GB2593583A

GB2593583A - A printing ink

Info

Publication number: GB2593583A
Application number: GB2101149.9A
Authority: GB
Inventors: Kite Matthew; Canning Sarah; 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: 2021-09-29
Anticipated expiration: 2041-01-28
Also published as: GB2593583B; GB202101149D0; GB202001151D0

Abstract

An inkjet ink comprises (i) a radiation-curable material and (ii) a photoinitiator package consisting of optionally in combination with The radiation-curable material may include an amine-modified (meth)acrylate oligomer and difunctional monomers, particularly 3-methyl-1,5-pentanediol diacrylate (3-MPDDA) and triethylene glycol divinyl ether (DVE-3). The ink may also contain a dispersed pigment. In another aspect, a method comprises inkjet printing the ink onto a substrate and curing the ink by exposure to a curing source. The curing source may comprise an actinic radiation source and/or a low energy electron beam radiation source. Preferably, the curing step comprises (i) partially curing the ink by exposing to a first curing source (e.g. a UV LED lamp) and (ii) fully curing the ink by exposing the ink to a second curing source (e.g. a source of low energy electron beam radiation). The substrate may be a food packaging.

Description

A printing ink This invention relates to a printing ink and in particular to an inkjet ink for printing onto food packaging.

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 viscosity requirements and physical film properties required, such as chemical and scratch resistance.

There is therefore a need in the art for inkjet inks which can be printed directly onto food packaging, have low migration properties and the required viscosity, without compromising the physical film properties required, such as chemical and scratch resistance.

Accordingly, the present invention provides an inkjet ink comprising: a radiation-curable material and a photoinitiator package, wherein the photoinitiator package consists of: S a+b+c = 1-20 or a+b+c = 1-20 in combination with Cl 0 d+e+f+g = 1-20 Cl 0 Surprisingly, it has been found that through careful selection of the photoinitiator package, a curable inkjet ink can be formulated that exhibits the physical film properties required but still has the desired low migration and viscosity required for inkjet application.

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

The inkjet ink comprises a photoinitiator package. The photoinitiator package consists of: which 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; or =P a+b+c = 1-20 a+b+c = 1-20 in combination with Cl 0 CH3 ^ / \ ---rigo 0 ilr 0 cH, cHseo "-0 irycH3 0 0 d+e-Ff-hg = 1-20 Cl 0 which is a polymeric thioxanthone-type photoinitiator and is known by the chemical name 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy (1-methylethylene)]} oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-ypoxy]acetylpoly[oxy (1-methylethylene)]}oxymethyl) propane or polymeric ITX.

Polymeric TPO-L has the CAS number 1834525-17-5 and is available commercially as Omnipol TPO sold by IGM. The total value of a, b and c of the chemical formula for polymeric TPO-L is equal to 120.

Polymeric ITX is available commercially as Speedcure 7010L® sold by Lambson®. Speedcure 7010L® additionally contains 2[2,2-bis(2-prop-2-enoyloxyethoxymethyDbutoxy]ethyl prop-2-enoate having the CAS number 28961-43-5. Speedcure 7010L® is a liquid at 20°C and is a solution of 1,3-diga-[1-chloro9-oxo-9H-thioxanthen-4-yDoxy]acetylpoly[oxy (1-methylethylene)]} oxy)-2,2-bis({a41-chloro-9-oxo-9H-thioxanthen-4-ypoxy]acetylpoly[oxy (1-methylethylene)]}oxymethyl) propane in trimethylolpropane ethoxylate triacrylate. The total value of d, e, f and g of the chemical formula for polymeric ITX is equal to 1-20. In a preferred embodiment, the value of d+e+f+g of the chemical formula for polymeric ITX is equal to 1-15.

The photoinitiator package consists of polymeric TPO-L as defined herein; or polymeric TPO-L and polymeric ITX as defined herein. By the photoinitiator package consisting of polymeric TPO-L; or polymeric TPO-L and polymeric ITX, it is meant that the only photoinitiators present in inkjet ink of the present invention are polymeric TPO-L, optionally in combination with polymeric ITX.

In a preferred embodiment, the inkjet ink of the present invention comprises polymeric TPO-L as the sole photoinitiator present in the inkjet ink.

In a preferred embodiment, the inkjet ink of the present invention comprises polymeric TPO-L and polymeric ITX as the only photoinitiators present in the inkjet ink.

It has surprisingly been found that an inkjet ink, which has the claimed photoinitiator package present in the inkjet ink, can achieve the required physical film properties and viscosity requirements, and is also suitable for application to food packaging.

Previously, components which are disadvantageous for food packaging owing to their migration properties were required in the inkjet ink in order to achieve the necessary physical film properties and viscosity. However, the inventors have found that utilising the specific photoinitiator package achieves reduced migration owing to the higher molecular weight of the polymeric photoinitiators, whilst achieving the necessary physical film properties and viscosity.

In a preferred embodiment, the inkjet ink of the present invention comprises 0.5 to 20% by weight, preferably 1 to 5% by weight, more preferably 2 to 3% by weight of the photoinitiator package, based on the total weight of the ink. As such, the total amount of photoinitiator present in the ink is preferably 0.5 to 20% by weight, preferably 1 to 5% by weight, more preferably 2 to 3% by weight, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink of the present invention comprises 0.5 to 10% by weight, preferably 1 to 5% by weight, more preferably 2 to 3% by weight of polymeric TPO-L, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink of the present invention comprises 0.5 to 10% by weight, preferably 1 to 5% by weight, more preferably 2 to 3% by weight of polymeric TPO-L, based on the total weight of the ink and 0.5 to 10% by weight, preferably 1 to 5% by weight, more preferably 2 to 3% by weight of polymeric ITX, based on the total weight of the ink.

The total amount of photoinitiator present in the inkjet ink will vary depending on the curing source used to cure the inkjet ink. The possible curing sources will be described in detail for the method of inkjet printing of the present invention. The formulator is free to add the required amount of photoinitiator for the chosen curing source. For example, higher amounts of photoinitiator are sometimes required for curing with a UV LED source when compared to curing with a low-energy electron beam.

The inkjet ink of the present invention comprises 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 ink of the present invention comprises 20 to 90% by weight of radiation-curable material, based on the total weight of the ink.

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

In a preferred embodiment, the inkjet ink of the present invention comprises one or more radiation-curable monomers. 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 of the present invention comprises one or more radiation-curable monomers 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 of the present invention comprises one or more difunctional monomers. In a particularly preferred embodiment, the inkjet ink of the present invention comprises at least two radiation-curable monomer having two or more functional groups.

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

In a preferred embodiment, the ink of the present invention comprises 5 to 90% by weight of radiation-curable monomers 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 of the present invention comprises one or more difunctional (meth)acrylate monomers.

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 of the present invention comprises 5 to 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 10 to 80% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink of the present invention comprises one or more multifunctional (meth)acrylate monomers.

Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include firi-, 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 ink of the present invention comprises 5 to 25% by weight of a multifunctional (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 10 to 80% by weight of a multifunctional (meth)acrylate monomer, based on the total weight of the ink.

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 of the present invention comprises one or more divinyl ether monomers, multifunctional vinyl ether monomers, divinyl ether (meth)acrylate monomers and/or multifunctional vinyl ether (meth)acrylate monomers. In a particularly preferred embodiment, the inkjet ink of the present invention comprises divinyl ether monomers.

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- (vinyloxymethypcyclohexylmethyl] glutarate, 1,4-butanediol divinyl ether and mixtures thereof.

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

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

Examples of 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 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), 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 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 /2° steel cone at 25°C with a shear rate of 25 s-1.

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

The inkjet ink of the present invention may further comprise one or more monofunctional monomers, 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 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 C5_10 aryl-or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

In a preferred embodiment, the inkjet ink comprises one or more monofunctional monomers present in 5-35% by weight, more preferably 10-30% by weight, most preferably 15-25% by weight, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink of the present invention comprises one or more monofunctional (meth)acrylate monomers, 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 monomers 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 monomers may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

In a preferred embodiment, the one or more monofunctional (meth)acrylate monomers comprise a cyclic monofunctional (meth)acrylate monomer.

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

The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (BOA), 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 one or more monofunctional (meth)acrylate monomers comprise an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

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

The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C6-C20 group. It may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear Ce-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 one or more monofunctional (meth)acrylate monomers are selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic IMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yOmethyl acrylate (MEDA/Medol10), 4-tert-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

In a preferred embodiment, the one or more monofunctional (meth)acrylate monomers are preferably selected from isobornyl acrylate (IBOA), cyclic TMP formal acrylate (CTFA), tetra hydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yOmethyl 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.

In a preferred embodiment, the inkjet ink comprises one or more monofunctional (meth)acrylate monomers present in 5-35% by weight, more preferably 10-30% by weight, most preferably 15-25% by weight, based on the total weight of the ink.

Tetrahydrofurfuryl acrylate (THFA) is often used to provide good adhesion to variety of substrates, as well as producing a flexible film which is less liable to cracking and delamination. A further advantage of THFA is that it can solubilise chlorinated polyolefins, which in turn provides good adhesion to 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 (TI-IFA), 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. 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, 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 one or more monofunctional (meth)acrylate monomers are the sole monofunctional monomers present in the ink.

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 N-acryloylmorpholine (ACMO).

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

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

Most preferably, the N-vinyl carbamate monomer is N-vinyl-5-methyl-2-oxazolidinone (known as NVMO or VMOX). It is available from BASF and has the following structure:

LO

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.

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

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

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

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

In a preferred embodiment, the inkjet ink 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 polyether acrylates.

More preferably, the radiation-curable oligomer is an amine-modified acrylate oligomer. A suitable amine-modified polyether acrylate oligomer is commercially available as CN3715LM.

Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance. However, for food packaging applications where quality and safety of the materials is a concern, the ink is preferably substantially free of bisphenol A based materials such as bisphenol A epoxy acrylates. Therefore, the ink is preferably substantially free of bisphenol A epoxy acrylates.

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

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

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

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

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

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

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

Preferably, the colouring agent is a dispersed pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF plc), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7.

Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

In one aspect the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7.

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

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

The present invention may also provide an inkjet ink set wherein at least one of the inks in the set is an inkjet ink of the present invention. Preferably, allot the inks in the set fall within the scope of the inkjet ink according to the present invention.

Usually, the inkjet ink set of the present invention is in the form of a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. The inks in a trichromatic set can be used to produce a wide range of colours and tones. Other inkjet ink sets may also be used, such as CMYK+white and light colours.

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 of the present invention 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.

Accordingly, the inkjet ink is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink comprises less than 5 wt% of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2 % by weight combined and most preferably less than 1% by weight combined, based on the total weight of the ink. 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 a preferred embodiment, the ink of the present invention comprises a surfactant. The surfactant controls the surface tension of the ink. Surfactants are well known in the art and a detailed description is not required. An example of a suitable surfactant is 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 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 inks of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The ink of the present invention is suitable for application 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 present invention may also provide a cartridge containing the inkjet ink as defined herein.

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

In the method of inkjet printing of the present invention, the 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.

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 present in the ink 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. 20 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.

The present invention may also provide a printed substrate having the ink as defined herein printed thereon. Preferably, the substrate is a food packaging.

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 ink of the present invention is cured by any means known in the art, such as exposure to actinic radiation and low-energy electron beam radiation.

It should be noted that the terms dry and "cure" are often used interchangeably in the art when referring to radiation-curable inkjet inks to mean the conversion of the inkjet ink from a liquid to solid by polymerisation and/or crosslinking of the radiation-curable material. Herein, however, by "drying" is meant the removal of the water by evaporation and by "curing" is meant the polymerisation and/or crosslinking of the radiation-curable material. Further details of the printing, drying and curing process are provided in WO 2011/021052.

In a preferred embodiment, the ink is cured by exposing the printed ink to a source of actinic radiation.

The source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources are well known in the art and a detailed description is not required. These include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source.

Preferably, the source of actinic radiation is a mercury discharge lamp and/or LEDs. When LEDs are used, these are preferably provided as an array of multiple LEDs.

The most common UV light source used to cure inkjet inks is a mercury discharge lamp. These lamps operate by creating a plasma between two electrodes in a high pressure mercury gas contained in a quartz envelope. Although these lamps have some drawbacks in terms of their operational characteristics, no other UV light source has yet managed to challenge their position in terms of UV output performance.

LEDs are increasingly used to cure inkjet inks. UV light is emitted from a UV LED light source. UV LED light sources comprise one or more LEDs and are well known in the art. Thus, a detailed

description is not required.

It will be understood that UV LED light sources emit radiation having a spread of wavelengths. The emission of UV LED light sources is identified by the wavelength which corresponds to the peak in the wavelength distribution. Compared to conventional mercury lamp UV sources, UV LED light sources emit UV radiation over a narrow range of wavelengths on the wavelength distribution. The width of the range of wavelengths on the wavelength distribution is called a wavelength band. LEDs therefore have a narrow wavelength output when compared to other sources of UV radiation. By a narrow wavelength band, it is meant that at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength within a wavelength band having a width of 50 nm or less, preferably, 30 nm or less, most preferably 15 nm or less.

In a preferred embodiment, at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

LEDs have a longer lifetime and exhibit no change in the power/wavelength output over time. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate.

In a preferred embodiment, the ink is cured by exposing the printed ink to low-energy electron beam (ebeam).

The source of low-energy electron beam (ebeam) can be any source of low-energy electron beam that is suitable for curing radiation-curable inks. Suitable low-energy electron beam radiation sources include commercially available ebeam curing units, such as the EB Lab from ebeam Technologies with energy of 80-300 keV and capable of delivering a typical dose of 30-50 kGy at line speeds of up to 30 m/min. By "low-energy" for the ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 70 kGy or less.

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

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

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

Advantageously, ebeam can be used to fully cure the ink of the present invention, even when polymeric TPO-L is the sole photoinitiator present in the ink and no other techniques are employed to improve curing.

In a preferred embodiment, the inkjet ink is initially partially cured (i.e. pinned) by exposure to a first curing source, followed by full cure by exposure to a second curing source. Preferably therefore, the method of the present invention comprises inkjet printing the inkjet ink of the present invention onto a substrate, and curing the ink by exposing the printed ink to a curing source, wherein curing the ink by exposing the printed ink to a curing source comprises the following steps in the following order: (i) partially curing the inkjet ink by exposing the ink to a first curing source, and (ii) fully curing the ink by exposing the printed ink to a second curing source.

The first and second curing source can be any curing source known in the art. Preferably, the first curing source for partially curing the inkjet ink is a UV LED lamp and the second curing source for fully curing the inkjet ink is a source of low-energy electron beam radiation.

By partial curing of the inkjet ink, it is meant that on exposure to a first curing source, partial curing of the ink below the surface of the printed film is achieved, which causes the ink to "set" on the substrate. This step is also known as "pinning". Without wishing to be bound by theory, it is believed that the first curing source penetrates the ink and partial cure of the ink throughout the printed film can be achieved. However, the surface of the film remains tacky after partial curing, often owing to the inhibiting effect of oxygen in the atmosphere adjacent to the printed ink surface. A second curing step comprising exposing the partially cured ink to a second curing source fully cures the ink by curing the ink film at the surface.

In a preferred embodiment, the step of curing the ink by exposing the printed ink to a curing source is performed in an oxygen-deficient atmosphere. An oxygen-deficient atmosphere is one having less oxygen than atmospheric air. For example, oxygen may be replaced with inert gases, such as carbon dioxide, nitrogen or argon. Employing an oxygen-deficient atmosphere during the curing step improves curing and this is known in the art. In this regard, curing of the inkjet ink, especially at the surface of the ink film, is hindered by the inhibiting effect of oxygen in the atmosphere adjacent to the printed ink surface, which is overcome in an oxygen-deficient atmosphere. Accordingly, the dose of radiation of the curing source required to cure the ink may be reduced and/or alternative sources of radiation can be used to achieve full cure, which would not achieve full cure without the oxygen-deficient atmosphere.

It is preferred that curing is performed in an oxygen-deficient atmosphere, preferably a nitrogen atmosphere, when polymeric TPO-L is the sole photoinitiator present in the ink. It is additionally preferred that curing is performed in an oxygen-deficient atmosphere, preferably a nitrogen atmosphere, when a mercury discharge lamp or LEDs are used as the curing source. It is particularly preferred that curing is performed in an oxygen-deficient atmosphere, preferably a nitrogen atmosphere, when polymeric TPO-L is the sole photoinitiator present in the ink and when a mercury discharge lamp or LEDs are used as the curing source. In this embodiment, blanketing the irradiated area with an inert gas facilitates full cure of the ink.

Curing in an oxygen-deficient atmosphere is well-known in the art and therefore a detailed explanation is not required.

For the avoidance of doubt, performing the step of curing the inkjet ink in an oxygen-deficient atmosphere is advantageous but not necessary to fully cure the ink. It is possible to fully cure the ink, without recourse to an oxygen-deficient atmosphere. Full curing is influenced by various other factors, including the dose of radiation, the source of radiation used, the components of the ink and others. In fact, it has been found that the particular photoinitiator package of the present invention aids in providing an improved cure (through and surface cure), whilst maintaining the required physical properties, viscosity and migration requirements for food packaging.

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 IS02049 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.

Partial through cure is achieved by a film which requires 20 IPA doubles rubs to break through the partially cured ink film to the substrate 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 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 Comparative ink 1, wt% Ink 2, wt% Ink 3, wt% 3-MPDDA 51.5 42.7 61.8 DVE-3 20.0 20.0 20.0 CN3715LM - 10.0 -UVP6600 7.0 lrgastab UV22 0.5 0.2 0.2 Cyan pigment dispersion 8.0 8.0 8.0 Omnipol TP 9.0 9.0 Speedcure 7010L 10.0 Esacure KIP 160 7.0 - - lrgacure 2959 3.0 - -Omnirad 819 2.0 Byk 307 1.0 0.1 1.0 Total 100.0 100.0 100.0 Viscosity at 25°C / mPa.s 13.3 14.5 8.7 3-MPDDA and DVE-3 are monomers, as defined herein. CN3715LM and UVP6600 are amine (meth)acrylate oligomers. 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.

Omnipol TP is a polymeric photoinitiator as defined herein from IGM. Speedcure 7010L is a polymeric photoinifiator from Lambson. Esacure KIP 160, lrgacure 2959 and Omnirad 819 (BAPO) are photoinitiators from IGM.

Byk 307 is a surfactant from Byk.

The viscosity of the inks were measured using a Brookfield DVIII LV 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 were drawn down onto a 220 micron PVC substrate (Genotherm) using a K bar applicator depositing a 12 micron wet film. The resulting films were cured using a Heraeus Noblelight UV mercury lamp of power rating 180 W/cm. The UV dose required for full cure, i.e. through and surface cure to provide a tack-free film and the required physical film properties, was recorded and the results are set out in Table 2.

Table 2

Comparative ink 1, wt% Ink 2, wt% Ink 3, wt% Dose / 325 360 Does not fully cure mJ/cm2 As can be seen from Table 2, comparative ink 1, which contains monomeric photoinitiators, requires a UV dose of 325 mJ/cm2 from a UV mercury lamp for full cure. Ink 2, which is an ink of the invention comprising both of the claimed polymeric photoinitiators, requires a UV dose of 360 mJ/cm2 from a mercury lamp for full cure.

Therefore, comparative ink 1 and ink 2 of the invention require a similar irradiation dose from a UV mercury lamp for full cure. Ink 2 is therefore able to match the cure response of comparative ink 1 and achieve full cure and the required physical film properties, by irradiation with a UV mercury lamp, without recourse to monomeric photoinitiators as used in comparative ink 1, which are disadvantageous for food packaging applications.

Ink 3, which is an ink of the invention comprising polymeric TPO-L but which does not contain polymeric ITX, does not fully cure on exposure to a UV mercury lamp alone. Although through cure is achieved, surface cure is not achieved on exposure to a UV mercury lamp alone in the absence of polymeric ITX.

Accordingly, in order to achieve full cure using a mercury lamp alone, both of the claimed polymeric photoinitiators must be present in the inkjet ink.

As is known in the art, surface cure using a UV mercury lamp is assisted by employing an oxygen-deficient, such as a nitrogen atmosphere. Accordingly, employing an oxygen-deficient atmosphere during curing of ink 3 with a UV mercury lamp will assist surface curing of ink 3 and hence permit full cure as discussed in Example 4 below.

Example 3

Each of the above ink formulations were printed in the same way as in Example 2 and the resulting films were cured using a Phoseon 20W LED lamp. The UV dose required for full cure, i.e. through and surface cure to provide a tack-free film and the required physical film properties, was recorded and the results are set out in Table 3.

Table 3

Comparative ink 1, wt% Ink 2, wt% Ink 3, wt% Dose! Does not fully cure 1130 Does not fully cure mJ/cm2 As can be seen from Table 3, it is only possible to fully cure ink 2 and achieve the required physical film properties using a UV LED lamp alone.

The irradiation dose required to achieve full cure of ink 2 using a UV LED lamp is relatively high. Although through cure of ink 2 was achieved with a lower dose of radiation from the UV LED lamp alone, surface cure (and hence full cure) required a relatively high dose. As is known in the art, it is possible to aid surface cure of inkjet inks by employing an oxygen-deficient atmosphere. As such, employing an oxygen-deficient atmosphere will reduce the required dose for full cure and achieve the required physical film properties.

In contrast, comparative ink 1 does not fully cure and does not have the required physical film properties on exposure to a UV LED lamp, even at high dose. Ink 2 of the present invention therefore has an improved cure response and physical film properties on exposure to a UV LED lamp when compared to comparative ink 1.

Regarding ink 3, full cure is not achieved on exposure to the UV LED lamp alone. Once again, through cure is achieved as shown in Example 4 but surface cure is not achieved for this ink on exposure to the UV LED lamp alone, in the absence of polymeric ITX. Accordingly, in order to achieve full cure and the required physical film properties using a UV LED lamp alone, both of the claimed polymeric photoinifiators must be present in the inkjet ink.

As is known in the art, surface cure using a UV LED lamp is assisted by employing an oxygen-deficient atmosphere. Accordingly, employing an oxygen-deficient atmosphere during curing of ink 3 with a UV LED lamp will assist surface curing of ink 3 and hence permit full cure and the required physical film properties as discussed in Example 4 below.

Example 4

This example has been carried out to show that it is possible to achieve full cure and the required physical film properties of ink 3 of the invention and as such, that an inkjet ink comprising polymeric TPO-L as the sole photoinitiator present in the ink achieves full cure and the required physical film properties.

Ink 3 was printed in the same way as in Example 2 and the resulting film was cured using a Phoseon 20 W LED lamp and through cure was assessed. The UV dose required for through cure, i.e. a film, which requires 20 IPA double rubs to break through the pinned ink film to the substrate, was recorded and the results are set out in Table 4.

Table 4

Ink 3, wt% Dose / mJ/cm2 470 As can be seen from Table 4, ink 3 which is an ink of the invention comprising polymeric TPO-L as the sole photoinitiator present in the ink, requires a UV dose of 470 mJ/cm2 from an LED lamp for through cure.

This shows that pinning with an LED lamp is achieved (i.e. through cure). As discussed above and as is known in the art, surface cure is assisted by employing an oxygen-deficient atmosphere. Accordingly, employing an oxygen-deficient atmosphere will assist surface curing of ink 3 and hence permit full cure and the required physical film properties. It is also known in the art that improved surface cure can be achieved when cured by ebeam. As such, full cure of ink 3 will also be achieved on curing by ebeam.

Therefore, full cure of inks 2-3 of the present invention and hence the required physical film properties can be achieved, at the required viscosity, without recourse to photoinitiators, which are disadvantageous for food packaging.

Claims

Claims 1. An inkjet ink comprising: a radiation-curable material and a photoinitiator package, wherein the photoinitiator package consists of:
Sa+b+c = 1-20 0 0 \ or \Nor: =p a+b+c = 1-20 in combination with Cl 0 d+e+f+g = 1-20 Cl 0 2. An inkjet ink as claimed in claim 1, wherein the radiation-curable material comprises one or more radiation-curable monomers.
3. An inkjet ink as claimed in claim 2, wherein the one or more radiation-curable monomers comprise one or more radiation-curable monomers having two or more functional groups.
4. An inkjet ink as claimed in claim 3, wherein the one or more radiation-curable monomers having two or more functional groups comprise one or more difunctional monomers.
S. An inkjet ink as claimed in any preceding claim, wherein the radiation-curable material comprises a radiation-curable oligomer, preferably a (meth)acrylate oligomer.
6. An inkjet ink as claimed in claim 6, wherein the radiation-curable oligomer is amine-modified.
7. An inkjet ink as claimed in any preceding claim, further comprising a colouring agent, preferably a dispersed pigment.
8. A cartridge containing the inkjet ink as claimed in any of the preceding claims.
9. A method of inkjet printing comprising: inkjet printing the inkjet ink as claimed in any of claims 1 to 7 onto a substrate and curing the ink by exposing the printed ink to a curing source.
10. A method of inkjet printing as claimed in claim 9, wherein the substrate is a food packaging.
11. A method of inkjet printing as claimed in any of claims 9 to 10, wherein the curing source is a source of actinic radiation and/or a source of low-energy electron beam radiation.
12. A method of inkjet printing as claimed in any of claims 9 to 11, wherein curing the ink by exposing the printed ink to a curing source comprises the following steps in the following order: (i) partially curing the inkjet ink by exposing the ink to a first curing source, (ii) fully curing the ink by exposing the printed ink to a second curing source.
13. A method of inkjet printing as claimed in claim 12, wherein the first curing source is a UV LED lamp and the second curing source is a source of low-energy electron beam radiation.
14. A printed substrate having the ink as claimed in any of claims 1 to 7 printed thereon.
15. A printed substrate as claimed in claim 14, wherein the substrate is food packaging.