Research Highlight 7 – A ‘KMH' Solution for Chemical Recycling of Mixed Plastic Waste Stream
Is it possible to recycle mixed plastic waste stream containing different polymers back into its constituent monomers? For example, recycling of PET bottles and PC goggles together in one-pot? Are you wondering like me? Well, Prof. Wim Thielemans and his reasearch team (Sustainable Materials Lab) at KU Leuven, Belgium have an answer to this. Their new found ‘KMH’ catalytic reagent can selectively and simultaneously depolymerize PET {poly(ethylene terephthalate)} and PC {polycarbonate} into its starting monomers – terephthalic acid, ethylene glycol (from PET) and bisphenol-A (from PC). Not only that, the Thielemans group also has a simple and one-time isolation strategy to separate these monomers from the depolymerized mixture. Their groundbreaking research is recently published in ‘ChemSusChem’ [1].
‘KMH’ – as the Thielemans team named it – is basically a solution of 1.25 M potassium hydroxide (KOH) in methanol. In their previous study, KMH instantaneously depolymerized flakes of a PET bottle via hydrolysis under microwave heating (1 min at 120 °C) [2]. In general, KMH (KOH–in–methanol) should result in potassium methoxide and undergo methanolysis of PET to yield dimethyl terephthalate (DMT), however, to their surprise the reaction followed temperature-driven hydrolysis (rather than diffusion-driven methanolysis) via KOH attack on carbonyl to produce terephthalic acid (TPA). The instantaneous hydrolysis observed for PET motivated the Thielemans team to this quest whether ‘KMH' can be equally efficient for depolymerization of PC or simultaneous depolymerization of PET and PC in a mixed waste stream, since PC is also a polymer susceptible to hydrolysis and widely used in our daily-use applications (plastic containers, toothbrushes, CDs & DVDs, safety goggles, multilayer barrier films, car headlamp lenses and so on).
The Thielemans group at first conducted separate studies using PC and PET pellets, and found that KMH can depolymerize PC at similar conditions, however PET pellets needed higher energy (in terms of longer reaction time, high temperature and KMH/polymer ratio) than flakes because of the particle thickness difference. Their findings also suggested that PET needed much high activation energy (131 kJ/mol) for depolymerization than PC (68 kJ/mol), because of its semi-crystalline nature. According to them, the unannealed PET (amorphous) has a tendency to form crystallites during reaction {for example when the reaction temperature reaches above its glass transition temperature (Tg: ~78 °C)}, which makes it difficult for ester hydrolysis as compared to PC (Tg: ~149 °C).
Because of the differences in the activation energy, for simultaneous depolymerization of PET and PC, the Thielemans team first and foremost targeted on finding ideal reaction conditions (temperature and time) that can fully depolymerize these polymers in a mixed stream, using pellets of PET:PC composition of 1:1. They found that a complete conversion can be achieved by employing 20 mL of KMH solution per gram of polymer in a sealed vial under microwave heating at 120 °C for 2 minutes. The major concern for Prof. Thielemans while dealing with the depolymerization of a mixed plastic stream was the stability of the produced monomers (resulted from PET & PC) under the reaction conditions, for example the monomers tendency to undergo repolymerization during reaction to produce oligomers. However, the team found that the produced monomers stayed stable and intact throughout the whole process. The complete depolymerization was achieved within 2 minutes, however further continued heating of the reaction mixture (microwave irradiation) with a residence time of up to 5 minutes did not result in any repolymerization or secondary reactions. This was the ‘breakthrough moment’ in their work.
Their isolation strategy for separation of produced monomers from the depolymerization mixture entails few simple steps – (i) at first the pH of the reaction mixture was changed to 4 by the addition of hydrochloric acid solution (HCl) which precipitates TPA as white powder (in alkaline pH it was soluble as potassium terephthalate). TPA can be easily filtered and used further by simple washing with distilled water and followed by ethanol (97% purity). (ii) The filtrate was then neutralized with KMH solution and evaporated on a laboratory rotary evaporator, and as methanol solution evaporates bisphenol-A (BPA) starts to crystallize in the solution. The resulting BPA crystals can be used as it is after filtration by simple washing with water and drying (98% purity). (iii) Finally, ethylene glycol (EG) was distilled out (under low pressure) from the remaining solution.
To find out whether their strategy will work on real-life scenarios, the Thielemans team also conducted systematic studies using PC/PET of compositions ranging from 10/90 to 80/20. They found that higher amount of PET in the system reduced depolymerization efficiency of PET, which is in accordance with its high activation energy. Since it is difficult to predict the actual composition of PC and PET in a real-life mixed waste stream, their challenge was to find reaction conditions that will work on any compositions of PC and PET. They discovered that a slight modification in the reaction procedure, that is by changing KMH/polymer ratio from 10 mL/0.5 g to 15 mL/0.5 g resulted in a complete conversion for all the studied compositions, and the resulting constituent monomers can be separated without any changes in the isolation procedure. Thus, the Thielemans team is confident that “30 mL of KMH solution per gram of polymer waste” can depolymerize a mixed plastic waste stream containing random compositions of PET and PC (under microwave heating at 120 °C for 2 minutes). To further demonstrate the efficiency of their strategy on complex mixtures of PET and PC, they extended their studies using PET:PC blend (1:1) which was prepared by solution casting method, and found that ‘KMH’ was equally powerful for simultaneous and selective depolymerization of the blend.
The highlighted work is the first report on one-pot simultaneous depolymerization (alkaline hydrolysis) of mixed stream containing PET and PC into its starting monomers, which can be used in repolymerization of these polymers to produce virgin-quality material. The industrial approach is to separate PC and PET from mixed waste stream either via manual (identification by sight) or mechanical sorting (density separation – magnetic/flotation, Near infrared) and then subject to mechanical recycling processes (grinding–washing–melting–pelletization) – however, in this case the polymer undergoes chain scission after each recycling loop, which leads to deterioration in material properties (low molecular weight, poor thermal and mechanical properties) – thus limiting their (re)use to downgrade applications. For example, a decrease in ductility of PET from ~310% to ~218% and ~2.9% was observed after first and third recycling respectively – thus PET bottles are generally recycled for making carpets or textile fabrics than bottle remanufacturing (recycling rate: 50-77%), and further recycling of such products are difficult due to mixed material compositions [3,4]. Previous academic efforts towards recycling of mixed plastic waste stream – PET/PC or PET/polylactide (PLA) – were focused on selective isolation of PET under PC or PLA depolymerization conditions and then it is subjected to a separate and second-step recycling process – that is, glycolysis of PET to produce the starting precursor ‘bis(2-hydroxyethyl) terephthalate’ (BHET) [5,6]. For instance, methanolysis of PET/PLA mixed plastic waste stream in the presence of zinc acetate catalyst allowed selective depolymerization of PLA into a liquid compound ‘methyl lactate', and under the same conditions PET remained unreactive, thus PET plastic pieces can be separated easily by simple filtration. This reactivity difference (in transesterification) was mainly attributed to steric hindrance in PET, high amount of mobile amorphous regions in PLA, and strong binding ability of PLA with the catalyst. This work was aimed to resolve PLA contamination in PET bale, which is a major concern in the recycling industry due to sorting difficulties of these polyesters from one another using current technologies [5]. Similarly, in case of PC/PET waste stream, glycolysis {using diol such as 2-((allyloxy)methyl)-2-ethyl-1,3-propane diol} of PC at low temperature in the presence of a TBD:MSA (1:1) organocatalyst yielded a functionalized cyclic carbonate and allowed separation of unreacted PET [6]. Prof. Thielemans key innovation here is the idea of depolymerizing multi-polymers in a mixed plastic waste stream back to its constituent monomers in a one-step process, and the beauty of their work is that the produced monomers can be isolated without any side reactions! Their strategy also stands out from the above-mentioned works in terms of high energy efficiency, fast process and only requires inexpensive reagents. Moreover, because of the simplicity in the procedure, existing small scale industries of ‘chemical recycling of PET' such as Loop Industries, ReTerra, PerPETual, etc., [7] can easily adapt Prof. Thielemans methodology and make chemolysis of PET-PC waste stream an industrial success.
Recently, plastic industry’s preference for chemical recycling over mechanical recycling is growing. This is because when dealing with a virgin polymer the manufacturers have many options at hand to modify the properties and makes it suitable for the desired product production. However, when it comes to mechanically recycled plastics, the product manufacturing is highly dependent upon the quality of the recyclate (generally poor quality) [7]. This emerging interest is further evident from the planned investment of ‘European Plastics Manufacturers' in chemical recycling technology (depolymerization + pyrolysis + gasification), which is 7.2 billion euros by 2030 with a target of producing 3.4 million tonnes (Mt) of recycled plastics via chemical recycling in 2030 (presently < 0.1 Mt) [8].
Prof. Thielemans work can encourage policymakers to put right strategies in place for waste management, for example collection of these plastic wastes in ‘one bin’ which could enhance their recycling rate, but at the same time ‘virgin quality material’ can be produced by using the resulting monomers, thereby achieving a circular economy. Currently PC based plastic wastes are not recycled. PC comes under SPI (Society of the Plastic Industry) ‘others category' SPI code 7 along with polyamide, polyurethane, polyurea, poly(methyl methacrylate), high performance thermoplastics such as polyether sulfone and thermosets such as epoxy resin [3] and is collected in a mixed bale. For example, in the United States it is collected in a 3-7 bale consisting of PVC (SPI code 3), LDPE (SPI code 4), PP (SPI code 5), PS (SPI code 6), and others (SPI code 7) [7]. Thus, PC is destined for either incineration or landfilling.
Waste management system has a major impact on recycling rates, according to Plastics Europes 2022 report [8] separate waste collection enhanced recycling rate 13.5 times higher because it allowed pre-sorting of waste. For example, 14.5 Mt of post-consumer waste collected out of 29.5 Mt in 2020 via separate waste collection resulted in 65% recycling, 27% energy recovery and 8% landfill, whereas the rest 15 Mt via mixed waste collection resulted in 5% recycling, 57% energy recovery, and 35% landfill due to contamination. Thus, putting proper waste management system in place like “collection of PC and PET waste in one stream” that is labelled for chemical recycling can enhance recycling rates of these plastics – especially now that we have a perfect solution to recycle them together – ‘Prof. Thielemans KMH strategy’.
Currently Prof. Thielemans group is focused on up-scaling of this process. They also have a plan to expand the scope of this study by inclusion of other condensation polymers in a mixed waste stream that can be depolymerized using ‘KMH’ to enhace the value. Prof. Thielemans work indicates that we are soon going to be in a world where all plastic wastes (condensation type: PET, PC, PLA, Nylon, etc.) are collected in ‘one bin’ and chemically recycled together in ‘one-pot’ using a ‘single catalytic procedure' to produce corresponding starting monomers. This may seem highly ambitious right now, however, with continuous innovation and efforts of researchers like ‘Prof. Thielemans and his team' we can be there soon, their work is ‘a proof of concept' that it is within our reach!
References:
[1]. Arias, J. J. R.; Barnard, E.; Thielemans, W. Ultra fast simultaneous and selective depolymerization of heterogeneous streams of polyethylene terephthalate and polycarbonate: Towards industrially feasible chemical recycling. ChemSusChem 2022, e202200625. (Featured Article!)
[2]. Arias, J. J. R.; Thielemans, W. Instantaneous hydrolysis of PET bottles: An efficient pathway for the chemical recycling of condensation polymers. Green Chem. 2021, 23, 9945-9946.
[3]. Rahimi, A.; García, J. M. Chemical recycling of waste plastics for new materials production. Nat. Rev. Chem. 2017, 1, 0046.
[4]. Sherwood, J. Closed-loop recycling of polymers using solvents. Johnson Matthey Technol. Rev. 2020, 64, 4-15.
[5]. Sánchez, A. C.; Collinson, S. R. The selective recycling of mixed plastic waste of polylactic acid and polyethylene terephthalate by control of process conditions. Eur. Polym. J. 2011, 47, 1970-1976.
[6]. Jehanno, C.; Demarteau, J.; D.Mantione, D.; Arno, M. C.; Ruipérez, F.; J.L. Hedrick, J. L.; A.P.Dove, A. P.; Sardon, H. Selective chemical upcycling of mixed plastics guided by a thermally stable organocatalyst. Angew. Chem. Int. Ed. 2021, 60, 67106717; Angew. Chem. 2021, 133, 6784-6791.
[7]. Billiet, S.; Trenor, S. R. 100th Anniversary of macromolecular science viewpoint: Needs for plastics packaging circularity. ACS Macro Lett. 2020, 9, 1376-1390.
[8]. Plastics Europe 2022 Report – The circular economy for plastics: A European overview. www.plasticseurope.org.