Yellow ink realizing image having excellent lightfastness and color development

A yellow ink composition for ink jet recording is provided which possesses excellent lightfastness and in addition can retain excellent color development for a long period of time. The yellow ink composition comprises both C.I. Pigment Yellow 74 and at least one second yellow pigment selected from the group consisting of C.I. Pigment Yellow 109, 110, 128, 150, and 154.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a yellow ink composition, and more
 particularly to a yellow ink composition suitable for ink jet recording.
 2. Background Art
 Various properties are required of images produced by ink compositions, and
 one of them is lightfastness. When color images are produced using ink
 compositions, it is common practice to use at least a magenta ink, a
 yellow ink, and a cyan ink. Each of the inks should meet property
 requirements that are generally required of ink compositions, for example,
 stability of composition or properties upon storage. In addition, the inks
 should produce sharp images on recording media, and the images produced on
 the recording media should be stable upon storage for a long period of
 time.
 In the production of color images using a plurality of ink compositions,
 presence of even one color having poor lightfastness causes a change in
 hue of the printed images. This results in remarkably deteriorated quality
 of color images. Therefore, more strictly controlled lightfastness is
 required of color ink compositions.
 In recent years, ink jet recording printers have begun to widely spread.
 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 images having
 high resolution and quality can be printed at a high speed by means of
 relatively inexpensive devices. In particular, color ink jet recording
 devices can provide images having improved quality, are used as output
 devices for photographs, and have become used as digital printing
 machines, plotters, CAD output devices and the like. Images printed by ink
 jet recording printers, which have become widely used, are considered
 usable in various forms. In particular, photograph-like prints produced by
 the ink jet recording printers are considered to be put as displays in
 places exposed to lights from fluorescent lamps or direct sunlight in
 outdoors and the like for a long period of time. Therefore, lightfastness
 is a very important property requirement for images produced by ink jet
 recording.
 C.I. Pigment Yellow 74 has hitherto been widely used as a colorant for
 yellow ink compositons. This pigment is superior especially in yellow
 color development. However, there is still room for improvement in
 lightfastness.
 Japanese Patent-Laid Open No. 25440/1998 proposes an ink composition, for
 ink jet recording, containing C.I. Pigment Yellow 154. This publication,
 however, discloses neither the addition of other pigments including C. I.
 Pigment Yellow 154 in combination with C.I. Pigment Yellow 74 to the ink
 composition nor advantages attained thereby.
 SUMMARY OF THE INVENTION
 The present inventors have now found that the lightfastness of a yellow ink
 containing C.I. Pigment Yellow 74 can be markedly improved by adding a
 second yellow pigment. The present invention has been made based on such
 finding.
 Accordingly, it is an object of the present invention to provide a yellow
 ink composition possessing excellent lightfastness and color development.
 The yellow ink composition according to the present invention comprises
 C.I. Pigment Yellow 74 in combination with at least one second yellow
 pigment selected from the group consisting of C.I. Pigment Yellow 109,
 110, 128, 150, and 154.
 DETAILED DESCRIPTION OF THE INVENTION
 The 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. The ink composition according to the present invention is
 preferably used in the ink jet recording method.
 The yellow ink composition according to the present invention can maintain
 excellent color development for a long period of time.
 According to the present invention, the second yellow pigment is selected
 from the group consisting of C.I. Pigment Yellow 109, 110, 128, 150, and
 154. The ratio of C.I. Pigment Yellow 74 to the second yellow pigment
 present in the ink composition is not particularly limited. However, the
 addition of C.I. Pigment Yellow 74 in a relatively large amount is
 preferred from the viewpoint of color development. The second yellow
 pigment is generally contained in an amount of 0.2 to 2 parts by weight
 based on the amount of C.I. Pigment Yellow 74. The upper limit of the
 amount of the second yellow pigment based on the amount of the C.I.
 Pigment Yellow 74 is preferably not more than 1 part by weight, more
 preferably not more than 0.8 part by weight.
 The content of the pigment in the ink composition of the present invention
 is preferably about 1 to 10% by weight, more preferably about 2 to 7% by
 weight, in terms of the total amount of C.I. Pigment Yellow 74 and the
 second yellow pigment.
 According to a preferred embodiment of the present invention, C.I. Pigment
 Yellow 74 and the second yellow pigment are added, to the ink, in the form
 of a pigment dispersion prepared by dispersing the pigment in an aqueous
 medium with the aid of a dispersant or a surfactant. Preferred dispersants
 include those commonly used in the preparation of a dispersion of a
 pigment, for example, a polymeric dispersant. In this connection, that the
 dispersant and the surfactant contained in the dispersion of the pigment
 function also as the dispersant and the surfactant for the ink composition
 will be apparent to a person having ordinary slcill in the art. Preferred
 examples of polymeric dispersants usable herein include naturally
 occurring polymers, and specific examples thereof include: proteins, such
 as glue, gelatin, casein, and albumin; naturally occurring rubbers, such
 as gum arabic and tragacanth; glucosides, such as saponin; alginic acid
 and alginic acid derivatives, such as propylene glycol alginate,
 triethanolamine alginate, and ammonium alginate; and cellulose
 derivatives, such as methyl cellulose, carboxymethyl cellulose,
 hydroxyethyl cellulose, and ethylhydroxy cellulose. Preferred polymeric
 dispersants usable herein include synthetic polymers, and examples thereof
 include polyvinyl alcohols, polyvinyl pyrrolidones, acrylic resins, such
 as polyacrylic acid, acrylic acid/acrylonitrile copolymer, potassium
 acrylate/acrylonitrile copolymer, vinyl acetate/acrylic ester copolymer,
 and acrylic acid/acrylic ester copolymer, styrene/acrylic resins, such as
 styrene/acrylic acid copolymer, styrene/methacrylic acid copolymer,
 styrene/methacrylic acid/acrylic ester copolymer,
 styrene/.alpha.-methylstyrene/acrylic acid copolymer, and
 styrene/.alpha.-methylstyrene/acrylic acid/acrylic ester copolymer,
 styrene/maleic acid copolymer, styrene/maleic anhydride copolymer,
 vinylnaphthalene/acrylic acid copolymer, vinylnaphthalene/maleic acid
 copolymer, and vinyl acetate copolymers, such as vinyl acetate/ethylene
 copolymer, vinyl acetate/fatty acid/vinylethylene copolymer, vinyl
 acetate/maleic ester copolymer, vinyl acetate/crotonic acid copolymer, and
 vinyl acetate/acrylic acid copolymer, and salts of the above polymers.
 Among them, a copolymer of a monomer having a hydrophobic group with a
 monomer having a hydrophilic group and a polymer of a monomer having both
 a hydrophobic group and a hydrophilic group in its molecular structure are
 particularly preferred.
 A preferred solvent usable in the ink composition according to the present
 invention comprises water and a water-soluble organic solvent. The
 water-soluble 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. The amount of the low-boiling organic
 solvent added is preferably 1 to 20% by weight, more preferably 1 to 10%
 by weight, based on the ink.
 According to a preferred embodiment of the present invention, the ink
 composition of the present invention further comprises a wetting agent
 comprising 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; and
 1,3-dimethyl-2-imidazolidinone.
 The amount of the wetting agent added is preferably 2 to 30% by weight,
 more preferably 5 to 20% by weight, based on the ink.
 The ink composition according to the present invention may contain a
 saccharide. Specific 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 "polysaccharides" 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 saccharides is suitably about 1 to 10% by
 weight based on the ink.
 The ink composition according to the present invention may further contain
 a surfactant. Surfactants usable herein include anionic surfactants (for
 example, sodium dodecylbenzenesulfonate, sodium laurylate, and an ammonium
 salt of polyoxyethylene alkyl ether sulfates), nonionic surfactants (for
 example, polyoxyethylene alkyl ethers, polyoxyethylene alkyl esters,
 polyoxyethylene sorbitan fatty acid esters, polyoxyethylene alkylphenyl
 ethers, polyoxyethylenealkylamines, and polyoxyethylenealkylamides), and
 acetylene glycols (OLFINE Y and Surfynol 82, 104, 440, 465, and 485 (all
 the above products being manufactured by Air Products and Chemicals Inc.).
 They may be used alone or in combination of two or more.
 If necessary, pH adjustors, preservatives, antimolds, phosphorus
 antioxidants and the like may be added to the ink composition of the
 present invention.
 The ink composition according to the present invention can be prepared by
 dispersing and mixing the above components using a suitablemethod.
 Preferably, the pigment, the polymeric dispersant, and water are first
 mixed together by means of a suitable dispergator (for example, a ball
 mill, a sand mill, an attrittor, a roll mill, an agitator mill, a Henschel
 mixer, a colloid mill, an ultrasonic homogenizer, a jet mill, or an
 angmill) to prepare a homogeneous pigment dispersion. Subsequently, water,
 a water-soluble organic solvent, a saccharide, a pH adjustor, a
 preservative, an antimold and the like are added to and satisfactorily
 dissolved in the pigment dispersion to prepare an ink solution. After
 thorough stirring, the ink solution is filtered to remove coarse particles
 and foreign matter causative of nozzle clogging to obtain a contemplated
 ink composition.

EXAMPLES
 The present invention will be described in more detail with reference to
 the following examples, though it is not limited to these examples only.
 In the following examples, "%" is by weight unless otherwise specified.
 Preparation of ink
 Pigment ink compositions having the following respective chemical
 compositions were prepared according to the following procedure. The
 pigment and the dispersant resin were first 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 Seisalcusho). Thereafter, the glass beads
 were removed, the other additives were added, and the mixture was then
 stirred at room temperature for 20 min. The mixture was filtered through a
 5 .mu.m membrane filter to prepare an ink composition.

Example 1
 C.I. Pigment Yellow 74 3.7%
 C.I. Pigment Yellow 154 2%
 Dispersant (styrene/acrylic acid copolymer) 1.8%
 (solid basis)
 Sucrose 0.7%
 Maltitol 6.3%
 Glycerin 10%
 Diethylene glycol 3%
 2-Pyrrolidone 2%
 Pure water Balance
 Example 2
 C.I. Pigment Yellow 74 3.3%
 C.I. Pigment Yellow 154 2.4%
 Dispersant (styrene/acrylic acid copolymer) 2.2%
 (solid basis)
 Sucrose 0.5%
 Maltitol 4.5%
 Glycerin 12%
 Diethylene glycol 3%
 Surfynol 465 1%
 Pure water Balance
 Example 3
 C.I. Pigment Yellow 74 4.0%
 C.I. Pigment Yellow 154 2.0%
 Dispersant (styrene/acrylic acid copolymer) 2.8%
 (solid basis)
 Glycerin 10%
 Diethylene glycol 3%
 Triethylene glycol monobutyl ether 5.0%
 Surfynol 465 1%
 Pure water Balance
 Example 4
 C.I. Pigment Yellow 74 3.2%
 C.I. Pigment Yellow 110 2.2%
 Dispersant (styrene/acrylic acid copolymer) 2.3%
 (solid basis)
 Glycerin 10%
 Ethylene glycol 5%
 Triethanolamine 0.7%
 Surfynol 465 1%
 Diethylene glycol monobutyl ether 8%
 Pure water Balance
 Example 5
 C.I. Pigment Yellow 74 3.8%
 C.I. Pigment Yellow 128 2.0%
 Dispersant (styrene/acrylic acid copolymer) 2.0%
 (solid basis)
 Glycerin 12%
 Diethylene glycol 2%
 Triethanolamine 0.8%
 Surfynol 465 0.8%
 Triethylene glycol monobutyl ether 5%
 Pure water Balance
 Example 6
 C.I. Pigment Yellow 74 2.8%
 C.I. Pigment Yellow 150 2.0%
 Dispersant (styrene/acrylic acid copolymer) 1.6%
 (solid basis)
 Glycerin 10%
 Diethylene glycol 5%
 Triethanolamine 0.8%
 Surfynol 465 1.0%
 Triethylene glycol monobutyl ether 10%
 Pure water Balance
 Comparative Example 1
 C.I. Pigment Yellow 74 4.5%
 Dispersant (styrene/acrylic acid copolymer) 1.8%
 (solid basis)
 Sucrose 0.3%
 Maltitol 2.7%
 Glycerin 15%
 Surfynol 465 1%
 Diethylene glycol monobutyl ether 8%
 Pure water Balance
 Comparative Example 2
 C.I. Pigment Yellow 138 4.2%
 Dispersant (styrene/acrylic acid copolymer) 2.1%
 (solid basis)
 Sucrose 0.4%
 Maltitol 3.6%
 Glycerin 14%
 Diethylene glycol 3%
 Surfynol 465 0.8%
 Pure water Balance
 Evaluation test on inks
 Properties of the ink compositions were evaluated as follows. Printing was
 carried out by means of an ink jet printer "MJ-930C" manufactured by Seiko
 Epson Corp. at a density of 360 dpi x 360 dpi with the weight of the ink
 ejected per dot being 0.040 .mu.g.
 Evaluation 1: Lightfastness
 Blotted images (100% duty) were printed on a specialty printing medium for
 ink jet recording (a specialty gloss film, manufactured by Seiko Epson
 Corp.). The lightfastness of the prints thus obtained was evaluated under
 the following conditions.
 The prints were first irradiated with light by means of a xenon
 weather-o-meter Ci35A (manufactured by ATLAS) under conditions of black
 panel temperature 63.degree. C., relative humidity 50%, and 340 nm
 ultraviolet irradiance 0.35 W/m.sup.2.
 The irradiation time was 284 hr at 360 kJ/m.sup.2 or 568 hr at 720
 kJ/m.sup.2.
 After the irradiation, the reflection density of the prints was measured
 with a spectrophotometer GRETAG SPM (manufactured by GRETAG) under
 conditions of light source D50, provision of no light source filter,
 absolute white as reference white, and angle of visibility 2.degree.. The
 percentage residual density was evaluated according to the following
 criteria.
 A: More than 90%
 B: 80 to less than 90%
 C: Less than 80%
 Evaluation 2: Evaluation of color development
 For the blotted images formed in connection with evaluation A, measurement
 was carried out with a spectrophotometer GRETAG SPM (manufactured by
 GRETAG) to determine the coordinates of the L*a*b* color system in the
 color difference indication method specified in CIE (commission
 International de l'Eclairage). From a* and b* thus obtained, the chroma C*
 was determined according to the following equation.
EQU C*=[(a*).sup.2 +(b*).sup.2 ].sup.1/2
 The results were evaluated according to the following criteria.
 A: C*&gt;110
 B: 110.gtoreq.C*.gtoreq.100
 C: C*&lt;100
 TABLE
 Results of evaluation
 Evaluation 1
 360KJ 720KJ Evaluation 2
 Example 1 A A A
 Example 2 A A A
 Example 3 A A A
 Example 4 A A A
 Example 5 A A A
 Example 6 A A A
 Comparative A C A
 Example 1
 Comparative B C C
 Example 2