Plastics are an essential part of our daily lives. One consequence of this is that about 30 million tons of plastic waste is collected in Europe every year. Roughly 85% of this is incinerated, exported or sent to landfill. Not only is this a waste of our precious resources, but it also contributes greatly to global carbon emissions. Therefore, it should come as no surprise that there is a huge demand for methods that allow for the reuse of plastic material. Traditionally, this is done with mechanical recycling methods. However, not every plastic material can be mechanically recycled. This is where the new chemical recycling sector comes in.
What is chemical recycling?
Chemical recycling is the umbrella term for “several emerging technologies in the waste treatment industry that allow for the recycling of plastics that are too difficult or uneconomical to recycle mechanically. Thus, chemical recycling is complementary to mechanical recycling in the sense that it allows for the further extraction of value from polymers that have exhausted their economic potential for mechanical processing. Whereas difficult-to-recycle plastic products were burned or landfilled in the past, chemical recycling serves as an alternative to reuse the plastics and even extract new-quality raw materials from them. These can then be reintroduced into the plastics supply chain.
Figure 1: The circle of recycling
In a more scientific way of writing, we state that: “Chemical recycling describes any technology that uses processes or chemical agents that directly affect the chemistry of polymers. There are three categories, based on the place in the plastics supply chain (see Figure 1). The categories correspond to the colored arrows. These are:
- Purification/Solvation
- Depolymerization
- Recycling of raw materials (thermal conversion)
Mechanical recycling is also shown in the figure. However, this does not drastically change the chemical structure of the material, but uses mechanical processing to prepare the waste polymers for reuse.
Solvation
Solvation is a process that uses solvents in which the plastic waste is dissolved. A series of purification steps are then performed to remove additives and contaminants from the polymer. When the selected polymer is fully dissolved, it can be crystallized. Therefore, it is important to choose a solvent that is selective for the intended polymer, or for the additives. The “end product” of this process is a purified polymer. The technique is especially suitable for PVC, PS, PE and PP.
Depolymerization
As the name implies, depolymerization is the opposite of polymerization. In this process, polymers are broken down into monomer or oligomer molecules (shorter polymer fragments). These monomers are exactly the same as the new monomers used to make the polymers. This means they can easily be reintroduced into the manufacturing process. The disadvantage of this technology is that it can only be applied to polycondensates, such as polyester (PET), polyamides (PA) and polyurethanes. However, most of the plastic waste stream consists of “additional” polymers, such as PP, PE and PVC.
Recycling into chemical building blocks
This technique consists of various thermal technologies that break down polymers into simpler molecules, which can serve as feedstock for the petrochemical industry. The outputs of this method of recycling are basic chemicals, such as hydrocarbons or syngas. These chemicals can then be further processed to make a polymer. There are three main processes: pyrolysis, gasification and hydrothermal treatment.
Pyrolysis
In this process, the polymers are broken down into simple hydrocarbons. This is done by heating the plastic in the absence of oxygen, also known as “thermal cracking. Using a distillation process, the hydrocarbon vapor can be separated into heavier and lighter fractions. These pyrolysis products can be processed in the same way as oil to make new polymers. Alternatively, after a few processing steps, it can be used as a fuel.
Gasification
This process involves heating mixed plastic waste material to very high temperatures, around 1,000 to 1,500 °C. This heating is done in the presence of a limited amount of oxygen, which causes the molecules to 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 fuels and fertilizers.
Additional 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 state. To put water in a near-critical state, it is heated to a temperature of 160 to 240 degrees Celsius and pressurized to keep it in a liquid state. This state of water makes it a good medium for dissolving organic compounds, such as plastic waste. HTT has been proposed as a solution for separating mixed waste into organic and inorganic compounds.
Delamination
Plastic packaging sometimes consists of several polymer layers, which we call “laminated. One can easily notice that such packaging is difficult to recycle because of the sealed layers that are not easily accessible to recycling techniques. However, several new processes are being experimented with to recycle this type of packaging as well. Although mechanical recycling techniques are also being proposed, chemical recycling also plays a major role here. The process by which polymer layers are separated from each other is called delamination and is promising given that many plastic packages are layered.
The future perspective
Over the next decade, the amount of plastic waste will nearly double, reaching 460 million tons by 2030. Currently, only 12% of plastic waste is recycled, but that could rise to 50% by 2030. This growth requires significant expansion of plastic waste collection infrastructure and more efficient sorting systems and mechanical recycling, complemented by chemical recycling infrastructure. Thus, chemical recycling could have a share in the growth of recycling. This is due to the ability of chemical recycling to handle polymer waste streams that mechanical recyclers cannot currently handle. Mixed polymer waste streams that cannot be mechanically recycled can be treated through recycling into chemical building blocks. Chemical recycling creates value in previously non-recyclable plastic waste by breaking down the plastics into petrochemical feedstocks, after which they can be reused as building blocks for new polymers. It bridges the gap between waste management companies and the petrochemical industry, creating a circular plastics value chain. Despite the promising future of chemical recycling, CHEM Trust is still concerned about the new technologies, as there is not much transparency about the hazardous/toxic byproducts that the chemical reactions can produce. To find out, further research needs to be conducted on these byproducts and also determine how to prevent their release into 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