Abstract
Low permeability of wood causes problems during drying of timber. This study evaluated the effects of microwave (MW) pretreatment on the conventional drying behavior and mechanical damages of Chinese fir lumber. MW pretreatment of lumber was performed at applied MW energy of 43 kWh/m3, and then, the samples were dried in a laboratory drying kiln. The results showed that the drying rate was effectively increased after MW pretreatment. The moisture content (MC) deviation in thickness and residual stress indexes of MW-pretreated samples were significantly decreased in comparison with the control samples, and the appearance quality of wood samples was not clearly affected by the MW pretreatment. Scanning electron microscope (SEM) micrographs demonstrated that pit membranes were damaged after MW pretreatment, and the micro-cracks in radial section as well as detachments between ray parenchyma cells and tracheids were also observed. Consequently, new pathways for moisture migration during drying process were formed after MW pretreatment, which contributed to the improved permeability of Chinese fir lumber and decreased drying time.
Highlights
Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is one of the most important plantation species in China
When the moisture content (MC) reached 10%, the drying time of quarter-sawn and flat-sawn samples with MW pretreatment were decreased by 9.51% and 12.61%, respectively, compared to the control group
The MC of MW-pretreated quarter-sawn and control samples were increased by 0.68% and 0.59%, while those of MW-pretreated flat-sawn and control samples were increased by 1.27 and 0.37%, respectively
Summary
Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is one of the most important plantation species in China. It has been widely used in timber construction and manufacturing of bridge, boat, and furniture [1]. The wood permeability is influenced by the conditions of cell tissues, chemical components, and physical properties of wood, and conditions of cell tissues are the most important factors [2]. The MW energy vaporizes the water in wood rapidly during the treatment and the increasing steam pressure causes varying degrees of damage to each cell tissue of wood. Ray cells, tyloses, and even main cell walls are ruptured to form new pathways for moisture migration, which lead to increased permeability and create favorable conditions
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