Devulcanization Technologies for Recycling of Tire-Derived Rubber: A Review

Abstract

In general, composite materials are difficult to recycle. Tires belong to this class of materials. On top, one of their main constitutents, vulcanized rubber, is as elastomer, which cannot be remolten and hence is particularly challenging to put to a new use. Today, the main end-of-life routes of tires and other rubber products are landfilling, incineration in e.g., cement plants, and grinding to a fine powder, generating huge quantities and indicating a lack of sustainable recycling of this valuable material. True feedstock recycling is not feasible for complex mixtures such as tires, but devulcanization can be done to reactivate the cross-linked polymer for material recycling in novel rubber products. Devulcanization, i.e., the breaking up of sulfur bonds by chemical, thermophysical, or biological means, is a promising route that has been investigated for more than 50 years. This review article presents an update on the state-of-the art in rubber devulcanization. The article addresses established devulcanization technologies and novel processes described in the scientific and patent literatures. On the one hand, tires have become high-tech products, where the simultaneous improvement of wet traction, rolling resistance, and abrasion resistance (the so-called “magic triangle”) is hard to achieve. On the other hand, recycling and sustainable end-of-life uses are becoming more and more important. It is expected that the public discussion of environmental impacts of thermoplastics will soon spill over to thermosets and elastomers. Therefore, the industry needs to develop and market solutions proactively. Every year, approximately 40 million tons of tires are discarded. Through the devulcanization of end-of-life tires (ELT), it is possible to produce new raw materials with good mechanical properties and a superior environmental footprint over virgin products. The devulcanization process has become an interesting technology that is able to support the circular economy concept.

Recycling Waste Tires into Ground Tire Rubber (GTR)/Rubber Compounds: A Review

“…However, devulcanization aims at S-S and C-S selective bonds scission for partial breakdown of the 3D network of the vulcanized tire to achieve plasticity, while reclamation is associated with the rubber backbone (C-C bonds) degradation to decrease the MW. However, the selective rupture of the crosslinked network is not possible without damage to the backbone chain since the required energies to break the S-S and C-S bonds (227 kJ/mol and 273 kJ/mol, respectively) are very close to the energy required to break the C-C bonds (348 kJ/mol) [66,92]. In this review, both reclaimed and devulcanized processes are associated with regeneration processes to produce RTR particles.…”

“…GTR regeneration processes are classified as physical and chemical processes in which thermo-mechanical [93], microwave-assisted [21,94], ultrasonic-based [65,95,96], and thermochemical processes [97] are broadly studied. A detailed description of these methods is reviewed in numerous papers and patent applications [28,74,90,92]. In general, physical regeneration uses external energy sources to breakdown the vulcanized structure, while chemical regeneration is based on organic and inorganic reclaiming agents to react with the crosslinked rubber.…”

“…In general, physical regeneration uses external energy sources to breakdown the vulcanized structure, while chemical regeneration is based on organic and inorganic reclaiming agents to react with the crosslinked rubber. Not only sulfide and mercaptan compounds are being used in chemical regeneration processes, but also non-sulfured agents have attracted attention due to ecological and economic benefits [92]. A brief description of the main GTR regeneration processes is presented next.…”

A New Multiparameter Model for Multiaxial Fatigue Life Prediction of Rubber Materials

“…However, the second possible plane of failure is defined by that plane where the product of shear stress and strain is maximum, and the fatigue life is determined by that product plus the product of normal stress and strain in that plane. They are defined in Equations (4) and (5).…”

“…Rubber materials are widely used in the automotive and aeronautics industries due to their mechanical properties, such as wear resistance, deformation capacity, or vibration isolation [1][2][3][4]. Although they are present in components as important as tires [5][6][7], they are also used in the manufacture of other critical components, such as seals, insulating joints, and engine ducts [8][9][10]. All these types of components are generally subjected to fluctuating loads that, in many cases, compromise their durability due to the phenomenon of structural degradation known as material fatigue [11].…”