GB2554817A

GB2554817A - Method of printing

Info

Publication number: GB2554817A
Application number: GB1716511.9A
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: 2016-10-10
Filing date: 2017-10-09
Publication date: 2018-04-11
Anticipated expiration: 2037-10-09
Also published as: GB2554817B; GB201716511D0; GB201617191D0

Abstract

A method of inkjet printing comprises printing an inkjet ink onto a substrate and exposing the ink to UV LED light to effect curing, wherein the ink comprises a (meth)acrylate monomer, a free 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 polyethyleneimine-polyester-fatty acid copolymer may comprise monomeric units of 12-hydroxyoctadecanoic acid, 2,2-iminobis[ethanamine], 2-oxepanone, and tetrahydro-2H-pyran-2-one and may be present in an amount of 0.5-1 wt.%. The comb-structured dispersant may comprise monomeric units of ethyleneimine and 2-oxepanone and may be present in an amount of 0.01-0.2 wt.%. Typically, the ink comprises 1-5 wt.% black pigment and 0.05-0.5 wt.% blue pigment. The photoinitiator may be a package comprising an acyl phosphine oxide and a thioxanthone. The ink may be prepared by combining a black pigment dispersion comprising the black pigment and the polyethyleneimine-polyester-fatty acid copolymer with a cyan pigment dispersion comprising the blue pigment and the comb-structured dispersant.

Description

(54) Title of the Invention: Method of printing

Abstract Title: Inkjet ink containing polyethyleneimine-based dispersants (57) A method of inkjet printing comprises printing an inkjet ink onto a substrate and exposing the ink to UV LED light to effect curing, wherein the ink comprises a (meth)acrylate monomer, a free 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 polyethyleneimine-polyester-fatty acid copolymer may comprise monomeric units of 12-hydroxyoctadecanoic acid, 2,2’-iminobis[ethanamine], 2oxepanone, and tetrahydro-2H-pyran-2-one and may be present in an amount of 0.5-1 wt.%. The comb-structured dispersant may comprise monomeric units of ethyleneimine and 2-oxepanone and may be present in an amount of 0.01-0.2 wt.%. Typically, the ink comprises 1-5 wt.% black pigment and 0.05-0.5 wt.% blue pigment. The photoinitiator may be a package comprising an acyl phosphine oxide and a thioxanthone. The ink may be prepared by combining a black pigment dispersion comprising the black pigment and the polyethyleneimine-polyester-fatty acid copolymer with a cyan pigment dispersion comprising the blue pigment and the comb-structured dispersant.

1/1

Fig. 1

Method of printing

The present invention relates to a method of printing, and in particular, a method of inkjet printing a black inkjet ink that has a high colour density and a vivid black colour utilising UV LED light. The present invention also relates to inks adapted for use in the method of the invention.

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 below 100 mPas at 25°C (although in most applications the viscosity should be below 50 mPas, and often below 25 mPas). Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature, which is often elevated to about 40°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.

In one common type of inkjet ink, this liquid is water - see for example the paper by Henry R. Kang in the Journal of Imaging Science, 35(3), pp. 179-188 (1991). In those systems, great effort must be made to ensure the inks do not dry in the head due to water evaporation. In another common type, the liquid is a low-boiling solvent or mixture of solvents - see, for example, EP 0 314 403 and EP 0 424 714. Unfortunately, inkjet inks that include a large proportion of water or solvent cannot be handled after printing until the inks have dried, either by evaporation of the solvent or its absorption into the substrate. This drying process is often slow and in many cases (for example, when printing on to a heat-sensitive substrate such as paper) cannot be accelerated.

Another type of inkjet ink contains radiation-curable material, such as radiation-curable monomers, which polymerise by irradiation with actinic radiation, commonly with ultraviolet light, in the presence of a photoinitiator. 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 exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures.

There are a number of sources of actinic radiation which are commonly used to cure inkjet inks which contain radiation-curable material. The most common source of radiation is a UV source. UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. Mercury discharge lamps, fluorescent tubes and flash lamps are most commonly used as the radiation source as they generate enough power to thoroughly cure the radiation-curable ink and hence achieve adequate through cure and surface cure. When LEDs are used, it is necessary to use an array of multiple LEDs in order to generate enough power to provide thorough curing of the ink. However, even with an array of multiple LEDs, inks which are cured by LEDs are prone to poor surface cure.

There is therefore a need in the art for a method of inkjet printing an inkjet ink comprising a radiation-curable material, where the source of actinic radiation used to cure the inkjet ink is UV LED light, which can achieve thorough curing of the ink. In addition to providing thorough curing of the ink, 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 also therefore a need in the art for a method of inkjet printing a black inkjet ink that has a high colour density and a vivid black colour, where the source of actinic radiation used to cure the inkjet ink is UV LED light, and where a thorough curing of the ink is also achieved.

Accordingly, the present invention provides a method of inkjet printing comprising: inkjet printing an inkjet ink onto a substrate, wherein the inkjet ink comprises a (meth)acrylate monomer, a free 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; and exposing the inkjet ink to UV LED light to cure the inkjet ink.

The inventors have surprisingly found that a method of inkjet printing, which utilises an inkjet ink that comprises the present blend of components and in particular, the two specific dispersants as defined herein, and which utilises UV LED light to cure the inkjet ink, 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, whilst achieving thorough cure of the ink.

The present invention also provides an inkjet ink adapted for use in the method of the invention. In particular, the present invention also provides an inkjet ink comprising a (meth)acrylate monomer, a free radical photoinitiator, a black pigment, a blue pigment, a polyethyleneiminepolyester-fatty acid copolymer dispersant and a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains, wherein the free radical photoinitiator comprises a photoinitiator package comprising a combination of an acyl phosphine oxide photoinitiator and a thioxanthone photoinitiator.

The inventors have found that an inkjet ink that comprises the present blend of components and in particular, the two specific dispersants as defined herein, and the specific blend of photoinitiators tailored for UV LED light, 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, whilst achieving thorough cure of the ink when utilising UV LED light to cure the inkjet ink.

The method of inkjet printing of the present invention comprises inkjet printing an inkjet ink onto a substrate.

The inkjet ink used in the method 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 acyclichydrocarbon 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), lauryi acrylate and mixtures thereof.

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

<

o

o

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

Phenoxyethyl acrylate (PEA) mol wt 192 g/mol

O

Isobornyl acrylate (IBOA) o

'0

Tetrahydrofurfuryl acrylate (THFA) mol wt 208 g/mol mol wt 156 g/mol

O

σ

Ό'

2-(2-Ethoxyethoxy)ethyl acrylate mol wt 188 g/mol

O.

R'

O.

Octadecyl acrylate (ODA) mol wt 200 g/mol

Tridecyl acrylate (TDA) mol 254 g/mol ^C10^H2l'

^C12^H25

Isodecyl acrylate (IDA) mol wt 212 g/mol

Lauryl acrylate 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 Ci_i₈ alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C_e.₁₀ aryl- or C₃.₁₈ cycloalkylsubstituted 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.

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

The ink used in the method 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 C^s alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C_e.₁₀ aryl- or C₃.₁₈ cycloalkylsubstituted 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 (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 used in the method 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 used in the method of the present invention 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 used in the method of the present invention comprises a free 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-2methyl-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).

Mixtures of free radical photoinitiators can be used and preferably, the ink used in the method of the present invention 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.

In a preferred embodiment, the photoinitiator present in the ink used in the method ofthe present invention is tailored for UV LED light and preferably comprises a photoinitiator package comprising two or more photoinitiators. By tailored for UV LED light, it is meant that the photoinitiators absorb the radiation which is emitted by the UV LED light source. Preferably, the photoinitiator present in the ink used in the method ofthe present invention absorbs radiation in a region of from 360 nm to 410 nm and absorbs sufficient radiation to cure the ink within a 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less bandwidth.

Preferably, the photoinitiator comprises a combination of an acyl phosphine oxide photoinitiator, such as TPO and BAPO, and a thioxanthone photoinitiator, such as ITX. In a preferred embodiment, the amount of acyl phosphine oxide photoinitiator present in the ink is 4-12% by weight and the amount of thioxanthone photoinitiator is 0.5-5.0% by weight, based on the total weight ofthe ink.

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

The ink used in the method 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, ofthe black pigment, based on the total weight ofthe ink.

The ink used in the method ofthe 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 ofthe ink.

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

The ink used in the method of the present invention comprises a polyethyleneimine-polyesterfatty acid copolymer dispersant. The polyethyleneimine-polyester-fatty acid copolymer dispersant preferably comprises the monomers 12-hydroxyoctadecanoic acid, 2,2'-iminobis[ethanamine], 2oxepanone 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

Ο

OH

N

H

NH

12-Hydroxyoctadecanoic acid mol wt 300.48 g/mol

2,2'-lminobis[ethanamine] mol wt 103.17 g/mol

2-Oxepanone mol wt 114.14 g/mol

Tetrahydro-2H-pyran-2-one mol wt 100.12 g/mol

In a preferred embodiment, the ink used in the method of the present invention comprises 0.51.0% 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 polyethyleneiminepolyester-fatty acid copolymer dispersant is EFKA PX 4731 available from BASF.

The ink used in the method 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 combstructured dispersant preferably comprises the monomers ethyleneimine and 2-oxepanone. 2Oxepanone has the above-depicted structure. Ethyleneimine has the following structure:

ΖΔ

Ethylene imine mol wt 43.07 g/mol

Ethylene imine is commonly available as polyethyleneimine.

In a preferred embodiment, the ink used in the method of the present invention comprises 0.010.2% 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 ink used in the method 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 used in the method 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 12° 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 hexafunctional 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 otthe ink.

The ink used in the method 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 used in the method otthe present 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 radiationcurable 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 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 used in the method of the present 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 used in the method ofthe present invention dries primarily by curing, i.e. by the polymerisation ofthe 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 ofthe 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 used in the method ofthe present invention is preferably substantially free of water and volatile organic solvents. 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 most preferably less than 1% by weight combined, based on the total weight ofthe ink. Some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components ofthe inks, but such low levels are tolerated.

The inks used in the method of the present invention 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® A11 from Rohm and Haas. The resin is preferably present at 1-5% by weight, based on the total weight ofthe ink.

The inkjet ink used in the method ofthe present invention 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 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 used in the method of the present invention 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. In a preferred embodiment, photosensitisers are added to the ink, which are selected to absorb strongly in the desired wavelength band of UV LED radiation source and are able to transfer energy to the photoinitiators of the ink.

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.

In the method of inkjet printing of the present invention, the inkjet ink is printed onto a substrate. Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto a substrate, on a roll-to-roll printer or a flat-bed printer. As discussed above, inkjet printing is well known in the art and a detailed description is not required.

The ink is jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image. The substrate is not limited. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene and polypropylene.

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.

In the method of the present invention, after inkjet printing the inkjet ink onto the substrate, the printed image is then exposed to UV LED light to cure the inkjet 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. More than one LED is often required to provide the required intensity of radiation to effect cure as currently available LEDs have a relatively low output compared with other conventionally used radiation sources, such as a typical mercury source. Arrangements of many LEDs may therefore be used and in particular arrays of LEDs may be required to provide the required intensity of radiation to effect a thorough cure of the printed ink film. Such arrangements and arrays of LEDs are well known in the art and a detailed discussion is not required.

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

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

Preferably, the wavelength ofthe UV LED source substantially matches the absorption profile of the ink. In a preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 360 nm to 410 nm. In a particularly preferred embodiment, the wavelength distribution ofthe UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm. The ink used in the method of the present invention has been specifically formulated to respond to the emission ofthe UV LED source.

In a particularly preferred embodiment, the wavelength distribution ofthe UV LED light peaks at a wavelength of from 360 nm to 410 nm, and at least 90%, preferably at least 95%, ofthe radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm, and at least 90%, preferably at least 95%, ofthe radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

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

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.

In a preferred embodiment, the exposure to UV LED light is performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink.

The present invention also provides an inkjet ink adapted for use in the method of the invention. In particular, the present invention provides an inkjet ink comprising a (meth)acrylate monomer, a free 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, wherein the free radical photoinitiator comprises a photoinitiator package comprising a combination of an acyl phosphine oxide and a thioxanthone photoinitiator.

The features of the inkjet ink of the invention include the features discussed hereinabove with respect to the ink used in the method of the present invention. Further preferred features are as identified above.

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

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 free 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 combstructured dispersant.

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.

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

Examples

Example 1

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
ITX (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 D7110 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
ITX (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 printed ink films were cured using a Phoseon FirePower 20 watt UV LED source which emitted radiation having a peak wavelength of 395nm. 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*) colourspace 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*	a*	b*
End user requirements	10.0	0.0	0.0
Ink 1	10.0	0.0	2.1
Ink 2 (comparative)	12.7	1.6	4.3

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.

Claims

1. A method of inkjet printing comprising: inkjet printing an inkjet ink onto a substrate, wherein the inkjet ink comprises a (meth)acrylate monomer, a free radical photoinitiator, a black pigment, a blue pigment, a polyethyleneimine-polyester-fatty acid copolymer dispersant and a combstructured dispersant having a polyethyleneimine backbone and polyester side chains; and exposing the inkjet ink to UV LED light to cure the inkjet ink.

2. A method of inkjet printing as claimed in claim 1, wherein the wavelength distribution of the UV LED light peaks at a wavelength of from 360 nm to 410 nm and at least 90%, preferably at least 95%, of the radiation has a wavelength in a band having a width of 50 nm or less.

3. A method of inkjet printing as claimed in claim 2, wherein the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm.

4. A method of inkjet printing as claimed in any preceding claim, wherein the polyethyleneimine-polyester-fatty acid copolymer dispersant comprises the monomers 12hydroxyoctadecanoic acid, 2,2'-iminobis[ethanamine], 2-oxepanone and tetrahydro-2H-pyran-2one.

5. A method of inkjet printing as claimed in any preceding claim, wherein the comb-structured dispersant comprises the monomers ethyleneimine and 2-oxepanone.

6. A method of inkjet printing as claimed in any preceding claim, wherein the ink comprises 15% by weight of the black pigment, based on the total weight of the ink.

7. A method of inkjet printing 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. A method of inkjet printing as claimed in any preceding claim, wherein the ratio by weight of the black pigment to the blue pigment is 10-20:1.

9. A method of inkjet printing as claimed in any preceding claim, wherein the ink comprises 0.51.0% by weight of the polyethyleneimine-polyester-fatty acid copolymer dispersant, based on the total weight of the ink.

10. A method of inkjet printing 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. A method of inkjet printing as claimed in any preceding claim, wherein the free radical photoinitiator comprises a photoinitiator package comprising a combination of an acyl phosphine oxide photoinitiator and a thioxanthone photoinitiator.

12. An inkjet ink comprising a (meth)acrylate monomer, a free radical photoinitiator, a black 5 pigment, a blue pigment, a polyethyleneimine-polyester-fatty acid copolymer dispersant and a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains, wherein the free radical photoinitiator comprises a photoinitiator package comprising a combination of an acyl phosphine oxide photoinitiator and a thioxanthone photoinitiator.

13. A cartridge containing the inkjet ink as claimed in claim 12.

10

14. A printed substrate having the ink as claimed in claim 12 printed thereon.

15. A method of preparing an inkjet ink as claimed in claim 12, wherein the (meth)acrylate monomer and the free 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

15 dispersion comprises the blue pigment and the comb-structured dispersant.

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