INKJET PRINTING SYSTEM

FIELD OF THE INVENTION:

The present invention relates to an inkjet printing system using an aqueous ink suitable for printing on various substrates, especially poorly- absorbing or impermeable substrates, which is capable of cooling such a substrate before or during deposition.

BACKGROUND OF THE INVENTION Inkjet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. It is used widely for commercial and business applications for printing on various substrates from paper to cable marking or wide format vinyl sheeting and across markets ranging from industrial labelling to short-run printing to desktop document and pictorial imaging. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous inkjet, ink is supplied under pressure through orifices that produce jets of ink which break up into a continuous stream of droplets which may be of different sizes. The droplets are subsequently sorted such that some droplets form the image whereas others are caught and recirculated. For example, droplets can be selectively charged as a means of sorting or their size can be selectively varied to allow them to be sorted by selective deflection using a stream of air. The droplets that have been caught can then be recycled from the catcher and redispersed within the bulk ink. In another process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant, which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium,

but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in either case. Commonly used carrier media include water, organic solvents such as alcohols, ketones or hydrocarbons, as well as mixtures of water and organic co-solvents, such as alcohols, esters and ketones.

An important characteristic of inkjet printing is the need to control the ink on the surface of the substrate onto which it is deposited. In the case of common inkjet recording elements, an important factor in achieving this is the absorption of significant portions of the ink, particularly the carrier medium, into some part of the substrate structure. As a consequence, the printed image can appear to be dry immediately after printing and the absorbed liquid can evaporate later. This allows organic solvents and co-solvents with low boiling points to be usefully incorporated into ink formulations, particularly for drop-on-demand inkjet printing.

One of the advantages of inkjet printing is that it is a non-contact method and can be used to print onto a wide range of surface topography. However, the nature of the surface, particularly its surface energy, can still present difficulties. The surface energy quantifies the disruption of chemical bonds that occurs when a surface is created. It is the interaction between the forces of cohesion and the forces of adhesion which determines whether or not wetting occurs. If complete wetting does not occur, then a bead of liquid will form with a contact angle which is a function of the surface energies of the system.

Successful printing is normally achieved by applying inks with a surface tension lower then the surface energy of the surface. Unfortunately water has a very high surface tension, which makes it particularly difficult to apply satisfactorily as droplets onto low energy, impermeable surfaces, such as plastic.

Thus, liquid absorption does not occur when printing onto impermeable substrates and in this case either a very fast drying process is applied, much more volatile organic solvents are used as a major component of the carrier medium, or the ink droplets undergo some kind of phase-change on the substrate. All of these practices have significant disadvantages. For example,

many impermeable substrates are heat-sensitive, many volatile organic compounds raise concerns about health and safety and phase-change inks produce significantly thicker printed layers because most of the ink droplet is solidified.

The deposition of aqueous inkjet inks can therefore be problematical as they are either not capable of sufficiently wetting the substrate or do not dry quickly enough at the speeds used in inkjet printing, especially in continuous inkjet printing wherein the time between successive drops is very significantly shorter than for drop-on demand inkjet systems. As a result the ink droplets can wick, bleed or coalesce on some substrates, even some kinds of paper, and particularly on such impermeable substrates. Moreover, this tendency is exacerbated by the fast print speeds, high ink coverages, low surface energy surfaces and overlapping drops.

This problem can be avoided to some extent by using certain kinds of ink, which are usually non-aqueous. For example, non-aqueous radiation curable inks have been disclosed in patent application WO 99/07796, whereby the droplets are 'cured' or solidified by a chemical reaction initiated by an exposure, for example, to ultra-violet radiation. However the printed image may be insufficiently hardened or curing may continue after the initial curing time under the radiation source. Other non-aqueous inks that work on a similar basis include

'phase-change' inks, such as 'hot-melt' inks, which incorporate waxes that are solid at room temperature and melt when heated to decrease viscosity. Ink which is heated and melted in a printhead is ejected and deposited on substrate, usually paper, where it solidifies on cooling on the substrate to form the printed images. An aqueous inkjet ink composition that can adhere to a wide range of surfaces, and in particular to impermeable substrates, is disclosed in co-pending WO PCT/GB2007/004891 , the disclosure of which is incorporated herein by reference. The compositions disclosed therein include discrete particles responsive to an external stimulus and having a lower viscosity in the printhead, such that the ink composition is conveniently jettable, and a higher viscosity in response to a stimulated change of conditions as the droplets are immobilised on the substrate.

Cooling of a substrate prior to or during printing has not been practised with aqueous inks as higher substrate temperatures can assist the evaporation of the often substantial quantities of ink carrier medium, also referred to as ink vehicle, in which the ink colorants are dissolved, dispersed or suspended. In the specific case of non-aqueous hot-melt inks, substrate cooling some time after printing is often necessary to solidify the hot-melt ink sufficiently rapidly to avoid off-set by means of a pinch roll coming in contact with the surface of the printed substrate. However, hot-melt ink applied to a substrate that has been cooled below the selected temperature prior to printing may solidify too quickly, thereby preventing sufficient penetration of the hot-melt ink into the substrate before it solidifies. For example, US patent nos. 4,751,528 and US 6,196,672 show heating and cooling arrangements for inkjet printers using non-aqueous hot- melt ink.

PROBLEM TO BE SOLVED BY THE INVENTION Aqueous inkjet inks as disclosed in co-pending

WO PCT/GB2007/004891 include stimulus-responsive particles that have first and second rheological states associated respectively with a lower first viscosity, enabling the composition to pass through an inkjet print head orifice, and a second higher viscosity, enabling immobilisation of droplets of the composition on a substrate therefor in response to a stimulated change of conditions.

The external stimulus causing the composition to change from one rheological state to another can be in response to temperature change, the temperature of the circulating ink and the substrate being on either side of the 'switching' temperature at which the change in rheological condition occurs. The substrate onto which the ink is deposited will be at a lower temperature than the ink passing through the inkjet printhead to ensure effective droplet immobilisation, such that the quality of the printed image is not impaired.

This droplet immobilisation could be achieved in such a cases by ensuring that the ink at least in the printhead was sufficiently warm to be in its first rheological, low viscosity, state and that the substrate were sufficiently cool to cause the ink to adopt its second, high viscosity, state when the ink was deposited. However, in some printing environments the ambient temperature

conditions may be such that the substrate is not sufficiently cool, the ink is not immobilised upon deposition and printing quality is adversely affected. The problem to be solved, therefore, is how to print using aqueous, thermally- responsive inks such that the ink is immobilised on the substrate in all ambient temperature conditions and that good printing quality can be maintained at all times.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an inkjet printing system comprising (a) at least one inkjet printing unit each comprising at least one inkjet printhead; (b) an ink-receiving element comprising a substrate; (c) an aqueous inkjet ink composition which comprises a polymeric compound comprising discrete particles responsive to a stimulated temperature change, and a functional material, wherein the functional material may be incorporated as part of the polymeric particles, the particles causing the composition to have a first rheological state and a different second rheological state in response to the stimulated change in temperature, the first rheological state being associated with a first lower viscosity of the composition, wherein the particles have a first lower volume, enabling the composition to pass through an inkjet printhead orifice and the second rheological state being associated with a second higher viscosity of the composition, wherein the particles have a second higher volume, enabling immobilisation of droplets of the composition on a substrate therefor; and (d) means for cooling the substrate prior to or during ink deposition onto the substrate, such that upon contact with the substrate, the ink composition undergoes a change in rheological state from the first to the second rheological state, enabling immobilisation of droplets of the ink composition on the substrate.

There is also provided an inkjet printing method comprising the steps of:

A) providing an inkjet printer including at least one inkjet printing unit each comprising at least one inkjet printhead, the printing unit being responsive to digital data signals;

B) loading the printer with an ink-receiving element comprising a substrate;

C) loading the printer with an aqueous inkjet ink composition as described above; D) cooling the substrate prior to or during ink deposition onto the substrate, such that upon contact with the substrate the ink composition undergoes a change in rheological state from the first to the second rheological state, enabling immobilization of droplets of the ink composition on the substrate; and E) printing on the ink-receiving element using the aqueous inkjet ink composition in response to the digital data signals.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention enables excellent image quality to be obtained under all environmental conditions, especially where printing is carried out in hot environments. More optimal, stable printing conditions can be more easily obtained.

In some cases it may be possible to modify the polymeric particles in the ink to obtain specific temperatures at which the ink assumes its first and second rheological states, but this facility is limited. Therefore, it is advantageous to be able to maintain a temperature differential between the substrate and the ink composition, with substrate temperatures specifically chosen to cause the ink to adopt its second, higher viscosity on contact with the substrate, rather than having it conform to a temperature defined by the prevailing, variable, ambient conditions.

Although it is possible to maintain the temperature differential by heating the ink composition rather than cooling the substrate, it is not always desirable to allow the temperature of the ink in the printhead, at which the ink adopts its first, lower viscosity, to be too high as this may cause other problems, such as ink instability, ink degradation or excessive evaporation. This is particularly the case in continuous inkjet printing when the ink is recirculated through the printhead. It is therefore advantageous to be able to cool the substrate to obtain a sufficient temperature differential when the ink in the printhead is. for example, merely warm, say in the range 30-50°C. Maintaining an elevated ink

temperature in the entire ink printing system requires energy and so it is advantageous, environmentally and economically, to be able to optimise this temperature in concert with the substrate temperature by controlling the substrate temperature. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows four embodiments of the invention each of which includes a printing unit (4) comprising at least one printhead, a substrate (1), means for cooling (2) the substrate and means for measuring and controlling (3) the temperature of the substrate (2) prior to or during printing. The cooling means is shown in Fig. IA as above the substrate and before the printing unit; in Fig. 1 B as below the substrate and before the printing unit; in Fig. 1 C as below the substrate and directly under the printing unit and in Fig. ID as integrated into the system such that it is above the substrate, coincident with the printing unit, but does not impair the operation of the printing unit. For example it could be incorporated into the printing unit.

Fig. 2 is a graph of hydrodynamic particle diameter v. temperature of a thermally-responsive particle (Curve A) and a latex polymer (Curve B).

Fig. 3 is a graph of viscosity v. shear stress at a range of temperatures from 10°C to 40°C of the thermally-responsive particles in water at a concentration of 4%w/w.

Fig.4 is a graph of elastic modulus v. stress at a frequency of 1 rad.s ^-1 at a range of temperatures from 10°C to 24°C of the thermally-responsive particles in water at a concentration of 4% w/w.

Fig. 5 is a comparison of the effect of substrate temperature on coalescence of dye-based ink droplets, which were wanned to 31°C and applied simultaneously onto an impermeable substrate, the comparative ink droplets lacking the thermally-responsive polymeric particles of the ink droplets in the inventive system.

Fig. 6 is a comparison of pigment-based inks warmed to 31°C and applied onto an impermeable substrates at 15°C, wherein Ink C is an ink

comprising thermally-responsive particles and Ink D is a comparative ink composition.

DETAILED DESCRIPTION OF THE INVENTION

An important characteristic of inkjet printing is the need to control the ink on the surface of the substrate onto which it is deposited. This is particularly difficult to achieve using aqueous inkjet inks when deposited, in particular, on poorly-absorbing or impermeable surfaces. However co-pending WO PCT/GB2007/004891 discloses an aqueous inkjet composition which comprises a polymeric compound comprising discrete particles responsive to an external stimulus, and a functional material, wherein the functional material may be incorporated as part of the polymeric particles, the particles causing the composition to have a first rheological state and a different second rheological state in response to the stimulated change in conditions, the first rheological state being associated with a first lower viscosity of the composition, wherein the particles have a first lower volume, enabling the composition to pass through an inkjet printhead orifice and the second rheological state being associated with a second higher viscosity of the composition, wherein the particles have a second higher volume, enabling immobilisation of droplets of the composition on a substrate therefor. In the case of common inkjet recording elements, an important factor in controlling ink on the surface of substrates is the absorption of significant portions of the ink, particularly the carrier medium, into some part of the substrate structure. Some porous substrates, defined herein and throughout the specification as 'poorly-absorbing', do not absorb ink quickly or completely and as a consequence undesirable printing artefacts result, particularly coalescence. Such artefacts are caused by unabsorbed or poorly-absorbed ink droplets interacting undesirably on or near the surface of the substrate.

Examples of this kind of poorly-absorbing substrate include many paper substrates designed more specifically for other forms of printing, such as lithographic, flexographic. gravure, letterpress, electrophotographic or thermal printing: printing processes in which smaller volumes of more concentrated, more viscosity ink, or different ink forms, are used, thereby avoiding the occurrence of

these kinds of artefacts. Other examples include some synthetic papers, such as Tyvek™ or Teslin™. The worst cases in this respect are substrates that are completely impermeable, where no ink is absorbed by the substrate and all the ink remains on the substrate surface. A 'functional material' is a material that provides a particular desired mechanical, electrical, magnetic or optical property. As used herein the term 'functional material' refers to a colorant, such as a pigment dispersion or dye solution, magnetic particles, conducting or semi-conducting particles, quantum dots, metal oxide, wax or non-' switching' polymer (as core polymer). Preferably the functional material, however, is a pigment dispersion or a dye solution.

As used herein with respect to viscosity and volume, the terms 'first lower' and 'second higher', refer to the viscosity and volume differentials of the composition in the printhead and on the substrate respectively.

Thus as disclosed therein polymer particles can be added to a functional material in order to alter the viscosity of the ink in response to an external stimulus, such that on jetting an ink droplet onto a substrate the droplet is immobilized thereon.

If the external stimulus is temperature change, the polymeric compound comprising discrete particles, (which may optionally include a functional material) that is responsive to this change is referred to herein as being 'thermally-responsive'.

Thus if the external stimulus causing the composition to change from one rheological state to another is temperature change, such that the polymeric particles are thermally-responsive, the substrate onto which the ink is deposited would normally be at a significantly lower temperature than the ink passing through the inkjet printhead, the difference between the temperature at which the ink adopts the first and second rheological state being defined herein as the temperature differential.

Means for creating the temperature differential could be provided by heating the ink before it is jetted from the printing unit onto a substrate. For example, any or all of the ink supply lines, ink reservoirs, droplet-catching system, pumping units, filtration units, recirculation units or printheads could be heated to

enable the ink to be supplied and jetted easily from the printhead as droplets. This would be less preferred, however, especially for continuous inkjet printing, wherein the ink is recirculated though the printhead, as the ink could, for example, degrade or be subject to excessive evaporation. It is preferred, therefore, to include a cooling arrangement in the printer which can facilitate a temperature differential under all ambient conditions by enabling the cooling of the ink -receiving element comprising the substrate. Without such cooling under some ambient conditions droplet immobilisation would not occur effectively and the quality of the printed image would be severely reduced.

Generally a temperature differential of at least 5°C is preferred, but preferably not greater than 50 °C. Preferably the temperature differential is in the range from 6 °C to 30 °C, and especially in the range 10 °C to 20 °C.

In some circumstances, particularly when ambient conditions are humid, it may also be necessary to control air humidity at least in a part of the printing system, so that moisture condensation does not occur when the substrate is cooled. This may require that some means of drying the air to an appropriate level is included in the system.

Fig. 1 illustrates the embodiment of a printing system comprising a printer including a static printing unit comprising at least one printhead, a means of conveying an ink-receiving element comprising a substrate (1), which may be a series of individual sheets or a continuous web, the temperature of which is measured and controlled by a detector (3) so that, as necessary, a means of cooling (2) the substrate can be operated such that the substrate is at the required temperature before or during deposition of the ink on the substrate by at least one inkjet printing unit (4). As the substrate can be cooled on either side, a means of cooling is also shown to act on the back of the substrate. Moreover since cooling can occur either before or during printing, the cooling means may be situated either before the printhead or coincident with the printhead on either side of the substrate. Whilst the cooling unit may be a separate unit, it is also possible to build it into the printing unit, the temperature detector unit or as part of any combination of these. In the same way, whilst the temperature detector may be a

separate unit, it is also possible to build the detector into the means of cooling the substrate or the printhead. It is also possible to operate the cooling means without a temperature detector, since it is not always necessary to accurately control the temperature of the substrate. The printing units are static, that is, unmoving, and print onto the moving substrate as it passes beneath. This is in contrast to the working of domestic printers in which both the printing unit and the substrate move. The printing units can themselves comprise a plurality of printheads, wherein the printheads can be any of the many variations of drop-on- demand (DOD) or continuous inkjet devices, or combinations of different types of such printhead devices. The printing units all deposit aqueous ink of the ink composition as described above but, within the defined composition, the inks in each printing unit may differ in some particular of formulation. For example, the colorant used in the ink deposited by a particular printing unit may vary so that colour printing can be achieved. This usually requires the facility to deposit patterns of, at least, cyan, magenta, yellow and preferably also black ink droplets, individually, but more often in various pattern combinations. Alternatively or additionally the colorant concentration can be selected to provide either a 'dark' or 'light' ink, as explained hereinafter. Other additional printing units can be included if it is necessary to print specific colours, for example, 'spot colours', or the colours needed to extend the colour gamut produced by the printer, for example, an orange or a violet.

Co-pending patent application entitled 'An inkjet printing system', attorney docket 94772, co-filed herewith, describes a printing unit comprising a plurality of printheads which can be arranged as close as is practicable in succession, without any drying interstation being required to be interposed therebetween. Details of the drying means, which is located only downstream of the plurality of printing units, as described therein, is incorporated herein by reference. In the embodiment of the present invention it is only necessary that the substrate temperature should be maintained at or below the temperature at which the rheological change of the ink of the present invention occurs.

However, since there may be benefits in controlling the substrate temperature within a certain range, to minimise energy consumption, to minimise the possibility of condensation on the substrate and to adjust the cooling means according to the thermal capacity of the substrate, it is preferred to implement a temperature control loop where the substrate temperature is measured.

Cooling can be achieved by a process of conduction, convection or, possibly less often, radiation. Convection occurs by the flow of a fluid and conduction occurs by the flow of thermal energy through a substance from a higher- to a lower-temperature region by atomic or molecular interactions. Typically in a practical cooling system of a substrate, as in the preferred embodiment of the invention, elements of both conduction and convection will be employed. For example, cooling of a hot solid can be provided simply by fluid impingement, that is liquid or gas impingement, especially chilled fluid impingement, which is known to be an efficient means for heat transfer and is used in many applications which require quick cooling of a surface. Heat is conducted from the interior of the solid to its surface where it transfers to the impinging fluid and is carried away by convection. If the hot solid is at a higher temperature than its surroundings then radiative cooling will also occur, although the associated heat flux will be small compared to the convective flux in this context. Additionally, a heat pump utilizing a Stirling cycle may be used to create a thermal difference which can then be employed to cool a system. Alternatively a thermoelectric device may be used wherein the Peltier effect creates a heat flux between the junction of two different types of materials. If one side of the junction is maintained at ambient temperature then the other side of the junction can be caused to cool. Heat will then typically be conducted to the cool side and convected from the hot side. Alternatively any known method of cooling the substrate surface to be printed such that sufficient heat flux from the ink drop is provided for may be used, such as, for example, a chilled roller, refrigeration chamber or pre-coated thin film evaporation. However other factors, such as the quantity of polymeric particles in the ink composition and the exact nature of the polymeric particles, also can

have some effect on the temperature differential needed to obtain the desired effect.

The change in rheological states of the thermally-responsive particles equates to differences in size or shape or more particularly volume, represented by equivalent spherical diameter of the particles in the inkjet printhead from that on the substrate, the term equivalent spherical diameter being used in its art recognized sense in recognition of particles that are not necessarily spherical. Thus the thermally-responsive particles are in a collapsed state in the inkjet printhead, having an equivalent spherical diameter considerably less than the diameter of the inkjet nozzle to prevent blockage and enable jetting, typically less then 0.5μm, preferably 0.3μm or less, more preferably 0.15μm or less and especially 0.1 μm to 0.05μm.

Since the temperature of the substrate will be lower, this causes an expansion of the thermally-responsive particles on hitting the substrate, as shown in Curve A in Fig. 2, and hence a rapid increase in viscosity, as shown in Fig. 3 This can be contrasted with the use of a non-thermally-responsive latex polymer (Curve B in Fig. 2) wherein no expansion and hence no such increase in viscosity occurred. The increase in viscosity reduces the tendency for the printed droplets to flow or coalesce with other printed droplets on the substrate surface, reducing the number of printed defects.

The 'switching temperature' can be fine-tuned to adapt to exterior conditions by appropriate selection of the thermally-responsive polymer particles and/or by the inclusion/exclusion or adjustment of concentration of other components in the composition. However it is desirable that the viscosity change from a lower to higher viscosity and a concomitant volume change from a lower to a higher volume induced by the temperature change occurs over as small a temperature range as possible.

This increase in viscosity is a factor of at least ten, preferably a factor of at least thirty, more preferably a factor of at least one hundred, and most preferably a factor of at least one thousand. The viscosity of the ink in the printhead corresponds to that determined at low shear (for example 10s ^-1 ) while

on the substrate the viscosity corresponds to that measured at low stress (for example 0.01 Pa).

Thus the viscosity of the composition in the printhead may typically have a viscosity similar to water, namely about 10 ^-3 Pa.s . The low shear viscosity on the substrate may, however, typically be about 10 ³ Pa.s. Fig. 4 shows an increase in yield stress as temperature falls, such that the suspensions have elastic properties at 24°C and below.

Fig. 5 illustrates the effect of reduction in image resolution resulting when two drops of comparative Ink A, i.e. not containing thermally- responsive polymer particles were warmed to 31°C, and applied simultaneously and in close proximity to the surface of an impermeable substrate, such that upon wetting out the two droplets touched, as described hereinafter in Example 5. When the droplets touched they coalesced immediately.

However when the procedure was repeated with Ink B, comprising thermally-responsive polymer particles, when the droplets touched, the degree of coalescence was found to depend on the temperature of the substrate surface and hence the temperature of the ink on that surface. The lower the temperature of the ink, the higher the ink viscosity became, thereby immobilising the droplets on the surface and preventing coalescence. Analogously Fig. 6 shows the result of a comparable experiment with pigment-based Inks C and D.

The thermally-responsive polymers for use in the printer system of the invention may be prepared, for example, by polymerization of monomers which will impart thermal sensitivity, such as N-alkylacrylamides, such as N-ethylacrylamide and N-isopropylacrylamide; N-alkylmethacrylamides, such as N-ethylmethacrylamide and N-isopropylmethacrylamide; vinylcaprolactam, vinyl methylethers, partially-substituted vinylalcohols, ethylene oxide-modified benzamide, N-acryloylpyrrolidone, N-acryloylpiperidine, N-vinylisobutyramide; hydroxyalkylacrylates, such as hydroxyethylacrylate; hydroxyalkylmefhacrylates, such as hydroxyethylmethacrylate; and copolymers thereof, by methods known in the art. The thermally-responsive polymer particles can also be prepared by micellisation of thermally-responsive polymers such as, for example, certain

hydroxyalkylcelluloses, aspartic acid, carrageenan, and copolymers thereof, and crosslinked while in micelles.

The polymerization may be initiated using a charged or chargeable initiator species, such as, for example, a salt of the persulfate anion, or with a neutral initiator species if a charged or chargeable co-monomer species is incorporated in the preparation, or alternatively by light or heat.

Alternatively copolymers of the thermally-responsive particles may be created by incorporating one or more other unsubstituted or substituted polymers such as, for example, polyacrylic acid, polylactic acid, polyalkylene oxides, such as polyethylene oxide and polypropylene oxide, polyacrylamides, polyacrylates, polyethyleneglycol methacrylate, polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl chloride, polystyrene, polyalkylene- imines, such as polyethyleneimine, polyurethane, polyester, polyurea, polycarbonate or polyolefϊnes. Introduction of a copolymer, such as polyacrylic acid or polyethyleneglycol methacrylate, may be useful to fine-tune the switching temperature and swellability.

Any polymeric acidic groups present may be partially or wholly neutralized by an appropriate base, such as, for example, sodium or potassium hydroxide, ammonia solution, alkanolamines such as methanolamine, dimethylethanolamine, triethylethanolamine or N-methylpropanolamine or alkylamines, such as triethylamine. Conversely, any amino groups present may be partially or wholly neutralized by appropriate acids, such as, for example, hydrochloric acid, nitric acid, sulfuric acid, acetic acid, propionic acid or citric acid. The copolymers may be random copolymers, block copolymers, comb copolymers, branched, star or dendritic copolymers.

Particularly preferred thermally-responsive polymers are for example, a poly-N-alkylacrylamide, especially poly-N-isopropylacrylamide, and a poly-N-alkylacrylamide-co-acrylic acid, especially poly-N-isopropylacrylamide- co-acrylic acid, poly-N-isopropylacrylamide-co-polyethyleneglycol methacrylate, polyhydroxyalkylcellulose, especially polyhydroxypropylcellulose, polyvinyl- caprolactam, polyvinylalkylethers or ethyleneoxide-propylene oxide block copolymers.

The number of monomer units in the thermally-responsive polymer particles may typically vary from about 20 to 1500k. For example the number of monomer units in poly-N-isopropylacrylamide is from 200-50Ok and in polyvinyl- caprolactam is from 20 to 1500k. Typically the ratio of hydrophobic moiety to hydrophilic moiety in the thermally-responsive polymer particles is about 50% although the ratio can be as high as 80%.

Generally a cross-linker may be required to maintain the shape of the polymer particle, although too high a concentration of cross-linker may inhibit the swellability in response to the temperature change. If there is an alternative way of maintaining particle architecture, such as a core particle in a thermally- responsive polymer shell, it may be possible in some instances, however, to exclude a cross-linker.

Suitable cross-linkers for this purpose are as disclosed in co- pending WO PCT/GB2007/004891 and in particular N,N'-methylenebis- acrylamide, N, N'-ethylenebisacrylamide, dihydroxyethylene bisacrylamide, N,N' bisacryloylpiperazine, ethylene glycol dimethacrylate, glycerin triacrylate, divinylbenzene, vinylsulfone or carbodiimides. The quantity of cross-linker should normally be in the range of about 0.1-10 mol%. The polymer particle may also be in the form of a core/shell particle wherein the polymer surrounds a core forming a shell, such that the functional material is itself incorporated into the polymer particles, as described in co-pending WO PCT/GB2007/004891. The polymer may be chemically bonded thereto, in which case a cross-linker would not normally be necessary, or physically associated therewith wherein the core is encapsulated within the polymer shell. The core could be functionalised or non-functionalised polystyrene, latex or silica, or titania, a hollow sphere, magnetic or conductive particles or could comprise an organic pigment.

The size and shape of the thermally-responsive polymer particles needs to be appropriate to the size and shape of the orifice, as well as any filters, through which it has to pass. Since the thermally- responsive particles are generally approximately spherical, these particles can be made about the same

equivalent spherical diameter as conventional pigment particles, knowing that those particles are proven to be reliably jettable.

When printing, the quantity of a functional material contained in an ink composition, for example a colorant, is defined by the printing purpose. For example, the colorant concentration could be selected such that a so-called 'dark' or 'light' ink were produced, where 'light' refers to an ink formulation containing a lower concentration of colorant, of similar hue, to a 'dark' ink. It is preferable that the quantity of functional material, such as a colorant, namely pigment or dye, in an ink composition is from about 0.1 wt% to about 30 wt%, more preferably from about 0.5 wt% to about 15 wt%, most preferably from about 0.5 wt% to about 10 wt%.

The amount of thermally-responsive polymer particles is determined experimentally and sufficient must be added for the purpose and in most cases the amount of polymer particles will be in the range of about 0.5 to about 20 vol. %. However, conveniently a form of the Krieger- Doughty equation, which relates the particle addition needed to the change in particle diameter, may be used, as described in co-pending WO PCT/GB2007/004891.

Although the ink composition is primarily water-based, it may be suitable in some instances to include a small amount of an organic solvent, for example up to 10% of a solvent such as, for example, ethanol or methylethyl ketone to improve drying speed on the substrate.

Generally humectants are employed in inkjet compositions to help prevent the ink from drying out in the printhead and to modify ink viscosity. However it can be a particular advantage of the present invention for a continuous inkjet system that a humectant may not be required. This can be particularly useful when printing onto impermeable media surfaces when the humectant can not be absorbed into the media but has to be removed by evaporation. Nevertheless, the addition of one or more humectants in the ink composition is not precluded. Examples of humectants which could be used, if required, are those described in co-pending WO PCT/GB2007/004891.

Surfactants may be added to the ink to adjust the surface tension to an appropriate level or to prevent aggregation of the thermally-responsive

polymer. The surfactants may be anionic, cationic or amphoteric but should normally be selected such that it is either uncharged (non-ionic), has no net charge (ampboteric) or matches the charge of the thermally-responsive polymer used. The most preferred surfactants include acetylene diol derivatives, such as Surfynol© 465 (available from Air Products Corp.) or alcohol ethoxylates, such as Tergitol® 15-S-5 (available from Dow Chemical Co.) The surfactants can be incorporated at levels of 0.01 to 1% of the ink composition.

Additional polymers, emulsions, latexes or biocides may be used in the inks for use in the present invention A. biocide may be added to the ink composition to suppress the growth of microorganisms such as moulds, fungi, etc . in aqueous inks. A preferred biocide for the ink composition employed in the present invention is Proxel® OXL (Avecia Corp.) at a final concentration of 0.0001-0.5 wt%, preferably 0.05-0 5 wt %

Additional additives which optionally may be present include thickeners, conductivity-enhancing agents, anti-kogation agents, drying agents, anti-corrosion agents, defoamers and penetrants, additional polymers, emulsions, latexes or btocides, all as described in co-pending WO PCT/GB2007/004891

In some instances it may be appropriate to include a binder, such as a styreneacrylic or polyurethane resin, to provide robustness to the ink, providing the resin does not cross-link in the orifices in the printhead.

The pH of the aqueous ink compositions employed in the invention may be adjusted by the addition of organic or inorganic acids or bases. Useful inks may have a preferred pH of from 2 io 11, preferably 7 to 9, depending upon the type o f dye being used. The inks used in the various inkjet printers and in accordance with the present invention comprising a functional material are preferably colorants and can be dye-based or pigment-based, although pigment-based inks are preferred since they provide enhanced image stability, especially light stability The method of preparing the pigment ink and examples of suitable pigment inks and dye-based inks are all contained m co-pending WO PCT/GB2007/004891. Preferably the dispersion is a pigment dispersion selected from IDIS™ 40, PNB 15-3 (cyan),

PR 122 (magenta) or PY74 (yellow) or carbon black. A suitable dye colorant is Duasyn™ KRL-SF.

Although the inkjet composition of the present invention can. also be used with conventional inkjet substrates used for ink jet receivers, as detailed hereunder, it is a particular advantage of the present invention that that it can be used for printing onto poorly-absorbing substrates, such as paper substrates designed and manufactured specifically for other forms of printing, for example, lithographic, flexographic, gravure, letterpress, electrographic or thermal printing printing, as hereinbefore described, or in particular 'low energy' impermeable substrates, such as, for example, polyethylene, polypropylene, polyvinylchloride, polymethylmethacrylate, polysryrene, polyurethane, polycarbonate, nylon, rubber, silicone, glass, diamond, borosilicates, silicon, germanium and metals such as aluminium, steel or copper.

Normally printing onto low energy substrates often involves the use of corona discharge treatment or prior treatment with primers to enable good adhesion to the substrate. It is a feature of this invention, that such pre-treatroejits are not usually necessary.

Conventional substrates include, for example, resin-coated paper, paper, polyesters, or microporous materials such as polyethylene polymer- containing material sold by PPG Industries, Inc., Pittsburgh, Pennsylvania under the trade name of Teslin ®, Tyvek ® synthetic paper (DuPont Corp.) and OPPalyte® films (Mobil Chemical Co.) and other composite films listed in U.S. Patent 5,244,861. Opaque supports include plain paper, coated paper, synthetic paper, photographic paper support, melt-extnision-coated paper and laminated paper, such as biaxially oriented support laminates. Biaxially oriented support laminates are described in U.S. Patents 5,853,965; 5,866,282; 5,874,205; 5,888,643; 5,888,681; 5,888,683 and 5,888,714. These biaxially oriented supports include a paper base and a biaxially oriented polyotefin sheet, typically polypropylene, laminated to one or both sides of the paper base. Polymeric supports also include cellulose derivatives, e.g., a cellulose ester, cellulose triacetate, cellulose diaceiate, cellulose acetate propionate, cellulose acetaie buryrate; polyesters, such as poly(ethylene

terephthalate), poly(ethylene naphthenate), poly(1,4-cyclo-hexanedimethylene terephthalate), poly(butylene terephthalate), and copolymers thereof; polyimides; polyamides; polycarbonates; polystyrene; polyolefins, such as polyethylene, polypropylene or polybutylene; polysulfones; polyacrylates; polyetherimides; polyvinyl chloride; polyvinylacetate; polyvinylamine; polyurethane; polyacryloxiitrile; polyacetal; polytetraftuoroethene; polyfluorovinylidene; polysiloxane; polycarboranes; polyisopreue; rubber and mixtures ihereof.

These materials can be coated or laminated onto other substrates or extruded as sheets or fibres; the latter can be woven or compressed into porous but hydrophobic substrates, such as Tyvek ®, and mixtures thereof. The papers listed above include a broad range of papers, from high end papers, such as photographic paper, to low end papers, such as newsprint-

When the support used in the invention is a paper support, it may have a thickness of from 50 to 1000 μm, preferably from 75 to 300 μm. Antioxidants, antistatic agents, plasticizers and other known additives may be incorporated into the support, if desired. However the invention is particularly suitable also for printing onto non-linear surfaces, such as food packets and food cans.

Further coating compositions may be applied to the substrate printed in accordance with the present invention by any number of well-known techniques, including dip-coating, wound-wire rod coating, doctor blade coating, rod coating, air knife coating, gravure and reverse-roll coating, slide coating, bead coating, extrusion coating, curtain coating and the like.

The patents and publications referred (o herein are incorporated by reference in their entirery.

The invention will now be described wirh reference to the following examples, which are however, in no way to be considered limiting thereof.

EXAMPLES Example 1. Preparation of Thermally-Responsive Polymer Particles.

15.8g N-isopropylacrylamide (obtainable from Aldrich), 0.301 g

N,N'-raethylenebisacrylamide (obtainable from Aldrich) and 0.3 Ig sodium

dodecyl sulfate (SDS) were added to a 11 double-walled glass reactor equipped with a mechanical stirrer and condenser. 900ral water was added and the mixture wanned to 40°C, purged with nitrogen for 30 rain, while being stirred at 500 rpm. The solution was then heated to 70°C and 0.60 g potassium persulfate initiator (dissolved in 100 ml deionised water which had been purged with nitrogen) was added quickly to the reactor. The mixture was stirred at 400 ipm at 70°C for 5h under nitrogen. The reaction mixture rapidly became opalescent then became white. The heating was switched off and the mixture left to cool down to room temperature. The reaction yielded a white latex which was filtered, then dialysed until the conductivity of the permeate was less than lOμS. The dialysed latex was freeze-dried yielding approximately 15 g of poly-N-isopropylacrylamide powder-

The particle size of the suspension of the thermally-responsive particles was measured as a function of temperature from 10 to 50°C by photon correlation, spectroscopy (PCS) using a Malvern Zetasizer™ 3000HS. A dilute sample of thermally-responsive particles was obtained directly from the preparation vessel and was diluted with ImM sodium chloride solution to obtain a count rate in the Zetasteer instrument of from 100,000 to 500,000 counts/sec. Fig. 2 shows the typical particle diameters for a suspension of thermally- responsive particles (represented by Curve A) and a thermally-unresponsive latex (represented by Curve B).

The viscosity of the suspension of thermally-responsive particles in water at a concentration of 4% w/w was measured using a Bohlin™ CS50 meometer with a bob-and-cup geometry (C2.3/26). The viscosity was measured as a function of shear stress from 10-40°C- A typical data set is shown in Fig.3 for a 4 %w/w suspension, showing that the low shear viscosity increases by four orders of magnitude on reducing the temperature from 34°C to 26°C. Above 34°C, the viscosity is close to that of water, namely about 10° Pa.s. and shows little change with temperature. In contrast, at temperatures of 24°C and below, studies in oscillatory shear show that the suspensions have elastic properties, as characterised by a yield stress that increases in value as temperature falls (Fig. 4)

The preparation of co-polymer particles which are thermally- responsive is described in co-pending WO PCT/GB2007/004891.

Example 2. Preparation of a typical Pigment Dispersion.

3Og Monarch 880 carbon black (Cabot) was mixed with 214.4g demineralised water, followed by 55.6g Joncryl™ HPD96DMEA dispersant (available from Johnson Polymer). Zirconia beads of 0.6-0.8mm diameter were added to the mixture, which was milled until the carbon black could not be milled down further. The zirconia beads were removed by filtration leaving a black dispersion with a mean particle size of 98nm.

Example 3. Preparation of a Dye-based Ink A comprising Thermally- Responsive Particles 13.3 g of the 15.0 wt% solution of a black dye, Duasyn™ KRL-SF, was mixed with 66.7 g of a 7.5 wt% aqueous solution of the thermally-responsive polymer particles from Example 1, 0.5 g of the fluorocarbon surfactant Zonyl™ FSN and sufficient demineralised water was added to form lOOg of ink. Example 4. Preparation of a Comparative Dye-based Ink B. 13.3 g of the 15.0 wt% solution of a black dye, Duasyn™ KRL-SF, was mixed with 0.5 g of the fluorocarbon surfactant Zonyl™ FSN and sufficient demineralised water was added to form lOOg of ink.

Example 5. Comparison of effect of Substrate Temperature on Coalescence of Dye-based Inks A and B Ink B was wanned to 31°C before two droplets were applied simultaneously onto the surface of an untreated polyethylene substrate such that upon wetting out the two droplets touched. When the droplets touched they coalesced immediately. After 1 min when coalescence was complete, the droplets were photographed. This procedure was repeated but the temperature of the substrate surface was varied. Three experimental runs were performed with the substrate surface temperature maintained at 30°C, 20°C and 15°C and the results shown in Fig 5.

In the same way, Ink A was warmed to 31 °C before two droplets were applied simultaneously onto the surface of an untreated polyethylene substrate such that upon wetting out the two droplets touched. When the droplets touched the degree of coalescence that occurred was found to depend on the temperature of the substrate surface and hence the temperature of the ink on that

surface. The lower the temperature of Ink A, the higher the ink viscosity became (see Fig. 2), thereby immobilising the droplets on the surface and preventing coalescence. After 1 min, when any coalescence that might have occurred was complete, the droplets were photographed. This procedure was repeated but the temperature of the substrate surface was varied. Three experimental runs were performed with the substrate surface temperature maintained at 30°C, 20°C and 15°C and the results shown in Fig. 5.

Example 6. Comparison of effect of Substrate Temperature on coalescence of Pigment-based Inks C and D These inks were made-up using the same procedures employed for

Examples 3 and 4, except that a pigment Idis 4OK (Evonik Degussa), was substituted in the same amount for the dye Duasyn KRL-SF to make Ink C, including a thermally-responsive polymer, and Comparative Ink D respectively. Ink D was warmed to 31 °C before two droplets were applied simultaneously onto the surface of an untreated polyethylene substrate previously cooled to 15°C, such that upon wetting out the two droplets touched. When the droplets touched they coalesced immediately. After 1 min the droplets were photographed.

In the same way, Ink C was warmed to 31 °C before two droplets were applied simultaneously onto the surface of an untreated polyethylene substrate previously cooled to 15°C, such that upon wetting out the two droplets touched. When the droplets touched the degree of coalescence was much reduced because of the increase in viscosity induced by the presence of the thermally- sensitive polymer particles. After 1 min., when any coalescence that might have occurred was complete, the droplets were photographed (Fig. 6).