WO2010079341A1

WO2010079341A1 - A printing ink

Info

Publication number: WO2010079341A1
Application number: PCT/GB2010/000032
Authority: WO
Inventors: Damian Ward; Angeligue Catherine Joyce Runacre
Original assignee: Sericol Limited
Priority date: 2009-01-09
Filing date: 2010-01-08
Publication date: 2010-07-15
Also published as: GB201107636D0; GB2476771A8; GB2476771B; WO2010079341A8; GB2476771A

Abstract

This invention relates to an ink-jet ink comprising at least one radiation-curable monomer, at least one Norrish type II radical photoinitiator and at least one colouring agent; wherein the ink has a viscosity of 200 mPas or less at 25°C, and wherein the only photoinitiators present in the ink are Norrish type II radical photoinitiators.

Description

A printing ink

This invention relates to a printing ink, and particularly to a printing ink which reduces or avoids printhead failure due to nozzle blockage.

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 200 mPas or less at 25⁰C although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 5- 15 mPas and ideally 10.5 mPas at the jetting temperature which is often elevated to about 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.

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.

In ink-jet printing, the inks are typically supplied in cartridges which are loaded into the printing machine. The inks are pumped from the cartridges to printheads where the inks are jetted via nozzles onto the substrate. However, in some printing machines, the nozzles may be exposed to ambient light from poorly shielded lamps or daylight which causes any residual ink in the nozzles to cure leading to blockage of the nozzles and ultimately head failure. There is therefore a requirement in the art for techniques to avoid this problem.

Accordingly, the present invention provides an ink-jet ink comprising at least one radiation-curable monomer, at least one Norrish type II radical photoinitiator and at least one colouring agent; wherein the ink has a viscosity of 200 mPas or less at 25°C, and wherein the only photoinitiators present in the ink are Norrish type II radical photoinitiators.

This ink-jet ink takes advantage of the oxygen present at the printhead nozzles to minimise undesired curing of the ink.

The ink-jet inks of the present invention dry primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. Such inks do not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink, although the presence of such components may be tolerated. Therefore, the ink-jet inks of the present invention are preferably substantially free of water and volatile organic solvents. This avoids the drawbacks of solvent-based inks as discussed hereinabove. However, trace amounts of volatile organic solvents present, for example, as part of commercially available pigment dispersions, or trace amounts of water inevitably present by absorption from the air may be tolerated in the ink provided they do not adversely affect the cure speed.

The ink of the present invention preferably includes a (meth)acrylate monomer. The (meth)acrylate monomer may be a monofunctional monomer, a multifunctional monomer or combinations thereof.

The monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Preferred examples include phenoxyethyl acrylate, cyclic TMP formal acrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, 2-(2-ethoxyethoxy)ethyl acrylate, octadecyl acrylate, tridecyl acrylate, isodecyl acrylate and lauryl acrylate.

Monofunctional (meth)acrylate monomers may be included at 1-90% by weight, preferably 10-80% by weight, more preferably 30-70% by weight, most preferably 40-60% by weight, based on the total weight of the ink.

Examples of the multifunctional acrylate monomers which may be included in the ink-jet inks include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylate, for example, tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tri(propylene glycol) triacrylate, neopentylglycol diacrylate, bis(pentaerythritol) hexaacrylate, 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 di- and trifunctional acrylates. Also preferred are those with a molecular weight greater than 200. A preferred combination of monomers is hexanediol diacrylate, dipropyleneglycol diacrylate and propoxylated neopentyl glycol diacrylate.

In addition, suitable multifunctional acrylate monomers include esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, trimethylolpropane trimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1 ,4-butanediol dimethacrylate. Mixtures of (meth)acrylates may also be used.

Multifunctional (meth)acrylate monomers may be included at 1-90% by weight, preferably 5-85% by weight, more preferably 40-80% , most preferably 50-70% by weight, based on the total weight of the ink. However, in alternative embodiments, the ink-jet ink of the present invention has less than 15% multifunctional acrylate or is substantially free of multifunctional acrylate.

(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 present invention may also contain α,β-unsaturated ether monomers, such as vinyl ethers. These monomers are known in the art and may be used to reduce the viscosity of the ink formulation. Typical vinyl ether monomers which may be used in the inks of the present invention are triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1 ,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether. Mixtures of vinyl ether monomers may be used.

The vinyl ether monomer is preferably 1-20% by weight, more preferably 7-15% by weight, based on the total weight of the ink. In a preferred embodiment, the ratio of acrylate monomer to vinyl ether monomer is from 4:1 and 15:1. See WO 02/061001 for further details of formulations containing α,β-unsaturated ether monomers in combination with acrylate monomers.

N- Vinyl amides and N-(meth)acryloyl amines may also be used in the inks. 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).

N-Vinyl amides and/or N-acryloyl amines may be included at 3-50% by weight, preferably 5-40% by weight, more preferably 10-30% by weight, based on the total weight of the ink. NVC is particularly preferred. It is possible to modify further the film properties of the ink-jet inks by inclusion of 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. Said oligomers have weight-average molecular weight from 500 to 8,000, preferably from 1,000 to 7,000 and most preferably from 2,000 to 6,000. The oligomers are preferably functional (i.e. reactive oligomers, in that they take part in the curing reaction. A suitable example is a urethane oligomer. The functionality is preferably 2-6 and most preferably the oligomers are difunctional.

Oligomers may be included at 1-30% by weight, preferably 2-20% by weight and more preferably 3-15% by weight, based on the total weight of the ink.

In a preferred embodiment, the ink-jet ink of the present invention comprises a monofunctional (meth)acrylate monomer, such as those described hereinabove; an N- vinyl amides and/or an N-(meth)acryloyl amine, such as those described hereinabove; and a functional oligomer, such as those described hereinabove. The ink of this embodiment preferably contains less than 15% multifunctional acrylate and more preferably is substantially free of multifunctional acrylate. Preferred monomer combinations are THFA/NVC, IBOA/NVC, PEA/NVC, CTFA/NVC, IBOA/ACMO and IBOA/NVP A particularly preferred example includes phenoxyethyl acrylate,

NVC and a difunctional oligomer.

Other preferred embodiments include: an ink comprising at least one monofunctional monomer, preferably at least one monofunctional (meth)acrylate monomer; an ink comprising at least one monofunctional (meth)acrylate monomer and at least one (monofunctional) monomer selected from an N-vinyl amide, an N-acryloyl amine, or a mixture thereof; an ink comprising at least one difunctional monomer, preferably at least one difunctional (meth)acrylate monomer; an ink comprising at least one difunctional (meth)acrylate monomer, and at least one (monofunctional) monomer selected from an N-vinyl amide, an N-acryloyl amine, or a mixture thereof; and an ink comprising at least one monofunctional monomer and at least one difunctional monomer, preferably comprising at least one monofunctional (meth)acrylate monomer and at least one difunctional (meth)acrylate monomer. The ink of the present invention also includes at least one Norrish type II radical photoinitiator. A Norrish type II radical photoinitiator is photoinitiator which undergoes the Norrish type II reaction when exposed to actinic radiation. The Norrish type II reaction is defined by IUPAC as the photochemical abstraction of a γ- hydrogen by an excited carbonyl compound to produce a 1 ,4-biradical as a primary photoproduct. Preferred Norrish type II radical photoinitiators are phenylbenzophenone, isopropylthioxanthenone (ITX) and combinations thereof.

Norrish type II radical photoinitiators are prone to oxygen inhibition. However, an advantage of the inks of the present invention is that this otherwise deleterious property of the inhibition can be used advantageously to prevent premature curing at the printhead. Curing at the printhead is reduced since the printhead is exposed both to air and light. Thus, the oxygen inhibition inherent in Norrish type II radical photoinitiators is used to prevent premature curing by the action of light in a location at which the ink is exposed to air and light. Using Norrish type II radical photoinitiators, curing in the printhead can be avoided for 48 hours or more.

Since the photoinitiators typically used in ink-jet inks cure on exposure to light within 1-2 hours, they should be avoided in the ink of the present invention. Accordingly, the only photoinitiators present in the ink of the present invention are Norrish type II radical photoinitiators. The ink is substantially free of all other photoinitiators (such as, 1 -hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4- morpholinophenyl)butan-l-one, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4- trimethylpentylphosphine oxide).

Preferably the Norrish type II radical photoinitiator is present from 1 to 20% by weight, preferably from 3 to 10% by weight, of the ink.

The ink of the present invention is cured by irradiating the ink with actinic radiation, preferably UV radiation. The ink is preferably cured by applying a dose of radiation of 50-2,000 mJ/cm², preferably 100-1,000 mJ/cm². This is a much higher dose that the ambient light to which the ink is exposed in the printhead and is sufficient to cure the ink even in the presence of oxygen. 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. The colouring agents are commercially available, 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).

A pigment is typically provided as a powder, having been milled and filtered to achieve an appropriate particle size. The provision of pigment in the form of a powder allows for the dispersion of the pigment through the ink. The average particle size (diameter) of the dispersed pigment will typically be 5 microns or less, preferably 1 micron or less and more preferably 0.5 microns or less.

The ink-jet ink of the present invention is preferably supplied as part of a set. Preferably all of the inks of the set are made up of the inks of the present invention. 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. Other sets are also available and the inks of the present invention may be used in any such sets.

The total proportion of pigment present is preferably from 0.5 to 15% by weight, more preferably from 0.5 to 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 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 present invention also provides a method of ink-jet printing using the above- described ink and a substrate having the cured inks thereon. The ink of the present invention is particularly suited to piezoelectric drop-on-demand ink-jet printing. Suitable substrates include styrene, PolyCarb (a polycarbonate), BannerPVC (a PVC) and VIVAK (a polyethylene terephthalate glycol modified). The inks of the present invention are preferably cured by ultraviolet irradiation and are suitable for application by ink -jet printing. The present invention further provides a set of ink-jet inks wherein one or more of the inks in the set is the ink of the present invention. Preferably all of the inks in the ink-jet ink set are inks of the present invention. The present invention also provides a cartridge containing the ink-jet ink as defined herein. The cartridges comprise an ink container and an ink delivery port which is suitable for connection with an ink-jet printer.

The ink-jet inks exhibit a desirable low viscosity, i.e. 100 mPas or less, preferably 50 mPas or less and most preferably 25 mPas or less at 25°C (although when ejected through the nozzles, the jetting temperature is often elevated to about 40⁰C). When using an HSS head, the ink may have a viscosity of 200 mPas or less when measured at 25⁰C. Viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as model LDV 1+ with the ULA spindle and cup arrangement at 25 ⁰C and a spindle speed of 20 rpm.

The inks of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

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

Examples

Example 1

Ink-jet ink formulations having the following compositions are prepared by mixing the components in the given amounts (percentages are by weight and are based on the total weight of the ink). Ink 1 (ink of the present invention):

Example 2

The inks prepared in Example 1 were tested for various properties.

Inks 1 and 2 both had viscosities of 22 mPas which is suitable for ink -jet printing. The viscosities were measured using a Brookfield model LDV 1+ with the ULA spindle and cup arrangement at 25⁰C. Cure speed was assessed by printing images on the Inca Spyder 320 flatbed UV inkjet machine, substrate 200 micron gloss PVC printed at 5 pass, 1500 mm/sec, 140% coverage and cured with two lamps set on medium power (providing a dose of 1 ,000 mJ/cm²).

The cure speeds for both inks were essentially the same. Films were prepared by drawing down 12 micron films using a no. 2 K-bar applicator on to a PVC substrate. Both inks were cured to a hard tack-free film at 35 m/min on a Svecia screen drier fitted with 2 x 80 W/cm medium pressure mercury lamps (giving a total dose of 200 mJ/cm²). However, ink 1 remained uncured in ambient light for over 48 hours (the inks was observed as being still wet), whereas ink 2 cured to a hard tack-free film within 2 hours.

Example 3

Images printed as above were cured using a Jenton International Ltd UV curing conveyor belt system, which allows a print to be exposed to varying levels of UV dose under a fixed lamp by varying the belt speed (speed can be varied between 0.2 - 150 metres/min). In this instance, the prints were cured using one Vzero lamp supplied by Integration Technology, capable of outputting 200 W/cm at full power.

The inks were cured at various UV doses and the degree of cure was assessed in each case. The degree of cure was assessed by rubbing the printed images with a soft cloth soaked in isopropyl alcohol. The IPA resistance of the ink is an indicator of the degree of crosslinking in the ink, and therefore the degree of cure. The number of IPA rubs before failure was recorded. The results are shown in Table 1. Figure 1 is a plot of the number of IPA rubs before failure vs. the UV dose for Ink 1 (Type 2) and Ink 2 (Type 1).

The resistance to IPA was used to calculate a cure rate. Cure rate was determined by first evaluating Ink 1 and Ink 2 to determine the IPA resistance of fully cured inks (full cure is considered achieved when subsequent exposure to UV light will not increase the ink resistance to IPA). The IPA resistance results obtained above were converted to a percentage compared to the maximum IPA resistance achieved and the results are shown in Table 1. Figure 2 is a plot of the cure rate vs. the UV dose for Ink 1 (Type 2) and Ink 2 (Type 1).

rable 1

Dose (mJ/cm2) 835 52 452 08 31549 248 64 205 91 174 84 157 91 148 19 136 14 133 52 94 1 88 94 87 7 80 1 79 8

Belt speed m/min 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Ink 1 IPA rubs 297 230 124 69 48 40 14 7 4 4 2 2 1 1 1

Ink 1 IPA % 100 77 44 41 75 23 23 16 16 13 47 4 71 2 36 1 35 1 35 0 67 067 0 34 034 0 34

Ink 2 IPA rubs 283 321 269 152 119 82 56 28 22 19 17 14 13 5 2

Ink 2 IPA % 88 16 100 00 83 80 47 35 37 07 25 55 17 45 8 72 6 85 5 92 5 30 4 36 4 05 1 56 0 62

Claims

1. An ink-jet ink comprising at least one radiation-curable monomer, at least one Norrish type II radical photoinitiator and at least one colouring agent; wherein the ink has a viscosity of 200 mPas or less at 25⁰C, and wherein the only photoinitiators present in the ink are Norrish type II radical photoinitiators.

2. An ink-jet ink as claimed in claim 1, wherein the Norrish type II radical photoinitiator is phenylbenzophenone, ITX or combinations thereof.

3. An ink-jet ink as claimed in claim 1 or 2, wherein the ink comprises at least one monofunctional monomer.

4. An ink-jet ink as claimed in claim 3, wherein the ink comprises at least one monofunctional (meth)acrylate monomer.

5. An ink-jet ink as claimed in any of claims 1 to 4, wherein the ink comprises at least one monofunctional (meth)acrylate monomer; and at least one (monofunctional) monomer selected from an N-vinyl amide, an N-acryloyl amine, or a mixture thereof.

6. An ink-jet ink as claimed in any preceding claim, wherein the ink comprises at least one difunctional monomer

7. An ink-jet ink as claimed in claim 6, wherein the ink comprises at least one difunctional (meth)acrylate monomer.

8. An ink-jet ink as claimed in any preceding claim, wherein the ink comprises at least one difunctional (meth)acrylate monomer; and at least one (monofunctional) monomer selected from an N-vinyl amide, an N-acryloyl amine, or a mixture thereof.

9. An ink-jet ink as claimed in any preceding claim, wherein the ink comprises at least one monofunctional monomer and at least one difunctional monomer.

10. An ink-jet ink as claimed in claim 9, wherein the ink comprises at least one monofunctional (meth)acrylate monomer and at least one difunctional (meth)acrylate monomer.

11. An ink-jet ink as claimed in any preceding claim, wherein the ink contains 15 wt% or less of multifunctional monomers based on the total weight of the ink.

12. An ink-jet ink as claimed in any preceding claim, wherein the ink further comprises a functional oligomer.

13. An ink-jet ink as claimed in any preceding claim, wherein the ink is substantially free of water and volatile organic solvents.

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

15. A substrate having the ink-jet ink as claimed in any of claims 1 to 13 printed thereon.

16. An ink-jet ink cartridge containing the ink-jet ink as claimed in any of claims 1 to 13.

17. An ink-jet ink set comprising a plurality of ink-jet inks, wherein each ink in the set is an ink as claimed in any of claims 1 to 13.