Note: Descriptions are shown in the official language in which they were submitted.
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2117102
ELECTROSTATIC IMAGE DEVELOPER AND
IMAGE FORMATION PROCESS USING SAME
FIELD OF THE INVENTION
The present invention relates to a single-component
type electrostatic image developer and a process for forming
an image using a single-component type electrostatic image
developer.
BACKGROUND OF THE INVENTION
The dry development process for use in various
electrostatic copying methods which have heretofore been put
into practical use can be roughly divided into two systems,
i.e., a two-component development system with a toner and a
carrier such as iron powder and a single-component
development system using only a toner.
Unlike the two-component development system, the
single-component development system requires no automatic
density adjustor and other mechanisms. Therefore, the
developing machine using such a single-component development
system can be compact. Further, the single-component
development system is free from stain on the carrier,
requiring no maintenance such as renewal of carrier. Thus,
the single-component development system has been employed in
low speed small-sized copying machines or printers, and
nowadyas even in middle or higher speed copying machines or
printers. Therefore, it has been desired to provide such a
single-component development system with more improvements in
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properties.
Because of the nature of the single component
development system (i.e., no carrier), unlike the two-
component development system, the single-component type
electrostatic image developer used therein tends to be
insufficient in charging the praticlulate toner. As a result,
the charge distribution of the particulate toner is liable to
be ununiform, causing a problem of the image hysteresis of a
copying cycle appearing on the subsequent cycle (ghost).
On the other hand, attempts have been made to utilize
the single-component development process using a magnetic
toner in easily preparing papers which can be read with a
magnetic ink character reader (MICR), such as personal
checks. This system is generally called an MICR system.
The MICR system is a system comprising reading a
magnetized image with a magnetic head. Because the magnetic
image is usually printed with a liquid magnetic ink, image
formation is not easy. While a printing system using the
aforesaid two-component development process has also been put
to practical use, this system requires a large-sized printing
machine and is still uneasy.
The magnetic toner which can be applied to an MICR
system should meet both suitability to an MICR and
suitability to a printer. More specifically, the magnetic
toner should provide a toner image having sufficient
magnetization enough to be read with a magnetic head; the
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toner should not cause untoward problems through repeated
friction with a magnetic head; the toner should be adaptable
to a conventional single-component magnetic toner
electrophotographic system; and the printed toner image
should have image quality at least equal to that obtained in
a conventional electrophotographic printing system.
In order to solve a problem of toner fall-off and
magnetic head contamination with the separated toner in the
MICR system, a few proposals have been made to date. For
example, it has been suggested to incorporate a polyolefin
into toner particles to provide a toner which can be used in
a medium- to high-speed double-side copying machine, which
has recently be developed for resources saving.
Incorporation of a polyolefin into toner particles aims at
improvements of slip properties and anti-smudge properties of
a toner image ("smudge" is a phenomenon that a fixed image is
strongly scraped and thereby stained). This method proved
effective to prevent smudging but still insufficient in
improving scratch resistance when applied to an MICR system
in which a fixed image is repeated scratched.
If a polyolefin is added in an amount higher than in
a conventional toner, a further improvement of scratch
resistance would be expected. However, substantially
incompatible with a toner, a polyolefin added in an increased
amount results in poor dispersibility, failing to obtain
sufficient scratch resistance in an actual MICR system.
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21471Q2
Moreover r in printing, a toner having an increased polyolefin
content contaminates a toner supporting member, reduces the
image density, causes background stains, and deteriorates
maintenance. Furthermore, because of the poor
dispersibility, it is difficult to uniformly disperse a
polyolefin in the toner, resulting in lowered chargeability
and non-uniform charge distribution.
In order to solve the above problems, many attempts
have been proposed to uniformize the charge distribution,
e.g, by adding an additive having a relatively low electrical
resistance such as titanium oxide to the particulate toner.
However, additives such as titanium oxide have a high
cohesiveness and thus are disadvantageous in that they adhere
to the photoreceptor when used in the electrophotographic
process. Thus, no single-component type electrostatic image
developers satisfying the foregoing requirements have yet
been available.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to
provide a single-component type electrostatic image developer
which provides a uniform toner chargeability distribution and
causes no ghost.
It is another object of the present invention to
provide a single-component type electrostatic image developer
which causes no problems concerning the toner cleanability
such as adhesion to the photoreceptor.
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It is still another object of the present invention
to provide a single-component type electrostatic image
developer which can be suitably used in the MICR system.
It is a further object of the present invention to
provide a process for forming an image using a single-
component type electrostatic image developer.
These and other objects of the present invention will
become more apparent from the following detailed description
and examples.
The inventors made extensive studies of the foregoing
problems. As a result, it was found that these objects can be
accomplished by attaching a particulate titanium oxide having
a predetermined particle diameter to the surface of a
particulate magnetic toner in a predetermined amount and
keeping the developer free of titanium oxide agglomerates
having a grain diameter of not less than a predetermined
value. Thus, the present invention has been worked out.
The present invention concerns a single-component
type electrostatic image developer, comprising a particulate
toner having a binder and a magnetic material incorporated
therein and a particulate titanium oxide having an average
primary particle diameter of 0.05 to 1.5 ~m in an amount of
0.1 to 1.5 % by weight based on the weight of said
particulate toner, the number of titanium oxide agglomerates
having a grain diameter of not less than 20 ~m among titanium
oxide agglomerates formed by said particulate titanium oxide
21471~2
being 0.
The present invention also concerns a process for the
formation of an image which comprises the steps of forming a
latent image on a latent image carrier, and then developing
said latent image with a single-component type developer
which has been provided in the form of layer on a developer
carrier.
DETAILED DESCRIPTION OF THE INVENTION
The single-component type electrostatic image
developer of the present invention will be further described
hereinafter.
The particulate toner constituting the single-
component type electrostatic image developer of the present
invention comprises at least a binder resin and a magnetic
material.
As the magnetic material there may be any known
magnetic material so far as it has heretofore been commonly
used. Examples of such a known magnetic material include
metal such as iron, cobalt and nickel, alloy thereof, metal
oxide such as Fe3O4, y-Fe2O3 and cobalt-added iron oxide, and
various ferrites such as MnZn ferrite and NiZn ferrite. These
magnetic materials preferably have a particle diameter of
0.05 to 0.5 ~m.
In the present invention, the content of the magnetic
material in the particulate toner is preferably from 20 to 70
% by weight, more preferably from 40 to 60 % by weight. If
2147102
the content of the magnetic material falls below 20 % by
weight, the chargeability of the toner may be difficult to
be controlled, causing an image density drop or ununiform
development particularly under a low-temperature and low-
humidity atmosphere. On the contrary, if the content of the
magnetic material exceeds 70 ~ by weight, the resulting toner
may exhibit a reduced fixability, causing practical
disadvantages.
As the binder resin there may be used a vinyl
polymer. Specific examples of such a vinyl polymer include
homopolymer or copolymer of one or more vinyl monomers.
Representative examples of such vinyl monomers include vinyl
aromatic compounds such as styrene, p-chlorostyrene and
vinylnaphthalene, halogenated vinyl compounds such as vinyl
chloride, vinyl bromide and vinyl fluoride, vinyl esters such
as vinyl acetate, vinyl propionate, vinyl benzoate, vinyl
butyrate, vinyl formate, vinyl stearate and vinyl caproate,
ethylenic monocarboxylic acids and esters thereof such as
acrylic acid, methacrylic acid, methyl acrylate, ethyl
acrylate, n-butyl acrylate, isobutyl acrylate, dodecyl
acrylate, n-octyl acrylate, 2-chloroethyl acrylate, phenyl
acrylate, methyl ~-chloroacrylate, methyl methacrylate, ethyl
methacrylate and butyl methacrylate, substituted ethylenic
monocarboxylic acids such as acrylonitrile, methacrylonitrile
and acrylamide, ethylenic dicarboxylic acids and esters
thereof such as dimethyl maleate, diethyl maleate and dibutyl
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21~7102
maleate, vinyl ketones such as vinyl methyl ketone, vinyl
hexyl ketone and methyl isopropenyl ketone, vinyl ethers such
as vinyl methyl ether, vinyl isobutyl ether and vinyl ethyl
ether, vinylidene halides such as vinylidene chloride and
vinylidene chlorofluoride, and N-vinyl compounds such as N-
vinylpyrrole, N-vinylcarbazole, N-vinylindole and N-
vinylpyrrolidone. Further, a resin such as polyester may be
used singly or in combination with the foregoing binder
resins.
The particulate toner of the present invention may
further contain polyolefin to prevent smudging and to improve
scratch resistance for the application to the MICR systerm.
The polyolefin is not particularly limited and may be
selected from conventionally known ones, including polyolefin
wax. Examples of polyolefin include polyethylene preferably
having a weight average molecular weight (Mw) of from 1,500
to 16,000, more preferably from 2,000 to 6,000, and
polypropylene preferably having a Mw of from 3,000 to 6,000,
more preferably from 4,000 to 6,000.
The polyolefin is preferably dispersed in the
particulate toner and contained in an amount of from l to 20
% by weight, particularly preferably from 2 to 15 % by weight
based on the weight of the toner. The polyolefin has a mean
dispersed particle diameter of from 0.01 to 0.5 ~m,
preferably from 0.01 to 0.3 ~m, more preferably from 0.02 to
0.2 ~m, and most preferably from 0.03 to 0.1 ~m. If the mean
214~1Q2
. ,
dispersed particle diameter of the polyolefin is less than
0.01 ~m, the toner has insufficient scratch resistance in the
MICR system. Polyolefin particles greater than 0.5 ~m
contaminate a toner supporting member in printing for a
prolonged period of time to reduce the developing properties.
The proportion of polyolefin dispersed particles of greater
than 2 ~m is preferably not more than 5%, more preferably not
more than 3%, and most preferably not more than 1%, based on
the total number of the polyolefin particles.
The coefficient of variation of polyolefin dispersed
particle size distribution is preferably from 20 to 40%, more
preferably from 25 to 40%, and most preferably from 25 to
35%. If the coefficient of variation is smaller than 20%,
smudges tend to become noticeable. If it is larger than 40%,
development maintenance tends to be deteriorated. With the
coefficient of variation falling within the above range,
development maintenance is improved, and anti-fog latitude is
broadened.
The coefficient of variation as above noted is
measured by slicing a toner particle with a cutter, such as a
microtome, to a thickness of 0.3 ~m, taking an electron
micrograph at a magnification of 9,000 with a,transmission
electron microscope, and analyzing about lO0 polyolefin
particles randomly selected by means of an image analyzer.
The coefficient of variation (%) is obtained from formula:
(standard deviation/mean dispersed particle diameter) x 100.
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2l47lo2
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Further, the single-component type electrostatic
image developer of the present invention may comprise various
substances incorporated therein for the purpose of
controlling the chargeability and electrical resistance
thereof or like purposes. Examples of these additives include
fluorinic surface active agents, dyes such as azo and
salicylic metal complex, high molecular acids such as
copolymer containing maleic acid as a monomeric component,
quaternary ammonium salts, azine dyes such as nigrosine, and
carbon black.
The particulate toner of the present invention can be
prepared by any conventional process, for example, by melt-
kneading a binder resin and a magnetic material with or
without additives, cooling the mixture, pulverizing the
cooled mixture and classifing to a desired particle size. In
the case of incorporating a polyolefin into toner particles
in a finely dispersed state, the polyolefin may previously be
ground to powder having a particle size of not greater than
10 ~m before it is melt-kneaded with a binder resin and other
toner components in a known kneading machine, such as an
extruder. All the components including the polyolefin may be
kneaded in the presence of water while suppressing a
temperature rise so as to apply sufficient shearing force to
the mixture. A dispersant, such as an oxidized wax, may be
used in combination.
In the present invention, the toner is preferably
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2l47lo2
produced by a process comprising previously preparing a
binder resin having a polyolefin finely dispersed therein and
mixing the resin with a magnetic material. According to this
process, the polyolefin can be finely dispersed efficiently.
The efficiency is further increased by using a graft or block
copolymer containing a polyolefin as a constituent component
-(hereinafter simply referred to as a polyolefin graft or
block copolymer) as a dispersant. It is preferred that the
polyolefin graft or block copolymer as a dispersant comprise
at least one of the monomer units constituting the binder
resin. In this case, the size distribution of the polyolefin
fine particles is further narrowed.
In carrying out the above-mentioned process in which
a binder resin having a polyolefin finely dispersed therein
is previously prepared, the polyolefin may be added to a
system for preparation of a binder resin, for example, during
polymerization of monomers or may be added to a binder resin
in a molten state and dispersed therein in the presence of
the above-mentioned polyolefin graft or block copolymer as a
dispersant to a dispersed particle size of not greater than
3 ~m. A part of the polyolefin may be added to the
polymerization system to form a polyolefin graft or block
copolymer, which serves as a dispersant. In this case, there
is obtained a binder resin containing polyolefin fine
particles with a narrow particle size distribution.
Dispersion of a polyolefin to a desired particle size
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214~102
is preferably effected in the following two-step process. In
the first step, a polyolefin is previously dispersed in a
binder resin to a particle size of not more than about 3 ~m.
The polyolefin may be dispersed in a system for producing a
binder resin, for example, at the time of polymerization or
in a molten binder resin by using a block or grafted polymer
as a dispersing agent. More specifically, a polyolefin
powder is dissolved in a solution containing a monomer for
the binder resin and graft polymerization is carried out
using a polymerization initiator. Alternatively, a
polyolefin powder is dissolved in a solution containing a
binder resin, together with a graft polymer of the monomer of
the binder resin and a vinyl monomer as a dispersing agent.
While the binder resin may be a resin having two molecular
weight distribution peaks corresponding to a low molecular
weight component and a high molecular weight component for
the sake of control of its fixing temperature, the polyolefin
is preferably incorporated in a system to constitute the low
molecular weight component of the binder resin in the same
manner as described above.
In the second step, a commonly employed procedure for
producing a toner is adopted. Prior to the second step, a
solvent used in the first step is removed from the resin
solution obtained in the first step. Dispersion of the
polyolefin in the second step is effected simultaneously with
blending and dispersing a magnetic material, etc. by means of
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21~7 lO~
a roll mill, an extruder, and so on. In the second step, the
polyolefin is finely dispersed to a final particle size of
not more than 0.5 ~m. An additional amount of a polyolefin,
either the same as or different from that added in the first
step, may be added in the second step. When the above-
mentioned two-molecular weight peak binder resin is used, the
polyolefin to be added in the second step is preferably added
to a resin solution of the first step together with a high
molecular weight component of the binder resin, which is
separately prepared by bulk polymerization, solution
polymerization and the like. Then, the solvent of the resin
solution is removed. In this case, the weight ratio of the
polyolefin added in the first step to that added in the
second step preferably ranges from 10:1 to 1:10 and more
preferably from 1:10 to 5:10.
The coefficient of variation as defined above can be
attained by the above-mentioned two-step process. The
coefficient of variation can be reduced by adding the
polyolefin(s) in the first and second steps in the preferred
weight ratio of 1:10 to 5:10.
The thus prepared binder resin having finely
dispersed therein a polyolefin is then blended with a
magnetic material and melt-kneaded by means of a roll mill,
an extruder or any other generally employed kneading machine.
The polyolefin particles are further reduced to 0.5 ~m or
less through this kneading step. The resulting mixture is
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ground and classified to obtain a toner having a desired
particle size. The grinding is preferably performed by a
mechanical means. The classification is preferably effected
by utilizing a co-under effect.
The thus prepared particulate toner of the present
invention preferably has an average particle size (D50) of
from 4 to 10 ~m. The particle size of the toner is measured
with a Coulter counter "TA-II" manufactured by Coulter
Counter Co. at an aperture of lO0 ~m. (The term "D50" means
the particle size of the particles at the time when the
cumulative value of the particles reaches 50 % of the total
volume of the measured particles.)
As the particulate titanium oxide which serves as the
other constituent component of the single-component type
electrostatic image developer of the present invention there
may be used a particulate titanium oxide having a specific
resistivity of from 1 x 107 to 2 x 108 Q-cm. It is necessary
that a particulate titanium oxide having a primary particle
diameter of from 0.05 to 1.5 ~m be contained in an amount of
from 0.1 to 1.5 % by weight, preferably from 0.3 to 1.2 % by
weight based on the weight of the particulate toner. If the
primary particle diameter and added amount of the particulate
titanium oxide deviate from the above defined range, ghost
cannot be inhibited.
In the present invention, the particulate titanium
oxide used in the single-component type electrostatic image
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2147102
developer forms agglomerates thereof in the developer. It is
necessary that the number of titanium oxide agglomerates
having a grain diameter of not less than 20 ~m among all the
titanium oxide agglomerates be 0. That is, it is necessary
that substantially no titanium oxide agglomerates having a
grain diameter of not less than 20 ~m be contained in the
developer. If the number of such titanium oxide agglomerates
is not 0, the developer is subject to adhesion to the
photoreceptor, i.e., filming.
The particulate titanium oxide can be crushed during
preparation so that the agglomeration thereof is reduced to
keep the 500-~m mesh sieve residue at not more than 30 ~ by
weight. In the present invention, a particulate titanium
oxide thus obtained can be obtained. If the 500-~m mesh sieve
residue exceeds 30 % by weight, the adhesion of the
particulate toner to the photoreceptor can hardly be
inhibited.
Further, the single-component type electrostatic
image developer of the present invention may comprise a
powder of other inorganic materials such as silica, powder of
an organic material such as aliphatic acid, derivative
thereof and metal salt thereof, powder of a resin such as
fluororesin, acrylic resin and styrene resin, or the like
incorporated therein in combination with the foregoing
particulate titanium oxide for the purpose of improving the
durability, fluidity or removability of the particulate
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214~12
toner.
The process for the formation of an image with the
single-component type electrostatic image developer of the
present invention will be described hereinafter. The process
for the formation of an image according to the present
invention comprises the steps of forming a latent image on a
latent image carrier, and then developing said latent image
with a developer on a developer carrier. The formation of a
latent image on the latent image carrier can be performed by
a known method. As the latent image carrier there may be used
an electrophotographic photoreceptor or dielectric. If an
electrophotographic photoreceptor is used as the latent image
carrier, it may be uniformly charged and then imagewise
exposed to light to form an electrostatic latent image
thereon.
The electrostatic latent image thus formed is then
subjected to development with a developer on a developer
carrier. In the present invention, the foregoing single-
component type electrostatic image developer is spread over
the developer carrier by means of a layer regulating member,
for example, to form a thin layer thereon. The thin layer of
the single-component type electrostatic image developer
formed on the developer carrier is then opposed to the latent
image carrier. In this manner, the particulate toner of the
developer charged by the layer regulating member flies and
adheres to the electrostatic latent image on the latent image
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carrier to develop the electrostatic latent image.
The present invention will be further described in
the following examples, but the present invention should not
be construed as being limited thereto. In the Examples, all
parts are by weight.
EXAMPLE 1
Styrene-n-butyl acrylate copolymer
(copolymerization ratio: 82/12;
Mw: 150,000) 46 parts
Magnetic material (magnetite having
a particle diameter of 0.2 ~m) 50 parts
Negatively chargeable charge
controller (azo chromium compound)2 parts
Polyethylene wax (Mn: 3,000) 2 parts
The foregoing materials were mixed in the form of
powder by means of a Henschel mixer. The mixture was then
heat-kneaded for 15 minutes by means of a two-roll mill which
had been adjusted to a temperature of 160 C. The mixture
thus kneaded was cooled, coarsely crushed, and then finely
crushed to obtain a crushed material having D50 of 8.5 ~m.
The crushed material was then classified to remove the finely
divided fractions and obtain a crushed material having D50 of
9.5 ~m. To 100 parts of the crushed material were then added
0.5 part of a hydrophobic colloidal silica and 0.8 part of a
particulate titanium oxide (primary particle diameter: 0.3
~m; sieve residue (defined below): 10 % by weight). The
mixture was stirred by a Henschel mixer, and then sieved
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through a 105-~m mesh sieve to remove the coarse grain
fractions and obtain a developer.
COMPARATIVE EXAMPLE 1
A developer was prepared in the same manner as in
Example 1 except that the particulate titanium oxide to be
used in Example 1 was replaced by a particulate titanium
oxide having the sieve residue of 50 % by weight.
COMPARATIVE EXAMPLE 2
A developer was prepared in the same manner as in
Example 1 except that no particulate titanium oxide was used.
The developers of Example 1 and Comparative Examples
1 and 2 were then subjected to printing test with a XP-11
type printer (manufactured by Fuji Xerox Co., Ltd.) having a
developing machine comprising a thin developer layer on a
developer carrier under an atmosphere of 10 C and 15 % RH.
For the evaluation of ghost, the difference in the image
density between the front end of a 1st solid black print
produced shortly after ten sheets of continuous printing and
the central part of the subsequent 2nd solid black print was
determined. For the evaluation of adhesion to the
photoreceptor (filming), the surface of the photoreceptor was
observed after 20,000 sheets of printing. In the criterion of
general evaluation, G (good) indicates good properties, F
(fair) indicates practically acceptable properties, and P
(poor) indicates practically unacceptable properties.
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Table 1
Example Amount Number of Sieve Ghost Filming General
No. of titanium residue (density (after evalu-
titanium oxide of tit- differe- 20,000 ation
oxide agglome- anium nce) sheets
(wt.%) rates of (wt.%) of prin-
not less ting)
than 20 ~m
1 0.8 0 10 0.02 None G
Comp. 0.8 2 50 0.03 Stripe- P
Ex.1 shaped
filming on
image
Comp. 0 0 - 0.20 None P
Ex.2
In the Table, the number of titanium oxide
agglomerates having a grain diameter of not less than 20 ~m
in the developer indicates the number of such titanium oxide
agglomerates in a 1,000 x magnification scanning electron
photomicrograph taken in a field of view. The sieve residue
of the particulate titanium oxide alone is measured by the
following method. 25 g of the sample is put on a 500-~m mesh
sieve mounted on a shaker (Octagon 200 manufactured by
Endecotts Corp.). The sample is then shaken at graduation 5
for 3 minutes. The sieve residue is then divided by the total
amount of the sample to determine the percent sieve residue
(by weight)(x 100).
As is apparent, the single-component type
electrostatic image developer of the present invention
provides a uniform chargeability distribution, making it
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2~rt 10~
possible to inhibit ghost, which is a problem of the
conventional magnetic toner. Further, problems concerning the
cleanability of the toner such as adhesion to the
photoreceptor can be solved.
While the invention has been described in detail and
with reference to specific embodiments thereof, it will be
apparent to one skilled in the art that various changes and
modifications can be made therein without departing from the
spirit and scope thereof.
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