Recent advances in the devulcanization technologies of industrially relevant sulfur-vulcanized elastomers

The interesting thermomechanical properties of rubber materials have favored their widespread application in many fields, with a consequent increase in the amount of worn synthetic and natural rubber discarded every year. However, the complex three-dimensional molecular structure of rubber complicates the recycling and degradation process, and the current rubber waste management measures (i.e., landfilling, incineration, rubber grinding) are found to be unsustainable or substantially inadequate. On the other hand, devulcanization technologies, thanks to the selective cleavage of crosslinks, represent a sustainable and feasible approach to obtain a material that can be reintroduced into the rubber value chain or reused in novel eco-sustainable thermoplastic or elastomeric blends. Hence, this review provides an overview of the current rubber waste management techniques and devulcanization technologies, highlighting the underlying devulcanization mechanisms, describing the pros and cons of each method, and presenting some literature examples. Since most of the research on devulcanization has been made on waste tires, this review mainly focuses on the most widely used rubber classes for this application, i.e., natural rubber (NR) and styrene-butadiene rubber (SBR), and the most common vulcanization technique, i.e., sulfur vulcanization. Considering the importance of the application of a circular economy approach, this work also reviews the applications of rubber devulcanizates, focusing on how devulcanized rubber can be compounded with different polymeric matrices to develop eco-sustainable polymer blends with suitable physical properties.