It is essential to recycle plastics to ensure a circular economy, but current recycling methods are often hampered by issues including people’s perceptions of recycling and economic and technological barriers.
Over the past few years the number of technical innovations to improve polymer recycling have substantially increased, and IDTechEx’s latest report, “Green Technology and Polymer Recycling 2020-2030: Technology for a Sustainable Circular Economy in Plastic Waste” by Dr Bryony Core presents key recycling processes that are helping to address the problems associated with waste plastic.
Polymer recycling can typically be categorised as either a physical or chemical process and further sub-divided in to the following three categories:
In a physical process, the polymer is separated from other polymers or impurities using a process that relies on a physical property such as melting point or solubility. Although the polymer may be chemically altered in the process, this is inadvertent, and not the principal route through which the polymer is purified. The established recycling method is secondary mechanical recycling, which relies upon the removal of impurities such as other polymers or non-recyclable contaminants manually or optically, before the polymer is ground down, melted and extruded into pellets.
The issues with mechanical recycling are that sorting prior to melting is imprecise and prone to impurity inclusion. Additionally, although steps have been taken to increase impurity removal with new optical sorting techniques such as near infrared, the primary route to reducing contamination relies upon the individuals producing the waste to sort it correctly into constituent materials at the point of generation. Even if the input polymer is free from contamination, the very act of heating to melt the polymer can impair the properties of the recycled output polymer, as heat can break down the polymer backbone for certain polymer types.
These issues, combined with other economic considerations, have acted as barriers to the widespread implementation of recycling; however, innovations in the field of polymer recycling are helping to address the technical hurdles to increasing recycled polymer quality.
Chemical methods of recycling includes both plastic to fuel conversion and depolymerisation. Chemical recycling relies on the transformation of the waste polymer into chemically different products via a reaction. The more mature technology, plastic to fuel conversion, uses mixed polymer waste, which is otherwise very challenging to recycle, and outputs fuel fractions using chemical reactions such as pyrolysis or gasification.
Depolymerisation, a relatively new method, is the reverse of how the polymer is created in the first place: The polymer is thermally or catalytically broken down, either into raw materials that can be used directly to resynthesise the polymer, or into other useful chemical intermediates.
Depolymerisation is particularly exciting as it offers a route to access the polymer without any of the downsides experienced in other recycling methods: The resultant polymer has identical properties and it is reasonably tolerant to contaminants.
Developing new processes to overcome the current barriers associated with polymer recycling will help to ensure a circular economy.