WO2025078827A1

WO2025078827A1 - Printing ink

Info

Publication number: WO2025078827A1
Application number: PCT/GB2024/052607
Authority: WO
Inventors: Gemma OSBORNE
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2023-10-13
Filing date: 2024-10-11
Publication date: 2025-04-17
Anticipated expiration: 2026-04-13
Also published as: GB202315724D0

Abstract

The present invention provides an inkjet ink comprising: a difunctional (meth)acrylate monomer; at least one of an N-vinyl amide monomer, an N-(meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer; and a resin comprising a structural unit of. The invention also provides a method of inkjet printing comprising inkjet printing the inkjet ink of the invention onto a substrate and curing the inkjet ink by exposing the inkjet ink to a curing source. The invention further provides a substrate having the inkjet ink of the invention printed thereon.

Description

Printing ink

The present invention relates to a printing ink and in particular, to an inkjet ink which has a desirable balance of properties.

In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate, which is moving relative to the reservoirs. The ejected ink forms an image on the substrate.

For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically 200 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 5-1 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 radiation-curable material, such as radiation-curable monomers and/or oligomers, which polymerise when cured. By “radiation-curable” is meant a material that polymerises and/or crosslinks upon irradiation, for example, when exposed to actinic radiation, 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 cured, a process which is more rapid than evaporation of solvent at moderate temperatures.

A colourless inkjet ink may be used to overcoat a coloured inkjet ink. Such a colourless inkjet ink acts as a varnish. In order to provide a highly decorated finish and good resistance, a varnish needs to have high gloss and good hardness. A high gloss varnish is generally formulated using a relatively high concentration of di- and multifunctional monomer. These monomers also provide the varnish with a high cure speed. However, such monomers typically have a detrimental effect on adhesion, flexibility and elongation.

Usually, poor adhesion is not a problem as adhesion will be provided by the underlying inkjet ink. However, there is an increasing demand for an inkjet ink that has the beneficial properties of a varnish but can be printed directly onto a substrate without compromising the adhesion, flexibility, elongation, and viscosity of the ink. There is therefore a need in the art for an inkjet ink that can provide good adhesion, flexibility and elongation without compromising the resistance properties, cure speed and viscosity of the ink.

Accordingly, the present invention provides an inkjet ink comprising: a difunctional (meth)acrylate monomer; at least one of an N-vinyl amide monomer, an N-(meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer; and a resin comprising a structural unit of

The inventors have surprisingly found that an inkjet ink containing the resin of the invention in combination with the monomer blend of the invention provides an ink with good adhesion, flexibility and elongation, without compromising the resistance properties, cure speed and viscosity of the ink.

By cure speed is meant the speed at which the actinic radiation source moves relative to the substrate. An advantage of a high cure speed is that a low dose per unit area is required to achieve a fully cured film.

The inkjet ink of the present invention comprises a difunctional (meth)acrylate monomer. The difunctional (meth)acrylate monomer in combination with the other components of the ink provides the ink with good adhesion, flexibility and elongation, without compromising the resistance properties, cure speed and viscosity of the ink.

As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers.

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 meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

Difunctional (meth)acrylate monomers are well known in the art. Examples include hexanediol diacrylate (HDDA), 1 ,8-octanediol diacrylate, 1 ,9-nonanediol diacrylate, 1 ,10-decanediol diacrylate (DDDA), 1 ,11-undecanediol diacrylate and 1 ,12-dodecanediol diacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA, PEG600DA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1 ,5- pentanediol diacrylate (3-MPDDA), propoxylated neopentylglycol diacrylate (NPGPODA), 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.

In a preferred embodiment, the difunctional (meth)acrylate monomer is selected from dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), 3-methyl-1 ,5-pentanediol diacrylate (3-MPDDA), propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof. These monomers are particularly preferred for adhesion and elongation. TPGDA is particularly preferred for adhesion and elongation.

Preferably, the difunctional (meth)acrylate monomer is present in a total amount of 5 to 25% by weight and more preferably 10 to 20% by weight, based on the total weight of the ink.

If TPGDA is present in the ink, TPGDA is preferably present in 5 to 12% by weight and more preferably 8 to 12% by weight, based on the total weight of the ink. This amount of TPGDA provides optimal elongation.

The inkjet ink may also comprise a difunctional monomer other than a difunctional (meth)acrylate monomer. Examples of a difunctional monomer other than a difunctional (meth)acrylate monomer include a divinyl ether monomer and/or a divinyl ether (meth) acrylate monomer. Mixtures of difunctional monomers may also be used.

Examples of a divinyl ether monomer include triethylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1 ,4-cyclohexanedimethanol divinyl ether, bis[4-(vinyloxy)butyl] 1 ,6- hexanediylbiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi- 4,1-phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4-(vinyloxymethyl)cyclohexylmethyl] glutarate, 1 ,4-butanediol divinyl ether and mixtures thereof. DVE-3 is particularly preferred.

Examples of a vinyl ether (meth)acrylate monomer include 2-(2-vinyloxy ethoxy)ethyl acrylate (VEEA), 2-(2-vinyloxy ethoxy)ethyl methacrylate (VEEM) and mixtures thereof.

Preferably, the difunctional monomer is present in a total amount of 5 to 25% by weight and more preferably 10 to 20% by weight, based on the total weight of the ink.

However, in a preferred embodiment, the difunctional (meth)acrylate monomer is the sole difunctional monomer present in the ink. In a preferred embodiment, the inkjet ink of the present invention comprises a monofunctional (meth)acrylate monomer. The monofunctional (meth)acrylate monomer in combination with the other components of the ink further contributes to providing the ink with good adhesion, flexibility and elongation, without compromising the resistance properties, cure speed and viscosity of the ink.

Monofunctional (meth) acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Mixtures of monofunctional (meth) acrylate monomers may be used.

The substituents of the monofunctional (meth)acrylate monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc.

The monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth) acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

In a preferred embodiment, the monofunctional (meth)acrylate monomer comprises a cyclic monofunctional (meth) acrylate monomer.

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 C3-18 cycloalkyl, Ce- 10 aryl and combinations thereof, any of which may substituted with alkyl (such as C1-18 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), isopropylidene glycerol acrylate (IPGA), 4-te/Y-butylcyclohexyl acrylate (TBCHA), 3,3,5- trimethylcyclohexyl acrylate (TMCHA), benzyl acrylate (BA) and mixtures thereof.

In a preferred embodiment, the monofunctional (meth)acrylate monomer comprises an acyclic- hydrocarbon monofunctional (meth)acrylate monomer.

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 C1-18 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 C6-C20 group. It may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth) acrylate monomer contains a linear C6-C20 group.

In a preferred embodiment, the 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/Medol-10), isopropylidene glycerol acrylate (IPGA), 4-fe/Y-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), benzyl acrylate (BA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof. PEA, CTFA, Medol-10 and mixtures thereof are particularly preferred.

Preferably, the monofunctional (meth)acrylate monomer is present in a total amount of 40 to 70% by weight, more preferably 45 to 65% by weight and most preferably 50 to 60% by weight, based on the total weight of the ink.

THFA is often used to provide good adhesion to variety of substrates, as well as producing a flexible film which is less liable to cracking and delamination. A further advantage of THFA is that it can solubilise chlorinated polyolefins, which in turn provides good adhesion to polyolefin substrates. However, THFA is a hazardous monomer and bears the GHS hazard statement H314 (Causes severe skin burns and eye damage). There is also growing evidence that it may damage fertility or the unborn child. Thus, there is an urgent need in the art to move away from THFA.

The ink will still function in the presence of THFA, in terms of its printing and curing properties. However, to avoid the hazardous nature of THFA, the ink preferably contains less than 2% by weight, more preferably less than 1 % by weight and most preferably is substantially free of THFA, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion. In other words, no THFA is intentionally added to the ink. However, minor amounts of THFA, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of THFA, more preferably less than 0.1 % by weight of THFA, most preferably less than 0.05% by weight of THFA, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of THFA. The inkjet ink may also comprise a multifunctional monomer. The multifunctional monomer may be a tri-, tetra-, penta- or hexa- functional monomer, i.e. the radiation curable monomer may have three, four, five or six functional groups. Examples of multifunctional monomer include multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers and multifunctional vinyl ether (meth)acrylate monomers. Mixtures of multifunctional monomers may also be used.

If present, the multifunctional monomer is present in a total amount of 5 to 25% by weight, based on the total weight of the ink.

The inkjet ink of the invention comprises at least one of an N-vinyl amide monomer, an N- (meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer. This component in combination with the other components of the ink provides the ink with good adhesion, flexibility and elongation, without compromising the resistance properties, cure speed and viscosity of the ink. In particular, the inventors have surprisingly found that this component provides the ink with good adhesion, whilst maintaining the cure speed of the ink.

In a preferred embodiment, the inkjet ink comprises an N-vinyl amide monomer.

N-Vinyl amide monomers are well-known monomers in the art. N-Vinyl amide monomers 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. If present, preferred examples are N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl formamide and N-vinyl acetamide.

In a preferred embodiment, the inkjet ink comprises NVC.

In a preferred embodiment, the inkjet ink comprises an N-(meth)acryloyl amine monomer.

Similarly, N-(meth)acryloyl amine monomers are also well-known in the art. N-(Meth)acryloyl amine monomers also have a vinyl group (optionally substituted with an a-methyl 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. If present, a preferred example is N-acryloylmorpholine (ACMO).

In a preferred embodiment, the inkjet ink comprises ACMO.

In a preferred embodiment, the inkjet ink comprises an N-vinyl amide monomer and/or an N- (meth)acryloyl amine monomer, preferably NVC and/or ACMO. In a preferred embodiment, the inkjet ink comprises an N-vinyl amide monomer and an N-

(meth)acryloyl amine monomer, preferably NVC and ACMO.

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 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).

If present, the N-vinyl carbamate monomer is preferably an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure:

in which R¹ to R⁴ 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 C1-18 alkyl, C3-18 cycloalkyl, Cs- aryl and combinations thereof, such as Cs- aryl- or C3-18 cycloalkyl-substituted C1-18 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 R¹ to R⁴ are independently selected from hydrogen or C1-10 alkyl. Further details may be found in WO 2015/022228 and US 4,831 ,153.

If present, the N-vinyl carbamate monomer is most preferably N-vinyl-5-methyl-2-oxazolidinone (known as NVMO or VMOX). 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 monomer is a racemate of NVMO. In another embodiment, the N-vinyl carbamate monomer is (/?)- 5-methyl-3-vinyl-1 ,3-oxazolidin-2-one. Alternatively, the N-vinyl carbamate monomer is (S)-5- methyl-3-vinyl-1 ,3-oxazolidin-2-one.

In a preferred embodiment, the inkjet ink comprises at least one of NVC, ACMO and/or NVMO.

In a preferred embodiment, the at least one of an N-vinyl amide monomer, an N-(meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer is present in a total amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

Preferably, the inkjet ink comprises an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer present in a total amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

More preferably, the inkjet ink comprises NVC and/or ACMO present in a total amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

Preferably, the inkjet ink comprises an N-vinyl amide monomer present in an amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

More preferably, the inkjet ink comprises NVC present in an amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

Preferably, the inkjet ink comprises an N-(meth)acryloyl amine monomer present in an amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink. More preferably, the inkjet ink comprises ACMO present in an amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

Preferably, the inkjet ink comprises an N-vinyl amide monomer and an N-(meth)acryloyl amine monomer present in a total amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

More preferably, the inkjet ink comprises NVC and ACMO present in a total amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

The inkjet ink may also comprise at least one N-vinyl monomer other than an N-vinyl amide monomer, an N-(meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole.

In a preferred embodiment, the inkjet ink comprises PEA, CTFA, Medol-10, TPGDA, and NVC and/or ACMO, preferably PEA, CTFA, Medol-10, TPGDA and NVC. In a particularly preferred embodiment, PEA, CTFA, Medol-10, TPGDA, NPGPODA, and NVC and/or ACMO, preferably PEA, CTFA, Medol-10, TPGDA, NPGPODA and NVC are the sole monomers present in the ink. The inventors have found that this blend of monomers is particularly preferred to provide the ink with good adhesion, flexibility and elongation, without compromising the resistance properties, cure speed and viscosity of the ink.

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 100mPas 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 12° steel cone at 25°C with a shear rate of 25 s ¹ .

The inkjet ink comprises a resin comprising a structural unit of

The resin in combination with the other components of the ink provides the ink with good adhesion, flexibility and elongation, without compromising the resistance properties, cure speed and viscosity of the ink. In particular, the inventors have surprisingly found that the resin provides the ink with good adhesion and elongation, whilst maintaining the cure speed of the ink. The resin comprises a structural unit of

or mixtures thereof.

The substituents of the structural unit of the resin may be cis- or trans- but preferably the resin is a cis- and trans- mixture.

The number of structural units of the resin, n, is 1-350, preferably 5-300, more preferably 10-250. In a particularly preferred embodiment, n is 50-350, more preferably 75-300 and most preferably 100-250.

The structural unit of the resin is derived from a monomer of diallyl 1 ,2-cyclohexanedicarboxylate, diallyl 1 ,3-cyclohexanedicarboxylate, diallyl 1 ,4-cyclohexanedicarboxylate or mixtures thereof. In one embodiment, the structural unit of the resin is derived from a monomer of diallyl 1 ,2- cyclohexanedicarboxylate. In another embodiment, the structural unit of the resin is derived from a monomer of diallyl 1 ,3-cyclohexanedicarboxylate. In a further embodiment, the structural unit of the resin is derived from a monomer of diallyl 1 ,4-cyclohexanedicarboxylate.

The resin may be a homopolymer of diallyl 1 ,2-cyclohexanedicarboxylate, diallyl 1 ,3- cyclohexanedicarboxylate or diallyl 1 ,4-cyclohexanedicarboxylate or the resin may be a copolymer of mixtures of 1 ,2-cyclohexanedicarboxylate, diallyl 1 ,3-cyclohexanedicarboxylate and diallyl 1 ,4- cyclohexanedicarboxylate. The homopolymer or copolymer may be linear or cyclic. If the resin is a copolymer of mixtures of 1 ,2-cyclohexanedicarboxylate, diallyl 1 ,3-cyclohexanedicarboxylate and diallyl 1 ,4-cyclohexanedicarboxylate, the monomers may be arranged in any order. For example, the copolymer may be a random copolymer or a block copolymer.

Alternatively, the resin may be a copolymer of diallyl 1 ,2-cyclohexanedicarboxylate, diallyl 1 ,3- cyclohexanedicarboxylate, diallyl 1 ,4-cyclohexanedicarboxylate or mixtures thereof in combination with additional monomers. In this case, the additional monomers are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The monomers may be arranged in any order. For example, the copolymer may be a random copolymer or a block copolymer.

In a preferred embodiment, the resin comprises a structural unit of n=1-350

In this embodiment, n is preferably 5-300, more preferably 10-250. In a particularly preferred embodiment, n is 50-350, more preferably 75-300 and most preferably 100-250.

In this embodiment, the resin may be a homopolymer of diallyl 1 ,2-cyclohexanedicarboxylate or the resin may be a copolymer of monomers comprising diallyl 1 ,2-cyclohexanedicarboxylate.

In a particularly preferred embodiment, the resin is a homopolymer of diallyl 1 ,2- cyclohexanedicarboxylate. In this embodiment, n is preferably 50-350, more preferably 75-300 and most preferably 100-250. The homopolymer of diallyl 1 ,2-cyclohexanedicarboxylate may be linear or cyclic.

If the resin is a copolymer of monomers comprising diallyl 1 ,2-cyclohexanedicarboxylate, the additional monomers are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. In a preferred embodiment, if the resin is a copolymer of monomers comprising diallyl 1 ,2-cyclohexanedicarboxylate, the resin is a copolymer of diallyl 1 ,2-cyclohexanedicarboxylate, styrene and butadiene.

If the resin is a copolymer of monomers comprising diallyl 1 ,2-cyclohexanedicarboxylate, the structural unit of n=1-350

and the structural units derived from the additional monomers may be arranged in any order. For example, the copolymer may be a random copolymer or a block copolymer.

If the resin is a copolymer of monomers comprising diallyl 1 ,2-cyclohexanedicarboxylate, n is preferably 1-250, more preferably 1-150 and most preferably 1-50.

In a preferred embodiment, the resin has a weight-average molecular weight of 20-60 KDa, more preferably 30-60 KDa, as determined by GPC with polystyrene standards. It is preferably solid at 25°C. It is preferably soluble in the radiation-curable components of the ink. The resin preferably has a glass transition temperature (Tg) of 40 to 75°C, preferably 45 to 70°C, as determined by Differential Scanning Calorimetry (DSC). For example, using a Perkin Elmer Diamond DSC. A resin of the present invention having such a glass transition temperature further improves the resistance properties of the cured ink film and in particular, the hardness of the cured ink film.

In a preferred embodiment, the resin is phthalate-free. In this regard, the resin is preferably free from the structural unit

By free is meant that the resin contains no structural units of the above formula.

Examples of preferred resins include RADPAR™ AD-032 and RADPAR™ AD-044 commercially available from Osaka Soda. RADPAR™ AD-032 has CAS number 30527-01-6.

In a preferred embodiment, the resin is present in an amount of 1 to 10% by weight, preferably 2 to 8% by weight, more preferably 4 to 6% by weight, based on the total weight of the ink.

The inkjet ink may also further comprise a resin other than the resin comprising a structural unit of

If present, the additional resin preferably has a weight-average molecular weight of 20-200 KDa and preferably 20-60 KDa, as determined by GPC with polystyrene standards. It is preferably solid at 25°C. It is preferably soluble in the radiation-curable components of the ink.

The additional resin, when present, is a passive (i.e. inert) resin, in the sense that it is not radiation curable and hence does not undergo cross-linking under the curing conditions to which the ink is subjected.

If present, the additional resin may further improve adhesion of the ink to the substrate. The additional resin, when present, is preferably present at 0.1-5% by weight, based on the total weight of the ink. However, in a preferred embodiment, the resin comprising a structural unit of

is the only resin in the ink.

The inkjet ink may further comprise a radiation-curable (i.e. polymerisable) oligomer, such as a (meth)acrylate oligomer. Any radiation-curable oligomer that is compatible with the other ink components is suitable for use in the ink. Preferably, the inkjet ink comprises a (meth)acrylate oligomer.

The term “radiation-curable oligomer” means a component formed from a monomer having two or more radiation-curable groups of the same functionality, wherein at least one of the radiation- curable groups has been reacted with another component to increase the molecular weight thereby forming the oligomer, and at least one of the radiation-curable groups is capable of polymerisation. For example, if the two or more radiation-curable groups are (meth)acrylate groups, the oligomer comprises reacted and unreacted (meth)acrylate groups. The reacted (meth)acrylate group may result from the reaction between a (meth)acrylate group and an amine.

Radiation-curable oligomers comprise a backbone and one or more radiation-curable groups. The backbone may comprise polyester, urethane, epoxy, polyether or amine functionality.

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

The oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality oligomers may be used. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is preferably multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 6.

Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide film-forming properties such as hardness or cure speed. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Preferred oligomers for inclusion in the ink of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique 12° steel cone at 60°C with a shear rate of 25 s ¹. The radiation-curable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers. The oligomer may include amine functionality, as the amine acts as an activator without the drawback of migration associated with low-molecular weight amines. When present, the radiation-curable oligomer is preferably amine modified. More preferably, when present, the radiation-curable oligomer is an amine-modified (meth)acrylate oligomer.

Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance.

The amount of radiation-curable oligomer, when present, is preferably 0.1-10% by weight, based on the total weight of the ink.

The inkjet ink may include a colouring agent or may be free of a colouring agent.

If present, the colouring agent may be either dissolved or dispersed in the liquid medium of the ink. The colouring agent can be any of a wide range of suitable colouring agents that would be known to the person skilled in the art.

Preferably, the colouring agent when present is a pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF pic), Cinquasia, I rgalite (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 , Pigment yellow 180 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.

If present, the colorant is preferably present in an amount of 0.2 to 20% by weight, preferably 0.3 to 15% 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.

In a preferred embodiment, the inkjet ink is free of a colouring agent. By free is meant no colouring agent is intentionally added to the ink. It can be difficult to formulate colourless inkjet inks that do not yellow and thus negatively affect the quality of the cured ink film. However, the inventors have surprisingly found that the blend of monomers in combination with the resin of the ink provides an ink with minimal yellowness.

In a preferred embodiment, the inkjet ink further comprises a photoinitiator.

Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), ethyl phenyl (2,4,6- trimethylbenzoyl) phosphinate (TPO-L), benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 2- benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzil dimethylketal, phenylbis(2,4,6- trimethylbenzoyl) phosphine oxide (BAPO), 2-isopropylthioxanthone (ITX), 2,4-diethylthioxanthone (DETX) or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Omnirad (from IGM) and Esacure (from Lamberti).

In a preferred embodiment, the inkjet ink comprises BAPO. Preferably, the inkjet ink comprises BAPO in 0.5 to 5.0% by weight, more preferably 1 .0 to 4.5% by weight and most preferably 2.0 to 4.0% by weight, based on the total weight of the ink. In a particularly preferred embodiment, BAPO is the sole photoinitiator present in the ink.

The inventors have found that the inclusion of BAPO in the inkjet ink of the invention is particularly preferred as it further improves adhesion.

The inkjet ink may also comprise one or more polymeric photoinitiators, such as Omnipol TP®.

Omnipol TP® is commercially available from IGM. It is a polymeric phosphine oxide photoinitiator, and is known by the chemical name polymeric ethyl (2,4,6-trimethylbenzoyl)-phenyl phosphinate or polymeric TPO-L. It has the following structure:

The total value of a, b and c of the chemical formula for polymeric TPO-L is equal to 1-20.

Mixtures of free radical photoinitiators can be used and in one embodiment, the ink comprises a plurality of free radical photoinitiators. The total number of free radical photoinitiators present in the ink is preferably from one to five.

TPO is a common photoinitiator but it is reprotoxic. TPO bears the GHS hazard statement H361 and is classified as a category 2 CMR substance with reproductive toxicity, which is suspected of damaging fertility or the unborn child.

The ink will still function in the presence of TPO, in terms of its printing and curing properties. However, to avoid the hazardous nature of TPO, the inkjet ink preferably contains less than 2% by weight, more preferably less than 1 % by weight and most preferably is substantially free of TPO, based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion. In other words, no TPO is intentionally added to the ink. However, minor amounts of TPO, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1 % by weight and most preferably less than 0.05% by weight of TPO, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of TPO.

If present, the photoinitiator is preferably present from 1 to 20% by weight, preferably from 5 to 15% by weight, based on the total weight of the ink. The presence of a photoinitiator is optional because the ink can cure without the presence of a photoinitiator by curing with a low-energy electron beam or curing by actinic radiation in an inert environment.

Therefore, the photoinitiator may be present in an amount of less than 20% by weight, preferably less than 5% by weight, more preferably less than 3%, more preferably less than 1 %, based on the total weight of the ink.

Therefore, in a preferred embodiment, no photoinitiator is intentionally added to the ink. However, minor amounts of photoinitiator, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of photoinitiator, more preferably less than 0.1 % by weight of photoinitiator and most preferably less than 0.05% by weight of photoinitiator, based on the total weight of the ink. The inkjet ink may also be free of photoinitiator.

However, an inkjet ink that is cured with a low-energy electron beam or actinic radiation in an inert environment may still contain a small amount of photoinitiator such as 1 to 5% by weight of a photoinitiator, based on the total weight of the ink. This is required if the ink is first pinned with actinic radiation.

By pinning is meant arresting the flow of the ink by treating the ink droplets quickly after they have impacted onto the substrate surface. Pinning provides a partial cure of the ink and thereby maximises image quality by controlling bleed and feathering between image areas. Pinning does not achieve full cure of the ink. By curing is meant fully curing the ink. Pinning leads to a marked increase in viscosity, whereas curing converts the inkjet ink from a liquid ink to a solid film. The dose of radiation used for pinning is generally lower than the dose required to cure the radiation- curable material fully.

The inkjet ink preferably 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. Preferably, the inkjet ink contains less than 5% by weight, more preferably less than 3% by weight, more preferably less than 2% by weight, more preferably less than 1 % by weight and most preferably the inkjet ink is substantially free of water and volatile organic solvent combined, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example 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. In other words, no water or a volatile organic solvent is intentionally added to the ink. However, minor amounts of water or a volatile organic solvent, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may contain less than 0.5% by weight, more preferably less than 0.1% by weight and most preferably less than 0.05% by weight of water and a volatile organic solvent combined, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of water and volatile organic solvents.

In a preferred embodiment, the inkjet ink comprises a surfactant. The surfactant controls the surface tension of the ink. Surfactants are well known in the art and a detailed description is not required. Examples of suitable surfactants include TegoRad 2010 (cross-linkable) and BYK307 (non-cross-linkable). TegoRad 2010 is particularly preferred. 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. Surface tension is also critical to ensuring stable jetting (nozzle plate wetting and sustainability). The surface tension is preferably in the range of 18-40 mNnr¹, more preferably 20-35 mNnr¹ and most preferably 20-30 mNnr¹.

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

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

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.

The inkjet ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less, more preferably 30 mPas or less and most preferably 20 mPas or less at 25°C. The inkjet ink preferably has a viscosity of 8 to 20 mPas and most preferably 12 to 16 mPas at 25°C. Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model DV1 .

The present invention also provides an inkjet ink set, wherein the inkjet ink set of the invention has at least one ink that falls within the scope of the inkjet ink according to 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 process set). This set is often termed CMYK. The inks in a process set can be used to produce a wide range of colours and tones. Inks containing lower amounts of pigment (so-called light inks) can also be included to improve the tonal range.

The present invention also provides a method of inkjet printing comprising inkjet printing the inkjet ink as defined herein onto a substrate and curing the inkjet ink by exposing the inkjet ink to a curing source.

In the method of inkjet printing of the present invention, the inkjet ink is inkjet printed onto a substrate. Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto a substrate, on a roll-to-roll printer or a flat-bed printer. As discussed above, inkjet printing is well known in the art and a detailed description is not required.

The ink is jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image.

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

Substrates include flexible and rigid substrates. Examples include substrates composed of polyvinyl chloride (PVC), polystyrene (PS), polyester, polyethylene terephthalate (PET), polyethylene terephthalate glycol modified (PETG), polyolefin (e.g. polyethylene, polypropylene or mixtures or copolymers thereof), polyester textile banner and soft signage (3P textile), polycarbonate (PC) and acrylic. Further substrates include all cellulosic materials such as paper and board, or their mixtures/blends with the aforementioned synthetic materials.

Acrylic is a particularly challenging substrate to print onto and achieve good adhesion. However, it has surprisingly been found that the inkjet ink of the present invention can achieve excellent adhesion to acrylic substrates.

When discussing the substrate, it is the surface which is most important, since it is the surface which is wetted by the ink. Thus, at least the surface of substrate is composed of the abovediscussed material.

The present invention also provides a printed substrate having the ink as defined herein printed thereon. In order to produce a high quality printed image a small jetted drop size is desirable. Preferably the inkjet ink is jetted at drop sizes below 90 picolitres, preferably below 35 picolitres, more preferably below 10 picolitres and most preferably below 5 picolitres.

To achieve compatibility with print heads that are capable of jetting drop sizes of 90 picolitres or less, a low viscosity ink is required. A viscosity of 30 mPas or less at 25°C is preferred, for example, 5 to 12 mPas, 18 to 20 mPas, or 24 to 26 mPas. 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 ink of the present invention is cured by any means known in the art, such as exposure to actinic radiation.

It should be noted that the terms “dry” and “cure” are often used interchangeably in the art when referring to radiation-curable inkjet inks to mean the conversion of the inkjet ink from a liquid to solid by polymerisation and/or crosslinking of the radiation-curable material. Herein, however, by “drying” is meant the removal of the water by evaporation and by “curing” is meant the polymerisation and/or crosslinking of the radiation-curable material. Further details of the printing, drying and curing process are provided in WO 201 1/021052.

In a preferred embodiment, the ink is cured by exposing the printed ink to a source of actinic radiation.

The source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources are well known in the art and a detailed description is not required. These include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof.

In a preferred embodiment, the source of actinic radiation is LEDs. It is particularly challenging to provide a high cure speed when curing with a UV LED curing source. However, the inventors have found that the inkjet ink maintains good adhesion and cure speed, even when the source of actinic radiation is LEDs.

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

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

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

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

The ink may also be cured by exposing the printed ink to low-energy electron beam (ebeam).

The source of low-energy electron beam (ebeam) can be any source of low-energy electron beam that is suitable for curing radiation-curable inks. Suitable low-energy electron beam radiation sources include commercially available ebeam curing units, such as the EB Lab from ebeam Technologies with energy of 80-300 keV and capable of delivering a typical dose of 30-50 kGy at line speeds of up to 30 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 70 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.

Ebeam curing is well-known in the art and therefore a detailed explanation of the curing method is not required. 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.

There is no restriction on the ebeam dose that is used to cure the inkjet inks of the present invention other than that the dose is sufficient to fully cure the ink. Preferably, the dose is more than 10 kGy, more preferably more than 20 kGy, more preferably more than 30 kGy and most preferably more than 40 kGy. Preferably, the dose is less than 100 kGy, more preferably less than 90 kGy, more preferably less than 80 kGy and most preferably less than 70 kGy. Preferably, the dose is more than 30 kGy but less than 70 kGy, more preferably more than 30 kGy but less than 60 kGy and most preferably, more than 30 kGy but 50 kGy or less. Doses above 50 kGy may cause damage to the substrate, particularly the substrates used for food packaging applications, and so doses of 50 kGy or less are preferred.

The energy associated with these doses is 80-300 keV, more preferably 70-200 keV and most preferably 100 keV.

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

Inkjet inks were prepared according to the formulations set out in Table 1. 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 1

3-MPDDA and NPGPDA are difunctional (meth)acrylate monomers. PEA, CTFA and Medol-10 are monofunctional (meth)acrylate monomers. NVC is an N-vinyl amide monomer. UV12 and MEHQ are stabilisers. OB184 is an optical brightening agent. BAPO is a photoinitiator. TegoRad 2010 is a slip aid. RAD PAR AD-032 and RAD PAR AD-044 are resins of the invention. BR113 and DM55 are comparative resins comprising methyl methacrylate structural units.

Inks 2 and 3 are inks of the invention as they contain a resin of the invention. Ink 1 is a comparative ink as it contains additional difunctional (meth)acrylate monomer NPGPODA instead of a resin. Inks 4-7 are comparative inks as they contain comparative resins.

The viscosity of the ink was measured using a Brookfield viscometer running at 20 rpm at 25°C.

Inks 1-7 were then assessed for hardness, adhesion, elongation, cure speed and colour. The tests and results are set out below.

Hardness

Inks 1-7 were drawn down in 12 pm films using a 12 pm wire wound K-bar onto an acrylic substrate. The ink films were then cured using a 20 W Honle 395 nm LED lamp, a belt speed of 25 m/min, two passes and the lamp power was set to 100%.

Hardness was assessed using a pencil hardness test according to ASTMD3363. The hardness of the corresponding grade of pencil which did not damage the cured ink film was recorded. The results are provided in Table 2.

Table 2

The ink film of Ink 2 of the invention was the hardest ink film and hence provides excellent resistance properties. Whilst the ink film of Ink 3 of the invention was the softest ink film, it was still sufficiently hard to provide good resistance properties. The inks films of comparative Inks 1 and 4- 7 were also sufficiently hard to provide good resistance properties. Adhesion

Inks 1-7 were drawn down in 12 pm films using a 12 pm wire wound K-bar onto an acrylic substrate and 24 pm films using a 24 pm wire wound K-bar onto a polycarbonate (PC) substrate. The ink films were then cured using a 20 W Honle 395 nm LED lamp, a belt speed of 25 m/min, two passes and the lamp power was set to 100%.

Adhesion was assessed by subjecting the prints to a cross hatch test at 0 hours. The film was cut using an Elcometer cross hatch testing kit. A piece of Nichiban adhesive tape was then firmly applied over the cut area and removed. The degree of film removed with the tape was then quantified as a percentage. 0% removal is excellent, 100% removal is poor. The results are shown in Table 3.

Table 3

Inks 2 and 3 of the invention and comparative Inks 4 and 5 showed good adhesion to acrylic and excellent adhesion to PC. In contrast, comparative Inks 1 , 6 and 7 showed poor adhesion to acrylic with comparative Inks 6 and 7 also showing inferior adhesion to PC.

Elongation

Inks 1-7 were drawn down in 12 pm films using a 12 pm wire wound K-bar onto a white self- adhesive vinyl (SAV) substrate. The ink films were then cured using a 20 W Honle 395 nm LED lamp, a belt speed of 25 m/min, two passes and the lamp powerwas set to 100%.

Elongation was assessed one to two hours after curing using an Instron 5544 tensile tester for digital print sample elongation tests, ISO 527 and ASTM D638. The results are provided in Table 4.

Table 4

Inks 2 and 3 of the invention and comparative Ink 7 showed excellent elongation. In contrast, comparative Inks 1 and 4-6 showed inferior elongation. Cure speed

Inks 1-7 were drawn down in 12 pm films using a 12 pm wire wound K-bar onto a white SAV substrate. The ink films were then cured using a 20 W Honle 395 nm LED lamp, a belt speed of 60 m/min and the lamp power was set to 25%.

Cure speed was assessed using the AMRL cure test by determining the number of passes required to cure each ink film, where cure is defined as the production of a tack-free film and no offset onto oriented photopaper (Epson, Glossy, 200 g/m²) or polypropylene (OPP) is observed. The results are shown in Table 5.

Table 5

Inks 2 and 3 of the invention typically required the least number of passes to achieve cure and hence these inks have the fastest cure speed. In contrast, comparative inks 1 and 4-7 typically required more passes to achieve cure and hence these inks cure more slowly.

Colour

Inks 1-7 were drawn down in 24 pm films using a 24 pm wire wound K-bar onto a white SAV substrate. The ink films were then cured using a 20 W Honle 395 nm LED lamp, a belt speed of 25 m/min, two passes and the lamp power was set to 100%.

Colour was assessed by determining L*, a* and b* values on a photospectrometer at 0 hours. The lightness, L*, represents the darkest black at L*=0, and the brightest white at L*=100. The colour channels, a* and b*, represents true neutral grey values at a*=0 and b*=0. The red/green opponent colours are represented along the a* axis, with green at negative a* values and red at positive a* values. The yellow/blue opponent colours are represented along the b* axis, with blue at negative b* values and yellow at positive b* values. The L*, a* and b* values of the inks are set out in Table 6.

Table 6

All of Inks 1-7 showed acceptable L*, a* and b* values and in particular b* values, meaning that the inks exhibited minimal yellowness. In summary, only Inks 2 and 3 of the invention showed the optimal balance of hardness, adhesion, elongation and cure speed whilst achieving low yellowness. All of comparative Inks 1 and 4-7 were inferior in at least one of adhesion, elongation and cure speed.

Example 2

Inkjet inks were prepared according to the formulations set out in Table 7. 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 7

3-MPDDA, NPGPDA, TPGDA and DPGDA are difunctional (meth)acrylate monomers. PEA, IBOA, CTFA and Medol-10 are monofunctional (meth)acrylate monomers. NVC is an N-vinyl amide monomer. UV12 and MEHQ are stabilisers. OB184 is an optical brightening agent. BAPO is a photoinitiator. TegoRad 2010 is a slip aid. RAD PAR AD-032 is a resin of the invention.

Inks 8-12 are inks of the invention and contain different difunctional (meth)acrylate monomers and amounts of these monomers. Ink 13-15 are comparative inks as they contain additional monofunctional (meth)acrylate monomer instead of at least one of an N-vinyl amide, an N- (meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer.

The surface tension was determined by first measuring the liquid density using a Sigma 702 tensiometer fitted with a Perspex ball. The liquid density difference between the sample and air was then calculated and entered into the tensiometer. The surface tension was then measured using the Sigma 702 tensiometer set to 25°C fitted with a platinum Du-Nouy ring set to pull mode. The surface tension according to the Huh-Mason equation was then recorded.

Inks 8-15 were then assessed for colour, adhesion, elongation and cure speed. The tests and results are set out below.

Colour

Inks 8-15 were drawn down in 40 pm films using a 40 pm wire wound K-bar onto a white SAV substrate. The ink films were then cured using a 20 W Baldwin 395 nm LED lamp, a belt speed of 25 m/min, two passes and the lamp power was set to 100%.

Colour was assessed in the same way as in Example 1 . The L*, a* and b* values of the inks are set out in Table 8.

Table 8

All of Inks 8-15 showed acceptable L*, a* and b* values and in particular b* values, meaning that the inks exhibited minimal yellowness. Adhesion

Inks 8-15 were drawn down in 24 pm films using a 24 pm wire wound K-bar onto acrylic and PC substrates. The ink films were then cured using a 20 W Baldwin 395 nm LED lamp, a belt speed of 25 m/min, two passes and the lamp powerwas set to 100%.

Adhesion was assessed in the same way as in Example 1 . The results are shown in Table 9.

Table 9

Inks 8-12 of the invention showed good/excellent adhesion to acrylic and Inks 8-12 of the invention showed excellent adhesion to PC. Comparative Inks 13-15 showed poor adhesion to acrylic with comparative Inks 15 and 16 also showing poor adhesion to PC.

Elongation

Inks 8-15 were drawn down in 12 pm films using a 12 pm wire wound K-bar onto a white SAV substrate. The ink films were then cured using a 20 W Baldwin 395 nm LED lamp, a belt speed of 25 m/min, two passes and the lamp power was set to 100%.

Elongation was assessed in the same way as in Example 1 . The results are provided in Table 11 .

Table 11

All of Inks 8-15 showed acceptable elongation.

Cure speed

Inks 8-15 were drawn down in 12 pm films using a 12 pm wire wound K-bar onto a white SAV substrate. The ink films were then cured using a 20 W Baldwin 395 nm LED lamp, a belt speed of 60 m/min and the lamp power was set to 20%.

Cure speed was assessed in the same way as in Example 1 . The results are shown in Table 12. Table 12

Inks 8-12 of the invention typically required the least number of passes to achieve cure and hence these inks have the fastest cure speed. In contrast, comparative Inks 13-15 typically required more passes to achieve cure and hence these inks cure more slowly.

In summary, only Inks 8-12 of the invention showed the optimal balance of adhesion, elongation and cure speed whilst achieving low yellowness. All of comparative Inks 13-15 were inferior in adhesion and cure speed.

Claims

1 . An inkjet ink comprising: a difunctional (meth)acrylate monomer; at least one of an N-vinyl amide monomer, an N-(meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer; and a resin comprising a structural unit of

2. An inkjet ink as claimed in claim 1 , wherein the resin is present in an amount of 1 to 10% by weight, preferably 2 to 8% by weight, more preferably 4 to 6% by weight, based on the total weight of the ink.

3. An inkjet ink as claimed in claim 1 or 2, wherein the resin comprises a structural unit of

4. An inkjet ink as claimed in any preceding claim, wherein the resin comprises a structural unit of

5. An inkjet ink as claimed in claim 4, wherein the resin is a homopolymer of diallyl 1 ,2- cyclohexanedicarboxylate.

6. An inkjet ink as claimed in any preceding claim, wherein the difunctional (meth)acrylate monomer is selected from dipropylene glycol diacrylate, tripropylene glycol diacrylate, 3-methyl-1 ,5- pentanediol diacrylate, propoxylated neopentylglycol diacrylate, and mixtures thereof.

7. An inkjet ink as claimed in any preceding claim, wherein the difunctional (meth)acrylate monomer is present in a total amount of 5 to 25% by weight, preferably 10 to 20% by weight, based on the total weight of the ink.

8. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer.

9. An inkjet ink as claimed in any preceding claim, wherein the at least one of an N-vinyl amide monomer, an N-(meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer is present in a total amount of 5 to 30% by weight, preferably 10 to 25% by weight, more preferably 15 to 20% by weight, based on the total weight of the ink.

10. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink further comprises a monofunctional (meth)acrylate monomer, preferably wherein the monofunctional (meth)acrylate monomer is selected from isobornyl acrylate, phenoxyethyl acrylate, cyclic TMP formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate, isopropylidene glycerol acrylate, 4-te/Y-butylcyclohexyl acrylate, 3,3,5-trimethylcyclohexyl acrylate, benzyl acrylate, octadecyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate, isodecyl acrylate, lauryl acrylate and mixtures thereof.

11. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink further comprises a photoinitiator, preferably phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide.

12. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a surfactant.

13. An inkjet ink as claimed in any preceding claim, wherein the ink is free from a colouring agent.

14. A method of inkjet printing comprising inkjet printing the inkjet ink as claimed in any preceding claim onto a substrate and curing the inkjet ink by exposing the inkjet ink to a curing source.

15. A substrate having the inkjet ink as claimed in claims 1 to 13 printed thereon.