A spreading model for the oxidation of polypropylene

A model for the heterogeneous oxidation of polypropylene (PP) is proposed in which it is considered that there is a small initial fraction, p(o), of oxidizing centres which have a high local rate of oxidation. Within these zones there is a free radical chain reaction producing secondary oxidation products, volatiles and chemiluminescence (CL) from peroxy radical termination reactions. These zones progressively spread (rate coefficient b/s(-1)) and the free radical reactions die away within the volume of the original zones, producing a measurable concentration of oxidation products (rate coefficient alpha/s(-1)). Analysis of the CL-time curve as representing the instantaneous infectious, fraction, p(i), in the spreading model enables the parameters p(o), b and alpha to be determined and profiles of the remaining fraction (p(r)) and dead or oxidized fraction (p(d)) constructed. Analysis of CL curves from 120 degrees C to 150 degrees C gives an activation energy for spreading in PP particles of 96kJ/mol. Both single particles and groups of particles of different types of PP have been examined and evidence is presented of rapid surface spreading of oxidation from particle to particle.