Stabilizing plastics - General principles

Today, polymers are mainly commodity synthetic materials used in the most diverse applications of everyday life. While the intrinsic characteristics of these polymers enable them to take on any form and be used in the most varied applications, they are also more or less sensitive to degradation during their polymerization phase, their storage, the mixing stage with additives (compounding), their shaping and/or during the service life of the processed part. This instability generally manifests itself in a loss of optical, rheological, mechanical and structural properties. Polymer stabilizers were developed early on to address this problem. Over the last few decades, researchers have continually tried to improve the intrinsic stability of polymers by minimizing the quantities of impurities present or the structural defects responsible for instability, but these efforts have brought only marginal improvement. Today, the most effective solutions are still the addition of additives in more or less complex combinations and in varying dosages. These additives offer formulators and processors great flexibility and, in some cases, increase the durability of certain polymers for several decades.

Knowledge of the nature and mechanisms of degradation is essential to understanding the choice of stabilization to be used. In this article, we deal mainly with two types of degradation and therefore of stabilization:

• thermal (and thermomechanical) degradation, mainly but not exclusively occurring during processing;

• weathering (thermal and photochemical) occurring during use of the finished part.

We are also referring to other types of degradation such as hydrolysis or chemical aggression.

All the major families of industrial polymers will be studied, with the exception of polyvinyl chloride (PVC). Given the importance of this polymer, it is the subject of a separate article on PVC stabilization [ AM3233 ].