The effects of surface extractives and lignin on the surface energy of eucalypt kraft pulp fibres

Abstract

This study focuses on the surface chemical composition, the surface energy, and the acid-base characteristics of lignin, cellulose, and eucalypt kraft pulps, and on the effects of changes in the surface chemical compositions on the dispersive components of the surface energy and acid-base characteristics of eucalypt pulp fibres. Surface chemical compositions, the dispersive component of the surface energy, and acid-base characteristics of Eucalyptus regnans soda lignin, cellulose, and bleached and unbleached eucalypt kraft pulps were investigated using X-ray photoelectron spectroscopy (XPS), inverse gas chromatography (IGC), and the Wilhelmy method. The dispersive component of lignin was found to be lower than that of cellulose. Measurements of the acid-base interactions of cellulose and lignin with acidic and basic probes showed that whereas cellulose is predominantly acidic, lignin is more evenly bipolar. XPS measurements of unbleached eucalypt kraft pulps indicated that there was a concentration of lignin and extractives at the surface. The presence of these materials on the fibre surface has the general effect of decreasing the surface energy of the fibres. Soxhlet extraction was employed to remove the extractives from the surface of the fibres so that the effect of the extractives and lignin on the surface energetics could be studied separately. Good correlations were found in most cases between the fibre surface energies obtained by IGC and single-fibre/water contact angles obtained by the Wilhelmy method. However, in one case, this correlation broke down; this was attributed to the high sensitivity of IGC to high-energy sites on the pulp surface.