Chemical Recycling Research Articles

Chemical Recycling of Cellulose Acetate Eyewear Industry Waste by Hydrothermal Treatment

Chemical Recycling of Cellulose Acetate Eyewear Industry Waste by Hydrothermal Treatment

Robust downstream technologies in polystyrene waste pyrolysis: Design and prospective life-cycle assessment of pyrolysis oil reintegration pathways

Chemical recycling of polystyrene waste to virgin styrene by fluidized bed pyrolysis and purification of pyrolysis oil in a distillation sequence is investigated. It is shown experimentally that pre-treatment of expanded polystyrene (EPS) waste by hot washing does not improve styrene yields and thus styrene content in the pyrolysis oil, suggesting that no elaborate waste pre-treatment is necessary for polystyrene chemical recycling. Based on experimental results, pyrolysis oils with styrene content from 30 wt.-% to 80 wt.-% are considered in an Aspen Plus® downstream simulation. Process robustness is evaluated in terms of operating parameter sensitivity such as reflux and distillate-to-feed ratio. It is shown that for the purification of highly variable feed compositions trade-offs such as more columns or higher RR must be made. Prospective LCA reveals that chemical recycling is preferable to incineration of polystyrene waste, although to varying degrees depending on critical impurities present in the pyrolysis oil.

Redesigned Nylon 6 Variants with Enhanced Recyclability, Ductility, and Transparency.

Geminal (gem-) disubstitution in heterocyclic monomers is an effective strategy to enhance polymer chemical recyclability by lowering their ceiling temperatures. However, the effects of specific substitution patterns on the monomer's reactivity and the resulting polymer's properties are largely unexplored. Here we show that, by systematically installing gem-dimethyl groups onto ϵ-caprolactam (monomer of nylon 6) from the α to ϵ positions, both the redesigned lactam monomer's reactivity and the resulting gem-nylon 6's properties are highly sensitive to the substitution position, with the monomers ranging from non-polymerizable to polymerizable and the gem-nylon properties ranging from inferior to far superior to the parent nylon 6. Remarkably, the nylon 6 with the gem-dimethyls substituted at the γ position is amorphous and optically transparent, with a higher Tg (by 30 °C), yield stress (by 1.5 MPa), ductility (by 3×), and lower depolymerization temperature (by 60 °C) than conventional nylon 6.

Redesigned Nylon 6 Variants with Enhanced Recyclability, Ductility, and Transparency

AbstractGeminal (gem−) disubstitution in heterocyclic monomers is an effective strategy to enhance polymer chemical recyclability by lowering their ceiling temperatures. However, the effects of specific substitution patterns on the monomer's reactivity and the resulting polymer's properties are largely unexplored. Here we show that, by systematically installing gem‐dimethyl groups onto ϵ‐caprolactam (monomer of nylon 6) from the α to ϵ positions, both the redesigned lactam monomer's reactivity and the resulting gem‐nylon 6’s properties are highly sensitive to the substitution position, with the monomers ranging from non‐polymerizable to polymerizable and the gem‐nylon properties ranging from inferior to far superior to the parent nylon 6. Remarkably, the nylon 6 with the gem‐dimethyls substituted at the γ position is amorphous and optically transparent, with a higher Tg (by 30 °C), yield stress (by 1.5 MPa), ductility (by 3×), and lower depolymerization temperature (by 60 °C) than conventional nylon 6.

Economic and environmental comparison of emerging plastic waste management technologies

Recovery of plastics may need to move beyond traditional mechanical methods and adopt emerging recycling processes including dissolution/precipitation, solvolysis, and pyrolysis. We investigate the costs and climate impacts of optimal solid waste management (SWM) strategies when deploying emerging recycling processes. Introducing a mix of emerging recycling technologies can reduce SWM system costs, increase plastic recycling rates, and potentially help SWM systems achieve net reductions in life cycle emissions. Recycling programs that rely solely on traditional mechanical recycling incur higher system costs, but can achieve the lowest life cycle emissions, regardless of whether the rejected plastic streams are landfilled or treated in waste-to-energy. In a future with increased recycling, SWM systems that utilize fully commercialized dissolution/precipitation and chemical recycling can further improve the cost advantage and the emission reduction potential. The sensitivity analysis demonstrates that enhancing waste collection and refining emerging recycling technologies can considerably increase the economic and environmental performance of SWM.

"Inverted" Cyclic(Alkyl)(Amino)Carbene (CAAC) Ruthenium Complex Catalyzed Isomerization Metathesis (ISOMET) of Long Chain Olefins to Propylene at Low Ethylene Pressure.

Isomerization Metathesis (ISOMET) reaction is an emerging tool for "open loop" chemical recycling of polyethylene to propylene. Novel, latent N-Alkyl substituted Cyclic(Alkyl)(Amino)Carbene (CAAC)-ruthenium catalysts (5a-Ru, 3b-Ru - 6c-Ru) are developed rendering "inverted" chemical structure while showing enhanced ISOMET activity in combination with (RuHCl)(CO)(PPh3)3(RuH) double bond isomerization co-catalyst. Systematic investigations reveal that the steric hindrance of the substituents on nitrogen and carbon atom adjacent to carbene moiety in the CAAC ligand have significantly improved the catalytic activity and robustness. In contrast to the NHC-Ru and CAAC-Ru catalyst systems known so far, these systems show higher isomerization metathesis (ISOMET) activity (TON: 7400) on the model compound 1-octadecene at as low as 3.0bar optimized pressure, using technical grade (3.0) ethylene. The propylene content formed in the gas phase can reach up to 20% by volume.

Accessing chemically recyclable polyamides via geminal dimethyl substitution

The development of new chemically recyclable polymers could serve as a means of reducing the plastic pollution and alleviating global energy crisis and monomer design strategy has driven the innovation in this field. In this contribution, α-dimethyl substituted caprolactam (α-DMCL) and γ-dimethyl substituted caprolactam (γ-DMCL) were prepared to evaluate the substitution position effect on chemical recyclability and material performance in polycaprolactam (PCL) system. The introduction of geminal dimethyl substitution to ε-caprolactam (CL) at α and γ position could change the thermal and mechanical properties of the resulting polyamides P(α-DMCL) and P(γ-DMCL). Remarkably, thermal depolymerization of P(γ-DMCL) in presence of potassium caprolactamate (CL-K) could be carried out at 200 °C and converted to γ-DMCL in 91% yield. The recovered γ-DMCL was capable of repolymerization to P(γ-DMCL) without a decrease in reactivity, demonstrating the proof-of-concept recyclability of P(γ-DMCL).

Polyurethane Foam Chemical Recycling: Fast Acidolysis with Maleic Acid and Full Recovery of Polyol.

Chemical recycling of polyurethane (PU) waste is essential to displace the need for virgin polyol production and enable sustainable PU production. Currently, less than 20% of PU waste is downcycled through rebinding to lower value products than the original PU. Chemical recycling of PU waste often requires significant input of materials like solvents and slow reaction rates. Here, we report the fast (<10 min) and solvent-free acidolysis of a model toluene diisocyanate (TDI)-based flexible polyurethane foam (PUF) at <200 °C using maleic acid (MA) with a recovery of recycled polyol (repolyol) in 95% isolated yield. After workup (hydrolysis of repolyl ester and separations), the repolyol exhibits favorable physical properties that are comparable to the virgin polyol; these include 54.1 mg KOH/g OH number and 624 cSt viscosity. Overall, 80% by weight of the input PUF is isolated into two clean-cut fractions containing the repolyol and toluene diamine (TDA). Finally, end-of-life (EOL) mattress PUF waste is recycled successfully with high recovery of repolyol using MA acidolysis. The solvent-free and fast acidolysis with MA demonstrated in this work with both model and EOL PUF provides a potential pathway for sustainable and closed-loop PU production.

Discrimination of plastic waste pyrolysis oil feedstocks using supercritical fluid chromatography

Advanced chemical recycling techniques provide new avenues for handling and recycling mixed plastic waste; pyrolysis is a prominent approach involving heating plastic waste in an oxygen-free environment to create pyrolysis oils. Pyrolysis oils must be thoroughly characterized before being refined into fuels and chemical feedstocks. Here, a method based on supercritical fluid chromatography with ultraviolet detection was developed to analyze plastic waste pyrolysis oils. Multiple stationary phases were examined, and 2-ethyl pyridine was chosen as the best stationary phase for resolving pyrolysis oil components. Different standards and different plastic waste pyrolysis oils were compared across the different stationary phases. Up to three columns were serially coupled to increase efficiency and column capacity. It was found that a general method using ethanol as a modifier and two 2-ethyl pyridine columns could effectively resolve plastic waste pyrolysis oils. The potential for differentiating polyethylene and polypropylene feedstocks was demonstrated using principal component analysis.

Closed-Loop Recyclable and Nonpersistent Polyethylene-like Polyesters.

ConspectusAliphatic polyesters based on long-chain monomers were synthesized for the first time almost a century ago. In fact, Carothers' seminal observations that founded the entire field of synthetic polymer fibers were made on such a polyester sample. However, as materials, they have evolved only over the past decade. This is driven by the corresponding monomers becoming practically available from advanced catalytic conversions of plant oils, and future prospects comprise a possible generation from third-generation feedstocks, such as microalgae or waste. Long-chain polyesters such as polyester-18.18 can be considered to be polyethylene chains with a low density of potential breakpoints in the chain. These do not compromise the crystalline structure or the material properties, which resemble linear high-density polyethylene (HDPE), and the materials can also be melt processed by injection molding, film or fiber extrusion, and filament deposition in additive manufacturing. At the same time, they enable closed-loop chemical recycling via solvolysis, which is also possible in mixed waste streams containing polyolefins and even poly(ethylene terephthalate). Recovered monomers possess a quality that enables the generation of recycled polyesters with properties on par with those of the virgin material. The (bio)degradability varies enormously with the constituent monomers. Polyesters based on short-chain diols and long-chain dicarboxylates fully mineralize under industrial composting conditions, despite their HDPE-like crystallinity and hydrophobicity. Fundamental studies of the morphology and thermal behavior of these polymers revealed the location of the in-chain groups and their peculiar role in structure formation during crystallization as well as during melting. All of the concepts outlined were extended to, and elaborated on further, by analogous long-chain aliphatic polymers with other in-chain groups such as carbonates and acetals. The title materials are a potential solution for much needed circular closed-loop recyclable plastics that also as a backstop if lost to the environment will not be persistent for many decades.

Semi-Crystalline and Amorphous Polyesters Derived from Biobased Tri-Aromatic Dicarboxylates and Containing Cleavable Acylhydrazone Units for Short-Loop Chemical Recycling

Semi-Crystalline and Amorphous Polyesters Derived from Biobased Tri-Aromatic Dicarboxylates and Containing Cleavable Acylhydrazone Units for Short-Loop Chemical Recycling

Material Separation from Polyester/Cotton Blended Fabrics Using Hydrothermal Treatment.

The production of textile products is increasing annually, and most of them are disposed of after use without recycling. One of the reasons for the low recycling percentage of discarded textile products is the difficulty of recycling as a single material as these products are produced from a combination of two or more materials. Therefore, a technology to separate materials is necessary to improve the recycling percentage of textile products and to build a sustainable recycling industry. The aim of this study was to separate the most common combination of materials, such as cotton/polyester, in an environmentally friendly technique using hydrothermal treatment with only water. Herein, the optimal treatment conditions for blended fabrics in a high-pressure reactor were studied. Moreover, cotton could be separated by treating the fabrics at 220 to 230 °C for 10 min while maintaining the shape of the fabrics. Additionally, polyester showed a melting point, confirming that polyester could be separated without decomposition into monomers, unlike common chemical recycling. The strength of the separated cotton and the molecular weight of the polyester were evaluated, and a kinetic analysis of the changes due to the treatment was conducted. The activation energy obtained from the Arrhenius plot was 111.8 kJ/mol for PET, which was smaller than 142.6 kJ/mol for cotton. This indicates that the decrease in the molecular weight of PET is more likely to occur than the change in the strength of cotton, suggesting the possibility of separating the materials from the kinetic analysis.

Chemical recycling – A key technology for the circular economy

Chemical recycling – A key technology for the circular economy

Investigation on the utilization of fiber clusters recycled from waste HFRP bars in concrete

The waste fiber reinforced polymer (FRP) bars are difficult to degrade naturally, and inevitably pose a significant challenge to environmental protection, resulting in the recycling of waste FRP bars is of great significance. Compared to chemical recycling and thermal recycling, mechanical recycling processes are more economical and environmental. In this study, the waste Hybrid FRP (HFRP) bars were mechanically recycled into fiber clusters for reinforcing concrete, and the influences of the fiber clusters on the mechanical properties of concrete were studied through compression test, splitting tensile test and four-point bending test. Meanwhile, Fourier transform infrared (FT-IR) spectroscopy and Scanning electron microscopy (SEM) were used to observe the interaction between the fiber clusters and the concrete, and acoustic emission (AE) technique was utilized to monitor the fracture characteristics inside the concrete. Results showed that the performance of mechanically recycled fiber clusters was retained to a certain extent and the alkaline environment of concrete produced slight damage to the fiber clusters. The addition of fiber clusters could obviously improve the mechanical properties of concrete, especially its energy absorption capacity under the splitting tensile and flexural loads. The absorbed energy EH/50 and peak flexural toughness of concrete with 2% fiber clusters were 97.24% and 126.47% higher than those of conventional concrete, respectively, and its flexural toughness index TP,150 was 163.16% higher than that of concrete containing 0.5% fiber clusters. As the volume fraction of fiber clusters increased, the number of AE event localization points and shear cracks mainly caused by fiber clusters pull-out and sliding increased. This study can bring enlightenment for the mechanical recycling of FRP bars into fiber clusters for concrete.

Pyrolysis-derived waste polypropylene oils in gas turbine engines: a comprehensive performance and emission study

Addressing the burgeoning issue of polymer waste management and disposal, chemical recycling, specifically the production of highquality oil, presents an enticing solution. This research paper delves into the process of plastic waste pyrolysis, focusing on polypropylene, and thoroughly examines the physico-chemical properties of the resulting pyrolytic oil. The oils, obtained from waste plastic pyrolysis (referred to as WPPO), are then blended with kerosene and utilized as fuel for a gas turbine engine. The primary objective of this investigation is to ascertain how the blend composition influences the performance and emission parameters of the micro gas turbine. In our findings, it was observed that all tested waste plastic pyrolysis blends displayed a trend towards escalating regulated emissions such as nitrogen oxides (NOx) with an average increase of 26% for polypropylene pyrolysis oil (PPO). The emission index (EI) for carbon monoxide (CO) was found to be relatively consistent across all fuel blends tested in this study. Interestingly, when considering the thrust specific fuel consumption (TSFC) within the EI calculation, blends of aviation kerosene and plastic oil showed lower values in comparison to the pure Jet A-1 fuel. Furthermore, an augmentation in the proportion of WPPO in the blends consequently led to an elevation in the exhaust gas temperature (an average increase of 8.7% for PPO). Interestingly, the fuel efficiency of the Jet engine, expressed as TSFC, demonstrated a decrease, with an average reduction of 13.8% observed for PPO.

Chemical Recycling and Physical Tuning of Necklace-Shaped Polydimethylsiloxanes Bearing Anthracene Dimer Units.

The problem of plastic waste in the environment calls for the development of new polymeric materials designed specifically for easy recycling at the end of their life cycle. Herein, a green polymer system comprising a series of necklace-shaped polydimethylsiloxanes bearing anthracene dimer units is developed. The polymers have low environmental impact and are easily recycled. Further, their flexibility and glass transition temperatures are easy to control. These necklace-shaped inorganic polymers are synthesized by photopolymerizing (dimerizing) anthracene-terminated oligo-dimethylsiloxane monomers. A key achievement of the present work is the successful chemical recovery of the monomers from the polymers through thermal depolymerization, enabling monomer-polymer recycling. By applying equilibrium polymerization with base catalysts, monomers with a controlled distributed chain length are synthesized from monomers with a constant chain length. The necklace-shaped polymers synthesized from these randomized monomers have amorphous structures and readily form transparent films. It is possible to modulate the thermal and mechanical properties of the polymers by controlling the average chain length of the polydimethylsiloxane between the anthracene dimers. This investigation presents a method for the synthesis and cyclic utilization of polymer materials with a wide range of applications, including plastics and elastomers.

Synthesis and Deconstruction of Polyethylene-type Materials.

Polyethylene deconstruction to reusable smaller molecules is hindered by the chemical inertness of its hydrocarbon chains. Pyrolysis and related approaches commonly require high temperatures, are energy-intensive, and yield mixtures of multiple classes of compounds. Selective cleavage reactions under mild conditions (<ca. 200 °C) are key to improve the efficacy of chemical recycling and upcycling approaches. These can be enabled by introduction of low densities of predetermined breaking points in the polyethylene chains during the step-growth or chain-growth synthetic construction of designed-for-recycling polyethylene-type materials. Alternatively, they can be accomplished by postpolymerization functionalization of postconsumer polyethylene waste via dehydrogenation and follow-up reactions or through oxidation to long-chain dicarboxylates. Deconstruction of litter under environmental conditions via the aforementioned break points can alleviate plastics' persistency, as a backstop to closed-loop recycling.

Chemical recycling of plastic waste for sustainable polymer manufacturing – A critical review

Chemical recycling has emerged as a promising approach to valorise non-recyclable, rejected, multi-layer, and contaminated plastic waste streams, such as packaging waste, aligning with the 3R's principles (reduce, reuse, and recycle). This paper critically evaluates various chemical recycling technologies, including pyrolysis, gasification, hydrothermal, and biological methods, which hold the potential to amend the global plastic pollution crisis while recovering valuable materials. Despite the potential of these methods, substantial challenges persist, including technological limitations, elevated capital costs, feedstock variability, energy consumption, and poor regulatory frameworks. The challenges and opportunities for future development are thoroughly discussed. Continuous research and development are vital for advancing chemical recycling technologies and transitioning towards a circular economy. A holistic approach, integrating not only chemical recycling, but also waste reduction, reuse, and other waste management strategies, is required to resolve the plastic waste crisis, and attain a more sustainable future.

Impact of Metal Impregnation of Commercial Zeolites in the Catalytic Pyrolysis of Real Mixture of Post-Consumer Plastic Waste

This work reports the study of the catalytic pyrolysis of rejected plastic fractions collected from municipal solid waste whose mechanical recovery is not plausible due to technical or poor conservation issues. The chemical recycling using catalytic pyrolysis was carried out over commercial zeolites formulas, i.e., HY and HZSM-5, in which Ni or Co metals were deposited at two different loadings (1 and 5%, wt.). The presence of these transition metals on the zeolitic supports impacted the total production of compounds existing in the liquid oil. The samples were characterized in terms of structural, chemical, and morphologic properties, and the production of different fuel fractions (gasoline, light cycle oil, and heavy cycle oil) was correlated with a combined parameter defined as a ratio of Acidity/BET area.

Chemical Recycling of Polycarbonates via Ammonolysis and Polycaprolactones by KOH‐Catalyzed Methanolysis

AbstractChemical recycling polymer waste is a highly required technology to follow the current carbon‐neutral trend. Here, we present practical recycling methods for poly (bisphenol A carbonate) (PC) and poly(ϵ‐caprolactone) (PCL). PC can be converted to bisphenol A and urea with excellent product yield at 80 °C via ammonolysis using ammonium salt (ammonium‐formate or ‐carbonate) instead of liquid ammonia that is usually used in previous PC recycling. PCL can be converted to methyl 6‐hydroxy‐hexanoate monomer with excellent yield at 80 °C via methanolysis catalyzed by KOH.