The influence of moisture on the stiffness of paper in terms of network mechanics

The variation with humidity of the longitudinal and transverse moduli of isolated pulp fibres has been evaluated using micromechanics. The decrease in the moduli of the fibres is assumed to be due to a softening of the hemicellulose phase. Using a network theory developed by Perkins and coworkers these moduli have been used to calculate the tensile modulus of isotropic low density paper sheets in the range of 0–95% relative humidity (RH). Comparison with experimental data on low density sheets reveals a satisfactory agreement up to about 75% RH. Above this value the stiffness of the sheets decreases at a rate faster than that predicted by the theory. This may be due to a gradual loss in band stiffness and/or a softening of disordered cellulosic material at high humidities. As predicted by the network theory, the difference in behaviour between sheets of different densities is not very marked in the RH-range.