Infinite regeneration of polyester
When polyester is depolymerised, the material is converted back to its fundamental building blocks. This chemical process can, theoretically, regenerate PET ad infinitum from bottles or polyester textiles to monomers that are virtually akin to petrochemical ones.
Take a batch of monomer molecules, string them together using a chemical reaction and they will form long polymer chains. Polymerisation is the broad term used to describe the making of plastics and synthetic textile filaments. It is also possible to reverse the process, that is depolymerise, to unlock the chain and convert the polymer back into its initial monomers. And repeat, ad infinitum.
This is the very much simplified recipe at work in the chemical recycling of polyester that a host of companies, large and small, are tackling head on. “It is always possible to invert the chemical polycondensation process to obtain the original monomers,” says Pierre Van Trimpont, a consultant at Belgian technical textile research centre Centexbel. “It is a promising field as it produces a secondary raw material that is comparable to a ‘virgin’ one. It is a process of regeneration more than of recycling.”
Among the many companies working on this, not all are depolymerising waste plastics and textiles, as it is a field that requires a high degree of purity of feedstock. Others are investigating a mix of mechanical and chemical processes that separate out various components, without breaking them down into their original monomers. These methods are believed to be better suited to blended textiles (see separate article on fibre separation technologies in this issue of WSA).
Depolymerisation is seen as complementary to the mechanical recycling of PET. The so-called (and again greatly simplified) method of “chop and wash” delivers recycled flakes of satisfactory quality only from clear or blue PET beverage bottles. In some cases, clothes can be a source, too, as long as they are made from 100% polyester. Though widely adopted, many consider rPET made from mechanically recycled post-consumer bottles a form of downcycling. Furthermore, two-thirds of global PET production, estimated by Textile Exchange to be around 55 million tonnes per year, goes into the making of polyester for apparel applications. Recycling processes for textiles would have a greater impact on reducing waste.
Just as there are several ways to polymerise monomers, there are various techniques to depolymerise a material. The most common involves glycolysis, methanolysis or hydrolysis. The process of disassembling polymer chains can also be executed by enzymes, a path being explored by French company Carbios, or by microwaves, as seen at Gr3n, in Italy.
The enzymatic recycling process in development for close to ten years at Carbios was the subject of a scientific article published in Nature this past April (“An engineered PET depolymerase to break down and recycle plastic bottles”). This is a high point for the company that sees publication in a peer-reviewed journal as a clear validation of its technology. “The system we are developing allows the infinite recycling of PET plastics and textiles. This is an improvement over mechanical recycling in which the polymer is degraded at each stage and at some point can no longer be recycled into anything useful,” Martin Stephan, Carbios deputy CEO, tells WSA.
Construction of a demonstration plant has begun in Saint-Fons near Lyon, the French Chemical Valley, and is expected to be in operation in 2021. It will allow the company to validate that the process can be scaled up to a PET plant with a capacity of up to 150,000 tonnes per year. The biotech company has formed a partnership with Novozymes, based in Denmark, that will scale up production of Carbios’ proprietary enzyme. It is a very specific enzyme that breaks down PET and leaves other materials untouched, says Mr Stephan. Once the enzymes have done their work, the PET monomers (terephthalic acid and monoethylene glycol) are then isolated, filtered and purified. “This makes our process highly tolerant to contaminants,” he says. Among its environmental advantages, it does not require solvents, water is recycled in a closed loop, and the process is conducted at what is called biological temperatures, around 72°C.
DEMETO (DE-polymerization by MicrowavE TechnolOgy), the process in development at Gr3n, has received funding from the EU Horizon 2020 programme, and it, too, has a demonstration plant under construction, with a capacity to treat 1,000 tonnes a year, says Maurizio Crippa, company founder and CEO. The microwaves are said to cut the polymer chains and remove contaminants that can then be separated out from the PET ingredients. This method, he says, increases the proportion of PET/PES that can be recycled as it can handle difficult to treat PET waste (such as plastic trays). DEMETO can handle any feedstock with a minimum polyester content of 70%, whatever the other components, including cotton, which comes out intact, or even PU, a challenging material to decompose. Used clothing can be microwaved without removing buttons, zippers or other non-textile elements, but does require shredding to a specific size. ”The 70% threshold is the minimum PET content to make our process economically viable,” says Mr Crippa. In his 15 years of research into the process, he says much time has been dedicated to the purification phases to obtain the original PET ingredients, namely ethylene glycol (EG) and terephtalic acid (PTA). This allows the company to generate output that Mr Crippa says is similar in specifications to monomers derived directly from oil. It is also a closed-loop system as it releases sodium chloride as a by-product, which is converted, using electricity, to produce the two chemicals needed for the process. “The by-product is a resource for further processing, closing the internal loop,” he says.
As depolymerisation is conceivably just a step away from polymerisation, some companies are investigating solutions that will enable them to tweak existing facilities into recycling or regeneration plants.
Eastman Chemical Company says it has modified the front end of its acetyls and cellulosics production processes to accept waste plastic through what it is calling Carbon Renewal Technology (CRT). Operations began in late 2019 in Kingsport, Tennessee, the company’s largest manufacturing site and world headquarters. Its “reforming” technique breaks down plastic waste and converts it into molecular building blocks such as carbon, oxygen and hydrogen. “We use these building blocks, which are indistinguishable from virgin, to produce new materials with certified recycled content. With this technology, we can recycle waste plastics an infinite number of times and produce new materials with no degradation in quality,” says Ruth Farrell, global marketing director for textiles at Eastman. This method, she adds, can recycle complex plastics that do not have good end-of-life options and potentially, in the future, textiles.
The company is also developing a chemical recycling process for PET, which it calls Polyester Renewal Technology (PRT). This year, it began operations to recycle clear PET bottles using glycolysis, but it plans to shift to methanolysis in 2022. “This plant will allow us to process a diverse stream of PET and PETG not suitable for mechanical recycling,” says Ms Farrell, citing green bottles and polyester fibres from carpets. Again, PRT produces the monomer building blocks of PET that can be used to make new materials.
CuRe Technology is developing a process it is calling ‘modular molecular recycling’. It is the result of a collaborative effort between recycled polyester maker Cumapol, Dufor (maker of speciality polyesters), DSM (maker of Dyneema) and its circular economy unit, DSM-Niaga, Morssinkhof Plastics and NHL Stenden, a polytechnic university. “Depending on the quality of the waste and the desired output, we choose the lowest-energy recycling system that will deliver the building blocks to make new PET,” says Josse Kunst, a chemical engineer, formerly at DSM and now chief commercial officer for CuRe. Each partner plays its part in the ecosystem, with waste preparation conducted by Morssinkhof Plastics and DSM providing its material science expertise.
The process in development at CuRe is based on partial glycolysis and involves 12 different steps, says Mr Kunst. This offers a glimpse of the very many stages required to revert a polymer back to its building blocks. After a series of collecting, sorting, cleaning, depolymerising and purifying stages, CuRe says it is working on developing a unique, continuous process that skips going back to the original monomers and directly repolymerises to polyester. “The CuRe process does not need to completely break down the polymer, this saves energy and solvents,” says Mr Kunst. “High temperatures affect viscosity and are better avoided. We keep the energy in the molecules, so to speak. As we don’t cool them down, we don’t need to heat them up again.” A pilot plant currently treats 20 kg per hour, and research is still being conducted on finalising the repolymerisation line, which should be in operation by the end of the year. Once that is done, the company says it may be able to convert a former polyester production plant to its molecular recycling method. The facility, based in the Netherlands, has a capacity to treat 25,000 tonnes and could be in operation in 2023.
“We are currently focusing on difficult- to-recycle plastics such as food trays and coloured bottles, before moving on to textiles,” says Mr Kunst. Clothing presents its own set of challenges due to the presence of coatings and Spandex, and the need for advanced sorting. “The pre-consumer waste generated by sports brands could already fill two factories today,” says Mr Kunst who urges the apparel industry to develop a global product passport system to simplify tracing and sorting.
At its Polygenta plant in India, PerPETual has been running full-scale chemical recycling of PET bottles by glycolysis since 2014. It now claims to offer high-quality PET that is competitively priced compared to conventional PET. “Our ester polymers are now the same quality as those produced from petrochemicals and they are even cheaper to produce; this has been a huge turning point,” says perPETual CEO, Vivek Tandon. He has seen a shift to chemical recycling in the past two or three years; this is he says is due in part to the ability of chemical recycling to remove contaminations and even bacteria within the plastic, resulting in consistent high quality.
From bottles to clothing
In addition to PET bottles, perPETual now also recycles polyester textiles at pilot scale. The depolymerisation and purification processes can actually be easier, says Mr Tandon, as dyes applied on textiles are typically on the surface, whereas in bottles the dyes are infused into the polymer. The company will however only recycle pure polyester clothing. “We don’t touch mixed fibres,” he says, “but we will recycle anything with 100% polyester.”
The chemical recycling of polyester clothing was first developed by Japanese company Teijin, which introduced Eco-Circle in 2002. Other Japanese companies have followed suit and also focus specifically on clothing as a feedstock. Jeplan, whose process is based on glycolysis and generates BHET, has plans to build a facility in France by 2022. Trading company Itochu, also based in Japan, has set up a circular supply chain for end-of-life polyester clothing based on a process that reverts polyester back to DMT and MEG. A factory with a capacity of 30,000 tonnes per year has been in operation in China since 2012. “The system both depolymerises and repolymerises PET. Dyes and contaminants are removed during the filtering and purification processes so as to obtain a level of purity nearly the same as virgin polyester,” says Sachiro Shimoda, manager for Itochu’s RENU project. Its system recycles waste textiles and clothes that have a minimum content of 93% polyester, with absolutely no PU, whilst zippers, buttons and metal elements must be removed, says company spokesperson Reira Miyatsugu. These time-consuming sorting and feedstock conversion operations add to the cost of the process. To make it economically viable, Itochu’s recycled RENU-branded polymers are made from a mixture of pre- and post-consumer waste and used clothing. The company is intent on developing a circular system and is looking to spread the technology worldwide.
“The chemical recycling of polyester has high potential,” says Daniël Verstraete, a consultant at Centexbel. But it remains costly; besides the necessary R&D investments, its energy use is higher than mechanical recycling, and the processing of end-of-life textiles is far from optimal, adding expenses to the making of a material that is very cheap to produce conventionally, he says. But the petrochemical polymer industry has a 50-year head start over these next-generation depolymerisation technologies, points out Martin Stephan at Carbios. It is, for many, a matter of time before the unmaking and remaking of polyester can compete head on with its petrochemical counterpart.