Coated carrier particles for use in electrophotographic process

Carrier particles for use in an electrophotographic process are prepared by coating the surface of the carrier particles with a perfluoro carboxylic acid in a polymeric binder. The resulting carriers are long lived and capable of imparting a positive triboelectric charge to electroscopic powders mixed therewith.

BACKGROUND OF THE INVENTION 
This invention relates to carriers for use in developer formulations which 
charge electroscopic powders triboelectrically. These carriers are useful 
in electrophotographic processes for developing latent electrostatic 
images in which a colored toner carried by the carrier particle is caused 
to be attracted from the carrier particle to develop the latent 
electrostatic image. 
In the electrophotographic process it is necessary to use a carrier for the 
toner in order to produce an electrostatic charge upon the toner 
particles. Various kinds of developing processes are known including 
cascade, powder cloud and magnetic brush processes. In each of these 
processes it is necessary that the carrier used have certain triboelectric 
properties so that it is capable of imparting to the toner particles an 
electrostatic charge of the proper polarity and magnitude. It has been 
noted that when a developer mix containing toner and carrier is allowed to 
stand in the developing unit of a copying machine the toner loses its 
electrostatic charge over a period of time. When the copying machine is 
turned on, the first several copies made are of relatively poor quality 
until the triboelectric charge on the toner is again built up. 
Recently, it has been found that the carrier particles can be coated with 
certain types of polymeric coatings to permit variation in the 
triboelectric properties thereof. One such method is disclosed in U.S. 
Pat. No. 3,811,880 to Luther C. Browning, assigned to the same assignee as 
this invention. 
Although polymeric coatings of this type enable a certain degree of control 
of the triboelectric properties of the developer mix, it has been found 
that in use in the environment of electrophotographic reproduction 
machines such carrier particles are subject to aging which limits their 
effectiveness. Wearing away and removal of part of the polymeric coating 
upon the surface of the carrier particles is another problem encountered. 
This may result in undesired abrasion of the photoconductive surface used 
for imaging and also cause bias shorting. 
Another problem inherent in the use of such polymeric coatings for carrier 
particles is the phenomenon known as "bound toner." Through a mechanism 
which is not clearly understood prolonged usage of developer mixes 
including polymeric coated carrier particles results in toner being 
adhered onto the surface of the coated carrier causing a decrease in the 
effectiveness of the toning process and hence in the overall development 
of the images being reproduced. 
It is also known to use carriers in which a perfluoro carboxylic acid or 
derivative thereof is adhered to the surface of uncoated carrier particles 
by chemical adsorption. Such chemically treated carrier particles will 
impart a positive electrostatic charge to toner particles with which they 
are mixed. Carriers of this type are disclosed in U.S. Pat. No. 3,922,381 
to Pabitra Datta, assigned to the same assignee as this invention. 
However, it has been found that in high speed electrophotographic 
reproduction machines the carrier particles coated with such perfluoro 
carboxylic acids are subject to abrasion, wear and flaking and the useful 
life thereof is impaired. 
OBJECTS 
It is accordingly an object of this invention to provide carriers for 
toners which are not subject to the disadvantages mentioned above. 
Another object of this invention is to provide carrier particles which have 
an enhanced longevity. 
Another object of this invention is to provide carrier particles which are 
capable of imparting desired triboelectric properties to various types of 
toners. 
It is another object of this invention to provide carrier particles which 
combine the desired features of the prior art polymer coated carriers and 
the perfluoro carboxylic acid chemically treated carriers in a manner such 
that the useful life of the carrier particles is enhanced, desirable 
triboelectric properties are provided and "bound toner" is reduced. 
It is a further object of this invention to provide a carrier which enables 
good copies to be made even though the developer mix containing the 
carrier and a toner is allowed to stand for a substantial period of time. 
Other objects and advantages of this invention will become apparent in the 
following detailed disclosure and description. 
SUMMARY OF THE INVENTION 
Carrier particles coated with a perfluoro carboxylic acid in a polymeric 
binder can be used in developer mixes in order to increase the useful life 
of the developer and also to provide desired triboelectric properties by 
imparting a positive triboelectric charge to electroscopic powders mixed 
therewith. Such carriers have a longevity which is significantly greater 
than untreated carrier particles or carrier particles treated with 
perfluoro carboxylic acids alone. A more durable carrier, a more efficient 
carrier with respect to triboelectric properties, a carrier having low 
surface energy and a low coefficient of friction and a carrier which does 
not need to be replenished as frequently as other types of carriers is 
thereby provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It has been found that the various problems encountered with previously 
available carriers are generally obviated by the use of a carrier particle 
to the surface of which is adhered a perfluoro carboxylic acid or 
derivative thereof in a polymeric binder. 
The perfluoro acids contemplated by this invention are those perfluorinated 
and substantially perfluorinated carboxylic acids, both aliphatic and 
aromatic, which have carbon chains of from 3 to 18 carbon atoms in length. 
Examplary of such acids are perfluoropropionic acid, perfluorobutyric 
acid, perfluorovaleric acid, perfluoroadipic acid, perfluoroheptanoic 
acid, perfluorooctanoic acid, perfluorononanoic acid, perfluorodecanoic 
acid, 11-H-eicosafluroundecanoic acid, as well as the higher molecular 
weight aliphatic acids and aromatic acids such as perfluorobenzoic acid. 
Polycarboxylic acids can also be used, for example, perfluorosuccinic acid 
or perfluoroglutaric acid. Derivatives of perfluoro carboxylic acids such 
as salts, esters and amides can also be used. 
As binders any of a wide variety of thermoplastic polymers which will not 
react with the carrier matrix materials can be used. Such materials 
increase the developer mix life up to about tenfold over that of carriers 
treated with perfluoro carboxylic acids alone and prevent toner filming 
and bias shorting. Some materials which are especially adapted for this 
purpose are vinyl resins, polycarbonate resins and other polyester resins, 
polyamide resins, polyurethane resins, epoxy resins, acrylic resins, 
novolak resins, heat curable silicone rubbers, fluorinated polymers and 
other thermoplastic polymers and copolymers. 
A preferred class of such resins are the polyamide resins produced by 
reaction of dicarboxylic acids or their esters with the polyamine 
compounds such as diamines. Preferred esters are the methyl, ethyl and 
propyl esters. Suitable polyamine compounds include ethylenediamine, 
tetramethylene diamine, pentamethylenediamine, piperazine and 
diethylenetriamine. One group of particularly suitable polyamides sold 
under the trademark "VERSAMID" by General Mills Company are prepared from 
dimer acids and polyamines, for example the condensation product of 
9,11-linoleic acid and 9,12-linoleic acid with ethylene diamine. Other 
suitable polyamide resins are available from the Krumbhaar Resin Division 
of Lawter Chemicals, Inc., under the trademark "POLYMID". 
As carrier matrix materials it is possible to use a wide variety of 
substances, for example glass beads, ceramic beads, grains of sand or 
metallic particles. Non-metallic matrix materials are useful where a 
cascade development system is utilized although metallic matrices can also 
be used in cascade development. Where a magnetic brush developing system 
is used it is necessary that the matrix be magnetic. For this purpose 
various irons and steels have been used, for example, spherical steel 
beads and irregularly shaped iron powders. 
The desired coating can be applied in a number of ways. In general, it is 
merely necessary to mix the carrier matrix material with a solution of the 
perfluoro carboxylic acid and polymeric binder. Ordinarily, adherence of 
the perfluoro acid and binder to the powder or bead occurs in a dilute 
solution of the perfluoro acid and binder in a suitable solvent such as an 
alcohol, ether, ketone, hydrocarbon or halogenated hydrocarbon at room 
temperature. 
Coating of the carrier matrix material is accomplished by a process which 
will thoroughly mix the carrier matrix particles and the solution of 
perfluoro acid and binder to achieve uniform coating of the carrier 
matrix. A fluidized bed coating apparatus has been found particularly 
adaptable to the coating operation although other coating techniques can 
also be used. In using the fluidized bed coating method, the carrier 
matrix material is loaded into the fluidized bed coating apparatus and air 
under pressure is then passed into the apparatus. The solution of 
perfluoro acid and binder is pumped through an atomizing spray nozzle at a 
rate such that uniform coatings occur. Spraying may be repeated as many 
times as desired to obtain the particular thickness of coating required. 
The resulting coated particles are finally dried in a fluidized bed oven. 
The matrix particles treated with a perfluoro acid and binder according to 
this invention are believed to constitute a thin film of the binder 
saturated with the perfluoro acid adhered to the surface of the matrix 
particle. A coating thickness, for example, of about from 1 micron to 3 
microns has been found to be particularly satisfactory for use with a wide 
variety of toners. 
In a preferred embodiment the perfluoro acid and resin binder are heated 
together to form a reaction product between the perfluoro acid and binder 
such that the adherence of the coating to the matrix particles is improved 
and the life of a developer mix into which such coated particles are 
incorporated is further increased. For this purpose the resin is melted 
and the perfluoro acid is added to the melted resin prior to dissolving in 
the solvent. 
The resulting carriers are found to have a longer useful life than prior 
art carriers in the environment of electrophotographic development. 
Because these carriers have a low surface energy, reduced toner filming, 
characterized as "bound toner" is realized. In addition the adherence of 
the film of perfluoro carboxylic acid in resin binder to the carrier 
matrix material reduces flaking, chipping and spalling of the carrier. 
Further, the triboelectric properties of the carriers of this invention are 
such that most toners will be charged with a positive polarity when used 
with these carriers. Even polytetrafluoroethylene (Teflon) and 
polyethylene can be charged positively using the carriers of this 
invention. Since most commercially available toners are less 
electronegative than Teflon or polyethylene they are charged with a 
positive polarity almost without exception. 
With the outstanding triboelectric properties of the carriers of this 
invention and the physical properties mentioned above, namely the low 
surface energy, the low coefficient of friction and the pronounced 
adherence of the perfluoro carboxylic acid film to the carrier matrix 
material these carriers represent a significant improvement over carriers 
which have been previously used. In addition to the long life of the 
carriers themselves the particular combination of properties increases the 
life of the photoconductor used in the electrophotographic process and 
also results in very high quality copies being produced. 
The durability and effectiveness of these carriers is greatly enhanced 
because of the coating of perfluoro carboxylic acid in resin binder upon 
the surface of the carrier matrix material. 
Another advantage of the carriers of this invention is that the toner 
filming or "bound toner" determined using various commercially available 
toners with these carriers is considerably less than in the case of 
carrier particles which have been coated with low surface energy polymers 
such as polyvinylidene fluoride or silicones, which show little if any 
improvement over uncoated carrier matrix material. The low percentage of 
"bound toner" indicates that the carriers of this invention can be used 
much more successfully than previously available carriers since their 
efficiency is not reduced by toner filming upon the carrier particles to 
the same extent as prior art carriers. 
This invention will be better understood by reference to the following 
examples which are intended to illustrate but not to unnecessarily limit 
the scope of the invention, which is defined in the claims appended 
hereto. 
EXAMPLE 1 
A 28 gram quantity of VERSAMID 935 polyamide resin supplied by General 
Mills Company was melted in a beaker at 130.degree. C. To the fluid 
polyamide resin was added 12 grams of perfluorooctanoic acid (PFOA). The 
molten mixture was stirred for 20 minutes, cooled and dissolved in a 
mixture of 600 grams of methylene chloride and 200 grams of n-propanol. 
The resulting solution was stirred until a clear solution was obtained. 
A 3 kilogram quantity of 175 micron average particle size spherical steel 
beads was cleaned with trichloroethane in an ultrasonic cleaner and then 
dried in a fluidized oven at 80.degree. C. The cleaned and dried beads 
were then loaded into a fluidized bed coating apparatus, air was 
introduced at 15 cubic feet per minute and the frequency of the apparatus 
was adjusted to 6,000 rpm. The solution of PFOA-polyamide reaction product 
was pumped through an atomizing spray nozzle. About 500 grams of the 
solution was applied in 60 spray cycles at 5 minute intervals. The 
resulting coated carrier particles were then dried in a fluidized bed 
dryer at 100.degree. C. for 2 hours. 
EXAMPLE 2 
A 4 kilogram quantity of 175 micron average particle size spherical steel 
beads was cleaned with trichloroethane in an ultrasonic cleaner and then 
dried in a fluidized oven at 80.degree. C. The cleaned and dried beads 
were then poured into a solution of 4 g. of perfluorooctanoic acid (PFOA) 
in 800 g. of methanol and stirred for 2 hours. The alcoholic solution was 
then decanted and the resulting PFOA coated carrier particles were washed 
with an equivalent amount of methanol and dried in a fluidized bed oven at 
60.degree. C. for 1 hour. 
EXAMPLE 3 
A quantity of 150 grams of the coated carrier particles of Example 1 was 
mixed with 2.2 grams of a toner containing copolymers of styrene and 
n-butyl methacrylate, maleic modified rosin, polyvinyl stearate and carbon 
black. The mixture was poured into the toning unit of an IBM II 
electrophotographic copier. Zinc oxide coated electrostatic paper was 
charged negatively by means of a corona discharge using a potential of 
5000 volts and exposed through a photographic transparency. The latent 
image was developed by passing through the toning unit. The optical 
density of the copies produced was determined by means of a Macbeth RD-519 
densitometer. 
A curve showing optical density as a function of the running time of the 
IBM II electrophotographic copier was plotted and is shown as Curve A of 
FIG. 1. One hour running time is equivalent to approximately 1,800 copies. 
Using this developer mix excellent copies were produced up to about 200 
hours running time, the equivalent of about 360,000 copies. 
By way of comparison the results using spherical steel beads coated with 
PFOA alone as described in Example 2 are plotted in Curve B of FIG. 1. 
Triboelectric properties of the toner subjected to frictional contact with 
the coated carrier particles of Example 1 were determined in the following 
manner. 
A sheet of toner about 1 mm. thick was formed upon a steel plate by melting 
the toner onto the metal. The resulting toner sheet was then gently rubbed 
in the carrier and the carrier was removed by shaking or lightly vacuuming 
the toner sheet. 
The triboelectric interaction between the toner sheet and carrier deposited 
a surface charge on the toner sheet. This surface charge caused a voltage 
drop across the toner sheet which was then measured with a noncontact 
voltmeter such as a Monroe Electronics "Isoprobe." 
For a sheet of dielectric of area A, thickness t, dielectric constant K, 
and bulk resistivity P, with a surface charge density of +(.sigma..sub.c) 
on the top surface and -(.sigma..sub.c) on the bottom surface the 
equivalent circuit is a capacitor of capacity 
EQU C=K.epsilon..sub.o A/t 
in parallel with a resistance 
EQU R=Pt/A 
with a voltage across the plates of: 
EQU V=(.sigma..sub.c)t/K.epsilon..sub.o 
where .epsilon..sub.o is the permittivity of free space. 
The surface charge density .sigma..sub.c associated with the toner-carrier 
triboelectric interaction was thus easily calculated from the measured 
voltage drop across the toner sheet, the dielectric constant and the 
thickness of the sheet. 
The sheet thickness was determined by measuring the thickness of the sheet 
plus metal plate with calipers and substracting the measured thickness of 
the bare metal plate. The sheet dielectric constant was determined in the 
standard manner by (1) measuring the capacitance of the sheet placed 
between electrodes of known area and, (2) dividing that value by the 
calculated unloaded capacitance of the electrodes separated by a space 
equal to the sheet thickness. 
Toner sheets made by carefully melting the toner powder onto the plate were 
often found to have edges slightly lower than the centers. In order to 
avoid any inaccuracies occasioned by the method of sheet preparation care 
was taken to measure the voltage only in the regions where the thickness 
was uniform and easy to measure. 
Using the above described technique the surface charge density, 
.sigma..sub.c, of the toner, due to triboelectric interaction between the 
toner sheet and the carrier of Example 1, was found to be 
+1.3.times.10.sup.-10 coulomb per square centimeter. 
By way of comparison the surface charge density of this toner was found to 
be -0.4.times.10.sup.-10 coulomb per square centimeter when charged with 
uncoated spherical steel beads and +1.3.times.10.sup.-10 coulomb per 
square centimeter when charged with spherical steel beads coated with PFOA 
alone as described in Example 2. 
EXAMPLE 4 
A quantity of 1500 grams of the coated carrier particles prepared according 
to Example 1 was mixed with 22.5 grams of a toner containing polyamide 
resin, maleic modified rosin, styrene-allyl alcohol copolymer, 
polyethylene, lithium stearate, carbon black and a positive orienting dye. 
The mixture of coated carrier particles and toner was poured into the 
toning unit of an Addressograph Multigraph Model 2000 electrostatic 
copier. Zinc oxide coated electrostatic paper was charged negatively and 
exposed to light from a tungsten filament light bulb through a transparent 
glass target. The latent image was developed by passing through the toning 
unit four times. The optical density of the copies produced was determined 
by means of a Macbeth RD-519 densitometer. 
A curve showing optical density as a function of the running time of the 
Addressograph Multigraph Model 2000 electrostatic copier was plotted and 
is shown as Curve C of FIG. 2. One hour running time is equivalent to 
approximately 1,200 copies. Using this developer mix excellent copies were 
produced up to about 300 hours of running time, the equivalent of about 
360,000 copies. 
By way of comparison, the results using uncoated spherical steel beads are 
plotted in curve D of FIG. 2. It can be seen from this curve that the 
optical density drops off very rapidly to an unacceptable level when 
uncoated carrier particles are used. 
Percentage of bound toner was determined using the analytical technique 
described in Denshi Shashin (Electrophotography) 10, 14 (1971). The 
analytical results were plotted and are shown in Curve E of FIG. 3 which 
shows the percentage of bound toner determined as a function of the 
running time. For comparison purposes the results using uncoated carrier 
particles are shown in Curve F of FIG. 3. 
EXAMPLE 5 
The procedure of Example 4 was followed except that a toner containing 
copolymers of styrene and butyl methacrylate, carbon black and a negative 
orienting dye was used. Positive images having an optical density of 1.1 
were obtained. 
EXAMPLE 6 
The coated carrier particles of Example 1 were mixed in a ratio of 24:1 
with a toner containing polyamide resin, maleic modified rosin, polyallyl 
alcohol, carbon black and a positive orienting dye and ball milled for 
various periods of time in an accelerated test for toner filming. 
The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Time, hr. Bound Toner, % 
______________________________________ 
0.5 0.04 
1 0.08 
4 0.08 
48 0.5 
______________________________________ 
EXAMPLE 7 
A 5 gram quantity of poly(methylvinylether/maleic anhydride) manufactured 
and sold under the trademark GANTREZ 119 by General Aniline & Film 
Corporation was mixed with 100 grams of tetrahydrofuran and 100 grams of 
methyl ethyl ketone in a beaker equipped with a magnetic stirrer. A 
quantity of 1.5 grams of PFOA was added and the mixture was stirred until 
completely dissolved. 
The resulting solution was applied to 2 kilograms of 175 micron average 
particle size spherical steel beads as described in Example 1. 
The surface charge density of various polymers and toners was measured as 
described in Example 4. The results are shown in Table 2. 
TABLE 2 
______________________________________ 
Surface Charge Density 
Polymer or Toner .times. 10.sup.10, coulomb/cm.sup.2 
______________________________________ 
Polytetrafluoroethylene 
+17.5 
Polyethylene +17.5 
Toner of Example 3 
+ 1.31 
Toner of Example 4 
+10.5 
Toner of Example 5 
+ 3.5 
Toner of Example 6 
+ 6.14 
______________________________________ 
EXAMPLE 8 
The procedure of Example 7 was repeated using 5 grams of a polycarbonate 
resin manufactured and sold under the trademark LEXAN by General Electric 
Company, 200 grams of chloroform and 1.5 grams of PFOA. The results are 
shown in Table 3. 
TABLE 3 
______________________________________ 
Surface Charge Density 
Polymer or Toner .times. 10.sup.10 coulomb/cm.sup.2 
______________________________________ 
Polytetrafluoroethylene 
+4.5 
Polyethylene +7.0 
Toner of Example 3 
+3.0 
Toner of Example 4 
+1.8 
Toner of Example 5 
+4.4 
Toner of Example 6 
+4.4 
______________________________________ 
EXAMPLE 9 
The procedure of Example 7 was repeated using 16 grams of a solvent 
solution of a curable dimethyl polysiloxane manufactured and sold under 
the trademark SS-4164 by General Electric Company, 100 grams of acetone, 
100 grams of tetrahydrofuran and 1.5 grams of PFOA. The dimethyl 
polysiloxane was catalyzed by adding 0.5 gram of General Electric catalyst 
SS-4163 C before coating on the steel beads. The results are shown in 
Table 4. 
TABLE 4 
______________________________________ 
Surface Charge Density 
Polymer or Toner .times. 10.sup.10 coulomb/cm.sup.2 
______________________________________ 
Polytetrafluoroethylene 
+1.0 
Polyethylene +8.7 
Toner of Example 3 
+0.3 
Toner of Example 4 
+3.0 
Toner of Example 5 
+2.6 
Toner of Example 6 
+2.2 
______________________________________ 
EXAMPLE 10 
The procedure of Example 7 was repeated using 5 grams of a polyurethane 
resin manufactured and sold under the trademark ESTANE by B. F. Goodrich 
Company, 100 grams of acetone, 100 grams of tetrahydrofuran and 1.5 grams 
of PFOA. The results are shown in Table 5. 
TABLE 5 
______________________________________ 
Surface Charge Density 
Polymer of Toner .times. 10.sup.10 coulomb/cm.sup.2 
______________________________________ 
Polytetrafluoroethylene 
+17.0 
Polyethylene +17.0 
Toner of Example 3 
+ 4.0 
Toner of Example 4 
+12.5 
Toner of Example 5 
+14.0 
Toner of Example 6 
+17.0 
______________________________________ 
It can thus be seen that the carriers of this invention can be used to 
produce good positive copies using a variety of toners. Particularly 
noteworthy are the apparent ability of the carriers of this invention to 
impart a positive charge to most types of toners and the long life of such 
carriers. 
This invention has been described with respect to a limited number of 
specific embodiments. However, it is intended that alternative 
compositions and methods may be used and it is to be understood that this 
invention is not to be limited except in accordance with the claims 
appended hereto.