EP0888222B1

EP0888222B1 - Method and apparatus for rendering an electrocoagulation image water-fast

Info

Publication number: EP0888222B1
Application number: EP97950434A
Authority: EP
Inventors: Adrien Castegnier
Original assignee: Toyo Ink Mfg Co Ltd
Current assignee: Toyo Ink Mfg Co Ltd
Priority date: 1996-12-30
Filing date: 1997-12-26
Publication date: 2002-09-04
Anticipated expiration: 2017-12-26
Also published as: CA2194129C; DE69715169T2; JP2000507177A; CA2194129A1; DE69715169D1; WO1998029256A1; EP0888222A1

Description

The present invention pertains to a method and an apparatus for electrocoagulation printing. More particularly, the invention relates to an improvement in the field of electrocoagulation printing, which includes tehcnique for rendering an electrocoagulation printed image water-fast.

In US-A 4,895,629 , there was described a highspeed 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 ink representative of an image are produced. These dots of colored, coagulated ink are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated ink onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the positive electrode is coated with an oily material prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated ink 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 an olefinic substance which is contained in the oily material 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 solution or a dispersion containing an electrolytically coagulable polymer, a liquid 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 ink, any remaining non-coagulated ink 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 ink which is thereafter transferred onto the substrate. The surface of the positive electrode is thereafter cleaned by means of, for example, a plurality of rotating brushes and a cleaning liquid to remove any residual coagulated ink and oily material adhered to the surface of the positive electrode.

When a polychromic image is desired, the negative and positive electrodes, the oily material 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 ink 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 ink 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.

The document US-A 4 555 320 discloses an electro-coagulation method of a colloid for reproducing an image which includes the steps of chemically setting or hardening the electro-coagulated colloid.

The document EP-A 0 235 700 discloses an electro-coagulation method of a colloid for reproducing an image, wherein the coagulated colloid may be coloured with a dye. The support on which the coagulated colloid may be transferred must be coated with a wetting agent the function of which is to be a good solvent for the dye which, by the solubilizing action of the "wetting agent", is transferred to the wetted support.

The document US-A 4,704,309 discloses a process for printing a substrate with an ink comprising an aqueous system of a colorant and a water dispersible polymer and contacting the ink, after drying, with a solution of one or more multivalent cation salts to impart to the ink a high resistance to water dispersibility.

The document WO 96/18507 discloses an electro-coagulation method of a colloid for reproducing an image wherein metal ions generated in excessive and, hence, undesirable amounts from the positive electrode during electrocoagulation printing are "trapped" and thereby prevented from causing undesirable crosslinking of said colloid during the electro-coagulation step. Said sequestering agent has intended effects in the electrifying step (i. e. in the step of coagulating the colloid by applying an electrical current to the electrolytically coagulable ink).

The document EP-A 776 768 (prior art according to Article 54 (3 + 4) EPC) discloses an electro-coagulation printing method using a specific record sheet wherein the material ((in)organic substances of claims 9, 18 and 27) effecting a crosslink to the coagulum are applied onto the substrate (record sheet) or even contained in the substrate in advance, i. e. before the coagulum is transferred to the substrate (sheet).

The inventor has observed that the colored, coagulated ink which has been transferred onto the substrate is not completely coagulated so that it can be redissolved if water is applied on the substrate. This, of course, is not acceptable for printed material.

It is therefore an object of the present invention to overcome the above drawbacks and to provide a method and an apparatus for rendering an electrocoagulation image waterfast.

In accordance with the present invention, there is provided an electrocoagulation printing method for printing an image on a substrate, comprising the steps of:

electrifying an ink being electrolytically coagulable, with use of a positive electrode and a negative electrode, and electrolytically coagulating the ink between the positive electrode and the negative electrode so as to form a dot of coagulated ink for representing the image;
transferring the dot of coagulated ink onto the substrate to represent the image; and treating the dot of coagulated ink with a crosslinking agent by applying an aqueous solution or dispersion containing the crosslinking agent to the dot of coagulated ink so as to cause crosslink to a crosslinkable component which is contained in the coagulated ink, thereby rendering the dot of coagulated ink water-fast.

Also in accordance with the present invention, there is provided an electrocoagulation printing apparatus for printing an image on a substrate, comprising:

a positive electrode and a negative electrode;
an electrifying device for electrifying an ink being electrolytically coagulable with use of the positive electrode and the negative electrode and electrolytically coagulating the ink between the positive electrode and the negative electrode so as to form a dot of coagulated ink for representing the image;
a transfer mechanism for transferring the dot of coagulated ink onto the substrate to represent the image;
a treatment device for applying a crosslinking agent onto the dot of coagulated ink after the dot is transferred onto the substrate so as to treat the dot of coagulated ink with a crosslinking agent to cause crosslink to a crosslinkable component which is contained in the coagulated ink, thereby rendering the dot of coagulated ink water-fast.

Finally, there is provided, in accordance with the present invention, a printed matter, comprising:

a substrate;
an image represented on the substrate with at least one ink dot which contains a coagulum of an electrolytically coagulable polymer and a multivalent metallic ion; and
a crosslinking agent applied onto the ink dot to cause crosslink to the coagulum.

The features and advantages of the method and the apparatus for electrocoagulation printing according to the present invention over the conventional method and apparatus will be more clearly understood from the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which like reference numerals designate the same or similar elements or sections throughout the figures thereof and in which:

Fig. 1, which does not show the invention but is given for a better understandig thereof, shows a schematic illustration for explanation of an electrocoagulation printing apparatus;

Fig. 2, which does not show the invention but is given for a better understandig thereof, shows a schematic illustration for explanation of another electrocoagulation printing apparatus; and

Fig. 3 shows a schematic illustration for explanation of an embodiment of the electrocoagulation printing apparatus according to the present invention.

The electrocoagulation printing ink contains a coloring agent, an electrolytically coagulable component, a soluble electrolyte, a liquid medium, etc. If the electrocoagulation printing ink is electrified between a positive electrode and a negative electrode, a metallic ion is electrolytically produced from the positive electrode. This metallic ion makes a chemical bond with the electrolytically coagulable component in the printing ink, which causes coagulation, involving the coloring agent, to make a dot of colored and coagulated ink. Namely, the printing ink is electrolytically coagulated by electrifying. Then, by transferring the dots of coagulated ink from the positive electrode onto the substrate so that the transferred dots represent the image, the electrocoagulation printing image is made on a substrate. However, in the presence of water, the dots of coagulated ink on the substrate are often dissolved into the water so that the image on the substrate is damaged.

In researches of the inventor on the coagulated ink, it has surprisingly been found, according to the invention, that by applying to the dots of colored, coagulated ink transferred onto the substrate a crosslinking agent, the printed image can be rendered water-fast. This should be considered because a crosslinkable component in the coagulated ink is further bonded by the crosslinking agent so that the dots of coagulated ink on the substrate are enhanced water-fastness. Specifically, the electrocoagulation printing ink generally contains a polymer as an electrolytically coagulable component, and the polymer makes crosslink with the crosslinking agent applied thereto.

Use can be made of inorganic crosslinking agents such as aluminum chloride, aluminum sulfate, chromic acid, chromic chloride, chromic sulfate, chromium potassium sulfate, ferric chloride, ferric sulfate and potassium permanganate. Aluminum chloride and aluminum sulfate are particularly preferred. Use can also be made of an organic crosslinking agent such as formaldehyde.

According to the present invention, the dots of colored, coagulated ink transferred onto the substrate are treated with the crosslinking agent by applying thereon an aqueous solution or dispersion containing the crosslinking agent. Preferably, the aqueous liquid is applied in the form of a mist. In such an embodiment, the crosslinking agent is preferably present in the aqueous liquid in an amount of about 1 to about 2 % by weight, based on the total weight of the liquid.

According to a prior art process, the dots of colored, coagulated ink were treated with the crosslinking agent by wetting the substrate with an aqueous solution or dispersion containing the crosslinking agent and drying the wet substrate prior to the transferring step so that when the dots of colored, coagulated ink were transferred onto the substrate in the transferring step, the crosslinking agent migrated from the substrate into the colored, coagulated ink to crosslink same.

Also according to the prior art, the dots of colored, coagulated ink were treated with the crosslinking agent by utilizing as substrate newspaper containing a metallic salt as the the crosslinking agent so that when the dots of colored, coagulated ink were transferred onto the newspaper in the transferring step, the crosslinking agent migrated from the newspaper into the colored, coagulated ink to crosslink same. The metallic salt as the crosslinking agent usually present in newspaper was aluminum sulfate, in said prior art.

Where a polychromic image is desired, the electrifying step and the transferring step 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 ink which are transferred at the respective transfer positions onto the substrate in superimposed relation to provide a polychromic image.

The positive electrode used can be in the form of a moving endless belt as described in the inventor's US Patent No. 4,661,222, or in the form of a revolving cylinder as described in the aforementioned US Patent No. 4,895,629 or in the inventor's US Patent No. 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 temperature of about 35-60 °C, preferably 35-45 °C, to increase the conductivity of the ink and the release of metal ions from. the positive electrode active surface into the ink in the electrifying step so that the metal ions cause coagulation of the ink and are released in a quantity sufficient to increase the optical density of the coaulated ink, thereby increasing coagulation efficiency in the electrifying step. It is also possible, by heating to the above-described temperature range, to increase the stickiness and hardness of the coagulated ink so that the dots of colored, coagulated ink remain coherent during their transfer in the transferring step, thereby enhancing transfer of the colored, coagulated ink onto the substrate. For example, the positive electrode active surface can be heated at the desired temperature and the ink applied on the heated positive electrode surface to cause a transfer of heat therefrom to the ink.

When use is made of a positive electrode of cylindrical configuration rotating at substantially constant speed about its central longitudinal axis, the electrifying step of the above electrocoagulation printing method can be suitably carried out through the steps of:

i) providing a plurality of negative electrolytically 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 from one another by a distance at least equal to the electrode gap;

ii) coating the positive electrode active surface with an oily material to form on the surface micro-droplets of the oily material;

iii) filling the electrode gap with the aforesaid electrocoagulation printing ink;

iv) electrically energizing selected ones of the negative electrodes to cause point-by-point selective coagulation and adherence of the ink onto the oily material-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 ink; and

v) removing any remaining non-coagulated ink 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 oily material prior to electrical energization of the negative electrodes weakens the adherence of the dots of coagulated ink to the positive electrode and also prevents an uncontrolled corrosion of the positive electrode. In addition, in a case of the oily material being an olefinic substance, 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.

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 metallic ions, especially multivalent ions, which then cause coagulation of the ink. Particularly, trivalent ions such as ferric ion and aluminum ion are suitable for causing coagulation of the ink.

The gap which is defined between the positive and negative electrodes can range from about 50 µm to about 100 µm, the smaller the electrode gap the sharper are the dots of coagulated ink produced. Where the electrode gap is of the order of 50 µm, the negative electrodes are the preferably spaced from one another by a distance of about 75 µm.

For the oily material which may be used to coat the surface of the positive electrode in the step ii), it is preferred to use olefinic substances. Examples of suitable olefinic substances include unsaturated fatty acids such as 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. A particulaly preferred olefinic substance for composing the oily material mainly contains oleic acid at a ratio of 50 % or more. The olefinic substance may be applied onto the positive electrode active surface in the form of an oily dispersion containing the metal oxide as dispersed phase. Examples of suitable metal oxides include aluminum oxide, ceric oxide, chromium oxide, cupric oxide, iron oxide, magnesium oxide, manganese dioxide, titanium dioxide and zinc oxide; chromium oxide is the preferred metal oxide. Depending on the type of metal oxide used, the amount of metal oxide may range from about 1 to about 50 % by weight, based on the total weight of the dispersion. In a case of using an oily dispersion, a particularly preferred dispersion contains about 75 % by weight of oleic acid or linoleic acid and about 25 % by weight of chromium oxide. Operating at a temperature of about 35-60 °C enables one to lower the concentration of metal oxide in the oily dispersion and thus to reduce wear of the positive electrode active surface.

The oily material containing the olefinic substance 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 material onto the ceramic coating to form on a surface thereof a film of the oily material uniformly covering the surface of the ceramic coating, the film of oily material breaking down into micro-droplets having substantially uniform size and distribution; and transferring the micro-droplets from the ceramic coating onto the positive electrode active surface. If the oily material completely covers throughout the surface of the positive electrode, the positive electrode is insulated so that electrifying is made substantially impossible. Therefore, the oily material is suitably applied in a form of micro-droplets. As explained in the inventor's US-A 5,449,392, 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 material which uniformly covers the surface of the ceramic coating and thereafter breaks down into micro-droplets 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 µm.

A particularly preferred oxide ceramic material forming the aforesaid ceramic coating comprises a fused mixture of 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 material is applied onto the ceramic coating by disposing an applicator roller parallel to the distribution roller and 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 material into the first nip, whereby the oily material 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 the positive electrode to form a third nip, 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-A 5,449,392.

Preferably, the applicator roller and the transfer roller are each provided with a peripheral covering of a resilient material which is resistant to attack by the oily material, 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.

In some instances, depending on the type of oily material used, the inventor has noted that the film of oily material only partially breaks down on the surface of the ceramic coating into the desired micro-droplets. Thus, in order to ensure that the film of oily material substantially completely breaks on the ceramic coating into micro-droplets having substantially uniform size and distribution, the step ii) of the electrocoagulation printing method of the invention is preferably carried out by providing first and second distribution rollers extending parallel to the positive cylindrical electrode and each having a peripheral coating comprising an oxide ceramic material, applying the oily material onto the ceramic coating of the first distribution roller to form on a surface thereof a film of the oily material uniformly covering the surface of the ceramic coating, the film of oily material at least partially breaking down into micro-droplets having substantially uniform size and distribution, transferring the at least partially broken film from the first distribution roller to the second distribution roller so as to cause the film to substantially completely break on the ceramic coating of the second distribution roller into the desired micro-droplets having substantially uniform size and distribution, and transferring the micro-droplets from the ceramic coating of the second distribution roller onto the positive electrode active surface. Preferably, the ceramic coatings of the first distribution roller and the second distribution roller comprise the same oxide ceramic material. Such an arrangement of rollers is described in the inventor's US-A 5,538,601.

According to a preferred embodiment, the oily material is applied onto the ceramic coating of the first distribution roller by disposing an applicator roller parallel to the first distribution roller and in pressure contact engagement therewith to form a first nip, and rotating the applicator roller and the first distribution roller in register while feeding the oily material into the first nip, whereby the oily material upon passing through the first nip forms a film uniformly covering the surface of the ceramic coating.

According to another preferred embodiment, the at least partially broken film of oily material is transferred from the first distribution roller to the second distribution roller and the micro-droplets are transferred from the second distribution roller to the positive electrode by disposing a first transfer roller between the first distribution roller and the second distribution roller in parallel relation thereto, positioning the first transfer roller in pressure contact engagement with the first distribution roller to form a second nip and in contact engagement with the second distribution roller to form a third nip, rotating the first distribution roller and the first transfer roller in register for transferring the at least partially broken film from the first distribution roller to the first transfer roller at the second nip, disposing a second transfer roller parallel to the second distribution roller and in pressure contact engagement therewith to form a fourth nip, positioning the second transfer roller in pressure contact engagement with the positive electrode to form a fifth nip, and rotating the second distribution roller, the second transfer roller and the positive electrode in register for transferring the at least partially broken film from the first transfer roller to the second distribution roller at the third nip, then transferring the micro-droplets from the second distribution roller to the second transfer roller at the fourth nip and thereafter transferring the micro-droplets from the second transfer roller to the positive electrode at the fifth nip. Such an arrangement of rollers is also described in the aforementioned US-A-5 308 601. Preferably, the applicator roller, first transfer roller and second transfer roller are each provided with a peripheral covering of a resilient material which is resistant to attack by the oily material.

The oily material-coated positive active surface is preferably polished to increase the adherence of the micro-droplets onto the positive electrode active surface, prior to the step iii). 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.

The step iii) of the above electrocoagulation printing method is advantageously carried out by continuously discharging the ink onto the positive 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 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 along the positive electrode active surface is collected and the collected ink is recirculated back to the fluid discharge means.

The ink generally used advantageously contains, as an electrolytically coagulable component, a linear high molecular weight compound, that is, a polymer having a weight average molecular weight between about 10,000 and about 1,000,000, preferably between 100,000 and 600,000. Moreover, the electrolytically coagulable compoenent suitably contains a reaction site which includes a functional group selected from the group consisting of an amino group, an amide group and a carboxy group, and which makes a chemical bond with the multivalent metallic ion produced from the positive electrode, especially a trivalent ion such as ferric ion and aluminum ion. Examples of suitable polymers for the printing ink include natural polymers such as albumin, gelatin, casein and agar, and synthetic polymers such as polyacrylic acid and polyacrylamide. A particularly preferred polymer is an anionic copolymer of acrylamide and acrylic acid having a weight average moecular weight of about 250,000 and sold by Cyanamid Inc. under the trade mark ACCOSTRENGTH 86. The polymer used can be in a liquid form of either solution or dispersion which includes coloid, etc. The polymer 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 ink. Water is preferably used as the medium for dispersing or dissolving the polymer to provide the desired ink.

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 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 6 % by weight, based on the total weight of the ink. The coloring agent can be a dye or a pigment. Examples of suitable dyes which may be used for a colored ink 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 V01 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 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 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 non-ionic dispersing agent sold by ICI Canada Inc. under the trade mark SOLSPERSE 27000. The pigment is preferably used in an amount of about 6.5 to about 12% by weight, and the dispersing agent in an amount of about 0.4 to about 6 % by weight, based on the total weight of the ink.

After coagulation of the ink, any remaining non-coagulated ink is removed from the positive electrode active surface to separate the coagulated ink from the non-coagulated ink, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated ink. Preferably, the non-coagulated ink thus removed is collected and mixed with the collected ink, and the collected non-coagulated ink in admixture with the collected ink is recirculated back to the aforesaid fluid discharge means.

The optical density of the dots of colored, coagulated ink 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, the substrate is 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. The transferring step is preferably carried out by providing at each transfer position a pressure roller extending parallel to the positive cylindrical electrode and in pressure contact engagement therewith to form a nip and permit the pressure roller to be driven by the positive electrode upon rotation thereof, and guiding the web so as to pass 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 ink from the positive electrode active surface onto the substrate. The pressure exerted between the positive electrode and the pressure-roller preferably ranges from about 4,903 MPa to about 9,806 MPa (50 to about 100 kg/cm²).

After the transferring step, the positive electrode active surface is generally cleaned to remove therefrom any remaining coagulated ink and oily material. According to a preferred embodiment, the positive electrode is rotatable in a predetermined direction and any remaining coagulated ink is removed from said 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 said positive electrode active surface, rotating the brush in a direction opposite to the direction of rotation of the positive electrode so as to cause said 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 positive electrode surface to cause a transfer of heat therefrom to the ink.

Fig. 1, which does not show the invention but is given for a better understanding thereof, shows an electrocoagulation printing apparatus. The electrocoagulation printing apparatus 1 comprises a cylindrical positive electrode 3, a negative electrode unit 5, an oil coating unit 7, an ink feeder 9, a remover unit 11, a transfer unit 13 and a crosslinking unit 15A (pre-treatment unit).

The cylindrical positive electrode 3 is made of stainless steal and has a continuous passivated surface which defines a positive electrode active surface. The negative electrode unit 5 has a plurality of negative electrodes which are 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 3 and spaced from the positive electrode active surface by a constant gap. In accordance with this structure, a plurality of couples of a positive electrode and a negative electrode for forming a plurality of dots of coagulated ink at the same time are constructed. The oil coating unit 7 has an application roller, distribution rollers and transfer rollers which are aligned in pressure contact engagement with each other, and the oily material is applied to the positive electrode 3 through the oil coating unit 7 in a form of micro-droplets, while the positive electrode 3 is rotated in a constant rotating speed. After polished by a rotating brush, the ink feeder 9 feeds the electrocoagulation printing ink on the positive electrode 3 with the micro-droplets of the oily material. By means of an electrifying device which includes an electric power supply and a controller unit for controlling the power supply, the ink on the positive electrode 3 is electrified with the positive electrode 3 and the negative electrodes of the negative electrode unit 5 in accordance with the image to be printed to form dots of coagulated ink necessary for representing the image. The ink being not coagulated is then removed by scraping with a rubber squeegee of the remover unit 11 and collected for repeated use.

The dots of the uncovered coagulated ink on the positive electrode 3 are brought into contact with the substrate S which is continuously transported by a rotating pressure roller of the transfer unit 13, thereby transferred onto the substrate S. Before transferring the dots of the coagulated ink onto the substrate S, an aqueous solution containing the crosslinking agent is sprayed on the substrate S by a sprayer 17 of the crosslinking unit 15A and the wetted substrate S is dried with drier 27. Therefore, when the dots of the coagulated ink are transferred on the substrate S, the crosslinking agent immediately migrates from the substrate S to the dots of the coagulated ink to cause crosslinking reaction to the dots of the coagulated ink, thereby the dots on the substrate are rendered fast to water. After the transfer of the dots of coagulated ink, the positive electrode 3 is cleaned with the cleaner unit 30 and repeatedly coated with the micro-droplets of the oily material by the oil coating unit 7.

Fig. 2 shows another embodiment of a prior art electrocoagulation printing apparatus. In this embodiment, the electrocoagulation printing apparatus 19 comprises a crosslinking unit 15B (pre-treatment unit) which has a roll coater 21 for applying the aqueous solution containing the crosslinking agent to the substrate S. In Fig. 2, the same references as those in Fig. 1 designate the same or similar elements or sections as those of Fig. 1, so description for such elements or sections are omitted here.

Fig. 3 shows an embodiment of the electrocoagulation printing apparatus of the present invention. In this embodiment, the electrocoagulation printing apparatus 23 comprises a crosslinking unit 15C (post-treatment unit) which includes a nozzle sprayer 25 for spraying the aqueous solution containing the crosslinking agent on the substrate S after the dots of coagulated ink are transferred thereon and a drier 27 for drying the substrate S wet with the aqueous solution containing the crosslinking agent. Also in Fig. 3, the same references as those in Fig. 1 designate the same or similar elements or sections as those of Fig. 1.

Claims

An electrocoagulation printing method for printing an image on a substrate (S), comprising the steps of:

electrifying an ink being electrolytically coagulable, with use of a positive electrode (3) and a negative electrode (5), and electrolytically coagulating the ink between the positive electrode (3) and the negative electrode (5) so as to form a dot of coagulated ink for representing the image;

transferring the dot of coagulated ink onto the substrate (S) to represent the image; and

treating the dot of coagulated ink with a crosslinking agent by applying an aqueous solution or dispersion containing the crosslinking agent to the dot of coagulated ink so as to cause crosslink to a crosslinkable component which is contained in the coagulated ink, thereby rendering the dot of coagulated ink water-fast.
The method as claimed in claim 1, wherein the positive electrode (3) is composed of a metallic material which is capable of electrolytically producing a multivalent metallic ion by electrifying, and the ink is an aqueous electrolytic liquid containing a colorant, an electrolyte and an electrolytically coagulable polymer containing a reaction site which is capable of forming a chemical bond with a multivalent metallic ion.
The method as claimed in claim 1 or 2, wherein the crosslinking agent includes an inorganic crosslinking agent which is selected from the group consisting of aluminum chloride and aluminum sulfate.
The method as claimed in any of the claims 1 to 3, further comprising, before the electrifying step, the step of: heating the positive electrode (3) and the ink at a temperature of approximately 35 to 60 °C, thereby increasing coagulation efficiency of the ink.
The method as claimed in claim 2, wherein the polymer of the ink has a weight average molecular weight of 10,000 to 1,000,000, the reaction site includes a functional group which is selected from the group consisting of an amino group, an amide group and a carboxyl group, and the multivalent metallic ion produced from the positive electrode (3) includes a trivalent ion which is selected from the group consisting of ferric ion and aluminum ion.
An electrocoagulation printing apparatus for printing an image on a substrate (S), comprising:

a positive electrode (3) and a negative electrode (5);

an electrifying device for electrifying an ink being electrolytically coagulable with use of the positive electrode (3) and the negative electrode (5) and electrolytically coagulating the ink between the positive electrode (3) and the negative electrode (5) so as to form a dot of coagulated ink for representing the image;

a transfer mechanism (13) for transferring the dot of coagulated ink onto the substrate to represent the image;

a treatment device (15 B) for applying a crosslinking agent onto the dot of coagulated ink after the dot is transferred onto the substrate (S) so as to treat the dot of coagulated ink with a crosslinking agent to cause crosslink to a crosslinkable component which is contained in the coagulated ink, thereby rendering the dot of coagulated ink water-fast.
The electrocoagulation printing apparatus as claimed in claim 6, wherein the crosslinking agent with which the treatment device (15 B) treats the dot of the coagulated ink is dissolved or dispersed in a liquid medium, and the electrocoagulation printing apparatus further comprises: a drier (27) for drying the liquid medium containing the crosslinking agent to remove the liquid medium, thereby settling the crosslinking agent on the substrate (S).
The electrocoagulation printing apparatus of claim 6 or 7, wherein the treatment device (15 B) includes a sprayer (25) for applying the crosslinking agent in the form of mist.
The electrocoagulation printing apparatus of claim 6, 7 or 8, wherein the treatment device (15 B) includes a roll coater (21) for coating the crosslinking agent.
A printed matter, comprising:

a substrate (S);

an image represented on the substrate with at least one ink dot which contains a coagulum of an electrolytically coagulable polymer and a multivalent metallic ion; and

a crosslinking agent applied onto the ink dot to cause crosslink to the coagulum.
The printed matter as claimed in claim 10, wherein the electrolytically coagulable polymer contains a reaction site which forms a chemical bond with a multivalent metallic ion.
The printed matter as claimed in claims 10 and 11, wherein the electrolytically coagulable polymer has a weight average molecular weight of 10,000 to 1,000,000, the reaction site includes a functional group which is selected from the group consisting of an amino group, an amide group and a carboxyl group, and the multivalent metallic ion includes a trivalent ion which is selected from the group consisting of ferric iron and aluminum ion.
The printed matter as claimed in claim 10, 11 or 12, wherein the crosslinking agent includes a compound which is selected from the group consisting of aluminum chloride and aluminium sulfate.