Chemical recycling: the second life of plastic waste

Chemical recycling of plastic

Chemical recycling

Chemical recycling of waste is the decomposition of the polymers that make up waste through heat, chemicals or catalysts. It is also called tertiary or advanced recycling because with chemical or physical processes it aims at the almost complete recovery of monomers, oligomers or other compounds. Through these chemical processes, it is possible to obtain a great variety of raw materials ranging from monomers to mixtures of compounds, mainly hydrocarbons, which become a new source of chemicals or fuels. Products derived from chemical recycling have properties and qualities similar to those of virgin raw materials.

Chemical recycling has significant advantages over mechanical recycling which instead gives rise to secondary materials of lower value. Hence the great interest in this method of recovering plastic waste which highlights the potential of chemical recycling as a sustainability process in the polymer chemical industry.

How the chemical recycling of plastic takes place

The chemical industry and research teams are working on a wide variety of procedures and treatments for the recycling of plastic and rubber waste. The goal is to define chemical processes that are sustainable from an environmental point of view and compatible with an economy of scale. The procedures developed are mainly:

Chemical depolymerization. The starting monomers are produced by reaction with agents.

Gasification. The process is done with oxygen and/or steam to produce synthesis gas.

Thermal decomposition of polymers by heating in an inert atmosphere.

Catalytic cracking. The polymer chains are broken down with a catalyst that promotes cleavage reactions.

Hydrogenation. The polymer is degraded by the combined actions of heat, hydrogen and catalysts.

Chemical depolymerization

It is a type of recycling that, with chemical agents, breaks down the polymer chain and brings it back to the starting monomers. In this way, you return exactly to the raw material used in the preparation of polymers. This can be used for the preparation of new plastics with chemical-physical characteristics identical to those produced with virgin raw material. The recovery of plastic by chemical depolymerization is the most established method of the chemical recycling of polymers and has been applied on an industrial scale for several years now. The disadvantage of this process is that it is limited to the recovery of condensation polymers such as polyesters, polyamides, polyacetates and polycarbonates. On the other hand, it is not useful in the recycling of most of the addition polymers, such as polyethylene, which represent 75% of total plastic waste.

 

Gasification

In principle, gasification was also studied as a method for obtaining synthesis gas, or syngas, from products derived from both biomass and organic solid residues. It has now also become one of the treatments for the reconversion of polymer waste. One of the advantages is that there is no need to separate the different polymers present in plastic waste. Even in many cases, plastic waste is gasified even if it is mixed with other solid waste.

If he resulting syngas is used only as a source of energy for combustion, then the recovery of the raw material is not fully realized and more CO2 is produced. Better to use gas for the synthesis of chemical substances, such as methanol, ammonia, hydrocarbons, acetic acid, and thus implement the principles of the circular economy.

To improve the convenience of chemical recycling by gasification, production plants should be located near the plants where waste is converted into gas. This requires a systemic approach to recycling to be carried out in harmony between the various players in the chemical industry.

Thermal decomposition or thermal cracking

This is a chemical recycling method that has been under consideration for some time. The first studies date back to the 70s and arise mainly from the study of the resistance of plastics to high temperatures. Two strands of chemical recycling can be identified through thermal decomposition:

  • pyrolysis with temperatures above 600 ° C
  • cracking with temperatures below 600 ° C.

Only in a few cases can the thermal decomposition of polymers be considered as a true depolymerization process. Polystyrene and polymethyl methacrylate are polymers that can be thermally degraded with the formation of high yields of the corresponding monomer. On the other hand, in most polymers thermal decomposition leads to a complex mixture of products, containing low concentrations of monomers and therefore of low-quality recycled material.

Catalytic cracking

It is a chemical recycling process that uses the combination of high temperatures with a catalyst, often acidic solids such as silica and alumina. Compared to the simple splitting of the polymer by thermal effect, catalytic cracking has several advantages:

  • lower temperatures
  • shorter time
  • higher quality of monomers.

This method also has several drawbacks to the catalysts which tend to become inactive over time. Carbon residues and toxic substances present in the mass of raw waste are deposited on them, which hinder the recovery of the catalysts and their reuse in subsequent chemical processes. For these reasons, catalytic cracking is mainly applied to relatively high purity waste.

Hydrogenation

The hydrogenation of plastic and rubber waste is an interesting alternative for breaking polymer chains. Compared to chemical recycling in the absence of hydrogen, hydrogenation leads to the formation of highly saturated products, which facilitates their use as fuels without further treatment. Furthermore, hydrogen promotes the removal of residues such as chlorine, nitrogen and sulfur that may be present in plastic waste.

The advantages of chemical recycling

The enhancement of the chemical recovery of plastic waste is slowed down more by economic considerations than by technical factors: it is considered not very profitable given the low price of the raw material to produce polymers. But in recent years, the environmental and social costs of waste are changing perspective. Economic analysts also see sustainability as a determining factor in companies' long-term prospects. The principles of the circular economy are increasingly affirmed and become the key to real economic development. To implement full circularity, chemical recycling is one of the fundamental steps for its countless advantages.

Values otherwise unused plastic waste: around 30 million tonnes of plastic waste is collected every year in Europe today. However, 85% of this is still incinerated or sent to landfill, wasting valuable resources.

Contaminated and/or mixed plastic waste is recycled which cannot be recovered by mechanical recycling.

The use of fossil resources is reduced: chemically recycled plastic waste can be reused as secondary raw materials for the production of new plastics and fewer extracted fossil resources will be required. Europe becomes less dependent on imports, as waste can be used as an immediately available resource.

CO2 emissions are reduced because the emissions associated with incineration and conventional production of raw materials are eliminated.

So in chemical recycling, plastic is reduced to its original molecular forms so that it can be transformed into completely new plastic materials. There is a significant demand for more development in this field - and this is where Covestro can offer its core chemical expertise. For example, the European research project PUReSmart aims to significantly improve the recycling of polyurethane foam.

Covestro promotes the recycling of plastic

Currently, chemical recycling processes exist on a small industrial scale and need further research efforts to achieve full economy of scale. This can only happen if a productive ecosystem is created supported by political and economic frameworks and by a collaboration between different industrial subjects.

With Green Recovery Europe has drawn up the guidelines for developing sustainable and for an economy that is circular and green. In this context, the chemical industry plays a leading role.

This is why Covestro has set itself the mission of designing plastic and its components so that they can be recycled as easily as possible. Over the next few years, Covestro will conduct extensive research on different recycling technologies, using its expertise in chemicals and its innovative capabilities. Through more than 20 research and development projects, Covestro is developing new and more efficient technologies and methods for recycling plastic. The approach is broad enough to incorporate the diversity of products and markets. The goal is to provide market-ready solutions as quickly as possible. By developing these new plastic recycling technologies, Covestro is paying particular attention to energy efficiency. In this way, the carbon footprint of the products will be reduced to make the business more sustainable.

Covestro is always at the forefront with its #PushingBoundaries vision which anticipated the crucial issues of these times by providing not only the right solutions but the best ones for growth and innovation.