The mechanical properties of single wood pulp fibers. III. The effect of drying stress on strength



An experimental study has been made of the effect of drying tension on the tensile strength of single fibers from several wood species pulped in various ways. Some pulps are strengthened by drying stress, whereas others show no response. These apparently contradictory results are explained in terms of the effect of drying stress on the morphologic features responsible for strength. In part I of this series, it was shown that fiber strength is controlled by the fibril angle of the S2 layer and the extent and severity of defects. These defects are mostly the local disturbances of fibril alignment termed “dislocations”, “slip-planes”, or “microcompressions”. It appears that drying stress strengthens fibers both by reducing the fibril angle and by aligning the fibrils in the dislocated regions. This is only possible, however, if the matrix between the fibrils can flow in shear under the applied stress. Thus, strengthening is possible for fibers of high hemicellulose content, the matrix of which flows readily in the water swollen state. Strengthening does not occur, however, for predried kraft fibers or alkali-extracted holocellulose, for which the matrix is well bonded and insufficiently swollen.