Basics on Degradation
Adhesives and their raw materials react like other organic materials with oxygen
in a process called "autoxidation".
Autoxidation is initiated by heat, light (high energy radiation), mechanical stress,
catalyst residues, or reaction with impurities. Free radicals are formed and react
in the presence of oxygen to form peroxy radicals, which further react with organic
material leading to hydroperoxides (ROOH).
As these reactions ultimately lead to a change in chemical composition and such
polymer properties as molecular weight, they have immediate impacts on properties
that determine the service life of an adhesive. Some results of adhesive degradation
are discoloration, viscosity changes, char formation, cracking and loss of adhesion.
The important aspect of this scheme is that once oxidation starts, it sets off a
chain reaction that accelerates degradation, unless stabilizers are used to interrupt
the oxidation cycle. Under standard aerobic conditions, polymer degradation generally
follows two predominant mechanisms. (See oxidation mechanism:
cross-linking and chain scission)
Antioxidants interrupt the degradation process in different ways, according to their
structure. The different mechanisms and families of antioxidants are described in