GB2594728A - A printing ink - Google Patents

Abstract

An inkjet ink comprises (i) at least one monofunctional (meth)acrylate monomer, which includes (3-ethyloxetane-3-yl)methyl acrylate (OXE-10) and/or (3-ethyloxetane-3-yl)methyl methacrylate (OXE-30) and (ii) at least one di- and/or multifunctional monomer. Typically, the ink comprises 10-50 wt.% OXE-10 and/or OXE-30 and 10-30 wt.% di- and/or multifunctional monomer(s). The di- or multifunctional monomer(s) may be 1,10-decanediol, hexanediol diacrylate, polyethylene glycol diacrylate, tripropylene glycol diacrylate, 3-methyl-1,5-pentanediol diacrylate, dipropylene glycol diacrylate, tricyclodecane dimethanol diacrylate, propoxylated neopentyl glycol diacrylate, trimethylolpropane triacrylate, di-trimethylolpropane tetraacrylate, di-pentaerythritol hexaacrylate, or ethoxylated trimethylolpropane triacrylate. The ink may further comprise an N-vinyl amide and/or an N-(meth)acryloyl amine as well as photoinitiator, colouring agent, radiation curable oligomer, and passive resin. The ink may be substantially free from tetrahydrofurfuryl acrylate. A preferred ink comprises OXE-10, isobornyl acrylate, N-vinylcaprolactam, and 1,10-decanediol diacrylate. The ink may be applied to a substrate composed of polyvinyl chloride or polystyrene.

Description

A printing ink This invention relates to a printing ink and in particular to an inkjet ink comprising one or more di-and/or multifunctional monomers having improved adhesion onto various substrates, including polyvinyl chloride and polystyrene.

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 200 mPas or less at 25°C, although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and most preferably between 7-11 mPas at the jetting temperature which is often elevated to, but not limited to 40-50°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 such as water or a low-boiling solvent or mixture of solvents.

Another type of inkjet ink contains unsaturated organic compounds, termed monomers and/or oligomers which polymerise when cured. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is cured, a process which is more rapid than evaporation of solvent at moderate temperatures.

Inks which cure by the polymerisation of monomers may contain a wide variety of monofunctional, difunctional and multifunctional monomers. The challenge is to provide the necessary printing properties, such as good adhesion, whilst providing a high-quality image, without compromising the jetting properties. This is made all the harder in inks which are formulated without the use of water or volatile organic solvents (which also have their own disadvantages). The printing of ink images onto particular substrates, such as polyvinyl chloride and polystyrene, is a significant challenge as it is difficult to gain adhesion thereto.

In order to improve the adhesion of an inkjet ink onto various substrates, including polyvinyl chloride and polystyrene, typically, the amount of certain monofunctional monomers, such as IBOA, would be increased, and the amount of di-and/or multifunctional monomers present in the ink would be reduced. Increasing the amount of certain monofunctional monomers, such as IBOA, would have the benefit of increasing adhesion of the inkjet ink onto the substrate but would reduce flexibility. Decreasing the amount of di-and/or multifunctional monomers present in the ink would also increase adhesion but would reduce the resistance properties of the ink.

Alternatively, in order to increase adhesion, THFA is typically included in the inkjet ink. However, although this would improve the adhesion of the inkjet ink to the substrate, THFA negatively affects health and safety. THFA is a hazardous monomer and bears the GHS hazard statement H314 (Causes severe skin burns and eye damage). There is also growing evidence that it may damage fertility or the unborn child. Thus, there is an urgent need in the art to move away from THEA.

There is therefore a need in the art for an inkjet ink comprising one or more di-and/or multifunctional monomers that has improved adhesion onto various substrates, including polyvinyl chloride and polystyrene, whilst maintaining a high-quality printed image with reduced health and safety concerns.

Accordingly, the present invention provides an inkjet ink comprising: one or more monofunctional (meth)acrylate monomers, including (3-ethyloxetane-3-yl)methyl acrylate (OXE-10) and/or (3-ethyloxetane-3-yl)methyl methacrylate (OXE-30); and one or more di-and/or multifunctional monomers.

The present invention provides an ink comprising one or more di-and/or multifunctional monomers that has improved adhesion onto various substrates, including polyvinyl chloride and polystyrene, whilst maintaining a high-quality printed image with reduced health and safety concerns.

Surprisingly, it has been found that the inclusion of one or more monofunctional (meth)acrylate monomers, including (3-ethyloxetane-3-yl)methyl acrylate (OXE-10) andlor (3-ethyloxetane-3-yl)methyl methacrylate (OXE-30) to an inkjet ink comprising one or more di-and/or multifunctional monomers improves the adhesion of the inkjet ink onto various substrates, without having to reduce the amount of the one or more di-and/or multifunctional monomers. Further, it has been found that the inclusion of one or more monofunctional (meth)acrylate monomers, including (3-ethyloxetane-3-yl)methyl acrylate (OXE-10) and/or (3-ethyloxetane-3-yl)methyl methacrylate (OXE-30) to an inkjet ink comprising one or more di-and/or multifunctional monomers improves the adhesion of the inkjet ink onto various substrates, without having to include other monofunctional monomers which have a negative impact on the printed ink image and/or the user's health and safety.

The inkjet ink of the present invention contains one or more monofunctional (meth)acrylate monomers. The one or more monofunctional (meth)acrylate monomers includes (3-ethyloxetane-3-yl)methyl acrylate (referred to herein as "OXE-10") and/or (3-ethyloxetane-3-yOmethyl methacrylate (referred to herein as "OXE-30").

OXE-10 has the formula: OXE-10 has been allocated CAS no. 41988-14-1, and has the chemical name (3-ethyloxetane-3-yl)methyl acrylate.

OXE-30 has the formula: OXE-30 has been allocated CAS no. 37674-57-0, and has the chemical name (3-ethyloxetane-3-yl)methyl methacrylate.

In a preferred embodiment, the inkjet ink of the present invention comprises OXE-10.

OXE-10 and/or OXE-30 is preferably present at 10-50% by weight, more preferably 15-35% by weight, based on the total weight of the ink.

Both OXE-10 and OXE-30 contain one (meth)acrylate group and one oxetane group. As such, OXE- 10 and OXE-30 are curable on exposure to actinic radiation and/or curing with a low-energy electron beam.

As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers.

For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and difunctional are intended to have their standard meanings, i.e. one or two groups, respectively, which take part in the polymerisation reaction on curing. Multifunctional (which does not include difunctional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing.

The inkjet ink of the present invention further comprises one or more di-and/or multifunctional monomers, such as a di-/multifunctional (meth)acrylate monomer, a di-/multifunctional vinyl ether or a di-/multifunctional vinyl ether (meth)acrylate monomer.

The substituents of the di-and/or multifunctional monomer are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CI-18 alkyl, C3-19 cycloalkyl, C6..to aryl and combinations thereof, such as C6.10 aryl-or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

Examples of the di-and/or multifunctional radiation-curable monomer include difunctional (meth)acrylate monomers. multifunctional (meth)acrylate monomers, divinyl ether monomers, multifunctional vinyl ether monomers and di-and/or multifunctional vinyl ether (meth)acrylate monomers. Mixtures of di-and/or multifunctional monomers may also be used.

Difuncfional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Examples include hexanediol diacrylate (HDDA), 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate (DDDA), 1,11-undecanediol diacrylate and 1,12-dodecanedial diacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA, PEGEOODA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1,5-pentanediol diacrylate (3MPDDA), and the acrylate esters of ethoxylated or propoxylated glycols and polyols: for example, propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof. Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate and 1,12-dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate and mixtures thereof.

Multifunctional monomers (tri-and higher-functional) are also well known in the art and a detailed description is therefore not required. Usually, the multifunctional (meth)acrylate monomer has a degree of functionality of four or more, e.g. 4-8.

Examples of the multifunctional monomers include trimethylolpropane triacrylate (TMPTA), ditrimethylolpropane tetraacrylate (DiTMPTA), pentaerythritol triacrylate, Iri(propylene glycol) triacrylate, di-pentaerythritol hexaacrylate (DPHA), and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate (EOTMPTA), and mixtures thereof.

Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as trimethylolpropane trimethacrylate. Mixtures of (meth)acrylates may also be used. A preferred multifunctional (meth)acrylate monomer is TMPTA.

The di-and/or multifunctional monomer, based on the total weight of the ink, may have at least one vinyl ether functional group.

Examples of divinyl ether monomers include Methylene glycol divinyl ether, diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether, bis[4-(vinyloxy)butyl] 1,6-hexanediythiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi-4,1-phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4- (vinyloxymethypcyclohexylmethyl] glutarate, 1,4-butanediol divinyl ether and mixtures thereof.

An example of a multifunctional vinyl ether monomer is tris[4-(vinyloxy)butyl] trimellitate.

Examples of vinyl ether (meth)acrylate monomers include 2-(2-vinyloxy ethoxy)ethyl acrylate (VEEA), 2-(2-vinyloxy ethoxy)ethyl methacrylate (VEEM) and mixtures thereof.

Surprisingly, it has been found that the inkjet ink of the present invention can tolerate one or more di-and/or multifunctional monomers, without compromising the adhesion of the inkjet ink onto various substrates. when one or more monofunctional (meth)acrylate monomers, including (3-ethyloxetane-3-yl)methyl acrylate (OXE-10) and/or (3-ethyloxetane-3-yl)methyl methacrylate (OXE-30) are present. The inclusion of one or more di-and/or multifunctional monomers in the inkjet ink of the present invention means that the cured ink image maintains good resistance properties. Together, the monomers provide the ink of the present invention with good film-forming properties, without having to restrict the amount of one or more di-and/or multifunctional monomers, and/or include monofunctional monomers which have a negative impact on the printed ink image/health and safety.

The di-and/or multifunctional monomer may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality monomers may be used.

Preferably, the one or more di-and/or multifunctional monomers is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3MPDDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA) and mixtures thereof.

In a preferred embodiment, the one or more di-and/or multifunctional monomers comprises difunctional monomers. When difunctional monomers are present, preferably, the one or more difunctional monomers comprises one or more difunctional (meth)acrylate monomers.

In a particularly preferred embodiment, the inkjet ink of the present invention comprises 1,10-decanediol diacrylate (DDDA).

The one or more di-and/or multifunctional monomers is preferably present at 10-30% by weight, more preferably 15-25% by weight, based on the total weight of the ink. More preferably, the one or more di-and/or multifunctional monomers comprises one or more difunctional monomers, wherein the one or more difunctional monomers are present at 10-30% by weight, more preferably 15-25% by weight, based on the total weight of the ink. Most preferably, the one or more di-and/or multifunctional monomers comprises one or more difunchonal (meth)acrylate monomers, wherein the one or more difunctional (meth)acrylate monomers are present at 10-30% by weight, more preferably 15-25% by weight. based on the total weight of the ink.

The ink may contain other monofunctional (meth)acrylate monomers, and in a preferred embodiment, the ink contains at least two monofunctional (meth)acrylate monomers, including OXE-10 and/or OXE30.

Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Mixtures of (meth)acrylate monomers may be used.

The substituents of the other monofunctional (meth)acrylate monomers are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The other monofunctional (meth)acrylate monomers may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

The substituents of the cyclic monofunctional (meth)acrylate monomer are typically cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms and/or substituted by alkyl. Non-limiting examples of substituents commonly used in the art include C3-18 cycloalkyl, C6-10 aryl and combinations thereof, any of which may substituted with alkyl (such as Ci.is alkyl) and/or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1.3-dioxolane-4-yOmethyl acrylate (MEDA/Medo1-10), 4-tert-butyloyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof. IBOA is particularly preferred.

The substituents of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is C1-18 alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C6-C20 group. It may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

Preferably, the inkjet ink of the present invention comprises IBOA.

The ink typically contains 10-700/o by weight of monofunctional (meth)acrylate monomer, including OXE10 and/or OXE-30, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink contains 5-35% by weight of monofunctional (meth)acrylate monomer, other than OXE-10 and/or OXE-30, based on the total weight of the ink.

Tetrahydrofurfuryl acrylate (THFA) is often used to provide good adhesion to variety of substrates, as well as producing a flexible film which is less liable to cracking and delamination. A further advantage of THFA is that it can solubilise chlorinated polyolefins, which in turn provides good adhesion to polyolefin substrates. However, THFA is a hazardous monomer and bears the GI-IS hazard statement H314 (Causes severe skin burns and eye damage). There is also growing evidence that it may damage fertility or the unborn child. Thus, there is an urgent need in the art to move away from THFA.

It has surprisingly been found that OXE-10 and/or OXE-30 can be used as a replacement for THFA in inks, which would otherwise require the presence of THFA The ink will still function in the presence of tetrahydrofurfuryl acrylate (THFA), in terms of its printing and curing properties. However, to avoid the hazardous nature of THFA, the ink is preferably substantially free of THFA. The ink preferably contains less than 5% by weight, more preferably less than 2% by weight, more preferably less than 1% by weight and most preferably 0% of THFA, based on the total weight of the ink.

The ink may further include at least one N-vinyl amide monomer and/or N-(meth)acryloyl amine monomer.

N-Vinyl amide monomers are well-known monomers in the art. N-Vinyl amide monomers have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl formamide and N-vinyl acetamide.

Similarly, N-acryloyl amine monomers are also well-known in the art. N-Acryloyl amine monomers also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO).

In a preferred embodiment, the inkjet ink comprises 10-30% by weight, more preferably 15-25% by weight. of an N-vinyl amide monomer, an N-acryloyl amine monomer or mixtures thereof, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises at least one of NVC and/or ACMO. N-Vinyl amide monomers are particularly preferred, and most preferably NVC.

The inkjet ink may also comprise one or more N-vinyl monomers other than an N-vinyl amide monomer and/or N-(meth)acryloyl amine monomer. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole.

In a preferred embodiment, the inkjet ink comprises 10-30% by weight, more preferably 15-25% by weight, of one or more N-vinyl monomers other than an N-vinyl amide monomer and/or N-(meth)acryloyl amine monomer, based on the total weight of the ink.

A particularly preferred monofunctional monomer combination for the present invention is OXE-10, IBOA and NVC. More preferably, OXE-10, IBOA and NVC are the sole monofunctional monomers present in the ink.

A particularly preferred monomer combination for the present invention is OXE-10, IBOA, NVC and DDDA. More preferably, OXE-10, IBOA, NVC and DDDA are the sole monomers present in the ink.

Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. Monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. They therefore preferably have a viscosity of less than 150 mPas at 25°C, more preferably less than 100mPas at 25°C and most preferably less than 20 mPas at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique 12' steel cone at 25°C with a shear rate of 25 s-1.

The ink may comprise a radiation-curable (i.e. polymerisable) oligomer, such as a (meth)acrylate oligomer. Any radiation-curable oligomer that is compatible with the other ink components is suitable for use in the ink.

The term "curable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 600. The molecular weight is preferably 4,000 or less. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

The oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality oligomers may be used. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is preferably multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 5.

Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The oligomer preferably comprises a urethane backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers. The oligomer may include amine functionality, as the amine acts as an activator without the drawback of migration associated with low-molecular weight amines.

Particularly preferred radiation-curable oligomers are urethane acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are di-, tri-, tetra-, penta-or hexafunctional urethane acrylates.

Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance.

The amount of radiation-curable oligomer, when present, is preferably 0.1-10% by weight, based on the total weight of the ink.

Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide film-forming properties such as hardness or cure speed. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Preferred oligomers for inclusion in the ink of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by TA. Instruments, which uses a 40 mm oblique! 7' steel cone at 60°C with a shear rate of 25 s The ink of the present invention may also include radiation-curable material which is capable of polymerising by cationic polyrnerisation, other than OXE-10 and/or OXE-30. Suitable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The radiation-curable material according to this embodiment may comprise a mixture of cationically curable monomer and oligomer. For example, the radiation-curable material may comprise a mixture of an epoxide oligomer and an oxetane monomer, which may be OXE-10 and/or OXE-30 or an additional oxetane monomer.

The ink may also contain a resin. The resin preferably has a weight-average molecular weight (Mw) of 10-50 KDa, and most preferably 15-35 KDa. The Mw may be measured by known techniques in the art, such as gel permeation chromatography (GPC), using a polystyrene standard. The resin preferably has a viscosity of 5-200 mPas at 25°C. It is preferably soluble in the liquid medium of the ink (the radiation-curable diluent and, when present, additionally the solvent).

The resin is a passive (i.e. inert) resin, in the sense that it is not radiation curable and hence does not undergo cross-linking under the curing conditions to which the ink is subjected.

The resin may improve adhesion of the ink to the substrate. It is preferably soluble in the ink. The resin is preferably present at 0.1-5% by weight, based on the total weight of the ink.

The ink of the present invention may also include a photoinitiator which under irradiation, for example using ultraviolet light, initiates the polymerisation of the radiation-curable diluent.

In a preferred embodiment, the inkjet ink of the present invention contains a photoinitiator. Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example. benzophenone, 1-hyd roxycyclohexyl phenyl ketone, 2-benzy1-2-dimethylamino-(4- morpholinophenyl)butan-1-one, benzil dimethylketal, bis(2,6-dimethylbenzoyI)-2,4,4-trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure, Darocur (from Ciba) and Lucirin (from BASF). The ink of the present invention is preferably cured by ultraviolet irradiation. In a preferred embodiment the radiation-curable material polymerises by free-radical polymerisation.

In one embodiment, the ink may also comprise a cationic photoinitiator.

Any suitable cationic initiator can be used, for example sulfonium or iodonium based systems. Non limiting examples include: Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SO from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa.

Preferably the photoinitiator is present from 1 to 20% by weight, preferably from 5 to 15% by weight, of the ink.

The presence of one or more photoinitiators is optional as it is not necessary to include a photoinitiator in the inkjet ink in order to achieve a thorough cure of the ink. This is because the ink can cure without the presence of one or more photoinitiators by curing with a low-energy electron beam.

In a preferred embodiment, the photoinitiator is present in an amount of 20% by weight or less, preferably 5% by weight or less, more preferably 3% or less, most preferably 1% or less, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink is substantially free of photoinitiator. By "substantially free" is meant that no photoinitiator is intentionally added to the ink. However, minor amounts of photoinitiator, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 1% by weight of photoinitiator, preferably less than 0.5% by weight of photoinitiator, more preferably less than 0.1010 by weight of photoinitiator, most preferably less than 0.05% by weight of photoinitiator, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of photoinitiator. An inkjet ink that is substantially free of photoinitiator is advantageous for various applications as there will be no unreacted photoinitiator or unreacted photoinitiator fragments present in the cured inkjet ink film. Photoinitiators create free radicals when exposed to radiation. These radicals react with reactive components of the ink (such as reactive monomers and oligomers). However, some photoinitiator and photoinitiator fragments will remain unreacted in the cured ink film and this is problematic for certain applications as such unreacted components can migrate into the substrate. In the ink of the present invention, photoinitiator is not necessary to achieve cure owing to curing with low-energy electron beam.

The ink-jet ink of the present invention may also include a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. The colouring agent can be any of a wide range of suitable colouring agents that would be known to the person skilled in the art.

Preferably, the colouring agent is a dispersed pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF plc), Cinquasia, lrgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK). The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

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 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 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 0.2-20% by weight, preferably 0.5-10% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink.

The present invention may also provide an inkjet ink set wherein at least one of the inks in the set is an inkjet ink of the present invention. Preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention.

Usually, the inkjet ink set of the present invention is in the form of a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. 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.

Print heads account for a significant portion of the cost of an entry level printer and it is therefore desirable to keep the number of print heads (and therefore the number of inks in the ink set) low. Reducing the number of print heads can reduce print quality and productivity. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity.

The inkjet ink used in the method of the present invention preferably dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. Accordingly. the inkjet ink is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink comprises less than 5 wt% of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2 % by weight combined and most preferably less than 1% by weight combined, based on the total weight of the ink. Some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated.

In a preferred embodiment, the ink of the present invention comprises a surfactant. The surfactant controls the surface tension of the ink. Surfactants are well known in the art and a detailed description is not required. An example of a suitable surfactant is BYK307. 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-' and more preferably 2535 mNm-1.

Other components of types known in the art may be present in the ink of the present invention to improve the properties or performance. These components may be, for example, defoamers, dispersants, synergists for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

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

The ink of the present invention is suitable for application by inkjet printing. The ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less and most preferably 30 mPas or less at 25°C. The ink most preferably has a viscosity of 20 to 30 mPas at 25aC. Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model LDV1+.

The present invention may also provide a cartridge containing the inkjet ink as defined herein.

The present invention also provides a method of inkjet printing, comprising jetting the ink as defined herein onto a substrate and curing the ink.

In the method of inkjet printing of the present invention, the inkjet ink is printed onto a substrate. Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto a substrate, on a roll-to-roll printer or a flat-bed printer. As discussed above, inkjet printing is well known in the art and a detailed description is not required.

The ink is jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image.

Substrates include those composed of polyvinyl chloride (PVC), polystyrene, polyester, polyethylene terephthalate (PET), polyethylene terephthalate glycol modified (PETG) and polyolefin (e.g. polyethylene, polypropylene or mixtures or copolymers thereof). In a preferred embodiment, the substrate is selected from polyvinyl chloride or polystyrene. When discussing the substrate, it is the surface which is most important, since it is the surface which is wetted by the ink. Thus, at least the surface of substrate is composed of the above-discussed material.

The present invention may also provide a printed substrate having the ink as defined herein printed thereon.

In order to produce a high quality printed image a small jetted drop size is desirable. Furthermore, small droplets have a higher surface area to volume ratio when compared to larger drop sizes, which facilitates evaporation of solvent from the jetted ink. Small drop sizes therefore offer advantages in drying speed. Preferably the inkjet ink is jetted at drop sizes below 90 picolitres, preferably below 35 picolitres and most preferably below 10 picolitres.

To achieve compatibility with print heads that are capable of jetting drop sizes of 90 picolitres or less, a low viscosity ink is required. A viscosity of 30 mPas or less at 25t is preferred, for example, 10 to 12 mPas, 18 to 20 mPas, or 24 to 26 mPas. Ink viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

The ink of the present invention is cured by any means known in the art, such as exposure to actinic radiation and low-energy electron beam radiation.

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 source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source. Preferably, the source of actinic radiation is a mercury discharge lamp and/or LEDs. When LEDs are used, these are preferably provided as an array of multiple LEDs.

The source of low-energy electron beam (ebeam) can be any source of low-energy electron beam that is suitable for curing radiation-curable inks. Suitable low-energy electron beam sources include commercially available ebeam curing lamps, such as a 280 mm comet ebeam curing lamp which has a penetrating voltage of 80 kV and is capable of delivering a dosage of 30 kGy at 100 m/min. By "low-energy" for the ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 50 kGy or less.

Ebeam curing is characterised by dose (energy per unit mass, measured in kilograys (kGy)) deposited in the substrate via electrons. Electron beam surface penetration depends upon the mass, density and thickness of the material being cured. Compared with UV penetration, electrons penetrate deeply through both lower and higher density materials. Unlike UV curing, photoinitiators are not required for ebeam curing to take place.

Ebeam curing is well-known in the art and therefore a detailed explanation of the curing method is not required. In order to cure the printed ink, the ink of the invention is exposed to the ebeam, which produces sufficient energy to instantaneously break chemical bonds and enable polymerisation or crossl in king.

The ink cures to form a relatively thin polymerised film. The ink of the present invention typically produces a printed film having a thickness of 1 to 20 pm, preferably 1 to 10 pm, for example 2 to 5 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

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

Examples

Example 1

Inks were prepared by mixing the components in the amounts shown in Table I. Amounts are given as weight percentages based on the total weight of the ink Table 1. Ink formulations Component Comparative ink 1, wt% Comparative ink 2, Ink 3, wt% wt% PEA 24.68 MEDOL-10 24.68 OXE-10 24.68 NVC 16.5 16.5 16.5 IBOA 11.9 11.9 11.9 DDDA 20 20 20 UV12 0.5 0.5 0.5 CN964A85 ITX 5 5 5 0.8 0.8 0.8 EPD 0.85 0.85 0.85 Benzophenone 2.88 2.88 2.88 Irgacure 184 1.88 1.88 1.88 TPO 8.01 8.01 8.01 Cyan pigment dispersion Byk 307 6 6 6 1 1 1 Total 100 100 100 PEA, MEDOL-10, OXE-10, NVC, 1130A and DDDA are monomers, as defined herein. UV12 is a stabiliser. CN964A85 is a urethane acrylate oligomer. ITX, EPD, benzophenone, lrgacure 184 and TPO are photoinitiators. The cyan pigment dispersion contains PEA (59.0 wt%), UV12 (1.0 wt%), Solsperse 32000 (10.0 wt%) and Heliogen blue (30.0 wt%). BYK307 is a surfactant.

Example 2

Each of the above ink formulations was coated on to a banner PVC substrate and a gloss high impact polystyrene substrate using a K 2 applicator bar (12 pm wet film). The resulting films were cured using a medium pressure mercury UV lamp with a combined UVA, UVB, UVC and UVV dose of 783 mJ/cm2 and a combined UVA, UVB, UVC, and UVV intensity of 3596 mW/cm2.

Adhesion of the inks to the substrates was measured by using a cross hatch tape removal test. The test is as follows. Score surface with an elcometeriblade to form a cross hatch area and apply 1502049 tape across the scored area. After applying pressure, remove the tape and assess for ink removal from the substrate. The inks are scored from 1 to 5, with 1 being poor ink adhesion to the substrate and 5 being excellent ink adhesion to the substrate. The results are set out in Table 2.

Table 2. Ink adhesion.

Substrate Adhesion, comparative Adhesion, comparative ink 2 Adhesion, ink 3 ink 1 PVC 1 1 5 Gloss high impact polystyrene 1 1 3 As can be seen from Table 2, ink 3 of the invention containing OXE-10 has excellent adhesion to PVC and acceptable adhesion to gloss high impact polystyrene. This is in contrast to comparative ink 1.

which contains PEA as opposed to OXE-10, and comparative ink 2, which contains MEDOL-10 as opposed to OXE-10. These comparative inks 1 and 2 have poor adhesion to both PVC and gloss high impact polystyrene.

Example 3

Each of the above ink formulations was coated on to a self-adhesive vinyl substrate using a K 2 applicator bar (12 pm wet film). The resulting films were passed under a medium pressure mercury UV lamp repeatedly until the ink film was cured. The combined UVA, UVB, UVC and UVV dose per pass was 80 mJ/cm2 and the combined UVA, U\/B, UVC and UVV intensity was 784 mW/cm2.

The cure was assessed by placing a piece of EPSON photo-paper on the print, rubbing the front of the photo-paper onto the ink film, and measuring the offset of ink upon removing the photo-paper. The ink is deemed fully cured when no ink offsets onto the photo-paper. If there is offset of ink onto the photo-paper, the ink film is placed under the lamp repeatedly until full cure is achieved. The results are set

out in Table 3

Table 3. Cure speed.

Cure speed Comparative ink 1 Comparative ink 2 Ink 3 Number of passes to cure 14 12 12 Dose to cure (mJ/cm2) 1120 960 960 As can be seen from Table 3, ink 3 of the invention containing OXE-10 has a comparable cure speed to the comparative inks. As such, the inclusion of OXE-10 does not negatively affect the cure speed of the ink.

Claims (15)

1. Claims 1. An inkjet ink comprising: one or more monofunctional (meth)acrylate monomers, including (3-ethyloxetane-3-yl)methyl acrylate (OXE-10) and/or (3-ethyloxetane-3-yl)methyl methacrylate (OXE-30); and one or more di-and/or multifunctional monomers.
2. 2. An inkjet ink as claimed in claim 1, wherein the OXE-10 and/or OXE 30 is present at 10-50% by weight. based on the total weight of the ink.
3. 3. An inkjet ink as claimed in claims 1 or 2, wherein the ink comprises OXE-10.
4. 4. An inkjet ink as claimed in any preceding claim, wherein the one or more di-and/or multifunctional monomers is present at 10-30% by weight, based on the total weight of the ink.
5. 5. An inkjet ink as claimed in any preceding claim, wherein the one or more di-and/or multifunctional monomers comprises one or more difunctional monomers, preferably wherein the one or more difunctional monomers are present at 10-30% by weight, more preferably 15-25% by weight, based on the total weight of the ink.
6. 6. An inkjet ink as claimed in any preceding claim, wherein the one or more di-and/or multifunctional monomers is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (I-IDDA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-Pentanediol Diacrylate (3MPDDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DPI-1A), ethoxylated trimethylolpropane triacrylate (EOTMPTA) and mixtures thereof.
7. 7. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises at least one N-vinyl amide monomer and/or N-(meth)acryloyl amine monomer.
8. 8. An inkjet ink as claimed in any preceding claim, wherein the ink comprises at least two monofunctional (meth)acrylate monomers, including OXE-10 and/or OXE 30.
9. 9. An inkjet ink as claimed in any preceding claim, wherein the ink comprises OXE-10, isobornyl acrylate (IBOA), N-vinyl caprolactam (NVC) and 1,10-decanediol diacrylate (DDDA).
10. 10. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a photoinitiator.
11. 11. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a colouring agent.
12. 12. An inkjet ink as claimed in any preceding claim, wherein the ink further comprises a radiation-curable oligomer and/or a resin.
13. 13. An inkjet ink as claimed in any preceding claim, wherein the ink is substantially free of tetrahydrofurfuryl acrylate (THFA).
14. 14. A method of inkjet printing, comprising jetting the ink as claimed in any preceding claim onto a substrate and curing the ink.
15. 15. A method as claimed in claim 14, wherein the substrate is composed of polyvinyl chloride or polystyrene.