Storage stable photosensitive diazo lithographic printing plates

Storage stable lithographic printing plates employing novel light sensitive, high molecular weight diazo polymers are herein provided. The novel diazo polymers of the instant invention have the generic formula: ##STR1## Where R is methyl or hydrogen PA1 X is methoxy, ethoxy or hydrogen PA1 Y is oxygen or sulfur PA1 Z is an anion of a sulfonic, carboxylic or phosphonic acid PA1 n is an integer from 8 to 970.

BACKGROUND OF THE INVENTION 
This invention relates to novel compositions of matter useful as 
lithographic photosensitizers. More specifically, the present invention 
relates to novel storage stable diazo photosensitizers useful in the 
manufacture of lithographic printing plates. Still more particularly the 
instant invention relates to novel lithographic photosensitizers which are 
optionally blended with a suitable dye and a compatible resin in an 
acceptable solvent system to form compositions which when applied to 
substrates form commercially satisfactory lithographic printing plates 
having extended shelf lives. 
It has been a problem in the past to produce commercially satisfactory 
lithographic printing plates which retain their ability to be effectively 
exposed and developed many years after their original manufacture. 
Planographic printing plates must be carefully prepared so as to maintain 
their ability to differentiate between hydrophilic and oleophilic areas on 
its surface. 
The prior art has described a plethora of photosensitive compositions which 
may be employed to produce such plates. One such example is the 
condensation product of paradiazo diphenyl amine with paraformaldehyde 
which has subsequently been reacted with a sulfonic acid. Typically, the 
shelf life of materials which have been used in the past to form such 
plates is less than two years and usually from about 1 year to 11/2 years. 
The present invention provides a lithographic printing plate employing 
certain specific photosensitizers wherein the obtained shelf life is in 
excess of five years. 
DESCRIPTION OF THE PRIOR ART 
It is known in the part to produce diazo type photosensitizers. In 
particular, U.S. Pat. No. 3,373,021 discloses diazo compounds which are 
relatively low molecular weight monomers, whereas the instant invention 
includes only high molecular weight polymers. 
SUMMARY OF THE INVENTION 
The present invention provides novel diazo type polymeric photosensitizers 
useful in the manufacture of storage stable lithographic printing plates. 
Characteristically said photosensitizers substantially extend the shelf 
life demonstrated by said plates as compared to other photosensitizers 
known to the prior art. The polymeric photosensitizers of the instant 
invention are represented by the formula: 
##STR2## 
where R is methyl or hydrogen 
X is methoxy, ethoxy or hydrogen 
Y is oxygen or sulfur 
Z is an anion of a sulfonic, carboxylic or phosphonic acid 
n is an integer from 8 to 970 
It is, therefore, a primary object of the present invention to provide 
novel diazo type polymeric photosensitizers useful for the production of 
lithographic printing plates. 
It is another object of the present invention to provide novel diazo type 
polymer photosensitizers useful for the production of lithographic 
printing plates having an extended shelf life. 
These and other objects of this invention will be in part discussed and in 
part apparent upon consideration of the detailed description of the 
preferred embodiment provided hereinafter. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the present invention, light sensitive, high molecular 
weight diazo polymers having the generic formula: 
##STR3## 
where R is methyl or hydrogen 
X is methoxy, ethoxy or hydrogen 
Y is oxygen or sulfur 
Z is an anion of a sulfonic, carboxylic or phosphonic acid 
n is an integer from 8 to 970 
are produced such that a polymer having a molecular weight of from about 
5,000 to about 500,000 is attained. 
In the above formula the anions usable within the context of the instant 
invention include anions which form a stable salt with the diazo resin and 
which render it soluble in organic solvents. These include the anions 
derived from organic carboxylic acids such as, decanoic acid and benzoic 
acid, organic phosphonic acids such as phenylphosphonic acid, and sulfonic 
acids, typical examples of which include aliphatic and aromatic sulfonic 
acids, such as methane sulfonic acid, chloroethanesulfonic acid, 
dodecanesulfonic acid, benzenesulfonic acid, toluenesulfonic acid, 
mesitylenesulfonic acid, and anthraquinonesulfonic acid. Preferred 
sulfonic acids are 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid, 
hydroquinonesulfonic acid, 4-acetylbenzenesulfonic acid and 
dimethyl-5-sulfoisophthalate. 
Generally in the production of the composition of the instant invention, a 
diazonium salt is treated with paraformaldehyde and sulfuric acid. The 
reaction product thus produced is then treated with a sulfonic, carboxylic 
or phosphonic acid to produce the desired condensation product herein 
described. This light sensitive composition may optionally be blended with 
such oleophilic binding resins as, polyamides, polyesters, polyethers, 
polycarbonates, polystyrenes, polyurethanes, polyvinyl chloride and its 
copolymers, polyvinylketals, polynitriles, polysulfones. The selection of 
particular ingredients and reaction conditions is made and controlled by 
the skilled worker to achieve a composition having the desired properties 
including molecular weight. 
In a series of preferred embodiments, it has been found beneficial to 
produce the composition of the instant invention such that in the formula 
set forth hereinabove, R is methyl, X is ethoxy, Y is sulfur, Z is 
2-hydroxy-4-methoxy benzophenone-5 sulfonic acid and n is selected such 
that the molecular weight of the polymer is from about 60,000 to about 
80,000.

The following specific examples illustrate the invention and methods of 
producing the polymers but it is to be understood that variations and 
modifications can be made therein without departing from the spirit and 
scope of the invention. 
EXAMPLE 1 
120 g of 2,5-diethoxy-4-tolylmercaptodiazobenzene zinc chloride double salt 
was added slowly to 385 ml of 98% sulfuric acid at 25.degree. C. and 
stirred for 15 minutes while hydrogen chloride evolved. 8.03 g of 
paraformaldehyde was added and the mixture was stirred for 4 hours at 
32.degree..+-.2.degree. C. The reaction product was poured into 2.9 liters 
of ice water with stirring and treated with a cold concentrated solution 
of 383 g of zinc chloride in water. The precipitate was recovered by 
suction filtration and the partially dry cake was dissolved in 3.8 liters 
of water, filtered, cooled with ice and treated with a solution of 96 g of 
2-hydroxy-4-methoxybenzophenone-5-sulfonic acid in water. The precipitate 
was recovered by filtration and air dried to obtain 175 g of brittle 
solid. 
EXAMPLE 2 
The light sensitive diazo resin from 
2,5-dimethoxy-4-tolymercaptodiazobenzene zinc chloride double salt was 
prepared according to example 1. A light sensitive coating solution 
incorporating this material was prepared by the following method. 
To 80 ml. methylene chloride, 56 ml. methanol, 35 ml. methyl cellosolve and 
3.5 ml. of dimethylformamide was added 2.0 g. of the above material, 1.0 
g. of Epon 1031 resin, 0.5 g. of Formvar 12/85 resin, 1.6 g. of a 60% 
solution of a polyester resin, 0.2 g. Basic blue dye, 0.02 g. of 
2-hydroxy-4-methoxybenzophenone-5-sulfonic acid available from GAF under 
the name MS-40, 0.03 g. methyl orange and 4 ml. of a phosphoric acid 
solution prepared by diluting 1 g. of concentrated phosphoric acid in 100 
ml. of methyl cellosolve. The materials were added to the solvent mixture 
while the mixture was stirred. Each material readily dissolved. 
The photosensitive solution was whirl-coated onto a grained anodized 
aluminum plate having a sodium silicate interlayer at a speed 80 rpm. 
During whirl-coating the aluminum plate was heated with electrical heating 
units. The temperature of the aluminum was approximately 70 to 80 degrees 
C. Within one minute the plate was dry and was kept in the whirl-coated 
and heated for a total of 5 minutes. This coating is identified as coating 
A. 
Coating B was prepared by the same procedure as Coating A except that only 
1 g. of the diazo resin was used. 
Coating C was prepared by the same procedure as Coating A except that a 
different diazo sensitizer was used. That sensitizer is the commonly used 
diazo resin for lithographic plates and is made by the condensation of 
p-diazodiphenylaminebisulfate with p-formaldehyde. The water soluble resin 
is then further reacted with MS-40 to form a sulfonate. 
All three coatings as soon as they were prepared were imaged through a 
negative in a Berkey Exposure Unit by a method well known in the art. 
After light exposure they were developed with a developer. The plates are 
negative-working and the non-exposed areas were removed. The three 
coatings all produced satisfactory images and were totally developed. The 
non-image areas were all hydrophilic. No coating remained on the aluminum 
in the non-light exposed areas. 
The developer used in this test was made according to the following 
formula, 
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Water 998 ml. 
Lithium benzoate 963 ml. 
Cyna-50 (Surfactant available from 
Mona Industries) 860 ml. 
Sodium Lauryl Sulfate 247 g. 
Sodium Citrate 247 g. 
n-Propanol 482 ml. 
Benzyl Alcohol 138 ml. 
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In developing the exposed coating a sufficient amount of developer is used 
to cover the entire plate. The developer is kept in contact with the plate 
for 30 seconds before the developer is rubbed into the plate gently by the 
use of a cotton swab or pad. The developer is then removed from the plate 
by the use of a squeegee. A second application of developer is applied to 
the plate and the process is repeated. 
After the development process, it is usually apparent if part of the 
coating remains in the unexposed areas. If any part of the coating 
remains, it would be colored. As a further test, in determining if the 
unexposed areas are totally removed the process of printing on a printing 
press is simulated. The plate is first treated with water and the excess 
water is removed by a squeegee. The wet plate is then rubbed gently with a 
cotton swab that contains a blotch of a typical lithographic ink. The 
image will accept the ink while the non-image areas will not accept ink. 
All three coatings did not accept ink in the non-light exposed areas. The 
coatings in these areas are therefore totally removed. 
One problem in the preparation of lithographic plates is that the light 
sensitive coatings are not sufficiently stable and when stored for an 
extended period of time at room temperature will undergo chemical changes. 
These changes manifest themselves by changing the solubility 
characteristics of the coating in the developer which is obviously 
undesirable and cause the coatings to be unuseable. 
To predict the long term stability properties of the 3 coatings, they were 
heated in an oven at 60 degrees C. for periods ranging from 5 days to 3 
weeks. The coatings were then exposed and developed as previously 
described. The results are summarized in the table below. 
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DAYS OF OVEN AGING AT 60 DEGREES C. 
5 Days 7 Days 14 Days 21 Days 
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Coating A Stable Stable Stable Stable 
Coating B Stable Stable Stable Stable 
Coating C Stable Unstable Unstable 
Unstable 
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A simulated oven aging of one day at 60 degrees C. translates to a 3 month 
shelf life. It can readily be seen that Coating C, which is known in the 
art, is stable for an estimated 11/4 year shelf life and unstable by 13/4 
years. However, the sensitizer of the present invention has been shown to 
be stable for over 51/4 years. 
As a series of preferred embodiments, a composition produced according to 
the present invention is applied at a coating weight of from about 10 
mg/sq.ft. to about 200 mg/sq.ft. on an aluminum sheet and dried. The thus 
formed coated film may then be exposed to ultraviolet radiation through a 
transparency in a manner well known to the art and developed using a 
suitable developer such as Polychrome's 922 developer. 
It is, of course, to be understood that the foregoing disclosure is 
intended to illustrate the invention and that numerous changes can be made 
in the ingredients, conditions and proportions set forth without departing 
from the scope of the invention as disclosed and defined in the claims 
appended hereafter.