Characteristics of out-of-plane rheological behaviour of paper

The dynamic strain behaviour of paper under z-directional compressive forces was investigated using a novel custom made testing rig. Laboratory hand sheets with same basis weights but different raw material composition were used. The plastic, elastic and creep components of strain caused by increasing stress and dwell time were determined and analysed. The results show that the logarithmic strain varied non-linearly with increasing stress level. The plastic deformation was causing the increase of the logarithmic strain. The increasing dwell time at constant stress caused also non-linear increase in strain. This was due to both increase in plastic component and decrease in elastic component. The results revealed also significant differences between mechanical and chemical pulp samples in both time and stress-dependent behaviour of strain probably due to the difference in the network structure. Large deformations occurred already in short time-scales.