WO2013175213A1

WO2013175213A1 - Ink- jet printing method

Info

Publication number: WO2013175213A1
Application number: PCT/GB2013/051345
Authority: WO
Inventors: Nigel Gould
Original assignee: Sericol Limited
Priority date: 2012-05-23
Filing date: 2013-05-22
Publication date: 2013-11-28
Also published as: GB2518551A; GB2518551B; GB201209195D0

Abstract

This invention relates to a method of single-pass reel-to-reel inkjet printing comprising the following steps in the following order: (i) providing an inkjet ink comprising at least 30% of a volatile organic solvent based on the total weight of the ink, a radiation-curable monomer and/or a radiation-curable oligomer, a photoinitiator and a dispersed pigment wherein the ink has a viscosity of 30 mPas or less at 25°C; (ii) depositing the ink on a substrate using an inkjet printer; (iii) partially curing the ink by exposing the ink to a first dose of radiation to pin the ink to the substrate; (iv) evaporating the solvent from the pinned ink; and (v) fully curing the ink by exposing the ink to a second dose of radiation.

Description

INK- JET PRINTING METHOD

The present invention relates to a printing method and in particular to an inkjet printing method for reel-to-reel printing.

Currently adhesive labels for applications such as shampoo bottles and the like are commonly printed using narrow web flexographic printing. The use of flexography seems an attractive technique for this application as thin flexible films with good product resistance are produced. It is also possible to produce good quality prints at the high production speeds needed in this segment. However, the technique does have some drawbacks. The preparation of the flexoplates required to print the image is not only rather time consuming but also requires specialist equipment, consequently many printers do not have the facility to produce these in- house. For label applications, it is often necessary to produce similar labels pattern but with variable information, such as language; however, the level of prepress preparation required before the print run limits the economically viable run lengths that can be produced. Consequently it would desirable to be able to print these types of label using inkjet printing, thereby allowing shorter runs of more bespoke labels to be economically produced. Whilst inkjet cannot at present match the production speed of a flexographic press, the far shorter computer- to-print time of the digital process could still yield acceptable total production times if the speed of the inkjet print process could be maximised.

In order to meet the print speeds necessary for this application, some changes to the methodology currently used for traditional graphic printing must be made. Rather than the image being built up by deposition of partially overlapping print swathes, i.e. multipass printing, the image must be printed in a continuous single action, i.e. single pass printing. The use of single pass printing greatly increases the potential speed of the inkjet printing process, but it also places greater demands on the nozzle reliability required of the ink printhead combination. As there are no overlapping swathes within a single pass print, any blocked nozzle or deviated jet will be immediately apparent in the printed article. As such, the need for printing reliability of the ink is greater than for multipass application. This need precludes the use of solvent-based or water-based inks which can have the potential for blocking heads leading to reduced nozzle reliability. A possibility is to use wholly non-volatile UV curable inkjet inks which have far better nozzle reliability; however, owing to further changes necessary in the system to meet the print speed requirements, their suitability is also rather limited. As discussed, flexography produces prints with good image quality and resistance properties at high speed. In order to meet the production speed requirements of a up to 100 m²/h printheads capable of operating at high frequencies must be used, for example Kyocera printheads or Samba printheads available from Fujifilm Dimatix. These high speed printheads are also capable of printing the small drop sizes required to meet the print quality requirements for the application. However, such printheads require low viscosity inks to function well, for example from 5 to 8 mPas at 25°C, which is below the range achievable with wholly UV curable inks without severely compromising the film properties of the final print. Consequently there exists the need for a low viscosity ink system with good nozzle reliability and final film properties for use in reel-to-reel printing. Accordingly, the present invention provides a method of single-pass reel-to-reel inkjet printing comprising the following steps in the following order:

(i) providing an inkjet ink comprising at least 30% of a volatile organic solvent based on the total weight of the ink, a radiation-curable monomer and/or a radiation-curable oligomer, a photoinitiator and a dispersed pigment wherein the ink has a viscosity of 30 mPas or less at 25°C;

(ii) depositing the ink on a substrate using an inkjet printer;

(iii) partially curing the ink by exposing the ink to a first dose of radiation to pin the ink to the substrate;

(iv) evaporating the solvent from the pinned ink; and

(v) fully curing the ink by exposing the ink to a second dose of radiation.

The present invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows an apparatus for reel-to-reel printing. The ink used in the present invention contains a radiation-curable (e.g. UV curable) monomer and/or a radiation-curable oligomer.

The monomer may be monofunctional and/or multifunctional and is preferably a monofunctional (meth)acrylate monomer and/or a multifunctional (meth)acrylate monomer. 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), lauryl acrylate and mixtures thereof.

Multifunctional (meth)acrylate monomers are also well known in the art and have a functionality of two or higher. Functionalities of two, three or four are preferred and preferably this monomer is a difunctional monomer. Examples of the multifunctional acrylate monomers that may be included in the ink 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 methacrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimeth aery late, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate and mixtures thereof. The monomers typically have a viscosity of less than 2 mPas at 25°C and they typically have a molecular weight of less than 450. The total amount of the monofunctional and/or multifunctional monomer, when present, is typically from 10 to 50 wt% based on the total weight of the ink. The ink may also contain a radiation-curable (i.e. polymerisable) oligomer, preferably a (meth)acrylate oligomer. The term "curable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 450. The molecular weight is preferably 4,000 or less. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 6. Preferred oligomers for use in the invention 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 s ^~1. UV-curable oligomers of this type are well known in the art. The oligomer is preferably based on bisphenol A, a polyester, a polyether or a urethane.

The total amount of the oligomer is preferably from 10 to 50 wt%, based on the total weight of the ink. In a particularly preferred embodiment, the ink contains the oligomer as the only curable material present in the ink.

(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.

In addition to the components described hereinabove, the ink also includes a photoinitiator which, under irradiation, for example by ultraviolet light, initiates the polymerisation of the monomers. Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators), such as benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 2-benzyl-2- dimethylamino-(4-morpholinophenyl)butan-1 -one, 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, Darocur (from Ciba) and Lucerin (from BASF). Preferably the photoinitiator is present from 1 to 20% by weight, more preferably from 5 to 15% by weight, based on the total weight of the ink.

The wavelength of the radiation and the nature of the photoinitiator system used must of course coincide. The ink is cured by irradiation with actinic radiation, such as UV, x-ray, electron beam etc, although UV curing is preferred.

The ink of the invention contains a volatile organic solvent. The volatile 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 of the invention 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 (hence the word "volatile"). 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.

On account of their low inherent viscosity, the inclusion of the solvent allows the formulation of inks with low viscosities yet still processing good film properties. This low viscosity allows the inks to be printed through high speed printheads such as those available from Kyocera and the Samba head from Dimatix. These heads not only allow the high speed printing required but also are capable of printing the small ink drop sizes necessary to reach the print quality targets for this application. Without the inclusion of solvent in the composition, the latitude for selection of monomer/oligomer functionality and molecular weight is low. By inclusion of solvent in the system, it is possible to use higher molecular weight and functionality materials which give higher crosslink densities and thus cured ink films with greater scratch and chemical resistance.

The inclusion of a volatile solvent in the composition also gives benefits in print quality by lowering the ink film thickness and consequently reducing gloss/matt banding commonly encountered with 100% UV systems due to areas of such prints having large variations in the ink film thickness.

The organic solvent is present in an amount of at least 30% by weight, more 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% by weight or less, more preferably 75% by weight or less 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 of the present invention 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. 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.

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 pigment is introduced into the inkjet ink formulation. The pigment 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 used in the present invention also includes a dispersed pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used. In one aspect 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 pigment 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.

Wholly UV curable inks have good printhead nozzle reliability, but are poor for print quality and film physical properties, as previously discussed. For solvent-based and latex inks the main problem is nozzle reliability. The ink systems are based on polymer solutions or dispersions, respectively. If the solvent or water evaporates from such systems, a high molecular weight solid resin is deposited on the printhead nozzle plate. As the deposited resins are solid and high in molecular weight, typically ten of thousands or more, they are not readily resolubilised by the ink. The build up of the resin deposits can lead to blocked or deviated nozzles and produce a severe deterioration in print quality. The inks used in the present invention do not require such high molecular weight solid resins, and may be formulated with lower molecular weight liquid materials. Typically the monomers and oligomers used in the ink of the present invention have molecular weights of 4,000 or less. In a particularly preferred embodiment, the ink does not contain any components having a molecular weight above 4,000. If solvent evaporates from such inks, a liquid deposit is formed which is readily resolubilised in the ink. If any small deposits form on the nozzle plate they are readily flushed away by the flow of ink through the nozzle and no deterioration in print quality is observed. Although the ink of the present invention cures by a free radical mechanism, the ink of the present invention may also be a so-called "hybrid" ink which cures by a radical and cationic mechanism. The ink-jet ink of the present invention, in one embodiment, therefore further comprises at least one cationically curable monomer, such as a vinyl ether, and at least one cationic photoinitiator, such as an iodonium or sulfonium salt, e.g. diphenyliodonium fluoride and triphenylsulfonium hexafluophosphate. Suitable cationic photoinitiators include the Union Carbide UVI-69-series, Deuteron UV 1240 and IJY2257, Ciba Irgacure 250 and CGI 552, IGM- C440, Rhodia 2047 and UV9380c.

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 for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

Suitable substrates include polyolefin substrates, such as polyethylene and polypropylene, e.g. PE85 Trans T/C, PE85 White or PP Top White, polyethylene terephthalate (PET) and paper. Polyolefin substrates represent the most difficult of these substrates on which to gain adhesion.

The ink-jet ink exhibits a desirable low viscosity, i.e. 30 mPas or less, preferably 20 mPas or less and most preferably 10 mPas or less at 25°C. A particularly preferred viscosity range is 2-8 mPas. Viscosity may be determined using a Brookfield DV-I + running at 20 rpm.

The inks used in the present invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill. The ink used in the present invention comprises both a solvent and a radiation-curable component and therefore dries by a combination of evaporation of the volatile organic solvent and curing of the radiation-curable component upon exposure to actinic radiation.

The ink can be used in printers that are suitable for printing conventional solvent-based inkjet inks, with the proviso that a source of actinic radiation is also provided. Often the printheads of inkjet printers for solvent-based inks are not heated or only heated to just above ambient temperatures. The inks of the present invention can optionally be jetted at ambient temperature or at just above ambient temperature, preferably at or below 35°C, at 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 present invention uses reel-to-reel printing. Reel-to-reel printing places particular demands on the inkjet ink. Fig. 1 shows an example of a reel to-reel printer of the present invention. A substrate 4 is tightly wound on a substrate reel 6. The substrate reel 6 is caused to move in order to deliver the substrate, via guide reels 8, to the inkjet printing station 10. The substrate moves in the print direction P shown by the arrow. At the printing station 10, the ink is applied by printhead 14 shown schematically in Fig. 1 . The stabilising reels 16 are positioned to provide a stable web onto which the ink is applied. As the substrate passes through the printing station 10, the ink is pinned by a UV source 18. Solvent is evaporated by a top down heater, 20 before the image is fully cured by UV source 22. The substrate 2 is subsequently accumulated on the receiving reel 12.

In one embodiment, the printing apparatus of the present invention comprises one or more drop- on-demand (piezo) 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 of the present invention comprises a means for evaporating solvent from the ink at the appropriate time after 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 of the invention may comprise one or more heaters.

When printing the ink of the present invention, 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 finally 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.

The solvent evaporation step is thought to be important because it is believed to provide further definition to 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.

Unlike standard solvent-based inks, once the solvent has evaporated, the ink is not expected to be completely solid. 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 a radiation source, the ink cures to form a relatively thin polymerised film. The ink of the present invention typically produces a printed film having a thickness of 1 to 20 μηι, preferably 1 to 10 μηι, for example 2 to 5 μηι. Film thicknesses can be measured using a confocal laser scanning microscope.

The source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source. When LEDs are used, these are preferably provided as an array of multiple LEDs. In a preferred embodiment a mercury discharge lamp is used, for example a medium-pressure mercury discharge lamp.

The source of actinic radiation for the initial pinning of the ink may be the same or different to the source of actinic radiation for performing the final cure of the ink.

Any of the sources of actinic radiation discussed herein may be used for the initial pinning of the inkjet ink. The dose of actinic radiation is lower than the dose required to cure the radiation- curable material fully, such as 1 -200 mJ/cm², preferably 1 -100 mJ/cm², and most preferably 1 -50 mJ/cm².

The wavelength of the pinning source is typically 200-700 nm, preferably 300-500 nm and most preferably 350-450 nm. 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 material thereby arresting the flow of the ink and improving the final image quality.

Following evaporation of the solvent, the composition is exposed to additional actinic radiation. That is, an additional dose of radiation to that required for pinning. 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.

High and medium-pressure mercury discharge lamps may be used.

LED sources that are currently available are relatively expensive and a printing apparatus comprising a LED source of UV radiation is unlikely to be suitable for use an entry level printer. However, development of UV LED sources for curing inks is ongoing and it is envisaged that the cost of LED sources will decrease significantly in the future. In this case, a printing apparatus according to the present invention that includes a source of actinic radiation comprising LEDs would be suitable for entry level printing systems. The invention will now be described with reference to the following examples, which are not intended to be limiting.

Examples Example 1

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

UV curable urethane acrylate oligomer having Mw 2,200 and functionality of six. The comparative ink, ink 3, was prepared using hexane diol diacrylate, which is the lowest viscosity difunctional acrylate monomer available at 6 mPas. Whilst lower viscosity monomers are available, these are monofunctional acrylates and their use would further compromise film toughness and chemical resistance which are important aspects for this ink application. The inks were prepared by dispersing all the ingredients with a high speed mixer. The viscosity was measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 30 rpm at 25 °C with spindle 00. It can be seen from the above results that the inks of the present invention are well within the desired viscosity range of 5-8 mPas at 25°C whilst the comparative ink is above. As can be seen the pigment dispersion contributes significantly to the final viscosity obtained, the ink viscosity being considerably higher than neat HDDA monomer. This problem would be further exacerbated with the remaining colours in the ink set, in particular the magenta and yellow where far higher pigment loadings are necessary to reach the optical densities required.

The inks were drawn down onto a commonly used label substrate, PE85 (uncoated polyethylene white substrate, thickness 85 microns) available from Fasson using at 12 micron K bar coater. The solvent-containing ink films were left for 2 mins at room temperature to allow the solvent to flash off before curing all the prints by passing through a conveyorised drier fitted with a 120 W/cm medium pressure mercury lamp at 50 m/min.

Table 2. Ink film properties

As can be seen from the results above, the surface cure of the comparative ink is poor; if this occurred in the reel-to-reel process, severe offsetting and blocking would occur making the composition completely unsuitable. Increasing the functionality of the monomer used to a trifunctional monomer would improve this property, but at the expense of even higher viscosity - for example, ethoxylated trimethylol propane triacrylate (e.g. Sartomer SR454) has a viscosity of 60-80 mPas. An additional problem of increasing the functionality of the UV-reactive component is film shrinkage. On the thin substrates used for label printing, ink film shrinkage after curing leads to curling of the label which in the worst cases can cause the applied label to fall off the container.

Claims

1 . A method of single-pass reel-to-reel inkjet printing comprising the following steps in the following order:

(ii) depositing the ink on a substrate using an inkjet printer;

(iv) evaporating the solvent from the pinned ink; and

(v) fully curing the ink by exposing the ink to a second dose of radiation.

2. A method of inkjet printing as claimed in claim 1 , wherein the solvent is present in an amount of at least 50% by weight, based on the total weight of the ink.

3. A method of inkjet printing as claimed in claim 1 or 2, wherein the solvent is selected from glycol ethers, glycol ether esters, alcohols, ketones, esters and pyrrolidones.

4. A method of inkjet printing as claimed in any preceding claim, wherein the oligomer is present and has a molecular weight of 4,000 or less.

5. A method of inkjet printing as claimed in any preceding claim, wherein the oligomer is present and has an average degree of functionality of 2 to 6.

6. A method of inkjet printing as claimed in any preceding claim, wherein the oligomer is present in an amount of 10 to 50 wt%, based on the total weight of the ink.

7. A method of inkjet printing as claimed in any preceding claim, wherein the ink contains the oligomer as the only curable material present in the ink.

8. A method of inkjet printing as claimed in any of claims 1 to 6, wherein the monomer is present and is a monofunctional (meth)acrylate monomer and/or a multifunctional (meth)acrylate monomer.

9. A method of inkjet printing as claimed in any preceding claim, wherein the ink has a viscosity of 5-8 mPas at 25°C

10. A method of inkjet printing as claimed in any preceding claim, wherein the method uses drop-on-demand printing.