Sustainability of self-healing polymers: A holistic perspective towards circularity in polymer networks

Permanent polymer networks present an important sustainability challenge. Irreversible covalent crosslinks impart these materials excellent mechanical properties, thermal and chemical resistance, yet also render them difficult to repair and to recycle. Self-healing mechanisms can extend the lifetime of thermosets and elastomers, improving their durability and making their lifecycle more sustainable. In addition to the lifetime extension, this paper reviews the sustainability of self-healing polymers from a holistic point of view. The entire lifecycle of self-healing polymers is critically assessed with reference to the green chemistry principles and sustainable development. The relation between the self-healing chemistries and the sustainability aspects of each of the phases of the lifecycle are discussed, starting from the feedstocks, monomer functionalisation and polymer synthesis, to processing and manufacturing as well as end-of-life considerations, i.e. recycling or (bio)degradation. The review provides a toolbox for the development of more sustainable thermosets, elastomers and their composites. It is of utmost importance to consider the entire lifecycle of self-healing materials, derived products and – by extension – any material or product. The self-healing ability and often related recyclability should primarily reduce the amount of new materials that are necessary to fulfill societal needs, by extending the lifetime of products and maximizing reprocessing into new products. Increasing healing efficiency and the number of healing cycles improves the overall environmental impact relative to the extended service lifetime. Renewable resources derived from biomass, recycling processes or waste streams should be the first choice to create new self-healing polymers. Finally, biodegradability can be considered as a complementary end-of-life scenario upon accidental loss of self-healing polymer to the environment, provided that the biodegradation does not start under the prospected use conditions of the self-healing polymers and products, but can be postponed until contact with stimuli present in the environment.