Note: Descriptions are shown in the official language in which they were submitted.
~a5~837
D/73069 BACKGROUND OF THE I~VENTION
This invention relates in general to electrostato-
graphy and in particular to a process for the preparation of
ferrite carrier materials, to the ferrite materials so prepared,
and to the use of said ferrite materials in electrostatography.
Ferrite materials are gaining ever increasing import-
ance in the electronics industry and in the electrostatographic
arts. Their use as low conductivity magnetic core materials and
as carrier materials in electrostatographic developer materials
is well known. Briefly, ferrites may be described in general as
compounds of magnetic oxides containing iron as a ma~or metallic
component. Thus, compounds of ferric oxide, Fe203, formed with
basic metallic oxides having the general formula MFe02 or MFe204
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 MgA1204. However, not all ferrites are magnetic
such as, for example, ZnFe204 and CdFe204. This lack of magnetic
property is due to the configuration of the ferrite lattice struc-
ture. Further, some ferrites, such as magnetobarite, BaFe12019,
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'~ ferrite has the opposite property;
it is easily magnetized and demagnetized. The "softer" the ferrite
material is, the better it is suited to various electrical devices
-2- ~ ~
105~ 7
D/73069 in which magnetization must be reversed very often per unit of
time. If one plots the characteristics of a "hard" ferrite and
a "soft" 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 hysteresis loop. A `'hard" ferrite has a wide
hysteresis loop and a "soft" ferrite has a narrow one. Since
each traversal of a loop represents energy lost, a narrow loop
is desirable in devices in which magnetization must be reversed
frequently.
The ferrite materials of main interest in the electrosta-
tographic arts are the "soft" ferrites. The 'Isoft'' ferrites may
further be characterized as being magnetic, polycrystalline,
highly resistive ceramic materials exemplified by intimate mix-
tures of nickel, manganese, magnesium, zinc, iron or other suitable
metal oxides with iron oxide. Upon firing or sintering, the oxide
mixtures assume a particular lattice structure which governs the
magnetic and electrical properties of the resulting ferrite.
The formation and development of images 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 2,297,691, involves placing a uniform
electrostatic charge on a photoconductive insulating layer, ex-
posing the layer to a light-and-shado~l image to dissipate the
charge on the areas of the layer exposed to the light and develop-
ing the electrostatic latent image by depositing on the image a
--3--
1054837
D/73069 finely-divided electroscopic material referred to in the art
as "toner". The toner will normally be attracted to those areas
of the layer which retain a charge, thereby forming a toner
image corresponding to the electrostatic latent image. This
powder image may then be transferred to a support surface such
as paper. The transferred image may subsequently be permanently
affixed to the support surface as by heat. Instead of latent
image formation by uniformly charging the photoconductive layer
and then exposing the layer to a light-and-shadow image, one
may form the latent image by directly charging the layer in
image configuration. The powder image may be fixed to the
photoconductive layer if elimination of the powder image transfer
step is desired. Other suitable fixing means such as solvent
or overcoating treatment may be substituted for the foregoing
heat fixing steps.
Several methods are known for applying the electro-
scopic particles to the electrostatic latent image to be de-
veloped. One development method, as disclosed by E. N. Wise
in U. S. Patent 2,618,552, is known as "cascade" development.
In this method, a developer material comprising relatively large
carrier particles having finely-divided toner particles electro-
statically 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 tribo-
electrically charge the toner particles to the desired polarity.
As the mixture cascades or rolls across the image bearing surface,
~.05~837
D/73069 the toner particles are electrostatically deposited and secured
to the charged portion of the latent image and are not deposited
on the uncharged or background portions of the image. Most of
the toner particles accidentally deposited in the background are
removed by the rolling carrier, due apparently, to the greater
electrostatic attraction between the toner and the carrier than
between the toner and the discharged background. The carrier
and excess toner are then recycled. This technique is extremely
good for the development of line copy images.
Another method of developing electrostatic latent
images is the "magnetic brush" development process as disclosed
for example, in U. S. Patent 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 brush-like configuration.
This "magnetic brush" is engaged with the electrostatic image-
bearing surface and the toner particles are drawn from the brush
to the latent image by electrostatic attraction. Thus, a devel-
oper mixture may be provided comprising a toner material and a
carrier material which consists of particles which are magneti-
cally attractable. Consequently, iron and magnetic ferrite
materials have been employed as the carrier material in the elec-
trostatographic arts.
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
lOS4837
D/73069 metallic constituents and causing the mixture to react at
elevated temperatures to form the desired structure. This
method requires extensive ball-milling of the oxides or car-
bonates, usually dispersed in a liquid, until an efficient
degree of mixing is obtained. The mixture is usually then
dried, granulated, 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.
This method is undesirable in that it results in ferrite
material of large crystallite or grain size having a high
temperature coefficient of permeability or decreased tempera-
ture stability. The wet method generally involves the forma-
tion 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, car-
bonates or oxalates. The product, after filtration and washing,
is then prefired, reground, sized, compacted with a binder, and
finally sintered or refired at temperatures above the pre-
2 sintering temperature. This method also has the disadvantage of
resulting in ferrite materials of large crystallite or grain
size having a high temperature coefficient of permeability or
decreased temperature stability. Both the dry and wet methods
have the further disadvantage of requiring compaction of the
product with a binder prior to final firing which is a time
consuming, expensive step and which limits the firing temperature
~054837
3069 and further causes bead to bead agglomeration and sticking of
beads to surfaces of sintering equipment.
Other techniques of producing magnetic powder are
known such as preparing a powdered alloy and mechanically disin-
tegrating the alloy to magnetic particles and blowing the magnetic
particles through a reducing gas flame at a temperature sufficient
to melt the particles to spherical form, and cooling and collect-
ing the particles so obtained as disclosed in U. S. Patent 2,186,659.
Even though this technique can produce spherical particles, to
avoid undesired reactions such as oxidation of the particles a
protective gas stream such as hydrogen or nitrogen is generally
re~uired. Further, the product coming from the ball mill must be
balled in a compressed gas flame and the ball material caught in
a liquid bath~ In addition, the balled material thus produced
must generally be mixed in a kneading machine with a binding
medium, such as an artificial resin that can be solidified. After
drying, the material must be compressed in a suitable manner.
Another process of preparing ferrite beads is
disclosed by A. Berg et al in Canadian Patent 1,000,477
is~ued November 30, 1976. In this patent, the
process comprises making ferrite materials by preparing a slurry
of meta~ oxides in a liquid, spray drying the slurry of metal
oxides to form metal oxide beads, and sintering the metal oxide
beads to form ferrite beads. However, it has been found that a
substantial portion of ferrite beads made by this process have a
characteristic void or dimple present in the dried particle whereas
.
O7_
~,~,
1054837
ferrite beads which are spherical in shape and continuous in
surface area are desired. Previous attempts to reduce or eliminate
voided areas in ferrite beads made by this process have not been
successful. Since previously known ferrite preparation processes
are deficient in one or more respects, there is a continuing need
for an improved ferrite production process and improved ferrite
materials.
Thus by one aspect of the present invention there is
provided a process for making substantially spherical, void-free
ferrite particles comprising preparing a slurry of ferrite forming
metal oxides in a liquid wherein said slurry has a ~iscosity at
about 23C of between about 55,000 centipoise and about 140,000
centipoise, spray drying said slurry of metal oxides to form
substatially spherical metal oxide beads to form ferrite particles
which are substan~ially shperical in shape, continuous in surface
area, and free of interior voids.
By another aspect of the present invention there is
provided a substantially shperical, void-free electrostatoyraphic
ferrite carrier particle, said ferrite carrier particle having
been made by a process comprising preparing a slurry of ferrite
forming metal oxides in alliquid wherein said slurry has a
viscosity at about 23C of between about 55,000 centipoise and
about 140,000 centipoise, spray-drying said slurry of metal
oxides to form substantially shperical metal oxide beads, and
sintering said substantially shperical rnetal oxide beads to form
ferrite particles which are substantially spherical in shape,
continuous in surface area, and free of interior voids.
By still another aspect of the present invention there
is provided an electrostatographic imaging process comprising
the steps of forming an electrostatic latent image on a surface
and developing said electrostatic latent imaye by contacting
said electrostatic latent image with a developer mixture
~ -8-
1054837
comprising finely-divided toner particles electrostatically
clinging to the surfaces of carrier particles having a particle
size of from about 30 to about 1,000 microns, each of said
carrier particles comprising carrier particles having been made
by a process comprising preparing a slurry of ferrite forming
metal oxides in a liquid wherein said slurry has a viscosity at
about 23C of between about 55,000 centipoise and about 140,000
centipoise, spray-drying said slurry of metal oxides to form
substantially shperical metal oxide beads, and sintering said
substantially spherical metal ,oxide beads to form ferrite
particles which are substantially spherical in shape, continuous
in surface area, and free of interior voids, 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 imag~.
-8a-
lOS4837
These and many other aspects will become more readily
apparent when the following specification is read and considered
in the light of the attendant drawings in which:
Fig. 1 is a perspective view of a ferrite-forming
metal oxide mixture spray-dried from a slurry having a viscosity
of about 9,000 centiposes at about 23C showing that the beads
have discontinuous surfaces and contain voids in their interiors.
Fig. 2 and Fig. 3 are perspective views of a ferxite-
forming metal oxide mixture spray-dried from a slurry having a
viscosity of about 131,000 centipoises at about 23C showing
respectively that the beads have void-free interiors and
continuous surface areas.
The foregoing features and others are accomplished,
generally speaking, by preparing a slurry of ferrite forming
metal oxides in a liquid wherein said slurry has a viscosity at
23C of between about 55,000 centipoise and about 140,000 centi-
poise, spray drying the slurry of metal oxides to form
substantially spherical metal oxide beads, and sintering the
substantially spherical metal oxide beads to form ferrite
particles which are substanitally spherical in shape, continuous
in surface area, and
_g_
l(~S4837
D/73069 free of interior voids. Thus, ferrite materials having the
aforementioned desired properties may be prepared by the process
of this invention wherein the viscosity of the slurry of ferrite
forming metal oxides at 23C is at least about 55,000 centipoise
and up to about 140,000 centipoise or about the practical limita-
tions of the equipment employed in preparing the ferrite materials
of this invention.
The metal oxide materials may be selected first on the
basis of desired ferrite properties. In a preferred embodiment
using a high speed mixer, the metal oxide starting materials
are slowly added to a make-up tank while a deflocculent is added
so that the solids are continually wetted out. A smooth, homo-
genous slurry is generally formed after approximately thirty
minutes of agitation depending upon the equipment capacity and
the size of the batch prepared. If the finished ferrite is to
be composed of several components for use as an electrostatographic
carrier particle, it is usually desirable to achieve an intimate
mixture of the metal oxide starting materials by this slurry
preparation process. The actual degree of mixing achieved may
be controlled by the choice of equipment used and selection of
specific equipment operating parameters and/or slurry conditions
such as mixing speed, mixing time, viscosity and temperature.
The metal oxide starting materials may be mixed in slurry form
in any one of the following types of equipment such a ball-mill,
vibrating pebble mill, high speed stirrer with counter turning
rotor and blades, impeller mixer, high speed dispersator, and
--10--
1~)54837 ~
D/73069 other conventional mixing equipment. As an alternative, one may
dry mix the metal oxide starting materials and combine the dry
mixture at a later time with a liquid medium. Following the
slurrying operation, it is generally preferred to screen the
slurries prior to spray drying in order to eliminate any large
solid particles which may be present as would plug a pressure
atomizer.
A spray dryer designed for either spray nozzle atom-
ization or spray machine-disc atomization or equivalent may be
employed to dry the slurry of metal oxide starting materials.
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 wheel.
The total systsm generally consists of a power-coolant-lubrication
console, power cables, fluid transport hoses, and a variable
speed motor drive with closed impeller. The high speed impeller
uses the energy of centrifugal force to atomize the slurry. The
particle size distribution obtained with this spray machine is
generally narrow. In addition, product characteristics may be
varied by the spinning atomizer design, speed and position in the
chamber relative to air entrance. Preferably, when employing the
spinning atomizer, the spray drying should have a large diameter
configuration to avoid sticking of the atomized metal oxide particles
to the dryer chamber walls. Slurries of metal oxides may be
atomized using two-fluid nozzles where the atomizing force is
pressurized air, single-fluid pressure nozzles where the atomizing
--11--
l()S4837,
D/73069 force is the pressure of the slurry itself released through an
orifice, and centrifugal atomization by a spinning wheel or other
suitable atomization method. The atomizing pressures, or the
speed of rotation in the case of wheel atomization, and the
slurry feed rates may be varied as a partial control of particle
size. It is also possible to control the particle size of the
spray dried metal oxide beads by varying the percentage of solids
in the feed slurry. The atomizing force and feed rate should be
adjusted to the configuration, size and volumetric air flow of
a given drying chamber in order that atomized particles do not
contact drying chamber surfaces while still wet. In accordance
with the process of this invention the percentage of solids in
the feed slurry may be varied from about ~0 to about 80 percent
by weight of oxides slurried in the liquid medium. If a defloc-
culent material is added to the metal oxide slurry, the concentra-
tion of deflocculent may be varied from about 0.01 to about 2.0
percent by weight of the oxide sollds. Although considerablelatitude exists in regard to the metal oxide particle sizes em-
ployed for the slurry, metal oxide particles having an average
particle size less than about-25 microns are preferred to avoid
high settling rates in the slurry.
It has been found that no binder material need be
added to the feed slurry in order to preserve the shape and inte-
grity of the atomized metal oxide beads formed during the spray
drying and collecting steps of the process of this invention.
The elimination of a binder material in the formation of spray
~C~S4837
D/73069 dried metal oxide beads has been found to provide a denser and
stronger ferrite material following sintering of the spray dried
beads. The elimination of binder material from spray dried metal
oxide beads is preferred because it has been found that binder
material promotes bead-to-bead agglomeration or adherence to equip-
ment surfaces during the sintering step. The spray dried metal
oxide beads may be collected in drying chambers of suitable size.
Spray dried metal oxide beads have been collected in a chamber 30
inches in diameter and 6 feet in height, with volumetric air flow
of about 250 cfm. With a system of this type, a product collection
rate of about 30 pounds per hour may be maintained. The same
metal oxide slurry may be dried in a chamber 12 feet in diameter
and 20 feet in height, with volumetric air flow of about 12,000
cfm. When employing this latter system, a product collection
rate of about 1500 pounds per hour of spray dried metal oxide
material may be maintained. It has been found that both types
of dryer systems will produce a spray dried metal oxide product
in the size range for a particular electrostatographic use, for
example, on the order of 50 to S00 microns. In addition, both
co-current and counter-current drying systems yield satisfactory
products. The temperature of the drying air may be varied from
about 400F to about 900F at the inlet and from about 200F to
about 700F at the outlet with satisfactory results.
Any suitable type of sintering furnace may be employed
in the sintering step of the process of this invention. Typical
sintering furnaces include a static furnace, a rotary kiln, a
1~4~37
D/73069 tunnel kiln, or an agitated bed furnace. The static furnace type
will generally provide for long residence times. The tunnel kiln
type of sintering furnace generally provides uniform product re-
action, consistent residence time and high capacity throughput.
During the sintering step, a special media such as a flow promoting
ingredient, for example, aluminum oxide, zirconium oxide, or
other materials may be added in combination with the metal oxide
beads in a quantity sufficient to minimize or avoid bead-to-bead
aggl~meration and bead'to furnace wall sticking. Generally, a
quantity of flow promoting ingredient in an amount of from about
0.5 parts by weight to about 2.0 parts by weight, based on the
weight of the metal oxide beads, provides satisfactory results.
Preferably, the flow promoting inyredient is added to the metal
oxide beads prior to sintering in the amount of about l part by
weight to about l part by weight of the metal oxide beads and is
larger than the size of the spray dried metal oxide beads because
bead-to-bead agglomeration and bead to furnace wall sticking is
substantially eliminated. Thus, if the spray dried beads are
about lOO microns, the flow promoting ingredient should be about
600 microns. Further, such a flow promoting ingredient may also
influence the electrostatographic properties of the ferrite carrier
material. In addition, to further avoid or minimize metal oxide
bead sticking to rotary furnace walls a scraping device may be
employed individually or in combination with the flow promoting
ingredient. In any event, the sintering of metal oxide beads
should be under controlled conditions as to preserve the shape
-:L~-
~05~837
D/73069 and particulate nature of the beads while providing a uniform
furnace residence time to produce maximum bead uniformity and
desired properties.
When the sintered ferrite material is to be employed
in the electrostatographic art, it is desirable that the ferrite
material when employed as a carrier possess certain basic properties.
The ferrite carrier should have uniform electrostatographic
properties such as triboelectric response, magnetic permeability,
and electrical conductivity as to meet machine performance require-
ments. The ferrite carrier should be substantially uniform in
size and sufficiently dense individual beads in order to minimizepossible bead sticking to the photoreceptor. The ferrite carrier
should have uniform surface characteristics with a minimum of
surface contamination. Finally, the ferrite carrier should be
of a uniform shape with maximum roundness and shpericity, continu-
ous in surface area, and free of interior voids.
Firing of the metal oxide spray dried beads at elevatedtemperatures to induce reaction of the ferrite components is
generally carried out at between about 1150 and about 1600C.
Actually, lower and higher temperatures may be used, but this is
dictated by the processing time, the furnace materials of construc-
tion generally available, the ferrite formulation and the resulting
strength of the fired bead. Generally, if a nickel-zinc ferrite
carrier material is fired at 1100C for less than one hour, the
carrier material may lack mechanical strength and sufficient solid
state reaction than if one chooses to fire at a higher temperature,
-15-
lOS~137
D/73069 for example, 1400C or 1500C. This is particu7arly importantwith respect to the resulting mechanical strength of the carrier
material. To achieve the desired electrostatographic carrier pro-
perties, based on firing, the firing time, the firing atmosphere,
and the temperature relationship is important to establish the
minimum firing conditions relative to the bead strength. Optimum
electrostatographic ferrite carrier properties are obtained at
sintering temperatures ranging from about 1200C to about 1300C
with a residence time of about 60 to about 180 minutes. The pre
ferred range of sintering temperatures is from about 1150C to
about 1500C with a residence time of about 10 to about 180 minutes
because the ferrite materials are magnetic, have a polycrystalline
spinel structure, are highly resistive, and provide the maximum
electrostatographic response. Satisfactory electrostatographic
ferrite carrier properties are also obtained at sintering tempera-
tures ranging from about 900C to about 1600C with a residence
time of about 5 minutes to about 5 hours. In any event, the
sintering conditions should be sufficient to provide the desired
polycrystalline spinel ferrite structure.
The firing atmosphere used is also important in that
it influences oxygen content and thus the oxidation state of the
metal ions present in the forming crystal structure. Here also,
the conductivity of the ferrite carrier is influenced by an oxygen
rich or deficient atmosphere. An example of the influence of the
firing atmosphere is clearly demonstrated in the preparation of a
ferrous-ferric ferrite from ferric oxide. When the material is
-16-
1~4~37
D/73069 fired in an oxidizing atmosphere. Such as an ambient or
oxygen-rich atmosphere, inferior magnetic properties are
obtained whereas firing in a suitable reducing atmosphere such
as one containing from about one to about eight percent by volume
of oxygen provides acceptable magnetic properties.
Any suitable size of sintering furnace may be em-
ployed in the sintering step of the process of this invention.
Static or tunnel furnaces are preferred because they generally
provide a consistent residence time and uniformity of product
reaction. Thus, metal oxide beads spray dried in accordance
with the process of this invention may be successfully pro-
cessed through a laboratory sized static furnace. Tonnage lots
may be processed in a tunnel kiln, gas or electric fired, at
high throughput rates. Where pre-sintering is desirable, the
preferred conditions consist of pre-sintering the spray dried
metal oxide beads in a furnace at about 900C to about 1300C
with about a 10 to 25 minute residence time because these condi-
tions provide bead strengthening and densification which assists
in preservation of bead shapeand integrity during the final
sintering step. This pre-sintering procedure provides suffi-
cient reaction time to insure desired electrostatographic and
magnetic properties of the ferrite carrier material following
the final sintering step. Following sintering, cooling with
about 12 to 15 hour residence time generally provides transition
from the firing temperature to that of the final cooling. This
method of cooling generally provides retention of desired
electrostatographic properties of ferrite carrier materials.
105~837
D/73069 Magnetic permeability, electrical conductivity, and triboelectri-
city can be controlled by controlling the cooling rate. For
example, the electrical resistivity may be decreased by two to
three orders of magnitude by rapid cooling.
Surprisingly, it has been found that no binder material
or additive other than a deflocculent need be mixed with the feed
slurry of the metal oxide starting materials. Spray dried spheri-
cal metal oxide beads formed in accordance with the process of
this invention unexpectedly retain their shape and integrity during
the gpray drying, collecting, classifying, and sintering steps.
The absnece of binder material benefits the process in that the
slurry is less apt to clog nozzle orifices under pressure and
also, equally important, the drying temperature is not thereby
limited. That is, when a binder material is present, the drying
temperature is usually limited to prevent loss of the binder by
oxidation. In addition, the use of higher drying temperatures
allows an increase in the slurry feed rate to the spray dryer.
If, however, a binder is employed it may comprise any suitable
fugitive film forming material. Typical fugitive film forming
binders include polyvinyl alcohol, dextrine, lignosulfonates and
methyl cellulose.
In accordance with the process of this invention it
has been found beneficial to employ a deflocculent when prepar-
ing the metal oxide slurry. Any suitable deflocculent may be
employed. Typical deflocculents include the ammonium or sodium
salt of polymethacrylic acid, pyrogallic acid, tannic acid, and
-18-
~054837/73069 humic acid and the salts of a mixed aryl hydroxy carboxylic
acid and a complex inorganic acid. The preferred deflocculent
is the sodium salt of polymethacrylic acid because it generally
promotes the preparation of a concentrated metal oxide slurry having
a solids content of up to about 80% by weight in water based on
the total weight of the slurry. Further, in spite of this remark-
ably high solids content, the metal oxide feed slurry may be
pumped to the spray dryer and atomized without clogging in a
pressure nozzle 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.
Any suitable pigmented or dyed electroscopic toner
material may be employed with the ferrite carrier materials pro-
duced in accordance with the process of this invention. Typicaltoner materials include: gum copal, gum sandarac, rosin, cumaron-
eindene resin, asphaltum, gilsonite, phenolformaldehyde resins,
rosin-modified phenolformaldehyde resins, methaxrylic resins,
polystyrene resins, polypropylene resins, epoxy resins, polyethy-
lene resins and mixtures thereof. The particular toner materialto 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 sur-
face of the carrier. Among the patents describing electroscopic
--19--
1~54~3~
D/73069 toner compositions are U. S. Patent 2,659,670 to Copley; U.S.
Patent 2,753,308 to Landrigan; U. S. Patent 3,079,342 to
Insalaco; U. S. Patent Reissue 25,136 to Carlson and U. S. Patent
2,788,288 to Reinfrank et al. These toner material generally
have an average particle diameter between about 1 and about 30
microns. Generally speaking, satisfactory results are obtained
when about 1 part toner is used with about 10 to about 200 parts
by weight of carrier.
Nickel-zinc ferrite and manganese-zinc ferrite carrier
materials produced in accordance with the process of this inven-
tion are preferred because they have triboelectric properties
which vary from 8 to 40 micro-coulombs per gram of toner depend-
ing on the specific toner used. Generally, the triboelectric
value of the ferrite carriers decreases as to the amount of
iron oxide present is increased. Increasing the iron content
beyond the stoichiometric amount of two moles per mole of diva-
lent metal and firing at temperatures above 1200C induces the
formation of divalent iron. The presence of divalent and tri-
valent iron causes an increase in the electrical conductivity of
the ferrite materials. Thus, the extent of divalent iron formed
and the conductivity of the ferrite and resulting developed
electrostatic latent image background desired may be controlled
within broad limits. Therefore, a ferrite carrier material having
high electrical conductivity generally provides a developed elec-
trostatic latent image with low background.
Generally, the ability to magnetically hold a ferrite
carrier material of the nickel-zinc ferrite type in a magnetic
-20-
i~S~1~37
D/73069 bxush configuration diminishes as the nickel to æinc ratio is
decreased in the composition. At the various firing conditions,
a significant loss in magnetic permeability is noted at nickel
to zinc ratios of less than about 0.3. In electrostatographic
machine evaluations it is found that the nickel-zinc ferrite
carrier materials provide optimum electrostatographic response
when the nickel to zinc molar ratio of about 0.3 or greater is
present in the ferrite formulations. In addition, ferrites
represented by MlM2Fex04+ prepared in accordance with the process
of this invention have satisfactory electrostatographic properties
when employed as carriers for electrostatographic developers when
Ml and M2 comprise between about 0.1 to about 0.9 moles of metal
oxide such as those described above and both Ml and M2 total 1.0,
and x comprises about 1.4 to about 4.0 moles of iron. All the
ferrite carriers exhibit magnetic permeability adequate for
magnetic brush operation when ~intered for about 5 minutes to
about 5 hours at temperatures between about 900C and about 1600C.
In accordance with the process of this invention, it is
possible to form spherical substantially void-free metal oxide
spray dried beads by atomizing and drying a slurry of metal oxide
starting materials without the addition of a binder material.
Thus, this process avoids the conventional requirement of mixing
a binder material such as an artificial resin with a metal oxide
slurry in order to form metal oxide beads -that are void-free,
continuous in surface area, and retain their particulate shape
and integrity after spray drying and sintering to convert them
-21-
lOS41~37
D/73069 to ferrites. In addition, this process avoids the step of press-ing or compacting the metal oxide mixtures prior to sintering.
Further, firing temperatures between about 900C and about 1600C
may be employed in sintering the spray dried metal oxide beads
without substantial bead-to-bead agglomeration since no binder
material is used. This process also permits storage of the spray
dried metal oxide beads prior to their sintering without problems
of caking, bead fracture, or significant loss of physical, chemi-
cal, and mechanical properties. In addition, sticking of beads
to the surfaces of sintering equipment is substantially avoided.
Further, ferrite materials produced according to this process have
been found to possess improved uniformity of particle size and
particle size distribution. The uniformity of particle size that
may be obtained by the process of this invention has been found
to provide ferrite carrier materials which have properties that
are extremely desirable when employed in electrostatographic
development processes. This process further provides economic
efficiency and simplicity in the production of ferrite materials,
avoids agglomeration and clogging problems in processing equipment
common to conventional methods of preparing ferrite materials,
removes restrictions imposed on conventional methods of preparing
ferrite materials, is capable of producing extremely small particle
size ferrite materials and ferrite materials of a desired size,
and is particularly advantageous in preparing ferrite materials
ranging from about 50 to 500 microns. Finally, this process may
be employed to form ferrite materials of various compositions and
characteristics.
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~05~837
3069 DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following examples further define, describe and
compare exemplary methods of preparing ferrite materials accord-
ing to the process of the present invention. Parts and percent-
ages are by weight unless otherwise indicated. The examples,
other than the control examples, are intended to illustrate the
various preferred embodiments of the present invention.
In the following examples, the unit employed for spray
drying is a Bowen Tower Laboratory Spray Dryer manufactured by
Bowen Engineering Incorporated, North Branch, ~ew Jersey. This
unit has a bottom chamber collector and a single cyclone collector.
This chamber collector is 30 inches in diameter and the vertical
chamber height is 6 feet. Nozzle atomization is upward with a
maximum vertical particle path height of about 8 feet. The incoming
air is heated by direct gas firing.
EXAMPLE I
A powdered metal oxide and water feed slurry comprising
about 100 pounds of about 62.6 percent ferric iron oxide having a
particle size of àbout 0.5 micron, about 26.6 percent zinc oxide
having a particle size of about 0.1 micron, about 10.8 percent
nickel oxide having a particle size of about 10 micronsj and about
43.2 pounds of water is prepared using a high speed dispersator.
About 0.825 percent by weight, based on the weight of the solids,
of Darvan 7 ~the sodium salt of a polymethacrylic acid available
2~ from the R. T. Vanderbilt Oompany~ is added to the oxide slurry
mixture~ The slurry mixture is about 70 percent by weight of
. ..
* ~rademark
-23_
T~ ~
i~5~837
D/73069 solids. The slurry is screened using 80 mesh seives to remove
non-wetted agglomerates characteristic of open tank mixing of
oxides. The slurry is found to have a viscosity of about 9,000
centipoises at about 23C. This slurry is then fed to the sprav
dryer at a feed rate of between about 45 and about 55 pounds per
hour, a drying air input temperature of about 490-500F~ and an
outlet temperature of about 310-330F The type of atomizer is
a single-fluid pressure nozzle and the atomizing force is about
160-165 psig provided by moving cavity pump. Spray dried metal
oxide beads of about lO0 microns are obtained. The dryer surfaces
are dry. The beads collected in the dryer chamber are a dry,
free-flowing powder, but are found to contain voids in the interior
of the beads and to have non-spherical surfaces as depicted in
Figure 1.
EXAMPLE II
A powdered metal oxide and water feed slurry comprising
about 5600 grams of about 62.6 percent ferric iron oxide having a
particle size of about 0.5 micron, about 26.6 percent zinc oxide
having a particle size of about 0.1 micron, about 10.8 percent
nickel oxide having a particle size of about lO microns, and about
1950 grams of water is prepared using a high speed dispersator.
About 0.29 percent by weight, based on the weight of the solids,
of Darvan 7 (the sodium salt of a polymethacrylic acid available
from the R~ T. Vanderbilt Company) is added to the oxide slurry
mixture. The slurry mixture is about 65 percent by weight of solids.
The slurry is screened using 80 mesh seives. The slurry is found
-24-
lOS4~37
D/73069 to have a viscosity of about 131,000 centipoises at about 23C.
'rhis slurry is then fed to the spray dryer at a feed rate of
between about 45 and about 55 pounds per hour, a drying air
input temperature of about 490-500F, and an outlet temperature
of about 310-330F. The type of atomizer is a single-fluid
pressure nozzle and the atomizing force is about 160 psig pro-
vided by moving cavity pump. Spherical spray dried metal oxide
beads of about 100 microns are obtained. The dryer surfaces are
dry. The beads collected in the dryer chamber are a dry, free-
flowing powder and are found to have void-free interiors and
continuous surface areas as depicted in Figure 2 and Figure 3,
respectively.
EXAMPLE III
A powdered metal oxide and water feed slurry compris-
ing about 9600 grams of about 62.6 percent ferric iron oxide
having a particle size of about 0.5 micron, about 26.6 percent
zinc oxide having a particle size of about 0.1 micron, about
10.8 percent nickel oxide having a particle size of about 10
microns, and about 2850 grams of water is prepared using a high
speed dispersator. About 0.33 percent by weight, based on the
weight of the solids, of Darvan 7 (the sodium salt of a poly-
methacrylic acid available from the R. T. Vanderbilt Co.) is
added to the oxide slurry mixture. The slurry mixture is a~out
65 percent by weight of solids. The slurry is screened using 80
mesh seives. The slurry is found to have a viscosity of about
17,000 centipoises at about 23C. This slurry is then fed to the
iO541~37
D/73069 spray dryer at a feed rate of between about 45 and about 55 pounds
per hour, a drying air input temperature of about 500F, and
an outlet temperature of about 310-330F. The type of atomizer
is a single-fluid pressure nozzle and the atomizing force is
about 125 psig provided by moving cavity pump. Spray dried
metal oxide beads of about 100 microns are obtained. The dryer
surfaces are dry. The beads collected in the dryer chamber are
a dry, free-flowing powder, but are found to contain voids in the
interior of the beads and to have non-spherical surfaces.
EXAMPLE IV
A powdered metal oxide and water feed slurry compris-
ing about 14,000 grams of about 62.6 percent ferric iron oxide
having a particle size of about 0.5 micron, about 26.6 percent
zinc oxide having a particle size of about 0.1 micron, about 10.8
percent nickel oxide having a particle size of about 10 microns,
and about 4800 grams of water is prepared using a high speed
dispersator. About 0.28 percent by weight, based on weight of
the solids, of Darvan 7 (the sodium salt of polymethacrylic acid
available from the R. T. Vanderbilt Co.) is added to the oxide
slurry mixture. The slurry mixture is about 65 percent b~
weight of solids. The slurry is screened using 80 mesh seives.
The slurry is found to have a viscosity of about 106,000 centi-
poises at about 23C. This slurry is then fed to the spray
dryer at a feed rate of between about 45 and about 55 pounds per
hour~- a drying air input temperature of about 500F, and an
outlet temperature of about 310-355F. The type of atomizer is
-26-
1054~37/73069 a single-fluid pressure nozzle and the atomizing force is about
175 psig provided by moving cavity pump. Spherical spray dried
metal oxide beads of about 100 microns are obtained. The dryer
su~faces are dry. The beads collected in the dryer chamber are a
dry, free-flowing powder and are found to have void-free interiors
and continuous surface areas.
EXAMPLE V
A powdered metal oxide and water feed slurry comprising
about 5700 grams of about 62.6 percent ferric iron oxide having
a particle size of about 0.5 micron, about 26.6 percent zinc oxide
having a particle size of about 0.1 micron, about 10.8 percent
nickel oxide having a particle size of about 10 microns, and about
1900 grams of water is prepared using a high speed dispersator.
About 0.75 percent by weight, based on the weight of the solids,
of Clay Deflocculent #5, (the ammonium salt of a mixed aryl hydroxy
carboxylic acid containing a small amount of sodium salt of a com-
plex inorganic acid, available from the R. T. Vanderbilt Co.) isadded to the oxide slurry mixture. The slurry mixture is about 65
percent by weight of solids. The slurry is screened using 80 mesh
seives. The slurry is found to have a viscosity of about 66,000
centipoises at about 23C. This slurry is then fed to the spray
dryer at a feed rate of between about 45 and about 55 pounds per
hour, a drying air input temperature of about 500F, and an outlet
temperature of about 310F. The type of atomizer is a single-fluid
pressure nozzle and the atomizing force is about 150 psig provided
by moving cavity pump. Spherical spray dried metal oxide beads of
~54837
D/73069 about 100 microns are obtained. The dryer surfaces are dry. The
beads collected in the dryer chamber are a dry, free-flowing powder
and are found to have v,oid-free interiors and continuous surface
areas.
EXAMPLE VI
A powdered metal oxide and water feed slurry comprising
about 21,000 grams of about 62.6 percent ferric iron oxide having
a particle size of about 0.5 micron, about 26.6 percent zinc oxide
having a particle size of about 0.1 micron, about 10.8 percent
nickel oxide having a particle size of about 10 microns, and about
7200 grams of water is prepared using a ball mill mixing device.
About 0.33 percent by weight, based on the weight of the solids,
of Darvan 7 (the sodium salt of a polymethacrylic acid available
from the R. T. Vanderbilt Co.) is added to the oxide slurry mix-
ture. The slurry mixture is about 64 percent by weight of solids.
The slurry did not require screening. The slurry is found to have
a viscosity of about 46,000 centipoises at about 23C. This slurry
is then fed to the spray dryer at a feed rate of between about 45
and about 55 pounds per hour, a drying air input temperature of
about 500F, and an outlet temperature of about 260F. The type
of atomizer is a single-fluid pressure nozzle and the atomizing
force is about 170 psig provided by moving cavity pump. Spherical
spray dried metal oxide beads of about 100 microns are obtained.
The dryer surfaces are dry. The beads collected in the dryer cham-
ber are a dry, free-flowing powder and are found to have smaller
voids in their interiors than those made with lower viscosity
slurries.
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~OS4~37
D/73069 EXAMPLE VII
A powdered metal oxide and water feed slurry compris-
ing about 21,000 grams of about 62.6 percent ferric iron oxide
having a particle size of about 0.5 micron, about 26.6 percent
zinc oxide having a particle size of about 0.1 micron, about 10.8
percent nickel oxide having a particle size of about 10 microns,
and about 7200 grams of water is prepared using a ball mill
mixing device. About 0.32 percent by weight, based on the weight
of the solids, of Darvan 7 (the sodium salt of a polymethacrylic
acid available from the R. T. Vanderbilt Co.) is added to the
oxide slurry mixture. The slurry mixture is about 65 percent
by weight of solids. The slurry did not require screening. The
slurry is found to have a viscosity of about 91,000 centipoises
at about 23C. This slurry is then fed to the spray dryer at a
feed rate of between about 45 and about 55 pounds per hour, a
drying air input temperature of about 500F, and an outlet temp-
erature of about 280-340F. The type of atomizer is a single-
fluid pressure nozzle and the atomizing force is about 170 psig
provided by moving cavity pump. Spherical spray dried metal
oxide beads of about 100 microns are obtained. The dryer surfaces
are dry. The beads collected in the dryer chamber are a dry, free-
flowing powder and are found to have void-free interiors and
continuous surface areas.
EXAMPLE VIII
A powdered metal oxide and water feed slurry comprising
about 42,000 grams of about 62.6 percent ferric iron oxide having
-29-
105~1~37
D/73069 a particle size of about 0.5 micron, about 26.6 percent zinc
oxide having a particle size of about 0.1 micron, about 10.8
percent nickel oxide having a particle size of about 10 microns,
and about 14,500 grams of water is prepared using a ball mill
mixing device. About 0.32 percent by weight, based on the weight
of the solids, of Darvan 7 (the sodium salt of a polymethacrylic
acid available from the R. T. Vanderbilt Co.) is added to the
oxide slurry mixture. The slurry mixture is about 65 percent by
weight of solids. The slurry did not require screening. The
slurry is found to have a viscosity of about 80,000 centipoises
at about 23C. This slurry is then fed to the spray dryer at a
feed rate of between about 45 and about 55 pounds per hour, a
drying air input temperature of about 500F, and an outlet tempera-
ture of about 310-330F. The type of atomizer is a single-fluid
pressure nozzle and the atomizing force is about 150 psig provided
by moving cavity pump. Spherical spray dried metal oxide beads of
about 100 microns are obtained. The dryer surfaces are dry. The
beads collected in the dryer chamber are a dry, free-flowing powder
and are found to have void-free interiors and continuous surface
areas.
EXAMPLE IX
Spray dried metal oxide beads prepared in accordance
with the process of Example VIII are placed in a direct electric-
fired static kiln. About 1 pound, per 1 pound of the metal oxide
beads, of aluminum oxide particles having a diameter of between
about 600 microns are included with the metal oxide beads.
-30-
10~4837
D/73069 Sintering of the oxide beads is conducted at a temperature of
about 1300C for about two hours. After cooling, the sintered
metal oxide beads are examined and found to have void-free
i~teriors and continuous surface areas.
Although specific materials and conditions are set forth
in the above exemplary processes of making ferrite materials by
the process of this invention, these are merely intended as
illustrations of the present inventionO There are other ferrite
materials, solvents, substituents and processes such as those
listed above which 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 reading of the present disclo-
sure. These are intended to be included within the scope of this
invention.
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