THE MECHANISM OF ACTION OF HINDERED AMINE STABILIZERS (HAS) AS LONG-TERM HEAT STABILIZERS

Hindered amine stabilizers (HAS) are promoted as long-term heat stabilizers. A difficulty for this application is that at low temperatures (below 100-degrees-C) the effectiveness of HAS is more pronounced than at high temperatures (150-degrees-C). In order to measure the effectiveness of HAS, prolonged experiments are necessary. Another difficulty is that for HAS-containing polypropylene (PP) the mechanical properties drop gradually, while for phenolic antioxidants a fast decline of the mechanical properties is observed after a long period of no change. These two difficulties are due to different mechanisms of action for phenolic antioxidants and HAS and to a change of the degradation mechanism of PP with temperature. At high temperatures the degradation of PP is mainly due to the oxidation of the polymer itself, while at low temperatures the oxidation of oxidation products is more important. This secondary oxidation results in peracids, via aldehydes, which determines the oxidation Tate at low temperatures. Model experiments show that HAS are capable of preventing this aldehyde oxidation, thereby giving good performance at low temperatures. These experiments also show that HAS are not able to prevent the oxidation of hydrocarbons, which takes place at high temperatures. The slow decline of the mechanical properties of HAS-containing PP is due to the oxidation of the polymer itself, which is not stopped by the HAS.