GB2606449A - Printing ink - Google Patents

Abstract

An inkjet ink having a viscosity of 30-100 mPa s at 40°C comprises (i) a radiation-curable oligomer, a passive thermoplastic resin, and/or a multifunctional monomer and (ii) 0.2-2 wt. % fumed silica, wherein if the ink comprises a passive thermoplastic resin, it also comprises a radiation-curable component. Typically, the ink comprises a non-wetting organic solvent and 0.5-1.5 wt.% fumed silica, especially hydrophilic silica. The radiation-curable component may be an oligomer or a multifunctional or other monomer, such as a difunctional (meth)acrylate. The oligomer may be an epoxy acrylate and the radiation-curable monomer may be hexanediol diacrylate. The ink may comprise 0.1-25 wt.% radiation-curable oligomer, 0.1-5 wt.% passive thermoplastic resin, 30-90 wt.% multifunctional monomer, and 1-20 wt.% photoinitiator. The ink preferably contains less than 5 wt.% water and less than 5 wt.% hyperdispersants. A method of inkjet printing the ink is also disclosed.

Description

Printing ink This invention relates to a printing ink, and in particular to an inkjet ink comprising fumed silica.

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

The inkjet ink must be formulated carefully to ensure the required jetting properties are achieved.

For example, the ink formulator must balance the ability to provide accurate placement of the inkjet ink droplets on to the substrate, whilst minimising satellite drop formation. One approach to provide this balance of properties is to provide an inkjet ink, which exhibits shear-thinning behaviour.

The inclusion of fumed silica to the inkjet ink has been considered to provide such shear-thinning properties to an inkjet ink. However, fumed silica has not found wide application in inkjet ink compositions owing to difficulties in formulating inks, which contain fumed silica.

There is therefore a need in the art for an inkjet ink, which comprises fumed silica and has improved jetting properties.

Accordingly, the present invention provides an inkjet ink comprising: at least one of a radiation-curable oligomer, a passive thermoplastic resin and a multifunctional monomer; 0.2 to 2.0% by weight of fumed silica, based on the total weight of the ink; with the proviso that when the inkjet ink comprises a passive thermoplastic resin, the inkjet ink further comprises a radiation-curable component; and wherein the ink has a viscosity of 30 to 100 mPas at 40°C.

It has surprisingly been found that it is possible to provide an inkjet ink, which comprises fumed silica and has improved jetting properties by providing an inkjet ink comprising: at least one of a radiation-curable oligomer, a passive thermoplastic resin and a multifunctional monomer; 0.2 to 2.0% by weight of fumed silica, based on the total weight of the ink; with the proviso that when the inkjet ink comprises a passive thermoplastic resin, the inkjet ink further comprises a radiation-curable component; and having a viscosity of 30 to 100 mPas at 40°C.

It has previously been considered that the inclusion of fumed silica in an inkjet ink, where the ink has such a high viscosity, as being unsuitable for inkjet printing as this provides poor jetting properties.

However, the inventors have found that by using a particular print head, it is possible to use such a high viscosity inkjet ink comprising: at least one of a radiation-curable oligomer, a passive thermoplastic resin and a multifunctional monomer; 0.2 to 2.0% by weight of fumed silica, based on the total weight of the ink; with the proviso that when the inkjet ink comprises a passive thermoplastic resin, the inkjet ink further comprises a radiation-curable component; without compromising the jetting properties of the inkjet ink.

The inkjet ink comprises 0.2 to 2.0% by weight of fumed silica, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises 0.5 to 1.5% by weight of fumed silica, based on the total weight of the ink. The fumed silica is dispersed in the inkjet ink.

Fumed silica (also known as pyrogenic silica) exists in the form of microscopic particles of amorphous silica fused into three-dimensional secondary particles, which then agglomerate into tertiary particles. The primary particle size is 5-50 nm. Fumed silica is commercially available, for example from Evonik under the trade name Aerosil0 and from Cabot under the trade name Cabo-sil®.

The fumed silica comprises hydrophilic fumed silica and/or hydrophobic fumed silica. Preferably, the fumed silica is hydrophilic fumed silica.

The inkjet inks of the present invention may comprise various components that are suitable for an inkjet ink and which are known to the skilled person in the art.

The inkjet ink comprises at least one of a radiation-curable oligomer, a passive thermoplastic resin and a multifunctional monomer. When the inkjet ink comprises a passive thermoplastic resin, the inkjet ink further comprises a radiation-curable component.

The inkjet ink may contain one or more radiation-curable oligomers. Any radiation-curable oligomer that is compatible with the other ink components is suitable for use in the ink, providing the viscosity requirements of the inkjet ink are met.

Preferably, the radiation-curable (i.e. polymerisable) oligomer is a (meth)acrylate oligomer.

The term "curable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 600. The molecular weight is preferably 4,000 or less. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

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

Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide film-forming properties such as hardness or cure speed. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25s* Such radiation-curable oligomers suitable for use in the present invention comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation polymerisable groups. The oligomer preferably comprises a urethane backbone. ;The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably, the oligomers are (meth)acrylate oligomers. Preferably, they are multifunctional and most preferably have a functionality of 2-6. Particularly preferred radiation-curable oligomers are di-, tri-, tetra-, penta-or hexa-functional acrylates. ;Particularly preferred radiation-curable materials are urethane acrylate oligomers as these have excellent compatibility with the inkjet ink comprising 0.2 to 2.0% by weight of fumed silica, based on the total weight of the ink. Furthermore, the urethane acrylate oligomers provide adhesion and elongation properties to the inkjet ink. Tr-, tetra-, penta-or hexafuncfional urethane acrylates, particularly hexafunctional urethane acrylates, are preferred as these yield films with good solvent resistance. For inks which cure solely by exposure to actinic radiation ("100% UV inks") lower functionality oligomers are preferred, typically di-or trifunctional. ;Other suitable examples of radiation-curable oligomers include epoxy-based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which provide fast cure speeds in the resulting inkjet ink of the present invention and provide cured films with good solvent resistance. ;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. In a preferred embodiment, the radiation-curable oligomer is amine modified. In a particularly preferred embodiment, the radiation-curable oligomer is an amine-modified (meth)acrylate oligomer. ;More preferably, the radiation-curable oligomer is 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. ;In one embodiment, the radiation-curable oligomer polymerises by free-radical polymerisation. In an alternative embodiment of the invention, the radiation-curable oligomer is capable of polymerising by cationic polymerisation. ;Any radiation-curable oligomer that is compatible with the ink components of the final inkjet ink is suitable for use in the inkjet ink of the present invention, providing the viscosity requirements of the inkjet ink are met. Thus, the ink formulator is able to select from a wide range of suitable oligomers. For the reasons discussed hereinbelow for the organic solvent and/or other radiation-curable monomer, in a preferred embodiment, the radiation-curable oligomer is a non-wetting radiation-curable oligomer. ;The amount of radiation-curable oligomer, when present, is preferably 0.1 to 25% by weight, based on the total weight of the ink. ;The inkjet ink may contain one or more passive (or "inert") thermoplastic 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. ;Any passive thermoplastic 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, providing the viscosity requirements of the inkjet ink are met. Thus, the ink formulator is able to select from a wide range of suitable passive thermoplastic resins. ;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. ;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. ;The resin, when present, is preferably present at 0.1 to 5% by weight, based on the total weight of the ink. ;When the inkjet ink comprises a passive thermoplastic resin, the inkjet ink further comprises a radiation-curable component. The radiation-curable component is not limited and may include the radiation-curable oligomer, the multifunctional monomer and/or other radiation-curable monomers discussed hereinbelow. In a preferred embodiment, the radiation-curable component comprises at least one of the radiation-curable oligomer and the multifunctional monomer. Other radiation-curable monomers include mono-and di-functional monomers discussed hereinbelow. ;The inkjet ink may contain one or more multifunctional monomers. ;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. ;The functional group of the multifunctional radiation-curable monomer, which may be 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, C6-10 aryl and combinations thereof, such as C6-10 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. ;Examples of multifunctional monomer include multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers and multifunctional vinyl ether (meth)acrylate monomers. Mixtures of multifunctional monomers may also be used. ;The multifunctional monomer used in the ink of the present invention preferably has a molecular weight of at least 300 to less than 600 and is therefore a high molecular weight monomer. ;In a preferred embodiment, the one or more multifunctional monomers comprise a tri-, tetra-, penta, hexa-, hepta-or octa-functional monomer, i.e. the radiation-curable monomer has three, four, five, six, seven or eight functional groups. Preferably, the one or more multifunctional monomers comprise a trifunctional and/or a tetrafunctional monomer. ;In a preferred embodiment, the one or more multifunctional monomers comprise a multifunctional (meth)acrylate monomer. Multifunctional (meth)acrylate monomers are also well known in the art and have a functionality of three or higher. ;Examples of the multifunctional (meth)acrylate monomers that may be included in the inkjet ink include trimethylolpropane triacrylate (TMPTA), dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, di-pentaerythritol hexaacrylate (DPHA), di-trimethylolpropane tetraacrylate (DiTMPTA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), glycerolpropoxy triacrylate (G PTA), propoxylated pentaerythritol triacrylate and ethoxylated pentaerythritol tetraacrylate (EOPETTA, also known as PPTTA), and mixtures thereof. 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. ;The multifunctional 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. ;An example of a multifunctional vinyl ether monomer is tris[4-(vinyloxy)butyl] trimellitate. ;The multifunctional monomer may comprise a cationically curable monomer. Suitable cationically curable materials include, oxetanes, cycloaphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The multifunctional monomer may comprise a mixture of cationically curable monomers. ;In a preferred embodiment, the multifunctional monomer has a viscosity of 60 mPas or above at 25°C, preferably 100 mPas or above at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 25°C with a shear rate of 25 s -1. ;Any multifunctional monomer that is compatible with the ink components of the final inkjet ink is suitable for use in the inkjet ink of the present invention, providing the viscosity requirements of the inkjet ink are met. Thus, the ink formulator is able to select from a wide range of suitable multifunctional monomers. For the reasons discussed hereinbelow for the organic solvent and/or other radiation-curable monomer, in a preferred embodiment, the multifunctional monomer is a non-wetting multifunctional monomer. ;The amount of multifunctional monomer, when present, is preferably 30 to 90% by weight, based on the total weight of the ink. More preferably, the inkjet ink comprises 50 to 80% by weight of one or more multifunctional monomers, based on the total weight of the ink. ;By "radiation-curable" is meant a material that contains functional groups that are capable of polymerising upon exposure to radiation. For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. ;The radiation-curable oligomer, passive thermoplastic resin and/or multifunctional monomer are preferably selected according to their compatibility with the inkjet ink comprising fumed silica of the present invention. ;A high amount of multifunctional monomer, radiation-curable oligomer and/or passive thermoplastic resin in an inkjet ink comprising fumed silica, which results in a high viscosity inkjet ink, have previously been seen as unsuitable for inkjet printing as it provides poor jetting properties. ;However, the present inventors have found that by using a particular print head, it is possible use such components, without compromising the jetting properties of the inkjet ink. ;In a preferred embodiment, the inkjet ink comprises at least two of a radiation-curable oligomer, a passive thermoplastic resin and/or a multifunctional monomer. In a preferred embodiment, the inkjet ink comprises a radiation-curable oligomer and a passive thermoplastic resin. Preferably, the inkjet ink comprises a passive thermoplastic resin and a multifunctional monomer. Preferably, the inkjet ink comprises a radiation-curable oligomer and a multifunctional monomer. ;The inkjet inks of the present invention may take the form of solely radiation-curable inks or hybrid inkjet inks. There is no limitation to the formulation of the inkjet inks of the present invention and may be formulated in any manner to provide the required inkjet ink properties. ;In one embodiment, the inkjet ink is a radiation-curable inkjet ink. Radiation-curable inkjet inks comprise a radiation-curable component and the ink cures by polymerisation of the radiation-curable component present. Accordingly, a solvent is not required. In such inks, the inks are preferably substantially free of water and volatile organic solvents, although some water will typically be absorbed by the ink from the air or volatile organic solvent may be present in the components of the inks (e.g. in the pigment dispersion), and such levels are tolerated. ;In a further embodiment, the inkjet ink is a hybrid inkjet ink. Hybrid inkjet inks comprise a radiation-curable component and an organic solvent. The organic solvent is in the form of a liquid at ambient temperatures and is capable of acting as a carrier for the remaining components of the ink. The organic solvent component of the ink may be a single solvent or a mixture of two or more solvents. ;As with known solvent-based inkjet inks, the organic solvent used in the ink of the present invention is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry. ;The inkjet ink therefore comprises a radiation-curable component. Accordingly, when the inkjet ink comprises a passive thermoplastic resin, the inkjet ink further comprises a radiation-curable component. The radiation-curable component is not limited, providing the viscosity requirements of the inkjet ink are met, and may include the radiation-curable oligomer, the multifunctional monomer and/or other radiation-curable monomers. ;In a preferred embodiment, the inkjet ink comprises one or more radiation-curable monomers, other than the multifunctional monomer. Other radiation-curable components include mono-and di-functional monomers. ;Monomers typically have a molecular weight of more than 200 and less than 600. Molecular weights (number average) can be calculated if the structure of the monomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards. ;The one or more other radiation-curable monomers may possess different degrees of functionality, and a mixture of monomers may be used. ;The inkjet ink of the present invention may comprise one or more monofunctional monomers, such as a monofunctional (meth)acrylate monomer. ;Monofunctional monomers are well-known in the art. A radiation-curable monofunctional monomer has one functional group, which takes part in the polymerisation reaction on curing. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation and is preferably selected from a (meth)acrylate group and a vinyl ether group. ;The substituents of the monofunctional 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, Cs_10 aryl and combinations thereof, such as C8-10 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. ;Monofunctional monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. The inkjet ink of the present invention has a high viscosity of 30 to 100 mPas at 40°C and as such, if present, the monofunctional monomers are limited to the extent that the viscosity requirements of the inkjet ink are met. ;As such, in a preferred embodiment, the amount of monofunctional monomer present in the inkjet ink is typically less than 20% by weight, based on the weight of the ink. ;If monofunctional monomers are present in the inkjet ink, the inkjet ink preferably comprises a monofunctional (meth)acrylate monomer, which are well-known in the art and are preferably the esters of acrylic acid. Mixtures of (meth)acrylates may also be used. ;The substituents of the monofunctional (meth)acrylate monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer. ;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, C13- 10 aryl and combinations thereof, any of which may be substituted with alkyl (such as C1-18 alkyl) and/or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tertbutylcyclohexyl acrylate (TBCHA), benzyl acrylate (BA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof. ;The substituents of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is C1-18 alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted. ;The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched Cs-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 C5-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. ;If monofunctional monomers are present in the inkjet ink, the inkjet ink preferably comprises at least one N-vinyl amide monomer and/or N-(meth)acryloyl amine monomer. ;N-Vinyl amide monomers are well-known monomers in the art. N-Vinyl amide monomers have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. 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). ;The inkjet ink may also comprise an N-vinyl monomer other than an N-vinyl amide monomer and/or N-(meth)acryloyl amine monomer. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole. ;The inkjet ink of the present invention may further contain one or more difunctional monomers. ;Preferably, the inkjet ink of the present invention comprises 5 to 90% by weight of one or more difunctional monomers, based on the total weight of the ink. More preferably, the inkjet ink comprises 30 to 80% by weight of one or more difunctional monomers, based on the total weight of the ink. ;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 subsfituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of subsfituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, CB-10 aryl and combinations thereof, such as C6-10 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. ;Examples of difunctional monomer include difunctional (meth)acrylate monomers, divinyl ether monomers and difunctional vinyl ether (meth)acrylate monomers. Mixtures of difunctional monomers may also be used In a preferred embodiment, the inkjet ink of the present invention comprises one or more difunctional (meth)acrylate monomers. ;Difunctional (meth)acrylate monomers are well known in the art. Examples include hexanediol diacrylate (HDDA), 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate (DDDA), 1,11-undecanediol diacrylate and 1,12-dodecanediol diacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA, PEG600DA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1,5-pentanediol diacrylate (3-MPDDA), propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof. Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate and 1,12-dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate and mixtures thereof. 3-MPDDA is particularly preferred. ;Preferably, the inkjet ink of the present invention comprises 5 to 90% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink. More preferably, the inkjet ink comprises 30 to 80% by weight of one or more difunctional (meth)acrylate monomers, based on the total weight of the ink. ;In a preferred embodiment, the inkjet ink of the present invention comprises one or more divinyl ether monomers and/or divinyl ether (meth)acrylate monomers. ;Examples of divinyl ether monomers 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)butyperephthalate, bis[4-(vinyloxymethyl)cyclohexylmethyl] glutarate, 1,4-butanediol divinyl ether and mixtures thereof. ;Examples of vinyl ether (meth)acrylate monomers include 2-(2-vinyloxy ethoxy)ethyl acrylate (VEEA), 2-(2-vinyloxy ethoxy)ethyl methacrylate (VEEM) and mixtures thereof. ;Preferably, the inkjet ink of the present invention comprises 5 to 90% by weight of one or more divinyl ether monomers and/or divinyl ether (meth)acrylate monomers, based on the total weight of the ink. More preferably, the inkjet ink comprises 30 to 80% by weight of one or more divinyl ether monomers and/or divinyl ether (meth)acrylate monomers, based on the total weight of the ink. ;The inkjet ink of the present invention may comprise a cationically curable monomer. Suitable cationically curable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The radiation-curable material according to this embodiment may comprise a mixture of cationically curable monomers. ;The inkjet ink of the present invention optionally comprises an organic solvent and is thus a hybrid inkjet ink. When the organic solvent is present, the inkjet ink of the present invention preferably comprises 30 to 90% by weight of an organic solvent, based on the total weight of the ink. ;The organic solvent and/or additional radiation-curable monomer comprises any organic solvent and/or radiation-curable monomers, other than multifunctional monomer, suitable for use in inkjet inks, providing the viscosity requirements of the inkjet ink are met. Thus, the ink formulator is able to select from a wide range of suitable organic solvents and/or radiation-curable monomers, which are well known in the art. ;Preferably, there is a difference in polarity between (i) the organic solvent and/or radiation-curable monomer and (ii) the fumed silica of the present invention, in order to allow for the 3D network (gel-like structure) to form. As the polarity of the organic solvent and/or radiation-curable monomer increases, the efficiency of the formation of the 3D network decreases when using a hydrophilic fumed silica. This can be compensated for by using an increased amount of fumed silica. ;The organic solvent and/or radiation-curable monomer is thus preferably a non-wetting organic solvent and/or a non-wetting radiation-curable monomer. Non-wetting means that the organic solvent and/or radiation-curable monomer is selected such that the fumed silica is not wetted by the organic solvent and/or radiation-curable monomer. The relative polarity of the fumed silica and the non-wetting organic solvent and/or non-wetting radiation-curable monomer should be suitable to prevent the fumed silica particles from becoming wetted by the non-wetting organic solvent and/or non-wetting radiation-curable monomer. ;If fumed silica is dispersed in an organic solvent and/or radiation-curable monomer, which possesses good wetting properties towards the surface of the fumed silica (i.e. the relative polarities of the solvent and the fumed silica are too close), then the fumed silica particles absorb molecules from the organic solvent and/or radiation-curable monomer until the surface energy is minimised relative to the environment. This results in the fumed silica particles being wetted. Such wetted particles which are densely covered by molecules identical to the surrounding organic solvent and/or radiation-curable monomer float in the medium independently of each other. This means that the 3D network previously described cannot form and results in only a minor increase in viscosity of the dispersion compared to the organic solvent and/or radiation-curable monomer alone. As a consequence and under the influence of gravity, the particles settle. This can be observed with hydrophilic fumed silica in hydrophilic fluids, such as alcohols, acetone and polar solvents. ;This is in marked contrast to fumed silica dispersed in a non-wetting organic solvent and/or non-wetting radiation-curable monomer. For example, hydrophilic fumed silica dispersed in non-polar solvents, such as mineral spirit. Such a combination results in an increase in viscosity of the dispersion. Likewise, the addition of hydrophobic fumed silica into polar solvents results in an increase in viscosity. In such cases, the polarity of the fumed silica and the non-wetting organic solvent and/or non-wetting radiation-curable monomer differ such that wetting of the silica particles is prevented, i.e. the molecules of the non-wetting organic solvent and/or non-wetting radiation-curable monomer cannot adsorb onto the surface of the fumed silica. Hence, the formation of a 3D network of the fumed silica particles may form, which results in an increase in viscosity and results in a gel-like structure. This 3D network forms in order to minimise the free energy of the surface of the fumed silica and it exhibits viscoelastic properties resulting in enhanced viscosity. Sedimentation is also inhibited because the solid particles are no longer free to sediment under the gravity. ;The non-welling solvent can be any solvent, which does not wet the fumed silica. The non-wetting radiation-curable monomer can be any radiation-curable monomer, which does not wet the fumed silica. ;Accordingly, the inkjet ink may comprise an organic solvent, which is in the form of a liquid at ambient temperatures and is capable of acting as a carrier for the remaining components of the ink. The organic solvent component of the ink may be a single solvent or a mixture of two or more solvents. The solvent can be selected from any solvent commonly used in the printing industry, such as glycol ethers, glycol ether esters, alcohols, ketones, esters, organic carbonates, lactones and pyrrolidones. Solvent should, however, be compatible with the 3D silica network, as previously discussed. ;In a preferred embodiment the organic solvent is a low toxicity and/or a low odour solvent. Solvents that have been given VOC exempt status by the United States Environmental Protection Agency or European Council are also preferred. ;The most preferred solvents are selected from glycol ethers and organic carbonates and mixtures thereof. Cyclic carbonates such as propylene carbonate and mixtures of propylene carbonate and one or more glycol ethers are particularly preferred. ;Alternative preferred solvents include lactones, which have been found to improve adhesion of the ink to PVC substrates. Mixtures of lactones and one or more glycol ethers, and mixtures of lactones, one or more glycol ethers and one or more organic carbonates are particularly preferred. Mixtures of gamma butyrolactone and one or more glycol ethers, and mixtures of gamma butyrolactone, one or more glycol ethers and propylene carbonate are particularly preferred. ;Dibasic esters and/or bio-solvents may be used. ;Dibasic esters are known solvents in the art. They can be described as di(Ci-C4alkyl) esters of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms having following general formula: RIO A OR2 in which A represents (CH2)ve, and R1 and R2 may be the same or different and represent C1-C4 alkyl which may be a linear or branched alkyl radical having 1 to 4 carbon atoms, preferably methyl or ethyl, and most preferably methyl. Mixtures of dibasic esters can be used. ;Bio-solvents, or solvent replacements from biological sources, have the potential to reduce dramatically the amount of environmentally-polluting VOCs released in to the atmosphere and have the further advantage that they are sustainable. Moreover, new methods of production of bio-solvents derived from biological feedstocks are being discovered, which allow bio-solvent production at lower cost and higher purity. ;Examples of bio-solvents include soy methyl ester, lactate esters, polyhydroxyalkanoates, terpenes and non-linear alcohols, and D-limonene. Soy methyl ester is prepared from soy. The fatty acid ester is produced by esterification of soy oil with methanol. Lactate esters preferably use fermentation-derived lactic acid which is reacted with methanol and/or ethanol to produce the ester. An example is ethyl lactate which is derived from corn (a renewable source) and is approved by the FDA for use as a food additive. Polyhydroxyalkanoates are linear polyesters which are derived from fermentation of sugars or lipids. Terpenes and non-linear alcohols may be derived from corn cobs/rice hulls. An example is D-limonene which may be extracted from citrus rinds. ;Other solvents may be included in the organic solvent component. A particularly common source of other solvents is derived from the way in which the colouring agent is introduced into the inkjet ink formulation. The colouring agent is usually prepared in the form of a pigment dispersion in a solvent, e.g. 2-ethylhexyl acetate. The solvent tends to be around 40 to 50% by weight of the pigment dispersion based on the total weight of the pigment dispersion. ;In a preferred embodiment, the inkjet ink of the present invention comprises 30 to 90% by weight of radiation-curable monomer, including multifunctional monomer, and/or organic solvent, based on the total weight of the ink. ;The inks of the invention may include one or more photoinitiators, which, under irradiation, for example by ultraviolet light, initiates the polymerisation of the monomers. ;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. ;When the inks of the invention include a free-radical polymerisable material, the photoinitiator system preferably includes a free-radical photoinitiator and when the inks include a cationic polymerisable material the photoinitiator system includes a cationic photoinitiator. When the inks comprise a combination of free-radical polymerisable and cationically polymerisable materials both a free-radical and cationic initiator are required. ;The free-radical photoinitiator can be selected from any of those known in the art. For example, benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2-benzy1-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzil dimethylketal, phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide or mixtures thereof Such photoinitiators are known and commercially available such as, for example, under the trade names Omnirad (from IGM) and Esacure (from Lamberti). ;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. ;The wavelength of the radiation and the nature of the photoinitiator system used must of course coincide. The ink is preferably cured by irradiation with actinic radiation, such as UV, x-ray, electron beam etc., although UV curing is preferred. ;In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulfonium or iodonium based systems. Non limiting examples include: Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa. Further suitable cationic photoinitiators are be sold under the Trade names of Irgacure 184, Irgacure 500, Darocure 1173, Irgacure 907, ITX, Lucerin TPO, Irgacure 369, Irgacure 1700, Darocure 4265, Irgacure 651, Irgacure 819, Irgacure 1000, Irgacure 1300, Esacure KT046, Esacure KIP150, Esacure KT37, Esacure EDB, H-Nu 470, H-Nu 470X, the Union Carbide UVI-69-series, Deuteron UV 1240 and IJY2257, Ciba Irgacure 250 and CGI 552, IGM-C440, Rhodia 2047 and UV9380c. ;Preferably, the photoinitiator if present, is present from 1 to 20% by weight, preferably from 5 to 15% by weight, based on the total weight of the ink. ;The presence of a photoinitiator is optional as it is not necessary to include a photoinitiator in the inkjet ink in order to achieve a thorough cure of the ink. This is 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, in a preferred embodiment, the photoinitiator is present in an amount of less than 20% by weight, preferably less than 5% by weight, more preferably less than 3%, more preferably less than 1%, based on the total weight of the ink. ;As such, the inkjet ink may be substantially free of photoinitiator. By "substantially free" is meant that no photoinitiator is intentionally added to the ink. However, minor amounts of photoinitiator, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of photoinitiator, more preferably less than 0.1% by weight of photoinitiator, most preferably less than 0.05% by weight of photoinitiator, based on the total weight of the ink. The inkjet ink may also be free of photoinitiator. ;However, an inkjet ink that is cured with a low-energy electron beam or actinic radiation in an inert environment may still contain a small amount of photoinitiator such as 1 to 5% by weight of a photoinitiator, based on the total weight of the ink. This is required if the ink is for example, 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 may be a colourless or white ink. By "colourless" is meant that the ink is substantially free of colorant such that no colour can be detected by the naked eye. Minor amounts of colorant that do not produce colour that can be detected by the eye can be tolerated, however. Colourless inks may also be described as "clear" or "water white". The white ink includes a dispersed white pigment. ;The inkjet ink of the present invention may also include a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersed pigment, of the types known in the art and commercially available such as, for example, under the trade-names Paliotol (available from BASF plc), 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 and Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used. ;In a preferred embodiment, the dispersible pigment is in the form of a solid dispersion in a vinyl resin. Such materials are available from BASF under the trade name of Microlith K. These colouring agents are preferred as they do not require the addition of hyperdispersant, which are preferably minimised in the inkjet ink of the present invention. ;In one aspect of the invention the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. ;Black: carbon black pigments such as Pigment black 7. ;The colorant is preferably present in an amount of 0.2-20% by weight, preferably 0.5-10% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink. ;Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an ink-jet nozzle, typically having a particle size less than 8 pm, preferably less than 5 pm, more preferably less than 1 pm and particularly preferably less than 0.5 pm. ;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. ;Other components of types known in the art may be present in the ink to improve the properties or performance. These components may be, for example, surfactants, defoamers, dispersants, synergists for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers. ;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. 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 mNm-1, more preferably 20-35 mNm-1 and most preferably 20-30 mNm-1. ;The ink does not require the presence of water to effect drying of the ink. ;Accordingly, the inkjet ink preferably comprises less than 5% by weight of water, based on the total weight of the ink. Preferably, the inkjet ink comprises less than 3% by weight of water, more preferably less than 2% by weight, more preferably less than 1% by weight, and most preferably the inkjet ink is substantially free of water, where the amounts are based on the total weight of the ink. ;By substantially free is meant that only small amounts will be present, for example some water will typically be absorbed by the ink from the air, but such low levels are tolerated. In other words, no water is intentionally added to the ink. However, minor amounts of water, 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, more preferably less than 0.1% by weight of water, most preferably less than 0.05% by weight of water, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of water. ;In a preferred embodiment, the amount of hyperdispersants is limited in the inkjet ink. In this regard, hyperdispersants wet the silica, which reduces the thixotropic nature of the inkjet ink. ;A hyperdispersant is defined as a polymer having an anchor group capable of adsorbing on to the surface of a particle in a colloidal system and a polymeric chain providing steric stabilisation so as to hold the particles apart and prevent flocculation of the particles. The definition in the context of an inkjet ink would therefore be a polymer having an anchor group capable of adsorbing on to the surface of the silica particles in an inkjet ink and a polymeric chain providing steric stabilisation so as to hold the silica particles apart and prevent flocculation of the silica particles. An example is an amine anchor group and a polyester chain. The hyperdispersant may have one or more anchor groups and one or more polymeric chains. Where the hyperdispersant contains multiple anchor groups and multiple polymer chains, it can form a so-called comb-structured dispersant. So-called "comb" polymers are a subset of branched polymers formed of a main chain with two or more three-way branch points defining linear side chains, i.e. it has the appearance of a comb. Examples of hyperdispersants include Solsperse® hyperdispersants available from Lubrizol. ;Accordingly, the inkjet ink preferably comprises less than 5% by weight of hyperdispersant, based on the total weight of the ink. Preferably, the inkjet ink comprises less than 3% by weight of hyperdispersant, more preferably less than 2% by weight, more preferably less than 1% by weight, and most preferably the inkjet ink is substantially free of hyperdispersant, where the amounts are based on the total weight of the ink. ;By substantially free is meant that only small amounts will be present. In other words, no hyperdispersant is intentionally added to the ink. However, minor amounts of hyperdispersant, 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 hyperdispersant, more preferably less than 0.1% by weight of hyperdispersant, most preferably less than 0.05% by weight of hyperdispersant, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of hyperdispersant. ;The ink of the present invention has a viscosity of 30 to 100 mPas at 40°C. 40°C is a preferred jetting temperature and as such, the inkjet ink of the present invention has a viscosity of 30 to 100 mPas at the jetting temperature of 40°C. Preferably, the ink of the present invention has a viscosity of 30 to 70 mPas at 40°C. ;Ink viscosity can be measured using an ARG2 rheometer manufactured by TA. Instruments, which uses a 60 mm diameter /10 aluminium cone at 40°C with a shear rate of 20 s-1. Accordingly, the ink viscosity can be measured using a rotational rheometer having a 60 mm diameter / 1° aluminium cone at 40°C with a shear rate of 20 s* It had previously been considered that such a high viscosity ink was unsuitable for inkjet printing as it was not previously possible to achieve the required jetting properties. However, the inventors have found that by using a particular print head, it is possible to achieve the required jetting properties for such a high viscosity ink. Such a high viscosity ink also has the advantage of improving satellite suppression, as well as improved edge definition.

The present invention may also provide a cartridge containing the inkjet ink as defined herein.

The present invention may also provide an inkjet ink set wherein at least one of the inks in the set is an inkjet ink of the present invention. Preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention.

Usually, the inkjet ink set of the present invention is in the form of a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. The inks in a trichromatic set can be used to produce a wide range of colours and tones. Other inkjet ink sets may also be used, such as CMYK+white and light colours.

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

The present invention also provides a method of inkjet printing comprising inkjet printing the ink as defined herein onto a substrate, curing the ink by exposing the printed ink to a curing source, and when the ink contains organic solvent, drying the ink to remove the solvent.

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

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

The inventors have found that by using a particular print head, it is possible forthe ink of the present invention to be jelled and maintain the required jetting properties. For example, Xaar® provide such high viscosity print heads with very high viscosity capabilities, which include Xaar® 2001 including a print head with high laydown technology. Xaar® 2001 with high laydown technology allows jetting of inkjet inks up to 100 cP at 40°C.

Suitable substrates include styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC), VIVAK (a polyethylene terephthalate glycol modified), polyolefin substrates, such as polyethylene and polypropylene, e.g. PE85 Trans TIC, PE85 White or PP Top White, polyethylene terephthalate (PET) and paper.

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 above-discussed 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.

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.

The ink may be cured in an inert atmosphere, e.g. using a gas such as nitrogen.

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 One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source.

Preferably, the source of actinic radiation is a mercury discharge lamp and/or LEDs. When LEDs are used, these are preferably provided as an array of multiple LEDs.

The most common UV light source used to cure inkjet inks is a mercury discharge lamp. These lamps operate by creating a plasma between two electrodes in a high pressure mercury gas contained in a quartz envelope. Although these lamps have some drawbacks in terms of their operational characteristics, no other UV light source has yet managed to challenge their position in terms of UV output performance.

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.

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.

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.

In a preferred embodiment, the exposure to a source of actinic radiation is performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink.

In a preferred embodiment, the ink is 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, photoinifiators 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 and so doses of 50 kGy or less are preferred.

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

The exposure to a source of low-energy electron beam (ebeam) is performed in an inert atmosphere, e.g. using a gas such as nitrogen.

When the ink contains organic solvent, the solvent must be evaporated after jetting. Accordingly, when the ink contains organic solvent, the ink is dried to remove the solvent. Evaporation of the solvent, when present, can occur simply by exposure of the inks to the atmosphere, but the inks may also be heated to accelerate evaporation.

The curing and drying step can occur in any order but in a preferred embodiment, when the ink contains organic solvent, the inkjet ink is first dried to remove the solvent and then cured by exposing the ink to a curing source.

The ink cures to form a relatively thin polymerised film. The ink of the present invention typically produces a printed film having a thickness of 1 to 20 pm, preferably 1 to 10 pm, for example 2 to 5 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

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

Examples

Example 1

An inkjet ink was prepared according to the formulation set out in Table 1. The inkjet ink formulation was 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

Component Ink 1 (wt%) Epoxy acrylate CN 104 A80 31.50 HDDA 47.00 UV12 1.00 Cab o sil M5 1.50 Omnirad 184 4.00 Omnirad TPO 6.00 BYK 307 0.10 Total 100.00 Epoxy acrylate CN 104 A80 is a radiation-curable oligomer. Cab o sil M5 is an untreated fumed silica having an average particle size of 0.2-0.3 microns. HDDA is a radiation-curable monomer as defined above. UV12 is a stabiliser. Omnirad 184 and Omnirad TPO are photoinitiators. BYK 307 is a surfactant.

The viscosity of the inkjet ink was 47.02 mPas at 40°C measured using an ARG2 rheometer manufactured by TA. Instruments, which uses a 60 mm diameter /10 aluminium cone at 40°C with a shear rate of 20 s-1. The inkjet ink thus has the required viscosity and the required jetting properties can be achieved.

Claims (15)

1. Claims 1. An inkjet ink comprising: at least one of a radiation-curable oligomer, a passive thermoplastic resin and a multifunctional monomer; 0.2 to 2.0% by weight of fumed silica, based on the total weight of the ink; with the proviso that when the inkjet ink comprises a passive thermoplastic resin, the inkjet ink further comprises a radiation-curable component; and wherein the ink has a viscosity of 30 to 100 mPas at 40°C.
2. 2. An inkjet ink as claimed in claim 1, wherein the ink comprises 0.5 to 1.5% by weight of fumed silica, based on the total weight of the ink.
3. 3. An inkjet ink as claimed in claims 1 or 2, wherein the radiation-curable component is selected from the radiation-curable oligomer, the multifunctional monomer and/or a radiation-curable monomer, other than a multifunctional monomer.
4. 4. An inkjet ink as claimed in claim 3, wherein the radiation-curable monomer comprises one or more difunctional monomers, preferably one or more difunctional (meth)acrylate monomers.
5. 5. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises an organic solvent, preferably a non-wetting organic solvent.
6. 6. An inkjet ink as claimed in any preceding claim, wherein the radiation-curable oligomer, the multifunctional monomer and/or the radiation-curable component is non-wetting.
7. 7. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 0.1 to 25% by weight of the radiation-curable oligomer, based on the total weight of the ink.
8. 8. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 0.1 to 5% by weight of the passive thermoplastic resin, based on the total weight of the ink.
9. 9. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 30 to 90% by weight of the multifunctional monomer, based on the total weight of the ink.
10. 10. An inkjet ink as claimed in any preceding claim, wherein the fumed silica is hydrophilic silica.
11. 11. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises one or more photoinitiators, preferably in an amount from 1 to 20% by weight, based on the total weight of the ink.
12. 12. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises less than 5% by weight of water, based on the total weight of the ink.
13. 13. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises less than 5% by weight of hyperdispersants, based on the total weight of the ink.
14. 14. A method of inkjet printing comprising inkjet printing the ink as claimed in any preceding claim onto a substrate, curing the ink by exposing the printed ink to a curing source, and when the ink contains organic solvent, drying the ink to remove the organic solvent.
15. 15. A printed substrate having the ink as claimed in claims Ito 13 printed thereon.