Written by Karina Zile Last updated on 3 June 2021
The plastic production process starts with base molecules which are turned into monomers, which are turned into polymers, which are turned into plastic pellets that are used to make plastic products. To recycle plastic waste into new plastic products, the material has to be broken down into one of these previous stages of the production process. The term physical recycling refers to plastic recycling methods where the polymers remain intact; chemical recycling refers to methods where the polymers are broken down.
Physical recycling methods include mechanical recycling (into pellets) and solvent-based recycling (into polymers). The advantages of these methods are that they generally produce high yields and are very energy efficient. One of the main disadvantages is that the additives present in the plastic are transferred to the recycled materials. Due to this accumulation of contaminants in the recycled material, physical recycling generally falls under cascading recycling (also referred to as downcycling) where the recycled material is of lower quality and functionality than the original material.
Chemical recycling methods include depolymerization (into monomers) and feedstock recycling (into base molecules). Chemical recycling plants don’t have to compete with virgin plastic producers, instead they can supply monomers or feedstock (base molecules) to them. The advantages of these methods are that re-assembled polymers are indistinguishable from virgin plastic, and that plastic waste can be converted into new plastics of another type. These methods fall under closed-loop recycling since no loss of quality occurs and there is no limit on how many times the same material can be recycled. The main disadvantages are low yields and high energy intensity.
Which is the best recycling method in each case depends on the plastic type, its contamination level and the goals set to be achieved. The costs and energy intensity of recycling increases “the further back you go” (see diagram above). If a recycling method requires a higher energy input than the original polymer had, then from an energy point of view it is best to make virgin plastic. The current recycling infrastructure for plastic waste is dominated by mechanical recycling, but it’s potential is limited to further increasing pellet quality in a financially sustainable way. Other recycling methods have a lot of potential and are expected to gain substantial market shares as complementary techniques to mechanical recycling.
Sorting plastic waste
Most plastics are incompatible with other types of plastics - in a molten state they don’t form a homogeneous mixture necessary to produce high quality recycled material. While inhomogeneous mix of incompatible plastics can be processed into thick products by direct extrusion (for example, plastic lumber), the quality of the material can be improved by at least sorting into groups of compatible plastics that form a homogeneous mix, if sorting into mono plastic streams is not possible. Methods for recycling incompatible plastics together, like mechano-chemical extrusion and using compatibilising additives, are an active area of research.
Plastic waste can be sorted using properties that differ between types of plastic: robotic sorting is based on differences in absorption of radiation, gravity separation is based on differences in density and settling velocity, electrostatic separation is based on differences in contact charging, thermoadhesive separation is based on differences in softening point, solvent extraction is based on differences in solubility, froth flotation is based on differences in wettability, etc.
Mechanical recycling (into pellets)
Mechanical recycling methods generally produce high yields and are very energy efficient. The two main limitations that generally prevent these methods from qualifying as closed-loop recycling are contaminant removal capabilities and polymer chain shortening. Due to degradation of the long polymer molecules the same material can only be recycled about 4-7 times before the material produced becomes low quality. Chain extension methods that would remove this limitation are an active area of research. The main steps of mechanical recycling are:
- Sorting waste by plastic type and shredding it into flakes.
- Washing the flakes to remove exterior contaminants. Different types of washing, in the order of increasing potential for contaminant removal, are cold wash, hot wash (contaminants are dissolved with caustic soda) and chemical wash (contaminants and pigments are dissolved with an organic liquid). Starting with uncontaminated plastic waste improves the quality of recycled material.
- Compounding - producing pellets by remelting. The inputs (washed plastic flakes 2-12mm in size and additives) are homogenised and passed through degassing and melt filtration to form pellets, the final product. Melt filtration removes non-meltable contaminants, while degassing removes other types of contaminants. Both of these processes are rather ineffective due to the high viscosity of the molten plastic.
Solvent-based recycling (into polymers)
Solvent-based recycling methods, generally produce very high yields, and better quality recycled material then mechanical recycling since both exterior and interior contaminants are removed. Contaminant removal efficiency is the highest for homogeneous low-viscosity molten plastics. The main steps of solvent-based recycling are:
- Sorting waste by plastic type and shredding it into flakes.
- Solvent-based purification (also referred to as dissolution) - plastic flakes are mixed with a solvent.
- Filtration to remove contaminants.
- Solvent removal. The solvent is then re-added to the next batch.
Depolymerization (into monomers)
The main steps of depolymerization are:
- Sorting waste by plastic type, washing and shredding it into clean, mono-type plastic flakes.
- Depolymerization to monomers, which can be a thermal or a chemical process. Depolymerization methods differ by types of reagents and catalysts added, and by process conditions, such as temperature, pressure and retention time.
- Separation and refining of monomers.
- Re-polymerization and compounding that results in additive free material that is identical to virgin material.
Pyrolysis is a thermal depolymerization process, which produces medium yields. Solvolysis is a chemical depolymerization process, which produces higher yields. Solvolysis can only be applied to types of plastic that allow depolymerization in a controlled way, such as PET, PA, PS and PMMA.
Feedstock recycling (into base molecules)
The main steps of feedstock recycling are:
- Sorting waste by plastic type, washing and shredding it.
- Decomposition. This could be cold decomposition using ultrasound, thermal or biological decomposition. Thermal decomposition methods include pyrolysis and gasification. Hydro pyrolysis, still in development, is a pyrolysis reaction in which hydrogen is introduced to remove unwanted elements, such as oxygen and nitrogen.
- Production of monomers, re-polymerization and compounding. The whole process results in low yields.
Pyrolysis and gasification
Both pyrolysis and gasification involve decomposition of plastic at 350-550℃ with exclusion of oxygen, and fractionation by distillation. The output is oil, gas and char in different ratios depending on the type of technology and the type of input plastic. The naphtha fraction of the output can be used for virgin plastic production.
Biological decomposition is a stimulated biological breakdown of plastic by enzymes produced by microbes or fungi. The output is base chemicals and biomass. The latter can be used to produce chemicals, for example lactic acid. Lactic acid can be converted to polylactic acid (PLA), a biodegradable plastic.