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. We examined a purposive sample of 40 peer-reviewed articles including seminal citations between 1990-2007 and recent publications between 2021-2025. The synthesis shows that thermal modification and acetylation can enhance hygroscopic and di-mensional 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. A descriptive particleboard case-study dataset covering 0–70% secondary-wood fractions shows decreasing MOR, MOE and IB and increasing thickness swelling with increasing recycled content. These observations are presented as only study-specific descriptive evidence, and not as evidence of a universal recycling threshold or formal EN 312 P2 conformity, as the current experimental documentation does not contain sufficient information on independent panel replication and inferential statistical analysis. Future work should consider design for disassembly, digital material traceability and multi-criteria sustainability assessment.