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.

The fluidity of wood remarkably increases during moisture changes. This phenomenon is termed mechano-sorptive creep. The mechanism of mechano-sorptive creep has been studied, including a previous report by our group. Here, creep tests in bending were carried out for wood during and immediately after adsorption of moisture and after a long moisture conditioning. The effects of the rate of moisture adsorption on creep were also examined. The results and conclusions are as follows: (I) Greater creep occurred immediately after the adsorption process as compared with that after a long moisture conditioning, whereas much greater creep occurred during the same adsorption process, similar to the case of drying. Therefore, during the changes in moisture, not only destabilization but also stabilization should occur simultaneously, so that the wood during the changing process is in a remarkably unstable state. (II) Smaller creep occurred immediately after a slower adsorption as compared with that immediately after a more rapid adsorption. This suggests that wood is more stabilized during a slower versus a more rapid adsorption process. However, difference in creep between the final stages of the slower and the more rapid adsorption process was scarcely found. This is considered to result from the difference in degree of stabilization caused by the different duration of both adsorptions. In other words, this result is only an outward appearance. (III) Greater creeps were recognized during larger changes in moisture content (Δu) during the adsorption processes corresponding to the drying process. Therefore, mechano-sorptive creep depends not only on Δu but also on the range of relative humidity (RH). Larger stabilization was found during the changing process of larger Δu.

SynopsisThe adsorption of 12 different nonionic water-soluble polymers, methylcellulose (MC), hydroxypropylcellulose (HPC), polygalactomannan (PGM), polyglycidol (PGD), polyacrylamide (PAAm), methylolated polyacrylamide (MPAAm), polymethacrylamide (PMAAm), three differently hydrolyzed poly(viny1 acetate)s (PVAs), poly(viny1) methyl ether) (PVME), and poly(viny1 pyrrolidone) (PVP) onto bleached kraft pulp (BK), unbleached kraft pulp (UK), and a groundwood pulp (MP) has been investigated under aqueous conditions with attention to the functional groups in the polymers and the chemical character of the pulps. It was found that the adsorption was often selective toward one of the pulps. In order to gain some additional information on these interactions, some adsorption experiments were also performed on acetylated pulps. Some polymers (PGD, PAAm, MPAAm, and PVA 124) having both proton donating and accepting capability were not adsorbed onto any of the pulps. This was attributed to intramolecular hydrogen bonding in these polymers. Some polymers (PVA 224, PVME, and PVP) were strongly adsorbed onto UK but not at all onto BK. It was suggested that in these cases the adsorptive interaction was between phenolic and/or catecholic groups in the lignin on the UK and proton accepting sites on the polymers. However, for three polymers (MC, PGM, and PMAAm), a more general type of hydrogen bonding interaction was considered. For some polymers (HPC, PMAAm, and PVA 420) having hydrophobic groups, it was suggested that hydrophobic interactions were important for the adsorption.

To better understand mechano-sorptive creep, creep behaviors were compared in wood samples during the drying process, immediately after drying, and after a long conditioning under constant humidity and temperature. Creep was greater in the sample tested immediately after drying than in the sample conditioned for a long time under relative humidity equal to that after drying, despite the fact that these samples had almost the same moisture content (MC). While the wood that has been moisture-conditioned for a long time is in a stable state, the wood tested immediately after the drying is presumed to be in an unstable state. Moreover, creep of the sample tested during the drying process was greater than that of the sample tested immediately after the drying. It has also been found that the instability decreased with time, indicating that stabilization and destabilization occur simultaneously during the drying process. In recent studies, a decrease in the elastic modulus and an increase in the fluidity of wood immediately after a change in MC or temperature have been reported. These findings are attributed to the instability caused by changes in MC or temperature. Based on the results of the present study and recent studies, we consider the increase in the fluidity of wood as the MC changes to be attributable to instability.

Abstrael Some properties of wood (hinoki: Charnaecyparis obtusa) moisture-conditioned by an adsorption process from a dry state and by two desorption processes (from a water-saturated state and from a state with a moisture content slightly below the fiber saturation point) were investigated. The moisture contents of wood conditioned by the adsorption process and by the desorption process continued to approach to one another for the moisture-conditioning period of over 50 weeks. Accordingly, sorption hysteresis should be regarded as a transitional phenomenon that occurs during the process of approaching the true equilibrium, which requires a long time. The wood conditioned by the desorption process beginning from a water-saturated state showed slightly smaller dimensions than those conditioned by the adsorption process with the same moisture content; however, the wood conditioned by the desorption process from a moisture content below the fiber saturation point showed slightly larger dimensions than those conditioned by the adsorption process. The wood conditioned by the adsorption process from a dry state showed a higher modulus of elasticity and modulus of rupture than did the wood conditioned from a water-saturated state with the same moisture content. The mechanical properties of the wood also varied based on the states at which the desorption process was started. This is a notable characteristic of the relation between the drying condition and the mechanical properties of wood.