Kinetic modelling of stabilization coupled with stabilizer loss by evaporation. Case of dithioester stabilized polyethylene

The thermal oxidation of medium density polyethylene (PE) films stabilized by various weight ratios of DiLaurylThioPropionate (DLTP) or DiStearylThioPropionate (DSTP) has been studied in air at 110 and 120 °C. DSC was used for the measurement of an induction time at 190 °C (directly linked to the concentration of residual stabilizing species) and FTIR spectrophotometry for the measurement of the total concentration of ester groups originating from unreacted and reacted stabilizer molecules. The growth in PE carbonyl groups is also recorded and the induction periods at the exposure temperatures are determined. The results show that the physical loss of these hydroperoxide decomposers cannot be neglected, and also imply that simple evaporation-reaction models are inadequate to predict the kinetic behaviour of these systems. A new model is tentatively proposed. It assumes the coexistence of two stabilizer phases: the insoluble (dispersed) phase as the stabilizer excess relatively to the solubility threshold, and the soluble phase from which evaporative loss and the chemical stabilization can occur. Both phases are in equilibrium where the insoluble phase acts as a reservoir for the soluble one.