GB2573207A - A printing ink - Google Patents

Abstract

A black inkjet ink comprises (i) N-vinyl amide monomer, N-acryloyl amine monomer and/or N-vinyl carbamate monomer, (ii) mono-, di- and/or multifunctional (meth)acrylate monomers, (iii) radical photoinitiator, and (iv) 2.0-4.5 wt.% dispersed black pigment and 0.1-0.8 wt.% dispersed cyan pigment, wherein the weight ratio of black pigment to cyan pigment is 4-9:1. Preferably, the N-vinyl amide monomer is N-vinyl caprolactam and the N-vinyl carbamate is N-vinyl-5-methyl-2-oxazolidinone. The black pigment may be carbon black and the cyan pigment may be Pigment Blue 15:4. The weight ratio of black pigment to cyan pigment may be 6-8:1, for example when the ink comprises 3 wt.% black pigment and 0.4 wt.% cyan pigment. The ink may also contain a radiation-curable oligomer and a comb-structured dispersant having a polyethyleneimine backbone with polyester side chains. Typically, the ink is substantially free of water and volatile organic solvents.

Description

A printing ink

This invention relates to a printing ink, in particular to an inkjet ink which provides a dense black colour in the image.

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 inks must flow rapidly from the printing heads and to ensure that this happens, they have low viscosities in use, typically below 100 mPas at 25°C, although in most applications the viscosity is below 50 mPas, and often below 25 mPas. The inks must also be resistant to drying or crusting in the reservoirs or nozzles.

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 and/or where a thin film is printed.

Recently, inkjet inks have been provided having higher amounts of black pigment to increase the density of the black colour in the image. Such inks may contain up to 6% by weight of black pigment. However, a drawback of this approach is that the cure response is adversely affected by the additional black pigment. GB 2538619 sets out an approach where a cyan pigment is added to a black ink in order to widen the colour gamut of the black ink. However, this document does not address the issue of the effect of widening the colour gamut on cure speed.

There remains therefore a need in the art for a black inkjet ink that has a high colour density and a vivid black colour, without compromising cure speed.

Accordingly, the present invention provides a dense-black inkjet ink comprising: an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer; mono-, di- and/or multifunctional (meth)acrylate monomers; a radical photoinitiator; 2.0-4.5% by weight of a dispersed black pigment and 0.1-0.8% by weight of a dispersed cyan pigment, the amounts being based on the total weight of the ink, wherein the weight ratio of the dispersed black pigment to the dispersed cyan pigment is 4-9:1.

This ink combines a dense black colour with a fast cure speed and robust film properties.

The inkjet ink of the present invention contains an N-vinyl amide monomer, N-acryloyl amine monomer and/or an N-vinyl carbamate monomer. Multiple monomers in each category or mixtures of monomers in different categories may be used. N-Vinyl amide monomers are well-known monomers in the art. N-Vinyl amide monomers have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers as discussed below. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP).

The most preferred monomer in this category is NVC. NVC is a well-known monomer in the art and has the following chemical structure:

N-vinyl caprolactam (NVC), mol wt 139 g/mol

Similarly, N-acryloyl amine monomers are also well-known in the art. N-Acryloyl amine monomers also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO). N-Vinyl carbamate monomers are defined by the following functionality:

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

In a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure:

in which R1 to R4 are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The substituents are typically hydrogen, alkyl, cycloaikyl, 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 cycloaikyl, Ce.10 aryl and combinations thereof, such as Ce.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. Preferably R1 to R4 are independently selected from hydrogen or C^w alkyl. Further details may be found in WO 2015/022228 and US 4,831,153.

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

molecular weight 127 g/mol NVMO has the IUPAC name 5-methyl-3-vinyl-1,3-oxazolidin-2-one and CAS number 3395-98-0. NVMO includes the racemate and both enantiomers. In one embodiment, the N-vinyl carbamate monomer is a racemate of NVMO. In another embodiment, the N-vinyl carbamate monomer is (R)-5-methyl-3-vinyl-1,3-oxazolidin-2-one. Alternatively, the N-vinyl carbamate monomer is (S)-5-methyl-3-vinyl-1,3-oxazolidin-2-one.

The inkjet ink preferably contains 10-50% by weight of the N-vinyl amide monomer, N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink. The amount refers to the total amount of all monomers within these categories.

Monomers typically have a molecular weight of less than 600 Da, preferably more than 200 Da and less than 450 Da. Monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. They therefore preferably have a viscosity of less than 150 mPas at 25°C, more preferably less than 100 mPas at 25°C and most preferably less than 20 mPas at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique 12° steel cone at 25°C with a shear rate of 25 s"1.

The ink also contains mono-, di- and/or multi-functional (meth)acrylate monomers.

The monofunctional (meth)acrylate monomers may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

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

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

The substituents of the cyclic monofunctional (meth)acrylate monomer are typically cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms and/or substituted by alkyl. Non-limiting examples of substituents commonly used in the art include C3.18 cycloalkyl, Ce.10 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.

The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA/Medol-10), 4-feri-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof.

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 CW8 alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C6-C20 group. It may be selected from octadecyl acrylate (ODA), 2-(2- ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

If present, the monofunctional (meth)acrylate monomers may be present at 5-25% by weight, based on the total weight of the ink. The amount refers to the total amount of all monomers within this category (i.e. monofunctional (meth)acrylate monomers).

The inkjet ink may also contain difunctional and/or multifunctional (meth)acrylate monomers. Preferably, the ink contains di- and/or multifunctional (meth)acrylate monomers, and more preferably contains a combination of di- and multifunctional (meth)acrylate monomers.

Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Difunctional has its standard meaning, i.e. two groups, which take part in the polymerisation reaction on curing.

Examples include hexanediol diacrylate, 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate (DDDA), 1,11-undecanediol diacrylate and 1,12-dodecanediol diacrylate, polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1,5-pentanediol diacrylate (3MPDDA), and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof. Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate and 1,12-dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate and mixtures thereof. Preferred difunctional (meth)acrylate monomers are NPGPODA and 3MPDDA, and the ink preferably contains NPGPODA and 3MPDDA.

The difunctional (meth)acrylate monomers is preferably present at 20-60% by weight, based on the total weight of the ink. The amount refers to the total amount of all monomers within this category (i.e. difunctional (meth)acrylate monomers).

Multifunctional (meth)acrylate monomers (tri- and higher-functional) are also 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. Usually, the multifunctional (meth)acrylate monomer has a degree of functionality of four or more, e.g. 4-8.

Examples of the multifunctional acrylate monomers include trimethylolpropane triacrylate (TMPTA), 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. A preferred multifunctional (meth)acrylate monomer is TMPTA.

The multifunctional (meth)acrylate monomers is preferably present at 1-20% by weight, more preferably 3-10% by weight, based on the total weight of the ink. The amount refers to the total amount of all monomers within this category (i.e. multifunctional (meth)acrylate monomers).

For the avoidance of doubt, (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 does not include difunctional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

In one embodiment the ink is substantially free of monomers other than the N-vinyl amide monomer, N-acryloyl amine monomer, N-vinyl carbamate monomer and di- and multifunctional monomers, meaning that only small amounts of other monomers will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion. Where additional monomers are included, they are preferably present in an amount of no more than 5 wt% and most preferably no more than 2 wt%, based on the total weight of the ink.

The inkjet ink may also contain a radiation-curable (i.e. polymerisable) oligomer, e.g. 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 often having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer typically has a molecular weight of at least 450 Da and more typically at least 600 Da (whereas monomers typically have a molecular weight below these values). The molecular weight is typically 4,000 Da 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 oligomer will also increase the viscosity of the ink and will require monomers to counteract that viscosity-increasing effect.

The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is typically multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is typically from 2-6.

Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide film-forming properties such as hardness or cure speed. They therefore typically have a viscosity of 150 mPas or above at 25°C, for example a viscosity of 0.5-10 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, and is usually (meth)acrylate. Polyester (meth)acrylate oligomers are preferred, e.g. CN2565 from Sartomer.

Typical radiation-curable oligomers are polyester acrylate oligomers as these have excellent adhesion and elongation properties, e.g. di-, tri-, tetra-, penta- or hexa-functional polyester acrylates.

The radiation-curable oligomer is preferably present at 1-20% by weight, more preferably 2-10% by weight, based on the total weight of the ink. The amount refers to the total amount of all material within this category.

The inkjet ink of the present invention is typically free of passive resin.

Passive (or “inert”) 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 is usually 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.

By substantially free is meant preferably less than 2% by weight, more preferably less than 1% by weight and most preferably less than 0.5% by weight, based on the total weight of the ink.

The inkjet ink of the present invention comprises a radical photoinitiator. The radical photoinitiator is a free radical photoinitiator. The 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 Lucirin (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 1-15% by weight, based on the total weight of the ink.

Mixtures of radical photoinitiators can be used and preferably, the ink comprises a plurality of radical photoinitiators. The total number of radical photoinitiators present is preferably from one to six, and more preferably, two or more radical photoinitiators are present in the ink.

The inkjet ink dries primarily by curing, i.e. by the polymerisation of the monomers and oligomers 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 is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink comprises less than 5% by weight combined 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 inkjet ink also comprises a dispersed black pigment combined with a smaller amount of dispersed cyan 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, e.g. Special Black 250 available from the Cary Company.

The cyan pigment is also dispersed in the liquid medium of the ink and is typically in the form of a powdered blue pigment. A preferred blue pigment is pigment blue 15:4, e.g. Heliogen Blue 7110 F available from BASF.

The ink contains 2.0-4.5% by weight of the dispersed black pigment, more preferably 3% by weight of the dispersed black pigment. It also contains 0.1-0.8% by weight of the dispersed cyan pigment more preferably 0.4% by weight of the dispersed cyan pigment. The amounts are based on the total weight of the ink.

The weight ratio of the dispersed black pigment to the dispersed cyan pigment is 4-9:1, preferably 6-8:1.

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

The pigments are preferably provided as pigment dispersions, and the pigments are typically commercially available in this form.

The black and cyan pigment dispersions contain the black or cyan pigment, respectively. The dispersions each comprise a mono-, di- and/or multifunctional (meth)acrylate monomer, the pigment and a dispersant. The dispersant is preferably a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains.

In a preferred embodiment, the black and cyan pigment dispersions contain the same monomer and/or dispersant and most preferably the same monomer and the same dispersant. This improves the dispersion of the pigments in the ink and increases the compatibility of the pigment dispersions. Preferably the monomer used for the pigment dispersions is the same as at least one of the monomers used as the ink vehicle.

Accordingly, the present invention also provides an inkjet ink obtainable by separately providing a black pigment dispersion and a cyan pigment dispersion, wherein the black pigment dispersion and the cyan pigment dispersion each comprise a mono-, di- and/or multifunctional (meth)acrylate monomer, the pigment and a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains, and mixing the pigment dispersions with the other components of the ink. The ink is characterised by an intimate mixture of the pigments and dispersant, and a more homogenous dispersal of the pigments throughout the ink.

The ink of the present invention preferably employs 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. Ethyleneimine and 2-oxepanone have the following structures:

Ethylene imine 2-oxepanone mol wt 43.07 g/mol mol wt 114.14 g/mol

In a preferred embodiment, the ink comprises 0.05-0.5% by weight, preferably 0.1-0.3% 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 inkjet ink of the present invention exhibits a desirable low viscosity (200 mPas or less, preferably 100 mPas or less and more preferably 30 mPas or less at 25°C). In a preferred embodiment, the viscosity of the inkjet ink is 10-30 mPas at 25°C.

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.

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 90 picolitres, preferably below 35 picolitres and most preferably below 10 picolitres.

The surface tension of the inkjet ink may be controlled by the addition of one or more surface active materials such as commercially available surfactants. A particularly preferred surfactant is BYK-307 from BYK-Chemie GmbH. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example,

plastic substrates. Too high a surface tension can lead to ink pooling and/or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks. The surface tension is preferably in the range of 20-40 mNm'1 and more preferably 21-32 mNm'1.

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, defoamers, dispersants, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

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

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

The substrate is not limited. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, and cellulosic materials. Mixtures/blends are included.

In the method of the present invention, after inkjet printing the inkjet ink onto the substrate, the printed image is then exposed to a UV radiation source, preferably UV LED light, to cure the inkjet ink.

The present invention also provides a substrate having the inkjet ink of the present invention printed thereon.

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

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 1-20 pm, preferably 1-10 pm, for example 2-5 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

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

Examples

Example 1

Inks were prepared by mixing the components in the amounts shown in Table 1. Amounts are given as weight percentages based on the total weight of the ink.

Table 1

NPGPODA is propoxylated neopentylglycol diacrylate, 3MPDDA is 3-methyl-1,5-pentanediol diacrylate, and TMPTA is trimethylolpropane triacrylate. They are di- and tri-functional acrylate monomers. CN2565 is a polyester acrylate oligomer. NVC is N-vinyl caprolactam. UV1 is a stabiliser. BAPO is bis-acylphosphine oxide and TPO is 2,4,6-trimethylbenzoyl-diphenylphosphine oxide. They are both phosphine oxide photoinitiators. ITX is isopropylthioxanthone, another photoinitiator. BYKJET 9151 is a wetting agent. DPHA is dipentaerythritol penta/hexa acrylate. BYK307 is a surfactant.

The pigment dispersions have the compositions set out in Table 2.

Table 2

UV12 is a stabiliser. Solsperse 32000 and 5000 are dispersants. SP Black 250 Fluffy and Heliogen Blue D7110 F are the black and blue pigments, respectively. It can be seen that the overall content of the dispersed pigments in the ink was 3% by weight black and 0.4% by weight cyan.

Example 2

Additional inks were prepared having the same formulation, except that the amounts of the pigment dispersions were changed, such that the amounts of the dispersed pigment in the ink were, 1.6% black (the current standard), 6% black, 3% black and 3% cyan, 4% black and 2% cyan, 2% black and 4% cyan, and 3% black and 1% cyan.

Example 3

The inks of Examples 1 and 2 were drawn down in a 12 pm film onto a onto a varnished card substrate. The ink was then cured using a Baldwin LED drier at 20% @ 40 m/min (117.49 mJ/cm2 per pass) and the dose to achieve full cure was measured. Full cure of the ink was determined by measuring the number of passes required to achieve no offset when using a varnished card substrate (here Epson Premium photo paper).

Further samples of the inks of Examples 1 and 2 were drawn down in varying film thicknesses and cured using the Baldwin LED drier at 40% @ 40m/min. The density and La*b* were measured for the films. The density was measured using a Vipdens densitometer and the La*b* was measured using a Gretag Macbeth Spectrophotometer.

The procedure was repeating using a commercially available ink containing 6% black pigment. The ink is promoted as being a high-density black ink.

The results are set out in Table 3.

co ω

CD Η

Looking to the first two columns of Table 3, the commercially available high-density black ink (6% K) is far denser than a standard black ink (1.6% K), but the standard black ink is faster curing (a lower dose is needed to achieve full cure).

Increasing the black pigmentation (see the inks containing 2, 3 and 4% K) does improve the density but in turn reduces the cure and shifts the hue into a different area (brown in colour). Addition of cyan pigmentation provides a benefit but adding too much adversely affects the colour. The correct ratio is key to obtaining the density needed whilst maintaining cure speed and keeping the hue in the correct gamut area. Hue is important with a dense black as it is required to appear black even at lower film weights. Gamut plot and print images were obtained to visualise the difference in hue (they are not reproduced herein because the information content requires colour reproduction).

Claims (15)

Claims

1. A dense-black inkjet ink comprising: an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer; mono-, di- and/or multifunctional (meth)acrylate monomers; a radical photoinitiator; 2.0-4.5% by weight of a dispersed black pigment and 0.1-0.8% by weight of a dispersed cyan pigment, the amounts being based on the total weight of the ink, wherein the weight ratio of the dispersed black pigment to the dispersed cyan pigment is 4-9:1.

2. An inkjet ink as claimed in claim 1, wherein the ink comprises an N-vinyl amide monomer and the N-vinyl amide monomer is N-vinyl caprolactam (NVC).

3. An inkjet ink as claimed in claim 1, wherein the ink comprises an N-vinyl carbamate monomer and the N-vinyl carbamate monomer is N-vinyl-5-methyl-2-oxazolidinone (NVMO).

4. An inkjet ink as claimed in any preceding claim, wherein the ink contains di- and/or multifunctional (meth)acrylate monomers.

5. An inkjet ink as claimed in claim 4, wherein the ink contains a combination of di- and multifunctional (meth)acrylate monomers.

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

7. An inkjet ink as claimed in any preceding claim, wherein the weight ratio of the dispersed black pigment to the dispersed cyan pigment is 6-8:1.

8. An inkjet ink as claimed in any preceding claim, wherein the black pigment is carbon black.

9. An inkjet ink as claimed in any preceding claim, wherein the cyan pigment is pigment blue 15:4.

10. An inkjet ink as claimed in any preceding claim, wherein the ink contains comb-structured dispersant having a polyethyleneimine backbone and polyester side chains.

11. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 3% by weight of the dispersed black pigment and 0.4% by weight of the dispersed cyan pigment, the amounts being based on the total weight of the ink.

12. An inkjet ink as claimed in any preceding claim, obtainable by separately providing a black pigment dispersion and a cyan pigment dispersion, wherein the black pigment dispersion and the cyan pigment dispersion each comprise a mono-, di- and/or multifunctional (meth)acrylate monomer, the pigment and a comb-structured dispersant having a polyethyleneimine backbone and polyester side chains, and mixing the pigment dispersions with the other components of the ink.

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

14. A method of inkjet printing, comprising printing the inkjet ink as claimed in any preceding claim on to a substrate and curing the ink.

15. A substrate having the inkjet ink as claimed in any of claims 1 to 13 printed thereon.