The present invention relates to a printing ink and in particular, an inkjet ink which can achieve full surface cure via exposure to UV radiation from a UV LED light source. The present invention also relates to a method of printing said ink.

In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have, in use, a low viscosity, typically below 100 mPas at 25°C (although in most applications the viscosity should be below 50 mPas, and often below 25 mPas). Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature, which is often elevated to about 40°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent. In one common type of inkjet ink, this liquid is water - see for example the paper by Henry R. Kang in the Journal of Imaging Science, 35(3), pp. 179-188 (1991). In those systems, great effort must be made to ensure the inks do not dry in the head due to water evaporation. In another common type, the liquid is a low-boiling solvent or mixture of solvents - see, for example, EP 0 314 403 and EP 0 424 714. Unfortunately, inkjet inks that include a large proportion of water or solvent cannot be handled after printing until the inks have dried, either by evaporation of the solvent or its absorption into the substrate. This drying process is often slow and in many cases (for example, when printing on to a heat-sensitive substrate such as paper) cannot be accelerated. Another type of inkjet ink contains radiation-curable material, such as radiation-curable monomers, which polymerise by irradiation with actinic radiation, commonly with ultraviolet light, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures.

There are a number of sources of actinic radiation which are commonly used to cure inkjet inks which contain radiation-curable material. The most common source of radiation is a UV source. UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. Mercury discharge lamps, fluorescent tubes and flash lamps are most commonly used as the radiation source as they generate enough power to thoroughly cure the radiation-curable ink and hence achieve adequate through cure and surface cure. Although these radiation sources have several drawbacks in their operational characteristics, no other UV light source has yet managed to challenge their position in terms of UV output performance.

LED UV light sources are an attractive alternative. In particular, when compared to, for example mercury discharge lamps (the most common UV light source used to cure inkjet inks), LEDs offer significant cost reduction, longer maintenance intervals, higher energy efficiency and are an ecologically friendlier solution. However, when LEDs are used, it is necessary to use an array of multiple LEDs in order to generate enough power to provide thorough curing of the ink. In fact, even with an array of multiple LEDs, inks which are cured by LEDs are prone to poor surface cure owing to the presence of oxygen in the atmosphere adjacent to the ink surface. This effect can be overcome by blanketing the irradiated area with an inert gas such as nitrogen during the cure process but, again, this adds considerably to the complexity and cost of the printer. There is also a danger of asphyxiation if the nitrogen builds up in the vicinity of the print machine.

There is therefore a need in the art for an inkjet ink which can undergo thorough curing, including full through and surface cure using a UV LED light source as the source of actinic radiation to cure the inkjet ink, without recourse to an inert atmosphere during the curing process.

Accordingly, the present invention provides an inkjet ink comprising: at least 30% by weight of a radiation-curable monomer having two or more functional groups, based on the total weight of the ink; 1 -20% by weight of a radiation-curable, amine-modified oligomer having two or more functional groups, based on the total weight of the ink; two or more photoinitiators; and a colouring agent; wherein at least 10% by weight of the radiation-curable monomer having two or more functional groups, based on the total weight of the ink, has at least one vinyl ether functional group. The inventors have surprisingly found that an inkjet ink that comprises the present blend of components in the specifically claimed amounts can achieve a thorough cure, including full surface cure, when exposed to a UV LED light source, without recourse to an inert atmosphere at the ink surface during curing. In particular, it has been found that the claimed radiation-curable monomer having two or more functional groups, where at least 10% by weight of the radiation- curable monomer having two or more functional groups, based on the total weight of the ink, has at least one vinyl ether functional group, in combination with the radiation-curable, amine- modified oligomer having two or more functional groups can achieve such advantages.

Without wishing to be bound by theory, the inventors have found that the claimed radiation- curable monomer having two or more functional groups, where at least 10% by weight of the radiation-curable monomer having two or more functional groups, based on the total weight of the ink, has at least one vinyl ether functional group, allows for the inclusion of the radiation-curable, amine-modified oligomer having two or more functional groups, which prevents oxygen inhibition at the surface of the printed ink film, allowing for full surface cure whilst using an UV LED light source to cure the ink and maintaining the required viscosity for an inkjet ink.

The present invention also provides a method of inkjet printing comprising inkjet printing the inkjet ink of the present invention onto a substrate and curing the ink by exposing the printed ink to a UV radiation source, and in particular, a UV LED radiation source.

The inkjet ink of the present invention comprises at least 30% by weight of a radiation-curable monomer having two or more functional groups, based on the total weight of the ink. In a preferred embodiment, the ink comprises at least 40% by weight, preferably at least 50% by weight, of a radiation-curable monomer having two or more functional groups, based on the total weight of the ink.

As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers. In the inkjet ink of the present invention, there must be at least 30% by weight of a radiation-curable monomer having two or more functional groups, based on the total weight of the ink. Radiation-curable monomer having two or more functional groups has its standard meaning, i.e. di or higher, that is two or more groups, respectively, which take part in the polymerisation reaction on curing.

In a preferred embodiment, the radiation-curable monomer having two or more functional groups is a di-, tri-, tetra-, penta- or hexa- functional monomer, i.e. the radiation curable monomer has two, three, four, five or six functional groups.

The functional group of the radiation-curable monomer having two or more functional groups, which is utilised in the ink of the present invention may be the same or different but must take part in the polymerisation reaction on curing. Examples of such functional groups include any groups that are capable of polymerising upon exposure to radiation and are preferably selected from a (meth)acrylate group and a vinyl ether group.

The radiation-curable monomer having two or more functional groups may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality monomers may be used. In a preferred embodiment of the invention, the radiation-curable monomer having two or more functional groups comprises a difunctional monomer, a trifunctional monomer and a hexafunctional monomer. Examples of the radiation-curable monomer having two or more functional groups include difunctional (meth)acrylate monomers, multifunctional (meth)acrylate monomers, divinyl ether monomers and vinyl ether (meth)acrylate monomers. Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Preferred examples include hexanediol diacrylate (HDDA), polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, neopentylglycol diacrylate, 3-methyl pentanediol diacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, and mixtures thereof.

In addition, suitable difunctional methacrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate and mixtures thereof.

Preferably, the difunctional (meth)acrylate monomer is selected from hexanediol diacrylate, propoxylated neopentyl glycol diacrylate, dipropylene glycol diacrylate, and mixtures thereof. Preferably, the ink of the present invention comprises 5-35% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink.

Multifunctional (which do not include difunctional) are well known in the art and a detailed description is therefore not required. Multifunctional has its standard meaning, i.e. tri or higher, that is three or more groups, respectively, which take part in the polymerisation reaction on curing.

In a preferred embodiment of the ink of the present invention, at least 5% by weight of the radiation-curable monomer having two or more functional groups, based on the total weight of the ink, has three or more functional groups. Put another way, at least 5% by weight of the radiation- curable monomer having two or more functional groups, based on the total weight of the ink, is a multifunctional monomer, which has three or more functional groups, which take part in the polymerisation reaction on curing. The substituents of the multifunctional monomers are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyi, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CMS alkyl, C _3-18 cycloalkyi, C ₆ . ₀ aryl and combinations thereof, such as C ₆ . ₀ aryl- or C _3-18 cycloalkyl- substituted CMS alkyl, any of which may be interrupted by 1 -1 0 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure. (The same groups may also be used for difunctional monomers.) Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet inks include trimethylolpropane triacrylate, dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate, and mixtures thereof. Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as trimethylolpropane trimethacrylate. Mixtures of (meth)acrylates may also be used. Preferably, the ink of the present invention comprises 5-25% by weight of a multifunctional (meth)acrylate monomer, based on the total weight of the ink.

In the inkjet ink of the present invention, at least 10% by weight of the radiation-curable monomer having two or more functional groups, based on the total weight of the ink, has at least one vinyl ether functional group. The inventors have found that the inclusion of such a monomer allows for the inclusion of the radiation-curable, amine-modified oligomer having two or more functional groups into the inkjet ink, whilst maintaining the required low viscosity for an inkjet ink.

Examples are well known in the art and include vinyl ethers such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1 ,4-cyclohexanedimethanol divinyl ether and 2-(2- vinyloxyethoxy)ethyl acrylate, bis[4-(vinyloxy)butyl] 1 ,6-hexanediylbiscarbamate, bis[4- (vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi-4,1 -phenylene), bis[4- (vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4-

(vinyloxymethyl)cyclohexylmethyl] glutarate, 1 ,4-butanediol divinyl ether, 1 ,4-butanediol vinyl ether, butyl vinyl ether, tert-butyl vinyl ether, 2-chloroethyl vinyl ether, 1 ,4-cyclohexanedimethanol divinyl ether, cyclohexyl vinyl ether, di(ethylene glycol) vinyl ether, diethyl vinyl orthoformate, dodecyl vinyl ether, ethylene glycol vinyl ether, 2-ethylhexyl vinyl ether, ethyl-1 -propenyl ether, ethyl vinyl ether, isobutyl vinyl ether, phenyl vinyl ether, propyl vinyl ether, and tris[4- (vinyloxy)butyl] trimellitate.

In a preferred embodiment, at least 10% by weight of the radiation-curable monomer having two or more functional groups, based on the total weight of the ink, has at least one vinyl ether functional group and has a viscosity of 5 mPas or less at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 25°C with a shear rate of 25 s . (Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate.

Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. Monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. They therefore preferably have a viscosity of less than 150 mPas at 25°C, more preferably less than l OOmPas at 25°C and most preferably less than 20 mPas at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 25°C with a shear rate of 25 s ^" .

The inkjet ink of the present invention further comprises 1 -20% by weight of a radiation-curable, amine-modified oligomer having two or more functional groups, based on the total weight of the ink. In a preferred embodiment, the ink comprises 1 -10% by weight, preferably 1 .0-4.0% by weight, of a radiation-curable, amine-modified oligomer having two or more functional groups, based on the total weight of the ink. The inventors have surprisingly found that the inclusion of such an oligomer into the ink of the invention, in combination with the other components of the ink, allows for full surface cure of the inkjet ink using a UV LED light source, without recourse to an inert environment during curing. Without wishing to be bound by theory, the oligomer prevents oxygen inhibition at the surface of the ink film, which allows for full cure, including surface cure, using a UV LED light source. This is particularly the case for low film weight prints, e.g. 6-8 μητ It has also been found that the inclusion of such an amine-modified oligomer permits surface cure, whilst maintaining adhesion of the printed film to the substrate.

Radiation-curable (i.e. polymerisable), amine-modified oligomers having two or more functional groups are well known in the art and a detailed description is hence not required.

The term "curable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation.

The oligomer preferably has a molecular weight of at least 450 and preferably at least 600 (whereas monomers typically have a molecular weight below these values). The molecular weight is preferably 4,000 or less. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer has two or more functional groups and is hence multifunctional, meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 6. Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Preferred oligomers for inclusion in the ink of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25 s .

Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and two or more radiation-curable groups. The oligomer preferably comprises a polyester backbone. The polymerisable groups can be any group that are capable of polymerising upon exposure to radiation. The functional groups are as discussed hereinabove for the radiation-curable monomer having two or more functional groups. Preferably the oligomers are (meth)acrylate oligomers. Particularly preferred radiation-curable oligomers are polyester acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are di-, tri-, tetra-, penta- or hexa- functional polyester acrylates, as these yield films with good solvent resistance.

More preferably, the radiation-curable oligomer is an amine-modified polyester acrylate oligomer. Such a radiation-curable oligomer is commercially available as Ebecryl 80.

In one embodiment the radiation-curable oligomer polymerises by free-radical polymerisation. Preferably, the radiation-curable oligomer cures upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film.

The inkjet ink of the present invention further comprises two or more photoinitiators.

The ink of the present invention preferably comprises free radical photoinitiators. Free radical photoinitiators can be selected from any of those known in the art. For example, benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 1 -[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1 - propane-1 -one, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1 -one, isopropyl thioxanthone, benzil dimethylketal, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, bis(2,4,6- trimethylbenzoyl)-phenylphosphine oxide and bis(2,6-dimethylbenzoyl)-2,4,4- trimethylpentylphosphine oxide. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure, Darocur and Lucirin (from BASF).

In a preferred embodiment, the photoinitiator present in the ink used in the method of the present invention is tailored for UV LED light. By tailored for UV LED light, it is meant that the photoinitiators absorb the radiation which is emitted by the UV LED light source. Preferably, the two or more photoinitiators present in the ink of the present invention absorbs radiation in a region of from 360 nm to 410 nm and absorbs sufficient radiation to cure the ink within a 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less bandwidth.

In a preferred embodiment, the two or more photoinitiators comprises a phosphine oxide photoinitiator, such as TPO and BAPO, and a thioxanthone photoinitiator, such as ITX. In a particularly preferred embodiment, the two or more photoinitiators comprises isopropyl thioxanthone (ITX) and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO).

In a preferred embodiment, the amount of phosphine oxide photoinitiator present in the ink is 2- 20% by weight and the amount of thioxanthone photoinitiator is 0.5-10% by weight, based on the total weight of the ink.

In a further preferred embodiment, the inkjet ink comprises three or more photoinitiators, and in a particularly preferred embodiment, the inkjet ink comprises three or more photoinitiators comprising isopropyl thioxanthone (ITX), 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO) and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide (Irgacure 819).

In a preferred embodiment, the two or more photoinitiators comprises a phosphine oxide photoinitiator and a benzophenone photoinitiator. In a further preferred embodiment, the two or more photoinitiators comprise 2-20% by weight, preferably 5-15% by weight, of a phosphine oxide photoinitiator, based on the total weight of the ink, and 1 -10% by weight, preferably 2-8% by weight, of a benzophenone photoinitiator.

Preferably, the total amount of photoinitiator present in the ink used in the method of the present invention is 1 -20% by weight, based on the total weight of the ink.

The inkjet ink of the present invention further comprises a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. The colouring agent can be any of a wide range of suitable colouring agents that would be known to the person skilled in the art. Preferably the colouring agent is a dispersible pigment, of the types known in the art and commercially available such as, for example, under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used. The total proportion of pigment present is preferably from 0.5 to 15% by weight, more preferably from 1 to 6% by weight, based on the total weight of the ink.

In a preferred embodiment, the ink of the present invention comprises a black colouring agent, and preferably a black pigment. The black pigment is dispersed in the liquid medium of the ink and is typically in the form of a powdered black pigment. A preferred black pigment is carbon black, more specifically MOGUL E available from Cabot Corporation. In a preferred embodiment, the ink comprises 1 -6% by weight of the black pigment, based on the total weight of the ink. The inkjet ink of the present invention may further comprise a monofunctional monomer, such as a monofunctional (meth)acrylate monomer. Monofunctional monomers are well known in the art and a detailed description is not required. A radiation-curable monofunctional monomer has one functional group, which takes part in the polymerisation reaction on curing. The polymerisable groups can be any group that are capable of polymerising upon exposure to radiation. The functional group is as discussed hereinabove for the radiation-curable monomer having two or more functional groups.

The substituents of the monofunctional monomers are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyi, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CMS alkyl, C _3-18 cycloalkyi, C ₆ . ₀ aryl and combinations thereof, such as C ₆ . ₀ aryl- or C _3-18 cycloalkyl- substituted CMS alkyl, any of which may be interrupted by 1 -10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

The amount of monofunctional monomers is not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. In a preferred embodiment, the ink of the present invention is substantially free of monofunctional monomers. Preferably, the inkjet ink comprises less than 5% by weight of monofunctional monomers, preferably less than 3% by weight, more preferably, less than 2% by weight and most preferably less than 1 % by weight, based on the total weight of the ink.

Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required.

Preferred examples include cyclic monofunctional (meth)acrylate monomers and acyclic- hydrocarbon monofunctional (meth)acrylate monomers. For example, phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 2-(2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

The preferred examples of monofunctional (meth)acrylate monomers have the following chemical structures:

Cyclic TMP formal acrylate (CTFA) Phenoxyethyl acrylate (PEA)

mol wt 192 g/mol

Isobornyl acrylate (IBOA) Tetrahydrofurfuryl acrylate (THFA)

mol wt 208 g/mol mol wt 156 g/mol

2-(2-Ethoxyethoxy)ethyl acrylate mol wt 188 g/mol

Octadecyl acrylate (ODA) Tridecyl acrylate (TDA)

mol wt 200 g/mol mol 254 g/mol

Isodecyl acrylate (IDA) Lauryl acrylate

mol wt 212 g/mol mol wt 240 g/mol The ink used in the method of the present invention may further comprise an α,β-unsaturated ether monofunctional monomer, which can polymerise by free-radical polymerisation. Examples are well known in the art and include vinyl ethers such as ethylene glycol monovinyl ether. The inkjet ink of the present invention may further comprise an N-vinyl amide and/or an N-acryloyl amine monomer.

N-Vinyl amides are well-known monomers in the art and a detailed description is therefore not required. N-vinyl amides have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP). Similarly, N-acryloyl amines are also well-known in the art. N-acryloyl amines also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO).

The ink of the present invention may further comprise radiation-curable (i.e. polymerisable) oligomers other than the radiation-curable, amine-modified oligomer having two or more functional groups present in the ink. For example, the ink may comprise a non-amine modified oligomer and/or a radiation-curable oligomer which has a degree of functionality of 1 . We would refer to our discussion hereinabove with respect to the radiation-curable, amine-modified oligomer having two or more functional groups as this applies to such other oligomers.

The ink of the present invention may also include radiation-curable material, which is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The radiation-curable material according to this embodiment may comprise a mixture of cationically curable monomer and oligomer. For example, the radiation-curable material may comprise a mixture of an epoxide oligomer and an oxetane monomer. In the embodiment where the ink comprises radiation-curable material, which polymerises by cationic polymerisation, the ink must also comprise a cationic photoinitiator.

In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulfonium or iodonium based systems. Non limiting examples include: Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa.

Preferably however, the ink of the present invention cures by free radical polymerisation only and hence the ink is substantially free of radiation-curable material, which polymerises by cationic polymerisation. The inkjet ink of the present invention dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. The absence of water and volatile organic solvents means that the ink does not need to be dried to remove the water/solvent. However, water and volatile organic solvents have a significant viscosity-lowering effect making formulation of the ink in the absence of such components significantly more challenging. Accordingly, the inkjet ink of the present invention is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink comprises less than 5% by weight of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2% by weight combined and most preferably less than 1 % by weight combined, based on the total weight of the ink. Some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated.

The inks of the present invention may comprise a passive (or "inert") thermoplastic resin. Passive resins are resins which do not enter into the curing process, i.e. the resin is free of functional groups which polymerise under the curing conditions to which the ink is exposed. In other words, resin is not a radiation-curable material. The resin may be selected from epoxy, polyester, vinyl, ketone, nitrocellulose, phenoxy or acrylate resins, or a mixture thereof and is preferably a poly(methyl (meth)acrylate) resin. The resin has a weight-average molecular weight of 70-200 KDa and preferably 100-150 KDa, as determined by GPC with polystyrene standards. A particularly preferred resin is Paraloid® A1 1 from Rohm and Haas. The resin is preferably present at 1 -5% by weight, based on the total weight of the ink.

The inkjet ink of the present invention exhibits a desirable low viscosity (200 mPas or less, preferably 100 mPas or less, more preferably 25 mPas or less and most preferably 15 mPas or less at 25 °C). The inks of the invention are thus capable of being printed and cured at line speeds in excess of 150 m/min, preferably in excess of 170 m/min up to 300 m/min.

In order to produce a high quality printed image a small jetted drop size is desirable. Furthermore, small droplets have a higher surface area to volume ratio when compared to larger drop sizes, which facilitates evaporation of solvent from the jetted ink. Small drop sizes therefore offer advantages in drying speed. Preferably the inkjet ink is jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 10 picolitres. To achieve compatibility with print heads that are capable of jetting drop sizes of 50 picolitres or less, a low viscosity ink is required. A viscosity range of 10 to 20 mPas at 25°C is preferred, more preferably 12 to 18 mPas at 25°C and most preferably 14 to 16 mPas at 25°C. Ink viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

Other components of types known in the art may be present in the ink of the present invention to improve the properties or performance. These components may be, for example, additional surfactants, defoamers, dispersants, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers. In a preferred embodiment, photosensitisers are added to the ink, which are selected to absorb strongly in the desired wavelength band of UV LED radiation source and are able to transfer energy to the photoinitiators of the ink.

Print heads account for a significant portion of the cost of an entry level printer and it is therefore desirable to keep the number of print heads (and therefore the number of inks in the ink set) low. Reducing the number of print heads can reduce print quality and productivity. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity.

The inkjet ink may be prepared by known methods such as stirring with a high-speed water- cooled stirrer, or milling on a horizontal bead-mill. The present invention also provides a method of inkjet printing the inkjet ink of the present invention. Specifically, the present invention provides a method of inkjet printing comprising inkjet printing the inkjet ink of the present invention onto a substrate and curing the ink by exposing the printed ink to a UV radiation source, which is preferably a UV LED radiation source. The inventors have surprisingly found that the ink of the present invention is particularly suitable as an ink which can be cured using a UV LED light source, whilst maintaining low viscosity, full cure, including surface cure, without recourse to an inert environment at the ink surface during curing. It is therefore possible to achieve the numerous advantages of LEDs, without having poor surface cure and/or the necessity for an inert environment during curing. In the method of inkjet printing of the present invention, the inkjet ink is printed onto a substrate. Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto a substrate, on a roll-to-roll printer or a flat-bed printer. As discussed above, inkjet printing is well known in the art and a detailed description is not required. The ink is jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image. The substrate is not limited. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, and all cellulosic materials or their mixtures/blends with the aforementioned synthetic materials.

A specific example of this is for printing QR codes and barcodes, e.g. for identification, coding or pricing information on packaging. In the method of the present invention, after inkjet printing the inkjet ink onto the substrate, the printed image is then exposed to a UV radiation source, preferably UV LED light, to cure the inkjet ink.

Any suitable radiation source may be used. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. In a preferred embodiment, a UV LED light source is used to cure the ink.

UV LED light is emitted from a UV LED light source. UV LED light sources comprise one or more LEDs and are well known in the art. Thus, a detailed description is not required.

It will be understood that UV LED light sources emit radiation having a spread of wavelengths. The emission of UV LED light sources is identified by the wavelength which corresponds to the peak in the wavelength distribution. Compared to conventional mercury lamp UV sources, UV LED light sources emit UV radiation over a narrow range of wavelengths on the wavelength distribution. The width of the range of wavelengths on the wavelength distribution is called a wavelength band. LEDs therefore have a narrow wavelength output when compared to other sources of UV radiation. By a narrow wavelength band, it is meant that at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength within a wavelength band having a width of 50 nm or less, preferably, 30 nm or less, most preferably 15 nm or less.

In a preferred embodiment, at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

Preferably, the wavelength of the UV LED source substantially matches the absorption profile of the ink. In a preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 360 nm to 410 nm. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm. The ink used in the method of the present invention has been specifically formulated to respond to the emission of the UV LED source.

In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 360 nm to 410 nm, and at least 90%, preferably at least 95%, of the radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm, and at least 90%, preferably at least 95%, of the radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

LEDs have a longer lifetime and exhibit no change in the power/wavelength output over time. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate.

Upon exposure to a radiation source, the ink cures to form a relatively thin polymerised film. The ink of the present invention typically produces a printed film having a thickness of 1 to 20 μηι, preferably 1 to 10 μηι, for example 2 to 5 μηι. Film thicknesses can be measured using a confocal laser scanning microscope.

The exposure to UV LED light may be performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink, although this is not required to achieve full cure, including surface cure owing to the components present in the ink of the present invention. The present invention also provides a cartridge containing the inkjet ink as defined herein. It also provides a printed substrate having the ink as defined herein printed thereon.

The invention will now be described with reference to the following examples, which are not intended to be limiting.

Examples

Example 1 An inkjet ink was prepared according to the formulation set out in Table 1 . The inkjet ink formulation was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1

The black pigment dispersion of ink 1 was prepared according to the formulation set out in Table 2. The dispersion was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the dispersion.

Table 2

The viscosity of ink was 14.3 mPas at 25°C. Example 2

An inkjet ink was prepared according to the formulation set out in Table 3. The inkjet ink formulation was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 3

The black pigment dispersion of ink 2 was prepared according to the formulation set out in Table 4. The dispersion was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the dispersion.

Table 4

The viscosity of ink was 17 mPas at 25°C. Example 3 (comparative)

An inkjet ink was prepared according to the formulation set out in Table 5. The inkjet ink formulation was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink. Table 5

Example 4

Inks 1 and 2 were drawn down on to 220 micron gloss PVC substrate using a no. 2 Kbar, depositing a 12 micron wet film. The cure was assessed on a conveyorised drier at a belt speed of 90 metres/minute using a UV LED light source. The UV dose and peak intensity to achieve full surface cure without the need for an inert atmosphere were noted. Full surface cure produced a film with no surface tackiness. The results are set out in Table 6.

Table 6

Inks 1 and 2 only required a single pass to achieve full surface cure. In comparison, ink 3 required two passes using the curing conditions of Table 6 to achieve full surface cure. These results show that the ink of the present invention enables full surface cure of low film weight prints without the need for an inert atmosphere.