GB2561985A - Printing ink - Google Patents

Abstract

An LED-curable inkjet ink comprises bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide in combination with one or more of 2-benzy1-2-dimethylarnino-1-(4-morpholinophenyl)-butan-1-one, 2-benzy1-2-dimethylamino-1-(4-piperidinylphenyl)-butan-1-one, and 1,3,5-trimethylbenzoyl phenyl phosphinate, wherein the ink comprises less than 0.5 wt.% isopropyl thioxanthone (ITX). The ink may comprise one or more monofunctional monomers, such as (meth)acrylate monomers in combination with an N-vinyl amide, N-acryloyl amine and/or N-vinyl carbamate monomer. The ink may further comprise a multi-functional monomer, an oligomer, and a colorant, such as a dispersed cyan or magenta pigment. A method of inkjet printing the ink onto a substrate and curing the ink by exposure to a UV LED radiation source is claimed. Use of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide with one or more of 2-benzy1-2-dimethylarnino-1-(4-morpholinophenyl)-butan-1-one, 2-benzy1-2-dimethylamino-1-(4-piperidinylphenyl)-butan-1-one, and 1,3,5-trimethylbenzoyl phenyl phosphinate in order to reduce yellow shift in an LED-curable inkjet ink is also disclosed.

Description

(71) Applicant(s):

Fujifilm Speciality Ink Systems Limited Patricia Way, Pysons Road Industrial Estate, BROADSTAIRS, Kent, CT10 2LE, United Kingdom (51) INT CL:

C09D 11/38 (2014.01) (56) Documents Cited:

EP 2471879 A1 US 20120127249 A1 US 20080213518 A1 (58) Field of Search:

INT CL C09D Other: WPI, EPODOC

JP 2006123542 A US 20110169902 A1 US 20070249750 A1 (72) Inventor(s):

Andrew Phillips Brian Woolrich (74) Agent and/or Address for Service:

Elkington and Fife LLP

Thavies Inn House, 3-4 Holborn Circus, London, EC1N 2HA, United Kingdom (54) Title of the Invention: Printing ink

Abstract Title: LED-curable inkjet ink containing photoinitiator blend (57) An LED-curable inkjet ink comprises bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide in combination with one or more of 2-benzy 1 -2-dimethylarnino-1 -(4-morpholinophenyl)-butan-1 -one, 2-benzy1 -2-dimethylamino-1 -(4piperidinylphenyl)-butan-1-one, and 1,3,5-trimethylbenzoyl phenyl phosphinate, wherein the ink comprises less than 0.5 wt.% isopropyl thioxanthone (ITX). The ink may comprise one or more monofunctional monomers, such as (meth)acrylate monomers in combination with an N-vinyl amide, N-acryloyl amine and/or N-vinyl carbamate monomer. The ink may further comprise a multi-functional monomer, an oligomer, and a colorant, such as a dispersed cyan or magenta pigment. A method of inkjet printing the ink onto a substrate and curing the ink by exposure to a UV LED radiation source is claimed. Use of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide with one or more of 2-benzy1-2-dimethylarnino-1-(4-morpholinophenyl)-butan-1-one, 2-benzy1-2-dimethylamino-1-(4piperidinylphenyl)-butan-1-one, and 1,3,5-trimethylbenzoyl phenyl phosphinate in order to reduce yellow shift in an LED-curable inkjet ink is also disclosed.

Printing ink

The present invention relates to a printing ink and in particular, an LED-curable inkjet ink which has a desirable balance of properties. The present invention also relates to a method of printing said ink.

In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have, in use, a low viscosity, typically below 100 mPas at 25°C (although in most applications the viscosity should be below 50 mPas, and often below 25 mPas). Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5-15 mPas and ideally 7-12 mPas at the jetting temperature, which is often elevated to about 40-50°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent.

In one common type of inkjet ink, this liquid is water - see for example the paper by Henry R. Kang in the Journal of Imaging Science, 35(3), pp. 179-188 (1991). In those systems, great effort must be made to ensure the inks do not dry in the head due to water evaporation. In another common type, the liquid is a low-boiling solvent or mixture of solvents - see, for example, EP 0 314 403 and EP 0 424 714. Unfortunately, inkjet inks that include a large proportion of water or solvent cannot be handled after printing until the inks have dried, either by evaporation of the solvent or its absorption into the substrate. This drying process is often slow and in many cases (for example, when printing on to a heat-sensitive substrate such as paper) cannot be accelerated.

Another type of inkjet ink contains radiation-curable material, such as radiation-curable monomers, which polymerise by irradiation with actinic radiation, 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.

There are a number of sources of actinic radiation which are commonly used to cure inkjet inks which contain radiation-curable material. The most common source of radiation is a UV source. UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof.

Mercury discharge lamps, fluorescent tubes and flash lamps are traditionally used as the radiation source as they have an impressive UV output performance. However, these radiation sources have several drawbacks in their operational characteristics, and LED UV light sources are an attractive alternative that are becoming increasingly popular. In particular, when compared to, for example mercury discharge lamps (the most common UV light source used to cure inkjet inks), LEDs offer consistent UV output, lower power consumption, higher energy efficiency, longer maintenance intervals and instant start up. Further, LEDs generate less heat, offer a significant cost reduction and are an environmentally friendlier solution. However, there remain a number of challenges when utilising LED UV light sources as the radiation source.

Compared to conventional mercury lamp UV sources, LEDs have a narrow spectral output. The UV output of LED lamps is essentially monochromatic and most commercial devices operate at 385, 395 or 405 nm. Therefore, the photoinitiator systems required for inks that will be cured using LED UV radiation are necessarily different to the photoinitiator systems required for inks that will be cured using a mercury lamp. Although a number of photoinitiators are available that absorb at the wavelength of LED UV radiation, which provides the ink with a fast cure speed, it has proven difficult to find photoinitiators for use in LED-curable inks that provide the ink with an appropriate balance of properties.

One suitable photoinitiator is the type II photoinitiator, isopropyl thioxanthone (ITX). ITX provides a suitable cure speed even on curing with an LED UV radiation source. However, a drawback of ITX is that it leads to colour instability in the cured ink over time, despite ITX having a highly effective response to LED UV radiation. Specifically, ITX gives rise to a yellow colour cast in the cured ink immediately after curing, which fades away over the following 24 hours (yellow shift).

Yellow shift is a known phenomenon in the art. It is when the yellow colour of the ink changes over a period of time, typically measured over 24 hours, and specifically, when the colour of the ink shifts towards positive or negative values on the b* axis on the CIELAB (L*a*b*) colour space system in the yellow quadrant, and therefore becomes increasingly or decreasingly yellow respectively. The amount of colour change is represented in the art by a so-called delta E (or AE)-value. A delta Evalue influenced by the shift on the b* axis (Ab*) in the yellow quadrant represents a change in the yellowness of the ink. A shift towards more positive values on the b* axis in the yellow quadrant is associated with an increase in the yellowness of the ink whereas a shift towards more negative values in the yellow quadrant is associated with a decrease in the yellowness of the ink. As a guide, a delta E-value of 1.0 is the minimum colour change detectable by the human eye.

Yellow shift is problematic for all colours as any change and instability of colour in the ink causes problems. The problem of yellow shift can be somewhat hidden in yellow and blacks inks but not in light-coloured inks (e.g. cyan and magenta). Cyan inks suffer from yellow shift the most because cyan is the opposite colour on the colour spectrum to yellow. Practically, yellow shift causes colour profiling issues and is particularly an issue for graphic art printers as it is not acceptable to wait long periods of time for the yellow shift to stabilise in the cured ink image before colour profiling.

By yellow shift post-cure, it is meant that yellow shift of the ink is only assessed post-cure, i.e. the b* values are only recorded post-cure, using a spectrophotometer. However, the b* values of the ink are also changing during and after printing. In this regard, the ink can be thought of as having an initial b* value just before printing, which then changes during and after printing, and during and after curing. The first b* value (b/) is recorded typically within one minute of curing. The second b* value (b₂*) is then recorded typically 24 hours later. Ab* is calculated by subtracting b/ from b₂*.

In one scenario, yellowing occurs immediately after curing and then fades away during the subsequent 24 hours - the colour of the cured ink images shifts towards negative values on the b* axis in the yellow quadrant. In this scenario, although the mechanism of yellow shift has not been confirmed, without wishing to be bound by theory, the inventors believe the photoinitiator breakdown products, which are generated during photocleaving, have a yellow chromophore, and then over a further period of time these unstable fragments further decompose to reduce the yellow coloration.

As well as being influenced by the choice of photoinitiator, yellow shift is also affected by storage temperature of the ink, concentrations of the photoinitiators and the choice of the binder or bulk material such as radiation-curable monomers.

There is therefore demand for LED-curable inks which do not contain high levels of ITX.

A suitable replacement photoinitiator in LED UV-curable inks is 2,4,6trimethylbenzoyldiphenylphosphine oxide (TPO). However, TPO has the drawback of being reprotoxic. Reprotoxic substances are those which cause adverse effects on sexual function and fertility in males and females, developmental toxicity in the offspring and effects through or via lactation. Reprotoxic substances are grouped with carcinogenic and mutagenic substances and are together known as CMR substances (CMRs). Classification of CMRs in the EU is based on the strength of evidence showing that they present one of the CMR types of hazards to human health. The EU legislation regarding Classification Labelling and Packaging of substances (the CLP Regulation 1272/2008) uses the following categories to define CMRs. Category 1A defines CMRs as those known to have CMR potential for humans, based largely on human evidence. Category 1B defines CMRs as those presumed to have CMR potential for humans, based largely on experimental animal data. Category 2 defines CMRs as those suspected to have CMR potential for humans. Evidence of adverse effects in the offspring due to transfer in the milk and/or on the quality of the milk and/or the substance is present in potentially toxic levels in breast milk, is also relevant in defining CMRs. Mixtures containing CMRs are further defined using similar categories. A mixture is defined as category 1A or 1B reprotoxic if it contains >0.3% reprotoxic category 1A or 1B substances. A mixture is defined as category 2 reprotoxic if it contains >0.3% reprotoxic category 2 substances. Further, mixtures containing >0.3% reprotoxic substances with effects on or via lactation are also relevant for defining mixtures containing reprotoxic substances.

Hazard codes can also be used to define reprotoxic substances. A substance having a hazard code of H360 is a category 1A or 1B CMR substance having reproductive toxicity and may damage fertility or the unborn child. A substance having a hazard code of H361 is a category 2 CMR substance having reproductive toxicity and is suspected of damaging fertility or the unborn child. A substance having a hazard code of H362 is linked to reproductive toxicity and effects on or via lactation; such a substance may cause harm to breast-fed children.

TPO has a hazard code of H361 and is classified as a category 2 CMR substance with reproductive toxicity, which is suspected of damaging fertility or the unborn child.

There is therefore a need in the art for an LED-curable inkjet ink that offers the desired balance of properties, namely a fast cure speed, is not reprotoxic and has reduced yellow shift post-cure.

Accordingly, the present invention provides an LED-curable inkjet ink comprising, as photoinitiators, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide in combination with one or more of 2-benzyl-2dimethylamino-1 -(4-morpholinophenyl)-butan-1 -one, 2-benzyl-2-dimethylamino-1 -(4piperidinylphenyl)-butan-1-one and 1,3,5-trimethylbenzoyl phenyl phosphinate, wherein the ink comprises less than 0.5% by weight of isopropyl thioxanthone, based on the total weight of the ink.

The inventors have surprisingly found that an LED-curable inkjet ink containing the blend of photoinitiators as claimed has the desired balance of properties, namely a fast cure speed, is not reprotoxic and has reduced yellow shift post-cure. In particular, it has been found that the use of the claimed blend of photoinitiators in an LED-curable inkjet ink reduces the degree of yellowing in the cured ink image to an acceptable level over 24 hours whilst maintaining a fast cure speed. It is surprising that the LED-curable inkjet ink of the invention can achieve such advantages without recourse to ITX in the blend of photoinitiators.

The inventors have found that the inclusion of the blend of photoinitiators as claimed in LED-curable inkjet inks reduces the yellow shift in the cured ink image to an acceptable level, i.e. wherein Ab* is an absolute value from 0.0 to 5.0, preferably from 0.0 to 3.5, and most preferably from 0.0 to 2.0, over 24 hours. An absolute value is the magnitude of a real number without regard to its sign. The first b* value (b/) is recorded within one minute of curing. The second b* value (b₂*) is then recorded 24 hours later. Ab* is calculated by subtracting b/ from b₂*. Ab* values vary with actinic radiation dose, so the Ab* values quoted herein are determined at a total dose per unit area defined as the minimum dose per unit area required to achieve a fully cured film, i.e. a tack-free film. A Ab* of 5.0 is the largest acceptable Ab* absolute value for any application of the present invention, a Ab* of 3.5 is the largest acceptable Ab* absolute value for moderately sensitive applications of the present invention and a Ab* of 2.0 is the largest acceptable Ab* absolute value for important sensitive graphic applications of the present invention. Therefore, the absolute value of Ab* acceptable will depend on the ultimate application of the cured ink image of the present invention.

As discussed above, yellow shift is known in the art. It is when the yellow colour of the ink changes over a period of time, typically measured over 24 hours, and specifically, when the colour of the ink shifts on the b* axis on the CIELAB (L*a*b*) colour space system in the yellow quadrant, and therefore becomes increasingly or decreasingly yellow respectively. The total shift along the b* axis is denoted as Ab* and is represented by delta E. The larger delta E, the larger change in colour. Delta E is therefore the measure of how far the yellow colour has changed along the b* axis over time, typically over 24 hours. The lightness, L*, represents the darkest black at L*=0, and the brightest white at /.*=100. The colour channels, a* and b*, represents true neutral grey values at a*=0 and b*=0. The red/green opponent colours are represented along the a* axis, with green at negative a* values and red at positive a* values. The yellow/blue opponent colours are represented along the b* axis, with blue at negative b* values and yellow at positive b* values. The formula for calculating delta E is as follows:

Δ/Γ

Thus, a yellow shift occurs when the colour of the ink shifts over 24 hours towards positive or negative values on the b* axis on the CIELAB (L*a*b*) colour space system in the yellow quadrant, and therefore becomes increasingly or decreasingly yellow respectively. The acceptable level of colour shift depends on the colour but a delta E of at least 1.0 is required to be visible to the human eye and a yellow shift is generally acceptable wherein Ab* is an absolute value from 0.0 to 5.0, preferably from 0.0 to 3.5, and most preferably from 0.0 to 2.0, over 24 hours, depending on the application of the present invention.

The present invention also provides a method of inkjet printing comprising inkjet printing the inkjet ink of the present invention onto a substrate and curing the ink by exposing the printed ink to a UV LED radiation source.

The present invention further provides the use of the photoinitiators as claimed for reducing yellow shift in an LED-curable inkjet ink.

The LED-curable inkjet ink of the present invention comprises the following photoinitiators: bis(2,4,6trimethylbenzoylj-phenylphosphine oxide in combination with one or more of 2-benzyl-2dimethylamino-1 -(4-morpholinophenyl)-butan-1 -one, 2-benzyl-2-dimethylamino-1 -(4piperidinylphenyl)-butan-1-one and 1,3,5-trimethylbenzoyl phenyl phosphinate.

The photoinitiator package as claimed is tailored for UV LED light, which means that the photoinitiators absorb the radiation which is emitted by the UV LED light source. Preferably, the blend of photoinitiators as claimed absorb radiation in a region of from 360 nm to 410 nm and absorb sufficient radiation to cure the ink within a 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less bandwidth.

Bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide is available commercially as Omnirad 819 sold by IGM. 2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one is available commercially as Omnirad 369 sold by IGM. 2-Benzyl-2-dimethylamino-1-(4-piperidinylphenyl)-butan-1-one is available commercially as Omnirad 264 sold by IGM. 1,3,5-Trimethylbenzoyl phenyl phosphinate is available commercially as Speedcure XKM sold by Lambson.

In a preferred embodiment, the total amount of photoinitiators present in the ink of the present invention is 1 to 20% by weight, more preferably 2 to 16% by weight, based on the total weight of the ink. In an even more preferred embodiment, the total amount of bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, 2-benzyl-2-dimethylamino-1 -(4-morpholinophenyl)-butan-1 -one, 2-benzyl-2dimethylamino-1-(4-piperidinylphenyl)-butan-1-one and 1,3,5-trimethylbenzoyl phenyl phosphinate present in the ink of the present invention is 0.1 to 20% by weight, more preferably 2 to 16% by weight, based on the total weight of the ink.

Preferably, the ink of the present invention comprises 0.5 to 10% by weight, preferably 1 to 5% by weight, of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, based on the total weight of the ink.

Preferably, the ink of the present invention comprises 0.5 to 20% by weight, preferably 1 to 15% by weight, of one of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, 2-benzyl-2dimethylamino-1-(4-piperidinylphenyl)-butan-1-one, 1,3,5-trimethylbenzoyl phenyl phosphinate or mixtures thereof, based on the total weight of the ink.

In a preferred embodiment, the ratio by weight of bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide to one of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, 2-benzyl-2-dimethylamino1-(4-piperidinylphenyl)-butan-1-one, 1,3,5-trimethylbenzoyl phenyl phosphinate or mixtures thereof is 1:1 to 1:10, more preferably 1:1.1 to 1:5.0. In one embodiment, the ratio by weight is 1:0.1 to 1:10.0.

Isopropyl thioxanthone is available commercially as Speedcure ITX from Lambson. In the present invention, the ink comprises less than 0.5% by weight of isopropyl thioxanthone, preferably less than 0.2% by weight of isopropyl thioxanthone, based on the total weight of the ink. More preferably the ink is free of isopropyl thioxanthone.

As discussed hereinabove, the inclusion of isopropyl thioxanthone helps to achieve a fast cure speed but results in an increased yellow shift. Replacing isopropyl thioxanthone with the claimed blend of photoinitiators has surprisingly been found to maintain cure speed whilst reducing yellow shift.

2,4-Diethylthioxanthone (DETX) behaves similarly to ITX in that it a highly effective response to LED UV radiation but causes yellow shift. DETX is available commercially as Speedcure DETX from Lambson. Therefore, in a preferred embodiment, the ink comprises less than 0.5% by weight of isopropyl thioxanthone combined with 2,4-diethylthioxanthone, more preferably less than 0.2% by weight of isopropyl thioxanthone combined with 2,4-diethylthioxanthone, based on the total weight of the ink. In other words, the total weight of isopropyl thioxanthone combined with 2,4diethylthioxanthone is preferably less than 0.5% by weight, more preferably less than 0.2% by weight, based on the total weight of the ink.

The photoinitiator package of the present invention is non-reprotoxic. Reprotoxic substances are defined hereinabove. In a preferred embodiment, the ink of the present invention comprises less than 0.5% by weight of 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO), preferably less than 0.2% by weight, based on the total weight of the ink. More preferably the ink is free of TPO.

In a preferred embodiment, the ink is substantially free from reprotoxic substances (category 1 and preferably categories 1 and 2). Preferably, the ink comprises less than 0.5% by weight of reprotoxic substances, based on the total weight of the ink. More preferably, the ink is free of such substances.

The claimed blend of photoinitiators in the ink of the present invention, namely bis(2,4,6trimethylbenzoyl)-phenylphosphine oxide in combination with one or more of 2-benzyl-2dimethylamino-1 -(4-morpholinophenyl)-butan-1 -one, 2-benzyl-2-dimethylamino-1 -(4piperidinylphenyl)-butan-1-one and 1,3,5-trimethylbenzoyl phenyl phosphinate, results in an ink with a fast cure speed, without recourse to isopropyl thioxanthone. Whether an ink has a fast cure speed or not can be defined by the following test.

An ink of the present invention is applied to 220 micron gloss white semi-rigid PVC with a K2 bar using a RK Coater automated drawdown machine. The inks is then cured using a Jenton conveyer fitted with a Phoseon 20W LED lamp running at full power at 60m/min speed. After the first pass under the lamp, a small strip of Epson Premium Photo Paper is applied to the printed ink, with the coated side of the paper facing the printed ink, and rubbed down. Any ink removal or surface marking of the printed ink indicates an incomplete cure. The printed ink is repeatedly passed under the lamp and tested in this way until there is no ink removal or surface marking of the printed ink, indicating full cure. An ink requiring three passes or less to achieve full cure is regarded as fast curing.

In one embodiment, the present invention provides an LED-curable inkjet ink wherein the only photoinitiators present in the ink are bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide and one or more of 2-benzyl-2-dimethylamino-1 -(4-morpholinophenyl)-butan-1 -one, 2-benzyl-2-dimethylamino-1 (4-piperidinylphenyl)-butan-1-one and 1,3,5-trimethylbenzoyl phenyl phosphinate.

The inkjet ink of the present invention is an LED-curable inkjet ink. It therefore comprises radiationcurable material. The radiation-curable material is not particularly limited and the formulator is free to include any such radiation-curable material in the ink of the present invention to improve the properties or performance of the ink. However, when the ink is restricted to containing non-reprotoxic substances, non-reprotoxic radiation-curable material must only be used in the ink. In general, the radiation-curable material can include any radiation-curable material readily available and known in the art in inkjet inks. By “radiation-curable” is meant a material that polymerises and/or crosslinks when exposed to actinic radiation, in the presence of a photoinitiator. By “LED-curable” is meant that the actinic radiation source is an LED or array of LEDs.

The amount of radiation-curable material is not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. In a preferred embodiment, the ink of the present invention comprises 30 to 95%, more preferably 50 to 90%, by weight of radiation-curable material, based on the total weight of the ink.

In a preferred embodiment, the LED-curable inkjet ink of the present invention comprises one or more monomers. As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers.

In a preferred embodiment, the LED-curable inkjet ink of the present invention comprises one or more monofunctional monomers, such as a monofunctional (meth)acrylate monomer. In a particularly preferred embodiment, the inkjet ink of the present invention comprises at least two monofunctional monomers.

In a preferred embodiment, the ink of the present invention comprises 20 to 90% by weight, preferably 50 to 80% by weight of monofunctional monomers, based on the total weight of the ink.

Monofunctional monomers are well known in the art. A radiation-curable monofunctional monomer has one functional group, which takes part in the polymerisation reaction on curing. The polymerisable groups can be any group that are capable of polymerising upon exposure to radiation and are preferably selected from a (meth)acrylate group and a vinyl ether group.

The substituents of the monofunctional monomers are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C^s alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C_e.₁₀ aryl- or C₃.₁₈ cycloalkyl-substituted Cm₈ alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure.

In a preferred embodiment, the ink of the present invention comprises one or more monofunctional (meth)acrylate monomers, which are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required. Mixtures of (meth)acrylates may also be used.

Preferred examples include cyclic monofunctional (meth)acrylate monomers and acyclic-hydrocarbon monofunctional (meth)acrylate monomers. For example, phenoxyethyl acrylate (PEA), 2-methyl-2ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol-10), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), 2-(2ethoxyethoxy)ethyl acrylate, octadecyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof.

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

Cyclic TMP formal acrylate (CTFA) Phenoxyethyl acrylate (PEA) mol wt 200 g/mol mol wt 192 g/mol

2-Methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (Medol-10) mol wt 200 g/mol

Isobornyl acrylate (IBOA) mol wt 208 g/mol

Tetrahydrofurfuryl acrylate (THFA) mol wt 156 g/mol

Ο

σ

Ό'

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

OR'

o

C13H27'

R — CgHi7 / C10H21 Octadecyl acrylate (ODA) mol wt 200 g/mol

Tridecyl acrylate (TDA) mol 254 g/mol

C-10H21'

o

C12H25'

Isodecyl acrylate (IDA) mol wt 212 g/mol

Lauryl acrylate mol wt 240 g/mol

In a preferred embodiment, the ink of the present invention comprises one or more monofunctional (meth)acrylate monomers, selected from PEA, CTFA, IBOA and Medol-10.

In a preferred embodiment, the ink of the present invention comprises 20 to 80% by weight, preferably 30 to 60% by weight of monofunctional (meth)acrylate monomers, based on the total weight of the ink.

In a preferred embodiment, the ink of the present invention comprises an N-vinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer. In a particularly preferred embodiment, the ink of the present invention comprises an N-vinyl amide monomer or an N-vinyl carbamate monomer. In one embodiment, the ink of the present invention comprises an N-vinyl amide monomer. In another embodiment, the ink of the present invention comprises an N-vinyl carbamate monomer.

N-Vinyl amide monomers are well-known monomers in the art and a detailed description is therefore not required. N-vinyl amide monomers have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP). Similarly, N-acryloyl amine monomers are also well-known in the art. N-acryloyl amine monomers also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO).

N-Vinyl carbamate monomers are defined by the following functionality:

o

The synthesis of N-vinyl carbamate monomers is known in the art. For example, vinyl isocyanate, formed by the Curtius rearrangement of acryloyl azide, can be reacted with an alcohol to form N-vinyl carbamates (Phosgenations - A Handbook by L. Cotarca and H. Eckert, John Wiley & Sons, 2003, 4.3.2.8, pages 212-213).

In a preferred embodiment, the N-vinyl carbamate monomer is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure:

R·

R· in which R¹ to R⁴ are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically hydrogen, alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C^s alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C₆-io aryl- °^r C3-18 cycloalkyl-substituted Cms alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. Preferably R¹ to R⁴ are independently selected from hydrogen or Cmo alkyl. Further details may be found in WO 2015/022228 and US 4,831,153.

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

'0 molecular weight 127 g/mol

NVMO has the IUPAC name 5-methyl-3-vinyl-1,3-oxazolidin-2-one and CAS number 3395-98-0. NVMO includes the racemate and both enantiomers. In one embodiment, the N-vinyl carbamate monomer is a racemate of NVMO. In another embodiment, the N-vinyl carbamate monomer is (R)-5methyl-3-vinyl-1,3-oxazolidin-2-one. Alternatively, the N-vinyl carbamate monomer is (S)-5-methyl-3vinyl-1,3-oxazolidin-2-one.

In a preferred embodiment, the ink of the present invention comprises NVC or NVMO. In one embodiment, the ink of the present invention comprises NVC. In another embodiment, the ink of the present invention comprises NVMO.

Preferably, the ink of the present invention comprises 5-30% by weight, more preferably 10-30% by weight and most preferably 10-25% by weight of an N-vinyl amide monomer, an N-(meth)acryloyl amine monomer and/or an N-vinyl carbamate monomer, based on the total weight of the ink.

Preferably, the inkjet ink of the present invention comprises 5-30% by weight of an N-vinyl amide monomer, based on the total weight of the ink. More preferably, the inkjet ink of the present invention comprises 10-30% by weight and most preferably 10-25% by weight of an N-vinyl amide monomer, based on the total weight of the ink

Preferably, the inkjet ink of the present invention comprises 5-30% by weight of an N-(meth)acryloyl amine monomer, based on the total weight of the ink. More preferably, the inkjet ink of the present invention comprises 10-30% by weight and most preferably 10-25% by weight of an N-(meth)acryloyl amine monomer, based on the total weight of the ink.

Preferably, the inkjet ink of the present invention comprises 5-30% by weight of an N-vinyl carbamate monomer, based on the total weight of the ink. More preferably, the inkjet ink of the present invention comprises 10-30% by weight and most preferably 10-25% by weight of an N-vinyl carbamate monomer, based on the total weight of the ink.

In a preferred embodiment, the LED-curable inkjet ink of the present invention comprises one or more monofunctional (meth)acrylate monomers in combination with an N-vinyl amide monomer, an Nacryloyl amine monomer and/or an N-vinyl carbamate monomer. In an even more preferred embodiment, the LED-curable inkjet ink of the present invention comprises two or more monofunctional (meth)acrylate monomers in combination with an N-vinyl amide monomer, an Nacryloyl amine monomer and/or an N-vinyl carbamate monomer. Such a blend of monomers further provides the ink with the desired balance of properties.

In a particular preferred embodiment, the LED-curable inkjet ink of the present invention comprises two or more monofunctional (meth)acrylate monomers, selected from PEA, CTFA, IBOA and Medol10, in combination with NVC or NVMO. In one embodiment, the LED-curable inkjet ink of the present invention comprises two or more monofunctional (meth)acrylate monomers, selected from PEA, CTFA, IBOA and Medol-10, in combination with NVC. In another embodiment, the LED-curable inkjet ink of the present invention comprises two or more monofunctional (meth)acrylate monomers, selected from PEA, CTFA, IBOA and Medol-10, in combination with NVMO.

In a preferred embodiment, the LED-curable inkjet ink of the present invention comprises one or more radiation-curable monomers having two or more functional groups. Radiation-curable monomer having two or more functional groups has its standard meaning, i.e. di or higher, that is two or more groups, respectively, which take part in the polymerisation reaction on curing.

In a preferred embodiment, the radiation-curable monomer having two or more functional groups is a di-, tri-, tetra-, penta- or hexa- functional monomer, i.e. the radiation curable monomer has two, three, four, five or six functional groups. In a particularly preferred embodiment, the inkjet ink of the present invention comprises a hexafunctional monomer.

The functional group of the radiation-curable monomer having two or more functional groups, which is utilised in the ink of the present invention may be the same or different but must take part in the polymerisation reaction on curing. Examples of such functional groups include any groups that are capable of polymerising upon exposure to radiation and are preferably selected from a (meth)acrylate group and a vinyl ether group.

The radiation-curable monomer having two or more functional groups may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality monomers may be used.

The substituents of the radiation-curable monomer having two or more functional groups are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C_H8 alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C_e.₁₀ aryl- or C₃_₁₈ cycloalkyl-substituted C^e alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure. (The same groups may also be used for difunctional monomers.)

In a preferred embodiment, the ink of the present invention comprises 1 to 20%, more preferably 2 to 10%, by weight of radiation-curable monomers having two or more functional groups, based on the total weight of the ink.

Examples of the radiation-curable monomer having two or more functional groups include difunctional (meth)acrylate monomers, multifunctional (meth)acrylate monomers, divinyl ether monomers and vinyl ether (meth)acrylate monomers. Mixtures of radiation-curable monomer having two or more functional groups may also be used.

Difunctional (meth)acrylate monomers are well known in the art and a detailed description is therefore not required. Preferred examples include decanediol diacrylate, hexanediol diacrylate (HDDA), tricyclodecanedimethanol diacrylate (TCDDMDA), polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, neopentylglycol diacrylate, 3-methyl pentanediol diacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, and mixtures thereof.

In addition, suitable difunctional methacrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as decanediol dimethacrylate, hexanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol di methacrylate and mixtures thereof.

Preferably, the difunctional (meth)acrylate monomer is selected from decanediol diacrylate, hexanediol diacrylate, propoxylated neopentyl glycol diacrylate, dipropylene glycol diacrylate, and mixtures thereof.

Preferably, the ink of the present invention comprises 5 to 25% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink. However, for some applications of the present invention, the amount present may be higher and in such a preferred embodiment, the ink of the present invention comprises 10 to 80% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink.

Multifunctional (which do not include difunctional) are well known in the art and a detailed description is therefore not required. Multifunctional has its standard meaning, i.e. tri or higher, that is three or more groups, respectively, which take part in the polymerisation reaction on curing.

Suitable multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet inks include trimethylolpropane triacrylate, dipentaerythritol triacrylate, tri(propylene glycol) triacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate, and mixtures thereof. Suitable multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as trimethylolpropane trimethacrylate. Mixtures of (meth)acrylates may also be used. Bis(pentaerythritol) hexaacrylate, also known as DPHA, is particularly preferred.

Preferably, the ink of the present invention comprises 5 to 25%, more preferably 1 to 20%, most preferably 2 to 10%, by weight of a multifunctional (meth)acrylate monomer, based on the total weight of the ink. However, for some applications of the present invention, the amount present may be higher and in such a preferred embodiment, the ink of the present invention comprises 10 to 80% by weight of a multifunctional (meth)acrylate monomer, based on the total weight of the ink.

The radiation-curable monomer having two or more functional groups, based on the total weight of the ink, may have at least one vinyl ether functional group. Examples are well known in the art and include vinyl ethers such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1,4cyclohexanedimethanol divinyl ether and 2-(2-vinyloxyethoxy)ethyl acrylate, bis[4-(vinyloxy)butyl] 1,6hexanediylbiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi-4,1phenylene), bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4(vinyloxymethyl)cyclohexylmethylj glutarate, 1,4-butanediol divinyl ether, 1,4-butanediol vinyl ether, butyl vinyl ether, tert-butyl vinyl ether, 2-chloroethyl vinyl ether, 1,4-cyclohexanedimethanol divinyl ether, cyclohexyl vinyl ether, di(ethylene glycol) vinyl ether, diethyl vinyl orthoformate, dodecyl vinyl ether, ethylene glycol vinyl ether, 2-ethylhexyl vinyl ether, ethyl-1-propenyl ether, ethyl vinyl ether, isobutyl vinyl ether, phenyl vinyl ether, propyl vinyl ether, and tris[4-(vinyloxy)butyl] trimellitate.

(Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate.

Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. Monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. They therefore preferably have a viscosity of less than 150 mPas at 25°C, more preferably less than lOOmPas at 25°C and most preferably less than 20 mPas at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique 12° steel cone at 25°C with a shear rate of 25 s¹.

Preferably, the LED-curable inkjet ink of the present invention comprises a radiation-curable (i.e. polymerisable) oligomer, such as a (meth)acrylate oligomer.

The term “curable oligomer” has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 450 Da and preferably at least 600 Da (whereas monomers typically have a molecular weight below these values). The molecular weight is preferably 4,000 Da or less. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

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

Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide filmforming properties such as hardness or cure speed. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Preferred oligomers for inclusion in the ink of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique I 2° steel cone at 60°C with a shear rate of 25 s ¹.

Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The oligomer preferably comprises a polyester backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers.

Particularly preferred radiation-curable oligomers are polyester acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are di-, tri-, tetra-, penta- or hexafunctional polyester acrylates, as these yield films with good solvent resistance.

More preferably, the radiation-curable oligomer is a polyester acrylate oligomer. Such radiationcurable oligomers are commercially available as CN964A85 from Arkema and UVP6600 from Kromachem.

Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance.

In one embodiment the radiation-curable oligomer polymerises by free-radical polymerisation. Preferably, the radiation-curable oligomer cures upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film.

The total amount of the oligomer is preferably from 1-20% by weight, based on the total weight of the ink. Preferably the oligomer is present from 2-10% by weight, based on the total weight of the ink.

The ink of the present invention may further comprise an α,β-unsaturated ether monomer, which can polymerise by free-radical polymerisation and may be useful for reducing the viscosity of the ink when used in combination with one or more (meth)acrylate monomers. Examples are well known in the art and include vinyl ethers such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1,4cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether. Mixtures of α,βunsaturated ether monomers may be used.

The ink of the present invention may also include radiation-curable material, which is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The radiation-curable material according to this embodiment may comprise a mixture of cationically curable monomer and oligomer. For example, the radiation-curable material may comprise a mixture of an epoxide oligomer and an oxetane monomer.

In the embodiment where the ink comprises radiation-curable material, which polymerises by cationic polymerisation, the ink must also comprise a cationic photoinitiator.

In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulfonium or iodonium based systems. Non limiting examples include: Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380C from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa.

Preferably however, the ink of the present invention cures by free radical polymerisation only and hence the ink is substantially free of radiation-curable material, which polymerises by cationic polymerisation.

The inkjet ink of the present invention further comprises a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. The colouring agent can be any of a wide range of suitable colouring agents that would be known to the person skilled in the art. Preferably the colouring agent is a dispersible pigment, of the types known in the art and commercially available such as, for example, under the trade-names Paliotol (available from BASF pic), Cinquasia, Irgalite (both available from Ciba Speciality Chemicals) and Hostaperm (available from Clariant UK).

The pigment may be of any desired colour such as, for example, Pigment Yellow 13, Pigment Yellow 83, Pigment Yellow 120, Pigment Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used.

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

The total proportion of pigment present is preferably from 0.5 to 15% by weight, more preferably from 1 to 10% by weight, based on the total weight of the ink.

In a preferred embodiment, the ink of the present invention comprises a cyan or a magenta colouring agent, and preferably a cyan or magenta pigment. The cyan or magenta pigment is dispersed in the liquid medium of the ink and is typically in the form of a powdered cyan or magenta pigment. A preferred blue pigment is Pigment Blue 15:4. A preferred magenta pigment depends on the user requirement. In a preferred embodiment, the ink comprises 1-10% by weight of the cyan or magenta pigment, based on the total weight of the ink. As previously discussed, yellow shift is particularly problematic for light-coloured inks such as magenta and cyan inks, and cyan inks especially as cyan is at the opposite end of the colour spectrum to yellow. Therefore, in an even more preferred embodiment, the ink of the present invention comprises a cyan colouring agent, and preferably a cyan pigment. Yellow shift is however a problem for all colours of inks, including black inks and other colours of the trichromatic process printing, including yellow. In yellow inks, yellow shift changes the yellow hue of the ink.

The inkjet ink of the present invention dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. The absence of water and volatile organic solvents means that the ink does not need to be dried to remove the water/solvent. However, water and volatile organic solvents have a significant viscositylowering effect making formulation of the ink in the absence of such components significantly more challenging.

Accordingly, the inkjet ink of the present invention is preferably substantially free of water and volatile organic solvents. Preferably, the inkjet ink comprises less than 5% by weight of water and volatile organic solvent combined, preferably less than 3% by weight combined, more preferably, less than 2% by weight combined and most preferably less than 1% by weight combined, based on the total weight of the ink. Some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated.

The inks of the present invention may comprise a passive (or “inert”) thermoplastic resin. Passive resins are resins which do not enter into the curing process, i.e. the resin is free of functional groups which polymerise under the curing conditions to which the ink is exposed. In other words, resin is not a radiation-curable material. The resin may be selected from epoxy, polyester, vinyl, ketone, nitrocellulose, phenoxy or acrylate resins, or a mixture thereof and is preferably a poly(methyl (meth)acrylate) resin. The resin has a weight-average molecular weight of 15-90 KDa and preferably 25-50 KDa, as determined by GPC with polystyrene standards. Particularly preferred resins are Paraloid® A11 from Rohm and Haas and BR-113 from Dianal Resins. The resin is preferably present at 1-5% by weight, based on the total weight of the ink.

Other components of types known in the art may be present in the ink of the present invention to improve the properties or performance. These components may be, for example, additional surfactants, defoamers, dispersants, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers. In a preferred embodiment, photosensitisers are added to the ink, which are selected to absorb strongly in the desired wavelength band of UV LED radiation source and are able to transfer energy to the photoinitiators of the ink.

In a preferred embodiment, the inkjet ink of the present invention further comprises a dispersant. The dispersant is not particularly limited and the formulator is free to include any dispersant in the ink of the present invention to improve the properties or performance of the ink. This dispersant can include any dispersant readily available and known in the art in inkjet inks. A particularly preferred dispersant is Solsperse® 32000 from Lubrizol Limited.

In a preferred embodiment, the inkjet ink of the present invention further comprises a stabiliser. The stabiliser is not particularly limited and the formulator is free to include any stabiliser in the ink of the present invention to improve the properties or performance of the ink. This stabiliser can include any stabiliser readily available and known in the art in inkjet inks. A particularly preferred stabiliser is Florstab UV2 from Kromachem Limited.

In a preferred embodiment, the inkjet ink of the present invention further comprises a surfactant. The surfactant is not particularly limited and the formulator is free to include any surfactant in the ink of the present invention to improve the properties or performance of the ink. This surfactant can include any surfactant readily available and known in the art in inkjet inks. A particularly preferred surfactant is BYK-307 from BYK-Chemie GmbH.

The amounts by weight provided herein are based on the total weight of the ink.

The inkjet ink of the present invention exhibits a desirable low viscosity (200 mPas or less, preferably 100 mPas or less and more preferably 30 mPas or less at 25 °C). In a preferred embodiment, the viscosity of the inkjet ink is 10 mPas to 30 mPas at 25 °C.

In order to produce a high quality printed image a small jetted drop size is desirable. Furthermore, small droplets have a higher surface area to volume ratio when compared to larger drop sizes, which facilitates evaporation of solvent from the jetted ink. Small drop sizes therefore offer advantages in drying speed. Preferably the inkjet ink is jetted at drop sizes below 90 picolitres, preferably below 35 picolitres and most preferably below 10 picolitres.

To achieve compatibility with print heads that are capable of jetting drop sizes of 90 picolitres or less, a low viscosity ink is required. A viscosity of 15 mPas or less at 25°C is preferred, for example, 10 to 12 mPas, 18 to 20 mPas, or 24 to 26 mPas.

Ink viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00.

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

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

The present invention also provides a method of inkjet printing the inkjet ink of the present invention. Specifically, the present invention provides a method of inkjet printing comprising inkjet printing the inkjet ink of the present invention onto a substrate and curing the ink by exposing the printed ink to a UV LED radiation source. The inventors have surprisingly found that the ink of the present invention is particularly suitable as an ink which can be cured using a UV LED light source, whilst minimising yellow shift and maintaining a fast cure speed.

In the method of inkjet printing of the present invention, the inkjet ink is printed onto a substrate. Printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto a substrate, on a roll-to-roll printer or a flat-bed printer. As discussed above, inkjet printing is well known in the art and a detailed description is not required.

The ink is jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image. The substrate is not limited. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene, polypropylene, and all cellulosic materials or their mixtures/blends with the aforementioned synthetic materials.

In the method of the present invention, after inkjet printing the inkjet ink onto the substrate, the printed image is then exposed to a UV LED radiation source to cure the inkjet ink. UV LED radiation sources are also known as UV LED light sources.

UV LED light is emitted from a UV LED light source. UV LED light sources comprise one or more LEDs and are well known in the art. Thus, a detailed description is not required.

It will be understood that UV LED light sources emit radiation having a spread of wavelengths. The emission of UV LED light sources is identified by the wavelength which corresponds to the peak in the wavelength distribution. Compared to conventional mercury lamp UV sources, UV LED light sources emit UV radiation over a narrow range of wavelengths on the wavelength distribution. The width of the range of wavelengths on the wavelength distribution is called a wavelength band. LEDs therefore have a narrow wavelength output when compared to other sources of UV radiation. By a narrow wavelength band, it is meant that at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength within a wavelength band having a width of 50 nm or less, preferably, 30 nm or less, most preferably 15 nm or less.

In a preferred embodiment, at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

Preferably, the wavelength of the UV LED source substantially matches the absorption profile of the ink. In a preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 380 nm to 410 nm. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 395 nm, 400 nm or 405 nm. The ink of the present invention is preferably formulated to respond to the emission of the UV LED source.

In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of from 380 nm to 410 nm, and at least 90%, preferably at least 95%, of the radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 395 nm, 400 nm or 405 nm, and at least 90%, preferably at least 95%, of the radiation has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less.

LEDs have a longer lifetime and exhibit no change in the power/wavelength output over time. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate.

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

The exposure to UV LED light may be performed in an inert atmosphere, e.g. using a gas such as nitrogen, in order to assist curing of the ink, although this is not required to achieve full cure, including surface cure owing to the components present in the ink of the present invention.

The present invention further provides the use of the photoinitiators: bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide in combination with one or more of 2-benzyl-2-dimethylamino-1-(4morpholinophenyl)-butan-1 -one, 2-benzyl-2-dimethylamino-1 -(4-piperidinylphenyl)-butan-1 -one and 1,3,5-trimethylbenzoyl phenyl phosphinate for reducing yellow shift in an LED-curable inkjet ink. By reducing yellow shift, it is meant that the change in the degree of yellowness of the ink post-cure is reduced and hence the ink is more colour stable. Colour profiling of the cured ink image can then be carried out more efficiently.

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

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

Examples

Example 1

Inkjet inks were prepared according to the formulations set out in Table 1. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1

Components	Supplier	Ink 1	Ink 2	Comparative ink 1	Comparative ink 2	Comparative ink 3
2-PEA	Arkema	23.4	16.4	23.7	17.2	21.4
CTFA	Arkema	20	20	20	20	20
IBOA	Arkema	12	12	12	12	12
NVC	Ashland	18	18	18	18	18
DPHA	Kromachem	2	3	2	2	3
UV2	Kromachem	0.2	0.2	0.2	0.2	0.2
UVP6600	Kromachem	5	5	5	5	5
CN964A85	Arkema	4	2	5	2	3
Cyan dispersion		8.6	8.6	8.6	8.6	8.6
Speedcure ITX	Lambson			0.5	1
TPO-L	BASF					6
Speedcure XKm	Lambson		12
Omnirad 369	IGM			1
Speedcure TPO	Lambson				14
Omnirad 819	IGM	2.8	2.8	4		2.8
Omnirad 264	IGM	4

The cyan pigment dispersion of the inks of Table 1 comprises 59% PEA, 1% stabiliser, 10% dispersant and 30% blue pigment. The dispersion was prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns. Amounts are given as weight percentages based on the total weight of the dispersion.

2-PEA, CTFA, IBOA and NVC are monofunctional monomers. DPHA is a multifunctional monomer. 10 UVP6600 is a multifunctional oligoamine resin. CN964A85 is an aliphatic polyester based urethane diacrylate oligomer blended with 15% SR306, tripropylene glycol diacrylate. UV2 is a stabiliser.

Speedcure ITX, TPO-L, Speedcure XKm, Omnirad 369, Speedcure TPO, Omnirad 819 and Omnirad 264 are photoinitiators.

Inks 1 and 2 and comparative inks 1 and 3 did not contain any substances having a hazard code of H360, H361 and H362. Therefore, inks 1 and 2 and comparative inks 1 and 3 were not classified as reprotoxic. Comparative ink 2 however did contain Speedcure TPO, which has a hazard code of H361. Therefore, comparative ink 2 was classified as reprotoxic.

Example 2

Inks 1 and 2 and comparative inks 1-3 were assessed for cure speed.

Inks 1 and 2 and comparative inks 1-3 were applied to 220 micron gloss white semi-rigid PVC with a K2 bar using a RK Coater automated drawdown machine. The inks were then cured using a Jenton conveyer fitted with a Phoseon 20W LED lamp running at full power at 60m/min speed. After the first pass under the lamp, a small strip of Epson Premium Photo Paper is applied to the printed ink, with the coated side of the paper facing the printed ink, and rubbed down. Any ink removal or surface marking of the printed ink indicates an incomplete cure. The printed ink is repeatedly passed under the lamp and tested in this way until there is no ink removal or surface marking of the printed ink, indicating full cure. An ink requiring three passes or less to achieve full cure is regarded as fast curing. The results are shown in Table 2.

Table 2

	Ink 1	Ink 2	Comparative ink 1	Comparative ink 2	Comparative ink 3
Number of passes required to achieve full cure	2	2	2	1	7

Example 3

Inks 1 and 2 and comparative inks 1-3 were assessed for short term yellow shift.

Inks 1 and 2 and comparative inks 1-3 were printed and fully cured as described in Example 2. The initial b* values within one minute of curing were recorded using a spectrophotometer. The printed inks were then left for 24 hours and the b* values recorded again. The results are shown in Table 3. The difference in b* represents the degree of yellow shift. A Ab* of less than two means that the inks have an acceptable degree of yellow shift after 24 hours for important sensitive graphic applications. However, a Ab* of more than two means that the inks have an unacceptable degree of yellow shift after 24 hours for important sensitive graphic applications.

Table 3

	Ink 1	Ink 2	Comparative ink 1	Comparative ink 2	Comparative ink 3
Ab* after 24 hours	1.6	1.5	3.5	5	0.6

Example 4

An ink is prepared according to the formulation set out in Table 4. The inkjet ink formulation is prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 4

Components	Supplier	Ink 3
2-PEA	Arkema	23.4
CTFA	Arkema	20
IBOA	Arkema	12
NVC	Ashland	18
DPHA	Kromachem	2
UV2	Kromachem	0.2
UVP6600	Kromachem	5
CN964A85	Arkema	4
Cyan dispersion		8.6
Omnirad 369	IGM	4
Omnirad 819	IGM	2.8

The cyan pigment dispersion is the same as that used in Example 1.

Ink 3 is not reprotoxic. Testing would show ink 3 to have a fast cure speed and an acceptable degree of yellow shift.

Example 5

Cyan inkjet inks were prepared according to the formulations set out in Table 5. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 5

Components	Ink 4	Ink 5	Comparative ink 4	Comparative ink 5	Comparative ink 6	Comparative ink 7
2-PEA	17.4	24.4	26.4	22.4	25.9	18.2
CTFA	20	20	20	20	20	20
IBOA	12	12	12	12	12	12
NVC	18	18	18	18	18	18
DPHA	2	2	2	2	2	2
UV2	0.2	0.2	0.2	0.2	0.2	0.2
UVP6600	5	5	5	5	5	5
CN964A85	2	5	5	3	5	2
Cyan dispersion	8.6	8.6	8.6	8.6	8.6	8.6
Speedcure ITX					0.5
TPO-L				6
Speedcure XKm	12
Omnirad 369		2
Speedcure TPO						14
Omnirad 819	2.8	2.8	2.8	2.8	2.8
Omnirad 264

The suppliers of the ink components are the same as those given in Example 1. The cyan pigment dispersion is also the same as that used in Example 1.

Inks 4 and 5 and comparative inks 4-6 did not contain any substances having a hazard code of H360, H361 and H362. Therefore, these inks were not classified as reprotoxic. Comparative ink 7 however did contain Speedcure TPO, which has a hazard code of H361. Therefore, comparative ink 7 was classified as reprotoxic.

Example 6

Inks 4 and 5 and comparative inks 4-7 were assessed for cure speed using the method described in Example 2. The results are shown in Table 6. An ink requiring three passes or less to achieve full cure is regarded as fast curing.

Table 6

	Ink 4	Ink 5	Comparative ink 4	Comparative ink 5	Comparative ink 6	Comparative ink 7
Number of passes required to achieve full cure	3	3	8	8	4	2

Example 7

Inks 4 and 5 and comparative inks 4-7 were assessed for short term colour shift using the method described in Example 3. The results are shown in Table 7. A Ab* of less than two means that the inks have an acceptable degree of yellow shift after 24 hours for important sensitive graphic applications. However, a Ab* of more than two means that the inks have an unacceptable degree of yellow shift after 24 hours for important sensitive graphic applications.

Table 7

	Ink 4	Ink 5	Comparative ink 4	Comparative ink 5	Comparative ink 6	Comparative ink 7
Ab* after 24 hours	1.3	0.3	0.5	0.9	3.7	1.0

Inks 4 and 5, in contrast to comparative inks 4-7, provide the desired combination of properties, 20 namely fast cure speed, acceptable yellow shift and the ink is not reprotoxic.

Example 8

Cyan inkjet inks were prepared according to the formulations set out in Table 8. The inkjet ink 25 formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 8

Components	Ink 6	Ink 7	Ink 8	Comparative ink 8	Comparative ink 9	Comparative ink 10
2-PEA	30.4	26.9	31.9	33.4	28.2	28.4
CTFA	20	20	20	20	20	20
IBOA	12	12	12	12	12	12
NVC	18	18	18	18	18	18
UV1	0.1	0.1	0.1	0.1	0.1	0.1
UVP6600	5	5	5	5	5	5
CN964A85	6.5	2	7	7	3	6
Cyan dispersion	1.2	1.2	1.2	1.2	1.2	1.2
Speedcure ITX				0.5		0.5
TPO-L						6
Speedcure XKm		12
Omnirad 369			2
Speedcure TPO					12
Omnirad 819	2.8	2.8	2.8	2.8	0.5	2.8
Omnirad 264	4

UV1 is a stabiliser from Kromachem. The suppliers of the other ink components are the same as those given in Example 1. The cyan pigment dispersion is also the same as that used in Example 1.

Inks 6-8 and comparative inks 8 and 10 did not contain any substances having a hazard code of H360, H361 and H362. Therefore, these inks were not classified as reprotoxic. Comparative ink 9 however did contain Speedcure TPO, which has a hazard code of H361. Therefore, comparative ink 9 was classified as reprotoxic.

Example 9

Inks 6-8 and comparative inks 8-10 were assessed for cure speed using the method described in Example 2. The results are shown in Table 9. An ink requiring three passes or less to achieve full cure is regarded as fast curing.

Table 9

	Ink 6	Ink 7	Ink 8	Comparative ink 8	Comparative ink 9	Comparative ink 10
Number of passes required to achieve full cure	3	3	3	9	2	5

Example 10

Inks 6-8 and comparative inks 8-10 were assessed for short term colour shift using the method described in Example 3. The results are shown in Table 10. A Ab* of less than two means that the inks have an acceptable degree of yellow shift after 24 hours for important sensitive graphic applications. However, a Ab* of more than two means that the inks have an unacceptable degree of yellow shift after 24 hours for important sensitive graphic applications.

Table 10

	Ink 6	Ink 7	Ink 8	Comparative ink 8	Comparative ink 9	Comparative ink 10
Ab* after 24 hours	0.7	1.8	0.5	5.9	1.9	2.1

Inks 6-8, in contrast to comparative inks 8-10, provide the desired combination of properties, namely fast cure speed, acceptable yellow shift and the ink is not reprotoxic.

Example 11 15

Magenta inkjet inks were prepared according to the formulations set out in Table 11. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 11

Components	Ink 9	Ink 10	Ink 11	Comparative ink 11	Comparative ink 12	Comparative ink 13
2-PEA	28.4	24.9	30.4	31.4	27.2	28.4
CTFA	20	20	20	20	20	20
IBOA	12	12	12	12	12	12
NVC	18	18	18	18	18	18
UV1	0.1	0.1	0.1	0.1	0.1	0.1
UVP6600	5	5	5	5	5	5
CN964A85	6.5	2	7	7	2	6.5
Magenta dispersion	3.2	3.2	3.2	3.2	3.2	3.2
Speedcure ITX				0.5
TPO-L						4
Speedcure XKm		12
Omnirad 369			1.5
Speedcure TPO					12
Omnirad 819	2.8	2.8	2.8	2.8	0.5	2.8
Omnirad 264	4

The suppliers of the ink components are the same as those given in Example 8.

The magenta pigment dispersion of the inks ofTable 11 comprises 56.5% PEA, 1.5% stabiliser, 12% dispersant and 30% magenta pigment. The dispersion was prepared by mixing the components in the given amounts and passing the mixture through a bead mill until the dispersion had a particle size of less than 0.3 microns. Amounts are given as weight percentages based on the total weight of the dispersion.

Inks 9-11 and comparative inks 11 and 13 did not contain any substances having a hazard code of H360, H361 and H362. Therefore, these inks were not classified as reprotoxic. Comparative ink 12 however did contain Speedcure TPO, which has a hazard code of H361. Therefore, comparative ink 12 was classified as reprotoxic.

Example 12

Inks 9-11 and comparative inks 11-13 were assessed for cure speed using the method described in Example 2. The results are shown in Table 12. An ink requiring three passes or less to achieve full cure is regarded as fast curing.

Table 12

	Ink 9	Ink 10	Ink 11	Comparative ink 11	Comparative ink 12	Comparative ink 13
Number of passes required to achieve full cure	3	3	3	1	2	5

Example 13

Inks 9-11 and comparative inks 11-13 were assessed for short term colour shift using the method described in Example 3. The results are shown in Table 13. A Ab* of less than two means that the inks have an acceptable degree of yellow shift after 24 hours for important sensitive graphic applications. However, a Ab* of more than two means that the inks have an unacceptable degree of yellow shift after 24 hours for important sensitive graphic applications.

Table 13

	Ink 9	Ink 10	Ink 11	Comparative ink 11	Comparative ink 12	Comparative ink 13
Ab* after 24 hours	0.9	1.1	0.7	5.9	0.8	1.0

Inks 9-11, in contrast to comparative inks 11-13, provide the desired combination of properties, namely fast cure speed, acceptable yellow shift and the ink is not reprotoxic.

Claims (15)

Claims

1. An LED-curable inkjet ink comprising, as photoinitiators, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide in combination with one or more of 2-benzyl-2-dimethylamino-1-(4morpholinophenyl)-butan-1 -one, 2-benzyl-2-dimethylamino-1 -(4-piperidinylphenyl)-butan-1 -one and 1,3,5-trimethylbenzoyl phenyl phosphinate, wherein the ink comprises less than 0.5% by weight of isopropyl thioxanthone, based on the total weight of the ink.

2. An LED-curable inkjet ink as claimed in claim 1, wherein the ink is free of isopropyl thioxanthone.

3. An LED-curable inkjet ink as claimed in claim 1 or 2, wherein the only photoinitiators present in the ink are bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide in combination with one or more of 2benzyl-2-dimethylamino-1 -(4-morpholinophenyl)-butan-1 -one, 2-benzyl-2-dimethylamino-1 -(4piperidinylphenyl)-butan-1-one and 1,3,5-trimethylbenzoyl phenyl phosphinate.

4. An LED-curable inkjet ink as claimed in any preceding claim, wherein the ink comprises radiation-curable material.

5. An LED-curable inkjet ink as claimed in claim 4, wherein the radiation-curable material comprises one or more monomers.

6. An LED-curable inkjet ink as claimed in claim 5, wherein the one or more monomers comprise one or more monofunctional monomers.

7. An LED-curable inkjet ink as claimed in claim 6, wherein the one or more monofunctional monomers comprise one or more monofunctional (meth)acrylate monomers in combination with an Nvinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer.

8. An LED-curable inkjet ink as claimed in claim 6 or 7, wherein the one or more monofunctional monomers comprise two or more monofunctional (meth)acrylate monomers in combination with an Nvinyl amide monomer, an N-acryloyl amine monomer and/or an N-vinyl carbamate monomer.

9. An LED-curable inkjet ink as claimed in claims 5 to 8, wherein the one or more monomers comprise one or more monomers having two or more functional groups.

10. An LED-curable inkjet ink as claimed in claims 4 to 9, wherein the radiation-curable material comprises an oligomer.

11. An LED-curable inkjet ink as claimed in any preceding claim, further comprising a colouring agent.

12. An LED-curable inkjet ink as claimed in claim 11, wherein the colouring agent is a dispersed pigment.

5

13. An LED-curable inkjet ink as claimed in claim 11 or 12, wherein the colouring agent is cyan or magenta.

14. A method of inkjet printing comprising inkjet printing the inkjet ink as claimed in any preceding claim onto a substrate and curing the ink by exposing the printed ink to a UV LED radiation source.

15. Use of the photoinitiators: bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide in combination with one or more of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one, 2-benzyl-2dimethylamino-1-(4-piperidinylphenyl)-butan-1-one and 1,3,5-trimethylbenzoyl phenyl phosphinate for reducing yellow shift in an LED-curable inkjet ink.

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Application No: GB1806622.5