WO2012022962A1

WO2012022962A1 - Printing Ink

Info

Publication number: WO2012022962A1
Application number: PCT/GB2011/051525
Authority: WO
Inventors: Nigel Gould; Jeremy Ward
Original assignee: Sericol Limited
Priority date: 2010-08-20
Filing date: 2011-08-12
Publication date: 2012-02-23
Also published as: GB201305038D0; GB2496571B; GB2496571A

Abstract

Printing Ink The invention is directed towards an inkjet ink comprising a radiation curable oligomer, at least 30 % by weight of organic solvent based on the total weight of the ink, 5 to 50 % by weight of water based on the total weight of the ink, a photoinitiator and optionally a colourant, and also to a method of inket printing comprising jetting said ink from a printhead onto a substrate;evaporating at least a portion of the solvent and/or water from the printed ink; andexposing the printed ink to actinic radiation to cure the radiation curable oligomer

Description

Printing Ink

This invention relates to a printing ink and in particular to an ink for use in inkjet printers.

Inkjet printing is becoming an increasing popular technique for the production of graphic images for advertising and the like. Two main ink chemistries are typically used: inks that dry by solvent evaporation 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.

EP 2 184 329 describes a curable ink useful for inkjet injection or serigraphy which has a surface tension between 30 and 55nM/m and a viscosity between lOcps and 35cps, comprising a UV- photocurable resin, a photoinitiator, a dye, between 15% and 60% by weight water, and between 0 and 40wt% solvent.

WO2008/053235 relates to compositions suitable for thermal inkjet printing comprising a mixture of curable materials, from 10 to 30wt% water, and one or more co-solvents for the curable materials. WO 2008/012512 relates to an activator composition suitable for thermal inkjet printing, comprising a single phase aqueous composition including one or more curable materials and one or more co-solvents, and an activator. WO 2007/036692 relates to a single phase aqueous composition suitable for thermal inkjet printing comprising a mixture of curable materials including at least two curable oligomers and one or more co-solvents.

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. Furthermore, the solvent used in solvent-based ink-jet inks needs to be volatile in order to allow the ink to dry in a reasonable time. This means that solvent-based ink-jet inks often comprise large amounts of volatile organic compounds (VOCs). VOCs are organic compounds that have a high vapour pressure and a low water solubility. A high vapour pressure means that the compounds are able enter the atmosphere where they have the potential to participate in photochemical reactions, causing atmospheric pollution and smog formation. Many countries set limits for the emission of VOCs but the precise definition of "VOC" varies. The United States Environmental Protection Agency (EPA), for example, defines a volatile organic compound (VOC) as "any organic compound that participates in atmospheric photochemical reactions except those designated by EPA as having negligible photochemical reactivity", whereas European Council Directive 1999/13/EC defines a VOC as "any organic compound having at 293.15 K a vapour pressure of 0.01 kPa or more, or having a corresponding volatility under the particular conditions of use".

It is therefore desirable to limit the VOC content of inks. However, this requirement is in conflict with the requirement for a solvent-based ink to include a large proportion of volatile solvent in order to allow fast drying.

There therefore exists a need for an alternative inkjet ink.

Accordingly, the present invention provides an inkjet ink comprising a radiation curable oligomer, at least 30 % by weight of organic solvent based on the total weight of the ink, 5 to 50 % by weight of water based on the total weight of the ink, a photoinitiator and optionally a colourant.

The organic solvent preferably has one or more of the following properties:

(a) an evaporation rate equal to or less than that of butyl acetate, preferably an evaporation rate compared to butyl acetate of 65 or less, more preferably an evaporation rate compared to butyl acetate of 23 or less, where the evaporation rate of butyl acetate is 100.

(b) a flash point of 20 °C or more, preferably 30 °C or more, more preferably 40 °C or more.

(c) a boiling point of 100°C or more, preferably 115°C or more, more preferably 140 °C or more.

It has been found that by carefully selecting the type of radiation curable material used and the amounts of solvent and water present in the ink, inkjet inks having desirable properties can be produced. In particular, the inkjet ink of the present invention is quick-drying and forms images with good image quality that have good adhesion to non-absorbent substrates. The presence of water in the ink of the present invention allows the VOC content to be reduced relative to currently available solvent-based inkjet inks.

By "radiation curable oligomer" is meant a material that comprises a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation polymerisable groups such that the oligomer polymerises or crosslinks when exposed to radiation, commonly ultraviolet light, in the presence of a photoinitiator. The radiation curable oligomer has a molecular weight of 500 g/mol or more, preferably 600 g/mol or more, for example 500 to 4000, more preferably 600 to 4000. Molecular weights can be calculated if the structure of the oligomer is known, or molecular weights can be measured using gel permeation chromatography using polystyrene standards. Thus, for polymeric materials, number average molecular weights can be obtained using gel permeation chromatography and polystyrene standards. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Mono, di, tri and higher functionality oligomers may be used. The radiation curable oligomer component of the ink can comprise a single radiation curable oligomer or a mixture of two or more radiation curable oligomers. Particularly preferred radiation curable oligomers include free radical polymerisable groups, preferably (meth)acrylate groups. Acrylate functional materials are most preferred.

(Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate.

In one embodiment the oligomer comprises two or more radical polymerisable groups. The radiation curable oligomer should be hydro lytically stable, that is it should not react with, or decompose in, water. The radiation curable oligomer should also be soluble or dispersible in the mixture of water and organic solvent provided in the ink. Suitable examples include water-soluble urethane acrylates, water-soluble epoxy acrylates, water-soluble polyester acrylates and water-soluble polyether acrylates. Water solubility can be introduced via ethylene oxide chains or via ionic character in the backbone. Examples of suitable oligomers include CN132, CN2560, SR602, CNSP061, CNSP066, all available from Sartomer. Other suitable examples include water-dispersible oligomers such as acrylate functional polyurethane dispersions such as those available from Cytec under the Ucecoat trade name.

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 ^_1.

In one embodiment the radiation curable oligomer is present in the composition in an amount of 2 % to 45 % by weight based on the total weight of the ink, preferably 5 to 35 % by weight, more preferably 8 to 25 % by weight, and most preferably 10 % to 18 % by weight.

The ink of the present invention may contain one or more radiation curable monomers having a molecular weight of 450 or less. Suitable monomers include mono- and multifunctional (meth)acrylates, vinyl ethers and N-vinyl amides. Such monomers are well known to the person skilled in the art and are readily available. The radiation curable monomer should be hydrolytically stable, that is it should not react with, or decompose in, water and should be soluble or dispersible in the mixture of water and organic solvent provided in the ink.

In one embodiment the ink of the present invention is substantially free of radiation curable material having a molecular weight of 450 or less although minor amounts can be tolerated. The ink of the present invention may comprise less than 10 % by weight, for example less than 5 % by weight of radiation-curable material having a molecular weight of 450 or less.

In an alternative embodiment of the invention the radiation curable oligomer is capable of polymerising by cationic polymerisation. Suitable materials include oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novo lacs and the like.

The ink can also comprise a combination of free radical polymerisable and cationically polymerisable oligomers. The ink of the invention contains an organic solvent. The organic solvent is in the form of a liquid at ambient temperature. The organic solvent does not include reactive groups that are able to crosslink on exposure to radiation. In other words, the organic solvent is not a radiation curable material. The organic solvent component of the inks 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. The solvent can be selected from any solvent commonly used in the printing industry, provided the solvent is soluble or at least partially miscible with water. Suitable examples include alcohols, glycol ethers, polyethylene glycol ethers, polypropylene glycol ethers, glycol ether esters, ketones, organic carbonates, pyrrolidones, esters, lactones and mixtures thereof.

The organic solvent is preferably present in an amount of greater than 40 % by weight, for example greater than 40 % to 80 % by weight, more preferably 42 % to 65 % by weight based on the total weight of the ink.

In a preferred embodiment the organic solvent is a low toxicity and/or a low odour solvent. Solvents that have been given VOC exempt status by the United States Environmental Protection Agency or European Council are also preferred.

The organic solvent is preferably selected from those having an evaporation rate equal to or less than that of butyl acetate. That is, where butyl acetate has an evaporation rate of 100, the relative evaporation rate of the organic solvents is preferably 100 or less, more preferably 65 or less, even more preferably 23 or less. The evaporation rate compared to butyl acetate is also preferably 0.1 or more, and more preferably is 0.4 or more. Thus, the evaporation rate compared to butyl acetate is preferably in the range of from 0.1 to 100, and is more preferably in the range of from 0.1 to 65 or from 0.4 to 100. Even more preferably, the evaporation rate compared to butyl acetate is in the range of from 0.4 to 65, and most preferably from 0.4 to 23. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such an evaporation rate.

The organic solvent is preferably selected from those having a boiling point of 100 °C or more, more preferably 115 °C or more, and even more preferably 140°C or more. The organic solvent also preferably has a boiling point of 260 °C or less, and more preferably 245 °C or less. Thus, the boiling point is preferably in the range of from 100 °C to 260 °C, more preferably in the range of from 115 °C to 260 °C and most preferably in the range of from 140 °C to 245 °C. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such a boiling point.

The organic solvent is preferably selected from those having a flash point of 20 °C or more, more preferably 30 °C or more, and most preferably 40 °C or more. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such a flash point.

In a preferred embodiment, the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate of 100 or less, and a flash point of 20 °C or more, and more preferably additionally having a boiling point of 100 °C or more. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such properties. In another preferred embodiment, the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate of 65 or less and a flash point of 30 °C or more, and more preferably additionally having a boiling point of 115 °C or more. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such properties.

In a further preferred embodiment, the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate in the range of from 0.1 to 100, and a flash point of 20 °C or more, and more preferably additionally having a boiling point in the range of from 100 °C to 260 °C. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such properties.

In a further preferred embodiment, the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate in the range of from 0.4 to 65, and a flash point of 30 °C or more, and more preferably additionally a boiling point in the range of from 115 °C to 245 °C. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such properties.

In a still further preferred embodiment, the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate in the range of from 0.4 to 23, and a flash point of 40 °C or more, and more preferably additionally a boiling point in the range of from 140 °C to 245 °C. Where the organic solvent comprises a mixture of solvents, each solvent in the mixture preferably has such properties. Preferred organic solvents are organic carbonates, preferably cyclic carbonates. Propylene carbonate is particularly preferred. Other preferred organic solvents include glycol ethers such as diethylene glycol diethyl ether and propylene glycol monopropyl ether, pyrrolidones, alcohols and lactones such as gamma butyro lactone. Preferably, the organic solvents are selected from propylene glycol monopropyl ether, diethylene glycol diethyl ether, propylene carbonate, gamma butyro lactone and mixtures thereof.

Table 1 below shows relevant flash point, evaporation rate and boiling point data of a number of organic solvents. Due to its high volatility, low flash point and low boiling point, methanol is not an organic solvent according to the present invention.

Table 1

In a preferred embodiment, the organic solvent is a mixture of i) an organic carbonate and/or lactone, and ii) one or more other solvents such that the carbonate and/or lactone is present in an amount of 15 to 40 %, preferably 20 to 30 % by weight based on the total weight of the ink, and the remaining organic solvent is present in an amount of 2 to 40 %, preferably 5 to 30 %, most preferably 5 to 25 % by weight based on the total weight of the ink. The organic carbonate is preferably a cyclic carbonate, most preferably propylene carbonate. The lactone is preferably gamma butyro lactone. The other solvent is preferably a glycol ether such as diethylene glycol diethyl ether.

The ink of the invention comprises 5 to 50 % by weight of water based on the total weight of the ink. Preferably the water is present in an amount of 5 to 40 %, more preferably 10 to 35 % based on the total weight of the ink. The water can be of any type suitable for use in inkjet inks such as deionised water.

Water may be introduced to the ink from commercially available colourants. The colouring agent is usually prepared in the form of a pigment dispersion in water. The water tends to be around 40 to 50 % by weight of the pigment dispersion based on the total weight of the pigment dispersion and a pigment dispersion can typically make up around 5 to 15 % by weight of the ink and sometimes more. The ink of the invention includes one or more photoinitiators. When the ink of the invention includes a free radical polymerisable material the photoinitiator system includes a free radical photoinitiator and when the ink includes a cationic polymerisable material the photoinitiator system includes a cationic photoinitiator. When the ink comprises a combination of free radical polymerisable and cationically polymerisable materials both a free radical and cationic initiator are required.

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

1 - [4-(2-Hydroxyethoxy)-phenyl] -2-hydroxy-2-methyl- 1 -propane- 1 -one,

2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-l-one, iso propyl

thioxanthone, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4- trimethylpentylphosphine oxide, (2,4,6-trimethylbenzoyl) diphenylphosphine 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).

In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulphonium or iodonium based systems. Non limiting examples include Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa.

Preferably the photoinitiator is present in an amount of 0.5 to 20 % by weight, preferably 1 to 10 % by weight, based on the total weight of the ink.

The ink of the present invention 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 in a colourless ink 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 of the invention 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 pigment that is dispersible in water using a suitable aqueous dispersing aid. Suitable pigments are 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.

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 μιη, preferably less than 5 μιη, more preferably less than 1 μιη and particularly preferably less than 0.5 μιη.

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, 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 resin can assist with flexibility and adhesion of the ink to the substrate and can increase the ink's drying efficiency. 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. The thermoplastic resin should be water compatible and soluble or dispersible in water. Examples of suitable water soluble thermoplastic resins include carboxyl functional materials such as Elvacite 2669 and 2776 from Lucite, which become fully water soluble when neutralised with a suitable amine, for example methyl diethanol amine or 2-Dimethylamino-2-Methyl-l-Propanol. Suitable water dispersible thermoplastic resins include water dispersible acrylic copolymers such as Joncryl 90 and 538 available from BASF.

The inkjet ink of the invention 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 water and organic solvent from the jetted ink. Small drop sizes therefore offer advantages in drying speed. Preferably the inkjet ink of the invention is capable of being jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 10 pico litres.

To achieve compatibility with print heads that are capable of jetting these small drop sizes a low viscosity ink is required. A viscosity of 10 mPa.s or less at 25 °C is preferred, for example, 2 to 10 mPa.s, 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 water and organic solvent in the ink of the invention 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 for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

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 according to the present invention. Preferably all of the inks in the ink set are inks according to the present invention. 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 of the present invention 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 of the present invention 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.

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 of the invention 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 set of the present invention optionally includes a colourless ink.

The ink set of the invention 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. A preferred ink set of the present invention comprises a cyan ink, a yellow ink, a magenta ink, a black ink, a colourless ink, a light cyan ink, a light magenta ink and an orange 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.

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 actinic radiation. Typically the printheads of inkjet printers for solvent-based inks are not externally heated. The ink of the current invention is therefore preferably jetted at below 35 °C, more preferably below degrees 30 °C and most preferably at about 25 °C, or ambient temperature. A printer that is suitable for printing a conventional solvent-based inkjet ink may be adapted before use in printing the ink 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 UV 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.

Preferably the ink of the invention is jetted in drop sizes of 50 picolitres or less, preferably 30 picolitres or less and most preferably 10 picolitres or less. When printing the inks of the present invention it is necessary to allow a significant portion of the water and organic solvent to evaporate before the radiation curable material is cured. This evaporation step is particularly important because it is believed to define the image quality. Furthermore, the loss of a significant portion of the ink through the evaporation of the water and organic 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.

The printed ink is dried by evaporating the water and organic solvent using known methods, preferably by heating the printed ink. 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 one example, the ink is jetted onto a preheated substrate that then moves over a heated platen.

The printed image may be heated for up to 25 minutes, however heat is typically applied for shorter periods, for example for less than 10 minutes, less than 9, 8 or 5 minutes and further less than 2 minutes. Typically the length of time the ink is heated is determined by the print speed because the print speed in a typical roll fed printer determines the speed at which the printed substrate moves through the 'heating zone' where solvent is evaporated. The slower the print speed, the longer the time period in which the ink is heated. Where only lower quality prints are required, for example for posters that will be viewed from a distance, the print speed can be increased so that the posters can be printed more quickly without any detriment. For higher quality printed images, a slower print speed may be required.

In order to maximise image quality and control bleed and feathering between image areas, it is preferable to arrest the flow of the inks by quickly evaporating the water and organic solvent from the ink droplets 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 water and organic solvent, within 5 seconds of impact, within 1 second or within 0.5 seconds of impact.

If too much water is added to the ink of the invention, a large amount of heat is required to dry the ink. This can adversely affect pinning efficiency and print quality and the high temperatures required can cause distortion of heat-sensitive substrates. The presence of water in an amount of 5 to 50 % by weight allows the ink to dry in a reasonable time using moderate heat, however, and produces films with good image quality.

Unlike standard solvent-based inks, once the water and organic solvent have evaporated, the ink is not expected to be dry. Rather, what remains on the surface is a high viscosity version of a radiation curable ink. The viscosity is sufficiently high to inhibit liquid flow and prevent image degradation in the timescale that is needed to post-cure the ink.

The printed ink is cured by exposure to actinic radiation, preferably UV radiation. Sources of UV radiation are known to the person skilled in the art and include mercury discharge lamps, fluorescent tubes, flash lamps and LEDs. The source of UV radiation is positioned downstream from the water and organic solvent removal step in order to allow the water and organic solvent to evaporate from the printed ink before the ink is cured. The UV source may be a fixed lamp, or the source can placed on a carriage that allows the radiation source to move across the print width, for example.

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 μιη, preferably 1 to 10 μιη, for example 2 to 5 μιη. Film thicknesses can be measured using a confocal microscope.

The inks are primarily designed for printing onto flexible substrates but the nature of the substrate is not limited and includes any substrate which may be subjected to inkjet printing. The inks of the present invention are particularly suited for printing onto self adhesive vinyl and banner grade PVC substrates. The present invention also provides a method of inkjet printing as described above, and a substrate having the printed ink thereon. The ink of the present invention is particularly suited to piezoelectric drop-on-demand inkjet printing.

The present invention further provides an inkjet ink cartridge containing an inkjet ink as defined herein. The cartridges comprise an ink container and an ink delivery port which is suitable for connection with an inkjet printer.

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

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

Example

A cyan inkjet ink formulation having the composition shown in Table 2 was prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

CNSP066 is a water soluble urethane acrylate, Irgacure 819 and 2959 are photoinitiators and Zonyl FSN is a surfactant.

Table 2

Component Amount

CNSP066 13.0

Propylene glycol methyl ether 23.0

Propylene carbonate 25.7

Deionised water 25.0

Irgacure 819 2.0

Irgacure 2959 1.0

Aqueous cyan pigment dispersion 10.0

Zonyl FSN 0.3 The above ink composition has a viscosity of 6.0 mPa.s measured using a Brookfield DV1 low- viscosity viscometer running at 20 rpm at 25 °C with spindle 00.

The ink composition contains 23 % by weight of VOC.

The ink was drawn onto self adhesive vinyl substrate using a no 2 Kbar, depositing a wet film weight of 12 microns. The film was dried in an oven at 60 °C for three minutes. The dried film was exposed to UV radiation using a conveyorised drier running at 20 metres per minute fitted with two 80 W/cm medium pressure mercury lamps. The water resistance of the film before and after UV curing was assessed by rubbing with a soft cloth soaked in water. The number of double rubs required to break through to the substrate was recorded in each case (100 maximum). The results are shown in Table 3. Table 3

Before UV cure After UV cure

Number of double rubs 2 100+

Claims

An inkjet ink comprising a radiation curable oligomer, at least 30 % by weight of organic solvent based on the total weight of the ink, 5 to 50 % by weight of water based on the total weight of the ink, a photoinitiator and optionally a colourant.

An inkjet ink according to claim 1 , in which the organic solvent has one or more of the following properties:

(a) an evaporation rate equal to or less than that of butyl acetate.

(b) a flash point of 20 °C or more.

(c) a boiling point of 100 °C or more.

The inkjet ink according to claim 1 or claim 2 in which the evaporation rate of the organic solvent compared to butyl acetate is in the range of from 0.1 to 100, where the evaporation rate of butyl acetate is 100.

The inkjet ink according to any one of claims 1 to 3, in which the boiling point of the organic solvent is in the range of from 100°C to 260°C.

The inket ink according to any one of claims 1 to 4 wherein the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate of 100 or less, and a flash point of 20 °C or more, and optionally additionally a boiling point of 100 °C or more.

The inkjet ink according to any one of claims 1 to 5, in which the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate in the range of from 0.1 to 100, and a flash point of 20 °C or more, and optionally additionally a boiling point in the range of from 100 °C to 260 °C.

7. The inket ink according to any one of claims 1 to 6 wherein the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate of 65 or less and a flash point of 30 °C or more, and optionally additionally a boiling point of 115 °C or more.

The inkjet ink according to any one of claims 1 to 7 wherein the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate in the range of from 0.4 to 65, and a flash point of 30 °C or more, and optionally additionally a boiling point in the range of from 115 °C to 245 °C.

The inkjet ink according to any one of claims 1 to 8 wherein the organic solvent is selected from those having a relative evaporation rate compared to butyl acetate in the range of from 0.4 to 23, and a flash point of 40 °C or more, and optionally additionally a boiling point in the range of from 140 °C to 245 °C.

An inkjet ink according to any one of claims 1 to 9, comprising greater than 40 % by weight of organic solvent based on the total weight of the ink.

An inkjet ink according to any one of claims 1 to 10, in which the organic solvent is non-curable.

The inkjet ink according to any one of claims 1 to 1 1 wherein the radiation curable oligomer comprises free radical polymerisable groups.

The inkjet ink according to claim 12, in which the radiation curable oligomer comprises (meth)acrylate groups.

The inkjet ink according to any one of claims 1 to 13 wherein the radiation curable oligomer is selected from water-soluble urethane acrylates, water- soluble epoxy acrylates, water-soluble polyester acrylates, water-soluble polyether acrylates, water-dispersible polyurethane acrylates or mixtures thereof.

15. The inkjet ink according to any one of claims 1 to 14 wherein the radiation curable oligomer has a viscosity of 0.5 to 20 Pa.s at 60 °C.

16. The inkjet ink according to any one of claims 1 to 15 wherein the radiation curable oligomer is present in the composition in an amount of 2 % to 45 % by weight based on the total weight of the ink.

17. The inkjet ink according to any one of claims 1 to 16 wherein the ink comprises less than 10 % by weight, more preferably less than 5 % by weight of radiation-curable material having a molecular weight of 450 or less based on the total weight of the ink.

18. The inkjet ink according to any one of claims 1 to 17 wherein the organic solvent is selected from alcohols, glycol ethers, polyethylene glycol ethers, polypropylene glycol ethers, glycol ether esters, ketones, organic carbonates, pyrrolidones, esters, lactones and mixtures thereof.

19. The inkjet ink according to any one of claims 1 to 18 wherein the organic solvent is present in an amount of greater than 40 % by weight, or greater than 40 % to 80 % by weight, or 42 % to 65 % by weight based on the total weight of the ink.

20. The inkjet ink according to any one of claims 1 to 19 wherein the organic solvent is selected from gamma butyro lactone, propylene carbonate, diethylene glycol diethyl ether, propylene glycol monopropyl ether and mixtures thereof.

21. The inkjet ink according to any one of claims 1 to 19 wherein the organic solvent comprises an organic carbonate and/or a lactone.

22. The inkjet ink according to claim 20 or claim 21 wherein the organic solvent comprises propylene carbonate and/or gamma butyrolactone.

23. The inkjet ink according to claim 21 wherein the organic solvent is a mixture of i) an organic carbonate and/or lactone, and ii) one or more other solvents such that the carbonate and/or lactone is present in an amount of 15 to 40 % by weight based on the total weight of the ink, and the remaining organic solvent is present in an amount of 2 to 40 % by weight based on the total weight of the ink.

24. The inkjet ink according to any one of claims 1 to 23 wherein the ink comprises 5 to 50 % by weight of water.

25. An ink set comprising a cyan ink, a magenta ink, a yellow ink and a black ink wherein at least one of the inks is an inkjet ink according to any one of claims 1 to 24. 26. A method of inkjet printing comprising: jetting the ink as defined in any one of claims 1 to 24 from a printhead onto a substrate; evaporating at least a portion of the solvent and/or water from the printed ink; and exposing the printed ink to actinic radiation to cure the radiation curable oligomer. 27. The method according to claim 26 wherein the ink is jetted at less than 35 °C, preferably less than 30 °C.

28. The method according to claim 26 or claim 27 wherein the printhead is not heated.

29. The method according to any of claims 26 to 28 wherein the ink is jetted in drop sizes of 50 picolitres or less.

30. The method according to any one of claims 26 to 29 wherein the portion of organic solvent and/or water is evaporated from the printed ink before the ink is cured.

31. The method according to any one of claims 26 to 30 wherein substantially all of the organic solvent and water is evaporated from the printed ink before the ink is cured.

32. The method according to any one of claims 26 to 31 wherein the printed ink is heated in order to evaporate the organic solvent and/or water within 5 seconds of the ink being jetted onto the substrate, preferably within 1 second, more preferably within 0.5 seconds.

33. The method according to any one of claims 26 to 32 wherein the time period between jetting the ink from the printhead onto the substrate and exposing the printed ink to actinic radiation is at least 1 second.

34. An inkjet cartridge comprising the ink as claimed any of claims 1 to 24.

35. A substrate having the ink as claimed in any of claims 1 to 24 printed thereon.