Converting Low-Value E-Waste into Phenolic Resole Resins: A Route to Value-Added, High-Performance Advanced Materials for Multifunctional Applications

This study presents a sustainable approach to resole-type phenolic resins using recycled pyrolysis oil derived from the nonmetallic fraction (NMF) of waste printed circuit boards (WPCBs). GC-MS confirmed phenol and phenolic derivatives in the recycled pyrolysis oil, providing suitable monomers. Among the tested conditions (300, 400, 500 °C), pyrolysis at 400 °C maximized both recycled pyrolysis oil yield and phenolic content. Resins were synthesized using recycled pyrolysis oil, pure phenol, and formaldehyde across recycled pyrolysis oil-to-phenol ratios from 100R-0P to 0R-100P, and their synthesis behavior, mechanical performance, thermal stability, and flame retardancy were comprehensively characterized. A novel mechanism is proposed, in which, under basic conditions, phenolic species form methylol phenols that condense via ether and methylene linkage, as confirmed by FTIR analysis, while nonphenolic and sterically hindered compounds limit apparent network rigidity. Flame retardancy was evaluated by the limiting oxygen index (LOI), which reached 42.3% for 0R-100P and 27.0% for 100R-0P. Formulations containing up to 40 wt % recycled pyrolysis oil retained over 82% of the tensile properties and 87% of impact resistance compared to the virgin resin. These findings highlight the feasibility of converting e-waste into value-added, high-performance, and flame-retardant thermosets, offering a viable route toward circular material innovation.