Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CARRIER MATERIAL FOR
ELECTROP~O OGRAPHIC DEVELOPERS
Field of the Invention:
This invention relates to carrier material for
electrophotographic developers, in particular two-component
developers comprising a toner with a carrier. In particular,
the invention relates to a carrier material for electrophoto-
graphic developers which comprises a substantially spherical
core material consisting essentially of hematite-containing
magnetite and having a resinous layer coated thereon.
Background of the Invention:
In electrophotography, the electrostatic image formed
on the photoconductor is developed by the magnetic brush
method using either a so called "one-component~ developer
or a ~two-component" developer. Usually, the two-component
developer system comprises a mixture of relatively fine
particles of a toner and relatively coarse particles of a
carrier. The toner particles are held on the carrier par-
ticles by the electrostatic forces of opposite polarities
which are generated by friction of the particles. When the
developer comes into contact with an electrostatic latent
image formed on the photosensitive plate, the toner parti-
cles are attracted by the image and thus make the latter
visible. The thus developed image is then transferred onto
a recording medium, such as a paper sheet. In the process,
therefore, the toner particles should be charged with an
accurately controlled amount of static electricity so that
they are preferentially attracted to the electrostatically
imaged area of the photosensitive plate.
Thus, in turn, the carrier which is used in combina-
tion with the toner should fullfil the following criteria:
The carrier should have an appropriate triboelectric prop-
erty which enables it to electrostatically hold the toner
particles and to transfer the held toner particles to the
electrostatic latent image on the photosensitive plate when
contacted. The carrier should have a-sufficient mechanical
strength to protect the carrier particles from breaking or
cracking. The carrier particles should e-hibit a good
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fluidity to facilitate their transfer on the sleeve of the
magnetic roll in the printing machine and also to facilitate
their mixing with the toner. The carrier particles should
be uniform in their electric and magnetic properties. The
carrier should be stable with respect to changes in the
environmental conditions such as temperature and, in
particular, humidity. The carrier particles should have a
sufficient durability to ensure an acceptable lifetime.
Although the quality of copies developed by electro-
photographic processes is largely governed by the type ofcopying machine employed, in particular the type of the
developing process on which the copying machine is based, it
is also known that the copy quality is affected to a signif-
icant extent by the developer used therein.
Therefore, various attempts to improve the carrier
and toner materials constituting the developers have been
made to ensure exact reproduction of an original.
Hitherto, carriers have been selected from a variety
of materials. An example which has been most widely used is
an iron (metallic) powder of irregular particulate form or
shape. Because the iron powder has a high level of satura-
tion magnetization and anisotropy, the magnetic brush (i.e.
a tuft of filaments comprising the carrier particles having
the toner particles theron, each of the filaments being
generally formed with the carrier particles chained together
in one length by the magnetic forces generated by and trans-
ferred from the magnetoroll, and each filament standing on
end from the surface of the sleeve of the magnetoroll) is
necessarily longer and more firm than those obtained with
other carrier materials of a lower saturation magnetization.
This phenomenon may be often observed where the iron powder
carrier is of a flat and irregular particulate shape. If
the magnetic brush is long and firm, the density of the
brush tuft will be low and hence it is difficult to produce
a fine copy having a good gradient. In spite of the poor
gradient of copy achieved by a developer comprising the iron
powder carrier of irregular particulate shape, such a devel-
oper may be advantageous in that it is possible to generate
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a high degree of chromic density therewith. Therefore, this
type of developer has been employed in a type of copying
machine which is provided with a relatively wide clearance
between the surface of the sleeve of the magnetoroll and the
surface of the photoconductor.
On the other hand, in order to develop a fine copy of
an improved gradient, a carrier comprising an iron powder of
a spherical particulate shape has been employed. Since the
iron particles are spherical and, thus, isotropic, the
disadvantages which are caused by the anisotropy of the
carrier comprising the irregularly shaped iron powder are
significantly reduced by use of the spherical iron carrier.
The magnetic brushes formed therefrom are remarkedly short
and dense. Accordingly, the developing torque required is
also reduced. However, this spherical iron powder is not
completely satisfactory; since the carrier again comprises
metallic iron and has a relatively high specific gravity and
hence a relatively large apparent density (usually greate~
than 4.0 g/cml), the particles tend to jump or fly away and
to cause difficulties in the process and machine. To pre-
vent the escape of the particles, it is necessary to adopt
a special and complicated design for the magnetoroll and
related parts of the copying machine. Further, the heavy
iron carrier may give rise to a high stress on the toner
particles when admixed together and may adversely affect the
working life of the developer. With this type of carrier,
although it is possible to develop a relatively high quality
of copies having an improved gradient, the copies generally
tend to have a relatively low degree of chromic density.
This undesirable tendency has been reduced or removed by
appropriate modifications of the machine or by regulation
thereof, for example, by increasing the electrostatic
potential charged on the surface of the photoconductor.
It has been proposed to use ferrite, an oxidic ferro-
magnetic material, as a carrier material in an attempt toimprove the copy quality and to prolong the working life of
developer. Because the ferrite carrier has a saturation
magnetization (about 40-70 emu/g) lower than that of iron
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powder (about 200 emu/g), the magnetic brush formed from the
former is soft and thus fine copies of an improved gradient
may be developed therewith. Further the ferrite carrier is
advantageous in that the stress exerted on the toner parti-
cles is significantly reduced and consequently the dura-
bility of developer is prolonged, since the ferrite carrier
has a specific gravity lower than that of metallic iron.
The developing torque required is also reduced. Notwith-
standing the above advantages, the ferrite carrier is used
only within a limited range of application, because the
electrical resistivity of ferrite is rather high. Further
the ferrite material is relatively sensitive to environ-
mental conditions so that the performance of the ferrite
carrier tends to be significantly influenced by such changes
lS in the environmental conditions as humidity variations and
the resulting copy quality tends to change over the period
of operation in which the ferrite carrier is used.
To improve the environmental stability of the ferrite
carrier, it has been proposed that the ferrite carrier par-
ticles be provided with a resinous coating layer, althoughthis further increases the undesirably high electrical
resistivity of the material, resulting in a further limita-
tion of the application range and additional production
costs.
Summary of the Invention:
An object of the present invention is to provide a
novel carrier material with which the problems associated
with the known carriers are obviated or substantially
removed.
Another object of the invention is to provide methods
for preparing this novel carrier.
A further object of the invention is to provide a
novel carrier which is effective in producing electro-
photographic copies of excellent quality, is stable in
respective of changes in environmental conditions, and has
increased durability.
Brief Explanation of the Invention:
Accordingly, the present invention provides a carrier
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material for electrophotographic developers which comprises
a substantially spherical core particle consisting of
hematite-containing magnetite and having a resinous layer
coated thereon.
The carrier material of the invention which is made
of hematite-containing magnetite has a lower electrical
resistivity than ferrite. This low resistivity provides a
wide room for increasing the resistivity over a wide range
by coating the material with a resinous insulating material.
Therefore, it is possible to accommodate the resistivity to
various types of copying machines and processes requiring
different optimal levels of resistivity with respect to the
carrier material.
The spherical shape of the carrier particles provides
the carrier with a desirable fluidity.
The present carrier material has a specific gravity
lower than iron and an apparent density comparable with that
of the ferrite carrier. Therefore, with the present carrier,
a relatively low level of developing torque is required in
the machine and a satisfactory developer working life is
provided.
The present carrier material has a saturation magne-
tization (as) within the range of about 40 - 80 emu/g
similar to that of ferrite and will form a soft and dense
magnetic brush resulting in development of a high quality
copy. The relatively low saturation magnetization will
reduce the torque required for cutting the top of the brush
during conveyance of the developer on the magnetic roll and,
thus, will contribute to extend the durability of the
material.
The hematite-containing magnetite particles which
may be used in the present invention may be prepared, for
example, in the following manners:
a) A homogeneous aqueous slurry comprising finely
divided hematite and magnetite is sprayed in a spray drier
to give granules, which are then calcined, lightly ground
and classified into a desired size fraction (for example,
20 - 200 ~).
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b) Spherical magnetite particles in a desired size range
are subjected to partial oxidization.
The carrier material of the invention exhibits a
saturation magnetization (as) in the range of about 40 -
s 80 emu/g. The saturation magnetization may be determined in
a magnetometer, for example a sample-vibrating type Magneto-
meter VSMP-l (ex. Toei Kogyo Co., Japan). It has been found
that the present product exhibits maximum saturation magne-
tization in a magnetic field of about 14.5 ROe.
If as of the particles is less than 40 emu/g, it is
difficult to prepare the particles in a homogeneous state.
In other words, though it is possible to prepare carrier
particles having a value ~f as as low as about 30 emu/g, the
respective particles are not equal to each other in their
magnetic properties. On the other hand, if the particles
have a ~5 exceeding 80 emu/g, they tend to form an undesir-
ably hard brush on the magnetic roll. In the both cases,
the particles are not suitable for the purpose of the
invention.
It is preferred that the size of the spherical core
particles is in the range of about 30 - 200 ~.
The spherical core particles having the preferred
ranges of the physical properties, i.e., a as ~f 40 -
80 emu/g and a diameter of 30 - 200 ~ may be coated by an
appropriate resinous material. The resinous material may be
applied to the core particles as a solution in an organic
solvent, such as methyl ethyl ketone, xylene, n-butanol,
methyl cyclohexane or toluene. The resinous solution may be
applied to the core particles, for example, by dipping the
particles in the solution or spraying the solution over a
fluidized mass of the core particles. When a thermosetting
resin is employed, the coated particles are subjected to a
suitable heat treatment to give the resin-coated carrier
material. Any resin may be used in the preparation of the
carrier material of the present invention provided that it
is effectively soluble in the common solvents.
The amount of the resinous material to be applied to
the core material is governed by the nature of the resin
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employed and the type of copying machine (and hence the
electrostatic and electrical re~istivity properties suitable
for the particle machine) for which the product carrier is
to be supplied. Generally, the amount of resin suitable for
the pu~pose of the invention will be in the range of 0.5 -
8%, preferably 1.5 - 6% by weight.
The resin-coated carrier material according to the
present invention may be used in conjunction with conven-
tional toners such as those, for example, made from a
natural resin, a synthetic resin, a blend of natural and
synthetic resins or such a material modified by incorpora-
tion of any appropriate additives.
~xamples
- The invention will be illustrated with reference to
the following non-limiting Examples.
ample 1
To 100 parts of magnetite powder, 30 parts of
hematite powder was added (by weight). The mixture charged
into a wet attrition mill and mixed thoroughly to give a
homogeneous aqueous slurry. Then the slurry was dried in
a spray-drier to form spherical granules of 30 - 150~ in
diameter. The granules were calcined at about 1200-C for
about 2 hours in a nitrogen atmosphere and allowed to cool.
The calcined mass was lightly groupd and classified to give
a fraction of 75 - 150 ~ in diameter.
- The thus prepared spherical particles containing
magnetite and hematite exhibited a saturation magnetization
(as) of 60 emu/g.
To 10 kq of the hematite-containing magnetite parti-
cles, 2 kg of a solution of a styrene/acryl copolymer resin(~MH 7015" produced by Fujikura Chemicals Co., Ltd.) in
toluene (a resin content of 10%) was spray-coated to give a
resin-coated carrier material according to the invention.
The carrier was mingled with a commercially available
toner for magnetic brush development (a negatively charge-
able toner adapted for use in ~Xerox 3870~ machine). The
thus obtained developer was used for developing an electro-
static latent image formed on a selenium photoconductive
* Trade mark
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surface to obtain a high quality copy showing an excellent
reproducibility in the intermediate tone range. It was
found that the initial copy quality was substantially
maintained even after reproduction of 60,000 copies.
Exa~Ele 2
An aqueous slurry of magnetite powder was spray-dried
in a spray drier to give spherical particles of 30 - 150
in diameter. After about 2 hours of calcination at about
1200~C in a nitrogen atmosphere, the product was cooled,
lightly ground and classified to give a fraction of 75 -
150 ~ in diameter.
The spherical magnetite particles were heat treated
at 300'C in a rotary kiln for 3 hours in air. Spherical
hematite-containing magnetite particles formed by partial
oxidation of the magnetite.
The obtained spherical hematite-containing magnetite
particles had a saturation magnetization of 45 emu/g.
To 10 kg of the spherical powder, 1 kg of the same
resin solution as in Example 1 was spray coated to give a
carrier powder according to the invention.
In the same manner as in Example 1, a developer was
prepared. Using this developer, an electrostatic latent
image on a selenium photoconductive surface was developed,
with a high quality copy of a good intermediate tone. No
substantial changes were seen in the copy quality after as
many as 60,000 times of development.
~QmparAtive ~Yample 1
An aqueous slurry of magnetite powder was spray-dried
in a spray drier to give spherical particles of 30 - 150
in diameter. After about 2 hours of calcination at about
1200~C in an nitrogen atmosphere, the product was cooled,
lightly ground and classified to give a powder of 75 - 150
in diameter.
The resulting spherical magnetite particles had a
saturation magnetization of 86 emu/g.
The powder was coated in the similar manner to that
in Example 1.
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The resulting carrier powder was used to prepare a
developer in the same manner as in Example 1. The developer
was subjected to the copying test as described in Example 1.
The resulting copy showed a high contrast with a poor inter-
5 mediate tone. The results were poor than those of Examples
1 and 2.
SQ~A rAt;ve ~xAmple 2
An aqueous slurry of magnetite powder was spray-dried
in a spray drier to give spherical particles of 30 - 150
in diameter. After about 2 hours of calcination at about
1200~C in a nitrogen atmosphere, the product was cooled,
lightly ground and classified to give a powder of 40 - 75
in diameter.
The spherical magnetite particles were heat treated
at 800~C in a rotary kiln for 2.5 hours in air to give
spherical particles of hematite-containing magnetite.
The obtained spherical particles of hematite-
containing magnetite had a saturation magnetization of
35 emu/g.
The spherical particle powder of hematite-containing
magnetite (10 kg) was coated with 1.5 kg of a solution of
acryl resin (~BR83~ produced by Mitsubishi Rayon Co., Ltd.)
in toluene (resin content: 6%) by spraying technique.
The thus-obtained carrier powder was mixed with a
toner for magnetic brush development (negatively-chargeable
toner, ~BD8811~ produced by Toshiba Electric Co., Ltd.) to
prepare a developer, with which an electric latent image on
a selenium photosensitive member was developed. An acci-
dental carrier deposition on the photoconductive surface
occurred. With such an accident, the carrier could not be
put into practical use.
The carrier material produced in this comparative
example exhibited a saturation magnetization in the range of
as low as 15 - 25 emu/g.
3s Example 3
An aqueous slurry of magnetite powder was spray-dried
in a spray drier to give spherical particles of 30 - 150
in diameter. After about 2 hours of calcination at about
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1200'C in a nitrogen atmosphere, the product was cooled,
- lightly ground and classified to give a powder of 40 - 75
in diameter.
The spherical magnetite particles were heat treated
at 400~C for 30 minutes to give spherical particles of
hematite-containing magnetite.
The obtained spherical particles of hematite-
containing magnetite had a saturation magnetization of
73 emu/g.
The spherical particle powder magnetite (10 kg) was
coated with 2.5 kg of a solution of acryl resin (~BR83~
produced by Mitsubishi Rayon Co., Ltd.) in toluene (resin
content: 6%) by spraying technique.
The thus-obtained carrier powder was mixed with a
toner for magnetic brush development (negatively-chargeable
toner, "BD8811" produced by Toshiba Electric Co., Ltd.) to
prepare a developer, with which an electric latent image on
a selenium photosensitive member was developed. An high
quality copy with a relatively good intermediate tone was
reproduced.
