Effect of microwave drying on drying kinetics, physico-mechanical properties, and microstructure of plantation-grown Hevea brasiliensis wood

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Abstract

We explored the microwave drying of rubber tree (Hevea brasiliensis) wood using a multifaceted approach that encompasses various aspects. The primary objective was to examine drying behavior, drying time, moisture distribution across the core and surface, and to evaluate drying stresses via the prong test. Static bending and compression parallel to the grain were tested to assess the impact of microwave treatments on mechanical properties. The drying process showed a nearly uniform moisture distribution within the wood’s core and on its surface, indicating well-controlled drying. Most notably, the dried wood had no observable drying-induced stresses, suggesting a promising application of microwave drying. However, the volumetric shrinkage (%) was higher in microwave-dried samples (5.65% and 6.51%) than in air-dried samples (4.16%). A reduction in modulus of elasticity (MOE), modulus of rupture (MOR), and maximum compressive strength (MCS) was observed in the microwave-dried wood. Compared to the air-dried samples, the maximum reductions recorded were 15% for MOE, 18% for MOR, and 15% for MCS. The examination under light microscopy showed that the wood microstructures, such as ray cells and vessel walls, had incurred damage. The diminished mechanical properties could likely be linked to these micro-cracks or damage in the microstructures. The results show that these microstructural changes may significantly increase wood’s permeability. We also attempted to calculate the energy consumption for different microwave treatments. These findings emphasize the need for a balanced approach to optimizing microwave drying methods to mitigate reductions in mechanical properties while capitalizing on the advantages of reduced drying time and controlled, uniform moisture distribution.

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