WO2018138492A1

WO2018138492A1 - Printing method and apparatus

Info

Publication number: WO2018138492A1
Application number: PCT/GB2018/050197
Authority: WO
Inventors: Matthew KITE; Kevin Burns
Original assignee: Fujifilm Speciality Ink Systems Limited
Priority date: 2017-01-25
Filing date: 2018-01-24
Publication date: 2018-08-02
Also published as: GB2572519A; GB201910440D0

Abstract

The present invention provides a method of printing an inkjet ink, wherein the inkjet ink comprises: at least 30%by weight of organic solvent based on the total weight of the ink, a radiation-curable material,and optionally a colourant, the method comprising:(i) inkjet printing the inkjet ink onto a substrate;(ii) evaporating at least a portion of the solvent from the printed ink; and(iii) exposing the printed ink to a low-energy electron beam to cure the radiation-curable material, wherein substantially all of the solvent is evaporated from the printed ink before the ink is cured. The present invention also provides an inkjet printing apparatus for printing a solvent-based inkjet ink comprising at least one printhead, a means for evaporating solvent from the printed ink and a low-energy electron beam source.

Description

Printing method and apparatus

The present invention relates to a method of printing and a printing apparatus. In particular, the invention relates to a method of inkjet printing and an inkjet printing apparatus.

Digital inkjet printing is becoming an increasingly popular method for the production of fine graphic images for advertising, due to its low implementation cost and versatility in comparison with traditional techniques such as lithographic and screen printing. Inkjet printers comprise one or more printheads that include a series of nozzles through which ink is ejected onto a substrate. The printheads are typically provided on a printer carriage that traverses the print width (moves back and forth across the substrate) during the printing process.

Three main ink chemistries are used: inks that dry by solvent evaporation, inks that dry by evaporation of water and inks that dry by exposure to ultraviolet radiation. Wide format solvent-based inkjet printers are an economic route into the industry as they are a relatively low cost option compared to the more complex machines employed for UV curing. Solvent-based inkjet printing also has other advantages. As well as the lower cost, the ink films produced are thinner (and therefore flexible) and yield a good quality natural looking image with a gloss finish. Furthermore, it is difficult to achieve very high pigment loadings in UV curable inks due to the high viscosity of the ink: if too much pigment is added, the ink becomes too viscous and cannot be jetted. In contrast, solvent-based inks include a high proportion of solvent and therefore have a lower viscosity, which means that higher pigment loadings can be tolerated. In addition, the printed film produced from solvent-based inkjet inks is formed predominantly of pigment along with comparatively few other solids that are included in the ink. The pigment is therefore largely unobscured, resulting in intense, vivid and vibrant colours and a large colour gamut.

However, there are some limitations to solvent-based inkjet technology. In particular, solvent-based inks may not adhere to certain types of substrate, particularly non-porous substrates such as plastics, and the cured films have poor resistance to solvents. This is also true of water-based inks, which are only really suitable for paper and board substrates. This limits their use in various applications, including food packaging applications, where plastics materials are more widely used.

Inkjet printing is an attractive technique for printing onto a wide-range of substrates on account of its flexibility and ease of use. However, food packaging represents a particular challenge on account of the strict limitations on the properties of materials which come into contact with food, including indirect additives like packaging inks. Specific exclusions include some volatile organic solvents, monomers, oligomers and unreacted photoinitiator fragments typically present in inkjet inks based on their odour and/or migration properties. There is therefore a need in the art for a method of printing utilising inkjet inks which are suitable for food packaging applications with low odour and low migration, without compromising the printing and drying/curing properties of the ink, maintaining a suitably low viscosity for inkjet printing, adhesion to a range of substrates, resistance to solvents, as well as maintaining the advantageous properties of solvent-based inkjet inks.

The present invention provides a method of printing an inkjet ink, wherein the inkjet ink comprises: at least 30% by weight of organic solvent based on the total weight of the ink, a radiation-curable material, and optionally a colourant, the method comprising:

(i) inkjet printing the inkjet ink onto a substrate;

(ii) evaporating at least a portion of the solvent from the printed ink; and

(iii) exposing the printed ink to a low-energy electron beam to cure the radiation-curable material, wherein substantially all of the solvent is evaporated from the printed ink before the ink is cured. The present invention further provides an inkjet printing apparatus for printing a solvent-based inkjet ink comprising at least one printhead, a means for evaporating solvent from the printed ink and a source of low-energy electron beam.

The present invention will now be described with reference to the accompanying drawing, in which:

Fig. 1 shows a perspective view of an exemplary embodiment of an inkjet printing apparatus according to the present invention.

Ink

The inks comprise a modified ink binder system. The presence of a radiation-curable material in the ink means that crosslinks can be formed in the dried ink film, leading to improved adhesion to a range of substrates and improved resistance to solvents. The presence of at least 30% by weight of organic solvent means that the advantageous properties of solvent-based inkjet inks are expected to be maintained, however.

By "radiation-curable material" is meant a material that polymerises or crosslinks when exposed to a low-energy electron beam. The radiation-curable material can comprise a monomer, an oligomer, or mixtures thereof. The monomers and/or oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality monomers and/or oligomers may be used.

Monomers typically have a molecular weight of less than 600 Daltons, preferably more than 200 Daltons and less than 450 Daltons. 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 l OOmPas 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 / 2° steel cone at 25°C with a shear rate of 25 s^" .

The term "curable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 450 Daltons and preferably at least 600 Daltons (whereas monomers typically have a molecular weight below these values). The molecular weight is preferably 4,000 Daltons or less. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured by Infinity 1260 supplied by Agilent technologies, using gel permeation chromatography calibrated against polystyrene standards. Thus, for polymeric materials, number average molecular weights can be obtained using gel permeation chromatography and polystyrene standards.

Preferably, the radiation-curable material comprises a radiation-curable oligomer.

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 polymerisable groups. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation.

In one embodiment the radiation-curable material polymerises by free radical polymerisation. Suitable free radical polymerisable monomers are well known in the art and include (meth)acrylates, α,β-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 (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA) and lauryl acrylate. PEA is particularly preferred.

Suitable multifunctional (meth)acrylate monomers include di-, tri- 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, polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentylglycol 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. α,β-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, 1 ,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether. Mixtures of α,β-unsaturated ether monomers may be used.

N-vinyl amide monomers, N-(meth)acryloyl amine monomers and N-vinyl carbamate monomers may also be used in the inks. 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 and a detailed description is therefore not required. 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. 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:

The synthesis of N-vinyl carbamate monomers is known in the art. For example, vinyl isocyanate, formed by the Curtius rearrangement of acryloyi 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 R 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, cycloalkyi, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CMS alkyl, C_3-18 cycloalkyi, C₆. ₀ aryl and combinations thereof, such as C₆. ₀ aryl- or C_3-18 cycloalkyl-substituted 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 R to R⁴ are independently selected from hydrogen or C_{1 -10} 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 lUPAC 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 is a racemate of NVMO. In another embodiment, the N-vinyl carbamate is (R)-5-methyl-3-vinyl-1 ,3- oxazolidin-2-one. Alternatively, the N-vinyl carbamate is (S)-5-methyl-3-vinyl-1 ,3-oxazolidin-2-one. 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. Particularly preferred radiation-curable materials are oligomers with free radical polymerisable groups, preferably (meth)acrylate groups. Acrylate functional oligomers are most preferred.

In one embodiment the oligomer comprises two or more radical polymerisable groups, preferably three or more, more preferably four or more. Oligomers comprising six polymerisable groups are particularly preferred.

The oligomer preferably comprises a urethane backbone.

Particularly preferred radiation-curable materials are urethane acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are tri-, tetra-, penta-, hexa- or higher 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.

The radiation-curable oligomer used in the preferred inks cures upon exposure to a low-energy electron beam to 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. 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 have a viscosity of 0.5 to 20 Pa.s at 60°C, more preferably 5 to 15 Pa.s at 60°C and most preferably 5 to 10 Pa.s 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 seconds .

In one embodiment the radiation-curable material comprises 50 to 100%, or 75 to 100% by weight of free radical curable oligomer, based on the total weight of radiation-curable material present in the ink.

In one embodiment the radiation-curable material comprises 50 to 100%, or 75 to 100% by weight of free radical curable oligomer and 1 to 50%, or 1 to 25% by weight of free radical curable monomer, based on the total weight of radiation-curable material present in the ink.

Preferably the ink comprises less than 20% by weight of (meth)acrylates with a molecular weight of less than 450 Daltons based on the total weight of the ink, or less than 10% by weight, more preferably less than 5% by weight. In a particularly preferred embodiment, the ink of the invention is substantially free of (meth)acrylates with a molecular weight of less than 450 Daltons.

In one embodiment the ink comprises less than 20% by weight of (meth)acrylates with a molecular weight of less than 600 Daltons based on the total weight of the ink, or less than 10% by weight, more preferably less than 5% by weight. In a particularly preferred embodiment, the ink of the invention is substantially free of (meth)acrylates with a molecular weight of less than 600 Daltons.

By "substantially free" is meant that no (meth)acrylate with a molecular weight of less than 450 Daltons or 600 Daltons, respectively, is intentionally added to the ink. However, minor amounts of (meth)acrylates with a molecular weight of less than 450 Daltons or 600 Daltons, respectively, that may be present as impurities in commercially available radiation-curable oligomers, for example, are tolerated.

The radiation-curable material is preferably present in the composition in an amount of 2% to 65% by weight, based on the total weight of the ink, more preferably 2 to 45% by weight, more preferably 5 to 35% by weight, more preferably 8 to 25% by weight, and most preferably 10% to 25% by weight.

Such amounts of radiation-curable oligomer and radiation-curable monomer, in combination with the organic solvent as claimed, are preferred as they are particularly advantageous when curing with low- energy electron beam. This is because lower molecular weight material, such as (meth)acrylates with a molecular weight of less than 450 Daltons, have been found to break up on exposure to low-energy electron beam. This is in contrast to higher molecular weight material, such as radiation-curable oligomer, which are more robust and as such do not break down and remain largely intact during electron bombardment during low-energy electron beam curing. The ink may optionally include one or more photoinitiators. When the ink includes a free radical polymerisable material the ink optionally includes a free radical photoinitiator.

The free radical photoinitiator can be selected from any of those known in the art. For example, benzophenone, 1 -hydroxycyclohexyl phenyl ketone,

1 -[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1 -propane-1 -one, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1 -one, iso propyl thioxanthone, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure and Darocur (from Ciba) and Lucerin (from BASF).

The presence of one or more photoinitiators is optional as it is not necessary to include a photoinitiator in the inkjet ink in order to achieve a thorough cure of the ink. This is because the ink can cure without the presence of one or more photoinitiators owing to curing with a low-energy electron beam.

It is surprising that low-energy electron beam can be used to cure the inkjet ink of the present invention. In this regard, it has previously been found that low-energy electron beam can break up the components of the ink, in particular monomers, which would have disastrous effects for inkjet inks applied to food packaging applications. However, the inkjet ink of the present invention has been found to be particularly suited for curing with low-energy electron beam.

In a preferred embodiment therefore, the photoinitiator is present in an amount of 20% by weight or less, preferably 5% by weight or less, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is substantially free of photoinitiator. By "substantially free" is meant that no photoinitiator is intentionally added to the ink. However, minor amounts of photoinitiator, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 1 % by weight of photoinitiator, more preferably less than 0.5% by weight of photoinitiator, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of photoinitiator. An inkjet ink that is substantially free of photoinitiator is advantageous in food packaging applications as there will be no unreacted photoinitiator or unreacted photoinitiator fragments present in the cured inkjet ink film. Photoinitiators create free radicals when exposed to radiation. These radicals react with reactive components of the ink (such as reactive monomers and oligomers). However, some photoinitiator and photoinitiator fragments will remain unreacted in the cured ink film and this is problematic for food packaging applications as such unreacted components can migrate into the substrate. In the ink of the present invention, photoinitiator is not necessary to achieve cure owing to curing with low-energy electron beam. The ink contains 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 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 and pyrrolidones.

The organic solvent is preferably present in an amount of at least 40% by weight, more preferably at least 45% by weight, and more preferably at least 50% by weight, for example 50 to 85% by weight, or 50% to 80% by weight based on the total weight of the ink. In a particularly preferred embodiment the organic solvent is present in an amount of at least 55% by weight, for example 60 to 85%, or 60% to 80% by weight based on the total weight of the ink. Known solvent-based inkjet inks dry solely by solvent evaporation with no crosslinking or polymerisation occurring. The film produced therefore has limited chemical resistance properties. In order to improve resistance of prints to common solvents such as alcohols and petrol, binder materials that have limited solubility in these solvents are added to the ink. The binder is typically in solid form at 25°C so that a solid printed film is produced when solvent is evaporated from the ink. Suitable binders such as vinyl chloride copolymer resins generally have poor solubility in all but the strongest of solvents such as glycol ether acetates and cyclohexanone, both of which are classified as "harmful" and have strong odours. In order to solubilise the binder, these solvents are generally added to the ink. The ink includes radiation-curable material that cures as the ink dries and it is not therefore necessary to include a binder in the ink in order to provide a printed film having improved solvent resistance. In one embodiment, the organic solvent is not therefore required to solubilise a binder such as a vinyl chloride copolymer resin, which means that the ink formulator has more freedom when selecting a suitable solvent or solvent mixture.

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, dibasic esters and/or bio-solvents may be used. Dibasic esters are known solvents in the art. They can be described as di(C -C₄ alkyl) esters of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms having following general formula:

in which A represents (CH₂)i-₆, and R and R² may be the same or different and represent C^-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 alcohol may be derived from corn cobs/rice hulls. An example is D-limonene which may be extracted from citrus rinds.

Other solvents may be included in the organic solvent component. A particularly common source of other solvents is derived from the way in which the colouring agent is introduced into the inkjet ink formulation. The colouring agent is usually prepared in the form of a pigment dispersion in a solvent, e.g. 2-ethylhexyl acetate. The solvent tends to be around 40 to 50% by weight of the pigment dispersion based on the total weight of the pigment dispersion and the pigment dispersion typically makes up around 5 to 15% by weight of the ink and sometimes more.

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 ink can be a coloured ink or a colourless ink.

By "colourless" is meant that the ink is substantially free of colourant such that no colour can be detected by the naked eye. Minor amounts of colourant that do not produce colour that can be detected by the eye can be tolerated, however. Typically, the amount of colourant present will be less than 0.3% by weight based on the total weight of the ink, preferably less than 0.1 %, more preferably less than 0.03%. Colourless inks may also be described as "clear" or "water white".

Coloured inks comprise at least one colouring agent. The colouring agent may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersible 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 of the invention 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 μηι, preferably less than 5 μηι, more preferably less than 1 μηι and particularly preferably less than 0.5 μηι.

The colourant is preferably present in an amount of 20 weight% or less, preferably 10 weight% or less, more preferably 8 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 weight%, or 25 weight% based on the total weight of the ink.

The ink can optionally contain a thermoplastic resin. The thermoplastic resin does not include reactive groups that are able to crosslink on exposure to radiation. In other words, thermoplastic resin is not a radiation-curable material. Suitable materials have molecular weights ranging from 10,000 to 100,000 Daltons as determined by GPC with polystyrene standards. The thermoplastic resin can be selected from epoxy, polyester, vinyl or (meth)acrylate resins, for example. Methacrylate copolymers are preferred. When present, the ink can comprise 1 to 5% by weight of thermoplastic resin, based on the total weight of the ink. The thermoplastic resin increases the viscosity of the ink film prior to curing, leading to improved print definition. The thermoplastic resin also decreases the glass transition temperature of the cured ink, giving greater film flexibility for applications such as vehicle side application.

In one embodiment, the ink comprises at least 50% by weight of organic solvent based on the total weight of the ink; a radiation-curable material, wherein the radiation-curable material comprises 50 to 100% by weight of free radical curable oligomer having a molecular weight of 600 to 4000 Daltons based on the total weight of radiation-curable material present in the ink; and optionally a colourant.

In one embodiment, the ink comprises at least 50% by weight of organic solvent based on the total weight of the ink; a radiation-curable material, wherein the radiation-curable material comprises 50 to 100% by weight of free radical curable oligomer having a molecular weight of 600 to 4000 Daltons and 1 to 50% by weight of free radical curable monomer having a molecular weight of 450 Daltons or less based on the total weight of radiation-curable material present in the ink; and optionally a colourant.

The inkjet ink exhibits a desirable low viscosity (200 mPa.s or less, preferably 100 mPa.s or less, more preferably 25 mPa.s or less, more preferably 10 mPa.s or less and most preferably 7 mPa.s or less at 25 °C). In order to produce a high quality printed image a small jetted drop size is desirable. Furthermore, small droplets have a higher surface area to volume ratio when compared to larger drop sizes, which facilitates evaporation of solvent from the jetted ink. Small drop sizes therefore offer advantages in drying speed. Preferably the inkjet ink is jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 10 picolitres.

To achieve compatibility with print heads that are capable of jetting drop sizes of 50 picolitres or less, a low viscosity ink is required. A viscosity of 10 mPa.s or less at 25°C is preferred, for example, 2 to 10 mPas, 4 to 8 mPa.s, or 5 to 7 mPa.s. It is problematic to achieve these low viscosities with conventional radiation-curable inks due to the relatively high viscosities of acrylate monomers and oligomers used in the compositions, but the presence of a significant amount of organic solvent in the ink allows these low viscosities to be achieved.

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, synergists, 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 ink allows control of the surface wetting of the ink 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 mNm^" and more preferably 21 -27 mNm^"1. The present invention also provides an ink set comprising a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set), wherein at least one of the inks is an ink as described hereinabove. Preferably all of the inks in the ink set are inks as described hereinabove. The inks in a trichromatic set can be used to produce a wide range of secondary colours and tones by overlaying the printed dots on white substrate.

The ink set can optionally include one or more light colour inks. Light colour versions of any colour ink can be used but preferred colours are light cyan, light magenta and light black. Particularly preferred are light cyan inks and light magenta inks. Light colour inks serve to extend the colour gamut and smooth the gradation from highlight to shadow areas of the printed image.

The ink set can optionally include one or more of a green ink, an orange ink and a violet ink. These colours further extend the gamut of colours that can be produced. Violet and orange inks are preferred, most preferred is orange ink. The ink set can optionally include a white ink. White ink can be used in two ways. When printing onto a transparent substrate, white ink can be printed over the image such that the image can be viewed from the reverse. Alternatively, white ink can be used to print a base coat onto a coloured substrate before the image is printed. Even with the range of inks detailed above, some colours can be particularly difficult to produce. Where it is essential that a printed colour is an exact match to a standard, such as a corporate colour, the ink set can optionally contain one or more inks having matched spot colours, which are designed to be printed in pure form with no overlaying. The ink can produce an image having a high gloss finish. This means that when the ink is printed on a substrate having low gloss, areas of the image that have high deposits of ink (for example where the image has deep colour or dark shading) have a significantly higher gloss level than areas of the image that have low deposits of ink (for example, where there is only light shading in the image). In other words, highlight areas of the print will have a lower gloss level than the shadow areas. Sharp lines can appear in the image where the transitions from heavy to light shading (e.g. from heavy gloss to low gloss) occur, which can lead to unattractive prints.

In order to provide an even finish and therefore improve the image quality, the entire print can optionally be coated with a colourless ink or varnish. Preferably, however, the ink is printed together with a colourless ink. The ink set therefore preferably includes a colourless ink.

The colourless ink is jetted at the same time as the coloured ink but the colourless ink is deposited in blank or highlight areas of the image that do not have high deposits of coloured ink. This means that the ink film covers the entire printed surface of the substrate, which results in prints with a more even finish across the print. The prints can also tend to have a more even ink film weight across the film, which improves the appearance of the prints because the surface topography is more even and the transitions between the areas of heavy coloured ink deposits to highlights are smoother. 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, however. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity. One preferred ink set comprises a cyan ink, a yellow ink, a magenta ink and a black ink. This limited combination of colours can achieve prints with a very high gloss that is even across the print, very good graduations of tone and a high colour gamut. Further variations of the above ink set can include the above ink set plus either one or more of a clear varnish, a metallic and a white ink. Another example of ink set is a cyan ink, a yellow ink, a magenta ink and a black ink, a colourless ink, a light cyan ink, a light magenta ink and an orange ink

When the ink is provided in an ink set, the surface tensions of the different inks in the ink set preferably differ by no more than 2 mNm^" , more preferably no more than 1 mNm^" and most preferably no more than 0.5 mNm^"1. Carefully balancing the surface tension of the different inks in this manner can lead to improvements in the quality and appearance of the printed image.

The ink set can optionally include one or more metallic effect inks. The use of metallic colours such as silver is becoming increasing popular in advertising images, for example.

Conventional solvent-based metallic inks can produce very bright metallic effects. The metallic pigments are in the form of flakes or platelets and these are randomly orientated in the undried liquid ink. In the case of solvent-containing inks, the flakes can align parallel to the print surface as the ink film thickness reduces as a result of solvent loss in the drying process. The alignment of metallic pigment flakes parallel with the print surface results in good reflectivity and metallic lustre. However, the films produced can often have very poor rub properties, which means that the pigment can be easily removed from the print surface. Radiation cured metallic inks generally have better rub properties but are often dull in appearance because the metallic pigment flakes do not have time to align during the rapid curing process.

Metallic inks overcome these problems because the inks dry in two stages, as discussed below. During the solvent evaporation step the metallic flakes have time to align, allowing a bright metallic effect to be produced in the final image. However, the curing stage yields a rub-resistant film.

Colourless inks may be used as a varnish. In one embodiment of the invention the colourless ink may be used as a varnish for a conventional solvent-based metallic effect ink. Metallic effect prints can be protected with known radiation curable varnishes but the high film weight produced when these materials are jetted dulls the metallic lustre of the prints and is deleterious to their appearance. The presence of a relatively large proportion of volatile solvent in the colourless inks allows a low film weight to be deposited, however. Typically, a radiation curable varnish would produce a 12 μηι film over the surface of the print. By using a colourless ink, the film weight can be reduced to 2 to 3 μητ The low film weight of the hybrid varnish has a far less deleterious affect on the appearance of the metallic print.

The inks are primarily designed for printing onto substrates suitable for food packaging but the nature of the substrate is not limited and includes any substrate which may be subjected to inkjet printing such as glass, metals, plastics and paper. Non limiting examples include, polyesters, fabric meshes, vinyl substrates, paper and the like. The inks are particularly suited for printing onto substrates suitable for food packaging. Food packaging is typically formed of flexible and rigid plastics (e.g. food-grade polystyrene and PE/PP films), paper and board (e.g. corrugated board). 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.

Printing Apparatus With conventional solvent-based inks, the printer productivity is governed by the system's ability to expel the bulk solvent. If too much wet ink is laid down on the media, the ink flows to blur the printed image. For this reason, solvents with a high vapour pressure are preferred in the ink. However, if the solvent vapour pressure is too high, ink drying on the printhead nozzle plate may lead to blocked nozzles. This compromise in solvent selection leads to a limitation in productivity.

Because of their lower productivity, the capital cost for solvent printers has to be relatively low to remain commercially viable. The internal mechanisms are therefore kept simple, with as few printheads as possible to produce a reasonable quality image. The low complexity makes these machines easy to operate and maintain. Over recent years, UV curable ink systems have largely replaced solvent ink printers in the higher productivity range, wide format graphics market. Unlike solvent printers, the ink deposited on the surface does not appreciably evaporate upon heating. Instead, the material is transformed into a solid through exposure to an energy source. In most cases, the energy source is an intense UV light, which causes photo-crosslinking of curable molecules in the presence of a photoinitiator to form a solid.

The greatest perceived benefit of UV curable printers is their ability to deliver high production rates. In most UV printers, the cure source is mounted on the shuttling printhead carriage, on one or both sides of the printhead cluster. In some cases, cure systems are also placed between printheads. With a typical separation distance of less than 100 mm between the print heads and cure unit, the maximum time between print and cure would be 0.1 s for a printhead carriage moving at 1 m/s. UV ink solidification times of less than one second compare favourably with solvent inks that can take several minutes to dry. Inkjet printers for UV curable inks are necessarily more complex and consequently more expensive than inkjet inks printers for solvent-based inks, however. Further, unreacted photoinitiator and unreacted photoinitiator fragments may remain in the cured ink film, which is problematic in food packaging applications.

The ink of the present invention can be printed using inkjet printers that are suitable for use with solvent-based inkjet inks, in combination with a source of low-energy electron beam (ebeam).

The features of printers that are suitable for printing solvent-based inkjet inks are well known to the person skilled in the art and include the features described below. As discussed above, printers suitable for printing solvent-based inkjet inks typically have a low capital cost, which means that the printers tend to have simple internal mechanisms. In practice, this means that inkjet printers suitable for printing solvent-based inks typically comprise gravity feed systems for delivering ink from the ink supply to the printhead. In contrast, UV printers use a pressurised header tank for delivering the ink to the printhead, which allows control of the meniscus position in the nozzle.

Since printheads account for a large proportion of the overall printer cost, inkjet printers suitable for printing solvent-based inkjet inks include the minimum number of printheads that is required to provide a high quality image. In any event, because solvent-based inkjet inks typically require longer to dry than UV inks, there is less advantage in using many printheads to apply large quantities of ink to the substrate because this causes the ink to pool and the image to blur.

Furthermore, printheads that are for printing solvent-based inkjet inks are not provided with a means for heating the ink because solvent-based inks have a low viscosity and do not therefore require heating at the printhead to produce a jettable viscosity (in contrast with UV curable inks). Thus, known solvent-based inks are jetted at ambient temperatures. Solvent-based inkjet inks are susceptible to drying on the nozzle plate due to evaporation of the solvent. Printers for solvent-based inkjet inks therefore typically include suction cups which can be used to cap the printheads when not in use, allowing a solvent vapour saturated environment to be established, which limits evaporation. Should a printhead become blocked, the suction cup can be used to pull a small volume of ink through the blockage, using a peristaltic pump, to recover performance after excess ink is removed using a wiper blade.

The ink of the present invention comprises both a solvent and a radiation-curable component and therefore dries by a combination of evaporation of the organic solvent and curing of the radiation- curable component upon exposure to an energy source.

The ink can surprisingly be used in printers that are suitable for printing conventional solvent-based inkjet inks, provided that a source of low-energy electron beam (ebeam) is also provided. Typically the printheads of inkjet printers for solvent-based inks are not externally heated. The inks can be jetted at ambient temperature, preferably below 35°C, or below 30°C or about 25°C, and are therefore compatible with the printheads and nozzles that are used to print solvent-based inkjet inks. The use of a printer that is for printing conventional solvent-based inkjet inks, particularly printheads, nozzles and ink delivery systems that are for use with conventional solvent-based inkjet inks, as the basis of the printing apparatus means that printing apparatus has a low capital cost.

A printer that is suitable for printing a conventional solvent-based inkjet ink may be adapted before use in printing the inks of the present invention. Depending on the exact nature of the ink and the location of the cure source, opaque ink feed components that are chemically compatible with the ink may be used and/or a screen filter film may be applied to the print window on the front of the apparatus. These are minor adaptations that would not have a significant effect on printer cost or performance.

In one embodiment, the printing apparatus comprises one or more piezo drop on demand printheads. Preferably the printheads are capable of jetting ink in drop sizes of 50 picolitres or less, more preferably 30 picolitres or less, particularly preferably 10 picolitres or less.

The printing apparatus comprises means for evaporating solvent from the ink once the ink has been applied to the substrate. Any means that is suitable for evaporating solvent from known solvent- based inkjet inks can be used in the apparatus of the invention. Examples are well known to the person skilled in the art and include dryers, heaters, air knives and combinations thereof.

In one embodiment, the solvent is removed by heating. Heat may be applied through the substrate and/or from above the substrate, for example by the use of heated plates (resistive heaters, inductive heaters) provided under the substrate or radiant heaters (heater bars, IR lamps, solid state IR) provided above the substrate. In a preferred embodiment, the ink can be jetted onto a preheated substrate that then moves over a heated platen. The apparatus may comprise one or more heaters.

When printing the ink, a significant portion of the solvent is preferably allowed to evaporate before the ink is cured. Preferably substantially all of the solvent is evaporated before the ink is cured. This is achieved by subjecting the printed ink to conditions that would typically dry conventional solvent- based inkjet inks. In the case of the ink of the present invention, such conditions will remove most of the solvent but it is expected that trace amounts of solvent will remain in the film given the presence of the radiation-curable component in the ink. For example, the ink may comprise less than 5% by weight of solvent after the solvent evaporation step but before the ink is cured, more preferably less than 2% by weight and most preferably less than 1 % by weight, based on the total weight of the ink.

The solvent evaporation step is thought to be important because it is believed to define the image quality. Thus, it is thought that the solvent evaporation step results in a printed image with high gloss, as would be expected for conventional solvent-based inks. Furthermore, the loss of a significant portion of the ink through the evaporation of the solvent leads to the formation of a printed film that is thinner than the film that would be produced by jetting an equivalent volume of known radiation- curable ink. This is advantageous because thinner films have improved flexibility. In order to maximise image quality, and control bleed and feathering between image areas it is preferable to arrest the flow of the ink by evaporating the organic solvent from the ink droplets quickly after they have impacted on the substrate surface, a process often referred to as pinning. To achieve a good quality image it is preferable that the inks are "thermally pinned", that is heated in order to evaporate the organic solvent, within 5 seconds of impact, preferably within 1 second and most preferably within 0.5 seconds.

Unlike standard solvent-based inks, once the solvent has evaporated, the ink is not expected to be fully dry. Rather, what remains on the surface is a high viscosity version of a radiation-curable ink. The viscosity is sufficiently high to inhibit or significantly hinder ink flow and prevent image degradation in the timescale that is needed to post-cure the ink. Upon exposure to the low-energy electron beam 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 μηι, preferably 1 to 10 μηι, for example 2 to 5 μηι. Film thicknesses can be measured using a confocal laser scanning microscope. In one embodiment the source of low-energy electron beam is positioned downstream from the means for evaporating solvent from the printed ink. In other words, the evaporating means and source of low-energy electron beam are positioned so that printed substrate is exposed to the means for evaporating solvent before it is exposed to the energy source, allowing evaporation of the solvent before the radiation-curable material is cured. In this embodiment, the one or more printheads and the low-energy electron beam source are positioned to create a delay between jetting of the ink onto the substrate and exposure of the printed ink to the low-energy electron beam, to allow for evaporation of the solvent before the ink is cured. Preferably the distance between the one or more printheads and the source of actinic radiation is at least 100 mm, preferably at least 200 mm, and more preferably at least 300 mm.

Preferably the time period between jetting the ink from the printhead onto the substrate and exposing the printed ink to the low-energy electron beam is at least 1 second, preferably at least 5 seconds, and more preferably at least 10 seconds. Typical time periods after jetting for exposure to the low- energy electron beam can range from between 1 to 5 minutes and longer.

The source of low-energy electron beam can be any source of low-energy electron beam that is suitable for curing radiation-curable inks. Suitable low-energy electron beam sources include commercially available ebeam curing lamps, such as a 280mm comet ebeam curing lamp which has a penetrating voltage of 80kV and is capable of delivering a dosage of 30 kGy at 100 m/min. By "low- energy" for the ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 50 kGy or less.

Ebeam curing is characterised by dose (energy per unit mass, measured in kilograys (kGy)) deposited in the substrate via electrons. Electron beam surface penetration depends upon the mass, density and thickness of the material being cured. Compared with UV penetration, electrons penetrate deeply through both lower and higher density materials. Unlike UV curing, photoinitiators are not required for ebeam curing to take place. Ebeam curing is well-known in the art and therefore a detailed explanation of the curing method is not required. In order to cure the printed ink, the ink of the invention is exposed to the ebeam, which produces sufficient energy to instantaneously break chemical bonds and enable polymerisation or crosslinking. The ink can be cured with or without a photoinitiator. As discussed above, a disadvantage of UV curing is that unreacted photoinitiator and unreacted photoinitiator fragments remain a part of any given UV inkjet ink formulation after curing occurs. Depending on the construction of the printed substrate, unreacted photoinitiator fragments along with any unreacted monomers and oligomers present can migrate into the substrate. Use of ebeam curing, with the option of using no photoinitiators, therefore helps to meet low migration requirements, which are particularly relevant for food packaging.

As previously discussed, it is surprising that low-energy electron beam can be used to cure inkjet inks suitable for application to food packaging as the components of the ink tend to break down on exposure to low-energy electron beam. However, it has been found that the inkjet ink of the present invention can be subjected to low-energy electron beam without breaking down the components of the ink. Therefore, the present invention is particularly suited to food packaging applications, especially as photoinitiators are not required to effect cure.

The source of low-energy electron beam could be situated off-line in a dedicated conveyor curing unit. Preferably, however, the source of radiation is situated in-line, which means that the substrate does not have to be removed from the printing apparatus between the heating and curing steps.

The low-energy electron beam source can be mobile, which means that the source is capable of moving back and forth across the print width, parallel with the movement of the printhead.

In one embodiment, the source of low-energy electron beam is placed on a carriage that allows the source of low-energy electron beam to traverse the print width. The carriage is placed downstream of the printer carriage in order to provide a delay between printing of the ink onto the substrate and exposure to the curing unit, allowing the solvent to evaporate before the curing step. In this embodiment the source of low-energy electron beam moves independently of the printer carriage and movement of the printhead does not therefore have to be slowed in order to provide adequate time for solvent evaporation before the curing step. Thus, overall productivity can be improved.

When the source of low-energy electron beam is provided on separate carriage, it is necessary to provide an additional carriage rail, motor and control systems. This adaptation can lead to large increases in equipment costs.

Preferably the source of low-energy electron beam is static. This means that the source does not move backwards and forwards across the print width of the substrate when in use. Instead the source of low-energy electron beam is fixed and the substrate moves relative to the source in the print direction.

A preferred printing apparatus that comprises a static source of low-energy electron beam located outside the print zone is expected to be economically attractive and therefore suitable for entry level wide format digital graphics use. This embodiment is therefore particularly preferred. By entry level is meant the simplest and cheapest printers that are suitable for wide format digital graphics use.

By locating the source of low-energy electron beam outside the print zone, and by avoiding the use of mobile sources, potentially expensive adaptations to the printing apparatus can be avoided. Furthermore, as discussed above, the separation of the print and curing zones is beneficial for printing the ink because this allows solvent to evaporate from the printed ink before the ink is cured by exposure to the low-energy electron beam source.

Static curing units preferably span the full print width, which is typically at least 1 .6 m for the smaller wide format graphics printers. Fig. 1 shows a perspective view of an exemplary embodiment of an inkjet printing apparatus according to the present invention. The apparatus includes a printer head (1), a heating unit (2) and a low-energy electron beam curing unit (3).

Printing method

(i) jetting the ink from a printhead onto a substrate;

(ii) evaporating at least a portion of the solvent from the printed ink; and

(iii) exposing the printed ink to a low-energy electron beam to cure the radiation-curable material. wherein substantially all of the solvent is evaporated from the printed ink before the ink is cured.

The total dose received by the ink printed on the substrate is inversely proportional to the speed that the substrate moves past the energy source. The use of a static source allows the printed ink to be exposed to the low-energy electron beam for longer periods than are achieved with traditional scanning type large format printers. Hence, the total dose provided by the low-energy electron beam source can exceed that provided by scanning type cure units.

The present invention provides methods of inkjet printing using the printing apparatus and inks as described above.

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

Examples

Example 1

Black, cyan, magenta and yellow inkjet ink formulations (Examples 1 to 1 1) having the compositions shown in Tables 1 and 2 were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

UVE2500-TP20 is an epoxy novolac acrylate oligomer available from Polymer Technologies, Nippon Gohsei 7630B is a hexafunctional urethane acrylate with a viscosity of 6.9 Pa.s at 60°C, and Tegoglide 410 is a polyether siloxane copolymer slip aid available from Evonik.

Optical density was measured using a GretagMacbeth SpectroEye Spectrophotometer. Table 1

Table 2

The inks of Examples 1 to 7 were drawn down onto self adhesive vinyl substrate (Profiscreen, Igepa) using a no. 2 Kbar, depositing a wet film weight of 12 microns. The ink films were dried in an oven at 60°C for three minutes. The dried films of Examples 1 to 7 were exposed to a low-energy electron beam using a 280 mm comet ebeam curing lamp which has a penetrating voltage of 80 kV and is capable of delivering a dosage of 30 kGy at 100 m/min. The relative solvent resistance of the cured prints was assessed by rubbing with a soft cloth soaked in isopropyl alcohol.

The inks of Examples 1 to 3 were printed on a self adhesive vinyl substrate (IMAGin™ JT5929P, MACtac®) using a Maxjet 220 printer, supplied by Mutoh. Primary colours were achieved by printing the inks of Examples 1 to 3 in a single layer. Secondary colours were achieved by overlaying a second colour on a first colour. Printing and thermal drying details are shown below:

Mode:

Print Mode: 540 x 720 DPI 4-Pass Var.

Heads: 1234

Dotsize: Normal (SML)

Height: Middle

Mode: Sign/Quality

Scan width: Data

Overprint: 1

Vacuum: Normal

Bidi: Selected

Distance: 0

Thickness: 210

Interval: 0

Screen: Speed Screen

The following heater settings were used for all of the print sampl

Table 3

After thermal drying, the inks were exposed to a low-energy electron beam. Inks were assessed for isopropyl alcohol resistance as described above.

Gloss was measured using a Tri-Glossmaster 20/60/85 available from Sheen instruments. The gloss was determined from both 20 degree and 60 degree angles.

The inks of the examples offer improved solvent resistance and higher gloss compared to the existing technology-based compositions. Example 2

Black, cyan, magenta and yellow inkjet ink formulations (Examples 12 to 15) having the compositions shown in Table 4 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 4

Genomer 4215 is an aliphatic polyester urethane acrylate resin available from Rahn AG. BYK 307 is a modified silicone surfactant from Blagden.

The inks of Examples 12 to 15 were printed onto clear biaxially-oriented polypropylene (BOPP) packaging material using a digital printer equipped with nitrogen-rich atmosphere and an electron beam curing unit. A four colour print was produced.

The print was dried in an oven at 60°C for three minutes to remove substantially all of the solvent. Finally, the print was cured by exposing the print to a low-energy electron beam under a nitrogen-rich atmosphere containing 230 ppm oxygen, using an ebeam curing lamp which has a penetrating voltage of 100 kV and is capable of delivering a dosage of 50 kGy at 30 m/min.

The cured film was assessed for solvent resistance, flexibility, gloss, the presence of migratable components, and odour. The relative solvent resistance of the cured prints was assessed by rubbing with a soft cloth soaked in isopropyl alcohol.

Flexibility of the film was assessed by folding the print in half, creasing and looking for the appearance of cracks. A good result would be if no cracks are visible to the naked eye. The film was tested for the presence of migratable components. The test is as follows. The print was cut and then placed into air tight migration cells with 10 mL of a solution of 95% ethanol in water as the food simulant. The cells were stored for 10 days at 40 °C before the ethanol was analysed for migratable species using liquid chromatography tandem mass spectrometry (LC-MS/MS). The migration results were as follows.

Finally, odour of the film was assessed. Samples of the film were placed into airtight sealable Kilner glass jars and stored at room temperature for 24 hours. After storage, the jars were opened and the odour emanating from each sample was assessed by a panel of testers. For each sample, the strength of the odour was given a rating from 1 to 5, using an unprinted sample as a control. A rating of 1 indicates a strong odour and a rating of 5 indicates no perceptible odour. A rating of 3 is deemed acceptable. The samples tested were all given a rating of 3 or less.

The inks of Example 2 provide the desired combination of properties, namely excellent solvent resistance, gloss and flexibility, and the ink has the desired low odour/migration properties. Such an ink is therefore particularly suited for application to food packaging.

Claims

1 . A method of printing an inkjet ink, wherein the inkjet ink comprises: at least 30% by weight of organic solvent based on the total weight of the ink, a radiation-curable material, and optionally a colourant, the method comprising:

(i) inkjet printing the inkjet ink onto a substrate;

(ii) evaporating at least a portion of the solvent from the printed ink; and

(iii) exposing the printed ink to a low-energy electron beam to cure the radiation-curable material, wherein substantially all of the solvent is evaporated from the printed ink before the ink is cured.

2. The method according to claim 1 , wherein the organic solvent is present in an amount of at least 50% by weight, preferably at least 55% by weight, based on the total weight of the ink.

3. The method according to claim 1 or claim 2, wherein the organic solvent is present in an amount of 50% to 85% by weight based on the total weight of the ink, preferably 60% to 80% by weight.

4. The method according to any of claims 1 to 3, wherein the solvent is selected from glycol ethers, organic carbonates, lactones and mixtures thereof.

5. The method according to any of claims 1 to 4, wherein the radiation-curable material is present in an amount of 2% to 65% by weight based on the total weight of the ink, preferably 2% to 45% by weight, more preferably 5 to 35% by weight, more preferably 8 to 25% by weight, and most preferably 10% to 25% by weight.

6. The method according to any of claims 1 to 5, wherein the radiation-curable material comprises a radiation-curable oligomer.

7. The method according to claim 6, wherein the radiation-curable material comprises 50 to 100%, preferably 75 to 100% by weight of free radical curable oligomer, based on the total weight of radiation-curable material present in the ink.

8. The method according to any of claims 1 to 7 which comprises less than 20% by weight of (meth)acrylates with a molecular weight of less than 450 Daltons based on the total weight of the ink, preferably less than 10% by weight, more preferably less than 5% by weight.

9. The method according to any of claims 1 to 8, wherein the solvent is evaporated by heating the printed ink.

10. The method according to claim 9, wherein the printed ink is heated in order to evaporate the organic solvent within 5 seconds of the ink being jetted onto the substrate, preferably within 1 second, more preferably within 0.5 seconds.

1 1 . The method according to any of claims 1 to 10, wherein the time period between jetting the ink onto the substrate and exposing the printed ink to a low-energy electron beam is at least 1 second, preferably at least 5 seconds, and more preferably at least 10 seconds.

12. The method according to according to any of claims 1 to 1 1 , wherein the substrate is food packaging.

13. An inkjet printing apparatus for printing a solvent-based inkjet ink comprising at least one printhead, a means for evaporating solvent from the printed ink and a low-energy electron beam source.