High temperature mechanical properties and thermal recovery of balsa wood

This article presents an experimental study into thermal softening and thermal recovery of the compression strength properties of structural balsa wood (Ochroma pyramidale). Balsa is a core material used in sandwich composite structures for applications where fire is an ever-present risk, such as ships and buildings. This article investigates the thermal softening response of balsa with increasing temperature, and the thermal recovery behavior when softened balsa is cooled following heating. Exposure to elevated temperatures was limited to a short time (15 min), representative of a fire or postfire scenario. The compression strength of balsa decreased progressively with increasing temperature from 20° to 250°C. The degradation rates in the strength properties over this temperature range were similar in the axial and radial directions of the balsa grains. Thermogravimetric analysis revealed only small mass losses (<2%) in this temperature range. Environmental scanning electron microscopy showed minor physical changes to the wood grain structure from 190° to 250°C, with holes beginning to form in the cell wall at 250°C. The reduction in compression properties is attributed mostly to thermal viscous softening of the hemicellulose and lignin in the cell walls. Post-heating tests revealed that thermal softening up to 250°C is fully reversible when balsa is cooled to room temperature. When balsa is heated to 250°C or higher, the post-heating strength properties are reduced significantly by decomposition processes of all wood constituents, which irreversibly degrade the wood microstructure. This study revealed that the balsa core in sandwich composite structures must remain below 200°-250°C when exposed to fire to avoid permanent heat damage.