GB2593796A - A method of printing - Google Patents

Abstract

A method of inkjet printing comprising (i) providing an inkjet ink comprising 5-40 wt.% monofunctional (meth)acrylate monomer, (ii) inkjet printing the ink onto a substrate to produce a printed surface and an unprinted surface, (iii) curing the printed surface by exposure to low energy electron beam radiation, and (iv) storing one or more substrates such that a printed surface is in contact with an unprinted surface. The one or more substrates may be food packaging and may be a single substrate stored as a roll or a plurality of substrates stored as a stack. The monofunctional (meth)acrylate is typically lauryl acrylate. The ink may also comprise a radiation-curable oligomer, e.g. amine (meth)acrylate oligomer, and 5-70 wt.% difunctional monomer, e.g. 3-methyl-1,5-pentanediol diacrylate (3-MPDDA). Steps (ii) and (iii) may involve movement of the substrate from a substrate reel to a receiving reel. The low energy electron beam radiation may be 10-100 kGy.

Description

A method of printing This invention relates to a method of printing and in particular to a method of printing an inkjet ink. The method of printing is suitable for food packaging applications.

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.

Reel-to-reel printing places particular demands on the inkjet ink. In reel-to-reel printing, a substrate moves from a substrate reel to an inkjet printing station where an inkjet ink is applied, and subsequently to a receiving reel. The accumulation of layer upon layer of printed substrate causes significant temperatures and pressures to develop in the receiving reel and, as a result, there can be a problem of so-called blocking, i.e. the ink has a tendency to cling to the unprinted side of the substrate as the receiving reel is unwound causing transfer of the ink or resistance to separation. Avoidance of blocking is therefore essential in reel-to-reel printing inks.

It has also been found that the same problem occurs in automated and semi-automated printing processes using a flat-bed inkjet printer where the substrate is loaded into the printer and the printer then prints and stacks the printed substrates upon one another. With automated processes, a significant pressure may build up under the weight of the printed substrates. Again, avoidance of blocking is therefore essential in printing inks designed for automated and semi-automated processes.

Blocking can be particularly problematic for food packaging applications. Food packaging represents a particular challenge on account of the strict safety limitations on the properties of materials which come into contact with food, including indirect additives like packaging inks. For printed food packaging, it is necessary to control and quantify the migration of the components of the printed image on the food packaging into the food products. The high pressures experienced when storing the substrate as a roll or a stack may result in the transfer of ink components between touching surfaces. If species from the print are transferred onto the unprinted side of the substrate (a process known as offset migration), it can ultimately lead to contamination of the foodstuff.

Offset migration is a particular concern for monofunctional monomers, compared to monomers having more than one functional group. Any unreacted functional groups in a monofunctional monomer that have not undergone crosslinking are susceptible to offset migration/extraction into food products. For example, if 90% of a monofunctional (meth)acrylate monomer undergoes crosslinking, 10% of the monofunctional (meth)acrylate monomer is available to migrate into food products. In contrast, only one functional group of a monomer having more than one functional group needs to undergo crosslinking in order for that monomer not to be available for migration into food products. For example, if there is full consumption of a difunctional (meth)acrylate monomer but only 50% conversion of the difunctional (meth)acrylate monomer (i.e. one (meth)acrylate group per molecule reacts), no difunctional (meth)acrylate monomer is available to migrate into food products despite crosslinking being incomplete.

Accordingly, monofunctional monomers often need to be minimised in inkjet inks for food packaging.

However, monofunctional monomers are beneficial and are often needed to meet the viscosity requirements of an inkjet ink and physical film properties required, such as flexibility.

There is therefore a need in the art for a method of inkjet printing which maintains the benefits of processes such as reel-to-reel printing but minimises blocking and offset migration for food packaging applications, without compromising the properties of the ink.

Accordingly, the present invention provides a method of inkjet printing comprising the following steps, in order: (i) providing an inkjet ink comprising 5-40% by weight of a monofunctional (meth)acrylate monomer, based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto one or more substrates to produce one or more substrates having a printed surface and an unprinted surface; (iii) curing the printed surface of the one or more substrates by exposure to a source of low-energy electron beam radiation; and (iv) storing the one or more substrates such that the printed surface of the one or more substrates is in contact with the unprinted surface of the one or more substrates.

Surprisingly, it has been found that it is possible to reduce blocking and offset migration in processes such as reel-to-reel printing for food packaging applications, without compromising the viscosity and physical film properties of the ink such as flexibility, using electron beam (ebeam) curing and 5-40% of a monofunctional (meth)acrylate monomer.

The present invention will now be described with reference to the drawing, in which Fig. 1 shows a printer for reel-to-reel printing.

The present invention is directed primarily to a method of inkjet printing suitable for use in reel-to-reel printing and with the printer described with reference to Fig. 1.

Fig. 1 shows a reel-to-reel printer 2. A substrate 4 is tightly wound on a substrate reel 6. The substrate reel 6 is caused to move in orderto deliver the substrate 4, via guide reels 8, to the inkjet printing station 10. The substrate 4 moves in the print direction P shown by the arrow. At the printing station 10, the ink is applied by printhead 14 shown schematically in Fig. 1. The stabilising reels 16 are positioned to provide a stable web onto which the ink is applied. As the substrate passes through the printing station 10, the ink is cured by a UV drier 18. The substrate 4 is subsequently accumulated on the receiving reel 12.

The method of the present invention comprises: (i) providing an inkjet ink comprising 5-40% by weight of a monofunctional (meth)acrylate monomer, based on the total weight of the ink.

The inkjet ink comprises a monofunctional (meth)acrylate monomer. The monofunctional (meth)acrylate monomer provides the cured ink film with the required physical film properties, including flexibility.

The inventors have surprisingly found that curing the inkjet ink by exposing the ink to a source of low-energy electron beam (ebeam) radiation, allows for the inkjet ink to contain a monofunctional (meth)acrylate monomer, which maintains the required viscosity of the ink and flexibility of the cured ink film, whilst achieving unexpectedly high resistance to blocking (transfer of the ink or resistance to separation after storage under pressure). High resistance to blocking translates into low offset migration of migratable species from the cured ink film onto the food contact side of the substrate and hence the method of the present invention is particularly advantageous for food packaging and compliance with food standards.

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, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and difunctional are intended to have their standard meanings, i.e. one or two groups, respectively, which take part in the polymerisation reaction on curing. Multifunctional (which does not include difunctional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Mixtures of monofunctional (meth)acrylate monomers may be used.

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

In a preferred 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, C610 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-tert-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof.

In a preferred embodiment, the cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), cyclic IMP 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) and mixtures thereof These are particularly preferred for use in food packaging applications where quality and safety of the materials is a concern.

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

In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), lauryl acrylate and mixtures thereof These are particularly preferred for use in food packaging applications. 10 In a preferred embodiment, the monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (BOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yOmethyl acrylate (MEDA/Medol-10), 4-tertbutylcyclohexyl 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.

In a preferred embodiment, the monofunctional (meth)acrylate monomer is preferably selected from isobornyl acrylate (IBOA), cyclic IMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tett-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), lauryl acrylate and mixtures thereof. These are particularly preferred for use in food packaging applications.

Lauryl acrylate is particularly preferred. Lauryl acrylate is preferred because it has a long straight chain that introduces flexibility into the cured ink film. However, because of its monofunctional nature, it is not crosslinked in the cured ink film and as such is prone to offset migration. As discussed below, the ebeam curing step of the method of the present invention reduces the amount of unreacted and migratable lauryl acrylate for food packaging applications.

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.

In a preferred embodiment, the monofunctional (meth)acrylate monomer has a molecular weight of 195 g/mol or higher. Such monomers are less prone to migration for food packaging applications.

In the present invention, the inkjet ink comprises a monofunctional (meth)acrylate monomer present in 5-40% by weight. The inventors have surprisingly found that an inkjet ink can contain this amount of monofunctional (meth)acrylate monomer when used in the method of printing of the present invention, to provide a cured ink film with good physical film properties, such as flexibility, whilst providing low migration.

Preferably, the monofunctional (meth)acrylate monomer is present in 10-40% by weight, most preferably 20-40% by weight, based on the total weight of 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 polyoleftn substrates. However, THFA is a hazardous monomer and bears the GHS hazard statement H314 (Causes severe skin burns and eye damage). There is also growing evidence that it may damage fertility or the unborn child. Thus, there is an urgent need in the art to move away from THFA.

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

By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion.

In other words, no THFA is intentionally added to the ink. However, minor amounts of THFA, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of THFA, more preferably less than 0.1% by weight of THFA, most preferably less than 0.05% by weight of THFA, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of THFA.

For food packaging applications, the Swiss Ordinance on Materials and Articles in Contact with Food (SR 817.023.21) sets out provisions for inks. Annex 10 lists permitted substances for the production of food packaging inks. Substances not listed should not be used for food packaging inks. Caution should still be used for some substances on the Swiss Ordinance list and there is some concern about the quality and safety of the monofunctional (meth)acrylate monomers isodecyl acrylate (IDA), octyl acrylate, phenoxyethyl acrylate (PEA) and 2-ethylhexyl acrylate (2-EHA).

The ink preferably contains less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free of each of IDA, octyl acrylate, PEA and 2-EHA, where the amounts are based on the total weight of the ink. More preferably, the ink contains less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free of IDA, octyl acrylate, PEA and 2-EHA in combination, 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 IDA, octyl acrylate, PEA and 2-EHA is intentionally added to the ink. However, minor amounts of IDA, octyl acrylate, PEA and 2-EHA, 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 each of IDA, octyl acrylate, PEA and 2-EHA, more preferably less than 0.1% by weight of each of IDA, octyl acrylate, PEA and 2-EHA, most preferably less than 0.05% by weight of each of IDA, octyl acrylate, PEA and 2-EHA, based on the total weight of the ink. Preferably, the ink may comprise less than 0.5% by weight of IDA, octyl acrylate, PEA and 2-EHA in combination, more preferably less than 0.1% by weight of IDA, octyl acrylate, PEA and 2-EHA in combination, most preferably less than 0.05% by weight of IDA, octyl acrylate, PEA and 2-EHA in combination, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of IDA, octyl acrylate, PEA and 2-EHA.

Preferably, the monofunctional (meth)acrylate monomer is the sole monofunctional monomer present in the ink.

However, the ink used in the method of the present invention may further 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 Nacryloylmorpholine (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).

If present, in a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure: 07NJ R2 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 subsfituents 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, C6_10 aryl and combinations thereof, such as C6_10 aryl-or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described subsfituents. 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.

If present, 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: N7L0 molecular weight 127 g/mol NVMO has the IUPAC name 5-methyl-3-vinyl-1,3-oxazolidin-2-one and CAS number 3395-98-0. NVMO includes the racemate and both enantiomers. In one embodiment, the N-vinyl carbamate monomer is a racemate of NVMO. In another embodiment, the N-vinyl carbamate monomer is (R)-5-methy1-3-viny1-1,3-oxazolidin-2-one. Alternatively, the N-vinyl carbamate monomer is (S)-5-methy1-3-viny1-1,3-oxazolidin-2-one.

If present, the inkjet ink preferably 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 one or more N-vinyl monomers 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 a di-and/or multifunctional monomer.

In a preferred embodiment, the di-and/or multifunctional monomer is a di-, Uri-, tetra-, penta-or hexa-functional monomer, i.e. the di-and/or multifunctional 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 functional group of the di-and/or multifunctional radiation-curable monomer may be the same or different but musttake 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 di-and/or multifunctional monomer may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality monomers may be used.

The substituents of the di-and/or multifunctional monomer are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, C8-10 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 5-90%, more preferably 15-85%, more preferably 30-80% and most preferably 40-75% by weight of a di-and/or multifunctional monomer, based on the total weight of the ink.

Examples of the di-and/or multifunctional 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 monomers may also be used.

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

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-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 5-70% by weight, more preferably 15-70% by weight, more preferably 30-70% by weight and most preferably 40-70% 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.

The inkjet ink may comprise a multifunctional (meth)acrylate monomer.

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

Preferably, the inkjet ink of the present invention comprises less than 5% by weight of a multifunctional (meth)acrylate monomer, based on the total weight of the ink. More preferably, the inkjet ink of the present invention comprises less than 4% by weight, more preferably less than 3% by weight, more preferably less than 2% by weight, less than 1% by weight and most preferably is substantially free of a multifunctional (meth)acrylate monomer, 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 multifunctional (meth)acrylate monomer is intentionally added to the ink. However, minor amounts of multifunctional (meth)acrylate monomers, 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 multifunctional (meth)acrylate monomers, more preferably less than 0.1% by weight of multifunctional (meth)acrylate monomers, most preferably less than 0.05% by weight of multifunctional (meth)acrylate monomers, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of multifunctional (meth)acrylate monomers.

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.

The inventors have surprisingly found that curing the inkjet ink by exposing the ink to a source of low-energy electron beam (ebeam) radiation, allows for the inkjet ink to contain a divinyl ether monomer or multifunctional vinyl ether monomer, which maintains the required viscosity of the ink, whilst achieving unexpectedly high resistance to blocking (transfer of the ink or resistance to separation after storage under pressure). High resistance to blocking translates into low offset migration of migratable species from the cured ink film onto the food contact side of the substrate and hence the method of the present invention is particularly advantageous for food packaging and compliance with food standards.

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 but vinyl ether groups are less reactive than acrylates on exposure to UV radiation. However, this lower reactivity can lead to unreacted DVE-3 molecules which are prone to offset migration. As discussed below, the ebeam curing step of the method of the present invention reduces the amount of unreacted and migratable DVE-3 for food packaging applications.

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, 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), di-pentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof.

In a preferred embodiment, the di-and/or multifunctional radiation-curable monomer is preferably selected from 1,10-decanediol diacrylate (DDDA), ethoxylated (5) hexanediol diacrylate (HD(E0)DA), polyethylene glycol (600) diacrylate (PEG600DA), tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), di-trimethylolpropane tetraacrylate (DiTMPTA), dipentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof. These are preferred for use in food packaging applications.

In a preferred embodiment, the difunctional monomer, when present, is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), polyethylene glycol diacrylate, 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.

In a preferred embodiment, the difunctional monomer, when present, is preferably selected from 1,10-decanediol diacrylate (DDDA), ethoxylated (5) hexanediol diacrylate (HD(E0)DA), polyethylene glycol (600) diacrylate (PEG600DA), tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof. These are preferred for use in food packaging applications.

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

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.

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

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-1.

In a preferred embodiment, 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. Preferably, the inkjet ink 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. Preferred oligomers for inclusion in the ink of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by TA. 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. Preferably the oligomers are (meth)acrylate oligomers. 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 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. However, for food packaging applications where quality and safety of the materials is a concern, the ink is preferably substantially free of bisphenol A based materials such as bisphenol A epoxy acrylates. Therefore, the ink is preferably substantially free of bisphenol A epoxy acrylates.

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

The amount of radiation-curable oligomer, when present, is preferably 0.1-10% 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 0.1-5% by weight, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink of the present invention also includes a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. The colouring agent can be any of a wide range of suitable colouring agents that would be known to the person skilled in the art.

Preferably, the colouring agent is a dispersed pigment, of the types known in the art and 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, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

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. 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.

Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 pm, preferably less than 5 pm, more preferably less than 1 pm and particularly preferably less than 0.5 pm.

The colorant is preferably present in an amount of 0.2-20% by weight, preferably 0.5-10% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink.

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 IGM) and Esacure (from Lambert).

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.

For food packaging applications, there is some concern about the negative odour/taint, migration potential and/or safety of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide.

Therefore, in a preferred embodiment, the ink preferably contains less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free of each of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide, 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 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1- hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide is intentionally added to the ink. However, minor amounts of each of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9Hthioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight, most preferably less than 0.05% by weight of each of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of each of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide.

More preferably, the ink contains less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9Hthioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide in combination, 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 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1- hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide is intentionally added to the ink. However, minor amounts of 2-hydroxy 2-methyl propiophenone, 2- (dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide in combination, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight, most preferably less than 0.05% by weight of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide in combination, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9Hthioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide.

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

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

Omnipol 910® is also commercially available from IGM. It is a piparazino-based aminoalkylphenone having the following structure: M \ ( ri

P \ -'>\D

a+b+c = 1-20 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- yDoxylacetylpoly[oxy(1-methylethylene)]) oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4- yl)oxylacetylpoly[oxy(1-methylethylene)]}oxymethyl) propane in trimethylolpropane ethoxylate triacrylate. 1,3-Di({a41-ch loro-9-oxo-9H-th ioxanth e n-4-yl)oxy]acetylpo ly[oxy(1-meth ylethyle n e)]} oxy)- 2,2-bis({a-[1-chloro-9-oxo-9H-thioxa nth e n-4-y0oxy]acetylpoly[oxy(1-m eth yleth ylen e)]}oxym eth yl) propane is known as polymeric ITX and has the following structure: Cl 0 0 0 H3d Th CH3 C H3 0 0 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 a+b+c+d = 1-20 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 photoinitiator 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 as such unreacted components can migrate into the substrate.

The inkjet ink preferably dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. Preferably, the inkjet ink comprises less than 5% by weight of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2% by weight combined, more preferably less than 1% by weight combined and most preferably the inkjet ink is substantially free of water and volatile organic solvents, where the amounts are based on the total weight of the ink.

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 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, dispersants, synergists, stabilisers against deterioration by heat or light other than an aerobic stabiliser, reodorants, flow or slip aids, biocides and identifying tracers.

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

The ink used in the method of the present invention is applied by inkjet printing. The ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less, more preferably 30 mPas or less and most preferably 20 mPas or less at 25°C. The ink most preferably has a viscosity of 8 to 20 mPas at 25°C. Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model DV1.

The method of the present invention further comprises: (ii) inkjet printing the inkjet ink onto one or more substrates to produce one or more substrates having a printed surface and an unprinted surface.

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.

Particularly preferred substrates are a food packaging. Food packaging is typically formed of flexible and rigid plastics (e.g. food-grade polystyrene and PE/PP films), paper and board (e.g. corrugated board). Printing onto a food packaging substrate represents a particular challenge on account of the strict safety limitations on the properties of materials which come into contact with food, including indirect additives like packaging inks. For printed food packaging, it is necessary to control and quantify the migration and/or odour of the components of the printed image on the food packaging into the food products. Specific exclusions based on their odour and/or migration properties include volatile organic solvents and many monomers typically used in UV curing inks. Preferably, the monomers of the ink used in the method of the present invention are suitable for food packaging applications.

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 a preferred embodiment, the substrate is a laminate carton material comprising the following layers, in order: an inner polyethylene layer; an aluminium layer; a board layer, and an outer polyethylene layer.

By inner is meant a surface of the substrate that would come into contact with food and by outer is meant a surface of the substrate that would come into contact with the inkjet ink used in the method of the present invention. More preferably, the outer polyethylene layer is corona treated to a surface tension of more than 45 dynes/cm using a Vetaphone unit. This provides improved adhesion of the ink.

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 method of the present invention further comprises: (iii) curing the printed surface of the one or more substrates by exposure 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 low-energy electron beam (ebeam) radiation, allows for the inkjet ink to contain an unusually high amount of a monofunctional (meth)acrylate monomer, which maintains the required viscosity of the ink and flexibility of the cured ink film, whilst achieving unexpectedly high resistance to blocking (transfer of the ink or resistance to separation after storage under pressure). High resistance to blocking translates into low offset migration of mig ratable species from the cured ink film onto the food contact side of the substrate and hence the method of the present invention is particularly advantageous for food packaging and compliance with food standards.

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

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

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

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, particularly the substrates used for food packaging applications, and so doses of 50 kGy or less are preferred. It is surprising that such a low dose can be used to cure an inkjet ink containing an unusually high amount of a monofunctional (meth)acrylate monomer and achieve such low migration of this monomer whilst still maintaining flexibility.

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

Advantageously, using ebeam curing significantly reduces the amount of unreacted and migratable monofunctional monomers as claimed relative to conventional UV curing (even when using a nitrogen blanket to prevent oxygen inhibition of cure). This allows greater formulation flexibility and the ink formulator is free to include a significant amount of the claimed monofunctional monomer in order to provide the ink with the required viscosity and beneficial ink film properties, such as flexibility.

Ebeam curing has the further advantage that the printed substrate is sterilised by the ebeam radiation, which is particularly useful for food packaging applications. In aseptic packaging processes, the printed substrate is sterilised before filling with the foodstuff. The sterilisation can be undertaken using, for example, aqueous H202 or ebeam radiation. The use of ebeam curing in the method of the present invention therefore has a twofold function: not only does it cure the inkjet ink but it also sterilises the substrate such that an additional sterilising step may not be required and the printed substrate can be transferred from a print/cure unit to a filling machine in a so-called "in-line" process.

Although a separate sterilising step is not required owing to the ebeam curing step, a separate sterilising step may still occur, especially if the printed substrate is stored before being filled. In this embodiment, the ebeam sterilising source may be the same as the ebeam curing source. There is no restriction on the ebeam dose that is used to sterilise the printed substrate other than that the dose is sufficient to sterilise the printed substrate. Suitable and preferred doses and energies for the sterilising source of low-energy ebeam radiation are the same as those given above for the curing source of low-energy ebeam radiation.

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 Ito 10 pm, for example 2 to 5 pm.

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

The method of the present invention further comprises: (iv) storing the one or more substrates such that the printed surface of the one or more substrates is in contact with the unprinted surface of the one or more substrates.

By storing is meant that the one or more substrates is not used "in-line". For example, for food packaging applications, an "in-line" process involves directly transferring the one or more substrates from a print/cure unit to a filling machine. By storing the one or more substrates, the printed surface of the one or more substrates comes into contact with the unprinted surface of the one or more substrates.

Storing the one or more substrates allows the one or more substrates to be used later but because the printed surface of the one or more substrates comes into contact with the unprinted surface of the one or more substrates, blocking and offset migration can occur as described above.

In a preferred embodiment, the one or more substrates is one substrate stored as a roll. A typical roll has a diameter of 1150 mm, a width of 214 mm and a weight of 170 kg. The centre of such a roll therefore experiences significant pressure, around 16,000 N/m2, and the printed substrate at and near the centre of the roll can suffer from significant blocking and offset migration. However, the method of the present invention minimises such blocking and offset migration, without compromising the viscosity and physical film properties of the ink.

The method of the present invention may be used in reel-to-reel printing. In a preferred embodiment, steps (ii) and (iii) of the method of the present invention occur as the substrate is caused to move from a substrate reel to a receiving reel.

The method of the present invention may also be used in a flat-bed inkjet printer in an automated or semi-automated process where the printed substrates are stacked one on top of another. In an alternative preferred embodiment, the one or more substrates is a plurality of substrates stored as a stack of printed substrates. The stack of printed substrates, especially near the bottom of the stack, can experience significant pressure and thus suffer from blocking and significant offset migration.

The process may be semi-automated in that the plurality of substrates are fed manually into the printer, or automated where the printer contains a substrate-storage facility having the plurality of substrates held therein.

In this embodiment, the method of the present invention involves (ii) inkjet printing the inkjet ink onto a plurality of substrates to produce a plurality of substrates having a printed surface and an unprinted surface; (iii) curing the printed surface of the plurality of substrates by exposure to a source of low-energy electron beam radiation; and (iv) storing the plurality of substrates as a stack of printed substrates such that the printed surface of the plurality of substrates is in contact with the unprinted surface of the plurality of substrates.

The present invention also provides a printed substrate obtainable by the method of the present invention. Preferably, the substrate is a food packaging.

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

Examples Example 1

Inkjet inks were prepared according to the formulations set out in 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, wt% Ink 2, wt% 3-MPDDA 47.46 62.16 Lauryl acrylate 25.00 28.80 CN3715LM 5.00 -lrgastab UV22 0.20 0.20 Cyan pigment dispersion 7.84 7.84 Speedcure 7010L 5.00 - Esacure KIP 160 5.00 - Omnirad 819 3.50 -Byk 307 1.00 1.00 Total 100.00 100.00 Viscosity at 25°C / mPa-s 10.8 6.2 3-MPDDA and lauryl acrylate are monomers, as defined herein. CN3715LM is an amine (meth)acrylate oligomer. Irgastab UV22 is a stabiliser from BASF.

The cyan pigment dispersion contains 30 wt% pigment, 20 wt% polymeric dispersing aid and 50 wt% DVE-3, based on the total weight of the pigment dispersion. The dispersion was prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns. Amounts are given as weight percentages based on the total weight of the dispersion.

Speedcure 7010L is a photoinitiator from Lambson. Esacure KIP 160 is a photoinitiator manufactured by Lamberti. Omnirad 819 (BAPO) is a photoinitiator from IGM.

Byk 307 is a surfactant from Byk.

The viscosity of the inks of Table 1 were measured using a Brookfield DV1 viscometer using the ULA spindle (00) and adaptor connected to a water bath set to 25°C and rpm 20-30. All of the inks have the required viscosity.

Example 2

Each of the above ink formulations were drawn down using a K bar applicator depositing a 12 micron wet film onto a carton material which had been corona-treated using a Vetaphone unit to a surface tension of more than 45 dynes/cm for improved adhesion. The resulting films were cured using the conditions discussed below.

Ink 1 was cured using a Heraeus Noblelight medium pressure mercury lamp of power rating 180 W/cm and a dose of 419 mJ/cm2 at a line speed of 60 m/min.

Ink 2 was cured using an EB Lab electron beam lab unit from ebeam Technologies (a division of COMET, Switzerland) at an energy of 100 key, a dose of 30 kGy, a line speed of 9 m/min and an 02 concentration of less than 300 ppm.

The cured ink films were then assessed for blocking. A 20 kg weight was placed on top of a stack of six cured ink films (three films of cured ink 1 and three films of cured ink 2), each cured ink film having a dimension of 11 cm x 11 cm. The cured ink films were orientated face-to-face (i.e. printed surface in contact with printed surface) and face-to-back (i.e. printed surface in contact with unprinted surface).

The pressure experienced by the cured ink films was similar to the pressure experienced at the centre of a roll when the substrate is stored in this way. In this regard, for a roll having: Diameter = 115.0 cm; Width = 21.4 cm; Core diameter = 15.1 cm (radius = 7.55 cm); and Weight = 170 kg, the pressure at the centre is the weight divided by the surface area of core = 170 / (2rz x 7.55 x 21.4) = 170 kg / 1015 cm2 = 0.167 kg/cm2 = 16,430 N/m2.

In the blocking test using a weight of 20 kg on a stack of cured inks film having a surface area of 11 cm x 11 cm: Pressure = 20 x 9.81 (acceleration under gravity) / 0.0121 = 16,214 N/m2, i.e. very close to the pressure at the centre of the roll described above.

The weight was removed after 24 hours at room temperature and the cured ink films separated and evidence of resistance to separation assessed. By resistance to separation is meant any noise, adhesion or ink transfer when separating each pair of surfaces. The results are shown in Table 2.

Table 2

Component Ink 1, wt% Ink 2, wt% Resistance to separation after 24hrs under pressure? Yes No For ink 1, the resistance to separation indicates significant interaction between the surfaces which would be accompanied by the transfer of migratable species from the ink (e.g. unreacted monomer) from one surface to another. For food packaging applications, this could lead to contamination of the foodstuff.

For ink 2, there is no resistance to separation. Therefore, offset migration would be less likely to occur and food compliance standards would be more likely to be met.

The cured ink film of ink 2 has good physical properties, particularly flexibility.

Accordingly, it has been surprisingly found that ebeam curing an inkjet ink containing a high amount of a monofunctional (meth)acrylate monomer results in reduced blocking and offset migration whilst maintaining flexibility of the cured ink film.

Claims (13)

1. Claims 1. A method of inkjet printing comprising the following steps, in order: (i) providing an inkjet ink comprising 5-40% by weight of a monofunctional (meth)acrylate monomer, based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto one or more substrates to produce one or more substrates having a printed surface and an unprinted surface; (iii) curing the printed surface of the one or more substrates by exposure to a source of low-energy electron beam radiation; and (iv) storing the one or more substrates such that the printed surface of the one or more substrates is in contact with the unprinted surface of the one or more substrates.
2. 2. A method as claimed in claim 1, wherein in step (iv), the one or more substrates is one substrate stored as a roll.
3. 3. A method as claimed in claim 2, wherein steps (i) and (iii) occur as the substrate is caused to move from a substrate reel to a receiving reel.
4. 4. A method as claimed in claim 1, wherein in step (iv), the one or more substrates is a plurality of substrates stored as a stack of printed substrates.
5. S. A method as claimed in any preceding claim, wherein the dose provided by the source of low-energy electron beam radiation is 10-10D kGy.
6. 6.A method as claimed in any preceding claim, wherein the one or more substrates is a food packaging.
7. 7. A method as claimed in any preceding claim, wherein the inkjet ink further comprises a di-and/or multifunctional monomer.
8. 8. A method as claimed in claim 7, wherein the inkjet ink further comprises a difunctional monomer.
9. 9.A method as claimed in claim 8, wherein the inkjet ink further comprises a difunctional (meth)acrylate monomer.
10. 10. A method as claimed in claim 9, wherein the difunctional (meth)acrylate monomer is present in 5- 70% by weight, based on the total weight of the ink.
11. 11. A method as claimed in any preceding claim, wherein the radiation-curable material comprises a radiation-curable oligomer.
12. 12. A method as claimed in any preceding claim, wherein the inkjet ink further comprises a colouring agent.
13. 13. A printed substrate obtainable by the method of any of claims 1-12.