CA2067608A1

CA2067608A1 - Solvent mixture for use in the development of negative-working recording layers

Info

Publication number: CA2067608A1
Application number: CA002067608A
Authority: CA
Inventors: Hans-Joachim Schlosser
Original assignee: Individual
Current assignee: Hoechst AG
Priority date: 1991-04-30
Filing date: 1992-04-29
Publication date: 1992-10-31
Also published as: EP0511585A2; ES2065102T3; EP0511585A3; DE59200889D1; EP0511585B1; JPH05210244A; DE4114060A1

Abstract

Abstract of the Disclosure The present invention relates to a solvent mixture comprising (a) at least one completely hydrogenated aromatic which has a flash point above 40°C and a boiling point in the range from 150 to 220°C; and (b) at least one aliphatic alcohol containing 4 to 9 carbon atoms.
The present invention relates to 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 said recording material with a developing solvent mixture according to the present invention.

Description

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.

2~67608 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 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 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 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 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.

20676~8 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.

Claims

1. A solvent mixture comprising (a) at least one completely hydrogenated aromatic compound which has a flash point above about 40°C and a boiling point in the range from about 150 to 220°C; and (b) at least one aliphatic alcohol containing 4 to 9 carbon atoms.

2. The solvent mixture according to Claim 1, wherein the content of said hydrogenated aromatic compound is about 60 to 95% by weight, and the content of said aliphatic alcohol is about 5 to 40% by weight, in each case relative to the total weight of the solvent mixture.

3. The solvent mixture according to Claim 2, wherein the content of said hydrogenated aromatic compound is preferably about 65 to 90% by weight, and the content of said aliphatic alcohol is preferably about 10 to 35% by weight, in each case relative to the total weight of the solvent mixture.

4. The solvent mixture according to Claim 1, which additionally contains a surface-active substance.

5. 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 said recording material with a developing solvent comprising: (i) at least one completely hydrogenated aromatic which has a flash point above about 40°C

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.

6. The process according to Claim 5, wherein the content in said solvent mixture of said hydrogenated aromatic compound is about 60 to 95% by weight, and the content in said solvent mixture of said aliphatic alcohol is about 5 to 40% by weight, in each case relative to the total weight of the solvent mixture.

7. The process according to Claim 6, wherein the content in said solvent mixture of said hydrogenated aromatic compound is preferably about 65 to 90% by weight, and the content of said aliphatic alcohol is preferably about 10 to 35% by weight, in each case relative to the total weight of the solvent mixture.

8. The process according to Claim 6, wherein said solvent mixture additionally contains a surface-active substance.