Indicator inks are an important part of the continued consumption of packaged goods. This study is focused on understanding the kinetics and reaction mechanism for a unique indicator ink that shows oxidation of indigotetrasulfonate (ITS) encapsulated in the polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMA). Sodium bisulfite was used as an initial oxygen scavenger to get the ITS into its reduced form. Fluorescence spectroscopy was the primary method used to determine the kinetic decay rates and interaction between the indigotetrasulfonate dye molecules and the PDADMA. The reduced form of the ITS displayed a significant fluorescence output compared to the oxidized form, making fluorescent studies a very selective and sensitive method for characterization. Varying concentrations of PDADMA, with constant concentrations of sodium bisulfite and ITS, were studied at temperatures of 30°C, 40°C, 50°C, 60°C and 70°C in order to gain a better understanding of the rate. The fluorescence emission was seen to decrease with each successive scan in time while the rate of decay was seen to increase along with the temperature. Polymer concentrations of 4.9%, 9.8% and 14.7% ink solutions were studied. Current results indicate that two separate kinetic regimens exist for the oxidation of the dye. The initial decay occurs over a 6 minute time scale while the slower decay occurs over a 3 hour time scale. For the 4.9% PDADMA solution, activation energies were calculated to be 260 kJ for the short scans and 358 kJ for the long scans. The 9.8% polymer solution had activations energies of 453 kJ and 128 kJ for the short and long scans, respectively. The initial rates showed higher activation energies with higher concentrations of the polyelectrolyte, however the long rates showed an opposite trend. The lower activation barrier indicates that the ITS molecules were not as tightly encapsulated at higher PDADMA concentrations, which is an unexpected result. The 14.7% polymer concentration in ink is still undergoing testing. These results will be presented in March.

Studies of kinetics and energy barriers to the oxidation of indigotetrasulfonate in a unique polyelectrolyte ink system

251st National Meeting (2016)

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