Chemical recycling: the missing link
Plastic recycling: environmental and economic challenges
Over the course of a century, plastics have continuously evolved and they have been extremely successful. Although they are currently the subject of much discussion, we should not forget how much they have changed and improved our daily lives - better food preservation, lighter cars, protection of property and people, and more. Each year, approximately 350 million tonnes of polymers are produced worldwide. Obviously, although plastic products have a very long life, they are not eternal, and a solution must be found for these plastics that have reached the end of their life. As we know, landfilling is the least desirable option. Energy recovery - the fact of using the plastics as an energy source - is a potential solution, but it is not entirely satisfactory because instead of simply being used a source of fuel, plastic waste could be a formidable source of new materials. The good news is that there is increasing awareness and ideas abound.
Plastics are very useful; thanks to them, food storage times have been considerably extended, vehicles have lost tens of kilograms and therefore consume less, buildings are better insulated... Many of them are recyclable so they can be considered as a resource rather than a waste.
A boost from chemical recycling
The process of mechanical recycling - which involves sorting, washing, granulating and then melting the material again to transform it into new products – is very effective for objects made of a single polymer, such as a polyethylene jug, or for those whose various components can be properly separated. However, the process is more complicated for those made up of several polymers or closely related materials (such as some multi-layer food packaging) or waste that is contaminated or contains undesirable substances. The truth of the matter is a little more complex, as it is sometimes possible to recycle different components into a mixture to make a new object. The only constraint is that these recycled materials often have lower performance and are therefore re-used for applications that are technically less demanding than the original ones but whose market is limited (street furniture, for example). That being said, today, 50% of all the various plastic packaging used by households can be fairly easily recycled.
A polymer, which is a component of all plastics, is made up of a long chain of molecules that repeats itself. Getting back to the monomer, the basic molecule, is one of the main objectives of chemical recycling.
It is therefore for the remaining 50%* that chemical recycling has garnered interest, particularly among manufacturers and many start-ups. Chemical recycling is a process that breaks down the polymer to transform it into shorter molecules. The aim is to create a monomer, the raw material for all polymers, in one or several steps. Perfecting this process would make it possible to produce a recycled polymer identical to the virgin polymer and thus be able to use it again in the most demanding applications, such as food packaging.
* source: Citeo
Chemical recycling is an essential way to meet the objectives of the European Union, in particular, which imposes increasingly high quotas of plastics to be recycled. These recycling obligations are not the only ones; there are others that take into account the integration of recycled materials into new objects, which obviously includes packaging. Thus, from 2030 onwards, beverage bottles will have to be made of at least 30% of recycled materials and in the vast majority of cases this will be PET and to a lesser extent HDPE.
The collection of PET bottles in Europe is rather satisfactory. This is good news because by 2030, 30% of this polymer, after recycling, will be reintroduced into new packaging.
The bottles intended for the food industry must meet some of the most stringent health standards. Thus, recycled polymers will be required to have exactly the same properties as virgin polymers. In this field, chemical recycling makes it possible to meet those requirements and it is for all these reasons that it is destined to see major developments in the coming years.
Polymers go in for rejuvenation
To understand the reactions involved in chemical recycling, it is important to remember that plastics are made up of long molecules called polymers. These molecules are in fact a chain whose links - monomers - are repeated throughout. The nature of the polymer therefore depends on the base monomer. The vast majority of plastics come from fossil resources, including gas and oil and by-products of fuel manufacturing processes such as ethylene or propylene. What we currently know as chemical recycling refers to two main technologies. Firstly, there is depolymerisation which is a process used to obtain the monomer directly by breaking the chain at specific points. Secondly, there is the process of cracking (by pyrolysis, gasification) which is also a cutting operation but is not selective and therefore leads to a multitude of products. Why several processes? Quite simply because some processes are more suitable than other depending on the nature of the polymer and its ability to break down into its basic elements. Thus, depolymerisation can be done with PS, PET or PMMA, while cracking will be necessary for recycling polyolefins (PE, PP).
Depolymerisation: back to basics for plastics
As its name suggests, depolymerisation consists of returning the polymer to the sate of a monomer by cutting it at specific locations. The aim, therefore, is to restore the plastics to their original state through a process that specialists call “plastic to monomer”. Three technologies are currently the subject of much research.
Thermal depolymerisation involves heating the plastics to a temperature of 450° to obtain a liquid solution containing over 95% of the original monomer. It is particularly effective for PMMA and PS. The Canadian Pyrowave company and the American Agylix company have developed processes that use this technology to recycle polystyrene (yoghurt pots, isothermal boxes, hangers, etc.) into styrene, which can be re-used to manufacture styrenic polymers.
Chemical depolymerisation (solvolysis) involves using a reagent to decompose the polymer matrix. The name of the technique depends on the reagent used: hydrolysis when the reagent is water, alcoholysis when it is an alcohol, and glycolysis when glycol is used. The latter is by far the most popular. The various chemical depolymerisation techniques are generally used on polymers derived from polycondensation such as PET and PA. PET, the polymer used to make water bottles in particular, is one of the polymers that is of greatest interest in the field of chemical recycling, as it could enable manufacturers in the sector to achieve the high levels of recycled material incorporation set by regulations or their own commitments.
Coca-Cola is unveiling the first ever sample bottle made using recovered and recycled marine plastics, demonstrating that, one day, even ocean debris could be used in recycled packaging for food or drinks.
Among the most notable examples, the Dutch company Ioniqa Technologie has approached Coca-Cola in order to be able to provide enough rPETs by 2030 to meet the "future standards" for plastic packaging. The Japanese company Jeplan also proposes to recycle PET from textiles or packaging by glycolysis, a solution that can be used to process opaque and coloured polymers. Finally, it is this technique that was developed by the Italian company Aquafil, now well known for its Econyl®, a polyamide made from fishing nets and used carpets.
For or years now, the specialised press has regularly reported on enzymatic depolymerisation and the famous gluttonous enzymes able to absorb unprecedented quantities of plastics. The term is slightly misleading because these enzymes do not feed on polymers but simply, and this is still quite an achievement, break down the molecular chain to return the plastic to a monomer. For the time being, this transformation is fulfilling all its promises for polymers such as PET and PLA. Its only flaw is its slowness since it takes several weeks for these enzymes to break down the chains.
Polymer-eating enzymes. In reality, enzymes simply break down the polymer chains to return to the base material. This technique is particularly effective for PET and PLA.
However, the French company Carbios recently announced that it had succeeded in genetically modifying the enzymes so that they would be able to "digest" the polymers in less than 20 hours in a tank maintained at 60°.
Cracking by pyrolysis, gasification: heat stroke for polymers
Although it is fairly easy to convert a monomer into a polymer, the reverse is not always so. In some other cases, the operation requires the involvement of pyrolysis technology. This process consists of cracking the polymer to obtain smaller molecules.
After being heated to high temperatures, the polymers are cracked and will then be transformed into a new virgin polymer.
To do this, the polymers are heated to temperatures between 350° and 650° in an oxygen void. In particular, the process produces an oil which, once transformed (in a steam cracker), will be re-used to form a new virgin polymer. Another technique, called gasification, consists in heating polymers very quickly to temperatures of up to 1,200° to obtain an even larger volume of gas.
The main difference with the pyrolysis technique is the much higher oxygen volume involved in gasification. This process results in the production of a synthesis gas. These techniques, in particular pyrolysis recycling, are currently being developed by many plastics companies: Sabic, BASF (Chemcycling), and Dow. Gasification technology is also the subject of a major project in the Netherlands in the Enerkem technology.
Dissolution: a separate process to melt polymers’ hearts
Dissolution is another process used to isolate molecular chains without breaking them. The process does not use chemicals to modify the polymer. In other words, this technique allows the polymer to be recycled without having to revert it to a monomer. This technology, which cannot therefore be strictly qualified as chemical recycling, has many advantages. The polymer is purified, and any additives or contaminants can be removed. On paper, it is almost the ideal process because it requires very little energy and the solvents used, such as acetone or styrene, can be easily recovered after use.
The German APK company recently announced that it had succeeded in creating a solvent capable of separating multilayer packaging to revert it to different polymers that are pure enough to be used in new packaging for food or cosmetics. In the United States, Purecycle Technologies has taken an interest in PP (polypropylene), one of the most widely used polymers in the world: it claims that it will be able to recycle 50,000 tonnes/year of PP by 2021 using chemical processes, even if it is particularly dirty. At Polystyvert, its Canadian neighbour, polystyrene is dissolved in an essential oil to restore in it the characteristics of a virgin resin. The tour de force is based on the purification process that eliminates all types of impurities. This is a major step forward, given that the grey polystyrene often used in the construction industry is very rarely recycled nowadays.