Fiber swelling favoured by lignin softening

Abstract

The effect of temperature on the uptake of water by various pulps has been studied at temperatures from 20 to 95°C. In the case of lignin-containing pulps, i.e. pulps which have been chlorite-delignified to various degrees, the swelling increases rapidly above a temperature of 60 to 75°C. For a CTMP where the softening temperature of the lignin has been lowered, a maximum in swelling is obtained at 50 to 60°C. When no lignin is present, as in cotton linters and holocellulose, the temperature has no effect on the swelling but the level of swelling in the holocellulose pulp is very high. It is suggested that the increase in swelling above 60 to 75°C is due to the softening of the lignin, the flow of which makes swelling possible. Below its softening temperature, the stiff lignin restricts the swelling which is primarily promoted by water uptake in the hemicelluloses. Some implications for pulping processes are discussed.