WO2009060235A1

WO2009060235A1 - Multi-functional thioxanthone photoinitiators

Info

Publication number: WO2009060235A1
Application number: PCT/GB2008/051033
Authority: WO
Inventors: David George Anderson; Kevin Harper; Alan Thomas Rose
Original assignee: Lambson Limited
Priority date: 2007-11-09
Filing date: 2008-11-06
Publication date: 2009-05-14
Also published as: DK2205582T3; GB0722067D0; GB2454579B; GB0820350D0; EP2205582A1; GB2454579A; EP2205582B1

Abstract

A photoinitiator compound and a method for preparing same is described. The photoinitiator compound is of formula O X A1 Q Y S O O x I wherein: X represents a halogen atom; Y represents an alkylyl or alkenylyl moiety; A1 represents a group which includes a -O- moiety; Q represents a linking atom or group; and x represents a number greater than 1, wherein moieties bonded to linking atom or group Q may be the same or different to one another. The photoinitiator may be used in combination with a polymerisable material and an amine synergist.

Description

PHOTOINITIATORS

This invention relates to photoinitiators and provides compounds per se, a method of making such compounds, a product and composition comprising such compounds and a method of forming a cured coating on a substrate.

Photoinitiators used in energy curable surface coating formulations are well known and need to possess a range of properties. For example, they should have good curing activity, be readily soluble in resin formulations and not significantly affect the viscosity or other properties of the formulations and they should be relatively easy to make. One additional important requirement is that there should be a low, or minimal, tendency of such photoinitiators to migrate out of cured coatings since this may taint products in contact with coatings and be a health hazard.

It is an object of the invention to address problems associated with photoinitiators.

The invention is based on the discovery of compounds which can be readily manufactured, are generally in a form (e.g. solid) which can easily be incorporated into formulations comprising curable resins, with little viscosity change of such resin formulations and, importantly, such photoinitiators have a very low tendency to migrate from cured films.

According to a first aspect of the invention, there is provided a photoinitiator compound of formula

wherein :

X represents a halogen atom;

Y represents an alkylyl or alkenylyl moiety;

A¹ represents a group which includes a -0- moiety;

Q represents a linking atom or group; and x represents a number greater than 1, wherein moieties bonded to linking atom or group Q may be the same or different to one another.

Unless otherwise stated herein any alkyl, alkenyl, alkylyl or alkenylyl moiety preferably includes carbon and hydrogen atoms only.

X may be selected from fluorine, chlorine, bromine and iodine atoms. X preferably represents a chlorine atom.

Y may represent a Ci_i₂, preferably a Ci_i₀, more preferably a Ci_₆ alkyl or alkenyl moiety.

Preferably, Y represents a moiety -(CH)_nR - wherein n is a number from 1 to 6 and R³ represents a hydrogen atom or an unsubstituted methyl or ethyl group. n is preferably in the range 1 to 3. Preferably, Y represents a -CH₂- moiety. Preferably, each y represents a -CH₂- moiety.

A¹ preferably includes an -0- moiety which is bonded to the carbonyl moiety adjacent moiety A¹ in the compound of formula I .

A¹ preferably includes a repeat unit; it may be an oligomeric or polymeric moiety.

A¹ preferably includes a repeat unit which includes an -O-CHR - moiety wherein R represents a hydrogen atom or a methyl or ethyl group.

Preferably, A¹ represents a group of formula

-[O (CHR²CHR¹) _a]_y-, -[O(CH₂)bCO]_y-, or - [0 (CH₂) _bC0] _(y_i)-

[0 (CHR²CHR¹) _a] -, where one of R¹ and R² represents a hydrogen atom and the other represents a hydrogen atom, a methyl group or an ethyl group, a is a number from 1 to 6, b is a number from 1 to 6 and y is a number from 1 to 10.

Preferably a is a number from 1 to 2.

Preferably b is a number from 4 to 5.

Preferably, A¹ represents a group of formula - [0 (CHR²CHR¹) _a] _y- where a is an integer from 1 to 2, and y is as defined above, preferably a number from 3 to 10. A¹ is more preferably a group of formula - [OCH₂CH₂] _y-, -[OCH₂CH₂CH₂CH₂J_y- or - [OCH (CH₃) CH₂] _y-, where y is as defined above, preferably a number from 3 to 10, or a group of formula - [0 (CH₂) _bC0] _y- or - [0 (CH₂) _bC0] _(y-i)- [0 (CHR²CHR¹) _a] -, where b is a number from 4 to 5 and y is as defined above, preferably a number from 3 to 10. Still more preferably, y is a number from 3 to 6.

Preferably, A¹ includes a moiety -0 [CH (CH₃) CH₂O] _z- wherein z is an integer.

Preferably, the compound of formula I includes on average 5 to 30, more preferably 5 to 20, especially 7 to 11 moieties of formula -0 [CH (CH₃) CH₂) ] _y- .

Q preferably includes a -0- moiety bonded to moiety A¹ in the compound of formula I. A -0-CH₂- moiety is preferably bonded to moiety A¹ in the compound of formula I via its -0- atom.

Preferably, when x is greater than 1, preferably x moieties bonded to Q by moiety A¹ are bonded to Q by -0- moieties which form part of Q. Each -0- moiety of Q may be directly bonded to a -CH₂ moiety. Each -0-CH₂- moiety of Q may be bonded to the same carbon atom.

Preferably, Q is a residue of a polyhydroxy compound, suitably having 2 to 6, preferably 3 to 5, especially 4 hydroxy groups .

Q may be a residue of ethylene glycol, propylene glycol, butylene glycol, glycerol, trimethylolpropane, di- trimethylolpropane, pentaerythritol or di-pentaerythritol .

Preferably, Q is a reside of a pentaerithritol .

The moiety Q-(A¹J_x- of the compound of formula I preferably has a molecular weight (g/mol) of less than 2000, preferably less than 1500, more preferably less than 1000, especially less than 800. Said molecular weight may be at least 200, preferably at least 400, more preferably at least 600.

Preferably, x is no greater than the number of hydroxy groups in the polyhydroxy compound of which Q is a residue .

Preferably, x is no greater than 4.

Said moiety -YCO-A¹- in the compound of formula I is preferably bonded to the thioxanthone moiety at a para position relative to the position of X.

Preferably, a plurality of moieties (preferably each moiety) bonded to the linking atom or group Q is the same.

Suitably, said compound of formula I has a melting point of at least 5O⁰C, preferably at least 6O⁰C, more preferably at least 7O⁰C. The melting point may be less than 200⁰C, or less than 100⁰C.

According to a second aspect of the invention, there is provided a method of making a photoinitiator compound of formula I of the first aspect, the method comprising:

(i) selecting an isolated salt of a compound comprising an anion of formula

(ii) treating said isolated salt with one or more reagents to produce said compound of formula I.

The method preferably includes a step of preparing a compound of formula IV and isolating a salt thereof which is preferably substantially pure.

The method preferably includes a step of forming an ester of the compound of formula IV. For example, the ester may be of formula

wherein R i s an alkyl group , especial ly an ethyl group .

X and Y in said moieties of formulae IV and V may be as described in accordance with the first aspect.

The method may involve treatment with a polyhydroxy compound which is arranged, after reaction, to define said moiety -A¹Q. A preferred polyhydroxy compound is a propoxylated pentaerithritol . The method may include comminuting a solid material produced in the process.

According to a third aspect of the invention, there is provided a product comprising or consisting of a compound according to the first aspect in powderous form.

According to a fourth aspect of the invention, there is provide a formulation comprising a compound according to the first aspect and at least one other material selected from: a polymerisable material; another photoinitiator, which is different to said compound of formula I; an amine synergist.

Said polymerisable material may comprise a monomer which is ethylenically unsaturated, such as an acrylate or a cationic monomer (e.g. epoxy or vinyl ether) .

The formulation of the fourth aspect may be in liquid form. It preferably includes a said polymerisable material. The compound of formula I may be dissolved or dispersed (preferably dissolved) in the polymerisable material.

According to a fifth aspect of the invention, there is provided a method of forming a cured coating on a substrate, the method comprising contacting the substrate with an uncured formulation arranged to provide said coating, wherein said uncured formulation includes a compound of formula I according to the first aspect and exposing the uncured formulation to radiation, for example UV radiation.

The uncured formulation may be as described according to the fourth aspect .

Said coating may comprise an ink.

Said coating may have an thickness in the range 1 to 500μm, preferably 1 to 200μm.

Any feature of any aspect of any invention or embodiment described herein may be combined with any feature of any aspect of any other invention or embodiment described herein mutatis mutandis .

The invention will now be described, by way of example, with reference to the following figures, wherein:

Figure 1 shows the UV spectrum of a photoinitiator of a preferred embodiment of the invention;

Figure 2 shows the UV spectra of films extracted with acetonitrile;

Figure 3 details certain materials referred to herein.

The following materials are referred to hereinafter:

Surfac N090 - a nonyl phenol condensate with nominally 9 moles of ethylene oxide; obtained from Surfachem Group, UK. Perstorp PS4360 - a propoxylated pentaerithritol having a degree of propoxylation of about 8.5

Vertec TIPT - a titanium alkoxide, tetra-iso-propyl titanate obtained from Johnson Matthey, UK. Chemcarb GCX - a wood-based carbon obtained from CPL Carbon Link, UK.

Speedcure 7005 - see Figure 3 (a) Speedcure 7040 - see Figure 3 (b)

In the description which follows examples 1 and 2 relate to preparation of a photoinitiator of a preferred embodiment of the invention; and thereafter the results of relevant tests undertaken on materials are described.

Example 1 - Preparation of polymeric thioxanthone photoinitiator (referred to herein as "thioxanthone photoinitiator A") - first method.

Step 1 - Preparation of l-chloro-4-hydroxythioxanthone

To a 250ml round-bottomed flask was charged 98% sulphuric acid (275.4g) and to this water (6.Ig) was added to reduce the acid strength to 96%. To this was added dithiobisbenzoic acid (30.6g; O.lOg mole) and the mixture stirred for 30 minutes. p-Chlorophenol (58.1g; 0.45g mole) was then added gradually over 1.5 hours, the temperature being maintained at 40-45⁰C. A 60-minute workout at this temperature yielded a mobile orange/red slurry. This was warmed slowly to 80-85⁰C and held for 2 hours, steady sulphur dioxide evolution occurring during this period. The mixture was finally warmed to 100⁰C and held for 30 minutes before then cooling the resultant deep red solution. This solution was poured slowly into water (269ml) at 5O⁰C, allowing the temperature to rise to 88⁰C. After self-cooling to 5O⁰C the mixture was filtered and the filter cake washed three times with water (100ml) . A buff coloured solid was obtained that was then oven-dried to constant weight.

Weight obtained = 40.9g (77.9% yield from DTBBA)

Step 2 - Preparation of l-chloro-4-hydroxythioxanthone, sodium salt

l-chloro-4-hydroxythioxanthone (52.5g; 0.20 g mole) prepared in step 1 was slurried in water (328ml) and Surfac NO90 (0.3g) added. This was heated to 80-85⁰C and then basified by adding 10% caustic soda solution (9Og; 0.225 g mole) over 30 minutes. The mixture was cooled to 5⁰C over 1.5 hours and held at this temperature for 30 minutes. The resulting slurry was filtered, the cake washed with cold water (80ml) and the sodium salt then dried to yield 36.4g.

Using the combined mother and wash liquors a second portion of l-chloro-4-hydroxythioxanthone (52.5g) was converted to sodium salt as above, but without the addition of extra surfactant. After drying 53.4g was obtained.

Combined yield from these two cycles was 89.8g (78.9% from l-chloro-4-hydroxythioxanthone) Step 3 - Preparation of l-chloro-4- carboxymethyoxythioxanthone, ethyl ester

l-chloro-4-hydroxythioxanthone, sodium salt (142.2g; 0.50Og mole) of step 2, methylethylketone (1000ml), dimethylsulphoxide (1Og) and ethylbromoacetate (96.Og; 0.575 g mole) were charged to a 2-litre flask and heated to reflux (80⁰C) . The initial red/orange colour of the slurry changed to a beige/brown as the reaction proceeded. After 3 hours at reflux extra methylethylketone (380ml) was added. The course of the reaction was followed by TLC. This involved placing a few drops of the mixture in a 7ml vial, adding a small volume of water and IN HCl to acidify. Sufficient acetone was then added to give a clear solution. The solution was spotted on a silica gel plate containing fluorescent indicator (254nm) and run in 1:1 acetone/petroleum ether. Viewed under a fluorescent lamp the product was seen to have an RF of approx. 0.51 and the chlorohydroxythioxanthone starting material an RF of 0.47. After 4.5 hours at reflux the reaction was complete.

The mixture was cooled to ambient temperature and sodium methoxide powder (4.04g; 0.075 g mole) added to react with any remaining ethylbromoacetate. After stirring for 1 hour the mixture was warmed to 70°C and held for a further 30 minutes. It was then filtered through a pre-warmed Buchner funnel to remove the by-product sodium bromide, the cake being washed with hot methylethylketone (30ml) . Approximately 52g of insolubles were collected.

The filtrate was then cooled over about 2 hours to 0-5°C and held for a further 30 minutes before filtering through a Buchner funnel to collect the precipitated product. The cake was given a small displacement wash with cold methylethylketone (30ml) and the product then dried at 70°C. The dried material was a yellowish khaki colour. First crop yield was 127.2g (73.0% from CHTX Na salt)

The volume of filtrate from product isolation was 1300ml. Using a rotary evaporator solvent was removed to reduce the volume to 300ml and this was then cooled to 0-5°C and held for 30 minutes. Filtration and drying of the cake collected gave a second crop of product, identical in appearance to the first crop.

Second crop yield was 16.2g (9.3% from CHTX Na salt)

On TLC in 1:1 acetone/petroleum ether both crops of material exhibit only one spot.

Overall yield was 143.4g (82.3% from CHTX Na salt)

Step 4 - Preparation of polymeric thioxathone photoinitiator (thioxanthone photoinitiator A)

The ethyl ester of Step 2 was transesterified with Perstorp PS4360 as represented and described below. Perstorp Polyol PS4360

Thioxanthone photoinitiator A

A 1-litre flask was charged with PS4360 (94.4g; 0.15 mole), CHTX-ethyl ester (198.6g; 0.57 moles) and toluene (15OmIs) . Any entrained water was azeotroped out of the system using a Dean and Stark apparatus. After cooling, the Dean and Stark apparatus was replaced by a 150mm vacuum jacketed Vigreux column, and still head, and the system was thoroughly purged with nitrogen. Vertec TIPT (0.5g) transesterification catalyst was added, and the reaction heated to steady reflux. The still base temperature was about 12O⁰C initially, the still head temperature was 79⁰C, indicating that ethanol was forming as expected. The ethanol was taken off slowly, such that a steady reflux was maintained, and the still head remained at 78°-79°C. Taking ethanol off too quickly causes the still head to rise as toluene is leached from the system. The reaction was monitored by TLC (1:2 acetone : petroleum ether 100-120) . During the last 5 hours of reaction, the head temperature intermittently reached HO⁰C. When this occurred, a further 0.5g TIPT was added, and the column was intermittently placed on total reflux until remaining ethanol formation brought the head temperature back down to 78⁰C, then take-off was resumed (i.e. intermittent take-off was used at the end of the reaction to ensure that most of the toluene stayed in the system) . However, after 18 Hrs, the rate of take-off was increased so that some of the toluene was removed, allowing the base temperature to increase and drive the reaction to completion. After 24 Hrs, 95.8g distillate had been removed and the final base temperature was 14O⁰C. A total reaction time of 24Hrs gave complete reaction as monitored by TLC.

After cooling, acetone (50OmIs) was added, together with sodium dihydrogen phosphate (3g in 3g water) , and the reaction mixture was refluxed for 30 minutes. This facilitated deactivation/removal of TITP catalyst. After cooling to 4O⁰C, anhydrous magnesium sulphate (6g) and Chemcarb GCX (2g) was added, and reflux continued for 1.5 Hrs, to dry and decolourise the solution. After cooling to 4O⁰C, the inorganics were filtered via a celite bed, and washed with acetone (lOOmls) . The acetone was removed initially at atmospheric pressure and then at up 95⁰C and 15mm Hg for 2 Hrs. The time taken was that required to ensure no solvent remained in the product melt, as signified by cessation of its bubbling due to solvent evaporation .

The yield was 264.8g of pale orange viscous oil (96.0% theory) .

Finally the product melt was poured onto a glass Petri dish, and allowed to set into a pale orange glass. This was removed, and powdered readily using a mortar and pestle, to give a pale yellow free-flowing solid.

The product had a melting point of 78-8O⁰C and an overall yield, based on the amount of l-chloro-4-hydroxthioanthone selected in step 1, of 64%.

Example 2 - Preparation of polymeric thioxanthone photoinitiator A - second method.

This method is based on the method described in US6960668B and is used to produce the same photoinitiator as that prepared in Example 1 and represented as the product of the reaction of Example 1, step 4.

Step 1 - Preparation of Thioxanthone 4-chlorophenoxyacetic acid was condensed with dithiobisbenzoic acid to yield the carboxymethoxy intermediate as the acid which was precipitated in water and isolated in a first filtration. This was then purified, in a second filtration, from a hot mixed solvent system before being used in the next stage of the process. The first filtration was reasonably quick but after the reslurry in the mixed solvent system it was extremely slow and a very low solids content cake was obtained. Consequently, a high proportion of materials in the liquor is dried back onto the filter cake. The yield at this point was 99.5%.

Step 2 - Preparation of polymeric thioxanthone photoinitiator A

The carboxylic acid intermediate prepared in Step 1 was esterified with the required polyol in toluene, using PTSA as catalyst. A solution of the product was obtained which was washed with dilute base and then water before the product was removed under vacuum to leave the product as a glassy solid. A yield of 61% was obtained. The acid intermediate is of low solubility in toluene (unlike the ester of Example 1 that dissolves readily on warming) and consequently an extended reaction time was needed to complete the esterification - in the region of 45 hours. Due to the presence of interfacial material the separations from the aqueous washes are slow.

Although the product was obtained as a glass that breaks up reasonably easily it tends to cake and would thus present handling problems. This indicates a slightly lower purity than from the Example 1 route.

Overall yield was 60.7%, the products melting point could not be determined as it began to soften at ~ 25⁰C.

Comparison of Examples 1 and 2

The Example 1 method appears to produce an advantageous product. It is preferred that the product is of sufficient quality that it may be ground, subsequently remain as a powder and show no signs of caking that would make handling problematic. This is the case for the Example 1 material but not for the example 2 material.

The good quality of the final product obtained is a culmination of the previous steps wherein the presence of impurities carrying forward is minimised by isolation of the intermediates. Although the isolations incur extra processing time and some yield loss they ensure that the final product is of the required quality.

Assessment of materials

Assessment 1 - Viscosity

The viscosity of a formulation comprising a typical resin mixture comprising Bisphenol A EO-2 (70%) and TPGDA (30%) (an acrylic acid ester) was assessed using a Brookified viscometer, under conditions of Spindle 2, 60rpm, 25⁰C. The viscosity of the same resin mixture containing, additionally, polymeric thioxanthone photoinitiator A (2%) was assessed in the same way. Results are tabulated below.

Formulation Viscosity (cp) resin mixture 190 resin mixture + polymeric 200 thioxanthone photoinitiator A

It will be noted that, advantageously, addition of the polymeric thioxanthone photoinitiator A affects the viscosity of the formulation to only a relatively small extent .

Assessment 2 - UV spectrum

The UV spectrum of the thioxanthone photoinitiator A was assess and is shown in figure 1. From the spectrum it is clear that thioxanthone photoinitiator A absorbs in appropriate regions so that it may be irradiated with an appropriate UV irradiation source.

Assessment 3 - cure testing

A formulation comprising polymeric photoinitiators ( including thioxanthone photoinitiator A) was compared to a formulation comprising equivalent monomeric photoinitiators in cure tests.

Formulations were made up in an epoxy acrylate resin comprising of 70 EO-2 bisphenol A diacrylate, 30% TPGDA and as near an equivalent molar amount of the various types of photoinitiators subjected to testing. The testing programme was carried out using a Dymac Hi5 belt curing machine equipped with a 90Ow mercury diffusion lamp. The test samples were cured in air for the number of passes displayed in the table below. Degree of cure was assessed by simple touch test for initial cure and thumb twist test for through cure. Belt speed in all cases was 16.5 m/min.

Results of the tests were as follows:

It will be appreciated from the results that a speed of cure of the polymeric photoinitiators comparable with equivalent monomeric photoinitiators can be achieved.

Assessment 4 - Migration test 1

A film comprising formulation no. 2 of Assessment 3 was cured by doubling the number of passes taken to achieve a pass on the thumb twist test. A migration test was then carried out by subjecting the film to a solution of 90% ethanol/10% water for 2 hours at 6O⁰C. The leachate was analysed by HPLC to determine the amount of migration of the photoinitiators . Results are tabulated below wherein migration is expressed as % of each material present in the cured formulation which has migrated.

Photoinitiator component Migration % Speedcure 7040 7.5

Speedcure 7005 6.8

Thioxanthone photoinitiator A 0

Assessment 5 - Migration test 2

Following a process similar to that described in Assessment 4, cured Aμm films of formulations 1 and 2 of Assessment 3 were extracted with acetonitrile . UV spectra for the acetonitrile extracts are provided in figure 2. It will be noted that, for the formulation 2, there is no absorbance at 380nm (compare figure 1) indicating that none of the thioxanthone photoinitiator A has been extracted into the acetonitrile.

Conclusion

It will be noted that thioxanthone photoinitiator A is readily handled and incorporated into resin formulation due to it being solid and, when introduced, it has a minimal effect on viscosity. The activity of the thioxanthone photoinitiator A is comparable to that of equivalent monomeric photoinitiators. Particularly advantageously, it is found that thioxanthone photoinitiator A exhibits negligible migration. The invention is not restricted to the details of the foregoing embodiment (s) . The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A photoinitiator compound of formula

wherein :

X represents a halogen atom; Y represents an alkylyl or alkenylyl moiety;

A¹ represents a group which includes a -0- moiety;

2. A compound according to claim 1, wherein X represents a chlorine atom.

3. A compound according to claim 1 or claim 2, wherein Y represents a moiety -(CH)_nR - wherein n is a number from 1 to 6 and R³ represents a hydrogen atom or an unsubstituted methyl or ethyl group.

4. A compound according to any preceding claim, wherein Y represents a -CH₂- moiety.

5. A compound according to any preceding claim, wherein A¹ represents a group of formula - [0 (CHR²CHR¹) _a] _y-, -[O(CH₂)bCO]_y-_f or -[O(CH₂)_bCO] _(y-_D - [0 (CHR²CHR¹) _a] -, where one of R¹ and R² represents a hydrogen atom and the other represents a hydrogen atom, a methyl group or an ethyl group, a is a number from 1 to 6, b is a number from 1 to 6 and y is a number from 1 to 10.

6. A compound according to any preceding claim, wherein A¹ is a group of formula -[OCH₂CH₂Jy-,

-[OCH₂CH₂CH₂CH₂J_y- or -[OCH(CH₃)CH₂J_y-, where y is a number from 1 to 10, or a group of formula - [0 (CH₂) _bC0] _y- or - [0 (CH₂) _bC0] (_y-i)- [0(CHR²CHR¹) _a] -, where b is a number from 4 to 5 and y is a number from 1 to 10.

7. A compound according to any preceding claim, wherein A¹ includes a moiety -[OCH(CH₃)CH₂J_y-, wherein y is a number from 1 to 10.

8. A compound according to any preceding claim, wherein Q includes a -0- moiety bonded to moiety A¹ in the compound of formula I .

9. A compound according to any preceding claim, wherein Q is a residue of a polyhydroxy compound.

10. A compound according to any preceding claim, wherein Q is a residue of ethylene glycol, propylene glycol, butylene glycol, glycerol, trimethylolpropane, di- trimethylolpropane, pentaerythritol or di-pentaerythritol .

11. A compound according to any preceding claim, wherein the moiety Q-(A¹J_x- of the compound of formula I has a molecular weight (g/mol) of less than 2000.

12. A compound according to any preceding claim, wherein said moiety -YCO-A¹- in the compound of formula I is bonded to the thioxanthone moiety at a para position relative to the position of X.

13. A method of making a photoinitiator compound of formula I of any preceding claim, the method comprising:

(i) selecting an isolated salt of a compound comprising an anion of formula

14. A formulation comprising a compound according to any of claims 1 to 12 and at least one other material selected from: a polymerisable material; another photoinitiator, which is different to said compound of formula I; and an amine synergist.

15. A method of forming a cured coating on a substrate, the method comprising contacting the substrate with an uncured formulation arranged to provide said coating, wherein said uncured formulation includes a compound of formula I according to any of claims 1 to 12, and exposing the uncured formulation to radiation.