Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-10-26
2002-12-31
Reddick, Judy M. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S284000, C524S320000, C524S321000, C524S403000, C524S417000, C524S419000, C524S429000, C524S519000, C524S520000, C524S521000, C524S555000, C524S560000
Reexamination Certificate
active
06500880
ABSTRACT:
TECHNICAL FIELD
The present invention relates to ink-jet inks employed in ink-jet printing. In particular, this invention relates to an ink system that utilizes the addition of specific polymers to conventional, non-self dispersing pigment-based ink. These polymers reduce, and in some cases even eliminate, color bleed problems typically associated with ink-jet printing.
BACKGROUND ART
Ink-jet printers offer a low cost, high quality, and comparatively noise-free option to other types of printers commonly used with computers. Such printers employ a resistor a element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an ink-jet pen.
In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber, which expels ink through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area-fill, and provides other print capabilities on the medium.
When some ink-jet inks are printed adjacent to each other a condition called “bleed” can appear. Bleed occurs as colors mix both on the surface of the paper being printed on and in the paper. The terms “bleed”, as used herein, are defined as follows: when inks of two different colors are printed next to each other, it is desired that the border between the two colors be clean and free from the invasion of one color into the other. When one color does invade into the other, the border between the two colors becomes ragged, and this is called bleed. This is in contradistinction to uses of the term “bleed” in the prior art, which often defines “bleed” in the context of ink of a single color following the fibers of the paper.
Prior solutions to bleed have largely involved the use of heated platens or other heat sources and/or special paper. Heated platens add cost to the printer. Special paper limits the user to a single paper, which is of a higher cost than a plain paper. Another way to reduce bleed involves increasing the penetration rate of the ink into the paper. However, increasing the penetration rate reduces the edge acuity (lowers the print quality of the ink). Nevertheless, this method is acceptable for printing color ink because of the lesser importance of color text quality. However, print quality is important for black ink. And hence, alternate bleed control mechanisms are needed.
U.S. Pat. No. 5,428,383 teaches a method to control bleed in multicolor ink-jet printing involving the use of multi-valent metal salts as precipitation agents in a first ink composition. The precipitation agent is designed to react with the coloring agent in a second ink composition.
A solution to the specific problem of black to yellow color bleed is disclosed in U.S. Pat. No. 5,198,023 wherein multivalent cations such as calcium chloride and magnesium chloride are added at concentrations ranging from about 1 to about 10 wt % to yellow cationic inks to prevent bleed between yellow and black inks. However, it has been determined that the addition of large amounts of multivalent cations to thermal ink-jet ink compositions may induce precipitation of dye salts, requiring further adjustments in the ink composition. U.S. Pat. No. 5,518,534 teaches the use of a first ink having a pigment dispersion and the second ink containing a salt of an organic acid or mineral acid wherein said salt reacts with said first ink to alleviate bleed between the inks. However, in this case, the pigment requires the presence of a dispersing agent, and both inks must have the same ionic character.
A method to control bleed is also disclosed by U.S. Pat. No. 5,730,790. The ink-jet ink is formulated to comprise at least one dye-based ink composition and at least one pigment-based ink. The dye-base ink also contains a cationic surfactant, and the pigment-base ink contains a negatively charged dispersant.
Another method of reducing bleed between ink-jet inks involves the use of pH-sensitive dyes as disclosed in U.S. Pat. No. 5,181,045. It discloses an ink having a pH-sensitive dye to prevent bleeding to an adjacent ink having an appropriate pH. Migration of the ink having the pH-sensitive dye is prevented by rendering the dye insoluble on the page by contact with the pH of the adjacent ink. This method requires a pH differential of about 4 or 5 units to completely control bleed. Accordingly, a pH not exceeding 4 would be required to effectively eliminate bleed from a pH-sensitive ink having a typical pH of about 8.
U.S. Pat. No. 5,679,143 builds upon the '045 case referenced above, but an organic acid component is added to the so-called target ink-jet ink composition, as opposed to the pH-sensitive ink composition. The organic acid component reduces the pH differential required to control bleed to about 3 units or less.
While each of the above has produced varying degrees of success, a need still remains for pigment-based (non self-dispersing) ink compositions for use in ink-jet printing which do not evidence bleed, as defined herein, when printed on plain papers, and yet which posses relatively long shelf life and other desirable properties of such inks.
DISCLOSURE OF INVENTION
In accordance with the invention, an ink set is provided in which certain pigment-based inks contain polymers which become insoluble under either specific and well defined pH conditions or when contacted with incompatible ionic substances. By forcing the polymers present to become insoluble on the page, migration of the colorant is inhibited, thereby helping to reduce bleed between different colors. Rendering the polymers insoluble is achieved by making the polymer come out of solution or to precipitate; this is done by contacting the polymer with another ink having an appropriate pH or one which contains ions that are incompatible with the polymers, or both.
The specific polymers disclosed herein are added to pigmented inks that are stabilized by a dispersant. Thus, in systems in which a dispersant yields a very stable dispersant but is not useful in the control of bleed, the specific polymers herein can be added in small quantities to enhance bleed performance. A further advantage of adding these specific polymers is that the concentration of acids and/or incompatible ions in the color inks can be significantly reduced. Lowered concentrations of the acids and ions in the color inks means that the ink will have better stability, better compatibility with pen materials, and will fire more reliably from pens. Thus, the combination of a dispersed pigment along with these bleed-reducing polymers provide for a very stable pigmented ink composition which have components which precipitate out of solution without adversely affecting the overall pigment's stability.
BEST MODES FOR CARRYING OUT THE INVENTION
It has long been known that certain colorant classes, especially pigments, are insoluble or become insoluble under specific and well defined pH conditions or in the presence of incompatible ions. Thus, the use of pigments in ink formulations has been limited to either 1) pigments in conjunction with dispersants or 2) self-dispersing pigments. Moreover, the dispersant must have two attributes. First, it must be able to stably disperse the pigment in the aqueous ink composition. Second, it is desirable for the dispersant to provide some measure of bleed control, or at the least not promote bleed. Industry has been challenged in formulating ink with the above limitations. However, now with this invention, a specific group of polymers has been found which precipitate under c
Hewlett-Packard Comapny
Jones Michael D.
Reddick Judy M.
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