Probing the chemical and surface chemical modification of vessel cell walls during bleaching of eucalyptus pulp

Abstract Eucalyptus pulp is increasingly used for the manufacture of printing papers. However, its major drawback, the vessel picking, is still waiting for a solution. The detailed features and the characteristic behaviour of vessel elements (VEs) in pulp and paper processes are poorly understood. This study focusses on the chemistry, surface chemistry and morphology of eucalyptus VEs. These properties were followed through the changes introduced by different stages of elemental chlorine-free (ECF) bleaching, by studying separated VEs. Microprobe X-ray photoelectron spectroscopy (μ-XPS) and field emission-scanning electron microscopy (FE-SEM) were applied to elucidate the surface chemical composition and morphological ultrastructure, respectively. The bulk chemical composition was investigated among others by Py-GC/MS. Lignin was detected in vessels still after completed bleaching sequence, whereas the fibres were lignin-free. The vessel lignin was mainly composed by syringyl-type units. Surface coverage by lignin and amount of surface anionic groups were practically unaffected by bleaching. The vessel cell wall structure was observed to be layered in a complex way with no particular orientation of cellulose fibrils, and the different layers seemed to be exfoliated during different bleaching stages.

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