GB1590125A

GB1590125A - Diazo resin containing photosesitive elements

Info

Publication number: GB1590125A
Application number: GB4632176A
Authority: GB
Original assignee: Coates Brothers and Co Ltd
Current assignee: Coates Brothers and Co Ltd
Priority date: 1976-11-08
Filing date: 1976-11-08
Publication date: 1981-05-28
Also published as: IT1116943B; FR2399047A2

Description

(54) DIAZO RESIN CONTAINING PHOTOSENSITIVE ELEMENTS (71) We, COATES BROTHERS & COMPANY LIMITED, a British Company of Easton Street, Rosebery Avenue, London WC1X ODP, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment: - This invention is concerned with improvements in and relating to photosensitive elements and compositions for the production thereof. More particularly, the invention is concerned with photosensitive elements comprising a substrate bearing a layer of a photosensitive composition of the socalled "photoresist" type, which may be employed, for example, in the preparation of deep etch, especially multimetal, lithographic printing plates, printed circuit boards or screens for screen printing.

Broadly, such photosensitive elements comprise a layer of a photosensitive composition comprising a resinous or filmforming component and a light sensitive component (which may in some cases be a substituent part of the film-forming component) applied to a substrate, such as a multimetal lithographic printing plate blank. In use the layer of photosensitive composition is subsequently exposed to light through an image (e.g. a photographic transparency) so that portions of the layer are exposed to light. Depending on the nature of the composition the exposed portions of the layer are rendered more or less soluble in certain solvents that are the unexposed portions and it is with this latter class of composition (i.e. those in which the exposed film is hardened) that the present invention is concerned. After exposure of the film it is "developed", i.e. the nonhardened (non-exposed in the present case) portions of the film are removed or washed away by the action of a suitable solvent to leave on the substrate a patterned image of hardened material which in the case of the preparation of a lithographic printing plate or printed circuit board, serves to protect the underlying metal against the action of the etchants which are subsequently used to etch out or dissolve all or a part of the unprotected metal. In the case of a printing screen for example, where the image is formed directly on the screen, no treatment subsequent to the development will be required.

A wide variety of resins and lightsensitive components have been used in the preparation of photoresist compositions.

Thus, for example, one group of compositions widely used in the preparation of lithographic plates comprises a hydrophilic polymer such as glue or gelatin rendered light-sensitive with a dichromate lightsensitive or light sensitizing component.

However, sensitized plates formed from such compositions have a very short storage or shelf life, commonly measured in days.

This problem was largely overcome by the development of the so-called "diazo resins" which may be described as the condensation products of a salt of p-diazo nitroso diphenyl (e.g. the double zinc chloride salt) with formaldehyde. The diazo resins are described, for example, in United States Patent No. 3,050,502 and in U.K. Patent Specification No. 418,011 and are commercially available. The diazo resins, which are photosensitive in themselves are commonly used in admixture with a water soluble polymer such as polyvinyl alcohol and coated or "presensitized" plates prepared therefrom generally have storage lives, in the dark, of several months. However, such plates suffer from the disadvantages that their resistance to strong etches is not as high as might be desired and, further, their performance varies depending upon the ambient relative humidity of the atmos phere in which they are employed.

In our Application No. 3663/74 (Serial No. 1488864) we have described a photosensitive element comprising a substrate coated with a layer of a photoresist composition comprising (a) an organic solvent soluble diazo resin and (b) a partially esterified copolymer of maleic anhydride and methyl vinyl ether.

It has now been found, in accordance with the present invention, that certain other free carboxyl group containing copolymers, as hereinafter defined may be used in place of the partially esterified copolymers specified in Application No. 3663/74 (Serial No.

1488864).

Accordingly, one embodiment of the present invention provides a photosensitive element comprising a substrate coated with a layer of a water-developable photoresist com osition comprising: (a an organic solvent soluble diazo resin; and (b) a free carboxyl group-containing copolymer of (1) an ethylenically unsaturated mono- or di-carboxylic acid containing a total of from 3 to 6 carbon atoms, and (2) an ethylenically unsaturated compound of the formula: CH2=CH- R in which R is a hydrogen atom, an alkyl group containing from 1 to 4 carbon atoms, or an alkoxy group containing from 2 to 4 carbon atoms.

The term "water-developable" as used herein with reference to the photoresist composition used in the elements of the invention means that the composition which has not been exposed to light (i.e. has not been cross-linked) may be removed, or "developed" by washing with water, if necessary with swabbing or light brushing to assist in the removal of the unexposed material. The exposed material on the other hand will not be removed by such treatment. The question of whether or not a particular diazo resin/copolymer composition is so "water-developable" may be really determined by simple testing.

Component (a) of the photoresist composition, namely the diazo resin, must be organic solvent soluble, i.e. it should be soluble in a substantially organic solvent, i.e. one containing not more than 20% by weight of water. The most commonly prepared or commercially available diazo resins (e.g. Diazo Resin No. 4 manufactured by the Fairmount Chemical Company) are water-soluble rather than organic solventsoluble salts, i.e. are, for example, the chlorides, sulphates or zinc chloride double salts. Diazo resin salts with certain other anions are, however, organic solventsoluble and especial mention may be made of the hexafluorophosphate, p-toluene sulphonate, fluoroborate, p-chlorobenzene sulphonate, trichloroacetate and dichloromaleate salts. Such salts may be prepared either by preparing the diazo resins using the appropriate salt of the p-diazo nitroso diphenyl or, more conveniently and more preferably, by converting a water-soluble diazo resin salt (e.g. sulphate, hydrochloride or zinc chloride double salt) to an organic solvent-soluble salt by a simple double decomposition reaction between the water-soluble salt and the appropriate acid.

This procedure has the advantage that suitable organic solvent-soluble resin may be prepared from commercially available water-soluble salts. Additionally, some commercially available diazo resin salts are themselves organic solvent-soluble, e.g. Solvent Soluble Negative Sensitizer No. 21-3 supplied by the Industrial Dyestuff Company. second The second component (b) of the photoresist compositions used in the present invention is a copolymer of an ethylenically unsaturated mono- or di-carboxylic acid containing a total of from 3 to 6 carbon atoms and an ethylenically unsaturated compound, copolymerizable therewith, of the formula CH2=CH-R where R is a hydrogen atom, an alkyl group containing from 1 to 4 carbon atoms or an alkoxy group containing from 2 to 4 carbon atoms.

Suitable unsaturated mono- and dicarboxylic acids include maleic acid, citraconic acid and acrylic acid; maleic acid being generally preferred.

The comonomer from which the copolymer is derived may be a vinyl alkyl ether wherein the alkyl group contains from 2 to 4 carbon atoms, such as ethyl vinyl ether, n-propyl vinyl ether or isobutyl vinyl ether.

It has been found that, in general, the use of such vinyl ethers, especially vinyl ether leads to a product which is more resistant to the stronger etches, such as zinc chloride etches, commonly used in the production of multimetal lithographic plates and that the resin also exhibits a greater dot hardness than do corresponding products formulated using a copolymer of methyl vinyl ether.

The vinyl alkyl ether may be employed as sole comonomer or may be employed in admixture with one or more other alkyl vinyl ethers containing from 1 to 4 carbon atoms in the alkyl group. Alternatively, the comonomer may be an olefin such as ethylene or propylene.

The relative proportions of the two comonomer components of the copolymer are preferably such that in the case of the dicarboxylic acids the molar ratio of unsaturated dicarboxylic acid to other comonom er(s) is from 7:3 to 3:7 more preferably about 1:1. In the case of the monocarboxylic acids the molar ratio of monocarboxylic acid to other comonomer(s) is preferably from 14:3 to 6:7, more preferably about 2:1. In other words, in the present preferred embodiment of the invention the average repeating unit of the copolymer contains two carboxylic acids to one unit derived from the comonomer.

As indicated above, the copolymer used in the photoresist compositions of the invention contains free carboxylic groups. It is not however always necessary or desirable that all of the carboxyl groups derived from the ethylenically unsaturated carboxylic acid be free carboxyl groups. Thus the carboxyl groups on the copolymer may be partially esterified or, where the ethylenically unsaturated component of the copolymer is a dicarboxylic acid, in the form of anhydride groups. Where the carboxyl groups of the copolymer are partially esterified they may be esterified with an alcohol, such as ethanol or methanol, or with an alkoxyalkanol such as methoxyethanol or ethoxyethanol.

Generally, the copolymers in which the carboxyl groups are partially esterified and/ or are partially in the form of anhydride groups are less water-soluble than the corresponding copolymers in which all the carboxyl groups are free and it is thus possible to alter the degree of watersolubility of any copolymer by adjusting the degree of esterification and/or anhydride group content.

Generally, the un-crosslinked copolymer should have some degree of water-solubility whereas the cross-linked polymer (i.e. that which has been exposed to radiation) should be water-insoluble to permit of development with aqueous development media. As indicated above the water-solubility of a polymer (and its cross-linked product) can be to some extent controlled by adjusting the degree of esterification and/or anhydride content of the polymer. Obviously, also, the water-solubility of the polymer will also depend upon the nature of the components from which it is prepared, their relative proportions and the degree of polymerization.

The copolymers may be prepared by any of the conventional techniques used in the production of copolymers of ethylenically unsaturated monomers and will generally be prepared by a free radical polymerization technique using a free radical catalyst, such as an organic peroxide (such as benzoyl peroxide) or an organic percarbonate (such as dicyclohexyl percarbonate). Conveniently the polymers are prepared by a solution polymerisation technique. The monomers employed in the preparation of the copolymers will comprise the ethylenically unsaturated acid (or, possibly, the anhydride thereof in the case of the dicarboxylic acids) and the ethylenically unsaturated comonomer.

When a mono-carboxylic acid or freecarboxyl group containing di-carboxylic acid is employed the resulting copolymer will of course contain free carboxyl groups and this copolymer may be converted to a partially esterified copolymer by reaction with an appropriate alcohol. When an anhydride of a dicarboxylic acid is employed the resulting copolymer will contain anhydride groups and these may be hydrolysed, in whole or in part, to free carboxyl groups by reaction with water. Alternatively, the anhydride group-containing copolymer may be first partially esterified by reaction with an appropriate alcohol and remaining anhydride groups then converted to free carboxyl groups by hydrolysing with water.

In some cases, the basic free carboxyl group-containing or anhydride groupcontaining copolymers may be commercially available and these commercially available polymers may serve as suitable starting materials for further partial esterification and/or hydrolysis.

The photosensitive elements of the invention are generally prepared by applying a solution of the diazo resin and copolymer in a substantially organic solvent (i.e. one containing not more than 20% by weight of water) to the substrate and allowing the composition to dry. Such solutions are possible as a further feature of the invention.

The substantially organic solvent is preferably 2-methoxy-ethanol but may be another water miscible organic solvent or may contain a minor amount of waterimmiscible diluent. It is sometimes found that a purely organic solvent fails to give complete solution and in such cases a small amount of water may be included in the solvent system. 2-Methoxy-ethanol preferably forms all or a major part of the organic solvent system for a number of reasons: thus, it is sufficiently volatile to evaporate off to leave a resin film in any subsequent film-forming operation and is, moreover, reasonably innoccuous and safe in use.

Accordingly, the esterification of the copolymer may be suitably carried out using an excess of 2-methoxy-ethanol, the excess providing a part at least of the organic solvent system.

Photo resist compositions suitably contain from 3 to 25, preferably from 7 to 15 parts by weight of diazo resin per 100 parts by weight of the copolymer. The total resin (diazo resin plus copolymer) concentration in the composition used in the preparation of the elements of the invention may be varied as desired in order to give them the most suitable viscosity for application to the substrate by the desired method.

The photo resist compositions of the invention may contain other compatible additives, especially dyestuffs to colour the compositions and suitable dyestuffs include, for example, magenta, methyl violet and rhodamine B200.

The photosensitive elements of the invention are as stated above, prepared by coating a composition comprising (a) diazo resin, (b) copolymer and (c) substantial organic solvent on to a substrate and subsequently drying the wet film, typically in a current of warm air. Any suitable coating method such as curtain or roller coating and any suitable substrate may be employed, e.g. a screen (for the preparation of a printing screen), a copper clad board (for the preparation of a printed circuit board) or a multimetal plate (for the preparation of a lithographic printing plate).

In the following description particular reference will be made to the preparation of a multimetal lithographic printing plate but it will be understood that in its general aspects the description will apply equally to the preparation of other objects by the general steps of imagewise exposing an element in accordance with the invention, to light; removing unexposed unhardened film from the substrate by washing with water and then, if desired, further treating the plate, e.g. with one or more etchants.

Thus, in the preparation of a lithographic printing plate a blank printing plate comprising a thin layer of chromium over a thin layer of copper electroplated on to a steel base is coated by means of a curtain coater with the solution of the copolymer and diazo resin, the concentration of resins having been adjusted so that the particular coating machine and coating speed employed yield a dry film thickness of 3.5 - 7 (typically 4) microns. The coating is dried in a current of warm air, all operations up to this point being carried out under "safelight" conditions (obviously, since the coating is photosensitive).

The presensitised plate is stored in the dark (for example by wrapping in a material opaque to light) until needed for use. It is then exposed image-wise in a printing down frame under a carbon arc or other source of UV-rich light. A typical exposure is for 3 minutes at a distance of 30 inches from a 2.5 KVA lamp. Those portions of the photo resist composition shielded from radiation in the image-wise exposure remain soluble and are removed by washing with water leaving an image "stencil" in waterinsoluble composition on the plate. This stencil provides a resist against the acid etch used in the next step which is to etch away the exposed chromium to expose the copper beneath. An advantage of the photo resist composition of the invention is that the heavy hygroscopic, zinc containing etches of the prior art need not be used. A typical etch may comprise 50% of calcium chloride, 8% of concentrated hydrochloric acid and 42% of water. After etching, the plate is washed with water and the stencil removed by chemical solution in an alkaline oxiding agent, typically alkaline permanganate. Excess permanganate is destroyed by washing with dilute hydrogen peroxide solution. The plate is then gummed up and used according to the established procedure of lithography.

A printed circuit may similarly be prepared using a wholly conventional sequence of steps. After imagewise exposure the still soluble composition is again washed away and then exposed, unwanted copper removed by etching for example with ferric chloride solution.

The preparation of a printing screen is still simpler and again wholly conventional. After imagewise exposure the still soluble composition is washed away and the screen is then ready for use after drying.

In order that the invention may be well understood the following Examples are given by way of illustration only.

Example I (a) A copolymer of maleic anhydride and ethyl vinyl ether was first prepared as follows. 100 g maleic anhydride, 70g ethyl vinyl ether and 0.5 g benzoyl peroxide in 600 g toluene were heated under reflux in a current of nitrogen at 65-700C for two and a half hours. The copolymer precipitated from solution and was filtered off and washed with toluene.

(b) 6g of the product from (a) above was dissolved in a mixture of 37g 2methoxyethanol and 7g water at 700C under reflux. Heating was continued until the solution could be diluted with water without causing precipitation and for 3 hours thereafter. Titration with alkali using a pH meter showed that ester and free carboxyl groups were presented in the ratio 35 : 65. Addition of 0.75g of the trichloracetate salts prepared from Diazo Resin No. 4 (ex Fairmount Chemical Co.) gave a clear solution which was used to coat a multimetal lithographic plate. This plate was processed in a zinc chloride etch after imagewise exposure and development with water and exhibited the advantages of greater dot hardness and etch resistance when compared with a plate prepared according to Example 1 of Application No. 3663/74 (Serial No. 1488864).

Example 2 A copolymer was prepared from 112g citraconic anhydride and 70g of ethyl vinyl ether following the procedure in Example 1(a).

6g of this product was dissolved in a mixture of 37g 2-methoxyl ethanol and 14g water and refluxed for 24 hours. The product was a clear solution which was used in the manner of Example 1(b) to prepare a lithographic plate with similar results.

Example 3 (a) A copolymer of acrylic acid and ethyl vinyl ether was first prepared as follows. 72g acrylic acid, 35g ethyl vinyl ether and 0.5g benzoyl peroxide in 600g toluene was heated under reflux in a stream of nitrogen at 65"C for two and a half hours.

The precipitated copolymer was filtered off and washed with toluene.

(b) A portion of the copolymer so obtained was dissolved in 2-methoxy ethanol and blended to a clear solution with 10% of its weight of the p-toluene sulphonate salt prepared from the Diazo Resin No. 4 of Example 1(b).

Used as a photosensitive coating on a multimetal lithographic plate, the product gave results similar to those of Example 1(b).

Example 4 A terpolymer of maleic anhydride, ethyl vinyl ether and isobutyl vinyl ether was prepared following the procedure of Example 1(a) from 100g maleic anhydride, 35g ethyl vinyl ether and 50g isobutyl vinyl ether.

Warmed with water this yielded a clear solution which, with the addition of 10% of its weight of the trichloracetate salt described in Example 1(b), gave a photosensitive coating which behaved in a similar manner.

Example 5 A similar copolymer to that described in Example 1(a) was prepared replacing the 0.5g benzoyl peroxide by 0.5g dicyclohexyl perdicarbonate. The reactants were held at room temperature under a stream of nitrogen for 16 hours. The precipitated polymer was filtered and washed with toluene.

The product was treated in the same way as Example 1(b) to produce a lithoplate coating. The resulting plate developed faster than the plate obtained in Example 1(b) but had softer resist dots.

Example 6 A partially esterified copolymer of ethylene/maleic anhydride was prepared from a commercially available Ethylenemaleic anhydride copolymer (EMA Grade 21 - Ex Monsanto Co.).

46g of EMA Grade 21 and 148g of 2-ethoxyethanol and 6g water were allowed to stand at room temperature for two weeks with occasional stirring.

709g of 2-methoxyethanol were added to the mixture together with 4.55g of the trichloracetate salt of the diazo resin of Example 1(b) and 0.27g Methyl Violet.

A sample of this coating was applied to a trimetal plate by whirling. This coating was developable, after image-wise exposure, with water but required swabbing to remove unexposed coating. The coating was sufficiently resistant to both calcium chloride based, and zinc chloride based etches to be suitable for such processes.

WHAT WE CLAIM IS: 1. A photosensitive element comprising a substrate coated with a layer of a waterdevelopable photosensitive composition comprising: rising: (a) an organic solvent-soluble photosensitive diazo resin; and (b) a free carboxyl group-containing copolymer of (1) an ethylenically unsaturated mono- or di-carboxylic acid containing a total of from 3 to 6 carbon atoms, and (2) an ethylenically unsaturated compound of the formula: CH2 = CH - R in which R is a hydrogen atom, an alkyl group containing from 1 to 4 carbon atoms or an alkoxy group containing from 2 to 4 carbon atoms.

2. A photosensitive element as claimed in claim 1 in which the diazo resin is in the form of a hexafluorophosphate, p-toluenesulphonate, fluoroborate, p-chlorobenzenesulphonate, trichloroacetate or dichloromaleate salt.

3. A photosensitive element as claimed in claim 1 or claim 2 in which the ethylenically unsaturated acid is maleic acid.

4. A photosensitive element as claimed in any one of the preceding claims in which the compound of the formula CH2 = CH R is ethyl vinyl ether.

5. A photosensitive element as claimed in any one of the preceding claims in which the copolymer is a copolymer of the said ethylenically unsaturated acid, a vinyl alkyl ether containing from 2 to 4 carbon atoms in the alkyl group and another different vinyl alkyl ether containing from 1 to 4 carbon atoms in the alkyl group.

6. A photosensitive element as claimed in any one of the preceding claims in which the molar ratio of unsaturated acid to other comonomer(s) is from 7:3 to 3:7 when the acid is a dicarboxylic acid and from 14:3 to 6:7 when the acid is a monocarboxylic acid.

7. A photosensitive element as claimed in any one of the preceding claims in which the copolymer is partially esterified with an alkanol or alkoxyalkanol.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 1(a). 6g of this product was dissolved in a mixture of 37g 2-methoxyl ethanol and 14g water and refluxed for 24 hours. The product was a clear solution which was used in the manner of Example 1(b) to prepare a lithographic plate with similar results. Example 3 (a) A copolymer of acrylic acid and ethyl vinyl ether was first prepared as follows. 72g acrylic acid, 35g ethyl vinyl ether and 0.5g benzoyl peroxide in 600g toluene was heated under reflux in a stream of nitrogen at 65"C for two and a half hours. The precipitated copolymer was filtered off and washed with toluene. (b) A portion of the copolymer so obtained was dissolved in 2-methoxy ethanol and blended to a clear solution with 10% of its weight of the p-toluene sulphonate salt prepared from the Diazo Resin No. 4 of Example 1(b). Used as a photosensitive coating on a multimetal lithographic plate, the product gave results similar to those of Example 1(b). Example 4 A terpolymer of maleic anhydride, ethyl vinyl ether and isobutyl vinyl ether was prepared following the procedure of Example 1(a) from 100g maleic anhydride, 35g ethyl vinyl ether and 50g isobutyl vinyl ether. Warmed with water this yielded a clear solution which, with the addition of 10% of its weight of the trichloracetate salt described in Example 1(b), gave a photosensitive coating which behaved in a similar manner. Example 5 A similar copolymer to that described in Example 1(a) was prepared replacing the 0.5g benzoyl peroxide by 0.5g dicyclohexyl perdicarbonate. The reactants were held at room temperature under a stream of nitrogen for 16 hours. The precipitated polymer was filtered and washed with toluene. The product was treated in the same way as Example 1(b) to produce a lithoplate coating. The resulting plate developed faster than the plate obtained in Example 1(b) but had softer resist dots. Example 6 A partially esterified copolymer of ethylene/maleic anhydride was prepared from a commercially available Ethylenemaleic anhydride copolymer (EMA Grade 21 - Ex Monsanto Co.). 46g of EMA Grade 21 and 148g of 2-ethoxyethanol and 6g water were allowed to stand at room temperature for two weeks with occasional stirring. 709g of 2-methoxyethanol were added to the mixture together with 4.55g of the trichloracetate salt of the diazo resin of Example 1(b) and 0.27g Methyl Violet. A sample of this coating was applied to a trimetal plate by whirling. This coating was developable, after image-wise exposure, with water but required swabbing to remove unexposed coating. The coating was sufficiently resistant to both calcium chloride based, and zinc chloride based etches to be suitable for such processes. WHAT WE CLAIM IS:

1. A photosensitive element comprising a substrate coated with a layer of a waterdevelopable photosensitive composition comprising: rising: (a) an organic solvent-soluble photosensitive diazo resin; and (b) a free carboxyl group-containing copolymer of (1) an ethylenically unsaturated mono- or di-carboxylic acid containing a total of from 3 to 6 carbon atoms, and (2) an ethylenically unsaturated compound of the formula: CH2 = CH - R in which R is a hydrogen atom, an alkyl group containing from 1 to 4 carbon atoms or an alkoxy group containing from 2 to 4 carbon atoms.

8. A photosensitive element as claimed

in claim 7 in which the alkanol or alkoxyalkanol is methanol, ethanol, methoxyethanol or ethoxyethanol.

9. A photosensitive element as claimed in any one of the preceding claims in which the unsaturated acid is a dicarboxylic acid and a portion of carboxyl groups in the copolymer are in the form of anhydride groups.

10. A photosensitive element as claimed in any one of the preceding claims containing from 3 to 25 parts by weight of the diazo resin per 100 parts by weight of the copolymer.

11. A photosensitive element as claimed in claim 10 containing from 7 to 15 parts by weight of the diazo resin per 100 parts by weight of the copolymer.

12. A photosensitive element as claimed in any one of the preceding claims in which the substrate is a copper clad board or a multimetal plate.

13. A photosensitive element as claimed in claim 1 substantially as hereinbefore described with reference to the Examples.

14. A composition for the preparation of a photosensitive element as claimed in any one of the preceding claims comprising an organic solvent-soluble diazo resin and a free carboxyl group-containing copolymer as defined in any one of claims 1 - 11 dissolved in a substantially organic solvent as herein defined.

15. A composition as claimed in claim 14 in which the organic solvent is 2 methoxy-ethanol.

16. A composition as claimed in claim 14 substantially as hereinbefore described with reference to the Examples.

17. A process for the preparation of a photosensitive element which comprises applying a layer of a composition as claimed in any one of claims 14-16 to a substrate and allowing it to dry.

18. A process for producing an image upon a substrate which comprises imagewise exposing a photosensitive element as claimed in any one of claims 1 - 13 to light and removing unexposed un-crosslinked coating therefrom by washing with water.

19. A process as claimed in claim 18 in which the substrate is a copper clad board or multimetal plate and the substrate is subsequently treated with one or more etchants.

20. A process as claimed in claim 18 substantially as hereinbefore described with reference to the Examples.