Systematic Considerations for the Thermodynamic Stability of Pigmented Inkjet Dispersions and Inks

Abstract

From the thermodynamic point of view, all pigmented inks including dispersions are thermodynamically unstable systems in nature. They are only meta-stable systems, or called kinetically stable systems due to some factors affected, such as electrostatic charge, viscosity, and interfacial potential difference. Therefore, for a well-performed pigmented inkjet ink system, a comprehensive thermodynamic stability (including particle size, viscosity, surface tension, pH etc.) is very important and required, which means the finished pigmented inkjet ink must possess the physical properties as follows, i.e. settle free and clog free, for a considerably long period. This paper attempts to provide a deep theoretic analysis and a comprehensive thermodynamic and kinetic consideration for inkjet inks and their dispersions by using quasi- (or pseudo-) equilibrium theory based on the experimental results. Under the guidance of the above theory, the paper finds that a proper dispersion for inkjet ink is a key for manufacturing excellent finished inkjet ink. There is a “gold cut” existing between conventional flexography and gravure dispersions and inkjet ink dispersions. Both inkjet dispersions and finished inkjet inks are superior in stability over the period of three years without obvious settling in the projected shelf life based on accelerating evaluations. Also, a critical particle size for the super stability of digital dispersions and inkjet inks has been found. The finished inkjet inks with ultra-fine particle size (< 0.3 μm) and narrow distribution provide a full range of color gamut, good water bleeding resistance and high UV light fading resistance. They can be utilized in both thermal and piezo drop-on-demand printing systems, including desk-jets, wide-formats, and industrial printers with high-performance printing quality. The invented dispersions and inks are currently in application for US patents and commercially available.