Note: Descriptions are shown in the official language in which they were submitted.
CA 02214606 1997-08-29
METHOD OF PREVENTING FORMATION OF UNDESIRABLE
BACKGROUND ON ELECTROCOAGULATION PRINTED IMAGES
The present invention pertains to improvements
in the field of electrocoagulation printing. More
particularly, the invention relates to a method of
preventing anode abrasion during electrocoagulation
printing.
In US Patent N° 4,895,629 of January 23, 1990,
Applicant has described a high-speed electrocoagulation
printing method anal apparatus in which use is made of a
positive electrode in the form of a revolving cylinder
having a passivated surface onto which dots of colored,
coagulated colloid representative of an image are
produced. These dots of colored, coagulated colloid are
thereafter contacted with a substrate such as paper to
cause transfer of the colored, coagulated colloid onto
the substrate and thereby imprint the substrate with the
image. As explained in this patent, the positive
electrode is coated with a dispersion containing an
olefinic substance and a metal oxide prior to electrical
energization of the negative electrodes in order to
weaken the adherence of the dots of coagulated colloid to
the positive electrode and also to prevent an
uncontrolled corrosion of the positive electrode. In
addition, gas generated as a result of electrolysis upon
energizing the negative electrodes is consumed by
reaction with the olefinic substance so that there is no
gas accumulation between the negative anal positive
electrodes.
The electrocoagulation printing ink which is
injected into the gap defined between the positive and
CA 02214606 1997-08-29
negative electrodes consists essentially of a. liquid
colloidal dispersion containing an electrolytically
coagulable colloid, a dispersing medium, a soluble
electrolyte and a coloring agent. Where the coloring
agent used is a pigment, a dispersing agent is added for
uniformly dispersing the pigment into the ink. After
coagulation of the colloid, any remaining non-coagulated
colloid is removed from the surface of the positive
electrode, for example, by scraping the surface with a
soft rubber squeegee, so as to fully uncover the colored,
coagulated colloid which is thereafter transferred onto
the substrate. The surface of the positive electrode is
thereafter cleaned by means of a-plurality of rotating
brushes and a cleaning liquid to remove any residual
coagulated colloid adhered to the surface of the positive
electrode.
When a polychromic image is desired, the
negative and positive electrodes, the positive electrode
coating device, ink injector, rubber squeegee and
positive electrode cleaning device are arranged to define
a printing unit and several printing units each using a
coloring agent of different color are disposed in tandem
relation to produce several differently colored images of
coagulated colloid which are transferred at respective
transfer stations onto the substrate in superimposed
relation to provide the desired polychromic image. Alter-
natively, the printing units can be arranged around a
single roller adapted to bring the substrate into contact
with the dots of colored, coagulated colloid produced by
each printing-unit, and the substrate which is in the
form of a continuous web is partially wrapped around the
roller and passed through the respective transfer
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stations for being imprinted with the differently colored
images in superimposed relation. -
Applicant has observed that the metal oxide
used in combination with the olefin substance for coating
the positive electrode causes abrasion and pitting of the
positive electrode so that it is necessary to regrind the
surface of such an electrode after every forty hours of
printing. This of course requires shutdown of the
printing apparatus and removal of the electrode. Where a
positive electrode made of stainless steel or aluminum is
utilized, Fe3+ or A13+ ions are released from. the surface
of the electrode as a result of the abrasion and pitting
thereof. As explained in Applicant's copending Canadian
application No. 2,138,190, these ions crosslink the
colloid contained in the ink, resulting in a viscosity
increase leading to an ultimate gelation of the ink.
Applicant has also observed that the metal
oxide in combination with the olefinic substance retain
on the surface of the positive electrode a film. of ink
which is transferred with the colored, coagulated colloid
onto the substrate during contact with same. Thus, when
black, cyan, magenta and yellow coloring agents are used
to provide a polychromic image, the residual films
containing these coloring agents upon being transferred
onto the substrate in superimposed relation create on the
printed image an undesirable colored background.
It is therefore an object of the present
invention to overcome the above drawbacks and to provide
a method of preventing anode abrasion during
electrocoagulation printing.
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It is another object of the invention to
prevent the formation of undesirable background on
electrocoagulation printed images.
5 In accordance with the present invention, there
is thus provided an electrocoagulation printing method
comprising the steps of:
a) providing a positive electrolytically inert
electrode having a continuous passivated surface moving
at substantially constant speed along a predetermined
path, the passivated surface defining a positive
electrode active surface;
15 b) coating the positive electrode active
surface with an olefinic substance and silica to form on
the surface micro-droplets of olefinic substance
containing the silica;
20 c) forming on the olefin and silica-coated
positive electrode active surface a plurality of dots of
colored, coagulated colloid representative of a desired
image, by elec:trocoagulation of an electrolytically
coagulable colloid present in an e:lectrocoagulation
25 printing ink comprising a liquid colloidal dispersion
containing the electrolytically coagulable colloid, a
dispersing medium, a soluble electrolyte and a coloring
agent; and
30 d) bringing a substrate into contact with the
dots of colored, coagulated colloid to cause transfer of
the colored, coagulated colloid from the positive
electrode active surface onto the substrate and thereby
imprint the substrate with the image.
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It has surprisingly been found, according to
the invention, that by replacing the aforesaid metal
oxide with silica, one eliminates the abrasion and
pitting of the positive electrode, without substantially
affecting passivation, so that the requirement to regrind
the surface of the positive electrode is significantly
reduced by at least 80g. Moreover, since there is no
longer any release of contaminant ions from the surface
of the positive electrode due to abrasion and pitting
thereof, the ink is stable and does not undergo an
undesirable increase in viscosity during
electrocoagulation printing. Thus, there is no longer any
need to utilize two separate inks, that is, a starting
ink and a replenishing ink having different
concentrations of sequestering agent, as proposed in the
aforementioned Canadian application No. 2,138,190, and
one may use only the starting ink which contains a
sequestering agent for complexing other contaminant ions.
In addition, the use of silica in step (b) of
the aforementioned electrocoagulation printing method
enables any remaining ink to be removed from the surface
of the positive electrode by the aforesaid rubber
squeegee, without altering the dots of colored,
coagulated colloid. Thus, there is no longer any need to
apply on the surface of the positive electrode between
steps (c) and (d) a liquid olefinic substance with a view
to preventing formation of undesirable background on the
printed image in step (d), as proposed in Applicant's
copending Canadian patent application No. 2,169,669.
Where a polychromic image is desired, steps
(b), (c) and (d) of the above electrocoagulation printing
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method are repeated several times to define a
corresponding number of printing stages arranged at
predetermined locations along the aforesaid path and each
using a coloring agent of different color, and to thereby
produce several differently colored images of coagulated
colloid which are transferred at the respective transfer
positions onto the substrate in superimposed relation to
provide a polychromic image. It is also possible to
repeat several times steps (a) through (d) to define a
corresponding number of printing stages arranged in
tandem relation and each using a coloring agent of
different color, and to thereby produce several
differently colored images of coagulated colloid which
are transferred at respective transfer positions onto the
substrate in superimposed relation to provide a
polychromic image, the substrate being in the form of a
continuous web which is passed through the respective
transfer positions for being imprinted with the colored
images at the printing stages. Alternatively, the
printing stages defined by repeating several times steps
(a) through (d) can be arranged around a single roller
adapted to bring the substrate into contact with the dots
of colored, coagulated colloid of each printing stage and
the substrate which is in the form of a continuous web is
partially wrapped around the roller and passed through
the respective transfer positions for being imprinted
with the colored images at the printing stages . The last
two arrangements are described in Applicant's US Patent
No. 4,895,629.
When a polychromic image of high definition is
desired, it is preferable to bring an endless non-
extendible belt moving at substantially the same speed as
the positive electrode active surface and having on one
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side thereof a colloid retaining surface adapted to
releasably retain dots of electrocoagulated colloid to
cause transfer of the differently colored images at the
respective transfer positions onto the colloid retaining
surface of such a belt in superimposed relation to
provide a polychromic image, and thereafter bring the
substrate into contact with the colloid retaining surface
of the belt to cause transfer of the polychromic image
from the colloid retaining surface onto the substrate and
to thereby imprint the substrate with the polychromic
image. As explained in Canadian patent application
No. 2, 214, 300 in the name of. the Applicant, filed August
29, 1997, by utilizing an endless non-extendable belt
having a r_olloi<:1 retaining surface such as a porous
surface on which dots of colored, coagulated colloid can
be transferred and by moving such a belt independently of
the positive electrode, from one printing unit to
another, so that the ~Yolloid retaining surface of the
belt contacts the colored, coagulated colloid in
sequence, .Lt is possible to significantly improve the
registration of the dif:Eerently colored images upon their
transfer onto the colloid retaining surface of the belt,
thereby providing a polychromic image of high definition
which can thereafter be transferred onto t:he paper web or
other substrate. For example, use can be made of a belt
comprising a plastic material having a porous coating of
silica.
Accordingly, the present invention also pro
vides, in another aspect thereof, a multicolor electroco
agulation printing method comprising the steps of:
a) providing a positive electrolytically inert
electrode having a continuous passivated surface moving
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CA 02214606 2002-O1-16
at substantially constant speed along a predetermined
path, the passivated surface defining a positive
electrode active surface;
b) coating t:he positive electrode active
surface with an olefinic substance and silica to form on
the surface micro-droplets of olefinic substance
containing the silica;
c) forming on the olefin and silica-coated
positive electrode active surface a plurality of dots of
colored, coagulated colloid representative of a desired
image, by electrocoagulation of an electrolytically
coagulable colloid present in an electrocoagulation
printing ink comprising a liquid colloidal dispersion
containing the el.ectrolytically coagulable colloid, a
dispersing medium, a soluble electrolyte and a coloring
agent;
d) bringing an endless non-extendable belt
moving at substantially the same speed as the positive
electrode and having on one side thereof a colloid
retaining surface adapted to selective=ly retain and
relase dots of electrocoagulated colloid, into contact
with the olefin and silica-coated positive electrode
active surface to cause transfer of the dots of colored,
coagulated colloid from the positive electrode active
surface onto the colloid retaining surface of the belt
and to thereby imprint the colloid retaining surface with
the image;
e} repeating steps (b), (c) and (d) several
times to define a corresponding number of printing stages
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CA 02214606 1997-08-29
arranged at predetermined locations along the path and
each using a coloring agent of different color, and to
thereby produce several differently colored images of
coagulated colloid which are transferred at respective
transfer positions onto the porous surface in
superimposed relation to provide a polychromic image; and
f) bringing a substrate into contact with the
porous surface of the belt to cause transfer of the
polychromic image from the porous surface onto the
substrate and to thereby imprint the substrate with the
polychromic image.
The positive electrode used can be in the form
of a moving endless belt as described in Applicant's US
Patent No. 4,661,222, or in the form of a revolving
cylinder as described in the aforementioned US Patent
Nos. 4,895,629 and 5,538,601. In the later case, the
printing stages are arranged around the positive
cylindrical electrode. Preferably, the positive electrode
active surface and the ink are maintained at a tem-
perature of about 35-60°C, preferably 40°C, to increase
the viscosity of the coagulated colloid in step (c) so
that the dots of colored, coagulated colloid remain
coherent during their transfer in step (d), thereby
enhancing transfer of the colored, coagulated colloid
onto the substrate. For example, the positive electrode
active surface can be heated at the desired temperature
and the ink applied on the heated electrode surface to
cause a transfer of heat therefrom to the ink.
Examples of suitable olefinic substances which
may be used to coat the surface of the positive electrode
in step (b) include unsaturated fatty acids such as
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CA 02214606 1997-08-29
arachidonic acid, linoleic acid, linolenic acid, oleic
acid and palmitoleic acid and unsaturated vegetable oils
such as corn oil, linseed oil, olive oil, peanut oil,
soybean oil and sunflower oil. Oleic acid is particularly
preferred. The olefinic substance is advantageously
applied onto the positive electrode active surface in the
form of an oily dispersion containing the silica as
dispersed phase. Use is preferably made of a precipitated
silica having a surface area of about 100 to 500 m2/g as
measured by the BET (Brunaver-Emmet-Teller) nitrogen
adsorption method. The surface of precipitated silica
contains silanol group (SiOH). A particularly preferred
silica is one having a BET surface area of about 400
m2/g, which is sold by Degussa Corporation under product
No . F'K500LS . The amount of silica may range from about 5
about 30~ by weight, based on the total weight of the
dispersion. A particularly preferred dispersion contains
about 92 . 5 wt. ~ of olefinic substance and about 7. 5 wt. ~
of silica.
The oily dispersion containing the olefinic
substance and the silica is advantageously applied onto
the positive electrode active surface by providing a
distribution roller extending parallel to the positive
cylindrical electrode and having a peripheral coating
comprising an oxide ceramic material, applying the oily
dispersion onto the ceramic coating to form on a surface
thereof a film. of the oily dispersion uniformly covering
the surface of the ceramic coating, the film. of oily
dispersion breaking down into micro-droplets containing
the olefinic substance in admixture with the silica and
having substantially uniform size and distribution, and
transferring the micro-droplets from the ceramic coating
onto the positive electrode active surface. As explained
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in Applicant's US Patent No. 5,449,392 of September 12,
1995, the use of a distribution roller having a,ceramic
coating comprising an oxide ceramic material enables one
to form on a surface of such a coating a film. of the oily
dispersion which uniformly covers the surface of the
ceramic coating and thereafter breaks down into micro-
droplets containing the olefinic substance in admixture
with the silica and having substantially uniform size and
distribution. The micro-droplets formed on the surface of
the ceramic coating and transferred onto the positive
electrode active surface generally have a size ranging
from about 1 to about 5 ~..
A particularly preferred oxide ceramic material
forming the aforesaid ceramic coating comprises a fused
mixture alumina and titania. Such a mixture may comprise
about 60 to about 90 weight ~ of alumina and about 10 to
about 40 weight ~ of titania.
According to a preferred embodiment of the
invention, the oily dispersion is applied onto the
ceramic coating by disposing an applicator roller
parallel to the distribution roller anal in pressure
contact engagement therewith to form a first nip, and
rotating the applicator roller and the distribution
roller in register while feeding the oily dispersion into
the first nip, whereby the oily dispersion upon passing
through the first nip forms a film uniformly covering the
surface of the ceramic coating. The micro-droplets are
advantageously transferred from the distribution roller
to the positive electrode by disposing a transfer roller
parallel to the distribution roller and in contact
engagement therewith to form a second nip, positioning
the transfer roller in pressure contact engagement with
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the positive electrode to form a third riip, and rotating
the transfer roller and the positive electrode in
register for transferring the micro-droplets from the
distribution roller to the transfer roller at the second
nip and thereafter transferring the micro-droplets from
the transfer roller to the positive electrode at the
third nip. Such an arrangement of rollers is described in
the aforementioned US Patent No. 5,449,392.
Preferably, the applicator roller and the
transfer roller are each provided with a peripheral
covering of a resilient materialwhich is resistant to
attack by the olefinic substance, such as a synthetic
rubber material. For example, use can be made of a
polyurethane having a Shore A hardness of about 50 to
about 70 in the case of the applicator roller, or a Shore
A hardness of about 60 to about 80 in the case of the
transfer roller.
When use is made of a positive electrode of
cylindrical configuration rotating at substantially
constant speed about its central longitudinal axis, step
(c) of the above electrocoagulation printing method is
carried out by:
i) providing a plurality of negative electro-
lytically inert electrodes electrically insulated from
one another and arranged in rectilinear alignment to
define a series of corresponding negative electrode
active surfaces disposed in a plane parallel to the
longitudinal axis of the positive electrode and spaced
from the positive electrode active surface by a constant
predetermined gap, the negative electrodes being spaced
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from one another by a distance at least equal .to the
electrode gap;
ii) filling the electrode gap with the afore-
said electrocoagulation printing ink;
iii) electrically energizing selected ones of
the negative electrodes to cause point-by-point selective
coagulation and adherence of the colloid onto the olefin
and silica-coated positive electrode active surface
opposite the electrode active surfaces of the energized
negative electrodes while the positive electrode is
rotating, thereby forming the dots of colored, coagulated
colloid; and
iv) removing any remaining non-coagulated
colloid from the positive electrode active surface.
As explained in US Patent No. 4,895,629,
spacing of the negative electrodes from one another by a
distance which is equal to or greater than the electrode
gap prevents the negative electrodes from undergoing edge
corrosion. On the other hand, coating of the positive
electrode with an olefinic substance and a silica prior
to electrical energization of the negative electrodes
weakens the adherence of the dots of coagulated colloid
to the positive - electrode and also prevents an
uncontrolled corrosion of the positive electrode. In
addition, gas generated as a result of electrolysis upon
energizing the negative electrodes is consumed by
reaction with the olefinic substance so that there is no
gas accumulation between the negative and positive
electrodes.
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Examples of suitable electrolytically inert
metals from which the positive and negative electrodes
can be made are stainless steel, platinum, chromium,
nickel and aluminum. The positive electrode is preferably
made of stainless steel, aluminum or tin so that upon
electrical energization of the negative electrodes,
dissolution of the passive oxide film on such an
electrode generates trivalent ions which then initiate
coagulation of the colloid.
The gap which is defined between the positive
and negative electrodes can range from about 50 ~.~. to
about 100 u, the smaller the electrode gap the sharper
are the dots of coagulated colloid produced. Where the
electrode gap is of the order of 50 ~., the negative
electrodes are preferably spaced from one another by a
distance of about 75 u.
The olefin and silica-coated positive active
surface is preferably polished to increase the adherence
of the micro-droplets onto the positive electrode active
surface, prior to step (c) (ii) . For example, use can be
made of a rotating brush provided with a plurality of
radially extending bristles made of horsehair and having
extremities contacting the surface of the positive
electrode. The friction caused by the bristles contacting
the surface upon rotation of the brush has been found to
increase the adherence of the micro-droplets onto the
positive electrode active surface.
Where the positive cylindrical electrode
extends vertically, step (c)(ii) of the above electro-
coagulation printing method is advantageously carried out
by continuously discharging the ink onto the positive
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electrode active surface from a fluid discharge means
disposed adjacent the electrode gap at a predetermined
height relative to the positive electrode and allowing
the ink to flow downwardly along the positive electrode
active surface, the ink being thus carried by the
positive electrode upon rotation thereof to the electrode
gap to fill same. Preferably, excess ink flowing
downwardly off the positive electrode active surface is
collected and the collected ink is recirculated back to
the fluid discharge means. ,
The colloid generally used is a linear colloid
of high molecular weight, that is, one having a weight
average molecular weight between about 10,000 and about
1,000,000, preferably between 100,000 and 600,000.
Examples of suitable colloids include natural polymers
such as albumin, gelatin, casein and agar, and synthetic
polymers such as polyacrylic acid, polyacrylamide and
polyvinyl alcohol. A particularly preferred colloid is an
anionic copolymer of acrylamide and acrylic acid having a
weight average molecular weight of about 250,000 and sold
by Cyanamid Inc. under the trade mark ACCOSTRENGTH 86.
The colloid is preferably used in an amount of about 6.5
to about 12~ by weight, and more preferably in an amount
of about 7~ by weight, based on the total weight of the
colloidal dispersion. Water is preferably used as the
medium for dispersing the colloid to provide the desired
colloidal dispersion.
The ink also contains a soluble electrolyte and
a coloring agent. Preferred electrolytes include alkali
metal halides and alkaline earth metal halides, such as
lithium chloride, sodium chloride, potassium chloride and
calcium chloride. Potassium chloride is particularly
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preferred. When operating at a temperature of about 35-
60°C, the electrolyte is preferably used in an amount of
about 4.5 to about 10~ by weight, based on. the total
weight of the dispersion. The coloring agent can be a dye
or a pigment. Examples of suitable dyes which may be used
to color the colloid are the water soluble dyes available
from HOECHST such a Duasyn Acid Black for coloring in
black and Duasyn Acid Blue for coloring in cyan, or those
available from RIEDEL=DEHAEN such as Anti-Halo Dye Blue
T. Pina for coloring in cyan, Anti-Halo Dye AC Magenta
Extra VOl Pina for coloring in magenta and-Anti-Halo Dye
Oxonol Yellow N. Pina for coloring in yellow. When using
a pigment as a coloring agent, use can be made of the
pigments which are available from CABOT CORD. such as
Carbon Black Monarch~ 120 for coloring in black, or those
available from HOECHST such as Hostaperm Blue B2G or B3G
for coloring in cyan, Permanent Rubine F6B or L6B for
coloring in magenta and Permanent Yellow DGR or DHG for
coloring in yellow. A dispersing agent is added for
uniformly dispersing the pigment into the ink. Examples
of suitable dispersing agents include the anionic
dispersing agent sold by Boehme Filatex Canada Inc. under
the trade mark CLOSPERSE 25000. The pigment is preferably
used in an amount of about 6.5 to about 15~ by weight,
and the dispersing agent in an -amount of about 0.1 to
about 0.1~ by weight, based on the total weight of the
ink.
After coagulation of the colloid, any remaining
non-coagulated colloid is removed from the positive
electrode active surface, for example, by scraping the
surface with a soft rubber squeegee, so as to fully
uncover the colored, coagulated colloid. Preferably, the
non-coagulated colloid thus removed is collected and
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mixed with the collected ink, and the collected non-
coagulated colloid in admixture with the collected ink is
recirculated back to the aforesaid fluid discharge means.
The optical density of the dots of colored,
coagulated colloid may be varied by varying the voltage
and/or pulse duration of the pulse-modulated signals
applied to the negative electrodes.
According to a preferred embodiment,- step _ ,(d),
is preferably carried out by providing at each transfer
position a pressure roller extending parallel to the
positive cylindrical electrode and pressed thereagainst
to form a nip and permit the pressure roller to be driven
by the positive electrode upon rotation thereof, and
passing the substrate or belt through the nip:
Preferably, the pressure roller is provided with a
peripheral covering of a synthetic rubber material such
as a polyurethane having a Shore A hardness of about 95.
A polyurethane covering with such a hardness has been
found to further improve transfer of the colored,
coagulated colloid from the positive electrode active
surface onto the porous surface of the belt. The pressure
exerted between the positive electrode anal the pressure
roller preferably ranges from about 50 to about 100
kg/cm2. In a particularly preferred embodiment, there
are at least two printing stages each including one such
pressure roller and wherein the pressure rollers are
arranged in pairs with the pressure rollers of each pair
being diametrically opposed to one another. The provision
of two pairs of diametrically opposed pressure rollers
arranged about the positive cylindrical electrode
prevents such an electrode from flexing since the forces
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exerted by the pressure rollers of each pair cancel each
other out.
After step (d), the positive electrode active
surface is generally cleaned to remove therefrom any
remaining coagulated colloid. According to a preferred
embodiment, the positive electrode is rotatable in a
predetermined direction and any remaining coagulated
colloid is removed from the positive electrode active
surface by providing an elongated rotatable brush
extending parallel to the longitudinal axis of the
positive electrode, the brush being provided with a
plurality of radially extending bristles made of
horsehair and having extremities contacting the positive
electrode active surface, rotating the brush in a
direction opposite to the direction of rotation of the
positive electrode so as to cause the bristles to
frictionally engage the positive electrode active
surface, and directing jets of cleaning liquid under
pressure against the positive electrode active surface,
from either side of the brush. In such an embodiment, the
positive electrode active surface and the ink are
preferably maintained at a temperature of about 35-60°C
by heating the cleaning liquid to thereby heat the
positive electrode active surface upon contacting same
and applying the ink on the heated electrode surface to
cause a transfer of heat therefrom to the ink.
Preferably, the electrocoagulation printing ink
contains water as the dispersing medium and the dots of
differently colored, coagulated colloid representative of
the polychromic image are moistened between the
aforementioned steps (e) and (f) so that the polychromic
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image is substantially completely transferred onto the
substrate in step {f).
According to another preferred embodiment, the
substrate is in the form of a continuous web and step (f)
is carried out by providing a support. roller and a
pressure roller extending parallel to the support roller
and pressed. thereagainst to form a nip through which the
belt is passed, the support roller and pressure roller
being driven by the belt upon movement thereof, and
guiding the web so as to pass through the nip between the
pressure roller and the porous surface of the belt for
imprinting the web with the polychromic image.
Preferably, the belt with the porous surface thereof
imprinted with the polychromic image is guided so as to
travel along a path extending in a plane intersecting the
longitudinal axis of the positive electrode at right
angles, thereby exposing the porous surface to permit
contacting thereof by the web. Where the longitudinal
axis of the positive electrode extends vertically, the
belt is preferably guided so as to travel along a
horizontal path with the porous surface facing
downwardly, the support roller and pressure roller having
rotation axes disposed in a plane extending perpendicular
to the horizontal path. Such an arrangement is described
in the aforementioned Canadian application No. 2,214,300.
After step (f) , the porous surface of the belt
is generally cleaned to remove therefrom any remaining
coagulated collaid. According to a preferred embodiment,
any remaining coagulated colloid is removed from the
porous surface of the belt by providing at least one
elongated rotatable brush disposed on the one side of the
belt and at least one support roller extending parallel
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to the brush and disposed on the opposite side of the
belt, the brush and support roller having rotation axes
disposed in a plane extending perpendicular to the belt,
the brush being provided with a plurality of radially
extending bristles made of horsehair and having
extremities contacting the porous surface, rotating the
brush in a direction opposite to the' direction of
movement oi= the belt. so as to cause t:he bristles to
frictionally engage the porous surface while supporting
l0 the belt with the support roller, directing jets of
cleaning liquid under pressure against the porous surface
from each side of the brush and removing the cleaning
liquid with any dislodged coagulated colloid from the
porous surface.
The method of the invention enables one to
eliminate the abrasion and pitting of. the positive
electrode, without substantially affecting passivation,
and to prevent the formation of undesirable background on
the printed image; .
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