Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-09-20
2004-04-06
Zalukaeva, Tatyana (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S307100, C526S319000, C526S306000, C526S310000, C526S312000, C526S322000, C526S324000, C526S329700
Reexamination Certificate
active
06716949
ABSTRACT:
TECHNICAL FIELD
The technical field relates to amphipathic polymeric particles that serve as dispersants and binders in ink compositions, ink compositions containing the same, and methods for making the particles and the inks. More specifically, the technical field relates to polymeric particles that increase the suspension stability, water fastness, smear fastness, and light fastness of inks.
BACKGROUND
Inks are among the oldest known technologies. Historians believe inks were utilized in China and Egypt as early as 2,500 B.C. Nonetheless, significant advances in the ink art continue to occur, especially when formulating compositions for use in more modem dispensers such as ink jet printers.
Inks for use in ink jet printers generally comprise an aqueous carrier and a colorant. The colorant can be a dye or a pigment—the distinction being that dyes are soluble in aqueous and/or organic solvents whereas pigments are relatively insoluble.
Inks containing soluble dyes, however, exhibit numerous problems. These problems include: poor water fastness; poor light fastness; clogging of the ink jet channels as a result of solvent evaporation, changes in the dye solubility, and/or dye crystallization; bleeding and feathering on the printed page; poor thermal stability; and chemical instability, including but not limited to poor oxidation resistance.
Many of these problems are minimized by replacing the dyes with pigments. In general, pigments have superior properties when compared to dyes, including good water fastness, good light fastness, thermal stability, oxidative stability and compatibility with paper. However, difficulties are encountered in maintaining the pigments in a stable and uniform suspension. If the pigments coagulate and/or fall out of suspension, the utility of the ink is greatly diminished, if not completely destroyed.
Polymeric dispersants are often employed to increase the shelf life of the pigment suspensions. Generally speaking, these dispersants contain hydrophobic groups that absorb onto the pigment particle surfaces through acid-base interactions, Van der Waals forces, or physical entanglement or entrapment. In addition, the dispersants contain hydrophilic groups that extend out into the aqueous medium. In this way, the dispersants associate the pigment with the aqueous carrier.
In the dispersant, large particles are undesirable since they clog the ink jet and are difficult to be suspend in water over a long period of time without settlement. Moreover, it is difficult to precisely control the identity, length, weight and distribution of the hydrophobic and hydrophilic groups in the polymer dispersant. When these properties are not controlled, the dispersant may not be able to fully cover the water-insoluble pigments to create an electrostatic layer that prevents aggregation. In some cases, the dispersant may even act as a flocculent which is the opposite desired effect.
Regardless of the colorant employed, the adherence of the ink on the substrate is always a major issue. Colorants must be chemically or physically bound to the treated surface, e.g., paper, in order to prevent bleeding, smearing or rubbing after the ink has dried. Accordingly, polymeric binders are often employed to chemically and/or physically entrap the colorant.
The present inventor has conducted a great deal of research in the field of inks. Much of this work is directed to polymeric dispersants and/or binders. Patents that have issued on this work include the following: U.S. Pat. Nos. 5,972,552; 5,973,025; 5,990,202; 6,027,844; 6,057,384; 6,090,193; 6,117,222; 6,248,161 B1; and 6,248,805 B1. However, there remains a need for inks, that can be used in ink jet printers, which exhibit improved shelf-life, water fastness, smear fastness, and light fastness.
SUMMARY
The invention is directed to amphipathic polymeric particles that serve as both the dispersant and the binder in water based inks. The particles have an average diameter of 50 to 400 nm with a pre-determined structure, making them ideal for inclusion in any ink marketed for ink jet printers.
In a preferred embodiment, the polymeric particles are formulated from a combination of hydrophilic and hydrophobic unsaturated monomers. Combining hydrophilic and hydrophobic moieties into the polymeric particles facilitates association between the ink's aqueous carrier and water insoluble components. This association, in turn, increases the stability of the suspension and, thereby, the shelf-life of the ink. When the ink is applied to a substrate, e.g., paper, the particles bind the colorants to the substrate by forming a film over the colorants. The film conveys superior durability, e.g., water fastness, smear fastness, and light fastness, to the inked image.
In another preferred embodiment, a water-soluble dye with a polymerizable functional group is formulated into the polymeric particles. The optical density of the dye is preserved since it lies on the outside of the particle in the water phase. The dye itself acts like a stabilizing group for the particle. The durability of the printed images is enhanced since the dye is trapped in the water-insoluble dispersant which forms a protective film upon removal of water.
In yet another preferred embodiment, the shear stability of these polymers may be improved by incorporating cross-linkers to an extent of about 1% by weight.
The invention is also directed to methods for making the aforementioned particles. A preferred method employs an emulsion of water-insoluble long chain acid containing monomers (convertible monomers) and hydrophobic monomers to generate polymers that can be stably suspended in water over a long period of time. Specifically, the convertible monomers are introduced into the emulsion in a hydrophobic form and incorporated into the polymers. The side chain acid groups of the incorporated convertible monomers are then converted to anionic salts by adjusting the pH of the solution to a basic range (pH>7). The acid-to-salt conversion changes the Zeta potential and net surface charge of the polymer particles, and increases the stability of the polymer particles in colloidal systems.
Another preferred method entails a combination of atom transfer radical polymerization (ATRP) and emulsion polymerization. By utilizing ATRP in the process, the molecular weight of the particles and the distribution of hydrophilic and hydrophobic moieties can be carefully controlled.
Finally, the invention is directed to an environmentally friendly, water based ink that contains a vehicle, a colorant, a surfactant, and the aforementioned polymeric particles. Due to the presence of the amphipathic polymeric particles, these inks exhibit improved dispersion and shear stability, shelf-life, water fastness, smear fastness, and light fastness.
Definitions
As defined herein, the term “water fastness” refers to the resistance of an impression to dilution or removal by water. A water fast ink has a reduced tendency to wick, feather or be washed away. Water fastness can be measured by wetting the printing area with water and determining the optical density (OD) in the neighboring areas (defined as “background OD”) before and after the exposure to water.
As defined herein, the term “smear fastness” refers to the resistance of an image to smear on contact with a hard object, such as the tip of a highlighter, under normal pressure. A smear is defined as the transfer of colorant from the printing area to the neighboring areas (background) by the object. Smear fastness can be measured by determining the change of the background OD after subjecting the printing area to a standard smearing force.
As defined herein, the term “light fastness” refers to the durability of a print when exposed to light. When an ink is light fast, it has fade resistance. It is generally thought that pigments have improved fade resistance over dyes but some of the newer dyes have shown that they can be comparable.
As defined herein, the term “shear stability” refers to the polymer particles' ability to maintain their original size
Hewlett--Packard Development Company, L.P.
Zalukaeva Tatyana
LandOfFree
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