WO2012110815A1

WO2012110815A1 - Method of ink- jet printing

Info

Publication number: WO2012110815A1
Application number: PCT/GB2012/050344
Authority: WO
Inventors: Yuichi HAYATA; Vincent Wright
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 2011-02-16
Filing date: 2012-02-15
Publication date: 2012-08-23
Anticipated expiration: 2013-08-16
Also published as: GB2501850B; GB201314673D0; GB201102686D0; GB2501850A

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)acry!ate 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 first exposure to actinic radiation of the ink in the final pass commences during a time interval of from 0.2 to 1.6 seconds from the ink impacting the substrate.

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-boi!ing 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 lays 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-cal!ed "multi-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 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 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 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.

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 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 increased colour bleeding in 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 of inkjet printing comprising printing a colourless ink in a multi-pass mode which results in a printed image with high gloss and reduced colour bleeding.

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 monofunctiona! (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 first exposure to actinic radiation of the ink in the final pass commences during a time interval of from 0.2 to 1.6 seconds from the ink impacting the substrate.

5 In this manner, the methods of the invention are able to produce images having high gloss with reduced colour bleeding in the printed images. Furthermore, the printed films obtainable from the method of the invention are flexible. It is believed that the method of the present invention is able to produce printed images having high gloss and with reduced colour bleeding due to the specified time interval from when the final portion of ink impacts the substrate to when the final 10 portion of ink is exposed to actinic radiation.

The present invention will now be described with reference to the accompanying drawing, in which Fig. 1 shows a perspective view of a printhead of an inkjet printing apparatus according to the present invention.

15

The ink

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

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 25 0.1 %, more preferably less than 0.03%. The colourless inks of the invention may also be described as "clear" or "water white". jVlonofunctional (meth)acrylate monomers are esters of (meth)acrylic acid and are well known in the art. Examples include a monomer selected from phenoxyethyl acrylate (PEA), cyclic TMP 0 formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), dicyclopentenyl oxyethyl acrylate, 2-{2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate, tridecyl acrylate, isodecyl acrylate (iso-decyl 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.

5

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 phenoxyethyl acrylate, cyclic TMP formal acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, dicyclopentenyl oxyethyl acrylate. 0 Preferred inks according to this embodiment comprise a monofunctional (meth)acrylate monomer that includes an aliphatic or aromatic cyclic group, such as phenoxyethyl acrylate, cyclic TMP forma! acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, dicyclopenteny! oxyethyl acrylate or combinations thereof. Preferably, the ink comprises a monofunctional meth(acrylate) monomer that includes cyclic TMP formal acrylate, phenoxyethyl 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 (tmeth)acrylate 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 alkyf group, preferably isodecyl acrylate.

In a preferred embodiment of the invention, the ink comprises a monofunctiona! (meth)acrylate monomer that includes 0 to 75% by weight of monofunctional (meth)acrylate monomer that includes a C₆ to C 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)acry!ate monomers, N-vinyl amides, N-{meth)acryloyl amines, α,β-unsaturated ether monomers and combinations thereof. Examples of the multifunctional {meth)acry!ate monomers which may be included in the inkjet inks include hexanediol diacrylate (HDDA), trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethylene glycol diacrylate, for example, tetraethylene glycol diacrylate}, dipropy!ene glycol diacrylate (DPGDA), tri(propylene glycol) triacrylate, neopentyl glycol diacrylate, bis(pentaerythritol} hexaacrylate, 3-methyl pentanedio! diacrylate {3-MPDA) and the acry!ate esters of ethoxylated or propoxyiated glycols and polyols, for example, propoxyiated neopentyl glycol diacrylate (NPGPODA), ethoxylated trimethylolpropane triacrylate, and mixtures thereof. Particularly preferred are di- and trifunctional acrylates. Also preferred are those with a molecular weight greater than 200. A preferred example is 3-methyt pentanediol diacrylate. In addition, suitable multifunctional (meth)acrylate monomers include esters of methacrylic acid {i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyleneg!ycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate. Mixtures of (meth)acryiates 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)acry!ate 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(acrylate) monomer that includes a C₆ to C₁₄ linear alky! group, such as hexanediol diacrylate, or nonanediol diacrylate.

(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.

N-Vinyf amides are well-known monomers in the art and a detailed description is therefore not required. N-Viny! 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 caprotactam (NVC) and N-vinyl pyrrolidone (NVP). NVC is particlalry preferred.

Combinations of NVC with the preferred (meth)acry!ate 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-acryloyl amines are also weli-known in the art. N-Acryloy! 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 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- acry!oylmorpholine (ACMO).

N-Viny! 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 vinyi ether monomers which may be used in the inks of the present invention are triethy!ene glycol divinyi ether, diethylene glycol divinyl ether, 1 ,4- cyciohexanedimethanol divinyl ether and ethylene glycol monoviny! ether. Mixtures of vinyl ether monomers may be used. Triethylene glycol divinyi 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)acrylate 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-methyl pentanediol diacrylate or hexanediol diacrylate and N-vinyl caprolactam. In a preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, 3-methyl 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 monofunctional (meth)acrylate monomer, hexanediol diacrylate and N-vinyl 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 oiigomer. 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 monofunctional (meth)acryiate monomer, N-vinyl caprolactam and an aliphatic urethane diacrylate oiigomer. In a preferred embodiment of the invention, the ink further comprises a multifunctional (meth)acrytate 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 monofunctional (meth) aery late monomer, 3-methyl pentanediol diacrylate, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer. In a preferred embodiment of the invention, the ink comprises phenoxyethyi acrylate as the monofunctional (meth)acrylate monomer, hexanediol diacrylate, N-vinyi 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 photoinitiator, 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

2-hydroxy-2-methyl-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 phenoxyethyi acrylate as the monofunctional (meth)acrylate monomer, 1-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 phenoxyethyi acrylate as the monofunctional (meth)acrylate monomer, 1- hydroxycyclohexyl phenyl ketone, 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 N-vinyl amide or an N-acryloyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyi acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyc!ohexyl phenyl ketone, N-viny! 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 acyi 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,-trimeihyI- penthylphosphineoxide and (2,4,6-trimethylbenzoyl} dipheny!phosphine oxide. Mixtures of two or more acyl phosphine oxide photoinitiators may be used. Preferably, the ink further comprises bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide.

In a preferred embodiment, the ink comprises an acyl phosphine oxide and further comprises an N-viny! 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- 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 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 (2,4,6-trimethyib8nzoyi}-phen^'y phosphiMeoxide or (2,4,6-tnmethyibenzoyl) diphenyipuGSphine 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- trimethylbenzoyl) 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.

In 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 acry!ate or phenoxyethyl acrylate as the monofunctional (metb)acrylate monomer, isopropyl thioxanthorie, 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 T P formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, isopropyl thioxanthone, N-vinyl caprolactam, 3-methyl pentanediol diacrylate or hexanediol diacrylate and an aliphatic urethane diacrylate oligomer. In a preferred embodiment, the ink comprises an alpha-amino ketone. Alpha amino ketones as photoinitiators are known in the art and include 2-benzyl-2-dimethylamino-{4- morpholinophenyl)butan-1-one, 2-methyl-1-[4-(methylthio)pheny!]-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-hydroxycyclohexy! phenyl ketone and bis {2,4,6- trimethylbenzoy!)-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-hydroxycyclohexy I phenyl ketone, bis (2,4,6-trimethylbenzoy!)- phenytphosphineoxide 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)acry!ate monomer, 1-hydroxycyciohexyl phenyl ketone, bis (2,4,6-trimethylbenzoyl)- phenylphosphineoxide or (2,4,6-trimethyibenzoyl) 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)acryiate monomer, 1-hydroxycyclohexyl phenyl ketone, bis (2,4,6- irimethylbenzoyl)-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-hydroxycyciohexy! phenyl ketone and isopropyl thioxanthone.

In a preferred embodiment, the ink comprises an aipha-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)acrylate monomer, 1- hydroxycyclohexyl phenyl ketone, isopropyl thioxanthone and N-vinyl caprolactam.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, a thioxanthone, an N-vinyi 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 -hydroxycyciohexyl phenyl ketone, isopropyl thioxanthone, N-vinyl caprolactam and an aliphatic urethane diacryiate oligomer.

In a preferred embodiment of the invention, the ink further comprises an alpha-hydroxy ketone, a thioxanthone, an N-vinyl amide or an N-acryloy! amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyl acrylate as the mo iofurictional (meth)acrylate monomer, l-hydroxycyciohexy! phenyl ketone, isopropyl thioxanthone, N-vinyl caprolactam, 3-methyl pentanediol diacryiate or hexanediol diacryiate and an aliphatic urethane diacryiate oligomer.

The ink of the invention preferably comprises an acyl phosphine oxide and a thioxanthone. Preferably, the ink further comprises bis (2,4,6-trimethy!benzoyl)-phenylphosphineoxide or (2,4,6- trimethylbenzoy!) 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 formal acrylate or phenoxyethyl acrylate as the monofunctional (meth)acrylate monomer, bis {2,4,6-trimethylbenzoy!)-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-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6- trimethylbenzoyl) diphenylphosphine oxide, isopropyl thioxanthone, N-vinyl caprolactam and an aliphatic urethane diacryiate 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-acryloyi amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyi acrylate as the monofunctiona! {meth)acrylate monomer, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6-trimethylbenzoyl) diphenylphosphine oxide, isopropy! thioxanthone, N-vinyl caprolactam, 3-methyi 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-hydroxycyclohexyl phenyl ketone, bis {2,4,6-trimethylbenzoyl)-phenylphosphineoxide or (2,4,6-trimethylbenzoy!) diphenylphosphine oxide and isopropyl thioxanthone.

In a preferred embodiment, the ink comprises an alpha-hydroxy ketone, 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 formal acrylate or phenoxyethyi acrylate as the monofunctional (meth)acrylate monomer, 1 -hydroxycyclohexyl phenyl ketone, 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 alpha-hydroxy ketone, an acyl phosphine oxide, a thioxanthone, an N-vinyl amide or an N-acry!oyi amine and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyi acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone, bis (2,4,6- trimethylbenzoy -phenylphosphineoxide 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 alpha-hydroxy ketone, an acyl phosphine oxide, a thioxanthone, an N-vinyl amide or an N-acryioyl amine, a multifunctional monomer and an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate or phenoxyethyi acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone, bis (2,4,6-trimethylbenzoyl)-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 benzit dtmethylketal. 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).

I I 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 photoinitiator selected from an alpha-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 monofunctiona! (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 Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

The inks of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooied 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-curable 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-cross!inking 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 reiates to the muiti-pass mode described hereinabove. Thai 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 the first exposure to actinic radiation of the ink in the final pass commences during a time interval of from 0.2 to 1.6 seconds from the ink impacting the substrate. Preferably the time interval is 0.3 to .3 seconds, most preferably 0.3 to 0.9 seconds.

Thus, a precise delay is provided between the ink impacting the substrate and the ink being exposed to actinic radiation. The subsequent duration of irradiation is 0.5 to 5.0 seconds. This exposure preferably cures the ink to provide a solid film. However, the initial exposure can be sufficient to arrest the flow of the ink and then the ink can be exposed one or more additional times in order to complete the cure.

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 will vary depending on the requirements of the final application of the printed image.

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 other than the final portion 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). In a preferred embodiment, all passes are exposed within the same time interval as the final portion.

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 until the fi!m 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 0 to 50. Suitable substrates include siyrene, PoiyCarb (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-curabie 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, the radiation source is integral with the print head. Preferably, as shown in Fig. 1 , the printhead comprises first and second radiation sources on the upstream and downstream of the jetting nozzles. In the forward direction, the ink is jetted from the nozzles and irradiated with the first radiation source, UV lamp A. In the return direction, the ink is jetted from the nozzles and irradiated with the second radiation source, UV lamp B.

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 a 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 AS5 is an a!iphatic urethane diacrylate oligomer diluted with 15% of TPGDA, available from Sartomer; Irgacure 184 is an a-hydroxy ketone photo initiator, Darocure TPO is an acyl phosphine photoinitiator, Irgacure 819 is a bis acyl phosphine photoinitiator, available from Ciba 5 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 trimethylolpropane formal acry!ate or SR531 available from Sartomer; PEA is phenoxyethyl acrylate or SR339 available from Sartomer; 3MPDA is 3-methyl 1 ,5-pentanedioldiacrylate or SR341 available from Sartomer; and HDDA is 1 ,6-hexanediol diacrylate or SR238 available from Sartomer.

Inkjet recording method

Inkjet recording was carried out on a recording medium using an experimental inkjet recording device having ten CA4 printheads manufactured by Toshiba Tec Co, Ltd. An image was printed by a shuttle scan system where a substrate is fixed on a table and a printhead and a UV lamp scan above the substrate. An ink supply system, which comprises a main tank, a supply pipe, an ink supply tank immediately before an inkjet printhead, a filter, inkjet printheads, and a section from the ink supply tank to the inkjet printhead, was thermally insulated and heated. Temperature sensors were provided on the ink supply tank and in the vicinity of the nozzles of the printhead,

— o - +,-,j

45°C. The printhead was driven so as to discharge multisize dots of 6 to 42 pL at a resolution of 600 x 600 dpi. The exposure system and the main scanning speed were adjusted such that UV light was focused to give an exposure area illumination intensity of 330 mW/cm at a favourable time after the ink impacts the recording medium. The UV lamp employed two Integration Technology SUB ZERO 085 lamp units with hydrogen bulbs powered by electronic ballasts with one lamp unit leading and one lamp unit trailing (placed on both sides of printhead unit). For the avoidance of doubt, dpi denotes the number of dots per inch (2.54 cm). The recording medium employed was an OK top coat (coated paper, manufactured by Oji paper Co., Ltd.) Cyan image printing

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 by the inkjet printing method described above. Table 1. Cyan ink formulation

Colourless ink printing

A colourless ink formulation of one of the colourless ink formulations of Table 2 was then printed onto the cyan image, changing the time interval from when the final portion of ink impacts the substrate to when the final portion of ink is exposed to actinic radiation.

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 g!oss meter (60 degree measurement} (Sheen Instruments Ltd.).

Bleeding

In order to test the bleeding of the printed inks, the ink formulations were printed over a 100 % solid cyan image. The edge of the image is observed by the naked eye and the bleeding level is checked.

5: No bleeding observed even by microscope (x10)

4: Slight bleeding observed but detected only by microscope (x10)

3: Bleeding is observed from 5 cm distance but not observed from 30 cm distance.

2: Bleeding is observed from 30 cm distance but not observed from 50 cm distance.

1 : Bleeding is observed from 50 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

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

Tamp was off when colourless ink was printed and is exposed more than 1 min after printing of the colourless inkjet ink.

Examples 1 to 10, where the time interval from when the final portion of ink hits the substrate to when the swath of ink is exposed to actinic radiation is more than 0.2 seconds (Lamp B, return direction) showed high gloss (gloss value 42 to 60). In contrast, Comparative Example 1 , which has a time interval of less than 0.2 seconds showed low gloss (gloss value: 30). This confirms that a time interval of 0.2 seconds or more is required for higher gloss.

Examples 1 to 10, where the time interval from when the final portion of ink hits the substrate to when the swath of ink is exposed to actinic radiation is less than 1.60 seconds showed less bleeding (Score 3 to 5, good for print quality). In contrast, Comparative Example 2, where the time interval is more than 1.60 seconds (actually 2.40 - 2.64 seconds) showed very bad bleeding. This confirms that a time interval of 1.6 seconds or less is required for a reduction in the bleeding.

As shown by Examples 1 to 10, to obtain a high quality varnish coated image and to have both a high gloss value and less bleeding, it is necessary to have a time interval of from 0.2 to 1.6 seconds from when the final portion of ink hits the substrate to when the swath of ink is exposed to actinic radiation.

Claims

1. 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 first exposure to actinic radiation of the ink in the final pass commences during a time interval of from 0.2 to 1.6 seconds from the ink impacting the substrate.

2. The method as claimed in claim 1 , wherein the time interval is from 0.3 to 1.3 seconds.

3. The method as claimed in claim 2, wherein the time interval is from 0.3 to 0.9 seconds.

4. 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.

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

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

7. The method as claimed in any preceding claim, wherein the monofunctional (meth)acrylate monomer is selected from phenoxyethyl 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, isodecyl acrylate {iso-decyl A), lauryl acrylate or combinations thereof.

8. 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-vinyl amides, N-(meth)acryloyl amines, a, β-un saturated ether monomers and combinations thereof.

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

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

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