A tire's multiple life-cycles
ELTs are already being collected and recycled, but researchers are opening to new and unexpected alternative solutions.
Used tires, especially if no longer useful or retreadable pose a double challenge: not only they fill huge landfills but the polymers used to make them are not easy to recycle. In the most advanced countries, ELT collection systems have long been used, as Pneurama readers are very well aware, but now also developing nations are gearing up to recover ELTs. A necessary activity seeing the environmental hazard they create: in fact, these ELTs are very resistant to sunlight, oxygen, ozone and even bacteria!
Most ELTs are recycled and used to create new materials or improve already existing ones. In 2016 about 2.5 million tons ELTs were collected in Europe alone with a 90% recycling rate; about 40% of them are burned while about 30% is shredded and reused in several other products.
Fractioning better than burning
Burning ELTs is not the best solution as this would create significant emission of pollutants, but fortunately the market offers a number of sophisticated processes to obtain new and highly flexible raw materials.
Pyrolisys represents an interesting method, a treatment that takes place at high temperatures (about 450 ° C) but in total absence of oxygen thus decomposing the ELTs without burning them. The result is: a mixture of combustible hydrocarbons (which is generally used, at least in part, to support the process), metal cables, used to reinforce the casing and ash that can be used as inert material for various applications.
Another promising treatment is solvolysis, that is, fractioning the different elements through solvents; this technique has been the subject of a recent research at the Technical University of Riga, Lithuania.
The research in question considers rubber granules obtained by crushing ELTs; the granules had 2 different average sizes: one sample was between 5 and 12 mm in diameter and the other was much smaller, with a maximum size of about 1 mm. Experiments concerned the mixing of melted rubber with bitumen to improve the latter’s characteristics.
Rubber in the oven
The process was performed in 2 different ways: in a traditional electric furnace or using, a rather unusual solution, a microwave oven.
The results were very different indeed: the traditional process required between 206 and 222 minutes (well over 3 hours) while the microwave oven reduced the required time to only 11 to 12 minutes! Energy consumption was also quite different: the traditional process required more than 3 kWh, while the experimental one used no more than 300 Wh.
The properties of the bitumen-rubber compound thus obtained, cannot be well-defined as these depend largely on the fillers and additives contained in the tires. In any case, this material can be used effectively on road surfaces or in waterproofing. The research also looked at this later aspect by looking at how effectively this compound sticks to concrete: the samples obtained through the microwave process showed superior adhesive qualities than the one obtained with the traditional method. However, despite the fact that the microwave system showed several advantages over the conventional furnace, the emission of a greater amount of smoke has been observed: not to serious, though, since the much shorter process guarantees a total amount of smoke that is considerably lower than the traditional method.
Building with rubber
Another use of ELTs is in the building sector, where rubber can be used as an inert additive to be added to cement instead of sand and / or gravel, which are quite difficult to obtain and whose extraction can often alter the ecosystem. A study published in Kuwait sums up the various experiments and concludes that adding rubber to the cement mix improves its characteristics and makes it particularly suitable for specific applications.
It turns out that rubber granules and dust obtained from ELTs could replace all other inert additives used in a cement structure: 30 mm granules can be used to replace gravel, smaller granules between 3 and 10 mm can replace sand and rubber powder, with dimensions of 1 mm or less can be used as filler.
Generally, as the percentage of rubber increases, the mechanical properties of the cement worsen to the point of dissuading builders from using it especially if severe conditions and structural stress are expected. A maximum percentage of 20% of the total amount of inert components, however, is beneficial because it reduces "shrinkage" as the cement dries, makes the structure lighter, decreases fragility and increases heat resistance and elasticity, preventing fractures and increasing the lifespan of the concrete.
Some studies also recommend adding small amounts of rubber to road pavements as this would greatly reduce the rolling noise of vehicles. Furthermore, rubber added to cement can be used in sectors such as roofing slabs, floors, car parks, driveways and prefabricated construction panels.
During these experiments a few problems have been found with the use of rubber aggregates in cement due to ELT conditions, as these can be full of grease, dirt, chemicals, dust and more. Long storage times can also degrade chemical elements by causing them react with other materials. Therefore, the need for a thorough cleaning process became clear: rubber granules would have to undergo a cleaning process with solvents and water to remove the impurities. If this precaution is not taken, the overall quality of the cement/rubber mix would be rather poor.
End-of-life tires can then “live” again in different, even unusual forms and society as a whole can "thank" these items as they turn out to be so necessary and useful even after their first “life”.