Surface chemical and morphological properties of mechanical pulp fines

Different types of fines, i.e. fibrils and flakes, were separated from thermomechanical pulp (IMP) fibers and their surface chemical and morphological properties were studied and compared to those of fibers. Fines contained more extractives and lignin than fibers, both on their surface and in the bulk. Fibrillar fines were especially rich in extractives and lignin, the latter indicating that they originated from primary wall rather than from secondary wall. Flakes had large amounts of lignin on their surfaces. Fibers contained more cellulose than did fines, with 50% of their surface covered with polysaccharides. The most common extractives on the surfaces of fibers and fines were fatty acids, probably present mainly as triglycerides, and sterols and steryl esters. Fines and fibers differed in their surface morphology. Fibrillar fines were largely covered with two different types of material, interpreted as being lignin and extractives. The surfaces of flake-like fines proved to be mainly covered with granular lignin and cellulose fibrils. On the fiber surfaces, areas with different microfibrillar orientations were found. In some areas the orientation was random as in the primary cell wall layer and in some areas the orientation was parallel to the fiber axis, indicating that S ₂ had been exposed during refining.