GB2113701A - Ink for planographic printing without the use of fountain solution and method of using the same - Google Patents

Abstract

Lithographic printing may be carried out without the use of fountain solution if the plate used has a non- absorbent hydrophilic surface and the ink is supplied to the plate in the form of a dispersion of a solution in a polyhydric alcohol of one or more inert deliquescent inorganic materials (e.g. a chloride of Li, Ca, Mg or Zn) in a continuous phase of lithographic printing ink comprising a chlorinated or cyclized rubber or a rosin-modified phenolic resin, a solvent or vehicle for the resin, and a pigment. After imaging, the plate is primed, and may then be used for printing without fountain solution.

Description

SPECIFICATION Ink for planographic printing without the use of fountain solution and method of using the same This invention relates to planographic printing without the use of fountain solution.

Conventional planographic printing using a lithographic plate involves alternately contacting the imaged plate with a printing ink which adheres preferentially to the oleophilic image on the plate and with so-called fountain solution the purpose of which is to keep the hydrophilic non-image areas of the plate in a moist state in which they will not accept the oleophilic ink. The adjustment of the supplies of ink and fountain solution to the plate requires a certain amount of skili and judgement on the part of the printer if high quality copies are to be obtained.While lithographic printing machines can be made so as to dispense the ink and the fountain solution automatically, the relative adjustment of the dispensing devices frequently requires alteration because of differences in operating conditions, and it has not in practice proved possible to devise a system which will produce high quality copies completely automatically.

Consequently, there has been a continuing search for methods for simplifying planographic printing, in particular by dispensing with the need for supplying fountain solution to the planographic printing plate. Basically, there have been two different approaches. In the first approach, the offset ink has been combined with a hydrophilic phase to form a two-phase system which is applied to the imaged planographic plate, the idea being that the hydrophilic phase replaces the fountain solution. Although the plate still requires to be primed before it is first used, lithographic inks formulated in this way are supposed to dispense with the need for fountain solution. The second approach involves formulating the ink and the plate in such a way that the ink has no tendency to adhere to the non-image areas of the plate even in the absence of any hydrophilic phase.Systems of this kind have usually depended upon the use of a planographic printing plate having a surface comprising a fluorinated compound and/or a silicone, and an ink containing a silicone oil. A specially designed or modified printing machine is usually required. With this method of operation, the planographic plate does not require to be primed and does not require fountain solution.

However, although a great deal of research has been carried out into both these methods, and some methods have been marketed on a small scale, no entirely satisfactory system has yet been devised. In particular, in the second method, the printing machine used has to have a specially modified inking roller system, and the plate itself is extremely sensitive to marking, e.g. with the fingers. The first method also has proved unreliable and it has not proved possible in practice to ensure that, under ordinary working conditions, the ink does not adhere, at least to a minor extent, to the non-image areas of the printing plate.

The present invention provides a novel lithographic ink formulation and a novel method of planographic printing using the same, which make it possible to obtain high quality copies reliably without the use of fountain solution and using a lithographic printing machine in which the only modification is the absence of a fountain supply system.

In the lithographic ink formulation of the present invention a hydrophilic phase is incorporated in the lithographic ink. As already mentioned, there have been many previous proposals to do this, but the prior proposals have proved unreliable in providing a continuous supply of the ink-repellent (i.e. hydrophilic) phase to the surface of the planographic printing plate.

Consequently scummy copies, i.e. copies with considerable transfer of ink to the non-image areas, have been obtained. In addition, in the known systems, when the lithographic ink formulation has been left to stand on the printing machine overnight, the hydrophilic phase has evaporated from the ink roller surface, thereby causing considerable difficulties when the printing process is re-started the following day. Scummy copies have invariably been obtained in these circumstances.

We have now discovered that by careful formulation of the lithographic ink and careful choice of the surface of the planographic printing plate, it is possible to avoid difficulties inherent in the prior art systems.

We have discovered that one of the causes of difficulty with the prior art systems is the continual loss of the hydrophilic phase from the ink formulation by transfer from the inking roller via the planographic printing plate onto the printed copies. By appropriate choice of the planographic printing plate surface, it is possible greatly to restrict the amount of hydrophilic phase which is taken up by the plate from the inking roller and thereby make it very much easier to ensure that there is always an adequate supply of the hydrophilic phase on the inking roller so that ink is not taken up by the non-image areas of the printing plate. This result is achieved by using a planographic printing plate with a hydrophilic but highly non-absorbent surface, which is not attacked (e.g. dissolved) by the hydrophilic phase.

However, use of non-absorbent planographic plates is not by itself sufficient. The hydrophilic phase of the lithographic ink formulation must itself have the correct composition. In particular, it must be formulated so that it substantially does not evaporate under ordinary conditions of use, and so that, when used, it is rapidly spread out over the hydrophilic, non-image areas of the plate.

This result is achieved in part by using as the hydrophilic phase a liquid polyhydric alcohol and preferably ethylene glycol, diethylene glycol or triethylene glycol, having an inert deliquescent inorganic material dissolved therein, preferably a chloride of lithium, magnesium, calcium or zinc.

The presence of the deliquescent material, and the high boiling point of the polyhydric alcohol ensure that substantially no evaporation of the hydrophilic phase takes place under ordinary conditions of use. For example, the hydrophilic phase does not dry out overnight in normal circumstances, so that the starting up problems mentioned above substantially do not occur. It is only necessary to re-prime the plate before the plate is re-used.

In addition, in order to ensure the proper balance between the lithographic ink itself and the hydrophilic phase simultaneously supplied to the planographic plate, it is essential to use, in the lithographic ink, an appropriate resinous vehicle for the pigment to ensure correct distribution of the ink to the image areas of the plate and of the hydrophilic phase to the non-image areas.

The present invention accordingly provides a novel lithographic ink formulation in the form of a dispersion in a continuous phase of lithographic printing ink of a solution in a liquid polyhydric alcohol of one or more inert deliquescent inorganic materials in sufficient amount substantially to delay evaporation of the solution on exposure of the said solution to ambient air, the printing ink itself comprising a chlorinated or cyclised rubber resin or a rosin-modified phenolic resin, a solvent or vehicle for the resin, and a pigment. The resin must be such as to ensure that the dispersed hydrophilic phase has substantially no tendency to separate out even if (as is preferred) no emulsifier is used.The dispersion must be such that under the shear action of the ink roller system during printing, the hydrophilic phase is readily released to maintain the nonimage areas of the planographic plate in a hydrophilic state in which they will not accept the lithographic ink. The stated rubber and phenolic resins satisfy these conditions.

The resin is dissolved in an appropriate solvent, which is preferably a high boiling point petroleum fraction (e.g. white spirit), or vehicle, preferably a low viscosity biown linseed oil or a linseed stand oil lithographic varnish. The resin may be dissolved with gentle heating. Conventional pigments, especially carbon black with or without a blue toner, are then ground into the varnish in known manner, for example on a conventional triple roll mill.

The proportions of the ingredients may be for example from 15 to 45% of the resin, from 35 to 65% of the solvent or vehicle, and from 10 to 40% of the pigment, these percentages being by weight based on the total weight of the lithographic ink.

The hydrophilic phase normally constitutes 25 to 45%, and preferably 35 to 40%, by weight of the formulation. It preferably comprises ethylene glycol having lithium chloride dissolved therein.

The proportion of the salt is such as substantially to delay evaporation of the solution on exposure to ambient air. Clearly this will depend inter alia on the temperature and humidity of the air, but under normal conditions (e.g. 1 5 to 250C and 4080% RH) the proportion may be from 5 to 20% by weight of the hydrophilic phase.

Since the hydrophilic phase does not contain water, the lithographic plate used with the lithographic ink formulation can be based on ordinary paper rather than the wet-strength paper which has to be used for lithographic paper plates which are exposed in use to water (i.e. fountain solution).

As already indicated, the lithographic ink should be capable of maintaining the hydrophilic phase in a stable dispersed state without the use of an emulsifier. The hydrophilic phase may be dispersed in the lithographic ink using a high speed stirrer or in other known manner.

The novel lithographic ink dispersion is used in the same way as known lithographic inks on a planographic printing machine without any separate supply of fountain solution.

As already explained, the plate used must have a non-absorbent surface. This result may be achieved in a number of different ways. Some known types of lithographic printing plate satisfy this condition, in particular those having an aluminium surface. Such a surface is both very hydrophilic and non-absorbent, and excellent results can be obtained using presensitized aluminium plates, imaged in known manner and primed in the manner described below, with the novel ink formulations of the present invention.

However, aluminium plates are relatively expensive and not appropriate for use in low-cost lithographic duplicating systems.

The preferred low-cost lithographic printing plate for use in the present invention comprises a layer of electrophotographic zinc oxide dispersed in a resinous binder and coated on an appropriate support sheet, e.g. of paper. Such sheets may be imaged electrophotographically and then primed with an aqueous conversion fluid comprising a ferrocyanide and other known constituents such as phosphate and organic acids in aqueous solution, so as to render the surface hydrophilic in the unimaged areas. The image remains oleophilic and an imaged lithographic plate is thus obtained.

The resinous layer containing the zinc oxide is non-absorbent, and such imaged plates are therefore especially useful in the present invention.

Lithographic plates of the aforesaid kind are known and are described in, for example, British Specification No. 1369868. Heretofore, however, they have been used for normal lithographic printing with separate application of printing ink and fountain solution. Suitable priming solutions are described in British Specification No.

1337164.

Other useful plates may be made by taking a sheet having a surface of cellulose acetate or polyvinyl acetate and then hydrolysing the surface to form thereon a very thin layer of cellulose or polyvinyl alcohol respectively. In this case the thinness of the layer ensures that there is substantially no uptake of hydrophilic phase from the ink formulation. Such plates may be made, for example, by laminating a thin film of cellulose acetate or polyvinyl acetate on a paper base sheet (preferably wet-strengthened) in known manner and then treating the surface of the laminate so obtained with caustic soda solution for a short period to hydrolyse the surface only of the laminate. This process can give good quality plates but is rather expensive to operate.

Other plates comprising a support sheet having a non-absorbent hydrophilic surface wetted by, but insoluble in, the hydrophilic phase of the ink formulation may be used.

Whatever type of non-absorbent plate is used, it is necessary to prime the plate after imaging and before use in order to wet the non-image areas of the plate. The priming liquid may be applied by hand with a swab but is preferably applied with a roller system dipping in the liquid and through which the plate to be primed is passed. Devices incorporating such priming rollers are preferably built into lithographic printing machines to provide for automatic priming of imaged plates.

The planographic printing plate which has been imaged in conventional manner, preferably in an electrostatic copier, and primed, preferably a zinc oxide offset electrostatic plate which has been imaged in an electrostatic copier and primed is then inked on the printing machine with the ink dispersion of the invention. The non-image areas of the plate are fully de-sensitised (unreceptive to ink) while the image areas take up ink from the ink dispersion. Printing can then be carried out without supply of fountain solution to the plate during the printing. A large number (e.g. 500 with a paper-based plate) of good quality copies can be obtained. If printing has to be stopped for any reason for more than a very short period, the plate requires to be re-primed before re-use.

The present invention thus makes possible a substantial simplification of planographic printing while still maintaining high quality results.

The following Examples illustrate the invention.

EXAMPLE 1 A lithographic ink dispersion in accordance with the present invention may be manufactured as follows.

The ink base is prepared by dissolving chlorinated rubber hydrocarbon resin 514 (Dryster Limited; 10 g) in a modified linseed oil having a viscosity of 7 poise at 250C (Plastybcol 7 of Younghusband 8 Stephens Limited; 40 g). Carbon black (Mogul L of Cabot Carbon; 1 5 g) is then ground into the varnish obtained. A blue toner may be included if desired.

The hydrophilic phase consists of ethylene glycol (85 g), lithium chloride (15 g), and optionally 25% aqueous phosphoric acid (8 g).

The lithographic ink dispersion is prepared by dispersing 40 g of the aforesaid hydrophilic phase in 65 g of the ink base which has previously been warmed to 70 to 800C. The hydrophilic phase is added slowly with stirring. When cold, the dispersion is ready for use.

EXAMPLE 2 A lithographic ink was prepared by dissolving an isomerised rubber resin (Plastoprene No.

1W/50 of Plastanol Ltd; 11.25 g) in a linseed stand oil (viscosity 5 poise, 38.75 g). Carbon black (Mogul L of Cabot Carbon; 1 5 g) was ground into the varnish obtained.

The hydrophilic phase was the same as in Example 1. 40 g of the hydrophilic phase were dispersed in the ink base which had been preheated to 70 to 800C. On cooling to ambient temperature, the ink dispersion is ready for use.

EXAMPLE 3 A lithographic ink is prepared by dissolving a chlorinated hydrocarbon resin 51 4 J (Dryster Limited; 11.25 g) in a modified linseed oil having a viscosity of 7 poise at 250C (Plastybcol 7 of Younghusband 8 Stephens Limited; 38.75 g).

Carbon black (Mogul L of Cabot Carbon; 1 5 g) is ground into the varnish obtained.

The hydrophilic phase consists of calcium chloride (anhydrous, 15 g) dissolved in ethylene glycol (85 g).

40 g of the hydrophilic phase is dispersed in the ink base after the latter has been warmed to 70 to 80 C. The dispersion obtained is ready for use when cold.

EXAMPLE 4 A lithographic ink is prepared by dissolving a low viscosity rosin-modified phenolic resin (Mitchanol 30 of Warwick Chemical Company Ltd; 13.5 g) in a modified linseed oil (Plastybcol 7 of Younghusband 8 Stephens Limited; 36.5 g).

Carbon black (Mogul L of Cabot Carbon; 1 5 g) was ground into the varnish obtained.

The hydrophilic phase was the same as in Example 1. 40 g of this hydrophilic phase were dispersed in the ink which had been pre-heated to 70 to 80"C. When cold, the dispersion was ready for use.

In the foregoing Examples, the lithium chloride can be replaced by an equal weight of calcium, magnesium or zinc chloride or a mixture thereof.

The ethylene glycol can be replaced by diethylene glycol or triethylene glycol. If either of these substitutions are made, the quality of copies obtained using the ink is slightly inferior and the emulsion stability is substantially reduced.

The aforesaid inks may be used for example, in the following manner. A zinc oxide offset electrostatic plate of known kind, e.g. as described in British Specification No. 1369868 is imaged in an electrostatic copier, care being taken that the fixation temperature is high enough to ensure that the toner image is securely bonded to the plate surface. The imaged plate is then primed with the following solution: Potassium ferrocyanide 30 g Potassium metabisulphite 5 9 Ammonium dihydrogen phosphate 50 g Gum arabic 20 g Citric acid 2 g Water 1850 g This primer solution is preferably applied using an automatic primer in which the imaged plate is fed by a roller system at a constant rate through a bath of the primer. The primed and imaged plate is then ready for use.

The plate is placed on a lithographic printing machine from which the fountain system has been removed and in which the ink supplied is one of the formulations described above. The image areas of the plate rapidly become thoroughly inked by the ink phase of the formulations while the non-imaged and primed areas are kept ink-free by the hydrophilic phase. Printed copies can then be obtained in the usual way, up to 500 copies of good quality being obtained without difficulty and at a high speed.

Claims (13)

1. A lithographic ink formulation in the form of a dispersion of a solution in a liquid polyhydric alcohol of one or more inert deliquescent inorganic materials in amount sufficient substantially to delay evaporation of the solution on exposure of the said solution to ambient air, in a continuous phase of lithographic printing ink comprising a chlorinated or cyclized rubber resin or a rosin-modified phenolic resin, a solvent or vehicle for the resin, and a pigment.

2. An ink formulation according to claim 1 in which the continuous ink phase comprises 1 5 to 45% of the resin, 35 to 65% of the solvent or vehicle, and 10 to 40% of pigment, these percentages being by weight based on the total weight of the lithographic ink.

3. An ink formulation according to claim 1 or 2 in which the said solution constitutes 25 to 45% by weight of the formulation.

4. An ink formulation according to claim 3 in which the said solution constitutes 35 to 40% by weight of the formulation.

5. An ink formulation according to any of claims 1 to 4 in which the said solution comprises one or more chlorides of lithium, calcium, magnesium or zinc dissolved in one or more of ethylene glycol, diethylene glycol ortriethylene glycol.

6. An ink formulation according to any of claims 1 to 4 in which the said solution comprises lithium chloride and is emulsifier-free.

7. An ink formulation according to claim 6 in which the said solution consists of 5 to 20% by weight of lithium chloride and 95 to 80% by weight of ethylene glycol.

8. An ink formulation according to claim 1 substantially as described in any one of the foregoing Examples.

9. Method of lithographic printing which comprises imaging a lithographic plate having a substantially non-absorbent surface, priming the imaged plate with a hydrophilic liquid which wets, but does not dissolve, and is not absorbed by, the surface of the plate, and then printing from the said plate without fountain solution using an ink formulation as claimed in any of claims 1 to 8.

1 0. Method according to claim 9 in which the plate has a surface of aluminium, superficially hydrolysed cellulose acetate, or superficially hydrolysed polyvinyL acetate.

11. Method according to claim 10 in which the image plate is primed with ethylene glycol containing up to 25% by weight of water.

12. Method according to claim 8 in which the plate is an electrophotographic sheet carrying a layer of zinc oxide dispersed in a resinous binder and the said plate is primed with an aqueous conversion fluid comprising a ferrocyanide.

13. Method according to claim 9 substantially as hereinbefore described.