Pigment dispersants, pigment dispersions and writing or recording pigment inks

Dispersants for organic pigments comprise compounds represented by the following formula (I): ##STR1## wherein X and X' each independently represent H, OH, alkoxy, primary, secondary or tertiary amino, or acylamino; Y represents an anthraquinonylamino, phenylamino or phenoxy group having H, OH, alkoxy, primary, secondary or tertiary amino, or acylamino at the 4-position or 5-position; A and B each independently represent alkyl, cycloalkyl or aryl, and at least one of A and B has at least one substituent group containing a basic nitrogen atom; and Z represents H, CN, halogen, alkyl, alkoxy, NO.sub.2, benzoylamino or 3-benzoyl, and the 3-benzoyl group may be fused together with X to form an acridone ring. Pigment dispersions making use of the above dispersants are suited for the production of writing or recording pigment inks.

BACKGROUND OF THE INVENTION
 a) Field of the Invention
 This invention relates to dispersants for organic pigments (hereinafter
 simply referred to as "pigments"), pigment dispersions and writing or
 recording pigment inks, and more specifically to pigment dispersants and
 also to pigment dispersions making use of the pigment dispersants, having
 excellent fluidity, dispersion stability, storage stability and the like
 and suited especially for the production of writing or recording pigment
 inks.
 b) Description of the Related Art
 Marking inks for use in writing instruments, which make use of bundled
 fiber tips or felt tips, have conventionally been made of resins, pigments
 and solvents, and as the pigments, dyes have been employed. As the
 solvents, ester or aromatic solvents have been used from the standpoint of
 solubility for the resins and also solubility for the dyes.
 It is however desired to avoid use of aromatic solvents, because they give
 deleterious effects on the health of workers (organic solvent
 intoxication) and they themselves are air pollutants. Although there is a
 move toward water-based writing inks, solvent-based maker pens are still
 employed widely. This can be attributed to the excellent writing
 performance of solvent-based inks on plastic films such as polyethylene
 films and also to the superb drying characteristics of the solvent-based
 inks shortly after written on plastic films.
 Ink-jet printing, on the other hand, is a digital printing controlled by a
 computer. Printing information signals are supplied to a printer directly
 from the computer, so that no plate-making is needed. Ink-jet printing is
 therefore suited particularly for the printing of various images in small
 numbers of copies, and recent advancements in ink-jet printers have made
 it possible to perform printing of a variety of highly-detailed large
 images.
 Ink-jet printing inks are required to have low viscosity and excellent
 stability. There are organic-solvent-based inks making use of dyes, but as
 pigment-type inks, water-based inks are used because ink-jet printing inks
 making use of pigments involve technical difficulties. However,
 water-based inks contain, as vehicles, resins at extremely low
 concentrations so that good color developments are not available.
 Water-based inks are also accompanied by a problem of adhesion to plastic
 films. Under these circumstances, there is a strong desire for the
 development of alcohol-based ink-jet printing inks making use of pigments
 and assuring good color developments.
 Incidentally, as is observed from paints and the like, the conventional
 technology expects much on the dispersing power of a resin for a pigment
 upon dispersing the pigment on a resin solution. When the dispersing power
 of the resin is insufficient for the pigment, a pigment dispersant
 (pigment treatment) (hereinafter simply referred to as "a pigment
 dispersant) has been employed. in general paints, sufficient dispersion of
 pigments is feasible with conventional dispersants.
 Compared with paints, however, writing or recording pigment inks are
 required to have extremely low viscosity and in addition, to have an
 extremely high degree of dispersion of pigments. Despite these
 requirements, use of conventional resins and dispersants is unable to
 achieve sufficient dispersion of pigments or leads to changes in the
 viscosity of pigment dispersions along the passage of time due to
 desorption of the dispersant from the pigments and insufficient
 compatibility between the pigments and the resins, thereby making it
 extremely difficult to obtain pigment dispersions which meet the property
 and performance requirements.
 If an alcoholic solvent such as ethanol can be used as a solvent in an
 organic-solvent-based pigment ink, deleterious effects on the health of
 users of writing instruments and people studying or working in the same
 environment as the users can be reduced, the problem of air pollution can
 be lessened owing to the avoidance of an organic solvent, and further, the
 problems of drying characteristics and wetting to plastic films, said
 problems being inherent to water-based inks, can also be eliminated.
 Moverover, alcoholic solvents are resources reproducible in the natural
 world so that use of such solvents is preferred. Nonetheless, with resins
 soluble in conventionally-known alcoholic solvents, it is still extremely
 difficult to obtain pigment inks satisfactory in the requirements for low
 viscosity, high dispersion and high dispersion stability even if
 dispersants are used.
 SUMMARY OF THE INVENTION
 An object of the present invention is therefore to provide a dispersant
 which is excellent in the dispersibility of pigments and permits
 production of a pigment dispersion excellent in the stability of
 viscosity. Another object of the present invention is to provide a pigment
 dispersion, writing ink or recording ink, which is using a pigment as a
 coloring matter, contains a resin--which is soluble in an organic solvent,
 especially an alcoholic solvent--in a sufficient. amount relative to the
 pigment, has a low viscosity, good pigment dispersibility and excellent
 viscosity stability, and gives less deleterious effects on the health of a
 user of a writing instrument or recording apparatus and people studying or
 working in the same environment as the user.
 To achieve the above-described objects, the present invention provides an
 dispersant for organic pigments, comprising a compound represented by the
 following formula (I):
 ##STR2##
 wherein X and X' each independently represent a hydrogen atom, a hydroxyl
 group, an alkoxy group, a primary, secondary or tertiary amino group, or
 an acylamino group; Y represents an anthraquinonylamino, phenylamino or
 phenoxy group having a hydrogen atom, a hydroxyl group, an alkoxy group, a
 primary, secondary or tertiary amino group, or an acylamino group at the
 4-position or 5-position thereof; A and B each independently represent an
 alkyl group, a cycloalkyl group or an aryl group, and at least one of A
 and B has at least one substituent group containing a basic nitrogen atom;
 and Z represents a hydrogen atom, a cyano group, a halogen atom, an alkyl
 group, an alkoxy group, a nitro group, a benzoylamino group or a 3-benzoyl
 group, and the 3-benzoyl group may be fused together with X to form an
 acridone ring; a pigment dispersion comprising an organic pigment, a
 dispersant, a film-forming resin and an organic solvent, wherein the
 dispersant is the above-described dispersant; a writing or recording
 pigment ink comprising the dispersion; and a writing instrument or
 recording apparatus comprising the ink (the pigment dispersion and the
 writing ink or recording ink will hereinafter be collectively called "the
 ink").
 The term "at least one substituent group containing a basic nitrogen atom"
 as used herein may mean a primary, secondary or tertiary amino group, a
 quaternary ammonium group or a pyridinium group, with a tertiary amino
 group being particularly preferred.
 The dispersant according to the present invention is useful as a dispersant
 for a variety of conventionally-known pigments, especially as a dispersant
 for various pigments used as coloring matters in inks, various paints,
 various printing inks, various, pigment printing agents and synthetic
 resins. In particular, use of the dispersant according to the present
 invention by adding it as a dispersant to red, green yellow and purple
 inks makes it possible to stably produce low-viscosity inks and eventually
 to obtain excellent inks.
 DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
 The present invention will hereinafter be described in further detail based
 on certain preferred embodiments. Dispersants according to the present
 invention are useful as dispersants for a variety of conventionally-known
 pigments used as coloring matters in various paints, various printing
 inks, various pigment printing agents and synthetic resins. An especially
 useful application is an application as dispersants for pigments in inks.
 Accordingly, the present invention will hereinafter be described by taking
 inks as representative examples.
 The dispersants according to the present invention can be prepared by the
 preparation process disclosed, for example, in JP 46-33232 B, JP 46-33233
 B, or JP 4-34518 B, or a process similar to the preparation process. As an
 example, a dispersant can be obtained by reacting 1 mole of
 1-amino-5-benzoylaminoanthraquinone, 1 mole of aniline or phenol and 1
 mole of cyanuric chloride at 130 to 160.degree. C. for 2 to 6 hours in an
 inert solvent such as o-dichlorobenzene, adding 1 mole of a polyamine
 containing at least one secondary amino group and at least one tertiary
 amino group and containing no primary amino group, and then reacting them
 at 150 to 170.degree. C. for 3 to 4 hours.
 Illustrative of the "polyamine containing at least one secondary amino
 group and at least one tertiary amino group and containing no primary
 amino group" employed in the above-described process are:
 N,N,N'-trimethyl-ethylenediamine,
 N,N-dimethyl-N'-ethyl-ethylenediamine,
 N,N-diethyl-N'-methyl-ethylenediamine,
 N,N-dimethyl-N'-ethyl-propylenediamine,
 N,N,N'-trimethyl-propylenediamine,
 N,N,N'-triethyl-propylenediamine,
 N,N,N'-trimethyl-hexamethylenediamine,
 N,N-diethyl-N'-methyl-p-phenylenediamine,
 N,N-dipropyl-N'-methyl-p-phenylenediamine,
 N,N,N'-trimethyl-p-phenylenediamine,
 N,N,N'-trimethyl-m-phenylenediamine,
 N,N,N'-triethyl-p-phenylenediamine,
 N,N-diethyl-N'-methyl-1,4-diaminocyclohexane,
 N,N-diethyl-N'-methyl-1,3-diaminocyclohexane,
 N,N,N'-trimethyl-1,4-diaminocyclohexane,
 N,N,N'-triethyl-1,4-diaminocyclohexane,
 N-methylpiperazine,
 N-ethylpiperazine,
 N-isobutylpiperazine,
 2-chlorophenylpiperazine,
 N-(2-pyridyl)piperazine,
 N-(4-pyridyl)piperazine, and
 Methylhomopiperazine.
 In addition to the above-described compounds, particularly preferred can
 include:
 N,N,N",N"-tetramethyldiethylenetriamine,
 N,N,N",N"-tetra(n-propyl)diethylenetriamine,
 N,N,N",N"-tetra(i-propyl)diethylenetriamine,
 N,N,N",N"-tetra(n-butyl)diethylenetriamine,
 N,N,N",N"-tetra(i-butyl)diethylenetriamine,
 N,N,N",N"-tetra(s-butyl)diethylenetriamine,
 N,N,N",N"-tetra(t-butyl)diethylenetriamine,
 3,3'-iminobis(N,N-dimethylpropylamine),
 3,3'-iminobis(N,N-diethylpropylamine),
 3,3'-iminobis[N,N-di(n-propyl)propylamine],
 3,3'-iminobis[N,N-di(i-propyl)propylamine],
 3,3'-iminobis[N,N-di(n-butyl)propylamine],
 3,3'-iminobis[N,N-di(i-butyl)propylamine],
 3,3'-iminobis[N,N-di(s-butyl)propylamine],
 3,3'-iminobis[N,N-di(t-butyl)propylamine],
 4,4'-iminobis(N,N-dimethylbutylamine),
 4,4'-iminobis(N,N-diethylbutylamine),
 2,9-dimethyl-2,5,9-triazadecane,
 2,12-dimethyl-2,6,12-triazatridecane,
 2,12-dimethyl-2,5,12-triazatridecane,
 2,16-dimethyl-2,9,16-triazaheptadecane,
 3-ethyl-10-methyl-3,6,10-triazaundecane,
 5,13-di(n-butyl)-5,9,13-triazaheptadecane,
 2,2'-dipicolylamine, and
 3,3'-dipicolylamine.
 Among the dispersants according to the present invention available by the
 above-described processes, preferred dispersants are compounds represented
 by the following formula (1), more preferred dispersants are compounds
 represented by the formula (2), and particularly preferred dispersants are
 compounds represented by the following formula (3):
 ##STR3##
 wherein X, X', Y and Z have the same meanings as defined above; R.sub.1 to
 R.sub.4 may be the same or different and each independently represent a
 substituted or unsubstituted alkyl or cycloalkyl group, and R.sub.1 and
 R.sub.2 and/or R.sub.3 and R.sub.4 may be fused together with the adjacent
 nitrogen atom thereof to form a heterocyclic ring which may additionally
 contain a further nitrogen atom, an oxygen atom or a sulfur atom; and
 R.sub.5 and R.sub.6 each independently represent an alkylene group, a
 cycloalkylene group or an arylene group.
 ##STR4##
 wherein X, X', Y, Z and R.sub.1 to R.sub.4 have the same meanings as
 defined above; and n and m each independently stand for an integer of from
 2 to 30.
 Incidentally, the aminoacyl groups in the above-described formula (I) and
 the above-described formulas (1) to (2), are groups represented by
 --NHCOR, in which R is a phenyl group, a methyl group, an ethyl group, a
 propyl group, a butyl group, or the like.
 ##STR5##
 wherein X and X' each independently represent a hydrogen atom or a
 benzoylamino group; Z represents a hydrogen atom; R.sub.1 to R.sub.4 may
 be the same or different and each independently represent a methyl group
 or an ethyl group; and n and m each independently stand for 2 or 3.
 Specific examples of preferred dispersants in the present invention can
 include, but are not limited to, the followings in which X represents a
 benzoylamino group:
 ##STR6##
 ##STR7##
 and the quaternary ammonium compounds of Specific Examples 1-6, 9-14 and
 17-19 and the pyridinium compounds of Specific Examples 7-8 and 15-16.
 Inks according to the present invention are formed of the above-described
 dispersants, pigments, film-forming resins and organic solvents.
 Conventionally-known pigments are all usable as pigments in the present
 invention. For example, pigments such as azo pigments, condensed azo
 pigments, anthraquinone pigments, perylene/perinone pigments,
 indigothioindigo pigments, isoindolinone pigments, azomethine pigments,
 azomethineazo pigments, quinacridone pigments, phthalocyanine blue,
 dioxazine violet, aniline black and carbon black can be used. Particularly
 preferred pigments can include C.I. pigment red (hereinafter abbreviated
 as "P.R.") 177, P.R. 254, P.R. 242, C.I. pigment green (hereinafter
 abbreviated as "P.G.") 36, C.I. pigment blue (hereinafter abbreviated as
 "P.B.") 15:2, P.B. 15:6, P.B. 60, C.I. pigment yellow (hereinafter
 abbreviated as "P.Y.") 138, P.Y. 185, P.Y. 150, P.Y. 139, C.I. pigment
 violet 23, C.I. pigment black (hereinafter abbreviated as "P.BL." 6, P.BL.
 7.
 Conventionally-known organic solvents employed in various paints, coating
 formulations, printing inks and the like are all usable as organic
 solvents in the present invention. When using inks according to the
 present invention as inks for writing instruments or recording
 apparatuses, use of alcoholic solvents is preferred. As alcoholic
 solvents, solvents containing alcohols the boiling points of which are
 150.degree. C. or lower are preferred. Alcohols amount to at least 10 wt.
 %, preferably to 50 to 100 wt. % of alcoholic solvents.
 Preferred examples of alcohols can include ethanol, propanol,
 methoxypropanol, ethoxypropanol, and propyloxyethanol. They can be used
 either singly or in combination. Solvents other than these alcohols can
 also be used to extents not contrary to the spirit of the present
 invention. For example, ethyl acetate, propyl acetate, cyclohexane,
 methylcyclohexane, ethyl-cyclohexane, methyl ethyl ketone, methyl propyl
 ketone and the like can be used in combination.
 The film-forming resin (hereinafter simply referred to as a "resin")
 employed in the present invention is required to be soluble in the
 above-described organic solvent, especially the alcoholic solvent
 described above. Such a resin comprises, as constituent monomers, an
 addition-polymerizing monomer containing a carboxyl group, a hydroxyl
 group or an amido group and a monomer addition-polymerizable with the
 addition-polymerizing monomer. A description will hereinafter be made
 about the monomers which make up the resin for use in the present
 invention.
 Examples of the carboxyl-containing monomer can include acrylic acid and
 methacrylic acid; fumaric acid, maleic acid, itaconic acid, alkyl
 monoesters thereof, and hydroxyalkyl monoesters thereof; and monoesters
 between hydroxyalkyl (meth)acrylates, such as hydroxyethyl (meth)acrylate
 and hydroxypropyl (meth)acrylate, and dibasic acids, for example, succinic
 anhydride, phthalic anhydride and cyclohexanedicarboxylic acid anhydride.
 Incidentally, acrylic acid and methacrylic acid will be called
 (meth)acrylic acid in the present invention.
 Examples of the hydroxyl-containing monomer can include hydroxyalkyl
 (meth)acrylates, for example, hydroxyethyl (meth)acrylate and hydropropyl
 (meth)acrylate; esters of monoesters between the above-described
 hydroxyalkyl (meth)acrylates and dibasic acids with dihydroxyalkyl
 compounds, for example, monoesters of monoesters between hydroxyethyl
 (meth)acrylate and hydroxypropyl (meth)acrylate and succinic acid,
 phthalic acid and cyclohexanecarboxylic acid with dihydroxyalkyl compounds
 such as ethylene glycol and propylene glycol. The carbon numbers of the
 alkyl groups in the above-described carboxyl- and hydroxyl-containing
 monomers range from about 1 to 12.
 Examples of the amido-containing monomer can include (meth)acrylamides and
 N-substituted (meth)acrylamides, for example, (meth)acrylic
 butoxymethylamide, N-tert-butylacrylamide, N-tert-butylmethacrylamide,
 diacetoneacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide,
 N,N-diethylacrylamide, N-methylol-(meth)acrylamide, and
 N-alkyloxyethyl(meth)acrylamide.
 In addition to the above-described monomers, addition-polymerizing monomers
 copolymerizable with the above-described respective monomers can be used
 to extents soluble in the alcoholic solvent in order to impart
 waterproofness, flexibility and/or other physical properties to the resin.
 Examples of such monomers can include (meth)acrylate esters, for example,
 methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl
 (meth)acrylate, octyl (meth)acrylate, lauryl (meth)acrylate, stearyl
 (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate,
 isobornyl (meth)acrylate, and tetrafulfuryl (meth)acrylate; styrene and
 styrene derivatives, for example, .alpha.-methylstyrene; dialkyl esters of
 dibasic acids, for example, dialkyl maleates, dialkyl fumarates, and
 dialkyl itaconates; and vinyl acetate, and (meth)acrylonitrile.
 The resin for use in the present invention can be obtained by using the
 above-described monomers and copolymerizing them, for example, by
 suspension polymerization or solution polymerization, which uses an
 alcoholic solvent, in the presence or absence of a conventionally-known
 radical polymerization initiator. As a resin for use in the present
 invention, it is preferred to use a resin in which a monomer between an
 alkyl (meth)acrylate and a dibasic acid--said monomer being represented by
 the following formula: CH.sub.2.dbd.C(R.sup.1)--COO--R.sup.2
 --O--CO--R.sup.3 --COOH in which R.sup.1 represents a hydrogen atom or a
 methyl group, R.sup.2 represents an alkylene chain which contains 2 to 6
 carbon atoms and may contain a branched chain, R.sup.3 represents an
 alkylene group which may contain a branched chain or unsaturated group--is
 copolymerized at least a part of a copolymer. A blend of the copolymer
 with a copolymer free of the above-described specific monomers may also be
 used. Whichever resin is used, it is preferred to contain units of a
 monoester monomer between hydroxyalkyl (meth)acrylate and a dibasic acid
 in a proportion of from 1 to 50 wt. %, preferably from 3 to 40 wt. % based
 on the whole resin. If the content of the monomer units is too low, the
 resin is not very effective for dispersing pigments. If the content of the
 monomer units is too high, on the other hand, writing matter or printed
 matter has lowered alkali resistance. It is therefore not preferred to
 contain the monomer units outside the above range.
 To obtain the resin by copolymerizing a mixture of the above-described
 monomers, the monoester of the hydroxyalkyl (meth)acrylate and the dibasic
 acid may account preferably for 1 to 50 wt. %, more preferably for 3 to 40
 wt. % of the monomer mixture. The proportion(s) of other monomer(s) can be
 suitably determined such that the resulting resin is soluble in the
 alcoholic solvent and can satisfy its performance requirements, and no
 particular limitation is imposed on their proportions. The number average
 molecular weight of the resin (the number average molecular weight as
 obtained by measuring the resin by GPC and converting the data with
 reference to the corresponding data of standard polystyrene) ranges
 generally from 2,000 to 100,000, preferably from 2,000 to 50,000. The acid
 value of the resin ranges generally from 0.5 to 300 mgKOH/g, preferably
 from 5 to 180 mgKOH/mg.
 The ink according to the present invention can be produced in a similar
 manner as in the production of conventional inks and paints by using the
 above-described pigment, dispersant, film-forming resin and organic
 solvent, and no particular limitation is imposed on the production method
 itself. Illustrative productions methods can include a method in which the
 pigment, subsequent to its advance treatment with the dispersant, is
 dispersed in a solution of the resin in an alcoholic solvent; and a method
 in which the untreated pigment and the dispersant are mixed with the resin
 solution and the pigment is subjected to dispersion processing in a
 disperser.
 When the method that the pigment, subsequent to its advance treatment with
 the dispersant, is dispersed in the solution of the resin in the alcoholic
 solvent is adopted, the treatment of the pigment can be practiced, for
 example, as will be described next. (1) Subsequent to dissolution of the
 pigment and the dispersant in sulfuric acid or the like, the resultant
 solution is poured into water. The thus-obtained mixture is alkalinized to
 have both of the pigment and dispersant precipitated as a solid solution.
 The solid solution is collected by filtration, washed with water, dried
 and then ground, whereby the pigment is obtained in a treated form. (2)
 The dispersant is converted into a salt with sulfuric acid, hydrochloric
 acid, acetic acid or the like. The salt is mixed with the pigment in water
 and, if necessary, the resultant mixture is subjected to dispersion
 processing in a disperser to have the dispersant adsorbed on pigment
 surfaces. The thus-treated pigment is caused to precipitate under alkaline
 condition, collected by filtration, washed with water, dried and then
 ground, whereby the pigment is obtained in the treated form. (3) The
 dispersant is dissolved in a liquid organic acid such as acetic acid,
 followed by the addition of the pigment. After the dispersant is allowed
 to be adsorbed on pigment surfaces with optional dispersing processing in
 a disperser, the thus-treated pigment is collected by filtration, washed
 with an alkaline solution, washed with water, dried and then ground,
 whereby the pigment is obtained in the treated form. The treated pigment
 obtained as described above is dispersed in the solution of the resin in
 an alcoholic solvent to produce the ink of the present invention.
 When the method that the untreated pigment and the dispersant are mixed
 with the resin solution and the pigment is subjected to dispersion
 processing in a disperser is adopted, the pigment and the dispersant are
 added to the solution of the resin in the alcoholic solvent and subsequent
 to optional provisional agitation, the mixture is dispersed in a disperser
 to obtain an ink. No particular limitation is imposed on the disperser
 which can be used in the present invention. Examples can include a
 kneader, an attritor, a ball mill, a sand mill and a horizontal disperser
 with tumbling means contained therein, said sand mill and horizontal
 disperser making use of glass, zircon or the like, and a colloid mill.
 Upon using the dispersant-treated pigment, it is also possible to further
 disperse the dispersant-treated pigment in a solid resin to obtain pigment
 chips and then to dissolve the pigment chips in an alcoholic solvent to
 obtain an ink. Subsequent to the dissolution of the pigment chips, a resin
 may be added as needed.
 Pigment chips can be obtained, for example, by one of the following
 methods: (1) A solid resin obtained by suspension polymerization or a
 solid resin collected from a resin solution obtained by solution
 polymerization and a dispersant-treated pigment are kneaded under heat by
 using a kneader, a Banbury mixer, a mixing roll, a three-roll mill or the
 like either singly or in combination such that the pigment is dispersed in
 the resin. The resultant pigment-dispersed resin is then ground or chopped
 to obtain chips. (2) A solution of a resin in a water-soluble solvent and
 a presscake of a dispersant-treated pigment are mixed in a kneader,
 followed by heating to the softening point of the resin or higher to
 remove water. If necessary, the pigment is dispersed by using a three-roll
 mill, an extruder or the like. The resultant pigment-dispersed resin is
 ground or chopped to obtain pigment chips. (3) A presscake of a
 dispersant-treated pigment and a solid resin are subjected to flushing at
 the softening temperature of the resin or higher.
 In the ink according to the present invention, the above-described
 dispersant may be used generally in a proportion of from 0.5 to 50 parts
 by weight, preferably in a proportion of from 1 to 30 parts by weight per
 100 parts by weight of the pigment. On the other hand, the pigment may be
 used generally in a proportion of from 5 to 500 parts by weight per 100
 parts by weight of the resin.
 The concentration of the pigment in the ink according to the present
 invention may be from 0.3 to 50 wt. % in general, with 0.5 to 30 wt. %
 being preferred, although it varies depending on the pigment. The
 viscosity of the ink may range generally from 1 to 50 mPa.multidot.s,
 preferably from 2 to 30 mPa.multidot.s. Depending on the construction of
 the writing instrument, a viscosity lower than 7 mPa.multidot.s may be
 required in some instances. It is particularly important for an ink to be
 equipped with excellent stability in viscosity along the passage of time.
 owing to the use of the above-described dispersant and resin along with a
 pigment, the ink according to the present invention has been imparted with
 excellent time-dependent stability in viscosity.
 A variety of additives can also be added to the ink of the present
 invention. Illustrative of such additives are fastness improvers such as
 ultraviolet absorbers and antioxidants; anti-settling agents; release
 agents and releasability improvers; perfumes and antimicrobial agents;
 plasticizers; and drying inhibitors. If necessary, one or more dyes can be
 added further. As a resin, a resin which has compatibility with the resin
 for use in the present invention may also be used in combination to an
 extent not lowering the dispersion stability of the pigment.
 In general, the thus-obtained ink can be used as is. It is however
 preferable to process the ink further in a centrifugal separator or an
 ultracentrifugal separator or through a filter, because removal of coarse
 particles of the pigment, which may exist in a trace amount in some
 instances, makes it possible to heighten the reliability of the writing
 instrument or recording apparatus.
 The writing instrument according to the present invention can be obtained
 by filling the ink of this invention, which has been obtained as described
 above, in a casing equipped with a porous tip. No particular limitation is
 imposed on the casing insofar as it has a size and shape convenient for
 writing as in the case of a variety of conventional writing instruments
 fitted with porous tips. The material of the casing, including its cap,
 can be a metal, a plastic or a composite. material thereof insofar as it
 is practically free from solvent permeation.
 No limitation is imposed on the structure or material of the porous tip
 insofar as it allows the ink to move from the interior of the casing to
 the free end of the tip as writing proceeds. However, excellent writing
 characteristics, durability and solvent resistance are required. Usable
 examples of fibers of bundled fiber tips and felt tips can include
 synthetic fibers such as polyester, polypropylene, nylon,
 polyacrylonitrile and vinylon fibers; cellulose and cellulose-derived
 regenerated fibers; and natural fibers such as wool, silk and cotton.
 Usable examples of open-cell foamed plastic products can include rigid or
 flexible urethane resin foams, foams of acetalized polyvinyl alcohol, and
 foamed of regenerated fibers. Usable examples of ink-occluding members can
 include bundles fibers, felted fibers, knitted fibers, and open-cell
 foamed plastic products.
 In the above description, inks making use of the dispersants according to
 the present invention have been referred to as representative examples.
 The present invention is however not limited only to inks. For example,
 the dispersants according to the present invention are useful as
 dispersants for a variety of conventionally-known pigments, and are useful
 as dispersants for various pigments employed as coloring matters in
 diverse paints, printing inks, pigment printing agents, synthetic resins
 and the like.

The present invention will next be described more specifically by Examples
 and Comparative Examples, in which the designations of "part" or "parts"
 and "%" are all by weight.
 EXAMPLE 1
 Added to 600 parts of o-dichlorobenzene were 62 parts of
 1-aminoanthraquinone and 25 parts of cyanuric chloride, followed by
 stirring at 130.degree. C. for 5 hours. After cooling, 50 parts of
 N,N,N",N"-tetraethyl-diethylenetriamine were added further, and the
 resultant mixture was stirred at 170.degree. C. for 3 hours. Subsequent to
 filtration, the thus-obtained filtercake was washed with ethanol and then
 dried, whereby the above-described specific example (1) was obtained as
 Dispersant 1.
 EXAMPLE 2
 In a similar manner as in Example 1, the above-described specific example
 (2) was obtained as Dispersant 2 by successively subjecting
 1-aminoanthraquinone and 3,3'-iminobis(N,N-dimethylpropylamine) to
 condensation reactions with cyanuric chloride.
 EXAMPLE 3
 In a similar manner as in Example 1, the above-described specific example
 (3) was obtained as Dispersant 3 by successively subjecting
 1-aminoanthraquinone and 3-ethyl-10-methyl-3,6,10-triazaundecane to
 condensation reactions with cyanuric chloride.
 EXAMPLE 4
 In a similar manner as in Example 1, the above-described specific example
 (4) was obtained as Dispersant 4 by successively subjecting
 1-amino-5-benzoylaminoanthraquinone and
 3,3'-iminobis(N,N-dimethylpropylamine) to condensation reactions with
 cyanuric chloride.
 EXAMPLE 5
 In a similar manner as in Example 1, the above-described specific example
 (5) was obtained as Dispersant 5 by successively subjecting
 1-amino-5-benzoylaminoanthraquinone, aniline and
 3,3'-imino-bis(N,N-dimethylpropylamine) to condensation reactions with
 cyanuric chloride.
 EXAMPLE 6
 In a similar manner as in Example 1, the above-described specific example
 (6) was obtained as Dispersant 6 by successively subjecting
 1-aminoanthraquinone and 5,13-di(n-butyl)-5,9,13-triazaheptadecane to
 condensation reactions with cyanuric chloride.
 EXAMPLE 7
 In a similar manner as in Example 1, the above-described specific example
 (7) was obtained as Dispersant 7 by successively subjecting
 1-aminoanthraquinone and 2,2'-dipicolylamine to condensation reactions
 with cyanuric chloride.
 EXAMPLE 8
 In a similar manner as in Example 1, the above-described specific example
 (8) was obtained as Dispersant 8 by successively subjecting
 1-aminoanthraquinone and 3,3'-dipicolylamine to condensation reactions
 with cyanuric chloride.
 EXAMPLE 9
 In a similar manner as in Example 1, the above-described specific example
 (9) was obtained as Dispersant 9 by successively subjecting
 1-amino-4-benzoylaminoanthraquinone and
 N,N",N"-tetraethyl-diethylenetriamine to condensation reactions with
 cyanuric chloride.
 EXAMPLE 10
 In a similar manner as in Example 1, the above-described specific example
 (10) was obtained as Dispersant 10 by successively subjecting
 1-amino-4-benzoylaminoanthraquinone and
 3,3'-iminobis(N,N-dimethylpropylamine) to condensation reactions with
 cyanuric chloride.
 EXAMPLE 11
 In a similar manner as in Example 1, the above-described specific example
 (11) was obtained as Dispersant 11 by successively subjecting
 1-amino-4-benzoylaminoanthraquinone and
 3-ethyl-10-methyl-3,6,10-triazaundecane to condensation reactions with
 cyanuric chloride.
 EXAMPLE 12
 In a similar manner as in Example 1, the above-described specific example
 (12) was obtained as Dispersant 12 by successively subjecting
 1-amino-4,5-dibenzoylaminoanthraquinone and
 3,3'-iminobis(N,N-dimethylpropylamine) to condensation reactions with
 cyanuric chloride.
 EXAMPLE 13
 In a similar manner as in Example 1, the above-described specific example
 (13) was obtained as Dispersant 13 by successively subjecting
 1-amino-4-benzoylaminoanthraquinone, aniline and
 3,3'-imino-bis(N,N-dimethylpropylamine) to condensation reactions with
 cyanuric chloride.
 EXAMPLE 14
 In a similar manner as in Example 1, the above-described specific example
 (14) was obtained as Dispersant 14 by successively subjecting
 1-amino-4-benzoylaminoanthraquinone and
 5,13-di(n-butyl)-5,9,13-triazaheptadecane to condensation reactions with
 cyanuric chloride.
 EXAMPLE 15
 In a similar manner as in Example 1, the above-described specific example
 (15) was obtained as Dispersant 15 by successively subjecting
 1-amino-4-benzoylaminoanthraquinone and 2,2'-dipicolylamine to
 -condensation reactions with cyanuric chloride.
 EXAMPLE 16
 In a similar manner as in Example 1, the above-described specific example
 (16) was obtained as Dispersant 16 by successively subjecting
 1-amino-4-benzoylaminoanthraquinone and 3,3'-dipicolylamine to
 condensation reactions with cyanuric chloride.
 EXAMPLE 16-2
 In a similar manner as in Example 1, the above-described specific example
 (17) was obtained as Dispersant 17 by successively subjecting
 1-amino-4-hydroxyanthraquinone and 3,3'-iminobis(N,N-dimethyl-propylamine)
 to condensation reactions with cyanuric chloride.
 EXAMPLE 16-3
 In a similar manner as in Example 1, the above-described specific example
 (18) was obtained as Dispersant 18 by successively subjecting
 1-amino-4-methoxyanthraquinone and 3,3'-iminobis(N,N-dimethyl-propylamine)
 to condensation reactions with cyanuric chloride.
 EXAMPLE 16-4
 In a similar manner as in Example 1, the above-described specific example
 (19) was obtained as Dispersant 19 by successively subjecting
 1-amino-acridone-1,2(2',4'-dichloro)acridone and
 3,3'-iminobis(N,N-dimethylpropylamine) to condensation reactions with
 cyanuric chloride.
 Synthesis Example of Film-Forming Resin
 Charged in a polymerizer were 75 parts of hydroxyethyl methacrylate, 25
 parts of .alpha.-methylstyrene, 100 parts of methyl methacrylate, 75 parts
 of acryloyloxyethyl phthalate, 75 parts of lauryl methacrylate, 150 parts
 of diacetoneacrylamide, 15 parts of azobisisobutyronitrile, 550 parts of
 ethanol, 150 parts of methylcyclohexane and 50 parts of ethyl acetate. A
 cooling coil was set, followed by polymerization at 75.degree. C. for 10
 hours. After cooling, the resin solution was taken out of the polymerizer.
 It was provided as a resin solution. The resin content was 40%, and the
 viscosity was 250 mPa.multidot.s. The number average molecular weight of
 the resin was 8,600 as measured by GPC and converted with reference to the
 corresponding data of standard polystyrene, and the acid value of the
 resin was 39.
 EXAMPLE 17
 Dispersed in a horizontal disperser with tumbling medium contained therein
 were 250 parts of the above-described resin solution, 50 parts of
 benzidine yellow (P.Y. 83), 4 parts of Dispersant (1) of Example 1, 536
 parts of ethanol, 60 parts of methylcyclohexane and 100 parts of
 ethoxypropanol. Coarse particles were then removed by ultracentrifugal
 separation, whereby an ink according to the present invention was
 obtained. The average particle size of the pigment was 95 nm, and the
 viscosity was 3.5 mPa.multidot.s. The ink was stored at 50.degree. C. for
 a week, but settling of the pigment was not observed. When its viscosity
 was measured, it was still 3.5 mPa.multidot.s, thereby indicating no
 change in viscosity.
 The ink was next filled in a pen casing equipped with a bundled fiber tip,
 and a writing test was conducted on a polyethylene film. Smooth writing
 was feasible. Further, 20 parts of ethoxypropanol and 5 parts of benzyl
 alcohol were added further to 100 parts of the ink. Using the
 thus-obtained ink, printing was performed by an ink-jet printer. Good
 printed matter was obtained.
 EXAMPLES 18-24
 Yellow inks were obtained in a similar manner as in Example 17 except that
 the dispersants of Examples 2-8 were used as dispersants, respectively.
 Those yellow inks were then ranked as in Example 17. The results are
 presented in Table 1 together with the results of Example 17. In the
 table, "A" indicates good in both writing characteristics and ink-jet
 printability, while "B" indicates insufficient writing characteristics or
 insufficient ink-jet printability.
 Comparative Example 1
 A yellow ink was obtained in a similar manner as in Example 17 except that
 the dispersant was not used. The ink was ranked as in Example 17. The
 results are also presented in Table 1.
 TABLE 1
 Viscosity stability
 of ink
 Shortly After Performance of ink
 after stored Writing Ink-jet
 Kind of formu- for a week character- print-
 dispersant lation (50.degree. C.) istics ability
 Example 17 Dispersant 1 3.5 3.5 A A
 Example 18 Dispersant 2 3.9 4.0 A A
 Example 19 Dispersant 3 3.5 3.6 A A
 Example 20 Dispersant 4 3.6 3.6 A A
 Example 21 Dispersant 5 3.4 3.5 A A
 Example 22 Dispersant 6 3.7 3.7 A A
 Example 23 Dispersant 7 3.9 4.0 A A
 Example 24 Dispersant 8 4.0 4.1 A A
 Comp. Ex. 1 None 62.1 89.3 B B
 EXAMPLE 25
 Ten (10) parts of Dispersant 2 were dissolved in 100 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of a red pigment (P.R. 254; pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 108 parts of red pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained surface-treated pigment was dispersed to a pigment
 content of 15% in a commercial melamine/alkyd paint by a ball mill. The
 colored paint so obtained had low viscosity, and showed fluidity
 substantially close to a Newtonian flow. Further, the red paint was mixed
 with a commercial white melamine/alkyd paint to formulate a pale red
 paint. Even after stored for a week, the pale red paint was still in a
 homogeneous form without any color separation.
 Comparative Example 2
 A paint was formulated in a similar manner as in Example 25 except that the
 red pigment (P.R. 254) was used without surface treatment with Dispersant
 2. The paint was ranked as in Example 25. The paint had high viscosity,
 and in the form of a mixed paint with a white paint, the red pigment
 underwent separation and settling through coagulation.
 EXAMPLE 26
 Eight (8) parts of Dispersant 3 were dissolved in 100 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of a red pigment (P.R. 254; pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 105 parts of red pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained pigment composition was dispersed in a commercial acrylic
 lacquer by a beads mill, whereby a red paint having a pigment content of
 13% was formulated. The paint was adjusted in viscosity with a lacquer
 thinner and was spray-painted on an iron plate. A plate coated in a red
 color and having a dry film thickness of 32 .mu.m was obtained. It was
 excellent in vividness, and showed a high degree of gloss.
 Comparative Example 3
 A paint was formulated in a similar manner as in Example 26 except that the
 red pigment (P.R. 254) was used without surface treatment with Dispersant
 3. The viscosity was adjusted likewise, and the paint was ranked as in
 Example 26. The surface of a coating on a plate coated in a red color was
 not smooth, and its gloss was low.
 EXAMPLE 27
 Dispersed in a horizontal disperser with tumbling medium contained therein
 were 250 parts of the resin solution prepared in the Synthesis Example of
 the film-forming resin, 50 parts of cyanine blue (P.B. 15-2), 4 parts of
 Dispersant (9) of Example 9, 536 parts of ethanol, 60 parts of
 methylcyclohexane and 100 parts of ethoxypropanol. Coarse particles were
 then removed by ultracentrifugal separation, whereby an ink according to
 the present invention was obtained. The average particle size of the
 pigment was 95 nm, and the viscosity was 3.8 mPa.multidot.s. The ink was
 stored at 50.degree. C. for a week, but settling of the pigment was not
 observed. When its viscosity was measured, it was still 3.8 mPa.multidot.s
 and accordingly, no change took place in viscosity.
 The ink was next filled in a pen casing equipped with a bundled fiber tip,
 and a writing test was conducted on a polyethylene film. Smooth writing
 was feasible. Further, 20 parts of ethoxypropanol and 5 parts of benzyl
 alcohol were added further to 100 parts of the ink. Using the
 thus-obtained ink, printing was performed by an ink-jet printer. Good
 printed matter was obtained.
 EXAMPLES 28-34
 Blue inks were obtained in a similar manner as in Example 27 except that
 the dispersants of Examples 10-16 were used as dispersants, respectively.
 Those blue inks were then ranked as in Example 27. The results are
 presented in Table 2 together with the results of Example 27. In the
 table, "A" indicates good in both writing characteristics and ink-jet
 printability, while "B" indicates insufficient writing characteristics or
 insufficient ink-jet printability.
 Comparative Example 4
 A blue ink was obtained in a similar manner as in Example 28 except that
 the dispersant was not used. The ink was ranked as in Example 28. The
 results are also presented in Table 2.
 TABLE 2
 Viscosity stability
 of ink
 Shortly After Performance of ink
 after stored Writing Ink-jet
 Kind of formu- for a week character- print-
 dispersant lation (50.degree. C.) istics ability
 Example 27 Dispersant 9 3.8 3.8 A A
 Example 28 Dispersant 10 4.3 4.3 A A
 Example 29 Dispersant 11 4.5 4.5 A A
 Example 30 Dispersant 12 5.0 5.0 A A
 Example 31 Dispersant 13 3.2 3.2 A A
 Example 32 Dispersant 14 4.1 4.1 A A
 Example 33 Dispersant 15 5.6 5.6 A A
 Example 34 Dispersant 16 5.8 5.8 A A
 Comp. Ex. 4 None 52.2 95.0 B B
 EXAMPLE 35
 Ten (10) parts of Dispersant 10 were dissolved in 100 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of a red pigment (P.R. 254; pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 108 parts of red pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained surface-treated pigment was dispersed to a pigment
 content of 15% in a commercial melamine/alkyd paint by a ball mill. The
 colored paint so obtained had low viscosity, and showed fluidity
 substantially close to a Newtonian flow. Further, the red paint was mixed
 with a commercial white melamine/alkyd paint to formulate a pale red
 paint. Even after stored for a week, the pale red paint was still in a
 homogeneous form without any color separation.
 Comparative Example 5
 A paint was formulated in a similar manner as in Example 35 except that the
 red pigment (P.R. 254) was used without surface treatment with Dispersant
 10. The paint was ranked as in Example 35. The paint had high viscosity,
 and in the form of a mixed paint with a white paint, the red pigment
 underwent separation and settling through coagulation.
 EXAMPLE 36
 Eight (8) parts of Dispersant 11 were dissolved in 100 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of a red pigment (P.R. 254; pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 105 parts of red pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained surface-treated pigment was dispersed in a commercial
 acrylic lacquer by a beads mill, whereby a red paint having a pigment
 content of 13% was formulated. The paint was adjusted in viscosity with a
 lacquer thinner and was spray-painted on an iron plate. A plate coated in
 a red color and having a dry film thickness of 32 .mu.m was obtained. It
 was excellent in vividness, and showed a high degree of gloss.
 Comparative Example 6
 A paint was formulated in a similar manner as in Example 36 except that the
 red pigment (P.R. 254) was used without surface treatment with Dispersant
 11. The viscosity was adjusted likewise, and the paint was ranked as in
 Example 36. The surface of a coating on a plate coated in a red color was
 not smooth, and its gloss was low.
 EXAMPLE 37
 Dispersed in a horizontal disperser with tumbling medium contained therein
 were 250 parts of the resin solution prepared in the Synthesis Example of
 the film-forming resin, 750 parts of C.I. pigment black, 4 parts of
 Dispersant (1) of Example 1, 536 parts of ethanol, 60 parts of
 methylcyclohexane and 100 parts of ethoxypropanol. Coarse particles were
 then removed by ultracentrifugal separation, whereby a black ink according
 to the present invention was obtained. The average particle size of the CB
 pigment was 95 nm, and the viscosity was 3.5 mPa.multidot.s. The black ink
 was stored at 50.degree. C. for a week, but settling of the CB pigment was
 not observed. When its viscosity was measured, it was still 3.5
 mPa.multidot.s and accordingly, no change took place in viscosity.
 The black ink was next filled in a pen casing equipped with a bundled fiber
 tip, and a writing test was conducted on a polyethylene film. Smooth
 writing was feasible. Further, 20 parts of ethoxypropanol and 5 parts of
 benzyl alcohol were added further to 100 parts of the black ink. Using the
 thus-obtained black ink, printing was performed by an ink-jet printer.
 Good printed matter was obtained.
 EXAMPLES 38-44
 Black inks were obtained in a similar manner as in Example 37 except that
 the dispersants of Examples 2-8 were used as dispersants, respectively.
 Those black inks were then ranked as in Example 37. The results are
 presented in Table 3 together with the results of Example 37. In the
 table, "A" indicates good in both writing characteristics and ink-jet
 printability, while "B" indicates insufficient writing characteristics or
 insufficient ink-jet printability.
 Comparative Example 7
 A black ink was obtained in a similar manner as in Example 37 except that
 the dispersant was not used. The ink was ranked as in Example 37. The
 results are also presented in Table 3.
 TABLE 3
 Viscosity stability
 of ink
 Shortly After Performance of ink
 after stored Writing Ink-jet
 Kind of formu- for a week character- print-
 dispersant lation (50.degree. C.) istics ability
 Example 37 Dispersant 1 3.5 3.5 A A
 Example 38 Dispersant 2 3.9 3.9 A A
 Example 39 Dispersant 3 3.8 3.9 A A
 Example 40 Dispersant 4 4.4 4.5 A A
 Example 41 Dispersant 5 4.1 4.1 A A
 Example 42 Dispersant 6 4.3 4.4 A A
 Example 43 Dispersant 7 3.9 3.9 A A
 Example 44 Dispersant 8 4.0 4.0 A A
 Comp. Ex. 7 None 63.5 89.0 B B
 EXAMPLE 45
 Ten (10) parts of Dispersant 2 were dissolved in 200 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of C.I. pigment black 7 (CB pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 108 parts of black pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained surface-treated CB pigment was dispersed to a CB pigment
 content of 15% in a commercial melamine/alkyd paint by a ball mill. The
 colored paint so obtained had low viscosity, and showed fluidity
 substantially close to a Newtonian flow. Further, the black paint was
 mixed with a commercial white melamine/alkyd paint to formulate a gray
 paint. Even after stored for a week, the gray paint was still in a
 homogeneous form without any color separation.
 Comparative Example 8
 A paint was formulated in a similar manner as in Example 45 except that the
 same CB pigment was used without surface treatment with Dispersant 2. The
 paint was ranked as in Example 45. The paint had high viscosity, and in
 the form of a mixed paint with a white paint, the CB pigment underwent
 separation and settling through coagulation.
 EXAMPLE 46
 Eight (8) parts of Dispersant 3 were dissolved in 100 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of C.I. pigment black 7 (CB pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 105 parts of black pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained surface-treated CB pigment was dispersed in a commercial
 acrylic lacquer by a beads mill, whereby a black paint having a CB pigment
 content of 13% was formulated. The paint was adjusted in viscosity with a
 lacquer thinner and was spray-painted on an iron plate. A plate coated in
 a black color and having a dry film thickness of 32 .mu.m was obtained. It
 was excellent in vividness, and showed a high degree of gloss.
 Comparative Example 9
 A paint was formulated in a similar manner as in Example 46 except that the
 same CB pigment was used without surface treatment with Dispersant 3. The
 viscosity was adjusted likewise, and the paint was ranked as in Example
 46. The surface of a coating on a plate coated in a black color was not
 smooth, and its gloss was low.
 EXAMPLE 47
 Dispersed in a horizontal disperser with tumbling medium contained therein
 were 250 parts of the resin solution prepared in the Synthesis Example of
 the film-forming resin, 750 parts of C.I. pigment black, 4 parts of
 Dispersant (9) of Example 9, 536 parts of ethanol, 60 parts of
 methylcyclohexane and 100 parts of ethoxypropanol. Coarse particles were
 then removed by ultracentrifugal separation, whereby a black ink according
 to the present invention was obtained. The average particle size of the CB
 pigment was 95 nm, and the viscosity was 3.5 mPa.multidot.s. The black ink
 was stored at 50.degree. C. for a week, but settling of the CB pigment was
 not observed. When its viscosity was measured, it was still 3.5
 mPa.multidot.s and accordingly, no change took place in viscosity.
 The black ink was next filled in a pen casing equipped with a bundled fiber
 tip, and a writing test was conducted on a polyethylene film. Smooth
 writing was feasible. Further, 20 parts of ethoxypropanol and 5 parts of
 benzyl alcohol were added further to 100 parts of the black ink. Using the
 thus-obtained black ink, printing was performed by an ink-jet printer.
 Good printed matter was obtained.
 EXAMPLES 48-54
 Black inks were obtained in a similar manner as in Example 47 except that
 the dispersants of Examples 10-16 were used as dispersants, respectively.
 Those black inks were then ranked as in Example 47. The results are
 presented in Table 4 together with the results of Example 47. In the
 table, "A" indicates good in both writing characteristics and ink-jet
 printability, while "B" indicates insufficient writing characteristics or
 insufficient ink-jet printability.
 TABLE 4
 Viscosity stability
 of ink
 Shortly After Performance of ink
 after stored Writing Ink-jet
 Kind of formu- for a week character- print-
 dispersant lation (50.degree. C.) istics ability
 Example 47 Dispersant 9 3.5 3.5 A A
 Example 48 Dispersant 10 3.9 3.9 A A
 Example 49 Dispersant 11 3.7 3.7 A A
 Example 50 Dispersant 12 3.8 3.8 A A
 Example 51 Dispersant 13 3.5 3.6 A A
 Example 52 Dispersant 14 3.8 3.8 A A
 Example 53 Dispersant 15 3.8 3.8 A A
 Example 54 Dispersant 16 3.7 3.8 A A
 Comp. Ex. 7 None 63.5 89.0 B B
 EXAMPLE 55
 Ten (10) parts of Dispersant 10 were dissolved in 200 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of C.I. pigment black 7 (CB pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 108 parts of black pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained surface-treated CB pigment was dispersed to a CB pigment
 content of 15% in a commercial melamine/alkyd paint by a ball mill. The
 colored paint so obtained had low viscosity, and showed fluidity
 substantially close to a Newtonian flow. Further, the black paint was
 mixed with a commercial white melamine/alkyd paint to formulate a gray
 paint. Even after stored for a week, the gray paint was still in a
 homogeneous form without any color separation.
 Comparative Example 11
 A paint was formulated in a similar manner as in Example 55 except that the
 same CB pigment was used without surface treatment with Dispersant 10. The
 paint was ranked as in Example 55. The paint had high viscosity, and in
 the form of a mixed paint with a white paint, the CB pigment underwent
 separation and settling through coagulation.
 EXAMPLE 56
 Eight (8) parts of Dispersant 11 were dissolved in 100 parts of an aqueous
 solution which contained 5 parts of glacial acetic acid. The resultant
 solution was added to a slurry of C.I. pigment black 7 (CB pigment
 content: 100 parts), followed by stirring for 60 minutes. A 10% aqueous
 solution of sodium hydroxide was then gradually added to adjust the pH of
 the system to 8.5. After the mixture was stirred for further 30 minutes,
 the resulting solid matter was collected by filtration, washed, dried at
 90.degree. C. and then ground, whereby 105 parts of black pigment powder
 surface-treated with the dispersant were obtained.
 The thus-obtained surface-treated CB pigment was dispersed in a commercial
 acrylic lacquer by a beads mill, whereby a black paint having a CB pigment
 content of 13% was formulated. The paint was adjusted in viscosity with a
 lacquer thinner and was spray-painted on an iron plate. A plate coated in
 a black color and having a dry film thickness of 32 .mu.m was obtained. It
 was excellent in vividness, and showed a high degree of gloss.
 Comparative Example 12
 A paint was formulated in a similar manner as in Example 56 except that the
 same CB pigment was used without surface treatment with Dispersant 11. The
 viscosity was adjusted likewise, and the paint was ranked as in Example
 56. The surface of a coating on a plate coated in a black color was not
 smooth, and its gloss was low.
 This application claims the priority of Japanese Patent Application No. HEI
 10-371915 filed Dec. 28, 1998, which is incorporated herein by reference.