Note: Descriptions are shown in the official language in which they were submitted.
20~7608
SOLVENT MIXTURE FOR USE IN THE DEVELOPMENT OF
NEGATIVE-WORKING RECORDING LAYERS
Backaround of the Invention
The present invention relates to a solvent mixture
for use in the development of negative-working recording
layers which comprise an elastomeric polymer, a
photopolymerizable monomer compatible therewith and a
photoinitiator. The invention further relates to a
process for the production of a relief printing form, in
which a recording material formed of a support and a
negative-working radiation-sensitive layer which contains
the above-indicated components as the essential
constituents, is a) irradiated imagewise and b) developed
by means of a solvent.
Tn the production of relief printing forms, the
solvent used should remove the non-crosslinked portions
of the recording layer as quickly as possible and without
leaving a residue. To have a relief printing form which
is soon ready for printing it is moreover necessary for
the solvent to be readily removable from the crosslinked
layer areas.
Different elastomeric polymers are found in
commercially available flexographic printing plates. It
is therefore important for a universally applicable
developing solvent to yield the same good results in the
development of the various printing plates of different
compositions which are in the market.
A universal solvent for use in the development of
negative-working radiation-sensitive layers which have
different polymer components has, up to the present, not
been available. In addition, the customary solvents
often have an unpleasant odor and/or are hazardous from
the point of view of toxicity.
Solvents hitherto used comprise, in particular, those
which have a low boiling point such as benzene, toluene,
: : .
2~67608
xylene, l,1,1-trichloroethane, 1,1,2,2-tetrachloroethane,
butanone and mixtures of these (cf. DE-C 22 15 090 = GB-A
1,366,769). A mixture of tetrachloroethylene and butanol
has also been employed in practice. However, the
halogenated hydrocarbons are toxicologically extremely
hazardous. Furthermore, considerable problems are
encountered in the disposal of spent solvents. According
to the official German "Technische Anleitung Luft (TA
Luft)" ("Technical Directions Air"), tetrachloroethylene
contained in the waste air must not exceed a maximum
value of 0.1 kg/h. Perchloroethylene ranks among the
group of substances which are reasonably suspected of
being cancerogenic (cf. "Liste der Maximalen
Arbeitsplatz-Ronzentrationen (MAX)" 1990 - "List of
Threshold Limit Values"). This definitely is a bar to
the further use of perchloroethylene. The chlorinated
hydrocarbons, in particular, also give rise to strong
swelling of the exp~sed areas of the layers and extensive
drying is therefore required to remove them again from
the plates. The non-chlorinated hydrocarbons such as
benzene, toluene and xylene likewise lead to strong
swelling of the layer and are moreover easily flammable.
owing to their low flash point, solvents of this kind can
only be used in explosion-proof processing e~uipment.
EP-A 0 365 98~ describes mixtures of higher-boiling
aromatic compounds which are less easily flammable. They
show a favorable developing behavior, but their intensive
aromatic odor is a disadvantage.
An even more intensive odor is given off by the
limonene-based solvents indicated in the examples of DE-A
36 00 116 (= US-A 4 806 452). Limonene, in addition, is
susceptible to atmospheric oxygen and irritates the skin.
An obnoxious odor which persists even in the dried
printing plates is also present in the carboxylic acid
esters described in WO 90/02359. These carboxylic acid
esters additionally have a low daily OEL (Occupational
Exposure Limit) of 50 ppm (Performance Fluids, Exxon
Chemical, cf. also: "Ein praktischer Leitfaden zum
3 20~76~8
Schutz des Menschen und seiner Umwelt" - "A practical
guide to the protection of man and his environment",
Exxon Chemical 1990, p. 79-83).
US-A 3,887,373 discloses a process for developing a
negative-working photoresist layer which contains
condensed polyisoprene as a binder and 2,6-bis-(4-
azidobenzylidene)-4-methylcyclohexane as a
photoinitiator. The developers used are saturated
petroleum fractions which predominantly comprise mixtures
lo formed of saturated aliphatic hydrocarbons with
cycloaliphatic hydrocarbons, in particular cyclohexane.
W0 90/13853 describes a process for the production
of relief printing forms which are crosslinked by
photopolymerization. Developing solvents used to wash
off the layer portions which remained uncrosslinked after
imagewise exposure comprise hydrogenated petroleum
fractions or mixtures of hydrogenated petroleum fractions
and alcohols. The hydrogenated petroleum fractions
preferably contain a naphthenic constituent, i.e. an 0.5
to 50% portion of cyclic hydrocarbons. The solvents
which are actually described have a naphthenic
constituent of 30 to 35%. These solvents are employed to
develop monolayér and multilayer plates containing a
three-block rubber formed of styrene/isoprene/styrene-
butadiene according to EP-A 0 027 612 (= US-A 4,320,188).
It was found, however, that flexographic printing plates
which contain a binder comprising a
styrenetisoprene/styrene block copolymer according to DE-
A 22 15 090 require a considerably longer development
time if these solvents are used. Depending upon the
block size and the molecular weight of the polymer, the
development time is up to twice as long as the time
required for developing flexographic printing plates
based upon the above-specified block copolymers,
according to EP-A 0 027 612. The developing solvents
described in W0 90/13853 are therefore not universally
applicable.
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A process for developing flexographic printing plates
with the aid of an emulsion of oil-in-water or of water-
in-oil is disclosed in DE-A 39 08 763. The organic phase
or a component of the organic phase can comprise, apart
from a plurality of other solvents, also hydrogenated
petroleum fractions and decalin. The emulsion can
furthermore contain alcohols such as pentanol or hexanol.
The developing process is intended for use with a
specific flexographic recording material, in which the
lo optically transparent top layer and the unexposed areas
of the relief-forming recording l~yer, which are to be
washed off, have different solubilities. This means that
either the top layer is soluble or at least swellable in
organic solvents and the non-irradiated areas of the
relief-forming layer are soluble or at least swellable in
water or aqueous solvents, or vice versa. Consequently,
the process i5 by no means universally applicable.
Therefore, a need exists for a developing solvent for
photopolymer layers, which is non-toxic, has a
sufficiently high flash point, causes the lowest possible
swelling of the crosslinked portions of the layer and can
thus be rapidly removed again from the developed plate,
reduces unpleasant odors to a minimum or is even odorless
and yields the highest possible speed of development
which is approximately the same for a very large number
of plate types conventionally used in practice.
Summary of the Invention
An object of the present invention was to provide a
developing solvent for photopolymer layers which is non-
toxic and reduces unpleasant odors to a minimum or is
even odorless.
Another object of the present invention was to
provide a developing solvent for photopolymer layers
which has a sufficiently high flash point.
A further object of the present inven~ion was to
provide a developing solvent for photopolymer layers
20~7~8
which causes the lowest possible swelling of the
crosslinked portio~s of the layer and thus can be rapidly
removed again from the developed plate.
Another object of the present invention was to
provide a developing solvent for photopolymer layers
which yields the highest possible speed of development
which is approximately the same for a very large number
of plate types conventionally used in practice.
An additional object of the present invention was to
provide a process for the production of a relief printing
form wherein a solvent mixture, having the aforementioned
characteristics, is used as the developing solvent in the
process of development of an irradiated recording
material comprising photopolymer layers.
In accomplishing the foregoing objects there has been
provided according to one aspect of the present invention
a solvent mixture for use in the development of negative-
working recording layers which comprise:
- an elastomeric polymer,
- a photopolymerizable monomer compatible therewith
and
- a photoinitiator,
the solvent mixture comprising
a~ at least one completely hydrogenated aromatic
compound which has a flash point abo~e a~out
40C and a boiling point in the range from
about 1~0 to 220OC and
b) at least one aliphatic alcohol containing 4 to
9 carbon atoms.
According to another aspect o~ the present invention
there has been provided a process for the production of
a relief printing form comprising the steps of: (a)
irradiating imagewise with actinic radiation a recording
material comprising a support and a negative-working
radiation-sensitive layer comprising an elastomeric
polymer, a photopolymerizable monomer compatible
therewith and a photoinitiator; and (b) developing the
irradiated recording material with a developing solvent
~6- 2067~08
comprising (i) at least one completely hydrogenated
aromatic compound which has a flash point above about 40
oc and a boiling point in the range from about 150 to 220
C; and (ii) at least one aliphatic alcohol containing 4
to 9 carbon atoms.
Further objects, features and advantages of the
invention will become apparent from the detailed
description of preferred embodiments which follows.
Detailed DescriPtion of the Preferred Embodiments
The content of completely hydrogenated aromatic(s)
is in general about 60 to 95% by weight, preferably about
to 90% by weight, and the content of aliphatic
alcohol(s) is about 5 to 40% by weight, preferably about
10 to 35% by weight, in each case relative to the total
weight of the solvent mixture.
Hydrogenated aromatics which are suitable for use in
the mixture according to the present invention include
~or example decahydronaphthalene and a mixture of
hydrogenated aromatics, commercially available under the
trade names Nappar 10 ~ (flash point 45 C, aniline point
54 C, density at 15 C 0.812 kgjdm3, viscosity at 25 C
1.13 mPa-s) and Nappar 11 ~ (flash point 60 C, aniline
point 42 ~C, density at 15 C 0.864 kg/dm3, viscosity at
25 C 2.17 mPa s (Exxon). A mixture of hydrogenated
aromatics is generally preferred. According to "TA-
Luft", Nappar 11 ~ may be present in the exhaust air in
an amount of up to 3 kg/hour. The OEL value indicated in
this connection is 300 ppm (Exxon Safety Data Sheet,
November 1990).
Particularly suitable (C4-Cg) alkanols comprise
butanol, pentanol, 2-ethylbutanol and 5-methylhexanol.
Surface-active substances, such as ethoxylated fatty
alcohols and alkane sulfonates, can optionally be added
to the developing solvent to increase the speed of
development.
Using the mixture according to the present invention
_7_ 2a~76~8
it is surprisingly possible to develop nearly all
commercially available flexographic printing plates in
approximately the same short time. Formation of the
relief is good, and drying time is short.
The commercial flexographic printing plates have in
general a multilayer structure. An about 0.7 to 6.7 mm
thick relief-~orming layer is covered by a thin polyamide
top layer having a thickness of about 5 ~m. The
polyamide layer prevents the film original from adhering
to the surface of the plate during exposure. To obtain
a plate which is capable of printing this layer must also
be dissolved. This is facilitated by the addition of
alcohols to the mixture according to the invention.
The invention further provides a process for the
production of a relief printing form, in which a
recording material formed of a support and a negative-
working radiation-sensitive layer which contains, as the
essential constituents, an elastomeric polymer, a
photopolymerizable monomer compatible therewith and a
photoinitiator is
a) irxadiated imagewise with actinic radiation and
b) developed with a developing solvent,
the process being characterized in that the developing
solvent used comprises the solvent mixture according to
the present invention.
The following examples and comparative examples serve
to illustrate the invention. They are not intended to be
in any sense limiting.
The types of plates used were as follows:
Type 1 plate
was prepared using a radiation-sensitive layer comprising
86.7% by weight of a three-block rubber formed of
styrene/isoprene/styrene, according
to DE-A 22 15 090 (1';% of styrene),
5.0% by weight of paraffin oil,
3.5% by weight of hexanediol diacrylate,
20676~8
--8--
2.5% by weight of hexanediol dimethacrylate,
1.5% by weight of benzil dimethyl ketal,
0.8% by weight of 2,6-di-tert.-butyl-4-methyl-
phenol,
0.01% by weight of Solvent Black (Color Index No. 26
150)
which was applied in a layer thickness of 2715 ~m to a
125 ~m thick polyester film as a support material and
covered with a 4 ~m thick top layer of polyamide.
The plate was first flood-exposed from the back for
9o s using a commercial fluorescent tube exposure unit,
followed by exposure from the front for 14 min through a
negative original placed in contact with the plate.
Type 2 plate
A commercial flexographic printing plate containing
a rubber binder according to EP-A O 027 612 (Nyloflex FAR
~) was first flood-exposed from the back for 72 s using
a commercial fluorescent tube exposure unit, followed by
exposure from the front for 14 min through a negative
original placed in contact with the plate.
Type 3 Plate
A commercial flexographic printing plate containing
a rubber binder according to DE-A 22 15 090 (Cyrel HLS O)
was first flood-exposed from the back for 75 s using a
commercial fluorescent tube exposure unit, followed by
exposure from the front for 17 min through a negative
original placed in contact with the plate.
Type 4 plate
A com~ercial flexographic printing plate containing
a rubber binder according to DE-A 22 15 090 (Cyrel HOS ~)
was first flood-exposed from the back for 70 s using a
commercial fluorescent tube exposure unit and then
.
.
20~7608
exposed from the front for 14 min through a negative
original placed in contact with the plate. The type ~
plate has a higher Shore-A hardness than the type 3
plate.
The plates were developed according to two variants:
Variant 1:
A commercial wash-off solution was used. The
flexographic printing plate was adhered to a metal plate
suspended on an excenter and pressed against a wash-off
brush. Using an alternating sense of rotation
(excentricity 2 cm), the plate was, in each case four
times in 10 s, moved horizontally against the stationary
brush.
Variant 2:
A commercial continuous wash-off apparatus for
flexographic printing plates was used. The plates were
first stripped by means of four oscillating brushes, then
rinsed in a rinsing zone and thereafter superficially
dried.
Example 1
A type 1 flexographic printing plate was exposed as
described above and then developed according to variant
1. The developer used was a mixture composed of 80% by
weight of decahydronaphthalene and 20% by weight of 5-
methylhexanol. After a developing time of 6.5 min at
room temperature the relief produced had a depth of 1 mm.
_ample 2
A type 2 flexographic printing plate was used. The
plate was developed as indicated in Example 1. The wash-
off time to obtain a relief depth of 1 mm was 7 min.
-lO- 2067608
Example 3
A type 1 flexographic printing plate was used. The
developing solvent employed was a mixture composed of 85%
by weight of a hydrogenated aromatics fraction (Nappar 11
~, Exxon, having a boiling range from 196 to 223C
according to ASTM D-86-82, an aromatics content below 2%,
a flash point of 68C and a content of hydrogenated
aromatics above 90%) and 15% by weight of 2-ethylbutanol.
Development was carried out according to variant 1. The
wash-off time to obtain a relief depth of 1 mm was 7.5
min. The printing form was dried for 2 hours at 65C,
then stored for 15 hours at room temperature, thereafter
post-treated with an aqueous bromine solution and again
dried. The completed printing form had an impeccable
surface and was odorless.
Example 4
A type 2 flexographic printing plate was used.
Development was carried out according to variant 1 using
the developing solvent of Example 3. The wash-off time
to obtain a relief depth of 1 mm was 7.5 min.
Exam~le 5
A type 1 flexographic printing plate was used.
Development was carried out according to variant 1 using
a developing solvent composed of 75% by weight of a
hydrogenated aromatics fraction (Nappar 11 ~) and 25% by
weight of 2-ethylbutanol. The wash-off time to obtain a
relief depth of 1 mm was 6.5 min.
Exam~le 6
A type 2 flexographic printing plate was used.
Development was carried out according to variant 1 using
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the developing solvent of Example 5. The wash-off time
to obtain a relief depth of 1 mm was 6.3 min.
Exam~le 7
A type 1 flexographic printing plate was used.
Development was carried out according to variant l using
a developing solvent composed of 80% by weight of a
hydrogenated aromatics fraction (Nappar 11 ~) and 20% by
weight of 5-methylhexanol. The wash-off time to obtain
a relief depth of 1 mm was 6.5 min.
Example 8
A type 2 flexographic printing plate was used.
Development was carried out according to variant 1 using
the developing solvent of Example 7. The wash-off time
to obtain a relief depth of 1 mm was 6.5 min. The
developed printing ~orm was dried for 2 hours at 65C,
then stored for 15 hours at room temperature, thereafter
post-treated with an aqueous bromine solution and again
dried. The completed printing form had an impeccable
surface and was odorless.
Example 9
A type 1 flexographic printing plate was used.
Development was carried out according to variant 2 using
a developing solvent composed of 85% by weight of a
hydrogenated aromatics fraction (Nappar 11 ~) and 15% by
weight of 5-methylhexanol. The throughput time to
produce the superficially dried plate totalled 12.4 min.
The flexographic printing form was dried for 2 hours
at 65C, then stored for 15 hours at room temperature,
post-treated with an aqueous bromine solution and again
dried. The completed printing form had an impeccable
surface and was odorless.
20676~8
-12-
Example lo
A type 2 flexographic printing plate was used.
Development was carried out according to variant 2 using
the developing solvent of Example 9. The throughput time
to produce the superficially dried plate totalled 11.5
min.
The flexographic printing form was dried for 2 hours
at 65~C, then stored for 15 hours at room temperature,
subjected to a conventional post-treatment with an
lo aqueous bromine solution a~d again dried. The completed
printing form had an impeccable surface and was odorless.
Exam~le 11
A type 3 flexographic printing plate was used.
Development was carried out according to variant 2 using
the developing solvent of Example 9. The throughput time
to produce the superficially dried plate totalled 13.5
min.
The flexographic printing form was dried for 2 hours
at 65C, then stored for 15 hours at room temperature,
post-treated with an aqueous bromine solution and again
dried. The completed printing form had an impeccable
surface and was odorless.
Exam~le 12
A type 4 flexographic printing form was used.
Development was carried out according to variant 2 using
the developing solvent of Example 9. The throughput time
to produce the superficially dried plate totalled 11 min.
The flexographic printing form was dried for 2 hours
at 65C, then stored for 15 hours at room temperature,
post-treated with an aqueous bromine solution and again
dried. The completed printing form had an impeccable
surface and was odorless.
20~76~8
-13-
Comparative Example 1
The procedure of Example 1 was followed. The
developing solvent used was a mixture of a hydrogenated
petroleum fraction (Exsol D 60 ~, Exxon, having a boiling
range from 181 to 216C according to EP-A 0 332 070) and
15% by weight of 2-ethylbutanol. The wash-off time was
17 min.
Comparative Example 2
Comparative Example 1 was followed. A type 2
printing plate was used. The wash-off time was 9 min.
Comparative Exam~le 3
Comparative Example 1 was followed. The wash-off
solvent used was a commercial developing solvent
according to EP-A 0 332 070 (Nylosolv 0). The wash-off
time was 15 min.
Comparative Exam~le 4
Comparative Example 3 was followed. A type 2
printing plate; was used. The wash-off time was 7 min.
Comparative Example 5
Example 9 was followed. The wash-off solvent of
Comparative Example 3 was used. Development was carried
out in accordance with manufacturer's specification at a
temperature of 30C to increase the wash-off speed. The
time of throughput through the apparatus to obtain a
relief depth of 1 mm was 26 min.
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-14-
Comparative Example 6
Comparative Example 5 was followed. A type 2
printing plate was used. The throughput time to obtain
a relief depth of 1 mm was 11 min.
Com~arative Exam~le 7
Comparative Example 5 was followed. A type 3
printing plate was used. The throughput time to obtain
a relief depth of 1 mm was 22 min.
Comparative Example 8
Comparative Example 5 was followed. A type 4
printing plate was used. The throughput time to obtain
a relief depth of 1 mm was 15 min.
