Plastics are an essential part of our daily lives. Because of this, about 30 million tonnes of plastic waste is collected every year in Europe. Roughly 85% of this is incinerated, exported or sent to a landfill. This is not only a waste of our precious resources, but it also contributes heavily to our global CO2 emissions. It may not come as a surprise that there is a huge demand for methods that provide the possibility for material reuse. Traditionally, this is achieved using mechanical recycling methods. However, not every plastic material can be recycled mechanically. This is where the new chemical recycling sector comes in.
What is chemical recycling?
Chemical recycling is the overarching term for ‘several emerging technologies in the waste management industry which allow plastics to be recycled, which are too difficult or uneconomic to recycle mechanically‘. Thus, chemical recycling is complementary to mechanical recycling in the sense that it enables further extraction of value from polymers that have exhausted their economic potential for mechanical processing. Whereas hard-to-recycle plastic products where incinerated or landfilled in the past, chemical recycling serves as an alternative for reusing the plastics and even extract virgin-quality raw materials from them. These can then be fed back into the plastics supply chain.
Figure 1: The recycling circle.
In a more scientific way of writing, we state that: ‘Chemical recycling describes any technology that utilises processes or chemical agents that directly affect the chemistry of the polymers’. There are three categories, based on the position in the plastics supply chain (see figure 1).
The categories correspond to the coloured arrows. These are:
- Purification/Dissolution
- Depolymerisation
- Feedstock (thermal conversion) recycling
Mechanical recycling is also shown in the figure. However, it does not significantly change the chemical structure of the material, but uses mechanical processing to prepare the waste polymers for reuse.
Dissolution
Dissolution is a process involving solvents in which the plastic waste is dissolved. After this, a series of purification steps are performed in order to remove additives and contaminants from the polymer. When fully dissolved, the target polymer can be selectively crystallized. Therefore it is important to select a solvent that is selective for either the target polymer or the additives. The ‘end product’ from this process is a purified plastic polymer. It is especially suitable for PVC, PS, PE and PP.
Depolymerisation
As the name suggests, depolymerisation is the opposite of polymerisation. With this process the polymers are degraded into monomer- or oligomer (shorter polymer fragments) molecules. These monomers are exactly the same as the virgin monomers that are used to create the polymers. This means that they can easily be fed back into the manufacturing process. The disadvantage of this technology is that it can only be applied to polycondensates, such polyester (PET), polyamides (PA) and polyurethanes. The majority of the plastic waste stream, however, is made up of ‘addition’ polymers, like PP, PE and PVC.
Feedstock Recycling
This technique consists out of different thermal process technologies that break down polymers into simpler molecules, which can be the feedstock for the petrochemical industry. The outputs of feedstock recycling are basic chemicals, like hydrocarbons or syngas. These chemicals can then be processed further to create a polymer. There are three main processes: pyrolysis, gasification and hydrothermal treatment.
Pyrolysis
In this process, the polymers are broken down into basic hydrocarbons. This is achieved by heating the plastic in the absence of oxygen, also referred to as ‘thermal cracking’. By using a distillation process, the hydrocarbon vapour can be divided into heavier and lighter fractions. These pyrolysis products can be processed in the same way as oil, in order to build new polymers. Alternatively, it can also be used as a fuel after some more processing steps.
Gasification
This process consists of heating mixed plastic waste materials to very high temperatures, around 1000 to 1500°C. This heating is done in the presence of a limited amount of oxygen, which ensures that the molecules break down into their simplest components, and produces a syngas. Syngas is a mixture of hydrogen, carbon monoxide and carbon dioxide. This gas in turn can be used to produce new chemicals, plastics or even fuel and fertilisers.
Supplementary processes
Hydrothermal treatment (HTT)
Hydrolysis is the chemical reaction in which a compound breaks down in the presence of water molecules in a near-critical condition. For water to be in a near-critical state, it is heated to a temperature of 160 to 240 degrees Celsius and pressurized in order to keep it in the liquid state. This state of the water makes it a good medium for dissolving organic compounds, like plastic waste. HTT has been proposed as a solution for the separation of mixed waste into organic and inorganic substances.
Delamination
Plastic packaging sometimes consists of different polymer layers, which we refer to as being ‘laminated’. One may easily notice that such packages are hard to recycle, because of the sealed off layers which are not easily accessed by recycling techniques. However, different new processes are being experimented with in order to recycle these types of packages as well. Although some mechanical recycling techniques have been proposed for this, chemical recycling plays a big role here as well. The process of separating polymer layers is called delamination and is quite promising considering the fact that a lot of plastic packaging is layered.
The future perspective
Within the next decade, the amount of plastic waste is set to almost double, reaching 460 million tonnes by 2030. Right now, only 12% of the plastic waste is recycled, but this could be scaled up to 50% in 2030. This level of growth requires a significant expansion of plastic waste collection infrastructure and more efficient sorting systems and mechanical recycling, complemented by the chemical recycling infrastructure.
Chemical recycling is thus proposed to have the potential for carrying a share in the growth of recycling. This is due to the ability of chemical recycling to treat polymer waste-streams that mechanical recyclers can currently not process. Mixed-polymer waste streams that do not have the potential to be mechanically recycled can be treated by feedstock recycling. Chemical recycling creates value in previously unrecyclable plastic waste, by breaking down the plastics into petrochemical feedstock, whereafter they can be reused as building blocks for new polymers. It creates a bridge between waste management companies and petrochemical industries, thereby creating a circular value chain for plastics.
Despite the promising future of chemical recycling, CHEM Trust still has its concern about the new technologies, since not much transparency is given about the hazardous/toxic by-products the chemical reactions may produce. In order to know this, further research into these side-products needs to be performed and it also needs to be determined how they can be prevented from entering the environment.
Sources:
https://www.bpf.co.uk/plastipedia/chemical-recycling-101.aspx
https://chemtrust.org/chemical-recycling/
https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/cssc.202002877