The effect of superheated steam vacuum drying (SSVD) on the drying time and mechanical properties of rubberwood was studied. Rubberwood boards with dimensions of 1000 mm × 76.2 mm × 25.4 mm were dried at 86.7–89.3 kPa vacuum pressure (14.6–12.0 kPa absolute) and temperatures of 60, 70, and 80°C. Superheated steam at 110°C was injected intermittently to relieve stress buildup in wood and eliminate cracking. The prong test was used to evaluate the initial acceptability of the dried wood and the mechanical properties of wood were measured. From this study, the total drying time was reduced from 168 h to less than 20 h (MC reduction from 0.80 to 0.06 db). In addition, compared to the reference values shown in the parentheses, the shear parallel-to-grain, the compression parallel-to-grain, the compression perpendicular-to-grain, the modulus of rupture (MOR), the modulus of elasticity (MOE), and the hardness for the optimum drying temperature of 70°C were 28.87 (11.0) MPa, 59.09 (32.0) MPa, 21.09 (5.0) MPa, 101.97 (66.0) MPa, 9838.5 (9240.0) MPa, and 6475 (4350) N, respectively. Thus, the vacuum-dried wood showed a 32% increase in hardness, a 12% increase in compression parallel-to-grain, and an 88% increase in shear parallel-to-grain.

A pilot-scale rubberwood dryer was constructed and injected with superheated steam and hot air to study the effect of the hybrid system on the drying rate and mechanical properties of the wood. A total of 300 pieces of rubberwood boards, each with dimensions of 1000 mm long × 76.2 mm wide × 25.4 mm thick, were stacked in 1.0 m × 1.0 m × 1.7 m (1.7 m³) pallet. The stack was impinged with alternating cycles of superheated steam and hot air. The time required for conventional drying was 168 hours, but the drying time for the hybrid system was only 64 hours, resulting in a 62% reduction. After being dried, the rubberwood boards were mechanically tested for static bending, compression strength, hardness, and shear strengths. From the mechanical tests, the hybrid drying system using superheated steam and hot air had no significant effect on the mean shear strength parallel-to-grain; however; the mean compression strength parallel-to-grain was reduced by 24.2% and the mean MOR by 21.4%. Nonetheless, the mean MOE was increased by 30.4% and the mean of hardness by 16.4%.