WO2014030015A1

WO2014030015A1 - A printing ink

Info

Publication number: WO2014030015A1
Application number: PCT/GB2013/052234
Authority: WO
Inventors: Ayesha RASHEED; Angelique Runacre
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 2012-08-23
Filing date: 2013-08-23
Publication date: 2014-02-27
Anticipated expiration: 2015-02-23
Also published as: GB201504818D0; GB201215062D0; GB2520465A; GB2520465B

Description

A Printing Ink

This invention relates to a printing ink, in particular to an ink which achieves a balance between cure speed at low dose of radiation and avoiding (or minimising) blocking.

In ink-jet 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, ink-jet 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 ink- jet 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, ink-jet 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 ink-jet ink contains unsaturated organic compounds, termed monomers, 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. In such ink-jet inks it is necessary to use monomers possessing a low viscosity. However, ink-jet inks largely based on curable components suffer from significant drawbacks. Substrates having ink-jet inks largely based on curable components printed thereon suffer from blocking, especially when the ink is cured at a low dose of radiation. Blocking occurs when an ink-jet ink based on curable components is inadequately cured after having been printed on to a substrate. On placement of a further printed substrate on to the substrate having ink-jet ink which is inadequately cured thereon, the substrates have a tendency to adhere to form a block. This adhesion of one substrate to another by this process is known in the art as "blocking". Blocking may also occur in roll-to-roll printing where the wrappings of the roll adhere to one another. In order to prevent blocking from occurring, a high dose of radiation is required. However, a high dose of radiation is expensive, has increased safety risks, increased heat dissipation, increased heat on the substrate, requires additional guards for heat escape and requires a large and heavy lamp. In addition to blocking, cure speed is reduced at low dose of radiation.

Ink-jet inks based on curable components can contain monofunctional and multifunctional components. The monofunctional components are typically soft, flexible and provide good adhesion properties while multifunctional components typically provide good cure and surface hardness. However, using either only monofunctional or multifunctional components in an ink can lead to problems. An ink containing solely multifunctional components can form films that suffer from embrittlement and inks solely containing monofunctional components tend to form films that are too soft and which may be prone to physical or chemical attack.

There is therefore a requirement in the art for inks which achieve a balance between cure speed at low dose of radiation and avoiding (or minimising) blocking without compromising the low viscosity and flexibility of the ink-jet ink.

Accordingly, the present invention provides an ink-jet ink comprising from 40 to 80 wt% of N- vinyl caprolactam (NVC), phenoxyethyl acrylate (PEA) and cyclic T P formal acrylate (CTFA) combined based on the total weight of the ink; 5 to 25 wt% of a multifunctional (meth)acrylate monomer, based on the total weight of the ink; 1 to 12 wt% of a radiation-curable oligomer, based on the total weight of the ink; a radical photoinitiator; and a colouring agent; wherein the molar ratio of NVC to PEA is 1.4-2.0:1.0 and the molar ratio of NVC to CTFA is 0.7- 2.8:1.0.

In an alternative embodiment, the present invention provides an ink-jet ink comprising from 40 to 80 wt% of N-vinyl caprolactam (NVC) and phenoxyethyl acrylate (PEA) or cyclic TMP formal acrylate (CTFA) combined, based on the total weight of the ink; 5 to 25 wt% of a multifunctional (meth)acrylate monomer, based on the total weight of the ink; 1 to 12 wt% of a radiation-curable oligomer, based on the total weight of the ink; a radical photoinitiator; and a colouring agent; wherein the molar ratio of NVC to PEA is 1 .4-2.0:1 .0 or the molar ratio of NVC to CTFA is 0.7-2.8:1 .0; and wherein PEA or CTFA is the sole monofunctional (meth)acrylate monomer in the ink.

The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a graph of the blocking performance against weight percentage of NVC for ink- jet inks;

Fig. 2 shows a bar chart of the blocking performance against NVC:PEA molar ratio for ink-jet inks;

Fig. 3 shows a graph of the dose against weight percentage of NVC based on the total weight of the ink;

Fig. 4 shows a bar chart of blocking performance against NVC:CTFA molar ratio for ink-jet inks;

Fig. 5 shows a graph of the blocking performance against weight percentage of multifunctional monomer for ink-jet inks of the present invention; and

Fig. 6 shows a graph of the blocking performance against weight percentage of radiation- curable oligomer for ink-jet inks of the present invention.

The ink-jet 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. Accordingly, the ink-jet ink of the present invention is preferably substantially free of water and volatile organic solvents. Preferably, the ink-jet ink of the present invention comprises less than 5 wt% of water and volatile organic solvent combined, based on the total weight of the ink.

As explained hereinabove, ink-jet inks largely based on curable components suffer from blocking, especially when the ink is cured at a low dose of radiation. Usually a high dose of radiation is required in order to prevent blocking from occurring in such inks. Furthermore, cure speed is reduced at low dose. However, it has now been found that a precisely formulated combination of NVC, PEA and CTFA provide a surprising synergistic effect, namely, reduced blocking at low dose (60-200 mJ/cm²), whilst maintaining adequate cure speeds and flexibility. The ink-jet ink of the present invention comprises N-vinyl caprolactam (NVC). NVC is a well- known N-vinyl amide 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. following chemical structure:

The ink-jet ink of the present invention further comprises PEA and CTFA. Phenoxyethyl acrylate (PEA) and cyclic T P formal acrylate (CTFA) have the following chemical structures:

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

mol wt 200 g/mol mol wt 192 g/mol

The total amount of the NVC, PEA and CTFA in combination is from 40 to 80 wt%, preferably 50 to 70 wt%, more preferably at least 60 wt%, based on the total weight of the ink. The total amount of NVC, PEA and CTFA in combination/combined means that the sum of all of the amounts of NVC, PEA and CTFA in the ink-jet ink is from 40 to 80 wt%, preferably 50 to 70 wt%, more preferably at least 60 wt%, based on the total weight of the ink. The molar ratio of NVC to PEA is 1.4-2.0:1.0, preferably 1.4-1.9:1 .0. A preferred molar ratio of NVC to PEA is 1 .44:1.0 and 1 .86:1 .0. A particularly preferred molar ratio of NVC to PEA is 1.38-1 .84:1.0.

As can be seen in Figs. 1 and 2, when the molar ratio of NVC to PEA (which is a monofunctional (meth)acrylate monomer) is below 0.9, the blocking performance is unsatisfactory. In addition, when the molar ratio of NVC to PEA is above 2.0, crystallisation issues occur in the ink-jet ink as NVC crystallises out of solution. In addition, as can be seen in Fig 3, a minimum dose of radiation is required when the ratio of NVC to PEA is 1.38:1 . The molar ratio of NVC to CTFA is 0.7-2.8:1.0, preferably 0.9-2.8:1 .0. A preferred molar ratio of NVC to CTFA is 0.97:1 .0 to 2.8:1 .0. A particularly preferred molar ratio of NVC to CTFA is 1.01 :1 .0 to 2.8:1.0.

As can be seen in Fig. 4 when the molar ratio of NVC to CTFA (which is a monofunctional (meth)acrylate monomer) is below 0.7, the blocking performance is unsatisfactory.

In a preferred embodiment, the molar ratio of NVC to PEA and CTFA combined is 0.7:1.0 to 3.0:1 .0. Preferably, the molar ratio of NVC to PEA and CTFA combined is 0.9:1 .0 to 2.0:1 .0. The molar ratio of NVC to monofunctional (meth)acrylate monomer is therefore 0.7 to 3.0. The upper limit to this ratio is preferably 2.0 or less. The lower limit to this ratio is preferably 0.9 or more and most preferably 1.2 or more. The molar ratio of NVC to PEA and CTFA in combination/combined means that the molar ratio of NVC to the sum of the molar ratios of PEA and CTFA in the ink-jet ink is 0.7:1 .0 to 3.0:1.0, preferably 0.9:1.0 to 2.0:1.0, more preferably 1 .2:1 .0 to 2.0:1 .0. In a particularly preferred embodiment, PEA and CTFA are the sole monofunctional (meth)acrylate monomers present in the ink.

The ink of the present invention further contains a multifunctional (meth)acrylate monomer. Multifunctional (meth)acrylate monomers are also well known in the art and have a functionality of two or higher. Functionalities of two, three or four are preferred and preferably this monomer is a difunctional or trifunctional monomer.

Examples of the multifunctional acrylate monomers that may be included in the ink include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentylglycol diacrylate, bis(pentaerythritol) hexaacrylate, 3-methyl pentanediol diacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate, and mixtures thereof.

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

Mixtures of (meth)acrylates may also be used.

Preferably, the multifunctional (meth)acrylate monomers is selected from propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate, and mixtures thereof.

The monomers typically have a viscosity of less than 2 mPas at 25°C and they typically have a molecular weight of more than 200 and less than 450.

The total amount of the multifunctional monomer is from 5 to 25 wt% based on the total weight of the ink. Preferably, the multifunctional monomer is present from 10 to 21 wt%, more preferably from 13 to 18 wt%, even more preferably from 13 to 17 wt% and most preferably from 15 to 17 wt%, based on the total weight of the ink.

As can be seen from Fig. 5, blocking performance is increased wherein the multifunctional monomer is present in the ink from 13 to 17 wt%, based on the total weight of the ink. The ink also contains a radiation-curable (i.e. polymerisable) oligomer, preferably a (meth)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 preferably has a molecular weight of at least 450. The molecularweight is preferably 4,000 or less. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is 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.

Preferred oligomers for use in the invention have a viscosity of 0.5 to 20 Pas at 60°C, more preferably 5 to 15 Pas at 60°C and most preferably 5 to 10 Pas at 60°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 ^"1.

UV-curable oligomers of this type are well known in the art. The oligomer is preferably based on bisphenol A, a polyester, a polyether or a urethane. The total amount of the oligomer is preferably from 1 to 12 wt%, based on the total weight of the ink. Preferably the oligomer is present from 2 to 10 wt%, more preferably from 4 to 8 wt%, most preferably from 6 to 9 wt%, based on the total weight of the ink.

As can be seen from Fig. 6, blocking performance is increased wherein the oligomer is present in the ink from 4 to 10 wt%, based on the total weight of the ink.

( eth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and multifunctional are also intended to have their standard meanings, i.e. one and two or more groups, respectively, which take part in the polymerisation reaction on curing.

In addition to the components described hereinabove, the ink also includes a photoinitiator which, under irradiation, for example by ultraviolet light, initiates the polymerisation of the monomers. Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators), such as benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2-benzyl- 2-dimethylamino-(4-morpholinophenyl)butan-1 -one, 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, Darocur (from Ciba) and Lucerin (from BASF). Preferably the photoinitiator is present from 1 to 20% by weight, more preferably from 5 to 15% by weight, most preferably from 4 to 10% by weight, based on the total weight of the ink.

The wavelength of the radiation and the nature of the photoinitiator system used must of course coincide. The ink is cured by irradiation with actinic radiation, such as UV, x-ray, electron beam etc, although UV curing is preferred.

The ink-jet ink of the present invention also includes a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersed 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.

In one aspect of the invention 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 ink-jet nozzle, typically having a particle size less than 8 pm, preferably less than 5 pm, more preferably less than 1 pm and particularly preferably less than 0.5 pm.

The pigment is preferably present in an amount of 20 weight % or less, preferably 15 weight %, based on the total weight of the ink. Preferably the total amount of pigment is present from 0.5 to 12% by weight, more preferably from 1 to 10% by weight. A higher concentration of pigment may be required for white inks, however, for example up to and including 30 weight %, or 25 weight % based on the total weight of the ink.

Although the ink of the present invention cures by a free radical mechanism, the ink of the present invention may also be a so-called "hybrid" ink which cures by a radical and cationic mechanism. The ink-jet ink of the present invention, in one embodiment, therefore further comprises a cationically curable monomer, such as a vinyl ether, and a cationic photoinitiator, such as an iodonium or sulfonium salt, e.g. diphenyliodonium fluoride and triphenylsulfonium hexafluophosphate. Suitable cationic photoinitiators are be sold under the Trade names of Irgacure 184, Irgacure 500, Darocure 1173, Irgacure 907, ITX, Lucerin TPO, Irgacure 369, Irgacure 1700, Darocure 4265, Irgacure 651 , Irgacure 819, Irgacure 1000, Irgacure 1300, Esacure KT046, Esacure KIP150, Esacure KT37, Esacure EDB, H-Nu 470, H-Nu 470X, the Union Carbide UVI-69-series, Deuteron UV 1240 and IJY2257, Ciba Irgacure 250 and CGI 552, IG -C440, Rhodia 2047 and UV9380c.

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, additional monofunctional (meth)acrylate monomers, surfactants, defoamers, dispersants, synergists for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Preferred additional monofunctional (meth)acrylate monomer examples include 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 additional monofunctional (meth)acrylate monomers have the following chemical structures:

Isobornyl acrylate (IBOA) Tetrahydrofurfuryl acrylate (THFA)

mol wt 208g/mol mol wt 156 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 substituents of the monofunctional 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, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C_1-18 alkyl, C₃. ₈ cycloalkyl, C₆. ₀ aryl and combinations thereof, such as C₆. ₀ aryl- or C₃.₁₈ cycloalkyl-substituted C_H8 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 present invention also provides a method of ink-jet printing using the above-described ink and a substrate having the cured ink thereon. Suitable substrates include styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC), VIVAK (a polyethylene terephthalate glycol modified), polyolefin substrates, such as polyethylene and polypropylene, e.g. PE85 Trans T/C, PE85 White or PP Top White, polyethylene terephthalate (PET) and paper. Polyolefin substrates represent the most difficult of these substrates on which to gain adhesion.

The ink of the present invention is preferably cured by ultraviolet irradiation and is suitable for application by ink-jet printing.

The ink-jet ink exhibits a desirable low viscosity (less than 100 mPas, preferably less than 50 mPas and most preferably less than 25 mPas at 25°C). Viscosity may be determined using a Brookfield DV-I + running at 20 rpm.

The inks of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

Examples

The invention will now be described, with reference to the following examples which are not intended to be limiting. Example 1

Inks were prepared by mixing the components in the amounts shown in Table 1. Amounts are given as weight percentages based on the total weight of the ink.

The dispersion comprises 30 wt% of pigment blue 15:4, based on the total weight of the dispersion, 60 wt% of propoxylated neopentylglycol diacrylate, based on the total weight of the dispersion and 10 % of dispersant (Solsperse 32000), based on the total weight of the dispersion.. Irg 184 is hydroxycyclohexyl phenyl ketone. TPO is diphenyl,-(2,4,6-trimethyl benzoyl) phosphine oxide.

The inks draw downs were prepared on PVC, cured under a UV-lamp and left overnight. Blocking performance was evaluated by visual/manual inspection using a scale 1 -5 (1 = Poor, 5 = Excellent).

As can be seen in Figs. 1 and 2, blocking performance is increased for the inks of the invention when compared to the comparative ink. Blocking performance is greater when the molar ratio of NVC to PEA is 1 .4-2.0:1.0. Cure speed was also evaluated for the inks of Table 1 . The comparative speed is rated using the scale (5= Excellent -1 = Poor) as shown in Table 2.

Table 2

The results are shown in Table 3 and Fig. 3. Viscosity values have also been provided.

Table 3

As can be seen in Table 3 and Fig. 3, the inks of the invention have a cure speed that is better than comparison ink 1. A low dose is required for inks of the invention and there is a dip in the dose required for an ink having a molar ratio of NVC: PEA of 1 .38:1 as shown in Fig. 3. A performance trend can be seen such that samples with lower concentrations of NVC show poor blocking performance. The inks of the invention also have a low viscosity. Example 2

Inks were prepared by mixing the components in the amounts shown in Table 4. Amounts are given as weight percentages based on the total weight of the ink. The effect of multifunctional monomer content was studied by changing multifunctional monomer concentration from 6.19 to 20.19% in an ink formulation having an NVC:PEA molar ratio of 1.38:1 , which has a good blocking performance and was selected as a reference formulation.

The dispersion, Irg 184 and TPO are as described for Example 1 .

As can be seen in Fig. 5, enhanced blocking performance is observed with the inks of the present invention, particularly, where multifunctional monomer is present in the ink from 5 to 25 wt%, preferably from 13 to 17 wt%, based on the total weight of the ink.

Example 3

Inks were prepared by mixing the components in the amounts shown in Table 5. Amounts are given as weight percentages based on the total weight of the ink. The effect of radiation- curable oligomer content was studied by changing radiation-curable oligomer concentration from 1 to 10% in an ink formulation having an NVC:PEA molar ratio of 1.38:1 , which has a good blocking performance and was selected as a reference formulation.

The dispersion, Irg 184 and TPO are as described for Example 1 .

As can be seen in Fig. 6, enhanced blocking performance is observed with the inks of the present invention, particularly, where radiation-curable oligomer is present in the ink from 1 to 12 wt%, preferably from 4 to 10 wt%, based on the total weight of the ink. The results suggested that preferably at least 4 wt% radiation-curable oligomer is required to achieve good blocking performance.

Example 4

Inks were prepared by mixing the components in the amounts shown in Table 6. Amounts are given as weight percentages based on the total weight of the ink.

Component PEA of Inks 2-5 of Table 1 was replaced by CTFA to prepare Inks 18-21 of Table 6 in order to compare the blocking performance between inks having a different monofunctional (meth)acrylate monomer present. The quantities of NVC for these inks were kept the same. There is however a slight difference in the molar ratios. Two further Inks 22- 23 were also prepared. The dispersion, Irg 184 and TPO are as described for Example 1.

As can be seen in Fig. 4, blocking performance is increased for the inks of the invention and provides a blocking performance comparable to Inks 4-5. Blocking performance is greater for the inks comprising CTFA when the molar ratio of NVC to CTFA is 0.7-2.8:1 .0.

Claims

1. An ink-jet ink comprising: from 40 to 80 wt% of N-vinyl caprolactam (NVC), phenoxyethyl acrylate (PEA) and cyclic TMP formal acrylate (CTFA) combined, based on the total weight of the ink; 5 to 25 wt% of a multifunctional (meth)acrylate monomer, based on the total weight of the ink; 1 to 12 wt% of a radiation-curable oligomer, based on the total weight of the ink; a radical photoinitiator; and a colouring agent; wherein the molar ratio of NVC to PEA is 1.4- 2.0:1.0 and the molar ratio of NVC to CTFA is 0.7-2.8:1 .0.

2. An ink-jet ink as claimed in claim 1 , wherein the total amount of NVC, PEA and CTFA combined is 50 to 70 wt% based on the total weight of the ink.

3. An ink-jet ink as claimed in any preceding claim, wherein the total amount of NVC, PEA and CTFA combined is at least 60 wt% based on the total weight of the ink.

4. An ink-jet ink as claimed in any preceding claim, wherein the molar ratio of NVC to PEA and CTFA combined is 0.7-3.0:1 .0.

5. An ink-jet ink as claimed in any preceding claim, wherein the molar ratio of NVC to PEA and CTFA combined is 0.9-2.0:1.0.

6. An ink-jet ink as claimed in any preceding claim, wherein the molar ratio of NVC to PEA and CTFA combined is 1 .2-2.0:1 .0.

7. An ink-jet ink as claimed in any preceding claim, wherein the molar ratio of NVC to PEA is 1.4-1 .9:1.0.

8. An ink-jet ink as claimed in any preceding claim, wherein the molar ratio of NVC to CTFA is 0.9-2.8:1.0

9. An ink-jet ink as claimed in any preceding claim, wherein the molar ratio of NVC to PEA is

I .38-1 .84:1.0. 10. An ink-jet ink as claimed in any preceding claim, wherein the molar ratio of NVC to CTFA is 1.01 -2 8:1.0.

I I . An ink-jet ink as claimed in any preceding claim, wherein the multifunctional (meth)acrylate monomer is selected from hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylate, dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentylglycol diacrylate, bis(pentaerythritol) hexa-acrylate, 3-methyl pentanediol diacrylate, the acrylate esters of ethoxylated or propoxylated glycols and polyols, and mixtures thereof.

12. An ink-jet ink as claimed in any preceding claim, wherein the multifunctional monomer is present from 13 to 21 wt%, based on the total weight of the ink.

13. An ink-jet ink as claimed in any preceding claim, wherein the radiation-curable oligomer is present from 4 to 10 wt%, based on the total weight of the ink.

14. An ink-jet ink as claimed in any preceding claim, wherein the ink-jet ink is substantially free of water and volatile organic solvents.

15. An ink-jet ink as claimed in any preceding claim, wherein the colouring agent is a dispersed pigment.

16. An ink-jet ink as claimed in any preceding claim, wherein PEA and CTFA are the sole monofunctional (meth)acrylate monomers in the ink.

17. An ink-jet ink comprising: from 40 to 80 wt% of N-vinyl caprolactam (NVC) and phenoxyethyl acrylate (PEA) or cyclic TMP formal acrylate (CTFA) combined, based on the total weight of the ink; 5 to 25 wt% of a multifunctional (meth)acrylate monomer, based on the total weight of the ink; 1 to 12 wt% of a radiation-curable oligomer, based on the total weight of the ink; a radical photoinitiator; and a colouring agent; wherein the molar ratio of NVC to PEA is 1.4-2.0:1 .0 or the molar ratio of NVC to CTFA is 0.7-2.8:1 .0; and wherein PEA or CTFA is the sole monofunctional (meth)acrylate monomer in the ink.

18. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in any preceding claim on to a substrate and curing the ink.

19. A substrate having the ink-jet ink as claimed in any of claims 1 to 17 printed thereon.