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Sustainable Design of Structural and Functional Polymers for a Circular Economy
Abstract
To achieve a sustainable circular economy, polymers need to start transitioning to recycled and biobased feedstock and accomplish CO2 emission neutrality. This is not only true for structural polymers, such as in packaging or engineering applications, but also for functional polymers in liquid formulations, such as adhesives, lubricants, thickeners or dispersants. At their end of life, polymers need to be either collected and recycled via a technical pathway, or be biodegradable if they are not collectable. Advances in polymer chemistry and applications, aided by computational material science, open the way to addressing these issues comprehensively by designing for recyclability and biodegradability. This review explores how scientific advances, together with emerging regulatory frameworks, societal expectations and economic boundary conditions, paint pathways for the transformation towards a circular economy of polymers.
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Carbon dioxide is inexpensive and abundant, and its prevalence as waste makes it attractive as a sustainable chemical feedstock. Although there are examples of copolymerizations of CO2 with high-energy monomers, the direct copolymerization of CO2 with olefins has not been reported. Here an alternative route to functionalizable, recyclable polyesters derived from CO2, butadiene and hydrogen via an intermediary lactone, 3-ethyl-6-vinyltetrahydro-2H-pyran-2-one, is described. Catalytic ring-opening polymerization of the lactone by 1,5,7-triazabicyclo[4.4.0]dec-5-ene yields polyesters with molar masses up to 13.6 kg mol⁻¹ and pendent vinyl side chains that can undergo post-polymerization functionalization. The polymer has a low ceiling temperature of 138 °C, allowing for facile chemical recycling, and is inherently biodegradable under aerobic aqueous conditions (OECD-301B protocol). These results show that a well-defined polyester can be derived from CO2, olefins and hydrogen, expanding access to new polymer feedstocks that were once considered unfeasible.
Scientific analysis and media coverage of rampant plastic pollution has taken a toll on the material's reputation in recent years, fueling talk of a “plastic crisis”. Brand owners have made ambitious pledges to overcome this crisis—but can voluntary commitments turn the tide? In this paper, we analyze the current flow of polyethylene terephthalate (PET) from production to recycling in the European Union (EU). We show that the pledged volume for recycled PET (rPET) to be used in the EU in 2025 amounts to 2.066 m tons, requiring the annual recycling growth rate to double in the next years compared to 2014–2018. Our results indicate that even widespread adoption of deposit return systems for bottles will not suffice, especially when increasing demand from other industries drives the price above the packaging producers’ willingness to pay. To realize the pledges, substantial investments and a regulatory framework for the targeted and sensible use of PET recyclate are necessary.
Following the circular economy hierarchy of reduce and reuse, recycling is the third layer to close the loops for materials and decouple their value from consumption. Polymeric materials (“plastics”) are in principle well suited for recycling, as they can be reprocessed with relatively low energy input as a material, cleaved back into their monomers or converted back to feedstock. Today, these approaches still fall short in quantitatively diverting waste towards reuse. This perspective describes the industry challenges for recycling back into high value applications of polymers, and the portfolio of existing and emerging technology solutions. Sustainable design of products and polymers, recycling technologies, appropriate business models, and enabling technologies converge at the frontier of plastics recycling, for a transformation of the industry towards circularity and greenhouse gas emission neutrality. Plastics today face the dual challenge of a global waste issue and lifecycle greenhouse gas emission contributions. Recycling both diverts plastics from waste and supports transitioning the whole industry to climate neutrality. The frontier of plastics recycling, from an industry perspective, is reconciling these goals with uncompromised material quality, to replace virgin plastics in their applications at scale.
Bioplastics — typically plastics manufactured from bio-based polymers — stand to contribute to more sustainable commercial plastic life cycles as part of a circular economy, in which virgin polymers are made from renewable or recycled raw materials. Carbon-neutral energy is used for production and products are reused or recycled at their end of life (EOL). In this Review, we assess the advantages and challenges of bioplastics in transitioning towards a circular economy. Compared with fossil-based plastics, bio-based plastics can have a lower carbon footprint and exhibit advantageous materials properties; moreover, they can be compatible with existing recycling streams and some offer biodegradation as an EOL scenario if performed in controlled or predictable environments. However, these benefits can have trade-offs, including negative agricultural impacts, competition with food production, unclear EOL management and higher costs. Emerging chemical and biological methods can enable the ‘upcycling’ of increasing volumes of heterogeneous plastic and bioplastic waste into higher-quality materials. To guide converters and consumers in their purchasing choices, existing (bio)plastic identification standards and life cycle assessment guidelines need revision and homogenization. Furthermore, clear regulation and financial incentives remain essential to scale from niche polymers to large-scale bioplastic market applications with truly sustainable impact. Plastics support modern life but are also associated with environmental pollution. This Review discusses technologies for the production and recycling of bioplastics as part of a more sustainable and circular economy.
Structural adhesives are commonly used to join dissimilar materials and are of particular interest in complex technological devices used in automotive, telecommunications, photonic devices, aerospace and sustainable power production and storage. Their strength, ease of application and stability are of significant help in the manufacture and service of the device, however, they can present significant issues at end of life, particularly when technology critical metals need to be recovered. In this paper, we highlight some of the issues, and discuss an alternative approach: the use of adhesives that can be debonded upon application of an external stimulus. The aim of this critical review is to consider the polymer systems which have been investigated and suggest some alternative strategies. It also aims to demonstrate the different stimuli that can be used to debond and comment on the applicability of these with a number of case studies. Finally, the service conditions and process economics are considered together with a discussion of the Green benefits of this type of methodology.
Additive manufacturing has become one of the forefront technologies in fabrication, enabling products impossible to manufacture before. Although many materials exist for additive manufacturing, most suffer from performance trade-offs. Current materials are designed with inefficient human-driven intuition-based methods, leaving them short of optimal solutions. We propose a machine learning approach to accelerating the discovery of additive manufacturing materials with optimal trade-offs in mechanical performance. A multiobjective optimization algorithm automatically guides the experimental design by proposing how to mix primary formulations to create better performing materials. The algorithm is coupled with a semiautonomous fabrication platform to substantially reduce the number of performed experiments and overall time to solution. Without prior knowledge of the primary formulations, the proposed methodology autonomously uncovers 12 optimal formulations and enlarges the discovered performance space 288 times after only 30 experimental iterations. This methodology could be easily generalized to other material design systems and enable automated discovery.
Plastic pollution accumulating in an area of the environment is considered “poorly reversible” if natural mineralization processes occurring there are slow and engineered remediation solutions are improbable. Should negative outcomes in these areas arise as a consequence of plastic pollution, they will be practically irreversible. Potential impacts from poorly reversible plastic pollution include changes to carbon and nutrient cycles; habitat changes within soils, sediments, and aquatic ecosystems; co-occurring biological impacts on endangered or keystone species; ecotoxicity; and related societal impacts. The rational response to the global threat posed by accumulating and poorly reversible plastic pollution is to rapidly reduce plastic emissions through reductions in consumption of virgin plastic materials, along with internationally coordinated strategies for waste management.
Our planet is flooded with plastics and the need for sustainable recycling strategies of polymers has become increasingly urgent. Enzyme‐based hydrolysis of post‐consumer plastic is an emerging strategy for closed‐loop recycling of polyethylene terephthalate (PET). The polyester hydrolase PHL7 isolated from a compost metagenome completely hydrolyzed amorphous PET films, releasing 91 mg of terephthalic acid per hour and mg of enzyme. Degradation rates of the PET film of 6.8 µm h ‐1 were monitored by vertical scanning interferometry. Structural analysis indicated the importance of leucine at position 210 for the extraordinarily high PET‐hydrolyzing activity of PHL7. Within 24 h, 0.6 mg enzyme g PET ‐1 completely degraded post‐consumer thermoform PET packaging in an aqueous buffer at 70°C without any energy‐intensive pretreatments. Terephthalic acid recovered from the enzymatic hydrolysate was used to synthesize virgin PET, demonstrating the potential of polyester hydrolases as catalysts in sustainable PET recycling processes with a low carbon footprint.
Pollution caused by plastic materials has a great impact on the environment. The biodegradation process is a good treatment solution for common polymers and biodegradation susceptible ones. The present work introduces new insight into the biodegradation process from a mathematical point of view, as it envisions a new empirical model for this complex process. The model is an exponential function with two different time constants and a time delay, which follows the weight loss profile of the polymer during the biodegradation process. Moreover, this function can be generated as the output variable of a dynamic exogenous system described through state equations. The newly developed models displayed a good fit against the experimental data, as shown by statistical indicators. In addition, the new empirical model was compared to kinetics models available in the literature and the correlation coefficients were closest to 1 for the new empirical model in all discussed cases. The mathematical operations were performed in the MATLAB Simulink environment.
We reviewed the current state of the art of poly(ethylene terephthalate) (PET) chemolysis used in the chemical recycling of PET.
Mechanical recycling of polymers downgrades them such that they are unusable after a few cycles. Alternatively, chemical recycling to monomer offers a means to recover the embodied chemical feedstocks for remanufacturing. However, only a limited number of commodity polymers may be chemically recycled, and the processes remain resource intensive. We use systems analysis to quantify the costs and life-cycle carbon footprints of virgin and chemically recycled polydiketoenamines (PDKs), next-generation polymers that depolymerize under ambient conditions in strong acid. The cost of producing virgin PDK resin using unoptimized processes is ~30-fold higher than recycling them, and the cost of recycled PDK resin ($1.5 kg ⁻¹ ) is on par with PET and HDPE, and below that of polyurethanes. Virgin resin production is carbon intensive (86 kg CO 2 e kg ⁻¹ ), while chemical recycling emits only 2 kg CO 2 e kg ⁻¹ . This cost and emissions disparity provides a strong incentive to recover and recycle future polymer waste.
Due to the depletion of fossil fuels, higher oil prices, and greenhouse gas emissions, the scientific community has been conducting an ongoing search for viable renewable alternatives to petroleum-based products, with the anticipation of increased adaptation in the coming years. New academic and industrial developments have encouraged the utilization of renewable resources for the development of ecofriendly and sustainable materials, and here, we focus on those advances that impact polyurethane (PU) materials. Vegetable oils, algae oils, and polysaccharides are included among the major renewable resources that have supported the development of sustainable PU precursors to date. Renewable feedstocks such as algae have the benefit of requiring only sunshine, carbon dioxide, and trace minerals to generate a sustainable biomass source, offering an improved carbon footprint to lessen environmental impacts. Incorporation of renewable content into commercially viable polymer materials, particularly PUs, has increasing and realistic potential. Biobased polyols can currently be purchased, and the potential to expand into new monomers offers exciting possibilities for new product development. This Review highlights the latest developments in PU chemistry from renewable raw materials, as well as the various biological precursors being employed in the synthesis of thermoset and thermoplastic PUs. We also provide an overview of literature reports that focus on biobased polyols and isocyanates, the two major precursors to PUs.
Methods to treat kinetic data for the biodegradation of different plastic materials are comparatively discussed. Different samples of commercial formulates were tested for aerobic biodegradation in compost, following the standard ISO14855. Starting from the raw data, the conversion vs. time entries were elaborated using relatively simple kinetic models, such as integrated kinetic equations of zero, first and second order, through the Wilkinson model, or using a Michaelis Menten approach, which was previously reported in the literature. The results were validated against the experimental data and allowed for computation of the time for half degradation of the substrate and, by extrapolation, estimation of the final biodegradation time for all the materials tested. In particular, the Michaelis Menten approach fails in describing all the reported kinetics as well the zeroth- and second-order kinetics. The biodegradation pattern of one sample was described in detail through a simple first-order kinetics. By contrast, other substrates followed a more complex pathway, with rapid partial degradation, subsequently slowing. Therefore, a more conservative kinetic interpolation was needed. The different possible patterns are discussed, with a guide to the application of the most suitable kinetic model.
Plastics are key components of almost any technology today. Although their production consumes substantial feedstock resources, plastics are largely disposed of after their service life. In terms of a circular economy1–8, reuse of post-consumer sorted polymers (‘mechanical recycling’) is hampered by deterioration of materials performance9,10. Chemical recycling1,11 via depolymerization to monomer offers an alternative that retains high-performance properties. The linear hydrocarbon chains of polyethylene¹² enable crystalline packing and provide excellent materials properties¹³. Their inert nature hinders chemical recycling, however, necessitating temperatures above 600 degrees Celsius and recovering ethylene with a yield of less than 10 per cent3,11,14. Here we show that renewable polycarbonates and polyesters with a low density of in-chain functional groups as break points in a polyethylene chain can be recycled chemically by solvolysis with a recovery rate of more than 96 per cent. At the same time, the break points do not disturb the crystalline polyethylene structure, and the desirable materials properties (like those of high-density polyethylene) are fully retained upon recycling. Processing can be performed by common injection moulding and the materials are well-suited for additive manufacturing, such as 3D printing. Selective removal from model polymer waste streams is possible. In our approach, the initial polymers result from polycondensation of long-chain building blocks, derived by state-of-the-art catalytic schemes from common plant oil feedstocks, or microalgae oils¹⁵. This allows closed-loop recycling of polyethylene-like materials.
A large portion of plastic produced each year is used to make single-use packaging and other short-lived consumer products that are discarded quickly, creating significant amounts of waste. It is important that such waste be managed appropriately in line with circular-economy principles. One option for managing plastic waste is chemical recycling via pyrolysis, which can convert it back into chemical feedstock that can then be used to manufacture virgin-quality polymers. However, given that this is an emerging technology not yet used widely in practice, it is not clear if pyrolysis of waste plastics is sustainable on a life cycle basis and how it compares to other plastics waste management options as well as to the production of virgin plastics. Therefore, this study uses life cycle assessment (LCA) to compare the environmental impacts of chemical recycling of mixed plastic waste (MPW) via pyrolysis with the established waste management alternatives: mechanical recycling and energy recovery. Three LCA studies have been carried out under three perspectives: waste, product and a combination of the two. To ensure robust comparisons, the impacts have been estimated using two impact assessment methods: Environmental footprint and ReCiPe. The results suggest that chemical recycling via pyrolysis has a 50% lower climate change impact and life cycle energy use than the energy recovery option. The climate change impact and energy use of pyrolysis and mechanical recycling of MPW are similar if the quality of the recyclate is taken into account. Furthermore, MPW recycled by pyrolysis has a significantly lower climate change impact (-0.45 vs 1.89 t CO2 eq./t plastic) than the equivalent made from virgin fossil resources. However, pyrolysis has significantly higher other impacts than mechanical recycling, energy recovery and production of virgin plastics. Sensitivity analyses show that some assumptions have notable effects on the results, including the assumed geographical region and its energy mix, carbon conversion efficiency of pyrolysis and recyclate quality. These results will be of interest to the chemical, plastics and waste industries, as well as to policy makers.
Although Staudinger realized makromoleküles had enormous potential, he likely did not anticipate the consequences of their universal adoption. With 6.3 billion metric tons of plastic waste now contaminating our land, water, and air, we are facing an environmental and public health crisis. Synthetic polymer chemists can help create a more sustainable future, but are we on the right path to do so? Herein, a comprehensive literature survey reveals that there has been an increased focus on “sustainable polymers” in recent years, but most papers focus on biomass-derived feedstocks. In contrast, there is less focus on polymer end-of-life fates. Moving forward, we suggest an increased emphasis on chemical recycling, which sees value in plastic waste and promotes a closed-loop plastic economy. To help keep us on the path to sustainability, the synthetic polymer community should routinely seek the systems perspective offered by life cycle assessment.
The degradation of synthetic polymers by marine microorganisms is not as well understood as the degradation of plastics in soil and compost. Here, we use metagenomics, metatran-scriptomics and metaproteomics to study the biodegradation of an aromatic-aliphatic copolyester blend by a marine microbial enrichment culture. The culture can use the plastic film as the sole carbon source, reaching maximum conversion to CO 2 and biomass in around 15 days. The consortium degrades the polymer synergistically, with different degradation steps being performed by different community members. We identify six putative PETase-like enzymes and four putative MHETase-like enzymes, with the potential to degrade aliphatic-aromatic polymers and their degradation products, respectively. Our results show that, although there are multiple genes and organisms with the potential to perform each degradation step, only a few are active during biodegradation.
The current global plastics economy is highly linear, with the exceptional performance and low carbon footprint of polymeric materials at odds with dramatic increases in plastic waste. Transitioning to a circular economy that retains plastic in its highest value condition is essential to reduce environmental impacts, promoting reduction, reuse, and recycling. Mechanical recycling is an essential tool in an environmentally and economically sustainable economy of plastics, but current mechanical recycling processes are limited by cost, degradation of mechanical properties, and inconsistent quality products. This review covers the current methods and challenges for the mechanical recycling of the five main packaging plastics: poly(ethylene terephthalate), polyethylene, polypropylene, polystyrene, and poly(vinyl chloride) through the lens of a circular economy. Their reprocessing induced degradation mechanisms are introduced and strategies to improve their recycling are discussed. Additionally, this review briefly examines approaches to improve polymer blending in mixed plastic waste streams and applications of lower quality recyclate.
The presence of plastic in the environment has sparked discussion amongst scientists, regulators and the general public as to how industrialization and consumerism is shaping our world. Here we discuss restrictions on the intentional use of primary microplastics: small solid polymer particles in applications ranging from agriculture to cosmetics. Microplastic hazards are uncertain, and actions are not similarly prioritized by all actors. In some instances, replacement is technically simple and easily justified, but in others substitutions may come with more uncertainty, performance questions and costs. Scientific impact assessment of primary microplastics compared to their alternatives relies on a number of factors, such as microplastic harm, existence of replacement materials and the quality, cost and hazards of alternative materials. Regulations need a precise focus and must be enforceable by these measurements. Policymakers must carefully evaluate under which contexts incentives to replace certain microplastics can stimulate innovation of new, more competitive and environmentally conscious materials.
PurposePlastic pollution in marine environments is a severe problem in the world due to misuse and mismanagement of the materials. Microplastics are a specific form of pollutants in this context and its handling is very difficult due to its very small size of particulates. Currently, the impacts of marine plastic debris are not considered. However, this type of particulates can be assessed like other emissions with the systematic and quantifiable approach in life cycle assessment (LCA). It was our goal to find and test first methodological approaches for including impacts of marine litter of microplastics to LCA.Methods
The Medellin Declaration on Marine Litter in Life Cycle Assessment and Management raised this issue in 2017 and called for LCA to address the challenges of marine litter. The present research paper focuses on how to integrate plastic debris impacts with focus on microplastics into LCA and gives a suggestion for an assessment approach. Based on a literature review, we considered various impacts to the marine environment of microplastics linked with their kinetics of the fragmentation and degradation. Subsequently, we developed a characterization LCA model for microplastics in the marine environment. We addressed therein the fate of microplastics and their specific eco-toxic effects to different organisms. We compared the impacts of different types of polymers as well and showed how these can be integrated in an assessment using the new characterization model.Results and discussionThe assessment of marine litter impacts in LCA was strongly dependent on the number of microplastic particles produced from the original litter over time. These impacts were derived from measurements of the number of microparticles, their densities in the marine environment and their impacts to different organisms. The new characterization model includes the relationship between fragmentation and degradation and can be used for impact assessments within LCA.Conclusion
The question where we did not find a finally satisfying solution is the issue of the length of the time horizon of the assessment or the discounting. Those are regarded as subjective and are encountered with sensitivity or scenario analysis. Results from different time horizons can be aggregated to one figure or can be compared separately. Further investigations should be taken for a better understanding of this issue and for concrete solutions because their influence on the results of life cycle assessments is often fundamental.
Thermosets—polymeric materials that adopt a permanent shape upon curing—have a key role in the modern plastics and rubber industries, comprising about 20 per cent of polymeric materials manufactured today, with a worldwide annual production of about 65 million tons1,2. The high density of crosslinks that gives thermosets their useful properties (for example, chemical and thermal resistance and tensile strength) comes at the expense of degradability and recyclability. Here, using the industrial thermoset polydicyclopentadiene as a model system, we show that when a small number of cleavable bonds are selectively installed within the strands of thermosets using a comonomer additive in otherwise traditional curing workflows, the resulting materials can display the same mechanical properties as the native material, but they can undergo triggered, mild degradation to yield soluble, recyclable products of controlled size and functionality. By contrast, installation of cleavable crosslinks, even at much higher loadings, does not produce degradable materials. These findings reveal that optimization of the cleavable bond location can be used as a design principle to achieve controlled thermoset degradation. Moreover, we introduce a class of recyclable thermosets poised for rapid deployment.
The goal of this research was to determine if we could develop commercially-relevant polyurethane products that can biodegrade in the natural environment. We developed polyester polyols from algae oils and formulated those into polyurethane foams that meet the standards for footwear, while maintaining a chemical structure that would allow them to biodegrade. These foams were incubated in compost and soil, and lost 30% and 71% mass, and 41% and 71.5% compression force, respectively, after 12 weeks. Several bacteria and fungi grew abundantly on the polyurethane and we were able to isolate microorganisms from compost and soil capable of growth with polyurethane as the sole carbon source. Scanning Electron Microscopy and Imaging Mass Spectrometry were used to visualize biodegradation activity. Enzymatic hydrolysis confirmed that breakdown products were reproductions of the original monomers. These results demonstrate that it is possible to create polyurethane products that have an end-of-life biodegradation option.
Der europäische Grüne Deal und die EU‐Chemikalienstrategie erfordern neue Prioritäten. Forschung darf dabei in ihrer gedanklichen Freiheit nicht eingeschränkt werden. Politik, Hochschulen, Studierende und Industrie müssen gemeinsam den Blick auf Nutzen und Wirkungen der Chemie schärfen.
There is an urgent need for new technologies to enable circularity for synthetic polymers, spurred by the accumulation of waste plastics in landfills and the environment and the contributions of plastics manufacturing to climate change. Chemical recycling is a promising means to convert waste plastics into molecular intermediates that can be remanufactured into new products. Given the growing interest in the development of new chemical recycling approaches, it is critical to evaluate the economics, energy use, greenhouse gas emissions, and other life cycle inventory metrics for emerging processes, relative to the incumbent, linear manufacturing practices employed today. Here we offer specific definitions for classes of chemical recycling and upcycling and describe general process concepts for the chemical recycling of mixed plastics waste. We present a framework for techno-economic analysis and life cycle assessment for both closed- and open-loop chemical recycling. Rigorous application of these process analysis tools will be required to enable impactful solutions for the plastics waste problem.
Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
The design and synthesis of chemically recyclable polymers, which can be reutilized as their starting monomers or new value-added chemicals, has provided a practical approach to address the end-of-use problem of polymer materials and a possible closed-loop method for polymer material usage. More and more attention has been paid to chemically recyclable polymers, which exhibit an increasing and prominent role in sustainable development. Nowadays, chemically recyclable polymers including polyesters, polythioesters, polycarbonates, polyacetals, polyamides, and so on have made significant achievements. Consequently, this minireview summarizes the examples for achieving the polymerization–depolymerization cycle to access chemically recyclable polymers, which are categorized into seven parts based on monomers.
The development of more sustainable material solutions is one of the key challenges today. New materials are subject to an overall risk assessment of which their impact on the environment is an important aspect. The biodegradability of a material may support a positive risk assessment. Water‐soluble and water‐dispersible polymers are widely used in daily life in a very broad range of applications. In anticipation of a further tightening of regulatory requirements and as a consequence of the introduction of sustainability targets in industry, in the next decade or so, industry targets to replace the currently used and predominantly non‐biodegradable high value water‐soluble and water‐dispersible polymers by biodegradable alternatives, wherever sustainably possible. This review provides an overview of the currently used water‐soluble and water‐dispersible polymers in detergent formulations for home care, in water treatment and in crop protection. These polymers have been engineered to near perfect performance at minimum costs. We address the challenges faced when introducing biodegradable alternatives in existing product value chains, and we highlight opportunities to further improve and fine‐tune the biodegradation of these materials. This overview is intended to contribute to the development of novel biodegradable water‐soluble and water‐dispersible polymers, both at academia and industry.
Zusammenfassung
de Die Windenergie hat sich in den letzten Jahrzehnten zu einem wichtigen Pfeiler der Stromproduktion entwickelt. Ein modernes Rotorblatt besteht aus Kompositmaterialien, die mit Hilfe von unterschiedlichen Verarbeitungstechnologien aufgebaut werden. Als Rohstoffe kommen Glas- und Carbonfasern, Tränkharze, Kernmaterialien und Strukturklebstoffe zum Einsatz, anschließend werden mehrere Coatingsschichten aufgebracht. Kleinere Reparaturen der Rotorblätter können mit Hilfe von schnell verarbeitbaren Systemen vor Ort durchgeführt werden. Nach Ende der Lebensdauer sind Entsorgungs- und zunehmend Recyclingkonzepte in Richtung einer nachhaltigen Kreislaufwirtschaft erforderlich.
Summary
en Wind energy has become an important pillar of electricity production in recent decades. A modern rotor blade consists of composite materials that are built up using different processing technologies. Glass and carbon fibers, impregnating resins, core materials and structural adhesives are used as raw materials, followed by application of several coating layers. Minor repairs of a rotor blade can be carried out on site using fast-curing systems. At the end of their service life, increasingly recycling concepts are required to establish a sustainable circular economy.
Mismanaged plastic waste is a major environmental concern, especially in countries of the Global South. Municipal solid waste management can not only alleviate environmental problems but also create jobs and promote local economic growth. However, providing appropriate waste management services is costly. The question is to what extent waste management policies that have proven to be successful in other geographies can help solve the challenge in developing countries. Specifically, the economics and financial flows along the value chain need to be known. In this paper, we shed light on these questions by presenting a novel, model-based method to elicit and assess the cost structure of the recycling sector in developing countries. We exemplify our method with plastics waste management in the Greater Accra Region of Ghana—an area particularly challenged by plastic waste. For this purpose, we surveyed over 80 participants of the waste management value chain and combined the data with insights from expert interviews and workshops. Based on this data, we built a bottom-up model with 67 parameters, reflecting all cost positions in the waste management value chain. We found that street waste pickers are the poorest and most vulnerable, earning only a fraction of the already low minimum wage. Middlemen and aggregators, while often being criticized for their earnings, also provide social-security-like services to waste pickers. In addition, formal and informal recyclers differ not only in earnings and size but also in recyclate quality where the informal sector provides higher quality.
Addition of keto groups to polyethylene helps it degrade while maintaining its properties
Polyethylene with a nickel’s worth of CO
The biggest problem with polyethylene, the most abundantly manufactured plastic, is that it doesn’t break down easily once it is discarded. Chemists have long sought to introduce small quantities of carbon monoxide (CO) into polyethylene chains to promote photodegradation, but too much CO tends to jump in and spoil the plastic’s other properties. Baur et al . report that nickel catalysts coordinated by bulky phosphinophenolate ligands can catalyze ethylene polymerization with approximately 1% CO incorporation, preserving tensile strength while promoting degradation under ultraviolet exposure (see the Perspective by Sobkowicz). —JSY
The bigger picture
Achieving the various goals of the global sustainable development agenda poses complex challenges for the chemical industry and society as a whole. Systemic innovation in chemical research and development, assessment, management, and education is required to facilitate a transition toward a sustainable future. Such an innovation can benefit, to a large extent, from the increased uptake and systematic adoption of digitalization and digital tools to optimize the management of entire chemical life cycles, from chemical supply chains and chemical manufacturing to use and end-of-life. Digitalization in chemistry will enable development of more flexible data exchange models, more transparent international and cross-sector chemical information transfer, and chemistries that are both safe and sustainable by design. With that, digitalization is key to a radical transformation to more sustainable and collaborative business models in the growing chemical industry sector.
Governments and corporations are increasingly adopting circular economy strategies to meet the need of decoupling growth from resource consumption. However, the world economy is far from being circular, with some of the reasons including lack of transparency, standardization, and data sharing. Digitalization can help overcome these challenges, thus making it a key enabler of the circular economy. This review looks at the concept of a digital product passport as a tool for implementing and scaling the circular economy. It discusses opportunities and challenges related to further development and adoption of digital product passports. Finally, it examines the battery passport, drafted in the EU Battery regulation, as one of the first examples of a digital product passport required by law. Digitalization is considered as a major driver of the transition to the circular economy. An emerging concept for digitalizing entire product life cycles, the digital product passport (DPP) presents an opportunity for circular economy adoption and scaling. This article discusses the challenges, benefits, and first legislation related to DPPs.
Reducing net emission
The great majority of plastics in current use are sourced from fossil fuels, with additional fossil fuels combusted to power their manufacture. Substantial research is focused on finding more sustainable building blocks for next-generation polymers. Meys et al . report a series of life cycle analyses suggesting that even the current varieties of commercial monomers could potentially be manufactured and polymerized with no net greenhouse gas emissions. The cycle relies on combining recycling of plastic waste with chemical reduction of carbon dioxide captured from incineration or derived from biomass. —JSY
Ionic liquids (ILs) have shown high catalytic activity in the degradation of poly(ethylene terephthalate) (PET), but the effects of the anions and cations, as well as the mechanism, remain ambiguous. Glycolysis is an important recycling method that converts waste PET into monomers through various chemical reactions. To reveal the role of ILs and the molecular mechanism of the glycolysis of PET, density functional theory (DFT) calculations have been carried out for the possible pathways for the generation of bis(hydroxyethyl)terephthalate (BHET) catalyzed by isolated anions/cations and ion pairs at different sites. The pathway with the lowest barrier for the glycolysis of PET is the cleavage of the C-O ester bond, which generates the BHET monomer. The synergistic effects of the cations and anions play a critical role in the glycolysis of PET. The cations mainly attack the carbonyl oxygen of PET to catalyze the reaction, and the anions mainly form strong H-bonds with PET and ethylene glycol (EG). In terms of the mechanism, the H-bonds render the hydroxyl oxygen of EG more electronegative. The cation coordinates the carbonyl oxygen of the ester, and the hydroxyl oxygen of EG attacks the ester group carbon of PET, with proton transfer to the carbonyl oxygen. A four-membered-ring transition state would be formed by PET, EG, and the IL catalyst, which regularly accelerates the degradation of PET. These results provide fundamental help in understanding the roles of ILs and the mechanism of IL-catalyzed PET degradation.
Tough recyclable polyacetals
Cyclic acetals such as dioxolane are appealing building blocks for recyclable plastics but have proven to be difficult to polymerize controllably. Abel et al . show that optimal pairing of a bromomethyl ether and indium or zinc Lewis acid produces polydioxolane with high tensile strength that may be advantageous for packaging applications. Heating this plastic in strong acid easily breaks it back down to its acetal monomer, which can then be recovered by distillation from mixed plastic waste streams in high yield. —JSY
Plastics pollution is causing an environmental crisis, prompting the development of new approaches for recycling, and upcycling. Here, we review challenges and opportunities in chemical and biological catalysis for plastics deconstruction, recycling, and upcycling. We stress the need for rigorous characterization and use of widely available substrates, such that catalyst performance can be compared across studies. Where appropriate, we draw parallels between catalysis on biomass and plastics, as both substrates are low-value, solid, recalcitrant polymers. Innovations in catalyst design and reaction engineering are needed to overcome kinetic and thermodynamic limitations of plastics deconstruction. Either chemical and biological catalysts will need to act interfacially, where catalysts function at a solid surface, or polymers will need to be solubilized or processed to smaller intermediates to facilitate improved catalyst–substrate interaction. Overall, developing catalyst-driven technologies for plastics deconstruction and upcycling is critical to incentivize improved plastics reclamation and reduce the severe global burden of plastic waste. Plastics are invaluable materials for modern society, although they result in the generation of large amounts of litter at the end of their life cycle. This Review explores the challenges and opportunities associated with the catalytic transformation of waste plastics, looking at both chemical and biological approaches to transforming such spent materials into a resource.
This paper briefly reviews the development history of polyethylene terephthalate (PET) and the recycling of PET. As one of the most promising way to degrade PET into oligomers and monomers that can be used for the production of high-quality PET, catalytic glycolysis is highlighted in this review. The developments on metal salt, metal oxide and ionic solvent catalysts for glycolysis of PET are systematically summarized, besides, the proposed catalytic mechanisms of ionic liquids (ILs) and deep eutectic solvents (DESs) are presented. The metallic catalysts show high catalytic performance but causing serious environmental pollution and high waste treatment costs, thereby it is proposed that metal-free catalysts, especially ILs and DESs can be the “greener” alternatives to address the PET waste problem. Additionally, the studies related to the glycolysis kinetics are discussed in this review, showing the results that PET glycolysis process consists of heterogeneous and homogeneous depolymerization, and different models should be used to investigate different depolymerization stages in order to obtain a more realistic picture.
The chemical industry is increasingly looking to develop bio-based alternatives to petroleum-based platform chemicals, in order to reduce dependence on diminishing fossil resources and to decrease GHG emissions. 5-Hydroxymethylfurfural (HMF) and 2,5-furandicarboxylic acid (FDCA) are two examples of bio-based chemicals which could allow for the synthesis of a wide range of chemicals and materials, particularly polymers, from renewable feedstocks. This review paper summarises and critically evaluates results from existing life cycle assessment (LCA) and technoeconomic analysis (TEA) studies of HMF and FDCA synthesis and, by doing this, provides several points of advice for future investigations and assessments of synthetic routes towards these bio-based products. Chemical considerations such as choice of solvent system, catalyst and energy production are reviewed; and methodological issues in LCA, such as treatment of biogenic carbon and allocation methods, are discussed. Overall, results suggest that the production of HMF and FDCA-based products may offer lower impacts from CO2 emissions than their fossil-based counterparts, but this often comes with an increase in environmental impacts in other impact categories. Higher operating costs from expensive fructose feedstocks and high energy demands also make HMF and FDCA less economically viable than current chemicals. Moving forwards, further investigation into different lignocellulosic feedstocks, energy production units and the development of new catalytic systems may help in making HMF and FDCA production more favourable than the production of fossil-based counterparts.
Hydrogels are commonly used as scaffolds for the preparation of three-dimensional tissue constructs and for the encapsulation and delivery of cells in regenerative medicine. Polyesters are an attractive class of polymers for hydrogel preparation. However, most polyesters have hydrophobic backbones and lack pendent groups that can be chemically functionalized. We describe here the development of water-soluble polyesters based on aspartic acid and poly(ethylene glycol) (PEG) (600 or 1500 g/mol), having pendent reactive amines. The reactivity of these amines with methacrylic anhydride, maleic anhydride, and itaconic anhydride was explored for the introduction of crosslinkable groups. The resulting methacrylamide-functionalized polymers were successfully crosslinked to form hydrogels using a redox-initiated free radical polymerization. The use of 10% (weight/volume) of polymer, and 10 mM of potassium persulfate and tetramethylethylenediamine led to high (> 97%) gel content, and compressive moduli of 13 – 21 kPa. Human adipose-derived stromal cells were encapsulated during the crosslinking process and exhibited greater than 80% viability in the hydrogels prepared from the polyester containing 600 g/mol PEG, with lower viability observed for the polymer containing 1500 g/mol PEG. These results support the potential for aspartic acid-based copolymers with short PEG chains in the backbone to serve as a platform for cell encapsulation, with additional opportunities for further functionalization available in the future.
Polyurethanes are highly resistant materials used for building insulation or automotive seats. The polyurethane end‐of‐life issue must be addressed by the development of efficient recycling techniques. Since conventional recycling processes are not suitable for thermosets, waste management of PU foam is particularly questioning. By coupling biological and chemical processes, this study aims at developing a green recycling pathway for PU foam using enzymes for depolymerization. For instance, enzymatic degradation of a PU foam synthesized with polycaprolactone and toluene diisocyanate led to a weight loss of 25 % after 24 h of incubation. The corresponding degradation products were recovered and identified as 6‐hydroxycaproic acid and a short acid‐terminated diurethane. An organo‐metallic catalyzed synthesis of second generation polymers from these building blocks was carried out. A polymer with a high average molar mass of 74 000 (Mw) was obtained by mixing 50 % of recycled building blocks and 50 % of neat 6‐hydroxycaproic acid. A poly(ester urethane) have been synthesized without the use of toxic and decrier polyisocyanates. It is the first time that a study offers the vision of a recycling loop starting from PU wastes and finishing with a second generation polymer in a full circular approach.
The feasibility of in-silico techniques, together with the computational framework, has been applied to predictive bioremediation to clean-up contaminants, toxicity evaluation, and possibilities for the degradation of complex recalcitrant compounds. Emerging contaminants from different industries have posed a significant hazard to the environment and public health. Given current bioremediation strategies, it is often a failure or inadequate for sustainable mitigation of hazardous pollutants. However, clear-cut vital information about biodegradation is quite incomplete from a conventional remediation techniques perspective. Lacking complete information on bio-transformed compounds leads to seeking alternative methods. Only scarce information about the transformed products and toxicity profile is available in the published literature. To fulfill this literature gap, various computer or in-silico technologies have emerged as alternating techniques, which are being recognized as in-silico approaches for bioremediation. Molecular docking, molecular dynamics simulation, and biodegradation pathways predictions are the vital part of predictive biodegradation, including the Quantitative Structure-Activity Relationship (QSAR), Quantitative structure-biodegradation relationship (QSBR) model system. Furthermore, machine learning (ML), artificial neural network (ANN), genetic algorithm (GA) based programs offer simultaneous biodegradation prediction along with toxicity and environmental fate prediction. Herein, we spotlight the feasibility of in-silico remediation approaches for various persistent, recalcitrant contaminants while traditional bioremediation fails to mitigate such pollutants. Such could be addressed by exploiting described model systems and algorithm-based programs. Furthermore, recent advances in QSAR modeling, algorithm, and dedicated biodegradation prediction system have been summarized with unique attributes.
The OASIS CATALOGIC software is distinctive in that it requires the simulation of microbial metabolism for prediction of biodegradation and fish liver metabolism for prediction of bioaccumulation. The reliability of these structure-activity predictions typically involves invoking OECD principles and ECHA practical guidance. Questions remain, especially whether some predictions for chemicals belonging to less than 100% of applicability domain (AD) still may be considered as reliable and what’s the tolerated threshold for belonging to a model’s AD. Here the reliability of the predictions associated with biodegradation and bioaccumulation endpoints obtained from OASIS CATALOGIC models are examined to clarify criteria for assessing the reliability of the predictions. In the end, the reliability of any prediction is based in large part on: 1) establishing a transparent stream of relevant information, 2) providing and documenting relevant in silico and experimental data, and 3) demonstrating metabolic similarity between chemicals. Moreover, when the metabolic transformation is fundamental, it is critical to establishing the adequacy of the simulated transformations as this is indispensable evidence for establishing the reliability of a prediction.
Bio-based diamines are considered to be a promising alternative to traditional fossil-fuel-based diamines, the important platform chemical for the synthesis of polymer materials. In this review, the current status of the art of the synthesis of aliphatic and aromatic diamines from renewable biomass are considered. In the case of aliphatic diamines, we describe strategies for biologically producing diamines with different carbon numbers including 1,3-diaminopropane, 1,4-butanediamine, 1,5-pentanediamine, 1,6-diaminohexane, 1,8-diaminooctane, 1,10-diaminodecane, and 1,12-diaminododecane. In addition, aromatic diamines produced from various kinds of renewable biomass, including lignin, cashew nut shell, and terpenoids, are reviewed here. Furthermore, the application of typical diamines in synthesis of polyurethane and polyamide are also reviewed.
Polymers (plastics) have transformed our lives by providing access to inexpensive and versatile materials with a variety of useful properties. While polymers have improved our lives in many ways, their longevity has created some unintended consequences. The extreme stability and durability of most commercial polymers, combined with the lack of equivalent degradable alternatives and ineffective collection and recycling policies, have led to an accumulation of polymers in landfills and oceans. This problem is reaching a critical threat to the environment, creating a demand for immediate action. Chemical recycling and upcycling involve the conversion of polymer materials into their original monomers, fuels or chemical precursors for value-added products. These approaches are the most promising for value-recovery of post-consumer polymer products; however, they are often cost-prohibitive in comparison to current recycling and disposal methods. Catalysts can be used to accelerate and improve product selectivity for chemical recycling and upcycling of polymers. This review aims to not only highlight and describe the tremendous efforts towards the development of improved catalysts for well-known chemical recycling processes, but also identify new promising methods for catalytic recycling or upcycling of the most abundant commercial polymers.
Thermosetting polyurethane (PU) foams, which cannot be recycled economically and efficiently due to their permanently crosslinked structures, have caused significant environmental concerns after service. To improve the sustainable development of the PU foam industry, herein, we report malleable PU foams that contain a dynamic disulfide bond. The disulfide exchange reaction under heat enables the rearrangement of the network topology of the PU foam, imparting malleability and thermal processability. The disulfide containing PU foams (PUSFs) have similar appearance and physical properties to common PU foams and were prepared using conventional foaming technology without any modification. The PUSFs can be easily recycled into PU films through thermal compression molding. The recycled PU films show excellent and tunable mechanical properties depending on the compositions of the original malleable PU foams. Furthermore, the PU film recycled from PU foam with a well-designed composition can be further reprocessed several times without obvious loss in mechanical proprieties and change in chemical structures. This investigation provides a novel methodology to recycle and reuse PU foams and is expected to promote the sustainable development of the PU foam industry.
The synthesis of functional and processable polyethylenes from simple vinyl monomers with controlled molecular weights and architectures has been a grand challenge of polymer chemistry. Post-polymerization modification of the homopolymers of ethylene is attractive for this purpose but has been hampered by the lack of efficient methods for the selective functionalization of C–H bonds in polyolefins. We report the selective, catalytic oxidation of C–H bonds in commodity polyethylenes with varying molecular weights and architectures. Remarkably, the functionalized materials, even at low levels of functionalization, exhibit physical properties that are absent in unmodified polyolefins, such as strong adhesion and the ability to be painted with common waterborne latex paint. Such observations indicate that selectively modified polyethylenes as described here may help to transform existing commodity plastics into more valuable and potentially more sustainable materials.
Sorting plastics according to their chemical composition is a crucial step in the recycling process, turning mixed plastics waste into a high‐value resource. Near‐infrared spectroscopy (NIRS) is an established method for plastics classification. Evolutionary progress in miniaturization of optical components has enabled the development of hand‐held spectrometers. Cloud‐based computing and centralized databases provide practically unlimited computing power for classification algorithms and enormous spectral databases. Here we discuss the basics of hand‐held NIRS and the essentials of the corresponding data analysis with a focus on plastics sorting.
Plastic solid waste (PSW) is an ever‐growing environmental challenge for our society, as it not only ends up in landfills but also in waterways and oceans and is consequently entering the food chain. A key strategy to overcome this problem while also preserving carbon resources is to use PSW as a feedstock, evolving towards a circular economy. To implement this, mechanical as well as chemical recycling technologies must be developed. Indeed, owing to the high volume of PSW generated each year, mechanical recycling alone is not adequate for addressing this global challenge. Because of this, chemical recycling via thermal and heterogeneous catalytic conversion has received growing attention. This process has the potential to take PSW and convert it into usable monomers, fuels, synthesis gas, and adsorbents under more sustainable conditions than thermal degradation. This Review highlights the recent research advances in catalytic technologies for PSW conversion and valorization. Elegantly wasted: Plastic solid waste (PSW) is a large environmental issue and results in the loss of valuable carbon resources. Heterogenous catalysts allow for the chemical recycling of PSW to chemical feedstocks. This Review offers a summary of catalysts that have been reported in the literature for PSW conversion and offers a perspective on future research directions that will assist in the transition to a circular economy.