A New pH Neutral Waterborne Dispersing Resin For Metallic And Organic Pigments

A new styrene-maleic anhydride-based dispersing resin is described and its utility in preparing waterbased metallic and organic pigment dispersions and printing inks is demonstrated. This resin contains amic acid functionality, groups that are possible to form only with anhydride-containing resins. A unique combination of properties is exhibited by this resin, including neutral pH, high water solubility, low molecular weight and high acid number. The amic acid resin functions as an efficient polymeric surfactant in a range of water-based dispersing and emulsifying applications. For example, it can be used to prepare high solids, low viscosity dispersions of organic pigments. In addition, the dispersing resin can be combined with a co-resin or an emulsified wax to produce a vehicle to disperse metallic pigments. Typical formulations and their properties are described in this paper. Background Dispersing resins are a crucial ingredient to waterbased ink formulations. Functions carried out by dispersing resins include wetting the pigment surfaces, facilitating the breakup of pigment agglomerates to maximize color development, stabilizing the dispersion of pigment particles and modifying the rheological properties of the dispersion and finished ink formulations. In addition to carrying out these functions, the dispersing resin must be compatible with the myriad of other ingredients that are used in preparing dispersions and inks. Defoamers, surfactants and emulsion solutions all are, or contain, surface active molecules that can potentially interact with the dispersing resin. The challenge to select the proper dispersing resin becomes even more difficult when the pigment, itself, can react or deteriorate from this interaction. Such is the case with metallic pigments, which present special issues for the preparation of stable water-based dispersions and inks. Metallic pigments are finally divided powders or platelets of a metal or a metal alloy. While metallic pigments have a unique appearance combining color and brilliance, they also have a unique reactivity compared to typical organic pigments. Compared to organic compounds, metals have a low reduction potential. The consequence of this is that in an oxidizing environment, the oxidant will be reduced and the metal will be oxidized in a so called redox couple reaction. The redox chemistries of the metals and alloys typically used in graphic arts are well known.1 For example, copper, which, either by itself or as its alloy with zinc, is the predominant metal in “gold” inks, has a reduction potential of -0.34V: (For comparison, actual gold has a reduction potential of -1.68V, which is why it is a much more stable, “noble” metal.) The low reduction potential for copper causes it to react even under relatively mild, ambient conditions. A shiny, new copper surface will rapidly acquire a tarnished patina due to reaction with atmospheric oxygen and carbon dioxide to form a coating of copper carbonate: 2 Cu + H2O + CO2 + O2 ® Cu2 (OH)2 CO3 In water solution, copper will react with trace hydroxide to form copper hydroxide, Cu(OH2). In the presence of ammonia, this solid reacts to form a bright blue complex: 4950 01/10 Cu ® Cu+2 + 2 egE = 0.34 V APPLICATION BULLETIN Oaklands Corporate Center, 468 Thomas Jones Way, Suite 100, Exton, PA 19341 Toll Free 877-871-2729