Improvements introduced in wetting systems for offset printing and a mechanism for their application

Abstract: The improvements consist in supplying the wetting system with a water based solution including a determined additive. This solution is subjected, through a mechanism forming a part of this invention, to a process of agitation by which it absorbs a high oxygen content. As a result of these improvements, the solution which wets the aluminium plate, from which the printing is done, acquires a high free oxygen content, increasing the water-ink interface and improving the quality of the printing.

Description: AIM OF THE INVENTION The present invention, as indicated in the title, refers to a solution aimed at eliminating the isopropyl alcohol, or isopropanol, contained in the solutions used in offset printing dampening systems, thus improving the printing specifications, and also includes a process and device which will allow for the preparation of the said dampening solution. BACKGROUND OF THE INVENTION The process of offset printing is based on certain physicochemical surface phenomena. The stage which precedes the application of the ink to the paper consists of phenomena of attraction and repulsion between a dampening solution and the ink, this allows for the formation of an image on an aluminium plate which is then directly transferred to a rubber blanket cylinder, and from there to the paper. In order to obtain satisfactory results, it is necessary that both the ink and the solution, which is supplied to the dampening system of the device, possess certain determined physicochemical features. The most widely used dampening solution consists of water, with certain additives providing the system with determined features, and one part, between 8 and 10 % of isopropyl alcohol. The use of isopropyl alcohol in the dampening solution ensures an interfacial tension between the water and the ink which allows for the achievement of a lesser degree of emulsification of the water in the ink, as a result of which the ink printed onto the paper benefits from a higher level of saturation than can be obtained with any other dampening system. The isopropyl alcohol, or isopropanol, provides the solution with other advantages, such as a greater viscosity allowing for a more uniform transference, a greater drying speed, and a lowering of the surface tension which ensures that the film of the solution on the printing plate is very thin, and that it has a lesser tendency to mechanical emulsification. Independently of the advantages which the addition of isopropyl alcohol to the solution used in the dampening system brings to offset printing, this product also presents certain important disadvantages, such as flammability and toxicity, which make it dangerous, and which have given rise to a generalised tendency towards the elimination of this product, to the extent that its use is completely, or partially, prohibited in some countries. The problem which is posed, therefore, is how to eliminate isopropyl alcohol from the solutions used for dampening and the formulation of additives, which has so far been attempted without success, that will allow for such an elimination while maintaining the water-ink interface, this being the principal problem. The need to reduce the surface tension, in order to avoid mechanical emulsification up to levels of water-alcohol, obliges the use of surface-active agents suited to that purpose, which are well known by those skilled in the art. However a greater concentration of these surface-active agents leads to a decrease of the interface, due to the affinity between the fatty acids of the ink and the hydrocarbon chains of these substances: this, together with the current impossibility of forming stable colloids which reduce the surface tension and maintain a greater interface, is the reason why alcohol cannot be eliminated from the dampening systems. The possibility to improve some features of lithographic inks by taking advantage of adsorption phenomena produced therein is generally known in the art. In this connection, the Patent N°. US-A-4 604 952, for instance, discloses the ability of lithographic inks to adsorb atmospheric oxygen, resulting in a reduction of their drying time. Also known are procedures and devices in which the production of adsorption phenomena is assisted by sprays, such as, e. g., the etching machine described in the Swiss Patent Application No. CH-A-597 363. Nevertheless, no reference can be actually found in the current art to the application of such techniques in the case of the dampening solutions used in dampening cylinders of offset printing machines. DESCRIPTION OF THE INVENTION The process, which is the object of the invention, consists in subjecting the solution to a process of oxygen adsorption, causing the formation of layers of vapour on the surface of the solution itself, in order to achieve that the whole of the film of the solution applied by the dampening system to a corresponding printing plate thereof acquires a high content of free oxygen. The formation of the vapour layers on the surface of the solution, and therefore the adsorption of the free oxygen by the same, are obtained by subjecting the solution to a process of high speed agitation, whereby the solution is sprayed onto a reduced volume of the solution itself, it being this reduced volume which supplies the dampening system, in a continuous manner. The dampening solution to be used in the present invention consists of water and of a concentrated additive, wherein the components and proportions of the concentrated additive are preferably as given in claim 3. The purpose of the additive consists in creating conditions in which, during the above mentioned high speed agitation, a high level of oxygen emulsification is produced on the solution and, at the same time, that this oxygen disappears at a determined speed from the body of the solution. The effecting of the supply of the dampening system with the above mentioned solution, previously subjected to the process of adsorption of oxygen, will ensure certain values of surface tension, interface tension and viscosity which provide a series of very important advantages with regard to the offset printing conditions obtained with conventional dampening systems. The most important advantages are: The complete elimination of isopropyl alcohol. A greater uniformity of the spreading of the ink on the printing plate. Obtaining a thinner and more homogeneous film of solution on the printing plate. A lower level of emulsification of the solution with the ink. A reduction of ink consumption, between 10 and 14%. More brilliant colours. A great ink drying speed. The possibility of increasing printing speed. Better wet on wet printing in multi-colour machines. A better printing contrast. Better printing with special screens, for example stochastic screens. The reduction or elimination of the use of antioffset powders. In order to obtain a significant adsorption of oxygen by the dampening solution, the invention also contemplates a device for the preparation of and feeding of the said dampening solution according'to the process provided by the present invention, characterised in that it includes a main tank to contain the solution, an outlet line leading to a pump which propels the solution towards a spray which then sprays it out in a jet against the walls of a reservoir, housed inside the main tank, this agitation causing the adsorption of oxygen by the solution; a feed line which connects the reservoir to the dampening system of the offset printing machine, and a return line, which returns the dampening solution from the dampening system to the main tank. The said reservoir, in which the process of the adsorption of oxygen by the solution is effected, presents a side overflow which determines the level of the solution subjected to the process of the adsorption of oxygen and contained in the said reservoir. According to a preferred embodiment of the device of the present invention, the mouth to the feed line is located inside the reservoir, just below the determined level of the overflow, in order that it will collect the surface of the solution which contains a higher oxygen content, this then being propelled towards the dampening system by a pump. DESCRIPTION OF THE DRAWINGS As a complement to this description, and in order to assist in a better understanding of the features of the invention, this present patent specification is accompanied, as an integral part of the same, by a drawing, as an illustration but that are not in any case limiting, which represent the following: Figure 1 shows a schematic view of the device employed to produce a significant agitation of the dampening solution, which ensures a high level of oxygen adsorption by the solution. PREFERABLE EMBODIMENT OF THE INVENTION The preferable embodiment requires that the solution used to supply the dampening system shall consist of water and an additive, the quantity of additive to be used being, between 3 and 5 %. The components and their relative amounts to be used to make up the additive will preferably be as indicated in claim 3. The invention also contemplates the use of a device causing the solution to adsorb oxygen, before being supplied to the dampening system of the offset printing machine. This device, the description of which is presented here only as an example, and in no way as a limitation, includes a main tank (1), which contains the solution; an outlet line (2), which includes a pump (3), to propel the solution towards a spray (4), which in turns sprays it against the walls of a reservoir (5), a feed line (6), fitted with a pump (7), which propels the solution from the reservoir(5) to the dampening system of the offset printing machine; and a line (8) through which the solution returns from the dampening system of the machine tot he main tank (1). The agitation caused by the jet of the solution, sprayed by the spray (4) against the walls of the reservoir (5), leads to the process of adsorption of oxygen by the solution. The reservoir (5) has a side overflow (51), through which the surplus solution pours out into the tank (1). The mouth of the feed line (7) is located just under the surface of the solution contained in the reservoir (5), in order to effect the supply to the dampening system, with the surface part of the solution containing the highest content of free oxygen. It is not considered that it is necessary to further extend this description, insofar as any expert in this material will be able to understand the invention, and the advantages which derive therefrom. The terms in which this description has been drawn up must in all cases de taken in the widest possible sense, and never as a limitation. The materials, form, size and disposition of the elements will be susceptible to variation, always insofar as this does not mean an alteration of the essential specifications of the invention, which are claimed in continuation.

Claims: A process for the preparation of a dampening solution to be used in a dampening system of an offset printing machine, the solution consisting of water and concentrated additive, characterised by subjecting the solution to a process of oxygen adsorption by high speed agitation, causing the formation of layers of vapour on the surface of the solution itself, in order to achieve that the whole of the film of the solution applied by the dampening system to a corresponding printing plate thereof acquires a high content of free oxygen. Process according to claim 1, characterised in that the formation of the vapour layers on the surface of the solution, and therefore the adsorption of the free oxygen by the same, are obtained by subjecting the solution to a process of high speed agitation, whereby the solution is sprayed onto a reduced volume of the solution itself, it being this reduced volume which supplies the dampening system, in a continuous manner. Process according to claim 1, , characterised in that the components and proportions of the concentrated additive are, from 300 to 600 ml of propanotriol, from 20 to 50 g of sodium citrate, from 8 to 24 ml of glacial acetic acid, from 0,5 to 2 g of 1-ascorbicacid and from 0,1 to 1 g of sodium metaphosphate. Process according to claim 3, characterised in that the amount of the additive contained in the dampening solution amounts to between 3 and 5 %. A device for the preparation and feeding of a dampening solution according to claims 3 and 4, and according to the process defined in claims 1 and 2, characterised in that it includes a main tank (1) to contain the solution, an outlet line (2) leading to a pump (3) which propels the solution towards a spray (4) which then sprays it out in a jet against the walls of a reservoir (5), housed inside the main tank (1), this agitation causing the adsorption of oxygen by the solution; a feed line (6) which connects the reservoir (5) to the dampening system of the offset printing machine, and a return line (8), which returns the dampening solution from the dampening system to the main tank. Device according to claim 5, characterised in that the reservoir (5), in which the process of the adsorption of oxygen by the solution is effected, presents a side overflow (51) which determines the level of the solution subjected to the process of the adsorption of oxygen and contained in the said reservoir (5). Device according to claim 5 or 6, characterised in that the mouth to the feed line (6) is located inside the reservoir (5), just below the determined level of the overflow (51), in order that it will collect the surface of the solution which contains a higher oxygen content, this then being propelled towards the dampening system by a pump (7).

Inventors: ROMERO SALVO GERARDO ANTONIO, ROMERO SALVO, GERARDO ANTONIO
Applicants: DELTA GRAF SA, DELTA-GRAF, S.A.
Application year: 1996
Main classification: B41N3
IPC codes: B41N3, B41J7, B41F7