Chemical characterization of mechanical pulp fines and fiber surface layers

The chemical compositions of TMP fines and material isolated from the surface layers of TMP long fibers were compared with that of the inner layers (i.e. bulk) of the long fibers. Both fines and fiber surface layers contained more extractives, lignin and pectins than the bulk fibers. The fiber surface layer material consisted mainly of fibrils, which were concluded to originate from the primary cell wall layer. These fibrils contained more cellulose and mannan and less xylan than the ray cell-rich fines fraction. The frequency of methylglucuronic acid groups in the primary wall fibril xylan was found to be about 30% lower than in the bulk fiber xylan. The outermost surfaces of the primary wall fibrils were very rich in extractives, lignin and protein. The lignin on the bulk fiber surfaces contained about 50% more guaiacyl units than lignin on the surface of the primary wall fibrils or of fines. The composition of extractives on the surface of bulk fibers was also different from that on the surface of fines and the primary wall fibrils. Both the fines and the primary wall fibrils were found to possess greater ability to contribute to surface charge, if treated with alkali, compared with the bulk fibers. Surface extractives as well as lignin were found as small patches all over the surfaces of TMP fibers and fines. Thus, the results showed differences in the distribution, structure and composition of protein, lignin, polysaccharides and wood extractives across the fiber wall.