Effects of freezing pretreatment on the drying rheological characteristics and collapse of Eucalyptus urophylla × Eucalyptus grandis wood

Abstract To mitigate collapse in Eucalyptus urophylla × Eucalyptus grandis wood induced by capillary tension and drying stress, this study examined the effects of freezing pretreatment (FP) on drying behavior, collapse, and rheological properties. Compared to control (C) samples dried at 65 °C with a reduced relative humidity, FP significantly enhanced the drying rate, particularly below the fiber saturation point (FSP), with an 8.7 % increase. FP also reduced tangential and radial shrinkage at FSP by 55 % and 52 %, respectively, suppressing collapse effectively. Rheological tests indicated that the FP wood exhibited lower elastic strain, viscoelastic creep strain, and mechano-sorptive creep strain throughout the drying process, along with a significant reduction in the residual drying stress. These improvements are primarily attributed to the microstructural changes induced by the FP, which reduce cell collapse and thus alleviate the development of internal stresses. This study confirms that the FP is an efficient pretreatment method that can significantly reduce drying defects and enhance the final product quality of eucalyptus wood.