Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
2001-08-03
2003-11-25
Dawson, Robert (Department: 1712)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C526S279000, C526S072000, C524S560000, C524S588000, C524S561000, C524S081000, C524S401000, C524S806000, C523S160000
Reexamination Certificate
active
06652634
ABSTRACT:
FIELD OF THE INVENTION
This invention belongs to the field of organic chemistry. In particular, it relates to dispersants useful in ink jet ink compositions.
BACKGROUND OF THE INVENTION
Ink jet printing is a non-impact method of printing that involves ejecting ink from a nozzle onto paper or other print media. The actual ink ejection method may occur via several processes including pressurized nozzles, electrostatic fields, piezoelectric elements within an ink nozzle, and heaters for vapor phase bubble formation.
The composition of the ink is traditionally comprised of deionized water, a water soluble organic solvent, and a colorant. The colorant may be a soluble dye or insoluble pigment. Several problems, however, are associated with soluble dyes that are not applicable to insoluble pigments. These problems include poor water-fastness, poor light-fastness, poor thermal stability, facile oxidation, dye crystallization, and ink bleeding and feathering on the print medium. To circumvent these problems, use of a pigment as the colorant is preferred. Pigments generally have better light-fast and water-fast properties, are more resistant to oxidation, and have higher thermal stability.
Use of a pigment instead of an aqueous dye presents solubility problems since the pigments are insoluble in aqueous media. As a result, the insoluble pigment is generally stabilized in a dispersion by a polymeric dispersant. Generally speaking most pigment inks stabilized by polymers in aqueous media are based on an electrosteric stabilizing mechanism in which a hydrophobic group in the dispersant acts as an anchor adsorbed on the pigment particle surface through an acid-base relation, electron donor/acceptor relation, Van der Waals forces, or physical absorption. The hydrophilic group is extended into the aqueous medium to keep the dispersant soluble. This results in a competition in the dispersing process between the pigment particle and the polymer, the polymer and the solvent, and the pigment particle and the solvent.
In order to form a stable polymeric dispersion, several factors are considered. First, the polymer must be firmly adsorbed to the pigment surface to withstand shear force and the competition of other chemical species. This requires a careful match of the polarity of the pigment surface to the hydrophobic group in the dispersant. Second, the physical dimensions of the hydrophobic group in the dispersant must be adequate to fully cover the pigment surface, otherwise, the adsorbed polymer will act as a flocculent. Third, an electrostatic layer of a requisite thickness around the particle is needed to repulse aggregation of particles within the aqueous medium.
Since carbon black and color pigments exhibit different hydrophilic-lipophilic balance (HLB) values
1
as well as different surface functional groups, the hydrophobic anchor groups that can be firmly adsorbed onto the carbon black surface do not interact well with the colored pigment surface. This poor interaction between the hydrophobic anchor and the colored pigment surface is further aggravated by the fact that the crystal structure of the pigment prevents full surface exposure of pigment particle functional groups. Van der Waals forces are not enough to stabilize the dispersion. This poor interaction between the hydrophobic anchor and the colored pigment allows for dispersion destabilization when other ink ingredients are introduced.
1
Hydrophilic-lipophilic balance values are measured on an index where the lowest value (1) is lipophilic (oil soluble) and the top value (typically 20 or 36) is hydrophilic (water soluble).
The present invention provides a novel polymer that exhibits favorable interactions with the surface functional groups of colored pigment particles. These favorable interactions serve to increase dispersion stability of the ink in comparison with current hydrophobic anchors. This increased dispersion stability facilitates the production of higher quality colored pigment inks for use in ink jet printing applications.
SUMMARY OF THE INVENTION
The present invention relates to graft copolymers, useful as dispersants in ink jet ink compositions, having an average weight molecular weight from 2,500 to 20,000, preferably of from about 3000 to 15,000. The graft copolymers comprise monomers having electron rich functional groups, which exhibit favorable interactions with the surface functional groups of colored pigment particles thereby better stabilizing the color pigment dispersion within the aqueous ink composition. The polymers comprise two structurally distinct segments: a hydrophilic segment and a hydrophobic segment.
The preferred hydrophilic segment is comprised of acrylamidoalkylsulfonic acid, styrene sulfonic acid, or other acid, amide or amine (quaternary amine) containing monomers, preferably a methacrylic acid (MAA) polymer, or a copolymer thereof. Different acid or amine groups may be used together.
The preferred hydrophobic segment comprises a polymer or copolymer containing electron rich functional groups comprised of a plurality of methacrylate derivatized monomers, preferably a substituted methacrylate ester monomer (a methacrylate ester where the alkyl group is replaced with a siloxyl substituent, an oligomeric siloxane), preferably comprising the formula:
wherein n ranges from 1 to 20, y is 0, 1, or 2, and at least one monomer comprising a segment of the formula:
wherein n is an integer from 0 to 50, m is an integer from 1 to 3, and each R
1
is independently selected from C
1
-C
9
alkyl, or aryl-C
1
-C
9
alkyl, provided that at least one of said R
1
is aryl-C
1
-C
9
alkyl. The hydrophobic segment may include a C
1
-C
20
alkythio functionality.
In a further embodiment of the present invention, a polymer comprising a monomeric hydrophobic head and a polymeric tail acts as a pigment dispersant stabilizing agent. The monomeric hydrophobic head comprises the formula:
wherein m is an integer from 1 to 3, X is a polymerizable group, preferably O, N or S, and each R
1
is independently selected from C
1
-C
9
alkyl, or aryl-C
1
-C
9
alkyl, provided that at least one of said R
1
is aryl-C
1
-C
9
alkyl. The tail comprises the formula:
—(OCH
2
CH
2
)
n
—O—; or
—[OCH(CH
3
)CH
2
]
n
—O—
wherein n is from 0 to 50 and is attached to a poly(methacrylic acid) or poly(methacrylic acid) derivative backbone. In a preferred embodiment, R
1
is a styrene functionality, X is ethylene glycol or propylene glycol, and the length of the hydrophilic tail is set to n=25. In a more preferred embodiment, the monomeric hydrophobic head is (ethylene glycol) 2,4,6-tris-(1-phenylethyl)phenyl ether, and the length of the hydrophobic tail is set to n=25.
The present invention also relates to aqueous ink compositions which include those polymeric dispersants. Specifically, the dispersant compositions of the present invention for use in ink jet printers comprises from about 0.5% to about 10% of an insoluble pigment, from about 0.2% to about 10% of the polymeric dispersant described above, and an aqueous carrier.
All percentages and ratios, used herein, are “by weight” unless otherwise specified. All molecular weights, used herein, are number average molecular weights unless otherwise specified. As used herein, “pigment” refers to an insoluble colorant (including organic and inorganic pigments.)
Further details and advantages of the present invention are set forth below in the following more detailed description.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to polymeric dispersants used to stabilize aqueous pigment ink compositions, as well as pigment ink compositions containing those dispersants. The polymers of the present invention have an average weight molecular weight of from about 2,500 to about 20,000, preferably of from about 3,000 to about 15,000. The polymers exhibit favorable interactions with the surface functional groups of colored pigment particles. These favorable interactions serve to increase dispersion stability of the ink in comparison with current hydrophob
Akers, Jr. Charles Edward
Butler Carla Marary
Franey Terence Edward
Sun Jing X.
Brady John A.
Kilpatrick & Stockton LLP
Lexmark International Inc.
Peng Kuo-Liang
LandOfFree
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