Comparison of dynamic sorption and hygroexpansion of wood by different cyclic hygrothermal changing effects II

Abstract

Dynamic wood sorption and hygroexpansion by different cyclic hygrothermal changing effects were investigated. Poplar ( populus euramericana Cv.) specimens, 20 mm in radial (R) and tangential (T) directions with two thicknesses of 4 and 10 mm along the grain, were subjected to three rounds under control of these two factors. In the first round, relative humidity changed sinusoidally between 75% and 45% with temperature constant at 20°C. In the second round, temperature changed sinusoidally between 5°C and 35°C with relative humidity constant at 60%. In the third round, relative humidity changed sinusoidally between 75% and 45%, simultaneously temperature changed between 5°C and 35°C. In each round, three sinusoidal change cyclic periods were operated: 1, 6, and 24 h. The measurement of moisture and dimensional responses showed the following results: (1) moisture and dimensional changes were both presented generally in sinusoidal mode. Their amplitudes were lower for thicker specimens exposed to shorter cyclic period. (2) Comparing with the effect brought by changing a single parameter, that is, changing relative humidity or temperature alone, changing relative humidity and temperature together brought greater impact on the specimens. Under a typical air-dry condition at 20°C and 60% relative humidity, this comparison was more obvious, especially for dimensional changes. (3) When the amplitudes resulted in changing relative humidity and changing temperature were superposed, its value was higher than which brought by changing relative humidity and temperature together with the maximum ratio of 1.48. (4) Moisture-content change (ΔMC) and variation of T-dimensional change (ΔT) were all linearly correlated with treating time. Less time was needed to reach a given ΔMC or ΔT, namely, larger ΔMC or ΔT could be attained within the same time by changing relative humidity and temperature together, where the greatest average rate of wood sorption and hygroexpansion could be observed. All these results indicated that in practice, sinusoidally controlling the changes in relative humidity and temperature together was most efficient to adjust the moisture content and deformation in wood processing and application.