Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1998-12-18
2001-07-17
Jagannathan, Vasu (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C523S160000, C523S161000, C525S212000
Reexamination Certificate
active
06262207
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to dispersants for hydrophobic particles including pigments in water-based systems. In particular, the inventive dispersant can be used with pigments such as those useful for inks adapted for ink jet printing applications.
BACKGROUND OF THE INVENTION
When hydrophobic particles are placed in water they tend to agglomerate. This tendency makes it difficult to obtain a dispersion of hydrophobic particles in a water-based system. To lessen this agglomeration tendency, a dispersant may be added to the mixture. If the dispersant is both hydrophobic and hydrophilic its hydrophobic part will be attracted to the hydrophobic particle surface and the hydrophilic part will be solvated by the water.
In recent years, the use of thermal ink jet printers in numerous applications has increased dramatically. Such printers use liquid-based inks, which are sprayed onto a receptor, typically a sheet of paper or film, to produce an image. By using basic ink colors (black, yellow, cyan, and magenta, or black, red, green and blue) as well as white pigments (such as TiO
2
) in various combinations and concentrations, virtually any color may be produced as part of the printed image. Additionally, ink jet technology is well-suited for high resolution graphic images, particularly those produced using electronic printing systems. Such systems typically employ computer technology to create, modify, and store images, text, graphics and the like.
Many of the inks that have been used in the past with ink jet and other printers are primarily comprised of dyes contained within organic-based carrier liquids. Although such inks may offer satisfactory performance in certain applications, the present trend is away from such systems, because such systems tend to produce images that lack the light stability and durability required for outdoor and similarly demanding applications. Additionally, the use of organic-based carrier liquids creates numerous environmental and material-handling complications. Rather, the printing industry has sought inks that are primarily water-based to thereby reduce or eliminate the problems associated with organic solvent-based systems.
Inks may comprise a suspension of pigment particles in a water-based carrier. However, the suspended pigments may tend to agglomerate. Because ink jet printers use very small jet nozzles (on the order of less than about 40 micrometers with droplet volumes on the order of picoliters) to provide high resolution images, pigment agglomerations may restrict or clog the printer heads. This effect is referred to herein as “plugging.” Additionally, in the case of thermal inkjet systems, the ink is subjected to high temperatures (approximately 350° C.) as a result of the heating element in the nozzle. Typically inks tend to agglomerate at a faster rate at higher temperatures, and may also settle onto, and coat, the heating elements of the printer heads. This causes decreased thermal efficiency of the print head which results in the formation of smaller ink droplets and lower image quality. This effect is commonly referred to as “kogation.”
To overcome the problems described above, some pigment particles have been stabilized (from agglomeration) with dispersants. In one approach, the dispersants were formed from surfactants having a hydrophilic portion and a hydrophobic portion, which hydrophobic segment adsorbed onto pigment surfaces of varying hydrophobicity. In another approach, copolymers having hydrophilic and hydrophobic segments were used. Examples of these approaches are described in U.S. Pat. No. 4,597,794 and U.S. Pat. No. 5,085,698.
In the approaches described above, the hydrophobic segments of the surfactant or polymer can adsorb onto pigments by hydrophobic interaction between the dispersant molecule and the organic-based pigments whose surfaces tend to be hydrophobic. This hydrophobic interaction is usually not very strong. Because of this weak attraction, the dispersant molecules may desorb from the pigment surface, thereby allowing the pigment particles to agglomerate. During printing, this can result in plugging of the printer head jet nozzles. Although heat produced in a thermal inkjet system may strengthen the adsorption of the dispersant on the hydrophobic pigment surface, thus enhancing the stability of the ink, plugging and kogation remain a problem.
SUMMARY OF THE INVENTION
In view of the foregoing, a need exists for dispersants that strongly adsorb to hydrophobic particles in aqueous media and that inhibit agglomeration. A need also exists for water-based inks that offer increased stability. The present invention addresses such needs.
One aspect of the invention provides a dispersant for dispersing hydrophobic particles in an aqueous system comprising an AB
n
structure wherein A is a hydrophilic polymeric segment containing ionizable moieties and is terminated with a multifinctional chain transfer agent (CTA), B is a nonpolar nonpolymerized hydrocarbon hydrophobic moiety, n is greater than one, and the B moieties form a forked segment at the CTA-terminated end of the A segment. The presence of multiple hydrophobic moieties can enhance the dispersant's interaction with particle surfaces.
Another embodiment of the invention relates to stable water-based systems in which the inventive dispersant is incorporated into a water-based dispersion of particles, the particles having hydrophobic surfaces. Such systems comprise an aqueous suspension of a) hydrophobic particles, and b) a dispersant comprising an AB
n
structure wherein A is a hydrophilic polymeric segment containing ionizable moieties and is terminated with a multifinctional chain transfer agent (CTA), B is a nonpolar nonpolymerized hydrocarbon hydrophobic moiety, n is greater than one, and the B moieties form a forked segment at the CTA-terminated end of the A segment.
A particular application of the present invention relates to stable water-based pigmented inks that meet the stringent requirements of ink jet and other printing applications. In these inks, an inventive dispersant is incorporated into a water-based pigment dispersion to eliminate or reduce the likelihood that the hydrophobic pigment particles will agglomerate.
As used in this invention:
“adsorb” refers to the adherence of an atom, ion, or molecule to the surface of another substance;
“dispersant/particle composite” means a dispersant molecule and the particle or particulate to which the dispersant is adsorbed; typically the composite will comprise layers of dispersant molecules with the hydrophobic particle;
“ionizable” means a neutral molecule comprising atoms capable of losing or gaining electrons, thereby acquiring a net electrical charge and becoming an ion, e.g., when placed in a polar medium such as water,
“forked” means having two or more hydrophobic moieties extending from the CTA-terminated end of the hydrophilic segment; the hydrophobic moieties may extend in different direction and are preferably able to move sufficiently to allow individual moieties of the dispersant molecule to adsorb to a location on a particle surface independent of the adsorption location of another moiety of the same dispersant molecule;
“hydrophilic polymeric segment” or “hydrophilic polymeric chain,” or “amphiphilic polymer (chain)” means a the portion of the dispersant, distinct from the hydrophobic segment, that is comprised of at least 60 weight % water-soluble monomer units and has at least one ionizable monomeric component;
“oligomer” means a polymer chain having approximately 10 to 50 monomer units;
“aliphatic group” or “aliphatic moiety” or “long chain aliphatic group” or “long chain aliphatic moiety” means a linear, branched, or cyclic, substituted or unsubstituted nonpolar hydrocarbon having approximately 6 to 100, preferably 8 to 20, carbon atoms;
“aromatic group” or “aromatic moiety” means a nonpolar hydrocarbon, having approximately 6 to 100, preferably 8 to 20, carbon atoms, comprising at least one cyclic structure containing at least one &pgr;-conjugated ring struc
Abuelyaman Ahmed Salih
Gaddam Babu N.
Rao Prabhakara Satyauolu
3M Innovative Properties Company
Gover Melanie
Jagannathan Vasu
Shosho Callie E.
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