Note: Descriptions are shown in the official language in which they were submitted.
CA 02282951 1999-09-15
ELECTROCOAGULATION PRINTING METHOD AND
APPARATUS PROVIDING ENHANCED IMAGE RESOLUTION
The present invention pertains to improvements
s in the field of electrocoagulation printing. More
particularly, the invention relates to an
electrocoagulation printing method and apparatus
providing enhanced image resolution.
~o In US Patent No. 4,895,629 of January 23, 1990,
Applicant has described a high-speed electrocoagulation
printing method and 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,
~S 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
zo 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
2s 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
3o gas accumulation between the negative and positive
electrodes.
The electrocoagulation printing ink which is
injected into the gap defined between the positive and
3s negative electrodes consists essentially of a liquid
colloidal dispersion containing an electrolytically
coagulable colloid, a dispersing medium, a soluble
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CA 02282951 1999-09-15
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
s 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
~o 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.
15 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
zo 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-
z5 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
3o roller and passed through the respective transfer
stations for being imprinted with the differently colored
images in superimposed relation.
The positive electrode which is used for
35 electrocoagulation printing must be made of an
electrolytically inert metal capable of releasing
trivalent ions so that upon electrical energization of
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CA 02282951 1999-09-15
the negative electrodes, dissolution of the passive oxide
film on such an electrode generates trivalent ions which
then initiate coagulation of the colloid. Examples of
suitable electrolytically inert metals include stainless
steels, aluminium and tin.
As explained in Applicant s Canadian patent
No. 2,138,190 of October 13, 1998, a breakdown of passive
oxide films occurs in the presence of electrolyte anions,
~o such as Cl-, Br- and I-, there being a gradual oxygen
displacement from the passive film by the halide anions
and a displacement of adsorbed oxygen from the metal
surface by the halide anions. The velocity of passive
film breakdown, once started, increases explosively in
the presence of an applied electric field. There is thus
formation of a soluble metal halide at the metal surface.
In other words, a local dissolution of the passive oxide
film occurs at the breakdown sites, which releases metal
ions into the electrolyte solution. Where a positive
zo electrode made of stainless steel or aluminium is
utilized in Applicant s electrocoagulation printing
method, dissolution of the passive oxide film on such an
electrode generates Fe3+ or A13+ ions. These trivalent
ions then initiate coagulation of the colloid.
As also explained in Applicant s US Patent
No. 4,895,629, the negative electrodes must be spaced
from one another by a distance which is equal to or
greater than the electrode gap in order to prevent the
3o negative electrodes from undergoing edge corrosion. This
considerably limits the resolution of the image printed
by electrocoagulation so that an image resolution of more
than about 200 lines per inch cannot be obtained.
Applicant has attempted to increase the image
resolution while satisfying the above minimum distance
between the negative electrodes by arranging the
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electrodes along two closely adjacent parallel rows with
the negative electrodes of one row being staggered with
respect to the negative electrodes of the other row. Upon
electrical energization of these electrodes, Applicant
s has observed that there is a grouping between the dots of
coagulated colloid formed opposite the electrode active
surfaces of the energized electrodes of one row and those
formed opposite the electrode active surfaces of the
energized electrodes of the other row, resulting in dots
~o having an elliptical configuration rather than the
desired circular configuration.
It is therefore an object of the present
invention to overcome the above drawbacks and to provide
~5 an improved electrocoagulation printing method and
apparatus enabling one to increase the resolution of the
image printed by electrocoagulation and to obtain an
image resolution as high as 400 lines per inch, or more.
zo According to one aspect of the invention, there
is provided an electrocoagulation printing method
comprising the steps of:
a) providing a positive electrolytically inert
z5 electrode having a continuous passivated surface moving
at substantially constant speed along a predetermined
path, the passivated surface defining a positive
electrode active surface;
3o b) forming on the 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
35 comprising a liquid colloidal dispersion containing the
electrolytically coagulable colloid, a dispersing medium,
a soluble electrolyte and a coloring agent; and
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CA 02282951 2002-09-18
c) bringing a substrate into contact with the
dots of colored, coagulated colloid to cause transfer of
the colored, coagulated colloid from the positive
s electrode active surface onto the substrate and thereby
imprint the substrate with the image;
the improvement wherein step (b) i.s carried out by:
~o i) providing a series of negative
electrolytically inert electrodes each having a surface
covered with a passive c>x:i.de film, the negative
electrodes being e~~ectri.cally insulated from one another
and arranged in rect_linear ~al.ignment so that the
surfaces thereof def.inc~ a plurality of corresponding
negative electrode active surfaces disposed in a plane
spaced from the positive e.lectr°c:~de active :>urface by a
constant predetermined gap, the negative electrodes being
spaced from one another by a distance smaller than the
2o electrode gap;
ii) coating the positive electrode active
surface with an olefinic substance to form on the surface
micro-droplets of olefin.ic substance:
iii) filling the electrode gap with the
aforesaid electrocoagulation printing ink;
iv) applying to the negative electrodes a bias
so voltage ranging from -1.5 to -2.5 vol.r_s;
v) applying to selected ornes of the negative
electrodes a trigger voltage sufficient to energize same
and cause point-by-point selective coagulation and
3s adherence of the colloid onto the olefin-coated positive
electrode active surface opposi.t.e the electrode active
surfaces of the energized electrodes while the positive
- 5 -
CA 02282951 1999-09-15
electrode active surface is moving, thereby forming the
dots of colored, coagulated colloid; and
vi) removing any remaining non-coagulated
colloid from the positive electrode active surface.
According to another aspect of the invention,
there is also provided an electrocoagulation printing
apparatus comprising:
- a positive electrolytically inert electrode
having a continuous passivated surface defining a
positive electrode active surface;
- means for moving the positive electrode
active surface at a substantially constant speed along a
predetermined path;
- means for forming on the positive electrode
2o 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
z5 electrolytically coagulable colloid, a dispersing medium,
a soluble electrolyte and a coloring agent; and
- means for bringing a substrate into contact
with the dots of colored, coagulated colloid to cause
3o transfer of the colored, coagulated colloid from the
positive electrode active surface onto the substrate and
thereby imprint the substrate with the image;
the improvement wherein the means for forming the dots of
35 colored, coagulated colloid comprise:
- 6 -
CA 02282951 2002-09-18
- a series of negative electrolytically inert
electrodes each having a surface covered with a passive
oxide film, the negative elec:troc~es being electrically
insulated from one another and arranged in rectilinear
alignment so that the surfaces thereof define a plurality
of corresponding negative elect:rodr~ active surfaces
disposed in a plane spaced from the positive electrode
active surface by a constant p:rec~c:termined gap, the
negative electrodes beincx spaced from one another by a
~o distance smaller than the electrode gap;
- means for coating the positive electrode
active surface with an olefinic subst<~nce to form on the
surface micro-droplets o.f olefi_nic substance;
- means for filling the electrode gap with the
electrocoagulation pr:~nting ink;
- means for applying to the negative electrodes
2o a bias voltage ranging from -1.5 to -2.5 volts;
- means for applying to selected ones of the
negative electrodes a trigger voltage sufficient to
energize same and cause point-by-point selective
coagulation and adherence of the colloid onto the olefin
coated positive electrode active surface opposite the
electrode active surfaces of the energized electrode
while said positive electrode active surface is moving,
thereby forming the dots of c:alared, ,~oagu:lated colloid;
3o and
- means ior_ :removing any remaining non-
coagulated colloid from the positive electrode active
surface.
Applicant has found quite unexpectedly that by
utilizing negative e:l.ectrolytically inert electrodes each
CA 02282951 2002-09-18
having a surface coated with a passive oxide film and
applying to these electrodes a bias voltage ranging from
-1.5 to -2.5 voles, the negative electrodes can be
positioned closer to one another withaut undergoing edge
s corrosion, thereby permitting the distance between the
electrodes to be smallex than the e=;_ectrode gap. If the
bias voltage is less than 1.5 volts, the passive oxide
film of each elect~zode upon being energized dissolves
into the ink, resu.Lting in a release of metal. ions and
~o formation of edge corrosion. 0n the other hand, if the
bias voltage is hig:~er tan -~ . 5 volts, such a voltage is
sufficient to trigger the electrocoagulation of the
colloid present in thc:: irk orz t~~:e anode. Thus, by
operating with a bias voltage c>f --1,5 to -2.5 volts,
preferably about -2 volts, and by positioning the
negative electrodes ~uff~_ci.ently close to one another, an
image resolution as high as 400 lines per inch, or more,
can be obtained without adverse effect.
~o Preferably, the negative electrodes each have a
cylindrical conf:igurati.on with a circular cross-section
and a diameter :rangi ng ~-rom about: f 0 ~m to about 50 um.
Electrodes having a diameter of about 20 um are
preferred. The gap which is defined between the positive
z5 and negative electrodes can range from about 35 um to
about 100 um, the smaller t_rle electrode gap the sharper
are the dots of coagulated coll;:id produced. Where the
electrode gap is of the order of 50 um, the negative
electrodes are preferably spaced from one another by a
:zo distance of about 30 urr; t:o about 40 ~Zm. On the other
hand, when the elecarode gap is of the order of 35 ~Zm,
the negative electrodes ar_e prefera~:~ 1y spaced from one
another by a distanr_:e of about. ~0 Vim.
:>s Examples of suitable electrolytically inert
metals from which the negative electrodes can be made
include chromium, ruickel, stainless steel and titanium;
g _
CA 02282951 2002-09-18
stainless steel is particularly preferred. The positive
electrode, on the other hand, can be made of stainless
steel, aluminum or l~i.n.
Coating of the positive electrode with an
olefinic substance prior to electrical energization of
the negative electrodes weakens tr~.e adherence of the dots
of coagulated colloid to the positive electrode and also
prevents an uncontrolled corrosion of the positive
to electrode. In addition, ga:~ generat:ed as a result of
electrolysis upon energizing the negative electrodes is
consumed by reacaion with the olefinic substance so that
there is no gas a~:cumu:'~ation between the negative and
positive electrodes. Applicant has found that it is no
n5 longer necessary to admix a meta,~ oxide with the olefin
substance; it is believed that the passive oxide film on
currently available electrode contains sufficient metal
oxide to act as catalyst for the desired reaction.
o Examples of suitable olefinic substances which
may be used to coat the surface of the positive electrode
in step (b) (ii) inc:l.ude unsaturated fatty acids such as
arachidonic acid, ~~.inoleic acid, linolenic acid, oleic
acid and palmitoleic acid and unsaturated vegetable oils
z5 such as corn oil, linseed oil, ol.i~,re oil, peanut oil,
soybean oil and sunflower oil. Oleic acid is particularly
preferred. The micro-droplets formed on the surface of
the positive elE=ctrode act:.ive s~.a:rf<~c:e generally have a
size ranging from about 1 to about 5 um.
~o
The olefin-coated positive active surface is
preferably polished to increase the adherence of the
micro-droplets onto the positive electrode active
surface, prior to step (b) (ii.) . Eor example, use can be
~5 made of a rotating brush provided with a plurality of
radially extending bristles made of riorsehair and having
extremities contact:i.ng the surface of the positive
CA 02282951 1999-09-15
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 a polychromic image is desired, steps (b)
and (c) of the above electrocoagulation printing method
are repeated several times to define a corresponding
number of printing stages arranged at predetermined
~o 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), (b) and (c) 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
zo 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
Zs transfer positions for being imprinted with the colored
images at the printing stages. Alternatively, the
printing stages defined by repeating several times steps
(a), (b) and (c) can be arranged around a single roller
adapted to bring the substrate into contact with the dots
30 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
35 two arrangements are described in US Patent No.
4,895,629.
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When a pr.~lychromic image of high definition is
desired, it is preferable to bring an endless non-
extensible belt moving at substantially the same speed as
the positive electrode active surface and having on one
s side thereof a colloid retaining surface adapted to
releasably retain dots of c~lec:trc:o:oagulated colloid, into
contact with the positive electrode active surface to
cause transfer of i:hne differently colored images at the
respective transfer positions onto the colloid retaining
~o surface of such a belt in superimposed relation to
provide a polychrom.ic _~.mage, and thereafter bring the
substrate into contact with the colloid retaining surface
of the belt to calase to ansfer c>- flue polychromic image
from the colloid retaining surfac°e onto the substrate and
~s to thereby imprint the substrate with the polychromic
image. As e:~plained in Applicant's copending Canadian
patent application No. :'.,214,300 filed August 29, 199'7,
by utilizing an endless non-extensible belt having a
colloid retaining surface such as a.~ porous surface on
zo which dots of colored, coagulated colloid can be
transferred and by moving such a bElt independently of
the positive electrode, from one priruting unit to
another, so that the colloid retaining surface of the
belt contacts the colored, c:oagalated colloid in
zs sequence, it is possib7_e to sir~nif icantly improve the
registration of the difft,rentiy colored images upon their
trans:Eer onto th.e col:Loi~ retain~.rlg :surface of the belt,
thereby providing a polychromic image of high definition
which can thereafter be ~.ransferred onta the paper web or
30 other substrate. For example, ~.zs:= can be made of a belt
comprising a plastic material having a porous coating of
silica.
Accordingly, the present invention also
ss provides, in a furthexv aspect thereof, an improved
multicolor el.ectrocoagulation printing method comprising
the st=eps of
-- 11 -
CA 02282951 2002-09-18
a) providing a positive electrolytically inert
electrode having a continuous passivated surface moving
at substantially constant speed al~~ng a predetermined
path, the passivated surface defining a positive
electrode active surface;
b) forming on the positive electrode active
surface a plurality of dots of colored, coagulated
~o colloid represent,~tive of a desired image, by
electrocoagulation of an electrolytically coagul.able
colloid present in an electroccagu.Lation printing ink
comprising a liquid coll.o:i.dal dispf~rsion containing the
electrolytically coagulable colloid, a dispersing medium,
a soluble electrolyte and a coloring agent;
c) bringing an endless non-extensible belt
moving at substantially the same speed as the positive
electrode active surface and havi.rug on one side thereof a
2o colloid retaining surface adapted to rel.easably retain
dots of electrocoagulated colloid, into contact with the
positive electrode active surface to cause transfer of
the dots of colored, coagu7.ated colloid from the positive
electrode active surface onto the colloid retaining
25 surface of the belt and to thereby imprint the colloid
retaining surface with the image;
d) repeating steps (b) and (c) several times to
define a corresponding n~zmber of printing stages arranged
:~o at predetermined lc~~atiot~s along the path and each using
a coloring agent: of dif:i=event co:l.or, to thereby produce
several differently colored images of coagulated colloid
which are transferred at respec.t.ive transfer positions
onto the colloid retaining surface in superimposed
35 relation to provide a polychromic image; and
- 12 -
CA 02282951 2002-09-18
e) bringing a substrate ~.rnto contact with the
colloid retaining surface of the belt to cause transfer
of the polychromi~~ image from tie colloid retaining
surface onto the substrate <rnd t:o thereby imprint the
s substrate with the polychromic image;
the improvement wherein step (b) is carried out as
defined above.
~o According to yet anothc>.r aspect of the
invention, there is provided an improved
electrocoagulation printing apparatus comprising:
- a positive electrolyt:ically inert electrode having a
t5 continuous passivated surface defivuing a positive
electrode active surface;
- means for moving the positive electrode active surface
at a substantia:Lly c~:onsi:ant speed a:1 ong a predetermined
zo path;
- an endless non-exten~~ible belt raving on one side
thereof a colloid retaining surface adapted to releasably
retain dots of elect~rocoagulated colloid;
?5
- means for moving the belt at substantially the same
speed as the positive electrode active surface;
- a plurality of printing units arranged at predetermined
:~o locations along 'the path, each printing unit comprising:
- means for forming on the positive electrode
active surface a plurality of dots of colored, coagulated
colloid representative of a desired image, by
a5 electrocoagulation of an electrol~;rtically coagulable
colloid present in an electrocoagulation printing ink
comprising a liquid colioi.dal d.i.spersion containing the
- 13 -
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electrolytically coagulable colloid, a dispersion medium,
a soluble electrolyte and a colori..ng ,gent, and
- means for bringing the belt into contact with
s the positive electrode active sur~Fc~ce at a respective
transfer station to cause transfer of the dots of
colored, coagulated colloid from t:hE:;; positive electrode
active surface onto the colloid retain.i.ng surface of the
belt and to imprint the colloid rei:aining surface with
to the image,
whereby to produce several differently colored images of
coagulated colloid which are transferred at the
respective transfer stations onto t~hue colloid retaining
i5 surface in superimposed a°elat.ion t;o provide a polychromic
image; and
- means for bringing a substrate into contact with the
colloid retaining surface of the belt to cause transfer
~o of the polychromic image from the colloid retaining
surface onto the substrate and to thereby imprint the
substrate with the polycr~rom~~c image;
the improvement wherein t:he means for forming the dots of
'S colored, coagulated colloid are as defined above.
The positive electrode t.ased can be in the form
of a moving endless belt as desca:i.bed in Applicant's US
Patent No. 4,661,222, or in the form of a revolving
.so cylinder as described in Applicant's US Patent
Nos. 4,895,629 and 5,533,60:1. In the latter case, the
printing stages or units 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 (b) sc
that the dots of colored, coagulated colloid remain
- 14 -
CA 02282951 2002-09-18
coherent during th~:i.r transfer in step (c), thereby
enhancing transfer of the colc_~r.ed, coagulated colloid
onto the substrate or be:Lt. For example, the positive
electrode active surfac~~ can be heated at the desired
s temperature and the ink applied on the heated electrode
surface to cause a transfer of heat therefrom to the ink.
Where the p~osit:ive cy)...indrical electrode
extends vertically, stem> (b) (i.i'~ cf the above electro-
~o coagulation printing method is advantageously carried out
by continuously discharging the ink onto the positive
electrode active surface°_ f=rcm a fluid discharge means
disposed adjacent the electrode gap ~.t a predetermined
height relative to the positive e~~ectrode and allowing
15 the ink to flow downwardly a~.ong t~~~ pos:itive electrode
active surface, thfe ink being trous carried by the
positive electrode upon rotation thereof to the electrode
gap to fill same. Preferably, excess ink flowing
downwardly of_f the posit::ive electrode active surface is
zo collected and the collected ink is recirculated back to
the fluid discharge means.
The colloid generally used is a linear colloid
of high molecular caeight, that is, one having a weight
25 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 pol.yac~r.yl is acid, polyacrylamide and
:~o polyvinyl alcohcl. A part~i.cularly 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.
Water is preferably used as the medium f_or dispersing the
colloid to provide the desired colloidal dispersion.
- 15 -
CA 02282951 2002-09-18
The in.k also contains a soluble electrolyte and
a coloring agent. Preferred electrolytes include alkali
metal halides and al.kal_~_ne earth met al halides, such as
lithium chloride, sodium chloride, potassium chloride and
s calcium chloride. Potassium ch~L.oride is particularly
preferred. The coloring agent can be a dye or a pigment.
Examples of suitable dyes which may be used to color the
colloid are t:he water soluble dyes available from HOECHST
such as Duasyn Acid Black for coloring in black and
~o Duasyn Acid Blue for col~:or~_ng in cyan, or those available
from RIEDEL-DEHAEN such ass Anti-Halo Dye Blue T. Pina for
coloring in cyan, Anti.-Halo Dye AC Magenta Extra V01 Pina
for coloring in magenta and Anti-Halo Dye Oxonol Yellow
N. Pina for coloring in yellow. When using a pigment as a
o coloring agent, use clan roe made c,>f the pigments which are
available from CABOT CORP. 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 Rubi.ne F6B or h6B for coloring in
zo magenta and Permanent Yellow DGR or DHG for coloring in
yellow. A dispersing agent is added for uniformly
dispersing the pigment into the .irak. Examples of suitable
dispersing agents include the anionic dispersing agent
sold by Boehme Filatex f.anada Inc::. l.xnder the trade mark
2s CLOSPERSE 25000.
After coagulation of the colloid, any remaining
non-coagulated colloid is removed from the positive
electrode active surface, for examp~_e, by scraping the
:.o surface with a soft rubber squeegee, so as to fully
uncover the colored, coagulated col:loi.d. Preferably, the
non-coagulated colloid thus remcwed is collected and
mixed with the collected ink, rind the collected non-
coagulated colloid in admixture with she collected ink is
.a recirculated :back to the aforesaic.~ fluid discharge means.
- 16 -
CA 02282951 2002-09-18
The optical density of trie dots of colored,
coagulated colloid may be vat:~ied by varying the voltage
and/or pulse duration of the pulse-modulated signals
applied to the negative electrode.
After step (c), the positive electrode active
surface is generally cleaned to r<emove therefrom any
remaining coagulated co:Lloid. According to a preferred
embodiment, the positive electrode i.s rotatable in a
io predetermined. direction and any remaining coagulated
colloid is removed froru tre positive electrode active
surface by providing an elongated rotatable brush
extending parallel to the longitudinal axis of the
positive electrode, the brush beir~:g provided with a
is plurality of rad:ially extending bristles made of
horsehair and having extremi.t.ies contacting the positive
electrode active surfa~~e, rotating the brush in a
direction opposite to the direction of rotation of the
positive electrode so as to cause the bristles to
fractionally engage the positive electrode active
surface, and directing jets of c.:Leaning liquid under
pressure against the positive electrode active surface,
from either side of true t:>rus~. zn such~ an embodiment, the
positive electrode actW a surft~cE: and the ink are
;~s preferably maintained at a temperature of about 35-60°C
by heating the cleaning .li<~uic~i to thereby heat the
positive electrode active surface upon contacting same
and applying the ink an true rieat.f,,d c-electrode surface to
cause a transfer of heat therefrom to the ink.
ao
Preferably, the elect.rocoagulati.on printing ink
contains water as the dispersing medium and the dots of
differently colored, coac:tulated colloLd representative of
the polychromic image are moistened between the
:.5 aforementioned steps (d) and (e) so t::.hat the polychromic
image is substantially complete.l.y transferred onto the
substrate in step ( e: ) .
- 17 -
CA 02282951 2002-09-18
Acc;ording to another preferred embodiment, the
substrate is in the form of a continuous web and step (e)
is carried out by providing <~ 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 su~:~port:. rol ~ ez and pressure roller
being driven by the belt upon movement thereof, and
guiding the web so as to pass througri the nip between the
to pressure roller and the porous surface of the belt for
imprinting the web with tie pc~lychromic image.
Preferably, the belt with the porou:~ surface thereof
imprinted with the polyc;hromi.c image is guided so as to
travel along a path extending in a ~:~lane intersecting the
is longitudinal axis of the positive electrode at right
angles, thereby exposin~~ 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
zo horizontal path with the porous surface facing
downwardly, the support r_ol.l.er 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 appl.ic~~tion No. 2,214,300.
~5
After step (e) , t:he porous s»rface of the belt
is generally cleaned to remove therefrom any remaining
coagulated colloid. According to a preferred embodiment,
any remaining coagulated colloid is removed from the
:SO porous surface of the belt by providing at least one
elong<~ted rotatable brusr~ disposed on the one side of the
belt and at least one support roller extending parallel
to the brush and disposed on the opposite side of the
belt, the brush and support roller raving rotation axes
a5 disposed in a plane extending per.penc~icular to the belt,
the brush being provided with a pl!arality of radially
extending bristles made of ho:r:~ehair and having
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CA 02282951 2002-09-18
extremities contacting the porous surface, rotating the
brush in a direction opposite to the direction of
movement of thE: belt; au as to cause the bristles to
frictionally engage the porous surface while supporting
the belt with the support roller, directing jets of
cleaning liquid under pressure against the porous surface
from either side of the brush and rf~moving the cleaning
liquid with any dislodged coagulated colloid from the
porous surface.
Further features and advantages of the
invention will become more readily apparent from the
description of preferred embodiment: as illustrated by
way of examples in the accompanying drawings, in which:
Figure 1 is a fragmentary sectional view of an
electrocoagulation printing apparatus according to a
preferred embodiment of the invention, showing a printing
head with a series of_ nectative electrodes;
zo
Figure 2 is a fragmentary Longitudinal view of
the printing head illustrated in c'ig. 1;
Figure 3 is a tragmentary sectional view of one
.?5 of the negative electrodes i.l.lustratec:~ in fig. 1; and
Figure 4 is a schematic diagram Showing how an
input signal of information is processed to reproduce an
image by electrocoac~ulation of a ~~:ollcoid.
so
Referring first to Fig. 1, there is illustrated
a positive electrode 10 in the f~7rm of a revolving
cylinder and havinc a passivated surface 1? defining a
positive electrode active surface-~ adapted to be coated
s5 with an olefinic substance by means of a positive
electrode coating device= (r?ot s~nowr.) . A device 14 is
provided for discharging an e_c.ectroc.~oagulation printing
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CA 02282951 2002-09-18
ink onto the surface 12. The electrocoagulation printing
ink consists of a colloidal dispersion containing an
electrolytically coagi.rlable colloid, a dispersing medium,
a soluble electrolyte and a coloring agent. A printing
s head 16 having a series of negative electrodes 18 is used
for electrocoagulating the col.loz.d contained in the ink
to form on positive electrode surface 12 dots of colored,
coagulated colloid representative of a desired image. As
shown in Fig. 2, the printing head 16 comprises a
to cylindrical electrode carrier 20 with t:he negative
e_Lectrodes 18 being a Lec:tri cal l y insulated from one
another and arranged in rectilinear alignment along the
length of the el.ect;rode carrier 20 t~o define a plurality
of corresponding negative act..i.ve surfaces 22. The
is printing head 16 is positioned relative to the positive
electrode 10 such that the surfaces 22 of the negative
electrodes 18 are disposed in a plane which is spaced
from the positive electrode surface 12 by a constant
predetermined gap 29. The electrodes 18 are also spaced
zo from one another by a distance smaller than the electrode
gap 24 to increase image resolution. The device 14 is
positioned adjacent the electrode gap 24 to fill same
with the electrocoagulat~on printing ink.
?s As shown in Fig. 3, the negative electrodes 18
each have a cylindrical body 26 made of an
electrolytically inert. metal and covered with a passive
oxide film 28. The end surface of the electrode body 26
covered with such a film defines the aforementioned
:;o negative electrode ~uc:tivE~ surface 22.
Figure 4 is a schematic diagram illustrating
how the negative electrodes 18 are energized in response
to an input signal of informat~_on 30 to form dots of
s5 colored, coagulated colloid represenr_ative of a desired
image. As shown, a bias circuit 32 is provided for
applying to the negative e.lectrcldes 18 a bias voltage
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CA 02282951 2002-09-18
ranging from -1.5 to -2.5 volts. A driver circuit 34 is
also used fo.r addressinG se:Lected ones of the electrodes
18 so as to apply a 'rigger voltage to the selected
electrodes and energise same. :~:uch an electrical
energizing causes point-by-point selective coagulation
and adherence of the colloid onto the olefin-coated
surface 12 of the pos~.t=ive ele:~tr.c:~de 10 opposite the
electrode active surfaces 22 while the electrode 10 is
rotating, thereby forming on the surface 12 a series of
to corresponding dots of colored, coagulated colloid.
A bias voltage within the above range ensures
that there is no dissolu'ion of the passive oxide film 28
into the ink and that there is no accidenta:L triggering
t5 of the electrocoagulation. Such a bias voltage also
enables the electrodes l8 to be spac:~ed from one another
by a distance which is smaller than the electrode gap 24,
thereby providing an image resolution as high as 400
lines per inch, or more.
zo
When it is desired to reproduce a polychromic
image, use is preferably made of a central processing
unit (CPU) for controlling t:he dxiver circuit associated
with each color pri:riting unit.
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