Working mechanism of adsorbed water on the vibrational properties of wood impregnated with extractives of pernambuco (Guilandina echinata Spreng.)

To clarify the lowering mechanism of loss tangent (tan delta) of sitka spru ce (Picea sitchensis Carr.) wood impregnated with extractives of pernambuco (Guilandina echinata Spreng. syn Caesalpinia echinata Lam.), we examined t he vibrational properties of the impregnated wood in relation to the adsorb ed water. The results obtained were as follows: (1) The equilibrium moistur e content (EMC) of impregnated sitka spruce decreased to some extent compar ed with untreated wood. (2) Frequency dependencies of tan delta at about 40 0-8000Hz showed that impregnated wood has much lower tan delta than untreat ed wood at around 9% moisture content (MC), except for the high-frequency r egion. At high relative humidity, impregnated wood has a minimum tan delta (at around 4000 Hz) and at other frequency ranges the tan delta of impregna ted wood did not differ considerably from that of untreated wood. (3) The a pparent activation energy of the mechanical relaxation process (Delta E) co ncerned with adsorbed water molecules was higher for impregnated specimens than for untreated ones at moderately high relative humidity, whereas at hi gh relative humidity the difference was not observed. Based on these result s, it is thought that the tan 6 of impregnated wood decreased at low relati ve humidity because of the formation of direct hydrogen bonds between impre gnated extractives and wood components. However. when the specimen is at hi gher relative humidity, the formation of direct hydrogen bonds are disturbe d by the existence of a large number of water molecules, and some extractiv es act as a plasticizer.