Advanced Wood Modification in the Circular Economy: A Critical Review of Performance, Machinability, and Recycling Pathways

Wood modification has been developed as a non-biocidal concept to overcome inherent material limitations such as dimensional instability and biological durability. But sustainability assessment must extend beyond initial functional performance to include machinability, interface compatibility, repairability and end-of-life (EoL) cascade potential. This critical narrative review integrates the mechanisms, processing dynamics and circular economy implications of 4 major modification routes: thermal treatment, acetylation, furfurylation and thermo-hydro-mechanical (THM) densification. The review uses a purposively selected core evidence set of 40 peer-reviewed articles, complemented by contextual references, standards, and foundational sources; coverage emphasizes seminal studies and recent literature. The synthesis shows that thermal modification and acetylation can enhance hygroscopic and dimensional stability through different mechanisms, but with different trade-offs in mechanical toughness and secondary processing. Furfurylation can improve hardness and decay resistance, but also increase brittleness and recycling difficulties. THM densification improves the local load-bearing capacity, but moisture-induced set-recovery is a major limitation. Cross-contamination in heterogeneous waste streams, residual chemical matrices, changed surface–energy interactions with adhesives, and limited standardization of sorting protocols are important barriers to circular material loops across the modification routes. Ultimately, this critical narrative review provides an integrated decision framework connecting cell wall modification chemistry, secondary processing dynamics (machinability and adhesive bonding), digital material passports for traceability, and cascading end-of-life (EoL) decision pathways. Future work should consider design for disassembly, digital material traceability and multi-criteria sustainability assessment.