This research deals with the study of long term behavior of elastomer housings used for the protection of the central epoxy and fiber glass composite rod of the electric insulators. Four types of industrial formulations having bases of polydimethylsiloxane (PDMS) or ethylene propylene diene (EPDM) are studied. The formulations were subjected to various types of ageings in order to simulate an exposure similar to natural degradation which the insulator experiences during its service life time. We have compared the evolution of properties of the different components of the formulations, to help us in identifying the influence of the polymer matrix and the different components on its aging. The Al(OH)3 filler was identified as the weakest component of the formulations in presence of nitric acid vapour. The formulations having a base of PDMS are not very sensitive to usual ageings (thermal and photochemical). No oxidation is detected after 10,000 hours of exposure. The formulation based on EPDM present at the same time a traditional oxidation of the dienic elastomer during photochemical and thermal ageings and the reactions of cross-linking. We have used the thermoporosimetry technique to calculate the pore size distribution of the polymer network giving us a new view about the cross-linking of the elastomers. Using this study we can rapidly characterize the period of induction before degradation and modification of the properties, an important post cross-linking is detected upon ageing for the EPDM and Siloxane formulations. We have carried out a fundamental study on uncross-linked polysiloxanes without fillers. It helps us to better understand the ageing behavior of silicones compared to the cross-linking and to study the structure property relationships. Polydimethylsiloxane do not oxidize but are degraded by cross-linking and chain scissions reactions upon thermal and photo-ageing. They are stable in presence of nitric acid vapours. However, polysiloxanes having a carbon chain in the backbone undergo oxidation, cross-linking and chain scission reactions upon thermal and photo-ageing. They are also degraded by nitric acid vapours. Lastly, we have tested new stabilized formulations in presence of nitric acid vapours and photochemical ageing.
