Solvolysis and oxidative liquefaction of the end-of-life composite wastes as an element of the circular economy assumptions

In pursuit of sustainable development, recycling composite waste is a crucial challenge as well as an opportunity to support the progress towards eco-friendly materials management and circular economy practices. The current study brings together two modern recycling techniques solvolysis and oxidative liquefaction to recycle five types of composite waste materials including wind turbine blades (WTBs), composite pipes, personal protective equipment (PPEs) used in the medical sector, photovoltaic (PV) panels and municipal solid waste (MSW). Following a well-structured experimental design for each type of waste the solvolysis was performed at a temperature range of 100-190 °C, process time within the range of 1-3 hours, and catalyst (TBD) concentration within the range of 0.015-0.025 mol, end with ethylene glycol and 1-methyl-2-pyrrolidinone in a 1:1 molar ratio, while depending on the material evaluated, the oxidative liquefaction process was conducted at temperatures between 250 and 350°C for WTB and between 200 and 300°C for PPE, MSW, and PV. Oxidant concentrations, reaction time and pressure (15-45%, 30-90 minutes, 20-40 bar for WTBs, 30-60%, 45 minutes, 30 bar for remaining waste types) were studied in addition to highly variant waste-to-liquid ratio from 5-25% for WTB, 3-7% for PPE and MSW, and 12.5-37.5 for PV. Prior to experimental investigation, surface morphologies and chemical studies of the waste material were performed by scanning electron microscope (SEM) and Fourier transform infrared FT-IR spectroscopy, while after experimentation total polymer degradation (TPD) was calculated and liquid products were analysed through gas chromatography with flame ionization detection (GC-FID). Following the solvolysis, 50–60% TPD was obtained, allowing for the recovery of glass fibres (GF) with average lengths of 50–60 mm and a maximum length of 650 mm. This allowed for the recovery of GFs with average lengths of 50–60 mm and a maximum length of 650 mm. In the case of oxidative liquefaction, total polymer degradation of 72-100%, 52-100% and 55-100% was achieved, depending on the investigated waste, i.e. WTB, PPE and MSW, respectively. The yield of oxygenated chemical compounds depended upon the types and composition of waste, but acetic acid was the main acid while WTB, PPE, and MSW, the liquid samples contained phenol also. Involved chemistry and possible degradation pathways of complex composite wastes are discussed in detail to enhance the recycling process and material recovery. • Solvolysis allows for fibre recovery from composite pipes with lower thermal input • Oxidative liquefaction converts intricate waste materials into oxygenated chemicals • Process optimization is crucial for energy efficiency and reaction selectivity • The KOH proportion in solvolysis reaction is crucial for enhanced degradation rates