant ferrite materials are deficient for the
aforementioned reasons and undoubtedly due to lack of control
of the surface properties of the finished product. More
particularly, past ferrite material preparation processes and
compositions were poorly controlled resulting in ferrites having
humidity sensitive surfacë properties. Further, past ferrite
preparation methods were deficient in the ability to control
another surface property of ~errites known as BET surface
area. BET surface area is measured as the area available for
the adsorption o~ mcasurable quantities of gases and reflects
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the surface roughness of the powder product. For use
as electrostatographic carrier materials, it is im-
portant to have the ability to control product BE~
area to whatever level is desired for maximum per-
formance, and the ability to produce powder with
uniform properties as to particle-to-particle. Since
previously known ferrite preparation processes are
deficient in one or more respects, there is a con-
tinuing need for an improved fexrite production pro-
cess and for improved ferrite materials.
SUMMARY OF THE INVENTION
; It is, therefore, an object of an aspect of this
invention to provide a ferrite manufacturing process
and resulting products which overcome the above noted
deficiencies.
It is an object of an aspect of this invention
to provide a ferrite manufacturing process whlch pro-
vides humidity-insensitive electrostatographic ferrite
carrier materials.
It is an object of an aspect of this invention
to provide a ferrit manufacturing process which limits
undesirable hygroscopic surface species on ferrite
particles.
It is an object of an aspect of this invention
to provide a ferrite manufacturing process which provldes
control of the surface properties of ferrite particles.
It is an object of an aspect of thïs invention
to provide a ferrite manufacturing process which pro-
vides improved ferrite carrier particles having more
uniform electrostatographic properties.
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It is an object of an aspect of this invention
to provide a ferrite manufacturing process which enables
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the contro]. of BET surface area of ferrite paxticles.
It is an object of an aspect of this invention
to provide a ferrite manufacturing process wherein
ferrite carrier materials having more uniform pro-
perties may be prepared.
It is an object of an aspect of this invention
to provide a ferrite manufacturing process which is
superior to known ferrite manufacturing processes and
ferrite carrier particles which are superior to known
ferrite carrier particles.
In accordance with one aspect of this invention
there is provided for preparing humidity-insensitive
: electrostatographic ferrite carrier materials comprising
blending an essentially stoichiometric mixture of
: ferrite forming metal oxides, calcining the blended
oxides in air at a temperature of up to about 2150 F.
: for up to about 30 minutes to provide said blended
oxides with a saturation magnetic moment of between
about 6 to about 30 electromagnetic units per gram,
milling the calcined oxides to a slurry to reduce the
particle size of said calcined oxides to between about
0.8 micron and about 1.6 microns, adding to said slurry
from about 0.02 to about 0.08 mole fraction of maganese
oxide and about 0.001 to about 0.100 mole fraction of
: copper oxide based on all divalent metal oxides, adding :~
~ a sodium-free deflocculant to said slurry in an amount ~-
of from about 0.5 to about 2.0 percent by weight based
on the weight of said metal oxides, adding water to said
.~ slurry to provide a slurry having about 80.0 percent
by ~eight of solids, pumping said slurry to hold/feed
tanks with continuous mixing while adding a binder
~ material to said slurry in an amount of from about
0.2 to about 1.5 percent by weight based
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on the weight of said metal oxldes, spray-drying said
slurry to form substantia]ly spherical beads, screening
said beads to obtain particles having a controlled size,
firing said particles in air at a temperature of up to
about 2500F. for about 4 to about 8 hours to provide
them with a saturation magnetic moment of about 48
electromagnetic units per gram, deagglomerating said
particles, and screening said particles to obtain ferrite
particles having surface sodium in an amount of less than
about 20 parts per million, surface zinc in an amount of
less than about 5,000 parts per million, and a BET surface
area of between about 170 cm /gram and about 500 cm2/gram.
In accordance with another aspect of this in-
vention there is provided an electrostatographic imaging
process comprising the steps of forming an electrostatic
latent image on a surface and developing said electro-
static latent image by contacting said electrostatic
latent image with a developer mixture comprising finely-
divided toner particles electrostatically clinging to
the surfaces of carrier particles, said carrier particles
comprising humidity-insensitive ferrite beads having an
average particle diameter from between about 30 to about
1,000 microns, said carrier particles having been prepared
by blending an essentially stoichiometric mixture of
ferrite forming metal oxides, calcining the blended oxides
in air at a temperature of up to about 2150 F. for up to
abou~ 30 minutes to provide said blended oxides with a
saturation magnetic moment of between about 6 to about 30
electromagnetic units per gram, milling the calcined
oxides in a slurry to reduce the particle size of said
calcined oxides to between about 0.8 microns and about
1.6 microns, adding to said slurry from about 0.02 to
about 0.08 mole fraction of manganese oxide and about
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0.001 to abou-t 0.100 mole fraction of copper oxi.ae based
on all divalent metal oxides, adding a sodium-free def-
locculant to said slurry in an amount of from about 0.5
to about 2~0 percent by weight based on the weight of said
metal oxides, adding water to said slurry to provide a
slurry having about 80.0 percent by weight of solids,
pumping said slurry to hold/feed tanks with continuous
mixing while adding a binder material to said slurry in
an amount of from about 0.2 to about 1.5 percent by weight
based on the weight of said metal oxides, spray-drying
said slurry to form substantially spherical beads, screening
said beads to obtain particles having a controlled size~
firing said particles in air at a temperature of up to
about 25000E. for about 4 to about 8 hours to provide them
with a saturation magnetic moment of about 48 electro-
magnetic units per gram, deagglomerating said particles,
and screening said particles to obtain ferrite particles
having surface sodium in an amount of less than about 20
parts per million, surface zinc in an amount of less than
about 5,000 parts per million, and a BET surface area of
between about 170 cm /gram and about 500 cm2/gram, whereby
at least a portion of said finely-divided toner particles
are attracted to and held on said surface in conformance
to said electrostatic latent image.
By way of added explanation, the foregoing
objects and others are accomplished, generally speaking,
by dry blending ferrite forming metal oxides, calcining ~:~
the blended oxides to provide them with a saturation -~
magnetic moment of about 6 to 30 electromagnetic units
per gram, milling the calcined oxides in a slurry to
reduce their partic].e si~e to between about 0.8 micron
and about 1.6 microns while adding manganese oxide,
copper oxide, a sodium-free deflocculant, and water to
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provide a slurry havinc~ about 80.0 percent by weight
of solids, pumping the milled slurry to hold/feed tan~s
with continuous mixing while adding a binder material
to the slurry, spray-drying the slurry to form sub-
stantially spherical beadsl screening the spray-dried
beads to obtain particles having controlled size, firing
the screened beads in air at a temperature of up to
about 2500F. for about 4 to about 8 hours, deag-
glomerating the fired beads, and screening the deag-
glomerated beads to obtain substantially spherical
ferrite beads having controlled surface species and
physical properties. Optionally, after dry blending
the ferrite forming metal oxides, the blend of oxides
may be pelletized in the presence of water. After
drying the pelletized oxides, they are then fed to a
calciner. Pelletizing the oxides, although not
necessary, is desirable to provide uniform, larger
particle size of the oxide blend for better flow and
heat transfer in rotary calciners. Where the oxide
blend is pelletized, the water may optionally be re-
moved from the pellets to avoid the evolution of gas in
the form of steam
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which would intcxfcrc with flow and feeding in rotary
calcincrs. In addition, the binder material may be added to
the mill durlng milling of the slurry instead o~ to the holding/
feeding tanks ~fter milling.
More particularly, the metal oxides are first selected
on the basis of desired errite composition. The metal oxides
are dry blended in a baffled, rotating drum or muller mixer or
similar equipment for a time sufficient to obtain a substantially
homogeneous mixture. The dry blended metal oxide mixture may then
be pelletized, typically with the addition of about 16.0 percent
by weight of water to the oxide mixure by employing a pelletizer,
a muller mixer, or a turbine mixer. Following pelletizing,
the metal oxide pellets of about 1/8 inch in diameter may
be dried before calcining. In the calcining step, the metal
oxide pellets are fired under conditions that complete about
10 to about 60 percent of the potential ferrite spinel structure.
~The percent reaction value may be higher or lower, but the
selected value should be coordinated with milling time and
final firing conditions to obtain a given BET sur~ace area
o~ the final ferrite product. Another consideration is that
the higher the percent reaction completion during calcining,
the more uniform the final product composition will be, but
the calcined material will be less reactive when fed to the
final firing step. ~hen following the aforementioned calcination
step, the calcined ferrite pellets obtain a saturation magnetic
moment of about 6 to about 30 electromagnetic units/gram of
material.
-Calcination may be performed in any suitable appara~us
such as rotary, inconel-type calciners indirectly fired with
electric globars. Such calciners typically have a 6 inch
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internal diametex, ~re about 90 inches long, and are operated
at about 7 r.p.m. However other calciners such as a rotary,
brick-lined calciner with direct gas firing operated at about
1900F. have been ound suitable. Generally, the maximum
calciner temperature is about 2150F. and the residence time of
the pellets is about 30 minutes. It has been found that calciner
dimensions and rotation usually determine residence time, and
residence time and operating temperature determine the magnetic
moment of the calcinate for a given oxide composition. An
ambient air temperature is suitable during calcination.
Obviously, other suitable process conditions and calciner designs
may be employed to obtain the same degree of product reaction.
Following calcination, the ferrite pellets are ball-
milled with steel media and water to obtain a slurry wherein
the calcined pellets are reduced to a controlled size of between
about 0.8 micron to 1.6 microns. The slurry typically contains
` about 80.0 percent by weight of solids. During this step,
manganese oxide, copper oxide, and a sodium-free deflocculant
are added to the siurry. Milling time is usually about 12 hours,
depending upon the particle size for a given slurry. The milled
slurry is then pumped to holding/feeding tanks with continuous
mixing. A binder material is usually added at this point to
the slurry, although the binder may be added to the mill as
previously indicated.
The next step in the process is to spray-dry the slurry.
A spray dryer designed for either spray nozzle atomization or
spray machine-disc atomization is employed to dry the slurry.
A particularly desirable type of spray machine is one that is
essentially a closed pump impeller driven by a variable speed
drive and is commonly termed a spinning atomizer, disc or
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whe~l. The high speed impel]er uses thc energ~ of centrifugalforce to atomize the slurry. The particle size distribution
obtained with this spray machine is generally narrow. Prefer-
ably, ~hen employing the spinning atomizer, the spray dryer
should have a large diameter configuration to avoid sticking of
the atomized particles to the dryer chamber walls. The atomizing
pressures, or the speed of rotation in the case of wheel
atomization, a~d the slurry feed rates may be varied as a partial
control of particle size.
After the slurry has been spray-dried, the spray-drieA
particles are screened and classified. Those particles which
are off~size are fed to a ball-mill for about 2 hours of re-
mill.~ng and then fed to the holding/feeding tank. For the
screening and classification step, any suitable production screens
may be employed. This apparatus typically contains stainless
bolting cloth.
Following classification, the partially fired particles
are fed to a firing kiln for final firing. Typically, the
particles are placed in refractory ceramic boxes called saggers
having a capacity of between about 10 and 12 pounds. The saggers
employed for firing are typically constructed from alumina
or cordierite and are stacked on kiln cars which are pushed
through a tunnel kiln. Firing has been performed in kilns
over 100 feet long, with peak temperature zones of 8 to 12
feet long. Peak temperature during firing of the particles may
be up to about 2500F. for up to about 4 hours. In a kiln
where the peak temperature during firing of the particles is
about 2350F., the residence time of the particles is up to about
8 hours. In any event, the requireA firing times for complete
reaction and densification of the particles are usuall~
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dependent on the size o~ the kiln, kiln car loading density,
and the firing temperature. Air is employed in the firing step
of the process of this invention. After cooling, the fired
ferrite particles are deagglomerated using a crusher and a
granulator. This is followed by a final screening step where
particles of a desired size are packaged and where off-size
particles are returned to the slurry mill for re-milling.
Thus, in accordance with this invention, improved
ferrite materials are provided by preparing ferrite matexials
by a process wherein the ferrite materials are sintered in the
absence of a flow-promoting ingredient such as aluminum oxide
or zirconium oxide particles. In addition, the improved ferrite
electrostatographic carrier materials are prepared by the use
ofan essentially stoichiometric starting composition selected
for control of bulk properties such as density and saturation
magnetic moment, and by the use of starting materials, including
a deflocculant, which are essentially sodium-free. The ferrite
products of this invention have been found to consistently have
surface sodium in an amount of less than about 5 parts per
- 20 million and surface zinc in an amount of less than about 50
parts per million. For true stoichiometry, ferric oxide would
have a molar ratio of 1.00 with respect to all divalent metal
oxides. The actual final formulation may have ferric oxide
present with a molar ratio of 1.00 + 0.06. Final formulation nay
differ from input formulation due to ferric oxide pick-up
during milling with steel media. Manganese is usually present
in the starting composition because it will assume higher
valency in cases of localized iron deficiency and allow
completion of a spinel structure rather than the formation of
excess zinc oxide and nickel oxide phases. Also, compositions
without manganese may result in powders with very high
surface zinc concentrations. Low surface zinc concentrations
and relatively low BET surface area are important properties
for satisfactory electrostatographic carrier performance.
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Thus, the mole fraction of manganese oxide of the divalent
metal oxides in the input composition is usua~ly between about
0.02 and about 0.08. Copper is usually present as one means
of controlling density, BET surface area, and magnetic moment
of the ferrite product.
Thus, ferrite composition uniformity is assured by
employing a balanced, essentially stoichiometric formulation.
Further, non-spinel phases are minimal. Composition uniformity
is further assured by employing ealcination in the process of
this invention wherein a second mixing and homogenizing of the
ferrite elem~nt~ is per~ormed In addition, because of the
composition uniformity and the signifieant reduction of surface
contaminants, the ferri$e materials of this invention may
be employed as electrostatographic carriers without a eoating
thereon, typically, a polymer coating material~ Polymer
coatings on prior ferrite materials served to cover dieleetri-
-; eally "lossy" surface contaminants which were sensitive to
humidity changes. However, where desired, the ferrite - -~
materials of this invention may be coated and will provide
satisfactorv performance in electrostatographic devices.
Another surface property of ferrite electrostatographic
carrier materials is very significant as to performance. This
surface property is BET surface area. BET surface area may
be described as the area available for the adsorption of
measurable quantities of gases per methods of Brunauer,
Emmett, and Teller, published in 1933. For a given carrier
coating process, the BET surfaee area of ferrite core is a
determinant of how well the polymer coating covers the carrier
29 surface and thus
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the amount o~ carricr surface which is active in c3cnerating
a triboelectric char~e when mixed with toner particlcs. It
has been found that with prior processes employincJ ~low-promo~ing
in~rcdients in the sintering stcp, férrite produc~ BET surface
area v~ried with the condition o~ the flow-promoting material.
Virgin flow-prornoting material extracted large amounts of sodium
and zinc oxide and thus provided a ferrite surface with very
high BET area. That is, ferrite BET surface area could he
as high as 350 cm /gram with fresh flow-promoting material and
as low as about 200 cm2/gram with totally spent flow-promoting
material at the same firing conditions. It has been found that
the elimination of flow-promoting material in the process of this
invention permits better control of BET surface area.
Further, and more specifically, the BET surface area
of the ferrite materials of this invention may be controlled by
(l) the input composition, (2) the reactivity of the product prior
to firing, (3) the densi~y of spray-dried powder, and (4) the
actual time and temperature employed for firing~ These variables
have been translated into controlled process conditions as to
(l) the percent of copper and iron present in the input composi-
tion; (2) the milled slurry particle size and the saturation
moment of the calcinate or slurry particles which af~ect the
reactivity of the powder before firing; (3) the porosity of
the powder before firing; and (4) the firing conditions of
soak temperature and time at the soak temperature. More
particularly, it has been found where copper is not present
in the input composition that densification of the powder product
and low BET surface area values are very difficult to achieve
and are very seldom obtained. In contrast, where more than
about 0.03 mole fraction of copper is prescnt, the powder
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product usually a~qlomeratcs scverely. Where values o~ iron
mole fraction arc much below stoichiometry, that is, less than
1.0, such generally promotc produc~ non-uni~ormi~y and non-
spinel phases and also result in high contents of surface zinc
species. Further/ iron mole fraction values much over
stoichiometry prevent good densification and low BET surface
area values. Another process variable which may be controlled
is the reactivity of the powder product before firing. This
may be done by: controlling the particle size of the milled
slurry and by controlling the saturation moment of the calcinate
or sluxried particles. Thus, it has been found that large
milled slurry particle sizes are unreactive in the final firing
step thereby promoting high BET surface area values. On the
other hand, small particle sizes are usually difficult to
disperse at about 80 percent slurry solids with sodium-free
deflocculants. In addition, low values of saturation moment
of the calcinate, that is, on the order of 0 to 8 indicate
low levels of ferrite pre-reaction in the powder product.
However, a high saturation moment reduces the reactivity of
the feed calcinate to the firing kiln and promotes high BET
surface area values to the powder product. Another process
variabIe having an effect on the BET surface area of the
powder product is the density of the spray-dried powder. In
general, lower density values indicate large porosity and
difficulty in densification during iring High density
values are acceptable, however tend to indicate high-spinel
~, . . .
calcinate which would be unreactive in final firing. The firing
conditions of soak temperature and time at soak temperature are
also important controlled process variables in that low soak
''
-17~
~1~7~)6
temperatures generally will not allo~ the powder to densify,
and short soak times likewise will not allow the powder to
dens.ify while long soak periods are uneconomical.
Therefore, in accordance with this invention, it has
been found that the following process conditions may be employed
for control of the BET surface area of ferrite materials. Thus,
as to the mole fraction of copper oxide relative to all divalent
metal oxides present in the input composition, a satisfactory
range of values is about 0.001 to about 0~100; the preferred
10 range is about 0.010 to about 0.050; and the optimum range
is from about 0.018 to about 0.025. As to the mole fraction
of ferric oxide relative to all divalent metal oxides present
in the input composition, a satisfactory range of values is
about 0.80 to about 1.05; the preferred range is about 0.90
to about 1.00; and the optimum range is from about 0.95 to
about 0.99. A satisfactory range of saturation moment of the
calcinate, in electromagnetic units per gram, is from about
0.1 to about 50; the preferred range is from about 5 to about
40; and the optimum range is from about 10 to about 30.
Satisfactory results are obtained when the milled slurry particle
size in microns is from about 0,5 to about 3.0, while the
preferred range is from about 0.5 to about 2.0, and the optimum
range is from about 0.8 to about 1.5. The bulk density of the
spray-dried powder in grams per cubic centimeter provides
satisfactory results when it is from about 1.4 to about 2.4;
the preferred range is from about 1.5 to about 1.8; and the
. optimum range is from about 1.55 to about 1.70. When employing
a tunnel kiln for firing, satisfactory results are obtained
when the soak temperature in degrees E'ahrenheit is between
30 about 1900 and about 2500; while the pre~erred range
-18-
1~76~0~
between about 2200 arld about 2500; and the optimum range is
between about 2350 and about 2500. Likewise, satisfactory
results are obtained when ~he time in hours at the soak
temperature is between about 2 and about 12; while the prcferred
range is between about 3 and abcut 10; and the optimwn range is
between about 4 and about 8.
It has been found that the aforementioned controls
affect densification, product uniformity, grain growth of the
ferrite materials during firing, magnetic moment, and the B~T
surface area of the powder product. The reactivity of ferrite
powder prepared for firing is a combination of the extent to
which spinel formation has occurred during calcining, and the
milled particle size of calcined powder before spray drying.
~here calcined material is milled for longer periods of time
before spray drying and firing, the ultimate particle size
in each dried powder bead will be smaller and promote faster
reaction and densification. Higher temperatures or longer
residence times during calcining produce more fully reacted cal-
cined powder which is then less reactive in final firing. The
production of milled slurries at 80 percent solids are preferred
as to achieve high density in spray dried powder, which promotes
lower BET area in fired powder. As previously indicated, the
BET surface area of ferrite powder is also affected by the
firing conditions of time and temperature, and the BET surface
area is reduced at higher firing temperatures and longer
residence time. It has been found that the B~T surface area is
a main factor regarding the triboelectric charge ~enerated at a
particular carrier coating weight. Thus, by this invention,
the BET surface area of ferrite carrier particles may be con-
trollcd to provide the proper triboelectric response to carrier
particles coated by practically any method.
-19-
764~;
~ hell a polymer coating is applicd to the ferrite
carrier materials of this invention, satisfactory results are
obtaincd in a high spced elcctrostatographic magnetic brush
developmen~ device whcre ~he B~T surface area of the uncoated
~errite materials is between about 170 cm2/gram and about 260
cm /gram. E~owever, it is preferred that the BET surface area
be between about 200 cm2/gram and about 250 cm2/gram because
the lower values present difficulties in application of coating
and the higher values require more coating material to achieve
effective charging surfaces. Optimum results are obtained in the
~forementioned electrostatographic device when the BET surface
area of the ferrite materials to be coated is between about
210 cm2/gram and about 230 cm2/gram. Where a polymer coating is
not applied to the fired ferrite powder particles, satisfactory
results may be obtained in a high speed electrostatographic
magnetic development device when the BET surface area of the
ferrite particles is between about 170 cm2/gram and about 500
cm2/gram.
- Any suitable type of sintering furnace may be employed
in the sintering step of this invention. Typical sintering
furnaces include a tunnel kiln, a batch furnace, a rotary kiln,
or an agitated bed furnace. The batch and tunnel furnace types
; are generally employed where long residence times are required.
Firing of the metal oxide spray dried beads at elevated
temperatures to complete reaction of the ferrite components is
generally carried out between about 2300F. and about 2500F.
Actually, lower and higher temperatures may be used, but this
is dictated by the processing time, the furnace materials of
construction generally available, the ferrite formulation, the
~20
,
,
:~31764/~)~
resulting strength of the fired bead, and the calcining
conditions. Generally, i~ a ferrite carrier material is
calcined at about 1900F ~or about 30 minutes, the calcinate
will obtain a saturation magnetic moment of about 10 ;
electromagnetic units/gram of calcinate, depending upon actua]
composition. Thus, to obtain a ferrite electrostatographic
; carrier material having the desired properties, the milled and
- spray dried calcinate is generally fired at about 2350F for
about 8 hours. The fired ferrite particles will then generally
possess a saturation magnetic moment of abo~t 48 emu/g and
BET surface area of about220 cm2/g, depending upon the selected
ferrite composition, and uniformity of all properties, particle
to particle.
The calcining and firing atmospheres of ambient air
are satisfactory to obtain the desired properties of the product
of this invention. Thus, the product usually needs not be
processed in a reducing gas flame, or a protective gas stream
~such as hydrogen or nitrogen, a low oxygen atmosphere, or a
neutral atmosphere such as nitrogen, or an inert gas atmosphere
containing less than 0.2 percent by volume of oxygen as in
prior art processes.
In accordance with this invention, any suitable
essentially sodium-free deflocculant may be added to the metal
oxide mixture during the milling and slurry preparation step.
Typical deflocculants include the ammonium salt of poly-
methacrylic acid, pyrogallic acid, tannic acid and humic acid,
; and the ammonium salts of tripolyphosphate and hexametaphosphate.
However, a deflocculant such as ammonium lignin sulfonate
29 (Orzan A* available from Crown Zellerbach Co.) is preferred
* trade mark
~21-
,':
:, . . .
~(~7fà~0~;
becausc it generally promotes the prcparation o~ a concentrated
calcined metal oxide slurry having a solids content o~ up to
about 80 perccllt by weicJht in ~ater based on the total
we~ght of the slurry. Further, in spite o~ this remarkably
high solids content, t~le metal oxide feed slurry may be pumped
to the spray dryer and atomized without clogging a pressure
noz~le or wheel atomizer. In addition, where about 50 to about
500 micron beads are desirable, the high solid content of the
metal oxide slurry contributes to attainment of such particle
sizes. Further, the high concentration of oxides reduces the
equipment and energy requirements necessary to form the particles.
The concentration of deflocculant added to the metal oxide slurry
may be varied from about 0.5 to aboùt 2.0 percent by weight based
on the weight of the oxide solids.
In addition, any suitable binder material may be added
to the milled slurry of metal oxides during milling in ball mills
or while the slurry is stored in holding tanks prior to the
-spray drying step. Typical binder materials include polyvinyl
alcohol, dextrin, lignosulfonates, and methyl cellulose. However,
a binder material such as gum arabic, or other natural and
synthetic acacia gums are preferred because they generally
better preserve the shape and integrity of the atomized metal
oxide beads formed during the spray drying and collecting steps
of this invention. Further, the binder material may also
assist the deflocculant in dispersing the high surface area
solids in the slurry. In addition, the presence of a binder
material has been found to prevent excessive "dusting", that is,
lack of particle cohesion, during the spray drying stepO The
amount of binder material employed with the slurry of milled
,
~ ~22-
1076~1~6
metal oxides is usually between about 0.2 to about 1.5 percent
by w~ight based on the weight o~ the oxide solids, however,
about 0.5 percent provides satisfactory results in most cases.
Any suitable substantially stoichiometric ferrite
forming metal oxide mixture consistent with desired surface
and physical properties may be employed as the starting
materials in this invention, Typical starting compositions,
on a molar basis, include: . .
(NiO 33Zn 67) 93Mn.05Cu.024( 2 3 96
(NiOo 3Zno 7)0 93MnQ~05Cu0~o2( 2 3 o.gg
NiOo 3Zn0.7 (Fe2 3 0-99
,39 nO0,68MnO0 03 Fe2o3
0.3ZnO0.7 (Fe~03)0 99 ~ CaO3 (1.5%mol)
(Lio 5Feo 5) ~ Fe2 3
0.5 0.5 0.3 0.7 2 3
MnO . Fe2O3 ~ CaO (1.5% mol)
0.38 n0,57Mn0,03CU0 07 . Fe2O3
NiOo 18ZnO0 45Mgoo.3Mnoo~05cuoo.o6 , Fe2o3
1.0
. 32Zn0 . 56CUo 09 Fe203
MgO0 5Zn0 3Mn0 05CUo,l ~ Fe2 31 0
When the ferrite composition is to be employed as a .,.
: carrier material for finely-divided toner particles in an
: electostatographic device, it has been found that the values of
surface sodium and zinc concentrations are related to starting
compositions. Thus, control of these values is desirable where
particular surface properties are required. Therefore, in
order to obtain less than 20 parts pex ~illion of surface sodium
32 in the ferrite particle, all ferrite raw materials should be
-23 ~
1~76a~)6
limited in sodium content. Likewise, the mole fraction of
manganese in the input composition
23A-
71~0~ .
is important as it a~ects acid cxtractable surface zinc
concentrations as previously indicated. ~lso, product
saturatiorl magnetic momen~ is affccted by the mole ratio of
the metals.
It is to be understood that numerous modifica~ions of
the above formulations may be obtaincd as is apparent. In any
event, the formulations of the starting oxide mixtures should be
selected so that after sintering the oxides, the resulting ferrite
composition will be substantially stoichiometric as described
above. The desired metal oxide materials may be selected on the
basis of desired processed ferrite properties and/or economics.
. .
- Any suitable polymer coating may be applied to
the ferrite carrier particles of this invention. Typical polymer
coating materials include natural resin, thermoplastic resin, or
partially cured thermosetting resin. Typical natural resins
include: caoutchouc, colophony, copal, dammar, dragon's blood,
jalop, 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, polymethyl-
styrene, polymethylmethacrylate, polyacrylonitrile, polyvinyl-
acetate, polyvinylalcohol, polyvinylbutyral, poly~inylchloride,
polyvinylcarbazole, polyvinyl ethers, and polyvinyl ketones;
fluorocarbons such as polytetrafluoroethylene, polyvinylfluoride,
~- polyvinylidene fluoride; and polychlorotrifluoroethylene; pol~-
amides such as polycaprolactamo and polyhexamethylene adipimide;
polyesters such as polyethylene terephthalate; polyurethanes;
polysulfides; polycarbonates; and mixtures thereof~ Typical
thermosetting resins include: phenolic resins SUCII as phenol
~ormaldchyde, phenol furfural and resorcinol formaldehyde; amino
.
lC~ 76406
resins such as urea formaldellyde and melamine formaldehyde;
polyester resins; epoxy resins, and mixtures ther~of. ~ styrene-
methacryla~c-or~anosilicon terpolymcr carrier coatin~ composition
such as described in U. S. Patent 3,526,533 is particularly
prcferred because of its excellerlt triboelectric characteristics.
The ferrite carrier materials of this invention may be coated
with carrier coating material by any conventional carricr coating
technique, such as, for example, the techni~ue described in U. S.
Patent 2,618,5~
An ultimate carrier particle diameter between about 50
microns to about 1,000 microns is generally preferred because the
carrier particles then possess sufficient density and inertia
to avoid adherence to the electrostatographic recording surface
during the development process. Adherence of carrier beads to
electrostatographic drums is undesirable because o~ the formation
of deep scratches on the surface during the imaging 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. Patent 3,186,838. Also print deletion occurs when
carrier beads adhere to electrostatographic imaging surfaces.
Any suitable electrostatographic carrier coating thick-
ness may be employed. However, a carrier coating having a
thickness at least sufficient to ~orm a thin continuous film on
the carrier bead is preferred 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, the carrier coating
material may comprisc from about 0.01 percent to about 1.0 percent
by weight based on the weight of the coated carriex particles.
Preferably, the electrostatographic carrier coatin~ material
. .
~5~
~ 76a~6
should comprise fxom about 0.3 perccnt to about 0.7 perccnt by
weight based on the weight of the coated carrier particles
becausc maximum durability, triboclectric response, and copy
quality are achieved. To achieve further variation in the
properties o the coating materials, well~known additives such
as plasticizers, reactive and non-reactlve polymers, dyes,
pigments, wetting agents, and mixtures thereof may be mixed with
the carrier coating material.
Any suitable piqmented or dyed electroscopic toner
material may be employed with the ferrite carrier materials
produced in accordance with 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 and mixtures thereof. The particular toner
material to be employed obviously depends upon the separation of
the toner particles from the ferrite carrier materials in the
triboelectric series. As is well known in the art, sufficient
separation should exist to permit the toner to electrostatically
cling to the surface of the carrier. ~tlong 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 No. 25,136 to Carlson
and U. S. Patent 2,788,288 to Rheinfrank et alO ~hese toner
materials generally have an averaqe particle diame~er between about
1 and about 30 microns. Generally speaking, satisfactory results
are obtaine~ when a~out 1 part koner is used with about 10 to
about 200 parts by weight of carrier.
-26
~7~i406
~ hen the fcrrite materials of this invcntion are mixcd
with fincly-divided toner materials and employed as a developer
mixture in an olectrostato~raphic device, it is found that the
developcr mixture is not sensitive to high humidity conditions
and consequent]y provide much lower levels of backgrouncl deposits
on developed electrostatic latent images, and also provide grea~ly
increased developer life which now results in their successful
commercial use in high speed electrostatographic devices.
DESCRIPTION OF T~l PREFERRED EMBODIMENTS
The following examples ~urther define, describe and
compare exemplary methods of preparing ferrite materials according
to the process of the present invention. Parts and percentages
are by weight unless otherwise indicated. The examples, other
than the control examples, are intended to illustrate the various
preferred embodimentc of the present invention.
In the following examples, the unit employed for spray
- drying is a large diameter spray dryer with centrifugal atomization.
Primary collection in this unit is in the dryer chamber, with
secondary collection in cyclones, filters, and scrubbers. The
dryer chamber is about 16 feet in diameter. Spinning wheel
atomization is downward from the top center of the dryer. The
incoming air is heated by direct gas firing.
XAMPLE I
A control composition comprising about 46 mole percent
of ferric iron oxide, about 38 mole percent of zinc oxide, and
about 16 mole percent of nickel oxide is added to a grinding mill
containing steel media. The sodium salt of a polymethacxylic
acid (Darvan 7, available from R. T. Vanderbilt Company) i5 addcd
to comprise about 0.8 percent by weight (solids basis). Water is
added such that the resulting slurry is 80 percent solids by weight.
trade ~nAr~
: '
~7
1(~764~?6
The slurr~ is miY~ed in thc ball mill for about ~ hours. The slurry
mixture is transfcrred to a holding tank where~rom it is fed to the
aforcmentioned spr~y dryer at a feed ratc of about 2500 pounds pcr
hour, a drying air input tempexature of about 575F., and an outlet
temperature of about 325F. ~fter spray drying, the substantially
spherical metal oxide beads are screened and the offsize material
is returned to.the slurry make-up tank for reprocessing. The
sized material having-an average particle diameter of about 100
microns is then blended with a flow-promoting ingredient, in this
case, aluminum oxide particles, of approximately 600 microns size
and in a weight ratio of about 1:1. The blend is then loaded into
saggers (alumina boxes) which hold about 12 to 14 pounds of material.
The saggers are stacked on kiln cars which are pushed through a
tunnel kiln and fired for about 4 hours at a peak temperature of
about 2300F. During the period of peak temperature firing and
dùring the period of cooling from peak temperature, oxygen in the
firing atmosphere is controlled to decrease from about 10 percent
to about 1 percent. After cooling, the saggers cake is deagglom-
erated with a crusher, followed by a coarse screen process to
separate ferrite particles from the aluminum oxide particles.
The aluminum oxide particles are returned to be blended and fired
again with metal oxide particles as many as 10 additional times.
The ferrite particles are screened again and classified as to
desirable particle size. Fired offsize material is discarded.
The classified ferrite particles were analyzed for surface species
of sodium and zinc and were found to contain from about 2 to about
80 ppm. of sodium and from about 500 to about 10,000 ppm. of zinc.
Analysis for sodium is performed by a water cxtraction method,
and for zinc by an acid extraction method. The saturation
- -2~-
'
~0~76406
.
magnetic moment of the fcrrite particles was ~ound to be about
55 elcctromagn~tic units/gram of ferritc material. Upon analysis
employing krypton gas as the adsorbate, the BET sur~ace arca of
the ferrite particles was found to range from about 180 to about
350 cm2/gram.
EX~MPLR II
The classlfied ferrite material of Example I was
employed as a carrier for a toner material in a high speed electro-
statographic magnetic brusll device for the development of electro-
static latent images, The toner material employed comprises a
copolymer of styrene and alkyl methacrylate with a carbon black
wherein the toner material has an average particle size of about
10 to about 15 microns. A polymer ma-terial comprising a styrene,
alkyl methacrylate, and an organosilicon as described in U. S.
3,526,533 is coated on the ferrite particles in a coating weight
of about 0.6 percent~ The coated ferrite carrier particles are
blended with the toner material in an amount of about 1 part
toner material per about 100 parts of carrier material. The
developer mixture is used to develop a selenium photoconductor
carrying an electrostatic latent image by the "magnetic brush"
development method described in U. S. Patent 2,874,063. The
magnetic field of the magnet causes alignment of the carrier and
toner into a brush~like configuration. The magnetic brush is
brought into developing configuration with the electrostatic
image bearing surface and toner particles are drawn from ~he
~arrier particles to the latent image by electrostatic attraction.
The ambient temperature was about 80F. and the relative humidity
was about 80 percent during development of the electrostatic
latent images. It is found tha~ the developer mixturc produces
-29-
~' .
,. . .. : : :
. , . ., . .
' '' ' , ~
1076~6
image background ]evels considcrably above the maximum value of
0.010 dcemcd acccptable as measurcd by a standard reference scale.
_X~MPL~ III
Ferrite electrostatographic carrier materials were
prepared by dry blending a quantity of metal oxides about 53 mole
percent of ferxic iron oxide, about 31 mole percent of zinc oxide,
and about 16 mDle percent of nickel oxide in a baffled, rotating
drum for about 20 minutes. The dry blended metal oxide mixture
was then pelletized in a turbine mixer to which was added about
15 percent by weight of water. Metal oxide pellets of about 1/8
inch diameter and finer in size were obtained and were dried in a
continuous belt dryer to about 2 percent by weight water. After
drying, the pellets were calcined for about 30 minutes in an air
atmosphere at a peak temperature of about 2150F. The calcined
pellets were found to have a saturation magnetic moment of about
30 electromagnetic units/gram of material. The calcined pellets
were then placed in a grinding mill containing steel media.
Additional metal oxides and ammonium lignin sulfonate were added
to the mill such that the total metal oxide composition in the
mill was about 96 mole percent calcined formulation, about 3
mole percent manganese oxide and about 1 percent copper oxide.
In addition, the ammonium lignin sulfonate constituted about 1
percent by weight of all solids. Water was then added to obtain
a slurry containing about 80 percent by weight of solids. After
milling for about 15 hours the pellets are reduced in size to
aout 1.5 microns and the slurry is transferred to hold/feed
tanks with continuous stirring where about 0.5 weight percent
5solids basis) of gum arabic, a soluble, natural acacia gum, is
added to the slurry.
,
~30~
, ' ' . :'' ;
107640~;
The slurry i5 then ~cd to the aforemcntioned spray dryer
unit where it is spray driecl into particles havin~ an average
diameter of about 100 microns. The feed rate to ~he spray dryer is
about 2500 pounds slurry per hours, the drying air input temperature
is about 575F~, and the outlet tem2erature is about 325F. A~ter
spray drying, the metal oxide bcads are screened and the offsize
material is fed to a separate milling device for reprocessing.
The sized material is then loaded into saggers which are stacked
on kiln cars and pushed through a tunnel kiln where the beads are
fired for about 4 hours in an air atmosphere at a peak temperature
of about 2450F. After cooling, the ferrite particles are
deagglomerated using a crusher and a granulator, and then screened
again to the desired particle size. In this case, ferrite particles
having an average particle diameter of about 100 microns were
selected. The offsize material was returned to the slurry mill
for reprocessing.
The classified ferrite particles were analyzed for surface
species of sodium and zinc and were found to contain, on ~he average,
about 2 ppm. of ~odium and about 15 ppm. of zinc. In addition,
the saturation magnetic moment of the ferrite particles was found
to be about 48 electromagnetic units/~r~m of material. Further~
the ferrite particles were analyzed ~or BET sur~ace area which was
found to be an average value of about 220 cm2/gram.
EXAMPLE IV
The classified ferrite particles of Example III were
employed as a carrier in a developer mixture as in Example II.
The ferrite particles were coated as in Example II and the toner
material was the same as in Example II. The carrier-toner ratio
was the same in Example II. The ambient temperature was about 80F.
and the relative humidity was about 80 percent durin~ devclopment
~31~
.
~7t~
of the clectrostatic latent images. It was found that the
devclopcr mixture produced imac3es o cxcellent quality with
sa~isfactory backyround levels w~ll bclow the maximum value of
0 010 deemed acceptable.
~X~MPL~ V
Ferrite electrostatographic carrier materials were
prepared by dry blending a quan-tity of metal oxides comprising
about 53 mole percent of ferric iron oxide, about 31 mole percent
of zinc oxide, and about 16 mole percent of nickel oxide in a
baf~led, rotating drum for about 20 minutes~ The dry blended
metal oxide mixture was then pelletized in a turbine mixer to
which was added about 15 percent by weight of water. Metal oxide
pellets of about 1/8 inch diameter and finer in size were obtained
and were dried in a continuous belt dryer to about 2 percent by
weight water. After drying, the pellets were calcined for about
30 minutes in an air atmosphere at a peak temperature of about
1900F. The calcined pellets were found to have a saturation
magnetic moment of about 7 electromagnetic units/gram of material.
The calclned pellets were then placed in a grinding mill containing
steel media. Additional metal oxides and ammonium lignin sulfonale
were added to the mill such that the total metal oxide composition
in the mill was about 96 mole percent calcined formulation, about
3 mole percent manganese oxide and about 1 percent copper oxide.
In addition, the ammonium lignin sulfonake constituted about 1
percent hy weight of all solids. Water was then added to obtain
a slurry containing about 80 percent by weight o solids. A~ter
milling for about 12 hours, the pellets are reduced in size to
about 1 micron and the slurry is transferred to holding/feeding
tanks with continuous stirring whcre about 0.5 weight percellt
(solids basis) of gum arabic, a soluble, natural acacia gum, lS
added to the slurry.
-32-
~07~406
The slurry is then fed to the aforementioncd spray dryer
unit where is is spray-dried into particles having an average
diameter o~ about 100 microns. The feed ra~e to the spray dryer
is about 2500 pounds o slurry per hour, the drying air input
temperature is about 575F, and the outlet temperature is about
325F. After spray drying, the metal oxide beads are screened
and the ofsize material is fed to a separate milling device for
reprocessing. The sized material is then loaded into saggers
which are stacked on kiln cars and pushed through a tunnel kiln
where the beads are fired for about 8 hours in an air atmosphere
at a peak temperature of about 2350F. After cooling, the ferrite
particles are deagglomerated using a crusher and a granulator,
and then screened again to the desired particle size. In this
case, ferrite particles having an average particle diameter of
about 100 microns were selected. The offsize material was returned
to the slurry mill for reprocessing.
The classified ferrite particles were analyzed for surface
species of sodium and zinc and were found to contain, on the average,
about 2 ppm. of sodium and about 25 ppm. of zinc. In addition,
the saturation magnetic moment of the ferrite particles was found
to be about 48 electromagnetic units/gram of material. Further,
the ferrite particles were analyzed for BET surface area which
was found to be an average value of about 220 cm2/gram.
EXAMPLE VI
The classified ferrite particles of Example V were
employed as a carrier in a developer mixture as in Example II.
The ferrite particles were coated as in Example II and the toner
material was the same as in Example II. The carrier-toner ratio
was the same as in Example II. The ambient temperature was about
80F. and the relative humidity was about 80 pcrcent during
.
-33
1~)7~4~06
devclopmcnt o~ the electros~atic latcnt images. It was found that
thc develop2r miY.ture produccd imagcs of exccllent quality with
satisfactory b~ckground lcvels well below the maximum value o
0.010 deemed acceptable.
EX~MPLE VII
Ferrite electrostatographic carrier material were
prepared by dry blending a quantity of metal oxides about 51 mole
pexcent of ferric iron oxide, about 34 mole percent of zinc oxide,
and about 15 mole percent of nickel oxide in a baffled, rotating
drum for about 20 minu-tes. The dry blended metal oxide mixture
was then pelletiæed in a turbine mixer to which was added about
15 percent by weight of water. Metal oxide pellets of about 1/8
inch diameter and finer in size were obtained and were dried
in a continuous belt dryer to about 2 percent by weight water.
After drying, the pellets were calcined for about 30 minutes in an
air atmosphere at a peak temperature of about 2150F. The calcined
pellets were found to have a saturation magnetic moment of about
30 electromagnetic units/gram of material. The calcined pellets
were then placed in a grinding mill containing steel media.
Manganese oxide and ammonium lignin sulfonate were added to the
mill such that the total metal oxide composition in the mill was
about 98.5 mole percent calcined formulation, and about 1.5 mole
percent manganese oxide. In addition, the ammonium lignin sulfonate
constituted about 1 percent by weight of all solids~ Water was
then added to obtain a slurry containing about 80 percent by weight
of solids. After milling for about 15 hours, the pellets are
reduced in size to about 1.5 microns and the slurry is transferred
to holding/feeding tanks with continuous stirring where about 0.5
weight percent (solids basis) of gum arabic, a soluble, natural
acacia gum, is added to the~ slurryO
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The slurry is thcn fcd to the aforementioned spray
dryer unit wh~re it is spray-dried into particles havin~ an
average diamet~r of about lOO micron's. The feed rate to the
spray dryer is about 2500 pounds of slurry per hour, the drying
air input temperature is about'575F., and the outlet temperature
is about 325F, After spray drying, the metal oxide beads are
screened and the offsize material is fed to a separate milling
device for reprocessing. The sized material is then loaded into
saggers which are stacked on kiln cars and pushed through a tunnel
kiln where the beads are fired for about 4 hours in an air atmos-
phe're at a peak temperature of about 2450F. After cooling, the
ferrite partlcles are deagglomerated using a crusher and a granulator,
and then screened again to the desired particle size. In this case,
ferrite particles having an average particle diameter of about 100
microns were selected~ The offsize material was returned to the
slurry mill for reprocessing.
The, classified ferrite particles were analyzed for surface
species of sodium and zinc and were found to contain, on the
average, about 3 ppm. of sodium and about 7 ppm. of zinc. In
addition, the saturation magnetic moment of the ferrite particles
was found to be about 36 electromagnetic units/gram of material.
~urther, the ferrite particles were analyzed for BET surface area
which was found to be an average value of about 250 cm?/gram.
EXAMPLE VIII
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The classified ferrite particles of Example VII were
employed as a carrier in a developer mixture as in Example II.
The ~errite particles were coated as in Example II and the toner
material was the same as in Example II. The carrier-toner ratio
was the same as in Example II. The ambient temperature was about
80F. and the relative humldity was about 80 percent during
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~ 076~06
devclopmcn~ of thc clectrostatic latcnt irnagcs. It was found
that the developcr mixture produced images of excellent quality
wi~h back~round levels well below the m~ximum value of 0.010
deemed acceptable.
EXAMPJJ~ IX
Ferrite ~lectrostatographic carrier material were
prepared by dry blending a quantity of metal oxides comprising
about 53 mole percent of ferric iron oxide, about 31 mole percent
of zinc oxide, and about 16 mole percent of nickel oxide in a muller
mixer for about 20 minutes. The blended powder was calcined for
about 30 minutes in an air atmosphere at a peak temperature of
about 1900F. The calcined material was found to have a saturation
~; magnetic moment of about 7 electromagnetic units/gram of material.
The calcined material was then placed in a grinding mill containing
steel media. Additional metal oxides, arnmonium lignin sulfonate,
and gum arabic were added to the mill such that the total metal
oxide composition in the mill was about 96 mole percent calcined
formulation, about 3 mole percent manganese oxide, and about 1
percent copper oxide. In addition, the ammonium lignin sulfonate
- constituted about 1 percent by weight of all solids, and the gum -
arabic constituted about 0.5 percent by weight of all solids.
Water was then added to obtain a slurry containing about 80
percent by weight of solids. After milling for about 12 hours,
the pellets are reduced in size to about 1 micron and the slurry
is tra~sferred to holding/feeding tanks with continuous stirring.
The slurry is then fed to the aforementioned spray
dryer unit where it is spray dried into particles having an average
diameter o~ about 100 microns. Thc feed rate to the spray dryer
is about 2500 pounds o~ slurry pcr hour, the drying air input
temperature is about 575F., and the ou~let temperature i5 about
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325F. After spray dxying, thc metal oxide beads are scrcened
and thc offsizc matcrial is fed to a separate millin~ device for
reprocessing. The sizcd mater.i.al is then loaded into sagcJers
which are stacked on ~iln cars and pushed through a tunnel kiln
whcre the beads are fired for about 8 hours in an air atmosphere
at a peak temperatur~ of about 2350F. A~ter cooling, the ferri~e
particles are deagglomerated using a crusher and a granulator, and
then screened again to the desired particle size. In this case,
ferrite particles having an average particle diameter of about 100
microns were selected. The offsize material was returned to the
slurry mill for reprocessing.
The classified ferrite particles were analyzed for surface
species of sodium and zinc and were fo~md to contain, on the average,
akout 2 ppm. of sodium and about 25 ppm. of zinc. In addition,
the saturation magnetic moment of the ferrite particles was found
to be about 48 electromagnetic units/gram of material. Further,
the ferrite particles were analyzed for BET surface area which ~as
found to be an average value of ahout 220 cm2/gram.
EXAMPLE X
The classified ferrite particles of Example IX were
employed as a carrier in a developer mixture as in Example II.
The ferrite particles were coated as in Example II and the toner
material was the same as in Example II. The carrier-toner ratio
was the same as in Example II. The ambient temperature was about
80F. and the relative humidity was about 80 percent during
development of the electrostatic latent images. It was found
that the developer mixture produced images of excellent quality
with background lcvcls well below the maximum value of 0.010
deemcd acceptable.
i~764~6
~ lthouyh specific materials and conditions ar~ set
orth in the abovc examplcs o making the errite matcrials of
this invcntion, these are merely intendcd as illustrations of
the present inventlon. These and other ferrite materials, solvents,
substituents, and processes, such as those listed above, may be
substituted for those in the Examples with similar results.
Other modifications of the present invention will
occur to those skilled in the art upon a readiny of the present
disclosure. These are intended to be included within the scope
of thio inveneion.
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