This invention relates to a printing ink, in particular to an inkjet ink which is suitable for low viscosity, high speed printing applications.

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. The inks must flow rapidly from the printing heads and to ensure that this happens, they have low viscosities in use, typically below 100 mPas at 25°C, although in most applications the viscosity is below 50 mPas, and often below 25 mPas. The inks must also be resistant to drying or crusting in the reservoirs or nozzles.

Recently, inkjet printheads have been developed to work with lower viscosity inks. Advantages of low viscosity printheads are high resolution, small drop size and high printing frequencies capable of single pass printing. This results in high quality images for graphic and industrial applications.

These developments in inkjet printheads have led to increased demand for ultra-low viscosity inkjet inks, which can still be cured by actinic radiation (typically UV radiation). This places particular demands on the formulator to reduce viscosity without recourse to non-curable materials like water or organic solvents. The inks are usually required to have a viscosity of less than 16 mPas at the jetting temperature.

Jetting temperatures tend to range from room temperature up to 50°C, but in most cases the manufactures recommend that the ink has a viscosity of less than 16 mPas at 25°C. It is also not uncommon for the ink to operate at a viscosity lower than the recommended printhead specification without issue.

Presently available inks that have been formulated for ultra-low viscosity applications tend to have to compromise on cure speed or film resistance, or both.

Another approach has been add tetrahydrofuran acrylate (THFAc) to reduce the viscosity. However, THFAc has recently been reclassified as a CMR substance (“carcinogenic, mutagenic and reprotoxic substances). It is therefore not a viable approach for most printing applications.

Therefore, there remains a need in the art for ultra-low viscosity inks that maintain a fast cure speed and robust film properties. It should also avoid CMR associated hazards and labelling. Accordingly, the present invention provides an ink-jet ink comprising: an N-vinyl amide, N- acryloyl amine monomer and/or N-vinyl carbamate; isopropylidene glycerol (meth)acrylate; optionally a radical photoinitiator; and optionally a dispersed pigment. This ink combines ultra-low viscosity, a fast cure speed and robust film properties.

The present invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows a graph of the cure properties versus NVC content for a range of inks of the invention and comparative inks.

The inkjet ink of the present invention contains an N-vinyl amide, N-acryloyl amine monomer and/or an N-vinyl carbamate. Multiple monomers in each category or mixtures of monomers in different categories may be used. N-Vinyl amides are well-known monomers in the art. 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 as discussed below. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP). The most preferred monomer in this category is NVC. NVC is a well-known monomer in the art and has the following chemical structure:

-vinyl caprolactam (NVC), mol wt 139 g/mol 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). N-Vinyl carbamates 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 acryloyl 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 ¹ 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, 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, C _3.18 cycloalkyl, C _6.10 aryl and combinations thereof, such as C _6-i o aryl- or C _3.18 cycloalkyl- substituted CM _S 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 ¹ to R ⁴ are independently selected from hydrogen or C,.i ₀ 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 IUPAC 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 preferably contains 10-60%, more preferably 12-40%, by weight of the N-vinyl amide, N-acryloyl amine monomer and/or an N-vinyl carbamate, based on the total weight of the ink. The amount refers to the total of all monomers within these categories.

The ink also contains isopropylidene glycerol (meth)acrylate, preferably isopropylidene glycerol acrylate (IPGA). IPGA has the following structure:

mol wt 132 g/mol

The inkjet ink preferably contains 10-80%, more preferably 20-50%, by weight of the isopropylidene glycerol (meth)acrylate, based on the total weight of the ink. The amount refers to the total of all monomers within this category.

In a preferred embodiment, the ink simultaneously contains 10-60%, more preferably 12-40%, by weight of the N-vinyl amide, N-acryloyl amine monomer and/or N-vinyl carbamate; and 20- 80%, more preferably 20-50%, by weight of the isopropylidene glycerol (meth)acrylate, where the amounts are based on the total weight of the ink.

In a further preferred embodiment, the ink comprises an N-vinyl amide and the N-vinyl amide is N-vinyl caprolactam (NVC) or the ink comprises an N-vinyl carbamate and the N-vinyl carbamate is N-vinyl-5-methyl-2-oxazolidinone (NVMO), and the isopropylidene glycerol (meth)acrylate is isopropylidene glycerol acrylate (IPGA).

The molar ratio of the N-vinyl amide, N-acryloyl amine monomer and/or N-vinyl carbamate to the isopropylidene glycerol (meth)acrylate is preferably 0.5-13:1 and most preferably is 0.9- 12:1 .

The inkjet ink may contain other monomers. In one embodiment, the ink further comprises one or more monomers selected from mono-, di- and/or multi-functional (meth)acrylate monomers. 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 ¹ .

The ink may contain additional monofunctional (meth)acrylate monomer. 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. Mixtures of (meth)acrylates may be used.

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 cycloalkyl, 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 cycloalkyl, C _6.10 aryl and combinations thereof, any of which may substituted with alkyl (such as C _MS 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.

The cyclic monofunctional (meth)acrylate monomer may be 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/Medol-10), 4- ferf-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof.

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 C _M6 alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C ₆ -C ₂₀ group. It may be selected from octa/decyl acrylate (ODA), 2-(2- ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

If present, the additional monofunctional (meth)acrylate monomers may be present at 5 to 25% by weight, based on the total weight of the ink. 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.

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 ,11-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.

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 monomers other than N-vinyl amide, N- acryloyl amine monomer, N-vinyl carbamate and isopropylidene glycerol (meth)acrylate, meaning that only small amounts will be present, for example as impurities in the monofunctional material or as a component in a commercially available pigment dispersion.

Where additional monomers are included, they are preferably present in an amount of no more than 30%, more preferably no more than 20%, more preferably 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. In one embodiment, the N-vinyl amide, N-acryloyl amine monomer and/or N-vinyl carbamate and the isopropylidene glycerol (meth)acrylate are the sole monomers present, and in another they are the sole polymerisable materials present in the ink.

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.

Ideally, the ink is free of CMR monomers like tetrahydrofuran acrylate (THFAc), and CMR substances in general.

The inkjet ink is preferably substantially free of radiation-curable (i.e. polymerisable) oligomer, e.g. 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 ¹ .

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 contain passive resin, although in some embodiments it is substantially free of 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 is usually 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.

Again, 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 usually 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 may also comprise 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 pm, preferably less than 5 pm, more preferably less than 1 pm and particularly preferably less than 0.5 pm.

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 present invention also includes non-pigmented varnishes, top coats and primers, which do not contain colouring agents.

The amounts by weight provided herein are based on the total weight of the ink.

The inkjet ink of the present invention preferably exhibits an ultra-low viscosity, e g. a viscosity of less than 16 mPas at 25°C, more preferably less than 10 mPas, and most preferably less than 8 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.

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.

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 ¹ and more preferably 21-32 mNm ¹ .

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 highspeed water-cooled stirrer, or milling on a horizontal bead-mill.

A suitable printer is a roll-fed UV printer, for example the Acuity Ultra from the Acuity series by Fujifilm.

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.

The present invention also provides a substrate having the ink-jet ink of the present invention printed thereon.

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.

The ink may also be cured using a low-energy electron beam (ebeam). The ebeam can be any source of low-energy electron beam that is suitable for curing radiation-curable inks. Suitable low-energy electron beam sources include commercially available ebeam curing lamps, such as a 280 mm Comet ebeam curing lamp which has a penetrating voltage of 80 kV and is capable of delivering a dosage of 30 kGy at 100 m/min. By“low-energy” for the ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 50 kGy or less.

Ebeam curing is characterised by dose (energy per unit mass, measured in kilograys (kGy)) deposited in the substrate via electrons. Electron beam surface penetration depends upon the mass, density and thickness of the material being cured. Compared with UV penetration, electrons penetrate deeply through both lower and higher density materials. Unlike UV curing, photoinitiators are not required for ebeam curing to take place.

In order to cure the printed ink, the ink of the invention is exposed to the ebeam, which produces sufficient energy to instantaneously break chemical bonds and enable polymerisation or crosslinking.

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 2012/110815.

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 pm, preferably 1 to 10 pm, for example 2 to 5 pm. 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

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. Table 1

TPO is diphenyl, -(2, 4, 6-trimethyl benzoyl) phosphine oxide. ITX is isopropylthioxanthone. Irg 184 is hydroxycyclohexyl phenyl ketone. They are photoinitiators.

The cyan dispersion contains 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.

BYK-307 is a surfactant.

The weight and molar ratios are set out in Table 2.

Table 2

The inks were drawn down onto self-adhesive vinyl substrates at a film weight of 12 pm, and cured under a under a 395 nm Baldwin lamp. The dose required for cure was measured. Full cure of the ink was determined by examining the cured surface using strips of photo paper and ensuring no tackiness was present. The results are set out in Fig. 1 . It can be seen that the inks containing IPGA/NVC exhibited better cure sensitivity than the inks containing NVC with PEA, IBOA or TMCHA.