The present invention relates to a printing ink and in particular to a printing ink containing a humectant and a method of printing said ink.

In inkjet 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 below 100 mPas at 25°C (although in most applications the viscosity should be below 50 mPas, and often below 25 mPas). Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature, which is often elevated to about 40°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent.

In one common type of inkjet ink, this liquid is water - see for example the paper by Henry R. Kang in the Journal of Imaging Science, 35(3), pp. 179-188 (1991). In those systems, great effort must be made to ensure the inks do not dry in the printhead due to water evaporation. Another type of inkjet ink contains radiation-curable material, such as radiation-curable monomers, which polymerise by irradiation with actinic radiation, commonly with ultraviolet light, in the presence of a photoinitiator.

There are a number of sources of actinic radiation which are commonly used to cure inkjet inks which contain radiation-curable material. The most common source of radiation is a UV source. UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof.

Printhead maintenance is required if the inks dry in the printhead. The open time of the printhead is defined as the maximum idle time before printhead maintenance is required. In order to lengthen the open time of the printhead and to improve the jetting reliability, humectants are commonly added to aqueous inks.

Radiation-curable humectants can be added to aqueous inks - see, for example, US 6,846,851 . However, after curing, UV-curable humectants form part of the final ink film, contributing to the solids content, film thickness and film shrinkage (owing to crosslinking), which has the potential to reduce the flexibility and adhesion of the film.

An alternative to UV-curable humectants are non-UV-curable humectants. Non-UV-curable humectants can also be added to aqueous inks to lengthen the open time and to improve the jetting reliability. Unlike UV-curable humectants, non-UV-curable humectants do not form part of the final ink film (i.e. do not undergo crosslinking within the film) and are lost from the printed ink film upon drying at elevated temperatures, leading to a thinner final ink film containing fewer solids, which in turn gives rise to a better print appearance and better film flexibility. The lower crosslink density of these films also leads to less film shrinkage and hence the potential for more flexible and adhesive films compared to ink films prepared using UV-curable humectants. However, there are disadvantages associated with such non-UV-curable humectants. In this regard, printed ink films containing non-UV- curable humectants typically require more thermal drying energy to dry than those containing UV- curable humectants. Further, in order to achieve the required printhead open time and jetting reliability using non-UV-curable humectants, it is often necessary to take further steps. The first approach involves the use of tickle pulses or other waveform modification, printhead design and lower frequency jetting, which causes complexity and efficiency issues. The second approach involves the use of large amounts of the non-UV-curable humectant and under typical drying conditions, a residual amount of non-UV-curable humectant remains in the final ink film, leading to lower film durability such as poorer solvent resistance.

There is therefore a need in the art to extend the printhead open time and to improve the jetting reliability of an aqueous inkjet ink using a humectant that does not have the associated disadvantages discussed above.

Accordingly, the present invention provides an inkjet ink comprising: an aqueous polyurethane (meth)acrylate dispersion, which is redispersible in water after thermal drying and before curing; more than 5% by weight but less than 15% by weight of a non-UV-curable humectant, based on the total weight of the ink; and a water-dispersible or water-soluble photoinitiator.

It has surprisingly been found that the aqueous polyurethane (meth)acrylate dispersion of the present invention, which is redispersible in water after thermal drying and before curing, in combination with more than 5% by weight but less than 15% by weight of a non-UV-curable humectant, based on the total weight of the ink, results in the optimal balance of printhead open time and jetting reliability, and properties of the ink, without recourse to complicated printer settings, extensive printhead maintenance regimes or elaborate system design, e.g. capping stations.

The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows the effect of the amount of non-UV-curable humectant on the thermal drying requirements;

Fig. 2 shows the effect of the amount of non-UV-curable humectant on the isopropyl alcohol resistance of the printed ink films;

Fig. 3 shows the effect of the amount of non-UV-curable humectant on the percentage change in weight over time; and

Fig. 4 shows the effect of the amount of non-UV-curable humectant on ink stability over time. The inkjet ink contains a non-UV-curable humectant, specifically the inkjet ink contains more than 5% by weight but less than 15% by weight of a non-UV-curable humectant, based on the total weight of the ink. Non-UV-curable humectants are known in the art. Humectants prevent loss of moisture and are typically added to inkjet inks to prevent the inks from drying in and blocking the nozzles of an inkjet printer. Blocked nozzles can lead to a poor quality printed image, especially if the printed image is formed during single-pass printing. Put another way, humectants are commonly added to inkjet inks to prolong the printhead open time (maximum idle time before printhead maintenance is required) and printhead lifetime and to improve jetting reliability.

The non-UV-curable humectant is a high boiling point organic compound with a low volatility, which ensures that it will remain in the ink and prevent the ink from drying in the printhead for long periods at printhead jetting temperatures. The non-UV-curable humectant preferably has a flash point above 90°C. The flash point of a material is defined as the lowest temperature at which vapours of the material will ignite. Measuring a flash point requires an ignition source. The flash point of the non- UV-curable humectant may be measured by PMCC (closed cup). Preferably, the non-UV-curable humectant has a molar mass above 50 g/mol, more preferably above 60 g/mol and most preferably above 70 g/mol. In a preferred embodiment, the non-UV-curable humectant has a boiling point above 160°C, more preferably above 170°C and most preferably, of 180°C or above. Preferably, the non- UV-curable humectant has a flash point above 90°C, a molar mass above 70 g/mol and a boiling point of 180°C or above.

The non-UV-curable humectant of the present invention is present in more than 5% by weight but less than 15% by weight, based on the total weight of the ink. In a preferred embodiment, the non-UV- curable humectant is present in more than 5% by weight but less than 14% by weight, based on the total weight of the ink. In an even more preferred embodiment, the non-UV-curable humectant is present in more than 5% by weight but 10% or less by weight, based on the total weight of the ink. Preferably, the non-UV-curable humectant of the present invention is present in 6% or more by weight but less than 15% by weight, more preferably 6% or more by weight but less than 14% by weight, and most preferably 6% or more by weight but 10% or less by weight, based on the total weight of the ink.

The preferred non-UV-curable humectants of the present invention are polyols, glycols, or lactams. Preferred polyols are selected from diols and glycerol. Preferred glycols are selected from (poly)ethylene glycol and (poly)propylene glycol. A preferred lactam is 2-pyrrolidinone. In a preferred embodiment, the non-UV-curable humectant is water soluble. The most preferred non-UV-curable humectant is a diol, preferably 2-methyl-1 ,3-propanediol. 2-Methyl-1 ,3-propanediol is not hazardous to health (2-methyl-1 ,3-propanediol is free from hazard codes) and is therefore suitable for a range of applications including food packaging. The more humectant that is present in an inkjet ink, the longer the printhead open time and the better the jetting reliability. However, if a large amount of non-UV-curable humectant is present in the ink, the properties of the printed ink film are affected. Such inks require more thermal drying energy in order to remove all of the non-UV-curable humectant from the ink, see Figs 1 and 2. If any non-UV- curable humectant remains, the printed ink films typically show poorer durability, e.g. solvent resistance - see Fig. 2 and Fig. 3 with accompanying Example 10. Further, the inventors have shown that the more non-UV-curable humectant that is present in the ink, the less stable the ink is, as shown by an increase in viscosity over time, see Fig. 4. The relatively small amount of the non-UV-curable humectant that is used in the ink of the present invention to achieve the required printhead open time and jetting reliability is made possible through the use of the water-redispersible PUD of the present invention, without relying on complicated printer settings such as tickle pulses and waveforms, extensive printhead maintenance regimes or elaborate system design (e.g. capping stations), and without compromising the properties of the ink. In order to achieve the same printhead open time and jetting reliability that is achieved using the ink of the present invention for a comparative ink containing a non-water-redispersible PUD, a larger amount of non-UV-curable humectant would be required. This would of course adversely affect the properties of the ink as described above. The inkjet ink contains an aqueous polyurethane (meth)acrylate dispersion, which is redispersible in water after thermal drying and before curing.

Aqueous PUDs are common components in aqueous inks. Aqueous PUD in dispersed form is a high molecular weight material suspended in an aqueous continuous phase and, as such, the viscosity and molecular weight are largely decoupled.

Aqueous PUDs suitable for the present invention are redispersible in water after thermal drying and before (UV) curing, meaning that it is easier to clean the inkjet printer. This property allows removal of any ink deposits from inkjet printheads that may build up during the printing process. The ink deposits are easily removable by water after thermal drying and before curing without the formation of solid particulates that could block the printhead nozzles.

It should be noted that resolubility and resoluble are terms often used in the art to mean redispersibility and redispersible, respectively.

In the present invention, redispersibility of the ink in water after thermal drying and before curing is controlled by the selection of the PUD. Most PUDs dry to a highly resilient film solely by water loss, where the final UV curing stage is only required to increase the final chemical resistance of the film. This characteristic makes the majority of PUDs unsuitable for inkjet application in that there is the potential for ink to build up inside or around the printhead nozzles that is very difficult or even impossible to remove, affecting the jettability of the ink.

The test to measure the suitability of an aqueous PUD for use in the ink of the present invention involves measuring the redispersibility of a PUD in water after thermal drying and before curing. In order to measure the redispersibility of a PUD in water after thermal drying and before curing, the aqueous PUD under test is blended with an aqueous pigment dispersion, such as Projet APD 1000 cyan pigment dispersion (available from Fujifilm imaging colorants) or Diamond D71 C cyan pigment dispersion (available from Diamond dispersions), to facilitate observation of film redispersion and removal. A surfactant, such as fluoro surfactants Capstone FS31 , Capstone FS30 or Capstone FS34 (available from Dupont), is added to reduce surface tension and to allow wetting onto a suitable test substrate. Suitable test substrates include 220 micron gloss PVC (Genotherm supplied by Klockner Pentaplast) and other non-absorbent self-adhesive vinyls, for example where good adhesion is possible. After mixing the components, the composition is coated onto a suitable test substrate to produce a wet film. The wet film is thermally dried (e.g. three minutes at 60°C in a convection oven or via a Tesoma IR belt drier which achieves 60°C on the substrate surface) and then cooled to room temperature. Redispersibilty of the thermally dried ink film in water can then be assessed by a water rub test. The water rub test is well known in the art. One takes a lint-free (cotton) cloth saturated in water. One then carries out a single rub where the saturated cloth is applied to one side of the dried PUD film and under light pressure, traverses the length of the dried PUD film in a single stroke. In order for the PUD to be suitable for use in the present invention, the PUD must be dispersible in water after thermal drying and before curing. Put anotherway, the PUD film should be cleanly removed from the substrate surface leaving no residual staining visible to the naked eye after a single rub. The presence of the pigment in the film helps to determine if this requirement has been met as the substrate should become visible when the colour is removed. Further, no particulate matter visible to the naked eye should be transferred to the wiping cloth or to the substrate at the end of the wiping area.

Therefore, in a preferred embodiment, the PUD of the invention after thermal drying and before curing can be redispersed in water in a single rub of the water rub test.

A PUD is water-dispersible after thermal drying and before curing if it maintains its water sensitivity/compatibility after thermal drying and before curing. In order to maintain this water sensitivity/compatibility in a PUD after thermal drying and before curing, it is necessary to maintain a water-sensitive functionalised PUD after thermal drying and before curing. Such functionality must be water-sensitive and therefore hydrophilic, and often includes ionic groups. An example of a PUD having ionic functionality which is maintained after thermal drying and before curing is a PUD which has carboxylic acid functional groups which are neutralised with an alkali metal hydroxide, such as NaOH, to produce a metal salt. Such a PUD maintains water dispersibility after thermal drying and before curing because the PUD salt is stable and compatible with water. An example of a PUD having non-ionic functionality which maintains oxygen functionality after thermal drying and before curing is a PUD having non-ionic blocks in the PUD chain of the polymer, such as polyether blocks.

Such redispersibility of the PUD in water after thermal drying and before curing allows for easy cleaning of the nozzles of an inkjet printer.

A PUD is not water-dispersible after thermal drying and before curing if it has reduced water sensitivity/compatibility after thermal drying. This occurs if the water-sensitive functional groups are lost during thermal drying of the ink to produce a thermally dried film. For example, in the case where a PUD has carboxylic acid functional groups and is neutralised with an amine salt as opposed to for example an alkali metal hydroxide, on thermal drying of an ink comprising such a functionalised PUD, this results in the breaking down of the amine salt. This amine salt is driven off during thermal drying and hence the ionic character of the PUD is lost and a PUD with carboxylic acid groups remains, which is not redispersible in water.

In the event that the PUD is not redispersible in water after thermal drying and before curing and therefore has lost its water sensitivity/compatibility after thermal drying, the dried film will not redisperse in water. This results in the production of shards of film, which is problematic in an inkjet nozzle.

Such PUDs for the present invention, which are redispersible in water after thermal drying and before curing, are available commercially, for example, from BASF. An example of preparing such a PUD is known in the art, see C.Y. Bai et al. "A new UV curable waterborne polyurethane: Effect of C=C content on the film properties", Progress in Organic Coatings, 2006, 55, 291 -295.

The PUD which is redispersible in water after thermal drying and before curing preferably has a number average molecular weight of over 1 ,200 Daltons. In a preferred embodiment, the PUD has a number average molecular weight of 1 ,200 to 20,000, preferably 1 ,500 to 10,000, and most preferably 2,000 to 5,000, as measured by Infinity 1260 supplied by Agilent technologies, using gel permeation chromatography calibrated against polystyrene standards.

Further, the aqueous PUD which is redispersible in water after thermal drying and before curing is in dispersed form and preferably has a particle size of less than 10 pm, more preferably less than 1 pm and most preferably less than 200 nm as measured by Zeta PALS provided by Brookhaven Instruments Corporation.

The aqueous PUD which is redispersible in water after thermal drying and before curing with actinic (preferably UV) radiation, is non-dispersible in water after curing with actinic (preferably UV) radiation. The aqueous PUD is crosslinkable when exposed to UV radiation as it is acrylate functionalised. This helps to provide the physical film properties required, such as chemical and scratch resistance.

Preferably, the ink of the present invention comprises 30 to 80%, more preferably 35 to 70% by weight of aqueous PUD which is redispersible in water after thermal drying, based on the total weight of the ink. In one preferred embodiment, the ink comprises 35 to 50% by weight of aqueous PUD which is redispersible in water after thermal drying, based on the total weight of the ink. In an alternative preferred embodiment, the ink comprises 50 to 60% by weight of aqueous PUD which is redispersible in water after thermal drying, based on the total weight of the ink. The amount of PUD in the present invention helps to ensure good film properties such as solvent resistance.

The ink of the present invention further comprises a water-dispersible or water-soluble photoinitiator. The free-radical, water-dispersible or water-soluble photoinitiator can be selected from any of those known in the art. For example, Irgacure 2959, 2-hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2- methylpropan-1 -one (PM10028), TPO-L (from BASF) and Irgacure 819 D (from BASF). The preferred photoinitiator of the present invention is 2-hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2- methylpropan-1 -one (PM10028).

Preferably the photoinitiator is present in an amount of 0.1 to 20% by weight, preferably 1 to 4% by weight, based on the total weight of the ink.

In one embodiment, the inkjet ink further comprises a surfactant to control the surface tension of the ink. Surfactants are well known in the art and a detailed description is not required. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example, plastic substrates. Too high a surface tension can lead to ink pooling and/or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks. Surface tension is also critical to ensuring stable jetting (nozzle plate wetting and sustainability). The surface tension is preferably in the range of 20-40 mNm ^"1 and more preferably 25-35 mNm ^"1 .

When present, the surfactant is preferably present in an amount of 0.01 to 5% by weight, based on the total weight of the ink.

In another embodiment, the inkjet ink further comprises a colouring agent such as a pigment or a dye. The colouring agent may be either dissolved or dispersed in the liquid medium of the ink. Alternatively, the ink may be colourless and free from colouring agents. In this embodiment, the ink may be suitable for use as a varnish.

Preferably, the colouring agent is a dispersible pigment, of the types known in the art and commercially available such as under the trade-names: Paliotol, Cinquasia and Irgalite (all available from BASF pic); Hostaperm and Hostajet (available from Clariant UK); and Cabojet (available from Cabot). 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 Red 122, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19 and Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used. Suitable dispersible pigments also include aqueous dispersions and are commercially available such as under the trade-names Diamond (Diamond Dispersions) and Project (Fujifilm Imaging Colorants).

In one aspect, the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 , Pigment yellow 155 and Pigment yellow 74. Magenta: quinacridone pigments, such as Pigment violet 19 and Pigment red 122 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7. Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 μιτι, preferably less than 5 μιτι, more preferably less than 1 pm and particularly preferably less than 0.5 pm.

The colorant is preferably present in an amount of 20% by weight or less, preferably 15% by weight or less, by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, however, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink.

The inks may be in the form of an ink set comprising a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). The inks in a trichromatic set can be used to produce a wide range of colours and tones. Other inkjet ink sets may also be used, such as CMYK+white and light colours.

The ink of the present invention optionally comprises a thickener, more preferably a synthetic thickener to facilitate the adjustment of the ink viscosity. Thickeners and more specifically, synthetic thickeners, are well known in the art and a detailed description is therefore not required.

In one embodiment, the ink contains 0.1 to 5% thickener, more preferably 0.1 to 2% thickener, by weight, based on the total weight of the ink.

In another embodiment, the ink is free from thickener.

In a preferred embodiment, the synthetic thickener when present, has Newtonian rheology. Preferably therefore, the synthetic thickener is not thixotropic and does not cause shear thinning of the ink. In a preferred embodiment, the thickener is either a polyether or a polyurethane thickener. Polyurethane thickeners are water-soluble polyurethane polymers which comprise the simultaneous presence of linear or branched polymers which contain hydrophiiic segments (for example, polyether chains containing at least 5 alkylene oxide units, preferably ethylene oxide units), hydrophobic segments (for example, hydrocarbon segments containing at least 6 carbon atoms) and urethane groups. Polyether thickeners are associative thickeners based on hydrophobically-modified polyether derivatives in solution in water. They are water soluble and/or water emulsifiable polymers with a segmented structure. The basic frameworks are polyethylene glycols with hydrophobic alcohols and diisocyanates or other linking groups. A preferred example of a thickener material suitable for use in the present invention is Rheovis PE1330 (supplied by BASF) which has an active content of 30%.

The inks of the invention comprise water. The water may come from the ink components such as that already contained in the PUD or the thickener and colouring agent may be added as aqueous dispersions, or may be additionally and separately added water. The total amount of water present in the ink of the present invention is preferably 40 to 85%, more preferably 60 to 75% by weight based on the total weight of the ink.

In another embodiment, the inkjet ink further comprises a stabiliser. Stabilisers are well known in the art and a detailed description is not required. Stabilisers protect the ink against deterioration by heat or light.

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, defoamers, dispersants, synergists for the photoinitiator, reodorants, flow or slip aids, biocides and identifying tracers. The inkjet ink exhibits a desirable low viscosity (100 mPas or less, more preferably 50 mPas or less and most preferably 35 mPas or less at 25°C). Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature, which is often elevated to about 40°C. In one embodiment, namely for single pass printing, the ink exhibits an even lower viscosity - 10 mPas or less at 25°C. In this embodiment, the ink has a viscosity of 6 mPas or less at the jetting temperature.

The ink of the present invention is preferably free from monomeric material with a level of ethoxylation of five or higher (5EO or higher), more preferably four or higher (4EO or higher) and most preferably three or higher (3EO or higher). Ethoxylation is a chemical reaction in which ethylene oxide units are added to alcohol functional groups. Monomeric material can often be present in aqueous polyurethane (meth)acrylate dispersions as a result of their process of manufacture.

The ink may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill to give a dispersion. Preferably, for ink combination, the ink is prepared using a low-impact stirrer. The ink of the present invention may be suitable for food packaging applications but the ink is not limited to this application. Other examples include soft signage applications, high productivity graphic arts applications and packaging other than food packaging applications.

The printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto packaging, such as food packaging, or a multiple-pass printer where the image is built up in print swathes. The inks are dried and exposed to actinic (often UV) radiation to cure the ink. Evaporation of the water can occur simply by exposure of the inks to the atmosphere, but the inks may also be heated to accelerate evaporation. The exposure to actinic radiation may be performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink.

Accordingly, the present invention further provides a method of inkjet printing comprising the following steps: inkjet printing the ink as defined herein onto a substrate and, in either order, evaporating the water and exposing the ink to actinic radiation to cure the ink.

It should be noted that the terms "dry" and "cure" are often used interchangeably in the art when referring to radiation-curable inkjet inks to mean the conversion of the inkjet ink from a liquid to solid by polymerisation and/or crosslinking of the radiation-curable material. Herein, however, by "drying" is meant the removal of the water and non-UV-curable humectant by evaporation and by "curing" is meant the polymerisation and/or crosslinking of the radiation-curable material. Further details of the printing, drying and curing process are provided in WO 2011/021052.

The present invention also provides a cartridge containing the inkjet ink as defined herein. It also provides a printed substrate having the ink as defined herein printed thereon. Suitable substrates are a food packaging. Food packaging is typically formed of flexible and rigid plastics (e.g. food-grade polystyrene and PE/PP films), paper and board (e.g. corrugated board). Other applications such as soft signage and textile printing require printing onto treated and untreated cotton and poly blends. Graphic art applications require media such as styrene, polycarbonate, PVC and polyethylene banner.

Any of the sources of actinic radiation discussed herein may be used for the irradiation of the inkjet ink. In one embodiment, a suitable dose would be greater than 200 mJ/cm ² , more preferably at least 300 mJ/cm ² and most preferably at least 500 mJ/cm ² . In this embodiment, the upper limit is less relevant and will be limited only by the commercial factor that more powerful radiation sources increase cost. A typical upper limit would be 5 J/cm ² . However, much lower doses can also be used and this is particularly the case for a method of single pass printing or when using a colourless ink as a varnish. In another embodiment, a suitable dose would be less than 200 mJ/cm ² , more preferably less than 150 mJ/cm ² and most preferably less than 100 mJ/cm ² . For example, a suitable dose could be 30 mJ/cm ² - although this dose is insufficient to fully cure the ink, it is enough for the final pass to have a sufficiently acceptable cure. Further details of the printing and curing process are provided in WO 2012/1 10815.

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

Examples

Example 1 (preparation of a aqueous PUD test samplel

To determine the suitability of the PUD for use in the ink of the present invention the following method was used:

First, the PUD under test is blended with an aqueous pigment dispersion to facilitate observation of film redispersion and removal. A surfactant is added to reduce surface tension and to allow wetting onto a suitable test substrate. The PUD test composition therefore comprises the components as set out in Table 1 .

Table 1

The components of Table 1 are accurately weighed into a mixing vessel and stirred with a flat bladed impeller stirrer at 800 rpm for 20 minutes to ensure the composition is fully homogeneous. After mixing, the composition is allowed to stand for 24 hours to deaerate. The PUD test composition is then coated onto a 220 micron gloss PVC (Genotherm, as supplied by Klockner Pentaplast) using a number 2 K bar. A wet film is deposited of approximately 12 microns.

The ink film is then dried by placing in an oven set at 60°C for three minutes. When the ink film has cooled to room temperature, the redispersibility of the thermally dried ink film is assessed. In this respect, the corner of a sheet of E Tork paper towel (supplied by Tork UK) is wetted with 1 ml of deionised water and placed over the tip of the index finger. The wetted corner of the paper towel is brought into contact with the thermally dried PUD film at the left hand side of the printed film and drawn across the printed film in single stroke with a light pressure. The stroke is continued until the wetted paper towel has completely traversed the printed film

In this case, the ink film is cleanly removed from the substrate surface leaving no residual staining visible to the naked eye. Further, there is no particulate matter, visible to the naked eye, transferred to the wiping paper or to the substrate at the end of the wiping area.

Example 2 (inks)

Inks, as detailed in Tables 2-8, were prepared by mixing the components in the given amounts using a high shear Silverson mixer. The components were added in the order that they are listed in Tables 2-8. Amounts are given as weight percentages based on the total weight of the ink.

Table 2. Formulation of ink 1 (of the invention)

*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one

After formulating the ink, the ink was allowed to cool to room temperature and the viscosity was measured using a Brookfield DVIII LV viscometer using the ULA spindle (00) and adaptor connected to a water bath set to 25°C and rpm 20-30. The sample was allowed to equilibrate to temperature by waiting five minutes before taking a reading, and taking the reading from the viscometer once the reading had stabilised. Ink 1 of Table 2 had a viscosity of 16.5 mPas at 25°C. Table 3. Formulation of comparative ink 2

*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one PEG400 diacrylate is a UV-curable humectant. The viscosity was measured as above. Ink 2 of Table 3 had a viscosity of 19.4 mPas at 25°C.

Table 4. Formulation of comparative ink 3

*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one Ucecoat 7655 is not redispersible in water after thermal drying but before UV curing.

The viscosity was measured as above. Ink 3 of Table 4 had a viscosity of 15.8 mPas at 25°C. Table 5. Formulation of comparative ink 4

*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one

Ucecoat 7655 is not redispersible in water after thermal drying but before UV curing, and PEG400 diacrylate is a UV-curable humectant.

The viscosity was measured as above. Ink 4 of Table 5 had a viscosity of 16.5 mPas at 25°C.

Table 6. Formulation of comparative ink 5

*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one

Ucecoat 7655 is not redispersible in water after thermal drying but before UV curing.

The viscosity was measured as above. Ink 5 of Table 6 had a viscosity of 20.1 mPas at 25°C. Table 7. Formulation of comparative ink 6

*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one

2-Methyl-1 ,3-propanediol is present in 20.0% by weight, based on the total weight of the ink, which is above the upper limit of the non-UV-curable humectant of the present invention.

The viscosity was measured as above. Ink 6 of Table 7 had a viscosity of 16.5 mPas at 25°C.

Table 8. Formulation of comparative ink 7

^* PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one

2-Methyl-1 ,3-propanediol is present in 1.0% by weight, based on the total weight of the ink, which is below the upper limit of the non-UV-curable humectant of the present invention.

The viscosity was measured as above. Ink 7 of Table 8 had a viscosity of 18.0 mPas at 25°C.

Example 3 (thicknesses of the inks)

The inks of Tables 2-8 were drawn down onto a 220 micron gloss PVC substrate (Genotherm, as supplied by Klockner Pentaplast) using a number 2 K bar depositing a 12 micron wet film. The films were thermally dried for three minutes using a Tesoma IR dryer set at 220°C, which achieved 60°C on the substrate surface. The temperature of the substrate surface was measured using thermotest thermal testing strips for ovens and driers (supplied by RS). The films were then cured on a belt at 25m/min in two passes under a standard medium pressure mercury arc lamp of 120 W/cm ² . The total dose used was 720 mJ/cm ² UVA-V lamp. The thicknesses of the films were then measured using a Moore and Rye Vernier caliper by subtracting the thickness of the substrate and the film from the thickness of the substrate. The thickness of each film was taken as the average of fourteen stacked prints. The results are shown in Table 9. Table 9

The presence of residual humectant in the dried and cured films affects the film thickness. Inks 2 and 4 both contain a UV-curable humectant not of the invention. Compared with ink 1 , which contains a non-UV-curable humectant of the invention, the films using inks 2 and 4 are thicker than ink 1. This is because UV-curable humectants remain part of the final ink film. A lower film thickness is desirable for graphic art and soft signage applications.

Inks 1 , 3, 6 and 7 all contain a non-UV-curable humectant, which is lost upon thermal drying, and hence the film thicknesses resulting from these inks are similarly thin. Ink 5 also contains a non-UV- humectant. However, ink 5 contains such a humectant in 20% by weight, based on the total weight of the ink, which is higher than the amount present in the ink of the invention. Some non-UV-curable humectant is still present in ink 5 after thermal drying and hence the film using ink 5 is thicker than the film using ink 3 (which also contains the comparative PUD but only 10% by weight of the non-UV- curable humectant) owing to residual non-UV-curable humectant.

Example 4 (open time)

The open times (maximum idle time possible before printhead maintenance required) of the inks of Tables 2-8 were investigated using a FDMX QSR-14 printhead. The results are shown in Table 10. Table 10

A long open time is advantageous as less frequent printhead maintenance is required. Ink 1 of the invention has a longer open time compared with inks 2-5 and 7 owing to the combination of 10.0% of a non-UV-curable humectant with a water-redispersible (meth)acrylate PUD of the invention. In contrast, inks 2-4 either contain a comparative non-UV-curable humectant or a comparative non- water-redispersible (meth)acrylate PUD. These combinations lead to shorter open times than ink 1 . Although ink 5 contains double the amount of non-UV-curable humectant as ink 1 , namely 20.0%, the open time of ink 5 is shorter. This is because ink 5 contains a comparative non-water-redispersible (meth)acrylate PUD and this component in combination with 20.0% of a non-UV-curable humectant results in a shorter open time. Ink 7 has a shorter open time than ink 1 as ink 7 only contains 1.0% of a non-UV-curable humectant in combination with a water-redispersible (meth)acrylate PUD of the invention. Ink 6, like ink 5, also contains 20.0% of a non-UV-curable humectant. However, in contrast, ink 6 contains a water-redispersible (meth)acrylate PUD of the invention. The combination of a higher amount of a non-UV-curable humectant with a water-redispersible (meth)acrylate PUD of the invention results in ink 6 having a longer open time than ink 1 . However, ink 6 shows poorer thermal properties than ink 1 , which can be seen in Table 13.

Example 5 (jetting sustainabilitv ^' )

The jetting sustainability (maximum frequency when less than three nozzles of the printhead are blocked) of the inks of Tables 2-8 were investigated using a FDMX QSR-14 printhead and 100% duty cycle for five minutes. The inks were jetted using a 5.5 microsecond pulse width with JSP. The jetting temperature varied between 35-45°C to achieve a jetting viscosity of 10-11 mPas. The voltage was optimised for each ink to provide a drop mass of 13 ng. The humidity was approximately 20%.

The results are shown in Table 11 . Table 11

As can be seen from Table 1 1 , inks 1 , 2, 6 and 7 all achieve a maximum frequency of >34 whereas inks 3-5 are inferior. Inks 1 , 2, 6 and 7 all contain a water-redispersible (meth)acrylate PUD of the invention whereas inks 3-5 all contain a comparative non-water-redispersible (meth)acrylate PUD.

Example 6 (water redispersibility)

The water redispersibility of the inks of Tables 2-8 after thermal drying but before curing was investigated.

The inks were drawn down as in Example 3 above. The films were thermally dried using a Tesoma IR dryer at 60°C as in Example 3. The redispersibility of the thermally dried film was checked by wiping a soft cloth soaked in water across the surface as described in Example 1 . The number of rubs required to break the surface of the film is shown in Table 12.

As can be seen from Table 12, inks 1 , 2, 6 and 7 all require only a single rub to break the surface of the film whereas inks 3-5 required five rubs to break the surface of the film. Inks 1 , 2, 6 and 7 all contain a water-redispersible (meth)acrylate PUD of the invention whereas inks 3-5 all contain a comparative non-water-redispersible (meth)acrylate PUD. Example 7 (thermal drying requirements ^' )

The thermal requirements of the inks of Tables 2-8 were assessed using a Tesoma IR dryer.

The inks were drawn down as in Example 3 above. The inks were thermally dried using a Tesoma IR dryer at 60°C and cured using a 720 mJ/cm ² UVA-V lamp as in Example 3. The inks were thermally dried for the amount of time that it took to achieve a specific solvent resistance, namely for the ink film to be removed using more than 50 isopropyl alcohol rubs. The results are shown in Table 13.

Table 13

As can be seen in Table 13, only comparative ink 6 has a longer Tesoma drying time than ink 1 of the invention. This is because ink 6 contains double the amount of humectant than ink 1 . Shorter Tesoma drying times are advantageous in that it takes less time for the ink to dry. However, the present invention provides a balance between open time and film properties. Therefore, although ink 1 has a long Tesoma drying time, it also has a long open time, meaning that printhead maintenance is required less frequently. Example 8 (effect of the amount of the non-UV-curable humectant on thermal drying requirements)

The amount of non-UV-curable humectant, 2-methyl-1 ,3-propanediol, in ink 1 of Table 2 was adjusted to provide ink formulations containing 0%, 5%, 10%, 15% and 25% of 2-methyl-1 ,3-propanediol. In order to accommodate the varying amount of non-UV-curable humectant, the amount of water was adjusted. These inks were used to investigate the effect of the amount of non-UV-curable humectant on the thermal drying requirements.

The inks were drawn down onto a 220 micron gloss PVC substrate (Genotherm, as supplied by Klockner Pentaplast) using a number 2 K bar depositing a 12 micron wet film. The films were thermally dried using a Tesoma IR dryer at a range of temperatures for three minutes to remove the humectant. The percentage change in weight of the ink films was measured as a function of temperature. The results are shown in Fig. 1 . Fig. 1 shows the set temperatures of the Tesoma IR dryer and not the temperatures of the substrate surface, which would have been lower.

As can be seen from Fig. 1 , all of the ink films decreased in weight as the water and non-UV-curable humectant was lost upon heating. The ink films decreased in approximately 75% in weight when all of the water and non-UV-curable humectant was lost. However, the inks containing higher levels of non-UV-curable humectant required higher temperatures to achieve the 75% decrease in film weight. This is because the more non-UV-curable humectant that is present in the ink, the more thermal energy is required to remove all of the humectant from the ink film. Therefore, there is a balance to be struck between extending printhead open time and improving jetting reliability, and the temperature required to remove all of the humectant from the ink film - this is achieved when the non-UV-curable humectant is present in more than 5% by weight but less than 15% by weight, based on the total weight of the ink. Example 9 (effect of the amount of non-UV-curable humectant on isopropyl alcohol resistance)

The inks of Example 8 were also used to assess the effect of the amount of non-UV-curable humectant on the isopropyl alcohol resistance of the printed ink films. The inks were drawn down onto a 220 micron gloss PVC substrate (Genotherm, as supplied by Klockner Pentaplast) using a number 2 K bar depositing a 12 micron wet film. The films were thermally dried using a Tesoma IR dryer at a range of temperatures for three minutes and cured using a 720 mJ/cm ² UVA-V lamp and. The number of isopropyl alcohol double rubs required to break the surfaces of the films was measured as a function of temperature. The results are shown in Fig. 2.

As can be seen from Fig. 2, the number of double rubs required to break the surface of the film increased as the non-UV-curable humectant was lost upon heating. When all of the non-UV-curable humectant is lost, it takes about 100 isopropyl alcohol double rubs to break the surface of the film. The inks containing higher amounts of non-UV-curable humectant required higher temperatures to achieve the 100 double rubs requirement. This is because the more non-UV-curable humectant that is present in an ink, the more thermal energy is required to remove all of the humectant from the ink film and achieve the necessary resistance. Therefore, there is a balance to be struck between extending printhead open time and improving jetting reliability, and the temperature required to remove all of the humectant from the ink film - this is achieved when the non-UV-curable humectant is present in more than 5% by weight but less than 15% by weight, based on the total weight of the ink. Example 10 (effect of the amount of non-UV-curable humectant on ink film durability)

To further investigate the effects of the amount of non-UV-curable humectant, the inks of Example 8 were printed, dried and cured as in Example 3, i.e. the printed films were dried at a constant temperature of 60°C in contrast to Examples 8 and 9.

The printed ink films were assessed for the percentage change in weight over time. The results are shown in Fig. 3. At the start of heating, all of the inks decrease in weight by the same amount owing to the evaporation of the water that is present in all of the inks. However, over time, the inks containing higher amounts of non-UV-curable humectant do not decrease in weight as much as the inks containing lower amounts of non-UV-curable humectant. At the end of the measured time, the difference in film weight between the ink containing no humectant and the ink containing 25% of the humectant, is 25%.

The residual amount of non-UV-curable humectant remaining in the ink films at the end of the measured affects the durability of the ink. This is apparent from the number of isopropyl alcohol double rubs required to break the surface of the films. The results are shown in Table 14 below.

Table 14

As can be seen from Table 14, the resistance of the ink films decreases as the amount of non-UV- curable humectant that was present in the ink before printing increases. Therefore, there is a balance to be struck between extending printhead open time and improving jetting reliability, and the properties of the final ink film such as solvent resistance - this is achieved when the non-UV-curable humectant is present in more than 5% by weight but less than 15% by weight, based on the total weight of the ink..

Example 11 (effect of the amount of non-UV-curable humectant on ink stability ^' )

The amount of non-UV-curable humectant, 2-methyl-1 ,3-propanediol, in ink 1 of Table 2 was adjusted to provide ink formulations containing 0%, 1 %, 2.5%, 5% and 10% of 2-methyl-1 ,3-propanediol. In order to accommodate the varying amount of non-UV-curable humectant, the amount of water was adjusted and in order to maintain a jetting similar viscosity, the amount of thickener was adjusted but the amount of thickener only varied between 0.4 and 0.6% by weight, based on the total weight of the ink. These inks were used to investigate the effect of the amount of non-UV-curable humectant on ink stability.

The inks were stored at 40°C for 16 weeks and the viscosities of the inks were measured every four weeks. The results are shown in Fig. 4.

The viscosity of the ink increases to a greater extent over time for inks containing higher amounts of non-UV-curable humectant than inks containing lower amounts of non-UV-curable humectant. An increase in viscosity corresponds to a decrease in ink stability. Therefore, the more non-UV-curable humectant that is present in the ink, the less stable the ink is. Therefore, there is a balance to be struck between extending printhead open time and improving jetting reliability, and the stability of the ink - this is achieved when the non-UV-curable humectant is present in more than 5% by weight but less than 15% by weight, based on the total weight of the ink.