Wood Products Thermal Degradation and Fire

Abstract

As wood reaches elevated temperatures, the different chemical components undergo thermal degradation that affect the performance of wood. The extent of these changes depends on the temperature level and length of time under exposure conditions. Permanent reductions in strength and modulus of elasticity can occur at temperatures >65 °C, with the amount depending on the temperature, pH of wood, moisture content, heating medium, exposure period, chemical treatment, and species (Winandy and Rowell, 2013). Studies relating strength reduction with temperature rise has indicated a kinetic basis(involving activation energy, pre-exponential factor, and order of reaction). If the specimens are tested while still hot after long term exposure (∼66 °C for 3 years), the percentage of strength reduction would be based on the combination of an immediate effect of temperature (∼14%) and a permanent effect (∼33%) for a total of around 47% strength loss (Green and Evans, 2008). The primary mechanism for this degradation is the production of acetic acid freed by the process of hydrolysis at long term ambient conditions that include varying moisture contents and by which the acetyl groups are split from their association with hemicelluloses in the wood (Packman, 1960). During hydrothermal treatments (as reported for up to 230 °C), the acetyl groups present in wood hemicelluloses hydrolyze to form acetic acid which provides the hydronium ions needed to self-catalyze the hydrolysis of wood, involving the hydrolytic reactions of ether and ester bonds (Garrote et al., 2002). Long term exposure to temperatures equal to or greater than 66 °C is not a concern in residential construction but may be an important consideration for certain industrial buildings. However, strength and stiffness degradation relationships with temperatures greater than 66 °C and varying time durations for wood under compression and tension has recently been utilized in building design for fire protection (Eurocode 5, 2004).

Citation

Dietenberger, M.A.; Hasburgh, L.E. 2016. Wood products thermal degradation and fire. Reference Module in Materials Science and Materials Engineering. 8 pp.