CA 02282128 1999-09-14
STAINLESS STEEL ANODE FOR
ELECTROCOAGULATION PRINTING
The present invention pertains to improvements
s in the field of electrocoagulation printing. More
particularly, the invention relates to an improved anode
for use in an electrocoagulation printing method and
apparatus.
~o In US Patent N° 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,
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
z5 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 02282128 1999-09-14
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-
25 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 02282128 1999-09-14
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 C1-, 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.
Stainless steels are preferred due to their low
cost and availability. These are iron alloys containing a
minimum of approximately 11 wt.% chromium. This amount of
chromium prevents the formation of rust in unpoluted
3o atmospheres. Their corrosion resistance is provided by
the aforesaid passive oxide film which is self-healing in
a wide variety of environments.
The stainless steels hitherto used by the
Applicant consisted of 12 to 20 wt.% Cr, 3 to 10 wt.% Ni,
0.5 to 2.5 wt.% Mo and 0.03 to 0.09 wt.% C, with the
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CA 02282128 1999-09-14
balance consisting of iron and unavoidable impurities.
Although such alloys give satisfactory results in respect
of electrocoagulation, Applicant has observed that they
do not have a hardness sufficient to withstand the harsh
s conditions encountered during cleaning of the positive
electrode, resulting in abrasion and pitting of such an
electrode. It is therefore necessary to regrind the
surface of the electrode after every forty hours of
printing. This, of course, requires shutdown of the
~o printing apparatus and removal of the electrode.
As it is known, many elements other than
chromium are added to iron to provide specific properties
or ease of fabrication. For example, nickel, nitrogen and
~5 molybdenum are added for corrosion resistance; carbon,
nitrogen and titanium for strength; sulfur and selenium
for machinability and nickel for formability and
toughness. Applicant has observed that a stainless steel
with a high carbon content adversely affects passivation.
2o A stainless steel with a high nickel content, on the
other hand, is difficult to clean so that a residual film
of ink containing non-coagulated colloid is left on the
surface of the positive electrode and is transferred with
the colored, coagulated colloid onto the substrate during
2s contacting 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
3o undesirable colored background.
It is therefore an object of the present
invention to overcome the above drawbacks and to provide
an improved stainless steel anode for use in an
35 electrocoagulation printing method and apparatus, that
can be thoroughly cleaned without undergoing abrasion
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CA 02282128 1999-09-14
and/or pitting during cleaning and has an alloy
composition which does not adversely affect passivation.
According to one aspect of the invention, there
s is provided an improved electrocoagulation printing
method comprising the steps of:
a) providing a positive electrolytically inert
electrode having a continuous passivated surface moving
~o at substantially constant speed along 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
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
2o electrolytically coagulable colloid, a dispersing medium,
a soluble electrolyte and a coloring agent; and
c) bringing a substrate into contact with the
dots of colored, coagulated colloid to cause transfer of
Zs 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 positive electrode is made of
3o an iron alloy consisting essentially of:
Cr: at least 20 wt.%
Ni: 5 to 15 wt.%
Si: 1 to 2 wt.%
35 Mn: 0.9 to 1.5 wt.%
C: 0.1 to 0.3 wt.%
balance: iron and unavoidable impurities.
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CA 02282128 1999-09-14
According to another aspect of the invention,
there is also provided an improved electrocoagulation
printing apparatus comprising:
- a positive electrolytically inert electrode
having a continuous passivated surface defining a
positive electrode active surface;
~o - means for moving the positive electrode
active surface at a substantially constant speed along a
predetermined path;
- means for forming on the positive electrode
~5 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
zo 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
z5 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 positive electrode is made of
3o an iron alloy consisting essentially of:
Cr: at least 20 wt.%
Ni: 5 to 15 wt.%
Si: 1 to 2 wt.%
35 Mn: 0.9 to 1.5 wt.%
C: 0.1 to 0.3 wt.%
balance: iron and unavoidable impurities.
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CA 02282128 1999-10-O1
App7_icant has found quite unexpectedly that a
stainless steel anode with the above alloy composition is
sufficiently hard so that it can be thoroughly cleaned
s without undergoing abrasion and/or pitting during
cleaning and that such an alloy composition does not
adversely affect passivation. The stainless steel must
have a chromium content of at least 20 wt.o since, when
the chromium content is lower than 20 wt. o, the passive
~o oxide film does not have sufficiently rapid self-healing
properties and there is a release of undesirable Fe+2
ions. A chromium content ranging between 20 and 30 wt.o
is preferred. The stainless steel must also have a nickel
content within the range of 5 to 15 wt . % since, when the
~s nickel content. is higher than 15 wt.%, the anode cannot
be thoroughly cleaned so that a residual film of ink
containing non-coagulated colloid is left on the surface
of the anode, leading to the formation of undesirable
background on the printed image. On the other hand, when
2o the nickel content is lower than 5 wt.%, the steel is not
sufficiently ductile and corrosion-resistant. A carbon
content within. the range of 0.1 to 0.3 wt. o is essential
since, when tine carbon content is higher than 0.3 wt.%,
passivation is adversely affected and, when the carbon
z5 content is lower than 0.1 wt.%, the steel is not
sufficiently hard. Manganese is an alloying agent added
for providing depassivation initiation sites, whereas
silicon is an alloying agent added for increasing the
resistance to chloride corrosion.
Where a pc>lychromic 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
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
CA 02282128 2002-09-18
which are transferred at: tYu_e ~especti.ve transfer
positions onto the subst rate in uperi;nposed relation to
provide a polyc:-.hrc>rriic image. 1t: is also possible to
repeat several t:imes steps (a), b) and (c) to define a
s corresponding number o_ pri.nt~irug stages arranged in
tandem relation and each using a coloring agent of
different color, and to l:h~-~:ceba produce several
differently colored images of_ c~c:m~gulated colloid which
are transferred at respective transfer positions onto the
~o substrate in superimposed. r_e'':ata..on t:o provide a
polychromic image, the wubst~rate being in the form of a
continuous web which i:; passed thxwugh the respective
transfer positions for Joez.ng impr~:inted with the colored
images at the printing stagfas. Alternatively, the
15 printing stages defined by r_epeat:ing several times steps
(a) , (b) and (c) c,aru be arranged ar:_~und a single roller
adapted to bring the substrate ins:c> c:ontact with the dots
of colored, coagulated colloid of each printing stage and
the substrate which i_s ii: t:he forr~u of a continuous web is
zo partially wr<~pped around the roller and passed through
the respective transfer ~:>ositiorjs for being imprinted
with the colored images at t:rne print W g stages. The last
two arrangements; arE: de.>cribed irn A~>plicant' s US Patent
No. 4,895,629.
zs
When a polychromic image oø~ high definition is
desired, it is preferable to bring an endless non-
extensible belt moving av: substantv:i_a:JLy the same speed as
the positive electrode active: st~r:face and having on one
3o side thereof a cc>.l l o:id reta.~.n:lng surface adapted to
releasably retain dots o:~ electrocoagulated colloid, into
contact with the positive e:Lect:rocJe active surface to
cause transfer of t:he d:ifferentl,r cc>>lored images at the
respective transfer positions onto tine colloid retaining
35 surface of such a belt i.n st.rpez:i.mposed relation to
provide a polychromic image, arc:~ thereafter bring the
substrate into c.ont<~c-~t with tJ-.e colloid retaining surface
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CA 02282128 2002-09-18
of the belt to cause transfer of the polychromic image
from the colloid regaining surface onto the substrate and
to thereby imprint the substrate with the polychromic
image. As explained in Applicant's c:opend:i_ng Canadian
s patent application No. 2,214,300 filed August 29, 1997,
by utilizing are. endless non-extens~ ble belt having a
colloid retaining tsurface such as a porous surface on
which dots of cc~l.orecl, coagulated colloid can be
transferred and by moving such a belt independently of
~o the positive electrode, from cane printing unit to
another, so that the colloid retaining surface of the
belt contacts the colored, coag~rlated colloid in
sequence, it is pc;ssible to sic~ni_fi.cantly improve the
registration of the differently colored images upon their
15 transfer onto the c~~..i..l.oicl ret.aini.ro~ ,urface of the belt,
thereby providing a polychromic i.rnage of high definition
which can thereafter be t: ran:>fer_r~~d onto the paper web or
other substrate. For example, use can be made of a belt
comprising a plastic: rrratr~ri.a1 havu.ng a porous coating of
zo silica.
Accordingly, the present invention also
provides, in a further aspect thereof, an improved
multicolor electrocoagulation pr~.nt;i.ng method comprising
2s the steps of:
a) providing a positive electrolytically inert
electrode having a c:ontp nuous pa.::,sivated surface moving
at substantially constant speed along a predetermined
3o path, the pas sivated si.zrface dev-ining a positive
electrode active surface;
b) forming on thue pos:i.,tivn electrode active
surface a plurality of dots of colored, coagulated
35 colloid representative of; a desired image, by
electrocoagulation of an electrolytically coagulable
collo:i.d present in an electrocos3gLrlation printing ink
_ c~ _
CA 02282128 2002-09-18
comprising a liquid colloidal dispersion containing the
electrolytically coagu_La>r~le c~ol~oid, a dispex:sing medium,
a soluble electrolyte and a c;olori_ng cogent;
c) bringing an endless non-extensible belt
moving at substantial:Ly the Name sptaed as the positive
electrode acting surface and having on one side thereof a
colloid retaining 4~urface adapted t,o releasably retain
dots of electrocoagulatec~ colloid, iruto contact with the
~o positive electrode active surface to cause transfer of
the dots of colored, coagulated cr>llo:~d 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;
d) repeating steps (b) and c) several times to
define a corresponding member o.f pr.in :ing stages arranged
at predetermined locations along the path and each using
a coloring agent of dift::erent_ colcor, to thereby produce
2'o several differently colored image::, <o coagulated colloid
which are transferred at respective transfer positions
onto the colloid retafining s~zrfacie in superimposed
relation to provide a polychromi.c :image; and
~5 e) bringing a sukastrate int=o 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 t::o thereby imprint the
substrate with the polychromic image;
the improvement wherein the positive c:>_lectrode is made of
an iron alloy consisting essentially of:
Cr: at least 20 wt.s
Ni: 5 to 15 wt.
Si: 1 to 2 wt °-
Mn : 0 . ~3 to 1 . 5 wt . °~s
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CA 02282128 2002-09-18
C: 0.1 t0 0.3 wt.'s
balance: iron and unavc>i.dak:>le impurities.
According too yet. another aspect of the
invention, there is prc_~vu ded an improved
electrocoagulation printing apparatus comprising:
- a positive electro.Lyti.cal_Ly inert: elect=rode having a
continuous pass>ivated surface defining a positive
'o electrode active surface;
- means for moving the positive electrode active surface
at a substantial..ly constant speed along a predetermined
path;
- an endless non-extensible be:Lr having on one side
thereof a colloid retaining surface adapted to releasably
retain dots of elect=rocoagu.Lated col__:Lc id;
a:o - means for moving the belts at sub standal7_y the same
speed as the positive electrode active surface;
- a plurality of printinct units arranged at predetermined
locations along the path, each print~_ng unit comprising:
c.5
- means for forming on the positive electrode
active surface a pl;_rrality of dots of colored, coagulated
colloid representative of: a dc.:sired image, by
elect r:ocoagulation of an elect:ro:Lytically coagulable
~o colloid present in an electroco.~gulation painting ink
comprising a liquid colloidal. di:>persion containing the
electrolytically coagulable colloid, ~-~ dispersion medium,
a soluble electrolyte and a colc~ri.rng agent, and
35 - mE=ans fc~r bringing the be:l_t into contact with
the positive electrode active si..irface at a respective
transfer station to cause transferv of the dots of
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CA 02282128 2002-09-18
colored, coagulated colloid from the positive electrode
active surface onto the coll.o id r.~eta:i._ning surface of the
belt and to imprint t:he~ cool. ioid retaining :>urface with
the image,
whereby to produce several c~i.ffex~ent ~ y colored images of
coagulated colloid which are transferred at the
respective transfer stations onto t:rce colloid retaining
surface in superimposed relation to provide a polychromic
~o image; and
- means for bringing a substrate inr_o contact with the
colloid retaining surface of. the belt to cause transfer
of the polychromic: image f rom tr~e~ colloid retaining
t5 surface onto the substrate and to thereby imprint the
substrate with tine polycrromic image
the improvement wherein said electrode is made of an iron
alloy consisting es~~enti~,lly of:
zo
Cr : at least 20 wt . o
Ni : 5 to 15 wt .
Si: 1. to 2 wt:. o
Mn: 0.9 to 1.5 wt.o
z5 C: 0.1 to 0.3 wt.'s
balance: iron and unavoida~>le impurities.
Use is preferably m~~de of an iron alloy
consisting essentially of== ~'.5 to 28 wt.o Cr, 8 to 11 wt.~
o Ni, lto2wt.° Si, (7.9t.o1..5wt.'<:Mnand0.1to0.2
wt.o C, the balance consisting of iron and unavoidable
impurities. A particularly preferred iron alloy consists
essentially of 26. 4 wt . o Cr, 9. 7 wt . '~ Ni, 1 . 08 wt . o Si,
0. 95 wt. o Mn and 0. 1.2 wt:. °, C, the k~alance consisting of
iron and unavoidab.Le impurities. Such an alloy has a
Brinell hardness of about 225. :Its is possible to increase
the Brinell hardness of this ~a.l.:loy up to about 325,
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CA 02282128 2002-09-18
without adversely affect:irug passivation, by subjecting
the alloy after c:ast.ing to a teat treatment at a
temperature of abou ~ 1120 °C ( 205() ° F') and to a subsequent
water quenching. The alloy t.hu.s treatE:d has an
s austenitic-ferritic structure.
The positive e:Lectrode used can be in the form
of a moving endless belt as c~esc:Y-ibed in Applicant's US
Patent No. 4,661,222, or in the form of a revolving
~o cylinder as described in Appl.i.cant's US Patent
Nos. 4,895,629 and 5,533,Fi01.. In the later case, the
printing stages or ur~,its are arrar;ged around t:.he positive
cylindrical electrode. Prefer.abl_y, th~a positive electrode
active surface and the ink are maintained at a tem-
perature of about ..5-60°C, preferably 40°C, to increase
the viscosity of tree coagulated ~::~olloid in step (b) so
that the dots of <~o.l.orec:l, coagulated colloid remain
coherent during their transfer in step (c), thereby
enhancing transfer of t: he col..ored, coag=alated colloid
a:o onto the substrate or belt. For example, the positive
electrode active surface cyan be heated at the desired
temperature and the ink applied on the heated electrode
surface to cause a transfer_ of heat: t:r~nerefrom to the ink.
When use i.s m;~de of a po;>itive electrode of
cylindrical configuration rotating at substantially
constant speed about: its central Longitudinal axis, step
(b) of the above electrocoa.gulation printing method is
carried out by:
i) providing a pluralit:.;r of negative electro-
lytically inert electrodes elec~ric~:~lly insulated from
one another and arranged in rectilinear alignment to
define a series of cc~rrespondi.rlg negative electrode
active surfaces disposed in a plarue parallel to the
longitudinal axis c;f thc> positive electrode and spaced
from the positive e:Lectrode active surface by a constant
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CA 02282128 2002-09-18
predetermined gap, the ruegatzve ~~lec°trodes being spaced
from one another by a distance at least equal to the
electrode gap;
s ii) co<~tir7g tine positive electrode active
surface with an olef.inic sub:>tance to form on the surface
micro-droplets of olefinic substance;
iii) filling the electrode gap with the afore-
~o said electrocoagulat:ion printing ~_rlk;
iv) electrically energi;_:ing selected ones of
the negative electrr_ades to cause point:-by-point selective
coagulation and adherenca~ of t:he col_l.o.id Into the olefin
coated positive e_Lect.rode active surface opposite the
electrode active ;=,urfaces of the energized negative
electrodes while, the p.;~si_t:ive t~ Lectrode i_s rotating,
thereby forming the c.~ots of c-.ol.o.re~d, coagulated colloid;
and
zo
v) removing any rerria:irn ng non-coagulated
colloid from the positive electrode active surface.
As explained in US Patent No. 4,895,629,
2s spacing of the negative e=lectrodes from one another by a
distance which is equal. to or greater than the electrode
gap prevents the negative electrodes 1-rom undergoing edge
corrosion. On the ether hand, coating of the positive
electrode with an o.l.efin:ic substaruc;e prior to electrical
3o energ:i.zation of tre negative elect.rcdes weakens the
adherence of the dots of coagulated colloid to the
positive elec;trc>de and also proven=s an uncontrolled
corrosion of the positive a ec.trode. Tn addition, gas
generated as a resu_Lt of electrolysis upon energizing the
35 negative elec:trcdes i s con:>umed by reaction with the
olefinic substance so that there is no gas accumulation
between the negative and positive. electrodes. Applicant
- 14 -
CA 02282128 2002-09-18
has found that it is no longer necessary to admix a metal
oxide with the olefin subst~anc~:e; it l s believed that the
passive oxide f:ilno on ci.zrrentl~; available electrodes
contains sufficient metal oxide to act as catalyst for
s the desired reaction.
Examples of suitable ele~,trolytically inert
metals from which =he positive and negative electrodes
can be made are stainless steel, platinum, chromium,
~o nickel and aluminum. The gap which is defined between the
posit.i_ve and negat i.ve eler_trode~~ can range from about
50 um to about 100 tern, the smaller true electrode gap the
sharper are the dots of coagu.ated colloid produced.
Where the electrode gap i.s cof t~°ie c::~rder of 50 um, the
~s negative electrodes are the preferably spaced from one
another by a distance of about 75 E.irn.
Examples of su__tab.le olefinic substances which
may be used to coat the :.urface oI thca positive electrode
zo in step (b)(ii) include unsaturated Latty acids such as
arachidonic acid, linoleic acid, linolenic acid, oleic
acid and palmitoleic: acid and un;~~.-3tuxated vegetable oils
such as corn of l, l inseed c:~i:l, c:oli.ve oi7_, peanut oil,
soybean oil and sunflower oil.. Oleic acid is particularly
zs preferred. The micro-droplets of olefinic substance
formed on the surface o:~ the positiT~e electrode active
surface generally have a size ranging from about 1 to
about 5 um.
so The olefin-coat::ed positive active surface is
preferably polished to increase true adherence of the
micro-droplets onto the positive electrode active
surface, prior to step (z~) (:ii) . F~'or example, use can be
made of a rotating brush provided with a plurality of
35 radially extending bristles made of horsehair and having
extremities cont:act~ing the surface of the positive
electrode. The fi:ict=ion caused by the bri_st.les contacting
- 15 -
CA 02282128 2002-09-18
the surface upon rotation of the brush has been found to
increase the adherence of the mi.cr,:>-droplets onto the
positive electrode active surface.
s Where the positive cylindrical electrode
extends vertically, step (b)(ii) of the above electro-
coagulation printinc metr~.od is advantageously
carried out
by continuously discharging the ink onto the positive
electrode active surface: from a f Lr.zid discharge means
~o disposed adjacent the e:,_ectrode chap at a predetermined
height relative to the positive ele~~trode and allowing
the ink to f.Low downwarc:lly a 1 ong the positive electrode
active surface, the ink being thus carried by the
positive electrode upon rotation t hereof to the electrode
~5 gap to fill same. Preferably, excess ink flowing
downwardly off the positive electrode
active surface is
collected and the c:oll.ected i.nk
i..s recirculated back to
the fluid discharge means.
zo The co:Llo_i.d 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 su.ii~able co:Llc>ids incluc:le natural polymers
z5 such as albumin, ge:Latin, casein <:jnd agar, and synthetic
polymers such as ~>olyacrylic acid, polyacrylamide and
polyvinyl alcohol. P. part::icul.arly preferred colloid is an
anionic copolymer of acrylamide and acrylic acid having a
weight average molecular weight of. abe>ut 250,000 and sold
3o by Cyanamid lnc . under i.he t: r_ ade ma r_ k ACCOSTRENGTH 8 6 .
Water is preferably used as t:he medium for dispersing the
colloid to provicie the desired co:l.:~oidal dispersion.
The ink also ccntai.ns a soluble electrolyte and
3s a coloring agent. Preferred electrol.~tes include alkali
metal halides and alkaline earth metal halides, such as
lithium chloride, sodium chloride, potassium chloride and
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CA 02282128 2002-09-18
calcium chloride. Potassium ~hlc>ri~:~e is particularly
preferred. The col.or__ng <-~gemt can be a dye or a pigment.
Examples of suitable dye;: which may ke used t.o color the
colloid are the water soluble dyes avai7_ab.le from HOECHST
s such a Duasyn Acid E37_ack for coloring in black and Duasyn
Acid Blue for coloring in cyan, car those available from
RIEDEL-DEHAEN such a.s P,nti-Halo Dye B7_ue T. Pina for
coloring in cyan, Anti-Halo Dye AC Magenta Extra VO1 Pina
for coloring in magenta and ~?,nti.-Halo Dye Oxonol Yellow
~o N. Pina for coloring in yellow. When ~zsing a pigment as a
coloring agent, use ca:l li>e made of the pigments which are
avail,~ible f rom CABOT CORf . such as O:arbon Black Monarch~
120 for coloring in black, ox tho:~e available from
HOECHST such as Hostaperm E3:Lue Es~G or B3G for coloring
in cyan, Permanent. Rubine ~:~6B ca.r_ L6B for coloring in
magenta and Permanent. Yellow DGR or D1-iG for coloring in
yellow. A dispersung cogent :i~> c~eided for uniformly
dispersing the p=figment into t:he ink. l~,xamples of suitable
dispersing agents i.nc:lude the aniani.c dispersing agent
zo sold by Boehme fi7_atex Canada Tnc. under the trade mark
CLOSPERSE 25000.
After coagulation of the co::l.loid, any remaining
non-coagulated collo:i.d is removed from the positive
z5 electrode active surface, for ex<ample, by scraping the
surface with a soft rubber_ squeegee, so as to fully
uncover the colored, coagu7_ated <~0:17_oid. Preferably, the
non-coagulated c:ol7.oid ',thus rernc>ved is collected and
mixed with the collected ink, and t:ze col:lec:ted non-
3o coagulated colloid in admixture with t:he collected ink is
recirculated back to the aforesaid fluid discharge means.
The optic<~l density of the dots of colored,
coagulated colloid may be varied by varying the voltage
35 and/or pulse duration of the pulse-modulated signals
applied to the negative electrodes.
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CA 02282128 2002-09-18
After step (c), the po:;itive electrode active
surface is generally cleaned t:o remove therefrom any
remaining coagulated colloid. _~<.cording too a preferred
embodiment, 1=he pos it:.ivc~ e:lE~ctr,~~de is rotatable in a
predetermined direca:ion anc~ any remaining coagulated
colloid is removed from t:he ~:~ositi~~re electrode active
surface by providing an ~~lori<~ated rotatable brush
extending parallel to the lonr~it=ac~inal axis of the
positive electrode, the brush k>ei.ng provided with a
plura7_ity o:fv radi ally E~xtE.mdi ruc~~ bristles made of
horsehair and having extremities :=ont.acting the positive
electrode active surface, rotating the brush in a
direction opposite to tkue direct::i.on of rotation of the
positive electrode so as to cJausfe the bristles to
15 frictionally engage the posit.i_ve electrode active
surface, and directing jets of c~_E:~aning liquid under
pressure against the po~~itive eiec:trode active surface,
from either side of the k.~rush. In such an embodiment, the
positive electrode act::i_ve surface and the ink are
zo preferably maintained at a temperature of about 35-60°C
by heating the cleanin:~ liquid to thereby heat the
posit.i_ve electrode active surface upon contacting same
and applying the ink on the heated electrode surface to
cause a transfer of heat therefrom t.o the ink.
Pre:Eerably, the elec:trocoagulation printing ink
contains water as the dispersing med~rum and the dots of
differently colored, coagulated colio::°._d representative of
the polychromic image are rru~ist:ened between the
3o aforementioned steps (d) arid (e) so that the polychromic
image is substanti~l:Ly :ornpletel~~- trvansferred onto the
substrate in step (e).
According to another prc~~fe~rred embodiment, the
substrate is in the form o.f a continucous web and step (e)
is carried out by providing a su~~port roller and a
pressure roller extending parallel. t.c the sub>port roller
- 18 -
CA 02282128 2002-09-18
and pressed thereaga:inst to farm <_~ n i p through which the
belt is passed, the support roller and pressure roller
being driven by the belt: capon movement thereof, and
guiding the web sa as to pass through the nip between the
s pressure roller and the porous s4arv=ace of the belt for
imprinting the web wi~~h the palychromic image.
Preferably, the belt with t:he ~orc.:us surface thereof
imprinted with the pol.ychromic~ image is guided so as to
travel along a path extending in a plane intersecting the
~o longit:udinal axis of the pc~sit:i_ve electrode at right
angles, thereby expasinct the par:aus surface to permit
contacting thereof by the web. Where the longitudinal
axis of the posit:ivE~ electrade e>xtenc'.s vertically, the
belt is preaerably guided so s~s to gravel along a
15 horizantal path wi th the pc:~z-aus surface facing
downwardly, the suppc>rt nolle:r an<~ pressure roller having
rotation axes dispotsed in a plane extending perpendicular
to the horizantal path. Such an <~rrarigement i_s described
in the aforementioned Canadian app!:ication No. 2,214,300.
ao
After step (e) , the porous surface of the belt
is generally cleaned to remove trierefrom any remaining
coagulated colloid. Acco~cdi.ng to ~ p~~eferred embodiment,
any remaining coagu3_ated colloids u::; removed from the
25 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 sl.rpport roller extending parallel
to the brush and disposed on t~hc~ o~:~posite side of the
belt, the brush and sups.>ort r:ol:lfer t,aving rotation axes
3o disposed in a plane extending perpendicular to the belt,
the brush being provident wvwth a pl~..aralit~y of radially
extending bristles mace of har~~ehair and having
extremities contacting the porous surface, rotating the
brush in a di:rect:ion opposite to the direction of
35 movement of the belt so a:~ to cause the bristles to
fricti_onally engage the porous surface while supporting
the belt with the support rol_:l_er, di_rect:ing jets of
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CA 02282128 2002-09-18
cleaning liquid under pressure against the porous surface
from either side of the brush and rE:moving the cleaning
liquid with any dislodged coagulated colloid from the
porous surface.
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