GB2593285A

GB2593285A - A method of printing

Info

Publication number: GB2593285A
Application number: GB2101147.3A
Authority: GB
Inventors: Ward Jeremy; Corfe Lee
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: GB2593285B; GB202001152D0; GB202101147D0

Abstract

A method of inkjet printing comprises (i) providing an inkjet ink comprising a radiation-curable material and 0.3-10 wt.% aerobic stabiliser, (ii) inkjet printing the ink onto a substrate, and (iii) curing the ink by exposure to low energy electron beam radiation in the presence of an oxygen-deficient atmosphere. Typically, the ink comprises 0.5-5 wt.% phenolic aerobic stabiliser selected from 4-methoxyphenol (MEHQ), 2,6-di-tert-butyl-4-methylphenol, benzene-1,4-diol, 2,6-di-tert-butyl-N,N-dimethylamino-p-cresol, and 4-tertbutylbenzene-1,2-diol. In a preferred embodiment, the ink comprises lauryl acrylate and 3-methyl-1,5-pentanediol diacrylate (3-MPDDA) as radiation-curable material and MEHQ as the aerobic stabiliser. A colouring agent may also be included in the ink. The ink may be cured using a dose of electron beam radiation of less than 30 kGy. The substrate may be a food packaging.

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 monomers and photoinitiators, cannot be used for printing onto food packaging because of their migration properties. However, such components are often needed to meet the requirements for inkjet printing.

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 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 a radiation-curable material and 0.3-10% by weight of an aerobic stabiliser, 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 the presence of an oxygen-deficient atmosphere.

Previously, it had not been appreciated that the curing step of a method of inkjet printing affects the choice of stabiliser that is included in the inkjet ink. In the present invention, the inventors have found that the inclusion of an aerobic stabiliser in the inkjet ink in combination with curing the inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation in the presence of an oxygen-deficient atmosphere has the advantage of stabilising the inkjet ink before curing but not impairing cure during the curing step.

Since the aerobic stabiliser does not impair cure during the curing step, the cured ink film contains a minimal amount of uncured and migratable radiation-curable material. Therefore, the method of the present invention is particularly suitable for food packaging applications.

Polymerisation of the radiation-curable material should occur only when the inkjet ink is cured by exposing the inkjet ink to a radiation source. The ink should remain liquid and free-flowing before and while it is being inkjet printed onto a substrate. However, the inkjet ink is at risk of polymerisation before curing. For example, radicals are often formed during purification of acrylated monomers and oligomers, when heating oligomers to make them free-flowing, when milling pigments, and during storage of the inkjet ink. These radicals can go on to react with the radiation-curable material in the inkjet ink and cause unwanted polymerisation.

Stabilisers are often added to inks to inhibit unwanted polymerisation. Stabilisers that are added to inhibit unwanted polymerisation during storage of the ink are sometimes known as in-can stabilisers.

Stabilisers can be described as being aerobic or anaerobic.

Aerobic stabilisers function in the presence of oxygen. Aerobic stabilisers are also known as antioxidants. Examples of aerobic stabilisers include phenolic stabilisers and aromatic amine stabilisers. They act by scavenging radicals and interrupt the oxidative degradation cycle.

Anaerobic stabilisers also act by scavenging radicals but they function in the absence of oxygen.

Examples of anaerobic stabilisers include phenothiazine, nitrosophenylhydroxylamine (NPHA)-based stabilisers such as the aluminium salt N-PAL, and quinone methide-based stabilisers. Florstab UV-12 commercially available from Kromachem and Genorad 16 commercially available from Rahn are both based on N-PAL. Irgastab UV-25 and Irgastab UV-22, commercially available from BASF, are based on quinone methide. For example, phenothiazine functions by hydrogen atom donation with subsequent radical scavenging.

The ink used in the method of the present invention contains an aerobic stabiliser.

In one embodiment, the aerobic stabiliser comprises a phenolic stabiliser and/or an aromatic amine stabiliser. In one embodiment, the aerobic stabiliser consists of a phenolic stabiliser and/or an aromatic amine stabiliser.

In one embodiment, the aerobic stabiliser comprises a phenolic stabiliser. In one embodiment, the aerobic stabiliser consists of a phenolic stabiliser.

Phenolic stabilisers function by donating the proton on the phenolic hydroxyl group to a peroxy radical. The resultant phenoxy radicals can react to form non-radical products as follows: Phenolic stabilisers are based on a phenol group which is typically substituted at the 2-and 6-positions. Substitution at these positions shields the resultant phenoxy radical to prevent initiation of a new polymerisation cycle but also decreases the radical scavenging rate.

In one embodiment, the phenolic stabiliser is:

OH

wherein RI, R2, R3 and R4 may be the same or different and represent hydrogen, hydroxy, 01-12 alkyl, 03-12 cycloalkyl, C2-12 acyl, C6-10 aryl or combinations thereof, wherein 01-12 alkyl, C3-12 cycloalkyl, 02-12 acyl and C6-10 aryl are optionally interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents, and wherein RI or R2 optionally form a cyclic structure with the phenoxy group; and wherein R5 represents hydrogen, hydroxy, 01-25 alkyl, 03-25 cycloalkyl, C2-25 acyl, C6-10 aryl, or combinations thereof, wherein C1-25 alkyl, 03-25 cycloalkyl, 02-25 acyl and C5-10 aryl are optionally interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents, and wherein R5 optionally acts as a linking group between two or more phenolic groups having R1, R2, R3 and R4 substituents as described above. If R5 acts as a linking group between two or more phenolic groups having R1, R2, R3 and R4 substituents as described above, the phenolic groups may be the same or different but are preferably the same.

In a preferred embodiment, R1 and R2 may be the same or different and represent hydrogen, hydroxy, 0112 alkyl, C2-12 acyl, C6-10 aryl or combinations thereof, wherein 01-12 alkyl, C2-12 acyl and C6_10 aryl are optionally interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents, and wherein RI or R2 optionally form a cyclic structure with the phenoxy group; R3 and R4 represent hydrogen; and R5 represents hydroxy, C1-12 alkyl or combinations thereof, wherein 01-12 alkyl is optionally interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents, and wherein R5 optionally acts as a linking group between two or more phenolic groups having R1, R2, R3 and R4 substituents as described above. If R5 acts as a linking group between two or more phenolic groups having R1, R2, R3 and R4 substituents as described above, the phenolic groups may be the same or different but are preferably the same.

Preferably, the phenolic stabiliser is selected form 4-methoxyphenol, 2,6-di-tert-butyl-4-methylphenol, benzene-1,4-diol, 2,6-di-tert-butyl-N,N-dimethylamino-p-cresol, 4-tert-butylbenzene-1,2-diol and mixtures thereof.

4-Methoxyphenol is also known as MEHQ. 2,6-Di-tert-butyl-4-methylphenol is also known as butyl hydroxy toluene or BHT. Benzene-1,4-diol is also known as hydroquinone or HQ. 2,6-Di-tert-butyl- N,N-dimethylamino-p-cresol is commercially available as IONOL® 103 from Oxiris. 4-tert-Butylbenzene-1,2-diol is also known as 4-tert-butylcatechol.

In a preferred embodiment, the phenolic stabiliser comprises MEHQ. In one embodiment, the phenolic stabiliser consists of MEHQ.

In one embodiment, the aerobic stabiliser comprises an aromatic amine stabiliser. In one embodiment, the aerobic stabiliser consists of an aromatic amine stabiliser.

Aromatic amine stabilisers function by scavenging radicals and reaction with oxygen, followed by reactions of the oxygenated amines with radicals. Examples of aromatic amine stabilisers include diphenylamine (DPA) and phenylenediamine (PPD) In the method of the present invention, the aerobic stabiliser is present in the ink in 0.3-10% by weight, based on the total weight of the ink. In a preferred embodiment, the aerobic stabiliser is present in the ink in 0.5-5% by weight, more preferably 0.5-2% by weight, most preferably 0.5-1.9% by weight, based on the total weight of the ink.

In a preferred embodiment, the ink comprises less than 2% by weight, more preferably less than 1% by weight and most preferably is substantially free of an anaerobic stabiliser, 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 anaerobic stabiliser is intentionally added to the ink. However, minor amounts of an anaerobic stabiliser, 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 an anaerobic stabiliser, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of an anaerobic stabiliser.

Anaerobic stabilisers impair cure when the inkjet ink is exposed to a source of low-energy electron beam radiation in the presence of an oxygen-deficient atmosphere and so it is advantageous to minimise the amount of anaerobic stabilisers present in the ink.

In a preferred embodiment, the aerobic stabiliser is the only stabiliser present in the ink.

The inkjet ink used in the method of the present invention comprises a radiation-curable material. The radiation-curable material is not particularly limited and the formulator is free to include any such radiation-curable material in the ink of the present invention to improve the properties or performance of the ink. This radiation-curable material can include any radiation-curable material readily available and known in the art in inkjet inks. By "radiation-curable" is meant a material that polymerises and/or crosslinks upon irradiation, for example, when exposed to actinic radiation, in the presence of a photoinitiator.

The amount of radiation-curable material is not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. In a preferred embodiment, the inkjet ink comprises 20 to 95% by weight of radiation-curable material, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises radiation-curable material that is suitable for printing onto food packaging.

In a preferred embodiment, the inkjet ink comprises a radiation-curable monomer. As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers.

In a preferred embodiment, the inkjet ink comprises a radiation-curable monomer having two or more functional groups. Radiation-curable monomer having two or more functional groups has its standard meaning, i.e. di or higher, that is two or more groups, respectively, which take part in the polymerisation reaction on curing.

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

For the avoidance of doubt, mono and difunctional are intended to have their standard meanings, i.e. one or two groups, respectively, which take part in the polymerisation reaction on curing. Multifunctional (which does not include difunctional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

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

The radiation-curable 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 radiation-curable 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 subsfituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, Cs-10 aryl and combinations thereof, such as Cs-10 aryl-or C3-18 cycloalkylsubstituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

In a preferred embodiment, the inkjet ink comprises 5 to 90% by weight of a radiation-curable monomer having two or more functional groups, based on the total weight of the ink.

Examples of the radiation-curable 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 radiation-curable monomer having two or more functional groups may also be used.

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

Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Examples include hexanediol diacrylate (HDDA), 1,8-octanediol diacrylate, 1,9- nonanediol diacrylate, 1,10-decanediol diacrylate (DDDA), 1,11-undecanediol diacrylate and 1,12-dodecanediol diacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA, PEG600DA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1,5-pentanediol diacrylate (3-MPDDA), and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate and 1,12-dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate and mixtures thereof. 3-MPDDA is particularly preferred.

Preferably, the inkjet ink comprises 5 to 70% by weight, more preferably 25-70% by weight, more preferably 40-70% by weight and most preferably 45-70% 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.

The amount of the multifunctional (meth)acrylate monomer, when present, is preferably 5-25% by weight, based on the total weight of the ink. Preferably however, 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, more preferably less than 1% by weight and most preferably is substantially free of a multifunctional (meth)acrylate monomer, where the amounts are based on the total weight of the ink.By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion. In other words, no multifunctional (meth)acrylate monomer is intentionally added to the ink. However, minor amounts of a multifunctional (meth)acrylate monomer, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight and most preferably less than 0.05% by weight of a multifunctional (meth)acrylate monomer, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of a multifunctional (meth)acrylate monomer.

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

Examples of a divinyl ether monomer include triethylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether, bis[4-(vinyloxy)butyl] 1,6-hexanediylbiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi-4,1-phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4-(vinylo>cymethyl)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.

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 radiation-curable 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), ditrimethylolpropane 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 radiation-curable 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), di-pentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane 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.

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 12° steel cone at 25°C with a shear rate of 25 5-1.

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

In a preferred embodiment, the inkjet ink may further comprise a monofunctional monomer, such as a monofunctional (meth)acrylate monomer.

Monofunctional monomers are well known in the art. A radiation-curable monofunctional monomer has one functional group, which takes part in the polymerisation reaction on curing. The polymerisable groups can be any group that are capable of polymerising upon exposure to radiation and are preferably selected from a (meth)acrylate group and a vinyl ether group.

The substituents of the monofunctional monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-16 alkyl, C3-18 cycloalkyl, C6-10 aryl and combinations thereof, such as C6-10 aryl-or C3-16 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.

In a preferred embodiment, the inkjet ink comprises a monofunctional monomer present in 10-40% by weight, more preferably 15-35% by weight, most preferably 20-30% by weight, based on the total weight of the ink.

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

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 03-18 cycloalkyl, C6-10 aryl and combinations thereof, any of which may substituted with alkyl (such as C1-18 alkyl) and/or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-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-tett-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 nitrogen further substituted.

The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched 06-020 group. It may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), la uryl acrylate and mixtures thereof. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear Ce-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.

In a preferred embodiment, the monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tettbutylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

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

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-tert-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.

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 inkjet ink comprises a monofunctional (meth)acrylate monomer present in 10-40% by weight, more preferably 15-35% by weight, most preferably 20-30% 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 delaminafion. A further advantage of THFA is that it can solubilise chlorinated polyolefins, which in turn provides good adhesion to polyolefin substrates. However, THFA is a hazardous monomer and bears the GHS hazard statement H314 (Causes severe skin burns and eye damage). There is also growing evidence that it may damage fertility or the unborn child. Thus, there is an urgent need in the art to move away from THFA.

The ink will still function in the presence of 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.

In a preferred embodiment, the radiation-curable material comprises a radiation-curable monomer having two or more functional groups and a monofunctional monomer. Preferably, the radiation-curable material consists of a radiation-curable monomer having two or more functional groups and a monofunctional monomer.

In a more preferred embodiment, the radiation-curable material comprises a difunctional monomer and a monofunctional monomer. Preferably, the radiation-curable material consists of a difunctional monomer and a monofunctional monomer.

In a most preferred embodiment, the radiation-curable material comprises a difunctional monomer and a monofunctional (meth)acrylate monomer. Preferably, the radiation-curable material consists of a difunctional monomer and a monofunctional (meth)acrylate monomer.

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

The ink 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.

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

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

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

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.

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

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.

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.

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 electron beam curing as used in the method of the present invention, the ink used in the method of the present invention may still contain 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, 1hydroxycyclohexyl 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.

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, 1hydroxycyclohexyl 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 9Hthioxanthen-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 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, where the amounts are based on the total weight of the ink.

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-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 9100 and Speedcure 70100.

Omnipol TP® 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: The total value of a, b and c of the chemical formula for polymeric TPO-L is equal to 1-20.

Omnipol 9100 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 Lambsona Speedcure 7010L® is a liquid at 20°C and is a solution of 1,3-di({a-p-chloro-9-oxo-9H-thioxanthen-4- yl)oxylacetyl poly[oxy(1-methylethylene)]) oxy)-2,2-bis(0[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxylacetylpoly[oxy (1-methylethylene)]}oxymethyl) propane in trimethylolpropane ethoxylate triacrylate. 1,3-Di({a[1-ch loro-9-oxo-9H4h ioxanth e n-4-yl)oxy]acetylpo ly[oxy(1-meth ylethyle n e)]). oxy)- 2,2-bis({a-[1-ch loro-9-oxo-9 H-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: a+b+c = 1-20 /a 0=P Cl 0 a+b+c+d = 1-20 Cl 0 The total value of a, b, c and d of the chemical formula for polymeric ITX is equal to 1-20. In a preferred embodiment, the value of a+b+c+d of the chemical formula for polymeric ITX is equal to 1-15.

The photoinitiator, when present, is preferably present in the ink in an amount of Ito 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 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 °A, by weight combined, more preferably less than 1% by weight combined and most preferably the inkjet ink is substantially free of water and volatile organic solvents, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated. In other words, no water or a volatile organic solvent is intentionally added to the ink. However, minor amounts of water or a volatile organic solvent, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of water or a volatile organic solvent, more preferably less than 0.1% by weight of water or a volatile organic solvent, most preferably less than 0.05% by weight of water or a volatile organic solvent, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of water or a volatile organic solvent.

In a preferred embodiment, the 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 high-speed 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.

Preferably, the ink used in the method of the present invention is stored in permeable packaging. This ensures that the aerobic stabiliser remains effective in preventing "in-can" polymerisation.

The method of the present invention further comprises: (ii) inkjet printing the inkjet ink 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.

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 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 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, 10 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 inkjet ink by exposing the inkjet ink to a source of low-energy electron beam radiation in the presence of an oxygen-deficient atmosphere.

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 key and capable of delivering a typical dose of 30-50 kGy at line speeds of up to 30 m/min. By "low-energy" for the abeam, 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 30 kGy. Such a low dose of ebeam radiation can be used to cure the ink because the aerobic stabiliser does not impair cure.

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

The ink is cured in the presence of an oxygen-deficient atmosphere, preferably a nitrogen atmosphere. Curing in the presence of an oxygen-deficient atmosphere is well-known in the art and therefore a detailed explanation is not required.

Advantageously, using ebeam curing significantly reduces the amount of unreacted and migratable radiation-curable material relative to conventional UV curing. This allows greater formulation flexibility and the ink formulator is free to include a significant amount of radiation-curable material such as radiation-curable monomers including monofunctional monomers in order to provide the ink with the required properties.

When investigating ebeam curing, the inventors found that the oxygen-deficient atmosphere used in ebeam curing could be used to tailor the nature of the stabiliser. Specifically, the inventors found that the use of an aerobic stabiliser does not impair cure of the inkjet ink in the oxygen-deficient atmosphere used in ebeam curing, whilst the aerobic stabiliser acts as a stabiliser for the inkjet ink before cure. The stabiliser used in the method of the present invention therefore ensures that a minimal amount of uncured and migratable radiation-curable material is present in the cured ink film and the method of the present invention is particularly suitable for food packaging applications.

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

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

Through cure is assessed by measuring adhesion using a cross hatch tape removal test and/or a finger nail scratch test. The cross hatch tape removal test is as follows. Score surface with an elcometer/blade to form a cross hatch area and apply 1502049 tape across the scored area. After applying pressure, remove the tape and assess for ink removal from the substrate. Through cure is achieved if the ink remains adhered to the substrate. Through cure is also achieved if the ink cannot be removed by finger nail scratch.

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

Preferably, the source of low-energy electron beam radiation is the only curing source used in the method of the present invention.

In a preferred embodiment, the method of the present invention consists of the following steps, in order: (i) providing an inkjet ink as described above; (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 presence of an oxygen-deficient atmosphere.

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

In order to determine whether cure is impaired, two inkjet inks are provided. One ink contains radiation-curable material and no aerobic stabiliser. The other ink contains radiation-curable material and 0.310% by weight of an aerobic stabiliser, based on the total weight of the ink. The inks are coated onto a suitable test substrate to produce wet films. The wet films are cured by exposure to a radiation source.

Curing of the ink can then be assessed by the dose required to achieve a cured ink film which requires greater than 100 doubles rubs of the IPA rub test. The IPA rub test is as described above.

If the ink containing the aerobic stabiliser requires the same dose as the ink not containing the aerobic stabiliser, cure is not impaired. However, if the aerobic stabiliser requires a higher dose than the ink not containing the aerobic stabiliser, cure is impaired.

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 Tables 1 and 2. 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. ebeam-curable inks Component Comparative Comparative Ink 3, wt% ink 1, wt% ink 2, wt% 3-MPDDA 62.4 61.9 61.9 Lauryl acrylate 28.8 28.3 28.3 Florstab UV12 1.0 MEHQ - - 1.0 Cyan pigment dispersion 7.8 7.8 7.8 Byk 307 1.0 1.0 1.0 Total 100.0 100.0 100.0 Viscosity at 25°C / mPa.s 5.8 6.0 5.7 Table 2. UV-curable inks Component Comparative Comparative Ink 6, wt% ink 4, wt% ink 5, wt% 3-MPDDA 55.4 54.9 54.9 Lauryl acrylate 21.8 21.3 21.3 Florstab UV12 - 1.0 -MEHQ 1.0 Cyan pigment dispersion 7.8 7.8 7.8 Esacure KIP 160 5.0 5.0 5.0 Omnirad 819 (BAPO) 4.0 4.0 4.0 Speedcure 7010L 5.0 5.0 5.0 Byk 307 1.0 1.0 1.0 Total 100.0 100.0 100.0 Viscosity at 25°C! mPa.s 10.3 10.2 10.5 3-MPDDA and lauryl acrylate are monomers, as defined herein. Florstab UV12 is an anaerobic stabiliser from Kromachem. MEHQ is an aerobic stabiliser.

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.

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

Byk 307 is a surfactant from Byk.

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

Example 2

Each of the above ink formulations was 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 Tables 3 and 4 and as discussed below.

Comparative inks 1 and 2 and ink 3 were cured using an EB Lab electron beam lab unit from ebeam Technologies (a division of COMET, Switzerland) at an energy of 100 keV, a line speed of 9 m/min and an 02 concentration of less than 200 ppm.

Curing of the ink was assessed by the dose required to achieve a cured ink film which requires greater than 100 doubles rubs of the IPA rub test. The IPA rub test is as described above.

The dose required was recorded and the results are set out in Table 3.

Table 3. ebeam dose required for full cure Comparative Comparative Ink 3, wt% ink 1, wt% ink 2, wt% Dose / kGy 20 30 20 Comparative inks 4 and 5 and ink 6 were cured using a medium pressure mercury lamp of power rating 135 W/cm, at a line speed of 30 m/min, in atmospheric air.

Again, curing of the ink was assessed by the dose required to achieve a cured ink film which requires greater than 100 doubles rubs of the IPA rub test. The dose required was recorded and the results are set out in Table 4.

Table 4. UV dose required for full cure Comparative Comparative Ink 6, wt% ink 4, wt% ink 5, wt% Dose / mJ/cm2 360 360 451 As can be seen from Table 3, comparative ink 1 does not contain any stabiliser and is fully cured using ebeam radiation at a dose of 20 kGy.

Comparative ink 2 contains an anaerobic stabiliser and it requires a higher dose of ebeam radiation for full cure than comparative ink 1. This shows that anaerobic stabilisers impair cure using ebeam radiation. The inventors believe that this is because anaerobic stabilisers function in the oxygen-deficient atmosphere required for ebeam curing.

Ink 3 contains an aerobic stabiliser and it requires the same dose of ebeam radiation for full cure as comparative ink 1 and a lower dose of ebeam radiation for full cure than comparative ink 2. This shows that aerobic stabilisers do not impair cure in an oxygen-deficient atmosphere, unlike anaerobic stabilisers. The inventors believe that this is because aerobic stabilisers do not function in the oxygen-deficient atmosphere required for ebeam curing.

Therefore, ink 3 is stabilised during storage by the presence of an aerobic stabiliser but the aerobic stabiliser does not inhibit cure in the presence of an oxygen-deficient atmosphere.

As can be seen from Table 4, comparative ink 4 does not contain any stabiliser and is fully cured using UV radiation at a dose of 360 mJ/cm2.

Comparative ink 5 contains an anaerobic stabiliser and it requires the same dose of UV radiation for full cure as comparative ink 4. This shows that anaerobic stabilisers do not impair cure using UV radiation, unlike using ebeam radiation. The inventors believe that this is because anaerobic stabilisers do not function in atmospheric air used in the UV curing.

Ink 6 contains an aerobic stabiliser and it requires a higher dose of UV radiation for full cure (451 mJ/cm2) than comparative inks 4 and 5. This shows that aerobic stabilisers impair cure using UV radiation, unlike using ebeam radiation. The inventors believe that this is because aerobic stabilisers function in atmospheric air used in the UV curing.

Therefore, the presence of the aerobic stabiliser in the ink in combination with curing the ink in the presence of an oxygen-deficient atmosphere ensures that the ink used in the method of the present invention is stable without compromising the cure of the ink. This is advantageous for food packaging applications.

Claims

Claims 1. A method of inkjet printing comprising the following steps, in order: (i) providing an inkjet ink comprising a radiation-curable material and 0.3-10% by weight of an aerobic stabiliser, 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 the presence of an oxygen-deficient atmosphere.
2. A method as claimed in claim 1, wherein the aerobic stabiliser is a phenolic stabiliser.
3. A method as claimed in claim 2, wherein the phenolic stabiliser is selected from 4-methoxyphenol, 2,6-di-tert-butyl-4-methylphenol, benzene-1,4-diol, 2,6-di-tert-butyl-N,N-dimethylamino-p-cresol, 4-tertbutylbenzene-1,2-diol and mixtures thereof.
4. A method as claimed in any preceding claim, wherein the aerobic stabiliser is present in 0.5-5% by weight, based on the total weight of the ink.
5. A method as claimed in any preceding claim, wherein the ink comprises less than 2% by weight of an anaerobic stabiliser, based on the total weight of the ink.
6. A method as claimed in claim 5, wherein the ink comprises less than 1% by weight of an anaerobic stabiliser, based on the total weight of the ink.
7. A method as claimed in any preceding claim, wherein the radiation-curable material comprises a radiation-curable monomer.
8. A method as claimed claim 7, wherein the radiation-curable monomer comprises a radiation-curable monomer having two or more functional groups.
9. A method as claimed in claim 8, wherein the radiation-curable monomer having two or more functional groups comprises a difuncfional monomer.
10. A method as claimed in any of claims 7-9, wherein the radiation-curable monomer comprises a monofuncfional monomer.
11. A method as claimed in any preceding claim, wherein the ink further comprises a colouring agent.
12. A method as claimed in any preceding claim, wherein the substrate is a food packaging.
13. A method as claimed in any preceding claim, wherein the ink is cured using a dose of ebeam radiation of less than 30 kGy.
14. A printed substrate obtainable by the method of any of claims 1-13.