GB2226150A

GB2226150A - A developer for use with lithographic printing plates

Info

Publication number: GB2226150A
Application number: GB8829241A
Authority: GB
Inventors: Nordisk Tidningsplat Ab
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-12-15
Filing date: 1988-12-15
Publication date: 1990-06-20
Also published as: GB8829241D0

Abstract

A developer for use with a lithographic printing plate which has a photosensitive layer comprising a photopolymerisable material, consists of an aqueous solution of an organic alkaline material together with a wetting agent. The organic alkaline material may be an amine such as tri-ethylamine, ethanolamine and tri-ethanolamine although the organic alkaline material may be a quaternary ammonium salt. The wetting agent may be a phosphate ester wetting agent such as potassium salt of an alkyl phosphate ester.

Description

DESCRIPTION OF INVENTION "Improvements in or relating to a developer for use with lithographic printing plates" This invention relates to a developer for use with lithographic printing plates, and to a method of developing lithographic printing plates.

Lithographic printing plates have been in use for many years. A typical lithographic printing plate consists of a substrate, such as aluminium, upon which is provided a coating of a photo-sensitive material.

When the photosensitive material has been selectively exposed to light or other actinic radiation, through an appropriate mask, part of the material (which may be the part exposed to light or which may be the part which has not been exposed to light) is soluble in an appropriate developer, and the remaining part is insoluble.

For example, for a simple printing plate which is developable with an alkali material, a simple alkaline solution of an appropriate pH will always give a reasonable result. The primary function of such a developer is to remove the soluble coating from the surface of the plate, and to dissolve or suspend the coating thus removed so it cannot re-deposit on the plate or stick to any part of an automatic machine in which the developer is used. Ideally the developer should treat the metal surface, left bare by the developer, so that it cannot accept printing ink nor be affected by finger marks or the like.

Ideally a developer of this type should function rapidly. In many automatic developing machines presently in use the maximum available developing time is around 20 seconds. The developer should not attack in any way the areas of coating which are not intended to be soluble. Even a partial attack can lead to poor dot reproduction, which can lead to very poor quality printing if photographs or the like are to be printed.

Ideally a developer should be stable in air, since the developer will be exposed to air when it is introduced to the machine. The developer should have a high dissolving power to enable a large number of lithographic plates to be developed in the same bath in an automatic machine without the need to replace the developer. The developer should provide as good a quality of developing when it is almost exhausted as when it is new.

The developer should, for obvious reasons, be non-toxic and should preferably have no odour. In order to minimise problems associated with the eventual disposal of the spent developer, the developer should be bio-degradable.

It has long been realised that a developer based on water will best fulfil the above functions, as well as being potentially inexpensive.

The commonest water-based or aqueous developers that have been proposed before are alkaline developers which make use of the fact that carboxylic acid groups are relatively easily introduced into light sensitive polymers, and also such carboxylic acid groups are associated with materials that are soluble in alkalies.

Aqueous alkaline developers as proposed previously are commonly used for positive working lightsensitive coatings. These coatings contain a naphthoquinone diazide derivative which, on exposure, decomposes to an indenecarboxylic acid. Thus the exposed area of a plate coated with such a diazide derivative becomes soluble in an alkaline on exposure to light.

Negative working, alkali-developable, coatings require the carboxylic acid group to be part of the original polymer, which then becomes insoluble by crosslinking. While such polymers have been known for a long time, they have not yet come into general commercial use.

It has recently been proposed to utilise, as a coating material for a lithographic plate, materials which photopolymeris and which can be developed in aqueous solutions.

Such a photopolymer material may be 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. One example of such a photopolymer comprises an ethyleneically unsaturated compound, an addition polyermisation initiator, and an organic macromolecular polymer binder which may contain acid groups, or salts thereof, said binder being alkali soluble.Alternatively the photopolymer may comprise at least one non-gaseous ethylenically un-saturated 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-poly- mer by photo-initiated addition polymerisation, an addition polymerisation initiator activated by actinic radiation and an organic macromolecular polymer binder which is alkali soluble. In this Specification such materials will be termed photopolymerisation materials of the type described.

It was initially thought that a conventional sodium silicate solution could be utilised as a developer for these photopolymer materials. However, this gave rise to problems in an automatic developing machine in that the reproduction quality deteriorated rapidly and insoluble matter quickly built up in the developer, requiring frequent changes of developer.

The present invention therefore seeks to provide an improved developer for use with a photopolymer coated lithographic plate.

According to this invention there is provided an aqueous developer for developing a lithographic printing plate having a photosensitive layer comprising a photopolymerisable material, said developer comprising an aqueous solution of an organic alkaline material together with a wetting agent.

Preferably the organic alkaline material is an amine, selected from the group comprising tri-ethylamine, ethanolamine and tri-ethanolamine, together with mixtures and combinations thereof, with tri-ethanolamine being most preferred.

Alternatively the organic alkaline material comprises one or more quaternary ammonium salts.

Preferably the wetting agent comprises a phosphate ester wetting agent, which may be the salt of an alkyl phosphate ester, preferably the potassium salt.

Alternatively the wetting agent may be the potassium salt of a phosphated alkylphenol ethylene oxide adduct.

Preferably the concentration of the organic alkaline material is between 2 and 20%, the preferred concentration of the organic alkaline material being between 5 and 10%.

Advantageously the concentration of the wetting agent is between 1 and 50%, the preferred concentration of the wetting agent being between 5 and 20%.

Conveniently the pH is within the range of 9 to 12%, the preferred pH being within the range of 9.5 to 11%.

According to another aspect of this invention there is provided a method of developing a photolithographic plate provided with a photopolymerisable coating, said method comprising the step of exposing selected parts of said plate to actinic radiation, and subsequently developing the plate with a developer as described above.

The invention also relates to a photolithographic plate provided with a photopolymerisable coating which has been selectively exposed to actinic radiation and subsequently developed using a developer as described above.

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.

The inventor has found, surprisingly, that an aqueous solution of an organic alkaline material may be used as a developer for a photopolymer material. In particular the present inventor has discovered that such an organic alkaline material in combination with a wetting agent provides particularly beneficial results.

The organic alkaline materials that may be utilised in the invention may be selected from a large group of such materials including amines and quaternary ammonium salts. However, quaternary ammonium salts can often be harmful in printing plate developers as these salts can react with the aluminium surface to make it ink-receptive. This is clearly not totally desirable.

Thus it is preferred to utilise the amines, but certain amines can be excluded from the list of preferred materials for reasons of odour, toxicity and stability.

The preferred materials are thus tri-ethylamine, ethanolamine and tri-ethanolamine, and mixtures thereof, with tri-ethanolamine being most preferred.

As far as the wetting agents are concerned, a number of classes of wetting agent have provided beneficial results, but the best results have been obtained utilising a phosphate ester wetting agent. The use of such a wetting agent in the developing solution considerably improved the speed of development and, surprisingly, also allowed the pH of the developer to be reduced from about 12 to about 10.5, while still giving perfectly adequate results. This of course leads to a much more stable developer since, at a lower pH, the developer is more stable in air.

Phosphate ester wetting agents, as a class, are described as salts or alkyl phosphate esters. It is preferred to utilise the potassium salts. Another type of phosphate ester wetting agent, which appears to work, is described as the potassium salt of a phosphated alkylphenol ethylene oxide aduct. Examples of suitable wetting agents are Berol 522 and Berol 733 from Berol Kemi AB. (Berol is a Trade Mark).

The combination of the organic alkaline material and the wetting agent has, surprisingly, provided a very beneficial and advantageous developer.

The organic alkaline material may have a concentration of from 2 to 20%, with the preferred range being from 5 to 10%, and the wetting agent preferably has a concentration of between 1 and 50%, with the preferred range being 5 to 20%. The pH range is preferably 9 to 12 with the range of 9.5 to 11 being most preferred.

The invention will be further described with reference to examples, some of which are comparative examples.

Example 1 An alumium plate, made of lithographic quality aluminium, is 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 ano dised for two minutes in an acid bath containing 7% phosphoric acid at a temperature of 15 0C, with a current density of 1.5 A/dm2.

As 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 9 C, 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 grained surface.

The plates were coated with an ultra violet curing photopolymer as supplied by Ciba-Geigy under des 2 ignation RT 605. The coating had a density of 1g/m2.

This is a coating of photopolymerisable material of the type described. The Ciba-Geigy material initially designated RT 605 is also commercially available, under designation XPH 1001/40, in combination with a sensitiser having the designation XSE 1003.

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.

Example 2 A plate in accordance with Example 1 was developed by sponging with a solution of: Tri-ethanolamine 10 grams Berol 733 (40% solution) 1 gram Water 100 ml pH 11.8 The plate developed in 25 seconds and, after washing, was treated with a 5% solution of dextrin in water and dried. The plate was subsequently placed on a printing press where clean sharp copies were produced within four revolutions of the printing cylinder.

Example 3 (Comparative Example) Example 2 was repeated except that the developer was: Tri-ethanolamine 10 grams Water 100 cc pH 12.

The plate was exposed and developed as before but this time the plate had to be sponged for 35 seconds before it was clear. A similar printing result was obtained as with Example 2.

Example 4 (Comparative Example Example 2 was repeated except that the developer was: Sodium metacylicate 5 grams Water 100 grams.

The plate was exposed and developed but the developer did not appear to dissolve the coating and small flakes could be seen suspended. Fairly hard rubbing was required to complete the development. When placed on a printing press only poor copies were produced until the 20th revolution of the printing cylinder.

Example 5 A plate as in Example 1 was placed in automatic developing machine together with a developer comprising: Tri-ethanolamine 500 grams Berol 522 500 grams Water 5 litres pH 10.4.

The plate was exposed and developed in the automatic developing machine, the development stage lasting 20 seconds. The machine gave a clean plate with good reproduction which printed as in Example 2. A further plate was passed through the automatic developing machine, without refreshing or replacing the developer. This process was repeated for 200 successive plates. There was no deterioration in the quality. of the plates.

Claims

CLAIMS:

1. An aqueous developer for developing a lithographic printing plate having a photosensitive layer comprising a photopolymerisable material, said developer comprising an aqueous solution of an organic alkaline material together with a wetting agent.

2. A developer according to Claim 1 wherein the organic alkaline material is an amine.

3. A developer according to Claim 2 wherein the amine is selected from the group comprising tri-ethylamine, ethanolamine and tri-ethanolamine, together with mixtures and combinations thereof.

4. A developer according to Claim 3 wherein the amine comprises tri-ethanolamine.

5. A developer according to Claim 1 wherein the organic alkaline material comprises one or more quaternary ammonium salts.

6. A developer according to any one of the preceding Claims wherein the wetting agent comprises a phosphate ester wetting agent.

7. A developer according to Claim 6 wherein the wetting agent is the salt of an alkyl phosphate ester.

8. A developer according to Claim 7 wherein said salt is a potassium salt.

9. A developer according to Claim 6 wherein the wetting agent is the potassium salt of a phosphated alkylphenol ethylene oxide adduct.

10. A developer according to any one of the preceding Claims wherein the concentration of the organic alkaline material is between 2 and 20%.

11. A developer according to Claim 10 wherein the concentration of the organic alkaline material is between 5 and 10%.

12. A developer according to any one of the preceding Claims wherein the concentration of the wetting agent is between 1 and 50%.

13. A developer according to Claim 12 wherein the concentration of the wetting agent is between 5 and 20%.

14. A developer according to any one of the preceding Claims wherein the pH is within the range of 9 to 12%.

15. A developer according to Claim 14 wherein the pH is within the range of 9.5 to 11%.

16. A developer substantially as herein described with reference to Example 2.

17. A developer substantially as herein described with reference to Example 5.

18. A method of developing a photolithographic plate provided with a photopolymerisable coating, said method comprising the step of exposing selected parts of said plate to actinic radiation, and subsequently developing the plate with a developer according to any one of Claims 1 to 15.

19. A photolithographic plate provided with a photopolymerisable coating which has been selectively exposed to actinic radiation and subsequently developed using a developer according to any one of Claims 1 to 15.

20. A method of developing a photolithographic plate substantially as herein described with reference to Example 2.

21. A lithographic plate, when developed, substantially as herein described with reference to Example 2.

22. A method of developing a lithographic plate substantially as herein described with reference to Example 5.

23. A lithographic plate whenever developed by methods substantially as herein described with reference to Example 5.

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