Frictional properties of native and chemically modified birch wood

The chemithermomechanical pulping of hardwoods has attracted increasing interest in the past decade. This study investigates the frictional behavior of both native and chemically treated birch under simulated chip-refining conditions (hot saturated steam, high sliding speed). Chip pretreatment, i.e. chemical impregnation and pre-heating done when birch chemithermomechanical pulp is produced, affects energy consumption during refining; the study found that frictional forces may be an important parameter in this respect. The coefficient of friction peaks at approximately 115°C for both native and extracted birch. It was shown that sulfonating native birch decreased its coefficient of friction over the temperature range investigated (100�170°C). The coefficient of friction had a broad and flat peak in the 130�150°C temperature interval for the specimens treated with 3% sodium sulfite at pH 13.5. Furthermore, it was clear that pH had a profound influence on the coefficient of friction. A high pH lowered the coefficient of friction, likely due to well lubricated contacts. The friction arising between birch (Betula verrucosa) and steel was shown to be higher than that between spruce (Picea abies) and steel. The higher stiffness and density of the birch wood, together with a more porous bulk structure leading to rough test surfaces, are some factors possibly accounting for these high friction coefficients. Also, the differences in the amount and composition of the extractive substances in birch and spruce may contribute to the differing frictional behaviors of these species.