Oxidation of Stressed Polymers as Studied by Chemiluminescence

Since the use of chemiluminescence (CL) is known to be a very sensitive technique for studying oxidation of polymers, its potential as a tool for studying the oxidation of stressed polymers has been thoroughly investigated. This report summarises six papers on SCL published by the authors. It can be divided into two parts. The first part deals with the construction of a specially designed SCL instrument and the origin of the luminescence emitted from some polymers during tensile testing. This luminescence was shown to have different sources reflected in the names: chemi luminescence, thermo luminescence, tribo luminescence. It could also be a superposition of two or all three phenomena. The chemiluminescence was shown to originate mainly from hydroperoxide decomposition due to stress-induced adiabatic heating, which after necking is localised in the travelling neck shoulders. Thus, this CL was not an effect of direct stress. However, some CL was also found to be due to radical formation at fracture, i.e. directly caused by stress. The thermoluminescence proved to be a result of radiation from fluorescent light tubes and was negligible if the specimens were kept in the dark prior to a measurement. The triboluminescence was found to be emitted from irradiated specimens in which stress helped to recombine separated charges. The second part of this review deals with how sub-yield stresses influence the oxidative degradation of polymers. In order to attain significant oxidation within a reasonable time, the process was accelerated by at elevated temperature. The materials studied were injection moulded parts of polyamide, polypropylene and high density polyethylene. Polyamide was also studied as film, in which there was no restricted oxygen diffusion. It was shown that stress did not change the course of oxidation in any of the materials.