Rubber recycling

Rubber, an ever present material in our everyday lives, from bike tyres to shoe soles, poses a significant environmental challenge throughout its life cycle. Thankfully, rubber recycling offers a promising alternative to reduce the impact of this waste.

Where does rubber come from? How is it manufactured?

Rubber, an elastic and durable material, originates from two principal sources: natural rubber and synthetic rubber.

Natural rubber mainly comes from the Latex of a tropical tree: the rubber tree found in tropical regions. A white, viscous liquid exudes from incisions made to the tree's bark:this is the Latex.
This liquid is in fact a sap rich in natural rubber, which the tree secretes to protect itself from physical damage. The incision, subsequently made, requires a great deal of skill from the rubber tapper to avoid damaging the tree.

This process is called rubber tapping, when the bark, partially cut through, is tapped. Just like we injure or cut ourselves, once the tree's bark is slit, drops of Latex very quickly exude from the wound (approximately 30 g per day) because of the very high internal pressure (10 to 15 atmospheres). This white milky liquid is collected in cups.
Latex then coagulates, transforming from a liquid state to a solid one, after an acid, such as acetic acid, is added.

In the plantations' factories, the harvested Latex is transferred into long steel vats to filter out unwanted impurities. . As a result of exposure to the air, the extracted liquid gets contaminated by other particles (detritus) that can turn it into a perishable, sticky substance due to the effects of temperature. To get natural rubber in solid form, the Latex of the natural rubber has to be coagulated.
The solid natural rubber will then be graded based on the dirt content percentage, the colour, and potentially the viscosity for CV (Constant Viscosity) grades.

Synthetic rubber, on the other hand, is derived from petrochemicals.
Simple molecules (monomers) join together to form long chains of molecules (polymers) that make up synthetic rubber.
It should be noted that synthetic elastomers are undergoing rapid development as a result of the rapidly expanding needs of the automotive industry.

To improve the resistance and elasticity of rubber, we then subject it to a vulcanisation process, which involves heating in the presence of a vulcanising agent.

Rubber waste management and used tyres, in particular, pose several environmental problems, ranging from soil and groundwater pollution to land use and biodiversity loss biodiversity, not to mention air pollution.

It is crucial to reduce these impacts by implementing sustainable management solutions, such as energy recovery (fuel derived from tyres), recovered materials (recovering metals and clothing fabric yarns) or recycling (transforming used rubber into granules to manufacture new products).
These options must not eclipse prevention: encouraging more sustainable design and manufacturing solutions.

Collection and sorting vary according to each country's regulations. Tyres currently represent the majority of rubber waste collected.
For shoes, sole disassembly remains a complicated challenge to industrialise at scale. Although this approach is technically possible, it is not widespread.It is mainly the choices made during the design phase of these products that can facilitate this process. However, what is actually happening is that sorting and recycling rely on existing waste stocks that often come from shoes designed without considering the end-of-life "disassembly" stage.

Recycling rubber helps to reduce the demand for oil, a raw material essential for manufacturing synthetic rubber. It, therefore, involves decreasing the consumption of this type of resource.
We can also observe a reduction in waste production: recycling reduces the amount of used products sent to landfill, which reduces, in turn, the amount of land used and the pollution risks associated with waste management. .
From an economic standpoint, recycling is part of circular economy dynamic, where resources are used optimally and waste minimised.

What are the main steps in the rubber recycling process?
Rubber recycling is a complicated process that involves several steps to transform the waste into new raw materials:
- Collection
- Sorting
- Shredding + micronization (finely shredded)
- According to the type of product, new sorting stage (between rubber, clothing, metal, etc, after the products are shredded)
- Devulcanization (as part of a chemical recycling process)
- Washing and drying
- Mixing and valorisation

What are the limitations of using recycled rubber?

Recycled rubber might perform less well than virgin rubber, especially resistance and elasticity. These limitations depend, in particular, on the type of recycling used:
- Mechanical recycling: a rubber shredding method to recover the material. Even though it has a lower environmental impact, it leads to a non-selective fragmentation that breaks down the fibres, reducing its mechanical performance. The elasticity and resistance to stretch are particularly affected.
- Chemical recycling: a method that helps preserve some rubber properties better but is more costly and has a higher environmental impact.

In both cases, recycled rubber remains an altered material, which does not recover the mechanical characteristics of virgin rubber, which can limit its durability and applications. Choosing between these recycling methods should be based on an all-encompassing analysis beyond carbon emissions alone, considering the product's end use and life cycle.

This process faces two significant issues: the lack of maturity of rubber recycling networks and toxicology requirements.

Lack of maturity of rubber recycling networks
Whether in France, Europe or worldwide, few organisations are effectively capable of collecting and processing end-of-life products. Despite the emergence of initiatives locally and worldwide, these remain limited and insufficient to meet the growing need for recycling.

Toxicology issues
Rubber contains molecules coming from raw materials or chemical transformations during manufacturing. These toxicology aspects must be analysed in detail because they vary according to the product's end use. A shoe, for example, in direct contact with the skin has to meet stricter standards than tyres or inner tubes, where requirements are less stringent.However, tyres – the primary source rubber to be recycled – often come from old stocks made under less stringent regulations. It can lead to discrepancies between current toxicology norms and the materials collected.
Another barrier is the regulatory framework and quality testing requirements, often aligned with the standards of the strictest countries. This mismatch between legal expectations and the characteristics of existing stocks complicates the development of viable rubber recycling networks.

DECATHLON uses natural and synthetic rubbers to manufacture its shoe soles, tyres and inner tubes on its bikes and to a lesser extent, the production of yoga mats, balls and other sports goods.

The company is now working to control the origin of its materials as much as possible and prefers rubbers that come from recycling or bio-based rubbers when no recycled alternative is available to reduce its environmental impact and increase its social value.

It is worth noting that since 2022, DECATHLON has used a bio-based rubber made from silica produced from rice husk combustion ash.A process used by several of its suppliers helps to get a raw material that is less impactful while reducing the amounts of rice waste, which traditionally generates soil pollution when not recycled.

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