Comparative Study on Characteristics of Vacuum and Conventional Kiln Drying of Non-Pith Heartwood of Rubberwood
This study compared the drying characteristics of non-heartwood rubberwood in vacuum drying (VD) and conventional kiln drying (CKD). The results show that overall, VD had a 21.6% higher drying rate than CKD. Especially below the fibre saturation point (FSP), VD’s rate was 1.56 times that of CKD because the low pressure in VD reduces the bound- water diffusion resistance. An ANOVA test shows that VD had significantly lower uniformity of moisture content (MC) in the longitudinal direction and higher uniformity of MC in the radial direction, especially when the MC drops below the FSP. VD also had significantly lower shrinkage ratios, colour change, and residual stress than CKD (p < 0.05). The greatest differences in shrinkage occurred at 20% MC. Overall, the colour change in VD was 30% lower, and the maximum decrease in the residual stress index in VD was 17.9% at 40% MC. These advantages of VD are likely related to its more uniform internal moisture gradient and low-oxygen environment.