It is time to refocus on recycling tyres

There are more vehicles in the world, and therefore there are more tires. Each year, hundreds of millions are discarded. The disposal of tires is a global issue.

Stockpiling used tires can pose serious risks, including uncontrollable Fires. Illegal dumping is another problem, as it leads to pollution and encourages pests and insect growth. Burning tyres releases harmful compounds.

The main focus has been on recycling tires, but this is not enough for the huge amount of waste tires that are generated every year. Retreading is an alternative, but it has major limitations. There are more eco-friendly recycling options being explored.

The composition of tires makes it difficult to find recycling methods that are effective. The tyres are composed of rubber, steel, carbon black and additives. They are hard to separate and break down, making reprocessing a challenge. This is why new technologies are needed.

The valorization of waste products (on an industrial scale) to produce useful products or energy is one avenue that has been explored. Rubber, carbon black, or steel, for example, can be recovered and used for other purposes. Energy can also be extracted from combustibles with a high calorific content.

Recycling can only go so far.

Recycling materials can take many forms. Civil engineering applications include road construction, manufacturing of shoe soles and swings, as well as shoring up embankments.

Sandals made from waste tires. Supplied

In Ethiopia and Indonesia, indigenous people produce shoes out of waste tires that would otherwise be disposed of. These enterprises create jobs in communities that are disadvantaged and reduce the amount of waste tires in landfills.

Alternatives such as incineration, retreading, and reclaiming rubber (by grinding and using chemical treatments to obtain rubber from old tires) have significant limitations. Incineration, for example, involves combustion, which releases harmful emissions. Retreading can reuse almost 90% of a tire, but the regulations and safety concerns for passenger vehicle tires make it an unprofitable market. The reclaimed rubber has its limitations. Its altered properties make it unsuitable to be used in tires.

The most promising ways to move forward

There are other eco-friendly methods to recycle used tires. The most important are:

The process of devulcanisation involves the breaking of the bonds that hold individual polymer chains and creating a network. Breaking the bonds can be done in a variety of ways, such as using microwaves or chemicals or biologically and ultrasonically.

Diagram of the devulcanisation procedure. Author: Kathryn Allan

Devulcanisation is the process of converting rubber waste into smaller pieces that can then be processed, mixed, and vulcanized. In addition to rubber, depending on the chosen system, oils, solvents, and sulfur can also be recovered.

The processes are causing a lot of problems and are researching to find solutions. It is important to find ways to reduce the chemicals used.

Grinding aims to reduce the size of waste tires into a fine crumb. The powdery particles can be mixed and used in plastics. This process allows the separation of steel and other fabrics. The steel and other materials can be recycled.

Grinding has a downside: the high temperatures required to grind the product can lead to a lower-quality end product. Temperature control can help to manage this. This can only be achieved by using large amounts of liquid nitrogen, which increases the cost.