A spreading model for the oxidation of polypropylene

Abstract

Abstraa:A model for the heterogeneous oxidation of polypropylene (PP) is proposed in which it is considered that there is a small initial fraction, po, 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-') and the free radical reactions die away within the volume of the original zones, producing a measurable concentration of oxidation products (rate coefficient a h 1 ) . Analysis of the CL-time curve as representing the instantaneous infectious, fraction, pi, in the spreading model enables the parameters p,, b and OL to be determined and profiles of the remaining fraction @, ) and dead or oxidized fraction (pd) constructed. Analysis of CL curves from 1200C to 150°C gives an activation energy for spreading in PP particles of 96kTlmol. 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.One of the contradictions in the study of the oxidation of polyolefins has been the success in applying a homogeneous free radical kinetic description of oxidation (typical of liquid hydrocarbons) to a solid polymer which, while above its glass transition temperature, is still well below the melting point. The kinetic analysis is usually based on oxygen uptake data or product accumulation curves measured spectroscopically (e.g. by infrared carbonyl index), which show features of an autoaccelerating chain reaction with an induction period, an exponential increase to a steady oxidation rate, followed by a decrease in rate at high extents of oxidation (ref 1). Models of various levels of complexity have been used to extract rate coefficients which may then be used to predict oxidation rates at different temperatures (ref 2 ).Investigations of the structure and morphology of solid polyolefins during oxidation have revealed that this apparently 'homogeneous' macromolecular oxidation has resulted in 0

Review on the thermo-oxidative degradation of polymers during processing and in service

“…Although the phenomenon of gas phase spreading already is known for a long time, the species causing this spreading is still under debate. George et al [62,63] showed that the normally observed overall oxidation is the sum of a lot of heterogeneous oxidations on a micro scale. To explain their results they used an "infectious spreading" model and concluded that the S-shape of the oxygen uptake curves found for PP is due to an increased amount of oxidizing polymer rather than to an increase in the oxidation rate [62,63].…”

“…George et al [62,63] showed that the normally observed overall oxidation is the sum of a lot of heterogeneous oxidations on a micro scale. To explain their results they used an "infectious spreading" model and concluded that the S-shape of the oxygen uptake curves found for PP is due to an increased amount of oxidizing polymer rather than to an increase in the oxidation rate [62,63]. This model was supported by the results of Gijsman and Hamskog [52] shown in Fig.…”

“…4. George et al [62,63] also showed that the changes on a macro scale are probably due to the sum of changes on a micro scale and might also take place in the induction period. Thus, what is observed macroscopically after the induction period might also take place on a micro scale within this period.…”

The classical kinetic model for radical chain oxidation of hydrocarbon substrates initiated by bimolecular hydroperoxide decomposition

“…Other kinetic schemes, as for instance those proposed to model heterogeneous oxidation [5], are not derived from a mechanistic scheme. This article deals with kinetic schemes derived from mechanistic schemes that involve the following two (often implicit) hypotheses.…”

Use of 9,10-diphenylanthracene as a contrast agent in chemiluminescence imaging: The observation of spreading of oxidative degradation in thin polypropylene films

“…Due to light scattering, the spatial resolution of CL imaging is in the order of 10 mm. Since the domain size has been theorised to be in the submicron region [11,19], the probability of observing heterogeneous oxidation is low, due to the large thickness of the films, because the number of domains that the detector can observe increases with increasing thickness. Although heterogeneous oxidation may be occurring this may be obscured for thicker films, because the probability of observing an unoxidised region from the top down is reduced with increasing film thickness.…”