BACKGROUND
Technical Field
The present invention relates to an electrostatic latent
image carrier and an electrostatic latent image developer used in an electrophotographic
method and in electrostatic recording.
Related Art
In an electrophotographic method, an electrostatic latent
image is formed on a latent image holding member (a photoreceptor) by charging and
exposure processes, this electrostatic latent image is developed with toner, the
developed image is transferred to a transfer target material, and fixing of the
image is conducted by heating or the like, thus forming the final image. Developers
that can be used in this type of electrophotographic method can be broadly classified
into one-component developers, in which a toner formed by dispersing a colorant
within a binder resin is used alone, and two-component developers that are formed
from a combination of the above type of toner and a carrier. Because the carrier
performs the functions of charging and transportation, two-component developers
offer excellent control, and are consequently in widespread use. A feature of two-component
developers is the separation of the developer functions, with the carrier performing
the functions of stirring, transportation and charging of the developer, and because
this separation yields more favorable control, two-component developers are currently
in widespread use.
In recent years, digitization processing has been employed
as a technique for achieving higher levels of image quality, and such digitization
has enabled more complex images to be processed rapidly. Furthermore, although a
laser beam is used in the process of forming the electrostatic latent image on the
latent image holding member, recent developments in exposure techniques using small-scale
laser beams have enabled the formation of more finely detailed latent images. As
a result of these types of image processing techniques, electrophotographic methods
are gradually expanding into fields such as convenience printing. Modern electrophotographic
apparatus also face continued demands for increased speeds and reductions in the
size of the apparatus. Particularly in the case of full color images, high quality
printing with image quality similar to that of silver halide photography is desirable.
Accordingly, in order to enable more finely detailed latent images to be faithfully
visualized over extended periods, maintaining the charge of the developer is very
important. In other words, further improvements are required in the charge retention
properties of the carrier that performs the charging function.
Furthermore, toner particles have been reduced in size
in order to yield higher image quality, and toners that include a low melting point
wax or the like are used to enable the fixed image to be drawn or written on with
a pen or the like . Particularly in the case of full color toners, toners in which
a resin with a low softening point and a low melting point wax have been incorporated
within the binder resin are widely used to improve the color reproducibility and
coloring properties of the toner. During charging of the developer, the desired
charge quantity is obtained by frictional charging between the toner and the carrier,
but when this type of toner is used, the toner component is prone to becoming spent
on the carrier surface as a result of factors such as friction between the toner
and the carrier, collisions between carrier particles, and mixing and temperature
increases inside the developing unit. This causes problems to arise, including a
deterioration in the ability of the carrier to impart charge to the toner and a
subsequent increase in the quantity of low-charge toner, which may lead to toner
fogging within areas outside of the latent image, as well as an increase in contamination
within the developing unit with ongoing use of the apparatus. Furthermore, in the
case of a toner that includes a wax or a low softening point resin, stress may cause
additives that have been added to the toner to become buried within the toner surface,
meaning they are unable to perform their intended functions. Examples of problems
that may arise include a deterioration in the image quality caused by image roughening
that arises from a reduction in toner fluidity, a deterioration in the developing
characteristics, or a deterioration in the transfer characteristics.
In order to improve the charging stability and extend the
lifespan of the charge, various investigations have been conducted into carrier
coating layers . From the viewpoint of improving the spent resistance, investigations
have focused on fluororesins, silicone-based resins and polyolefin-based resins
that exhibit excellent releasability. Coated carriers that employ these types of
high releasability resins have certainly proven to be an effective tool in extending
the lifespan of the charge. However, in order to target further improvements in
the lifespan of the charge, the coating layer needs to be made structurally thicker.
In such cases, the carrier develops high resistance, making it unable to undergo
rapid charge exchange under conditions of low temperature and low humidity, and
causing a deterioration in the initial charge-up. This leads to a deterioration
in the charge-up and toner addition characteristics, causing the toner charge distribution
to widen. As a result, the image density falls, and scattering and fogging of the
low charge toner occurs. Furthermore, edge effects caused by the carrier resistance
also arise.
Furthermore, burying of the aforementioned additives, namely
external additives, and the aforementioned problem of the toner component becoming
spent are caused by collisions or friction between the toner and the carrier. Accordingly,
tests have been conducted into addressing this problem by investigating the shapes
of the core particles within the carrier and the carrier particles themselves. In
particular, roughnesses are being conducted into techniques in which a core material
with an uneven surface is used as the core material within the above core particles.
A technique has been proposed in which a small quantity
of a resin coating layer is provided on top of a core material that contains fine
pores within the surface, and the resulting pores within the carrier surface increase
the surface area, thereby improving the efficiency with which the carrier is able
to impart charge to the toner (for example, see
Japanese Patent Laid-Open Publication No. Hei 03-160463
, and Japanese Patent Laid-Open Publication
No. Hei 02-108065
).
However, toner particles have reduced in size in recent
years, and if the types of pores described above are provided in the carrier surface,
then there is a possibility that toner particles caught between carrier particles
may be subjected to additional stress, or that the problem of the toner component
becoming spent may actually be accelerated. Furthermore, because structurally large
protrusions exist at the carrier surface, there is a possibility that friction between
carrier particles may increase the like lihood of separation of the resin coating
layer. As a result, there is a possibility that the charge-imparting properties
of the carrier itself may suffer a dramatic deterioration.
Furthermore,
Japanese Patent Laid-Open Publication No. Hei 07-98521
discloses an electrophotographic carrier in which the particle size of
the carrier and the carrier content are both specified, and for which the specific
surface area S1 of the carrier determined by an air permeation method,
and the specific surface area S2 of the carrier calculated using a formula
satisfy the condition: 1.2 ≤ S1/S2 ≤ 2.0, and
it is suggested that this configuration enables rapid startup of the frictional
charging between the toner and the carrier. Furthermore,
Japanese Patent Laid-Open Publication No. 2000-172019
discloses a resin-coated carrier formed by coating a carrier core material
with a coating layer of a resin, wherein the particle size of the carrier and the
carrier content are both specified, the BET specific surface area SW1 of the carrier
core material from which the coating layer has been removed, and the BET specific
surface area SW2 of the resin-coated carrier satisfy the condition: 80 ≤
SW1-SW2 ≤ 650 (cm2/g), the shape factor SF-1 of the resin-coated
carrier satisfies 110 ≤ SF-1 ≤ 160, and the shape factor SF-2 of the
resin-coated carrier satisfies 105 ≤ SF-2 ≤ 150.
(wherein, ML represents the absolute maximum length of a carrier particle, and
A represents the projected area of the carrier particle)
Furthermore,
Japanese Patent Laid-Open Publication No. 2005-134708
proposes a magnetic carrier which, inorderto improve the spent resistance
and fluidity, and enable a stable image to be retained over an extended period,
includes a magnetic core and multiple resins, wherein the particle size and absolute
specific gravity are specified, the specific surface area falls within a range from
0.080 to 0.300 m2/g, and the ratio (B/A) between the BET specific surface
area A of the magnetic carrier and the BET specific surface area B of the magnetic
core is within a range from 1.3 to 15.0.
However, although using a core material with an uneven
surface and reducing the carrier absolute specific gravity enables a reduction in
the collision energy between both toner and carrier particles and between carrier
particles, and also results in some improvement in the spent resistance, an adequate
level of magnetism may not be attainable depending on the BET specific surface area
of the core material. Furthermore, because the surface shape of the core material
is not controlled, the carrier surface is randomly rough, meaning there is a possibility
of either a deterioration in the spent resistance, or a deterioration in the fluidity.
In recent years, miniaturization of the developing unit has progressed significantly,
and if stress inside the unit is high, then the expected effects may not be achievable.
Moreover, in those cases where a recently adopted toner density control method that
employs magnetic permeability sensors is used, lower magnetism and a deterioration
in fluidity may make control of the toner impossible.
The present invention addresses the problems outlined above,
wherein by using a core material and carrier that have been subj ected to a high
degree of surf ace control, stress on the toner is minimized, excellent toner spent
characteristics and fluidity are achieved, and even when used inside a small developing
unit, no difference in toner density occurs inside the unit, enabling a high level
of image quality to be maintained over an extended period.
SUMMARY
As a result of detailed investigations of the problems
described above, the inventors of the present invention discovered that by adopting
the configuration of the present invention described below, the effects described
above could be achieved, and they were thus able to complete the present invention.
- (1) According to an aspect of the present invention, there is provided an electrostatic
latent image carrier having core particles and a resin coating layer that coats
the surface of the core particles, wherein the surface roughness of the core particles
exhibits a surface roughness Sm that satisfies the expression Sm ≤ 2.0 µm
and a surface roughness Ra (compliant with JIS B0601) that satisfies the expression
Ra ≥ 0.1 µm, the surface roughness Ra (compliant with JIS B0601) of
the electrostatic latent image carrier satisfies the expression Ra ≤ 0.5
µm, and the sphericity of the electrostatic latent image carrier is 0.975 or
higher. Ra is also referred to as the "centerline average roughness".
- (2) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according to aspect (1) above, wherein the
core exposure ratio at the surface of the electrostatic latent image carrier is
2% or lower.
- (3) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according to aspect (1) above, wherein a
core of the carrier is represented by a formula shown below:
(MO)X(Fe2O3)Y
(wherein, M comprises one or more metals selected from the group consisting of Cu,
Zn, Fe, Mg, Mn, Ca, Li, Ti, Ni, Sn, Sr, Al, Ba, Co and Mo; and X and Y represent
molar ratios, wherein X+Y = 1.00) .
- (4) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according toaspect (3) above, wherein M represents
one or more metals selected from the group consisting of Li, Mg, Ca, Mn, Sr, and
Sn, and a combined quantity of any other M components is no higher than approximately
1% by weight.
- (5) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according to aspect (1) above, wherein when
a magnetization &sgr; of the core particles is measured within a magnetic field
of 1 kOe, using a VSM (vibrating sample method) measuring apparatus and employing
a BH tracer method, a resulting magnetization value &sgr;1000 is within a range
from approximately 45 to 90 Am2/kg (emu/g).
- (6) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according to aspect (1) above, wherein an
average particle size of the core particles is within a range from approximately
10 to 100 µm.
- (7) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according to aspect (1) above, wherein an
electrical resistance of the carrier under a measurement electric field of 5, 000
V/cm is within a range from approximately 1 × 105 to 1 × 1014
&OHgr;-cm.
- (8) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according to aspect (1) above, wherein a
dynamic electrical resistance of the carrier, when measured in the form of a magnetic
brush under an electric field of 1.04 V/cm, is within a range from approximately
1 × 103 to 1 × 1013 &OHgr;-cm.
- (9) According to another aspect of the present invention, there is provided
the electrostatic latent image carrier according to aspect (1) above, wherein a
thickness of the resin coating layer is within a range from approximately 0.1 to
5 µm,
- (10) According to another aspect of the present invention, there is provided
an electrostatic latent image developer that includes a toner and a carrier, wherein
the carrier is the electrostatic latent image carrier according to aspect (1) above.
- (11) According to another aspect of the present invention, there is provided
the electrostatic latent image developer according to aspect (10) above, wherein
a volume average particle size of the toner is within a range from approximately
3 to 9 µm.
- (12) According to another aspect of the present invention, there is provided
the electrostatic latent image developer according to aspect (10) above, wherein
an average value of a shape factor SF1 for the toner is approximately 100 or greater,
but no higher than approximately 135.
- (13) According to another aspect of the present invention, there is provided
the electrostatic latent image developer according to aspect (10) above, wherein
a volume average particle size of a colorant of the toner is within a range from
approximately 0.01 to 1 µm.
- (14) According to another aspect of the present invention, there is provided
the electrostatic latent image developer according to aspect (10) above, wherein
a proportion of the toner is within a range from approximately 1 to 15% by weight
of the entire developer.
- (15) According to another aspect of the present invention, there is provided
an image forming apparatus, comprising a latent image forming unit that forms an
electrostatic latent image on a surface of a latent image holding member, a developing
unit that develops the electrostatic latent image formed on the surface of the latent
image holding member using a developer supported on a developer carrier, thereby
forming a developed image, a transfer unit that transfers the developed image formed
on the surface of the latent image holding member to a surface of a transfer target,
and a fixing unit that heat fixes an image that has been transferred to the surface
of the transfer target, wherein the developer uses the electrostatic latent image
carrier disclosed in aspect (1) above.
- (16) According to another aspect of the present invention, there is provided
an electrostatic latent image developer, comprising a toner and a carrier, wherein
the carrier is the electrostatic latent image carrier according to aspect (2) above.
By prescribing the surface roughness of the core particles
in the manner described above, the present invention eliminates internal voids,
and yields core particles with irregularities only at the particle surface. By using
core particles with this type of structure, a resin coating layer with a high coating
ratio can be formed, meaning reductions in the charge-imparting ability of the carrier
can be suppressed. Furthermore, by using core particles of the above prescription,
reductions in the level of magnetism can be alleviated, the transportation properties
of the resulting carrier can be improved, and magnetic permeability toner density
control can also be improved.
Furthermore, by covering essentially the entire surface
of the core particles with the resin coating layer, and minimizing the irregularities
on the carrier surface, not only is the frictional energy able to be reduced, but
the anchoring effect on the resin coating layer provided by the core particle is
able to function more effectively, meaning separation of the resin coating layer
can be suppressed.
In addition, by employing the carrier shape described above,
not only can charge be imparted more efficiently to the toner, but stress between
carrier particles and stress inside the developing unit can also be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiment(s) of the present invention will be described
in detail based on the following figures, wherein:
- Fig. 1 is a schematic illustration showing a sample configuration of an image
forming apparatus that uses an image forming method of the present invention to
form an image; and
- Fig. 2 is a laser microscope photograph showing particle surfaces.
DETAILED DESCRIPTION
As follows is a more detailed description of the present
invention.
[Electrostatic Latent Image Carrier]
As follows is a description of an electrostatic latent
image carrier of the present invention. In the following description, the term "electrostatic
latent image carrier" may be abbreviated as simply "carrier".
A carrier of the present invention has core particles and
a resin coating layer that coats the surface of the core particles, wherein the
surface roughness of the core particles exhibits a surface roughness Sm that satisfies
the expression Sm ≤ 2.0 µm and a surface roughness Ra (compliant with
JIS B0601) that satisfies the expression Ra ≥ 0.1 µm, the surface roughness
Ra (compliant with JIS B0601) of the electrostatic latent image carrier satisfies
the expression Ra ≤ 0.5 µm, and the sphericity of the electrostatic
latent image carrier is 0.975 or higher. Ra is also referred to as the "centerline
average roughness".
In the present invention, measurement of Ra and Sm is conducted
in accordance with JIS B0601 . In the examples described below, measurements are
conducted using the measuring device described below.
The sphericity is measured using the LPF measurement mode
of a FPIA-3000 device (manufactured by Sysmex Corporation). To conduct the measurement,
0.03 g of the carrier is dispersed in a 25% by weight aqueous solution of ethylene
glycol, and the average sphericity is determined by analyzing particles other than
those with a particle size of either less than 10 µm or greater than 50 µm.
In the present invention, the raw material for the core
particles prior to baking is ground more finely than in conventional production
methods, thereby increasing the packing ratio within the core particles of the raw
material, and the temperature is also applied more uniformly during the baking stage,
enabling a more uniform surface to be obtained. Moreover, the core particles of
the present invention can be prepared by controlling the crystal growth by grinding
and dispersing the raw material more finely, and applying the temperature in a uniform
manner. One method that can be used to apply a uniform temperature involves the
use of a rotary kiln.
Although any of the conventionally used materials can be
used as the core particles, the use of either ferrite or magnetite is particularly
desirable. Examples of other known core particles include iron powder. Because iron
powder has a large specific gravity, it is more likely to cause deterioration of
the toner, and consequently ferrite and magnetite offer higher levels of stability.
Examples of ferrite include the materials represented by the general formula shown
below.
(MO)X(Fe2O3)Y
(wherein, M includes at least one metal selected from a group including Cu, Zn,
Fe, Mg, Mn, Ca, Li, Ti, Ni, Sn, Sr, Al, Ba, Co and Mo; and X and Y represent molar
ratios, wherein X+Y = 1.00)
Ferrite particles in which the aforementioned M is one
or more metals selected from a group including Li, Mg, Ca, Mn, Sr and Sn, and the
quantity of any other components is no higher than 1% by weight are preferred. If
Cu, Zn or Ni elements are added, then the resistance is more likely to be low, making
the ferrite prone to charge leakage. Furthermore, the ferrite also tends to become
more difficult to coat, and the environmental dependency also tends to deteriorate.
In addition, because these elements are heavymetals, the stress applied to the carrier
tends to increase, which may have an adverse effect on the lifespan of the carrier.
Furthermore, from the viewpoint of safety, ferrites that include added Mn or Mg
have recently become widespread. A ferrite core material is ideal, and the raw materials
for the core particles include Fe2O3 as an essential component,
together with the magnetic fine particles that are incorporated within the fine
magnetic particle-dispersed resin core, examples of which include ferromagnetic
iron oxide powders such as magnetite and maghemite, spinel ferrite powders that
contain one or more metals other than iron (such as Mn, Ni, Zn, Mg and Cu), magnetoplumbite
ferrite powders such as barium ferrite, and fine particulate powders of iron or
iron alloys that are surface-coated with an oxide film.
Specific examples of the core particles include iron oxides
such as magnetite, &ggr;-iron oxide, Mn-Zn ferrite, Ni-Zn ferrite, Mn-Mg ferrite,
Li ferrite, and Cu-Zn ferrite. Of these, the low cost magnetite is particularly
favorable.
In those cases where a ferrite core material is used as
the core particles, an example of a suitable production method for the ferrite core
material involves first blending appropriate quantities of each of the oxides, subsequently
grinding and mixing the oxides for 8 to 10 hours in a wet ball mill, drying the
resulting mixture, and then conducting preliminary baking in a rotary kiln or the
like at a temperature of 800 to 1,000°C for a period of 8 to 10 hours. Subsequently,
the prebaked product is dispersed in water, and ground in a ball mill or the like
until the average particle size falls within a range from 0.3 to 1. 2 µm. The
resulting slurry is granulated and dried using a spray dryer or the like, subsequently
held at a temperature of 800 to 1, 200°C for a period of 4 to 8 hours under
a controlled oxygen concentration environment in order to regulate the magnetic
properties and resistance, and then ground and classified to yield the desired particle
size distribution. In the present invention, the use of a rotary electric kiln is
desirable in terms of achieving a uniform shape for the surface of the core particles.
The surface roughness of the core particles used in the
present invention exhibits a surface roughness Sm that satisfies the expression
Sm ≤ 2.0 µm and a surface roughness Ra (compliant with JIS B0601) that
satisfies the expression Ra ≥ 0.1 µm. Prescribing the surface roughness
of the core particles in this manner eliminates internal voids, yielding core particles
with irregularities only at the particle surface. By employing core particles with
this type of structure, a resin coating layer with a high coating ratio can be formed,
meaning reductions in the charge-imparting ability of the carrier can be suppressed.
Furthermore, by using core particles of the above prescription, reductions in the
level of magnetism can be alleviated, the transportation properties of the resulting
carrier can be improved, and magnetic permeability-based toner density control can
also be improved.
Furthermore, if the surface roughness of the core particles
is such that the surface roughness Sm exceeds 2.0 µm, then during production
of the core particles, voids are more likely to develop inside the core particles,
increasing the likelihood of difficulties arising in the subsequent formation of
the resin coating layer. Furthermore, if the surface roughness Ra (compliant with
JIS B0601) of the core particles is less than 0.1. µm, then the anchoring effect
on the resin coating layer that is subsequently coated onto the surface of the core
particles weakens, meaning that when the particles are used as a developer, not
only is the resin coating layer prone to separation from the core particles, but
the specific gravity of the carrier particles also increases, making it impossible
to achieve the targeted reduction in specific gravity, and preventing the manifestation
of the desired reduction in collision energy.
In addition, the surface roughness Ra (compliant with JIS
B0601) of the carrier that includes a resin coating layer formed on the surface
of the core particles satisfies the expression Ra ≤ 0.5 µm, and the
sphericity of the carrier is 0.975 or higher. Furthermore, the core exposure ratio
at the surface of the carrier is 2% or lower.
In this manner, by covering essentially the entire surface
of the core particles with the resin coating layer, and minimizing the irregularities
on the carrier surface, not only is the frictional energy able to be reduced, but
the anchoring effect on the resin coating layer provided by the core particle is
able to function more effectively, meaning separation of the resin coating layer
can be suppressed. By employing the carrier shape described above, not only can
charge be imparted more efficiently to the toner, but stress between carrier particles
and stress inside the developing unit can be reduced.
If the surface roughness Ra (compliant with JIS B0601)
of the carrier surface exceeds 0.5 µm, then the toner component becomes prone
to scraping by the carrier surface, and accumulation and fusion of the toner component
within recesses on the carrier may exacerbate the toner spent problem.
Furthermore, the sphericity of the carrier is 0.975 or
higher, and the closer this value is to 1, the closer the carrier particles are
to a true spherical shape, and furthermore, the larger the surface roughness value
becomes, the more likely the existence of fine irregularities within the surface.
By adjusting the sphericity of the carrier to 0.975 or higher, thereby bringing
the shape closer to a true spherical shape, the fluidity of the carrier is improved,
enabling a more uniform resin coating layer to be formed, and enabling suppression
of aggregation of the core particles, thereby improving the production yield. As
described above, the sphericity is measured using the LPF measurement mode of a
FPIA-3000 device (manufactured by Sysmex Corporation).
Furthermore, the core exposure ratio at the surface of
the carrier is 2% or lower. In cases such as the present invention, where core particles
with surface irregularities are used, the exposed portions of the core that occur
at the carrier surface are usually protrusions. In those cases where factors such
as stress inside the developing unit cause the resin coating layer of the carrier
to separate, the exposed core portions that exist at the carrier surface act as
nuclei for this Separation of the resin coating layer. If the core exposure ratio
exceeds 2%, then the number of locations for potential separation of the resin coating
layer increases, meaning the resin coating layer is more likely to undergo separation
upon extended use. In other words, the charging function of the carrier deteriorates.
By ensuring that the core particles used in the present
invention have fine irregularities at the particle surface, the resin coating layer
can be firmly fixed to the particles by an anchoring effect, meaning separation
of the coating layer from the carrier can be prevented. Furthermore, by ensuring
that the surface of the core particles exhibits the surface roughness described
above and includes protrusions, an electrical path can be formed via these protrusions
in those cases where the toner density is high, meaning the resistance value of
the developer is less likely to vary with variations in the toner density.
The magnetization &sgr; of the core particles of the
present invention is measured within a magnetic field of 1 kOe, using a VSM (vibrating
sample method) measuring apparatus and employing a BH tracer method, and the resulting
magnetization value &sgr;1000 is typically within a range from 45 to 90 Am2/kg
(emu/g), and preferably from 45 to 70 Am2/kg (emu/g). If the value of
&sgr;1000 is less than 50 Am2/kg (emu/g), then the magnetic adsorption
to the developing roller weakens, which can cause the particles to adhere to the
photoreceptor, causing undesirable image defects. In contrast, if the value of &sgr;1000
exceeds 90 Am2/kg (emu/g), then the magnetic brush becomes overly hard,
which increases the likelihood of the particles rubbing overly strongly against
the photoreceptor, generating undesirable scratches.
The average particle size of the core particles of the
present invention is typically within a range from 10 to 100 µm, and is preferably
from 20 to 50 µm. If the average particle size is smaller than 10 µm,
then the developer is prone to flying off the developing unit, whereas if the average
particle size exceeds 100 µm, achieving a satisfactory image density becomes
impossible.
The electrical resistance of the carrier with the formed
resin coating layer, when the measurement electric field is 5, 000 V/cm, is typically
within a range from 1 × 105 to 1 × 1014 &OHgr;-cm,
and is preferably from 1 × 109 to 1 × 1012 &OHgr;-cm.
The charge of the carrier with the formed resin coating
layer is preferably within a range from 15 to 50 µC/g. If this carrier charge
is less than 15 µC/g, then toner staining of non-image areas can occur (known
as fogging), increasing the possibility that a high quality color image will be
unobtainable, whereas if the carrier charge exceeds 50 µC/g, achieving a satisfactory
image density may become problematic.
If the electrical resistance of the carrier with the formed
resin coating layer is less than 1 × 105 &OHgr;-cm, then the charge
is able to migrate more readily from the carrier surface, meaning image defects
such as brush marks become more likely, and if the printer is left standing idle,
with no print operation conducted for a certain period, then the charge may undergo
an excessive decrease, causing scumming or the like on the first page that is printed
on recommencement of printing. If the electrical resistance of the carrier with
the formed resin coating layer exceeds 1 × 1014 &OHgr;-cm, then
not only is a favorable solid image unattainable, but if printing is conducted continuously
for multiple copies, then the toner charge becomes excessively high, causing a reduction
in the image density.
When measured in the form of a magnetic brush, the dynamic
electrical resistance of the carrier under an electric field of 104 V/cm
is typically within a range from 1 × 103 to 1 × 1013&OHgr;-cm,
and is preferably from 1 × 105 to 1 × 1012 &OHgr;-cm.
If the dynamic electrical resistance is less than 1 × 103 &OHgr;-cm,
then the likelihood of image defects such as brush marks increases, whereas if the
electrical resistance exceeds 1 × 1013 &OHgr;-cm, then achieving
a favorable solid image may become problematic. An electric field of 103
V/cm is similar to the developing electric field within an actual apparatus, and
this is the reason that the above dynamic electrical resistance is measured under
a field of this strength.
From the above description it can be ascertained that the
dynamic electrical resistance on mixing the carrier and the toner is preferably
within a range from 1 × 105 to 1 × 1013 &OHgr;-cm
under an electric field of 103 V/cm. If this dynamic electrical resistance
is less than 1 × 105 &OHgr;-cm, then various problems can arise,
including scumming caused by a reduction in the toner charge when left standing
following printing, or broadening of line images and a resulting deterioration in
resolution caused by over-development. If the dynamic electrical resistance exceeds
1 × 1013 &OHgr;-cm, then a deterioration in the developing characteristics
of the edges of solid images may make achieving a high quality image impossible.
The dynamic electrical resistance of the carrier is determined
in the manner described below. Namely, approximately 30 cm3 of the carrier
is deposited on the developing roller (the magnetic field on the surface of the
developing roller sleeve generates 1 kOe) to form a magnetic brush, and a planar
electrode with a surface area of 3 cm2 is positioned facing the developing
roller with a spacing of 2.5 mm therebetween. A voltage is then applied between
the developing roller and the planar electrode while the developing roller is rotated
at a rotational speed of 120 rpm, and the resulting current is measured. The thus
obtained current-voltage relationship is then used to determine the dynamic electrical
resistance using Ohm's law. It is well known that a relationship represented by
the expression ln (I/V) ∝V× 1/2 applies between the applied voltage
V and the current I, In cases where the dynamic electrical resistance is very small,
as is the case in the carrier used in the present invention, a high electric field
of 103 V/cm or greater may produce a very large current, making measurement
impossible. In such cases, three or more measurements are conducted under lower
electric fields, and a least squares method is then used to extrapolate the value
to an electric field of 104 V/cm using the relationship mentioned above.
Examples of the coating resin formed on top of the core
particles include polyolefin-based resins such as polyethylene and polypropylene;
polyvinyl-based and polyvinylidene-based resins such as polystyrene, acrylic resins,
polyacrylonitrile, polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl
chloride, polyvinyl carbazole, polyvinyl ether, and polyvinyl ketone; copolymers
of vinyl chloride and vinyl acetate; copolymers of styrene and acrylic acid; straight
silicon resins formed from organosiloxane linkages, or modified products thereof;
fluororesins such as polytetrafluoroethylene, polyvinyl fluoride, polyvinylidene
fluoride, and polychlorotrifluoroethylene; polyester; polyurethane; polycarbonate;
amino resins such as urea-formaldehyde resin; and epoxy resins. These resins may
be used either alone, or as a mixture of multiple resins.
The thickness of the resin coating layer is typically within
a range from 0.1 to 5 µm, and preferably from 0.3 to 3 µm. If the thickness
of the resin coating layer is less than 0.1 µm, then forming a uniform and
smooth coating layer on the surface of the core particles becomes difficult. In
contrast, if the thickness exceeds 5 µm, then aggregation of carrier particles
tends to occur, making it difficult to obtain a uniform carrier.
Suitable methods of forming the resin coating layer on
the core particles include immersion methods in which the core particles are immersed
in a resin coating layer-forming solution, spray methods in which a resin coating
layer-forming solution is sprayed onto the core particles, fluidized bed methods
in which a resin coating layer-forming solution is atomized while the core particles
are maintained in a floating state using an air flow, and kneader coater methods
in which the core particles and a resin coating layer-forming solution are mixed
together in a kneader coater and the solvent is subsequently removed.
There are no particular restrictions on the solvent used
within the resin coating layer-forming solution, provided it is capable of dissolving
the aforementioned coating resin, and suitable solvents include aromatic hydrocarbons
such as toluene and xylene, ketones such as acetone and methyl ethyl ketone, and
ethers such as tetrahydrofuran and dioxane. Furthermore, suitable methods of dispersing
the conductive powder include methods using a sand mill, dyno mill or homomixer.
[Electrostatic Latent Image Developer]
An electrostatic latent image developer used in the present
invention is a two-component developer that contains a toner and a carrier. The
toner described below may be either a magnetic toner or a non-magnetic toner. Below,
the term "electrostatic latent image developer" may be abbreviated as simply "developer".
In the present invention, the toner can be prepared using
a so-called aggregation fusion method that includes: a first step of heating a dispersion
containing at least dispersed resin particles at a temperature no higher than the
glass transition temperature of the resin particles, thereby forming aggregate particles
and producing an aggregate particle dispersion, a second step of adding and mixing
a fine particle dispersion containing dispersed fine particles with the aggregate
particle dispersion, thereby causing the fine particles to adhere to the aggregate
particles and generate adhered particles, and a third step of heating and fusing
the adhered particles.
The characteristics of such a toner include a comparatively
round particle shape, a narrow particle size distribution, a comparatively uniform
toner surface with high chargeability, and a favorably narrow charge distribution.
Accordingly, an electrostatic latent image developer obtained
by mixing the toner with the aforementioned carrier exhibits extremely good fluidity
and developing properties, meaning a developer is obtained that is ideal as a high
quality color developer.
Examples of other toners that can be used include polymer
toners, solution-suspension toners, emulsification-aggregation toners, and kneading/grinding/classification/spheronization
type toners.
The following description focuses on the case in which
an emulsification-aggregation toner is used in the developer.
In an exemplary embodiment of the present invention, aggregation
and fusion are conducted using fine resin particles and fine particles of a yellow,
magenta, cyan or black pigment respectively, thus yielding a series of colored toners.
Furthermore, the volume average particle size for each toner is within a range from
approximately 3 to 9 µm, and the average value of the shape factor SF1 is at
least 100 but no higher than 135. The shape factor SF1 can be calculated from the
formula shown below.
In this formula, ML represents the average value of the absolute maximum length
of the particles, and A represents the projected area of particles, and these values
are converted to numerical form mainly by analyzing a microscope image or a scanning
electron microscope image using an image analyzer.
As disclosed in
Japanese Patent Laid-Open Publication No. Hei10-026842
, Japanese Patent Laid-Open Publication No. Hei
10-133423
, Japanese Patent Laid-Open Publication
No.Hei 10-198070
and
Japanese Patent Laid-Open Publication No. Hei 11-231570
, these toners can be prepared by a method of producing toner for an electrostatic
latent image developer that includes: a first step of heating a dispersion containing
at least dispersed resin particles at a temperature no higher than the glass transition
temperature of the resin particles, thereby forming aggregate particles and producing
an aggregate particle dispersion, a second step of adding and mixing a fine particle
dispersion containing dispersed fine particles with the aggregate particle dispersion,
thereby causing the fine particles to adhere to the aggregate particles and generate
adhered particles, and a third step of heating and fusing the adhered particles.
The volume average particle size, particle shape and particle
size distribution can be adjusted by adjusting factors such as the conditions during
preparation of the aggregate particle dispersion, the conditions during formation
of the adhered particles, and the conditions during heating and fusion of the adhered
particles.
The dispersion described above is prepared by dispersing
at least resin particles. These resin particles are particles formed from a resin.
Examples of this resin include the various thermoplastic binder resins, and specific
examples include homopolymers or copolymers of styrenes such as styrene, para-chlorostyrene
and &agr;-methylstyrene (namely, styrene-based resins) ; homopolymers or copolymers
of esters having a vinyl group such as methyl acrylate, ethyl acrylate, n-propyl
acrylate, n-butyl acrylate, lauryl acrylate, 2-ethylhexyl acrylate, methyl methacrylate,
ethyl methacrylate, n-propyl methacrylate, lauryl methacrylate and 2-ethylhexyl
methacrylate (namely, vinyl-based resins); homopolymers or copolymers of vinyl nitriles
such as acrylonitrile and methacrylonitrile (vinyl-based resins); homopolymers or
copolymers of vinyl ethers such as vinyl methyl ether and vinyl isobutyl ether (vinyl-based
resins) ; homopolymers or copolymers of vinyl ketones such as vinyl methyl ketone,
vinyl ethyl ketone and vinyl isopropenyl ketone, (vinyl-based resins) ; homopolymers
or copolymers of olefins such as ethylene, propylene, butadiene and isoprene (namely,
olefin-based resins); non-vinyl condensation resins such as epoxy resins, polyester
resins, polyurethane resins, polyamide resins, cellulose resins and polyether resins,
and graft polymers of these non-vinyl condensation resins and vinyl-based monomers.
These resins may be used either alone, or in combinations of two or more different
resins.
Of these resins, styrene-based reins, vinyl-based resins,
polyester resins and olefin-based resins are preferred, and copolymers of styrene
and n-butyl acrylate, poly (n-butyl acrylate), copolymers of bisphenol A and fumaric
acid, and copolymers of styrene and an olefin are particularly desirable.
The average particle size of the resin particles is typically
no greater than 1 µm, and is preferably within a range from 0.01 to 1 µm.
If this average particle size exceeds 1 µm, then the particle size distribution
of the final product electrostatic latent image toner broadens, which leads to the
generation of free particles, and tends to result in a deterioration in the performance
and reliability of the toner. In contrast, if the average particle size falls within
the above range, then not only can the above drawbacks be avoided, but other advantages
are also realized, including a reduction in uneven distribution within the toner,
more favorable dispersion within the toner, and less variation in the performance
and reliability of the toner. The average particle size can be measured, for example,
using a laser diffraction method (LA-700, manufactured by Horiba, Ltd.).
Examples of suitable colorants include pigments such as
carbon black, chrome yellow, hansa yellow, benzidine yellow, threne yellow, quinoline
yellow, permanent orange GTR, pyrazolone orange, vulkan orange, watchung red, permanent
red, brilliant carmine 3B, brilliant carmine 6B, Dupont oil red, pyrazolone red,
lithol red, rhodamine Blake, lake red C, rosebengal, aniline blue, ultramarine blue,
calco oil blue, methylene blue chloride, phthalocyanine blue, phthalocyanine green
and malachite green oxalate; and dyes such as acridine-based dyes, xanthene-based
dyes, azo-based dyes, benzoquinone-based dyes, azine-based dyes, anthraquinone-based
dyes, dioxazine-based dyes, thiazine-based dyes, azamethine-based dyes, indigo-based
dyes, thioindigo-based dyes, phthalocyanine-based dyes, aniline black-based dyes,
polymethine-based dyes, triphenylmethane-based dyes, diphenylmethane-based dyes
and thiazole-based dyes. These colorants may be used either alone, or in combinations
of two or more different colorants.
The average particle size of the colorant is typically
no greater than 1 µm, and is preferably within a range from 0.01 to 1 µm.
If this average particle size exceeds 1 µm, then the particle size distribution
of the final product electrostatic latent image toner broadens, which leads to the
generation of free particles, and tends to result in a deterioration in the performance
and reliability of the toner. In contrast, if the average particle size falls within
the above range, then not only can the above drawbacks be avoided, but other advantages
are also realized, including a reduction in uneven distribution within the toner,
more favorable dispersion within the toner, and less variation in the performance
and reliability of the toner. The average particle size can be measured, for example,
using a laser diffraction method (LA-700, manufactured by Horiba, Ltd.).
Depending on the purpose of the present invention, other
components may also be dispersed within the aforementioned dispersion, including
release agents, internal additives, charge control agents, inorganic particles,
lubricants and abrasives. In such cases, these other particles may simply be dispersed
in the dispersion containing the dispersed resin particles, or a separate dispersion
formed by dispersing the other particles may be mixed with the dispersion containing
the dispersed resin particles.
Examples of suitable release agents include low molecular
weight polyolefins such as polyethylene, polypropylene and polybutene; silicones
that exhibit a softening point under heating; fatty acid amides such as oleyl amide,
erucyl amide, ricinoleyl amide and stearyl amide; vegetable waxes such as carnauba
wax, rice wax, candelilla wax, Japan wax and jojoba oil; animal waxes such as beeswax;
mineral or petroleum waxes such as montan wax, ozokerite, ceresin, paraffin wax,
microcrystalline wax and Fischer-Tropsch wax; as well as modified products of the
above.
These waxes can easily be converted to fine particles of
no more than 1 µm by dispersing the wax in water together with an ionic surfactant
and a polymer electrolyte such as a polymeric acid or polymeric base, heating the
dispersion to a temperature at least as high as the melting point of the wax, and
then processing the dispersion using a homogenizer or pressure discharge disperser
capable of imparting a powerful shearing force.
Examples of the aforementioned charge control agents include
quaternary ammonium salts, nigrosine-based compounds, dyes formed from complexes
of aluminum, iron or chromium, and triphenylmethane-based pigments. In the present
invention, from the viewpoints of enabling more favorable control of the ionic strength,
which effects the level of safety during aggregation and fusion, and reducing wastewater
contamination, the charge control agent is preferably a material that is substantially
insoluble in water.
Examples of the aforementioned inorganic particles include
those particles that are typically used as external additives for the toner surface,
such as silica, alumina, titania, calcium carbonate, magnesium carbonate, calcium
phosphate and cerium oxide. Examples of the aforementioned lubricants include fatty
acid amides such as ethylene bis stearamide and oleyl amide, and fatty acid metal
salts such as zinc stearate and calcium stearate. Examples of the aforementioned
abrasives include the previously mentioned silica, alumina and cerium oxide.
In a method of producing the toner, the resin fine particle
dispersion and colorant dispersion and the like described above are mixed together
to prepare a uniform mixed particle dispersion, and an inorganic metal salt that
is soluble in the dispersion medium is then added and mixed, thereby forming the
desired aggregate particles. During this process, the resin fine particles, the
colorant, and any inorganic fine particles that are added as necessary may either
be added in a single batch, or may be divided into portions so that the fine particles
are added in stages, thereby enabling the aggregate particles to be imparted with
a core shell structure, or a structure in which the component concentration varies
across the radial direction of the particles. In such cases, the resin fine particle
dispersion, the colorant particle dispersion, and the release agent fine particle
dispersion and the like are mixed together and dispersed, and the aggregate particles
are grown until a certain particle size is achieved. If required, an additional
resin fine particle dispersion or the like may then be added in order to adhere
these additional resin fine particles to the surface of the aggregate particles.
By coating the surface of the aggregate particles, the additional resin fine particles
can prevent the exposure of the colorant or the release agent at the toner surface,
thereby effectively suppressing charge irregularities or non-uniform charging caused
by such exposure.
In the above aggregation step of forming the aggregate
particles, a bivalent or higher inorganic metal salt is used as a coagulant, and
a trivalent or higher salt, and particularly a tetravalent salt, is preferred. The
cohesive force of the inorganic metal salt increases with increasing valency, enabling
the aggregation process to be controlled with favorable stability, and as a result,
an excellent particle size distribution with minimal non-aggregated material can
be obtained. Examples of suitable tetravalent or higher inorganic metal salt polymers
that can be used include polyaluminum chloride and polyaluminum hydroxide.
Following preparation of aggregate particles of the desired
particle size in this manner, the target toner particles can be obtained by fusing
the aggregate particles by heating at a temperature at least as high as the glass
transition temperature of the resin. By appropriate selection of the fusion heating
conditions, the toner shape can be controlled to yield amorphous through to spherical
particles. By conducting fusion at a high temperature over an extended period, the
shape of the toner particles moves closer to a true spherical shape.
The average particle size of the toner is typically no
higher than 10 µm, and is preferably within a range from 3 to 9 µm.
When a developer is prepared by mixing together a toner
and a carrier, the proportion of the toner is typically within a range from 1 to
15% by weight, and preferably from 3 to 12% by weight of the entire developer.
If the proportion of toner is less than 1% by weight, then
achieving a satisfactory image density may become difficult, and achieving uniform
solid printing may also be difficult. In contrast, if the proportion of toner exceeds
15% by weight, then because the toner coating ratio on the carrier surface exceeds
100%, the charge quantity falls (with the absolute value of the average charge quantity
falling to less than 15 µC/g), and toner staining (fogging) occurs within non-image
areas, making it more difficult to achieve a high quality color image. For example,
if the toner proportion exceeds 15% by weight, then because the toner coating ratio
on the carrier surface approaches 100%, the resistance of the developer increases
dramatically and becomes difficult to maintain within the range from 1 × 105
to 1 × 108
&OHgr;.cm, which increases the likelihood of blurring at the image edges, and
makes obtaining a favorable high quality color image more difficult.
In a low humidity environment, if the toner proportion
is less than 1% by weight, then the developer is prone to developing a very high
charge (with the absolute value of the average charge quantity exceeding 25 µC/g),
which may make it impossible to achieve a satisfactory image density. Accordingly,
depending on the environment, the proportion of toner is preferably selected so
that the absolute value of the charge quantity falls within a range from 15 to 50
µC/g.
[Image Forming Method]
As follows is a description of an image forming method
according to an exemplary embodiment of the present invention.
An image forming method of the present invention includes:
forming an electrostatic latent image on the surface of a latent image holding member;
developing the electrostatic latent image formed on the surface of the latent image
holding member using a developer supported on a developer carrier, thereby forming
a toner image; transferring the toner image formed on the surface of the latent
image holding member to the surface of a transfer target; and heat fixing the toner
image that has been transferred to the surface of the transfer target, wherein the
developer contains at least an electrophotographic carrier according to the present
invention.
Each of the above steps can use conventional processes
from known image forming methods.
An electrophotographic photoreceptor or a dielectric recording
material may be used as the latent image holding member. In the case of an electrophotographic
photoreceptor, the surface of the electrophotographic photoreceptor is charged uniformly
using a corotron charger or a contact charger or the like, and is then exposed to
form an electrostatic latent image (the latent image-forming step). Subsequently,
toner particles are adhered to the electrostatic latent image by bringing the image
either into contact with, or into close proximity to, a developing roller with a
developer layer formed on the surface thereof, thereby forming a toner image on
the electrophotographic photoreceptor (the developing step). The thus formed toner
image is then transferred to the surface of a transfer target material such as a
sheet of paper using a corotron charger or the like (the transfer step). The toner
image that has been transferred to the surface of the transfer target is subsequently
subjected to heat fixing using a fixing device, thereby forming the final toner
image.
During heat fixing by the above fixing device, a release
agent is usually supplied to the fixing member of the above fixing device in order
to prevent offset problems and the like.
In order to achieve favorable releasability at the surface
of the roller or belt that functions as the fixing member within the fixing device,
the use of a material that exhibits a low surface energy is desirable. Furthermore,
there are no particular restrictions on the method used for supplying the release
agent, and suitable methods include a pad system that uses a pad impregnated with
the liquid release agent, a web system, a roller system, and a non-contact shower
system (a spray system), although of these, a web system or roller system is preferred.
These systems offer the advantages that the release agent can be supplied uniformly,
and the quantity of release agent supplied can be readily controlled. If a shower
system is used, then a separate blade or the like should be used to ensure that
the release agent is supplied uniformly across the entire fixing member.
Fig. 1 is a schematic illustration showing a sample configuration
of an image forming apparatus that forms an image using an image forming method
according to the present invention. The image forming apparatus 200 shown in the
drawing includes four electrophotographic photoreceptors 401a to 401d positioned
in a mutually parallel arrangement along an intermediate transfer belt 409 inside
a housing 400. These electrophotographic photoreceptors 401a to 401d are configured
so that, for example, the electrophotographic photoreceptor 401a is capable of forming
a yellow image, the electrophotographic photoreceptor 401b is capable of forming
a magenta image, the electrophotographic photoreceptor 401c is capable of forming
a cyan image, and the electrophotographic photoreceptor 401d is capable of forming
a black image.
The electrophotographic photoreceptors 401a to 401d are
each capable of rotating in a predetermined direction (in a counterclockwise direction
within the plane of the drawing), and around this rotational direction there are
provided charging rollers 402a to 402d, developing units 404a to 404d, primary transfer
rollers 410a to 410d, and cleaning blades 415a to 415d. The four colored toners,
namely the black, yellow, magenta and cyan toners housed within the toner cartridges
405a to 405d can be supplied to the developing units 404a to 404d respectively.
Furthermore, the primary transfer rollers 410a to 410d contact the electrophotographic
photoreceptors 401a to 401d respectively across the intermediate transfer belt 409.
An exposure unit 403 is also positioned at a predetermined
location inside the housing 400, and the light beam emitted from the exposure unit
403 is able to be irradiated onto the surfaces of the charged electrophotographic
photoreceptors 401a to 401d. Accordingly, rotating the electrophotographic photoreceptors
401a to 401d enables the processes of charging, exposure, developing, primary transfer
and cleaning to be conducted in sequence, thereby transferring and superimposing
the toner image for each color onto the intermediate transfer belt 409.
In this description, the charging rollers 402a to 402d
are used for bringing a conductive member (the charging roller) into contact with
the surface of the respective electrophotographic photoreceptor 401a to 401d, thereby
applying a uniform voltage to the photoreceptor and charging the photoreceptor surface
to a predetermined potential (the charging step). Besides the charging rollers shown
in this exemplary embodiment, charging may also be conducted using contact charging
systems that employ charging brushes, charging films or charging tubes. Furthermore,
charging may also be conducted using non-contact systems that employ a corotron
or a scorotron.
The exposure unit 403 may employ an optical device that
enables a light source such as a semiconductor laser, an LED (light emitting diode)
or a liquid crystal shutter to be irradiated onto the surface of the electrophotographic
photoreceptors 401a to 401d with a desired image pattern. Of these possibilities,
if an exposure unit that is capable of irradiating incoherent light is used, then
the generation of interference patterns between the conductive base material and
the photosensitive layer of the electrophotographic photoreceptors 401a to 401d
can be prevented.
For the developing units 404a to 404d, typical developing
units that use the aforementioned two-component electrostatic latent image developer
to conduct developing via either a contact or non-contact process may be used (the
developing step). There are no particular restrictions on these types of developing
units, provided they use a two-component electrostatic latent image developer, and
appropriate conventional units may be selected in accordance with the desired purpose.
In the primary transfer step, a primary transfer bias of
the reverse polarity to the toner supported on the image holding member is applied
to the primary transfer rollers 410a to 410d, thereby effecting sequential primary
transfer of each of the colored toners to the intermediate transfer belt 409.
The cleaning blades 415 to 415d are used for removing residual
toner adhered to the surfaces of the electrophotographic photoreceptors following
the transfer step, and the resulting surface-cleaned electrophotographic photoreceptors
are then reused within the above image forming process. Suitable materials for the
cleaning blades include urethane rubbers, neoprene rubbers and silicone rubbers,
The intermediate transfer belt 409 is supported at a predetermined
level of tension by a drive roller 406, a backup roller 408 and a tension roller
407, and can be rotated without slack by rotation of these rollers. Furthermore,
a secondary transfer roller 413 is positioned so as to contact the backup roller
408 across the intermediate transfer belt 409.
By applying a secondary transfer bias of the reverse polarity
to the toner on the intermediate transfer belt to the secondary transfer roller
413, the toner undergoes secondary transfer from the intermediate transfer belt
to the recording medium. After passing between the backup roller 408 and the secondary
transfer roller 413, the intermediate transfer belt 409 is surface-cleaned by either
a cleaning blade 416 positioned near the driver roller 406 or a charge neutralizing
device (not shown in the drawing), and is then reused in the next image forming
process. Furthermore, a tray (a transfer target medium tray) 411 is provided at
a predetermined position within the housing 400, and a transfer target medium 500
such as paper stored within this tray 411 is fed by feed rollers 412 between the
intermediate transfer belt 409 and the secondary transfer roller 413, and then between
two mutually contacting fixing rollers 414, before being discharged from the housing
400.
EXAMPLES
As follows is a description of specifics of the present
invention based on a series of examples and comparative examples.
[Production of Core Particles A]
MnO, MgO and Fe2O3 are mixed together
thoroughly in quantities of 29 parts by weight, 1 part by weight and 70 parts by
weight respectively, and this raw material mixture is mixed and ground for 10 hours
in a wet ball mill, and then finely ground and dispersed using a rotary kiln. The
mixture is then subjected to preliminary baking at 900°C for 1 hour in the
rotary kiln. The resulting prebaked product is then ground for a further 10 hours
in a wet ball mill, yielding an oxide slurry with an average particle size of 0.8
µm. To the thus obtained slurry is added suitable quantities of a dispersant
and polyvinyl alcohol (0.3% by weight relative to 100% by weight of the oxide slurry),
and following granulation and drying using a spray dryer, full baking is conducted
in a rotary electric kiln, by holding the product under conditions including a temperature
of 1100°C and an oxygen concentration of 0.3% for a period of 7 hours. The
resulting ferrite particles are subjected to magnetic concentration, and are then
mixed to yield core particles A. The core particles A have an Sm value of 1.06 µm
and an Ra value of 0.39 µm.
[Production of Core Particles B]
Li2O, MgO, CaO and Fe2O3
are mixed together thoroughly in quantities of 15 parts by weight, 7 parts by weight,
3 parts by weight and 75 parts by weight respectively, and this raw material mixture
is mixed and ground for 10 hours in a wet ball mill, and then finely ground and
dispersed using a rotary kiln- The mixture is then subjected to preliminary baking
at 900°C for 1 hour in the rotary kiln. The resulting prebaked product is then
ground for a further 10 hours in a wet ball mill, yielding an oxide slurry with
an average particle size of 0.8 µm. To the thus obtained slurry is added suitable
quantities of a dispersant and polyvinyl alcohol (0.3% by weight relative to 100%
by weight of the oxide slurry), and following granulation and drying using a spray
dryer, full baking is conducted in a rotary electric kiln, by holding the product
under conditions including a temperature of 1100°C and an oxygen concentration
of 0.3% for a period of 7 hours. The resulting ferrite particles are subjected to
magnetic concentration, and are then mixed to yield core particles B. The core particles
B have an Sm value of 1.52 µm and an Ra value of 0.62 µm.
[Production of Core Particles C]
MnO, MgO and Fe2O3 are mixed together
thoroughly in quantities of 29 parts by weight, 1 part by weight and 70 parts by
weight respectively, and this raw material mixture is mixed and ground for 10 hours
in a wet ball mill, and then finely ground and dispersed using a rotary kiln. The
mixture is then subjected to preliminary baking at 900°C for 1 hour in the
rotary kiln. The resulting prebaked product is then ground for a further 8 hours
in a wet ball mill, yielding an oxide slurry with an average particle size of 1.8
µm. To the thus obtained slurry is added suitable quantities of a dispersant
and polyvinyl alcohol (0.3% by weight relative to 100% by weight of the oxide slurry),
and following granulation and drying using a spray dryer, full baking is conducted
in a rotary electric kiln, by holding the product under conditions including a temperature
of 1100°C and an oxygen concentration of 0.3% for a period of 7 hours. The
resulting ferrite particles are subjected to magnetic concentration, and are then
mixed to yield core particles C. The core particles C have an Sm value of 1.91 µm
and an Ra value of 0.85 µm,
[Production of Core Particles D]
MnO, MgO and Fe2O3 are mixed together
thoroughly in quantities of 29 parts by weight, 1 part by weight and 70 parts by
weight respectively, and this raw material mixture is mixed and ground for 10 hours
in a wet ball mill, and then finely ground and dispersed using a rotary kiln. The
mixture is then subjected to preliminary baking at 900°C for 1 hour in the
rotary kiln. The resulting prebaked product is then ground for a further 10 hours
in a wet ball mill, yielding an oxide slurry with an average particle size of 0.8
µm. To the thus obtained slurry is added suitable quantities of a dispersant
and polyvinyl alcohol (0.3% by weight relative to 100% by weight of the oxide slurry),
and following granulation and drying using a spray dryer, full baking is conducted
in a rotary electric kiln, by holding the product under conditions including a temperature
of 1300°C and an oxygen concentration of 0.3% for a period of 7 hours. The
resulting ferrite particles are subjected to magnetic concentration, and are then
mixed to yield core particles D. The core particles D have an Sm value of 0.84 µm
and an Ra value of 4.39 µm.
[Production of Carrier A]
A resin coating layer-forming raw material solution A containing
the components listed below is stirred and dispersed for 60 minutes with a stirrer,
thus forming a resin coating layer-forming raw material solution A. Subsequently,
this resin coating layer-forming raw material solution A and 100 parts by weight
of the core particles A are placed inside a vacuum deaerat ion kneader, and following
stirring for 30 minutes at 70°C, the pressure is reduced and the mixture is
deaerated and dried. The resulting product is then passed through a 75 µm mesh,
yielding a carrier A. The thus obtained carrier A has an Ra value of 0.22 and a
sphericity of 0.993, and the core exposure ratio at the surface of the carrier A
is 2%.
<Resin coating layer-forming raw material solution A>
Toluene:
18 parts by weight
Styrene-methacrylate copolymer (component ratio 30:70)
4.5 parts by weight
Carbon black (Regal 330, manufactured by Cabot Corporation)
0.7 parts by weight
[Production of Carrier B]
A resin coating layer-forming raw material solution B containing
the components listed below is stirred and dispersed for 60 minutes with a stirrer,
thus forming a resin coating layer-forming raw material solution B, this resin coating
layer-forming raw material solution B and 100 parts by weight of the core particles
B are then stirred together for 30 minutes, and the pressure is subsequently reduced
and the mixture is deaerated and dried. The resulting product is then passed through
a 75 µm mesh, yielding a carrier B. The thus obtained carrier B has an Ra value
of 0.45 and a sphericity of 0.982, and the core exposure ratio at the surface of
the carrier B is 2%.
<Resin coating layer-forming raw material solution B>
Methanol:
20 parts by weight
&ggr;-aminotriethoxysilane (KBE903, manufactured by Shin-Etsu Chemical
Co., Ltd.)
2.2 parts by weight
Carbon black (Regal 330, manufactured by Cabot Corporation)
0.34 parts by weight
[Production of Carrier C]
A resin coating layer-forming raw material solution C containing
the components listed below is stirred and dispersed for 60 minutes with a stirrer,
thus forming a resin coating layer-forming raw material solution C. Subsequently,
this resin coating layer-forming raw material solution C and 100 parts by weight
of the core particles A are placed inside a vacuum deaeration kneader, and following
stirring for 30 minutes at 70°C, the pressure is reduced and the mixture is
deaerated and dried. The resulting product is then passed through a 75 µm mesh,
yielding a carrier C. The thus obtained carrier C has an Ra value of 0.31 and a
sphericity of 0.972, and the core exposure ratio at the surface of the carrier C
is 4.3%.
<Resin coating layer-forming raw material solution C>
Toluene:
8.6 parts by weight
Styrene-methacrylate copolymer (component ratio 30:70)
1.30 parts by weight
Carbon black (Regal 330, manufactured by Cabot Corporation) 0
.20 parts by weight
[Production of Carrier D]
A resin coating layer-forming raw material solution A containing
the components listed above is stirred and dispersed for 60 minutes with a stirrer,
thus forming a resin coating layer-forming raw material solution A. Subsequently,
this resin coating layer-forming raw material solution A and 100 parts by weight
of the core particles C are placed inside a vacuum deaeration kneader, and following
stirring for 30 minutes at 70°C, the pressure is reduced and the mixture is
deaerated and dried. The resulting product is then passed through a 75 µm mesh,
yielding a carrier D. The thus obtained carrier D has an Ra value of 0.65 and a
sphericity of 0.991, and the core exposure ratio at the surface of the carrier A
is 3.6%.
[Production of Carrier E]
A resin coating layer-forming raw material solution B containing
the components listed above is stirred and dispersed for 60 minutes with a stirrer,
thus forming a resin coating layer-forming raw material solution B. Subsequently,
this resin coating layer-forming raw material solution B and 100 parts by weight
of the core particles D are placed inside a vacuum deaeration kneader, and following
stirring for 30 minutes at 70°C, the pressure is reduced and the mixture is
deaerated and dried. The resulting product is then passed through a 75 µm mesh,
yielding a carrier E. The thus obtained carrier E has an Ra value of 0.72 and a
sphericity of 0.973, and the core exposure ratio at the surface of the carrier E
is 5%.
[Production of Toner A]
A detailed description of one example of preparing a toner
of the present invention is presented below, although the present invention is in
no way restricted by the following example.
<Preparation of Resin Fine Particle Dispersion>
Styrene
296 parts by weight
n-butyl acrylate
104 parts by weight
Acrylic acid
6 parts by weight
Dodecanethiol
10 parts by weight
Divinyl adipate
1.6 parts by weight
(All these components are manufactured by Wako Pure Chemical Industries, Ltd.)
A mixture prepared by mixing and dissolving the above components
is added to a solution containing 12 parts by weight of a non-ionic surfactant (Nonipol
400, manufactured by Sanyo Chemical Industries, Ltd.) and 8 parts by weight of an
anionic surfactant (Neogen SC, manufactured by Dai-ichi Kogyo Seiyaku Co. , Ltd.)
dissolved in 610 parts by weight of ion-exchanged water, and following dispersion
and emulsification within the flask, 50 parts by weight of ion-exchanged water containing
8 parts by weight of ammonium persulfate (manufactured by Wako Pure Chemical Industries,
Ltd.) dissolved therein is added gradually while the mixture in the flask is stirred
slowly for 10 minutes, and the flask is then flushed with nitrogen for 20 minutes
at a rate of 0.1 liters/minute. Subsequently, the flask is placed in an oil bath
and the internal temperature of the system is heated to 70°C with constant
stirring, and the emulsion polymerization is then allowed to progress at this temperature
for 5 hours, yielding a resin fine particle dispersion with an average particle
size of 200 nm and a solid fraction concentration of 40%. A sample prepared by placing
a portion of this dispersion in an oven at 100°C to remove the moisture is
measured using a DSC (differential scanning calorimeter), and reveals a glass transition
temperature of 53°C and a weight average molecular weight of 32,000.
<Preparation of Colorant Dispersion (K)>
Carbon black (Regal 330, manufactured by Cabot Corporation)
100 parts by weight
Anionic surfactant (Neogen RK, manufactured by Dai-ichi Kogyo Seiyaku Co.,
Ltd.)
10 parts by weight
Ion-exchanged water
490 parts by weight
The above components are mixed together and dissolved,
and then dispersed for 10 minutes using a homogenizer (Ultraturrax, manufactured
by IKA Works Inc.), thereby yielding a colorant dispersion (K).
<Preparation of Release Agent Particle Dispersion>
Paraffin wax (HNP-9, manufactured by Nippon Seiro Co., Ltd.)
100 parts by weight
Anionic surfactant (Lipal 860K, manufactured by Lion Corporation)
10 parts by weight
Ion-exchanged water
390 parts by weight
The above components are mixed together and dissolved,
dispersed using a homogenizer (Ultraturrax, manufactured by IKA Works Inc.), and
then subjected to further dispersion treatment using a pressure discharge homogenizer,
thereby yielding a release agent particle dispersion containing dispersed particles
of a release agent (paraffin wax) with a center diameter of 220 nm.
(Production of Black Toner)
Resin fine particle dispersion
320 parts by weight
Colorant dispersion (K)
80 parts by weight
Release agent particle dispersion
96 parts by weight
Aluminum sulfate (manufactured by Wako Pure Chemical Industries, Ltd.)
1.5 parts by weight
Ion-exchanged water
1270 parts by weight
The above components are combined in a round-bottom stainless
steel flask fitted with a temperature-regulating jacket, subsequently dispersed
for 5 minutes at 5,000 rpm using a homogenizer (Ultraturrax T50, manufactured by
IKA Works Inc.), and then transferred to another flask and stirred for 20 minutes
at 25°C using a 4-blade paddle. Subsequently, with the flask contents undergoing
constant stirring, the flask is heated with a mantle heater at a rate of temperature
increase of 1°C/minute until the contents reach a temperature of 48°C,
and this temperature of 48°C is maintained for 20 minutes. An additional 80
parts by weight of the resin particle dispersion is then added gently, and after
holding the resulting mixture at 48°C for a further 30 minutes, a 1N aqueous
solution of sodium hydroxide is added to adjust the pH to 6.5.
Subsequently, the temperature is raised to 95°C at
a rate of 1°C/minute and then held at that temperature for 30 minutes. The
pH of the system is then adjusted to 4.8 by adding a 0.1N aqueous solution of nitric
acid, and the resulting mixture is then allowed to stand at 95°C for a period
of two hours. The aforementioned 1N aqueous solution of sodium hydroxide is then
once again added to adjust the pH to 6.5, and the system is then allowed to stand
for a further 5 hours at 95°C, The temperature is then cooled to 30°C
at a rate of 5°C/minute.
The resulting toner particle dispersion is filtered, and
then (A) 2, 000 parts byweightof 35°C ion-exchanged water is added to the resulting
toner particles, (B) the mixture is stirred for 20 minutes, and then (C) the mixture
is filtered. The operations from (A) to (C) are repeated 5 times, and the toner
particles on the filter are then transferred to a vacuum dryer, and dried for 10
hours at 45°C under a pressure of no more than 1,000 Pa. The reason that a
pressure of no more than 1,000 Pa is specified is that the toner particles contain
moisture, which may be frozen in the initial stages of drying, even at 45°C,
and because this moisture then undergoes sublimation during the drying process,
the internal pressure within the reduced pressure dryer does not remain constant.
However, at the completion of the drying process, this pressure stabilizes at 100
Pa. After returning the inside of the dryer to normal pressure, the resulting toner
matrix particles are removed, 1.5 parts of a silica external additive (RY-50, manufactured
by Nippon Aerosil Co., Ltd.) is added to 100 parts of the toner matrix particles,
and the resulting mixture is blended for 3 minutes at 3, 000 rpm in a Henschel mixer,
thereby yielding a black toner.
The thus obtained black toner has a D50v value of 5.7 µm,
a GSDp value of 1.23, an acid value of 28 mgKOH/g, and a glass transition temperature
of 53°C.
[Production of Toner B]
Using 87 parts of a binder resin (a bisphenol A polyester),
8 parts of carbon black (BPL, manufactured by Cabot Corporation), 1 part of a charge
control agent (TRH, manufactured by Hodogaya Chemical Co., Ltd.), and 4 parts of
polypropylene wax (660P, manufactured by Sanyo Chemical Industries, Ltd.) , toner
particles with an average particle size of 7.5 µm are prepared using a kneading-grinding
method. To 100 parts of these toner particles is then added 1 part of a colloidal
silica (R972, manufactured by Nippon Aerosil Co. , Ltd.), and the resulting mixture
is blended in a Henschel mixer, yielding a toner B.
[Developer]
100 parts by weight samples of the aforementioned carriers
A to E are blended with 8.5 parts by weight of one of the aforementioned toners
A and B, thereby producing developers of the examples 1 to 3 and the comparative
examples 1 to 3, as shown in Table 1.
<Evaluation Methods>
[Surface Roughness of Core Material and Carrier]
Using a laser microscope (VK-9500, an ultra-deep color
3D profile measuring microscope, manufactured by Keyence Corporation), the Sm and
Ra values are measured for a particle surface area of 12 × 12 µm, and
in each case, the average of 50 measured values is reported as the numerical value.
Fig. 2 is an example of a photograph from the above laser microscope showing the
surfaces of core particles and carrier particles, and the values of Sm and Ra are
determined from a curve showing the relationship between measurement locations on
the photograph and the corresponding surface roughness.
[Carrier Sphericity]
The various characteristic values are measured using the
LPF measurement mode of a FPIA-3000 device (manufactured by Sysmex Corporation).
A sample is prepared by adding and mixing 200 mg of the carrier particles with 30
ml of an aqueous solution of ethylene glycol, removing the supernatant aqueous solution,
and using the residue as the measurement sample. The average sphericity is determined
by analyzing particles other than those with a particle size of either less than
10 µm or greater than 50 µm.
[Core Exposure at Carrier Surface]
Using an X-ray photoelectron spectrophotometer (JPS-9000MX)
manufactured by Jasco Corporation, measurement is conducted using a MgK&agr; X-ray
source, an output of 10 kV, and an analysis area of 10 × 10 mm. The peak intensities
for each measured element are used to determine the respective atom concentration
levels at the particle surface. Calculation of the surface atom concentration levels
is conducted using the relative photosensitive factors provided by Jasco Corporation.
The peak intensity of each of the measured elements is proportional to the quantity
of atoms of that element that exist within the analysis area. In the present invention,
an approximation of the amount of core exposure at the carrier surface is calculated
by determining the ratio between the intensity of the peak derived from iron atoms
at the carrier surface, and the intensity of the peak derived from iron atoms at
the surface of the core particles.
Furthermore, in order to measure the amount of core exposure
at the carrier surface within a developer, the developer is placed in a container
such as a beaker, a suitable quantity of a surfactant solution (such as a 0.2% by
weight aqueous solution of polyoxyethylene octylphenyl ether) is added, the carrier
is held within the bottom of the container by holding a magnet beneath the container,
and the toner alone is washed away. This operation is continued until the supernatant
liquid becomes colorless and transparent. A suitable quantity of ethanol is then
added to remove any surfactant adhered to the carrier surface. Subsequently, the
carrier from which the toner has been removed is dried in a dryer, and the above
method can then be used to measure the amount of core exposure at the carrier surface.
[Image Evaluation]
Using the modified DocuCentre Color 400 apparatus (manufactured
by Fuji Xerox Co., Ltd.) shown in Fig. 1, print tests are conducted under high temperature
conditions (35°C, 80% RH), by printing 50, 000 copies with an image area of
10% and 3, 000 copies with an image area of 5%. The image is then evaluated in terms
of image density Shade, fogging, and toner density. The magnetic permeability setting
Vs is set so as to yield a toner density of 9%. Control of the toner density is
conducted so that when the difference between the sensor detected value V and the
set value Vs, namely &Dgr; = Vs - V is positive, the toner density is adjudged
to be satisfactory, and toner supplementation is stopped, whereas when the difference
&Dgr; is negative, the toner density is adjudged to be insufficient, and toner
supplementation is started, with the control process designed to limit the value
of &Dgr;.
Furthermore, the image forming method used includes: forming
an electrostatic latent image on the surface of an electrostatic latent image holding
member, developing the electrostatic latent image using a developer, thereby forming
a toner image, transferring the toner image to the surface of a transfer target,
cleaning any residual toner from the latent image holding member with an elastic
cleaning blade, and heat fixing the toner image, and the process speed is set to
350 mm/second.
<Evaluation of Image Density Shade>
A predetermined number of copies are printed with each
developer under predetermined conditions, the developer is then left to stand overnight,
an image having a 2 cm × 5 cm patch is then copied, and 5 locations within
the patch are then measured using an image densitometer (X-Rite 404A, manufactured
by X-Rite, Inc.). A developer for which the difference between the maximum measured
value and the minimum measured value is less than 0.5 is evaluated using the symbol
A, a difference of at least 0.5 but less than 0.8 is evaluated using the symbol
B, and a difference of 0.8 or greater is evaluated using the symbol C.
<Fogging>
Each developer is used to print 10,000 copies under predetermined
conditions, and the number of copies printed at the point where fogging starts to
occur is evaluated visually. A developer for which no fogging occurred even after
10,000 copies is evaluated using the symbol A, a developer for which fogging occurred
after between 9, 000 and 10,000 copies is evaluated using the symbol B, a developer
for which fogging occurred after between 6,000 and 9,000 copies is evaluated using
the symbol C, and all other cases are evaluated using the symbol D.
<Evaluation of Toner Density>
A sample of the developer to be measured, with a weight
of approximately 0.30 ± 0.05 g, is measured by the blow-off method using a
charge measuring device (TB200, manufactured by Toshiba Corporation). Using the
evaluation conditions described above, the toner density is measured every 100,000
copies. A measured density value within ±1.0%. of the set value is evaluated
using the symbol A, a measured value within ±1.5% of the set value is evaluated
using the symbol B, and all other cases are evaluated using the symbol C.
From the print tests above it is evident that carriers
and developers of the present invention are resistant to carrier adhesion under
all manner of environments, and are able to provide a combination of high image
quality, in which image quality deterioration caused by localized degradation of
the latent image holding member is prevented, and favorable reliability.
An electrostatic latent image carrier and an electrostatic
latent image developer of the present invention, and an image forming method that
uses these materials can be ideally employed within a method of visualizing image
information via an electrostatic latent image, such as an electrophotographic method.
The foregoing description of the exemplary embodiments
of the present invention has been provided for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the invention to the
precise forms disclosed. Obviously, many modifications and variations will be apparent
to practitioners skilled in the art. The exemplary embodiments were chosen and described
in order to best explain the principles of the invention and its practical applications,
thereby enabling others skilled in the art to understand the invention for various
embodiments and with the various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be defined by the following
claims and their equivalents.