AFM nanoindentation of pulp fibers and thin cellulose films at varying relative humidity

In papermaking, the formation of bonds between single pulp fibers is influenced by the hardness of the fibers in their wet state. In this work, transversal hardness and modulus of pulp fibers have been studied via atomic force microscopy-based nanoindentation in dependence on relative humidity (RH). Additionally, the change in hardness of cellulose and xylan/cellulose model films was also investigated as a function of swelling in the presence of water and calcium chloride (CaCl2) solution. The hardness of pulp fibers is decreasing slowly from 240 MPa at 5% RH to 90 MPa at 80% RH and exhibits a distinct decrease to 2.7 MPa at the fully wet state. The hardness in water is reduced by a factor of almost 100 compared with the dry state; therefore, a form change is easily possible and facilitates the formation of hydrogen bonds on the fiber surfaces. The investigations on the model films reveal that pure cellulose hardens in the CaCl2 solution, compared with distilled water, whereas xylan on cellulose is becoming softer