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 spruce (Picea sitchensis Carr.) wood impregnated with extractives of pernambuco (Guilandina echinata Spreng. syn Caesalpinia echinata Lam.), we examined the vibrational properties of the impregnated wood in relation to the adsorbed water. The results obtained were as follows: (1) The equilibrium moisture content (EMC) of impregnated sitka spruce decreased to some extent compared with untreated wood. (2) Frequency dependencies of tan delta at about 400-8000Hz showed that impregnated wood has much lower tan delta than untreated wood at around 9% moisture content (MC), except for the high-frequency region. At high relative humidity, impregnated wood has a minimum tan delta (at around 4000 Hz) and at other frequency ranges the tan delta of impregnated wood did not differ considerably from that of untreated wood. (3) The apparent activation energy of the mechanical relaxation process (Delta E) concerned with adsorbed water molecules was higher for impregnated specimens than for untreated ones at moderately high relative humidity, whereas at high relative humidity the difference was not observed. Based on these results, it is thought that the tan 6 of impregnated wood decreased at low relative humidity because of the formation of direct hydrogen bonds between impregnated extractives and wood components. However. when the specimen is at higher relative humidity, the formation of direct hydrogen bonds are disturbed by the existence of a large number of water molecules, and some extractives act as a plasticizer.