WO2009098509A1

WO2009098509A1 - A printing ink

Info

Publication number: WO2009098509A1
Application number: PCT/GB2009/050105
Authority: WO
Inventors: Angelique Catherine Joyce Runacre; Matthew Roger Brooks
Original assignee: Sericol Limited
Priority date: 2008-02-04
Filing date: 2009-02-04
Publication date: 2009-08-13
Also published as: GB2469407A; GB201012352D0; GB2469407B

Abstract

This invention relates to an ink-jet ink substantially free of water and volatile organic solvent. The ink comprises at least one radiation-curable monomer, at least one photoinitiator, Tego dispers (685) and a dispersed pigment selected from: Pigment Yellow (120), Pigment Yellow (138), Pigment Yellow (150), Pigment Yellow (151), Pigment Yellow (155), Pigment Yellow (180), Pigment Yellow (194) and combinations thereof; Pigment Green (7); Pigment Orange (36); Pigment Violet (19), Pigment Violet (22), Pigment Violet (23), Pigment Violet (122) and combinations thereof; Pigment Blue (15:1) Pigment Blue (15:2) Pigment Blue (15:3), Pigment Blue (15:6) and combinations thereof; Carbon Black; and Pigment White (6). The ink is particularly stable.

Description

A printing ink

This invention relates to a printing ink and in particular to a printing ink containing a dispersed pigment and a dispersant.

In ink-jet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically at or below 50 mPas at 25°C, or using an HSS head, at or below 200 mPas when measured at 25⁰C (although when ejected through the nozzles, the jetting temperature is often elevated to about 40⁰C).

Ink-jet inks are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent. In one common type of ink-jet ink this liquid is water (see for example the paper by Henry R. Kang in the Journal of Imaging Science 1991, 35(3), pp. 179-188). In another common type the liquid is a low-boiling solvent or mixture of solvents (see, for example, EP 0 314 403 and EP 0424 714).

Another type of ink-jet ink contains unsaturated organic compounds, termed monomers, which polymerise by irradiation, commonly with ultraviolet light, in the presence of a photo initiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures. In such ink-jet inks it is necessary to use monomers possessing a low viscosity.

Inks also contain a colouring agent. The colouring agent may generally be dissolved or dispersed in the liquid medium of the ink. The colouring agents are commercially available, for example under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). When the colouring agent is a dispersed pigment, the pigment dispersion produces an increase in the viscosity of the ink. In many cases, this viscosity increase can be substantial. Even in cases where a stable dispersion at an appropriate viscosity is achieved, the viscosity and particle size can increase with time leading to an ink which is unsuitable for long-term storage and hence cannot be a commercially viable ink.

Numerous approaches have been proposed to increase the stability of pigment dispersions in inks, and particularly ink-jet inks where a low viscosity is critical to the jetting of the ink. Solvents and water may be used either to stabilise the pigment directly, or to reduce the viscosity of the ink whilst using a separate component to stabilise the dispersion. However, solvents and water are unsuitable for most radiation-curing inks. The reactive diluents can be selected to improve the pigment dispersion, but this places unacceptable limitations on other properties of the ink, such as cure speed, flexibility etc. Commercially available surfactants, wetting agents, dye synergists and dispersants may also be used to increase stability. The pigments themselves have also been treated to increase their dispersiblity, e.g. the commercially available non-flocculating grade (RL-NF) of Hostaperm Violet RL with Solsperse 32000. However, certain pigments remain particularly problematic to disperse and hence there remains in the art a need for improved pigment dispersions.

Accordingly, the present invention provides an ink-jet ink substantially free of water and volatile organic solvent comprising at least one radiation-curable monomer, at least one photoinitiator, Tego dispers 685, and a dispersed pigment selected from: Pigment Yellow 120, Pigment Yellow 138, Pigment Yellow 150, Pigment Yellow 151, Pigment Yellow 155, Pigment Yellow 180, Pigment Yellow 194 and combinations thereof; Pigment Green 7; Pigment Orange 36;

Pigment Violet 19, Pigment Violet 22, Pigment Violet 23, Pigment Violet 122 and combinations thereof; Pigment Blue 15:1 Pigment Blue 15:2 Pigment Blue 15:3, Pigment Blue 15:6 and combinations thereof; Carbon Black; and Pigment White 6. This combination provides a surprisingly stable ink despite the presence of pigments which have been found to be otherwise difficult to formulate in a stable ink.

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

Figs. Ia- Id show the variation in the viscosity and particle size of inks A, B₃ C and D when held at 4O⁰C and room temperature for 4 weeks; and

Figs. 2a-2d show the variation in the viscosity and particle size of inks C, D, E and F when held at 4O⁰C and room temperature for 4 weeks.

The ink-jet ink of the present invention contains at least one radiation-curable monomer, at least one photoinitiator, a dispersed pigment and a dispersant. It dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a radiation-curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink, although the presence of such components may be tolerated. The ink-jet ink of the present invention is therefore substantially free of water and volatile organic solvent.

The ink of the present invention preferably includes at least one (meth)acrylate monomer as the radiation-curable monomer. The at least one (meth)acrylate monomer is selected from a monofunctional monomer, a multifunctional monomer and combinations thereof. (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.

Examples of the multifunctional acrylate monomers which may be included in the ink-jet inks include hexanediol diacrylate (HDDA), trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylate, for example, tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate (dPGDA), 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 (POnPGDA), ethoxylated trimethylolpropane triacrylate, and mixtures thereof. Particularly preferred are difunctional acrylates. Also preferred are those with a molecular weight greater than 200. Preferred multifunctional monomers are hexanediol diacrylate, dipropyleneglycol diacrylate, propoxylated neopentyl glycol diacrylate and combinations thereof.

In addition, suitable multifunctional acrylate monomers 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.

Multifunctional (meth)acrylate monomers may be included at 1-90% by weight, preferably 5-85% by weight, more preferably 40-80% , most preferably 50-70% by weight, based on the total weight of the ink.

The monofunctional (meth)acrylate monomers are also well known in the art and are preferably the esters of acrylic acid. Preferred examples include phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, 2- (2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate, tridecyl acrylate, isodecyl acrylate and lauryl acrylate.

Monofunctional (meth)acrylate monomers may be included at 1-90% by weight, preferably 3-80% by weight, more preferably 5-30% by weight, most preferably 10- 20% by weight, based on the total weight of the ink.

The inks of the present invention may also contain as a radiation-curable monomer, at least one α,β-unsaturated ether monomer, such as a vinyl ether. These monomers are known in the art and may be used to reduce the viscosity of the ink formulation. Typical vinyl ether monomers which may be used in the inks of the present invention are triethylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1,4- cyclohexanedimethanol divinyl ether and ethylene glycol mono vinyl ether. Mixtures of vinyl ether monomers may be used. The vinyl ether monomer is preferably 1-20% by weight, more preferably 7-15% by weight, based on the total weight of the ink. In a preferred embodiment, the ratio of acrylate monomer to vinyl ether monomer is from 4:1 and 15: 1. See WO 02/061001 for further details of formulations containing α,β-unsaturated ether monomers in combination with acrylate monomers.

N- Vinyl amides and N-(meth)acryloyl amines may also be used in the inks as a radiation-curable monomer (regarding the nomenclature, since the term "acryloyl" incorporates a carbonyl group, the amide is actually named as an amine). These monomers are well-known in the art. Preferred examples of N-vinyl amides are N- vinyl caprolactam (NVC) and N-vinyl pyrrolidone; and of N-(meth)acryϊoyl amines, N-acryloylmorpholine. NVC is particularly preferred.

N- Vinyl amides and/or N-acryloyl amines may be included at 3-50% by weight, preferably 5-30% by weight, more preferably 10-20% by weight, based on the total weight of the ink.

It is possible to modify further the film properties of the ink-jet inks by inclusion of oligomers or inert resins, such as thermoplastic acrylics. However, it should be noted that in the case of oligomers and multifunctional monomers the flexibility may be adversely affected and also that some adjustments to stoichiometry may be required to retain optimum cure speed. Said oligomers have weight-average molecular weight from 500 to 8,000, preferably from 1,000 to 7,000 and most preferably from 1 ,500 to 5,000.

Oligomers may be included at 1-30% by weight, preferably 2-20% by weight and more preferably 4-15% by weight, based on the total weight of the ink.

In addition to the monomers described above, the compositions include a photoinitiator, which, under irradiation by, for example, ultraviolet light, initiates the polymerisation of the monomers. Preferred are photoinitiator s which produce free radicals on irradiation (free radical photoinitiators) such as, for example, benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4- morpholinophenyl)butan-l-one, benzil dimethylketal, bis(2,6-dimethylbenzoyI)-2,4,4- trimethylpentylphosphine oxide, ethyl 4-(dimethylamine) benzoate (EDB) or mixtures thereof. Such photo initiators 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 25% by weight, more preferably from 5 to 15% by weight, of the ink.

The ink also includes a dispersed pigment. The dispersed pigment is an essential feature of the ink and provides the required colour properties. The pigment is typically provided as a powder, having been milled and filtered to achieve an appropriate particle size. The provision of pigment in the form of a powder allows for the dispersion of the pigment through the ink. The average particle size (diameter) of the dispersed pigment will typically be 5 microns or less, preferably 1 micron or less and more preferably 0.5 microns or less. However, the inclusion of a dispersed pigment leads to an increase in the viscosity of the ink. The viscosity of the ink can be controlled by a careful selection of the components of the ink. However, an ink containing a dispersed pigment will tend to an increase in viscosity and particle size on storage.

It has been found that certain pigments which are otherwise difficult to formulate successfully, can be stabilised by using a particular dispersant, namely Tego® Dispers 685, available from Tego (Degussa). The dispersant of the present invention is a polymer comprising a polyester-containing polyamine and a polyethylene oxide. The polyamine is based on the following repeat unit:

The substituent, X, represents hydrogen or a radical having both a polyester region and an alkyl chain (preferably C_5-20, more preferably C_10-I₅ and most preferably C₁]). The radical preferably has the formula:

where m = 4 or 5.

Although the alkyl chain is shown having 11 carbon atoms, the alkyl chain may be C5-20, preferably Ci 0-15-

The polyethylene oxide has the following structure:

where q = 8-12.

The values of n and p are selected such that the polymer has a weight-average molecular weight of around 50,000 to 100,000 and particularly 67,200 as measured by GPC.

This dispersant is preferably present at 0.2 to 10% by weight, more preferably 0.5 to 5% by weight and most preferably 1 to 3% by weight, based on the total weight of the ink.

The pigments of the present invention, which have been found to be particularly well stabilised by Tego® Dispers 685, are (as defined by their CI (Colour Index International) number):

Pigment Yellow 120 (a benzimidazolone, e.g. Novaperm Yellow H2G), Pigment Yellow 138, Pigment Yellow 150 (an azo metal complex), Pigment Yellow 151, Pigment Yellow 155 (a bisarylacetarylide), Pigment Yellow 180, Pigment Yellow 194, and combinations thereof; Pigment Green 7; Pigment Orange 36; Pigment Violet 19 (e.g. Hostaperm InkJet E5B02), Pigment Violet 22, Pigment Violet 23, Pigment Violet 122 and combinations thereof; and

Pigment Blue 15:1 Pigment Blue 15:2 Pigment Blue 15:3, Pigment Blue 15:6 and combinations thereof (copper phthalocyanines);

Carbon Black (preferably Pigment Black 6, Pigment Black 7, Pigment Black 8 and combinations thereof); and

Pigment White 6 (titanium dioxide, such as rutile white and/or anatase white, e.g. Kronos 2300).

Blends of the above pigments are also included in the present invention. For example, Quinacridone blends of pigment violet such as Cinquasia Magenta RT-355D and Cromophtal Jet 2BC (a blend of Pigment Violet 19 and Pigment Violet 22).

Preferred pigments are Pigment Yellow 120, Pigment Yellow 155, Pigment Green 7, Pigment Orange 36, Pigment Violet 19, Pigment Violet 22, Pigment Violet 23, and a combination of Pigment Violet 19 and Pigment Violet 22, and particularly preferably Pigment Yellow 120.

In a preferred embodiment, the ink-jet inks of the present invention are provided as an ink-jet ink set. The set is preferably based on the CMYK system, and may also contain orange, green and violet inks as well as light versions of the CMYK inks, and also white. The set will include at least one of the inks of the present invention.

The total proportion of pigment present in the ink is preferably from 0.5 to 15% by weight, more preferably from 1 to 5% by weight, based on the total weight of the ink, with the proviso that, the light versions of the inks will always contain less pigment than the full colour version of the ink, e.g. light magenta contains less pigment than magenta. In a preferred embodiment of the present invention, the ink comprises by way of monomers at least one monofunctional (meth)acrylate monomer and at least one monofunctional N-vinyl amide monomer wherein the molar ratio of the at least one monofunctional (meth)acrylate monomer to the at least one monofunctional N-vinyl amide monomer is from 0.5 to 15, preferably from 1.0 to 10 and most preferably from 1.5 to 6.0.

It has also been found that certain monomers also tend to destabilise the pigment dispersion. These monomers include dipropyleneglycol diacrylate (dPGDA), phenoxyethyl acrylate (PEA) and N-vinyl caprolactam (NVC). A particularly problematic combination is an ink containing PEA and NVC.

In a preferred embodiment, the ink contains at least one monomer selected from dPGDA, HDDA, PEA and NVC, preferably PEA and NVC or dPGDA and HDDA, and a pigment selected from Pigment Yellow 120, Pigment Yellow 155, Pigment Green 7, Pigment Orange 36, Pigment Violet 19, Pigment Violet 22, Pigment Violet 23, and a combination of Pigment Violet 19 and Pigment Violet 22, and preferably Pigment Yellow 120.

A particularly preferred ink contains 10-50 wt% PEA, 10-50 wt% NVC, 1-20 wt% HDDA and at least one of the pigments of the present invention.

Although the ink of the present invention preferably cures by a free radical mechanism, the ink of the present invention may also cure by a cationic mechanism, or 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 cationicaϊly 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 triphenyl sulfonium hexafluophosphate. Suitable cationic photoinitiators include the Union Carbide UVl-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.

The present invention also provides a method of ink-jet printing using the above- described ink and a substrate having the cured inks thereon. The inks of the present invention are particularly suited to piezoelectric drop-on-demand ink-jet printing. Suitable substrates include styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC) and VIVAK (a polyethylene terephthalate glycol modified). The inks of the present invention are preferably cured by ultraviolet irradiation and are suitable for application by ink-jet printing. The present invention further provides an ink-jet ink cartridge containing the ink of the present invention. The cartridges comprise an ink container and an ink delivery port which is suitable for connection with an ink-jet printer.

The ink-jet inks exhibit a desirable low viscosity (100 mPas or less, preferably 80 mPas or less, more preferably 50 mPas or less and most preferably 25 mPas or less at 25⁰C). Viscosity may be measured using a Brookfield DVl low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

The inks of the 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.

Examples

The invention will now be described, by way of example, with reference to the following example (parts given are by weight).

Example 1

An ink-jet ink was prepared employing a DPGDA-based system using Tego Dispers 685 for stabilising PY 120. The formula is set out in Table 1. The dispersant is prepared based on Tego Dispers 685, SR9003 (NPGPODA) and the pigment and then combined with the other components of the ink. Table 1. DPGDA-based UV-curable yellow ink-jet ink using Tego Dispers 685 in an NPG(PO)DA dispersion with components shown as a percentage by weight.

This ink was compared to a commercially available ink-jet ink employing a DPGDA- based system using the standard dispersant Dysperbyk 168 for PY 120 having the formula set out in Table 2. The dispersant is prepared based on Dysperbyk 168, a part of the Rapi-Cure DVE3 and the pigment and then combined with the other components of the ink.

Table 2. DPGDA-based UV-curable yellow ink-jet ink using Dysperbyk 168 in a Rapi-Cure DVE3 dispersion.

The physical properties of the inks were measured using the following techniques. Viscosity

The target viscosity for the inks was within a range of 20 to 30 mPas. The ink sample was measured using a Brookfield DVl + Rheometer, UL-A Spindle and Adapter kit and a temperature-controlled water bath to ensure a constant temperature of 25^CC. After ensuring the Brookfield DVl rheometer was level, calibrated and zeroed prior to measuring samples (once switched on, the machine prompts with instructions), a 16 cm³ sample was measured via a syringe and added to the rheometer cup. Spindle 00 was selected. The spindle was placed inside the cup and attached to the rheometer, via a hook. The rheometer motor was switched on and the speed selected using touch pad buttons. A rotational speed of 20 rpm was selected. The water bath temperature was checked and the sample allowed to equilibrate to the measuring temperature over a period of five minutes. Once a stable reading was obtained, the sample measurement was ended and the equipment cleaned,

Particle Size (Malvern Laser Diffraction)

Particle size measurements were conducted using a Malvern Mastersizer 2000 Particle Size Analyser.

Once switched on, the Dispersion Unit was flushed and filled several times using the correct diluent/dispersion medium. Depending on the sample under analysis, the correct optical model was selected (or a new model constructed based upon the refracted indices of the dispersing medium and the particulate matter to be evaluated).

With the dispersion unit operating at 2200 rpm and filled with the required dispersing medium, the laser was aligned and a background reading taken to measure and allow for the particles present in the dispersion medium and the electrical interference from the laser, when conducting the sample measurement, and subsequent statistical calculations. A small quantity of sample was added to the dispersion unit until the range of the laser was obscured by 15-20% (indicated by the computer). Once the correct amount of dispersion was added, the sample and obscuration level was allowed to stabilise for 2 minutes and a measurement taken. Using the Mie theory for particle size and the extent of laser diffraction caused by the sample, the results were presented both graphically and in a table showing the percentage of sample at different diameters, for both by number and by volume. The D_v90 (90% of particle volume less than size figure quoted) and D_n90 (90% of number of particles less than size figure quoted) were used.

Particle Size (Filtration)

A side-armed flask and stainless steel filter funnel were attached to a vacuum pump and the vacuum regulated to 400 mbar. A Uttipor N66 NCG025 100 removal rating, 3 micron filter was placed across the funnel. Ink samples were not diluted. Samples of 10 cm³ and 50 cm³ of the test product were injected via a syringe into the funnel and the time taken for the product to pass across the filter membrane was noted. The ideal filtration time was stated to be 10-40 seconds for 10 cm³ and less than 300 seconds for 50cm³. If the filter blocked before 50 cm³ had passed through the filter, an indication of failure was given, as the amount or size of particles present could block the in-line filter in an ink-jet printhead.

Cure

A Svecia Dryer with two lamps on full power was used to assess cure performance. The ink was applied at a wet film weight of 12 microns and then passed under the UV lamps at 35 m/min. The ink was then checked for cure using the conventional scratch, finger rub and tape adhesion tests.

The initial values for the inks set out in Tables 1 and 2 are shown in Table 3.

Table 3. Physical property data for Tego 685-based inks versus standard Dysperbyk 168 using different dispersant media.

The initial viscosities were both within the required range of 22 ± 0.5 mPas. The change in viscosity and particle size for the inks is shown in the attached Figs. Ia to Id. Fig. Ia compares the viscosity of the inks when held at 4O⁰C for 4 weeks. Fig. Ib compares the viscosity of the inks when held at room temperature for 4 weeks. It is apparent that the ink of the present invention shows a significantly more stable viscosity profile. Similar results may be seen by comparing the inks in Figs. Ic and 1 d which show the change in particle size of the inks over 4 weeks when held at 40⁰C and room temperature, respectively.

Example 2

Further ink-jet inks were prepared to explore the role of the dispersant following the procedure set out in Example 1. The formulations are set out in Tables 4 and 5. The corresponding inks have essentially the same formulations, other than the dispersant.

Table 4. A DPGDA-based UV-curable yellow ink-jet ink using Tego Dispers 685 or Dysperbyk 168 in a PEA dispersion with components shown as % by weight.

Table 5. An NVC/PEA-based UV-curable yellow ink-jet ink using Tego Dispers 685 or Dysperbyk 168 in a PEA dispersion with components shown as % by weight.

The initial values for the viscosity and particle sizes of inks C and E set out in Tables 4 and 5 are shown in Table 6.

Table 6. Physical property data for Tego Dispers 685-based inks using PEA as the dispersant medium.

The variation in the viscosity and particle size of the inks when held at 4O⁰C for 4 weeks is shown in Figs. 2a and 2b, and Figs 2c and 2d, respectively. The inks of the present invention are clearly superior in both respects compared to those using the conventional dispersant.

Example 3

Dispersions were formed using Dysperbyk 168 for dispersion 1 (comparative) and Tego Dispers 685 for dispersion 2 (invention). The dispersions are set out in Table 7.

Table 7. Dispersions incorporating Dysperbyk 168 or Tego Dispers 685 with components shown as a percentage by weight.

The dispersions were then formulated into ink-jet inks. The formulations of inks Gl- G3 (comparative) and H1-H3 (invention) are set out in Table 8.

Table 8. Ink-jet inks incorporating dispersions A and B from Table 7 with components shown as a percentage by weight..

The viscosities and particle sizes measured for inks G1-G3 and H1-H3 are set out in Table 9. The initial values and the values on storage at 4O⁰C and at room temperature (RT) were measured. Table 9. Physical property data for inks G1-G3 and H1-H3 from Table 8.

Viscosity is in units of mPas; particle size is in units of microns.

² Comparative

³ Invention

Inks Gl and Hl are based on the monomers dPGDA and DVE- 3 and incorporate an oligomer, Ebecryl 657. The inks initially show a viscosity of 21.8 mPas and 22.5 mPas, respectively. On storage, the viscosity of ink Gl increases significantly and shows a value of 28.7 mPas after 4 weeks at 4O⁰C. In contrast, ink Hl shows greater stability, with the viscosity only increasing slightly to 23.4 mPas on storage for 4 weeks at 40⁰C. At room temperature, little increase is seen in ink Hl even after 4 weeks. Similar results are shown for the particle size.

Inks G2 and H2 include the monomers dPGDA and DVE-3 as well as HDDA. The oligomer is Craynor CN964 A85. The inks initially show a viscosity of 25,9 mPas and 22,5 niPas, respectively. On storage at 4O⁰C, the viscosity of ink G2 increases significantly after 4 weeks whereas the increase for ink H2 is markedly less. At room temperature, no increase is seen in ink H2even after 4 weeks whereas an increase of

2.3 mPas to 27.2 mPas is seen for ink G2. Similar results are shown for the particle size.

Inks G3 and H3 are based on the monomer combination PEA and NVC, with a small quantity of HDDA, It has been found that the combination of PEA and NVC leads to particularly unstable dispersions of a number of pigments of the present invention. In the presence of Dysperbyk 168, ink G3 became completely unstable after a few days at 4O⁰C and in less than 4 weeks at room temperature. However, ink H3 in the presence of Tego Dispers 685 increased only slightly in viscosity after 4 weeks at 4O⁰C and hardly any increase was seen even after 4 weeks at room temperature. Similar results are shown for the particle size.

Claims

1. An ink-jet ink substantially free of water and volatile organic solvent comprising at least one radiation-curable monomer, at least one photoinitiator, Tego dispers 685, and a dispersed pigment selected from:

Pigment Yellow 120, Pigment Yellow 138, Pigment Yellow 150, Pigment Yellow

151, Pigment Yellow 155, Pigment Yellow 180, Pigment Yellow 194 and combinations thereof;

Pigment Green 7; Pigment Orange 36;

Pigment Violet 19, Pigment Violet 22, Pigment Violet 23, Pigment Violet 122 and combinations thereof;

Pigment Blue 15: 1 Pigment Blue 15:2 Pigment Blue 15:3, Pigment Blue 15:6 and combinations thereof; Carbon Black; and

Pigment White 6,

2. An ink-jet ink as claimed in claim 1, wherein the at least one radiation-curable monomer is selected from a (meth)acrylate monomer, an α,β-unsaturated ether monomer, an N-vinyl amide, an N-(meth)acryloyI amine, and combinations thereof.

3. An ink-jet ink as claimed in claim 1 or 2, wherein the at least one radiation- curable monomer includes hexanediol diacrylate, dipropyleneglycol diacrylate, propoxylated neopentyl glycol diacrylate, phenoxyethyl acrylate or combinations thereof.

4. An ink-jet ink as claimed in any preceding claim, wherein the at least one radiation-curable monomer includes triethylene glycol divinyl ether.

5. An ink-jet ink as claimed in any preceding claim, wherein the at least one radiation-curable monomer includes N-vinyl caprolactam.

6. An ink-jet ink as claimed in any preceding claim, wherein the at least one radiation-curable monomer includes phenoxyethyl acrylate and N-vinyl caprolactam.

7. An ink-jet ink as claimed in any preceding claim, wherein the at least one radiation-curable monomer includes 10-50% by weight of phenoxyethyl acrylate, 10- 50% by weight of N-vinyl caprolactam and 1-20% by weight of hexanediol diacrylate, the weights being based on the total weight of the ink.

8. An ink-jet ink as claimed in any preceding claim, wherein the dispersed pigment is selected from Pigment Yellow 120, Pigment Yellow 155, Pigment Green 7, Pigment Orange 36, Pigment Violet 19, Pigment Violet 22, Pigment Violet 23, and a combination of Pigment Violet 19 and Pigment Violet 22.

9. An ink-jet ink as claimed in any preceding claim, wherein the dispersed pigment is present at 0.5 to 15% by weight, based on the total weight of the ink,

10. An ink-jet ink as claimed in any preceding claim, wherein the Tego Dispers 685 is present at 0.2 to 10% by weight, based on the total weight of the ink.

11. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in any preceding claim on to a substrate and curing the ink.

12, A substrate having the ink-jet ink as claimed in any of claims 1 to 10 printed thereon.

13. An ink-jet ink cartridge containing the ink-jet ink as claimed in any of claims 1 to 10.