Production, recovery, recycling and reuse of tires

It seems incredible but in 2019 around 3 billion tires were sold worldwide, a huge volume of raw materials used, whose final product must be recycled in compliance with the rules of the circular economy.

We are used to seeing tires in our daily lives, on cars, bikes, motorbikes and on all the other means of mobility that we encounter every day, but we must also think, in a circular economy perspective, how to give a second life to tires and how to make the product we use become sustainable .

To do this we should know a bit of product history, how it is made and what methods we know today for their disposal.

History

The history of tires can be traced back to the patent filed in London by Scotsman Robert William Thomson in 1846, well before the spread of cars, trucks, couriers and motorcycles. Those were also the years in which natural rubber entered the industrial world, ( see article ), and we tried to model it in different shapes at various temperatures, to test its consistency in hot and cold temperatures.

In the United States, in the same period, Charles GoodYear , after long studies started in 1839 that dealt with the reactions between rubber with sulfur , was able to patent in 1844, a compound through which it was possible to manage the elastic deformations of rubber under the effect of temperatures.

But the invention of the two researchers remained a dead letter until the inner tube was invented which could bear the weight and twisting of the moving vehicle inside the tire. The advantages of the “air” wheel were summarized in a lower force necessary for the movement of the vehicle, more silent operation, greater comfort and greater maneuverability.

Despite these undoubted successes, from an industrial point of view it was not followed and the air wheel was quickly forgotten.

It had to wait until 1888 when John Boyd Dunlop again patented an air tire for bicycles and the following year he applied it to the bicycle of William Hume , a mediocre cyclist, who with this revolutionary bicycle won, in Belfast, all three competitions at to which he had subscribed.

The success was such that series production of these bicycles equipped with the revolutionary tire began.

The studies at that time did not focus only on the tires, but also on the rims that were to contain them, on the compounds to thicken parts in which the rolling efforts were greater, on the problems related to the overheating of the warping threads and, finally, to protect the rotates by punctures.

In 1912 the tires changed from light colors to black , as it was discovered that the addition of smoke black to the compound increased the wear resistance of the rubber.

During these years the production of cars increased and the adoption of the air tire is attributable to the Michelin brothers , who tested it in the Paris-Brest-Paris race of 1891, who won with only 5 punctures.

Rubber vulcanization process

The heart of the tire production process lies in the principle of vulcanizing the raw material, which consists of heating the rubber with sulfur.

The vulcanization between polyisoprene and sulfur causes a modification of the molecular structure of the polymer creating an increase in the elasticity and tensile strength of the product, reducing abrasiveness and initial tackiness.

The vulcanization process is achieved through a mixture of elastomer, sulfur and other chemical additives such as accelerators, activators, reinforcers, antioxidants, inhibitors and anti-aging agents.

Tire recycling

As we saw earlier, in 2019 alone, around 3 billion tires were sold, which will wear out over a certain period of time, and then be replaced with other new items.

This has been happening every year for years, so as to generate an immense quantity of used tires that until recently ended up in landfills or in storage centers , because their recycling was complicated due to the mix of components that the product contains.

Today we have some tire recovery processes that can reduce the pressure between production and recycling. We can list three recycling processes:

Mechanical shredding

The process involves coarse shredding of tires with sizes around 70 or 100 mm. on each side, then going through the removal process of the metal parts, the granulation, with further cleaning and the final micronization process in which the product will be clean and divided into different particle sizes.

Cryogenic process

The process involves a first phase of coarse shredding of the tires with relative removal of the metal parts. Subsequently the ground is subjected to a cooling with liquid nitrogen, in order to recreate a crystalline and fragile structure that easily allows a new fine shredding. The resulting material is then treated through the pulverization process with hammer or disc mills.

Electro-thermal process

The process involves the first mechanical dimensional reduction of the product to then be inserted in vertical magnetic induction ovens. In these ovens the metal parts are detached from the rubber under the effect of a temperature of about 700 degrees. At the end of this operation, the rubber part is collected and sent for de-vulcanization which consists in returning the recovered element to a chemical form similar to the original elastomer, through thermochemical processes in autoclaves.

What are the characteristics of the recycled raw material and what are its uses?

The recycled rubber granules are used for the realization of resinous-rubbery conglomerates using them in 60/70% mix, using cold molding machines.

As for the powder , its use can be combined with doughs with virgin elastomer and used through die-casting processes or other types of hot pressing.

However, it should be noted that the recycling of tires is still very small compared to the total harvest, which increases disposal costs, leaving the problem of their post-life management open.

The main applications of granules and dust can be found in the production of draining surfaces for fields with synthetic grass, asphalts, surfaces that cushion falls in play heads, non-slip floors, acoustic insulation, accessories for urban furniture, mattresses for farms and other articles.

New studies on recycling

Currently, the ongoing studies, in order to try to increase the percentage of reuse of used tires, focus on the processes of decomposition of the chemical bonds that the elastomer, sulfur and additives create between them.

Researchers at Mc Master University have experimentally developed a system that can cut horizontal polymer bonds by breaking the mesh that chemically holds the various components, bringing them back to their primary state.

See more information on the history of recycling

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