WO2020030921A1

WO2020030921A1 - Printing ink

Info

Publication number: WO2020030921A1
Application number: PCT/GB2019/052237
Authority: WO
Inventors: Angelique Runacre; Brian Rowatt
Original assignee: Fujifilm Speciality Ink Systems Limited
Priority date: 2018-08-09
Filing date: 2019-08-09
Publication date: 2020-02-13
Also published as: GB201812989D0; GB2590839A; GB202102160D0; GB2590839B

Abstract

This invention relates to an inkjet ink comprising: a non-redispersible aqueous polyurethane (meth)acrylate dispersion; a polyoxyalkylene alkyl/alkenyl ether surfactant; and a water-dispersible or water-soluble photoinitiator. The ink has improved redispersibility in water after thermal drying and before curing.

Description

Printing ink

The invention relates to a printing ink and in particular to an inkjet ink which exhibits good redispersibility in water after thermal drying and before curing.

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. The inks must flow rapidly from the printing heads and to ensure that this happens they have low viscosities in use, 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 addition, ink deposits may build up inside or around the printhead nozzles that are very difficult or even impossible to remove, affecting the jettability of the ink. Blocked nozzles can lead to a poor quality printed image, especially if the printed image is formed during single-pass printing.

In order to lengthen the open time of the printhead and to improve the jetting reliability, it is desirable for an ink to be redispersible in water after thermal drying and before curing. 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 is also desirable to include an aqueous polyurethane (meth)acrylate dispersion (PUD) in the ink, because this component helps to provide the physical film properties required, such as chemical and scratch resistance and film toughness. PUDs are known in the art, including for application in the wood-coating industry. However, most PUDs, particularly PUDs used in the wood-coating industry, are“non-redispersible”, meaning that they are not redispersible in water after thermal drying and before curing, and instead need to dry to a resilient film via water loss during an initial thermal drying process. The benefit of the PUD drying to a resilient film via water loss is that it limits potential for damage during transportation of coated materials prior to (UV) cure which gives a final boost to the film toughness and chemical resistance. However, this physical drying prior to (UV) cure is highly undesirable for the inkjet process and would lead to inks drying into printhead nozzles that would become permanently blocked. 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.

WO 2015/18963 identifies PUDs which are redispersible in water after thermal drying and before curing. However, it would be useful if alternative non-redispersible PUDs could also be used in inkjet inks, particularly non-redispersible PUDs used in the wood coating industry.

There is therefore a need in the art to increase the redispersibility, and hence extend the printhead open time and improve the jetting reliability, of an aqueous inkjet ink containing a non-redispersible aqueous polyurethane (meth)acrylate dispersion, whilst maintaining the required viscosity and without compromising the physical film properties required, such as chemical and scratch resistance and film toughness.

Accordingly, the present invention provides an inkjet ink comprising: a non-redispersible aqueous polyurethane (meth)acrylate dispersion; a polyoxyalkylene alkyl/alkenyl ether surfactant; and a water- dispersible or water-soluble photoinitiator.

The inventors have surprisingly found that the inclusion of a polyoxyalkylene alkyl/alkenyl ether surfactant increases the redispersibility of inks containing non-redispersible aqueous polyurethane (meth)acrylate dispersions. The non-redispersible aqueous polyurethane (meth)acrylate dispersion, in combination with a polyoxyalkylene alkyl/alkenyl ether surfactant, can therefore be used to formulate UV curable inkjet inks of the present invention that have the desired open time and jetting reliability, but, still exhibit both the physical film properties, such as chemical and scratch resistance and film toughness, and viscosity required for inkjet application.

The inkjet ink contains a non-redispersible aqueous polyurethane (meth)acrylate dispersion (PUD). 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 of the ink of the invention are non-redispersible, meaning that they are not redispersible 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.

A PUD which is redispersible in water after thermal drying and before curing can be redispersed in water in a single rub of the water rub test. In this case, the PUD film is 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.

In contrast, the non-redispersible PUD of the ink of the invention after thermal drying and before curing cannot be redispersed in water in a single rub of the water rub test. Put another way, the non- redispersible PUD film is not cleanly removed from the substrate surface (leaving no residual staining visible to the naked eye) after a single rub. The non-redispersible PUD of the ink of the invention can only be redispersed in water in 2 or more rubs of the water rub test, more preferably 5 or more rubs, even more preferably 10 or more rubs and most preferably 15 or more rubs.

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

In contrast, a non-redispersible PUD has reduced water sensitivity/compatibility after thermal drying and before curing. 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.

Suitable non-redispersible PUDs that can be used in the ink of the present invention are available commercially. These include Ucecoat 2803 and IRR 929 (available from Allnex), NeoRad R-448, NeoRad R-550, NeoRad R-540XP and Halwedrol UV 65/40 W (available from DSM), Alberdingk Lux 399 and Alberdingk Lux 260 VP (available from Alberdingk Boley) and Bayhydrol UV 2282 and Bayhydrol UV 2689/1 (available from Covestro). Ucecoat 2803 is preferred.

In contrast, examples of redispersible PUDs which are not used in the ink of the invention are Laromer UA9122 (available from BASF), IRR 811 (also known as Ucecoat 2382), Ucecoat 2801 and Ucecoat 2802 (available from Allnex), and NeoRad R-441 (modified to remove the monomer content) which is available from DSM. An example of preparing such a redispersible 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 non-redispersible PUD has a number average molecular weight of over 500 Daltons. In a preferred embodiment, the PUD has a number average molecular weight of over 500 Daltons to 20,000 Daltons, preferably over 500 Daltons to 10,000 Daltons, as measured by Infinity 1260 supplied by Agilent technologies, using gel permeation chromatography calibrated against polystyrene standards. Further, the non-redispersible PUD 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 a Helos instrument provided by Sympatec GmBH.

The non-redispersible PUD is also non-dispersible in water after curing with actinic (preferably UV) radiation. The non-redispersible 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 and film toughness.

Preferably, the ink of the present invention comprises 30-95%, preferably 30-80%, more preferably 35-70% by weight of the non-redispersible PUD, based on the total weight of the ink. In one preferred embodiment, the ink comprises 35-50% by weight of the non-redispersible PUD, based on the total weight of the ink. In an alternative preferred embodiment, the ink comprises 50-60% by weight of the non-redispersible PUD, based on the total weight of the ink. The amount of non-redispersible PUD in the present invention helps to ensure good film properties such as solvent resistance and film toughness. For the avoidance of doubt, the amount of non-redispersible PUD includes the water contained in the PUD.

The inkjet ink of the invention contains a polyoxyalkylene alkyl/alkenyl ether surfactant.

The polyoxyalkylene alkyl/alkenyl ether surfactant is a polyoxyalkylene alkyl ether surfactant or a polyoxyalkylene alkenyl ether surfactant. Either or both of these classes of surfactants may be included in the inkjet ink of the invention. Polyoxyalkylene alkyl/alkenyl ether surfactants are nonionic surfactants and are known in the art. They have the following chemical formula:

R-0-(AO)_n-H (I) wherein R is an alkyl group or an alkenyl group, A is (CH₂)_X, wherein x is an integer of 1 or more, and n is an integer of 2 or more. The alkyl group is an acyclic saturated hydrocarbon, which is a straight- chain or a branched-chain compound. The alkenyl group is an acyclic hydrocarbon containing at least one carbon-carbon double bond, which is a straight-chain or branched-chain compound. The alkyl or alkenyl groups may be unsubstituted, or they may interrupted by heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted.

In a preferred embodiment, the alkyl or alkenyl group (R of formula (I)) has 5 to 30 carbon atoms, more preferably 8 to 25 carbon atoms and most preferably 10 to 20 carbon atoms.

Preferably, the polyoxyalkylene moiety of the polyoxyalkylene alkyl/alkenyl ether surfactant is one of polyoxyethylene, polyoxypropylene or polyoxybutylene, more preferably polyoxyethylene or polyoxypropylene, and most preferably polyoxyethylene. In other words, x of formula (I) is preferably 2 to 4, more preferably 2 to 3 and most preferably 2.

In a preferred embodiment, n of formula (I) is an integer of 2 to 40, more preferably 4 to 35 and most preferably 6 to 30.

The polyoxyalkylene alkyl/alkenyl ether surfactant is a high melting 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 polyoxyalkylene alkyl/alkenyl ether surfactant preferably has a melting point above -10°C and more preferably above -5°C. The polyoxyalkylene alkyl/alkenyl ether surfactant preferably has a flash point above 90°C, more preferably above 120°C and most preferably above 150°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 polyoxyalkylene alkyl/alkenyl ether surfactant may be measured by the Cleveland open-cup method.

The hydrophilic-lipophilic balance (HLB value) of a non ionic surfactant is a measure of the degree to which it is hydrophilic (water soluble) or lipophilic (oil soluble). The HLB value can be calculated using the following equation (known as Griffin’s method):

HLB = 20 x M_h/M (II) where M_h is the molecular mass of the hydrophilic portion of the molecule, and M is the molecular mass of the whole molecule, giving a result on a scale of 0 to 20. An HLB value of 0 corresponds to a completely lipophilic/hydrophobic molecule, and an HLB value of 20 corresponds to a completely hydrophilic/lipophobic molecule.

The HLB value of the polyoxyalkylene alkyl/alkenyl ether surfactant is preferably 4 to 20, and more preferably 10 to 18.

It has surprisingly been found that including a polyoxyalkylene alkyl/alkenyl ether surfactant improves the redispersibility of the non-redispersible PUD.

Without wishing to be bound by theory, it is believed that the alkoxylated material of the polyoxyalkylene alkyl/alkenyl ether surfactant works its way into the polyurethane (meth)acrylate polymer network and plasticises it, increasing the plasticity and therefore decreasing the viscosity of the PUD. This has an effect of wetting of the polyurethane backbone of the polyurethane (meth)acrylate polymer network, therefore increasing the redispersibility of the polyurethane (meth)acrylate dispersion. Suitable polyoxyalkylene alkyl ether surfactants of formula (I) are commercially available from the Kao Corporation. These include Emulgen 707 (a mixture of polyoxyethylene (7) sec-C^-C^ alkyl ethers, CAS no. 68131-40-8), Emulgen 108 (polyoxyethylene (6) lauryl ether, CAS no. 9002-92-0), Emulgen 21 OP (polyoxyethylene cetyl-stearyl ethers, CAS no. 68439-49-6), Emulgen 320P (polyoxyethylene (13) stearyl ether, CAS no. 9005-00-9), Emulgen 1 108 (polyoxyethylene (8) alkyl ethers), Emulgen 2020G-HA (polyoxyethylene (20) octyldodecyl ether, CAS no. 32128-65-7) and Emulgen 2025G (polyoxyethylene (25) octyldodecyl ether, CAS no. 32128-65-7).

Suitable polyoxyalkylene alkenyl ether surfactants of formula (I) are commercially available from the Kao Corporation. These include Emulgen 409PV (polyoxyethylene (9) oleyl ether, CAS no. 9004-98- 2), Emulgen 420 (polyoxyethylene (13) oleyl ether, CAS no. 9004-98-2) and Emulgen 430 (polyoxyethylene (30) oleyl ether, CAS no. 9004-98-2).

The most preferred polyoxyalkylene alkyl/alkenyl ether surfactants are Emulgen 707, Emulgen 409PV and Emulgen 420. Emulgen 707 is particularly preferred.

The more polyoxyalkylene alkyl/alkenyl ether surfactant 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 polyoxyalkylene alkyl/alkenyl ether surfactant is present in the ink, cure performance and hence the properties of the printed ink film may be affected.

Accordingly, in a preferred embodiment, the polyoxyalkylene alkyl/alkenyl ether surfactant is present in 0.1-5% by weight, more preferably 0.1 -2% by weight, and most preferably 0.1-1 % by weight, based on the total weight of the ink. The amounts refer to the total amount of polyoxyalkylene alkyl ether and polyoxyalkylene alkenyl ether.

The ink of the invention containing the non-redispersible PUD and the polyoxyalkylene alkyl/alkenyl ether surfactant has improved redispersibility in water after thermal drying and before curing compared to a comparative ink which does not contain the polyoxyalkylene alkyl/alkenyl ether surfactant.

In order to measure the redispersibility of the ink, the following test is used. A non-redispersible PUD is blended with a water-dispersible or water-soluble photoinitiator, such as 2-hydroxy-1 -{4-[2-(2- hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one (PM10028, available from BASF) and, in the case of an ink of the invention, a polyoxyalkylene alkyl/alkenyl ether surfactant, such as Emulgen 707 (available from the Kao Corporation). 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. Other components may be added to the ink, for example a humectant such as 2-methyl- 1 , 3-propanediol (available from Perstorp) and/or a thickener such as Rheovis PE1330 (supplied by BASF). After mixing the components, the composition is coated onto a suitable test substrate to produce a wet film. 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. 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. Redispersibility of the thermally dried ink film in water can then be assessed by a water rub test.

The water rub test for the thermally dried ink film is as follows. 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. The number of rubs required to break the surface of the film is recorded.

It has surprisingly been found that the ink of the invention containing the non-redispersible PUD and the polyoxyalkylene alkyl/alkenyl ether surfactant is more redispersible than a comparative ink containing the non-redispersible PUD, but without the polyoxyalkylene alkyl/alkenyl ether surfactant. Put another way, the ink of the invention is redispersible in water after thermal drying and before curing in fewer rubs of the above water rub test than a comparative ink without the polyoxyalkylene alkyl/alkenyl ether surfactant. By way of example, an ink of the invention may be redispersible in 2 rubs of the water rub test, but a comparative ink where the polyoxyalkylene alkyl/alkenyl ether surfactant is absent is only redispersible in 4 rubs of the water rub test.

In a preferred embodiment, the ink of the invention, containing the non-redispersible PUD in combination with the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing can be redispersed in water in less than 15 rubs of the water rub test, more preferably less than 10 rubs, more preferably less than 5 rubs, even more preferably less than 2 rubs and most preferably 1 rub.

In contrast, a comparative ink, containing the non-redispersible PUD but without the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing can only be redispersed in water in 2 or more rubs of the water rub test, more preferably 5 or more rubs, even more preferably 10 or more rubs and most preferably 15 or more rubs.

Put another way, by way of example, a comparative ink containing the non-redispersible PUD but without the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing may only be redispersed in water in 15 rubs of the water rub test, whereas an ink of the invention containing the same non-redispersible PUD and the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing is redispersible in water in less than 15 rubs, preferably less than 10 rubs, more preferably less than 5 rubs, even more preferably less than 2 rubs and most preferably 1 rub. Again, by way of example, a comparative ink containing the non-redispersible PUD but without the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing may only be redispersed in water in 10 rubs of the water rub test, whereas an ink of the invention containing the same non-redispersible PUD and the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing is redispersible in water in less than 10 rubs, preferably less than 5 rubs, more preferably less than 2 rubs and most preferably 1 rub. As a further example, a comparative ink containing the non-redispersible PUD but without the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing may only be redispersed in water in 5 rubs of the water rub test, whereas an ink of the invention containing the same non-redispersible PUD and the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing is redispersible in water in less than 5 rubs, preferably less than 2 rubs and most preferably 1 rub. To provide an additional example, a comparative ink containing the non-redispersible PUD but without the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing may only be redispersed in water in 2 rubs of the water rub test, whereas an ink of the invention containing the same non-redispersible PUD and the polyoxyalkylene alkyl/alkenyl ether surfactant, after thermal drying and before curing is redispersible in water in 1 rub.

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 ink further comprises an inorganic silicate. Inorganic silicates are known in the art - see, for example, EP 2 298 555 A2 and US 201 1/00691 14 A1.

Printhead members may be modified in order to be liquid-repellent, thereby maintaining ink ejection performance. For example, the surface of a printhead member may be treated using a fluorine-based surface treating agent. However, if the printhead is used over a long period of time, the liquid- repellency is known to be slowly reduced as a result of contact with ink.

On the other hand, a nozzle plate may contain silicon or silicon dioxide, particularly in order to form fine nozzles (ejection ports) with accuracy. However, corrosion of printheads equipped with a silicon nozzle plate is known to occur as a result of contact with ink. Even when the silicon nozzle plate surface has been modified to be liquid-repellent, corrosion is known to occur as a result of penetration of the ink and a decrease in the liquid repellency. A decrease in the liquid-repellency of the nozzle plate affects the ejection performance.

When the ink includes both a non-redispersible PUD and an inorganic silicate, the ink has excellent ejection restorability (jetting reliability), and shape deformation (corrosion) of printhead members and the decrease in liquid-repellence may be suppressed. As a result, even when printing is carried out over a long time period, the occurrence of deterioration in the ejection performance or of ink leakage is greatly reduced. In turn, this can reduce the occurrence of a disturbance in the direction of ink ejection and consequent deterioration in the quality of the printed image.

Although the ink contains a non-redispersible PUD which may cause corrosion of printhead members that are brought into contact with the ink, the combination of a non-redispersible PUD and an inorganic silicate may more effectively suppress corrosion or the decrease in liquid-repellency of the printhead member, even when used in a printhead having a nozzle plate which has been modified to become liquid-repellent or formed from silicon as described above.

The inorganic silicate may be selected from silicic acid and salts of silicic acid. In particular, anhydrous silicic acid (silica), alkali metal salts of silicic acid such as sodium silicate and potassium silicate and alkali earth metal salts of silicic acid such as calcium silicate and magnesium silicate are preferred.

When the inorganic silicate is silicic acid, it is preferably colloidal silica. Colloidal silica is a colloid formed from microparticles of an inorganic oxide containing silicon, wherein the microparticles have an average particle diameter of several hundred nanometres or less. Colloidal silica contains silicon dioxide (including hydrates of silicon dioxide) as the main component. Aluminates such as sodium aluminate and potassium aluminate may also be present as minor components in the colloid. Further, inorganic salts such as sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium hydroxide or organic salts such as tetramethylammonium hydroxide may be present as stabiliser(s) for the colloid.

The dispersion medium of the colloidal silica may be water, an organic solvent, or a mixture thereof. When an organic solvent is used, the organic solvent may be a water-soluble organic solvent or a non-water-soluble organic solvent, but a water-soluble organic solvent is preferred. Examples of the organic solvent include methanol, ethanol, isopropyl alcohol, and n-propanol. Colloidal silica dispersed in water is known as an aqueous sol, while a colloidal silica dispersed in an organic solvent is known as an organosol.

The average particle size of the colloidal silica is preferably adjusted to 1 -100 nm, more preferably from 1-25 nm, even more preferably from 3-25 nm, and most preferably from 5-20 nm. The average particle size of the colloidal silica is a volume average particle size, which can be determined by light scattering methods or laser diffraction methods known in the art. When colloidal silica is used, the average particle diameter is determined by a light scattering method.

When the average particle size is 25 nm or less, damage (for example, a decrease in liquid- repellency) caused by the ink to printhead members, such as a base material, a protective film and a liquid-repellent film, may be more effectively suppressed. That is, the attacking properties of the ink may be suppressed. In general, reducing the average particle size leads to an increase in the total surface area of the particles. Without wishing to be bound by theory, it is believed that by limiting the average particle size to 25 nm or less, the resulting total surface area of the particles leads to more effective suppression of damages to the members constituting an inkjet head. Moreover, an average particle size of 25 nm or less is preferred in view of jettability of the ink and the abrasive effect of the particles. When the average particle size is 1 nm or more, colloidal silica having increased productivity and reduced variation in performance can be obtained.

Preferably, from the viewpoint of suppressing a decrease in the liquid-repellency of printhead members and improving jettability of the ink, the ink contains colloidal silica having a volume average particle size of 1 -25 nm (more preferably 3-25 nm) in an amount of 0.005-0.5% by weight based on the total weight of the ink, and more preferably colloidal silica having a volume average particle size of 5-20 nm in an amount of 0.005%-0.5% by weight based on the total weight of the ink.

The colloidal silica particles may have a spherical shape, a long shape, a needle-like shape, or a shape like a string of beads. Preferably, the colloidal silica particles are spherical, from the viewpoint of jettability of the ink.

The colloidal silica can be produced by any method known in the art, such as via thermal decomposition of silicon tetrachloride (Aerosil synthesis) or by using water glass. Alternatively, colloidal silica can be produced via a liquid phase synthesis method including hydrolysis of an alkoxide (see, for example, "Seni to Kogyo", vol. 60, No. 7, page 376, 2004).

Alternatively, a commercially available colloidal silica dispersion can be used. Specific examples of commercially available products include LUDOX AM, LUDOX AS, LUDOX LS, LUDOX TM, LUDOX HS (manufactured by E.l. Du Pont de Nemours & Co.), SNOWTEX S, SNOWTEX XS, SNOWTEX 20, SNOWTEX 30, SNOWTEX 40, SNOWTEX N, SNOWTEX C and SNOWTEX O (available from Nissan Chemical Industries, Ltd.) SYTON C-30, SYTON ZOO (manufactured by Monsanto Co.) NALCOAG-1 060, NALCOAG-ID21 to 64 (manufactured by Nalco Chem Co.); METHANOL SOL, IPA SOL, MEK SOL, TOLUENE SOL (manufactured by Fuso Chemical Co., Ltd.) CATALOID-S, CATALOID-F120, CATALOID SI-350, CATALOID SI.-500, CATALOID SI-30, CATALOID S-20L, CATALOID S-20H, CATALOID S-30L, CATALOID S-30H, CATALOID SI-40, OSCAL-1432 (isopropyl alcohol sol) (manufactured by Nikki Chemical Co., Ltd.); and ADELITE (manufactured by Asahi Denka Kogyo K.K.). Examples of commercially available bead-shaped colloidal silica include SNOWTEX ST-UP, SNOWTEX PS-S, SNOWTEX PS- , SNOWTEX ST-OUP, SNOWTEX PS-SO, and SNOWTEX PS-MO (manufactured by Nissan Chemical Industries, Ltd.). SNOWTEX XS, SNOWTEX 40 and SNOWTEX O are preferred.

The pH of the commercially available colloidal silica dispersion may need to be adjusted to be acidic or alkaline. This is because the stable dispersion region of colloidal silica exists on the acidic side or on the alkaline side. It is necessary to take the pH of the stable dispersion region of the colloidal silica and the pH of the ink composition into consideration before the colloidal silica dispersion is added.

When the inorganic silicate is a salt of silicic acid, it is preferably an alkali solution of an alkali metal salt of silicic acid. Suitable alkali metal salts include alkali metal salts of metasilicic acid or orthosilicic acid. The salts of silicic acid may be used alone or in any combination thereof.

Particularly preferred salts of silicic acid are sodium silicate and potassium silicate. Sodium and potassium salts of silicic acid have improved dispersibility in the ink and a low odour compared to salts of other monovalent cations, particularly ammonium salts of silicic acid.

The alkali metal salt of silicic acid is preferably one or more compounds represented by the following chemical formula: x(M₂0)^«y(Si0₂) (III) wherein M is sodium or potassium, x is an integer of 1 or 2 and y is an integer of 1 to 4. The alkali metal salt of metasilicic acid is represented by formula (III) when x=1 and y=1 and the alkali metal salt of orthosilicic acid is represented by formula (III) when x=2 and y=1. Both classes of alkali metal salts of silicic acid are soluble in water.

Alkali metal salts of silicic acid may be obtained by reacting silica with alkali metal carbonates or alkali metal hydroxides.

When the inorganic silicate is included, the total amount of the inorganic silicate may be 0.0001 %- 10% by weight, preferably 0.0005-0.5% by weight, more preferably 0.001 -0.5% by weight, even more preferably 0.005-0.5% by weight, and most preferably 0.01 -0.3% by weight, based on the total weight of the ink. It is desirable to include the inorganic silicate to suppress the decrease in liquid-repellence of printhead members. However, if too much inorganic silicate is included, this may affect the jettability of the ink and in the case of colloidal silica, the colloid particles may be abrasive to the printhead.

The ink of the present invention preferably further comprises a humectant, which is well-known in the art. The humectant can be any material that acts to retain water in the system. Where a humectant is used, the humectant is preferably a non-UV-curable humectant. Examples of suitable non-UV- curable humectants 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.

In a preferred embodiment, the ink contains 1-20% by weight of humectant, more preferably 5-15% 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, in addition to the polyoxyalkylene alkyl/alkenyl ether surfactant of the present invention. Surfactants are well known in the art and a detailed description is not required. An example of a suitable surfactant is a fluoro surfactant, such as Capstone FS31 (available from Dupont). 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 (excluding the polyoxyalkylene alkyl/alkenyl ether surfactant) is preferably present in an amount of 0.01 to 2% by weight, based on the total weight of the ink. The amount does not include the total amount of polyoxyalkylene alkyl/alkenyl ether surfactant.

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 pm, preferably less than 5 pm, 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 -5% by weight of thickener, more preferably 0.1-2% 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 hydrophilic 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 that already contained in components of the ink, such as the PUD, an aqueous dispersion of the colourant and/or an aqueous dispersion of the thickener, or the water may be additionally and separately added water. The total amount of water present in the ink of the present invention is preferably 40-95%, more preferably 40- 85% and most preferably 60-75% 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 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.

Examples of suitable substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, self-adhesive vinyl and cellulosic materials. Cellulosic materials include paper and board (e.g corrugated board). Mixtures/blends are included.

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 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. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, self-adhesive vinyl and cellulosic materials. Applications for the ink include soft signage applications, high productivity graphic arts applications and packaging including food packaging applications. Food packaging is typically formed of flexible and rigid plastics (e.g. 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 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/cm², more preferably at least 300 mJ/cm² and most preferably at least 500 mJ/cm². 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/cm². 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/cm², more preferably less than 150 mJ/cm² and most preferably less than 100 mJ/cm². For example, a suitable dose could be 30 mJ/cm² - 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 an aqueous PUD test sample)

To determine if the PUDs used in the examples are non-redispersible, 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 fluoro 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 substrate (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 a Tesoma IR dryer set at 220°C (so that 60°C is achieved at the substrate surface) for three minutes.

When the ink film has cooled to room temperature, the redispersibility of the thermally dried ink film is assessed by the water rub test. 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 the case of a redispersible PUD, the ink film is cleanly removed from the substrate surface after a single rub of the water rub test, leaving no residual staining visible to the naked eye. Such a redispersible PUD is not used in the ink of the invention.

In contrast, a non-redispersible PUD used in the ink of the invention is only redispersible in 2 or more rubs of the water rub test.

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 (comparative ink)

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

Laromer UA9122 is redispersible in water after thermal drying and before curing in a single rub of the water rub test of Example 1 . Laromer UA9122 is therefore a redispersible PUD.

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 1 1.6 mPas at 25°C. Table 3. Formulations of ink 2 (comparative ink) and ink 3 (comparative ink)

NeoRad R-441 (modified to remove the monomer content) is redispersible in water after thermal drying and before curing in a single rub of the water rub test of Example 1 . NeoRad R-441 is therefore a redispersible PUD.

The viscosities of inks 2 and 3 of Table 3 were measured as above. Inks 2 and 3 had viscosities of 13.5 mPas and 13.8 mPas, respectively at 25°C.

Table 4. Formulations of ink 4 (comparative ink) and ink 5 (ink of the invention)

*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one NeoRad R-540XP is a non-redispersible PUD according to the water rub test of Example 1 .

The viscosities of inks 4 and 5 of Table 4 were measured as above. Inks 4 and 5 had viscosities of 12.7 mPas and 13.0 mPas, respectively, at 25°C.

Table 5. Formulations of ink 6 (comparative ink) and ink 7 (ink of the invention)

^*PM10028 = 2-Hydroxy-1 -{4-[2-(2-hydroxyethoxy)ethoxy]phenyl}-2-methylpropan-1 -one Ucecoat 2803 is a non-redispersible PUD according to the water rub test of Example 1 .

The viscosities of inks 6 and 7 of Table 5 were measured as above. Inks 6 and 7 had viscosities of 5.0 mPas and 5.3 mPas, respectively, at 25°C.

Table 6. Formulations of ink 8 (comparative ink) and ink 9 (ink of the invention)

IRR 929 is a non-redispersible PUD according to the water rub test of Example 1 .

The viscosities of inks 8 and 9 of Table 6 were measured as above. Inks 8 and 9 had viscosities of 5.0 mPas and 5.3 mPas, respectively, at 25°C.

Table 7. Formulations of comparative ink 10 and inks 11-14 (inks of the invention)

Table 8. Formulations of inks 15-19 (inks of the invention)

Example 3 (water redispersibility)

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

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 (so that 60°C is achieved at the substrate surface), as detailed in Table 9 below. 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 9.

Table 9. Redispersibility after thermal drying at 220°C

As can be seen from Table 9, comparative inks 1 and 2 only require a single water rub to break the surface of the film, even without the polyoxyalkylene alkyl ether surfactant (Emulgen 707). This is because these inks contain a water-redispersible PUD (Laromer UA9122 and NeoRad R-441 , respectively). As noted above, Laromer UA9122 and NeoRad R-441 (modified to remove the monomer content) are redispersible in water after thermal drying and before curing in a single rub of the water rub test of Example 1.

Comparative inks 4, 6 and 8 all contain PUDs which, after thermal drying and before curing, cannot be redispersed in water in a single rub of the water rub test. Inks 5, 7 and 9 are identical to comparative inks 4, 6 and 8, respectively, except that these inks contain a polyoxyalkylene alkyl ether surfactant (Emulgen 707). Adding the polyoxyalkylene alkyl ether surfactant improves the redispersibility, as evidenced by the water rub results above.

Comparative ink 10 and inks 11 -19 each contain the same PUD (Ucecoat 2803). Comparative ink 10 does not contain a polyoxyalkylene alkyl/alkenyl ether surfactant. Inks 11 -19 contain 0.39% of a polyoxyalkylene alkyl/alkenyl ether surfactant (Emulgen 108, Emulgen 21 OP, Emulgen 320P, Emulgen 409PV, Emulgen 420, Emulgen 430, Emulgen 1108, Emulgen 2020G-HA and Emulgen 2025G, respectively). In all cases, including a polyoxyalkylene alkyl/alkenyl ether surfactant improves the redispersibility with respect to comparative ink 10, as evidenced by the water rub results above.

By decreasing the amount of water rubs required to break the surface of the film, any dried ink residue would be more readily removed from the printhead nozzle plate by cleaning with water. The ready dissolution of a dried ink film prevents the risk of blocking of jets with particulate matter.

Further, the water redispersibility results of Table 9 for the inks of Tables 2-8 are reflective of the water redispersibility results that would be obtained for the PUDs used in the inks of Tables 2-8 according to the test of Example 1 (without/with the polyoxyalkylene alkyl/alkenyl ether surfactant).

Example 4 (effect of polyoxyalkylene alkyl/alkenyl ether surfactant on water/isopropyl alcohol rub resistance)

The water/isopropyl rub resistance of the inks of Tables 2-6 after thermal drying and curing was investigated.

The inks of Tables 2-6 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 (so that 60°C is achieved at the substrate surface), as detailed in Table 10 below. The films were then cured using a Jenton UV manicure set on a belt at 25 m/min (passed twice) under a standard medium pressure mercury arc lamp of 4 W/cm², achieving an exposure of about 800 mJ. The number of a) water double rubs and b) isopropyl double rubs required to break the surface of the films was measured. The results are shown in Table 10.

As can be seen from Table 10, the number of double rubs required to break the surface of the film generally remained constant for a given PUD, whether-or-not the polyoxyalkylene alkyl/alkenyl ether surfactant was present or absent. However, the number of double rubs sometimes decreased when the polyoxyalkylene alkyl/alkenyl ether surfactant was included. Despite this, all of the inks have an adequate water and isopropyl rub resistance, and therefore adequate film toughness, as required for inkjet inks.

Example 5 (effect of the inorganic silicate on contact angle)

The effect of colloidal silica on attack of the ink formulation on a silicon nozzle plate material was investigated. The contact angle of deionised water on the nozzle plate material was measured to be 105°.

The nozzle plate material was then immersed for four weeks at 40°C in ink compositions as set out in Table 11 below. The soaked material was drip dried for 24 hours, and the contact angle of de-ionised water on the dried coating surface was then recorded. A change in contact angle indicates a change in the state of the nozzle plate material (and hence the nozzle plate). The results are shown in Table 11.

Table 11. Contact angle after immersion in the ink for four weeks and drying

Ink 7 contains 0.05% by weight of commercially available colloidal silica (Snowtex XS), whereas Ucecoat 2803 does not contain any colloidal silica. As can be seen from Table 11 , including colloidal silica suppresses the decrease in contact angle, and hence the change in state of the nozzle plate. Further, the change in contact angle for Ucecoat 2803 (neat) is reflective of change in contact angle that would be obtained for ink 7 without Snowtex XS.

Claims

1. An inkjet ink comprising: a non-redispersible aqueous polyurethane (meth)acrylate dispersion; a polyoxyalkylene alkyl/alkenyl ether surfactant; and a water-dispersible or water-soluble photoinitiator.

2. An inkjet ink as claimed in claim 1 , wherein the ink further comprises an inorganic silicate.

3. An inkjet ink as claimed in claim 2, wherein the inorganic silicate is selected from colloidal silica or an alkali metal, preferably colloidal silica.

4. An inkjet ink as claimed in any one of claims 1 to 3, wherein the polyoxyalkylene alkyl/alkenyl ether surfactant is present in 0.1-5% by weight, preferably 0.1-2% by weight and more preferably 0.1- 1 % by weight, based on the total weight of the ink.

5. An inkjet ink as claimed in any one of claims 1 to 4, wherein the polyoxyalkylene alkyl/alkenyl ether surfactant is a polyoxyethylene alkyl/alkenyl ether surfactant.

6. An inkjet ink as claimed in any preceding claim, wherein the polyurethane (meth)acrylate dispersion is present in 30-95% by weight, based on the total weight of the ink.

7. An inkjet ink as claimed in any preceding claim, wherein the polyurethane (meth)acrylate dispersion has a number average molecular weight of over 500 Daltons to 20,000 Daltons.

8. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a humectant.

9. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a synthetic thickener.

10. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a colouring agent.

11. An inkjet ink as claimed in claim 10, wherein the colouring agent is a dispersible pigment.

12. An inkjet ink as claimed in any preceding claim, wherein the ink comprises 40-95% by weight of water, based on the total weight of the ink.

13. A cartridge containing the inkjet ink as claimed in any preceding claim.

14. A printed substrate having the ink as claimed in any of claims 1 to 12 printed thereon.

15. A method of inkjet printing comprising inkjet printing the inkjet ink as claimed in any of claims 1 to 12 onto a substrate, drying and curing the ink.