Inkjet ink

Abstract

The present invention provides an inkjet ink comprising: 10% by weight or more of isobornyl acrylate; and 5% by weight or less of difunctional (meth)acrylate monomer; wherein the total amount of all monofunctional monomer in the inkjet ink is 65% by weight or more, and wherein the inkjet ink has a viscosity of 15 mPas or less at 25°C, where the amounts by weight are based on the total weight of the ink. 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] INKJET INK

[0002] The present invention relates to a printing ink and in particular to a low viscosity inkjet ink which has improved adhesion.

[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] The inkjet ink often 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.

[0005] 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, in order to achieve the required jetting performance, when ejected through the nozzles, the ink should have a viscosity of less than 25 mPas, preferably 5-15 mPas and most preferably between 7-13 mPas at the jetting temperature.

[0006] In order to achieve such a low viscosity in use, the jetting temperature may be elevated to, but not limited to 40-50°C, for example, using heated print heads. Inks often have a much higher viscosity at ambient temperature.

[0007] There is however increased demand for non-heated print heads, which means that inks which are jettable at ambient temperature are required. Inks which have a low, jettable viscosity at 25°C are known in the art. However, these inks often suffer from limited adhesion to substrates, particularly substrates such as acrylic, polystyrene and polycarbonate substrates, which are known to be difficult to establish improved adhesion thereto. This is because the ink components that are required to achieve low viscosity at ambient temperature do not provide the required adhesion.

[0008] In order to improve adhesion to more difficult substrates, including acrylic, polystyrene and polycarbonate substrates, components such as multifunctional monomers and resins are often included in the inkjet ink. However, as well as improved adhesion, these components also result in increased viscosity at 25°C. This means that they suffer from poor jetting properties at 25°C, for example when using non-heated print heads.

[0009] There is therefore a need in the art for a low viscosity inkjet ink at 25°C, which has improved adhesion.

[0010] Accordingly, the present invention provides an inkjet ink comprising: 10% by weight or more of isobornyl acrylate; and 5% by weight or less of difunctional (meth)acrylate monomer; wherein the total amount of all monofunctional monomer in the inkjet ink is 65% by weight or more, and wherein the inkjet ink has a viscosity of 15 mPas or less at 25°C, where the amounts by weight are based on the total weight of the ink.

[0011] The inventors have surprisingly found that the inkjet ink of the present invention having the specific blend of components has a low viscosity at 25°C, as well as improved adhesion. The ink of the invention is thus suitable for jetting at ambient temperature, without recourse to heated print heads, and for printing onto and having improved adhesion to more difficult substrates, including acrylic, polystyrene and polycarbonate substrates.

[0012] The inkjet ink of the present invention comprises 10% by weight or more of isobornyl acrylate (IBOA), based on the total weight of the ink.

[0013] Isobornyl acrylate is a monofunctional (meth)acrylate monomer having the following structure:

[0014] The inventors have surprisingly found that the inclusion of 10% by weight or more of IBOA in the inkjet ink of the present invention, based on the total weight of the ink, allows for the provision of a low viscosity inkjet ink at 25°C, having a viscosity of 15 mPas or less at 25°C, as well as improved adhesion.

[0015] In a preferred embodiment, the inkjet ink comprises 10 to 30% by weight, preferably 12 to 20% by weight, of isobornyl acrylate, based on the total weight of the ink.

[0016] The inkjet ink of the present invention comprises 5% by weight or less of difunctional (meth)acrylate monomer, based on the total weight of the ink. In other words, the total amount of all difunctional (meth)acrylate monomers in the inkjet ink is 5% by weight or less, based on the total weight of the ink.

[0017] 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. Examples of functional groups that are capable of polymerising upon exposure to radiation include a (meth)acrylate group and a vinyl ether group. 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 / 2° steel cone at 25°C with a shear rate of 25 s1.

[0018] A difunctional (meth)acrylate monomer has two functional groups, specifically (meth)acrylate groups, which take part in the polymerisation reaction on curing.

[0019] Forthe avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and / or methacrylate.

[0020] Difunctional (meth)acrylate monomers are well known in the art. 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- MPDDA), 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.

[0021] Difunctional (meth)acrylate monomers are often included in inkjet inks in order to increase adhesion to the substrate. The inventors have surprisingly found that it is possible to achieve improved adhesion despite restricting the amount of difunctional (meth)acrylate monomer to 5% by weight or less, based on the total weight of the ink.

[0022] Difunctional (meth)acrylate monomers are typically added to inkjet inks to increase adhesion, and can also result in an increase in the viscosity of the inkjet ink. The inkjet ink of the present invention has a low viscosity of 15 mPas or less at 25°C and as such, the amount and type of difunctional (meth)acrylate monomers, when present, are limited to the extent that the viscosity requirements of the inkjet ink are met.

[0023] The inkjet ink of the present invention may comprise a surfactant. In a preferred embodiment, if present, the inkjet ink comprises O.O1 to 5 % by weight of surfactant, based on the total weight of the ink.

[0024] The surfactant controls the surface tension of the ink. Surfactants are well-known in the art and a detailed description is not required. Examples of surfactants include non-crosslinking surfactants and crosslinking surfactants. An example of a suitable non-crosslinking surfactant is BYK307. An example of a suitable crosslinking surfactant is Tegorad 2010. 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 mNm1, more preferably 20-38 rnNm1.

[0025] The total amount of all monofunctional monomer in the inkjet ink is 65% by weight or more, preferably 67% by weight or more, more preferably 70% by weight or more, based on the total weight of the ink. For the avoidance of doubt, the total amount of all monofunctional monomer in the inkjet ink includes all monofunctional monomers, including IBOA.

[0026] Monofunctional monomers are well known in the art. A radiation-curable monofunctional monomer has one functional group, which takes part in the polymerisation reaction on curing. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation and is preferably selected from a (meth)acrylate group and a vinyl ether group.

[0027] The substituents of the monofunctional monomer are not limited other than 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 ofwhich may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include Ci- alkyl, C3-18 cycloalkyl, CB-W aryl and combinations thereof, such as CMO aryl- or Cs-is cycloalkyl-substituted C1-18 alkyl, any ofwhich 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.

[0028] In a preferred embodiment, the inkjet ink comprises one or more additional monofunctional (meth)acrylate monomers, which are well known in the art and are preferably the esters of acrylic acid. Mixtures of (meth)acrylates may also be used. By additional monofunctional (meth)acrylate monomer it is meant a monofunctional (meth)acrylate monomer, other than IBOA.

[0029] The monofunctional (meth)acrylate monomer may be an additional cyclic monofunctional (meth)acrylate monomer and / or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer. In one embodiment, the monofunctional (meth) acrylate monomer comprises an additional cyclic monofunctional (meth)acrylate monomer. By additional cyclic monofunctional (meth)acrylate monomer, it is meant a cyclic monofunctional (meth) acrylate monomer, other than IBOA.

[0030] 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. Nonlimiting examples of substituents commonly used in the art include C3-18 cycloalkyl, Cs-w 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. The substituents may together also form a cyclic structure.

[0031] The additional cyclic monofunctional (meth)acrylate monomer may be selected from phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetra hydrofurfuryl 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), isopropylidene glycerol acrylate (IPGA) and mixtures thereof. Medol-10, IPGA or a mixture thereof is particularly preferred. The inventors have found that the inclusion of Medol-10, IPGA or a mixture thereof in the inkjet ink of the present invention further improves adhesion.

[0032] In a preferred embodiment, the inkjet ink comprises a monofunctional monomer selected from PEA, NVC, Medol-10, CTFA, IPGA and mixtures thereof. The inkjet ink preferably comprises PEA, NVC, Medol-10, CTFA and IPGA.

[0033] In a more preferred embodiment, the inkjet ink comprises a monofunctional monomer selected from PEA, NVC, Medol-10, CTFA and mixtures thereof. The inkjet ink preferably comprises PEA, NVC, Medol-10 and CTFA.

[0034] Preferably, the inkjet ink comprises 35 to 70% by weight, preferably 38 to 65% by weight in total of PEA, NVC, Medol-10, CTFA and IPGA. More preferably, the inkjet ink comprises 35 to 70% by weight, preferably 38 to 65% by weight in total of PEA, NVC, Medol-10 and CTFA.

[0035] In a preferred embodiment, the inkjet ink comprises Medol-10, IPGA or a mixture thereof, preferably present in a total amount of 5 to 20% by weight, more preferably present in a total amount of 10 to 18% by weight, based on the total weight of the ink.

[0036] In one preferred embodiment, the inkjet ink comprises Medol-10, preferably present in 5 to 20% by weight, more preferably present in 10 to 18% by weight, based on the total weight of the ink. In another preferred embodiment, the inkjet ink comprises IPGA, preferably present in 5 to 20% by weight, more preferably present in 10 to 18% by weight, based on the total weight of the ink. In a further preferred embodiment, the inkjet ink comprises Medol-10 and IPGA, preferably present in a total amount of 5 to 20% by weight, more preferably present in a total amount of 10 to 18% by weight, based on the total weight of the ink.

[0037] In one embodiment, the monofunctional (meth)acrylate monomer comprises an acyclic-hydrocarbon monofunctional (meth)acrylate monomer.

[0038] 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 nitrogen further substituted.

[0039] 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.

[0040] In a preferred embodiment, the inkjet ink comprises an N-vinyl amide monomer and / or an N- (meth)acryloyl amine monomer.

[0041] 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.

[0042] Similarly, 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. A preferred example is N-acryloylmorpholine (ACMO).

[0043] In a preferred embodiment, the inkjet ink comprises 10 to 30% by weight, more preferably 15 to 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. Most preferably, the ink comprises 10 to 30% by weight, preferably 15 to 25% by weight, of an N-vinyl amide monomer, based on the total weight of the ink.

[0044] 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.

[0045] Preferably, the inkjet ink comprises NVC, preferably present in 10 to 30% by weight, more preferably 15 to 25% by weight, based on the total weight of the ink. Monofunctional monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. The inkjet ink of the present invention has a low viscosity of 15 mPas or less at 25°C and as such, the amount and type of monofunctional monomers are limited to the extent that the viscosity requirements of the inkjet ink are met.

[0046] The inkjet ink of the present invention has a viscosity of 15 mPas or less at 25°C. In a preferred embodiment, the inkjet ink has a viscosity of 14 mPas or less at 25°C. In a preferred embodiment, the inkjet ink has a viscosity of 13 mPas or less at 25°C, preferably the inkjet ink has a viscosity of 7 to 13 mPas at 25°C.

[0047] The ink of the present invention has a viscosity of 15 mPas or less at 25°C. As such, the inkjet ink of the present invention has a low viscosity of 15 mPas or less at an ambient jetting temperature of 25°C.

[0048] Ink viscosity can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 60 mm diameter / 1 ° aluminium cone at 25°C with a shear rate of 25 s1. Accordingly, the ink viscosity can be measured using a rotational rheometer having a 60 mm diameter / 1 ° aluminium cone at 25°C with a shear rate of 25 s1.

[0049] The low viscosity at 25°C of the inkjet ink of the present invention means that the ink can be jetted without recourse to heated print heads at ambient temperature, and can still achieve the required jetting properties. The inventors have surprisingly found however that the low viscosity inkjet ink of the present invention also achieves improved adhesion.

[0050] The ink of the present invention can thus have the required jetting properties at 25°C, whilst achieving improved adhesion.

[0051] Resins are often included in inkjet inks in order to increase adhesion to the substrate. The inventors have surprisingly found that it is possible to achieve improved adhesion, whilst also restricting the amount of resin present in the ink.

[0052] Resins are often referred to as passive (or “inert”) 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. Example resins include epoxy, polyester, vinyl, ketone, nitrocellulose, phenoxy and acrylate resins. A resin has a weight-average molecular weight of 70-200 KDa and more often from 100-150 KDa, as determined by GPC with polystyrene standards.

[0053] Resins are typically added to inkjet inks to increase adhesion, also resulting in an increase in the viscosity of the inkjet ink. The inkjet ink of the present invention has a low viscosity of 15 mPas or less at 25°C and as such, the amount and type of resin, when present, are limited to the extent that the viscosity requirements of the inkjet ink are met.

[0054] In a preferred embodiment, the inkjet ink comprises less than 1% by weight, preferably less than 0.80% by weight, more preferably less than 0.5% by weight, and is more preferably free of resin, where the amounts by weight are based on the total weight of the ink. In other words, the total amount of resin in the inkjet ink is less than 1 % by weight, preferably less than 0.80% by weight, more preferably less than 0.5% by weight, and is more preferably free of resin, where the amounts by weight are based on the total weight of the ink.

[0055] Multifunctional monomers are often included in inkjet inks in order to increase adhesion to the substrate. The inventors have surprisingly found that it is possible to achieve improved adhesion, whilst also restricting the amount of multifunctional monomers present in the ink.

[0056] Multifunctional monomers are typically added to inkjet inks to increase adhesion, also resulting in an increase in the viscosity of the inkjet ink. The inkjet ink of the present invention has a low viscosity of 15 mPas or less at 25°C and as such, the amount and type of multifunctional monomers, when present, are limited to the extent that the viscosity requirements of the inkjet ink are met.

[0057] In a preferred embodiment, the inkjet ink comprises less than 1% by weight, preferably less than 0.5% by weight, and is more preferably free of multifunctional monomer, where the amounts by weight are based on the total weight of the ink. In other words, the total amount of all multifunctional monomer in the inkjet ink is less than 1% by weight, preferably less than 0.5% by weight, and is more preferably free of all multifunctional monomer, where the amounts by weight are based on the total weight of the ink.

[0058] Examples of the multifunctional monomer include multifunctional (meth)acrylate monomers, multifunctional vinyl ether monomers and multifunctional vinyl ether (meth)acrylate monomers.

[0059] Multifunctional (meth)acrylate monomers (which do not include difunctional (meth) acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-functional 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 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.

[0060] An example of a multifunctional vinyl ether monomer is tris[4-(vinyloxy)butyl] trimellitate. In a preferred embodiment, the inkjet ink comprises a combined amount of multifunctional monomer and resin of less than 2% by weight, preferably less than 1 % by weight, more preferably less than 0.5% by weight, more preferably free of multifunctional monomers and resin, where the amounts are based on the total weight of the ink. In other words, the total amount of all resin and multifunctional monomer in the inkjet ink is less than 1 % by weight, preferably less than 0.5% by weight, and is more preferably free of all resin and multifunctional monomer, where the amounts by weight are based on the total weight of the ink.

[0061] Radiation-curable oligomers are often included in inkjet inks in order to increase adhesion to the substrate. The inventors have surprisingly found that it is possible to achieve improved adhesion, whilst also restricting the amount of radiation-curable oligomer present in the ink.

[0062] 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 molecularweight 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.

[0063] 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.

[0064] 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 / 2° steel cone at 60°C with a shear rate of 25 s1.

[0065] Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups.

[0066] 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.

[0067] Radiation-curable oligomers are typically added to inkjet inks to increase adhesion, also resulting in an increase in the viscosity of the inkjet ink. The inkjet ink of the present invention has a low viscosity of 15 mPas or less at 25°C and as such, the amount and type of radiation-curable oligomer, when present, are limited to the extent that the viscosity requirements of the inkjet ink are met. In a preferred embodiment, the inkjet ink comprises less than 1% by weight, preferably less than 0.5% by weight, and is more preferably free of radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink. In other words, the total amount of all radiation-curable oligomer in the inkjet ink is less than 1 % by weight, preferably less than 0.5% by weight, and is more preferably free of all radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink.

[0068] In a preferred embodiment, the inkjet ink comprises less than 1% by weight, preferably less than 0.80% by weight, preferably less than 0.5% by weight, and is more preferably free of resin, and less than 1 % by weight, preferably less than 0.5% by weight, and is more preferably free of radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink. In a preferred embodiment, the inkjet ink comprises less than 0.80% by weight of resin and less than 1 % by weight of radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink. In a particularly preferred embodiment, the inkjet ink comprises less than 0.5% by weight of resin and less than 1 % by weight of radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink.

[0069] In a preferred embodiment, the inkjet ink comprises a combined amount of resin and radiation-curable oligomer of less than 2% by weight, preferably less than 1 % by weight, more preferably less than 0.5% by weight, more preferably free of resin and radiation-curable oligomer, where the amounts are based on the total weight of the ink. In other words, the total amount of all resin and radiation-curable oligomer in the inkjet ink is less than 1 % by weight, preferably less than 0.5% by weight, and is more preferably free of all resin and radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink.

[0070] In a preferred embodiment, the inkjet ink comprises a combined amount of multifunctional monomer and radiation-curable oligomer of less than 2% by weight, preferably less than 1% by weight, more preferably less than 0.5% by weight, more preferably free of multifunctional monomers and radiation- curable oligomer, where the amounts are based on the total weight of the ink. In other words, the total amount of all multifunctional monomer and radiation-curable oligomer in the inkjet ink is less than 1% by weight, preferably less than 0.5% by weight, and is more preferably free of all multifunctional monomer and radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink.

[0071] In a preferred embodiment, the inkjet ink comprises a combined amount of multifunctional monomer, resin and radiation-curable oligomer of less than 2% by weight, preferably less than 1% by weight, more preferably less than 0.5% by weight, more preferably free of multifunctional monomers, resin and radiation-curable oligomer, where the amounts are based on the total weight of the ink. In other words, the total amount of all resin, multifunctional monomer and radiation-curable oligomer in the inkjet ink is less than 1% by weight, preferably less than 0.5% by weight, and is more preferably free of all resin, multifunctional monomer and radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink.

[0072] In a preferred embodiment, the inkjet ink of the present invention comprises one or more divinyl ether monomers and / or divinyl ether (meth)acrylate monomers.

[0073] The inkjet ink of the present invention has a low viscosity of 15 mPas or less at 25°C and as such, the amount and type of one or more divinyl ether monomers and / or divinyl ether (meth)acrylate monomers, when present, are limited to the extent that the viscosity requirements of the inkjet ink are met.

[0074] Examples of divinyl ether monomers 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- phenylenejbiscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4- (vinyloxymethyl)cyclohexylmethyl] glutarate, 1 ,4-butanediol divinyl ether and mixtures thereof. DVE-3 is particularly preferred.

[0075] Examples of vinyl ether (meth)acrylate monomers include 2-(2-vinyloxy ethoxyjethyl acrylate (VEEA), 2-(2-vinyloxy ethoxyjethyl methacrylate (VEEM) and mixtures thereof.

[0076] When present, the inkjet ink of the present invention preferably comprises 1 to 12% by weight of one or more divinyl ether monomers and / or divinyl ether (meth)acrylate monomers, based on the total weight of the ink. More preferably, the inkjet ink comprises 1 to 12% by weight of DVE-3, based on the total weight of the ink.

[0077] In a preferred embodiment, the inkjet ink of the present invention also includes 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.

[0078] Preferably, the colouring agent is a pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (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. Often, pigments are commercially available as dispersions in monomer or solvent.

[0079] 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.

[0080] 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.

[0081] The colorant is preferably present in an amount of 0.2-20% by weight, preferably 0.5-15% 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.

[0082] 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.

[0083] 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. For example, the inkjet ink set of the present invention may additionally include orange, green and / or violet inks. A well-known inkjet ink set useful for the present invention is a so-called “CMYKOG” inkjet ink set, which includes a cyan ink, a magenta ink, a yellow ink, a black ink, an orange ink and a green ink.

[0084] Other suitable colouring agents include dyes. The dyes include but are not limited to azo dyes, anthraquinone dyes, xanthene dyes, azine dyes, and combinations thereof.

[0085] If the ink is cured by exposure to a source of actinic radiation without an inert environment, one or more photoinitiators will be required. If the ink is cured by exposure to a source of low-energy electron beam radiation or a source of actinic radiation in an inert environment, the ink may still contain a photoinitiator, although photoinitiators are not required.

[0086] In a preferred embodiment, the ink of the present invention further comprises one or more photoinitiators.

[0087] Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4- morpholinophenyl)butan-1-one, benzil dimethylketal, phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, ethyl(2,4,6-trimethylbenzoyl)-phenyl phosphinate or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Omnirad (from IGM) and Esacure (from Lamberti).

[0088] Mixtures of free radical photoinitiators can be used and preferably, the ink comprises a plurality of free radical photoinitiators. The total number of free radical photoinitiators present is preferably from one to five, and more preferably, two or more free radical photoinitiators are present in the ink.

[0089] Preferably, the photoinitiator if present, is present from 1 to 20% by weight, preferably from 5 to 15% by weight, based on the total weight of the ink.

[0090] The presence of a photoinitiator 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 a photoinitiator by curing with a low-energy electron beam or curing by actinic radiation in an inert environment.

[0091] Therefore, in a preferred embodiment, the photoinitiator is present in an amount of less than 20% by weight, preferably less than 5% by weight, more preferably less than 3%, more preferably less than 1 %, based on the total weight of the ink.

[0092] Therefore, in a preferred embodiment, 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 0.5% by weight of photoinitiator, more preferably less than 0.1% by weight of photoinitiator, most preferably less than 0.05% by weight of photoinitiator, based on the total weight of the ink. The inkjet ink may also be free of photoinitiator.

[0093] However, an inkjet ink that is cured with a low-energy electron beam or actinic radiation in an inert environment may still contain a small amount of photoinitiator, such as 1 to 5% by weight of a photoinitiator, based on the total weight of the ink. This is required if the ink is for example, first pinned with actinic radiation.

[0094] By pinning is meant arresting the flow of the ink by treating the ink droplets quickly after they have impacted onto the substrate surface. Pinning provides a partial cure of the ink and thereby maximises image quality by controlling bleed and feathering between image areas. Pinning does not achieve full cure of the ink. By curing is meant fully curing the ink. Pinning leads to a marked increase in viscosity, whereas curing converts the inkjet ink from a liquid ink to a solid film. The dose of radiation used for pinning is generally lower than the dose required to cure the radiation-curable material fully. The inkjet ink 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.

[0095] 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.

[0096] 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 of water 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.

[0097] 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, additional surfactants, defoamers, dispersants, synergists, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

[0098] The amounts by weight provided herein are based on the total weight of the ink.

[0099] The ink or inkjet ink sets 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.

[0100] 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. Preferably, the substrate is an acrylic, polystyrene or polycarbonate substrate.

[0101] 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. The ink is jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image.

[0102] 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. Non-heated print heads helps to reduce the running costs. The inventors have surprisingly found that it is possible to use non-heated print heads when using the low viscosity inkjet ink of the present invention, whilst maintaining jetting properties and improved adhesion.

[0103] Substrates include those for packaging applications and in particular, flexible packaging applications. Examples include substrates composed of acrylic, polyvinyl chloride (PVC), polystyrene, polyester, polyethylene terephthalate (PET), polyethylene terephthalate glycol modified (PETG) and polyolefin (e.g. polyethylene, polypropylene or mixtures or copolymers thereof). Further substrates include all cellulosic materials such as paper and board, or their mixtures / blends with the aforementioned synthetic materials.

[0104] It can be difficult to achieve adhesion on substrates such as acrylic, polystyrene and polycarbonate. Often components, such as multifunctional monomers and resins, are required to achieve the required adhesion, which can increase viscosity. The inventors have surprisingly found that the inkjet ink of the present invention, which has the desirable low viscosity, also has improved adhesion to substrates, including acrylic, polystyrene and polycarbonate.

[0105] 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.

[0106] The present invention may also provide a printed substrate having the ink as defined herein printed thereon. Preferably, the substrate is an acrylic, polystyrene or polycarbonate substrate.

[0107] 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 and most preferably below 10 picolitres.

[0108] 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.

[0109] 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.

[0110] In a preferred embodiment, the ink is cured by exposing the printed ink to a source of actinic radiation.

[0111] 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

[0112] Preferably, the source of actinic radiation is LEDs. 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.

[0113] There are many advantages of using LEDs as the UV source. In this regard, LEDs are cost effective, have long maintenance intervals, have high energy efficiency and are an environmentally friendly option. 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.

[0114] 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.

[0115] 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.

[0116] The ink may also be cured by exposing the printed ink to low-energy electron beam (ebeam).

[0117] 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 radiation sources include commercially available ebeam curing units, such as the EB Lab from ebeam Technologies with energy of 80-300 keV and capable of delivering a typical dose of 30-50 kGy at line speeds of up to 30 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 70 kGy or less.

[0118] 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.

[0119] 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 crosslinking.

[0120] There is no restriction on the ebeam dose that is used to cure the inkjet inks of the present invention other than that the dose is sufficient to fully cure the ink. Preferably, the dose is more than 10 kGy, more preferably more than 20 kGy, more preferably more than 30 kGy and most preferably more than 40 kGy. Preferably, the dose is less than 100 kGy, more preferably less than 90 kGy, more preferably less than 80 kGy and most preferably less than 70 kGy. Preferably, the dose is more than 30 kGy but less than 70 kGy, more preferably more than 30 kGy but less than 60 kGy and most preferably, more than 30 kGy but 50 kGy or less. Doses above 50 kGy may cause damage to the substrate, particularly the substrates used for food packaging applications, and so doses of 50 kGy or less are preferred.

[0121] The energy associated with these doses is 80-300 keV, more preferably 70-200 keV and most preferably 100 keV.

[0122] 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.

[0123] The invention will now be described with reference to the following examples, which are not intended to be limiting. Examples

[0124] Example 1 Inkjet inks were prepared according to the formulations set out in Table 1. The 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.

[0125] Table 1

[0126] PEA, IBOA, NVC, Medol-10 and CTFA are monofunctional monomers. DDDA and HDDA are difunctional (meth)acrylate monomers. DVE-3 is a divinyl ether monomer. UV12 is a stabiliser. BT40279 is a stabiliser. TPO, ITX and BAPO are photoinitiators. BYK307 is a surfactant. The cyan pigment dispersion contains 30% pigment, 20% polymeric dispersing aid and 50% divinyl ether monomer. The magenta pigment dispersion contains 30% pigment, 42% polymeric dispersing aid and 28% divinyl ether monomer. The yellow pigment dispersion 1 contains 30.0% pigment, 22.5% polymeric dispersing aid and 47.5% monofunctional monomer. The yellow pigment dispersion 2 contains 33.9% pigment, 6.8% polymeric dispersing aid, 1.1% stabiliser and 58.2% monofunctional monomer. The black pigment dispersion contains 40.0% pigment, 30.0% polymeric dispersing aid, 0.9% synergist and 29.1% divinyl ether monomer. Amounts are given as weight percentages based on the total weight of the dispersion.

[0127] Inks 1 to 4 are comparative inks and Inks 5 to 9 are inks of the invention. The viscosity of the inks was measured using a Brookfield DV I Viscometer at 25°C. Inks 1 to 9 each have a viscosity at 25°C of between 10 to 13 mPa.s.

[0128] The inks of Table 1 were assessed for adhesion.

[0129] In order to assess adhesion, the inkjet inks of Table 1 were drawn down in a 12 pm film using a 12 pm wire wound K-bar onto each of the substrates listed in Table 3. The 12 pm drawdowns were then passed under a Baldwin 395nM lamp to provide a cured ink film.

[0130] The adhesion was assessed by subjecting the prints to a cross hatch test 24 hours after curing, during which the film is cut using an Elcometer 107 cross hatch testing kit and a multi-tooth cutter. Specifically, the film was cut by scoring a vertical 20-30 mm line on the film and then scoring a horizontal 20-30 mm line across the vertical line. The surface of the film was brushed and a piece of tape (scotch 600) firmly applied over the cross hatch formed in the direction of the second cut. The tape was then removed, holding the tape at a 60° angle to the substrate and sharply pulling it. The degree of film removed with the tape was then assessed on a scale of 0 to 5 as shown in Table 2. The results are set out in Table 3.

[0131] Table 2 Table 3

[0132] As can be seen, Inks 5 to 9 have improved adhesion results compared to comparative Inks 1 to 4. The inventors have surprisingly found that the low viscosity inkjet ink of the invention, having 10% by weight or more of isobornyl acrylate, 5% by weight or less of difunctional (meth)acrylate monomer, and a surfactant, where the total amount of all monofunctional monomer in the inkjet ink is 65% by weight or more, has improved adhesion to a range of substrates.

[0133] Example 2

[0134] Inkjet inks were prepared according to the formulations set out in Table 4. The 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.

[0135] Table 4

[0136] PEA, IBOA, NVC and Medol-10 are monofunctional monomers. HDDA is a difunctional (meth)acrylate monomer. UV12 is a stabiliser. BT40279 is a stabiliser. TPO, ITX and BAPO are photoinitiators. BYK307 is a surfactant.

[0137] The pigment dispersions are the same pigment dispersions used in Example 1 .

[0138] Inks 10 to 13 are further examples of the invention. The viscosity of the inks was measured using a Brookfield DV I viscosmeter at 25°C. Inks 10 to 13 each have a viscosity at 25°C of between 10 to 13 mPa.s.

[0139] The inks of Table 4 were assessed for adhesion. In order to assess adhesion, the inkjet inks of Table 4 were drawn down in a 12 pm film using a 12 pm wire wound K-bar onto each of the substrates listed in Table 6. The 12 pm drawdowns were then passed under a Baldwin 395nM lamp to provide a cured ink film. The adhesion was assessed by subjecting the prints to a cross hatch test 24 hours after curing, during which the film is cut using an Elcometer 107 cross hatch testing kit and a multi-tooth cutter. Specifically, the film was cut by scoring a vertical 20-30 mm line on the film and then scoring a horizontal 20-30 mm line across the vertical line. The surface of the film was brushed and a piece of tape (scotch 600) firmly applied over the cross hatch formed in the direction of the second cut. The tape was then removed, holding the tape at a 60° angle to the substrate and sharply pulling it. The degree of film removed with the tape was then assessed on a scale of 0 to 5 as shown in Table 5. The results are set out in Table 6.

[0140] Table 5

[0141] Table 6

[0142] As can be seen, Inks 10 to 13 of the invention have excellent adhesion to a range substrates. The inclusion of Medol-10 as a monofunctional monomer present in the ink provides further improvement in the adhesion results.

[0143] Example 3

[0144] Inkjet inks were prepared according to the formulations set out in Table 7. The 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 7

[0145] PEA, IBOA, NVC and Medol-10 are monofunctional monomers. HDDA is a difunctional (meth)acrylate monomer. DVE-3 is a divinyl ether monomer. UV12 is a stabiliser. BT40279 is a stabiliser. TPO and ITX are photoinitiators. BYK307 is a surfactant.

[0146] The yellow pigment dispersion 2 is the same yellow pigment dispersion 2 used in Example 1 .

[0147] Inks 14 to 16 are further examples of the invention. The viscosity of the inks was measured using a Brookfield DV I Viscometer at 25°C. Inks 14 to 16 each have a viscosity at 25°C of between 10 to 13 mPa.s.

[0148] The inks of Table 7 were assessed for adhesion.

[0149] In order to assess adhesion, the inkjet inks of Table 7 were drawn down in a 12 pm film using a 12 pm wire wound K-bar onto each of the substrates listed in Table 9. The 12 pm drawdowns were then passed under a Baldwin 395nM lamp to provide a cured ink film.

[0150] The adhesion was assessed by subjecting the prints to a cross hatch test 24 hours after curing, during which the film is cut using an Elcometer 107 cross hatch testing kit and a multi-tooth cutter. Specifically, the film was cut by scoring a vertical 20-30 mm line on the film and then scoring a horizontal 20-30 mm line across the vertical line. The surface of the film was brushed and a piece of tape (scotch 600) firmly applied over the cross hatch formed in the direction of the second cut. The tape was then removed, holding the tape at a 60° angle to the substrate and sharply pulling it. The degree of film removed with the tape was then assessed on a scale of 0 to 5 as shown in Table 8. The results are set out in Table 9. Table 8

[0151] Table 9

[0152] As can be seen, Inks 14 to 16 of the invention have excellent adhesion to a range substrates.

[0153] Example 4

[0154] Inkjet inks were prepared according to the formulations set out in Table 10. The 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.

[0155] Table 10

[0156] PEA, NVC, IBOA, CTFA and Medol-10 are monofunctional monomers. HDDA and DDDA are difunctional (meth)acrylate monomers. DVE-3 is a divinyl ether monomer. UV12 and BT40279 are stabilisers. TPO, BAPO and ITX are photoinitiators.

[0157] The pigment dispersions are the same pigment dispersions used in Example 1 .

[0158] Inks 17 to 20 are further examples of the invention. The viscosity of the inks was measured using a Brookfield DV I viscosmeter at 25°C. Inks 17 to 20 each have a viscosity at 25°C of between 10 to 13 mPa.s.

[0159] The inks of Table 10 were assessed for adhesion. In order to assess adhesion, the inkjet inks of Table 10 were drawn down in a 12 m film using a 12 pm wire wound K-bar onto each of the substrates listed in Table 12. The 12 pm drawdowns were then passed under a Baldwin 395nM lamp to provide a cured ink film.

[0160] The adhesion was assessed by subjecting the prints to a cross hatch test 24 hours after curing, during which the film is cut using an Elcometer 107 cross hatch testing kit and a multi-tooth cutter. Specifically, the film was cut by scoring a vertical 20-30 mm line on the film and then scoring a horizontal 20-30 mm line across the vertical line. The surface of the film was brushed and a piece of tape (scotch 600) firmly applied over the cross hatch formed in the direction of the second cut. The tape was then removed, holding the tape at a 60° angle to the substrate and sharply pulling it. The degree of film removed with the tape was then assessed on a scale of 0 to 5 as shown in Table 11 . The results are set out in Table 12.

[0161] Table 11

[0162] Table 12

[0163] As can be seen, Inks 17 to 20 of the invention have excellent adhesion to a range substrates.

[0164] Example 5

[0165] Inkjet inks were prepared according to the formulations set out in Table 13. The 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 13

[0166] PEA, NVC, IBOA and IPGA are monofunctional monomers. HDDA is a difunctional (meth)acrylate monomer. UV12 and BT40279 are stabilisers. TPO, BAPO and ITX are photoinitiators. BYK307 is a surfactant.

[0167] The pigment dispersions are the same pigment dispersions used in Example 1 .

[0168] Inks 21 to 24 are further examples of the invention. The viscosity of the inks was measured using a Brookfield DV I viscosmeter at 25°C. Inks 21 to 24 each have a viscosity at 25°C of between 10 to 13 mPa.s.

[0169] The inks of Table 13 were assessed for adhesion. In order to assess adhesion, the inkjet inks of Table 13 were drawn down in a 12 pm film using a 12 pm wire wound K-bar onto each of the substrates listed in Table 15. The 12 pm drawdowns were then passed under a Baldwin 395nM lamp to provide a cured ink film. The adhesion was assessed by subjecting the prints to a cross hatch test 24 hours after curing, during which the film is cut using an Elcometer 107 cross hatch testing kit and a multi-tooth cutter. Specifically, the film was cut by scoring a vertical 20-30 mm line on the film and then scoring a horizontal 20-30 mm line across the vertical line. The surface of the film was brushed and a piece of tape (scotch 600) firmly applied over the cross hatch formed in the direction of the second cut. The tape was then removed, holding the tape at a 60° angle to the substrate and sharply pulling it. The degree of film removed with the tape was then assessed on a scale of O to 5 as shown in Table 14. The results are set out in Table 15.

[0170] Table 14

[0171] Table 15

[0172] As can be seen, Inks 21 to 24 of the invention have excellent adhesion to a range substrates. The inclusion of IPGA as a monofunctional monomer present in the ink provides further improvement in the adhesion results.

Claims

Claims1. An inkjet ink comprising: 10% by weight or more of isobornyl acrylate; and 5% by weight or less of difunctional (meth)acrylate monomer; wherein the total amount of all monofunctional monomer in the inkjet ink is 65% by weight or more, and wherein the inkjet ink has a viscosity of 15 mPas or less at 25°C, where the amounts by weight are based on the total weight of the ink.

2. An inkjet ink as claimed in claim 1 , wherein the inkjet ink comprises less than 1 % by weight, preferably less than 0.5% by weight, and is more preferably free of resin, where the amounts by weight are based on the total weight of the ink.

3. An inkjet ink as claimed in claims 1 or 2, wherein the inkjet ink comprises less than 1% by weight, preferably less than 0.5% by weight, and is more preferably free of multifunctional monomer, where the amounts by weight are based on the total weight of the ink.

4. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises less than 1% by weight, preferably less than 0.5% by weight, and is more preferably free of radiation-curable oligomer, where the amounts by weight are based on the total weight of the ink.

5. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises 10 to 30% by weight, preferably 12 to 20% by weight, of isobornyl acrylate, based on the total weight of the ink.

6. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises a surfactant, preferably present in 0.01 to 5 % by weight, based on the total weight of the ink.

7. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink has a viscosity of 13 mPas or less at 25°C, preferably wherein the inkjet ink has a viscosity of 7 to 13 mPas at 25°C.

8. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises an N-vinyl amide monomer and / or an N-(meth)acryloyl amine monomer.

9. An inkjet ink as claimed in claim 7, wherein the inkjet ink comprises N-vinyl caprolactam, preferably present in 10 to 30% by weight, based on the total weight of the ink.

10. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises a monofunctional monomer selected from phenoxyethyl acrylate, N-vinyl caprolactam, (2-methyl-2-ethyl- 1 ,3-dioxolane-4-yl)methyl acrylate, cyclic TMP formal acrylate, isopropylidene glycerol acrylate and mixtures thereof.

11. An inkjet ink as claimed in any preceding claim, wherein the inkjet ink comprises (2-methyl-2- ethyl-1 ,3-dioxolane-4-yl)methyl acrylate, isopropylidene glycerol acrylate or a mixture thereof, preferably present in a total amount of 5 to 20% by weight, more preferably present in a total amount of 10 to 18% by weight, based on the total weight of the ink.

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

13. A method of inkjet printing as claimed in claim 12, wherein the substrate is an acrylic, polystyrene or polycarbonate substrate.

14. A substrate having the inkjet ink as claimed in claims 1 to 11 printed thereon.

15. A substrate as claimed in claim 14, wherein the substrate is an acrylic, polystyrene or polycarbonate substrate.

Images