INFLUENCE OF SOLVENT HYSTERESIS IN THERMOCHROMIC OFFSET PRINTS ON THE THERMOCHROMISM EFFECT

"The colorimetric values of prints of three thermochromic offset inks on four different paper substrates – bulky, synthetic, filter (neat cellulose) and recycled in one heating-cooling cycle – were determined. Prints that are cooled are lighter than those that are heated at the same temperature. This phenomenon is a consequence of the hysteresis of the solvent inside or outside the microcapsules. The solidification temperature of the solvent is lower than the melting point of the solvent. Due to solvent hysteresis, the concentration of solvent in the solid aggregate state at the same temperature is lower when the prints are cooled than when they are heated. Therefore, the liquid solvent prevents the interaction of dyes and developers and the formation of their colored complex, i.e. color development, and the prints are lighter, i.e. less colored. The colorimetric cooling curves are therefore shifted to the left relative to the heating colorimetric curves. As regards certain paper substrates, for example, when the printing paper is very compressible, as in the case of bulky paper, the deformations of many microcapsules are so great that they become completely inactive in terms of color development."