Pigmented aqueous inks and ink set for ink jet printing

Incremental printing of symbolic information – Ink jet – Fluid or fluid source handling means

Reexamination Certificate

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C347S096000, C106S031600, C106S031130

Reexamination Certificate

active

06565202

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to improved black pigmented ink jet inks, and in particular to a black ink jet ink and an ink set showing an improved tonal appearance.
BACKGROUND OF THE INVENTION
Ink jet printing is a non-impact method that in response to a digital signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers have found broad application as output for personal computers in the office and the home. There are several classes of ink jet printer, for instance thermal drop-on-demand printers, piezo drop-on-demand printers, and continuous ink jet printers.
Ink jet printers generally use aqueous inks, and such inks commonly employ dyes as colorants. Several suitable water soluble dyes which have been perfected for ink jet use are known, but there are some deficiencies to the use of these dyes. For instance they remain soluble in water on the printed page and are therefore subject to print defects such as feathering and to leaching by water, for example rain water and coffee. Furthermore, many of the dyes in use have poor light fastness and fade even on exposure to fluorescent lighting used in offices. There is particular difficulty in finding a good black dye to fulfill all these requirements and also provide a neutral black appearance. It is especially noticeable when printing monochrome, or so called black and white images, that gray areas of the image can appear somewhat blue, violet, or brown in shade rather than neutral. A blue shaded image is described as cold toned, and a brown shaded image as warm toned. Although under certain circumstances toned images may be desired, a neutral or nearly neutral shade is normally preferred, and it has become common to use a mixture of black dyes such as those disclosed in PCT WO 00/46309 partly in an attempt to provide the neutral appearance.
However the other difficulties remain, and consequently it is becoming increasingly common to use a black pigment such as carbon black in ink jet inks. The term pigment describes a colorant which is substantially insoluble in the aqueous ink medium. It is expected that prints produced using pigmented inks will display improved fastness to washing and weathering and better light stability. In addition carbon black shows a more neutral tone than dye based black inks.
There is interest in using ink jet prints as a replacement for conventional photographic monochrome, or black and white images. Monochrome prints cannot be produced to a high quality on a standard four color ink jet printer. The alternatives for monochrome printing on a standard four color printer are either to print a monochrome image using all four colors, or to print with the black ink only. Printing with colored inks can result in a overall color cast to the image, and in addition the individual colored dots become visible in the light areas of the print. The alternative, printing with the black ink only, produces grainy prints because effectively the dot resolution of a 600 dpi (dots per inch) printer is reduced to 150 dpi, and this is most noticeable in the light areas of the print.
It is known that the use of more than one black ink (commonly four) of different dilutions (i.e. gray inks) can address these problems. For example it is known to use a conventional black ink in the “black” channel of the printer, and somewhat dilute black inks to feed the three color channels, with the most dilute ink being in the “yellow” channel. Such inks may be printed without modification of the control software to produce a monochrome image. The use of all four ink cartridges allows full use of the dot resolution of the printer, but the image still suffers defects, for example that individual dots are visible in low density areas. Furthermore the overall image quality does not equal that of conventional photographic prints.
A better solution is to modify the controlling software and use four inks of increasing dilution. Such a system is commercially available under the trade name ILFORD Archiva MonoKrome Ink. By correct choice of the strength of each of the four inks and the design of the software a high quality image may be produced. Furthermore, the visibility of the dots in the light areas of the print can be reduced by printing such areas with light gray inks, thus increasing the number of dots printed for a given density and reducing the visual impact of any given dot, and the black density may be increased by overprinting one ink with another. One limitation of this approach however is that the image tone is dictated by the color of the inks, and in particular that the lightest, or most dilute inks tend to provide brown, or warm toned images even though carbon black is used as pigment. This effect is the factor which limits the degree of dilution of the most dilute ink. On the other hand, it is generally desirable to increase the dilution of this ink to improve the image quality.
The use of a combination of carbon black with a dye in a black aqueous ink jet ink is known according to European Patent Applications 0 724 968 A, 0 913 438 A, and 1 002 839 A. The use of a combination of carbon black with cyan and magenta pigments is known according to U.S. Pat. No. 5,803,958. However these inks are not described for improving the tone of black and white images. We have found a particular combination of pigments which is useful for providing monochrome images from aqueous ink jet inks.
SUMMARY OF THE INVENTION
In the present invention, these purposes, as well as others which will be apparent, are achieved generally by providing a pigmented aqueous ink set which comprises a black ink and at least one dilute ink, wherein at least one of the inks comprises carbon black and Pigment Blue 60. The ratio of black pigment to blue pigment may be between 10:1 and 1:2 and depends on the particular grade of the pigments, the ink formulation, the printing medium and the desired final tone.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a pigmented aqueous ink set comprising:
(a) A concentrated black ink having a pigment concentration of up to 10% by weight; and
(b) at least one dilute ink (preferably a gray ink) having a pigment concentration below that of the concentrated ink; wherein all the inks additionally comprise an aqueous carrier medium and at least one of the set of inks comprises a combination of carbon black and Pigment Blue 60.
Carbon black is listed in the Pigments and Solvent Dyes section of the Colour Index International, published by the Society of Dyers and Colourists in 1997 as C. I. Pigment Black 7. Examples include furnace black, lamp black, acetylene black, and channel black. It is also possible to use a processed pigment such as self dispersing carbon black or graft carbon black having a surface treated with a resin or the like. Preferably the pigment concentration in the black ink is between about 3% and about 5%.
C. I. Pigment Blue 60 is an indanthrone of the following structure.
Optionally and preferably each ink may also comprise at least one dispersant for the pigments. The purpose of the dispersant is to stabilize the particles and prevent flocculation, aggregation, and settling of the ink. Suitable dispersants for pigmented ink jet inks are well known in the art, and include polymeric dispersants as well as some non-polymeric compounds of the surfactant type. Suitable dispersants for this invention include macromolecular polyionic dispersants, for example copolymers of styrene with acrylic, methacrylic, or maleic acids; various types of poly(ethylene oxide) condensates such as alkyl polyethylene oxide ethers and sulfate or phosphate esters thereof; and surfactants such as sarcosinate compounds. The ink may contain up to 400% of the dispersant by weight on the pigment, but preferably between about 10% and about 100% by weight on the pigment, and most preferably approximatel

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