عنوان مقاله [English]
Background and objectives: The wood is used for its aesthetic appearance and at the same time its high mechanical strength, but it also has some disadvantages that limit its application. Wood modification is a science to improve undesirable properties of wood. Thermal modification is a useful method to develop dimensional stability and biological resistance. Heat treatment degrades the fire resistance of wood. The main disadvantage of wood is its flammability. Lignocellulosics burn because the cell wall polymers undergo pyrolysis reactions with increasing temperature to give off flammable gasses. Primary impregnation of wood with borates has been found to reduce flammability of thermally modified wood. Boron compounds work efficiently as fire retardant for cellulosic materials. Boron compounds are recognized as inexpensive, easily applicable and environmentally safe preservatives. The aim of this study was to investigate the effect of heat treatment and primary impregnation of Fir wood with borax on the fire resistance, thermal behavior and dimensional stability.
Materials and methods: Specimens were prepared to dimensions of 150 (L) ×100 (T) ×20 (R) mm and were impregnated with %7 aqueous solution of borax for 40 minutes at the pressure of 4 bar. Heat treatment was carried out at the temperatures of 170 ◦C and 190 ◦C in oven for 3 hours. Specimens were prepared according to ISO 11925 standard specifications to measure fire retarding properties including ignition time, glowing time, carbonized area and mass loss within the burning process. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) were carried out under air at a heating rate of 10 °C/min. The temperature was heated from the room temperature up to 600 °C. Long-term dimensional stability were measured.
Results: Heat treatment of specimens caused mass loss. The impregnation of wood with borax reduced the mass loss during heat treatment. Heat treatment decreased ignition time. Borax postponed the ignition time of thermally modified wood.TGA thermograms showed that residual char amount was increased in borax preimpregnated-heat treated specimens compared with thermally modified specimens. Char provided insulation and also inhibited propagation of heat. DSC thermograms determined that for borax preimpregnated-heat treated specimens rate of formation of the volatile products reached its maximum at a lower temperature in comparison to thermally modified specimens. During the heat exposure borax formed glassy films on the wood that may inhibit mass transfer of combustible gases. Long-term immersion results confirmed that volumetric swelling and water absorption of thermally modified specimens decreased due to enhancement of hydrophobicity. Volumetric swelling and water absorption of borax preimpregnated specimens increased due to enhancement of hydrophylicity.
Conclusion: Thermally modified wood was susceptible to burning and its fire resistance was decreased. Primary impregnation of Fir wood with borax postponed ignition time and glowing time and eventually increased fire resistance. The lowest carbonized area and mass loss was obtained in borax preimpregnated-heat treated specimens. Thermal analysis showed sharp mass loss for thermally modified wood in comparison to borax preimpregnated-heat treated specimens. Borax increased the resistance of thermally modified wood to thermal degradation and altered pyrolysis route of wood and leaded to the inhibiton of the flame spread. In other words borax decreased the Tmax (maximum degradation temperature). Borax preimpregnated-heat treated specimens of Fir wood decreased dimensional stability in comparison to thermally modified wood.
Key words: Fire resistance, Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Dimensional stability.