The thermal softening of isolated samples of lignin, hemicellulose and cellulose has been investigated by observation of the thermally induced collapse of a column of powder under constant gravitational load. Softening temperatures of lignins ranged from 127-193°C. Birch xylan and pine glucomannan softened at 167° and 181’C, respectively. Sorption of water by lignin and hemicellulose caused pronounced decrease of the softening temperature-in some cases,to as low as 54°C. Softening points of both dry and moist lignins or hemicelluloses have been shown to correlate with the temperature at which the sample develops adhesive properties. The softening and adhesive behaviour has been explained in terms of the concept of the glass transition for amorphous polymers. Sorbed water is considered to act as a low molecular weight diluent in plasticising the polymer chains and lowering the glass transition temperature.
Celluloses were found to soften at temperatures greater than 230°C. In contrast to lignin and hemicellulose, sorption of water by the cellulose had negligible effect on the softening temperature. This difference was probably due to the crystalline nature of cellulose and indicated that water did not plasticise individual cellulose chains at the molecular level.