FIELD OF THE INVENTION
-
This invention relates to a recording method and an apparatus
for use in the process of Direct Electrostatic Printing (DEP), in
which an image is created upon a receiving substrate by creating a
flow of toner particles from a toner bearing surface to the image
receiving substrate and image-wise modulating the flow of toner
particles by means of an electronically addressable printhead
structure.
BACKGROUND OF THE INVENTION
-
In DEP (Direct Electrostatic Printing) toner particles are
deposited directly in an image-wise way on a receiving substrate,
the latter not bearing any image-wise latent electrostatic image.
-
This makes the method different from classical electrography, in
which a latent electrostatic image on a charge retentive surface is
developed by a suitable material to make the latent image visible,
or from electrophotography in which an additional step and
additional member is introduced to create the latent electrostatic
image (photoconductor and charging/exposure cycle).
-
A DEP device is disclosed in e.g. US-A-3 689 935. This document
discloses an electrostatic line printer having a multi-layered
particle modulator or printhead structure comprising :
- a layer of insulating material, called isolation layer ;
- a shield electrode consisting of a continuous layer of
conductive material on one side of the isolation layer ;
- a plurality of control electrodes formed by a segmented layer
of conductive material on the other side of the isolation layer ; and
- at least one row of apertures.
-
Each control electrode is formed around one aperture and is
isolated from each other control electrode.
-
Selected electric potentials are applied to each of the control
electrodes while a fixed potential is applied to the shield
electrode. An overall applied propulsion field between a toner
delivery means and a support for a toner receiving substrate
projects charged toner particles through a row of apertures of the
printhead structure. The intensity of the particle stream is
modulated according to the pattern of potentials applied to the
control electrodes. The modulated stream of charged particles
impinges upon a receiving substrate, interposed in the modulated
particle stream. The receiving substrate is transported in a
direction perpendicular to the printhead structure, to provide a
line-by-line scan printing. The shield electrode may face the toner
delivery means and the control electrodes may face the receiving
substrate. A DC-field is applied between the printhead structure and
a single back electrode on the receiving substrate. This propulsion
field is responsible for the attraction of toner to the receiving
substrate that is placed between the printhead structure and the
back electrode.
-
One of the problems with this type of printing devices is that
charged toner particles can accumulate upon the printhead structure
and in the printing apertures. Due to this problem the achievable
printing density does not remain constant in the time, while the
charged toner particles accumulated on the printhead structure may
change the electrical field wherein the charged toner particles are
propelled towards the substrate and the toner particles accumulated
in the printing apertures can physically block the toner passage.
-
Several disclosures concerning devices that can clean up a
printhead structure after it has been smudged with toner particles
are known in the art.
-
In other disclosures, ways and means are disclosed to prevent
the smudging of the printhead structure in the first place. In,
e.g., US-A-4 755 837 and US-A-4 814 796 it is disclosed that the
presence of Wrong Sign Toner (WST) is the main cause of accumulation
of toner particles upon said printhead structure and in the printing
apertures. Wrong sign toner particles are particles that have a sign
different from that of the majority of the particles. Therefore
they respond to the applied electrical fields for creating a flow of
charged toner particles to the substrate in an opposite way than the
majority of the toner particles. In these disclosures it has been
described that the problem of wrong signed toner can be solved when
in a device for direct electrostatic printing the flow of toner
particles towards the substrate originates from the surface of a
conveyer for charged toner particles (hereinafter indicated as
"charged toner conveyer" or CTC) whereon well behaved (i.e. wherein
no wrong sign toner is present) charged toner particles are
deposited by using a magnetic brush comprising two-component
developer.
-
An other way to avoid the presence of wrong signed toner is to
use a magnetic brush with two-component developer in which the toner
particles are charged to a high charge-to-mass ratio (µC/g) for
bringing charged toner particles to the surface of the CTC.
However, a high charge-to-mass ratio leads to a high sticking force
of charged particles to electrode surfaces of opposite polarity so
that printing at high speed with sufficient density becomes
problematic. It is, e.g. indicated in EP-A-811 894 that a higher
charge-to-mass ratio can also lead to an unevenness in image parts
of maximum and moderate density.
-
In US-A-5 337 124 and EP-A-740 224 a toner application module
for electrophotographic and electrographic printing has been
described in which two different magnetic brushes are used, one to
supply toner particles to a charged toner conveying roller and one
recuperate them from said roller. This is a system wherein a
pushing magnetic brush brings the toner particles to the surface of
the CTC and after the surface of the CTC has passed near the
printing apertures a pulling magnetic brush is used to clean the
surface of the CTC.
-
In European application 98202607, filed on August 3, 1998,
European Application 98203008, filed on September 8, 1998 and
European Application 98203873, filed on November 16, 1998 a toner
application module has been described in which a single magnetic
brush and a cleaning blade are used, said one magnetic brush to
supply toner particles to a charged toner conveying roller and said
cleaning blade to recuperate them from said roller. This is a
system wherein the magnetic brush brings the toner particles to the
surface of the CTC and after the surface of the CTC has passed near
the printing apertures a cleaning blade is used to clean the surface
of the CTC. In this configuration charged toner particles are only
applied once to the surface of said charged toner conveyor, and as a
result the charge to mass ratio of the charged toner particles can't
be enhanced by multiple contacts with carrier hairs touching said
charged toner particles multiple times before they are propelled
through said printhead apertures. Since the charge to mass ratio is
not increasing as a function of printing time, the resulting
printing density is not decreasing as a function of printing time.
However, said charged toner particles may not be damaged by the
recuperation process and have to be recycled, making the system
complex and expensive.
-
In all of these prior art applicator designs, the long term
stability of the charge-to-mass ratio during the complete printing
process is not easily achieved in an economical way. Thus there is
still a need for less expensive DEEP devices making it possible to
print at elevated speed with no or very low toner accumulation upon
said printhead structure and with a reliable and constant flow of
well behaved charged toner particles from said toner application
module.
OBJECTS AND SUMMARY OF THE INVENTION
-
It is an object of the invention to provide a DEP device, i.e. a
device for direct electrostatic printing, that can print at high
speed with low clogging of the printing apertures and with high and
constant maximum density over a long period of time.
-
A further object of the invention is to provide an inexpensive
DEP device that can print at high speed with low clogging of the
printing apertures and with high and constant maximum density over a
long period of time.
-
Further objects and advantages of the invention will become
clear from the detailed description herein after.
-
The object of the invention is realised by providing a method
for Direct Electrostatic Printing (DEP) comprising the steps of :
- providing charged toner particles having an average charge to mass
ratio Q1/m on an outer sleeve of a charged toner conveyor,
- adapting said toner particles so that after 30 seconds of printing
said average charge to mass reaches a value of Q2/m, said value
after 1 hour of printing changing by a factor of not more than
1.400,
- creating an electric field between said conveyor and an image
receiving member, for attracting charged said toner particles to
said image receiving member, and
- a printhead structure, placed between said conveyor and an image
receiving member and having printing apertures coupled to control
electrodes coupled to a control voltage, being image wise modulated
for image-wise depositing said charged toner particles on said image
receiving member.
-
In a preferred embodiment, in said step of adapting said toner
particles both a charge control agent (CCA) and a charge limitation
agent (CLA) are incorporated in said toner particles.
BRIEF DESCRIPTION OF THE DRAWINGS
-
Figure 1 shows schematically a DEP device according to the
present invention using toner particles comprising a charge control
agent (CCA) and a charge limitation agent (CLA) in a magnetic brush
applying said charged toner particles to a charged toner conveyor.
DETAILED DESCRIPTION OF THE INVENTION
-
It is known in the art of DEP (direct electrostatic printing),
that toner accumulation on the printhead structure and in the
printing apertures can partially or completely block said printing
apertures, leading to white stripes of missing dots. Adherence of
toner particles can be avoided by using charged toner particles
having a high charge to mass ratio and a low population of wrong
sign toner (WST), i.e. toner particles having a polarity that is
opposite to the polarity of most of the charged toner particles.
-
However, it is known in the literature, that choosing toner
particles with a high charge to mass ratio, lead to lowering the
image density. Moreover, when in a DEP device charged toner
particles are transferred to the surface of a charged toner
conveyor, then it is observed that the charge to mass ratio of the
charged toner particles present upon the charged toner conveyor
(CTC) has the tendency to increase as the printing time increases.
The reason for this is that not all charged toner particles that
pass under the printing apertures are used in the printing so that
some of them are carried back to the place where the surface of the
CTC is loaded with fresh toner, so that the friction between the
toner particles on the surface of the CTC and the fresh toner
particles can increase the charge on the toner particles. This is
especially so when the surface of the CTC is loaded with charged
toner particles from a magnetic brush carrying magnetic carrier
particles and non-magnetic toner particles. The reason for this is
that the carrier hairs of the magnetic brush do further contact the
charged toner particles present upon the surface of the charged
toner conveyor, increasing there charge to mass ratio, unless the
are consumed for image formation and are propelled through said
printing apertures. So, if image parts of low or no image density
frequently occur in an image, then said charged toner particles are
brought in contact with the carrier hairs of said magnetic brush,
multiple times before they are consumed, leading to enhanced charge
to mass ratio and resulting lowered image density.
-
For that reason, it has been described in the literature, to
apply said charged toner particles to said charged toner conveyor
for just a single revolution of said charged toner conveyor, and
removing all the charged toner particles that have not been used in
the printing process before they can contact the carrier hairs of
the magnetic brush for a second time. This concept works perfectly
well, but it requires expensive toner recuperation and recirculation
components, and is very limiting towards the charged
toner particles that may not be destroyed by the recuperation
mechanism.
-
It has been found that said expensive recuperation means (and
resulting partially toner destruction) can be avoided, by using a
toner that is adapted in charging kinetics and charging limits, so
that after 30 seconds of printing the average charge to mass reaches
a value of Q2/m, said value after 1 hour of printing changing by a
factor of not more than 1.400. Preferably said value changes by a
factor of not more than 1.300
-
The use of such toner particles is beneficial in any DEP device
wherein charged toner particles are loaded on a CTC from a toner
source. Thus DEP devices wherein charged toner particles are loaded
on a CTC via a toner dispensing part of a non-magnetic development
system held at a distance of the CTC, wherein charged toner
particles are loaded on a CTC via a toner dispensing part of a non-magnetic
development system held in contact with the CTC, and
devices wherein charged toner particles are loaded on a CTC via a
magnetic brush containing magnetic carrier particles and non-magnetic
carrier particles can all beneficially be operated in a
method according to this invention using toner particles with
adapted charging properties. It is especially beneficial to operate
DEP devices, wherein charged toner particles are loaded on a CTC via
a magnetic brush containing magnetic carrier particles and non-magnetic
carrier particles, in a method according to this invention
using toner particles with adapted charging properties.
-
Said toner particles are adapted via incorporation of charge
control agents (CCA) to obtain very fast charging kinetics, while
their charge to mass ratio is limited to a certain value by
incorporation of charge limitation agents (CLA). The combination of
charge control agents (CCA) and charge limitation agents (CLA)
provide very useful properties: i.e. the toner particles are charged
very rapidly to a minimum charge to mass ratio, and after transfer
of said toner particles to the charged toner conveyor a layer of
charged toner particles is provided with sufficient charge to mass
ratio, so as to prevent toner adhesion to said printhead structure
as a result of wrong sign toner or toner with a much too small
charge to mass ratio. On the other hand, if in an image no or very
low toner consumption is required, then said charged toner particles
remain on the surface of said charged toner conveyor for a long time
during which multiple new contacts with fresh toner particles and/or
with carrier particles of the carrier hairs in the magnetic brush
take place, but due to the charge limiting agents incorporated in
said toner particles, the resulting charge to mass ratio of said
toner particles upon said charged toner conveyor do not exceed the
threshold value. As a result, even without toner recuperation and
recycling unit, constant charge to mass ratio of charged toner
particles upon said charged toner conveyor is obtained, resulting in
image densities which remain constant in time and do not drop as a
function of printing time as in the prior art description which do
not utilise an expensive toner recuperation and recycling unit.
-
The toner particles with adapted charging properties for use in
the method of this invention can further incorporate any ingredient
known in the art, e.g., toner resin, a colorant, metal oxides, etc..
The toner particles with adapted charging properties for use in
the method of this invention can be toner particles with a negative
charge as well as toner particles with a positive charge. When
toner particles with negative charge are needed, charge control
agents for inducing or enhancing a negative chargeability are used.
Charge control agents are well known in the art of preparation of
toner particles, for toner particles with positive charge, mainly
ammonium compounds, pyridinium compounds, triphenylmethane, cationic
dyes, negrosine dyes, etc. are used. for toner particles with
positive charge, mainly metal complexes, phenylsilicates,
naphthylsilicates, azo compounds, cationic polymers, modified
ammonium compounds etc. are used. Charge control agents for
positive charging are commercially available through e.g. Ciba-Geigy
of Switserland under trade name CG 14-146, CG 16-569, BASF of
Germany under trade name NEPTUNSCHWARZ X60, Orient Chemical of Japan
under trade name BONTRON P51, etc. Charge control agents for
negative charging are commercially available through e.g. Clariant
of Germany under trade name NCS LP 2145, NCS VP 2145, COPY CHARGE
NCA, Orient Chemical of Japan under trade name BONTRON E82, BONTRON
S34, BONTRON S44, BONTRON F21, etc.
-
The charge limitation agent (CLA) used for adapting the
charging properties of toner particles used in the method of this
invention are meso-ionic compounds.
-
Meso-ionic compounds as referred to in the present invention
are a group of compounds defined by W. Baker and W.D. Ollis as "5-or
6-membered heterocyclic compounds which cannot be represented
satisfactorily by any one covalent or polar structure and possesses
a sextet of p-electrons in association with the atoms comprising the
ring. The ring bears a fractional positive charge balanced by a
corresponding negative charge located on a covalently attached atom
or group of atoms" as described in Quart. Rev., Vol. 11, p. 15
(1957) and Advances in Heterocyclic Chemistry, Vol. 19, P. 4 (1976).
-
Preferred meso-ionic compounds are those represented
by formula (I):
wherein M represents a 5- or 6-membered heterocyclic ring
composed of at least one member selected from the group consisting
of a carbon atom, an oxygen atom, a sulphur atom and a selenium
atom; and A
- represents -O
-, -S
- or -N
--R, wherein R represents an
alkyl group (preferably having 1 to 6 carbon atoms), a cycloalkyl
group (preferably having 3 to 6 carbon atoms), an alkenyl group
(preferably having 2 to 6 carbon atoms) an alkynyl group (preferably
having 2 to 6 carbon atoms), an aralkyl group, an aryl group
(preferably having 6 to 12 carbon atoms), or a heterocyclic group
(preferably having 1 to 6 carbon atoms).
-
In formula (I), examples of the 5-membered heterocyclic ring as
represented by M include an imidazolium ring, a pyrazolium ring, an
oxazolium ring, an isoxazolium ring, a thiazolium ring, an
isothiazolium ring, a 1,3-dithiol ring, a 1,3,4- or 1,2,3
oxadiazolium ring, a 1,3,2-oxathiazolium ring, a 1,2,3-triazolium
ring, a 1,3,4-triazolium ring, a 1,3,4-, 1,2,3- or 1,2,4-thiadiazolium
ring, a 1,2,3,4-oxatriazolium ring, a 1,2,3,4-tetrazolium
ring and a 1,2,3,4-thiatriazolium ring. Meso-ionic
compounds are known for use in the fixing step of a photographic
process as disclosed in EP-A-431 568. Triazolium thiolate meso-ionic
compounds are well known in silver halide photography and are
used e.g. for increasing temperature latitude as disclosed in
JP-A-60-117240, for reducing fog as disclosed in US-A-4 615 970, in
preparing silver halide emulsions as disclosed in US-A-4 631 253, in
bleach etching baths as disclosed in EP-A-321 839, to prevent
pressure marks as disclosed in US-A-4 624 913, in EP-A-554 585 for
enhancing the printing properties and especially the printing
endurance of a lithographic printing plate according to the DTR-process,
etc.. From these disclosures it can not be inferred that
the use of such compounds in dry toner particles in combination with
a CCA would enhance the charging properties of the toner particles.
-
Preferred meso-ionic compounds for use in toner particles
useful in the method of this invention correspond to the formula :
wherein R
1 and R
2 each independently represents an
unsubstituted or substituted alkyl group, alkenyl group, cycloalkyl
group, aralkyl group, aryl group or heterocyclic group, A represents
an unsubstituted or substituted alkyl group, alkenyl group,
cycloalkyl group, aralkyl group, aryl group, heterocyclic group or -
NR
3R
4 and R
3 and R
4 each independently represents hydrogen, an alkyl
group or aryl group and wherein R
1 and R
2 or R
1 and A or R
3 and R
4
can combine with each other to form a 5- or 6-membered ring.
-
Specific examples of 1,2,4-triazolium-3-thiolates suitable for
use in accordance with the present invention are shown in table 1.
-
Preferably at least 0.5 % by weight of said CLA is present in
the toner particles, more preferably at least 1 % by weight.
-
It is preferred in this invention not-only to prevent the
presence of wrong sign toner and toner with a much too high charge
to mass ratio, but also to use toner particles with a narrow charge
distribution, i.e. the charge of the toner particles shows a
distribution wherein the coefficient of variability (ν), i.e. the
ratio of the standard deviation to the average value, is equal to or
lower than 0.4 preferably lower than 0.3. The charge distribution
of the toner particles is measured by an apparatus sold by Dr. R.
Epping PES-Laboratorium D-8056 Neufahrn, Germany under the name "q-meter.
In, e.g., US-A-5 569 567, US-A-5 622 803 and US-A-5 532 097
it is disclosed how to prepare both negatively and positively
chargeable toner particles with narrow charge distribution. It is a
preferred embodiment of the invention to use toner particles
prepared according to the method described in these disclosures,
that are incorporated herein by reference.
-
The invention thus not only encompasses a method for direct
electrostatic printing, but also dry toner particles wherein a
charge control agent (CCA) and a charge limitation agent (CLA) are
incorporated. Preferably said CLA is a meso-ionic compound as
described above. Preferably at least 0.5 % by weight of said CLA is
present in the toner particles, more preferably at least 1 % by
weight.
-
The invention also encompasses a device for direct electrostatic
printing comprising :
- a charged toner conveyor bearing charged toner particles
applied to it from a toner dispensing part carrying toner particles
that have a charge control agent (CCA) and a charge limitation agent
(CLA),
- one or more voltage sources for creating an electric field between
the conveyor and an image receiving member, for forming a flow of
charged toner particles to the image receiving member, and
- a printhead structure, placed in the flow and having printing
apertures coupled to control electrodes coupled to a control
voltage, being image wise modulated for image-wise depositing toner
particles on the image receiving member.
-
Preferably said toner dispensing part is the sleeve of a
magnetic brush carrying magnetic carrier particles and non-magnetic
toner particles that have a charge control agent (CCA) and a charge
limitation agent (CLA). The CLA is preferably a meso-ionic compound
as described above.
Description of the DEP device
-
A non limitative example of a device according to this invention
is shown in fig 1. It comprises :
- (i) a means for bringing non magnetic charged toner particles
(102a)to the surface of a charged toner conveyer (104), comprising a
container (101) for two component developer (102), with non-magnetic
toner particles (102a) and magnetic carrier particles (102b), a
magnetic brush (103), with a magnetic core (103a) and a non-magnetic
sleeve (103b). Said sleeve is equipped with a means (not shown in
the figure) for rotating said sleeve, in the direction of arrow B so
that the surface of the sleeve has a linear speed LSM. Said sleeve
is coupled to a DC-voltage source DC1 and an AC-voltage source AC1
for jumping charged toner particles upon the surface of said charged
toner conveyer (104) from said two component developer. The charged
toner conveyer (104) is equipped with a means (not shown in the
figure) for rotating said it, in the direction of arrow C, which is
opposite to the direction of rotation of the sleeve of the magnetic
brush, so that the toner bearing surface of it (104) has a linear
speed LSC. The CTC is rotated so that the charged toner particles
on its surface are brought in the development zone where a flow of
charged toner particles (111) can be propelled while the magnetic
brush (103) is located upstream of the development zone.
- (ii) a back electrode (105) coupled to a DC-voltage source DC4,
for maintaining said back electrode at a voltage different from the
voltage (DC1/AC1) applied to the surface of the CTC, for forming an
electrical propulsion field wherein a flow (111) of charged toner
particles is created from the surface of the CTC towards the back
electrode.
- (iii) a printhead structure (106), made from a plastic
insulating film, coated on both sides with a metallic film. The
printhead structure (106) comprises one continuous electrode
surface, hereinafter called "shield electrode" (106b) coupled to a
voltage source DC5 facing in the shown embodiment the CTC and a
complex addressable electrode structure, hereinafter called "control
electrode" (106a) around printing apertures (107) and coupled to a
variable voltage DC3/AC3. Said printhead structure is placed in
said flow of toner particles so that by applying a varying voltage
DC3/AC3 to the control electrode the flow of toner particles towards
the back electrode can be image-wise modulated in the development
zone. Said printing apertures are arranged in an array structure
for which the total number of rows can be chosen according to the
field of application.
- (iv) conveyer means (108) to convey an image receiving member (a
substrate) (109) between said printhead structure (106) and said
back electrode (105) in the direction indicated by arrow A at a
linear speed LSS,
- (v) a spacer means (110), placed upon the front side of said
printhead structure in order to keep the development nip between
said charged toner conveyor and said printhead structure dynamically
constant,
- (vi) means for fixing (112) said toner onto said image
receiving member.
-
-
The location and/or form of the shield electrode (106b) and the
control electrode (106a) can, in other embodiments of a device for a
DEP method using toner particles according to the present invention,
be different from the location shown in fig. 1.
-
Although in fig. 1 an embodiment of a DEP device using two
electrodes (106a and 106b) on printhead 106 is shown, it is possible
to implement a DEP device, using toner particles according to the
present invention using devices with different constructions of the
printhead (106). It is, e.g. possible to implement a DEP device
having a printhead comprising only one electrode structure as well
as a device having a printhead comprising more than two electrode
structures. The apertures in these printhead structures can have a
constant diameter, or can have a broader entrance or exit diameter.
-
The back electrode (105) of this DEP device can also be made to
co-operate with the printhead structure, said back electrode being
constructed from different styli or wires that are galvanically
isolated and connected to a voltage source as disclosed in e.g
US-A-4,568,955 and US-A-4,733,256. The back electrode, co-operating
with the printhead structure, can also comprise one or more flexible
PCB's (Printed Circuit Board).
-
Between said printhead structure (106) and the charged toner
conveyer (104) as well as between the control electrode around the
apertures (107) and the back electrode (105) behind the toner
receiving member (109) as well as on the single electrode surface or
between the plural electrode surfaces of said printhead structure
(106) different electrical fields are applied. In the specific
embodiment of a DEP device according to the present invention, shown
in fig 1. voltage DC1/AC1 is applied to the sleeve of the magnetic
brush 103, voltage DC2/AC2 is applied to the surface of the charged
toner conveyer 104, voltage DC5 to the shield electrode 106b,
voltages DC3/AC30 up to DC3/AC3n for the control electrode (106a).
The value of DC3/AC3 is selected, according to the modulation of the
image forming signals, between the values DC3/AC30 and DC3/AC3n, on
a time basis or grey-level basis. Voltage DC4 is applied to the back
electrode behind the toner receiving member. In other embodiments
of the present invention multiple voltages DC50 to DC5n and/or DC40
to DC4n can be used.
-
The magnetic brush 103 preferentially used in a DEP device
according to the present invention is of the type with stationary
core and rotating sleeve.
-
In a DEP device, according to a preferred embodiment of the
present invention, any type of known carrier particles and toner
particles can successfully be used. It is however preferred to use
"soft" magnetic carrier particles. "Soft" magnetic carrier
particles useful in a DEP device according to a preferred embodiment
of the present invention are soft ferrite carrier particles. Such
soft ferrite particles exhibit only a small amount of remanent
behaviour, characterised in coercivity values ranging from about 4
kA/m up to 20 kA/m (50 up to 250 Oe). Further very useful soft
magnetic carrier particles, for use in a DEP device according to a
preferred embodiment of the present invention, are composite carrier
particles, comprising a resin binder and a mixture of two magnetites
having a different particle size as described in EP-B 289 663. The
particle size of both magnetites will vary between 0.05 and 3 µm.
The carrier particles have preferably an average volume diameter
(dv50) between 10 and 300 µm, preferably between 20 and 100 µm.
More detailed descriptions of carrier particles, as mentioned above,
can be found in EP-A-675 417.
-
It is preferred to use in a DEP device according to the present
invention, toner particles with an absolute average charge over mass
ratio (|q/m|) corresponding to 5 µC/g ≤ |q/m| ≤ 15 µC/g, preferably
to 8 µC/g ≤ |q/m| ≤ 11 µC/g. The charge to mass ratio of the toner
particles is measured by mixing the toner particles with carrier
particles, and after 15 min of charging the q/m-ratio is measured as
described in US-A-5 880 760. Said toner particles were pulled under
vacuum from said CTC-roller to an accurately weighted filter paper
(weight was WP in g), which was shielded in a Faraday cage. The
amount of charge that arrived, after about 5 minutes vacuum pulling
and after an accurate surface area of said CTC-roller was cleaned
from said toner particles, at said filter paper was measured with a
Coulomb meter in µC. The filter paper with the toner particles was
weighted again, giving weight WPT in g. The charge to mass ratio
was then determined as µC/(WPT-WP). In this disclosure the charge
to mass ratio is taken as the absolute value, as a DEP device
according to this invention can function either with negatively
charged toner particles or with positively charged toner particles
depending on the polarity of the potential difference between
DC1/AC1 and DC4. Preferably the toner particles used in a device
according to the present invention have an average volume diameter
(dv50) between 1 and 20 µm, more preferably between 3 and 15 µm.
More detailed descriptions of toner particles, as mentioned above,
can be found in EP A 675 417 that is incorporated herein by
reference.
-
A DEP device making use of the above mentioned marking toner
particles can be addressed in a way that enables it to give black
and white. It can thus be operated in a "binary way", useful for
black and white text and graphics and useful for classical bi-level
half-toning to render continuous tone images.
-
A DEP device according to the present invention is especially
suited for rendering an image with a plurality of grey levels. Grey
level printing can be controlled by either an amplitude modulation
of the voltage DC3/AC3 applied on the control electrode 106a or by a
time modulation of DC3/AC3. By changing the duty cycle of the time
modulation at a specific frequency, it is possible to print
accurately fine differences in grey levels. It is also possible to
control the grey level printing by a combination of an amplitude
modulation and a time modulation of the voltage DC3/AC3, applied on
the control electrode.
-
The combination of a high spatial resolution and of the multiple
grey level capabilities typical for DEP, opens the way for
multilevel half-toning techniques, such as e.g. described in
EP-A-634 862 with title "Screening method for a rendering device
having restricted density resolution". This enables the DEP device,
according to the present invention, to render high quality images.
EXAMPLES
The carrier particles
-
A macroscopic "soft" ferrite carrier consisting of a MgZn-ferrite
with average particle size 50 µm, a magnetisation at
saturation of 36 Tm3/kg (29 emu/g) was provided with a 1 µm thick
acrylic coating. The material showed virtually no remanence.
The toner particles
Comparative toner (CT1)
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The toner used for the experiment had the following
composition : 97 parts of a co-polyester resin of fumaric acid and
bispropoxylated bisphenol A, having an acid value of 18 and volume
resistivity of 5.1 x 1016 ohm.cm was melt-blended for 30 minutes at
110° C in a laboratory kneader with 3 parts of Cu-phthalocyanine
pigment (Colour Index PB 15:3). A resistivity decreasing substance -
having the following formula : (CH3)3N+C16H33 Br- was added in a
quantity of 0.5 % with respect to,the binder, as described in
WO-A-94/027192.
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After cooling, the solidified mass was pulverised and milled
using an ALPINE Fliessbettgegenstrahlmühle type 100AFG (trade name)
and further classified using an ALPINE multiplex zig-zag classifier
type 100MZR (trade name). The average particle size was measured by
Coulter Counter model Multisizer (trade name), was found to be 6.3
µm by number and 8.2 µm by volume. In order to improve the
flowability of the toner mass, the toner particles were mixed with
0.5 % of hydrophobic colloidal silica particles (BET-value 130 m2/g)
and hydrophobic colloidal titaniumoxide particles.
Comparative toner 2 (CT2)
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The second comparative toner was equal to comparative toner 1
(CT1) except for the addition of 3 % by weight of a charge control
agent (CCA), COPY CHARGE NCA, (trade name of Clariant) to the bulk
of the particles.
Comparative toner 3 (CT3)
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The third comparative toner was equal to comparative toner 1
(CT1) except for the addition of 2 % by weight of a charge
limitation agent (CLA), with formula
to the bulk of the toner particles.
Invention toner 1 (IT1)
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The first invention toner was equal to comparative toner 1 (CT1)
except for the addition of 2 % by weight of a charge control agent
(CCA), COPY CHARGE NCA, (trade name of Clariant) and the addition of
0.25 % by weight of a charge limitation agent (CLA), with formula
to the bulk of the toner particles.
Invention toner 2 (IT2)
-
The second invention toner was equal to the first one except for
the addition of 0.5 % by weight of the same charge limitation agent
(CLA).
Invention toner 3 (IT3)
-
The third invention toner was equal to the first one except for
the addition of 1 % by weight of the same charge limitation agent
(CLA).
Invention toner 4 (IT4)
-
The fourth invention toner was equal to the first one except for
the addition of 2 % by weight of the same charge limitation agent
(CLA).
Invention toner 5 (IT5)
-
The fifth invention toner was equal to the fourth one except for
the toner resin, instead of 97 parts of a co-polyester resin of
fumaric acid and bispropoxylated bisphenol A, AG23, an experimental
hybrid resin comprising polyester and polystyrene, provided by KAO
Corp. Of Japan, was used.
Invention toner 6 (IT6)
-
The sixth invention toner was equal to the fourth one except for
the toner resin, instead of 97 parts of a co-polyester resin of
fumaric acid and bispropoxylated bisphenol A, AG11, an experimental
linear polyester, provided by KAO Corp. of Japan,was used.
The developer
-
An electrostatographic developer was prepared by mixing said
mixture of toner particles and colloidal silica in a 5 % ratio
(wt/wt) with silicon coated carrier particles.
The developers were used in a DEP device as described hereinbelow
The printhead structure (106)
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A printhead structure (106) was made from a polyimide film of 50
µm thickness, double sided coated with a 5 µm thick copper film. The
printhead structure (106) had two rows of printing apertures. On the
back side of the printhead structure, facing the image receiving
member, a rectangular shaped control electrode (106a) was arranged
around each aperture. Each of said control electrodes was connected
over 2 MΩ resistors to a HV 507 (trade name) high voltage switching
IC, commercially available through Supertex, USA, that was powered
from a high voltage power amplifier. The printing apertures were
rectangular shaped with dimensions of 360 by 120 µm. The dimension
of the central part of the rectangular shaped copper control
electrodes was 500 by 260 µm. The apertures were spaced so to
obtain a resolution of 33 dots/cm (85 dpi). On the front side of
the printhead structure, facing the charged toner conveyer roller, a
common shield electrode (106b) was arranged around the aperture zone
leaving a free polyimide zone of 1620 µm. Said printhead structure
was fabricated in the following way. First of all the control and
shield electrode pattern was etched by conventional copper etching
techniques. The apertures were made by a step and repeat focused
excimer laser making use of the control electrode patterns as
focusing aid. After excimer burning the printhead structure was
cleaned by a short isotropic plasma etching cleaning. Finally a
thin coating of PLASTIK70, commercially available from Kontakt
Chemie, was applied over the control electrode side of said
printhead structure.
The charged toner conveyer (CTC)
-
The CTC was a cylinder with a sleeve made of aluminium, coated
with TEFLON (trade name of Du Pont, Wilmington, USA) with a surface
roughness of 0.3 µm (Ra-value) and a diameter of 30 mm.
The printing engine
-
Charged toner particles were propelled to this conveyer from a
stationary core (103a)/rotating sleeve (103b) type magnetic brush
(103) comprising two mixing rods and one metering roller. One rod
was used to transport the developer through the unit, the other one
to mix toner with developer.
-
The magnetic brush 103 was constituted of the so called magnetic
roller, which in this case contained inside the roller assembly a
stationary magnetic core (103a), having three magnetic poles with an
open position (no magnetic poles present) to enable used developer
to fall off from the magnetic roller (open position was one quarter
of the perimeter and located at the position opposite to said CTC
(104).
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The sleeve (103b) of said magnetic brush had a diameter of 20 mm and
was made of stainless steel roughened with a fine grain to assist in
transport (Ra=3 µm) and showed an external magnetic field strength
in the zone between said magnetic brush and said CTC of 0.045 T,
measured at the outer surface of the sleeve of the magnetic brush.
The magnetic brush was connected to a DC power supply (DC1) of +140
V. The surface of the charged toper conveyor was positioned at 750
µm from the surface of the magnetic brush, and said surface of said
charged toner conveyor was connected to an AC power supply (AC2)
with a sinusoidally oscillating field of 1800 V (peak to peak) at a
frequency of 3.0 kHz with +100 V DC-offset (DC2). The surface of
said charged toner conveyor was set via PU spacers means at 260 µm
from said printhead structure. The shield electrode was connected
to a DC power supply (DC5) of +100 V. The control electrodes were
connected to a (image-wise selected) DC power source of 0 or +280 V.
The back electrode was placed at 1000 µm from the back side of the
printhead structure and was connected to a DC power supply of +1250
V. The receiving substrate was moved at a linear speed of 3 m/min,
the linear speed of the charged toner conveyor was 6 m/min, and the
linear speed of the magnetic brush was 30 m/min.
PRINTING EXAMPLES 1-9 (PE1-9)
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In these experiments the toner particles as described above (CT1
to CT3 and IT1 to IT6) were used in a developer as described above.
After putting a developer comprising 5 % of said toner particles in
said magnetic brush, the printing engine was started and the charge
to mass ratio of the charged toner particles present upon the sleeve
of the charged toner conveyor was measured at different times. The
data of these measurements are tabulated in table 1. Also indicated
in table 1 is a criterion for drop in maximum printing density. An
"OK" meant that the maximum density after 1 minute of printing and 1
hour of printing did not differ by more than 10 %, when it differed
not more than 20 % the quotation ACC (acceptable was given).
| Measured charge to mass ratio after 30 seconds to 1 hour, and change in maximum printing density between 1 minute and 1 hour. |
| Toner used | 0.5 | 1 | 2' | 5' | 12' | 30' | 60' | Q05/Q60 | DELTA-D |
| CT1 | 9,3 | 12,0 | 12,0 | na | 12,3 | 20,3 | 21,4 | 2.301 | NOK |
| CT2 | 12,4 | 13,6 | 13,7 | na | 14,3 | 16,4 | 17,5 | 1.411 | NOK |
| CT3 | 7,9 | 8,6 | 9,2 | na | 9,8 | 10,6 | 11,5 | 1.456 | NOK |
| IT1 | 12.6 | na | na | 13.0, | na | 15.0 | 19.7 | 1.563 | NOK |
| IT2 | 10.9 | na | na | 12.1 | na | 13.5 | 15.2 | 1.394 | ACC |
| IT3 | 8.4 | na | na | 9.8 | na | 10.0 | 10.5 | 1.250 | OK |
| IT4 | 11,2 | 10,8 | 10,9 | na | 11,3 | 12,3 | 11,2 | 1.000 | OK |
| IT5 | 11.9 | na | na | 10.3 | na | 11.2 | 11.7 | 0.983 | OK |
| IT6 | 9.2 | na | na | 8.8 | na | 8.9 | 10.8 | 1.174 | OK |
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From the data in table 1 it is clear that only the combination
of combining both a charge controlling agent (CCA) enhancing the
charging kinetics, AND a charge limiting agent (CLA) limiting the
maximum charge to mass ratio, an acceptable printing reliability
regarding maximum printing density can be obtained. If no CLA is
used (as in comparative examples CT1 and CT2) the charge to mass
ratio of the charged toner particles present upon the CTC increases
to -21 and -17 µC/g (factor 2.301 and 1.411 higher than starting
value) resulting in considerably reduced maximum image density. If
no CCA is present (as in comparative example CT3) the initial charge
to mass ratio at the beginning of the experiment (and every time the
toner concentration adjusting mechanism adds new toner particles to
the developer) is too low, leading to wrong sign toner, toner
accumulation upon the printhead structure and dusting in the
magnetic brush. Here the charge to mass ratio levels off at -11.5
µC/g but since the initial value is only -7.9 µC/g a change by a
factor of 1.456 is observed between the beginning and end of the
printing sequence.
-
It is only when at least 0.5 of the CLA is present that the
printing results become acceptable and from at least 1 % of CLA on
the results become good.
-
It must be clear for those skilled in the art that many other
implementations of charging and limiting operations can be provided
without departing from the spirit of the present invention.