CIWM Early Careers Ambassador Dr. Bryan Ng, founder and CEO of Repolywise, a start-up company transforming waste plastics to new plastics, demystifies the process of chemical recycling for plastics.
Only 9% of the plastic that has ever been produced has been recycled. The most frequently seen approach to recycle plastics is mechanical recycling, which shreds plastic waste into small pieces before re-melting (extruding) them into new products – but it has a few limitations.
Firstly, the waste stream needs to be thoroughly sorted. Secondly, due to unavoidable contamination during the sorting step, this can reduce the quality of recycled plastics, and cannot always be used for food contact.
The value proposition of chemical recycling to the waste management industry is to transform plastics into chemical feedstock as fossil fuel substitutes. This is done using more energy than for mechanical recycling, with the upside of virgin-grade recycled plastics coming when they reach end-markets.
What is the chemical recycling process?
Plastics produced from fossil fuels are called virgin-grade plastics, the cleanest and highest grade possible. In the production process, fossil fuels will be cracked (Step 1) into monomers. These monomers are small molecules such as ethylene, propylene, or ethanediol. Through polymerization, the monomers will be transformed into plastics (Step 2).
After consumption (Step 3) they become waste plastics, which we throw into our “mixed recycling” bin every day. Some portion of the mixed recycling stream will then be sorted and sent for mechanical recycling (Step 4).
The -lysis suffix from various chemical recycling technologies originates from Greek, meaning “loosening, dissolving”. When applied to the context of chemical recycling it indicates what has driven the degradation of plastics.
Pyrolysis is the most well-known example of chemical recycling. It is a process using heat to break the strong carbon-carbon bonds dominant in most of mainstream plastics (HDPE, LDPE, PP). This is performed normally in the absence of oxygen to avoid combustion.
The major product of such processes is an oily substance (pyrolysis oil), which is intended to be a substitute for naphtha. Commonly the pyrolysis oil are mixed with fossil-derived naphtha to be cracked into monomers for new plastics production.
Solvolysis, hydrolysis, enzymolysis are processes involving solvent, water and enzymes respectively. They targeting carbon-nitrogen or carbon-oxygen bonds (amide or ester linkage) present in polyester (e.g. PET) and polyamides (e.g. Kevlar). Common output of these processes are monomers directly useful for the production of new plastics.
Hydrogenolysis uses hydrogen molecules to break the carbon-carbon bonds present in mainstream plastics (HDPE, LDPE, PP).
It is also the technology I am working on – an “atomic scissors” to selectively degrade common plastics into propane, which can be used to build virgin-grade PP. It is a process of reduced energy costs and improved efficiency.
Chemical recycling technologies are designed to be complimentary to mechanical recycling, as it addresses some of the limitations of mechanical recycling.
It would take some time for the technologies to upscale and commercialise, before a fully circular economy can be created for plastics where no plastics need to be landfilled or incinerated.
