WO2017013422A1 - Process for preparing encapsulated pigment dispersion and use - Google Patents

Abstract

A process for preparing an encapsulated pigment dispersion suitable for use in an ink jet printing ink comprising the following steps in the order I) followed by II): I)providing a dispersion comprising a pigment, a liquid medium and a dispersant having a WAMW of up to 50,000 Daltons obtained by copolymerising a monomer composition comprising components a) and b): a) from 75 to 97 parts of one or more hydrophobic ethylenically unsaturated monomers comprising at least 50 parts benzyl (meth) acrylate; and b) from 3 to 25 parts one or more hydrophilic ethylenically unsaturated monomers having one or more ionic group(s); wherein the parts are by weight and the sum of the parts a) and b) add up to 100; II) cross-linking the dispersant in the presence of the pigment and the liquid medium.

Description

PROCESS FOR PREPARING ENCAPSULATED PIGMENT DISPERSION AND USE

The present invention relates to a process for preparing encapsulated pigment dispersions, to the encapsulated pigment dispersions obtainable by the process, to ink jet printing inks which contain such dispersions and to the use of the process for preparing ink jet printing inks.

Inks used in ink jet printing fall into two main categories, depending on the type of colorant they contain. Dye-based inks typically comprise a dye dissolved in a liquid medium. Pigment-based inks typically comprise a pigment dispersed in a liquid medium. Pigment-based inks typically have better ozone-fastness and light-fastness than dye-based inks. However, because the pigment is in the form of a particulate dispersion there is a tendency for pigment particles to agglomerate or flocculate whilst the ink is being stored and/or whilst the ink is being used (e.g. printed). Such agglomeration or flocculation is highly undesirable, particularly when the ink is intended for use in a printer having very small nozzles which are susceptible to blockage by any oversized particulate matter. Thus, in the ink jet field a great deal of effort has been spent attempting to increase the stability of pigment dispersions. It is particularly difficult to provide pigment inks having good stability when the liquid medium comprises large amounts of water-miscible organic solvents and relatively smaller amounts of water.

It is also desirable to provide pigment inks which offer high optical density (OD) especially when printed onto plain paper.

EP 2,342,005 describes a process for preparing pigment dispersions from dispersants having a weight average molecular weight of more than 82,000. The present invention seeks to provide a process which results in pigment dispersions having good performance in ink jet printing, e.g. good storage stability even when stored at high temperatures.

According to a first aspect of the present invention there is provided a process for preparing an encapsulated pigment dispersion suitable for use in an ink jet printing ink comprising the following steps in the order I) followed by II):

I) providing a dispersion comprising a pigment, a liquid medium and a dispersant having a weight average molecular weight ("WAMW") of up to 50,000 Daltons (Da) obtained by copolymerising a monomer composition comprising components a) and b): and

a) from 75 to 97 parts of one or more hydrophobic ethylenically unsaturated monomers comprising at least 50 parts benzyl (meth) acrylate; and b) from 3 to 25 parts one or more hydrophilic ethylenically unsaturated monomers having one or more ionic group(s);

wherein the parts are by weight and the sum of the parts a) and b) add up to 100; II) cross-linking the dispersant in the presence of the pigment and the liquid medium.

In this description the words "a" and "an" mean one or more unless indicated otherwise. Thus, for example, "a" pigment includes the possibility of there being more than one pigment, similarly "a" dispersant includes the possibility of there being more than one dispersant.

The dispersant used in step I) preferably has a WAMW in Daltons of 1 ,000 to 50,000, more preferably 5,000 to 45,000 and especially 10,000 to 38,000.

The WAMW of dispersant may be determined by gel permeation chromatography ("GPC"), preferably by comparison with polymers of known WAMW. Preferably the GPC uses Ν,Ν-dimethylformamide (DMF) as eluent. For convenience one may employ two Plgel "Mixed D" columns for the GPC, connected in sequence, and a single detector (Refractive Index).

In one embodiment the dispersion in step I) can be provided by a process comprising dispersing a pigment in a liquid medium in the presence of a dispersant having the abovementioned composition. Dispersion can be performed by any suitable method, including for example bead milling, bead shaking, ultrasonic treatment, homogenizing and/or microfluidizing. A preferred method for dispersing a pigment in liquid medium comprises bead milling. Typically, bead milling is performed using a composition comprising milling beads, a dispersant, a liquid medium and a relatively high proportion of pigment (often around 15-45% by weight relative to the weight of the liquid medium). After milling, the milling beads are removed, typically by filtration. The milled dispersion (mill-base) may be diluted with more of the liquid medium which optionally contains further dispersant, which may be the same as or different to the dispersant included in the aforementioned composition.

The pigment may comprise and preferably is an inorganic or organic pigment material or mixture thereof which is insoluble in the liquid medium.

Preferred organic pigments include, for example, any of the classes of pigments described in the Colour Index International, Third Edition, (1971 ) and subsequent revisions of, and supplements thereto, under the chapters headed "Pigments". Examples of organic pigments include those from the azo (including disazo and condensed azo), thioindigo, indanthrone, isoindanthrone, anthanthrone, anthraquinone, isodibenzanthrone, triphendioxazine, quinacridone and phthalocyanine series, especially copper phthalocyanine and its nuclear halogenated derivatives, and also lakes of acid, basic and mordant dyes. Preferred organic pigments incldue phthalocyanines, especially copper phthalocyanine pigments, azo pigments, indanthrone, anthanthrone and quinacridone pigments. Preferred inorganic pigments include carbon black, titanium dioxide, aluminium oxide, iron oxide and silicon dioxide.

In the case of carbon black pigments, these may be prepared in such a fashion that some of the carbon black surface has oxidized groups (e.g. carboxylic acid and/or hydroxy groups). However, the amount of such groups is preferably not so high that the carbon black may be dispersed in water without the aid of the dispersant.

Preferably, the pigment is a cyan, magenta, yellow, orange, violet, green or black pigment.

The pigment may be a single chemical species or a mixture comprising two or more chemical species (e.g. a mixture comprising two or more different pigments). In other words, two or more different pigments may be used in the process of the present invention. Where two or more pigments are used these may be of the same colour or shade or they may have a different colour or shade to each other.

Preferably the pigment is not dispersible in an aqueous liquid medium without the aid of a dispersant, i.e. the presence of a dispersant is required to facilitate dispersion. Preferably, the pigment is not chemically surface treated, for example the pigment is preferably free from ionic groups covalently bonded to its surface (e.g. free from -CO2H and -SO3H groups).

Preferably the liquid medium is aqueous i.e. it is or comprises water. The aqueous liquid medium may optionally contain one or more water-miscible organic solvents.

When the liquid medium comprises a mixture of water and one or more water-miscible organic solvents, the weight ratio of water to the total amountof water-miscible organic solvents is preferably from 1 : 1 to 100:1 , more preferably from 2: 1 to 50: 1 and especially from 3:1 to 20: 1.

A preferred liquid medium comprises:

(a) from 50 to 100 parts, more preferably 75 to 100 parts water; and

(b) from 0 to 50 parts, more preferably 0 to 25 parts in total of one or more water-miscible organic solvents;

wherein the parts are by weight and the sum of the parts (a) and (b) = 100.

In one embodiment the only liquid in the liquid medium is water.

The liquid medium may contain further components in addition to the water and water-miscible organic solvents, for example biocides, surfactants, water- immiscible organic solvent(s), further dispersant(s) and so on.

The water-miscible organic solvent can be used to increase the solubility of the dispersant in the aqueous liquid medium. Preferably the liquid medium has a viscosity of less than 100 mPa.s, more preferably less than 50 mPa.s, when measured at 25°C.

Although the number of parts of a) and b) add up to 100 this merely fixes the amount of each of components a) and b) relative to each other and does not rule out the presence of further components other than a) and b) in the composition, for example one or more hydrophilic ethylenically unsaturated monomers having a hydrophilic non-ionic group may be present as component c).

The dispersant preferably has a WAMW of 1000 to 50,000 Da, more preferably 5,000 to 45,000 Da, especially 10,000 to 38,000 Da.

Preferably the said dispersant is obtained by copolymerising a monomer composition comprising components a) to c):

a) from 75 to 97 parts of one or more hydrophobic ethylenically unsaturated monomers comprising at least 50 parts benzyl (meth) acrylate; b) from 3 to 25 parts one or more hydrophilic ethylenically unsaturated monomers having one or more ionic group(s); and

c) from 0 to 2 parts of one or more hydrophilic ethylenically unsaturated monomers having a hydrophilic non-ionic group; and

wherein the parts are by weight and the sum of the parts a) to c) add up to 100.

The preference for the dispersant to be obtained by copolymerising a monomer composition comprising c) from 0 to 2 parts of one or more hydrophilic ethylenically unsaturated monomers having a hydrophilic non-ionic group can alternatively be expressed as the monomer composition being free from hydrophilic ethylenically unsaturated monomers having a hydrophilic non-ionic group or containing up to 2 parts in total of hydrophilic ethylenically unsaturated monomer(s) having a hydrophilic non-ionic group.

Preferred dispersants have graft, comb or star structures, more preferably a linear structure.

The dispersant is a copolymer. Preferred copolymers are block copolymers (e.g. its monomer units are distributed throughout the copolymer in blocks such as AAAA-BBBB), more preferably the dispersant is a random copolymer (e.g. its monomer units are distributed randomly/statistically throughout the copolymer).

The dispersion referred to in step I) of the present process optionally comprises two or more dispersants, where one or all of such dispersants are as defined above. This it is possible to utilise one or more dispersants defined in the statement of invention above and one or more further dispersants which are not as defined in the statement of invention above. Preferably all the dispersants used I step I) are as defined in the statement of invention above. Dispersants used in the process according to the first aspect of the present invention may be made by any suitable means. A preferred method is free radical polymerisation. Suitable free radical polymerisation methods include suspension, emulsion, dispersion and preferably solution polymerisation. Preferably the dispersant is prepared by the solution polymerisation of a monomer composition comprising components a), b) and c) (when present) in the presence of an aqueous or organic liquid carrier.

The term hydrophobic means more hydrophobic than the hydrophilic monomers in components b) and c) (when present). Preferably, the hydrophobic monomers have no hydrophilic groups, e.g. the hydrophobic ethylenically unsaturated monomers are free from ionic or non-ionic water-dispersing groups. For example, the hydrophobic ethylenically unsaturated monomersare preferably free from acidic groups and free from polyethyleneoxy groups.

Preferably the hydrophobic ethylenically unsaturated monomers have a calculated Log P value of at least 1 , more preferably from 1 to 6, especially from 2 to 6.

A review by Mannhold, R. and Dross, K. (Quant. Struct-Act. Relat. 15, 403- 409, 1996) describes 14 methods for calculating Log P values of compounds and especially drugs. From this review we prefer the "fragmental methods" and especially the fragmental method implemented by ACD labs software. The calculated Log P of a monomer may be calculated using commercially available computer software, for example using the Log P DB software version 7.04 or a later version of such software (which is available from Advanced Chemistry Development Inc (ACD labs)). Any ionic or ionisable groups are calculated in their neutral (unionised) form. A higher log P value corresponds to a more hydrophobic monomer. We have found the inclusion of such monomers aids in adsorbing the dispersant onto the pigment surface and in providing encapsulated pigment dispersions which when printed onto plain paper have good optical density.

Preferred hydrophobic ethylenically unsaturated monomers include styrenic monomers (e.g. styrene, alpha methyl styrene), aromatic (meth)acrylates

(especially benzyl (meth)acrylate), C-i-30-hydrocarbyl (meth)acrylates, butadiene, (meth)acrylates containing poly(C3_-4)alkylene oxide groups, (meth)acrylates containing alkylsiloxane or fluorinated alkyl groups and vinyl naphthalene.

Preferably the amount of hydrophobic ethylenically unsaturated monomer(s) comprising at least 50 parts benzyl (meth) acrylate used to make the dispersant is from 75 to 97 parts, more preferably from 77 to 97 parts, especially from 80 to 93 parts and most especially from 82 to 91 parts by weight.

Dispersants having the defined WAMW and comprising at least 50 parts of benzyl (meth)acrylate per 100 parts of monomers (i.e. dispersants containing at least 50wt% benzyl (meth)acrylate) provide encapsulated pigment dispersions with good stability and good OD when printed onto plain paper.

Component a) preferably comprises at least 60 parts, more preferably at least 70 and especially at least 80 parts by weight of benzyl (meth)acylate. The remainder required to obtain the overall preferred amounts of hydrophobic monomers may be provided by one or more hydrophobic monomers other than benzyl (meth)acrylate, e.g. selected from the hydrophobic ethylenically unsaturated monomers described above. Preferably the benzyl (meth)acrylate is benzyl methacrylate (rather than benzyl acrylate)

In a preferred embodiment the hydrophobic ethylenically unsaturated monomers used as component a) consist of benzyl (meth)acrylate, more preferably benzyl methacrylate.

Preferably, the hydrophilic ethylenically unsaturated monomers used as component b) have a calculated Log P value of less than 1 , more preferably from 0.99 to 2, especially from 0.99 to 0 and most especially from 0.99 to 0.5, when calculated in the un-neutralised (e.g. free acid) form.

Preferably the ionic groups present in the monomers in component b) are cationic or, more preferably, anionic.

Preferably, the hydrophilic ethylenically unsaturated monomers of component b) each have one or more anionic groups, more preferably one or more acidic anionic group.

Preferred acidic anionic groups include sulphonic acid, phosphonic acid and especially carboxylic acid. Acidic sulfates, phosphates and polyphosphates may also be used as the acidic anionic groups.

Thus, component b) preferably comprise one or more the hydrophilic ethylenically unsaturated monomers having one or more carboxylic acid groups.

Preferred hydrophilic ethylenically unsaturated monomers having one or more carboxylic acid groups (as ionic group) include beta carboxyl ethyl acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, more preferably acrylic acid and especially methacrylic acid. Preferably, these ethylenically unsaturated monomers having ionic group(s) when polymerised provide all of the ionic groups present in the dispersant.

In a preferred embodiment component b) is or comprises methacrylic acid. Preferably, the amount of hydrophilic ethylenically unsaturated monomers having one or more ionic group(s) used to make the dispersant is from 3 to 25 parts, more preferably from 3 to 23 parts, especially from 7 to 22 parts and most especially from 9 to 18 parts by weight. This is especially so when component b) comprises or more preferably is methacrylic acid.

For the purposes of the present invention a monomer having both ionic and non-ionic hydrophilic groups is considered to belong to component c). Thus, all the ethylenically unsaturated monomers in component b) are free from hydrophilic non-ionic groups.

The dispersant is preferably obtained from copolymerising the abovementioned components a) and b) in the specified amounts and from 0 to 2 wt% of hydrophilic ethylenically unsaturated monomers having a hydrophilic non- ionic group.

Preferably the dispersant is obtained by polymerisation of the aforementioned monomer composition comprising components a), b) and optionally c) in the presence of d) one or more chain transfer agents ("CTAs"), especially where at least one of the CTAs has one or more hydrophilic groups.

The use of CTAs during free radical polymerisation is known in the art and is described for example in Chapter 12 ('Control of Free-Radical Polymerization by Chain Transfer Methods' by Chiefari and Rizzardo; pp639 to 690) in the textbook "Handbook of Free-Radical Polymerization" Edited by Davis and Matyjaszewski (Wiley Interscience 2002, ISBN 0-471 -39274-X).

One function of the CTA is to allow control of the chain lengths during the synthesis which results in reduction of the molecular weight of the polymer obtained when compared to that made when no CTA is present.

Preferred CTAs contain one or more sulphur atoms. Such CTAs include thiols (including di- and higher-functional thiols), polysulfides (especially disulfides) and thioethers, thioesters and thiocarbamates.

Preferred CTAs have at least one, more preferably only one -SH (thiol) group.

Preferred CTAs have just one hydrophilic group. For CTAs a hydrophilic group is other than the chain transfer group. Thus for example -SH is not considered to be a hydrophilic group for a CTA.

The CTAs preferably have one or more hydrophilic groups selected from non-ionic, cationic and especially anionic groups. The possible hydrophilic groups are as previously described for the monomers.

Preferred hydrophilic anionic group(s) on the CTA includes sulfonic acid, phosphonic acid and especially carboxylic acid group(s).

Preferred hydrophilic non-ionic group(s) on the CTA include hydroxy and polyethyleneoxy.

Preferred CTAs having one or more hydrophilic groups include:

i) mercapto acids, preferably thioglycolic acid, mercaptoundecanoic acid, thiolactic acid, thiobutyric acid, thiomalic acid, thiomalonic acid, thioadipic acid, 2-mercapto ethane sulfonic acid, and especially 3- mercapto propionic acid; ii) mercapto alcohols, preferably 2-mercaptoethanol, mercaptopropanol, mercapto butanol, 3-mercapto-1 ,2-propanediol, thioglycerol, ethylene glycol mono thio glycolate;

iii) mercapto amines and especially cysteine.

Of all of the above a preferred CTA having one or more hydrophilic groups is 3-mercaptopropionic acid.

Preferably, the CTA is not acrolein or (meth)acrolein.

In some cases component d) (the CTA) may contain one or more CTA having no hydrophilic groups (of course other than the chain transfer group itself). These may be simply referred to as hydrophobic CTAs. When present preferred "hydrophobic" CTAs include mercaptans, e.g. octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-hexadecyl mercaptan, n-tetradecyl mercaptan and t-tetradecyl mercaptan, butyl 3-mercaptopropionate; xanthogenndisulfides, e.g. dimethyl xanthogenndisulfide, diethyl xanthogenndisulfide and diisopropyl xanthogenndisulfide; thiuram disulfides, e.g. tetramethyl thiuram disulfide, tetraethyl thiuram disulfide and tetrabutyl thiuram disulfide; halogenated hydrocarbons, e.g. carbon tetrachloride and ethylene bromide; hydrocarbons e.g. pentaphenylethane; unsaturated cyclic hydrocarbon compounds such as, 2- ethylhexyl thioglycolate, terpinolene, alpha terpinene, gamma terpinene, diterpene, alpha methylstyrene dimer, 9, 10-dihydroanthracene, 1 ,4-dihydronaphthalene, indene and 1 ,4-cyclohexadiene; unsaturated heterocyclic compounds such as xanthene and 2,5-dihydrofuran; certain transition metal chelate compounds e.g. low-spin bis(glyoximato) cobalt(ll) complexes; and the like. These hydrophobic CTAs can be used alone or in admixture of at least two kinds.

It is preferred that in component d) the ratio of the total number of moles of the CTAs having one or more hydrophilic groups to the total number of moles of all the CTAs having no hydrophilic groups is 1 : 1 , more preferably 5: 1 , especially 10: 1 and it is especially preferred that component d) comprises only CTAs which have one or more hydrophilic groups. In this way all the CTA that binds to the copolymer provides CTA groups which have hydrophilic groups. Again, the chain transfer groups are not counted as hydrophilic groups.

The optimum total amounts of CTA(s) to be used in a composition will depend on the magnitude of molecular weight desired, the chemical structure(s) of the CTA(s) used and their reactivity towards the monomers present in the composition. One may determine the optimum amount of total CTA(s) to be used experimentally in order to obtain a polymer of desired WAMW, e.g. by first polymerising the composition without any CTA present, then repeating the polymerisation using increasing amounts of CTA. The WAMW of each polymer can then measured and the data used to generate an (x,y) plot of the molar amount of CTA used against the WAMW obtained: the resulting line can then be used to ascertain how much CTA is required in order to obtain a polymer with a particular value of WAMW.

The optional CTA d) may be present in the composition used to form the dispersant in an amount from 0.1 to 15%, especially from 1 to 10% and most especially from 1 to 5% by weight based on the total weight of polymerisable components present in the composition.

Preferably the monomer composition further comprises e) an initiator, especially a free-radical initiator. Preferably the initiator is thermally activated, i.e. a thermal initiator.

Examples of free radical initiators include azo compounds such as 2,2'- azobis(isobutyronitrile), 2,2'-azobis-(2-methyl)butanenitrile,4,4'-azobis(4- cyanovaleric acid), 2-(t-butylazo)-2-cyanopropane, and 2,2'-azobis[2-methyl-N- hydroxyethyl)]-propionamide. Other soluble free radical initiators may also be used, examples of which include peroxy compounds such as benzoyl peroxide, lauroyl peroxide, hydrogen peroxide, and sodium, potassium and ammonium persulphates. Redox initiator systems can also be used, examples of which include redox pairs such as ammonium persulphate and sodium metabisulphite.

The initiator e) may be present in the composition used to form the dispersant in an amount from 0.1 to 15%, especially from 1 to 10% and most especially from 1 to 7% by weight based on the total weight of polymerisable components present in the composition. When the dispersant is obtained from a composition containing a large amount of CTA it is also sometimes advantageous to also increase the levels of initiator.

Preferably, one or more of the monomers in optional component c) have calculated Log P values of less than 1 , more preferably from 0.99 to 2 .

The presence of 2 parts or less (2wt% or less) of the monomers defined in component c) can result in improved OD when inks derived from the dispersants are printed on plain paper.

Preferably the monomer composition comprises less than 1 part, more preferably less than 0.5 parts, especially less than 0.1 parts and most especially 0 parts of hydrophilic ethylenically unsaturated monomers having a hydrophilic non- ionic group.

Examples of hydrophilic non-ionic groups include polyethyleneoxy, polyacrylamide, polyvinyl pyrrolidone, hydroxy functional celluloses and poly vinyl alcohol. The most common ethylenically unsaturated monomer having a hydrophilic non-ionic group is polyethyleneoxy (meth) acrylate.

In embodiments where the residue of component c) is present in the dispersant (for example 2 parts by weight of component c) then in one embodiment the amount of component c) is deducted from the preferred amounts of component a). In this way the amounts of all the components a) to c) still adds to 100. Thus for examples where 2 parts by weight of component c) is present the preferred amounts of component a) expressed above would become from 73 to 95 (i.e. 75-2 to 97-2), more preferably from 75 to 95 (i.e. 77-2 to 97-2), especially from 78 to 91 (i.e. 80-2 to 93-2) and most especially from 80 to 89 (i.e. 82-2 to 91 -2) parts by weight of component a). In an another embodiment it is possible to deduct the amount of component c) from the preferred amounts of component b) so that again the sum of the amounts of components a) to c) adds to 100 parts by weight.

In view of the foregoing a preferred dispersant is obtained by copolymerising a monomer composition comprising the ethylenically unsaturated monomers a) to c):

a) from 75 to 97 parts of one or more hydrophibic ethylenically unsaturated monomers comprising at least 50 parts of benzyl methacrylate;

b) from 3 to 25 parts of methacrylic acid; and

c) 0 parts of hydrophilic ethylenically unsaturated monomers having a hydrophilic non-ionic group;

wherein the parts are by weight and the sum of the parts a) to c) add up to 100.

When the components used to make the dispersant comprise a CTA the dispersant will typically comprise residues of the CTA.

It is preferred that the only hydrophobic ethylenically unsaturated monomer in component a) is benzyl methacrylate.

More preferably the dispersant is obtained by copolymerising a monomer composition comprising the ethylenically unsaturated monomers a) to c):

a) from 80 to 93 parts of one or more hydrophobic ethylenically unsaturated monomers comprising at least 50 parts benzyl methacrylate;

b) from 7 to 22 parts of methacrylic acid;

c) 0 parts of hydrophilic ethylenically unsaturated monomers having a hydrophilic non-ionic group

wherein the parts are by weight and the sum of the parts a) to c) add up to 100.

Preferably, the dispersant has at least 0.35mmoles, more preferably at least 0.9mmoles, even more preferably at least 1 .15mmoles and especially at least 1 .3mmoles of ionic groups per g of dispersant.

Preferably, the dispersant has in order of increasing preference no more than 2.65mmoles, 2.3mmoles, 2.15mmoles, 2.0mmoles, 1 .75mmoles and 1 .40 mmoles of ionic groups per g of dispersant. In one embodiment the dispersant comprises 1 .2 to 2.0, more preferably 1 .3 to 1 .7 mmoles of ionic groups per g of dispersant.

Preferred dispersants have, for example, from 0.9 to 2.65mmoles, especially from 1 .0 to 2.3mmoles and most preferably from 1 .0 to 2.0mmoles in total of ionic groups per gram of dispersant. Such dispersants work particularly well in the present invention and can be used to provide pigment inks which offer particularly good optical density on plain paper and have good stability.

The amount of ionic groups may be established by any suitable method a preferred method is a titrimetric method, for example acid/base titration.

Preferably, all the ionic groups present in the dispersant are anionic

(especially acidic). It is especially preferred that all the ionic groups present in the dispersant are selected from -CO2H, -SO3H and -PO3H2 groups and salts thereof. Most preferably, all the ionic groups present in the dispersant are -CO2H groups or a salt thereof. When all the ionic groups are -CO2H groups or a salt thereof the dispersant can be used to prepare inks having particularly good optical density on plain paper. Thus, it is preferred that the above amounts of mmoles of ionic groups corresponds directly with the preferred amounts of mmoles of carboxylic acid groups present in the dispersant.

The dispersant optionally contains one or more groups which enable the dispersant to self-cross-link in step II).

In one embodiment the dispersant comprises ethylenically unsaturated groups (especially vinyl groups). Such groups enable the dispersant to self- crosslink in step II) e.g. in the presence of an initiator (especially a free radical initiator).

In another embodiment the dispersant can be self cross-linked using one or more ionic group(s) (as described in component b) and one or more groups which cross-link with the ionic group(s). For example, the dispersant may comprise carboxylic acid ionic groups and epoxy groups, thereby creating a self- crosslinkable dispersant.

The self cross-linking reaction is preferably performed by heating the dispersion.

The dispersant is preferably adsorbed onto the pigment when step II) is performed.

Although it is possible that the dispersant chemically bonds to pigment surface this is not preferred.

Preferably the process further comprises the step of preparing the dispersant by copolymerising said monomer composition in the absence of a pigment (i.e. preferably the monomer composition is free from pigments). Preferably the dispersion in step I) has, in order of increasing preference, a sodium chloride critical coagulation concentration (CCC) of no more than 2.0M, no more than 1 .8M, no more than 1 .6M, no more than 1 .4M, no more than 1 .2M, no more than 1 .0M and no more than 0.8M.

Preferably the dispersion in step I) has a CCC of at least 0.1 M, more preferably at least 0.25M and especially at least 0.35M.

In preferred embodiments the CCC of the dispersion in step I) is from 0.1 to 2.0M, more preferably from 0.10 to 1 .8M, even more preferably from 0.20 to 1 .6M and especially from 0.30 to 0.8M.

The CCC is preferably measured by performing the following steps in the order i) to v):

i) adjusting the concentration of pigment in the dispersion referred to in step I) to 10% by weight by adding or removing water;

ii) preparing a test sample by mixing two drops of the adjusted dispersion prepared in step i) and 1 .5g of a solution of sodium chloride in water having a molarity of 0.5M;

iii) storing the test sample prepared in step ii) for 24 hours at a temperature of 25°C;

iv) visually assessing the test sample to see if there is significant precipitation at the bottom of the sample;

v) repeating steps i) to iv) using sodium chloride solutions of higher or lower molarity, until the lowest molarity of the sodium chloride solution is established at which the visual assessment referred to in step iv) reveals a significant precipitation at the bottom of the sample, this molarity being the CCC.

By significant precipitation we mean most or all of the pigment initially present in the test sample has precipitated. Mere traces of precipitate pigment are not regarded as being significant precipitation. By using gravimetric or light transmittance methods it is possible to more accurately measure the degree of precipitation, however, for most purposes a visual assessment is sufficiently accurate and reliable.

In step v) we have found that using sodium chloride solutions of higher or lower molarity to the extent of, for example, 0.05M or 0.1 M, will generally be suitable, depending on the accuracy required.

Experimentally, it is often expedient to simply prepare a large number of samples each having a different concentration of sodium chloride in order to quickly establish the CCC.

We have found that dispersions having the abovementioned CCC values tend to provide prints on plain paper having good optical density. The pigment in the dispersion referred to in step I) is preferably in the form of particles having an average particle size of no more than 1 micron, more preferably from 10 to 1000nm, especially from 50 to 500nm and most especially from 50 to 300nm. The average particle size is preferably measured by a light 5 scattering technique. Preferably the average particle size is a Z-average or volume average size.

Preferably the pH of the dispersion in step I) is from 5 to 12, more preferably from 7 to 1 1 .

In step II) the dispersant may be self-cross-linked, cross-linked using a 10 cross-linking agent or a combination of the two. In any case it is preferred that the cross-linking reaction links the dispersant molecules by covalent bonds.

The cross-linking reaction utilises any of the pairs of groups described in PCT patent publication WO 2005/061087 at page 6, Table 1 wherein "reactive groups in the compound" in column 2 can be read as reactive groups in the cross- i s linking agent.

Preferred cross-linking agents include those having isocyanate, aziridine, n- methylol, carbodiimide, oxetane, oxazoline and especially epoxy groups. These reactive groups are particularly useful for cross-linkingh dispersants comprising one or more carboxylic acid groups. A preferred cross-linking agent has epoxy 20 groups preferably as the sole crosslinkable groups) no other cross-linking groups.

In a preferred embodiment the cross-linking in step II) is effected by an epoxy cross-linking agent and component b) is or comprises one or more hydrophilic ethylenically unsaturated monomers having one or more carboxylic acid groups.

25 Preferably, the cross-linking in step II) is performed by a process comprising heating the dispersion (preferably in the presence of a cross-linking agent), preferably to a temperature in the range 40 to 100°C. To accelerate or promote the cross-linking reaction it is sometimes useful to perform cross-linking in the presence of a catalyst.

30 The pH of the dispersion used in step II) is preferably from 5 to 13, especially from 7 to 12.

When the cross-linking in step II) is performed in the presence of a crosslinking agent comprising epoxy groups it is preferred that the crosslinking is performed in the presence of a borate salt and/or boric acid.

35 Preferably, the cross-linking in step step II) is performed by a process comprising mixing a composition comprising the following components in the specified proportions:

(a) 30 to 99.7 parts, preferably 50 to 97 parts, of the liquid medium;

(b) 0.1 to 50 parts, preferably 1 to 30 parts, of the pigment; (c) 0.1 to 30 parts, preferably 1 to 30 parts, of the dispersant; and

(d) 0.001 to 30 parts, preferably 0.01 to 10 parts, of a cross-linking agent; wherein the parts are by weight.

Preferably, the encapsulated pigment dispersion resulting from the process of the present invention has a CCC of no more than 2.0M. The CCC of the encapsulated pigment dispersion resulting from the process of the present invention is preferably from 0.1 to 2.0M, more preferably from 0.10 to 1 .8M, especially from 0.20 to 1.6M and most preferably from 0.30 to 1 .0M.

The process according to first aspect of the present invention may additionally comprise the step of removing some or all of the liquid medium from the resultant pigment dispersion. The liquid medium may be removed by methods such as evaporation and filtration. In this way the pigment dispersion may be concentrated or converted into the form of a dry solid. When the liquid medium comprises a mixture of water and a water-miscible organic solvent it may be desirable to selectively remove the water-miscible organic solvent. This may be performed by for example distillation or by membrane treatment.

Preferably the process according to the first aspect of the present invention further comprises the step of purifying the encapsulated pigment dispersion. Preferably, the purification process is performed after step II). The purification can be by any suitable method including microfiltration, deionizer resins, centrifugation followed by decantation and washing. A preferred purification method is membrane filtration especially ultrafiltration using an ultrafiltration membrane. Preferred ultrafiltration membranes have a pore size of about 0.1 microns. Preferably, the dispersion after step II) is washed with from 5 to 50 volumes of purified water based on the volume of the dispersion. Preferably, the water used in the ultrafiltration process is deionized, distilled or has been purified by reverse osmosis. A preferred method of assessing when the dispersion has been sufficiently purified is to measure the conductivity of the permeate stream from the ultrafiltration stage and to continue adding further volumes of pure water until the permeate stream has a conductivity of less than Ι ΟΟμβ/αη, more preferably less than δθμβ/αη. The ultrafiltration is preferably performed on a dispersion which has from 10 to 15% by weight of pigment in the dispersion. We have found that purification can in some instances provide final dispersions and inks having further improved OD when printed onto plain paper.

It is preferred that the process of the present invention further comprises adding one or more additives to the encapsulated pigment dispersion, preferably selected from viscosity modifiers, pH buffers, metal chelating agents, surfactants, corrosion inhibitors, biocides, dyes, water miscible organic solvent(s) and/or kogation reducing additives. Preferably any additives are added after step II). According to a second aspect of the present invention there is provided a dispersion comprising an encapsulated pigment obtained or obtainable by the process according to the first aspect of the present invention.

Preferably the dispersion comprises water and 5 to 60wt%, more preferably 10 to 40wt%, especially 12 to 30wt%, of the encapsulated pigment obtained or obtainable by the process according to the first aspect of the present invention.

The encapsulated pigment dispersion according to the second aspect of the present invention and the process according to the first aspect of the present invention may be used to prepare an ink, especially an ink jet printing ink. Thus a third aspect of the present invention provides an ink jet printing ink comprising an encapsulated pigment dispersion according to the second aspect of the present invention. Preferably, the dispersion according to the second aspect of the present invention and/or the ink according to the third aspect of the present invention have a viscosity of less than 50 mPa.s, more preferably less than 30 mPa.s and especially less than 15 mPa.s, when measured at a temperature of 25°C.

Preferably, the ink has a surface tension of 20 to 65 dynes/cm, more preferably 30 to 60 dynes/cm, when measured at a temperature of 25°C.

The pH of the ink is preferably from 4 to 1 1 , more preferably from 7 to 10. When the ink is to be used as in ink jet printing, the ink preferably has a concentration of halide ions of less than 500 parts per million, more preferably less than 100 parts per million. It is especially preferred that the ink has less than 100 parts per million, more preferably less than 50 parts per million of divalent and trivalent metals. Parts per million as used above refers to parts by weight relative to the total weight of the ink. These low concentrations of ions in the resultant ink can be achieved by the abovementioned purification step.

Preferably the process for making the ink includes a step for removing particles having any particulate matter from the dispersion or the ink having a diameter of more than 1 micron, for example by filtration or centrifugation. Preferably less than 10%, more preferably less than 2% and especially less than

1 % by weight of particles present in the dispersion or the ink have a diameter greater than 1 micron.

Preferably the amount of pigment in the ink is from 0.1 to 15%, more preferably from 1 to 10% and especially from 3 to 10% by weight relative to the total weight of the ink.

Preferably the ink contains water and organic solvent in the weight ratio of 99:1 to 1 :99, more preferably 99: 1 to 50:50 and especially 95:5 to 70:30.

Preferred organic solvents are water-miscible organic solvents and mixtures of such solvents. Preferred water-miscible organic solvents include d-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; water-miscible ethers, preferably tetrahydrofuran and dioxane; diols, preferably diols having from 2 to 12 carbon atoms, for example pentane-1 ,5-diol, ethylene glycol, propylene glycol, butylene glycol, pentylene glycol, hexylene glycol and thiodiglycol and oligo- and poly-alkyleneglycols, preferably diethylene glycol, triethylene glycol, polyethylene glycol and polypropylene glycol; triols, preferably glycerol and 1 ,2,6-hexanetriol; mono-C-i-4-alkyl ethers of diols, preferably mono-Ci-₄-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)ethoxy]ethanol, 2-[2-(2- ethoxyethoxy)-ethoxy]-ethanol and ethyleneglycol monoallylether; cyclic amides, preferably 2-pyrrolidone, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, caprolactam and 1 ,3-dimethylimidazolidone; cyclic esters, preferably caprolactone; sulphoxides, preferably dimethyl sulphoxide and sulpholane. Preferably, the liquid medium comprises water and 2 or more, especially from 2 to 8, water-miscible organic solvents.

Especially preferred water-miscible organic solvents for the ink are cyclic amides, especially 2-pyrrolidone, N-methyl-pyrrolidone and N-ethyl-pyrrolidone; diols, especially 1 ,5-pentane diol, ethyleneglycol, thiodiglycol, diethyleneglycol and triethyleneglycol; and mono- Ci_-4-alkyl and di- Ci_-4-alkyl ethers of diols, more preferably mono- Ci_-4-alkyl ethers of diols having 2 to 12 carbon atoms, especially 2-methoxy-2-ethoxy-2-ethoxyethanol.

Examples of further suitable ink media comprising a mixture of water and one or more organic solvents are described in US 4,963,189, US 4,703,1 13, US 4,626,284 and EP 4,251 , 50A.

The ink jet printing ink can be included in the chamber of an ink jet printer cartridge to provide an ink jet printer cartridge containing the ink of the present invention.

The process of the present invention can be used to prepare encapsulated pigment dispersions which are particularly suitable for use in an ink jet printing inks. In addition the encapsulated pigment dispersions may be used in other inks, paints, tints, cosmetics, thermoplastics and thermosets.

According to a fourth aspect of the present invention there is provided the use of the process according to the first aspect of the present invention for preparing an ink jet printing ink. Preferably this use is for the purpose of providing an ink jet printing ink which has good and/or improved storage stability, e.g. an ink jet printing ink which demonstrates no sedimentation when stored at 60°C for 4 weeks. The storage stability can be determined by measuring how the viscosity of the ink varies over time, e.g. at elevated temperatures. Low viscosity change indicates high storage stability. Also a slow or no increase in average particle size over time also indicates good storage stability.

The ink jet printing inks containing encapsulated pigment dispersions prepared by the process of the present invention may in some embodiments be used with papers which comprise fixing agents to improve, for example, wet fastness, optical density or to reduce colour bleeding. In another embodiment ink jet printing inks containing encapsulated pigment dispersions prepared by the process of the present invention may be used with jettable fixing agents. For example, an ink jet printer cartridge might comprise an ink as described above in one chamber and a liquid comprising a fixing agent in a further chamber. In this way the ink jet printer may apply the ink and the fixing agent to a substrate.

Fixing agents are well known in the art and include such things as metal salts, acids and cationic materials.

The invention is further illustrated by the following Examples in which all parts and percentages are by weight unless otherwise stated. "Parts" and "parts of" mean the number of parts of the relevant component. Examples

WAMW Measurement

The WAMW were measured using a Waters GPC apparatus comprising a 2695 Separations Module, a 2414 Refractive Index Detectro unit and two x Plgel Mixed D 5 micron 300 mm x 7.5 mm columns (obtained from Agilent Technologies) fitted with a guard column (PLgel 5pm Guard 50 x 7.5 mm) using an eluent (DMF(GPC grade) containing 1wt% each of acetic acid and triethylamine). The flow rate used was 1 .0 mL/min and the injection volume was in microlitres. Samples were prepared directly from the polymer solutions as follows: about 50 mg of the polymer solution under evaluation at was accurately (on a four-place analytical balance) weighed in to a suitable glass vial (weight taken was adjusted according to the polymer concentration in the solution so that weight of the polymer in the sample was about 20 mg). To this was added freshly prepared eluent as solvent (10ml, having the composition described above) and the vial was left for a minimum of six hours at ambient temperature (18 to 25°C) to allow for complete dissolution of the sample, thus providing a solution for analysis having a polymer concentration of about 2.0 mg polymer per ml_ solvent. Once dissolved the sample solutions for analysis were passed through a disposable 0.2 micron PFTE Autovial™ syringless filter device (obtained from GE Healthcare) into a 2 ml_ sample vial which was sealed (crimp-capped) immediately after filling. The samples were run in duplicate (100 microlitre injection) using a flow rate of 1 .0 mL/min and a column oven temperature of 50°C. For each sample, duplicate injections were run in direct succession for comparative purposes. At the beginning and the end of each sample run set an injection of EasiCal® PS-2 (0.58 to 400 kDa range; one injection each of the A and B polystyrene sample mixtures) standards (obtained from Agilent Technologies) were run, that is to say the first and last samples in any set of analyses were injections from the reference standard samples of known WAMW and narrow poly dispersities. Raw data was processed/manipulated using the Waters Empower 2 software package.

Particle Size Measurement

The Z-average particle size of the dispersions at the end of milling were determined by dynamic light scattering using a Malvern ZS90 Zetasizer instrument. The dispersions were diluted with HPLC Grade water to a pigment concentration of approximately 0.0005% by weight before being added to disposable cuvette for the analysis. Instrument settings were as follows:

Measurement type = size

Material = pigment type used

Dispersant = water

General options - default (Mark - Houwink)

Temperature = 25°C with 2 minute equilibration time.

Cell = DT0012

Measurement = automatic number of measurements = 3

Data processing model - general purpose (normal resolution)

Two samples were made for each dispersion and a standard sample

(Nanosphere™ Size Standards, 200nm; available from Fisher) was run with each set of measurements carried out.

Viscosity Measurement

The viscosity measurements described in Table 2 below were measured using a Brookfield LV-DVII viscometer and a Brookfield Viscosity Standard, ~5cP. The viscosity of the inks were measured 'as is'. The instrument settings were as follows:

Temperature = 25°C +/- 1 °C with 30 minute equilibration time.

Ultra-low adaptor (ULA) spindle.

The spindle was rotated for 15 minutes at a speed of 60 revolutions per minute before measuring the viscosity at the same rotation speed. The torque range for measurements was 10-100% and the measurements were made in duplicate. The averages of 2 readings of within +/- 0.2cP are included in Table 2. Sedimentation Test

The sedimentation test referred to in Table 3 below was performed as follows: Each ink under evaluation was sealed in transparent, plastic flat-bottomed bottle and stored in an oven at 60 °C for 4 weeks. The bottles were then removed from the oven and turned upside down to stand on their lids. After cooling overnight each bottle was then turned upright and the lid was immediately removed then visually inspected to determine the presence of any sediment which indicates that the ink formulation was not stable. If sediment was present the ink was scored "Yes" (poor storage stability). If there was no sediment present the ink was scored "No" (good storage stability).

The following chemical abbreviations are used below:

TRB2 is a C. I. Pigment Blue 15:3 from Dainichiseika.

Nipex 170 is a C. I. Pigment Black 7 from Orion.

Yellow 7413 is C. I. Pigment Yellow 74 from Sanyo.

EX-321 is Denacol™ EX-321 obtained from Nagase ChemteX, with weight per epoxy of 140.

1 . Preparation of Dispersants

Dispersant (1 ) (WAMW 26,209 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate

(612.3 parts), methacrylic acid (167.7 parts), 3-mercaptopropionic acid (15.6 parts) and dipropylene glycol (288.8 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (13.8 parts) and dipropylene glycol (172.5 parts). Dipropylene glycol (752.7 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively. The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C. The resultant polymer was designated as Dispersant (1 ) and had the following properties:

- number average molecular weight ("NAMW") of 16,910 Da;

- WAMW of 26,209 Da;

- polydispersity of 1 .55;

- acid value corresponding to about 2.59 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 78.5:21 .5 relative to each other by weight.

Dispersant (2) (WAMW 31 ,625 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate

(572.2 parts), methacrylic acid (84.8 parts), n-butyl 3-mercaptopropionate (15.3 parts) and dipropylene glycol (243.7 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (10.8 parts) and dipropylene glycol (145.6 parts).

Dipropylene glycol (635.2 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively.

The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Dispersant (2) and had the following properties:

- NAMW of 19,709 Da;

- WAMW of 31 ,625 Da;

- polydispersity of 1 .60;

- acid value corresponding to about 1 .44 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 87.1 : 12.9 relative to each other by weight. Dispersant (3) (WAMW 22,078 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate (544 parts), methacrylic acid (1 13 parts), n-butyl 3-mercaptopropionate (21 .6 parts) and dipropylene glycol (246.2 parts). An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (1 1 .2 parts) and dipropylene glycol (147 parts).

Dipropylene glycol (641 .5 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively. The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Dispersant (3) and had the following properties:

- NAMW of 14,664 Da;

- WAMW of 22,078 Da;

- polydispersity of 1 .51 ;

- acid value corresponding to about 1 .90 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 82.8: 17.2 relative to each other by weight.

Dispersant (4) (WAMW 35,659 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate (368.28 parts), methacrylic acid (54.54 parts), 3-mercaptopropionic acid (7.61 parts) and dipropylene glycol (171 .08 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (6.93 parts) and dipropylene glycol (103.22 parts).

Dipropylene glycol (401.75 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively. The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C. The resultant polymer was designated as Dispersant (4) and had the following properties:

- NAMW of 23, 150 Da;

- WAMW of 35,659 Da;

- polydispersity of 1 .54;

- acid value corresponding to about 1 .61 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 87.1 : 12.9 relative to each other by weight.

Dispersant (5) (WAMW 10,336 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate

(202.25 parts), methacrylic acid (47.75 parts), 3-mercaptopropionic acid (15.06 parts) and dipropylene glycol (96.1 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (4.33 parts) and dipropylene glycol (28.1 parts).

Dipropylene glycol (250.5 parts) was heated to 95°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 95°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively.

The reactor vessel contents were maintained at 95°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Dispersant (5) and had the following properties:

- NAMW of 7,279 Da;

- WAMW of 10,336 Da;

- polydispersity of 1.42;

- acid value corresponding to about 2.59 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 80.9: 19.1 relative to each other by weight. Dispersant (6) (WAMW 40,597 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate (679.4 parts), methacrylic acid (100.6 parts), 3-mercaptopropionic acid (14 parts) and dipropylene glycol (287.9 parts). An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (12.8 parts) and dipropylene glycol (172 parts).

Dipropylene glycol (750.3 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively. The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Dispersant (6) and had the following properties:

- NAMW of 25,701 Da;

- WAMW of 40,597 Da;

- polydispersity of 1 .58;

- acid value corresponding to about 1 .61 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 87.1 : 12.9 relative to each other by weight.

Comparative Dispersant (1 ) (WAMW 82,400 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate (672.2 parts), methacrylic acid (184.3 parts), butyl 3-mercaptopropionate (5.55 parts) and dipropylene glycol (327.4 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (15.1 parts) and dipropylene glycol (197.1 parts).

Dipropylene glycol (81 1 .1 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively. The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Comparative Dispersant (1 ) and had the following properties:

- NAMW of 47,535 Da;

- WAMW of 82,400 Da;

polydispersity of 1 .73; acid value corresponding to about 2.50 mmoles of acid groups/g of dispersant; and

benzyl methacrylate and methacrylic acid in the proportions 78.5:21 .5 relative to each other by weight.

Comparative Dispersant (2) (WAMW 68, 105 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate

(723 parts), methacrylic acid (150.6 parts), butyl 3-mercaptopropionate (5.76 parts) and dipropylene glycol (318.6 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (14.88 parts) and dipropylene glycol (190.2 parts).

Dipropylene glycol (830.4 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively.

The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Comparative Dispersant (2) and had the following properties:

- NAMW of 40,067 Da;

- WAMW of 68, 105 Da;

- polydispersity of 1 .70;

- acid value corresponding to about 2.00 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 82.8: 17.2 relative to each other by weight. Comparative Dispersant (3) (WAMW 67, 177 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate (215.5 parts), methacrylic acid (34.5 parts), 3-mercaptopropionic acid (0.99 parts) and dipropylene glycol (91 .0 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (4.13 parts) and dipropylene glycol (54.4 parts).

Dipropylene glycol (237.3 parts) was heated to 95°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 95°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively. The reactor vessel contents were maintained at 95°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Comparative Dispersant (3) and had the following properties:

- NAMW of 37,324 Da;

- WAMW of 67, 177 Da;

- polydispersity of 1 .80;

- acid value corresponding to about 1 .61 mmoles of acid groups/g of dispersant; and

- benzyl methacrylate and methacrylic acid in the proportions 86.2: 13.8 relative to each other by weight. Comparative Dispersant (4) (WAMW 97,718 Da)

A monomer feed composition was prepared by mixing benzyl methacrylate (215.5 parts), methacrylic acid (34.5 parts), 3-mercaptopropionic acid (0.99 parts) and dipropylene glycol (91.0 parts).

An initiator feed composition was prepared by mixing tert-butyl peroxy 2- ethyhexanoate (4.13 parts) and dipropylene glycol (54.4 parts).

Dipropylene glycol (237.3 parts) was heated to 85°C in a reactor vessel, continuously stirred and purged with a nitrogen gas atmosphere. The monomer feed and the initiator feed compositions were slowly fed into the reactor vessel whilst stirring the contents, maintaining the temperature at 85°C and maintaining the nitrogen atmosphere. The monomer feed and the initiator feed were both started simultaneously but fed into the reactor over 4 and 5 hours respectively. The reactor vessel contents were maintained at 85°C while the feeds were added and then held at this temperature for a further 2 hours before cooling to 25°C.

The resultant polymer was designated as Comparative Dispersant (4) and had the following properties:

- NAMW of 52,445 Da;

- WAMW of 97,718 Da;

- polydispersity of 1 .86;

- acid value corresponding to about 1 .61 mmoles of acid groups/g of dispersant; and

benzyl methacrylate and methacrylic acid in the proportions 86.2:13.8 relative to each other by weight.

2. Preparation of Dispersant solutions The Dispersants (1 ) to (6) and Comparative Dispersants (1 ) to (4) described above (in an amount containing 300 parts polymer by weight) were each independently diluted with dipropylene glycol (about 150 parts) and the resultant solutions were each neutralised with 50wt% aqueous potassium hydroxide solution to provide homogeneous aqueous solutions having a pH in the range 8 to 9 and were made up with water where necessary to give 1000 parts in total of dispersant solution. This resulted in Dispersant Solutions (1 ) to (6) and Comparative Dispersant Solutions (1 ) to (4) respectively which each contained approximately 300 parts (approximately 30wt%) of the relevant dispersant.

3. Preparation of Mill-bases and Comparative Mill-bases

Pigment powder and the relevant Dispersant solution were mixed together in the amounts shown in Table 1 to form pre-mixtures. Water (to provide a suitable viscosity for mixing and milling) was added to each pre-mixture as required to achieve the milling strength (wt% pigment) indicated in Table 1 .

The resultant mixtures were mixed thoroughly using a high shear mixer for 90 minutes. After mixing the mixtures were transferred to a horizontal bead mill containing 1 mm beads. The mixtures were then milled until the particles present in the dispersions had the desired Z-average particle size. (In Table 1 , Comparative Mill-base 4 had a high Z-average and a high Milling Specific Energy. This reflects the fact that after extensive milling it was not possible to reduce the particle size down to a similar size to the other Examples. The higher particle size of the pigment in Comparative Mill-base CMB4 indicates that Comparative Dispersant Solution 4 was not able to fully stabilise the pigment dispersion).

The Milling Specific Energy referred to in Table 1 represents the total energy consumed per kilogram of 100% pigment (kW.h/kg) during the course of milling of the pigment to the desired particle size; it is defined as the electrical power (kW) drawn by the mill motor, multiplied by the total time (h) taken to mill the pigment, divided by the mass (kg) of 100% pigment milled in that time.

The resultant mill-bases MB1 to MB4 and Comparative Mill-bases CMB1 to CMB4 were discharged from the mill.

4. Cross-linking the dispersant to prepare the Encapsulated Pigment Dispersions

The Mill-bases MB1 to MB4 and Comparative Mill-bases CMB1 to CMB4 described above were adjusted to a pigment content of about 10% by weight by the addition of water and boric acid solution (3 wt% in water). The boric acid solution acted as a pH buffer. The dispersants in each of the mill-bases MB1 to MB4 and Comparative Mill-bases CMB1 to CMB4 were then cross-linked by heating with trimethylolpropane polyglycidyl ether (Denacol™ EX-321 ) at 70°C for 6 hours. This cross-linked the carboxylic acid groups in the dispersants and thereby encapsulated the pigment.

The crosslinking of the mill-bases MB1 to MB4 and Comparative Mill-bases CMB1 to CMB4 resulted in, respectively, Encapsulated Pigment Dispersions 1 to 4 ("ECPD1 " to "ECPD4") and Comparative Encapsulated Pigment Dispersions 1 to 4 ("Comparative ECPD1 " to " Comparative ECPD4").

5. Purification of ECPD1 to EPCD4 and Comparative ECPD1 to Comparative to EPCD4 by Ultrafiltration

ECPD1 to EPCD4 and Comparative ECPD1 to Comparative to EPCD4 were each purified by means of ultrafiltration using membrane having a 0.1 micron pore size. The encapsulated pigment dispersions were diafiltered with approximately 10 to 40 wash volumes of pure deionized water per 1 volume of the encapsulated pigment dispersion. The ultrafiltration membrane was then used to concentrate the encapsulated pigment dispersions back to a pigment content of around 10 to 15% by weight to give Purified ECPD1 to EPCD4 and Purified Comparative ECPD1 to Purified Comparative EPCD4, as indicated in Table 1 below:

Table 1 :

Purified Purified Purified Purified Purified

Encapsulated ECPD 1 ECPD 2 ECPD 3 ECPD 4

Dispersion

Mill-base MB1 MB2 MB3 MB4

Pigment TRB2 Nipex 170 Yellow 7413 Yellow 7413

Dispersant Dispersant 3 Dispersant 5 Dispersant 6 Dispersant 4

Parts of Pigment 100 100 100 100

Parts of 150 133 133 133

Dispersant

solution

Milling strength 20 19 20 20

(wt% pigment)

Z-average particle 80 1 15 103 109 size (nm)

Milling Specific 4.6 2.3 9.8 13.5

Energy (KWh/kg)^**

Parts of Boric Acid 66.67 83.33 12.33 12.33

3wt% solution

Parts of EX 321 4.5 2.8 0.84 0.84

Pigment content of 12.0 10.1 10.5 10.5 purified ECPD

(wt%)

Table 1 continued:

Notes to Tablel :

^* Comparative Dispersant 4 was unable to fully stabilise the pigment dispersion (see above for further explanation).

*^* See above for an explanation of Milling Specific Energy.

*^** Milled at slightly lower strength as became too viscous at standard 20%.

6. Preparation of Inks 1 and 2 and Comparative Inks 1 and 2 and Stability Results

Inks 1 and 2 and Comparative Inks 1 and 2 were prepared by mixing the ingredients shown below in Table 2 in the number of parts indicated. Table 2 provides the viscosity of the inks initially (0 weeks) and after 4 weeks at 60 C. A large change in viscosity indicates poor storage stability whereas zero or a small change indicates good storage stability:

Table 2 - Ink Formulations

Note to Table 2: ^T means calculated on a 100wt% pigment basis. Thus, for instance, to achieve 4 parts of 100wt% pigment there was used 33.33 parts of purified ECPD1 .

The results in Table 2 show that Inks 1 and 2 have improved storage stability over Comparative Inks 1 and 2.

7. Preparation and Testing of High Strength Inks 3 and 4 and High Strength Comparative Inks 3 and 4

High strength Inks 3 and 4 and High strength Comparative Inks 3 and 4 were prepared by mixing the ingredients shown in Table 3 below in the number of parts by weight indicated. The resultant inks contained approximately 10.0 wt% pigment. Table 3 - Ink Formulations and Sedimentation Test Results

As can be seen from Table 3 above, Inks 3 and 4 derived from low WAMW dispersants according to the present invention had better storage stability than the Comparative inks derived from higher WAMW dispersants.

8. Effect of WAMW on Dispersion Stability

The viscosity of the purified dispersions ECPD 2 and purified Comparative ECPD 2 prior to storage (0 weeks) and after 4 weeks storage at 60 °C were measured and the results are shown in Table 4 below. A large change in viscosity indicates poor storage stability whereas zero or a small change indicates good storage stability:

Table 4

As can be seen from Table 4 above, the viscosity of purified ECPD2 derived from a dispersant having a WAMW of 10,336 changed much less than the viscosity of the Comparative dispersion derived from a dispersant having a WAMW of 68, 105. Thus Purified ECPD2 had better storage stability than Purified Comparative ECPD2.

9. Further inks

The further inks described in Tables A and B may also be prepared. In place of PECPD1 used in Tables A and B there may also be used purified encapsulated pigment dispersions derived in an identical manner from Dispersant (2), Dispersant (3) and Dispersant (4). Numbers quoted in the third column onwards refer to the number of parts of the relevant ink components. All parts are by weight. The inks may be applied to paper by thermal, piezo or Memjet ink jet printing.

The following abbreviations are used in Tables A and B:

PECPD1 = Purified ECPD1 derived from Dispersant (3) having a WAMW of

22,078 Da

PG = propylene glycol

DEG = diethylene glycol

NMP = N-methyl pyrrolidone

DMK = dimethylketone

IPA = isopropanol

MEOH = methanol

2P = 2-pyrrolidone

MIBK = methylisobutyl ketone

P12 = propane-1 ,2-diol

BDL = butane-2,3-diol

Surf = Surfynol™ 465 from Airproducts

PHO = Na₂HP0₄ and

TBT = tertiary butanol TDG = thiodiglycol

GLY = Glycerol

nBDPG = mono-n-butyl ether of dipropylene glycol nBDEG = mono-n-butyl ether of diethylene glycol nBTEG = mono-n-butyl ether of triethylene glycol

Claims

1 . A process for preparing an encapsulated pigment dispersion suitable for use in an ink jet printing ink comprising the following steps in the order I) followed by II):

I) providing a dispersion comprising a pigment, a liquid medium and a dispersant having a WAMW of up to 50,000 Daltons obtained by copolymerising a monomer composition comprising components a) and b):

a) from 75 to 97 parts of one or more hydrophobic ethylenically unsaturated monomers comprising at least 50 parts benzyl (meth) acrylate; and

b) from 3 to 25 parts one or more hydrophilic ethylenically unsaturated monomers having one or more ionic group(s);

wherein the parts are by weight and the sum of the parts a) and b) add up to 100;

II) cross-linking the dispersant in the presence of the pigment and the liquid medium.

2. A process according to claim 1 wherein the dispersant is obtained by copolymerising a monomer composition comprising components a) to c):

a) from 75 to 97 parts of one or more hydrophobic ethylenically unsaturated monomers comprising at least 50 parts benzyl (meth) acrylate;

b) from 3 to 25 parts one or more hydrophilic ethylenically unsaturated monomers having one or more ionic group(s);

c) from 0 to 2 parts of one or more hydrophilic ethylenically unsaturated monomers having a hydrophilic non-ionic group; and

wherein the parts are by weight and the sum of the parts a) to c) add up to 100.

3. A process according to claim 1 or 2 wherein the dispersant has a WAMW of 10,000 to 38,000.

4. A process according to any one of the preceding claims wherein component a) comprises at least 70 parts of benzyl (meth) acrylate.

5. A process according to any one of the preceding claims wherein component a) consists of benzyl (meth) acrylate.

6. A process according to any one of the preceding claims wherein component a) consists of benzyl methacrylate.

7. A process according to any one of the preceding claims wherein the one or more hydrophihc ethylenically unsaturated monomers having one or more ionic group(s) comprise one or more hydrophihc ethylenically unsaturated monomers having one or more carboxylic acid groups.

8. A process according to any one of the preceding claims wherein component b) comprises methacrylic acid.

9. A process according to any one of the preceding claims wherein component b) consists of methacrylic acid.

10. A process according to any one of the preceding claims wherein the monomer composition is free from hydrophihc ethylenically unsaturated monomers having hydrophihc non-ionic group(s).

1 1. A process according to any one of the preceding claims wherein the dispersant is obtained by copolymerising a monomer composition comprising components a) to c):

a) from 80 to 93 parts of one or more hydrophobic monomers comprising at least 50 parts benzyl methacrylate;

b) from 7 to 22 parts of methacrylic acid;

c) 0 parts of hydrophihc ethylenically unsaturated monomers having a hydrophihc non-ionic group;

wherein the parts are by weight and the sum of the parts a) to c) add up to 100.

12. A process according to any one of the preceding claims wherein the cross- linking in step II) is effected by an epoxy cross-linking agent and component b) is or comprises one or more monomers having one or more carboxylic acid groups.

13. A process according to any one of the preceding claims wherein the liquid medium is or comprises water.

14. A process according to any one of the preceding claims wherein the dispersion provided in step I) have a sodium chloride critical coagulation concentration of no more than 2.0M.

15. A process according to any one of the preceding claims wherein the resulting encapsulated pigment dispersion has a sodium chloride critical coagulation concentration of no more than 2.0M.

16. A process according to claim 14 wherein the resulting encapsulated pigment dispersion has a sodium chloride critical coagulation concentration of from 0.2M to 1 .6M.

17. A process according to any one of the preceding claims further comprising the step of purifying the encapsulated pigment dispersion.

18. A process according to any one of the preceding claims which further comprises adding one or more additives to the encapsulated pigment dispersion, said additives being selected from viscosity modifiers, pH buffers, metal chelating agents, surfactants, corrosion inhibitors, biocides, dyes, water-miscible organic solvent(s) and kogation reducing additives.

19. A process according to any one of the preceding claims wherein the monomer composition further comprises d) 0.1 to 15% of chain transfer agent(s), by weight, based on the total weight of polymerisable components present in the composition.

20. An encapsulated pigment dispersion obtained or obtainable by a process according to any one of claims 1 to 19.

21. An ink jet printing ink which comprises an encapsulated pigment dispersion according to claim 20 or which has been prepared by a process according to anyone of claims 1 to 19.

22. An ink jet printer cartridge comprising a chamber and an ink jet printing ink wherein the ink jet printing ink is present in the chamber and is according to claim 20.

23. Use of a process according to any one of claims 1 to 19 for preparing an ink jet printing ink.

24. Use according to claim 23 for the technical purpose of providing an ink jet printing ink which has good and/or improved storage stability.