Thermal Softening and Degradation of Wood and Bark

A thermogravimetric analyzer was modified for the study of thermal softening of several Pacific Northwest woods and barks under constant load at a heating rate of 16 C/min. Several stages of thermal softening were found in barks and wood. Regardless of species, oven-dry samples start to soften at 180 C, with termination at about 500 C. The maximum rate of softening occurred at 380 C with an additional softening at 280 C for bark and 320 C for wood of hardwood species. An increase of moisture content decreased the softening temperature. When the moisture content of either material was higher than 10%, a new maximum rate of softening appeared at 160 C, while the 280 C, 320 C and 380 C maxima were retained. The absolute softening of wood and bark at 160 C increased with increasing moisture content to a limit at about 30%. In conjunction with results from infrared spectrum, X-ray diffraction and differential thermal analysis, the heating of oven-dry wood and bark was found to exhibit neither physical nor chemical changes at less than 200 C. The softening of wood and bark in the presence of water at temperature less than 200 C must occur only in the amorphous regions, with water serving as a plasticizer. Softening of wood and bark at more than 200 C is a combined response of physical and chemical degradations. These thermal responses of wood and bark, particularly bark, are expected to be important to the strength, dimensional stability, water resistance and fire-retardant properties of composite products.