GB2080962A - Light-sensitive Coatings or Lithographic Plates - Google Patents

Abstract

It has been found that several groups of substances, known per se as vat dyes, can undergo colour changes on change of pH caused by exposure to actinic or ultra-violet light. These indoanilines and indonaphthylamines are derived from oxidative coupling of N-substituted p-phenylene diamines, or N-substituted 1,4-naphthalene diamines, with phenol, naphthol, hydroxyquinoline or their derivatives, and are present as indicators in light sensitive coatings containing naphthoquinone diazide esters.

Description

SPECIFICATION Light-sensitive Coatings for Lithographic Plates This invention relates to light-sensitive coatings for lithographic plates and to lithographic plates having such coatings.

The treatment of lithographic plates for use in printing processes includes the exposure of a plate to an image (generally in the presence of actinic or ultra-violet light), followed by developing and optionally fixing the image using a desensitising solution. Subsequently, the image is inked-up ready for printing.

It is well known to those skilled in the art that provision should be made for the "image" portion of the plate to be ink-sensitive and for the "non-image" portion to be ink-repellent. It is often desirable that a visual change shall take place in the exposed area of a plate so that the degree of completeness of exposure can be determined and details of the transposed image can be clearly seen by the operator.

This is especially helpful for registering the image when performing repeat exposures on the same plate.

A convenient method of providing such a visual change is by incorporating a substance which acts as a pH indicator under the conditions of exposure into the coating. Many attempts at such a method have been reported, for example, in U.K. Patent Specifications 1,1 70,458, 1,041,463, 1,347,759 and 1,039,475. The disclosures in the aforesaid Patents are, in general, confined to the use of a combination of one or more diazo-compounds with one or more substances known per se as pH indicators.

We have now unexpectedly found that several groups of substances, known per sue as vat dyes, can undergo colour-changes under conditions of changing pH.

The substances which are the subject of the present invention have been found to have the following advantages: (i) The visual image produced on the exposed plate appears as a positive of the orginal (as opposed to the negative image resulting from the use of indicators described in the aforementioned U.K. Specifications 1,039,475, 1,041,463 and 1,347,759). This greatly facilitates the operator's task of inspection and registration.

(ii) The positive colour-change leads, after removal of the exposed coating during development of the plate, to a dark-coloured, well-contrasted printing image. This also aids the operator when inspecting and correcting any faults in the image.

(iii) The substances are insoluble in water but readily soluble in a wide range of other solvents, making them easy to incorporate in, and ideally compatible with, the light-sensitive coatings. By contrast, the water-soluble indicators disclosed in the aforementioned U.K.

Specifications 1 ,039,475 and 1,041,463 tend to reduce the resistance of the coatings to water during the printing process and hence tend to weaken the image.

These substances are derived from oxidative coupling of N-substituted p-phenylene diamines, or N-substituted 1 ,4-naphthalene diamines, with phenol, naphthol, hydroxyquinoline or their derivatives.

Such substances will hereinafter be referred to as phenol-indoanilines, naphthol-indoanilines, phenol-indonaphthylamines, naphthol-indonaphthylamines, hydroxyquinoline-indoanilines and hydroxyquinoline-indonaphthylamines respectively.

Accordingly, the present invention provides a light-sensitive coating for a lithographic plate, said coating comprising a light-reactive mixture which includes one or more naphthoquinone diazide esters and a substance which will impart a colour change to the said coating on exposure to light, said substance being selected from the group consisting of (a) phenol-indoanilines (b) naphthol-indoanilines (c) hydroxyquinoline-indoanilines (d) phenol-indonaphthylamines (e) naphthol-indonaphthylamines, and (f) hydroxyquinoline-indonaphthylamines (all as hereinbefore defined).

The present invention also provides a lithographic plate having a coating as described in the immediately-preceding paragraph.

The substances used in accordance with the present invention to provide a colour-change on exposure of the plates may be represented by the following general formulae: (a) Phenol-indoanilines

In the above general formulae, R, and R2 (which may be the same or different groups) can be methyl, ethyl, n-propyl, n-butyl, hydroxyethyl or methane sulphonamido-ethyl. Rr and R2 can also be linked to form a heterocycle, e.g. pyrrolidino-, piperidino- or morpholino-.

R,,,, Rn and R, (which again may be the same or different groups) can be hydrogen, methyl, ethyl, hydroxy, methoxy, ethoxy, halogen (e.g. F, Cl or Br), sulphonic acid ester or amide, or carboxylic acid ester or amide.

Rn and Ry can also be a fused-aromatic group forming an anthracene or phenanthrene nucleus.

Rx can be hydrogen, methyl, ethyl, higher alkyl (up to 5 carbon atoms), methoxy, ethoxy, a methylene group linking two molecules, halogen (e.g. F, Cl or Br), benzyl or aryl.

In the case of the phenol-indoanilines (a) or the phenol-indonaphthylamines (d), R, cannot be entirely hydrogen but must be substituted with one of the alkyl or alkoxy groups listed above, preferably in the ortho position relative to the quinonoid group.

In the practice of the present invention, the substance which imparts a colour change to the coating on exposure of a lithographic plate is dissolved in a compatible solvent, mixed with one or more appropriate naphthoquinone diazide esters and subsequently mixed with a carrier material such as a novolac resin before being spread on to a prepared plate.

We have found that the concentration of the substance producing a colour change may be in the range 1% to 100% (wiz, calculated on the naphthoquinone diazide content), suitably 2% to 40% and preferably 2% to 20%.

The actual concentration range selected will depend on the intensity of colour of the substance used. For example, in the naphthol-indoaniline having the formula

when R is hydrogen the blue colour is approximately 50% more intense than when R is methyl. The presence of a methyl group also affects the colour shade-which becomes greener.

The present invention will be illustrated by way of the following Examples.

Example 1 A naphthol-indoaniline dye (as hereinbefore defined), available under the description COLOUR INDEX 49700 and consisting essentially of N-(pdimethylaminophenyl)-1 ,#naphthoquinoneimine was used as the substance imparting a colour change to a lithographic plate coating.

The dye has the following structural formula:

The following formulation was prepared: Note Ingredient Amount (g) (a) Novolac resin 7.5 (b) Naphthoquinone diazide ester 2.5 (c) Naphthol-indoaniline dye 0.1 Notes (a) A cresol-formaldehyde resin, available from C.P.C. (U.K.) Limited as KELREZ 40130. (KELREZ is a Registered Trade Mark).

(b) The p-tolyl aster of 1 ,2-naphthoqulnone diazide-(2)-5-sulphonic acid (available from Ward Blenkinsop Limited "M.DIAZ.P-TOLYL ESTER)." (c) COLOUR INDEX 49700 (see above) dissolved in methyl ethyl ketone to give a total volume of 100 ml.

The mixture was coated (to a wet-film depth of approximately 25 microns) on to an aluminium plate which had previously been electrochemically grained, anodised and sealed. The coated plate was dried (at 65-700C for 5 minute,s) and then exposed through a contact-positive film in the presence of ultraviolet light. The exposure was sufficient to render the coating soluble in an alkaline developer, consisting of a mixture of sodium metasilicate and trisodlum phosphate dissolved in water to give a pH in the range 12.5 to 13.0.

The coating changed colour from blue/green to pale orange/grey with a contrast sufficient to see the fine detail transposed from the original contact positive. The image produced was also a positive of the original.

A range of dye concentrations from 2% to 20% WAW (calculated on the content of naphthoquinone diazide ester) were evaluated and they all produced a colour change, although the optimum dye concentration for best contrast was in the range 4% to 8%. It was found possible to increase the concentration of dye beyond 20% by increasing the concentration of naphthoquinonediazide ester in the coating.

Example 2 A phenol-indoaniline dye (as hereinbefore defined) was prepared by coupling a novolac resin (KELREZ 40130~see Example 1 above) with N,N-diethyl-3-methyl-p-phenylene diamine. The reaction was adjusted so that only about 50% of the phenolic groups in the resin were coupled to form dye molecules.

The idealised structure of the dye moiety in the resin is

The following formulation was prepared: Note Ingredient Amoung (g) (a) Novolac resin 7.5 (b) Naphthoquinone diazide ester 2.5 (c) Phenol-indoaniline dye 0.2 Notes (a)# See Example 1 (above) (b) See Example 1 (above)Example f (c) Dissolved in a 50:50 mixture (by volume) of methyl ethyl ketone and 2-ethoxyethyl acetate to give a total volume of 100 ml.

The mixture was coated, dried and exposed as described in Example 1 (above).

The coating changed colour on exposure from blue/green to pale grey, with sufficient contrast to see the fine detail transposed from the original contact-positive film. The image produced was a positive of the original.

Claims (1)

1. Claims

   1. A light-sensitive coating for a lithographic plate, said coating comprising a light-reactive mixture which includes one or more naphthoquinone diazide esters and a substance which will impart a colour change to the said coating on exposure to light, said substance being selected from the group consisting of: (a) phenol-indoanilines (b) naphthol-indoanilines (c) hydroxyquinoline-indoanilines (d) phenol-indonaphthylamines (e) naphthol-indonaphthylamines, and (f) hydroxyquinolineindonaphthylamines (all as hereinbefore defined).

   2. A coating according to Claim 1, in which the concentration of the said substance in the coating is from 1% to 100% WIW, calculated on the naphthoquinone diazjde content.

   3. A coating according to Claim 2, in which the concentration of the said substance is from 2% to 40% WAW, calculated on the naphthoquinone diazide content.

   4. A coating according to Claim 2 or 3, in which the concentration of the said substance is from 2% to 20% W/W, calculated on the naphthoquinone diazide content.

   5. A lithographic plate having a light-sensitive coating according to any one of Claims 1 to 4.

   6. A coating for a lithographic plate, substantially as hereinbefore described with reference to either of the Examples.

   New Claims or Amendments to Claims filed on 24/7/80.

   Superseded Claims 1.

   New or Amended Claims:~

   1. A light-sensitive coating for a lithographic plate, said coating comprising a light-reactive mixture which includes one or more naphthoquinone diazides and a substance which will impart a colour change to the said coating on exposure to light, said substance being selected from the group consisting of: (a) phenol-indoanilines (b) naphthol-indoanilines (c) hydroxyquinoline-indoanilines (d) phenol-indonaphthylamines (e) naphthol-indonaphthylamines, and (f) hydroxyquinolineindonaphthylamines (all as hereinbefore defined).