Positive-working light sensitive diazo materials with azo dye

A positive working light-sensitive composition comprises in admixture (a) a first component which is an azo dye formed by reacting a p-amino-benzene diazonium salt and a compound including the grouping ##STR1## and (b) a second component which includes a o-quinone diazide and/or a diazonium compound and which, on exposure to light produces an acidic light decomposition product which is capable of reacting with the azo dye to produce a material having a color different to that of the azo dye. The composition exhibits a color change on exposure to light and is useful for the manufacture of light sensitive plates for use in lithographic printing plate production.

This invention relates to positive working light-sensitive compositions and 
is concerned with such compositions which are suitable for, for example, 
the production of lithographic printing plates by photo-mechanical 
methods. 
Photo-mechanical methods of preparing printing plates comprise producing a 
light-sensitive plate by coating a light-sensitive composition onto a 
suitable substrate e.g. of anodised aluminium, image-wise exposing the 
light-sensitive plate so that the light-struck areas of the composition 
and the non-light struck areas of the composition have differing 
solubility in a liquid and then developing the image-wise exposed plate 
with the liquid to selectively remove the more soluble of the areas. In 
the case where the light-sensitive composition is negative working, the 
non-light-struck areas are removed whereas the light-struck areas are 
removed in the case where the light-sensitive composition is positive 
working. Those areas of the composition remaining on the substrate after 
development constitute the final ink-receptive printing image of the 
printing plate and the water-receptive non-printing areas of the printing 
plate are constituted by the surface of the substrate revealed on 
development. 
In these procedures it is desirable for a colour change to take place in 
the light-struck areas so as to render them visible: this is especially 
the case in step-and-repeat work where it is necessary to align successive 
images on the plate. In some cases the colour change inherent in the 
light-sensitive system used (e.g. in azide sensitised layers) may be 
sufficient in itself. In other cases, although some colour change occurs 
on image-wise exposure to light, this is rarely sufficient in practice to 
enable the image to be visibly inspected. This is particularly the case 
with light-sensitive compositions based on quinone diazides or diazonium 
salts. 
Light-sensitive compositions comprising positive working quinone diazides, 
usually in admixture with a novolak resin, are used to a large extent in 
printing plate manufacture but, as aforementioned, they suffer from the 
disadvantage that they do not ordinarily undergo an adequate colour change 
on exposure to light. On exposure to light of light-sensitive compositions 
based on such quinone diazides, the quinone diazide is converted to an 
indene carboxylic acid [via a ketene intermediate: Cf, O Sus, Annalen, 
556, 65-84 (1944)], thus rendering the composition soluble in and 
developable by an aqueous alkaline developer. Although there is a slight 
colour change from the original yellow colour on exposure, even with the 
aid of the addition of a contrast-providing dye, for example a violet dye, 
it is difficult to see the light-struck areas in the yellow safe-light 
that is usually employed in the working area. 
Although a number of dyes are known which, when incorporated in 
light-sensitive coatings are capable of reacting with acidic light 
decompositions products of materials in the coating to give brightly 
coloured non-image areas on exposure, these are generally yellow in colour 
and hence the final image constituted by the non-light struck areas of the 
coating will also be yellow and thus difficult to see under the yellow 
safelight. The incorporation in the light-sensitive composition, of an 
additional dye having a stronger colour can improve the "visibility" of 
the final image but also tends to reduce the contrast between the light 
struck and non-light struck areas and may adversely affect the sensitivity 
and developability of the plate. 
According to the present invention, there is provided a positive working 
light sensitive composition which exhibits a colour change upon exposure 
to light, which composition comprises in admixture (a) a first component 
which is an azo dye formed by reacting a p-amino-benzene diazonium salt 
and a compound including the grouping 
##STR2## 
and (b) a second component which includes an o-quinone diazide and/or a 
diazonium compound and which, on exposure to light, produces an acidic 
light decomposition product which is capable of reacting with the azo dye 
to produce a material having a colour different to that of the azo dye. 
The o-quinone diazide or diazonium compound of the second component may be 
such as to produce the desired light decomposition product. Alternatively, 
or additionally, the second component may include a substance, other than 
the o-quinone diazide or diazonium compound, which produces the desired 
acidic light decomposition product. Examples of such substances are 
halogenated hydrocarbons, such as iodo form, and sulphonyl halides. 
The p-amino benzene diazonium salt may be unsubstituted but preferably it 
has substituent groups on the amino nitrogen atom and possibly also in the 
aromatic ring. 
Examples of p-amino benzene diazonium salts which may be used to form the 
azo dye are: 
##STR3## 
where X is a suitable anion such as Cl or (ZnCl.sub.4.sup.2-)1/2 
Said compound used to form the azo dye by reaction with the p-amino benzene 
diazonium salt may be 3-hydroxy-2-naphthoic acid or a suitable derivative 
thereof. Examples of such derivatives are: 
______________________________________ 
(i) 
##STR4## methyl 3-hydroxy- 2-naphthoate 
(ii) 
##STR5## 3-hydroxy-2- naphthamide 
(iii) 
##STR6## 3-hydroxy-2- naphthanilide 
(iv) 
##STR7## 3-hydroxy-2- naphth-o-toluidide 
(v) 
##STR8## 3-hydroxy-7- methoxy-2- naphthanilide 
(vi) 
##STR9## 3-hydroxy-2-[N- (2-amino-ethyl]- naphthamide 
(vii) 
##STR10## 3-hydroxy-2- naphthoic acid piperidide 
(viii) 
##STR11## 3-hydroxy-2- naphth-p- anisidide 
(ix) 
##STR12## 4-chloro-2,5- dimethoxy-anilide of 3-hydroxy- 
2-naphthoic acid 
______________________________________ 
The azo dyes used in the compositions of the present invention have the 
general formula: 
##STR13## 
The azo dyes have a colour which is readily visible in yellow safelight but 
are not light-sensitive, or not sufficiently so that any fading reaction 
which does take place on exposure to light provides an adequate colour 
change. Of the contrary, they react with the acidic light decomposition 
products of the second component of the composition to form a substance of 
different colour to that of the azo dye. The light-struck areas thus 
exhibit a colour change (usually by fading to a much paler shade) but the 
colour of these areas remains sufficiently strong to enable the platemaker 
to see clearly all areas of the composition which have been struck by 
light and which need to be removed completely during development. Thus, 
both the light struck areas and the non-light struck areas are 
sufficiently coloured to allow the presence of any residual material left 
after development to be detected and the quality of the final image to be 
ascertained under the yellow safelight. It has also been found that the 
compositions of the present invention have a reduced tendency to stain the 
non-printing areas when used in lithographic printing plate production. 
In order to facilitate coating a substrate with the composition to form a 
light sensitive plate or to improve upon the properties of a printing 
plate produced from such a light sensitive plate, it may be desirable to 
include, in the composition of the present invention, materials such as a 
novolak resin, other non-light sensitive polymers, plasticisers or 
stabilisers.

The following Examples illustrate the invention: 
EXAMPLE 1 
3-Hydroxy-2-naphth-o-toluidide (20 g) was dissolved in dimethylformamide 
(400 ml) and the resultant solution was then filtered and subsequently 
added dropwise to a stirred solution of sodium hydroxide (2.73 g) in water 
(400 ml). Sodium carbonate (12.25 g) was added and the pH adjusted to 12 
to 13. Para-N,N-diethylaminobenzene diazonium zinc chloride double salt 
(20 g) in a 1% solution of citric acid (150 ml) was introduced slowly and 
with stirring into the toluidide solution, previously cooled to 
0.degree.-5.degree. C., over a period of 30 minutes. The mixture was 
stirred for a further 3 hours and filtered. The residue was washed with 
water and then treated with dilute hydrochloric acid to yield the azo dye 
3-hydroxy-4-(p-N,N-diethylaminophenylazo)-2-naphth-o-toluidide. 
A light sensitive coating composition was prepared comprising a first 
component consisting of 0.16 g of the above azo dye; a second component 
consisting of 1.0 g of the product obtained by the esterification of 1 
mole 2,3,4-trihydroxybenzo phenone with 2 moles of 
naphthoquinone-(1,2)diazide-(2)-5-sulphonyl chloride and 0.16 g of 
4-dimethyl aminobenzene diazonium tetrafluoroborate; 5.7 g. of a 
cresol-formaldehyde novolak resin; and 80 ml of acetone. 
The composition was applied by means of a whirler to the surface of grained 
and anodised aluminium lithographic plate base and the coated plate was 
then dried. The resultant light-sensitive plate was exposed for 2 minutes 
in a printing down frame and in contact with a positive transparency to a 
4000 watt pulsed xenon lamp at a distance of 0.65 meters. During exposure 
the colour of the plate was seen to change from a dark green to a pink in 
the light-struck areas. 
The exposed plate was developed with 8% sodium metasilicate solution to 
remove the light struck areas and leave a final image constituted by the 
non-light struck areas which were readily visible in yellow safelight. 
When mounted on a lithographic printing press the developed plate provided 
many satisfactory copies. 
EXAMPLE 2 
Example 1 was repeated four times but using as the dye the following 
compounds in turn: 
i. 
3-hydroxy-4-(p-N,N-diethylaminophenylazo)-2-[N-(2-aminoethyl)]-naphthamide 
. Colour change: green to pink. 
ii. 
3-hydroxy-4-(p-N,N-diethylamino-3-chlorophenylazo)-2-naphth-o-toluidide. C 
olour change: brown red to pink. 
iii. the 4-chloro-2,5-dimethoxyanilide of 
4-(diphenylamine-4-azo)-3-hydroxy-2-naphthoic acid. Colour change: green 
to grey. 
iv. 3-hydroxy-4-(p-diethylaminophenylazo)-2-naphthoic acid. Colour change: 
green to pink. 
EXAMPLE 3 
A light sensitive plate was produced and processed as in Example 1 but 
using as the light-sensitive composition a mixture comprising a first 
component consisting of 0.05 g. of 
3-hydroxy-4-(p-N,N-diethylaminophenylazo)-2-naphth-p-anisidide; a second 
component consisting of 2 g. of diazo RO 220 and 0.05 g. of 
4-dimethylaminobenzene diazonium tetrafluoro-borate; and 25 ml of acetone. 
Diazo RO 220 is a light-sensitive 
naphthoquinone(1,2)-diazide(2)-5-sulphonic acid ester of a resin. 
The colour change was similar to that obtained in Example 1. 
EXAMPLE 4 
Example 3 was repeated using 0.1 g. iodoform in place of the 
4-dimethylamino benzene diazonium tetrafluoroborate. A similar colour 
change was obtained. 
EXAMPLE 5 
A light sensitive plate was produced and processed as in Example 1 but 
using as the light sensitive composition a mixture comprising a first 
component consisting of 0.25 g. of the 4-chloro-2,5-dimethoxy anilide of 
4-(4.sup.1 -morpholino-2.sup.1, 5.sup.1 
-diethoxyphenylazo)-3-hydroxy-2-naphthoic acid; a second component 
consisting of 1.65 g. of diphenylamine-4-diazonium fluoroborate; 12 g. of 
cresol-formaldehyde novolak resin; 180 ml of acetone; and 100 ml of ethyl 
methyl ketone 
After exposure the plate was developed in 1% sodium hydroxide solution. 
During exposure the colour of the plate was seen to change from green to 
pink. 
EXAMPLE 6 
A light sensitive plate was produced and processed as in Example 1 but 
using as the light sensitive composition a mixture comprising a first 
component consisting of 0.16 g of 
3-hydroxy-4-(p-N-ethyl-N-hydroxyethyl-phenylazo)-2-naphthpiperidide; a 
second component consisting of 0.32 g. of 4-chloro-3-nitro-benzene 
sulphonyl chloride and 1 g of the product obtained by the esterification 
of 1 mole of 2,4-dihydroxybenzophenone with 2 moles of 
naphthoquinone-(1,2-diazide-(2)-5-sulphonyl chloride; 5.7 g. of 
cresol-formaldehyde resin; and 80 ml of acetone. 
The colour change during exposure was again green to pink