Synergistic Effects of Heating Platens’ Temperature and Compression Ratio on the Periodic Hot-Press Drying of Chinese Fir Lumber

Abstract

The effects of periodic hot-press drying on drying behavior and mechanical damage to Chinese fir lumber were investigated by taking the heating platens’ temperature (TP) and compression ratio (Rc) as experimental factors. The temperature and pressure inside lumber were analyzed during drying process. The results were as follows. The drying rate of lumber was significantly increased with increasing TP and Rc. Scanning electron microscope (SEM) micrographs showed that bordered pit membranes, cross-field pits, middle lamella between adjacent cells, and tracheid walls were damaged after drying, and the damage became more severe with higher TP and Rc. Detachments between ray parenchyma cells and tracheids were observed at 170 °C. Nitrogen-adsorption measurement results demonstrated that more cell wall pores in the 2.5~6.2 nm pore diameter range were generated at higher TP, resulting in an enlarged specific surface area and pore volume of cell walls. These structural changes contributed to accelerating moisture migration and decreasing the drying time. Furthermore, fluctuating pressure inside lumber was the main driving force leading to moisture migration and cell tissue damage in lumber during drying. The influence of TP on internal temperature (TM) and pressure (PM) was greater than Rc. With the increase in TP from 130 to 170 °C at the Rc of 10%, the maximum TM and PM were increased by 30.90% and 39.84%, respectively. However, TP should not be too high to prevent the formation of macro-cracks caused by high pressure, which may significantly affect wood’s mechanical properties. These results provide theoretical support for periodic hot-press drying processes’ improvement and high-value utilization of Chinese fir.