To investigate micropores and mesopores in the cell walls of dry wood, CO 2 gas and N 2 gas adsorption onto dry wood were measured at ice-water temperature (273 K) and liquid nitrogen temperature (77 K). CO 2 gas adsorption isotherms obtained were used for determining micropore volumes smaller than 0.6 nm by the HK method (Horvath-Kawazoe method), and N 2 gas adsorption isotherms obtained were used for determining the mesopore volume between 2 nm and 50 nm by the Barrett-Joyner-Halenda (BJH) method. Micropores and mesopores existed in cell walls of dry wood, and the cumulative pore volume was much larger for micropores than for mesopores. Micropores in the cell wall of dry wood decreased with elevating heat treatment temperature, and the decreased micropore was reproducible by wetting and drying. Mesopores did not decrease so much with elevating heat treatment temperature. Micropore volumes for the softwood Hinoki and the hardwood Buna were compared. A larger amount of micropores existed in hardwood Buna than in softwood Hinoki, and this relationship was considered to correspond to the difference in thermal softening properties for lignin in water-swollen Hinoki and Buna. This result probably indicates that micropores in the cell walls of dry wood relate to the structure of lignin.

To clarify effects of lignin on the thermal-softening properties of water-swollen wood, dynamic viscoelasticities of water-swollen wood were measured in the temperature range from 5 to 100. The results obtained were as follows. 1) Delignified wood and moso bamboo did not show thermal softening around 60~80 which was found for untreated wood and moso bamboo. From this result, it was found that the thermal softening of lingo-cellulosic materials around 60~80 were attributable to lignin. Decrease in storage elastic moduli (E') of wood and moso bamboo from 20 to 100 remarkably decreased with decreasing in their lignin contents. From this result, it can be concluded that the degree of thermal softening around 60~80 largely depends on lignin contents. 2) The thermal-softening temperatures were different among many kind of wood species (softwood, Japanese hardwood, Tropical hardwood) and between an untreated katsura (Cercidiphyllum japonicum) sample and a sample slightly delignified. Consequently, it is deduced that the thermal-softening behaviors of water-swollen wood were largely affected by cross-linking of lignin. 3) The thermal softening behaviors of katsura were quite different among specimens experienced cooling with different rates. This suggests that the thermal-softening behaviors of water-swollen wood largely depend on the conformation of lignin.

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To clarify the effects of lignin on the fi xation of bending deformation by cooling, cooling set for delignifi ed woods with various lignin residues were investigated to compare with mechanical and dynamic viscoelastic properties. Bending tests showed that steep reductions occurred in the modulus of elasticity and modulus of rupture with delignifi cation during the initial stage of delignifi cation. The dynamic viscoelastic measurements revealed that the peak temperature of tan d due to micro-Brownian motion of lignin was reduced with delignifi cation, and the peak disappeared in the temperature range of 5°-100°C for the specimens that had lost more than 21% of their weight. On the other hand, no clear change in residual set was found in the range of 0%-15% of weight loss in spite of a marked reduction in lignin content. Subsequently, set decreased steeply for the specimens delignifi ed beyond 15% of weight loss. It was suggested that cooling set is not determined solely b y lignin content but is infl uenced by changes in the quality of lignin due to delignifi cation. Lignin quality affects the balance of the elastic potential to recover from deformation and its viscosity, which is an indication of resistance against fl ow.

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