This invention relates to electrophotographic imaging with liquid
developers, and more specifically to the composition of a liquid developer having
a charge director mixture.
Background of the Invention
Liquid developers are known having a pigment-containing resin or
resin mixture, an inert organic liquid vehicle, and a charge director. This invention
comprises a selection of materials to provide optimum results and is believed
applicable with any inert pigment or coloring matter. The vehicle of this invention
is mineral oil, which is known for use as such a vehicle. The resin is a mixture
of two resins which have been employed or mentioned in prior art as resins for
such a liquid toner, but not in combination. The toner charge director is a combination
including materials which have been employed or mentioned in prior art as charge
directors, but not in the combination of this invention.
U.S. Patent No. 5,047,307 to Landa et al at column 7 discloses mineral
oil as a vehicle for such liquid toners. Similarly, U.S. Patent No. 4,886,729 to
Grushkin et al at column 4 is illustrative of other teachings of such use of mineral
U.S. Patent No. 4,925,763 to Tsubuko et al teaches ionomeric resins
to toner with some blends, and with a separately added lauryl acrylate-acrylic
acid resin in liquid toners. U.S. Patent No. 5,034,299 to Houle et al at column
3 mentions Surlyn (trademark) ionomer resin and blends including that resin as
the resin in such a liquid toner. U.S. Patent No. 4,772,528 to Larson teaches blends
of resins and mentions Surlyn in a list of suitable resins. Similarly, U.S. Patent
No. 4,923,778 to Blair et al at column 4 and U.S. Patent No. 4,707,429 to Trout
at column 4 are illustrative of other teachings of such use of Surlyn resin. This
invention employs a Surlyn resin blended with a Nucrel (trademark) resin, a similar,
but non-ionic resin. Nucrel resin, not combined with Surlyn resin, appears prominently
in the liquid toner compositions disclosed in U.S. 5,019,477 to Felder. Similarly,
Nucrel resin alone is disclosed in the Example of U.S. Patent No. 4,891,286 to
Gibson. The added acrylate-acrylic acid resin of the foregoing patent 4,925,763
has an acid component but in this patent the ionomeric component is mixed with
the pigment and finely devided before being kneaded with the acrylate-acrylic
The foregoing U.S. Patent No. 4,707,429 to Trout teaches the Surlyn
resin, lecithin as a charge control agent, calcium Petronate (trademark) as a charge
control agent, and aluminum tristearate as an additive to such liquid toner compositions.
The foregoing patent 4,772,528 discloses lecithin and calcium petronate and barium
petronate as charge control agents. Lecithin is widely taught as a charge control
agent. U.S. Patent No. 4,897,332 to Gibson teaches the combination of lecithin
and alkylated N-vinyl pyrrolidone in combination as a charge control agent. Further
illustrative teachings of lecithin, calcium Petronate, and aluminum tristearate
appear in U.S. Patent No. 5,066,821 to Houle et al at column 6. The foregoing
U.S. Patent No. 5,047,307 to Landa et al at column 19 is further illustrative of
a use of calcium Petronate. The subject invention employs lecithin, aluminum tristearate
(optional), and calcium Petronate in combination with other material not included
in the foregoing patents.
Disclosure of the Invention
The liquid toner of the present invention comprises mineral oil as
a vehicle, fine particles of a thorough blend of ionomeric resin and a resin which
is an acid form of an ionomeric resin, pigment, aluminum tristearate (optional),
and a separate charge control mixture added to the final mixture of the final
materials. The charge control mixture is lecithin, N-methyl-2-pyrrolidone, and
calcium Petronate as a 10% - 30% solution in mineral oil. This charge control
mixture is selected to optimize speed of recovery of charge properties during continual
Best Mode for Carrying Out the Invention
Sixty-four and eight-tenths (64.8) parts by weight Surlyn 9020, an
ionomeric resin, a trademarked product of Du Pont Co., is mixed with forty-one
and one-tenth (41.1) parts by weight Nucrel 599 resin, a trademarked product of
Du Pont Co., an acid form of resin of otherwise the same structure as the Surlyn
resins. Surlyn 9020 is a sodium ionomer having melt flow index of 1.0 and nominal
density of 0.95. Thirty-five parts by weight of this combination of solids mixture
is mixed with 65 parts by weight of Peneteck (trademark) mineral oil. Alternatively,
Surlyn 7940, closely similar to Surlyn 9020, is employed as above described with
Nucrel 699. Surlyn 7940 is a lithium ionomer having melt flow index of 2.6 and
nominal density of 0.94. This produces a somewhat harder toner.
Peneteck mineral oil is described by the manufacturer as a food grade
white mineral oil. It is highly purified to remove all aromatics and odor producing
impurities. Analysis shows it consists of a mixture of straight chained and branched
alkanes. The straight chain portion consists of about 25% of the total and is predominantly
a mixture of C-14 to C-18 alkanes. The remaining 75% is a mixture of branched
alkanes ranging predominantly from C-16 to C-19 has low volatility.
This mixture is mixed thoroughly as by double planetary mixing or
screw extrusion at elevated temperatures (140-160°C) to a mixture in which the
resins are thoroughly mixed and plasticized with mineral oil, and all of the mineral
oil is completely incorporated into the resulting solid. Where extrusion is employed,
the resulting solid may be extruded into cold water with chopping at the nozzle
of the extruder. This product from the extruder is pellets having the general
appearance of grains of rice. The solid is 21.6% Surlyn resin 14.4% Nucrel resin
and 64% mineral oil. This may be termed the "plasticized product."
This product is ground one step further, if needed, in a high speed
blender. This added step is needed in the case of the double planetary mixer. In
the case of the extruder, the resulting product is in pellets that can go directly
into an attritor.
To this product is added pigments, aluminum tristearate, and additional
of the mineral oil in an amount to bring the total mineral oil content to 80 to
90% by weight. This is size reduced in an attritor for 6 to 14 hours until the
final mixture has volume averaged particle diameter measured using a Shimadzu centrifugal
particle size analyzer of about 1 to 3 µm (microns). Preferred attrition is by
Union Process model 1S attritor. (The attritor has a one gallon (3,78 l) fluid
capacity.) For that attritor, attritor speed may be 200 to 400 rpm. Attritor temperature
is 20 to 70°C. Attritor temperature affects morphology and attritor speed affects
the time to achieve desired particle size. Preferred temperature is 50°C. The resulting
product remains ten to twenty percent solids.
Total solids from the foregoing in each of black and three substractive
colors for color imaging are as follows:
Solids by Weight %
NBS6157 violet dye
Toyo FG7341 cyan pigment
Mobay ER8616 magenta dye
Toyo FG1310 yellow dye
After the attrition is completed, the mixture is diluted with the
mineral oil to two percent solids with stirring. To each of the foregoing color
formulas is added a mixture of lecithin, 2-methyl-N-pyrrolidone, and calcium Petronate
in a 10-30% solution with the mineral oil until conductivity of approximately 50
picomhos/cm is reached.
This mixture is for charge control. It is formulated as follows:
20 grams of lecithin is dissolved in 500 ml of the mineral oil by stirring at room
temperature, after the lecithin is dissolved, 15 ml of N-methyl-2-pyrrolidone is
added with continued stirring. This causes an obvious turbidity in the solution.
Then 20 grams of calcium petronate is added and dissolved with stirring. Finally,
the mixture is centrifuged sufficiently to result in a clear, stable solution
which can be decanted from any residue.
Roughly 2 ml of this charge director is required to charge 100 ml
of the foregoing resin mixture to the 50 picomhos/cm.
The resulting toner is a negative liquid toner providing good-resolution
printing and fixing at relatively moderate temperatures. The mineral oil vehicle
does not vaporize well and is readily condensed, thus permitting use as a toner
without significant environmental disturbance.
Important parameters to selected are transfer efficiency, fuse grade
and optical density. Transfer efficiency is the movement of images from surface
to surface since this is done to move an image from a photoconductor surface to
an intermediate roller where three colors and black are accumulated in registration,
and then to a print roller. Transfer efficiency is measured by direct observation
of extent of transfer in typical operation.
Fuse grade is a measure of permanence of final printing. It is determined
by measuring the resistance of print in a typical operation to rubbing and scratching.
Optical density is measured on a colored block using a standard OD
meter with different filters for different colors.
The foregoing formulas result from a selection balancing good results
for the foregoing properties, as well as the necessary properties for liquid toning.
The charge control agent is selected to maximize recovery speed of
electrical properties as toner is continually circulated is active imaging system.
Since mineral oil is a heavy vehicle, maximization of edge definition may be a
more dominated design objective, and the charge control agent would then differ.
An important property of the charge director is termed "percent recovery."
That is the percent of the original current generator after a high voltage spike
is applied at 90 second intervals. The percent recover is obtained using the following
The sample is placed in a cell with electrode diameters of 25 mm
and a spacing of 1 mm. The voltage applied is 2000 volts for 7 seconds. The current
vs time graph is displayed on an oscilloscope and the current maximum is recorded.
The sample is allowed to "relax" for 1.5 minutes whereupon a second voltage of
2000 volts is applied. The current maximum is again recorded. The ratio of the
second current to the first is the measure of "% recovery."
The electrodes must be cleaned with IPA and allowed to totally dry
The following table illustrates the importance of the presence of
N-methyl-2-pyrrolidone (NM2P) to the % recovery. To a stock solution of 4% lecithin
and 4% calcium Petronate, NM2P was added. The resulting charge director is used
to charge the toner to a 50pmhos/cm level and percent recovery was measured.
% NM2P Added
Some of the data might lead one to speculate that the role of the
NM2P may be nothing more than to select the correct solubility fraction of lecithin
since the addition of NM2P does cause the solution to go cloudy and some lecithin
is centrifuged out. To test that hypothesis a charge director was made up with
Isopar-H (trademark isoparaffin) instead of the mineral oil so that after the
centrifuge step the Isopar paraffin and the NM2P could be removed with evaporation.
Then mineral oil can be added back, which results in a charge director containing
the proper fraction of lecithin but without the presence of NM2P.
The following procedure was followed: 2 grams lecithin was dissolved
with heat in Isopar-H paraffin. 2 grams of calcium Petronate and 1.5 ml NM2P were
added. Stirring was continued in an ice bath to complete the precipitation of
the lecithin. The sample was centrifuged and tested to show the initial percent
recovery. Then the isopar and NM2P were evaporated off and 50 ml mineral oil was
replaced. This sample was tested.
Also, to 25 ml of the mineral oil sample, 0.75 ml of NM2P was added
back therefore the right solubility fraction of lecithin was confirmed.
The surprising result was that the sample which had the NM2P removed
by evaporation did not have a stable conductivity. That is, a significant drop
was observed over the first few hours of the dilution. The following table illustrates
this property. 2 ml of each charge director was added to 100 ml the mineral oil
and the conductivity was monitored with time.
This data suggests that the NM2P may play a role in micelle stabilization.
The following table contains the results from the % recovery testing
of these samples.
TO T=1:30 (current)
Initial sample in Isopar-H
Sample with mineral oil replacement w/o NM2P
Sample with mineral 1.66 oil replacement + NM2P (remained clear)
The solubility of lecithin may contribute to the variation seen between
80 and 90% recovery BUT the presence of the NM2P does more than just select this
solubility fraction. In addition to the % recovery being low for the sample without
the NM2P, a surprising result of conductivity instability was observed. This suggests
that the NM2P plays a role in stabilizing the micelles, that's why the conductivity
drops so drastically without it.
Variations within the scope of the claims of this invention will