WO2008061954A1

WO2008061954A1 - Novel co-initiators

Info

Publication number: WO2008061954A1
Application number: PCT/EP2007/062494
Authority: WO
Inventors: Johan Loccufier; Roland Claes
Original assignee: Agfa Graphics Nv
Priority date: 2006-11-23
Filing date: 2007-11-19
Publication date: 2008-05-29

Abstract

A novel co-initiator is disclosed characterized in that said CO- initiator has a structure according to Formula I wherein B represents a structural moiety comprising at least two aliphatic tertiary amines; L represents a divalent linking group positioning the nitrogen atom of the aromatic amine and the nitrogen of at least one aliphatic amine of the structural moiety B in a 1-9 to 1-23 position; with the proviso that at least one aromatic and at least two aliphatic amine each have an alf a-hydrogen.

Description

NOVEL CO- INITIATORS

[DESCRIPTION]

FIELD OF THE INVENTION

The present invention relates to a new type of co- initiator and its use in radiation curable compositions such as varnishes, lacquers, coating compositions and printing inks, e.g. radiation curable inkjet inks.

BACKGROUND OF THE INVENTION

A radiation curable composition, e.g. a radiation curable inkjet ink, typically comprises a photo-initiator. Upon exposure of said composition, the photo- initiator initiates the curing, i.e. the polymerization, of said composition. Free radical photo- initiators can be classified as Norrish Type I or Norrish Type II photo- initiators . A Type I photo- initiator undergoes photo-cleavage to yield free radicals while a Type II photo- initiator produces free radicals through an abstraction process, in most cases hydrogen-abstraction. While a Type I photo- initiator produces free radicals through an unimolecular bond cleavage reaction, a Type II photo-initiator forms free radicals in the presence of a second molecule, the so called co-initiator also referred to as polymerization synergist.

A first problem for free-radical polymerization processes, especially with Type I photo- initiating systems, is the inhibition of the polymerization, i.e. decrease of the curing speed, by oxygen. Inhibition by oxygen results not only in a decreased overall curing speed, but can also result in an inhomogeneous curing, e.g. poor surface curing resulting in tacky surfaces. Amines are known in the art as co-initiators for Type II photo- initiators and to suppress oxygen inhibition of free radical polymerization processes. The role of amines in curing systems has been reviewed, for example, by R. S. Davidson in "Radiation Curing in Polymer Science and Technology, Volume III", edited by J. P.

Fouassier et al . , Elsevier Applied Science, 1993, pages 153-176.

According to WO 02/46323, ethyl-4-dimethylaminobenzoate or an acryl amine may serve as oxygen scavengers in UV curable inks for printing on Printed Circuit Boards. US 3 963 771 and US 4 045 416 disclose amine acrylates, produced by the reaction of a polyacrylate ester, preferably a diacrylate ester, with an amine having at least one hydrogen attached to the nitrogen atom. Said amine acrylates provide a fast curing speed to radiation curable coating compositions. In the radiation curable coating compositions of US 5 777 024 and US 2004/0254293 (a) tertiary aliphatic amines, e.g. diethanol amine, (b) aromatic amines, e.g. ethyl-4-dimethylaminobenzoate, (c) (meth) acrylated amines and (d) amino- functional (meth) acrylate resin or oligomer blends may be used as co- initiators . According to WO 06/041289 Type-II photo-initiators are used in combination with (a) a monomer tertiary amine compound, (b) an oligomer or polymer tertiary amine compound, (c) a polymerizable amino acrylate compound or (d) a polymerized amino acrylate compound or mixtures thereof, in radiation curable compositions suitable for use as coating layers in optical fibre technology.

WO 99/03930 discloses a radiation curable coating composition containing an aliphatic maleimide as photo-active component and a tertiary amine, said amine providing a higher curing speed to the composition. The tertiary amine may be aliphatic (e.g. triethyl- amine) or aromatic (e.g. ethyl-4-dimethylaminobenzoate).

A second problem, associated with co-initiators, e.g. amines, present in a radiation curable composition, may arise when unreacted co- initiator remains in the cured composition. Hydrogen transfer from an amine co-initiator to a Type II photo-initiator is rarely quantitative. The unreacted co-initiator remains mobile in the cured composition and may adversely affect the physical properties of the cured composition or may diffuse out of the cured composition. If the radiation curable composition is printed upon food packaging, extraction of the unreacted co-initiator, also referred to as "extractables", into the food may cause health risks. In WO 03/091288 a new class of amine co-initiators, in combination with Type I and/or Type II photo- initiators, is disclosed, said amine co-initiators being trialkylamines having a total of 10 to about 36 carbon atoms, wherein at least one alkyl group has a chain length of at least 8 carbon atoms . According to WO 03/091288 said amine co-initiators, when used in combination with a Type II photo- initiator , provide less "extractables" after curing compared to conventional amines as e.g. N-methyl-N,N-diethanolamine .

WO 99/07746 discloses a radiation curable composition containing a radiation curable resin, a photo-initiator and an amine as Co- initiator, characterized in that said amine is a compound containing at least one tertiary amino group and wherein at least one substituent of the tertiary amino group is an aliphatic chain containing at least one electron-withdrawing group. Said amine compound is preferably a branched, highly branched or star- shaped dendrimer comprising at least one tertiary amino group. Said co- initiators, in combination with Type II photo- initiators, provide a higher curing speed and less "extractables" .

EP-A 1 616 922 discloses a radiation curable composition containing a radiation curable resin, a photo- initiator and a co- initiator characterized in that said co-initiator comprises a dendritic polymer core with at least one co-initiating functional group, e.g. aliphatic or aromatic amines, as an end group on the polymer core. The radiation curable compositions according to EP-A 1 616 922 are characterized by a high curing speed and a low amount of "extractables" .

EP-A 1 147 094 discloses multi-functional reactive amine acrylates having a low viscosity, prepared by the reaction of multifunctional (meth) acrylates with cyclic secondary amines. These amine acrylates provide a good curing speed, a higher thermal stability and less "extractables" to radiation curable compositions. OBJECTS OF THE INVENTION

It is an object of the present invention to provide a new class of very effective co-initiators. Another object of the present invention is to provide a new class of very effective co-initiators which provides a high curing speed to radiation curable compositions, e.g. curable inkjet inks, said compositions comprising a Type I and/or Type II photo- initiator. It is another object of the present invention to provide a new class of co-initiators which provides a low amount of extractables to said cured compositions.

These and other objects of the present invention will become apparent form the description hereinafter.

SUMMARY OF THE INVENTION

Aliphatic and aromatic amines are well known co-initiators for Type II photo-initiators . It was found that combining an aromatic and at least two aliphatic tertiary amine into a single molecule results in a new class of co-initiators which, in combination with a type II photo-initiator, provides a high curing speed and a low amount of "extractables" to radiation curable compositions. Surprisingly, said new class of co-initiators also provides a high curing speed in combination with a Type I photo-initiator to radiation curable compositions

The objects of the present invention are realized with a co- initiator characterized in that said co-initiator has a structure according to Formula I,

Formula I

wherein

B represents a structural moiety comprising at least two aliphatic tertiary amines;

L represents a divalent linking group positioning the nitrogen atom of the aromatic amine and the nitrogen of at least one aliphatic amine of the structural moiety B in a 1-9 to 1-23 position;

X represents 0, S or NR7 ;

Rl, R2 , R3 and R4 independently represent a hydrogen or a substituent ;

R5 and R6 independently represent an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted

(hetero) aralkyl group, an optionally substituted cycloalkyl group or an optionally substituted (hetero) aryl group; R7 represents a hydrogen, an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted

(hetero) aralkyl group, an optionally substituted cycloalkyl group or an optionally substituted (hetero) aryl group; any two groups selected from R2 , R4 , R5 and R6 may represent the necessary atoms to form a ring;

Rl and R2 , R3 and R4 may represent the necessary atoms to form a ring; Rl or R3 may together with R7 represent the necessary atoms to form a ring ;

R7 may represent the necessary atoms to form a ring with an atom of L;

R7 may represent the necessary atoms to form a ring with an atom of B; an atom of L may form a ring with an atom of B; with the proviso that at least one aromatic and at least two aliphatic amine each have an alfa-hydrogen.

Preferred embodiments of the present invention are defined in the dependent claims.

DETAILED DESCRIPTION OF THE INVENTION

Co- initiator

The co-initiator, also referred to as polymerization synergist, has a structure according to Formula I,

Formula I

wherein

L represents a divalent linking group positioning the nitrogen atom of the aromatic amine and the nitrogen of at least one aliphatic amine of the structural moiety B in a 1-9 to 1-23 position ;

X represents 0, S or NR7 ;

Rl, R2 , R3 and R4 independently represent a hydrogen or a substituent ; R5 and R6 independently represent an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted

(hetero) aralkyl group, an optionally substituted cycloalkyl group or an optionally substituted (hetero) aryl group; any two groups selected from R2, R4 , R5 and R6 may represent the necessary atoms to form a ring,-

Rl and R2 , R3 and R4 may represent the necessary atoms to form a ring;

Rl or R3 may together with R7 represent the necessary atoms to form a ring;

R7 may represent the necessary atoms to form a ring with an atom of L;

An aromatic tertiary amine means an amine wherein at least one substituent is an aromatic group, here a phenyl group, directly bound to the N-atom of the amine.

An aliphatic amine means an amine wherein all substituents are aliphatic groups directly bound to the N-atom of the amine.

The divalent linking group L comprises at least one carbon atom. The nitrogen atom of the aromatic amine and the nitrogen of at least one aliphatic amine of the structural moiety B are in a 1-9 position when the divalent linking group L comprises one carbon atom.

The divalent linking group L in Formula I positions the nitrogen atom of the aromatic amine and the nitrogen of at least one aliphatic amine of the structural moiety B in a 1-9 to 1-23 position, preferably in a 1-9 to 1-18 position, more preferably in a 1-9 to 1-15 position.

The structural moiety in Formula I comprises two, three or more tertiary aliphatic amines. Preferably all tertiary aliphatic amines each have at least one alfa-hydrogen. In a preferred embodiment the structural moiety in Formula I comprises three aliphatic amines.

In a more preferred embodiment the co- initiator has a structure according to Formula II

Formula II

wherein

L and Rl to R6 have the same meaning as in Formula I;

L' represents a divalent linking group positioning the at least two aliphatic amines in a 1-4 to a 1-10 position;

Y represents 0, S or NRIl; R8 to RlO independently represent an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero) aralkyl group, an optionally substituted cycloalkyl group; RIl represents a hydrogen, an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero) aralkyl group, an optionally substituted cycloalkyl group or an optionally substituted (hetero) aryl group; any two groups selected from R8 to RlO may represent the necessary atoms to form a ring; Rl and R2 , R3 and R4, R8 and RIl, Rl and RIl, R3 and RIl may represent the necessary atoms to form a ring; R8 may represent the necessary atoms to form a ring with an atom of L or L' ;

R9 or RlO may represent the necessary atoms to form a ring with an atom of L' ;

RIl may represent the necessary atoms to form a ring with an atom of L; with the proviso that at least one aromatic and at least two aliphatic amine each have an alfa-hydrogen.

In a further preferred embodiment the co- initiator has a structure according to Formula III

Formula III

wherein

L, L', R5 , R6 , R8 to RlO have the same meaning as in Formula I and II;

In another preferred embodiment the co- initiator has a structure according to Formula IV

Formula IV

wherein s L, R5 and R6 have the same meaning as in Formula I;

L" and L" ' are divalent linking groups representing the necessary atoms to form with the two nitrogen atoms a 5 to 8 membered saturated heterocycle;

R12 represent an optionally substituted alkyl group, ano optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero) aralkyl group, an optionally substituted cycloalkyl group,-

Preferably, L" and L" ' in Formula IV represent the necessary5 atoms to form with the two nitrogen atoms a 6 membered saturated heterocycle, more preferably a piperazine ring.

The alkyl group referred to in Formulae I to IV means all variants possible for each number of carbon atoms in the alkyl0 group, i.e. for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary butyl; for five carbon atoms: n-pentyl, 1, 1-dimethyl -propyl , 2 , 2-dimethylpropyl and 2- methyl-buthyl ; etc.

Preferably, R5 and R6 in Formulae I, II, III and IV represent5 an optionally substituted lower alkyl group. A lower alkyl group means an alkyl group as defined above comprising 1 up to 6 carbon atoms (Cl to C6 alkyl group) . More preferably R5 and R6 represent an optionally substituted alkyl group with less than 4 carbon atoms (Cl to C4 alkyl group) . Most preferably R5 and R6 represent a methylo group. Rl to R4 in Formulae I and II preferably represent an optionally substituted alkyl group, more preferably an optionally substituted lower alkyl group, most preferably a hydrogen.

The substituents referred to in Formulae I to IV may be halogens, ethers, R-CO-NR'-, RCOO-, amines and tioethers wherein R represents an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero) aralkyl group or an optionally substituted (hetero) aryl group and wherein R' represents an alkyl, aralkyl , alkenyl , alkynyl or aryl group .

The Co-initiator according to Formulae I to IV preferably has a Molecular Weight of less than 1000, more preferably less than 850, most preferably less than 750, particularly preferred less than 650.

Some examples of co-initiators of the present invention are given in table 1.

Table 1

Two or more different co- initiators according to the present invention may be used in radiation curable compositions. A combination of one or more co-initiators according to the present invention and one or more co- initiator known in the art may also be used in radiation curable compositions.

The total co-initiator amount in the radiation curable compositions is preferably from 0.01 to 20 % by weight, more preferably from 0.05 to 15 % by weight, most preferably from 0.1 to 10 % by weight relative to the total weight of said curable composition.

Radiation curable composition

The co- initiator according to the present invention can be used in any radiation curable composition such as a varnish, a lacquer, a coating composition and a printing ink. Preferably, the radiation curable composition according to the present invention is a radiation curable inkjet ink, most preferably a UV-curable inkjet ink.

Besides the co- initiators described above, the radiation curable inkjet ink typically comprises a photo- initiator , a radiation curable compound and preferably at least one colorant, i.e. a pigment or dye. The radiation curable inkjet ink may further comprise one or more dispersant (s) , dispersion synergist (s) , inhibitor (s) , surfactant (s) .

The viscosity of the curable inkjet ink at a shear rate of 100 s and a jetting temperature between 25 and 70 ⁰C is preferably lower than 35 mPa.s, more preferably lower than 15 mPa.s, most preferably between 2 and 10 mPa.s.

The curable pigment inkjet ink preferably does not contain an evaporable component, but sometimes, it can be advantageous to incorporate a small amount of an organic solvent in said inks to improve adhesion to the surface of the ink-receiver after UV-curing. In this case, the added solvent may be any amount in the range that does not cause problems of solvent resistance and VOC (Volatile Organic Compound), and preferably between 0.1-10.0 % by weight, more preferably between 0.1-5.0 % by weight relative to the total weight of the curable pigment inkjet ink. Photo-Initiator

A radiation curable composition typically comprises one or more photo-initiators. The photo- initiator initiates the polymerization upon exposure of said composition. Upon absorption of said radiation, the photo- initiator produces free radicals, thereby inducing polymerization of the curable compounds (monomers, oligomers or (pre) polymers) of said composition. When polyfunctional monomers or oligomers are present as curable compounds, said free radicals may also initiate cross linking.

The photo-initiator may be a Norrish Type I photo-initiator, a Norrish Type II photo- initiator or a combination thereof.

A preferred Norrish type I-initiator is selected from the group consisting of benzoinethers, benzil ketals, α, α-dialkoxy- acetophenones, α-hydroxyalkylphenones, α-aminoalkylphenones, acylphosphine oxides, acylphosphine sulphides, α-haloketones, α- halosulfones and α-halophenylglyoxalates . Particularly preferred are acyl or bisacyl phosphine oxides.

A preferred Norrish type II-initiator is selected from the group consisting of benzophenones , thioxanthones, 1 , 2-diketones and anthraquinones . Suitable Type I and Type II photo- initiators are disclosed by J. V. Crivello et al . in "Photoinitiators for Free Radical, Cationic & Anionic Photopolymerisation 2^nd edition" , Volume III of the Wiley/SITA Series In Surface Coatings Tecnology, edited by G. Bradley and published in 1998 by John Wiley and Sons Ltd London, pages 276 to 294.

Specific examples of photo- initiators may include, but are not limited to, the following compounds or combinations thereof: benzophenone and substituted benzophenones, 1-hydroxycyclohexyl phenyl ketone, thioxanthones such as isopropylthioxanthone, 2- hydroxy-2-methyl-l-phenylpropan-l-one, 2-benzyl-2-dimethylamino- (4- morpholinophenyl) butan-1-one, benzil dimethylketal, bis (2,6- dimethylbenzoyl) 2 , 4, 4-trimethylpentylphosphine oxide, 2,4,6- trimethylbenzoyldiphenylphosphine oxide, 2 -methyl -1- [4- (methylthio) phenyl] -2-morpholinopropan-l-one, 2 , 2-dimethoxy-l, 2-diphenylethan-l- one or 5 , 7-diiodo-3 -butoxy-6-fluorone . Suitable commercial photo- initiators include Irgacure 184, Irgacure 500, Irgacure 907, Irgacure 369, Irgacure 1700, Irgacure 651, Irgacure 819, Irgacure 1000, Irgacure 1300, Irgacure 1870, Darocur 1173, Darocur 2959, Darocur 4265 and Darocur ITX available from CIBA SPECIALTY CHEMICALS, Lucerin TPO available from BASF AG, Esacure KT046, Esacure KIP150, Esacure KT37 and Esacure EDB available from LAMBERTI, H-Nu 470 and H-Nu 470X available from SPECTRA GROUP Ltd..

Since the photo- initiator generates radicals upon absorption of radiation, the photo-initiator is typically chosen as a function of the type of radiation used to cure the curable composition. If two or more types of radiation are used to cure the radiation curable composition, two or more types of photo-initiators may be present.

A preferred amount of initiator is preferably from 0.3 to 50 % weight, more preferably from 1 to 25 % by weight, most preferably form 2 to 15 procent by weight relative to the total weight of the radiation curable composition.

The radiation curable composition may comprise one or more sensitizers. Upon exposing said composition, the sensitizers may transfer energy to the photo- initiator (s) resulting in free radical formation. Suitable sensitizers include photoreducible xanthene, fluorene, benzoxanthene, benzothioxanthene, thiazine, oxazine, coumarin, pyronine, porphyrin, acridine, azo, diazo, cyanine, merocyanine, diarylmethyl, triarylmethyl , anthraquinone, phenylenediamine, benzimidazole, fluorochrome, quinoline, tetrazole, naphthol, benzidine, rhodamine, indigo and/or indanthrene dyes. The amount of the sensitizer is preferably from 0.01 to 15 % by weight, more preferably from 0.05 to 5 % by weight relative of the total weight of said curable composition. Curable compound

The curable compounds are typically monomers, oligomers or prepolymers or a combination thereof. The monomers, oligomers and/or prepolymers comprise one, two, three or more polymerizable groups. A mixture of monomers, oligomers and/or prepolymers, having different amounts of polymerizable groups may be used.

The viscosity of the curable composition can be adjusted by varying the ratio between the monomers and oligomers.

Any polymerizable compound commonly known in the art may be employed. Particularly preferred for use as a radiation curable compound in the radiation curable inkjet ink are monofunctional and/or polyfunctional acrylate monomers, oligomers or prepolymers, such as isoamyl acrylate, stearyl acrylate, lauryl acrylate, octyl acrylate, decyl acrylate, isoamylstyl acrylate, isostearyl acrylate, 2-ethylhexyl-diglycol acrylate, 2-hydroxybutyl acrylate, 2- acryloyloxyethylhexahydrophthalic acid, butoxyethyl acrylate, ethoxydiethylene glycol acrylate, methoxydiethylene glycol acrylate, methoxypolyethylene glycol acrylate, methoxypropylene glycol acrylate, phenoxyethyl acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxy-3-phenoxypropyl acrylate, vinyl ether acrylate, 2-acryloyloxyethylsuccinic acid, 2-acryloyloxyethylphthalic acid, 2- acryloxyethyl-2-hydroxyethyl-phthalic acid, lactone modified flexible acrylate, and t -butylcyclohexyl acrylate, triethylene glycol diacrylate, tetraethylene glycol diacrylate, polyethylene glycol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, polypropylene glycol diacrylate, 1, 4-butanediol diacrylate, 1 , 6-hexanediol diacrylate, 1, 9-nonanediol diacrylate, neopentyl glycol diacrylate, dimethylol-tricyclodecane diacrylate, bisphenol A EO (ethylene oxide) adduct diacrylate, bisphenol A PO (propylene oxide) adduct diacrylate, hydroxypivalate neopentyl glycol diacrylate, propoxylated neopentyl glycol diacrylate, alkoxylated dimethyloltricyclodecane diacrylate and poly- tetramethylene glycol diacrylate, trimethylolpropane triacrylate, EO modified trimethylolpropane triacrylate, tri (propylene glycol) triacrylate, caprolactone modified trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerithritol tetraacrylate, penta- erythritolethoxy tetraacrylate, dipentaerythritol hexaacrylate, ditrimethylolpropane tetraacrylate, glycerinpropoxy triacrylate, and caprolactam modified dipentaerythritol hexaacrylate, or an N-vinyl- amide such as, N-vinylcaprolactam or N- vinylformamide ; or acrylamide or a substituted acrylamide, such as acryloylmorpholine . Other suitable monofunctional acrylates include caprolactone acrylate, cyclic trimethylolpropane formal acrylate, ethoxylated nonyl phenol acrylate, isodecyl acrylate, isooctyl acrylate, octyldecyl acrylate, alkoxylated phenol acrylate, tridecyl acrylate and alkoxylated cyclohexanone dimethanol diacrylate.

Other suitable difunctional acrylates include alkoxylated cyclohexanone dimethanol diacrylate, alkoxylated hexanediol diacrylate, dioxane glycol diacrylate, dioxane glycol diacrylate, cyclohexanone dimethanol diacrylate, diethylene glycol diacrylate and neopentyl glycol diacrylate .

Other suitable trifunctional acrylates include propoxylated glycerine triacrylate and propoxylated trimethylolpropane triacrylate .

Other higher functional acrylates include di -trimethylolpropane tetraacrylate, dipentaerythritol pentaacrylate, ethoxylated pentaeryhtitol tetraacrylate, methoxylated glycol acrylates and acrylate esters.

Furthermore, methacrylates corresponding to the above-mentioned acrylates may be used with these acrylates. Of the methacrylates, methoxypolyethylene glycol methacrylate, methoxytriethylene glycol methacrylate, hydroxyethyl methacrylate, phenoxyethyl methacrylate, cyclohexyl methacrylate, tetraethylene glycol dimethacrylate, and polyethylene glycol dimethacrylate are preferred due to their relatively high sensitivity and higher adhesion to an ink-receiver surface .

Furthermore, the inkjet inks may also contain polymerizable oligomers. Examples of these polymerizable oligomers include epoxy acrylates, aliphatic urethane acrylates, aromatic urethane acrylates, polyester acrylates, and straight-chained acrylic oligomers .

Suitable examples of styrene compounds are styrene, p- methylstyrene, p-methoxystyrene, β-methylstyrene, p-methyl-β- methylstyrene, cx-methylstyrene and p-methoxy-β-methylstyrene .

Suitable examples of vinylnaphthalene compounds are 1- vinylnaphthalene, α-methyl-1-vinylnaphthalene, β-methyl-1- vinylnaphthalene , 4-methyl-l-vinylnaphthalene and 4-methoxy-l- vinylnaphthalene . Suitable examples of N-vinyl heterocyclic compounds are N- vinylcarbazole, N-vinylpyrrolidone, N-vinylindole, N-vinylpyrrole, N-vinylphenothiazine, N-vinylacetoanilide, N-vinylethylacetoamide, N-vinylsuccinimide, N-vinylphthalimide, N-vinylcaprolactam and N- vinylimidazole . Examples of vinyl ethers having at least one vinyl ether group include ethyl vinyl ether, n-butyl vinyl ether, isobutyl vinyl ether, octadecyl vinyl ether, cyclohexyl vinyl ether, butanediol divinyl ether, hydroxyl butyl vinyl ether, cyclohexane dimethanol monovinyl ether, phenyl vinyl ether, p-methylphenyl vinyl ether, p- methoxyphenyl vinyl ether, α-methylphenyl vinyl ether, β-methyl - isobutyl vinyl ether and β-chloroisobutyl vinyl ether, diethylene- glycol divinyl ether, triethylene glycol divinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, dodecyl vinyl ether, diethylene glycol monovinyl ether, cyclohexanedimethanol divinyl ether, 4- (vinyloxy) butyl benzoate, bis [4- (vinyl oxy) butyl] adipate, bis [4- (vinyl oxy) butyl] succinate, 4- (vinyloxy methyl) cyclohexyl-methyl benzoate, bis [4- (vinyloxy) butyl] isophthalate, bis [4- (vinyl - oxymethyl) cyclohexylmethyl] glutarate, tris [4- (vinyloxy) -butyl] trimellitate, 4- (vinyloxy) butyl steatite, bis [4- (vinyloxy) -butyl] hexanediylbiscarbamate, bis [4- (vinyloxy) methyl] -cyclohexyl] methyl] terephthalate, bis [4- (vinyloxy) methyl] cyclohexyl] methyl] isophthalate, bis [4- (vinyloxy) butyl] (4 -methyl-1 , 3-phenylene) - biscarbamate, bis [4-vinyloxy) butyl] (methylene di-4, 1-phenylene) biscarbamate and 3-amino-l-propanol vinyl ether. Colorants

The radiation curable inkjet ink preferably comprises at least one colorant. Colorants may be dyes, but are preferably pigments or s a combination thereof. The pigment can also be a precipitated dye having at least one carboxylic acid group or a salt thereof.

The pigment can be black, white, cyan, magenta, yellow, red, orange, violet, blue, green, brown, mixtures thereof, and the like.

The pigment may be chosen from those disclosed by HERBST, Willyo et al . "Industrial Organic Pigments, Production, Properties,

Applications", 3^rd edition, Wiley - VCH, 2004 (ISBN 3527305769).

Particular preferred pigments are C.I. Pigment Yellow 1, 3, 10, 12, 13, 14, 17, 55, 65, 73, 74, 75, 83, 93, 97, 109, 111, 120, 128, 138, 139, 150, 151, 154, 155, 175, 180, 181, 185, 194 and 213. s Particular preferred pigments are C.I. Pigment Red 17, 22, 23, 41, 48:1, 48:2, 49:1, 49:2, 52:1, 57:1, 81:1, 81:3, 88, 112, 122, 144, 146, 149, 169,170, 175, 176, 184, 185, 188, 202, 206, 207, 210, 216, 221, 248, 251, 254, 264 and 282.

Particular preferred pigments are C.I. Pigment Violet 1, 2, 19,0 23, 32, 37 and 39.

Particular preferred pigments are C.I. Pigment Blue 15:1, 15:2, 15:3, 15:4, 16, 56, 61 and (bridged) aluminium phthalocyanine pigments .

Particular preferred pigments are C.I. Pigment Orange 5, 13,5 16, 34, 40, 43, 59, 66, 67, 69, 71 and 73.

Particular preferred pigments are C.I. Pigment Green 7 and 36. Particular preferred pigments are C.I. Pigment Brown 6 and 7. Suitable pigments also include mixed crystals of the above particular preferred pigments. In mixed crystals, also referred too as solid solutions, the molecules of the components enter into the same crystal lattice, usually, but not always, that of one of the components. The x-ray diffraction pattern of the resulting crystalline solid is characteristic of that solid and can be clearly differentiated from the pattern of a physical mixture of the same5 components in the same proportion. In such physical mixtures, the x- ray pattern of each of the components can be distinguished, and the disappearance of many of these lines is one of the criteria of the formation of solid solutions. A commercially available example is Cinquasia Magenta RT-355-D from Ciba Specialty Chemicals, which is a mixed crystal of C.I. Pigment Violet 19 and C.I. Pigment Red 202. Under certain conditions different quinacridones mix with each other to form solid solutions

For the black ink, suitable pigment materials include carbon blacks such as Regal 400R, Mogul L, Elftex 320 from Cabot Co., or Carbon Black FW18, Special Black 250, Special Black 350, Special Black 550, Printex 25, Printex 35, Printex 55, Printex 150T from DEGUSSA Co., and C.I. Pigment Black 7 and C.I. Pigment Black 11. It is also possible to make mixtures of pigments in a nonaqueous inkjet ink. For example, carbon black generally exhibits a warm brownish black tone, while a neutral black tone is generally preferred. A neutral black inkjet ink can be obtained, for example, by mixing carbon black with a cyan, a magenta or a cyan and magenta pigment into the ink, as for example described in EP-A 1 593 718.

The inkjet application may also require one or more spot colours, for example for packaging inkjet printing or textile inkjet printing. Silver and gold are often desired colours for inkjet poster printing and point-of -sales displays. Particular preferred pigments are C.I. Pigment Metal 1, 2 and 3. Illustrative examples of the inorganic pigments include titanium oxide, barium sulfate, calcium carbonate, zinc oxide, lead sulfate, yellow lead, zinc yellow, red iron oxide (III), cadmium red, ultramarine blue,

Prussian blue, chromium oxide green, cobalt green, amber, titanium black and synthetic iron black. Titanium dioxide is a preferred pigment for a white ink.

Pigment particles in pigmented inkjet ink should be sufficiently small to permit free flow of the ink through the inkjet -printing device, especially at the ejecting nozzles. It is also desirable to use small particles for maximum colour strength and to slow down sedimentation.

The average particle size of the pigment in pigmented inkjet ink should be between 0.005 and 15 μm. Preferably, the average pigment particle size is between 0.005 and 5 μm, more preferably between 0.005 and 1 μm, particularly preferably between 0.005 and 0.3 μm and most preferably between 0.040 and 0.150 μm. Larger pigment particle sizes may be used as long as the objectives of the present invention are achieved, s

Dispersant

The radiation curable inkjet ink may further comprise ao dispersant in order to obtain a stable dispersion of the pigment (s) in the inkjet ink.

Preferred dispersants are disclosed in the unpublished EP-A 06 122 098 (filed 11 October 2006) , paragraph [51] to [87] . 5

Dispersion synergist

The radiation curable inkjet ink may further comprise a dispersion synergist. A dispersion synergist is used for improving0 the dispersion quality and stability of the pigment dispersions in the inkjet ink. A mixture of dispersion synergists may be used to further improve dispersion stability.

The dispersion synergist usually consists of an anionic part and a cationic part. The anionic part of the dispersion synergist5 exhibitis often a certain molecular similarity with the colour pigment and the cationic part of the dispersion synergist consists of one or more protons and/or cations to compensate the charge of the anionic part of the dispersion synergist .

The curable magenta inkjet ink according to the present 0 invention preferably contains a dispersion synergist.

Preferred dispersion synergyst are disclosed in the unpublished EP-As 05 111 356, 05 111 357, 05 111 358 and 05 111 360 (all filed 28 November 2005) . 5 Inhibitors

The radiation curable composition may also contain a polymerization inhibitor, also referred to as stabilizers. Suitable polymerization inhibitors include phenol type antioxidants, hindered amine light stabilizers, phosphor type antioxidants, hydroquinone monomethyl ether, hydroquinone, t-butylcatechol or pyrogallol.

Suitable commercial inhibitors are, for example, Sumilizer GA- 80, Sumilizer GM and Sumilizer GS produced by Sumitomo Chemical Co. Ltd.; Genorad 16, Genorad 18 and Genorad 20 from Rahn AG; Irgastab UVlO and Irgastab UV22, Tinuvin 460 and CGS20 from Ciba Specialty Chemicals; Floorstab UV range (UV-I, UV-2, UV-5 and UV- 8) from Kromachem Ltd, Additol S range (SlOO, SIlO, S120 and S130) from Cytec Surface Specialties. Since excessive addition of these polymerization inhibitors will lower the ink sensitivity to curing, it is preferred that the amount capable of preventing polymerization is determined prior to blending. The amount of a polymerization inhibitor is preferably lower than 5 % by weight, more preferably lower than 2.5 % by weight relative the total weight of the inkjet ink composition.

Surfactant .

The curable pigment inkjet inks may furhter contain a surfactant. The surfactant (s) may be anionic, cationic, non-ionic, or zwitter- ionic and are usually added in a total quantity below 20 % by weight relative to the total ink weight and particularly in a total quantity below 10 % by weight relative to the total ink weight .

A fluorinated or silicone comprising surfactant may be used, preferably comprising a cross-linkable group such as silicone modified acrylates, silicone modified methacrylates , acrylated siloxanes, polyether modified acrylic modified siloxanes, fluorinated acrylates, and fluorinated methacrylates. Polymerizable monomers having surface-active effects can be mono-, di-, tri- or higher functional (meth) acrylates or mixtures thereof.

Preparation of an inkjet ink

The curable inkjet inks may be prepared by milling the pigment in the dispersion medium, preferably in the presence of a polymeric dispersant . Mixing apparatuses may include a pressure kneader, an open kneader, a planetary mixer, a dissolver and a Dalton Universal Mixer. Suitable milling and dispersion apparatuses are a ball mill, a pearl mill, a colloid mill, a high-speed disperser, double rollers, a bead mill, a paint conditioner, and triple rollers. The dispersions may also be prepared using ultrasonic energy.

Many different types of materials may be used as milling media, such as glasses, ceramics, metals, and plastics. In a preferred embodiment, the grinding media can comprise particles, preferably substantially spherical in shape, e.g. beads consisting essentially of a polymeric resin or yttrium stabilized zirconium oxide beads.

In the process of mixing, milling and dispersion, each process is performed with cooling to prevent build up of heat, and as much as possible under light conditions in which actinic radiation has been substantially excluded. The curable pigment inkjet ink may contain more than one pigment, the inkjet ink may be prepared using separate dispersions for each pigment, or alternatively several pigments may be mixed and co-milled in preparing the dispersion.

The dispersion process can be carried out in a continuous, batch or semi -batch mode.

The preferred amounts and ratios of the ingredients of the mill grind will vary widely depending upon the specific materials and the intended applications. The contents of the milling mixture comprise the mill grind and the milling media. The mill grind comprises pigment, polymeric dispersant and a liquid carrier. For inkjet inks, the pigment is usually present in the mill grind at 1 to 50 % by weight, excluding the milling media. The weight ratio of pigment over polymeric dispersant is 20:1 to 1:2.

The milling time can vary widely and depends upon the pigment, the selected mechanical means and residence conditions, the initial and desired final particle size, etc. In the present invention pigment dispersions with an average particle size of less than 150 nm may be prepared .

After milling is completed, the milling media is separated from the milled particulate product (in either a dry or liquid dispersion form) using conventional separation techniques, such as filtration, sieving through a mesh screen, and the like. Often the sieve is built into the mill, e.g. for a bead mill. The milled pigment concentrate is preferably separated from the milling media by filtration. In general it is desirable to make the inkjet inks in the form of a concentrated mill grind, which is subsequently diluted to the appropriate concentration for use in the inkjet printing system.

This technique permits preparation of a greater quantity of pigmented ink from the equipment. By dilution, the inkjet ink is adjusted to the desired viscosity, surface tension, colour, hue, saturation density, and print area coverage for the particular application.

In preparing the inkjet ink, it is preferred that one or more degassing steps are performed for removing air or gas bubbles from the ink. Degassing is preferably performed by heating and/or reduced pressure. The degassing step(s) can be performed on the concentrated pigment dispersion and/or on the final inkjet ink composition.

Printing means

The curable inkjet ink may be jetted by one or more printing heads ejecting small droplets of ink in a controlled manner through nozzles onto an ink-receiver surface, which is moving relative to the printing head(s) . A preferred printing head for the inkjet printing system is a piezoelectric head. Piezoelectric inkjet printing is based on the movement of a piezoelectric ceramic transducer when a voltage is applied thereto. The application of a voltage changes the shape of the piezoelectric ceramic transducer in the printing head creating a void, which is then filled with ink. When the voltage is again removed, the ceramic expands to its original shape, ejecting a drop of ink from the print head. However the inkjet printing method according to the present invention is not restricted to piezoelectric inkjet printing. Other inkjet printing heads can be used and include various types, such as a continuous type and thermal, electrostatic and acoustic drop on demand type.

At high printing speeds, the inks must be ejected readily from the printing heads, which puts a number of constraints on the physical properties of the ink, e.g. a low viscosity at the jetting temperature, which may vary from 25 ⁰C to 110 °C, a surface energy such that the printing head nozzle can form the necessary small droplets, a homogenous ink capable of rapid conversion to a dry printed area. The inkjet printing head normally scans back and forth in a transversal direction across the moving ink-receiver surface. Often the inkjet print head does not print on the way back. Bi-directional printing is preferred for obtaining a high areal throughput. Another preferred printing method is by a "single pass printing process", which can be performed by using page wide inkjet printing heads or multiple staggered inkjet printing heads which cover the entire width of the ink-receiver surface. In a single pass printing process the inkjet printing heads usually remain stationary and the ink- receiver surface is transported under the inkjet printing heads.

Curing means

The radiation curable composition, preferably the radiation curable inkjet ink, can be cured by exposing to actinic radiation and/or by electron beam curing. Preferably the curing is performed by an overall exposure to actinic radiation or by an overall electron beam curing.

The curing means may be arranged in combination with the print head of the inkjet printer, travelling therewith so that the curable liquid is exposed to curing radiation very shortly after been jetted.

In such an arrangement it can be difficult to provide a small enough radiation source connected to and travelling with the print head. Therefore, a static fixed radiation source may be employed, e.g. a source of curing UV- light, connected to the radiation source by means of flexible radiation conductive means such as a fibre optic bundle or an internally reflective flexible tube.

Alternatively, the actinic radiation may be supplied from a fixed source to the radiation head by an arrangement of mirrors including a mirror upon the radiation head.

The source of radiation arranged not to move with the print head, may also be an elongated radiation source extending transversely across the ink-receiver surface to be cured and adjacent the transverse path of the print head so that the subsequent rows of images formed by the print head are passed, stepwise or continually, beneath that radiation source.

The radiation source is preferably a ultraviolet radiation source. Ultraviolet radiation or light is also referred to as UV- radiation or UV- light. Any UV-light source, as long as part of the emitted light is absorbed by the photo- initiator or the photo- initiator system, may be employed as a radiation source, such as a high or low pressure mercury lamp, a cold cathode tube, a black light, an ultraviolet LED, an ultraviolet laser or a flash light. Of these, the preferred source is one exhibiting a relatively long wavelength UV-contribution having a dominant wavelength of 300-400 nm. Specifically, a UV-A light source is preferred due to the reduced light scattering therewith resulting in more efficient interior curing.

UV radiation is generally classed as UV-A, UV-B, and UV-C as follows :

UV-A: 400 nm to 320 nm UV-B: 320 nm to 290 nm

UV-C: 290 nm to 100 nm.

Furthermore, it is possible to cure the image using, consecutively or simultaneously, two light sources of differing wavelength or illuminance. For example, the first UV- source can be selected to be rich in UV-C, in particular in the range of 260 nm- 200 nm. The second UV- source can then be rich in UV-A, e.g. a gallium-doped lamp, or a different lamp high in both UV-A and UV-B. The use of two UV- sources has been found to have advantages e.g. a fast curing speed.

For facilitating curing, the inkjet printer often includes one or more oxygen depletion units. The oxygen depletion units place a blanket of nitrogen or other relatively inert gas (e.g. CO₂), with adjustable position and adjustable inert gas concentration, in order to reduce the oxygen concentration in the curing environment.

Residual oxygen levels are usually maintained as low as 200 ppm, but are generally in the range of 200 ppm to 1200 ppm.

EXAMPLES

Materials

All materials used in the examples were readily available from standard sources such as Aldrich Chemical Co. (Belgium) and Acros (Belgium) unless otherwise specified.

• Solsperse 39000 is a polyethyleneimine-polyester dispersant from NOVEON . • Solsperse 32000 is a polyethyleneimine-polyester dispersant from NOVEON .

• Solsperse 5000 is a sulfonated copper phthalocyanine dispersion synergist from NOVEON.

• DPGDA is a difunctional acrylate monomer available from CYTEC SURFACE SPECIALTIES • Sartomer SR9003 is a difunctional acrylate monomer available from SARTOMER .

• Craynor CN386 is an amine modified acrylate synergist from SARTOMER . • Darocur TPO is a type I photo-initiator from CIBA SPECIALTY CHEMICALS .

• Genocure ITX is a type II photo-initiator from RAHN AG.

• Genocure EPD is an amine synergist from RAHN AG.

• Genorad 16 is a stabilizer for UV-curable compositions available from RAHN AG.

• Byk333 is a surfactant available from BYK CHEMIE Gmbh. In the examples Byk333 is used as a 1 wt% solution in DPGDA.

• PB15:3 is an abbreviation for the cyan pigment Sun Fast Blue 15:3 from SUN CHEMICAL CORPORATION. • PB15:4 is an abbreviation for the cyan pigment Hostaperm Blue P-BFS available from CLARIANT.

Measurements methods

Curing speed, viscosity and amount of extractables were used to evaluate the UV curable inkjet ink.

• Curing speed: the percentage of the maximum output of the lamp was taken as a measure for the curing speed, the lower the number the higher the curing speed. A sample was considered fully cured at the moment scratching with a Q-tip caused no visual damage.

• viscosity: the viscosity of the curable compositions was measured with a Brookfield DV-II+ at 25 ⁰C and a shear rate 3RPM.

• amount of extractables: a sample of 30 mm in diameter was taken from the coated and cured curable compositions. The sample was put in a beaker of 50 ml and extracted three times with 3 ml acetonitrile using ultrasound. The acetonitrile extracts were combined and the volume adjusted to 10 ml. Reference samples were made by dissolving 1 mg of the reference compound (the co- initiator) in 50 ml acetonitrile . The samples were analyzed on a Alltime C18 5μm HPLC column (150 x 3.2 mm) equipped with a Alltime Cl8 5μm precolumn (7.5 x 3.2 mm) (from Alltech) . An amount of 5μl of the reference samples and 10 μl of the extraction samples were injected. A gradient elution was used starting from acetonitrile/water 80/20 going to 100 % acetonitrile over 12 minutes and switching again to acetonitrile/water 80/20 over one minute followed by equilibration of the column for 7 minutes. The total peak area compared to the reference samples was taken as a measure for the amount of co- initiator extracted from the cured composition.

EXAMPLE 1

Synthesis of the co- initiators according to the present invention

Synthesis of COINI-01

COINI-01 has been prepared according to scheme 1.

Scheme 1

After cooling a solution of 2.8 g of 2- { [2- (dimethylamino) ethyl] methylamino} ethanol and 1.9 g of triethylamine in 25 ml ethyl acetate, 3.5 g of 4- (dimethylamino) benzoylchloride was slowly added while the temperature was kept between 0 ⁰C and 5 ⁰C, inducing precipitation of triethylamine hydrochloride. The reaction mixture was allowed to stir for 15 minutes at 0 ⁰C and for 24 hours at room temperature. The reaction mixture was filtered and the precipitate washed with ethyl acetate. The filtrate was extracted with a 2/5 mixture of a 1 M aqueous solution of sodium carbonate and distilled water (pH = + 10) . The organic layer was separated, dried over MgSO₄ and the solvent was evaporated under reduced pressure. The product was purified on a Prochrom LC80 Column using ethanol/water/ammonium acetate (50/50/0.2 M) as eluent . After evaporation of ethanol, the pH of the aqueous layer was brought to 11 with sodium carbonate, and extracted with ethyl acetate. The organic layer was separated and dried over MgSO₄. After evaporation of the solvent 1.0 g of a colourless oil was obtained.

Synthesis of COINI-02 to COINI-05

COINI-02 to COINI-05 were prepared according to scheme 2

(1) (2) (3)

Scheme 2

The different aliphatic amines (2) used in scheme 2 to obtain the different co-initiators (3) are given in Table 2.

Table 2

_ -DT _

wherein Me = Methyl, Et = Ethyl

5.0 g of compound (1), prepared according to example 2 disclosed in WO 1988/009783 (Biopolymers Limited) , was heated to 80 ⁰C, followed by a dropwise addition of 0.025 moles of the amine (2) . Upon completion of the addition of the amine, the mixture was stirred for 1 hour at 80 ⁰C. The reaction mixture was diluted with 150 ml of ethylacetate and extracted twice with 50 ml distilled water. Finally the organic layer was dried over anhydrous magnesium sulfate. After evaporation of the solvent, COINI-02 to COINI-05 (3) were obtained as a yellow oil.

Synthesis of COINI-06

COINI- 06 has been prepared according to scheme 3

Scheme 3

20 g of compound (1) was heated to 75 ⁰C followed by a drop wise addition of 0.098 moles of the amine (4). Upon completion of the addition of the amine (4) , the mixture was stirred for 1 hour at 75 ⁰C. The reaction mixture was diluted with 150 ml of ethyl acetate and extracted twice with 100 ml of distilled water. The organic layer was dried over anhydrous magnesium sulfate. Upon evaporation of the solvent, a yellow oil was obtained. The crude COINI -6 (5) was purified by preparative column chromatography on a Prochrom LC80, using a gradient elution from methylene chloride to methylene chloride/ methanol 80/20 on Kromasil Si 60 A 10 μm. Synthesis of the comparative co-initiators COMPCOINI-01 to 03

The comparative co- initiators COMPCOIN- 01 to COMPCOINI -03 have a structure according to Formula I but the structural moiety B comprises only one aliphatic amine while the structural moiety of the inventive co- initiators comprises at least two aliphatic amines.

Synthesis of COMPCOINI-01

COMPCOINI-01 has been prepared according to scheme 4.

Scheme 4

A reaction mixture containing 12.0 g N,N-dimethylaminoethanol (7), 8.7 g ethyl-4-dimethylaminobenzoate (6) and 0.146 g of a 30 wt% solution of sodium methylate (NaOMe) in methanol was heated to 136 ⁰C and stirred for about 24 hours. Meanwhile the formed ethanol was distilled off. The mixture was filtered to remove the formed precipitate. The filtrate was diluted with 70 ml of toluene and extracted with 50 ml of distilled water. The organic layer was separated, washed with water and dried over MgSO₄. After evaporation of the solvent, the residue was recrystallized from n-hexane. After filtration, 2.9 g of COMPCOINI-01 (8) was obtained as a pale yellow solid. Synthesis of COMPCOINI-02

COMPCOINI-02 has been prepared according to scheme 5.

Scheme 5

A reaction mixture containing 15.9 g of N,N-diethyl ethanolamine (9), 8.7 g of ethyl-4-dimethylaminobenzoate (6) and 0.146 g of a 30 wt% solution of sodium methylate (NaOMe) in methanol was heated to 150 ⁰C and stirred for about 24 hours. Meanwhile the formed ethanol was distilled off. The mixture was filtered to remove the formed precipitate. The filtrate was diluted with 70 ml of toluene (70 ml) and extracted with 50 ml of distilled water. The organic layer was separated, washed with water and dried over MgSO₄. After evaporation of the solvent, the product was purified on a Flash Column using as eluent dichloromethane/ethanol (90/10). 1.97 g of COMPCOINI-02 (10) was obtained as a yellow oil.

Synthesis of COMPCOINI -03

COMPCOINI - 03 has been prepared according to scheme 6 .

Scheme 6 A reaction mixture containing 17.4 g of N-piperidine ethanol (10), 8.7 g of ethyl-4-dimethylaminobenzoaat (6) and 0.67 g of dibutyltin oxide (Bu₂SnO) was heated at 160 ⁰C and stirred for about 8 hours. Meanwhile the formed ethanol was distilled off. The reaction mixture was diluted with 70 ml of toluene and extracted with 70 ml of distilled water. The aqueous layer was separated and extracted with toluene. The organic layers were collected and dried over MgSO₄. After evaporation of the solvent, the product was purified on a Flash Column using dichloromethane/ethanol (90/10) as eluent, to obtain 10.2 g of COMPCOINI-03 (12) as a pale yellow solid.

EXAMPLE 2

This example illustrates the high curing provided to an inkjet ink, comprising a Type I photo- initiator, by the inventive co- initiators .

Preparation of inkjet inks INV-Ol to INV-05 and COMP-Ol to COMP-03

Preparation of the concentrated cyan dispersion I

The concentrated cyan dispersion I was prepared by mixing the pigment, the dispersant, the dispersion synergist and the stabilizer with a dissolver and treating this mixture with an Eiger bead mill. The composition of the cyan dispersion I is given in Table 3.

Table 3

Preparation of the UV curable inkjet ink

The composition of the UV curable inkjet inks INV-Ol to INV- 04 and COMP-01 to COMP-03 is given in Table 4. The inks were made by adding the ink components from Table 4 to the concentrated cyan pigment dispersion I under stirring at room temperature. The weight % (wt%) is based on the total weight of the radiation curable composition. Each sample comprises an equivalent amount (on molar basis) of co-initiator

Table 4

The UV curable inkjet ink compositions INV-01 to INV- 04 and

COMP-01 to COMP-03 were coated on an unsubbed 100 μm PET substrate, using a bar coater and a 10 μm wired bar. Each coated layer was cured on a Fusion DRSE-120 conveyer, equipped with a Fusion VPS/1600 lamp (D-bulb) , which transported the samples under the UV-lamp on a conveyer belt at a speed of 20 m/min. The curing speed and viscosity, determined as described above, are summarized in Table 5

Table 5

From Table 5, it is clear that all inventive UV curable inkjet ink compositions (INV-01 to INV-04) showed a high curing speed while none of the comparative UV curable inkjet ink compositions (COMP-Ol to COMP-03) were cured, even at 10 m/min (= 200 %) . Even the comparative example (COMP-03, comprising a mixture of the two types of comparative co- initiators, did not cure under ambient atmosphere, clearly illustrating the high efficiency of the co-initiators of the present invention in combination with a Type I photo- initiator .

EXAMPLE 3

This example illustrates the high curing speed and/or low viscosity and the low amount of extractables provided to inkjet inks, comprising a Type II photo-initiator, by co-initiators according to the present invention. Preparation of inkjet inks INV- 05 to INV-08 and COMP-04 to COMP-07

Preparation of the concentrated cyan dispersion II

The concentrated cyan dispersion II was prepared by mixing the pigment, the dispersant, the dispersion synergist with a dissolver and treating this mixture with an Eiger bead mill. The composition of the cyan dispersions II is given in Table 6.

Table 6

Preparation of the UV curable inkjet ink

The composition of the UV curable inkjet inks INV-05 to INV-08 and COMP-04 to COMP-07 is given in Table 7. The inks were made by adding the ink components from Table 7 to the concentrated cyan pigment dispersion II under stirring at room temperature. The weight % (wt%) was based on the total weight of the radiation curable composition. Each sample comprises an equivalent amount (on molar basis) of co-initiator

Table 7

The UV curable inkjet ink compositions INV- 05 to INV- 08 and COMP-04 to COMP-07 were coated on an unsubbed 100 μm PET substrate, using a bar coater and a 10 μm wired bar. Each coated layer was cured on a Fusion DRSE- 120 conveyer, equipped with a Fusion VPS/1600 lamp (D-bulb) , which transported the samples under the UV-lamp on a conveyer belt at a speed of 20 m/min. The curing speed, viscosity and extractables are summarized in Table 8.

Table 8

From Table 8, it is clear that the inventive UV curable inkjet inks (INV-05 to INV- 08) exhibited a lower amount of extractables and a higher curing speed compared to the comparative example COMP- 04 to COMP-07.

Claims

[CLAIMS]

1. A co-initiator characterized in that said co-initiator has a structure according to Formula I,

Formula I

wherein

L represents a divalent linking group positioning the nitrogen atom of the aromatic amine and the nitrogen of at least one aliphatic amine of the structural moiety B in a 1-9 to 1-23 position; X represents O, S or NR7 ;

Rl, R2 , R3 and R4 independently represent a hydrogen or a substituent ;

(hetero) aralkyl group, an optionally substituted cycloalkyl group or an optionally substituted (hetero) aryl group;

R7 represents a hydrogen, an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted

(hetero) aralkyl group, an optionally substituted cycloalkyl group or an optionally substituted (hetero) aryl group; any two groups selected from R2 , R4 , R5 and R6 may represent the necessary atoms to form a ring; Rl and R2 , R3 and R4 may represent the necessary atoms to form a ring;

Rl or R3 may together with R7 represent the necessary atoms to form a ring; s R7 may represent the necessary atoms to form a ring with an atom of L;

R7 may represent the necessary atoms to form a ring with an atom of B; an atom of L may form a ring with an atom of B; o with the proviso that at least one aromatic and at least two aliphatic amine each have an alfa-hydrogen.

2. The co-initiator according to claim 1 wherein said co-initiator has a structure according Formula II,

Formula II

wherein o L and Rl to R6 have the same meaning as in Formula I;

Y represents 0, S or NRIl;

R8 to RlO independently represent an optionally substituted alkyl5 group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted

(hetero) aralkyl group, an optionally substituted cycloalkyl group ;

RIl represents a hydrogen, an optionally substituted alkyl group,0 an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero) aralkyl group, an optionally substituted cycloalkyl group or an optionally substituted (hetero) aryl group; any two groups selected from R8 to RlO may represent the necessary atoms to form a ring; Rl and R2 , R3 and R4 , R8 and RIl, Rl and RIl, R3 and RIl may represent the necessary atoms to form a ring;

R8 may represent the necessary atoms to form a ring with an atom of L or L' ;

R9 or RlO may represent the necessary atoms to form a ring with an atom of L' ;

3. The co-initiator according to any of the preceding claims wherein said co-initiator has a structure according Formula III,

Formula III

wherein

L, L', R5 , R6 and R8 to RlO have the same meaning as in Formula I or II; with the proviso that at least one aromatic and at least two aliphatic amine each have an alfa-hydrogen.

4. The co-initiator according to any of the preceding claims wherein said CO- initiator has a structure according Formula IV

Formula IV

wherein

L, R5 and R6 have the same meaning as in Formula I; L" and L" ' represent the necessary atoms to form with the two nitrogen atoms a 5 to 8 membered saturated heterocycle; R12 represent an optionally substituted alkyl group, an optionally substituted alkenyl group, an optionally substituted alkynyl group, an optionally substituted (hetero) aralkyl group, an optionally substituted cycloalkyl group; with the proviso that at least one aromatic and at least two aliphatic amine each have an alfa-hydrogen.

5. The co-initiator according to claim 1 wherein said structural moiety B comprises at least three aliphatic amines.

6. The co-initiator according to any of the preceding claims wherein R5 and R6 in Formula I to IV represent a methyl group.

7. A composition comprising the co-initiator according to any of the preceding claims.

8. The composition according to claim 7 wherein said composition is a coating composition used as a varnish, a lacquer or a primer.

9. The composition according to claim 7 wherein said composition is a UV curable inkjet ink.

10. The UV curable inkjet ink according to claim 9 further comprising a Type I or Type II photo- initiator .