GB2593797A - A method of printing - Google Patents

Abstract

A method of inkjet printing comprises (i) providing an inkjet ink containing radiation-curable material, 8 wt.% dispersed organic pigment, and <5 wt.% combined water and volatile organic solvent, (ii) inkjet printing the ink onto a substrate to produce a film, and (iii) curing the film by exposure to low energy electron beam radiation. The ink may comprise 10-40 wt.% monofunctional monomer and/or 40-80 wt.% difunctional monomer as radiation-curable materials. Monofunctional monomers may include lauryl acrylate, isobornyl acrylate, phenoxyethyl acrylate, cyclic-TMP formal acrylate, tetrahydrofurfuryl acrylate, (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate, 4-tert-butylcyclohexyl acrylate, 3,3,5-trimethylcyclohexyl acrylate, octadecyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate, or isodecyl acrylate. The difunctional monomer may be 3-methyl-1,5-pentanediol diacrylate, triethylene glycol divinyl ether, 1,10-decanediol diacrylate, hexanediol diacrylate, polyethylene glycol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate, tricyclodecane dimethanol diacrylate, propoxylated neopentyl glycol diacrylate, trimethylolpropane triacrylate, di-trimethylolpropane tetraacrylate, dipentaerythritol hexaacrylate, ethoxylated trimethylolpropane triacrylate, or ethoxylated pentaerythritol tetraacrylate. The ink is typically free of photoinitiators.

Description

A method of printing This invention relates to a method of printing and in particular to a method of printing an inkjet ink.

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

For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically 200 mPas or less at 25°C, although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and most preferably between 7-11 mPas at the jetting temperature, which is often elevated to, but not limited to 40-50°C (the ink might have a much higher viscosity at ambient temperature).

The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent such as water or a low-boiling solvent or mixture of solvents.

Another type of inkjet ink contains unsaturated organic compounds, termed monomers and/or oligomers, which polymerise when cured. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is cured, a process which is more rapid than evaporation of solvent at moderate temperatures.

Inkjet inks are often formulated to contain a colouring agent such as a dispersed pigment. It is often desirable to increase the colour density of a printed ink film and one way to do this is to increase the concentration of the dispersed pigment present in the ink. Organic pigments are particularly preferred because of their brightness. However, inks containing a high of amount of a dispersed organic pigment are difficult to fully cure because the dispersed organic pigment does not contain any functional groups that are capable of polymerising upon exposure to radiation.

Achieving through cure is particularly challenging and the resultant inks films tend to suffer from problems including poor adhesion and resistance.

There is therefore a need in the art to provide a method of inkjet printing that is able to fully cure an inkjet ink containing a high amount of a dispersed organic pigment to produce an inkjet ink film that has desirable properties including good adhesion and resistance.

Accordingly, the present invention provides a method of inkjet printing comprising the following steps, in order: (i) providing an inkjet ink comprising a radiation-curable material, 8% by weight or more of a dispersed organic pigment, and less than 5% by weight of water and volatile organic solvent combined, where the amounts are based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto a substrate to produce an inkjet ink film; and (iii) curing the inkjet ink film by exposing the inkjet ink film to a source of low-energy electron beam radiation.

The inventors have found that low-energy electron beam (ebeam) radiation is able to fully cure an inkjet ink containing 8% by weight or more of a dispersed organic pigment. It is surprising that ebeam radiation is able to fully cure an inkjet ink containing such a high amount of a dispersed organic pigment. Without wishing to be bound by theory, the inventors believe that ebeam radiation ionises the dispersed organic pigment, which polymerises and crosslinks with the radiation-curable material present in the inkjet ink, to produce a fully cured inkjet ink film with desirable properties including good adhesion and resistance.

The method of the present invention comprises (i) providing an inkjet ink comprising a radiation-curable material, 8% by weight or more of a dispersed organic pigment, and less than 5% by weight of water and volatile organic solvent combined, where the amounts are based on the total weight of the ink.

The inkjet ink used in the method of the present invention comprises a radiation-curable material The radiation-curable material is not particularly limited and the formulator is free to include any such radiation-curable material in the inkjet ink to improve the properties or performance of the ink.

This radiation-curable material can include any radiation-curable material readily available and known in the art in inkjet inks. By "radiation-curable" is meant a material that contains functional groups that are capable of polymerising upon exposure to radiation.

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 90% 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.

Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. Monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. They therefore preferably have a viscosity of less than 150 mPas at 25°C, more preferably less than 100mPas at 25°C and most preferably less than 20 mPas at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique /2° steel cone at 25°C with a shear rate of 25 s-l.

In a preferred embodiment, the inkjet ink comprises a monofunctional monomer, 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, C6-10 aryl and combinations thereof, such as C6-10 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.

In a preferred embodiment, the inkjet ink comprises a monofunctional monomer present in 10-40% by weight, more preferably 10-35% by weight, most preferably 10-30% by weight, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises a monofunctional (meth)acrylate monomer, which are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required. Mixtures of (meth)acrylates may also be used.

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

The substituents of the monofunctional (meth)acrylate monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

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

The substituents of the cyclic monofunctional (meth)acrylate monomer are typically cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms and/or substituted by alkyl. Non-limiting examples of substituents commonly used in the art include C3-18 cycloalkyl, C6- 10 aryl and combinations thereof, any of which may substituted with alkyl (such as C1-18 alkyl) and/or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tertbutylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof.

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

The substituents of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is C1-18 alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C6-C20 group. It may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear Ce-C2o group.

In a preferred embodiment, the monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetra hyd rofu rfu ryl acrylate (TH FA), (2-methyl-2-ethyl-1,3-d ioxo la n e-4-yl)methyl acrylate (MEDA/Medol-10), 4-tert-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

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

Preferably, the monofunctional (meth)acrylate monomer comprises lauryl acrylate. Preferably, lauryl acrylate is the sole monofunctional (meth)acrylate monomer present in the ink and more preferably, lauryl acrylate is the sole monofunctional monomer present in the ink.

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

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

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

In a preferred embodiment, the inkjet ink comprises a monofunctional (meth)acrylate monomer present in 10-40% by weight, more preferably 10-35% by weight, most preferably 10-30% by weight, based on the total weight of the ink.

Preferably, when the inkjet ink comprises a monofunctional (meth)acrylate monomer, the monofunctional (meth)acrylate monomer is the sole monofunctional monomer present in the ink. However, the ink may comprise a monofunctional monomer other than the monofunctional (meth)acrylate monomer.

The ink may include at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate 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).

N-Vinyl carbamate monomers are defined by the following functionality: The synthesis of N-vinyl carbamate monomers is known in the art. For example, vinyl isocyanate, formed by the Curtius rearrangement of acryloyl azide, can be reacted with an alcohol to form N-vinyl carbamates (Phosgenations -A Handbook by L. Cotarca and H. Eckert, John Wiley & Sons, 2003, 4.3.2.8, pages 212-213).

In a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure: in which R1 to R4 are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically hydrogen, alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, C8-10 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. Preferably R1 to R4 are independently selected from hydrogen or C1_10 alkyl. Further details may be found in WO 2015/022228 and US 4,831,153.

Most preferably, the N-vinyl carbamate monomer is N-vinyl-5-methyl-2-oxazolidinone (known as NVMO or VMOX). It is available from BASF and has the following structure: molecular weight 127 g/mol NVMO has the 1UPAC name 5-methyl-3-vinyl-1,3-oxazolidin-2-one and CAS number 3395-98-0. NVMO includes the racemate and both enantiomers. In one embodiment, the N-vinyl carbamate monomer is a racemate of NVMO. In another embodiment, the N-vinyl carbamate monomer is (R)- 5-methyl-3-vinyl-1,3-oxazolidin-2-one. Alternatively, the N-vinyl carbamate monomer is (S)-5-methy1-3-viny1-1,3-oxazolidin-2-one.

In a preferred embodiment, the inkjet ink comprises 10-30% by weight, more preferably 15-25% by weight, of an N-vinyl amide monomer, an N-acryloyl amine monomer, an N-vinyl carbamate monomer or mixtures thereof, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises at least one of NVC, ACMO and/or NVMO. N-Vinyl amide monomers are particularly preferred, and most preferably NVC.

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

In a preferred embodiment, the inkjet ink comprises 10-30% by weight, more preferably 15-25% by weight, of an N-vinyl monomer other than an N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises a di-and/or multifunctional monomer.

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 difuncfional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

In a preferred embodiment, the di-and/or multifunctional monomer is a di-, tri-, tetra-, penta-or hexa-functional monomer, i.e. the radiation curable monomer has two, three, four, five or six functional groups. In a particularly preferred embodiment, the inkjet ink comprises a difunctional monomer. In a particularly preferred embodiment, the inkjet ink comprises at least two di-and/or multifunctional radiation-curable monomers and more preferably, at least two difunctional monomers.

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

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

The substituents of the di-and/or 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 C8-10 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 40 to 80% by weight of a di-and/or multifunctional radiation-curable monomer, more preferably 45 to 75% by weight and most preferably 50 to 70% by weight, based on the total weight of the ink.

In a particularly preferred embodiment, the inkjet ink comprises 40 to 80% by weight of a difunctional radiation-curable monomer, more preferably 45 to 75% by weight and most preferably 50 to 70% by weight, based on the total weight of the ink.

The inkjet ink may comprise a multifunctional monomer. The amount of the multifunctional monomer, when present, is preferably 5-25% by weight, based on the total weight of the ink.

Examples of the di-and/or multifunctional radiation-curable monomer 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 radiation-curable monomer may also be used.

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

Difunctional (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-MPDDA), and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof. Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate and 1,12-dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate and mixtures thereof. 3-MPDDA is particularly preferred.

Preferably, 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 a difunctional (meth)acrylate monomer, based on the total weight of the ink. However, for some applications of the present invention, the amount present may be higher and in such a preferred embodiment, the ink comprises up to 80% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink.

Examples of multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta-and octa-functional monomers.

Examples of the multifunctional acrylate monomers that may be included in the inkjet ink include trimethylolpropane triacrylate, dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate and ethoxylated pentaerythritol tetraacrylate (EOPETTA, also known as PPTTA), and mixtures thereof 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 di-and/or multifunctional radiation-curable monomer may have at least one vinyl ether functional group In a preferred embodiment, the inkjet ink 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 inkjet ink 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] (methylenedi- 4,1-phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4-(vinyloxymethyl)cyclohexylmethyl] glutarate, 1,4-butanediol divinyl ether and mixtures thereof.

Triethylene glycol divinyl ether (DVE-3) is particularly preferred. DVE-3 is preferred because of its low viscosity. It has a lower viscosity than the equivalent acrylate monomer because the vinyl ether groups have fewer polar interactions than acrylates.

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 di-and/or multifunctional radiation-curable monomer is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), polyethylene glycol diacrylate (preferably PEG600DA), tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), di-trimethylolpropane tetraacrylate (DiTMPTA), dipentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof.

In a preferred embodiment, the difunctional monomer, when present, is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), polyethylene glycol diacrylate (preferably PEG600DA), tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof.

Preferably, the difunctional monomer, when present, comprises 3-methyl 1,5-pentanediol diacrylate (3-MPDDA) and triethylene glycol divinyl ether (DVE-3). More preferably, 3-methyl 1,5-pentanediol diacrylate (3-MPDDA) and triethylene glycol divinyl ether (DVE-3) are the sole difunctional monomers present in the ink.

In a preferred embodiment, the ink comprises a di-and/or multifunctional monomer and a monofunctional monomer. More preferably, the radiation-curable material consists of a di-and/or multifunctional radiation-curable monomer and a monofunctional monomer.

In a particularly preferred embodiment, the ink comprises a difunctional monomer and a monofunctional monomer. More preferably, the radiation-curable material consists of a difunctional monomer and a monofunctional monomer.

Even more preferably, the ink comprises a difunctional monomer and a monofunctional (meth)acrylate monomer. More preferably, the radiation-curable material consists of a difunctional monomer and a monofunctional (meth)acrylate monomer.

In a preferred embodiment, when the ink comprises a monofunctional (meth)acrylate monomer and a difunctional monomer, the monofunctional (meth)acrylate monomer comprises lauryl acrylate and the difunctional monomer comprises 3-MPDDA.

A particularly preferred monomer combination for the present invention is lauryl acrylate, 3-MPDDA and DVE-3.

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

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. Preferred oligomers for inclusion in the ink of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomerviscosifies 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 25 s -1.

Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups.

The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. In a preferred embodiment, the radiation-curable oligomer is a (meth)acrylate oligomer. 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.

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.

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 10% by weight, based on the total weight of the ink.

The ink may also contain a resin. The resin preferably has a weight-average molecular weight (Mw) of 10-50 KDa, and most preferably 15-35 KDa. The Mw may be measured by known techniques in the art, such as gel permeation chromatography (GPC), using a polystyrene standard. The resin is preferably solid at 25°C. It is preferably soluble in the liquid medium of the ink (the radiation-curable diluent and, when present, additionally the solvent).

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

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

The inkjet ink used in the method of the present invention further comprises 8% by weight or more of a dispersed organic pigment, based on the total weight of the ink.

The inkjet ink contains a high amount of a dispersed organic pigment. A dispersed organic pigment is an example of a non-radiation-curable material. By "non-radiation-curable" is meant a material that does not contain any functional groups that are capable of polymerising upon exposure to radiation such as a (meth)acrylate group and a vinyl ether group. The inkjet ink used in the method of the present invention therefore contains a high amount of non-radiation-curable material, which makes it difficult to achieve full cure. However, the inventors have surprisingly found that ebeam radiation is able to fully cure an inkjet ink containing a high amount of non-radiation-curable material. Without wishing to be bound by theory, it is believed that ebeam radiation ionises the non-radiation-curable material, which polymerises and crosslinks with the radiation-curable material present in the inkjet ink, to produce a fully cured inkjet ink film with desirable properties including good adhesion and resistance.

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

The organic pigment can be any of a wide range of suitable organic pigments that would be known to the person skilled in the art.

Organic 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 organic pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7, SP Black 250 Fluffy. Especially useful are black and the colours required for trichromafic process printing. Mixtures of organic pigments may be used.

In one aspect the following organic 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? and SP Black 250 Fluffy.

In a preferred embodiment, the dispersed organic pigment comprises a black dispersed organic pigment. More preferably, the sole dispersed organic pigment present in the ink is a black dispersed organic pigment. Achieving full cure for black inkjet inks is particularly challenging and the method of the present invention is particularly effective at fully curing such inks.

Organic 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 dispersed organic pigment is present in the inkjet ink in an amount of 10% by weight or more, more preferably 12% by weight or more, based on the total weight of the ink. The dispersed organic pigment is preferably present in an amount of up to and including 20% by weight, more preferably up to and including 15% by weight, based on the total weight of the ink.

More preferably, the dispersed organic pigment is present in 10 to 20% by weight, more preferably 10 to 15% by weight, based on the total weight of the ink. In a particularly preferred embodiment, the dispersed organic pigment is present in 12 to 20% by weight, more preferably 12 to 15% by weight, based on the total weight of the ink.

Preferably, the inkjet ink used in the method of the present invention further comprises a pigment dispersant and/or a synergist for the dispersed organic 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. A particular preferred pigment dispersant is Byk 168. In another preferred embodiment, the inkjet ink comprises a synergist. A particularly preferred synergist is Solsperse 50005. In a particularly preferred embodiment, the inkjet ink comprises a pigment dispersant and a synergist.

When present, the pigment dispersant is preferably present in Ito 20% by weight, more preferably to 15% by weight, based on the total weight of the ink.

When present, the synergist is preferably present in 0.1 to 1% by weight, more preferably 0.2 to 0.5% by weight, based on the total weight of the ink.

When present, pigment dispersants and synergists also contribute to the amount of non-radiationcurable material present in the inkjet ink, i.e. pigment dispersants and synergists are non-radiationcurable. Therefore, when a pigment dispersant and/or a synergist is present in the ink, it is particularly surprising that the ink of the present invention can be fully cured, when the ink contains such a high amount of non-radiation-curable material including 8% by weight of a dispersed organic pigment, based on the total weight of the ink, in combination with a pigment dispersant and/or a synergist. Again, it is believed that ebeam radiation ionises the non-radiation-curable material, which polymerises and crosslinks with the radiation-curable material present in the inkjet ink, to produce a fully cured inkjet ink film with desirable properties including good adhesion and resistance.

The inkjet used in the method of the present invention comprises less than 5% by weight of water and volatile organic solvents combined, based on the total weight of the ink. The ink therefore primarily dries 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.

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

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.

In a preferred embodiment the radiation-curable material polymerises by free-radical polymerisation Although photoinitiators are not required for ebeam curing as used in the method of the present invention, the ink used in the method of the present invention may comprise a photoinitiator. This is required if the ink is first pinned with actinic radiation.

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

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 1GM) and Esacure (from Lamberti).

Mixtures of free radical photoinitiators can be used and if present, the ink preferably 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.

Polymeric photoinitiators are preferred. Examples include Omnipol IP®, Omnipol 910® and Speedcure 70100.

Omnipol IP® is commercially available from 1GM. It is a polymeric phosphine oxide photoinitiator, and is known by the chemical name polymeric ethyl (2,4,6-trimethylbenzoy1)-phenyl phosphinate or polymeric TPO-L. It has the following structure: a+b+c = 1 -2 0 The total value of a, b and c of the chemical formula for polymeric TPO-L is equal to 1-20.

Omnipol 910® is also commercially available from 1GM. It is a piparazino-based aminoalkylphenone having the following structure: The value of n of the chemical formula for Omnipol 910® is equal to 1-10.

Speedcure 7010L® is a particularly preferred photoinitiator for inclusion in the ink used in the method of the present invention. Speedcure 7010L® is commercially available from Lambson®.

Speedcure 7010L® is a liquid at 20°C and is a solution of 1,3-di({a41-chloro-9-oxo-9H-thioxanthen- 4-yDoxy]acetylpoly[oxy(1-methylethylene)]) oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy (1-methylethylene)]}oxymethyl) propane in trimethylolpropane ethoxylate triacrylate. 1,3-Di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy (1-methylethylen e)]).

oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yDoxylacetylpoly[oxy (1-methylethylene)]}oxymethyl) propane is known as polymeric ITX and has the following structure: Cl 0 a+b+c+d = 1-20 Cl 0 The total value of a, b, c and d of the chemical formula for polymeric ITX is equal to 1-20. In a preferred embodiment, the value of a+b+c+d of the chemical formula for polymeric ITX is equal to 1-15.

The photoinitiator, when present, is preferably present in the ink in an amount of 1 to 10% by weight, based on the total weight of the ink. Preferably however, the ink comprises 5% or less by weight of a photoinitiator, based on the total weight of the ink. Preferably, the ink comprises 4% or less by weight, 3% or less by weight, 2% or less by weight, or 1% or less by weight of a photoinitiator, based on the total weight of the ink. Most preferably, the inkjet ink is substantially free of a photoinitiator.

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 photoinitiator is intentionally added to the ink. However, minor amounts of a 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, more preferably less than 0.1% by weight and most preferably less than 0.05% by weight of a photoinitiator, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of a photoinitiator.

An inkjet ink that is substantially free of photoinitiators is advantageous for various applications as there will be no unreacted photoinitiator or unreacted photoinitiator fragments present in the cured inkjet ink film. Photoinitiators create free radicals when exposed to radiation. These radicals react with reactive components of the ink (such as reactive monomers and oligomers). However, some photoinitiator and photoinitiator fragments will remain unreacted in the cured ink film and this is problematic for certain applications, such as food packaging, as such unreacted components can migrate into the substrate.

In a preferred embodiment, the inkjet ink comprises a surfactant. The surfactant controls the surface tension of the ink. Surfactants are well known in the art and a detailed description is not required. 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.

Other components of types known in the art may be present in the ink of the present invention to improve the properties or performance. These components may be, for example, additional surfactants, defoamers, stabilisers against deterioration by heat or light other than an aerobic stabiliser, reodorants, flow or slip aids, biocides and identifying tracers.

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

The ink may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill The ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less, more preferably 30 mPas or less and most preferably 20 mPas or less at 25°C. The ink most preferably has a viscosity of 5 to 20 mPas at 25°C. Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model DV1.

The method of the present invention further comprises: (ii) inkjet printing the inkjet ink onto a substrate to produce an inkjet ink film.

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 one or more substrates to form a printed image.

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

Substrates include those for packaging applications and in particular, flexible packaging applications. Examples include substrates composed of polyvinyl chloride (PVC), polystyrene, polyester, polyethylene terephthalate (PET), polyethylene terephthalate glycol modified (PETG) and polyolefin (e.g. polyethylene, polypropylene or mixtures or copolymers thereof). Further substrates include all cellulosic materials such as paper and board, or their mixtures/blends with the aforementioned synthetic materials.

When discussing the one or more substrates, 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 the one or more substrates 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.

To achieve compatibility with print heads that are capable of jetting drop sizes of 90 picolitres or less, a low viscosity ink is required. A viscosity of 30 mPas or less at 25°C is preferred, for example, 8 to 12 mPas, 18 to 20 mPas, or 24 to 26 mPas. Ink viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

The inkjet ink is printed onto a substrate to produce an uncured inkjet ink film. The resultant inkjet ink film can have any thickness and a typical inkjet ink film has a thickness up to and including 50 pm. Relatively thin inkjet ink films such as those having a thickness of less than 12 pm are particularly useful for further increasing the colour density of an inkjet ink film. Further, a low thickness inkjet ink film has reduced problems of shrinkage and provides a more flexible cured inkjet ink film.

However, in a preferred embodiment, the inkjet ink film has a thickness of 12 pm or more. Such inkjet ink films cure to form a relatively thick polymerised film. Inkjet ink films having a thickness of 12 pm or more are particularly useful for back-fit applications where a high ink deposit is required to flow and fill any pin holes on the substrate that would otherwise transmit light. However, achieving full cure for inkjet ink films having a thickness of 12 pm or more is particularly challenging and the method of the present invention is particularly effective at fully curing such ink films.

When the inkjet ink film has a thickness of 12 pm or more, the inkjet ink film preferably has a thickness of up to 50 pm, preferably 12 to 40 pm.

Film thicknesses can be measured using a confocal laser scanning microscope.

The method of the present invention further comprises: (iii) curing the inkjet ink film by exposing the inkjet ink film to a source of low-energy electron beam radiation.

The inventors have surprisingly found that curing the inkjet ink by exposing the ink to a source of ebeam radiation, allows for the inkjet ink to contain an unusually high amount of a dispersed organic pigment, which produces inkjet ink films with high colour density, whilst achieving full cure of the inkjet ink and thus unexpectedly good properties of the cured inkjet ink film such as adhesion and resistance.

Assessment of the degree of curing is well known in the art. Full cure requires through and surface cure.

Surface cure is achieved on providing a tack-free film, e.g. no transfer to photopaper. A suitable test is as follows. A strip of glossy photopaper having a glossy surface and a non-glossy surface, such as Epson glossy (200 gsm, 3 star) photopaper, is placed onto the surface of the printed substrate with the glossy surface of the glossy photopaper in contact with the surface of the printed substrate. Light pressure is applied to the non-glossy surface of the photopaper to ensure good contact between the glossy surface of the glossy photopaper and the surface of the printed substrate. The strip of glossy photopaper is removed and the glossy surface of the glossy photopaper examined for evidence of ink transfer. A well surface cured ink shows no evidence of transfer to the glossy photopaper.

Through cure is assessed by the isopropyl alcohol (IPA) rub test. This test is as follows. Using a lint-free (cotton) cloth saturated in IPA, a double rub is applied to the surface of the printed substrate under light pressure, traversing the length of the surface of the printed substrate in a back and forth motion. The number of double rubs is counted until the substrate is visible. A well through cured ink requires greater than 100 doubles rubs of the IPA rub test.

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 source of ebeam radiation can be any source of low-energy electron beam radiation 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 key and capable of delivering a typical dose of 30-50 kGy at line speeds of up to 30 m/min. By "low-energy" for the ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 70 kGy or less.

Ebeam curing is characterised by dose (energy per unit mass, measured in kilograys (kGy)) deposited in the substrate via electrons. Electron beam surface penetration depends upon the mass, density and thickness of the material being cured. Compared with UV penetration, electrons penetrate deeply through both lower and higher density materials. Unlike UV curing, photoinitiators are not required for ebeam curing to take place.

Ebeam curing is well-known in the art and therefore a detailed explanation of the curing method is not required. In order to cure the printed ink, the ink of the invention is exposed to the ebeam, which produces sufficient energy to instantaneously break chemical bonds and enable polymerisation or crosslinking.

There is no restriction on the ebeam dose that is used to cure the inkjet inks of the present invention other than that the dose is sufficient to fully cure the ink. Preferably, the dose is more than 10 kGy, more preferably more than 20 kGy and most preferably more than 25 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 25 kGy but less than 70 kGy, more preferably more than 25 kGy but less than 60 kGy and most preferably, more than 25 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 inkjet ink film cures to form a polymerised film. The inkjet ink film of the present invention typically produces a polymerised film having a thickness of up to 50 pm, preferably 12 to 40 pm.

Film thicknesses can be measured using a confocal laser scanning microscope.

The present invention also provides a printed substrate obtainable by the method of the present invention. It has surprisingly been found that the printed substrate of the present invention having the inkjet ink of the present invention printed and cured thereon, has improved properties such as adhesion and resistance, despite containing 8% by weight or more of a dispersed organic pigment, based on the total weight of the ink. Owing to the high amount of dispersed organic pigment present in the inkjet ink, the printed substrate of the present invention also has a high colour density.

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 Ink 5 Ink 6 (comparative) (invention) (invention) (comparative) (invention) (invention) 3-MPDDA (difunctional monomer) 61.3 56.3 51.3 54.3 49.3 44.3 Lauryl acrylate (monofunctional monomer) 27.7 22.7 17.7 20.7 15.7 10.7 DVE-3 2.9 5.8 8.7 2.9 5.8 8.7 (difunctional monomer) Byk 168 (pigment dispersant) 3.0 6.0 9.0 3.0 6.0 9.0 Solsperse 5000S (synergist) 0.1 0.2 0.3 0.1 0.2 0.3 SP Black 250 Fluffy (black dispersed organic pigment) 4.0 8.0 12.0 4.0 8.0 12.0 Omnirad 819 (photoinitiator) - - - 4.0 4.0 4.0 Esacure KIP 160 5.0 5.0 5.0 (photo initiator) Speedcure 7010L 5.0 5.0 5.0 (photo initiator) Byk 307 (surfactant) 0.1 1.0 1.0 1.0 1.0 1.0 Total 100.0 100.0 100.0 100.0 100.0 100.0 Viscosity/mPa.s 5.9 6.9 8.3 9.8 12.3 15.6 The viscosity of the ink was measured using Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00. All of the inkjet inks have a viscosity of less than 20 mPa.s and so have an ink-jettable viscosity.

The inks of Table 1 were drawn down on Leneta card (a coated board) using 12, 24 and 40 pm wire wound bars (K-bar) as shown in Tables 2 and 3. The pigment content is given in % by weight, based on the total weight of the ink.

Inks Ito 3 were cured with ebeam radiation having a dose of 30 kGy and an energy of 100 keV, and using a conveyor speed of 9 m/min.

Inks 4 to 6 were cured with UV radiation using a Phoseon 180 VV/cm lamp at full power and a dose of 700 mJ/cm2.

The colour density of the 12 pm ink film of Ink 1 containing 4.0% by weight of a dispersed organic pigment, based on the total weight of the ink, was measured as 2.7 using a Vipdens Densitometer. This is "saturated", i.e. a higher value could not be measured with this instrument for the other combinations of dispersed organic pigment content and ink film thickness.

S

The cured ink films were then assessed for surface cure and through cure. The results are shown in Tables 2 and 3.

Surface cure was assessed by ink transfer onto glossy photopaper with no colour transfer meaning full surface cure. This test is as follows. A strip of Epson glossy (200 gsm, 3 star) photopaper having a glossy surface and a non-glossy surface is placed onto the surface of the printed substrate with the glossy surface of the glossy photopaper in contact with the surface of the printed substrate. Light pressure is applied to the non-glossy surface of the photopaper to ensure good contact between the glossy surface of the glossy photopaper and the surface of the printed substrate. The strip of glossy photopaper is removed and the glossy surface of the glossy photopaper examined for evidence of ink transfer. A well surface cured ink shows no evidence of transfer to the glossy photopaper Through cure was assessed by the isopropyl alcohol (IPA) rub test. This test is as follows. Using a lint-free (cotton) cloth saturated in IPA, a double rub is applied to the surface of the printed substrate under light pressure, traversing the length of the surface of the printed substrate in a back and forth motion. The number of double rubs is counted until the substrate is visible. Full through cure was achieved if breakthrough required greater than 100 double rubs.

Table 2.

Ink Dispersed organic pigment content Ink film Ebeam cure thickness/pm Surface cure Through cure 1 4.0 12 Pass Pass 24 Pass Pass Pass Pass 2 8.0 12 Pass Pass 24 Pass Pass Pass Pass 3 12.0 12 Pass Pass 24 Pass Pass Pass Pass

Table 3.

Ink Dispersed organic pigment content Ink film UV cure thickness/pm Surface cure Through cure 4 4.0 12 Pass Pass 24 Pass Fail Pass Fail 8.0 12 Pass Fail 24 Pass Fail Pass Fail 6 12.0 12 Pass Fail 24 Pass Fail Pass Fail As can be seen from Table 2, Inks 1 to 3 are fully cured using ebeam radiation at a range of film thicknesses.

In contrast, Table 3 shows that only Ink 4 containing 4.0% by weight of a dispersed organic pigment, based on the total weight of the ink, and printed at a film thickness of 12 pm, is fully cured using UV radiation. None of the ink films printed using Inks 5 and 6, which contain 8% by weight or more of a dispersed organic pigment, based on the total weight of the ink, are fully cured using UV radiation. In particular, these inks are not fully through cured using UV radiation owing to their high dispersed organic pigment content.

The inventors have therefore surprisingly found that it is possible to fully cure an ink containing 8% by weight or more of a dispersed organic pigment, based on the total weight of the ink, using ebeam radiation.

Claims (1)

1. Claims 1. A method of inkjet printing comprising the following steps, in order: (i) providing an inkjet ink comprising a radiation-curable material, 8% by weight or more of a dispersed organic pigment, and less than 5% by weight of water and volatile organic solvent combined, where the amounts are based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto a substrate to produce an inkjet ink film; and (iii) curing the inkjet ink film by exposing the inkjet ink film to a source of low-energy electron beam radiation. 10 2. A method as claimed in claim 1, wherein the radiation-curable material comprises a radiation-curable monomer.3. A method as claimed in claim 2, wherein the radiation-curable monomer comprises a monofunctional monomer, preferably a monofunctional (meth)acrylate monomer.4. A method as claimed in claim 3, wherein the monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tert-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.5. A method as claimed in claims 3 or 4, wherein the monofunctional monomer is present in 10- 40% by weight, based on the total weight of the ink.6. A method as claimed in any of claims 2 to 5, wherein the radiation-curable monomer comprises a di-and/or multifunctional monomer, preferably a difunctional monomer.7. A method as claimed in claim 6, wherein the difunctional monomer comprises a difunctional (meth)acrylate monomer.8. A method as claimed in claim 6 or 7, wherein the di-and/or multifunctional monomer is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), polyethylene glycol diacrylate (preferably PEG600DA), tripropylene glycol diacrylate (TPGDA), 3-methyl-1,5-pentanediol diacrylate (3-MPDDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), di-trimethylolpropane tetraacrylate (DiTMPTA), dipentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof.9. A method as claimed in any of claims 6 to 8, wherein the di-and/or multifunctional monomer is present in 40-80% by weight, based on the total weight of the ink.10. A method as claimed in claim 2, wherein the radiation-curable material comprises launil acrylate and 3-MPDDA.11. A method as claimed in any preceding claim, wherein the dispersed organic pigment comprises a black dispersed organic pigment.12. A method as claimed in any preceding claim, wherein the inkjet ink further comprises a pigment dispersant and/or a synergist.13. A method as claimed in any preceding claim, wherein the inkjet ink is substantially free of photoinitiators.14. A method as claimed in any preceding claim, wherein the inkjet ink film has a thickness of 12 pm or more.15. A printed substrate obtainable by the method of any of claims 1 to 14.