GB2488231A

GB2488231A - Colourless inkjet printing ink

Info

Publication number: GB2488231A
Application number: GB1202611.8A
Authority: GB
Inventors: Brian Woolrich; Yuichi Hayata; Vincent Wright
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 2011-02-16
Filing date: 2012-02-15
Publication date: 2012-08-22
Anticipated expiration: 2032-02-15
Also published as: GB2488231B; GB201102685D0; GB201202611D0

Abstract

A colourless inkjet ink composition comprises of at least 30% by weight of monofunctional (meth)acrylate monomer based on the total weight of the ink; and at least 3% by weight of photoinitiator based on the total weight of the ink, wherein the photoinitiator comprises an alpha-hydroxy ketone, and one or more of a bis-acyl phosphine oxide, a thioxanthone and an alpha-amino ketone. The monofunctional (meth)acrylate monomer can be selected from cyclic TMP formal acrylate (CTFA), phenoxyethyl acrylate (PEA), 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.

Description

Printing ink This invention relates to a printing ink, and particularly to 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 mFas 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.

lnkjet 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 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-called "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 moves to the next position. 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.

Furthermore, the surface of the printed image, particularly images formed from solvent-based inkjet inks, can have little solvent resistance and be susceptible to scratching and abrasion.

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, or to a lesser but appreciable extent in a single-pass mode, produces visible swath lines 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 a colourless ink that can be applied in a multi-pass mode with reduced swath lines in the printed image.

Accordingly, the present invention provides a colourless inkjet ink comprising: (a) at least 30% by weight of a monofunctional (meth)acrylate monomer based on the total weight oftheink;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 bis-acyl phosphine oxide, a thioxanthone and an alpha-amino ketone.

In this manner, the inks of the invention are able to produce images having high gloss with a reduced visibility of swath lines in the printed images. Furthermore, the inks of the invention can be jetted reliably because they are not susceptible to polymerisation in the printer nozzle when exposed to stray low-level radiation at the printhead. Printed films produced from the inks of the invention are flexible.

It is believed that the inks of the present invention are able to produce printed images having high gloss and with reduced visibility of swath lines due to the presence of the alpha-hydroxy ketone photoinitiator and at least one of a bis-acyl phosphine oxide, a thioxanthone and an alpha-amino ketone in the ink.

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".

Monofunctional (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 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), Iauryl 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 phenoxyethyl 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 phenoxyethyl acrylate, cyclic TMP formal acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate or dicyclopentenyl oxyethyl acrylate.

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)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 C to 014 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)acrylate monomer that includes a C to 014 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 and a, 3-unsaturated ether monomers.

Examples of the multifunctional (meth)acrylate monomers which may be included in the inkjet inks include hexanediol d iacrylate (HDDA), trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethylene glycol diacrylate, for example, tetraethylene glycol diacrylate), dipropylene glycol diacrylate (DPG DA), tri(propylene glycol) triacrylate, neopentyl glycol diacrylate, bis(pentaerythritol) hexaacrylate, 3-methyl 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 trifunctional 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 trimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol d imethacrylate, I,4-butanediol dimethacrylate. Mixtures of (meth)acrylates may also be used.

When present in the ink of the invention, multifunctional (meth)acrylate monomers may be included in an amount of 10 to 50% by weight based on the total weight of the ink, for example 10 to 40%.

In a preferred embodiment of the invention, the ink comprises a multifunctional meth(acrylate) monomer that includes a 06 to 014 linear alkyl 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-Vinyl amides are well-known monomers in the art and a detailed description is therefore not required. N-Vinyl amides have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to (meth)acrylate monomers. Preferred examples include N-vinyl amides having an aliphatic or aromatic cyclic group. The cyclic group may optionally include one or more heteroatoms such as oxygen or nitrogen. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP).

Similarly, N-acryloyl amines are also well-known in the art. N-Acryloyl amines also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to (meth)acrylate monomers. Preferred examples include N-acryloyl amines having 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 2 to 40% by weight, preferably 3 to 15% by weight, more preferably 3 to 10% by weight based on the total weight of the ink. NVC is particularly preferred.

The inks of the present invention may also contain cxj3-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 divinyl ether, 1,4-cyclohexanedimethanol divinyl 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, o43-unsaturated ether monomers are preferably provided in an amount of I to 20% by weight, more preferably 2 to 10% 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 30:1.

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 and N-vinyl caprolactam. In a preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer, 3-methyl pentanediol diacrylate and N-vinyl caprolactam. 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 a oj3-unsatu rated ether monomer. Preferably, the ink comprises 3-methyl pentanediol diacrylate, N-vinyl caprolactam and triethylene glycol divinyl ether. In a preferred embodiment of the invention, the ink comprises cyclic TMP formal acrylate as the nionofunctional (meth)acrylate monomer, 3-methyl pentanediol diacrylate, N-vinyl caprolactam and triethylene glycol divinyl ether.

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. 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. Preferably, the ink comprises 3-methyl pentanediol diacrylate, N-vinyl caprolactam and an aliphatic urethane diacrylate oligomer. In a preferred embodiment of the invention, the ink comprises cyclic IMP formal acrylate as the monofunctional (meth)acrylate monomer, 3-methyl pentaned iol 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 photoinitiator. Firstly, 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-i-one and 1- [4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-i-one. Mixtures of two or more alpha-hydroxy ketone photoinitiators may be used.

Secondly, the ink of the invention also comprises one or more of a bis-acyl phosphine oxide, a thioxanthone and an alpha-amino ketone.

By bis-acyl phosphine oxide photoinitiator is meant a photoinitiator that includes two acyl phosphine groups. Examples include bis (2,6-dimethylbenzoyl)-2,4,4- trimethylpentylphosphineoxide, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide and bis (2,6-dimethoxybenzoyl)-2,3,3,-trimethyl-penthylphosphineoxide. Mixtures of two or more bis-acyl phosphine oxide photoinitiators may be used.

Thioxanthones as photoinitiators are known in the art and include isopropyl thioxanthone.

Mixtures of two or more thioxanthones may be used.

Alpha-amino ketones as photoinitiators are known in the art and include 2-benzyl-2- dimethylamino-(4-morpholinophenyl)butan-1 -one, 2-methyl-I -[4-(methylthio)phenyl]-2- morpholinopropane-1-one and 2-dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholin-4-yl-phenyl) -butan-I-one. Mixtures of two more alpha-amino ketones may be used.

The ink of the invention preferably comprises a bis-acyl phosphine oxide. Preferably, the total amount of bis-acyl phosphine oxide in the ink is I to 4% by weight based on the total weight of the ink. In a preferred embodiment, the ink comprises an alpha-hydroxy ketone and a bis-acyl phosphine oxide and further comprises a multifunctional (meth)acrylate monomer and an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone as the alpha-hydroxy ketone, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide, 3-methyl pentaned iol diacrylate and N-vinyl caprolactam. In a preferred embodiment of the invention, the ink comprises an alpha-hydroxy ketone and a bis-acyl phosphine oxide and further comprises a multifunctional (meth)acrylate monomer, an N-vinyl amide or an N-acryloyl amine and a o,13-unsaturated ether monomer or an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone as the alp ha-hydroxy keto n e, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide, 3-methyl pentaned iol diacrylate, N-vinyl caprolactam and triethylene glycol divinyl ether or an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention, the ink comprises a bis-acyl phosphine oxide and a thioxanthone. In a preferred embodiment, the ink comprises an alpha-hydroxy ketone, a bis-acyl phosphine oxide and a thioxanthone and further comprises a multifunctional (meth)acrylate monomer and an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone as the alpha-hydroxy ketone, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide, isopropyl thioxanthone, 3-methyl pentanediol diacrylate and N-vinyl caprolactam. In a preferred embodiment of the invention, the ink comprises an aipha-hydroxy ketone, a bis-acyl phosphine oxide and a thioxanthone and further comprises a multifunctional (meth)acrylate monomer, an N-vinyl amide or an N-acryloyl amine and a oj3-unsaturated ether monomer or an oligomer.

Preferably, the ink comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone as the alpha-hydroxy ketone, bis (2,4,6-trimethylbenzoyl)-phenylphosphineoxide, isopropyl thioxanthone, 3-methyl pentanediol diacrylate, N-vinyl caprolactam and triethylene glycol divinyl ether or an aliphatic urethane diacrylate oligomer.

In a preferred embodiment of the invention the ink comprises an alpha-amino ketone. In a preferred embodiment, the ink comprises an alpha-hydroxy ketone and an alpha-amino ketone and further comprises a multifunctional (meth)acrylate monomer and an N-vinyl amide or an N-acryloyl amine. Preferably, the ink comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer, 1-hydroxycyclohexyl phenyl ketone as the alpha-hydroxy ketone, 2- benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1 -one or 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-one, 3-methyl pentanediol diacrylate and N-vinyl caprolactam. I n a preferred embodiment of the invention, the ink comprises an alpha-hydroxy ketone and a alpha-amino ketone and further comprises a multifunctional (meth)acrylate monomer, an N-vinyl amide or an N-acryloyl amine and a aj3-unsaturated ether monomer or an oligomer. Preferably, the ink comprises cyclic TMP formal acrylate as the monofunctional (meth)acrylate monomer, 1- hydroxycyclohexyl phenyl ketone as the alpha-hydroxy ketone, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one or 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-one, 3-methyl pentanediol diacrylate, N-vinyl caprolactam and triethylene glycol divinyl ether or an aliphatic urethane d iacrylate oligomer.

The photoinitiator component of the ink of the present invention may also comprise a mixture of at least one alpha-hydroxy ketone photoinitiator, one or more of a bis-acyl phosphine oxide, a thioxanthone and an alpha-amino ketone and 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, benzil dimethylketal and (2,4,6-trimethylbenzoyl) diphenylphosphine oxide. 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 alpha-hydroxy ketone photoinitiator is present in an amount of 25 to 100% by weight based on the total weight of photoinitiators present, more preferably 30 to 70% by weight.

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 present invention also provides a method of inkjet printing using the above-described ink and a substrate having the cured inks thereon. The ink of the present invention is particularly suited to multi-mode inkjet printing. Suitable substrates include styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC) and VIVAK (a polyethylene terephthalate glycol modified).

The present invention provides a method of inkjet printing comprising printing the colourless inkjet ink as described hereinabove on to a substrate and exposing the ink to actinic radiation.

The ink of the present invention is located over or under 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 ink of the present invention is cured by exposing the printed ink to actinic radiation. The inks of the present invention are preferably cured by ultraviolet irradiation and are suitable for application by inkjet printing. For the avoidance of doubt, the monomers described herein are therefore radiation-curable monomers. The present invention further provides a set of inkjet inks wherein at least one of the inks in the set is the ink of the present invention. The present invention also provides a cartridge containing the inkjet ink as defined herein. The cartridges comprise an ink container and an ink delivery port which is suitable for connection with an inkjet printer. The present invention also provides a substrate having the ink as defined hereinabove printed on.

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 DVI 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 invention will now be described, by way of reference to the following examples.

Examples

Inkjet ink formulations according to the present invention (Examples 1 to 8) and comparative formulations (Comparative Examples I to 4) having the compositions shown in Table I were prepared by mixing the components in the given amounts. Amounts are given 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; Irgacure 369 and Irgacure 907 are alpha amino-ketone initiators, available from Ciba Specialty chemicals; Esacure ITX is isopropyl thioxanthone, available from Lamberti; Firstcure ST-i is radical inhibitor available from Albemarle Corporation.

A 100% solid colour image (10 cm x 30 cm) was produced by printing a cyan ink formulation having the composition shown in Table I onto Avery permanent 400 (a PVC substrate available from Avery) using a Acuity Advance UV inkjet printer from Fujifilm Co. Ltd. The colourless ink formulations were then printed onto the cyan image. 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 H bulbs powered by electronic ballasts with one lamp unit leading and one lamp unit trailing. Radiation intensity was measured by addition of the total peak area of UVA, UVB and UVC radiation and checked using a UV Power Map system (Electronic Instrumentation & Technology Inc.).

Table 1.

Composition Description wt%

NVC Monomer 24.5 PEA Monomer 39.5 NPGPODA Monomer 15.5 CN964 A85 Oligomer 4 Irgacure 184 Initiator 3 DarocureTPO Initiator 9.5 Firstcure ST-i Inhibitor 0.5 Irgaliteblue GLVO (PB1S:3) Pigment 2.5 Solsperse 32000 Dispersant 1 Total 100 The printed images were tested for gloss, visibility of swath line, cure performance and flexibility as described below. Gloss

The gloss values were obtained using a gloss meter (60 degree measurement) (Sheen Instruments Ltd.) and scored as follows: 5: More than 60 4:50 to 60 3: 35 to 49 2:20 to 34 1: Less than 20 Swath line Visibility of swath line is checked by observation from different distance 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/mm 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 2.

9 9 9 9 9 _____ 9 9 9 P 9 Agqixaj ____ V V V 9 V 9 9 V 9 9 aiflO ___ C 3 9 9 9 9 9 9 g OUIIIIIeMS V 9 ___ 9... 9 9 _____________ P0 P0 90 VU VU VU P0 P0 PG 90 L.-.LseJnaIslld _______ 9 3V 3V _______ ______ _____ I. I. _____ I Odi anooie 9L I C 9 9 S C C Ut P9LOJfl36J[ ______ ______ ______ _____ o'9 sv z's zt üdsoqd _______ _______ _______ wo wo (euoquexoq4) XII Wfl3GS3 ________ ________ ________ ________ -C ______ _____ _____ _____ _____ (euo;eN ou!we eqde) £06 einot6 _______ _______ ________ _______ _______ ______ ______ _____ _____ _____ _____ (ouocnj ouiwe eqdje) 69 eino6j 96 ______ ______ 9 ______ _____ _____ ____ ____ ____ 9 99VP96N0 _______ 35 3.5 _______ Zt ZE Zt Zt 15 3C ____ SaAO UI. _____ _____ _____ _____ ____ ____ ___ ___ ____ VOOd9dN 9VC 691. L9L 26P_ 6CL 621. 0CL 951. LVL 6PL SUP -VJJI) P'23 IL II 93 14._iL_ IL LI. IL IL 93 VOdIiNC 96 92 92 U 95 9 92 92 92 92 OL DAN p I. dwo dwo dwo *dwo 4 _____ 9 P ____ _____ i. aidwexa 3eqe As can be seen from Comparative Examples 1-4 in Table 2, colourless inks comprising an alpha-hydroxy ketone and/or a mono acyl phosphine oxide as photoinitiators provide a printed and cured film with high gloss, good cure and flexibility. However, such colourless inks also provide visible swath lines on the printed and cured film. By using alpha-hydroxy ketone and at least one of bis-acyl phosphine oxide, alpha-amino ketone and thioxanthone photoinitiators, the gloss, cure and flexibility of the cured and printed films are retained and the visibility of the swath lines are considerably reduced or removed completely.

Claims

Claims 1. A colourless inkjet ink comprising: (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 bis-acyl phosphine oxide, a thioxanthone and an alpha-amino ketone.
2. The colourless inkjet ink as claimed in claim 1, 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.
3. The colourless inkjet ink as claimed in claim 2, wherein the monofunctional (meth)acrylate monomer is cyclic TMP formal acrylate (CTFA).
4. The colourless inkjet ink as claimed in any preceding claim, further comprising one or more monomers selected from multifunctional (meth)acrylate monomers, N-vinyl amides, N- (meth)acryloyl amines and a, 13-unsaturated ether monomers.
5. The colourless inkjet ink as claimed in claim 4, comprising 10 to 50% by weight of multifunctional (meth)acrylate monomer, based on the total weight of the ink.
6. The colourless inkjet ink as claimed in claim 4 or 5, comprising 2 to 40% by weight of N-vinyl amide and/or N-acryloyl amine based on the total weight of the ink.
7. The colourless inkjet ink as claimed in claims 4 to 6, comprising 1 to 20% by weight of a, 3-unsaturated ether monomer, based on the total weight of the ink.
8. The colourless inkjet ink as claimed in any preceding claim, wherein the ink comprises cyclic TMP formal acrylate, 3-methyl pentanediol diacrylate and N-vinyl caprolactam.
9. The colourless inkjet ink as claimed in any preceding claim further comprising an oligomer or inert resin.
10. The colourless inkjet ink as claimed in any preceding claim, wherein the ink is substantially free of water and volatile organic solvents.
11. The colourless inkjet ink as claimed in any preceding claim, wherein the ink comprises an alpha-hydroxy ketone, and a bis-acyl phosphine oxide and wherein 1 to 4% by weight of a bis-acyl phosphine oxide is present in the ink, based on the total weight of the ink.
12. A method of inkjet printing comprising printing the colourless inkjet ink as claimed in any preceding claim on to a substrate and exposing the ink to actinic radiation.
13. An inkjet cartridge comprising the ink as claimed in any of claims 1 to 11.
14. A substrate having the ink as claimed in any of claims 1 to 11 printed thereon.
15. The substrate as claimed in claim 14, wherein the colourless inkjet ink is located over or under a printed image formed by inkjet printing an inkjet ink that comprises a colorant.