Carrier for electrostatic image developer and process for the production thereof

A carrier of developer for developing an electrostatic image and a method for producing the carrier are disclosed. The carrier comprises core particles each having a resin layer covering the surface of said core particle. The resin layer is formed by dry process in which the core particles are stirred together with fine resin particles with no solvent for the resin to form the resin layer on the surface of each core particle. The fine resin particles comprise a homopolymer or a copolymer comprising a fluorinated acrylate repeating unit and are formed by emulsified polymerization carried out in the presence of a surfactant comprising an alkylbenzene sulfonic compound, and the residual amount of the surfactant in the fine resin particles is within the range of from 60 ppm to 10,000 ppm. The carrier is improved in adhesion between the surface of the core and the resin layer and satisfactory in friction electricity providing capability.

FIELD OF THE INVENTION 
The present invention relates to a carrier of developer for developing an 
electrostatic image, which is prepared by forming a resin coating layer on 
the surface of core particles. 
BACKGROUND OF THE INVENTION 
Two-component developers used in electrophotography comprise a toner and a 
carrier, and the carrier is used for the purpose of imparting a proper 
amount of polar frictional electricity to the toner. 
As such a carrier, a resin coated carrier having a resin coating layer on 
the core particle surface is used for various reasons including 
enhancement of the frictional electricity providing capability. 
It is known to skilled persons of the art that a resin coating layer 
comprising a fluorine-containing resin has favorable properties, and that 
a fluorinated acrylate polymer is particularly preferred as a material for 
the coating layer, because (1) this polymer has a glass transition point 
higher than that of other fluorine-containing resins and, therefore, are 
less liable to melt and stick to the surface of carriers, and (2) 
electrification can be easily controlled by varying the number of fluorine 
atoms contained in the branched chain. 
As a resin-coated carrier using such a fluorinated acrylate polymer, 
Japanese Pat. O.P.I. Pub. No. 235964/1988 discloses an electrostatic image 
developing carrier prepared by coating the surface of core particles with 
fine particles of a fluorinated acrylate polymer by use of a dry process. 
However, these electrostatic image developing carriers have the problems 
described below. 
(1) Resins comprising fluorinated acrylates are generally poor in adhesion 
to the core particles and low in film forming property. Accordingly, when 
used in forming a resin coating layer by a dry process, fine particles of 
these resins cannot be adhered and extended properly to the surface of 
core particles, yielding a carrier of poor durability. 
(2) Fine resin particles used to form a resin coating layer can be prepared 
by, for example, emulsion polymerization. According to a study of the 
present inventors, however, resin fine particles obtained by the usual 
emulsion polymerization do not always form a satisfactory resin coating 
layer and often give a carrier poor in capability of providing a toner 
with an adequate frictional electricity. When such a toner is used, 
formation of sharp images is prevented and, moreover, troubles such as 
fogging and scattering of toner are apt to be caused in the process of 
image formation. 
SUMMARY OF THE INVENTION 
The first object of the invention is to provide an electrostatic image 
developing carrier excellent in adhesion between the surface of core 
particles and a coating layer of resin with high film forming property 
(extensibility of fine particles) and satisfactory in friction electricity 
providing capability. The second object of the invention is to provide a 
method for producing an electrostatic image developing carrier having the 
above-mentioned excellent properties by a simple and sure means. 
The present inventors it is found by the inventers that a carrier having a 
satisfactory adhesion between the surface of core particles and a resin 
coating layer, an excellent film forming property and an adequate 
frictional electrification providing capability can be obtained by use of 
fluorinated alkyl acrylate polymer fine particles prepared by carrying out 
an emulsion polymerization in the presence of a specific surfactant and by 
regulating the amount of residual surfactant contained in the fine resin 
particles within a prescribed range. 
The carrier of the invention comprises core particles each having a resin 
layer covering the surface of said core particle. The resin layer is 
formed by dry process in which the core particles are stirred together 
with fine resin particles to form the resin, any solvent for resin is not 
used at this time. The fine resin particles comprise a homopolymer or a 
copolymer comprising a fluorinated acrylate repeating unit, which are 
formed by emulsified polymerization carried out in the presence of a 
surfactant comprising an alkylbenzene sulfonic compound, and the residual 
amount of the surfactant in said fine resin particles is within the range 
of from 60 ppm to 10,000 ppm. It is preferable that the core particles and 
the fine resin particles are placed in a rotor blade mixer and stirred at 
a stirring blade peripheral speed of 4 m/sec to 12 m/sec. 
Further, it is preferable that the resin coating layer be formed by a dry 
process using fine resin particles having a primary particle size of 50 to 
500 .mu.m and a BET specific surface area of 10 to 120 m.sup.2 /g. 
Because the fine resin particles used to form the resin coating layer 
comprise a fluorinated acrylate (co)polymer obtained by an emulsion 
polymerization conducted in the presence of a specific surfactant, and the 
amount of the surfactant retained in the fine resin particles is regulated 
within a prescribed range, the primary particle size and BET specific 
surface area of the fine resin particles are each controlled within a 
range suitable to the formation of the resin coating layer; therefore, the 
adhesion between the core particle surface and the resin coating layer as 
well as the film forming property are markedly improved and, moreover, the 
frictional electricity providing capability is satisfactorily developed, 
as shown by the results of the examples described later. 
Regulating the peripheral speed of stirring blades of the rotor blade mixer 
(the intensity of stirring) within a prescribed range improves the 
adhesion between the core particle surface and the resin coating layer 
and, thereby, allows the foregoing carrier to be produced advantageously.

DETAILED DESCRIPTION OF THE INVENTION 
Constituents of the Carrier 
The carrier of the invention is a resin coated carrier comprising a core 
particle and a resin coating layer formed on the surface of the core 
particles. 
As core particles of the carrier, particles of a magnetic substance are 
used. 
Suitable magnetic substances are those strongly magnetized by a magnetic 
field in its direction such as iron, ferrite or magnetite. "Ferrite" is a 
general term for iron-containing magnetic oxides and not limited to spinel 
type ferrites represented by the chemical formula MO.Fe.sub.2 O.sub.3, 
where M represents a divalent metal such as nickel, copper, zinc, 
manganese, magnesium or lithium. 
Taking into account the capability of providing a toner with frictional 
electricity and the adhesion of a carrier to a photoreceptor, the size of 
core particles is preferably 20 to 200 .mu.m, more preferably 30 to 120 
.mu.m, in weight average particle sizes. The weight average particle size 
of the carrier is a value determined by a dry process using a Microtrack 
Type 7981-OX made by Leeds & Northrup Co. 
On the other hand, the resin coating layer is formed on the surface of such 
core particles by a dry process using fine resin particles. 
The term "dry process" used here means a process for the formation of the 
resin coating layer on the core particle surface, without the use of any 
solvent for the resin, by mixing and stirring core particles and fine 
resin particles and applying mechanical impact force repeatedly to them. 
Materials of the Fine Resin Particles 
The fine resin particles used in the invention comprise a fluorinated 
acrylate homopolymer or copolymer. 
The fluorinated acrylate homopolymer means a polymer comprising a 
fluorinated acrylate repeating unit alone (hereinafter occasionally 
referred to as a F--Ac repeating unit), and the fluorinated acrylate 
copolymer is copolymers comprising the foregoing F--Ac repeating unit and 
another repeating unit. 
The F--Ac repeating unit which forms the fluorinated acrylate polymer or 
copolymer includes the unit represented by the following formula 1. 
##STR1## 
where R.sup.1 represents a hydrogen atom or a methyl group, and R.sup.2 
represents a residue formed by removing a hydrogen atom from the hydroxyl 
group of an alcohol compound containing an alkyl group in which at least 
one hydrogen atom is substituted by a fluorine atom. 
As residues represented by R.sup.2 in the foregoing F--Ac repeating unit of 
formula 1, preferable are --O(CH.sub.2).sub.n C.sub.m F.sub.2m+1 (n: an 
integer of 1 to 8, m: an integer of 1 to 19) and --O(CH.sub.2).sub.p 
(CF.sub.2).sub.q H (p: an integer of 1 to 8, q: an integer of 1 to 19); 
particularly preferred are --OCH.sub.2 CF.sub.3, --OCH.sub.2 
(CF.sub.2).sub.2 H and --OCH.sub.2 CF.sub.2 CF.sub.3. 
The fluorinated acrylate copolymer is composed of the foregoing F--Ac 
repeating unit and another repeating unit. Such another repeating unit may 
be a repeating unit derived from a monomer such as an aliphatic olefine, 
halogenated aliphatic olefine, conjugated diene type aliphatic diolefine, 
aromatic vinyl compound, nitrogen-containing vinyl compound or alkyl 
(meth)acrylate, and may be used in combination of two or more types to 
form the fluorinated acrylate copolymer. 
Typical examples of the monomer used to introduce another repeating unit 
include the monomers illustrated in Japanese Pat. O.P.I. Pub. No. 
33562/1989. Among them, styrene, methylstyrene and alkyl (meth)acrylates 
are particularly preferred from the viewpoint of controlling 
electrification amount and film forming property. 
In the embodiment of the invention, the content of the F--Ac repeating unit 
in the fluorinated acrylate copolymer is preferably not less than 50 wt %; 
more preferably, the content is not less than 55 wt %. 
When the content of the F--Ac repeating unit is more than 50 wt %, the 
initial electrification amount of a toner can be usually controlled within 
the range of 10 to 40 .mu.C/g and is not reduced by repeated uses; 
accordingly, the durability of the developer is improved and, thereby, 
sharp developed images can be formed constantly from the beginning to the 
end of copying. 
Preparation of Fine Resin Particles 
The first feature of the carrier of the invention is that the fine resin 
particles are prepared by an emulsion polymerization conducted in the 
presence of a surfactant containing an alkylbenzene sulfonic acid 
compound. 
By use of an alkylbenzene sulfonic acid compound as surfactant, the primary 
particle size and BET specific surface area of the fine resin particles 
can be controlled within the range suitable for the formation of the resin 
coating layer. 
Usable alkylbenzene sulfonic acid compounds include alkylbenzene sulfonates 
of the following formula (a) and alkylbenzene sulfonates of the following 
formula (b); preferred among them are alkylbenzene disulfonates. 
##STR2## 
where M represents an alkali metal or an alkali earth metal atom, 
preferably a sodium atom or a potassium atom, and n is an integer of 12 to 
22, preferably an integer of 15 to 19. 
Besides the foregoing alkylbenzene sulfonic acid compounds used as 
essential surfactants in the invention, other types of surfactants may be 
jointly employed. Such surfactants are not particularly limited to 
specific ones and may be selected from conventional ones. However, it is 
preferred that the amount of the alkylbenzene sulfonic acid compound be 
not less than 50 wt % of the total amount of the surfactants used in the 
emulsion polymerization. 
Size of Fine Resin Particles 
The primary particle size of the fine resin particles is preferably 50 to 
500 nm. 
The term "primary particle size" used here means the size of fine particles 
dispersed in the emulsion after completion of the emulsion polymerization. 
The BET specific surface area of the fine resin particles is preferably 10 
to 120 m.sup.2 /g. When the BET specific surface area is kept within this 
range at the time of mixing with core particles, the fine resin particles 
are easily pulverized into primary particles having a good film forming 
property, facilitating the formation of a uniform resin coating layer. 
The fine resin particles prepared by the emulsion polymerization form 
aggregates on drying. The size of such aggregates is preferably 1 to 50 
.mu.m in view of miscibility with core particles and film forming 
property. 
Residual Amount of Surfactant 
The second feature of the carrier of the invention is that the residual 
amount of the foregoing surfactant in the fine resin particles is 
regulated within the range of 60 to 10,000 ppm. 
As an outcome of study by the inventors, it is found that the objects of 
the invention cannot be achieved adequately only by use of a surfactant 
containing an alkylbenzene sulfonic acid compound. 
That is, the adhesion between the core particle surface and the resin 
coating layer as well as the film forming property can be improved and, 
thereby, an electrostatic image developing carrier having a satisfactory 
frictional electricity providing capability can be prepared by regulating 
the residual amount of the surfactant retained in the fine resin 
particles. 
In the invention, the residual amount of the surfactant can be controlled 
by adjusting the amount of the surfactant used in the emulsion 
polymerization and the washing conditions of the fine resin particles. 
Formation of Resin Coating Layer 
The resin coating layer constituting the carrier of the invention is formed 
by a dry process which uses the foregoing fine resin particles. 
In a preferred example of the dry process, core particles and fine resin 
particles are uniformly mixed, for example, in a conventional mixer, the 
mixture is placed, for example, in a modified apparatus of conventional 
rotor blade mixer and, then, mechanical impact force is repeatedly applied 
to the mixer for 5 to 40 minutes to form a resin coating layer on the core 
particle surface. 
In a particularly preferred example, this mixture is stirred for 5 to 15 
minutes at room temperature and then stirred further within a temperature 
range of (Tg-15).degree. C. to (Tg+15).degree. C. to form a resin coating 
layer, where Tg is a glass transition point of the fine resin particles 
contained in the mixture. 
In order to give a proper resistivity to the carrier, the amount of the 
fine resin particles to be blended is controlled within the range of 
preferably 0.3 to 10 wt %, more preferably 0.5 to 5 wt % of the core 
particles. 
FIG. 1 illustrates an example of the horizontal rotor blade mixer suitable 
for the production of the carrier of the invention, in which stock feed 
opening 12 equipped with feeding valve 13, and filter 14 and check opening 
15 are provided on top lid 11 of mixing chamber 10. 
A carrier stock introduced from stock feed opening 12 via feeding valve 13 
is stirred by rotor blades 18a, 18b and 18c of horizontal rotator 18 
driven by motor 22 and, thereby, subjected to mechanical impact force. As 
shown in FIG. 2, horizontal rotator 18 comprises center piece 18d which 
rotates in the arrow direction, and three rotor blades 18a, 18b and 18c 
provided symmetrically with respect to center piece 18d. These rotor 
blades each have inclined planes running upwards obliquely at an angle of 
.theta..degree. from bottom 10a of mixing chamber 10, as shown in FIGS. 3 
and 4. Accordingly, a carrier stock introduced is driven upwards by these 
rotor blades. The driven carrier stock strikes against the inclined upper 
inner wall or the vertical lower inner wall of mixing chamber 10 and falls 
into the rotating area of rotor blades 18a, 18b and 18c of horizontal 
rotator 18. In mixing chamber 10, vertical rotator 19 is provided above 
horizontal rotator 18. This vertical rotator 19 has two rotor blades, 
which strike the carrier stock having rebounded from the inner walls of 
mixing chamber 10 while rotating vertically. This vertical rotator 19 
performs the functions of accelerating stirring of the carrier stock and 
preventing aggregation thereof. 
As described above, the carrier stock is subjected to mechanical impact 
force while colliding repeatedly with horizontal rotator 18, vertical 
rotator 19 and the inner walls of mixing chamber 10 or collisions among 
carrier stock particles, so that the fine resin particles are caught, 
spread and fixed on the core particles to form a resin coating layer. The 
resin coated carrier so prepared is taken out from product discharge 
opening 20 with the aid of discharge valve 21. 
Jacket 17 functions as a heating means at the time of stirring the carrier 
stock and as a cooling means after completion of the stirring. The outer 
wall of mixing chamber 10 is covered with jacket 17 up to about 3/4 of its 
height, or the height at which vertical rotator 19 is mounted. The 
temperature of a carrier stock is measured with thermometer 16. 
Vertical rotator 19 is not necessarily indispensable and used when occasion 
demands; therefore, there may be provided horizontal rotator 18 alone. 
Peripheral Speed of the Rotor Blades 
The feature of the production process of carriers according to the 
invention is that a resin coating layer is formed by the steps of 
introducing core particles and fine resin particles into such a rotor 
blade mixer as described above and stirring them at a stirring intensity 
which gives rotor blade peripheral speed of 4 to 12 m/sec, preferably 6 to 
10 m/sec. 
Regulating the peripheral speed of rotor blades in a rotor blade mixer 
(stirring intensity) within a prescribed range improves the adhesion 
between core particles and resin coating layers and, thereby, makes it 
possible to prepare the carrier of the invention advantageously. 
The carrier of the invention is made up into a two-component developer by 
being mixed with a toner. A mixing ratio of giving a toner content of 1 to 
10 wt % is preferred. Since toner types are not particularly limited, 
conventional toners can be used. 
EXAMPLES 
Examples of the invention are described hereunder together with comparative 
examples. Parts in the following description are parts by weight. 
Preparation of Fine Resin Particles 
Fine Resin Particle A 
1,1,1-trifluoroethyl methacrylate was emulsion polymerized in the presence 
of a surfactant comprising a sodium alkylbenzene sulfonate of the 
following formula 1. Then, the resultant fluorinated methacrylate polymer 
was separated from the polymerization medium by ultrafiltration, washed 
and spray dried to obtain resin fine particle A. 
The primary particle size, BET specific surface area and residual amount of 
surfactant of fine resin particle A were measured by the following 
methods: 
Primary Particle Size 
Using the emulsion immediately after the emulsion polymerization, 
measurement was made with an LPA-3000 particle size distribution measuring 
equipment made by Otsuka Denshi Co. 
BET Specific Surface Area 
Measurement was made with a FlowSorb II-2300 BET specific surface area 
measuring equipment made by Shimadzu Corp. 
Residual Amount of Surfactant 
Fine resin particle A was accurately weighed out and dissolved in methyl 
ethyl ketone, followed by addition of methanol to precipitate the resin 
component. Then, the supernatant was filtered. The filtrate was condensed 
and analyzed for the surfactant by high performance liquid chromatography 
under the following conditions: 
Column: TSK-GEL LS-410 ODS 5 .mu.m 
Detector: Differential refractometer 
Eluate: acetonitrile--0.05 mol/1 NaCl solution (50/50) 
Flow velocity: 1.5 ml/min 
The evaluation results are shown in Table 1. 
Fine Resin Particles B to D, Fine Resin Particles e to j 
Inventive fine resin particles B to D and comparative fine resin particles 
e to j were prepared in the same manner as fine resin particle A except 
that the surfactants shown in Table 1 were used in place of compound 
formula 1. The resultant fine resin particles were each measured for the 
primary particle size, BET specific surface area and residual amount of 
the surfactant as shown in Table 1. 
For fine resin particles e and f, the residual amount of the surfactant was 
adjusted by varying the amount of surfactant used in the emulsion 
polymerization and the number of washing times. 
For fine resin particles g to j, the residual amount of the surfactant was 
measured by properly selecting a column packing for high performance 
liquid chromatography, a detection method, and an eluate. 
TABLE 1 
______________________________________ 
Residual Primary 
BET 
Sur- Amount of Particle 
Specific 
Fine Resin factant Surfactant 
Size Surface 
Particle Type (ppm) (nm) Area (m.sup.2 /g) 
______________________________________ 
Invention 
A (1) 60 400 20 
Invention 
B (1) 500 300 88 
Invention 
C (1) 2500 250 106 
Invention 
D (1) 10000 80 120 
Comparison 
e (1) 50 600 9 
Comparison 
f (1) 10500 40 150 
Comparison 
g (2) 2970 280 62 
Comparison 
h (3) 2960 240 80 
Comparison 
i (4) 2400 280 66 
Comparison 
j (5) 2560 200 58 
______________________________________ 
Sufactant 
##STR3## 
(2) C.sub.12 H.sub.25 OSO.sub.3 Na 
(3) C.sub.17 H.sub.35 COONa 
##STR4## 
##STR5## 
Example 1 
______________________________________ 
Core particles 100 parts 
(ferrite particles of 80 .mu.m average particle size) 
Fine resin particle A 2 parts 
______________________________________ 
The above carrier stock was introduced into a horizontal rotor blade mixer 
and stirred at a horizontal rotor blade peripheral speed of 8 m/sec for 10 
minutes at 22.degree. C. It was then heated to 90.degree. C. and stirred 
for another 40 minutes to obtain a carrier of the invention. 
Examples 2 to 4 
Carriers of the invention were prepared in the same manner as in Example 1, 
except that fine resin particles B to D were used by turns in place of 
fine resin particle A. 
&lt;Comparative Example 1&gt; 
A comparative carrier was prepared in the same manner as in Example 1, 
except that fine resin particle e was used in place of resin fine particle 
A. 
&lt;Comparative Examples 2 to 6&gt; 
Comparative carriers were prepared in the same manner as in Example 1, 
except that fine resin particles f to j were used by turns in place of 
fine resin particle A. 
These comparative carriers were so weak in adhesion of fine resin particles 
to core resin particles that a large number of fine resin particles were 
left uncombined. 
&lt;Evaluation&gt; 
The carriers of the invention and those for comparison prepared as above 
were evaluated for the following points: 
(1) Evaluation of adhesion between Core Particles and Resin Coating Layers 
In a 50-ml beaker were placed 4 g of carrier, the surfactant and water as 
dispersion medium. After stirring the mixture for 30 seconds in a 
supersonic homogenizer of 150 W output, the carrier was observed on a 
scanning electron microscope and rated as A when no peeling was found 
between core particles and resin coating layers and as B when peeling was 
found. 
(2) Electrification Amount of Toner 
Measurement was made by the usual blow-off method using a 350-mesh screen, 
under the conditions of blow pressure 0.2 kg/cm.sup.2 and blow time of 6 
seconds. 
(3) Fog Density 
Two-component developers were first prepared by mixing the respective 
carriers prepared in the above Examples and Comparative Examples with a 
toner for an Electrophotographic Copier U-bix 1017, at a mixing ratio to 
give a toner content of 5 wt %. 
Using these two-component developers, an actual copying test was run on an 
Electrophotographic Copier U-bix 4045 made by Konica Corp. For each copied 
image, a density relative to the white density was determined using a 
Sakura Densitometer manufactured by Konic Corp. A fog can be visually 
perceived when its density exceeds 0.005. 
(4) Scattering of Toner 
After making 50,000 copies for each of the two-component developers 
prepared in (3) on the Electrophotographic Copier U-bix 4045, the inside 
of the copier was checked for contamination with the toner. Appearance of 
the contamination is shown by the following sign. 
A: No contamination was observed after 50,000 copies 
B: Contamination was observed after 10,000 copies 
C: Contamination was observed after 2,000 copies 
The results of the above evaluations are shown in Table 2. 
TABLE 2 
______________________________________ 
Electri- 
Type of fication 
Fine Resin Peeling Amount Fog Scatter- 
Particle of resin 
of Toner Densi- 
ing of 
for Coating layer (.mu.C/g) ty Toner 
______________________________________ 
Example 1 
Fine resin 
A 27.5 0.000 A 
particle A 
Example 2 
Fine resin 
A 25.4 0.000 A 
particle B 
Example 3 
Fine resin 
A 23.1 0.001 A 
particle C 
Example 4 
Fine resin 
A 23.5 0.001 A 
particle D 
Comp. Fine resin 
B 14.1 0.009 B 
Example 1 
particle e 
Comp. Fine resin 
B 12.5 0.010 B 
Example 2 
particle f 
Comp. Fine resin 
B 4.2 0.016 C 
Example 3 
particle g 
Comp. Fine resin 
B 5.0 0.016 C 
Example 4 
particle h 
Comp. Fine resin 
B 3.3 0.018 C 
Example 5 
particle i 
Comp. Fine resin 
B 2.1 0.019 C 
Example 6 
particle j 
______________________________________ 
It can be understood from the above results that the carriers of the 
invention are excellent in adhesion between core particles and resin 
coating layers and capable of providing a toner with an adequate 
frictional electricity. And, by use of a two-component developer 
comprising the carrier of the invention, fogging and scattering of toners 
can be prevented. Accordingly, it is clear that the carriers of the 
invention are far superior to the comparative carriers.