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Innovations in the recycling of rubber waste

07.03.2017

Each year, 7 million Mg of waste tires is discarded as a hazardous waste, which disposal is one of the current challenges of recycling. Innovative and prospective method of rubber waste disposal is thermo-mechanical regeneration of rubber, which can be carried out in twin-screw extruders.

The development of waste rubber recycling is accelerated not only by law (since 2003 waste rubber storage is prohibited, and EU guidelines impose the size of recycling the „Cleaning act” of 7th February 2003). Recovery of rubber waste is also motivated by economics and ecology, as its devulcanization gives material of good quality and wide applications.

Regeneration of rubber waste – devulcanization

Regeneration of rubber waste is a process of converting the rubber by thermal, mechanical or chemical energy. During this process, the crosslinked structure of rubber is disrupted by tearing of chemical bounds. This allows for processing, shaping and vulcanization of regenerated rubber. There are different methods of rubber waste regeneration, varying with the source of energy.

Heating of the vulcanized rubber leads to disrupt of chemical bounds between atoms in chains of crosslinked structure. This leads to breaking of both cross-linking bounds with sulphur (S-S and C-S bounds) as well as bounds in the main chain of polymer. The disruption of C-C bounds in main chain is negative phenomenon and leads to degradation of the rubber, and in consequence to deterioration of the material. The disruption of bounds with sulfur is the aim of pursued vulcanization, because it leads to disconnection of crosslinked chains and restoration of plastic properties of material.

 

 

Picture 1 Scheme of bounds present in the cross-linked rubber. Marked with red the sulfur bridges are broken in the process of devulcanization.

The traditional thermal regeneration thus leads to a recycled material with significantly inferior properties of raw rubber, because it results in shorter main chains of polymer. Regarding to this problem thermo-mechanical regeneration of waste rubber in twin-screw extruders has been developed.

Innovative solutions

Use of twin-screw extruders in thermo-mechanical regeneration of waste tire rubber is quite new method of rubber recycling. The technology was developed by Toyota R&D Division.

In thermo-mechanical method, the chemical reactions, that leads to disruption of chemical bounds in polymer chain can be controlled by parameters of extruder. To achieve satisfactory quality of regenerate one should find correct settings of extruder, especially temperature, shear forces and internal pressure.

In technological process of tire rubber waste regeneration following steps can be distinguished:

  • Separation of metal and textile cord, cutting of rubber
  • 1-2mm pellets achieved by use of granulator
  • Thermo-mechanical regeneration on twin-screw extruder (for laboratory scale EHP 2x 20 ZAMAK MERCATOR can be used)

Twin-screw extruder

In thermo-mechanical regeneration one should use twin-screw extruders of solid construction and special modular design counter-rotating screws, providing optimal shear forces in plasticizing zone. The screw is build of transporting and kneading segments. The kneading segments, consisting of cam disks, allowing to change the intensity of grinding and distributive mixing. Numerous studies documented by scientific publications, show that in the laboratory scale process of thermo-mechanical recycling of rubber waste the twin-screw extruder EHP 2x20 ZAMAK MERCATOR with special design of screws works very well. [1] [2]

The regeneration process is continuous, and extruder can be divided into pulverising and devulcanizing zones. In pulverizing zone feeded pellets of 2 mm diameter are being fragmented under shear forces and are heated to temperature of devulcanization. The material goes through the following part of extruder in time that ensures efficient devulcanization. In this zone polymer chains are stretched on kneading elements which enhances plasticisation.

Picture 2 Scheme of the thermo-mechanical regeneration process.

Applications of regenerated rubber

Thanks to outstanding quality of the rubber regenerated via thermo-mechanical process, it can be used as a filler in new tires lowering the cost of manufacture (even with 10% of additive, new rubber maintain a standard strength). By the given method also thermoplastic elastomers can be produced, with an adapted extruder.

Regenerated rubber show similar properties to those of native rubber, so it can be used as substitute for a more expensive raw material.

Literature

[1] Krzysztof Formela, Investigating the combined impact of plasticizer and shear force on the efficiency of low temperature reclaiming of ground tire rubber (GTR), Polymer Degradation and Stability, 125 (2016), 1-11

[2] Michał Gągol, Investigation of volatile low molecular weight compounds formed during continuous reclaiming of ground tire rubber, Polymer Degradation and Stability, 119 (2015), 113-120

[3] Kenzo Fukumori, Material recycling technology of crosslinked rubber waste, R&D Review of Toyota CRDL, Vol.38 No.1

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