Pigment-based ink composition having excellent lightfastness

A yellow ink composition comprising C.I. Pigment Yellow 74 is provided which possesses excellent properties, especially excellent lightfastness. The yellow ink composition comprises a pigment C.I. Pigment Yellow 74 and a styrene-(meth)acrylic acid-based, water-soluble resin in an amount of not less than 0.4 part by weight in terms of solid content based on the pigment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a pigment-based yellow ink composition, 
particularly a yellow ink composition suitable for ink jet recording. 
2. Background Art 
Ink jet recording is a printing method wherein droplets of an ink 
composition are ejected and deposited onto a recording medium, such as 
paper, to conduct printing. This method has a feature that an image having 
high resolution and quality can be printed at a high speed by means of a 
relatively inexpensive apparatus. In general, the ink composition used in 
the ink jet recording comprises water as a main component and, added 
thereto, a colorant and a wetting agent, such as glycerin, for preventing 
clogging and other purposes. Water-soluble dyes, by virtue of high chroma 
of the colorant, abundance in types of usable colorants, solubility in 
water and other advantageous properties, have been extensively used as the 
colorant in the ink composition for ink jet recording. The dyes, however, 
are essentially soluble in water, and hence are often poor in 
waterfastness and other various properties. 
Pigments, as compared with the dyes, have superior waterfastness. This has 
recently led to studies on utilization of pigments as a colorant in the 
ink composition for ink jet recording from the viewpoint of improving the 
waterfastness of the printed image. However, among various pigments, only 
limited pigments can exhibit color reproduction and other properties 
comparable to dyes. For yellow pigments, use of C.I. Pigment Yellow 74 can 
be expected by virtue of high color reproduction. 
C.I. Pigment Yellow 74, however, often has poor lightfastness that is a 
disadvantageous property commonly observed in yellow pigments, and, hence, 
techniques for improving the lightfastness of this pigment have been 
desired in the art. 
SUMMARY OF THE INVENTION 
The present inventors have now found that the lightfastness and other 
properties of the ink composition containing C.I. Pigment Yellow 74 can be 
improved by adding a specific resin to the ink composition. The present 
invention has been made based on such finding. 
Accordingly, an object of the present invention is to provide an ink 
composition comprising C.I. Pigment Yellow 74 that has excellent 
properties. 
The yellow ink composition according to the present invention comprises: 
C.I. Pigment Yellow 74 as a colorant; and a styrene-(meth)acrylic 
acid-based, water-soluble resin, the content of the resin being not less 
than 0.4 part by weight in terms of solid content based on the colorant. 
DETAILED DESCRIPTION OF THE INVENTION 
The yellow ink composition according to the present invention may be used 
in recording methods using an ink composition. Recording methods using an 
ink composition include, for example, an ink jet recording method, a 
recording method using writing utensils, such as pens, and other various 
printing methods. Particularly preferably, the ink composition according 
to the present invention is used in the ink jet recording method. 
Basically, the yellow ink composition according to the present invention 
comprises a colorant and a water-soluble resin, wherein the colorant is 
C.I. Pigment Yellow 74 while the water-soluble resin is a 
styrene-(meth)acrylic acid-based, water-soluble resin. The amount of the 
water-soluble resin added is not less than 0.4 part by weight, preferably 
0.4 to 1.5 parts by weight, more preferably 0.5 to 1.2 parts by weight, in 
terms of solid content based on the colorant. At the outset, the yellow 
ink composition according to the present invention has excellent 
lightfastness. As described above, it has been pointed out that C.I. 
Pigment Yellow 74 has poor lightfastness. The yellow ink composition 
according to the present invention, however, has good and improved 
lightfastness. Further, the yellow ink composition according to the 
present invention is excellent in various properties required of ink 
compositions, especially various properties required of ink compositions 
for ink jet recording, as well as in lightfastness. Specifically, the 
present invention provides a yellow ink composition having excellent image 
fixation and hue (chroma). 
In the yellow ink composition according to the present invention, the 
styrene-(meth)acrylic acid-based, water-soluble resin substantially 
functions as a dispersant for C.I. Pigment Yellow 74 as the colorant. As 
described below, the colorant is preferably added as a pigment dispersion 
to the ink composition. The styrene-(meth)acrylic acid-based, 
water-soluble resin may be added as a dispersant for the pigment 
dispersion to the ink composition. On the other hand, independently of the 
dispersant for the pigment dispersion or additionally, the 
styrene-(meth)acrylic acid-based, water-soluble resin may be added to the 
ink composition. 
According to the present invention, C.I. Pigment Yellow 74 may be added in 
such a suitable amount that a desired image density can be realized. The 
amount of C.I. Pigment Yellow 74 added, however, is preferably 0.3 to 8% 
by weight, more preferably 2 to 4% by weight, based on the ink 
composition. 
According to a preferred embodiment of the present invention, C.I. Pigment 
Yellow 74 is added, to the ink, as a pigment dispersion prepared by 
dispersing this pigment in an aqueous medium with the aid of a dispersant 
or a surfactant. The average diameter of fines particles of the pigment is 
preferably not more than 200 nm, more preferably not more than 120 nm. 
The styrene-(meth)acrylic acid-based, water-soluble resin used in the 
present invention may be of random polymer or block polymer. The molecular 
weight thereof is preferably about 3,000 to 30,000, more preferably about 
5,000 to 15,000. The ratio of the segment derived from the styrene monomer 
to the segment derived from the (meth)acrylic acid may be determined using 
an acid value as an index. Thus, according to a preferred embodiment of 
the present invention, the acid value of this resin is preferably about 50 
to 200, more preferably about 70 to 150. 
Preferably, the ink composition according to the present invention contains 
an organic solvent. The organic solvent is preferably a low-boiling 
organic solvent, and preferred examples thereof include methanol, ethanol, 
n-propyl alcohol, iso-propyl alcohol, n-butanol, sec-butanol, 
tert-butanol, iso-butanol, and n-pentanol. Monohydric alcohols are 
particularly preferred. The low-boiling organic solvent has the effect of 
shortening the time taken for drying the ink composition. 
According to a preferred embodiment of the present invention, the ink 
composition used in the present invention further comprises a high-boiling 
organic solvent. Preferred examples of high-boiling organic solvents 
usable herein include: polyhydric alcohols, such as ethylene glycol, 
diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene 
glycol, propylene glycol, butylene glycol, 1,2,6-hexanetriol, thioglycol, 
hexylene glycol, glycerin, trimethylolethane, and trimethylolpropane; 
alkyl ethers of polyhydric alcohols, such as ethylene glycol monoethyl 
ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl 
ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl 
ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl 
ether, and triethylene glycol monobutyl ether; urea; 2-pyrrolidone; 
N-methyl-2-pyrrolidone; 1,3-dimethyl-2-imidazolidinone; and 
triethanolamine. 
The amount of the low-boiling organic solvent added is preferably in the 
range of from 0.5 to 10% by weight, more preferably in the range of from 
1.5 to 6% by weight, based on the ink composition. The amount of the 
high-boiling organic solvent added is preferably 0.5 to 40% by weight, 
more preferably 2 to 20% by weight, based the ink composition. 
According to a preferred embodiment of the present invention, the ink 
composition according to the present invention may contain a surfactant. 
Preferred examples of surfactants usable herein include: anionic 
surfactants (for example, sodium dodecylbenzenesulfonate, sodium laurate 
and an ammonium salt of a polyoxyethylene alkyl ether sulfate); nonionic 
surfactants (for example, a polyoxyethylene alkyl ether, a polyoxyethylene 
alkyl ester, a polyoxyethylene sorbitan fatty acid ester, a 
polyoxyethylene alkyl phenyl ether, a polyoxyethylene alkylamine, and a 
polyoxyethylene alkylamide). They may be used alone or as a mixture of two 
or more. Further, it is also possible to use acetylene glycol (OLFINE Y 
and Surfynol 82, 104, 440, 465, 485, and TG (all the above products being 
manufactured by Air Products and Chemicals Inc.). 
According to a preferred embodiment of the present invention, the ink 
composition according to the present invention may contain a saccharide. 
Examples of saccharides usable herein include monosaccharides, 
disaccharides, oligosaccharides (including trisaccharides and 
tetrasaccharides), and other polysaccharides, preferably glucose, mannose, 
fructose, ribose, xylose, arabinose, galactose, aldonic acid, glucitol, 
sorbitol, maltose, cellobiose, lactose, sucrose, trehalose, and 
maltotriose. The term "polysaccharide" used herein means saccharides in 
the broad sense as including substances which exist widely in the world of 
nature, such as alginic acid, .alpha.-cyclodextrin, and cellulose. 
Derivatives of these saccharides usable herein include reducing sugars of 
the above saccharides (for example, sugar alcohols represented by the 
general formula HOCH.sub.2 (CHOH).sub.n CH.sub.2 OH, wherein n is an 
integer of 2 to 5), oxidizing sugars (for example, aldonic acid or uronic 
acid), amino acids, and thiosugars. Among them, sugar alcohols are 
particularly preferred, and specific examples thereof include maltitol and 
sorbitol. 
The content of the saccharide is preferably about 0.1 to 40% by weight, 
preferably 0.5 to 30% by weight, based on the ink composition. 
If necessary, pH adjustors, preservatives, antimolds and the like may be 
added.

[EXAMPLES] 
Preparation of Pigment Ink Compositions 
Pigment ink compositions comprising the following ingredients were prepared 
as follows. At the outset, a pigment and a dispersant resin were mixed 
together, and the mixture, together with glass beads (diameter: 1.7 mm, 
amount: 1.5 times (by weight) larger than the mixture), was dispersed for 
2 hr in a sand mill (manufactured by Yasukawa Seisakusho). Thereafter, the 
glass beads were removed, the other additives were added, and the mixture 
was stirred at room temperature for 20 min. The mixture was filtered 
through a 5 .mu.m membrane filter to prepare an ink composition. 
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Example 1 
C.I. Pigment Yellow 74 (particle 3 wt % 
diameter 110 nm) 
Styrene/acrylic acid copolymer, 1.5 wt % 
(molecular weight 7000, acid value 80) 
(solid content) 
Sucrose 0.7 wt % 
Maltitol 6.3 wt % 
Glycerin 10 wt % 
2-Pyrrolidone 2 wt % 
Ethanol 4 wt % 
Pure water Balance 
Example 2 
C.I. Pigment Yellow 74 (particle 3 wt % 
diameter 80 nm) 
Styrene/acrylic acid copolymer, 2 wt % 
(molecular weight 7000, acid value 
(solid content) 
120) 
Sucrose 0.7 wt % 
Maltitol 6.3 wt % 
Glycerin 10 wt % 
Triethylene glycol monobutyl ether 5 wt % 
Surfynol 465 1 wt % 
Pure water Balance 
Example 3 
C.I. Pigment Yellow 74 (particle 3 wt % 
diameter 100 nm) 
Styrene/acrylic acid copolymer, 1.3 wt % 
(molecular weight 7000, acid value 
(solid content) 
130) 
Sucrose 0.7 wt % 
Maltitol 6.3 wt % 
Glycerin 10 wt % 
Diethylene glycol 3 wt % 
Surfynol 465 1 wt % 
Pure water Balance 
Comparative Example 1 
C.I. Pigment Yellow 74 (particle 3 wt % 
diameter 140 nm) 
Styrene/acrylic acid copolymer, 0.8 wt % 
(molecular weight 7000, acid value 80) 
(solid content) 
Sucrose 0.7 wt % 
Maltitol 6.3 wt % 
Glycerin 10 wt % 
Diethylene glycol 3 wt % 
Surfynol 465 1 wt % 
Pure water Balance 
Comparative Example 2 
C.I. Pigment Yellow 74 (particle 3 wt % 
diameter 120 nm) 
Acrylic acid copolymer, 1.5 wt % 
(molecular weight 10000, acid value 
(solid content) 
200) 
Sucrose 0.7 wt % 
Maltitol 6.3 wt % 
Glycerin 7 wt % 
Diethylene glycol monobutyl ether 3 wt % 
Diethylene glycol 3 wt % 
Surfynol 465 1 wt % 
Pure water Balance 
Comparative Example 3 
C.I. Pigment Yellow 138 (particle 3 wt % 
diameter 60 nm) 
Styrene-acrylic acid copolymer, 1.5 wt % 
(molecular weight 7000, acid value 
(solid content) 
140) 
Sucrose 0.7 wt % 
Maltitol 6.3 wt % 
Glycerin 10 wt % 
Diethylene glycol 3 wt % 
Surfynol 465 1 wt % 
Pure water Balance 
Comparative Example 4 
C.I. Pigment Yellow 13 (particle 3 wt % 
diameter 100 nm) 
Styrene/acrylic acid copolymer, 1.5 wt % 
(molecular weight 7000, acid value 
(solid content) 
100) 
Sucrose 0.7 wt % 
Maltitol 6.3 wt % 
Glycerin 10 wt % 
2-Pyrrolidone 2 wt % 
Ethanol 4 wt % 
Pure water Balance 
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Evaluation Test on Ink Compositions 
Prints obtained by printing the ink composition by means of an ink jet 
printer MJ-930C manufactured by Seiko Epson Corp. were evaluated. The 
printing was carried out at a density of 360 dpi.times.360 dpi with the 
weight of the ink ejected per dot being 0.040 .mu.g. 
Evaluation 1: Lightfastness 
A blotted image was printed on a specialty printing medium for ink jet 
recording (a specialty gloss film, manufactured by Seiko Epson Corp.). 
The print was irradiated with light for 500 hr by means of a xenon 
fadeometer. In this case, the color of the print before exposure and the 
color of the print after exposure were measured with a Macbeth CE-7000 
spectrophotometer (manufactured by Macbeth) and expressed in terms of the 
L*a*b* color system of the color difference indication method specified in 
CIE (Commission International de l'Eclairage), and a color difference 
.increment.E*ab between the initial color of the printed image and the 
color of the printed image after the light irradiation was determined from 
the measured L*a*b* values by the following equation. The lightfastness 
was evaluated in terms of the change in hue according to the following 
criteria. 
Color difference: 
EQU .increment.E*ab=[(.increment.L*).sup.2 +(.increment.a*).sup.2 
+(.increment.b*).sup.2 ].sup.1/2 
Evaluation criteria: 
A: .increment.E*ab.ltoreq.10 
B: .increment.E*ab&gt;10 
The results were as summarized in the following table. 
Evaluation 2: Fixation 
Blotted images and characters were printed on a specialty printing medium 
for ink jet recording as used in Evaluation 1. The print was rubbed with a 
finger in an environment of temperature 25.degree. C. and humidity 50% and 
then visually inspected for smearing of the print and separation of the 
colorant. The results were evaluated according to the following criteria. 
A: Neither smearing of the print nor separation of the colorant occurred. 
B: Slight smearing of the print occurred with the colorant not separated. 
C: Both smearing of the print and separation of the colorant occurred. 
The results were as summarized in the following table. 
Evaluation 3: Hue 
A blotted image was printed on a specialty recording medium for ink jet 
recording (a specialty gloss film, manufactured by Seiko Epson Corp.). The 
color was measured with a Macbeth CE-7000 spectrophotometer (manufactured 
by Macbeth) to determine the coordinates of L*a*b* color system of the 
color difference indication method specified in CIE (Commission 
International de l'Eclairage). The chroma of the printed image defined by 
the following equation was determined from the measured a*b* values, and 
the results were evaluated according to the following criteria. 
Chroma: 
EQU C*=[(a*).sup.2 +(b*).sup.2 ].sup.1/2 
Evaluation Criteria: 
A: C*.gtoreq.95 
B: 90.ltoreq.C*&lt;95 
C: C*&lt;90 
The results were as summarized in the following table. 
TABLE 1 
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Evaluation 1 
Evaluation 2 
Evaluation 3 
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Example 1 A A A 
Example 2 A A A 
Example 3 A A A 
Comparative B B B 
Example 1 
Comparative B C B 
Example 2 
Comparative B A C 
Example 3 
Comparative B A B 
Example 4 
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