Chemical characterization of various types of mechanical pulp fines

Extracted thermomechanical pulp from spruce was fractionated into microfines, fibrils, flakes, ray cells and fibres using centrifugation, sedimentation and filtration steps. The chemical composition and the amount of anionic groups were determined. The various types of fines differed greatly in chemical composition. The content of cellulose was higher in the fibrils and the microfines than in the flakes and the ray cells, whereas the content of lignin was lower. The flakes and the ray cells contained much arabinogalactans, xylans and pectins, but only little galactoglucomannans. The fibrils contained much arabinogalactans and pectins. The increase in the amount of anionic groups during alkaline treatment was to a great extent due to the demethylation of pectins, corroborated by the amount of methanol released. It can be suggested that the fibrils originate mainly from the primary wall and the outer secondary wall. The flakes originate primarily from the middle lamella and the primary wall.