GB2539840A - Transparent compositions - Google Patents

Abstract

A transparent composition having a boiling point of 140-250°C, a viscosity of 100-2000cP when measured at 25°C, which composition comprises 45-65wt.% radiation-curable oligomer having a molecular weight of 500-4000; 25-40wt.% organic solvent and less than 5wt.% (meth)acrylates having a molecular weight below 600. The composition may also comprise 0-4wt.% silicone acrylate, preferably 0.25-2wt.%. Preferably the composition comprises 0-8wt.% photoinitiator, more preferably 4-8wt.%. The composition may also be colourless and/or odourless. The composition preferably has a flash point of 50-120°C. The organic solvent may be organic carbonate and a glycol ether, preferably propylene carbonate and diethylene glycol diethyl ether. An amine synergist may also be included in the composition. The transparent compositions may be used in printing processes, particularly as a protective layer applied to an inkjet printed image.

Description

TRANSPARENT COMPOSITIONS

This invention relates to transparent compositions suitable for use in a printing process.

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. However the images can also suffer from a number of problems such as poor solvent resistance, scratch resistance and weak adhesion to the substrate. If the substrate is mechanically deformed, e.g. stretched, the image can crack and/or lose its glossiness.

Protecting the image by applying a self-adhesive film is possible. However this adds considerably to the time and cost of preparing the print and considerably reduces its mechanical flexibility.

To address the above problems, solvent-free ultraviolet ("UV") curable ink jet inks have been proposed. Such UV curable inks can provide images having good solvent resistance. However, the high viscosity of UV curable inks results in large drop sizes and therefore relatively low image resolution compared to conventional solvent-and water-based inks. Furthermore, the resultant images can be rather thick and prominent, leading to a rough texture as the print stands out from the substrate. This can in some cases lead to an inflexible, printed substrate.

Therefore there exists a need for a printing process which results in the high resolution images having good image durability, robustness and even gloss, ideally without making the printed substrate inflexible. Ideally the good gloss is not compromised by cracking when the printed substrate is bent.

According to a first aspect of the present invention there is provided a printing process comprising the steps of: i) ink jet printing an ink onto a substrate to form an image thereon; ii) applying a transparent composition comprising at least 10wW0 organic solvent and a radiation-curable material to the image; iii) evaporating at least a part of the organic solvent from the composition; and iv) curing the radiation-curable material; wherein the composition has a viscosity of at least 100 cp when measured at 35 25°C.

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, 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".

The ink may be ink jet printed onto the substrate using any of the known types of printing techniques, for example thermal, piezo and paddle-type ink jet printing. 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 ink may be any type of ink but is preferably a radiation-curable ink or, more preferably, a solvent-based ink or an aqueous ink.

Radiation-curable inks typically comprise 0.1 to 1 Owt% colorant and 10 to 99.9wt% radiation curable components. These are typically called "100% UV cure inks" Unpublished International patent application No. PCT/GB2010/051384 describes radiation-curable inks comprising at least 30wt% of organic solvent.

Solvent-based inks and aqueous inks are preferred over 100% UV cure inks because they are capable of providing higher resolution images.

Preferred solvent-based inks typically comprise 0.1 to 10wt% colorant, 10 to 99wt% organic solvent and less than 5wt% water. Suitable solvent-based inks are available commercially from a number of sources, including FUJIFILM Speciality Ink Systems Limited in the UK.

Preferred aqueous inks comprise 0.1 to 10wt% colorant, 10 to 75wt% organic solvent, 0 to 50wt% radiation-curable material (usually Owt%) and at least 5wt% water, preferably 20 to 70wt% water. Suitable water-based inks are available commercially from a number of sources, including FUJIFILM Imaging Colorants Inc., USA.

Typically the liquid medium in solvent-based inks comprises one or more organic solvents and no or little (e.g. <5wr/o) water. In contrast, the liquid medium in water-based inks comprises one or more organic solvents and more than 5wt% 30 water, typically >20wt% or even >40wt% water.

In order to maximise image quality, and control bleed and feathering between image areas it is preferable to arrest the flow of the ink by evaporating any solvent therefrom quickly after they have impacted on the substrate surface, a process often referred to as pinning. To achieve a good quality image it is preferable that the inks are thermally 'pinned', that is heated in order to evaporate any solvent, within 5 seconds of impact, preferably within 1 second and most preferably within 0.5 seconds. Thus the process optionally comprises the additional step of evaporating at least a part of the organic solvent from an ink before the applying the transparent composition thereto.

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.

Preferably the composition is applied to the image by a contact printing method. Contact printing processes can be distinguished from non-contact printing processes such as jetting where the printhead fires a fluid onto a substrate without making contact with substrate.

Suitable contact printing methods include screen printing and, more preferably, roll-to-roll coating using roll-to-roll coating apparatus. Preferred roll-to-roll coating methods include air knife coating, anilox coating, curtain coating, flexo coating, gap coating, gravure coating, immersion (dip) coating, knife-over-roll coating, metering rod coating (e.g. K-bar and/or Meyer bar coating), reverse roll coating, rotary screen coating, silk screen coating and slot die (extrusion) coating.

Of these various roll-to-roll coating methods, the application by means of metering rod is preferred, especially using a Meyer bar or K-bar.

The feature whereby the transparent composition has a viscosity of at least 100 cP at 25°C is of great assistance for contact printing. Applying transparent compositions of lower viscosity by contact printing runs the risk of the composition running off the substrate and potentially fouling the interior of the printing device.

The transparent composition preferably has a viscosity of 100 to 2,000 cP, more preferably 200 to 1,000 cP and especially 300 to 800 cP.

The composition may be applied to just the imaged area of the substrate or, more preferably, to both imaged and any non-imaged areas of the substrate. The latter preference arises because the resultant substrate has a finish of uniform gloss.

If desired the process optionally includes the step of embossing a relief pattern onto the composition, and/or the partially dried composition and/or the partially cured radiation-curable material. This enables attractive surface effects to be achieved, for example replicating a canvass texture or brush strokes to make a picture appear more like a genuine painting.

Preferably sufficient curable-composition 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.

As the transparent composition is required to have a viscosity of at least 100cP, the identity and amount of organic solvent is selected to match this criteria, taking account of the properties of other components of the composition. Typically however the composition comprises 15 to 55wt% organic solvent, more preferably 20 to 47wt%, especially 25 to 40wt%.

The transparent composition preferably has a boiling point of at least 140 C, especially 140 to 250 C. This range is preferred because transparent compositions having a low boiling point are more likely to increase in viscosity during storage due to evaporation of organic solvents therefrom, while transparent composition having a very high boiling point may extend the time taken to perform the evaporation step and this reduce productivity of the printer. Bearing these factors in mind, the transparent composition preferably comprises at most small amounts (e.g. <10wt% based on the total weight of organic solvent) of organic solvents having a boiling point below 140''C or above 250"C.

The transparent composition preferably has a flash point of at least 50 °C, especially 50 to 120 ' C. This range is preferred because transparent compositions having a lower flash point present an increased flammability risk, while transparent composition having a higher flash point may take longer to dry. Bearing these factors in mind, the transparent composition preferably comprises at most small amounts (e.g. <10wt% based on the total weight of organic solvent) of organic solvents having a flash point below 50'C or above 120 C. The transparent composition preferably has a relative evaporation rate of at least 1, especially 1 to 35, relative to n-butyl acetate's evaporation rate of 100. This range is preferred because transparent compositions having an evaporation rate above 35 may increase in viscosity during storage due to evaporation of organic solvents therefrom, while transparent compositions having an having an evaporation rate below 1 may extend the time taken to perform the evaporation step and this reduce productivity of the printer. Bearing these factors in mind, the transparent composition preferably comprises at most small amounts (e.g. <10wt% based on the total weight of organic solvent) of organic solvents having an evaporation rate below 1 or above 35, relative to n-butyl acetate's evaporation rate of 100.

Preferably the organic solvent is selected such that the composition does not require any toxicity labelling.

In one embodiment all of the organic solvents present in the transparent composition have a boiling point of 140 to 250C.

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

The most preferred organic solvents are selected from glycol ethers, cyclic lactones, organic carbonates, dibasic esters, bio-solvents and mixtures comprising two or more thereof.

Preferred glycol ethers include diethylene glycol diethyl ether, DowanolTM DPM, ethoxy propanol, propoxy propanol and butoxy propanol.

Preferred organic carbonates include propylene carbonate.

Preferred cyclic lactones include y-butyrolactone, y-valerolactone and 5-valerolactone.

The preferred organic solvent comprises an organic carbonate (especially propylene carbonate) and a glycol ether (especially diethylene glycol diethyl ether).

Organic solvents comprising y-butyrolactone and one or more glycol ethers and/or propylene carbonate are also preferred.

Dibasic esters typically comprise a di(Ci_4 alkyl) ester of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms, e.g. of the following formula: wherein: A represents (CH2)1-6; and R1 and R2 are the same or different and represent C14 -alkyl, preferably methyl or ethyl, and most preferably methyl.

Mixtures comprising two or more dibasic esters can also be used. Bio-solvents, or solvent replacements from biological sources, may also be used. These 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.

The radiation curable material optionally comprises a monomer of molecular weight 450 or less, an oligomer, or a mixture comprising two or more thereof.

The monomers and/or oligomers may possess different degrees of functionality, and a mixture including two or more monomers and/or oligomers selected from mono-, di-, tri-and higher-functionality monomers and/or oligomers may be used.

Preferably the radiation curable material comprises a radiation curable oligomer. Preferred 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 ethylenically unsaturated groups. The ethylenically unsaturated groups can be any group capable of polymerising upon exposure to radiation.

Preferred radiation curable oligomers have a molecular weight of 500 to 4,000, more preferably 600 to 4,000. Molecular weights can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards of known molecular weight.

In one embodiment the radiation curable material polymerises by free radical polymerisation. Suitable free radical polymerisable monomers are well 10 known in the art and include ethylenically unsaturated groups such as (meth)acrylates, a,-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 [FDA), isodecyl acrylate (IDA) and lauryl acrylate. PEA is particularly preferred.

Suitable multifunctional (meth)acrylate monomers include di-, tri-and tetra-functional monomers. Examples of the multifunctional acrylate monomers that may be used 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.

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 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. Ethylenically a,p-unsaturated monomers can polymerise by free radical polymerisation and may be useful for reducing the viscosity of the composition 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 a,3-unsaturated monomers may also be used.

N-vinyl amides and N-(meth)acryloyl amines may also be used in the transparent compositions. 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).

Particularly preferred radiation curable materials are oligomers having ethylenically unsaturated groups, preferably (meth)acrylate groups. Acrylate-functional oligomers are most preferred.

In one embodiment the oligomer comprises two or more ethylenically unsaturated groups, preferably three or four such groupsThe oligomer preferably comprises a urethane backbone.

Particularly preferred radiation curable materials are urethane acrylate oligomers as these have excellent adhesion and elongation properties. Most preferred are di, tri-and tetra-, -functional urethane acrylates, particularly di and trifunctional urethane acrylates as these yield films with a good balance between film flexibility and solvent resistance.

Other suitable examples of radiation curable oligomers include polyester acrylates and 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.

The radiation curable oligomer used in the preferred transparent compositions 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.

Preferred oligomers for use in the invention have a viscosity of 1 to 20 Pa.s at 60°C, more preferably 5 to 20 Pa.s at 60°C and most preferably 5 to 15 Pa.s at 35 60°C.

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 40 mm oblique/2° steel cone at 60°C with a shear rate of 25 seconds-1.

In one embodiment the radiation curable material comprises 50 to 100wt%, or 75 to 100wt% of free radical curable oligomer and 0 to 20wt%, preferably 0 to 10wt% of free radical curable monomer, based on the total weight of radiation curable material present in the transparent composition.

Although the process steps may be performed in the order i), H), Hi) then iv), additional steps may be included in the process if desired. For example, a drying step is preferably included between steps i) and ii) to dry the ink before the transparent composition is applied to the image.

Preferably the transparent composition comprises less than 20wt% of (meth)acrylates having a molecular weight below 450 based on the total weight of the transparent composition, or less than lOwt%, more preferably less than 5wP/0. In a particularly preferred embodiment, the transparent composition is substantially free of (meth)acrylates with a molecular weight of less than 450. In one embodiment the transparent composition comprises less than 20wP/0 of (meth)acrylates having a molecular weight below 600 based on the total weight of the transparent composition, or less than lOwt%, more preferably less than 5wr/o. In a particularly preferred embodiment, the transparent composition is substantially free of (meth)acrylates with a molecular weight of less than 600.

By "substantially free" is meant that no (meth)acrylate having a molecular weight below 450 or 600, respectively, is intentionally added to the transparent composition. However, minor amounts of (meth)acrylates having a molecular weight below 450 or 600, respectively, that may be present as impurities in commercially available radiation curable oligomers, for example, are tolerated.

The above preferences arise because low molecular weight (meth)acrylates can be volatile and therefore give rise to unpleasant odours during the evaporation step.

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 novolac resins 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.

In one embodiment the radiation curable material comprises 0 to 40wt% of cationically curable oligomer and 60 to 100wr/o of cationically curable monomer based on the total weight of radiation curable material present in the transparent composition.

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

The radiation curable material is preferably present in the transparent composition in an amount of 20 to 75wt%, based on the total weight of the composition, more preferably 40 to 70wt%, especially 45 to 65wtch. All wt% are on a 100% solids basis, e.g. non-curable diluents are ignored.

Preferably the composition further comprises a silicone acrylate, especially silicone acrylates having at least two acrylate groups. Examples of suitable silicone acrylates include Tegoradml 2100, SartomerTM CN9800, EbecrylTM 350 (from Cytec) and mixtures comprising two or more thereof.

The preferred amount of silicone acrylate is 0 to 4wt%, more preferably 0.1 to 3wt%, especially 0.25 to 2.5wt%, based on the total weight of the composition.

For free radical systems, curing rates may be increased by including an amine synergist in the transparent composition. Suitable amine synergists are e.g. free alkyl amines such as triethylamine, methyldiethanol amine, triethanol amine; aromatic amine such as 2-ethylhexy1-4-dimethylaminobenzoate, ethyl-4-dimethylaminobenzoate and also polymeric amines as polyallylamine and its derivatives. Curable amine synergists such as ethylenically unsaturated amines (e.g. acrylated amines) are preferable since their use will give less odour due to their ability to be incorporated into the resultant protective coating by curing.

When present, the amount of amine synergists is preferably from 0.1- 10wt% based on the weight of curable material in the composition, more preferably from 0.3-3wt%.

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 as described in W02007/018425, page 20, line 15 to page 22, line 6, which are incorporated herein by reference thereto. Silicone surfactants are particularly preferred, for example silicone surfactants sold under the BYK trade mark, e.g. BYKTm 307.

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-methy1-1-phenyl propan-1-one, 2-hydroxy-2-methy1-1-(4-tert-butyl-) phenylpropan-1-one, 2-hydroxy-[4--(2- hydroxypropoxy)pheny1]-2-methylpropan-1-one, 2-hydroxy-144-(2-hydroxyethoxy)pheny1]-2-methyl propan-1-one, 1-hydroxycyclohexylphenylketone and oligo[2-hydroxy-2-methyl-1-{4-(1-methylvinyl)phenyllpropanone], 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-trimethylbenzoyI)-phenylphosphine oxide, benzophenone, 1-hydroxycyclohexyl phenyl ketone, benzil dimethylketal, bis(2,6-dimethylbenzoyI)-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 lrgacureTM, DarocurTM and LucerinTM (from BASF).

Preferably the amount of photoinitiator present in the composition is 0 to 10 8wt%, more preferably between 0.1 to 5wt%, especially 0.5 to 4wt%.

Additionally the composition may contain further ingredients, e.g. a stabilizer, wax, extender powder pH controllers, preservatives, viscosity modifiers, stabilisers, dispersing agents, inhibitors, antifoam agents, organic/inorganic salts, anionic, cationic, non-ionic and/or amphoteric surfactants and the like in accordance with the object to be achieved.

The step of evaporating at least a part of the solvent from the transparent composition may be used to fix the radiation-curable material on the substrate prior to the curing step. This embodiment is quite different from conventional printing of radiation-curable inks where the ink is fixed in position on the substrate, or 'pinned' as it is sometimes called, by radiation-curing rather than drying.

Bearing this in mind, in a preferred embodiment, step iii) comprises evaporating sufficient solvent from the transparent composition to fix the position of the radiation-curable material on the substrate prior to curing step iv).

The evaporation step is preferably performed at a temperature in the range 45 to 110 -C, more preferably 50 to 100C.

The evaporation step is preferably performed for from 0.5 to 10 minutes, more preferably 0.5 to 5 minutes, especially 0.5 to 3 minutes, depending on the temperature used, the amount of solvent to be evaporated and the volatility of the organic solvent.

Any means that is suitable for evaporating solvent from the composition can be used in the process and apparatus of the invention. Examples include dryers, heaters, air knives and combinations thereof.

In one embodiment, the solvent is evaporated by heating. Heat may be applied to either side or both sides of the substrate, for example by the use of heated plates (resistive heaters, inductive heaters) provided on the opposite side of the substrate to the image or radiant heaters (heater bars, IR lamps, solid state IR) provided on the same side as the printed image. In a preferred embodiment, the transparent composition is applied to a preheated, printed substrate that then moves over a heated platen. The evaporation may be performed using one or more heaters.

Preferably a significant portion of the organic solvent is evaporated from the composition before the radiation-curable material is cured. Preferably at least 50% or more preferably substantially all of the organic solvent is evaporated before the radiation-curable material is cured. This may be achieved by subjecting the transparent composition present on the image to conditions that would typically dry conventional solvent-based inkjet ink. Such conditions will remove most of the solvent but it is expected that trace amounts of solvent may remain in the film which forms over the image from the transparent composition.

The solvent evaporation step may be used to provide the printed image with high and even gloss, even when the ink and substrate are of the type which do not normally provide good gloss properties, e.g. a conventional aqueous ink printed onto plain paper. Furthermore, the loss of a significant portion of the transparent composition through the evaporation of the solvent leads to the formation of a transparent film over the image that is thinner than a film that would be produced by applying an equivalent volume of solvent-free radiation curable varnish onto the image. This is advantageous because thinner films result and the printed substrate has improved flexibility.

Applying the transparent composition to the image by a contact printing method has an advantage over application through a printhead in that the risk of the transparent composition curing inside or over the printhead (e.g. due to stray light) and blocking the printhead is eliminated.

After step iii) has been performed, the non-volatile components present on the image may not be fully dry. Rather, what remains on the surface of the image is often a high viscosity version of the transparent composition. 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 radiation curable material. Upon exposure to a radiation source, the radiation curable material cures to form a relatively thin polymerised film. The transparent composition of the present invention typically produces a film having a thickness of 5 to 40 pm, preferably 6 to 33 pm, for example 7 to 26 pm. Film thicknesses can be measured using a confocal laser scanning microscope.

In one embodiment the radiation source is positioned downstream from a means for evaporating solvent from the transparent composition. In other words an evaporating means and the radiation source are positioned so that printed substrate is exposed to the means for evaporating solvent before it is exposed to radiation, allowing evaporation of the at least some (and preferably most or all) of the solvent before the radiation curable material is cured.

The source of radiation may be any source which provides the wavelength and intensity of radiation necessary to cure the curable material. 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 10 source having linear power density below 1W/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 having a fast conveyor belt 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. The low linear power density and low peak irradiance of these lamps suggests that they would not provide effective curing of radiation curable inkjet inks. It has surprisingly been found that low pressure mercury lamps can be used to cure the transparent compositions according to the present invention.

A single low pressure mercury lamp or two or more low pressure mercury lamps can be used for the curing step.

The IUPAC Compendium of Chemical Terminology (PAC, 2007, 79, 293 "Glossary of terms used in photochemistry", 3rd edition (IUPAC Recommendations 2006), doi:10.1351/pac200779030293) describes a low pressure mercury lamp as a: "resonance lamp that contains mercury vapour at pressures of about 0.1 Pa (0.75 x10-3 Tom 1 Torr = 133.3 Pa). At 25°C, such a lamp emits mainly at 253.7 and 184.9nm.

Low pressure mercury lamps are used extensively in the water purification industry and are therefore widely available.

As mentioned above, low pressure mercury lamps predominantly emit UV radiation with a peak wavelength of around 254 nm but the wavelength of the radiation can be varied by coating the internal surface of the lamp with a phosphor.

In a preferred embodiment of the lamp, there is no such phosphor coating. In the method of the present invention the lamp preferably emits radiation with a peak wavelength of around 254 nm, or put another way, the natural or unaltered wavelength of radiation emitted by mercury vapour in a low pressure lamp environment.

The use of a phosphor coating can lead to a reduction in lamp luminous efficiency. The preferred phosphor-free lamps used according to the invention have an efficiency exceeding 45% for UVC generation, however. This high efficiency helps to minimise the cure unit running costs. In low pressure mercury lamps the UV output varies with temperature.

In a preferred embodiment, the low pressure mercury lamp is an amalgam lamp. In amalgam lamps an amalgam of mercury, typically with bismuth and/or indium, is used instead of pure mercury. Other suitable materials that are compatible with, or are capable of forming an amalgam with mercury could be used instead of bismuth or indium, however. Amalgam lamps have much the same spectral output as conventional low pressure mercury lamps.

Typically, amalgam lamps can run at temperatures up to 140°C with linear 10 power densities exceeding 380 mW/cm and such lamps can achieve outputs that equate to approximately five times the output of a conventional low pressure mercury lamp.

The combination of the increased radiation and heat generated by the amalgam lamp offers a useful advantage in drying and curing the inks used in the present invention when compared to regular low pressure mercury lamps.

In an embodiment of the invention the cure lamp linear power density is below 2000 mW/cm, preferably 200 mW/cm to 1500 mW/cm, more preferably 380 mW/cm to 1,500 mW/cm. In a more preferred embodiment the linear power density is 380 mW/cm to 1,200 mW/cm and in a most preferred embodiments either 380 to 1000 mW/cm or 500 to 1000 mW/cm.

Standard low pressure mercury lamps have current densities not exceeding 0.45 Amps/cm whereas amalgam lamps have current densities above this level.

The temperature of the amalgam lamp may be controlled in order to allow the optimal UV light output to be maintained. Temperature control can be achieved by immersing the lamp in water within a quartz sleeve. As well as providing electrical insulation against the water, the air gap around the lamp prevents overcooling by the water. By controlling the water flow past the lamps, the optimal lamp temperature can be maintained for maximum UV output. While convenient, this method is not preferred as it incurs the additional cost of a chiller.

In a preferred embodiment air is blown across the low pressure mercury lamp(s) to control the lamp temperature. In a further preferred embodiment, forced air that has been warmed by the lamp(s) is directed over the surface of transparent composition to evaporate organic solvent therefrom, thereby performing or assisting with the performance of step iii) of the present process.

For example, one or more fans can be positioned at the rear of the lamp reflector in order to extract and transport excess warm air upstream in the print process to assist in drying of the transparent composition, thus increasing the efficiency of the printer.

Thus in a preferred embodiment the process comprises the steps of: i) ink jet printing a solvent-based or aqueous ink onto a substrate to form an image thereon; ii) evaporating at least 50% of the solvent from the solvent-based or at least 50% of the water from the aqueous ink (as the case may be) to give a dry or partially dried image; iii) applying a transparent composition comprising at least 10wt% organic solvent and a radiation-curable material to the dry or partially dried image by means of a metering rod; iv) evaporating at least 50% of the organic solvent from the composition; and v) UV curing the radiation-curable material using an irradiation source having maximum linear power density of below 1W/cm; wherein the composition has a viscosity of at least 100 cp when measured at 25°C and the radiation-curable material comprises a radiation curable oligomer having a molecular weight of 500 to 4,000.

Preferably the transparent composition used in this preferred process is substantially colourless and/or odourless. The radiation-curable material present therein is preferably as described above in relation to the first aspect of the present invention, especially a polyester and/or urethane having more than one ethylenically unsaturated group.

The process preferably employs a transparent composition as defined below in the second aspect of the present invention.

The transparent composition may be applied to the image shortly after the image has been printed (e.g. within 30 minutes of the image being printed) or there may be a delay before the transparent composition is applied to the image (e.g. of at least 30 minutes).

In view of the foregoing preferences, a second aspect of the present invention provides a transparent composition having a boiling point of 140 to 250 C and a viscosity of 100 to 2,000 cP when measured at 25°C, which comprises: a) 45 to 65wt% radiation-curable oligomer having a molecular weight of 500 to 4,000; b) 25 to 40wt°/0 of organic solvent; c) 0 to 4 (especially 0.25 to 2) wt% of silicone acrylate; d) 0 to 8 (especially 4 to 8) wt% of photoinitiator; and e) less than 3.1wt% of (meth)acrylates having a molecular weight below 600 The preferred features of the composition according to the second aspect of the present invention (e.g. preferred amounts of components a) to e), viscosity, identity of preferred solvents, flash point, evaporation rate and oligomers etc.) are as described above in relation to the process of the present invention.

According to a third aspect of the present invention there is provided a ink jet printer comprising: (a) a printhead for printing an image on a substrate; (b) a means for contact printing the aforementioned transparent composition onto the image; (c) a means for evaporating solvent from the composition; and (d) a radiation source for curing the radiation-curable material. The means for contact printing the transparent composition onto the image preferably comprises a roll coater, or a bar coater, especially a bar coater comprising a metering rod. Metering rods typically comprise a rod and a wire wound around the rod (often in a spiral pattern), wherein grooves between adjacent windings of the wire define the thickness of composition which will be applied to the image.

Preferably the ink jet printer is constructed such that the means for contact printing the substrate is downstream from the printhead, the means for evaporating solvent is downstream from printhead and the radiation source is downstream from the means for evaporating solvent. If desired the printer may comprise a first drying means which is downstream from the printhead (for drying ink) and a second drying means which is downstream from the means for contact printing the aforementioned transparent composition onto the image (for drying the transparent composition).

Suitable means for contact printing include a screen printing unit, more preferably, a roll-to-roll coating unit. Preferred roll-to-roll coating units perform air knife coating, anilox coating, curtain coating, flexo coating, gap coating, gravure coating, immersion (dip) coating, knife-over-roll coating, metering rod coating (e.g. K-bar and/or Meyer bar coating), reverse roll coating, rotary screen coating, silk screen coating or slot die (extrusion) coating. Of these various units, a roll-to-roll coating unit is preferred, the application of the transparent composition preferably being by means of metering rod, especially using a Meyer bar or K-bar.

While printers comprising a means for evaporating solvent from conventional solvent-based inks are known, and printers comprising a means for radiation curing radiation-curable inks are known, it is believed that 'hybrid' ink jet printers comprising both of such means in combination with a means for contact printing a transparent composition onto the image are novel.

Suitable means for evaporating solvent from the composition are those used in conventional ink jet printers used for printing solvent-based inks.

Suitable radiation sources include those used in conventional ink jet printers used for printing radiation-curable inks. As mentioned above, the most preferred radiation source is a low pressure mercury lamp.

The process, composition and printer of the present invention may be used to provide printed images of high resolution with good image durability and robustness. The compositions provide the substrates with even gloss without unduly reducing the substrate flexibility. The printed, coated substrate achievable by the present invention have a low likelihood of cracking or losing their gloss when bent in the substrate, making the invention particularly useful for preparing imaged flexible items which retain durability without compromising flexibility.

The invention is illustrated by the following non-limiting examples.

Examples 1 and 2

Transparent, curable compositions TC1 and TC2 were prepared by mixing the ingredients described in Table 1 in the dark. The viscosity of the resultant compositions is shown in the final row of Table 1:

Table 1

Component Example 1 (TC1) Example 2 (TC2) y-butyrolactone 9.9 9.9 Diethylene glycol diethyl ether 19.0 25.4 UVP6000TM (trifunctional polyester acrylate from Polymer technology Ltd) 64.1 0 Genomer T" 4215 (a difunctional urethane acrylate from Rahn) 0 57.7 Irgacure nil 184 4.0 4.0 Benzophenone 2.0 2.0 Tegorad nil 2100 0.5 0.5 Sartomer T" CN9800 0.5 0.5 Viscosity @ 25 ' C 650 cP 780 cP Notes: UVP@000TM, Genomer TM 4215, Tegorad TrVi 2100 and SartomerTM CN9800 are radiation-curable materials, with the latter two also being silicone acrylates.

Irgacure TM 184 and Benzophenone are photoinitiators.

y-butyrolactone and diethylene glycol diethyl ether are organic solvents.

Example 3

A printed substrate was prepared by printing Mactac JT5929 P self adhesive vinyl (which comprises a backing sheet) with 15 x 15 cm2 solid rectangles using solvent-based inks obtained from Fujifilm specialist ink systems.

Curable composition TC1 from Example 1 was applied to the printed substrate by a contact printing using a number 2 Kbar to give a wet coating of 12 micron thickness. The theoretical dry film thickness was 8.5 microns.

The composition was dried for 3 minutes at 60 C before curing on a conveyored drier fitted 2x 80w/cm medium pressure mercury lamps employing a 10 belt speed of 20 m/m in to provide a UV dose of approximately 400 mj/cm2.

Example 4

The general method of Example 2 was repeated except that curable composition TC2 was used in place of TC1 and number 4 Kbar was used as contact coater in place of a number 2 Kbar to give a wet coating of 40 microns thickness. The theoretical dry film thickness was 25.9 microns The drying and curing were performed in an identical manner to Example 3.

Comparative Example 1 Example 1 was repeated except that no curable composition was applied to the image. Consequently there was no curable composition to dry and no curing step.

Test Results The coated images produced in Examples 3 and 4 were compared to the uncoated virgin images for solvent resistance, adhesion, elongation and scratch resistance. The tests were performed as follows and the results are in Table 2: Solvent resistance -each test sample was rubbed with a soft cloth impregnated 30 with isopropyl alcohol, the number of double rubs being taken to remove the image was noted.

Scratch resistance -each test sample was scratched by fingernail and the relative ease with which the image was damage was scored 1 to 5, with 1 indicating an image which was easily damaged and 5 indicating an image which suffered little or no damage.

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

Elongation -Each test sample was cut into rectangular sections of size 150 mm x mm. The backing sheet was removed from each rectangular section and the short edge of the ink image was adhered to a large aluminium panel. The opposite end of the image was gripped between two small aluminium panels and the print heated to 60 C using an hot air gun. The image was then elongated to a total length of by 33% to 200 mm and smoothed onto the surface of the large panel using a felt squeegee. The elongated image was checked for signs of film cracking and loss of gloss and scored 1 for substantial cracking and loss of gloss and 5 for no visible cracking or loss of gloss.

Table 2

Sample Solvent resistance Scratch resistance Adhesion Elongation Control (Comparative Example 1) 1 3 5 5

Example 3 >100 5 5 5

Example 4 >100 5 5 5

As can be seen from the above results, the solvent resistance and scratch resistance for Examples 3 and 4 were significantly better than the control which had not been coated with TC1 or TC2, without detriment to the film flexibility and 20 adhesion.

Example 5

The method of Example 1 was repeated except that instead of curing with a medium pressure mercury lamp, there was used a low pressure mercury lamp (NNI50/26 XL, supplied by Heraeus). The coated print was irradiated by fixing it to the table of an x-y movable platform at a distance of 15 mm from the lamp and moving the coated print at a speed of 5mm/second.

Example 6

The method of Example 1 was repeated except that the coated print was moved at a speed of 10mm/second.

The solvent resistance for Comparative Example 1 and Examples 5 and 6, measured in "double rubs" as described above, are shown in Table 3 below:

Table 3

Sample Solvent resistance Control (Comparative Example 1) 1 Example 5 (5 mm/second) >100 Example 6 (10 mm/second) 65