WO2005080513A1

WO2005080513A1 - Ink jet ink composition comprising glycerol

Info

Publication number: WO2005080513A1
Application number: PCT/GB2005/000519
Authority: WO
Inventors: Kirsten Jane Holland; Mark Holbrook; Richard John Stuart Allan Cordwell; Vicky Riding
Original assignee: Fujifilm Imaging Colorants Limited
Priority date: 2004-02-17
Filing date: 2005-02-14
Publication date: 2005-09-01
Also published as: GB0403424D0; TW200540231A

Abstract

An ink composition comprising: a) from 0.1 to 20 parts colloidal polymer; b) from 0.1 to 20 parts colorant; c) from 25 to 80 parts glycerol; d) from 0 to 40 parts of an organic solvent other than glycerol; and e) from 20 to 45 parts water; wherein all parts are by weight and the ink composition has a viscosity of less than 30 mPa.s at 25 °C. The ink composition is especially suitable for use as an ink jet printing ink and can be printed with acceptable printer operability and latency characteristics to provide prints having good wet-fastness and dry rub-fastness. Where the colorant is a pigment, good pigment dispersion stability is also provided.

Description

INK JET INK COMPOSITION COMPRISING GLYCEROL

The present invention relates to ink compositions and especially to ink compositions for use in ink jet printing ("UP"). UP is a non-impact printing technique in which droplets of ink are ejected through a fine nozzle onto a substrate without bringing the nozzle into contact with the substrate. There are many demanding performance requirements for UP and inks used in UP. For example, they desirably provide sharp, non-feathered images having good water-fastness, dry rub-fastness, light-fastness, ozone-fastness and optical density. The inks are often required to dry quickly when applied to a substrate to prevent smudging, but they should not form a crust over the tip of an ink jet nozzle because this will stop the printer from working. The inks should also be stable for storage over time without decomposing or forming a precipitate which could block the fine nozzle. In the case of P inks containing dyes the wet-fastness of the final printed image is often poor. In the case of UP inks containing pigments, the dry rub-fastness of the printed image is often poor. Soluble polymers have been incorporated into ink jet ink formulations in an attempt to address these deficiencies. WO 97/49774 and WO

97/47697 describe the use of such soluble polymers. UP inks containing water-soluble polymers often result in a slight deterioration in printer operability and any improvement in wet-fastness or dry rub fastness is usually small. Ink jet printer operability is the ability to print reliably and to print an image of good print quality over long print runs. Problems in operability can result, for example, in feint lines or even missing lines. Any problems in printer operability are often even more pronounced when the ink jet printer is restarted after a period of rest. The ability to restart printing effectively is often referred to as "latency". Ink jet inks comprising a colloidal polymer are known, however the presence of the colloidal polymer in the known inks tends to result in a pronounced deterioration of printer operability and especially latency. EP 0,412,548, US 5,207,324 and EP 0,882,771 A2 describe P inks containing colloidal polymers. Thus there is a need to provide ink compositions with good wet-fastness and dry rub-fastness with acceptable printer operability and latency characteristics. According to the first aspect of the present invention there is provided an ink composition comprising: a) from 0.1 to 20 parts colloidal polymer; b) from 0.1 to 20 parts colorant; c) from 25 to 80 parts glycerol; d) from 0 to 40 parts of an organic solvent other than glycerol; and e) from 20 to 45 parts water; wherein all parts are by weight and the ink composition has a viscosity of less than 30 mPa.s at 25°C. Preferably the sum of all the parts a) to e) equals 100. The colloidal polymer must be in a colloidal state in the ink compositions of the present invention. Preferably, the colloidal polymer has a Z-average particle size of not more than

1 μm, more preferably from 10nm to 500nm and especially from 50nm to 350nm. The particle size can be measured by any means known in the art though a preferred method is by photon correlation spectrospcopy (PCS). PCS devices are available from companies such as Malvern and Coulter. The colloidal polymer can be made by any means known in the art, preferably by dispersion of a polymer in water or more preferably by emulsion polymerisation in water. The polymer may be natural or synthetic. Preferred examples of natural polymers include cellulose, alginate, protein (especially casein and whey) and fat (long chain gycerides) polymers. Preferred examples of synthetic polymers are polyester and polyurethane, more preferably vinyl polymers. Preferred vinyl polymers are (meth)acrylate polymers. A preferred (meth)acrylate polymer is Neocryl XK-52 supplied by Avecia BV. The polymer may be partially cross-linked, but is preferably not cross-linked as this results in printed images exhibiting better wet-fastness and dry rub-fastness. In the case where the polymer is not cross-linked the number average molecular weight of the polymer is preferably from 5,000 to 1 ,000,000, more preferably from 10,000 to 500,000 and especially from 50,000 to 500,000. Preferably the molecular weight is measured by gel permeation chromatography. In the case where the polymer is cross-linked it is preferred that not more than 70% of the polymer by weight is cross-linked. A preferred method for measuring the amount of cross-linking is soxhlet extraction. Cross-linking more than this amount tends to impair the ability of the polymer to bind the colorant to the substrate and to improve wet-fastness and dry rub-fastness. The polymer may be branched but is preferably linear. Inks of the present invention advantageously facilitate the use of relatively large amounts of colloidal polymer whilst minimising the reduction in printer operability and latency. The colloidal polymer is preferably added in to the ink in the form of a liquid colloid and especially an aqueous colloid. The amount of colloidal polymer in the ink is based on the weight of the polymer only in the ink, i.e. not the weight of the colloid (including the liquid) added to the ink. The amount of colloidal polymer is preferably from 1 to 15 parts and more preferably from 2 to 10 parts by weight. The amount of colloidal polymer is of course calculated on a solids basis, thus for example 10 parts of a 10% w/w aqueous colloidal polymer would constitute 9 parts of water and 1 part of colloidal polymer. As colorants there may be mentioned pigments, dyes and mixtures thereof. Preferred dyes include azo, diazo, xanthene, anthraquinone, triaryl methane, azine, thiazino, phthalocyanine and nigrosine types. Dyes may be water-soluble, or disperse (i.e. solvent soluble) examples of which may be selected from The Colour Index especially the 3^rd edition, 1982, pages 147-263. Dyes can be metalised or non-metalised. Mixtures of dyes can be used. The colorant is preferably a pigment. The pigment may be organic or inorganic. Preferred organic pigments include phthalocyanines, anthraquinones, perylenes, carbazole, monoazo and disazobenzimidazolones, isoindolinones, monoazonaphthols, diarylidepyrazolones, rhodamines, indigoids, quinacridones, diazopyranthrones, dinitranilines, pyrazolones, dianisidines, pyranthrones, tetrachloroisoindolinones, dioxazines, monoazo acrylides, anthrapyrimides and mixtures thereof. Preferred inorganic pigments include carbon black, titania, iron oxide, zinc oxide, silicon dioxide and mixtures thereof. Examples of carbon black pigments include Regal 400R, Mogul™ L, Elftex™ 320, Cabojet™ 200 and 300 from Cabot Co., or Carbon Black FW18, Special Black 250, Special Black 350, Special Black 550, Printex™ 25, Printex™ 35, Printex™ 55 and Printex™ 150T from Degussa Co., CW1 and CW2 from Orient and Pigment Black 7. Further examples of pigments include CI. Pigment Yellow 17, CI. Pigment Blue

27. C.I. Pigment Red 49:2, CI. Pigment Red 81 :1 , CI. Pigment Red 81 :3, CI. Pigment Red 81 :x, CI. Pigment Yellow 83, CI. Pigment Red 57:1 , CI. Pigment Red 49:1 , CI. Pigment Violet 23, CI. Pigment Green 7, CI. Pigment Blue 61 , CI. Pigment Red 48:1 , CI. Pigment Red 52:1 , C.I. Pigment Violet 1 , CI. Pigment White 6, CI. Pigment Blue 15, CI. Pigment Yellow 12, CI. Pigment Blue 56, C.I. Pigment Orange 5, CI. Pigment Black 7, CI. Pigment Yellow 14, CI. Pigment Red 48:2, CI. Pigment Blue 15:3, CI. Pigment Yellow 1 , CI. Pigment Yellow 3, CI. Pigment Yellow 13, CI. Pigment Orange 16, CI. Pigment Yellow 55, CI. Pigment Red 41 , CI. Pigment Orange 34, CI. Pigment Blue 62, CI. Pigment Red 22, CI. Pigment Red 170, CI. Pigment Red 88, CI. Pigment Yellow 151 , CI. Pigment Red 184, CI. Pigment Blue 1 :2, CI. Pigment Red 3, CI. Pigment Blue 15:1 , CI. Pigment Blue 15:3, CI. Pigment Blue 15:4, CI. Pigment Red 23, CI. Pigment Red 112, CI. Pigment Yellow 126, CI. Pigment Red 169, CI. Pigment Orange 13, CI. Pigment Red 1-10, 12, CI. Pigment Blue 1 :X, CI. Pigment Yellow 42, CI. Pigment Red 101 , CI. Pigment Brown 6, CI. Pigment Brown 7, CI. Pigment Brown 7:X, CI. Pigment Black 11 , CI. Pigment Metal 1 , CI. Pigment Metal 2, CI. Pigment Yellow 128, CI. Pigment Yellow 93, CI. Pigment Yellow 74, CI. Pigment Yellow 138, CI. Pigment Yellow 139, CI. Pigment Yellow 154, CI. Pigment Yellow 185, CI. Pigment Yellow 180, CI. Pigment Red 122, CI. Pigment Red 184, and bridged aluminium phthalocyanine pigments. Further pigments are listed in The Colour Index and updates thereof, especially the 3^rd edition, 1982, pages 6-146. The especially preferred pigments are Pigment Yellow 128, 93, 17, 74, 138, 139, 154, 185, 180; Pigment Red 122, 57:1, 184; Pigment Blue 15:3, 15:4 and carbon black. Mixtures of pigments can be used. We have found that effectively stabilising a pigment dispersed in an ink comprising only 20 to 45 parts water and significant amounts (e.g. at least 20 parts) of organic solvent is particularly difficult. The presence of such large amounts of organic solvent and the small amounts of water tends, in many cases, to destabilise the pigment dispersion. This often results in agglomeration and/or flocculation of the pigment in the ink. Any agglomerated or flocculated pigment tends to give rise to ink jet printing problems such as operability and latency. We have surprisingly found that the inks of the present invention can comprise a pigment dispersion and said dispersion exhibits good stability. Without wishing to be bound by theory we believe that the presence of glycerol and water, at the amounts specified, is particularly important in maintaining the stability of the pigment dispersion. As mentioned hereinbefore printed images from ink compositions wherein the colorant is a pigment often exhibit poor rubfastness. The presence of the colloidal polymer provides good rub fastness to the printed ink composition. The pigment is preferably self-dispersing, for example surface treated carbon blacks carrying carboxy, sulphonic acid or ethyleneoxy groups. Self-dispersing pigments have further improved stability (against flocculation and aggregation) in the ink compositions of the present invention. Preferred self-dispersing carbon blacks are available from Cabojet under the tradename Cabojet™, preferred examples include Cabojet™ 200 and especially Cabojet™ 300. A preferred method for measuring the stability of the pigment dispersed in the ink composition is to measure the Z-average particle size before, during and after storage at a given temperature. The Z-average particle size may be measured by any suitable technique but it is preferably measured by means of a photon correlation spectroscopy device, especially a Malvern Zetasizer™. A preferred storage temperature is 60°C This temperature readily differentiates those inks wherein the pigment is unstable from those inks wherein the pigment is stable. At a storage temperature of 60°C inks comprising unstable pigment dispersions tend to show an increase in the Z-average particle size of greater than 10% within a week. In contrast, the inks of the present invention comprising pigment dispersions show less than 10% increase in the Z-average particle size even when stored for many weeks. Preferably, the Z-average particle size of the pigment dispersed in the ink composition is no more than 10% larger than the Z-average particle size of the pigment dispersed in the ink prior to storage after a period of 2 weeks, more preferably 8 weeks and especially 28 weeks. The pigment is preferably in the form of particles which are small enough to pass through the very fine nozzles used in the printheads of ink jet printers. Typically these nozzles are half the diameter of a human hair. Thus, the Z-average particle size of the pigment is preferably 0.003 to 15 μm, more preferably 0.004 to 5 μm and more preferably from 0.005 to 1 μm. Best colour characteristics and optical densities are generally obtained with pigments having a Z average particle size significantly smaller than 1 micron. In general, achieving satisfactory stability of the pigment dispersion is more difficult as the particle size of the pigment decreases. For ink compositions of the present invention we have found that the pigment dispersion remains stable even when the pigment particle size is very much less than a micron is size. Preferably, the pigment has a Z-averaged particle size of from 5nm to 1000nm, more preferably from 30nm to 500nm and especially from 50nm to 300nm. Pigment particle sizes outside these ranges may be used where printheads having particularly large nozzle diameters are used. Very fine dispersions of pigments and methods for their preparation are disclosed in, for example, EP 0 776 952, US 5,538,548, US 5,443,628, EP 0 259 130, US 5,285,064, EP 0 429 828 and EP 0 526 198. The ink optionally further contains a dispersant. When the colorant is a water- soluble or disperse dye a dispersant is often not necessary because the dye may be soluble in the formulation. Similarly, when the pigment is self-dispersible (e.g. the pigment carries dispersing groups or is surface modified to be self-dispersible) no further dispersant is needed. However, when the colorant is insoluble or partly soluble a dispersant is preferably included in the ink to assist storage stability. Suitable dispersants include, for example, polyester, polyurethane and polyacrylate dispersants, especially those in the form of high molecular weight block copolymer. Examples of dispersants include Disperbyk™ (ex BYK Chemie), Tego™ (ex Degussa) and Solsperse™ (ex Lubrizol) dispersants. A detailed list of non-polymeric as well as some polymeric dispersants appears in, for example, McCutcheon's Functional Materials, North American Edition, Manufacturing Confectioner Publishing Co., Glen Rock, N.J., pp.110-129 (1990), the entire disclosure of which is incorporated herein by reference. When the ink contains a dispersant this is preferably present in an amount of 50 to 150% by weight relative to the weight of colorant. The pigment and dispersant are preferably mixed together and then the resultant mixture is mixed with the remaining components of the ink. For example, a mixture of a pigment and a dispersant may be made by milling pigment in a small amount of water and/or glycerol with a milling media, e.g. glass beads. The pigment dispersion is then typically screened and "let down" by slowly adding the remaining ink components. The amount of colorant is preferably from 0.5 to 15 parts and more preferably from 1 to 10 parts by weight. Glycerol is also known by the names glycerine and 1 ,2,3-trihydroxy propane. The amount of glycerol is preferably from 25 to 75 parts, more preferably from 30 to 70 parts, especially from 30 to 60 parts and . more especially from 35 to 60 parts by weight. These preferred ranges in the amounts of glycerol further minimise operability problems which would otherwise result from the presence of the colloidal polymer. In the case where the sum of all the parts a) to e) equals 100 the amount of glycerol can be no more than 79.8 parts. Preferably, in this case, the amount of glycerol is from 25 to 74.8 parts, more preferably from 30 to 69.8 parts, especially from 30 to 59.8 parts and more especially from 35 to 59.8 parts by weight. Preferably the organic solvent other than glycerol is water-miscible. Preferred organic solvent other than glycerol which are water-miscible include

Ci-₆-alkanols (preferably methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, n-pentanol, cyclopentanol and cyclohexanol), linear amides (preferably dimethylformamide or dimethylacetamide), ketones and ketone-alcohols (preferably acetone, methyl ether ketone, cyclohexanone and diacetone alcohol), triols (other than glycerol such as 1 ,2,6-hexanetriol), mono-C_M-alkyl ethers of diols (preferably mono-C^- alkyl ethers of diols having 2 to 12 carbon atoms, especially 2-methoxyethanol, 2-(2- methoxyethoxy)ethanol, 2-(2-ethoxyethoxy)-ethanol, 2-[2-(2-methoxyethoxy) ethoxyjethanol, 2-[2-(2-ethoxyethoxy)-ethoxy]-ethanol and ethyleneglycol monoallylether), ethers, (preferably tetrahydrofuran and dioxane), cyclic esters (preferably caprolactone), sulphoxides (preferably dimethyl sulphoxide) and sulpholane. More preferably the organic solvent other than glycerol is a glycol, especially an optionally substituted pyrrolidinone and more especially a mixture thereof. The amount of organic solvent other than glycerol is preferably from 0.1 to 30 parts, more preferably from 1 to 20 parts and especially from 2 to 10 parts by weight. The optionally substituted pyrrolidinone may be unsubstituted or substituted at one or more positions, although it is preferably unsubstituted or carries only one substituent. The substituent is preferably an amino, ester, amide, carboxylic acid, thiol, hydroxy, halide (especially F.CI.Br or I) group, or of the formula -RX wherein R is C.₆ alkylene and X is H, amino, ester, amide, carboxylic acid, thiol, hydroxy or halide (especially F, CI, Br or I) group. Preferred substituted pyrrolidinones are N-substituted pyrrolidinones. Preferred examples of optionally substituted pyrrolidinones include N-cyclohexyl pyrrolidinone, N-(2-hydroxy ethyl) pyrrolidinone, especially N-methyl pyrroldinone and 2- pyrrolidinone and mixtures thereof. More than one type of optionally substituted pyrrolidinone may be present in the ink. It is preferred that some optionally substituted pyrrolidinone is included in the inks of the present invention as this results in further improvements in latency and printer operability. Preferred glycols have from 2 to 12 carbon atoms, for example propylene glycol, butylene glycol, pentylene glycol, hexylene glycol and thiodiglycol and oligo- and poly- alkyleneglycols, more preferably diethylene glycol, triethylene glycol, polyethylene glycol and polypropylene glycol and especially ethylene glycol. The amount of water is preferably from 30 to 45, more preferably from 35 to 45 parts by weight. The number of parts of water include any water added as part of another component. Whilst not wishing to be limited by theory we believed that an important feature of the present invention is the requirement for the specific amounts of both water and glycerol which we have found facilitate the use of a colloidal polymer whilst maintaining acceptable printer operability and latency. A preferred ink composition of the present invention comprises: a) from 2 to 10 parts colloidal polymer; b) from 1 to 10 parts colorant; c) from 25 to 80, more preferably from 30 to 70 parts glycerol; d) from 0.1 to 30, more preferably from 1 to 20 parts of an organic solvent other than glycerol; and e) from 20 to 45, more preferably from 30 to 45 parts water; wherein all parts are by weight and the ink composition has a viscosity of less than

30mPa.s at 25°C Preferably the sum of all the parts a) to e) equals 100. In the case where the sum of all the parts a) to e) equals 100 the amount of glycerol can be no more than 76.9. Preferably, in this case, the amount of glycerol is from 30 to 66.9 parts. The inks of the present invention may of course further comprise components in addition to a) to e) for example additives suitable for or known in UP inks. Such additives include one or more selected from wetting additives, surfactants, antifoaming agents, antikogation aids, biocides, rheology modifiers, levelling agents, pH buffers, chelating agents and the like. Preferably the ink composition contains less than 500ppm, more preferably less than 250ppm, especially less than 100pm, more especially less than 10ppm in total of divalent and trivalent metal ions. Preferably the ink composition has been filtered through a filter having a mean pore size below 10μm, more preferably below 3μm, especially below 2μm, more especially below 1μm. This filtration removes particulate matter that could otherwise block the fine nozzles found in many ink-jet printers. The viscosity of the ink composition is preferably less than 25 mPa.s and more preferably less than 20 mPa.s at 25°C The ink viscosity is preferably at least 1 mPa.s. The surface tension of the ink is preferably in the range 20 to 65 dynes/cm, more preferably in the range 25 to 60 dynes /cm. We have found that printed images obtained from ink compositions, according to the present invention, have good wet-fastness and/ or dry rub-fastness. This is achieved with minimal reduction in printer operability and latency. According to a second aspect of the present invention there is provided a process for printing an image on a substrate comprising applying an ink composition according to the first aspect of the present invention to the substrate, preferably by means of an ink jet printer. According to a third aspect of the present invention there is provided a paper, a plastics film or a textile material printed with an ink composition according to the first aspect of the present invention by means of a process according the second aspect of the present invention. Preferred papers are those typically used in ink jet printing including for example Photo Paper Pro (PR101 ), Photo Paper Plus (PP101 ), Glossy Photo Paper (GP401 ), Semi Gloss Paper (SG101), Matte Photo Paper (MP101), (all available from Canon); Premium Glossy Photo Paper, Premium Semi gloss Photo Paper, ColorLife™, Photo Paper, Photo Quality Glossy Paper, Double-sided Matte Paper, Matte Paper Heavyweight, Photo Quality Inkjet Paper, Bright White Inkjet Paper, Premium Plain Paper, (all available from Seiko Epson Corp); HP All-ln-One Printing Paper, HP Everyday Inkjet Paper, HP Everyday Photo Paper Semi-glossy, HP Office Paper, HP Photo Paper, HP Premium High-Gloss Film, HP Premium Paper, HP Premium Photo Paper, HP Premium Plus Photo Paper, HP Printing Paper, HP Superior Inkjet Paper, (all available from Hewlett Packard Inc.); Everyday Glossy Photo Paper, Premium Glossy Photo Paper, (both available from Lexmark Inc.); Matte Paper, Ultima Picture Paper, Premium Picture Paper, Picture Paper, Everyday Picture Paper (available from Kodak Inc.). Preferably the paper is selected from those suitable for lithographic, gravure and flexographic printing. Examples of such papers include Citation ™ or Focus™ grades sold by Mead Westvaco. We have found that the inks of the present invention can be printed onto substrates suitable for lithograthic, gravure and flexographic printing to give good wet fastness, dry rub-fastness, good optical density and image quality. This is especially surprising given that these substrates do not have an ink jet ink receptive layer and were not designed to be printed on with an ink jet ink. It is preferred that the papers have no ink jet ink receptive layer on their surface. Examples of papers having no ink jet ink receptive coating include newsprint (for example Norm C™ and Brite C™ manufactured by UPM) or book paper. We have found that the inks of the present invention can be printed onto newsprint and exhibit a surprisingly reduced tendency to strike through or penetrate the substrate. This is even so when the newsprint has a light weight, for example less than 60g per m². Preferred plastic films are transparent polymeric films, especially those suitable for use as overhead projector slides, for example polyesters (especially polyethylene terephthalate), polycarbonates, polyimides, polystyrenes, polyether sulphones, cellulose diacetate and cellulose triacetate films. According to a fourth aspect of the present invention there is provided an ink jet printer cartridge comprising a chamber and an ink composition wherein the ink composition is present in the chamber and the ink composition is as defined in the first aspect of the present invention. An ink jet printing head may be integral with said printer cartridge or it may be separate from the printer cartridge and provided with means such that the ink may be delivered to the ink jet printing head from the printer cartridge. We have found that the ink compositions according to the first aspect of the present invention can be fixed onto the substrate with fixing agents. The fixing is seen to be especially effective with the ink compositions of the present invention and especially notable improvements in strikethrough, edge acuity and wet-fastness are achievable. Preferably the fixing agent is a metal salt and more preferably a cationic compound. Preferred cationic compounds are quaternary ammonium compounds, polyamines, polyimines, guanidines and biguanidines including salts thereof. The salt may be in the form of a halide (CI, I, Br or F), nitrate, sulphate, hydroxide, carbonate or bicarbonate. The cationic compound is preferably polymeric. Preferred polymeric cationic compounds include the polyethyleneimine, polydiallyldimethylammonium, polyhexamethylene biguanidine and polyhexamethylene guanidine and salts thereof. The fixing agent may be a single fixing agent of a mixture of fixing agents. The fixing agent may be incorporated into the substrate but is more preferably applied to the substrate by means of a liquid composition comprising the fixing agent. According to a fifth aspect of the present invention there is provided a set of compositions comprising an ink composition according to the first aspect of the present invention and a separate liquid composition comprising a fixing agent. The liquid composition comprising the fixing agent may be applied to the substrate by any suitable means such as spraying, coating or dipping but the liquid composition comprising the fixing agent is preferably applied by means of an ink jet printer. The application of the liquid composition comprising the fixing agent may be before, during or after the application of the ink according to the first aspect of the present invention. A preferred process for applying the ink compositions according to the first aspect of the present invention and the liquid composition comprising the fixing agent is by the process as described in WO 02/45971 which is incorporated herein by reference thereto. A preferred process comprises the steps (a) and (b) in any order or simultaneously: (a) applying an ink composition according to the first aspect of the present invention to a substrate by means of an ink-jet printer; and (b) applying a liquid composition comprising a fixing agent to the substrate by means of an ink-jet printer. The invention is further illustrated by the following examples in which all parts and percentages are by weight unless specified otherwise.

Examples

Preparation of Ink compositions Pigment based inks Ink compositions 1 to 9 were prepared by mixing the components shown in Table 1. The numbers in Table 1 represent the amounts in parts by weight. Components e), d) and c) were charged to a vessel and stirred at a temperature of 25°O The wetting additive, rheology modifier and dispersant were added to the vessel and stirred until they had dissolved. The pH of the resultant mixture was adjusted to approximately 7 with amino methyl propanol. Component b) was added to a separate vessel. The mixture prepared above was then slowly added to component b) whilst stirring. The resultant coloured mixture was stirred for a further 30minutes. Component a) was added to a further vessel and then neutralised to a pH of approximately 7 with amino methyl propanol. The coloured mixture prepared above was then slowly added to the vessel whilst stirring. The resultant ink was stirred for a further 30mins to ensure complete mixing.

Table 1

Acryjet™ are pigment dispersions in water supplied by Rohm & Haas having a solids content of approximately 17% pigment. Neocryl™ XK-52 is a colloidal polymer in water supplied by Avecia BV having a solids content of approximately 40% polymer. Tego Wet™ 510 is a silicone free wetting additive supplied by Tego Chemie having a solids content of 100%. Tego Dispers™ 651 is a dispersant additive supplied by Tego Chemie having a solids content of 100%. PEG 10,000 is a polyethylene glycol rheology modifier having a molecular weight of 10,000 and a solids content of 100%. Cabot™ IJX 157 is a black pigment dispersion in water supplied by Cabot having a solids content of approximately 15% pigment. Cabojet™ 300 is a self-dispersing carbon black pigment dispersion in water having a solids content of 15% by weight. The amounts of materials in Table 1 are indicated as supplied, thus for example the amount of Neocryl™ XK-52 in Ink 1 is 40/100x12.56 i.e. 5.024 parts of active polymer. Accounting for the water content of the various components shown in Table 1 the total water content (number of parts) in each ink composition was as tabulated in Table 2.

Table 2

TWC - Total water content.

Ink compositions 1 to 4 had a viscosity of 10 mPa.s at 25°C and ink compositions 5 to 9 had a viscosity of 10 mPa.s at 35°O

Dye based inks A series of inks 10 to 12 were prepared in the same manner as the pigment based inks 2 to 4 except that the dyes were used in place of pigments. For ink 10 Projet™ Yellow 1G (ex Avecia) was used in place of the Acryjet™ Yellow 747 in ink 2. For ink 11 Projet™ Fast Magenta 2 (ex Avecia) was used in place of Acryjet™ Magenta 127. For ink 12 Projet Cyan 1 (ex Avecia) was used in place of Acryjet TM Cyan 157.

Preparation of Comparative Ink (CI) compositions Comparative Ink compositions, CI 1 to 9, were prepared by mixing the components shown in Table 3 using the same method as that described in Inks 1 to 9 except that the only optional additive present was the wetting additive (Tego Wet 510). The numbers in Table 3 represent the amounts in parts by weight. Table 3

Accounting for the water content of the various components shown in Table 3 the total water content (number of parts) in each ink composition was as tabulated in Table 4. The amounts indicated in Table 3 are as supplied, thus for example the amount of Acryjet Cyan 157 in comparative example CM is 17/100x17.6 i.e. 2.992 parts of solid pigment.

Table 4

Testing of Inks The stability of the pigment dispersion in the inks was assessed by measuring any trend in pigment particle size. The initial Z-average particle size of each ink was measured using a Malvern Zetasizer 3000HS. The Z-average particle size was remeasured each week after storage at a temperature of 60°O The results were as tabulated below. Table 5 shows the results for the inks of the present invention, whilst table 6 shows the results for the comparative inks. Table 5

All inks showed excellent pigment stability at 60°O No ink demonstrated an increase in particle size of greater than 10% after 1 week of storage at a temperature of 60°C.

Table 6

All the comparative inks showed much inferior pigment stability. All the comparative inks demonstrated an increase in particle size of greater than 10% after 1 week of storage at a temperature of 60°O Comparative examples CM to CI8 show that omitting the glycerol and replacing it with propylene glycol/water resulted in much inferior ink stability. To put it another way glycerol offers an advantage in pigment stability not exhibited by similar organic solvents such as propylene glycol. Comparative example CI9 showed that where too little water is added (outside the scope of the present invention) the pigment dispersion stability is much reduced. Wet and Dry rub-fastness results Inks 2 to 4 and 10 to 12 were printed onto newsprint of different types using a Spectra SE industrial piezo printhead attached to a Spectra Apollo P nthead Support Kit. Each ink was printed to give an image in the form of a rectangular block. The printed image was allowed to dry for 24hrs. The printed image was then tested for fastness by attempting to smear the printed ink with a thumb from the block printed area into an unprinted area of the newsprint. This was done with a dry thumb (dry rub-fastness) and a wet thumb (wet rub-fastness). The amount of ink transferred to the unprinted area was visually assessed and qualified using the scale: Excellent = no ink transferred, Very good = barely perceivable ink transferred, Good = Very slight ink transferred, Moderate = Noticeable ink transferred, Poor = extensive ink transferred. The results are tabulated in Table 7.

Table 7

As can be seen in Table 7 printed inks 2 to 4 and 10 to 12 demonstrated good to excellent wet rub-fastness. Printed inks 2 to 4 and 10 to 12 (i.e. both pigment and dye based) also had excellent dry rub-fastness for all the above newsprint substrates.

The excellent dry and wet-rubfastness is especially surprising given that newsprint has no ink jet ink receptive coating and is a lightweight paper (i.e. less than 60g/m²).

Operability Two aspects of operability were tested, these were continuous printing (jet sustainability) and printing after a period of rest (latency). Jet Sustainability Ink 9 was printed using a Spectra SX printhead controlled by an Apollo printhead support kit. The printhead was primed and purged with Ink 9 to ensure the printhead was jetting well. Ink 9 was then printed continuously for 30 minutes at a jetting frequency of 5kHz and then 20kHz. During this time the number of nozzles in the printhead which stopped firing, or which fired improperly were recorded. The results are tabulated in Table 8.

Table 8

Table 8 shows that the even at high jetting frequencies the jet sustainability of the inks of the present invention were very good.

Latency After the above jet sustainability tests were completed the printer was switched off for different periods of time as indicated in columns 2 to 4 of Table 9. The printer was then restarted at a jetting frequency of 5kHz and the number of nozzles which had stopped firing or which fired improperly was again recorded. The results are tabulated in Table 9. Table 9

As can be seen in Table 9 the inks of the present invention showed especially good latency characteristics.

Claims

1. An ink composition comprising: a) from 0.1 to 20 parts colloidal polymer; b) from 0.1 to 20 parts colorant; c) from 25 to 80 parts glycerol; d) from 0 to 40 parts of an organic solvent other than glycerol; and e) from 20 to 45 parts water; wherein all parts are by weight and the ink composition has a viscosity of less than 30 mPa.s at 25°O

2. An ink composition according to claim 1 wherein the organic solvent other than glycerol is a glycol, an optionally substituted pyrrolidinone or a mixture thereof.

3. An ink composition according to any one of the preceding claims wherein the organic solvent other than glycerol is 2-pyrrolidinone or N-methyl pyrrolidinone or mixtures thereof.

4. An ink composition according to any one of the preceding claims further comprising one or more components selected from wetting additives, surfactants, antifoaming agents, antikogation aids, biocides, rheology modifiers, levelling agents, pH buffers and chelating agents.

5. An ink composition according to any one of the preceding claims wherein the colloidal polymer is a vinyl polymer.

6. An ink composition according to claim 5 wherein the colloidal polymer is a (meth)acrylate polymer.

7. An ink composition according to any one of the preceding claims wherein not more than 70% of the colloidal polymer is cross-linked.

8. An ink composition according to any one of the preceding claims wherein the colloidal polymer is not cross-linked.

9. An ink composition according to any one of the preceding claims wherein the amount of colloidal polymer is from 1 to 15 parts by weight.

10. An ink composition according to any one of the preceding claims wherein the amount of glycerol is from 30 to 70 parts by weight.

11. An ink composition according to any one of the preceding claims wherein the amount of organic solvent other than glycerol is from 1 to 20 parts by weight.

12. An ink composition according to any one of the preceding claims wherein the amount of water is from 30 to 45 parts by weight.

13. An ink composition according to any one of the preceding claims which additionally comprises a dispersant.

14. An ink composition comprising: a) from 2 to 10 parts colloidal polymer; b) from 1 to 10 parts colorant; c) from 25 to 80 parts glycerol; d) from 0.1 to 30 parts of an organic solvent other than glycerol; and e) from 20 to 45 parts water; wherein all parts are by weight and the ink has a viscosity of less than 30 mPa.s at 25°C

15. An ink composition according to claim 14 wherein the organic solvent other than glycerol is a glycol, an optionally substituted pyrrolidinone or mixture thereof.

16. An ink composition according to claim 14 wherein the organic solvent other than glycerol is 2-pyrrolidinone or N-methyl pyrrolidinone or a mixture thereof.

17. A set of compositions comprising an ink composition according to any one of the preceding claims and a separate liquid composition comprising a fixing agent.

18. A process for printing an image on a substrate comprising applying an ink composition according to any one claims 1 to 16 to the substrate.

19. A process according to claim 18 wherein the printing is achieved by means of an ink jet printer.

20. A process according to claim 18 or claim 19 additionally comprising the step of applying a liquid composition comprising a fixing agent to the substrate wherein the application of the liquid composition comprising the fixing agent is before, during or after the application of the ink composition.

21. A process according to claim 20 wherein the liquid composition comprising the fixing agent is applied to the substrate by means of an ink jet printer.

22. A paper, a plastic film or a textile material printed with an ink composition according to any one of claims 1 to 16 by means of a process according to claim 18 or claim 19.

23. An ink jet printer cartridge comprising a chamber and an ink composition wherein the ink composition is present in the chamber and the ink composition is according to any one of claims 1 to 16.