Aqueous ink of pigment type

An aqueous pigment ink is provided made of a pigment, a dispersant, water, and a water-soluble solvent, wherein the dispersant contains a copolymer of a monomer (a) represented by formula (A), and one or more monomers (b) selected from the group consisting of compounds represented by formulae (B) and (C): ##STR1## Wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, AO, m.sub.1, m.sub.2, n, M.sub.1, M.sub.2 and Y are as defined herein that gives improved jettability, storage stability and printing characteristics.

BACKGROUND OF THE INVENTION 
The present invention relates to an aqueous ink of a pigment type that 
permits printing without causing clogging at the head portion of an ink 
jet printer and is excellent in storage stability. 
DESCRIPTION OF THE RELATED ART 
Aqueous inks are used in many cases as inks for printing and writing 
utensils because of the ease with which they can be produced and handled. 
For example, due to the recent development and spread of computers, 
printing apparatuses have also spread. Aqueous inks are commonly used in 
such printing apparatuses. The ink jet recording technique is a typical 
recording technique for these printing apparatuses. Ink jet recording is a 
recording technique wherein ink droplets are expelled from a very small 
nozzle directly onto a recording medium to allow the ink droplets to 
impact on the recording medium, thereby obtaining images, including 
characters. Lately, this technique has been widely used because this 
technique not only provides the advantage that the printer used is low in 
noise and good in operability but also provides the advantage that images 
can be colored easily and plain paper can be used as a recording medium. 
Widely used techniques for jetting an ink in personal printers include the 
piezo-electric technique, which uses a piezo-electric element for a 
printing head, and the thermal jet technique, which uses a heater of a 
heating resistor element for a printing head. 
Conventional inks used in the above-mentioned ink jet recording include 
inks of a dye type and a pigment type. Unfortunately, the conventional 
inks of a dye type are poor in water resistance and light resistance, and 
the conventional inks of a pigment type tend to clog the nozzle of the 
head portion and the pigment element precipitates when the ink is stored. 
To solve the nozzle clogging problem, in the inks used for ink jet 
printers, generally a water-soluble dye is used. However, when using a 
water-soluble dye, while the ink hardly clogs the nozzle, the ink is poor 
in water resistance and light resistance and, in the case of a thermal jet 
technique, the dye is oxidized by excess heat at the printing head causing 
the ink to be scorched easily onto the printing head. 
Further, for the inks of a pigment type, in order to prevent the ink from 
clogging the nozzle and enhance storage stability of the ink, the 
following methods have been proposed: (1) a method wherein a combination 
of a dispersant and a carbon black having a pH of 7 or over is used 
(Japanese Patent Application Laid-Open 4-189876), (2) a method wherein a 
pigment is emulsified with a core material (Japanese Patent Application 
Laid-Open 5-9421), and (3) a method wherein a pigment and insoluble resin 
particles are bonded with an organic silane coupling agent (Japanese 
Patent Application Laid-Open 5-65443). However, these methods are not 
still satisfactory. 
SUMMARY OF THE INVENTION 
Therefore, one object of the present invention is to provide an aqueous ink 
of a pigment type (hereafter called an "aqueous pigment ink") that permits 
printing without causing clogging at the head portion of an ink jet 
printer. 
A further object of the present invention is to provide an aqueous pigment 
ink that provides enhanced storage stability of the ink. 
A further object of the present invention is to provide an aqueous pigment 
ink that exhibits excellent water resistance and light resistance. 
These and other objects of the present invention have been satisfied by the 
discovery of an aqueous pigment ink comprising a pigment, a dispersant, 
water and a water-soluble solvent, wherein the dispersant comprises a 
copolymer of a monomer (a) represented by general formula (A), and one or 
more monomers (b) selected from the group consisting of compounds 
represented by general formulae (B) and (C): 
##STR2## 
wherein R.sub.1 and R.sub.2, which may be the same or different, each 
independently represent a hydrogen atom or an alkyl group having 1 to 3 
carbon atoms, m.sub.t represents an integer of 0 to 2, AO represents an 
oxyalkylene group having 2 to 4 carbon atoms, n represents an integer of 1 
to 300, and X represents a hydrogen atom or an alkyl group having 1 to 3 
carbon atoms, 
##STR3## 
wherein R.sub.3, R.sub.4 and R.sub.5, which may be the same or different, 
each represent a hydrogen atom, an alkyl group having 1 to 3 carbon atoms 
or (CH.sub.2)m.sub.2 COOM.sub.2, R.sub.6 represents a hydrogen atom or an 
alkyl group having 1 to 3 carbon atoms, M.sub.1, M.sub.2 and Y, which may 
be the same or different, each independently represent a hydrogen atom, an 
alkaline metal, an alkaline earth metal, an ammonium, an alkylammonium or 
a substituted alkylammonium, and m.sub.2 represents an integer of 0 to 2, 
with the compounds represented by general formula (B) including acid 
anhydrides thereof. 
DETAILED DESCRIPTION OF THE INVENTION 
Now, the aqueous pigment ink of a pigment type of the present invention 
will be described in detail. 
The dispersant used in the aqueous pigment ink of the present invention is 
a copolymer of a monomer (a) represented by the above-mentioned general 
formula (A), and one or more monomers (b) selected from the group 
consisting of compounds represented by the above-mentioned general 
formulae (B) and (C). 
In the above-mentioned general formula (A), R.sub.1 and R.sub.2, which may 
be the same or different, each independently represents a hydrogen atom or 
an alkyl group having 1 to 3 carbon atoms, preferably a hydrogen atom or a 
methyl group. In particular, it is preferred that both R.sub.1 and R.sub.2 
are the same and represent a hydrogen atom, or alternatively that R.sub.1 
represents a hydrogen atom and R.sub.2 represents a methyl group. Further, 
m.sub.1 represents an integer of 0 to 2, preferably 0 to 1. Further, AO 
represents an oxyalkylene group having 2 to 4 carbon atoms, preferably an 
oxyalkylene group having 2 to 3 carbon atoms and more preferably an 
oxyethylene group. Further, n represents an integer of 1 to 300, 
preferably an integer of 2 to 150. X represents a hydrogen atom or an 
alkyl group having 1 to 3 carbon atoms, preferably a hydrogen atom, a 
methyl group or an ethyl group. 
Preferably the above-mentioned monomer (a) is made up of 0.1 to 10 mol % of 
a monomer represented by the above-mentioned general formula (A) wherein n 
(the number of introduced oxyalkylene groups) is 1 or 2 (the monomer is 
hereinafter referred to as "monomer a.sub.1 "), and 90 to 99.9 mol % of a 
monomer represented by above-mentioned general formula (A) wherein n is 3 
or more (the monomer is hereinafter referred to as "monomer a.sub.2 "). By 
using a copolymer that satisfies these conditions, an aqueous pigment ink 
can be provided that is further improved in pigment dispersibility. In 
addition, the viscosity of the ink can be improved, thereby improving the 
flying of the ink droplets. Therefore, an aqueous pigment ink that is 
particularly suitable as an ink for ink jet recording can be obtained. 
More preferably, the proportion of the monomer a.sub.2 in the 
above-mentioned monomer (a) is 92 to 98 mol % (that is, the proportion of 
the monomer a.sub.1 is 2 to 8 mol %), and most preferably 93 to 97 mol % 
(that is, the proportion of the monomer a.sub.1 is 3 to 7 mol %). 
The above-mentioned monomer a.sub.2 is preferably a monomer, wherein n is 3 
to 150 and more preferably a monomer wherein n is 4 to 130. 
Specific examples of the preferred compounds represented by the 
above-mentioned general formula (A) include esterified compounds formed 
between a polyalkylene glycol in which one terminal thereof is blocked 
with an alkyl group, such as methoxypolyethylene glycol, 
methoxypolyethylene polypropylene glycol, ethoxypolyethylene glycol, 
ethoxypolyethylene polypropylene glycol, propoxypolyethylene glycol, and 
propoxypolyethylene polypropylene glycol, and acrylic acid, methacrylic 
acid or a dehydrogenated (oxidized) reaction product of a fatty acid; and 
adducts of ethylene oxide, propylene oxide or butylene oxide (preferably 
ethylene oxide or propylene oxide) to acrylic acid, methacrylic acid, or a 
dehydrogenated (oxidized) reaction product of a fatty acid. In particular, 
esterified compounds formed between a polyalkylene glycol in which one 
terminal thereof is blocked with an alkyl group and acrylic acid, 
methacrylic acid or a dehydrogenated (oxidized) reaction product of a 
fatty acid are preferred. With respect to the adducts of ethylene oxide 
and propylene oxide, any of random adducts, block adducts or alternating 
adducts can be used. Preferably the addition molar ratio of the ethylene 
oxide to the propylene oxide in these adducts is from 10:90 to 90:10, and 
more preferably from 10:90 to 60:40. 
The compound represented by the above-mentioned general formula (B) 
includes unsaturated monocarboxylic acids and their salts, and unsaturated 
dicarboxylic acids and their salts. Specific examples include acrylic 
acid, methacrylic acid, crotonic acid, metal salts of these acids, and 
unsaturated dicarboxylic acid monomers such as maleic anhydride, maleic 
acid, itaconic anhydride, itaconic acid, citraconic anhydride, citraconic 
acid, fumaric acid, or alkaline metal salts, alkaline earth metal salts, 
ammonium salts, and amine salts, such as alkyl amine salts and substituted 
alkyl amine salts, of these acids. In particular, acrylic acid, 
methacrylic acid, crotonic acid and metal salts of these acids are 
preferred. The preferred carbon number of the alkyl group of the amine 
salts is from 1 to 3. Preferred examples of the substituted alkyl amine 
include monoethanol amine, diethanol amine and triethanol amine. 
Specific examples of the preferred compounds represented by the 
above-mentioned formula (C) include allylsulfonic acid, methallylsulfonic 
acid, or alkaline metal salts, alkaline earth metal salts, ammonium salts, 
and amine salts such as alkyl amine salts and substituted alkyl amine 
salts thereof. The preferred carbon number of the alkyl group of the amine 
salts and the preferred examples of the substituted alkyl amine are the 
same as those compounds represented by the above-mentioned general formula 
(B). 
The dispersant obtained by copolymerizing the monomer (a) represented by 
general formula (A), and one or more monomers (b) selected from the group 
consisting of compounds represented by general formulae (B) and (C) 
preferably includes a copolymer obtained by copolymerizing the monomer (a) 
represented by general formula (A), and the monomer (b) represented by 
general formula (B). 
The preparation method of the copolymers is not particularly limited so 
long as the function of the obtained copolymer as a dispersant is not 
impaired, and conventionally known preparation methods can be used. 
Specific examples include solvent polymerization methods described in 
Japanese Patent Application Laid-Open 59-162163, Japanese Patent 
Application Laid-Open 62-119147 corresponding to U.S. Pat. Nos. 4,870,120 
and 5,137,945, Japanese Patent Application Laid-Open 62-78137 and Japanese 
Patent Application Laid-Open 62-70250, all of which are hereby 
incorporated by reference. 
Examples of solvents used in the above-mentioned solvent polymerization 
methods include water; mono to tetrahydric alcohols such as methanol, 
ethanol and isopropanol; benzene, toluene and xylene; aliphatic 
hydrocarbons such as cyclohexane and n-hexane; ethyl acetate; acetone; and 
methyl ethyl ketone. Taking the handling and reaction facilities into 
account, water, methanol, ethanol and isopropanol are preferably used. 
In the solvent polymerization method wherein an aqueous solvent is used as 
a polymerization initiator, a water-soluble polymerization initiator is 
used, such as ammonium salts and alkaline metal salts of persulfuric acids 
(i.e., peroxosulfuric acid and peroxodisulfuric acid) and hydrogen 
peroxide. In the solvent polymerization method wherein a solvent other 
than an aqueous solvent is used, for example, benzoyl peroxide or lauroyl 
peroxide is used as a polymerization initiator. 
In combination with the polymerization initiator, it is possible to use a 
polymerization promoter, such as sodium hydrogensulfite, mercaptoethanol, 
and amine compounds. 
The copolymer preferably has a molar ratio of the monomer (a) to the 
monomer (b) (a)/(b)! of from 0.1/100 to 100/100, and more preferably from 
1/100 to 70/100. By bringing the molar ratio between the monomers (a) and 
(b) to fall in the above range, the dispersant favorably exhibits 
excellent dispersibility, storage stability of dispersibility and water 
solubility. 
In the copolymer, other copolymerizable monomers may be used additionally 
as a copolymerization component in a range that will not impair the 
function of the dispersant. Such a monomer includes, for example, 
(meth)acrylonitrile, (meth)acrylamide, styrene, and styrene sulfonic acid. 
The above monomers (a) and (b), can be copolymerized by random 
copolymerization, block copolymerization, alternating copolymerization, or 
graft copolymerization. The type of polymer thus produced is not 
particularly restricted, so long as the function of the dispersant of the 
obtained copolymer is not impaired. 
The weight-average molecular weight of the copolymer is preferably from 
5,000 to 200,000, more preferably from 5,000 to 120,000 and most 
preferably from 5,000 to 70,000. If the copolymer has a high molecular 
weight, the viscosity becomes high thereby making the viscosity of the 
resulting ink high, and thereby leading to a fear that the performance of 
the ink will be lowered. 
The above weight-average molecular weight can be obtained by gel permeation 
chromatography and calibrated in terms of poly (sodium styrene sulfonate). 
In the above-mentioned copolymers, a copolymer of a polyalkylene glycol 
monoester monomer, in which preferably 1 to 300 oxyalkylene groups, more 
preferably 2 to 150 oxyalkylene groups, and most preferably 4 to 130 
oxyalkylene groups, are introduced per molecule particularly a 
polyalkylene glycol ester or an alkoxypolyalkylene glycol ester of 
(meth)acrylic acid, more particularly a polyalkylene glycol ester of 
(meth)acrylic acid!, and a (meth)acrylic acid or (meth)acrylate monomer is 
desirable in view of the dispersibility and the dispersion stability of 
the pigment. 
In the aqueous pigment ink of the present invention, by allowing the 
above-mentioned copolymer serving as a dispersant to be contained together 
with the pigment, nozzle clogging does not occur, particularly when 
printing is carried out by using an ink jet printer, so that a high 
storage stability of the ink can be obtained. Although the reason is not 
sufficiently clear and the present inventors do not wish to be bound by 
any particular theory of action, the mode of operation is believed to be 
as follows: 
The copolymer in the ink surrounds the surface of each particle of the 
pigment in such a way that the charged sites in the molecules (e.g., the 
carbonyl sites) are adsorbed on the surface of each particle of the 
pigment with the side chains, i.e., the polyoxyalkylene groups, directed 
outward. Since the chain of the polyoxyalkylene group is long and is 
highly likely to cause steric hindrance, the agglomeration of particles of 
the pigment is hampered by the polyoxyalkylene groups. As a result, it 
seems that clogging at the nozzle is prevented and the storage stability 
of the ink is enhanced. Since the steric hindrance by the polyoxyalkylene 
groups is particularly improved when the pH of the aqueous pigment ink of 
the present invention is high, it is preferable that the pH of the aqueous 
pigment ink of the present invention should be set within a high range as 
described later. 
In contrast, styrene copolymers, such as a styrene/acrylic acid copolymer, 
or vinylnaphthalene copolymers, such as a vinylnaphthalene/acrylic acid 
copolymer, that are conventionally used as a dispersant surround the 
surface of each particle of the pigment in such a way that the lipophilic 
groups in the molecules (e.g., the aromatic rings of the copolymer) are 
adsorbed on the surface of each particle of the pigment with the 
hydrophilic groups (e.g., the carbonyl sites of the copolymer) directed 
outward. The electric repulsive forces between the hydrophilic groups on 
the surfaces of particles of the pigment prevents the particles of the 
pigment from agglomerating, thereby enhancing the dispersibility of the 
pigment. However, it seems that since there is a limit on the electric 
repulsive forces between the hydrophilic groups and the hydrophilic group 
is not likely to exhibit steric hindrance, the particles of the pigment 
are apt to agglomerate and as a result the storage stability of the ink is 
lowered. 
In the present invention, an additional dispersant other than the 
above-mentioned copolymers can be used in combination. Examples of the 
additional dispersant include anionic surface-active agents, such as a 
surface active agent selected from the group consisting of alkyl sulfonate 
salts, alkylbenzene sulfonate salts, alkylnaphthalene sulfonate salts, 
alkane or olefin sulfonate salts, alkyl sulfate ester salts, 
polyoxyethylene alkyl sulfate ester salts, alkylallyl ether sulfate ester 
salts, alkyl phosphate salts, alkyldiphenyl ether disulfonate salts, ether 
carboxylate, alkyl sulfosuccinate salts, .alpha.-sulfo fatty acid ether 
and .alpha.-sulfo fatty acid salts; a condensate of a higher fatty acid 
and an amino acid; and naphthenic acid salt. Preferred surface active 
agents include alkyl (in particular, linear alkyl) benzene sulfonate 
salts, alkane (in particular, secondary alkane) or olefin (in particular, 
.alpha.-olefin) sulfonate salts, alkylsulfonate ester salts, 
polyoxyethylene alkyl sulfonate ester salts, polyoxyethylene alkylallyl 
ether sulfate ester salts (in particular, polyoxyethylene alkyl ether 
sulfate ester salts), alkyl (in particular, monoalkyl) phosphate salts, 
ether carboxylate, alkyl sulfosuccinate salts, .alpha.-sulfo fatty acid 
ester and .alpha.-sulfo fatty acid salts. More preferably, alkyl sulfonate 
salts, alkyl (in particular, linear alkyl) benzene sulfonate salts, 
polyoxyethylene alkyl sulfonate ester salts, polyoxyethylene alkylallyl 
ether sulfate ester salts (in particular, polyoxyethylene alkyl ether 
sulfate ester salts) and alkyl sulfate ester salts can be used. Unless 
otherwise specified, within the context of the present invention, the term 
"alkyl" or "alkane" includes C.sub.1 -C.sub.20 Compounds, preferably 
C.sub.1 -C.sub.14, more preferably C.sub.1 -C.sub.8. These agents can be 
used singly or in a combination of two or more thereof. 
As cationic surface-active agents, aliphatic amine salts, quaternary 
ammonium salts, sulfonium salts, and phosphonium salts can be used. These 
agents can be used singly or in a combination of two or more thereof. 
As non-ionic surface-active agents, those selected from the group 
consisting of polyoxyethylene alkyl ethers, polyoxyethylene alkylallyl 
ethers, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid 
esters, polyoxyethylene sorbitol fatty acid esters, glycerin fatty acid 
esters, polyoxyethylene fatty acid esters and alkyl (poly)glycosides can 
be used. Preferred non-ionic surface-active agents include those selected 
from the group consisting of polyoxyethylene alkyl ethers and 
polyoxyethylene alkylallyl ethers. These agents can be used singly or in a 
combination of two or more thereof. 
As amphoteric surface-active agents, one or more amino acid type or betaine 
type compounds can be used. 
Also, polymetric dispersants can be used. The polymetric dispersants 
include, for example, proteins such as gelatin and casein; natural rubbers 
such as gum arabic; glucoxides such as saponin; cellulose derivatives such 
alkyl cellulose, carboxyalkyl, cellulose and hydroxyalkyl cellulose; 
lignin sulfonates; natural polymers such as shellac; anionic polymers such 
as polyacrylate salts, styrene/acrylic acid copolymer salts, 
vinylnaphtalene/acrylic acid copolymer salts, styrene/maleic acid 
copolymer salts, vinylnaphtalene/maleic acid copolymer salts, salts of 
.beta.-naphtalenesulfonic acid/formaldehyde condensates, and 
polyphosphoric acids; and nonionic polymers such as polyvinyl alcohol, 
polyvinyl pyrrolidone, and polyalkylene glycol. These polymeric 
dispersants can be used singly or in a combination of two or more thereof. 
When the above-mentioned additional dispersants are used, in view of the 
printing density of the printed images and the improvement in the 
dispersion stability under long-term storage, it is preferable that 
anionic surface-active agents should be used, or alternatively, anionic 
surface-active agents and nonionic surface-active agents should be used in 
combination with the amount of each surface-active agent being 0.01 to 5% 
by weight (preferably 0.01 to 3% by weight and more preferably 0.05 to 2% 
by weight) and with the weight ratio of the anionic surface-active agent 
to the nonionic surface-active agent being from 100:1 to 1:100 (preferably 
from 90:1 to 1:70, and more preferably from 70:1 to 1:50). 
Further, when an anionic surface-active agent is used as the 
above-mentioned additional dispersant, the copolymer and the anionic 
surface-active agent are preferably formulated in a weight ratio of the 
copolymer to the anionic surface-active agent in a range of from 1:2 to 
30:1. If the amount of the copolymer exceeds the above range, the 
wettability of the copolymer for the pigment becomes unsatisfactory and 
the dispersibility sometimes lowers. On the other hand, if the amount of 
the anionic surface-active agent exceeds the above range, the frothing 
tendency of the ink increases and therefore the handleability of the ink 
can become poor. More preferably, the ratio of the copolymer to the 
anionic surface-active agent is from 1:1 to 15:1, and most preferably from 
1:1 to 10:1. 
When the additional dispersant is used in combination, the additional 
dispersant is preferably contained at an amount of 1 to 10,000 parts by 
weight, more preferably 10 to 1,000 parts by weight, based on 100 parts by 
weight of the copolymer. 
Any conventional pigment can be used in the aqueous pigment ink of the 
present invention, so long as it is a pigment. Examples of suitably 
pigments include carbon black; C.I. Pigment Yellow 1,5, 12, 14, 17, 24, 
42, 53, 83, 95, 97, 98 and 100; C.I. Pigment Red 1,3,4,5, 17, 22, 31, 48, 
48:3, 49, 53, 63, 64, 81:1, 83, 88, 101, and 122; C.I. Pigment Blue 1, 15, 
15:3, 16, 27, 28, 29, 56, 60 and 63; and C.I. Pigment Black 1,7 and 11, 
with carbon black being particularly preferred. 
In the present invention, the term "pigment" refers to a solid in the form 
of fine particles that is insoluble in water, solvents, oils, and the 
like, whereas the term "dye" refers to a compound that can dye water or 
solvents when it is dissolved in water or the solvents. "Color Chemical 
Jiten" (first impression, issued in March, 1988, edited by CMC KK, 
Yukigousei Kagaku-kyokai), pp.45-!. 
Therefore, within the context of the present invention, pigments and dyes 
are different from one another. 
In order to improve the storage stability of the ink, preferably the 
average particle diameter of the pigment is 0.05 to 3 .mu.m, and more 
preferably 0.05 to 1 .mu.m. 
With respect to the distribution of the particle diameter, the present 
pigment is preferably made up of 60 to 98% by volume of particles having a 
particle diameter of less than 0.2 .mu.m, 0.5 to 40% by volume of 
particles having a particle diameter of 0.2 .mu.m or more but less than 
0.6 .mu.m, and 5% by volume or less of particles having a particle 
diameter of 0.6 to 3 .mu.m. By using the pigment having the above particle 
diameter distribution, particularly in an ink for ink jet recording, a 
very small amount of coarse particles forms a coarse coating on the 
printer head, and as a result, the pigment particles are prevented from 
further sticking to the printer head, thereby to improving the long-term 
jetting stability of the ink. Further, when particles having a narrow 
particle distribution are used, the particles are likely to agglomerate. 
To the contrary, by using the pigment particles having the above particle 
diameter distribution, since the particles are prevented from 
agglomerating, the dispersibility of the pigment particles is further 
improved, thus further enhancing the long-term storage stability of the 
ink under high temperatures. 
More preferably, the pigment is made up of 75 to 98% by volume of particles 
having a particle diameter of less than 0.2 .mu.m, 0.5 to 20% by volume of 
particles having a particle diameter of 0.2 .mu.m or more but less than 
0.6 .mu.m, and 3% by volume or less of particles having a particle 
diameter of 0.6 to 3 .mu.m, and most preferably is made up of 85 to 98% by 
volume of particles having a particle diameter of less than 0.2 .mu.m, 0.5 
to 10% by volume of particles having a particle diameter of 0.2 .mu.m or 
more but less than 0.6 .mu.m, and 1.5% by volume or less of particles 
having a particle diameter of 0.6 to 3 .mu.m. 
The particle diameter distribution of the pigment can be measured, for 
example, by using a centrifugal settling type distribution measuring 
apparatus(manufactured by Shimadzu Corporation). 
When carbon black is used as the above-mentioned additional dispersant 
pigment, in seeking to improve the dispersion stability after long-term 
storage and to improve the printing quality, the carbon black is 
preferably one having a volatile content of 4 to 25% by weight, more 
preferably 9 to 25% by weight, and most preferably 9.5 to 20% by weight. 
The volatile content is the value measured according to the description in 
Japanese Patent Application Laid-Open 3-210373, page 4, left lower column 
which is hereby incorporated by reference. It is also preferable to use 
carbon black having an oil absorption (JIS K 6221-1982) of 250 to 900 
g/100 g, more preferably of 250 to 800 g/100 g, and most preferably 300 to 
700 g/100 g. 
When color printing is performed by the subtractive color mixing method 
using the aqueous pigment ink of the present invention, an aqueous pigment 
ink set comprising four inks containing a black pigment, a yellow pigment, 
a magenta pigment, and a cyan pigment, respectively, is preferably used. 
In this case, if carbon black is used as the black pigment, the yellow 
pigment is preferably a pigment selected from among C.I. Pigment Yellow 17 
and 83, the magenta pigment is preferably a pigment selected from among 
C.I. Pigment Red 48:3, 83 and 122, and the cyan pigment is preferably C.I. 
Pigment Blue 15:3. This combination of pigments is preferable because 
black printed images which are excellent in black tone formed by the black 
ink only and processed black formed by mixing the three colors of the 
yellow ink, the magenta ink, and the cyan ink can be obtained and the 
color reproducibility is improved. 
In the aqueous pigment inks of the present invention, the weight ratio of 
the pigment to the copolymer (pigment/copolymer) is generally from 1/10 to 
10/1, preferably from 1/7 to 7/1, more preferably from 1/5 to 5/1. If the 
weight ratio is smaller than 1/10, in some cases the staining power of the 
pigment degrades, thereby lowering the performance of the ink. If the 
weight ratio is larger than 10/1, insufficient dispersion may take place, 
so that the pigment fails to be present in a stable state. 
The above-mentioned water soluble solvent for use in the aqueous pigment 
ink of the present invention is used as a wetting agent or a humectant. 
Examples of the water soluble solvent include glycols, such as ethylene 
glycol, propylene glycol, diethylene glycol, triethylene glycol, 
tetraethylene glycol, and polyethylene glycol; glycerin; ethers of 
polyhydric alcohols, such as diethylene glycol diethyl ether, diethylene 
glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol 
monoethyl ether, ethylene glycol monobutyl ether, methyl carbitol, ethyl 
carbitol, butyl carbitol, ethyl carbitol acetate, diethyl carbitol, 
triethylene glycol monomethyl ether, triethylene glycol monoethyl ether 
and propylene glycol monomethyl ether; acetates; thiodiglycol; 
nitrogen-containing compounds, such as N-methyl-2-pyrrolidone, 
1,3-dimethylimidazolidinone, triethanol amine, formamide, and dimethyl 
formamide; and dimethyl sulfoxide. These water soluble solvents can be 
used singly or in a combination of two or more thereof. 
In the aqueous pigment ink of the present invention (particularly in the 
case where it is used as an ink for ink jet recording), ion exchanged 
water is preferably used. 
In addition to the above-described components, various components described 
below may be added to the aqueous pigment ink of the present invention if 
desired. 
A polypropylene glycol can be added in order to balance between improved 
permeability of the ink into recording paper and prevention of 
strike-through of the ink (exudation of the ink into the backside of the 
recording paper). Preferably, the polypropylene glycol has a molecular 
weight of 400 to 3,000, and more preferably 400 to 2,000. When present, 
the polypropylene glycol is used in an amount of 0.01 to 3% by weight, and 
more preferably 0.01 to 1% by weight, in the aqueous pigment ink of the 
present invention. 
Further, an aliphatic monohydric alcohol can be added to the aqueous 
pigment ink of the present invention in order to improve the drying 
properties of the ink. As the aliphatic monohydric alcohol, at least one 
of ethyl alcohol, 1-propyl alcohol, and 2-propyl alcohol is preferably 
used with particular preference given to 1-propyl alcohol. When present, 
the aliphatic monohydric alcohol is used preferably in an amount of 0.01 
to 10% by weight, and more preferably 0.1 to 8% by weight, in the aqueous 
pigment ink of the present invention. By adjusting the ratio of the 
copolymer to the aliphatic monohydric alcohol to be from 1:10 to 1:1, the 
drying properties of the ink are further improved. 
The intermittent jetting properties of the present aqueous pigment ink can 
be improved, particularly in the case wherein the ink is used in ink jet 
recording by adding a polyethylene glycol disulfate salt having a 
molecular weight of 400 to 3,500 (preferably 400 to 2,000). Preferably, 
the polyethylene glycol disulfate salt is used in an amount of 0.01 to 3% 
by weight, and more preferably 0.01 to 1% by weight, in the aqueous 
pigment ink of the present invention. 
Similarly, in order to improve the intermittent jetting properties, a 
saccharide can be used in the aqueous pigment ink of the present 
invention. As the saccharide, preferably at least one of D-glucose, 
fructose, maltose, and saccharose is used, with particular preference 
given to saccharose. The saccharide is used preferably in an amount of 0.1 
to 5% by weight, and more preferably 0.1 to 3% by weight, in the aqueous 
pigment ink of the present invention. By adjusting the ratio of the 
copolymer to the saccharide to be from 20:1 to 21:5, the intermittent 
jetting properties are further improved. 
In order to further improve the intermittent jetting properties, a 
nitrogen-containing heterocyclic compound, such as N-methyl-2-pyrrolidone 
and 1,3-dimethyl-2-imidazolidinone, can be added to in the aqueous pigment 
ink of the present invention. The nitrogen-containing heterocyclic 
compound is used preferably in an amount of 0.01 to 10% by weight, and 
more preferably 0.1 to 5% by weight. 
Further, the intermittent jetting properties can be improved by adding an 
aprotic polar solvent to the aqueous pigment ink of the present invention. 
As the aprotic polar solvent, at least one of dimethyl sulfoxide and 
sulfolane is preferably used. The aprotic polar solvent is preferably used 
in an amount of 0.01 to 10% by weight, and more preferably 0.1 to 5% by 
weight, in the aqueous pigment ink of the present invention. 
When these polyethylene glycol disulfate salts, saccharides, 
nitrogen-containing heterocyclic compounds, and aprotic polar solvents 
that can improve the intermittent jetting properties are used in 
combination with a pigment having an average particle diameter of 0.05 to 
3 .mu.m, the intermittent jetting properties are even further improved. 
In order to improve the fixability of the present aqueous pigment ink, an 
emulsion, such as an acrylic emulsion, an aqueous urethane emulsion, a 
styrene/butadiene emulsion, or a polyester emulsion, can be added. 
Preferably, an emulsion of an acryl/urethane copolymer is used, more 
preferably an emulsion of an acryl/urethane block copolymer (a copolymer 
having a main chain of an acrylic chain with urethane grafted as side 
chains). The weight-average molecular weight of the copolymer used in the 
emulsion is preferably 10,000 to 300,000, and more preferably 20,000 to 
250,000. The emulsion is present in the aqueous pigment ink of the present 
invention in an amount of 0.01 to 10% by weight, and more preferably 0.05 
to 5% by weight, in terms of solid content. 
Similarly, in order to improve the fixability of the ink, a 
polyethyleneimine having a weight-average molecular weight of 800 to 
300,000 can be added to the aqueous pigment ink of the present invention. 
As the polyethyleneimine, at least one of a polyethyleneimine grafted as 
side chains to a main chain of an acrylic chain, a polyethyleneimine that 
has been subjected to hydroxylation, and an amphoterically ionized 
polyethyleneimine is preferably used, with particular preference given to 
a polyethyleneimine that has been subjected to hydroxylation. The 
polyethyleneimine is used preferably in an amount of 0.01 to 10% by 
weight, more preferably 0.05 to 5% by weight, in the aqueous pigment ink 
of the present invention. The polyethyleneimine grafted as side chains to 
a main chain of an acrylic chain can be synthesized, for example, by a 
method wherein a polyacrylic acid is converted with thionyl chloride to a 
polyacrylic acid chloride and the polyacrylic acid chloride is completely 
reacted with a polyethyleneimine. The polyethyleneimine that has been 
subjected to hydroxylation can be synthesized, for example, by a method 
wherein a polyacrylic acid is converted with thionyl chloride to an acid 
chloride, and after the excess of the polyacrylic acid is reacted with a 
polyethyleneimine, the excess of the acid chloride is hydrolyzed back to 
the acid. The amphoterically ionized polyethyleneimine can be synthesized, 
for example, by a method wherein a polyacrylic acid is converted with 
thionyl chloride to a polyacrylic acid chloride and the polyacrylic acid 
chloride is reacted with part of a polyethyleneimine. 
Other components including conventionally known various additives such as 
antifoaming agent of a silicone type or the like, a mildewproofing agent 
of a chloromethyl phenol type or the like, and/or a chelate agent such as 
EDTA, or an oxygen absorbing agent such as sulfite may be added to the 
aqueous pigment ink of the present invention. 
Although the formulation of the aqueous pigment ink of the present 
invention is not particularly limited, generally the components are used 
in the following ranges on the basis of the total weight of the ink of the 
present invention: 
______________________________________ 
Pigment: 1 to 20% by weight, preferably 2 to 10% by 
weight 
Dispersant: 0.3 to 30% by weight, preferably 1 to 15% by 
weight 
Water-soluble solvent: 
1 to 30% by weight, preferably 5 to 20% by 
weight 
Water: 40 to 97.7% by weight, preferably 75 to 92% 
by weight 
Other components: 
0 to 10% by weight, preferably 1 to 10% by 
weight 
______________________________________ 
Within the context of the present invention, the aqueous pigment ink 
composition comprises the composition resulting from admixture of the 
recited components, including any resultant reaction products formed by 
interaction or reaction of the added components. 
The preparation method of the aqueous pigment ink of the present invention 
by mixing the above-mentioned components is not particularly limited. The 
components can be mixed by using a conventionally known apparatus such as 
a ball mill, a sand mill, an attritor, a basket mill and a roll mill. 
In the preparation of the aqueous pigment ink of the present invention, it 
is preferable to remove coarse particles. For instance, an ink that does 
not cause clogging can be obtained by subjecting the ink obtained after 
formulation to a centrifugal machine to remove particles preferably of 
3,000 nm or more in size, more preferably 2,000 nm or more, most 
preferably of 1,000 nm or more. 
The viscosity (25.degree. C.) of the thus obtained aqueous pigment ink of 
the present invention is generally 1 to 10 cps, preferably 1 to 5 cps, in 
order to improve the stability of jetting, particularly in the case 
wherein the ink is used in ink jet recording. 
The surface tension (25.degree. C.) of the aqueous pigment ink of the 
present invention is generally 25 to 50 dyne/cm, preferably 30 to 45 
dyne/cm, in view of the permeability into the material to be printed. 
To bring the viscosity to within the above range, for example, it is 
recommended that the pH of the aqueous pigment ink of the present 
invention be adjusted to such a value that the thickness of the adsorption 
layer (a layer formed on the surface of the pigment particle by 
adsorption) of the copolymer may be optimized, or alternatively, that the 
solid content in the ink is optimized. 
To bring the surface tension to within the above range, for example, it is 
recommended that the above various surface-active agents are used in the 
ink. 
The aqueous pigment ink of the present invention preferably has a specific 
heat of more than 4.00 J/gK but 4.15 J/gK or less, and a heat conductivity 
of 0.1.times.10.sup.-3 to 50.times.10.sup.-3 W/(cm.deg) in order to 
prevent clogging of the ink due to drying at the tip of a printer head and 
to prevent scorching of the ink due to heat energy at the time of 
printing. In this case, the pigment is preferably formulated in an amount 
of 2 to 5% by weight and the copolymer is formulated in an amount of 0.5 
to 6% by weight. To bring the specific heat and the heat conductivity of 
the ink to within the above ranges, it is recommended that the amounts of 
the water (preferably ion exchanged water) and the water soluble solvent 
are suitably adjusted. 
To improve the storage stability of the ink, the printing density of the 
printed images, and particularly the printing density of black, the 
aqueous pigment ink of the present invention preferably has a pH of 9.5 to 
13.0, more preferably 10.0 to 12.0. To bring the pH to within the above 
range, it is recommended that a pH adjuster, such as an amino-alcohol, be 
added to the aqueous pigment ink of the present invention. 
As the amino-alcohol, a monoethanol amine (e.g., 2-aminoethanol) or a 
triamino ethanol is preferably used. 
To improve the fixability, particularly to materials on which recording 
will be made, such as plastics (in particular sheets for OHP), metals, and 
cloth, the aqueous pigment ink is preferably formulated so that the 
water-soluble resin component the above copolymer as well as the above 
emulsion and the above polyethyleneimine (both are resin components for 
fixing the ink) which will be used as desired! in the supernatant liquid 
of the ink, after settling the pigment by an ultracentrifugation operation 
(50,000 G, 4 hours), is 2 to 7% by weight. 
The aqueous pigment ink of the present invention is suitable for ink jet 
recording, and when it is used for ink jet recording, any printer that 
employs the ink jet recording system can be used. For example, printers 
that employ the piezo-electric recording technique that uses a 
piezo-electric element for a printing head and printers that employ the 
thermal jet recording technique that uses thermal energy generated by a 
heating resistor element in the printing head or the like can be used. 
Further, the aqueous pigment ink of the present invention can also be used 
as an ink for writing utensils, such as fountain pens, ball-point pens, 
Magic Markers, and felt-tipped pens.

EXAMPLES 
Having generally described this invention, a further understanding can be 
obtained by reference to certain specific examples which are provided 
herein for purposes of illustration only and are not intended to be 
limiting unless otherwise specified. In the following Examples, all the 
parts represent parts by weight unless otherwise stated. 
Example 1 
______________________________________ 
Carbon black: 15 parts 
Copolymer of polyethylene glycol 
3 parts 
monoacrylate in which 45 mol of 
oxyethylene groups were introduced, 
and sodium acrylate the molar ratio 
of the monomers (the former/the latter) = 2/8!: 
Monoethanol amine: 1 part 
Diethylene glycol: 5 parts 
Ion exchanged water: 77 parts 
______________________________________ 
The above components were dispersed for 4 hours by a sand mill to obtain 
Dispersion A. 
______________________________________ 
Dispersion A: 33 parts 
Diethylene glycol: 
4 parts 
Ion exchanged water: 
63 parts 
______________________________________ 
After the components were formulated, the formulation was processed by a 
centrifugal machine at 4,000 rpm for 20 min to remove coarse particles, 
thereby obtaining an ink. This aqueous pigment ink was excellent in 
storage stability. This ink was used for printing by using a thermal jet 
printer BJ-10 Lite manufactured by Canon Inc. that employed a heater of a 
heating resistor element as a source of the thermal energy, and good 
printed images were obtained without causing clogging at the head portion. 
Further, it was found that these printed images were excellent in water 
resistance and light resistance. The results are shown in Table 1. 
Example 2 
______________________________________ 
C.I. Pigment Red 81:1: 15 parts 
Copolymer of polyethylene glycol 
3 parts 
monomethacrylate in which 4 mol of 
oxyethylene groups were introduced, 
and sodium methacrylate the molar 
ratio of the monomers the former/the latter) = 1/9!: 
Diethylene glycol: 5 parts 
Ion exchanged water: 77 parts 
______________________________________ 
The above components were dispersed for 4 hours by a sand mill to obtain 
Dispersion B. 
______________________________________ 
Dispersion B: 33 parts 
Diethylene glycol: 
4 parts 
Ion exchanged water: 
63 parts 
______________________________________ 
After the components were formulated, the formulation method shown in 
Example 1 was followed to obtain an ink. This aqueous pigment ink was 
excellent in storage stability. When this ink was used for printing in the 
same manner as in Example 1, good printed images were obtained without 
causing clogging at the head portion. Further it was found that these 
printed images were excellent in water resistance and light resistance. 
The results are shown in Table 1. 
Example 3 
______________________________________ 
C.I. Pigment Blue 15:1: 15 parts 
Copolymer of polyethylene glycol 
4 parts 
monoacrylate in which 130 mol of 
oxyethylene groups were introduced, 
and sodium acrylate the molar ratio 
of the monomers (the former/the latter) = 3/7!: 
Monoethanol amine: 1 part 
Diethylene glycol: 6 parts 
Ion exchanged water: 74 parts 
______________________________________ 
The components were dispersed for 4 hours by a sand mill to obtain 
______________________________________ 
Dispersion C: 33 parts 
Diethylene glycol: 
6 parts 
Ion exchanged water: 
61 parts 
______________________________________ 
After the components were formulated, the preparation method shown in 
Example 1 was followed to obtain an ink. The ink was particularly 
excellent in storage stability. When this ink was used for printing in the 
same manner as in Example 1, good printed images were obtained without 
causing clogging at the head portion. Further it was found that these 
printed images were excellent in water resistance and light resistance. 
The results are shown in Table 1. 
Comparative Example 1 
______________________________________ 
Carbon black: 15 parts 
Copolymer of styrene and sodium acrylate 
4 parts 
the molar ratio of the monomers 
(the former/the latter) = 3/7!: 
Diethylene glycol: 6 parts 
Ion exchanged water: 75 parts 
______________________________________ 
The above components were dispersed for 4 hours by a sand mill to obtain 
Dispersion D. 
______________________________________ 
Dispersion D: 33 parts 
Glycerin: 5 parts 
Ion exchanged water: 
62 parts 
______________________________________ 
After the components were formulated, the formulation method shown in 
Example 1 was followed to obtain an ink. The ink had a problem in storage 
stability, and precipitation was observed after storage test performed for 
one month at 40.degree. C. When this ink was used for printing in the same 
manner as in Example 1, clogging occurred and good printed images were not 
obtained. These printed images were, however, excellent in water 
resistance and light resistance. The results are shown in Table 1. 
Comparative Example 2 
______________________________________ 
C.I. pigment blue 15:3: 
18 parts 
Copolymer of styrene and sodium maleate 
6 parts 
the molar ratio of the monomers 
(the former/the latter) = 1/1!: 
Glycerin: 6 parts 
Ion exchanged water: 70 parts 
______________________________________ 
The above components were dispersed for 4 hours by a sand mill to obtain 
Dispersion E. 
______________________________________ 
Dispersion E: 33 parts 
Diethylene glycol: 
12 parts 
Ion exchanged water: 
55 parts 
______________________________________ 
After the components were formulated, the formulation method shown in 
Example 1 was followed to obtain an ink. The ink had a problem in storage 
stability, precipitation was observed after storage test performed for one 
month at 40.degree. C. When this ink was used for printing in the same 
manner as in Example 1, clogging occurred and good printed images were not 
obtained, although these printed images were excellent in water resistance 
and light resistance. The results are shown in Table 1. 
Comparative Example 3 
______________________________________ 
Food Black 2 (dye): 5 parts 
Diethylene glycol: 6 parts 
Ion exchanged water: 89 parts 
______________________________________ 
After the components were formulated, the preparation method shown in 
Example 1 was followed to obtain an ink of a dye type. When this ink was 
used for printing in the same manner as in Example 1, although good 
printed images were obtained, the printed images were poor in water 
resistance and light resistance. The results are shown in Table 1. 
Comparative Example 4 
______________________________________ 
C.I Pigment yellow 5: 
15 parts 
Homopolymer of Sodium acrylate: 
6 parts 
Glycerin: 6 parts 
Ion exchanged water: 73 parts 
______________________________________ 
The above components were dispersed for 4 hours by a sand mill to obtain 
Dispersion F. 
______________________________________ 
Dispersion F: 33 parts 
Glycerin: 5 parts 
Ion exchanged water: 
62 parts 
______________________________________ 
After the components were formulated, the formulation method shown in 
Example 1 was followed to obtain an ink. The ink was excellent in storage 
stability. However, when this ink was used for printing in the same manner 
as in Example 1, clogging occurred at the head portion and good printed 
images were not obtained. Further it was found that these printed images 
were poor in water resistance and light resistance. The results are shown 
in Table 1. 
TABLE 1 
______________________________________ 
Comparative 
Examples Examples 
1 2 3 1 2 3 4 
______________________________________ 
Storage Stability 
G G G P P G G 
Occurrence of Clogging 
G G G P P G G 
Water Resistance 
G G G P P P P 
Light Resistance 
G G G G G P G 
______________________________________ 
Storage Stability: Stored at 40.degree. C. for one month. 
G: No precipitation was observed. 
P: Precipitation was observed. 
Clogging: 
G: All the dots were recovered by one cleaning operation. 
P: Two or more cleaning operations were required for recovering 
all the dots. 
Water Resistance: Subjecting the printed image to flowing 
water for 60 seconds. 
G: Change in optical density was within 0 to 5. 
P: Change in optical density was 5 or more. 
Light Resistance: Subjecting the printed image to a weather 
meter for 8 hours. 
G: Change in optical density was within 0 to 5. 
P: Change in optical density was 5 or more. 
As is apparent from the above results, the aqueous pigment ink of the 
present invention permits printing to be conducted without causing 
clogging and exhibits excellent storage stability, particularly when 
printing is carried out by using an ink jet printer, and therefore the 
intended object is well attained. 
This application is based on Japanese Patent Application 7-343,032, filed 
with the Japanese Patent Office on Dec. 28, 1995, the entire contents of 
which are hereby incorporated by reference. 
Obviously, additional modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.