THEREBY PRODUCED

THE PRESENT INVENTION relates to the production of a printing plate and more particularly relates to the pro¬ duction of a printing plate which may be used in a lith¬ ographic 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, is to have hydrophobia 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

and 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 porous 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 various photopolymers as the resin layer. Such photopolymers have 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.

The photopolymers that have been proposed for use as coatings for lithographic plates fall into two general types. The first type comprises polymeric materials that will, under exposure to ultra-violet light cross-link to produce three-dimensional structures that are insoluble in the developer. Usually a photosensitiser has to be present for this cross-linking to occur. Such materials are exemplified by the polyvinyl cinnamates. The second type of photopolymer comprises a mixture of resin binder and a monomeric material which polymerises by a chain reaction when exposed to ultra-violet light in the presence of a photoinitator. Polyacrylates are often

used as the resin binder, while the monomer, which is usually present in equal quantity to the resin binder, is typified by glycerol tri-acrylate.

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.

Subsequently, as a substitute for the use the 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 photo¬ polymers.

Recently it has been proposed to provide photopoly¬ mers which can be developed in aqueous solutions.

Such photopolymers generally incorporate a carboxylic acid group or some other group so that the material is soluble in water or an aqueous alkali. Such photopolymers may either cross-link, or may be monomers in combination with resin binders. However, it has been found that if a photopolymer of the cross-linking type is completely and readily soluble in water or in an aqueous

alkali, then the photopolymer is not of use as a lithographic coating material since, even after cross- linking has been effected, the photopolymer retains an affinity for the water, or the alkali typically used in a _c fountain solution, which is used to prepare a lithographic plate before the printing process. Thus the coating will not be sufficiently ink receptive. However, if a photopolymer is utilised that is not completely soluble in water or an alkali solution, again

,« difficulties can arise in that it has been found that when such materials are utilised, it is dififcult to achieve complete removal of the material from the anodised plate surface during the developing process. This can result in the condition known as "scumming" in

,c which the non-image area of the lithographic plate becomes partially ink receptive, thus providing ink on the printed sheet in regions where there should not be any ink. The monomer type of photopolymers, which are developable in aqueous solutions, that have been proposed

20 fo ^r use ^{as a} lithographic coating have included relatively high proportions of monomers that are not soluble in water or in alkali. It has been found that these monomers have a tendency to be retained on the anodised aluminium surface after development, which can

25 also lead to "scumming".

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

_Q 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

35 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 app¬ ropriate graining technique may be used including a brush draining technique.

Preferably the photopolymer is an ultra-violet curing polymer. .The photopolymer may contain unsaturated groupings, which may enable the polymer to cross-link when exposed to ultra-violet light, and may also contain carboxylic acid or quatenary ammonium functional groups to enable the polymer to be developable in an aqueous solution.

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 ethy- leinically unsaturated compound containing at least two terminal ethylenic groups, having a boiling point greater than 100°C 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 15°C or less, the anodising being carried out at a current density in excess of 1 A/dm .

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 or mixtures incorporating one or more of these acids or their salts.

Thus the electrolyte in each bath may comprise phosphoric acid of, for example, between 5 and 10? con¬ centration. Alternatively each bath may comprise sulphuric acid of, for example, between 5 and 10% con- centration. 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 0% concentration and the sulphuric acid is between 5 and 10% concentration.

The anodising in the first step may be carried out

2 at a current density of 1 to 4 A/dm , and in the second step may be carried out at a current density of 1 to 4

2 A/dm and in the third step may be carried out at a

2 current density of 1 to 4 A/dm

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, as a lithographic printing plate, which is to be used with a photopolymer to be developed in an aqueous solution, 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 preferably 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 that any remaining monomer or other material which is insoluble will be retained in the hydrophilic region to produce "scumming".

The present applicants believe that any insoluble monomer or other material remaining after development of the photopolymer tends to adhere to the porous surface layer on the prior proposed plates primarily because that surface is porous. However, in the present invention, because the surface is not porous, there is no tendency for the insoluble monomer or other material to cling to the surface, and it can thus be totally washed away, thus ensuring that no "scumming" occurs.

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.

Whilst a plate as described above may be used with many types of aqueously developed photopolymers, a typical photopolymer for this purpose will include unsaturated groups which enable the polymer to cross-link an exposure to ultra-violet light, and will include one or more functional groups, such as carboxylic acid groups or quatenary ammonium groups to enable the polymer to be developed in an aqueous solution.

Thus typically the photopolymer may comprise an ethylenically unsaturated compound. If such a compound is utilised it may be necessary to incorporate an

addition polymerisation initiator. Such a photopolmer may also be utilised in conjunction with an organic macro-molecular polymer binder which may contain acid groups or salts thereof, said binder being alkali soluble.

One example of the suitable photopolymer is a non- gaseous ethyleneically unsaturated compound, or a combination of two or more such compounds, containing at least two terminal ethyleneic groups, having a boiling point greater than 100°C at normal atmospheric pressure and being capable of forming a polymer by photo- initiated. Such a photopolymer may include an addition polymerisation initiator which is activated by actinic radiation, together with an organic macromolecular polymer binder which is alkali soluble.

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 cleaned 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 a three stage process.

In a first step of process the plate is anodised for 2 minutes in an acid bath containing 1% phosphoric acid at a temperature of 15°C, with a current density of 1.5 A/dm ² .

As a second step, in a separate bath containing 8.5/5 phosphoric acid at a temperature of 12°C, the plate is anodised for 2.5 minutes at a current density of 2 A/dm .

In a third bath, containing 5.8? phosphoric acid, at a temperature of 9°C, the plate is anodised for 1.75

2 minutes with a current density of 2.5 A/dm .

The resultant plate had a hard non-porous grained 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 4

The procedure of Example 1 was repeated at a 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 Can 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 photo¬ polymer.

Copolymer of styrene/maleic anhydride partially esterified with iso-butanol

(Mw 40,000) 10 Parts

Ethylene glycol di-acrylate

Benzil 0.6 Michlers Ketone 0.6

Methyl Ethyle Ketone (Solvent) 100

The plates again ran satisfactorily on a web offset press.

Example 7

Plates made in accordance with Examples 5 and 6 were stored for five months before developing in accordance with the procedures of Example 5. The plates again provided a satisfactory result.

The Ciba-Geigy material initially designated RT 605 is now commercially available, under designation XPH 1001/40, in combination with a sensitiser having the designation XSE 1003.