Drying behavior and property changes of water-saturated Eucalyptus urophylla × E . grandis wood subjected to freeze-thaw pretreatment

Conventional drying-induced shrinkage defects greatly limit the application of plantation-grown eucalyptus wood, and freeze-thaw (FT) treatment can ameliorate this issue by modifying the wood’s properties and drying characteristics. This study investigated the effects of FT core parameters (freezing temperature, freezing duration, cycle number) on Eucalyptus urophylla × E. grandis, systematically assessing drying rate, shrinkage variation, water absorption, moisture absorption rate, and equilibrium moisture content (EMC). Results showed that FT significantly improved drying efficiency, especially below the fiber saturation point (FSP): the average and overall drying rates increased by 10.9% and 14.9%, respectively. FT also enhanced drying stability by suppressing shrinkage, reducing the average area shrinkage strain by 19.6%. Moreover, FT effectively increased wood permeability, with a 31.8% rise in water absorption, a maximum 9.1% increase in moisture absorption rate, and a slight EMC elevation; freezing temperature, duration, and their interaction were the main influencing factors. After statistical analyses, water-absorption weight-gain rate and drying dimensional change rate were selected as key evaluation indices. Comprehensive evaluation determined the optimal FT condition for Eucalyptus urophylla × E. grandis to be −20 °C for 3 h with 2 cycles, which achieves a balance of improved drying efficiency, reduced shrinkage defects, and optimized permeability.