Ultrastructural organisation of the wood cell wall can explain modifications caused in fibers during the pulping process

Abstract

Pulp fibers ought their technical properties to their origin as wood fibers, consisting of a polymer composite whose architecture largely relies on the organization of cellulose microfibrils, and to the treatments they underwent during pulping process. The alterations occurring in the wood fiber during pulping and subsequent treatments were investigated at ultrastructural scale, in transmission electron microscopy. Different types of virgin pulps, from stone ground to chemical pulps, were investigated. Characteristic morphological features related to the type of pulping process could be distinguished. The delamination and micro-fibrillation patterns brought about in CTMP and kraft pulp fibers may be explained by the lamellar organization of the native wood fiber secondary wall and by the susceptibility of the matrix polymers to mechanical and chemical treatments. The implication of hemicelluloses in the aggregation of cellulose microfibrils and their behaviour during pulping were visualized by immuno-gold labelling of xylans. In most pulps, drying induced irreversible effects such as partial re-association of internal microfibrils, imparting more compactness, but also more stiffness to the fiber walls. Another typical effect of drying is the loss of individualized microfibrils at the fiber wall surface, which modifies inter-fiber bonding. Drying, which causes hornification, also decreases the amount of fines. These various effects of pulping and drying may be explained by the ultrastructural organization of the wall matrix polymers around the cellulose microfibrils.