Carrier particles for use in a developer for developing latent electrostatic images comprise organic tin compound, silicone resin and conductive material

Carrier particles for use in a two-component dry-type developer for developing latent electrostatic images comprising a core particle and a silicone resin layer coated on the core particle, the silicone resin layer comprising a silicone resin, an organic tin compound and finely-divided electroconductive particles.

BACKGROUND OF THE INVENTION 
The present invention relates to carrier particles of a two-component 
dry-type developer for developing latent electrostatic images to visible 
images for use in electrophotography, electrostatic recording methods and 
electrostatic printing methods. More particularly, it relates to carrier 
particles coated with a silicone resin layer comprising a silicone resin, 
an organic tin compound and finely-divided electroconductive particles. 
Conventionally, as the methods for developing latent electrostatic images 
with toner, for example, a cascade development method (U.S. Pat. No. 
2,618,552) and a magnetic brush development method (U.S. Pat. No. 
2,874,063) are known. In either method, a two-component developer is 
employed, which comprises carrier particles and toner particles, and the 
toner particles are usually much smaller than the carrier particles and 
are triboelectrically attracted to the carrier particles and are held on 
the surface thereof. The electric attraction between the toner particles 
and the carrier particles is caused by the friction between the toner 
particles and the carrier particles. When the toner particles held on the 
carrier particles are brought near or into contact with a latent 
electrostatic image, the electric field of the latent electrostatic image 
works on the toner particles to separate the toner particles from the 
carrier particles, overcoming the bonding force between the toner 
particles and the carrier particles. As a result, the toner particles are 
attracted towards the latent electrostatic image, so that the latent 
electrostatic image is developed to a visible toner image. In this case, 
it is necessary that the toner particles be charged with an appropriate 
polarity and with an exact amount of electric charge, so that the toner 
particles are preferentially and exactly attracted to the desired areas to 
be developed on the photoconductor. 
In a conventional two-component type developer, it is apt to occur that the 
surface of the carrier particles is eventually covered with the toner 
particles to form a toner film layer on the carrier particles in the 
course of the mechanical mixing of the toner particles with the carrier 
particles in the development apparatus. Once this phenomenon takes place, 
which is generally referred to as the "spent phenomenon", the toner 
particles gradually accumulate on the carrier particles, so that the 
triboelectric charging between the carrier particles and the toner 
particles is replaced by the triboelectric charging among the toner 
particles. In the end, the triboelectric charging characteristics of the 
developer significantly deteriorate, so that a considerable amount of the 
toner particles are deposited on the background of the copy images. As a 
matter of course, when this occurs, the copy quality is considerably 
degraded. In the end, it is necessary to replace the entire developer by a 
new developer. 
In order to prevent the spent phenomenon, a method of coating the surface 
of carrier particles with a variety of resins has been proposed. However, 
resins capable of satisfactorily preventing the spent phenomenon have not 
been discovered. At one extreme, for instance, carrier particles coated 
with a styrene - methacrylate copolymer or polystyrene are excellent in 
the triboelectric charging properties. However, the surface energy of such 
carrier particles is comparatively so high that the carrier particles are 
easily covered with the toner particles while in use. In other words, the 
spent phenomenon occurs easily and, accordingly, the life of such 
developer is short. 
The above-mentioned "spent phenomenon", is greatly reduced with carriers 
coated with a polytetrafluoroethylene polymer, since such carriers have a 
low surface energy. However, since the polytetrafluoroethylene polymer is 
positioned on the extreme negative side in the triboelectric series, the 
carriers coated with the polytetrafluoroethylene polymer cannot be 
employed when toner particles must be charged to a negative polarity. 
In order to eliminate the above shortcomings, it has been proposed to coat 
the carrier particles with a material having a low surface energy, for 
example, with a silicone resin as proposed in Japanese Patent Publication 
No. 44-27879 and Japanese Laid-Open Patent Application No. 50-2543. In 
this method, the deposition of the toner particles on the carrier 
particles can be prevented. However, the silicone resin easily wears away 
and lacks mechanical strength. Therefore, when the carrier particles 
coated with silicone resin are used for continuous copying over an 
extended period of time, the core materials of the carrier particles are 
exposed since the silicone resin layer wears away by the collision among 
the carrier particles themselves and between the carrier particles and the 
mechanical parts of the development apparatus. As a result, the 
triboelectric charging between the toner particles and the silicone resin 
is eventually replaced by the triboelectric charging between the toner 
particles and the core materials of the carrier particles. Once this takes 
place, the triboelectric characteristics of the developer cannot be 
maintained constantly. Accordingly the copy image quality is significantly 
degraded. Further, since most of the resins for use in coating the carrier 
particles have high electric resistivity, when carrier particles coated 
with such resins are used in the developer, problems such as edge 
development, a significant decrease in image density and no image 
formation could occur. 
Such coated carrier particles can be improved by decreasing the electric 
resistance of the coated layer of the carrier particles, for instance, by 
dispersing an electroconductive material in the coated layer of the 
carrier particles. 
More specifically, when the carrier particles are provided with a certain 
electroconductivity, the carrier particles work as a development 
electrode, so that development of latent electrostatic images can be 
carried out as if development electrodes were positioned in close contact 
with the electrophotographic photoconductor. The result is that not only 
line images, but also solid images can be reproduced faithfully to the 
original images. 
Conventionally, as such electroconductive materials for use in the coated 
layer of the carrier particles, for example, carbon and tin oxide are 
employed. However, when such electroconductive materials are employed in 
the coated layer of the carrier particles, the electric resistivity of the 
carrier particles is so decreased that the electric charge generated in 
the toner particles leaks through the carrier particles which are in 
contact with the toner particles, so that the toner particles cannot 
maintain a predetermined necessary amount of electric charge for 
development. 
In order to develop latent electrostatic images formed on a photoconductor 
with a toner, the toner must maintain a predetermined quantity of electric 
charge. It is generally said that the quantity of electric charge ranging 
from 10 .mu.C/g to 20 .mu.C/g is suitable, since when the electric charge 
quantity is less than 10 .mu.C/g, fogging appears in the developed images 
or the developer is caused to scatter from the development apparatus. On 
the other hand, when the electric charge quantity is more than 20 .mu.C/g, 
an image density which is sufficiently high for faithful reproduction of 
original images cannot be obtained. 
When the electric resistivity of the carrier particles is decreased, the 
quantity of electric charge that can be generated in the toner particles 
also decreases. When carrier particles with an electroconductive material 
dispersed therein are employed, it is necessary to appropriately adjust 
the quantity of electric charge to be generated in the toner. In order to 
do this, a charge controlling agent, for instance, a dye conventionally 
employed as charge control agent, is added to the toner, by dissolving 
such a dye in a solvent together with a resin component of the toner, or 
by kneading the dye together with a resin component of the toner. 
Dyes employed as such charge controlling agent are generally expensive, and 
when a small amount of the dye is employed, it does not work effectively 
as charge controlling agent, while when a large amount of the dye is 
employed in order to increase the electric charge quantity, it becomes 
extremely difficult to uniformly disperse the dye throughout the resin and 
when such a toner is employed for an extended period of time, the 
development characteristics are significantly degraded while in use and 
high image quality cannot be obtained in a stable manner. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide carrier 
particles for use in a two-component dry-type developer for developing 
latent electrostatic images to visible toner images, which carrier 
particles are capable of retaining high charging performance and are not 
eventually subjected to the spent phenomenon while in use. Thus the 
developers using the carrier particles according to the present invention 
are capable of yielding high quality developed images without 
deterioration for an extended period of time. 
The above object of the present invention is attained by the carrier 
particles coated with a silicone resin layer comprising a silicone resin, 
an organic tin compound and finely-divided electroconductive particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Examples of a silicone resin for use in the silicone resin layer of the 
carrier particles according to the present invention are as follows: 
(1) Silicone varnishes, for example, TSR 115, TSR 114, TSR 102, TRS 103, YR 
3061, TSR 110, TSR 116, TSR 117, TSR 108, TSR 109, TSR 180, TSR 181, TSR 
187, TSR 144 and TSR 165 (manufactured by Toshiba); and KR 271, KR 272, KR 
275, KR 280, KR 282, KR 267, KR 269, KR 211 and KR 212 (manufactured by 
Shinetsu Silicone Co., Ltd.), 
(2) Alkyd-modified silicone varnishes, for example, TSR 184 and TSR 185 
(manufactured by Tohiba), 
(3) Epoxy-modified silicone varnishes, for example, TSR 194 and YS 54 
(manufactued by Toshiba) 
(4) Polyester-modified silicone varnishes, for example, TSR 187 
(manufactured by Toshiba), 
(5) Acryl-modified silicone varnishes, for example, TSR 170 and TRS 171 
(manufactured by Toshiba) 
(6) Urethane-modified silicone varnishes, for example, TSR 175 
(manufactured by Toshiba), and 
(7) Reactive silicone varnishes, for example, KA 1008, KBE 1003, KBC 1003, 
KBM 303, KBM 403, KBM 503, KBM 602 and KBM 603 (manufactured by Shin-etsu 
Silicone Co., Ltd.) 
Examples of an organic tin compound for use in the present invention are as 
follows: 
EQU No. 1 R.sub.2.sup.1 Sn(OCOR.sup.2).sub.2 
wherein R.sup.1 and R.sup.2 each represent an alkyl group having 1 to 10 
carbon atoms. 
##STR1## 
wherein R represents an alkyl group having 1 to 10 carbon atoms. 
In the present invention, by changing the amount of the organic tin 
compound employed in the silicone resin coated layer of the carrier 
particles, the quantity of electric charge that can be generated in the 
toner can be easily adjusted as desired, with the desired polarity. 
Further, since the silicone resins for use in the present invention 
include silanol groups (--SiOH) prior to the hardening thereof by 
application of heat, it is considered that the above organic tin compounds 
also serve as hardening catalyst for the silicone resins. 
Examples of the electroconductive material for use in the present invention 
are organic materials, for example, carbon black such as furnace black, 
acetylene black and channel black; and inorganic materials, for example, 
borides, carbides, nitrides, oxides and silicides. 
a. Examples of borides: 
chromium boride, hafnium boride, molybdenum boride, niobium boride, 
tantalum boride, titanium boride and zirconium boride. 
b. Examples of carbides: 
boron carbide, hafnium carbide, molybdenum carbide, niobium carbide, 
silicon carbide, thallium carbide, titanium carbide, uranium carbide, 
vanadium carbide, tungsten carbide and zirconium carbide. 
c. Examples of nitrides: 
boron nitride, niobium nitride, thallium nitride, titanium nitride, 
vanadium nitride and zirconium nitride. 
d. Examples of oxides: 
chromium oxide, lead oxide, tin oxide, vanadium oxide, molybdenum oxide, 
bismuth oxide, iron oxide (Fe.sub.3 O.sub.4), niobium oxide, osmium oxide, 
platinum oxide, rhenium oxide, ruthenium oxide, titanium oxide and 
tungsten oxide. 
e. Examples of silicides: 
molybdenum silicide, niobium silicide, thallium silicide, titanium 
silicide, vanadium silicide and tungsten silicide. 
The above compounds have specific volume resistivities of 10.sup.-1 
.OMEGA.cm or less and are representative materials suitable for use in the 
present invention. In particular, they are effective for adjusting the 
electric resistivity of the silicone resin layer by use of a small amount 
thereof. 
It is preferable that the particle size of these compounds be 1 .mu.m or 
less, more preferably 0.5 .mu.m or less. Further, it is preferable that 
the amount of the above compounds be in the range of 1 wt. % to 50 wt. %, 
more preferably in the range of 2 wt. % to 30 wt. %, to the entire weight 
of the coating layer resin of the carrier particles according to the 
present invention. 
For preparation of silicone resin coated carrier particles according to the 
present invention, the above-mentioned silicone resin, the organic tin 
compound and the electroconductive material are well dispersed in an 
appropriate organic solvent in a homogenizer to prepare a coating liquid, 
followed by coating the core particles with the coating liquid by 
immersing the core particles in the coating liquid, by spraying the 
coating liquid on the core particles or by a fluidized bed process. 
As the organic solvent for dispersing the silicone resin, the organic tin 
compound and the electroconductive material, any solvents can be employed 
as long as the silicone resin can be dissolved therein. Specific examples 
of such solvents are alcohols such as methanol, ethanol and isopropanol; 
aromatic hydrocarbons such as toluene and xylene; ketones such as acetone 
and methyl ethyl ketone; and tetrahydrofuran and dioxane, and mixtures of 
the above. 
After the core particles are coated with the coating liquid as mentioned 
above, the core particles are dried and heated, so that the coated 
silicone resin layer is hardened on the core particles, thus the carrier 
particles according to the present invention are prepared. 
When drying the above coating liquid coated core particles, it is effective 
to add to the coating liquid as a drying acceleration agent a metal salt 
such as lead octylate, lead naphthenate, iron octylate, iron naphthenate, 
cobalt octylate, cobalt naphthenate, manganese octylate, manganese 
naphthenate, zinc octylate and zinc naphthenate, or an organic amine such 
as ethanol amine. 
As a resin component of the toner to be used in combination with the 
carrier particles according to the present invention, homopolymers, 
copolymers and mixtures thereof of styrenes such as p-chlorostyrene; 
halogenated vinyl monomers such as vinyl chloride, vinyl bromide and vinyl 
fluoride; vinyl esters such as vinyl propionate, acetate, vinyl benzoate 
and vinyl butyrate; .alpha.-methylene fatty acid monocarboxylic acid 
esters such as methyl acrylate, ethyl acrylate, n-butyl acrylate, 
iso-butyl acrylate, dodecyl acrylate, n-octyl acrylate, 2-chloroethyl 
acrylate, phenyl acrylate, methyl .alpha.-chloroacylate, methyl 
methacrylate, ethyl methacrylate and butyl methacrylate; acrylonitrile, 
methacrylonitrile and acrylamide; vinyl ethers such as vinyl methyl ether, 
vinyl iso-butyl ether and vinyl ethyl ether; vinyl ketones such as vinyl 
methyl ketone, vinyl hexyl ketone and methyl iso-propenyl ketone; and 
N-vinyl compounds such as N-vinylpyrrole, N-vinyl-carbazole, N-vinylindole 
and N-vinylpyrrolidone. 
In addition to the above resins, thermofusible non-vinyl-type resins such 
as rosin-modified phenol-formaldehyde resin, oil-modified epoxy resin, 
polyurethane resin, cellulose resin and polyether resin can be employed 
alone or in combination with the above mentioned vinyl-type resins as the 
resins for the toner. 
As the coloring agents for use in the toner, the following can be employed: 
carbon black, Nigrosine dye, Aniline Blue, Calconyl Blue, Chrome Yellow, 
Ultramarine Yellow, Methylene Blue, Du Pont Oil Red, Quinoline Yellow, 
Methylene Blue Chloride, Phthalocyanine Blue, Malachilte Green Oxalate, 
Lamp Black, and Rose Bengale, and mixtures of the above. It is necessary 
that these pigments be contained in the toner particles in an effective 
amount for producing clear visible images. 
As the materials for the core particles of the carrier particles according 
to the present invention, for example, non-metallic materials such as sand 
and glass, metals and alloys such as cobalt, iron, copper, nickel, zinc, 
aluminum, brass and bronze, and oxides thereof can be employed. In 
addition to the above, any materials which are employed as core materials 
in the conventional carrier particles can be employed. 
It is preferable that the particle size of such core particles be in the 
range of from 50 .mu.m to 1000 .mu.m, more preferably in the range of from 
100 .mu.m to 500 .mu.m. 
Embodiments of the present invention will now be explained by referring to 
the following examples. 
EXAMPLE 1 
Preparation of Carrier Coating Liquid 
A mixture of the following components was dispersed to prepare a carrier 
coating liquid: 
______________________________________ 
Parts by Weight 
______________________________________ 
Toluene 100 
Silicone Varnish (KR-271 
100 
commercially available 
from Shin-etsu Silicone 
Co., Ltd.) 
Carbon black (Ketjen Black EC 
0.4 
commercially available 
from Lion Akzo Co., Ltd.) 
Dibutyl tin dilaurate 
0.5 
______________________________________ 
Preparation of Carrier Particles No. 1 
The above carrier coating liquid and 1000 parts of iron powder having an 
average particle size of 100 .mu.m were mixed in a fluidized bed of a 
fluidized granulation dryer apparatus. The mixture was then dried in the 
atmosphere at 90.degree. C., followed by allowing the mixture to stand in 
an electric heating furnace at 200.degree. C. for 30 minutes, so that the 
silicone resin was hardened, whereby carrier particles No. 1 according to 
the present invention were prepared. 
Preparation of Toner 
A toner for use with the above prepared carrier particles No. 1 was 
prepared as follows: 
A mixture of the following components was kneaded under application of heat 
at a temperature of 160.degree. C. by a roll mill: 
______________________________________ 
Parts by Weight 
______________________________________ 
Styrene-n-butylmethacrylate 
100 
copolymer (commercially 
available under the name of 
Himer SBM 73 from Sanyo 
Chemical Industries, Ltd.) 
Nigrosine dye (commercially 
1 
available under the name of 
Spirit Black SB from 
Oriental Chemical Industrial 
Ltd.) 
Carbon black 10 
______________________________________ 
After cooling the kneaded mixture, it was roughly crushed by a speed mill. 
Thereafter the mixture was finely divided by a jet mill and was then 
subjected to classification by a pneumatic classifier, whereby a toner 
having an average particle size of 6 .mu.m was prepared. 
Preparation of Two-Component Dry-Type Developer No. 1 
100 parts of the carrier particles No. 1 and 2.5 parts of the above 
prepared toner were mixed, whereby a two-component dry-type developer No. 
1 was prepared. 
The specific volume resistivity of the carrier particles was 
1.2.times.10.sup.10 .OMEGA.cm and the quantity of electric charge 
generated in the toner was 20 .mu.C/g. 
Image Formation Tests 
Image formation tests were carried out by a commercially available copying 
machine, using the two-component dry-type developer No. 1. Copies with 
excellent image quality, with clear reproduction of line images, solid 
images and half-tone images, were obtained. 
Comparative Example 1 
Example 1 was repeated except that dibutyl tin dilaurate serving as an 
organic tin compound was eliminated from the composition of the carrier 
coating liquid, whereby comparative carrier particles No. 1 were prepared. 
In the same manner as in Example 1, a comparative developer No. 1 was 
prepared by mixing with the toner particles prepared in Example 1. 
The electric resistivity of the comparative developer No. 1 was 
1.1.times.10.sup.10 .OMEGA.cm and the quantity of electric charge 
generated in the toner particles was 3 .mu.C/g. 
Image formation tests were carried out in the same manner as in Example 1 
using the comparative developer No. 1. The result was that images with 
considerable fogging were formed. 
Comparative Example 2 
Example 1 was repeated except that carbon black was eliminated from the 
composition of the carrier coating liquid, whereby comparative carrier 
particles No. 2 were prepared. 
In the same manner as in Example 1, a comparative developer No. 1 was 
prepared by mixing with the toner particles prepared in Example 1. 
The electric resistivity of the comparative developer No. 2 was 
1.1.times.10.sup.14 .OMEGA.cm and the quantity of electric charge 
generated in the toner particles was 30 .mu.C/g. 
Image formation tests were carried out in the same manner as in Example 1 
using the comparative developer No. 2. The result was that images free 
from fogging were formed, but the reproduction of half-tone images was 
considerably pocr due to the edge effects. 
The comparison between Example 1 and Comparative Example 1 indicates that 
when carbon black was added in order to improve the reproduction of line 
images and halftone images, the quantity of electric charges generated in 
the toner particles decreased, but the addition of the organic tin 
catalyst increased the quantity of electric charges in the toner 
particles, so that proper images were formed. 
In Comparative Example 2, since the electric resistance of the carrier 
particles was so high that the quantity of electric charge generated in 
the toner particles was large and the edge effects were found to be 
enhanced. 
In short, the quantity of electric charges generated in the toner particles 
increased as the amount of the organic tin compound increased, while the 
quantity of electric charges generated in the toner particles decreased as 
the amount of carbon black increased since the electric resistivity of the 
carrier layer decreased. 
FIG. 1 is a graph showing the relationship between the the quantity of 
carbon added as an electroconductive material and the quantity of electric 
charge of a developer of the type explained in Example 1. 
FIG. 2 is a graph showing the relationship between the quantity of carbon 
added as an electroconductive material and the electric resistivity of the 
developer. 
FIG. 3 is a graph showing the relationship between the quantity of the 
organic tin compound (dibutyl tin dilaurate) and the quantity of electric 
charge of the developer. 
FIG. 4 is a graph showning the relationship between the quantity of the 
organic tin compound and the electric resistivity of the developer. 
As can be seen from the above graphs, when the quantity of carbon 
increased, both the quantity of electric charges and the electric 
resistivity of the developer decreased. In contrast to this, when the 
amount of the organic tin compound increased, the quantity of electric 
charge of the developer increased, but the electric resistivity of the 
developer did not much increase. Therefore, the electric resistivity of 
the developer can be adjusted as desired by changing the quantity of the 
electroconductive material and the quantity of electric charge of the 
developer can also be changed by changing the quantity of the organic tin 
compound, whereby the quantity of electric charges and the electric 
resistivity of the developer can be obtained as desired. 
EXAMPLE 2 
Example 1 was repeated except that dibutyl tin dilaurate serving as organic 
tin compound and carbon black serving as the electroconductive material 
were respectively replaced by the previously mentioned organic tin 
compound No. 1 and titanium oxide, whereby carrier particles No. 2 
according to the present invention were prepared and a developer No. 2 was 
also prepared by mixing the carrier particles No. 2 with the toner 
prepared in Example 1. 
EXAMPLE 3 
Example 1 was repeated except that dibutyl tin dilaurate serving as organic 
tin compound and carbon black serving as the electroconductive material 
were respectively replaced by the previously mentioned organic tin 
compound No. 5 and silicon carbide, whereby carrier particles No. 3 
according to the present invention were prepared and a developer No. 3 was 
also prepared by mixing the carrier particles No. 3 with the toner 
prepared in Example 1. 
EXAMPLE 4 
Example 1 was repeated except that dibutyl tin dilaurate serving as organic 
tin compound and carbon black serving as the electroconductive material 
were respectively replaced by the previously mentioned organic tin 
compound No. 8 and iron oxide, whereby carrier particles No. 4 according 
to the present invention were prepared and a developer No. 4 was also 
prepared by mixing the carrier particles No. 4 with the toner prepared in 
Example 1. 
With respect to each of the above developers No. 2 through No. 4, the 
relationship between the quantity of the electroconductive material and 
the quantity of electric charges of each developer, the relationship 
between the quantity of the electroconductive material and the electric 
resistivity of the developer, the relationship between the quantity of the 
organic tin compound and the quantity of electric charges of each 
developer, and the relationship between the quantity of the organic tin 
compound and the electric resistivity of the developer were investigated. 
Almost the same results were obtained as those shown in FIGS. 1 through 4. 
(1) According to the present invention, since the carrier coating layer 
comprises a silicone resin having a low surface energy, toner particles 
hardly adhere and fix to the carrier particles. 
(2) Generally silicone resin has a shortcoming of easily wearing away. 
However, in the present invention, an electroconductive material is 
dispersed in the silicone resin and the electroconductive material works 
as a filler in the silicone resin, so that the silicone resin does not 
easily wear away. 
(3) The silicone resin, prior to the hardening, includes silanol groups 
(--SiOH) and is reactive with inorganic materials. Therefore, the silicone 
resin is capable of closely adhering to the core material of carrier 
particles (for instance, iron and ferrite) without particular treatment. 
Tetrafluoroethylene is known as a material having low surface energy, 
which is similar to silicone resin in this sense. However, unlike the 
silicone resin, tetrafluoroethylene does not adhere to the core material 
of the carrier particles. Therefore, it is necessary to treat the core 
material, for instance, with a coupling agent or a pre-coating material, 
prior to the coating with tetrafluoroethyelene or to subject the core 
material to a complex pretreatment for assuring close adhesion of 
tetrafluoroethylene to the core material. 
(4) Since the silicone resin, prior to the hardening, includes silanol 
groups (--SiOH), electroconductive inorganic materials can be well 
dispersed in the silicone resin. Further, since it has methyl groups, 
electroconductive organic materials such as carbon black can be well 
dispersed in the silicone resin. 
(5) When silanol groups (--SiOH) undergoes a condensation reaction by the 
presence of an organic tin compound, the quantity of electric charge 
generated in the toner particles can be adjusted as desired by changing 
the amount of the organic tin compound. The exact mechanism of the organic 
tin compound having an effect on the quantity of electric charge generated 
in the toner particles is unknown. However, tin is detected in the carrier 
coating layer after the silicone resin is hardened. 
(6) The resistivity of the coating layer of the carrier particles can be 
adjusted by changing the quantity of an electroconductive material and the 
quantity of electric charge generated in the toner particles can be 
adjusted by changing the quantity of an organic tin compound. Therefore, 
the resistivity of the carrier particles can be adjusted to the same 
extent as that of uncoated carrier particles, although the carrier 
particles according to the present invention are coated with the silicone 
resin having high electric resistivity.