GB2202957A

GB2202957A - Lithographic printing plate

Info

Publication number: GB2202957A
Application number: GB08702917A
Authority: GB
Inventors: Maria Schyllander; Jan-Ake Falt
Original assignee: Nordisk Tidningsplat AB
Current assignee: Nordisk Tidningsplat AB
Priority date: 1987-02-10
Filing date: 1987-02-10
Publication date: 1988-10-05
Also published as: GB8702917D0; WO1988006102A1; EP0301073A1

Abstract

A lithographic printing plate consists of an aluminium substrate with a hard non-porous surface which is coated with a photopolymerisable material to be developed with an aqueous developer. The non-porous surface is grained and thus has hydrophilic properties whilst not retaining insoluble monomers or other material generated during development of the photo-polymerisable coating material.

Description

DEeCRIPTIOtJ OF INVENTION Improvements in or relating to the Production of a~ printing plate THE PRESENT INVENTION relates to the production of a printing plate and more particularly relates to the production of a printing plate which may be used in a lithographic printing press, for example in planographic printing.

Many different types of printing plate have been proposed for use in lithographic printing.

It is most common now to utilise an aluminium substrate for such printing plates, since this material is readily and cheaply available, and has many desired properties, such as flexibility and dimensional stability.

When fabricating a lithographic printing plate it is desired to be able to produce a plate which has two areas with different properties. One area is to have hydrophilic properties, whereas another area, having a shape corresponding to the image to be printed, has hydrophobic properties.

This has been achieved in many different ways, but in a typical prior proposed lithographic plate, an aluminium substrate is anodised in such a way that a hard but porous surface is provided. This surface is hydrophilic. A coating of a photosensitive resin is then applied to the plate, totally covering the porous surface. The resin may be exposed to an optical image arid may then be developed, with part of the resin being left in position on the plate, and the remainder being washed away. A plate is then provided which has an area in which the surface of the anodised aluminium sheet is exposed as a hydrophilic layer, and the remaining area or areas, which correspond to the image to be printed, have resin as a hydrophobic layer. Such plates are well known and do not require further description at this stage.

Various materials have been proposed for use as the resin layer.

Many years ago it was proposed to utilise a photopolymer as the resin layer. Such a photopolymer has desirable properties, in that a plate made with such a photopolymer may have a relatively long shelf-life (that is to say the plate may be stored in the unexposed state for a considerable period of time, without the plate deteriorating in any way) and also such a plate may be utilised in a very long print run. Such photopolymers may provide adequate printing with a print run 3 or 4 times as long as may be obtained with other equivalent photosensitive coating materials. However, the use of such photopolymers was not totally satisfactory, since the photopolymers originally proposed had to be developed and washed away with the assistance of organic solvents.It was subsequently found that these organic solvents represented a major health hazard to people exposed to them in their working life, and it is for this reason that it is no longer common practice to utilise photopolymers which require organic solvents as developers.

As a substitute for the use of photopolymers and organic solvents, materials of the diazo-type have been used extensively, one major advantage being that these materials can be developed and washed away using only aqueous solvents. However, whilst these materials provide adquate results when used for printing, the results are not of the quality obtained with photopolymers. The shelf life of such plates may be limited, and also the extent of a print run may also be limited.

There is thus still a requirement for the quality that was previously obtained utilising photopolymers.

Recently it has been proposed to provide photopolymers which can be developed in an aqueous solution.

Thus a carboxylic acid polymer may be utilised which can be developed in alkaline conditions. However, such photopolymers, that have been proposed to date, include monomers which are not soluble in aqueous conditions.

It has been found that these monomers tend to adhere to the porous anodised surface, which has been retained to ensure that the printing plate has the required hydrophilic properties in the area where no printing image is to be produced, and this has lead to "spotting" or "fogging". It is believed that, due to the porous nature of the hyrophilic anodised surface, the monomers tend to adhere to that surface when the photopolymer is developed.

The present invention seeks to obviate or reduce the disadvantages of the prior art.

According to the broadest aspect of this invention there is provided a method of producing a lithographic printing plate, said method comprising the steps of treating an alumimium substrate to provide a hard substantially non-porous surface, and coating the surface with a photopolymerisable material adapted to be developed with an aqueous developer.

Preferably the plate is grained. The plate may be grained before the substrate is treated to provide the hard substantially non-porous surface, and any appropriate graining technique may be used including a brush draining technique.

Preferably the photopolymer is an ultra-violet curing polymer.

-In one embodiment of the invention the photopolymer comprises an ethyleneically unsaturated compound, an addition polymerisation initiator, and an organic macromolecular polymer binder which may contain acid groups or salts thereof, said binder being alkali soluble. In another embodiment of the invention said photopolymer comprises at least one non-gaseous ethyleinically unsaturated compound containing at least two terminal ethylenic groups, having a boiling point greater than 1000C at normal atmospheric pressure and being capable of forming a polymer by photo initiated addition polymerisation, an addition polymerisation initiator activate by actinic radiation and an organic macromolecular polymer binder which is alkali soluble.

Preferably the plate is anodised in an electrolyte, the electrolyte being at a temperature of 150C or less, the anodising being carried out at a current density in excess of 1 A/dm2.

Advantageously the anodising is carried in a plurality of separate steps, and each step may be carried out in a separate electrolyte bath. The or each electrolyte may consist of or comprise sulphuric acid, phosphoric acid, oxylic acid, boric acid or their salts cr m-xtures incorporating one or more of these acids or their salts.

Thur the electrolyte in each bath may comprise phosphoric acid of, for example, between 5 and 10% con cerstration. Alternatively each bath may comprise sulphuric acid of, for example, between 5 and 10% concentration. Alternatively again the electrolyte in one or more of the baths may be phosphoric acid and the electrolyte in one or more of the baths may be sulphuric acid, and in such a case the phosphoric acid is between 5 and 10% concentration and the sulphuric acid is between 5 and 10% concentration.

The anodising in the first step may be carried out at a current density of 1 to 4 A/dm2, and in the second step may be carried out at a current density of 1 to 4 A/dm2 and in the third step may be carried out at a current density of 1 to 4 A/dm2 The invention also relates to a printing plate when made by a method in accordance with the invention.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described by way of example.

In the invention the surprising step is taken of providing an aluminium sheet with a hydrophilic surface which is not porous. Up till now it has been believed that it is essential to provide a porous hydrophilic surface for the aluminium to present the desired hydrophilic properties. However, the present applicants have now discovered that if a non-porous, but grained, surface is utilised, the aluminium will present an adequate hydrophilic property, but the plate may be utilised with a photopolymer which is developed in an aqueous medium, with virtually no risk of any remaining monomer which is insoluble will be retained in the hy drop}s c region to produce "spotting" or tfogging".

The graining of the plate may be achieved in several alternative ways. While a conventional brush graining technique is preferred an electrochemical graining process may be used, or even a chemical graining process.

It is thus believed that embodiments of the invention will, because they incorporate a photopolymer, have a long shelf life, and may be developed to provide excellent printing characteristics, and will also be able to be utilised in very long print runs.

Thus it is believed that embodiments of the invention will be able to overcome all the disadvantages experienced with prior art proposals.

The invention will now be described with reference to the specific examples.

Example 1 An aluminium plate, made of lithographic quality aluminium, initially grained with a brush graining process in which a fine quartz powder, dispensed in water, is sprayed onto a cleared surface of the aluminium and is brushed into the surface with a cylindrical nylon brush which rotates and oscillates simultaneously. Several brushes are used in series to give an even non-directional grain. Subsequently the plate is anodised in â three stage process.

In a first step of process the plate is anodised for 2 minutes in an acid bath containing 7% phosphoric acid at a temperature of 150C, with a current density of 1.5 A/dm2.

Ae a second step, in a separate bath containing 8.5 phosphoric acid at a temperature of 120C, the plate is anodised for 2.5 minutes at a current density of 2 A/dm2 In a third bath, containing 5.8% phosphoric acid, at a temperature of 90C, the plate is anodised for 1.75 minutes with a current density of 2.5 A/dm2.

The resultant plate had a hard non-porous surface.

Example 2 The procedure of Example 1 was repeated using 7% sulphuric acid in each of the three baths.

Example 3 The procedure of Example 1 was repeated at a temperature of 5 C.

Example II The procedure of Example 1 was repeated at a 0 temperature of 10 C.

Example 5 Plates in accordance with Examples 1 to 4 were each coated with an ultra violet curing photopolymer as supplied by Ciba Geigy under designation RT 605.

The plates were subsequently exposed to an ultra violet light source through a 21-step Lithographic Technical Foundation exposure wedge to yield a solid seven, i.e. so that the photopolymerisable material under the first seven steps polymerised sufficiently to resist subsequent removal by development.

Subsequently the plate was developed by washing out the unexposed parts of the coating using a developer which is a 5% solution of sodium metalylicate (hydrated) in de-mineralised water.

The plate was covered with the developer and allowed to soak for 30 seconds. The unexposed areas of the photopolymerisable were removed by sponging. The developed plate was then rinsed with water and dried.

The plate showed good ink/water characteristics. The exposed photopolymerised areas readily accepted inks while the areas of the support from which the unexposed photopolymerisable had been removed accepted water and fountain solution (an aqueous solution used widely in lithographic printing) satisfactorily. The plate ran satisfactorily on a web offset press using a black printing ink and fountain solution, and printed a large number of copies.

Example 6 The procedure of Example 5 was repeated using the following mixture as the ultra-violet curing photopolymer.

Copolymer of styrene/maleic anhydride partially esterified with iso-butanol (Mw 40,000) 10 Parts Ethylene glycol di-acrylate 5 1? Benzil 0.6 Michlers Ketone 0.6 Methyl Ethyle Ketone (Solvent) 100 Example 7 Plates made in accordance with Examples 5 and 6 were stored for five months before developing. The plates were then exposed, and developed. The plates again provided a satisfactory result.

Claims

CLAIM:

1. A method of producing a lithographic printing plate, said method comprising the steps of treating an alumimium substrate to provide a hard substantially nonporous surface, and coating the surface with a photopolymerisable material adapted to be developed with an aqueous developer.

2. A method according to claim 1 wherein the said plate is grained.

3. A method according to claim 1 or claim 2 wherein the photopolymer is an ultra-violet curing polymer.

4. A method according to claim 1, 2 or 3 wherein the photopolymer comprises an ethyleneically unsaturated compound, an addition polymerisation initiator, and an organic macromolecular polymer binder which may contain acid groups or salts thereof, said binder being alkali soluble.

5. A method according to claim 1, 2 or 3 wherein said photopolymer comprises at least one non-gaseous ethyleinically unsaturated compound containing at least two terminal ethylenic groups, having a boiling point greater than 1000C at normal atmospheric pressure and being capable of forming a polymer by photo initiated addition polymerisation, an addition polymerisation initiator activate by actinic radiation and an organic macromolecular polymer binder which is alkali soluble.

6. A method according to any one of the preceding claims wherein the plate is anodised in an electrolyte, the electrolyte being at a temperature of 150C or less, the anodising being carried out at a current density in excess of 1 A/dm2

7. A method according to claim 6 wherein the anodising is carried in a plurality of separate steps.

8. A method according to claim 7 where each step is carried out in a separate electrolyte bath.

9. A method according to any one of claims 4 to 6 wherein the or each electrolyte consists of or comprises sulphuric acid, phosphoric acid, oxalic acid, boric acid, or their salts, or mixtures incorporating one or more of these acids or their salts.

10. A method according to claim 8 or 9 wherein the electrolyte in each bath comprises phoshoric acid.

11. A method according to claim 10 wherein the phosphoric acid is phosphoric acid between 5 and 10% concentration.

12. A method according to claim 8 wherein the electrolyte in each bath comprises sulphuric acid.

13. A method according to claim 12 wherein the sulphuric acid is sulphuric acid between 5 and 10% concentration.

A A method according to any one of the preceding claims wherein the electrolyte in one or more of the baths is phosphoric acid and the electrolyte in one or more of the baths is sulphuric acid.

15. A method according to claim 14 wherein the phosphoric acid is between 5 and 10% concentration and the sulphuric acid is between 5 and 10% concentration.

16. A method according to any one of claims 10 to 15 wherein the anodising in the first step is carried out at a current density of 1 to 4 A/dm2.

17. A method according to claim 16 wherein the anodising in the second step is carried out at a current density of 1 to 4A/dm2.

18. A method according to claim 17 wherein the anodising in the third step is carried out at a current density of 1 to 4 A/dm2.

19. A method according to claim 1 in accordance with Example 5.

20. A method according to claim 1 in accordance with Example 6.

21. A printing plate when made by the method according to any one of the preceding claims.

22. Any novel feature or combination of features disclosed herein.