GB2606448A - Printing ink - Google Patents

Abstract

An inkjet ink having a viscosity of 30-100 mPa s at 40°C comprises radiation-curable material, ≥15 wt.% pigment, <5 wt.% monofunctional monomer excluding lauryl acrylate, and <5 wt.% water and volatile organic solvent combined. The ink may contain 10-45 wt.% difunctional monomers, 10-40 wt.% multifunctional monomers, and 0.1-25 wt.% radiation-curable oligomer. The di- and/or multifunctional monomers may be 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), ethoxylated (5) hexanediol diacrylate (HD(EO)DA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl-1,5-pentanediol diacrylate (3-MPDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, propoxylated trimethylolpropane triacrylate (PoTMPTA), glycerolpropoxy triacrylate (GPTA), propoxylated pentaerythritol triacrylate, di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), or triethylene glycol divinyl ether (DVE-3). Typically, the ink comprises 5-15 wt.% lauryl acrylate and 1-20 wt.% photoinitiators. The pigment is preferably a white pigment like TiO2.

Description

Printing ink This invention relates to a printing ink, and in particular to an inkjet ink with a high colour density and/or high opacity without compromising the stability and the cure speed of the ink.

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.

Inkjet inks are often formulated to contain a colouring agent such as a pigment. It is desirable to include a high amount of pigment in the ink in order to achieve a high colour density and/or high opacity with a low film weight. Low film weight products are desirable for flexible packaging. However, it can be difficult to incorporate a high amount of pigment in an inkjet ink because such inks are prone to poor jetting and stability properties.

It is particularly difficult to incorporate a high amount of pigment into a white inkjet ink, particularly a white inkjet ink containing the white pigment Ti02. In this regard, white inkjet inks containing the white pigment TiO2 require a high pigment loading to achieve a workable opacity. However, the high specific gravity of TiO2 increases the risk of sedimentation in white inkjet inks.

A number of ways to try and maximise colour density and/or opacity whilst minimising sedimentation in inkjet inks are known in the art such as stabilising the pigment and/or using complex ink delivery systems. However, stabilising the pigment restricts the ink formulation and using complex ink delivery systems adds to the cost and complexity of printing apparatus design and maintenance. The restriction on the ink formulation and the use of complex ink delivery systems has a negative impact on the cure speed of the ink. By cure speed is meant the speed at which the actinic radiation source moves relative to the substrate.

There is therefore a need in the art for an inkjet ink with a high colour density and/or high opacity without compromising the stability and the cure speed of the ink.

Accordingly, the present invention provides an inkjet ink comprising: a radiation-curable material; 15% by weight or more of a pigment, based on the total weight of the ink; less than 5% by weight of monofunctional monomer other than lauryl acrylate, based on the total weight of the ink; and less than 5% by weight of water and volatile organic solvent combined, based on the total weight of the ink, 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 with a high colour density and/or high opacity without compromising the stability and the cure speed of the ink by providing an inkjet ink comprising: a radiation-curable material; 15% by weight or more of a pigment, based on the total weight of the ink; less than 5% by weight of monofunctional monomer other than lauryl acrylate, based on the total weight of the ink; and less than 5% by weight of water and volatile organic solvent combined, based on the total weight of the ink, wherein the ink has a viscosity of 30 to 100 mPas at 40°C.

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An inkjet ink having such a blend of components and such a high viscosity has previously been considered unsuitable for inkjet printing as it 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 a radiation-curable material; 15% by weight or more of a pigment, based on the total weight of the ink; less than 5% by weight of monofunctional monomer other than lauryl acrylate, based on the total weight of the ink; and less than 5% by weight of water and volatile organic solvent combined, based on the total weight of the ink, without compromising the jetting properties or the colour density/opacity, stability and cure speed of the ink.

The inkjet ink comprises a radiation-curable material.

The radiation-curable material is not particularly limited apart from the ink comprises less than 5% by weight of monofunctional monomer other than lauryl acrylate, based on the total weight of the ink. The formulator is free to include any such radiation-curable material in the ink of the present invention to improve the properties or performance of the ink. This radiation-curable material can include any such radiation-curable material readily available and known in the art. By "radiation-curable" is meant a material that polymerises and/or crosslinks upon irradiation, for example, when exposed to actinic radiation, in the presence of a photoinitiator.

The amount of radiation-curable material is not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. In a preferred embodiment, the inkjet ink comprises 20 to 80% by weight of radiation-curable material, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises a radiation-curable monomer. 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 difuncfional 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 difuncfional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. 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 inkjet ink of the present invention has a high viscosity of 30 to 100 mPas at 40°C and as such, the amount and type of monomers are limited to the extent that the viscosity requirements of the inkjet ink are met.

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

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

In a preferred embodiment, the radiation-curable material comprises one or more di-and/or multifunctional radiation-curable monomers.

In a preferred embodiment, the radiation-curable material comprises one or more difunctional monomers In a preferred embodiment, the radiation-curable material comprises one or more multifunctional monomers In a particularly preferred embodiment, the radiation-curable material comprises at least two di-and/or multifunctional radiation-curable monomers and more preferably, one or more difunctional monomers and one or more multifunctional monomers.

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

The substituents of the one or more di-and/or multifunctional monomers 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 CB-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.

In a preferred embodiment, the inkjet ink comprises 30 to 70% by weight, more preferably 35 to 65% by weight and most preferably 40 to 60% by weight, of one or more di-and/or multifunctional monomers, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises 10 to 45% by weight, more preferably 15 to 40% by weight and most preferably 20 to 35% by weight, of one or more difunctional monomers, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises 10 to 40% by weight, more preferably 15 to 35% by weight and most preferably 20 to 30% by weight, of one or more multifunctional monomers, based on the total weight of the ink.

Examples of the one or more di-and/or multifunctional monomers include difunctional (meth)acrylate monomers, multifunctional (meth)acrylate monomers, divinyl ether monomers, multifunctional vinyl ether monomers and di-and/or multifunctional vinyl ether (meth)acrylate monomers. Mixtures of di-and/or multifunctional monomers may also be used.

In a preferred embodiment, the radiation-curable material comprises a (meth)acrylate monomer, more preferably one or more di-and/or multifunctional (meth)acrylate monomers.

In a preferred embodiment, the radiation-curable material comprises one or more difunctional (meth)acrylate monomers.

In a preferred embodiment, the radiation-curable material comprises one or more multifunctional (meth)acrylate monomers.

In a particularly preferred embodiment, the radiation-curable material comprises at least two di-and/or multifunctional (meth)acrylate monomers and more preferably, one or more difunctional (meth)acrylate monomers and one or more multifunctional (meth)acrylate monomers.

For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate.

Difuncfional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. 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 Alkoxylated monomers such as NPGPODA are particularly effective at stabilising the high amount of pigment present in the inkjet ink of the present invention.

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-MPDA is particularly preferred.

Preferably, the inkjet ink comprises 5 to 35% by weight, more preferably 10 to 30% by weight, of one or more difunctional (meth)acrylate monomers, based on the total weight of the ink.

Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta-and oda-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet inks include trimethylolpropane triacrylate (TMPTA), dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, di-pentaerythritol hexaacrylate (DPHA), di-trimethylolpropane tetraacrylate (DiTMPTA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), propoxylated trimethylolpropane triacrylate (PoTMPTA), glycerolpropoxy triacrylate (GPTA), propoxylated pentaerythritol 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. EOTMPTA is particularly preferred.

Preferably, the inkjet ink comprises 10 to 35% by weight, more preferably 15 to 30% by weight, and most preferably 20 to 25% by weight, of one or more multifunctional (meth)acrylate monomers, based on the total weight of the ink.

The one or more di-and/or multifunctional monomers may have at least one vinyl ether functional group.

In a preferred embodiment, the radiation-curable material comprises a divinyl ether monomer, a multifunctional vinyl ether monomer, a divinyl ether (meth)acrylate monomer and/or a multifunctional vinyl ether (meth)acrylate monomer. In a particularly preferred embodiment, the radiation-curable material comprises a divinyl ether monomer.

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] (methylenedi4,1-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 Triethylene glycol divinyl ether (DVE-3) is particularly preferred.

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

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

In a preferred embodiment, the one or more di-and/or multifunctional monomers are selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), ethoxylated (5) hexanediol diacrylate (HD(E0)DA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, propoxylated trimethylolpropane triacrylate (PoTMPTA), glycerolpropoxy triacrylate (GPTA), propoxylated pentaerythritol triacrylate, di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DR-IA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof. A preferred polyethylene glycol diacrylate is polyethylene glycol (600) diacrylate (PEG600DA).

In a preferred embodiment, the one or more difunctional monomers, when present, are selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), ethoxylated (5) hexanediol diacrylate (HD(E0)DA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof A preferred polyethylene glycol diacrylate is polyethylene glycol (600) diacrylate (PEG600DA).

In a preferred embodiment, the one or more multifunctional monomers, when present, are selected from trimethylolpropane triacrylate (TMPTA), dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, propoxylated trimethylolpropane triacrylate (PoTMPTA), glycerolpropoxy triacrylate (GPTA), propoxylated pentaerythritol triacrylate, di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA) and mixtures thereof.

Preferably, the radiation-curable material comprises 3-methyl 1,5-pentanediol diacrylate (3-MPDA) and ethoxylated trimethylolpropane triacrylate (EOTMPTA).

The inkjet ink comprises less than 5% by weight of monofunctional monomer other than lauryl acrylate. In other words, the ink comprises less than 5% by weight of all monofunctional monomers combined, other than lauryl acrylate. The restriction on the monofunctional monomer content of the ink means that the required colour density/opacity can be achieved at a faster cure speed, without compromising the viscosity and the stability of the ink.

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 groups can be any group that are capable of polymerising upon exposure to radiation such as a (meth)acrylate group and a vinyl ether group.

Lauryl acrylate has the following chemical structure: 0 mol wt 240 g/mol The substituents of the restricted monofunctional monomer 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, Ce-io 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.

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 other than lauryl acrylate are limited to the extent that the viscosity requirements of the inkjet ink are met.

Examples of the restricted monofunctional monomer include monofunctional (meth)acrylate monomers, N-vinyl amide monomers, N-(meth)acryloyl amine monomers, N-vinyl monomers other than N-vinyl amide monomers and N-(meth)acryloyl amine monomers, and mixtures thereof.

Monofunctional (meth)acrylate monomers are well known in the art and include the esters of acrylic acid. A detailed description is therefore not required.

Monofunctional (meth)acrylate monomers include cyclic monofunctional (meth)acrylate monomers and/or acyclic-hydrocarbon monofunctional (meth)acrylate monomers.

The substituents of the restricted 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, C6-10 aryl and combinations thereof, any of which may substituted with alkyl (such as Cl -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 restricted cyclic monofunctional (meth)acrylate monomer includes 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), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), benzyl acrylate (BA) and mixtures thereof.

Lauryl acrylate is an acyclic-hydrocarbon monofunctional (meth)acrylate monomer. The substituents of the restricted acyclic-hydrocarbon monofunctional (meth)acrylate monomer other than lauryl acrylate are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is Ci-is alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

The restricted acyclic-hydrocarbon monofunctional (meth)acrylate monomer may contain a linear or branched C6-C20 group. It includes octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA) and mixtures thereof.

Restricted monofunctional monomers also include N-vinyl amide monomers and N-(meth)acryloyl amine monomers.

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 mannerto the (meth)acrylate monomers. Examples include 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. An example is Nacryloylmorpholine (ACMO).

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N-Vinyl monomers other than N-vinyl amide monomers and N-(meth)acryloyl amine monomers are also restricted monofunctional monomers. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole.

In one embodiment, the radiation-curable material comprises lauryl acrylate. Lauryl acrylate is a monofunctional monomer that is tolerated in the inkjet ink of the present invention because it has a long straight chain that introduces flexibility into the cured ink film but it does not substantially adversely affect the cure speed of the ink.

Preferably, the inkjet ink comprises less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free, of monofunctional monomer other than lauryl acrylate, where the amounts are based on the total weight of the ink.

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

When present, lauryl acrylate is preferably present in an amount from 5 to 15% by weight, based on the total weight of the ink.

Preferably, when present, lauryl acrylate is the sole monofunctional monomer present in the ink.

In an alternative preferred embodiment, the ink comprises less than 5% by weight of monofunctional monomer, based on the total weight of the ink. All monofunctional monomers are taken into account in this embodiment, including lauryl acrylate. In other words, preferably, the ink comprises less than 5% by weight of all monofunctional monomer combined, based on the total weight of the ink.

Preferably, the inkjet ink comprises less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free, of monofunctional monomer, where the amounts are based on the total weight of the ink.

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

In a preferred embodiment, the radiation-curable material comprises a radiation-curable (i.e. polymerisable) oligomer. 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 material comprises 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-curable groups. The radiation-curable oligomer preferably comprises a polyester backbone. A polyester (meth)acrylate oligomer is particularly effective at stabilising the high amount of pigment present in the inkjet ink of the present invention The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably, the radiation-curable oligomer is a (meth)acrylate oligomer. The 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.

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

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 a particularly preferred embodiment, the radiation-curable material comprises an amine-modified polyester acrylate oligomer.

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

The amount of radiation-curable oligomer, when present, is preferably 0.1 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.

The inkjet ink dries primarily by curing, i.e. by the polymerisation of the radiation-curable material 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 and the inkjet ink comprises less than 5% by weight of water and volatile organic solvent combined, based on the total weight of the ink. Preferably, the inkjet ink comprises less than 3% by weight combined, more preferably, less than 2% by weight combined and 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.

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 of water 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.

The inkjet ink comprises 15% by weight or more of a pigment, based on the total weight of the ink.

The inkjet ink contains a high amount of pigment. Achieving a high colour density and/or high opacity for inkjet inks without compromising the stability and the cure speed of the ink is challenging and the present invention achieves this balance using the blend of components present in the ink together with the high viscosity of the ink.

A high amount of pigment in an inkjet ink, which results in a high viscosity inkjet ink, has previously been seen as unsuitable for inkjet printing as it provides poor jetting properties.

However, the inventors have found that by using a particular print head, it is possible to use a high amount of pigment, without compromising the jetting properties of the inkjet ink, along with the colour density/opacity, stability and cure speed of the ink.

The pigment is dispersed in the liquid medium of the ink.

The pigment can be any type of pigment selected from a wide range of pigments that are known to the person skilled in the art, providing the viscosity requirements of the inkjet ink are met.

All pigments have a characteristic specific gravity, which is the ratio of the density of the pigment at 20°C and 1 atm to the density of water as the reference material at 4°C and 1 atm.

The specific gravity of water is 1.0; anything that floats has a specific gravity less than 1. Denser pigments have a specific gravity much greater than 1, which means they sink in solution.

Examples of pigments include inorganic pigments and organic pigments.

In a preferred embodiment, the pigment comprises an inorganic pigment. More preferably, the sole pigment present in the ink is an inorganic pigment. Inorganic pigments typically have a specific gravity of 2 to 6.

In a preferred embodiment, the pigment comprises an organic pigment. More preferably, the sole pigment present in the ink is an organic pigment. Organic pigments typically have a specific gravity of less than 2.

Pigment particles having a particle size suitable for inkjet printing and having a specific gravity of more than 3 are prone to sedimentation. However, the blend of components present in the inkjet ink of the present invention together with the high viscosity of the ink is particularly effective at stabilising such pigments. In a preferred embodiment, the pigment comprises a pigment having a specific gravity of more than 3, more preferably more than 3.5. In a particularly preferred embodiment, the sole pigment present in the ink is a pigment having a specific gravity of more than 3, more preferably more than 3.5.

Pigments are commercially available such as 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, Novoperm Orange HL 70, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7, SP Black 250 Fluffy, Kronos 2300, RDI-S, RDDI, Kronos 2190 and Kronos 2064. Especially useful are black, the colours required for trichromatic process printing and white. Mixtures of pigments may be used.

In one aspect the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Orange: benzimidazolone pigments such as Novoperm Orange HL 70.

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 and SP Black 250 Fluffy. White: TiO2 pigments, such as Kronos 2300, RDI-S, RDDI, Kronos 2190 and Kronos 2064.

In a preferred embodiment, the pigment comprises a cyan, magenta, yellow and/or black pigment.

More preferably, the pigment present in the ink is a cyan, magenta, yellow and/or black pigment.

In an alternative preferred embodiment, the pigment comprises a white pigment. More preferably, the sole pigment present in the ink is a white pigment.

In a particularly preferred embodiment, the pigment comprises a white TiO2 pigment. More preferably, the sole pigment present in the ink is a white TiO2 pigment.

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.

In a preferred embodiment, the pigment is present in the inkjet ink in an amount of 18% by weight or more, more preferably 20% by weight or more, most preferably 21% by weight or more, based on the total weight of the ink. The pigment is preferably present in an amount of up to and including 40% by weight, more preferably up to and including 35% by weight, most preferably up to an including 30% by weight, based on the total weight of the ink.

More preferably, the pigment is present in 18 to 40% by weight, more preferably 18 to 35% by weight, most preferably 18 to 30% by weight, based on the total weight of the ink. In a particularly preferred embodiment, the pigment is present in 20 to 40% by weight, more preferably 20 to 35% by weight, most preferably 20 to 30% by weight, based on the total weight of the ink. Most preferably, the pigment is present in 21 to 40% by weight, more preferably 21 to 35% by weight, most preferably 21 to 30% by weight, based on the total weight of the ink.

Preferably, the inkjet ink of the present invention further comprises a pigment dispersant and/or a stabiliser for the pigment. Such components are known in the art and a detailed discussion is not required.

In a preferred embodiment, the inkjet ink comprises a pigment dispersant. In another preferred embodiment, the inkjet ink comprises a stabiliser. In a particularly preferred embodiment, the inkjet ink comprises a pigment dispersant and a stabiliser.

When present, the pigment dispersant is preferably present in 0.5 to 3.5% by weight, more preferably 2 to 3% by weight, based on the total weight of the ink.

When present, the stabiliser is preferably present in 0.1 to 2% by weight, more preferably 0.5 to 1.5% by weight, based on the total weight of the ink.

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.

In a preferred embodiment, the ink of the present invention further comprises one or more photoinitiators.

Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, 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).

Polymeric photoinitiators are also preferred. Examples include polymeric thioxanthone photoinitiators, polymeric benzophenone photoinitiators, polymeric phosphine oxide photoinitiators and polymeric alpha-amino ketone photoinitiators.

In a preferred embodiment, the ink further comprises one or more polymeric phosphine oxide photoinitiators.

By a polymeric phosphine oxide photoinifiator is meant a photoinifiator comprising two or more phosphine oxide units connected by a phosphine oxide linking unit.

The phosphine oxide unit has the structure: RI I R3 R2, with the proviso that at least one of RI, R2 and R3 is C=01R4, and one of RI, R2, R3 and R4 is the phosphine oxide linking unit.

The polymeric phosphine oxide photoinitiator is therefore a polymeric acylphosphine oxide photoinitiator. The phosphine oxide unit can contain one, two or three C=0R4 substituents. Preferably, the phosphine oxide unit contains one or two C=0R4substituents, more preferably one C=0R4 substituent.

Otherwise, IR1, R2, R3 and R4 are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. Preferably however, RI, R2 and R3 are independently hydrogen, halogen, 01-12 alkyl, C2-12 alkenyl, 02-12 alkynyl, C7-12 aralkyl, 07-12 alkaryl, C6-12 aryl, 05-12 heteroaryl, C1-12 alkoxy, combinations thereof, C=0R4 or the phosphine oxide linking unit, with the proviso that at least one of R1, R2 and R3 is C=0R4, and one of RI, R2, R3 and R4 is the phosphine oxide linking unit.

Preferably, R4 is C1-12 alkyl, 02-12 alkenyl, 02-12 alkynyl, 07-12 aralkyl, 07-12 alkaryl, C6-12 aryl, C5-12 heteroaryl, C1-12 alkoxy, combinations thereof or the phosphine oxide linking unit. More preferably, R4 is C6-12 aryl.

More preferably, RI is C=0R4, R2 is C5-12 aryl and R3 is the phosphine oxide linking unit, wherein R4 is C6-12 aryl. Most preferably, IR1 is 0=01R4, R2 is phenyl and R3 is the phosphine oxide linking unit, wherein R4 is 2,4,6-trimethylphenyl.

In a preferred embodiment, the polymeric phosphine oxide photoinitiator has two or more phosphine oxide units, more preferably three or more phosphine oxide units. Preferably, the polymeric phosphine oxide photoinitiator has up to six phosphine oxide units. Preferably, the polymeric phosphine oxide photoinitiator has two to six phosphine oxide units, more preferably two to four phosphine oxide units.

The two or more phosphine oxide units may be the same or different but preferably they are the same.

The phosphine oxide linking unit is a polymer. The polymer is not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. Preferably, the phosphine oxide linking unit has a core based on a polyhydroxy compound. Non-limiting examples of a polyhydroxy compound include ethylene glycol, propylene glycol, butylene glycol, glycerol, trimethylolpropane, di-trimethylolpropane, pentaerythritol and di-pentaerythritol.

An optional repeating monomer unit may be present between the core based on a polyhydroxy compound and the two or more phosphine oxide units The repeating monomer unit is not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. Non-limiting examples include (OCH2CH2),1, (OCH(C1-13)CH2),,, or (OCH2CH2CH2CH2),,, wherein n is a value from 1 to 10. For example, the core may be based on glycerol bonded to repeating ethylene glycol monomer units via a condensation reaction between the hydroxy groups, which is then connected to the two or more phosphine oxide units by an oxygen of glycerol or an oxygen of the repeating ethylene glycol monomer units.

Preferably, the phosphine oxide linking unit is (OCH2CH2)Y, (OCH(CH3)0H2),,Y, or (OCH2CH2CH2CH2)nY, wherein n is a value from 0 to 10 and Y is 0, OCH2CH(OZ)CH2OZ, OCH2C(CH2OZ)(CH2CH3)CH20Z, OCH2C(CH2OZ)(CH2CH3)CH2OCH2C(CH2OZ)(CH2CH3)CH2OZ, OCH2C(CH2OZ)2CH2OZ or OCH2C(0H20Z)2CH2OCH2C(CH2OZ)20Z, wherein Z is (CI-12C1-120)n, (CH2CH(CH3)0),, or (CH2CH2CH2CH20),, and n is as defined above, and wherein the total value of n in the phosphine oxide linking unit is a value from 1 to 20.

In a preferred embodiment, the phosphine oxide linking unit is (OCH2CH2),1Y, wherein n is a value from 0 to 10 and Y is OCH2CH(OZ)CH2OZ, wherein Z is (CH2CH20),, and n is as defined above, and wherein the total value of n in the phosphine oxide linking unit is a value from 1 to 20.

In a particularly preferred embodiment, R1 is C=0R4, R2 is Ct3-12 aryl and R3 is the phosphine oxide linking unit, wherein R4 is C6-12 aryl and wherein the phosphine oxide linking unit is (OCH2CH2)0Y, wherein n is a value from 0 to 10 and Y is OCH2CH(OZ)CH2OZ, wherein Z is (CH2CH20),, and n is as defined above, and wherein the total value of n in the phosphine oxide linking unit is a value from 1 to 20. More preferably, R1 is C=0R4, R2 is phenyl and R3 is the phosphine oxide linking unit, wherein R4 is 2,4,6-trimethylphenyl and wherein the phosphine oxide linking unit is (OCH2CH2),1Y, wherein n is a value from 0 to 10 and Y is OCH2CH(OZ)CH2OZ, wherein Z is (CH2CH20),, and n is as defined above, and wherein the total value of n in the phosphine oxide linking unit is a value from 1 to 20.

Preferably, the one or more polymeric phosphine oxide photoinitiators have a molecular weight of more than 500 g/mol, more preferably more than 600 g/mol and most preferably more than 700 g/mol. Preferably, the one or more polymeric phosphine oxide photoinitiators have a molecular weight of less than 2000 g/mol, more preferably less than 1800 g/mol and most preferably less than 1500 g/mol. In a preferred embodiment, the one or more polymeric phosphine oxide photoinitiators have a molecular weight of more than 500 g/mol to less than 2000 g/mol, more preferably more than 600 g/mol to less than 2000 g/mol and most preferably more than 700 g/mol to less than 2000 g/mol. Molecular weights (number average) can be calculated if the structure of the photoinitiator is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

A preferred example of a polymeric phosphine oxide is Omnipol TPO.

Omnipol TPO is commercially available from IGM with CAS number 1834525-17-5. It is known by the chemical name polymeric ethyl (2,4,6-trimethylbenzoy1)-phenyl phosphinate and is also known as polymeric TPO-L. It has the following structure: The total value of a, b and c of the chemical formula for polymeric TPO-L is equal to 1-20.

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

Preferably, the 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.

0 0 0) 11 II / /a b 0 =p a+b+c = 1-20

P

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.

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 BYK 307. 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.

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: additional surfactants; defoamers; dispersants other than a pigment dispersant; synergists for the photoinitiator; stabilisers against deterioration by heat or light, other than a stabiliser for the pigment; reodorants; flow or slip aids; biocides; and identifying tracers.

The ink of the present invention may also include radiation-curable material, which is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. Preferably however, the ink of the present invention cures by free radical polymerisation only and hence the ink is substantially free of, preferably free of, radiation-curable material, which polymerises by cationic polymerisation.

By substantially free is meant that only small amounts will be present, for example some radiation-curable material, which polymerises by cationic polymerisation, may be present as impurities in commercially available inkjet ink components, but such low levels are tolerated. In other words, no radiation-curable material, which polymerises by cationic polymerisation, is intentionally added to the ink. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight, most preferably less than 0.05% by weight of radiation-curable material, which polymerises by cationic polymerisation, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of radiation-curable material, which polymerises by cationic polymerisation.

In the embodiment where the ink comprises radiation-curable material, which polymerises by cationic polymerisation, the ink must also comprise a cationic photoinitiator.

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.

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 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 T.A. Instruments, which uses a 60 mm diameter /10 aluminium cone at 40°C with a shear rate of 20s1. 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 20s1.

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, without compromising the colour density/opacity, stability, and cure speed of the ink.

In particular, such a high viscosity ink tolerates a high pigment loading and the high viscosity contributes to reduced sedimentation in the ink and an increased stability profile of the ink. The ink of the present invention therefore has a longer shelf life and requires a lower cost and simpler ink delivery system, facilitating faster cure speeds.

There is also the opportunity to select from a wider range of components when formulating such a high viscosity ink to improve film properties and cure speed. For example, high molecular weight material can be included in the ink to provide additional performance benefits such as hexafuncfional resins to further boost cure speed. Lower cost raw materials can also be included in the ink leading to cost benefits.

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

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 jetted and maintain the required jetting properties, without compromising the colour density/opacity, stability and cure speed of the ink. 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.

Using the ink of the present invention, high resolution print heads with small drops can provide opaque images having a reduced film weight and an improved quality.

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.

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 keV, more preferably 70-200 keV and most preferably 100 keV.

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.

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 4 to 10 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

It is surprising that such a thin film with the required colour density and/or opacity can be produced using inkjet printing. The blend of components present in the ink together with the high viscosity of the ink means that high colour density and/or high opacity images can be produced without the need for additional print bars, larger drop print heads, multi-pulse, multi-pass and/or complex ink delivery systems. Using the inkjet ink of the present invention, high colour density and/or high opacity images can be produced at lower dpi (fewer print heads) and faster cure speeds. Therefore, thinner films can be produced with a lower film weight, without compromising the colour density/opacity of the printed image. Thinner films with a lower film weight have improved flexibility and adhesion to the substrate.

The present invention may also provide a method of single-pass inkjet ink printing comprising inkjet printing the ink as claimed in any preceding claim onto a substrate and curing the ink by exposing the printed ink to a curing source. Single-pass is a term of the art and a detailed description is not required. Such a method surprisingly provides the required colour density/opacity and matches the colour density/opacity achieved by analogue printing processes at similar single-pass print speeds.

The present invention also provides a printed substrate having the ink of the present invention printed thereon. The printed substrate has a high colour density and/or high opacity but a lower film weight than would be expected using inkjet printing.

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

Exam Wes

Example 1

Inkjet inks were prepared according to the formulations set out in Table 1. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1

Component Ink 1 Ink 2 Ink 3 Ink 4 (comparative) (invention) (invention) (invention) 3-MPDA 45.80 17.00 12.00 DVE-3 17.00 7.00 EOTMPTA 22.00 22.00 22.00 Lauryl acrylate 10.00 UVP 6600 15.00 White pigment dispersion 22.5 40.00 40.00 40.00 Omnirad 819 4.00 3.00 3.00 3.00 Irgacure 2959 3.00 3.00 3.00 3.00 KIP160 5.20 2.50 2.50 2.50 Omnipol TP 10.00 10.00 Irgastab UV22 0.50 0.50 0.50 0.50 BYK 307 1.00 1.00 1.00 1.00 Tego Dispers 685 1.00 1.00 1.00 1.00 Total 100.00 100.00 100.00 100.00 Viscosity (cP at 25°C) 12.0 63.25 89.34 118.34 Sedimentation rate (Turbiscan global TS!) 3.40 2.30 1.80 1.95 3-MPDA, DVE-3, EOTMPTA and lauryl acrylate are radiation-curable monomers as described above. UVP 6600 is a radiation-curable oligomer as described above.

The white pigment dispersion contains 60.00% Kronos 2300 (TiO2 pigment), 35.64% difuncfional (meth)acrylate monomer, 3.36% dispersing additive and 1.00% stabiliser additive. The dispersion was prepared by mixing the components in the given amounts on a stirrer to a Dv90 particle size of less than 0.5 pm. Amounts are given as weight percentages based on the total weight of the dispersion.

Omnirad 819, Irgacure 2959, KIP160 and Omnipol TP are photoinitiators. Irgastab UV22 is a stabiliser. BYK 307 is a surfactant. Tego Dispers 685 is a pigment dispersant.

The viscosity of the inkjet inks were measured using an ARG2 rheometer manufactured by T.A.

Instruments, which uses a 60 mm diameter! 1° aluminium cone at 25°C with a shear rate of 20 s1. Ink 1 (comparative) would have a viscosity below 30 mPas at 40°C and Inks 2-4 (invention) would have a viscosity of 30 to 100 mPas at 40°C.

The sedimentation rate of the inkjet inks was measured by calculating the Turbiscan global TSI. The sample was loaded into a glass jar in the Turbiscan and held at 40°C for four days. The rate of separation/sedimentation was measured.

Inks 2-4 of the invention have a lower sedimentation rate than comparative ink 1. Therefore, inks 2-4 of the invention are more stable than comparative ink 1 despite containing a higher amount of pigment.

Ink 1 (comparative) also has a surface tension of 21.6 dynes/cm at 25°C, a particle size of 0.39 pm (Dv90 at 25°C) and an optical density of 0.37.

Inks 1-4 were applied to Leneta card using a K2 applicator bar (12 pm wet film). The resulting films were then run on a Jenton mini-cure machine set at 100% power, under a mercury lamp. The lamp used was an UV-Hg H bulb with an intensity of 2800mW/ cm2. The cure speed was varied to determine the maximum speed at which the ink would produce a fully cured film, i.e. a tack-free film. At 50 m/min, the dose was 261 mJ/cm2 and at 70 m/min, the dose was 186 mJ/cm2.

The cure speeds required to achieve fully cured films are set out in Table 2.

Table 2

Ink 1 Ink 2 Ink 3 Ink 4 (comparative) (invention) (invention) (invention) Cure speed (m/min) 50 45 70 C45 The cure speed for ink 3 of the invention is faster than the cure speed for comparative ink 1 and the cure speeds for inks 2 and 4 of the invention are similar to the cure speed for comparative ink 1. Therefore, the cure speed is not adversely affected by the high amount of pigment present in the ink of the invention.

The opacity of the cured films was measured using a Sheen Opac opacity reflectometer 310. The instrument was calibrated to a pair of standards at high opacity and medium opacity. The opacity was measured for the white ink drawn down over the black portion of the Leneta card. The results are set out in Table 3.

Table 3

Ink 1 Ink 2 Ink 3 Ink 4 (comparative) (invention) (invention) (invention) Opacity 34.2 48.5 50.8 49.3 Inks 2-4 would therefore provide a higher opacity image than comparative ink 1.

In summary, inks 2-4 of the invention provide images which are more opaque than the ones provided by comparative ink 1, without compromising the stability and cure speed of the ink.

Claims (15)

1. Claims 1. An inkjet ink comprising: a radiation-curable material; 15% by weight or more of a pigment, based on the total weight of the ink; less than 5% by weight of monofunctional monomer other than lauryl acrylate, based on the total weight of the ink; and less than 5% by weight of water and volatile organic solvent combined, based on the total weight of the ink, 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 radiation-curable material comprises a radiation-curable monomer.
3. 3. An inkjet ink as claimed in claim 2, wherein the radiation-curable monomer comprises one or more di-and/or multifunctional monomers.
4. 4. An inkjet ink as claimed in claim 3, wherein the one or more di-and/or multifunctional monomers are present in an amount from 30 to 70% by weight, based on the total weight of the ink.
5. 5. An inkjet ink as claimed in claims 3 or 4, wherein the radiation-curable monomer comprises one or more difunctional monomers, preferably one or more difunctional (meth)acrylate monomers.
6. 6. An inkjet ink as claimed in claim 5, wherein the one or more difunctional monomers are present in an amount from 10 to 45% by weight, based on the total weight of the ink.
7. 7. An inkjet ink as claimed in any of claims 3 to 6, wherein the radiation-curable monomer comprises one or more multifunctional monomers, preferably one or more multifunctional (meth)acrylate monomers
8. 8. An inkjet ink as claimed in claim 7, wherein the one or more multifunctional monomers are present in an amount from 10 to 40% by weight, based on the total weight of the ink. 30
9. 9. An inkjet ink as claimed in any of claims 3 to 8, wherein the one or more di-and/or multifunctional monomers are selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), ethoxylated (5) hexanediol diacrylate (HD(E0)DA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, propoxylated trimethylolpropane triacrylate (PoTMPTA), glycerolpropoxy triacrylate (GPTA), propoxylated pentaerythritol triacrylate, di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof.
10. 10. An inkjet ink as claimed in any preceding claim, wherein the radiation-curable material comprises lauryl acrylate, preferably in an amount from 5 to 15% by weight, based on the total weight of the ink.
11. 11. An inkjet ink as claimed in any preceding claim, wherein the radiation-curable material comprises a radiation-curable oligomer, preferably in an amount from 0.1 to 25% by weight, based on the total weight of the ink.
12. 12. An inkjet ink as claimed in any preceding claim, wherein the pigment comprises a white pigment.
13. 13. 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.
14. 14. A method of inkjet printing comprising inkjet printing the ink as claimed in any preceding claim onto a substrate and curing the ink by exposing the printed ink to a curing source.
15. 15. A printed substrate having the ink as claimed in claims 1 to 14 printed thereon.