Rubbers are elastomers, thermosets, characterized by large deformability and the capability to recover their shapes upon removals of applied loads. Industrial rubbers divide into natural rubber (NR) and synthetic rubbers. The former derives from natural rubber latex, mainly constituted by poly(cis-1,4-isoprene), an unsaturated hydrocarbon, produced by over 2,000 plant species of the Compositae, Moraceae, and Apocynaceae families, but the commercially utilized latex is made almost entirely from Hevea brasiliensis. Synthetic rubbers are synthesized monomers from petroleum-based hydrocarbons, and the most commonly produced are ethylene propylene diene monomer and styrene-butadiene rubber, chloroprene, and isobutylene (EPDM, SBR, CR, IS).

The demand for rubber products has grown steadily over the last years. Global NR production in 2018 amounted to almost 13.9 million metric tons (mt), the vast majority of which (91%) was produced by the Asia Pacific region, while Europe, the Middle East, and Africa (EMEA) contributed approximately 6.5%. Natural rubber is largely used to manufacture tires and also, single-use gloves, the production of which has tremendously grown to meet the rising healthcare demands linked to the recent pandemic. Synthetic rubbers are used primarily in the transportation industry to produce tires, with an overall demand reaching the 3.9 billion units last year. This conspicuous and ever-increasing rubber production demonstrates the versatility and the widespread application of this class of materials, but it also poses the challenge of how to manage the growing amount of rubber waste.

Rubber scrap (from post-industrial and post-consumer sources) is a sizeable component of the total solid waste around the world. The majority of such scrap derives from the transportation industry (tires of automobiles, trucks, off-road vehicles, and motorcycles), but other sources of rubber waste include clothing, footwear, gaskets, and furniture. In 2020, the United States generated 9.7 million tons of rubber waste, out of which 5.5 mt were landfilled (55%), 2.9 mt were combusted for energy recovery (27%), and only 2.1 mt were recycled (17%). Most of such applications glue crumb rubber (or GTR: Ground Tire Rubber) with epoxy of polyurethane (PU).

Recycling rubber waste is challenging, and efforts in improving related policies and technologies continue to evolve to increase sustainability and the circularity of the Rubber Industry.

As most rubber is used in tires, all of the research on devulcanization targeted waste tires, made of NR and SBR and the most common sulfur vulcanization [4, 5]. The following provides a concise technical overview on the most important devulcanization technologies on sulfur-vulcanized rubbers such as NR and SBR. Considering the importance of adopting a circular economy, rubber devulcanizate applications concentrate on how devulcanized rubber can be compounded with different polymeric matrices to develop eco-sustainable polymer blends with suitable physical properties.

Any procedure in which vulcanized rubber waste is transformed to be mixed with virgin rubber, processed, and vulcanized again, is called reclaiming or reclamation. Reclaiming has a high potential to recycle rubber waste products, as it allows to break the starting permanent tridimensional network and decrease the molecular weight sufficiently to achieve plasticity and reprocessing of the scrap rubber. However, reclaiming mostly leads to degradation, with cleavage of carbon-carbon (C-C) bonds on the rubber backbone making it impossible to reobtain the initial macromolecules. So devulcanization is much more desirable, to selectively cleavage the intermolecular sulfidic such as carbon-sulfur (C-S) and/or sulfur-sulfur (S-S) bonds, brealing down the tridimensional network without involving main chain scissions and degradation of the polymer.