Collapse recovery in kiln-dried eucalyptus boards after steaming or hot-water treatment

Conventional drying of Eucalyptus delegatensis and Eucalyptus nitens boards remains challenging due to collapse. Steaming and hot-water treatments were introduced in the present study to recover kiln-dried boards with severe collapse from these two species. The 30 mm-thick E. delegatensis samples were steamed for 4, 6, 10, and 12 h at 100 °C and hot-water treated at 70, 80, and 90 °C for 2, 4, 6, 8, and 10 h, respectively. The 30 and 50 mm-thick E. nitens samples were subjected to steaming for 6, 10, 12, 16 and 24 h at 95 °C, and hot-water treatment at 70, 80, and 90 °C for 8, 10, and 12 h, respectively. The collapse intensity was assessed by the defined “collapse depth”, and the recovery effectiveness was evaluated through the collapse recovery percentage. For 30 mm-thick E. delegatensis, the optimal hot-water treatment condition for collapse recovery was 6 h at 80 °C. In contrast, 50 mm-thick E. nitens boards showed the best recovery at 8 h and 90 °C. The optimal steaming conditions for 30 mm-thick boards of both species were 6 h at 95 °C, while for 50 mm-thick E. nitens boards, the best outcome was achieved after 12 h at 95 °C. The maximum recovery percentage from both treatments was comparable, achieving 85% or higher; however, the overall effectiveness of collapse recovery from steaming was lower than that of hot-water treatment. Collapsed cell structures were examined under light and scanning electron microscopes. The vessels, fibers, vasicentric tracheids and rays were all deformed to different degrees in severely collapsed regions, but the fundamental cell wall structures remained largely intact. Cell wall integrity, heat, and moisture appear to be the main factors limiting the collapse recovery.