WO2018146494A1

WO2018146494A1 - Printing method and ink

Info

Publication number: WO2018146494A1
Application number: PCT/GB2018/050385
Authority: WO
Inventors: Lee CORFE
Original assignee: Fujifilm Speciality Ink Systems Limited
Priority date: 2017-02-13
Filing date: 2018-02-13
Publication date: 2018-08-16
Also published as: GB201911602D0; GB2573927B; GB2573927A

Abstract

The present invention provides a method of inkjet printing comprising the steps of: (i) providing an inkjet ink comprising: more than 5% by weight of an N-vinyl amidemonomer,an N-acryloyl amine monomerand/or an N-vinyl carbamate monomer, based on the total weight of the ink; 20 to 55% by weight of isobornyl acrylate, based on the total weight of the ink; a difunctional (meth)acrylate monomer selected from 1,4-butanediol diacrylate, 3-methyl-1,5-pentanediol diacrylate, 1,6- hexanediol diacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate, 1,11-undecanediol dimethacrylate, 1,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate; a radical photoinitiator; optionally a colorant; and optionallya passive resin; (ii) inkjet printing the inkjet ink onto a polypropylene substrate; and (iii) curing the inkjet ink. The present invention also provides an inkjet ink comprising: more than 5% by weight of an N-vinyl amidemonomer,an N-acryloyl amine monomerand/or an N-vinyl carbamate monomer, based on the total weight of the ink; more than 35% by weight of isobornyl acrylate, based on the total weight of the ink; a difunctional (meth)acrylate monomer selected from 1,4- butanediol diacrylate, 3-methyl-1,5-pentanediol diacrylate, 1,6-hexanediol diacrylate, 1,8- octanediol dimethacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate, 1,11- undecanediol dimethacrylate, 1,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate; a radical photoinitiator; optionally a colorant; and optionally a passive resin.

Description

Printing method and ink

The present invention relates to a method of printing, and in particular to a method of inkjet printing on to polypropylene. The invention further provides a printing ink, and in particular an inkjet ink that provides a good adhesion to polypropylene

In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. The resulting image should be as high quality as possible.

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 7-1 1 mPas at the jetting temperature which is often elevated to, but not limited to 40-50°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent such as water or a low-boiling solvent or mixture of solvents.

Another type of inkjet ink contains unsaturated organic compounds, termed monomers and/or oligomers which polymerise by irradiation, commonly with ultraviolet light, 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 exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures.

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

There is therefore a need in the art for a method of inkjet printing on to polypropylene substrates, where good adhesion is achieved between the ink image and the polypropylene substrate. There is also a need in the art for an inkjet ink that has good adhesion onto polypropylene substrates. Accordingly, the present invention provides a method of inkjet printing comprising the steps of:

(i) providing an inkjet ink comprising: more than 5% by weight of an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink; 20 to 55% by weight of isobornyl acrylate, based on the total weight of the ink; a difunctional (meth)acrylate monomer selected from 1 ,4-butanediol diacrylate, 3-methyl-1 ,5- pentanediol diacrylate, 1 ,6-hexanediol diacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9-nonanediol diacrylate, 1 ,10-decanediol diacrylate, 1 ,1 1 -undecanediol dimethacrylate, 1 ,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate; a radical photoinitiator; optionally a colorant; and optionally a passive resin;

(ii) inkjet printing the inkjet ink onto a polypropylene substrate; and

(iii) curing the inkjet ink.

The present invention further provides an inkjet ink comprising: more than 5% by weight of an N- vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink; more than 35% by weight of isobornyl acrylate, based on the total weight of the ink; a difunctional (meth)acrylate monomer selected from 1 ,4-butanediol diacrylate, 3-methyl-1 ,5-pentanediol diacrylate, 1 ,6-hexanediol diacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9-nonanediol diacrylate, 1 ,10-decanediol diacrylate, 1 ,1 1 -undecanediol dimethacrylate, 1 ,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate; a radical photoinitiator; optionally a colorant; and optionally a passive resin.

The inventors have surprisingly found that the method of inkjet printing the inkjet ink, and the inkjet ink of the present invention, where the inkjet ink comprises the specific blend of monomers, in the specific amount, onto a polypropylene substrate provides the necessary adhesion to the polypropylene substrate.

The inkjet ink comprises more than 5% by weight of an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink. The most preferred monomers in this class are an N-vinyl amide monomer or an N-vinyl carbamate monomer.

N-Vinyl amide monomers are well-known monomers in the art. N-Vinyl amide monomers have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP). Similarly, N-acryloyl amine monomers are also well-known in the art. N-Acryloyl amine monomers also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO). N-Vinyl carbamate monomers are defined by the following functionality:

The synthesis of N-vinyl carbamate monomers is known in the art. For example, vinyl isocyanate, formed by the Curtius rearrangement of acryloyi 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 R to R⁴ are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically 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 CMS alkyl, C_3-18 cycloalkyl, C₆. ₀ aryl and combinations thereof, such as C₆. ₀ aryl- or C_{3-1 8} cycloalkyl-substituted CMS 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. Preferably R to R⁴ are independently selected from hydrogen or C_{1 -10} alkyl. Further details may be found in WO 2015/022228 and US 4,831 ,153.

Most preferably, the N-vinyl carbamate monomer is N-vinyl-5-methyl-2-oxazolidinone (NVMO). It is available from BASF and has the following structure:

molecular weight 127 g/mol

NVMO has the lUPAC 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.

The most preferred N-vinyl amide monomer, N-acryloyl amine monomer and/or N-vinyl carbamate monomers are N-vinyl caprolactam (NVC) or NVMO. NVC is a well-known monomer in the art having the following chemical structure:

N-vinyl caprolactam (NVC)

mol wt 139 g/mol

In one embodiment, the N-vinyl amide monomer, N-acryloyl amine monomer and/or N-vinyl carbamate monomer present in the ink is NVC. In another embodiment, the N-vinyl amide monomer, N-acryloyl amine monomer and/or N-vinyl carbamate monomer present in the ink is NVMO.

In a preferred embodiment, the inkjet ink comprises more than 10% by weight of an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink. In a preferred embodiment, the inkjet ink comprises 6-35% by weight, more preferably 8-25% by weight and most preferably 10-22% by weight, of an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises more than 5% by weight of NVC or NVMO, based on the total weight of the ink. Preferably, the inkjet ink comprises more than 10% by weight of NVC or NVMO, based on the total weight of the ink. In a preferred embodiment, the inkjet ink comprises 6-35% by weight, more preferably 8-25% by weight and most preferably 10- 22% by weight, of NVC or NVMO, based on the total weight of the ink. The inkjet ink comprises 20 to 55% by weight of isobornyl acrylate, based on the total weight of the ink. Isobornyl acrylate (IBOA) is a well-known monomer in the art. It is a monofunctional (meth)acrylate monomer having the following chemical structure:

Isobornyl acrylate (IBOA)

mol wt 208 g/mol

In a preferred embodiment, the inkjet ink comprises more than 35% by weight, preferably 36-55% by weight, more preferably 36-50% by weight, of isobornyl acrylate, based on the total weight of the ink.

The inventors have surprisingly found that the specific blend of monomers, in the specific amount, allows for the inkjet ink to be printed onto a polypropylene substrate, whilst achieving good adhesion to these substrates. It is surprising that such a high amount of IBOA in combination with an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer in the claimed amount allows for such adhesion to polypropylene substrates.

The inkjet ink may further comprise further monofunctional (meth)acrylate monomer, other than IBOA. The further monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth)acrylate monomer and/or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer. In a preferred embodiment, the further monofunctional (meth)acrylate monomer, other than IBOA, is a cyclic monofunctional (meth)acrylate monomer.

Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required.

Monomers typically have a molecular weight of less than 600 Daltons, preferably more than 200 Daltons and less than 450 Daltons. 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 l OOmPas 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 s

The substituents of the further monofunctional (meth)acrylate monomer, other than IBOA, are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc.

The substituents of the further cyclic monofunctional (meth)acrylate monomer, other than IBOA, 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 C_3-18 cycloalkyl, C₆. ₀ aryl and combinations thereof, any of which may substituted with alkyl (such as C_{M S} 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.

Preferably, the further cyclic monofunctional (meth)acrylate monomer, other than IBOA, is selected from phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA) and mixtures thereof. PEA is particularly preferred. The preferred examples of cyclic monofunctional (meth)acrylate monomers have the following chemical structures:

Phenoxyethyl acrylate (PEA), mol wt 192 g/mol

Cyclic TMP formal acrylate (CTFA), mol wt 200 g/mol

Tetrahydrofurfuryl acrylate (THFA)

mol wt 156 g/mol

(2-Methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA)

mol wt 208.4 g/mol Mixtures of (meth)acrylate monomers may be used.

Preferably, the inkjet ink comprises 2-20% by weight, more preferably 5-15% by weight, of further cyclic monofunctional (meth)acrylate monomer, other than IBOA, based on the total weight of the ink.

Preferably, the inkjet ink comprises 20-65% by weight, more preferably 36-60% by weight, of cyclic monofunctional (meth)acrylate monomers in total, including IBOA, based on the total weight of the ink. In a preferred embodiment, the inkjet ink comprises at least two cyclic monofunctional (meth)acrylate monomers, one of which is IBOA.

In a preferred embodiment, the inkjet ink comprises a second cyclic monofunctional (meth)acrylate monomer selected from cyclic TMP formal acrylate (CTFA), phenoxyethyl acrylate (PEA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate (MEDA) and mixtures thereof.

The most preferred is PEA. In a particularly preferred embodiment, the only cyclic monofunctional (meth)acrylate monomers present in the ink are PEA and IBOA. In a further preferred embodiment, the ink contains no other monofunctional (meth)acrylate monomers, i.e. it is substantially free of acyclic-hydrocarbon monofunctional (meth)acrylate monomers. In a particularly preferred embodiment, the only monofunctional (meth)acrylate monomers present in the ink are PEA and IBOA. 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 CMS alkyl, which may be interrupted by 1 -10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted. Preferably, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C₆-C₂₀ group. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer is selected from octa/decyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

The preferred examples of acyclic-hydrocarbon monofunctional (meth)acrylate monomers have the following chemical structures:

Octadecyl acrylate (ODA) Tridecyl acrylate (TDA)

mol wt 200 g/mol mol 254 g/mol

Isodecyl acrylate (IDA) Lauryl acrylate

mol wt 212 g/mol mol wt 240 g/mol

2-(2-Ethoxyethoxy)ethyl acrylate, mol wt 188 g/mol

Mixtures of (meth)acrylates may be used.

Preferably, the inkjet ink comprises 1 -20% by weight, preferably 1 -10% by weight of an acycli hydrocarbon monofunctional (meth)acrylate monomer, based on the total weight of the ink.

The inkjet ink further comprises a difunctional (meth)acrylate monomer. Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Difunctional has its standard meaning, i.e. two groups, which take part in the polymerisation reaction on curing.

The substituents of the difunctional monomers are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyi, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CMS alkyl, C_3-18 cycloalkyi, C₆. ₀ aryl and combinations thereof, such as C₆. ₀ aryl- or C_3-18 cycloalkyl- substituted CMS alkyl, any of which may be interrupted by 1 -10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above-described substituents. The substituents may together also form a cyclic structure.

Examples of difunctional monomers include hexanediol diacrylate, 1 ,8-octanediol diacrylate, 1 ,9- nonanediol diacrylate, 1 ,10-decanediol diacrylate, 1 ,1 1 -undecanediol diacrylate and 1 ,12- dodecanediol diacrylate, polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tripropylene glycol diacrylate (TPGDA), neopentylglycol diacrylate, 3-methyl pentanediol diacrylate, tricyclodecane dimethanol diacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, and mixtures thereof.

In addition, suitable difunctional methacrylate monomers include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9- nonanediol dimethacrylate, 1 ,10-decanediol dimethacrylate, tricyclodecane dimethanol diacrylate, 1 ,1 1 -undecanediol dimethacrylate and 1 ,12-dodecanediol dimethacrylate. triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate and mixtures thereof.

The difunctional (meth)acrylate monomer of the present invention is selected from 1 ,4-butanediol diacrylate, 3-methyl-1 ,5-pentanediol diacrylate, 1 ,6-hexanediol diacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9-nonanediol diacrylate, 1 ,10-decanediol diacrylate, 1 ,1 1 -undecanediol dimethacrylate, 1 ,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate. These difunctional monomers are particularly advantageous with respect to adhesion to polypropylene.

Preferably, the difunctional (meth)acrylate monomer is selected from TCDDMDA, DDDA and DPGDA. These monomers are particularly advantageous with respect to adhesion onto polypropylene.

TCDDMDA is the most preferred difunctional monomer. TCDDMDA is particularly advantageous with respect to blocking resistance.

In a preferred embodiment, the inkjet ink comprises 1 -10%, preferably 1 -9% by weight, more preferably 2-5% by weight of difunctional (meth)acrylate monomer, based on the total weight of the ink.

Mixtures of (meth)acrylate monomers may also be used. It has surprisingly been found that difunctional (meth)acrylate monomer can be included in the inkjet ink of the present invention, and as such can achieve advantages associated with the inclusion of difunctional (meth)acrylate monomer, such as improved gloss and blocking, without having a detrimental effect on the other printing and jetting properties of the ink, whilst maintaining a high-quality image with good adhesion onto polypropylene substrates.

The inkjet ink may further comprise multifunctional (meth)acrylate monomer. Multifunctional (meth)acrylate monomer (which do not include difunctional) are well known in the art and a detailed description is therefore not required. Multifunctional has its standard meaning, i.e. tri or higher, that is three or more groups, respectively, which take part in the polymerisation reaction on curing. Preferably, the multifunctional (meth)acrylate monomer has a degree of functionality of four or more, more preferably a degree of functionality of from 4-8.

The substituents of the multifunctional monomers are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyi, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CMS alkyl, C_3-18 cycloalkyi, C₆. ₀ aryl and combinations thereof, such as C₆. ₀ aryl- or C_3-18 cycloalkyl- substituted CMS alkyl, any of which may be interrupted by 1 -1 0 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.

Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet inks include trimethylolpropane triacrylate, pentaerythritol 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 mixtures thereof. Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as trimethylolpropane trimethacrylate. Mixtures of (meth)acrylates may also be used.

Multifunctional (meth)acrylate monomer may be present in an amount of 1 -1 0% by weight, preferably 2-8% by weight, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink comprises low levels of multifunctional monomers (including multifunctional (meth)acrylate monomers), i.e. no more than 5% by weight, more preferably no more than 2% , based on the total weight of the ink. The ink is preferably substantially free of multifunctional monomers (including multifunctional (meth)acrylate monomers). Preferably, the ink comprises less than 2% by weight, more preferably less than 1 % by weight and most preferably less than 0.5% by weight, of multifunctional monomers, based on the total weight of the ink.

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

In a preferred embodiment, the inkjet ink of the present invention further comprises a passive resin.

Passive (or "inert") resins are resins which do not enter into the curing process, i.e. the resin is free of functional groups which polymerise under the curing conditions to which the ink is exposed. In other words, resin is not a radiation-curable material. 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. The resin has a weight-average molecular weight of 70-200 KDa and preferably 100-150 KDa, as determined by GPC with polystyrene standards.

The total amount of the passive resin is preferably from 0.1 -15.0 wt%, more preferably 0.2-10.0 wt%, based on the total weight of the ink. It has surprisingly been found that including a passive resin in the inkjet ink helps to improve the adhesion of the inkjet ink onto a polypropylene substrate. The inclusion of the passive resin in the inkjet ink, in combination with the specific blend of monomers in the specific amounts provides a Crosshatch adhesion, which allows for improved adhesion onto polypropylene substrates.

The ink of the present invention comprises a radical photoinitiator. The free-radical photoinitiator can be selected from any of those known in the art. For example, benzophenone, 1 - hydroxycyclohexyl phenyl ketone, 1 -[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1 -propane- 1 -one, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1 -one, isopropyl thioxanthone, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure and Darocur (from Ciba) and Lucerin (from BASF). Preferred photoinitiators are selected from bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide, 1 -hydroxycyclohexyl phenyl ketone and mixtures thereof. Preferably, the photoinitiator is present in an amount of 1 -20% by weight, preferably 2-15% by weight, based on the total weight of the ink.

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 six, and more preferably, two or more free radical photoinitiators are present in the ink.

The surface tension of the ink is preferably in the range of 20-40 mNm^" and more preferably 21 - 32 mNm^"1.

The inkjet inks may also contain a radiation-curable (i.e. polymerisable) 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. The oligomer preferably has a molecular weight of at least 450 Daltons and preferably at least 600 Daltons (whereas monomers typically have a molecular weight below these values). The molecular weight is preferably 4,000 Daltons 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. Thus, for polymeric materials, number average molecular weights can be obtained using gel permeation chromatography and polystyrene standards.

The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is preferably multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 6.

Preferred oligomers for inclusion in the ink have a viscosity of 0.5 to 10 Pas at 50°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 s ^~1.

Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The oligomer preferably comprises a polyester backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers, e.g. polyester aery late oligomers. Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance. In one embodiment the radiation-curable oligomer polymerises by free-radical polymerisation. Preferably, the radiation-curable oligomer cures upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film.

The total amount of the oligomer and/or passive resin is preferably from 1 -15 wt%, more preferably 2-10 wt%, based on the total weight of the ink.

The inkjet ink 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. The absence of water and volatile organic solvents means that the ink does not need to be dried to remove the water/solvent. However, water and volatile organic solvents have a significant viscosity-lowering effect making formulation of the ink in the absence of such components significantly more challenging. Accordingly, the inkjet ink is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink comprises less than 5% by weight combined of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2 % by weight combined and most preferably less than 1 % by weight combined, based on the total weight of the ink. Some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated.

The inkjet ink may also comprise a colouring agent. The colouring agent may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersible pigment, of the types known in the art and commercially available such as under the trade-names Paliotol (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.

In one aspect the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7.

Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 μηι, preferably less than 5 μηι, more preferably less than 1 μηι and particularly preferably less than 0.5 μηι.

The colorant is preferably present in an amount of 0.2-20% by weight, preferably 0.5-10% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink

The amounts by weight provided herein are based on the total weight of the ink. The inkjet ink exhibits a desirable low viscosity (200 mPas or less, preferably 100 mPas or less, more preferably 50 mPas or less at 25°C).

In order to produce a high quality printed image a small jetted drop size is desirable, particularly for high resolution images. Preferably the inkjet ink of the invention is jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 20 picolitres.

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.

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

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

The present invention also provides an inkjet ink, wherein at least one of the inks of the set is an inkjet ink comprising: more than 5% by weight of an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink; more than 35% by weight of isobornyl acrylate, based on the total weight of the ink; a difunctional (meth)acrylate monomer selected from 1 ,4-butanediol diacrylate, 3-methyl-1 ,5-pentanediol diacrylate, 1 ,6-hexanediol diacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9-nonanediol diacrylate, 1 ,10-decanediol diacrylate, 1 ,1 1 -undecanediol dimethacrylate, 1 ,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate; a radical photoinitiator; optionally a colorant; and optionally a passive resin.. Preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention.

Usually, the inkjet ink set of the present invention is in the form of a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. The inks in a trichromatic set can be used to produce a wide range of colours and tones.

The ink or inkjet ink sets may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The present invention also provides a cartridge containing the inkjet ink or inkjet ink set as defined herein. It also provides a printed substrate having the ink or inkjet ink set as defined herein printed thereon.

The inks of the present invention may advantageously be printed onto low surface energy substrates, by which is meant substrates having a surface energy of 25-50 mN/m (25-50 dyne/cm). Examples of substrates include those composed of polycarbonate, polyethylene terephthalate (PET), PMMA, PVC, polystyrene, polyethylene and polypropylene. In a preferred embodiment, the substrate is preferably polypropylene.

The present invention also provides a method of inkjet printing comprising: (i) providing an inkjet ink comprising: more than 5% by weight of an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink; 20 to 55% by weight of isobornyl acrylate, based on the total weight of the ink; a difunctional (meth)acrylate monomer selected from 1 ,4-butanediol diacrylate, 3-methyl-1 ,5-pentanediol diacrylate, 1 ,6- hexanediol diacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9-nonanediol diacrylate, 1 ,10-decanediol diacrylate, 1 ,1 1 -undecanediol dimethacrylate, 1 ,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate; a radical photoinitiator; optionally colorant; and optionally a passive resin; (ii) inkjet printing the inkjet ink onto a polypropylene substrate; and (iii) curing the inkjet ink.

Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto the substrate, on a roll-to-roll printer or a flat-bed printer. The inks or inkjet ink set are exposed to actinic (often UV) radiation to cure the ink. The exposure to actinic radiation may be performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink. Any of the sources of actinic radiation discussed herein may be used for the irradiation of the inkjet ink. A suitable dose would be greater than 200 mJ/cm², more preferably at least 300 mJ/cm² and most preferably at least 500 mJ/cm². The upper limit is less relevant and will be limited only by the commercial factor that more powerful radiation sources increase cost. A typical upper limit would be 5 J/cm². Further details of the printing and curing process are provided in WO 2012/1 10815.

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

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

Example 1

Inkjet inks were prepared according to the formulations set out in Tables 1 and 2. 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 1. Inks 1-4

UV12 is a stabiliser. BR1 13 is a passive resin. CN964A85 is a urethane acrylate. ITX, BP, IRG184, TPO and IRG379 are photoinitiators. EDB is an amine synergist. BT40279 is a stabiliser. BYK307 is a surfactant. The cyan, magenta, yellow and black pigment dispersions of the inks of Tables 1 and 2 were prepared according to the following formulations. The cyan pigment dispersion comprises 59% PEA, 1 % stabiliser, 10% dispersant and 30% blue pigment. The magenta pigment dispersion comprises 56.5% PEA, 1 .5% stabiliser, 12% dispersant and 30% magenta pigment. The yellow pigment dispersion comprises 58% PEA, 1 % stabiliser, 7% dispersant and 34% yellow pigment. The black pigment dispersion comprises 46.5% PEA, 1 .5% stabiliser, 12% dispersant and 40% black pigment. The dispersions were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the dispersion.

The inks of Tables 1 and 2 were printed and cured onto various substrates as listed in Tables 3 and 4 using an Acuity 550 inkjet printer. Inks 1 -8 were tested for adhesion. Cross hatch tape adhesion was tested according to ISO2409. A score of 5 in cross hatch tape adhesion shows excellent adhesion, 3 indicates borderline adhesion and 1 is poor adhesion. The results can be seen in Tables 3 and 4. Table 3.

As can be seen from the results, the inclusion of a passive resin improves adhesion. Example 2

Reference inkjet inks 9-72 were prepared according to the following formulations. 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 5. Reference inks 9-12 containing 10% IBOA and 0% DDDA

Reference inks 13-16 containing 20% IBOA and 0% DDDA were prepared according to the formulations set out in Table 5 except that inks 13-16 contained 10% more IBOA and 10% less PEA than inks 9-12, respectively.

Reference inks 17-20 containing 30% IBOA and 0% DDDA were prepared according to the formulations set out in Table 5 except that inks 17-20 contained 20% more IBOA and 20% less PEA than inks 9-12, respectively.

Reference inks 21 -24 containing 40% IBOA and 0% DDDA were prepared according to the formulations set out in Table 5 except that inks 21 -24 contained 30% more IBOA and 30% less PEA than inks 9-12, respectively. Table 6. Inks 25-28 containing 10% IBOA and 5% DDDA

Inks 29-32 containing 20% IBOA and 5% DDDA were prepared according to the formulations set out in Table 6 except that inks 29-32 contained 10% more IBOA and 10% less PEA than inks 25- 28, respectively.

Inks 33-36 containing 30% IBOA and 5% DDDA were prepared according to the formulations set out in Table 6 except that inks 33-36 contained 20% more IBOA and 20% less PEA than inks 25- 28, respectively.

Table 7. Inks 37-40 containing 40% IBOA and 5% DDDA

Inks 45-48 containing 20% IBOA and 10% DDDA were prepared according to the formulations set out in Table 8 except that inks 45-48 contained 10% more IBOA and 10% less PEA than inks 41 -44, respectively. Inks 49-52 containing 30% IBOA and 10% DDDA were prepared according to the formulations set out in Table 8 except that inks 49-52 contained 20% more IBOA and 20% less PEA than inks 41 -44, respectively.

Table 9. Inks 53-56 containing 40% IBOA and 10% DDDA

Table 10. Inks 57-60 containing 10% IBOA and 15% DDDA

Inks 61 -64 containing 20% IBOA and 15% DDDA were prepared according to the formulations set out in Table 10 except that inks 61 -64 contained 10% more IBOA and 10% less PEA than inks 57-60, respectively.

Table 1 1 . Inks 65-69 containing 30% IBOA and 15% DDDA

The cyan, magenta, yellow and black pigment dispersions of inks 9-72 were the same as those used in Example 1 .

Reference inks 9-24 and inks 25-72 were printed and cured onto polypropylene substrates using an Acuity HD3545 inkjet printer in production print mode (4 pass). Inks 9-72 which comprise varying IBOA content and varying DDDA content were tested for adhesion. Cross hatch tape adhesion was tested according to ISO 2409. Once again, a score of 5 in cross hatch tape adhesion shows excellent adhesion, 3 indicates borderline adhesion and 1 is poor adhesion. The results can be seen in Table 13. Each result represents an average result for the inks containing a specific amount of IBOA and a specific amount of DDDA.

Table 13.

As can be seen from the results, at least 20% by weight of IBOA, based on the total weight of the ink, is required to provide the necessary adhesion. Preferably, more than 30% by weight, and preferably more than 35 to 40% by weight of IBOA, based on the total weight of the ink, is required for adhesion to polypropylene. A difunctional (meth)acrylate monomer is present (to help with blocking and gloss) and a high level of IBOA allows for the inclusion of such difunctional (meth)acrylate monomer. The difunctional (meth)acrylate monomer is preferably present in 1 to 10% by weight, based on the total weight of the ink.

Example 3

Inkjet inks 73-76 were prepared according to the formulations set out in Table 14. 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.

The inks of Table 14 each contain the passive resin BR1 13, approximately 40% by weight of IBOA, and 5% by weight of the difunctional monomer TCDDMDA, based on the total weight of the ink. Inks 73-76 contain different pigment dispersions (cyan, magenta, yellow and black, respectively). The cyan, magenta, yellow and black pigment dispersions were the same as those used in Example 1 . Table 14. CYMK inks of the invention.

The inks of Table 14 were printed and cured using an Inca Onset X3 inkjet printer onto various polypropylene substrates in 3-pass Satin mode (130% relative ink density). Again, cross hatch tape adhesion was tested according to ISO2409. A score of 5 in cross hatch tape adhesion shows excellent adhesion, 3 indicates borderline adhesion and 1 is poor adhesion. The results can be seen in Table 15.

Table 15.

As can be seen, the ink set of the invention exhibits excellent adhesion to various polypropylene substrates.

Example 4

Inkjet inks 77-79 were prepared according to the formulations set out in Table 16. 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.

The inks of Table 16 each contain the passive resin BR1 13, 40% by weight of IBOA, 5% by weight of the difunctional monomer DPGDA and 20% by weight of an N-vinyl amide monomer, N- acryloyl amine monomer and/or an N-vinyl carbamate monomer selected from NVC, ACMO and NVMO, based on the total weight of the ink. The cyan pigment dispersion was the same as the cyan pigment dispersion of Example 1 .

Table 16. Inks 77-79 of the invention

The inks of Table 16 were printed and cured using an Inca Onset X3 inkjet printer onto various polypropylene substrates in 3-pass Satin mode (130% relative ink density). Again, cross hatch tape adhesion was tested according to ISO2409. A score of 5 in cross hatch tape adhesion shows excellent adhesion, 3 indicates borderline adhesion and 1 is poor adhesion. The results can be seen in Table 17.

Table 17.

As can be seen, the inks exhibit excellent adhesion to polypropylene substrates. Example 5

Inkjet inks 80 and 81 were prepared according to the formulations set out in Table 18. 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. The inks of Table 18 each contain the passive resin BR1 13, 40% by weight of IBOA, and 5% by weight of a difunctional monomer selected from DPGDA and neopentyl glycol propoxylated diacrylate (NPGPODA), based on the total weight of the ink. The cyan pigment dispersion was the same as the cyan pigment dispersion of Example 1 .

Table 18.

The inks of Table 18 were printed and cured using an Inca Onset X3 inkjet printer onto a polypropylene substrate in 3-pass Satin mode (130% relative ink density). Again, cross hatch tape adhesion was tested according to ISO2409. A score of 5 in cross hatch tape adhesion shows excellent adhesion, 3 indicates borderline adhesion and 1 is poor adhesion. The results can be seen in Table 19.

Table 19.

As can be seen, the inclusion of DPGDA leads to superior adhesion onto polypropylene compared to NPGPODA.

Claims

1 . A method of inkjet printing comprising the steps of:

(ii) inkjet printing the inkjet ink onto a polypropylene substrate; and

(iii) curing the inkjet ink.

2. A method of inkjet printing as claimed in claim 1 , wherein the inkjet ink comprises more than 35% by weight of isobornyl acrylate, based on the total weight of the ink.

3. An inkjet ink comprising: more than 5% by weight of an N-vinyl amide monomer, an N- acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink; more than 35% by weight of isobornyl acrylate, based on the total weight of the ink; a difunctional (meth)acrylate monomer selected from 1 ,4-butanediol diacrylate, 3-methyl-1 ,5- pentanediol diacrylate, 1 ,6-hexanediol diacrylate, 1 ,8-octanediol dimethacrylate, 1 ,9-nonanediol diacrylate, 1 ,10-decanediol diacrylate, 1 ,1 1 -undecanediol dimethacrylate, 1 ,12-dodecanediol dimethacrylate, tricyclodecane dimethanol diacrylate, tripropylene glycol diacrylate, dipropylene glycol diacrylate and tetraethylene glycol diacrylate; a radical photoinitiator; optionally a colorant; and optionally a passive resin.

4. A method of inkjet printing as claimed in claims 1 or 2 or an inkjet ink as claimed in claim 3, wherein the N-vinyl amide monomer, the N-acryloyl amine monomer and/or the N-vinyl carbamate monomer is NVC or NVMO.

5. A method of inkjet printing as claimed in any of claims 1 , 2 or 4 or an inkjet ink as claimed in claims 3 or 4, wherein the ink contains 1 to 10% by weight of the difunctional (meth)acrylate monomer.

6. A method of inkjet printing as claimed in any of claims 1 , 2, 4 or 5 or an inkjet ink as claimed in any of claims 3-5, wherein the difunctional (meth)acrylate monomer is selected from 1 ,10-decanediol diacrylate (DDDA), tricyclodecane dimethanol diacrylate (TCDDMDA) and dipropylene glycol diacrylate (DPGDA).

7. A method of inkjet printing as claimed in any of claims 1 , 2 or 4-6 or an inkjet ink as claimed in any of claims 3-6, wherein the difunctional (meth)acrylate monomer is tricyclodecane dimethanol diacrylate (TCDDMDA).

8. A method of inkjet printing as claimed in any of claims 1 , 2 or 4-7 or an inkjet ink as claimed in any of claims 3-7, wherein the colorant is a dispersed pigment.

9. A method of inkjet printing as claimed in any of claims 1 , 2 or 4-8 or an inkjet ink as claimed in any of claims 3-8, wherein the ink is substantially free of water and volatile organic solvents.

10. An inkjet ink set wherein at least one of the inks in the set, preferably all of the inks in the set, is an inkjet ink as claimed in any of claims 3-9.

1 1 . A cartridge containing the inkjet ink or the inkjet ink set as claimed in any of claims 3-10.

12. A printed substrate having the ink or the inkjet ink set as claimed in any of claims 3-10 printed thereon.

13. A printed substrate according to claim 12, wherein the substrate is polypropylene.