Magnetic developing process and toner containing high coercive force magnetic powder

Disclosed is an electrostatic image developing process using a magnet roll and a magnetic toner of specified flux density.

This invention relates to a developing process of electrostatic latent 
images which uses single component magnetic toner, particularly to a 
magnet brush developing process. 
BACKGROUND OF THE INVENTION 
In a magnet-brush development process, developer powder, which includes 
magnetic material, stored in a developer vessel is conveyed to a 
development zone and attracted to a magnet roll. Image-bearing material, 
positioned adjacent the magnet roll, may be composed of a highly resistive 
polyester sheet, photoconductive selenium, an electrically insulating film 
overlying a layer of photoconductive cadmium sulfide disposed in an 
insulating binder, a thin film of polyvinylcarbazole or 
poly-N-vinylcarbazole, a layer of the mixture of photoconductive zinc 
oxide and an insulating resin binder, or the like, as known in the art. 
The developer powder is supplied from the developer vessel through a gap of 
predetermined size onto the magnet roll and, according to the rotation of 
the magnet roll, the developer powder rotates or tumbles along the roll to 
the development zone. At least at the development zone, the developer 
powder forms a magnet-brush on the magnet roll and the magnet-brush rubs 
the surface of the image-bearing material to adhere the toner material of 
the developer powder to electric pattern images on the surface. For 
purposes of this application, electric pattern images include 
electrostatic images, capacitive images, and electrically conductive 
images. For convenience of explanation, the latent electrostatic images 
will be used as representative in this specification. 
In some previous developing processes the development, there has been used 
an admixture of ferromagnetic carrier particles and toner particles. The 
ferromagnetic carrier particles are resin-coated-iron beads and the toner 
particles are a mixture of pigment and binder. The carrier particles and 
the toner particles are triboelectrically charged to the opposite polarity 
by blending them. The materials of the carrier particles and the toner 
particles are selected to cause a charge on the toner opposite to the 
charge of the electrostatic latent image on the image-bearing material. 
The admixture is stored in the developer vessel in which the toner 
particles adhere to the surfaces of the carrier particles by the 
triboelectric charge and is then conveyed on the surface of the magnet 
roll as the roll rotates. The admixture forms a magnet-brush at the 
development zone and, when the brush rubs the latent image, the toner 
particles adhere to the latent image by the electrostatic attraction force 
between the charge of the latent image and the charge of the toner, but 
the carrier particles remain on the magnetic roll by the magnetic 
attraction force between the carrier and the roll. After the development 
the admixture, less the adhered toner, returns to the developer vessel and 
is supplied new toner. 
On the other hand, a single component magnetic toner has been improved to 
be used in the magnet-brush development and has the advantage that it is 
not necessary to use the carrier particles or to mix them. Although such a 
magnetic toner is referred to as "single component" or "one component," 
the name does not mean that the toner consists of only one component, but 
the toner comprises mainly one kind of particles composed of fine magnetic 
particles, organic binder, pigment, carbon black and flow agents. No 
so-called "carrier" is required. 
A toner containing magnetic material is shown in Giaimo, Jr. U.S. Pat. No. 
2,890,968, Copper U.S. Pat. No. 3,345,294 and Strong U.S. Pat. No. 
3,925,219. Giaimo, Jr. teaches that two kinds of magnetic powder are mixed 
so that one kind of the magnetic powder is charged triboelectrically to a 
polarity while the other has an opposite charge and that the mixture in 
conveyed to a photoreceptor with latent images by rotation of a magnet 
roll to form a magnet brush on the surface of magnet roll and, by 
attraction force between the charge of the magnetic powder and that of the 
latent images the latent images are developed visible. One of those 
powders consists of polystyrene, carbon black, Nigrosine and magnetite 
while the other consists of Vinsol, Carmine dye and magnetite. 
Cooper discloses a developer mixture of ferromagnetic carrier particles and 
tones particles containing carbon black, magnetite and resin. The content 
of magnetite is 28.75% by weight. 
Since both Giaimo, Jr. and Cooper are used with carrier particles, those 
are triboelectrically chargeable. 
Single component magnetic toner is, for example, disclosed in Strong. The 
magnetic toner of Strong is composed of wax and ethylene/vinyl acetate 
copolymer as a resin and magnetite of 60 weight %. Instead of magnetite, 
Strong suggests barium ferrite, nickle zinc ferrite, chromium oxide, 
nickle oxide, etc may be useable. When the toner is conveyed to a position 
close to latent images, an electric charge of opposite polarity to the 
electric charge of the latent images is induced in the toner by subjecting 
the toner to the electric field of the latent images, so the toner is 
attracted to the latent images to adhere the latent images. 
The structure of the magnet roll is well-known and is shown, for example, 
in Anderson, U.S. Pat. No. 3,455,276. Anderson refers to a magnet roll as 
a magnetically responsive powder applicator, which comprises a shaft of 
high magnetic permeability material, a plurality of elongate, generally 
sector-shaped in cross section, magnetic members formed of fine grain, 
permanent magnet material dispersed in a non-magnetic matrix, which 
members are positioned to define a circular array around the shaft, the 
alternate, outer faces of adjacent members being oppositely polarized. 
In development with admixture of ferromagnetic carrier particles and toner 
particles, it is usual that a magnet roll having a magnetic force of 
600-1,300 gauss on a shell surface is used. The carrier particles which 
have toner particles triboelectrically adhered on them are magnetically 
held and conveyed by a magnet roll and form magnet brush along magnetic 
flux lines. A relatively weak magnetic flux density of the magnet roll 
causes white spots on a copy paper because carrier particles are 
transfered together with toner particles to a photoreceptor. By this 
reason, a magnet roll having a relatively strong magnetic force with such 
"two coomponent" toners. 
On the other hand, when a single component magnetic toner is used in 
development, a magnet roll having a relatively weak magnetic force on the 
shell surface is used. In development processes using single component 
toners, only when the electrostatic attraction force between latent images 
and toner becomes larger than magnetic attraction force between the 
magnetic toner and the magnet roll, are the toner particles removed from 
the shell surface of the magnet roll and transferred to the latent images. 
For this reason, if the magnetic force of the magnet roll is too strong, 
development might not occur. But, when the magnetic force is too weak, 
toner is attracted by a very small electric potential on a photoreceptor, 
and the background of the copy paper becomes dark from the transfer of 
unwanted toner. 
A large magnetic brush formed on a magnet roll causes blackness of 
developed images, i.e. a diffuse reflection density, to increase. The 
large magnetic brush is formed by a large magnetic force of the magnet 
roll. Also, the magnitude of the magnetic brush depends on magnetic 
properties of magnetic toner. 
The adherence of toner to the background of the copy paper discussed above 
concerns magnetic characteristics of the magnetic toner. 
As a result, the quality of developed images depends on the magnetic 
characteristics of the magnetic toner and the magnetic force of the magnet 
roll. 
The toner utilized in these reproducing steps in a "plain paper copier" 
(PPC) system ordinarily includes magnetic powder and a resin. The magnetic 
properties, particle size and electric resistance of the magnetic toner, 
as a whole, and the content ratio between the magnetic powder and the 
resin form important factors for determining the quality of the images 
reproduced. Particularly, in the above developing step, the magnetic 
properties of the magnetic toner greatly affect the developing 
performance. Increases in the magnetic force of the magnetic toner tend to 
improve the developing property. While an increase in the magnetic powder 
content of the magnetic toner generally increases the magnetic force of 
the toner and improves the developing property of the toner, an increase 
in the magnetic powder content in the magnetic toner, however, results in 
a roughness of the fixed images caused by the magnetic powder thereon in 
the course of the fixing step. Images of a satisfactory quality are, 
therefore, not obtained. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to provide a developing process 
of electrostatic latent images for developing the images to highly black 
copy which has low background. 
The present invention is accomplished by a developing process of 
electrostatic latent images comprising: 
providing a magnet roll which includes a cylindrical rotatable shell and a 
rotatable permanent magnet member positioned coaxially within the shell, 
the permanent magnet member having a plurality of adjacent axially 
extending magnetic poles causing a magnetic field pattern of at least 400 
gauss as its peak value on the shell surface; 
supplying single component magnetic toner which consists essentially of at 
least a resin, which is solid at ambient temperature and rendered molten 
under heating, coloring material and at most 40%, by weight, of 
ferromagnetic powder, the magnetic toner having a saturated magnetic flux 
density 4.pi.Is of a value between 300 and 1,000 gauss and a magnetic 
property, in the relation between the saturated magnetic flux density 4#Is 
and the coercive force iHc of the magnetic toner, defined by the region 
above a line connecting a point where iHc is 1,000 oersted for 4#Is at 300 
gauss and a point where iHc is 200 oersted for 4.pi.Is at 300 gauss and 
below a line connecting a point where iHc is 400 oersted for 4.pi.Is at 
300 gauss and a point where iHc is 400 oersted for 4.pi.Is at 1,000 gauss; 
conveying the single component magnetic toner on the shell surface to an 
image-bearing material having the electrostatic latent images by rotating 
at least one of the shell and the permanent magnet member to form a magnet 
brush of the single component magnet toner; 
inducing an electric charge on the magnetic toner in the magnet brush on 
the shell surface adjacent the image-bearing material by subjecting to an 
electrical field due to an electric charge of the electrostatic latent 
images; and 
rubbing the image-bearing material by the magnet brush to adhere the 
charged magnetic toner onto the electrostatic latent images on the 
image-bearing material by the attraction force between the charge induced 
in the toner and that of the electrostatic latent images. 
The magnet roll used in the present invention preferably shows a magnetic 
flux density of between 400 and 1,500 gauss as its peak value. The more 
preferable range of the flux density is 600-1,200 gauss. 
The magnetic toner used in the present invention preferably consists 
essentially of, by weight, magnetic powder of 20-55%, a plastic binder of 
80-45% and carbon black of 0.2-6%. The magnetic powder may be barium 
ferrite powder, strontium ferrite powder or cobalt powder. The plastic 
binder may be epoxy resin, ethylenvinyl acetate copolymer or wax. Instead 
of the carbon black, nickel powder may be used. The more preferable 
content of the magnetic powder is 25 to 40%.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring to FIG. 2, single component magnetic, toner 2 is stored in toner 
vessel 1 which has an opening 11 at the position opposite to a magnet roll 
3. The toner 2 is supplyed onto the surface of a shell 31 of the magnet 
roll 3 through the opening 11. 
The magnet roll 3 has a permanent magnet 32 held on a shaft 33 inside the 
non-magnetic cylindrical shell 31. The permanent magnet 32 is secured on 
the shaft 33 and the shell 31 rotates relatively to the magnet 32. Both 
the shell and the magnet rotate. When the shell 31 rotates clockwise, or 
when the magnet 32 rotates counter clockwise, the toner is transported 
clockwise. 
An image-bearing drum 4 is juxtaposed with the magnet roll 3 and the 
image-bearing material 41 is disposed on a peripheral surface of a 
conductive backing 42. Electrostatic latent images are formed by a 
conventional process on the image-bearing material 41. 
FIG. 1 shows the magnetic properties of the single component magnetic toner 
to be used in the present invention. The toner has saturated magnetic flux 
density 4.pi.Is of a value between 300 and 1,000 gauss and a magnetic 
property, in the relation between the saturated magnetic flux density 
4.pi.Is and the coercive force iHc of the magnetic toner, defined by the 
region above a line connecting a point where iHc is 1,000 oersted for 
4.pi.Is at 300 gauss and a point where iHc is 200 oersted for 4.pi.Is at 
300 gauss and below a line connecting a point where iHc is 400 oersted for 
4.pi.Is at 300 gauss and a point where iHc is 400 oersted for 4.pi.Is at 
1,000 gauss, shown as a hatched area ABCD in FIG. 1. 
The magnetic toner which has been supplied on the shell surface of the 
magnet roll 3 from the toner vessel 1 is conveyed under a doctor blade 12 
in the direction of the image-bearing material 41 by rotation of the shell 
31 or the permanent magnet 32. A magnetic brush of the toner is formed 
along magnetic flux lines of the permanent magnet 32 on the shell surface. 
When the magnet brush reaches the latent images on the image bearing 
material, an electric charge is induced in the toner subjected to the 
electric field due to the electric charge of the latent images. The 
induced charge has a polarity opposite to that of the latent images. The 
charge of the toner is attracted electrically to the latent images so that 
the toner adheres to the latent images which become visible. 
The toner images may be fixed directly on the image-bearing material such 
as in a "coated paper copier" (CPC) process. Alternatively, the toner 
images may be transfered to another material, i.e. a plain paper, and 
fixed thereon by pressure or heat in a PPC process. 
The permanent magnet 32 shown in FIG. 2 has been magnetized to have eight 
adjacent axially elongated magnetic poles symmetrically on the peripheral 
surface. The magnetic flux density distribution on the shell surface has 
four north poles and four south poles and a magnetic flux density of about 
550 gauss at the peaks, as shown in FIG. 3. THe magnetic force of the 
magnet roll and the magnetic properties of the magnetic toner affect the 
force of attracting and holding the toner to the shell surface. The 
increase of magnetic flux density on the shell increases the attraction 
force and reduces toner amount transferred and adhered to the latent 
images. When a saturated magnetic flux density 4.pi.Is of toner, or the 
content of ferromagnetic powder in the toner, increases, toner amount 
adhered to latent images reduces. So, in order to obtain clear background, 
it is useful to use a strong magnet and a toner having a large magnetic 
flux density. 
However, it is not practicable to use a magnet having too strong magnetic 
force. In general, an isotropic barium ferrite magnet exhibits magnetic 
flux density of 400-800 gauss and an anisotropic barium ferrite magnet has 
magnetic flux density of 900-1,300 gauss. A rare earth-cobalt magnet is 
relatively expensive, but shows a high magnetic flux density of about 
2,000 gauss. 
Magnetic flux density of between 400 and 1500 gauss is suitable to a 
development of electrostatic latent images with inductively chargeable, 
single component magnetic toner. It is preferred to combine a magnet roll 
having surface magnetic flux density of 400 gauss with magnetic toner with 
magnetic powder of 55%. Surface magnetic flux density of 1,500 gauss goes 
nicely with toner with 20% magnetic powder. 
When magnetic toner with 25-40% magnetic powder is used, an isotropic 
barium ferrite gives toner images of high quality. It is most preferable 
that a magnet roll with magnetic flux density of 600-1,200 gauss is 
combined with magnetic toner having magnetic powder of 25-40%. 
As toner to be used in the process of the present invention, the followings 
were prepared. 
Toner A 
Polyester resin (PS No. 1; prepared by Hitachi Chemicals) of 50 weight 
parts and magnetite (MTA740; prepared by Toda Industry) of 50 weight parts 
were pre-mixed by a super mixer. The mixed powder was heated too 
150.degree.-200.degree. C. and blended by a needer at the temperature, and 
then cooled and became solid. The solid material was pulverized by a jet 
mill and speroidized at a temperature of 100.degree.-200.degree. C. Carbon 
black of 1 weight % was added to the particles and mixed by a mixer to be 
fixed on the particle surface. The particles were classified to select 
particle size of 5-30 .mu.m. 
Toner B 
Epoxy resin (Epicot 2057GP; Shell Chemicals) of 75 weight parts and barium 
ferrite magnet powder (YBM-IB; Hitachi Metals) of 25 weight parts were 
used and treated as in the process described in Toner A. 
Toner C 
Stylene (Himer ST95; Sanyo Chemicals) of 60 weight parts and barium ferrite 
magnet powder (YBM-3; Hitachi Metals) of 40 weight parts were treated as in 
the process described in Toner A. 
Toner D 
Epoxy resin (Epicot 1004; Shell Chemicals) of 70 weight parts, barium 
ferrite magnet powder (YBM-2B; Hitachi Metals) of 15 weight parts and 
magnetite (EPT500; Toda Industry) of 15 weight parts were treated as in 
the process described in Toner A. 
Toner E 
Epoxy resin (Epicot 1004; Shell Chemicals) of 30 weight parts, and 
magnetite (EPT 500; Toda Industry) of 70 weight parts were treated as in 
the process described in Toner A. 
Toner F 
Epoxy resin (Epicot 2057GP; Shell Chemicals) of 80 weight parts and barium 
ferrite magnet powder of 20 weight parts were treated as in the process 
described in Toner A. 
Evaluation of toner images where were developed by using Toners A to F is 
shown in the following table. 
TABLE 
__________________________________________________________________________ 
Experiment Condition 
Magnetic 
Toner Properties force 
Particle Conductivity Photoconductor 
of mag. 
Image Quality 
size at 4000V/cm 
4.pi.Is 
iHc Voltage 
roll Develop- 
Toner 
Toner 
(.mu.m) 
(ohm/cm) 
(gauss) 
(Oe) 
Material 
(V) (gauss) 
ment Scattering 
Fixability 
__________________________________________________________________________ 
A 5-30 5 .times. 10.sup.-13 
1,000 
370 
Se 800 850 
good no good 
B " 8 .times. 10.sup.-14 
310 2,050 
Se 600 1,300 
good little 
good 
C " 1 .times. 10.sup.-15 
310 4,000 
Se 650 1,200 
good no good 
D " 3 .times. 10.sup.-15 
480 1,760 
ZnO -300 1,200 
good little 
good 
E " 2 .times. 10.sup.-14 
1,700 
130 
Se 1,000 
500 
good no bad 
F " 3 .times. 10.sup.-15 
124 4,000 
ZnO -300 1,200 
bad yes good 
__________________________________________________________________________ 
It is apparent from this Table that Toner E gave images that had been well 
developed but was poorly fixed and was nonsmooth, since the toner contains 
large amount of magnetic powder and has high saturated magnetic flux 
density. Toner F which contains a small amount of magnetic powder gave a 
good fixability but a poor developability and a large toner scattering on 
the background because it has low saturated magnetic flux density. 
By contrast, Toners A to D contain suitable amount of magnetic powder and 
have high saturated flux density, so they gave a good developability and 
fixability and no toner scattering on the background.