GB2521746A

GB2521746A - Printing apparatus

Info

Publication number: GB2521746A
Application number: GB1419380.9A
Authority: GB
Inventors: Robert Fassam; Nigel Gould
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 2013-10-31
Filing date: 2014-10-31
Publication date: 2015-07-01
Anticipated expiration: 2034-10-31
Also published as: GB201319285D0; GB2521746B; GB201419380D0

Abstract

An inkjet printing apparatus for printing a radiation-curable inkjet ink on to a substrate 11 comprises at least one printhead (1, Fig.1), wherein the printhead (1, Fig.1) moves with respect to a substrate 11 in a direction of travel 4; and a source of actinic radiation 3 downstream from the printhead (1, Fig.1) in relation to the direction of travel 4, which also moves with respect to the substrate 11 in the direction of travel 4. The source of actinic radiation 3 has a length and a width 6, wherein the length is equal to or greater than the width 6. The width 6 extends in a second direction 5 and the second direction 5 defines an acute angle 7 with respect to the direction of travel 4. The present invention also relates to a method of printing a radiation curable inkjet ink using the apparatus of the present invention.

Description

Intellectual Property Office Application No. GB1419380.9 RTI\4 Date:22 April 20t5 The following terms are registered trade marks and should be read as such wherever they occur in this document: Rhodorsil Uvacure Esacure Irga cure Darocur Lucirin Paliotol Cinquasia Irgalite Hostaperm Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo Printing apparatus The present invention relates to a printing apparatus and a method of printing. In particular the invention relates to a printing apparatus suitable for printing radiation-curable inkjet inks and methods of printing the same.

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. lnkjet 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.

Usually, the substrate remains stationary whilst the printhead traverses the substrate depositing ink droplets on to the substrate. After traversing the substrate, the direction of travel reverses and the process is repeated building up the image in a series of swathes. At the point after which the print carriage reverses when the swath is complete, the substrate is moved on in readiness for the next ink deposit and thus the next swath of ink.

There are three inkjet printing systems which have dominated the digital wide format graphics production industry, based on different ink technologies, namely thermal, radiation-curable and a combination of thermal/radiation-curable.

Thermally cured inks tend to be solvent-based, where the solvent is mainly either a volatile organic solvent or water. After printing onto a substrate the majority of the ink evaporates, usually with the assistance of one or more heaters, to leave a dry film on the surface.

With solvent inks, the printer productivity is governed by the system's ability to expel the bulk solvent.

If too much wet ink is laid down on the media, the ink flows to blur the printed image. For this reason, solvents with a high vapour pressure are preferred in the ink. However, if the solvent vapour pressure is too high, ink drying on the printhead nozzle plate may lead to blocked nozzles. This compromise in solvent selection leads to a limitation in productivity and because 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.

Radiation-curable inks (usually UV-curable inks) have largely replaced solvent-based-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 actinic radiation. In most cases, the actinic radiation is an intense UV light, which causes photo-crosslinking of curable molecules in the presence of a photoinitiatorto form a solid.

The greatest perceived benefit of radiation-curable printers is their ability to deliver high production rates. In most radiation-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 printheads and cure unit, the maximum time between print and cure would be 0.1 seconds for a printhead carriage moving at 1 mIs. Radiation-curable ink solidification times of less than one second compare favourably with solvent-based inks that can take several minutes to dry. lnkjet printers for radiation-curable inks are necessarily more complex and consequently more expensive than inkjet inks for solvent-based inks.

Hybrid solvent/radiation-curable inks comprise an organic solvent, a radiation-curable material and a photoinitiator and hence comprise 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 organic solvent means that the advantageous properties of solvent-based inkjet inks are maintained. It is also possible to achieve low film weight and high print quality.

The conventional printing system for hybrid solventlradiation-curable inks combines both evaporating at least a portion of the solvent from the printed ink and exposing the printed ink to actinic radiation to cure the radiation-curable component of the ink. Thus, it is usual initially to pin the printed solvent/radiation-curable hybrid inks image by thermally evaporating solvent from deposited ink droplets, which results in a viscosity rise and thus restricts the ink flow. Once this pinning process is completed the image is fixed by exposing the printed image to actinic radiation.

The current solvent/radiation-curable printer uses a low pressure mercury lamp curing system to fix the image and give the inks their final resistance properties. The cure lamp is downstream from the printing and image pinning area and runs parallel to the direction of the printhead covering the entire swath of the print. As a consequence of the printing, pinning and fixing process, the medium moves under the cure lamp in a series of steps. Put another way, the medium moves under the lamp, stops and then is moved on when the print swath is completed.

However, the stepwise motion of the print under the curing lamp can give rise to uneven cure owing to variations in radiation intensity dependent on the relative position of the ink to the radiation source.

Specifically, the ink under the radiation source can be exposed to higher intensity radiation than that at either side. In areas of low deposit of ink, this does not give rise to any visible effect. However, in areas of heavy ink deposit, such as dark or shadow areas of the print, this gives rise to differential gloss. The ink receiving a higher dose of radiation is crosslinked to a greater extent and is glossier in appearance. This gives rise to banding in the print, which is deleterious to the overall print quality and appearance of the print. In theory, this problem could be overcome by using a higher power actinic-radiation source. However, there is a limitation to the maximum linear output that can be achieved.

There therefore exists a need for a printing apparatus that is capable of providing increased actinic radiation to the printed image, without the need of using a higher power actinic-radiation source, and providing such radiation in an improved, uniform manner across the print to try to reduce banding in the printed image and try to improve print quality.

Surprisingly, the inventors have found that this is provided by an inkjet printing apparatus of the present invention.

Accordingly, the present invention provides an inkjet printing apparatus for printing a radiation-curable inkjet ink on to a substrate comprising at least one printhead, wherein the printhead moves with respect to a substrate in a direction of travel; and a source of actinic radiation downstream from the printhead in relation to the direction of travel, which also moves with respect to the substrate in the direction of travel, wherein the source of actinic radiation has a length and a width, wherein the length is equal to or greater than the width and wherein the width extends in a second direction and wherein the second direction defines an acute angle with respect to the direction of travel.

The present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows a comparative arrangement of an inkjet printing apparatus; Fig. 2 shows an arrangement of an inkjet printing apparatus according to the present invention; Fig. 3 shows an arrangement of an inkjet printing apparatus according to the present invention; Fig. 4 shows an arrangement of an inkjet printing apparatus according to the present invention; Fig. 5 shows the orientation of the source of actinic radiation; Fig. 6 shows the orientation of the source of actinic radiation; and Fig. 7 shows multiple sources of actinic radiation in a zig-zag configuration.

It has surprisingly been found that the inkjet printing apparatus of the present invention having a defined orientation of the source of actinic radiation 3 increases the dose of actinic radiation received by the printed ink film in an improved, uniform manner across the print to provide an improved cure, which reduces differences in gloss and banding in the print. This improves print quality without the need of a higher power actinic-radiation source.

The printing apparatus of the present invention is suitable for printing a radiation-curable inkjet ink and preferably, a hybrid solvent/radiation-curable inkjet ink. The features of printers that are suitable for printing radiation-curable and hybrid solvent/radiation-curable inkjet inks are well known to the person skilled in the art and a detailed description is therefore not required.

The inkjet printing apparatus of the present invention for printing radiation-curable inkjet inks on to a substrate 11 comprises at least one printhead 1. Printheads are well known in the art and are conventional to the skilled person and therefore a detailed description is not required.

The printhead 1 moves with respect to a substrate 11 in a direction of travel 4 such that there is relative movement between the printhead 1 and the substrate 11. The direction that the printhead 1 moves relative to the substrate 11 defines the direction of travel 4. Thus the resultant direction of the printhead 1 defines the direction of travel 4. The relative movement between the printhead 1 and the substrate 11 can arise from the printhead 1 traversing a length of the substrate 11, while the substrate 11 remains stationary; the printhead 1 remaining stationary, while the substrate 11 moves relative to the printhead 1; or movement of both the substrate 11 and the printhead 1, where there is resultant direction of movement of the printhead 1 relative to the substrate 11. Preferably, the printhead 1 traverses the substrate 11 in a direction of travel 4 and the substrate 1 remains stationary.

The printhead 1 has a length and a width. Preferably, the length is greater than the width, such that the printhead 1 is elongate and has an elongate axis. Preferably, the printhead 1 is rectangular in shape. In a preferred embodiment, the elongate axis of the printhead is aligned perpendicularly to the direction of travel 4. Preferably, the width extends in the direction of travel 4.

The inkjet printing apparatus of the present invention further comprises a source of actinic radiation 3.

The source of actinic radiation 3 can be any source of actinic radiation that is suitable for curing radiation-curable inks and hybrid solvent/radiation-curable inkjet inks but is preferably a UV source.

Suitable UV sources include mercury discharge lamps, low pressure mercury lamps, low pressure mercury amalgam lamps, fluorescent tubes, light emitting diodes (LED5), 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 3 is a source that does not generate ozone when in use.

The source of actinic radiation 3 is situated downstream from the printhead 1 in relation to the direction of travel 4. The source of actinic radiation 3 is positioned downstream from the printhead 1 in the direction of travel 4 such that the printed substrate 11 is first exposed to the printhead I before it is exposed to radiation, allowing printing of the inkjet ink before the radiation-curable ink is cured by the source of actinic radiation 3.

In a preferred embodiment, the one or more printheads I and the source of actinic radiation 3 are positioned to create a delay between jetting of the ink onto the substrate 11 and exposure of the printed ink to radiation, to allow for evaporation of the optional solvent in the ink before the ink is cured. Preferably the distance between the one or more printheads and the source of actinic radiation is at least 100 mm, preferably at least 200 mm, and more preferably at least 300 mm.

Preferably the time period between jetting the ink from the printhead 1 onto the substrate 11 and exposing the printed ink to radiation is at least 1 second, preferably at least 5 seconds, and more preferably at least 10 seconds.

The source of actinic radiation 3 also moves with respect to the substrate 11 in the direction of travel 4. The direction of travel 4 is as defined hereinabove. Therefore, as discussed hereinabove for the relative movement between the printhead 1 and the substrate 11, there is also relative movement between the source of actinic radiation 3 and the substrate 11 in the direction of travel 4. The relative movement between the source of actinic radiation 3 and the substrate 11 such that the source of actinic radiation 3 moves with respect to the substrate 11 in the direction of travel 4 can arise from: the source of actinic radiation 3 traversing a length of the substrate 11 in the direction of travel 4, while the substrate 11 remains stationary; the source of actinic radiation 3 remaining stationary, while the substrate 11 moves relative to the source of actinic radiation 3 such that there is relative movement of the source of actinic radiation 3 in the direction of travel 4; movement of both the substrate 11 and the source of actinic radiation 3, where there is resultant direction of movement of the source of actinic radiation 3 relative to the substrate 11 in the direction of travel 4. Preferably the source of actinic radiation 3 traverses the substrate 11 in the direction of travel 4 and the substrate 11 remains stationary.

The source of actinic radiation 3 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 3 is situated in-line, which means that the substrate 11 does not have to be removed from the printing apparatus between the heating and curing steps.

The radiation source 3 is preferably capable of moving back and forth across the print width, parallel with the movement of the printhead 1.

The source of actinic radiation 3 may be placed on a carriage that allows the source of actinic radiation 3 to traverse the print width. The carriage is placed downstream of the printer carriage in order to provide a delay between printing of the ink onto the substrate 11 and exposure to the curing unit. The source of actinic radiation 3 may move independently of the printer carriage.

When the source of radiation 3 is provided on separate carriage, it is necessary to provide an additional carriage rail, motor and control systems.

When the source of actinic radiation 3 is provided in the print zone of the printer, light contamination at the printhead 1, which could lead to premature curing in the nozzle, must be avoided. Adaptations to prevent light contamination are known in the art.

The source of actinic radiation 3 is 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 1 can move and therefore the region in which ink is applied to the substrate 11.

The source of actinic radiation 3 may be static. This means that the source does not move backwards and forwards across the print width of the substrate 11 when in use. Instead the source of actinic radiation 3 is fixed and the substrate 11 moves relative to the source in the print direction.

Static curing units preferably span the full print width, which is typically at least 1.6 m for the smaller wide-format graphics printers.

The source of actinic radiation 3 has a length and a width 6, wherein the length is equal to or greater than the width 6. Preferably, the length is greater than the width 6 and therefore, the source of actinic radiation 3 is preferably elongate and has an elongate axis. Preferably, the source of actinic radiation 3 is rectangular in shape. The width 6 of the source of actinic radiation 3 is defined as the shortest dimension i.e. the dimension of the source of actinic radiation that has the shortest actinic radiation output.

In the inkjet printing apparatus of the present invention, the width 6 of the source of actinic radiation 3 extends in a second direction 5, wherein the second direction 5 defines an acute angle 7 with respect to the direction of travel 4.

The width 6 of the source of actinic radiation 3 defines an axis. This axis extends in a second direction 5. The second direction 5 and the direction of travel 4 define an acute angle 7. This orientation of the source of actinic radiation 3 relative to the direction of travel 4 increases the dose of actinic radiation received by the printed ink film in an improved, uniform manner across the print, when compared to a printing apparatus where there is no acute angle. This allows for a longer lamp to be used covering the same width of substrate, when compared to a printing apparatus where there is no such acute angle.

In a printing apparatus which is known in the art as shown in Fig. 1, the width of the source of actinic radiation extends in a direction that is parallel to the direction of travel of the printhead. This means that each point on the substrate is exposed to a dose of radiation defined by the width of the source of actinic radiation. In order to increase the dose of radiation in which each point on the substrate is exposed thereto, it is necessary to increase the power output of the source of actinic radiation. The power output of the source of actinic radiation is proportional to the source of actinic radiation's internal diameter and enhancement of the radiation output compared to the IR output can therefore be achieved by using a large internal diameter source of actinic radiation with a large power supply.

However, this is expensive and requires expensive cooling systems and there is a limit to the maximum linear output that can be achieved.

In the present invention, the width 6 of the source of actinic radiation 3 extends in a second direction 5, where the second direction 5 defines an acute angle 7 with respect to the direction of travel 4. This means that each point on the substrate 11 is exposed to a dose of radiation defined by a distance 8 that is greater than the width 6 of the source of actinic radiation 3. This means that each point on the substrate 11 is exposed to an increased dose of radiation.

As can be seen in Fig. 5, assuming a right angle as shown, the distance which defines the dose of radiation that each point on the substrate is exposed 8 to is a distance that is equal to (±) width! cos (acute angle), which is a distance that is greater than the width 6.

There is an optimum range for the acute angle 7 between the second direction 5 and the direction of travel 4. If the angle is at 0° and hence, the width 6 extends in a second direction 5 which is parallel to the direction of travel 4, the benefit of the invention will not occur and there will be no increase in the dose of actinic radiation received by the ink image. If on the other hand, the angle is too large, the breadth of the print that can be accommodated is too restricted and hence is not commercially attractive. Further, the dimensions of the cure system will be too large to be easily accommodated in the machine design. Preferably, the acute angle is from ito 35° and more preferably from S to 20°.

The source of actinic radiation 3 spans a length on the substrate 9. The acute angle also defines a distance on the substrate 10 as shown Figs. 2, 3 4 and 6. In a preferred embodiment, the ratio between the length of span of the source of radiation on the substrate 9 and the distance on the substrate defined by the acute angle 10 is 10:3 to 10:1. Fig. 2 shows a preferred embodiment of the invention wherein the ratio between the length of span of the source of actinic radiation on the substrate 9 and the distance defined by the acute angle 10 is 10:3. This provides an acute angle of 16.7°. Fig. 3 also shows a preferred embodiment of the invention wherein the ratio between the length of span of the source of actinic radiation on the substrate 9 and the distance defined by the acute angle 10 is 10:2. This provides an acute angle of 11.3°. Fig. 3 also shows a preferred embodiment of the invention wherein the ratio between the length of span of the source of actinic radiation on the substrate 9 and the distance defined by the acute angle 10 is 10:1. This provides an acute angle of 5.7°.

In a preferred embodiment, the printhead I is elongate and has an elongate axis, and the source of actinic radiation 3 is elongate and has an elongate axis.

In a still preferred embodiment, the elongate axis of the source of actinic radiation 3 is at an acute angle to the elongate axis of the printhead 1. The acute angle is as described hereinabove.

In a preferred embodiment, the inkjet printing apparatus of the present invention comprises two or more sources of actinic radiation 3. Preferably, the two or more sources of actinic radiation each have a length and a width 6, wherein each of the lengths are equal to or greater than the corresponding widths 6 and wherein each width 6 extends in a second direction 5 and wherein the second direction 5 defines an acute angle 7 with respect to the direction of travel 4 as shown in Fig. 7.

The length and width 6 of the sources of actinic radiation 3 are as defined hereinabove. The second direction 5, acute angle 7 and direction of travel 4 are also as defined hereinabove.

More preferably, the printhead 1 has a length and a width, wherein the width of the printhead extends in the direction of travel 4, and wherein the multiple sources of actinic radiation 3 span the length of the printhead on the substrate. Once again, the length and width of the printhead 1 and the direction of travel 4 are as defined hereinabove. In a preferred embodiment, the multiple sources of actinic radiation 3 are in a zig-zag configuration as shown in Fig. 7.

As is known in the art, a printing apparatus for a radiation-curable ink comprises a source of actinic radiation to cure the radiation-curable component of the inkjet ink. A printing apparatus for a hybrid solvent/radiation-curable inkjet ink that comprises both a solvent and a radiation-curable component comprises a dryer for evaporating solvent from the inkjet ink and a source of actinic radiation for curing of the radiation-curable component of the ink upon exposure to actinic radiation. Therefore, the printing apparatus of the present invention optionally comprises a dryer 2 for evaporating solvent from the ink once the ink has been applied to the substrate.

In a preferred embodiment, the inkjet printing apparatus of the present invention may further comprise a dryer 2 for evaporating solvent from the printed ink downstream from the printhead 1 in relation to the direction of travel 4 and upstream from the source of actinic radiation 3 in relation to the direction of travel 4. This is particularly preferred when the inkjet printing apparatus is used for printing hybrid solvent/radiation-curable inkjet inks as discussed hereinbelow.

Preferably therefore, the source of actinic radiation 3 is positioned downstream from the dryer 2 for evaporating solvent from the printed ink, which is located downstream from the printhead 1. In other words the printhead 1, dryer 2 and source of actinic radiation 3 are positioned so that printed substrate 11 is exposed to printhead 1, then the dryer 2 for evaporating solvent before it is exposed to radiation, allowing printing of the ink onto the substrate, evaporation of the solvent before the radiation-curable material is cured.

Any dryer 2 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 heaters, air knives and combinations thereof In a preferred 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 provided under the substrate or radiant heaters provided above the substrate. In one example, the ink can be jelled onto a preheated substrate that then moves over a heated platen. The apparatus of the invention may comprise one or more heaters.

When printing hybrid solvent/radiation-curable inkjet inks, 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 cured. This is achieved by subjecting the printed ink to conditions that would typically dry conventional solvent-based ink jet inks. In the case of hybrid solvent1radiation-curable inkjet inks, 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 an optional but preferred step because it is believed to define the image quality for hybrid solvent/radiation-curable inkjet ink. 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 inkthrough the evaporation of the solvent leads to the formation of a printed film that is thinner than the film that would be produced by jelling 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 hybrid solvent/radiation-curable ink is not expected to be fully dry. 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.

In a preferred embodiment, the inkjet printing apparatus of the present invention may further comprise an additional source of actinic radiation for pinning the printed ink downstream from the printhead I in relation to the direction of travel 4 and upstream from the source of actinic radiation 3 in relation to the direction of travel 4. This is particularly preferred when the inkjet printing apparatus is used for printing radiation-curable inkjet inks, which are substantially free from solvents and water.

Substantially free from solvent and water means that the ink comprises less than 5% by weight of water and solvent, more preferably less than 2% by weight of water and solvent and most preferably less than 1% by weight of water and solvent, based on the total weight of the ink.

Preferably therefore, the source of actinic radiation 3 is positioned downstream from the additional source of actinic radiation for pinning the printed ink, which is located downstream from the printhead 1. In other words the printhead 1, additional source of actinic radiation and source of actinic radiation 3 are positioned so that printed substrate 11 is exposed to printhead 1, then the additional source of actinic radiation 2 for pinning the ink, allowing printing of the ink onto the substrate, pinning of the ink before the radiation-curable material is fully cured.

The additional source of actinic radiation can be any source of actinic radiation for pinning and may be positioned in any orientation. Preferably, the additional source of actinic radiation is as described above for the source of actinic radiation.

When printing radiation-curable inkjet inks, which are substantially free of solvent and water, the ink is preferably pinned to the substrate before the ink is fully cured. This is achieved by subjecting the printed ink to conditions of actinic radiation that does not fully cure the ink but merely pins the ink to the substrate. This is well known in the art and a detailed description is not required. The pinning step is an optional but preferred step because it is believed to define the image quality for radiation-curable inkjet ink. Thus, it is thought that the pinning step results in a printed image with high gloss.

Unlike fully curing, once the pinning step has occurred, the radiation-curable ink is not expected to be fully cured. 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 printing apparatus of the present invention is suitable for printing radiation-curable inkjet inks and more preferably, hybrid solvent/radiation-curable inkjet inks. The inkjet printing apparatus of the present invention may further comprise a radiation-curable inkjet ink and more preferably a hybrid solvent/radiation-curable inkjet ink. Radiation-curable inkjet inks and hybrid solvent/radiation-curable inkjet inks are known in the art and a detailed description herein is not required.

The ink used in the present invention comprises a radiation-curable material.

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

The radiation-curable material may optionally comprise a radiation-curable oligomer. The oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tn 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 tn-, 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 cures upon exposure to radiation in the presence of a photoinitiatorto 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 other ink components and that is capable of curing to form a crosslinked, solid film is suitable for use in the ink. Thus, the ink formulator is able to select from a wide range of suitable oligomers.

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 seconds1.

The ink optionally contains radiation-curable monomers as the radiation-curable material. Suitable free-radical polymerisable monomers are well known in the art and include (meth)acrylates, a,-unsaturated ethers, vinyl amides and mixtures thereof.

The monomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tn and higher functionality monomers/oligomers may be used.

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-, tn-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 diaclylate), 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.

In an alternative embodiment of the invention, the radiation-curable material is capable of polymerising by cationic polymerisation. Suitable materials include, oxetanes and epoxides (e.g. cycloaliphatic epoxides, bisphenol A epoxides and epoxy novolacs). 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.

0,13-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 a,I3-unsaturated ether monomers may be used.

N-Vinyl amides, such as N-vinyl caprolactam (NVC) and N-vinyl pyrrolidone (NVP), and N- (meth)acryloyl amines, such as N-acryloylmorpholine (ACMO), may also be used in the ink. 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. 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.

Preferably the ink comprises 10 to 30% by weight of radiation-curable material based on the total weight of the ink, preferably at least 12% by weight, more preferably 10 to 20% by weight, based on the total weight of the ink.

The ink includes one or more photoinitiators. If the ink includes a free-radical polymerisable material, the photoinitiator system includes a free-radical photoinitiator.

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-i -propane-i -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 P12074 from Rhodia; MC AA, MC BB, MC CC, MC CC PF, MC SD from Siber Hegner; UV9380c from Alfa Chemicals; Uvacure 1590 from UCS Chemicals; and Esacure 1064 from Lamberti spa.

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

Preferably, the inkjet printing apparatus of the present invention comprises a hybrid solvent/radiation-curable inkjet ink, which comprises an organic solvent, a radiation-curable component and a photoinitiator.

The ink therefore preferably 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 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, orpanic carbonates, lactones and pyrrolidones.

The organic solvent is preferably present in an amount of at least 30% by weight, preferably at least 50% by weight, preferably at least 60% 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 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 as the optional solvent present in the ink.

Dibasic esters are known solvents in the art. They can be described as di(C1-C4 alkyl) esters of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms having following general formula: R1 OLA11OR2 in which A represents (CH2)15, and R1 and R2 may be the same or different and represent C1-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.

The ink may also contain a passive (or "men') 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 1,500-200,000, as determined by GFC with polystyrene standards as previously described hereinabove.

The ink 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 plc), 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 pm, preferably less than 5 pm, more preferably less than 1 pm 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%, or25 weight% based on the total weight of the ink.

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 and most preferably 10 mPa.s or less, at 25°C). 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.

The inkjet ink preferably comprises a radiation-curable material, a photoinitiator, a colorant and a solvent.

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 printing apparatus of the present invention is primarily designed for printing onto flexible substrates but the nature of the substrate 11 is not limited and includes any substrate which may be subjected to inkjet printing. The printing apparatus of the present invention is particularly suited for printing onto self adhesive vinyl and banner grade PVC substrates.

The present invention also provides the use of the apparatus as described above in a method of inkjet printing and a method of inkjet printing using the printing apparatus and inks as described above.

The method of printing a radiation-curable inkjet ink as described above, using the apparatus as described above comprises jetting the ink from the printhead 1 onto a substrate 11; and exposing the printed ink to actinic radiation to cure the radiation-curable ink.

Preferably, the radiation-curable inkjet ink is a hybrid solvent/radiation-curable inkjet ink and the method further comprises the step of evaporating at least a portion of the solvent from the printed ink.

In a preferred embodiment, the portion of solvent is evaporated from the printed before the ink is cured.

Preferably, the time period between jetting the ink from the printhead 1 onto the substrate 11 and exposing the printed ink to actinic radiation is at least 1 second, preferably at least 5 seconds, and more preferably at least 10 seconds.

Claims

Claims 1. An inkjet printing apparatus for printing a radiation-curable inkjet ink on to a substrate (11) comprising: at least one printhead (1), wherein the printhead (1) moves with respect to a substrate (11) in a direction of travel (4); and a source of actinic radiation (3) downstream from the printhead (1) in relation to the direction of travel (4), which also moves with respect to the substrate (11) in the direction of travel (4), wherein the source of actinic radiation (3) has a length and a width (6), wherein the length is equal to or greater than the width (6) and wherein the width (6) extends in a second direction (5) and wherein the second direction (5) defines an acute angle (7) with respect to the direction of travel (4).
2. An inkjet printing apparatus according to claim 1, wherein the acute angle (7) is from ito 35°.
3. An inkjet printing apparatus according to claim 2, wherein the acute angle (7) is from 5 to 20°.
4. An inkjet printing apparatus according to any preceding claim, wherein the printhead (1) is elongate and has an elongate axis.
5. An inkjet printing apparatus according to any preceding claim, wherein the source of actinic radiation (3) is elongate and has an elongate axis.
6. An inkjet printing apparatus according to any preceding claim, further comprising: a dryer (2) for evaporating solvent from the printed ink downstream from the printhead (1) in relation to the direction of travel (4) and upstream of the source of actinic radiation (3) in relation to the direction of travel (4).
7. An inkjet printing apparatus according to any preceding claim, further comprising multiple sources of actinic radiation (3).
8. An inkjet printing apparatus according to claim 7, wherein the multiple sources of actinic radiation (3) each have a length and a width (6), wherein each of the lengths are equal to or greater than the corresponding widths (6) and wherein each width (6) extends in a second direction (5) and wherein the second direction (5) defines an acute angle (7) with respect to the direction of travel (4).
9. An inkjet printing apparatus according to claim 8, wherein the printhead (1) has a length and a width, wherein the width of the printhead extends in the direction of travel (4), and wherein the multiple sources of actinic radiation (3) span the length of the printhead on the substrate.
10. An inkjet printing apparatus according to any one of claims 7 to 9, wherein the multiple sources of actinic radiation (3) are in a zig-zag configuration.
11. The inkjet printing apparatus according to any preceding claim wherein the source of actinic radiation (3) is a UV source, wherein the UV source is preferably selected from mercury discharge lamps, low pressure mercury lamps, low pressure mercury amalgam lamps, LEDs, flash lamps, fluorescent lamps and combinations thereof.
12. The ink]et printing apparatus according to any preceding claim, further comprising a hybrid solvent/radiation-curable ink]et ink.
13. The inkjet printing apparatus according to claim 12 wherein the ink comprises at least 30% by weight of organic solvent based on the total weight of the ink, a radiation-curable material, a photoinitiator and a colourant.
14. A method of printing a radiation-curable inkjet ink using the apparatus according to any one of claims ito 13, the method comprising: jetting the ink from the printhead (1) onto a substrate (11); and exposing the printed ink to actinic radiation to cure the radiation-curable ink.
15. The method according to claim 15, wherein the radiation-curable inkjet ink is a hybrid solvent/radiation-curable inkjet ink and the method further comprises the step of evaporating at least a portion of the solvent from the printed ink.