Influence of organic pigment particles on millbase flow of nitro-cellulose/alcohol-rich liquid inks
Millbase flow of nitro-cellulose/alcohol-based inks deteriorates above a critical pigment weight fraction in the ink which differs from pigment to pigment. Below this critical concentration the inks are flocculation-resistant by virtue of a stabilizing adsorbed layer of NC resin on the pigment particle surfaces. The pigment particles are large aggregates of crystals, from which additives such as abietyl resin have been shown to dissolve off into the ink medium. The aggregates are associated with liquid medium immobilized within the shear boundary near the pigment/medium interface (includes adsorbed layers and liquid within pores). This results in the effective pigment volume fraction in the ink being higher than that calculated from the weight fraction. When considered in terms of the effective volume fraction, most of the pigments have the same critical concentration of about 0.2–0.25. This indicates that deterioration in flow at higher concentrations is due in the first instance to crowding of particles, rather than to the failure of the mechanism of flocculation resistance per se. The robustness of the flocculation resistance is indicated by a low rate of increase of the strength of the structure with an increase in the effective volume fraction above the critical concentration. Robustness is important in maintaining manageable flow at high concentrations. Increasing the proportion of ethyl acetate in the solvent blend can adversely affect the millbase rheology of dispersions of some pigments in NC ink media due to induced flocculation and structure formation. The improved solvency of the blend for NC resin has been shown to reduce the amount of NC resin adsorbed on the particles of an ester-sensitive pigment. Other pigments give inks with rheology insensitive to ester content. This is attributed to more specific interaction of the NC resin with the pigment particle surfaces. The effect of temperature on flocculation resistance is consistent with these views.