Compressive-molding of wood by high-pressure steam-treatment: Part 2. Mechanism of permanent fixation

Permanent fixation of the compressively transformed shape of Harigiri (Kalopanax pictus Nakai) specimens was attempted by steaming them, compressively transforming their shapes, and processing them with high-pressure steam again. Also, the mechanism of the shape fixation was examined. The results are summarized as follows: (1) Compressively transformed Harigiri specimens, after being processed with high-pressure steam, did not recover to their original shape with heat and moisture, thus achieving permanent fixation of compressed shape. (2) The conditions under which the fixed specimens would not recover to their original shape were found to be steaming with saturated steam at 200 degrees C for 4 min or longer or at 180 degrees C for 8 min or longer, within the scope of the current experiment. (3) Hemicellulose and lignin do not affect the fixation of compressive transformation. (4) The fixation of compressive transformation is caused by a structural change of cellulose. (5) The mechanism of fixation of compressive transformation is supposed to be that the inner stress is released because the paracrystalline region of cellulose, which is distorted by compressive transformation, is partially hydrolyzed. Further, steam-rearrangement of hydrolyzed constituent into crystalline region occurs, keeping the transformed shape intact.