CHEMICAL PULP BEATING RELATED TO FIBER STRUCTURE

The influence of fiber structure and chemical composition was studied using bleached kraft pulps from southern pine, silver birch, Eucalyptus grandis, and Eucalyptus rostrata.;The response of the pulps to beating was correlated with structural parameters such as fiber cross sectional dimensions, and thickness and microfibrillar angles of the individual cell wall layers. The ultrastructural, geometrical and chemical data allowed the estimation of fiber flexibility using a two-dimensional mechanical analysis of the cell wall. The results of such analysis obtained for the fibers of the four species could explain their conformability in the sheet structure and provided a better understanding of the beatabilities of the corresponding pulps.;The advancement of beating in the PFI mill with two loads for fines-free and whole pulps was monitored by changes in specific surface, specific volume, fines content, the dynamic drainage rates, wet web strength, and paper properties of the four pulps. General patterns were observed for the changes in the internal and external structure of the fibers with beating energy. The main effect of beating was shown to be the promotion of internal wall delamination, which depends on the fiber's ultrastructure and chemical composition. At a macroscopic level the extent of fiber interaction and floc formation were also identified as important phenomena additionally responsible for the behavior of different pulps during beating, and should deserve further study.;It was concluded that the controlling factors of the response of a pulp to beating are the degree of association of microfibrils, their width and number (as indicated by the packing density of the cell wall) and the fibrillar angles of the S(,1) and S(,2) layers. These characteristics could explain well why the fibers from the two eucalypt species and of southern pine, with steep microfibrillar helices and high cell wall density require more energy to beat to a desired wet web or dry paper strength than birch fibers.