The structure-activity relationship of fire retardant phosphorus compounds in wood

Sawdust of Scots Pine sapwood was chemically modified with various alkyl- and phenylchlorophosphorus compounds. The formation of covalent bonds was confirmed with solid state CP-MAS 13C NMR.According to thermogravimetric analysis (TGA), all phosphorus compounds decreased the temperature for the maximum rate of pyrolysis (from 350 °C to max. 240 °C) and increased the char formation (from 25% to max. 54%). Variation of the alkyl groups (C2-C8) had no significant effect. Phenylphosphates decrease the temperature of pyrolysis more efficiently than the alkyl analogues, due to higher thermal stability. The order in which the phenylphosphorus compounds affect the pyrolysis of the modified sawdust is consistent with their acidity order: organophosphate > organophosphonate ≫ organophosphinate.All phosphorus compounds used in this study reduce the equilibrium moisture content (EMC). Whereas the results obtained with the dialkyl phosphates are relatively poor, significant reductions in EMC can be achieved with the phenylphosphorus compounds.