WO2013034880A2

WO2013034880A2 - Radiation curable inks

Info

Publication number: WO2013034880A2
Application number: PCT/GB2012/051800
Authority: WO
Inventors: Nigel Gould
Original assignee: Sericol Limited
Priority date: 2011-09-06
Filing date: 2012-07-26
Publication date: 2013-03-14
Also published as: GB2511927B; GB201115363D0; WO2013034880A3; GB2511927A; GB201402012D0

Abstract

A radiation-curable ink comprising: (a)a polyurethane having ethylenically unsaturated groups; (b)a water-soluble triacrylate; (c)a colorant; (d)a liquid medium comprising water and organic solvent; and (e)optionally a photoinitiator; and (f)optionally a surfactant; wherein the weight of component (d) is greater than the combined weight of components (a), (b), (c), (e) and (f).

Description

RADIATION CURABLE INKS

This invention relates to radiation curable inks and to their use in ink jet printing.

Many ink jet printing processes employed for the production of large format printed images use solvent- or water-based inks. Such inks have a number of attractive properties, e.g. they can form a small drop size and therefore a high resolution image.

Radiation curable ink jet inks are known. However, many of such inks can cause firing problems in ink jet printers, possibly due to the formation of an insoluble film which blocks the fine nozzles of ink jet printheads.

Conventional aqueous inks typically provide prints suffering from low gloss and/or poor adhesion to plastics substrates.

Therefore there exists a need for aqueous, radiation-curable inks having a low tendency to block printheads and which can provide prints having good gloss, good optical density, water and solvent rub resistance and adhesion to plastics substrates.

US 201 1/0104453 describes aqueous liquid compositions containing a water-soluble monomer, a photoinitiator and a polymer emulsion. The only two Examples containing an organic solvent provided prints which suffered from poor pencil hardness (4B or less), indicating that a weak film was formed from these compositions. This reference does propose any solution to the problems of solvent or water rub resistance.

According to a first aspect of the present invention there is provided a radiation-curable ink comprising:

(a) a polyurethane having ethylenically unsaturated groups;

(b) a water-soluble triacrylate;

(c) a colorant;

(d) a liquid medium comprising water and organic solvent; and

(e) optionally a photoinitiator; and

(f) optionally a surfactant;

wherein the weight of component (d) is greater than the combined weight of components (a), (b), (c), (e) and (f).

In this specification (including its claims), the verb "comprise" and its conjugations is used in its non-limiting sense to mean that items following the word are included (unless optional), but items not specifically mentioned are not excluded. In addition, reference to a feature by the indefinite article "a" or "an" does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements. For example "having one" means having one and only one (not including two or more). The indefinite article "a" or "an" thus usually means "at least one".

Preferably the weight of component (d) is from 0.1 to 40 wt% (preferably 0.5 to 30wt%) greater than the combined weight of components (a), (b), (c), (e) and (f).

Preferably the polyurethane having ethylenically unsaturated groups is a polyurethane having one or more acrylate groups, i.e. an acrylated polyurethane.

Component (a) preferably has an MFFT of below 0°C.

Preferably component (a) is water-dispersible and in fact may be included in the ink as a dispersion in water. When component (a) is included in the ink as a dispersion in water the water is not included when calculating the amount of component (a) present. For example, 10g of a 35% solids aqueous dispersion of polyurethane having ethylenically unsaturated groups would count as 3.5g of polyurethane having ethylenically unsaturated groups.

Polyurethanes having ethylenically unsaturated groups are commercially available as aqueous dispersions from a number of sources for use in wood and floor finishes. For example, Cytec sell such polyurethane dispersions under the Ucecoat trade mark. Examples of commercially available dispersions comprising a polyurethane having ethylenically unsaturated groups include Alberding™ Lux 399 and Lux 515 from Alberdingk Boley, Laromer™ 8949 from BASF and Ucecoat™ 7571 , 7655, 7689, 7699 and 7890 from Cytec, with Ucecoat™ 7655 being preferred.

In one embodiment component (a) is a filtered polyurethane having ethylenically unsaturated groups. In this way one may remove particles which might otherwise block the fine nozzles used in ink jet printheads.

Preferably the composition comprises 2 to 20, more preferably 3 to 15 parts by weight of component (a).

Suitable water-soluble triacrylates include alkoxlated triacrylates, especially alkokylated trimethylolpropane triacrylates, e.g. ethoxylated and/or propoxylated triacrylates.

Examples of water-soluble triacrylates include ethoxylated pentaerythritol triacrylate, ethoxylated trimethylolpropane triacrylate (e.g. 10 to 20, especially 12 to 18, more especially 15 EO trimethylolpropane triacrylate (EO refers to ethylene oxide and 15 refers to the number of ethylene oxide groups, e.g. 15 EO refers to - (CH₂CH₂O)i₅ ^" and 12 EO refers to -(CH₂CH₂O)i₂ ^") and mixtures thereof.

Preferably the composition comprises 2 to 15, more preferably 3 to 15 parts by weight of component (b).

Generally speaking the weight ratio of component (a):(b) is preferably at least 2:1 , more preferably 2:1 to 10:1 , especially 2:1 to 9:1 and more especially 2:1 to 8:1 . This preference is based on the observation that the prints arising from such inks have particularly good properties, for example in terms of their wet rub- fastness, hardness, adhesion and low water-solubility..

The colorant is preferably a pigment-based colorant, especially a colorant comprising a coloured, water-insoluble core and a polymer (e.g . acrylate polymer) shell having water-dispersing groups (e.g. carboxy and/or sulpho groups). Such colorants may be obtained, if desired, by dispersing a pigment in a liquid medium comprising a dispersant having carboxy groups and a crosslinking agent and cross-linking the dispersant with the crosslinking agent, thereby forming a polymer shell having water-dispersing groups around a pigment core. Suitable methods for making such colorants are described in more detail in, for example, WO 2006/064193 and WO 201 1 /008810.

Suitable colorants are also available commercially, in the form of aqueous dispersions, from FUJIFILM Imaging Colorants, e.g. the Pro-Jet APD range of pigment-based colorants. Examples include Pro-Jet™ APD 1000 yellow, magenta cyan and black.

When component (c) is included in the ink as an aqueous dispersion the water is not included when calculating the amount of component (c) present.

Preferably the composition comprises 0.05 to 10, more preferably 0.25 to 5 parts by weight of component (c).

Preferably component (d) comprises water and organic solvent in a weight ratio of 1 :1 to 19:1 , more preferably 1 :1 to 9:1 and especially 3:2 to 9:1 .

In a preferred embodiment the ink comprises at least 50wt% of water.

The organic solvent may be a single organic solvent or more than one organic solvent, preferably one or more water-miscible organic solvents. As will be understood, organic solvents are not radiation-curable and provide the function helping to solubilising one or more of the ink components. Sometimes organic solvents are referred to as inert organic solvents, reflecting the fact that they are not radiation-curable.

Preferred organic solvents comprise water-miscible amides, e.g. optionally substituted cyclic and/or straight chain water-miscible amides and combination comprising two or more thereof.

Examples of suitable amides include pyrrolidones (e.g. 2-pyrrolidone), N- alkyl pyrollidones (e.g. N-ethyl pyrrolidone), Ν,Ν-dialkyl alkylamides (e.g. N,N- dimethyl ethylamide), alkoxylated Ν,Ν-alkyl alkylamides (e.g. Ci_-4-alkoxy-C i_-4alkyl-

CON(Ci_-4-alkyl)₂, especially 3-methoxy-N,N-dimethylpropanamide) and mixtures comprising two or more thereof.

Preferably the organic solvent is selected such that the composition does not require any toxicity labeling. Preferably the composition comprises 60 to 92, more preferably 70 to 90 parts by weight of component (d).

When UV light is used to cure the cure the curable material the composition preferably contains one or more photoinitiators. Whilst any commercially photoinitiators can be used which matches the radiation, those with a low tendency for yellowing are preferred. Examples of suitable photoinitators include alpha-hydroxyalkylphenones, such as 2-hydroxy-2-methyl-1 -phenyl propan-1 -one, 2-hydroxy-2-methyl-1 -(4-tert-butyl-) phenylpropan-1 -one, 2-hydroxy-[4^'-(2- hydroxypropoxy)phenyl]-2-methylpropan-1 -one, 2-hydroxy-1 -[4-(2- hydroxyethoxy)phenyl]-2-methyl propan-1 -one, 1 -hydroxycyclohexylphenylketone and oligo[2-hydroxy-2-methyl-1 -{4-(1 -methylvinyl)phenyl}propanone], alpha- aminoalkylphenones, alpha-sulfonylalkylphenones and acylphosphine oxides such as 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, ethyl-2,4,6- trimethylbenzoylphenylphosphinate and bis(2,4,6-trimethylbenzoyl)- phenylphosphine oxide, benzophenone, 1 -hydroxycyclohexyl phenyl ketone, benzil dimethyl ketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide and mixtures comprising two or more thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure™, Darocur™ and Lucerin™ (from BASF). An especially preferred photoinitiator is Irgacure™ 2959.

Preferably the composition comprises 0.1 to 7.5, more preferably 0.5 to 7 parts by weight of component (e).

Where desired, a surfactant or combination of surfactants may be included in the composition as a wetting agent or to adjust surface tension. Commercially available surfactants may be utilized, including radiation-curable surfactants. Surfactants suitable for use in the composition include non-ionic surfactants, ionic surfactants, amphoteric surfactants and combinations thereof. Preferred surfactants are fluorinated surfactants, for example Zonyl™ FSN.

Preferably the composition comprises 0.01 to 2.5, more preferably 0.025 to 1 .5 parts by weight of component (f).

In view of the foregoing, a preferred radiation-curable ink according to the invention comprises:

2 to 20 (more preferably 3 to 15) parts of component (a);

2 to 15 (more preferably 3 to 15) parts of component (b);

0.05 to 10 (more preferably 0.25 to 5) parts of component (c);

60 to 92 (more preferably 70 to 90) parts of component (d);

0.1 to 7.5 (more preferably 0.5 to 7) parts of component (e); and

0.01 to 2.5 (more preferably 0.025 to 1 .5) parts of component (f);

wherein all parts are by weight. A particularly preferred ink preferred ink according to the invention comprises:

(a) acrylated polyurethane having an MFFT of below 0°C;

(b) 12 to 18 (preferably 15) EO trimethylolpropane triacrylate;

(c) 0.05 to 10 parts of pigment comprising a coloured, water-insoluble core and a polymer shell having water-dispersing groups;

(d) over 65 parts (preferably over 70 parts) of a liquid medium comprising water and 15 to 25 parts (preferably 17 to 23 parts) of organic solvent comprising 2-pyrollidone, N-ethyl pyrollidone and/or 3-methoxy-N,N- dimethylpropanamide;

(e) 0.5 to 7.0 parts of photoinitiator; and

(f) 0.025 to 1 .5 parts of surfactant;

wherein:

(i) the total number of parts of (a) + (b) is from 13 to 19 (preferably 15 to 17); and

(ii) all parts are by weight.

In the above particularly preferred ink, the weight ratio of (a):(b) is preferably 0.9:1 to 2.7:1 . More specifically, when the organic solvent is N-ethyl pyrrolidone the ratio of (a) to (b) is preferably 0.9:1 to 1 :0.9. When the organic solvent is 2-pyrrolidone the ratio of (a) to (b) is preferably 1 :1 .5 to 2.7:1 . When the organic solvent is 3-methoxy-N,N-dimethylpropanamide the ratio of (a) to (b) is preferably 1 :1 .5 to 2.7:1 except when the colorant is black in which case the ratio of (a) to (b) is preferably 1 :1 .5 to 2.3:1 .

Additionally the ink may contain further ingredients, e.g. a stabilizer, pH controller, preservative, viscosity modifier, dispersing agent, inhibitor, antifoam agent, organic/inorganic salt, anionic, cationic, non-ionic and/or amphoteric surfactants and the like in accordance with the object to be achieved.

Preferably the ink is free from acrylic polymers, e.g. free from copolymers comprising acrylic acid.

The radiation-curable ink preferably has a viscosity of 1 to 20 cP, more preferably 2 to 10 cP and especially 3 to 9 cP. Viscosities mentioned in this specification can be measured by any suitable technique, e.g. using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40mm oblique/2° steel cone at 60°C with a shear rate of 25 seconds^"1.

Preferably sufficient ink is applied to the image to provide a coating of cured radiation-curable material of depth 4 microns or greater, especially 7 to 26 microns. The inks of the present invention can be used in ink jet printers to provide images having good properties, e.g. a good optical density, water rub resistance, solvent rub resistance and good adhesion.

According to a second aspect of the present invention there is provided a process for printing an image on a substrate comprising applying an ink according to the first aspect of the present invention to the substrate by means of an ink jet printer and curing the ink.

The ink jet printer may be any of the known types, for example thermal, piezo and paddle-type ink jet printer. Thermal printheads are commonly used in HP and Canon printers, while piezo printheads are common in Epson printers. Paddle-type printers are disclosed in the numerous patents filed by Silverbrook.

The identity of the image is not critical to the present invention. For example, the image may be text, numbers, a picture or a combination of two or more thereof. The image may cover all or just a part of the substrate and may be any colour or combination of colours.

The ink may be cured by irradiating it. The source of radiation may be any source which provides the wavelength and intensity of radiation necessary to cure the ink. A typical example of a UV light source for curing is an H-bulb with an maximum linear power density of 600 Watts/inch (240 W/cm) as supplied by Fusion UV Systems which has emission maxima around 220nm, 255nm, 300nm, 310nm, 365nm, 405nm, 435nm, 550nm and 580nm. Alternatives are the V-bulb and the D-bulb which have a different emission spectrum with main emissions between 350 and 450nm and above 400nm respectively.

In a preferred embodiment the curing is performed using an irradiation source having linear power density below 1 W/cm, more preferably below 0.5W/cm.

An irradiation source having its emissions maximum in the range 253 to 254nm is especially preferred.

Preferably the irradiation is performed using a low pressure mercury or amalgam lamp. Low pressure mercury and amalgam lamps have a much lower linear power density than the typical 80 to 240 W/cm of medium pressure mercury lamps and the spectral profile is different. Historically such lamps are typically used for disinfection purposes.

Examples of low pressure mercury lamps include low output class (0.02 to below 0.1 W/cm²), standard output class (0.1 to below 0.17 W/cm²), high output class (0.17 to below 0.4 W/cm²) and ultra-high output class (0.4 to below 1 W/cm²). The figures in W/cm² may be measured at a distance of either 0.3 metres or 1 metre from the lamp, although it will be understood that during the process the actual distance between the lamp and the transparent composition will almost certainly be different from this distance.

Crystec Technology Trading GmbH provide low pressure mercury lamps in all of the aforementioned output classes, e.g. the "UVL" low output range of lamps, the "SUV" standard output range of lamps, the "EUV" high output range of lamps and the FUV400U ultra-high output lamp. Low pressure mercury lamps may also be obtained from Heraeus Noblelight.

The choice of low pressure mercury lamp depends to some extent on the speed required for cure, with fast printing methods typically requiring a higher linear power density lamp than slower printing methods. Furthermore, when the lamp is included in an ink jet printer a lower linear power density lamp is preferred to avoid the printer from over-heating and to avoid the need to include cooling fans within the printer.

Low pressure mercury lamps are preferred over medium pressure mercury lamps because they are much more efficient in the present process. Approximately 35% of the energy input is converted to UV radiation, 85% of which has a wavelength of 254 nm (UVC). These lamps therefore generate less heat in use than medium pressure mercury lamps, which means that they are more economical to run and less likely to damage temperature-sensitive substrates. Furthermore, low pressure mercury lamps can be manufactured in such a way as not to generate ozone in use and are therefore safer to use than medium pressure mercury lamps.

Although low pressure mercury lamps are used extensively in the water purification industry, they have not yet found widespread application in the printing industry.

Typical medium pressure mercury lamps have linear power density in the range of 80 to 240 W/cm. In contrast, the maximum linear power density for low pressure mercury lamps is around 30 to 440 mW/cm, which means that the peak irradiance of low pressure mercury lamps is also low.

A single low pressure mercury lamp or two or more low pressure mercury lamps can be used for curing the ink. Low pressure mercury lamps are used extensively in the water purification industry and are therefore widely available.

According to a third aspect of the present invention there is provided a ink jet printer cartridge comprising an chamber and an ink, wherein the ink is present in the chamber and is as defined in the first aspect of the present invention.

The invention is illustrated by the following non-limiting examples. The following ingredients were used in the Examples:

Sartomer™ SR9035 is 15 EO trimethylolpropane triacrylate

Irgacure™ 2959 is a photoinitiator Ucecoat 7655 is a 35% solids dispersion in water of an acrylated polyurethane having an MFFT of below 0°C

Pro-Jet™ APD 1000 yellow, magenta, cyan and black are colorants, available from FUJIFILM Imaging Colorants. Each has a 14wt% pigment solids content except for the magenta which has a 16.6% solids content.

Zonyl™ FSN is a fluorosurfactant

Examples 1 to 4

Inks were prepared by mixing the ingredients described in Table 1 .

For the purpose of determining the properties of the ink, each ink was drawn down onto Mactac JT5929P self adhesive PVC using a 12 micron K bar, dried for 3 minutes in an oven then cured by passing them through a UV cure unit fitted 1 x 80w/cm medium pressure mercury lamp at a speed of 20 metres/min. The resultant cured inks were then tested as follows and the results are shown in Table 1 :

Print optical density was measured by spectrophotometry using a Gretag Macbeth Spectroeye.

Solvent rub resistance - each test sample was rubbed with a soft cloth impregnated with isopropyl alcohol, the number of double rubs being taken to remove the cured ink was noted.

Water rub resistance - each test sample was rubbed with a soft cloth impregnated with water, the number of double rubs being taken to remove the cured ink was noted.

Crosshatch Tape Adhesion - 3M scotch tape was securely applied to each test sample and removed with a sharp tug. The degree of cured ink removal was scored 1 for complete removal and 5 for no visible removal.

Table 1

Table 2

Table 3

Claims

1 . A radiation-curable ink comprising:

(a) a polyurethane having ethylenically unsaturated groups;

(b) a water-soluble triacrylate;

(c) a colorant;

(d) a liquid medium comprising water and organic solvent; and

(e) optionally a photoinitiator; and

(f) optionally a surfactant;

2. An ink according to claim 1 wherein component (d) comprises water and organic solvent in a weight ratio of 1 :1 to 19:1 .

3. An ink according to claim 1 or 2 which comprises at least 50wt% of water.

4. An ink according to any one of the preceding claims wherein the weight ratio of component (a):(b) is at least 2:1 .

5. An ink according to any one of the preceding claims wherein the organic solvent comprises an optionally substituted, cyclic water-miscible amides, and/or an optionally substituted straight chain water-miscible amides.

6. An ink according to any one of the preceding claims wherein the organic solvent comprises a pyrrolidone, an N-alkyl pyrollidone, an Ν,Ν-dalkyl alkylamide, an alkoxylated N, N-alkyl alkylamides or a mixture comprising two or more thereof.

7. An ink according to any one of the preceding claims wherein the organic solvent comprises 2-pyrrolidone, N-ethyl pyrrolidone, N,N- dimethyl ethylamide, 3- methoxy-N,N-dimethylpropanamide or a mixture comprising two or more thereof.

8. An ink according to any one of the preceding claims wherein component (a) comprises an acrylated polyurethane.

9. An ink according to any one of the preceding claims wherein component (a) comprises a polyurethane having ethylenically unsaturated groups and has an MFFT of below 0°C.

10. An ink according to any one of the preceding claims wherein component (b) comprises an alkokylated trimethylolpropane triacrylate.

1 1 . An ink according to any one of the preceding claims wherein component (b) comprises one or more 12 to 18 EO trimethylolpropane triacrylate.

12. An ink according to any one of the preceding claims wherein component (c) comprises a coloured, water-insoluble core and a polymer shell having water- dispersing groups.

13. A radiation-curable according to any one of the preceding claims comprising:

2 to 20 parts of component (a);

2 to 15 parts of component (b);

0.05 to 10 parts of component (c);

60 to 92 parts of component (d);

0.1 to 7.5 parts of component (e); and

0.01 to 2.5 parts of component (f);

wherein all parts are by weight.

14. A radiation-curable according to any one of claims 1 to 13 comprising:

(a) an acrylated polyurethane having an MFFT of below 0°C;

(b) a 12 to 18 EO trimethylolpropane triacrylate;

(d) over 65 parts of a liquid medium comprising water and 15 to 25 parts of organic solvent comprising 2-pyrollidone, N-ethyl pyrollidone and/or 3- methoxy-N,N-dimethylpropanamide;

(e) 0.5 to 7.0 parts of photoinitiator; and

(f) 0.025 to 1 .5 parts of surfactant;

wherein:

(i) the total number of parts of (a) + (b) is from 13 to 19; and

(ii) all parts are by weight.

15. A process for printing a substrate comprising applying thereto an ink according to any one of the preceding claims, drying and then irradiating the ink.

16. A process according to claim 15 which comprises irradiating the ink with UV light.

17. A process according to claim 15 or 16 wherein the ink is applied to the substrate by means of an ink jet printer.

18. An ink jet printer cartridge comprising a chamber and an ink, wherein the ink is present in the chamber and the ink is as defined in any one of claims 1 to 14.

19. An ink jet printer comprising a means for irradiating UV light and an ink according to any one of claims 1 to 14 and/or a cartridge according to claim 18.