How is plastic recycled?

The most common plastic recycling method is mechanical recycling. If you have a relatively clean, well-sorted stream, this is the preferred option in terms of cost, impact and complexity.

It uses mechanical processes like washing, grinding, melting and compounding. It does not significantly change the chemical structure of the material, so plastics like PET (polyethylene terephthalate), HDPE (high-density polyethylene) and PP can be reused directly. The downside of mechanical recycling is the presence of contaminants. In addition, recycling loops lower the quality of the products.

We develop novel strategies to mechanically recycle more challenging waste streams, such as composites from the automotive sector. Currently, thermal conversion techniques are widely used, resulting in low-quality glass or carbon fibres. At TNO (BMC) and within the R&D Hub for plastic waste processing, we set out to develop cost-effective technologies that preserve the quality of these complex materials and their components, including fibres and plastic material. This involves innovative separation, recycling and decompounding technologies.

Dissolution is a physical recycling method that dissolves only the desired polymer, and not the unwanted polymers, additives or contaminants.

Several dissolution technologies for plastics recycling have been developed on pilot and market scale. TNO have proven that with its Möbius technology high-quality polymers can be obtained and directly compounded into plastic products. It involves the selective dissolution of the target polymers using an organic solvent, and a combination of separation technologies (e.g. filtration, adsorption) to remove any contaminants, additives and undesired polymers. It delivers higher quality products than mechanical recycling and is more suitable for complex waste flows. However, the use and recovery of organic solvents typically result in higher processing costs.

Depolymerisation is used when polymers are broken down into smaller units – the original building blocks:

• Enzymolysis (a biochemical process) involves the splitting or cleavage of a substance into smaller parts through the action of an enzyme.
• Solvolysis (a chemical process) involves a chemical reaction in which a solvent, such as water or alcohol, works as a reagent and is present in excess compared to the quantity required for the reaction itself. As a result of the chemical reaction, the polymer is split into smaller entities, either oligomers or monomers, the original building blocks.

After purification, both can be used to produce new polymers, with the same quality as virgin materials.

Thermochemical conversion is the breaking down of polymers at high-temperature, either in the absence or presence of oxygen. This produces an assortment of molecules that can be used in the petrochemical industry as the building blocks for chemicals, polymers and/or fuels.

Incineration combined with Carbon Capture and Utilisation (CCU);

Incineration should be used in combination with full heat integration and Carbon Capture and Utilisation for maximum circularity. TNO has developed various carbon capture and utilisation technologies to optimise the elimination of CO2 and the sequestration of carbon.

Gasification thermochemical process

Gasification is the most versatile solution for highly complex waste streams not suited to chemical recycling. This technology converts waste into syngas (CO+H2) using a limited amount of oxygen. Syngas is a very versatile raw material for a large variety of chemicals and fuels. TNO uses different technologies to convert syngas into highly valuable products> nog linken nieuwe.

Pyrolysis

One widely investigated technology is pyrolysis, in which plastics are heated in the absence of oxygen to crack the chains of polymers. The process yields a complex mixture of hydrocarbons in solid, liquid and/or gas form, depending on the conditions and the quality of the feedstock. Technologies vary depending on the input used and the products targeted.

Most pyrolysis technologies aim to convert polyolefin-rich waste into pyrolysis oil. After further treatment and separation, this oil (naphtha) can be used in a steam-cracking process to produce olefins, the building blocks for polymers.

In this field, many organizations explore the use of a catalyst. By using catalysts, milder conditions like a lower temperature can be used and more selective products are obtained.

Another example is the TNO Milena Technology , a thermal cracking technology that applies a higher temperature and a specially designed process. It produces a range of high-value chemicals such as olefins, BTX and syngas, depending on the composition of the waste input. Generally, thermal cracking can deal with much more contaminated waste streams compared to pyrolysis techniques. As such, cascading of technologies becomes a feasible option.