AU614675B2 - Developing solvent for layers which are crosslinkable by photopolymerization and process for the production of relief forms - Google Patents

Description

1' A "bur Ref: 298080 FORM AUSTRALIA Patents Act COMPLLTE SPECIFICATION

(ORIGINAL)

Application Number: Lodged: Complete Specification Lodgea: Acc-epted: Published: woo* o* a e.g.

*0 *0* 0@ a boom0 Priority: Related Art: Applicant HOECHST AKTIENGESELLSCHAFT 6230 Frankfurt/main Federal Republic of Germany ARTHUR S. CAVE CO.

Patent Trade Mark Attornerys Lev.- l 10, 10 Barrack Street SYDNEY NSW 2000 Address for Service: ego, 00 S. 0 00 0 0 @0 0*0* 0 @0CC 0 @00000 o C 00

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COO 0 *0 0 C 0 4* Complete specification for the invention entitled "Developing solvent for layers crosslinkable by photopolymerization for the production of relief forms".

whi ch are and process The following statement is a full description of best method of performing it known to me:this invention, including the 1- 406 lU/gs PHOTOPOLYMERIZATION AND PROCESS FOR THE PRODUCTION OF RELIEF FORMS Background of the Invention i 5 The invention relates to a developing solvent for layers which are crosslinkable by photopolymerization and contain a binder based on an elastomeric polymer, a photopolymerizable monomer compatible therewith and a photoinitiator.

It has long been known to produce relief forms, in particular flexographic printing plates, by means of photopolymerization-crosslinkable layers which are exposed imagewise and thereafter developed *0 .and washed out.

For the production of these forms, the photopolymer layer is exposed imagewise to actinic light; a relief can then be formed by washing off the non-exposed and thus non-crosslinked portions of the layer, using a developing solvent. The 0, 20 developing solvent should dissolve the noncrosslinked portions of the layer as quickly as possible, and the solvent must be removable with r I I: JLP A *see of *0 0 0 e *e as*0 as 4 4, *000 00 4, 4, *o 0 *o 4, O 0 greatest possible ease from the crosslinked portions of the layer, so that the plate is rapidly dried.

For this reason, low-boiling developing solvents presently are generally used.

Examples of preferred solvents which are described in DE-A 22 15 090 include methyl ethyl ketone, benzene, toluene, xylene, carbon tetrachloride, trichloroethane, tri- chloroethylene, methyl chloroform and tetrachloroethane, and mixtures of these.

It is a disadvantage of the above-indicated chlorinated hydrocarbons that they are toxic and give rise to disposal problems. If chlorinated hydrocarbons are used as developing solvents, the portions of the layer which are to be removed swell very strongly while still on the plate and as the concentration of dissolved components of the noncrosslinked layer portions increases in the developing solvent, viscosity of the solvent rises sharply. As a result, the capacity of the developing solvent is very limited and the solvent is rendered useless, already at a solids content of A further consequence of the strong increase in viscosity is that plates develop very slowly even 25 at less than 5% solids content. Of the nonchlorinated hydrocarbons specified in DE-A 22 15 090 benzene, toluene and xylene are mentioned. These solvents have the disadvantage of being easily flammable, moreover, they lead to severe swelling of the layer and, consequently, to slow developing and drying of the plate. Additionally, due to their -2- 'j. s low flash-point, these solvents cannot be used in developing apparatuses which are not explosion-proof.

DE-A 36 00 116 describes relatively highly boiling developing solvents which contain hydrocarbons, alcohols or ketones which are branched, or cyclic with one or three olefinic double bonds, or saturated, or cycloaliphatic with one to three olefinic double bonds. Monoterpenes are mentioned in particular. Limonene which is used in the example may have good developing 0 characteristics, but is sensitive to oxygen and S. irritates the skin, properties which should not be present in a developing solvent. It is also a 15 disadvantage of limonene that it permits only very slow development and is difficult to remove from the layer.

Summary of the Invention a.

It is therefore an object of the present 20 invention to provide a developing solvent for photopolymer layers, which is non-toxic, has a high flash-point, can be removed from the layer as 25 completely and quickly as possible, without giving rise to swelling of the layer and shows the smallest possible increase in viscosity when taking up components of i-3- -3- _1 0 4 4 W4 0 4 0 the layer, so that it is possible to develop quickly and faultlessly, irrespective of high capacity reouirements.

2 It is also an object of the invention to provide a process for the production of photopolymerization-crosslinked relief forms which employs the developing solvent according to the present invention.

In accomplishing these objects there has been provided a developing solvent, comprising, an ester of the general formula I 0 O C

O-R

2 wherein R' and R 2 are the same or different and denote

(C

i

C

13 )n-alkyl or iso-alkyl, C5, C 6 or

C

12 -cycloalkyl, (C 1 alk-C 6 -aryl, C 6 or C,,-aryl, provided that the sum of carbon atoms in R 1 and R 2 is from 6 to 13.

Further objects, features and advantages of the present invention will become apparent from the detailei description of preferred embodiments that follows.

-4- 1 1 Detailed Description of the Preferred Embodiments Preference is given to those esters of the general formula I, in which the sum of carbon atoms in R 1 and R 2 is from 6 to Particularly preferred are esters of the general formula I, in which

R

1 denotes (Cl-C,,)n-alkyl, in particular (Ci- C) n-alkyl, C 12 -cycloalkyl, alkaryl, in particular benzyl or phenyl, and 10 R 2 denotes (C-C)n-alkyl or iso-alkyl, provided So*. that R' does not stand for alkaryl or phenyl, 0 (Cl-C,)n-alkyl or iso-alkyl, in particular (Cl-C)n-alkyl, provided that R' is benzyl or phenyl and benzyl or phenyl, provided that R' 15 is n-alkyl or C 5 iso-alkyl.

Examples of particularly preferred esters corresponding to the general formula I are: Amyl acetate, isoamyl acetate, hexyl acetate, cyclohexyl acetate, ethylhexyl acetate, nonyl 20 acetate, isononyl acetate, propyl propionate, propyl butyrate, butyl butyrate, isobutyi butyrate, benzyl **0 acetate, methyl benzoate.

The esters of the general formula I may be contained in the developing solvent of the present *o 25 invention, either individually or in the form of a mixture of various esters corresponding to the general formula I. Preferred are developing solvents which contain only one ester of the general formula I.

The developing solvent according to the invention comprises from about 55 to 100% by weight, particularly from about 55 to 90% by weight, of ester of the general formula I.

Apart from the esters of the general formula I, the developing solvent of this invention may contain other solvents as additions, particularly if the polymer layer carries an additional polyamide-containing protective layer. In this event, alcohols, especially relatively highly boiling alcohols, are preferred for use as additional solvents.

The principal constituents of the So photopolymerization-crosslinkable layers which are 15 to be developed using the developing solvent of the present invention comprise a binder based on an elastomeric polymer, a photopolymerizable monomer which is compatible with the binder, and a photoinitiator. The layers may also comprise e. 20 various binders, monomers or photoinitiators.

Additions which may be contained in the layers include dyes, pigments, anti-halation agents, antioxidants, plasticizers, antiozonants, crosslinking agents, regulat- rs, fillers, levelling 25 agents and other agents which improve the action of 4 e *s the layers.

Further auxiliary agents which may be added to the above-described layer are, for example, inhibitors to prevent thermal polymerization, such as hydroquinone and its derivatives, 2,6-di-tert.

-butyl-p-cresol, nitrophenols, nitrosamines, such as -6- ~r~a a~ N-nitrosodiphenylamine, or salts of N-nitrosocyclohexylhydroxylamine, the alkali metal or aluminum salts thereof.

Particularly preferred layers which are crosslinkable by photopolymerization are those which contain binders comprising polymers of conjugated aliphatic dienes, the monomer units of which have 4 to 5 carbon atoms. Among these the following are particularly mentioned: natural rubber, homu" polymers of butadiene and isoprene, copolymers of butadiene and isoprene, copolymers of butadiene and/or isoprene with other monomers, such as styrene, vinyl toluene, acrylonitrile, or c. (meth)acrylic alkyl esters, for example, nitrile 15 rubbers according to EP-A 064 564, random copolymers of styrene/butadiene, styrene/isoprene, and styrene/isoprene/butadiene, or block polymers of styrene monomers and butadiene and/or isoprene having a styrene content of about 10 to 50% by 4 S 20 weight. Elastomers of this kind are described in DE-B 22 15 090, DE-A 24 56 439, DE-A 29 42 183, and DE-A 21 38 582.

S*a The layers which are crosslinkable by photopolymerization generally contain from about to 95% by weight, preferably from about 30 to 95% by Sa weight, of binder.

Preferred monomers having 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 -7- 1 ml monohydric or polyhydric alcohols, such as ethylene glycol, di-, tri-, tetra- or polyethylene glycols, the latter preferably having from about 10 to ethylene glycol units, 1,3- propane diol, 1,6-hexane diol, dodecane diol, glycerol, 1,1,1-trimethylol propane, 1,2,4-butanetriol, or pentaerythritol, for example, ethylene glycol monomethacrylate, 1,3-propanediol monomethacrylate, hexanediul diacrylate, hexanediol dimethacrylate, 2-ethyl-hexylacrylate, lauryl methacrylate, stearyl methacrylate, glycerol mono or diacrylate, 1,2,4-butanetriol monomethacrylate, pentaerythritol triacrylate, polyethylene glycol methyl ether acrylate, tetradecaethylene glycol dimethacrylate or 15 the triether of glycerol and 3 mols of N-methylol 0 acrylamide or methacrylamide. The amount of monomers contained in the layer is, in general, from about 1 to by weight, preferably from about 2 to by weight, of the non-volatile constituents of 20 the composition.

SPhotoinitiators which can be used are the known compounds which exhibit sufficient thermal stability in the processing of recording materials o and which form a sufficient number of free radicals, 25 when exposing and thereby initiating polymerization of the monomers. The photoinitiators should absorb light in the wavelength region from about 250 to 500 *nm, forming radicals in the process. Examples of preferred photoinitiators are acyloins and the derivatives thereof, such as benzoin, benzoin alkylethers, for example, benzoin isopropyl ether, -8- St 0* 6 0 0 .s 0* 0 o00.

0O e e0 0 o 0 i0 *0 vicinal diketones and the derivatives thereof, for example, benzil, benzil acetals such as benzil dimethyl ketal, fluorenones, thioxanthones, polynuclear quinones, acridines and quinoxalines; and also trichloromethyl-s-triazines, 2-halomethyl-4-vinyl-l,3,4-oxadiazole derivatives, halogen-containing oxazoles substituted by trichloromethyl groups, carbonyl methylene heterocycles containing trihalomethyl groups, according to DE-A 33 33 450, acyl phosphine oxide compounds as described, for example, in DE-A 31 33 419 and other phosphorus containing photoinitiators, for example, the 6-acyl-(6H)-dibenz-[c,e]- [1,2]-oxaphosphorin-6-oxides, in particular 15 -trimethylbenzoyl)-(6H)-dibenz-[c,e][1,2]oxaphosphorin-6-oxide described in German patent application P 38 27 735.2. The photoinitiators can also be used in combination with one another or with coinitiators or activators, respectively, for 20 example, with Michler's ketone and its derivatives or with 2-alkvl anthraquinones.

The amount of photoinitiator is, in general, from about 0.01 to 10% by weight, preferably from about 0.5 to 5% by weight, of the layer.

25 The compositions which are crosslinkable by photopolymerization can be used for the preparation of relief and flexographic printing plates by way of casting from a solution or extruding and calendering to form layers having a thickness of about 0.02 to 10 mm, preferably about 0.2 to 6 mm. The layer can be laminated to the surface of an appropriate -9j J support or a solution of the composition can be applied to a layer support.

The above-indicated layers are not only used for the production of relief printing plates, but also for the prcduction of, for example, planographic printing plates, gravure cylinders, screen printing stencils, and photoresists. Depending on the intended application, suitable supports comprise, for example, polyester films, steel or aluminum sheets, copper cylinders, supports for screen printing stencils, foam layers, rubberelastic supports, or circuit boards. It may also be advantageous to apply a covering or protective layer, for example, a thin layer of polyvinyl 15 alcohol or polyamide, or a peelable covering film, for example, of polyethylene terephthalate, to the photosensitive recording layer. Moreover, precoating of the support may be advantageous. The additional layer between the support and the photosensitive layer may act, for example, as an anti-halation layer or as an adhesive layer.

The invention also relates to a process for the production of photopoly'nerization-crosslinked 4 Srelief forms. In the process, the layers which are 25 crosslinkable by photopolymerization are imagewise S* exposed to the actinic light of light sources such S* as mercury vapor lamps or fluorescent tubes, the emitted wavelength ranging between about 230 and 450 nm, preferably between about 300 and 420 nm. The non-exposed and thus non-crosslinked portions of the layer are removed with the aid of the developing 0 s0o 0 0 00 0 0 00 solvent according to the present invention, by spraying, washing or brushing. The developed relief forms are appropriately dried at temperatures up to abc-.. and may be post-exposed to actinic light, either simultaneously or subsequently.

The photopolymerization-crosslinked relief forms according to the invention are advantageously used in the production of printing forms, especially letterpress and relief printing forms, which are particularly suitable for flexographic printing.

The invention is explained by the examples below.

Example 1 A commercial flexographic printing plate based on a styrene-isoprene-styrene three-block 15 polymer (®Cyrel HL) as an elastomer having a layer thickness of 2.8 umm was first subjected to overall exposure from the back for 76 seconds, using a commercial fluorescent-tube exposure apparatus and thereafter exposed imagewise for 12 minutes, through 20 a negative transparency placed in contact with it.

After removing the covering layer, the exposed plate was developed in a commercial developing rpparatus equipped with brushes, using hexyl acetate containing 15% by weight butanol. A washing off 25 time of 5 minutes yielded the best results.

The flexographic printing form was dried for 1 hour at 50 0 C and stored for 15 hours at room temperature. Following a conventional post-treatment with an aqueous bromine solution, a -11- 'r t flexographic printing form of excellent quality was obtained.

Example 2 0 Goes 00 90000 o 6 0900e 0 9000 A commercial plate corresponding to Example 1 was exposed overall from the back for 76 seconds and thereafter exposed for 12 minutes from the front without using a negative transparency. After removing the covering layer, the plate (diameter mm) was placed for 10 minutes in 50 ml of hexyl 10 acetate. It was then wiped, dried for 1 ho'ur at C and stored for 17 hours. After storage, an increase in weight of the plate of only 0.42% could be determined, which was due to non-evaporated developing solvent.

Example 3 0 O 0 0 9 o o b000 0 @000 s 0 006

S@

*b 9 *a0 0 SO4

SO

S0 The procedure of Example 2 was followed, but the developing solvent used was cyclohexyl acetate.

The increase in weight of the plate after drying and storage was 1.4%.

20 Example 4 The procedure of Example 2 was followed, with the exception that butyl butyrate was used as the developing solvent. The increase in weight of the plate after drying and storage was only 1.05%.

-12- I -W Wii Example 5 (Comparative Example) A commercial plate as in Example 2 was processed in accordance with the instruction given in Example 2, with the exception that xylene was used as the developing solvent. The increase in weight of the plate after drying and storage under the conditions indicated in Example 2 was, however, 1.92%. This means that a relatively large proportion of the solvent was still present in the 10 plate.

0 3 Example 6 (Comparative Example) A commercial plate was treated as described S, in Example 2. However, the developing solvent used was limonene. The increase in weight of the plate after drying and storage was 1.92%.

Example 7

*O

To determine the capability (capacity) of developi. solvents to take up components from non-crosslinked regions of a styrene-isoprenestyrene three-block polymer (®Cyrel HL) the viscosities of the corresponding developing solvents were measured at specific solids contents produced by components from the layer after development thereof. Table 1 shows the viscosity values determined in an Ubbelohde viscometer at 25 C, for solids contents of 5, 7.5 and 10% by weight.

-13-

._I

'A

Although the developing solvents of the present invention still exhibited a viscosity which allowed a very high de veloping speed at a solids content of by weight, viscosity of perchlo':oethylene at a solids content of only 5% by weight was already so high that rapid developmnent was no longer ensured.

Table 1 iDeveloping Solvent Viscosities (cstj at Different solids contents by weight) 55 7 .5 I butyl butyrate 9.1 22 40.6 hexyl acetate 13 28 68 Cperchloroethylene 37.1 120.8 333.5 *see ,,,too scs -14-

Claims (4)

1. A propess for the prod ction of photopolymerization- crosslinked relief forms, Wh~ich comprises the steps of imagewise exposing to actinic light a layer which is crosslinkable by photopolyme rization and contajins a binder based on an elastomeric poi jmer, a photopolyinerizable monomer compatible therewith4 and a photoinitiator, and washing off the non-crosslinked portions of said layer with a developing solvent compri ing an ester of the general formula I (1) where in R Iand R 2are the same (C 1 -C 13 )n-alkyl or iso-alkyl C 2 -cycloalkyl, (C 1 -C 3 )alk-C C 10 aryl, provided that the and R2is from 6 to 13.

2. A process as recited in steps of, drying at temperat relief form, and simultaneou exposing said relief form to

3. A process as recited in contains from about 55 to 10 about 45 to 0% of an alcohol

4. A process as recited in contains from about 55 to 90 about 45 to 10% of an alcoho A process as -recited in. alcohol is a relatively highl or different and denote ,C 5 1 C 6or 6 aryl, C 6 or sum of carbon atoms inR claim 1, further comprising the ures of up to about 120*C said sly or subseq-uently post- actinic light. claim 1, wherein said solvent 0% by weight of said ester, and claim 1, wherein said oolvent Sby weight of said ester and claim 3 or 4 wherein the said ly boiling alcohol. DATED this 21st day of Augus4, 4991. AKTIENGESELL$CHAFT By Its Patent Attorneys ARTIMR S. CAVE