Carrier for development of electrostatic latent images

The present invention provide a carrier for development of electrostatic latent images, comprising magnetic powder dispersed in binder resin, the carrier having a mean particle size in a range of 30 to 80 .mu.m and satisfying the following relational expression: EQU (x).sup.2 /.phi..sup.2 .gtoreq.9.0 wherein x represents mean particle size of the carrier and .phi..sup.2 represents variance of particle size distribution. The carrier of the present invention is superior in chargeability and fluidity and free from occurrence of carrier adhesion even when used in combination with small particle size toner and which can form excellent copy images.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to carriers used in developers for 
development of electrostatic latent images and, more particularly, to a 
carrier having magnetic powder dispersed in binder resin. 
2. Description of the Prior Art 
As developers for use with electrophotographic copying machines or 
printers, there have been known two-component developers composed of a 
toner and a magnetic carrier such as iron powder. In any developing method 
using such a two-component developer, the magnetic strength among carrier 
particles is so strong that ears of the magnetic brush harden, causing a 
problem that white lines may appear in black-solid images. Also, the iron 
powder carrier itself is low in volume electrical resistivity. Therefore, 
when the toner concentration in the developer has lowered due to 
continuous use or the like, electrical charges on the electrostatic latent 
image supporting member may escape via the carrier so that the latent 
image is disordered, causing defects or other damages in copy images, or 
electrical charges may be injected from the developing sleeve to the 
carrier so that the carrier adheres to the image portion. Further, if a 
hard carrier such as iron powder has adhered to the electrostatic latent 
image supporting member, the surface of the electrostatic latent image 
support member may be damaged when residual toner is removed. 
To solve the above problems, a binder type carrier has been proposed in 
which magnetic fine powder is dispersed in binder resin. The binder type 
carrier is generally low in magnetization level within a magnetic field, 
compared with iron powder carrier or the like, so that the ears of the 
magnetic brush become soft. Thus, the binder type carrier has an advantage 
that excellent images free from white lines due to carrier can be 
obtained. 
However, even with the use of the binder type carrier, especially when it 
is used in combination with a toner having a particle size as small as 3 
to 9 .mu.m, there may arise some problems that the chargeability of toner 
is insufficient or the fluidity of developer is insufficient. Moreover, 
there may occur carrier adhesion that the carrier adheres to non-image 
portions of the electrostatic latent image supporting member, making image 
noise when developed, as still another problem. 
SUMMARY OF THE INVENTION 
The present invention is to provide a carrier which is superior in 
chargeability and fluidity and free from occurrence of carrier adhesion 
even when used in combination with small particle size toner and which can 
form excellent copy images. 
The present invention relates to a carrier for development of electrostatic 
latent images, comprising magnetic powder dispersed in binder resin, the 
carrier having a mean particle size in a range of 30 to 80 .mu.m and 
satisfying the following relational expression: 
EQU (x).sup.2 /.phi..sup.2 .gtoreq.9.0 
wherein x represents mean particle size of the carrier and .phi..sup.2 
represents variance of particle size distribution.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a carrier which is superior in chargeability 
and fluidity and free from occurrence of carrier adhesion even when used 
in combination with small particle size toner and which can form excellent 
copy images. 
The present inventors have found that the aforementioned problems of 
insufficient chargeability and fluidity as well as carrier adhesion, which 
would be involved when small particle size toner and the binder type 
carrier are used in combination, can be attributed to the contents of 
small particle size carrier-particles and large particle size 
carrier-particles contained in the carrier. 
The present invention has accomplished the above object by controlling a 
particle size distribution of carrier to a specified range. 
The present invention relates to a carrier for development of electrostatic 
latent images, having magnetic powder dispersed in binder resin, the 
carrier being characterized in that its mean particle size is in a range 
of 30 to 80 .mu.m and the following relational expression is satisfied: 
EQU (x).sup.2 /.phi..sup.2 .gtoreq.9.0 
wherein x represents mean particle size and .phi..sup.2 represents variance 
of particle size distribution. 
The carrier of the present invention has a value of (x)/.sup.2 /.phi..sup.2 
wherein x represents mean particle size, .phi..sup.2 represents variance 
of particle size distribution) being not smaller than 9.0, preferably not 
smaller than 10.0. When the value is smaller than 9.0, small particle size 
carriers and large particle size carriers increase in their proportions, 
resulting in insufficient chargeability and fluidity as well as occurrence 
of carrier adhesion. 
The carrier of the present invention has a mean particle size in the range 
of 30 to 80 .mu.m, preferably 30 to 70 .mu.m. When the mean particle size 
of the carrier is smaller than 30 .mu.m, carrier adhesion to the 
electrostatic latent image supporting member is likely to occur. When it 
is larger than 80 .mu.m, brushing nonuniformities may take place such as 
in ordinary iron powder carrier, resulting in unclear copy images, and 
moreover use of the carrier in combination with small particle size toner 
having a mean particle size of 3 to 9 .mu.m may easily incur insufficient 
charge amounts of toner. 
Examples of the binder resin used for the carrier of the present invention 
are polystyrene resins, poly(metha)acrylic resins, styrene-acrylic 
copolymer resins, polyolefin resins, polyester resins, epoxy resins, and 
the like. 
Examples of the magnetic powder used for the carrier of the present 
invention are such metals as iron, nickel and cobalt, alloys or mixtures 
of these metals with such metals as zinc, antimony, aluminum, lead, tin, 
bismuth, beryllium, manganese, selenium, tungsten, zirconium and vanadium, 
mixtures thereof with such metal oxides as iron oxide, titanium oxide and 
magnesium oxide, and ferromagnetic ferrite, magnetite and their mixtures. 
The particle size of these magnetic powders is desirably not greater than 5 
.mu.m, preferably not greater than 2 .mu.m, and more preferably 0.1 to 1 
.mu.m in primary particle size, from the viewpoint of uniform dispersion 
in the binder resin. 
A blending ratio of the binder resin to the magnetic powder is 100 to 900 
parts by weight, preferably 400 to 800 parts by weight, and more 
preferably 500 to 700 parts by weight of magnetic powder on the basis of 
100 parts by weight of the resin. When the blending ratio of magnetic 
powder is more than 900 parts by weight, the magnetic powder forms 
secondary powder without being uniformly dispersed, so that the carrier 
becomes brittle. On the other hand, when the blending ratio of magnetic 
powder is less than 100 parts by weight, sufficient magnetism cannot be 
obtained. 
The carrier of the present invention may also contain a dispersing agent, 
such as carbon black, silica, titania and alumina. The dispersing agent, 
if contained, allows the uniform dispersibility of magnetic powder in the 
binder resin to be improved. A content of the dispersing agent is 
preferably 0.01 to 3% by weight relative to the carrier. 
The carrier of the present invention may be prepared, for example, by a 
method in which the binder resin and the magnetic powder are mixed and 
heated at a specified mixing ratio and after cooling, the mixture is 
pulverized and classified, or by a method in which the binder resin is 
dissolved into a solvent and, after the magnetic powder is dispersed into 
the resin solution, the resultant is spray-dried. 
When the carrier is prepared by the above mixing and pulverizing process, a 
jet mill as shown in FIG. 1 is commonly used as the mill for use in the 
step of pulverizing particles. 
In the jet mill in FIG. 1, coarsely pulverized particles 1 are accelerated 
by a high speed air stream spouting from a jet nozzle 2 to intensely 
collide against a collision plate 3, thus being pulverized. 
When such a jet mill is used to prepare the above-described carrier, a high 
content ratio of the magnetic powder makes it difficult to pulverize the 
particles into uniform particle size. The collision plate of such a jet 
mill is conventionally a collision plate whose surface for pulverization 
of particles is flat as illustrated in FIG. 2 or another whose surface for 
pulverization of particles is conical as illustrated in FIG. 3. When the 
collision plate of FIG. 2 is used for the aforementioned pulverization of 
carrier, the pulverizability is very successful but overpulverization may 
occur, causing generation of a large amount of fine powder, and resulting 
in a wide particle size distribution. When the collision plate of FIG. 3 
is used, the particle size distribution is rather narrow but a poor 
pulverizability results in less yield per unit time. 
Thus, when a collision plate of a shape as shown in FIG. 4 or FIG. 5 is 
used, especially when the collision plate as illustrated in FIG. 4 is 
used, it has been found that a narrow particle size distribution is 
obtained while the pulverizability can be maintained. That is, use of a 
collision plate having the shape of FIG. 4 is effective to the 
pulverization of particles whose specific gravity is rather greater, like 
the carrier of the present invention, in terms of control of the particle 
size distribution of given particles and the pulverizing efficiency. 
Values of 8 and d of the collision plate are set to proper ones depending 
on hardness and size of the object materials to be pulverized. In 
addition, a collision plate of FIG. 4 with 
100.degree..ltoreq..theta.140.degree. and 6 mm.ltoreq.d.ltoreq.16 mm is 
desirably used for the preparation of the carrier of the present 
invention. 
Further, the carrier of the present invention may be heated after the 
classifying step. The heating process is desirably a process of 
instantaneous heating by spouting the carrier into an air stream. The 
equipment for such heating may be, for example, Surfusing System (made by 
Nihon Pneumatic Kogyo K.K.) or the like. The heating temperature is 
preferably in the range of about 150.degree. to 350.degree. C. 
Such heating process allows the carrier to be modified in its surface 
state. Thus, a carrier can be obtained which has such an excellent 
durability that the magnetic powder will not be separated even when the 
carrier is subjected to continuous use. 
The toner used in combination with the carrier of the present invention may 
be a known toner which has a mean particle size of 2 to 20 .mu.m. In 
particular, when a small particle size toner having a mean particle size 
of 3 to 9 .mu.m is used in combination with the carrier of the present 
invention, a remarkable effect can be exerted so that the problems of 
insufficient fluidity and poor chargeability in small particle size toners 
can be successfully resolved. 
Concrete examples of the present invention are now described hereinbelow, 
but the scope of the present invention is not limited to these examples. 
Preparation Example of Carrier 1 
One hundred parts by weight of polyester resin (Tafton NE-1110, made by Kao 
K.K.), 500 parts by weight of ferrite powder (MFP-2, made by TDK K.K.), 2 
parts by weight of carbon black (Ketchen Black EC, made by Lion Yushi 
K.K.), 1.5 parts by weight of silica (#200, made by Nihon Aerosil K.K.) 
were well mixed by means of a Henschel mixer. The mixture was melt and 
kneaded by a pressure kneader. The kneaded mixture was cooled and then 
coarsely pulverized by a feather mill. Thereafter, by using a jet mill 
(model IDS-II) loaded with a collision plate (.theta.=120.degree., d=8 mm) 
of FIG. 4 as the collision plate, the mixture was finely pulverized at a 
milling air pressure of 2.5 kg.multidot.f/cm.sup.2, and classified in 
Multiplex. Thus, Carrier 1 with a mean particle size of 69.5 .mu.m and a 
value of (x).sup.2 /.phi..sup.2 of 11.33 was obtained. 
Particle size distribution of the resulting carrier is shown in Table 1 and 
FIG. 6. The abscissa axis in FIG. 6 represents the channel of Table 1. 
Preparation Example of Carrier 2 
In the same way as in the preparation example of Carrier 1 except that 600 
parts by weight of ferrite powder was added and that the milling air 
pressure was 3.5 kg.multidot.f/cm.sup.2, Carrier 2 with a mean particle 
size of 43 .mu.m and a value of (x).sup.2 /.phi..sup.2 of 12.43 was 
obtained. Particle size distribution of the resulting carrier is shown in 
Table 1 and FIG. 7. 
Preparation Example of Carrier 3 
In the same way as in the preparation example of Carrier 1 except that 700 
parts by weight of ferrite powder was added and that the milling air 
pressure was 4.5 kg.multidot.f/cm.sup.2, Carrier 3 with a mean particle 
size of 33 .mu.m and a value of (x).sup.2 /.phi..sup.2 of 16.60 was 
obtained. Particle size distribution of the resulting carrier is shown in 
Table 1 and FIG. 8. 
Preparation Example of Carrier 4 
In the same way as in the preparation example of Carrier 1 except that the 
collision plate of FIG. 2 was used, carrier 4 with a mean particle size of 
71 .mu.m and a value of (X).sup.2 /.phi..sup.2 of 5.93 was obtained. 
Particle size distribution of the resulting carrier is shown in Table 1 and 
FIG. 9. 
Preparation Example of Carrier 5 
In the same way as in the preparation example of Carrier 4 except that the 
milling air pressure was 3.5 kg.multidot.f/cm.sup.2, Carrier 5 with a mean 
particle size of 46 .mu.m and a value of (X).sup.2 /.phi..sup.2 of 6.85 
was obtained. Particle size distribution of the resulting carrier is shown 
in Table 1 and FIG. 10. 
Preparation Example of Carrier 6 
In the same way as in the preparation example of Carrier 4 except that the 
milling air pressure was 4.5 kg.multidot.f/cm.sup.2, Carrier 6 with a mean 
particle size of 31 .mu.m and a value of (X).sup.2 /.phi..sup.2 of 4.85 
was obtained. Particle size distribution of the resulting carrier is shown 
in Table 1 and FIG. 11. 
TABLE 1 
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Channel 
1 2 3 4 5 6 7 8 
8.00.about.10.1 
10.1.about.12.7 
12.7.about.16.0 
16.0.about.20.2 
20.2.about.25.4 
25.4.about.32.0 
32.0.about.40.3 
40.3.about.50.8 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
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Carrier 1 
0.0 0.0 0.0 0.0 0.0 2.0 5.9 12.3 
Carrier 2 
0.0 0.0 0.0 1.1 3.4 11.3 22.6 32.3 
Carrier 3 
0.0 0.0 0.5 3.5 9.0 24.8 36.9 22.8 
Carrier 4 
0.0 0.0 0.0 0.6 1.5 3.8 6.5 9.8 
Carrier 5 
0.0 0.0 0.5 2.3 4.2 10.0 17.4 25.9 
Carrier 6 
0.0 0.9 3.1 8.7 13.9 24.3 26.2 14.5 
__________________________________________________________________________ 
Channel 
9 10 11 12 13 14 15 16 
8.00.about.10.1 
10.1.about.12.7 
12.7.about.16.0 
16.0.about.20.2 
20.2.about.25.4 
25.4.about.32.0 
32.0.about.40.3 
40.3.about.50.8 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
(.mu.m) 
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Carrier 1 
19.1 30.5 24.4 6.0 0.0 0.0 0.0 0.0 
Carrier 2 
22.0 7.1 0.2 0.0 0.0 0.0 0.0 0.0 
Carrier 3 
2.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
Carrier 4 
13.9 22.5 21.5 14.9 3.6 1.6 0.0 0.0 
Carrier 5 
22.5 11.4 4.1 1.7 0.0 0.0 0.0 0.0 
Carrier 6 
3.9 1.5 2.2 0.7 0.0 0.0 0.0 0.0 
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Component Parts by weight 
______________________________________ 
styrene-n-butylmethacrylate 
100 
(softening point: 132.degree. C., 
glass transition temperature: 
60.degree. C.) 
carbon black 8 
(MA #8, made by Mitsubishi 
Kasei Kogyo K.K.) 
Nigrosine dye 5 
(Bontron N-01, made by Orient 
Kagaku Kogyo K.K.) 
______________________________________ 
The above materials were well mixed by a ball mill, and kneaded on a 
three-roll heated at 140.degree. C. The kneaded mixture was left for 
cooling. After cooling the mixture was coarsely pulverized into a mean 
particle size of 2 mm by a hammer mill. Then it was pulverized into a mean 
particle size of 11 .mu.m by Criptron and further finely pulverized by a 
jet mill. Then the resulting powder was air-classified. Thus a toner with 
a mean particle size of 8.5 .mu.m was obtained. 
Evaluation of Physical Properties 
(1) Particle Size of Carrier 
For measurement of mean particle size of Carriers, the Coulter Counter 
TA-II model (made by Coulter Counter Co.) was used, and relative weight 
distribution for each particle size was measured by a 500 .mu.m aperture 
tube. 
(2) Variation of Electrical Charge Amount of Toner due to Stirring Strength 
The above obtained toner (100 parts by weight) was mixed with Colloidal 
Silica R974 (0.1 part by weight) (made by Nihon Aerosil K.K.). This 
resulting toner was mixed with Carriers 1 through 6 at a toner-mixing 
ratio of 5% for 10 minutes by using a Vial Rotator to prepare developers. 
The developers were subjected to measure a charge amount of toner Q.sub.2 
(.mu.C/g) under conditions of 25.degree. C. and a humidity of 65%. Next, 
the developers were strongly stirred for 30 minutes by a paint 
conditioner, and then a charge amount of toner Q.sub.2 (.mu.C/g) was 
measured. A value of .vertline.Q.sub.1 -Q.sub.2 .vertline. as the 
variation of charge amount due to stirring strength is shown in Table 2. 
(3) Carrier Adhesion 
Each developer prepared in the above step (2) was evaluated practically by 
a copying machine EP-5400 (made by Minolta Camera K.K.). Results are shown 
in Table 2. 
In Table 2, carrier adhesion was evaluated visually by checking carriers 
adhered onto copy images. The mark o shows that no carrier adhesion had 
occurred, .DELTA. shows that carrier adhesion had occurred, but at such a 
level that it would not matter practically, and x shows that carrier 
adhesion is noticeable and problematic as image noise. 
(4) Fluidity of Developer 
Each developer prepared in the above step (2) was set in a developing unit 
for a copying machine EP-5400 (made by Minolta Camera K.K.). The transfer 
screw within the developing unit was adjusted so that the developer would 
not be unbalanced in the longer direction of the developing unit after a 
10 minute idle rotation. The developing unit adjusted in this way was 
mounted to the copying machine and subjected to durability test with 
respect to copy. After 10,000 times of copy with regard to a black-solid 
image, image density was measured at two points 20 cm away from each other 
in the direction perpendicular to direction of paper path, and difference 
in density in the longer direction of the developing unit due to unbalance 
of the developer was measured. The image density was measured by Macbeth 
Reflective Densitometer. 
This value was evaluated to be ranked as follows. 
The symbol "o" represents that a value of the difference was 0.05 or less. 
The symbol ".DELTA." represents that a value of the difference was greater 
than 0.05 to smaller than 0.1. The symbol "x" represents that a value of 
the difference was greater than 0.1. The results are shown in Table 2. 
TABLE 2 
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Mean particle 
Variation of 
Carrier 
Carrier 
size x(.mu.m) 
(x).sup.2 /.sigma..sup.2 
charge amount 
adhesion 
Fluidity 
__________________________________________________________________________ 
Example 1 
1 69.5 11.33 
3.5 .smallcircle. 
.smallcircle. 
Example 2 
2 43.0 12.43 
2.8 .smallcircle. 
.smallcircle. 
Example 3 
3 33.0 16.60 
1.0 .smallcircle. 
.smallcircle. 
Comparative 
4 71.0 5.93 
6.8 .DELTA. 
x 
Example 1 
Comparative 
5 46.0 6.85 
5.2 x x 
Example 2 
Comparative 
6 31.0 4.85 
4.9 x x 
Example 3 
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