WO2025078829A1

WO2025078829A1 - Printing ink

Info

Publication number: WO2025078829A1
Application number: PCT/GB2024/052609
Authority: WO
Inventors: Andrew Phillips
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2023-10-12
Filing date: 2024-10-11
Publication date: 2025-04-17
Anticipated expiration: 2026-04-12
Also published as: GB202315672D0

Abstract

The present invention provides an inkjet ink comprising: a cyclic monofunctional (meth)acrylate monomer; an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer; a cyan pigment or a magenta pigment; and an oligomer having the following formula. The invention also provides a method of inkjet printing the inkjet ink of the invention onto a substrate and curing the ink.

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 7-11 mPas at the jetting temperature which is often elevated to, but not limited to 40-50°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent such as water or a low-boiling solvent or mixture of solvents.

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

Inks, which cure by the polymerisation of monomers and/or oligomers, may contain a wide variety of monomers and/or oligomers, including monofunctional, difunctional and multifunctional monomers, and radiation-curable oligomers. The challenge is to provide the necessary printing properties, including rapid curing (achieving both surface and through cure), whilst providing a high-quality image, without compromising the jetting properties.

For high-speed printing applications, cure speeds of 60 m/min or above are required. By cure speed is meant the speed at which the source of radiation moves relative to the substrate.

In order to increase the cure speed, inkjet inks are often formulated with at least one of difunctional (meth)acrylate monomers, multifunctional (meth)acrylate monomers and/or radiation- curable oligomers. However, inks produced from these materials are usually high viscosity, inflexible and suffer from shrinkage of the cured film. In order to offset such issues, monofunctional monomers can be added, but this reduces the cure speed of the ink as they are slow curing. It is particularly challenging to achieve such a balance of properties, including high cure speed, flexibility in the cured film and low viscosity, when using LEDs as the source of radiation. This is because LEDs have a narrow wavelength output and reduced energy output when compared to other sources of radiation, including mercury discharge lamps and flash lamps.

However, LEDs offer a number of advantages as the source of radiation, when compared to, for example, mercury discharge lamps (the most common UV light source used to cure inkjet inks). In this regard, LEDs offer significant cost reduction, longer maintenance intervals, higher energy efficiency and are an environmentally friendlier solution. LEDs have a longer lifetime and exhibit no change in the power/wavelength output over time. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate.

Cyan and magenta inks are particularly challenging. In this regard, it is often difficult to achieve an acceptable cure for UV cyan inks, and a higher pigment loading is often needed for magenta inks, which leads to reduced latitude for formulating magenta inks.

There is therefore a need in the art to provide an inkjet ink, which has a high UV LED cure speed, good surface and through cure, whilst maintaining a jettable viscosity and the required flexibility of the cured film.

Accordingly, the present invention provides an inkjet ink comprising: a cyclic monofunctional (meth)acrylate monomer; an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer; a cyan pigment or a magenta pigment; and an oligomer having the following formula:

The inventors have surprisingly found that the inclusion of the claimed oligomer in a cyan or magenta inkjet ink of the present invention, having the specific blend of components, provides improved LED cure speed, surface and through cure, whilst maintaining a jettable viscosity and the required flexibility of the cured film. By cure speed is meant the speed at which the source of radiation moves relative to the substrate.

Such improved LED cure speed, surface and through cure, is not observed when using other blends of components, including other oligomers, which are marketed as faster curing when compared to that claimed.

The inkjet ink of the present invention comprises a cyclic monofunctional (meth)acrylate monomer. The inventors have found that the inclusion of a cyclic monofunctional (meth)acrylate monomer in the inkjet ink of the present invention contributes to the low viscosity of the ink and improved cure speed, surface and through cure, and flexibility, particularly when using an LED cure source.

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, 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. Examples of groups that are capable of polymerising upon exposure to radiation include a (meth)acrylate group and a vinyl ether group.

(Meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required. For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate.

Monomers typically have a molecular weight of less than 600, preferably more than 200. 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. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique I 2° steel cone at 25°C with a shear rate of 25 s ¹.

Cyclic monofunctional (meth)acrylate monomers are well known in the art. The substituents of the cyclic monofunctional (meth)acrylate monomer 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 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, Cs- aryl and combinations thereof, any of which may substituted with alkyl (such as Ci-is alkyl) and/or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above-described substituents. The substituents may together also form a cyclic structure.

Preferably, the cyclic monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-te/Y- butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), benzyl acrylate (BA), isopropylidene glycerol acrylate (IPGA) and mixtures thereof.

In a particularly preferred embodiment, the cyclic monofunctional (meth)acrylate monomer comprises isobornyl acrylate (IBOA) and/or phenoxyethyl acrylate (PEA).

In a preferred embodiment, wherein the inkjet ink is a magenta inkjet ink and comprises a magenta pigment, the cyclic monofunctional (meth) acrylate monomer comprises (2-methyl-2- ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10). It has surprisingly been found that the inclusion of this monomer reduces the viscosity and improves the flexibility of the cured film, whilst maintaining the required cure speed, through cure and surface cure, for a magenta inkjet ink.

Therefore, for the magenta ink, the cyclic monofunctional (meth)acrylate monomer preferably comprises (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10) and optionally isobornyl acrylate (IBOA) and/or phenoxyethyl acrylate (PEA). Preferably, for the magenta ink, the cyclic monofunctional (meth)acrylate monomer comprises (2-methyl-2-ethyl-1 ,3-dioxolane-4- yl)methyl acrylate (MEDA/Medol-10), isobornyl acrylate (IBOA) and phenoxyethyl acrylate (PEA).

Preferably, the ink comprises 10 to 60% by weight, preferably 25 to 50% by weight, of cyclic monofunctional (meth) acrylate monomer, based on the total weight of the ink.

Preferably, the ink comprises 10 to 60% by weight, preferably 25 to 50% by weight, of PEA, IBOA or a combination thereof, based on the total weight of the ink.

For the magenta ink, the ink preferably comprises 10 to 60% by weight, preferably 25 to 50% by weight, of PEA, IBOA, Medol-10 or a combination thereof, based on the total weight of the ink.

The ink may also comprise an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

When present, 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.

When present, the acyclic-hydrocarbon monofunctional (meth) acrylate monomer may contain 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.

Lauryl acrylate is particularly preferred. Lauryl acrylate is preferred because it has a long straight chain that introduces flexibility into the cured ink film.

When present, the ink preferably comprises 1 to 20% by weight, preferably 1 to 10% by weight of acyclic-hydrocarbon monofunctional (meth)acrylate monomer, based on the total weight of the ink.

The inkjet ink of the present invention comprises an N-vinyl amide monomer and/or an N- (meth)acryloyl amine monomer. The inventors have found that the inclusion of an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer in the inkjet ink of the present invention contributes to the low viscosity of the ink and improved cure speed, surface and through cure and flexibility, particularly when using an LED cure source.

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. Preferred examples are N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl formamide and N-vinyl acetamide.

Similarly, N-acryloyl amine monomers are also well-known in the art. N-Acryloyl amine monomers 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).

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

Preferably, the inkjet ink comprises an N-vinyl amide monomer, preferably NVC.

Preferably, the inkjet ink comprises an N-(meth)acryloyl amine monomer, preferably ACMO. In a preferred embodiment, the inkjet ink comprises 5 to 35% by weight, more preferably 10 to 30% by weight and most preferably 15 to 25% by weight in total of an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises 5 to 35% by weight, more preferably 10 to 30% by weight and most preferably 15 to 25% by weight of an N-vinyl amide monomer, based on the total weight of the ink. More preferably, the inkjet ink comprises 5 to 35% by weight, more preferably 10 to 30% by weight and most preferably 15 to 25% by weight of NVC, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises 5 to 35% by weight, more preferably 10 to 30% by weight and most preferably 15 to 25% by weight of an N-(meth)acryloyl amine monomer, based on the total weight of the ink. Most preferably, the inkjet ink comprises 5 to 35% by weight, more preferably 10 to 30% by weight and most preferably 15 to 25% by weight of ACMO, based on the total weight of the ink.

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

In a preferred embodiment, the inkjet ink comprises 50 to 80% by weight in total, more preferably 60 to 75% by weight in total of monofunctional monomers including cyclic monofunctional (meth)acrylate monomer, and an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer, based on the total weight of the ink.

The inkjet ink of the present invention comprises an oligomer having the following formula:

Radiation-curable (i.e. polymerisable) oligomers are discussed generally below. As can be seen from the structure, the radiation-curable oligomer used in the ink of the present invention is a diacrylate oligomer, is amine-modified and has a polyether backbone. Accordingly, it is an amine-modified polyether acrylate oligomer.

It has surprisingly been found that the inclusion of this specific oligomer in the cyan or magenta inkjet inks of the present invention having the specific blend of components claimed, provides an inkjet ink, which has a high UV LED cure speed, good surface and through cure, whilst maintaining a jettable viscosity and the required flexibility of the cured film.

Such improved LED cure speed, surface and through cure, is not observed when using other blends of components, including other oligomers, which have higher functionality and are thus marketed as faster curing when compared to that claimed.

In this regard, it has surprisingly been found that the inclusion of the claimed oligomer, which is an amine-modified diacrylate oligomer, has a faster LED cure speed, and improved through and surface cure, in the cyan or magenta inkjet ink having the specific blend of components as claimed, when compared to a cyan or magenta ink including an amine-modified tetraacrylate oligomer. Of course, it would be expected that a higher functionality oligomer would provide increased cure speed and provide improved surface and through cure, and this is suggested by the manufacturer. However, surprisingly, in the cyan and magenta inks having the specific blend of components as claimed, this is not the case and the inclusion of the specific amine-modified diacrylate oligomer as claimed provides improved LED cure speed, and improved through and surface cure, whilst maintaining a jettable viscosity and required flexibility of the cure film.

More specifically, it has been observed that a cyan or magenta inkjet ink having the specific blend of components and an oligomer having the following formula:

has improved cure speed, through cure and surface cure, whilst maintaining a jettable viscosity and required flexibility of the cured film, when compared to a cyan or magenta ink including an oligomer having the following formula:

Thus, the present invention may further provide the use of an oligomer having the following formula:

for increasing the cure speed of a cyan or magenta inkjet ink, preferably LED cure speed.

Preferably, the present invention provides the use of an oligomer having the following formula:

for increasing the cure speed (preferably LED cure speed) of a cyan or magenta inkjet ink, wherein the cure speed is increased relative to a cyan or magenta inkjet ink, respectively, which does not comprise this oligomer but comprises an oligomer having the following formula:

In a preferred embodiment, the ink comprises 0.5 to 5% by weight of the oligomer having the following formula:

based on the total weight of the ink.

The oligomer used in the ink of the present invention is commercially available as Ebecryl 81 from

Allnex. Ebecryl 81 is provided as a mixture comprising the oligomer having the formula:

and a trifunctional (meth)acrylate monomer of formula:

The amount of trifunctional (meth)acrylate monomer present in Ebecryl 81 is typically 45 to 55% by weight, based on the total amount of Ebecryl 81 .

Accordingly, when using Ebecryl 81 as the source of the claimed oligomer used in the inkjet ink of the present invention, the ink preferably comprises 1 to 10% by weight of Ebecryl 81 , based on the total weight of the ink.

The inkjet ink of the present invention may further comprise additional radiation-curable oligomer, other than the oligomer having the formula:

Any additional radiation-curable oligomer that is compatible with the other ink components is suitable for use in the ink. Preferably, the inkjet ink comprises an additional (meth)acrylate oligomer, other than

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.

In the case of the oligomer having the following formula:

the radiation-curable oligomer comprises one reacted acrylate group and two unreacted acrylate groups.

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

Oligomers typically have 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 additional 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 additional oligomer determines the degree of crosslinking and hence the properties of the cured ink. The additional 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 additional 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 I 2° steel cone at 60°C with a shear rate of 25 s ¹.

The polymerisable group of the additional oligomer can be any group that is capable of polymerising upon exposure to radiation. Preferably the additional oligomers are (meth)acrylate oligomers.

The additional oligomer may include amine functionality, as the amine acts as an activator without the drawback of migration associated with low-molecular weight amines. Amines improve reactivity and help mitigate oxygen inhibition. Including amine modification in the oligomer adds to the functionality of the oligomer without requiring amines as a separate component. This enables greater formulation latitude for optimised photoinitiators, additional radiation-curable material and/ or other components. Therefore, when present, the additional radiation-curable oligomer is preferably amine-modified. More preferably, when present, the additional radiation- curable oligomer is an amine-modified (meth)acrylate oligomer (in that an amine has reacted with a (meth)acrylate group), other than the oligomer having the formula:

Other suitable examples of additional 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 additional radiation-curable oligomer, when present, is preferably 0.1 to 10% by weight, based on the total weight of the ink.

In a preferred embodiment, the amount of radiation-curable oligomer in total, is 0.5 to 10% by weight, preferably 0.5 to 5% by weight, based on the total weight of the ink.

In a preferred embodiment however, no additional radiation-curable oligomer is present in the ink.

Accordingly, in a preferred embodiment, the inkjet ink comprises less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free of radiation-curable oligomer, other than the oligomer having the formula

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. In other words, no radiation-curable oligomer, other than

is intentionally added to the ink. However, minor amounts of oligomer, other than

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, most preferably less than 0.05% by weight of radiation-curable oligomer, other than

based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of radiation- curable oligomer, other than

In a preferred embodiment therefore, the oligomer having the formula

is the only radiation-curable oligomer present in the ink.

In a preferred embodiment, the inkjet ink of the present invention comprises a difunctional monomer and/or multifunctional monomer. The inclusion of a difunctional monomer and/or multifunctional monomer in the inkjet ink of the invention further improves the cure speed of the ink.

In a preferred embodiment, the inkjet ink comprises 0.5 to 20% by weight in total of difunctional monomer and/or multifunctional monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises a difunctional monomer.

The functional group of the difunctional monomer may be the same or different but must take part in the polymerisation reaction on curing. Examples of such functional groups include any groups that are capable of polymerising upon exposure to radiation and are preferably selected from a (meth)acrylate group and a vinyl ether group.

The substituents of the difunctional monomer are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, CB- aryl and combinations thereof, such as Cs- aryl- or C3-18 cycloalkylsubstituted 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. The substituents may together also form a cyclic structure.

In a preferred embodiment, the inkjet ink comprises 0.5 to 20% by weight, more preferably 5 to 15% by weight of difunctional monomer, based on the total weight of the ink.

Examples of a difunctional monomer include difunctional (meth)acrylate monomers, divinyl ether monomers, and difunctional vinyl ether (meth)acrylate monomers. Mixtures of difunctional monomers may be used. In a preferred embodiment, the difunctional monomers comprise difunctional (meth)acrylate monomers. By difunctional (meth)acrylate monomers, it is meant difunctional monomers in which the only radiation-curable functional groups present in the monomer are (meth)acrylate groups. In other words, the two groups which take part in the polymerisation reaction on curing are both (meth)acrylate groups. Such difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required.

In a preferred embodiment, the inkjet ink comprises a difunctional (meth)acrylate monomer.

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-MPDA), and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, 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.

Preferably, the inkjet ink comprises 0.5 to 20% by weight, more preferably 5 to 15% by weight of difunctional (meth)acrylate monomer, based on the total weight of the ink.

In a particularly preferred embodiment, the inkjet ink comprises 1 ,10-decanediol diacrylate (DDDA). Preferably, the inkjet ink comprises 0.5 to 20% by weight, more preferably 5 to 15% by weight of DDDA, based on the total weight of the ink.

The inventors have surprisingly found that the inclusion of DDDA in the cyan or magenta inkjet ink as claimed, having the specific blend of components, including the oligomer, provides improved flexibility, whilst maintaining improved cure speed, through and surface cure, and jetting viscosity.

In one embodiment, when present, the difunctional monomers comprise divinyl ether monomers. Such difunctional monomers are well known in the art and a detailed description is not required. Examples include triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1 ,4- cyclohexanedimethanol divinyl ether and mixtures thereof. In one embodiment, when present, the difunctional monomers comprise vinyl ether (meth)acrylate monomers. Such difunctional monomers are well known in the art and a detailed description is not required. Examples include 2-(2-vinyloxy ethoxy)ethyl acrylate (“VEEA”), 2-(2- vinyloxy ethoxy)ethyl methacrylate (“VEEM”) and mixtures thereof.

In a preferred embodiment, the ink comprises 10% or less of difunctional monomers, other than difunctional (meth)acrylate monomers. In one embodiment, no further difunctional monomers, other than difunctional (meth)acrylate monomers, are present in the ink.

In a preferred embodiment, the inkjet ink comprises a multifunctional radiation-curable monomer.

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

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

The multifunctional radiation-curable monomer may possess different degrees of functionality, and a mixture including combinations of tri and higher functionality monomers may be used.

The substituents of the multifunctional radiation-curable monomer are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, CB- 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. The substituents may together also form a cyclic structure.

In a preferred embodiment, the inkjet ink comprises 0.5 to 10.0% by weight of a multifunctional radiation-curable monomer, based on the total weight of the ink.

Examples of multifunctional radiation-curable monomer include multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers and multifunctional vinyl ether (meth)acrylate monomers. Mixtures of multifunctional radiation-curable monomers may also be used. In a preferred embodiment, the multifunctional monomers comprise multifunctional (meth)acrylate monomers. By multifunctional (meth)acrylate monomers, it is meant multifunctional monomers in which the only radiation-curable functional groups present in the monomer are (meth)acrylate groups. In other words, the three or more groups which take part in the polymerisation reaction on curing are all (meth)acrylate groups. Such multifunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required.

In a preferred embodiment, the inkjet ink comprises a multifunctional (meth)acrylate monomer.

Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet inks include trimethylolpropane triacrylate, dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate and ethoxylated pentaerythritol tetraacrylate (EOPETTA, also known as PPTTA), 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.

In a preferred embodiment, the multifunctional monomer comprises di(pentaerythritol) hexaacrylate (DPHA) and/or

The inkjet ink preferably comprises 0.5 to 10.0% by weight of multifunctional (meth)acrylate monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises 0.5 to 10.0% by weight of

based on the total weight of the ink.

The inventors have surprisingly found that the inclusion of this trifunctional monomer in the cyan or magenta inkjet ink as claimed, having the specific blend of components, including the oligomer, provides improved cure speed, surface and through cure, whilst maintaining flexibility of the cured film and jetting viscosity.

Preferably, the ratio of the amount by weight of oligomer having the formula:

to the amount by weight of the trifunctional monomer having the formula:

is 0.5 to 1 .5 : 1 , preferably 0.8 to 1 .25 : 1 . This ratio provides an improved balance of properties, including improved cure speed.

In a preferred embodiment, the inkjet ink comprises 0.5 to 10.0% by weight of di(pentaerythritol) hexaacrylate (DPHA), based on the total weight of the ink.

The inventors have surprisingly found that the inclusion of this monomer increases cure speed, through and surface cure, whilst maintaining the flexibility and viscosity, for a cyan inkjet ink. Therefore, for the cyan ink, the multifunctional (meth)acrylate monomer preferably comprises di (penta erythritol) hexaacrylate (DPHA).

In a preferred embodiment, wherein the inkjet ink is a cyan inkjet ink and comprises a cyan pigment, the inkjet ink further comprises a multifunctional (meth)acrylate monomer, preferably wherein the multifunctional (meth)acrylate monomer comprises di(pentaerythritol) hexaacrylate (DPHA).

The multifunctional radiation-curable monomer may have at least one vinyl ether functional group.

In a preferred embodiment, the inkjet ink comprises a multifunctional vinyl ether monomer and/or a multifunctional vinyl ether (meth)acrylate monomer. Such multifunctional monomers are well known in the art and a detailed description is not required.

An example of a multifunctional vinyl ether monomer is tris[4-(vinyloxy)butyl] trimellitate.

In a preferred embodiment, the ink comprises 10% or less of multifunctional monomers, other than multifunctional (meth)acrylate monomers. In one embodiment, no further multifunctional monomers, other than multifunctional (meth)acrylate monomers, are present in the ink.

In a preferred embodiment, the difunctional and/or multifunctional radiation-curable monomer is selected from 1 ,10-decanediol diacrylate (DDDA), di (penta erythritol) hexaacrylate (DPHA),

mixtures thereof.

In a preferred embodiment, the inkjet ink of the present invention comprises a difunctional (meth)acrylate monomer and/or multifunctional (meth) acrylate monomer.

In a preferred embodiment, the inkjet ink comprises 0.5 to 20% by weight in total of difunctional (meth)acrylate monomer and/or multifunctional (meth)acrylate monomer, based on the total weight of the ink.

The ink may also contain a resin. The resin preferably has a weight-average molecular weight of 20-200 KDa and preferably 20-60 KDa, as determined by GPC with polystyrene standards. The resin is preferably solid at 25°C. It is preferably soluble in the liquid medium of the ink (the radiation-curable diluent and, when present, additionally the solvent).

The resin 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.

The resin may improve adhesion of the ink to the substrate. It is preferably soluble in the ink. The resin, when present, is preferably present at 0.1 to 5% by weight, based on the total weight of the ink.

The inkjet ink of the present invention comprises a cyan pigment or a magenta pigment. The ink of the invention is therefore a cyan or magenta ink.

It has surprisingly been found that the inclusion of the claimed oligomer in a cyan or magenta inkjet ink of the present invention, having the specific blend of components, provides improved LED cure speed, surface and through cure, whilst maintaining a jettable viscosity and the required flexibility of the cured film. Such improvement has been shown to be the case, particularly for cyan and magenta inks.

The cyan or magenta pigment is a dispersed pigment. Examples are well known in the art and are commercially available such as under the trade-names Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). Mixtures of pigments may be used.

The cyan pigment is preferably a phthalocyanine pigment, such as Phthalocyanine blue 15.4.

In a preferred embodiment, the inkjet ink comprises a cyan pigment and the difunctional monomer and/or multifunctional monomer comprises di(pentaerythritol) hexaacrylate (DPHA).

The magenta pigment is preferably a quinacridone pigment, including mixed crystal quinacridones such as Cromophtal Jet magenta 2BC, Magenta L4540 and Cinquasia RT-355D.

In a preferred embodiment, the inkjet ink comprises a magenta pigment and the cyclic monofunctional monomer comprises (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10).

The inks can be categorised on the CIELAB (L*a*b*) colour space system. 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.

Preferably, the cyan ink has a* value from -60 to -10 and b* value from -70 to -20, preferably a* value from -55 to -15 and b* value from -65 to -25, more preferably a* value from -50 to -20 and b* value from -60 to -30. The L* value will depend on the lightness of the cyan ink. The cyan ink may have L* value from 35 to 69, preferably 40 to 67, more preferably 42 to 65. Or the cyan ink may be a lighter cyan and have L* value from 71 to 105, preferably 73 to 100, more preferably 75 to 95.

Preferably, the magenta ink has a* value from 55 to 105 and b* value from -40 to 10, preferably a* value from 60 to 100 and b* value from -35 to 5, more preferably a* value from 65 to 95 and b* value from -30 to 0, most preferably a* value from 70 to 90 and b* value from -25 to -5. The L* value will depend on the lightness of the magenta ink. The magenta ink may have L* value from 30 to 64, preferably 35 to 62, more preferably 40 to 60. Or the magenta ink may be a lighter magenta and have L* value from 66 to 100, preferably 68 to 95, more preferably 70 to 90.

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 cyan pigment or magenta pigment is preferably present in an amount of 0.2 to 20% by weight, preferably 0.5 to 15% by weight, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink further comprises one or more photoinitiators.

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), 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) and mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Omnirad (from IGM) and Esacure (from Lamberti).

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 preferably, the ink comprises a plurality of free radical photoinitiators. The total number of free radical photoinitiators present is preferably from one to five, and more preferably, two or more free radical photoinitiators are present in the ink.

Preferably, the one or more photoinitiators if present, are present from 1 to 20% by weight in total, preferably from 5 to 15% by weight in total, 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 one or more photoinitiator may be present in an amount of less than 20% by weight in total, preferably less than 5% by weight in total, more preferably less than 3% by weight in total, more preferably less than 1% by weight in total, 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 in total of photoinitiator, more preferably less than 0.1% by weight in total of photoinitiator and most preferably less than 0.05% by weight in total 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 in total of one or more photoinitiators, 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.

The ink preferably contains less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1 % by weight of water and a volatile organic solvent, based on the total weight of the ink. Preferably, the inkjet ink is substantially free of water and volatile organic solvents. 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 water and a volatile organic solvent is intentionally added to the ink. However, minor amounts of water and a volatile organic solvent, 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 water and a volatile organic solvent, more preferably less than 0.1% by weight of water and a volatile organic solvent, most preferably less than 0.05% by weight of water and a volatile organic solvent, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of water and a volatile organic solvent.

In a preferred embodiment, the ink of the present invention 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. An example of a suitable surfactant is BYK307. In a preferred embodiment, the inkjet ink comprises an acrylated surfactant. Acrylated surfactants are particularly preferred as they can be partially included in the crosslink network on cure. Preferred examples of acrylated surfactants are commercially available as Tego Rad 2010 and Tego Rad 2300.

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 mNm ¹ and most preferably 20-30 mNm ¹.

Preferably, the inkjet ink comprises 0.1 to 1.5% by weight in total, more preferably 0.1-1 .2% by weight in total of surfactant, based on the total weight of the ink.

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 or light, reodorants, flow or slip aids, biocides and identifying tracers.

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

The 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 ink most preferably has a viscosity of 8 to 20 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 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

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, the magenta and cyan ink of the ink 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. Other inkjet ink sets may also be used, such as CMYK+white and light colours. For example, the inkjet ink set of the present invention may additionally include orange, green and/or violet inks. A well-known inkjet ink set useful for the present invention is a so-called “CMYKOG” inkjet ink set, which includes a cyan ink, a magenta ink, a yellow ink, a black ink, an orange ink and a green 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 of the present invention is particularly suitable for high-speed printing applications. The present invention also provides a method of inkjet printing using the above-described ink or ink set and a substrate having the ink or ink set cured thereon.

Accordingly, the present invention further provides a method of inkjet printing comprising inkjet printing the inkjet ink or inkjet ink set as defined herein onto a substrate and curing the ink.

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. 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 one or more substrates 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.

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 may also provide 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 and most preferably below 10 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, 8 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 and low-energy electron beam 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 2011/021052.

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.

Preferably, the source of actinic radiation is LEDs. LEDs are increasingly used to cure inkjet inks. UV 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.

There are many advantages of using LEDs as the UV source. In this regard, LEDs are cost effective, have long maintenance intervals, have high energy efficiency and are an environmentally friendly option. LEDs have a longer lifetime and exhibit no change in the power/wavelength output over time. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate.

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

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

Preferably, the wavelength of the UV LED source substantially matches the absorption profile of the ink. In a preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 360 nm to 410 nm. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm.

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

The inventors have surprisingly found that the inkjet ink of the present invention is particularly suitable for curing by a UV LED source. It is often a challenge to achieve fast cure speed and through and surface cure, when using a UV LED source as the curing source. However, the inventors have found that the cyan and magenta inkjet inks as claimed, having the specific blend of components, including the oligomer as claimed, achieves high cure speed, through cure and surface cure, whilst maintaining a jettable viscosity and flexibility of the cured film.

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 method of the present invention may comprise a step of pinning the ink by exposing the ink to a first dose of radiation, followed by a step of curing the ink by exposing to a second dose of radiation.

Accordingly, in a preferred embodiment, the method of the present invention comprises inkjet printing the inkjet ink or inkjet ink set as defined herein onto a substrate, exposing the inkjet ink or inkjet ink set to a first dose of radiation to pin (partial-cure) the ink and exposing the inkjet ink or inkjet ink set to a second dose of radiation to fully cure the ink.

In other words, following the partial-cure of the inkjet ink through exposure to a first dose of radiation, the inkjet ink is fully cured by exposing the ink to a second dose of radiation. That is, an additional dose of radiation to that required for pinning.

Any source of radiation as described above may be used to pin or cure the ink. A combination of sources may be used.

In a preferred embodiment, the method of the present invention comprises pinning the inkjet ink or inkjet ink set of the present invention by exposing the inkjet ink or inkjet ink set to a first dose of radiation from an LED source and fully curing the inkjet ink or inkjet set of the present invention by exposing the inkjet ink or inkjet ink set to a second dose of radiation from an LED source.

The terms “partial-cure” and “full-cure” refer to the degree of curing of the inkjet ink.

By partial-cure (also known as pinning) is meant arresting the flow of the ink by exposing the inkjet ink to a low dose of radiation quickly after the ink has impacted onto the substrate surface. Pinning provides a partial-cure of the ink and hence does not achieve full-cure of the ink. Partialcure does not result in full surface cure. The partially cured ink will typically be liquid or a tacky film.

By full-cure is meant fully curing the ink such that the surface is fully cured. Full curing of the ink can be assessed by placing a piece of Epson photo paper face down on the cured ink film. The back of the photo paper is rubbed by hand in both directions 10 times. The photo paper is then pulled away from the cured ink film. If the ink film is fully cured, the paper will separate with no offset and no noise. If the ink film is not fully cured, the ink film will offset to the photo paper and/or noise will be observed. The ink film will offset to the photo paper and/or noise will be observed if force is required to separate the partially cured film and the photo paper.

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 for each LED source used for pinning is lower than the dose for each mercury lamp required to cure the ink fully at a constant print speed.

It is preferable to arrest the flow of the ink by pinning the ink droplets quickly after they have impacted on the substrate surface. To achieve a good quality image it is preferable that the inks are pinned within 5 seconds of impact, preferably within 1 second and more preferably within 0.5 seconds and most preferably within 0.1 seconds.

The delay between pinning and providing a final cure of the ink is less critical than the initial pinning of the ink, but is typically at least 1 minute after jetting.

The pinning and/or curing steps may be performed in an inert atmosphere, e.g. using a gas such as nitrogen.

In a preferred embodiment, the method of inkjet printing of the present invention comprises inkjet printing the inkjet ink from a printing head onto a substrate, wherein the printing head moves relative to the substrate at a speed of 60 m/min or more. The method is thus a method of highspeed printing. High-speed printing is a term of the art. Preferably, in the method of the present invention, the printing head moves relative to the substrate at a speed of 60 m/min or more, preferably 80 m/min or more, more preferably 100 m/min or more. Typically, the printing head can move relative to the substrate at a speed of up to 300 m/min. But for some applications, the speed can be even higher than 300 m/min.

In a preferred embodiment, the printing head moves relative to the substrate at a speed of 60 to 200 m/min, preferably 60 to 150 m/min, most preferably 60 to 100 m/min.

The advantage of high-speed printing is that a low dose per unit area is required to achieve a fully cured film. The inventors have surprisingly found that the inkjet ink of the present invention, having the specific blend of components provides fast curing, through and surface cure, whilst maintaining the required jetting viscosity and flexibility of the cured film.

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 24 pm, preferably 1 to 12 pm, for example 2 to 6 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 Tables 1 and 2. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1

Table 2

PEA, IBOA, Medol-10, DPHA, DDDA, NVC and PEG600DA are monomers, as defined herein. Ebecryl 81 comprises an oligomer having the following formula:

Ebecryl 80 comprises an oligomer having the following formula:

UV-12 is a stabiliser. OH Tempo is a polymerisation inhibitor and a stabiliser. TPO, Omnirad 184, ITX and Bapo are photoinitiators. TegoRad 2010 is a surfactant.

The cyan pigment dispersion contains 10.00% of a dispersant, 1.00% of a stabiliser, 59.00% PEA and 30.00% of a blue pigment. The magenta pigment dispersion contains 12.00% of a dispersant, 1.50% of a stabiliser, 56.50% PEA and 30.00% of a magenta pigment.

The pigment dispersions were prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns. Amounts are given as weight percentages based on the total weight of the dispersion.

Inks 1-2, 4-7 and 9-10 are comparative inks as they do not comprise an oligomer having the following formula:

Inks 1-2 and 6-7 are commercially available comparative inks.

Inks 3 and 8 are inks of the invention.

Inks 1-10 were then assessed for surface cure and through cure.

The tests and the results are set out below.

Surface cure

Inks 1-10 were drawn down in 12 pm films using a K2 bar onto SAV (Avery 400).

The 12 pm drawdowns were cured using a 16 W Honle 395 nm LED lamp, a belt speed of 60 m/min and the lamp power was set to 20%.

In order to test the surface cure of each print, a strip of Epson paper was applied, glossy side down, across each of the prints. The back of the strip of Epson paper is then rubbed with a piece of Tork Wipe 10 times (10 double rubs). A double rub is where the wipe is applied to one side of the Epson paper and under light pressure, traverses the length of the Epson paper in a single stroke, and then traverses back again in a single stroke. The strip of Epson paper is then removed and the glossy side is examined for evidence of off-setting from the print.

If no off-setting occurs, full cure is indicated and the dose to achieve full surface cure was recorded. If off-setting occurs, the print is passed through the curing unit and retested with a fresh strip of Epson paper. This was repeated until full cure is achieved and the dose to achieve full surface cure was recorded, or if full cure is not achieved, the highest dose applied was recorded. The results are set out in Table 3. Table 3

A pass for surface cure was recorded when full surface cure is achieved at the dose specified. A fail for surface cure was recorded when full surface cure is not achieved at the dose specified.

As can be seen from Table 3, inks 3 and 8, which are inks of the invention, pass and achieve surface cure at a dose of 410 and 154 mJ/cm², respectively. This is in marked contrast to current commercially available inks 1 , 2, 6 and 7, which fail at the dose specified. Comparative inks 4 and 9, which comprise an alternative oligomer to that used in the ink of the present invention, pass and fail for surface cure, respectively. However, comparative ink 4 passes for surface cure at a higher dose when compared to comparable ink 3 of the present invention, and comparative ink 9 fails for surface cure at a higher dose when compared to comparable ink 8 of the present invention. This is surprising as the oligomer used in the ink of the present invention (Ebecryl 81) is an amine-modified diacrylate oligomer and the oligomer present in comparative inks 4 and 9 (Ebecryl 80) is an amine-modified tetraacrylate oligomer. It would therefore be expected that, and in fact the manufacturer teaches that, an inkjet ink comprising Ebecryl 80 exhibits improved cure over Ebecryl 81. The present inventors have surprisingly found however that for cyan and magenta inks comprising the particular blend of components as claimed, including the oligomer as claimed, this achieves improved surface cure when compared to comparative inks.

Comparative inks 5 and 10, which do not comprise an oligomer as claimed and have additional TPO, fail for surface cure at a higher dose than the comparable inks of the invention.

Through cure

Inks 1-10 were drawn down in 12 pm films using a K2 bar onto SAV (Avery 400). The 12 pm drawdowns for inks 1-5 were cured using a 16 W Honle 395 nm LED lamp, a belt speed of 60 m/min and the lamp power was set to 40%, which gives a dose of 133 mJ/cm² at intensity 2918 mW/cm².

The 12 pm drawdowns for inks 6-10 were cured using a 16 W Honle 395 nm LED lamp, a belt speed of 70 m/min and the lamp power was set to 20%, which gives a dose of 59 mJ/cm² at intensity 1565 mW/cm². In order to test the through cure of each print, isopropanol was applied to a Tork Wipe. The isopropanol Tork Wipe was then rubbed backwards and forwards across the print. After each rub, the wipe is inspected for colour transfer from the print to the wipe. The number of isopropanol rubs were recorded until noticeable colour transfer occurs or until 100 isopropanol rubs are executed, whichever occurs first. The results are set out in Table 4.

Table 4

A pass for through cure was recorded when no noticeable colour transfer occurs from the print to the wipe, after 100 isopropanol rubs. A fail for through cure was recorded when noticeable colour transfer occurs from the print to the wipe (before 100 isopropanol rubs) and the number of isopropanol rubs required for noticeable colour transfer is recorded. As can be seen from Table 3, inks 3 and 8, which are inks of the invention, pass and maintain through cure at 100 isopropanol rubs. This is in marked contrast to current commercially available inks 1 , 2, 6 and 7, which fail at the number of isopropanol rubs specified for each.

Comparative ink 4, which comprises an alternative oligomer to that used in the ink of the present invention, fails for through cure after 10 isopropanol rubs. This is surprising as the oligomer used in the ink of the present invention (Ebecryl 81) is an amine-modified diacrylate oligomer and the oligomer present in comparative ink 4 (Ebecryl 80) is an amine-modified tetraacrylate oligomer. It would therefore be expected that, and in fact the manufacturer teaches that, an inkjet ink comprising Ebecryl 80 exhibits improved cure over Ebecryl 81. The present inventors have surprisingly found however that for cyan and magenta inks comprising the particular blend of components as claimed, including the oligomer as claimed, this achieves improved through cure when compared to comparative inks.

For completeness, comparative ink 5, which does not comprise an oligomer as claimed and uses additional TPO, fails for through cure at 64 isopropanol rubs. Comparative inks 9 and 10 also achieve a pass for through cure after 100 isopropanol rubs, but do not pass for surface cure.

Viscosity and flexibility Inks 1-10 have a low, jettable viscosity and show flexibility of the cured film.

Claims

1. An inkjet ink comprising: a cyclic monofunctional (meth)acrylate monomer; an N-vinyl amide monomer and/or an N-(meth)acryloyl amine monomer; a cyan pigment or a magenta pigment; and an oligomer having the following formula:

2. An inkjet ink as claimed in claim 1 , wherein the ink comprises 10 to 60% by weight of cyclic monofunctional (meth) acrylate monomer, based on the total weight of the ink.

3. An inkjet ink as claimed in claims 1 or 2, wherein the cyclic monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1 ,3-dioxolane-4- yl)methyl acrylate (MEDA/Medol-10), 4-te/Y-butylcyclohexyl acrylate (TBCHA), 3,3,5- trimethylcyclohexyl acrylate (TMCHA), benzyl acrylate (BA) and mixtures thereof, preferably wherein the cyclic monofunctional (meth)acrylate monomer comprises isobornyl acrylate (IBOA) and/or phenoxyethyl acrylate (PEA).

4. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 5 to 35% by weight in total of N-vinyl amide monomer and/or N-(meth)acryloyl amine monomer, based on the total weight of the ink.

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

6. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 0.5 to 5% by weight of the oligomer having the following formula:

based on the total weight of the ink.

7. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a difunctional monomer and/or multifunctional monomer, preferably a difunctional and/or multifunctional (meth)acrylate monomer.

8. An inkjet ink as claimed in claim 7, wherein the ink comprises 0.5 to 20% by weight in total of difunctional monomer and/or multifunctional monomer, based on the total weight of the ink.

9. An inkjet ink as claimed in claims 7 or 8, wherein the difunctional monomer and/or multifunctional monomer comprises 1 ,10-decanediol diacrylate (DDDA), di (penta erythritol) hexaacrylate (DPH A),

or mixtures thereof.

10. An inkjet ink as claimed in claim 9, wherein the ink comprises 0.5 to 10% by weight of

based on the total weight of the ink.

11. An inkjet ink as claimed in claims 9 or 10, wherein the ink comprises 5 to 15% by weight of 1 ,10-decanediol diacrylate (DDDA), based on the total weight of the ink.

12. An inkjet ink as claimed in any of claims 7 to 11 , wherein the inkjet in comprises a cyan pigment and the difunctional monomer and/or multifunctional monomer comprises di (penta erythritol) hexaacrylate (DPHA).

13. An inkjet ink as claimed in any of claims 1 to 11 , wherein the inkjet ink comprises a magenta pigment and the cyclic monofunctional monomer comprises (2-methyl-2-ethyl-1 ,3-dioxolane-4- yl)methyl acrylate (MEDA/Medol-10) .

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

15. A method of inkjet printing comprising inkjet printing the inkjet ink as claimed in any of claims 1 to14 onto a substrate and curing the ink, preferably curing the ink by an LED source.