This invention relates to the production of a pre- sensitized lithographic printing plate.

Pre-sensitized lithographic printing plates are usually formed from a web of an aluminium substrate which is supplied in roll form, and which passes continuously through successive treatment stages (including an anodising stage) which prepare the surface of the web to receive a suitable light sensitive coating, and then the web is cut- up into printing plates.

The present invention has-been developed primarily, though not exclusively, with a view to providing an improved coating composition, and an improved anodising treatment of the web, so as to achieve production of a pre- sensitized lithographic printing plate having improved performance.

In this specification, the reference to an "aluminium substrate" is intended to include aluminium and aluminium alloy substrates of the type suitable for use in the production of lithographic printing plates.

It is known from Fuji German specification DE 3633456 to provide a resin which forms a light sensitive coating on an aluminium substrate, and which comprises a photo crosslinkable polymer having a maleimidic functional group. Various examples of resins are disclosed but, as stated in the specification, there are problems with resins of this type in achieving reliable adhesion of the light sensitive coating formed from the resin, and the disclosure teaches a solution to this problem by the use of a negatively functioning diazo resin. While this does indeed help in overcoming problems of improper adhesion i.e. peeling- of the coating, there are well known difficulties with diazo resins in that they are not thermally stable and also they have a short shelf life, especially when used in hotter countries.

The diazo resin therefore loses its effective properties fairly rapidly, with consequent adverse effect on the subsequent development process.

It is also generally known to use acid anodising of substrates to improve the properties of the substrate in the following -areas: (a) water balance on the printing press; (b) coating release during development; (c) durability of substrate; and (d) substrate colour.

However, the present invention is based on the surprising discovery that the difficulties of loss of adhesion encountered with resins of the general type disclosed in DE 3633456 can be overcome in a different way and which can realise additional technical advantages as regards to sensitivity.

Accordingly, the present invention provides a method for producing a lithographic printing plate which comprises a light sensitive coating applied to an aluminium substrate, in which the method includes an anodising step in which the aluminium substrate is anodised using phosphoric acid present in a range from 100% to 30% by and sulphuric acid present in a range from 0% to 70% by volume of the same normality, and a coating step in which the anodised aluminium substrate is coated with a resin which comprises a photo crosslinkable polymer having a maleimidic functional group plus a sensitiser.

A lithographic printing plate made in accordance with the method of the present invention is developable using an aqueous developing medium. A significant loss of light sensitivity is normally encountered when dye stuffs are added to the polymer resins containing dialkylmaleimidyl functional groups. It has been surprisingly found that by using a mixed acid anodising medium it is possible to overcome this problem by significantly reducing this loss of light sensitivity. This improvement in the sensitivity of the coating is regarded as a highly desirable property for a lithographic printing plate.

Preferably the coating composition also includes one or more of a sensitiser, a dye-stuff, a photo-oxidiser and a surfactant.

Preferably the mixed acid medium comprises a mixture of sulphuric and phosphoric acids in a total acid concentration from IN to 20N. Preferably the ratio of sulphuric acid to phosphoric acid is in the range from 30/70 V/V to 50/50 V/V and more preferably is about 40/60 V/V respectively. The preferred total acid concentration is in the range from 3N to 9N.

Preferably the photo crosslinkable polymer is an acrylic polymer. Preferably the crosslinkable groups are dimethylmaleimidyl groups.

Typically the polymer may be formed by co-polymerising a dimethylmaleimid 1-functional ised acrylic or methacrylic monomer (DMI-monomer) with one or more acrylic or methacrylic comonomers. The DMI-monomer may be represented by the formula

in which the group R may be, for instance, alkylene or oxyalkylene.. Examples of R are ⁽ CH2 ⁾ 2> ⁽ CH2 ⁾ 2~0- ⁽ CH2 ⁾ 2 ^anc * (CH2)g. the latter being particularly preferred-.

An example of a photo-oxidiser which may be used in connection with the present invention is a triazine derivative, for instance, one containing a chalcogenide substituent. Examples of such ^' photo-oxidisers are described in pending British patent application No 8627059. They have the general formula

wherein R is an alkyl group or an aryl group, X is S, Se or Te, n and m are 0, or each is an integer of from 1 to 3, the total sum of n + m not being greater than 3 and p and q are 0, or each is an integer of from 1 to 3, the total sum of p + q not being greater than 3.

In carrying out the method of the present invention, the following steps will normally be involved: degreasing, water rinse, acid treatment, water rinse, graining, water rinse, acid or alkali treatment, water rinse, anodising, water rinse, post-anodic treatment, water rinse, drying oven, coating and drying oven.

The various processing conditions and the formulations of the materials used in each stage are in accordance with standard procedures well known to those skilled in the art.

While the preferred ratio of sulphuric acid to phosphoric acid in the anodising treatment came in the range 30/70 to 50/50, it has been found in tests that advantageous results can be obtained for both adhesion and improved sensitivity when the phosphoric acid is present from 100% down to 30% and the sulphuric acid present from 0% to 70% by volumes of the same normality.

The use of greater proportions of phosphoric acid tend to form a less durable coating, but for many uses this may be quite acceptable e.g. for proofing purposes.

Experiments ^" 1)

Formulae

DMI (referred to above)

Thioxanthone sensitizers

Photooxidizer

Dye stuffs

Surfactant

100.0% 100% Using formulae A and B, 2 plates were manufactured at

1. Ogm coating weight.

These formulae were evaluated on the following substrates:

1) sulphuric acid anodized

2) sulphuric / phosphoric (40/60 by volume of same normality) acid anodized

3) phosphoric acid anodized.

The plates were all exposed to the same quantity of light. Then developed in a commercial developer available from Horsell Graphic Industries designated C 1346 for one minute at 20°C and the results were then evaluated.

The results were assessed using a stouffer grey scale to determine sensitivity, and in which S apparently represents the highest fully cured step on the wedge, and C apparently represents the lowest totally uncured step on the wedge.

Results formulae Substrate A B

1 S6-C15 S6-C15

2 S9-C17 S9-C17

3 S13-C19 S14-C19 NB good adhesion was seen on all plates.

2) Formulae

A B

DMI monomer 91% DMI monomer 91%

Thioxanthone sensitizer 9% Ketocoumarin 9%

Again processing conditions, coating weights and substrate designations same as Experiment 1.

Results Substrate A B

1 S6-C15 S4-C13

2 S9-C17 S9-C17

3 S14-C19 S15-C19 NB Good adhesion was seen on all plates.

As can be seen from the above two experiments, a considerable improvement in sensitivity can be achieved by the use of mixed and anodising with sulphuric and phosphoric acid while still retaining good adhesion and a durable anodised substrate.