WO2015140538A1

WO2015140538A1 - Printing ink

Info

Publication number: WO2015140538A1
Application number: PCT/GB2015/050779
Authority: WO
Inventors: Matthew KITE; Andrew Phillips; Karen SANDWELL
Original assignee: Fujifilm Speciality Ink Systems Limited
Priority date: 2014-03-17
Filing date: 2015-03-17
Publication date: 2015-09-24
Also published as: GB2558530B; GB2558530A; GB201617327D0

Abstract

The present invention provides a white inkjet ink comprising: a monofunctional (meth)acrylate monomer; a difunctional and/or multifunctional (meth)acrylate monomer; an N-vinyl amide and/or N-acryloyl amine monomer; a radical photoinitiator; a dispersed white pigment; and 1 -5% by weight, based on the total weight of the ink, of a surfactant having the following structure: (I) wherein the value of m is 1-5 and the value of n is such that the ratio of acrylate groups to methyl groups is from 1:20 to 1:50.

Description

Printing ink

The present invention relates to a printing ink, and in particular to a white inkjet ink that provides optimal properties for overprinting coloured inks.

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. The resulting image should be as high quality as possible.

In order to provide a high quality final image, a high degree of spread when the ink is jetted onto the substrate is required. The inks must also flow rapidly from the printing heads. In order to achieve this, it is necessary to formulate an ink either having a low viscosity (typically below 100 mPas at 25°C although in most applications the viscosity should be below 50 mPas, and often below 25 mPas) by using low surface tension components or alternatively, formulate an ink that has a surfactant to reduce ink surface tension. If the surface tension of the ink is not reduced, this can lead to ink pooling and/or a mottled appearance in high coverage areas of the print.

It is not always possible to use only low surface tension components in an ink, especially as higher surface tension components are often required to provide necessary advantageous properties of an ink. Therefore, surfactants are often used to reduce the surface tension of an ink. Reduction of the surface tension of inks allows control of the surface wetting of inks on various substrates, for example, plastic substrates. Inks that show good wetting on a substrate provide good quality images and low reticulation. However, the inclusion of excess surfactant can also cause problems. In this regard, if the ink has a too much surfactant present in the ink, this means that the surface tension will be too low, which can lead to excessive ink spread, high reticulation and excessive ink bleed between different coloured inks. This leads to imperfection in the printed image. It is important therefore to get the correct balance of surface tension. Particular issues arise when a white ink has to be printed onto coloured inks. An example is in the production of shrink wrap labels. Colour inks are printed onto a clear film, such as polypropylene or polyethylene (the image will be viewed through the clear film from underneath). The coloured inks are successively pinned and then the white ink is applied over the coloured inks and the inks are all fully cured. A suitable printing apparatus for this process is the Graphium printing press from FFEI. A label is formed from the film, which is then heated and shrunk over the article to be labelled, such as a bottle. The image is viewed through the film and the white ink acts as a backing white. It is particularly useful where a dense white is required, such as for barcodes on labels. It has been found that surfactant-containing white inks suffer from excessive ink spread, but removing the surfactant causes reticulation issues. There is therefore a need in the art for a white inkjet ink that has the correct balance of surface tension, which is necessary to provide good surface wetting and does not have excessive ink bleed/spread, in order to produce a good quality image.

Accordingly, the present invention provides a white inkjet ink comprising: a monofunctional (meth)acrylate monomer; a difunctional and/or multifunctional (meth)acrylate monomer; an N- vinyl amide and/or N-acryloyl amine monomer; a radical photoinitiator; a dispersed white pigment; and 1 -5% by weight, based on the total weight of the ink, of a surfactant having the following structure:

wherein the value of m is 1 -5 and the value of n is such that the ratio of acrylate groups to methyl groups is from 1 :20 to 1 :50.

The inventors have surprisingly found that an inkjet ink that comprises the specific blend of components and in particular, the specific surfactant as defined herein can successfully provide the correct balance of surface tension, despite the surfactant being present in the ink at such a high amount. The ink of the invention has the necessary surface wetting but does not have excessive ink bleed/spread, despite the high amount of surfactant, and hence provides a high quality image on the substrate. The inkjet ink of the present invention comprises an N-vinyl amide and/or N-(meth)acryloyl amine monomer.

N-Vinyl amides are well-known monomers in the art and a detailed description is therefore not required. N-vinyl amides have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP). Similarly, N-acryloyl amines are also well-known in the art. N-acryloyl amines also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO). Preferably, the inkjet ink comprises 10-30% by weight of an N-vinyl amide and/or N- (meth)acryloyl amine monomer, based on the total weight of the ink.

The inkjet ink of the present invention comprises a monofunctional (meth)acrylate monomer. The monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer. Preferably both a cyclic monofunctional (meth)acrylate monomer and an acyclic-hydrocarbon monofunctional (meth)acrylate monomer are present. Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required.

Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. They typically have a viscosity of less than 2 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 60°C with a shear rate of 25 s^" .

The substituents of the monofunctional (meth)acrylate monomer are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc.

The substituents 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 C_3-18 cycloalkyl, C₆. ₀ aryl and combinations thereof, any of which may substituted with alkyl (such as CMS 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.

Preferably, the cyclic monofunctional (meth)acrylate monomer is selected from phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA) and mixtures thereof. PEA is particularly preferred. The preferred examples of cyclic monofunctional (meth)acrylate monomers have the following chemical structures:

Phenoxyethyl acrylate (PEA), mol wt 192 g/mol

Cyclic TMP formal acrylate (CTFA), mol wt 200 g/mol

Isobornyl acrylate (IBOA) Tetrahydrofurfuryl acrylate (THFA)

mol wt 208g/mol mol wt 156 g/mol

Mixtures of (meth)acrylates may be used.

Preferably, the ink comprises 25-50% by weight, preferably 25-40% by weight of a cyclic monofunctional (meth)acrylate monomer, based on the total weight of the ink.

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 CMS alkyl, which may be interrupted by 1 -10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

Preferably, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C₆-C₂₀ group. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer is selected from octa/decyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

The preferred examples of acyclic-hydrocarbon monofunctional (meth)acrylate monomers have the following chemical structures:

Octadecyl acrylate (ODA) Tridecyl acrylate (TDA)

mol wt 200 g/mol mol 254 g/mol

Isodecyl acrylate (IDA) Lauryl acrylate

mol wt 212 g/mol mol wt 240 g/mol

2-(2-Ethoxyethoxy)ethyl acrylate, mol wt 188 g/mol Mixtures of (meth)acrylates may be used.

Preferably, the ink comprises 1 -20% by weight, preferably 1 -10% by weight of an acyclic- hydrocarbon monofunctional (meth)acrylate monomer, based on the total weight of the ink.

The ink of the present invention further comprises a multifunctional (meth)acrylate monomer, and/or a difunctional (meth)acrylate monomers. It preferably contains a difunctional (meth)acrylate monomers.

Multifunctional (which do not include difunctional) are well known in the art and a detailed description is therefore not required. Multifunctional has its standard meaning, i.e. tri or higher, that is three or more groups, respectively, which take part in the polymerisation reaction on curing. Preferably, the multifunctional (meth)acrylate monomer has a degree of functionality of four or more, more preferably a degree of functionality of from 4-8.

The substituents of the multifunctional monomers are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyi, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CMS alkyl, C_3-18 cycloalkyi, C₆. ₀ aryl and combinations thereof, such as C₆. ₀ aryl- or C_3-18 cycloalkyl- substituted CMS alkyl, any of which may be interrupted by 1 -10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure. (The same groups may also be used for difunctional monomers.)

Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet inks include trimethylolpropane triacrylate, pentaerythritol triacrylate, tri(propylene glycol) triacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate, and mixtures thereof. Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as trimethylolpropane trimethacrylate. Mixtures of (meth)acrylates may also be used. When present, the multifunctional (meth)acrylate monomer is preferably present in an amount of 1 -10% by weight, preferably 2-8% by weight, based on the total weight of the ink.

Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. The substituents of the difunctional monomers 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 as set out for the multifunctional (meth)acrylate monomers. Preferred examples include hexanediol diacrylate (HDDA), polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, neopentylglycol diacrylate, 3-methyl pentanediol diacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, and mixtures thereof.

In addition, suitable difunctional methacrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate and mixtures thereof.

Preferably, the ink comprises 1 -15%, more preferably 2-10% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink. Preferably, the difunctional (meth)acrylate monomer is selected from hexanediol diacrylate, propoxylated neopentyl glycol diacrylate, dipropylene glycol diacrylate, and mixtures thereof.

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

The ink of the present invention comprises a radical photoinitiator. The free-radical photoinitiator can be selected from any of those known in the art. For example, benzophenone, 1 - hydroxycyclohexyl phenyl ketone, 1 -[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1 -propane- 1 -one, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1 -one, isopropyl thioxanthone, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure and Darocur (from Ciba) and Lucerin (from BASF). Preferred photoinitiators are selected from bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide, 1 -hydroxycyclohexyl phenyl ketone and mixtures thereof.

Preferably, the photoinitiator is present in an amount of 1 -20% by weight, preferably 2-15% by weight, based on the total weight of the ink.

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.

The inkjet ink of the present invention comprises a surfactant having the following structure:

The surfactant is available commercially as Tego Rad 2300. By Tego Rad 2300 herein, we mean surfactant having the following structure:

Tego Rad 2300 has a value for m of 1 -5, preferably 1 -3 and more preferably about 1 . Tego Rad 2300 has a value for n such that the ratio of acrylate groups to methyl groups is from 1 :20 to 1 :50, preferably 1 :30 to 1 :40. In a preferred embodiment, the value of n is such that the ratio of acrylate groups to methyl groups is 1 :35. Preferably, Tego Rad 2300 has a value for n of 13-15 and more preferably about 14.

By acrylate groups, it is meant a group having the following structure which is directly bonded to the siloxane chain:

Siloxane Chairu

By methyl groups, it is meant a methyl group which is directly bonded to the siloxane chain, and for the avoidance of doubt includes the end groups.

Tego Rad 2300, like most polymeric material, is a blend of polymers that falls within the general definition of the surfactant as claimed. Tego Rad 2300 has a modal molecular weight by weight of from 3,500 to 4,500 as determined by GPC using polystyrene standards. More specifically, as determined using the following conditions:

An amount of 0.1 g of Tego Rad 2300 is dissolved in 10 mL of toluene repeated in triplicate. The injection size is 70 μΙ_. The mobile phase is 100% toluene (HPLC grade). The same solvent in mobile phase is used to make sample dilution. The detector used is a refractive index refractometer using the difference in refractive index of the pure solvent and the solvent with the Tego Rad 2300 dissolved in it. The calibration standards were polystyrene narrow calibration standards. The column used was a Minimix D column which is used to separate molecular masses by size.

The inkjet ink of the present invention has 1 -5% by weight, preferably 1 .5-5% by weight, more preferably 1 .5-3% by weight of the surfactant having the given structure (i.e. Tego Rad 2300), based on the total weight of the ink. In a preferred embodiment the surfactant having the given structure (i.e. Tego Rad 2300) is the sole surfactant present in the ink.

The inventors have surprisingly found that Tego Rad 2300 can be included in the inkjet ink of the present invention at a high concentration, which provides an inkjet ink having good surface wetting on various substrates, which provides good quality images and low reticulation. It can do so however, without having excessive ink spread, high reticulation and excessive ink bleed between different coloured inks, which is usually expected when including surfactants at high concentration. A good balance of ink surface tension is hence achieved despite including a high concentration of Tego Rad 2300. The high concentration of Tego Rad 2300 surprisingly restricts the ink flow, controls the spread of ink and provides a high quality image on the substrate.

The surface tension of the ink is preferably in the range of 20-32 mNm^" and more preferably 21 - 27 mNm^"1.

The ink of the present invention also comprises a dispersed white pigment. The pigment is dispersed in the liquid medium of the ink. The pigment is preferably titanium dioxide, such as Kronos® 2300 available from Kronos Ltd.

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 μηι, preferably less than 5 μηι, more preferably less than 1 μηι and particularly preferably less than 0.5 μηι. The white pigment is preferably present in an amount of 5-30% by weight, more preferably 10- 20% by weight, based on the total weight of the ink.

The ink of the present invention may further comprise a radiation-curable (i.e. polymerisable) oligomer, such as a (meth)acrylate oligomer.

The term "curable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation . The oligomer preferably has a molecular weight of at least 450 and preferably 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 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.

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 1 2° steel cone at 60°C with a shear rate of 25 s ^~1.

Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The oligomer preferably comprises a polyester backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers. Particularly preferred radiation-curable oligomers are polyester acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are di-, tri-, tetra-, penta- or hexa- functional polyester acrylates, as these yield films with good solvent resistance.

More preferably, the radiation-curable oligomer is an amine-modified polyester acrylate oligomer. Such a radiation-curable oligomer is commercially available as Ebercryl 80.

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.

In one embodiment the radiation-curable oligomer polymerises by free-radical polymerisation. Preferably, the radiation-curable oligomer cures upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film.

The total amount of the oligomer is preferably from 1 -15 wt%, based on the total weight of the ink. Preferably the oligomer is present from 2-10 wt%, based on the total weight of the ink. The ink of the present invention may further comprise an α,β-unsaturated ether monomer, which can polymerise by free-radical polymerisation and may be useful for reducing the viscosity of the ink when used in combination with one or more (meth)acrylate monomers. Examples are well known in the art and include vinyl ethers such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1 ,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether. Mixtures of α,β-unsaturated ether monomers may be used.

The ink of the invention may also include radiation-curable material, which is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The radiation-curable material according to this embodiment may comprise a mixture of cationically curable monomer and oligomer. For example, the radiation-curable material may comprise a mixture of an epoxide oligomer and an oxetane monomer.

In the embodiment where the ink of the present invention comprises radiation-curable material, which polymerises by cationic polymerisation, the ink must also comprise a cationic photoinitiator.

In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulfonium or iodonium based systems. Non limiting examples include: Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa. Preferably however, the ink of the invention cures by free-radical polymerisation only and hence the ink is substantially free of radiation-curable material, which polymerises by cationic polymerisation.

The inkjet ink of the present invention dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. The absence of water and volatile organic solvents means that the ink does not need to be dried to remove the water/solvent. However, water and volatile organic solvents have a significant viscosity-lowering effect making formulation of the ink in the absence of such components significantly more challenging.

Accordingly, the inkjet ink of the present invention is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink of the present invention comprises less than 5 wt% of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2 % by weight combined and most preferably less than 1 % by weight combined, based on the total weight of the ink. Some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated.

The inks may comprise a passive (or "inert") thermoplastic resin. Passive resins are resins which do not enter into the curing process, i.e. the resin is free of functional groups which polymerise under the curing conditions to which the ink is exposed. In other words, resin is not a radiation- curable material. The resin may be selected from epoxy, polyester, vinyl, ketone, nitrocellulose, phenoxy or acrylate resins, or a mixture thereof and is preferably a poly(methyl (meth)acrylate) resin. The resin has a weight-average molecular weight of 70-200 KDa and preferably 100-150 KDa, as determined by GPC with polystyrene standards. A particularly preferred resin is Paraloid® A1 1 from Rohm and Haas. The resin is preferably present at 1 -5% by weight, based on the total weight of the ink.

The amounts by weight provided herein are based on the total weight of the ink.

The inkjet ink exhibits a desirable low viscosity (200 mPas or less, preferably 100 mPas or less, more preferably 25 mPas or less, more preferably 10 mPas or less and most preferably 7 mPas or less at 25 °C).

In order to produce a high quality printed image a small jetted drop size is desirable. Furthermore, small droplets have a higher surface area to volume ratio when compared to larger drop sizes, which facilitates evaporation of solvent from the jetted ink. Small drop sizes therefore offer advantages in drying speed. Preferably the inkjet ink of the invention is jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 10 picolitres.

To achieve compatibility with print heads that are capable of jetting drop sizes of 50 picolitres or less, a low viscosity ink is required. A viscosity of 15 mPas or less at 25°C is preferred, for example, 2 to 12 mPas, 8 to 1 1 mPas, or 10 to 1 1 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.

Other components of types known in the art may be present in the ink to improve the properties or performance. These components may be, for example, additional surfactants, defoamers, dispersants, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

Print heads account for a significant portion of the cost of an entry level printer and it is therefore desirable to keep the number of print heads (and therefore the number of inks in the ink set) low. Reducing the number of print heads can reduce print quality and productivity. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity.

The white inkjet ink of the present invention is typically used in conjunction with a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. The inks in a trichromatic set can be used to produce a wide range of colours and tones. They are formulated using standard techniques known in the art.

In a preferred embodiment of the present invention, the coloured inks are formulated in the same manner as the white ink of the present invention, except that colouring agents are used. The colouring agent may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersible pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF pic), 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.

The colorant is preferably present in an amount of 20% by weight or less, preferably 2-10% by weight, based on the total weight of the ink. This is a lower concentration than that required for the white 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 present invention also provides a method of inkjet printing using the above-described white ink or ink set and a substrate having the white ink cured thereon. Accordingly, the present invention further provides a method of inkjet printing comprising inkjet printing the white inkjet ink 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. A specific example of this is for label decoration and production, more particularly where labels are printed on webs of transparent or coloured substrates. The label images can be printed onto rolls of substrate and then the individual labels cut out from the web or roll of printed substrate material at the end of the process. The inks are exposed to actinic (often UV) radiation to cure the ink. 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.

In a preferred embodiment, the present invention provides a method of inkjet printing comprising inkjet printing one or more coloured inks onto the substrate, pinning the one or more coloured inks after the or each colour is applied, inkjet printing the white ink onto the one or more pinned coloured inks, and curing the inks.

The white ink is particularly useful when printing onto transparent ("clear") substrates. By "transparent" is meant that the image can be seen through the substrate when viewed with the naked eye.

In another preferred embodiment, the inks are printed onto a flexible substrate. In this embodiment, it is important that the inks possess the required flexibility, so that, for example, the inks do not become brittle and unsuitable for applications where there is flexure.

The present invention also provides a cartridge containing the inkjet ink as defined herein. It also provides a printed substrate having the ink as defined herein printed thereon. It further provides a printed substrate having one or more coloured inks printed thereon, and the white ink as defined herein printed over the one or more coloured inks. Examples of substrates include those composed of polyethylene and polypropylene.

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 2012/1 10815. Upon exposure to a radiation source, 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 μηι, preferably 1 to 10 μηι, for example 2 to 5 μηι. Film thicknesses can be measured using a confocal laser scanning microscope. The invention will now be described with reference to the following examples, which are not intended to be limiting.

Examples Example 1

A white inkjet ink was prepared according to the formulation set out in Table 1 below. The white inkjet ink formulation was 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 .

The white dispersion of the ink of Table 1 was prepared according to the formulation set out in Table 2. The white dispersion was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink. Table 2.

Example 2

Coloured inkjet inks were prepared according to the formulations set out in Table 3. 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. The inks are not examples of inks of the present invention, but may be used in conjunction with the white inks of the present invention. Table 3.

Ebecryl 80 is an amine-modified polyester acrylate oligomer which is commercially available from Cytec. BT40279 is a stabilizer for the yellow dispersion. Irgacure 184 is 1 -hydroxycyclohexyl phenyl ketone. TPO is bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide.

The cyan, magenta, yellow and black dispersions of the inks of Table 3 were prepared according to the formulations set out in Table 4. The colour dispersions 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 2.

Example 3 The four coloured inks were drawn down onto a polypropylene film using a K bar applicator and LED pinned after each colour. The white ink is then applied over the other inks. All inks are then cured using a medium-pressure mercury lamp.

The print images of the inks are observed and the image quality is assessed. The results show no reticulation of the printed film. A label is formed from the film, heated and shrunk over a bottle. The image can be viewed through the film.

Claims

1 . A white inkjet ink comprising: a monofunctional (meth)acrylate monomer; a difunctional and/or multifunctional (meth)acrylate monomer; an N-vinyl amide and/or N-acryloyl amine monomer; a radical photoinitiator; a dispersed white pigment; and 1 -5% by weight, based on the total weight of the ink, of a surfactant having the following structure:

2. An inkjet ink as claimed in claim 1 , wherein the ink contains 1 .5-3% by weight, based on the total weight of the ink, of the surfactant having the given structure.

3. An inkjet ink as claimed in claim 1 or 2, wherein the surfactant having the given structure is the sole surfactant present in the ink.

4. An inkjet ink as claimed in any preceding claim, wherein the monofunctional (meth)acrylate monomer comprises a cyclic monofunctional (meth)acrylate monomer and an acyclic- hydrocarbon monofunctional (meth)acrylate monomer.

5. An inkjet ink as claimed in any preceding claim, wherein the ink comprises a difunctional (meth)acrylate monomer.

6. An inkjet ink as claimed in any preceding claim, wherein the ink comprises a radiation- curable oligomer.

7. An inkjet ink as claimed in any preceding claim, wherein the ink is substantially free of water and volatile organic solvents.

8. A cartridge containing the inkjet ink as claimed in any preceding claim.

9. A printed substrate having one or more coloured inks printed thereon, and the white ink as claimed in any of claims 1 -7 printed over the one or more coloured inks.

10. A method of inkjet printing comprising inkjet printing the white ink as claimed in any of claims 1 -7 onto a substrate and curing the ink.

1 1 . A method of inkjet printing according to claim 9, comprising inkjet printing one or more coloured inks onto the substrate, pinning the one or more coloured inks after the or each colour is applied, inkjet printing the white ink onto the one or more pinned coloured inks, and curing the inks.

12. The printed substrate as claimed in claim 8, or the method as claimed in claims 9 or 10, wherein the substrate is a transparent substrate.

13. The printed substrate or the method as claimed in claim 1 1 , wherein the substrate is a polypropylene film or a polyethylene film.