Solvent-based separation and recycling of waste plastics: A review

Upcycling polyamide containing post-consumer Tetra Pak carton packaging to valuable chemicals and recyclable polymer

Billion tons of post-consumer Tetra Pak cartons are discarded annually as land and ocean wastes, creating significant environmental problems and resource losses. Recycling of the carton wastes is hindered by its multi-material compositions and low values of the recycled products. In this study, a novel upcycling of the cartons was investigated. A post-consumer carton consisting of paper, polyolefin, and polyamide was directly converted in 210-230 °C tetrahydrofuran containing 10-20 mM acid to produce up to 19.2% of levoglucosenone and 8.6% of furfural by selectively decomposing paper fraction. The remaining solids containing mostly intact polyethylene and polyamide but also a smaller fraction of paper-derived char were separated using a solvent-dissolution method. The xylene-soluble fraction was a recycled polymer similar to the original polyethylene, which was verified by its functional groups, the composition of the pyrolysis products, and the melt rheology results. The xylene-insoluble fraction was a mixture of polyamide and paper-derived char. Upon pyrolysis, caprolactam was produced as the only major vapor product. The remaining, thermally stable paper-derived char could be used as a high-quality solid fuel. Overall, the demonstrated recycling method could potentially maximize the values of the products recovered from carton wastes.

Toward polymer upcycling-adding value and tackling circularity

Plastics have revolutionized modern life, but have created a global waste crisis driven by our reliance and demand for low-cost, disposable materials. New approaches are vital to address challenges related to plastics waste heterogeneity, along with the property reductions induced by mechanical recycling. Chemical recycling and upcycling of polymers may enable circularity through separation strategies, chemistries that promote closed-loop recycling inherent to macromolecular design, and transformative processes that shift the life-cycle landscape. Polymer upcycling schemes may enable lower-energy pathways and minimal environmental impacts compared with traditional mechanical and chemical recycling. The emergence of industrial adoption of recycling and upcycling approaches is encouraging, solidifying the critical role for these strategies in addressing the fate of plastics and driving advances in next-generation materials design.

Value-added products from thermochemical treatments of contaminated e-waste plastics

The rise of electronic waste (e-waste) generation around the globe has become a major concern in recent times and its recycling is mostly focused on the recovery of valuable metals, such as gold, silver, and copper, etc. However, e-waste consists of a significant weight fraction of plastics (25-30%) which are either discarded or incinerated. There is a growing need for recycling of these e-waste plastics. The majority of them are made from high-quality polymers (composites), such as acrylonitrile butadiene styrene (ABS), high impact polystyrene (HIPS), polycarbonate (PC), polyamide (PA), polypropylene (PP) and epoxies. These plastics are often contaminated with hazardous materials, such as brominated flame retardants (BFRs) and heavy metals (such as Pb and Hg). Under any thermal stress (thermal degradation), the Br present in the e-waste plastics produces environmentally hazardous pollutants, such as hydrogen bromide or polybrominated diphenyl ethers/furans (PBDE/Fs). The discarded plastics can lead to the leaching of toxins into the environment. It is important to remove the toxins from the e-waste plastics before recycling. This review article gives a detailed account of e-waste plastics recycling and recovery using thermochemical processes, such as extraction (at elevated temperature), incineration (combustion), hydrolysis, and pyrolysis (catalytic/non catalytic). A basic framework of the existing processes has been established by reviewing the most interesting findings in recent times and the prospects that they open in the field recycling of e-waste plastics.

Catalytic methods for chemical recycling or upcycling of commercial polymers

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.

Recycling of Aseptic Beverage Cartons: A Review

Aseptic beverage cartons are multilayer polymer-coated paperboards with a layer of aluminum foil. Due to their multilayer structure it is commonly assumed that they cannot be recycled. This is not the case and this review details the multifarious processes that are used to recycle aseptic beverage cartons. Hydrapulping to recover the paper fibers that constitute 75% of the carton is the most widespread process, followed by the manufacture of construction materials such as boards and tiles which utilize the complete carton. A range of mechanical, chemical and thermal processes are used to separate the PolyAl (polyethylene and aluminum) residual that remains after the paper fibers have been recovered. The simplest process involves agglutination followed by extrusion to obtain pellets that can then be used in industrial and consumer products or combined with other materials such as lignocellulosic wastes. Chemical approaches involve the solubilization of polyethylene and the removal of aluminum. Various thermal processes have also been investigated and a novel microwave-induced pyrolysis process appears the most commercially viable. It is concluded that the focus in future years is likely to be on recycling cartons into construction materials where there is a theoretical yield of 100% compared with 75% for hydrapulping.

On-line spectroscopic study of brominated flame retardant extraction in supercritical CO2

Removal of brominated flame retardants (BFRs) from polymers before disposal or recycling will alleviate negative environmental effects and ensure safe usage of recycled products. Extraction of BFRs in supercritical CO₂ is appealing but also presents challenges to industries due to limited solubility and lack of kinetic studies. For a more comprehensive evaluation of supercritical extraction potentialities, we (i) developed an on-line pressure apparatus that is compatible with both the FTIR and UV-vis spectrometers to enable kinetic and thermodynamic studies; (ii) studied kinetic extraction involving three conventional and two novel BFRs as well as three typical polymeric matrix. Solubilities were determined using the gravimetric method or X-ray fluorescence. FTIR exhibited a superior applicability compared to UV-vis in the following BFR extraction's time-dependency binary and ternary systems. We observed that faster stirring speed, higher temperature, and finer particle size can accelerate the overall extraction kinetics. In binary systems, it took less than 2 h to achieve equilibrium for each BFR at 60 °C, 25 MPa and 1000 rpm. In the presence of polymeric matrix, slower extraction kinetics were observed due to the occurrence of competitive dissolution and molecular diffusion within the matrix. Mathematical models derived from irreversible desorption and Fick's diffusion laws fitted well with the observed extraction kinetics of BFRs, thus enabling us to identify the rate-determining step. The high solubilization rate coefficients that we measured for BFRs revealed that the dynamic extraction process in up-scaling design could compensate for the low solubility with flowing supercritical CO₂.

Beyond Mechanical Recycling: Giving New Life to Plastic Waste

Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re-extrusion. Various chemical recycling processes have great potential to enhance recycling rates. In this Review, a summary of the various chemical recycling routes and assessment via life-cycle analysis is complemented by an extensive list of processes developed by companies active in chemical recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, that is, by stricter regulation. This Review aims to inspire both science and innovation for the production of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.

Properties of wood composite plastics made from predominant Low Density Polyethylene (LDPE) plastics and their degradability in nature

To address concerns over plastics in the global environment, this project produced three wood plastics composites (WPCs) which could divert plastics from the waste stream into new materials. The three materials made had a ratio of 85%:15%, 90%:10%, and 95%:5% low density polyethylene (LDPE) to wood powder and were produced using the dissolution method. Physical and mechanical properties of each WPC were evaluated according to Japanese Industrial Standard (JIS) A 5908:2003. Their degradation in nature was evaluated through a graveyard test and assay test conducted in Coptotermes curvignathus termites. Results showed that density, moisture content, thickness swelling and water absorption of the WPCs fulfilled the JIS standard. The mechanical properties of these composites also met the JIS standard, particularly their modulus of elasticity (MOE). Modulus of rupture (MOR) and internal bonding (IB) showed in lower values, depending on the proportion of wood filler they contained. Discoloration of the WPCs was observed after burial in the soil with spectra alteration of attenuated transmission reflectance (ATR) in the band of 500-1000 cm-1 which could be assigned to detach the interphase between wood and plastics. As termite bait, the WPCs decreased in weight, even though the mass loss was comparatively small. Micro Confocal Raman Imaging Spectrometer revealed that termite guts from insects feeding on WPCs contained small amounts of LDPE. This indicated termite can consume plastics in the form of WPCs. Thus WPCs made predominantly of plastics can be degraded in nature. While producing WPCs can assist in decreasing plastics litter in the environment, the eventual fate of the LDPE in termites is still unknown.

Transforming polystyrene waste into 3D hierarchically porous carbon for high-performance supercapacitors

Polystyrene (PS) is usually discarded as a solid waste after a short lifespan. Thus the disposal of waste PS is an inevitably worldwide issue because of their stable and non-biodegradable nature. Herein, a facile method was proposed to carbonize PS waste into novel three-dimensional (3D) hierarchically porous carbon using Fe₂O₃ particles as both catalyst and template. Furthermore, KOH activation was applied to generate microporous and mesopores on the wall of macropores. As a result, the obtained 3D hierarchically porous carbon exhibits a high specific capacitance of 284.1 F g^-1 at 0.5 A g^-1 and good rate performance of 198 F g^-1 at 20 A g^-1 in a three-electrode device. Moreover, the assembled symmetrical capacitor displays a high energy density of 19.2 W h kg^-1 at the power density of 200.7 W kg^-1 in aqueous electrolyte. Therefore, the present research develops a sustainable way to recycle waste plastics into 3D hierarchically porous carbon for supercapacitors.

Plastics as a materials system in a circular economy

Plastics have transformed our modern world. With a range of outstanding properties, they are used in an ever-widening range of applications. However, the linear economy of their use means that a large volume of plastics is discarded after use. It is believed that approximately 80% of the estimated total 6.3 Bt of plastics ever produced have been discarded, representing not only a huge loss of valuable resources, but mismanaged waste is also the origin of an ever-increasing environmental disaster. Strategies to prevent loss of materials resources and damage to the environment are elements of a circular plastics economy that aims to maintain plastics at their highest value for the longest time possible and at the same time improve the economy and prevent detrimental environmental impact. The latter in particular is driving recent changes in policies and legislation across the world that are rapidly being introduced in order to solve these environmental issues. The achievement of a circular economy will require not only innovative technical developments, but also major economic investment and changes to business practice coupled with significant changes in social behaviour. This paper summarizes the complex and highly interrelated technical issues and provides an overview of the major challenges, potential solutions and opportunities required to achieve and operate a circular plastics economy. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Co-etching effect to convert waste polyethylene terephthalate into hierarchical porous carbon toward excellent capacitive energy storage

With the ever-increasing consumption of polyethylene terephthalate (PET) related products, how to recycle the waste PET still remains as a great challenge for the sustainable development. Converting waste PET into porous carbon material has been emerged as a promising way to address this issue. Recently, the microporous carbon derived from waste PET has drawn considerable attention in adsorption field, but its electrochemical application is still impeded by low specific surface area (SSA <1500 m² g^-1) and small meso-/macropores volume (<0.2 cm³ g^-1). Herein, hierarchical porous carbon (HPC) is successfully prepared from waste PET. The obtained HPC possesses a high SSA (2238 m² g^-1) and a large meso-/macropores volume (0.51 cm³ g^-1). The formation mechanism of hierarchical porous structure is proposed: co-etching effect of sp²/sp³ hybridized carbon produces micropores and meso-/macropores, respectively. In a three-electrode configuration, HPC based electrode achieves an outstanding capacitance of 413 F g^-1, while the traditional microporous carbon exhibits a low capacitance of 142 F g^-1. The fabricated symmetric supercapacitor shows a high energy density of 25 Wh kg^-1. This work provides a good reference to convert waste plastics into hierarchical porous carbon.

Bimetallic carbon nanotube encapsulated Fe-Ni catalysts from fast pyrolysis of waste plastics and their oxygen reduction properties

Carbon-based bimetallic electrocatalysts were obtained by catalytic pyrolysis of waste plastics with Fe-Ni-based catalysts and were used as efficient oxygen reduction reaction (ORR) catalysts in this study. The prepared iron-nickel alloy nanoparticles encapsulated in oxidized carbon nanotubes (FeNi-OCNTs) are solid products with a unique structure. Moreover, the chemical composition and structural features of FeNi-OCNTs were determined. The iron-nickel alloy nanoparticles were wrapped in carbon layers, and the carbon nanotubes had an outer diameter of 20-50 nm and micron-scale lengths. FeNi-OCNT with a Fe/Ni ratio of 1:2 (FeNi-OCNT12) exhibited remarkable electrochemical performance as an ORR catalyst with a positive onset potential of 1.01 V (vs. RHE) and a half-wave potential of 0.87 V (vs. RHE), which were comparable to those of a commercial 20% Pt/C catalyst. Furthermore, FeNi-OCNT12 exhibited promising long-term stability and higher tolerance to methanol than the commercial 20% Pt/C catalyst in an alkaline medium. These properties were attributable to the protective OCNT coating of the iron-nickel alloy nanoparticles.

Characterization of whey protein-based films incorporated with natamycin and nanoemulsion of α-tocopherol

Food packaging materials are commonly derived from petroleum that increases global contamination; this raises the interest to evaluate raw material from renewable sources such as whey protein for the development of packaging materials, especially to produce active films. This research aimed to evaluate whey protein-based film properties when natamycin, nanoemulsioned α-tocopherol, or both were added. An oil-in-water (O/W) nanoemulsion of antioxidant (α-tocopherol) was prepared by microfluidization technique. Four films were prepared with different levels of natamycin and nanoemulsified α-tocopherol and were characterized in terms of physicochemical, mechanical, optical-properties, water vapor barrier, FTIR, microstructure, antioxidant and antimicrobial activity. The natamycin, nanoemulsified α-tocopherol, or both did not modify the moisture content of the films. Moreover lead to a significant reduction of tensile strength and elastic modulus, while presenting growth in the elongation at break. Film opacity, the total color difference, the UV-Vis light barrier, and the water vapor permeability values increased when compounds were incorporated into the film. The microstructure studies showed uniformly distributed porosity throughout the films. The addition of nanoemulsioned α-tocopherol into whey protein-based films provoked antioxidant activity and the addition of natamycin produced films with effectivity against C. albicans, P. chrysogenum, and S. cerevisiae, allowing develop a material appropriate for use as active food packaging.

Preparation of Iron- and Nitrogen-Codoped Carbon Nanotubes from Waste Plastics Pyrolysis for the Oxygen Reduction Reaction

A novel method for the preparation of iron- and nitrogen-codoped carbon nanotubes (Fe-N-CNTs) is proposed, based on the catalytic pyrolysis of waste plastics. First, carbon nanotubes are produced from pyrolysis of plastic waste over Fe-Al₂ O₃ ; then, Fe-CNTs and melamine are heated together in an inert atmosphere. Different co-pyrolysis temperatures are tested to optimize the electrocatalyst production. A high doping temperature improves the degree of graphite formation and promotes the conversion of nitrogen into a more stable form. Compared with commercial Pt/C, the electrocatalyst obtained from pyrolysis at 850 °C shows remarkable properties, with an onset potential of 0.943 V versus RHE and a half-wave potential of 0.811 V versus RHE, and even better stability and anti-poisoning properties. In addition, zinc-air battery tests are performed, and the optimized catalyst exhibits a high maximum power density.

Degradation of Plastics under Anaerobic Conditions: A Short Review

Plastic waste is an issue of global concern because of the environmental impact of its accumulation in waste management systems and ecosystems. Biodegradability was proposed as a solution to overcome this problem; however, most biodegradable plastics were designed to degrade under aerobic conditions, ideally fulfilled in a composting plant. These new plastics could arrive to anaerobic environments, purposely or frequently, because of their mismanagement at the end of their useful life. This review analyzes the behavior of biodegradable and conventional plastics under anaerobic conditions, specifically in anaerobic digestion systems and landfills. A review was performed in order to identify: (a) the environmental conditions found in anaerobic digestion processes and landfills, as well as the mechanisms for degradation in those environments; (b) the experimental methods used for the assessment of biodegradation in anaerobic conditions; and (c) the extent of the biodegradation process for different plastics. Results show a remarkable variability of the biodegradation rate depending on the type of plastic and experimental conditions, with clearly better performance in anaerobic digestion systems, where temperature, water content, and inoculum are strictly controlled. The majority of the studied plastics showed that thermophilic conditions increase degradation. It should not be assumed that plastics designed to be degraded aerobically will biodegrade under anaerobic conditions, and an exact match must be done between the specific plastics and the end of life options that they will face.

Regeneration and Recovery of Plastics

Plastics are the most essential commodities in the world and are replacing the metals, wood, glass, cements and other materials in some application areas. Plastics are easy handling, resource-saving, non-toxic with esthetically attractive. Oil and gas are the major feedstocks for plastics, besides sugar and corn as renewable feedstocks. Plastics represent a serious waste-handling problem, with only 10%–12% of the plastic waste generated worldwide being regenerated/recycled. After disposal, plastic waste takes very long time to decompose naturally and impact the environment during the degradation process. The above situation has necessitated the invention of efficient technologies for regeneration and recovery of plastics all over the world.

Supercritical fluid extraction for purification of waxes derived from polyethylene and polypropylene plastics

Polyethylene (PE) and polypropylene (PP) feedstock contain various additives, such as fillers and colorants, which either degrade or carry through the depolymerization process; thereby causing intense dark colors and a pungent petroleum odor. The combination of color and odor imposes several challenges, limiting the potential markets of the wax products. This study put emphasis on the development of an innovative and environmentally sustainable process based on supercritical fluid extraction (SCFE) to remove organic and inorganic contaminants that cause color and odor in waxes derived from recycled polymers. In terms of organic impurity removal, for PE 81% and for PP 97% removal efficiency was achieved. The color of PE and PP in terms of lightness under CIELAB (lightness, green-red, blue-yellow) color space was improved by 13 and 40 units, respectively. The purified waxes could be utilized in a variety of market segments, including color masterbatch, roofing shingles, rubber, and coatings. Compared with traditional purification technologies based on solvent extraction and absorbent filters, SCFE process offers exceptional advantages, including fast reaction rates, little liquid waste, ease of separation of solutes, and fewer separation stages. This novel process enables producing high-value water white waxes from reclaimed polymeric feedstock with a focus on clean technologies and enhanced resource efficiency.

Dissolution of adhesive resins present in plastic waste to recover polyolefin by sink-float separation processes

This study investigated the dissolution of adhesive resins present in polyolefin films that cause plastic materials to adhere to each other. The process of dissolution was made by the use of ethyl acetate and followed by separation through the sink-float process. The objective was to separate and characterize polyolefin films from plastic solid waste derived from recycled post-consumer paper. Through these procedures, 6% polyethylene of high-density (HDPE), 14% polyethylene of low-density (LDPE) and 39% polypropylene (PP) were separated and recovered from plastic waste. Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analyzes (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) were conducted to determine the chemical, thermal and mechanical properties of the recovered polymers and to establish a comparison with standard commercial polymers. It demonstrated that recovered material kept their chemical, thermal, and mechanical properties. This process indicates possible economic viability considering the demand, the market value of the PP, and the required investment to be implemented in the recycling process that could be amortized in a short period of time. Moreover, the organic solvent used in the dissolution process can be easily recovered by distillation.

Depolymerization of waste poly(methyl methacrylate) scraps and purification of depolymerized products

A big challenge for the civilization in energy saving/waste management can be "the regeneration of monomers from the waste plastics followed by their re-polymerization" using an ideal recycling method. Herein, we investigate the thermal depolymerization of poly(methyl methacrylate) (PMMA) using thermogravimetric analysis coupled with mass spectrometry (TGA-MS). In this process, the polymer chains were decomposed to methyl methacrylate (MMA) in high yield and the degradation species were thoroughly characterized. The obtained MMA contained traces of byproducts. Firstly, the byproducts were found to be nonpolymerizable, secondly, their presence interrupt the polymerization reaction, and thirdly, they reduce the quality of re-polymerized PMMA (r-PMMA). This study reclaims that besides the main byproduct (methyl isobutyrate), traces of methyl pyruvate and 2,3-butanedione were also formed during the thermal depolymerization of PMMA. The formed 2,3-butanedione was found to be responsible for the unpleasant smell in the recovered MMA that also found itself in the r-PMMA. Further, the generated byproducts were eliminated from the r-PMMA by a dissolution/re-precipitation method. The structural characterizations of the recycled and purified PMMA were carried out by Fourier-transform-infrared spectroscopy (FT-IR), Hydrogen-1 (¹H)- and Carbon-13 (¹³C)-nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). The chemical properties of the r-PMMA and purified PMMA proved to be similar to that of the virgin commercial PMMA. This study can provide an effective and practical prototype for the recycling of waste PMMA scraps and thus reduction in pollution caused by the landfilling of waste PMMA scraps.