Raw material recycling in view

Dealing with used plastics is difficult, especially when they end up in the environment in an uncontrolled way. Neither regulations nor existing waste disposal and recycling measures seem to be able to get a grip on the problem. However, new raw material and chemical recycling processes come just at the right time.

One world is not enough. We need at least two to satisfy our hunger for raw materials. According to the Global Footprint Network, we reached our "Earth Overshoot Day", the day when we have already consumed our available annual renewable resources, on August 22^nd.  The time we still had something left in this global account at the end of the year was 1961. Since then, this day moves earlier and earlier each year.  Although certainly connected to the growing world population, the consumption-oriented lifestyle of our highly industrialised world is the driving force. The consumption of resources is accompanied by an increase in waste, which we pass on from generation to generation like a dowry. Waste plastics or plastics and rubber made from petroleum are an main aspect of this negative inheritance.

It is true that used plastics and other polymer products that have exceeded their use horizon do not generally pose an immediate danger to life and limb, unlike radioactive material from nuclear reactors. Nevertheless, plastics do leave their mark. The "mid ocean garbage patches" are symbolic, as is the problem of microplastics, which has often been discussed in the media. These micro particles are created when products made of plastic and rubber crumble into tiny pieces under external influences. They thendisperse on land, at sea and in the air, and in this way ultimately also enter the food chain.

The property profile of synthetic materials can be specifically adjusted to their later use by means of additives, auxiliary or reinforcing materials and with the appropriate technology. If desired, polymer materials can be as hard as steel and yet much lighter in weight, as flexible as wood fibre, but much more resistant to breakage. Plastics can be foamed, pressed into sheets and shapes or spun into fibres and, depending on the additive, they have an antistatic, disinfectant, less flammable or current-conducting effect. Many good arguments speak for the use of plastics, which ultimately also explains the steep career that polymer materials have had over the past 100 years. On top of that, they are cheap to produce, usually much cheaper and not infrequently more efficient than natural materials, which is why plastics have become the material of choice in so many applications.

They are now plentiful and extremely durable. Therefore, when manufacturing any new polymer product, its fate after use, i.e. when the end of its useful life has been reached, needs to be considered and greatly improved.

When the first humans in earlier times killed a mammoth to satisfy their hunger, little or nothing of the animal remained as waste in the end: what was not eaten was used as clothing, tools and toys or weapons - our modern society can only dream of such a virtually perfect eco-balance. 

Today, when we go food shopping, products are often wrapped in plastic foil. Its transparency gives the deceptive  impression of being made of a single material.

But even seemingly simple films are often complex, multi-layered polymer products. Plastics processed in them cannot be separated from each other using common processes and techniques. This limits their recycling capacity as raw materials in the production of new polymer products of equivalent quality.

Where possible, plastics are "driven in circles". In the case of PET bottles, for example, this is the case to a certain extent because this "packaging" is made of only one plastic, namely polyethylene terephthalate (PET). PET is a thermoplastic, and thus deformable under the influence of heat, from the polyester family. Since all plastics, regardless of their type, are usually made from petroleum, they possess its energy potential. Waste plastics’ potential can therefore be realized thermally or energetically  to generate heat in a combined heat and power plant.

The Waste Framework Directive 2008/98/EC (WFD) of the European Union (EU) lays down a general order of priority for waste management measures, according to which (mechanical) recycling is generally more advantageous than energy recovery (Article 4 of the WFD). However, recycling is not an end in itself, according to the background paper "Chemical Recycling" of the German Federal Environment Agency (UBA) [2]. "Rather, it must correspond to the measure of the best possible protection of humans and the environment, whereby the entire life cycle of the waste must be considered." For this very reason, plastics recycling should be as high-quality as possible with the aim of "recovering substances from the waste stream that replace primary materials and, at best, primary plastics, thereby saving resources".

In other words, preference should be given to the recycling of used plastics in the course of manufacturing high-quality products (upcycling). At this point, the process of chemical or raw material recycling is of great importance.

The idea behind feedstock or chemical recycling can be summed up in a nutshell: It is about recovering the basic chemical building blocks (monomers) processed in plastics (polymers) so that they can be reused one-to-one. This process is called depolymerisation, following the production of plastics in the course of polymerisation. It is carried out by means of suitable chemical reactions, whereby the hydrocarbon-based monomers are linked together in various ways.

Raw material or chemical recycling basically means nothing more than recovering the monomers in their pure form. The principle is similar to that of the Danish toy classic Lego: different structures can be assembled from the same differently coloured and differently sized plastic bricks. What was just an imperial star cruiser in the science fiction saga "Star Wars" can be arranged into a cruise ship, a roller coaster or the International Space Station in the next moment.

In the course of feedstock recycling, plastics are broken down into their original monomers. One restriction: clean monomers can only be produced from unmixed plastic waste. One way to obtain monomers is through pyrolysis. Homogeneous material that is as clean as possible is heated to 400 to 800 degrees Celsius in the absence of oxygen - a characteristic of pyrolysis. Old plastics are not burnt (oxidised), but thermally broken up (cracked) and broken down into petrochemically interesting substances (fragmented). This produces oils or waxes which, after a few purification steps, can be used again in the chemical industry as raw materials.

The Frenchman Antoine Laurent de Lavoisier (1743-1794), who was well known to chemists for his work, produced so-called water gas from coal heated in air in combination with water vapour. This combustible gas mixture of carbon monoxide and hydrogen is also suitable for the production of chemicals. At the beginning of the 19th century, the process of coal gasification was further developed in England and used to produce luminous or generator gas containing carbon monoxide, which in the further course of history was used as town gas, first for street lighting and later also for indoor lighting. At the beginning of the 20th century, it was used as a heat source for cookers and ovens.

Finally, a further development step took place in the course of coal hydrogenation, in which hydrogen was added to carbon, which led to the formation of various gaseous hydrocarbon compounds such as methane, propane, ethane or butane - all of which are important starting materials for the production of chemicals and also plastics.

Since polymers are compounds rich in carbon, old plastics are also suitable for use in gasification processes to produce a synthesis gas that can be used as a basic material for the production of new plastics.

Waste plastics can also be directly liquefied and recycled. in the course of "solvolysis". Here, the material reacts with a solvent, causing chemical bonds to break down and the polymers to be broken down into monomers. Variable pressure and temperature are used in this process.

However, used plastics can also be "oiled" without the use of solvents, namely by direct thermal or catalytic decomposition in a stirred tank. The target product is a liquid phase in which the monomers are present.

"The potential advantages of all raw material or chemical recycling processes follows    the possibility of removing pollutants from the cycle, the use of plastic fractions that cannot be recycled and the wide and the wide range of uses as a raw material in the chemical industry.” is how the authors of the Federal Environment Agency introduce the conclusion of their background report "Chemical Recycling" [2]. "Furthermore, feedstock recycling processes could be a sensible alternative for plastic waste that cannot be recycled and has so far been used for energy recovery. Like mechanical recovery processes, they are part of recycling and come before energy recovery in the waste hierarchy. Successful implementation of feedstock/chemical recycling could make plastics production more sustainable overall as a complement to mechanical recycling processes - for wastes that cannot be sent for mechanical recycling."

The waste management sector recycles almost all of the plastic waste it collects. In 2019, it recycled 46 per cent of all collected plastic waste for mechanical recycling and less than 1 per cent for feedstock recycling. 53 per cent of the waste was recycled for energy. From a climate and environmental protection perspective, it is important to recycle more plastic waste for mechanical recycling. [Further information: Federal Environment Agency]

[1] Earth Overshoot Day 1970-2020, https://www.overshootday.org/newsroom/past-earth-overshoot-days/
[2] Federal Environment Agency, Chemical Recycling, July 2020 (https://www.umweltbundesamt.de/sites/default/files/medien/1410/publikationen/2020-07-17_hgp_chemisches-recycling_online.pdf)