THIOXANTHONE DERIVATIVES

Abstract: The invention provides thioxanthone derivatives of general formula wherein one of R², R³ and R⁴ is a group of formula in which one of R⁵, R⁶ and R⁷ is an alkyl or a benzyl group, the others being alkyl groups and A⁻ represents an anion, and R¹ and the others of R², R³, and R⁴ are independently selected from hydrogen atoms, alkyl groups and alkoxy groups; and the use of such derivatives as photoinitiators, particularly in aqueous photopolymerisable compositions for the production of stencils for screen printing.

Description: This invention relates to thioxanthone derivatives, their preparation, photopolymerisable compositions containing them, use of such compositions in the production of stencils for screen printing, screens for screen printing coated with such compositions, and the use of thioxanthone derivatives as photoinitiators. Three main photosensitive systems are used in screen printing, which are usually referred to as direct, indirect and direct/indirect methods. The prior art photoinitiators include certain dichromate compounds, polymeric diazonium salts and a leuco sulphuric acid ester of an indigo or thioindigo dye. When these photoinitiators are employed in any of the screen printing methods they suffer the disadvantage of being sparingly soluble in water so necessitating the addition of a solution of a peroxide or employment of large quantities of humectant or use of two pack systems. In order to overcome these difficulties, EP-A-81280 discloses that certain thioxanthones containing anionic groups such as carboxylic acid or sulphonic acid groups, have useful solubility in water of a suitable pH and can be used as photoinitiators in screen printing. These thioxanthones have the formula where R¹ is halogen, alkyl, alkoxy, alkylthio, nitro, amino, alkylamino, dialkylamino, hydroxyalkylamino, alkanoylamino, benzoylamino, N-alkanoyl-N-benzoylamino, sulphonamido or acetyl, R² is alkylene of 1 to 4 carbon atoms, n is 0, 1 or 2, m is 1 or 2, and A is -COOH, -SO₃H, -OSO₃H, or -OCO-X-COOH (where X is such that HOOC-X-COOH is a di- or tri- carboxylic acid of up to 8 carbon atoms), the aforesaid alkyl, alkoxy, and alkenyl residues containing up to 4 carbon atoms each, and water-soluble salts thereof. There have now been discovered certain novel thioxanthone derivatives which are very water-soluble over a wide pH range. According to the present invention there are provided thioxanthone derivatives of general formula wherein one of R², R³ and R⁴ is a group of formula in which one of R⁵, R⁶ and R⁷ is an alkyl or a benzyl group, the others being alkyl groups and A⁻ represents an anion, and R¹ and the others of R², R³, and R⁴ are independently selected from hydrogen atoms, alkyl groups and alkoxy groups. An alkyl or alkoxy group R¹, R², R³ or R⁴ is preferably a C 1-6 alkyl or C 1-6 alkoxy group, more preferably a C 1-4 alkyl or C 1-4 alkoxy (e.g. methoxy) group. In preferred embodiments of the invention, R¹ and said others of R², R³ and R⁴ are independently selected from hydrogen atoms and methyl groups. It is preferred for one of R⁵, R⁶ and R⁷ to be a C 1-20 alkyl group (e.g. a cetyl group) or a benzyl group, the others being C 1-6 alkyl, preferably C 1-4 alkyl, groups. In preferred embodiments of the invention, R⁵, R⁶ and R⁷ are all methyl groups. A⁻ may represent ½ of a divalent anion, e.g. ½SO₄²⁻, but is preferably a monovalent anion e.g. a halide ion. A preferably represents a chlorine or bromine atom, and is very conveniently a chlorine atom. Thioxanthone derivatives of formula I wherein R² or R⁴ represents the group of formula II are preferred. The invention further provides a process for the preparation of a thioxanthone derivative of formula I as defined above which comprises reacting a hydroxythioxanthone corresponding to formula I wherein in place of the group of formula II is a hydroxy group, with a compound of formula wherein R⁵, R⁶, R⁷ and A⁻ are all as defined for the group of formula II, R⁸ is a group -OH and R⁹ is a leaving group, or R⁸ and R⁹ together represent an oxygen atom, in the presence of a base. A leaving group R⁹ may conveniently be a halogen atom. Preferably when R⁸ is a group -OH, R⁹ is a chlorine atom. The hydroxythioxanthones corresponding to formula I wherein in place of the group of formula II is a hydroxy group are either known compounds, preparable by known methods, or they may be prepared by methods analogous to known processes. Suitable methods will be apparent to those skilled in the art by reference to, for example, EP-A-81280; DE-A-2504642; Czechoslovakian Patent No. 145,016; R.N. Sen and S.C. Sen Gupta, J. Ind. Chem. Soc., 1929, 6 , 273; H. Christopher and S. Smiles, J.C.S., 1911, 99 , 2050-1; and A. Mustafa and O.H. Hishmat, J.A.C.S., 1957, 79 , 2227-8. Reaction of the hydroxythioxanthone with the compound of formula III may conveniently be effected using an alkanol e.g. a C 1-3 alkanol, conveniently ethanol, or a polar aprotic solvent such as N,N-dimethylformamide as solvent, at temperatures in the range 60°C to reflux temperature. Reaction at reflux temperature is very convenient. Convenient bases include alkali metal (e.g. sodium or potassium) alkoxides and hydroxides. Further in accordance with the invention there is provided an aqueous photopolymerisable composition comprising an aqueous dispersion or solution of at least one polymerisable monomer and, as photoinitiator, a thioxanthone derivative of formula I. The invention also extends to the use of such a composition in the production of a stencil for screen printing, and to screens for screen printing coated with such a composition and dried. Suitable such aqueous photopolymerisable compositions comprise compositions as described in EP-A-81280, substituting a thioxanthone derivative of formula I for the photoinitiator thioxanthone thereof. Accordingly the disclosure of EP-A-81280 is incorporated herein by reference. The invention also comprises the use of a derivative of formula I as photoinitiator. Compounds of formula I possess the property of having useful absorption of actinic light above 330nm. In addition to their use a photoinitiators as described above in photopolymerisable compositions, they may also be employed in the manufacture of water based U.V. curable inks. The invention will be further understood from the following Examples, of which Examples 1 to 6 relate to the preparation of compounds of formula I, and Example 7 demonstrates efficacy of the compounds as photoinitiators. Example 1 2-Hydroxy-3-(9-oxo-9H-thioxanthen-4-yloxy)-N,N,N-trimethyl­ propanaminium chloride 4-Hydroxythioxanthone (5.7g) (mp 284.5 to 286.5°C, prepared by methods analogous to those of DE-A-2 504 642 and Czechoslovakian Patent No. 145,016) and glycidyl trimethylammonium chloride (5.44g, 70% assay) together with absolute ethanol (95ml) were stirred and brought to reflux whereupon the pH was adjusted to ca. 9 by the addition of a few drops of ethanolic sodium ethoxide solution (3% w/v). After four minutes refluxing a clear solution resulted and after a further three hours refluxing a crystalline solid appeared. After refluxing for a further three and one half hours, water (7mls) was added and the pH was adjusted to ca. 3 by the addition of a few drops of 2N isopropanolic hydrogen chloride. Hot filtration followed by ice bath cooling of the filtrate and filtration gave the crude title compound which was washed with isopropanol and dried in an air oven at 60°C. The dull pale green solid (8.58g; mp 267 to 268°C) was recrystallised from a mixture of ethanol (90mls) and water (7mls), washed with ethanol and dried, giving 2-hydroxy-3-(9-oxo-9H-thioxanthen-4-yloxy)-­ N,N,N-trimethyl-1-propanaminium chloride (7.03g; 87.8%) as light green crystals, mp 268.5-269.2°C. Analysis: Calc. for C₁₉H₂₂ClNO₃S: 0.25H₂O; C 59.36; H 5.90, N 3.64; Cl 9.22; S 8.34 Found: C 59.53; H 5.76; N 3.63; Cl 9.48; S 8.40. An aqueous solution of this compound has an absorption maximum at 391nm with an E (1%; 1cm) of 149. Example 2 2-Hydroxy-3-(1-methyl-9-oxo-9H-thioxanthen-4-yloxy)-N,N,N-­ trimethyl-1-propanaminium chloride 4-Hydroxy-1-methylthioxanthone (6.05g), (prepared by a method analogous to that of R.N. Sen and S.C. Sen Gupta, J. Ind. Chem. Soc., 1929, 6 , 2730) and glycidyl trimethylammonium chloride (5.44g; 70% assay) together with absolute alcohol (60mls) were stirred and brought to reflux whereupon the pH was adjusted to ca. 9 by the addition of a few drops of ethanolic sodium ethoxide solution (3% w/v). After two minutes refluxing a clear solution resulted and after a further two hours refluxing a crystalline solid appeared. This was redissolved by the addition of water (13mls) and refluxing continued for an additional four hours. After adjusting the pH to ca. 4 by the addition of a few drops of 2N isopropanolic hydrogen chloride hot filtration followed by ice bath cooling of the filtrate and filtration gave the crude title compound which was washed with ethanol and vacuum dried. The yellow solid (6.37g; mp 269 to 270.5°C) was recrystallised from ethanol containing a little water, washed with ethanol and dried in an air oven at 70°C giving 2-hydroxy-3-(1-methyl-9-oxo-9H-thioxanthen-4-yloxy)-N,N,N-­ trimethyl-1-propanaminium chloride (4.5g; 45.6%) as yellow crystals (mp 271 to 272°C). Analysis: Calc. for C₂₀H₂₄ClNO₃S: C 60.98; H 6.14; N 3.56; Cl 9.00; S 8.14 Found : C 60.70; H 6.15; N 3.48; Cl 9.01; S 8.13 An aqueous solution of this compound has an absorption maximum at 390nm with an E (1%; 1cm) of 134.5. Example 3 2-Hydroxy-3-(9-oxo-9H-thioxanthen-2-yloxy)-N,N,N-trimethyl-1-­ propanaminium chloride Sodium (1.36g) was reacted with absolute ethanol (120mls) and 2-hydroxy-thioxanthone (11.4g) (prepared by a method analogous to that of H. Christopher and S. Smiles, J.C.S., 1911, 99 , 2050-2051) was added. To the stirred refluxing mixture (3-chloro-2-hydroxypropyl)trimethyl ammonium chloride (10.7g) was added and the mixture refluxed overnight. After adjusting the pH to ca. 4 via the addition of a few drops of 2N isopropanolic hydrogen chloride solution, hot filtration followed by ice bath cooling of the filtrate and filtration gave the crude title compound which was washed with methanol and vacuum dried. The yellow solid (15.3g; mp 245 to 246.5°C) was recrystallised from a mixture of ethanol (150mls) and methanol (40mls), washed with ethanol and vacuum dried giving 2-hydroxy-3-(9-oxo-9H-thioxanthen-2-yloxy)-N,N,N-trimethyl-1-­ propanaminium chloride (14.5g; 72.8%) as yellow crystals, mp 245 to 246.5°C. Analysis: Calc. for C₁₉H₂₂ClNO₃S: 1H₂O: C 57.35; H 6.08; N 3.52; Cl 8.91; S 8.06. Found : C 57.73; H 6.01; N 3.40; Cl 8.77; S 8.02. An aqueous solution of this compound has an absorption maximum at 404nm with an E (1%, 1cm) of 135. Example 4 2-Hydroxy-3-(4-methyl-9-oxo-9H-thioxanthen-2-yloxy)-­ N,N,N-trimethyl-1-propanaminium chloride 2-Hydroxy-4-methylthioxanthone (6.5g) (prepared by a method analogous to that of A. Mustafa and O.H. Hishmat, J.A.C.S., 1957, 79 , 2227-8) and glycidyl trimethylammonium chloride (5.12g; 80% assay) together with absolute alcohol (70mls) were stirred and brought to reflux, whereupon the pH was adjusted to ca. 9 by the addition of a few drops of ethanolic sodium ethoxide solution (3%w/v). After two hours refluxing a clear solution resulted and after a further 18 hours reflux a crystalline solid appeared. Solution was effected by the addition of methanol (70mls) followed by water (2mls) and the mixture was refluxed overnight. After adjusting the pH to ca. 4 by the addition of a few drops of 2N isopropanolic hydrogen chloride, hot filtration followed by ice bath cooling of the filtrate and subsequent filtration gave the crude title compound which was washed twice with ethanol and vacuum dried. The yellow solid (9.6g; mp 247 to 253°C) was recrystallised from a mixture of ethanol (60mls), methanol (60mls) and water (6mls), washed with ethanol and vacuum dried giving 2-hydroxy-3-(4-methyl-9-oxo-9H-thioxanthen-2-yloxy)-N,N,N-­ trimethyl-1-propanaminium chloride (8.0g; 77.7%) as yellow crystals, mp 253.3 to 255°C. Analysis: Calc. C₂₀H₂₄ClNO₃S: 1H₂O: C 58.33; H 6.36; N 3.40; Cl 8.61; S 7.78. Found C 58.65; H 6.54; N 3.27; Cl 8.51; S 7.73. An aqueous solution of this compound has an absorption maximum at 404nm with an E (1%; 1cm) of 144. Example 5 2-Hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-N,N,N-­ trimethyl-1-propanaminium chloride 2-Hydroxy-3,4-dimethylthioxanthone (mp 309-310°C after recrystallisation from 70% v/v dimethylformamide/water; microanalysis:- Calcd. for C₁₅H₁₂O₂S: C 70.30; H 4.72; S 12.51. Found C 70.22; H 4.73; S 12.37) was prepared by adding 2,3-dimethylphenol (24.4g) during two hours to a stirred mixture of 2,2′-dithiobisbenzoic acid (15.3g) and conc. sulphuric acid (150mls) whilst the temperature was kept at 15 ± 5°C via ice bath cooling. The temperature was then raised to 65°C and maintained at 65 ± 5°C for one hours after which the mixture was allowed to cool for 16 hours, before being quenched into water (500mls) at 95 ± 5°C and then stirred for thirty minutes prior to filtration. The residue was washed twice with water and then slurried in water (250 mls) at 70°C and sodium bicarbonate (44g) was slowly added until the pH became 8. After boiling for ten minutes the mixture was filtered and the residue washed sequentially with water (X3) then isopropanol (X1). Drying in an air oven at 60°C gave crude 2-hydroxy-3,4-dimethyl­ thioxanthone (18.59g) as dull yellow crystalline solid, mp 298 to 302°C. 2-Hydroxy-3,4-dimethylthioxanthone (6.4g) was dissolved in dimethylformamide (40mls) at 104°C and after hot filtration, glycidyltrimethylammonium chloride (5.44g; 70% assay) as a slurry in dimethylformamide (10mls) was added to the filtrate at 100°C. The pH was adjusted to ca.9 by the addition of a few drops of ethanolic sodium hydroxide solution (3% w/v) and after twelve minutes stirring at 100°c a crystalline solid appeared. After a further six hours stirring at 100°C the pH was adjusted to ca.6 by the addition of a few drops of 2N isopropanolic hydrogen chloride and then the mixture was cooled in an ice bath. Filtration followed by sequential washing of the residue with dimethylformamide (X1) and isopropanol (X3) and then air drying at 70°C gave 2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-­ thioxanthen-2-yloxy)-N,N,N-trimethyl-1-propanaminium chloride (7.57g; 72.4%) as a pale yellow solid, mp 271.5 to 273.5°C. Analysis: Calc. for C₂₁H₂₆ClNO₃: 0.75 H₂O. C 59.82; H 6.58; N 3.36; Cl 8.41; S 7.61. Found C 59.90; H 6.55; N 3.23; Cl 8.53; S 7.69. An aqueous solution of this compound has an absorption maximum at 402nm with an E (1%; 1cm) of 115. Example 6 2-Hydroxy-3-(1,3,4-trimethyl-9-oxo-9H-thioxanthen-2- ylox­ y)-N,N,N-trimethyl-1-propanaminium chloride 2-Hydroxy-1,3,4-trimethylthioxanthone was prepared by adding 2,3,6-trimethylphenol (18.6g) during two hours to a stirred mixture of 2,2′-dithiobisbenzoic acid (30.6g) and conc. sulphuric acid (300mls) whilst the temperature was kept at 12.5 ± 2.5°C via ice bath cooling. After heating at 75 ± 5°C for one hour the mixture was cooled to 50°C prior to stirring into hot water (2 litres). Filtration, followed by water washing and drying in an air oven at 70°C gave the crude product which was recrystallised from a mixture of dimethylformamide (140mls) and water (60mls). Filtration followed by washing (H₂0) and drying at 70°C gave 2-hydroxy-1,3,4-trimethyl­ thioxanthone (38.4g; 71.1%) as khaki crystals, mp 201 to 204°C. 2-Hydroxy-1,3,4-trimethylthioxanthone (6.75g) and glycidyl-­ trimethylammonium chloride (6.44; 70% assay) together with absolute alcohol (60mls) were stirred and brought to reflux whereupon the pH was adjusted to ca. 9 via the addition of a few drops of ethanolic sodium ethoxide solution (3% w/v). After refluxing for one hour a clear solution resulted. The mixture was refluxed for 16 hours and methanol (30mls) plus water (2mls) were added to effect homogenity. After adjusting the pH to ca.3 by the addition of a few drops of 2N isopropanolic hydrogen chloride solution hot filtration followed by ice bath cooling of the filtrate and subsequent filtration gave the crude title compound which was washed thrice with ethanol and vacuum dried. The fawn solid (9.21g; mp 258.5 to 260.5°C) was recrystallised from a mixture of ethanol (75mls) and methanol (90mls), washed with ethanol and vacuum dried giving 2-hydroxy-3-(1,3,4-­ trimethyl-9-oxo-9H-thioxanthen-2-yloxy)-N,N,N-trimethyl­ propanaminium chloride (6.5g, 61.0%) as fawn crystals, mp 264.5 to 265.5°C. Analysis: Calc. for C₂₂H₂₈ClNO₃S: 0.25H₂O: C 61.94; H 6.74; N 3.28; Cl 8.30; S 7.52. Found: C 62.26; H 6.76; N 3.23; Cl 8.17; S 7.42. An aqueous solution of this compound has an absorption maximum at 390nm with an E (1%; 1cm) of 111. Example 7 Evaluation of the Efficiency of the Compounds given in Examples 1 to 6 as Water Soluble Photoinitiators Each of the compounds described in the previous examples was investigated as follows. A sample (0.33g) was dissolved in water (0.8mls) and N,N-dimethylaminoethanol (0.7mls) added. This solution was stirred into "Azocol S" (Trade Mark) emulsion (30g) and then N,N′-methylenebisacrylamide (1g) added. After stirring for five minutes the mixture was allowed to stand for 16 hours prior to coating into orange polyester screen (110 threads/cm) with two coats on the direct side and one on the reverse. The coated screen was air dried and then irradiated by light which had passed first through a piece of plate glass (5mm thick) and second through a film containing a positive image. The screen, film and glass in the form of a tight sandwich sat directly below two 375 watt photographic lamps whose bottom surfaces were positioned at a distance of 35 cms from the top surface of the piece of glass. After irradiating for a given time period the screens were gently rinsed with running cold water until an image began to form and then such treatment continued for a further two minutes. The screens were then air dried and the stencils examined. By using stepwise increasing exposure times the minimum time needed to give a good quality stencil was determined and these times are reproduced in the table below. For comparison a standard sensitized "Azocol S" emulsion was prepared according to the manufacturers instructions and after processing as described above its minimum exposure time was determined. Also the sodium salt of 2-(3-sulphopropoxy)- thioxanthone (Example 2 of EP-A-81280) was tested as above (comparative A) for comparison purposes. These results prove the efficiency of the compounds of the present invention as water soluble photoinitiators. "Azocol S" emulsion is a partially hydrolysed polyvinyl acetate emulsion in water. The recommended photosensitiser provided is a diazonium salt of paraphenylene diamine/formaldehyde cross-linked polymer.

Claims: 1. A thioxanthone derivative of general formula wherein one of R², R³ and R⁴ is a group of formula in which one of R⁵, R⁶ and R⁷ is an alkyl or a benzyl group, the others being alkyl groups and A⁻represents an anion, and R¹ and the others of R², R³, and R⁴ are independently selected from hydrogen atoms, alkyl groups and alkoxy groups. 2. A derivative according to Claim 1 wherein R¹ and said others of R², R³ and R⁴ are independently selected from hydrogen atoms and methyl groups. 3. A derivative according to Claim 1 or 2 wherein R⁵, R⁶ and R⁷ are all methyl groups. 4. A derivative according to Claim 1, 2 or 3 wherein A represents a chlorine or bromine atom. 5. A derivative according to any of Claims 1 to 4 wherein R² or R⁴ represents the group of formula II. 6. A process for the preparation of a thioxanthone derivative of formula I as defined in any of Claims 1 to 5 which comprises reacting a hydroxythioxanthone corresponding to formula I wherein in place of the group of formula II is a hydroxy group, with a compound of formula wherein R⁵, R⁶, R⁷ and A⁻ are all as defined for the group of formula II, R⁸ is a group -OH and R⁹ is a leaving group, or R⁸ and R⁹ together represent an oxygen atom, in the presence of a base. 7. A process according to Claim 6 wherein when R⁸ is a group -OH, R⁹ is a chlorine atom. 8. An aqueous photopolymerisable composition comprising an aqueous dispersion or solution of at least one polymerisable monomer and, as photoinitiator, a thioxanthone derivative of formula I according to any of Claims 1 to 5. 9. The use of a composition according to Claim 8 in the production of a stencil for screen printing. 10. A screen for screen printing coated with a composition according to Claim 8 and dried.

Inventors: GREEN, PETER NICHOLL, GREEN, WILLIAM ARTHUR, GWANE, GEORGE
Applicants: WARD BLENKINSOP AND COMPANY LIMITED
Application year: 1986
Main classification: C07D335
IPC codes: C08F2, G03F7, C07D335