Mechanical fiber separation under torsional stress

Summary Spruce wood (Picea abies) samples were pretreated at different temperatures and, in some cases, with sodium sulfite solution under normal conditions for the chemimechanical pulping of wood chips. The pretreated samples were simultaneously subjected to torsion and compression stresses at temperatures ranging from 20 to 90 °C. The load-deformation relationship was analyzed at each temperature. The failure zones were studied using the scanning electron microscope technique. The results showed that the maximum torque decreased with increasing temperature under the deformation conditions applied. Sulfonation of the samples gave a similar effect, although to a smaller extent. The elastoplasticity of the samples, as viewed in terms of the twist angle at failure, was also affected by the pretreatment. While axial compression had a major effect, sulfonation only caused a marginal change. Microscopic studies of the failure zone showed that when the temperature increased, the fracture plane traveled around fibers instead of through them, thus causing less fiber damage. Sulfonation improved fiber separation and decreased fiber damage. The separation of ray cells from the fiber tracheids was improved, particularly by sulfonation. Increasing temperature and sulfonation changed the fracture plane from the secondary wall of fibers towards the more lignin enriched primary wall and middle lamella. Received 29 January 1997