Converting the old into a virgin plastic
To understand the principle of chemical recycling, it is important to recall that all plastics are composed of one or more basic molecules, called monomers, which repeat to form a long chain: a polymer. In the majority of cases, the monomer(s) is/are obtained from gases or oils from oil refining operations.
There are currently three main families of technologies used to recycle plastics other than that of mechanical recycling. Although they are all different, these three types of technologies have a common objective: re-using end-of-life plastics in one or another of the main stages of their initial manufacturing process; that is to say, reprocessing them either into raw materials for the petrochemicals industry, basic monomers or purified polymers. This is kind of a return to the source for plastics. In this way, chemical recycling makes it possible to manufacture polymers with properties equivalent to those of virgin resins.
It is also the achievement of a paradigm shift. These technologies aim to overcome the limitations of mechanical recycling; not to replace it, but rather to complement it. In this way, it would be possible, in the not-too-distant future, for most plastic waste to enter a recycling chain. That waste will then constitute a precious resource that will have to be collected and sorted in the same way as other materials that are already the stars of the recycling industry.
Chemical recycling is not intended to replace mechanical recycling, which proves its effectiveness every day. Chemical recycling becomes interesting in the case of polymers that are more complex to recycle. It should therefore be seen as a complement to mechanical recycling.
This technology could also put an end to issues surrounding the presence of certain substances, especially those known as "legacy" substances, additives that were authorised in the past but are no longer authorised (or may no longer be authorised in the future), which may limit the applications for recycled plastics. This advantage specific to chemical recycling should open the way to a circular economy for all plastics used in areas as sensitive as food, medical applications and toys, which are all subject to the most stringent health regulations.
In terms of resources, using both chemical and mechanical recycling should reduce the polymer production industry’s reliance on fossil fuels, even though it is generally accepted that plastics only account for 4% of oil consumption. It is also a key way to meet emerging regulatory requirements for the integration of recycled content in consumer products.
Mechanical recycling is well suited and already widely used for many end-of-life plastic products. They are sorted, shredded, washed and re-melted into granules or flakes, which are then transformed into new products. This relatively simple technology, which has not yet reached its full potential, works very well for objects made from a single polymer, such as a polyethylene canister, or for those where it is possible to easily separate the different components, such as used window and door frames. However, it is not as effective for items where several polymers are closely bonded, or combined with other materials, and are difficult to separate. This is the case for some multi-layer food packaging or residues from sorting or shredding certain plastic waste. This is where the three chemical recycling technologies come into their own.
The rule of three
Currently, three innovative technologies are available to complement mechanical recycling.
Conversion: from a polymer to its hydrocarbon raw material
Most polymers are derived from hydrocarbon raw materials (gases or oils). The objective of conversion is to return them to their raw material state.
Conversion is a process that enables plastic waste to be transformed into hydrocarbons. Using heat and little or no oxygen, plastic waste is "cracked" to obtain either oil (pyrolysis) or gas (gasification). After purification, the oil and gas can be supplied to refineries or cracking plants whose products are then used to make new plastics, among other things.
This technique is increasingly being used. It is suited to almost all types of waste, especially that made up of several polymers or materials, such as certain multi-layer food packaging. It is also very effective in ridding polymers of all their impurities.
Depolymerisation: from a polymer to a monomer
This operation consists of breaking the chemical bonds of the polymer in order to obtain the monomer, either using chemical agents (solvents, water, alcohols) or heat.
As the name suggests, a polymer is a repeating chain of monomers. The aim of depolymerisation is to break this chain down in order to obtain the base monomer.
Dissolution: from polymer to purified polymer
Dissolution makes it possible to dissolve the polymer without altering its chemical structure. The aim is to remove all impurities.
As the name suggests, dissolution is a process in which polymers are dissolved most often in a solvent bath. This technology does not alter the polymer’s chemical structure, but allows it to be separated from certain additives, impurities and other materials. This recycling principle is based on the solvency of the polymers which are the sole component to be dissolved, while the other elements remain in a solid state. As in the case of mechanical recycling, the polymer thus recovered is used to manufacture new objects without having to be polymerised again beforehand.
All these innovative technologies help to reduce the environmental footprint of plastics. They are often still at the industrial pilot stage and require major investments. As a result, they still need to be optimised and reach a larger scale of production to demonstrate their full potential in terms of technical and economic performance.