GB2520595A

GB2520595A - Printing method

Info

Publication number: GB2520595A
Application number: GB1416417.2A
Authority: GB
Inventors: Nigel Gould; Barry Mcgregor
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 2013-09-17
Filing date: 2014-09-17
Publication date: 2015-05-27
Anticipated expiration: 2034-09-17
Also published as: GB201316523D0; GB201416417D0; GB2520595B

Abstract

A method of ink jet printing comprises: providing a multi chromatic inkjet ink set; jetting the ink onto a substrate; applying a first dose of radiation to pin the ink on the substrate; and exposing the pinned ink to a second, higher dose of radiation to form a fully cured image; wherein each ink in the set includes a radiation curable monomer and/or oligomer; a photoinitiator, and a pigment; and wherein, at the wavelength of the first dose of radiation, at least one ink has a pigment with a higher radiation absorption and a photo initiator which absorbs more strongly compared to the pigment and photo initiator in at least one other ink and/or at least one ink has a pigment with a lower radiation absorption and a photo initiator which absorbs less strongly compared to the pigment and photo initiator in at least one other ink; so that the pinning responses of the inks are consistent. The inks may include a solvent which is dried between the two doses of radiation. The radiation doses are preferably supplied by LEDs or a mercury lamp and the radiation is preferably UV. A multi chromatic ink jet set and individually coloured inks are also disclosed.

Description

Printing method The present invention relates to a printing method, and particularly to a method for improving the pinning response of inkjet ink sets.

The image quality of radiation-curable inkjet inks can be controlled by arresting the flow of deposited ink droplets by exposing them to a low dose of actinic radiation, typically UV radiation, immediately after jetting but prior to full cure of the ink. This is termed "pinning".

The dose of radiation is sufficient to increase the viscosity of composition preventing flow into non-image areas and bleed between colours but does not fully cure the composition. After pinning, the inks are then exposed to secondary cure source of higher power which completes the cure process.

Pinning is often used in single-pass radiation-curable inkjet systems where the inks are deposited from a stationary head onto the substrate which is transported under the head. The image is commonly pinned using UV LED lamps, the level of pinning being critical in controlling the quality of the image produced. Whilst under-pinning results in inter-colour bleed, over-pinning may also result in poor image quality. Too little drop spread results in reduced colour density with the substrate remaining exposed. Over-pinning can also result in low gloss in areas of higher ink deposit through the generation of a roughed surface thereby reducing specular reflection.

In solvent/radiation-curable hybrid inks, LEDs can also be used to pin ink droplets before the solvent is removed giving improved control of the ink drop spread. This is particularly beneficial when the hybrid inks are being printed on less receptive substrates, where the normal pinning mechanisms are hindered by the nature of the substrate. This occurs where solvent absorption by the substrate is reduced through poor solvency of the substrate, or where the evaporation the rate of the solvent is slowed owing to the substrate's high heat capacity preventing the substrate from reaching a sufficiently high temperature to evaporate the solvent.

As with conventional radiation-curable inkjet inks, under pinning results in poor print quality through inter-colour bleed, but over-pinning is also problematic. If the droplet is pinned to too great an extent, solvent can become trapped in the matrix. This trapped solvent both reduces the gloss of the ink film and is detrimental to the physical properties of the film, such as scratch and solvent resistance.

There remains a need in the art for inkjet inks and/or printing methods which reduce the adverse effects of under-and over-pinning.

Accordingly, the present invention provides a method of inkjet printing comprising the following steps, in order: (i) providing a multi-chromatic inkjet ink set, wherein each ink in the set comprises a radiation-curable monomer and/or oligomer, a photoinitiator package, a dispersed pigment and optionally a solvent; (ii) jetting the inkjet ink set onto a substrate; (iU) exposing the printed inkjet ink set to a first dose of actinic radiation to pin the jetted ink on the substrate; (iv) when the ink contains a solvent, drying the ink to remove the solvent; and (v) exposing the pinned inkjet ink set to a second, higher, dose of actinic radiation to form a fully cured image; wherein at least one of the inks in the inkjet ink set has a pigment with a higher radiation absorption at the wavelength of the first dose of radiation compared to the pigment in at least one other ink in the inkjet ink set and contains a photoinitiator package which absorbs more strongly at the wavelength of the first dose of radiation than that of the at least one other ink in the inkjet ink set and/or at least one of the inks in the inkjet ink set has a pigment with a lower radiation absorption at the wavelength of the first dose of radiation compared to the pigment in at least one other ink in the inkjet ink set and contains a photoinitiator package which absorbs less strongly at the wavelength of the first dose of radiation than that of the at least one other ink in the inkjet ink set, in order that the inks in the inkjet ink set have a consistent pinning response.

It has been found that in both types of inks, radiation-curable and hybrid inks, the pinning process is complicated by the fact that the different ink colours in the inkjet ink set (e.g. cyan, magenta, yellow and black) respond differently to the pinning dose of radiation applied. The colour orderfor reactivity towards the UV LED pinning lamps most often used, 395 or 385 nm, has been found to be magenta, cyan, yellow and black, with the black being the lowest reactivity. This is particularly problematic when the colours are all pinned concurrently with the same LED source.

This has been addressed by the present invention by balancing the reactivity of the colour set towards the pinning radiation, such as UV light generated from 385 and 395 nm LED sources.

Thus, by careful selection of the photoinitiator blend it is possible to balance the relative cure speed of the inkjet ink set. This is particularly advantageous when the cure sources used for the pinning and final cure stages of the printing process have differing wavelength outputs as this gives the greatest formulation latitude For example a 385/395 nm LED UV source for pinning and a mercury lamp for final cure.

The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a schematic representation of a printhead for use with the present invention; Fig. 2 shows the relative bleed between differential colour areas for a cyan standard ink set (upper image) and an ink set of the present invention (lower image); Fig. 3 shows the relative bleed between differential colour areas for a magenta standard ink set (upper image) and an ink set of the present invention (lower image); Fig. 4 shows the relative bleed between differential colour areas for a yellow standard ink set (upper image) and an ink set of the present invention (lower image); and Fig. 5 shows the relative bleed between differential colour areas for a black standard ink set (upper image) and an ink set of the present invention (lower image).

The inks of the present invention comprise a radiation-curable oligomer and/or monomer and a photoinitiator package so that polymers can be formed in the dried ink film.

By "radiation-curable" is meant a material that polymerises or crosslinks when exposed to actinic radiation, commonly ultraviolet light, in the presence of a photoinitiator.

The radiation-curable material may include a radiation-curable oligomer, either alone or as a mixture with a radiation-curable monomer. The monomers/oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tn and higher functionality monomers/oligomers may be used.

Radiation-curable oligomers suitable for use in the present invention comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The oligomer preferably comprises a urethane backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation.

Preferably the oligomers are (meth)acrylate oligomers. Preferably they are multifunctional and most preferably have a functionality of 2-6.

Particularly preferred radiation-curable oligomers are urethane acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are tn-, tetra-, penta-or hexa-functional urethane acrylates, particularly hexafunctional urethane acrylates as these yield films with good solvent resistance.

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.

Oligomers typically have a molecular weight of 600 to 4,000. 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.

In one embodiment the radiation-curable oligomer polymerises by free-radical polymerisation.

The radiation-curable oligomer used in the ink of the invention cures upon exposure to radiation in the presence of a photoinitiatorto form a crosslinked, solid film. The resulting film has good adhesion to substrates and good solvent resistance. Any radiation-curable oligomer that is compatible with the remaining ink components and that is capable of curing to form a crosslinked, solid film is suitable for use in the ink of the present invention. Thus, the ink formulator is able to select from a wide range of suitable oligomers. In particular, the oligomer can be a low molecular weight material that is in liquid form at 25°C. This is beneficial when aiming to produce a low viscosity ink. Furthermore, the use of a low molecular weight, liquid oligomer is advantageous when formulating the ink because low molecular weight liquid oligomers are likely to be miscible in a wide range of solvents.

Preferred oligomers for use in the invention are dependent on the type of ink being used. For solvent/radiation-curable hybrid inks, the oligomers preferably have a viscosity of 0.5 to 20 Pas at 60°C, more preferably 5 to 15 Pas at 60°C and most preferably 5 to 10 Pas at 60°C.

Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A.

Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25 seconds1.

The total amount of the radiation-curable oligomer present in such ink is preferably 5 to 30% by weight based on the total weight of the ink, more preferably 10 to 25% by weight, and most preferably 15% to 20% by weight.

For solely radiation-curable inks, i.e. those which are substantially free of volatile organic solvents, the oligomers preferably have a viscosity of 0.5 to 10 Pas at 50°C, more preferably.

Again, oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A.

The total amount of the radiation-curable oligomer present in such ink is preferably 0.1 to 10% by weight based on the total weight of the ink.

The inks may also contain radiation-curable monomers. Suitable free-radical polymerisable monomers are well known in the art and include (meth)acrylates, aj3-unsaturated ethers, vinyl amides and mixtures thereof Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Preferred examples include phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 2-(2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate (aDA), tridecyl acrylate (TDA), isodecyl acrylate (IDA) and lauryl acrylate.

Suitable multifunctional (meth)acrylate monomers include di-, tn-and tetra-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the ink-jet inks include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate), dipropylene glycol diacrylate, tri(propylene glycol) triacrylate, neopentyl glycol diacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate, and mixtures thereof Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate. Mixtures of (meth)acrylates may also be used.

(Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and multifunctional are also intended to have their standard meanings, i.e. one and two or more groups, respectively, which take part in the polymerisation reaction on curing.

a43-Unsaturated ether monomers 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, I,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether. Mixtures of a,p-unsaturated ether monomers may be used.

N-Vinyl amides and N-(meth)acryloyl amines may also be used in the ink of the invention. 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).

Monomers typically have a molecular weight of less than 600.

The total amount of the radiation-curable oligomer and radiation-curable monomer present in the ink is 10 to 65% by weight based on the total weight of the ink.

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

The radiation-curable material can also comprise a combination of free-radical polymerisable and cationically polymerisable materials.

The inks may also contains 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 100-1 50 KDa, as determined by GPC with polystyrene standards. A particularly preferred resin is Paraloid® All from Rohm and Haas. The resin is present at 1- 5% by weight, based on the total weight of the ink.

The inks may be solely radiation-curable inks, in which case they contain curable monomers and/or oligomers, as previously described. Alternatively, they may be hybrid solvent/radiation-curable inks.

In the case of solely radiation-curable inks, often termed "UV inks", the ink cures ("dries") by the polymerisation of the monomers and/or oligomers present. Accordingly, a solvent is not required. In such inks, the inks are preferably substantially free of water and volatile organic solvents, although some water will typically be absorbed by the ink from the air or volatile organic solvent may be present in the components of the inks (e.g. in the pigment dispersion), and such levels are tolerated. For example, the ink may comprise less than 10% by weight of water and volatile organic solvents (in total), more preferably less than 5% by weight of water and most preferably less than 2% by weight of water, based on the total weight of the ink.

In the case of hybrid inks, the inks of the invention additionally contain an organic solvent.

The organic solvent is in the form of a liquid at ambient temperatures and is capable of acting as a carrier for the remaining components of the ink. The organic solvent component of the ink may be a single solvent or a mixture of two or more solvents. As with known solvent-based inkjet inks, the organic solvent used in the ink of the present invention is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry. The solvent can be selected from any solvent commonly used in the printing industry, such as glycol ethers, glycol ether esters, alcohols, ketones, esters, organic carbonates, lactones and pyrrolidones.

The organic solvent is present in an amount of at least 30% by weight, preferably at least 50% by weight, and most preferably at least 60% by weight based on the total weight of the ink. The upper limit is typically 85% or 75% by weight based on the total weight of the ink.

In a preferred embodiment the organic solvent is a low toxicity and/or a low odour solvent.

Solvents that have been given VOC exempt status by the United States Environmental Protection Agency or European Council are also preferred.

The most preferred solvents are selected from glycol ethers and organic carbonates and mixtures thereof. Cyclic carbonates such as propylene carbonate and mixtures of propylene carbonate and one or more glycol ethers are particularly preferred.

Alternative preferred solvents include lactones, which have been found to improve adhesion of the ink to PVC substrates. Mixtures of lactones and one or more glycol ethers, and mixtures of lactones, one or more glycol ethers and one or more organic carbonates are particularly preferred. Mixtures of gamma butyrolactone and one or more glycol ethers, and mixtures of gamma butyrolactone, one or more glycol ethers and propylene carbonate are particularly preferred.

In another embodiment of the invention, dibasic esters and/or bio-solvents may be used.

Dibasic esters are known solvents in the art. They can be described as di(C1-C4 alkyl) esters of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms having following general formula: R1 O11A)OR2 in which A represents (CH2)1.6, and R' and R2 may be the same or different and represent C1-C4 alkyl which may be a linear or branched alkyl radical having 1 to 4 carbon atoms, preferably methyl or ethyl, and most preferably methyl. Mixtures of dibasic esters can be used.

Bio-solvents, or solvent replacements from biological sources, have the potential to reduce dramatically the amount of environmentally-polluting VOCs released in to the atmosphere and have the further advantage that they are sustainable. Moreover, new methods of production of bio-solvents derived from biological feedstocks are being discovered, which allow bio-solvent production at lower cost and higher purity.

Examples of bio-solvents include soy methyl ester, lactate esters, polyhydroxyalkanoates, terpenes and non-linear alcohols, and D-limonene. Soy methyl ester is prepared from soy.

The fatty acid ester is produced by esterification of soy oil with methanol. Lactate esters preferably use fermentation-derived lactic acid which is reacted with methanol and/or ethanol to produce the ester. An example is ethyl lactate which is derived from corn (a renewable source) and is approved by the FDA for use as a food additive. Polyhydroxyalkanoates are linear polyesters which are derived from fermentation of sugars or lipids. Terpenes and non-linear alcohols may be derived from corn cobs/rice hulls. An example is D-limonene which may be extracted from citrus rinds.

The ink is preferably substantially free of water, although some water will typically be absorbed by the ink from the air or be present as impurities in the components of the inks, and such levels are tolerated. For example, the ink may comprise less than 5% by weight of water, more preferably less than 2% by weight of water and most preferably less than 1% by weight of water, based on the total weight of the ink.

The inks of the present invention also contain a dispersed/dispersible pigment. of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF plc), 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 pigment is preferably present in an amount of 20% by weight or less, preferably 10% by weight or less, more preferably 8% by weight or less and most preferably 2 to 5% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, however, for example up to and including 30% by weight, or 25% by weight based on the total weight of the ink.

The inks are provided 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). The inks in a trichromatic set can be used to produce a wide range of colours and tones.

However, when a multi-chromatic inkjet ink set is pinned, the inks tend to behave differently on account of their differing radiation absorptions. This is a function of the pigment being used, in that some pigments absorb more strongly than others at the wavelength of the radiation being used to pin the inks.

This is addressed by the inks of the present invention. In any set there will be multiple colours and hence the inks will have different radiation absorptions, i.e. at least one of the inks in the inkjet ink set will have a pigment with a higher radiation absorption than the pigment in at least one other ink. Similarly, at least one of the inks in the inkjet ink will have a pigment with a lower radiation absorption than the pigment in at least one other ink. The inks of the present invention modify the initiator package to smooth out these differences and provide a more consistent pinning response. This means that the dot spread will be essentially the same for all of the inks leading to a better printed image.

The radiation absorption of the ink may be determined as follows. Place 0.3 g of the ink into a centrifuge tube, add 4 mL of acetonitrile and shake to disperse the ink. Centrifuge for 10 minutes at 10,000-30,000 rpm to separate the sample into supernatant liquid and solid pigment. Discard the supernatant liquid and repeat the process twice more. Take the resulting solid and disperse into acetonitrile, the exact concentration being required being assessed via a series of iterative steps until an adequate absorption signal was obtained. The prepared sample is then scanned from 200 nm to 450 nm to assess the key absorption wavelength for the pigment. Any suitable UVNis spectrophotometer can be used, e.g. a lambda 40 from Perkin Elmer.

In one embodiment, the at least one ink in the inkjet ink set having a pigment with a higher radiation absorption at the wavelength of the first dose of radiation is black and/or yellow and the at least one ink having a pigment with a lower radiation absorption at the wavelength of the first dose of radiation is cyan and/or magenta.

The inks of the invention thus include a photoinitiator package. When the ink of the invention includes a free-radical polymerisable material the photoinitiator package includes a free-radical photoinitiator and when the inks include a cationic polymerisable material the photoinitiator package includes a cationic photoinitiator. When the inks comprise a combination of free-radical polymerisable and cationically polymerisable materials both a free-radical and cationic initiator are required. The inks are preferably free-radical polymerisable.

The free-radical photoinitiator can be selected from those known in the art, with the proviso that the absorptions are tailored for the radiation absorptions of the given inks. A high radiation absorption results in increased absorption of radiation by the dispersed pigment and therefore such inks require an enhanced photoinitiator package. Conversely, a low radiation absorption results in reduced absorption of radiation by the dispersed pigment and therefore such inks require an attenuated photoinitiator package in order to bring the pinning response into line with the other inks. The nature of the photoinitiator package will also depend on the wavelength of the pinning radiation. The absorption must be enhanced or attenuated for the wavelength of the pinning radiation. For a CMYK set, photoinitiator packages preferably absorb at the wavelength of the pinning radiation in the artier: black > yellow > cyan > magenta.

By way of an example, a pinning radiation often used is an LED having an output at 385 and 395 nm. Acyl phosphine oxide photoinitiators absorb strongly at these wavelengths. The most photoinitiator for use at this wavelength is bis acyl phosphine oxide (available as Irgacure 819).

The sensitivity to the UV output from such LEDs can be further enhanced by addition of a thioxanthone photoinitiator. Non limiting examples include ispopropyl thioxanthone (ITX), 1-chloro-4-propoxythioxanthone (CPTX), 2-chlorothoxanthone (CTX) and diethyl thioxanthone (DETX). Polymeric thioxanthones, such as Speedcure 7010 from Lambsons, are also

suitable.

This approach works well with black and yellow inks which have a high radiation absorption at 385/395 nm and so require an enhanced photoinitiator package.

For some colours, in particular, cyan and magenta, it may be necessary to reduce the sensitivity to the LED output in order to balance speed with the less reactive inks in the four colour set. To reduce the sensitivity to 385 and 395 UV LED sources, a photoinitiator that does not absorb in this range should be used as the photoinitiator. Any photoinitiators that do not absorb in the output region are suitable, but most preferred are ci-hydroxy ketones. Non-limiting examples include Irgacure 184, Darocure 1173 and Irgacure 127.

Thus, the cyan and magenta inks have a photoinitiator package which absorbs less strongly at the wavelength of the pinning radiation than the photoinitiator package of the yellow and black inks. As exemplified, the photoinitiator package of the black and yellow inks contain an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and the photoinitiator package of the cyan and magenta inks contain an a-hydroxy ketone photo initiator.

The absorption of the photoinitiator package may be determined by measuring the absorption spectrum of the photoinitiator package in a suitable solvent, such as acetonitrile, and noting the wavelength of the absorption maxima. That maxima can then be used to assess the suitability of the photoinitiator package for either attenuation or enhancement of the absorption at the given wavelength, i.e. the wavelength of the pinning radiation.

Wavelength!photoinitiator combinations may also be obtained directly from readily available commercial literature.

Preferably the photoinitiator is present in an amount of 1 to 20% by weight, preferably 4 to 15% by weight, based on the total weight of the ink.

Thus, the present invention also provides a multi-chromatic ink]et ink set, wherein each ink in the set comprises a radiation-curable monomer and/or oligomer, a photoinitiator package, a dispersed pigment and optionally a solvent; wherein the pigments in the inks in the inkjet ink set have different radiation absorptions at a given wavelength of radiation and at least one ink in the inkjet ink set has a pigment with a higher radiation absorption than the pigment in at least one other ink and contains a photoinitiator package which absorbs more strongly at the given wavelength than the at least one other ink in the inkjet ink set and/or at least one ink in the inkjet ink set has a pigment with a lower radiation absorption than the pigment in at least one other ink and contains a photoinitiator package which absorbs more strongly at the given wavelength than the at least one other ink in the inkjet ink set. The given radiation will be the radiation used for pinning the inks.

In a preferred embodiment, the multi-chromatic inkjet ink set is a CMYK set and the cyan and magenta inks have a photoinitiator package which absorbs less strongly at the given wavelength of radiation than the photoinitiator package of the yellow and black inks.

Preferably, photoinitiator packages absorb at the given wavelength of radiation in the order black> yellow > cyan > magenta. As an example, the photoinitiator package of the black and yellow inks contain an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and the photoinitiator package of the cyan and magenta inks contain an a-hydroxy ketone photoinitiator.

The individual inks in the inkjet ink set are also characteristic of the present invention because they specifically combine the pigment having the above-described radiation absorption together with a photoinitiator package specifically provided for that radiation absorption.

Accordingly, the present invention also provides the following inks: a black inkjet ink comprising a radiation-curable monomer and/or oligomer, a photoinitiator package containing an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and a dispersed black pigment; a yellow inkjet ink comprising a radiation-curable monomer and/or oligomer, a photoinitiator package containing an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and a dispersed yellow pigment; a cyan inkjet ink comprising a radiation-curable monomer andlor oligomer, a photoinitiator package containing an ci-hydroxy ketone photoinitiator and a dispersed cyan pigment; and a magenta inkjet ink comprising a radiation-curable monomer and/or oligomer, a photoinitiator package containing an ci-hydroxy ketone photoinitiator and a dispersed magenta pigment.

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

In order to produce a high quality printed image a small jetted drop size is desirable.

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

To achieve compatibility with print heads that are capable of jetting drop sizes of 50 picolitres or less, a low viscosity ink is required. A viscosity of 15 mPas or less at 25°C is preferred, for example, 2 to 12 mPas, 8 to 11 mPas, or 10 to 11 mPas.

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

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

In one aspect of the invention the surface tension of the ink is controlled by the addition of one or more surface active materials such as commercially available surfactants. 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 20-32 mNm1 and more preferably 21-27 mNm1.

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

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

The printing is performed by inkjet printing using the following steps.

The inks are jetted onto a substrate. The inkjet printer is typically a roll-to-roll printer or flat-bed printer. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene and polypropylene.

The inks are then exposed to a first dose of actinic radiation to pin the jelled ink on the substrate. Any suitable source of actinic radiation may be used for providing the first (pinning) dose of radiation of the inkjet ink, but it is preferably an LED. The dose of actinic radiation is lower than the dose required to cure the ink fully, and is typically 1-200 mJ/cm2, preferably 1-mJ/cm2, more preferably 1-50 mJ/cm2 and most preferably about 35 mJ/cm2. The wavelength of the pinning source is typically 200-700 nm, preferably 300-500 nm and most preferably 350-450 nm. In one embodiment, the source has a wavelength of 385 and 395 nm. Preferably, the radiation source for the first dose of actinic radiation (i.e. the pinning radiation) is an LED It is preferable to arrest the flow of the ink by pinning the ink droplets quickly after they have impacted on the substrate surface. To achieve a good quality image it is preferable that the inks are pinned within 5 seconds of impact, preferably within 1 second and most preferably within 0.5 seconds. As a result of the pinning, the viscosity of the ink is increased by polymerisation and/or crosslinking of the radiation-curable oligomer and/or monomer thereby arresting the flow of the ink and improving the final image quality.

After pinning, the pinned ink is exposed to a second, higher, dose of actinic radiation to form a fully cured image. That is, an additional dose of radiation to that required for pinning. Again, any suitable radiation source may be used. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LED5), flash lamps and combinations thereof.

When LEDs are used, these are preferably provided as an array of multiple LEDs. Preferably, the radiation source for the second dose of actinic radiation (i.e. the radiation for the final cure) is a medium-pressure mercury lamp.

The dose required to achieve the final cure will be higher than the pinning dose. The dose provided results in the formation of a solid film. 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. The delay between pinning and providing a final cure of the ink is less critical than the initial pinning of the ink, but is typically at least 1 minute after jetting.

Usually, the wavelengths of the first and second doses of actinic radiation are different.

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

When the ink contains a solvent, the solvent must be evaporated afterjetting. Evaporation of the solvent, when present, can occur simply by exposure of the inks to the atmosphere, but the inks may also be heated to accelerate evaporation. Evaporation of the solvent occurs after pinning. It may be any stage after pinning, but is preferable prior to final cure.

It should be noted that the terms "dry" and "cure" are often used interchangeably in the art when referring to radiation-curable inkjet inks to mean the conversion of the inkjet ink from a liquid to solid by polymerisation and/or crosslinking of the radiation-curable material. Herein, however, by "drying" is meant the removal of the solvent by evaporation and by "curing" is meant the polymerisation and/or crosslinking of the radiation-curable material.

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

Examples

Examrle 1 lnkjet inks were prepared according to the formulations set out in Tables I and 2. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1. Standard hybrid UV/solvent inkjet ink set 1 (comparative) Component Cyan Magenta fellow Black InklA lnklB lnklC lnklD Gamma butyrolactone 16.3 16.0 16.0 16.4 (solvent) Diethylene glycol diethyl ether 50.4 47.0 17.9 49.4 (solvent) Genomer42ls 10.5 6.7 7.0 9.9 (oligome r) Nippon Gohsei UV7630B 10.2 12.5 12.5 10.2 (oligome r) Cyan pigment dispersion 6.0. -(pig ment) Magenta pigment dispersion -11.2 -(pig ment) Yellow pigment dispersion -. 10.0 -(pig ment) Black pigment dispersion -. . 7.5 (pigment) Irgacure 819 4.0 4.0 1.0 4.0 (photoinitiator) Irgacure 2959 2.0 2.0 2.0 2.0 (photoinitiator) Byk 331 0.1 0.1 0.1 0.1 (surfactant) UV12 0.5 0.5 0.5 0.5 (stabiliser) Total 100.0 100.0 100.0 100.0 Genomer 4215 is a difunctional urethane acrylate with a viscosity of 15 Pa.s at 60°C. Nippon Gohsei UV7630B is a hexafunctional urethane acrylate oligomer with a viscosity of 6.9 Fa.s at 60°C. BYK 331 is a polyether-modified polydimethylsiloxane and reduces surface tension.

Table 2. Hybrid UV/solvent inkjet ink set 2 with adjusted LED pin response (invention) Component Cyan Magenta (ellow Black Ink 2A Ink 2B Ink 2C Ink 2D Gamma butyrolactone 16.3 16.0 16.0 16.4 (solvent) Diethylene glycol diethyl ether 50.4 47.0 16.9 46.4 (solvent) Genomer42l5 10.5 6.7 7.0 9.9 (oligome r) Nippon Gohsei UV7630B 10.2 12.5 12.5 10.2 (oligome r) Cyan pigment dispersion 6.0 -(pig ment) Magenta pigment dispersion -11.2 -(pig ment) Yellow pigment dispersion -. 10.0 -(pig ment) Black pigment dispersion -. 7.5 (pigment) Irgacure 819 --1.0 4.0 (photoinitiator) Irgacure 184 4.0 1.0 - (photoinitiator) Isopropyl thioxanthone -. 1.0 1.0 (sensitiseo Irgacure 2959 2.0 2.0 2.0 2.0 (photoinitiator) Byk 331 0.1 0.1 0.1 0.1 (surfactant) UV12 0.5 0.5 0.5 0.5 (stabiliser) 100.0 100.0 100.0 100.0 The output of a 395 nm LED is principally in a narrow band either side of the maxima at 395 nm. Irgacure 819 absorbs strongly in this region, but the amount of UV energy that is available to the photoinitiator is strongly influenced by the absorption of the pigment in this region. Cyan and magenta are both relatively transparent to UV light of 395 nm; however, both yellow and black pigments absorb strongly in this region. It has been found possible to increase the sensitivity of the ink set 1 above to 395/385 nm LED light by the addition of isopropyl thioxanthone (ITX). Whilst not wishing to be bound by theory it is believed that the ITX is acting as a sensitiserto enhance the reactivity towards the available UV light.

Conversely it has been found that replacing the Irgacure 819 in ink set 1 with a photoinitiator that only weakly absorbs around 385/395 nm, i.e. Irgacure 184, it is possible to reduce the pinning response.

Example 2

The inks of both sets were printed on a hybrid solventfUV printer fitted with Ricoh generation 4 printheads. As shown in Fig. 1, the machine was modified with two 385 nm LED lamps fitted either side of the printhead, generating 5 mJ/cm2 per pass.

The print mode used was 600x600 four-pass bi-directional at high speed (approximately 16 m2Ih). The platen heaters were set at 40°C140°C/56°C (minimum setting). The substrate used was Avery MFI 3500 self-adhesive vinyl.

For each print set the relative bleed between differential colour areas was assessed and the results are shown in Figs. 2 to 5. Increased ink bleed is seen with the example set 2 images for cyan and magenta and reduced ink bleed is seen with the example set 2 images for yellow and black.

It has been shown to be possible to adjust the response of the individual colours to the LED pin lamp. Here, the pin speed of the yellow and black were increased by the addition of 1% ITX whilst the pin speed of the magenta and cyan can be retarded by replacement or partial replacement of the Irgacure 819 with Irgacure 184. Using this approach, it was possible to match the pin response between the colours allowing selection of an appropriate power LED to achieve equivalent pinning for all colours The exact degree of photoinitiator package modification will depend on several factors, namely the power of the LED lamp, the relative radiation absorption of the colours and the pigment selected in each case. However, the experimental results clearly show that using the strategy of the present invention, it is possible to adjust the reactivity between colours by photoinitiator selection to achieve an ink set with a balanced pin response.

It should be noted that the response to the final cure remains relatively unaffected because of its different spectral output. Hence, the changes are not detrimental to the final cure response with the medium pressure mercury lamp and have no adverse effects on the final film properties.

Claims

Claims 1. A method of inkjet printing comprising the following steps, in order: (i) providing a multi-chromatic inkjet ink set, wherein each ink in the set comprises a radiation-curable monomer and/or oligomer, a photoinitiator package, a dispersed pigment and optionally a solvent; (ii) jetting the inkjet ink set onto a substrate; (iii) exposing the printed inkjet ink set to a first dose of actinic radiation to pin the jetted ink on the substrate; (iv) when the ink contains a solvent, drying the ink to remove the solvent; and (v) exposing the pinned inkjet ink set to a second, higher, dose of actinic radiation to form a fully cured image; wherein at least one of the inks in the inkjet ink set has a pigment with a higher radiation absorption at the wavelength of the first dose of radiation compared to the pigment in at least one other ink in the inkjet ink set and contains a photoinitiator package which absorbs more strongly at the wavelength of the first dose of radiation than that of the at least one other ink in the inkjet ink set and/or at least one of the inks in the inkjet ink set has a pigment with a lower radiation absorption at the wavelength of the first dose of radiation compared to the pigment in at least one other ink in the inkjet ink set and contains a photoinitiator package which absorbs less strongly at the wavelength of the first dose of radiation than that of the at least one other ink in the inkjet ink set, in order that the inks in the inkjet ink set have a consistent pinning response.
2. A method as claimed in claim 1, wherein the wavelengths of the first and second doses of actinic radiation are different.
3. A method as claimed in claim 1 or 2, wherein the radiation source for the first dose of actinic radiation is an LED.
4. A method as claimed in preceding claim, wherein the radiation source for the second dose of actinic radiation is a medium-pressure mercury lamp.
5. A method as claimed in preceding claim, wherein the wavelength of the first dose of actinic radiation is 385/395 nm.
6. A method as claimed in preceding claim, wherein the multi-chromatic inkjet ink set is a CMYK set and the cyan and magenta inks have a photoinitiator package which absorbs less strongly at the wavelength of the first dose of radiation than the photoinitiator package of the yellow and black inks.
7. A method as claimed in claim 6, wherein the multi-chromatic inkjet ink set is a CMYK set and the photoinitiator package of the black and yellow inks contain an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and the photoinitiator package of the cyan and magenta inks contain an a-hydroxy ketone photoinitiator.
8. A method as claimed in claim 6 or 7, wherein the multi-chromatic ink]et ink set is a CMYK set and the photoinitiator packages absorb at the wavelength of the first dose of radiation in the order: black> yellow> cyan > magenta.
9. A multi-chromatic inkjet ink set, wherein each ink in the set comprises a radiation-curable monomer and/or oligomer, a photoinitiator package, a dispersed pigment and optionally a solvent; wherein the pigments in the inks in the inkjet ink set have different radiation absorptions at a given wavelength of radiation and at least one ink in the inkjet ink set has a pigment with a higher radiation absorption than the pigment in at least one other ink and contains a photoinitiator package which absorbs more strongly at the given wavelength than the at least one other ink in the inkjet ink set and/or at least one ink in the inkjet ink set has a pigment with a lower radiation absorption than the pigment in at least one other ink and contains a photoinitiator package which absorbs more strongly at the given wavelength than the at least one other ink in the inkjet ink set.
10. An inkjet ink set as claimed in claim 9, wherein the multi-chromatic inkjet ink set is a CMYK set and the cyan and magenta inks have a photoinitiator package which absorbs less strongly at the given wavelength of radiation than the photoinitiator package of the yellow and black inks.
11. An inkjet ink set as claimed in claim 9 or 10, wherein the multi-chromatic inkjet ink set is a OMYK set and the photoinitiator packages absorb at the given wavelength of radiation in the order: black> yellow> cyan > magenta.
12. An inkjet ink set as claimed in any of claims 9 to 11, wherein the photoinitiator package of the black and yellow inks contain an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and the photoinitiator package of the cyan and magenta inks contain an o-hydroxy ketone photoinitiator.
13. A black inkjet ink comprising a radiation-curable monomer and/or oligomer, a photoinitiator package containing an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and a dispersed black pigment.
14. A yellow inkjet ink comprising a radiation-curable monomer and/or oligomer, a photoinitiator package containing an acyl phosphine oxide photoinitiator and optionally a thioxanthone sensitiser and a dispersed yellow pigment.
15. A cyan inkjet ink comprising a radiation-curable monomer and/or oligomer, a photoinitiator package containing an ci-hydroxy ketone photoinitiator and a dispersed cyan pigment.
16. A magenta inkjet ink comprising a radiation-curable monomer and/or oligomer, a photoinitiator package containing an ci-hydroxy ketone photoinitiator and a dispersed magenta pigment.