Thermal Degradation of Starch in Production of Environmentally Benign Flocculants, 2. Experiments and Process Identification

Experiments were carried out to produce biodegradable flocculants by phophorylation and heat treatment of corn starch at elevated temperatures in the dry state. The temporal change of molecular weight distribution was measured by HPSEC/MALLS/RI. To elucidate the mechanism and kinetics of degradation during this treatment a Markov-chain stochastic model was used. For this, five molecular weight intervals were defined between >32 10 6 and 2 10 6 Da with exponentially decreasing spans. The probabilities of mass fraction transitions among these intervals at different processing temperatures were determined by fitting the calculated molecular weight distributions to the experimental data. Examining the transition probabilities between various MW intervals, it was concluded that degradation took place gradually by multiple splits of molecules, resulting in stepwise transition of fragments from higher intervals to lower ones. From the values of the transition probabilities between different molecular weight intervals, and considering the evolution of molecular weight distributions during the process, it was concluded that an end-scission mechanism did not play a role in the studied system under the applied conditions. The degradation probability of starch molecules diminished with decreasing molecular weight and increased with the temperature of treatment. These dependences were described by an exponential relationship.