U.S. patent number 3,627,682 [Application Number 04/767,977] was granted by the patent office on 1971-12-14 for encapsulated particulate binary magnetic toners for developing images.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Joseph P. Hall, Jr., George J. Young.
United States Patent |
3,627,682 |
Hall, Jr. , et al. |
December 14, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
ENCAPSULATED PARTICULATE BINARY MAGNETIC TONERS FOR DEVELOPING
IMAGES
Abstract
Described and claimed are flowable, particulate, binary toners
for developing magnetic images comprising a particulate hard
magnetic material, e.g., Fe.sub.3 O.sub.4 or CrO.sub.2, and a
particulate soft magnetic material, e.g., Fe, each type of material
being present in substantially each toner particle.
Inventors: |
Hall, Jr.; Joseph P.
(Shavertown, PA), Young; George J. (Dallas, PA) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
25081146 |
Appl.
No.: |
04/767,977 |
Filed: |
October 16, 1968 |
Current U.S.
Class: |
430/106.2;
252/62.54; 430/106.3; 430/111.41; 430/115; 430/116; 252/62.53 |
Current CPC
Class: |
G03G
5/16 (20130101); G03G 9/0832 (20130101); G03G
9/0833 (20130101); G03G 9/083 (20130101) |
Current International
Class: |
G03G
9/083 (20060101); G03G 5/16 (20060101); G03g
009/02 () |
Field of
Search: |
;252/62.1,62.53,62.54
;117/93.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lesmes; George F.
Assistant Examiner: Brammer; J. P.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A flowable particulate magnetic toner substantially each
particle of which comprises at least one finely divided
magnetically hard material having a remanence of at least about 20
percent of the saturation magnetization, and at least one finely
divided magnetically soft material having a remanence of less than
about 5 percent of the saturation magnetization, said hard and soft
materials being substantially uniformly dispersed in a compatible
nonmagnetic thermoplastic resin present at about 20 percent to 40
percent by weight of the entire composition, the weight ratio of
magnetically hard to magnetically soft material being in the range
1:6 to 4:1.
2. The magnetic toner of claim 1 wherein the particles are
substantially spherical.
3. The magnetic toner of claim 1 wherein the weight ratio of
magnetically hard to magnetically soft material is about 1:1.
4. The magnetic toner of claim 1 wherein the diameter of the
flowable particles ranges from about 3 to about 20 microns.
5. The magnetic toner of claim 1 wherein the resin is a
polyamide.
6. The magnetic toner of claim 1 containing additionally from about
1 to 5 percent of a color control agent selected from the group
consisting of carbon black, pigments and dyes.
7. The magnetic toner of claim 6 wherein the color control agent is
black.
8. The magnetic toner of claim 1 wherein the magnetically hard
material is a magnetic iron oxide.
9. The magnetic toner of claim 1 wherein the magnetically hard
material is magnetic chromium oxide.
10. The magnetic toner of claim 1 wherein the magnetically soft
material is iron.
11. The magnetic toner of claim 1 wherein the magnetically hard
material is Fe.sub.3 O.sub.4 and the magnetically soft material is
iron.
12. The magnetic toner of claim 1 wherein the magnetically hard
material is magnetic chromium oxide and the magnetically soft
material is iron.
13. The magnetic toner of claim 1 carrying up to about 2 percent by
weight of a dispersing agent.
14. The magnetic toner of claim 1 wherein the dispersing agent is
adhered to the outside of the particle.
15. The magnetic toner of claim 1 wherein the dispersing agent is
distributed throughout the particles.
16. A dispersion of the magnetic toner of claim 1 in an inert,
liquid dispersions medium.
17. The dispersion of claim 16 wherein the dispersions medium is a
polar solvent.
18. The dispersion of claim 16 wherein the dispersions medium is
water.
19. The dispersion of claim 16 wherein the dispersions medium is
hydrocarbon.
20. The dispersion of claim 16 wherein the dispersions medium is a
halogenated hydrocarbon.
21. A dry mixture of the magnetic toner of claim 1 with an inert
solid, particulate carrier.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to, and has as its principal object
provision of, improved encapsulated particulate magnetic pigments
of developers for developing magnetic images, which particulate
substances contain a hard magnetic material and a soft magnetic
material essentially within each particle.
Magnetic developers are frequently called "toners" in the art. The
term "toner" will accordingly be used in the following discussion
to refer to a particulate material capable of developing or making
visible a magnetic image. The novel toners of this invention are
particularly useful in the readout step of the thermomagnetic
copying processes disclosed in the Nacci Belgian Pat. No. 672,017
and thermomagnetic imaging processes disclosed in the Nacci Belgian
Pat. No. 672,018. See also the copending, coassigned Nacci U.S.
application Ser. No. 682,234, filed Nov. 13, 1967, now
abandoned.
2. Description of the Prior Art
As shown, for example, by Atkinson et al. U.S. Pat. No. 2,826,634,
the use of iron or iron oxide particles, either alone or
encapsulated in low-melting resins or binders, for developing
magnetic images, is well known to the art. These toners have been
successfully used to develop magnetic images recorded on magnetic
tapes, films, drums and printing plates. The encapsulating resin or
binder may aid in transferring the decorated magnetic image (or the
developing pigment) to paper and can further be heated, pressed or
vapor softened or subjected to combinations of these treatments to
attach physically or fix the magnetic particles to the surface
fibers of the copy paper. In general, images on copy paper using
either iron- or iron oxide-based magnetic particles have been of
low-optical density or unpleasing appearance due to the difficulty
of hiding the natural metallic luster of iron or due to the feeble
pickup of iron oxide particles resulting from their tendency to
form agglomerates that are in an internally magnetically satisfied
state, relatively unresponsive to the weak fields emanating from
the surface of magnetic images recorded on films, tapes, printing
plates or other magnetic storage media.
The present invention provides improvements in toners. In
particular, it provides toners with desirable printing
characteristics For example, slurries made from these toners,
although settling rapidly, are easily reslurried and are
nonreactive chemically with the dispersing medium (such as, the
rusting of iron by water) so that they print equally well even
after long periods of storage. The dispersions have less tendency
to agglomerate due to magnetic forces than dispersions containing
either hard or soft magnetic materials used alone. Also the optical
reflectance density of the ultimate copy and its appearance in
terms of image crispness and sharpness are generally improved with
these toners that both pack more tightly on the magnetic image
(because of lack of magnetic agglomeration) to give superior
areawise coverage on the ultimate copy surface and are more
strongly attracted to the magnetic image.
THE DRAWING
The achievement of the above-noted and other objects of the
invention will be evident from the remainder of the specification
and from the drawing (essentially FIG. 6 of the above-mentioned
application of Nacci, Ser. No. 682,234) wherein:
The figure shows a schematic view of apparatus for the
thermographic repoduction of documents in which the magnetic toners
of this invention are particularly useful. In the figure, a
magnetized thermomagnetic copying member in the form of a film 35
is stretched over the surface of a transparent drum 20 which is
driven in the direction indicated in the arrow. The document which
is to be copied, 21, is fed through the machine in stationary
relationship with the copying member by friction applied by the
flexible belt 22 which holds the document in contact with the
copying member and moves synchronously therewith over the idling
rollers 23 and 24. At the center of drum 20 is positioned a Xenon
lamp 25 which emits flashes of light at high intensity with a
duration of the order of a millisecond over the surface of the
member in contact with the document as defined by the stationary
mask or shield 36. Each flash forms a magnetic image of the
document on the copying member. The copying member returns to its
initial thermal state in about 0.5 second, and the flashes are
spaced in time at somewhat longer intervals. The speed of document
feed and drum rotation is maintained so that each portion of the
document is exposed to the radiation at least once while in contact
with the copying member.
The magnetic image can be developed by padding on a toner of this
invention by the padding roll 26 which dips in a bath 27 containing
the toner in suspension. Surplus toner is removed by wiping means
28. The image is then transferred to paper 38 which is fed from a
roll 29, passing over the idling roll 30 and thence in contact with
the recording member by pressure roll 31, when the image is
transferred. The toner particles are then fused to the copying
paper by a band of heaters 32, and the paper is removed on the roll
33.
Once the document has passed through the machine forming the
magnetic image, a large number of copies can be made by continued
rotation of drum 20, since the image is substantially permanent.
The image can be destroyed and the magnetic recording member
returned to its uniformly magnetized state ready to copy further
documents by operation of the magnetic head 34 .
DESCRIPTION OF THE INVENTION
In accordance with the present invention, it has now been found
that a surprising improvement in the quality of images printed on
copy paper, transparent plastic sheets and the like can be effected
through the use of resin-encapsulated magnetic pigments that
contain binary mixtures of at least one each of a magnetically hard
and a magnetically soft powder material that may optionally contain
opacity control agents, release agents, and the like. The
magnetically soft powder material may be iron or another
high-permeability, low-remanence material, such as certain of the
ferrites, e.g., (Zn, Mn)Fe.sub.2 O.sub.4, or permalloys, while the
magnetically hard material may be an iron oxide, preferably
Fe.sub.3 O.sub.4, .nu.-Fe.sub.2 O.sub.3, other of the ferrites,
e.g., BaFe.sub.12 O.sub.19, or chromium dioxide. The ratio of the
hard to soft components may vary considerably but is preferably in
the range of 1:6 to 4:1 by weight. The encapsulating resin is
generally present in the order of 20-40 percent, preferably 20-30
percent, by weight of the final composition. Small amounts of the
order of 1-5 percent of additives, such as carbon black or black or
colored dyes, for a blacker or colored copy, and stearamide or
silicone derivatives for easier transfer to paper may be added if
desired. For reasons given later, particle diameter in the range of
3-20 microns is preferred.
The improved binary magnetic toners of this invention are prepared
by a process in which a hard and soft magnetic material are mixed
together in desired proportions with an encapsulating resin in a
solvent for the resin, ball-milled and spray dried. More
specifically, the respective hard and soft magnetic powders (cf.
seq. for meaning of "hard" and "soft") in ratio of 1:6 to 4:1, but
preferably approximately equal quantities by weight, are intimately
mixed and dispersed, preferably by ball-milling for a length of
time extending up to 17 hours or more at about 60 percent by weight
nonvolatiles content, in a multicomponent dispersions system
consisting of:
1. An organic liquid selected on the basis of its chemical
inertness toward the magnetic materials, its volatility
characteristics, and its ability to dissolve the encapsulating
polymer;
2. A readily fusible, organosoluble organic binder or encapsulating
resin; and
3. Optionally (a) additives such as carbon black, pigments or dyes
to control color, (b) agents such as stearamide or silicones to
promote easy release during magnetic image transfer, and (c) agents
such as conductive carbons or electron donors and acceptors to
control the electrostatic properties of the toner particles.
The dispersion resulting from the steps above is separated from the
ceramic balls, sand, or other grinding means, diluted, and spray
dried at a nonvolatiles content of about 20 percent by weight. The
diluent is a compatible organic liquid, usually the same solvent
employed during preparation of the dispersion. Spray drying is
accomplished by conventional means, e.g., by dropping the diluted
dispersion onto a disk rotating at high speed or by using a
conventional spray drying nozzle. The droplets are dried in a
chamber through which a heated gas is flowing, obtained, for
example, by combustion of natural gas in air. Gas flow and
temperature are adjusted in known manner to remove solvent quickly,
leaving discrete, free-flowing, approximately spherical toner
particles, preferably about 3-20 microns in size.
Spray-dried toners tend to have the magnetic particles buried in
the resin spheres. Occasionally it is advantageous to have these
particles at the surface of the toner spheres. This may be
accomplished by the additional step of abrading the spheres by
fluid-energy milling (jet pulverizing) to expose the magnetic
particles.
The toners prepared as above may be applied to imaged magnetic
films either from a dispersion, that is, such as an ink, or in the
dry state. In the former case, a nonsolvent dispersion medium such
as water, etc., is generally employed. In the latter case, the
toner is conveniently mixed with an inert solid, particulate
material such as polystyrene beads. In either case, the toners can
be transferred from the imaged magnetic film to an ultimate copy
which can be further treated as by heat if desired.
The "magnetically hard" and "magnetically soft" materials which
form the basis of the present toners are substances which are,
respectively, permanently magnetizable and substantially
nonpermanently magnetizable under similar conditions below the
Curie point of the materials (cf. British Pat. No. 1,108,102). A
magnetically hard material, as the term is used here, has a
coercivity of at least 40 Oe and exhibits a significant remanence.
The latter is 20 percent or more of the saturation magnetization
and the material can be used to fashion a permanent magnet.
Magnetically soft material has low coercivity, e.g., of the order
of an oested or less when in bulk form, and high permeability,
permitting saturation to be obtained with a small applied field.
More importantly, the soft material exhibits a remanence of less
than 5 percent of the saturation magnetization. The ideal magnetic
properties for soft materials are found in high permeability,
low-loss compositions used in transformers, but such properties are
seldom realized in small-diameter particles. Nevertheless, as long
as the remanence is low, these particles serve very well even
though they are not, strictly speaking, high-permeability
materials.
"Soft" magnetic materials are discussed widely in the literature,
e.g., by E. W. Lee and R. L. Lynch, Advances in Physics, supplement
to Philosophical Magazine 8 (July 1959), pp. 292-348. This high
permeability implies a narrow hysteresis loop, a low-energy
product, (BH).sub.max, and low-hysterisis loss. Such materials are
used in transformers and motors. Examples are soft iron,
silicon-iron alloys, and the permalloys, i.e., magnetic alloys of
nickel and iron. Certain ferrites (such as Mn.sub.0.5 Zn.sub.0.5
Fe.sub.2 O.sub.4) can also be used but their low magnetizations
generally give an inferior toner. Preferred as soft magnetic
materials for use in this invention are iron-based pigments such as
carbonyl iron, iron flakes and iron alloys.
Many magnetic materials usually designated as soft may become hard
and show high-coercive force when prepared as fine particles.
Geometric factors, including size and shape of the particle, are
important. For example, iron is normally considered a "soft"
magnetic material with a coercivity of a fraction of an oersted.
However, small iron particles composed of single magnetic domains
with lengths great compared to their diameters can be expected to
show coercivities of the order of 10.sup.3 -10.sup. 4 Oe. In this
case, high coercivity is due to shape anisotropy. For some other
materials, such as manganese bismuthide or cobalt, high coercivity
for single domain particles may be the result of magnetocrystalline
anisotropy arising from an easy direction of magnetization along a
particular crystalline direction. Even fine nickel particles should
show a high coercivity under uniaxial stress. Many normally "soft"
magnetic materials not in single domain form can be made to exhibit
a high coercivity after being subjected to cold work or other
similar treatments designed to introduce defects or internal
strains which serve to pin or block movement of domain walls.
Further discussion of "Hard Magnetic Materials" can be found in the
article by that title by E. P. Wohlforth, Advances in Physics,
supplement to Philosophical Magazine 8 (Apr. 1959), pp. 87-224 and
in the book by R. M. Bozorth on "Ferromagnetism," D. Van Nostrand
and Company, Princeton, New Jersey (1951), particularly the section
on fine particles, pp. 828-834.
"Hard" magnetic materials are usually characterized by having a
high-intrinsic coercivity, iH.sub.c, ranging from a few tens of
oersteds, e.g., 40 Oe, to several hundred or even several thousand
oersteds and a relatively high remanence, B.sub.r, when the
materials are removed from a magnetic field. Accordingly, these
"hard" materials will in general have a high-energy product
(BH).sub.max, ranging as high as several tens of thousands of
joules/m.sup.3. Such materials are of relatively low permeability
and require high fields for magnetic saturation.
Examples of hard magnetic materials include the permanent magnetic
materials, such as the "Alnicos," the "Lodexes" (acicular
iron-cobalt alloys encased in lead or plastic, manufactured by the
General Electric Company), the "Indox" barium ferrite compositions,
and materials used in tape recording, magnetic discs, and magnetic
printing inks. These latter materials include .nu.-iron oxide
(Fe.sub.2 O.sub.3), magnetite (black Fe.sub.3 O.sub.4), chi-iron
carbide and chromium dioxide (CrO.sub.2).
Preferred Fe.sub.3 O.sub.4 particles include the commercially
available types "3000" and "4000" (Wright Industries), the much
less expensive naturally occurring and modified magnetites, and
acicular forms of Fe.sub.3 O.sub.4, such as the IRN 100 sold by C.
K. Williams and Company. Other hard magnetic iron oxides can be
used such as .nu.-Fe.sub.2 O.sub.3, especially when reddish or
brownish toner particles are desired. The recently available black,
magnetically hard pigment CrO.sub.2 is especially preferred because
of its magnetic properties.
The ratio of the hard to soft component may vary considerably but
is preferably in the range 1:6 to 4:1 by weight, a ratio of about
1:1 being more preferred. It is preferred that the soft magnetic
material be less than 5 microns in maximum dimension and that the
hard magnetic material have a maximum dimension not exceeding about
1 micron.
A variety of organic solvents may be used as solvents for binders
and dispersion media for the hard and soft magnetic materials
during toner preparation. Use of a 1:1 mixture of
n-propylalcohol:toluene and n-propyl alcohol:xylene is described in
the examples. The particular organic solvent selected depends
primarily upon its ability to dissolve the binder.
It is important, moreover, that the solid toner particles of this
invention be free-flowing with a sticking temperature above that
encountered in shipment and storage. Stick temperature is related
to thermoplasticity, i.e., the melting or fusion temperature of the
binder, and if too low, the toner particles may become cemented or
sintered together, thereby interfering with their use in image
development systems. In wet systems, binders of somewhat lower
sticking temperature may sometimes be used provided the particles
are protected by a film of the dispersing medium. It will be
appreciated that free-flowing characteristics are particularly
important for toners used in dry imaging methods, e.g., the cascade
method described in table I and in examples 5 and 6 below.
Particles precoated with or containing surfactants (see below) for
use in liquid development systems should also be free-flowing to
facilitate complete dispersion and to prevent clogging of
replenishment devices.
Binders are preferably of the thermoplastic type in order to permit
fixing to the paper by melting or fusion. Certain waxy polymers are
also useful and can be caused to flow into paper by pressure.
Preferred binders include the low molecular weight polyamides and
ethylene/vinyl acetate copolymers. Of these, for ease of spray
drying, ease in image development from aqueous or nonaqueous
dispersions and ease of fixing to paper by fusion, the
low-molecular weight polyamides are especially preferred. The
binder (encapsulating resin) is generally present in the order of
20 -40 percent, preferably 20 -30 percent, by weight of the final
composition.
Binders and solvents other than those described in the examples may
be used. Preferred binders readily melt to fluid liquids, but have
a sticking temperature above about 60.degree. C. In some instances,
the solutions must be kept warm to prevent gelation prior to spray
drying. The binders are readily soluble in organic liquids that are
sufficiently volatile for spray drying, and they are chemically
resistant toward the hard and soft magnetic materials present in
the toners. The binder serves to fix the developed image to paper
and hence is a functional part of the toner. Binders and solvents
in addition to those described in the examples include:
1. Copolymers with functional groups, e.g.,
styrene/dimethylaminomethacrylate copolymer dissolved in xylene or
toluene; ehtylene-methacrylic acid copolymer dissolved in 7:2 parts
by weight toluene: Triclene trichloroethylene; and
styrene/acrylonitrile copolymer dissolved in toluene.
2. Copolymers containing vinyl acetate, e.g., ethylene/vinyl
acetate copolymer dissolved in xylene; and mixtures of
ethylene/vinyl acetate copolymer and waxes dissolved in
toluene.
3. Hydrocarbon types, e.g., "Paraflint," or "Tervan" 2,865
hydrocarbon waxes dissolved in toluene or xylene; Piccolastic D125
styrene resin dissolved in toluene; and styrene/indene copolymer
dissolved in toluene or xylene.
4. Fluorine-containing copolymers, e.g.,
tetrafluoroethylene/vinylidene fluoride/vinyl butyrate copolymer
dissolved in methyl ethyl ketone, and tetrafluoroethylene/vinyl
acetate copolymer dissolved in cyclohexanone.
Additives to enhance the functional behavior of the toners include
color and opacity control agents, surface modifiers, release
agents, materials to increase affinity for the copy paper, and the
like.
Since toners are normally used to give black copy on white paper,
carbon black or a black dye such as "Nigrosine" SSB may be added to
give a more pleasing appearance, enhance reflectance optical
density, and hide the metallic gray of particulate iron in the
toners and exert control over electrostatic properties of the
toners. Other dyes and pigments may be added to give a range of
colored toners; for many of these .nu.-Fe.sub.2 O.sub.3
(yellow-red) is used in preference to Fe.sub.3 O.sub.4 (black) as
the hard magnetic species along with dyes or pigments. Conductive
carbons such as acetylene blacks or graphite and certain electron
donors or acceptors may be used to control electrostatic properties
of the toner particles. Stearamide or silicones may be added to
promote easy release during the magnetic image transfer to paper.
Other modifications to the surface of the toner particles to
enhance these properties are well within the state of the art.
Small amounts of the order of 1-5 percent of additives, such as
carbon black or colored dyes, for a blacker or colored copy, and
stearamide or silicon derivatives for easier transfer to paper are
usually adequate.
Other additives of particular importance where the toners are to be
applied to a magnetic image from a liquid dispersion are
surfactants. These materials aid greatly in dispersing the binary
toners in water. A preferred surfactant is "Lakeseal" (see
below).
The surfactant or dispersing agent can be added to the toner before
it is spray dried, in which case it is distributed throughout the
particle, or the particles can be coated with the agent after spray
drying, in which case it is carried only on the surface of the
particle. Note example 7, below. Both types of toner particles,
i.e., those carrying up to about 2 percent by weight of surfactant
distributed throughout the particles or those carrying it held on
the surface, constitute additional aspects of the invention.
As noted above, in the case of dry development of the toner
particles, other additives may be desirable. Thus, the buildup of
static charges between toner particles and the image-bearing
magnetic film can lead to high-toner pickup in background or
nonimage areas. This can be controlled by use of conductive
recording media and conductive toner particles or carriers;
conductive carbon particles, for example, added to the toner help
to dissipate the electrical charges. Carries, e.g., polystyrene or
glass beads, which give a preferred charge orientation, are also
useful in preventing agglomeration as a result of static in dry
development systems.
The liquid used for preparing toner dispersions in "wet"
development systems plays an important role. Preferably, it should
be of high density, low viscosity and low-surface tension,
nonflammable or with a high-flash point to minimize fire hazards,
nontoxic, easily volatile with a low heat of vaporization, cheap
and readily available, nonreactive with and exerting no undesired
solvent action on the image-bearing magnetic storage medium or with
the toner binder or the toner pigments, and polar to aid in
dissipating static charges induced by the cyclical processes in a
copier or duplicator. Compromises, of course, must be made in
selecting a liquid to meet as many of these conflicting
requirements as possible. Liquids that have been used successfully
include Freon 113 (1,1,2-trichlorotrifluoroethane, b.p.
47.6.degree. C.), certain alcohols (methanol), saturated
hydrocarbons such as hexane or higher boiling petroleum fractions,
e.g., "Isopar-G" (Humble Oil Co.) or Du Pont No. 711 odorless paint
thinner and water. Especially preferred among these liquids are
water or high-boiling hydrocarbon fractions containing minor
amounts of dissolved species such as methanol or antistatic
agents.
Water-based toner slurries, which contain nearly 50 weight percent
solids, were used for developing magnetic images in the majority of
the examples below. In such systems, the role of the dispersing
agent used with the toner is of great importance. Investigation of
a number of commercially available dispersants including
representative soaps, anionic, cationic, and nonionic surfactants,
both pure and in so-called "built" formulations showed that most of
these were operable, but most presented difficulty after aging for
ca. 10 days in that the toner tended to stick to the image-carrying
magnetic film, making cleaning difficult. An especially preferred
dispersing agent for aqueous systems is the commercially available
built detergent laboratory glass cleaner sold under the name of
"Lakeseal" discussed below. "Lakeseal" dispersions of the preferred
toners have been found to function satisfactorily after long
periods of use and storage times in excess of 6 months.
The type of toner used will vary depending on the nature of the
ultimate copy sheet, i.e., where the image is to reside finally.
Thus, as described in detail in the examples, if the final residual
copy sheet contains an, or is, adhesive, the image can be
successfully transferred from the imaged magnetic surface by simple
pressure after the image is developed using suitable magnetic toner
powder. On the other hand, if it is desired to transfer the image
to a nonadhesive copy support e.g., conventional white bond paper
or a clear film for projection, a different type of toner particle
may be used. In such a permutation, the magnetic particles,
whatever their nature, will have been previously coated with a
thermoplastic material, such as a relatively low-melting polymer or
copolymer which will be inert to magnetic fields. Thus in this
permutation, the developing toner powder will be imagewise
attracted to the imagewise recorded magnetic record and then can be
transferred by pressure, fusion, or a combined pressure/fusion step
to white bond paper or clear plastic film, or the like, thereby
resulting in a fixed or permanent image, the fixing being due to
the thermoplasticity of the coating on the developing toner
particles on heating.
Image resolution, uniformity and quality are functions of the
magnetic and electrostatic properties of the toner and also of its
particle size and size distribution. For high resolution the toner
particles should be small (3--5 microns in longest dimension), but
there should be, and normally will be, larger particles present
also. The response of the toner to the magnetic fields of the image
will be controlled in part by proper adjustment of the range of
coercivity and saturation magnetization of the toner particles as
well as the electrostatic properties which may be used as a bias to
aid in development and in the final toner transfer step.
If toner particles are smaller than about 1 micron in diameter,
they are attracted to surfaces with or without magnetic images and
adhere tenaciously by Van der Waal's forces or electrostatic
attraction. Particles much larger than 20 microns in size are too
easily removed by fluid drag forces or gravity. Further, magnetic
forces drop off approximately as the cube of the separation, and
appreciable magnetic signal fields from the image areas extend no
more than 25-40 microns from the surface of the film. Furthermore,
in the case of large particles, much of the magnetic material on
the side of the particle farthest removed from the magnetic image
is beyond the range of effective magnetic attraction. Accordingly,
the optimum particle size for image development should be in the
range 3-20 microns.
Particle size distribution as reported herein is based on a 200
particle count of toners dispersed in a viscous liquid, using an
optical microscope at 400X. The average size reported is the
arithmetic mean based on this count.
Microscopic examination of a number of spray-dried toners has shown
them to consist of nearly spherical particles, occasionally with a
somewhat roughened or wrinkled surface. Photomicrographs of
particle cross sections have shown a major number of particles to
contain one to several iron particles surrounded by a relatively
uniform oxide/binder matrix.
Throughout this specification, values in the more conveniently
measured electromagnetic units/gram (emu/g.) are reported rather
than B values in gauss. Thus, .sigma..sub.4,400 Oe or .sigma..sub.s
is used to denote magnetization of the toner particles in a 4,400
Oe field, corresponding to magnetization in a saturating field,
often designated as B.sub.s. Remanence values, .sigma..sub.r,
correspond to remanence magnetization values designated as B.sub.r
in induction units.
The sigma values employed herein are defined on pp. 5-8 of
Bozorth's "Ferromagnetism," D. Van Nostrand Co., New York (1951).
These sigma values are determined in fields of 4,400 oersteds on
apparatus similar to that described by T. R. Bardell on pp. 226-228
of "Magnetic Materials In The Electrical Industry," Philosophical
Library, New York (1955). The definition of intrinsic coercive
force is given in Special Technical Publication No. 85 of the
American Society for Testing Materials entitled "Symposium on
Magnetic Testing" (1948), pp. 191-198. The values for the intrinsic
coercive force given herein are determined on a DC ballistic-type
apparatus which is a modified form of the apparatus described by
Davis and Hartenhiem in the Review of Scientific Instruments, 7,
147 (1936).
As noted above, one advantage of the present invention is the
improvement in optical reflectance obtained when the toners are
developed. Reflection optical density of toner images transferred
to paper gives a measure of the darkness or blackness of an image.
Reflection optical density is the logarithmn of the reciprocal of
the fraction of incident light reflected from a given area. For
example, for a reflection optical density of 1.0, one-tenth of the
incident light is reflected:
Typical printing in books, magazines, etc. will have a reflection
optical density of 1.0-1.4. A very black image that reflected only
one-thousandth of the incident light would have a reflection
optical density of 3:
The reflectance optical density of white paper is usually in the
range 0.1-0.14 and the images produced by many commercial copying
processes are in the range of 0.5-0.8.
EMBODIMENTS OF THE INVENTION
There follow some nonlimiting examples illustrative of the
invention in detail. In these examples, spray-drying operations
were carried out below the lower explosive limit of the mixtures
involved. Caution, of course, is necessary when mixtures of metals
and/or oxides, especially when finely divided, and organic
materials are exposed to high temperatures.
The designation, source, identity, and properties of iron, iron
oxide, binders, and other materials used in the examples are given
immediately hereinafter.
The term "carbonyl iron" refers to essentially pure iron powder
produced commercially by the General Aniline and Film Corporation
by pyrolysis of iron carbonyl:
Aver. Particle .sigma..sub.s .sigma..sub.r Desig- Size in Microns
nation (Based on Wgt.) (emu/g.) (emu/g.)
__________________________________________________________________________
GS- 6 5 199 1.0 SF 3 199 0.5 L 20 196 3.3
__________________________________________________________________________
black magnetic oxide of iron (ferroso-ferric oxide, Fe.sub.3
O.sub.4) employed had a modified cubic crystal structure and the
properties: ##SPC1##
Carbon blacks used in the examples were:
1. Raven 30, a product of the Columbian Carbon Company, is an all
purpose carbon black of high-tinting strength and low-vehicle
demand. It has an arithmetic mean particle diameter of 25
millimicrons, a surface area of 82 square meters per gram, an oil
absorption by the Venuto method of 90 gallons/100lbs., a pH of 8, a
fixed carbon content of 99 percent, and a covering power of 102
(tinting strength index).
2. Statex R, also produced commercially by the Columbian Carbon
Company, is a high-abrasion furnace black with an arithmetic mean
particle diameter of 26 millimicrons, a surface area of 100 square
meters per gram, and an oil absorption of 14 gallons/100
pounds.
3. Darco carbon black refers to Grade G-60, produced commercially
by Atlas Chemical Industries, Inc., as a premium grade of powdered
activated carbon used for decolorizing, purifying and refining. It
is made by activating lignite with heat and steam.
4. "Permanent Black," made by the General Aniline and Film
Corporation is a finely divided carbon black pigment.
A description of other previously undescribed mater materials used
in the examples follows immediately hereinafter.
Versamid 930 is a low-molecular weight polyamide resin with
molecular weight of about 3,100, an inherent viscosity of 0.24, and
a softening ("stick") temperature of 105-115.degree. C. available
from the Chemical Division of General Mills. Polyamide resins of
this type are described in U.S. Pat. No. 2,450,940, J. C. Cowan, L.
B. Falkenburg, H. M. Teeter, and P. S. Skell to the U.S.A., in the
Handbook of Material Trade Names by Zimmerman & Levine, p. 257,
Supp. I, in "Polyamide Resins," D. E. Floyd, Reinhold Plastics
Application Series (1958), and in General Mills Bulletin No.
11--D-3. They are prepared by condensing polyamides such as
ethylene diamine with polymeric fatty acids, e.g., dilinoleic acid,
derived by polymerization of natural, oleaginous materials of
animal and vegetable origin.
Tween 20 made by Atlas Chemical Industries, Inc. is a
polyoxyethylene sorbitan monolaurate, a nonionic surfactant with
approximately 20 polyoxyethylene units in the chain and a
hydrophilic-lipophilic balance of 16.7 ["Emulsions: Theory and
Practice," Amer. Chem. Soc. Monograph, p. 238 (1966)].
"450 H" is a coumarone/indene resin with a stick temperature of
90.degree.-100.degree. C. marketed by the Pennsylvania Industrial
Chemicals Corporation.
"Lakeseal," sold by Peck's Products Company of St. Louis, Missouri
for use as a laboratory glass cleaner, is an especially preferred
agent for dispersing toners in aqueous systems. "Lakeseal" is a
"built" detergent consisting of sodium phosphates, sodium
carbonates, and biodegradable anionic and nonionic surfactants; the
detergent contains appreciable quantities of inert materials
resulting from its method of manufacture.
EXAMPLE 1
A. preparation of Pigment/Binder Dispersions
A preferred magnetic toner was made up from 40 weight percent of
carbonyl iron (particle size approximately 2-5 microns), 39 weight
percent of Fe.sub.3 O.sub.4 (particle size 0.03-0.06 micron), 1
weight percent of carbon black and 20 weight percent of a polyamide
resin with molecular weight of ca. 3,100. Carbonyl iron powder,
type GS-6 or type SF, was used as the soft magnetic material. The
black Fe.sub.3 O.sub.4 pigment was grade "4,000". The carbon black
was Raven 30 carbon black, and the polyamide resin was Versamid
930.
The low-molecular weight polyamide resin was dissolved in a 1:1
mixture by weight of n-propyl alcohol and toluene to give a
moderately viscous (100-200 centipoise) solution. The specified
ingredients were mixed in proper quantity to give the weight ratio
of nonvolatiles shown in table I. A ceramic ball-mill was selected
of such a size that when the ball-mill was about one-half to
two-thirds full of high-density stone balls, the above ingredients
including the solvent just covered the balls, and the mixture was
ball-milled at 60 percent nonvolatiles to break up agglomerates.
Ball-milling was carried out for 17 hours. After discharging the
ball-mill and diluting with more solvent to reduce the total solids
(pigments plus resin) to approximately 20 weight percent, the
dispersion was ready for spray drying.
B. preparation of Magnetic Toner Particles by Spray Drying
Spray-drying apparatus manufactured by Bowen Engineering, Inc. of
North Branch, New Jersey, was used. Precautions were taken to stir
the pigment/binder solvent dispersions and maintain a uniform feed
composition. The procedure consisted in atomizing a dispersion
(prepared as described under Part A) by dropping it onto a disc
rotating at 30,000 r.p.m. into a chamber through which heated air
was swirling at a high velocity. [Bifluid nozzle atomization in
which a stream of slurry containing the pigments, binders, and
solvents is atomized by a second stream of air as it leaves the
nozzle, or other well-known means may also be used for
atomization.] The exact temperature and air velocity depend mainly
on the stick point of the resin and boiling point of the solvent.
In the following toner preparation involving use of a rotating
disc, the inlet gas temperature was 350.degree.-363.degree. F.,
obtained by combustion of natural gas in air. Total gas volume was
250 standard cu. ft./minute. Under appropriate conditions, the
solvent was quickly removed from the dispersed droplets, leaving
discrete toner particles of pigmented resin. The particles were
classified to some extent by a cyclone collection system. Toner
adhering to the sides of the chamber and that from the first or
bottom cyclone separator were removed by brushing into a bottle,
combined, air dried a few days to remove residual solvent,
designated as main fraction, and evaluated.
The main fraction of nearly spherical spray-dried particles had an
average particle size of approximately 10 microns with a range of
from 5-20 microns. The magnetic properties of the final
encapsulated pigment were as follows: .sub.i H.sub.c, 85 Oe;
saturation magnetization, .sigma..sub.s, measured at 4,400 Oe, 110
emu/g.; and remanent magnetization, .sigma..sub.r, 9 emu/g.
Magnetic toners prepared by the above procedure and with the
specified composition are designated by the letter "A" in table I.
For comparison, samples of the same carbonyl iron were dispersed in
the same polyamide resin and spray dried by the same procedure,
these samples are designated by the letter "B". In addition, an
iron oxide toner was prepared in a similar fashion except that
coumarone/indene resin of stick temperature 90.degree.-100.degree.
C., commercially available as "450 H" from Pennsylvania Industrial
Chemical Corporation, was used as binder; the toner is designated
by the letter "C" in table I.
C. preparation of Thermomagnetic Recorded Images
Toner evaluation comparisons were made on chromium dioxide magnetic
tapes prepared as described in Cox, U.S. Pat. No. 3,278,263. A
1,500-cycles/second sine wave signal was recorded on this 1/4 inch
wide tape at 7-1/2 inch per second using a full width recording
head. This is equivalent to 1,500/7.5 or 200 cycles/inch or 400
flux reversals or bits/inch of tape. The flux reversals were
recorded at various levels from near saturation to a small fraction
of saturation. The signal level was determined in arbitrary units
by playing back the recorded tape at 7-1/2inch/second on a tape
recorder and amplifying the signal from the playback head; the
signal level is given in volts as a subheading in table I
immediately above optical density data discussed hereinafter.
The signalled tapes were then heated to 85.degree. C. and exposed
through a photographic transparency bearing printed text to a xenon
flash lamp discharge as taught by Belgian Pat. No. 672,018. The
flux reversals recorded on the CrO.sub.2 tape were erased by the
flash exposure in areas corresponding to the clear areas of the
transparency but remained as recorded where protected by those
opaque areas of the transparency bearing the text. The magnetic
images thus created by thermomagnetic recording were developed by
the magnetic toner particles as discussed below.
D. preparation of Magnetic Toner Particle Dispersions
The toner particles were dispersed in a liquid medium for most of
these tests. The toner dispersions were made from 0.75 g. of the
magnetic toner particles in 150 ml. of a nonsolvent liquid, that
is, hexane, methanol or water containing a small amount of Tween 20
surfactant. Ultrasonic agitation using a 10-minute exposure on a
laboratory unit (General Ultrasonic Company, Model 400 ) operated
at 22-52 kilohertz tuned to give maximum coupling was used in all
cases. The resulting dispersions could be maintained by gentle
stirring.
E. development of Magnetic Images
The chromium dioxide tapes bearing the thermomagnetically recorded
images were mounted on slides and immersed for 6 seconds in the
stirred dispersion from above. This 6-second time was found to be
sufficient to permit the magnetic image to attract an equilibrium
amount of toner. The slides were then carefully removed from the
toner slurry and allowed to drain and dry. The dry toner particles
were then stripped from the chromium dioxide tape by using an
adhesive-coated, transparent tape that was subsequently transferred
to ordinary bond paper. The reflection optical density of the
transferred image was then determined using a standard device for
measuring optical density, i.e., a Welch "Densichron."
For dry development, 1 g. of the magnetic toner was mechanically
mixed with 5 g. of commercially available polystyrene beads
(100-150 mesh, Koppers Company) containing 0.01 g. of Darco carbon
black to improve powder flow characteristics. In this case the
thermomagnetically recorded image was developed by pouring the
toner mixture containing the polystyrene beads over the chromium
dioxide tape mounted in a trough at a 35.degree. angle to the
horizontal.
F. interpretation of Table I
Comparison of optical densities at corresponding tape output
voltages shows a definite advantage, particularly at intermediate
output levels, for the A-1 or A-2 toner formulations containing
both iron and iron oxide over the corresponding B-1 or B-3
formulations containing only iron as the magnetic pigment at the
same resin concentration, whether dry or in a liquid system. Higher
iron concentration (B-5) or lower iron concentration (B-2, B-4,
B-6) were also inferior. The carbonyl iron used made comparatively
little difference. ##SPC2##
EXAMPLE 2
A cured, filled, magnetized CrO.sub.2 line pattern film embossed in
the surface of polyurethane coated on 5-mil polyester film and
prepared according to the copending coassigned application of Nacci
Ser. No. 636,955, filed May 8, 1967, now abandoned was
reflex-imaged to an original containing representative line text,
including both type and graph forms. The image was developed using
a machine simplified as described in example 4. The toner employed
was a plastic-coated particulate magnetic composition (average size
10 microns) composed, by weight, of 40 percent of a commercially
available carbonyl iron (GS-6), 39 percent of a commercially
available iron oxide (Fe.sub.3 O.sub.4 -Mapico Black), 20 percent
of Versamid 930 and 1 percent of a commercially available Raven 30
carbon black, which formulation was prepared in the desired
particulate form by spray drying from a 50:50 blend by volume of
xylene and n-propanol. The spray-dried toner had an arithmetic mean
particle size of 9.2 microns, coercivity .sub.i H.sub.c of 85 Oe,
.sigma..sub.s measured at 4,400 Oe of 107 emu/g., and .sigma..sub.r
of 5.8 emu/g.
The slurry or dispersion used in the printing machine was prepared
by mixing 170 parts of the above toner, 8 parts of a commercially
available laboratory detergent ("Lakeseal" Laboratory Glass
Cleaner), and 400 parts of warm (50.degree. C.) water, which
mixture was finally dispersed by 10 minutes of ultrasonic agitation
with stirring. Two such dispersions were combined and allowed to
settle, and 520 parts of toner-free supernatant liquid was decanted
therefrom for later use. The developer tank of the printing machine
was charged with the remainder of the above slurry (dispersion)
after agitation, and the reflex film described in the initial
paragraph of this example was used as the printing master. Five
thousand copies were run off at the rate of 12 per minute, at a
transfer pressure of 44 lbs./linear inch using apparatus similar to
that depicted in the drawing.
During the printing run the operating level of the developer slurry
was maintained by readding the 520 parts of supernatant liquid
previously decanted from the original dispersion preparation, 290
ml. of an aqueous solution containing 1 percent "Lakeseal" and 0.5
percent aryl alkyl sulfonate (G3,300 of Atlas Chemical Industries,
Inc.) dispersing agent and 100 ml. of water. The following
tabulation shows the reflection optical density of the printed
images as a function of the number of copies printed versus that of
the first:
Start
During the run, 159 g. of toner was consumed in printing an image
recorded on an area 9-1/4.times.2-1/4 inch.
The decrease in copy density during the run was the result of
slight loss of chromium dioxide form the film. This was
demonstrated by checking the development and transfer with a
separate but similar imaged film at the start and end of the 5,000
copy run; the reflection optical density of the printing from this
film was 0.85 at both times showing substantially no loss in
printing properties of the toner slurry.
EXAMPLE 3
A 480-line per inch chromium dioxide pattern incised in the surface
of a5-mil thick polycarbonate (General Electric's "Lexan" ) film in
the manner described in the above-mentioned copending application
of Nacci, Ser. No. 636,955, was magnetized by passing it over the
pole pieces of a bar magnet of approximately 1,500 gauss average
field strength. The film was next imagewise demagnetized by reflex
imaging against a printed text using multiple flashes about 0.72
second apart of a xenon lamp operating at 1,790 volts and 128
microfarads while both the original and the magnetized copying
member were passing in intimate contact over a 5-inch diameter
polymethyl methacrylate driven drum with the xenon lamp in a
reflector inside with intimate pressure being maintained by an
external polyurethane foam belt as in the drawing.
The resultant magnetic image was developed with an aqueous slurry
containing a magnetic pigment mixture of average particle size 10
microns composed of 25 percent of a commercially available
low-melting polyamide (Versamid 930 ), 43.8 percent of a
commercially available Fe.sub.3 O.sub.4 ("3,000"), 29.2 percent of
a commercially available carbonyl iron (GS-6), 2 percent of Raven
30 carbon black and 0.4 percent by total weight of a commercially
available stearamide, which formulation was prepared by spray
drying the above ingredients from a 50:50 by weight blend of xylene
and n-propanol. The thus developed sheet was washed gently in water
to remove toner from the background and air dried.
In another case the toner was applied to the CrO.sub.2 film,
mounted on a rotary drum, by a fountain and excess toner from the
background was removed by wiping means. These toner images on the
CrO.sub.2 films were transferred to and fused by pressure and heat
(150.degree. C.) onto the surface of a standard imaging paper
normally used in A.B. Dick duplicators. The imaging papers having
the fused toner on them were placed on an A.B. Dick Litho-Offset
duplicator, washed with the prescribed etchant to remove the
protective coating and to make the background hydrophilic. Printing
was with regular litho ink, offset-blanket, and water roll. The
to-be-printed areas were wet with the oil-based ink and the
background, kept moist by the hydrophilic surface, was free of ink.
The imaging obtained on a good commercial grade of lithopaper gave
printings of the text of the original positives showing good
resolution and fidelity for all the letters.
EXAMPLE 4
Toners were prepared by ball-milling the ingredients and spray
drying the dispersion as described in example 1. The compositions
of the final toner particles were as shown in table II. These
toners were prepared under closely similar conditions, and were
dispersed in water using "Lakeseal" dispersing agent and ultrasonic
agitation as described in example 2. In testing the toner
dispersions a simplified apparatus similar to that of the drawing
was used. The simplified apparatus had no provision for
magnetization and exposure and did not have a transparent drum.
Instead, the CrO.sub.2 -containing reflex film was magnetized by
passing it over the pole pieces of a permanent magnet, exposed
using a Xenon flash lamp through an image-bearing photographic
transparency, and affixed to the film drum using a double-sided
adhesive tape.
The magnetic image on the CrO.sub.2 film was developed by padding
on toner by means of the flock-covered padding roll 26 of the
figure. The particles were carried by the flock covering and
elevated to the nip with the film drum where they were squeezed
between the surface of the flock-covered roll and the film drum.
Both the roll and the film drum were driven at the same surface
speed. Excess toner was removed and the developed image was
transferred to copy.
The toners in table II were tested as nearly as possible under
identical conditions. Some minor variations were made in operating
conditions to optimize performance for each individual toner. The
compositions, magnetic properties, particle sizes, and reflectance
optical density of the copies are tabulated. ##SPC3##
EXAMPLE 5
Using a reflex film and apparatus similar to that of example 4, but
without the flock-covered padding roll, a magnetic image was
developed by pouring or cascading toner onto the film drum at 2
o'clock position with the machine running at 1.5 inches per second.
Excess toner fell off at the 6 o'clock position. The toner was that
coded A-2 in table I to which, however, 0.5 percent by weight
Cab-O-Sil, a finely divided silica of about 15-20 millimicrons
particle size sold by the Cabot Corporation, had been added to
improve its flow properties. After transfer to paper the developed
image had an average reflectance optical density of 0.65.
EXAMPLE 6
A magnetic toner was prepared by spray drying a mixture containing
22.5 percent of carnauba wax, 33.75 percent Mapico Black Fe.sub.3
O.sub.4, 33.75percent GS-6 carbonyl iron and 10 percent of the
cationic modifier Aliquat 207 dimethyldistearylammonium chloride,
(percentage by weight). The spray-dried product was sieved through
a 400 -mesh screen after adding 0.5 Cab-O-Sil silica to render the
toner free flowing. The particles were predominantly 5-10 micron
spheres. This toner was termed JCS-1. Another toner termed JCS-2
was prepared, which was identical in preparation and composition to
JCS-1, except that the magnetic component consisted entirely of
Mapico Black, which comprised 67.5 percent of the toner.
Toner properties of the two compositions were compared by cascading
each against the magnetically imaged surface of a thermomagnetic
film exposed in the manner described in the first paragraph of
example 4. The developed images were printed by pressure contact
with paper. The toner designated as JCS-1 was clearly superior,
giving a reflectance optical density of 0.86 in the image area as
compared to 0.50 for the toner JCS-2.
EXAMPLE 7
Spray-dried toner (120 g. ), prepared as described in example 1
from 40 percent GS-6 carbonyl iron, 39 percent Mapico Black
Fe.sub.3 O.sub.4 1 percent "Permanent Black", and 20 percent
Versamid 930, was dispersed in 400 ml. of 2 percent "Lakeseal" in
water, evaporated to dryness in an oven at 50.degree. C., and
gently crushed to pass a 40-mesh screen. The resulting
surfactant-coated particles dispersed easily in water and served
both to prepare dispersions for use in printing machines as in
example 2 and to replenish toner removed from a dispersion by image
development. By additions of the powder and deionized water, a
single dispersion was used to print about 55,000 copies.
Other surface active agents may also be employed, usually at a
concentration of 2 percent or less by weight based on toner, to
provide self-dispersing properties. These include Atlas "G B3300" ,
an anionic, general purpose, branched-chain alkyl aryl sulfonate
surfactant, DuPont "Merpol" SE, a general purpose, nonionic,
ethylene oxide condensate, and DuPont "Product BCO", a C-cetyl
betaine amphoteric surfactant
EXAMPLE 8
Spray-dried toner consisting of 40 percent GS-6 carbonyl iron, 39
percent Mapico Black Fe.sub.3 O.sub.4, 1percent "Permanent Black",
and 20 binder (percentage by weight) was prepared as described in
example 1. The "binder" consisted of a mixture of 20 parts of Elvax
polyethylene/vinyl acetate, 10 parts of carnauba wax, and 1 part of
an alkyl aryl sulfonate surfactant. The toner dispersed in water
upon stirring without further addition of dispersant, and the
resulting dispersion was successfully used in the manner described
in example 4 in image development and transfer operations.
Many of the disadvantages apparent for the prior art toners do not
exist for the binary toners of this invention. First of all, not
only does the hard magnetic material contribute to the magnetic
properties of the product, but also it is the primary color agent
of toner; e.g., toners and their decorations containing only
approximately one-fourth by weight of Fe.sub.3 O.sub.4 or
CrO.sub.2, the rest being Fe and resin, are very black. During the
resin encapsulation process the finely divided hard magnetic
material, which is close to single domain in size, has sufficient
magnetic moment to magnetically interlock all of the magnetic
particles including the soft magnetic material into a loose
resin-impregnated three dimensional network. This network
effectively resists any tendency for segregation of resin from the
magnetic material. Any slight tendency of the large particles of
soft magnetic material to be less than uniformly distributed among
the final toner particles is of small consequence because all
particles uniformly contain the vary finely divided hard magnetic
material and so no nonmagnetic toner particles exist.
The mixed toners possess a desirable balance of magnetic
properties. Their magnetic moments in the fields close to a
magnetically imaged surface are somewhat less than for soft
magnetic toners but are significantly higher than the remanent
moments of hard magnetic toners. The remanent moments of the mixed
toners are modest. They are large enough to result in an attractive
force on the toner at some distance from an imaged surface; yet,
they are not large enough to make magnetic flocculation of the
toner a problem. Also considerable flexibility in tailoring the
magnetic properties to the needs of an application exists via
adjustment of the ratios of the two magnetic materials. The
superior performance of the mixed toners in a practical
application, which results from their desirable combination of
properties, is clearly evident in the data of tables I and II. The
optical densities of visual images formed magnetically using these
toners are consistently the highest obtained.
Since obvious modifications and equivalents will be evident to
those skilled in the art, we propose to be bound solely by the
appended claims.
* * * * *