Improving the Fire Performance of Chemically Modified Pine (P. Sylvestris) Sapwood by an Integrated Modification Approach

Natural wood has been widely used for construction and building purposes. However, wood is susceptible to wood-destroying organisms moisture-induced dimensional changes and fire. Although chemical wood modification technologies with thermosetting resins like phenol-formaldehyde (PF) may compensate durability and swelling issues, the latter may not enhance fire resistance. To address this issue, Scots pine sapwood (Pinus sylvestris L.) has been impregnated with a formulation of a PF resin and a phosphate-nitrogen-based flame retardant. The reaction to fire of the treated material was assessed through thermogravimetry analysis, Bunsen burner test (Pries and Mai), small burner box (ISO 11925–2) and cone calorimeter measurements (ISO 5660–1). Results indicated that pure PF resin treatment slightly improved thermal stability and reduced flame spread. However, it led to a significant increase in heat release peak and smoke production. In contrast, incorporating the flame retardant with PF resin demonstrated notable improvements in fire resistance independent of the test method applied. Compared to pure PF-treated wood, thermogravimetry analysis revealed a 31% increase in final char residue. The Bunsen burner test exhibited great self-extinguishing properties and the small burner box test demonstrated a 56% reduction in flame height. Cone calorimeter tests confirmed a substantially inhibited heat release with no ignition during the test. Notably, the total smoke production was also significantly reduced, even lower than the untreated wood. These findings emphasize the effectiveness of the combined PF resin and phosphate-nitrogen-based flame retardant treatment in enhancing the fire resistance of wood.