Devulcanization - Thermoplastic polymers, elastomers and additives

Until now vulcanization was considered an irreversible reaction. But some companies have developed a patented solution, the "devulcanización", which allows to reverse the vulcanization process of rubber. Devulcanization is a potential method to recycle rubber from waste tires. As the name implies, in the process of devulcanization, the structure of vulcanized waste rubber is modified. Historically and in practice, in the concept of rubber recovery, devulcanization consists of the breaking of intermolecular bonds of the chemical network, such as carbon-sulfur (CS) and / or sulfur-sulfur (SS), with a greater shortening of the chains. The resulting material can be revulcanized or transformed into useful products. The devulcanized rubber is a highly valued form of residual rubber. The processes of devulcanization are varied but can be classified into two categories. On the one hand, the physical processes in which the cross-linked material undergoes a tension (mechanical stress, radiation ...) that leads to the breaking of the weaker chemical connections and, therefore, to the destruction of the cross-linking to finally be able to reuse the material. On the other hand, the chemical processes in which devulcanization is performed by the addition of chemical agents. In addition, the use of supercritical fluid (supercritical CO2) seems to favor the distribution of devulcanization agents within the rubber matrix, which improves its action. Therefore, it is possible to recover the devulcanized material as scrap and then use it at a rate of 10-20%. This allows us to incorporate similar amounts to the waste rate generated by the process without compromising the characteristics of the compound. The devulcanization in biological processes is also studied. Some fungi can degrade the sulfur that is present in the elastomer but in a too long period of time (around 200 days). The degradation can also be enzymatic through the use of lipid peroxidase. Most processes are a mixture of chemical and physical processes such as, for example, trituration with a ball combined with a phenolic antioxidant, but also the use of sulfur to destroy the sulfur bonds.

Thermomechanical solutions: these processes are not applied mainly to all types of elastomers (only NR / SBR / NBR / EPDM) or to all vulcanization systems (only sulfur) and lead to a significant increase in the temperature of the material that can affect some mechanical characteristics of the compound. Mechanochemical solutions: the cost of catalysts is affecting competitiveness, a new ingredient is added to the recipe that limits the applications and the ecological restrictions that lead to the laws that limit most of these chemicals in Western countries. So far, it is by no means to offer a solution to a significant part of the total waste rubber and, therefore, most of its potential value is lost.