End-of-life of silicone polymers

Silicones, polymers structured around the Si–O siloxane bond, are widely used in domestic consumer goods, but also on an industrial scale. Depending on their degree of polymerization and the chemical groups grafted onto the siloxane backbone, they can be found as fluids (in cosmetics and detergents, for example), as elastomers (used as seals in a wide range of applications), or as resins. Around 4 million tonnes of silicones are produced worldwide every year.

Depending on their use and physico-chemical properties, these materials are directed towards different waste treatment channels and have a specific impact on environmental compartments (soil, atmosphere, wastewater, etc.). This article reviews the current state of knowledge concerning the end-of-life of these polymers, from their disposal routes to the environmental impacts generated, via the mechanisms of their degradation.

The vast majority of scientific data available on the subject is supplied directly by silicone manufacturers or industry groups (such as CES, the European Silicones Centre). The environmental fate of these materials was extensively studied in the 1990s and into the early 2000s. Indeed, in the wake of the market's rapid expansion, manufacturers communicated widely to reassure consumers that degradation products were non-toxic to them and to the environment. Since then, numerous ecotoxicity tests have been carried out for regulatory purposes, but very little end-of-life data has been published. Today, the study of silicones in the environment has focused mainly on the analysis and treatment of VOCSi (Volatile Organic Silicon Compounds) in biogas.

After a description of silicone materials, the link between physico-chemical properties, applications and distribution in waste treatment channels is established. Following a description of the processes (biotic and abiotic) by which silicones degrade in the environment, particular attention is paid to their fate according to the environmental compartments in which they are found.