Effects of water-soluble extractives on the vibrational properties and color of hygrothermally treated spruce wood

Sitka spruce wood specimens were heated at 120–140 °C and different relative humidity values (RHh), and their specific dynamic Young’s moduli (E′/ρ), mechanical loss tangents (tanδ), and CIELAB color parameters were measured at 25 °C and 60% relative humidity before and after the removal of water-soluble extractives. The E′/ρ and tanδ values were significantly decreased and increased, respectively, with increases in mass loss by heating at 100% RHh. After the removal of water-soluble extractives, the decreased E′/ρ was increased and the increased tanδ was decreased. This suggested that water-soluble sugars, i.e., depolymerization products of hemicelluloses, acted as plasticizers to decrease E′/ρ and increase tanδ values of hygrothermally treated wood, particularly for wood heated in humid conditions. By heating at 60–75% RHh, the E′/ρ was slightly increased and tanδ was almost unchanged. The water-soluble extractives had little effect on the color of the hygrothermally treated wood, irrespective of RHh.