広葉樹 (ダケカンバ) 蒸解における木材細胞壁の細孔構造と溶出リグニンの分子量特性

SO₂+MgSO₄二成分蒸解法に関する研究 (第8報)

抄録

In this paper, delignification mechanism in acid sulfite processes, i. e., SO₂ MgSO₄, bicomponent and Mg base sulfite, has been discussed in some detail by comparing cooking results of birch wood with those of spruce wood.
A birch wood (40-80 mesh meal extracted with ethanol-benzene) was digested by t wo liquors under the same conditions as those of cooking of spruce wood previously conducted. Differences in the rate of delignification and in the selectivity of delignification reaction were found between two systems and they were investigated in terms of changes of development of porous structure in wood cell wall, and molecular weight distribution and average molecular weight. (Mn) of dissolved lignin during cooking.
In an initial stage of cooking up to 35% delignification, the rate of delignification was faster in Mg-base than in the bicomponent, but there was no difference in the selectivity of delignification between them, showing that delignification proceeded in proportion to dissolution of carbohydrates. From the results that they were almost equal in the value of Mn, this would he explained by the situation that pore width of cell wall which increased linearly with dissolution of carbohydrates played a role of controlling the size of soluble lignin fragment. And it confirms the validity of the earlier kinetic studies, similarly in the case of the previous digestion of spruce wood.
In higher degree of delignification, development of porous structure of cell wall appeared to be based on the behavior of dissolution of carbohydrates rather than the amount of it, since a marked difference in an increase of median pore width (MPW) of the digested meal was observed between two systems. The pore width was thought to act no longer as a barrier of dissolution of lignin fragment in Mg-base, while in the bicomponent, the relation between the pore width and the size of dissolving lignin molecule was considered to be still closely. The tendency of an increase of MPW could be a good measure representing the extent of collapse not only of cell wall tissue but also of lignin gel network, and therefore, it would be explicable for the different rate of sulfonation and delignification between two liquors.
As the cell wall tissue for hardwood is denser than that for softwood, it is speculated that the tissue of hardwood must be destroyed more steadily and highly to let the delignification reaction advance smoothly. It should be accompanied by an excess degradation of soluble lignin fragment as well as lignin gel network. This point of view would be accepted as an interpretation of the result that MPW is much high, but Mn is low for birch wood, in comparison of those for spruce wood.