Surface lignin and extractives on hardwood RDH kraft pulp chemically characterized by ToF-SIMS

Abstract

Abstract The chemical characteristics of lignin and extractives on the outermost surface of hardwood rapid displacement heating (RDH) kraft pulp were studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Changes in these surface compounds induced by totally chlorine-free (TCF) bleaching were also revealed. The results suggest that the surface lignin remaining on unbleached RDH pulp was remnant from native wood lignin, which was never released from wood fibers, but had undergone structural changes during kraft cooking. Less than half of the surface lignin was removed during bleaching, with the oxygen stage and the final peroxide stage being the most effective. The main wood extractives on the hardwood RDH kraft pulp surface were sterols and steryl esters. TCF bleaching strongly reduced their abundance. Bleaching also removed fatty acids, but less effectively than neutral extractives. Fatty acid sodium salts were partly removed during TCF bleaching, in contrast to fatty acid calcium salts, which were very stable. ToF-SIMS has the unique capability to give valuable chemical information about the complex mixture of compounds on the pulp surfaces. Thus, it complements other surface-sensitive analytical techniques such as electron spectroscopy for chemical analysis (ESCA), atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). ToF-SIMS can give detailed chemical information about the effects of process conditions on pulp surface characteristics.