Method of making cationic pigment slurries

Details Method of making cationic pigment slurries Method of making cationic pigment slurries

2000-11-14

2003-09-09

Lovering, Richard D. (Department: 1712)

Colloid systems and wetting agents; subcombinations thereof; pro

Continuous liquid or supercritical phase: colloid systems;...

Aqueous continuous liquid phase and discontinuous phase...

C106S287170, C427S395000, C428S702000, C524S430000

Reexamination Certificate

active

06617362

ABSTRACT:

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION
The invention relates to a method of preparing stable slurries for use in coating color formulations that are applied to a base stock in the manufacture of paper or paperboard materials. The slurries include one or more alumina pigments,a nonionic wetting agent or nonionic polymer and a cationic interfacial modifier. The coating colors formed using such slurries may be applied to a paper or paperboard stock during a manufacturing process and dried on the surface thereof to form a smooth, printable surface with good porosity, such as is needed for subsequent printing applications. In comparison to conventional pigment dispersions, the slurries may be prepared in advance and stably maintained under ambient conditions for relatively long periods without gelling, flocculation or other deterioration. In this regard, a high solids content of greater than 70% weight may be achieved while maintaining the viscosity at a level that permits the slurry to be easily poured.
BACKGROUND OF THE INVENTION
Coating formulations or “colors” used in the manufacture of printable paper or paperboard products are usually applied to at least one side of a base stock to improve surface smoothness and to provide uniform ink reception and other generally acceptable printing properties. Typically, such formulations include pigments, binders and other additives such as dispersants, viscosity modifiers, lubricants, and chemicals that modify foaming tendencies, microbial susceptibility, pH or physical characteristics such as color. Porous pigments, for example alumina, are important components of such coating formulations because they contribute to surface porosity of the paper, which, in turn, aids ink drainage. The pigments also reduce surface irregularities to provide a smoother product surface. In this regard, a higher proportion of porous pigment produces a corresponding improvement in the smoothness, porosity and ink-receptivity of the coating. These characteristics are particularly desirable for certain end-use applications of the coated paper product, such as inkjet printing. A commonly observed drawback, however, is that the particulate nature of the pigment limits the amount that can effectively be included in the coating formulation during the manufacturing process.
The pigment is usually added during makedown of the coating formulation as a dispersion or slurry of finely divided particles, as this mode of addition is more convenient for bulk manufacturing. The use of such dispersions is, however, limited by the amount of pigment that is actually suspended in the dispersing medium at the time that it is added to the coating formulation. Because the pigments are insoluble, the pigment particles will settle out, resulting in a reduced amount of pigment per unit volume in the dispersion and the coating color. As a consequence, it is difficult to control the total solids content in the dispersion itself and in the final coating formulation. Another disadvantage is that the particles may react with certain components of the dispersion and aggregate to form a gel or sol of high viscosity, which may cause an undesirable increase in the viscosity or otherwise affect the rheological properties of the coating color.
Inventive efforts have therefore focused on the preparation of stable dispersions of pigments to be incorporated into the coating color in a manner that allows maximum compatibility with other components of the coating formulation. Such dispersions have been prepared using an acid-based dispersant as a means of stabilizing the pigment particles in suspension. For example, U.S. Pat. No. 3,935,023 (Derolf) discloses aqueous dispersions of alumina in a hydrochloric acid-containing solution. The dispersions so formed have an alumina content of from about 18-26% weight, based on the total weight of solids, or about 54-66% parts by weight (pbw) of Al
2
O
3
per pbw of hydrochloric acid, and a specific surface area of pigment, as determined by the Brunauer, Emmett and Teller (BET) method, of 150-250 m
2
/gram. The patent points out that in order to achieve dispersions of 18-26% by weight, it is necessary to maintain an Al
2
O
3
to HCl ratio of 54-65 pbw to 1 pbw of HCl, and that when less than 53 pbw of Al
2
O
3
per pbw of HCl is used, the shelf life of the resulting product is drastically limited. Accordingly, the acid is a necessary component of such formulations. Acids, however, tend to be hazardous and corrosive materials, and are less effective towards achieving a dispersant effect. Moreover, it has been determined experimentally that using acids as the primary dispersion means will not produce slurries having the desired high level of solids as well as an acceptable viscosity.
U.S. Pat. No. 5,518,660 (Wei) describes preparing low-viscosity colloidal dispersions of sub-micrometer alumina particles for use in ceramics manufacture that contain 0.1 to 5% by weight of a semicarbazide hydrochloride acidic compound as a dispersing agent. In this regard, the semicarbazide compound dissociates in water by releasing the HCl ligand, which, in turn acidizes the dispersion to avoid agglomeration and thereby maintain a low viscosity. However, as noted previously, the use of acidic compounds in the dispersion is accompanied by the drawbacks of corrosivity and hazardous nature of such materials.
As an alternative, coating colors have been prepared by incorporating the solid pigment directly into the color formulation. For example, Japanese Patent Abstract No. JP 9314985A2 describes an aqueous coating formulation for inkjet or electrophotographic printing containing alumina hydrate, ethylenediamine-tetraacetic acid sodium as a chelating agent, an imidazolium compound as a cationic component and polyacrylamide. The process described by the patent abstract eliminates the step of dispersing the alumina hydrate and adding it to the color formulation as a slurry. Direct introduction of a solid pigment during preparation of the color formulation as reported in this abstract is undesirable, however, because additional steps are then required to homogenize the coating formulation and to maintain the desired rheological properties. These additional steps would increase manufacturing costs. In addition, the coating formulation may still be susceptible to the deterioration observed with respect to many dispersions, such as flocculation and gellation.
Accordingly, a need exists in the art for a method of preparing alumina pigment slurries having high pigment solids content, and which may be stably maintained over long storage periods. In addition, it is desirable that such slurries be highly compatible with ingredients used in coating formulations for paper and paperboard manufacture, while also being particularly useful in coatings for printing applications such as inkjet or electrographic printing.
SUMMARY OF THE INVENTION
A solution to the problems evident in the prior art is found in the present invention, which relates to stable pigment slurries that are prepared without an acid-based dispersant, and methods for using such slurries in paper or paperboard manufacturing applications. The slurries may advantageously be used in the manufacture of ink-receptive coatings for printing applications including, but not limited to the manufacture of inkjet paper coatings.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
According to the present invention, a pigment slurry that is suitable for use in manufacturing paper and paperboard coating formulations may be prepared and maintained over relatively long periods without flocculation, gellation or other deterioration. The slurries of the invention may be used in a variety of applications including, but not limited to, paper and paperboard coating color formulations. The slurries are formed by dispersing and homogenizing an alumina pigment, a nonionic wetting agent or nonionic polymer and a cationic interfacial modifier in an aqueous medium.
As used herein, the term “alumina pigment” includes particu

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