Structural and Thermal Properties of Leuco-dye Based Mixed-developer Composite

Taiwo K. Fagbemigun and Jürgen Odermatt, University of Hamburg

Insufficient colour density and low shelf life of image formed are some of the challenges limiting the use of certain organic acids as developers in leuco-dye based rewritable thermal printing systems. The use of mixture of two or more developers in such system may lead to improved properties. In this study, the differences between the structural, thermal and colour properties of leuco-dye based ternary composites prepared with single developer and a mixture of developers were evaluated. Bio-based weak organic acids (ascorbic acid, ascorbyl palmitate, citric acid and salicylic acid) which produce heat-initiated colour in combination with fluoran dye, 2-Anilino-6-dibutylamino-3-methylfluoran, (ODB-2) were utilized as developers; while dodecanoic acid acts as an accelerating agent in the three-component system. In total, ten composites – four single-developer composites and six mixed-developer composites were prepared and characterized.

Thermal properties including enthalpy change, and temperatures characterizing the various phase transitions were determined by Differential Scanning Calorimetry. Chemical structure and reflection properties were analysed by FTIR and UV-VIS. Colour parameters such as – reflectance, trichromatic values and visual colour densities were all computed in CIE lab space from spectroscopic values. The results of the FTIR analyses confirm the opening of the lactone ring of the fluoran dye in all the composites. Visual colour observation shows a significant difference in colour intensities. While the absorption (%) of the mixed-developer composites were found to be higher than the single-developer system, the reflectance did not follow this same trend. Thermal data shows variation in the enthalpies of solid-liquid, liquid- solid and solid-solid transitions. This paper contributes to the advancement of studies in the search for alternatives to Bisphenol a and analogues in thermal printing paper.