Optimization of Polyethylene Glycol Impregnation in Two Stages for Artificially Aged Waterlogged Wood and Performance Evaluation of the Treated Wood After Freeze Drying

Considering differences in the penetration and retention of polyethylene glycol with different molecular weights in waterlogged wood structures at different scales, artificially aged waterlogged wood served as the model material. A Box–Behnken response surface design was employed, using low-molecular-weight PEG with molecular weights of 400–800 and solution mass fractions of 29–41 wt.% and high-molecular-weight PEG with molecular weights of 4000–8000 and solution mass fractions of 28–44 wt.% as the independent variables, with volume shrinkage after freeze drying as the response variable for optimization. The quadratic model showed good fit and predictive reliability, with significant interactions between molecular weight and solution mass fraction in both ranges. The optimal two-stage treatment was 35.7 wt.% polyethylene glycol 600 followed by 37.5 wt.% polyethylene glycol 6000, with predicted and experimental volume shrinkage values of 2.34% and 2.43%, respectively. Polyethylene glycol 600 was distributed mainly in the cell wall regions, whereas polyethylene glycol 6000 was distributed in both the cell wall and some cell lumen regions, with relatively pronounced enrichment in some lumina; after two-stage treatment, polyethylene glycol exhibited a more uniform spatial distribution within the wood cellular structure. The optimized treatment achieved an anti-shrink efficiency of 82.91% relative to the water control group, a moisture uptake of 14.3% after 230 h at 85% relative humidity, and a bending strength of 12.97 MPa, 67.79% higher than the water control. Overall, the treatment improved dimensional stability after freeze drying while maintaining favorable moisture stability and relative bending performance.