Wood, a ubiquitous material in furniture and construction, is limited by its natural flammability. Existing wood flame retardant technologies are often ineffective and lack environmental sustainability. Ionic liquids (ILs), known for their non-flammability and non-volatility, offer a green solvent solution to these challenges. In this study, we synthesized a novel phosphorus-containing, polymerisable IL. The IL was impregnated into the wood and polymerised in situ, yielding PIL-modified wood. Raman spectroscopy and SEM confirmed the IL's in-situ polymerisation in the wood and its effect on swelling the cell walls. Notably, wood treated with 30% PIL (IL30W) showed a 74.2% decrease in total heat release (THR) compared to untreated wood (UW). The condensed-phase flame retardant mechanism analysis reveals that PIL-treated wood expands during heating, transitioning into wood foam with a linear expansion growth rate of up to 84%. This macroscopically manifests as an increase in wood pores, improving thermal and oxygen insulation, and thus enhancing flame retardancy. This represents a plastification intumescent flame-retardant mechanism, and the observed changes in PIL-treated wood upon heating could offer new insights for developing novel wood-based functional materials.

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