GB2569628A

GB2569628A - A printing ink

Info

Publication number: GB2569628A
Application number: GB1721639.1A
Authority: GB
Inventors: Corfe Lee
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2017-12-21
Filing date: 2017-12-21
Publication date: 2019-06-26
Also published as: GB201721639D0

Abstract

An inkjet ink comprises monofunctional (meth)acrylate monomer(s), N-vinyl amide and/or N-acryloyl amine and/or N-vinyl carbamate monomer, 1-20 wt.% radiation-curable hyperbranched acrylate oligomer having a functionality of 25-35 and a viscosity of 1-10 Pas at 50°C, passive resin, dispersed pigment, and radical photoinitiator, wherein the ink viscosity is <100 mPas at 25°C. Typically, the ink contains <10 wt.% difunctional and multifunctional monomers (e.g. dipropyleneglycol diacrylate, tricyclodecane dimethanol diacrylate) and <5 wt.% water and volatile organic solvents combined. The only monomers present may be the monofunctional (meth)acrylate(s), N-vinyl amide, N-acryloyl amine, and N-vinyl carbamate. The ink may comprise 5-20 wt.% N-vinyl amide (e.g. N-vinyl caprolactam, N-vinyl pyrrolidone), N-acryloyl amine (e.g. N-acryloyl morpholine) and/or N-vinyl carbamate (e.g. N-vinyl-5-methyl-2-oxazolidinone) and 30-80 wt.% monofunctional (meth)acrylate(s), especially ≥30 wt.% isobornyl acrylate and 5-30 wt.% phenoxyethyl acrylate. A method of printing comprises jetting the ink onto a substrate and exposing to actinic radiation to effect curing. The substrate may be polypropylene or corona-treated polystyrene.

Description

A printing ink

The present invention relates to a printing ink, and in particular to an inkjet for printing onto polypropylene and corona-treated polystyrene substrates.

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.

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 100 mPas or less at 25°C, although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and most preferably between 7-11 mPas at the jetting temperature which is often elevated to, but not limited to 40-50°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent such as water or a low-boiling solvent or mixture of solvents.

Another type of inkjet ink contains unsaturated organic compounds, termed monomers and/or oligomers which polymerise by irradiation, 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.

Inks which cure by the polymerisation of monomers may contain a wide variety of monofunctional, difunctional and multifunctional monomers. The challenge is to provide the necessary printing properties, such as good adhesion and rapid curing, whilst providing a high-quality image, without compromising the jetting properties. In particular, poor adhesion and poor cure response can lead to so-called “blocking”. Blocking occurs when one substrate is laid upon another, or where the substrate is contained on a reel, and the ink remains tacky causing the layers of printed substrate to stick together resulting in an unusable block of substrates adhered to one another by the ink.

Formulating to avoid these problems is made all the harder in inks which are formulated without relying on water or volatile organic solvents as carriers for the ink (which also have their own disadvantages associated with their removal after jetting).

There is therefore a need in the art for an inkjet ink that provides the required balance of properties, including the avoidance of blocking, particularly with substrates like polypropylene and corona-treated polystyrene substrates.

Accordingly, the present invention provides an inkjet ink comprising: one or more monofunctional (meth)acrylate monomers; an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an Nvinyl carbamate monomer; a passive resin; 1-20% by weight of a radiation-curable hyperbranched acrylate oligomer having a functionality of 25-35 and a viscosity of 1-10 Pas at 50°C, based on the total weight of the ink; a dispersed pigment; and a radical photoinitiator, where the ink has a viscosity of less than 100 mPas at 25°C.

The inventors have surprisingly found that the oligomer of the ink markedly reduces blocking after printing.

The invention relates to an inkjet ink based on one or more monofunctional (meth)acrylate monomers; an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer; a passive resin; a dispersed pigment; and a radical photoinitiator. As is usual for inkjet inks, the ink has a viscosity of less than 100 mPas at 25°C. Blocking is markedly reduced by the incorporation of 120% by weight of a radiation-curable hyperbranched acrylate oligomer having a functionality of 25-35 and a viscosity of 1-10 Pas at 50°C, based on the total weight of the ink.

The inkjet ink contains one or more monofunctional (meth)acrylate monomers. The monofunctional (meth)acrylate monomers may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclichydrocarbon monofunctional (meth)acrylate monomer.

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

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

Mixtures of (meth)acrylates may be used.

The ink preferably contains 30-80% by weight of the one or more monofunctional (meth)acrylate monomers in total, based on the total weight of the ink, and more preferably 40-70% by weight, based on the total weight of the ink.

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, C_e.₁₀aryl and combinations thereof, any of which may substituted with alkyl (such as alkyl) and/or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

Preferably, the cyclic monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetra hydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA) and mixtures thereof. IBOA and PEA are particularly preferred. The preferred examples of cyclic monofunctional (meth)acrylate monomers have the following chemical structures:

Isobornyl acrylate (IBOA) mol wt 208 g/mol

Tetrahydrofurfuryl acrylate (THFA) mol wt 156 g/mol

Phenoxyethyl acrylate (PEA) mol wt 192 g/mol (2-Methyl-2-Ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA) mol wt 208.4 g/mol

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

In a preferred embodiment, the inkjet ink comprises only cyclic monofunctional (meth)acrylate monomers as the monofunctional (meth)acrylate monomers present. Preferably there are at least two cyclic monofunctional (meth)acrylate monomers present, more preferably one of which is IBOA and most preferably the ink contains IBOA and PEA.

In a preferred embodiment, the ink contains 30-80% by weight of cyclic monofunctional (meth)acrylate monomers, based on the total weight of the ink, and more preferably 40-70% by weight, based on the total weight of the ink. The ink preferably contains at least 30% by weight of IBOA, based on the total weight of the ink. It preferably contains 5-30% by weight of PEA, based on the total weight of the ink.

The substituents of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

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

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

Octadecyl acrylate (ODA) mol wt 200 g/mol

Tridecyl acrylate (TDA) mol 254 g/mol

Isodecyl acrylate (IDA) mol wt 212 g/mol

Lauryl acrylate mol wt 240 g/mol

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

In a particularly preferred embodiment, the only cyclic monofunctional (meth)acrylate monomers present in the ink are PEA and IBOA. In a further preferred embodiment, the ink contains no other monofunctional (meth)acrylate monomers, i.e. it is substantially free of acyclic-hydrocarbon monofunctional (meth)acrylate monomers. In a particularly preferred embodiment, the only monofunctional (meth)acrylate monomers present in the ink are PEA and IBOA.

The inkjet ink further comprises an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer. The most preferred monomers in this class are an N-vinyl amide monomer or an N-vinyl carbamate monomer.

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). 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:

o

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 ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically hydrogen, alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C_e.₁₀ aryl- or C₃.₁₈ cycloalkyl-substituted 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 Cmo 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:

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)-5methyl-3-vinyl-1,3-oxazolidin-2-one. Alternatively, the N-vinyl carbamate monomer is (S)-5-methyl-3vinyl-1,3-oxazolidin-2-one.

The most preferred N-vinyl amide monomer, N-acryloyl amine monomer and/or N-vinyl carbamate monomer is N-vinyl caprolactam (NVC) or NVMO. NVC is a well-known monomer in the art and has the following chemical structure:

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

In one embodiment, the N-vinyl amide monomer, N-acryloyl amine monomer and/or N-vinyl carbamate monomer present in the ink is NVC. In another embodiment, the N-vinyl amide monomer, N-acryloyl amine monomer and/or N-vinyl carbamate monomer present in the ink is NVMO.

The inkjet ink preferably contains 5-20% by weight of the N-vinyl amide monomer, the N-acryloyl amine monomer and/or the N-vinyl carbamate monomer, based on the total weight of the ink.

The inkjet ink of the present invention preferably contains less than 10% by weight in total of difunctional and multifunctional monomers, based on the total weight of the ink. More preferably, it contains less than 5% by weight and most preferably less than 1% by weight in total of difunctional and multifunctional monomers, based on the total weight of the ink.

Indeed, in a preferred embodiment, the only monomers present in the ink are the one or more monofunctional (meth)acrylate monomers; and the N-vinyl amide monomer, the N-acryloyl amine monomer and/or the N-vinyl carbamate monomer.

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, 1,10decanediol diacrylate, 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 pentanediol diacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, 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.

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, 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.

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 meanings, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

The inkjet ink of the present invention comprises a 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 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.

The total amount of the passive resin is preferably from 0.1-15% by weight, more preferably 0.2-10% by weight, based on the total weight of the ink.

The passive resin in the inkjet ink helps to improve the adhesion of the inkjet ink onto a polypropylene or corona-treated polystyrene substrate.

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)butan1-one, isopropyl thioxanthone, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4trimethylpentylphosphine 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)-2,4,4trimethylpentylphosphine 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 also contains a radiation-curable (i.e. polymerisable) hyperbranched acrylate oligomer.

The terms “curable” and “oligomer” are used in their standard senses, namely that the component has a higher molecular weight and viscosity than the monomers present, but is not a polymer in that it 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). Molecular weights (number average) can be measured using gel permeation chromatography using polystyrene standards.

The oligomer is hyperbranched and so has a dendritic structure. That is, it has three or more branching points, and preferably has four or more branching points. The hyperbranched, dendritic structure means that it is spherical in shape. This provides a lower viscosity than non-branched, linear oligomers.

The hyperbranched oligomer thus has a viscosity of 1-10 Pas, more preferably 3-7 Pas and most preferably about 4 Pas, all measured at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique I 2° steel cone at 60°C with a shear rate of 25 s ¹.

The degree of functionality of the oligomer determines the degree of crosslinking. The oligomer of the present invention has a functionality of 25-35, more preferably 28-32. The functionality represents an average degree of functionality for the oligomer.

The hyperbranched oligomer may be a polyester acrylate, a polyurethane acrylate, a polyepoxy acrylate or polyether acrylate. The oligomer is preferably a polyether acrylate.

A preferred hyperbranched oligomer of the present invention is BDT4330 (from Dymax Corp), which is a dendritic polyether acrylate having an average degree of functionality of 30 and a viscosity of 4,000 mPas at 50°C.

Preferably no other oligomers 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 pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

In one aspect the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7.

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

The colorant is preferably present in an amount of 0.2-20% by weight, preferably 0.5-10% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink

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

The inkjet ink exhibits a desirable low viscosity (100 mPas or less, more preferably 50 mPas or less at 25°C).

In order to produce a high quality printed image a small jetted drop size is desirable, particularly for high resolution images. Preferably the inkjet ink of the invention is jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 20 picolitres.

Ink viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

The surface tension of the inkjet ink may controlled by the addition of one or more surface active materials such as commercially available surfactants. Surfactants are well known in the art and a detailed description is not required. 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 mNrri¹.

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 also provides an inkjet ink set wherein at least one of the inks in the set is an inkjet ink of the present invention. Preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention.

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

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.

A suitable printer is a flatbed UV printer, for example from the Onset series from Inca Digital.

The inks of the present invention may advantageously be printed onto low surface energy substrates, by which is meant substrates having a surface energy of 25-50 mN/m (25-50 dyne/cm). Examples of substrates include those composed of polycarbonate, polyethylene terephthalate (PET), PMMA, PVC, polystyrene, polyethylene and polypropylene. In a preferred embodiment, the substrate is polypropylene or corona-treated polystyrene.

The present invention also provides a printed substrate having the ink or inkjet ink set as defined herein printed thereon. Preferably, the substrate is polypropylene or corona-treated polystyrene.

The present invention also provides a method of inkjet printing comprising jetting the ink of the present invention onto a substrate and curing the ink by exposing the ink to actinic radiation. The substrate is preferably as defined hereinabove.

Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto the substrate, on a roll-to-roll printer or a flat-bed printer. 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.

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

Examples

Example 1

Four inks were prepared. The inks had formulations as shown in Table 1.

Table 1. Ink formulations.

Component	InkA	Ink B	InkC	Ink D
IBOA	40.00	40	40.00	40.00
NVC	16.50	16.50	16.50	16.50
PEA	14.18	19.18	14.18	14.18
UV12	0.50	0.50	0.50	0.50
BDT4330	5.00	-	-	-
DPGDA	-	-	5.00	-
TCDDMDA	-	-	-	5.00
BR113	2.40	2.40	2.40	2.40
ITX	0.80	0.80	0.80	0.80
EDB	0.85	0.85	0.85	0.85
BP	2.88	2.88	2.88	2.88
IRG184	1.88	1.88	1.88	1.88
TPO	8.01	8.01	8.01	8.01
Cyan pigment dispersion	6.00	6.00	6.00	6.00
BYK307	1.00	1.00	1.00	1.00
Total	100.00	100.00	100.00	100.00

UV12 is a stabiliser. BR113 is a passive resin. ITX, EDB, BP, IRG184 and TPO are photoinitiators. BYK307 is a surfactant.

The cyan pigment dispersion of the inks of Table 1 comprises 59% PEA, 1% stabiliser, 10% dispersant and 30% blue pigment. The dispersion was prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns. Amounts are given as weight percentages based on the total weight of the 10 dispersion.

The inks were drawn down onto a substrate using a K-barto a thickness of 12 pm.

Blocking conditions were simulated using the dose and intensity of the last pass of a multi-pass 15 printer since the last cure pass generally governs blocking performance. The dose and intensity settings used for the drawdowns are set out in Table 2.

Table 2. Dose and intensity settings

	Total UVA/B/C/V
Dose (mJ/cm^z)	91
Intensity (mW/cnf)	1990

Adhesion conditions were simulated using the dose and intensity of the total dose of a multi-pass printer. The full cure conditions generally govern adhesion performance. The dose and intensity settings used for the drawdowns are set out in Table 3.

Table 3. Dose and intensity settings

	Total UVA/B/C/V
Dose (mJ/cm^z)	873.6
Intensity (mW/cnf)	1990

The blocking results are set out in Table 4 and the adhesion results are set out in Table 5. Unacceptably poor results are highlighted by underlining.

Table 4. Blocking results

Blocking (noise/offset)
Face-face noise	3	1	2	3
Face-face offset	5	1	3	5
Face-back noise	3	1	2	3
Face-face offset	4	1	3	3

Table 5. Adhesion results

Iso tape cross hatch adhesion
3 mm fluted polypropylene adhesion	5	5	5	5
0.08 (2.03 mm) screen corona treated styrene adhesion	5	5	4	2

Ink A (invention, 5% BDT-4330) shows good blocking and good adhesion. Ink B (comparative, no multifunctional material) shows extremely poor blocking but good adhesion. Ink C (comparative, 5% DPGDA) shows poor blocking but good adhesion. Ink D (comparative, 5% TCDDMDA) shows good blocking but reduced adhesion. The results show that ink A demonstrates blocking improvement whilst maintaining adhesion over that of the inks containing DPGDA and TCDDMDA.

Claims

1. An inkjet ink comprising: one or more monofunctional (meth)acrylate monomers; an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer; a passive resin; 1-20% by weight of a radiation-curable hyperbranched acrylate oligomer having a functionality of 25-35 and a viscosity of 1-10 Pas at 50°C, based on the total weight of the ink; a dispersed pigment; and a radical photoinitiator, where the ink has a viscosity of less than 100 mPas at 25°C.

2. An inkjet ink as claimed in claim 1, wherein the ink contains less than 10% by weight in total of difunctional and multifunctional monomers, based on the total weight of the ink.

3. An inkjet ink as claimed in claim 1 or 2, wherein the only monomers present are the one or more monofunctional (meth)acrylate monomers; and the N-vinyl amide monomer, the N-acryloyl amine monomer and/or the N-vinyl carbamate monomer.

4. An inkjet ink as claimed in any preceding claim, wherein the ink contains less than 5% by weight combined of water and volatile organic solvents, based on the total weight of the ink.

5. An inkjet ink as claimed in any preceding claim, wherein the ink contains 30-80% by weight of the one or more monofunctional (meth)acrylate monomers in total, based on the total weight of the ink.

6. An inkjet ink as claimed in any preceding claim, wherein the ink contains at least 30% by weight of IBOA, based on the total weight of the ink.

7. An inkjet ink as claimed in any preceding claim, wherein the ink contains 5-30% by weight of PEA, based on the total weight of the ink.

8. An inkjet ink as claimed in any preceding claim, wherein the ink contains 5-20% by weight of the N-vinyl amide monomer, the N-acryloyl amine monomer and/or the N-vinyl carbamate monomer, based on the total weight of the ink.

9. A method of inkjet printing comprising jetting the ink as claimed in any preceding claim onto a substrate and curing the ink by exposing the ink to actinic radiation.

10. A method as claimed in claim 9, wherein the substrate is a polypropylene or a corona-treated polystyrene substrate.