Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2002-04-11
2004-06-29
Mayekar, Kishor (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S483000, C204S508000, C101SDIG029
Reexamination Certificate
active
06755950
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to improvements in the field of electrocoagulation printing. More particularly, the invention relates to an electrocoagulation printing method providing an image having enhanced optical density.
In U.S. Pat. No. 4,895,629 of Jan. 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 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 and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined between the positive and 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. Alternatively, 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 stations for being imprinted with the differently colored images in superimposed relation.
Moreover, instead of providing a polychromic image in which the differently colored images are superimposed, it is possible to form on the olefin-coated positive electrode surface a plurality of colored pixels representative of a desired polychromic image, each pixel comprising juxtaposed dots of differently colored, coagulated colloid. To this end, a single positive electrode coating device as well as a single positive electrode cleaning device are utilized and the negative electrodes, ink injector and rubber squeegee are arranged to define a printing unit. The negative electrodes each have a cylindrical configuration with a predetermined cross-sectional dimension. Several printing units are disposed around the positive cylindrical electrode. The printing units each use a coloring of different color so as to form a plurality of dots of differently colored, coagulated colloid on the olefin-coated positive electrode surface, the distance between the negative electrodes of each printing unit being at least three times the cross-sectional dimension of each negative electrode to permit juxtaposition of the dots of differently colored, coagulated colloid, whereby to form the aforesaid pixels. These colored pixels are thereafter transferred from the positive electrode surface onto a substrate at a single transfer station so as to imprint the substrate with the polychromic image. Such an arrangement is described in Applicant's U.S. patent application Ser. No. 09/934,467, now U.S. Pat. No. 6,551,481 the teaching of which is incorporated herein by reference.
The optical density of each dot of colored, coagulated colloid can be varied by varying either the voltage applied to the negative electrodes to energize same or the period of time during which such a voltage is applied. Varying the voltage of selected ones of the negative electrodes causes corrosion of adjacent electrodes. Varying the period of time during which the voltage is applied to the negative electrodes, on the other hand, is limited by a threshold value at which there is an undesirable gas generation at the negative electrodes. For example, in the case of an electrocoagulation printing ink having an electrolytic conductivity of 100 mS at 30° C., this threshold value is 4 microseconds. Thus, if the period of time during which the voltage is applied to the negative electrodes is longer than 4 microseconds, there is an undesirable gas generation at the negative electrodes, which adversely affects the electrical signal and may lead to a complete blocking thereof.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to overcome the above drawbacks and to provide an electrocoagulation printing method enabling one to increase the optical density of the image printed by electrocoagulation.
According to one aspect of the invention, there is 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;
b) coating the positive electrode active surface with an olefinic substance to form on the surface micro-droplets of olefinic substance;
c) forming on the olefin-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 electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent; and
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.
Step (c) of the above method is carried out by:
i) providing a series of negative electrolytically inert electrodes each having a surface, the negative electrodes being electrically insulated from one another and arranged in rectilinear alignment so that the surfaces thereof define a plurality of corresponding negative electrode active surfaces disposed in a plane spaced from the positive electrode active surface by a constant predetermined gap;
ii) filling the electrode gap with the aforesaid electrocoagulation printing ink;
iii) applying to selected ones of the negative electrodes a trigger signal of a 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 electrodes while the positive elec
(Ogilvy Renault)
Elcorsy Technology Inc.
Mayekar Kishor
LandOfFree
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