Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
1999-06-04
2001-04-03
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C106S031650
Reexamination Certificate
active
06210474
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for preparing a pigmented ink jet ink which employs a certain milling media.
BACKGROUND OF THE INVENTION
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
A dye is a colorant which is molecularly dispersed or solvated by a carrier. The carrier can be a liquid or a solid at room temperature. A commonly used carrier is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Water-based pigmented inks are prepared by incorporating the pigment in the continuous water phase by a milling and dispersing process.
U.S. Pat. No. 5,679,138 relates to a process for preparing an ink jet ink containing nanoparticles of organic pigments. This process employs milling media of polymeric resins such as cross-linked polystyrene. There is a problem with using this media, however, in that the jetting reliability is not as good as one would like it to be.
U.S. Pat. No. 5,755,861 relates to a process for preparing an ink jet ink which has an improved jetting reliability by removing trace metal impurities from the ink. The metal impurities are removed from the ink by a cation-exchange treatment using a cation-exchange resin. There is a problem with this process, however, in that it requires that the ink be passed through a column packed with an ion exchange resin which adds another step and expense to the process.
It is an object of this invention to provide a process for preparing an ink jet ink having an improved jetting reliability. It is another object of the invention to provide a process for preparing an ink jet ink which will remove trace metal impurities without requiring a treatment with an ion exchange resin.
SUMMARY OF THE INVENTION
These and other objects are achieved in accordance with this invention which relates to a process for making an ink jet ink comprising:
a) providing an organic pigment dispersion containing a pigment and a carrier;
b) mixing the pigment dispersion with rigid milling media having an average size less than about 100 &mgr;m;
c) introducing the mixture from step b) into a high speed mill;
d) milling the mixture from step c) until a pigment particle size distribution is obtained wherein about 50% by weight of the pigment particles have a particle size less than about 100 nanometers;
e) separating the milling media from the mixture milled in step d); and
f) diluting the mixture from step d) to obtain the ink jet ink;
wherein the milling media comprises negatively-charged polymeric beads which are crosslinked sufficiently to prevent swelling of the polymeric beads to levels greater than or equal to about 50 volume % within 4 hours at 25° C. in the carrier.
By use of the invention, an ink jet ink can be obtained which has an improved jetting reliability by removing trace metal impurities without requiring a treatment with an ion exchange resin.
DETAILED DESCRIPTION OF THE INVENTION
As noted above, the milling media which can be used in the invention comprises negatively-charged beads of a polymer which are crosslinked sufficiently to prevent swelling of the beads to levels greater than or equal to 50 volume % within 4 hours at 25° C. in the carrier used for the milling process. This swelling value was obtained from a study of the effectiveness of crosslinked polymers as milling media as disclosed in U.S. Pat. No. 5,902,711.
The negatively-charged groups on the polymer beads which can be used include strong acids, such as those containing sulfonic acid groups, and weak acids, such as those containing carboxylic acid groups. The polymer beads can be used either in the hydrogen or salt form, with the sodium salt form being preferred.
It has been found that crosslinking of negatively-charged polymer beads sufficiently to prevent swelling to levels greater than or equal to 50 volume % within 4 hours at 25° C. in the carrier used for a milling process provides a milling media which unexpectedly both removes undesirable metal ions and reduces the particle size of a pigment without fracturing of the media.
In a preferred embodiment of the invention, the negatively-charged polymeric beads comprise the monovalent salt of sulfonated polystyrene or a polycarboxylic acid, such as methacrylic acid, which has been crosslinked. In another preferred embodiment, the polymer is cross-linked with divinylbenzene.
In general, the milling media of the invention comprise negatively-charged polymeric beads which are of sufficient hardness and friability to enable them to avoid being chipped or crushed during the milling process. The preferred method of making the polymeric beads is by suspension polymerization of acrylic or styrenic monomers, optionally followed by chemical treatment, such as sulfonation, as required to provide the negative charge. Methyl methacrylate, methacrylic acid and styrene are preferred monomers because they are inexpensive, commercially available materials which make acceptable milling media. Other acrylic and styrenic monomers are also useful.
In accordance with the invention, the negatively-charged polymeric beads are sufficiently crosslinked to prevent 50 vol. % swelling of the polymer in the carrier within 4 hours of contact. Any co-monomer with more than one ethylenically unsaturated group can be used in the preparation of the polymeric beads to provide the crosslinking functionality, such as divinylbenzene or ethylene glycol dimethacrylate. While only a few weight percent crosslinker may be sufficient to make a polymer insoluble in a carrier for an ink jet ink, typically a significantly higher level will be required to prevent substantial swelling of the negatively-charged beads in such carriers. The amount of crosslinking monomer required to be incorporated into the polymer to restrict swelling to less than 50 vol. % will depend upon the composition of the carrier and of the polymeric beads. In general, however, it will be advantageous to provide at least about 10 mole %, more preferably at least 20 mole %, and most preferably at least about 25 mole % crosslinking monomer. Polymers, which contain negative charges or to which negative charges will be added, having the following formula are preferred:
(A)
X
(B)
y
(I)
where A is derived from one or more monofunctional ethylenically unsaturated monomers, B is derived from one or more monomers which contains at least two ethylenically unsaturated groups, x is from 0 to about 90 mole %, and y is from about 10 to 100 mole %, preferably from about 20 to 100 mole %, and most preferably from about 25 to 100 mole %. If less than about 10 mole % crosslinking monomer is included, the polymeric beads may not be sufficiently crosslinked to limit swelling in many carriers to less than 50 vol. %. In general, the higher the mole % of crosslinking monomer in the polymer, the more resistant it will be to swelling in carriers, and the more effective it will be in reducing the pigment particle size without contaminating the ink with fractured polymer beads.
Suitable ethylenically unsaturated monomers which can be
Bennett James R.
Bugner Douglas E.
Guistina Robert A.
Romano, Jr. Charles E.
Smith Dennis E.
Cole Harold E.
Eastman Kodak Company
Faison Veronica F.
Klemanski Helene
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