Printing ink

Abstract

The present invention relates to an inkjet ink comprising: at least one N-vinyl amide monomer, N- (meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; one or more difunctional (meth)acrylate monomers; one or more divinyl ether monomers; and one or more radiation-curable oligomers, wherein the ink is free of benzophenone. The present invention also relates to a method of inkjet printing.

Description

[0001] Printing ink

[0002] The present invention relates to a printing ink and in particular to an inkjet ink which has a desirable balance of properties. The present invention also relates to a method of printing the ink.

[0003] 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.

[0004] 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 7-1 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.

[0005] Another type of inkjet ink contains unsaturated organic compounds, termed monomers and / or oligomers which polymerise by irradiation, commonly with ultraviolet light, often in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures.

[0006] Inkjet inks have found wide application based on their ability to be digitally printed, together with a useful balance of properties. However, there remains a challenge to provide the necessary printing properties, including good adhesion and rapid curing, whilst providing a high-quality image, without compromising the jetting properties. This is particularly difficult when printing onto self-adhesive vinyl (SAV) substrates.

[0007] The challenge is to achieve the required adhesion of the ink on SAV substrates, whilst maintaining the other required properties, including cure speed, image quality and jetting. There are inks available in the art, which can provide this balance of properties, including Uvijet OB inks available from Fujifilm. However, the inks include benzophenone as a photoinitiator. Unfortunately, for health and safety reasons, there are growing demands for the use of this photoinitiator to be avoided. The present inventors have found that removal of benzophenone is problematic because its absence surprisingly impacts the other properties of the inks. Accordingly, there is a need in the art for an inkjet ink which has a desirable balance of properties, including cure speed and adhesion to difficult substates, particularly SAV substrates, without recourse to the inclusion of benzophenone.

[0008] Accordingly, the present invention provides an inkjet ink comprising: at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; one or more difunctional (meth)acrylate monomers; one or more divinyl ether monomers; and one or more radiation-curable oligomers, wherein the ink is free of benzophenone.

[0009] The inventors have surprisingly found that the specific blend of components provides an inkjet ink with the desired balance of properties including cure speed and adhesion to difficult substates, including SAV substrates, without recourse to the inclusion of benzophenone.

[0010] In this regard, on removal of benzophenone from inks suitable for printing onto SAV substrates, this unsurprisingly had a negative impact on the cure speed. The inventors looked to adjust the photoinitiator package in order to overcome the reduced cure speed. However, on doing so, cure speed improved, but surprisingly, this had a negative impact on adhesion to substrates, including SAV substrates.

[0011] In order to try to improve adhesion, a number of blends of components were used, including the inclusion of PEA, which has previously been shown to improve adhesion on SAV substrates. However, this did not improve adhesion.

[0012] The inventors of the present invention surprisingly found that on inclusion of at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomerto an inkjet ink having the specific blend of components, including one or more difunctional (meth)acrylate monomers, one or more divinyl ether monomers and one or more radiation-curable oligomers, the required balance of properties was provided, including cure speed and adhesion onto a number of substrates, including an SAV substrate, without recourse to the inclusion of benzophenone.

[0013] The ink comprises at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer.

[0014] In a preferred embodiment, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer is present in a total amount of 5 to 20% by weight, preferably 7 to 15% by weight, based on the total weight of the ink. In a preferred embodiment, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer comprises an N-vinyl amide monomer.

[0015] 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 discussed below. Preferred examples are N- vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl formamide and N- vinyl acetamide. N-vinyl caprolactam (NVC) is particularly preferred.

[0016] In a preferred embodiment, the inkjet ink comprises 5 to 20% by weight, preferably 7 to 15% by weight, of one or more N-vinyl amide monomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 5 to 20% by weight, preferably 7 to 15% by weight, of NVC, based on the total weight of the ink.

[0017] In a preferred embodiment, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer comprises an N-(meth)acryloyl amine monomer.

[0018] N-(meth)acryloyl amine monomers are also well-known in the art. N-(meth)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 discussed below. A preferred example is N-acryloylmorpholine (ACMO).

[0019] In a preferred embodiment, the inkjet ink comprises 5 to 20% by weight, preferably 7 to 15% by weight, of one or more N-(meth)acryloyl amine monomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 5 to 20% by weight, preferably 7 to 15% by weight, of ACMO, based on the total weight of the ink.

[0020] In a preferred embodiment, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer comprises an N-vinyl carbamate monomer.

[0021] N- Vinyl carbamate monomers are defined by the following functionality:

[0022] 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,

[0023] 2003, 4.3.2.8, pages 212-213).

[0024] In a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure: in which R1to R4are 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 R1to R4are independently selected from hydrogen or C1-10 alkyl. Further details may be found in WO 2015 / 022228 and US 4,831 ,153.

[0025] Most preferably, the N-vinyl carbamate monomer is 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

[0026] 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 5 to 20% by weight, preferably 7 to 15% by weight, of one or more N-vinyl carbamate monomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 5 to 20% by weight, preferably 7 to 15% by weight, of NVMO, based on the total weight of the ink.

[0027] The inkjet ink may also comprise one or more N-vinyl monomers other than an N-vinyl amide monomer and N-vinyl carbamate monomer. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole.

[0028] In a preferred embodiment, the inkjet ink comprises 5 to 20% by weight in total, preferably 7 to 15% by weight in total, of one or more N-vinyl monomers other than an N-vinyl amide monomer and N- vinyl carbamate monomer, based on the total weight of the ink.

[0029] In a preferred embodiment, the inkjet ink comprises 5 to 20% by weight in total, preferably 7 to 15% by weight in total, of all N-vinyl monomers and N-(meth)acryloyl amine monomers, based on the total weight of the ink.

[0030] N-(meth)acryloyl amine monomers are particularly preferred. Accordingly, in a preferred embodiment, the inkjet ink comprises one or more N-(meth)acryloyl amine monomers and no N- vinyl monomers are present in the ink.

[0031] Therefore, in a preferred embodiment, the inkjet ink contains less than 5% by weight in total, preferably less than 2% by weight in total, more preferably less than 1 % by weight in total, and most preferably is substantially free of N-vinyl monomer, where the amounts are based on the total weight of the ink.

[0032] 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 N-vinyl monomer is intentionally added to the ink. However, minor amounts of N-vinyl monomer, 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 in total, more preferably less than 0.1 % by weight in total and most preferably less than 0.05% by weight in total of N-vinyl monomer, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of N-vinyl monomer.

[0033] The inventors have found that the inclusion of N-(meth)acryloyl amine monomer, preferably ACMO, in the specific blend of components in the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including adhesion to difficult substrates, including SAV substrates, and cure speed, without recourse to benzophenone.

[0034] The inkjet ink comprises one or more difunctional (meth)acrylate monomers. As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers. Forthe 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. Polymerisable groups can be any group that are capable of polymerising upon exposure to radiation. Examples of functional groups that are capable of polymerising upon exposure to radiation include a (meth)acrylate group and a vinyl ether group.

[0035] 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 s1.

[0036] (Meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and / or methacrylate.

[0037] A difunctional (meth)acrylate monomer has two functional groups, specifically (meth)acrylate groups, which take part in the polymerisation reaction on curing. In other words, a difunctional (meth)acrylate monomer is a difunctional monomer in which the only radiation-curable functional groups present in the monomer are (meth)acrylate groups.

[0038] Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Mixtures of difunctional (meth)acrylates may be used. 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 ,1 1-undecanediol dimethacrylate and 1 ,12- dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate and mixtures thereof. The inventors have found that the inclusion of one or more difunctional (meth)acrylate monomers, particularly hexanediol diacrylate (HDDA) and / or dipropylene glycol diacrylate (DPGDA)in the specific blend of components in the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including adhesion to difficult substrates, including SAV substrates, and cure speed, without recourse to benzophenone.

[0039] In a preferred embodiment, the one or more difunctional (meth)acrylate monomers comprise hexanediol diacrylate (HDDA) and / or dipropylene glycol diacrylate (DPGDA).

[0040] In a preferred embodiment, the inkjet ink comprises 40 to 65% by weight, preferably 45 to 60% by weight, of one or more difunctional (meth)acrylate monomers in total, based on the total weight of the ink.

[0041] In a preferred embodiment, the inkjet ink comprises hexanediol diacrylate (HDDA) and / or dipropylene glycol diacrylate (DPGDA) in a total amount of 40 to 65% by weight, preferably 45 to 60% by weight, based on the total weight of the ink. Preferably, the inkjet ink comprises hexanediol diacrylate (HDDA) and dipropylene glycol diacrylate (DPGDA) in a total amount of 40 to 65% by weight, preferably 45 to 60% by weight, based on the total weight of the ink.

[0042] In a preferred embodiment, the one or more difunctional (meth)acrylate monomers comprise hexanediol diacrylate (HDDA) and dipropylene glycol diacrylate (DPGDA), and the weight ratio of hexanediol diacrylate (HDDA) to dipropylene glycol diacrylate (DPGDA) is 0.5-2:1 , more preferably 0.8-1 .2:1 , and most preferably around 1 :1. Accordingly, in a preferred embodiment, there is around equal amounts, by weight, of hexanediol diacrylate (HDDA) and dipropylene glycol diacrylate (DPGDA), present in the ink, based on the total weight of the ink. In a preferred embodiment, the inkjet ink comprises 20-30% by weight of hexanediol diacrylate (HDDA) and 20-30% by weight of dipropylene glycol diacrylate (DPGDA), based on the total weight of the ink.

[0043] The inkjet ink of the present invention comprises one or more divinyl ether monomers.

[0044] Divinyl ether monomers are well known in the art and a detailed description is not required. Mixtures of divinyl ether monomers may 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. A preferred example is triethylene glycol divinyl ether (DVE-3). In a preferred embodiment, the one or more divinyl ether monomers comprise triethylene glycol divinyl ether (DVE-3).

[0045] The inventors have found that the inclusion of one or more divinyl ether monomers, particularly DVE-3, in the specific blend of components in the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including adhesion to difficult substrates, including SAV substrates, and cure speed, without recourse to benzophenone.

[0046] In a preferred embodiment, the inkjet ink comprises 5 to 20% by weight, preferably 7 to 15% by weight, of one or more divinyl ether monomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 5 to 20% by weight, preferably 7 to 15% by weight, of triethylene glycol divinyl ether (DVE-3), based on the total weight of the ink.

[0047] The inkjet ink may comprise other difunctional monomers, including vinyl ether (meth)acrylate monomers. Vinyl ether (meth)acrylate 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.

[0048] In a preferred embodiment, the total amount of all difunctional monomers present in the ink is 45 to 85% by weight, preferably 52 to 75% by weight, based on the total weight of the ink.

[0049] The inkjet ink comprises one or more radiation-curable (i.e. polymerisable) oligomers, 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. Mixtures of radiation-curable oligomers may be used.

[0050] The term “curable oligomer” has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 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.

[0051] 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.

[0052] 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 s1.

[0053] Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. In a preferred embodiment, the one or more radiation-curable oligomers comprise a radiation-curable oligomer comprising a urethane backbone.

[0054] The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. In a preferred embodiment, the one or more radiation-curable oligomers comprise a (meth)acrylate oligomer.

[0055] The inventors have found that the inclusion of one or more radiation-curable oligomers, particularly a polyurethane (meth)acrylate oligomer, in the specific blend of components in the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including adhesion to difficult substrates, including SAV substrates, and cure speed, without recourse to benzophenone.

[0056] In a preferred embodiment, the one or more radiation-curable oligomers comprise a polyurethane (meth)acrylate oligomer.

[0057] The radiation-curable oligomer may include amine functionality, as the amine acts as an activator without the drawback of migration associated with low-molecular weight amines. Amines are commonly used in radiation-curable inkjet inks to improve reactivity and to help mitigate oxygen inhibition. Including amine modification in the (meth)acrylate oligomer adds to the functionality of the (meth)acrylate oligomer without requiring amines as a separate component. This enables greater formulation latitude for optimised photoinitiators, additional radiation-curable material and / or other components. In a preferred embodiment, the one or more radiation-curable oligomers comprise an amine-modified oligomer. In a preferred embodiment, the one or more radiation- curable oligomers comprise an amine-modified (meth)acrylate oligomer.

[0058] Particularly preferred radiation-curable oligomers are di-, tri-, tetra-, penta- or hexa-functional acrylates.

[0059] Preferably, the one or more radiation-curable oligomers comprise an amine-modified acrylate oligomer. A suitable amine-modified polyester acrylate oligomer is commercially available as UVP6600. A suitable amine-modified polyether acrylate oligomer is commercially available as CN3715LM. 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.

[0060] In a preferred embodiment, the inkjet ink comprises 0.5 to 5% by weight, preferably 1 to 4% by weight, of one or more radiation-curable oligomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 0.5 to 5% by weight, preferably 1 to 4% by weight, of one or more polyurethane radiation-curable oligomers in total, based on the total weight of the ink. More preferably, the inkjet ink comprises 0.5 to 5% by weight, preferably 1 to 4% by weight, of one or more polyurethane (meth)acrylate oligomers in total, based on the total weight of the ink.

[0061] The inkjet ink may comprise one or more a monofunctional (meth)acrylate monomers, which are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required. A monofunctional (meth)acrylate monomer has one functional group (a (meth)acrylate group), which takes part in the polymerisation reaction on curing. Mixtures of (meth)acrylates may also be used.

[0062] 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 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.

[0063] The amount of monofunctional (meth)acrylate monomer, when present, is preferably 1-30% by weight in total, more preferably 5-25% by weight in total, based on the total weight of the ink.

[0064] The monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth) acrylate monomer and / or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

[0065] When present, the monofunctional (meth) acrylate monomer may comprise a cyclic monofunctional (meth)acrylate monomer.

[0066] 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, CB- 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.

[0067] The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA / Medol-10), 4-te / Y- butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), benzyl acrylate (BA) and mixtures thereof. Preferably, the cyclic monofunctional (meth)acrylate monomer comprises benzyl acrylate (BA).

[0068] When present, the monofunctional (meth)acrylate monomer may comprise an acyclic-hydrocarbon monofunctional (meth) acrylate monomer.

[0069] 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.

[0070] 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. Preferably, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer comprises lauryl acrylate.

[0071] In a preferred embodiment, when present, 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), 4-te / 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.

[0072] Tetrahydrofurfuryl acrylate (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 tetrahydrofurfuryl acrylate (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.

[0073] 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.

[0074] In a preferred embodiment, the ink contains less than 2% by weight in total, more preferably less than 1 % by weight in total and most preferably is substantially free of monofunctional (meth)acrylate monomer, where the amounts are based on the total weight of the ink.

[0075] 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 monofunctional (meth) acrylate monomer is intentionally added to the ink. However, minor amounts of monofunctional (meth)acrylate monomer, 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 monofunctional (meth)acrylate monomer, preferably less than 0.1 % by weight in total of monofunctional (meth)acrylate monomer, more preferably less than 0.05% by weight in total of monofunctional (meth)acrylate monomer, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of monofunctional (meth)acrylate monomer.

[0076] The inkjet ink may further comprise one or more multifunctional monomers. 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.

[0077] The functional group of multifunctional 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.

[0078] The multifunctional 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 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.

[0079] In a preferred embodiment, the inkjet ink comprises 15% by weight or less in total, preferably 10% by weight or less in total of multifunctional monomer, based on the total weight of the ink. In a particularly preferred embodiment, the inkjet ink comprises 0.5 to 15.0% by weight in total, preferably 0.5 to 10.0% by weight in total of multifunctional monomer, based on the total weight of the ink.

[0080] Examples of the 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.

[0081] When present, the one or more multifunctional monomers preferably comprise a (meth)acrylate monomer. As such, in a preferred embodiment, the inkjet ink comprises one or more multifunctional (meth)acrylate monomers.

[0082] Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-fu notional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet inks include trimethylolpropane triacrylate, dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, di (penta erythritol) hexaacrylate (DPHA), 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. DPHA is particularly preferred.

[0083] Preferably, the inkjet ink comprises 5% by weight or less in total, more preferably 4% by weight or less in total and most preferably 2% by weight or less in total of multifunctional (meth)acrylate monomer, based on the total weight of the ink. Preferably, the inkjet ink comprises 0.5 to 5.0% by weight in total, more preferably 0.5 to 4.0% by weight in total and most preferably 0.5 to 2.0% by weight in total of one or more multifunctional (meth)acrylate monomers, based on the total weight of the ink.

[0084] When present, the multifunctional monomer may have at least one vinyl ether functional group. Examples include a multifunctional vinyl ether monomer and / or a multifunctional vinyl ether (meth)acrylate monomer.

[0085] An example of a multifunctional vinyl ether monomer is tris[4-(vinyloxy)butyl] trimellitate.

[0086] The inkjet ink may contain one or more passive resins. Passive resins are resins which are not radiation-curable and hence do not undergo crosslinking under the curing conditions to which the ink is exposed. In other words, resin is not a radiation-curable material.

[0087] Any passive resin that is compatible with the ink components of the final inkjet ink is suitable for use in the inkjet ink of the present invention. Thus, the ink formulator is able to select from a wide range of suitable passive thermoplastic resins.

[0088] The resin may be selected from epoxy, polyester, vinyl, ketone, nitrocellulose, phenoxy or acrylate resins, or a mixture thereof and is preferably a poly(methyl (meth)acrylate) resin. Methacrylate copolymers are preferred.

[0089] The resin 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 may improve adhesion of the ink to the substrate.

[0090] The resin, when present, is preferably present at 0.1 to 5% by weight, based on the total weight of the ink.

[0091] If the ink is cured by exposure to a source of actinic radiation without an inert environment, one or more photoinitiators will be required. If the ink is cured by exposure to a source of low-energy electron beam radiation or a source of actinic radiation in an inert environment, the ink may still contain a photoinitiator, although photoinitiators are not required.

[0092] In a preferred embodiment, the inkjet ink further comprises one or more photoinitiators.

[0093] 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).

[0094] The inkjet ink may also comprise one or more polymeric photoinitiators, such as Omnipol TP®.

[0095] 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:

[0096] The total value of a, b and c of the chemical formula for polymeric TPO-L is equal to 1-20.

[0097] 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.

[0098] Unfortunately, for health and safety reasons, there are growing demands for the use of benzophenone to be avoided. Accordingly, the inkjet ink of the present invention is free from benzophenone.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] The inkjet ink of the present invention 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.

[0105] Accordingly, the inkjet ink preferably comprises less than 5% by weight of water and volatile organic solvents combined, based on the total weight of the ink. Preferably, the inkjet ink comprises less than 3% by weight of water and volatile organic solvent combined, more preferably less than 2% by weight combined, more preferably less than 1 % by weight combined, and most preferably the inkjet ink is substantially free of water and volatile organic solvents, where the amounts are based on the total weight of the ink.

[0106] 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 comprise less than 0.5% by weight of water or a volatile organic solvent, more preferably less than 0.1% by weight ofwater or a volatile organic solvent, most preferably less than 0.05% by weight of water or a volatile organic solvent, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of water or a volatile organic solvent.

[0107] In a preferred embodiment, the inkjet ink of the present invention also includes a colouring agent, which 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.

[0108] Preferably, the colouring agent 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, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] In a particularly preferred embodiment, the inkjet ink is a cyan, magenta or yellow inkjet ink. The inventors of the present invention have found that the specific blend of components in the inkjet ink of the present invention is particularly advantageous in cyan, magenta and yellow inks for enhancing adhesion to difficult substrates, including SAV substrates, and cure speed, without recourse to benzophenone. The present invention may also provide 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, at least one of the cyan, magenta and yellow inks in the set falls within the scope of the inkjet ink of the present invention. More preferably, the cyan, magenta and yellow inks in the set fall within the scope of the inkjet ink of the present invention. Most preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention.

[0113] 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.

[0114] Other suitable colouring agents include dyes. The dyes include but are not limited to azo dyes, anthraquinone dyes, xanthene dyes, azine dyes, and combinations thereof.

[0115] 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.

[0116] 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 mNnr1and most preferably 20-30 mNnr1.

[0117] Preferably, the inkjet ink comprises 0.1 to 1.5% by weight in total, more preferably 0.1-0.9% by weight in total of surfactant, based on the total weight of the ink.

[0118] 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, defoamers, dispersants, synergists, stabilisers against deterioration by heat or light, 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.

[0119] In a preferred embodiment, the inkjet ink consists of: at least one N-vinyl amide monomer, N- (meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; one or more difunctional (meth)acrylate monomers; one or more divinyl ether monomers; one or more radiation-curable oligomers; and optionally a photoinitiator, a colouring agent, a passive resin, a surfactant, a defoamer, a dispersant, a synergist, a stabiliser against deterioration by heat or light, a reodorant, a flow or slip aid, a biocide, an identifying tracer, a whitening agent, and mixtures thereof, wherein the ink is free from benzophenone.

[0120] In a particularly preferred embodiment, the inkjet ink consists of an N-(meth)acryloyl amine monomer; one or more difunctional (meth)acrylate monomers; one or more divinyl ether monomers; one or more radiation-curable oligomers; and optionally a photoinitiator, a colouring agent, a passive resin, a surfactant, a defoamer, a dispersant, a synergist, a stabiliser against deterioration by heat or light, a reodorant, a flow or slip aid, a biocide, an identifying tracer, a whitening agent, and mixtures thereof, wherein the ink is free from benzophenone.

[0121] 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.

[0122] 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 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] Substrates include flexible and rigid substrates. Examples include substrates composed of self- adhesive vinyl (SAV), 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. The inkjet ink of the present invention is also particularly suitable for printing onto skis.

[0127] Self-adhesive vinyl substrates are particularly difficult substrates to print onto and achieve the required adhesion, whilst maintaining the other required balance of properties, including cure speed, image quality and jetting. However, it has surprisingly been found that the specific blend of components of the inkjet ink of the present invention can achieve excellent adhesion to SAV substrates, whilst maintaining the required balance of properties, including cure speed, without recourse to benzophenone.

[0128] 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.

[0129] The present invention may also provide a printed substrate having the ink as defined herein printed thereon. Preferably, the substrate is a self-adhesive vinyl substrate.

[0130] 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.

[0131] 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.

[0132] 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 201 1 / 021052.

[0133] In a preferred embodiment, the ink is cured by exposing the printed ink to a source of actinic radiation.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] 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. The ink may also be cured by exposing the printed ink to low-energy electron beam (ebeam).

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] The energy associated with these doses is 80-300 keV, more preferably 70-200 keV and most preferably 100 keV.

[0144] 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.

[0145] The invention will now be described with reference to the following examples, which are not intended to be limiting. Examples

[0146] Example 1

[0147] 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.

[0148] Table 1 (comparative inks)

[0149] DPGDA and HDDA are difunctional (meth) acrylate monomers. DVE-3 is a divinyl ether monomer. UV12 is a stabiliser. Ebecryl 230 is an aliphatic urethane diacrylate. CN964 A85 is a urethane acrylate oligomer. Omnirad 184, TPO and benzophenone are photoinitiators. BYK 307 is a surfactant.

[0150] The cyan pigment dispersion contains 30.0% blue pigment, 10.0% polymeric dispersing aid, 59.0% difunctional (meth)acrylate monomer and 1.0% stabiliser. The magenta pigment dispersion contains 30.0% magenta pigment, 42.0% polymeric dispersing aid and 28.0% divinyl ether monomer. The yellow pigment dispersion contains 30.0% pigment, 22.50% dispersing aid and 47.50% divinyl ether monomer. The pigment dispersions were prepared by mixing the components in the given amounts and passing the mixtures through a bead mill until the dispersions had a particle size of less than 0.3 microns. Amounts are given as weight percentages based on the total weight of the dispersion.

[0151] Inks 1-3 are comparative examples as they do not comprise at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer. They also comprise benzophenone, which is avoided for health and safety concerns.

[0152] The inkjet inks were tested for adhesion to an SAV substrate and good adhesion and cure speed was achieved. The adhesion and cure assessment are detailed below.

[0153] Example 2

[0154] Inkjet inks were prepared according to the formulations set out in Table 2. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

[0155] Table 2 (comparative inks)

[0156] The components are as described for Example 1. KIP 160 is a photoinitiator. CN3715 is an monofunctional acrylate amine synergist. Inks 4-6 are comparative examples as they do not comprise at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer. Inks 4-6 mirror inks 1-3, with the exception that benzophenone has been removed, which has been replaced with Omnirad 184 in the cyan ink, KIP 160 in the magenta ink and a monofunctional acrylate amine synergist in the yellow ink. On removal of the benzophenone and replacement with alternative photoinitiators or a monofunctional acrylate amine synergist, cure speed was maintained and was comparable to inks 1-3. However, poor adhesion to an SAV substrate was surprisingly observed for all of inks 4- 7 when compared to inks 1-3. The adhesion and cure assessment are detailed below.

[0157] Example 3

[0158] Inkjet inks were prepared according to the formulations set out in Table 3. 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.

[0159] Table 3 (comparative inks)

[0160] The components are as described for Examples 1 and 2. PEA is a monofunctional (meth)acrylate monomer. CN3715 is a monofunctional acrylated amine synergist.

[0161] Inks 7-9 are comparative examples as they do not comprise at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer. In comparison to inks 1-3, inks 7-9 do not comprise benzophenone, and PEA was added to try to increase adhesion. The cure speed of inks 7-9 was comparable to inks 1-3 but poor adhesion to an SAV substrate was achieved for inks 7-9 when compared to inks 1-3. The adhesion and cure assessment are detailed below.

[0162] Example 4

[0163] Inkjet inks were prepared according to the formulations set out in Table 4. 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.

[0164] Table 4 (inks of the invention)

[0165] The components are as described for Examples 1-3.

[0166] Inks 10-12 are inks of the invention as they comprise at least one N-vinyl amide monomer, N- (meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; a difunctional (meth)acrylate monomer; a divinyl ether monomer; and a radiation-curable oligomer and are free from benzophenone.

[0167] Inks 10-12 were tested for adhesion to an SAV substrate and good adhesion and cure speed was achieved, without recourse to benzophenone. Surprisingly therefore, on addition of at least one N- vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer to the specific blend of components as claimed, good adhesion to an SAV substrate, and good cure speed was achieved, without the presence of benzophenone. The adhesion and cure assessment are detailed below.

[0168] Test results

[0169] Adhesion assessment

[0170] In order to assess adhesion, the inkjet inks of Examples 1 to 4 were each drawn down in 12 pm films using a 12 pm wire wound K-bar onto an SAV substrate. The ink films were then passed under a mercury lamp at 50% power at a speed of 40 m / min, which gives a dose of 148 mJ / cm2and an intensity of 1487 mW / cm2per pass, using a Jenton UV curing system, to provide a cured ink film.

[0171] The adhesion was assessed by subjecting the prints to cross hatch test after at least 24 hours. The film was cut using an Elcometer 107 cross hatch testing kit. A piece of ISO 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.

[0172] Inks 1-3 were used as the control cyan, magenta and yellow inks, respectively, as they are the current industry standard displaying good adhesion to SAV substrates. They were awarded a pass.

[0173] The amount of ink removed during the tests for inks 4, 7 and 10 were compared to ink 1. The amount of ink removed during the tests for inks 5, 8 and 11 were compared to ink 2. And the amount of ink removed during the tests for inks 6, 9 and 12 were compared to ink 3. If the inks showed a comparable ink removal to inks 1 , 2 or 3, they were awarded a pass. If more ink was removed with the tape than inks 1 , 2 or 3, they were awarded a fail.

[0174] The results are provided in Table 5.

[0175] Cure assessment

[0176] In order to assess cure, the inkjet inks of Examples 1 to 4 were each drawn down in 12 pm films using a 12 pm wire wound K-bar onto an SAV substrate. The ink films were then passed under a mercury lamp at 50% power at a speed of 40 m / min, which gives a dose of 148 mJ / cm2and an intensity of 1487 mW / cm2per pass, using a Jenton UV curing system, to provide a cured ink film.

[0177] At least 24 hours after curing, the degree of cure was assessed by applying a strip of photo-paper over the cured ink film. The ink film was deemed to be cured and awarded a pass, if no tack (or removal of ink) was observed on the photo-paper. The ink film was deemed not to have cured and awarded a fail, if tack (or removal of ink) was observed on the photo-paper. The results are set out in Table 5. Results

[0178] Table 5

Claims

Claims1. An inkjet ink comprising: at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; one or more difunctional (meth)acrylate monomers; one or more divinyl ether monomers; and one or more radiation-curable oligomers, wherein the ink is free of benzophenone.

2. An inkjet ink as claimed in claim 1 , wherein the at least one N-vinyl amide monomer, N- (meth)acryloyl amine monomer and / or N-vinyl carbamate monomer is present in a total amount of 5 to 20% by weight, preferably 7 to 15% by weight, based on the total weight of the ink.

3. An inkjet ink as claimed in claims 1 or 2, wherein the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer comprises an N- (meth)acryloyl amine monomer.

4. An inkjet ink as claimed in any preceding claim, wherein the N-(meth)acryloyl amine monomer comprises N-acryloylmorpholine.

5. An inkjet ink as claimed in any preceding claim, wherein the one or more difunctional (meth)acrylate monomers are present in a total amount of 40 to 65% by weight, preferably 45 to 60% by weight, based on the total weight of the ink.

6. An inkjet ink as claimed in any preceding claim, wherein the one or more difunctional (meth)acrylate monomers comprise hexanediol diacrylate and / or dipropylene glycol diacrylate.

7. An inkjet ink as claimed in claim 6, wherein the weight ratio of hexanediol diacrylate to dipropylene glycol diacrylate is 0.5-2:1 , preferably 0.8-1 .2:1 , and more preferably around 1 :1.

8. An inkjet ink as claimed in any preceding claim, wherein the one or more divinyl ether monomers are present in a total amount of 5 to 20% by weight, preferably 7 to 15% by weight, based on the total weight of the ink.

9. An inkjet ink as claimed in any preceding claim, wherein the one or more divinyl ether monomers comprise triethylene glycol divinyl ether.

10. An inkjet ink as claimed in any preceding claim, wherein the one or more radiation-curable oligomers are present in a total amount of 0.5 to 5%, preferably 1 to 4% by weight, based on the total weight of the ink.

11. An inkjet ink as claimed in any preceding claim, wherein the one or more radiation-curable oligomers comprise a polyurethane radiation-curable oligomer, preferably a polyurethane (meth)acrylate oligomer.

12. An inkjet ink as claimed in any preceding claim, wherein the ink comprises a colouring agent, preferably a dispersed pigment.

13. An inkjet ink as claimed in any preceding claim, wherein the ink is a cyan, magenta or yellow ink.

14. An inkjet ink as claimed in any preceding claim, wherein the ink consists of: at least one N- vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; one or more difunctional (meth)acrylate monomers; one or more divinyl ether monomers; one or more radiation-curable oligomers; and optionally a photoinitiator, a colouring agent, a passive resin, a surfactant, a defoamer, a dispersant, a synergist, a stabiliser against deterioration by heat or light, a reodorant, a flow or slip aid, a biocide, an identifying tracer, a whitening agent, and mixtures thereof, wherein the ink is free from benzophenone.

15. 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, preferably wherein the substrate is a self-adhesive vinyl substrate.

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