GB2593798A

GB2593798A - Printing Ink

Info

Publication number: GB2593798A
Application number: GB2101154.9A
Authority: GB
Inventors: Ward Jeremy; Canning Sarah; Wade Francesca
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: 2021-10-06
Anticipated expiration: 2041-01-28
Also published as: GB202101154D0; GB2593798B; GB202001157D0

Abstract

An inkjet ink comprises (i) 5-40 wt.% one or more first multifunctional monomers having an alkoxylated backbone, (ii) 20-70 wt.% one or more second multifunctional monomers having a logPoctanol/water value of at least 4.0, and (iii) less than 5 wt.% monofunctional monomers, wherein the total amount of multifunctional monomers is at least 50 wt.%. Typically, the first multifunctional monomer is difunctional and has an ethoxylated backbone, e.g. triethylene glycol divinyl ether (DVE-3). The second multifunctional monomer may also be difunctional and may be 1,10-decanediol diacrylate (1,10-DDDA). The ink is normally free of monofunctional monomers and photoinitiators. A preferred ink comprises 10-30 wt.% first multifunctional monomers, 30-70 wt.% second multifunctional monomers, and 1-10 wt.% additional multifunctional monomers. Preferably, the ink components, monomers, and multifunctional monomers in the ink each have weighted mean logPoctanol/water values of at least 3.2. The inkjet ink may be used on food packaging.

Description

Printing ink The present invention relates to a printing ink, and in particular to an inkjet ink that is suitable for 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 can be printed onto a variety of substrates. Food packaging represents a particular challenge. There are a number of key requirements for food packaging, which include flexibility and water resistance of the cured ink, and low migration of the components of the printed ink.

It is relatively straightforward to provide flexibility in the cured ink by using low viscosity monofuncfional monomers. However, such monofuncfional monomers are prone to migration, which should be avoided for food packaging.

In this regard, there are strict safety limitations on the properties of materials that 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.

In order to reduce migration of the components of the printed ink for food packaging applications, inkjet inks using high functionality components, such as multifunctional monomers, are preferred.

High functionality components are more likely to react, which means that they will not be available for migration. However, inkjet inks using high functionality components, such as multifunctional monomers, often suffer from reduced flexibility of the printed ink.

There are very few high functionality components, such as multifunctional monomers, available that permit the required balance of flexibility in combination with low migration. And such multifunctional monomers, which provide the balance of flexibility and low migration, often suffer from poor water resistance. Water resistance is a key requirement in food packaging applications, where the printed material is often sterilised in aqueous hydrogen peroxide or the final packaged product is stored in the fridge.

There is therefore a need in the art to provide an inkjet ink, which balances low migration, improved flexibility and water resistance of the printed ink.

Accordingly, the present invention provides an inkjet ink comprising: 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone; 20-70% by weight of one or more multifunctional monomers each having a logP oclanol/water value of 4.0 or more; less than 5% by weight of monofunctional monomer; wherein the total amount of all multifunctional monomers in the inkjet ink is at least 50% by weight, where the amounts are based on the total weight of the ink.

The inventors have surprisingly found that an inkjet ink, which comprises 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone, 20-70% by weight of one or more multifunctional monomers each having a logPodanouwaier value of 4.0 or more, less than 5% by weight of monofuncfional monomer, where the total amount of all multifunctional monomers in the inkjet ink is at least 50% by weight, based on the total weight of the ink, provides the necessary balance of reduced migration, improved flexibility and water resistance of the printed ink.

Monomers typically have a molecular weight of less than 600 Daltons, preferably more than 200 Daltons and less than 450 Daltons. 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.

As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers. Monofunctional is intended to have its standard meaning, i.e. one group, which takes part in the polymerisation reaction on curing. Multifunctional is intended to have its standard meaning, i.e. two or more groups, which take part in the polymerisation reaction on curing.

The inkjet ink of the present invention comprises 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone.

In a preferred embodiment, the one or more multifunctional monomers each having an alkoxylated backbone is a di-, tri-, tetra-, penta-or hexa-functional monomer, i.e. the one or more multifunctional monomers each having an alkoxylated backbone has two, three, four, five or six functional groups. In a particularly preferred embodiment, the one or more multifunctional monomers each having an alkoxylated backbone comprises a difunctional monomer. In a particularly preferred embodiment, the one or more multifunctional monomers each having an alkoxylated backbone comprises at least two multifunctional monomers and more preferably, at least two difunctional monomers.

The one or more multifunctional monomers each having an alkoxylated backbone may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality monomers may be used.

The functional group of the one or more multifunctional monomers each having an alkoxylated backbone 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 one or more multifunctional monomers each having an alkoxylated backbone may be selected from multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers, multifunctional vinyl ether (meth)acrylate monomers and mixtures thereof.

The multifunctional monomer having an alkoxylated backbone is composed of two or more polymerisable groups connected by an alkoxylated backbone. Accordingly, if the functional groups are (meth)acrylate groups, the multifunctional monomer having an alkoxylated backbone is composed of two or more esters of acrylic or methacrylic acid in which the alcohol moieties are connected by an alkoxylated backbone. Likewise, if the functional groups are vinyl ether groups, the multifunctional monomer having an alkoxylated backbone is composed of two or more ethers of vinyl alcohol in which the alcohol moieties are connected by an alkoxylated backbone. Similarly, if the functional groups are vinyl ether and (meth)acrylate groups, the multifunctional monomer having an alkoxylated backbone is composed of one or more esters of acrylic or methacrylic acid and one or more ethers of vinyl alcohol in which the alcohol moieties are connected by an alkoxylated backbone.

The backbone is alkoxylated in that the backbone contains carbon atoms interrupted by oxygen atoms. The alkoxylated backbone is preferably a C2-50 backbone interrupted by 1-20 oxygen atoms and more preferably a C4-40 backbone interrupted by 2-15 oxygen atoms.

In a preferred embodiment, the alkoxylated backbone is an ethoxylated or propoxylated backbone. Preferably, the one or more multifunctional monomers each having an alkoxylated backbone comprises a multifunctional monomer having an ethoxylated backbone. A multifunctional monomer having an ethoxylated backbone contains a number of -CH2CH20-units. More preferably, the one or more multifunctional monomers each having an alkoxylated backbone comprises a multifunctional monomer having an ethoxylated backbone, wherein the ethoxylated backbone comprises 1-20 -CH2CH20-units, preferably 2-15 -CH2CH20-units.

The alkoxylated backbone may be linear or branched, but is preferably linear.

The alkoxylated backbone may be saturated or unsaturated, but saturated is preferred.

Preferred examples of multifunctional (meth)acrylate monomers having an alkoxylated backbone include the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA and PEG600DA), ethoxylated trimethylolpropane triacrylate (for example TMP3EOTA, TMP6EOTA, TMP9EOTA and TMP15EOTA), ethoxylated 1,6-hexanediol diacrylate (for example HD(5E0)DA and HD(2E0)DA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tripropylene glycol triacrylate, propoxylated neopentylglycol diacrylate (NPGPODA), propoxylated pentaerythritol triacrylate, ethoxylated pentaerythritol tetraacrylate (EOPETTA), di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DPHA) and mixtures thereof Preferred examples of multifunctional vinyl ether monomers having an alkoxylated backbone include triethylene glycol divinyl ether (DVE-3) and diethylene glycol divinyl ether (DVE-2).

DVE-3 is particularly preferred. DVE-3 is preferred because of its low viscosity.

In a preferred embodiment, the inkjet ink comprises 10-30% by weight of the one or more multifunctional monomers each having an alkoxylated backbone, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises 10-30% by weight of multifunctional (meth)acrylate monomers having an alkoxylated backbone, multifunctional vinyl ether monomers having an alkoxylated backbone or mixtures thereof Inkjet inks comprising one or more multifunctional monomers each having an alkoxylated backbone in the claimed amount, provide an ink film having various advantageous properties, including improved flexibility. However, such inkjet inks are often susceptible to hydrolysis on account of the high polarity, hydrophilic nature of the monomers. Accordingly, they suffer from poor water resistance, which is essential for food packaging.

In this regard, for food packaging applications, the printed material is often sterilised in aqueous hydrogen peroxide. For an ink with poor water resistance, on sterilisation in aqueous hydrogen peroxide, the surface of the print is hydrolysed. This results in pigment and other components of the ink being released, which should be avoided.

The inventors have surprisingly found however that the addition of 20-70% by weight of one or more multifunctional monomers each having a logP oclanol/vvater value of 4.0 or more to an inkjet ink having 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone, having less than 5% by weight of monofunctional monomer, where the total amount of all multifunctional monomers in the inkjet ink is at least 50% by weight, where the amounts are based on the total weight of the ink, provides improved water resistance, whilst surprisingly maintaining the other advantageous properties of the ink, such as flexibility of the ink film.

Without wishing to be bound by theory, the inventors have found that 20-70% by weight of one or more multifunctional monomers each having a logP * oclanol/water value of 4.0 or more, based on the total weight of the ink, protects the cured ink against hydrolysis/degradation. This is surprisingly achieved whilst maintaining the other desirable properties, such as flexibility.

The inkjet ink of the present invention comprises 20-70% by weight of one or more multifunctional monomers each having a logP * octanol/water value of 4.0 or more, based on the total weight of the ink.

LogP is known in the art and a detailed description is not required. LogP is a measure of the polarity of a compound. The partition coefficient (P) is the ratio of concentrations of a compound in a mixture of two immiscible phases at equilibrium. Therefore, this ratio is a measure of the difference in solubility of the compound in these two phases. Both phases are usually solvents. The two solvents have different polarity and the most common solvents are water and 1-octanol (octanol). Water is the polar solvent and octanol is the non-polar solvent.

LogP is the logarithm of the ratio of the concentrations of a solute between the two solvents, specifically for un-ionised solutes. Accordingly, logP value is a measure of the lipophilicity or hydrophilicity. When the two phases are water and octanol: -1 tirst iardaead catirtiA 1. 101 eilat/er log pix: To measure the partition coefficient of ionisable solutes, the pH of the aqueous phase is adjusted such that the predominant form of the compound in solution is the un-ionised. A number of methods of measuring partition coefficients have been developed including the shake-flask, reverse phase HPLC and pH-metric techniques.

Partition coefficients are also widely available in the art and may also be theoretical and based on the structure of the compound. For example, partition coefficients may be calculated using software, such as ChemSketch, or online on websites, such as the Chemicalize website. The partition coefficients discussed herein are theoretical values that were determined using the Chemicalize website.

The logPodanomate, value defines the relative polarity/hydrophilicity of components of the ink. High log Poctanol/Water values, such as a logPoetanawater value of 4.0 or more, indicate low polarity.

Multifunctional monomers having a logP * octanoliwater value of 4.0 or more are low polarity multifunctional monomers. Accordingly, the inkjet ink of the present invention comprises 20-70% by weight of one or more multifunctional monomers each having a low polarity, based on the total weight of the ink.

The one or more multifunctional monomers each have a logP octanol/water value of 4.0 or more, preferably 4.5 or more. Preferably, the one or more multifunctional monomers each having a log PodanoMater value of 4.0 or more, preferably 4.5 or more, each have a logPodanouwater value of less than 9.0.

In a preferred embodiment, the one or more multifunctional monomers each having a logP octanol/water value of 4.0 or more is a di-, tri-, tetra-, penta-or hexa-functional monomer, i.e. the one or more multifunctional monomers each having a logPcdanovweie, value of 4.0 or more has two, three, four, five or six functional groups. In a particularly preferred embodiment, the one or more multifunctional monomers each having a logP oclanol/water value of 4.0 or more comprises a difunctional monomer. In a particularly preferred embodiment, the one or more multifunctional monomers each having a 10gPocianol/Water value of 4.0 or more comprises at least two multifunctional monomers and more preferably, at least two difunctional monomers.

The functional group of the one or more multifunctional monomers each having a logP octanol/water value of 4.0 or more 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 one or more multifunctional monomers each having a logPodanouwate, value of 4.0 or more may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality monomers may be used.

The one or more multifunctional monomers each having a logP octanol/water value of 4.0 or more may be selected from multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers, multifunctional vinyl ether (meth)acrylate monomers and mixtures thereof. Multifunctional monomers having a logPodanov,mter value of 4.0 or more are well known in the art and a detailed description is therefore not required.

Preferred examples of multifunctional monomers having a logP octanonwater value of 4.0 or more include 1,10-decanediol diacrylate (1,10-DDDA) and 1,9-nonanediol diacrylate (1,9-NDDA). 1,10-DDDA is particularly preferred. 1,10-DDDA has a lo g. P octanol/water value of 4.9.

In a preferred embodiment, the inkjet ink comprises 30-70% by weight of the one or more multifunctional monomers of the one or more multifunctional monomers each having a log P oda nol/water value of 4.0 or more, based on the total weight of the ink.

The inkjet ink of the present invention may comprise 1-10% by weight of, preferably 1-5% by weight of, one or more additional multifunctional monomers other than the one or more multifunctional monomers each having an alkoxylated backbone and the one or more multifunctional monomers each having a logPoctanomaier value of 4.0 or more, based on the total weight of the ink. Accordingly, the one or more additional multifunctional monomers are distinct from the one or more multifunctional monomers each having an alkoxylated backbone and the one or more multifunctional monomers each having a logP octanol/water value of 4.0 or more.

Preferred examples of the one or more additional multifunctional monomers is a di-, tri-, tetra-, penta-or hexa-functional monomer, i.e. the one or more additional multifunctional monomers has two, three, four, five or six functional groups. The one or more additional multifunctional monomers preferably comprises a difunctional monomer.

The functional group of the one or more additional multifunctional monomers 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 one or more additional multifunctional monomers 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 one or more additional multifunctional monomers cannot be alkoxylated and the one or more additional multifunctional monomers cannot have a logP * octanolAvater value of 4.0 or more. Otherwise, the substituents of the one or more additional multifunctional monomers are only limited 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 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

The one or more additional multifunctional monomers may be selected from multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers, multifunctional vinyl ether (meth)acrylate monomers and mixtures thereof. Multifunctional monomers of the one or more additional multifunctional monomers are well known in the art and a detailed description is therefore not required.

Multifunctional (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, 1,11-undecanediol diacrylate, 1,12-dodecanediol diacrylate, tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1,5-pentanediol diacrylate (3-MPDDA) 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, 1,11-undecanediol dimethacrylate, 1,12-dodecanediol dimethacrylate, 1,4-butanediol dimethacrylate, trimethylolpropane trimethacrylate, and mixtures thereof.

Multifunctional vinyl ether monomers are well known in the art and a detailed description is not therefore required. Examples of multifunctional vinyl ether monomers include divinyl ether monomers such as 1,4-cyclohexanedimethanol divinyl ether, bis[4-(vinyloxy)butyl] 1,6-hexanediylbiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi- 4,1-phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4-(vinyloxymethyl)cyclohexylmethyl] glutarate, tris[4-(vinyloxy)butyl] trimellitate and mixtures thereof.

In a preferred embodiment, the one or more additional multifunctional monomers is preferably selected from 3-methyl 1,5-pentanediol diacrylate (3-MPDDA), tricyclodecane dimethanol diacrylate (TCDDMDA) and mixtures thereof. More preferably, the one or more additional multifunctional monomers comprises 3-methyl 1,5-pentanediol diacrylate (3-MPDDA). These are particularly preferred for use in food packaging applications.

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

The total amount of all multifunctional monomers in the inkjet ink is at least 50% by weight, based on the total weight of the ink. This is the total amount of all multifunctional monomers present in the inkjet ink as a whole, which includes the one or more multifunctional monomers each having an alkoxylated backbone, the one or more multifunctional monomers each having a logPoctanouwate, value of 4.0 or more, and the optional one or more additional multifunctional monomers. In a preferred embodiment, the total amount of all multifunctional monomers in the inkjet ink is at least 60% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, based on the total weight of the ink.

The inventors have found that this high total amount of all multifunctional monomers provides reduced migration. High functionality components are more likely to react, which means that they will not be available for migration.

Further, the inventors have found that this high total amount of all multifunctional monomers, where the inkjet ink comprises 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone, 20-70% by weight of one or more multifunctional monomers each having a log Podanolfwater value of 4.0 or more, and less than 5% by weight of monofunctional monomer, where the amounts are based on the total weight of the ink, provides improved water resistance. In this regard, such a high amount of all multifunctional monomers, in combination with the claimed blend of multifunctional monomers in the claimed amounts, and limited monofuncfional monomer content, leads to a less polar inkjet ink. Specifically, the weighted mean logP octanol/water value of the components of the ink is increased. This leads to improved water resistance, whilst surprisingly maintaining the other advantageous properties of the ink, such as flexibility of the ink film.

By weighted mean oda nol/water value of logP the components of the ink, it is meant the mean of the * logPodanouwater values of the components of the ink, which is weighted based on the percentage of each component in the ink, where the percentage of each component in the ink is a weight percent based on the total weight of the ink. Put another way, the weighted mean of the logloodanouwate, values of the components of the ink is the sum of each logPoctanomate, value for each component multiplied by the percent by weight of each component divided by the sum of the percent by weight of each component.

In a preferred embodiment, the weighted mean logP oda no Uwater value of the components in the inkjet ink of the present invention is 3.2 or more, preferably 3.5 or more.

In a preferred embodiment, the weighted mean logP octanol/water value of the monomers present in the inkjet ink of the present invention is 3.2 or more, preferably 3.5 or more.

By weighted mean logP octa nolAvater value of the monomers of the ink, it is meant the mean of the logPodanouwater values of the monomers of the ink, which is weighted based on the percentage of each monomer in the ink, where the percentage of each monomer in the ink is a weight percent based on the total weight of the ink. Put another way, the weighted mean of the logP oda nolAvaler values of the monomers of the ink is the sum of each logP oda n olfwater value for each monomer multiplied by the percent by weight of each monomer divided by the sum of the percent by weight of each monomer.

In a preferred embodiment, the weighted mean logPodanouwater value for the multifunctional monomers present in the inkjet ink of the present invention is 3.2 or more, preferably 3.5 or more.

By weighted mean logP oda nol/water value of the multifunctional monomers of the ink, it is meant the mean of the logPoctanoimater values of the multifunctional monomers of the ink, which is weighted based on the percentage of each multifunctional monomer in the ink, where the percentage of each multifunctional monomer in the ink is a weight percent based on the total weight of the ink. Put another way, the weighted mean of the logPodanow,ater values of the multifunctional monomers of the ink is the sum of each logPodandmater value for each multifunctional monomer multiplied by the percent by weight of each multifunctional monomer divided by the sum of the percent by weight of each multifunctional monomer.

In a preferred embodiment, the inkjet ink comprises less than 10% by weight, preferably less than 5% by weight, based on the total weight of the ink, of components that reduce the weighted mean log Pocianol/water value of the inkjet ink should it be included in the ink, other than the one or more multifunctional monomers each having an alkoxylated backbone.

In a preferred embodiment, the inkjet ink is a water-resistant inkjet ink, meaning that they are water-resistant after curing.

In order to measure the water resistance of the inkjet ink after curing, an aqueous pigment dispersion is added to the inkjet ink to facilitate observation of water resistance of the inkjet ink film after curing. The inkjet ink is coated onto a substrate to produce a wet film and then cured to produce a cured ink film. The cured ink film is then submerged in a tray of boiling water for two hours. Water resistance of the cured ink film can then be assessed by a water rub test.

The water rub test is well known in the art. One takes a lint-free (cotton) cloth, and while the cured ink film is still submerged in the water, one carries out a total of five double rubs. A double rub is where the cloth is applied to one side of the cured ink film and under light pressure, traverses the length of the cured ink film in a single stroke and then traverses back again in a single stroke.

An inkjet ink is water-resistant if no noticeable colour transfer is transferred from the cured ink film to the cloth in five double rubs of the water rub test. Put another way, in a preferred embodiment, the surface of the cured inkjet ink of the present invention resists hydrolysation.

The inkjet ink of the present invention may further comprise one or more monofunctional monomers. Monofunctional monomers are well known in the art and a detailed description is not required.

However, on account of their monofunctional nature, monofunctional monomers are more prone to migration. Accordingly, the inkjet ink of the present invention comprises less than 5% by weight of monofunctional monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink contains less than 2% by weight, more preferably less than 1% by weight, more preferably is substantially free of monofunctional monomer. This has the advantage of reduced migration as low functionality components are more likely to migrate.

Surprisingly, the inkjet ink of the present invention maintains flexibility, without recourse to such monofunctional monomers.

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 monofunctional monomer is intentionally added to the ink. However, minor amounts of monofunctional monomers, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of monofunctional monomers, more preferably less than 0.1% by weight of monofunctional monomers, most preferably less than 0.05% by weight of monofunctional monomers, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of monofunctional monomers.

If present, preferably the inkjet ink comprises one or more monofunctional monomers in 1-4% by weight, based on the total weight of the ink.

If present, the one or more monofunctional monomers may be any monofunctional monomers known in the art. In a preferred embodiment, if present, the one or more monofunctional monomers each have a logPodanovweier value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. Preferably, if present the one or more monofunctional monomers each have a logP oclanoliwater value of less than 9.0.

In a preferred embodiment, if present, the one or more monofunctional monomers each have a molecular weight of 195 g/mol or higher. Such monomers are less prone to migration for food packaging applications.

If present the one or more monofunctional monomers are preferably selected from a monofunctional (meth)acrylate monomer, N-vinyl amide monomer, N-(meth)acryloyl amine monomer, N-vinyl carbamate monomer and combinations thereof.

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

The substituents of the one or more monofunctional (meth)acrylate monomer if present are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. If present, the one or more monofunctional (meth)acrylate monomers may be a cyclic 20 monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

In a preferred embodiment, if present the one or more monofunctional (meth)acrylate monomer comprises a cyclic monofunctional (meth)acrylate monomer.

The substituents of the cyclic monofunctional (meth)acrylate monomer are typically cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms and/or substituted by alkyl. Non-limiting examples of substituents commonly used in the art include C3-18 cycloalkyl, Cs-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-tertbutylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof.

In a preferred embodiment, if present the cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tertbutylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof These are preferred for use in food packaging applications where the quality and safety of the materials is a concern.

In a preferred embodiment, if present the one or more monofunctional (meth)acrylate 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 C5-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 C5-C20 group.

In a preferred embodiment, if present 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 preferred for use in food packaging applications.

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

In a preferred embodiment, if present the monofunctional (meth)acrylate monomer is preferably selected from isobornyl acrylate (BOA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-tertbutylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), lauryl acrylate and mixtures thereof. These are preferred for use in food packaging applications. Lauryl acrylate is particularly preferred. Lauryl acrylate is preferred because it has a long straight chain that introduces flexibility into the cured ink film.

In a preferred embodiment, if present the one or more monofunctional (meth)acrylate monomers each have a logP octanol/water value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. Preferably, if present, the one or more monofunctional (meth)acrylate monomers each have a logPodnvater value of less than 9.0. Preferably, if present the monofunctional (meth)acrylate monomer comprises lauryl acrylate. Lauryl acrylate has a logPodandmater value of 5.8. 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, if present the one or more monofunctional (meth)acrylate monomer each have a molecular weight of 195 g/mol or higher. Such monomers are less prone to migration for food packaging applications.

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 (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, more 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, more 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, more 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, if present the monofunctional monomer is one or more monofunctional (meth)acrylate monomers. As such, if present monofunctional (meth)acrylate monomers are the sole monofunctional monomer present in the ink.

If present, the one or more monofunctional monomers may include at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer.

If present, in a preferred embodiment, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer each have a logPodanouwate, value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. If present, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer each have a logP octanolhvater value less than 9.0.

In a preferred embodiment, if present the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer each have a molecular weight of 195 g/mol or higher. Such monomers are less prone to migration for food packaging applications.

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. If present, 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 mannerto the (meth)acrylate monomers. If present, a preferred example is N-acryloylmorpholine (ACMO).

N-Vinyl carbamate monomers are defined by the following functionality: 0 N tie-C The synthesis of N-vinyl carbamate monomers is known in the art. For example, vinyl isocyanate, formed by the Curtius rearrangement of acryloyl azide, can be reacted with an alcohol to form N-vinyl carbamates (Phosgenations -A Handbook by L. Cotarca and H. Eckert, John Wiley & Sons, 2003, 4.3.2.8, pages 212-213).

If present, in a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone.

N-Vinyl oxazolidinones have the following structure: in which R1 to R4 are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically hydrogen, alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, C5-10 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 RI 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: molecular weight 127 g/mol NVMO has the 1UPAC name 5-methyl-3-vinyl-1,3-oxazolidin-2-one and CAS number 3395-98-0.

NVMO includes the racemate and both enantiomers. In one embodiment, the N-vinyl carbamate monomer is a racemate of NVMO. In another embodiment, the N-vinyl carbamate monomer is (R)-5-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, ACM0 and/or NVMO. N-Vinyl amide monomers are particularly preferred, and most preferably NVC.

If present, the one or more monofunctional monomers may include 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.

If present, the one or more N-vinyl monomers other than an N-vinyl amide monomer, N(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer preferably each have a 10gPodanolfwaler value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. Preferably, if present, the one or more N-vinyl monomers other than an N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer preferably each have a logP octanolthater value of 9.0 or less.

In a preferred embodiment, if present the one or more N-vinyl monomers other than an N-vinyl amide monomer, N-(meth)acryloyl amine monomer and/or N-vinyl carbamate monomer each have a molecular weight of 195 g/mol or higher. Such monomers are less prone to migration for food packaging applications.

In a preferred embodiment, the inkjet ink comprises less than 2% by weight, more preferably less than 1% by weight, more preferably is substantially free of monofunctional monomer having a log PocianolAgater value of less than 3.0.

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 monofunctional monomer having a logP octanolfwater value of less than 3.0 is intentionally added to the ink. However, minor amounts of monofunctional monomers having a logPoctanoinvater value of less than 3.0, 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 monofunctional monomers having a logP octanoVwaler value of less than 3.0, more preferably less than 0.1% by weight of monofunctional monomers having a logPocianouwater value of less than 3.0, most preferably less than 0.05% by weight of monofunctional monomers having a logP octanol/water value of less than 3.0, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of monofunctional monomers having a logPocianoimater value of less than 3.0.

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. In a preferred embodiment, the radiation-curable oligomer has a logP octa nol/water value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. Preferably, the radiation-curable oligomer has a logP octanoliwater value of less than 9.0.

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 T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25 s Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The oligomer preferably comprises a polyester, urethane or a polyester-based urethane backbone.

The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers. The oligomer may include amine functionality, as the amine acts as an activator without the drawback of migration associated with low-molecular weight amines. In a preferred embodiment, the radiation-curable oligomer is amine modified. In a particularly preferred embodiment, the radiation-curable oligomer is an amine-modified (meth)acrylate oligomer.

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

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, in the present invention, the ink preferably contains less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, and more preferably is substantially free of bisphenol A epoxy acrylates, 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 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.

In a preferred embodiment, the inkjet ink comprises less than 5% by weight, preferably less than 2% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, more preferably is substantially free of radiation-curable oligomer having a logP oda noVwater value of less than 3.0.

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 radiation-curable oligomer having a logPodanovwater value of less than 3.0 is intentionally added to the ink. However, minor amounts of radiation-curable oligomer having a logPodanawater value of less than 3.0, 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 radiation-curable oligomer having a logP less than 3.0, more preferably less * oda nol/water value of than 0.1% by weight of radiation-curable oligomer having a logP oda noVwater value of less than 3.0, most preferably less than 0.05% by weight of radiation-curable oligomer having a logP octa nol/water value of less than 3.0, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of radiation-curable oligomer having a logP octanol/water value of less than 3.0.

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, if present the passive resin has a logP * oda nol/water value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. Preferably, if present, the passive resin has a logPodanoinvaler value of 9.0 or less.

In a preferred embodiment, the inkjet ink comprises less than 5% by weight, preferably less than 3% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, more preferably is substantially free of passive resin having a logP * odanol/water value of less than 3.0. 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 passive resin having a logP oda noliwater value of less than 3.0 is intentionally added to the ink. However, minor amounts of passive resin having a logP oda nol/water value of less than 3.0, 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 passive resin having a logP octanoltwater value of less than 3.0, more preferably less than 0.1% by weight of passive resin having a logPoctanater value of less than 3.0, most preferably less than 0.05% by weight of passive resin having a logP oda nol/vvater value of less than 3.0, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of passive resin having a logP oda nol/water value of less than 3.0.

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 0.5-15% 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 weighted mean logP oda nolMater value for all components present in the colourant is 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.001 more. In a preferred embodiment, the weighted mean logP ocianol/water value for all components present in the colourant is 9.0 or less.

In a preferred embodiment, the inkjet ink comprises less than 5% by weight, preferably less than 2% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, more preferably is substantially free of colourant where the weighted mean log Pootanouwater value for all components present in the colourant is less than 3.0.

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 colourant where the weighted mean logP odanawater value for all components present in the colourant is less than 3.0 is intentionally added to the ink. However, minor amounts of colourant where the weighted mean logPodanouwater value for all components present in the colourant is less than 3.0, 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 colourant where the weighted mean logPoconouwater value for all components present in the colourant is less than 3.0, more preferably less than 0.1% by weight of colourant where the weighted mean logP octanol/water value for all components present in the colourant is less than 3.0, most preferably less than 0.05% by weight of colourant where the weighted mean logP ocianol/water value for all components present in the colourant is less than 3.0, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of colourant where the weighted mean logPodanawater value for all components present in the colourant is less than 3.0.

In a preferred embodiment the radiation-curable material polymerises by free-radical polymerisation.

If the ink is cured by exposure to a source of actinic radiation without an inert environment, one or more photoinitiators will be required. If the ink is cured by exposure to a source of low-energy electron beam radiation or a source of actinic radiation in an inert environment, the ink may still contain a photoinitiator, although photoinitiators are not required.

In a preferred embodiment, the ink of the present invention further comprises a photoinifiator.

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 Lamberti).

Mixtures of free radical photoinitiators can be used and preferably, the ink 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, most preferably is substantially free of each of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion. In other words, no 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide is intentionally added to the ink. However, minor amounts of each of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 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, 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-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, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of each of 2-S 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.

More preferably, the ink contains less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, 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-hydrwwcyclohexyl 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 in combination, where the amounts are based on the total weight of the ink.

By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion. In other words, no 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2- methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide is intentionally added to the ink. However, minor amounts of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide in combination, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight, most preferably less than 0.05% by weight of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2-isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide in combination, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of 2-hydroxy 2-methyl propiophenone, 2-(dimethylamino)ethyl benzoate, benzophenone, 2-methyl benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2,2-dimethoxy 2-phenyl acetophenone, 2-methyl 4'-(methylthio) 2-morpholino-propiophenone, 2- isopropyl 9H-thioxanthen-9-one (2-ITX), 4-isopropyl 9H-thioxanthen-9-one (4-ITX), 2,4-diethyl 9H-thioxanthen-9-one and diphenyl (2,4,6-trimethyl benzoyl) phosphine oxide.

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

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

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

Speedcure 7010L® is a particularly preferred photoinitiator for inclusion in the ink used in the method of the present invention. Speedcure 7010L® is commercially available from Lambson®. Speedcure 7010L® is a liquid at 20°C and is a solution of 1,3-di({a41-chloro-9-oxo-9H-thioxanthen- 4-yDoxy]acetylpoly[oxy(1-methylethylene)]} oxy)-2,2-bis({a-[1-ch loro-9-oxo-9H-th ioxanthe n-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane in trimethylolpropane ethoxylate triacrylate. 1,3-Di({a-[1-ch lo ro-9-oxo-9H-th ioxa nthe n-4-yl)oxy] acetyl poly[oxy(1-methylethyle n e)]} oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yDoxylacetylpoly[oxy (1-methylethylene)]}oxymethyl) propane is known as polymeric ITX and has the following structure: Cl 0 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.

Preferably, the photoinitiator if present, is present from 1 to 20% by weight, preferably from 5 to 15% by weight, of the ink.

The presence of a photoinitiator is optional as it is not necessary to include a photoinitiator in the inkjet ink in order to achieve a thorough cure of the ink. This is because the ink can cure without the presence of a photoinitiator by curing with a low-energy electron beam or curing by actinic radiation in an inert environment.

Therefore, in a preferred embodiment, the photoinitiator is present in an amount of less than 20% by weight, preferably less than 5% by weight, more preferably less than 3%, more preferably less than 1%, based on the total weight of the ink.

a+b+c+d = 1-20 Cl 0 a CH3 0 0-cy

CH

ici".CH3 0 0 Most preferably, the inkjet ink is substantially free of photoinitiator. By "substantially free" is meant that no photoinitiator is intentionally added to the ink. However, minor amounts of 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 of photoinitiator, more preferably less than 0.1% by weight of photoinitiator, most preferably less than 0.05% by weight of photoinitiator, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of photoinitiator.

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

Further, an inkjet ink that is substantially free of photoinitiator is also advantageous in providing improved water resistance. In this regard, photoinitiators are often polar in nature. For example, Omnirad 2959 has a logP octanol/water value of 0.9. Thus, the inclusion of one or more polar photoinitiators decreases the weighted mean logPodanater value for all components present in the inkjet ink of the present invention, which decreases the water resistance of the inkjet ink.

In a preferred embodiment, the inkjet ink comprises less than 5% by weight, preferably less than 2% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight, more preferably is substantially free of photoinitiators having a logPodenate, value of less than 3.0.

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 photoinitiators having a logPodanouwater value of less than 3.0 is intentionally added to the ink. However, minor amounts of photoinitiators having a logP octanol/water value of less than 3.0, 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 photoinitiators having a logPodanouwaie, value of less than 3.0, more preferably less than 0.1% by weight of photoinitiators having a logP octanol/water value of less than 3.0, most preferably less than 0.05% by weight of photoinitiators having a log Poctanouwater value of less than 3.0, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of photoinitiators having a log Pocianoliwater value of less than 3.0.

In a preferred embodiment, the inkjet ink comprises one or more photoinitiators having a log Pocianoliwater value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. Preferably, the one or more photoinitiators each have a logP octa nal/water value of 9.0 or less. Preferred examples include KIP 160 and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide. Phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide has a logPocianawater value of 7.2 and KIP 160 has a logP 3.0. The inclusion of such photoinitiators is * octanol/water value of advantageous as it is increases the weighted mean logPocianouwater value for all components present in the inkjet ink of the present invention, which increases the water resistance of the inkjet ink.

However, an inkjet ink that is cured with a low-energy electron beam or actinic radiation in an inert environment may still contain less than 5% by weight of a photoinitiator, based on the total weight of the ink. 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.

The inkjet ink preferably dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. Preferably, the inkjet ink comprises less than 5% by weight of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2% by weight combined and 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. Such other components where appropriate, preferably each have a logP octa nol/waler value of 3.0 or more, preferably 4.0 or more, more preferably 4.5 or more, most preferably 5.0 or more. Such other components where appropriate, preferably each have a logP oda nol/water value of less than 9.0 The amounts by weight provided herein are based on the total weight of the ink.

The ink or inkjet ink sets may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less, more preferably 30 mPas or less and most preferably 20 mPas or less at 25°C. The ink most preferably has a viscosity of 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 also provides an inkjet ink set, wherein the inkjet ink set of the invention has at least one ink that falls within the scope of the inkjet ink according to the present invention. Preferably, all of 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 trichromafic set can be used to produce a wide range of colours and tones.

The present invention also provides a method of inkjet printing using the above-described ink or ink set and a substrate having the ink or ink set cured thereon.

Accordingly, the present invention further provides a method of inkjet printing comprising inkjet printing the inkjet ink or inkjet ink set as defined herein onto a substrate and curing the ink.

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. Inkjet printing is well known in the art and a detailed description is not required.

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

Print heads account for a significant portion of the cost of an entry level printer and it is therefore desirable to keep the number of print heads (and therefore the number of inks in the ink set) low.

Reducing the number of print heads can reduce print quality and productivity. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity.

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

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

Preferably, the monomers of the ink used in the method of the present invention are suitable for food packaging applications.

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

In a preferred embodiment, the substrate is a laminate carton material comprising the following layers, in order: an inner polyethylene layer; an aluminium layer; a board layer, and an outer polyethylene layer. By inner is meant a surface of the substrate that would come into contact with food and by outer is meant a surface of the substrate that would come into contact with the inkjet ink used in the method of the present invention. More preferably, the outer polyethylene layer is corona treated to a surface tension of more than 45 dynes/cm using a Vetaphone unit. This provides improved adhesion of the ink.

In order to produce a high quality printed image a small jetted drop size is desirable. Preferably the inkjet ink is jetted at drop sizes below 90 picolitres, preferably below 35 picolitres and most preferably below 10 picolitres.

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

The inks or inkjet ink set are exposed to actinic (often UV) radiation or a source of low-energy electron beam radiation to cure the ink.

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.

Any suitable radiation source may be used. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof In a preferred embodiment, a UV LED light source is used to cure the ink.

UV LED 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.

Any of the sources of actinic radiation discussed herein may be used for the irradiation of the inkjet ink. A suitable dose would be greater than 200 mJ/cm2, more preferably at least 300 mJ/cm2 and most preferably at least 500 mJ/cm2. The upper limit is less relevant and will be limited only by the commercial factor that more powerful radiation sources increase cost. A typical upper limit would be 5 J/cm2. Further details of the printing and curing process are provided in WO 2012/110815.

The exposure to actinic radiation may be performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink. This has the advantage that the amount of photoinitiators present in the ink can be significantly reduced, and particularly the amount of photoinitiators required for surface cure of the ink.

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 ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 70 kGy or less.

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

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

There is no restriction on the ebeam dose that is used to cure the inkjet inks of the present invention other than that the dose is sufficient to fully cure the ink. Preferably, the dose is more than 10 kGy, more preferably more than 20 kGy and most preferably more than 25 kGy. Preferably, the dose is less than 100 kGy, more preferably less than 90 kGy, more preferably less than 80 kGy and most preferably less than 70 kGy. Preferably, the dose is more than 25 kGy but less than 70 kGy, more preferably more than 25 kGy but less than 60 kGy and most preferably, more than 25 kGy but 50 kGy or less. Doses above 50 kGy may cause damage to the substrate, 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 key, more preferably 70-200 key and most preferably 100 keV.

Advantageously, using ebeam curing significantly reduces the amount of unreacted and migratable monomers as claimed relative to conventional UV curing (even when using a nitrogen blanket to prevent oxygen inhibition of cure). This allows the formulator greater formulation flexibility to include components in the ink which would usually be avoided because of issues with migration.

Accordingly, using ebeam curing is preferred to reduce migration and to allow for the inclusion of components which improve properties of the inkjet ink and printed film, including flexibility. Ebeam also has the advantage that photoinitiators are optional.

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

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

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

The present invention may also provide a printed substrate obtainable by the method of the present invention. Preferably, the substrate is a food packaging. It has surprisingly been found that the printed substrate of the present invention having the inkjet ink of the present invention printed and cured thereon, has reduced migration and improved flexibility and hence reduced cracking of the printed ink, and improved water resistance. This is particularly beneficial for food packaging.

The present invention may also provide a cartridge containing the inkjet ink or inkjet ink set as defined herein.

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

Examples

Example 1

Inkjet inks were prepared according to the formulations set out in Table 1. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1.

Component Ink 1 Ink 2 Ink 3 Ink 4 (invention) (comparative) (comparative) (comparative) 3-MPDDA (difunctional monomer) 20.00 81.90 51.50 DVE-3 (difunctional monomer) 20.00 20.00 1,10-DDDA (difunctional monomer) 61.90 61.90 UVP6600 (oligomer) 7.00 UV22 (stabiliser) 0.50 Cyan pigment dispersion A 8.00 Cyan pigment dispersion B 8.00 8.00 8.00 Omnirad 819 (photoinitiator) 3.00 3.00 3.00 2.00 Omnirad 2959 (photoinitiator) 2.00 2.00 2.00 3.00 Esacure KIP 160 (photoinitiator) 5.00 5.00 5.00 7.00 Byk 307 (surfactant) 0.10 0.10 0.10 1.00 Total 100.00 100.00 100.00 100.00 Viscosity/mPa.s 12.2 14.9 9.7 13.3 UV dose required to cure/mJcm-2 1257 838 838 325 Flexibility No cracks Cracks Cracks No cracks Water resistance test Pass Pass Fail Fail DVE-3 and DDDA are monomers, as defined herein. DVE-3 has a logPocianouwaier value of 1.0. 1,10-DDDA has logPodenovwater value of 4.9. 3-MPDDA is 3-methyl-1,5-pentanediol diacrylate and has a log Podanolfwater value of 3.0. UVP6600 is a polyester acrylate oligomer.

Cyan pigment dispersion A contains 20% hyperdispersant, 50% DVE-3 and 30% cyan 15:4 pigment (total 100%). Cyan pigment B contains 10% hyperdispersant, 1% UV12 stabiliser, 59% NPGPODA and 30% cyan 15:4 pigment (total 100%). NPGPODA has a logP * oda nol/water value of 2.7. The dispersions were each 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.

The viscosity of the ink was measured at 25°C using a digital Brookfield DV1 viscometer with a thermostatically controlled cup and spindle arrangement. All of the inkjet inks have a viscosity of less than 20 mPa.s and so have an ink-jettable viscosity.

The inks of Table 1 were drawn down in a 12 pm film using a 12 pm wire wound K-bar onto a substrate that has been corona-treated to >45 dynes/cm. The inks were then cured using a medium pressure Hg Heraeus Noblelight UV lamp of power rating 180 W/cm providing a UV dose of 1128 mJ/cm. All inks had good cure response.

The cured ink films were then assessed for flexibility and water resistance.

To assess for flexibility, the cured ink films were stored for 25 hours after curing. They were subsequently folded through 90 degrees. The fold was then examined for cracks with a magnifying glass. On folding, ink 1 and comparative ink 4 have no cracking. In contrast, comparative inks 2 and 3 have cracking along the fold. A reduction in cracking shows improved flexibility of ink 1 and comparative ink 4 when compared to comparative inks 2 and 3.

To assess water resistance, after curing, the cured ink films were submerged in a tray of boiling water for two hours. Water resistance of the cured ink film can then be assessed by a water rub test.

The cloth was then examined for any cyan colour transfer. An inkjet ink is water-resistant if no noticeable colour transfer is transferred from the cured ink film to the cloth in five double rubs of the water rub test. Noticeable colour transfer was judged as a fail. Ink 1 and comparative ink 2 passed the test of water resistance with no noticeable colour transfer from the ink film to the cloth. In contrast, comparative inks 3 and 4 failed the test of water resistance with noticeable colour transfer from the ink film to the cloth.

As can be seen from Table 1, ink 1, which has 5-40% by weight of a one or more multifunctional monomers each having an alkoxylated backbone and 20-70% by weight of one or more multifunctional monomers each having a logPoctanovwater value of 4.0 or more, and has a total amount of all multifunctional monomers in the inkjet ink of at least 50% by weight, where the amounts are based on the total weight of the ink, has both excellent flexibility and water resistance. Comparative ink 2, which has 20-70% by weight of one or more multifunctional monomers each having a logPodanouwate, value of 4.0 or more and has a total amount of all multifunctional monomers in the inkjet ink of at least 50% by weight, but does not include one or more multifunctional monomers each having an alkoxylated backbone, has excellent water resistance but has poor flexibility.

Comparative ink 3, which includes neither one or more multifunctional monomers each having an alkoxylated backbone nor one or more multifunctional monomers each having a logPocianouwater value of 4.0 or more, but has a total amount of all multifunctional monomers in the inkjet ink of at least 50% by weight, based on the total weight of the ink, has both poor water resistance and flexibility. Comparative ink 4, which includes 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone and has a total amount of all multifunctional monomers in the inkjet ink of at least 50% by weight, based on the total weight of the ink but does not include one or more multifunctional monomers each having a logPodanobwaie, value of 4.0 or more, has poor water resistance but excellent flexibility.

The inventors have found therefore, that an ink of the invention, which has 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone; 20-70% by weight of one or more multifunctional monomers each having a logP * octanol/water value of 4.0 or more; and wherein the total amount of all multifunctional monomers in the inkjet ink is at least 50% by weight; where the amounts are based on the total weight of the ink, has both excellent flexibility and water resistance.

As can be seen, the addition of high polarity DVE-3 to an ink having 1,10-DDDA, increases flexibility, and increases the polarity of the ink (decreases the weighted mean logP odanoVvvater of the ink) but still maintains improved water resistance. The addition of low polarity 1,10-DDDA to an ink having DVE-3, decreases the polarity of the ink (increases the overall logP octanol/water of the ink) but still maintains flexibility.

All of the inks of the examples comprise at least 50% by weight of multifunctional monomer, based on the total weight of the ink and are suitable for food packaging, showing low migration.

Claims

Claims 1. An inkjet ink comprising: 5-40% by weight of one or more multifunctional monomers each having an alkoxylated backbone; 20-70% by weight of one or more multifunctional monomers each having a logPoctenomate, value of 4.0 or more; less than 5% by weight of monofunctional monomer; wherein the total amount of all multifunctional monomers in the inkjet ink is at least 50% by weight, where the amounts are based on the total weight of the ink.
2. An inkjet ink as claimed in claim 1, wherein the inkjet ink comprises 1-10% by weight of one or more additional multifunctional monomers other than the one or more multifunctional monomers each having an alkoxylated backbone and the one or more multifunctional monomers each having a logP oclanolfwaler value of 4.0 or more, based on the total weight of the ink.
3. An inkjet ink as claimed in claims 1 or 2, wherein the inkjet ink comprises 10-30% by weight of the one or more multifunctional monomers each having an alkoxylated backbone, based on the total weight of the ink.
4. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises 30-70% by weight of the one or more multifunctional monomers each having a logP octanol/water value of 4.0 or more, based on the total weight of the ink.
5. An inkjet ink as claimed in any preceding claim, wherein the one or more multifunctional monomers each having an alkoxylated backbone comprises a multifunctional monomer having an ethoxylated backbone.
6. An inkjet ink as claimed in any preceding claim, wherein the total amount of all multifunctional monomers in the inkjet ink is at least 60% by weight, preferably at least 70% by weight, more preferably at least 80% by weight, based on the total weight of the ink.
7. An inkjet ink as claimed in any preceding claim, wherein the one or more multifunctional monomers each having an alkoxylated backbone comprises a difunctional monomer, preferably triethylene glycol divinyl ether (DVE-3).
8. An inkjet ink as claimed in any preceding claim, where the one or more multifunctional monomers each having a logP * octanolhvater value of 4.0 or more comprises a difuncfional monomer, preferably 1,10-decanediol diacrylate (1,10-DDDA).
9. An inkjet ink as claimed in any preceding claim, wherein the weighted mean logP octanol/water value of the components present in the ink is 3.2 or more, preferably 3.5 or more.
10. An inkjet ink as claimed in any preceding claim, wherein the weighted mean logPodanoimaie, value of the monomers present in the ink is 3.2 or more, preferably 3.5 or more.
11. An inkjet ink as claimed in any preceding claim, wherein the weighted mean logP oclanolAvater value of the multifunctional monomers present in the ink is 3.2 or more, preferably 3.5 or more.
12. An inkjet ink as claimed in any preceding claim, further comprising a colourant, preferably a dispersed pigment.
13. An inkjet ink as claimed in any preceding claim, wherein the ink is substantially free of monofunctional monomer and/or photoinitiators.
14. A method of inkjet printing comprising inkjet printing the inkjet ink as claimed in any preceding claim onto a substrate and curing the ink.