Recycling and high-value utilization of polyethylene terephthalate wastes: A review

Polyethelene terephthalate (PET) is a well-known thermoplastic, and recycling PET waste is important for the natural environment and human health. This study provides a comprehensive overview of the recycling and reuse of PET waste through energy recovery and physical, chemical, and biological recycling. This article summarizes the recycling methods and the high-value products derived from PET waste, specifically detailing the research progress on regenerated PET prepared by the mechanical recycling of fiber/yarn, fabric, and composite materials, and introduces the application of PET nanofibers recycled by physical dissolution and electrospinning in fields such as filtration, adsorption, electronics, and antibacterial materials. This article explains the energy recovery of PET through thermal decomposition and comprehensively discusses various chemical recycling methods, including the reaction mechanisms, catalysts, conversion efficiencies, and reaction products, with a brief introduction to PET biodegradation using hydrolytic enzymes provided. The analysis and comparison of various recycling methods indicated that the mechanical recycling method yielded PET products with a wide range of applications in composite materials. Electrospinning is a highly promising recycling strategy for fabricating recycled PET nanofibers. Compared to other methods, physical recycling has advantages such as low cost, low energy consumption, high value, simple processing, and environmental friendliness, making it the preferred choice for the recycling and high-value utilization of waste PET.

One-Pot Syntheses of PET-Based Plasticizer and Tetramethyl Thiuram Monosulfide (TMTS) as Vulcanization Accelerator for Rubber Production

Styrene-butadiene (SBR) and acrylonitrile-butadiene (NBR) rubber blends with tetramethyl thiuram disulfide (TMTD) and tetramethyl thiuram monosulfide (TMTS) accelerators and environmentally friendly plasticizers, obtained from PET recycling and biobased resources (LA/PG/PET/EG/LA), were prepared. The mechanical properties of the obtained rubber products were tested and compared with those of commercial dioctyl terephthalate (DOTP). TMTS was prepared by simple and efficient one-pot synthesis from dimethylamine, carbon disulfide, potassium cyanide, and ammonium chloride as catalysts in recycled isopropanol/water azeotrope as solvent. In a comparative study, methoxide, ethoxide, iodide, and amide ions were also used. The two-step reaction mechanism of TMTS synthesis involves the oxidation of the amine salt of dimethyldithiocarbamic acid to TMTD by hydrogen peroxide and sulfur elimination from the TMTD disulfide bond. Potassium cyanide appears to be the most efficient nucleophile. The simplicity of operation, mild reaction conditions, solvent recycling, high yields, and applicability to the industrial level are the advantages of this process. Shore hardness, tensile strength, and compression test results of vulcanized blends before and after aging showed similar properties for both accelerators, while somewhat better results were obtained with LA/PG/PET/EG/LA plasticizer.

Effect of Wood Dust Fibre Treatments Reinforcement on the Properties of Recycled Polypropylene Composite (r-WoPPC) Filament for Fused Deposition Modelling (FDM)

The efficacy of wood dust fibre treatment on the property of wood dust reinforced recycled polypropylene composite (r-WoPPC) filament was investigated. The wood dust fibre was treated using alkali, silane, and NaOH-silane. The treated wood fibre was incorporated with r-PP using a twin-screw extruder to produce filament. The silane treatment on wood dust fibre enhances interfacial bonding between wood fibre and recycled PP; hence, a filament has the highest wire pull strength, which is 35.2% higher compared to untreated and alkaline-treated wood dust filament. It is because silanol in silane forms a siloxane bond that acts as a coupling agent that improves interfacial bonding between wood dust fibre and recycled PP. The SEM micrograph of the fracture structure reveals that treated silane has strong interfacial bonding between wood dust fibre and recycled PP, having minimal void, gap, and good fibre adhesion. The water absorption test results indicate that filament with treated wood dust absorbs less water than filament with untreated wood because the treatment minimizes the gap between wood fibres and recycled PP. The FTIR analysis identified the presence of silane on the wood dust surface for silane-treated wood dust. The DSC studies suggest that the temperature range 167-170 °C be used in the extrusion machine to produce r-WoPPC filament. As a result, r-WoPPc filaments containing silane-treated wood dust have better mechanical properties and have a greater potential for usage in FDM applications.

Kinetics Study of Hydrothermal Degradation of PET Waste into Useful Products

Kinetics of hydrothermal degradation of colorless polyethylene terephthalate (PET) waste was studied at two temperatures (300 °C and 350 °C) and reaction times from 1 to 240 min. PET waste was decomposed in subcritical water (SubCW) by hydrolysis to terephthalic acid (TPA) and ethylene glycol (EG) as the main products. This was followed by further degradation of TPA to benzoic acid by decarboxylation and degradation of EG to acetaldehyde by a dehydration reaction. Furthermore, by-products such as isophthalic acid (IPA) and 1,4-dioxane were also detected in the reaction mixture. Taking into account these most represented products, a simplified kinetic model describing the degradation of PET has been developed, considering irreversible consecutive reactions that take place as parallel in reaction mixture. The reaction rate constants (k1–k6) for the individual reactions were calculated and it was observed that all reactions follow first-order kinetics.

Special Issue “Green Synthesis Processes of Polymers & Composites”

Undoubtedly, polymers and composites are the most important materials in the late XXth and early XXIst century [...]