Nel mezzo del cammin di nostra vita / mi ritrovai per una selva di plastico...
Dante Alighieri: Beginning of La Divina Commedia , slightly updated.
Illustration: Gustave Doré with help from Andreas Møllebjerg.
The overview
Among the strongest and most durable plastics exist a group called thermosets, used for wind turbines, airplanes, and insulation foams. Common for these plastics are that they cannot be melted, and thus, not recycled through conventional means. However, unlike regular plastics, most thermosets consist of chemical linkages reminiscent of nature’s own linkages as found in food sources such as sugar or protein. Using nature’s own approach of degradation, En’Zync will attempt to develop enzymes, found in bacteria and fungi, which are able to break down thermosets into molecular units in a controlled manner. These molecular units, called monomers, can be used in the production of new thermosets and will offer an unprecedented method for thermoset recycling. This ambitious goal is made possible only through interdisciplinary collaboration between material specialists, molecular biologists, and computational experts.
Enzyme activity against thermosets has been reported previously [1,2]. However, the structural complexity and robustness of thermosets combined with limited understanding of both enzyme-polymer substrate interactions and mechanisms of thermoset deconstruction pose significant fundamental challenges for enzymatic thermoset recycling.
Daniel Otzen and the EnZync team have received DKK 57,003,755 for the project.
Some more details
En´Zync will focus on the group of plastics known as thermosets. These materials make up approx. 15% of all plastic production (>50 mio tons). They are high-performance and very difficult-to-replace materials with poor recycling rates [3]. Thermoset materials find numerous applications in products ranging from shoes and mattresses over medical devices to wind turbine blades and the aviation and space industries. Thermosets display exceptional strength, durability, and 3D stability thanks to a dense chemical crosslinking network within the polymer matrix. Crosslinking can be tuned to match exact requirement and performance specifications. (Their non-crosslinked analogues, i.e. thermoplastic materials, are less durable and softer at higher temperatures, but also more readily recyclable since they can be melted, dissolved and even depolymerized for recycling). Combined, these features endow thermosets with highly desirable unique properties but at the same time make them very hard to recycle. Consequently, growth in consumption of thermosets such as polyurethane and epoxy outstrips current recycling rates, resulting in accumulating thermoset waste [4].
Recycling potential: Today’s recycling of thermosets relies on a narrow set of technologies, mostly providing products of low quality or value. Research initiatives aimed at recycling thermosets into high value products are generally in their infancy. We propose to solve the challenge of enzymatic recycling of thermosets. We expect that this will lead to a tipping point. If we can crack the most difficult plastic recycling challenge, we will have broken ground for other plastics to follow. En’Zync will convert thermosets to precursors for new polymers, both for thermoset production and “drop-in” chemicals for existing and emerging chemical industries. The precursors can enter the production line as building blocks more efficiently than from fossil-based crude oil, i.e. several production steps later.
Strategy and risk mitigation: Thermosets’ high content of heteroatoms and labile chemical bonds are potential targets for enzymatic cleavage. En’Zync will base enzymatic recycling of thermosets on current knowledge about enzymes able to degrade non-crosslinked versions of polyurethane (PUR) [2, 5] to establish a novel library of thermoset-targeting enzymes. To address crosslinking challenges, increase options and thus minimize risks, En’Zync will apply a four-stage “funnel” method in which we initially screen broadly for enzymes based on readily accessible substrates and stepwise increase operational criteria. Thus, we will start with a large number of enzyme candidates which will be whittled down based on ever more stringent performance demands. Ongoing work at low TRL-levels within chemical recycling of thermosets [6] may also inspire to chemical-based pretreatment of thermosets prior to enzymatic steps.
En´Zync will tackle the thermosets polyurethane (PUR), unsaturated polyester resin (UPR) and amino-cured epoxy. PUR and UPR are expected to be degraded by hydrolases [2, 6] while approaches to the latter, almost unexplored system, will be sought with inspiration from recent research [7,8] on lignin degradation by oxidoreductases.
The fundamental knowledge accumulated in En’Zync within: 1) structural descriptions of enzymes and their Michaelis complexes, 2) mechanistic insight, 3) (physical and chemical properties of) plastic surfaces, and 4) thermoset pretreatment processes hold the potential to outcompete classical chemical recycling methods.
By combining the current advances within enzymatic conversion of natural polymers like cellulose, and plastics like polyester (polyethylene terephthalate, PET) [9-11], this target is also realistic.
EnZync is built up around 3 work packages:
Work Package 1 – Materials: Substrates and Enzymes
The aim is to develop model substrates (DTI) to screen for enzymes in bacteria (iNANO) and fungi (DTU), followed by detailed computational analyses of how these enzymes interact with plastics (Porto).
Work Package 2 – Structure, kinetics and mechanism
Kinetic analyses of the enzymes and substrates from WP1 (DTU and iNANO) will deliver parameters that ultimately single out the rates of different steps, and hence elucidate rate-limiting steps in the catalytic cycle. We will use X-ray crystallography to study structures of substrate-free enzymes and complexes with non-cleavable substrate-analogues and TS analogues from WP1 (DTU and AU). Building on these structural results, the enzyme reaction mechanisms will be determined computationally (Porto) and potentially novel analogs for TS and intermediates will be identified, synthesized (DTI) and verified experimentally with the available crystals (DTU).
Work Package 3 – Enzyme and material dynamics
The substrates from WP1 will be used to link changes in physical and chemical properties of the substrate with accessibility to enzymatic conversion (iNANO). Enzymatic changes to material properties (shape, size, film thickness) and release of larger fragments from these NPPs will be characterized using a broad range of in-house techniques.
Strategic outlook
En’Zync will provide a fundamental understanding of the key interactions of enzymes with thermosets relevant for other polymeric and insoluble substrates. This will lead to completely new tools towards recycling of both man-made and natural materials. For thermosets in particular, enzymatic recycling will support a circular and sustainable future for the global plastics industry. Specific break-down products of interest are methylene dianiline and toluene dianiline (from PUR), bisphenol A (from epoxy) and phthalic acid (from UPR), currently only available as petro-based chemicals without any current scalable biobased routes to these precursors.
Extrapolation of available CRISPR gene editing tools and directed evolution will help upgrade En’Zync’s tools to produce more efficient and scalable solutions. En’Zync will also offer solutions for selective microplastic removal from wastewater streams and generation of biofuels and chemicals from biomasses like lignin. Enzymes from En’Zync may be polymer-embedded in future applications to fine-tune degradation towards a more processive mechanism of degradation [12]. Finally, En’Zync holds the potential to reveal completely new enzymes and mechanistic pathways that can directly impact our future chemical infrastructure.
REFERENCES
[1] Eliaz, N. et al. (2018) Microbial Degradation of Epoxy, Materials, 11, 2123
[2] Magnin, A. et al. (2021) Breakthrough in polyurethane bio-recycling: An efficient laccase-mediated system for degradation of different types of polyurethanes, Waste Manage., 132, 23-30
[3] Source: EuRIC AISBL – Recycling: Bridging Circular Economy & Climate Policy
[4] https://www.recycling-magazine.com/2020/07/31/chemical-recycling-of-polyurethane-waste/
[5] Gamerith, C. et al. (2016). Improving enzymatic polyurethane hydrolysis by tuning enzyme sorption. Polym. Degrad., 132, 69-77
[6] Innovation Foundation projects: RePURpose, CETEC and Dreamwind
[7] Chen, C.-C., et al. (2020), Enzymatic degradation of plant biomass and synthetic polymers, Nat. Rev. Chem., 4, 114-126
[8] Datta, R. et al. (2017), Enzymatic Degradation of Lignin in Soil: A Review, Sustainability, 9, 1163
[9] Tournier, V., et al. (2020) An engineered PET depolymerase to break down and recycle plastic bottles, Nature, 580, 216-219
[10] Bååth, J. A., Borch, K., Jensen, K., Brask, J., & Westh, P. (2021). Comparative biochemistry of four polyester (PET) hydrolases. ChemBioChem, 22(9), 1627-1637.
[11] Badino, S. F., Bååth, J. A., Borch, K., Jensen, K., & Westh, P. (2021). Adsorption of enzymes with hydrolytic activity on polyethylene terephthalate. Enzyme and Microbial Technology, 152, 109937.
[12] Zanni, R., Galvez-Llompart, M., Galvez, J., & Garcia-Domenech, R. (2014). QSAR multi-target in drug discovery: a review. Current Computer-Aided Drug Design, 10(2), 129-136.
