WO2010029351A1

WO2010029351A1 - A printing ink

Info

Publication number: WO2010029351A1
Application number: PCT/GB2009/051147
Authority: WO
Inventors: Angelique Runacre; Damian Ward
Original assignee: Sericol Limited
Priority date: 2008-09-09
Filing date: 2009-09-09
Publication date: 2010-03-18
Also published as: GB2474816A; GB0816458D0; GB201103911D0; GB2474816B

Abstract

This invention provides an ink-jet ink comprising a radiation-curable monomer, 1 to 10 % by weight of a solvent based on the total weight of the ink, a photoinitiator and a colouring agent, wherein the solvent has a higher surface tension than the monomer. The ink provides an improved colour gamut.

Description

A Printing Ink

This invention relates to a printing ink for use in ink-jet printers. In particular, this invention concerns ink-jet inks that comprise a solvent and are cured using ultraviolet radiation.

In ink-jet 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 no more than 200 mPas at 25⁰C although in most applications the viscosity should be no more than 50 mPas, and often no more than 25 mPas. Typically, when ejected through the nozzles, the ink has a viscosity of no more than 25 mPas, preferably 5-15 mPas and ideally 10.5 mPas at the jetting temperature which is often elevated to about 40⁰C (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, ink-jet 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 ink-jet 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, ink-jet 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 ink-jet ink contains unsaturated organic compounds, termed monomers, which polymerise by irradiation, commonly with ultraviolet light, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process which is more rapid than evaporation of solvent at moderate temperatures. In such ink-jet inks it is necessary to use monomers possessing a low viscosity.

Ink-jet ink sets typically use the CMYK colour space, i.e. the ink-jet ink set contains cyan, magenta, yellow and black inks. This ink-jet ink set has also been expanded to the hexachrome set which, in addition to CMYK, also includes orange and green inks. However, even these expanded ink sets provide a limited representation of the visible colour space. There is therefore a requirement in the art for ink-jet inks which achieve a better representation of the visible colour space without compromising the printing properties of the inks.

Accordingly, the present invention provides an ink-jet ink comprising a radiation- curable monomer, 1 to 10% by weight of a solvent based on the total weight of the ink, a photoinitiator and a colouring agent, wherein the solvent has a higher surface tension than the monomer.

The present invention will now be described with reference to Figure 1 which shows the colour gamut of inks with and without the inclusion of 2% of a high surface tension solvent (reproduced in B&W from a colour image).

The ink-jet 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.

By "radiation curable monomer" is meant a monomer that polymerises or crosslinks when exposed to radiation, commonly ultraviolet light, in the presence of a photoinitiator.

The radiation curable monomer can comprise one monomer or a mixture of two or more monomers. The radiation curable monomer may comprise monofunctional monomers, multifunctional monomers and mixtures thereof. In one embodiment, the radiation-curable monomer comprises at least one monofunctional monomer.

Any monofunctional monomer that is suitable for use in an ink-jet ink maybe used in the ink of the invention and such monomers are well known in the art. The monofunctional monomer is preferably selected from N- vinyl amides, N- acryloylamines, monofunctional (meth)acrylates and mixtures thereof.

N-Vinyl amides are well-known monomers in the art and a detailed description is therefore not required. N- Vinyl amides have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers.

Preferred examples are N-vinyl caprolactam (NVC) and N- vinyl pyrrolidone (NVP):

Similarly, N-acryloyl amines are also well-known in the art. N-Acryloyl amines also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. Regarding the nomenclature, since the term "acryloyl" incorporates a carbonyl group, the amide is actually named as an amine.

A preferred example is N-acryloylmorpholine (ACMO):

In one embodiment the ink comprises 10 to 40% by weight of N-vinyl amide based on the total weight of the ink, preferably 20 to 30% by weight.

Monofunctional (meth)acrylate monomers are also well known in the art and are preferably the esters of acrylic acid. Preferred examples include:

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

Isobornyl acrylate (IBOA) Tetrahydrofurfuryl acrylate (THFA) mol wt 208g/mol mol wt 156 g/mol

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

R = C₈H₁₇ / C₁₀H₂₁ Octadecyl acrylate (ODA) Tridecyl acrylate (TDA) mol wt 200 g/mol mol 254 g/mol

Isodecyl acrylate (IDA) Lauryl acrylate mol wt 212 g/mol mol wt 240 g/mol

The substituents of the monofunctional monomers 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_1-Jg alkyl, C_3-Ig cycloalkyl, C₆. io aryl and combinations thereof, such as C₆-_I0 aryl- or C_3-Is cycloalkyl-substituted Ci_-I8 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 as exemplified by ACMO.

The total amount of the at least one monofunctional monomer is preferably at least 50 wt%, more preferably at least 60 wt% and most preferably at least 70 wt%, based on the total weight of the ink.

The at least one monofunctional monomer preferably comprises an N-vinyl amide, a monofunctional (meth)acrylate or a mixture thereof, most preferably a mixture thereof. A particularly preferred ink of the invention comprises a mixture of NVC and a monofunctional acrylate, particularly NVC and PEA. The weight ratio of the monofunctional (meth)acrylate monomer to the N-vinyl amide is preferably from 0.2 to 4, more preferably 0.5 to 3, and most preferably 1 to 2.5.

The present invention also contains a solvent in order to improve the spreading of the ink-jet ink drops on the substrate. The solvent has a higher surface tension than the monomers present in the ink. Ink-jet inks presently available tend to provide a relatively narrow colour gamut which can arise when jetted drops of the ink fail to spread sufficiently. Obtaining a wide colour gamut in multipass UV ink-jet printing relies upon the ability of the drops to spread effectively over the inks which have been previously printed on the substrate. This is because an insufficient drop spread can result in areas of substrate showing through the printed image, resulting in a drop in colour intensity. The inclusion of a solvent having a surface tension higher than that of the monomers in the ink helps to elevate the surface energy of the cured ink film, promoting the ink drop spread for those subsequently dropped thereon, enhancing the colour gamut achievable.

The surface tension of the solvent is higher than that of the monomers in the system. When the ink of the invention comprises more than one radiation curable monomer the surface tension of the solvent is higher than that of the monomer having the highest surface tension. In one embodiment the solvent has a surface tension of at least one, preferably at least two and more preferably at least 3 dynes/cm higher than the monomers.

Surface tension can be measured using the Du Noϋy ring method at 25 °C, for example using the automated KSV Sigma 702 instrument supplied by KSV Instruments Ltd,

Although not wishing to be bound by theory, it is believed that the inks of the present invention provide a better colour gamut as a result of the improved contact angle that the ink droplets have on printing. Once the ink is applied to the substrate it is believed that the improved surface energy characteristics of the ink give rise to better contact angle and thus wider colour gamut in the print. It should be noted that too much wetting (low surface tension) leads to too great a dot gain (spreading of printed ink dots) which reduces print definition.

The solvent can comprise a single solvent or a mixture of two or more solvents that each have a surface tension higher than that of the monomers.

The solvent is preferably propylene carbonate, glycerol or a mixture thereof.

The solvent is present in an amount of 1 to 10% by weight, preferably 1 to 7% by weight, more preferably 1 to 5% by weight, more preferably 1 to 3% by weight and most preferably 2 to 3% by weight, based on the total weight of the ink. This is significantly less than would be found in a conventional, solvent-based, ink-jet ink. It is therefore conceptually akin to a curable, solvent-free (including water-free) ink-jet ink, albeit with a small amount of solvent added. Minor amounts of solvents that do not have a surface tension higher than that of the monomer in the ink can be tolerated. Such solvents are preferably present in an amount of 1% by weight or less based on the total weight of the ink. Preferably the ink is substantially free of solvents that do not have a surface tension that is higher than that of the monomer.

The ink is preferably substantially free of water. That is, the ink does not dry by the evaporation of water and no water is added to the ink. However, traces of water, for example absorbed from the air, are tolerated in the ink.

In one embodiment, the ink contains an oligomer which is preferably present in an amount of 1 to 30%, preferably 1 to 20 wt%, more preferably 5 to 15 wt% based on the total weight of the ink. Oligomeric materials (e.g. acrylate oligomers) are known in the art and typically have a molecular weight above 500, more preferably above 1000. The upper limit is less relevant, but is preferably less than 10,000, more preferably less than 5,000. The ink of the present invention preferably comprises an oligomer, most preferably a urethane (meth)acrylate such as a difunctional urethane acrylate available from Sartomer. The oligomer is preferably a functional oligomer meanmg that it has functional groups which take part in the curing reaction. The oligomer preferably has 2-6 functional groups and is most preferably difunctional.

It is possible to modify the film properties of the ink-jet inks by inclusion of multifunctional monomers. However, it should be noted that the flexibility may be adversely affected and also that some adjustments to stoichiometry may be required to retain optimum cure speed.

Examples of the multifunctional acrylate monomers which may be included in the ink-jet ink formulation include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyieneglycol diacrylate, for example, tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentyl glycol diacrylate, bis(pentaerythritol) hexa-acrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentyl glycol diacrylate, ethoxylated trimethylolpropane triacrylate, and mixtures thereof. Particularly preferred are difunctional acrylates with a molecular weight greater than 200.

In addition, suitable multifunctional methacrylate monomers include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyl eneglycol dimethacylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate.

Mixtures of (meth)acrylates may also be used.

In one embodiment the ink is substantially free of multifunctional monomer, meaning that only trace amounts will be present, for example as impurities in the monofunctional material or as a component in a commercially available pigment dispersion. Where multifunctional monomer is included, the multifunctional monomer is preferably present in an amount of no more than 15 wt%, preferably no more than 10 wt%, more preferably no more than 7 wt%, more preferably no more than 5 wt% and most preferably no more than 2 wt% based on the total weight of the ink. The multifunctional monomer which is limited in amount may be any multifunctional monomer which could be involved in the curing reaction, such as a multifunctional (meth)acrylate monomer or a multifunctional vinyl ether.

In addition to the monomers described above, the compositions include a photo initiator, which, under irradiation by, for example, ultraviolet light, initiates the polymerisation of the monomers. Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4- morpholmophenyl)butan-l-one, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4- trimethylpentylphosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure, Darocur (from Ciba) and Lucerin (from BASF).

Preferably the photoinitiator is present from 1 to 20% by weight, preferably from 4 to 10% by weight, of the ink. The ink-jet ink of the present invention also includes a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. 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 Red 9, Pigment Red 184, Pigment Blue 15:3, Pigment Green 7, Pigment Violet 19, Pigment Black 7, pigment orange 36 or pigment orange 43. White pigments may include titanium dioxide either ruyile or anatase, for example pigment white 6 (a rutile titanium oxide supplied by Kronos). Further inorganic pigments include synthetic iron oxides and chrome pigments (for example red oxide and light yellow chrome supplied by colourjet). Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used. Alternatively, the colouring agent may be a dye such as Orasol (available from Ciba Speciality Chemicals or Telajet (available from Clariant UK), for example, solvent black 27 or 29, solvent red 233, solvent yellow 146 and solvent blue 136. Additionally, water soluble dyes such as Duasyn (supplied by Clariant UK) or Hostafϊne (Ciba Speciality Chemicals) may also be included.

The total proportion of pigment present is preferably from 0.5 to 20% by weight, more preferably 0.5 to 15% by weight, more preferably from 2 to 8% by weight for the colour inks and for the white ink the total proportion of pigment present is preferably from 5 to 40% by weight, more preferably 8 to 20% by weight.

Although the ink of the present invention preferably cures by a free radical mechanism, the ink of the present invention may also be a cationically cured ink or indeed a so-called "hybrid" ink which cures by a radical and cationic mechanism. The ink-jet ink of the present invention, in one embodiment, therefore further comprises at least one cationically curable monomer, such as a vinyl ether, and at least one cationic photo initiator, such as an iodonium or sulfonium salt, e.g. diphenyliodonium fluoride and triphenylsulfonium hexafiuophosphate. Suitable cationic photoinitiators are be sold under the Trade names of lrgacure 184, Irgacure 500, Darocure 1 173, Irgacure 907, ITX, Lucerin TPO, Irgacure 369, Irgacure 1700, Darocure 4265, Irgacure 651, Irgacure 819, Irgacure 1000, Irgacure 1300, Esacure KT046, Esacure KlP 150, Esacure KT37, Esacure EDB, H-Nu 470 and H-Nu 470X.

Other components of types known in the art may be present in the ink to improve the properties or performance. These components may be, for example, surfactants, defoamers, dispersants, synergists for the photoinitiator, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers.

The ink of the present invention is preferably provided as part of an ink-jet ink set consisting of a plurality of inks. The set preferably comprises or consists of CMYK inks. Other combinations may be contemplated.

The ink of the present invention is preferably cured by ultraviolet irradiation and is suitable for application by ink-jet printing.

The present invention also provides a method of ink-jet printing using the above- described ink or set, an ink-jet cartridge containing the ink and a substrate having the cured ink thereon. The nature of the substrate is not limited and includes any substrate which may be subjected to ink-jet printing. However, the inks of the present invention are particularly suited for printing onto substrates such as polycarbonate (e.g. PolyCarb), PVC (e.g. BannerPVC), polypropylene, polyethylene, polyethylene terephthalate), acrylic and styrene a polyethylene terephthalate glycol modified (e.g. VIVAK).

The ink-jet ink exhibits a desirable low viscosity (preferably no more than 200 mPas, more preferably no more than 50 mPas and most preferably no more than 25 mPas at 25⁰C). Viscosity may be measured using a Brookfield viscometer DVl low- viscosity viscometer running at 20 rpm at 25⁰C with spindle 00.

The inks of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill. (Meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and/or methacrylate. Mono and multifunctional are also intended to have their standard meanings, i.e. one and two or more groups, respectively, which take part in the polymerisation reaction on curing. Throughout the application amounts are provided as weight percentages unless otherwise stated.

The invention will now be described, by way of example, with reference to the following examples.

Examples

Example 1

A CMYK ink-jet ink set was prepared, the inks having the following formulations'

Cyan:

Magenta:

Yellow:

Black:

Example 2

A further CMYK ink-jet ink set was prepared, the inks having the following formulations:

Cyan:

Magenta:

Yellow:

Black:

Example 3

The inks from Examples 1 and 2 were printed onto optically brightened coated paper using an Inca Spyder 320, 10 pass, 100% pass mode, uni directional, 1 lamp high power ink-jet printer. By way of comparison, the same inks were printed in the absence of the solvent.

The results are provided as a gamut plot, see the Figure I₅ which is reproduced in black and white. The ink set of Example 1 is the outer plot (white), the ink set of Example 2 is the next plot (yellow) in and the comparative ink set is the inner plot (red). This example shows that a significant gain in colour gamut can be achieved by incorporating 2% of propylene carbonate or glycerol to a UV ink-jet solvent-free ink colour gamut.

Claims

1. An ink-jet ink comprising a radiation-curable monomer, 1 to 10% by weight of a solvent based on the total weight of the ink, a photoinitiator and a colouring agent, wherein the solvent has a higher surface tension than the monomer,

2. An ink-jet ink as claimed in claim 1, wherein the monomer comprises a monofunctional monomer.

3. An ink-jet ink as claimed in claim 2, wherein the monomer is selected from an N- vinyl amide, an N-acryloylamine, a monofunctional (meth)acrylate or a mixture thereof.

4. An ink-jet ink as claimed in claims 2 or 3, wherein the monofunctional monomer is selected from phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 2-(2- ethoxyethoxy)ethyl acrylate, octa/decyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate, N- vinyl caprolactam (NVC)₅ N-vinyl pyrrol idone (NVP) and N-acryloylmorpholine (ACMO), and mixtures thereof.

5. An ink-jet ink as claimed in any of claims 2 to 4, wherein the total amount of the monofunctional monomer is preferably at least 50 wt%, more preferably at least 60 wt% and most preferably at least 70 wt%, based on the total weight of the ink.

6. An ink-jet ink as claimed in any of claims 2 to 5, wherein the monofunctional monomer comprises a mixture of an N-vinyl amide and a monofunctional

(meth)acrylate.

7. An ink-jet ink as claimed in claim 6 wherein the weight ratio of the monofunctional (meth)acrylate monomer to the N-vinyl amide is preferably from 0.2 to 4, more preferably 0.5 to 3, and most preferably 1 to 2.5.

8. An ink-jet ink as claimed in any one of claims 1 to 7 comprising 10 to 40% by weight of N-vinyl amide, based on the total weight of the ink, preferably 20 to 30% by weight.

9. An ink-jet ink as claimed any preceding claim, comprising a mixture of NVC and a monofunctional acrylate, preferably NVC and PEA.

10. An ink-jet ink as claimed in any preceding claim, wherein the photoinitiator is a free radical photoinitiator.

11. An ink-jet ink as claimed in any preceding claim, wherein the ink additionally comprises at least one oligomer.

12. An ink-jet ink as claimed in claim 11, wherein the oligomer is a urethane (meth)acrylate oligomer.

13. An ink-jet ink as claimed in any preceding claim, wherein the solvent has a surface tension 3 dynes/cm higher than the monomer.

14. An ink-jet ink as claimed in any preceding claim, wherein the solvent is propylene carbonate, glycerol, or a mixture thereof.

15. An ink-jet ink as claimed in any preceding claim, wherein the solvent is present in an amount of 1 to 7% by weight, preferably 1 to 5% by weight.

16. An ink-jet ink as claimed in any preceding claim, wherein the solvent is present in an amount of 1 to 3% by weight, preferably 2 to 3% by weight.

17. An ink-jet ink as claimed in claim 10 or claim 1 1 wherein the at least one oligomer is present in an amount of 1 to 20 wt%, more preferably 5 to 15 wt%, based on the total weight of the ink.

18. An ink-jet ink as claimed in any preceding claim, wherein the ink contains no more than 10 wt% of multifunctional monomers based on the total weight of the ink.

19. An ink-jet ink as claimed in any preceding claim, wherein the ink is substantially free of water.

20. An ink-jet ink set consisting of a plurality of inks, wherein one or more of the inks is an ink as claimed in any preceding claim.

21 , An ink-jet ink set according to claim 20 wherein the set comprises CMYK inks.

22. A method of ink-jet printing, comprising printing the ink-jet ink as claimed in any in any of claims 1 to 19 or the set claimed in claims 20 or 21 on to a substrate and curing the ink.

23. An ink-jet cartridge comprising an ink as claimed in any of claims 1 to 19.

24. A substrate having the ink-jet ink as claimed in any of claims 1 to 18 or the set claimed in claims 20 or 21 printed thereon.