Torque measurement as a tool to monitor the breakdown of cassava starch gels, by the effect of Fenton's initiator for graft copolymerization

Abstract

The use of a combined stirrer-torque meter allows for monitoring viscosity during polymerizations and other reactions, much faster and more continuous than off-line viscosity measurements. This method was applied in previous research about the grafting of acrylic acid onto cassava starch using Fenton’s initiator. In this article the method is used to assess the effect of Fenton's initiator on gels of cassava starch in the absence of the monomer. The intention here is to activate the starch selectively first, which could improve graft selectivity. The interaction of a redox graft copolymerization initiator with starch gels when there is no monomer in the system yet, is hardly ever addressed in grafting literature. Remarkably, the present experimental study shows a rapid viscosity decrease of the starch gel, in the order of 70% in less than 2 min, at conditions reflecting graft copolymerization. This result must be considered a major setback for the application of pre-initiation. Torque measurement also allows to identify the important steps in the reaction of Fenton's reagent with starch without the need to do other, more intensive analyses on the chemistry. For example, it could be concluded that the second stage of Fenton's reaction, the slower decomposition of hydrogen peroxide by Fe3+ ions, accounts for another 20% loss of the original gel viscosity but over a longer period, some 30–60 min. Possible further decrease over a longer period is too slow or small relative to the measuring accuracy of the torque meter. FTIR analyses show the occurrence of peaks at 1730–1740 cm−1 in starch which has been subjected to reaction with Fenton's. These peaks are in the range of the vibrational frequencies associated to CO bonds, that are not present in the original starch. This provides at least a strong indication for oxidative degradation of the starch chains.