Chemical aging of polymers - Degradation kinetics

Chemical ageing is any ageing process involving changes in the chemical structure of macromolecules. In a neutral environment, macromolecules can react as a result of their own instability (generally at temperatures above ambient) or under the effect of UV or ionizing radiation.

With rare exceptions, "anaerobic" aging processes are radical, and therefore likely to play a role in initiating oxidation processes. As soon as the polymer is exposed to oxygen, the latter tends to react rapidly with the radicals, giving a different direction to the aging process and making it highly self-accelerating. Of course, the relative importance of the anaerobic and oxidative aging processes depends on the oxygen concentration, which in turn depends on the oxygen partial pressure in the atmosphere and the distance between the volume element in question and the sample surface, with oxidation possibly being controlled by oxygen diffusion in the polymer.

Polymers are capable of reacting with all kinds of oxidizing (e.g. chlorinated disinfectants) and non-oxidizing substances. The most common reactive species, apart from oxygen, is water. However, while virtually all industrial polymers, with the exception of polytetrafluoroethylene, are susceptible to oxidation, only a few chemical families, essentially polyesters and polyamides, are reactive towards water. These are ionic processes, possibly catalyzed by bases or acids. Like oxidation processes, they may be kinetically governed by water diffusion within the material.

Since oxidation is the most important aging process, this dossier is structured in two parts.

The first is devoted to "anaerobic" processes, among which we distinguish :

• hydrolysis;

• depolymerization ;

• sequential elimination of small molecules;

• radiochemical processes ;

• photochemical processes.

The second part, devoted to oxidation, includes seven paragraphs dedicated respectively to :

• to the general oxidation mechanism;

• kinetic aspects;

• to radio-oxidative ageing ;

• photo-oxidative ageing ;

• to cooxidation;

• to oxidation heterogeneity;

• the effects of oxidation on physical properties ;

• to lifespan prediction.