Overview and challenges
Plastics are widely used in a number of applications: packaging, vehicles, household appliances, building, agriculture, etc. As a result, plastic waste can be found in all post-consumption sectors.
Plastics are generally made up of:
- different polymers (polyolefins, polyesters, polyurethanes, etc.),
- which contain additives: addition of dyes, pigments, stabilizers, plastifiers, etc.).
Given the plethora of sectors, plastics recycling remains a complex task. Nevertheless, in view of the quantity of plastic used and their considerable growth potential, the stakes are high.
Mechanical plastics recycling
Today, most recycling is carried out using mechanical recycling processes. These integrate regeneration operations (sorting/grading, washing and/or grinding, and/or extrusion) to be able to recover the polymer(s) present in the items processed.
Depending on the case, the recycled plastic material (RPM) is sometimes considered to be “degraded” compared to the equivalent virgin polymers in terms of intrinsic properties (purity, mechanical properties, etc.).
These modifications are the result of:
- the processes involved in the production of plastic objects (e.g. incorporation of additives)
- and the regeneration process itself (e.g. modification of the molar mass of the recyclate).
In such conditions, so-called closed-loop recycling is rare:
- closed loop means that the recycled polymer is used for the production of the same type of object as the one it is derived from
- a high proportion of the recycled polymer is often redirected to other recycling loops, for the production of lower value-added products: this is the so-called open loop.
The “household packaging” sector, initiated in the 1990s, is a good example of what can be achieved in terms of mechanical recycling, with, in particular, the roll-out of collection and sorting infrastructures adapted to specific plastics. This sector concerns products containing polyethylene, polypropylene and PET (PolyEthylene Terephthalate). The latter is a plastic used in the manufacture of bottles for mineral water and sparkling drinks as well as food containers.
|Bales packaged in sorting centers|
|Washing, grinding and sorting to produce raw materials for recycling such as high-purity flakes|
|Extrusion of the flakes to produce granules|
|The granules are then reused in plastics processing or fiber production, combined with virgin raw materials, primarily of fossil origin|
|These are technically straightforward operations|
|They can be conducted on a small scale: extruders generally process between 500 and 10,000 t/year|
|The sectors are operational and well developed|
Constraints to be overcome
Sorting: to adequately control the mechanical properties of the recycled product, plastic flows have to be meticulously sorted with a very high degree of material purity
Color: if the recycled plastic is colored, it is impossible to remove the color. Just as it is impossible to remove other additives incorporated in virgin polymers prior to the production of objects: pigments, stabilizers, plastifiers, etc. The RPM is often gray
Decontamination: it depends on the nature of the plastics, which sometimes contain undesirable ingredients (exogenous substances related to their use or plastic degradation, such as acetaldehyde for PET). Also with respect to PET, processes have been developed to decontaminate it. However, it is very difficult to purify some other plastics, such as polyolefins, meaning they cannot be returned to food-grade products.
Aging: mechanical recycling exposes the plastic to high temperatures (often in excess of 200°C), which accelerates aging: indefinite mechanical recycling of plastics is not feasible.
Chemical recycling has a role to play to convert used plastics beyond mechanical recycling.
Contribution of chemistry to plastics recycling
As with mechanical recycling, chemical recycling requires a number of preparation steps (sorting or grading/washing/grinding, etc.) prior to treatment.
Different chemical treatments
A plastic is often made up of a mixture of polymers and additives. The polymer comprises very long molecular chains, themselves made up of identical basic building blocks: monomers. Depending on the nature of these plastics, chemical recycling can take a variety of forms:
- dissolution: this makes it possible to extract the polymer chains from the other components of the plastics. A solvent is used to recover the polymer, or to purify it. This method is employed for PVC (Poly Vinyl Chloride) and polyolefins,
- depolymerization: the aim is to convert the polymers into monomers, which can then be purified. This method is primarily employed for PET and concerns less than 5% of PET currently recycled,
- conversion: this involves the thermal or thermocatalytic cracking of plastics. This method does not lead to the production of monomers: it leads to hydrocarbon cuts containing very broad mixtures of molecules, useful for fuel production.
Chemical recycling is thus employed to break down the plastic and recover the polymers or its fragments, purified or otherwise to remove additives, depending on needs and purification options.
Technically speaking, these processes are well known, but a number of major challenges need to be overcome before they can be made sustainable on an industrial scale. They require more complex implementation than mechanical recycling and economic models remain to be found in a context that is complicated by the evolving and heterogeneous conditions present from one country to another.
Challenges to overcome
The future of chemical recycling will depend on the capacity of the sector’s players to overcome the following challenges:
- Optimizing processes as a function of available feeds: the materials to be processed are evolving (in packaging, appearance of opaque PET and containers in recycled plastics reserves, extension of sorting requirements, etc.) and it is necessary to focus on:
- the quality of the reserves to be processed,
- the costs of the resource,
- available quantities,
- the sectors in place.
- Adapting processes to target products: products resulting from recycling must meet market specifications, hence it is essential to take into account:
- the value of the product as a function of its quality,
- and the cost of ensuring the product meets specifications, associated with its value.
- Recycling in economically sustainable conditions: as a function of the cost of feeds and conversion costs, and in the current absence of incentive mechanisms, the cost of the product must be competitive relative to that of a product derived from virgin materials,
- Reducing production costs by focusing on large-scale recycling operations, providing the resource is available in sufficient quantities.
To develop chemical recycling, it is necessary to:
- precisely identify the need and position of the technological solution in the value chain,
- conduct tests on dedicated units,
- invest to support R&D and demonstration pilot units.
A bigger production capacity makes it possible to reduce the costs of converting plastics reserves into oil cuts: if the value of the end product is limited, a large production scale reduces the cost of chemical recycling. Unlike mechanical recycling, production units with capacities above 10 or 20 kt/year must be the aim in low added-value markets. However, given the scattered nature of reserves and bearing in mind transport costs, units with a capacity in excess of 40-50 kt/year will probably struggle to find supplies. The size of facilities in line with economic profitability is a real challenge for the development of the chemical recycling of plastics.
It takes between 5 and 10 years to develop a process prior to the launch of the first industrial unit. A unit costs roughly €1 million per 1,000 tons of plastic converted. More than 25 Mt of plastic waste is currently produced annually in Europe.
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