Why rubber waste is so difficult to recover

• An industrial rubber part is usually single-use and becomes waste once defective or worn, according to our partner The Conversation.

• But recycling them is complicated by the fact that the rubber they are made of has gone from a deformable state with no mechanical strength to a hard state that is impossible to melt.

• This analysis was conducted by Marie-Pierre Deffarges, mechanical research engineer, Nicolas Berton, teacher-researcher in chemistry, and Stéphane Méo, professor at the University of Mechanics.

Sometimes without even realizing it, we are surrounded by rubber!

Natural and synthetic, we find it in our homes, cars, buildings, planes, hospitals and many other places, due to its very specific properties.

Nowadays, tires and industrial rubber are the two main sectors consuming and transforming rubber raw materials.

An industrial rubber part is usually single-use.

Defective or worn, it becomes waste.

In France, in 2020, approximately 508 kilotons of tires are collected and then processed.

In addition, the health crisis and the war in Ukraine have caused problems with supplies and raw materials, with the consequent increase in prices and lead times.

These events intensify the need for rubber recycling.

With such omnipresence, the question about the circular economy quickly arises.

What are our means of action to facilitate the recovery of all our waste containing rubber?

Currently, waste management must comply with binding regulations.

“Material” recovery and “energy” recovery are two processing methods applied to rubber waste.

It is then possible to use, for example, the waste after treatment as a raw material intended for a new manufacture or the energy generated following its degradation.

​Why is rubber so essential?

Waterproof, elastic, highly deformable (sometimes up to 600%), shock-absorbing or rebounding, the properties of each rubber are dependent on its chemical nature and the various ingredients that compose it.

They compete to best adapt to the specifications of the product to be manufactured.

For the same application, several rubbers can be chosen and therefore the performance will be significantly different.

For example, a silicone pacifier, compared to a natural rubber pacifier, is tasteless, has a longer lifespan but is more fragile to baby teething.

Natural rubber has been known for centuries: the Maya people already used it.

Its raw material is of plant origin: latex, a milky product extracted from the rubber tree.

It is harvested by tapping directly on the bark of this tree.

The first form of synthetic rubber appeared at the end of the 19th century.

It is mainly obtained from hydrocarbon derivatives or belongs to the family of silicones.

Rubber consumption in 2020 is distributed between 47% for natural rubber against 53% for synthetic rubber in the world.

Very complex, rubber is a mixture of gums (pure rubber(s), vulcanizing agents, “fillers” such as carbon black and other ingredients depending on the desired properties.

​Difficult to recover used rubber... but not impossible!

The major obstacle to its recycling lies in the irreversible nature of its manufacture.

It is ready for use after having undergone a chemical process, which changes it from an "unvulcanized" or "raw" state, i.e. soft, deformable and without mechanical strength, to a " vulcanized” or “cooked”, that is to say hard and impossible to melt.

Vulcanization is described by one or more crosslinking reactions.

Rubber is a polymer described schematically by long macromolecular chains held together by the existence of secondary bonds.

The vulcanization reaction is triggered in the presence of vulcanizing or cross-linking agent(s).

These agents consist of molecules which react on certain sites of the polymer chain to create chemical bridges which are strong covalent bonds.

The agent therefore makes it possible to link the macromolecular chains together, which makes the material hard and resistant.

Historically, the first vulcanizing agent is sulfur and the term, in English (

) comes from the word

, the sulfur-spitting god.

Since then, the term "vulcanization" has been generalized to the crosslinking of all rubbers.

The amount of chemical bridges (or crosslinking nodes) created is higher for “thermosettings” than for elastomers.

A rubber stretches up to a certain limit given by these nodes and has the ability to return to its initial shape by elastic recovery.

These three-dimensional networks are the characteristic chemical structures of rubbers and provide the material with cohesion on a macroscopic scale.

​Rubber regeneration is growing

One solution to enable the recycling of rubber consists of incorporating the recovered materials into new formulas: the recycled product is reduced to crumbs and can be used directly as a raw material.

“Regeneration” is a process integrated into “material recovery”.

In the presence of sulfur, the objective is to break the carbon-sulfur, sulfur-sulfur and/or carbon-carbon chemical bonds by chemical, thermal and/or mechanical means.

The rubber is found in a regenerated state (breaks in carbon-carbon bonds) and/or devulcanized (breaks in sulfur-carbon and sulfur-sulfur bonds).

When the carbon-carbon bonds are broken, the macromolecular chain is cut, which causes losses of mechanical properties on a macroscopic scale.

If the targeted bonds are the carbon-sulfur and sulfur-sulfur bonds, the rubber partially regains its ability to "vulcanize".

Our "RUBBER" file

Our research teams in partnership with an industrialist have studied the behavior of regenerated rubbers and the modifications they can cause when they are incorporated into a new rubber mixture.

Our first results recently obtained and unpublished to date are promising and confirm that recycled materials have properties that make them usable by choosing the appropriate rate of regenerated material to incorporate depending on the industrial application.

Re-treated, our old rubbers have a bright future!

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This analysis was written by Marie-Pierre Deffarges, mechanical research engineer, Nicolas Berton, teacher-researcher in chemistry, and Stéphane Méo, professor at the University of Mechanics (all three at the University of Tours).

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Declaration of interests

• Planet

• Video

• The Conversation

• Recycling

• Rubber

• Waste

• Plastic

• Carbon