Two component dispersant for wet milling process

Details Two component dispersant for wet milling process Two component dispersant for wet milling process

2001-10-01

2002-09-24

Jagannathan, Vasu (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S522000, C524S523000, C523S160000

Reexamination Certificate

active

06455628

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an improved wet milling dispersion process using a unique combination of a block copolymer and a random copolymer of similar composition as the dispersant. The process provides higher loading of particulate solids, with smaller particle size and shorter processing times. The dispersions are particularly suited for preparing pigmented inks for ink-jet printers.
BACKGROUND OF THE INVENTION
Water-based pigment dispersions are well known in the art, and have been used commercially for applying films, such as paints, to various substrates. The pigment dispersions generally are prepared by a dry method, such as 2-roll or 3-roll milling, or a wet method, such as media milling. The dry milling method includes a milling step wherein the dispersant and the pigment are intimately mixed and milled utilizing mechanical forces to cause particle size reduction and adsorption of the polymer to the pigment surface; a grinding step wherein the pigment dispersion is reduced to small chips; and an inversion step where the chips are dissolved in an aqueous carrier medium. In the wet milling method, particle size reduction and dispersion stabilization are conducted in one step in the presence of the aqueous carrier medium.
The dispersant used in a wet milling process must effectively wet the pigment surface and achieve a stable pigment dispersion. Random copolymer dispersants have been disclosed for this purpose, wherein the resulting aqueous dispersion is used as an ink-jet printer ink, in U.S. Pat. No. 4,597,794 (Ohta et al.) assigned to Canon. Block copolymer dispersants, having a hydrophobic block that links to the pigment particles and a hydrophilic block, also are disclosed for this purpose in U.S. Pat. No. 5,085,698 (Ma et al.) assigned to DuPont.
Block copolymer dispersants offer improved dispersion stability (compared to random polymer dispersants) because they provide both a charge double layer and steric stabilization. The block copolymers are difficult to manufacture, however, and tend to form stable micelles in the dispersion due to their structure. These micelles, with the hydrophobic pigment-binding segments buried in the core, do not wet the pigment surface as effectively as may be desired. Also, the block copolymers tend to have a high viscosity, which hinders the milling process and requires a reduction in pigment loading for successful milling to occur.
Surfactants may be added to facilitate pigment wetting and to reduce dispersion viscosity, thereby addressing the problems discussed above. The addition of surfactants, however, tends to change other physical properties of the dispersion (such as surface tension), and may render the dispersion unsuitable for a desired application. For example, the surfactant may adversely affect dispersion stability; and/or, when the resulting dispersion is used as an ink, the ink tends to have a low surface tension that causes image feathering and general poor print quality.
Accordingly, a need continues for an improved dispersion process for preparing aqueous dispersions in general, and in particular for preparing aqueous pigmented inks that are particularly suited for use in ink-jet printers and contain high-loading levels of small pigment particles.
SUMMARY OF THE INVENTION
It now has been found that the presence of certain random polymers in aqueous particulate dispersions having a block copolymer dispersant enhance wetting of the particles, and enable the particulate loading to be increased without adversely affecting physical properties such as surface tension. Accordingly, in one aspect the invention provides a dispersion having an aqueous carrier medium, at least one particulate solid, and a polymeric dispersant that is a mixture of a block copolymer and a random copolymer, wherein (i) the block and random copolymers are prepared from substantially the same monomers; and (ii) the random copolymer is present in the amount of 1 to 100 parts by weight per 100 parts, by weight, of the block copolymer.
In another aspect, the invention provides a process for preparing an aqueous dispersion by wet milling an aqueous carrier medium, at least one particulate solid, and a polymeric dispersant that is a mixture of a block copolymer and a random copolymer, wherein (i) the block and random copolymers are prepared from substantially the same monomers; and (ii) the random copolymer is present in the amount of 1 to 100 parts by weight per 100 parts, by weight, of the block copolymer. The process achieves high loadings of the particulate solid (e.g., a pigment or disperse dye) during a short time, which improves productivity.
DETAILED DESCRIPTION OF THE INVENTION
The invention provides a unique mixture of a block copolymer to provide the dispersion stability, and a random copolymer of substantially the same composition to enhance the wetting of the surface of the particulate solid. This unique mixture of dispersants allows a higher loading of particulate solid for higher productivity, without adversely affecting the physical properties of the dispersion such as surface tension. For example, the resulting dispersion may contain up to 50% or 60% dispersed solids, based on total weight of the dispersion. This loading of particulate solids is up to twice the amount that can be achieved when the block dispersants are used alone. Resulting pigment dispersions are particularly suited for use with ink jet printers in general, and thermal ink jet printers in particular.
Aqueous Carrier Medium
The aqueous carrier medium is water or a mixture of water or at least one water soluble organic component. Deionized water is commonly used. The organic component may be an organic solvent, polymeric binder, thickener, thixotropic agent, coating aid, etc.
For ink jet inks, the aqueous carrier medium is typically a mixture of water and at least one water-soluble organic solvent. Representative examples of water-soluble organic solvents are disclosed in U.S. Pat. No. 5,085,698. Selection of a suitable mixture of water and water soluble organic solvent depends upon requirements of the specific application, such as desired surface tension and viscosity, the selected pigment, desired drying time, and the type of media substrate onto which the coating or ink will be printed. A mixture of diethylene glycol and deionized water is preferred as the aqueous carrier medium for ink jet inks, with the composition typically containing between 30% and 95% (preferably 60% to 95%) water by weight, based on the total weight of the mixture.
The amount of aqueous carrier medium in the ink is in the range of approximately 70 to 99.8%, preferably at least 94%, based on the total weight of the ink when the particulate solid is an organic pigment, and approximately 25 to 99.8% when the particulate solid is an inorganic pigment.
Particulate Solids
The particulate solid may be an insoluble colorant (such as a pigment or disperse dye), colloidal silver halide, metallic flake, a herbicide, an insecticide, or biomaterials (such as drugs) depending upon the particular application of the dispersion. For example, if the intended use is in an ink or a paint, the particulate solid is an aqueous carrier medium insoluble colorant such as a pigment, disperse dye, or a mixture thereof.
The particulate solid selected must be capable of binding with the hydrophobic portion of the block copolymer. Preferably, the particulate solids have “binding sites” that permit binding with the polymer. Most of the above-mentioned particulate solids have very specific functional groups on their surfaces.
For example, all carbon blacks have chemisorbed oxygen complexes, primarily acidic in nature (e.g. carboxylic, quinonic, lactonic or phenolic groups) on their surfaces to varying degrees, depending on the conditions of manufacture. These acidic groups provide binding sites for dispersants having basic functional groups, such as amine groups. Other pigments have basic surfaces. The pigment itself may contain functional groups, or the surfaces may be modified by compounds containing

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