Chemical recycling is a collective name for various processes that each reintroduce the plastic at a different point in the chain: polymers are broken down into monomers or molecules that form the basis for e.g. new plastics. The various chemical recycling processes each have their own requirements with regard to the input stream of plastic (packaging materials) and result in different outputs. Three main techniques, each of which encompasses various processes, are depolymerisation, pyrolysis and gasification. These three techniques offer a solution for the deteriorating quality of the polymer chains after each cycle of mechanical recycling. A fourth technique is solvolysis, which makes it possible to e.g. separate polymers from other materials. This is a form of physical recycling because the polymers are kept intact.
In legislation, a distinction is made between the chemical recycling of plastic (packaging materials) where the output of the process is reused for the production of products, materials or substances with the original or a different purpose and the chemical processing of plastic (packaging materials) where the output is used as fuel, filler material or for energy recovery. Legally speaking, the latter is not a form of recycling.
Considerations for chemical recycling
- Chemical recycling makes it possible to separate various types of plastic in a single product or packaging or separate plastic from other materials or resources.
- The outputs of chemical recycling processes can often be used for the production of new food-grade plastic packaging materials. This is important for major manufacturers with ambitions regarding the use of recycled plastics in their packaging materials.
- Chemical recycling can handle relatively complex streams of different materials and any adherent moisture and contaminants.
- In pure plastic streams, chemical recycling can handle difficult forms of contamination or pollution (e.g. additives, odours and colours).
- At the moment, chemical recycling processes are only utilised at a small scale (<10 kiloton input).
- Not all chemical recycling processes are classified as recycling by the Packaging Waste Fund.
- A sound business case requires installations of sufficient size and supply security for a suitable volume of input streams. At the moment, this is not feasible for all techniques. International collaboration can help to achieve an economically viable scale and guarantee sufficient input material.
Did you know?
Chemical recycling is not a new form of recycling. Some processes rely on techniques that were developed in the 1930s but never utilised on a large scale before. Since mechanical recycling is currently unable to process all plastic waste streams and with a growing focus on the quality of the recyclate, the possibilities of chemical recycling are becoming more popular again. It appears to be a promising next step towards further closing the plastic packaging chain.
What is chemical recycling?
To understand what chemical recycling does, it is important to have some insight into the “DNA” of plastic. Plastics are all chemical compounds that are produced with the help of non-natural chemical processes. They consist of monomers, i.e. molecules that, in turn, consist of carbon and hydrogen atoms. Monomers are extracted from naphtha. Naphtha is produced during the distillation process of crude oil.
When monomers are bound together, they form chains (polymers/granulate). Polymers can be used to make a wide range of plastics. With chemical recycling, the various elements of plastic are brought back to their original form. When this granulate is reheated, it can be used to produce all kinds of plastic packaging materials.
There are various chemical recycling processes, each of which reintroduces the plastic into the chain at a different stage. Most people lack any in-depth knowledge of these processes, how they work exactly, what their possibilities and limitations are and how they contribute to a circular economy.
Techniques for chemical recycling
Three main techniques, each of which encompasses various processes, are depolymerisation, pyrolysis and gasification. The fourth technique, solvolysis (a form of physical recycling), offers a solution for e.g. separating polymers from other materials. Chemical recycling also offers a solution for the deteriorating quality of the polymer chains after each cycle of mechanical recycling. See figure 1 for an overview of these four main techniques in the plastic chain and the possible input streams of each technique.
Solvolysis (physical process)
Formally speaking, solvolysis is not a chemical recycling technique but a physical process, because the polymers are left intact. By exposing plastics to a solvent, composites (products that consist of multiple materials) as well as any additives (e.g. fillers) can be separated from each other. This results in a polymer that can then be removed from the solution. The solvent can be reused, as can the additives. The benefit of solvolysis is that any impurities are removed from the collected plastic packaging waste. The downside is that the method can only be used to process materials with a relatively high degree of purity (no more than ten percent contamination).
Officially, solvolysis is not classified as chemical recycling, because dissolution is a physical process. Nevertheless, this recycling technique is included here, because it is a markedly different process than mechanical recycling.
Examples of (potential) applications include the separation of metal and plastic in e.g. electronic waste and the recycling of car components and other composites. The polystyrene loop is a practical example. Polystyrene that is used in the construction sector often contains a brominated substance that acts as a flame retardant. Using solvolysis, the bromine is separated from the polystyrene. Both materials can then be reused.
A condition for recycling via solvolysis is that a collection system must be in place for the packaging materials or products in question. It must also be known what plastics are involved and the right kind of solvent must be available. During this process, the polymer remains the polymer. Because of this, solvolysis is somewhat similar to current forms of mechanical recycling. For certain plastics, this process therefore fits in a circular economy. Solvolysis is a relatively energy-efficient process.
During chemical depolymerisation, polymers are broken up with the help of a solvent and heat and reduced to monomers. These monomers must then be polymerised in order to produce new plastics with them. The benefit of this form of chemical recycling is that impurities can be removed from the collected plastic. The downside is that only input that consists of fairly pure materials is suitable for depolymerisation.
One example of this process is the chemical depolymerisation of PET. The input material consists of so-called PET flakes, which have been shredded and washed. During the depolymerisation process, the polymers are reduced to monomers. This is done using solvents and heat. This process is suitable for a specific group of plastics: the polycondensation plastics. The monomers are then reintroduced into the polymerisation process in order to form polymers. After this process, any impurities, e.g. colourants or the PE of a multilayer, will have been removed from the product.
Depolymerisation is technically feasible for polyesters from packaging materials or textile. The industry is hard at work to roll the process out to the next level: producing several batches of monomers of 1,000 litres each. Financing poses a challenge here. There is a very high chance that the polymers that enter the depolymerisation process are eventually turned back into polymers. Little is currently known about the process’ energy consumption, although it takes place at a low temperature. One application is the production of packaging materials, although - as is true for all polymers - there are many other possible applications. The advantage compared to mechanical recycling is that the potential end product is of food-grade quality. It is also possible to remove any colourants, thereby creating a transparent material.
With pyrolysis, the polymers are broken up by heating them without oxygen at a temperature of 500-800 degrees. The result is an oily substance or a gas, depending on the input. This can be used to produce fuel or serve as a raw material for the plastic industry. The feedstock is more varied here; pyrolysis is suitable for polyolefins such as PE, PP or PS. Pyrolysis is not necessarily suitable for the stream of mixed plastics, which contains other plastics such as PET and PVC in addition to polyolefins.
A distinction is made between regular pyrolysis and catalytic pyrolysis. The end products of regular pyrolysis can be used for the production of transport fuels. The end products of catalytic pyrolysis become naphtha. After additional refining, this can be used to create new monomers for the production of plastics. Whether or not a collected plastic packaging will be turned back into a polymer therefore depends on the choice of process.
There are several pilot installations for pyrolysis, primarily in Japan and now also in Canada. At the moment, the output of the pyrolysis process is mostly fuel. For a closed plastic chain, it is important to make the shift towards the production of more naphtha. With the help of catalysts, this shift is gradually taking place. The temperature at which pyrolysis occurs is higher than for depolymerisation or solvolysis.
With gasification, plastics are heated with a small amount of oxygen at a temperature of 700-900 degrees, which produces a gas. This gas can be used as a raw material for the chemical industry or as an alternative to cokes for the production of steel. The applications of the gas in the chemical industry are as broad as oil-based applications, e.g. for the production of methanol or fertilisers (ammonia or urea). The feedstock for syngas can include more than plastics alone: the system can process all carbonaceous products, including wood and paper. The goal of this process is the recovery of carbon.
The chance of the input material being used to produce new polymers is as small as for a barrel of oil. Energy is lost during this process, because it involves a small degree of incineration.
Situation in various countries
In the Netherlands, various chemical recycling initiatives (at the pilot level) are in development:
- Ioniqa (depolymerisation): reduces PET packaging waste to pure raw materials, removing any colourants and other contaminants. Capacity realised in 2019: 9 kiloton output BHET.
- Waste-to-Chemicals (gasification): Air Liquide, AkzoNobel Specialty Chemicals, Enerkem and Havenbedrijf Rotterdam want to build an installation in which carbonaceous waste is turned into methanol. In addition to plastic packaging waste, the facility will also process biomass, diapers, paper, etcetera. Capacity: circa 200 kiloton output (methanol).
- IGE Solutions (pyrolysis): processes, among other things, plastic packaging streams that are hard or impossible to recycle mechanically and would otherwise be sent to a waste-to-energy plant. Capacity: 24 kiloton output (fuels).
- PolyStyreneLoop (solvolysis): has set up a pilot facility that turns EPS into, among other things, PS. Capacity pilot facility: 3 kiloton PS.
- Cumapol (depolymerisation): is working on a pilot facility for PET recycling for, among other things, packaging materials. Capacity after successful pilot: 25 kiloton output BHET/PET.
The waste stage
The use of packaging materials is subject of European legislation. On 20 December 1994, the European Parliament and the Council of the European Union introduced the Directive 94/62/EG (hereinafter: Directive) for packaging materials and packaging waste. This Directive was subsequently revised on 22 May 2018. The goal of this Directive is to limit the use of packaging materials and stimulate recycling, reuse and other useful applications for packaging waste.
All EU Member States are required to implement the Directive in their own national legislation. Every Member State has its own way of doing so. Packforward started to give an overview of the way the different Member States implemented the Directive, but the overview is not completed yet. You can find more information for the Netherlands, information about other countries will follow soon.
Despite the efforts made with regard to collecting, sorting and recycling packaging waste, new raw materials will have to flow into the packaging chain in order to safeguard the quality of the material and compensate for the loss of material in the chain. For a growing number of the new raw materials, steps are being taken towards a circular economy, e.g. by making use of biobased materials.