Thioxanthone derivatives

Abstract: Thioxanthone derivatives of the general formula I wherein R¹ wherein R¹ represents a halogen atom or a C₁₋₁₀ alkoxy group, R² and R⁴ each represent a hydrogen atom, a halogen atom or a C₁-C₆ alkyl group, X represents oxygen or sulphur, and R³ represents a C₁-C₁₀ alkyl group optionally substituted by a hydroxy group which hydroxy group may be alkylated by an optionally substituted C₁₋₆ alkyl, C₁₋₆ alkenyl or a benzyl group or acylated by a C₁₋₈ alkanoyl or C₁₋₈ alkenoyl group, a C₁₋₁₀ alkenyl group, a C₃-C₆ cycloalkyl group, a C₁-C₈ alkanoyl group, a C₁₋₈ alkenoyl group, an optionally substituted phenyl group, an optionally substituted benzyl group or an optionally substituted benzoyl group. Process for the preparation of the above thioxanthones, their use as photoinitiators, photopolymerisable compositions containing them, the use of such compositions for the production of photocured surface coating and products on which these photocured compositions have been applied.

Description: The invention relates to thioxanthone derivatives, their preparation, their use as photoinitiators, photopolymerisable compositions containing them, the use of such compositions for the production of photocured surface coatings and products on which these photocured compositions have been applied. Prior art photoinitiators include substituted thioxanthones such as 2-chlorothioxanthone and, in particular, 2-isopropyl-thioxanthone. These compounds are widely used in the field of pigmented photocurable surface coatings and inks. Two important techniques in which these photoinitiators are used are lithographic printing and screen printing. The lithographic printing process involves the separation of a (photocurable) ink/water phase to form an image which is transferred via a system of rollers to the substrate, which is usually paper or card, followed by photocuring of the ink layer on the substrate. The process requires high line speeds to be efficient and thus a fast photocurable reaction is essential. The screen printing process involves passing (a photocurable) ink through a screen mesh which is negatively blocked out so that a positive image appears on the substrate under the mesh, followed by photocuring of the ink layer on the substrate. This process gives rise to heavier, thicker depositions of the ink on the substrate when compared with other printing processes, the heavier, thicker depositions resulting in brighter, stronger colours. The screen process is used for posters, banners, signs etc. and the process requires good depth hardness to be developed during photocuring to give the ink layer on the substrate good stability. We have now found that certain thioxanthone derivatives (some of which are novel compounds) give increased photocuring speed in the lithographic printing process and increased depth cure in the screen printing process. One aspect of the present invention comprises the use of a thioxanthone derivative as initiator in a photocurable composition, characterized in that the said derivative is of the general formula I:- wherein R 2 and R 4 each represent H, a halogen atom or a C 1-6 alkyl group; and R 3 is a C 1-10 alkyl group optionally substituted by a hydroxy group, which hydroxy group may be alkylated by a C 1-6 alkyl, alkenyl containing up to 6 carbon atoms or a benzyl group, or acylated by a alkanoyl containing up to 8 carbon atoms or alkenoyl group containing up to 8 carbon atoms; a alkenyl group containing up to 10 carbon atoms, a C 3-6 cycloakyl group, a alkanoyl group containing up to 8 carbon atoms, a alkenoyl group containing up to 8 carbon atoms, a phenyl group, a benzyl group, benzoyl group or a benzoylmethyl group. R 2 and R 4 each suitably represent a hydrogen, fluorine, chlorine or bromine atom or a C 1-4 alkyl group, preferably a hydrogen, chlorine or bromine atom or a methyl group, more preferably a hydrogen atom or a methyl group. R 2 especially represents a hydrogen atom or a methyl group and R 4 especially represents a hydrogen or chlorine atom or a methyl group, preferably a hydrogen atom. R 3 especially represents a C 1-6 alkyl or a alkenyl group containing up to 6 carbon atoms, a alkanol group containing up to 5 carbon atoms, a alkenoyl group containing up to 5 carbon atoms or a phenyl, benzyl or benzoyl group, preferably a propyl group. When R 3 represents an alkyl group, this group may be substituted by a hydroxy group, which hydroxy group may be alkylated by a C 1-4 alkyl or a benzyl group or acylated by a alkanoyl containing up to 5 carbon atoms or alkenoyl group containing up to 5 carbon atoms. According to another aspect of the invention there are provided, as novel chemical compounds, 1-chloro-4-benzoylmethoxy thioxanthone and compounds of general formula I the use of which is described herein, provided that R 3 is other than a methyl group, a methoxymethyl group or a benzyl group. Compounds for use in the present invention may be prepared conveniently by a two step process from readily available materials. In the first step a substituted hydroxy- thioxanthone is prepared by ring closure from 2, 2'- dithiobisbenzoic acid or 2-mercapto benzoic acid or a derivative thereof and a phenolic- compound. In the second step the ether is formed by reacting the hydroxy group with an alkylating or acylating agent. The alkylating compound is preferably an alkyl halide, more preferably an alkyl bromide or an alkyl iodide, and the acylating compound is preferably an acyl halide, more preferably an acyl chloride. The derivative of 2-mercapto benzoic acid is suitably the acid chloride. Another aspect of the invention relates to photocurable compositions comprising a solution or dispersion of at least one polymerisable monomer or oligomer and, as photoinitiator, a thioxanthone derivative characterized in that the said derivative is of the general formula I as described hereinbefore, or is 1-chloro-4-benzoylmethoxy thioxanthone. The composition usually will comprise 0.5 to 10 % by weight of photoinitiator. Suitable photopolymerisable prepolymers include acrylated epoxides, polyesters and polyurethanes. The prepolymers are usually combined with monomers for viscosity control. Suitable monomers are low weight acrylates, which may be mono-, di- or tri-functional. To increase the photosensitivity still further it is preferred to add a tertiary nitrogen compound to the composition, suitably in an amount between 0.5 and 8%wt, preferably between 1 and 4%wt. Suitable compounds are triethanolamine, methyl-diethanolamine, ethyldiethanolamine and esters of dimethylamino benzoic acid. These compounds behave as co-initiators or accelerators for the primary photoinitiator and increase the efficiency and speed of the crosslinking process. The invention also relates to the use of the above described compositions in the production of photocurable surface coatings, preferably in a lithographic printing process or a screen printing process in view of their increased photocure speeds and their increased depth hardness over the normally used thioxanthone derivatives. Certain thioxanthone derivatives stated to be useful as photoinitiators are described in European Application 0 430 881 A2; these derivative all contain a phenoxy group in the 1-position. United Kingdom Application GB 2 161 482A describes the use as photoinitiators of certain thioxanthone derivatives, and other thioxanthone derivatives stated to be useful as activators or sensitizers are described in United States Patent No. 4 101 558; none of the derivatives described in either of these publications contains a substituent in the 1-position or contains a an alkoxy substituent in the 4-position. The invention is illustrated by the following examples. Example 1 1-Chloro-4-Hydroxythioxanthone 2,2'-Dithiobisbenzoic acid (15.3 g; 0.05 mol) was stirred in concentrated sulphuric acid (150 ml) and 4-chlorophenol (38.5;0.3) added over two hours at 10-20°C. After stirring for a further hour the temperature was raised to 70-80° for 2 hours. The mixture was cooled and quenched on to water (500 ml) and the resulting slurry cooled. The solid was filtered off, washed well with cold water and dried at 60° to give the title compound (18.3g; 69.7%) of mp 260-265°C. Example 2 1-Chloro-4-n-propoxythioxanthone 1-Chloro-4-hydroxythioxanthone (7.8g; 0.03 mol) prepared as above and potassium carbonate (5.0g; 0.036 mol) was stirred and refluxed for 10 minutes in acetone (50 ml). n-Propyl bromide (5.5g; 0.045 mol) was added and the resulting mixture heated under reflux for 16 hours. The mixture was then cooled and quenched on to water (250 ml). The solid was filtered and washed with water the resulting damp solid recrystallised from ethanol (50 ml) to afford the title compound (6.9g; 75.5%) of mp 102-3°. Analysis C 16 H 13 ClO 2 S=304.777 Requires C 63.05%; H 4.30%; Cl 11.63%; S 10.52% Found C 63.14%; H 4.37%; Cl 11.66%; S 10.63% Example 3 1,3-Dichloro-4-n-propoxythioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 104-105°. Analysis C 16 H 12 Cl 2 O 2 S = 339.24 Requires C 56.64% H 3.57% Cl 20.90% S 9.45% Found C 56.52 H 3.61% Cl 20.38% S 9.47% Example 4 4-Benzyloxy-1-chlorothioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 139-40°. Analysis C 20 H 13 ClO 2 S Requires C 68.08% H 3.71% Cl 10.05% S 9.09% Found C 68.34% H 3.85% Cl 10.13% S 9.17 Example 5 4-Isoamyloxy-1-chlorothioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 96-97°. Analysis C 18 H 17 ClO 2 S=332.84 Requires C 64.95% H 5.14% Cl 10.65% S 9.63% Found C 64.80% H 5.15% Cl 10.67% S 9.81% Example 6 1-Chloro-3-methyl-4-n-propoxythioxanthone Similarly prepared in Examples 1 and 2, the title compound was prepared of mp 101-103°. Analysis C 17 H 15 ClO 2 S=318.81 Requires C 64.04% H 4.74% Cl 11.12% S 10.06% Found C 63.78% H 4.79% Cl 11.19% S 10.06% Example 7 4-Allyloxy-1-chlorothioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 135-136° Analysis C 16 H 11 ClO 2 S=302.77 Requires C 63.47% H 3.66% Found C 63.7% H 3.71% Example 8 1-Chloro-4-octyloxythioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 85-87.5°. Analysis c 21 H 23 ClO 2 S=374.92 Requires C 67.27% H 6.18% Found C 67.08% H 6.14% Example 9 1-Chloro-2-methyl-4-n-propoxythioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 116.5-117.5°. Analysis C 17 H 15 ClO 2 S=318.81 Requires C 64.04% H 4.74% Cl 11.12% Found C 64.23% H 4.79% Example 10 1-Chloro-4-isopropoxythioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 91-93°. Analysis C 16 H 13 ClO 2 S=304.78 Requires C 63.05% H 4.30% Cl 11.63% S 10.52% Found C 63.15% H 4.32% Example 11 4-(Benzoylmethoxy)-1-chlorothioxanthone Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 174-176°. Analysis C 21 H 13 ClO 3 S=380.84 Requires C 66.22% H 3.44% Cl 9.31% S 8.42% Found C 66.39% H 3.54% Cl 9.29% S 8.27% Example 12 2-(1-Chloro-4-thioxanthenyloxy)ethylpropenoate Similarly prepared as in Examples 1 and 2, the title compound was prepared of mp 90-91°. Analysis C 18 H 13 ClO 4 S=360.81 Requires C 59.92% H 3.63% Cl 9.83% Found C 59.91% H 3.70% Example 13 4-Acetyloxy-1-chlorothioxanthone 1-Chloro-4-hydroxythioxanthone as prepared in Example 1 (13.1g) and pyridine (10g) was stirred in acetone (50ml) at 50°. Acetyl chloride (4.3g) was added over 0.25h and the mixture stirred and heated under reflux for a further hour. The mixture was cooled to 20° and quenched with water (150ml). The resulting solution was extracted with chloroform (50ml), separated and the organic phase washed twice with water. The organic phase was evaporated to dryness under reduced pressure and the residue crystallised from methanol with a carbon treatment to afford the title compound, 4.1g (27.0%)' as a pale yellow solid of mp 141-42°. Analysis C 15 H 9 ClO 3 S=304.745 Requires C 59.11% H 2.98% Cl 11.63% S 10.52% Found C 59.10% H 3.06% Cl 11.59% S 10.41% Example 14 4-Benzoyloxy-1-chlorothioxanthone 1-Chloro-4-hydroxythioxanthone as prepared in Example 1 (6.6g), potassium carbonate (4.1g), 2-butanone (60ml) and water (6ml) were heated under reflux. Benzoyl chloride (5.3g) was added over 20 minutes and the mixture heated under reflux for 3 hours. A solid precipitated from solution during the heating period. The mixture was cooled, quenched with water (120ml), the solid filtered and recrystallised from 2-butanone with a charcoal treatment. The title compound (6.8g; 74.0%) was obtained of mp 177-78°. Analysis C 20 H 11 ClO 3 S=366.81 Requires C 65.48% H 3.02% Cl 9.67% S 8.74% Found C 65.46% H 3.13% Cl 9.77% S 8.70% Example 15 1-Chloro-4-thioxanthenyl propenoate Similarly prepared as in Example 13, the title compound was obtained in 48.1% yield as a pale yellow solid of mp 164-165°. Analysis C 16 H 9 ClO 3 S=316.76 Requires C 60.67% H 2.86% Cl 11.19% S 10.12% Found C 60.57% H 2.94% Cl 11.42% S 10.01% Example 16 Comparison of the efficiency of the compounds synthesised with Isopropylthioxanthone A method of assessing photoinitiator efficiency involves measuring the cure speed of a coating by counting the number of passes through a Primarc Minicure Unit to give a coating hardness measured by 2H pencil scratch. The minimum number of passes required to produce a scratch free surface is a measure of the cure speed; the lower the number of passes the higher the cure speed. A blue urethane based screen ink was loaded with 2 molar % photoinitiator and 3% methyldiethanolamine and a 12µ coating of ink made on glass slides by a No. 2 Bar Coatere. A uv Minicure Unit composed of a single 80 Watts cm -1 lamp was run with a belt speed of 60 rpm and the coated glass slide passed through the machine a number of times until a 2H pencil no longer marked the surface. The number of passes was recorded as a measure of the cure speed. Example Number Long Wave Abs (nm) E(1cm; 1%) Number of Passes 2H 2-Isopropylthioxanthone (for comparison) 383 207 13 2 387 175 6 3 383 161 7 4 390 167 5 5 390 173 6 6 383 175 6 7 390 186 6 8 392 161 6 9 395 175 5 10 392 177 5 11 390 170 10 12 390 153 6 Example 17 Comparison of the efficiency of the compound of Example 2 with a known photoinitiator A blue litho ink was loaded with 3% benzophenone, 3% of ethyl p-dimethylaminobenzoate as amine synergist and 3% photoinitiator. The ink was coated at 1.5 thickness on to white card using a PIP Proofer and then passed through a Minicure UV unit running at 60 rpm for the minimum number of passes to give a hard tack free surface as defined by the Thumb Twist test. The number of passes was then related to the belt to give a cure speed. Comparison was made with isopropylthioxanthone which is widely used in litho inks. Cure Speed 2-Isopropyithioxanthone 18.6 ms -1 1-Chloro-4-propoxythioxanthone 23.2 ms -1 In screen inks the hardness of the coating is measured to give some measure to the depth of cure achieved for thick coatings. Hardness is measured by a Koenig Pendulum Hardness machine where a steel ball attacked to a pendulum is placed on the surface coating. The oscillation of the pendulum is measured to follow the damping effect of the surface coating on the rocking steel ball. A hard surface will have little damping effect compared with a soft surface and will lead to higher oscillation figures. An epoxy acrylate resin was loaded with 2% benzophenone, 3% methyldiethanolamine as co-initiator and 2% initiator. This was coated at 50 thickness on to glass slides and cured in the Minicure unit at 2.5 ms -1 . The coatings were allowed to stand for 24 hours and the Pendulum Hardness measured. An average of 5 readings taken on different parts of the surface was noted. Comparison was made with 2-isopropyl-thioxanthone as photoinitiator. Pendulum Hardness 2-Isopropylthioxanthone 93s 1-Chloro-4-propoxythioxanthone 115s The above figures represent a clear advance over known photoinitiators and illustrate the invention.

Claims: The use of a thioxanthone derivative as photoinitiator in a photocurable composition, characterized in that the said derivative is of the general formula I: wherein R 2 and R 4 each represent H, a halogen atom or a C 1-6 alkyl group; and R 3 is a C 1-10 alkyl group optionally substituted by a hydroxy group, which hydroxy group may be alkylated by a C 1-6 alkyl, alkenyl containing up to 6 carbon atoms or a benzyl group, or acylated by a alkanoyl containing up to 8 carbon atoms or alkenoyl group containing up to 8 carbon atoms; a alkenyl group containing up to 10 carbon atoms, a C 3-6 cycloakyl group, a alkanoyl group containing up to 8 carbon atoms, a alkenoyl group containing up to 8 carbon atoms, a phenyl group, a benzyl group, a benzoyl group or characterized in that the said derivative is 1-chloro-4-benzoylmethoxythioxanthone A use according to Claim 1 characterized in that each of R 2 and R 4 is H, Cl, Br or a C 1-4 alkyl group. A use according to Claim 2 characterized in that R 2 is H or a methyl group and R 4 is H, Cl or a methyl group. A use according to any of the preceding claims characterized in that R 3 is a C 1-6 alkyl group. A use according to Claim 4 characterized in that R 3 is a propyl group. A use according to Claim 5 characterized in that the derivative is 1-chloro-4-n-propoxythioxanthone. A photocurable composition comprising a solution or dispersion of at least one polymerisable monomer or oligomer and, as photoinitiator, a derivative the use of which is claimed in any of the preceding claims. A thioxanthone derivative, the use of which is claimed in any of Claims 1 to 6, provided that R 3 is other than a methyl group, a methoxymethyl group or a benzyl group. A derivative according to Claim 8 which is 1-chloro-4-n-propoxythioxanthone.

Inventors: GREEN WILLIAM ARTHUR, TIMMS ALLAN WILLIAM, GREEN, WILLIAM ARTHUR, TIMMS, ALLAN WILLIAM
Applicants: LAMBSON FINE CHEMICALS LIMITED
Application year: 1992
Main classification: C07D335
IPC codes: G03F7, C08F2, C08K5, C07D335