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Explaining chemical recycling processes

Chemical recycling is an umbrella term that includes a variety of technologies, each with their own process characteristics, input requirements and outputs.

Few in the plastics industry will not have heard of chemical recycling but that simple term covers a

huge range of quite different technologies. Today’s chemical recycling technologies can be classified into three broad concepts: dissolution, depolymeri-

sation, and thermal cracking.

These three approaches differ, at a conceptual level at least, in the type of materials they can handle, the amount of “chemistry” involved, and the product that results.

Dissolution technologies use carefully selected solvents to dissolve the polymer from the mixed

waste, allowing insoluble contaminants such as fillers and pigments to be filtered out. The dis-

solved polymer can then be precipitated and recovered from the solvent, which is reused. This is

a physical process — the chemical composition and structure of the polymer is unchanged.

As a result, many of its proponents consider it to be closer to mechanical than chemical recycling and promote it accordingly, using terms such as solvent-based purification or physical or material recycling. The key to success in dissolution is the selection

of a solvent that recovers only the target polymer. This means it is best suited for use with relatively

homogenous waste streams. A number of pilot projects are already well advanced — Purecycle

Technologies in the US, for instance, is targeting polypropylene with a technology licensed from

P&G while Canada’s Polystyvert is focusing its efforts on polystyrene.

The need for a relatively homogenous waste stream does not necessarily mean that dissolution

technologies are suitable only for mono-material plastic waste. Germany’s APK, for example, is

developing its technology to recover LDPE and PA from multi-layer films.

In theory, at least, dissolution exposes the polymer to less thermal and physical stress during

the recovery process than conventional mechanical recycling. However, the recovered polymer is likely to require compounding or pelletising to make it suitable for further use, which may mitigate that benefit to some extent. In addition, the cost of the numerous processing steps — pre-treatment,

dissolution, filtration, precipitation, solvent removal and reformulation — is likely to make dissolution

most attractive for processing of mono-material waste streams with a relatively high level of

contaminants that would be difficult to remove mechanically otherwise.

Depolymerisation is certainly a chemical recycling process, typically using heat (and often a

catalyst) to convert a polymer back to its building block monomers — for this reason it is sometimes

referred to as monomer recovery. It is most suitable for use with step-growth polymers such as PET,

which are polymerised by polycondensation.

A number of companies are developing various processes to depolymerise PET, with pilot projects

underway at Carbios in France, CuRe Technology and Ioniqa in the Netherlands, Rittec in Germany, and BP Infinia, Eastman and Loop Industries in

North America.

Depolymerisation of polycondensation polymers typically involves reintroducing the molecular

component that was eliminated during the original polymerisation process. Several solvolytic processes are being investigated to do this, including hydrolysis, glycolysis, methanolysis and transesterification. These are all multi-step processes that include pre-treatment of the waste, followed by depolymerisation, monomer recovery, repolymerisation, and finally extrusion and pelletising.

Solvolytic depolymerisation techniques are not suitable for use with polymers produced by

chain-growth or polyaddition reactions, such as PE, PP and PS.

However, some companies — including Pyrowave in Canada and Agilyx in the US — are

working with alternative thermal depolymerisation technologies that are capable of converting PS

polymer back to styrene monomer.