Etching solution for etching of photopolymeric films

Etching solution for dot etching photopolymerized materials which comprises an organic polymeric binder having alkali soluble groups, monomer and photoinitiator, the material having an optical density in the actinic region of at least 1.0, wherein a water-soluble salt of a water-insoluble polymeric polycarboxylic acid whose acid strength is the same or less than that of the binder is present as the etching agent.

DESCRIPTION 
1. Technical Field 
This invention relates to solutions for etching photopolymerizable 
elements. More particularly, this invention relates to etching solutions 
for dot etching photopolymerized elements having an alkali-soluble 
polymeric binder. 
2. Background Art 
Photopolymerizable reproduction materials or elements have found numerous 
applications in the area of photographic reproduction technology. Thus, 
they are used primarily for the preparation of printing plates and in the 
area of photofabrication for the preparation of printed circuits. 
Recently, photopolymerizable reproduction materials which can be used in 
graphic arts for the preparation of line and screen contacts have become 
known. These materials are suitable for replacing litho films based on 
silver halide used up to now for this purpose. Such reproduction materials 
are, e.g., described in U.S. Pat. No. 4,173,673. There is special 
practical significance attached to materials which can be developed in 
aqueous alkaline solutions. A preferred area of application of these 
materials is the preparation of screen negatives and positives, as well as 
screened color separations for reprography, especially for the preparation 
of printing plates. An important prerequisite for the use of these 
photopolymerizable reproduction materials is that the screened images 
obtained be dot etchable, i.e., the size of the obtained dots can be 
reduced in selected areas in order to be able to make necessary tone value 
and/or color corrections. 
According to U.S. Pat. No. 4,173,673 etching of the polymeric screen dots 
can take place by treatment with the developer solution, whereby the 
developing process is carried out until there is an essential 
under-etching of the exposed areas. Subsequently, then, the polymerized 
screen dot is reduced in size by mechanical abrasion. 
Depending on the polymeric binder present in the photopolymerizable 
composition, suitable developer solutions are either organic solvents or 
aqueous alkali solutions. It is disadvantageous that it is not possible to 
etch in a controlled manner with this so-called solvent etching method, 
i.e., to obtain a uniform dot reduction on larger areas, because the 
degree of dot reduction greatly depends on the time of application as well 
as the amount of etching solution used. Additionally, with longer 
application times of the developer solutions on the exposed areas of the 
image there is a strong swelling of the screen dots which frequently leads 
to considerable losses of density. 
Therefore, it is an object of the present invention to provide etching 
solutions which make it possible to etch in a controlled manner dot 
etchable photopolymerized materials which contain a polymeric binder with 
alkali soluble groups and whose optical density in the actinic range is at 
least 1.0. 
DISCLOSURE OF THE INVENTION 
In accordance with this invention there is provided an etching solution for 
dot etching photopolymerized materials which comprise an organic polymeric 
binder having alkali soluble groups, a polymer of an ethylenically 
unsaturated monomeric compound, and a photoinitiator or photoinitiator 
system, the photopolymerized material having an optical density in the 
actinic region of the spectrum of at least 1.0, the improvement wherein 
the etching solution contains as the etching agent a water-soluble salt of 
a water-insoluble polymeric polycarboxylic acid whose acid strength is the 
same or less than that of the organic polymeric binder present in the 
photopolymerized material. 
According to a preferred embodiment of the invention, the etching solutions 
can contain an excess of free alkali. It is possible with the help of the 
described etching solutions to carry out etching in a controlled manner, 
i.e., to achieve uniform dot reduction on larger areas. The maximum 
density of the screen image is not influenced by these etching solutions. 
The superior action of these etching solutions is based on the fact that 
they lose their etching ability after a certain application time. It is 
believed that there is an ion exchange between the etching agent and the 
salt-forming groups of the polymeric binder whereby water-soluble alkali 
salts of the binder necessary for etching are formed. As a result of this 
ion exchange, a phase of the water-insoluble free polymeric polycarboxylic 
acid in the contact range between the polymerized dot and the etching 
solution is formed, which acts as a diffusion barrier for further alkali 
from the etching solution into the binder, so that the etching process 
comes to a stop. 
Suitable polymeric polycarboxylic acids for the preparation of the etching 
solutions are, e.g., vinyl addition polymers which contain free carboxylic 
acid groups and are prepared from 30 to 94 mol-% of one or several alkyl 
acrylates and 70 to 6 mol-% of one or several .alpha.,.beta.-ethylenically 
unsaturated carboxylic acids, preferably of 61 to 94 mol-% of two alkyl 
acrylates and 39 to 6 mol-% of an .alpha.,.beta.-ethylenically unsaturated 
carboxylic acid. Suitable alkyl acrylates include: methyl acrylate, ethyl 
acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl 
methacrylate, and butyl methacrylate, etc. Suitable 
.alpha.,.beta.-ethylenically unsaturated carboxylic acids, by way of 
example, are acrylic acid and methacrylic acid. Also suitable are 
copolymers comprising a vinyl monomer of the styrene type and an 
unsaturated monomer containing carboxyl groups, copolymers of vinyl 
acetate and crotonic acid, copolymers of ethyl acrylate, methyl 
methacrylate, and acrylic acid, copolymers of methyl methacrylate and 
methacrylic acid, copolymers of methyl methacrylate, ethyl acrylate, and 
hydrogen maleate, copolymers of vinyl chloride, vinyl acetate, and maleic 
acid, copolymers of styrene and maleic acid anhydride or maleic acid 
semiesters as well as copolymers of methyl methacrylate, ethyl acrylate, 
and methacrylic acid. 
To prepare the etching solutions, the polymeric polycarboxylic acid is 
first dispersed in water followed by the addition of the alkali solution, 
and these are stirred to complete dissolution. The amount of alkali used 
is equivalent to or exceeds the acid number of the polycarboxylic acid. A 
specific etching solution according to the invention can be prepared as 
follows: 
The polymer is dispersed in water and, if desired, up to a 30% excess of 
alkali solution is added than corresponds to the acid number of the 
binder. 
The acid strength of the polymeric polycarboxylic acid can be measured by 
titration in alcoholic solution. 500 mg of the polymer are dissolved in 75 
ml ethanol and are titrated with 0.1 N aqueous NaOH solution up to the 
equivalence (end) point, the pH being monitored with a glass electrode 
during titration. The pH at which half of the carboxyl groups is 
neutralized can be read off from the plot of the pH against the volume of 
the standard solution added. This pH corresponds to the pK.sub.a -value of 
the carboxylic acid (Houben-Weyl, 4th Edition 1955, Vol. 3/2, p. 166). For 
the actual carrying out of the etching process, the etching solution is 
applied, e.g., with a brush onto the image areas to be etched and after 
the etching process has come to a stop, it is removed by means of an 
absorbent paper. Then the etchable part of the screen dots is removed by 
light rubbing with a sponge and water. The end point of the etching 
process can be easily recognized when an indicator is added to the etching 
solution whose end point corresponds to the equivalence (end) point of the 
polycarboxylic acid used in the etching solution. After the color change 
the reaction is substantially complete. The etching process can be 
repeated as often as necessary until the desired degree of etching is 
achieved. Using the described etching solutions postexposed materials dot 
reductions up to 9% are achieved, while with materials, which were not 
subjected to any postexposure, dot reductions up to 25% are possible. 
According to a preferred embodiment the etching solution can contain an 
excess of free alkali. In this case, the etching process takes place in 
two stages. In the first stage the free alkali is active and the coating 
is etched as if by a solvent. After consumption of the excess alkali, the 
controlled etching begins until the ion exchange is finished and the 
etching process comes to an end. The concentration of the free alkali, of 
course, must be prechosen to avoid achieving the desired degree of etching 
by solvent etching alone. The optimal amount of free alkali necessary for 
achieving the required degree of etching can be easily determined by a 
series of tests of the etching solutions with different contents of free 
alkali. 
The use of the combination of free alkali/alkali salt of a polymeric 
polycarboxylic acid makes it possible for those skilled in the art to 
prepare a series of etching solutions in a simple manner, by which any 
desired degree of etching can be achieved without the danger of over- or 
under-etching. Preferred etchable materials within the scope of the 
invention consist essentially of a support film and a photopolymerizable 
layer, which has as essential ingredients an aqueous alkaline developable 
organic polymeric binder, an ethylenically unsaturated monomer and an 
initiator or initiator system and has an optical density of at least 1.0 
in the actinic region of the spectrum. Suitable aqueous alkaline 
developable binders are widely known. These binders generally contain 
functional groups soluble in alkali, such as acid anhydride-, carboxyl-, 
or sulfonic acid groups. Examples are: arylic acid and/or methacrylic acid 
polymers and/or their copolymers with other suitable monomers, e.g., 
acrylic acid ester or other acryl derivatives; vinyl compounds, such as 
vinyl ether, vinyl acetate, or their saponification product, styrene, 
vinyl pyrrolidone; butadiene, and related monomers; polyacrylic acid 
anhydrides; copolymers of maleic acid anhydride, maleic acid, maleic acid 
semiesters, -semiamides and/or anhydrides and derivatives of related 
compounds, such as itaconic acid, with suitable comonomers, such as 
styrene, vinyl ethers, vinyl acetates, etc.; polystyrene sulfonic acid 
and/or their copolymers; cellulose derivatives, such as e.g., cellulose 
phthalate or succinate, alginic acid and their derivatives. Suitable 
binders, are furthermore polymeric polycarboxylic acids that can also be 
used as etching agents. Suitable monomers which are polymerized can be 
taken from U.S. Pat. No. 4,173,673 which is incorporated by reference. To 
obtain an optical density of at least 1.0, necessary to form a 
polymerization gradient which makes possible the under-etching of the 
polymeric screen image, ultraviolet stable dyes and/or pigments are added 
to the light sensitive layer. Colloidal carbon is especially preferred as 
a pigment. Useful ultraviolet absorbers are likewise widely known, e.g., 
from U.S. Pat. No. 4,173,673. 2,2'-dihydroxy-4-methoxy-benzophenone has 
proven to be especially useful.

EXAMPLES 
The following examples illustrate the invention wherein the parts and 
percentages are by weight. The molecular weights are number average 
molecular weights Mn. The Mn for the polymers described herein can be 
determined by gel permeation chromatography employing a polybutadiene 
standard or other standard known to those skilled in the art. 
EXAMPLE 1 
A photopolymerizable material of the following composition was prepared: 
______________________________________ 
Ingredient Amount (%) 
______________________________________ 
(a) Terpolymer comprising ethyl 
16.2 
acrylate, methyl methacrylate, 
and acrylic acid (56/37/7), 
MW ca. 260,000, acid number 
76-85, pK.sub.a in ethanol 8.3 
(b) 1:1 Copolymer comprising styrene 
30.3 
and maleic acid anhydride partially 
esterified with sec. butanol, 
MW ca. 10,000, acid number 190, 
pK.sub.a in ethanol 8.3 
(c) 2,2'-Bis (2-chlorophenyl)-4,4',5,5' 
5.8 
tetraphenyl-bisimidazole 
(d) 4,4'-Bis (dimethylamino) 
2.3 
benzophenone 
(e) Nonionic surface active 
0.1 
fluorohydrocarbon, FC-430 .RTM. 
manufactured by 3M Co. 
(f) Trimethylol propane triacrylate 
27.2 
(g) Mixed esters comprising triethylene 
1.7 
glycol dicaproate and diacrylate 
(h) Colloidal carbon 16.4 
______________________________________ 
To prepare the coating solution first a 10% solid solution of components 
(b) to (g) was prepared in methylene chloride. Components (a) and (h) were 
likewise dispersed in methylene chloride using a high-speed mixer. To this 
dispersion was then added the solution of components (b) to (g). The 
coating solution then was applied onto a polyethylene terephthalate 
support film, which was provided with an intermediate layer according to 
the data of Example 3 of U.S. Pat. No. 4,173,673 so that the layer 
thickness after drying was 6 .mu.m. The optical density in the visible 
range was 3.2 and in the spectral range from 300 to 500 nm it was 4.5. An 
overcoat of the following composition was applied onto the light-sensitive 
layer: 
______________________________________ 
Ingredient Amount (g) 
______________________________________ 
Distilled water 122.5 
Polyvinyl alcohol (saponified 98%, 
2.25 
low-molecular weight) 
Colloidal silica (30% 2.70 
solution) 
Polyoxyethylene surfactant 
0.45 
______________________________________ 
The layer thickness of the dried coating was 2 .mu.m. The material then was 
exposed for 7 seconds at a distance of 65 cm using a metal halide lamp 
(1000 W) through a 50% screen used as an original. After exposure the 
coating layer was removed by washing with water and the material was 
developed for 5 seconds at 30.degree. C. using a solution of the following 
composition: 
52.5 g K.sub.2 CO.sub.3 
3.1 g KHCO.sub.3 
water to 1 liter. 
The unexposed material was then removed by water spray. An exact negative 
copy of the screen cover sheet was obtained. The dried material was then 
postexposed in the conventional manner from the front and the back and 
then was treated with an etching solution of the following composition: 
______________________________________ 
7.5 g 1:1 Copolymer comprising styrene and maleic 
acid anhydride, partially esterified 
with sec. butanol, MW ca. 10,000, acid 
number 190, pK.sub.a in ethanol 8.3 
50 ml Distilled water 
16.5 ml NaOH, 2N 
______________________________________ 
To carry out the etching, the etching solution was applied to the material 
with a brush and, at the end of the process the excess etchant was 
absorbed with an absorbent paper. The etchable portion of the dots was 
removed by light rubbing with a sponge and water. 
The achieved dot reductions of the original 50% dot are summarized in Table 
1: 
TABLE 1 
______________________________________ 
Application 
time of the Dot Size 
Etching After Optical Density in 
Solution Etching the visible 
(Sec) (%) range 
______________________________________ 
1 .times. 15 45 3.2 
2 .times. 15 41 3.2 
1 .times. 30 45 3.2 
2 .times. 30 41 3.2 
______________________________________ 
It is evident from Table 1, that the degree of etching attained is 
independent of the duration of action of the etching solution and that the 
etching process comes to a stop after a certain duration of action. It is 
moreover evident, that optical density is not influenced. 
EXAMPLE 2 
An exposed and developed material prepared according to the procedure of 
Example 1 was etched according to the procedure of Example 1 with an 
etching solution of the following composition: 
______________________________________ 
7.5 g Terpolymer comprising ethyl acrylate, 
methyl methacrylate, and acrylic acid 
(56/37/7), MW ca. 260,000, acid number 
76-85, pK.sub.a in ethanol 8.3 
50.0 ml Distilled water 
6.7 ml NaOH, 2N 
______________________________________ 
Dot reductions obtained are summarized in Table 2: 
TABLE 2 
______________________________________ 
Application 
time of the Dot Size 
Etching After Optical Density in 
Solution Etching the visible 
(Sec) (%) range 
______________________________________ 
1 .times. 15 47 3.2 
2 .times. 15 44 3.2 
1 .times. 30 46 3.2 
2 .times. 30 42 3.2 
______________________________________ 
It is evident from the table, that the etching process is finished after a 
certain duration of action and an overetching is practically excluded. It 
is moreover evident, that optical density is not influenced. 
EXAMPLE 3 
Various screen dot sizes using suitable screen cover sheets were exposed 
onto a photopolymerizable reproduction material according to Example 1. 
The material was developed according to the procedure of Example 1 and 
then was etched with an etching solution of the following composition 
containing an indicator: 
______________________________________ 
133.0 ml Distilled water 
15.0 g 1:1 Copolymer comprising styrene and maleic 
acid anhydride, partially esterified with 
sec. butanol, MW ca. 10,000, acid 
number 190, pK.sub.a in ethanol 8.3 
2.78 g NaOH, solid 
100.0 mg Phenolphthaleine 
______________________________________ 
A decoloration was observed after application of the etching solution, 
which came to a stop after ca. 30 seconds and indicated the end of the 
etching process. 
The obtained dot reductions are summarized in Table 3: 
TABLE 3 
______________________________________ 
Original Dot Sizes (%) 
Dot Sizes after Etching (%) 
______________________________________ 
5 3 
40 32 
50 40 
60 52 
90 86 
95 93 
98 97 
______________________________________ 
It is evident from the table, that in etching different dot sizes there is 
a similar result as in etching silver halide films, i.e., average tones 
are reduced more strongly than the two ends of the tone scale. It is 
moreover advantageous, that sharp dots next to high tone values can be 
etched without the danger that the sharp dots are destroyed. 
EXAMPLE 4 
An exposed and developed material according to the procedure of Example 1 
was etched according to the procedure of Example 1 with etching solutions 
which contained a varied content of free alkali. The etching solutions 
used had the following composition: 
______________________________________ 
To each 66.5 
ml H.sub.2 O distilled and 
7.5 g 1:1 Copolymer comprising styrene and 
maleic acid anhydride, partially 
esterified with sec. butanol, MW 
10,000, acid number 190, pK.sub.a in 
ethanol 8.3 
______________________________________ 
were added 
______________________________________ 
(A) 1.01 g NaOH, solid 
(B) 1.15 g NaOH, solid 
(C) 1.32 g NaOH, solid. 
______________________________________ 
The dot reductions obtained are summarized in Table 4: 
TABLE 4 
______________________________________ 
Dot Size After Etching With 
Application 
Solution Solution Solution 
time (sec) 
(A) (%) (B) (%) (C) (%) 
______________________________________ 
1 .times. 30 
48 47 46 
2 .times. 30 
48 45 43 
1 .times. 60 
48 46 45 
2 .times. 60 
47 43 40 
______________________________________ 
It is evident from the table that the degree of dot reduction per 
application cycle can be predetermined by the composition of the etching 
solution without the danger of overetching the dots. 
EXAMPLE 5 
An exposed and developed material prepared according to the procedure of 
Exaple 1 was etched according to the procedure of Example 1 with etching 
solutions of the following composition: 
______________________________________ 
Etching Solution (A) 
65.0 ml H.sub.2 O distilled 
7.5 g Copolymer comprising methyl methacrylate, 
ethyl acrylate, methacrylic acid 
(48/27/25), 2.4 .multidot. 10.sup.-2 mol-COOH, acid 
number 178, pK.sub.a in ethanol 8.7 
12.0 ml NaOH, 2N (2.4 .multidot. 10.sup.-2 mol) 
Etching solution (B) 
65.0 ml H.sub.2 O distilled 
7.5 g 1:1 Copolymer comprising styrene and maleic 
acid anhydride, partially esterified with 
secondary butanol, 2.54 .multidot. 10.sup.-2 mol-COOH, 
acid number 190, pK.sub.2 in ethanol 8.3 
1.01 g NaOH, solid (2.54 .multidot. 10.sup.-2 mol) 
Etching solution (C) 
65.0 ml H.sub.2 O distilled 
7.5 g Copolymer comprising methyl methacrylate, 
acrylic acid, t-octyl acryl amide, hydroxy 
propyl methacrylate, t-butyl amino-ethyl 
methacrylate (35/16/40/5/4), 1.34 .multidot. 10.sup.-2 
mol-COOH, acid number 100, pK.sub.a in ethanol 
8.2 
6.7 ml NaOH, 2N (1.34 .multidot. 10.sup.-2 mol) 
______________________________________ 
The dot reductions obtained with the individual etching solutions are 
summarized in Table 5: 
TABLE 5 
______________________________________ 
Dot Size After Etching With 
Application 
Solution Solution Solution 
Time (sec) 
(A) (%) (B) (%) (C) (%) 
______________________________________ 
30 49 48 50 
60 49 48 50 
90 49 48 50 
______________________________________ 
It is evident from the table, that the alkali salts of polymeric 
polycarboxylic acids, whose acid strength is the same or less than that of 
the binder, cause controlled etching, while the alkali salts of polymeric 
polycarboxylic acids whose acid strength is greater than that of the 
binder are not effective, i.e., there is no etching action. 
EXAMPLE 6 
A material according to the procedure of Example 1 was used with the sole 
difference that the material was not postexposed. This material then was 
etched according to the procedure of Example 1 with the following etching 
solutions: 
A: Etching solution (A) of Example 5, to which was added 1 ml 2 N NaOH, so 
that the excess of free alkali was 8.3% 
B: Etching solution (C) of Example 5, to which was added 0.55 ml 2 N NaOH, 
so that the excess of free alkali was 8.3%. 
The dot reductions obtained with the two etching solutions are summarized 
in Table 6: 
TABLE 6 
______________________________________ 
Dot Size After Etching With 
Application Solution Solution 
Time (sec) A (%) C (%) 
______________________________________ 
30 11 2 
60 12 5 
90 14 10 
______________________________________ 
It is evident from the table, that the etching process using Solution A 
comes to a stop after a short time, i.e., the etching is controlled, while 
with Solution C there is a pure solvent etching, i.e., the degree of 
etching depends very much on the application time of the etching solution.