The present invention relates to a printing method and in particular to an inkjet printing method using a low-intensity radiation source.

Digital inkjet printing is becoming an increasingly popular method for the production of fine graphic images for advertising, owing 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 ultraviolet 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 on account of particle-particle interactions between the fine pigment particles 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.

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 energy source is typically any source of actinic radiation that is suitable for curing radiation curable inks but is preferably a UV source. Suitable UV sources include light emitting diodes (LEDs), flash lamps, fluorescent tubes, mercury discharge lamps, and combinations thereof.

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. 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.

Flash lamps operate by discharge breakdown of an inert gas, such as xenon or krypton, between two tungsten electrodes. Flash lamps 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.

UV fluorescent lamps find wide application.

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.

Low-pressure mercury discharge lamps would be highly desirable as they are inexpensive, do not generate ozone and do not heat the substrate. However, in order to tolerate the low intensity of actinic radiation, the inkjet inks must contain (meth)acrylate monomers and/or oligomers having a high acrylate functionality in order to achieve sufficient reactivity to give well-cured coatings. The drawback of such an approach is that high functionality acrylates generally have a high viscosity and in order to achieve an ink jettable viscosity it is necessary to add organic solvents (or water) to reduce the viscosity. An example of such an approach is discussed in WO 201 1/021052. WO 201 1 /021052 discloses an inkjet ink which may be cured using a low-pressure mercury discharge lamp which contains a radiation-curable material and at least 30% of volatile organic solvent.

Whilst such an ink is commercially useful, there remains a need in the art for inkjet inks which cure using a low-pressure mercury discharge lamp, but which do not require the presence of volatile organic solvents (or water). Accordingly, the present invention provides method of inkjet printing comprising the following steps:

(i) providing an inkjet ink comprising a multifunctional (meth)acrylate monomer and/or oligomer, an N-vinyl amide and/or N-(meth)acryloyl amide, a free radical photoinitiator and a tertiary amine which may be present as a separate component or covalently bonded to the multifunctional (meth)acrylate monomer and/or oligomer, wherein the ink contains less than 5 wt% of water and volatile organic solvents based on the total weight of the ink;

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

(iii) curing the ink with a radiation source which emits all of its peak intensities within a wavelength of 230-460 nm and with a total power output of 20-40 mW/cm ² within the wavelength of 230-460 nm.

The present invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows a section view of a low-pressure mercury lamp provided with a reflective coating.

The ink-jet ink of the present invention dries by curing, i.e. by the polymerisation of the monomers and/or oligomers present, as discussed hereinabove, and hence is a radiation- curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink, although the presence of small amounts of such components may be tolerated, e.g. by absorption of atmospheric moisture or the presence of residual solvent in commercially available ink components. Thus, the ink-jet ink of the present invention contains less than 5% by weight of water and volatile organic solvents, wherein the wt% represents the combination of water and volatile organic solvents and is based on the total weight of the ink.

The ink of the present invention may contain monofunctional (meth)acrylate monomers. onofunctional (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. PEA is particularly preferred. The total amount of the monofunctional (meth)acrylate monomer is from 10 to 50 wt% based on the total weight of the ink

The ink of the present invention also comprises a multifunctional (meth)acrylate monomer and/or oligomer. Such monomers/oligomers are well known in the art and have a functionality of two or higher. Multifunctional (meth)acrylate monomers typically have a viscosity of less than 2 Pas at 25°C and a molecular weight of less than 450. Functionalities of two, three or four are preferred and preferably this monomer is a difunctional monomer. Examples of the multifunctional acrylate monomers that may be included in the ink-jet inks include hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyethyleneglycol diacrylate (for example tetraethyleneglycol diacrylate), dipropyleneglycol diacrylate, tri(propylene glycol) diiacrylate, 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 methacrylate 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.

The total amount of the multifunctional (meth)acrylate monomer is from 10 to 70 wt% based on the total weight of the ink The ink also contains a polymerisable (i.e. curable) (meth)acrylate oligomer. The term "polymerisable oligomer" has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units which is capable of further polymerisation. The oligomer is a curable, e.g. UV-curable, (meth)acrylate. The oligomer preferably has a molecular weight of at least 450. The molecular weight is preferably 4,000 or less, more preferably from 2,000 or less and most preferably 1500 or less. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 6, more preferably 2 to 4 and most preferably 3.

UV-curable oligomers of this type are well known in the art. The oligomer is preferably based on bisphenol A, a polyester, a polyether or a urethane.

The total amount of the polymerisable (meth)acrylate oligomer is preferably from 3 to 30 wt%, based on the total weight of the ink.

(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. Monomers may be distinguished from the oligomers on account of the lack of repeat units. The monomers typically have a molecular weight of less than 450 and more often less than 300.

The ink of the present invention also contains tertiary amine. It has surprisingly been found that the presence of a tertiary amine allows the ink to be cured using a low intensity radiation source.

The amine may be covalently bound to the (meth)acrylate monomer or oligomer (termed herein an "amine-modified (meth)acrylate monomer or oligomer"), or it may be present as a separate component (i.e. one which is not covalently bound to the (meth)acrylate monomer or oligomer).

Suitable amine-modified (meth)acrylate monomers or oligomers may be prepared by a pseudo-Michael addition reaction between a multifunctional (meth)acrylate monomer or oligomer (i.e. those with two or more (meth)acrylate groups) and a primary or secondary amine. Such a reaction may be represented as follows:

amine

pseudo-Mchael addition

amine modified monomer/oligomer wherein R ¹ represents the remainder of the multifunctional (meth)acrylate monomer or oligomer (which may be a C _1-8 -alkyl, C _1-8 -alkanol, C ₃ . ₈ -cycloalkyl Cs-e-cycloalkyl-C s-alkyl, phenyl, polyester oligomer or polyether oligomer, substituted with a further 1 -6 (meth)acrylate groups), and

R ² and R ³ may be the same or different and represent C^-alkyl, C,. ₈ -alkanol, C ₃ . ₈ -cycloalkyl C _{3 8} -cycloalkyl-C _{1 8} -alkyl or phenyl (the alkyl groups may be straight chain or branched). Secondary amines include diethylamine (R ² = -CH ₂ CH ₃ , R ³ = -CH ₃ ) and primary amines include monoethanolamine (R ² = -H, R ³ = -CH ₂ OH). If a primary amine is used then the amine can react with two acrylate groups to link two (meth)acrylate monomer/oligomers together.

All of the multifunctional (meth)acrylate monomers and/or oligomers may be amine modified, or the ink may contain a mixture of multifunctional (meth)acrylate monomers and/or oligomers and amine-modified multifunctional (meth)acrylate monomers and/or oligomers.

Alternatively, a tertiary amine may be added as a separate component to the multifunctional (meth)acrylate monomer or oligomer. The nature of the tertiary amine is not limited. An example is a benzoate containing an -N(Ci _-6 -alkyl) ₂ group. The amino group may be directly attached to the benzene ring, preferably at the 4-positon, and the alcohol moiety of the benzoate is based on a C _1-8 -alcohol, wherein the alkyl chain of the alcohol is optionally interrupted by 1-3 oxygen atoms. The amino group may alternatively be attached to the afore-defined alcohol moiety. The following examples are preferred: 2-butoxyethyl 4- (dimethylamino)benzoate, 2-(dimethylamino)ethyl benzoate, ethyl 4-dimethylaminobenzoate, 2-ethylhexyl 4-dimethylaminobenzoate and 2-ethylhexyl 4-dimethylaminobenzoate.

Another example is N(Ci _-g -alkyl) ₃ , wherein one or more of the alkyl groups is optionally substituted with hydroxy groups. Preferably one or two of the alkyl groups is a methyl group. An example is methyl diethanol amine. Where the amine is added as a separate component, it is preferably present in an amount from 1 to 15 wt%, based on the total weight of the ink.

N-vinyl amides and N-(meth)acryloyl amides may also be used in the inks of the invention. N- vinyl amides are well-known monomers in the art. 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 amides are also well-known in the art. N-acryloyl amides also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. Regarding the nomenclature, since this compound incorporates a carbonyl group adjacent to the nitrogen atom, it is referred to as an amide rather than an amine. A preferred example is N- acryloylmorpholine (ACMO):

Thus, the monomer selected from an N-vinyl amide, an N-acryloyl amide or a mixture thereof, is preferably selected from N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N- acryloylmorpholine (ACMO) and mixtures thereof. The monomer selected from an N-vinyl amide, an N-acryloyl amide or a mixture thereof, is preferably present in the ink at a total amount of from 10 to 40 wt% based on the total weight of the ink.

The ink used in the present invention preferably contains a combination of a monofunctional (meth)acrylate monomer, a multifunctional (meth)acrylate monomer, an N-vinyl amide and an amine-modified multifunctional (meth)acrylate oligomer. A particularly preferred ink contains PEA as the monofunctional (meth)acrylate monomer and NVC as the N-vinyl amide.

In addition to the components described hereinabove, the compositions include a photoinitiator which, under irradiation, for example by ultraviolet light, initiates the polymerisation of the monomers. The photoinitiators produce free radicals on irradiation (free radical photoinitiators). Examples include benzophenone, 1 -hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1 -one, benzil dimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide, 2,4,6-trimethylbenzoyl- diphenyl phosphine oxide, 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone , isopropyl thioxanthone or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Irgacure, Darocur (from Ciba) and Lucirin (from BASF). Preferably the photoinitiator is present from 1 to 20% by weight, more preferably from 5 to 15% by weight, based on the total weight of the ink.

The wavelength of the radiation and the nature of the photoinitiator system used must of course coincide. The ink is cured by irradiation with actinic radiation, such as UV, x-ray, electron beam etc, although UV curing is preferred.

The ink of the present invention can be a coloured ink or a colourless ink. By "colourless" is meant that the ink is substantially free of colourant such that no colour can be detected by the naked eye. Minor amounts of colourant that do not produce colour that can be detected by the eye can be tolerated, however. Typically the amount of colourant 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".

The ink-jet ink of the present invention may include a colouring agent, which may be either dissolved or dispersed in the liquid medium of the ink. Preferably the colouring agent is a dispersed 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 of the invention 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 pm, preferably less than 5 μηι, more preferably less than 1 μηι and particularly preferably less than 0.5 pm.

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. Although the ink of the present invention cures by a free radical mechanism, the ink of the present invention may also be a so-called "hybrid" ink which cures by a radical and cationic mechanism. The ink-jet ink of the present invention, in one embodiment, therefore further comprises at least one cationically curable monomer, such as a vinyl ether, and at least one cationic photoinitiator, such as an iodonium or sulfonium salt, e.g. diphenyliodonium fluoride and triphenylsulfonium hexafluophosphate. Suitable cationic photoinitiators include the Union Carbide UVI-69-series, Deuteron UV 1240 and IJY2257, Ciba Irgacure 250 and CGI 552, IGM-C440, Rhodia 2047 and UV9380c.

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.

Suitable substrates include polyolefin substrates, such as polyethylene and polypropylene, e.g. PE85 Trans T/C, PE85 White or PP Top White, polyethylene terephthalate (PET) and paper. Polyolefin substrates represent the most difficult of these substrates on which to gain adhesion.

The ink-jet ink exhibits a desirable low viscosity, i.e. 50 mPas or less, preferably 30 mPas or less and most preferably 28 mPas or less at 25°C. Viscosity may be determined using a Brookfield DV-I + running at 20 rpm.

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

The ink of the present invention is cured using a low intensity radiation source. That radiation source which emits all of its peak intensities within a wavelength of 230-460 nm, more preferably 250-445 nm. Moreover, within those ranges, the radiation source emits a total power output of 10-100 mW/cm ² preferably 15-50 mW/cm ² , more preferably 20-40 mW/cm ² , most preferably 25-35 mW/cm ² .

The peak intensities are preferably found at 250-260, 280-320, 320-390 and 395-445 nm. The power output at these ranges is preferably 10-20 mW/cm ² at 250-260 nm, 3-5 mW/cm ² at 280-320 nm, 3.2-5.2 mW/cm ² at 320-390 nm and 5-15 mW/cm ² at 395-445 nm.

To achieve these outputs, the radiation source is preferably a low-pressure mercury lamp.

Low-pressure mercury lamps are much more efficient than medium-pressure mercury lamps. 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 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. Typical medium-pressure mercury lamps have an output in the range of 80 to 240 W/cm, which is significantly higher than the maximum output for low-pressure mercury lamps.

The lUPAC Compendium of Chemical Terminology (PAC, 2007, 79, 293 "Glossary of terms used in photochemistry", 3rd edition (lUPAC 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 ^χ 10 ^~3 Torr; 1 Torr = 133.3 Pa). At 25°C, such a lamp emits mainly at 253.7 and 184.9 nm. They are also called germicidal lamps. There are cold- and hot-cathode as well as cooled electrodeless (excited by microwaves) low-pressure mercury lamps. The Wood lamp is a low-pressure mercury arc with an added fluorescent layer that emits in the UV-A spectral region (315-400 nm)."

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. When the lamp is first switched on the liquid mercury starts to vaporise and as the temperature increases, the vapour pressure of the mercury reaches an optimum level and the output of UVC radiation reaches a maximum. As the temperature of the lamp increases further the vapour pressure continues to rise, reducing the UVC output. Low-pressure mercury lamps are therefore operated at an optimum temperature at which maximum UVC output can be achieved and this temperature is typically around 25-40°C for standard low pressure lamps. This limit on the operating temperature limits the energy input, however, because the lamp temperature can be raised above the optimum temperature if the energy input is too high. Limiting the energy input limits the maximum UV output achievable. The maximum UV output achievable from a low-pressure mercury lamp is therefore limited by the operating temperature and the energy input. Standard low-pressure mercury lamps have linear power densities of less than 380 mW/cm in their normal configuration. However, U-shaped lamps can have effective total power densities of up to twice this, for example 650 mW/cm.

In a preferred embodiment of the invention, 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 liquid 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 the same spectral output as conventional low-pressure mercury lamps. In operation, the amalgam gradually releases mercury vapour as the temperature increases, but vapour is reabsorbed if the pressure becomes too high. This self- regulation means that the optimum mercury vapour pressure is achieved at a higher temperature, approximately 80-160°C, for example 83°C, depending on the type of lamp and manufacturer. Amalgam lamps therefore operate at a higher optimum temperature than standard low-pressure mercury lamps, which means that higher energy inputs can be tolerated. A higher energy input leads to an accompanying increase in UVC output, which remains stable during extended operation of the lamp.

Typically, amalgam lamps can run at temperatures up to 140°C with linear 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. 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.

The low-pressure mercury lamp is preferably used together with auxiliary ballast electronics in order to regulate the current through the lamp. Many types of ballast are available. Preferred for use in this invention are electronic ballasts that convert input mains frequency to frequencies greater than the relaxation time of the ionised plasma in the lamp, thereby maintaining optimal light output.

In a more preferred embodiment, an electronic ballast operating in rapid or instant start mode is provided wherein electrodes of the low-pressure mercury lamp may be pre-warmed before ignition in order to reduce electrode damage caused by frequent switching. Though more expensive to implement than cold-start methods, pre-heating is preferred because the preferred amalgam lamp of the present invention is high power, operates at high temperature and in use is likely to be frequently switched.

Low-pressure mercury lamps emit light in all directions. For efficient UV curing of printed images, the lamp is therefore preferably used in conjunction with at least one reflector to ensure that the majority of emitted UV light is efficiently directed to the printed surface. The reflector is preferably made of a material that efficiently reflects the UV light with minimal loss, for example aluminium, which has a reflective efficiency of greater than 80%. To prevent hazing of the mirror finish during long term UV exposure, pre-anodised aluminium is preferred, such as 320G available from Alanod. This material is easily formed into curved or faceted shapes by rolling or bending to provide efficient reflectors. In one embodiment the reflector preferably has en elliptical shape such that the radiation directed at the printed substrate is focussed to a narrow line, thereby increasing the peak irradiance at the printed substrate. "Elliptical reflector" is a term known in the art.

The finite diameter of the low-pressure mercury lamp prevents all of the emitted light from originating at the focus of the ellipse. In a preferred embodiment low-pressure mercury lamps with diameter below 30 mm, preferably below 20 mm and more preferably below 10 mm are therefore used in combination with an elliptical reflector, in order to increase the peak irradiance at the substrate even further. In one embodiment, the bulb of the low-pressure mercury lamp is partially coated with a reflective coating such that the radiation produced by the bulb is directed towards the print surface. Fig. 1 is a section view of a low-pressure mercury lamp that is provided with a reflective coating. The lamp (1) comprises a bulb (3) that produces the UV radiation. The bulb is mounted within a reflector (5). The bulb surface that is orientated away from the print surface (7) is coated with a reflective coating (9), which directs radiation (10) emitted from the bulb towards the print surface (7) and therefore improves lamp efficiency. Furthermore, the presence of the reflective coating allows gaps (1 1) in the reflector (5) to be provided, allowing cooling of the lamp. The reflective material can be any material that reflects UVC radiation, and the coating can be can be applied by painting or vacuum deposition, for example. The total UV dose received by the ink printed on the substrate is inversely proportional to the speed that the substrate moves past the lamp. Although the low-pressure mercury lamps used according to the preferred embodiment of the present invention have a relatively low power output when compared to medium pressure mercury lamps, the use of a static lamp allows the printed ink to be exposed to the radiation from the lamp for longer periods than are achieved with traditional scanning type large format printers. Hence, the total dose provided by the low pressure lamps can exceed that provided by scanning type cure units using higher output lamps. The envelope of a low-pressure mercury lamp is typically made from fused quartz, which allows production of lamps with lengths exceeding one meter. To ensure even curing across the full print width using a static in-line cure unit, it is preferable to provide a lamp with an arc length exceeding the print width by several centimetres to counter the emission variance near the electrodes. Together with the electrode encapsulation, the final lamp length could approach 3 m in some cases. This length of lamp is achievable for envelopes with a wide diameter. However, narrower lamps would be more fragile and require additional support along their length, which could interfere with the irradiance profile. In this case, it may be preferable to use several smaller lamps in a castellated or staggered arrangement to achieve full width curing.

An advantage of the approach taken by the present invention is that the above-described radiation source may be used as the only source of energy for drying and curing the ink to a solid film (additional sources may be present to providing an initial pinning of the inkjet droplets, but pinning does not provide a solid film. This provides for a lower cost inkjet printer. Accordingly, the present invention also provides an inkjet printer comprising a radiation source which emits all of its peak intensities within a wavelength of 230-460 nm and with a total power output of 10-100 mW/cm ² within the wavelength of 230-460 nm, as the sole means for drying and curing the ink to a solid film. Other preferred features of this radiation source are as described hereinabove.

The invention will now be described with reference to the following examples, which are not intended to be limiting. Examples

Example 1

Inks 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. Acrylate functionality (f) vs reactivity for acrylate esters and amine-modified acrylate esters.

The output of the lamp is shown in Table 2.

Table 2. Intensity vs wavelength of the low-pressure mercury lamp used.

The inks were formulated to achieve a viscosity of about 25 mPas at 25°C which is a typical viscosity for an inkjet ink used in heated print heads. Full cure is defined as being tack free to touch (surface cure) and resistant to 100 IPA double rubs (through cure). It is apparent that simply increasing the functionality of the oligomer/monomer is not sufficient to gain cure with this low power lamp. However, the use of an amine-modified acrylate ester enables cure to be achieved despite the functionality being modest, i.e. f = 2.9 compared to f = 3 or f = 4 for the TMPTA and PPTTA-containing inks. Example 2

Inks 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 3. Acrylate functionality (f) vs reactivity for acrylate esters and amine-modified acrylate esters.

Inks 1 -4 are inks of the invention and contain an amine-modified acrylate. Inks 5-9 are comparative inks which do not contain an amine-modified acrylate. The inks were formulated to achieve a viscosity of about 25 mPas at 25°C. Full cure is defined as being tack free to touch (surface cure) and resistant to 100 IPA double rubs (through cure). It is apparent that:

- NVC plays an important role as replacing the monofunctional NVC/PEA blend with HDDA (f = 2) results in a non-curing ink (compare ink 1 with ink 10) (an all PEA-based system also does not work).

- To achieve cure, the amine may be either chemically attached to the acrylate (inks 1 ,2,3) or separate (chemically not attached) (ink 4).

- An amine provides both a jettable viscosity (approximately 25 mPas (cP) at 25°C) and a full cure (inks 1 -4). Without the use of the amine, cure can be achieved but at a viscosity that is not practically ink-jettable (ink 9).

- Ink 4 fully cures when exposed to the low-pressure mercury lamp, whilst ink 7, without the tertiary amine, remains uncured.

Example 3

Inks were prepared as set out in Table 3. The inks were subjected to actinic radiation from a low-pressure amalgam lamp, 0.25 m/min and a medium-pressure Hg lamp, 1 x 80 W/cm lamp, belt speed 55 m/min. The low-pressure lamp was used at the maximum printing speed of the printer and hence lowest dose achievable. The medium-pressure lamp was used at the lowest intensity and dose achievable for this lamp corresponding to the lowest power setting and highest belt speed. The results are set out below.

Table 4. Lamp type vs cure response.

The two lamps have very different outputs, both in terms of wavelength distribution and power. Using the standard medium-pressure mercury lamp on the lowest power setting and at the highest speed it is not possible to differentiate between inks 3 and 7 whereas the inks behave very differently under the low-pressure amalgam lamp