GB2349153A

GB2349153A - Pigmented ink jet inks

Info

Publication number: GB2349153A
Application number: GB9908710A
Authority: GB
Inventors: Peter Edward Rose; Nicholas Alexander Walker
Original assignee: Ilford Imaging UK Ltd
Current assignee: Ilford Imaging UK Ltd
Priority date: 1999-04-17
Filing date: 1999-04-17
Publication date: 2000-10-25
Anticipated expiration: 2019-04-17
Also published as: GB2349153B; GB9908710D0

Abstract

A pigmented aqueous ink jet ink composition is provided which comprises two dispersants characterised by:- <SL> <LI>(a) A first dispersant selected from a ballasted poly(ethylene oxide) dispersant, a dispersant containing a triple bond, an acrylate resin dispersant, a sarcosinate dispersant, or a naphthalene sulphonic acid dispersant, and <LI>(b) A second dispersant which is a styrene acrylate copolymer. </SL> It is preferred that the ink is prepared by dispersing the pigment in an aqueous carrier medium in the presence of the first dispersant to form a concentrated dispersion, and then diluting this to produce the ink by adding an aqueous alkaline solution of the styrene acrylic co-polymer second dispersant, together with other components and optional co-solvents.

Description

Field of the Invention This invention relates to aqueous pigmented inks, to their preparation, and to their use in ink jet printers.

Background of the Invention Ink jet printing is a non-impact method that in response to a digital signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printing has found broad application in industry as well as for output from personal computers in the office and the home. There are several classes of ink jet printer, for instance thermal drop-on-demand printers, piezo drop-on-demand printers, and continuous ink jet printers.

Ink jet printers, particularly those for use in the home or office, generally use aqueous inks, and such inks commonly employ dyes as colorants. However the use of dyes introduces certain disadvantages, particularly the problems of poor light stability and poor fastness to washing and weathering of the prints produced. It is expected that these disadvantages can be overcome by the use of pigments as colorants in ink jet inks, and there is therefore considerable interest in the preparation of such pigmented inks. The term pigment describes a colorant which is essentially insoluble in the aqueous ink medium.

Various classes of pigment are catalogued and classified in the Pigments and Solvent Dyes section of the Colour Index International, published by the Society of Dyers and Colorists in 1997. Many pigments described by the Colour Index are suitable for ink jet use, together with certain compounds classified by the Colour Index as vat or disperse dyes as well as numerus other insoluble coloured compounds which are not included in the Colour Index such as, for example, the cyan pigments described in United States t Patent Specification 4 311 775.

Pigments need to be finely dispersed for use in inks, and this requirement is particularly important for use in jetting inks because of the fine nozzles used in ink jet printers. The dispersing step may be accomplished using one of several well known techniques, for example in a horizontal mini mill, a ball mill, an attritor, or by passing the mixture through a plurality of nozzles within a liquid jet interaction chamber at a liquid pressure of at least 1000 psi as described in United States Patent Specification 5 026 427 to produce a uniform dispersion of the pigment particles.

It is common to formulate dispersions of pigments for use in jetting inks by first dispersing the pigment in a suitable medium, frequently water, in the presence of one or more stabilisers to stabilise and disperse the fine particles, and then letting this dispersion down into the ink with more water, together with organic cosolvents and other additives. This technique is generally necessary because the viscosity needed for the dispersion stage is normally greater than that desired in the ink. By dilution, the ink is adjusted to the desired viscosity, colour, hue, saturation density, and print area coverage for the particular application.

The stabiliser used in the dispersion is commonly known as a dispersant or surfactant. Suitable dispersants useful for pigmented aqueous inks are well known, and include macromolecular polyionic dispersants, for example polyester ionomers such as those disclosed in United States Patent 4 597 794 and European Patent Application 0 776 953, acrylate polymers, such as acrylic acid polymers and copolymers of styrene with acrylic or maleic acids, for example those disclosed by United States Patent Specifications 5 085 698,5 172 133, and 5 821 283 and European Patent Applications 0 819 738, and 0 826 752, various types of poly (ethylene oxide) condensates such as those disclosed in United States Patent Specification 5 837 043 and sulphate or phosphate esters thereof such as those disclosed in United States Patent Specification 5 707 433 and European Patent Application 0 848 045, and simple surfactants such as the azlactone dispersants disclosed in United States Patent Specification 5 840 106, sodium N-methyl-N-oleyl taurate as disclosed in United States Patent Specification 5 651 813, or sodium dodecyl sulphate.

Hereinafter such components which are part of the stabilisation system for the pigment will be referred to as dispersants whether they are present during the dispersion stage or added subsequently.

The properties of the ink greatly depend on the stability of the dispersed particles, and the stabilisation system is of crucial importance in achieving this. For example, it can be found that particles which are well stabilise in a concentrated dispersion lose this stability when let down into an ink, and that the particles in the ink flocculate or aggregate on storage. This can sometimes be due to simple dilution of the stabiliser, but more frequently results from interactions with the organic cosolvents or other additives which are commonly present in the final ink. However these cosolvents and other additives need to be present in the ink, either for it to be used succesfully in an ink jet printer or to give acceptable properties to the final image. For instance, a major concern with all ink jet printing devices is pluggage of nozzles during operation and between operations, and it is well known that nozzle pluggage with an aqueous ink can be reduced or eliminated by an addition of a proportion of between 5 and 50%, commonly around 10 to 25%, of a relatively involatile organic cosolvent or mixture of cosolvents. Various simple aliphatic polyols, such as glycerol, ethylene glycol, propylene glycol, and diethylene glycol are known to be useful for this purpose for dyed ink jet inks.

In addition, certain water soluble polymers, particularly condensates of ethylene oxide, are also known to be useful, generally in combination with the low molecular weight organic cosolvents.

However the situation is more complicated in pigmented ink jet inks as these additives can destabilise dispersions and cause instability, flocculation, or aggregation on storage. Therefore a need still exists for dispersions which will produce inks which are highly stable towards aggregation or flocculation on storage, which print successfully through an ink jet printer, and which give good properties to the final image, without introducing other disadvantages.

We have found that pigmented inks formulated with a mixture of dispersants are particularly suitable for ink jet use.

Summarv of the Invention The invention provides a pigmented aqueous ink jet ink composition which comprises two dispersants characterised by: (a) A first dispersant selected from a ballasted poly (ethylene oxide) dispersant, a dispersant containing a triple bond, an acrylate resin dispersant, a sarcosinate dispersant, or a naphthalene sulphonic acid dispersant, and (b) A second dispersant which is a styrene acrylate copolymer.

The first dispersant may be an ionic or a non-ionic dispersant. By a ballasted poly (ethylene oxide) dispersant is meant a dispersant which comprises a block copolymer of poly (ethylene oxide) with a hydrophobic comonomer or a dispersant which comprises a poly (ethylene oxide) chain and a ballasting group which comprises at least twelve carbon atoms or a phosphate or sulphate ester of such compounds. By an acrylate resin dispersant is meant a copolymer of acrylic, methacrylic, or maleic acids with at least one vinylically unsaturated hydrophobic comonomer. By a dispersant containing a triple bond is meant a dispersant which is based on a substituted 2-butyne-1, 4-diol and ethylene oxide condensates of such diols. By a sarcosinate dispersant is meant a dispersant which comprises the reaction product of sarcosine with a ballasting acyl group. By a naphthalene sulphonic acid dispersant is meant a dispersant which comprises a naphthalene ring which is substituted with at least one sulphonic acid substituent and at least one hydrophobic substituent.

Suitable ballasted poly (ethylene oxide) dispersants include block copolymers of ethylene oxide with propylene oxide, ethylene oxide condensates of aliphatic alcohols such as poly (ethylene oxide) cetyl ether, ethylene oxide condensates of substituted phenols, such as p-nonyl phenol, and phosphate or sulphate esters of these compounds. Suitable acrylate resin first dispersants include copolymers of acrylic, methacrylic, or maleic acids with styrene or an acrylate ester and terpolymers of these monomers. Suitable such acrylate resin dispersants are characterised by an acid number of between about 180 and about 300 and a molecular weight of between about 2000 and about 20000.

Suitable first dispersants containing a triple bond include ethylene oxide condensates of 2,4,7,9-tetramethyl-5-decyne-4,7-diol. Suitable naphthalene sulphonic acid dispersants include di-isopropyl naphthalene sulphonic acid and salts thereof. Suitable sarcosinate dispersants have the formula : R-CO-NMe-CH2-COOM In which R is a ballasting group comprising at least ten carbon atoms and M is hydrogen, an alkali metal cation, an ammonium cation or a substituted ammonium cation.

Many dispersants of these classes are commercially available.

The styrene acrylate second dispersant is a copolymer of styrene with acrylic or methacrylic acids, together with optionally additional vinylically unsaturated comonomers such as acrylate esters. If the first dispersant is also a styrene acrylate copolymer then the second dispersant is of lower molecular weight and lower acid number than the first dispersant. Preferably the second dispersant is characterised by an acid number of between about 150 and about 250, a glass transition temperature between about 70 and about 90 , and a molecular weight of between about 2000 and about 10000, and most preferably the acid number is between 150 and 200 and the molecular weight is about 5000. Many suitable styrene acrylate dispersants are commercially available, and a particularly useful example has a molecular weight of between 2000 and 6000, a Tg of 70 and an acid number of 190. This polymer will hereinafter be referred to as dispersant A.

The styrene acrylate copolymer first and second dispersants are either supplied commercially in the form of concentrated solutions in aqueous base, or are supplied 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, triethanolamine, 2-amino-2-methyl-l-propanol or 1- (dimethylamino)-2- propanol. It is also possible to use mixtures of such bases to dissolve the styrene acrylate copolymer.

According to a preferred aspect of the invention the ink is prepared by dispersing the pigment in an aqueous carrier medium in the presence of the first dispersant to form a concentrated dispersion, and then diluting this to produce the ink by adding an aqueous alkaline solution of the styrene acrylic co-polymer second dispersant, together with other components and optional co-solvents.

According to a second aspect of the invention both the first and second dispersants are present when the pigment is dispersed in the aqueous carrier medium to form the concentrated dispersion, which is then diluted to form the ink with water together with other components and optional co-solvents.

According to a third aspect of the invention the ink is prepared by dispersing the pigment in an aqueous carrier medium in the presence of the second dispersant to form a concentrated dispersion, and the first dispersant is then added when this dispersion is diluted to produce the ink, together with other components and optional co-solvents, but this is not preferred.

The ink may contain up to approximately 30% pigment by weight. Preferably it will be in the range of approximately 0.1 to 15%, and most preferably approximately 1 to 8%, by weight of the total ink composition for most thermal ink jet printing applications.

The ink may contain up to about 400% by weight on pigment of the first and second dispersants combined. The amount of each dispersant separately will generally be in the range 1 to 100% by weight on pigment, and preferably approximately 10 to 50% by weight on pigment for most thermal inkjet applications.

The particle diameter of the pigment is preferably 101lu or less, particularly preferably 1 Fm or less.

Specific examples of pigments for a black ink include carbon black (C. I.

Pigment Black 7), such as furnace black, lamp black, acetylene black, and channel black, and o-nitroaniline black (C. I. Pigment Black 1), of which carbon black is preferred. Specific examples of the pigment for a coloured ink include C. I. Pigment Yellow 1,3,12,13,14,17,24,34,35,37,53,55,74,81, 83,93,95,97,98,100,101,104,108,109,110,117,120,128,138,153,154, 155,180, and 185, C. I. Pigment Orange 5,13,16,17,34,36,43,51,70, and 71, 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: 1,60: 1,63: 1,63: 2,64: 1,81,83,88,104,105,106,112,114,122, 123,146,149,166,168,170,172,177,178,179,184,188,190,193,202, 209, and 219, C. I. Pigment Violet 1, 3,5: 1,16,19,23, and 38, C. I. Pigment Blue 1,2,15: 1,15: 2,15: 3,15: 4,15: 6,16,17: 1,56,60, and 63, and C. I.

Pigment Green 1,4,7,8,10,17,18, and 36. Preferred examples of pigments for a coloured ink include Pigment Yellow 13,74,128,154,155, and 180, Pigment Orange 34,43, and 71, Pigment Red 122,202, and 209, Pigment Violet 19, Pigment Blue 15: 3,15: 4, and 16, and Pigment Green 7. It is also possible to mix more than one pigment in the inks of the invention.

Many of the above pigments are available commercially in finely divided or predispersed forms which are specifically aimed at ink jet use. It is preferred to use such available forms in the inks of this invention. It is also possible to use processed pigments such as graft carbon having a surface treated with a resin or the like.

The aqueous carrier medium is water or a mixture of water and at least one water soluble organic solvent. Deionized water is commonly used. Preferably the pigment dispersion is prepared in water, and the organic cosolvent or cosolvents are added when this is diluted to produce the ink. Selection of a suitable mixture of water and water soluble organic solvent depends on the requirements of the specific application, such as the selected pigment, the desired surface tension and viscosity, the drying time of the pigmented ink jet ink, and the type of substrate onto which the ink will be printed.

Representative examples of water-soluble organic solvents that may be selected include: (1) alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, sec-butyl alcohol, t-butyl alcohol, iso-butyl alcohol, furfuryl alcohol, and tetrahydrofurfuryl alcohol; (2) ketones or ketoalcohols such as acetone, methyl ethyl ketone and diacetone alcohol; (3) ethers, such as tetrahydrofuran, dioxolane and dioxane; (4) esters, such as ethyl lactate, (5) polyhydric alcohols, such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, glycerol, 1,2-hexanediol, 1,5-pentanediol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, trimethylolethane, trimethylolpropane, neopentyl glycol, and thiodiethanol; (6) lower alkyl mono-or di-ethers derived from alkylene glycols, such as ethylene glycol monomethyl (or-ethyl) ether, diethylene glycol mono-methyl (or-ethyl) ether, ethylene glycol mono allyl ether, propylene glycol mono-methyl (or-ethyl) ether, triethylene glycol mono-methyl (or-ethyl) ether and diethylene glycol di-methyl (or-ethyl) ether; (7) nitrogen containing organic compounds such as urea, pyrrolidone, N-methyl-2-pyrrolidone, and 1, 3-dimethyl-2-imidazolidinone; and (8) sulphur-containing compounds such as dimethyl sulfoxide and tetramethylenesulfone.

A preferred aqueous carrier medium for the ink comprises a mixture of water and at least one water soluble organic solvent having at least two hydroxyl groups such as diethylene glycol or thiodiethanol. Another preferred aqueous carrier medium comprises a mixture of water together with at least one water soluble organic solvent having at least two hydroxyl groups and at least one nitrogen containing water soluble organic compound such as urea, pyrrolidone, or N-methyl pyrrolidone.

The aqueous composition may contain up to 50% of the organic cosolvent or cosolvent mixture. In the case of cosolvent mixtures there is preferably up to about 10% of each of the organic cosolvents, and most preferably between about 5% and about 10% of each organic cosolvent.

The ink compositions of the invention may also contain other components which are advantageously added to aqueous 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 pigmented ink jet ink compositions of this invention are suited for use in ink jet printers in general, and thermal ink jet printers in particular. The following examples will serve to illustrate the invention: Example 1.

Magenta Ink 1 according to the invention was prepared by the following steps: Preparation of a Pigment Dispersion l Og of a commercially available finely divided sample of the magenta pigment CI Pigment Red 122 was mixed using a Silverson rotor stator mixer with 1 Og of a 10% solution of a commercially available modified styrene acrylate dispersant which has an acid number of 214 and an average molecular weight of 17000 which had been neutralised using potassium hydroxide. This dispersant is available from SC Johnson.

The resultant mixture was then passed through a Microfluidiser Model 11 OY at 12000 psi for 30 mins to produce a pigment dispersion. The dispersion was then centrifuged at 3000 rpm for 20 minutes to remove oversized particles.

The average particle size was less than 200nm.

Preparation of the ink Magenta Ink 1 was then prepared from this dispersion according to the formulation shown in table 1.

Table 1.

Amount (grams) Component Parts by Weight Pigment Dispersion 35 Thiodiglycol 10 Urea 5 Polyethylene Glycol MW300 10 10% Solution of Acrylate Dispersant A 7 Deionised Water 33 The solution of acrylate dispersant A was prepared from a commercially available styrene acrylate copolymer having an acid number of 190, a Tg of 70 and a molecular weight of 2000 to 6000 by neutralising with potassium hydroxide. This dispersant is available from BF Goodrich. The pH of this solution was 9.2.

Ink 2. (Invention) A magenta pigment dispersion was prepared in the same manner as for Ink 1, except that 15g of the 10% solution of the styrene acrylate dispersant which had been neutralised with potassium hydroxide was used. The ink from this dispersion was formulated with the components listed in table 1.

Ink 3. (Invention) A magenta pigment dispersion was prepared in the same manner as for Ink 1, except that 20g of the 10% solution of the styrene acrylate dispersant which had been neutralised with potassium hydroxide was used. The ink from this dispersion was formulated with the components listed in table 1.

Ink 4 (Comparison).

A magenta pigment dispersion was prepared in the same manner as for Ink 1, but using 15g of a 10% solution of the styrene acrylate dispersant A which had been neutralised using potassium hydroxide. The ink from this dispersion was formulated with the components listed in table 2 (i. e. without the addition of the second dispersant).

Table 2.

Amount (grams) Component Parts by Weight PigmentDispersion 35 Thiodiglycol 10 Urea 5 PEG300 10 Deionised Water 33 Ink 5 (Comparison).

A magenta pigment dispersion was prepared exactly as described for Ink 3.

The ink from this dispersion was formulated with the components listed in table 2 (i. e. without without the addition of the second dispersant).

Ink 6 (Comparison).

A magenta pigment dispersion was prepared in the same manner as for Ink 1, but this time the pigment was dispersed with 20g of a 10% solution of a styrene acrylate resin dispersant having an acid number of 240, a Tg of 102 , and an average molecular weight of about 15500 which had been neutralised using potassium hydroxide. This dispersant is available from SC Johnson.

The ink from this dispersion was formulated with the components listed in table 2.

Ink 7 (Comparison).

A magenta pigment dispersion was prepared in the same manner as for Ink 1, but this time the pigment was dispersed with 20g of a 10% solution of a styrene acrylate resin dispersant having an acid number of 215, a Tg of 95 , and an average molecular weight of about 8500 which had been neutralised using potassium hydroxide. This dispersant is available from SC Johnson. The ink from this dispersion was formulated with the components listed in table 2.

Ink 8 (Comparison).

A magenta pigment dispersion was prepared in the same manner as for Ink 1, but this time the pigment was dispersed with 20g of a 10% solution of a styrene acrylate resin dispersant having a Tg of 70 and an average molecular weight of about 11000 which had been neutralised using potassium hydroxide.

This dispersant is available from BF Goodrich. The ink from this dispersion was formulated with the components listed in table 2.

Ink 9 (Comparison).

A magenta pigment dispersion was prepared in the same manner as for Ink 1, but this time the pigment was dispersed with 20g of a 10% solution of a styrene acrylate resin dispersant having an acid number of 210, a Tg of 120 and an average molecular weight of about 11000 which had been neutralised using potassium hydroxide. This dispersant is available from BF Goodrich.

The nine inks were tested as follows : Ink Stability Test 5g of each ink formulation was placed into small sealed tube and then stored in a water bath at 70 C. The inks were inspected regularly for evidence of flocculation or instability, and the time before any evidence was seen was recorded. This test is a indication of long term ink stability.

Ink Runnability Test 500g of each ink formulation was loaded into a print cartridge and ink reservoir of an Encad Novajet Pro 42E Inkjet printer. A test pattern was printed which contained solid 100% colour blocks from which the optical density measurements were taken. This represented the print density at the start of the test. A solid panel of 100% colour approximately 40.8" by 108" was then printed. This sequence was then repeated until all the ink was used or until the printhead had failed or blocked. The loss in density of the test pattern during the course of the test was calculated, except for any inks which blocked the nozzles during the course of the test. A good ink will print for 500g with minimal loss in density or print quality.

The results of these tests are shown in Table 3.

Table 3.

Formulation Ink Stability at 70 C Ink Runnability Loss in Density Ink 1 (Invention) > 7days < 0. I Ink 2 (Invention) > 7days < 0. 1 Ink 3 (Invention) > 7days < 0. 1 Ink 4 (Comparison) 2 days < 0. 1 Ink 5 (Comparison) > 7 days > 0.3 Ink 6 (Comparison) > 7 days > 0.3 Ink 7 (Comparison) > 7 days Print head blocked before 75g Ink 8 (Comparison) > 7 days Print head blocked before 25g Ink 9 (Comparison) > 7 days > 0.3 The inventive inks 1-3 show excellent stability and run successfully with negligible change in density of the resulting print, while the comparison ink 4 has poor stability and the comparison inks 5-10 which lack the second dispersant show poor runnability.

Example 2.

Four further magenta inks were prepared.

Ink 10. (Invention) A magenta pigment dispersion was prepared in the same manner as for Ink 1 of Example 1, but this time using 3g of a 10% solution of a commercially available aromatic ethoxylate dispersant. This dispersant is available from Zeneca Colours. The ink from this dispersion was formulated with the components listed in table 4.

Table 4.

Amount (grams) Component Parts by Weight Pigment Dispersion 35 Thiodiglycol 10 Urea 5 Polyethylene Glycol MW300 10 10% Solution of Acrylate Dispersant A 5. 25 Deionised Water 34. 75 Ink 11. (Invention) A magenta pigment dispersion was prepared in the same manner as for Ink 10, but this time using 3g of a commercially available anionic/nonionic blend dispersant based on 2,4,7,9-tetramethyl-5-decyne-4,7-diol. This dispersant is available from Air Products. The ink from this dispersion was formulated with the components listed in table 4.

Ink 12 (Comparison) A magenta pigment dispersion was prepared exactly as described for Ink 10.

The ink from this was formulated with the components listed in table 2 (i. e. without the second dispersant).

Ink 13 (Comparison) A magenta pigment dispersion was prepared exactly as described for Ink 11.

These two inks were subjected to the 70 C stability test as described in Example 1. They were also tested according to the following decap test in comparison with Inks 1, 2, and 3 from Example 1.

Decap Test Separate HP51626a print cartridges were loaded with 20g of each ink and mounted onto an HP DJ400 printer. Several test patterns were printed which contained solid 100% colour blocks. Each cartridge was then removed from the printer and left uncapped for 7 days, then re-loaded into the printer and the test pattern which contained a solid 100% colour block was again printed. An ink with good decap properties would start printing immediately, a bad ink would not print at all or only a proportion of the nozzles would fire. The results of these tests are shown in Table 5.

Table 5.

Formulation Ink Stability at 70 C Decap Performance Ink 1 (Invention) > 7days Excellent Ink 2 (Invention) > 7days Excellent Ink 3 (Invention) > 7days Excellent Ink 10 (Invention) 6 days Excellent Ink 11 (Invention) 7 days V Good Ink 12 (Comparison) 4 days Bad Ink 13 (Comparison) 7days Bad The inventive inks show acceptable stability and excellent decap performance.

The comparative inks lacking the second dispersant show poor decap performance and ink 12 also shows poor thermal stability.

The inventive inks in examples 1 and 2 also show excellent print quality.

Example 3.

Two yellow inks were prepared.

Ink 14 (Invention).

A yellow pigment dispersion was prepared in the same manner as in example 1, but this time lOg of a commercially available finely divided sample of CI Pigment Yellow 155 was dispersed with 2g of the triple bonded dispersant blend used for Ink 11. The ink from this dispersion was formulated with the components listed in table 6.

Table 6.

Amount (grams) Component Parts by Weight Pigment Dispersion 35 Thiodiglycol 10 Urea 5 Polyethylene Glycol MW300 10 10% Solution of Acrylate Dispersant A 10. 6 Deionised Water 29. 4 Ink 15 (Comparison).

A yellow pigment dispersion was prepared exactly as described for ink 14, and the ink was formulated from this using the components listed in table 2 (i. e. without the second dispersant).

The two inks were tested using the decap test described in Example 2, and by the following print density test.

Print Density Test Separate HP51626a print cartridges were loaded with 20g of each ink and mounted onto an HP DJ400 printer. A test pattern containing a solid 100% colour block was printed with each ink and the print density measured.

The results are given in table 7.

Table 7.

Formulation Decap Performance Print Density Ink 14 (Invention Excellent 1. 29 Ink 15 (Comparison) Very Poor 1. 23 It will be seen that the inventive ink shows superior decap performance and print density.

Example 4.

Two further magenta inks were prepared.

Ink 16 (Invention).

A magenta pigment dispersion was prepared in the same manner as in Example 1, but using as the dispersant 3g of Sodium N-lauroyl sarcosinate.

The ink was formulated from this dispersion using the components listed in table 4.

Ink 17 (Comparison) A magenta pigment dispersion was prepared exactly as described for ink 16.

The ink was formulated from this dispersion using the components listed in table 2.

These two inks were tested according to the print density test shown in Example 3 in comparison with inks 10,11,12, and 13 from Example 2. The results are given in table 8.

Table 8.

Formulation Density Ink 10 (Invention) 1. 38 Ink 12 (Comparison for 10) 1. 1 Ink 11 (Invention) 1.35 Ink 13 (Comparison for 11) 1.02 Ink 16 (Invention) 1. 21 Ink 17 (Comparison for 16) 1. 14 It is seen that the density obtained from the inventive inks is greater that that obtained from the comparison inks.

Claims

Claims 1. A pigmented aqueous ink jet ink composition which comprises two dispersants characterised by: (a) A first dispersant selected from a ballasted poly (ethylene oxide) dispersant, a dispersant containing a triple bond, an acrylate resin dispersant, a sarcosinate dispersant, or a naphthalene sulphonic acid dispersant, and (b) A second dispersant which is a styrene acrylate copolymer.
2. A pigmented aqueous ink jet ink composition according to Claim 1 in which the second dispersant is a styrene acrylate copolymer is characterised by an acid number of between about 150 and about 250, a glass transition temperature between about 70 and about 90 , and a molecular weight of between about 2000 and about 10000.
3. A pigmented aqueous ink jet ink composition according to Claim 2 in which the second dispersant is a styrene acrylate copolymer is characterised by a molecular weight of between 2000 and 6000, a Tg of 70 and an acid number of 190.
4. A pigmented aqueous ink jet ink composition according to Claim 1 in which the first dispersant is a block copolymer of ethylene oxide with propylene oxide.
5. A pigmented aqueous ink jet ink composition according to Claim 1 in which the first dispersant is an ethylene oxide condensate of a ballested aliphatic alcohol.
6. A pigmented aqueous ink jet ink composition according to Claim 1 in which the first dispersant is an ethylene oxide condensate of a ballasted phenols.
7. A pigmented aqueous ink jet ink composition according to Claim 1 in which the first dispersant is an acrylate resin copolymer characterised by an acid number of between about 180 and about 300 and a molecular weight of between about 2000 and about 20000.
8. A pigmented aqueous ink jet ink composition according to Claim 1 in which the first dispersant is an ethylene oxide condensate of 2,4,7,9-tetramethyl-5-decyne-4,7-diol.
9. A pigmented aqueous ink jet ink composition according to Claim 1 in which the first dispersant is a naphthalene sulphonic acid dispersant.
10. A pigmented aqueous ink jet ink composition according to Claim 1 in which the first dispersant is a sarcosinate compound of the formula : R-CO-NMe-CH2-COOM In which R is a ballasting group comprising at least ten carbon atoms and M is hydrogen, an alkali metal cation, an ammonium cation or a substituted ammonium cation.
11. A method of preparing a pigmented aqueous ink jet ink according to Claim 1 which involves dispersing a pigment in an aqueous carrier medium in the presence of the first dispersant to form a concentrated dispersion, and then diluting this to produce the ink by adding an aqueous alkaline solution of the second dispersant.