Background of the Invention
1. Field of the Invention
[0001] The present invention relates to a method for manufacturing a resin-coated carrier
for an electrophotographic developer for use in a copying machine, a printer or the
like, and to an electrophotographic developer using therein the carrier obtained by
said method.
2. Prior Art
[0002] A two-component type developer used for electrophotography is composed of a toner
and a carrier. When the developer is used, the carrier is stirred and mixed with the
toner in a development box to give a desired charge to the toner and then carries
the thus charged toner onto electrostatic latent images on a photosensitive material
to develop the latent images thereby forming toner images. The carrier thus used remains
on a magnet roll and is then returned again to the development box when it is again
stirred and mixed with a fresh toner for repeated use as a developer.
[0003] Accordingly, the carrier is required to always provide the toner with the desired
charge during the use of the carrier.
[0004] Further, in substitution for many of carriers, there has recently been generally
used a resin-coated carrier prepared by coating the surface of a carrier core with
various resins to obatin a carrier which is improved in durability and resistance
to environmental dependence.
[0005] However, conventional developers which have heretofore been used will cause various
inconveniences with respect to images developed, due to collision of carrier particles
with each other by stirring, or friction between the development box and the carrier
particles. One of the inconveniences is that the toner particles are fusion adhered
onto the surface of the carrier particles by friction to cause useless consumption
of the toner (the useless consumption being hereinafter refered to as "spent"-phenomenon)
thereby widening the distiribution of charge amount between the toner and carrier
with the result that toner scattering and image fog arise during the use of the developer
for testing the service life thereof. Further, in a case where a resin-coated carrier
is included instead of the above carrier in the developer, the resin of the outermost
layer of the coated resin falls off to expose the internal resin layer thereby varying
a charge amount between the toner and carrier with the result that such toner scattering
and image fogging as above arise especially under high-temperature and high-humidity
conditions, during the use of the developer. When the developer service life test
is further conducted, the resin layer is peeled off to expose the carrier core thereby
varying the carrier resistance with the result that the developer cannot maintain
its performance of making images which are substantially equality to the initially
made image and it finally ends its survice life.
[0006] In a case where a developer including the resincoated carrier is used, a "spent"-phenomenon
with the toner tends to decrease as compared with the use of the uncoated carrier
in the developer and, however, so-called granulation of the resin-coated carrier particles
arise due to the carrier core particles being adhered to one another with the resin
at the time of resin coating. The granulated resin-coated carrier particles are then
separated one by one from one another thereby varying the fluidity of the developer
during printing or copying, with the result that the toner concentration is varied,
the density of images be obtained decreases due to the toner scattering caused by
excessive concentration of the toner and to lessening of the toner density, and image
deficiencies are very often incurred by adhesion of the carrier particles to the image.
[0007] There have heretofore been proposed various resin-coated carriers in order to improve
carriers in durability. For example, there have been proposed carriers prepared by
coating carrier cores on the surface with a fluorocarbon-based polymer (for example,
Japanese Patent Application Laid-Open Gazette No. Sho. 54-126,040). Although the resin-coated
carrier so proposed was stable in charging characteristics at its initial, but it
allowed a developer containing it to vary in fluidity due to the use of the carrier
for a long time thereby to form images having insufficient density, impair a photosensitive
material due to adhesion of the carrier thereto and consequently incur defects as
to images, thus the resin-coated carrier having been overall estimated to have a short
service life.
[0008] Further, carriers coated with a silicone resin or the like have been proposed (Japanese
Patent Application Laid Open Gazette No. Sho. 57-78,552, etc.). This silicone resin-coated
carrier has an advantage that the use thereof makes it difficult to cause a "spent"-phenomenon
because of its low-surface energy and, however, it causes fogging and toner scattering
due to its gradual decrease in charge level with the result that a developer having
the silicone resin-coated carrier has a short service life.
[0009] Recently, a carrier has been required to have satisfactory durability and a developer
including such a carrier is ultimately required to be maintenance-free for use, as
mentioned above. However, the service life of the resin-coated carrier is still short
and, therefore, there has remained an outstanding problem as to difficulties in prolonging
the service life of a developer containing such a carrier. Further, there has also
remained a problem as to a still high cost for coating a carrier core with a resin.
[0010] The present invention was made to solve the problems of the above mentioned prior
art, and an object of the present invention is to provide a method for manufacturing
resin-coated carriers for an electrophotographic developer which are excellent in
durability, resistance to environment without changing their electrifiability for
the toner, and producibility, as well as in economy, and also to provide an electrographic
developer including the carrier obtained by said method.
Summary of the Invention
[0011] The present inventors made various intensive studies to make sure what factors constitute
a bar to solving the above problems and, as the result of their studies, they found
that the occurrence of peeling of the resin of the resin-coated carrier and the granulation
thereof are the biggest factors. As the result of their further studies, they also
found that a further big factor is a method for heating a resin coated on a carrier
core in the process of preparation of the resin-coated carrier. The present invention
is based on this finding.
[0012] The method for manufacturing a resin-coated carrier for an electrophotographic developer
of the present invention is characterized in that it comprises the steps of coating
the surface of a carrier core which is a dielectric material, with a resin which is
also a dielectric material and then baking the thus obtained resin coated carrier
core by means of microwave heating to obtain a resin-coated carrier.
DETAILED EXPLANATION OF THE INVENTION
[0013] The present invention will now be explained hereunder in more detail.
[0014] The carrier core to be used in the present invention may be a dielectric material.
Examples -thereof are ferrite, magnetite and ceramics, among which ferrite is preferred
and Cu-Zn ferrite is particularly preferred. Therefore, since metals such as iron
and copper are not a dielectric material, they cannot be used in the present invention.
The mean particle diameter of the carrier core is in the range of preferably 15 to
200 µm, more preferably 20 to 150 µm. When the mean particle diameter of a carrier
used is less than 15 µm, the carrier will exhibit its severe adhesion. On the other
hand, when the mean particle diameter of a carrier is larger than 200 µm, the density
of solid area of the resulting image will be deficient.
[0015] In the present invention, the surface of the carrier core which is a dielectric material
is coated with a desired synthetic resin.
[0016] The resins to be used in the present invention are not particularly limited, and
examples thereof are fluorine-based resins, silicone-based resins, acrylstyrene resins,
alkyd resins, polyolefin resins, polyester resins and epoxy resins, and mixtures thereof.
[0017] In addition to the above, the typical usable resins include fluorine-based resins
such as polyvinylidene fluoride and vinylidene fluoride/ tetrafluoroethylene copolymers;
silicone-based resins such as cold-setting and thermosetting silicone resins which
are used in the form of silicone varnish, and silane coupling agents of low-degree
polymerization; acryl/styrene-based resins such as acryl-based resins alone, styrene-based
resins alone, and copolymers of acryl-based monomer and styrene-based monomer, the
acryl-based resins used herein refer to acrylic or methacrylic acid homopolymers,
copolymers of said acid and other monomer, homopolymers of acrylic or methacrylic
acid ester having functional groups, copolymers of said ester with other monomer;
and each of these polymers, into which is introduced a thermosetting resin such as
a melamine resin, or an isocyanate compound. Among these resins, acryl-based resins
are the best from the standpoint of adhesion to the carrier cores.
[0018] The amount of such a resin coated is preferably 0.05 to 10.0% by weight, more preferably
0.1 to 7.0% by weight, of the resulting resin-coated carrier. When the amount of the
resin used is less than 0.05% by weight, a uniform coating layer cannot be formed
on the carrier surface. When the amount of the resin used is more than 10.0% by weight,
the resulting coating layer will be too thick and the resulting resin-coated carrier
particles will be remarkably granulated even if they are baked by microwave heating.
[0019] In order to adjust charges and resistivities, conductive materials such as carbon
black may be added to the above resins.
[0020] In the present invention, the resin is generally dissolved in a solvent and then
coated on the surface of the carrier core. The solvents used herein may be ones which
can easily dissolve the resins therein, and examples thereof are toluene, xylene,
Celsolve butyl acetate, methyl ethyl ketone, methyl isobutyl ketone and methanol.
The resin diluted with the solvent may be applied by any method such as a dipping
method, spray method, brushing method or kneading method, and the solvent so applied
is then volatalized in the air.
[0021] Furthermore, the surface of the carrier core may be coated with the resin by a dry
method instead of a wet method using the solvent.
[0022] In the present invention, the micro-wave heating is applied to the resin, which is
a dielectric material, coated on the surface of a carrier core, thereby baking the
resin thereon. In this case, it is necessary to heat the coated resin to a temperature
higher than that at which the resin is cured by the use of microwave heating, and,
to this end, it is also necessary to adjust a time for irradiating the microwave.
The irradiation time is generally from 30 seconds to 10 minutes.
[0023] When the irradiation time is shorter than 30 seconds, the adhesion of the resin to
the carrier core is poor since the amount of heat applied is insufficient. On the
other hand, when the irradiation time is longer than 10 minutes, the durability of
the resin-coated carrier is deteriorated since the resin is overheated thereby causing
the decomposition thereof.
[0024] The microwave used for microwave heating in the present invention is among electromagnetic
waves having a frequency ranging from 300 MHz to 30 GHz and has a frequency of 915±25
MHz or 2450±50 GHz with the latter being prefered.
[0025] In the above manner, the resin is coated on the surface of the carrier core, baked
(or cured) and then cooled to obtain a resin-coated carrier.
Function
[0026] Conventional heating used at the time of carrier cores with a resin has been effected
by flame and heated air originated from combustion, a burner, an electric heater or
vapor or the like. These heating means have been used to heat the external surface
of the thus coated resin by their heat irradiation from the outside and then heat
the inside, that is the carrier core, by thermal conductivity. In brief, conventional
heating is external heating.
[0027] In contrast, the present invention adopts an internal heating method using high-frequency
heating (radio heating), in which method a material to be heated generates heat from
itself (from the carrier core in this case) as a heating medium. This internal heating
will enable the adhesion of the resin layer to carrier core to be improved or enhanced
since the resin and the carrier core themselves generate heat uniformly from their
insides. Further, the microwave heating system will enable resin coatings to be baked
in a short time.
[0028] Thus, the conventional heating apparatus needs a longer baking time and a higher
cost to manufacture resin-coated carriers, while the present invention makes it possible
to remarkably shorten a baking time as compared with the conventional one, reduce
their production cost and render their producibility excellent.
[0029] The present invention will be better understood by the following Examples and Comparative
Examples.
Example 1
[0030] Cu-Zn ferrite particles (produced under the tradename "F-150" by Powdertech Co.,
Ltd., Japan; mean particle diameter: 80 µm; saturation magnetization: 65 emu/g) were
used as a carrier core. On the other hand, 88 % by weight of an acryl-styrene based
resin and 12 % by weight of a melamine based resin were dissolved in a toluene solvent
to prepare a coating resin composition. The thus prepared coating composition was
coated on the carrier core by using a fluidized-bed so that the resin so coated amounted
to 3.0 % by weight based on the total weight of the resulting resin-coated carrier.
The thus coated carrier core was then baked for 3 minutes by irradiating microwave
having a wavelength of 2450MHz by using a microwave heating apparatus to obtain a
resin-coated carrier. The temperature of this stage was 150°C.
[0031] These characteristics of the resin-coated carriers obtained and treating conditions
used in Examples and Comparative Examples are summarized in Table 1.
[0032] The resin-coated carrier obtained in Example 1 and a toner were used to prepare a
developer which was then subjected to a developer service life test in which 100,000
sheets were to be copied by a commercially available copying machine to test the resin-coated
carrier for its image evaluation. The toner used in this test was a commercially available
black toner and a toner concentration used was 4.5 % by weight.
[0033] The image evaluation was effected for image density, for on the image, carrier adhesion
and toner scattering which were observed at the initial stage of this developer service
life test and after the end thereof, as well as for environmental variation of electric
charge, resistance variation of the carrier and amount of "spent", after which an
overall estimation was carried out. The test results are shown in the following table
2. In Table 2, the symbols, ⓞ , ○ , Δ , and X represent "excellent", "good", "slightly
poor" and "poor", respectively. Methods for evaluating these items are indicated hereunder.
[Image evaluation of carrier]
[0034]
(1) Image density (ID)
The developer was used in copying under a proper exposure condition to obtain copies
which were then evaluated for image density. More particularly, the initial and final
test copies were evaluated at their solid image portions for image density by using
a SAKURA densitometer, and the image densities found were ranged.
(2) Fog on image
As for fogs on the image, test copies were evaluated at their white grounds for toner
fog on the image, and the fogs found were ranged.
(3) Carrier adhesion
As in the case of the above toner fog, test copies were evaluated at their white grounds
for carrier adhesion level, and the levels found were ranged.
(4) Toner scattering
Test copies were eveluated for degree of toner scattering which stains the copies,
soil of an electrification charger and undesirable condition of toner scattering,
and these results found were then ranked.
(5) Environmental variation of charge
One portion of the developer was maintained at 10 °C and 15% humidity for 24 hours
to find its amount of charge (QLL) at the end of the maintenance, while another portion
of the same developer was also maintained at 30 °C and 85% humidity for 24 hours to
find its amount of charge (QHH) at the end of said maintenance. The difference (ΔQ)
was calculated and ranked for evaluating environmental variation of charge.

(6) Carrier resistance variation
The inherent resistance (R1) of the resin-coated carrier at the initial stage and
that (R2) thereof after the end of the developer service life test of copying 100,000
sheets were measured respectively, and the difference (ΔR) therebetween was calculated
from the said resistances found. The variation of the carrier resistance was evaluated
by ranking the difference thus obtained.

(7) Amount of "spent"
The amount of toner (amount of "spent") adhered to the carrier after the end of the
developer service life test of copying 100,000 sheets was measured and ranked for
evaluating the amount of "spent".
(8) Overall evaluation
The image evaluations (image density, fog on the image, carrier adhesion, toner scattering,
environmental variation of electrification or charge, carrier resistance variation
and amount of "spent") obtained in the developer service life test of 100,000 sheets
were overall evaluated and ranked.
Example 2
[0035] Cu-Zn ferrite particles (produced under the tradename "FL-100" by Powdertech Co.,
Ltd., Japan; mean particle diameter: 120 µm; saturation magnetization: 55 emu/g) were
used as a carrier core. On the other hand, a silicone resin (produced under the tradename
"SR-2411" by the Toray-Dow Corning silicone Co., Ltd.,) was coated in the same manner
as in Example 1 so that said resin was coated in an amount of 1.0 % by weight based
on the total weight of the resulting resin-coated carrier. Furthermore, the thus obtained
coated carrier core was baked for 6 minutes by irradiating thereto microwave having
a wavelength of 2450MHz by using a microwave heating apparatus to obtain a resin-coated
carrier. The temperature of this stage was 200°C.
[0036] The characteristics of the carrier core used in this Example and treating conditions
are summarized in Table 1.
[0037] A developer was prepared from the thus obtained resin-coated carrier and the same
toner as used in Example 1 so that the toner concentration was 4.0 % by weight. The
thus obtained developer was subjected to a developer service life test under the same
conditions as Example 1 by using a commercially available copying machine to conduct
image evaluation. The image evaluation in this case is shown in table 2.
Example 3
[0038] Cu-Zn Ferrite particles (produced under the tradename "F-1030" by Powdertech Co.,
Ltd., Japan; mean particle diameter: 100 µm; saturation magnetization: 65emu/g) were
used as a carrier core. On the other hand, a fluorine-acryl resin (1:1 mixed resin
of polyvinylidene fluoride and acryl-styrene resin) was coated in the same manner
as in Example 1 so that said resin was coated in an amount of 0.3 % by weight based
on the total weight of the resulting resin-coated carrier. Furthermore, the thus obtained
coated carrier core was baked for 4 minutes by irradiating thereto microwave having
a wavelength of 2450MHz by using a microwave heating apparatus to obtain a resin-coated
carrier. The temperature of this stage was 170°C.
[0039] The characteristics of the carrier core used in this Example and treating conditions
are summarized in Table 1.
[0040] A developer was prepared from the thus obtained resin-coated carrier and the same
toner as used in Example 1 so that the developer had a toner concentration of 4.0
% by weight. The thus obtained developer was subjected to a developer service life
test under the same conditions as Example 1 by using a commercially available copying
machine to carry out image evaluation of the resin-coated carrier. The image evaluation
in this case is shown in Table 2.
Example 4
[0041] Cu-Zn ferrite particles (produced under the tradename "F-150" by Powdertech Co.,
Ltd., Japan; mean particle diameter: 80 µm; saturation magnetization: 65 emu/g) were
used as a carrier core. On the other hand, 88 % by weight of an acryl-styrene based
resin and 12 % by weight of a melamine based resin were dissolved in a toluene solvent
to prepare a coating resin composition, and said carrier core was coated with said
resin composition in the same manner as in Example 1 so that said resin was coated
in an amount of 8.0 % by weight based on the total weight of the resulting resin-coated
carrier. Furthermore, the thus obtained coated carrier core was baked for 6 minutes
by irradiating thereto microwave having a wavelength of 915MHz by using a microwave
heating apparatus to obtain a resin-coated carrier. The temperature of this stage
was 150°C.
[0042] The characteristics of the carrier core used in this Example and treating conditions
are summarized in Table 1.
[0043] A developer was prepared from the thus obtained resin-coated carrier and the same
toner as used in Example 1 so that the developer had a toner concentration of 4.5
% by weight. The thus obtained developer was subjected to a developer service life
test under the same conditions as Example 1 by using a commercially av-ailable copying
machine to conduct image evaluation of the resin-coated carrier. The image evaluation
in this case was shown in Table 2.
Comparative Example 1
[0044] The procedure of Example 1 was followed except that baking of the resin at 150 °C
for 60 minutes in an electric furnace was substituted for the microwave heating of
2450MHz for 3 minutes, to obtain a comparative resin-coated carrier.
[0045] The characteristics of the carrier core used in this Comparative Example and treating
conditions are summarized in Table 1.
[0046] A developer was prepared from the thus obtained resin coated carrier and the same
toner as used in Example 1 so that the developer had a toner concentration of 4.5
% by weight of a toner concentration. The thus obtained developer was subjected to
a developer service life test under the same conditions as Example 1 by using a commercially
available copying machine to conduct image evaluation of the resin-coated carrier.
The image evaluation in this case is shown in Table 2.
Comparative Example 2
[0047] The procedure of Example 2 was followed except that baking of the resin at 200 °C
for 120 minutes in a thermostat was substituted for the microwave heating of 2450MHz
for 6 minutes, to obtain a comparative resin-coated carrier.
[0048] The characteristics of the carrier core used in this Comparative Example and treating
conditions are summarized in Table 1.
[0049] A developer was prepared from the thus obtained resin-coated carrier and the same
toner as used in Example 1 so that the developer had a toner concentration of 4.0
% by weight. The thus obtained developer was subjected to a developer service life
test under the same conditions as Example 1 by using a commercially available copying
machine to conduct image evaluation of the resin-coated carrier. The image evaluation
in this case is shown in Table 2.
Comparative Example 3
[0050] The procedure of Example 3 was followed except that baking of the resin at 170 °C
for 90 minutes in an the electric furnace was substituted for the microwave heating
of 2450MHz for 4 minutes, to obtain a comparative resin-coated carrier.
[0051] The characteristics of the carrier core used in this Comparative Example and treating
conditions are summarized in Table 1.
[0052] A developer was prepared from the thus obtained resin-coated carrier and the same
toner as used in Example 1 so that the developer had a toner concentration of 4.0
% by weight. The thus obtained developer was subjected to a developer service life
test under the same conditions as Example 1 by using a commercially available copying
machine to carry out image evaluation of the comparative resin-coated carrier. The
image evaluation in this case was shown in Table 2.
Comparative Example 4
[0053] The procedure of Example 4 was followed except that baking of the resin at 150 °C
for 60 minutes in an the electric furnace was substituted for the microwave heating
of 915MHz for 3 minutes, to obtain a comparative resin-coated carrier.
[0054] The characteristics of the carrier core used in this Comparative Example and treating
conditions are summarized in Table 1.
[0055] A developer was prepared from the thus obtained comparative resin-coated carrier
and the same toner as used in Example 1 so that the developer had a toner concentration
of 4.5 % by weight. The thus obtained developer was subjected to a developer service
life test under the same conditions as Example 1 by using a commercially available
copying machine to carry out image evaluation of the comparative resin-coated carrier.
The image evaluation in this case is shown in table 2.

(Effect of the Invention)
[0056] The resin-coated carriers obtained by the method of the present invention have excellent
advantages that a "spent"-phenomenon (amount of "spent") of the toner appreciated
on hardening or curring the coated resin is very slight since the adhesion between
the carrier core and the resin becomes firm and steady due to the internal heating
by using microwave heating in the baking step after the carrier core was coated with
the resin, and they also have excellent durability since the peeling or falling of
the coated resin layer off the carrier surface decreases. Further, the resincoated
carrier can maintain stable charging performance even if the environment varies. Accordingly,
a developer using therein such a resin-coated carrier as above can remarkably improve
itself in service life and image characteristics. In addition to this, the method
of the present invention is excellent in economy and productivity since the heating
time is short as previously mentioned.
1. A method for manufacturing a resin-coated carrier for an electrophotographic developer
comprising the steps of:
a) coating a resin on the surface of a carrier core, the resin and carrier core being
independently formed of dielectric material, to obtain a resin coated carrier, and
b) baking the thus obtained resin-coated carrier by means of microwave heating, to
obtain the resincoated carrier.
2. A method according to claim 1, wherein said microwave heating is carried out for 30
seconds to 10 minutes.
3. An electrophotographic developer including the resin-coated carried obtained by the
method of claim 1 or 2.
4. A resin-coated carrier for use as an electrophotographic developer, said resin-coated
carrier comprising a carrier core substantially formed of dielectric material and
a resin coating substantially formed of dielectric material, wherein the carrier core
and the resin coating are bonded together.
5. A resin-coated carrier as claimed in claim 4 wherein the carrier core is formed from
ferrite or magnetite.
6. A resin-coated carrier as claimed in claim 5 wherein the ferrite is Cu-Zn ferrite.
7. A resin-coated carrier as claimed in any of claims 4, 5 or 6 wherein the mean particle
diameter of the carrier core is from not less than 15 µm to not more than 200 µm.
8. A resin-coated carrier as claimed in any of claims 4 to 7 wherein the resin is selected
from one or more of; fluorine-based resins, silicone-based resins, acryl-styrene resins,
alkyd resins, polyolefin resins, and polyester resins.
9. A resin-coated carrier as claimed in any of claims 4 to 8 wherein the resin-coated
carrier comprises 0.05 to 10.0% by weight of the resin.
10. A resin-coated carrier as claimed in any of claims 4 to 9 in which the resin further
includes conductive material.