Chemical Recycling Research Articles

Textile Recycling: Efficient Polyester Recovery from Polycotton Blends Using the Heated High-Ethanol Alkaline Aqueous Process

Textile production has doubled in the last 20 years, but only 1% is recycled into new fibers. It is the third largest contributor to water pollution and land use, accounting for 10% of global carbon emissions and 20% of clean water pollution. A key challenge in textile recycling is blended yarns, such as polycotton blends, which consist of polyester and cotton. Chemical recycling offers a solution, in particular, alkali treatment, which hydrolyzes polyester (PET) into its components while preserving cotton fibers. However, conventional methods require high temperatures, long durations, or catalysts. Our study presents, for the first time, the heated high-ethanol alkaline aqueous (HHeAA) process that efficiently hydrolyzes PET from polycotton at lower temperatures and without a catalyst. A near-complete PET hydrolysis was achieved in 20 min at 90 °C, while similar results were obtained at 70 °C and 80 °C with longer reaction times. The process was successfully scaled at 90 °C for 20 min, and complete PET hydrolysis was achieved, with a significantly reduced liquid-to-solid ratio, from 40 to 7 (L per kg), signifying its potential to be implemented in an industrial context. Additionally, the cotton maintained most of its properties after the treatment. This method provides a more sustainable and efficient approach to polycotton recycling.

Chemically Recyclable, Reprocessable, and Mechanically Robust Reversible Cross-Linked Polyurea Plastics for Fully Recyclable Aramid Fiber Reinforced Composites.

Aramid fiber reinforced composites (AFRCs) have received increasing attention because of their excellent comprehensive performance including high mechanical strength, high modulus, and light weight. However, full recycling of AFs from ARCFs is difficult to achieve. Herein, fully recyclable ARCFs are fabricated using reversible cross-linked polyurea plastics (PUHA) as the matrix. PUHA plastics are fabricated by cross-linking linear polyurea using hemiaminal groups. By changing the main chain structures, two types of PUHA plastics are prepared with excellent mechanical performance, which is comparable to that of traditional engineering plastics. PUHA plastics can be reprocessed at least five times without losing their original mechanical properties because of the dynamic exchangeability of the hemiaminal groups. Meanwhile, PUHA plastics can be rapidly depolymerized into linear polyurea under acidic conditions. When PUHA plastics are used as a matrix to fabricate AFRCs, the AFRCs exhibit excellent mechanical strength. Moreover, due to the simple chemical recycling ability of PUHA plastics, AFRCs can be fully decomposed into intact AFs and linear polyurea with high purity. This work presents the use of reversible cross-linked polyurea plastics in the fabrication of fully recyclable AFRCs and provides the future direction of developing fully recyclable and high-performance fiber-reinforced composites.

Evolution of Copolymers of Epoxides and CO2: Catalysts, Monomers, Architectures, and Applications.

The copolymerization of CO2 and epoxides presents a transformative approach to converting greenhouse gases into aliphatic polycarbonates (CO2-PCs), thereby reducing the polymer industry's dependence on fossil resources. Over the past 50 years, a wide array of metallic catalysts, both heterogeneous and homogeneous, have been developed to achieve precise control over polymer selectivity, sequence, regio-, and stereoselectivity. This review details the evolution of metal-based catalysts, with a particular focus on the emergence of organoborane catalysts, and explores how these catalysts effectively address kinetic and thermodynamic challenges in CO2/epoxides copoly2merization. Advances in the synthesis of CO2-PCs with varied sequence and chain architectures through diverse polymerization protocols are examined, alongside the applications of functional CO2-PCs produced by incorporating different epoxides. The review also underscores the contributions of computational techniques to our understanding of copolymerization mechanisms and highlights recent advances in the closed-loop chemical recycling of CO2-sourced polycarbonates. Finally, the industrialization efforts of CO2-PCs are discussed, offering readers a comprehensive understanding of the evolution and future potential of epoxide copolymerization with CO2.

Upcycling polyethylene into closed-loop recyclable polymers through titanosilicate catalyzed C-H oxidation and in-chain heteroatom insertion

Polyolefins are the most widely produced type of plastics owing to their low production cost and favorable properties. Their polymer backbone consists solely of inert C-C bonds, making them resistant and durable materials. Although this is an extremely useful attribute during their use phase, it complicates chemical recycling. In this work, different types of polyethylenes (PEs) are converted into ketone-functionalized PEs with up to 3.4% functionalized carbon atoms, in mild conditions (≤100 °C), using a titanosilicate catalyst and tert-butyl hydroperoxide as the oxidant. Subsequently, the introduced ketones are exploited as sites for heteroatom insertion. Through Baeyer-Villiger oxidation, in-chain esters are produced with yields up to 73%. Alternatively, the ketones can be converted into the corresponding oxime, which can undergo a Beckmann rearrangement to obtain in-chain amides, with yields up to 75%. These transformations allow access to polymers that are amenable to solvolysis, thereby enhancing their potential for chemical recycling.

Depolymerization of Waste Polycarbonates to Value-Added Products.

Additive free aminolysis method developed for the depolymerization/upcycling of polycarbonates. We report here chemical recycling of polycarbonate under ambient conditions to get its monomer bisphenol A, monoaminocarbamate and biscarbamates in 1 : 2 : 1 ratio respectively. By employing the secondary amine as the aminating reagent, facilitates the depolymerization to work under additive/catalyst free conditions. The developed method deals with depolymerization of waste polycarbonates and works even with late-stage amine derivatives such as amoxapine and desloratadine which are drugs molecules known to treat neurotic disorders and allergies respectively. The reaction can be scaled up and works with similar efficacy which depicts the efficiency of the depolymerization of wasteend-of-life polycarbonate plastic waste. The biscarbamate and bisphenol-A was further subjected for the post functionalization to obtain amides and phenol in good yields.

Overcoming barriers to proactive plastic recycling toward a sustainable future

The plastics sector, accounting for a significant portion of global emissions, presents a challenge and an opportunity in achieving carbon neutrality. Despite Japan's commendable polyethylene terephthalate (PET) bottle recycling rates, most plastics are thermally recycled, creating environmental issues. This study proposes an evaluation framework to enhance recycling, aligned with end-user preferences and fostering a circular plastics economy. Employing a mixed-methods approach, this study conducts fieldwork including interviews with plastic recyclers and analysis of industry data. A weighted sum multicriteria analysis integrating end-user preferences, recycling effectiveness, and market dynamics is utilized. Systemic, process, and policy challenges were shown to hinder sustainable recycling practices, while varying willingness to pay, emission and cost reduction potentials, among acceptability and sectoral diversity informed priority plastic types for recycling. Multicriteria analysis showed that although PET is favored by end users, Polyoxymethylene (POM) emerges as a potential priority target for manufacturers and recyclers. Sensitivity analysis underscores the potential impact of establishing or enhancing willingness to pay (WTP) toward certain plastic types. Moreover, manufacturer and recycler evaluations suggest a broader willingness to recycle plastics than previously assumed. The proposed evaluation framework offers insights toward plastic recycling strategies. Policy interventions such as sustained subsidies for recyclers, market incentives leveraging WTP preferences, and technological advances, including chemical recycling and the broadening of plastic type recycling in line with user and manufacturer preferences, could all contribute to promoting sustainable plastic recycling practices.

Upcycling of Polyamide Wastes to Tertiary Amines Using Mo Single Atoms and Rh Nanoparticles.

The pursuit of sustainable practices through the chemical recycling of polyamide wastes holds significant potential, particularly in enabling the recovery of a range of nitrogen-containing compounds. Herein, we report a novel strategy to upcycle polyamide wastes to tertiary amines with assistance of H2 in acetic acid under mild conditions (e.g., 180 ºC), which is achieved over anatase TiO2 supported Mo single atoms and Rh nanoparticles. In this protocol, the polyamide is first converted into diacetamide intermediates via acidolysis, which are subsequent hydrogenated into corresponding carboxylic acid monomers and tertiary amines in 100% selectivity. It is verified that Mo single atom and Rh nanoparticles work together to activate both amide bonds of the diacetamide intermediate, and synergistically catalyze its hydrodeoxygenation to form tertiary amine, but this catalyst is ineffective for hydrogenation of carboxylic acid. This work presents an effective way to reconstruct various polyamide wastes into tertiary amines and carboxylic acids, which may have promising application potential.

Alkyl-Substituted Polycaprolactone Poly(urethane-urea)s as Mechanically Competitive and Chemically Recyclable Materials.

We report the mechanical performance and chemical recycling advantages of implementing alkyl-substituted poly(ε-caprolactones) (PCLs) as soft segments in thermoplastic poly(urethane-urea) (TPUU) materials. Poly(4-methylcaprolactone) (P4MCL) and poly(4-propylcaprolactone) (P4PrCL) were prepared, reacted with isophorone diisocyanate, and chain-extended with water to form TPUUs. The resulting materials' tensile properties were similar or superior to a commercially available polyester thermoplastic poly(urethane) and had superior elastic recovery properties compared to a PCL analogue due to the noncrystalline nature of P4MCL and P4PrCL. Additionally, monomers were recovered from the TPUU materials in high yields via ring-closing depolymerization using a reactive distillation approach at an elevated temperature and a reduced pressure (240-260 °C, 25-140 mTorr) with zinc chloride (ZnCl2) as the catalyst. The thermodynamics of polymerization were estimated using Van't Hoff analyses for 4MCL and 4PrCL; these results indicated that the propyl group in 4PrCL results in a lower practical ceiling temperature (Tc) for P4PrCL.

Synergistic effect of diatomaceous earth as a catalyst and terephthalic acid as a co-catalyst on chemical recycling of polyethylene terephthalate via hydrolysis

Polyethylene terephthalate (PET) is one of the most useful commodity polymers that have been used extensively in packaging and textile industry and hence it has become a concerning waste. In this work, diatomaceous earth (DE), terephthalic acid (TPA) and distilled water were used to hydrolyze PET waste from drinking water bottles into TPA and ethylene glycol (EG). TPA can be used as one of the monomers for the synthesis of PET or other industrial applications such as for the synthesis of alkyd resins. Here DE was used as a solid catalyst which can be separated by a simple filtration after the TPA removal, in contrast to the majority of earlier efforts, which used aqueous acid or base catalysts that are difficult to separate. Additionally, commercial TPA, which is used as co-catalyst, can easily be substituted by produced TPA from the PET hydrolysis. The conversion of PET and the yield of TPA, using 5.0 wt % of DE and 3.0 wt % of commercial TPA with a PET:H2O mass ratio of 1.0:3.0, at 185 °C and 11.2 bar was 100 % and 95.40 % respectively, obtained in 2.5 h. TPA product was isolated from the reaction mixture by basic wash and subsequent neutralization and it was characterized by infrared spectroscopy, nuclear magnetic resonance, differential scanning calorimetry, X-ray diffraction and thermogravimetric analysis. The effects of reaction parameters such as reaction temperature, reaction time, mass ratio of PET:H2O and the catalysts loading on the yield of TPA were studied. The optimized reaction was repeated over seven consecutive reaction cycles using recovered DE to further demonstrate the effectiveness of reused DE and it was found that the outcomes were unaltered. Due to DE's affordability and accessibility, this reaction is a green and convenient way to handle the staggering accumulation of PET waste.

Thermogravimetric study on thermal degradation kinetics and polymer interactions in mixed thermoplastics

AbstractPotential interactions during thermal degradation of polymer blends significantly influence product yields and their composition. Therefore, chemical recycling of plastic waste requires fundamental understanding of feedstock dependency for effective process design. This study investigates the pyrolysis of polymer blends (HDPE, LDPE, PP, PS, ABS, PET, PA6, PVC) through thermogravimetric experiments at different heating rates. Sample homogeneity’s impact on interactions is analyzed using particles, powder, coextruded blends, and samples in crucibles with separated compartments. A kinetic model is presented to support the experimental findings, assuming linear superposition of individual polymer kinetics. A proposed grouping of thermoplastics, reflecting their degradation behavior and potential interactions, correlates with the polymer structure. Observed interactions, particularly in blends of heteroatom-containing polymers (N, O, Cl), are accelerated reactions and coke formation. Hence, the model accurately predicts the degradation of heteroatom-free polymer mixtures but encounters challenges with more complex blends. This comprehensive study emphasizes the importance of feedstock composition for future pyrolytic polymer recycling. Graphical abstract

Recent advances in chemical recycling of polyoxymethylene waste

Recent advances in chemical recycling of polyoxymethylene waste

Bio-derived epoxy thermosets incorporating imine linkages: Towards sustainable and advanced polymer materials

In the search for sustainable and eco-friendly alternatives, Schiff-based epoxy thermosets are emerging as a promising strategy due to the inherent dynamic and reversible nature of their imine functional groups, which enables thermal reprocessability and chemical recyclability. This article explores the synthesis of Schiff-based epoxy thermosets using sustainable approaches. Starting from vanillin or syringaldehyde, the synthesis involves the incorporation of imine linkages into the epoxy network. A new strategy has been employed by self-polymerization without the use of additional curing agents. This approach conducts to novel thermosets with superior material properties that exceed those reported in previous studies. Thermo-mechanical investigations showed that the designed thermosets have high mechanical properties, with storage moduli at room temperature ranging between 1.5–2.2 GPa, and glass transition values between 138 and 249 °C. Thermal analyses allowed to evaluate the Limit Oxygen Index (LOI) which ranged from 33 % to 36 %, demonstrating excellent flame-retardant properties without the addition of supplementary additives. Their reduced density (0.74–1.09 g/cm3) makes them suitable for applications where weight considerations are critical. These results obtained results place the designed Schiff-based materials as promising candidates for a wide range of high-end applications, particularly in industries that prioritize eco-friendly processes and materials.

Influence of castor oil‐derived bio‐resin on mechanical and visco‐elastic properties of dynamic covalent imine bond based epoxy and its composite

AbstractThe widespread use of thermoset composites has raised significant environmental and economic concerns due to their non‐recyclable nature. This study addresses these issues by developing vitrimers and their composites featuring dynamic imine bonds synthesized from vanillin, 3,3′‐dimethyl‐4,4′‐diaminodicyclohexylmethane, and epoxy. To enhance toughness and bio‐content in the vitrimer and its composites, a green reactive monomer, epoxy methyl ricinoleate (EMR), was synthesized via the transesterification of epoxidized castor oil (ECO) and incorporated as the reactive diluent. The effect of EMR on bio‐resin mechanical, thermo‐mechanical, and relaxation times (τ) of the vitrimer was examined. The inclusion of EMR reduced the relaxation time, indicating improved dynamic bond exchange efficiency. EMR incorporation decreased the relaxation time, suggesting enhanced efficiency in the interchange of dynamic bonds. In addition, the incorporation of 20% EMR increased the impact strength of the epoxy vitrimer by 37.8%. The study also details the manufacturing and characterization of greener composites made with these vitrimers and flax fabric, which were evaluated for various mechanical and thermo‐mechanical properties. Morphological analysis using a scanning electron microscope revealed good interfacial adhesion between the fiber and matrix. Furthermore, the flax fiber vitrimer‐based composite demonstrated effective chemical recyclability. The results indicate that these composites possess mechanical and thermal‐mechanical properties well‐suited for lightweight engineering applications.Highlights Recyclable vitrimers were developed from vanillin and epoxy. Toughness and bio‐content were enhanced by adding EMR. Relaxation time was reduced by EMR, improving bond exchange efficiency. Epoxy vitrimer impact strength was increased by 37.8% with 20% EMR. Green composites with vitrimers and flax fabric were manufactured.

Degradation Mechanism and Enhanced Stability of Organolithium for Chemical Lithiation

AbstractOrganolithium solutions, especially Li‐arene solutions (LASs) with high reactivity and controllable redox potentials, have gained significant attention because of their wide applications in chemical lithiation, liquid anodes, and battery recycling. However, the sudden loss of reactivity when stored or applied at room temperature is still puzzling and inhibits the application of LASs. In this work, the degradation mechanism of LASs is fully investigated and revealed by combining various experimental characterization and computational simulations. A hierarchical reaction mechanism for lithium biphenyl/2‐methyl tetrahydrofuran (Li‐Bp‐2MT), a lithiation solution used for most anodes, explains degradation and side product formation. Specifically, the dimerization of the active component Li1Bp[2MT]1 forms an inactive dimer that is irreversibly reduced in the presence of locally accumulated highly reductive Li0. This reaction mechanism reveals the atomic origins of lithiation solution deactivation and accounts for all solid and gaseous byproducts. LiH is identified as the dominating solid byproduct, indicating irreversible destruction of the active components and facilitating side reactions producing H2 and CH4. Based on reaction mechanism insights, modifying molecular interactions and reaction kinetics are experimentally shown to inhibit Li0 aggregation kinetics, enhancing long‐term prelithiation performance. This research provides comprehensive guidelines for practical applications of LASs.

More Efficient Chemical Recycling of Poly(ethylene terephthalate) by Intercepting Intermediates.

The research presented in this paper offers insight into the availability of intermediates in the depolymerization of Polyethylene Terephthalate (PET) to be used as feedstocks to create value added products.Monitoring the dispersity, molecular weight, end groups, and crystallinity of reaction intermediates during the heterogeneous depolymerization of PET offer insight into the mechanism by which the polymer chains evolve during the reaction. Our results show dispersity decreases and crystallinity increases, while the yield of insoluble PET remains high early in the reaction. Our interpretation of this data depicts a mechanism where chain scission targets amorphous tie-chains between crystalline phases. Targeting the tie-chains lowers the Mnof the polymer without changing the amount of recovered polymer flake. Chain scission of tie-chains and isolating highly crystalline PET lowers the dispersity ((Đ)) of the polymer chains, as the size of the crystalline lamellae guide the molecular weight of the depolymerized oligomers.When sufficient end groups of PET chains are converted to alcohol groups, the PET flakes break apart into highly crystalline and less disperse polymer. Our results also demonstrate that the oligomeric depolymerization intermediates are readily repolymerized, offering new opportunities to more effectively and efficiently chemically recycle PET.

A simulation-based techno-economic-safety-social-environmental decision-making framework for prioritizing plastic waste treatment processes

Chemical recycling technologies hold promising potential in converting mixed plastic waste into energy and chemicals. This study proposed a simulation-based quantitative life cycle sustainability assessment framework for the potential plastic waste valorization processes. Sustainability metrics including environment, economy, safety, society, and energy were quantified and comprehensively investigated. Based on the Bayesian best-worst weighting and vector-based ranking methods, the normalized and weighted life cycle sustainability scores were used to prioritize the best process. Gasification-based technology was found superior than incineration-based technology, especially in the economic, techno-energetic, and social aspects. However, incinerated-based process with direct CO2 emissions performed better in the environmental aspects compared with the process integrating with carbon capture and utilization unit. Additional environmental burdens were brought to the system when incorporating the carbon capture and methanol synthesis process. Increased system complexity might bring unexpected environmental burdens, and sometimes even outweighed the benefits. Various weights were assigned to the multi-criteria decision-making model, and the gasification-based process consistently ranked first across all scenarios.

Sustainable Energy Application of Pyrolytic Oils from Plastic Waste in Gas Turbine Engines: Performance, Environmental, and Economic Analysis

The rapid accumulation of polymer waste presents a significant environmental challenge, necessitating innovative waste management and resource recovery strategies. This study investigates the potential of chemical recycling via pyrolysis of plastic waste, specifically polystyrene (PS) and polypropylene (PP), to produce high-quality pyrolytic oils (WPPOs) for use as alternative fuels. The physicochemical properties of these oils were analyzed, and their performance in a gas turbine engine was evaluated. The results show that WPPOs increase NOx emissions by 61% for PSO and 26% for PPO, while CO emissions rise by 25% for PSO. Exhaust gas temperatures increase by 12.2% for PSO and 8.7% for PPO. Thrust-specific fuel consumption (TSFC) decreases by 13.8% for PPO, with negligible changes for PSO. The environmental-economic analysis indicates that using WPPO results in a 68.2% increase in environmental impact for PS100 and 64% for PP100, with energy emission indexes rising by 101% for PS100 and 57.8% for PP100, compared to JET A. Although WPPO reduces fuel costs by 15%, it significantly elevates emissions of CO2, CO, and NOx. This research advances the understanding of integrating waste plastic pyrolysis into energy systems, promoting a circular economy while balancing environmental challenges.

Assessment of recycling and repair methods for discontinuous glass fiber reinforced vitrimer composites with reclaimed parts

In the context of fiber-reinforced polymer composites, the pursuit of sustainability is of paramount importance to promote environmentally responsible practices. Vitrimer materials have recently gained significant attention for their potential to enable repair and recycling, rendering them an attractive choice for manufacturing eco-friendly composites while maintaining mechanical strength akin to a thermoset resin like epoxy. This study specifically focuses on glass fibers, and delves into the examination of mechanical properties using both chemical and mechanical recycling methods for quasi-isotropic, randomly dispersed discontinuous glass fiber-reinforced vitrimer composites. It places specific emphasis on the optimization of factors to promote sustainable production. The findings indicate that chemical recycling methods are remarkably effective, resulting in the almost complete restoration of mechanical properties when appropriately selected process parameters are applied. Additionally, this study investigates diverse repair techniques, with the overlap repair method exhibiting the highest degree of recovery. These findings underscore the remarkable potential of glass fiber reinforced vitrimer composites, characterized by their high mechanical properties, for applications in various industries, particularly in structural contexts. Overall, this study highlights the significant reduction in composite waste achievable through recycling and repair strategies, aligning with the broader goal of advancing sustainability within the domain of engineering and manufacturing.

Targeted valorization of waste polycarbonate into bisphenol A dimethyl ether

The chemical recycling of waste polycarbonate (PC) plastic to produce high value-added chemicals represents a sustainable and economical approach to its management. Facile methods that utilize the unique structure of the bisphenol A (BPA) unit are expected to be more low-carbon and cost-effective. However, there are few established routes for converting BPA into valuable derivatives while preserving its structure. In this work, we developed a new pathway for the targeted valorization of PC waste into a multifunctional value-added chemical, bisphenol A dimethyl ether (BPAME). Using ionic liquids as catalysts, low-toxicity and environmentally friendly dimethyl carbonate (DMC) as a methyl source, and N-methylpyrrolidone as a co-solvent, we successfully achieved 100 % depolymerization of PC and an impressive 99 % yield of BPAME under mild conditions (150 °C, 6 h). Kinetic studies indicated that the methylation of BPA is the rate-determining step. Moreover, this innovative pathway not only transformed commercial PC waste into BPAME with exceptional selectivity (99 %) and high yield (≥92 %), but also facilitated the efficient methylation of aromatic phenols and amines. This valorization strategy for PC waste disposal offers a promising option for industrial applications.

Kinetic modelling and mechanism elucidation of catalyst-free dimethyl terephthalate hydrolysis process intensification by reactive distillation

Polyethylene terephthalate (PET) is a widely used thermoplastic polymer in the packaging and materials industries, and is predicted to have continuous market growth in the future. Methods such as glycolysis and methanolysis of PET have been identified as promising approaches in sustainable chemical recycling of PET. The latter produces dimethyl terephthalate (DMT), which can be easily hydrolyzed to terephthalic acid (TPA) at elevated temperatures and pressures, and further reused for PET production. The aim of this study was to develop a kinetic model for the hydrolysis of dimethyl terephthalate, using the experimental data. Various reaction conditions (temperature, pressure, time) have been investigated and used to calculate the activation energies and reaction rate constants, while the variation in reactor setup with methanol (MeOH) removal was conducted to show its detrimental effect on the efficiency of the reaction and selectivity towards terephthalic acid. The first reaction step of DMT to monomethyl terephthalate (MMT) is relatively irreversible, while the second reaction step of MMT to TPA ceases, as soon as the equilibrium is reached. Highest yields of TPA (76.7 %) were obtained after 40 min at 265 °C, with MeOH removal, and the activation energy was calculated to be 95 and 64 kJ mol−1 for the first and second hydrolysis reaction step, respectively.