Effects of thermal modification on physical and mechanical properties of Mozambican Brachystegia spiciformis and Julbernardia globiflora wood

Abstract

AbstractMozambique’s large pool of tropical hardwoods is hampered by the prevalence of low-grade tree species along with a lack of cost-effective processing technologies to improve timber properties. Brachystegia spiciformis and Julbernadia globiflora are the most abundant tree species in terms of volume in the country, but with limited use due to their low timber quality. In this study, thermal modification at three different temperatures (215 °C; 230 °C; 245 °C) was applied for 2 h to the timber of both species, followed by measurement of a set of physical and mechanical wood properties. The results show that the originally light-coloured sapwood of both tree species darkened gradually as the intensity of thermal modification increased. Additionally, from untreated samples to the highest thermal treatment level, timber of B. spiciformis incurred a maximum mass loss of 27%, while oven-dry density was reduced from 0.65 to 0.56 g/cm3 and equilibrium moisture content (EMC) changed from 7 to 3%. Timber of J. globiflora had a mass loss of 23% after the highest treatment level, an oven-dry density reduction of 0.81 to 0.74 g/cm3 and an EMC decrease from 8 to 3%. The changes in mechanical properties from reference samples to the highest thermal treatment level were also significant. For B. spiciformis, MOE decreased by 10.2%, MOR by 50.8%, compression strength parallel to the grain by 29.2% and Brinell hardness by 23.5%. Timber of J. globiflora followed the same trend with an MOE decrease by 6.9%, an MOR decrease by 53.2% and a decrease in compression strength parallel to the grain by 21.9%. All tested wood properties showed significant responses to thermal modification after the most intensive treatment level had been applied. Despite the degradation of mechanical properties in both species, an optimal combination of temperature and treatment time could be achieved. The recorded changes of the tested wood properties in both species could increase the range of applications; the new colour resembled that of highly sought-after tropical hardwoods.