Effects of acetylation on moisture sorption of wood under cyclically changing conditions of relative humidity

Dynamic moisture changes affect practical wood processing and wood utilization. To evaluate the effect of acetylation on dynamic moisture sorption of wood, acetylation was controlled at two levels by adjusting the reaction time. A cyclically changing condition was created where the relative humidity changed from 45 to 75% sinusoidally at 25 °C. Moisture content of wood was automatically recorded in time. The two-level acetylated wood exhibited 12.3% and 17.8% mass gain and 7.7% and 9.4% volume increase respectively. Moisture content against time for acetylated wood changed in a similar sinusoidal curve to relative humidity. Higher acetylation resulted in a greater decrease in dynamic moisture sorption as indicated by over 54% decrease in moisture content, about 26% decrease in amplitude and about 23% decrease in moisture sorption coefficient. This was mainly because higher acetylation caused larger cell wall bulking of wood. More hydroxyl group substitution also reduced the dynamic interaction of water molecules and wood in a certain time. The phase lag was reduced by acetylation mainly due to the decrease in the amplitude. Higher acetylation led to lower sorption hysteresis, since the swelling of wood after higher acetylation was lower and less relaxation time was needed for the matrix during moisture sorption. Moreover, acetylation decreased hydrophobicity of wood and facilitated more evaporation of water molecules during desorption. Consequently, the sorption hysteresis decreased. The results can help understand the dynamic interactions between acetylated wood and water and better control wood acetylation according to practical processing requirements.