Conformability of wood fiber surface determined by AFM indentation

Mechanical properties of wet fiber surfaces of hardwood chemi-thermo-mechanical pulp and softwood kraft pulp were measured using an AFM as a nanoindetation device. Elastic modulus of wet fiber surfaces was observed in the range of 30 MPa to 1.6 GPa, which is significantly affected by chemical treatment of pulping and bleaching as well as mechanical refining. Plastic yield stress of wet fiber surfaces measured varies from a few MPa to about 100 MPa, and is also highly correlated to chemical and mechanical treatments of these fibers. Plasticity index shows no difference among once-dried hardwood and softwood pulp fibers, but it increases after refining. The dynamic responses of wet fiber surfaces to external loads were evaluated through creep analysis. Even creeps were observed on all wet fiber surfaces, deformation caused by creep is relatively small. The major portion of deformation still comes from instantaneous elastic and plastic deformations. Creep of wet fiber surfaces is a fast dynamic process when subjected to a load change, most of it occurs within several seconds after the initial contact.