WO2011053609A1

WO2011053609A1 - Aqueous ink composition

Info

Publication number: WO2011053609A1
Application number: PCT/US2010/054179
Authority: WO
Inventors: John Martin Higgins; Danuta Gibson; Andrew Michael Howe; Andrew Clarke; Trevor Wear; Stephanie Veronique Desrousseaux
Original assignee: Estman Kodak Company
Priority date: 2009-10-30
Filing date: 2010-10-27
Publication date: 2011-05-05
Also published as: GB0919025D0

Abstract

The incorporation of a polyol redispersant, being an alcohol having at least four hydroxyl groups which are preferably underivatised, e.g. a saccharide such as sucrose, into an aqueous ink composition which comprises property- enhancing polymers (e.g. for robustness, waterfastness and adhesion) substantially enhances the redispersability of residues of the ink composition. Such use and ink compositions are described, which find particular application in continuous inkjet printing (where redispersiblity of residues is essential) and which are useful for printing on impermeable substrates whereby such ink compositions can be advantageously absent a liquid humectant.

Description

AQUEOUS INK COMPOSITION

FIELD OF THE INVENTION

The present invention relates to an aqueous composition suitable for inkjet printing systems and particularly for continuous inkjet printing systems. Further the present invention relates to the use of certain polyols in polymer- containing compositions, such as aqueous inkjet inks, for enhancing aqueous redispersibility.

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. Polymers in various forms may also be included to provide or improve properties such as image robustness, water-fastness or adhesion to the substrate.

An important characteristic of inkjet ink is the ability to re-dissolve or re-disperse ink that has dried on one of the surfaces of the printer. This is particularly necessary in continuous inkjet printing systems when the ink droplets are sorted and a portion has to be caught and re-circulated, rather than deposited onto the substrate.

In EP-A-0859036 an anionic dye is incorporated into an ink containing black pigment to aid redispersibility of the ink. However, the efficacy of this method would be very dependant on the types of polymer incorporated in the ink and the pigment dispersion. Indeed the examples contained in the patent do not incorporate any polymer at all and do not mention the type of material used to disperse the pigment - only that it was not redispersible. Anionic dyes can interact very strongly with other ink additives, such as rheology modifiers so are often not suitable.

US-B-7,445,312 describes the effect of humectants, such as polyols with a vapour pressure of 0.1 mm Hg or less, incorporated with a base into a porous platen of an inkjet printer (i.e. not in the ink). This arrangement allows printing right up to the edges of paper by preventing the formation of piled dried ink deposits where the ink has overflowed on to the platen itself. There is no disclosure of the redispersal of inks and this arrangement would not help to redispersed ink deposits formed around the printhead orifices. US-A- 2006/119680 describes an inkjet recording ink comprising water, a colorant and a surfactant in an amount of at least 0.3 wt%, which further comprises a sucrose derivative having from 1 to 20 alkylene oxide groups pendant thereon, of which at least 1 in 5 is an ethylene oxide group, which sucrose derivative is present in the ink in an amount of 0.5 to 40 wt% of the ink as a whole. The benefits of the composition are said to be reduced curling when printing on cellulose-containing receivers, enhanced responsiveness to high drive frequency, improved storage stability, and improved "sticking resistance" i.e. the clogging of printheads by colourant in the ink (demonstrated on printheads which have been left to dry at 35°C and 10% relative humidity for a week). US-A-

2006/119680 does not refer to the problems associated with property-enhancing polymer-containing inks nor to redispersibility of such dried ink deposits.

It is desirable to have an inkjet ink formulation which enables redispersion of polymer-containing ink residues from surfaces of the printer and printhead.

An optional characteristic of an inkjet ink is to store it in a freeze- dried form. However, if the ink contains certain types of polymers, aqueous reconstitution can be problematic. It is desirable to provide an inkjet ink formulation that can be readily reconstituted for use in an inkjet printer.

FR-A-2, 810,258 describes a procedure for enabling the long-term storage of electrophoretic gels containing agarose or certain polyacrylamides, whereby the fragile gels are conserved and may be rapidly rehydrated for use in medical or analytical electrophoresis (typically in separating proteins and nucleic acids). The procedure comprises incubating the agarose or polyacrylamide in a trehalose solution immediately after formation and then dehydrating the resulting solution. The dehydrated formulation may be stored at ambient temperature for a long duration without damaging the gel and may be rehydrated to form the gel prior to use. PROBLEM TO BE SOLVED BY THE INVENTION

There is a need for an ink that can be easily removed, e.g. with the carrier solvent (aqueous), from the surfaces of an inkjet printer (or other printing apparatus such as flexographic) if it has dried there inappropriately. Removal can be difficult to achieve if the ink comprises materials, especially polymers, incorporated to provide or improve the physical properties of the ink when it dries, for example, robustness, adhesion or waterfastness. Failure to remove dried ink deposits from printers, particularly from the surfaces associated with the generation, jetting, catching and recirculation of inkjet droplets, for example, especially in continuous inkjet printing, can degrade print quality or even prevent printing altogether.

There is a further need to provide a dehydrated or freeze-dried ink formulation which comprises polymer materials for enhancing ink properties such as robustness, adhesion or waterfastness, yet is readily reconstituted into its aqueous formulation.

Further, it is desirable to identify the use of a material for such redisperisiblity of ink residues and/or aqueous reconstitution of dehydrated ink formulations which material may be used in a broad range of inks without modifying the components of the ink.

SUMMARY OF THE INVENTION

According to a first aspect, the invention provides the use of a polyol redispersant to enhance the resdispersibility of residues of an aqueous ink composition comprising a property-enhancing polymer, by incorporating the polyol redispersant into said aqueous ink composition.

In a second aspect of the invention, there is provided the use of a polyol redispersant to enhance the aqueous reconstitution of a lyophilised, anhydrous, or dehydrated property-enhancing polymer-containing aqueous ink composition, by incorporating said polyol redispersant into said aqueous ink composition prior to lyophilisation or dehydration.

In a third aspect of the invention, there is provided an aqueous ink composition comprising an aqueous ink medium, a functional component, a property-enhancing polymer and a polyol redispersant. In a fourth aspect of the invention, there is provided a freeze dried ink composition capable of use as an aqueous ink composition upon aqueous reconstitution, which composition comprises a functional component, a property- enhancing polymer and a polyol redispersant.

In a fifth aspect of the invention, there is provided an inkjet printing method, said method comprising the steps of:

A) providing an inkjet printer that is responsive to digital data signals;

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

C) loading the printer with an aqueous ink composition as described above; and

D) printing on the ink-receiving element using the aqueous ink composition in response to the digital data signals.

In a sixth aspect of the invention, there is provided the use of an ink composition as defined above in continuous inkjet printing.

ADVANTAGEOUS EFFECT OF THE INVENTION

The properties of printing ink compositions, such as inkjet inks, may be modified and improved by the incorporation of certain polymeric materials, for example to improve the physical properties of the dried ink on the substrate on which it has been printed. However, the same properties are often disadvantageous when the ink dries on a surface in the printer itself. Such dried ink on the printer needs to be easily removed by wetting with water or the ink itself, perhaps the most desirable option, or by a special cleaning fluid. It is very difficult to obtain exactly the right balance of these contrary properties from the polymeric material alone. The use of a polyol redispersant in an ink composition in accordance with the present invention enables such composition to comprise a property-enhancing polymer without causing problems with immovable residues within the printer itself. In particular, the use of a polyol redispersant to provide the properties of re-dispersibility or re-dissolution allows:

(i) Easy selection of a polymeric material for robustness, adhesion, water-fastness or whatever other property might be required.

(ii) The exact balance of properties to be obtained by varying the

quantities and relative proportions of polymeric material and polyol, e.g. a sugar.

(iii) The use of a material, e.g. a sugar, that is very low cost, presents no risk to health or to the environment and is readily available.

Further, by incorporating a polyol redispersant into an inkjet composition comprising property-enhancing polymers, such composition may be freeze-dried for storage and transport whilst the polyol redispersant, in addition to providing redispersant properties to the ink in use, enhances the rate of aqueous reconstitution of the freeze-dried composition.

DETAILED DESCRIPTION OF THE INVENTION

The use of a polyol redispersant and the aqueous ink composition and dehydrate thereof used in the invention find particular application in continuous inkjet printing, although use in a drop-on-demand inkjet printing system and in other printing systems, such as flexographic printing, are also included within the scope.

The ink compositions of the present invention find application in various printing methods and as compositions suitable or adapted for such methods of printing. Accordingly, where appropriate, ink compositions discussed herein may be used in a variety of methods or include inks adapted for use in inkjet printing (drop-on-demand or continuous inkjet printing), flexographic printing, gravure printing, lithographic printing, etc. Such adapted ink

compositions are thereby disclosed herein. In continuous inkjet printing, 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 continuous inkjet systems, the incorporation of polymeric additives to improve adhesion to a substrate, for example, may cause residues that form in the capture plate and recirculation system to be difficult to remove. Incorporation of a polyol redispersant in a manner of the present invention helps to address that problem. It may also make cleaning the printhead to remove residues a straightforward, rapid and automatable process.

In 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. Residues may form about an inkjet printhead when not regularly used. A composition of the present invention may reduce the propensity of this to occur, but moreover makes the residue readily removable by a simple process.

An aqueous ink composition according to the invention comprises an aqueous medium, a functional component, a property-enhancing polymer and a polyol redispersant. The present inventors have found that the incorporation of the polyol redispersant into this composition provides a substantial improvement in redispersibility of such polymer-containing inkjet compositions.

By aqueous composition, it is meant that the composition comprises a medium, being a solvent or carrier fluid, that comprises water in an amount of at least 50% by weight, more preferably an amount of at least 75%, still more preferably at least 90% and most preferably at least 95%. Optionally, an aqueous composition may comprise minor amounts of organic solvents such as alcohols, esters and ketones. Preferably, the composition is purely aqueous. A purely aqueous composition comprises a carrier fluid consisting essentially of water. By using a polyol redispersant in the context of the present invention, such non-aqueous additives to an ink, such as ketones, esters and liquid alcohols may be eliminated or incorporated in reduced quantities.

A functional component in the sense of the present invention is a component that provides a particular desired mechanical, electrical, magnetic or optical property. As used herein the term 'functional component' preferably refers to a colorant, such as a pigment, which is dispersed in a carrier fluid, or a dye, dispersed and/or dissolved in the carrier fluid, magnetic particles (e.g. for barcoding), conducting or semi-conducting particles, quantum dots, metal oxide or wax. Optionally, in one particular embodiment, the functional component and the property-enhancing polymer may the same component, but is preferably a separate component. Preferably the functional component, however, is a pigment dispersed in the carrier fluid or a dye dispersed and/or dissolved in the carrier fluid.

Accordingly, therefore, the composition is preferably a pigment- or dye-based ink composition. Whilst the capability to be applied by inkjet is a preferred feature of the compositions of the present invention, especially an inkjet ink such as a dye- or pigment-based inkjet ink and the use of the dispersant in the inkjet printing process, it should be noted that the essence of the invention (i.e. the use of a polyol redispersant to improve redisperibility of a property-enhancing polymer-containing composition or to enhance the aqueous reconstitution of a dehydrated property-enhancing polymer-containing composition) finds application in a printing ink suitable for and applicable by any individual or multiple methods. Accordingly, where herein reference is made to an 'ink composition', and where 'an inkjet composition' is referred to this may be substituted with 'an ink composition' more generally where the context allows, the ink composition may be further specified as inkjet ink, flexographic ink, gravure ink etc, whereby inks suitable for respective printing methods are included. A polyol as used herein means an alcohol comprising at least four hydroxyl groups. A polyol may include, for example, a mono-saccharide, di- saccharide, an oligo-saccharide such as a tri-saccharide, sugar alcohol or other short chain polymeric polyol. Optionally, a polyol as used herein may include polyol derivatives, which may have other hydrophilic groups incorporated therein, such as ethylene oxide derivatives of hydroxyl groups.

A polyol redispersant in the context of the present invention is a polyol as referred to above that is capable of redispersing in aqueous media (e.g. the polyol redispersant should be water-soluble) a property-enhancing polymer- containing composition, which polyol redispersant is characterized by having a molecular weight of 1000 Da or less, preferably 500 Da or less and by being a solid at 0°C, more preferably a solid at 20°C, still more preferably a solid at 30°C, yet more preferably a solid at 45°C and most preferably a solid at 60°C.

The polyol redispersant is preferably selected from one or a combination of a mono-saccharide, di-saccharide, tri-saccharide, sugar alcohol, derivatives such as alkylene oxide, especially ethylene oxide, derivatives of any one of the aforementioned, or a short chain polymeric polyol. Preferably, the polyol redispersant is a saccharide and more preferably, the polyol redispersant is selected from one or a combination of mono- and di-saccharides. Especially preferred polyol redispersants are such polyols having at least 4 free hydroxyl groups and most preferred are saccharides with no derivatised hydroxyl groups.

Mono-saccharides include, for example, pentoses such as ribose and hexoses such as galactose, mannose, fructose and glucose. Preferred monosaccharides are hexoses such as fructose and glucose. Di-saccharides include any combination of two of the same or different mono-saccharides (such as the monosaccharides referred to above). Preferred examples of di-saccharides in the context of the present invention include sucrose, lactulose, maltose, trehalose, cellobiose for example.

Sugar alcohols may include for example sorbitol, mannitol and xylitol. The preferred polyol for use as a polyol redispersant in accordance with the present invention is sucrose.

The property-enhancing polymer may be a polymer that modifies the properties of the inkjet or ink composition to improve the physical properties for the purpose of the composition. Typically, the property-enhancing polymer is for improving the physical properties of an ink when it dries, such as the robustness, adhesion or waterfastness.

The property-enhancing polymer may be defined as being a polymeric material having multiple sites for internal-hydrogen bonding.

Such polymers may be for example, acrylic polymers, acrylamide polymers and the like.

The need for adhesion-enhancing polymer in an ink composition (e.g. an inkjet composition) is particularly pertinent in the application of aqueous- based inks onto low surface-energy substrates or impermeable substrates, in which there is little or no absorption of an aqueous ink. Rapid drying processes, UV- curable inks and phase-change inks may be used in assist in printing onto impermeable substrates. In one embodiment the property-enhancing polymer is a stimulus-responsive polymer or microgel, preferably water-swellable (or aqueous) microgel. Such microgels may be capable of adjusting the rheology of an ink in response to a range of stimuli, but typically are temperature responsive.

The microgel may typically be incorporated into the ink

composition used in the invention in any suitable proportion to achieve the desired property-enhancing effect, depending upon the precise nature of the printing ink, the substrate, the microgel itself and the intended printing conditions. For example, the aqueous printing ink may comprise a microgel particulate in an amount of from 0.1 to 50% by weight of the aqueous ink composition, more preferably from 1 to 40%, still more preferably at least 2%. For use inkjet inks, it is more preferable still that the microgel is present in an amount from 2 to 30% and most preferably 2.5 to 20%. For use in other inks, such as for flexographic printing, for example, it may be preferable that the microgel is present in greater proportions by weight of the ink, such as from 5 to 30% and more preferably from 10 to 25%. The microgel may be prepared from any suitable monomer units that will form the corresponding microgel, typically by polymerisation, co- polymerisation, block polymerisation or otherwise.

The property-enhancing polymer used may alternatively be formed in other configurations than pure polymer particles capable of forming microgels, which have the desired property-enhancing capability.

A microgel may typically be prepared, for example, by polymerisation of monomers such as N-alkylacrylamides, such as N-ethyl- acrylamide and N-isopropylacrylamide, N-alkylmethacrylamides, such as N-ethyl- methacrylamide and N-isopropylmethacrylamide, vinylcaprolactam, vinyl methyl- ethers, partially substituted vinylalcohols, ethylene oxide modified benzamide, N- acryloylpyrrolidone, N-acryloylpiperidine, N-vinylisobutyrarnide, hydroxy- alkylacrylates, such as hydroxyethylacrylate, hydroxyalkylmethacrylates, such as hydroxyethylmethacrylate, and copolymers thereof, by methods known in the art.

Optionally, a microgel may be prepared by micellisation of polymers and crosslinked while in micelles. This method applies to such polymers as, for example, certain hydroxyalkyl-celluloses, aspartic acid, carrageenan, and copolymers thereof. Alternatively copolymers of the carrier- swellable polymer 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 polyolefm.

Examples of microgels include those disclosed in WO-A-

2008/075049. Preferably, the property-enhancing polymer is substantially biologically inert (e.g. it is preferred that it is non-proteinaceous and not a ribonucleic molecule).

Each of the aforementioned polymers may be provided as the property-enhancing polymer of the compositions used in the invention (whether or not in the form of microgels or other classification).

Accordingly, the property-enhancing polymer may comprise, for example, substituted or unsubstituted polymers or mixed, branched or block copolymers of any of the following monomers: acrylic acid, lactic acid, alkylene oxides, such as ethylene oxide and propylene oxide, acrylamides, acrylates, ethylene glycol methacrylate, vinyl alcohol, vinyl acetate, vinylpyrrolidone, vinyl chloride, styrene, alkylene-imines, such as ethyleneimine, urethane, urea, or olefin. However, it is preferred, as mentioned above, that the property-enhancing polymer is capable of providing one or more of robustness, adhesion or water fastness and more preferable that the property-enhancing polymer is a stimulus- responsive microgel of the type defined in earlier paragraphs.

Preferred property-enhancing 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.

Without being bound by theory, it is believed that the polyol redispersants used according to the present invention are effective by temporarily occupying hydrogen bonding sites on the polymers that would otherwise form polymer inter- or intra-molecular hydrogen bonds. Such internal macromolecular hydrogen bonding, it is believed, makes aqueous reconstitution of a formulation difficult since the equilibrium for multiple hydrogen bond sites in such an internal system lies on the bonded side. The polyol redisdpersant in forming hydrogen bonds in a dried polymer-containing formulation enables displacement of those hydrogen bonds with water much more rapidly - it effectively shifts the equilibrium for rehydration strongly in favour of rehydration, thereby enabling an un-zipping effect.

The polyol dispersant in accordance with the present invention may be incorporated in any effective amount to achieved the desired degree of redispersibility (or the desired degree of enhanced aqueous reconstitution of a dehydrated composition), which amount may be achieved by a balance between the amount and relative properties of the property-enhancing polymer and the polyol redispersant, and other components of the ink composition.

Preferably, however, the polyol redispersant is present in an amount such that the ratio by weight of polyol redispersant to property-enhancing polymer is from 1 : 10 to 5 : 1 , more preferably 1 : 10 to 4 : 1 , still more preferably 1 : 8 to 2: 1 , optionally the ratio is at least 1 :5 or 1 :4 or even 1 :2, and overall the ratio is ideally no more than 1 : 1.

The polyol redispersant may be present in the aqueous ink composition in an amount that enables it to produce its beneficial effect. This may depend primarily on the loading and nature of the property-enhancing polymer, which are respectively related to the type of printing and the purpose of the polymer. For flexographic and gravure printing, for example, a much higher loading of property-enhancing polymer in the ink is likely and as such a greater amount of polyol may be required in line with the respective ratios referred to above. The amount of polyol redispersant present in such an ink may therefore be taken as corresponding, for example, to the ranges for inkjet below, but adjusting the lower limits by 5x (greater) and the upper limits by lOx (greater) to allow for the different make up. Preferably, for inkjet inks, such as continuous inkjet inks, the polyol redispersant is present in the aqueous ink composition in an amount of from 0.1-10 wt%, more preferably 0.2-8 wt%, still more preferably 0.5-5 wt%, more preferably 1-3 wt% and most preferably 2 wt%. However, the most appropriate amount of polyol redispersant incorporated as a proportion of the aqueous ink composition will be apparent to the person skilled in the art having regard to the amount of property-enhancing polymer present and the preferred amounts of polyol redisperant relative to property-enhancing polymer mentioned above.

As mentioned above, the ink may contain a colorant, if appropriate to the application for which it is employed, which may be a pigment or a dye. However, it is believed that greater benefits may be achieved in incorporating a polyol redispersant in a property-enhancing polymer-containing pigmented ink.

Additional polymers, emulsions or latexes may be used in the inks of the present invention. Any homopolymer or copolymer can be used in the present invention provided it can be stabilized in an aqueous medium, e.g. an aqueous basic medium, and is generally classified as either water-soluble, water- reducible or water-dispersible.

Although the ink composition is primarily water-based, it maybe 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 may employed in ink compositions, for example to help prevent the ink from drying out in an inkjet printhead and to modify ink viscosity. However it can be a particular advantage of the present invention, e.g. when applied to a continuous inkjet system, that a liquid humectant (such as glycerol) may not be used. This can be particularly useful when printing onto impermeable media surfaces when the humectant cannot 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 although the levels of liquid humectants employed can be significantly reduced and may be very low. It is a preferred feature of this invention that polyol redispersants which are solid at 30°C are incorporated into the ink in place of liquid humectants. Optionally, the solid polyol can also be incorporated into the ink as a component of a freeze-dried polymer-containing composition - i.e. where an aqueous solution of solid polyol redispersant and property-enhancing polymer is created and freeze-dried. It is, therefore, provided as a further aspect of the invention a continuous inkjet aqueous ink composition (i.e. an aqueous ink composition for use in continuous inkjet printing methods) comprising an aqueous medium, a functional component and a polyol redispersant, which polyol redispersant is a solid at 30°C. Preferably the continuous inkjet aqueous ink composition is absent a liquid humectant, especially a high-boiling liquid humectant, such as glycerol, or comprises a liquid humectant, e.g. a high-boiling liquid humectant such as glycerol, in an amount of no more than 0.5 wt% of the ink composition, preferably no more than 0.1 wt% and more preferably no more than 0.05 wt%, e.g. 0.02 wt% or less. Preferably, the continuous inkjet aqueous ink composition according to this aspect of the invention further comprises a property-enhancing polymer as defined above. Other preferred features described elsewhere herein in respect of other aspects of the invention may also be incorporated into the composition according to this aspect. In a further, but related, aspect of the invention, there is provided a method of continuous inkjet printing comprising providing a continuous inkjet printing apparatus responsive to digital data signals, providing the continuous inkjet printing apparatus with an ink-receiving element, which optionally comprises an impermeable print surface or hydrophobic surface, supplying or loading the continuous inkjet printing apparatus with a continuous inkjet aqueous ink as previously defined and printing on the ink-receiving element using the continuous inkjet aqueous ink composition in response to digital data signals.

In each aspect of the invention, surfactants may be added to the ink to adjust the surface tension to an appropriate level. The surfactants may be anionic: for example, salts of fatty acids, salts of dialkylsulfosuccimc acid, salts of alkyl and aryl sulfonates; they may be nonionic: for example, polyoxy ethylene alkyl ethers, acetylene diols and their derivatives, copolymers of polyoxyethylene and polyoxypropylene, alcohol alkoxylates, sugar-based derivatives; they may be cationic, such as alkyl amines, quaternary ammonium salts; or they may be amphoteric: for example, betaines. 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 company). The surfactants may typically be incorporated at levels of 0.01 to 1% of the ink composition. Preferably, however, the surfactant is not a polymer surfactant.

A biocide may be added to the ink composition employed in the invention to suppress the growth of microorganisms such as molds, fungi, etc. in aqueous inks. Preferred biocides for the ink composition employed in the present invention are Proxel® GXL (Arch chemicals.) or Kordek MLX (Rohm & Haas) 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. In some instances it may be appropriate to include a binder, such as a styrene acrylic or polyurethane resin, to provide robustness to the ink, providing the resin does not crosslink 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 to 11, preferably 3.5 to 9 and most preferably 7 to 9, depending upon the type of functional component, e.g. dye, being used.

The inks comprising a colorant used in the various inkjet printers and in accordance with the present invention may be either dye -based or pigment- based, although pigment-based inks are preferred since they provide enhanced image stability, especially light stability and stability against loss of image quality as a result of exposure to oxidising atmospheric gases. The process of preparing inks from pigments commonly involves two steps: (a) a dispersing or milling step to break up the pigment to the primary particle, and (b) a dilution step in which the dispersed pigment concentrate from step (a) is diluted with a carrier and other addenda to a working strength ink. In the milling step, the pigment is usually suspended in a carrier (typically the same carrier as that in the finished ink) along with rigid, inert milling media.

Mechanical energy is supplied to this pigment concentrate, and the collisions between the milling media and the pigment cause the pigment to deaggregate into its primary particles. A dispersant or stabilizer, or both, may be added to the dispersed pigment concentrate to facilitate deaggregation, maintain particle stability, and retard particle reagglomeration and settling.

The dispersant is an optional ingredient used to prepare the dispersed pigment concentrate. Dispersants which could be used in the present invention include sodium dodecyl sulfate, acrylic and styrene-acrylic copolymers, such as those disclosed in U. S. Patent. Nos. 5,085,698 and 5,172,133, and sulfonated polyesters and styrenics, such as those disclosed in U.S. Patent. No. 4,597,794. Other patents referred to above in connection with pigment availability also disclose a wide variety of dispersant to select from. Non-ionic dispersants could also be used to disperse pigment particles. Dispersants may not be necessary if the pigment particles themselves are stable against flocculation and settling. Self-dispersing pigments are an example of pigments that do not require a dispersant; these types of pigments are well known in the art of inkjet printing. These may typically have carboxylate, sulfonate or polymeric groups grafted onto the pigment particle surface.

Pigment particles useful for inkjet ink embodiments of the invention may have any particle size that can be jetted through an inkjet printhead. The pigment particles may have a mean particle size of up to 0.5 μιη. Preferably, the pigment particles have a mean particle size of less than 0.3 μιη, more preferably less than 0.15 μιη. A wide variety of organic and inorganic pigments, alone or in combination, may be selected for use in the inks of the present invention.

Pigments that may be used in the invention include those disclosed in, for example, U.S. Patent Nos. 5,026,427; 5,086,698; 5,141,556; 5,160,370; and 5,169,436. The exact choice of pigments will depend upon the specific application and performance requirements such as color reproduction and image stability.

Pigments suitable for use in the present invention include, for example, azo pigments, monoazo pigments, disazo pigments, azo pigment lakes, β-Naphthol pigments, Naphthol AS pigments, benzimidazolone pigments, disazo condensation pigments, metal complex pigments, isoindolinone and isoindoline pigments, polycyclic pigments, phthalocyanine pigments, quinacridone pigments, perylene and perinone pigments, thioindigo pigments, anthrapyrimidone pigments, flavanthrone pigments, anthanthrone pigments, dioxazine pigments,

triarylcarbonium pigments, quinophthalone pigments, diketopyrrolo pyrrole pigments, titanium oxide, iron oxide, and especially carbon black.

Particularly preferred pigments for use in this invention are, for example, PNB15-3, (cyan), PR122 (magenta). PY74 (yellow) and especially Carbon K (black).

The pigment used in preferred ink compositions used in the invention is present in any effective amount, depending in part on the type of printing ink, but which is generally from 0.1 to 30 wt%, preferably from 0.5 to 15 wt%, more preferably 0.5 to 10 wt%, especially in inkjet ink.

Alternatively the colorants which could be used in this invention could be dyes including water-soluble dyes. The dyes can be photochromic, thermochromic or fluorescent.

After formulation, inks are often stored for a long time before use, e.g. in an inkjet printer. When polymers and pigments are incorporated into an ink, long-term unwanted interactions between the polymer and the pigment can cause pigment particles to agglomerate causing blockage of the orifices of the printhead when the ink is eventually used. A biocide may be added to prevent biological fouling of the ink and printhead. Long term storage may instead be achieved by dehydrating or freeze-drying the aqueous inkjet composition.

Reconstituting freeze-dried polymer is fraught with problems, however, the polymer could be denatured during freeze-drying or cross-linked further than desired; and rehydrating water-soluble polymers can take an unacceptably long time for many industrial applications. However, the inventors have further found that the incorporation of polyol redispersants enables rapid aqueous reconstitution of a lyophilised or freeze-dried ink composition.

It is thus a further aspect of the invention that a polyol redispersant as defined above may be used to enhance the aqueous reconstitution of a lyophilised, anhydrous, or dehydrated property-enhancing polymer-containing aqueous ink composition, especially an inkjet ink composition, by incorporating said polyol redispersant into said aqueous ink composition prior to lyophilisation or dehydration. Typically such enhancement is an increase in the rate of aqueous reconstitution. The invention therefore further relates to a lyophilised composition comprising a pigment, a property-enhancing polymer and a polyol redispersant, preferably comprising the property-enhancing polymer and polyol redispersant in the respective amounts mentioned above.

In a still further aspect of the invention, there is provided a composition comprising polyNIPAM and a polyol redispersant such as sucrose.

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

EXAMPLES

EXAMPLE 1

This Example demonstrates the effect of polyol redispersants on the redispersion of freeze-dried polyNIPAM polymer particles.

Preparation of comparative freeze dried polymer X.

A suspension of poly-N-isopropylacrylamide (polyNIPAM) polymeric particles was prepared as described in patent application WO-A- 2008/075049. The aqueous suspension of polymeric particles was freeze dried to constant weight under vacuum (<100 mm Hg), so that all water was removed and white, fluffy material was formed.

Preparation of freeze dried polymer Y.

A suspension of PolyNIPAM polymeric particles was prepared as described in patent application WO-A-2008/075049. This was concentrated to 5.45% polyNIPAM polymer particles. 2.5 g sucrose was added to 27.5 ml of the suspension of polyNIPAM polymer particles in demineralised water and stirred until dissolved, so that the mixture contained 5% polyNIPAM and 0.25 moles/1 sucrose in solution. This modified suspension was freeze-dried under vacuum (<100 mm Hg) to constant weight forming a white crystalline solid.

Redispersion of freeze-dried polymer X and Y.

The rate at which the freeze-dried polymer samples dispersed completely was measured by weighing a small sample of freeze-dried material into a vial. Then, while stirring at room temperature, sufficient demineralised water or aqueous solution of polyol was added to each sample to disperse the material and create 25 ml of a 5% suspension of polyNIPAM polymeric particles. Complete dispersion was determined to have occurred when the suspension was completely clear. Table 1

The results in Table 1 show that only the example of the invention, sample 6, that was formed from the polyNIPAM polymer which had been freeze-dried with sucrose (i.e. polymer Y) redispersed in demineralised water in less than a day at room temperature. In fact, redispersion in this example was very rapid. The addition of a polyol solution to the freeze-dried polyNIPAM (polymer X) only slightly affected the rate at which the freeze-dried polymer could be re-dispersed - at 2.5 days this rate would not be called rapid and would not be acceptable.

In addition, only the inventive example, sample 6, could be filtered through a 5 micron pore filter (a Minisart filter obtained from Sartorius was used for this purpose). This filter blocking suggests that in spite of apparent redispersion of freeze-dried polyNIPAM polymer particles, there were still aggregates of polymer particles present in the other samples. This poor filtration behaviour would prevent the use of polymer X in an inkjet printhead because of blocking of the printing orifices. This poor redispersion behaviour apparently contradicts the findings by M.J. Snowden et al. (Colloids and Surfaces A.

Physicochemical and Engineering aspects: 1998, vol.137, p.155- 164) who found no detrimental effect on the polyNIPAM microgel by the freeze-drying (or lyophilisation) process. EXAMPLE 2

This Example illustrates the redispersibility of residues of an ink and the effect thereon of incorporating a polyol redispersant in accordance with the present invention.

Preparation of Inventive Ink Yl.

An ink was made by taking 16 ml of the sample of polymer Y (sample 6 in the example above) and adding 2 ml of IDIS 40 carbon black dispersion (manufactured by Evonik-Degussa, at 30% solids). This was stirred to ensure thorough mixing of the polymer particles and the carbon black dispersion. Then 0.23 ml of a 10% solution of sodium dodecyl sulphate was added, followed by a further 1.76 ml of demineralised water and 0.01 ml of Kordek™ MLX biocide (from Rohm & Haas) which were stirred at room temperature. For comparison, an ink was also made by taking 16 ml of a concentrated (i.e. not previously freeze-dried) sample of polyNIPAM (containing 5% of the

polyNIPAM particles, but no sucrose) which was treated in the same way. The presence of the polyol did not affect the rheological properties of the two inks, which were investigated under steady shear and oscillatory shear between 10°C and 50°C.

Further examples below illustrate the effect on ink redispersibility when solid polyol is incorporated into an ink. All of the inks described below were formulated to contain 4 wt% of a polymeric material incorporated to provide or improve the physical properties of the ink when it dries, such as robustness, adhesion or waterfastness, together with a 0 wt%>, 0.5 wt%>, 1.0 wt%>, 2.0 wt%> or 4 wt%> of a second material, sucrose, to provide or improve the contrary physical properties of re-dispersibility or re-dissolution. Some of the inks also contained 4 wt% of a colorant, a carbon-black pigment, IDIS40.

Preparation of Inventive Ink Al, A2 and A3.

3.81 g of a self-crosslinking acrylic emulsion, manufactured by

BASF as Joncryl FLX5000, with a quoted solids percentage of 42 wt%, was mixed with 1 g, 2 g or 4 g of a 20 wt% solution of sucrose (hereinafter used interchangeably with "sugar") and sufficient demineralised water to obtain a total weight of 40 g. These mixtures were designated as Inventive inks Al, A2 and A3 respectively.

The redispersibility or re-dissolution of these inks was assessed using the following method. 1 g sample of ink was measured into a pre-weighed disposable weighing boat (Weighing Boat 1) and allowed to dry completely under ambient conditions. 1 g of demineralised water was added to the dried ink and swirled around briefly in the weighing boat so that ink and water could mix, before leaving to stand. Five minutes after the water addition the supernatant liquid, now comprised of a mixture of water and dissolved ink, was poured off into another pre-weighed disposable weighing boat (Weighing Boat 2). The contents of both boats were allowed to dry completely under ambient conditions and then weighed.

From the data so obtained it was possible to determine (1) the quantity of dried ink solids weighed into Weighing Boat 1 and (2) the quantity of dried ink solids re-dissolved in Weighing Boat 1 by the addition of water and transferred to Weighing Boat 2. The quantity of dried ink solids that was redispersed (2) was expressed as a percentage of the dried ink solids deposited initially (1) and the results given in Table 2.

Preparation of Comparative Ink A.

A comparative ink, Comparative Ink A, was prepared in the same way as Inventive Inks Al, A2 and A3, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as Inventive Ink Al, A2 and A3.

Preparation of Inventive Ink Bl, B2 and B3.

3.25 g of a pH neutral, film-forming styrene-acrylic emulsion, manufactured by BASF as Joncryl™ LMV7030, with a quoted solids percentage of 49.3 wt%, was mixed with 1 g , 2 g or 4 g of a 20 wt% solution of sucrose and sufficient demineralised water to obtain a total weight of 40 g. These mixtures were designated as Inventive inks Bl, B2 and B3 respectively. The redispersibility or re-dissolution of these inks was assessed using the following method. 1 g sample of ink was measured into a pre-weighed disposable weighing boat (Weighing Boat 1) and allowed to dry completely under ambient conditions. 1 g of demineralised water was added to the dried ink and swirled around briefly in the weighing boat so that ink and water could mix, before leaving to stand. Five minutes after the water addition the supernatant liquid, now comprised of a mixture of water and dissolved ink, was poured off into another pre-weighed disposable weighing boat (Weighing Boat 2). The contents of both boats were allowed to dry completely under ambient conditions and then weighed.

Preparation of Comparative Ink B.

A comparative ink, Comparative Ink B, was prepared in the same way as Inventive Inks Bl, B2 and B3, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as Inventive Ink Bl, B2 and B3.

Preparation of Inventive Ink CI, C2 and C3.

3.40 g of a glycol ether- free acrylic emulsion, manufactured by

BASF as Joncryl™ EC02124, with a quoted solids percentage of 47 wt%, was mixed with 1 g , 2 g or 4 g of a 20 wt% solution of sucrose and sufficient demineralised water to obtain a total weight of 40 g. These mixtures were designated as Inventive inks CI, C2 and C3 respectively.

From the data so obtained it was possible to determine (1) the quantity of dried ink solids weighed into Weighing Boat 1 and (2) the quantity of dried ink solids re-dissolved in Weighing Boat 1 by the addition of water and transferred to Weighing Boat 2. The quantity of dried ink solids that was redispersed (2) was expressed as a percentage of the dried ink solids deposited initially (1) and the results given in Table 2. Preparation of Comparative Ink C.

A comparative ink, Comparative Ink C, was prepared in the same way as Inventive Inks CI, C2 and C3, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as Inventive Ink CI, C2 and C3.

Preparation of Inventive Ink Dl, D2 and D3.

3.56 g of a hard non- film-forming styrene-acrylic emulsion, manufactured by BASF as Joncryl™ DFC3050E, with a quoted solids percentage of 45 wt%, was mixed with 1 g , 2 g or 4 g of a 20 wt% solution of sucrose and sufficient demineralised water to obtain a total weight of 40 g. These mixtures were designated as Inventive inks D 1 , D2 and D3 respectively.

Preparation of Comparative Ink D.

A comparative ink, Comparative Ink D, was prepared in the same way as Inventive Inks Dl, D2 and D3, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as Inventive Ink Dl, D2 and D3.

Table 2

Preparation of Comparative Ink El and E2.

5.33 g of a carbon-black pigment dispersion, manufactured by Degussa as IDIS40, with a quoted solids % of 30%, was mixed with 4 g or 8 g of a 20 wt% solution of sucrose and sufficient demineralised water to obtain a total weight of 40 g. These mixtures contain no polymer and were designated as Comparative inks El and E2 respectively. The redispersibility or re-dissolution of these inks was assessed using the following method. 1 g sample of ink was measured into a pre-weighed disposable weighing boat (Weighing Boat 1) and allowed to dry completely under ambient conditions. 1 g of demineralised water was added to the dried ink and swirled around briefly in the weighing boat so that ink and water could mix, before leaving to stand. Five minutes after the water addition the supernatant liquid, now comprised of a mixture of water and dissolved ink, was poured off into another pre-weighed disposable weighing boat (Weighing Boat 2). The contents of both boats were allowed to dry completely under ambient conditions and then weighed.

From the data so obtained it was possible to determine (1) the quantity of dried ink solids weighed into Weighing Boat 1 and (2) the quantity of dried ink solids re-dissolved in Weighing Boat 1 by the addition of water and transferred to Weighing Boat 2. The quantity of dried ink solids that was redispersed (2) was expressed as a percentage of the dried ink solids deposited initially (1) and the results given in Table 3.

Preparation of Comparative Ink E.

A comparative ink, Comparative Ink E, was prepared in the same way as Comparative Inks El and E2, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as

Comparative Ink El and E2.

Preparation of Inventive Ink Fl and F2.

3.81 g of a self-crosslinking acrylic emulsion, manufactured by

BASF as Joncryl™ FLX5000, with a quoted solids percentage of 42 wt%, was mixed with 5.33 g of a carbon-black pigment dispersion, manufactured by Degussa as IDIS40, with a quoted solids % of 30%, and 4 g or 8 g of a 20 wt% solution of sucrose and sufficient demineralised water to obtain a total weight of 40 g. These mixtures were designated as Inventive inks Fl and F2 respectively. The redispersibility or re-dissolution of these inks was assessed using the following method. 1 g sample of ink was measured into a pre-weighed disposable weighing boat (Weighing Boat 1) and allowed to dry completely under ambient conditions. 1 g of demineralised water was added to the dried ink and swirled around briefly in the weighing boat so that ink and water could mix, before leaving to stand. Five minutes after the water addition the supernatant liquid, now comprised of a mixture of water and dissolved ink, was poured off into another pre-weighed disposable weighing boat (Weighing Boat 2). The contents of both boats were allowed to dry completely under ambient conditions and then weighed.

Preparation of Comparative Ink E.

A comparative ink, Comparative Ink F, was prepared in the same way as Inventive Inks Fl and F2, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as Inventive Ink Fl and F2.

Preparation of Inventive Ink Gl and G2.

3.25 g of a pH neutral, film-forming styrene-acrylic emulsion, manufactured by BASF as Joncryl™ LMV7030, with a quoted solids percentage of 49.3 wt%, was mixed with 5.33 g of a carbon-black pigment dispersion, manufactured by Degussa as IDIS40, with a quoted solids % of 30%, and 4 g or 8 g of a 20wt% solution of sucrose and sufficient demineralised water to obtain a total weight of 40 g. These mixtures were designated as Inventive inks Gl and G2 respectively. The redispersibility or re-dissolution of these inks was assessed using the following method. 1 g sample of ink was measured into a pre-weighed disposable weighing boat (Weighing Boat 1) and allowed to dry completely under ambient conditions. 1 g of demineralised water was added to the dried ink and swirled around briefly in the weighing boat so that ink and water could mix, before leaving to stand. Five minutes after the water addition the supernatant liquid, now comprised of a mixture of water and dissolved ink, was poured off into another pre-weighed disposable weighing boat (Weighing Boat 2). The contents of both boats were allowed to dry completely under ambient conditions and then weighed.

Preparation of Comparative Ink G.

A comparative ink, Comparative Ink G, was prepared in the same way as Inventive Inks Gl and G2, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as Inventive Ink Gl and G2.

Preparation of Inventive Ink HI and H2.

3.40 g of a glycol ether- free acrylic emulsion, manufactured by

BASF as Joncryl™ EC02124, with a quoted solids percentage of 47 wt%, was mixed with 5.33 g of a carbon-black pigment, manufactured by Degussa as IDIS40, with a quoted solids % of 30%, and 4 g or 8 g of a 20 wt% solution of sucrose and sufficient demineralised water to obtain a total weight of 40 g. These mixtures were designated as Inventive inks HI, H2 and H3 respectively. The redispersibility or re-dissolution of these inks was assessed using the following method. 1 g sample of ink was measured into a pre-weighed disposable weighing boat (Weighing Boat 1) and allowed to dry completely under ambient conditions. 1 g of demineralised water was added to the dried ink and swirled around briefly in the weighing boat so that ink and water could mix, before leaving to stand. Five minutes after the water addition the supernatant liquid, now comprised of a mixture of water and dissolved ink, was poured off into another pre-weighed disposable weighing boat (Weighing Boat 2). The contents of both boats were allowed to dry completely under ambient conditions and then weighed.

Preparation of Comparative Ink H.

A comparative ink, Comparative Ink H, was prepared in the same way as Inventive Inks HI and H2, except no sugar was incorporated into the ink. The redispersibility of this sample was assessed in the same way as Inventive Ink HI and H2.

Table 3

In all cases where a polymeric material was incorporated into the ink to provide or improve the physical properties of the ink when it dries, such as robustness, adhesion or waterfastness, the addition of sucrose improved significantly the redispersibility of the ink.

Claims

CLAIMS:

1. Use of a polyol redispersant to enhance the redispersibility of residues of an aqueous ink composition comprising at least one property-enhancing polymer, by incorporating said polyol redispersant into said aqueous ink composition.

2. A use as claimed in claim 1, wherein the aqueous ink composition is an aqueous inkjet ink composition.

3. A use as claimed in claim 2, wherein the aqueous inkjet ink composition is a continuous inkjet aqueous ink composition.

4. A use as claimed in claim 1, wherein the aqueous ink composition is a flexographic printing ink composition.

5. A use as claimed in any one of claims 1 to 4 wherein the at least one property-enhancing polymer contained within the aqueous ink composition is a polymer for enhancing the robustness, adhesion or waterfastness of a printed composition.

6. A use as claimed in any one of the preceding claims, wherein the polymer is selected from acrylic emulsions, acrylic-styrene emulsions, or polymers of acrylic acid, acrylamides or combinations and derivatives thereof.

7. A use as claimed in any one of the preceding claims, wherein the polymer is for improving adhesion of the aqueous ink composition composition to an impermeable substrate.

8. A use as claimed in any one of the preceding claims, wherein the polyol redispersant is a polyol selected from mono-saccharides, di-saccharides, tri- saccharides, sugar alcohols and combinations and derivatives thereof.

9. A use as claimed in any one of the preceding claims, wherein the polyol redispersant is further characterized by having a melting point of at least 20°C.

10. A use as claimed in any one of the preceding claims, wherein the polyol redispersant is sucrose.

11. A use as claimed in any one of the preceding, wherein the polyol redispersant is employed in an amount as a ratio by weight of the property- enhancing polymer of from 1 : 10 to 5 : 1.

12. A use as claimed in claim 11, wherein the polyol redispersant is employed in an amount as a ratio by weight of the property-enhancing polymer of from 1 :8 to 2: 1.

13. A use as claimed in any one of the preceding claims in which the polyol redispersant is employed in the aqueous ink composition in an amount of from 0.1 to 10% by weight of the aqueous aqueous ink composition.

14. A use as claimed in claim 13 in which the polyol redispersant is employed in an amount of from 1 to 3% by weight of the aqueous ink composition.

15. Use of a polyol redispersant to enhance the aqueous reconstitution of a lyophilised, anhydrous, or dehydrated property-enhancing polymer-containing aqueous ink composition, by incorporating said polyol redispersant into said aqueous composition prior to lyophilisation or dehydration.

16. A use as claimed in claim 15, wherein the polyol redispersant, property- enhancing polymer and relative proportions thereof are as further defined in any one of claims 1 to 12.

17. An aqueous ink composition comprising an aqueous medium, a functional component, a property-enhancing polymer and a polyol redispersant.

18. An aqueous ink composition as claimed in claim 17, wherein the polyol redispersant is a saccharide having underivatised hydroxyl groups and is a solid at 30°C.

19. An aqueous ink composition as claimed in claim 16 or claim 17, wherein the polyol redispersant, property-enhancing polymer and relative proportions thereof are as further defined in any one of claims 1 to 14.

20. An aqueous ink composition as claimed in any one of claims 16 to 19 adapted for use in continuous inkjet printing, which further comprises a liquid humectant in an amount of no more than 0.1 wt% of the ink composition.

21. An aqueous ink composition as claimed in any one of claims 16 to 19 adapted for use in continuous inkjet printing, which is substantially absent a liquid humectant.

22. A freeze-dried or dehydrated ink composition comprising a functional component, a property-enhancing polymer and a polyol redispersant, which freeze-dried or dehydrated composition is capable of aqueous reconstitution.

23. A composition as claimed in claim 22, wherein the property-enhancing polymer and polyol redispersant and relative proportions thereof are

independently as further defined in any one of claims 1 to 14.

24. A composition as claimed in any one of claims 17 to 23, wherein the functional component is a pigment as a colourant.

25. An inkjet printing method, said method comprising the steps of:

A) providing an inkjet printer that is responsive to digital data signals;

B) loading the printer with an ink-receiving element comprising a support; C) loading the printer with an aqueous inkjet composition as described above; and

D) printing on the ink-receiving element using the aqueous inkjet ink composition in response to the digital data signals.

26. Use of an ink composition as defined in any one of claims 17 to 21 and 24 in continuous inkjet printing.