GB2593284A

GB2593284A - A method of printing

Info

Publication number: GB2593284A
Application number: GB2101143.2A
Authority: GB
Inventors: Ward Jeremy
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2020-01-28
Filing date: 2021-01-28
Publication date: 2021-09-22
Anticipated expiration: 2041-01-28
Also published as: GB2593284B; GB202001148D0; GB202101143D0

Abstract

A method of printing an inkjet ink comprises (i) providing an inkjet ink containing 5-40 wt.% monofunctional (meth)acrylate monomer having a linear, acyclic, saturated, aliphatic C1-30 substituent, optionally interrupted by 1-10 oxygen atoms and/or terminated by OH, (ii) inkjet printing the ink onto a substrate, and (iii) curing the ink by exposure to low energy electron beam radiation. Typically, the monofunctional (meth)acrylate monomer has a molecular weight of at least 195 g/mol and may be selected from lauryl acrylate, octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), or 4-hydroxybutyl acrylate. The ink may further comprise a difunctional monomer such as 3-methyl-1,5-pentandiol diacrylate (3-MPDDA), a radiation-curable amine-modified (meth)acrylate oligomer, and a dispersed pigment. The substrate is preferably a food packaging. The printed ink may be cured by exposure to low energy electron beam radiation at a dose of more than 30 kGy but less than 70 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.

Inkjet inks are printed onto a variety of substrates. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, and many more. Different substrates are suitable for different applications. A particular challenge is inkjet printing onto substrates for food packaging.

In this respect, 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. Many components readily used for inkjet inks, including volatile organic solvents, many monofunctional monomers and photoinifiators, cannot be used for printing onto food packaging because of their migration properties.

However, such components are often needed to meet the viscosity requirements and physical film properties required, such as flexibility.

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 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, which has negative consequences on physical film properties of the ink.

There is therefore a need in the art for a method of printing which minimises the amount of migratable species present in the cured ink film, without compromising the viscosity and the physical film properties of the ink, such as flexibility.

Accordingly, the present invention provides a method of printing an inkjet ink comprising: (i) providing an inkjet ink comprising: 5-40% by weight of a monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto a substrate; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.

Surprisingly, it has been found that using an inkjet ink containing a specific amount and type of a monofunctional (meth)acrylate monomer, and using a low-energy electron beam to cure the ink, a method of printing can be provided which minimises the amount of migratable species present in the cured ink film, without compromising the viscosity and the physical film properties of the ink, such as flexibility. The cured ink film is thus suitable for food packaging applications.

The present invention also provides a printed substrate obtainable by the method of printing of the present invention.

The method of printing comprises: (i) providing an inkjet ink comprising: 5-40% by weight of a monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic CI-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, based on the total weight of the ink.

The inkjet ink comprises a monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH. Mixtures of monofunctional (meth)acrylate monomers having a linear acyclic saturated aliphatic Ci-Cso substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, may be used.

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.

The monofunctional (meth)acrylate monomer has a linear acyclic saturated aliphatic Ci-Csosubstituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH. Such a monomer is composed of an ester of acrylic or methacrylic acid in which the alcohol substituent is a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, i.e. the radical covalently bonded to the (meth)acrylate unit is a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH.

The Ci-Cao substituent has a linear acyclic saturated aliphatic backbone. The Ci-C30 substituent is linear and acyclic in that it contains no branching and does not contain any cyclic groups. The Ci-C30 substituent is saturated and aliphatic in that it contains no unsaturation and no aryl groups.

The linear acyclic saturated aliphatic Ci-C30 substituent is optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH.

If the linear acyclic saturated aliphatic Ci-C30 substituent is interrupted by 1-10 oxygen atoms, and/or terminated by OH, in one embodiment, the linear acyclic saturated aliphatic Ci-C30 substituent is interrupted by 1-10 oxygen atoms and terminated by OH. In another embodiment, the linear acyclic saturated aliphatic Cl-C30 substituent is interrupted by 1-10 oxygen atoms or terminated by OH. In another embodiment, the linear acyclic saturated aliphatic Cl-C30 substituent is interrupted by 1-10 oxygen atoms. In another embodiment, the linear acyclic saturated aliphatic Ci-C30 substituent is terminated by OH.

In a preferred embodiment, the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, is optionally interrupted by 1-8 oxygen atoms, and/or optionally terminated by OH. More preferably, the monofunctional (meth)acrylate monomer having linear acyclic saturated aliphatic C1-C30 substituent, is optionally interrupted by 1-6 oxygen atoms, and/or optionally terminated by OH.

If the linear acyclic saturated aliphatic Ci-C30 substituent is interrupted by 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, and/or terminated by OH, in one embodiment, the linear acyclic saturated aliphatic Ci-C30 substituent is interrupted by 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, and terminated by OH. In another embodiment, the linear acyclic saturated aliphatic Ci-C30 substituent is interrupted by 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, or terminated by OH. In another embodiment, the linear acyclic saturated aliphatic Ci-C30 substituent is interrupted by 1-8 oxygen atoms, more preferably 1-6 oxygen atoms.

In a preferred embodiment, the monofunctional (meth)acrylate monomer is a linear acyclic saturated aliphatic C2-C20 substituent, more preferably a C3-C15 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH. In both of these embodiments, the linear acyclic saturated aliphatic substituent is preferably optionally interrupted by 1-8 oxygen atoms, and/or optionally terminated by OH, and more preferably optionally interrupted by 1-6 oxygen atoms, and/or optionally terminated by OH.

If the linear acyclic saturated aliphatic C2-C20 substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, and/or terminated by OH, in one embodiment, the linear acyclic saturated aliphatic C2-C20 substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, and terminated by OH. In another embodiment, the linear acyclic saturated aliphatic C2-C20 substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, or terminated by OH. In another embodiment, the linear acyclic saturated aliphatic C2-C20 substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms.

If the linear acyclic saturated aliphatic C3-C15 substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, and/or terminated by OH, in one embodiment, the linear acyclic saturated aliphatic C3-Cis substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, and terminated by OH. In another embodiment, the linear acyclic saturated aliphatic Ca-Cis substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms, or terminated by OH. In another embodiment, the linear acyclic saturated aliphatic C3-Cis substituent is interrupted by 1-10 oxygen atoms, preferably 1-8 oxygen atoms, more preferably 1-6 oxygen atoms.

The monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic C1-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, provides the cured ink film with the required physical film properties, including flexibility.

In a preferred embodiment, the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, is selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), lauryl acrylate, 4-hydroxybutyl 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. However, because of its monofunctional nature, it is not crosslinked in the cured ink film and as such is prone to 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 ink contains lauryl acrylate. Preferably, lauryl acrylate is the sole monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, present in the ink, more preferably, lauryl acrylate is the sole monofunctional (meth)acrylate monomer present in the ink and most preferably, lauryl acrylate is the sole monofunctional monomer present in the ink.

Therefore, in a preferred embodiment, the method of printing an inkjet ink of the present invention comprises: (i) providing an inkjet ink comprising: 5-40% by weight, based on the total weight of the ink, of lauryl acrylate, wherein lauryl acrylate is the sole monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, present in the ink; (ii) inkjet printing the inkjet ink onto a substrate, preferably wherein the substrate is a food packaging; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.

In a particularly preferred embodiment, the method of printing an inkjet ink of the present invention comprises: (i) providing an inkjet ink comprising: 5-40% by weight, based on the total weight of the ink, of lauryl acrylate, wherein lauryl acrylate is the sole monofunctional (meth)acrylate monomer present in the ink, more preferably sole monofunctional monomer present in the ink; (ii) inkjet printing the inkjet ink onto a substrate, preferably wherein the substrate is a food packaging; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.

In a preferred embodiment, the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, has a molecular weight of 195 g/mol or higher. Such monomers are less prone to migration for food packaging applications.

The monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Cl-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, is present in 5-40% by weight, based on the total weight of the ink. The inventors have surprisingly found that an inkjet ink can contain this amount of this type 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 having a linear acyclic saturated aliphatic CiC30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, is present in 10-35% by weight, more preferably 15-30% by weight, based on the total weight of the ink.

Therefore, in a preferred embodiment, the method of printing an inkjet ink of the present invention comprises: (i) providing an inkjet ink comprising: 15-35% by weight, preferably 15-30% by weight, of a monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic CI-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto a substrate; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.

In a preferred embodiment, the method of printing an inkjet ink of the present invention comprises: (i) providing an inkjet ink comprising: 10-35% by weight, preferably 15-30% by weight, of a monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto a substrate, wherein the substrate is a food packaging; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.

In a particularly preferred embodiment, the method of printing an inkjet ink of the present invention comprises: (i) providing an inkjet ink comprising: 10-35% by weight, preferably 15-30% by weight, based on the total weight of the ink, of lauryl acrylate, wherein lauryl acrylate is the sole monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, present in the ink; (ii) inkjet printing the inkjet ink onto a substrate, preferably wherein the substrate is a food packaging; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.

Preferably, the method of printing an inkjet ink of the present invention comprises: (i) providing an inkjet ink comprising: 10-35% by weight, preferably 15-30% by weight, based on the total weight of the ink, of lauryl acrylate, wherein lauryl acrylate is the sole monofunctional (meth)acrylate monomer present in the ink, more preferably sole monofunctional monomer present in the ink; (ii) inkjet printing the inkjet ink onto a substrate, preferably wherein the substrate is a food packaging; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation. 10 The ink used in the method of the present invention may further comprise one or more additional monofunctional monomers, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH.

The one or more additional monofunctional monomers, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, may be any monofunctional monomers known in the art, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Cl-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH.

The substituents of the other monofunctional monomers 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 C3-10 cycloalkyl-substituted C1-16 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.

The ink may comprise an additional monofunctional (meth)acrylate monomer, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH.

If present, the other 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), isodecyl acrylate (IDA) and mixtures thereof.

In a preferred embodiment, the other monofunctional (meth)acrylate monomer is 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.

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, 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, based on the total weight of the ink. 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, 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.

The ink used in the method of the present invention may further comprise 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: *ONI-77C L\ 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: 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, C6_10 aryl and combinations thereof, such as C6-10 aryl-or Ca-is 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.

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:

N VLO

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.

However, the ink preferably contains less than 5% by weight, more preferably less than 4% by weight, more preferably less than 3% by weight, more preferably less than 2% by weight and most preferably less than 1% by weight of a monofunctional monomer, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH. More preferably, the ink contains less than 5% by weight, more preferably less than 4% by weight, more preferably less than 3% by weight, more preferably less than 2% by weight and most preferably less than 1% by weight of a monofunctional monomer, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH.

In a preferred embodiment, the inkjet ink is substantially free of monofunctional (meth)acrylate monomers, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Cl-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH. As such, the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-Cso substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH is preferably the sole monofunctional (meth)acrylate monomer present in the ink and more preferably, the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH is the sole monofunctional monomer present in the ink. This has the advantage of reduced migration as low functionality components are more likely to migrate. Surprisingly, the inkjet ink of the present invention maintains flexibility, without recourse to such additional monofunctional monomers.

In other words, no monofunctional monomer, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, is intentionally added to the ink. However, minor amounts of monofunctional monomers, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic C1-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, 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 monofunctional monomers, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Cl-C30 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of monofunctional monomers, other than the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic C1-030 substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH.

In a preferred embodiment, the ink used in the method of the present invention comprises a monomer having two or more functional groups.

The functional group of the monomer having two or more functional groups 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 monomer having two or more functional groups 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 monomer having two or more functional groups 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, Ca-10 aryl and combinations thereof, such as C8-10 aryl-or 03-18 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 ink of the present invention comprises 40-75% by weight of monomers having two or more functional groups, based on the total weight of the ink.

Examples of the monomer having two or more functional groups 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 monomers having two or more functional groups may also be used.

In a preferred embodiment, the ink used in the method of the present invention comprises a difunctional monomer, preferably 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, 40 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 of the present invention comprises 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 of the present invention comprises 50-80% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink.

The inkjet ink of the present invention may comprise one or more multifunctional (meth)acrylate monomers.

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. 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, more preferably 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, more preferably less than 0.1% by weight and 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 monomer having two or more functional groups, based on the total weight of the ink, may have at least one vinyl ether functional group.

In a preferred embodiment, the ink used in the method of the present invention may comprise 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 ink used in the method of the present invention comprises a divinyl ether monomer.

Examples of divinyl ether monomers include triethylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether, bis[4-(vinyloxy)butyl] 1,6- hexanediylbiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi-4,1- phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4- (vinylcorymethyl)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 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 vinyl ether (meth)acrylate monomers include 2-(2-vinyloxy ethoxy)ethyl acrylate (VEEA), 2-(2-vinyloxy ethoxy)ethyl methacrylate (VEEM) and mixtures thereof.

In a preferred embodiment, the monomer having two or more functional groups 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 monomer having two or more functional groups 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 trimethylol propane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof. These are particularly preferred for use in food packaging applications. A preferred NPGPODA is propoxylated (2) neopentyl glycol diacrylate. A preferred EOTMPTA is ethoxylated (3) trimethylolpropane triacrylate.

In a preferred embodiment, the difunctional 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), triethylene glycol divinyl ether (DVE-3) and mixtures thereof.

In a preferred embodiment, the difunctional 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), triethylene glycol divinyl ether (DVE-3) and mixtures thereof. These are particularly preferred for use in food packaging applications.

Preferably, the difunctional monomer comprises 3-methyl 1,5-pentanediol diacrylate (3-MPDDA) and triethylene glycol divinyl ether (DVE-3). 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.

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 25s-1.

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

In a preferred embodiment, the ink used in the method 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.

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 used in the method of the present invention 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 ink used in the method 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 9Hthioxa nthen-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-morpholinopropiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-IDC), 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 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide.

Polymeric photoinitiators are preferred. Examples include Omnipol TP®, Omnipol 9100 and Speedcure 70100.

Omnipol IP® is commercially available from IGM. It is a polymeric phosphine oxide photoinitiator, and is known by the chemical name polymeric ethyl (2,4,6-trimethylbenzoyI)-phenyl phosphinate or polymeric TPO-L. It has the following structure: a+b+c = 1-20 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: 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({a-[1-chloro-9-oxo-9H-thioxanthen-4- yl)oxylacetyl poly[oxy(1-methylethylene)]) oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yboxy]acetylpoly[oxy (1-methylethylene)]loxymethyl) propane in trimethylolpropane ethoxylate triacrylate. 1, 3-Di({a-[1-ch loro-9-oxo-9H-th ioxa nth e n-4-yboxy]a cetylpo ly[oxy(1-meth ylethyle n e)]) oxy)-2,2-bis({a-[1-ch loro-9-oxo-9 I-1-th ioxa nth e n-4-yl)oxy]acetylpoly[oxy(1-m eth yleth ylen e)]}oxym eth yl) 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.

When the ink comprises a photoinitiator, polymeric ITX is preferably the sole photoinitiator present in the ink.

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 photoinitiator is advantageous for various applications as there will be no unreacted photoinitiator or unreacted photoinitiator fragments present in the cured inkjet ink film. Photoinifiators 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. In the ink of the present invention, photoinitiator is not necessary to achieve cure owing to curing with low-energy electron beam.

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.

The ink used in the method of the present invention 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 ink used in the method of the present invention comprises a surfactant. The surfactant controls the surface tension of the ink. Surfactants are well known in the art and a detailed description is not required. An example of a suitable surfactant is BYK307. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example, plastic substrates. Too high a surface tension can lead to ink pooling and/or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks. Surface tension is also critical to ensuring stable jetting (nozzle plate wetting and sustainability). The surface tension is preferably in the range of 18-40 mNm-1, more preferably 20-35 mNm-, and most preferably 20-30 mNm-1.

Other components of types known in the art may be present in the ink used in the method 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, reodorants, flow or slip aids, biocides and identifying tracers.

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

The ink used in 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 a substrate; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.

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

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

The substrate is not limited. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, and all cellulosic materials or their mixtures/blends with the aforementioned synthetic materials.

Particularly preferred substrates are a substrate for 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).

When discussing the substrate, it is the surface which is most important, since it is the surface which is wetted by the ink. Thus, at least the surface of substrate is composed of the above-discussed material.

In 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, 01 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 that is inkjet printed onto the substrate is cured by exposing the inkjet ink to a source of low-energy electron beam (ebeam) 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 the specific monofunctional (meth)acrylate monomer as claimed, which maintains the required viscosity of the ink and flexibility of the cured ink film, whilst achieving unexpectedly low migration, which is particularly advantageous for food packaging.

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, more preferably more than 30 kGy and most preferably more than 40 kGy. Preferably, the dose is less than 100 kGy, more preferably less than 90 kGy, more preferably less than 80 kGy and most preferably less than 70 kGy. Preferably, the dose is more than 30 kGy but less than 70 kGy, more preferably more than 30 kGy but less than 60 kGy and most preferably, more than 30 kGy but 50 kGy or less. Doses above 50 kGy may cause damage to the substrate, particularly the substrates used for food packaging applications, and so doses of 50 kGy or less are preferred. 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 having a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, and achieve such low migration of this monomer whilst still maintaining flexibility.

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

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

The 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. Ink formulations Component Ink 1, wt% Ink 2, wt% 3-MPDDA 47.5 59.0 Lauryl acrylate 25.0 25.0 CN3715LM 5.0 5.0 lrgastab UV22 0.2 0.2 Cyan pigment dispersion 7.8 7.8 Omnirad 819 3.5 Esacure KIP 160 5.0 Speedcure 7010L 5.0 2.0 Byk 307 1.0 1.0 Total 100.0 100.0 Viscosity at 25°C / mPa.s 10.4 7.0 Ink 1 was formulated for UV curing and ink 2 was formulated for ebeam curing. However, both inks 1 and 2 contain the same amount of lauryl acrylate.

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.

Omnirad 819 (BAPO) and Esacure KIP 160 are photoinitiators from IGM. Speedcure 7010L is a photoinitiator from Lambson.

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 set out in Table 2 and discussed below.

Ink 1 was cured using a medium pressure mercury lamp of power rating 135 W/cm, at a line speed of 30 m/min, either in atmospheric air or using a nitrogen blanket (02 concentration less than 100 ppm).

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 keV, a dose of 50 kGy, a line speed of 9 m/min and an 02 concentration of less than 200 ppm The cured inks were cut to provide circular samples having a diameter of 8 cm and a surface area of 0.5 dm2. The migration test cell was constructed from a stainless steel ring (diameter 8 cm, height 2 cm) sandwiched between (i) the circular sample described above (print side up), forming the base of the sandwich and (ii) unprinted substrate that was wrapped in aluminium foil, forming the top of the sandwich. This sandwich was clamped between circular alloy plates connected by three bolts tightened by wing nuts. 15 cms of 95% w/w ethanol was measured from a pipette into the cell via a spout welded to the stainless steel ring and the aperture was then sealed shut with a Teflon plug. The sealed cells were stored at 40°C for 24 hours and then samples of the ethanol were taken and analysed using GC-MS and LC-MS. The concentrations of lauryl acrylate, 3-MPDDA, DVE-3, CN3715LM, Omnirad 819, Esacure KIP 160 and Speedcure 7010 for each cured ink were determined and the results are set out in Table 2.

Table 2. Extraction of migratable species from cured ink films Curing Concentration of species extracted into ethanol (mg/dm2 conditions Lauryl 3-MPDDA DVE-3 CN3715LM Omnirad Esacure Speed acrylate 819 KIP160 -cure Ink 1 UV, no N2 0.75 0.13 0.06 n.d. 1.33 4.00 n.d.

Ink 1 UV, N2 0.69 0.10 0.02 n.d. 1.19 1.19 n.d.

Ink 2 Electron 0.06 n.d. n.d. n.d. n.d. n.d. n.d.

beam, N2 n.d. = not detected Table 2 shows that although 3-MPDDA is present in the inkjet ink in a much higher amount than lauryl acrylate and 3-MPDDA has a lower molecular weight than lauryl acrylate, less 3-MPDDA migrates than lauryl acrylate for all cure sources. In this regard, for both of the inks and all of the curing conditions, a higher concentration of the monofunctional monomer lauryl acrylate is observed when compared to the difunctional monomer 3-MPDDA. This supports that monofunctional monomers are more susceptible to migration than difunctional monomers, despite the relative molecular weights.

Table 2 further shows that curing by ebeam results in a much lower migration of the claimed monofunctional monomer than curing the ink using UV (both with and without nitrogen). In particular, curing the ink using ebeam radiation results in a ten-fold reduction in the amount of extracted monofunctional (meth)acrylate monomer lauryl acrylate compared to curing the ink using UV radiation (with and without nitrogen).

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 having a linear acyclic saturated aliphatic Ci-Csosubsfituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, results in reduced migration whilst maintaining flexibility of the cured ink film.

Claims

Claims 1. A method of printing an inkjet ink comprising: (i) providing an inkjet ink comprising: 5-40% by weight of a monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, based on the total weight of the ink; (ii) inkjet printing the inkjet ink onto a substrate; and (iii) curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation.
2. A method as claimed in claim 1, wherein the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, is selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), lauryl acrylate, 4-hydroxybutyl acrylate and mixtures thereof.
3. A method as claimed in claims 1 or 2, wherein the ink contains lauryl acrylate.
4. A method as claimed in any preceding claim, wherein the monofunctional (meth)acrylate monomer having a linear acyclic saturated aliphatic Ci-Cao substituent, optionally interrupted by 1-10 oxygen atoms, and/or optionally terminated by OH, has a molecular weight of 195 g/mol or higher.
5. A method as claimed in any preceding claim, wherein the ink further comprises a monomer having two or more functional groups.
6. A method as claimed in claim 5, wherein the monomer having two or more functional groups comprises a difunctional monomer.
7. A method as claimed in any preceding claim, wherein the ink further comprises a radiation-curable oligomer, preferably a (meth)acrylate oligomer.
8. A method as claimed in claim 7, wherein the radiation-curable oligomer is amine-modified.
9. A method as claimed in any preceding claim, wherein the ink further comprises a colouring agent, preferably a dispersed pigment.
10. A method as claimed in any preceding claim, wherein the substrate is a food packaging.
11. A method as claimed in any preceding claim, wherein the printed ink is exposed to a source of low-energy electron beam radiation at a dose of more than 30 kGy but less than 70 kGy.
12. A method as claimed in claim 11, wherein the dose is more than 30 kGy but less than 60 kGy.
13. A method as claimed in claim 12, wherein the dose is more than 30 kGy but 50 kGy or less.
14. A printed substrate obtainable by the method of any of claims 1-13.