WO2009133402A1

WO2009133402A1 - A printing ink

Info

Publication number: WO2009133402A1
Application number: PCT/GB2009/050446
Authority: WO
Inventors: Yuuichi Hayata; Takehiko Sato; Vincent Wright; Bridgette Blaskett
Original assignee: Sericol Limited
Priority date: 2008-04-30
Filing date: 2009-04-30
Publication date: 2009-11-05
Also published as: GB0807869D0

Abstract

This invention relates to method for treating an ink-jet ink comprising: providing an ink-jet ink which comprises a radiation-curable monomer, a photoinitiator and a dispersed pigment; and subjecting the ink to ultrasound and/or centrifugation.

Description

A printing ink

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

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 25⁰C, 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 4O⁰C).

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 0 424 714).

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 photoinitiator. 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). However, it has been found that when the colouring agent is a dispersed pigment, the presence of agglomerated pigment particles can lead to nozzle blockage when the ink is printed.

Ink-jet printing machines include an ink path from an ink supply (usually a cartridge) to a nozzle in the printhead from which ink drops are ejected. Ink drop ejection is controlled by pressurising ink in the ink path with an actuator, which may be for example a piezoelectric deflector, a thermal bubble jet generator or an electrostatically deflected element. In high performance printheads, the nozzle openings are very narrow and typically have a diameter of 50 microns or less.

The printing accuracy of ink-jet printing machines is influenced by a number of factors, including the size and velocity uniformity of the drops ejected by the nozzles in the printhead. The drop size and drop velocity uniformity are in turn influenced by a number of factors. One important factor is the contamination of the ink flow paths with pigment particles, and particularly agglomerates of pigment particles. This is a particular problem at the nozzle and often leads to blockage which can stop the ink from ejecting completely.

Accordingly, the present invention provides a method for treating an ink-jet ink comprising: providing an ink-jet ink which comprises a radiation-curable monomer, a photoinitiator and a dispersed pigment; and subjecting the ink to ultrasound and/or centrifugation.

It has been found that by employing these techniques, nozzle blockage can be significantly reduced, thereby increasing printer efficiency by reducing the frequency of costly stoppages. Surprisingly, it has been found that a major factor in the blocking of the nozzles is the release of air bubbles from the aggregated pigment particles as the particles pass through the printhead.

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

Fig. 1 shows an ultrasonic processor; and

Fig. 2 shows a graph of count versus particle size for inks falling outside (top curve) and within the present invention (lower three curves). The ink-jet ink of the present invention contains at least one radiation-curable monomer, at least one photoinitiator and a dispersed pigment. 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 preferably 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, ϊ ,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 Methylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1,4- cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl 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 of the present invention, the ink comprises by way of monomers at least one monofunctional (meth)acrylate monomer and at least one α,β-unsaturated ether monomer, such as a vinyl ether. The ratio of acrylate monomer to vinyl ether monomer is preferably 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)acryloyl 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 or polyesters. 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 photoinitiators 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-dimethylbenzoyl) -2,4,4 - trimethylpentylphosphine oxide, ethyl 4-(dimethylamine) benzoate (EDB) 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 25% by weight, 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, filtered and subjected to ultrasound or centrifugation 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 (Dv50) of the dispersed pigment will typically be 5 microns or less, preferably 1 micron or less and more preferably 0.5 microns or less. In a preferred embodiment, the Dv50 is 0.1-0.4 microns.

Pigments which have been found to be particularly difficult to disperse 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 1, 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 Cromophtal Jet 2BC and Cinquasia Magenta RT-355D (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, Pigment Yellow 155, or Pigment Violet 19 optionally with Pigment Violet 22. 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.

As explained hereinabove, it has been found that the dispersed pigment, particularly those pigments explicitly recited hereinabove have a tendency to form aggregates which in turn tend to block the nozzle(s) of the printing head. However, careful inspection of the nozzles reveals that they tend to be blocked by air bubbles which are released from the aggregated pigment particles during the jetting process. The air bubbles block the nozzles, for example, by absorbing the piezo-impulse from the printhead and hence no impulse is imparted to the ink. It has been found that by pretreating the ink prior to jetting, the aggregates are broken down thereby releasing a substantially part of the entrapped air.

The aggregates can be disrupted thereby reducing the particle size of the dispersed pigment by subjecting the ink to ultrasound and/or the aggregates can be separated from the remainder of the ink by centrifugation. Although both techniques can be used subsequently or simultaneously, for economic reasons more usually only one technique will be employed for any given ink batch. Thus, according to the present invention the ink-jet ink is provided and then subjected to ultrasound and/or centrifugation. Preferably, the ink is then transferred to a suitable container, for example an ink-jet ink cartridge.

The ultrasound and/or centrifugation is preferably performed at 60⁰C or less, more preferably 10-50⁰C and most preferably 25-50⁰C to avoid or at least minimise chemical degradation of the ink. The ink is usually subjected to ultrasound and/or centrifugation for at least 0.5 seconds. The upper limit to the time is less critical but will not usually be in excess of 30 mins. Preferably the treatment is for 0.5-10 seconds, most preferably 1-5 seconds. The treatment may be batch-wise or continuous. In the continuous process, the feeding speed is preferably from 0.5 to 10 L/min, more preferably from 1 to 5 L/min. The continuous process may be driven by any suitable feeding pump, such as a Watson Marlow 62Oi.

Fig. 1 shows an ultrasonic processor 1 suitable for a continuous ultrasound treatment. The ink 2 is introduced into the processor 1 via entrance port 3 at a flow rate F of 2 L/min. The capacity within the processor 1 is around 100 mL. The processor 1 includes a probe 4 and as the ink 2 passes the probe 4 it comes into contact with the ultrasonically active area 5 of the probe 4. At this part of the processor 1 the ink 2 is subjected to the ultrasound. The processor 1 also contains water cooling 6 to prevent excessive rises in temperature leading to degradation of the ink 2. After treatment, the ink 2 passes from the processor 1 through exit port 7.

For centrifugation, suitable parameters are: for continuous flow, a flow rate of 100 L/h, a chamber volume of about 5 L (hence a duration of 3 min) and a rotational speed of 8,400 rpm; and for batch processing, 10-30 min at 9,000 rpm.

The ultrasound used is required to have a sufficient frequency and power to cause deagglomeration of agglomerated pigment particles in the ink. The frequency is preferably 10-100 KHz, more preferably 15-50 KHz and most preferably about 20 KHz. The power is preferably 100-3,000 W, more preferably 1,000-2,000 W and most preferably about 1,500 W. A suitable ultrasonic processor is the VCF- 1500 (1,50OW, 20 KHz) from Sonics Sc Materials Inc. A plurality of ultrasonic processors may also be used.

The centrifugation uses centripetal force to separate agglomerated particles from the remainder of the ink (the supernatant) and the centrifugation need to generate a sufficient force to achieve this separation. The speed is preferably 2,000-20,000 rpm, more preferably 3,000-15,000 rpm, more preferably 4,000-14,000, more preferably 5,000-12,000 and most preferably about 8,400 rpm. A suitable centrifuge separator is the SA-7, Westfalia Separator. The resultant (treated) inks are thus characterised by a reduced particle size in the dispersed pigment and in particular a significantly lower amount in larger agglomerates which have a tendency to produce nozzle blockage, In this regard, the ink is structurally different from an ink merely milled and filtered since it will have narrower range of particles sizes and particularly will have fewer particles having a diameter greater than 1 micron. This is shown graphically in Fig. 2, which shows a graph of counts against particle size for the large particle fragment (one micron or greater) of a number of inks. The upper most curve is for an unprocessed ink, i.e. an ink which has been milled and filtered but not subjected to any further treatment; the next curve is an ink which has been milled and filtered and then treated with ultrasound at a flow rate 120 L/h; the next curve is an ink which has been milled and filtered and then treated by centrifugation at a flow rate of 100 L/h; and the lower curve is an ink which has been milled and filtered and then treated by centrifuge circulation for 4 hours.

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 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 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 (e.g. Dysperbyk, Tego Dispers, Solsperse etc.) 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 container, e.g. a 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 (200 mPas or less, preferably 100 mPas or less, more preferably 50 mPas or less and most preferably 25 mPas or less at 25°C). 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 examples (parts given are by weight).

Example 1

An ink-jet ink (assigned the code IM-100) was prepared by combining the components shown in Table 1 in a Silverson mechanical stirrer. Table 1. Formulation of ink-jet ink IM-100.

Other coloured inks were prepared by varying the pigment. The pigment dispersion varied depending on the colour of the ink, as follows:

Yellow ink IMlOl was prepared using Novoperm Yellow H2G (Clariant).

Cyan ink IM 102 was prepared using Irgalite Blue GLVO (Ciba).

Black ink IMl 03 was prepared using Special Black 250 (Evonik).

Example 2

The inks prepared in Example 1 were subjected to ultrasound treatment using an ultrasonic processor: Model: VCF- 1500 (1,50OW, 20 KHz) from Sonics & Materials Inc. The processor employed a Watson Marlow 62Oi feeding pump.

Magenta ink IM200 was prepared by feeding IMlOO through the ultrasonic processor. The feeding speed was 2 L/min. The ink inlet temperature was 21⁰C and the outlet temperature was 3O⁰C. Yellow ink IM201 was prepared by feeding IMlOl through the ultrasonic processor at the same ink inlet and outlet temperatures as IMlOO.

Cyan ink IM202 was also prepared by feeding IMl 02 through the ultrasonic processor. The ink inlet temperature was 2O⁰C and the outlet temperature was 29⁰C.

Black ink IM203 was prepared by feeding IMl 03 through the ultrasonic processor. The ink inlet and outlet temperatures were also 2O⁰C and 29°C, respectively.

Example 3

The jetting properties of the inks were determined using a UV inkjet printer: LuxelJET UV250GT, from Fujifilm Graphic Systems Co. Ltd. The resolution was 600*450 dpi and the print size was 2m(W)* Im(L), The number of nozzles lost was checked during 10 continuous prints and the results are shown in Table 2.

Table 2. Jetting properties of the inks.

It is apparent that the nozzle loss for the treated inks was significantly lower than that for the untreated inks. Example 4

Further samples of the inks prepared in Example 1 were subjected to centrifugation using a centrifuge separator: Model SA-7, Westfalia Separator with a Watson Marlow 62Oi feeding pump.

Magenta ink IM300 was prepared by feeding IMlOO through the centrifuge separator. The feeding speed was 1.7 L/min. The rotational speed of the centrifuge separator was 8,400 rpm. The amount of wasted high density liquid was 1 % of the initial amount of IMlOO.

Yellow ink IM301, cyan ink IM302 and black ink IM303 were prepared in an analogous manner. The amount of wasted high density liquid in each case was also 1% of the initial amount of IM1## inks.

Example 5

The jetting properties of the inks were determined using a UV inkjet printer: LuxelJET UV250GT, from Fujifilm Graphic Systems Co. Ltd. The resolution was 600*450 dpi and the print size was 2m(W)* Im(L). The number of nozzles lost was checked during 10 continuous prints and the results are shown in Table 3.

Table 3. Jetting properties of the inks.

It is apparent that the nozzle loss for the treated inks was lower than that for the untreated inks.

Example 6

An ink-jet ink (assigned the code IM-400) was prepared by combining the components shown in Table 4 in a Silverson mechanical stirrer.

Table 4. Formulation of ink-jet ink IM-400.

Yellow ink IM-401 was prepared using Novoperm Yellow 4G01 (Clariant).

Cyan ink IM-402 was prepared using Irgalite Blue GLVO (Ciba).

Black ink IM-403 was prepared using Special Black 250 (Evonik). Example 7

The inks prepared in Example 6 were subjected to ultrasound treatment using an ultrasonic processor: Model: VCF-1500 (1,50OW, 20 KHz) from Sonics & Materials Inc. The processor employed a Watson Marlow 62Oi feeding pump.

Magenta ink IM500 was prepared by feeding IM-400 through the ultrasonic processor. The feeding speed was 2 L/min. The ink inlet temperature was 2O⁰C and the outlet temperature was 34⁰C.

Yellow ink IM501 was prepared by feeding IM-401 through the ultrasonic processor at the same ink inlet and outlet temperatures as IM500. The ink inlet temperature was 18⁰C and the outlet temperature was 30⁰C.

Cyan ink IM502 was also prepared by feeding IM-402 through the ultrasonic processor. The ink inlet temperature was 21⁰C and the outlet temperature was 32⁰C.

Black ink IM503 was prepared by feeding IM-403 through the ultrasonic processor. The ink inlet and outlet temperatures were also 21⁰C and 31⁰C, respectively.

Example 8

The jetting properties of the inks were determined using a jetting rig on that CA4 print-head (Toshiba TEC co.) was placed with following conditions:

Frequency: 6.2 kHz (7 drop)

Voltage: 25.5 V

Head temperature: 45⁰C.

The number of nozzle outages was checked after continuous jetting for 15 min and the results are shown in Table 5. Table 5. Jetting properties of the inks.

It is apparent that the nozzle loss for the treated inks was significantly lower than that for the untreated inks.

Example 9

Further samples of the inks prepared as IM-501 were subjected to centrifugation using a centrifuge separator: Model SA-I, Westfalia Separator with a Watson Marlow 62Oi feeding pump.

Magenta ink IM601 was prepared by feeding IM501 through the centrifuge separator. The feeding speed was 0.5 L/min. The rotational speed of the centrifuge separator was 8,400 rpm. The amount of wasted high density liquid was 1 % of the initial amount of IM501.

Example 10

Frequency: 6.2 kHz (7 drop) Driving Voltage: 25.5 V Head temperature: 45°C The number of nozzle outages was checked after continuous jetting for 15min and the results shown in Table 6.

Table 6. Jetting properties of the inks.

Claims

1. A method for treating an ink-jet ink comprising: providing an ink-jet ink which comprises a radiation-curable monomer, a photoinitiator and a dispersed pigment; and subjecting the ink to ultrasound and/or centrifugation.

2. A method as claimed in claim 1, wherein the ink is subjected to ultrasound and/or centrifugation at 60⁰C or less.

3. A method as claimed in claim 2, wherein the ink is subjected to ultrasound and/or centrifugation at 10-50⁰C.

4. A method as claimed in any preceding claim, wherein the ink is subjected to ultrasound and the ultrasound is at a frequency of 10-100 KHz.

5. A method as claimed in any preceding claim, wherein the ink is subjected to ultrasound and the ultrasound has a power of 100-3,000 W.

6. A method as claimed in any preceding claim, wherein the ink is subjected to centrifugation and the centrifugation is at a speed of 2,000-20,000 rpm.

7. A method as claimed in any preceding claim, wherein the ink-jet ink is substantially free of water and volatile organic solvent.

8. A method as claimed in any preceding claim, wherein dispersed pigment is 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.

9. A method 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.

10. A method as claimed in any preceding claim, wherein the radiation-curable monomer is selected from a (meth)acrylate monomer, an α,β-unsaturated ether monomer, an N- vinyl amide, an N-(meth)acryloyl amine, and combinations thereof.

11. A method as claimed in any preceding claim, wherein the radiation-curable monomer includes hexanediol diacrylate, dipropyleneglycol diacrylate, propoxylated neopentyl glycol diacrylate, phenoxyethyl acrylate or combinations thereof.

12. A method 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.

13. A treated ink-jet ink obtainable by the method claimed in any preceding claim.

14. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in claim 13 on to a substrate and curing the ink.

15. A substrate having the ink-jet ink as claimed in claim 13 printed thereon.

16. An ink-jet ink container containing the ink-jet ink as claimed in claim 13.

17. An ink-jet ink container as claimed in claim 16, wherein the container is an ink-jet ink cartridge.