Thermal-Softening Properties of Water-Swollen Wood VI. The change of Thermal-Softening Properties due to lignification with moso bamboo as a model material

The temperature dependencies of dynamic elastic modulus (E'), loss modulus (E "), loss tangent (tan delta) and nominal strain (epsilon) in tangential and longitudinal directions for juvenile green moso bamboo (Phyllostachys h eterocycla) were investigated in the temperature range from 5 degrees C to 100 degrees C, The values of E', E " and a decreased remarkably in 50 simil ar to 65 degrees C and a sharp peak of tan delta was observed at about 60 d egrees C in unlignified specimens. However, these remarkable changes were n ot observed in the specimen experienced the heating history at 100 degrees C. The temperature range in which these values remarkably changed hardly va ried with measurement frequency. The amounts of these changes in E' and eps ilon of 50 similar to 65 degrees C was reduced when specimens were soaked i n water at 4 degrees C for 20 days, and these changes were not observed in specimens soaked in water at 4 degrees C for 40 days. This phenomenon was o bserved in tangential and longitudinal directions. The amount of this chang e in E' and epsilon decreased and this phenomenon was observed at lower tem perature in slightly lignified specimens. These changes of E' and epsilon w ere not recognized in further lignified specimens. To explain these results, we postulate a temporal fixation mechanism that a large amount of strain caused by exceedingly fast growth of the moso bambo o is fixed by some bonding. This temporal fixation should be released by he ating history or by long term soaking, and should be tightened permanently by lignification.