Thermal degradation and fire performance of soybean oil-modified Norway spruce: evaluating the impact of triphenyl phosphate impregnation

While thermal modification using refined soybean oil (RSO) enhances wood’s dimensional stability and hydrophobicity, its fire characteristics remain under-explored. This study addresses this literature gap by investigating a multi-step modification to mitigate the flammability risks of oil-treated Norway spruce (Picea abies, L.). Wood was pretreated with 15 mass% triphenyl phosphate (TPP) prior to RSO thermal modification. The combined treatment achieved a 15.3% mean mass percentage gain (MPG), of which TPP accounted for approximately 92% (mean MPGTPP = 14.1 ± 2.4%), with retained soybean oil contributing only approximately 8%, confirming that the treated wood was predominantly TPP-loaded. This treatment significantly reduced equilibrium moisture content from 9.1 to 4.6%. Thermogravimetric analysis demonstrated altered thermal degradation pathways, shifting char oxidation to higher temperatures. Cone calorimetry revealed that while oil treatment alone increased the first peak heat release rate (pHRR1) by 72%, incorporating TPP mitigated this effect. Specifically, TPP reduced pHRR1 by 8% and total heat release (THR) by 17% relative to the oil-treated control, effectively restoring THR to levels statistically identical to untreated wood, although the limiting oxygen index of the treated wood (22.0%) remained below that of untreated wood (24.0%), indicating that the overall fire classification was not improved. This heat reduction is driven by gas-phase radical scavenging, evidenced by a depressed effective heat of combustion (EHC), which consequently prompted a 77% increase in total CO yield and more than doubled total smoke release. Ultimately, while secondary TPP treatment successfully manages the volatile fuel load of the oil medium without exceeding the fire performance of untreated wood, the resulting smoke and toxicity trade-offs favour its suitability primarily for exterior applications.