Among various color-recording methods, use of an ink-jet printer is a typical one, in which ink droplets are produced to be deposited onto various record-receiving objects (paper, a film, and fabric, for example) to achieve recording. The ink-jet printer does not involve direct contact between a recording head and the record-receiving object, and therefore operates quietly without making much noise. In addition, having such features that size reduction and speed enhancement are easily accomplished, the ink-jet printer is rapidly achieving widespread use in recent years and is expected to be used even more widely in the future.
As an ink for fountain pens, felt-tipped pens, and the like and as an ink for ink-jet printing, an aqueous ink containing a water-soluble coloring matter dissolved in an aqueous medium is conventionally used. The aqueous ink generally contains a water-soluble organic solvent to prevent ink clogging from occurring at the tip of a pen or in an ink-discharging nozzle. This ink is required, for example, to give a recorded image with adequate density, to cause no clogging at the tip of a pen or in a nozzle, to dry well on a record-receiving material, bleed little, and to have excellent storage stability. In addition, the water-soluble coloring matter contained therein is required to have excellent solubility particularly in water and excellent solubility in the water-soluble organic solvent contained in the ink. Furthermore, an image to be obtained is required to have image-fastness such as water resistance and light resistance, well-reproduced colors, and well-rendered colors. It is also crucial for the image to have not only excellent image-fastness but also well-balanced colors.
Due to advancement in ink-jet printing technology in recent years, printing speed in ink-jet printing is being dramatically enhanced. This trend is causing movement toward use of ink-jet printers instead of laser printers, which operate on electronic toner, for printing documents on plain paper, which is a major application of printing for commercial purposes and in office environment. With their advantages of operability on any kinds of recording paper and low prices of the devices, ink-jet printers are achieving widespread use in small- to medium-sized office environment, particularly in SOHO. When ink-jet printers are used for printing on plain paper, focus tends to be put on color development properties and water resistance, among other properties required of printed articles. To satisfy such requirement for performance, use of a pigment ink has been suggested. The pigment ink, however, is not in the state of solution but in the state of dispersion containing solid pigments dispersed therein, and therefore tends to cause problems such as poor storage stability of the pigment ink and clogging in a print-head nozzle, compared to a dye ink. In addition, when the pigment ink is used, the printed image often has a problem of poor resistance to scuffing. In contrast, the dye ink contains dyes as coloring matter components dissolved in the ink, and therefore the above-mentioned problems unique to the pigment ink are less likely to occur compared to the pigment ink. However, the dye ink is generally and significantly inferior to the pigment ink particularly in color development properties, light resistance, and water resistance, and the improvement is strongly desired.
By the way, when an image or text information on a color monitor of a computer is recorded in color using an ink-jet printer, subtractive color mixture with inks of four colors, namely, yellow (Y), magenta (M), cyan (C), and black (K), is generally employed to produce a recorded image in color. In order for subtractive color mixture to faithfully reproduce the image that has been yielded by additive color mixture with red (R), green (G), and blue (B) on a CRT (cathode-ray tube) display or the like, each of the coloring matters, particularly Y, M, and C, contained in the ink is desired to have a hue similar to the hue of the corresponding standard color and to be vivid. To be vivid here generally refers to have high chroma. When three primary colors of Y, M, and C have low chroma, the color range produced with a single color or color mixture is small, which may not satisfy the intended color range. Because of this reason, development of coloring matters with high chroma as well as inks containing such coloring matters is desired.
As for the properties of the ink, not only long-term storage stability and high density in a recorded image but also excellent fastness of the printed image, such as water resistance, moisture resistance, light resistance, and gas resistance, are required.
The light resistance here refers to fastness of the recorded image to light such as sunlight and indoor light.
The moisture resistance here refers to resistance of a coloring matter in a recorded image to phenomena such as discoloration due to excess moisture present primarily in the atmosphere in the surrounding environments and bleeding and escaping of the coloring matter from the printed image. The lack of moisture resistance is regarded as a critical problem left to be solved for ink-jet recorded articles, which are exhibited in various environments in recent years. However, an ink that adequately satisfies the requirement from the market has not yet been developed.
The gas resistance refers to resistance to phenomena where a coloring matter (dye) in a recorded image on or within a record-receiving material reacts with a gas that is present in the air and has oxidizing action (also called an oxidizing gas) to cause discoloration and color fading of the recorded image. Ozone gas, among other oxidizing gases, is regarded as a major factor that facilitates the phenomena of discoloration and color fading of the ink-jet recorded image. Because the phenomena of discoloration and color fading are characteristic to the ink-jet recorded image, improvement in ozone-gas resistance is a serious technical objective to achieve in the field.
As a method of obtaining a photograph-quality printed image by ink-jet recording, an ink-receiving layer is provided on the surface of a record-receiving material. The ink-receiving layer provided for this purpose often contains a white porous inorganic substance for facilitating ink drying and reducing bleeding of a coloring matter to obtain a high-quality image. However, the record-receiving material thus treated, in particular, often receives discoloration and color fading due to ozone gas described above. Along with the recent widespread use of digital cameras and color printers, images captured with digital cameras and the like are increasingly and more often printed at home with photograph quality. Due to this trend, discoloration and color fading of a recorded image due to an oxidizing gas described above are regarded as problematic.
A black ink, among other ink compositions, is a key ink used in both monochrome and full-color printed images. It is difficult from technical viewpoints, however, to develop a coloring matter capable of producing an excellent black color with neutral hues in the deep-color gamut and the light-color gamut, high print density, and low light-source dependency of hues. Much effort that has been made on research and development has only achieved a few coloring matters that have adequate performance. Because of this reason, a black ink is generally formulated by adding a plurality of different coloring matters. An ink as a mixture of a plurality of coloring matters, however, has disadvantages such as 1) variation in hues depending on a medium (a record-receiving material) and 2) serious discoloration, among others, caused by degradation of the coloring matters due to light and ozone gas, compared to an ink only containing a single coloring matter.
A black ink composition for ink-jet printing that is excellent in a wide range of durability of a printed article is suggested, for example, in Patent Document 1. This ink composition is extremely excellent in the black hue and is greatly improved in fastness of an image on a printed article, but it needs further improvement particularly in ozone resistance. So far, no product that adequately fulfills the requirement from the market has been developed.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H09-217018    Patent Document 2: Japanese Unexamined Patent Application, Publication No. H10-306221    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H11-70729    Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2001-139836    Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2003-192930    Patent Document 6: Japanese Unexamined Patent Application, Publication No. 2003-321627    Patent Document 7: Japanese Unexamined Patent Application, Publication No. 2004-285351    Patent Document 8: PCT International Publication No. WO2004/087815    Patent Document 9: PCT International Publication No. WO2004/104108    Patent Document 10: Japanese Unexamined Patent Application, Publication No. 2005-036222    Patent Document 11: PCT International Publication No. WO2005/033211    Patent Document 12: PCT International Publication No. WO2005/097912    Patent Document 13: PCT International Publication No. WO2006/001274    Patent Document 14: PCT International Publication No. WO2006/051850    Patent Document 15: PCT International Publication No. WO2006/075706    Patent Document 16: PCT International Publication No. WO2007/077931    Patent Document 17: PCT International Publication No. WO2007/091631    Patent Document 18: PCT International Publication No. WO2007/116933    Patent Document 19: PCT International Publication No. WO2008/018495    Patent Document 20: PCT International Publication No. WO2008/066062    Patent Document 21: PCT International Publication No. WO2008/111635    Patent Document 22: Japanese Unexamined Patent Application, Publication No. 2009-084346    Patent Document 23: PCT International Publication No. WO2009/060654    Patent Document 24: Japanese Unexamined Patent Application, Publication No. 2010-006969    Patent Document 25: PCT International Publication No. WO2010/125903    Patent Document 26: PCT International Publication No. WO2010/143619    Patent Document 27: PCT International Publication No. WO2011/122426    Patent Document 28: PCT International Publication No. WO2012/002317    Patent Document 29: PCT International Publication No. WO2012/050061    Patent Document 30: PCT International Publication No. WO2012/081640    Patent Document 31: Japanese Patent No. 3346755 Patent Document 32: Japanese Patent No. 4100880    Patent Document 33: Japanese Patent No. 4420267    Patent Document 34: Japanese Patent No. 5337716    Non-Patent Document 1: Yutaka Hosoda, “Theory manufacturing dye chemistry”, Gihodo, pp. 190-193