The present invention relates to a printing ink, and in particular to an inkjet ink formulated without using NVC.

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 onto a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. The resulting image should be as high quality as possible.

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 100 mPas or less at 25°C, although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and most preferably between 7-1 1 mPas at the jetting temperature which is often elevated to, but not limited to 40-50°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 such as water or a low-boiling solvent or mixture of solvents.

Another type of inkjet ink contains unsaturated organic compounds, termed monomers and/or oligomers which polymerise by irradiation, 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.

Inks which cure by the polymerisation of monomers may contain a wide variety of monofunctional, difunctional and multifunctional monomers. The challenge is to provide the necessary printing properties, such as good adhesion and rapid curing, whilst providing a high-quality image, without compromising the jetting properties. These types of ink can have significant amounts of the monomer content replaced with acrylate oligomers or inert thermoplastic resins whose higher molecular weight leads to a reduction in the number of bonds that must be formed during the curing process. When each link is formed the bond length between the repeat units reduces leading to shrinkage of the cured film and unless this is controlled stress is imparted to the substrate. With plastic substrates this film shrinkage can lead to severe embrittlement of the printed article and post print finishing, such as guillotining, becomes problematic.

Traditionally UV ink-jet inks are formulated with difunctional and multifunctional acrylate monomers in order to achieve adequate cure speeds. Inks produced from these types of materials suffer badly from film shrinkage and consequent substrate embrittlement. Theoretically it should be possible to reduce shrinkage by use of wholly monofunctional acrylate or vinyl monomer based systems, however this approach has generally been avoided due to very low cure speeds associated with monofunctional monomers. N-Vinyl caprolactam (NVC) is commonly added to ink-jet inks to assist in achieving this balance of properties. NVC is a well-known monomer in the art and has the following chemical structure:

N-vinyl caprolactam (NVC), mol wt 139 g/mol

However, safety concerns have been raised in respect of NVC and precautions have to be taken during its use.

WO 2015/022228 relates to inkjet inks containing N-vinyl oxazolidinones instead of NVC and N- acryloylmorpholine (ACMO). WO 2015/022228 shows that some properties of inkjet inks containing N-vinyl oxazolidinones are superior to inkjet inks containing NVC and ACMO. However, there still remains a need for an inkjet ink that provides the required balance of properties, in particular an inkjet ink that provides a balance between cure speed and film shrinkage, without recourse to NVC.

Accordingly, the present invention provides an inkjet ink comprising: one or more (meth)acrylate monomers; an N-vinyl carbamate monomer; a dispersed pigment; and a radical photoinitiator, wherein the ink has a viscosity of less than 100 mPas at 25°C, and wherein the molar ratio of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer is from 1 .1 to 3.5. As explained hereinabove incorporating significant amounts of monofunctional monomers in inks has traditionally led to very poor UV cure response and hence multifunctional monomers have had to be added to boost cure. It has now been found that, at certain molar ratios, combinations of one or more (meth)acrylate monomers with an N-vinyl carbamate monomer provide a surprising synergistic effect, namely higher cure speeds are observed than for either of the component monomers when taken alone. This effect is particularly beneficial in ink-jet inks formulated with monofunctional monomers allowing cure speeds which are similar or even better than those observed with difunctional and even trifunctional (meth)acrylate monomer-based inks.

The invention relates to an inkjet ink based on an N-vinyl carbamate monomer. It has been found that this class of monomer is a useful replacement for NVC. It has similar printing properties, but has a more acceptable safety profile. N-Vinyl carbamate monomers are defined by the following functionality:

The synthesis of N-vinyl carbamate monomers is known in the art. For example, vinyl isocyanate, formed by the Curtius rearrangement of acryloyi azide, can be reacted with an alcohol to form N-vinyl carbamates (Phosgenations - A Handbook by L. Cotarca and H. Eckert, John Wiley & Sons, 2003, 4.3.2.8, pages 212-213).

In a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure:

in which R to R ⁴ 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 hydrogen, 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. Preferably R to R ⁴ are independently selected from hydrogen or C _{1 -10} alkyl. Further details may be found in WO 2015/022228 and US 4,831 ,153. Most preferably, the N-vinyl carbamate monomer is N-vinyl-5-methyl-2-oxazolidinone (NVMO). It is available from BASF and has the following structure: molecular weight 127 g/mol

NVMO has the lUPAC name 5-methyl-3-vinyl-1 ,3-oxazolidin-2-one and CAS number 3395-98-0. NVMO includes the racemate and both enantiomers. In one embodiment, the N-vinyl carbamate is a racemate of NVMO. In another embodiment, the N-vinyl carbamate is (R)-5-methyl-3-vinyl-1 ,3- oxazolidin-2-one. Alternatively, the N-vinyl carbamate is (S)-5-methyl-3-vinyl-1 ,3-oxazolidin-2-one.

The inkjet ink contains one or more (meth)acrylate monomers. An example is a monofunctional (meth)acrylate monomer. In one embodiment, the one or more (meth)acrylate monomers includes one or more monofunctional (meth)acrylate monomers. The monofunctional (meth)acrylate monomers may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid.

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 ^" .

Mixtures of (meth)acrylates may be used.

The ink preferably contains 10-90% by weight of the one or more monofunctional (meth)acrylate monomers in total, based on the total weight of the ink, and more preferably 30-85% by weight, based on the total weight of the ink. The substituents of the monofunctional (meth)acrylate 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 of the cyclic monofunctional (meth)acrylate monomer are typically cycloalkyi, aryl and combinations thereof, any of which may be interrupted by heteroatoms and/or substituted by alkyl. Non-limiting examples of substituents commonly used in the art include C _3-18 cycloalkyl, C ₆ . ₀ aryl and combinations thereof, any of which may substituted with alkyl (such as CMS alkyl) and/or 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.

Preferably, the cyclic monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA), 4-fe/f-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof. IBOA, PEA and MEDA are particularly preferred. PEA is most preferred and in one embodiment, the one or more (meth)acrylate monomers includes PEA. The preferred examples of cyclic monofunctional (meth)acrylate monomers have the following chemical structures:

Isobornyl acrylate (IBOA) Tetrahydrofurfuryl acrylate (THFA)

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

Phenoxyethyl acrylate (PEA) (2-Methyl-2-Ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA) mol wt 208.4 g/mol

Cyclic TMP formal acrylate (CTFA)

mol wt 200 g/mol

In a preferred embodiment, the inkjet ink comprises only cyclic monofunctional (meth)acrylate monomers as the monofunctional (meth)acrylate monomers present. IBOA and PEA are preferred examples. PEA is particularly preferred. In a preferred embodiment, the ink contains 10-90% by weight of cyclic monofunctional (meth)acrylate monomers, based on the total weight of the ink, and more preferably 30-85% by weight, based on the total weight of the ink.

The substituents of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is CMS alkyl, which may be interrupted by 1 -10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

Preferably, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C ₆ -C ₂₀ group. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer is selected from octa/decyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

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

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

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

The inkjet ink is preferably substantially free of N-vinyl amide monomers and N-acryloyl amines. 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. Typical 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 typical example is N-acryloylmorpholine (ACMO). By substantially free is meant preferably less than 2% by weight, more preferably less than 1 % by weight and most preferably less than 0.5% by weight.

The inkjet ink is preferably substantially free of NVC. It is also preferably substantially free of N-vinyl pyrrolidone (NVP). N-Acryloylmorpholine (ACMO) may be used, but in one embodiment, the ink is also substantially free of ACMO. By substantially free is meant preferably less than 2% by weight, more preferably less than 1 % by weight and most preferably less than 0.5% by weight. The amounts apply individually to these components, but in a preferred embodiment, the total amount of NVC, NVP and ACMO is less than these values.

The molar ratio of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer is from 1 .1 to 3.5. Preferably, the upper limit in the molar ratio is 3.0, more preferably 2.5, and most preferably 2.0. Preferably, the lower limit in the molar ratio is 1 .2, more preferably 1 .3, and most preferably 1 .4.

To calculate the molar ratio of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer, the double bond equivalent molecular weight and not the molecular weight of the molecule must be taken into account. For monofunctional (meth)acrylate monomers, the double bond equivalent molecular weight is the same as the molecular weight of the molecule. However, for difunctional and multifunctional (meth)acrylate monomers, the double bond equivalent molecular weight is calculated by dividing the molecular weight of the molecule by the number of double bonds in the molecule. For example, see Table 1 :

Table 1 . Calculation of double bond equivalent molecular weight

(Meth)acrylate and N-vinyl carbamate monomers, when combined, offer significant improvements in cure response over the non-combined monomers. All of the combinations exhibit a minimum in the dose of UV light required for cure when cure response is plotted against monomer blend composition. The depth of these minima is dependent on the individual cure speeds of the component monomers: the faster curing the acrylate monomers when taken alone, the deeper the minimum in UV dose required. Many blends exhibit faster cure speeds than DPGDA and in some cases speeds equivalent to TMPTA. These optimised blends provide significant advantages: high cure speeds can be achieved without compromising film properties such as flexibility, which is often sacrificed when multifunctional acrylates are employed. In particular, blends containing monofunctional (meth)acrylate monomers, for which the molar ratio of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer is from 1 .1 to 3.5 have an increased cure speed and are highly flexible, with low shrinkage and hence good adhesion. Such inks have significant performance advantages compared to inks containing only monofunctional (meth)acrylate monomers or inks containing monofunctional (meth)acrylate and N-vinyl carbamate monomers, wherein the molar ratio of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer is either below 1 .1 or above 3.5. When the molar ratio of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer is from 1 .1 to 3.5, it is possible to formulate inks based solely on monofunctional monomers that have cure speeds similar to those based on difunctional or multifunctional (meth)acrylate monomers and that also possess high film flexibility and adhesion to a wide range of plastic substrates.

In one embodiment, when the one or more (meth)acrylate monomers includes one or more monofunctional (meth)acrylate monomers, the molar ratio of the number of (meth)acrylate groups in the one or more monofunctional (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer is from 1.1 to 3.5. Preferably, the upper limit in the molar ratio is 3.0, more preferably 2.5, and most preferably 2.0. Preferably, the lower limit in the molar ratio is 1 .2, more preferably 1 .3, and most preferably 1 .4.

However, the increase in cure speed by controlling the molar ratio of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer can also be seen for blends containing difunctional and multifunctional (meth)acrylate monomers.

The inkjet ink may also contain difunctional and multifunctional (meth)acrylate monomers. If present, they may be present in the same amounts as the monofunctional (meth)acrylate monomers. Although, in a preferred embodiment, the total amount of difunctional and multifunctional (meth)acrylate monomers is 5 to 25% by weight, based on the total weight of the ink. In one embodiment, the one or more (meth)acrylate monomers includes one or more difunctional (meth)acrylate monomers.

Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Difunctional has its standard meaning, i.e. two groups, which take part in the polymerisation reaction on curing.

Examples include hexanediol diacrylate, 1 ,8-octanediol diacrylate, 1 ,9-nonanediol diacrylate, 1 ,10- decanediol diacrylate (DDDA), 1 ,1 1 -undecanediol diacrylate and 1 ,12-dodecanediol diacrylate, polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), 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. Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9-nonanediol dimethacrylate, 1 ,10-decanediol dimethacrylate, 1 ,1 1 -undecanediol dimethacrylate and 1 ,12-dodecanediol dimethacrylate. triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate and mixtures thereof. A preferred difunctional (meth)acrylate monomer is DPGDA. In one embodiment, the amount of difunctional (meth)acrylate monomers present in the ink is restricted. Preferably, the amount of difunctional (meth)acrylate monomers present in the ink is less than 10% by weight, more preferably less than 5% by weight, based on the total weight of the ink.

It is possible to modify further the film properties of the ink-jet inks by inclusion of multifunctional monomers, oligomers or inert resins, such as thermoplastic acrylics. However, it should be noted that in the case of oligomers and multifunctional monomers the flexibility may be adversely affected and also that some adjustments to stoichiometry may be required to retain optimum cure speed.

In one embodiment, the one or more (meth)acrylate monomers includes one or more multifunctional (meth)acrylate monomers.

Multifunctional (meth)acrylate monomers (tri- and higher-functional) are also 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. Usually, the multifunctional (meth)acrylate monomer has a degree of functionality of four or more, e.g. 4-8.

Examples of the multifunctional acrylate monomers include trimethylolpropane triacrylate, pentaerythritol 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. A preferred multifunctional (meth)acrylate monomer is trimethylol propane triacrylate.

In one embodiment the ink is substantially free of multifunctional monomer, meaning that only trace amounts will be present, for example as impurities in the monofunctional material or as a component in a commercially available pigment dispersion. Where multifunctional monomer is included, the multifunctional monomer is present in an amount of no more than 15 wt%, preferably no more than 10 wt%, more preferably no more than 7 wt%, more preferably no more than 5 wt% and most preferably no more than 2 wt% based on the total weight of the ink. The multifunctional monomer which is limited in amount may be any multifunctional monomer which could be involved in the curing reaction, such as a multifunctional (meth)acrylate monomer or a multifunctional vinyl ether. For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and difunctional are intended to have their standard meanings, i.e. one or two groups, respectively, which take part in the polymerisation reaction on curing. Multifunctional (which does not include difunctional) is intended to have its standard meanings, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

In a preferred embodiment, the (meth)acrylate monomers are the only source of acrylate functionality in the ink.

The inkjet ink is therefore preferably substantially free of radiation-curable (i.e. polymerisable) oligomer, particularly acrylate oligomer.

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 typically has a molecular weight of at least 450 Da and more typically at least 600 Da (whereas monomers typically have a molecular weight below these values). The molecular weight is typically 4,000 Da 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 is typically multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is typically from 2 to 6. Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide film- forming properties such as hardness or cure speed. They therefore typically have a viscosity of 150 mPas or above at 25°C, for example 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 one or more radiation-curable groups. The oligomer typically comprises a polyester backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation, and is usually (meth)acrylate.

Typical radiation-curable oligomers are polyester acrylate oligomers as these have excellent adhesion and elongation properties, e.g. di-, tri-, tetra-, penta- or hexa-functional polyester acrylates. By substantially free is meant preferably less than 2% by weight, more preferably less than 1 % by weight and most preferably less than 0.5% by weight.

The inkjet ink of the present invention may also contain a passive resin. Passive (or "inert") 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.

The total amount of the passive resin is preferably from 0.1 -15% by weight, more preferably 0.2-10% by weight, based on the total weight of the ink. The passive resin in the inkjet ink helps to improve the adhesion of the inkjet ink onto a polypropylene or corona-treated polystyrene substrate.

In an embodiment the ink is substantially free of oligomeric and polymeric material meaning that only trace amounts will be present. The total amount is preferably, less than 2% by weight and more preferably less than 1 % by weight and most preferably less than 0.5% by weight.

The inkjet ink of the present invention comprises a radical photoinitiator. The radical photoinitiator is a free radical photoinitiator. The radical photoinitiator 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, bis(2,6-dimethylbenzoyl)-2,4,4- trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure and Darocur (from Ciba) and Lucirin (from BASF). Preferred photoinitiators are selected from bis(2,6-dimethylbenzoyl)-2,4,4- trimethylpentylphosphine oxide, 1 -hydroxycyclohexyl phenyl ketone and mixtures thereof.

Preferably, the photoinitiator is present in an amount of 1 -20% by weight, preferably 1 -15% by weight, based on the total weight of the ink. Mixtures of radical photoinitiators can be used and preferably, the ink comprises a plurality of radical photoinitiators. The total number of radical photoinitiators present is preferably from one to six, and more preferably, two or more radical photoinitiators are present in the ink.

The inkjet ink dries primarily by curing, i.e. by the polymerisation of the monomers and oligomers 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 is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink comprises less than 5% by weight combined 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 inkjet ink also comprises a dispersed pigment, of the types known in the art and commercially available such as 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.

In one aspect the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7.

Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 μηι, preferably less than 5 μηι, more preferably less than 1 μηι and particularly preferably less than 0.5 μηι.

The colorant is preferably present in an amount of 0.2-20% by weight, preferably 0.5-10% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink

The amounts by weight provided herein are based on the total weight of the ink. The inkjet ink exhibits a desirable low viscosity (100 mPas or less, more preferably 50 mPas or less at 25°C).

In order to produce a high quality printed image a small jetted drop size is desirable, particularly for high resolution images. Preferably the inkjet ink of the invention is jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 20 picolitres.

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.

The surface tension of the inkjet ink may be controlled by the addition of one or more surface active materials such as commercially available surfactants. Surfactants are well known in the art and a detailed description is not required. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example, plastic substrates. Too high a surface tension can lead to ink pooling and/or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks. The surface tension is preferably in the range of 20-40 mNm ^" and more preferably 21 -32 mNm ^" .

Other components of types known in the art may be present in the ink to improve the properties or performance. These components may be, for example, defoamers, dispersants, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

The present invention may also provide an inkjet ink set wherein at least one of the inks in the set is an inkjet ink of the present invention. Preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention. Usually, the inkjet ink set of the present invention is in the form of a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. The inks in a trichromatic set can be used to produce a wide range of colours and tones.

The ink or inkjet ink sets may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill. A suitable printer is a flatbed UV printer, for example from the Onset series from Inca Digital.

The substrate is not limited. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, and cellulosic materials. Mixtures/blends are included.

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.

Any of the sources of actinic radiation discussed herein may be used for the irradiation of the inkjet ink. A suitable dose would be greater than 200 mJ/cm ² , more preferably at least 300 mJ/cm ² and most preferably at least 500 mJ/cm ² . The upper limit is less relevant and will be limited only by the commercial factor that more powerful radiation sources increase cost. A typical upper limit would be 5 J/cm ² . Further details of the printing and curing process are provided in WO 2012/1 10815.

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

Example 1 A cyan ink-jet ink formulation of the present invention is prepared by combining the following components:

Pigment dispersion * 4.53

N-Vinyl-5-methyl-2-oxazolidinone 30.70

Phenoxyethyl acrylate 51 .16

Firstcure ST-1 0.8

Irgacure 184 1 .88

Acyl phosphine oxide 8.01

Benzophenone 2.82

Byk 307 0.1

*Pigment dispersion:

SOLSPERSE 32000 10.00

FIRSTCURE ST - 1 1 .00

SARTOMERSR 9003 59.00 (propoxylated NPGDA - difunctional)

IRGALITE BLUE GLVO 30.00

The ink is drawn down onto to 220 micron gloss PVC using a 12 micron K bar applicator. The film is cured using a Svecia UV drier fitted with two independently switchable 80 W/cm lamps.

The ink has good printing and curing properties and an acceptable safety profile.

Example 2

Inkjet inks were prepared according to the formulations as set out in Table 2. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink. Table 2

The cyan pigment dispersion of the inks of Table 2 comprises 59% PEA, 1 % stabiliser, 10% dispersant and 30% blue pigment. The dispersion was prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns. Amounts are given as weight percentages based on the total weight of the dispersion.

PEA is a monofunctional (meth)acrylate monomer. NVMO is an N-vinyl carbamate monomer. UV12 is a stabiliser. Irgacure 184, TPO and benzophenone are photoinitiators. Byk307 is a surfactant.

Inks 1 and 2 were then blended in the ratios given in Table 3. The molar ratios of the number of (meth)acrylate groups in the one or more (meth)acrylate monomers to the number of vinyl groups in the N-vinyl carbamate monomer in the resultant ink blends is also given in Table 3.

The ink blends were drawn down onto to 220 micron gloss PVC using a 12 micron K bar applicator. The films were cured using a Jenton UV drier fitted with one 80 W/cm lamp at 100% power using a belt speed of 25 m/min. The ink blends were cured using 1 -4 passes and the number of passes used for each ink blend is shown in Table 3. The cure for each ink blend was then assessed and the results are shown in Table 3.

Table 3

As can be seen from Table 3, a molar ratio of the number of acrylate groups in PEA to the number of vinyl groups in NVMO of 0.66 or less results in an incompletely cured ink, even when 2-4 passes are used. Similarly, a molar ratio of the number of acrylate groups in PEA to the number of vinyl groups in NVMO of 5.50 or more results in an incompletely cured ink, even when two passes are used. In contrast, an ink of the present invention undergoes complete curing using just a single pass.

Example 3

An ink of the invention is prepared having the parts by weight of PEA and NVMO, based on the total weight of the ink, given in Table 4. Table 4 shows the calculation of the molar ratio of the number of acrylate groups in PEA to the number of vinyl groups in NVMO. Table 4

The ink has good printing and curing properties and an acceptable safety profile. Example 4

An ink of the invention is prepared having the parts by weight of DPGDA and NVMO, based on the total weight of the ink, given in Table 5. Table 5 shows the calculation of the molar ratio of the number of acrylate groups in DPGDA to the number of vinyl groups in NVMO.

Table 5

The ink has good printing and curing properties and an acceptable safety profile.