GB2561984A

GB2561984A - Printing ink

Info

Publication number: GB2561984A
Application number: GB1806621.7A
Authority: GB
Inventors: Phillips Andrew
Original assignee: Fujifilm Speciality Ink Systems Ltd
Current assignee: Fujifilm Speciality Ink Systems Ltd
Priority date: 2017-04-24
Filing date: 2018-04-24
Publication date: 2018-10-31
Anticipated expiration: 2038-04-24
Also published as: GB201806621D0; GB2561984B

Abstract

An LED-curable inkjet ink comprises N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone, phenoxyethyl acrylate, isobornyl acrylate, and compound (I), i.e. 1,3-di({α-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({α-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl)propane. Preferably, the ink comprises 5-30 wt.% N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone, 10-40 wt.% phenoxyethyl acrylate, and 5-40 wt.% isobornyl acrylate. The ink may contain one or more difunctional monomers, such as 1,10-decanediol diacrylate. Additional photoinitiators may also be included, especially a phosphine oxide photoinitiator like 2,4,6-trimethylbenzoyl diphenylphosphine oxide (TPO). Typically, the ink does not contain isopropyl thioxanthone (ITX). A colouring agent may be present, e.g. a dispersed cyan pigment. A printing method comprises inkjet printing an ink onto a substrate and curing the printed ink by exposure to a UV LED radiation source in the absence of an oxygen-deficient atmosphere, wherein the ink comprises 5-30 wt.% N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone and compound (I). Use of compound (I) to reduce colour shift, especially yellow shift, in an LED-curable inkjet ink is also disclosed. Compound (I)

Description

(54) Title of the Invention: Printing ink Abstract Title: LED-curable inkjet ink (57) An LED-curable inkjet ink comprises N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone, phenoxyethyl acrylate, isobornyl acrylate, and compound (I), i.e. 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy (1-methylethylene)]}oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]} oxymethyl)propane. Preferably, the ink comprises 5-30 wt.% N-vinyl caprolactam or N-vinyl-5-methyl-2oxazolidinone, 10-40 wt.% phenoxyethyl acrylate, and 5-40 wt.% isobornyl acrylate. The ink may contain one or more difunctional monomers, such as 1,10-decanediol diacrylate. Additional photoinitiators may also be included, especially a phosphine oxide photoinitiator like 2,4,6-trimethylbenzoyl diphenylphosphine oxide (TPO). Typically, the ink does not contain isopropyl thioxanthone (ITX). A colouring agent may be present, e.g. a dispersed cyan pigment. A printing method comprises inkjet printing an ink onto a substrate and curing the printed ink by exposure to a UV LED radiation source in the absence of an oxygen-deficient atmosphere, wherein the ink comprises 5-30 wt.% N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone and compound (I). Use of compound (I) to reduce colour shift, especially yellow shift, in an LED-curable inkjet ink is also disclosed.

Compound (I)

GB 2561984 A continuation (74) Agent and/or Address for Service:

Elkington and Fife LLP

Thavies Inn House, 3-4 Holborn Circus, London, EC1N 2HA, United Kingdom

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Printing ink

The present invention relates to a printing ink and in particular, an LED-curable inkjet ink which has a reduced colour shift. 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. Amongst other considerations, monomers to include in radiation-curable inks are selected based on the film-forming properties that they confer to the ink. However, such monomers may cause problems with colour shift of the inks post-cure and in particular, problems with yellow shift of the inks post-cure. This is particularly problematic in white and colourless inks where yellowing is most visible. Colour shift and yellow shift is discussed hereinbelow. One way to overcome this problem is to minimise the amount of any problematic monomers that are present but as a result, the film-forming properties of the inks suffer.

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. In particular, when compared to, for example mercury discharge lamps (the most common UV light source used to cure inkjet inks), LEDs offer significant cost reduction, longer maintenance intervals, higher energy efficiency and are an environmentally friendlier solution. However, there are a number of challenges when utilising LED UV light sources as the radiation source.

When LEDs are used, it is necessary to use an array of multiple LEDs in order to generate enough power to provide thorough curing of the ink. In fact, even with an array of multiple LEDs, inks which are cured by LEDs are prone to poor surface cure owing to the presence of oxygen in the atmosphere adjacent to the ink surface, and the spectral output of LEDs. 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. LEDs emit radiation in the UVA region having a long wavelength over a narrow range of wavelengths, which although suitable for depth cure, provides a particular challenge for effective surface cure in normal atmospheric conditions.

Poor surface cure can be reduced in a number of ways. First, it can be overcome by blanketing the irradiated area with an inert gas such as nitrogen during the cure process but this adds considerably to the complexity and cost of the printer. Secondly, it can be overcome by working the LEDs at high power, providing more energy to react with the photoinitiators and oxygen, or the total amount of photoinitiators may be increased, thus allowing more radicals to be generated. However, working the LEDs at high power generates heat, which must be removed and both approaches add to the cost and complexity of the process - regard must be had to limitations on the media range, power consumption, ink viscosity and ink stability. Thirdly, a blend of photoinitiators may be used, including photoinitiators that function throughout the ink resulting in through cure and those that work at the surface of the ink, to obtain adequate surface cure. For example, a type II photoinitiator such as ITX (and hydrogen donating species) in conjunction with type I photoinitiators such as phosphine oxides may be used. Reduced oxygen inhibition has been reported using type II photoinitiators owing to the presence of hydrogen donors which can decrease the concentration of molecular oxygen or react with generated peroxy radicals to reinitiate the polymerisation reaction (see the paper entitled “Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization” by Samuel Clark Ligon et al in Chemical Reviews, 114(1), pp. 557-589 (2014)). Further, ITX can absorb radiation at longer wavelengths, as well as generating its own radicals, absorbing light and using this energy to sensitise further type I photoinitiators, which act at shorter wavelengths.

However, such blends of photoinitiators in the LED-curable inkjet inks suffer from colour shift postcure. In particular, LED-curable inkjet inks often require a photoinitiator blend comprising ITX, which is a photoinitiator that works at the surface of the ink film during curing, and results in adequate surface cure. However, the inclusion of ITX in LED-curable inkjet inks increases the problem with colour shift post-cure and particularly the problem with yellow shift post-cure. Removal of ITX from the blend of photoinitiators reduces the colour shift post-cure issue but results in poor surface cure.

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

This is problematic for all colours as any change and instability of colour in the ink causes problems. However, this is particularly problematic for cyan inks, which is the opposite colour on the colour spectrum to yellow. Practically, colour 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 colour to stabilise in the cured ink image before colour profiling.

By colour shift post-cure, it is meant that colour shift of the ink is only assessed post-cure, i.e. the L*a*b* values are only recorded post-cure, using a spectrophotometer. However, the L*a*b* values of the ink are also changing during and after printing. In this regard, the ink can be thought of as having initial L*a*b* values just before printing, which then change during and after printing, and during and after curing. The first L*a*b* values (L/, a/ and b/) are recorded typically within one minute of curing. The second L*a*b* values (L₂*, a₂* and b₂*) are then recorded typically 24 hours later. The formula for calculating delta E is as follows:

Δ£ ^::::: y α·>

Ϊ.

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, colour 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.

In addition to reducing colour shift post-cure and maintaining the required surface cure, it is of course also necessary to maintain the necessary film-forming properties of the inkjet inks, such as acceptable gamut, gloss, print quality, balance of adhesion, blocking resistance, film toughness, low embrittlement, the correct balance of surface tension, good surface wetting, without excessive ink bleed/spread, in order to produce a good quality image.

There is therefore a need in the art for an LED-curable inkjet ink which has reduced colour shift postcure, maintains the required surface cure and has the necessary film-forming properties of an inkjet ink.

Accordingly, the present invention provides an LED-curable inkjet ink comprising: N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone; phenoxyethyl acrylate; isobornyl acrylate; and

The inventors have surprisingly found that an LED-curable inkjet ink that comprises the specific blend of monomers and the specific polymeric thioxanthone-type photoinitiator as claimed has reduced colour shift in the cured ink image, maintains the required surface cure of the cured ink image and has the necessary film-forming properties. In particular, it has been found that the use of the claimed photoinitiator in an LED-curable inkjet ink reduces the level of colour shift in the cured ink image to an acceptable level over 24 hours and maintains surface cure and film-forming properties. 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 this specific photoinitiator in LED-curable inkjet inks reduces the colour shift in the cured ink image to an acceptable level (wherein ΔΕ is an absolute value from 0.0 to 5.0, preferably from 0.0 to 3.5 and more preferably from 0.0 to 2.0, over 24 hours) whilst maintaining film-forming properties and surface cure. An absolute value is the magnitude of a real number without regard to its sign. The first L*a*b* values (L/, a/ and b/) are recorded within one minute of curing. The second L*a*b* values (L₂*, a₂* and b₂*) are then recorded 24 hours later. Delta E is calculated using the formula hereinabove. Delta E values vary with actinic radiation dose, so the delta E 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 delta E of 5.0 is the largest acceptable delta E absolute value for any application of the present invention, a delta E of 3.5 is the largest acceptable delta E absolute value for moderately sensitive applications of the present invention and a delta E of 2.0 is the largest acceptable delta E absolute value for important sensitive graphic applications of the present invention. Therefore, the absolute value of delta E acceptable will depend on the ultimate application of the cured ink image of the present invention. This is in marked contrast to other known LED-curable inkjet inks which have a much higher colour shift, and in particular, much more than the acceptable delta E absolute value of the present invention over 24 hours when achieving acceptable surface cure.

As discussed above, colour shift is known in the art. It is when the colour of the ink changes over a period of time, typically measured over 24 hours. The larger delta E on the CIELAB (L*a*b*) colour space system, the larger change in colour. Delta E is therefore the measure of how far the colour has changed over time, typically over 24 hours. The lightness, L*, represents the darkest black at L*=0, and the brightest white at Z_*=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 total shift along the b* axis is denoted as Ab* and is represented by delta E. Hence, yellow shift occurs when the colour of the ink shifts towards positive or negative values on the b* axis in the yellow quadrant, and therefore becomes increasingly ordecreasingly yellow respectively.

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

The present invention also provides a method of inkjet printing comprising:

(i) inkjet printing an LED-curable inkjet ink onto a substrate, wherein the ink comprises 5-30% by weight of N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone, based on the total weight of the ink, and

(ii) curing the ink by exposing the printed ink to a UV LED radiation source in the absence of an oxygen-deficient atmosphere.

It has further surprisingly been found that the required surface cure of the cured ink image can be achieved using LEDs in the absence of an oxygen-deficient atmosphere, e.g. air, without recourse to one of the options discussed above to achieve adequate surface cure. For example, when using the method of the present invention, it is not necessary to blanket the irradiated area with an inert gas such as nitrogen during the cure process, work the LEDs at high power, provide more energy to react with the photoinitiators and oxygen, increase the total amount of photoinitiators, or use a blend of photoinitiators including ITX, in order to achieve the required surface cure. Thus, the method of the present invention minimises the complexity and cost of the printer required when using LEDs, whilst achieving a cured ink image with reduced colour shift and the required surface cure and film-forming properties.

The present invention further provides the use of a photoinitiator according to the following chemical formula for reducing colour shift in an LED-curable inkjet ink:

Cl ο

The present invention will now be described with reference to the drawings, in which Fig. 1 shows < graph showing colour shift over 24 hours.

The LED-curable inkjet ink of the present invention comprises: N-vinyl caprolactam or N-vinyl-5 methyl-2-oxazolidinone; phenoxyethyl acrylate; isobornyl acrylate; and

The LED-curable inkjet ink comprises:

This is a polymeric thioxanthone-type photoinitiator and is also known by the following chemical 5 name: 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]} oxy)-2,2bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane.

This photoinitiator which is utilised in the LED-curable inkjet ink of the present invention is available commercially as Speedcure 701OL® sold by Lambson®. Speedcure 7010L® additionally contains 2[2,2-bis(2-prop-2-enoyloxyethoxymethyl)butoxy]ethyl prop-2-enoate having the CAS number 2896110 43-5.

Speedcure 701 OL® is a liquid at 20°C and is a solution of 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4yl)oxy]acetylpoly[oxy(1-methylethylene)]} oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane in 2-[2,2-bis(2-prop-215 enoyloxyethoxymethyl)butoxy]ethyl prop-2-enoate.

In a preferred embodiment, the ink of the present invention comprises 0.5 to 15% by weight, preferably 1 to 5% by weight, more preferably 2 to 3% by weight of 1,3-di({a-[1-chloro-9-oxo-9Hthioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({a-[1-chloro-9-oxo-9H20 thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane, based on the total weight of the ink.

The inkjet ink of the present invention is an LED-curable inkjet ink. It therefore comprises radiation curable-material. 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.

The LED-curable inkjet ink of the invention comprises radiation-curable monomers. As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers.

The LED-curable inkjet ink of the invention comprises: monofunctional monomers N-vinyl caprolactam (NVC) or N-vinyl-5-methyl-2-oxazolidinone (NVMO); phenoxyethyl acrylate (PEA); and isobornyl acrylate (IBOA).

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 LED-curable inkjet ink of the invention comprises monofunctional (meth)acrylate monomers PEA and IBOA. Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required. PEA and IBOA have the following chemical structures:

Phenoxyethyl acrylate (PEA) mol wt 192 g/mol

Isobornyl acrylate (IBOA) mol wt 208 g/mol

Preferably, PEA is present in 10-40% by weight, more preferably 15-35% by weight, based on the total weight of the ink. Preferably, IBOA is present in 5-40% by weight, more preferably 5-30% by weight, based on the total weight of the ink.

The LED-curable inkjet ink of the invention also comprises N-vinyl amide monomer NVC or N-vinyl carbamate monomer NVMO. In one embodiment, the LED-curable inkjet ink of the invention comprises NVC. In another embodiment, the LED-curable inkjet ink of the invention comprises NVMO.

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.

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).

NVMO is an N-vinyl oxazolidinone. N-Vinyl oxazolidinones have the following structure:

0'

N

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 alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C_e.₁₀ aryl- or C₃.₁₈ cycloalkyl-substituted C^s 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.

NVMO 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, NVMO is a racemate. In another embodiment, NVMO is (/?)-5-methyl-3-vinyl-1,3-oxazolidin-2-one. Alternatively, NVMO is (S)5-methyl-3-vinyl-1,3-oxazolidin-2-one.

Preferably, NVC or NVMO is present in 5-30% by weight, more preferably 10-25% by weight, based on the total weight of the ink. In a preferred embodiment, PEA, IBOA and NVC/NVMO are each present in their preferred amounts.

NVC/NVMO, PEA and IBOA provide the ink with the required film-forming properties such as acceptable gamut, gloss, print quality, balance of adhesion, blocking resistance, film toughness, low embrittlement, the correct balance of surface tension, good surface wetting, without excessive ink bleed/spread, in order to produce a good quality image.

However, NVC and NVMO in particular are known to cause colour shift post-cure and particularly yellow shift post-cure. This is particularly problematic in white and colourless inks where yellowing is most visible. In the present invention, it has surprisingly been found that the inclusion of the specific polymeric thioxanthone-type photoinitiator as claimed into an inkjet ink containing NVC/NVMO achieves a reduction in colour shift and maintains the required film-forming properties.

The LED-curable inkjet ink of the invention may also comprise additional radiation-curable material, other than NVC/NVMO, PEA and IBOA. This additional 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. This radiation-curable material can include any radiation-curable material readily available and known in the art in inkjet inks, other than NVC/NVMO, PEA and IBOA.

The amount of additional 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 total amount of radiation-curable material, including NVC/NVMO, PEA and IBOA, present in the ink of the present invention is 20 to 80% by weight, based on the total weight of the ink.

The LED-curable inkjet ink of the invention may also comprise additional monofunctional monomers, other than NVC/NVMO, PEA and IBOA. The substituents of the additional 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 Cpw alkyl, C₃.₁₈ cycloalkyl, C_e.₁₀ aryl and combinations thereof, such as C_e.₁₀aryl- or C₃.₁₈ cycloalkyl-substituted C^s 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 total amount of monofunctional monomers, including NVC/NVMO, PEA and IBOA, present in the ink of the present invention is 20 to 80% by weight, preferably 30 to 60% by weight, based on the total weight of the ink.

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

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

o

o.

o

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

o

,O

Cyclic TMP formal acrylate (CTFA) mol wt 200 g/mol o

Tetrahydrofurfuryl acrylate (THFA) mol wt 156 g/mol

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

O

Octadecyl acrylate (ODA) mol wt 200 g/mol

Tridecyl acrylate (TDA) mol 254 g/mol

O

O

Isodecyl acrylate (IDA) mol wt 212 g/mol

Lauryl acrylate mol wt 240 g/mol

In a preferred embodiment, the total amount of monofunctional (meth)acrylate monomers, including PEA and IBOA, present in the ink of the present invention is 20 to 80% by weight, preferably 25 to 50% by weight, based on the total weight of the ink.

Preferably, the LED-curable inkjet ink of the present invention comprises one or more additional monofunctional (meth)acrylate monomers, other than NVC/NVMO, PEA and IBOA, selected from Medol-10 and CTFA.

The LED-curable inkjet ink of the invention may comprise an N-vinyl amide monomer other than NVC, an N-vinyl carbamate monomer other than NVMO and/or an N-acryloyl amine monomer. A preferred example of an additional N-vinyl amide monomer otherthan NVC is N-vinyl pyrrolidone (NVP).

Like N-vinyl amide monomers, 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).

Preferably, the LED-curable inkjet ink of the present invention comprises 5-30% by weight of an Nvinyl amide monomer otherthan NVC, more preferably 10-30% by weight, most preferably 10-25% by weight, based on the total weight of the ink.

Preferably, the LED-curable inkjet ink of the present invention comprises 5-30% by weight of an Nvinyl carbamate monomer other than NVMO, more preferably 10-30% by weight, most preferably 1025% by weight, based on the total weight of the ink.

Preferably, the LED-curable inkjet ink of the present invention comprises 5-30% by weight of an Nacryloyl amine monomer, more preferably 10-30% by weight, most preferably 10-25% by weight, based on the total weight of the ink.

Preferably, the total amount of N-vinyl amide monomers, N-vinyl carbamate monomers and/or Nacryloyl amine monomers, including NVC/NVMO, present in the ink of the present invention is 5-30% by weight, more preferably 10-30% by weight, most preferably 10-25% by weight, 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 radiation-curable monomers having two or more functional groups. The one or more radiation-curable monomers having two or more functional groups further provide the ink with good film-forming properties.

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 one or more difunctional monomers.

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^s 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 5 to 35% by weight of radiationcurable 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. In a preferred embodiment, the inkjet ink of the present invention comprises one or more difunctional (meth)acrylate monomers.

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 dimethacrylate 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. In a particularly preferred embodiment, the one or more difunctional (meth)acrylate monomers comprises 1,10-decanediol diacrylate (DDDA).

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.

Preferably, the ink of the present invention comprises 5 to 25% 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)cyclohexylmethyl] 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 100mPas 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.

The ink of the present invention may further comprise 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 and preferably at least 600 (whereas monomers typically have a molecular weight below these values). The molecular weight is preferably 4,000 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 / 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 an amine-modified polyester acrylate oligomer. Such a radiation-curable oligomer is commercially available as Ebecryl 80.

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 1-15% by weight, based on the total weight of the ink. Preferably the oligomer is present from 2-5% 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 is LED-curable and comprises one or more photo initiators and in particular, the specific polymeric thioxanthone-type photoinitiator as claimed, namely 1,3-di({a-[1 chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({a-[1-chloro-9oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane. In a preferred embodiment, the inkjet ink of the present invention comprises one or more additional photoinitiators. In other words, the inkjet ink of the present invention comprises 1,3-di({a-[1-chloro-9-oxo-9Hthioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({a-[1-chloro-9-oxo-9H18 thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane and one or more further photoinitiators.

Preferably, the ink of the present invention preferably comprises one or more additional free radical photoinitiators. Free radical photoinitiators can be selected from any of those known in the art. For example, benzophenone, 1-hydroxycyclohexyl phenyl ketone, 1-[4-(2-hydroxyethoxy)-phenyl]-2hydroxy-2-methyl-1-propane-1-one, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzil dimethylketal, 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide and bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure, Darocurand Lucirin (from BASF).

In a preferred embodiment, the additional one or more photoinitiators present in the ink of the present invention is tailored for UV LED light. By tailored for UV LED light, it is meant that the photoinitiators absorb the radiation which is emitted by the UV LED light source. Preferably, the one or more additional photoinitiators present in the ink of the present invention absorbs radiation in a region of from 360 nm to 410 nm and absorbs sufficient radiation to cure the ink within a 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less bandwidth.

In a preferred embodiment, the one or more additional photoinitiators comprises a phosphine oxide photoinitiator, such as TPO and BAPO. In a particularly preferred embodiment, the one or more additional photoinitiators comprises 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO). In one embodiment, the LED-curable inkjet ink of the present invention comprises a photoinitiator package consisting of 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1methylethylene)]}oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1methylethylene)]}oxymethyl) propane and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide.

In a preferred embodiment, the inkjet ink of the present invention is substantially free of isopropyl thioxanthone. Preferably, the inkjet ink comprises less than 2% by weight of isopropyl thioxanthone, preferably less than 1% by weight of isopropyl thioxanthone, based on the total weight of the ink. As discussed above, the inclusion of isopropyl thioxanthone helps to achieve improved surface cure but results in an increased yellow shift. Replacing isopropyl thioxanthone with the claimed photoinitiator has surprisingly been found to maintain surface cure whilst reducing yellow shift.

In a preferred embodiment, the amount of one or more phosphine oxide photoinitiator present in the ink is 1 to 20% by weight, based on the total weight of the ink.

In a further preferred embodiment, the inkjet ink comprises two or more additional photoinitiators.

Preferably, the total amount of photoinitiator, including the polymeric thioxanthone type photoinitiator, namely 1,3-di({a-[1 -chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1 -methylethylene)]} oxy)-2,2bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yl)oxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane, present in the ink of the present invention is 1-20% by weight, based on the total weight of the ink.

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 colouring agent, and preferably a cyan pigment. The cyan pigment is dispersed in the liquid medium of the ink and is typically in the form of a powdered cyan pigment. A preferred blue pigment is Heliogen Blue 7110 F available from BASF. In a preferred embodiment, the ink comprises 1-10% by weight of the cyan pigment, based on the total weight of the ink. As previously discussed, yellow shift is particularly problematic for cyan inks, as cyan is at the opposite end of the colour spectrum to yellow. Yellow shift is however a problem for all colours of inks, including black inks and other colours of the trichromatic process printing, including magenta and 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 viscosity20 lowering 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 70-200 KDa and preferably 100-150 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 UV-12 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 30 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 comprising:

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(ii) curing the ink by exposing the printed ink to a UV LED radiation source in the absence of an oxygen-deficient atmosphere. 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 colour shift and maintaining surface cure and film-forming properties, without recourse to curing in an oxygen-deficient atmosphere.

In the method of inkjet printing of the present invention, an inkjet ink is printed onto a substrate. The inkjet ink used in the method of the present invention comprises 5-30% by weight of N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone, based on the total weight of the ink. As discussed above, NVC and NVMO in particular are known to cause colour shift post-cure and particularly yellow shift post-cure. This is particularly problematic in white and colourless inks where yellowing is most visible. In the present invention, it has surprisingly been found that the inclusion of the specific polymeric thioxanthone-type photoinitiator as claimed into an inkjet ink containing NVC/NVMO achieves a reduction in colour shift and maintains the required film-forming properties.

Preferably, the inkjet ink used in the method of the present invention comprises 10-25% by weight of NVC or NVMO, based on the total weight of the ink.

The inkjet ink used in the method of the present invention may also comprise any of the components discussed above for the inkjet ink of the present invention.

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 360 nm to 410 nm. In a particularly preferred embodiment, the wavelength distribution of the UV LED light peaks at a wavelength of around 365 nm, 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 360 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 365 nm, 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 is performed in the absence of an oxygen-deficient atmosphere. An oxygen-deficient atmosphere is one having less oxygen than atmospheric air. For example, oxygen may be replaced with inert gases, such as carbon dioxide, nitrogen or argon. In the method of the present invention, an oxygen-deficient atmosphere is not required to achieve full cure, including surface cure owing to the components present in the ink of the present invention. This minimises the complexity and cost of the printer required. Preferably, the exposure to UV LED light is performed in air.

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Cl O n = 1-20

By reducing colour shift, it is meant that the change in the degree of colour shift 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	Ink 1	Ink 2	Comparative ink 1	Comparative ink 2
PEA	29.86	30.86	31.86	24.68
IBOA	11.90	11.90	11.90	11.90
DDDA	20.00	20.00	20.00	20.00
NVC	16.50	16.50	16.50	16.50
UV12	0.40	0.40	0.40	0.50
CN964 A85				5.00
BR113	1.34	1.34	1.34
Cyan Dispersion	6.00	6.00	6.00	6.00
ITX			1.00	0.80
Speedcure 7010L	3.00	2.00
Speedcure EDB				0.85
Benzophenone				2.88
Irgacure 184				1.88
TPO	10.00	10.00	10.00	8.01
Byk 307	1.00	1.00	1.00	1.00

PEA, IBOA and NVC are monofunctional monomers. DDDA is a difunctional monomer. UV 12 is a stabiliser. CN964A85 is an aliphatic polyester based urethane diacrylate oligomer blended with 15%

SR306, tripropylene glycol diacrylate. BR113 is a passive resin. ITX, Speedcure 7010L, Speedcure EDB, benzophenone, Irgacure 184 and TPO are photoinitiators. Byk307 is a surfactant.

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.

Example 2

Inks 1 and 2, and comparative inks 1 and 2 were drawn down onto self-adhesive PVC. The inks were then cured using a Phoseon 20W LED lamp set at 40% power at 20 m/min speed, passed twice. Colour shift was then measured on a photospectrometer for L*, a* and b* values over a period of 24 hours and the delta E values found. The principal change in the L*, a* and b* values is on the b* axis in the yellow quadrant and therefore, the delta E values are an indicative measure of Ab*. All inks had adequate surface cure post-cure. The results of the colour shift of the inks are set out in Table 2.

Table 2

		Ink 1	Ink 2	Comparative ink 1	Comparative ink 2
Number of passes until tack-free film achieved		8	10	9	8
Dose		888.8 mJ/cnT	1111 mJ/ cnT	999.9 mJ/cnT	640 mJ/crrY
Delta E	30 mins	2.04	1.74	6.15	2.27
	1 hour	2.74	1.83	10.22	3.76
	24 hours	3.22	2.55	10.89	7.36

The results are also set out in Figure 1. The target delta E absolute value for an acceptable colour shift is from 0.0 to 5.0 after 24 hours, preferably from 0.0 to 3.5 and more preferably from 0.0 to 2.0, depending on the application of the present invention. Figure 1 shows that inks 1 and 2 both have an acceptable colour shift after 24 hours. This is in marked contrast to comparative inks 1 and 2, which have a colour shift much higher than the target, 10.89 and 7.36, respectively after 24 hours.

Inks 1 and 2 also had good film-forming properties.

Example 3

Inkjet inks are prepared according to the formulations set out in Table 3. The inkjet ink formulations are prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 3

Components	Ink 3	Ink 4
PEA	29.86	30.86
IBOA	11.90	11.90
DDDA	20.00	20.00
NVMO	16.50	16.50
UV12	0.40	0.40
CN964 A85
BR113	1.34	1.34
Cyan Dispersion	6.00	6.00
ITX
Speedcure 7010L	3.00	2.00
Speedcure EDB
Benzophenone
Irgacure 184
TPO	10.00	10.00
Byk 307	1.00	1.00

NVMO is a monofunctional monomer. The cyan pigment dispersion of the inks of Table 3 is the same 10 as the dispersion used in the inks of Table 1.

Example 4

Inks 3 and 4 are printed and cured as in Example 2. The colour shift after 24 hours for inks 3 and 4 is 15 acceptable. The film-forming properties for inks 3 and 4 are also good.

Claims

1. An LED-curable inkjet ink comprising: N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone; phenoxyethyl acrylate; isobornyl acrylate; and

2. An LED-curable inkjet ink as claimed in claim 1, further comprising one or more radiation-curable monomers having two or more functional groups.

10

3. An LED-curable inkjet ink as claimed in claim 2, wherein the one or more radiation-curable monomers having two or more functional groups comprises one or more difunctional monomers.

4. An LED-curable inkjet ink as claimed in claim 3, wherein the one or more difunctional monomers comprises one or more difunctional (meth)acrylate monomers.

5. An LED-curable inkjet ink as claimed in claim 4, wherein the one or more difunctional (meth)acrylate monomers comprises 1,10-decanediol diacrylate.

6. An LED-curable inkjet ink as claimed in any preceding claim, wherein N-vinyl caprolactam or N20 vinyl-5-methyl-2-oxazolidinone is present in 5-30% by weight, based on the total weight of the ink.

7. An LED-curable inkjet ink as claimed in any preceding claim, wherein phenoxyethyl acrylate is present in 10-40% by weight, based on the total weight of the ink.

8. An LED-curable inkjet ink as claimed in any preceding claim, wherein isobornyl acrylate is present in 5-40% by weight, based on the total weight of the ink.

9. An LED-curable inkjet ink as claimed in any preceding claim, further comprising one or more additional photoinitiators.

10. An LED-curable inkjet ink as claimed in claim 9, wherein the one or more additional photoinitiators comprises a phosphine oxide photoinitiator, preferably 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO).

11. An LED-curable inkjet ink as claimed in any preceding claim, wherein the ink is substantially free of isopropyl thioxanthone.

12. An LED-curable inkjet ink as claimed in any preceding claim, further comprising a colouring agent, preferably wherein the colouring agent is a dispersed pigment.

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

14. A method of inkjet printing comprising:

(i) inkjet printing an LED-curable inkjet ink onto a substrate, wherein the ink comprises 5-30% by weight of N-vinyl caprolactam or N-vinyl-5-methyl-2-oxazolidinone, based on the total weight of the ink, and oxygen-deficient atmosphere.

15. Use of a photoinitiator according to the following chemical formula for reducing colour shift in an LED-curable inkjet ink:

Intellectual

Property

Office

Application No: GB1806621.7 Examiner: Dr Paul Minton