Effect of hot-water extraction on the hygroscopicity, shrinkage, and swelling of Paulownia tomentosa wood

Liquid water and water vapor absorption on each separate surface and all surfaces at once, as well as the oven-dry shrinkage before and after the hot-water extraction of the wood, were examined using Korean Standards. The extractives content was examined using American Society for Testing and Materials (ASTM) standards. The highest water absorption and moisture adsorption were found in the transverse sections of both unextracted and extracted wood samples. There was no change in the equilibrium moisture content (EMC) in the unextracted vs. extracted wood samples at a relative humidity (RH) of 75%. However, the EMC in the extracted sample was higher than that in the unextracted sample at a RH of 90%. Swelling per 1% moisture content remained unchanged in the radial direction before and after extraction, while it increased in the tangential direction. The linear and volumetric shrinkage, as well as the coefficient of anisotropy, considerably increased after extraction. There was no difference in the extractive content between sapwood and heartwood. In conclusion, it was confirmed that the extractives in wood considerably affect the hygroscopicity and shrinkage of Paulownia wood.