WO2014041346A1

WO2014041346A1 - Printing ink

Info

Publication number: WO2014041346A1
Application number: PCT/GB2013/052373
Authority: WO
Inventors: Daniel LIPSCOMBE
Original assignee: Sericol Limited
Priority date: 2012-09-11
Filing date: 2013-09-11
Publication date: 2014-03-20
Also published as: GB201506137D0; GB201216159D0; GB2520902A; GB2520902B

Abstract

The present invention provides a hybrid inkjet ink comprising: (i) at least 30% by weight of an organic solvent based on the total weight of the ink; (ii) one or more radiation-curable oligomers; (iii) optionally one or more radiation-curable monomers; (iv) a photoinitiator; (v) optionally a colorant; (vi) 1 -5% by weight, based on the total weight of the ink, of a passive thermoplastic resin having a weight-average molecular weight of 70-200 KDa; and (vii) 2-10% by weight, based on the total weight of the ink, of polymeric beads having a particle size of 0.5-1.5 μm.

Description

Printing ink

The present invention relates to a printing ink, and particularly to a hybrid inkjet ink providing a matt finish.

Digital inkjet printing is becoming an increasingly popular method for the production of fine graphic images for advertising, due to its low implementation cost and versatility in comparison with traditional techniques such as lithographic and screen printing. Inkjet printers comprise one or more printheads that include a series of nozzles through which ink is ejected onto a substrate. The printheads are typically provided on a printer carriage that traverses the print width (moves back and forth across the substrate) during the printing process.

Two main ink chemistries are used: inks that dry by solvent evaporation and inks that dry by exposure to actinic radiation (typically UV radiation). Wide-format solvent-based inkjet printers are an economic route into the industry as they are a relatively low-cost option compared to the more complex machines employed for UV curing. Solvent-based inkjet printing also has other advantages. As well as the lower cost, the ink films produced are thinner (and therefore flexible) and yield a good quality natural looking image with a gloss finish. Furthermore, it is difficult to achieve very high pigment loadings in UV curable inks owing to the high viscosity of the ink: if too much pigment is added, the ink becomes too viscous and cannot be jetted. In contrast, solvent-based inks include a high proportion of solvent and therefore have a lower viscosity, which means that higher pigment loadings can be tolerated. In addition, the printed film produced from solvent-based inkjet inks is formed predominantly of pigment along with comparatively few other solids that are included in the ink. The pigment is therefore largely unobscured, resulting in intense, vivid and vibrant colours and a large colour gamut.

However, there are some limitations to solvent-based inkjet technology. In particular, solvent- based inks may not adhere to certain types of substrate, particularly non-porous substrates such as plastics, and the cured films have poor resistance to solvents. However, the printing of high-quality low-intercolour-bleed inkjet images with good mechanical and chemical resistance properties onto non-solvent-receptive substrates is a requirement in many industrial printing applications. In addition, inkjet inks capable of being printed at small drop sizes and hence producing the required high image quality have a number of formulation constraints, including the requirement for low viscosity in order to be printed through these low drop volume printheads. This is easily achievable with solvent-based ink compositions owing to the inherently low viscosity of the organic solvents used. However, these types of ink often have poor chemical and scratch resistance and can have difficulty in drying on these non-solvent-receptive materials.

To give adequate head stability, solvent-based inkjet inks are typically formulated with relatively low evaporation rate solvents and the inks rely on both evaporation and imbibition into the substrate to give adequate pinning of the ink droplets to fix the image quality (the term "pinning" is used in the art to mean arresting the flow of the ink by treating the ink droplets quickly after they have impacted onto the substrate surface). If the solvent is not able to penetrate into the substrate after deposition of the ink droplet, the rate of viscosity increase is too slow resulting in excessive bleed. If faster evaporating solvents are used in an attempt to overcome this problem head stability can be compromised through solvent loss leading to build up of dried ink deposits on the head face plate. In addition the use of faster solvent blends can also give rise to undesirable Marangoni effects, where faster evaporation at the edge of the ink deposit gives rise to a surface tension gradient which drives a bulk flow to the print edges (the so-called "coffee stain effect").

Conventional UV-curable inkjet inks have excellent head stability and typically have better mechanical and chemical resistance properties than solvent-based inks. Image quality is less affected by the nature of the substrate as the droplet is cured or partially pinned by exposure to ultraviolet light immediately after deposition. However, the inherently higher viscosity of the radiation-curable materials greatly restricts the formulation latitude and in practice inks with suitably low viscosities have poor mechanical and chemical resistance properties.

Hybrid radiation-curable/solvent-containing inkjet inks (see, for example, WO 201 1/021052) can overcome most of the above limitations and allow UV-curable inks to be formulated to meet the low viscosity requirements (previously met by purely solvent-based inks) whilst still maintaining the chemical resistance and mechanical properties (as previously provided mainly by UV-curable inks) required for these industrial applications. However, in common with purely solvent-based inks, there are limitations on the finish to the image produced.

Traditional UV inkjet printers produce prints with a matt finish owing to the way in which the ink is cured almost immediately after it is jetted onto the substrate. The ink droplets cannot flow before curing so the surface of the cured film is microscopically rough and so appears matt.

With radiation-curable/solvent-containing inkjet inks, the printed ink film is not cured immediately after printing, but curing occurs some time after printing. This delay in curing allows the ink droplets to flow producing a smooth film surface which after curing gives a glossy appearance. In some graphic display applications the glossy nature of the cured print is undesirable and a matt finish is required. The conventional way in which curable systems have been matted involves the addition of relatively large particles (typically greater than 10 μηι) which, owing to their large size, protrude from the coating surface and so disrupt the reflected light, producing a matt finish. However, on account of the small diameter of the nozzles used by the printheads in inkjet printers, any particles incorporated into an inkjet ink or ink must be small enough so as to avoid any blockage of the nozzles (in the region of 1 μηι or less).

Owing to the large particle size of commercially available matting agents combined with the size requirements for inkjet printing, no inkjet-printable matt ink currently exists in the market place.

Accordingly, the present invention provides a hybrid inkjet ink comprising: (i) at least 30% by weight of an organic solvent based on the total weight of the ink; (ii) one or more radiation- curable oligomers; (iii) optionally one or more radiation-curable monomers; (iv) a photoinitiator; (v) optionally a colorant; (vi) 1 -5% by weight, based on the total weight of the ink, of a passive thermoplastic resin having a weight-average molecular weight of 70-200 KDa; and (vii) 2-10% by weight, based on the total weight of the ink, of polymeric beads having a particle size of 0.5-1 .5 μηι.

Thus, it has surprisingly been found that a matting effect may be achieved by using a low volume of a high molecular weight resin combined with sub-micron particles, dispersed in a solvent. When printed onto a substrate, and the solvent evaporated, there is so little resin in the ink that a layer of resin is deposited which is thin enough to allow the particles to protrude from the coating surface and so disrupt the reflected light, giving a matt finish.

The ink of the present invention comprises a modified ink binder system. The presence of a radiation-curable material and a photoinitiator in the ink means that crosslinked polymers can be formed in the dried ink film, leading to improved adhesion to a range of substrates and improved resistance to solvents. The presence of at least 30% by weight of organic solvent means that the advantageous properties of solvent-based inkjet inks are maintained.

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

The radiation-curable material includes a radiation-curable oligomer, either alone or as a mixture with a radiation-curable monomer. The monomers/oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality monomers/oligomers may be used. Radiation-curable oligomers suitable for use in the present invention comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation polymerisable groups. The oligomer preferably comprises a urethane backbone. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. Preferably the oligomers are (meth)acrylate oligomers. Preferably they are multifunctional and most preferably have a functionality of 2-6.

Particularly preferred radiation-curable materials are urethane acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are tri-, tetra-, penta-, hexa- or higher functional urethane acrylates, particularly hexafunctional urethane acrylates as these yield films with good solvent resistance.

Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance.

Preferred oligomers have a molecular weight of 450 to 4,000, more preferably 600 to 4,000. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

In one embodiment the radiation-curable oligomer polymerises by free-radical polymerisation.

The radiation-curable oligomer used in the ink of the invention cures upon exposure to radiation in the presence of a photoinitiator to form a crosslinked, solid film. The resulting film has good adhesion to substrates and good solvent resistance. Any radiation-curable oligomer that is compatible with the remaining ink components and that is capable of curing to form a crosslinked, solid film is suitable for use in the ink of the present invention. Thus, the ink formulator is able to select from a wide range of suitable oligomers. In particular, the oligomer can be a low molecular weight material that is in liquid form at 25°C. This is beneficial when aiming to produce a low viscosity ink. Furthermore, the use of a low molecular weight, liquid oligomer is advantageous when formulating the ink because low molecular weight liquid oligomers are likely to be miscible in a wide range of solvents. Preferred oligomers for use in the invention have a viscosity of 0.5 to 20 Pa.s at 60°C, more preferably 5 to 15 Pa.s at 60°C and most preferably 5 to 10 Pa.s at 60°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25 seconds^" . The total amount of the radiation-curable oligomer present in the ink is preferably 5 to 30% by weight based on the total weight of the ink, more preferably 10 to 25% by weight, and most preferably 15% to 20% by weight. If the amount is too high, the polymeric beads become enrobed by the film and cannot effect the required light scattering.

In an alternative embodiment of the invention, the radiation-curable material is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes, cycloaliphatic epoxides, bisphenol A epoxides, epoxy novolacs and the like. The radiation-curable material according to this embodiment may comprise a mixture of cationically curable monomer and oligomer. For example, the radiation-curable material may comprise a mixture of an epoxide oligomer and an oxetane monomer.

The radiation-curable material can also comprise a combination of free-radical polymerisable and cationically polymerisable materials.

The ink optionally contains radiation-curable monomers. Suitable free-radical polymerisable monomers are well known in the art and include (meth)acrylates, α,β-unsaturated ethers, vinyl amides and mixtures thereof. Monofunctional (meth)acrylate monomers are well known in the art and are preferably the esters of acrylic acid. Preferred examples include phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), isobornyl acrylate (IBOA), tetrahydrofurfuryl acrylate (THFA), 2-(2- ethoxyethoxy)ethyl acrylate, octadecyl acrylate (ODA), tridecyl acrylate (TDA), isodecyl acrylate (IDA) and lauryl acrylate.

Suitable multifunctional (meth)acrylate monomers include di-, tri- and tetra-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the ink-jet inks include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate), dipropylene glycol diacrylate, tri(propylene glycol) triacrylate, neopentyl glycol 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. Suitable multifunctional (meth)acrylate monomers also 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. (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. α,β-Unsaturated ether monomers can polymerise by free-radical polymerisation and may be useful for reducing the viscosity of the ink when used in combination with one or more (meth)acrylate monomers. Examples are well known in the art and include vinyl ethers such as triethylene glycol divinyl ether, diethylene glycol divinyl ether, 1 ,4-cyclohexanedimethanol divinyl ether and ethylene glycol monovinyl ether. Mixtures of α,β-unsaturated ether monomers may be used.

N-Vinyl amides and N-(meth)acryloyl amines may also be used in the ink of the invention. 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. A preferred example is N- acryloylmorpholine (ACMO).

Preferably the ink comprises less than 20% by weight of radiation-curable monomers (e.g. having a molecular weight of less than 450) based on the total weight of the ink, or less than 10% by weight, more preferably less than 5% by weight.

The total amount of the radiation-curable oligomer and radiation-curable monomer present in the ink is 10 to 65% by weight based on the total weight of the ink. The ink of the invention includes one or more photoinitiators. When the ink of the invention includes a free-radical polymerisable material the photoinitiator system includes a free-radical photoinitiator and when the inks include a cationic polymerisable material the photoinitiator system includes a cationic photoinitiator. When the inks comprise a combination of free- radical polymerisable and cationically polymerisable materials both a free-radical and cationic initiator are required. The ink is preferably free-radical polymerisable.

The free-radical photoinitiator can be selected from any of those known in the art. For example, benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 1 -[4-(2-hydroxyethoxy)-phenyl]- 2-hydroxy-2-methyl-1 -propane-1 -one, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan- 1 -one, isopropyl thioxanthone, 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 and Darocur (from Ciba) and Lucerin (from BASF). In the case of a cationically curable system, any suitable cationic initiator can be used, for example sulfonium or iodonium based systems. Non limiting examples include: Rhodorsil PI 2074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCB Chemicals; and Esacure 1064 from Lamberti spa.

Preferably the photoinitiator is present in an amount of 1 to 20% by weight, preferably 4 to 10% by weight, based on the total weight of the ink.

The ink also contains a passive (or "inert") thermoplastic resin. Passive resins are resins which do not enter into the curing process, i.e. the resin is free of functional groups which polymerise under the curing conditions to which the ink is exposed. In other words, resin is not a radiation-curable material. The resin may be selected from epoxy, polyester, vinyl, ketone, nitrocellulose, phenoxy or acrylate resins, or a mixture thereof and is preferably a poly(methyl (meth)acrylate) resin. The resin has a weight-average molecular weight of 70- 200 KDa and preferably 100-150 KDa, as determined by GPC with polystyrene standards. A particularly preferred resin is Paraloid® A1 1 from Rohm and Haas. The resin is present at 1 - 5% by weight, based on the total weight of the ink.

It has been found that the combination of the oligomer and the passive thermoplastic resin provide the necessary binding properties to hold the polymeric beads in place on the substrate after the ink has cured/dried to a solid film, whilst providing an appropriate viscosity for jetting, and providing the required stability to the ink suspension on storage, i.e. prior to jetting. The ink of the invention contains an organic solvent. The organic solvent is in the form of a liquid at ambient temperatures and is capable of acting as a carrier for the remaining components of the ink. The organic solvent component of the ink may be a single solvent or a mixture of two or more solvents. As with known solvent-based inkjet inks, the organic solvent used in the ink of the present invention is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry. The solvent can be selected from any solvent commonly used in the printing industry, such as glycol ethers, glycol ether esters, alcohols, ketones, esters, organic carbonates, lactones and pyrrolidones. The organic solvent is present in an amount of at least 30% by weight, preferably at least 50% by weight, and most preferably at least 60% by weight based on the total weight of the ink. The upper limit is typically 85% or 75% by weight based on the total weight of the ink. Known solvent-based inkjet inks dry solely by solvent evaporation with no crosslinking or polymerisation occurring. The film produced therefore has limited chemical resistance properties. In order to improve resistance of prints to common solvents such as alcohols and petrol, binder materials that have limited solubility in these solvents are added to the ink. The binder is typically in solid form at 25°C so that a solid printed film is produced when solvent is evaporated from the ink. Suitable binders such as vinyl chloride copolymer resins generally have poor solubility in all but the strongest of solvents such as glycol ether acetates and cyclohexanone, both of which are classified as "harmful" and have strong odours. In order to solubilise the binder, these solvents are generally added to the ink. The ink of the present invention includes radiation-curable material that cures as the ink dries and it is not therefore necessary to include a binder in the ink in order to provide a printed film having improved solvent resistance. In one embodiment of the invention the organic solvent is not therefore required to solubilise a binder such as a vinyl chloride copolymer resin, which means that the ink formulator has more freedom when selecting a suitable solvent or solvent mixture.

In a preferred embodiment the organic solvent is a low toxicity and/or a low odour solvent. Solvents that have been given VOC exempt status by the United States Environmental Protection Agency or European Council are also preferred.

The most preferred solvents are selected from glycol ethers and organic carbonates and mixtures thereof. Cyclic carbonates such as propylene carbonate and mixtures of propylene carbonate and one or more glycol ethers are particularly preferred. Alternative preferred solvents include lactones, which have been found to improve adhesion of the ink to PVC substrates. Mixtures of lactones and one or more glycol ethers, and mixtures of lactones, one or more glycol ethers and one or more organic carbonates are particularly preferred. Mixtures of gamma butyrolactone and one or more glycol ethers, and mixtures of gamma butyrolactone, one or more glycol ethers and propylene carbonate are particularly preferred.

In another embodiment of the invention, dibasic esters and/or bio-solvents may be used. Dibasic esters are known solvents in the art. They can be described as di(C -C₄ alkyl) esters of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms having following general formula:

in which A represents (CH₂) i -₆, and R and R² may be the same or different and represent C C₄ alkyl which may be a linear or branched alkyl radical having 1 to 4 carbon atoms, preferably methyl or ethyl, and most preferably methyl. Mixtures of dibasic esters can be used.

Bio-solvents, or solvent replacements from biological sources, have the potential to reduce dramatically the amount of environmentally-polluting VOCs released in to the atmosphere and have the further advantage that they are sustainable. Moreover, new methods of production of bio-solvents derived from biological feedstocks are being discovered, which allow bio- solvent production at lower cost and higher purity.

Examples of bio-solvents include soy methyl ester, lactate esters, polyhydroxyalkanoates, terpenes and non-linear alcohols, and D-limonene. Soy methyl ester is prepared from soy. The fatty acid ester is produced by esterification of soy oil with methanol. Lactate esters preferably use fermentation-derived lactic acid which is reacted with methanol and/or ethanol to produce the ester. An example is ethyl lactate which is derived from corn (a renewable source) and is approved by the FDA for use as a food additive. Polyhydroxyalkanoates are linear polyesters which are derived from fermentation of sugars or lipids. Terpenes and non- linear alcohols may be derived from corn cobs/rice hulls. An example is D-limonene which may be extracted from citrus rinds.

Other solvents may be included in the organic solvent component. A particularly common source of other solvents is derived from the way in which the colouring agent is introduced into the inkjet ink formulation. The colouring agent is usually prepared in the form of a pigment dispersion in a solvent, e.g. 2-ethylhexyl acetate. The solvent tends to be around 40 to 50% by weight of the pigment dispersion based on the total weight of the pigment dispersion and the pigment dispersion typically makes up around 5 to 15% by weight of the ink and sometimes more.

The ink is preferably substantially free of water, although some water will typically be absorbed by the ink from the air or be present as impurities in the components of the inks, and such levels are tolerated. For example, the ink may comprise less than 5% by weight of water, more preferably less than 2% by weight of water and most preferably less than 1 % by weight of water, based on the total weight of the ink.

In order to provide the matt finish, the ink also contains polymeric beads. Such beads are known in the art. They are spherical and have a particle size (average geometric diameter) of 0.5-1 .5 μηι and preferably 0.7-1.0 μηι. The beads are typically formed of cross-linked polymethyl(meth)acrylate, cross-linked polybutyl(meth)acrylate or cross-linked polystyrene. The beads are present at 2-10% by weight, based on the total weight of the ink, preferably 3- 7% by weight.

The ink of the present invention may be a coloured or a colourless ink. By "colourless" is meant that the ink is free of colorant such that no colour can be detected by the naked eye. Minor amounts of colorant that do not produce colour that can be detected by the eye can be tolerated, however. Typically the amount of colorant present will be less than 0.3% by weight based on the total weight of the ink, preferably less than 0.1 %, more preferably less than 0.03%. Colourless inks may also be described as "clear" or "water white". Colourless inks may also be used as a varnish, where it is applied over a coloured ink. For the avoidance of doubt, coloured inks include white inks. The coloured inks comprise at least one colouring agent. The colouring agent 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 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. Especially useful are black and the colours required for trichromatic process printing. Mixtures of pigments may be used. In one aspect the following pigments are preferred. Cyan: phthalocyanine pigments such as Phthalocyanine blue 15.4. Yellow: azo pigments such as Pigment yellow 120, Pigment yellow 151 and Pigment yellow 155. Magenta: quinacridone pigments, such as Pigment violet 19 or mixed crystal quinacridones such as Cromophtal Jet magenta 2BC and Cinquasia RT-355D. Black: carbon black pigments such as Pigment black 7.

Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 μηι, preferably less than 5 μηι, more preferably less than 1 μηι and particularly preferably less than 0.5 μηι. The colorant is preferably present in an amount of 20 weight% or less, preferably 10 weight% or less, more preferably 8 weight% or less and most preferably 2 to 5% by weight, based on the total weight of the ink. A higher concentration of pigment may be required for white inks, however, for example up to and including 30 weight%, or 25 weight% based on the total weight of the ink.

The inks may be in the form of an ink set comprising a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). The inks in a trichromatic set can be used to produce a wide range of colours and tones.

The inkjet ink exhibits a desirable low viscosity (200 mPa.s or less, preferably 100 mPa.s or less, more preferably 25 mPa.s or less, more preferably 10 mPa.s or less and most preferably 7 mPa.s or less at 25 °C). In order to produce a high quality printed image a small jetted drop size is desirable. Furthermore, small droplets have a higher surface area to volume ratio when compared to larger drop sizes, which facilitates evaporation of solvent from the jetted ink. Small drop sizes therefore offer advantages in drying speed. Preferably the inkjet ink of the invention is jetted at drop sizes below 50 picolitres, preferably below 30 picolitres and most preferably below 10 picolitres.

To achieve compatibility with print heads that are capable of jetting drop sizes of 50 picolitres or less, a low viscosity ink is required. A viscosity of 15 mPa.s or less at 25°C is preferred, for example, 2 to 12 mPas, 8 to 1 1 mPa.s, or 10 to 1 1 mPa.s.

It is problematic to achieve these low viscosities with conventional radiation-curable inks owing to the relatively high viscosities of acrylate monomers and oligomers used in the compositions, but the presence of a significant amount of organic solvent in the ink of the invention allows these low viscosities to be achieved.

Ink viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25 °C with spindle 00. 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. In one aspect of the invention the surface tension of the ink is controlled by the addition of one or more surface active materials such as commercially available surfactants. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example, plastic substrates. Too high a surface tension can lead to ink pooling and/or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks. The surface tension is preferably in the range of 20-32 mNm^" and more preferably 21 -27 mNm^" .

Print heads account for a significant portion of the cost of an entry level printer and it is therefore desirable to keep the number of print heads (and therefore the number of inks in the ink set) low. Reducing the number of print heads can reduce print quality and productivity. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity. Examples of substrates include those composed of PVC, polyester, polyethylene terephthalate (PET), PETG, polyethylene and polypropylene.

The ink may be prepared by known methods such as stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill.

The ink may be used as a coloured ink or as a varnish. The present invention therefore provides two methods of using the ink.

Firstly, the present invention provides a method of inkjet printing comprising the following steps, in order:

(i) providing an ink as claimed in any preceding claim, wherein the ink contains a colorant;

(ii) inkjet printing the ink onto a substrate; and

(iii) evaporating the solvent from the ink, and exposing the ink to actinic radiation to cure the ink to form an image.

Secondly, the present invention provides a method of inkjet printing comprising the following steps, in order:

(i) forming an image on a substrate;

(ii) providing an ink as claimed in any of claims 1 to 6, wherein the ink is free of colorant; (iii) inkjet printing the ink over the image formed in step (i); and

(iv) evaporating the solvent from the ink, and exposing the ink to actinic radiation to cure the ink.

The printing is preferably all performed by inkjet printing, e.g. on a roll-to-roll printer or flat-bed printer. Evaporation of the solvent can occur simply by exposure of the inks to the atmosphere, but the inks may also be heated to accelerate evaporation. In addition, the inks are exposed to actinic radiation to cure the ink. The ink is then applied over the thus-formed image. It should be noted that the terms "dry" and "cure" are often used interchangeably in the art when referring to radiation-curable inkjet inks to mean the conversion of the inkjet ink from a liquid to solid by polymerisation and/or crosslinking of the radiation-curable material. Herein, however, by "drying" is meant the removal of the solvent by evaporation and by "curing" is meant the polymerisation and/or crosslinking of the radiation-curable material.

On account of their lower productivity, the capital cost for solvent printers has to be relatively low to remain commercially viable. The internal mechanisms are therefore kept simple, with as few printheads as possible to produce a reasonable quality image. The low complexity makes these machines easy to operate and maintain.

Over recent years, UV curable ink systems have largely replaced solvent ink printers in the higher productivity range, wide format graphics market. Unlike solvent printers, the ink deposited on the surface does not appreciably evaporate upon heating. Instead, the material is transformed into a solid through exposure to an energy source. In most cases, the energy source is an intense UV light, which causes photo-crosslinking of curable molecules in the presence of a photoinitiator to form a solid.

The greatest perceived benefit of UV curable printers is their ability to deliver high production rates. In most UV printers, the cure source is mounted on the shuttling printhead carriage, on one or both sides of the printhead cluster. In some cases, cure systems are also placed between printheads. With a typical separation distance of less than 100 mm between the print heads and cure unit, the maximum time between print and cure would be 0.1 s for a printhead carriage moving at 1 m/s. UV ink solidification times of less than one second compare favourably with solvent inks that can take several minutes to dry. Inkjet printers for UV curable inks are necessarily more complex and consequently more expensive than inkjet inks printers for solvent-based inks, however.

The ink of the present invention can be printed using inkjet printers that are suitable for use with solvent-based inkjet inks, in combination with a source of actinic radiation.

The features of printers that are suitable for printing solvent-based inkjet inks are well known to the person skilled in the art and include the features described below.

As discussed above, printers suitable for printing solvent-based inkjet inks typically have a low capital cost, which means that the printers tend to have simple internal mechanisms. In practice, this means that inkjet printers suitable for printing solvent-based inks typically comprise gravity feed systems for delivering ink from the ink supply to the printhead. In contrast, UV printers use a pressurised header tank for delivering the ink to the printhead, which allows control of the meniscus position in the nozzle.

Since printheads account for a large proportion of the overall printer cost, inkjet printers suitable for printing solvent-based inkjet inks include the minimum number of printheads that is required to provide a high quality image. In any event, because solvent-based inkjet inks typically require longer to dry than UV inks, there is less advantage in using many printheads to apply large quantities of ink to the substrate because this causes the ink to pool and the image to blur.

Furthermore, printheads that are for printing solvent-based inkjet inks are not provided with a means for heating the ink because solvent-based inks have a low viscosity and do not therefore require heating at the printhead to produce a jettable viscosity (in contrast with UV curable inks). Thus, known solvent-based inks are jetted at ambient temperatures.

Solvent-based inkjet inks are susceptible to drying on the nozzle plate due to evaporation of the solvent. Printers for solvent-based inkjet inks therefore typically include suction cups which can be used to cap the printheads when not in use, allowing a solvent vapour saturated environment to be established, which limits evaporation. Should a printhead become blocked, the suction cup can be used to pull a small volume of ink through the blockage, using a peristaltic pump, to recover performance after excess ink is removed using a wiper blade.

Typically the printheads of inkjet printers for solvent-based inks are not externally heated. The ink of the present invention can be jetted at ambient temperature, preferably below 35°C, or below 30°C or about 25°C, and are therefore compatible with the printheads and nozzles that are used to print solvent-based inkjet inks. The use of a printer that is for printing conventional solvent-based inkjet inks, particularly printheads, nozzles and ink delivery systems that are for use with conventional solvent-based inkjet inks, as the basis of the printing apparatus of the invention means that printing apparatus of the invention has a low capital cost. A printer that is suitable for printing a conventional solvent-based inkjet ink may be adapted before use in the present invention. Depending on the exact nature of the ink and the location of the cure source, opaque ink feed components that are chemically compatible with the ink may be used and/or a UV screen filter film may be applied to the print window on the front of the apparatus. These are minor adaptations that would not have a significant effect on printer cost or performance. In one embodiment, the printing apparatus of the present invention comprises one or more piezo drop on demand printheads. Preferably the printheads are capable of jetting ink in drop sizes of 50 picolitres or less, more preferably 30 picolitres or less, particularly preferably 10 picolitres or less.

The printing apparatus of the present invention comprises means for evaporating solvent from the ink at the appropriate time after the ink has been applied to the substrate. Any means that is suitable for evaporating solvent from known solvent-based inkjet inks can be used in the apparatus of the invention. Examples are well known to the person skilled in the art and include dryers, heaters, air knives and combinations thereof.

In one embodiment, the solvent is removed by heating. Heat may be applied through the substrate and/or from above the substrate, for example by the use of heated plates (resistive heaters, inductive heaters) provided under the substrate or radiant heaters (heater bars, IR lamps, solid state IR) provided above the substrate. In a preferred embodiment, the ink can be jetted onto a preheated substrate that then moves over a heated platen. The apparatus of the invention may comprise one or more heaters. When printing the ink of the present invention, a significant portion of the solvent is preferably allowed to evaporate before the ink is cured. Preferably substantially all of the solvent is evaporated before the ink is finally cured. This is achieved by subjecting the printed ink to conditions that would typically dry conventional solvent-based inkjet inks. In the case of the ink of the present invention, such conditions will remove most of the solvent but it is expected that trace amounts of solvent will remain in the film given the presence of the radiation- curable component in the ink.

The solvent evaporation step is thought to be important because it is believed to provide further definition to the image quality. Thus, it is thought that the solvent evaporation step results in a printed image with high gloss, as would be expected for conventional solvent- based inks. Furthermore, the loss of a significant portion of the ink through the evaporation of the solvent leads to the formation of a printed film that is thinner than the film that would be produced by jetting an equivalent volume of known radiation-curable ink. This is advantageous because thinner films have improved flexibility.

Unlike standard solvent-based inks, once the solvent has evaporated, the ink is not expected to be completely solid. Rather, what remains on the surface is a high viscosity version of a radiation-curable ink. The viscosity is sufficiently high to inhibit or significantly hinder ink flow and prevent image degradation in the timescale that is needed to post-cure the ink. Upon exposure to a radiation source, the ink cures to form a relatively thin polymerised film. The ink of the present invention typically produces a printed film having a thickness of 1 to 20 μηι, preferably 1 to 10 μηι, for example 2 to 5 μηι. Film thicknesses can be measured using a confocal laser scanning microscope. The source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source. When LEDs are used, these are preferably provided as an array of multiple LEDs.

Preferably the source of actinic radiation is a source that does not generate ozone when in use. The source of actinic radiation for the initial pinning of the ink may be the same or different to the source of actinic radiation for performing the final cure of the ink.

The source of UV radiation could be situated off-line in a dedicated conveyor UV curing unit, such as the SUVD Svecia UV Dryer. Preferably, however, the source of radiation is situated in-line, which means that the substrate does not have to be removed from the printing apparatus between the heating and curing steps.

The radiation source can be mobile, which means that the source is capable of moving back and forth across the print width, parallel with the movement of the printhead.

In one embodiment, one or more sources of actinic radiation are placed on a carriage that allows the source of actinic radiation to traverse the print width. The carriage may be placed up and downstream of the printer carriage in allow irradiation before and after evaporation of the solvent. In this embodiment the source of actinic radiation moves independently of the printer carriage and movement of the printhead does not therefore have to be slowed in order to provide adequate time for solvent evaporation. Thus, overall productivity can be improved.

When the source of radiation is provided on separate carriage, it is necessary to provide an additional carriage rail, motor and control systems. This adaptation can lead to large increases in equipment costs.

Preferably the source of radiation is static. This means that the source does not move backwards and forwards across the print width of the substrate when in use. Instead the source of actinic radiation is fixed and the substrate moves relative to the source in the print direction. When the source of actinic radiation is provided in the print zone of the printer, light contamination at the printhead, which could lead to premature curing in the nozzle, must be avoided. Adaptations to prevent light contamination, such as lamp shutters, give rise to additional costs. The source of radiation is therefore preferably located outside the print zone of the printing apparatus. By print zone is meant the region of the printing apparatus in which the printhead can move and therefore the region in which ink is applied to the substrate.

A preferred printing apparatus according to the present invention that comprises a static source of radiation located outside the print zone is expected to be economically attractive and therefore suitable for entry level wide format digital graphics use. This embodiment is therefore particularly preferred. By entry level is meant the simplest and cheapest printers that are suitable for wide format digital graphics use. By locating the source of actinic radiation outside the print zone, and by avoiding the use of mobile radiation sources, potentially expensive adaptations to the printing apparatus can be avoided.

Static curing units preferably span the full print width, which is typically at least 1 .6 m for the smaller wide format graphics printers. Fluorescent tubes, mercury discharge lamps, and light emitting diodes can be used as static curing units.

Any of the sources of actinic radiation discussed herein may be used for the initial irradiation of the inkjet ink. The dose of actinic radiation is lower than the dose required to cure the radiation-curable material fully, namely 1 -200 mJ/cm², preferably 1 -100 mJ/cm², more preferably 1 -50 mJ/cm² and most preferably about 35 mJ/cm².

The wavelength of the pinning source is typically 200-700 nm, preferably 300-500 nm and most preferably 350-450 nm.

It is preferable to arrest the flow of the ink by pinning the ink droplets quickly after they have impacted on the substrate surface. To achieve a good quality image it is preferable that the inks are pinned within 5 seconds of impact, preferably within 1 second and most preferably within 0.5 seconds. As a result of the pinning, the viscosity of the ink is increased by polymerisation and/or crosslinking of the radiation-curable material thereby arresting the flow of the ink and improving the final image quality.

Following evaporation of the solvent, the composition is exposed to additional actinic radiation. That is, an additional dose of radiation to that required for pinning. The dose required to achieve the final cure will be higher than the pinning dose. The dose provided results in the formation of a solid film. A suitable dose would be greater than 200 mJ/cm², more preferably at least 300 mJ/cm² and most preferably at least 500 mJ/cm². The upper limit is less relevant and will be limited only by the commercial factor that more powerful radiation sources increase cost. A typical upper limit would be 5 J/cm². The delay between evaporating the solvent and providing a final cure of the ink is less critical than the initial pinning of the ink, but is typically at least 1 minute after jetting.

Particularly when the colourless or white hybrid inkjet ink is inkjet printed, it is beneficial to pin the ink to the substrate. The presence of this colourless or white layer means that it is less important for the coloured ink to be pinned. This is because the colourless or white layer provides a more receptive surface than the substrate. In a preferred embodiment, the coloured ink is not pinned prior to full cure. That is, the only exposure to actinic radiation by the coloured ink is the dose required to achieve full cure. High and medium pressure mercury discharge lamps can be relatively expensive to operate. The lamp units themselves can be heavy and expensive and often additional shielding is required to prevent unintentional UV exposure to the operator. Extraction is also required to remove ozone that is produced by the lamps. Furthermore, where high discharge currents are involved for high output lamps, electronic ballast is required because the resistance of the gas used in the lamp changes during use. High and medium pressure mercury discharge lamps are not therefore preferred UV sources according to the present invention.

LED sources that are currently available are relatively expensive and a printing apparatus comprising a LED source of UV radiation is unlikely to be suitable for use an entry level printer. Thus, a source of actinic radiation comprising currently available LEDs is not preferred. However, development of UV LED sources for curing inks is on-going and it is envisaged that the cost of LED sources will decrease significantly in the future. In this case, a printing apparatus according to the present invention that includes a source of actinic radiation comprising LEDs would be suitable for entry level printing systems.

In one embodiment of the invention, the source of radiation comprises a UV fluorescent lamp.

In another embodiment of the invention the source of radiation comprises one or more flash lamps. Flash lamps operate by discharge breakdown of an inert gas, such as xenon or krypton, between two tungsten electrodes. Unlike mercury discharge lamps, flash lamps do not need to operate at high temperature. Flash lamps also have the advantage of switching on instantaneously, with no thermal stabilisation time. The envelope material can also be doped, to prevent the transmission of wavelengths that would generate harmful ozone. Flash lamps are therefore economical to operate and therefore suitable for use in entry level printers. The invention will now be described with reference to the following examples, which are not intended to be limiting. Examples

Example 1

Inkjet inks were prepared according to the formulations set out in Table 1 . The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink.

Table 1 .

Gamma butyrolactone and diethylene glycol diethyl ether are organic solvents. Nippon Gohsei UV7630B is a hexafunctional urethane acrylate oligomer with a viscosity of 6.9 Pa.s at 60°C. Irgacure 819 and Irgacure 2959 are free-radical photoinitiators. Paraloid A 1 1 is a methyl methacrylate resin having a weight-average molecular weight of 125,000. Techpolymer BMSA 18GN are spherical cross-linked polybutylmethacrylate beads having a particle size (average geometric diameter) of 0.839 μηι. Kronos 2300 is a white pigment (titanium dioxide). BYK 331 is a polyether-modified polydimethylsiloxane and reduces surface tension. Example 2

Inks 1 and 2, as well as a commercially available white ink, Mimaki solvent white SS21 , were analysed for the distribution of fine particles using an analytical centrifuge (a Lumisizer). The Lumisizer provides a measure of the light extinction profile. As the sample is spun in the centrifuge, a light is shone through it. As the particles sediment, more light is able to pass through the sample. The rate of the light extinction curve caused by centrifugal segregation gives an accurate prediction of the long term stability/sedimentation of the sample. The samples were analysed for the extinction variation over the entire sample length of the centrifugation.

White inks are known to separate upon storage on account of the heavy nature of the titanium dioxide pigment particles. This level of separation is considered to be commercially acceptable. The results for particulate settling are set out in Table 2.

Table 2.

The Lumisizer data show that the rate of sedimentation of the polymer beads in ink 1 is about the same as a commercially available white ink (SS21) and the chemically closest ink, ink 2.

Example 3

Ink 1 was coated onto a print from a Mimaki JV400 printer with a 12 μηι RK K-hand coater. The gloss units were measured using a Sheen Instruments Tri-glossmaster. The results are shown in Table 3.

Table 3.

This shows the matt effect provided by ink 1 .

Claims

1 . A hybrid inkjet ink comprising: (i) at least 30% by weight of an organic solvent based on the total weight of the ink; (ii) one or more radiation-curable oligomers; (iii) optionally one or more radiation-curable monomers; (iv) a photoinitiator; (v) optionally a colorant; (vi) 1 -5% by weight, based on the total weight of the ink, of a passive thermoplastic resin having a weight- average molecular weight of 70-200 KDa; and (vii) 2-10% by weight, based on the total weight of the ink, of polymeric beads having a particle size of 0.5-1 .5 μηι.

2. An ink as claimed in claim 1 , wherein the ink comprises less than 5% by weight of water based on the total weight of the ink.

3. An ink as claimed in claim 1 or 2, wherein the total amount of the radiation-curable oligomer and radiation-curable monomer present in the ink is 2 to 65% by weight based on the total weight of the ink.

4. An ink as claimed in any preceding claim, wherein the ink comprises less than 20% by weight of radiation-curable monomers based on the total weight of the ink.

5. An ink as claimed in any preceding claim, wherein the radiation-curable oligomer is multifunctional.

6. An ink as claimed in any preceding claim, wherein the radiation-curable oligomer is a (meth)acrylate oligomer.

7. A method of inkjet printing comprising the following steps, in order:

(ii) inkjet printing the ink onto a substrate; and

8. A method of inkjet printing comprising the following steps, in order:

(i) forming an image on a substrate;

(ii) providing an ink as claimed in any of claims 1 to 6, wherein the ink is free of colorant;

(iii) inkjet printing the ink over the image formed in step (i); and

9. A method as claimed in claims 7 or 8, wherein the printing is performed on a roll-to-roll printer or flat-bed printer.