Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~Z~05~
PHOTOCURABLE ELASTOMERIC MIXTURE AND RECORDING
MATERIAL OBTAINED THEREFROM FOR PRODUCING
RELIEF PRINTING PLATES
Backqround of the Invention
The invention relates to a photocurable
negative-working mixture and a recording material
obtained therefrom which is suitable for producing
elastic relief printing plates.
Mixtures of the said class are known and
normally contain an elastomeric binder, a radical-
polymerizable compound ar,d a polymerization
initiator activatable by actinic radiation. Mixtures
of this type, which are described, for example, in
DE-B-2,215,090 (= US-~-4,423,13~), can be developed
after exposure only with organic solvents to form
the relief image.
Although photosensitive copying materials
which can be aqueously developed and are based on
partially hydroly~ed polyvinyl acetate are described
for producing relief printing plates in US-A-
Z()~58~
4,042,386 and DE-B-1,902,63s/l,sl7,917 (= US-A-
3,630,746) and 2,402,808 (= US-A 3,936,254), an
addition of water, which makes a subsequent drying
process necessary, is required for the purpose of
homogenization and layer formation. Furthermore,
these relief printing ~orms do not have the
elasticity or low Shore A hardness re~uired for
flexographic printing.
DE-A-3,541,162 (= AU 86~65,535) and EP-B-
0,080,664 (= US-~-4,493,807) describe internally
plasticized partially hydrolyzed polyvinyl acetates
as binders for photosensitive copying materials.
Although these graft polymers, which are obtained by
polymerization of vinyl esters in the presence of
polyalkylene oxides and subsequent partial
hydrolysis, are more elastic than pure partially
hydrolyzed polyvinyl acetates, water addition is
li~ewise necessary in processing the corresponding
photopolymer mixtures to f`orm laminar copying
materials.
German Patent Application P 38 24 146.3,
which is not prepublished, describes a photocurable
elastomeric mixture for producing relief printing
plates which contains, as elastomeric binder, a
graft polymer which has, as grafting base, a polymer
of diol components and diisocyanate components
containing at least two urethane groups in the
molecule onto which are grafted polymer chains
composed of units of carboxylic acid vinyl esters
containing 3 to 20 carbon atoms and/or their
hydrolysis products and/or of further ethylenically
~S87~
unsaturated monomers and/or their hydrolysis
products. The photocurable elastomeric mixtures
which contain said binders can be developed after
imagewise exposure with water or a~ueous solutions.
The stability of the flexographic printing plate in
the printing operation appears to be worth
improving.
It is assumed that (since water is employed
in printing) the cured regions of the flexographic
printing plate could also be attacked by water.
Summary of the Invention
Accordingly, it is an object of the present
invention to provide a photosensitive elastomeric
mixture which can be developed, not in water, but in
aqueous alkaline solution.
Another object of the present invention is to
pro~ide an improved photocurable recordinq material.
In accomplishing the foregoing objectives,
there has been provided, in accordance with one
aspect of the present invention, a photocurable
elastomeric mixture which comprises: a radical-
polvmerizable compound compatible with the binder
and containing at least one terminal ethylenically
unsaturated group and having a boiling point at
normal pressure above 100C; a compound or
combination of compounds capable of initiating the
polymerization of the polymerizable compound on
exposure to actinic light; and an elastomeric binder
comprising a graft polymer which comprises the
~ J
esterlfication product of a hydroxyl-containing graft polymer
with an acylating reagent, esterified in the melt in
homogeneous phase, wherein said hydroxyl-containing
graft polymer has a graft base which comprises a
polymer comprising diol components and diisocyanate
components, said polymer having at least two
urethane groups in the molecule, onto which graft
base have been grafted a plurality of polymer chains
comprising units of vinyl esters of carboxylic
acids, said vinyl esters having 3 to 20 carbon
atoms, and/or hydrolysis products thereof.
In accordance with another aspect of the
present invention, there is provided a photocurable
recordin~ material which comprises a coating base and
a photocurable coating comprising the foregoing
mixture.
Othar objects, features and advantages of the
present invention will become apparent to those
skilled in the art from the following detailed
description~ It should be unders~ood, however, that
the detailed descripkion and specific examples,
while indicating preferred embodiments of the
present invention, are given by way of illustration
and not limitation. Many changes and modifications
within the scope of the presen~ invention may be
made without departing from the spirit thereof, and
the invention includes all such modifications.
2~C~5~
Detailed Descri~tion of the Preferred Embodiments
Graft polymers containing hydroxyl groups are
understood to be those compounds which result from
grafting onto a grafting base carboxylic acid vinyl
esters and/or their hydrolysis products and
optionally further monomer units from ethylenically
unsaturated polymerizable compounds and their
hydrolysis products, wherein the graft polymer is
esterified with an acylating agent in a homogeneous
phase in the melt. Partially esterified graft
polymers are particularly preferred. The percentage
of grafted-on components is about 10 to 99% by
weight, preferably 30 to 90% by weight, in
particular 40 to 80% by weight, based on the total
graft polymer.
As grafting bases, mention is made of those
which are composed of polyurethanes containing at
least 2 urethane groups in the molecule, the number
of urethane groups per gra~ting base molecule not
being subject to any particular limitation in the
upward direction and having in general values higher
than 2.
In principle, any diol which can be used in
polyurethane synthesis can be employed. Preferred
are cycloaliphatic diols such as, for e~ample,
cyclohexanediols and also aliphatic diols containing
preferably 2 to 12 carbon atoms. Polyether diols,
for example polypropylene oxides, polybutylene
oxides, copolymers of ethylene oxides, propylene
oxide, butylene oxide, pre~erably their block
z()osa7~i
copolymers are furthermore preferred, polyethylene
oxides being particularly preferred.
Preferably, polyether diols, in particular
polyethylene glycols, having molecular weights
between about 200 and 10,000 are used, polyethylene
glycols having molecular weights between 400 and
1,500 being particularly preferred. The polyether
diols are optionally employed in combination with
low-molecular aliphatic diols such as, for example,
1,4-butanediol, 1,3-propanediol, ethylene glycol or
diethylene glycol. Preferably the molar ratio of
polyether diol to low-molecular aliphatic diol is
from about 1:0.1 to 1:0.7.
Aromatic diisocyanates, for example m- and
p-xylylene diisocyanate, tolylene 2,4-diisocyanate,
tolylene 2,6-diisocyanate or mixtures of the last
two isomers, naphthylene l,5-diisoGyanate,
diphenylmethane-4,4'-diisocyanate and phenylbenzyl-
4,4'-diisocyanate are used as diisocyanate
components.
Preferably aliphatic and~or cycloaliphatic
diisocyanates are employed. Preferred aliphatic
diisocyanates are, for example, those containing 2
to 12 carbon atoms in the aliphatic radical, for
example ethylene diisocyanate, propylene
diisocyanate, tetramethylene diisocyanate and 2,2,4-
trimethylhexamethylene diisocyanate. Preferred
cycloaliphatic diisocyanates are, for example, 1,4-
diisocyanatocyclohexane, dicyclohexylmethane-
30 diisocyanate and isophorone diisocyanate.
2005~3~7~
Particularly preferred is the use of hexamethylene
diisocyanate and isophorone diisocyanate.
The molar ratio of diol component to
diisocyanate component is preferably between about
l:o.ss and 1:0.5, in particular between 1:0.98 and
1:0.7. The mean molecular weights of the
polyurethanes are preferably between about 200 and
100,000, in particular between l,300 and 50,000,
particularly pre~erably between 3,000 and 25,000.
To regulate the molecular weight in the pxeparation
of the polyurethanes, use may be made of
monohyclroxyl compounds such as, for example,
methanol, ethanol or other aliphatic alcohols, in
particular those of the general formula CnH2~+20 where
n = 1 to 4, but also semi-esterified and semi-
etherified diols or diol components. In addition,
monoisocyanates are used to control the molecular
weight.
It should be pointed out that, for example,
with a specified molecular weight for the grafting
base, the number of its urethane ~roups is directly
dependent on the molecular weights of the diol
components or diisocyanate components used.
Carboxylic acid vinyl esters containing 3 to
20 carbon atoms, preferably containing 4 to 14
carbon atoms are employed for grafting onto the
polyurethane. Particularly preferred are vinyl
acetate and/or vinyl propionate, in particular vinyl
acetate. Mixtures of vinyl acetate and~or vinyl
propionate and vinyl versatate are furthermore
preferred. In particular, with partial or complete
~o~
hydrolysis of the products following the graft poly-
merization, the concomitant use of vinyl propionate
in addition to vinyl acetate in grafting is
advantageous. In addition, copolymerizable mixtures
of carboxylic acid vinyl esters, preferably mixtures
of vinyl acetate and vinyl versatate, can be
grafted, the proportion of vinyl versatate being
about 0.2 to 10% by weight, preferably 0.5 to 5% by
weight, based on the proportion of vinyl acetate.
Grafting with various carboxylic acid vinyl esters
in the form of block copolymers, optionally in
combination with further ethylenically unsaturated
and copolymerizable monomers, may also be
advantageous.
Furthermore, the carboxylic acid vinyl esters
can also be grafted together with other
ethylenically unsaturat2d and copolymerizable
monomers such as, for example, maleic acid, itaconic
acid, mesaconic acid, crotonic acid, acrylic acid or
their esters.
Grafting is carried out using ~rafting
catalysts whic.h initiate radical chains. Preferably,
any radical ~ormer which is soluble in the monomers,
the monomer mixture or the monomer solution is
suitable. In particular mention may be made of
organic percompounds such as peroxides and
percarbonates and organic azo compounds.
~zobisisobutyronitrile and particularly dibenzoyl
peroxide are pre~erred. The grafting reaction is
~0 carried out in the presence of pre-ferably about
0.013 to 1.3 mol ~, in particular 0.026 to 0.27 mol
~0~5~'7~
~, of a radical-forming catalyst, based on the
quantity of monomer.
The graft polymers containing hydroxyl groups
obtained can be converted into partially or fully
hydrolyzed products by hydrolysis, alcoholysis or
transesterification, the degree of hydrolysis being
at least 1 mol %, preferably about 70 to 99 mol %,
based on the number of moles of hydrolyzable monomer
units in the graft polymer.
The process for preparing the graft polymers
containing hydroxyl groups is described in German
Patent Application P 37 32 089.
The process in which the graft polymers
containing hydroxyl groups mentioned in P 37 32 089
are reacted with an acylating reagent and also the
products resulting therefrom axe described in the
slmultaneously filed German ~'atent Application P 38
43 505.5, corresponding to U.S. Application No.
As acylating reagents for preparing said
graft polymers containing hydroxyl groups, mention
is made there, in particular, of carboxylic acid
anhydrides. Preferably, alkanecarboxylic,
alkenecarbo~ylic and/or acrylic acid anhydrides
containing 2 to 12 carbon atoms are used. Mention
may be made, for example, of succinic anhydride,
maleic anhydride, citraconic anhydride, glutaric an-
hydride, phthalic anhydride, cis-~-cyclohexene-1,2-
dicarboxylic anhydride, acrylic anhydride,
methacrylic anhydride, crotonic anhydride and
itaconic anhydride. ~ixtures of carboxylic acid
anhydrides may also be usedO
2()~5~37~
The degree of reaction of free hydroxyl
groups of the graft polymer with the said acylating
reagents for preparing the esterified product
depends on its desired properties, in particular the
solution properties of the product. Either
hydrolyzed or partially hydrolyzed graft polymers
may be employed as starting material for the
esterification reaction. Partially hydrolyzed graft
polymers are, in general, understood to be products
which are hydrolyzed to an extent of about 80 to 90
mol % (degree of hydrolysis).
Less usual are partially hydrolyzed products
which have a degree of hydrolysis of about 50 to
60%.
In general, the degree of reaction of the
graft polymer containing hydroxyl groups with the
acylating reagent may be so described that the
weight ratio of graft polymer to acylating reagent,
particularly if a carboxylic acid anhydride is used,
is about 1:0.01 to 1:5, pre~erably 1:0.05 to 1:2.
The basis of this specificat:ion is ~ormed by the
partially hydrolyzed gra~t polymers which are
hydrolyzed to an extent of about 80 to 90%.
Finally, the reaction must be adjusted in
such a way that the resultin~ product can be used as
binder in a photosensitive mixture. This implies
that it is possible to develop the coating in
aqueous alkali (in particular, in 1% strength
aqueous soda solution), but equally that the
developed relief form is not destroyed by water.
--10--
2(~C~58~7S
The mean molecular weight of the at least
partially esterified graft polymers containing
hydroxyl groups is preferably between about 200 and
100,000, in particular between 1,300 and 50,000,
particularly preferably between 3,000 and 25,000.
The acid number of the product should be in the
range from about 80 to 250 mg KOH/g of poly~er, in
particular from 100 to 200 mg KOH/g of polymer.
According to the method mentioned in the
simultaneously filed German Patent Application P 38
43 505.5, the graft polymer containing hydroxyl
groups is fused either without acylating agent or
simultaneously with the acylating agent, preferably
without adding a solvent. Carboxylic acid
anhydrides are preferably used as acylating
reagents. The melting point is, in general, about 80
to 200C, in particular 100 to 180C, and also
particularly preferably 120 to 160C.
The reaction is carried out without adding a
catalyst which may result in coloration of the
product, in particular of amines. The reaction time
may be made extremely short and is in the region of
a few minutes. Preferably, the reaction is already
complete after a reaction time of less than 10 min.
~ven if the acylatiny reagent is not added
simultaneously with the fusion, the esterification
can be carried out in a conventional reaction vessel
or processing apparatus for carryin~ out thermo-
plastic reactions. Such apparatuses are, for
example, two roll mills, kneadin~ machines and
extrusion appliances. Surprisingly, it was found
;~6j5~7~:~
that, for example, for a planned ma~imum of 50%
reaction of the secondary hydroxyl groups contained
in the graft polymer (by using a corresponding molar
ratio of the starting materials), the reaction has
already proceeded to completion after a few minutes
even without using the catalyst. The product is
obtained in a homogeneous phase. It does not have
any locally varying degrees of reaction in the graft
polymer which otherwise have to be tolerated; the
product has consequently reacted uniformly and can
therefore be prepared reproducibly.
The photosensitive mixtures according to the
invention contain, in general, about 20 to 95,
preferably 30 to 70, ~ by weight of binder. They
furthermore contain at least one radical-
polymerizable olefinically unsaturated compound and
also at least one photoinitiator.
Suitable monomers ~aving one or more
polymerizable olefinic double bonds are, in
particular, esters and amides of acrylic and
methacrylic acid. Examples are the compatible mono-
and diacrylates and mono- and dimethacrylates of
monohydric or polyhydric alcohols such as ethylene
glycol, di~, tri-, tetra- or polyethylene glycols,
Z5 the latter preferably containing 10 to 15 ethylene
glycol units, 1,3-propanediol, ~lycerol, 1,1,1-
trimethylolpropane, 1,2,4-butanetriol or
pentaerythritol, for example ethylene glycol
monomethacrylate, 1,3-propanediol monomethacrylate,
glycerol mono- and diacrylate, 1,2,~-butanetriol
monomethacrylate, pentaerythritol triacrylate,
-12-
x~c~sa~s
polyethylene glycol methyl ether acrylate,
tetradecaethylene glycol dimethacrylate or the
triether formed from glycerol and 3 mols of N-
methylolacrylamide or -methacrylamide. The quantity
of monome~ is, in general, about 5 to 70, preferably
about 10 to 50, % by weight of the nonvolatile
constituents of the mixture.
Suitable photoinitiators for the mixtures
according to the invention are the known compounds
which have an adequate thermal stability during the
processing of the copying materials and also an
adequate radical formation on exposure to initiate
the polymerization of the monomers. They should
absorb light in the wavelength range from
approximately 250 to approximately 500 nm to form
radicals. Examples of suitable photoinitiators are
acyloins and their derivatives such as benzoin,
benzoin alkyl ethers, for example benzoin isopropyl
ether, vicinal diketones and their derivatives, for
example benzil, benzil acetals such as
benzildimethyl ketal, fluorenones, thioxanthones,
polynuclear quinones, acridines and quinoxalines;
also trichloromethyl-s-triazines, 2-halomethyl-4-
styryl-1,3,4-oxadiazole deri~atives, halooxazoles
substituted with trichloromethyl groups, carbonyl-
methyleneheterocyclics according to DE-A 33 33 450
containing trihalomethyl groups, acylphosphine oxide
compounds such as are described, for example, in DE-
A-31 33 419, and other phosphorus-containing photo-
initiators, for example 6-acyl-(6H)-dibenz~c,e]
[1,2]oxaphosphorine-6-oxides, in particular the 6-
-13-
2~C)58~
(2,4,6-trimethylbenzoyl)-(6H)-dibenz{c,e][1,2]-
oxaphosphorine- 6-oxide described in the older
German Patent Application P 38 27 735.2. The
photoinitiators may also be used in combination
with one another or with coinitiators or activators,
for example ~ith Michler's ketone and its
derivatives or 2-alkylanthraquinones. The quantity
of photoinitiator is, in general, about 0.01 to lO,
preferably about 0.5 to 5, ~ by weight of the
recording material.
It is often advantageous to add still further
auxiliaries and additives to the recording material,
for example thermal polymerization inhibitors such
as hydroquinone and its derivatives, 2,6-di-tert.-
butyl-p-cresol, nitrophenols, nitrosamines such as
N-nitrosodiphenylamine or salts of N-nitroso-
cyclohexylhydroxylamine, for example its alkali
metal or aluminum salts. Further common additives
are dyestuffs, pigments, proc~essing auxiliaries and
plasticizers.
The mixtures accordinq to the invention can
be used for producing relie~ printing pla~es and
flexographic printing plates by casting from
solution or extrusion and calendering to ~orm
coatings having a thickness of about 0.02 to 6 mm,
preferably from 0.2 to 2 mm. The coating may be
laminated onto the surface of a suitable base or a
solution of the mixtures according to the invention
can be applied to a coating base.
The mixtures according to the invention can
be used not only for producing relief printing
plates, but also, for example, for producin~
lithographic printing plates, gravure cylinders,
screen printing stencils and photoresists.
Suitable substrates are, depending on the
intended use, for example, polyester films,
steel or aluminum sheets, copper cylinders, bases
for screen printing stencils, ~oam layers, rubber-elastic bases
or p.c. boards. It may also be advantageous to
apply a topcoating or protective coating, for
example a thin coating of polyvinyl alcohol or a
peelable top film, for example of polyethylene
glycol terephthalate, to the photosensitiverecording
coating. Furthermore, a preliminary coating of the
base may be advantageous. The additional coating
between the base and the photosensitive coating may
be effective, for example, as an antihalation
coating or as an adhesive coating.
The recording materials according to the
invention can be subjected to imagewise exposure
with actinic light from light sources such as
mercury vapor lamps or fluorescent tubes, the
emitted wavelength preferably being between 300 and
420 nm. The unexposed and uncrosslinked coating
areas can be removed by spraying, washing or
brushing with aqueous alkaline solutions~ for
example of wetting or emulsifying agents. The
crosslinked coating areas are resistant to
water. Small quantities of foam inhibitors or
organic solvents miscible with water, for example
low aliphatic alcohols, may also be added to the
solution. The quantitative proportion of organic
-15-
2~0~ 5
solvents is, in general, less than 10%, preferably
less than 5~. Advantageously, the developed relief
forms are dried at temperatures up to 120C and
o~tionally post-exposed at the same time or later to
actinic light.
The recording materials according to the
invention are suitable in particular for producing
printing forms, especially letterpress or relief
printing forms which are particularly suitable for
flexographic printing.
The invention is explained by the examples
below. The limiting viscosity numbers JO [ml/g] were
determined at 25C in an Ostwald viscometer. The
degr~es of hydrolysis (in mol %) relate to the
hydrolyzable monomer units in the unhydrolyzed graft
polymer.
Preparation Examples:
Example 1
Preparation of the graft polymer containing hydroxyl
20 groups:
a) Synthesis of thP polyurethane grafting base
3,000 g of polyethylene glycol 600 and 193.1
g of 1,4-butanediol are heated together with
1.5 ml of 1,~-dimethylpiperazine to 75~C and
1,428.2 g of isophorone diisocyanate are
added in the course of 6 hours. The reaction
-16-
2C~58'7~
is then allowed to proceed for two hours at
80C.
The limiting viscosity number JO in methanol
of the polyurethane prepared in this manner
is 10.6 ml/g.
b) Synthesis of the graft polymer
l,500 g of the polyurethane grafting base are
heated to 80C and a mixture of 3,480 g of
vinyl acetate and 26.1 g of diben~oyl
peroxide 175% strength) is added in the
course of 5 hours while stirring. After
further reaction for 45 minutes at 80C, the
unreacted vinyl acetate is removed by
azeotropic distillation with methanol. The
limiting viscosity number JO in THF of the
graft polymer prepared in this manner is 17.5
ml/g.
c) Synthesis of the graft polymer containing
hydroxyl groups
172.1 g of a 10% strength methanolic NaOH an~
172.1 g of water are added to 9,178 g of a
50% strength methanolic solution of the graft
polymer at room temperature. The mixture gels
after some time and is then granulated. The
granulated material obtained is suspended in
methanol and a quantity of acetic acid
equivalent to the NaOH is added. The product
is filtered off, washed several times with
methanol and dried. The limiting viscosity
number JO in water is 10.1 ml/g, the degree
of hydrolysis is 85.8% and the proportion of
vinyl alcohol groups is 42.2%.
Example 2
Esterification of the graft polymer containing
hydroxyl groups:
After 100 g of the partially hydrolyzed graft
polymer described in Example lc) have been fused at
140C in a two-roll mill 47.8 g of succinic
anhydride are added at this temperature. After
reacting for 6 min. at 140C, the reaction is
complete; a colorless homogeneous product is
obtained which has an acid number of 181 mg KOH/g of
polymer. The product is not soluble in water, but
it dissolves to form a clear solution both in a 1%
strength aqueous soda solution and also in ethanol.
Exam~le 3
Esterification of the graft polymer containing
hydroxyl groups:
50 g of the partially hydrolyzed graft polymer
described in Example lc) are fused in a two-roll
mill at 140C. Than 23.5 g o~ maleic anhydride and
-18-
200~ 3'75
0.8 g of 2,6-di-tert.-butyl-4-methylphenol are added
at 140C. After 4 min. the reaction is complete.
The product obtained is colorless and has an acid
number of 133 mg XOH/g of polymer. It is only
sparingly soluble in water, producing turbidity, but
a clear solution is obtained in 1% strength aqueous
soda solution or in ethanol.
Example 4
Esterification of the graft polymer containing
hydroxyl groups:
200 g of the partially hydrolyzed graft polymer
described in Example lc), 9~ g of maleic anhydride
and 3.2 g of 2,6-di-tert.-butyl-4-methylphanol are
mixed and then jointly extruded at 140C and with a
retention time of 3 min. in a counter-rotating twin-
screw extruder having a screw length to screw
dianleter ratio o~ 15. The colorless homogeneous
extrudate has an acid number o~ 1~2 mg KOH/g of
polymer and, in addition, exhibits the same
properties as the polymer from Example 3.
Working examples:
ExamPle 1
73.15 pbw (parts by weight) of the graft polymer
containing hydroxyl groups reacted with succinic
anhydride (Example 2) are premixed with 20.0 pbw of
--19--
20~5~37S
triethylene glycol dimethacrylate, 4.0 pbw of
trimethylolpropane triacrylate, 2.0 pbw of benzil
dimethyl ketal, 0.8 pbw of 2,6-di-tert.-butyl-4-
methylphenol and 0.05 pbw of 2,4-
dihydroxybenzophenone, and extruded in a singlescrew extruder (Brabender Plasticorder) at 140C and
150 rev/min via a flat film extrusion die to form a
transparent melt. The extrudate is hot-pressed in
a plate press for one minute at lOO~C and 15 bar to
form a 0.6 mm thick photopolymer coating between a
0.125 mm thick polyester film and a 0.3 mm thick
anodically oxidized aluminum sheet which is provided
with a polyurethane adhesive coating. After peeling
off the polyester film, imagewise exposure is
carried out for five minutes in a commercial UVA
flat-bed exposure apparatus and development is carried
out for two minutes with 1~ strength aqueous soda
solution to produce a relief printing plate having
a Shore A hardness of 90 which is resistant to water
(storage for 24 hours at 25C).
If development is carried out with pure water, it is
only possible to obtain incompletely developed
relief coatings.
Example 2 (Comparison exam~le)
73.15 pbw of a graft polymer containing hydroxyl
groups (according to Example lc) which has not been
reacted with a carboxylic acid anhydride are
processed similarly to Example 1 to produce a
-20-
20C~5~
photopolymer coating. After exposure, development
is carried out with water. The relief printing
plate obtained is destroyed during several hours of
storage in water thermostated to 25C.
Example 3
57.8 pbw of the ~inder described in Example 3 are
homogenized for 2 minutes in a two-roll mill at
130C with 35.8 pbw of 2-hydroxypropyl methacrylate,
4.0 pbw of trimethylolpropane triacrylate, 2.0 pbw
of benzildimethyl ketal and 0.8 pbw of 2,6-di-
tert.-butyl-4-methylphenol, and the mixture is
processed similarly to Example 1 to produce a relief
printing plate having a Shore A hardness of 92. It
was possible to develop the plate well with 1%
strength aqueous soda solution. In addition, the
relief coating is resistant to pure water (storage
~or 24 hours at 25C).
Example 4 (Comparison example~
57~8 pbw of a graft polymer containing hydroxyl
groups (according to Example lc) which has not been
reacted with a carboxylic acid anhydride are
processed similarly to Example 3 to produce a
photopolymer coating. After exposure, development
is carried out with water. The relief printing
plate obtained is destroyed on storing for several
hours in water thermostated to 25C.
~21-
