Printing ink

Abstract

The present invention provides a white inkjet ink comprising: at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; a white pigment; and 4.0 to 7.0% by weight of ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, based on the total weight of the ink, wherein the ink is free of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide. The present invention further provides a method of inkjet printing comprising inkjet printing the inkjet ink onto a substrate and curing the inkjet ink by exposing the inkjet ink to a curing source. Further, the invention provides a substrate having the inkjet ink printed thereon.

Description

[0001] Printing ink

[0002] The present invention relates to a printing ink and in particular, to a white inkjet ink which can achieve a desirable balance of properties.

[0003] 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.

[0004] 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.

[0005] Another type of inkjet ink contains radiation-curable material, such as radiation-curable monomers and / or oligomers, which polymerise when cured. By “radiation-curable” is meant a material that polymerises and / or crosslinks upon irradiation, for example, when exposed to actinic radiation, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is cured, a process which is more rapid than evaporation of solvent at moderate temperatures.

[0006] Inkjet inks are often formulated to contain a colouring agent such as a pigment. It is desirable to include a high amount of pigment in the ink in order to achieve a high colour density and / or high opacity. However, it can be difficult to incorporate a high amount of pigment in an inkjet ink because such inks are prone to poor jetting and stability properties.

[0007] It is particularly difficult to incorporate a high amount of pigment into a white inkjet ink, particularly a white inkjet ink containing the white pigment TiC>2. In this regard, white inkjet inks containing the white pigment TiC>2 require a high pigment loading to achieve a workable opacity. However, the high specific gravity of TiC>2 increases the risk of sedimentation in white inkjet inks.

[0008] A number of ways to try and maximise colour density and / or opacity whilst minimising sedimentation in inkjet inks are known in the art such as stabilising the pigment and / or using complex ink delivery systems. However, stabilising the pigment restricts the ink formulation and using complex ink delivery systems adds to the cost and complexity of printing apparatus design and maintenance. The restriction on the ink formulation and the use of complex ink delivery systems has a negative impact on the cure of the ink.

[0009] Yellowing is also a problem in white inkjet inks and components that lead to yellowing must be restricted in white inkjet inks. It is therefore apparent that white inkjet ink formulations require very different considerations to other inkjet inks.

[0010] A common photoinitiator for inclusion in a radiation-curable inkjet ink is diphenyl(2,4,6- trimethylbenzoyljphosphine oxide (TPO). TPO provides excellent cure and adhesion. However, TPO is hazardous. It currently has a hazard statement of H361 f - suspected of damaging fertility - and will soon be updated to having the more hazardous hazard statement of H360 - may damage fertility or the unborn child. TPO is also potentially a substance of very high concern (SVHC).

[0011] Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) is an alternative photoinitiator to TPO. However, BAPO is highly undesirable in white inkjet inks as it leads to yellowing.

[0012] There is therefore a need in the art for a white inkjet ink which contains an alternative photoinitiator to TPO and BAPO and maintains a desirable balance of properties such as cure, adhesion and low yellowing.

[0013] Accordingly, the present invention provides a white inkjet ink comprising: at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; a white pigment; and 4.0 to 7.0% by weight of ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, based on the total weight of the ink, wherein the ink is free of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide.

[0014] The inventors have surprisingly found that including at least one N-vinyl amide monomer, N- (meth)acryloyl amine monomer and / or N-vinyl carbamate monomer in combination with 4.0 to 7.0% by weight of ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate (TPO-L), based on the total weight of the ink, in a white inkjet ink containing a white pigment, wherein the ink is free of diphenyl(2,4,6- trimethylbenzoyl)phosphine oxide (TPO) and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), provides a white inkjet ink having a desirable balance of properties such as cure, adhesion and low yellowing, without recourse to TPO and BAPO. The ink of the present invention is therefore safer than one containing TPO and less discoloured than one containing BAPO.

[0015] It is surprising that only 4.0 to 7.0% by weight of TPO-L is required to achieve the optimum balance of cure and adhesion when a higher amount of TPO would be required to achieve a similar balance of cure and adhesion. Typically, increasing acylphosphine oxide concentration in inks increases cured. It would therefore be expected that increasing the amount of TPO-L above 7.0% would increase cure. However, the inventors have surprisingly found that increasing the amount of TPO-L above 7.0% actually decreases cure. A lower concentration of photoinitiator present in the ink leads to cost saving and a lower viscosity ink, which is useful when a low viscosity ink is required.

[0016] The white inkjet ink of the invention comprises at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer.

[0017] The at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer in combination with 4.0 to 7.0% by weight of ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate (TPO-L), based on the total weight of the ink, in a white inkjet ink containing a white pigment, provides a white inkjet ink having a desirable balance of properties such as cure, adhesion and low yellowing, without recourse to TPO and BAPO. In particular, the inclusion of at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer in the inkjet ink of the invention increases cure and adhesion.

[0018] Preferably, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer is present in a total amount of 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, based on the total weight of the ink.

[0019] In a preferred embodiment, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer comprises an N-vinyl amide monomer and / or an N-(meth)acryloyl amine monomer. Put another way, the ink of the invention preferably comprises an N-vinyl amide monomer and / or an N-(meth)acryloyl amine monomer.

[0020] Preferably, the N-vinyl amide monomer and / or the N-(meth)acryloyl amine monomer is present in a total amount of 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, based on the total weight of the ink.

[0021] 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 discussed below. Preferred examples are N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl formamide and N-vinyl acetamide. N- vinyl caprolactam (NVC) is particularly preferred.

[0022] N-(Meth)acryloyl amine monomers are also well-known in the art. N-(Meth)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 discussed below. A preferred example is N-acryloylmorpholine (ACMO). In a preferred embodiment, the N-vinyl amide monomer, when present, comprises NVC and the N- (meth)acryloyl amine monomer, when present, comprises ACMO. Preferably, the ink of the invention preferably comprises NVC and / or ACMO. In one preferred embodiment, the inkjet ink of the invention comprises NVC. In another preferred embodiment, the inkjet ink of the invention comprises ACMO. In an alternative preferred embodiment, the inkjet ink of the invention comprises NVC and ACMO.

[0023] Preferably, the ink comprises NVC and / or ACMO in a total amount of 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, based on the total weight of the ink.

[0024] In a preferred embodiment, the inkjet ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, of one or more N-vinyl amide monomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, of NVC, based on the total weight of the ink.

[0025] In a preferred embodiment, the inkjet ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, of one or more N-(meth)acryloyl amine monomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, of ACMO, based on the total weight of the ink.

[0026] In a preferred embodiment, the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer comprises an N-vinyl carbamate monomer.

[0027] N-Vinyl carbamate monomers are defined by the following functionality:

[0028] The synthesis of N-vinyl carbamate monomers is known in the art. For example, vinyl isocyanate, formed by the Curtius rearrangement of acryloyl azide, can be reacted with an alcohol to form N-vinyl carbamates (Phosgenations - A Handbook by L. Cotarca and H. Eckert, John Wiley & Sons, 2003, 4.3.2.8, pages 212-213). In a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure: in which R1to R4are 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 hydrogen, 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 C1-18 alkyl, C3-18 cycloalkyl, CB- aryl and combinations thereof, such as Cs- aryl- or Cs-is 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 abovedescribed substituents. Preferably R1to R4are independently selected from hydrogen or C1-10 alkyl. Further details may be found in WO 2015 / 022228 and US 4,831 ,153.

[0029] Most preferably, the N-vinyl carbamate monomer is N-vinyl-5-methyl-2-oxazolidinone (known as NVMO or VMOX). It is available from BASF and has the following structure: molecular weight 127 g / mol

[0030] NVMO has the IUPAC name 5-methyl-3-vinyl-1 ,3-oxazolidin-2-one and CAS number 3395-98-0. NVMO includes the racemate and both enantiomers. In one embodiment, the N-vinyl carbamate monomer is a racemate of NVMO. In another embodiment, the N-vinyl carbamate monomer is (R)-5- methyl-3-vinyl-1 ,3-oxazolidin-2-one. Alternatively, the N-vinyl carbamate monomer is (S)-5-methyl-3- vinyl-1 ,3-oxazolidin-2-one.

[0031] In a preferred embodiment, the ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight of one or more N-vinyl carbamate monomers in total, based on the total weight of the ink. Preferably, the inkjet ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight, of NVMO, based on the total weight of the ink.

[0032] The inkjet ink may also comprise one or more N-vinyl monomers other than an N-vinyl amide monomer and an N-vinyl carbamate monomer. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole.

[0033] In a preferred embodiment, the inkjet ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight in total, of one or more N-vinyl monomers other than an N-vinyl amide monomer and an N-vinyl carbamate monomer, based on the total weight of the ink.

[0034] Preferably, the inkjet ink comprises 5 to 35% by weight, more preferably 5 to 30% by weight and most preferably 5 to 25% by weight in total, of all N-vinyl monomers and N-(meth)acryloyl amine monomers, based on the total weight of the ink.

[0035] In a preferred embodiment, the inkjet ink further comprises one or more mono- and / or difunctional (meth)acrylate monomers. The inventors have found that the inclusion of one or more mono- and / or difunctional (meth)acrylate monomers in the specific blend of components of the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including cure, adhesion and low yellowing, without recourse to TPO and BAPO. In particular, the inclusion of one or more mono- and / or difunctional (meth)acrylate monomers in the inkjet ink of the invention provides the highest adhesion and flexibility compared to tri- and higher functionality monomers.

[0036] 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, 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.

[0037] (Meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and / or methacrylate.

[0038] 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 10OmPas 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 s1. Preferably, the inkjet ink comprises one or more mono- and / or difunctional (meth)acrylate monomers in a total amount of 15 to 70% by weight, more preferably 25 to 60% by weight and most preferably 30 to 55% by weight, based on the weight of the ink.

[0039] In a preferred embodiment, the inkjet ink comprises one or more monofunctional (meth)acrylate monomers. The inventors have found that the inclusion of one or more monofunctional (meth)acrylate monomers in the specific blend of components of the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including cure, adhesion and low yellowing, without recourse to TPO and BAPO. In particular, monofunctional (meth)acrylate monomers synergise with the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer in the inkjet ink of the invention to increase cure speed significantly. And the inclusion of one or more monofunctional (meth)acrylate monomers in the inkjet ink of the invention provides the highest flexibility compared to di-, tri- and higher functionality monomers.

[0040] The substituents of the one or more monofunctional (meth)acrylate monomers are not limited otherthan by the constraints imposed by the use in an inkjet 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 C1-18 alkyl, C3-18 cycloalkyl, Cs- aryl and combinations thereof, such as CB- aryl- or C3-18 cycloalkylsubstituted 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.

[0041] Preferably, the inkjet ink comprises one or more monofunctional (meth)acrylate monomers in a total amount of 5 to 70% by weight, more preferably 15 to 60% by weight and most preferably 20 to 55% by weight, based on the weight of the ink.

[0042] The one or more monofunctional (meth) acrylate monomers may be one or more cyclic monofunctional (meth)acrylate monomers and / or one or more acyclic-hydrocarbon monofunctional (meth)acrylate monomers.

[0043] In a preferred embodiment, the one or more monofunctional (meth)acrylate monomers comprise one or more cyclic monofunctional (meth)acrylate monomers. The inventors have found that the inclusion of one or more cyclic monofunctional (meth)acrylate monomers in the specific blend of components of the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including cure, adhesion and low yellowing, without recourse to TPO and BAPO.

[0044] The substituents of the one or more cyclic monofunctional (meth)acrylate monomers 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, CB- aryl and combinations thereof, any of which may substituted with alkyl (such as C1-18 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.

[0045] Preferably, the inkjet ink comprises one or more cyclic monofunctional (meth)acrylate monomers in a total amount of 5 to 70% by weight, more preferably 15 to 60% by weight and most preferably 20 to 55% by weight, based on the weight of the ink.

[0046] In a preferred embodiment, the one or more cyclic monofunctional (meth)acrylate monomers, when present, are selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA / Medol-10), 4-te / Y-butylcyclohexyl acrylate (TBCHA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof.

[0047] More preferably, the one or more cyclic monofunctional (meth)acrylate monomers, when present, are selected from IBOA, PEA, CTFA and mixtures thereof. The inventors have found that the inclusion of IBOA, PEA, CTFA and mixtures thereof in the specific blend of components of the inkjet ink of the invention is particularly preferred as it further improves the properties of the ink, including cure, adhesion and low yellowing, without recourse to TPO and BAPO.

[0048] In a particularly preferred embodiment, the inkjet ink comprises IBOA.

[0049] THFA is often used to provide good adhesion to a 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 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 THFA.

[0050] 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 inkjet ink preferably contains less than 2% by weight, more preferably less than 1 % by weight and most preferably is substantially free of THFA, where the amounts are based on the total weight of the ink.

[0051] By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present or as a component in a commercially available pigment dispersion. In other words, no THFA is intentionally added to the ink. However, minor amounts of THFA, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of THFA, more preferably less than 0.1 % by weight of THFA, most preferably less than 0.05% by weight of THFA, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of THFA.

[0052] In a preferred embodiment, the one or more monofunctional (meth)acrylate monomers comprise an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

[0053] The substituents of the one or more acyclic-hydrocarbon monofunctional (meth)acrylate monomers 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 nitrogen further substituted.

[0054] The one or more acyclic-hydrocarbon monofunctional (meth)acrylate monomers contain a linear or branched C6-C20 group. They may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof. In a preferred embodiment, the one or more acyclic-hydrocarbon monofunctional (meth)acrylate monomers contain a linear C6-C20 group.

[0055] Preferably however, the inkjet ink comprises one or more cyclic monofunctional (meth)acrylate monomers and no acyclic-hydrocarbon monofunctional (meth)acrylate monomers are present in the ink. Such acyclic-hydrocarbon monofunctional (meth)acrylate monomers are typically slow curing.

[0056] Therefore, in a preferred embodiment, the inkjet ink contains less than 5% by weight in total, preferably less than 2% by weight in total, more preferably less than 1 % by weight in total, and most preferably is substantially free of acyclic-hydrocarbon monofunctional (meth)acrylate monomers, where the amounts are based on the total weight of the ink.

[0057] By substantially free is meant that only small amounts will be present, for example as impurities in the radiation-curable materials present. In other words, no acyclic-hydrocarbon monofunctional (meth)acrylate monomers are intentionally added to the ink. However, minor amounts of acyclic- hydrocarbon monofunctional (meth)acrylate monomers, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may contain less than 0.5% by weight in total, more preferably less than 0.1 % by weight in total and most preferably less than 0.05% by weight in total of acyclic-hydrocarbon monofunctional (meth)acrylate monomers, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of acyclic-hydrocarbon monofunctional (meth)acrylate monomers.

[0058] In a preferred embodiment, the one or more monofunctional (meth)acrylate monomers are selected from IBOA, PEA, CTFA and mixtures thereof.

[0059] In a preferred embodiment, the inkjet ink comprises one or more difunctional (meth)acrylate monomers. Difunctional (meth) acrylate monomers are well known in the art and a detailed description is not required. Mixtures of difunctional (meth)acrylates may be used. 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-dodecanediol diacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA, PEG600DA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1 ,5-pentanediol diacrylate (3- MPDA), 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 di meth acrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate and mixtures thereof.

[0060] In a preferred embodiment, the one or more difunctional (meth)acrylate monomers comprise DDDA. More preferably, DDDA is the only difunctional (meth)acrylate monomer present in the ink.

[0061] Preferably, the inkjet ink preferably comprises one or more difunctional (meth)acrylate monomers in a total amount of 1 to 20% by weight, more preferably 5 to 15% by weight, based on the weight of the ink.

[0062] In a preferred embodiment, the inkjet ink of the invention comprises one or more monofunctional (meth)acrylate monomers and one or more difunctional (meth)acrylate monomers.

[0063] Preferably, the inkjet ink comprises one or more monofunctional (meth)acrylate monomers and one or more difunctional (meth)acrylate monomers in a total amount of 15 to 70% by weight, more preferably 25 to 60% by weight and most preferably 30 to 55% by weight, based on the weight of the ink.

[0064] The inkjet ink may comprise difunctional monomers other than a difunctional (meth)acrylate monomer, including divinyl ether monomers, vinyl ether (meth)acrylate monomers and mixtures thereof.

[0065] Divinyl ether monomers are well known in the art and a detailed description is not required. Mixtures of divinyl ether monomers may be used. Examples of a divinyl ether monomer include triethylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1 ,4-cyclohexanedimethanol divinyl ether, bis[4- (vinyloxy)butyl] 1 ,6-hexanediylbiscarbamate, 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-(vinyloxymethyl)cyclohexylmethyl] glutarate, 1 ,4-butanediol divinyl ether and mixtures thereof. A preferred example is triethylene glycol divinyl ether (DVE-3). Vinyl ether (meth)acrylate monomers are well known in the art and a detailed description is not required. Examples include 2-(2-vinyloxy ethoxy)ethyl acrylate (“VEEA”), 2-(2-vinyloxy ethoxy)ethyl methacrylate (“VEEM”) and mixtures thereof.

[0066] Preferably, when present, DDDA is the only difunctional monomer present in the ink.

[0067] In a preferred embodiment, the inkjet ink of the invention comprises IBOA in combination with PEA and / or CTFA, and optionally DDDA. More preferably, the inkjet ink of the invention comprises NVC and / or ACMO, IBOA in combination with PEA and / or CTFA, and optionally DDDA. Most preferably, the inkjet ink of the invention comprises NVC, IBOA in combination with PEA and / or CTFA, and optionally DDDA.

[0068] In a particularly preferred embodiment, IBOA in combination with PEA and / or CTFA are the only monofunctional (meth)acrylate monomers present in the ink, and, when present, DDDA is the only difunctional monomer present in the ink. More preferably, NVC and / or ACMO, IBOA in combination with PEA and / or CTFA are the only monofunctional monomers present in the ink, and, when present, DDDA is the only difunctional monomer present in the ink. Most preferably, NVC, IBOA in combination with PEA and / or CTFA are the only monofunctional monomers present in the ink, and, when present, DDDA is the only difunctional monomer present in the ink.

[0069] The inkjet ink may comprise a multifunctional monomer to increase solvent resistance and / or prevent blocking.

[0070] Examples of multifunctional monomers include multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers and multifunctional vinyl ether (meth)acrylate monomers.

[0071] Multifunctional (meth)acrylate monomers (which do not include difunctional (meth) acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-fu notional monomers. Examples of multifunctional acrylate monomers include trimethylolpropane triacrylate, dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate (EOTMPTA) and ethoxylated pentaerythritol tetraacrylate (EOPETTA, also known as PPTTA). Multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as trimethylolpropane trimethacrylate. EOTMPTA is particularly preferred.

[0072] An example of a multifunctional vinyl ether monomer is tris[4-(vinyloxy)butyl] trimellitate.

[0073] The multifunctional monomer, when present, is preferably present in 0.1 to 5% by weight, more preferably 1 to 5% by weight, based on the total weight of the ink.

[0074] The inkjet ink may comprise a radiation-curable oligomer. 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. Oligomers have a molecularweight 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.

[0075] Oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality oligomers are often used. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink.

[0076] Oligomers typically have a viscosity of 150 mPas or above at 25°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique 12° steel cone at 60°C with a shear rate of 25 s1. The oligomer may modify the viscosity of the ink.

[0077] Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups.

[0078] The polymerisable group can be any group that is capable of polymerising upon exposure to radiation, for example, (meth)acrylate oligomers. Oligomers may include amine functionality, for example, an amine-modified (meth)acrylate oligomer. An amine-modified polyester acrylate oligomer is commercially available as UVP6600. An amine-modified polyether acrylate oligomer is commercially available as CN3715LM.

[0079] A particularly preferred radiation-curable oligomer is UVP6505, which is a polyester acrylate oligomer and is used to increase the viscosity of the ink.

[0080] The oligomer, when present, is preferably present in 0.1 to 5% by weight, more preferably 1 to 5% by weight, based on the total weight of the ink.

[0081] The inkjet ink may contain one or more passive resins. Passive resins are resins which are not radiation-curable and hence do not undergo crosslinking under the curing conditions to which the ink is exposed. In other words, resin is not a radiation-curable material.

[0082] Any passive resin that is compatible with the ink components of the final inkjet ink is suitable for use in the inkjet ink of the present invention. Thus, the ink formulator is able to select from a wide range of suitable passive thermoplastic resins.

[0083] The resin may be selected from epoxy, polyester, vinyl, ketone, nitrocellulose, phenoxy or acrylate resins, or a mixture thereof and is preferably a poly(methyl (meth)acrylate) resin. Methacrylate copolymers are preferred. The resin has a weight- average molecular weight of 20-200 KDa and preferably 20-60 KDa, as determined by GPC with polystyrene standards. The resin is preferably solid 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 may improve adhesion of the ink to the substrate.

[0084] The resin, when present, is preferably present in 0.1 to 5% by weight, more preferably 1 to 5% by weight, based on the total weight of the ink.

[0085] The white inkjet ink of the invention comprises a white pigment.

[0086] The white pigment is dispersed in the liquid medium of the ink. The pigment can be selected from a wide range of suitable pigments that would be known to the person skilled in the art. Preferably the white pigment is titanium dioxide, such as Kronos® 2300 available from Kronos Ltd.

[0087] 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.

[0088] In a preferred embodiment, the inkjet ink comprises 5 to 30% by weight, preferably 10 to 20% by weight, of white pigment, based on the total weight of the ink.

[0089] The inkjet ink is a white inkjet ink and as such, is preferably free of any coloured pigments. By coloured pigments, it is meant any coloured pigment, other than white pigment.

[0090] However, minor amounts of coloured pigment, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1 % by weight and most preferably less than 0.05% by weight of coloured pigments, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of coloured pigments.

[0091] The white inkjet ink of the invention comprises 4.0 to 7.0% by weight of ethyl phenyl (2,4,6- trimethylbenzoyl) phosphinate, wherein the ink is free of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide.

[0092] Ethyl (2,4,6-trimethylbenzoyl) phenyl phosphinate is a phosphine oxide photoinitiator and is known commercially as TPO-L. It has the following structure:

[0093] The inventors have surprisingly found that including 4.0 to 7.0% by weight of ethyl phenyl (2,4,6- trimethylbenzoyl) phosphinate (TPO-L) in combination with at least one N-vinyl amide monomer, N- (meth)acryloyl amine monomer and / or N-vinyl carbamate monomer, based on the total weight of the ink, in a white inkjet ink containing a white pigment, wherein the ink is free of diphenyl(2,4,6- trimethylbenzoyl)phosphine oxide (TPO) and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), provides a white inkjet ink having a desirable balance of properties such as cure, adhesion and low yellowing, without recourse to TPO and BAPO. The ink of the present invention is therefore safer than one containing TPO and less discoloured than one containing BAPO.

[0094] It is surprising that only 4.0 to 7.0% by weight of TPO-L is required to achieve the optimum balance of cure and adhesion when a higher amount of TPO would be required to achieve a similar balance of cure and adhesion.

[0095] Typically, increasing acylphosphine oxide concentration in inks increases cured. It would therefore be expected that increasing the amount of TPO-L above 7.0% would increase cure. However, the inventors have surprisingly found that increasing the amount of TPO-L above 7.0% actually decreases cure. A lower concentration of photoinitiator present in the ink leads to cost saving and a lower viscosity ink, which is useful when a low viscosity ink is required.

[0096] However, it is still the case that if the TPO-L concentration is too low, poor adhesion can be observed on certain media. Therefore, a minimum amount of 4.0% by weight of TPO-L, based on the total weight of the ink, is required.

[0097] In a preferred embodiment, the inkjet ink comprises TPO-L in an amount of 4.0 to 6.0% by weight, more preferably 4.5 to 5.5% by weight, based on the total weight of the ink. In a particularly preferred embodiment, the inkjet ink comprises TPO-L in an amount of 5.0% by weight, based on the total weight of the ink.

[0098] Preferably, the inkjet ink comprises one or more photoinitiators other than TPO-L. The one or more additional photoinitiators are not particularly limited apart from they cannot include TPO and BAPO, which are restricted photoinitiators of the inkjet ink of the invention. By an additional photoinitiator is meant a photoinitiator other than TPO-L. Preferred one or more additional photoinitiators are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, isopropylthioxanthone (ITX), 1 ,3-di({a-[1 - chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1 -methylethylene)]} oxy)-2,2-bis({a-[1-chloro-9- oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1 -methylethylene)]}oxymethyl) propane (Speedcure 7010), benzophenone, 1 -hydroxycyclohexyl phenyl ketone (Omnirad 184), 2-benzyl-2-dimethylamino- (4-morpholinophenyl)butan-1-one, benzil dimethylketal, phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, 1-[4-(2-hydroxyethoxyl)-phenyl]-2-hydroxy-2-methylpropanone (Omnirad 2959), 2-hydroxy-1-[4- [4-(2-hydroxy-2-methylpropanoyl)phenoxy]phenyl]-2-methylpropan-1-one (KIP160) and mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Omnirad (from IGM) and Esacure (from Lamberti).

[0099] Mixtures of free radical photoinitiators can be used and preferably, the inkjet ink comprises a plurality of free radical photoinitiators. The total number of free radical photoinitiators present is preferably from two to five, and more preferably, two free radical photoinitiators are present in the ink.

[0100] In a preferred embodiment, the one or more photoinitiators other than ethyl phenyl (2,4,6- trimethylbenzoyl) phosphinate are selected from 2-hydroxy-1-[4-[4-(2-hydroxy-2- methylpropanoyl)phenoxy]phenyl]-2-methylpropan-1-one (KIP160), 1 -hydroxycyclohexyl phenyl ketone (Omnirad 184), 1-[4-(2-hydroxyethoxyl)-phenyl]-2-hydroxy-2-methylpropanone (Omnirad 2959) and mixtures thereof. More preferably, the one or more photoinitiators other than ethyl phenyl (2,4,6- trimethylbenzoyl) phosphinate are selected from 2-hydroxy-1-[4-[4-(2-hydroxy-2- methylpropanoyl)phenoxy]phenyl]-2-methylpropan-1-one (KIP160) and / or 1 -hydroxycyclohexyl phenyl ketone (Omnirad 184).

[0101] The inventors have found that the inclusion of one or more additional photoinitiators in the specific blend of components of the inkjet ink of the invention, particularly Omnirad 184, KIP 160, Omnirad 2959 and mixtures thereof, is preferred as it further improves the properties of the ink, including cure, adhesion and low yellowing, without recourse to TPO and BAPO. Omnirad 184, KIP 160, Omnirad 2959 all provide good cure when using mercury lamp UV sources.

[0102] Preferably, TPO-L in combination with KIP160 and / or BAPO are the only photoinitiators present in the ink.

[0103] In a preferred embodiment, the one or more additional photoinitiators are present in a total amount of 1 to 8% by weight, more preferably 2 to 6% by weight and most preferably 3 to 5% by weight, based on the total weight of the ink.

[0104] When one or more additional photoinitiators are present, the weight ratio of the one or more additional photoinitiators to TPO-L is preferably 1 :1-2, more preferably 1 :1-1 .75 and most preferably 1 :1-1 .5. In a particularly preferred embodiment, the weight ratio of the one or more additional photoinitiators to TPO- L is 1 :1 .25. The inventors have found that the inclusion of one or more additional photoinitiators in the specific blend of components of the inkjet ink of the invention in this ratio is preferred as it further improves the properties of the ink, including cure, adhesion and low yellowing, without recourse to TPO and BAPO.

[0105] In a preferred embodiment, the inkjet ink contains a total amount of 5.0 to 15.0% by weight, more preferably 6 to 12% by weight, most preferably 8 to 10% by weight of photoinitiator including ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, based on the total weight of the ink. It is surprising that such a small amount of photoinitiator is required to achieve the optimum balance of cure and adhesion.

[0106] The inkjet ink of the 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.

[0107] Accordingly, the inkjet ink preferably comprises less than 5% by weight of water and volatile organic solvents combined, based on the total weight of the ink. Preferably, the inkjet ink comprises less than 3% by weight of water and volatile organic solvent combined, more preferably less than 2 % by weight combined, more preferably less than 1 % by weight combined, and most preferably the inkjet ink is substantially free of water and volatile organic solvents, where the amounts are based on the total weight of the ink.

[0108] By substantially free is meant that only small amounts will be present, for example 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 other words, no water or a volatile organic solvent is intentionally added to the ink. However, minor amounts of water or a volatile organic solvent, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight of water or a volatile organic solvent, more preferably less than 0.1 % by weight of water or a volatile organic solvent, most preferably less than 0.05% by weight ofwater or a volatile organic solvent, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of water or a volatile organic solvent.

[0109] In a preferred embodiment, the inkjet ink of the 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.

[0110] 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 18-40 mNnr1, more preferably 20-35 mNnr1and most preferably 20-30 mNnr1.

[0111] In a preferred embodiment, the inkjet ink of the invention comprises a whitening agent. The whitening agent intensifies the degree of whiteness of the ink. Whitening agents are well-known in the art and a detailed description is not required. An example of a suitable whitening agent is SpeedBright OB184, which is a 2,5-bis(tert-butyl-2-benzoxazol-2-yl) thiophene and acts as a fluorescent whitening agent.

[0112] 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, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

[0113] The amounts by weight provided herein are based on the total weight of the ink.

[0114] In a preferred embodiment, the inkjet ink of the invention consists of at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; a white pigment; and 4.0 to 7.0% by weight of ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, based on the total weight of the ink; and optionally one or more mono- and / or difunctional (meth) acrylate monomers, a passive resin, one or more photoinitiators other than ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, a surfactant, a whitening agent, a defoamer, a dispersant, a synergist, a stabiliser against deterioration by heat or light, a reodorant, a flow or slip aid, a biocide, an identifying tracer, and mixtures thereof.

[0115] In a preferred embodiment, the inkjet ink of the invention consists of at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; a white pigment; and 4.0 to 7.0% by weight of ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, based on the total weight of the ink; and optionally one or more mono- and / or difunctional (meth) acrylate monomers, a passive resin, one or more photoinitiators other than ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, a surfactant, a whitening agent, a stabiliser against deterioration by heat or light, and mixtures thereof.

[0116] The preferred features of these inks are as described above.

[0117] The inkjet ink of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

[0118] The inkjet ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less, more preferably 30 mPas or less and most preferably 20 mPas or less at 25°C. The inkjet ink most preferably has a viscosity of 8 to 20 mPas at 25°C. Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00. The present invention also provides a method of inkjet printing comprising inkjet printing the inkjet ink as defined herein onto a substrate and curing the inkjet ink by exposing the inkjet ink to a curing source.

[0119] In the method of inkjet printing of the present invention, the inkjet ink is inkjet 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.

[0120] The ink is jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image.

[0121] 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.

[0122] Substrates include flexible and rigid substrates. Examples include substrates composed of self- adhesive vinyl (SAV), polyvinyl chloride (PVC), polystyrene (PS), high impact polystyrene (HIPS) polyester, polyethylene terephthalate (PET), polyethylene terephthalate glycol modified (PETG), polyolefin (e.g. polyethylene, polypropylene or mixtures or copolymers thereof), polyester textile banner and soft signage (3P textile), polycarbonate (PC) and acrylic. Further substrates include all cellulosic materials such as paper and board, or their mixtures / blends with the aforementioned synthetic materials.

[0123] 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.

[0124] The present invention may also provide a substrate having the inkjet ink as defined herein printed thereon.

[0125] In order to produce a high-quality printed image a small jetted drop size is desirable. Preferably, the inkjet ink is jetted at drop sizes below 90 picolitres, preferably below 35 picolitres, more preferably below 10 picolitres and most preferably below 5 picolitres.

[0126] 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 25°C is preferred, for example, 5 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.

[0127] 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.

[0128] 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 201 1 / 021052.

[0129] In a preferred embodiment, the ink is cured by exposing the printed ink to a source of actinic radiation.

[0130] 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 are well known in the art and a detailed description is not required. These include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof.

[0131] 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.

[0132] The most common UV light source used to cure inkjet inks is a mercury discharge lamp. These lamps operate by creating a plasma between two electrodes in a high pressure mercury gas contained in a quartz envelope. Although these lamps have some drawbacks in terms of their operational characteristics, no other UV light source has yet managed to challenge their position in terms of UV output performance.

[0133] LEDs are increasingly used to cure inkjet inks. UV light is emitted from a UV LED light source. UV LED light sources comprise one or more LEDs and are well-known in the art. Thus, a detailed description is not required.

[0134] It will be understood that UV LED light sources emit radiation having a spread of wavelengths. The emission of UV LED light sources is identified by the wavelength which corresponds to the peak in the wavelength distribution. Compared to conventional mercury lamp UV sources, UV LED light sources emit UV radiation over a narrow range of wavelengths on the wavelength distribution. The width of the range of wavelengths on the wavelength distribution is called a wavelength band. LEDs therefore have a narrow wavelength output when compared to other sources of UV radiation. By a narrow wavelength band, it is meant that at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength within a wavelength band having a width of 50 nm or less, preferably, 30 nm or less, most preferably 15 nm or less.

[0135] In a preferred embodiment, at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

[0136] Preferably, the wavelength of the UV LED source substantially matches the absorption profile of the ink. In a preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 360 nm to 410 nm. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm.

[0137] In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 360 nm to 410 nm, and at least 90%, preferably at least 95%, of the radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 395 nm, 400 nm or 405 nm, and at least 90%, preferably at least 95%, of the radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

[0138] LEDs have a longer lifetime and exhibit no change in the power / wavelength output over time. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate.

[0139] 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 24 pm, preferably 1 to 12 pm, for example 2 to 6 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

[0140] The invention will now be described with reference to the following examples, which are not intended to be limiting.

[0141] Examples

[0142] Example 1

[0143] White inkjet inks were prepared according to the formulations set out in Table 1. The white inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink. Table 1

[0144] PEA and IBOA are monofunctional (meth)acrylate monomers. DDDA is a difunctional (meth)acrylate monomer. NVC is an N-vinyl amide monomer. UV12 is a stabiliser. BR113 is an acrylic resin solution in PEA (79% PEA, 20% Dianal BR113, 1% UV12, based on the total weight of the acrylic resin solution). SpeedBright OB184 is a 2,5-bis(tert-butyl-2-benzoxazol-2-yl) thiophene and acts as a fluorescent whitening agent. KIP160, TPO, TPO-L and ITX are photoinitiators.

[0145] The white pigment dispersion contains 50% pigment, 3.53% polymeric dispersing aid, 1% UV12 and 45.47% PEA. Amounts are given as weight percentages based on the total weight of the dispersion.

[0146] Ink 1 is a comparative ink because it contains 8.50% by weight of TPO instead of 4.0 to 7.0% by weight of TPO-L, where the amounts are based on the total weight of the ink.

[0147] Ink 2 is a comparative ink because it contains TPO-L in more than 7.0% by weight, specifically in 10.00% by weight, where the amounts are based on the total weight of the ink.

[0148] Ink 3 is an ink of the invention that contains 4.0 to 7.0% by weight of TPO-L, based on the total weight of the ink.

[0149] The inks of Table 1 were then assessed for cure and adhesion. The tests and results are set out below. Cure

[0150] In order to assess for cure, the inkjet inks of Table 1 were printed onto a self-adhesive vinyl substrate and cured using a HgUV lamp at a dose of 211 mJ / cm2and an intensity of 2735 mW / cm2. The number of HgUV lamp passes until no offset onto photopaper was recorded and the dose to cure was calculated. The results are shown in Table 2.

[0151] Table 2

[0152] As can be seen, Ink 3 of the invention has improved cure compared to comparative Inks 1 and 2. It is surprising that comparative Ink 2 has worse cure than Ink 3 of the invention when comparative Ink 2 contains a higher amount of TPO-L than Ink 3 of the invention.

[0153] Adhesion

[0154] In order to assess adhesion, the inkjet inks of Table 1 were each printed onto gloss PVC, matt HIPS, PETG and SAV substrates, and cured using a HgUV lamp at a dose of 1000 mJ / cm2and an intensity of 2735 mW / cm2. The cured ink films were assessed for adhesion by ISO2409 crosshatch adhesion, where a score of 1 is poor and a score of 5 is excellent. The results are shown in Table 3.

[0155] Table 3

[0156] As can be seen, Ink 3 of the invention has improved adhesion compared to comparative Inks 1 and 2. It is surprising that comparative Ink 2 has worse adhesion results than Ink 3 of the invention when comparative Ink 2 contains a higher amount of TPO-L than Ink 3 of the invention.

[0157] Example 2

[0158] White inkjet inks were prepared according to the formulations set out in Table 4. The white inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink. Table 4

[0159] NVC is an N-vinyl amide monomer. CTFA and IBOA are monofunctional (meth)acrylate monomers. UV12 is a stabiliser. UVP6505 is a polyester acrylate oligomer. Omnirad 184, TPO-L and TPO are photoinitiators. SpeedBright OB184 is a 2,5-bis(tert-butyl-2-benzoxazol-2-yl) thiophene and acts as a fluorescent whitening agent.

[0160] The white pigment dispersion contains 50% pigment, 3.53% polymeric dispersing aid, 1% UV12 and 45.47% PEA. Amounts are given as weight percentages based on the total weight of the dispersion.

[0161] Ink 4 is a comparative ink because it contains 8.50% by weight of TPO instead of 4.0 to 7.0% by weight of TPO-L, where the amounts are based on the total weight of the ink.

[0162] Ink 5 is a comparative ink because it contains TPO-L in less than 4.0% by weight, specifically in 2.50% by weight, where the amounts are based on the total weight of the ink.

[0163] Ink 6 is a comparative ink because it contains TPO-L in more than 7.0% by weight, specifically in 7.50% by weight, where the amounts are based on the total weight of the ink.

[0164] Ink 7 is a comparative ink because it contains TPO-L in more than 7.0% by weight, specifically in 10.00% by weight, where the amounts are based on the total weight of the ink. Ink 8 is an ink of the invention that contains 4.0 to 7.0% by weight of TPO-L, based on the total weight of the ink.

[0165] The inks of Table 4 were then assessed for cure and adhesion. The tests and results are set out below.

[0166] Cure

[0167] In order to assess for cure, the inkjet inks of Table 4 were printed onto a self-adhesive vinyl substrate and cured using a HgUV lamp at a dose of 211 mJ / cm2and an intensity of 2735 mW / cm2. The number of HgUV lamp passes until no offset onto photopaper was recorded and the dose to cure was calculated. The results are shown in Table 5.

[0168] Table 5

[0169] As can be seen, comparative Ink 5 and Ink 8 of the invention have improved / equivalent cure compared to comparative Inks 4, 6 and 7. It is surprising that comparative Inks 6 and 7 have worse cure than Ink 8 of the invention when comparative Inks 6 and 7 contain a higher amount of TPO-L than Ink 8 of the invention. It is also surprising that Ink 8 of the invention has worse cure than comparative Ink 5 when ink 8 of the invention contains a higher amount of TPO-L than comparative ink 5. However, as can be seen from the adhesion results below, too low an amount of TPO-L leads to worse adhesion.

[0170] Adhesion

[0171] In order to assess adhesion, the inkjet inks of Table 4 were each printed onto gloss PVC, matt HIPS, PETG and SAV substrates, and cured using a HgUV lamp at a dose of 1000 mJ / cm2and an intensity of 2735 mW / cm2. The cured ink films were assessed for adhesion by ISO2409 crosshatch adhesion, where a score of 1 is poor and a score of 5 is excellent. The results are shown in Table 6. Table 6

[0172] As can be seen, Ink 8 of the invention has improved / equivalent adhesion compared to comparative Inks 4 and 5. Although comparative ink 5 has better cure than ink 8 of the invention, comparative ink 5 has worse adhesion than ink 8 of the invention. Therefore, including TPO-L in an amount of 4.0 to 7.0% by weight, based on the total weight of the ink, provides the ink of the invention with the optimal balance of cure and adhesion, without recourse to TPO.

Claims

Claims1. A white inkjet ink comprising: at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer; a white pigment; and 4.0 to 7.0% by weight of ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, based on the total weight of the ink, wherein the ink is free of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide.

2. A white inkjet ink as claimed in claim 1 , wherein the at least one N-vinyl amide monomer, N- (meth)acryloyl amine monomer and / or N-vinyl carbamate monomer is present in a total amount of 5 to 35% by weight, based on the total weight of the ink.

3. A white inkjet ink as claimed in claim 1 or 2, wherein the at least one N-vinyl amide monomer, N-(meth)acryloyl amine monomer and / or N-vinyl carbamate monomer comprises an N-vinyl amide monomer and / or an N-(meth)acryloyl amine monomer.

4. A white inkjet ink as claimed in any preceding claim, wherein the N-vinyl amide monomer, when present, comprises N-vinyl caprolactam and the N-(meth)acryloyl amine monomer, when present, comprises N-acryloylmorpholine.

5. A white inkjet ink as claimed in any preceding claim, wherein the ink further comprises one or more mono-and / or difunctional (meth) acrylate monomers.

6. A white inkjet ink as claimed in claim 5, wherein the one or more mono- and / or difunctional (meth)acrylate monomers are present in a total amount of 15 to 70% by weight, based on the total weight of the ink.

7. A white inkjet ink as claimed in claim 5 or 6, wherein the one or more mono- and / or difunctional (meth)acrylate monomers comprise one or more monofunctional (meth)acrylate monomers.

8. A white inkjet ink as claimed in claim 7, wherein the one or more monofunctional (meth)acrylate monomers comprise one or more cyclic monofunctional (meth)acrylate monomers.

9. A white inkjet ink as claimed in claim 8, wherein the one or more cyclic monofunctional (meth)acrylate monomers are selected from isobornyl acrylate, phenoxyethyl acrylate, cyclic TMP formal acrylate and mixtures thereof.

10. A white inkjet ink as claimed in any preceding claim, wherein the ink comprises 5 to 30% by weight of the white pigment, based on the total weight of the ink.

11. A white inkjet ink as claimed in any preceding claim, wherein the ink further comprises one or more photoinitiators other than ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate.

12. A white inkjet ink as claimed in claim 11 , wherein the one or more photoinitiators other than ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate are selected from 2-hydroxy-1-[4-[4-(2-hydroxy-2- methylpropanoyl)phenoxy]phenyl]-2-methylpropan-1-one, 1 -hydroxycyclohexyl phenyl ketone, 1-[4-(2- hydroxyethoxyl)-phenyl]-2-hydroxy-2-methylpropanone and mixtures thereof .

13. A white inkjet ink as claimed in claim 11 or 12, wherein the ink contains a total amount of 5.0 to 15.0% by weight of photoinitiator including ethyl phenyl (2,4,6-trimethylbenzoyl) phosphinate, based on the total weight of the ink.

14. A method of inkjet printing comprising inkjet printing the ink as claimed in any preceding claim onto a substrate and curing the inkjet ink by exposing the inkjet ink to a curing source.

15. A substrate having the inkjet ink as claimed in claims 1 to 13 printed thereon.

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