WO2018042194A1

WO2018042194A1 - A method of printing

Info

Publication number: WO2018042194A1
Application number: PCT/GB2017/052556
Authority: WO
Inventors: Brian Woolrich; Sean Slater
Original assignee: Fujifilm Speciality Ink Systems Limited
Priority date: 2016-09-02
Filing date: 2017-09-01
Publication date: 2018-03-08
Also published as: GB201904610D0; GB2569256B; GB2569256A

Abstract

The present invention provides a method of inkjet printing comprising the following steps, in order: (i) providing a substrate; (ii) jetting a primer onto the substrate, wherein the primer comprises two or more monofunctional monomers and one or more photoinitiators, wherein the two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer; (iii) jetting an inkjet ink onto the primer, wherein the inkjet ink comprises one or more multifunctional monomers, a colourant, one or more photoinitiators and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink; and (iv) curing the primer and the inkjet ink. The present invention also provides a printed substrate obtainable by the method of the present invention, a primer as used in the method of the present invention, and an ink set comprising a primer as used in the present invention and an inkjet ink as used in the method of the present invention.

Description

A method of printing

This invention relates to a method of printing and in particular to a method of printing utilising inks that have high gloss and excellent colour gamut, and that provide hard cured films with solvent-resistant properties.

UV-curable digital inkjet inks that have high gloss and excellent colour gamut, and that provide hard cured films with solvent-resistant properties are known in the art. These inks typically contain one or more multifunctional monomers and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink. A minimum of 15% multifunctional monomers and optionally multifunctional oligomers is required to provide sufficient crosslinking in the printed ink film upon curing and hence hard cured films with solvent-resistant properties. Further, multifunctional monomers and multifunctional oligomers have the properties required to provide a printed ink film with a high gloss and excellent colour density. All of these properties result in a printed ink film with a highly desirable combination of excellent appearance and physical properties.

However, the same multifunctional monomers and oligomers used in the inks lead to shrinkage in the printed ink film upon curing. Shrinkage leads to poor adhesion between the printed ink film and the underlying substrate, especially when printing onto difficult substrates such as acrylic and polyolefins. One consequence of this poor adhesion is that printed substrates cannot be successfully cut or routed into finished shapes without damage to the printed ink film. There therefore remains a need in the art for a method of printing utilising inks that have high gloss and excellent colour gamut, and that provide hard cured films with solvent-resistant properties in which good adhesion to the underlying substrate can be achieved.

Accordingly, the present invention provides a method of inkjet printing comprising the following steps, in order: (i) providing a substrate; (ii) jetting a primer onto the substrate, wherein the primer comprises two or more monofunctional monomers and one or more photoinitiators, wherein the two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer; (iii) jetting an inkjet ink onto the primer, wherein the inkjet ink comprises one or more multifunctional monomers, a colourant, one or more photoinitiators and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink; and (iv) curing the primer and the inkjet ink. The present invention further provides: a printed substrate obtainable by the method; a primer as defined in relation to the method; and an ink set comprising a primer as defined in relation to the method and an inkjet ink as defined in relation to the method. Thus, the present invention provides a method of printing utilising inks that have high gloss and excellent colour gamut, and that provide hard cured films with solvent-resistant properties in which good adhesion to the underlying substrate is achieved.

The method of printing of the present invention is a method of inkjet printing. 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. 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 below 100 mPas at 25°C (although in most applications the viscosity should be below 50 mPas, and often below 25 mPas). Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas and preferably 5-15 mPas at the jetting temperature, which is often elevated to about 35 to 50°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying and curing in the reservoirs or nozzles. The method of the present invention includes step (i), providing a substrate. A wide variety of substrates are suitable for use in the method, including substrates that have previously been difficult to print onto using the inkjet ink that is used in the method. Suitable substrates include polycarbonate (PC), fluted polypropylene (FPP), polypropylene (PP), self-adhesive vinyl (SAV), acrylic and high impact polystyrene (HIPS).

The method of the present invention includes step (ii), jetting a primer onto the substrate, wherein the primer comprises two or more monofunctional monomers and one or more photoinitiators, wherein the two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer.

The primer of the present invention, which is jetted in step (ii) of the method of the present invention, comprises two or more monofunctional monomers and one or more photoinitiators, wherein the two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer.

Such a primer may be prepared by known methods such as, for example, mixing with a high-speed stirrer.

The primer of the present invention, which is jetted in step (ii) of the method of the present invention, is a radiation-curable (often termed "UV") primer. By "radiation-curable" is meant a material that polymerises or crosslinks when exposed to actinic radiation, commonly ultraviolet light, in the presence of a photoinitiator. The primer does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. Thus, a radiation-curable primer is typically free of volatile organic solvent and water. Preferably, the primer comprises less than 5% by weight of volatile organic solvent and water 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 primer. Some water will typically be absorbed by the primer from the air and solvents may be present as impurities in the components of the primer, but such low levels are tolerated.

The primer of the present invention, which is jetted in step (ii) of the method of the present invention, is based on two or more monofunctional monomers. By "monofunctional" is meant that the monomers have only one functional group which takes part in a polymerisation reaction during curing. As the primer is based on monofunctional monomers, the primer exhibits a particularly low viscosity of 5-15 mPas at 25°C, preferably 5 or more mPas at 25°C but less than 10 mPas at 25°C. The primer is suitable for using with particularly low viscosity inkjet inks, and preferably the viscosity of the primer matches the viscosity of the inkjet ink ± 20%, more preferably the viscosity of the primer matches the viscosity of the inkjet ink ± 10%. Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model LDV1 +.

The two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer. In a preferred embodiment, the two or more monofunctional monomers are present in at least 90% by weight, based on the total weight of the primer. The two or more monofunctional monomers are preferably present in no more than 95% by weight, based on the total weight of the primer. A combination of monofunctional monomers facilitates good adhesion between the substrate and the overlying inkjet ink. This means that the inkjet ink can now be printed onto substrates that were previously difficult to gain adherence to, such as polycarbonate, acrylic and polypropylene, and the final printed substrates can now be print finished.

The two or more monofunctional monomers are selected from monofunctional (meth)acrylate monomers, N-vinyl amides, N-(meth)acryloyl amines, α,β-unsaturated ether monomers and combinations thereof. Preferably the two or more monofunctional monomers include monofunctional (meth)acrylate monomers.

Monofunctional (meth)acrylate monomers are esters of (meth)acrylic acid and are well known in the art. The substituents of the monofunctional monomers are not limited other than by the constraints imposed by the use in primer, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include CMS alkyl, C_3-18 cycloalkyl, C₆_ ₁₀ 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 include a monomer selected from phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), dicyclopentenyl oxyethyl acrylate, octadecyl acrylate, trimethyl cyclohexyl acrylate (TMCHA), tertiobutyl cyclohexyl acrylate (TBCHA) or combinations thereof. The primer of the invention can include a mixture of two or more monofunctional (meth)acrylate monomers. A mixture of two or more monofunctional (meth)acrylate monomers is particularly preferred.

In a particularly preferred embodiment, the primer includes a cyclic monofunctional (meth)acrylate. That is, the radical covalently bonded to the (meth)acrylate unit is cyclic. The cyclic radical may be saturated or unsaturated, including aromatic. Preferred cyclic monofunctional (meth)acrylates for the primer are phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA) or mixtures thereof. The structures are set out hereinbelow. The cyclic nature of these monomers is immediately apparent from the structures. Most preferably, the monofunctional (meth)acrylates present are exclusively cyclic, i.e. they are the sole monofunctional (meth)acrylate(s) present, such that the only monofunctional monomers present in the primer are cyclic. A particularly preferred combination is a saturated and an unsaturated, particular aromatic, monofunctional (meth)acrylate monomer.

Phenoxyethyl acrylate (PEA) Cyclic TMP formal acrylate (CTFA)

mol wt 192 g/mol mol wt 200 g/mol

Isobornyl acrylate (IBOA) Tetrahydrofurfuryl acrylate (THFA)

mol wt 208g/mol mol wt 156 g/mol The two or more monofunctional monomers may include two or more N-vinyl amides, N- (meth)acryloyl amines, α,β-unsaturated ether monomers and combinations thereof.

N-Vinyl amides are well-known monomers in the art and a detailed description is therefore not required. N-Vinyl amides have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to (meth)acrylate monomers. Preferred examples include N-vinyl amides having an aliphatic or aromatic cyclic group. The cyclic group may optionally include one or more heteroatoms such as oxygen or nitrogen. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP). NVC is particularly preferred.

Combinations of NVC with the (meth)acrylate monomers set out hereinabove are particularly preferred.

Similarly, N-acryloyl amines are also well-known in the art. N-Acryloyl amines 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 (meth)acrylate monomers. Preferred examples include N-acryloyl amines having a cyclic group. The cyclic group may optionally include one or more heteroatoms such as oxygen or nitrogen. A preferred example is N-acryloylmorpholine (ACMO). The two or more monofunctional monomers may also include α,β-unsaturated ether monomers, such as vinyl ethers. These monomers are known in the art and may be used to reduce the viscosity of the primer. A typical vinyl ether monomer which may be used in the primer of the present invention is ethylene glycol monovinyl ether. Mixtures of vinyl ether monomers may be used. Preferably, the two or more monofunctional monomers each have a glass transition temperature (Tg) of more than -15°C, more preferably more than 0°C. The Tg may be measured by DSC with a heating ramp of 10°C/min. If the one or monofunctional monomers each have a glass transition temperature (Tg) of more than -15°C or more preferably more than 0°C, the cured films are harder and hence more durable. In this regard, it has been found that the monofunctional monomers have a tendency to migrate into the overlying inkjet ink and the migration of any monofunctional monomers with a Tg of less than 0°C from the primer into the inkjet ink has an adverse effect on the hardness of the overlying printed inkjet ink film. For example, 2-(2-ethoxyethoxy)ethyl acrylate (EOEOEA) has a Tg of -70°C and as the amount of EOEOEA in the primer increases, the overlying printed inkjet ink film becomes softer.

In one embodiment of the invention, and for applications where product safety is an important consideration, the primer preferably comprises less than 20% by weight of THFA based on the total weight of the primer, more preferably less than 10% by weight of THFA based on the total weight of the primer and more preferably, the primer is substantially free from THFA. Preferably, the primer comprises 2% by weight or less of multifunctional monomers. The inclusion of multifunctional monomers in the primer causes shrinkage and thus has an adverse effect on the adhesion between the substrate and the overlying inkjet ink. However, 2% by weight or less of multifunctional monomers in the primer is tolerated and provides cohesion to the primer.

The multifunctional monomer which is limited in amount may be any multifunctional monomer which could be involved in the curing reaction, such as a multifunctional (meth)acrylate monomer or a multifunctional vinyl ether. Examples of the multifunctional acrylate monomers whose level is minimised include hexanediol diacrylate, polyethylene glycol diacrylate (for example tetraethyleneglycol diacrylate), dipropylene glycol diacrylate, neopentylglycol diacrylate, 3-methyl pentanediol diacrylate, the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, 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 combinations thereof. In addition, multifunctional acrylate monomers include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate, trimethylolpropane trimeth acrylate and combinations thereof.

Unless purified, CTFA may contain approximately 5% trimethylolpropane triacrylate (TMPTA), a multifunctional monomer, as a reaction by-product. As the amount of CTFA in the primer increases and hence the amount of multifunctional monomers in the primer increases, the adhesion between the substrate and the overlying inkjet ink decreases. Therefore, the amount of CTFA that is present in the primer needs to be controlled and preferably, the primer comprises 30% by weight or less of CTFA, based on the total weight of the primer.

(Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and multifunctional are also intended to have their standard meanings, i.e. one and two or more groups, respectively, which take part in the polymerisation reaction on curing.

Preferably, the primer does not contain a radiation-curable oligomer. However, between 1 % and 5% by weight of a radiation-curable oligomer, based on the total weight of the primer, can be tolerated.

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 weight-average molecular weight of at least 600 Da. The weight-average molecular weight is preferably below 3,000 Da. Molecular weights (weight-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. Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation.

The primer of the present invention, which is jetted in step (ii) of the method of the present invention, preferably avoids the inclusion of a resin, since resins increase the viscosity of the primer. Examples of resins to be excluded are epoxy resins, polyester resins, vinyl resins, ketone resins, aldehyde resins, nitrocellulose resins, phenoxy resins and polyolefin resins. Resins typically have a weight- average molecular weight of 3,000 Da or above, as determined by gel permeation chromatography (GPC) with polystyrene standards.

Some of the multifunctional monomer of the inkjet ink that is jetted in step (iii) of the method of the present invention can migrate into the primer and cause shrinkage of the primer. The combination of monomers of the primer act as a spacer to separate the radiation-curable material present in the primer so that any shrinkage is distributed across the whole primer layer. In this way, adhesion is maintained. Further, the careful selection of monofunctional monomers in the primer allows a small amount of multifunctional monomer in the primer to be tolerated. The primer of the present invention, which is jetted in step (ii) of the method of the present invention, contains one or more photoinitiators, which under irradiation, for example using ultraviolet light, initiate the polymerisation of the radiation-curable material. 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, 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 (from BASF), Darocur (from Ciba) and Lucirin (from BASF). The primer of the present invention is preferably cured by ultraviolet irradiation. In a preferred embodiment, the radiation-curable material polymerises by free-radical polymerisation.

Preferably, the photoinitiator is present from 3 to 20% by weight, preferably from 4 to 10% by weight, of the primer.

Other components of types known in the art may be present in the primer to improve the properties or performance. These components may be, for example, surfactants, defoamers, dispersants, sensitisers, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides, optical brighteners and identifying tracers. The surfactant assists with wetting of the substrate surface by the primer, but it can be detrimental to the bonding process of the overlying inkjet ink and so is preferably present at less than 2% by weight, more preferably less than 1 %, based on the total weight of the primer. Most preferably, surfactant is absent from the primer. The method of the present invention includes step (iii) jetting an inkjet ink onto the primer, wherein the inkjet ink comprises one or more multifunctional monomers, a colourant, one or more photoinitiators and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink.

The inkjet ink of the present invention, which is jetted in step (iii) of the method of the present invention, comprises one or more multifunctional monomers, a colorant, one or more photoinitiators and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink.

The inkjet ink can be prepared by methods well known in the art and so a detailed discussion is not required. The inkjet ink, like the primer, is radiation-curable as discussed above. Preferably, the inkjet ink comprises less than 5% by weight of volatile organic solvent and water 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 inkjet ink. The total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight, based on the total weight of the ink. Preferably, the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 40% by weight, more preferably at least 70% by weight, based on the total weight of the ink. Preferably, the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is no more than 85% by weight, based on the total weight of the ink.

The one or more multifunctional monomers and optionally one or more multifunctional oligomers provide sufficient crosslinking in the printed ink film upon curing and hence the hard and solvent- resistant properties of the ink. Further, the multifunctional monomers and multifunctional oligomers have the polarity values required to provide a printed ink film with a high gloss and excellent colour density. All of these properties result in a printed ink film with a highly desirable combination of excellent appearance and physical properties.

The multifunctional monomers may be any multifunctional monomer which could be involved in the curing reaction, such as a multifunctional (meth)acrylate monomer, a multifunctional vinyl ether or a multifunctional vinyl (meth)acrylate.

Multifunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Preferred examples include hexanediol diacrylate, polyethylene glycol diacrylate (for example tetraethyleneglycol diacrylate), dipropylene glycol diacrylate, neopentylglycol diacrylate, 3-methyl pentanediol diacrylate, the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, 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 combinations thereof.

In addition, suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate, trimethylolpropane trimeth acrylate and combinations thereof. Mixtures of (meth)acrylates may also be used.

Multifunctional vinyl ethers may also be present in the inkjet ink. Examples are well known in the art and include vinyl ethers such as triethylene glycol divinyl ether, diethylene glycol divinyl ether and 1 ,4- cyclohexanedimethanol divinyl ether. Mixtures of multifunctional vinyl ethers can be used alone or in combination with multifunctional (meth)acrylate monomers. Triethylene glycol divinyl ether is particularly preferred.

Multifunctional monomers containing both an acrylate and a vinyl ether reactive group may also be present in the inkjet ink. An example is 2-(2-vinyloxyethoxy) ethyl acrylate (VEEA).

Monomers typically have a molecular weight of less than 600, preferably more than 150 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 100 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 radiation-curable material of the ink used in the method of the present invention may comprise a radiation-curable (i.e. polymerisable) multifunctional oligomer, such as a multifunctional (meth)acrylate oligomer. Any radiation-curable multifunctional oligomer that is compatible with the other ink components is suitable for use in the ink. Thus, the ink formulator is able to select from a wide range of suitable oligomers.

Oligomers are defined hereinabove in relation to the primer. Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Preferred oligomers for inclusion in the ink of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25 s ^~1. The multifunctional oligomers may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality oligomers may be used. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is 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.

Radiation-curable multifunctional oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The backbone will be selected depending on the specific requirements of the printed ink, such as print flexibility, durability etc. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the multifunctional oligomers are (meth)acrylate 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 a preferred embodiment, the one or more multifunctional monomers and optionally one or more multifunctional oligomers comprise multifunctional (meth)acrylates. Multifunctional (meth)acrylates present in the inkjet ink have a strong affinity to the underlying primer, particularly a primer in which the one or more monofunctional monomers are (meth)acrylate monomers. Preferred multifunctional monomers for use in the inkjet ink of the present invention include dipropylene glycol diacrylate (DPGDA), neopentylglycol diacrylate (NPGPODA), triethylene glycol divinyl ether (DVE-3) and combinations thereof. In one embodiment, the one or more multifunctional monomers and optionally one or more multifunctional oligomers polymerise by free-radical polymerisation. The one or more multifunctional monomers and optionally one or more multifunctional oligomers cure upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film. Monofunctional monomers may also be present in the inkjet ink. Monofunctional monomers are defined hereinabove in relation to the primer. Monofunctional (meth)acrylate monomers such as PEA and IBOA may be present in 1 -50% by weight, based on the total weight of the ink. N-Vinyl amides such as NVC may be present in 1 -20% by weight, based on the total weight of the ink. Combinations of NVC with the (meth)acrylate monomers set out hereinabove are particularly preferred.

The inkjet ink of the present invention, which is jetted in step (iii) of the method of the present invention, comprises a colourant. 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 155, Pigment Yellow 120, Pigment Yellow 13, Pigment Yellow 83, Pigment Red 9, Pigment Red 122, Pigment Red 184, Pigment Orange 34, 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 colourant is preferably present in an amount of 10% by weight or less, preferably 5% by weight or less, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, however, for example up to and including 30% by weight, or 15% by weight based on the total weight of the ink.

The inks may be 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). Process colour inks can be used to produce a wide range of colours and tones.

The inkjet ink of the present invention, which is jetted in step (iii) of the method of the present invention, comprises one or more photoinitiators. Suitable photoinitiators are discussed above in relation to the primer. Preferably, the photoinitiator is present from 3 to 20% by weight, preferably from 4 to 18% by weight, of the inkjet ink.

The inkjet ink exhibits a desirable low viscosity, less than 100 mPas, preferably 50 mPas or less and most preferably 30 mPas or less at 25°C. The inkjet ink most preferably has a viscosity of 10 to 30 mPas at 25°C.

Other components of types known in the art may be present in the inkjet ink to improve the properties or performance. These components may be, for example, surfactants, defoamers, dispersants, sensitisers, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers. The method of the present invention includes step (iv), curing the primer and the inkjet ink.

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

A suitable cure 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.

In one embodiment, there is no pinning or curing step applied to the primer, and jetting of the primer onto the substrate is immediately followed by jetting of the inkjet ink onto the primer with no exposure of the primer to actinic radiation prior to jetting the inkjet ink onto the primer. In a preferred embodiment, there is an additional step of pinning after jetting the primer onto the substrate and before jetting the inkjet ink onto the primer. In this embodiment, the method of the present invention comprises the following steps, in order: (i) providing a substrate; (ii) jetting a primer onto the substrate, wherein the primer comprises two or more monofunctional monomers and one or more photoinitiators, wherein the two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer; (iii) pinning the primer; (iv) jetting an inkjet ink onto the primer, wherein the inkjet ink comprises one or more multifunctional monomers, a colourant, one or more photoinitiators and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink; and (v) curing the primer and the inkjet ink.

Pinning is achieved by exposure to actinic radiation. Suitable sources of actinic radiation are discussed hereinabove in relation to the curing of the primer and the inkjet ink.

The dose of actinic radiation for pinning is lower than the dose required for (full) cure, and is typically 1 -200 mJ/cm², preferably 1 -100 mJ/cm² and more preferably 1 -50 mJ/cm². Medium pressure mercury lamps may be used as the pinning source. LED lamps may also be used as the pinning source and in this embodiment, the source preferably has a wavelength of 385 or 395 nm.

In one embodiment, there is an additional step of curing the primer after jetting the primer onto the substrate and before jetting the inkjet ink onto the primer. In this embodiment, the method of the present invention comprises the following steps, in order: (i) providing a substrate; (ii) jetting a primer onto the substrate, wherein the primer comprises two or more monofunctional monomers and one or more photoinitiators, wherein the two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer; (iii) curing the primer; (iv) jetting an inkjet ink onto the primer, wherein the inkjet ink comprises one or more multifunctional monomers, a colourant, one or more photoinitiators and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink; and (v) curing the inkjet ink. The present invention also provides a printed substrate obtainable by the method of the present invention as discussed above.

The present invention also provides a primer as defined hereinabove in relation to the method of the present invention.

The present invention also provides an ink set comprising a primer as defined hereinabove in relation to the method of the present invention and an inkjet ink as defined hereinabove in relation to the method of the present invention. The invention will now be described with reference to the following examples, which are not intended to be limiting.

Examples Example 1

A primer of the invention is prepared. The primer (primer 1) has the formulation as shown in Table 1 .

Table 1 . Formulation of primer 1

2-PEA, IBOA, CTFA are cyclic monofunctional monomers. Florstab UV-12 is a stabiliser. Irgacure 184 and Irgacure 819 are photoinitiators. The primer is prepared by first weighing the monofunctional monomers into a suitable mixing vessel, placing the vessel under the mixing head of a Silverson stirrer and starting the stirrer. The temperature is monitored throughout to ensure that the temperature does not exceed 40°C. The remaining components are added to the mixture and the mixture stirred and heated to 40°C to ensure complete dissolution.

Example 2

Inkjet inks of the invention were prepared. The inkjet inks (inkjet inks 1 -4) had the formulations as shown in Table 2.

Table 2. Formulations of inkjet inks 1 -4

DVE-3 and DPGDA are multifunctional monomers. UV-12 is a stabiliser. Irgacure 184, Lucirin TPO and Irgacure 2959 are photoinitiators. The cyan, magenta, yellow and black dispersions had the compositions as shown in Table 3. Table 3. Compositions of the cyan, magenta, yellow and black dispersions

Example 3 Primer 1 is applied to polycarbonate (PC), polyvinyl chloride (PVC), acrylic and fluted polypropylene (FPP) substrates via drawdown using an RK coater and a K2 bar (12 μηι wet film). The primed substrates are cured using a Jenton dryer fitted with an Oce mercury lamp from an Arizona 550 at power level 7 at 40 m/min with two passes. Inkjet ink 1 containing a cyan pigment is applied either over primer 1 or directly onto the substrates via drawdown using an RK coater and a K2 bar (12 μηι wet film). The printed ink films are cured using a Jenton dryer fitted with an Oce mercury lamp from an Arizona 550 at power level 7 at 40 m/min with four passes.

The substrates are provided by Stephen Webster Plastics (SWP). The FPP is 3 mm thick with the trade name Correx. The acrylic is supplied to SWP by Altuglas. The PC is 2 mm 10 year Lexan Red. The PVC is a 220 micron gloss PVC.

The printed substrates are assessed for adhesion and hardness.

Adhesion is tested using a cross hatch cutter (Sheen Instruments) following ASTM D3359. More than 16 hours after printing, the cross hatch cutter is used to cut through the ink film in a grid pattern. Scotch 610 tape is then applied to the cut surface and pressed down by hand. The tape is sharply removed in an upward direction and the percentage of ink removed is assigned a score of 5-0, wherein 5 = no ink loss, 4 = 0-5% ink loss, 3 = 5-15% ink loss, 2 = 15-30% ink loss, 1 = 30-60% ink loss and 0 = >60% ink loss.

Hardness is tested using a Taber Reciprocating Abraser (Abrader), Model 5900 fitted with a round wire loop stylus. The printed substrates are subjected to five cycles at 30 cycles per minute using three weights of 1 , 2 and 2.5 N. Each printed substrate is tested shortly after printing and more than 16 hours after printing. The printed substrates are assigned a score of 5-1 for each weight, wherein 5 = no visible damage, 4 = faint mark visible, 3 = mark visible but not worn through to underlying substrate, 2 = partially worn through to underlying substrate and 1 = fully worn through to underlying substrate. The scores for each weight are added together to give a final score of 15-3.

All of the inkjet inks that are printed directly onto the substrates show poor adhesion. In contrast, all of the inkjet inks that are printed onto the primer show good adhesion. The hardness is not affected by the use of the primer.

Example 4

Primer 2 has the formulation as shown in Table 4.

Table 4. Formulation of primer 2

Primer 2 is prepared as in Example 1 .

Example 5

Primer 2 is applied onto FPP, PC, self-adhesive vinyl (SAV) and acrylic and cured as in Example 3. Inkjet ink 1 is applied over the primer and cured as in Example 3.

The SAV is Avery 400 Permanent, 0.5 mm thick.

The printed substrates are assessed for adhesion and hardness as in Example 3. The printed substrate containing primer 2 provides acceptable scores. This provides a workable balance between adhesion and viscosity, where particularly low viscosity primer is desirable. The hardness is not affected by the use of the primer.

Claims

1 . A method of inkjet printing comprising the following steps, in order:

(i) providing a substrate;

(ii) jetting a primer onto the substrate, wherein the primer comprises two or more monofunctional monomers and one or more photoinitiators, wherein the two or more monofunctional monomers are present in at least 85% by weight, based on the total weight of the primer;

(iii) jetting an inkjet ink onto the primer, wherein the inkjet ink comprises one or more multifunctional monomers, a colourant, one or more photoinitiators and optionally one or more multifunctional oligomers, wherein the total weight of the multifunctional monomers combined with, when present, the multifunctional oligomers, is at least 15% by weight based on the total weight of the ink; and

(iv) curing the primer and the inkjet ink.

2. A method as claimed in claim 1 , wherein the primer has a viscosity of 5-15 mPas at 25°C.

3. A method as claimed in claims 1 or 2, wherein the primer comprises 2% by weight or less of multifunctional monomers, based on the total weight of the primer.

4. A method as claimed in any preceding claim, wherein the two or more monofunctional monomers each have a glass transition temperature of more than -15°C.

5. A method as claimed in any preceding claim, wherein the two or more monofunctional monomers include two or more monofunctional (meth)acrylate monomers.

6. A method as claimed in claim 5, wherein the two or more monofunctional (meth)acrylate monomers include two or more cyclic monofunctional (meth)acrylate monomers.

7. A method as claimed in claim 6, wherein the two or more cyclic monofunctional (meth)acrylate monomers include a saturated cyclic monofunctional (meth)acrylate monomer and an unsaturated cyclic monofunctional (meth)acrylate monomer.

8. A method as claimed in any preceding claim, wherein the only monofunctional monomers present in the primer are cyclic.

9. A method as claimed in any preceding claim, wherein the primer contains phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA) and isobornyl acrylate (IBOA) as monofunctional monomers.

10. A method as claimed in any preceding claim, further comprising an additional step of pinning the primer after jetting the primer onto the substrate and before jetting the inkjet ink onto the primer.

1 1 . A method as claimed in any of claims 1 -9, further comprising an additional step of curing the primer after jetting the primer onto the substrate and before jetting the inkjet ink onto the primer.

12. A printed substrate obtainable by the method as claimed in any preceding claim.

13. A primer as defined in any of claims 1 -9.

14. An ink set comprising a primer as defined in any of claims 1 -9 and an inkjet ink as defined in claim 1 .