Utilising plastic waste in a sensible way

Fruits and vegetables, toothpastes, body lotions and shampoos, toys, jewellery and clothing, even electrical appliances, sporting goods and cars: nowadays hardly any part of our lives is completely plastic-free.

The very versatile plastics have been on the rise for 70 years—and the majority of these is packaging . This means that the amount of plastic waste is increasing. Plastic waste must no longer be disposed of in landfills. Owing to ecological and economic reasons plastic waste must be utilised in a sensible way. Three processes are used to do this: mechanical recycling, raw material recycling and energy generation. The processes can also be organised in two groups: recycling and energy generation.

According to the German Environment Agency (Umweltbundesamt, UBA) 99% of the plastic waste generated in Germany can be reutilised with the help of these processes (in 2015, energy generation accounted for 53%, mechanical recycling for 45% and raw material recycling for 1%). But what do these processes entail?

Mechanical recycling

Mechanical recycling refers to the traditional method of recycling: old plastics are converted into new raw materials or new mouldings. However, this form of recycling is only suitable for so-called thermoplastics. The most well-known are the following five varieties: polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and polyamide (PA). They make up about 74% of all produced plastics and, once they are shaped into a plastic, they can be melted down and shaped again.

With regard to mechanical recycling it is important to distinguish between single-variety waste, waste of similar varieties and mixed waste. Single-variety waste is plastic waste made out of the same thermoplastics. Plastic waste of similar varieties is waste consisting of plastics that fall into one product group, for example polypropylene. Mixed waste refers to plastic waste that consists of different varieties of thermoplastics.

During the reutilisation process, the mixing of different plastics significantly reduces the quality and mechanical properties of the new product. This is why mechanical recycling is used almost exclusively in the case of single-variety materials; for example polystyrene foam (also known as styrofoam), which is used for the thermal insulation of buildings and for food packaging. Thanks to mechanical recycling the materials can then be used, for example, for the production of polystyrene concrete and bricks.

Mechanical recycling is generally the preferred process. According to the Packaging Ordinance, at least 60% of the plastic waste is to be reutilised and at least 60% of that waste via mechanical recycling. The Packaging Act, which comes into force on 1 January 2019 and replaces the Ordinance, significantly increases the mechanical recycling share. 90% of the plastic waste will have to be recycled, of which 65% will have to go through mechanical recycling. This represents an increase from the current 36% of all the plastic waste, which has to go through mechanical recycling, to 58.5%.

Raw material recycling

During the process of raw material recycling, plastics are broken down into their original monomers or other materials that can be used from a petrochemical point of view, such as high-quality oils and liquid gases. Monomers can only be produced out of single-variety plastic waste.

The pyrolysis process is used for this kind of recycling. During this process the plastics are broken down at temperatures between 400 and 800 degrees Celsius under the exclusion of oxygen. The waste is not burned, it is broken down into petrochemical substances. This produces, for example, methanol and synthesis gases, which can then be used as an alternative for fossil raw materials.

A different form of raw material recycling is hydrogenation. During this process, gaseous and liquid intermediate products are obtained by breaking down the plastics with the help of heat. Subsequently, the intermediate products react with the hydrogen generated during the processing prior to the hydrogenation. This process results in the creation of hydrocarbons with a low molecular weight—oil and gas. A high pressure (400 to 1,500 bars) and a high temperature (380 to 500 degrees Celsius) are needed for this process.

Raw material recycling is used whenever mechanical recycling is not possible, for example in the case of products containing mixed varieties of plastics. In these cases, an attempt is made to recover at least the components with a lower molecular weight and utilise them as raw materials. However, as this form of recycling accounts for only 1% out of the 99% of plastic waste being reutilised, this process is hardly ever used. When it comes to pyrolysis, for example, the production of end products out of the corresponding raw material distillates is much easier and cheaper.

Energy generation

Energy generation (also referred to as thermal utilisation) refers to the burning of plastics with two goals in mind: energetic utilisation or energy generation. It is not recycling in its strict sense because no materials are processed and reused or reintroduced into the economic cycle in a new form.

During the energy generation process, the plastics are burnt in plants at very high temperatures, for example in power stations, blast furnaces or cement mills. Owing to the high temperatures the plastic can be burned completely and almost without producing harmful substances.

Energy generation comes into play when the waste cannot be processed through mechanical recycling or raw material recycling. Its advantage lies in the fact that polymers are energy-rich compounds. The energy used during their production is largely stored in these materials in the form of chemical energy. The heating value of waste corresponds approximately to that of coal.

With a share of more than 50%, energy generation is used for a large portion of the overall reutilisation of plastic waste. This is based on the fact that, as of 2005, the disposal of untreated household waste in landfills is no longer permitted in Germany. Any material that cannot be recycled now has to be burnt. Today, only the smallest fraction of waste, which cannot be reutilised, can be stored in “safer landfills”.

According to the UBA, around 5.88 million tons of plastic waste were reutilised in 2015, while in 1994 the corresponding figure had reached 1.4 million tons.