WO2009074833A1

WO2009074833A1 - A printing ink

Info

Publication number: WO2009074833A1
Application number: PCT/GB2008/051179
Authority: WO
Inventors: Samuel Lawton
Original assignee: Sericol Limited
Priority date: 2007-12-13
Filing date: 2008-12-12
Publication date: 2009-06-18
Also published as: GB2468104B; GB0724350D0; GB201011571D0; GB2468104A

Abstract

This invention relates to an ink comprising at least one acrylate monomer that comprises a non-bridged cyclic group, at least one photoinhibitor and at least one photochromic dye.

Description

A PRINTING INK

This invention relates to a printing ink and in particular to a photochromic ink which is cured by irradiation.

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 below 200 mPas at 25⁰C although in most applications the viscosity should be below 100 mPas or 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 10.5 mPas at the jetting temperature which is often elevated to about 4O⁰C (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. Photochromic dyes are dyes that undergo a reversible colour change upon exposure to radiation, for example UV light. The colour change may be from colourless to coloured, or from one colour to another colour. The change in colour is due to structural changes in the dye molecule caused by the radiation. Photochromic dyes are currently used in Transitions® spectacle lenses, in special effect toys and in inks for use in speciality clothing and security purposes. For example, identification marks can be applied to a document using a colourless photochromic ink. Upon exposure to radiation, the identification mark becomes visible, allowing authentication of the document.

There exists a need for improved photochromic inks that can be printed using existing printers in order to produce a printed film having photochromic properties.

Accordingly, the present invention provides an ink comprising at least one acrylate monomer that comprises a non-bridged cyclic group, at least one photoinitiator and at least one photochromic dye.

The "acrylate monomer that comprises a non-bridged cyclic group" is a compound of the following formula

wherein R is any group that includes a non-bridged cyclic group. "Non-bridged cyclic group" means a cyclic group that does not include a bridge. The IUPAC Compendium of Chemical Technology defines a bridge as "a valence bond or an atom or an unbranched chain of atoms connecting two different parts of a molecule". As used herein, non-bridged cyclic group therefore means a cyclic group that includes no valence bond or atom or unbranched chain of atoms connecting two different parts of the cyclic group. The non-bridged cyclic group is therefore monocyclic. An example of a cyclic group that does include a bridge is the isobornyl group. Examples of non-bridged cyclic groups include saturated or partially saturated carbocyclic rings having, for example, 3 to 8 carbon atoms and which may optionally include one or more heteroatoms selected from N, O and S; monocyclic aryl groups wherein aryl means a carbocyclic aromatic ring, and monocylic heterocyclic aromatic rings containing one or more heteroatoms selected from N, O and S.

The non-bridged cyclic group is preferably a C3-C7 cycloalkyl group, a phenyl group or a 3- to 5-membered heterocycle.

By C3-C7 cycloalkyl is meant a saturated monocyclic ring structure containing 3 to 7 carbon atoms. By 5- to 7-membered heterocycle is meant a 5- to 7- membered monocyclic heterocyclic ring which is either saturated or aromatic. Each heterocycle consists of 3 or more carbon atoms and 1 to 3 heteroatoms independently selected from O, N and S. Saturated rings comprising one or more oxygen heteroatoms are preferred.

The non-bridged cyclic group may be attached to the ester moiety of the acrylate by a chemical bond, or by a hydrocarbyl linking group wherein one or more carbon atoms may be replaced with a heteroatom, for example a straight or branched chain Ci-C₆ alkylene group, or a C₂-C₆ alkylene group in which one of the carbon atoms is replaced with -O- .

Preferred acrylate monomers that comprise a non-bridged cyclic group include phenoxyethyl acrylate (PEA), tetrahydrofurfuryl acrylate (THFA) and cyclic TMP formal acrylate (CTFA). These monomers are well known in the art and have the structures shown below:

PEA is particularly preferred. The ink of the invention may comprise one acrylate monomer that comprises a non- bridged cyclic group or a mixture of two or more acrylate monomers that comprise a non-bridged cyclic group. Preferably the ink of the invention comprises one acrylate monomer that comprises a non-bridged cyclic group.

The ink of the present invention may optionally comprise one or more mono functional monomers in addition to the acrylate monomer that comprises a non-bridged cyclic group. Examples of mono functional monomers include mono functional (meth)acrylate monomers, N-vinyl amides and N-acryloyl amines. Suitable monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Preferred examples include:

Isobornyl acrylate (IBOA) mol wt 208g/mol

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

R — CsHn / 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

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. Examples include N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP):

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. An example is N-acryloylmorpholine (ACMO):

In a preferred embodiment, the ink of the invention comprises one or more additional mono functional monomers selected from N-vinyl amides and N-acryloyl amines, preferably NVC.

In the ink of the present invention the at least one acrylate monomer that comprises a non-bridged cyclic group is preferably present in an amount of 40 to 95 wt%, and one or more additional mono functional monomers are present in an amount of 0 to 40 wt%, based on the total weight of the ink.

In one preferred embodiment, the ink of the invention comprises at least 50 wt% of at least one acrylate monomer that comprises a non-bridged cyclic group, more preferably at least 70 wt%, more preferably at least 80 wt% and most preferably at least 85 wt%, based on the total weight of the ink. In preferred inks of this embodiment, the mono functional monomer component of the ink consists essentially of at least one acrylate monomer that comprises a non-bridged cyclic group.

In an alternative embodiment, inks of this invention comprise 40 to 70 wt% of at least one acrylate monomer that comprises a non-bridged cyclic group and 20 to 40 wt% of one or more additional mono functional monomers, based on the total weight of the ink.

In one preferred embodiment the ink of the present invention comprises PEA and NVC. Preferred inks of this embodiment comprise 40 to 70 wt% PEA and 20 to 40 wt% NVC, based on the total weight of the ink.

The 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, although the presence of such components may be tolerated. Preferably, however, the ink of the present invention is substantially free of water and volatile organic solvents.

It is possible to modify the film properties of the inks by inclusion of multifunctional monomers, oligomers or inert resins, such as thermoplastic acrylics. However, it should be noted that in the case of oligomers and multifunctional monomers 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 formulation include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylate, for example, tetraethyleneglycol diacrylate, dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentylglycol 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 acrylate monomers include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimacrylate, triethyleneglycol dimethacrylate, diethyleneglycol 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 mono functional material or as a component in a commercially available pigment dispersion. Where multifunctional monomer is included, the multifunctional monomer is 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 an embodiment the ink is substantially free of oligomeric and polymeric material meaning that only trace amounts will be present. Where oligomeric or polymeric material is included, the oligomeric and polymeric material is present in an amount of no more than 20 wt%, more preferably no more than 10 wt%, most preferably no more than 5 wt% based on the total weight of the ink. Oligomeric and polymeric materials (e.g. acrylate oligomers and inert thermoplastic resins, respectively) 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. In addition to the monomers described above, the ink of the invention includes a photoinitiator, 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 -hydro xycyc Io hexyl phenyl ketone, 2-benzyl-2-(dimethylamino)-l-[4-(4- morpholinyl)phenyl]-l-butanone, 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.

In a preferred embodiment the ink of the present invention comprises at least one α- amino ketone photoinitiator. By α-amino ketone photoinitiator is meant a radical photoinitiator that comprises the following moiety, where the dashed lines represent bonds to other atoms or chemical groups:

Preferably, the α-amino ketone photoinitiator is an α-amino phenyl ketone. Preferred examples of α-amino ketone photoinitiators include 2-benzyl-2-(dimethylamino)-l-[4- (4-morpholinyl)phenyl]-l-butanone, and 2-methyl-l-[4-(methyl thio)phenyl]- 2-(4-morpholinyl)-l-propanone. Such photoinitiators are known and commercially available under the trade names Irgacure 369 and Irgacure 907 (from Ciba).

In one preferred embodiment, the photoinitiator component of the ink of the invention consists essentially of at least one α-amino ketone photoinitiator. It has surprisingly been found that the use of an α-amino ketone photoinitiator in the inks of the invention is particularly advantageous. In particular, inks according to this preferred embodiment of the invention show a more pronounced photochromic effect than inks comprising other commonly-used free radical photoinitiators. This means that the inks according the preferred embodiment can be printed in relatively thin films, while still providing a useful photochromic effect. In particular, the inks of this preferred embodiment can be printed in thicknesses of 5 to 10 microns. This is advantageous because films having the required photochromic effect can be printed in a single pass, which means that film production is efficient. For inks comprising less preferred photoinitiators, films having the required photochromic effect can be produced by increasing the printed film thickness.

Although not wishing to be bound by theory, it is believed that the conversion of α- amino ketone photoinitiators into free radicals upon irradiation is more efficient than the conversion of other commonly-used free radical photoinitiators. This means that printed inks that are cured by irradiating α-amino ketone photoinitiators will tend to include less unreacted photoinitiator than inks that have been cured using other free radical photoinitiators. It is believed that unconverted free radical photoinitiator remaining in the printed ink can absorb radiation when the printed ink is irradiated with the intention of changing the colour of the photochromic dye. This means that the intensity of the radiation available to the photochromic dye is reduced, which in turn means that the photochromic effect is less marked.

The photoinitiator may optionally be used together with an amine synergist. Amine synergists are well known in the art and include methyldiethanolamine (MDA) and ethyl 4-dimethylaminobenzoate (EPD). When an amine synergist is present, it is preferably used in an amount of 1 to 10 % by weight, based on the total weight of the ink.

The ink of the present invention also comprises at least one photochromic dye. Photochromic dyes are materials that undergo a reversible colour change when exposed to radiation, preferably UV radiation. Suitable photochromic dyes are known and include spiropyrans, azobenzenes, stilbenes, naphthopyrans, spiro-oxazines, chalcones, diarylethenes and fulgides. The ink of the present invention is preferably capable of reversibly changing colour many times. If the radiation used to cure the ink of the present invention is of the same wavelength as the radiation that causes the colour change in the photochromic dye, exposure to radiation in the cure step may cause degradation of the photochromic dye. The photochromic dye used in the ink of the invention is therefore preferably fatigue-resistant. By "fatigue-resistant" is meant that the photochromic dye does not degrade significantly upon exposure to radiation, particularly UV radiation. Preferred fatigue-resistant photochromic dyes are spiro- naphthoxazines and naphthopyrans.

Spiro-naphthoxazines and naphthopyrans are known and are commercially available for example under the name Reversacol from James Robinson Ltd. Examples include Palatinate Purple, Sea Green, Aqua Green and Plum Red.

The at least one photochromic dye is preferably present in an amount of 0.01 wt% to 20 wt% based on the total weight of the ink, preferably 0.1 to 10 wt%, most preferably 0.5 to 5 wt%.

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, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers. For example, the inks of the invention may contain a silicone surface additive in an amount of 0.05 to 3 wt %, based on the total weight of the ink.

The present invention also provides a method of printing using the above-described ink and a substrate having the cured ink thereon. Suitable substrates include styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC) and VIVAK (a polyethylene terephthalate glycol modified). The inks of the invention can be printed on optical media such as DVDs and high density (HD)-DVDs, which are made from polycarbonate, and CDs and Blu-ray Discs (BDs), which are also made from polycarbonate but which have a coating on the outer layer. The ink of the present invention is preferably cured by ultraviolet irradiation. Although the ink of the invention can be used for flexographic printing, the ink of the present invention is preferably an ink-jet ink and is suitable for application by ink-jet printing.

The inks of the present invention can be used for security purposes in the optical media market. For example, a small logo could be printed on a DVD using colourless photochromic ink when the DVD is produced. Further down the supply chain the DVD can be irradiated with UV light which would cause the ink to become visible, allowing the user to verify that the DVD is genuine.

The ink exhibits a desirable low viscosity (less than 200 mPas, preferably less than 100 mPas, more preferably 50 mPas and most preferably less than 25 mPas at 25⁰C). Viscosity may be measured using a digital Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model LDV 1+ with the ULA spindle and cup arrangement and a spindle speed of 20 rpm.

(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.

The inks of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, milling on a horizontal bead-mill or ultrasonification. Throughout the text amounts are provided in weight percentages unless stated otherwise.

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

Examples

The inks of the examples comprise the following materials:

Palatinate Purple (provided as a 5 wt% solution in PEA). PEA - phenoxy ethyl acrylate.

Irgacure 907 - 2-methyl-l-[4-(methylthio)phenyl]-2-(4-morpholinyl)-l-propanone. Irgacure 369 - 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-l-one. Irgacure 184 - 1-hydroxycyclohexyl phenyl ketone. TPO - diphenyl (2,4,6-trimethylbenzoyl) phospine oxide. Darocur 1173 - 2-hydroxy-2-methyl-l -phenyl- 1-propanone. ITX - isopropylthioxanthone. BAPO - bis acyl phosphine oxide. MDA - methyl diethanol amine. EPD - ethyl 4 - dimethyl amino benzoate.

TPO-XL - TPO photoinitiator available from BASF.

Byk 307 - silicone surface additive available from BYK-Chemie (provided as a 1 wt% solution in PEA).

Inks were prepared as 5 g samples in 30 ml glass jars. Raw materials were weighed on a 3 decimal place balance. Each sample was ultrasonicated for approximately 5 minutes.

Inks having the compositions shown in Table 1 were prepared. Amounts are shown in weight percentages:

Table 1. Ink formulations

Drawdowns of the inks of Examples 1 to 11 were produced on self-adhesive backed PVC using a Maya bar 10 and a RK Print-Coat Instruments Ltd. drawdown machine operating at speed 4. The resulting drawdown thickness was 8 microns. Each drawdown was cured at 30 m/min with 2 lamps on full power until a reasonable cure was achieved (one to three passes). The inks of Examples 1, 2 and 4 produced drawdowns having a slight red tint. The remaining inks produced light green/yellow drawdowns.

To examine the resulting photochromic effect, the drawdowns were exposed to bright sunlight. The inks changed colour to a dark blue (the name Palatinate Purple is misleading, as often the resulting colour is a blue). The inks were rated from 1 to 5 based on colour intensity, with a rating of 5 indicating the most intense colour. The results are shown in Table 2.

Table 2. Photochromic effect.

Claims

1. An ink comprising at least one acrylate monomer that comprises a non-bridged cyclic group, at least one photoinitiator and at least one photochromic dye.

2. The ink of claim 1 wherein said non-bridged cyclic group is C3-C7 cycloalkyl, phenyl or a 3- to 5- membered heterocycle.

3. The ink of claim 1 or claim 2 wherein said at least one acrylate monomer that comprises a non-bridged cyclic group is phenoxy ethyl acrylate (PEA), tetrahydrofurfuryl acrylate (THFA), cyclic TMP formal acrylate (CTFA), or a mixture thereof.

4. The ink of any preceding claim wherein said at least one acrylate monomer that comprises a non-bridged cyclic group is PEA.

5. The ink of any preceding claim wherein said at least one acrylate monomer that comprises a non-bridged cyclic group is present in an amount of 40 to 95 wt%, based on the total weight of the ink.

6. The ink of any preceding claim wherein said at least one acrylate monomer that comprises a non-bridged cyclic group is present in an amount of at least 50 wt%, preferably at least 70 wt%, more preferably at least 80 wt% and most preferably at least 85 wt%, based on the total weight of the ink.

7. The ink of any one of claims 1 to 5 additionally comprising one or more additional mono functional monomers.

8. The ink of claim 7 wherein said one or more additional mono functional monomers are present in an amount up to and including 40 wt%, based on the total weight of the ink, preferably 20 to 40 wt%.

9. The ink of claim 7 or claim 8 wherein said one or more additional mono functional monomers are selected fromN-vinyl amides and N-acryloyl amines.

10. The ink of claim 9 wherein said additional monofunctional monomer is N- vinyl capro lactam (NVC).

11. The ink of claim 10 comprising 40 to 70 wt% of PEA and 20 to 40 wt% of NVC, based on the total weight of the ink.

12. The ink of any preceding claim wherein said photoinitiator comprises at least one α-amino ketone radical photoinitiator.

13. The ink of claim 12 wherein said at least one α-amino ketone radical photoinitiator is selected from 2-benzyl-2-(dimethylamino)-l-[4-(4- morpholinyl)phenyl] - 1 -butanone and 2-methyl- 1 - [4-(methyl thio)phenyl] - 2-(4-morpholinyl)- 1 -propanone.

14. The ink of any preceding claim wherein said photoinitiator is present in an amount of 1 to 20% by weight based on the total weight of the ink, preferably 4 to 10% by weight.

15. The ink of any preceding claim additionally comprising an amine synergist.

16. The ink of any preceding claim wherein said at least one photochromic dye is selected from spiro-naphthoxazines and naphthopyrans.

17. The ink of any preceding claim wherein said at least one photochromic dye is present in an amount of 0.01 wt % to 20 wt% based on the total weight of the ink, preferably 0.1 to 10 wt%, most preferably 0.5 to 5 wt%.

18. The ink of any preceding claim having a viscosity of 200 mPas or less at 25⁰C, preferably 100 mPas or less.

19. A method of printing, comprising printing the ink as claimed in any preceding claim on to a substrate and curing the ink.

20. The method according to claim 20 wherein said ink is ink-jet printed.

21. The method according to claim 19 or 20 wherein the substrate is selected from a CD, DVD, HD-DVD or BD.

22. A substrate having the ink as claimed in any one of claims 1 to 18 printed thereon.

23. A substrate according to claim 22 selected from a CD, DVD, HD-DVD or BD.

24. An ink-jet ink cartridge containing the ink as claimed in any one of claims 1 to 18.