What is the Devulcanization Process for Tire Recycling

If we think that every car, motorcycle, truck, bus or any other wheeled vehicle uses tires for an average period of 1-2 years, and then replaces them with new tires, we can begin to understand how many used tires there can be in the world.

If we then take a quick account of how many millions of vehicles on wheels circulate on earth, we can easily multiply the number of vehicles times the average number of tires fitted, resulting in an astonishing number.

This astonishing number every 1-2 years expresses the waste, in the form of used tires, with which we have to deal, waste that if not treated correctly and put back into circulation, they dangerously burden the environment.

If the ELTs (exhausted tires) are collected and managed correctly, they can become a resource because they contain rubber, steel and textile fibers which, through a process of recycling can generate secondary raw materials.

In particular, the rubber powders and granules obtained from the volumetric reduction of tires, if subjected to mechanical, chemical, thermal or ultrasonic irradiated, undergo a process defined devulcanization, with variable results depending on the starting material and the technology used, as reported by a recent study by the mechanical and structural engineering department of the University of Brescia.

It allows to regain a second raw material by breaking the bonds created between the polymer chains during the vulcanization phase.

This process of the rubber constituting the tires is not only a potential method of recycling them, but represents, at present, the most promising approach to solve the difficulties related to the problem of environmental impact caused by the huge quantities of tires at the end of their life.

Through the process called devulcanization, the rubber is brought back to a chemical structure close to that of the starting elastomer; this allows it to be added to normal compounds.

Devulcanization is generally carried out in an autoclave using thermochemical processes using the joint action of temperature, pressure and chemical additives.

The composition of the recycled tires is very similar to that of the virgin material of origin.

In the form of granulate or powder, it can become part of the compounds used by the industry for numerous applications.

The concept of "circular economy" currently assumes a predominant value as the substances of which the products are made will be increasingly treated as a resource equal to raw materials and no longer just disposed of.

The perspective is therefore aimed at enhancing the activities aimed at the reuse of end-of-life tires (ELTs).

DEVULCANIZATION

Devulcanization is the process through which we try to separate the chemical bonds between rubber and sulfur, created thanks to vulcanization, and responsible for the elastic properties and mechanical resistance which make rubber a very popular material.

Devulcanization involves the use of chemical, thermal and mechanical processes that are highly polluting, as they could release toxic gases into the environment; in addition, they require large energy consumption.

Due to the use of chemical additives or high temperatures, there is a high risk that the polymer chains that make up the rubber itself break down, which would be denatured losing all its chemical and physical characteristics.

In particular, the different modes currently used for this process are listed below:

Chemistry: a quantity of chemical reagents at specific high temperatures and pressures is added to the rubber powder. Once the process is finished, the residues are rinsed, filtered and dried to eliminate any unwanted chemical impurities.

Different devulcanizing agents can be used and, depending on the type chosen and the characteristics of the raw material used, different substances will be obtained leaving the reactor.

For example, by using disulfides in the process it could be possible to obtain the formation of hydrogen sulphide (H2S), methyl or other thiols (organic compounds similar to alcohols in which the oxygen atom has been replaced by a sulfur atom, thus having the general formula R-SH: the functional group SH is called both as a thiol group and as a sulfanyl group).

Since tire manufacturing uses zinc oxide and zinc carbonate, chemical devulcanization could also produce metal particles suspended in the air; therefore before release into the atmosphere it is necessary to provide for a specific treatment.

Ultrasounds: in this method the residues are loaded at the head of a hopper and then introduced into an extruder which heats and softens the rubber through a mechanical action .

Subsequently the material is subjected to the action of ultrasonic waves with an exposure of a few seconds. The synergistic activity of ultrasonic energy, heat, pressure and mechanical action contributes to the devulcanization of the rubber.

The temperature reached in this process is about 110 ° C so it will generate a lower vapor emission and since no chemical additives are used to break the bonds of the sulfur formed in vulcanization, no dangerous emissions into the atmosphere will occur either. However, the gaseous residues will still be treated with activated carbon filters.

Microwave: this methodology uses controlled microwave energy to devulcanize the elastomers containing sulfur.

The material to be subjected to this process must be sufficiently polar to be able to accept energy at such a speed as to be able to generate the heat necessary to devulcanize the rubber.

Biological: certain microorganisms are used to attack the sulfur bonds formed during the vulcanization of natural rubber.

The biological contact time required for this process varies between 10 and a few hundred days.

These microorganisms, being equipped with desulphurizing metabolic pathways, are able to selectively break the sulfur bridges present in the vulcanized rubber, without affecting the polymer chain.

The microorganisms used, in fact, are equipped with metabolic pathways, in which specific enzymes selectively intervene to catalyze the breakdown of carbon-sulfur and sulfur-sulfur bonds of the rubber, without affecting the constituent polymers of the elastomer itself, which therefore is not destroyed.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.

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