Effect of fire-retardant treatment of wood prior to thermo-mechanical densification

Wood densification itself does not, in general, improve the fire-retardant properties sufficiently to reach the standard requirements. The object of this study was to enhance the fire-retardant properties of thermo-mechanically densified wood without any loss of moisture stability and hardness. Scots pine sapwood was pretreated before densification by impregnation with a fire retardant (FR) consisting of ammonium dihydrogen phosphate and urea and then cured in-situ by hot pressing at 150°C or 210°C. Densified specimens without FR were used as a control. Set-recovery, fire retardancy in an open flame test, and Brinell hardness were determined. The set-recovery was slightly reduced as a result of the FR treatment, but the pressing temperature and time had a much greater influence. In the open flame test, specimens without FR-treated ignited within 15−50 s of exposure to the flame, whereas all the FR-treated specimens exhibited ignition resistance over the 10 min duration of the test. Water-soaking cycles had no impact on the ignition resistance in these groups, indicating a strong resistance to water leaching of FR after pressing at 210°C for 60 min. The hardness increased due to the presence of FR after pressing at 210°C, but sharply decreased after water immersion.