WO2012110814A1

WO2012110814A1 - Method of ink- jet printing

Info

Publication number: WO2012110814A1
Application number: PCT/GB2012/050343
Authority: WO
Inventors: Yuichi HAYATA; Vincent Wright
Original assignee: Sericol Limited
Priority date: 2011-02-16
Filing date: 2012-02-15
Publication date: 2012-08-23
Also published as: GB2501048A; GB201314671D0; GB201102687D0; GB2501048B

Abstract

The present invention provides a method of inkjet printing comprising: (a) providing a colourless inkjet ink comprising at least 30% by weight of a monofunctional (meth)acrylate monomer based on the total weight of the ink and a photoinitiator; (b) jetting the ink on to a substrate; and (c) exposing the inkjet ink to actinic radiation, wherein the ink is printed in a multi-pass mode, and the amount of ink that is applied to the substrate on each pass is from 2.0 to 11.0 g/m².

Description

METHOD OF INK- JET PRINTING

This invention relates to a printing ink, and particularly to a method of inkjet printing a colourless printing ink.

In inkjet printing, minute droplets of 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 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 ideally 10.5 mPas at the jetting temperature which is often elevated to about 40°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, 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. In such inkjet inks it is necessary to use monomers possessing a low viscosity,

Inkjet inks for printing coloured images include a colouring agent that is typically selected from dyes and pigments.

In inkjet printing, the inkjet printhead moves relative to the substrate from one side of the substrate to another laying down the ink on the substrate as it traverses the print width. This movement of the printhead relative to the substrate is termed a single pass of the inkjet head relative to the substrate. The ink that is applied to the substrate during this single pass is termed a "swath". All of the ink for that swath is applied in one pass of the printhead. Having printed this first swath, the printhead then indexes downward one unit (i.e. moves to a second position) and fays down a second swath of ink in a second single pass adjacent to the first swath. The process is repeated in third and subsequent passes until the multiple swaths of ink on the substrate form the desired image on the substrate. This process is termed a "single pass mode". A potential drawback of the single pass mode is that the substrate can flood with ink causing a flow of ink which leads to an uneven application. To prevent the substrate from flooding with ink and to avoid the consequential uneven application, a so-called "mu!ti-pass mode" is used. Multi-pass mode occurs when not all of the ink required for each swath of ink on a substrate is applied during one pass of the printhead over the substrate. In multi-pass mode, the ink is applied in portions on each pass. The printhead moves in a 5 forwards and backwards direction relative to the substrate. A portion of the total amount of ink is applied to the substrate on each pass of the printhead until a final pass is reached, where the final portion of the total amount of ink is applied. Completing the final pass of the printhead over the substrate therefore lays down a complete swath of ink. After the swath has been laid down, the printhead indexes downward one unit. The process is then repeated to form a second and 10 subsequent swathes of ink. This results in the application of ink in an even manner preventing unwanted flow of the ink.

Printed images having a high gloss are preferred for a number of applications, such as photographic printing. Although inkjet inks comprising a dye colorant can be used to print high 15 gloss images on high gloss substrates, such inks are susceptible to fading when exposed to the atmosphere and/or light. Images formed from inks comprising pigment are less susceptible to fading but it can be difficult to achieve images with uniform gloss when using these inks. In other words, it can be difficult to achieve the same levels of gloss in inked areas of the image and "white" areas, or areas that are not inked.

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Colourless inks have therefore been developed for printing prior to or subsequently to printing of a coloured image in order to improve properties of the printed image, such as adhesion to the substrate, scratch resistance, abrasion resistance, hardness, gloss and resistance to fading. Colourless inks can also be printed together (typically through the same printhead) with coloured 25 inks, particularly inks comprising pigment, in order to provide uniform gloss for photographic applications.

Colourless inks are preferably applied in multi-pass mode for the reasons outlined hereinabove. However, the application of colourless inks in a multi-pass mode produces visible swath lines in

30 the printed image. This adversely affects the quality of the final image, in this regard we refer to Fig.1 which is a reproduction of a photograph of an image printed using the multi-pass mode with visible swath lines. The ink was a conventional colourless ink printed onto a substrate which had previously been printed with a cyan ink. Therefore, there remains a need in the art for an approach to inkjet printing colourless inks in a multi-pass mode with reduced swath lines in the 5 printed image.

Accordingly, the present invention provides a method of inkjet printing comprising:

(a) providing a colourless inkjet ink comprising at least 30% by weight of a monofunctional (meth)acrylate monomer based on the total weight of the ink and a photoinitiator;

0 (b) jetting the ink on to a substrate; and (c) exposing the inkjet ink to actinic radiation, wherein the ink is printed in a multi-pass mode, and the amount of ink that is applied to the substrate on each pass is from 2.0 to 1 1.0 g/m².

In this manner, the method of the invention is able to produce images having high gloss with reduced swath lines in the printed images. Furthermore, the printed films obtainable from the method of the invention are flexible.

The ink The ink of the present invention is a colourless inkjet ink comprising at least 30% by weight of a monofunctional (meth)acrylate monomer based on the total weight of the ink and a photoinitiator.

By "colourless" is meant that the ink of the invention is substantially free of colorant such that no colour can be detected by the naked eye. Minor amounts of colorant that do not produce colour that can be detected by the eye can be tolerated, however. Typically the amount of colorant present will be less than 0.3% by weight based on the total weight of the ink, preferably less than 0.1 %, more preferably less than 0.03%. The colourless inks of the invention may also be described as "clear" or "water white". rvlonofunctionai (meth)acryiate monomers are esters of (meih)acryiic acid and are wall known in the art. Examples include a monomer selected from phenoxyethyi acrylate {PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), dicyclopentenyl oxyethyl acrylate, 2-{2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate, tridecyl acrylate, isodecy! acrylate (iso-decy! A), lauryl acrylate or combinations thereof. The ink of the invention can include one monofunctional (meth)acrylate monomer or a mixture of two or more monofunctional (meth)acrylate monomers.

In a preferred embodiment of the invention, the monofunctional (meth)acrylate monomer has an aliphatic or aromatic cyclic group. The cyclic group may optionally include one or more heteroatoms such as oxygen or nitrogen. Examples include phenoxyethyi acrylate, cyclic TMP formal acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, dicyclopentenyl oxyethyl acrylate. Preferred inks according to this embodiment comprise a monofunctional (meth)acrylate monomer that includes an aliphatic or aromatic cyclic group, such as phenoxyethyi acrylate, cyclic TMP formal acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, dicyclopentenyl oxyethyl acrylate or combinations thereof. Preferably, the ink comprises a monofunctional meth(acrylate) monomer that includes cyclic TMP formal acrylate, phenoxyethyi acrylate or mixtures thereof.

The ink of the invention preferably comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer. The ink of the invention preferably comprises 35 to 80% by weight of monofunctional (meth}acryiate monomer, based on the total weight of the ink.

In preferred embodiment of the invention, the ink comprises a monofunctional (meth)acrylate monomer that has a C₆ to C₁₄ linear alkyl group, preferably isodecyl acrylate.

In a preferred embodiment of the invention, the ink comprises a monofunctional (meth)acrylate monomer that includes 0 to 75% by weight of monofunctional (meth)acry!ate monomer that includes a C₆ to C₁₄ linear alkyl group and 25 to 100% by weight of a monofunctional (meth)acrylate monomer that includes an aliphatic or aromatic cyclic group, based on the total weight of monofunctional (meth)acrylate monomer present in the ink.

The ink of the present invention may optionally include one or more additional monomers that are suitable for use in radiation curable inkjet inks. Examples include multifunctional (meth)acrylate monomers, N-vinyl amides, N-(meth)acryloyl amines, α,β-unsaturated ether monomers and combinations thereof.

Examples of the multifunctional (meth)acrylate monomers which may be included in the inkjet inks include hexanediol diacrylate (HDDA), trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethylene glycol diacrylate, for example, tetraethytene glycol dfacrylate), dipropylene glycol diacrylate (DPGDA), tri(propylene glycol) triacrylate, neopentyl glycol diacrylate, bis(pentaerythritol) hexaacrylate, 3-methy! pentanediol diacrylate (3-MPDA) and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate (NPGPODA), ethoxylated trimethylolpropane triacrylate, and mixtures thereof. Particularly preferred are di- and trtfunctionaf acrylates. Also preferred are those with a molecular weight greater than 200. A preferred example is 3-methyl pentanediol diacrylate.

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

In a preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate, phenoxyethyl acrylate or a combination thereof as the monofunctional (meth)acrylate monomer and 3-methyl pentanediol diacrylate or hexanediol diacrylate.

When present in the ink of the invention, multifunctional (meth) acrylate monomers may be included in an amount of 15 to 50% by weight based on the total weight of the ink, for example 20 to 40%. In a preferred embodiment of the invention, the ink comprises a multifunctional meth(acryiate) monomer that includes a C₆ to C₁₄ linear alkyl group, such as hexanediol diacrylate, or nonanediol diacryiate. (Meth)acryiate 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.

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)acry!ate 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-viny! caprolactam (NVC) and N-vinyl pyrroiidone (NVP). NVC is particlalry preferred.

Combinations of NVC with the preferred (meth)acrylate monomers set out hereinabove are particularly preferred. In one such preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate, phenoxyethyl acrylate or a combination thereof as the monofunctional (meth)acrylate monomer and N-vinyl caprolactam.

Similarly, N-acry!oyl 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)acry!ate monomers. Preferred examples include N- acryloyl amines having an aliphatic or aromatic cyclic group. The cyclic group may optionally include one or more heteroatoms such as oxygen or nitrogen. A preferred example is N- acryloylmorpholine (ACMO).

N-Vinyl amides and/or N-acryloyl amines may be included at 3 to 40% by weight, preferably 5 to 30% by weight, more preferably 8 to 18% by weight based on the total weight of the ink. NVC is particularly preferred.

The inks of the present invention may also contain 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 ink formulation. Typical vinyl ether monomers which may be used in the inks of the present invention are triethylene glycol divinyl ether, diethylene glycol diviny! ether, 1 ,4- cyclohexanedimethano! divinyj ether and ethylene glycol monovinyl ether. Mixtures of vinyl ether monomers may be used. Triethylene glycol divinyl ether is preferred. When present in the ink, α,β-unsaturated ether monomers are preferably provided in an amount of 1 to 20% by weight, more preferably 7 to 15% by weight, based on the total weight of the ink. The weight ratio of (meth)acryiate monomer to vinyl ether monomer is from 4: 1 and 15: 1. In a preferred embodiment of the invention, the ink further comprises a multifunctional (meth)acrylate monomer and an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises 3-methyf pentanediol diacry!ate or hexanedio! diacrylate and N-vinyl caprolactam. In a preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctionaf (meth)acry!ate monomer, 3-methyi pentanediol diacrylate and N-vinyl caprolactam. In a preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctionai (meth)acrylate monomer, hexanediol diacrylate and N-viny! caprolactam.

It is possible to modify the film properties of the inkjet inks by inclusion of oligomers or inert resins, such as thermoplastic acrylics. Said oligomers have a weight-average molecular weight from 500 to 8,000, preferably from 1 ,000 to 7,000 and most preferably from 2,000 to 6,000. The oligomers are preferably functional (i.e. reactive oligomers), in that they take part in the curing reaction. A suitable example is a urethane oligomer. The functionality is preferably 2 to 6 and most preferably the oligomers are difunctional. Oligomers may be included at 1 to 30% by weight, preferably 2 to 20% by weight and more preferably 3 to 15% by weight, based on the total weight of the ink.

In a preferred embodiment, the inkjet ink of the present invention further comprises an N-vinyl amide monomer or N-acryloyl amine monomer and a functional oligomer as defined above. Preferably, the ink comprises N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer. In a preferred embodiment, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctionai (meth)acrylate monomer, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer. In a preferred embodiment of the invention, the ink further comprises a multifunctional (meth)acrylate monomer, an N-vinyl amide or an N-acryloyl amine and an oligomer. In a preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate as the monofunctionai (meth)acrylate monomer, 3-methy! pentanediol diacrylate, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer. In a preferred embodiment of the invention, the ink comprises phenoxyethyl acrylate as the monofunctionai (meth)a cry late monomer, hexanediol diacrylate, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer.

The inkjet inks of the present invention dry primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence are curable inks. Such inks do not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink, although the presence of such components may be tolerated. Therefore, the inkjet inks of the present invention are preferably substantially free of water and volatile organic solvents. However, trace amounts of volatile organic solvents present or trace amounts of water inevitably present by absorption from the air may be tolerated in the ink provided they do not adversely affect the cure speed.

The ink of the invention comprises a free radical photo initiator, such as an alpha-hydroxy ketone, an acyl phosphine oxide, a thioxanthone, an alpha-amino ketones and any other suitable photoinitiator.

In a preferred embodiment of the invention, the ink of the invention comprises an alpha-hydroxy ketone photoinitiator. Such photoinitiators are known in the art and include 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1 -phenyl propane-1-one and 1-[4-(2-hydroxyethoxy)-phenyl]- 2-hydroxy-2-methy!-1-propane-1 -one. Mixtures of two or more alpha-hydroxy ketone photoinitiators may be used. Preferably, the ink further comprises 1-hydroxycyclohexyl phenyl ketone.

In a preferred embodiment, the ink comprises an alpha-hydroxy ketone and further comprises an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyeihyi acryiate as the monofunctional (fneth)acryiate monomer, -hydroxycyclohexyl phenyl ketone and N-vinyl caprolactam.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, an N-vinyl amide or an N-acryloyl amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1- hydroxycyclohexyl phenyl ketone, N-vinyl caprolactam and an aliphatic urethane diacry!ate oligomer.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, an N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone, N-vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanediol diacrylate and an aliphatic urethane diacrylate oligomer. In a preferred embodiment of the invention, the ink comprises an acyl phosphine oxide. By acyl phosphine oxide is meant a photoinitiator that includes an acyl phosphine group. Examples include bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphineoxide, bis (2,4,6- trimethylbenzoyl)-phenylphosphineoxide, bis (2,6-dimethoxybenzoyl)-2,3,3,-trimethyl- penthylphosphineoxide and (2,4,6-trimethy!benzoyl) diphenylphosphine oxide. Mixtures of two or more acyl phosphine oxide photoinitiators may be used. Preferably, the ink further comprises bis (2,4,6-trimethylbenzoyl)-pheny[phosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide.

In a preferred embodiment, the ink comprises an acyl phosphine oxide and further comprises an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, bis (2,4,6- trimethylbenzoy -phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide and N-vinyl caprolactam. In a preferred embodiment of the invention, the ink further comprises an acyl phosphine oxide, an N-vinyl amide or an N-acryloyl amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, bis (2₁4,6-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6-trimetfiylbenzoyf) diphenylphosphine oxide, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink further comprises an acyl phosphine oxide, an N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6- trimethyibenzoyl) diphenylphosphine oxide, N-vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanediol diacrylate and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink comprises a thioxanthone. Thioxanthones as photoinitiators are known in the art and include isopropyl thioxanthone. Mixtures of two or more thioxanthones may be used. Preferably, the ink further comprises isopropyl thioxanthone.

!n a preferred embodiment, the ink comprises a thioxanthone and further comprises an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, isopropyl thioxanthone and N-vinyl caprolactam.

In a preferred embodiment of the invention, the ink further comprises a thioxanthone, an N-vinyl amide or an N-acryloyl amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, isopropyl thioxanthone, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink further comprises a thioxanthone, an N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP format acrylate or phenoxyethyl acrylate as the monofunctional {meih)acrylate monomer, isopropyl thioxanthone, N-vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanedio! diacrylate and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment, the ink comprises an alpha-amino ketone. Alpha amino ketones as phototnitiators are known in the art and include 2-benzyi-2-dimethy!amino-(4- morpholinophenyl)butan-1-one, 2-methyl-1-[4-(methylthio)pheny!J-2-morpholinopropane-1-one and 2-dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholin-4-yl-phenyl}-butan-1-one. Mixtures of two more alpha-amino ketones may be used. The ink of the invention preferably comprises an alpha-hydroxy ketone and an acyl phosphine oxide. Preferably, the ink further comprises 1-hydroxycyclohexyf phenyl ketone and bis (2,4,6- trimethylbenzoyl)-phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide.

In a preferred embodiment, the ink comprises an alpha-hydroxy ketone and an acyl phosphine oxide and further comprises an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1 -hydroxycyclohexyl phenyl ketone, bis (2,4,6-trimethylbenzoyl)- phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide and N-vinyl caprolactam.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, an acyl phosphine oxide, an N-vinyl amide or an N-acryloyl amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1 -hydroxycyclohexyi phenyl ketone, bis (2,4,6-trimethylbenzoyl)- phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, an acyl phosphine oxide, an N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone, bis (2,4,6- trimethylbenzoyi)-phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide, N- vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanediol diacrylate and an aliphatic urethane diacrylate oligomer.

The ink of the invention preferably comprises an alpha-hydroxy ketone and a thioxanthone. Preferably, the ink further comprises 1-hydroxycyclohexyl phenyl ketone and isopropyl thioxanthone. In a preferred embodiment, the ink comprises an afpha-hydroxy ketone and a thioxanthone and further comprises an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acry!ate monomer, 1- hydroxycyc!ohexyl phenyl ketone, isopropyl thioxanthone and N-vinyl caprolactam.

In a preferred embodiment of the invention, the ink further comprises an atpha-hydroxy ketone, a thioxanthone, an N-vinyl amide or an N-acryloyl amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional {meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone, isopropyl thioxanthone, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer. in a preferred embodiment of the invention, the ink further comprises an aipha-hydroxy ketone, a thioxanthone, an N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone, isopropyl thioxanthone, N-vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanedio! diacrylate and an aliphatic urethane diacrylate oligomer.

The ink of the invention preferably comprises an acyl phosphine oxide and a thioxanthone. Preferably, the ink further comprises bis (2,4,6-trimethyibenzoyi)-phenyiphosphineoxide or (2,4, 6- trimethylbenzoyl) diphenylphosphine oxide and isopropyl thioxanthone.

In a preferred embodiment, the ink comprises an acyl phosphine oxide and a thioxanthone and further comprises an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP forma! acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6-trimethylbenzoyl} diphenylphosphine oxide, isopropyl thioxanthone and N-vinyl caprolactam.

In a preferred embodiment of the invention, the ink further comprises an acyl phosphine oxide, a thioxanthone, an N-vinyl amide or an N-acryloyl amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, bis (2,4,6-trimethy!benzoyl)-phenyiphosphineoxide or (2,4,6- trimethylbenzoyl) diphenylphosphine oxide, isopropyl thioxanthone, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink further comprises an acyl phosphine oxide, a thioxanthone, an N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, bis (2,4,6-trimethylbenzoyl)-phenyIphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide, isopropyl thioxanthone, N-vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanediol diacrylate and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink comprises an alpha-hydroxy ketone, an acyl phosphine oxide and a thioxanthone. Preferably, the ink further comprises 1-hydroxycyclohex l phenyl ketone, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide and isopropyl thioxanthone.

In a preferred embodiment, the ink comprises an alpha-hydroxy ketone, an acyi phosphine oxide and a thioxanthone and further comprises an N-viny! amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethy! acrylate as the monofunctional (meth)acry!ate monomer, 1-hydroxycyclohexyl phenyl ketone, bis (2,4,6-trimethy!benzoyl)- phenylphosphineoxide or (2,4,6-trimethylbenzoyl} diphenylphosphine oxide, isopropyl thioxanthone and N-vinyl caprolactam.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, an acyl phosphine oxide, a thioxanthone, an N-vinyl amide or an N-acryioyl amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone, bis (2,4,6- irimeihylberizoyij-pheriylphosphineoxide or (2,4,6-trimethyiberizoyl) diphenylphosphine oxide, isopropyl thioxanthone, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, an acyl phosphine oxide, a thioxanthone, an N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 1 -hydroxycyc!ohexyl phenyl ketone, bis (2,4,6-trimethylbenzoy!)-phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide, isopropyl thioxanthone, N-vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanediol diacrylate and an aliphatic urethane diacrylate oligomer.

The photoinitiator component of the ink of the present invention may also comprise one or more other free radical photoinitiators. The other free radical photoinitiator(s) can be selected from any of those known in the art for example, benzophenone and benzil dimethylketal. The photoinitiators named above are known and commercially available such as, for example, under the trade names Irgacure and Darocur {from Ciba) and Lucerin (from BASF).

Preferably the total amount of photoinitiator in the ink is 3 to 20% by weight, preferably 3 to 15% by weight, based on the total weight of the ink. Preferably the acyl phosphine oxide is present in an amount of 25 to 100% by weight based on the total weight of photoinitiators present, more preferably 30 to 80% by weight. In a preferred embodiment, the ink comprises at least 3% of a photo initiator selected from an a!pha-hydroxy ketone, an acyl phosphine oxide, a thioxanthone and a combination thereof. In a preferred embodiment of the invention, the ink comprises:

(a) at least 30% by weight of a monofunctional (meth)acrylate monomer based on the total weight of the ink; and

(b) at least 3% by weight of a photoinitiator based on the total weight of the ink, wherein the photoinitiator comprises an alpha-hydroxy ketone, and one or more of a acyl phosphine oxide and a thioxanthone.

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, radical inhibitors, surfactants, defoamers, dispersants, synergists for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

The inkjet ink of the invention exhibits a desirable low viscosity (200 mPas or less, preferably 100 mPas or less, more preferably 30 mPas or less at 25°C). Preferably the viscosity of the ink of the invention is between 10 mPas and 30 mPas at 25°C. Viscosity may be measured using a Brookfieid viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

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

The ink of the present invention is cured by exposing the printed ink to actinic radiation, e.g. UV radiation. For the avoidance of doubt, the monomers and oligomers described herein are therefore radiation-curable monomers and radiation-curab!e oligomers. The inks described herein may be applied in the form of an inkjet ink set. The inks are typically provided in a cartridge. The cartridges comprise an ink container and an ink delivery port which is suitable for connection with an inkjet printer.

The printing method Over recent years, radiation-curable inkjet inks have largely replaced solvent-based inks in the higher productivity range, wide format graphics market. Unlike solvent printers, the ink deposited on the surface does not appreciably evaporate upon heating. Instead, the material is transformed into a solid through exposure to an energy source. In most cases, the energy source is a UV light. The exposure to actinic radiation causes photo-crossiinking of curable molecules in the presence of a photoinitiator to form a solid film. The greatest perceived benefit of UV curable printers is their ability to deliver high production rates. In most UV printers, the cure source is mounted on the shuttling printhead carriage, on one or both sides of the printhead cluster. In some cases, cure systems are also placed between printheads. With a typical separation distance of less than 100 mm between the print heads and cure unit, the maximum time between print and cure would be 0.1 s for a printhead carriage moving at 1 m/s. UV ink solidification times of less than one second compare favourably with solvent inks that can take several minutes to dry. The ink of the present invention can be printed using Inkjet printers that are suitable for use with radiation-curable inkjet inks.

As explained hereinabove, inkjet inks may be printed in a single- or multi-pass mode. The present invention relates to the multi-pass mode described hereinabove. That is, in the method of the present invention, the ink is exposed to actinic radiation after jetting onto the substrate and, in the multi-pass mode, the inkjet printhead moves relative to the substrate from one side of the substrate to another applying a portion of the ink to the substrate in a first pass of the printhead, one or more further passes of the printhead optionally occur in which one or more further portions of ink are applied to the substrate over the one or more portions of ink previously applied to the substrate, until a final pass is applied where a final portion of the ink is applied, forming a swath of ink.

A key feature of the present invention is that each portion of ink that is applied to the substrate on each pass is from 2.0 to 1 1.0 g/m². Preferably, each portion of ink that is applied to the substrate on each pass is from 2,4 to 10.4 g/m², more preferably from 2.8 to 8.4 g/m^z and most preferably from 3.1 to 6.0 g/m²,

The total number of passes will depend on the properties of the ink and substrate, and on the requirement for the final image. Typically, the number of passes for each swath will be from 2 to 20, more probably from 5 to 15. The total amount of ink applied to the substrate for each swath of ink is preferably from 12.0 to 66.0 g/m², more preferably from 14.4 to 56.0 g/m², more preferably from 16.8 to 50.4 g/m² and most preferably from 18.6 to 36.0 g/m² of the inkjet ink.

The ink may be exposed to actinic radiation between every pass, or some portions of ink may be applied before the previous portion has been cured. Preferably, every portion is exposed to actinic radiation after each pass. Preferably the portions of ink are exposed to actinic radiation within 5 seconds, more preferably within 1 second after the ink impacting the substrate (or impacting the previous portion of ink for subsequent passes). After a first swath of ink has been applied to the substrate, the printhead moves to a second part of the substrate (i.e. indexes downward one unit) and applies a second swath adjacent to the first swath, thereby building up the colourless ink over the substrate, or the image already printed on the substrate. The process continues with the printhead moving to a third and then subsequent part of the substrate (i.e. indexes downward in successive units) and applies a third and then subsequent swath adjacent to the preceding swath untii the film is complete. The total number of swaths applied will depend on the width of the printhead and on the size of the substrate. Typically, the number of swaths will be from 2-100, more preferably from 10 to 50. Suitable substrates include styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC) and VIVAK (a polyethylene terephthalate glycol modified).

The method of the present invention is typically used to provide a colourless ink film over a printed image. Preferably the ink of the invention is located over a coloured image that has been formed by inkjet printing a coloured inkjet ink. The ink of the invention may also be printed at the same time as one or more coloured inkjet inks, preferably inkjet inks comprising a pigment.

The features of printers that are suitable for printing radiation-curable inkjet inks in a multi-pass mode are well known to the person skilled in the art.

Such printers use a pressurised header tank for delivering the ink to the printhead, which allows control of the meniscus position in the nozzle. UV printers usually require heating at the printhead to produce a jettable viscosity of the UV curable inks. In one embodiment, the printing apparatus of the present invention comprises one or more piezo drop on demand printheads. In a preferred embodiment of the invention, the average drop size for inkjet printing is from 20 to 95 pL. Preferably the printheads are capable of jetting ink in drop sizes of from 20 to 75 pL, more preferably 25 to ,55 pL, particularly preferably 25 to 45 pL.

The ink of the present invention comprises a radiation-curable component and therefore requires curing of the radiation-curable component upon 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.

Preferably the source of actinic radiation is a source that does not generate ozone when in use.

The source of UV radiation could be situated off-line in a dedicated conveyor UV curing unit, such as the SUVD Svecia UV Dryer. Preferably, however, the source of radiation is situated in-line, which means that the substrate does not have to be removed from the printing apparatus between the heating and curing steps.

The source of radiation can be static. This means that the source does not move backwards and forwards across the print width of the substrate when in use. Instead the source of actinic radiation is fixed and the substrate moves relative to the source in the print direction. However, the radiation source is preferably mobile, which means that the source is capable of moving back and forth across the print width, parallel with the movement of the printhead. in a preferred embodiment, one or more sources of actinic radiation are placed on a carriage that allows the source of actinic radiation to traverse the print width. The carriage may be placed up and downstream of the printer carriage to allow irradiation of each portion of the ink. In this embodiment the source of actinic radiation moves independently of the printer carriage and movement of the printhead.

When the source of radiation is provided on separate carriage, it is necessary to provide an additional carriage rail, motor and control systems. This adaptation can lead to large increases in equipment costs. Suitable radiation sources known in the art include 3 high and medium pressure mercury discharge lamp, an LED including an array of LEDs, a UV fluorescent lamp or a flash lamp.

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

Examples

Inkjet ink formulations of cyan ink in Tablel and colourless inks A, B, C and D in Table 2 were prepared by mixing the components in the given amounts. Amounts are provided as weight percentages.

CN964 A85 is an aliphatic urethane diacrylate oligomer diluted with 15% of TPGDA, available from Sartomer; Irgacure 184 is an alpha-hydroxy ketone photoinitiator, Darocure TPO is an acyl phosphine photoinitiator, Irgacure 819 is a bis-acyl phosphine photoinitiator, available from Ciba Specialty chemicals; Esacure ITX is isopropyl thioxanthone available from Lamberti; Firstcure ST- 1 is a radical inhibitor available from Albemarle Corporation.

NVC is N-vinyl caprolactam available from BASF; CTFA is cyclic trimethy!olpropane formal acrylate or SR531 available from Sartomer; PEA is phenoxyethyl acrylate or SR339 available from Sartomer; 3 PDA is 3-methy! 1 ,5-pentanedioldiacryiate or SR341 available from Sartomer; and HDDA is 1 ,6-hexanediol diacrylate or SR238 available from Sartomer.

A 100% solid colour image (10 cm x 30 cm) was produced by printing a cyan ink formulation having the composition shown in Table 1 onto Avery permanent 400 (a PVC substrate available from Avery) using a Acuity Advance UV inkjet printer from Fujifiim Co. Ltd.

Table 1. Cyan ink formulation.

A colourless ink formulation of one of the colourless ink formulations of Table 2 was then printed onto the cyan image, changing the amount of each portion of ink that was applied to the substrate on each pass. Drop size, resolution and the number of printheads were adjusted to achieve the amount of ink to be applied in one pass.

The print heads were CA4 printheads manufactured by Toshiba Tec Co, Ltd, The print resolution was 600^*450 dpi and the print size was 2 m (width) x 1 m (length). The prints were cured using two Integration Technology SUB ZERO 085 lamp units with hydrogen bulbs powered by electronic ballasts with one tamp unit leading and one lamp unit trailing. The radiation intensity was 500 mW/cm², which was measured through the addition of the total peak area of UVA, UVB and UVC radiation and checked using a UV Power Map system (Electronic Instrumentation & Technology Inc.). Scan speed and lamp position was set so that lamp was exposed to jetted ink within 1.0 sec after jetting. Table 2, Colourless ink formulations

Gloss

In order to test the gloss of the printed inks, the ink formulations were printed over a 100% solid cyan image. The gloss values were obtained using a gloss meter (60 degree measurement) (Sheen Instruments Ltd.) and scored as follows:

5: More than 70

4: 60 to 70

3: 50 to 59

2: 35 to 49

1 : Less than 35

Swath line

Visibility of swath line was checked by observation from different distances and scores as follows. 5: Not recognisable from 10 cm distance

4: Slightly recognisable from 10 cm distance but not recognisable from 50 cm distance

3: Recognisable from 10 cm distance but not recognisable from 50 cm distance

2: Recognisable from 50 cm distance but not recognisable from 100 cm distance

1 : Recognisable from 100 cm distance but not recognisable from 200 cm distance

Cure performance

The printed and cured films were touched by hand and the film tackiness was scored as follows: 5: no tack

4: slight tacky

3: tacky 2: slight wet

1 : wet

Flexibility

The printed and cured films were extended at 3 cm/min using an INSTRON 5544 instrument (Instron Limited). The percentage extension at which cracks were generated on the cured film was noted and scored as shown below:

5: More than 50%

4: 40% to 50%

3: 30% to 39%

2: 20% to 29%

1 : Less than 20%

The results are shown in Table 3.

Table 3.

Examples 1 to 12, where each portion of colourless ink that was applied to the substrate on each pass was from 2,0 to 11.0 g/m^z of the inkjet ink, showed high gloss (gloss score 2 to 5). In contrast, Comparative Example 2, where each portion of colourless ink that was applied to the substrate on each pass was less than 2 g/m², showed low gloss (gloss score:1). This clearly shows that at least 2.0 g/m² of the inkjet ink for each portion of colourless ink that was applied to the substrate on each pass was required to achieve a high gloss.

Examples 1 to 10 ink, where each portion of colourless ink that was applied to the substrate on each pass was from 2.0 to 11.0 g/m² of the inkjet ink, showed a reduction in swath lines (Score 2 to 5, good for print quality). In contrast, Comparative Example 1 , where each portion of colourless ink that was applied to the substrate on each pass was more than 1 1.0 g/m², showed visible swath lines, which was not good for print quality. This clearly shows that no more than 1 1.0 g/m² of the inkjet ink for each portion of colourless ink that was applied to the substrate on each pass was required to achieve a reduction in visible swath lines.

As shown by Examples 1 to 10, to obtain a high quality varnish coated image and to have both a high gloss value and a reduction in swath lines, 2.0 to 11.0 g/m² of the inkjet ink for each portion of colourless ink that was applied to the substrate on each pass was required.

Claims

1. A method of Inkjet printing comprising:

(a) providing a coiourless inkjet ink comprising at least 30% by weight of a monofunctional (meth)acrylate monomer based on the total weight of the ink and a photoinitiator;

(b) jetting the ink on to a substrate; and

(c) exposing the inkjet ink to actinic radiation, wherein the ink is printed in a multi-pass mode, and the amount of ink that is applied to the substrate on each pass is from 2.0 to 1 1.0 g/m²

2. The method as claimed in any preceding claim wherein the total amount of ink applied to the substrate to form a swath of ink is from 12.0 to 66.0 g/m².

3. The method as claimed in any preceding claim, wherein the ink is exposed to actinic radiation after each and every pass, prior to applying a subsequent portion of ink in a subsequent pass.

4. The method as claimed in any preceding claim, wherein the number of passes for each swath is from 2 to 20.

5. The method as claimed in any preceding claim, wherein number of swaths pnnted onto the substrate is from 2-100,

6. The method as claimed in any preceding claim, wherein the monofunctional (meth)acrylate monomer is selected from phenoxyethy! acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), dicyclopentenyl oxyethyl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate, tridecyl acrylate, isodecyi acrylate (iso-decyl A), !auryl acrylate or combinations thereof.

7. The method as claimed in any preceding claim, wherein the ink further comprises one or more radiation curable monomers selected from multifunctional (meth)acrylate monomers, N-vinyi amides, N-(meth)acryloyl amines, α,β-unsaturated ether monomers and combinations thereof.

8. The method as claimed in any preceding claim, wherein the inkjet ink comprises an oligomer or inert resin.

9. The method as claimed in any preceding claim, wherein the inkjet ink is substantially free of water and volatile organic solvents.

10. A substrate having an ink applied thereto obtainable by the method as claimed in any preceding claim.