Chemical Recycling Methods Research Articles

Chemical Recycling of Epoxy Thermosets: From Sources to Wastes

As one of the most widely used thermosets due to its excellent performances, epoxy resin (EP) is widely used in various fields and often employed as a component of composite actuator devices, strengthening their mechanical properties. However, the expanding production of EP inevitably leads to the accumulation of waste end-of-life equipment and the corresponding increasingly serious environmental problems. This review summarizes the recycling strategies of EP, divided into two perspectives: recycling from wastes and sources. Chemical recycling is expected to be the future of waste EP treatment, and we discuss the chemical recycling methods of existing waste EP based on different mechanisms, including the selective cleavage of ester bonds, C–N bonds, and C–O bonds. On the other hand, epoxy vitrimer networks based on various dynamic covalent linkages are also outlined, which can respond to multiple external stimuli and provide materials with recyclability from the origin. Therefore, the use of epoxy vitrimer actuators can prevent waste generation throughout the whole lifecycle. We present some issues of concern in both waste-based and source-based recycling strategies and emphasize the significance of scaling-up. Finally, we summarized the current situation and present some future perspectives with the aim of making practical contributions to environmental issues.

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.

Characterization and Optimization of Biocatalysts for New Recycling Technologies

While the term “plastic” refers to a variety of chemically distinct compounds, these materials are all prized for one key property: incredible chemical stability. Consequently, plastics have become a crutch for modern society, both industrially and at-home. However, mechanical recycling initiatives have proven largely ineffective and uneconomical, which has led to a widespread accumulation of plastics and microplastics in both terrestrial and aquatic ecosystems. The limitations of physical recycling have increased interest in alternative protocols like chemical recycling methods. This project explores an intriguing biochemical approach which involves enzymes that can catalyze the degradation of specific synthetic polymers. This study investigates five novel nylon-degrading enzymes, previously isolated from thermostable organisms. The primary objective of this research is to develop and refine procedures for the expression and purification of these nylonases. The enzymes were overexpressed in Escherichia coli hosts and subsequently purified using advanced chromatographic techniques, including Fast Protein Liquid Chromatography (FPLC) and size exclusion chromatography (SEC). The optimization of these purification procedures is critical, as it ensures that subsequent assays are conducted with highly purified enzyme samples, while minimizing contaminants and improving sample yield. Future research directions will involve a detailed mechanistic examination of the enzymatic autocleavage and the nylon degradation. An additional goal aims to crystallize the proteins to analyze their active sites. This comprehensive approach is expected to advance our understanding of enzymatic plastic degradation and contribute to the development of more effective recycling technologies.

Synthesis and properties of biodegradable PBAT prepared from PBT chemically recycled resources

The production of synthetic polymer materials has engendered substantial environmental hazards stemming from waste generation. Employing chemical recycling methods constitutes a viable approach for attaining sustained recycling of synthetic raw materials. In the present study, bishydroxybutyl terephthalate (BHBT), a precursor for the synthesis of poly(butylene adipate-co-terephthalate) (PBAT), was created by chemically alcoholyzing polybutylene terephthalate (PBT). Further, bis(4-hydroxybutyl) adipate (BHAT), which was produced via esterification with adipic acid (AA) and 1,4-butanediol (BDO), and BHBT condensation polymerization were combined to create biodegradable PBAT. Additionally, the effects of the recycled BHBT and BHAT segment ratio on the structures and properties of biodegradable PBAT were investigated. The results indicate that the synthesis of repolymerized PBAT was accomplished successfully. As the proportion of butylene terephthalate (BT) segments increased, several notable improvements were observed: an increase in melting temperature, enhanced thermal stability, elevated tensile strength, and a reduction in the rate of biodegradation. The melting point was 139.51 °C, the tensile strength was 22.62 MPa, and the 30-day biodegradation rate was 23.32 % when the percentage of BT segments was 60 %. In the present study, a method for producing biodegradable polyesters through the process of alcoholysis and repolymerization of recalcitrant polyester waste was proposed. It provides a new solution for the efficient recycling of waste polymer materials.

Hybrid recycling methods for fiber reinforced polymer composites: A review

Fibrous polymer composites are integral materials utilized in various sectors due to their unparalleled material properties. However, their widespread application has led to significant volumes of end-of-life material, necessitating sustainable disposal strategies. Conventional disposal methods, landfilling, and incineration exhibit shortcomings in both fiber recovery efficiency and environmental impact. Conversely, recycling presents a viable avenue for fiber recovery, facilitating its reuse in composite fabrication alongside allied industries like construction. This article provides a comprehensive examination of mechanical, thermal, and chemical recycling methods tailored for fibrous composites made of thermoset resins. In addition to traditional approaches, hybrid techniques emerge as compelling recycling prospects, amalgamating various traditional methods to enhance overall process efficiency while mitigating the limitations. This article stands out for its in-depth exploration of the intricate methodologies underpinning hybrid recycling methods. The article also delineates the range of products produced from recycling, encompassing fibers, liquids, and gaseous byproducts. In addition, a dedicated section outlines the potential applications of recovered fibers within the composite industry and their integration into cutting-edge additive manufacturing processes. Through a holistic examination of recycling strategies and their downstream implications, this article underscores the imperative for sustainable practices in the utilization and management of fibrous polymer composites.

From Waste to Energy: Enhancing Fuel and Hydrogen Production through Pyrolysis and In-Line Reforming of Plastic Wastes

Plastics have become integral to modern life, playing crucial roles in diverse industries such as agriculture, electronics, automotive, packaging, and construction. However, their excessive use and inadequate management have had adverse environmental impacts, posing threats to terrestrial and marine ecosystems. Consequently, researchers are increasingly searching for more sustainable ways of managing plastic wastes. Pyrolysis, a chemical recycling method, holds promise for producing valuable fuel sustainably. This study explores the process of the pyrolysis of plastic and incorporates recent advancements. Additionally, the study investigates the integration of reforming into the pyrolysis process to improve hydrogen production. Hydrogen, a clean and eco-friendly fuel, holds significance in transport engines, power generation, fuel cells, and as a major commodity chemical. Key process parameters influencing the final products for pyrolysis and in-line reforming are evaluated. In light of fossil fuel depletion and climate change, the pyrolysis and in-line reforming strategy for hydrogen production is anticipated to gain prominence in the future. Amongst the various strategies studied, the pyrolysis and in-line steam reforming process is identified as the most effective method for optimising hydrogen production from plastic wastes.

Covestro becomes shareholder in company BioBTX

Covestro announces it has become a shareholder in company BioBTX, a company recovering raw materials by recycling plastic waste, by investing a mid-single-digit million Euro amount. A demonstration plant is planned for the new chemical recycling method.

A life cycle assessment of the laboratory—scale oxidative liquefaction as the chemical recycling method of the end-of-life wind turbine blades

According to the latest reports, estimated values of 50,000–66 000 t of end-of-life wind turbine blades (WTB) are expected to be decommissioned in Europe in 2025–2030, posing a significant threat from the environmental and waste management perspectives. This study aims to present the preliminary Life Cycle Assessment (LCA) with sensitivity and uncertainty analysis of the lab-scale oxidative liquefaction process of the WTB, as the original method to recover the high-quality glass fibers with simultaneous production of the secondary chemicals: phenols, ketones, acids, and fatty acids, from the oxidation of the epoxy resin from the polymer matrix. The LCA is based on the experimental results of the oxidative liquefaction process carried out on a laboratory scale using a Parr 500 ml batch reactor, at two different conditions sets for the functional unit (FU) of 1 kg of treated WTB. Each of the analyzed scenarios resulted in higher impact indicators compared to the landfilling. The highest quality fibers were obtained at 350 °C and 40 wt % H2O2 content resulted in 5.52 ± 1.20 kgCO2 eq Climate change impact and 97.8 ± 20.6 MJ of Resource use, fossil per kg of recycled WTB. The lowest quality fiber recovered in char, yet well separated from the matrix obtained at 250 °C and the lowest H2O2 content resulted in 0.0953 ± 0.487 kgCO2 eq Climate change impact and 8.84 ± 7.90 MJ of Resource use, fossil per kg of recycled WTB. The hot spot and sensitivity analysis indicated, that the oxidizer for the process – hydrogen peroxide, when acquired as a shelf product causes a significant burden on the whole process, with sensitivity ratios on the total impact indicators varying across the categories from 0.56 to 0.99. Substitution of H2O2 with theoretical 0-input oxidizer allowed to significantly lower environmental load of the recycling process, which in all of the analyzed scenarios presented environmental benefits compared to landfilling with recovery of the glass fiber and secondary chemicals.

Searching for the Achilles' Heel of Urethane Linkage-An Energetic Perspective.

A sudden increase in polyurethane (PU) production necessitates viable recycling methods for the waste generated. PU is one of the most important plastic materials with a wide range of applications; however, the stability of the urethane linkage is a major issue in chemical recycling. In this work, termination reactions of a model urethane molecule, namely methyl N-phenyl carbamate (MPCate), are investigated using G3MP2B3 composite quantum chemical method. Our main goal was to gain insights into the energetic profile of urethane bond termination and find an applicable chemical recycling method. Hydrogenation, hydrolysis, methanolysis, peroxidation, glycolysis, ammonolysis, reduction with methylamine and termination by dimethyl phosphite were explored in both gas and condensed phases. Out of these chemicals, degradation by H2, H2O2 and CH3NH2 revealed promising results with lower activation barriers and exergonic pathways, especially in water solvation. Implementing these effective PU recycling methods can also have significant economic benefits since the obtained products from the reactions are industrially relevant substances. For example, aniline and dimethyl carbonate could be reusable in polymer technologies serving as potential methods for circular economy. As further potential transformations, several ionizations of MPCate were also examined including electron capture and detachment, protonation/deprotonation and reaction with OH-. Alkaline digestion against the model urethane MPCate was found to be promising due to the relatively low activation energy. In an ideal case, the transformation of the urethane bond could be an enzymatic process; therefore, potential enzymes, such as lipoxygenase, were also considered for the catalysis of peroxidation, and lipases for methanolysis.

Tuning of molecular weight and chemical composition of polyols obtained from Poly(ethylene terephthalate) waste recycling through the application of organocatalysts in an upcycling route

High molecular weight liquid polyols were produced from complex Poly (ethylene terephthalate) (PET) waste (low molecular weight PET waste, monolayer compounds, and coloured PET bottles) in one-step synthesis using mild reaction conditions. For the first time, a wide range of upcycled polyols were obtained by modulating the reaction conditions to control the chemical composition and molecular weight, in order to revalorize the same PET waste in different polyols.A new versatile, efficient, selective and sustainable chemical recycling method was developed, employing cheap reagents (ethylene carbonate) and basic organocatalysts, under mild reaction conditions (moderate temperatures and atmospheric pressure). Chemically, the polyols showed a singular composition, consisting of three types of repeating units, two coming from PET and one belonging to the ethylene carbonate ring-opening. Depending on the depolymerisation conditions of the PET waste and the catalyst used, the ratio between these moieties varied considerably, giving rise to a wide portfolio of liquid polyols. In addition, the ability of the organocatalyst to introduce the carbonate unit into the polyol chain was analysed as a function of the number of ethylene carbonate units used.As a proof of concept, these new liquid secondary raw materials served as a starting point for the synthesis of polyurethanes under standard conditions. Ultimately, the final objective of this upcycling process will be the application of a wide range of high-quality polyols as soft segments to produce more sustainable and high-performance polyurethanes (flexible foams, coatings, sealants, adhesives, elastomers).

Convenient Size Analysis of Nanoplastics on a Microelectrode.

Chemical recycling is a promising approach to reduce plastic pollution. Timely and accurate size analysis of produced nanoplastics is necessary to monitor the process and assess the quality of chemical recycling. In this work, a sandwich-type microelectrode sensor was developed for the size assessment of nanoplastics. β-Mercaptoethylamine was modified on the microelectrode to enhance its surface positive charge density. Polystyrene (PS) nanoplastics were captured on the sensor through electrostatic interactions. Ferrocene was used as an electrochemical beacon and attached to PS via hydrophobic interactions. The results show a nonlinear dependence of the sensor's current response on the PS particle size. The size resolving ability of the microelectrode is mainly attributed to the small size of the electrode and the resulting attenuation of the electric field strength. For mixed samples with different particle sizes, this method can provide accurate average particle sizes. Through an effective pretreatment process, the method can be applied to PS nanoplastics with different surface properties, ensuring its application in evaluating different chemical recycling methods.

Multi-color polymer carbon dots synthesized from waste polyolefins through phenylenediamine-assisted hydrothermal processing

The large accumulation and low recycling rates of polyolefin waste have posed a threat to the environment and human health. The shortage of chemical recycling methods for polyolefins strongly demands the development of new and sustainable treatment technologies for hydrocarbon plastics to improve their waste management. In this study, polyethylene (PE) and polypropylene (PP) were utilized for the preparation of multi-color polymer carbon dots (PCDs) via a two-step hydrothermal (HT) synthesis involving (i) thermo-oxidative degradation of polyolefins to precursors containing plentiful oxygen-based functional groups, and (ii) modification with phenylenediamine (PDA). The fluorescence of PCDs depends on the structure of isomeric PDA and PCDs modified by ortho-, meta-, and para-PDA emit blue, green, and yellow color fluorescence, respectively. The formation mechanism of PCDs, involving dehydrative condensation and amination of PE or PP-derived precursors by PDA, was proposed. The obtained PCDs were utilized for the detection and quantification of Fe3+ ions at ppm concentrations. The proposed strategy here aims to broaden the scope of the chemical recycling methods for polyolefin plastic waste as well as to develop a conversion route of polyolefin to value-added materials.

Degradation Product-Promoted Depolymerization Strategy for Chemical Recycling of Poly(bisphenol A carbonate)

The accumulation of waste plastics has a severe impact on the environment, and therefore, the development of efficient chemical recycling methods has become an extremely important task. In this regard, a new strategy of degradation product-promoted depolymerization process was proposed. Using N,N′-dimethyl-ethylenediamine (DMEDA) as a depolymerization reagent, an efficient chemical recycling of poly(bisphenol A carbonate) (BPA-PC or PC) material was achieved under mild conditions. The degradation product 1,3-dimethyl-2-imidazolidinone (DMI) was proven to be a critical factor in facilitating the depolymerization process. This strategy does not require catalysts or auxiliary solvents, making it a truly green process. This method improves the recycling efficiency of PC and promotes the development of plastic reutilization.

Revealing the long way towards lead-free plastic in China through dynamic material flow analysis of lead salt heat stabilizers in PVC products

The use of lead salt heat stabilizers (LHSs) in plastics are restricted or even prohibited owing to their high toxicity. However, little information is available on its industrial metabolism. A dynamic material flow analysis of LHSs indicates that China consumed 2952 kt LHSs between 1958 and 2020. Moreover, at present, 1020 kt of these is being used in polyvinyl chloride (PVC) products, 1007 kt is in landfills, and 681 kt is in the environment. For China to realize lead-free primary plastics by 2040, 2050, and 2060, the LHSs consumption should be reduced by at least 13 %, 10 %, and 9 % annually, respectively. Certain LHSs would gain new life with PVC recycling. Therefore, approximately 30 years would be required for LHSs to disappear completely after their use is stopped. With improvements in the plastic recycling rate, it is necessary to use chemical recycling and other methods to separate the hazardous additives such as LHSs and reduce the circulation of toxic substances.

Shaping a federal strategy for chemical recycling: Moving toward sensible applications of emerging technologies in US plastic waste management

AbstractPlastic waste management requires a diverse portfolio of strategies to avoid the most damaging environmental and social impacts of the plastic lifecycle and meet current national and international policy goals. The limitations of existing mechanical recycling methods for processing plastic waste have motivated the development and commercialization of chemical recycling processes. The novelty and diversity of such pathways beg critical questions of how these emerging technologies will fit within existing policy frameworks and contribute to recycling, plastic pollution, and waste management policy objectives. This work provides technical background into mechanical and chemical recycling methods, and policy background into differing modern approaches to chemical recycling at various levels of governance. Finally, this work summarizes key conflicts and crafts recommendations for future policy action on chemical recycling in three main areas: (1) defining an objective and identifying sensible applications for chemical recycling technologies, (2) filling gaps in existing knowledge and upholding environmental justice protections, and (3) implementing a suite of policies to complement chemical recycling's role in plastic waste management strategy.

Boosting the kinetics of PET glycolysis

Glycolysis is the most promising chemical recycling method to depolymerize poly(ethylene terephthalate) (PET) with ethylene glycol (EG) into the monomer bis(2-hydroxyethyl) terephthalate (BHET).

Study on Low-Energy Chemical Recycling Method for Polyesters

Study on Low-Energy Chemical Recycling Method for Polyesters

Improvement in BHETA yield by aminolysis and its application in hot melt adhesive

Polyethylene terephthalate (PET) recycling has been carried out by various methods, e.g., mechanical recycling, chemical recycling, and energy recovery method. In this study, chemical recycling of PET was carried out by aminolysis using ethanolamine and converted into bis(2-hydroxyethyl) terephthalamide (BHETA). We have studied the effective polymeric catalysis of the aminolytic depolymerization of waste polyethylene terephthalate (PET) using sulfated polyborate producing a broad range of crystalline terephthalamides. The reaction was performed by varying the PET: ethanolamine ratio, reaction time and catalyst. Recycled PET bottles, film, and fibres were used for conversion. A yield of 95 % was obtained for PET: ethanolamine ratio of 1:4 (w/w), with 4 h reaction time, at 160oC with sulfated polyborate as a catalyst. BHETA was characterized with FTIR, 1H,13C NMR, and DSC analysis. BHETA further gets reacted with dimer acid along with Ethylenediamine (EDA) at different molar ratios which produces polyesteramide (PEA) based hot melt adhesive (HMA). Thermal, mechanical, and rheological tests are performed and compared with traditional HMA. The effect on the properties of the hot melt adhesive, such as mechanical properties: tensile strength, shore D hardness, elongation at break; thermal properties: glass transition temperature (Tg), melting temperature (Tm), the heat of crystallization (Hc), crystallization temperature (Tc); adhesion properties: lap shear strength (LSS) and T-peel strength (TPS); rheological properties and degree of crystallinity were investigated. It was observed that the mechanical and thermal properties are increased by optimizing the concentration of BHETA. These HMA materials may have the capacity of being eco-friendly and have high adhesion properties.

Deconstructable and Chemically Recyclable High-Density Polyethylene (HDPE)-Like Materials Based on Si-O Bonds.

Polyethylene (PE) is the most widely produced synthetic polymer. By installing chemically cleavable bonds into the backbone of PE, it is possible to produce chemically deconstructable PE derivatives; to date, however, such designs have primarily relied on carbonyl- and olefin-related functional groups. Bifunctional silyl ethers (BSEs; SiR2(OR'2)) could expand the functional scope of PE mimics as they possess strong Si-O bonds, excellent oxidative stabilities, and facile chemical tunability. Here, we report BSE-containing high-density polyethylene (HDPE)-like materials synthesized through a one-pot catalytic ring-opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si-substituents, providing handles to control thermomechanical properties while maintaining the high thermal stability of HDPE. Two methods for chemical recycling of HDPE mimics are introduced, including a circular approach that leverages acid-catalyzed Si-O bond exchange with 1-propanol. Additionally, despite the fact that the starting HDPE mimics were synthesized by chain-growth polymerization (ROMP), we show that it is possible to recover the molar mass and dispersity of recycled HDPE products using step-growth Si-O bond formation or exchange, generating high molecular weight recycled HDPE products with mechanical properties similar to commercial HDPE.

Thermally Robust yet Deconstructable and Chemically Recyclable High‐Density Polyethylene (HDPE)‐Like Materials Based on Si−O Bonds

AbstractPolyethylene (PE) is the most widely produced synthetic polymer. By installing chemically cleavable bonds into the backbone of PE, it is possible to produce chemically deconstructable PE derivatives; to date, however, such designs have primarily relied on carbonyl‐ and olefin‐related functional groups. Bifunctional silyl ethers (BSEs; SiR2(OR′2)) could expand the functional scope of PE mimics as they possess strong Si−O bonds and facile chemical tunability. Here, we report BSE‐containing high‐density polyethylene (HDPE)‐like materials synthesized through a one‐pot catalytic ring‐opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si‐substituents, providing handles to control thermomechanical properties. Two methods for chemical recycling of HDPE mimics are introduced, including a circular approach that leverages acid‐catalyzed Si−O bond exchange with 1‐propanol. Additionally, despite the fact that the starting HDPE mimics were synthesized by chain‐growth polymerization (ROMP), we show that it is possible to recover the molar mass and dispersity of recycled HDPE products using step‐growth Si−O bond formation or exchange, generating high molecular weight recycled HDPE products with mechanical properties similar to commercial HDPE.