BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates to electrostatic recording and more particularly, to a method
for making magnetic toners for electrostatic development in electrophotography, electrostatograpy,
electrostatic printing and the like.
Description of the Prior Art
[0002] Magnetic toners are usually composed of binder resins, magnetic materials, resistance-adjusting
agents and optionally colorants and fluidizing agents. These magnetic toners have
the advantage that no control in concentration of magnetic toner is required because
no carrier is contained and use of the toner allows a simpler construction or mechanism
of developing device. In order to suitably control the resistance of magnetic toner,
it is necessary to disperse conductive particles such as of carbon black in individual
magnetic toner particles or to form a conductive layer on the surface of individual
particles. The control is easier in the latter case.
[0003] Typical methods of forming a conductive layer include:
a) Mixing conductive particles and magnetic toner matrix particles together in a stream
of hot air such as in fluidized drying furnace thereby depositing the conductive particles
on the surface of each matrix particle to form a conductive layer thereon; and
b) Mixing conductive particles and magnetic matrix particles in a rotary drum to form
a conductive layer on each matrix particle.
[0004] However, these methods have several drawbacks that mere mixing of magnetic toner
matrix particles and conductive particles will not permit sufficient deposition of
the conductive particles on the matrix particles, it being thus difficult to form
a stable, uniform conductive layer on each matrix particle, that the amount of treatment
per unit hour is relatively small, and that it is rather difficult to obtain a magnetic
toner of constant resistance. The application of the magnetic toners produced by these
methods results in low image density and frequent occurrence of fogging, leading to
the lowering of image quality.
[0005] The above defect is emphasized especially in magnetic roll developments using insulating
magnetic rolls such as, for example, an anodized aluminium sleeve, a plastic resin
sleeve and the like.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a method for making a magnetic
toner which overcomes the drawbacks of the prior art techniques and in which the toner
of stable quality can readily be prepared in large quantities.
[0007] It is another object of the invention to provide a method for making a magnetic toner
exhibiting low resistivity and good flowability whereby the toner ensures much reduced
occurrence of fogging phenomenon and very excellent image quality and density even
when applied in an insulating magnetic roll developing system.
[0008] It is a further object of the invention to provide a method for making a magnetic
toner in which conductive particles uniformly deposited on and dispersed in the surface
of individual magnetic matrix particles which have been softened but not molten thereby
forming a uniform tanacious coating on each particle of the matrix.
[0009] It is a still further object of the invention to provide a magnetic toner which can
give excellent results when used to develop in any known magnetic roll developing
systems utilizing rotation of sleeve, rotation of magnet and simultaneous rotation
of both magnet and sleeve.
[0010] The above objects can be achieved according to the present invention by a method
which comprises agitating magnetic matrix particles, each comprising at least a binder
material and a magnetic material, in a high speed mixer until the particles are frictionally
heated to a temperature between the melting point and the softening point of the binder
material, adding a predetermined amount of conductive particles to the heated matrix
particles, further agitating the mixture to permit the conductive particles to deposit
on the surface of the individual matrix particles as a tenacious coating, and classifying
the resulting particles to have a predetermined range of size. In the high speed mixer,
the matrix particles are agitated and heated by means of an agitator fitted with a
rotor. The peripheral speed of the rotor should preferably be in the range of from
200 m/min to 2000 m/min in order to realize the intended level of the temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011]
Fig. 1 is a perspective schematic view of a cell for measuring the resistivity of
magnetic toner; and
Fig. 2 is a perspective schemativ view of an instrument for measuring the flowability
of magnetic toner.
PREFERRED EMBODIMENT OF THE INVENTION
[0012] According to the method of the invention, magnetic toner particles are first agitated
and mixed together in any known types of high speed mixer having an agitator until
they are heated to a temperature between the melting point and the softening point
of a binder material contained in the matrix particles. The actual temperature level
to which the matrix particles must be heated depends on the binder material used but
is generally in the range of approximately 30 - 80°C, preferably 40 - 60°C and most
preferably 45 - 50°C, at which an ordinary binder resin or material used for this
purpose, e.g. an epoxy resin, a styrene resin, polyethylene wax or the like, can be
softened. The reason why the particles are heated during the mixing is due to the
fact that when agitated at high speed, high shearing force of the agitator is exerted
on the particles, so that heat generates by fricational force among the particles
and between the particles and the rotor of agitator and wall surfaces of the mixer.
The high speed mixer is, for example, Super Mixer made by Kawada Mfg Co., Ltd., Henchel
Mixer made by Mitsui-Miike Mfg. Co., Ltd., or the like. As a matter of course, any
mixers which can yield such a high shearing force as mentioned above may be used in
the practice of the invention. The agitation is carried out under conditions of vigorous
agitation with an agitator fitted with a rotor whose peripheral speed is in the range
of from 200 m/min to 2000 m/min whereby the generation of heat becomes sufficient
to attain a desired level of temperature. Smaller peripheral speeds may not cause
the matrix particles to be softened, which makes is difficult to firmly deposit on
the matrix particles conductive particles of smaller sizes than the matrix particles.
On the other hand, larger peripheral speeds show the tendency that among conductive
particles which have once deposited on the matrix particles, the particles of smaller
sizes than the matrix particles are liable to fall off and thus a uniform coarting
cannot be obtained. It will be noted that prior to the agitation, the matrix particles
may be classified by a suitable means to improve a yield of final product. In this
case, the size of the particles is generally in the range of from 5 to 60 microns,
preferably 10 to 44 microns.
[0013] To the thus heated matrix particles are immediately added conductive particles serving
as a resistance adjuster, followed by high speed mixing or agitation under the same
agitating conditions as in the first agitation to uniformly disperse the both particles.
As a result, the conductive particles adhere to and deposit on the individual softened
matrix particles to form a tenacious coating of the conductive particles on the surface
of each matrix psarticle. The conductive particles are usually added in an amount
ranging from 1 to 5 wt% of the matrix particles charge. The amount is varied, within
the above-defined range, depending on an intended resistivity level of the final magnetic
toner product.
[0014] The conductive particles are made of any of conductive materials, and carbon black
is used for general purpose because of its availability and inexpensiveness.
[0015] Aside from the resistance adjuster, any known additives such as charge-controlling
particles may be added together with the resistance adjuster after heating of the
matrix particles.
[0016] The magnetic toner thus obtained in accordance with the method of the present invention
are classified to have a predetermined size of from 5 to 60 microns, preferably 10
to 44 microns. This magnetic toner can give good results when fixed on recording paper
by any known fixing systems including 1) fixing by heating, 2) fixing by application
of pressure, and 3) fixing by application of heat and pressure in combination.
[0017] The present invention is described in more detail by way of examples and comparative
examples.
[Example 11
[0018] A starting material for toner composed of 50 parts by weight of an epoxy resin (Epikote
No. 1004, by Shell Chem. Co.), and 50 parts by weight of iron oxide (Magnetite EPT-500,
by Toda Ind. Co., Ltd.) was kneaded in a biaxial kneader and reduced into particles
with a size below 2 mm by means of the Rotoplex powdering machine (Itoman Engineering
Model 8/16), followed by finely powdering in a pin mill (Alpine : 160 z). The resulting
powder was classified by means of a wind power classifier (Alpine 100 MZR) to have
a size ranging from 10 to 44 microns, and then agitated in a high speed agitated mixer
(Super Mixer SMG-20, by Kawada Mfg. Co.) until it was self-heated up to 45°C. Immediately,
carbon black to be the resistance adjuster (Carbon Black #44, by Mitsubishi Chem.
Co., Ltd.) was added in an amount of 3 wt% of the magnetic matrix particles, followed
by agitating at 1900 r.p.m. for 30 seconds thereby coating or depositing the conductive
particles on the surface of the individual matrix particles in a uniform and tenacious
manner (which treatment is hereinafter referred to as surface coating). The resulting
particles were again subjected to the wind power classifier to have a size of from
10 to 44 microns.
[0019] This magnetic toner was placed in a cell shown in Fig. 1 to measure its resistivity.
The resistivity was found to be 2.00 x 10
3 ohms-cm. In Fig. 1, indicated at 1 are copper electrodes each having a length of
1 cm, a width of 1 cm and a thickness of 0.03 cm, the electrodes being spaced from
each other at a distance of 1 cm, at 2 is a glass cell having an inner wall dimension
of 1 cm in length, 1.06 cm in width and 3 cm in height, and at 3 are covered wires
each connected to the electrode at one end and also to one of terminals of the Wheatstone
bridge at the other end. The magnetic toner is charged into the cell to a certain
level for the measurement.
[0020] The magnetic toner was then subjected to the measurement of flowability using an
instrument shown in Fig. 2, which includes a brass plate 4 having a thickness of 0.15
cm and formed with through-holes 7 of different sizes indicated in the figure, a ring
5 having an inner diameter of 0.8 cm and a height of 1 cm, and a frame 6 supporting
the plate 4. In measuring operation, the ring 5 is placed just on an arbitrary through-hole
and a magnetic toner to be measured is charged into the ring 5. The flowability is
represented by a diameter of the smallest through-hole 7 through which the charged
toner starts to drop. The magnetic toner obtained in this example showed a flowability
of 0.6 mm.
[0021] Further, the magnetic toner particles were subjected to the measurement of angle
of repose by a powder tester (Model PT-E, by Hosokawa Micron Co., Ltd.). The angle
of repose which is a measure for flowability was found to be 3
10.
[0022] The magnetic toner was used to develop by the magnetic roll developing techniques
in which magnets were rotated with respect to aluminium and insulating sleeves and
then thermally fixed thereby obtaining high quality visible images of high density
which were completely free of any fogging. Similar excellent results were also obtained
by other magnetic roll developing systems including the sleeve rotation system and
the sleeve and magnet simultaneous rotation system.
[0023] To confirm the reproducibility, the procedure of Example 1 was exactly repeated five
times. The values of resistivity, flowability and angle of repose are shown in Table
1, revealing that good reproducibility is obtained. The results of the development
and fixation were also excellent similar for Example 1.
[Comparative Example 11
[0024] 600 cc of the magnetic matrix particles obtained in Examle 1 were charged into a
1 liter wide mouth bottle, to which was added the resistance adjuster in an amount
of 2 wt% based on the matrix particles, followed by the surface coating treatment
on a shaker for 30 minutes. The resulting magnetic toner was subjected to the wind
power classifier to have a size of 10 - 44 microns. The magnetic toner had a resistivity
of 8 x 10
4 ohms-cm, a flowability of 0.9 mm, and an angle of repose of 34 degrees when measured
in the same manner as in Example 1.
[0025] Similarly, 2.5 liters of the magnetic matrix particles obtained in Example 1 was
charged into a 5 liters ball mill pot, to which was added the carbon black resistance
adjuster in an amount of 2 wt% based on the matrix particles. The matrix particles
were surface coated by shaking for 3 hours and then classified by means of the wind
power classifier to have a size of from 10 to 44 microns. The thus classified magnetic
toner had a resistivity of 2 x 10
5 ohms-cm, a flowability of 1.2 mm and an angle of repose of 35 degrees on measurement
in the same manner as in Example 1.
[0026] The both magnetic toners were applied as usual and developed by magnetic roll developing
techniques and thermally fixed, with the result that there could be obtained in both
cases high quality visible images of high density which were free of any fogging when
developed using an conductive aluminium sleeve. However, the development using an
insulating magnetic roll resulted in generation of fogging phenomenon with the image
being low in density and having a reduced commercial value.
[0027] Then, the reproducibility test was conducted repeating, five times, the respective
procedures of Comparative Example 1 using the shaker and the ball pot mill. The resistivities,
flowabilities and angles of repose of the resulting magnetic toners were so fluctuated
as shown in Tables 2 and 3.
[Example 2]
[0028] Example 1 was repeated using a starting material for toner composed of 40 parts by
weight of a styrene resin (
Picolastic
D-125, Esso), 10 parts by weight of low molecular weight polypropylene (Biscall 550P,
by Sanyo Chem. Co., Ltd) and 50 parts by weight of iron oxide (Magnetite EPT 500,
by Toda Ind. Co., Ltd.), thereby obtaining a magnetic toner.
[0029] The characteristics of this magnetic toner were measured in the same manner as in
Example 1. As a result, it had a resistivity of 1.5 x 10
3 ohms-cm, a flowability of 0.5 mm, and an angle of repose of 30 degrees.
[0030] The magnetic toner was used for development by magnetic roll developing techniques
and fixed by a heat roll. In both developing systems using conductive and insulating
rolls, there were obtained high quality visible images of high density free of any
fogging involved.
[Comparative Example 2]
[0031] The procedure of Comparative Example 1 was repeated using the magnetic toner matrix
particles obtained in Example 2. The resulting magnetic toner was subjected to the
measurement of its characteristics in the same manner as in Example 1 and found to
have a resistivity of 9 x 10
4 ohms-cm, a flowability of 1.0 mm and an angle of repose of 35 degrees.
[0032] Further, 2.5 liters of the magnetic toner matrix particles obtained in Example 2
were charged into a 5 liters ball mill pot, to which was added the carbon black resistance
adjuster in an amount of 2 wt% based on the matrix particles, followed by surface
coating of the matrix particles with the carbon black for 3 hours. The resulting magnetic
toner was classified by a wind power classifier to have a size of from 10 to 44 microns.
When measured in the same manner as in Example 1, the magnetic toner had a resistivity
of 1.5 x 10
5 ohms-cm, a flowability of 1.2 mm, and an angle of repose of 35 degrees. The magnetic
toner was used for development by magnetic roll developing techniques and fixed with
a heat roll. Although a high quality visible image of high density which was free
of any fogging was obtained by the developing method using the conductive aluminium
sleeve, the image obtained using the insulating magnetic roll suffered fogging with
its density being low, and had thus little commercial value.
[Example 3]
[0033] Example 1 was repeated using a starting material for toner composed of 30 parts by
weight of polyethylene wax (Hi-wax 200P, Mitsui Petroleum Chem. Co., Ltd.), 10 parts
by weight of EVA (Evaflex #260, by Mitsui Polychemical Co., Ltd.) and 60 parts by
weight of iron oxide (Magnetite EPT-500, by Toda Ind. Co., Ltd.).
[0034] The resulting magnetic toner was subjected to the measurement of characteristics,
revealing that it had a resistivity of 1.8 x 10
3 ohms-cm, a flowability of 0.6 mm and an angle of repose of 31 degrees.
[0035] The magnetic toner was used for development by mangetic roll techniques and fixed
by a press fixing roll thereby obtaining high quality visible images of high density
free of any fogging in both the conductive and insulating roll developing systems.
[Comparative Example 3]
[0036] The magnetic tonner matrix particles obtained in Example 3 were used and treated
in the same manner as in Comparative Example 1 using a shaker and a ball mill to obtain
two types of magnetic toner. The magnetic toner treated by the shaker had a resistivity
of 7 x 10
4 ohms-cm, a flowability of 1.1 mm and an angle of repose of 35 degrees and the magnetic
toner obtained in the ball mill had a resistivity of 5 x 10
5 ohms-cm, a flowability of 1.2 mm and an angle of repose of 35 degrees.
[0037] The both magnetic toners were used for development by magnetic roll developing techniques
and fixed by a press fixing roll. Although high quality visible images of high density
which were completely free of any fogging were obtained by the developing method using
the conductive aluminium sleeve, the images obtaind by the insulating magnetic roll
suffered fogging with their density being low, and had thus little commercial value.
[0038] It should be noted that appropriate binder materials, magnetic materials and resistance
adjusting material other than those set forth in these examples can be used as long
as they are ordinarily used for this purpose. These will not be set forth since they
are well known.