Effect of Heat Treatment on Color Changes, Dimensional Stability, and Mechanical Properties of Wood

Abstract

Properties of fast-growing timbers with low durability can be improved by thermal modification. Thermal modification is an eco-friendly method of improving durability of wood. In this work, specimens of rubberwood (Hevea brasiliensis) and silver oak (Grevillea robusta) were thermally modified in vacuum between 210 to 240°C for 1 to 8 hours, and their weight loss, color, and chemical changes evaluated. Rate of thermal degradation was determined from weight loss data. The color of the modified wood darkened and was uniform throughout. CIE lightness color coordinate (L*) decreased with treatment severity, while chroma coordinates a* and b* increased initially, but later decreased with increased process severity. FTIR analysis showed degradation of cell wall polymers resulting in generation of structures which are responsible for color darkening of thermally modified wood. Mechanical properties (bending strength, MOR, and bending stiffness, MOE) of heat-treated wood decreased. A decrease in hydroxyl groups reduced the hygroscopic nature, resulting in increased dimensional stability of thermally modified wood.