Abstract
Microwave (MW) treatment is an effective method to increase refractory wood permeability, thereby reducing drying time and defects. The extent of modification depends on the damage extent of the wood microstructure. In this study, MW intensities of 43 kWh/m3 (low intensity) and 57 kWh/m3(high intensity) were adopted to treat Chinese fir lumber. Microstructural changes in wood samples were observed using scanning electron microscopy (SEM) and pore structure was characterized using mercury intrusion porosimetry (MIP). Results were as follows: After low-intensity MW treatment, parts of the bordered pit membranes in tracheids were damaged, and micro-fibrils on the margo were ruptured, while the torus basically remained intact. Micro-cracks were observed at both ends of the cross-field pit apertures, propagating to the cell walls of tracheids. The middle lamellar between ray parenchyma cells and longitudinal tracheids cracked, and the width of cracks was in the range of 1–25 μm. After high-intensity MW treatment, damage to the wood microstructure was more severe than that in the low-intensity MW treatment, with macro-cracks having a width range of 100–130 μm being generated. In addition, on the fracture surface of macro-cracks, the bordered pit membranes in tracheids fell off, cross-field pit membranes disappeared and the ray parenchyma cells were seriously damaged, exhibiting fracture of the tracheid walls. Both low-intensity and high-intensity MW treatment can increase the pore diameter corresponding to the margo capillaries (peak value increased from 674.7 nm to 831.8 nm and 1047.6 nm, respectively). The number of pores in the tracheid lumen diameter range also significantly increased. These results provide a theoretical support forMW treatment processes’ improvement and high-value utilization of Chinese fir.
Highlights
As an important wood property, permeability reflects the ease of fluid migration in wood under a pressure gradient
Mercury intrusion porosimetry (MIP) is a commonly used method for measuring pore size and distribution of wood [14,15,16,17]. He et al [18] studied the effect of MW treatment on pore structure of Chinese fir by mercury intrusion porosimetry (MIP) and demonstrated that MW enlarges pore diameter in the pit opening range
Previous studies have highlighted that bordered pit membranes in heartwood of Chinese fir were Previous studies have highlighted that bordered pit membranes in heartwood of Chinese fir were usually covered with amorphous materials, forming a completely encrusted pit membrane and usually covered with amorphous materials, forming a completely encrusted pit membrane and blocking blocking openings on the margo, thereby seriously affecting permeability [20]
Summary
As an important wood property, permeability reflects the ease of fluid migration in wood under a pressure gradient. Microwave (MW) treatment can quickly vaporize water inside wood During this process, rapidly increased steam pressure can damage some weak tissues of anatomical structures, such as pit membranes and ray parenchyma cells. Investigations examining microstructural changes of Chinese fir under different MW intensities are important to reveal the MW treatment mechanisms and to optimize MW process parameters. Mercury intrusion porosimetry (MIP) is a commonly used method for measuring pore size and distribution of wood [14,15,16,17] He et al [18] studied the effect of MW treatment on pore structure of Chinese fir by MIP and demonstrated that MW enlarges pore diameter in the pit opening range. Variations in pore structure, associated with microstructural changes, can further reveal the mechanism for MW treatment increasing wood permeability. The results of this study will provide a reference to improve MW treatment processes for rapid drying of Chinese fir and provide theoretical support for its high-value utilization
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