Devulcanization aims at selectively cleavage the intermolecular sulfidic such as carbon- sulfur (C-S) and/or sulfur-sulfur (S-S) bonds, breaking down the tridimensional network without involving main chain scissions and degradation of the polymer.

Mechanical/thermo-mechanical devulcanization: Thermo-mechanical devulcanization is the most widely used in industry. Rubber crumb is subjected to mechanical shearing and high temperature, approximatively 200 C, brought by internal friction and from outside sources. Solvents like water, oils, hexane, or supercritical fluids can be added before or during grinding to facilitate the process, solubilize the small chains, and swollen the rubber.

Thermo-mechanical devulcanization has been largely studied during the last decades because it generally yields high devulcanization throughputs. The most common mechanical and thermomechanical devulcanization technologies can be divided into batch mixers or continuous extruders, but other equipment has also been tried.

Batch mixers: Devulcanization based on batch mixers is relatively simple, low-cost, and environmentally friendly, generally performed without external heating or chemicals. Batch mixers divide into (i) open, two-roll mills or Brabender mixers, and (ii) close, Lancaster- Banbury or internal batch mixers. Crumb rubber is devulcanized through intense mechanical shearing for minutes, in which the temperature can reach 250 C. To prevent excessive heating and degradation of the polymer chains, a technique has recently been proposed that involves a water-cooled two-roll mixing mill, to devulcanize CB-filled NR. Still, the resultant degree of devulcanization is quite low, between 20 and 37.8%.

Extruders: Common mechanical devulcanization is Ficker's or a single/twin-screw extruder method, widely used as extruders are commonly and readily available in rubber manufacturing and allow for continuous high-yield devulcanization. The fundamental importance of optimizing screw speed and barrel temperature for proper devulcanization is highlighted in many research works correlating processing parameters with devulcanization quality and yield and the response surface methodology (RSM). It has generally been observed that the selectivity of crosslink scission decreases as temperature increases, with a consequent increase in the C-C bond cleavage and a reduction in the mechanical performance of the revulcanizate. So the quality of the devulcanization increases at moderate operating conditions.

Other approaches: Researchers devulcanized ground rubber from tires in a unique metallic- cone-like high shear mixer. In this setup, cooled by a stream of cold water, one cone is static while the other rotates and pressurizes the system. The instrument is fed with large chunks of GTR, for size reduction and decrease in crosslink density. However, such reactions mainly occur at the rubber particle surface, classifying this technique as a rubber surface activation phenomenon rather than a proper bulk devulcanization.

Another devulcanization technology is the High-Pressure High Temperature Sintering (HPHTS), which allows to recycle vulcanized rubber powder by applying heat and pressure. The applied pressure compresses the particles and increases the inter-particle contact, while temperature (between 80 and 240 C) promotes cleavage of crosslink bonds. This in turn allows the formation of new covalent bonds at the particle interface, thereby sintering the particles into a single piece. Researchers found that the mechanical properties of the sintered rubber compare with those of conventionally manufactured rubbers.

Thermo-mechanical devulcanization can be implemented with different devulcanizing agents, to further promote scission of sulfidic bonds. In these cases, the process is called mechano-chemical devulcanization.