GB2407323A - Aqueous ink set for ink jet printers - Google Patents

Abstract

An aqueous ink set for ink jet printing comprising: <SL> <LI>1) a first ink comprising at least one pigment dispersed in an aqueous carrier medium; and <LI>2) a second ink comprising at least one colorant, boric acid or salt thereof, and an aqueous carrier medium; </SL> wherein intercolour bleed between the first ink and the second ink is reduced when the second ink is printed adjacent to, on top of, or underneath the first ink on the substrate. The inks may further comprise a dispersant and water soluble organic cosolvent. Also shown is an ink-jet printing process using the aqueous ink set.

Description

Aqueous Ink Set for Ink Jet Printers

Field of the Invention

This invention relates to an aqueous ink set for ink jet printers. In particular it relates to an ink set exhibiting improved bleed performance.

Backeround of the Invention The ink jet process is a non-contact printing process in which droplets of a recording fluid, the ink, are formed by forcing the fluid through a tiny nozzle (or a series of nozzles) in response to digital signals and deposited on a recording material to be printed such as paper or transparent film. This method is important because it permits high speed recording with limited noise or other disadvantages. In recent years, ink jet printers have found broad application as output for personal computers in the office and the home in applications such as desktop document production and pictorial imaging. There are several classes of ink jet printers, for example thermal and piezo drop on demand printers and continuous ink jet printers. In a thermal printer, the droplets are ejected on demand from tiny ink reservoirs by heating the ink to form bubbles which expand to expel the ink as the print head scans the recording material. In a piew drop on demand printer the droplets are expelled by flexure of a piezoelectric element controlled by the computer. In the continuous printing method, a continuous stream of electrostatically charged droplets may be produced; the trajectories of the droplets may be controlled by means of electrically controlled deflection plates to hit the desired spot on the paper, and unused droplets may be collected into a reservoir for recycling. Drop-on-demand systems do not require ink recovery, charging, or deflection, and hence are much simpler than the continuous type and more common in home and office use.

Ink jet printers for the home or office generally utilise aqueous inks. Typically, for most uses a set of four inks is provided comprising the three subtractive primary colours cyan, magenta, and yellow, together with a black ink (commonly referred to as CMYK). It is also known to use ink sets comprising additional inks, for example additional dilute ("light") inks such as dilute cyan and magenta inks or inks of additional colours such red, blue, orange, violet, or green inks. The colorants may be dyes or pigments.

One of the limiting factors of the ink jet system is print quality on socalled plain, or uncoated paper. One problem which can arise when a multicoloured element is desired in which a printing liquid of one colour is placed in abutting relationship to a printing liquid of another colour is the tendency of inks to "bleed". As used herein, the terms bleed or bleeding refer to intercolour bleed caused by ink mobility at the boundary between two areas printed in different colours. Bleeding may cause loss of resolution due to an increase or decrease in the width of thin lines of one colour printed against a background of another colour, loss of edge acuity because the line of demarcation between the two colours is blurred, or loss of colour purity and formation of undesired coloured fringes at a boundary between two adjacent coloured areas, such as when blue and yellow mix to produce green. The more contrasting the two adjacent liquids are in colour (such as black and yellow), the greater the visual effect of the bleed especially if the black ink bleeds into the yellow ink.

Bleed is a particular problem in ink jet printing because ink jet printers have the capability of printing two or more coloured inks in simultaneous (or near simultaneous) fashion and because the relatively low viscosity inks used therein tend to spread and mix readily. Bleed is more prevalent if there is a difference in surface tension of the inks involved. Bleeding performance has become more of a problem as ink jet printers have developed and become faster, and also as more ink is laid down for example when dense full colour images are formed.

Thus interest in increasing resistance to bleed has increased. It is possible to improve image quality and resistance to bleed by printing on to special ink receiving materials comprising a substrate having one or more ink receiving layers coated thereon, but these materials are more expensive and less readily available than plain paper, and thus there is interest in methods for improving bleed performance on plain paper.

Several methods of improving bleeding performance, including performance on plain paper, are known. The simplest is to delay application of the second printing liquid until the first printing liquid is completely dry. However this method is inefficient because of the increase of time necessary to generate multi-coloured prints, and neither is it particularly effective.

Another method is to provide the essentials of an ink receiving layer in the form of an additional (normally colourless) ink laid down immediately before printing the coloured inks rather than as part of a coated material. The additional ink acts to precipitate or destabilise the colorant or other components in at least one of the coloured inks and hence prevent mixing of the inks. For example, United States Patent 6,547, 382 discloses an ink set capable of achieving improved image quality on commercial plain paper comprising an ink and a performance improving liquid. However this type of system is complicated and not capable of general use because it requires additional hardware and software.

Thus there has developed an interest in providing combinations of two or more inks which will mutually precipitate or otherwise destabilise at any boundaries and thus provide a barrier to mixing and bleeding of the inks. In particular, it is known that use of inks having different pH conditions can precipitate components in one or both inks and thus improve bleed performance. For instance, United States Patent 5,181,045 describes certain dyes which become insoluble under specific and well- defined pH conditions. The dye is forced out of solution from the ink by contact with another ink having the appropriate pH (either higher or lower than that of the first ink). By forcing a dye to become insoluble on the page, migration of the dye is inhibited, thereby helping to reduce bleed between inks of different colours. However this method is limited to a specific group of dyes as colorants, and also inks having a low or acidic pH may tend to be undesirably corrosive towards components of the printing apparatus.

It is also known that use of cationic components in one ink and anionic components in another ink may improve bleed by mutual precipitation at boundaries between the two inks. For example, United States Patent 5,198, 023 provides an ink set in which bleed between yellow and black inks is reduced by using a cationic yellow dye in the yellow ink and an anionic dye in the black ink. Bleed is further reduced by adding a multivalent precipitating agent to the yellow ink.

Similarly, United States Patent 5,428,383 discloses a method for controlling colour bleed in multi-colour thermal ink jet printing systems wherein one of the ink compositions contains a precipitating agent such as a multivalent metal salt designed to react with the anionic colorant in another ink composition. As a result, when the two ink compositions come in contact on the printed image, a precipitate is formed from the colorant in the other ink composition which prevents migration thereof and colour bleed problems.

Also, United States Patent 5,518,534 discloses an ink set for alleviating bleed in multicolour printed elements employing a first ink and a second ink, each containing an aqueous carrier medium and a colorant, the colorant in the first ink being a pigment dispersion and the second ink containing a multivalent metal salt of an organic acid or mineral acid.

Further, United States Patent 5,570,594 describes a method whereby bleed between two adjacent inks on a printing medium may be reduced by selecting inks having the same ionic character, at least one ink containing an aqueous carrier medium and a pigment dispersion wherein the dispersant is a quaternised amine containing polymeric dispersant, and wherein the second ink contains an aqueous carrier medium, a colorant and optionally a polymer, wherein the colorant or the polymer has a sulphonate or phosphonate group. The cationically stabilised pigment in the first ink precipitates with the anionic polymer in the second ink.

United States Patent 6,086,198 provides dye and pigment based inks which become insoluble under specific and well-defined conditions and additives. By reacting a quaternised or neutralized amine dispersant in a pigmentcontaining ink with an anion in a second ink that quantitatively combines with the dispersant, the pigment dispersion is destabilized, resulting in inhibition of the migration of pigment on the print medium, thereby helping to reduce bleed between black ink and colour inks and between different colour inks.

United States Patent 6,383,277 describes an ink set for utilisation in ink jet printers to reduce bleeding at contact lines of the different colours during printing, comprising a first ink containing a watery ink medium, a first colour pigment dispersed in the ink medium, and optionally, a first dispersing medium, whereby the first colour pigment and/or the first dispersing medium carry ionic groups, a second ink containing a watery ink medium and a dye soluble in the ink medium, whereby the dye acts as precipitation means for the first colour pigment.

Further, United States Patent Application 2003/130374 A discloses a set of inks for printing multicolour images in an ink jet printer, said ink set comprising (1) a first ink having a first colour and comprising water, a first colorant, and at least one of (a) a cationic polymer, (b) a cationic surfactant, or (c) an inorganic salt the cation of which has a tetraphenylboride salt that is substantially insoluble in water; and (2) a second ink having a second colour different from the first colour and comprising water, a second colorant, and sodium tetraphenylboride.

However, there remain disadvantages with all these methods. For instance certain cationic components may present toxicity concerns, and inks comprising such components may also be incompatible with parts of the printing system. Thus there is still a need for an ink set to provide improved bleed performance without these disadvantages.

Summary of the Invention

According to this invention there is provided an aqueous ink set for ink jet printers comprising: 1) a first ink comprising at least one pigment dispersed in an aqueous carrier medium; and 2) a second ink comprising at least one colorant, boric acid or salt thereof, and an aqueous carrier medium; wherein intercolour bleed between the first ink and the second ink is reduced when the second ink is printed adjacent to, on top of, or underneath the first ink on the substrate.

The term pigment describes a colorant which is substantially insoluble in the aqueous ink medium. Many pigments are listed in the Pigments and Solvent Dyes section of the Colour Index International, published by the Society of Dyers and Colourists in 1997.

Suitable pigments for the first ink of the invention include those classified by the Colour Index as C.I. Pigment Black 1; C.I. Pigment Black 7; C.I. Pigment Yellow 1, 3, 12, 13, 14, 17, 24, 55, 74, 81, 83, 87, 95, 97, 98, 100, 101, 104, 108, 109, 110, 117, 120, 138, 151, 154, 155, 180, and 213; C.I. Pigment Orange 5, 13, 16, 17, 36, 43, 51, 71, and 73; C.I. Pigment Red 1, 2, 3, 9, 17, 22, 23, 31, 38, 48:1, 48:2, 48:3, 48:4, 49:1, 52:2, 53:1, 57:1, 60:1, 63:1, 63:2, 64:1, 81, 83, 88, 112, 114, 122, 123, 146, 149, 166, 168, 170, 175, 176, 178, 179, 184, 185, 188, 190, 202, 207, 209, and 222; C.I. Pigment Violet 1, 3, 5:1, 19, 23,35, and 37; C.I. Pigment Blue 1, 2, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 56, 60, 76, 79, and 80; and C.I. Pigment Green 1, 4, 7, 8, 10, and 36. However almost any suitable insoluble coloured compound may be used in the first ink of the invention. It is also possible to mix more than one pigment in the first ink of the invention.

Preferably the first ink of the invention is a black ink, and most preferably it is a black ink comprising carbon black as pigment. Carbon black is listed in the Colour Index as C.I. Pigment Black 7. Examples include gas black, furnace black, lamp black, acetylene black, and channel black.

The pigment concentration of the first ink depends on a number of factors including the printer to be used, the type of substrate onto which the ink will be printed, and the desired visual result. However in general the first ink may contain up to approximately 10% of pigment by weight, preferably between approximately 2% and approximately 5%.

Optionally the first ink may also comprise at least one dispersant for the pigment. The purpose of the dispersant is to stabilise the particles and prevent flocculation, aggregation, and settling of the ink. Suitable dispersants for pigmented ink jet inks are well known in the art, and include polymeric dispersants as well as some non-polymeric compounds of the surfactant type. Suitable dispersants for the inks of this invention include macromolecular polyionic dispersants, for example copolymers of styrene with acrylic, methacrylic, or maleic acids; various types of poly(ethylene oxide) condensates such as alkyl polyethylene oxide ethers and sulphate or phosphate esters thereof; and surfactants such as sarcosinate compounds. A preferred dispersant for the inks of the invention is a styrene/acrylate copolymer, by which is meant a copolymer comprising styrene or a substituted styrene; acrylic, methacrylic, or maleic acids; and optionally with additional vinylically unsaturated comonomers such as acrylate esters.

Preferably the dispersant is characterized by an acid number of between about 150 and about 250, a glass transition temperature between approximately 50 C and approximately 150 C, and a molecular weight between approximately 1500 and approximately 25000, and most preferably the molecular weight is approximately 1800 - 15000. Many suitable styrene acrylate dispersants are commercially available.

Such styrene acrylate copolymer dispersants may be supplied commercially in the form of concentrated solutions in aqueous base, or as solids which are to be dissolved in water in the presence of a base. Suitable bases for dissolving such dispersants include sodium or potassium hydroxide, ammonia, or an organic amine base such as ethanolamine, diethanolamine, triethanolamine, dimethylaminoethanol, 1-(dimethylamino)-2-propanol, or 2arnino-2- methyl-l-propanol. It is also possible to use mixtures of such bases to dissolve the styrene acrylate copolymer.

There may be present up to 400% of the dispersant by weight on the pigment, but preferably between about 10% and about 100% by weight on the pigment, and most preferably approximately 20 to 50% by weight on the pigment, depending on the pigment and dispersant used and other properties desired of the ink.

Further, instead of a dispersant for the pigment it is also possible to use processed pigments having a surface treatment of the particles such as so-called self dispersing carbon black and graft carbon black. By a self-dispersing carbon black is meant a carbon black pigment, the particles of which have been modified by surface oxidation or by incorporation of groups such as, for example, carboxylic or sulphonic acid groups at the surface such that the pigment may be stably dispersed in an aqueous formulation without a dispersant. Such processed carbon black pigments are known and commercially available from suppliers such as Cabot Corporation and Orient Corporation. It is also possible to use a combination of a self-dispersing pigment and a pigment dispersed with the aid of a dispersant in the first ink of the invention.

The colorant of the second ink is not particularly limited subject to being compatible with the boric acid or salt thereof component of the ink, and may be a dye, a pigment, or a mixture thereof. Preferably the colorant of the second ink is a water-soluble dye, and most preferably it is an anionic dye. Many suitable dyes for ink jet inks are known, and include anthraquinone dyes, monoazo dyes, disazo dyes, phthalocyanine dyes, and triphenodioxazine dyes. Specific examples of suitable dyes include Food dyes such as Food Black 1, Food Black 2, Food Red 40, Food Blue 1, Food Yellow 7, and the like; Acid dyes such as Acid Black 1, 7, 9, 24, 26, 48, 52, 58, 60, 61, 63, 92, 107, 109, 118, 119, 131, 140, 155, 156, 172, 194, and the like, Acid Red 1, 8, 32, 35, 37, 52, 57, 92, 115, 119, 154, 249, 254, 256, 289, and the like, Acid Blue 1, 7, 9, 25, 40, 45, 62, 78, 80, 92, 102, 104, 113, 117, 127, 158, 175, 183, 193, 209, 249,andthelike, AcidYellow3,7, 17, 19, 23, 25, 29, 38, 42, 49, 56, 59, 61, 72, 73, 114, 128, 151, and the like; Direct dyes such as Direct Black 4, 14, 17, 22, 27, 38, 51, 112, 117, 154, 168, and the like, Direct Blue 1, 6, 8, 14, 15, 25, 71, 76, 78, 80, 86, 87, 90, 106, 108, 123, 163, 165, 199, 226, and the like, Direct Red 1, 2, 16, 23, 24, 28, 39, 62, 72, 236, and the like, or Direct Yellow 4, 11, 12, 27, 28, 33, 34, 39, 50, 58, 86, 100, 106, 107, 118, 127, 132, 142, 157, and the like; certain Reactive dyes and pacified reactive dyes such as Reactive Black 31, Reactive Red 4, 23, 31, 56, 180, Reactive Yellow 37, and the like; various other dyes such as the ILFORD yellow dye Y-104 and the ILFORD magenta dye M-377; and mixtures comprising such dyes. It is preferred to use dyes such as are commercially available in high purity with low salt content suitable for ink jet use.

Preferably the second ink has a chromatic colour such as yellow, magenta, or cyan, and most preferably it is a yellow ink. Preferred yellow dyes for the second ink include Tartrazine (Acid Yellow 23), Direct Yellow 86, Direct Yellow 132, Reactive Yellow 37, ILFORD yellow Y-104, and mixtures thereof.

The colorant may be present in the second ink composition in any desired or effective amount depending on a number of factors including the colorant, the printer to be used, the type of substrate onto which the ink will be printed, and the desired visual result.

Typically, a dye will be present in the second ink composition from about 0.05 to about 15% by weight of the total ink composition, preferably from about 0.1 to about 10% by weight of the ink, and more preferably from about 1 to about 5% by weight of the ink, although the amount can be outside of these ranges.

The inks of the present invention comprise an aqueous carrier medium. By an aqueous carrier medium is meant a liquid solvent vehicle which is water or is predominantly water. Deionised water is commonly used. The liquid vehicle may consist solely of water, but it is common in aqueous ink jet inks to employ a carrier medium which is a mixture comprising water and at least one relatively involatile water miscible organic cosolvent.

The organic solvent component(s) generally serve as humectant to prevent drying out of the ink in the nozzles of the printer, but may also serve as cosolvent, viscosity regulating agent, ink penetration additive, wetting agent, levelling agent or drying agent.

The water miscible organic cosolvent may be any organic solvent which has sufficient solubility in water. Representative examples of organic solvents which may be selected include: (l) alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, neopentyl alcohol, lO 2-ethyl hexyl alcohol, benzyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; (2) ketones or ketoalcohols such as acetone, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, methyl pentyl ketone, cyclopentanone, cyclohexanone and diacetone alcohol; (3) ethers, such as tetrahydrofuran and dioxan; (4) esters, such as ethyl lactate; (5) polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, butylene glycol, glycerol, pentaerythritol, 1,3-propanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 1,2,4-butanetriol, 1,5- pentanediol, 2,4-pentanediol, 1,2,5-pentanetriol, 1,2-hexanediol, 1,6-hexanediol, 1,2, 6-hexanetriol, I,2-cyclohexanediol, 1,4-cyclohexanediol, 2-methyl-2,4pentanediol, 3-methyl- 1,5 pentanediol, 2-ethyl- 1,3 -hexanediol, 2,2,4trimethyl- 1,3 -pentanediol, I, I, I -tris (hydroxymethyl)-ethane, 1, 1,1-tris-(hydroxymethyl)-propane, and neopentyl glycol; (6) mono-or all-ethers derived from alkylene glycols, such as ethylene glycol monomethyl (or -ethyl, - propyl, or - butyl) ether, diethylene glycol mono-methyl (or -ethyl, - propyl, or - butyl) ether, triethylene glycol mono-methyl (or -ethyl, - propyl, or - butyl) ether, propylene glycol mono-methyl (or -ethyl) ether, dipropylene glycol mono-methyl (or -ethyl) ether, ethylene glycol monophenyl ether, ethylene glycol all- methyl (or -ethyl) ether, diethylene glycol dimethyl (or -ethyl) ether, propylene glycol dimethyl (or -ethyl) ether and dipropylene glycol dimethyl (or -ethyl) ether; (7) sulphur-containing compounds such as tetramethylene sulphone, dimethyl sulphoxide, bis-(2-hydroxyethyl) sulphone, and thiodiglycol; and (8) nitrogen containing organic compounds such as pyrrolidone, 1-methyl-2- pyrrolidone, 1-(2-hydroxyethyl)-2-pyrrolidinone, 1-cyclohexyl-2-pyrrolidone, dimethylformamide, dimethylacetamide, 1,3-dimethyl-2-imidazolidinone, urea, and tetramethylurea.

Selection of a suitable mixture of water and water miscible organic cosolvent depends on the requirements of the specific printer and application, such as the desired surface tension and viscosity of the inks, the drying time, and the type of substrate onto which the inks will be printed. A mixture of water and at least one water soluble organic solvent having at least 2 hydroxyl groups is preferred. Therefore preferred organic cosolvents include for example ethylene glycol, ethylene glycol condensates such as diethylene glycol, triethylene glycol, polyethylene glycol, and ethoxylated glycerol, propylene glycol, glycerol, and mixtures comprising these solvents. Preferred nitrogen containing cosolvents include pyrrolidone and 1-methyl-2-pyrrolidone. The cosolvent or cosolvents in the first ink and the second ink of the invention may be the same or different.

The aqueous ink compositions may contain up to 50% of the organic cosolvent or mixture of organic cosolvents. Preferably the inks of the inventive set comprise up to 10% of each organic cosolvent, and most preferably between approximately 1% and approximately 10% of each of a mixture of at least two organic cosolvents, although the amount can be outside these ranges.

The term boric acid includes metaboric acid, orthoboric acid, tetraboric acid, and pentaboric acid. Suitable salts for use in the second ink of the invention include in particular salts of alkali metals such as sodium, potassium, or lithium, and ammonium or substituted ammonium salts. Specific preferred salts include sodium tetraborate and sodium metaborate, of which the tetraborate as borax is especially preferred. The content of the boric acid or salt thereof in the ink may be up to approximately 5% by weight of the ink, preferably up to approximately 1% by weight, most preferably between approximately 0.1% and 0.5% by weight ofthe ink.

The first and second inks of the invention may also comprise other components which are advantageously added to aqueous ink jet inks, such as surfactants, viscosity modifiers, and biocides. In addition, sequestering agents such as EDTA may also be included to eliminate deleterious effects of heavy metal impurities.

The surface tension of the first and second inks may be adjusted to the desired value by appropriate addition of surfactant. Depending on the use, surface tension is typically in the range from approximately 20 dyne/cm to approximately 60 dyne/cm. In one especially preferred embodiment, the first ink comprises substantially no deliberate addition of surfactant and has surface tension between approximately 50 dyne/cm and approximately 60 dyne/cm, and the surface tension of the second ink is between approximately 25 dyne/cm and approximately 35 dyne/cm.

The surfactant for use in the second ink and, if desired, the first ink of the invention is not particularly limited. Suitable surfactants for ink jet use are well known; examples include nonionic surface active agents such as alkylene oxide derivatives, for example polyethylene glycol alkyl or alkylaryl ethers, polyethylene glycol esters, polyethylene glycol ethers of acetylene dials, or polyethylene glycol/ polypropylene glycol condensates; glycidol derivatives such as alkylphenol polyglycidols; aliphatic esters of polyhydric alcohols or sucrose; anionic surfactants such as dialkyl sulphosuccinates, N-acyl-N- alkyltaurines, N-acylsarcosinates, alkylsulphonates, alkylbenzenesulphonates, alkylnaphthalenesulphonates, and surfactants comprising a sulphuric acid ester group or a phosphoric acid ester group, such as alkyl sulphuric acid esters, alkyl phosphoric acid esters and sulphated or phosphated polyethylene glycol alkyl or alkylaryl ethers. Many suitable surfactants are well known and commercially available. A particularly suitable surfactant is sodium dioctyl sulphosuccinate.

The pH of the first ink may be controlled by the quantity of base added to the dispersant or by appropriate addition of acid or base to the final ink. The pH of the second ink may be controlled by appropriate selection of boric acid or salt thereof, or by appropriate addition of acid or base to the final ink. The pH of the inks is not particularly limited, but is generally between about 7 and about 10, preferably between approximately 8 and 9.

The viscosity of the first and second ink compositions of the present invention should be suitable for the ink jet printer used, typically no greater than approximately 20 cP. For a thermal drop-on-demand ink jet printer, ink viscosity may be in the range from approximately 1 cP to approximately 10 cP at room temperature, especially between approximately 2 cP and approximately 3 cP. For a piezo drop-on-demand printer the viscosity of the ink may be up to approximately 20 cP, especially in the range from approximately 3 cP to approximately 4 cP for a desk top printer. For a continuous ink jet printer viscosity of the ink is preferably in the range from approximately 1 cP to approximately 2 cP.

The first and second ink compositions can be prepared by any suitable process. Typically, a second ink comprising a dye as colorant may be prepared by simple mixing of the ingredients, heating if desired, with moderate shaking or stirring until an homogeneous mixture is obtained, and then filtering the mixture to obtain the ink.

The pigmented first ink and a pigmented second ink may be prepared by a process involving the steps of: (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 is diluted with a liquid vehicle and other addenda to a working strength ink. In the dispersing step, the pigment is usually dispersed in a liquid carrier medium which is either water or the same carrier medium as that in the finished ink, optionally in the presence of one or more dispersants. The dispersing step may be accomplished by many well known methods, for example by milling in a suitable grinding mill such as a horizontal mini mill, a ball mill, a roll mill, an attritor, or an airjet mill. Mechanical energy is supplied to this pigment dispersion, and the collisions between the milling media and the pigment cause the pigment to deaggregate into its primary particles.

Alternatively the pigment may be dispersed using a homogenizer or by passing the dispersing mixture through a plurality of nozzles within a liquid jet interaction chamber at a liquid pressure of at least lOOO psi (6 89 MPa) as described in United States Patent Specification 5,026,427 to produce a uniform dispersion of the pigment particles. The pigment may be used in the form of a dry powder. However pigments are

often supplied commercially as a concentrated aqueous pigment dispersion, and this invention is also useful for pigments supplied as such dispersions, which commonly include dispersants and other cosolvents as well as water. Many pigments are available commercially in finely divided forms which are specifically aimed at ink jet use, and it is preferred to use such available forms in the inks of this invention.

Alternatively the pigment may be supplied in the form of a water wet presscake. In presscake form, the pigment is not aggregated to the extent that it is in dry form and thus the pigment does not require as much deaggregation in the process of preparing the inks from dry pigments.

A suitable average particle size of the pigment in the ink is up to approximately 10 Em, preferably up to I Am, most preferably 200 rim or less, especially from 50 nm to 200 nm.

Optionally and preferably, the ink set of the invention may also comprise additional inks.

According to one particularly preferred embodiment of the invention, there is provided a four colour CMYK ink set comprising: 1) a first ink comprising carbon black pigment as colorant dispersed in an aqueous carrier medium; 2) a second ink comprising a yellow dye, boric acid or salt thereof, and an aqueous carrier medium, 3) a third ink comprising a magenta dye and an aqueous carrier medium; and 4) a fourth ink comprising a cyan dye and an aqueous carrier medium, wherein intercolour bleed between the first ink and the second ink is reduced when the second ink is printed adjacent to, on top of, or underneath the first ink on the substrate.

Optionally, there may also be included in the ink set at least one additional ink such as a dilute cyan, dilute magenta, dilute yellow, dilute black, red, blue, orange, violet, or green ink. The magenta and cyan inks, together with any optional additional inks may also comprise the boric acid or salt thereof component of the second ink of the inventive ink set to minimise bleed between the black and coloured inks. The other components of the inks of this aspect of the invention are as hereinbefore described. Preferred dyes for the cyan and optional dilute cyan inks include Acid Blue 9, Direct Blue 86, and Direct Blue 199, of which Direct Blue 199 is especially preferred. Preferred dyes for the magenta and optional dilute magenta inks include Acid Red 52, Acid Red 249, Reactive Red 180, ILFORD magenta M-377, and mixtures thereof.

In another embodiment of the present invention, there is provided a process of printing using the ink set of the invention. Therefore according to this aspect of the invention there is provided a multicolour ink jet printing process which comprises: I) incorporating into an ink jet printer a first ink having a colour and comprising a pigment as colorant dispersed in an aqueous carrier medium; 2) incorporating into an ink jet printer a second ink having a colour different from the colour of the first ink and comprising a colorant, boric acid or salt thereof, and an aqueous carrier medium; 3) causing droplets of the first ink to be ejected in an imagewise pattern onto a substrate; and 4) causing droplets of the second ink to be ejected in an imagewise pattern onto the substrate; wherein intercolour bleed between the first ink and the second ink is reduced when the second ink is printed adjacent to, on top of, or underneath the first ink on the substrate.

The substrate may be any of the printing media commonly used for inkjet printing, for example so-called plain paper, neutral plain paper, acid plain paper, sized paper, bond paper, wet strength paper, recycled paper, fine art paper, coated paper, cardboard, canvas, fabrics, textile products, inorganic substrates such as metals and wood, plastics, polymeric films such as self-adhesive vinyl, scrim vinyl, overhead transparencies, or white polyester, and bonded nonwoven polyolefin filmfibril sheets such as polyethylene or polypropylene banner materials and the like. In a preferred embodiment, the process entails printing onto a porous or ink absorbent substrate, and in an especially preferred embodiment, the substrate is plain paper, by which is meant conventional plain paper of ordinary office quality.

The ink jet printer may be a continuous ink jet printer, a piezoelectric drop-on-demand printer, or a thermal or bubble jet drop-on-demand printer. In a particularly preferred embodiment, the ink jet printer is a thermal drop-on-demand printer. The printer may be a desk top printer or a wide format ink jet printer.

Brief Description of the Drawines

The attached Figures show enlargements of parts of the test prints printed using the inventive and comparative ink sets of Example 3 which follows. In figure I for the comparative ink set it is seen that the printed black line across the field of view has a very irregular boundary, with unacceptable bleed into the yellow print areas. In figure 2 for the inventive ink set it is seen that the black line shows a much smoother boundary and a more regular appearance, with significantly reduced bleed.

The following examples will serve to illustrate the invention:

Example 1.

Preparation of Black Pipment Disoersion A 20% by weight black aqueous pigment dispersion was prepared using a Microfluidiser Model M210C at 205 MPa. The black pigment was a commercial sample from Degussa under the trade name S170. 20% by weight on pigment of a commercially available styrene/ acrylate dispersant characterized by an acid number of 215, a Tg of 95 C, and a molecular weight of approximately 8500 neutralized with potassium hydroxide. This dispersant is available from Johnson Polymer. The dispersion was then diluted to a pigment concentration of 10% by addition of water and centrifuged to remove oversized particles.

Preparation of inks A black first ink composition was prepared from the above dispersion by dilution according to the formulations shown in table 1.

Table 1.

Component % by Weight Black Dispersion 45.4 Polyethylene glycol mw 300 10 Diethylene Glycol 10 EG1 4 Deionised water 30.6 EGI is an ethoxylated glycerol with a degree of ethoxylation approximately 26. The average particle size of the pigment was measured as 96.6 rim using a Polymer Laboratories LSP particle size analyser. The pH of the ink was 8.50, the surface tension 58 dyne/cm, and the viscosity 3.55 cP.

A yellow second ink composition for an inventive ink set was prepared according to the formulation shown in table 2.

Table 2.

Component % by Weight ILFORD yellow dye Y-104 3 Ethylene Glycol 4 Propylene Glycol 2 Glycerol 8 2-Pyrrolidone 1.2 Surfactant 0.5 Borax 0.15 Deionised water 81. 15 The surfactant was sodium dioctyl sulphosuccinate as a 70% solution in propylene glycol. The pH of the ink was 8.58, the surface tension 28 dyne/cm, and the viscosity 1.59 cP.

Evaluation The inks were loaded into ink cartridges for a HP990 desktop thermal inkjet printer, and test charts composed of thin black lines abutting solid yellow printed areas on both sides were printed on plain inkjet paper using the best quality print setting. This test chart is designed to make intercolour bleed of the black ink from the printed lines into the adjacent yellow regions readily visible. High density prints were seen, with little evidence of bleed of the black ink into the yellow areas.

Comparison A yellow second ink composition for a comparison ink set was prepared according to the formulation shown in table 3.

Table 3.

Component % by Weight ILFORD yellow dye Y-104 3 Ethylene Glycol Propylene Glycol Glycerol 8 2-Pyrrolidone 1.2 Surfactant 0.5 Deionised water 81.3 This formulation only differs from the second ink of the inventive ink set by omission of the borax. The pH of the comparison second ink was 8.70, the surface tension 28 dyne/cm, and the viscosity 1.60 cP. A comparison ink set was made up using a black ink prepared according to table 1 and the comparative yellow ink prepared according to table 3. The comparison ink set was loaded into a printer and printed as above. An image of good density was seen, but significant bleed of the black ink into the yellow printed areas was observed.

As a measure of bleed, the line width of 0.2 mm black lines of the test charts for the inventive and comparative ink sets printed on the chart was measured using a Personal Image Analysis System available from Quality Engineering Associates. The greater the measured line width the greater the bleed of the black ink into the yellow printed areas.

The results are given in table 4.

Table 4.

Ink Set Line Width Invention 0.333 mm Comparison 0.472 mm It is seen that the measured line width of the black line of the print from the inventive ink set is considerably less than that of the comparative ink set, showing significantly less bleed. In addition the line from the inventive set had a considerably smoother and less ragged appearance.

Example 2.

A black first ink composition was prepared according to the formulation shown in table 5 using a black pigment dispersion prepared as in example 1. EG1 and the surfactant are as used above. The pH of the ink was 7.94, the surface tension 56.5 dyne/cm, and the average pigment particle size 98.5 nm.

Table 5.

Component % by Weight Black Dispersion 40 Polyethylene Glycol mw 300 7.5 Diethylene Glycol 7.5 EGl 3 Surfactant 0.05 Deionised water 30.6 A yellow second ink composition for an inventive ink set was prepared according to the formulation shown in table 6. The pH of the ink was 8.48, the surface tension 28.2 dyne/cm, and the viscosity 1.60 cP.

Table 6.

Component % by Weight ILFORD yellow dye Y-104 3 Ethylene Glycol 4 Propylene Glycol 2 Glycerol 8 2-Pyrrolidone 1.2 Surfactant 0.5 Ammonium Pentaborate 0.09 Deionised water 81.21 An inventive ink set was made up comprising a first ink formulated according to table 5 and a second ink formulated according to table 6. A comparative ink set was made up comprising a black first ink formulated according to table 5 and a yellow second ink formulated according to table 3. Thus the comparative set only differs from the inventive ink set by omission of the pentaborate from the yellow ink.

The inventive and comparative ink sets were loaded into the printer and test charts printed as in Example 1. The bleed of the inventive and comparative ink sets was measured as in example 1, and the results are given in table 7.

Table 7.

Ink Set Line Width | Invention 0.517 mm Comparison 0.618 mm It is seen that the measured line width of the black line of the print from the inventive ink set is considerably less than that of the print from the comparative set, showing significantly less bleed. In addition the line from the inventive ink set had a considerably smoother and less ragged appearance.

Examole 3.

An inventive ink set was made up comprising a black first ink formulated according to table 5 and a yellow second ink formulated according to table 2. The ink set was loaded into a printer and a test chart printed as in Example 1. The bleed of the inks was measured as in example I and compared with the comparative ink set from example 2, and the results are given in table 8.

Table 8.

Ink Set Line Width Invention 0.447 mm Comparison 0.618 mm It is seen that the measured line width of the black line of print from the inventive ink set is considerably less than that of the print from the comparative set, showing significantly less bleed.

The accompanying figures show microscopic enlargements of the black lines printed against a yellow background of the inventive and comparative ink sets of this example.

The black area on the right of the figures as shown is the limit of the field of view of the microscope. The print direction was down the field of view, that is to say the yellow fill print area which appears as the grey region at the top of the field of view was printed first, immediately followed by the black line across the field of view, and then by the yellow fill area which appears as the grey region at the bottom of the field of view.

In figure 1 for the comparative ink set it is seen that the printed black line across the field of view has a very irregular appearance, with unacceptable bleed into the yellow print area at the top of the field of view and some bleed into the yellow area at the bottom of the field of view. In figure 2 for the inventive ink set it is seen that the black line shows a much smoother and more regular appearance, with no bleed of the black ink in to the lower yellow area, and significantly reduced bleed into the upper yellow area. An indication of the enlargement of these images is that the width of the black printed line of figure 2 was measured at 0.447mm.

Example 4.

A comparison ink set was made up using a black ink composition prepared as in example 1 and a commercially available dye based yellow ink available under the trade name ILFORD Archiva Yellow. An inventive ink set was made up using the same inks, but by adding 0.1% Boric acid by weight to the commercial yellow ink composition. The comparison and inventive ink sets were loaded into a printer and a test charts printed as in Example 1. The bleed was measured as in Example 1, and the results are given in table 9.

Table 9.

Ink Set Line Width Invention 0.267 mm Comparison 0.319 mm It is seen that the measured line width of the black line of the print from the inventive ink set is considerably less than that of the print from the comparative set, showing significantly less bleed. In addition the line from the inventive ink set had a considerably smoother and less ragged appearance.

Example 5.

Preparation of Black Piament Dispersion A 15% by weight black aqueous pigment dispersion was prepared using a laboratory Microfluidiser Model M110F for 30minutes at 14,000PSI (96 MPa). The black pigment was a commercial sample from Degussa under the trade name Nipex 150. 6% by weight on pigment of a commercially available styrene/ acrylate dispersant characterized by an acid number of 240, a Tg of 57 C, and a molecular weight of approximately 1800 neutralized with potassium hydroxide. This dispersant is available from Johnson Polymer. The dispersion was centrifuged to remove oversized particles.

Preparation of inks An inventive black first ink composition was prepared from the above dispersion by dilution according to the formulations shown in table 10. The pH of the ink was 7.99, the surface tension 56.5 dyne/cm, and the average pigment particle size 158.4 nm.

Table 10.

Component % by Weight Black Dispersion 26.7 Polyethylene Glycol mw 300 Diethylene Glycol 5 EG1 4 Surfactant 0.05 Deionised water 59.3 A comparison ink set was made up using the black ink composition prepared above and a commercially available dye based yellow ink available under the trade name ILFORD Archiva Yellow. An inventive ink set was made up using the same inks, but by adding 0.1% Boric acid by weight to the commercial yellow ink composition. The comparison and inventive ink sets were loaded into a printer and a test charts printed as in Example 1.

The bleed was measured as in Example l, and the results are given in table 11.

Table 11.

Ink Set Line Width Invention 0.269 mm Comparison 0.368 mm It is seen that the measured line width of the black line of the print from the inventive ink set is considerably less than that of the print from the comparative set, showing significantly less bleed. In addition the line from the inventive ink set had a considerably smoother and less ragged appearance.

Example 6.

A black ink composition was prepared according to the formulation shown in table 12 using the components as described above and a black pigment dispersion prepared as in example 5. The pH of the ink was 7.92, the surface tension 58 dyne/cm, the viscosity 2.79 cP, and the average pigment particle size 155 nm.

Table 12.

Component % by Weight Black Dispersion 32.6 Polyethylene Glycol mw 300 7. 5 Diethylene Glycol 7.5 EG1 3 Surfactant 0.05 Deionised water 42 A comparison ink set was made up using the black ink composition prepared above and a commercially available dye based yellow ink available under the trade name ILFORD Archiva Yellow. An inventive ink set was made up using the same inks, but by adding 0.1% Boric acid by weight to the commercial yellow ink composition. The comparison and inventive ink sets were loaded into a printer and a test charts printed as in Example 1.

The bleed was measured as in Example 1, and the results are given in table 13.

Table 13.

Ink Set Line Width Invention 0.326 mm Comparison 0.374 mm It is seen that the measured line width of the black line of the print from the inventive ink set is considerably less than that of the print from the comparative set, showing significantly less bleed. In addition the line from the inventive ink set had a considerably smoother and less ragged appearance.

Claims (11)

1. Claims:- 1. An aqueous ink set for ink jet printers comprising: 1) a

   first ink comprising at least one pigment dispersed in an aqueous carrier medium; and 2) a second ink comprising at least one colorant, boric acid or salt thereof, and an aqueous carrier medium; wherein intercolour bleed between the first ink and the second ink is reduced when the second ink is printed adjacent to, on top of, or underneath the first ink on the substrata.
2. 2. An aqueous ink jet ink set according to claim 1 wherein the first ink also comprises at least one dispersant for the pigment.
3. 3. An aqueous ink jet ink set according to claim 2 wherein the dispersant in the first ink is a styrene/ acrylate copolymer characterized by an acid number between 150 and 250, a glass transition temperature between 50 C and 150 C, and a molecular weight between 1500 and 25000.
4. 4. An aqueous ink jet ink set according to claim 1 wherein the pigment of the first ink is a self dispersing carbon black pigment.
5. 5. An aqueous ink jet ink composition according to any of claims 1 - 4 wherein the colorant of the second ink is a water-soluble dye.
6. 6. An aqueous ink jet ink set according to any of claims 1 - 4 wherein the colorant of the second ink is a pigment.
7. 7. An aqueous ink jet ink set according to claim 5 wherein the dye is a yellow dye selected from Acid Yellow 23, Direct Yellow 86, Direct Yellow 132, Reactive Yellow 37, ILFORD yellow Y-104, and mixtures thereof.
8. 8. An aqueous ink jet ink set according to any of the preceding claims wherein the aqueous carrier medium in any of the inks comprises a mixture of water and at least one relatively involatile water soluble organic cosolvent.
9. 9. An aqueous ink set according to any of the preceding claims wherein the first ink comprises substantially no deliberate addition of surfactant and has a surface tension between 50 dyne/cm and 60 dyne/cm, and the second ink comprises at least one surfactant and has a surface tension between 25 dyne/cm and 35 dyne/cm.
10. 10. An ink jet printing process which comprises: 1) incorporating into an ink jet printer a first ink having a colour and comprising a pigment as colorant dispersed in an aqueous carrier medium; 2) incorporating into an ink jet printer a second ink having a colour different from the colour of the first ink and comprising a colorant, boric acid or salt thereof, and an aqueous carrier medium; 3) causing droplets of the first ink to be ejected in an imagewise pattern onto a printing medium; and 4) causing droplets of the second ink to be ejected in an imagewise pattern onto the printing medium; wherein intercolour bleed between the first ink and the second ink is reduced when the second ink is printed adjacent to, on top of, or underneath the first ink on the substrate.
11. 11. A process for printing according to claim 10 wherein the printing medium is selected from plain paper, recycled paper, neutral plain paper, and acid plain paper.