Long term thermo-oxidative degradation and stabilization of polypropylene (PP) and the implications for its recyclability

Degradation and stabilization of pristine polypropylene (PP) have been extensively studied especially from the 1980s until the first years of the 21st century. As a result of the increasing importance of the recycling of PP, its degradation and stabilization became again relevant. Differently from virgin grades, post-consumer PP presents distinct chemical moieties formed due to degradation, affecting its thermo-oxidative stability. To support studies on the mechanical recycling of PP, this review presents an outline of the chemical reactions leading to PP's degradation and the mechanisms of action of the most relevant stabilizers. Emphasis is given to the heterogeneous degradation of PP and its infectious spreading, in addition to the influence of stabilizers on this phenomenon, as these characteristics are of major importance for the proper stabilization of PP during its first lifetime cycle, which allows its recyclability.