Bulk and surface composition of ECF bleached hardwood kraft pulp fibres

ECF (DEDED)-bleached kraft pulps were prepared from five hardwoods: Acacia mangium (Indonesia), Betula pendula (Scandinavia), Eucalyptus globulus (Portugal), Eucalyptus urograndis (Brazil) and Eucalyptus grandis (Brazil). The bleached pulps were investigated by detailed bulk chemical analysis including lignin content, carboxyl groups content and composition of carbohydrates and lipophilic extractives. The five pulps showed remarkable differences in xylan/cellulose ratio, content of carboxyl groups and, particularly, in the amount and composition of lipophilic extractives. Such differences were assigned to the different wood compositions and to the diverse pulping and bleaching conditions required to fully bleach the pulps. The surface composition of pulps fibres was analysed by X-ray Photoelectron Spectroscopy (XPS) and by Time-of-Flight Secondary Ions Mass Spectrometry (ToF-SIMS). The XPS analysis showed that the extractives content of the surface is much higher than in the inner parts of fibres. The Acacia pulp had a surface coverage of extractives around five times higher than the other pulps investigated. The ToF-SIMS spectra showed that fatty acids are the dominant extractives of the surface of Acacia fibres while sterols dominate at the surface of Eucalyptus fibres. The influence of the diverse compositions on the behaviour of fibres in papermaking is discussed.