Pigment composition and preparation process thereof

A pigment composition is composed of an intimate mixture of an organic pigment and a polymer. The content of the organic pigment is 75-95 wt. % based on the whole pigment composition. Monomers which form the polymer include 0-40 wt. % of styrene, 20-70 wt. % of a methacrylic ester and 20-50 wt. % of methacrylic acid. The total proportion of styrene, the methacrylic ester and methacrylic acid is at least 85 wt. % of the entire monomers. The pigment composition is prepared by mixing the organic pigment with an aqueous alkaline solution, which contains the polymer and an alkali metal salt, alkali metal hydroxide, lower amine or ammonium, to form an aqueous dispersion. The organic pigment and polymer are then subjected to coprecipitation, preferably, by lowering the pH of the aqueous dispersion beyond 7.0.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention: 
This invention relates to an easily water-dispersible pigment composition, 
and more specifically to a pigment composition dispersible readily by its 
addition to alkaline water and subsequent slight mixing and hence useful, 
especially, for the preparation of water-base inks and water-base paints. 
(2) Description of the Prior Art: 
A number of proposals have heretofore been made on pigment compositions 
each of which can be readily dispersed by adding same in an aqueous medium 
or vehicle and then merely mixing the resultant aqueous mixture. These 
readily-dispersible prior-art pigment compositions are generally mixtures 
of pigments and surfactants kneaded together or mixtures of pigments and 
water-soluble polymers kneaded together. 
Readily-dispersible conventional pigment compositions of the 
above-mentioned sort employ the surfactants or water-soluble polymers in 
relatively large amounts relative to their corresponding pigments so that 
they can be readily dispersed in aqueous media or vehicles. Where the 
pigment is an organic pigment, a surfactant or water-soluble polymer is 
used in an amount accounting for at least 40 wt. % of a pigment 
composition to be prepared. When such a readily-dispersible pigment 
composition is added and dispersed in an aqueous medium or vehicle for use 
in the preparation of a water-base ink or water-base paint, one or more of 
various polymers are also used as binders in addition to the pigment 
composition. These polymers are selected suitably depending on what use to 
be made of the resulting water-base ink or paint. Since surfactants or 
water-soluble polymers are contained in high proportions in their 
corresponding pigment compositions, these surfactants or water-soluble 
polymers amount to significant proportions in dispersions and affect 
deleteriously on the properties of polymers incorporated as binders, 
resulting in development of various problems. These problems can be solved 
by lowering the proportion of a surfactant or water-soluble polymer to be 
employed in a pigment composition. Use of such a surfactant or 
water-soluble polymer in any amounts below 40 wt. % will however lead to 
significant loss of the high dispersibility of the pigment in the pigment 
composition so that sufficient dispersion will no longer be attained by 
its simple mixing in an aqueous medium or vehicle. 
Under the circumstances, there is thus a strong demand for an 
easily-dispersible pigment composition which contains a dispersant in such 
a small amount that the properties of a resin added as a binder in an 
aqueous medium or vehicle are not deleteriously affected. 
SUMMARY OF THE INVENTION 
The present inventors have carried out an extensive investigation with a 
view toward meeting the above-mentioned demand. As a result, it has been 
found that when a specific water-soluble polymer is used as a dispersant 
for an organic pigment and the polymer and organic pigment are converted 
into a pigment composition by a special preparation process, the resultant 
pigment composition can fully meet the above-mentioned demand, leading to 
completion of this invention. 
In one aspect of this invention, there is thus provided a pigment 
composition composed of an intimate mixture of an organic pigment and a 
polymer, characterized in that the content of the organic pigment is 75-95 
wt. % based on the whole pigment composition, monomers which form the 
polymer include 0-40 wt. % of styrene, 20-70 wt. % of a methacrylic ester 
and 20-50 wt. % of methacrylic acid, and the total proportion of styrene, 
the methacrylic ester and methacrylic acid is at least 85 wt. % of the 
entire monomers. 
In another aspect of this invention, there is also provided a process for 
preparing a pigment composition by mixing 75-95 wt. %, based on the whole 
pigment composition, of an organic pigment with an aqueous solution 
containing 25-5 wt. %, based on the whole pigment composition, of a 
polymer to form an aqueous dispersion and then subjecting the organic 
pigment and polymer to coprecipitation, characterized in that monomers 
which form the polymer include 0-40 wt. % of styrene, 20-70 wt. % of a 
methacrylic ester and 20-50 wt. % of methacrylic acid, and the total 
proportion of styrene, the methacrylic ester and methacrylic acid is at 
least 85 wt. % of the entire monomers, and the aqueous solution is an 
aqueous alkaline solution of an alkali metal salt, an alkali metal 
hydroxide, a lower amine or ammonium. 
The pigment composition of this invention can be easily dispersed as fine 
particles in an aqueous alkaline medium or vehicle by simply adding and 
mixing the composition with the medium or vehicle. In conventional 
preparation of water-base paints or inks, it was necessary to carry out a 
dispersing step for a long period of time under very strict conditions. 
Use of the pigment composition of this invention permits the provision of 
a sufficient aqueous dispersion of the pigment composition only by simple 
mixing and is hence very economical. 
Moreover, the pigment composition contains the polymer as a dispersant in 
an amount far smaller than the amounts of dispersants in conventional 
pigment compositions. Corollary to this, the polymer gives less adverse 
effects to the properties of a binder resin which is used upon preparation 
of a water-base ink or paint. The pigment composition of this invention is 
therefore useful as a colorant for water-base inks and paints which are 
employed for various applications. 
The above and other objects, features and advantages of this invention will 
become apparent from the following description and the appended claims. 
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The pigment useful in the practice of the present invention is an organic 
pigment. It is one of features of the present invention and an important 
advantage of this invention that such an organic pigment is used in an 
amount ranging from 75 wt. % to 95 wt. % based on the whole pigment 
composition. Among conventional pigment compositions which are dispersible 
readily in aqueous media or vehicles and contain organic pigments as their 
pigments, none have been known to have such a high pigment content. 
The polymer employed as a principal feature in the present invention is a 
water-soluble acrylic polymer. Although there have been known some reports 
dealing with use of water-soluble acrylic polymers as dispersants for 
organic pigments, the objects of the present invention were not fully 
attained with such conventional water-soluble acrylic polymers. The 
water-soluble acrylic polymer useful in the practice of this invention is 
characterized in that it has specific monomer proportions. Owing 
principally to these features, the objects of this invention have now been 
achieved. 
The water-soluble acrylic polymer to be employed in the present invention 
features that styrene, a methacrylic ester and methacrylic acid are 
polymerized in proportions of 0-40 wt. %, 20-70 wt. % and 20-50 wt. % 
respectively and when one or more additional monomers are employed besides 
the above-described monomers, the sum of the first-mentioned three types 
of monomers amounts to at least 85 wt. % of the entire monomers. Further, 
the monomer proportions most preferable in the present invention include 
10-30 wt. % of styrene, 10-55 wt. % of a methacrylic ester (with the 
methyl ester being particularly preferred), 10-30 wt. % of the methacrylic 
acid ester of an aliphatic alcohol having at least 6 carbon atoms, and 
25-50 wt. % of methacrylic acid. The sum of these monomers amounts to at 
least 90 wt. % of the entire monomers. 
The above-described dispersant useful in the practice of this invention can 
be obtained by any polymerization process such as a usual mixed 
polymerization process, e.g., solution polymerization, suspension 
polymerization, emulsion polymerization, or the like. The polymeric 
dispersant of this invention, which is obtained in the above-described 
manner, may be of any polymerization degree. In view of the objects of 
this invention, the polymeric dispersant may preferably have a molecular 
weight in a range of 5,000-100,000 and a glass transition point of 
100.degree. C. or higher. 
In the present invention, the pigment to be dispersed by the 
above-described polymer is an organic pigment. Any conventionally-known 
organic pigments may be successfully used in the present invention, 
including azo pigments, phthalocyanine pigments, anthraquinone pigments, 
perylene/perinone pigments, quinacridone pigments and dioxazine pigments 
by way of example. In the pigment composition of this invention, the 
organic pigment is used in such a amount that it accounts for 75-95 wt. % 
of the whole pigment composition with 80-90 wt. % being most preferred. It 
is another advantage of this invention that the pigment is used in such a 
high proportion. Owing to the use of a specific polymeric dispersant such 
as that mentioned above, good dispersibility is still assured even at such 
a high pigment concentration. Unlike the present invention, conventional 
techniques were not able to impart sufficient dispersibility to pigments 
in aqueous media or vehicles when the pigments were used in such high 
proportions as described above. 
The essential components of the pigment composition of this invention have 
been described above. The pigment composition of this invention is 
prepared in the following manner. Namely, the above-described specific 
polymeric dispersant is dissolved in an aqueous alkaline solution of a 
alkali metal hydroxide, alkali metal carbonate, ammonia, lower amine or 
the like, preferably, to a concentration of about 3-10 wt. %. A desired 
pigment is then added in the thus-prepared solution, followed by 
dispersion of the pigment by means of a desirable disperser, for example, 
ball mill, sand mill, speed line mill or the like. Thereafter, an organic 
or inorganic acid is added to the resultant dispersion to lower the pH of 
the dispersion beyond 7.0, whereby the water-soluble acrylic polymer as a 
binder is allowed to deposit while entrapping the dispersed pigment 
therein, in other words, to undergo coprecipitation with the dispersed 
pigment. Any conventionally-known acid may be used as the acid employed 
for the above-described pH adjustment. It is an organic acid such as 
acetic acid that is most preferable for it. It is suitable to effect the 
pH adjustment to such a degree that the pH of the dispersion falls within 
a range of about 3 to 4.5. The deposited pigment composition is separated 
from the aqueous medium by filtration. The pigment composition of this 
invention may of course be in such a wet form as that obtained above. If 
needed, it may be dried at a temperature of about 90.degree.-120.degree. 
C. for about 4-24 hours to obtain the pigment composition of this 
invention in a powdery or particulate form.

The present invention will hereinafter be described more specifically by 
the following Examples, in which all designations of "part" or "parts" and 
"%" mean part or parts by weight and wt. %. It should however be borne in 
mind that the present invention is not necessarily limited to or by the 
following Examples. 
EXAMPLE 1 
Using 2.5 parts of azobisisobutyronitrile (hereinafter abbreviated as 
"AIBN" for the sake of brevity) as a polymerization initiator, 20 parts of 
styrene, 25 parts of methyl methacrylate, 20 parts of 2-ethylhexyl 
methacrylate and 35 parts of methacrylic acid were polymerized at 
76.degree. C.-80.degree. C. for 5 hours in 150 parts of ethanol as a 
polymerization solvent. The polymerization mixture was then cooled to 
50.degree. C., followed by addition of 24 parts of a 28% solution of 
ammonia in water and 56.8 parts of water to obtain an aqueous dispersant 
solution useful in the practice of this invention. 
Then, 34 parts of C.I. Pigment Blue 15-3, 20 parts of the above-obtained 
aqueous dispersant solution and 46 parts of water were subjected to a 
dispersion treatment with four passes through a horizontal- and 
continuous-type glass-bead disperser. 
Five hundred parts of water were added to and mixed with one hundred parts 
of the thus-obtained dispersion of the blue pigment. Fifty parts of a 10% 
solution of acetic acid in water were thereafter added with stirring to 
the diluted dispersion, so that a pigment composition of this invention 
was caused to deposit. The acidified mixture was then heated to 70.degree. 
C., followed by its filtration to collect the deposit. The deposit was 
washed with hot water of 70.degree. C.-80.degree. C. until the pH of the 
washing reached 6.5 or higher. 
The thus-washed solid matter was dried at 120.degree. C. for 24 hours and 
then ground to obtain the pigment composition of this invention in its 
final form. Upon mixing the resultant pigment composition with the 
following composition for 30 minutes in a dissolver, it was readily 
dispersed in water to provide an aqueous fine pigment dispersion having 
stable and excellent quality. 
______________________________________ 
Parts 
______________________________________ 
Water 59 
Polyoxyethylene octylphenyl 
1 
ether (HLB: 13.1) 
28% aqueous ammonia 2.5 
Above-obtained pigment composition 
37.5 
of this invention 
______________________________________ 
The pigment dispersion was useful in coloring an water-base emulsion paint 
as described below. 
Preparation of titanium white dispersion: 
______________________________________ 
Parts 
______________________________________ 
Rutile-type titanium white 
50 
Water 44.8 
"Demol EP" (trade name; product of 
2 
Kao-Atlas Co., Ltd., now, Kao 
Corporation) 
10% Aq. hydroxyethylcellulose soln. 
3 
Defoaming agent 0.2 
______________________________________ 
The above components were dispersed for 10 hours in a porcelain-made ball 
mill. 
______________________________________ 
Parts 
______________________________________ 
Aqueous titanium white 40 
obtained above 
Acrylic emulsion for water-base 
57 
paints (solid content: 46%) 
"Texanol" (trade mark; product of 
3 
Eastman Kodak Company) 
Blue pigment dispersion 
2 
obtained above 
______________________________________ 
The above components were proportioned and mixed to obtain a water-base 
paint. Panels painted with the water-base paint had a vivid light blue 
color of excellent gloss. 
EXAMPLE 2 
Using 2.5 parts of AIBN as a polymerization initiator, 10 parts of styrene, 
55 parts of methyl methacrylate, 10 parts of lauryl methacrylate and 25 
parts of methacrylic acid were polymerized at 81.degree. C.-85.degree. C. 
for 5 hours in 100 parts of isopropyl alcohol as a polymerization solvent. 
The polymerization mixture was then cooled to 50.degree. C., followed by 
addition of 15 parts of a 28% solution of ammonia in water and 35 parts of 
water to obtain an aqueous dispersant solution useful in the practice of 
this invention. 
Then, 36 parts of C.I. Pigment Yellow 14, 10 parts of the above-obtained 
aqueous dispersant solution and 54 parts of water were subjected to a 
dispersion treatment for 24 hours in a ball mill. Three parts of acetic 
acid were then added to 500 parts of hot water of 70.degree. C., followed 
by an addition with stirring of 100 parts of the above-obtained 
dispersion, so that a pigment composition of this invention was caused to 
deposit. Filtration, hot-water washing, drying and grinding were 
thereafter carried out in the same manner as in Example 1, thereby 
obtaining the pigment composition of this invention in its final form. 
To a liquid mixture of 50 parts of water and 10 parts of isopropyl alcohol, 
38 parts of the above-obtained pigment composition were added. After 
thorough deaeration, 2 parts of diethanolamine were added and the 
resultant mixture was stirred for about 30 minutes to obtain a dispersion. 
In accordance with the following composition, this dispersion was 
successfully employed for the preparation of a water-base flexographic 
ink. 
______________________________________ 
Parts 
______________________________________ 
Yellow pigment dispersion 
44 
obtained above 
35% Aq. soln. of "Johncryl 678" 
39 
(trade name; product of Johnson 
Company Ltd.) prepared in a manner 
to be described subsequently 
Water 11 
"Johnwax 26" (trade name; product 
5 
of Johnson Company Ltd.) 
Defoaming agent 0.2 
______________________________________ 
Printed matter making use of the above-described water-base flexographic 
ink were good, especially, in gloss. 
Incidentally, the 35% aq. soln. of "Johncryl 678" was prepared in 
accordance with the following composition. 
______________________________________ 
Parts 
______________________________________ 
"Johncryl 678" 35 
28% Aqueous ammonia 
7.5 
Ethylene glycol 1.5 
Isopropyl alcohol 3 
Water 53 
Total 100 
______________________________________ 
EXAMPLE 3 
After thoroughly stirring 112.5 parts of a 32%-water paste of C.I. Pigment 
Red 48-2 and 13.3 parts of the aqueous dispersant solution of Example 1 in 
a dissolver, the resultant mixture was subjected to a dispersion treatment 
in a sand mill. Five hundred parts of water were added to 125.8 parts of 
the above-obtained dispersion of the red pigment, and the resultant 
mixture was heated to 70.degree. C. With stirring, 30 parts of a 10% 
aqueous solution of acetic acid were added to cause a pigment composition 
of this invention to deposit. Filtration, hot-water washing, drying and 
grinding were thereafter carried out in the same manner as in Example 1, 
thereby obtaining the pigment composition of this invention in its final 
form. 
______________________________________ 
Parts 
______________________________________ 
Water 58.8 
Polyoxyethylene octylphenyl 
1 
ether (HLB: 13.1) 
Aqueous ammonia 2.5 
Defoaming agent 0.2 
The above-obtained pigment 
37.5 
composition of this invention 
______________________________________ 
The above components were next stirred for about 30 minutes in a dissolver 
to obtain a dispersion. 
In accordance with the following composition, this dispersion was 
successfully employed for the preparation of a water-base flexographic 
ink. 
______________________________________ 
Parts 
______________________________________ 
Red pigment dispersion 
44 
obtained above 
Acrylic-styrene base emulsion 
56 
(solid content: 43%) 
______________________________________ 
The above flexographic ink was superb, particularly, in gloss and 
transparency. It was able to provide attracting wrapping and packaging 
materials even when employed for transparent film-laminated paper, to say 
nothing of ordinary paper. 
EXAMPLE 4 
Using 2 parts of AIBN as a polymerization initiator, 20 parts of styrene, 
20 parts of methyl methacrylate, 20 parts of 2-ethylhexyl methacrylate, 10 
parts of N,N-dimethylaminoethyl methacrylate and 30 parts of methacrylic 
acid were polymerized at 125.degree. C.-130.degree. C. for 4 hours in 100 
parts of ethylene glycol monoethyl ether as a polymerization solvent. The 
polymerization mixture was then cooled to 80.degree. C., followed by 
addition of 36 parts of diethanolamine and 14 parts of water. The 
resultant mixture was stirred to obtain an aqueous dispersant solution 
useful in the practice of this invention. 
______________________________________ 
Parts 
______________________________________ 
50% Paste of C.I. Pigment 
64 
Green 7 in ether glycol 
Aqueous dispersant solution 
20 
obtained above 
Water 16 
______________________________________ 
The above components were proportioned and mixed. The resultant mixture was 
given two passes through the same horizontal-type disperser as that 
employed in Example 1. 
Six parts of acetic acid were then added to 500 parts of hot water of 
70.degree. C., followed by an addition with stirring of the above-obtained 
dispersion to cause a pigment composition of this invention to deposit. 
Filtration, hot-water washing, drying and grinding were thereafter carried 
out in the same manner as in Example 1, thereby obtaining the pigment 
composition of this invention in its final form. 
The pigment composition of this invention was readily dispersed in the 
following manner. 
______________________________________ 
Parts 
______________________________________ 
Water 45 
Isopropyl alcohol 20 
The above-obtained pigment 
32 
composition of this invention 
Diethanolamine 3 
______________________________________ 
By stirring the above mixture for 30 minutes in a dissolver, a dispersion 
was obtained. In accordance with the following composition, this 
dispersion was successfully employed for the preparation of a waterbase 
flexographic ink. 
______________________________________ 
Parts 
______________________________________ 
Above-obtained dispersion 
59 
Water 9 
35% Aq. "Johncryl 678" solution 
32 
prepared in Example 2 
______________________________________ 
The water-base flexographic ink obtained by mixing the above components had 
extremely good gloss and transparency. Attracting printed matter were 
obtained upon printing of paper laminated with aluminum foil or aluminized 
paper with the flexographic ink. 
EXAMPLE 5 
Using 2.5 parts of AIBN as a polymerization initiator, 30 parts of styrene, 
10 parts of methyl methacrylate, 10 parts of lauryl methacrylate and 50 
parts of methacrylic acid were polymerized at 125.degree. C.-130.degree. 
C. for 4 hours in 100 parts of ethylene glycol monoethyl ether as a 
polymerization solvent. The polymerization mixture was then cooled to 
80.degree. C., followed by addition of 60 parts of diethanolamine and 73 
parts of water to obtain an aqueous dispersant solution useful in the 
practice of this invention. 
______________________________________ 
Parts 
______________________________________ 
C.I. Pigment Blue 15-3 
34 
Aqueous dispersant solution 
20 
obtained above 
Water 46 
______________________________________ 
The above components were proportioned and mixed, followed by a dispersion 
treatment through the same horizontal-type disperser as that employed in 
Example 1. 
Five hundred parts of water were added to 100 parts of the dispersion. The 
resultant mixture was heated to 70.degree. C., followed by an addition of 
75 parts of a 10% aqueous solution of acetic acid to cause a pigment 
composition of this invention to deposit. Filtration, hot-water washing, 
drying and grinding were thereafter carried out in the same manner as in 
Example 1, thereby obtaining the pigment composition of this invention in 
its final form. 
The pigment composition was useful in coloring a powdered water-base paint 
as described below. 
______________________________________ 
Parts 
______________________________________ 
Precipitated barium sulfate 
50 
Pigment composition obtained above 
1 
Polyvinyl alcohol of low 
5 
polymerization degree 
Polyoxyethylene octylphenyl ether 
0.2 
Diethanolamine 0.2 
______________________________________ 
The above components were mixed intimately. The mixing is preferably 
effected by a high-speed mixer such as Henschel mixer. Alternatively, a 
grinder may be used subsequent to mild mixing. 
A water-base paint was obtained by adding 50 parts of water to 50 parts of 
the above composition and then mixing the resultant mixture with simple 
mixing means such as a mixing rod or the like. Since the waterproofness of 
this paint is not sufficient, it is desirable to use a crosslinking agent 
or a powdered readily-dispersible resin emulsion depending on what use to 
be made. 
REFERENTIAL EXAMPLE 
Using 3.5 parts of AIBN as a polymerization initiator, 20 parts of styrene, 
25 parts of methyl methacrylate, 20 parts of butyl acrylate and 35 parts 
of methacrylic acid were polymerized at 81.degree. C.-85.degree. C. for 5 
hours in 100 parts of isopropyl alcohol as a polymerization solvent. The 
polymerization mixture was added with 21 parts of 28% aqueous ammonia and 
29 parts of water to obtain an aqueous dispersant solution for the sake of 
comparison. 
______________________________________ 
Parts 
______________________________________ 
C.I. Pigment Blue 15-3 
34 
Aqueous dispersant solution 
15 
obtained above 
Water 51 
______________________________________ 
The above components were subjected to a dispersion treatment through the 
same horizontal-type disperser as that employed in Example 1. Five hundred 
parts of water were added to 100 parts of the above-obtained dispersion of 
the blue pigment. While stirring the resultant mixture, 50 parts of a 10% 
aqueous solution of acetic acid were added to cause a referential pigment 
composition to deposit. The deposit was collected by filtration, washed 
with water and then dried at 120.degree. C. for 24 hours, followed further 
by its grinding to obtain the referential pigment composition in its final 
form. 
______________________________________ 
Parts 
______________________________________ 
Water 59 
Polyoxyethylene octylphenyl 
1 
ether (HLB: 13.1) 
Aqueous ammonia 2.5 
Pigment composition obtained above 
37.5 
______________________________________ 
The above components were stirred for 30 minutes in a dissolver but a great 
deal of insoluble matter was observed. 
Having now fully described the invention, it will be apparent for one of 
ordinary skill in the art that many changes and modifications can be made 
thereto without departing from the spirit or scope of the invention as set 
forth therein.