GB2538619A

GB2538619A - Printing ink

Info

Publication number: GB2538619A
Application number: GB1606585.6A
Authority: GB
Inventors: Paszke Gary; Sandwell Karen; Phillips Andrew
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2015-04-16
Filing date: 2016-04-15
Publication date: 2016-11-23
Anticipated expiration: 2036-04-15
Also published as: GB2538619B

Abstract

An inkjet ink comprising a (meth)acrylate monomer, a radical photoinitiator, a black pigment, a blue pigment, a polyethyleneimine-polyester-fatty acid copolymer dispersant and a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains. Preferably, the polyethyleneimine-polyester-fatty acid copolymer dispersant comprises the monomers 12-hydroxyoctadecanoic acid, 2,2-iminobis[ethanamine], 2-oxepanone and tetrahydro-2H-pyran-2-one. Preferably, the comb-structured dispersant comprises the monomers ethyleneimine and 2-oxepanone. The (meth)acrylate monomer may include a cyclic monofunctional (meth)acrylate monomer, an acyclic-hydrocarbon monofunctional (meth)acrylate monomer, a difunctional (meth)acrylate monomer and a multifunctional (meth)acrylate monomer. The ink may further comprise a silicone polyether acrylate surfactant. Preferably, the inkjet ink is substantially free of water and volatile organic solvents. Also disclosed are a cartridge comprising said inkjet ink and a printed substrate having said inkjet ink printed thereon. A method of preparing said inkjet ink is also disclosed, wherein the (meth)acrylate monomer and radical photoinitiator are mixed with a black pigment dispersion and a cyan pigment dispersion, wherein the black pigment dispersion comprises the black pigment and the polyethyleneimine-polyester-fatty acid copolymer dispersant and wherein the cyan pigment dispersion comprises the blue pigment and the comb-structured dispersant.

Description

Intellectual Property Office Application No. GII1606585.6 RTM Date:15 September 2016 The following terms are registered trade marks and should be read as such wherever they occur in this document: Irgacure Darocur Lucirin Mogul Heliogen Efka Solsperse Ebecryl Rhodorsil Tego Intellectual Property Office is an operating name of the Patent Office www.gov.uk /ipo Printing ink The present invention relates to a printing ink and in particular to a black inkjet ink that has a high colour density and a vivid black colour.

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 onto 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 image, a high quality ink is required that has a high colour density and a vivid colour when the ink is jetted onto the substrate. Black inkjet inks are widely used in inkjet printing. However, current black inkjet inks often appear weak when printed and have a brown undertone. This is particularly apparent when printing a large area of black ink.

There is therefore a need in the art for a black inkjet ink that has a high colour density and a vivid black colour.

Accordingly, the present invention provides an inkjet ink comprising a (meth)acrylate monomer, a radical photoinitiator, a black pigment, a blue pigment, a polyethyleneimine-polyester-fatty acid copolymer dispersant and a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains.

The inventors have surprisingly found that an inkjet ink that comprises the present blend of components and in particular, the two specific dispersants as defined herein can successfully facilitate the combination of a black and blue pigment to form a black ink with a high colour density and a vivid black colour.

The inkjet ink of the present invention comprises a (meth)acrylate monomer.

The monomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality monomers may be used.

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.

Preferred examples include cyclic monofunctional (meth)acrylate monomers and acyclic-hydrocarbon monofunctional (meth)acrylate monomers. For example, phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 2-(2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

The preferred examples of monofunctional (meth)acrylate monomers have the following chemical structures: Cyclic TMP formal acrylate (CTFA) Phenoxyethyl acrylate (PEA) mol wt 200 g/mol mol wt 192 g/mol Isobornyl acrylate (IBOA) Tetrahydrofurfuryl acrylate (THFA) mol wt 208 g/mol mol wt 156 g/mol 2-(2-Ethoxyethoxy)ethyl acrylate mol wt 188 g/mol C13"27 0 0 R = C81-117 / C1oH21 Octadecyl acrylate (ODA) Tridecyl acrylate (TDA) mol wt 200 g/mol mol 254 g/mol 0 0 C10, 21 C12,125 0 0 Isodecyl acrylate (IDA) Lauryl acrylate mol wt 212 g/mol mol wt 240 g/mol The substituents of the monofunctional monomers are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1.15 alkyl, C3_18 cycloalkyl, C6_10 aryl and combinations thereof, such as C6.10 aryl-or C3_13 cycloalkyl-substituted C1.15 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.

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. 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 inkjet ink of the invention may further comprise a difunctional (meth)acrylate monomer.

Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. 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 difunctional (meth)acrylate monomer is selected from hexanediol diacrylate, propoxylated neopentyl glycol diacrylate, dipropylene glycol diacrylate, and mixtures thereof.

Preferably, the ink comprises 5-15% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink.

The ink of the present invention may further comprise a multifunctional (meth)acrylate monomer, which does not include 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, 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, C8_10 aryl and combinations thereof, such as Ca10 aryl-or C3_18 cycloalkylsubstituted C1_18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure. (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 (HDDA), 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.

Preferably, the multifunctional (meth)acrylate monomer is present in an amount of 1-10% by weight, preferably 2-8% by weight, based on the total weight of the ink.

The total amount of the (meth)acrylate monomer is from 25 to 70 wt% based on the total weight of the ink. Preferably, the radiation-curable (meth)acrylate monomer is present from 35 to 65 wt%, more preferably from 40 to 60 wt%, even more preferably at least 50 wt%, based on the total weight of the ink.

In a preferred embodiment, the (meth)acrylate monomer includes a cyclic monofunctional (meth)acrylate monomer, an acyclic-hydrocarbon monofunctional (meth)acrylate monomer, a difunctional (meth)acrylate monomer and a multifunctional (meth)acrylate monomer.

The inkjet ink of the present invention may further comprise an N-vinyl amide and/or an N-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 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-hydroxyethwry)-phenyl]-2-hydroxy-2-methyl-1-propane1-one, 2-benzy1-2-dimethylamino-(4-morpholinophenybbutan-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 ink of the present invention comprises a black pigment. The black pigment is dispersed in the liquid medium of the ink and is typically in the form of a powdered black pigment. A preferred black pigment is carbon black, more specifically MOGUL E available from Cabot Corporation. In a preferred embodiment, the ink comprises 1-5% by weight, preferably 2-4% by weight, of the black pigment, based on the total weight of the ink.

The ink of the present invention comprises a blue pigment. The blue pigment is dispersed in the liquid medium of the ink and is typically in the form of a powdered blue pigment. A preferred blue pigment is Heliogen Blue 7110 F available from BASF. In a preferred embodiment, the ink comprises 0.05-0.50% by weight, preferably 0.10-0.30%, of the blue pigment, based on the total weight of the ink.

The blue pigment is present in the ink in a small amount compared to the black pigment. Preferably, the ratio by weight of the black pigment to the blue pigment is 10-20:1.

The ink of the present invention comprises a polyethyleneimine-polyester-fatty acid copolymer dispersant. The polyethyleneimine-polyester-fatty acid copolymer dispersant preferably comprises the monomers 12-hydroxyoctadecanoic acid, 2,2'-iminobis[ethanamine], 2-oxepanone and tetrahydro-2H-pyran-2-one. 12-Hydroxyoctadecanoic acid, 2,2'-iminobis[ethanamine], 2-oxepanone and tetrahydro-2H-pyran-2-one have the following structures: OH 0 H 2N H 2 H3C H io OH 2,2'-lminobis[ethanamine] mol wt 103.17 g/mol 12-Hydroxyoctadecanoic acid mol wt 300.48 g/mol 0 0 2-Oxepanone Tetrahydro-2H-pyran-2-one mol wt 114.14 g/mol mol wt 100.12 g/mol In a preferred embodiment, the ink comprises 0.5-1.0% by weight, preferably 0.6-0.9% by weight, of the polyethyleneimine-polyester-fatty acid copolymer dispersant, based on the total weight of the ink. A preferred polyethyleneimine-polyester-fatty acid copolymer dispersant is EFKA PX 4731 available from BASF.

The ink of the present invention comprises a comb-structured dispersant. The comb-structured dispersant is a copolymer. So-called "comb" polymers are a subset of branched polymers formed of a main chain with two or more three-way branch points defining linear side chains, i.e. it has the appearance of a comb. The comb-structured dispersant has a polyethyleneimine backbone and polyester side chains. The polyethyleneimine backbone can be thought of as the shaft of a comb and the polyester side chains can be thought of as the teeth of a comb. The polyethyleneimine backbone is an anchoring group that provides strong adsorption onto the pigment surface and the polyester side chains provide stabilisation. The comb-structured dispersant preferably comprises the monomers ethyleneimine and 2-oxepanone. 2-Oxepanone has the above-depicted structure. Ethyleneimine has the following structure:

H N

Ethylene imine mol wt 43.07 g/mol Ethylene imine is commonly available as polyethyleneimine.

In a preferred embodiment, the ink comprises 0.01-0.2% by weight, preferably 0.02-0.1% by weight, of the comb-structured dispersant, based on the total weight of the ink. A preferred comb-structured dispersant is Solsperse 32000 available from Lubrizol.

The two specific dispersants of the invention aid in the distribution and the compatibility of the black and blue pigments of the ink. The particular blend of components of the ink and in particular, the two specific dispersants with the black and blue pigments results in a black ink with a high colour density and a vivid black colour. When the black ink is applied to a substrate, the ink is more opaque and dense compared to conventional single pigment black inks. Further, the black ink has an enhanced black colour and the colour gamut of the black is wider compared to conventional single pigment black inks that often have a brown undertone and are weak in appearance.

The ink of the present invention may further comprise a surfactant having the following structure: 4-4-0) ( o 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.

A particularly preferred example of the surfactant is available commercially as TEGO Rad 2300.

By TEGO Rad 2300 herein, we mean surfactant having the following structure: --SH4-0 S ) ( 0 TH*70-TH-

OH

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.

OH

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 Chain

OH

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: 0.1 g of TEGO Rad 2300 is dissolved in 10 mL of toluene repeated in triplicate. The injection size is 70 pL. 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 preferably has 1.0-5.0% by weight, more preferably 2.03.0% by weight, most preferably 2.5% by weight of TEGO Rad 2300, based on the total weight of the ink.

The inventors have found that TEGO Rad 2300 can be included in the inkjet ink of the present invention to provide improved dot spread, stability and colour gamut.

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 (whereas monomers typically have a molecular weight below these values). 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 / 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 Ebecryl 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% by weight, based on the total weight of the ink. Preferably the oligomer is present from 2-10% by weight, based on the total weight of the ink.

The ink of the present invention may further comprise an a,[3-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 a13-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 Lambert 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% 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 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® All from Rohm and Haas. The resin is preferably present at 1-5% by weight, 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 11 mPas, or 10 to 11 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 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.

Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, 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 2012/110815.

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 pm, preferably 1 to 10 pm, for example 2 to 5 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

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 ink and a substrate having the ink cured thereon. Accordingly, the present invention further provides a method of inkjet printing comprising inkjet printing the 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, on a roll-to-roll printer or a flat-bed printer. A specific example of this is for label decoration and production, more particularly where labels are printed on webs of white, 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 or inkjet ink set 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 black pigment is formulated as a black pigment dispersion wherein the black pigment dispersion comprises the black pigment and the polyethyleneiminepolyester-fatty acid copolymer dispersant. Similarly, the blue pigment is formulated as a cyan pigment dispersion wherein the cyan pigment dispersion comprises the blue pigment and the comb-structured dispersant. The black and blue pigment dispersion are then mixed with the other components of the ink of the invention to form the ink.

Accordingly, the present invention further comprises a method of preparing an inkjet ink as defined herein, wherein the (meth)acrylate monomer and the radical photoinitiator are mixed with a black pigment dispersion and a cyan pigment dispersion, wherein the black pigment dispersion comprises the black pigment and the polyethyleneimine-polyester-fatty acid copolymer dispersant and wherein the cyan pigment dispersion comprises the blue pigment and the comb-structured dispersant.

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

Examples

Example1

An inkjet ink was prepared according to the formulation set out in Table 1. The 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

Component Ink 1 CTFA (monofunctional (meth)acrylate monomer) 36.00 NVC (N-vinyl amide) 16.00 IDA (monofunctional (meth)acrylate monomer) 7.50 HDDA (difunctional (meth)acrylate monomer) 7.25 DPHA (multifunctional (meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Black pigment dispersion 5.65 Cyan pigment dispersion 0.50 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.00 The black and cyan pigment dispersions of ink 1 were prepared according to the formulations set out in Table 2. The dispersions were prepared by mixing the components in the given amounts.

Amounts are given as weight percentages based on the total weight of the dispersion.

Table 2

Component Black pigment dispersion Cyan pigment dispersion Solsperse 32000 (dispersant) - 10.00 EFKA PX 4731 (dispersant) 12.00 -UV12 (stabiliser) 1.50 1.00 Sartomer 339C (PEA) 46.50 59.00 Mogul E (pigment) 40.00 Heliogen blue D 7110 F (pigment) - 30.00 Example 2 (comparative) An inkjet ink was prepared according to the formulation set out in Table 3. The 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 3

Component Ink 2 (comparative) CTFA (monofunctional (meth)acrylate monomer) 35.80 NVC (N-vinyl amide) 16.00 IDA (monofunctional (meth)acrylate monomer) 7.50 HDDA (difunctional (meth)acrylate monomer) 8.30 DPHA (multifunctional (meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Black pigment dispersion 5.30 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.00 The black pigment dispersion of ink 2 (comparative) is the same as the black pigment dispersion of ink 1.

Example 3

Ink 1 and ink 2 (comparative) were drawn down onto Leneta card substrates using a K2 applicator bar. The results are shown in Fig. 1; which clearly shows that ink 1 has a higher colour density and an enhanced black colour than ink 2 (comparative).

Example 4

The following colour data were obtained using an i1 spectrophotometer. The end user requirements for a black ink and the properties of ink 1 and ink 2 (comparative) are shown in Table 4. The figures are based on the CIELAB (L*a*b*) colour space system. The lightness, L", represents the darkest black at L*=0, and the brightest white at L*=100. The colour channels, a* and b", represents true neutral grey values at a*=0 and b"=0. The red/green opponent colours are represented along the a" axis, with green at negative a" values and red at positive a" values. The yellow/blue opponent colours are represented along the b* axis, with blue at negative b* values and yellow at positive b" values.

Table 4

L" e if End user requirements 10.0 0.0 0.0 Ink 1 10.0 0.0 2.1 Ink 2 12.7 1.6 4.3 (comparative) As can be seen from Table 4, for a black ink, ink 1 of the present invention more closely matches the end user requirements than comparative ink 2.

Example 5

An inkjet ink was prepared according to the formulation set out in Table 5. The 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 5

Component Ink 3 CTFA (monofunctional(meth)acrylate monomer) 34.00 NVC (N-vinyl amide) 16.00 IDA (monofunctional(meth)acrylate monomer) 7.50 HDDA (difunctional(meth)acrylate monomer) 8.75 DPHA (mutifunctional(meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Solsperse 32000 (dispersant) 0.05 EFKA PX 4731 (dispersant) 0.67 UV-12 (stabiliser) 0.10 Sartomer 339C (PEA) 2.92 Mogul E (pigment) 2.26 Heliogen blue D 7110 (pigment) 0.15 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.50

Example 6

An inkjet ink was prepared according to the formulation set out in Table 6. The 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 6

Component Ink 4 CTFA (monofunctional(meth)acrylate monomer) 33.15 NVC (N-vinyl amide) 16.00 IDA (monofunctional(meth)acrylate monomer) 7.50 HDDA (difunctional(meth)acrylate monomer) 8.75 DPHA (mutifunctional(meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Solsperse 32000 (dispersant) 0.10 EFKA PX 4731 (dispersant) 0.71 UV-12 (stabiliser) 0.11 Sartomer 339C (PEA) 3.38 Mogul E (pigment) 2.40 Heliogen blue D 7110 (pigment) 0.30 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.50

Example 7

An inkjet ink was prepared according to the formulation set out in Table 7. The 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 7

Component Ink 5 CTFA (monofunctional(meth)acrylate monomer) 33.15 NVC (N-vinyl amide) 16.00 IDA (monofunctional(meth)acrylate monomer) 7.50 HDDA (difunctional(meth)acrylate monomer) 8.75 DPHA (mutifunctional(meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Solsperse 32000 (dispersant) 0.35 EFKA PX 4731 (dispersant) 0.42 UV-12 (stabiliser) 0.09 Sartomer 339C (PEA) 3.69 Mogul E (pigment) 1.40 Heliogen blue D 7110 (pigment) 1.05 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.50

Example 8

An inkjet ink was prepared according to the formulation set out in Table 8. The 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 8

Component Ink 6 CTFA (monofunctional(meth)acrylate monomer) 33.15 NVC (N-vinyl amide) 16.00 IDA (monofunctional(meth)acrylate monomer) 7.50 HDDA (difunctional(meth)acrylate monomer) 8.75 DPHA (mutifunctional(meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Solsperse 32000 (dispersant) 0.60 EFKA PX 4731 (dispersant) 0.12 UV-12 (stabiliser) 0.10 Sartomer 339C (PEA) 3.98 Mogul E (pigment) 0.40 Heliogen blue D 7110 (pigment) 1.80 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.50

Example 9

An inkjet ink was prepared according to the formulation set out in Table 9. The 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 9

Component Ink 7 CTFA (monofunctional(meth)acrylate monomer) 31.15 NVC (N-vinyl amide) 16.00 IDA (monofunctional(meth)acrylate monomer) 7.50 HDDA (difunctional(meth)acrylate monomer) 8.75 DPHA (mutifunctional(meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Solsperse 32000 (dispersant) 0.20 EFKA PX 4731 (dispersant) 0.84 UV-12 (stabiliser) 0.13 Sartomer 339C (PEA) 4.43 Mogul E (pigment) 2.80 Heliogen blue D 7110 (pigment) 0.60 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.50

Example 10

An inkjet ink was prepared according to the formulation set out in Table 10. The 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 10

Component Ink 8 CTFA (monofunctional(meth)acrylate monomer) 25.15 NVC (N-vinyl amide) 16.00 IDA (monofunctional(meth)acrylate monomer) 7.50 HDDA (difunctional(meth)acrylate monomer) 8.75 DPHA (mutifunctional(meth)acrylate monomer) 4.00 UV-1 (stabiliser) 0.10 Ebecryl 80 (multifunctional oligomer) 9.00 Solsperse 32000 (dispersant) 0.50 EFKA PX 4731 (dispersant) 1.20 UV-12 (stabiliser) 0.20 Sartomer 339C (PEA) 7.60 Mogul E (pigment) 4.00 Heliogen blue D 7110 (pigment) 1.50 Irgacure 184 (photoinitiator) 4.00 TPO (photoinitiator) 8.00 TEGO Rad 2300 (surfactant) 2.50

Example 11

Inks 3-8 were each cured, to determine the variation of cure response with the amount of black pigment dispersion and cyan pigment dispersion present in the ink. Amounts are given as weight percentages based on the total weight of the ink. As can be seen from Table 11, inks 3-7 more closely match the end user requirements than ink 8.

Ink Black pigment dispersion Cyan pigment dispersion Cure response 3 5.65 0.50 Good 4 6.00 1.00 Good 3.50 3.50 Good 6 1.00 6.00 Good 7 7.00 2.00 Good 8 10.00 5.00 Poor -adhesion impaired

Claims

Claims 1. An inkjet ink comprising a (meth)acrylate monomer, a radical photoinitiator, a black pigment, a blue pigment, a polyethyleneimine-polyester-fatty acid copolymer dispersant and a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains.
2. An inkjet ink as claimed in claim 1, wherein the ink further comprises a surfactant having the following structure: -Si-E0 Si) 0 nOwherein 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.
3. An inkjet ink as claimed in claims 1 or 2, wherein the polyethyleneimine-polyester-fatty acid copolymer dispersant comprises the monomers 12-hydroxyoctadecanoic acid, 2,2'-iminobis[ethanamine], 2-oxepanone and tetrahydro-2H-pyran-2-one.
4. An inkjet ink as claimed in any preceding claim, wherein the comb-structured dispersant comprises the monomers ethyleneimine and 2-oxepanone.
5. An inkjet ink as claimed in any preceding claim, wherein the (meth)acrylate monomer includes a cyclic monofunctional (meth)acrylate monomer, an acyclic-hydrocarbon monofunctional (meth)acrylate monomer, a difunctional (meth)acrylate monomer and a multifunctional (meth)acrylate monomer.
6. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 1-5% by weight of the black pigment, based on the total weight of the ink.
7. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 0.05-0.50% by weight of the blue pigment, based on the total weight of the ink.
8. An inkjet ink as claimed in any preceding claim, wherein the ratio by weight of the black pigment to the blue pigment is 10-20:1.OH
9. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 0.5-1.0% by weight of the polyethyleneimine-polyester-fatty acid copolymer dispersant, based on the total weight of the ink.
10. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 0.01-0.2% by weight of the comb-structured dispersant, based on the total weight of the ink.
11. An inkjet ink as claimed in any preceding claim, wherein the ink is substantially free of water and volatile organic solvents.
12. A cartridge containing the inkjet ink as claimed in any preceding claim.
13. A printed substrate having the ink as claimed in any of claims 1-11 printed thereon.
14. A method of inkjet printing comprising inkjet printing the inkjet ink as claimed in any of claims 1-11 onto a substrate and curing the ink.
15. A method of preparing an inkjet ink as claimed in any of claims 1-11, wherein the (meth)acrylate monomer and the radical photoinitiator are mixed with a black pigment dispersion and a cyan pigment dispersion, wherein the black pigment dispersion comprises the black pigment and the polyethyleneimine-polyester-fatty acid copolymer dispersant and wherein the cyan pigment dispersion comprises the blue pigment and the comb-structured dispersant.